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Phosphatidylinositolphosphate phosphatase activities and cancer

Open AccessPublished:August 24, 2015DOI:https://doi.org/10.1194/jlr.R059154
      Signaling through the phosphoinositide 3-kinase pathways mediates the actions of a plethora of hormones, growth factors, cytokines, and neurotransmitters upon their target cells following receptor occupation. Overactivation of these pathways has been implicated in a number of pathologies, in particular a range of malignancies. The tight regulation of signaling pathways necessitates the involvement of both stimulatory and terminating enzymes; inappropriate activation of a pathway can thus result from activation or inhibition of the two signaling arms. The focus of this review is to discuss, in detail, the activities of the identified families of phosphoinositide phosphatase expressed in humans, and how they regulate the levels of phosphoinositides implicated in promoting malignancy.
      A series of tightly regulated lipid kinase and phosphatase enzyme activities coordinately convert phosphatidylinositol (PtdIns) into a network of phosphorylated derivatives, collectively called phosphoinositides. These differ in the number and position of the phosphate groups on the inositol head group: three PtdIns monophosphate isomers [PtdIns(3)P, PtdIns(4)P, and PtdIns(5)P], three PtdIns bisphosphate isomers [PtdIns(3,4)P2, PtdIns(3,5)P2, and PtdIns(4,5)P2], and a single trisphosphate isomer [PtdIns(3,4,5)P3] [reviewed in (
      • Rudge S.A.
      • Wakelam M.J.
      SnapShot: lipid kinases and phosphatases.
      ,
      • Rudge S.A.
      • Wakelam M.J.
      SnapShot: lipid kinase and phosphatase reaction pathways.
      )].
      Phosphoinositide signaling is an extensively studied topic with clear evidence linking the pathway from the kinase to activation of AKT and TOR to a number of disease states. While these include malignancies, there is also strong evidence for other diseases, such as overgrowth syndromes. Most studies have focused upon the importance of the kinase pathways in regulating phosphoinositide signaling; however, all signaling pathways are subject to negative as well as positive control, pointing to a key role for inositol phospholipid phosphatases in cell and tissue regulation in health and disease. This review focuses upon this aspect of regulation, considering the importance of the many phosphatases that can act upon phosphoinositides, and we further consider the importance of each in malignancy.

      PHOSPHOINOSITIDE 3-KINASES

      In mammals, the phosphoinositide 3-kinase (PI3K) isoforms responsible for phosphorylating the 3-hydroxyl group of the inositol headgroup of PtdIns, PtdIns(4)P, and PtdIns(4,5)P2 have been divided into three classes: class I, class II, and class III.
      In response to activation of cell surface receptors, class I PI3Ks are responsible for phosphorylating PtdIns(4,5)P2 to generate PtdIns(3,4,5)P3 (
      • Hawkins P.T.
      • Stephens L.R.
      PI3K signalling in inflammation.
      ). There are four class I PI3Ks that exist as heterodimers of regulatory and catalytic subunits. The four distinct catalytic subunits, p110α, -β, -γ, and -δ, are further divided into class IA (p110α, -β, and -δ) and class IB (p110γ). Class IA PI3Ks bind the SH2 domain containing the p85 family of regulatory subunits (p85α, p85β, and p55) that bind to protein tyrosine phosphate residues in activated receptors, thus relieving p85-mediated inhibition of p110 catalytic subunits and facilitating access to PI3K lipid substrates in cell membranes. The single class IB PI3K, p110γ, can bind p84 or p101 regulatory subunits that mediate heterotrimeric G-protein βγ-subunit activation of p110γ (
      • Stephens L.R.
      • Eguinoa A.
      • Erdjument-Bromage H.
      • Lui M.
      • Cooke F.
      • Coadwell J.
      • Smrcka A.S.
      • Thelen M.
      • Cadwallader K.
      • Tempst P.
      • et al.
      The G beta gamma sensitivity of a PI3K is dependent upon a tightly associated adaptor, p101.
      ,
      • Voigt P.
      • Dorner M.B.
      • Schaefer M.
      Characterization of p87PIKAP, a novel regulatory subunit of phosphoinositide 3-kinase gamma that is highly expressed in heart and interacts with PDE3B.
      ).
      The class II PI3Ks, PI3K-C2α, PI3K-C2β, and PI3K-C2γ, phosphorylate PtdIns to generate PtdIns(3)P in cells [reviewed in (
      • Maffucci T.
      • Falasca M.
      New insight into the intracellular roles of class II phosphoinositide 3-kinases.
      )]. There is some evidence that class II PI3Ks can be regulated by activated cell surface receptors, but because they do not have regulatory subunits, the mechanism is unclear (
      • Maffucci T.
      • Falasca M.
      New insight into the intracellular roles of class II phosphoinositide 3-kinases.
      ). Furthermore PI3K-C2α has been reported to phosphorylate PtdIns(4)P to generate PtdIns(3,4)P2 (
      • Posor Y.
      • Eichhorn-Gruenig M.
      • Puchkov D.
      • Schoneberg J.
      • Ullrich A.
      • Lampe A.
      • Muller R.
      • Zarbakhsh S.
      • Gulluni F.
      • Hirsch E.
      • et al.
      Spatiotemporal control of endocytosis by phosphatidylinositol-3,4-bisphosphate.
      ).
      The sole class III PI3K in mammals, Vps34, phosphorylates PtdIns in cells to generate PtdIns(3)P [reviewed in (
      • Vanhaesebroeck B.
      • Guillermet-Guibert J.
      • Graupera M.
      • Bilanges B.
      The emerging mechanisms of isoform-specific PI3K signalling.
      )]. Vps34 is regulated by at least three distinct regulatory complexes (
      • Vanhaesebroeck B.
      • Guillermet-Guibert J.
      • Graupera M.
      • Bilanges B.
      The emerging mechanisms of isoform-specific PI3K signalling.
      ).

      PI3K AND CANCER

      Gene amplification of the genes encoding all four p110 catalytic subunits have been reported in several human cancers, including ovarian, prostate, lung, thyroid, cervical, and glioblastoma [reviewed in (
      • Kok K.
      • Nock G.E.
      • Verrall E.A.G.
      • Mitchell M.P.
      • Hommes D.W.
      • Peppelenbosch M.P.
      • Vanhaesebroeck B.
      Regulation of p110delta PI 3-kinase gene expression.
      )]. Furthermore, the gene encoding p110α, PIK3CA, is frequently mutated in human cancers such as colorectal, breast, lung, and glioblastoma (
      • Samuels Y.
      • Wang Z.
      • Bardelli A.
      • Silliman N.
      • Ptak J.
      • Szabo S.
      • Yan H.
      • Gazdar A.
      • Powell S.M.
      • Riggins G.J.
      • et al.
      High frequency of mutations of the PIK3CA gene in human cancers.
      ). The consequence of gene amplification and the mutations in p110 subunits is the increase in the basal activity of PI3Ks.
      Somatic mutations have also been identified in the gene encoding the p85α subunit, PIK3R1, in colon, breast, pancreatic, and glioblastoma cancers (
      • Jaiswal B.S.
      • Janakiraman V.
      • Kljavin N.M.
      • Chaudhuri S.
      • Stern H.M.
      • Wang W.
      • Kan Z.
      • Dbouk H.A.
      • Peters B.A.
      • Waring P.
      • et al.
      Somatic mutations in p85α promote tumorigenesis through class IA PI3K activation.
      ). The p85α mutants interact with the p110α, -β, and -δ subunits, but because they have an impaired ability to inhibit the activity of the catalytic subunits, the basal activity of class IA PI3Ks is increased (
      • Jaiswal B.S.
      • Janakiraman V.
      • Kljavin N.M.
      • Chaudhuri S.
      • Stern H.M.
      • Wang W.
      • Kan Z.
      • Dbouk H.A.
      • Peters B.A.
      • Waring P.
      • et al.
      Somatic mutations in p85α promote tumorigenesis through class IA PI3K activation.
      ).
      To date, there have been no somatic mutations in the genes encoding the class II and class III PI3Ks in human cancer.

      PI3K SIGNALING PATHWAYS IN CANCER

      Since the discovery that the serine/threonine kinase, AKT/PKB, is an effector of PI3K in the mid-nineties (
      • Andjelković M.
      • Jakubowicz T.
      • Cron P.
      • Ming X.F.
      • Han J.W.
      • Hemmings B.A.
      Activation and phosphorylation of a pleckstrin homology domain containing protein kinase (RAC-PK/PKB) promoted by serum and protein phosphatase inhibitors.
      ,
      • Burgering B.M.
      • Coffer P.J.
      Protein kinase B (c-Akt) in phosphatidylinositol-3-OH kinase signal transduction.
      ,
      • Cross D.A.
      • Alessi D.R.
      • Cohen P.
      • Andjelkovic M.
      • Hemmings B.A.
      Inhibition of glycogen synthase kinase-3 by insulin mediated by protein kinase B.
      ,
      • Franke T.F.
      • Yang S.I.
      • Chan T.O.
      • Datta K.
      • Kazlauskas A.
      • Morrison D.K.
      • Kaplan D.R.
      • Tsichlis P.N.
      The protein kinase encoded by the Akt proto-oncogene is a target of the PDGF-activated phosphatidylinositol 3-kinase.
      ,
      • Kohn A.D.
      • Kovacina K.S.
      • Roth R.A.
      Insulin stimulates the kinase activity of RAC-PK, a pleckstrin homology domain containing ser/thr kinase.
      ), the enzyme has been identified as the key transducer of oncogenic signaling in numerous human cancers. Activating mutants of AKT have been identified in human cancers such as breast, colorectal, and ovarian (
      • Carpten J.D.
      • Faber A.L.
      • Horn C.
      • Donoho G.P.
      • Briggs S.L.
      • Robbins C.M.
      • Hostetter G.
      • Boguslawski S.
      • Moses T.Y.
      • Savage S.
      • et al.
      A transforming mutation in the pleckstrin homology domain of AKT1 in cancer.
      ). The AKT gene family comprises three forms, of which AKT1 and AKT2 are widely expressed, but AKT3 expression appears to be limited to brain tissue. AKT phosphorylates upwards of 200 target proteins that regulate gene expression, protein synthesis, cell cycle progression, cytoskeleton organization, and cell metabolism (
      • Toker A.
      Achieving specifity in AKT signaling in cancer.
      ).
      PtdIns(3,4,5)P3 and PtdIns(3,4)P2 generated in response to growth factor stimulation recruit AKT/PKB to the plasma membrane by binding to its N-terminal pleckstrin homology (PH) domain (
      • Frech M.
      • Andjelkovic M.
      • Ingley E.
      • Reddy K.K.
      • Falck J.R.
      • Hemmings B.A.
      High affinity binding of inositol phosphates and phosphoinositides to the pleckstrin homology domain of RAC/protein kinase B and their influence on kinase activity.
      ,
      • James S.R.
      • Downes C.P.
      • Gigg R.
      • Grove S.J.
      • Holmes A.B.
      • Alessi D.R.
      Specific binding of the Akt-1 protein kinase to phosphatidylinositol 3,4,5-trisphosphate without subsequent activation.
      ). This interaction has been shown to activate AKT in vitro (
      • Frech M.
      • Andjelkovic M.
      • Ingley E.
      • Reddy K.K.
      • Falck J.R.
      • Hemmings B.A.
      High affinity binding of inositol phosphates and phosphoinositides to the pleckstrin homology domain of RAC/protein kinase B and their influence on kinase activity.
      ,
      • Franke T.F.
      • Kaplan D.R.
      • Cantley L.C.
      • Toker A.
      Direct regulation of the Akt protooncogene product by phosphatidylinositol-3,4-bisphosphate.
      ,
      • Klippel A.
      • Kavanagh W.M.
      • Pot D.A.
      • Williams L.T.
      A specific product of phosphatidylinositol 3-kinase directly activates the protein kinase Akt through its pleckstrin homology domain.
      ). However, other studies have favored a model in which PtdIns(3,4,5)P3 and PtdIns(3,4)P2 binding does not activate AKT (
      • James S.R.
      • Downes C.P.
      • Gigg R.
      • Grove S.J.
      • Holmes A.B.
      • Alessi D.R.
      Specific binding of the Akt-1 protein kinase to phosphatidylinositol 3,4,5-trisphosphate without subsequent activation.
      ), but instead recruits the enzyme to the membrane, altering its conformation and allowing the kinase to be fully activated by subsequent phosphorylation at Thr308 and Ser473.
      AKT is phosphorylated at Thr308 in the activation loop by a PH domain containing 3-phosphoinositide-dependent kinase 1 (PDK1) (
      • Alessi D.R.
      • Deak M.
      • Casamayor A.
      • Caudwell F.B.
      • Morrice N.
      • Norman D.G.
      • Gaffney P.R.
      • Reese C.B.
      • MacDougall C.N.
      • Harbison D.
      • et al.
      Phosphoinositide-dependent protein kinase-1 (PDK1): structural and functional homology with the Drosophila DSTPK61 kinase.
      ,
      • Alessi D.R.
      • James S.R.
      • Downes C.P.
      • Holmes A.B.
      • Gaffney P.R.
      • Reese C.B.
      • Cohen P.
      Characterization of a 3-phosphoinositide-dependent protein kinase which phosphorylates and activates protein kinase Balpha.
      ,
      • Stephens L.
      • Anderson K.
      • Stokoe D.
      • Erdjument-Bromage H.
      • Painter G.F.
      • Holmes A.B.
      • Gaffney P.R.
      • Reese C.B.
      • McCormick F.
      • Tempst P.
      • et al.
      Protein kinase B kinases that mediate phosphatidylinositol 3,4,5-trisphosphate-dependent activation of protein kinase B.
      ,
      • Stokoe D.
      • Stephens L.R.
      • Copeland T.
      • Gaffney P.R.J.
      • Reese C.B.
      • Painter G.F.
      • Holmes A.B.
      • McCormick F.
      • Hawkins P.T.
      Dual role of phosphatidylinositol-3,4,5-trisphosphate in the activation of protein kinase B.
      ). PDK1 only phosphorylates AKT in the presence of PtdIns(3,4,5)P3 or PtdIns(3,4)P2 (
      • Alessi D.R.
      • Deak M.
      • Casamayor A.
      • Caudwell F.B.
      • Morrice N.
      • Norman D.G.
      • Gaffney P.R.
      • Reese C.B.
      • MacDougall C.N.
      • Harbison D.
      • et al.
      Phosphoinositide-dependent protein kinase-1 (PDK1): structural and functional homology with the Drosophila DSTPK61 kinase.
      ,
      • Alessi D.R.
      • James S.R.
      • Downes C.P.
      • Holmes A.B.
      • Gaffney P.R.
      • Reese C.B.
      • Cohen P.
      Characterization of a 3-phosphoinositide-dependent protein kinase which phosphorylates and activates protein kinase Balpha.
      ,
      • Stephens L.
      • Anderson K.
      • Stokoe D.
      • Erdjument-Bromage H.
      • Painter G.F.
      • Holmes A.B.
      • Gaffney P.R.
      • Reese C.B.
      • McCormick F.
      • Tempst P.
      • et al.
      Protein kinase B kinases that mediate phosphatidylinositol 3,4,5-trisphosphate-dependent activation of protein kinase B.
      ,
      • Stokoe D.
      • Stephens L.R.
      • Copeland T.
      • Gaffney P.R.J.
      • Reese C.B.
      • Painter G.F.
      • Holmes A.B.
      • McCormick F.
      • Hawkins P.T.
      Dual role of phosphatidylinositol-3,4,5-trisphosphate in the activation of protein kinase B.
      ). The C-terminal PH domain of PDK1 binds to PtdIns(3,4,5)P3 and PtdIns(3,4)P2, with weaker binding to PtdIns(4,5)P2, resulting in PDK1 binding to the plasma membrane (
      • Stephens L.
      • Anderson K.
      • Stokoe D.
      • Erdjument-Bromage H.
      • Painter G.F.
      • Holmes A.B.
      • Gaffney P.R.
      • Reese C.B.
      • McCormick F.
      • Tempst P.
      • et al.
      Protein kinase B kinases that mediate phosphatidylinositol 3,4,5-trisphosphate-dependent activation of protein kinase B.
      ,
      • Currie R.A.
      • Walker K.S.
      • Gray A.
      • Deak M.
      • Casamayor A.
      • Downes C.P.
      Role of phosphatidylinositol 3,4-5-trisphosphate in regulating the activity and localization of 3-phosphoinositide-dependent protein kinase-1.
      ). AKT does not bind PtdIns(4,5)P2 (
      • Currie R.A.
      • Walker K.S.
      • Gray A.
      • Deak M.
      • Casamayor A.
      • Downes C.P.
      Role of phosphatidylinositol 3,4-5-trisphosphate in regulating the activity and localization of 3-phosphoinositide-dependent protein kinase-1.
      ). AKT is further activated by phosphorylation at Ser473 in the hydrophobic motif by mTORC2 (
      • Sarbassov D.D.
      • Guertin D.A.
      • Ali S.M.
      • Sabatini D.M.
      Phosphorylation and regulation of Akt/PKB by the rictor-mTOR complex.
      ). However, little is known about the upstream activators of mTORC2 in response to growth factors. Interestingly, one report has demonstrated that PtdIns(3,4,5)P3 can directly activate mTORC2 activity in vitro (
      • Gan X.
      • Wang J.
      • Su B.
      • Wu D.
      Evidence for direct activation of mTORC2 kinase activity by phosphatidylinositol 3,4,5-trisphosphate.
      ).
      There is evidence that PDK1 can be recruited to membranes independently of PtdIns(3,4,5)P3 or PtdIns(3,4)P2 through its binding to the activated insulin receptor via the adaptor protein, Grb14 (
      • King C.C.
      • Newton A.C.
      The adaptor protein Grb14 regulates the localization of 3-phosphoinositide-dependent kinase-1.
      ). Furthermore, the PH domain of PDK1 exhibits a greater binding affinity for PtdIns(3,4,5)P3, PtdIns(3,4)P2, and PtdIns(4,5)P2 than the binding affinity of the PH domain of AKT for PtdIns(3,4,5)P3 (
      • Stephens L.
      • Anderson K.
      • Stokoe D.
      • Erdjument-Bromage H.
      • Painter G.F.
      • Holmes A.B.
      • Gaffney P.R.
      • Reese C.B.
      • McCormick F.
      • Tempst P.
      • et al.
      Protein kinase B kinases that mediate phosphatidylinositol 3,4,5-trisphosphate-dependent activation of protein kinase B.
      ,
      • Currie R.A.
      • Walker K.S.
      • Gray A.
      • Deak M.
      • Casamayor A.
      • Downes C.P.
      Role of phosphatidylinositol 3,4-5-trisphosphate in regulating the activity and localization of 3-phosphoinositide-dependent protein kinase-1.
      ). Indeed, the binding affinity of PDK1 for PtdIns(4,5)P2 is comparable to the affinity of AKT toward PtdIns(3,4,5)P3 (
      • Currie R.A.
      • Walker K.S.
      • Gray A.
      • Deak M.
      • Casamayor A.
      • Downes C.P.
      Role of phosphatidylinositol 3,4-5-trisphosphate in regulating the activity and localization of 3-phosphoinositide-dependent protein kinase-1.
      ). Therefore, AKT membrane localization and activation is potentially more dependent on higher PtdIns(3,4,5)P3 and PtdIns(3,4)P2 levels than the levels necessary for PDK1 localization. In this model, PtdIns(3,4,5)P3 and PtdIns(3,4)P2 are considered to have redundant abilities to activate AKT and PDK1; however, one study has suggested that both PtdIns(3,4,5)P3 and PtdIns(3,4)P2 are required for full activation of AKT (
      • Scheid M.P.
      • Marignani P.A.
      • Woodgett J.R.
      Multiple phosphoinositide 3-kinase-dependent steps in activation of protein kinase B.
      ).
      Not all PI3K signals in cancer are transduced by AKT phosphorylation. The levels of phosphorylated AKT in PIK3CA mutant cell lines and human breast tumors are low and anchorage-dependent growth is less dependent on AKT (
      • Vasudevan K.M.
      • Barbie D.A.
      • Davies M.A.
      • Rabinovsky R.
      • McNear C.J.
      • Kim J.J.
      • Hennessy B.T.
      • Tseng H.
      • Pochanard P.
      • Kim S.Y.
      • et al.
      AKT-independent signaling downstream of oncogenic PIK3CA mutations in human cancer.
      ). However, PDK1 is highly expressed in PIK3CA mutant breast tissue, and PDK1 expression is required for anchorage-independent growth in PIK3CA mutant cell lines (
      • Vasudevan K.M.
      • Barbie D.A.
      • Davies M.A.
      • Rabinovsky R.
      • McNear C.J.
      • Kim J.J.
      • Hennessy B.T.
      • Tseng H.
      • Pochanard P.
      • Kim S.Y.
      • et al.
      AKT-independent signaling downstream of oncogenic PIK3CA mutations in human cancer.
      ). PDK1 is not solely an AKT kinase; it phosphorylates the activation segment of at least 23 AGC kinases [reviewed in (
      • Pearce L.R.
      • Komander D.
      • Alessi D.R.
      The nuts and bolts of AGC protein kinases.
      )]. One of these kinases, the serum and glucocorticoid-regulated kinase 3 (SGK3) is phosphorylated by PDK1 at the activation loop (
      • Kobayashi T.
      • Cohen P.
      Activation of serum- and glucocorticoid-regulated protein kinase by agonists that activate phosphatidylinositide 3-kinase is mediated by 3-phosphoinositide-dependent protein kinase-1 (PDK1) and PDK2.
      ) and mTORC2 at the hydrophobic motif (
      • García-Martínez J.M.
      • Alessi D.R.
      mTOR complex 2 (mTORC2) controls hydrophobic motif phosphorylation and activation of serum- and glucocorticoid-induced protein kinase 1 (SGK1).
      ). SGK3 is amplified and hyperactivated in PIK3CA-mutant breast cancer (
      • Gasser J.A.
      • Inuzuka H.
      • Lau A.W.
      • Wei W.
      • Beroukhim R.
      • Toker A.
      SGK3 mediates INPP4B-dependent PI3K signaling in breast cancer.
      ), and is required for AKT-independent viability (
      • Vasudevan K.M.
      • Barbie D.A.
      • Davies M.A.
      • Rabinovsky R.
      • McNear C.J.
      • Kim J.J.
      • Hennessy B.T.
      • Tseng H.
      • Pochanard P.
      • Kim S.Y.
      • et al.
      AKT-independent signaling downstream of oncogenic PIK3CA mutations in human cancer.
      ), anchorage-independent growth, and spheroid growth (
      • Gasser J.A.
      • Inuzuka H.
      • Lau A.W.
      • Wei W.
      • Beroukhim R.
      • Toker A.
      SGK3 mediates INPP4B-dependent PI3K signaling in breast cancer.
      ).

      PHOSPHOINOSITIDE PHOSPHATASE ACTIVITIES AND CANCER

      Over the last 20 years, there has been a tremendous amount of research that has demonstrated that PI3K/AKT signaling is constitutively activated in a large number of human cancers as a result of gene mutation or amplification of PI3K and AKT. However, the functional loss of the phosphoinositide phosphatases has also been reported to drive the progression of human cancer. There are phosphatases which remove the phosphate group from each of the 3-, 4-, and 5-positions of PtdIns(3,4,5)P3 and related phosphoinositides. Considering the key role the PI3K/AKT pathway plays in malignancy, changes in the activities of these enzymes will have distinct effects upon the onset and progression of cancer. Thus, this review will highlight the phosphoinositide phosphatase activities expressed in humans and consider their involvement in cancer, recognizing that most data relate to the 3- and 5-phosphatases.

      PHOSPHOINOSITIDE 3-PHOSPHATASES

      Mammalian phosphoinositide 3-phosphatases can be divided into two major groups: one group comprised of phosphatase and tensin homolog deleted on chromosome 10 (PTEN) and the transmembrane phosphatase with tensin homology (TPTE) and PTEN homologous inositol lipid phosphatase (TPIP) proteins, and the other group made up of myotubularin (MTM) and MTM-related (MTMR) proteins.

      PTEN AND TPIP PROTEINS

      PTEN, also known as mutated in multiple advanced cancers (MMAC), was first identified as a candidate tumor suppressor gene on chromosome 10q23 in 1997 (
      • Li J.
      • Yen C.
      • Liaw D.
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      • Bose S.
      • Wang S.I.
      • Puc J.
      • Miliaresis C.
      • Rodgers L.
      • McCombie R.
      • et al.
      PTEN, a putative protein tyrosine phosphatase gene mutated in human brain, breast, and prostate cancer.
      ,
      • Steck P.A.
      • Pershouse M.A.
      • Jasser S.A.
      • Yung W.K.
      • Lin H.
      • Ligon A.H.
      • Langford L.A.
      • Baumgard M.L.
      • Hattier T.
      • Davis T.
      • et al.
      Identification of a candidate tumour suppressor gene, MMAC1, at chromosome 10q23.3 that is mutated in multiple advanced cancers.
      ). PTEN is the best known and most described of phosphoinositide phosphatases that play a role in human malignancy. Somatic homozygous mutations and deletions of PTEN have been detected in glioblastoma, prostate, kidney, melanoma, lung, endometrial, bladder, and breast cancer cell lines and primary tumors (
      • Steck P.A.
      • Pershouse M.A.
      • Jasser S.A.
      • Yung W.K.
      • Lin H.
      • Ligon A.H.
      • Langford L.A.
      • Baumgard M.L.
      • Hattier T.
      • Davis T.
      • et al.
      Identification of a candidate tumour suppressor gene, MMAC1, at chromosome 10q23.3 that is mutated in multiple advanced cancers.
      ,
      • Cairns P.
      • Evron E.
      • Okami K.
      • Halachmi N.
      • Esteller M.
      • Herman J.G.
      • Bose S.
      • Wang S.I.
      • Parsons R.
      • Sidransky D.
      Point mutation and homozygous deletion of PTEN/MMAC1 in primary bladder cancers.
      ,
      • Guldberg P.
      • Thor Straten P.
      • Birck A.
      • Ahrenkiel V.
      • Kirkin A.F.
      • Zeuthen J.
      Disruption of the MMAC1/PTEN gene by deletion or mutation is a frequent event in malignant melanoma.
      ,
      • Kohno T.
      • Takahashi M.
      • Manda R.
      • Yokota J.
      Inactivation of the PTEN/MMAC1/TEP1 gene in human lung cancers.
      ,
      • Li L.
      • Ernsting B.R.
      • Wishart M.J.
      • Lohse D.L.
      • Dixon J.E.
      A family of putative tumor suppressors is structurally and functionally conserved in humans and yeast.
      ,
      • Tashiro H.
      • Blazes M.S.
      • Wu R.
      • Cho K.R.
      • Bose S.
      • Wang S.I.
      • Li J.
      • Parsons R.
      • Ellenson L.H.
      Mutations in PTEN are frequent in endometrial carcinoma but rare in other common gynecological malignancies.
      ). That same year, it was reported that germline mutations of PTEN were associated with Cowden's disease, an autosomal dominant cancer predisposition syndrome that has an associated increased risk of cancer (
      • Liaw D.
      • Marsh D.J.
      • Li J.
      • Dahia P.L.
      • Wang S.I.
      • Zheng Z.
      • Bose S.
      • Call K.M.
      • Tsou H.C.
      • Peacocke M.
      • et al.
      Germline mutations of the PTEN gene in Cowden disease, an inherited breast and thyroid cancer syndrome.
      ,
      • Nelen M.R.
      • van Staveren W.C.
      • Peeters E.A.
      • Hassel M.B.
      • Gorlin R.J.
      • Hamm H.
      • Lindboe C.F.
      • Fryns J.P.
      • Sijmons R.H.
      • Woods D.G.
      • et al.
      Germline mutations in the PTEN/MMAC1 gene in patients with Cowden disease.
      ). Homozygous PTEN knockout mice are embryonic lethal, but PTEN/− mice are viable (
      • Di Cristofano A.
      • Pesce B.
      • Cordon-Cardo C.
      • Pandolfi P.P.
      Pten is essential for embryonic development and tumour suppression.
      ,
      • Podsypanina K.
      • Ellenson L.H.
      • Nemes A.
      • Gu J.
      • Tamura M.
      • Yamada K.M.
      • Cordon-Cardo C.
      • Catoretti G.
      • Fisher P.E.
      • Parsons R.
      Mutation of Pten/Mmac1 in mice causes neoplasia in multiple organ systems.
      ,
      • Suzuki A.
      • de la Pompa J.L.
      • Stambolic V.
      • Elia A.J.
      • Sasaki T.
      • del Barco Barrantes I.
      • Ho A.
      • Wakeham A.
      • Itie A.
      • Khoo W.
      • et al.
      High cancer susceptibility and embryonic lethality associated with mutation of the PTEN tumor suppressor gene in mice.
      ). Nevertheless, consistent with the tumor suppressor function of PTEN and the situation observed in Cowden's disease patients, heterozygous PTEN/− mice exhibited a higher frequency of spontaneous tumor formation compared with wild-type mice (
      • Di Cristofano A.
      • Pesce B.
      • Cordon-Cardo C.
      • Pandolfi P.P.
      Pten is essential for embryonic development and tumour suppression.
      ,
      • Podsypanina K.
      • Ellenson L.H.
      • Nemes A.
      • Gu J.
      • Tamura M.
      • Yamada K.M.
      • Cordon-Cardo C.
      • Catoretti G.
      • Fisher P.E.
      • Parsons R.
      Mutation of Pten/Mmac1 in mice causes neoplasia in multiple organ systems.
      ,
      • Suzuki A.
      • de la Pompa J.L.
      • Stambolic V.
      • Elia A.J.
      • Sasaki T.
      • del Barco Barrantes I.
      • Ho A.
      • Wakeham A.
      • Itie A.
      • Khoo W.
      • et al.
      High cancer susceptibility and embryonic lethality associated with mutation of the PTEN tumor suppressor gene in mice.
      ).
      Due to its similarity to a subfamily of protein tyrosine phosphatases, the dual specificity phosphatases, in particular the VH1-like family (
      • Alonso A.
      • Burkhalter S.
      • Sasin J.
      • Tautz L.
      • Bogetz J.
      • Huynh H.
      • Bremer M.C.D.
      • Holsinger L.J.
      • Godzik A.
      • Mustelin T.
      The minimal essential core of a cysteine-based protein-tyrosine phosphatase revealed by a novel 16-kDa VH1-like phosphatase, VHZ.
      ), PTEN was initially predicted to be a dual specificity protein phosphatase (
      • Steck P.A.
      • Pershouse M.A.
      • Jasser S.A.
      • Yung W.K.
      • Lin H.
      • Ligon A.H.
      • Langford L.A.
      • Baumgard M.L.
      • Hattier T.
      • Davis T.
      • et al.
      Identification of a candidate tumour suppressor gene, MMAC1, at chromosome 10q23.3 that is mutated in multiple advanced cancers.
      ,
      • Li L.
      • Ernsting B.R.
      • Wishart M.J.
      • Lohse D.L.
      • Dixon J.E.
      A family of putative tumor suppressors is structurally and functionally conserved in humans and yeast.
      ,
      • Li D.M.
      • Sun H.
      TEP1, encoded by a candidate tumor suppressor locus, is a novel protein tyrosine phosphatase regulated by transforming growth factor beta.
      ). Subsequent work demonstrated that recombinant PTEN exhibited tyrosine phosphatase (
      • Li L.
      • Ernsting B.R.
      • Wishart M.J.
      • Lohse D.L.
      • Dixon J.E.
      A family of putative tumor suppressors is structurally and functionally conserved in humans and yeast.
      ,
      • Li D.M.
      • Sun H.
      TEP1, encoded by a candidate tumor suppressor locus, is a novel protein tyrosine phosphatase regulated by transforming growth factor beta.
      ,
      • Myers M.P.
      • Stolarov J.P.
      • Eng C.
      • Li J.
      • Wang S.I.
      • Wigler M.H.
      • Parsons R.
      • Tonks N.K.
      P-TEN, the tumor suppressor from human chromosome 10q23, is a dual-specificity phosphatase.
      ) and serine/threonine phosphatase activity in vitro (
      • Myers M.P.
      • Stolarov J.P.
      • Eng C.
      • Li J.
      • Wang S.I.
      • Wigler M.H.
      • Parsons R.
      • Tonks N.K.
      P-TEN, the tumor suppressor from human chromosome 10q23, is a dual-specificity phosphatase.
      ). Importantly, Myers et al. (
      • Myers M.P.
      • Stolarov J.P.
      • Eng C.
      • Li J.
      • Wang S.I.
      • Wigler M.H.
      • Parsons R.
      • Tonks N.K.
      P-TEN, the tumor suppressor from human chromosome 10q23, is a dual-specificity phosphatase.
      ) demonstrated that mutation of the essential cysteine from the invariant signature motif HCxxGxxR found in all the members of the protein tyrosine phosphatase superfamily, PTEN C124S, resulted in loss of tyrosine phosphatase activity in vitro. Consistent with the in vitro results, overexpression of PTEN in NIH-3T3 cells reduced tyrosine phosphorylation of focal adhesion kinase in vitro and in vivo (
      • Tamura M.
      • Gu J.
      • Matsumoto K.
      • Aota S.
      • Parsons R.
      • Yamada K.M.
      Inhibition of cell migration, spreading, and focal adhesions by tumor suppressor PTEN.
      ). Furthermore, when the phosphatase inactive PTEN C124S mutant was overexpressed in NIH-3T3 and glioblastoma U-89 cell lines, focal adhesion kinase tyrosine dephosphorylation, cell spreading, invasion, and growth were attenuated (
      • Tamura M.
      • Gu J.
      • Matsumoto K.
      • Aota S.
      • Parsons R.
      • Yamada K.M.
      Inhibition of cell migration, spreading, and focal adhesions by tumor suppressor PTEN.
      ,
      • Tamura M.
      • Gu J.
      • Takino T.
      • Yamada K.M.
      Tumor suppressor PTEN inhibition of cell invasion, migration, and growth: differential involvement of focal adhesion kinase and p130Cas.
      ).
      Maehama and Dixon (
      • Maehama T.
      • Dixon J.E.
      The tumour suppressor, PTEN/MMAC1, dephosphorylates the lipid second messenger, phosphatidylinositol 3,4,5-trisphosphate.
      ) suggested that PTEN could dephosphosphorylate negatively charged substrates other than tyrosine and/or serine/threonine phosphoproteins. They based this on the observation that the in vitro catalytic rate activity of PTEN toward protein and peptide substrates was low (
      • Li L.
      • Ernsting B.R.
      • Wishart M.J.
      • Lohse D.L.
      • Dixon J.E.
      A family of putative tumor suppressors is structurally and functionally conserved in humans and yeast.
      ,
      • Li D.M.
      • Sun H.
      TEP1, encoded by a candidate tumor suppressor locus, is a novel protein tyrosine phosphatase regulated by transforming growth factor beta.
      ,
      • Myers M.P.
      • Stolarov J.P.
      • Eng C.
      • Li J.
      • Wang S.I.
      • Wigler M.H.
      • Parsons R.
      • Tonks N.K.
      P-TEN, the tumor suppressor from human chromosome 10q23, is a dual-specificity phosphatase.
      ), but was far greater toward highly negatively charged and multi-phosphorylated tyrosine peptide substrates such as polyGlu4Tyr1 (
      • Myers M.P.
      • Stolarov J.P.
      • Eng C.
      • Li J.
      • Wang S.I.
      • Wigler M.H.
      • Parsons R.
      • Tonks N.K.
      P-TEN, the tumor suppressor from human chromosome 10q23, is a dual-specificity phosphatase.
      ). When PTEN was overexpressed in HEK293 cells, the levels of insulin-stimulated PtdIns(3,4,5)P3 were found to be significantly reduced, whereas overexpression of the catalytically inactive PTEN C124S mutant resulted in an increase in cellular PtdIns(3,4,5)P3 in the absence of insulin stimulation (
      • Maehama T.
      • Dixon J.E.
      The tumour suppressor, PTEN/MMAC1, dephosphorylates the lipid second messenger, phosphatidylinositol 3,4,5-trisphosphate.
      ). Furthermore, recombinant PTEN dephosphorylated PtdIns(3,4,5)P3 to PtdIns(4,5)P2 and Ins(1,3,4,5)P4 to Ins(1,4,5)P3 in vitro (
      • Maehama T.
      • Dixon J.E.
      The tumour suppressor, PTEN/MMAC1, dephosphorylates the lipid second messenger, phosphatidylinositol 3,4,5-trisphosphate.
      ), confirming that PTEN was indeed a lipid phosphatase with activity toward phosphates at the 3-position of the inositol ring.
      Myers et al. (
      • Myers M.P.
      • Pass I.
      • Batty I.H.
      • Van der Kaay J.
      • Stolarov J.P.
      • Hemmings B.A.
      • Wigler M.H.
      • Downes C.P.
      • Tonks N.K.
      The lipid phosphatase activity of PTEN is critical for its tumor supressor function.
      ) confirmed that recombinant PTEN catalyzed the dephosphorylation of PtdIns(3,4,5)P3 to PtdIns(4,5)P2 and Ins(1,3,4,5)P4 to Ins(1,4,5)P3 in vitro. However, they also demonstrated that in addition to PtdIns(3,4,5)P3, PTEN was able to dephosphorylate PtdIns(3)P and PtdIns(3,5)P2 at the 3-position in vitro (
      • Myers M.P.
      • Pass I.
      • Batty I.H.
      • Van der Kaay J.
      • Stolarov J.P.
      • Hemmings B.A.
      • Wigler M.H.
      • Downes C.P.
      • Tonks N.K.
      The lipid phosphatase activity of PTEN is critical for its tumor supressor function.
      ), in the order of preference PtdIns(3,4,5)P3 = PtdIns(3,4)P2 > PtdIns(3)P > Ins(1,3,4,5)P4 (
      • Myers M.P.
      • Pass I.
      • Batty I.H.
      • Van der Kaay J.
      • Stolarov J.P.
      • Hemmings B.A.
      • Wigler M.H.
      • Downes C.P.
      • Tonks N.K.
      The lipid phosphatase activity of PTEN is critical for its tumor supressor function.
      ).
      Interestingly, it was demonstrated that the dephosphorylation activity of the PTEN mutant found in Cowden's disease, PTEN G129E (
      • Liaw D.
      • Marsh D.J.
      • Li J.
      • Dahia P.L.
      • Wang S.I.
      • Zheng Z.
      • Bose S.
      • Call K.M.
      • Tsou H.C.
      • Peacocke M.
      • et al.
      Germline mutations of the PTEN gene in Cowden disease, an inherited breast and thyroid cancer syndrome.
      ), against phosphotyrosine peptides is unaltered (
      • Myers M.P.
      • Stolarov J.P.
      • Eng C.
      • Li J.
      • Wang S.I.
      • Wigler M.H.
      • Parsons R.
      • Tonks N.K.
      P-TEN, the tumor suppressor from human chromosome 10q23, is a dual-specificity phosphatase.
      ), but the activity is significantly reduced against PtdIns(3,4,5)P3 (
      • Myers M.P.
      • Pass I.
      • Batty I.H.
      • Van der Kaay J.
      • Stolarov J.P.
      • Hemmings B.A.
      • Wigler M.H.
      • Downes C.P.
      • Tonks N.K.
      The lipid phosphatase activity of PTEN is critical for its tumor supressor function.
      ).
      This result strongly suggested that it is the lipid, rather than the protein phosphatase activity of PTEN that is required for its tumor suppressor function, and that loss of PtdIns(3,4,5)P3-phosphatase activity results in Cowden's disease (
      • Myers M.P.
      • Pass I.
      • Batty I.H.
      • Van der Kaay J.
      • Stolarov J.P.
      • Hemmings B.A.
      • Wigler M.H.
      • Downes C.P.
      • Tonks N.K.
      The lipid phosphatase activity of PTEN is critical for its tumor supressor function.
      ). Furthermore, in immortalized mouse embryonic fibroblasts isolated from PTEN −/− mice, the levels of PtdIns(3,4,5)P3 are elevated and AKT is hyperactivated (
      • Stambolic V.
      • Suzuki A.
      • de la Pompa J.L.
      • Brothers G.M.
      • Mirtsos C.
      • Sasaki T.
      • Ruland J.
      • Penninger J.M.
      • Siderovski D.P.
      • Mak T.W.
      Negative regulation of PKB/Akt-dependent cell survival by the tumor suppressor PTEN.
      ), and in heterozygous PTEN/− mice, the elevated frequency of spontaneous tumor formation is associated with an increased activity of AKT (
      • Suzuki A.
      • de la Pompa J.L.
      • Stambolic V.
      • Elia A.J.
      • Sasaki T.
      • del Barco Barrantes I.
      • Ho A.
      • Wakeham A.
      • Itie A.
      • Khoo W.
      • et al.
      High cancer susceptibility and embryonic lethality associated with mutation of the PTEN tumor suppressor gene in mice.
      ). These and subsequent studies have shown that the mechanism underlying the tumor suppressor function of PTEN is the regulation of the cellular pool of PtdIns(3,4,5)P3 levels that controls AKT-mediated signaling [reviewed in (
      • Worby C.A.
      • Dixon J.E.
      PTEN.
      )].
      In the Cowden PTEN G129E mutant, the protein phosphatase activity is attenuated, but the PtdIns(3,4,5)P3 phosphatase activity is intact. Thus, the significance of the PTEN protein phosphatase activity for PTEN tumor suppressor function is not clear.
      Nevertheless, it has been suggested that this protein phosphatase activity may be relevant to various physiological functions of PTEN, such as regulating cell migration and invasion. In contrast, PTEN Y138C, found in small cell lung carcinoma, dephosphorylates PtdIns(3,4,5)P3, but has no protein phosphatase activity (
      • Tibarewal P.
      • Zilidis G.
      • Spinelli L.
      • Schurch N.
      • Maccario H.
      • Gray A.
      • Perera N.M.
      • Davidson L.
      • Barton G.J.
      • Leslie N.R.
      PTEN protein phosphatase activity correlates with control of gene expression and invasion, a tumor-suppressing phenotype, but not with AKT activity.
      ). When overexpressed in a glioblastoma cell line, PtdIns(3,4,5)P3 levels and AKT phosphorylation are reduced, but cellular invasion into matrigel is not suppressed (
      • Tibarewal P.
      • Zilidis G.
      • Spinelli L.
      • Schurch N.
      • Maccario H.
      • Gray A.
      • Perera N.M.
      • Davidson L.
      • Barton G.J.
      • Leslie N.R.
      PTEN protein phosphatase activity correlates with control of gene expression and invasion, a tumor-suppressing phenotype, but not with AKT activity.
      ), arguing against a role for the protein dephosphorylating activity of PTEN in this function. PTEN Y138C displays increased phosphorylation at Thr366. It has therefore been hypothesized to alter PTEN binding to other proteins, and consequently to altering the localization of PtdIns(3,4,5)P3 dephosphorylation (
      • Tibarewal P.
      • Zilidis G.
      • Spinelli L.
      • Schurch N.
      • Maccario H.
      • Gray A.
      • Perera N.M.
      • Davidson L.
      • Barton G.J.
      • Leslie N.R.
      PTEN protein phosphatase activity correlates with control of gene expression and invasion, a tumor-suppressing phenotype, but not with AKT activity.
      ), thereby affecting the physiological consequence of a local change in PtdIns(3,4,5)P3 concentration. The PTEN G129E mutant also displays increased phosphorylation at Thr366. This has been proposed to provide a mechanism whereby the PTEN protein phosphatase activity is required to auto-dephosphorylate PTEN and thereby control lipid phosphatase activity in vivo (
      • Tibarewal P.
      • Zilidis G.
      • Spinelli L.
      • Schurch N.
      • Maccario H.
      • Gray A.
      • Perera N.M.
      • Davidson L.
      • Barton G.J.
      • Leslie N.R.
      PTEN protein phosphatase activity correlates with control of gene expression and invasion, a tumor-suppressing phenotype, but not with AKT activity.
      ). Of potential importance, however, remains the demonstration that PTEN is a protein tyrosine phosphatase for IRS1, both in vitro and in vivo (
      • Shi Y.
      • Wang J.
      • Chandarlapaty S.
      • Cross J.
      • Thompson C.
      • Rosen N.
      • Jiang X.
      PTEN is a protein tyrosine phosphatase for IRS1.
      ).
      In addition to post-translational modification, PTEN activity can be regulated by intracellular localization and recruitment. In particular, PTEN has been demonstrated in both nuclear and mitochondrial compartments, in addition to being found in the cytoplasm and being translocated to the plasma membrane. The nuclear localization of PTEN appears to be regulated by sumoylation of PTEN (
      • Bassi C.
      • Ho J.
      • Srikumar T.
      • Dowling R.J.O.
      • Gorrini C.
      • Miller S.J.
      • Mak T.W.
      • Neel B.G.
      • Raught B.
      • Stambolic V.
      Nuclear PTEN controls DNA repair and sensitivity to genotoxic stress.
      ). The function of nuclear PTEN activity is not fully clear, there are reports of nuclear PI3K activity (
      • Gimm O.
      • Attié-Bitach T.
      • Lees J.A.
      • Vekemans M.
      • Eng C.
      Expression of the PTEN tumour suppressor protein during human development.
      ), and phosphorylated nuclear AKT has been detected in thyroid adenomas and carcinomas with an increase in phosphorylation detectable in PTEN-null tumors (
      • He X.
      • Saji M.
      • Radhakrishnan D.
      • Romigh T.
      • Ngeow J.
      • Yu Q.
      • Wang Y.
      • Ringel M.D.
      • Eng C.
      PTEN lipid phosphatase activity and proper subcellular localization are necessary and sufficient for down-regulating AKT phosphorylation in the nucleus in Cowden syndrome.
      ). However, He et al. (
      • He X.
      • Saji M.
      • Radhakrishnan D.
      • Romigh T.
      • Ngeow J.
      • Yu Q.
      • Wang Y.
      • Ringel M.D.
      • Eng C.
      PTEN lipid phosphatase activity and proper subcellular localization are necessary and sufficient for down-regulating AKT phosphorylation in the nucleus in Cowden syndrome.
      ) demonstrated that PTEN suppresses translocation of phosphorylated AKT, suggesting that it is the non-nuclear rather than the nuclear PTEN that is critical in cancer. Nevertheless, there is clearly a signaling role for nuclear PtdIns(3,4,5)P3. Blind et al. (
      • Blind R.D.
      • Sablin E.P.
      • Kuchenbecker K.M.
      • Chiu H-J.
      • Deacon A.M.
      • Das D.
      • Fletterick R.J.
      • Ingraham H.A.
      The signaling phospholipid PIP3 creates a new interaction surface on the nuclear receptor SF-1. Proc.
      ) have demonstrated that PtdIns(3,4,5)P3 binds to and stabilizes the nuclear receptor SF-1 structure, explaining the PI3K-dependent increase of SF-1 activity (
      • Lin B.C.
      • Suzawa M.
      • Blind R.D.
      • Tobias S.C.
      • Bulun S.E.
      • Scanlan T.S.
      • Ingraham H.A.
      Stimulating the GPR30 estrogen receptor with a novel tamoxifen analogue activates SF-1 and promotes endometrial cell proliferation.
      ). This group has, however, questioned the importance of PI3K in this process by demonstrating that PtdIns(4,5)P2-associated SF-1 can be modified by IMPK, as well as PTEN. A further role for nuclear PTEN is suggested by its interaction with histone H1 via its C2 domain, thereby regulating chromatin condensation and gene expression (
      • Chen Z.H.
      • Zhu M.
      • Yang J.
      • Liang H.
      • He J.
      • He S.
      • Wang P.
      • Kang X.
      • McNutt M.A.
      • Yin Y.
      • et al.
      PTEN interacts with histone H1 and controls chromatin condensation.
      ). The PTEN-histone H1 interaction is not dependent on PTEN phosphatase activity (
      • Chen Z.H.
      • Zhu M.
      • Yang J.
      • Liang H.
      • He J.
      • He S.
      • Wang P.
      • Kang X.
      • McNutt M.A.
      • Yin Y.
      • et al.
      PTEN interacts with histone H1 and controls chromatin condensation.
      ), and is thus mediated by protein-protein interaction; the regulation of this process and its regulatory significance in cancer tissues remains unclear.
      Mitochondrial PTEN has also been reported, though it has been shown that this is a 70 kDa PTENα which has a 173 amino acid elongated N-terminal region generated through an alternate translation initiation site (
      • Liang H.
      • He S.
      • Yang J.
      • Jia X.
      • Wang P.
      • Chen X.
      • Zhang Z.
      • Zou X.
      • McNutt M.A.
      • Shen W.H.
      • et al.
      PTENα, a PTEN isoform translated through alternative initiation, regulates mitochondrial function and energy metabolism.
      ). This form of PTEN is detectable in both the cytoplasm and mitochondria, and in the latter localization it can activate cytochrome oxidase and thus mitochondrial oxidative metabolism. It appears that PTENα physically associates with cytochrome oxidase and that mutagenesis demonstrates that the phosphatase activity is necessary for activation of the oxidase. However, while it has been suggested that cytochrome oxidase activity can be regulated by phosphorylation, with the hypo-phosphorylated form showing greater activity (
      • Lee I.
      • Salomon A.R.
      • Ficarro S.
      • Mathes I.
      • Lottspeich F.
      • Grossman L.I.
      • Hüttemann M.
      cAMP-dependent tyrosine phosphorylation of subunit I inhibits cytochrome c oxidase activity.
      ), the mutation in PTENα, C297S, that reduces oxidase activity, is equivalent to C124S in PTEN, and thus regulates lipid rather than protein phosphatase activity. The presence of PtdIns(4,5)P2 in mitochondria has been reported (
      • Rosivatz E.
      • Woscholski R.
      Removal or masking of phosphatidylinositol(4,5)bisphosphate from the outer mitochondrial membrane causes mitochondrial fragmentation.
      ); however, there is a paucity of evidence for a mitochondrial PI3K, which nevertheless may exist and there could therefore be an as yet unrecognized PtdIns(3,4,5)P3 signaling pathway in mitochondria which could be important in malignancy where energy metabolism is increased.
      Hopkins et al. (
      • Hopkins B.D.
      • Fine B.
      • Steinbach N.
      • Dendy M.
      • Rapp Z.
      • Shaw J.
      • Pappas K.
      • Yu J.S.
      • Hodakoski C.
      • Mense S.
      • et al.
      A secreted PTEN phosphatase that enters cells to alter signaling and survival.
      ) have also reported a longer form of PTEN, which they named PTEN-long. The additional 174 N-terminal amino acids of PTEN-long share homology to the poly-basic residues of the cell-penetrating element of the HIV transactivator of transcription (TAT) protein (
      • Frankel A.D.
      • Pabo C.O.
      Cellular uptake of the tat protein from human immunodeficiency virus.
      ,
      • Green M.
      • Loewenstein P.M.
      Autonomous functional domains of chemically synthesized human immunodeficiency virus tat trans-activator protein.
      ). Consequently, PTEN-long is a membrane-permeable enzyme that is secreted and can enter other cells to antagonize PI3K-AKT signaling (
      • Hopkins B.D.
      • Fine B.
      • Steinbach N.
      • Dendy M.
      • Rapp Z.
      • Shaw J.
      • Pappas K.
      • Yu J.S.
      • Hodakoski C.
      • Mense S.
      • et al.
      A secreted PTEN phosphatase that enters cells to alter signaling and survival.
      ). PTEN-long is most likely identical to PTENα however, as discussed by Liang et al. (
      • Liang H.
      • He S.
      • Yang J.
      • Jia X.
      • Wang P.
      • Chen X.
      • Zhang Z.
      • Zou X.
      • McNutt M.A.
      • Shen W.H.
      • et al.
      PTENα, a PTEN isoform translated through alternative initiation, regulates mitochondrial function and energy metabolism.
      ), due to the absence of peptide sequence data for PTEN-long, this remains to be definitely proven. Furthermore, Liang et al. (
      • Liang H.
      • He S.
      • Yang J.
      • Jia X.
      • Wang P.
      • Chen X.
      • Zhang Z.
      • Zou X.
      • McNutt M.A.
      • Shen W.H.
      • et al.
      PTENα, a PTEN isoform translated through alternative initiation, regulates mitochondrial function and energy metabolism.
      ) have proposed, based on sequence analysis and the CUG initiation mechanism, that multiple longer forms of PTEN are likely to be expressed. PTEN-long is not the only PtdIns(3,4,5)P3 phosphatase that has been reported to be secreted from cells, PTEN has also been reported to be secreted from cells inside microvesicles, called exosomes, allowing PTEN to enter and antagonize PI3K-AKT signaling in other cells (
      • Putz U.
      • Howitt J.
      • Doan A.
      • Goh C-P.
      • Low L-H.
      • Silke J.
      • Tan S-S.
      The tumor suppressor PTEN is exported in exosomes and has phosphatase activity in recipient cells.
      ). However, it remains unclear what the physiological function of PTEN and PTEN-long transfer between cells fulfils.
      Phosphorylation of residues S380, T382, T383, and S385 in the C terminus of PTEN maintains the phosphatase as a monomer (
      • Papa A.
      • Wan L.
      • Bonora M.
      • Salmena L.
      • Song M.S.
      • Hobbs R.M.
      • Lunardi A.
      • Webster K.
      • Ng C.
      • Newton R.H.
      • et al.
      Cancer-associated PTEN mutants act in a dominant-negative manner to suppress PTEN protein function.
      ,
      • Vazquez F.
      • Grossman S.R.
      • Takahashi Y.
      • Rokas M.V.
      • Nakamura N.
      • Sellers W.R.
      Phosphorylation of the PTEN tail acts as an inhibitory switch by preventing its recruitment into a protein complex.
      ). However, when these residues are not phosphorylated, PTEN exists as a dimer that has greater PtdIns(3,4,5)P3 phosphatase activity than the monomer (
      • Papa A.
      • Wan L.
      • Bonora M.
      • Salmena L.
      • Song M.S.
      • Hobbs R.M.
      • Lunardi A.
      • Webster K.
      • Ng C.
      • Newton R.H.
      • et al.
      Cancer-associated PTEN mutants act in a dominant-negative manner to suppress PTEN protein function.
      ). Furthermore, catalytically inactive mutants of PTEN heterodimerize with wild-type PTEN, resulting in the inhibition of the PtdIns(3,4,5)P3 phosphatase activity in a dominant-negative manner (
      • Papa A.
      • Wan L.
      • Bonora M.
      • Salmena L.
      • Song M.S.
      • Hobbs R.M.
      • Lunardi A.
      • Webster K.
      • Ng C.
      • Newton R.H.
      • et al.
      Cancer-associated PTEN mutants act in a dominant-negative manner to suppress PTEN protein function.
      ). This, in turn, results in hyper-activation of PI3K-AKT signaling and increased tumor formation in mice (
      • Papa A.
      • Wan L.
      • Bonora M.
      • Salmena L.
      • Song M.S.
      • Hobbs R.M.
      • Lunardi A.
      • Webster K.
      • Ng C.
      • Newton R.H.
      • et al.
      Cancer-associated PTEN mutants act in a dominant-negative manner to suppress PTEN protein function.
      ). The implications for the clinical outcome of patients that express both wild-type and mutant PTEN proteins are significant based on the dominant-negative effect of the mutant protein. For example, patients with one mutant gene expressing an inactive mutant of PTEN could conceivably have a significant reduction in the PtdIns(3,4,5)P3 phosphatase activity of the remaining wild-type PTEN, whereas patients expressing PTEN mutants that result in reduced protein levels, would not (
      • Papa A.
      • Wan L.
      • Bonora M.
      • Salmena L.
      • Song M.S.
      • Hobbs R.M.
      • Lunardi A.
      • Webster K.
      • Ng C.
      • Newton R.H.
      • et al.
      Cancer-associated PTEN mutants act in a dominant-negative manner to suppress PTEN protein function.
      ).
      The dominant-negative effects of cancer-associated PTEN mutants are most likely not just confined to the same cell. The discovery that PTEN is sorted into exosomes, and that PTEN-long is secreted PtdIns(3,4,5)P3 phosphatases that enter other cells to regulate PI3K-AKT signaling, raises the intriguing possibility that catalytically inactive mutants of PTEN could also be transferred from cancer cells to stromal cells. This cell-to-cell communication of cancer-associated PTEN mutants, and the dominant-negative manner in which they can inhibit the catalytic activity of wild-type PTEN could further accentuate tumor progression by hyperactivating PI3K-AKT signaling in the surrounding stromal cells, thus facilitating the formation of a tumor microenvironment that is required to support tumor growth.
      Human cells express a single TPTE (
      • Chen H.
      • Rossier C.
      • Morris M.A.
      • Scott H.S.
      • Gos A.
      • Bairoch A.
      • Antonarakis S.E.
      A testis-specific gene, TPTE, encodes a putative transmembrane tyrosine phosphatase and maps to the pericentromeric region of human chromosomes 21 and 13, and to chromosomes 15, 22 and Y.
      ). However, the human enzyme lacks PtdIns(3)P phosphatase activity (
      • Walker S.M.
      • Downes C.P.
      • Leslie N.R.
      TPIP: a novel phosphoinositide 3-phosphatase.
      ). In contrast, mouse TPTE has been demonstrated to exhibit phosphatase activity in vitro toward PtdIns(3)P, PtdIns(3,4)P2, PtdIns(3,5)P2, and PtdIns(3,4,5)P3 (
      • Wu Y.
      • Dowbenko D.
      • Pisabarro M.T.
      • Dillard-Telm L.
      • Koeppen H.
      • Laskey L.A.
      PTEN 2, a Golgi-associated testis-specific homologue of the PTEN tumor suppressor lipid phosphatase.
      ). TPIP exists as four splice variants (TPIPα, -β, -γ, and -C2) (
      • Walker S.M.
      • Downes C.P.
      • Leslie N.R.
      TPIP: a novel phosphoinositide 3-phosphatase.
      ,
      • Mishra R.R.I.
      • Chaudhary J.K.
      • Rath P.C.
      Cell cycle arrest and apoptosis by expression of a novel TPIP (TPIP-C2) cDNA encoding a C2-domain in HEK-293 cells.
      ,
      • Tapparel C.
      • Reymond A.
      • Gidardet C.
      • Guillou L.
      • Lyle R.
      • Lamon C.
      • Hutter P.
      • Antonarakis S.E.
      The TPTE gene family: cellular expression, subcellular localisation and alternative splicing.
      ). TPIPα, but not TPIPβ, dephosphorylates PtdIns(3)P, PtdIns(3,4)P2, PtdIns(3,5)P2, and PtdIns(3,4,5)P3 in vitro (
      • Walker S.M.
      • Downes C.P.
      • Leslie N.R.
      TPIP: a novel phosphoinositide 3-phosphatase.
      ), and in contrast to PTEN, TPIPα does not dephosphorylate Ins(1,3,4,5)P4 in vitro (
      • Walker S.M.
      • Downes C.P.
      • Leslie N.R.
      TPIP: a novel phosphoinositide 3-phosphatase.
      ). To date, there are no reports about whether either TPIPγ or TPIPC2 have phosphatase activity. Although both TPTE and TPIP phosphatases have the capacity to dephosphorylate PtdIns(3,4,5)P3, there are no reports, to date, that the genes encoding either activity are mutated or overexpressed in a human cancer; thus, it is likely that these enzymes play a limited role in regulating PtdIns(3,4,5)P3 levels physiologically and their normal physiological role(s) remains open to question.
      The PTEN-like phosphatase (PTPMT1) was first identified from database searches using the PTEN active site as a query (
      • Pagliarini D.J.
      • Worby C.A.
      • Dixon J.E.
      A PTEN-like phosphatase with a novel substrate specificity.
      ). The recombinant protein exhibited phosphatase activity against PtdIns(5)P, but no other phosphoinositide (
      • Pagliarini D.J.
      • Worby C.A.
      • Dixon J.E.
      A PTEN-like phosphatase with a novel substrate specificity.
      ). However, other reports have demonstrated that PTPMT1 can dephosphorylate PtdIns(3,5)P2, PtdIns(3,4)P2, and PtdIns(5)P in vitro (
      • Shen J.
      • Liu X.
      • Yu W.M.
      • Liu J.
      • Nibbelink M.G.
      • Guo C.
      • Finkel T.
      • Qu C.K.
      A critical role of mitochondrial phosphatase Ptpmt1 in embryogenesis reveals a mitochondrial metabolic stress-induced differentiation checkpoint in embryonic stem cells.
      ). Furthermore, PtdIns(5)P is not dephosphorylated in vivo by this enzyme (
      • Zhang J.
      • Guan Z.
      • Murphy A.N.
      • Wiley S.E.
      • Perkins G.A.
      • Worby C.A.
      • Engel J.L.
      • Heacock P.
      • Nguyen O.K.
      • Wang J.H.
      • et al.
      Mitochondrial phosphatase PTPMT1 is essential for cardiolipin biosynthesis.
      ), but rather phosphatidylglycerolphosphate is the physiological substrate (
      • Zhang J.
      • Guan Z.
      • Murphy A.N.
      • Wiley S.E.
      • Perkins G.A.
      • Worby C.A.
      • Engel J.L.
      • Heacock P.
      • Nguyen O.K.
      • Wang J.H.
      • et al.
      Mitochondrial phosphatase PTPMT1 is essential for cardiolipin biosynthesis.
      ,
      • Pagliarini D.J.
      • Wiley S.E.
      • Kimple M.E.
      • Dixon J.R.
      • Kelly P.
      • Worby C.A.
      • Casey P.J.
      • Dixon J.E.
      Involvement of a mitochondrial phosphatase in the regulation of ATP production and insulin secretion in pancreatic beta cells.
      ); this is a lipid structurally very similar to PtdIns(5)P (
      • Xiao J.
      • Engel J.L.
      • Zhang J.
      • Chen M.J.
      • Manning G.
      • Dixon J.E.
      Structural and functional analysis of PTPMT1, a phosphatase required for cardiolipin synthesis.
      ). This finding questions the importance of PTPMT1 in regulating the levels of cellular phosphoinositides.
      However, Shen et al. (
      • Shen J.
      • Liu X.
      • Yu W.M.
      • Liu J.
      • Nibbelink M.G.
      • Guo C.
      • Finkel T.
      • Qu C.K.
      A critical role of mitochondrial phosphatase Ptpmt1 in embryogenesis reveals a mitochondrial metabolic stress-induced differentiation checkpoint in embryonic stem cells.
      ) have reported that PtdIns(3,5)P2 can be dephosphorylated in vivo by the phosphatase. Knockdown of PTPMT1 in cancer cells causes apoptotic death. This is probably brought about through the dual specificity phosphatase being mitochondrially located and its inhibition affects cardiolipin levels leading to modulation of energy metabolism. Nevertheless, this result could suggest that inhibition of this phosphatase could be used in the treatment of cancer (
      • Niemi N.M.
      • Lanning N.J.
      • Westrate L.M.
      • MacKeigan J.P.
      Downregulation of the mitochondrial phosphatase PTPMT1 is sufficient to promote cancer cell death.
      ); however, there are no reports that PTPMT1 is mutated in cancer tissues.

      MTM AND RELATED PROTEINS

      MTM1 was first identified as the gene mutated in X-linked recessive myotubular myopathy (
      • Laporte J.
      • Hu L.J.
      • Kretz C.
      • Mandel J.L.
      • Kioschis P.
      • Coy J.F.
      • Klauck S.M.
      • Poustka A.
      • Dahl N.
      A gene mutated in X-linked myotubular myopathy defines a new putative tyrosine phosphatase family conserved in yeast.
      ). MTM1 was first predicted to be a dual-specificity tyrosine phosphatase (
      • Laporte J.
      • Hu L.J.
      • Kretz C.
      • Mandel J.L.
      • Kioschis P.
      • Coy J.F.
      • Klauck S.M.
      • Poustka A.
      • Dahl N.
      A gene mutated in X-linked myotubular myopathy defines a new putative tyrosine phosphatase family conserved in yeast.
      ), and initial in vitro assays with recombinant protein reported that MTM1 exhibited both phosphotyrosine (
      • Laporte J.
      • Hu L.J.
      • Kretz C.
      • Mandel J.L.
      • Kioschis P.
      • Coy J.F.
      • Klauck S.M.
      • Poustka A.
      • Dahl N.
      A gene mutated in X-linked myotubular myopathy defines a new putative tyrosine phosphatase family conserved in yeast.
      ,
      • Cui X.
      • De Vivo I.
      • Slany R.
      • Miyamoto A.
      • Firestein R.
      • Cleary M.L.
      Association of SET domain and myotubularin-related proteins modulates growth control.
      ) and phosphoserine (
      • Cui X.
      • De Vivo I.
      • Slany R.
      • Miyamoto A.
      • Firestein R.
      • Cleary M.L.
      Association of SET domain and myotubularin-related proteins modulates growth control.
      ) dephosphorylating activity. However, Taylor, Maehama, and Dixon (
      • Taylor G.S.
      • Maehama T.
      • Dixon J.E.
      Myotubularin, a protein tyrosine phosphatase mutated in myotubular myopathy, dephosphorylates the lipid second messenger, phosphatidylinositol 3-phosphate.
      ) noted that the catalytic activity of MTM1 toward artificial phosphotyrosine substrates was poor, and reported that the active site of MTM1 shares some similarity to the active site region of suppressor of actin (Sac)1, a phosphoinositide phosphatase. Based on these observations, the authors tested the capability of recombinant MTM1 in dephosphorylating phosphoinositides and inositol phosphates, and reported that the enzyme exhibited greatest activity toward PtdIns(3)P and Ins(1,3)P2 (
      • Taylor G.S.
      • Maehama T.
      • Dixon J.E.
      Myotubularin, a protein tyrosine phosphatase mutated in myotubular myopathy, dephosphorylates the lipid second messenger, phosphatidylinositol 3-phosphate.
      ). However, the soluble inositol phosphate substrate is dephosphorylated with a 10- to 20-fold reduced rate compared with the lipid substrate, which is thus likely to be the authentic substrate in cells (
      • Taylor G.S.
      • Maehama T.
      • Dixon J.E.
      Myotubularin, a protein tyrosine phosphatase mutated in myotubular myopathy, dephosphorylates the lipid second messenger, phosphatidylinositol 3-phosphate.
      ,
      • Kim S.A.
      • Taylor G.S.
      • Torgersen K.M.
      • Dixon J.E.
      Myotubularin and MTMR2, phosphatidylinositol 3-phosphatases mutated in myotubular myopathy and type 4B Charcot-Marie-Tooth disease.
      ). Blondeau et al. (
      • Blondeau F.
      • Laporte J.
      • Bodin S.
      • Superti-Furga G.
      • Payrastre B.
      • Mandel J.L.
      Myotubularin, a phosphatase deficient in myotubular myopathy, acts on phosphatidylinositol 3-kinase and phosphatidylinositol 3-phosphate pathway.
      ) also demonstrated that MTM1 dephosphorylated PtdIns(3)P in vitro and, based on cell expression studies using Schizosaccharomyces pombe, reported that MTM1 dephosphorylated PtdIns(3,5)P2 in vivo. Subsequent studies demonstrated that MTM1 could dephosphorylate PtdIns(3)P (
      • Kim S.A.
      • Taylor G.S.
      • Torgersen K.M.
      • Dixon J.E.
      Myotubularin and MTMR2, phosphatidylinositol 3-phosphatases mutated in myotubular myopathy and type 4B Charcot-Marie-Tooth disease.
      ,
      • Cao C.
      • Backer J.M.
      • Laporte J.
      • Bedrick E.J.
      • Wandinger-Ness A.
      Sequential actions of myotubularin lipid phosphatases regulate endosomal PI(3)P and growth factor receptor trafficking.
      ,
      • Schaletzky J.
      • Dove S.K.
      • Short B.
      • Lorenzo O.
      • Clague M.J.
      • Barr F.A.
      Phosphatidylinositol-5-phosphate activation and conserved substrate specificity of the myotubularin phosphatidylinositol 3-phosphatases.
      ,
      • Tronchère H.
      • Laporte J.
      • Pendaries C.
      • Chaussade C.
      • Liaubet L.
      • Pirola L.
      • Mandel J.L.
      • Payrastre B.
      Production of phosphatidylinositol 5-phosphate by the phosphoinositide 3-phosphatase myotubularin in mammalian cells.
      ) and PtdIns(3,5)P2 (
      • Schaletzky J.
      • Dove S.K.
      • Short B.
      • Lorenzo O.
      • Clague M.J.
      • Barr F.A.
      Phosphatidylinositol-5-phosphate activation and conserved substrate specificity of the myotubularin phosphatidylinositol 3-phosphatases.
      ,
      • Tronchère H.
      • Laporte J.
      • Pendaries C.
      • Chaussade C.
      • Liaubet L.
      • Pirola L.
      • Mandel J.L.
      • Payrastre B.
      Production of phosphatidylinositol 5-phosphate by the phosphoinositide 3-phosphatase myotubularin in mammalian cells.
      ), both in vitro and in vivo.
      The MTMR proteins are close homologs of MTM. MTMR1 can dephosphorylate both PtdIns(3)P (
      • Kim S.A.
      • Taylor G.S.
      • Torgersen K.M.
      • Dixon J.E.
      Myotubularin and MTMR2, phosphatidylinositol 3-phosphatases mutated in myotubular myopathy and type 4B Charcot-Marie-Tooth disease.
      ,
      • Tronchère H.
      • Laporte J.
      • Pendaries C.
      • Chaussade C.
      • Liaubet L.
      • Pirola L.
      • Mandel J.L.
      • Payrastre B.
      Production of phosphatidylinositol 5-phosphate by the phosphoinositide 3-phosphatase myotubularin in mammalian cells.
      ) and PtdIns(3,5)P2 (
      • Tronchère H.
      • Laporte J.
      • Pendaries C.
      • Chaussade C.
      • Liaubet L.
      • Pirola L.
      • Mandel J.L.
      • Payrastre B.
      Production of phosphatidylinositol 5-phosphate by the phosphoinositide 3-phosphatase myotubularin in mammalian cells.
      ) in in vitro assays. There are also reports that MTMR1 dephosphorylates both lipids in vivo. MTMR2 is also able to dephosphorylate PtdIns(3)P in vitro (
      • Kim S.A.
      • Taylor G.S.
      • Torgersen K.M.
      • Dixon J.E.
      Myotubularin and MTMR2, phosphatidylinositol 3-phosphatases mutated in myotubular myopathy and type 4B Charcot-Marie-Tooth disease.
      ,
      • Cao C.
      • Backer J.M.
      • Laporte J.
      • Bedrick E.J.
      • Wandinger-Ness A.
      Sequential actions of myotubularin lipid phosphatases regulate endosomal PI(3)P and growth factor receptor trafficking.
      ,
      • Berger P.
      • Berger I.
      • Schaffitzel C.
      • Tersar K.
      • Volkmer B.
      • Suter U.
      Multi-level regulation of myotubularin-related protein-2 phosphatase activity by myotubularin-related protein-13/set-binding factor-2.
      ,
      • Berger P.
      • Bonneick S.
      • Willi S.
      • Wymann M.
      • Suter U.
      Loss of phosphatase activity in myotubularin-related protein 2 is associated with Charcot-Marie-Tooth disease type 4B1.
      ) and in vivo (
      • Cao C.
      • Backer J.M.
      • Laporte J.
      • Bedrick E.J.
      • Wandinger-Ness A.
      Sequential actions of myotubularin lipid phosphatases regulate endosomal PI(3)P and growth factor receptor trafficking.
      ) and has also been shown to dephosphorylate PtdIns(3,5)P2 in vitro (
      • Berger P.
      • Berger I.
      • Schaffitzel C.
      • Tersar K.
      • Volkmer B.
      • Suter U.
      Multi-level regulation of myotubularin-related protein-2 phosphatase activity by myotubularin-related protein-13/set-binding factor-2.
      ,
      • Berger P.
      • Bonneick S.
      • Willi S.
      • Wymann M.
      • Suter U.
      Loss of phosphatase activity in myotubularin-related protein 2 is associated with Charcot-Marie-Tooth disease type 4B1.
      ). In addition, MTMR2 can dephosphorylate Ins(1,3)P2, but with a 10-fold lower activity than when dephosphorylating the lipid substrates, making its physiological relevance questionable (
      • Kim S.A.
      • Taylor G.S.
      • Torgersen K.M.
      • Dixon J.E.
      Myotubularin and MTMR2, phosphatidylinositol 3-phosphatases mutated in myotubular myopathy and type 4B Charcot-Marie-Tooth disease.
      ). MTMR3 dephosphorylates PtdIns(3)P in vitro (
      • Kim S.A.
      • Taylor G.S.
      • Torgersen K.M.
      • Dixon J.E.
      Myotubularin and MTMR2, phosphatidylinositol 3-phosphatases mutated in myotubular myopathy and type 4B Charcot-Marie-Tooth disease.
      ,
      • Lorenzo O.
      • Urbé S.C.
      • Clague M.J.
      Analysis of phosphoinositide binding domain properties within the myotubularin-related protein MTMR3.
      ,
      • Walker D.M.
      • Urbé S.
      • Dove S.K.
      • Tenza D.
      • Raposo G.
      • Clague M.J.
      Characterization of MTMR3. An inositol lipid 3-phosphatase with novel substrate specificity.
      ,
      • Zhao R.
      • Qi Y.
      • Chen J.
      • Zhao Z.J.
      FYVE-DSP2, a FYVE domain-containing dual specificity protein phosphatase that dephosphorylates phosphotidylinositol 3-phosphate.
      ) and in vivo (
      • Walker D.M.
      • Urbé S.
      • Dove S.K.
      • Tenza D.
      • Raposo G.
      • Clague M.J.
      Characterization of MTMR3. An inositol lipid 3-phosphatase with novel substrate specificity.
      ), and PtdIns(3,5)P2 in vitro and in vivo (
      • Lorenzo O.
      • Urbé S.C.
      • Clague M.J.
      Analysis of phosphoinositide binding domain properties within the myotubularin-related protein MTMR3.
      ,
      • Walker D.M.
      • Urbé S.
      • Dove S.K.
      • Tenza D.
      • Raposo G.
      • Clague M.J.
      Characterization of MTMR3. An inositol lipid 3-phosphatase with novel substrate specificity.
      ). MTMR4 has been reported to dephosphorylate PtdIns(3)P in vitro (
      • Zhao R.
      • Qi Y.
      • Chen J.
      • Zhao Z.J.
      FYVE-DSP2, a FYVE domain-containing dual specificity protein phosphatase that dephosphorylates phosphotidylinositol 3-phosphate.
      ) and in vivo (
      • Naughtin M.J.
      • Sheffield D.A.
      • Rahman P.
      • Hughes W.E.
      • Gurung R.
      • Stow J.L.
      • Nandurkar H.H.
      • Dyson J.M.
      • Mitchell C.A.
      The myotubularin phosphatase MTMR4 regulates sorting from early endosomes.
      ). There are, however, no reports that PtdIns(3,5)P2 can be a substrate. MTMR6 dephosphorylates PtdIns(3)P in vitro (
      • Kim S.A.
      • Taylor G.S.
      • Torgersen K.M.
      • Dixon J.E.
      Myotubularin and MTMR2, phosphatidylinositol 3-phosphatases mutated in myotubular myopathy and type 4B Charcot-Marie-Tooth disease.
      ,
      • Schaletzky J.
      • Dove S.K.
      • Short B.
      • Lorenzo O.
      • Clague M.J.
      • Barr F.A.
      Phosphatidylinositol-5-phosphate activation and conserved substrate specificity of the myotubularin phosphatidylinositol 3-phosphatases.
      ,
      • Zou J.
      • Zhang C.
      • Marjanovic J.
      • Kisseleva M.V.
      • Majerus P.W.
      • Wilson M.P.
      Myotubularin-related protein (MTMR) 9 determines the enzymatic activity, substrate specificity and role in autophagy of MTMR8.
      ), and PtdIns(3,5)P2 both in vitro (
      • Schaletzky J.
      • Dove S.K.
      • Short B.
      • Lorenzo O.
      • Clague M.J.
      • Barr F.A.
      Phosphatidylinositol-5-phosphate activation and conserved substrate specificity of the myotubularin phosphatidylinositol 3-phosphatases.
      ,
      • Zou J.
      • Zhang C.
      • Marjanovic J.
      • Kisseleva M.V.
      • Majerus P.W.
      • Wilson M.P.
      Myotubularin-related protein (MTMR) 9 determines the enzymatic activity, substrate specificity and role in autophagy of MTMR8.
      ) and in vivo (
      • Zou J.
      • Zhang C.
      • Marjanovic J.
      • Kisseleva M.V.
      • Majerus P.W.
      • Wilson M.P.
      Myotubularin-related protein (MTMR) 9 determines the enzymatic activity, substrate specificity and role in autophagy of MTMR8.
      ). Nevertheless, there are no reports that MTMR6 can dephosphorylate PtdIns(3)P in vivo. The catalytically inactive MTMR9 is able, however, to regulate the activity of MTMR6 and, as such, determines the substrate specificity in vivo (
      • Zou J.
      • Zhang C.
      • Marjanovic J.
      • Kisseleva M.V.
      • Majerus P.W.
      • Wilson M.P.
      Myotubularin-related protein (MTMR) 9 determines the enzymatic activity, substrate specificity and role in autophagy of MTMR8.
      ). MTMR7 dephosphorylates PtdIns(3)P in vitro (
      • Mochizuki Y.
      • Majerus P.W.
      Characterization of myotubularin-related protein 7 and its binding partner, myotubularin-related protein 9.
      ), and MTMR9 has been shown to increase the in vitro activity (
      • Mochizuki Y.
      • Majerus P.W.
      Characterization of myotubularin-related protein 7 and its binding partner, myotubularin-related protein 9.
      ). MTMR8 dephosphorylates PtdIns(3)P and Ptd(3,5)P2 in vitro, but has only been shown to be active against PtdIns(3)P in vivo (
      • Zou J.
      • Zhang C.
      • Marjanovic J.
      • Kisseleva M.V.
      • Majerus P.W.
      • Wilson M.P.
      Myotubularin-related protein (MTMR) 9 determines the enzymatic activity, substrate specificity and role in autophagy of MTMR8.
      ). MTMR9 regulates activity of MTMR8 in vitro and in vivo, and determines substrate specificity in vivo (
      • Zou J.
      • Zhang C.
      • Marjanovic J.
      • Kisseleva M.V.
      • Majerus P.W.
      • Wilson M.P.
      Myotubularin-related protein (MTMR) 9 determines the enzymatic activity, substrate specificity and role in autophagy of MTMR8.
      ); it is thus a regulatory component rather than an activity, per se. MTMR14 dephosphorylates PtdIns(3)P and PtdIns(3,5)P2 in vitro, but only PtdIns(3)P in vivo (
      • Tosch V.
      • Rohde H.M.
      • Tronchere H.
      • Zanoteli E.
      • Monroy N.
      • Kretz C.
      • Dondaine N.
      • Payrastre B.
      • Mandel J.L.
      • Laporte J.
      A novel PtdIns3P and PtdIns(3,5)P2 phosphatase with an inactivating variant in centronuclear myopathy.
      ). These PtdIns(3)P and PtdIns(3,5)P2 substrate specificities, with no reported activities against PtdIns(3,4,5)P3, PtdIns(3,4)P2, or PtdIns(4,5)P2, make the importance of these phosphoinositide phosphatases in the regulation of cell proliferation and malignancy questionable, and indeed no tumors have been reported to exhibit mutation or altered expression of the MTMRs.

      PHOSPHOINOSITIDE 4-PHOSPHATASE ACTIVITIES AND CANCER

      Dephosphorylation of phosphoinositides at the 4-position is relatively infrequent. Many of the enzymes that catalyze this reaction were identified primarily as inositol polyphosphate phosphatases (INPPs) and, subsequently, as lipid phosphatases.
      INPP4A dephosphorylates PtdIns(3,4)P2 at the 4-position in vitro (
      • Norris F.A.
      • Auethavekiat V.
      • Majerus P.W.
      The isolation and characterization of cDNA encoding human and rat brain inositol polyphosphate 4-phosphatase.
      ) and in vivo (
      • Ivetac I.
      • Munday A.D.
      • Kisseleva M.V.
      • Zhang X-M.
      • Luff S.
      • Tiganis T.
      • Whisstock J.C.
      • Rowe T.
      • Majerus P.W.
      • Mitchell C.A.
      The type Iα inositol polyphosphate 4-phosphatase generates and terminates phosphoinositide 3-kinase signals on endosomes and the plasma membrane.
      ), and, to lesser extent, demonstrates activity against Ins(3,4)P2 and Ins(1,3,4)P3 in vitro (
      • Norris F.A.
      • Auethavekiat V.
      • Majerus P.W.
      The isolation and characterization of cDNA encoding human and rat brain inositol polyphosphate 4-phosphatase.
      ). The C2 domain of INNP4A is able to bind PtdIns(3,4)P2, PtdIns(4)P, and phosphatidylserine (
      • Ivetac I.
      • Munday A.D.
      • Kisseleva M.V.
      • Zhang X-M.
      • Luff S.
      • Tiganis T.
      • Whisstock J.C.
      • Rowe T.
      • Majerus P.W.
      • Mitchell C.A.
      The type Iα inositol polyphosphate 4-phosphatase generates and terminates phosphoinositide 3-kinase signals on endosomes and the plasma membrane.
      ). Furthermore, recombinant INPP4A lacking the C2 domain exhibits significantly reduced activity toward PtdIns(3,4)P2 in vitro, suggesting that this domain regulates substrate binding (
      • Ivetac I.
      • Munday A.D.
      • Kisseleva M.V.
      • Zhang X-M.
      • Luff S.
      • Tiganis T.
      • Whisstock J.C.
      • Rowe T.
      • Majerus P.W.
      • Mitchell C.A.
      The type Iα inositol polyphosphate 4-phosphatase generates and terminates phosphoinositide 3-kinase signals on endosomes and the plasma membrane.
      ).
      INPP4A overexpression in cells has been shown to reduce PtdIns(3,4)P2 (
      • Ivetac I.
      • Munday A.D.
      • Kisseleva M.V.
      • Zhang X-M.
      • Luff S.
      • Tiganis T.
      • Whisstock J.C.
      • Rowe T.
      • Majerus P.W.
      • Mitchell C.A.
      The type Iα inositol polyphosphate 4-phosphatase generates and terminates phosphoinositide 3-kinase signals on endosomes and the plasma membrane.
      ), suggesting that INPP4A might function to suppress AKT activity. Subsequent work using INPP4A −/− mouse embryonic fibroblasts (MEFs) demonstrated that the level of phosphorylation of AKT at both Thr308 and Ser473 in response to EGF was elevated compared with INPP4A +/+ MEFs (
      • Ivetac I.
      • Gurung R.
      • Hakim S.
      • Horan K.A.
      • Sheffield D.A.
      • Binge L.C.
      • Majerus P.W.
      • Tiganis T.
      • Mitchell C.A.
      Regulation of PI3K/AKT signaling and cellular transformation by inositol polyphosphate 4-phosphatase-1.
      ).
      Furthermore, loss of INPP4A resulted in increased cell growth, decreased apoptosis, and increased anchorage-independent growth, and formed tumors in nude mice, in keeping with activation of the AKT signaling cascade (
      • Ivetac I.
      • Gurung R.
      • Hakim S.
      • Horan K.A.
      • Sheffield D.A.
      • Binge L.C.
      • Majerus P.W.
      • Tiganis T.
      • Mitchell C.A.
      Regulation of PI3K/AKT signaling and cellular transformation by inositol polyphosphate 4-phosphatase-1.
      ). INPP4A is thus similar in effect to PTEN in regulating the AKT pathway and cell growth and malignancy.
      INPP4B also dephosphorylates PtdIns(3,4)P2 at the 4-position, and to lesser extent both Ins(3,4)P2 and Ins(1,3,4)P3 in vitro (
      • Norris F.A.
      • Atkins R.C.
      • Majerus P.W.
      The cDNA cloning and characterization of inositol polyphosphate 4-phosphatase type II. Evidence for conserved alternative splicing in the 4-phosphatase family.
      ) and PtdIns(3,4)P2 in vivo (
      • Norris F.A.
      • Atkins R.C.
      • Majerus P.W.
      Inositol polyphosphate 4-phosphatase is inactivated by calpain-mediated proteolysis in stimulated human platelets.
      ). The C2 domain of INNP4B selectively binds PtdIns(3,4,5)P3 and phosphatidic acid, suggesting that these lipids may play a role in regulating the localization and/or activity of INPP4B in vivo (
      • Ferron M.
      • Vacher J.
      Characterization of the murine Inpp4b gene and identification of a novel isoform.
      ). However, this hypothesis remains to be tested. More recently, INNP4B has been demonstrated to dephosphorylate PtdIns(3,4,5)P3 and PtdIns(3,4)P2 in vitro, showing a greater preference for PtdIns(3,4)P2 (
      • Gewinner C.
      • Wang Z.C.
      • Richardson A.L.
      • Teruya-Feldstein J.
      • Etemadmoghadam D.
      • Bowtell D.
      • Barretina J.
      • Lin W.M.
      • Rameh L.
      • Salmena L.
      • et al.
      Evidence that inositol polyphosphate 4-phosphatase type II is a tumor suppressor that inhibits PI3K signaling.
      ). Furthermore, from cellular expression experiments, INPP4B has been shown to selectively dephosphorylate PtdIns(3,4)P2 in vivo (
      • Gewinner C.
      • Wang Z.C.
      • Richardson A.L.
      • Teruya-Feldstein J.
      • Etemadmoghadam D.
      • Bowtell D.
      • Barretina J.
      • Lin W.M.
      • Rameh L.
      • Salmena L.
      • et al.
      Evidence that inositol polyphosphate 4-phosphatase type II is a tumor suppressor that inhibits PI3K signaling.
      ). The knockdown of INPP4B in a human mammary epithelial cell line resulted in anchorage-independent growth, increased cell migration, and elevated AKT phosphorylation in response to insulin (
      • Gewinner C.
      • Wang Z.C.
      • Richardson A.L.
      • Teruya-Feldstein J.
      • Etemadmoghadam D.
      • Bowtell D.
      • Barretina J.
      • Lin W.M.
      • Rameh L.
      • Salmena L.
      • et al.
      Evidence that inositol polyphosphate 4-phosphatase type II is a tumor suppressor that inhibits PI3K signaling.
      ). Furthermore, increased INPP4B expression resulted in reduced tumor growth in a xenograft mouse model; whereas, loss of INPP4B expression has been found to correlate with poor patient survival in both breast and ovarian cancer (
      • Gewinner C.
      • Wang Z.C.
      • Richardson A.L.
      • Teruya-Feldstein J.
      • Etemadmoghadam D.
      • Bowtell D.
      • Barretina J.
      • Lin W.M.
      • Rameh L.
      • Salmena L.
      • et al.
      Evidence that inositol polyphosphate 4-phosphatase type II is a tumor suppressor that inhibits PI3K signaling.
      ). The phosphatidic acid binding of INPP4B, if inhibitory or localizing, could further suggest a cooperative effect with increased phospholipase D activity, which has also been suggested to play a role in breast cancer, particularly metastasis (
      • Scott S.A.
      • Selvy P.E.
      • Buck J.R.
      • Cho H.P.
      • Criswell T.L.
      • Thomas A.L.
      • Armstrong M.D.
      • Arteaga C.L.
      • Lindsley C.W.
      • Brown H.A.
      Design of isoform-selective phospholipase D inhibitors that modulate cancer cell invasiveness.
      ).
      Fedele et al. (
      • Fedele C.G.
      • Ooms L.M.
      • Ho M.
      • Vieusseux J.
      • O'Toole S.A.
      • Millar E.K.
      • Lopez-Knowles E.
      • Sriratana A.
      • Gurung R.
      • Baglietto L.
      • et al.
      Inositol polyphosphate 4-phosphatase II regulates PI3K/Akt signaling and is lost in human basal-like breast cancers.
      ) reported that decreased INPP4B expression in human breast cancer cell lines elevates AKT signaling and results in increased tumor formation. In keeping with this, INPP4B expression is frequently lost in human breast cancer (
      • Fedele C.G.
      • Ooms L.M.
      • Ho M.
      • Vieusseux J.
      • O'Toole S.A.
      • Millar E.K.
      • Lopez-Knowles E.
      • Sriratana A.
      • Gurung R.
      • Baglietto L.
      • et al.
      Inositol polyphosphate 4-phosphatase II regulates PI3K/Akt signaling and is lost in human basal-like breast cancers.
      ). A reduction in expression of INPP4B in prostate cancer cell lines increases AKT phosphorylation and stimulates cell proliferation (
      • Hodgson M.C.
      • Shao L.J.
      • Frolov A.
      • Li R.
      • Peterson L.E.
      • Ayala G.
      • Ittmann M.M.
      • Weigel N.L.
      • Agoulnik I.U.
      Decreased expression and androgen regulation of the tumor suppressor gene INPP4B in prostate cancer.
      ), whereas overexpression of INPP4B inhibits prostate cancer cell invasion (
      • Hodgson M.C.
      • Deryugina E.I.
      • Suarez E.
      • Lopez S.M.
      • Lin D.
      • Xue H.
      • Gorlov I.P.
      • Wang Y.
      • Agoulnik I.U.
      INPP4B suppresses prostate cancer cell invasion.
      ). INPP4B expression is reduced in prostate cancer, and patients with lower expression of INPP4B show a decrease in recurrence-free survival (
      • Hodgson M.C.
      • Shao L.J.
      • Frolov A.
      • Li R.
      • Peterson L.E.
      • Ayala G.
      • Ittmann M.M.
      • Weigel N.L.
      • Agoulnik I.U.
      Decreased expression and androgen regulation of the tumor suppressor gene INPP4B in prostate cancer.
      ,
      • Rynkiewicz N.K.
      • Fedele C.G.
      • Chiam K.
      • Gupta R.
      • Kench J.G.
      • Ooms L.M.
      • McLean C.A.
      • Giles G.G.
      • Horvath L.G.
      • Mitchell C.A.
      INPP4B is highly expressed in prostate intermediate cells and its loss of expression in prostate carcinoma predicts for recurrence and poor long term survival.
      ). These results demonstrate that INPP4B, and probably INPP4A, is a tumor suppressor that inhibits PI3K/AKT signaling.
      TMEM55A (also known as type II PtdIns 4,5-bisphosphate 4-phosphatase) and TMEM55B (also known as type I PtdIns 4,5-bisphosphate 4-phosphatase) both contain two putative transmembrane domains at the C terminus, and they dephosphorylate PtdIns(4,5)P2 at the 4-position in vitro (
      • Ungewickell A.
      • Hugge C.
      • Kisseleva M.
      • Chang S.C.
      • Zou J.
      • Feng Y.
      • Galyov E.E.
      • Wilson M.
      • Majerus P.W.
      The identification and characterization of two phosphatidylinositol-4,5-bisphosphate 4-phosphatases.
      ). TMEM55B has been further reported to dephosphorylate PtdIns(4,5)P2 in vivo (
      • Ungewickell A.
      • Hugge C.
      • Kisseleva M.
      • Chang S.C.
      • Zou J.
      • Feng Y.
      • Galyov E.E.
      • Wilson M.
      • Majerus P.W.
      The identification and characterization of two phosphatidylinositol-4,5-bisphosphate 4-phosphatases.
      ,
      • Zou J.
      • Marjanovic J.
      • Kisseleva M.V.
      • Wilson M.
      • Majerus P.W.
      Type I phosphatidylinositol-4,5-bisphosphate 4-phosphatase regulates stress-induced apoptosis.
      ). In keeping with TMEM55 being unlikely to modulate the AKT pathway, there are no reports of TMEM55 involvement in cancer.

      PHOSPHOINOSITIDE 5-PHOSPHATASE ACTIVITIES AND CANCER

      The phosphoinositide 5-phosphatases are grouped into four types (I–IV).
      The 40 kDa type I inositol polyphosphate 5-phosphatase, also known as INPP5A, dephosphorylates Ins(1,4,5)P3 and Ins(1,3,4,5)P4 at the 5-position in vitro, but has no activity toward phosphoinositides (
      • De Smedt F.
      • Verjans B.
      • Mailleux P.
      • Erneux C.
      Cloning and expression of human brain type I inositol 1,4,5-trisphosphate 5-phosphatase. High levels of mRNA in cerebellar Purkinje cells.
      ,
      • Laxminarayan K.M.
      • Chan B.K.
      • Tetaz T.
      • Bird P.I.
      • Mitchell C.A.
      Characterization of a cDNA encoding the 43-kDa membrane-associated inositol-polyphosphate 5-phosphatase.
      ,
      • Laxminarayan K.M.
      • Matzaris M.
      • Speed C.J.
      • Mitchell C.A.
      Purification and characterization of a 43-kDa membrane-associated inositol polyphosphate 5-phosphatase from human placenta.
      ). There have been a number of studies that suggested a link between INPP5A activity and cancer. However, siRNA knockdown of INPP5A resulted in increased cellular levels of Ins(1,4,5)P3 and Ins(1,3,4,5)P4, leading to cell transformation and tumor formation in nude mice (
      • Speed C.J.
      • Little P.J.
      • Hayman J.A.
      • Mitchell C.A.
      Underexpression of the 43 kDa inositol polyphosphate 5-phosphatase is associated with cellular transformation.
      ). In addition, INPP5A expression is reduced in human cutaneous squamous cell carcinoma tumors (
      • Sekulic A.
      • Kim S.Y.
      • Hostetter G.
      • Savage S.
      • Einspahr J.G.
      • Prasad A.
      • Sagerman P.
      • Curiel-Lewandrowski C.
      • Krouse R.
      • Bowden G.T.
      • et al.
      Loss of inositol polyphosphate 5-phosphatase is an early event in development of cutaneous squamous cell carcinoma.
      ). Together these observations suggest that, if important, INPP5A will play a tumor suppressing rather than malignancy promoting role.
      The type II 5-phosphatases include INPP5B, oculocerebrorenal syndrome (OCRL) (INPP5F), synaptojanin 1 (SYNJ1/INPP5G), synaptojanin 2 (SYNJ2/INPP5H), INPP5J, and INPP5K [skeletal and muscle enriched INPP (SKIP)].
      INPP5B, also known as 75 kDa type II inositol polyphosphate 5-phosphatase, dephosphorylates PtdIns(3,4,5)P3 (
      • Jackson S.P.
      • Schoenwaelder S.M.
      • Matzaris M.
      • Brown S.
      • Mitchell C.A.
      Phosphatidylinositol 3,4,5-trisphosphate is a substrate for the 75 kDa inositol polyphosphate 5-phosphatase and a novel 5-phosphatase which forms a complex with the p85/p110 form of phosphoinositide 3-kinase.
      ,
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ), PtdIns(4,5)P2 (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ,
      • Jefferson A.B.
      • Majerus P.W.
      Properties of type II inositol polyphosphate 5-phosphatase.
      ,
      • Matzaris M.
      • Jackson S.P.
      • Laxminarayan K.M.
      • Speed C.J.
      • Mitchell C.A.
      Identification and characterization of the phosphatidylinositol-(4, 5)-bisphosphate 5-phosphatase in human platelets.
      ), Ins(1,4,5)P3 (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ,
      • Jefferson A.B.
      • Majerus P.W.
      Properties of type II inositol polyphosphate 5-phosphatase.
      ,
      • Matzaris M.
      • Jackson S.P.
      • Laxminarayan K.M.
      • Speed C.J.
      • Mitchell C.A.
      Identification and characterization of the phosphatidylinositol-(4, 5)-bisphosphate 5-phosphatase in human platelets.
      ), and Ins(1,3,4,5)P4 (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ) in vitro. Furthermore, INPP5B has been reported to dephosphorylate PtdIns(4,5)P2 in vivo (
      • Jefferson A.B.
      • Majerus P.W.
      Properties of type II inositol polyphosphate 5-phosphatase.
      ). INPP5B does not dephosphorylate PtdIns(3,5)P2 (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ). INPP5B has a similar domain organization to OCRL/INPP5F. However, INPP5B does not contain clathrin binding sites, and it has a C-terminal CAAX prenylation sequence (
      • Jefferson A.B.
      • Majerus P.W.
      Properties of type II inositol polyphosphate 5-phosphatase.
      ). To date no human disease or cancer has been associated with INPP5B.
      Lowe syndrome, also known as OCRL, is a rare human X-linked developmental disorder that affects brain, kidney, and eye function [reviewed in (
      • Mehta Z.B.
      • Pietka G.
      • Lowe M.
      The cellular and physiological functions of the Lowe syndrome protein OCRL1.
      )]. Mutations in the OCRL gene were demonstrated to be responsible for Lowe syndrome, and the OCRL protein was found to share similarity with INPP5B (
      • Attree O.
      • Olivos I.M.
      • Okabe I.
      • Bailey L.C.
      • Nelson D.L.
      • Lewis R.A.
      • McInnes R.R.
      • Nussbaum R.L.
      The Lowe's oculocerebrorenal syndrome gene encodes a protein highly homologous to inositol polyphosphate-5-phosphatase.
      ).
      Patients diagnosed with a related X-linked disorder, called Dent-2 disease, also carry mutations in OCRL (
      • Hoopes R.R.J.
      • Shrimpton A.E.
      • Knohl S.J.
      • Hueber P.
      • Hoppe B.
      • Matyus J.
      • Simckes A.
      • Tasic V.
      • Toenshoff B.
      • Suchy S.F.
      • et al.
      Dent disease with mutations in OCRL1.
      ).
      OCRL dephosphorylates Ins(1,4,5)P3 and Ins(1,3,4,5)P4 at the 5-position, and PtdIns(4,5)P2 in vitro, with a greater preference for the lipid substrate (
      • Zhang X.
      • Jefferson A.B.
      • Auethavekiat V.
      • Majerus P.W.
      The protein deficient in Lowe syndrome is a phosphatidylinositol-4,5-bisphosphate 5-phosphatase.
      ). Subsequent studies demonstrated that OCRL can also dephosphorylate PtdIns(3,4,5)P3 in vitro (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ). However, PtdIns(3,5)P2 is a very poor substrate for INPP5B in vitro (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ). Studies using cell lines isolated from patients diagnosed with Lowe syndrome have shown that OCRL dephosphorylates PtdIns(4,5)P2 in vivo (
      • Zhang X.
      • Hartz P.A.
      • Philip E.
      • Racusen L.C.
      • Majerus P.W.
      Cell lines from kidney proximal tubules of a patient with Lowe syndrome lack OCRL inositol polyphosphate 5-phosphatase and accumulate phosphatidylinositol 4,5-bisphosphate.
      ). Despite its similarity in elevating phosphoinositide signaling, there is surprisingly no association between OCRL and cancer.
      SYNJ1/INPP5G contains two phosphoinositide phosphatase domains: a Sac1-like phosphatase domain and an inositol 5-phosphatase domain (
      • Woscholski R.
      • Finan P.M.
      • Radley E.
      • Totty N.F.
      • Sterling A.E.
      • Hsuan J.J.
      • Waterfield M.D.
      • Parker P.J.
      Synaptojanin is the major constitutively active phosphatidylinositol-3,4,5-trisphosphate 5-phosphatase in rodent brain.
      ,
      • McPherson P.S.
      • Garcia E.P.
      • Slepnev V.I.
      • David C.
      • Zhang X.
      • Grabs D.
      • Sossin W.S.
      • Bauerfeind R.
      • Nemoto Y.
      • De Camilli P.
      A presynaptic inositol-5-phosphatase.
      ). SYNJ1 dephosphorylates PtdIns(4,5)P2 (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ,
      • McPherson P.S.
      • Garcia E.P.
      • Slepnev V.I.
      • David C.
      • Zhang X.
      • Grabs D.
      • Sossin W.S.
      • Bauerfeind R.
      • Nemoto Y.
      • De Camilli P.
      A presynaptic inositol-5-phosphatase.
      ,
      • Khvotchev M.
      • Sudhof T.C.
      Developmentally regulated alternative splicing in a novel synaptojanin.
      ,
      • Woscholski R.
      • Waterfield M.D.
      • Parker P.J.
      Purification and biochemical characterization of a mammalian phosphatidylinositol 3,4,5-trisphospate 5-phosphatase.
      ) and PtdIns(3,4,5)P3 (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ,
      • Woscholski R.
      • Finan P.M.
      • Radley E.
      • Totty N.F.
      • Sterling A.E.
      • Hsuan J.J.
      • Waterfield M.D.
      • Parker P.J.
      Synaptojanin is the major constitutively active phosphatidylinositol-3,4,5-trisphosphate 5-phosphatase in rodent brain.
      ,
      • Woscholski R.
      • Waterfield M.D.
      • Parker P.J.
      Purification and biochemical characterization of a mammalian phosphatidylinositol 3,4,5-trisphospate 5-phosphatase.
      ) in vitro. PtdIns(3,5)P2 is a poor substrate for SYNJ1 (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ). Furthermore, SYNJ1 can dephosphorylate Ins(1,4,5)P3 (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ,
      • McPherson P.S.
      • Garcia E.P.
      • Slepnev V.I.
      • David C.
      • Zhang X.
      • Grabs D.
      • Sossin W.S.
      • Bauerfeind R.
      • Nemoto Y.
      • De Camilli P.
      A presynaptic inositol-5-phosphatase.
      ), and to a lesser extent Ins(1,3,4,5)P4 (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ), in vitro. The isolated inositol 5-phosphatase domain of SYNJ1 dephosphorylates PtdIns(3,4,5)P3, PtdIns(4,5)P2, PtdIns(3,5)P2, Ins(1,4,5)P3, and Ins(1,3,4,5)P4 in vitro (
      • Tsujishita Y.
      • Guo S.
      • Stolz L.E.
      • York J.D.
      • Hurley J.H.
      Specificity determinants in phosphoinositide dephosphorylation: crystal structure of an archetypal inositol polyphosphate 5-phosphatase.
      ), and the isolated Sac1 domain of SYNJ1 can dephosphorylate PtdIns(3)P, PtdIns(4)P, and PtdIns(3,5)P2 in vitro (
      • Guo S.
      • Stolz L.E.
      • Lemrow S.M.
      • York J.D.
      SAC1-like domains of yeast SAC1, INP52, INP53 and of human synaptojanin encode polyphosphoinositide phosphatases.
      ).
      Consistent with these studies of the activities of the individual phosphatase domains, full length SYNJ1 has been reported to dephosphorylate both PtdIns(3)P and PtdIns(4)P, in addition to PtdIns(4,5)P2, in vitro (
      • Krebs C.E.
      • Karkheiran S.
      • Powell J.C.
      • Cao M.
      • Makarov V.
      • Darvish H.
      • Di Paolo G.
      • Walker R.H.
      • Shahidi G.A.
      • Buxbaum J.D.
      • et al.
      The Sac1 domain of SYNJ1 identified mutated in a family with early-onset progressive Parkinsonism with generalized seizures.
      ,
      • Mani M.
      • Lee S.Y.
      • Lucast L.
      • Cremona O.
      • Di Paolo G.
      • De Camilli P.
      • Ryan T.A.
      The dual phosphatase activity of synaptojanin1 is required for both efficient synaptic vesicle endocytosis and reavailability at nerve terminals.
      ). Furthermore, inactivating mutants in the Sac1 domain of SYNJ1 results in loss of activity toward PtdIns(3)P and PtdIns(4)P, but not PtdIns(4,5)P2, in vitro (
      • Krebs C.E.
      • Karkheiran S.
      • Powell J.C.
      • Cao M.
      • Makarov V.
      • Darvish H.
      • Di Paolo G.
      • Walker R.H.
      • Shahidi G.A.
      • Buxbaum J.D.
      • et al.
      The Sac1 domain of SYNJ1 identified mutated in a family with early-onset progressive Parkinsonism with generalized seizures.
      ,
      • Mani M.
      • Lee S.Y.
      • Lucast L.
      • Cremona O.
      • Di Paolo G.
      • De Camilli P.
      • Ryan T.A.
      The dual phosphatase activity of synaptojanin1 is required for both efficient synaptic vesicle endocytosis and reavailability at nerve terminals.
      ). Conversely, inactivating mutants in the inositol 5-phosphatase domain of SYNJ1 results in loss of activity toward PtdIns(4,5)P2, but not PtdIns(3)P and PtdIns(4)P (
      • Mani M.
      • Lee S.Y.
      • Lucast L.
      • Cremona O.
      • Di Paolo G.
      • De Camilli P.
      • Ryan T.A.
      The dual phosphatase activity of synaptojanin1 is required for both efficient synaptic vesicle endocytosis and reavailability at nerve terminals.
      ). The study of these mutants has established that both the Sac1 and the inositol 5-phosphatase domain are required for SYNJ1 function in endocytic recycling of synaptic vesicles (
      • Mani M.
      • Lee S.Y.
      • Lucast L.
      • Cremona O.
      • Di Paolo G.
      • De Camilli P.
      • Ryan T.A.
      The dual phosphatase activity of synaptojanin1 is required for both efficient synaptic vesicle endocytosis and reavailability at nerve terminals.
      ). PtdIns(4,5)P2 levels are elevated in SYNJ1 −/− primary cortical neurons compared with wild-type control cells; however, PtdIns(4)P levels are not altered (
      • Cremona O.
      • Di Paolo G.
      • Wenk M.R.
      • Luthi A.
      • Kim W.T.
      • Takei K.
      • Daniell L.
      • Nemoto Y.
      • Shears S.B.
      • Flavell R.A.
      • et al.
      Essential role of phosphoinositide metabolism in synaptic vesicle recycling.
      ). Elevated PtdIns(3,4,5)P3 levels in SYNJ1 whole brain tissue have only been observed when brain lysates were incubated with 32P-ATP (
      • Cremona O.
      • Di Paolo G.
      • Wenk M.R.
      • Luthi A.
      • Kim W.T.
      • Takei K.
      • Daniell L.
      • Nemoto Y.
      • Shears S.B.
      • Flavell R.A.
      • et al.
      Essential role of phosphoinositide metabolism in synaptic vesicle recycling.
      ).
      SYNJ2/INPP5H is a highly homologous enzyme to SYNJ1 and also contains a Sac1 phosphoinositide phosphatase domain and an inositol 5-phosphatase domain (
      • Khvotchev M.
      • Sudhof T.C.
      Developmentally regulated alternative splicing in a novel synaptojanin.
      ,
      • Nemoto Y.
      • Arribas M.
      • Haffner C.
      • De Camilli P.
      Synaptojanin 2, a novel synaptojanin isoform with a distinct targeting domain and expression pattern.
      ). In vitro, SYNJ2 has been reported to dephosphorylate PtdIns(3,4,5)P2 (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ) and PtdIns(4,5)P2 (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ,
      • Khvotchev M.
      • Sudhof T.C.
      Developmentally regulated alternative splicing in a novel synaptojanin.
      ). PtdIns(3,5)P2 is a poor substrate for SYNJ2 (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ). It has also been proposed that SYNJ2 can also dephosphorylate Ins(1,3,5)P3 and Ins(1,3,4,5)P4 in vitro (
      • Nemoto Y.
      • Arribas M.
      • Haffner C.
      • De Camilli P.
      Synaptojanin 2, a novel synaptojanin isoform with a distinct targeting domain and expression pattern.
      ); however, other groups have reported that Ins(1,3,5)P3 and Ins(1,3,4,5)P4 are not substrates for SYNJ2 (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ). The isolated Sac1 domains of SYNJ2 dephosphorylate PtdIns(3)P, PtdIns(4)P, and PtdIns(3,5)P2 in vitro (
      • Nemoto Y.
      • Wenk M.R.
      • Watanabe M.
      • Daniell L.
      • Murakami T.
      • Ringstad N.
      • Yamada H.
      • Takei K.
      • De Camilli P.
      Identification and characterization of a synaptojanin 2 splice isoform predominantly expressed in nerve terminals.
      ). However, it remains to be established whether full length SYNJ2 can dephosphorylate PtdIns(3)P or PtdIns(4)P. Furthermore, the substrate selectivity of SYNJ2 in cells has not been reported.
      There is no reported evidence that SYNJ1 and SYNJ2 are associated with cancer. However, SYNJ2 interacts with Rac1, and siRNA knockdown of either SYNJ2 or Rac1 in glioblastoma cells resulted in reduced cell migration and invasion (
      • Chuang Y.Y.
      • Tran N.L.
      • Rusk N.
      • Nakada M.
      • Berens M.E.
      • Symons M.
      Role of synaptojanin 2 in glioma cell migration and invasion.
      ), suggesting a possible, if as yet ill-defined, role in malignancy.
      INPP5J, also known as proline-rich inositol polyphosphate 5-phosphatase, was identified through sequence similarity to the conserved residues in the phosphoinositide 5-phosphatase catalytic domains (
      • Mochizuki Y.
      • Takenawa T.
      Novel inositol polyphosphate 5-phosphatase localizes at membrane ruffles.
      ). INPP5J dephosphorylates at the 5-position on the inositol ring of PtdIns(3,4,5)P3 (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ,
      • Ooms L.M.
      • Fedele C.G.
      • Astle M.V.
      • Ivetac I.
      • Cheung V.
      • Pearson R.B.
      • Layton M.J.
      • Forrai A.
      • Nandurkar H.H.
      • Mitchell C.A.
      The inositol polyphosphate 5phosphatase, PIPP, is a novel regulator of phosphoinositide 3-kinase-dependent neurite elongation.
      ), PtdIns(4,5)P2 (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ,
      • Mochizuki Y.
      • Takenawa T.
      Novel inositol polyphosphate 5-phosphatase localizes at membrane ruffles.
      ), Ins(1,4,5)P3, and Ins(1,3,4,5)P4 in vitro (
      • Mochizuki Y.
      • Takenawa T.
      Novel inositol polyphosphate 5-phosphatase localizes at membrane ruffles.
      ). Cellular studies, in which INPP5J was downregulated through the use of siRNA, showed recruitment of PtdIns(3,4,5)P3-binding PH domains to the plasma membrane of growth factor-stimulated COS-1 cells, thereby suggesting that INPP5J dephosphorylates PtdIns(3,4,5)P3 in vivo, pointing to this being an authentic lipid phosphatase (
      • Ooms L.M.
      • Fedele C.G.
      • Astle M.V.
      • Ivetac I.
      • Cheung V.
      • Pearson R.B.
      • Layton M.J.
      • Forrai A.
      • Nandurkar H.H.
      • Mitchell C.A.
      The inositol polyphosphate 5phosphatase, PIPP, is a novel regulator of phosphoinositide 3-kinase-dependent neurite elongation.
      ).
      Overexpression of INPP5J in PC12 cells decreased PtdIns(3,4,5)P2 levels (as determined by the cellular localization of a PtdIns(3,4,5)P3-specifc binding GFP-PH domain), and consequently decreased AKT Ser473 phosphorylation (
      • Ooms L.M.
      • Fedele C.G.
      • Astle M.V.
      • Ivetac I.
      • Cheung V.
      • Pearson R.B.
      • Layton M.J.
      • Forrai A.
      • Nandurkar H.H.
      • Mitchell C.A.
      The inositol polyphosphate 5phosphatase, PIPP, is a novel regulator of phosphoinositide 3-kinase-dependent neurite elongation.
      ). This study demonstrated that INPP5J is a negative regulator of PI3K/AKT signaling (
      • Ooms L.M.
      • Fedele C.G.
      • Astle M.V.
      • Ivetac I.
      • Cheung V.
      • Pearson R.B.
      • Layton M.J.
      • Forrai A.
      • Nandurkar H.H.
      • Mitchell C.A.
      The inositol polyphosphate 5phosphatase, PIPP, is a novel regulator of phosphoinositide 3-kinase-dependent neurite elongation.
      ). Subsequent studies have identified INPP5J to have a tumor suppressor function in melanoma (
      • Ye Y.
      • Jin L.
      • Wilmott J.S.
      • Hu W.L.
      • Yosufi B.
      • Thorne R.F.
      • Liu T.
      • Rizos H.
      • Yan X.G.
      • Dong L.
      • et al.
      PI(4,5)P2 5-phosphatase A regulates PI3K/Akt signalling and has a tumour suppressive role in human melanoma.
      ). INPP5J expression is downregulated in melanoma tissue, and the overexpression of INPP5J inhibited AKT activation (
      • Ye Y.
      • Jin L.
      • Wilmott J.S.
      • Hu W.L.
      • Yosufi B.
      • Thorne R.F.
      • Liu T.
      • Rizos H.
      • Yan X.G.
      • Dong L.
      • et al.
      PI(4,5)P2 5-phosphatase A regulates PI3K/Akt signalling and has a tumour suppressive role in human melanoma.
      ). Furthermore, overexpression of INPP5J inhibited cell proliferation, and ablated melanoma cell survival in vitro and melanoma growth in a xenograft mouse model (
      • Ye Y.
      • Jin L.
      • Wilmott J.S.
      • Hu W.L.
      • Yosufi B.
      • Thorne R.F.
      • Liu T.
      • Rizos H.
      • Yan X.G.
      • Dong L.
      • et al.
      PI(4,5)P2 5-phosphatase A regulates PI3K/Akt signalling and has a tumour suppressive role in human melanoma.
      ).
      INPP5J expression has been shown to be downregulated in esophageal squamous cell carcinoma (ESCC) tissue, compared with normal tissue (
      • Lin C.
      • Liu A.
      • Zhu J.
      • Zhang X.
      • Wu G.
      • Ren P.
      • Wu J.
      • Li M.
      • Li J.
      • Song L.
      miR-508 sustains phosphoinositide signalling and promotes aggressive phenotype of oesophageal squamous cell carcinoma.
      ). Furthermore overexpression of INPP5J in ESCC cell lines results in decreased pAKT Ser473, with decreased rates of both cell proliferation and anchorage-independent growth (
      • Lin C.
      • Liu A.
      • Zhu J.
      • Zhang X.
      • Wu G.
      • Ren P.
      • Wu J.
      • Li M.
      • Li J.
      • Song L.
      miR-508 sustains phosphoinositide signalling and promotes aggressive phenotype of oesophageal squamous cell carcinoma.
      ).
      Moreover, ESCC cell lines overexpressing INPP5J exhibited diminished tumor formation in a xenograft mouse model, compared with vector control cell lines with reduced pAKT Ser473 levels detected in the tumor tissue (
      • Lin C.
      • Liu A.
      • Zhu J.
      • Zhang X.
      • Wu G.
      • Ren P.
      • Wu J.
      • Li M.
      • Li J.
      • Song L.
      miR-508 sustains phosphoinositide signalling and promotes aggressive phenotype of oesophageal squamous cell carcinoma.
      ). This demonstrates, therefore, that INPP5J overexpression inhibits ESCC tumorigenicity in vitro and in vivo.
      INPP5K, also known as SKIP, was identified on the basis of sequence homology to other 5-phosphatases (
      • Ijuin T.
      • Mochizuki Y.
      • Fukami K.
      • Funaki M.
      • Asano T.
      • Takenawa T.
      Identification and characterization of a novel inositol polyphosphate 5-phosphatase.
      ). INPP5K dephosphorylates PtdIns(3,4,5)P3 at the 5-position (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ,
      • Ijuin T.
      • Mochizuki Y.
      • Fukami K.
      • Funaki M.
      • Asano T.
      • Takenawa T.
      Identification and characterization of a novel inositol polyphosphate 5-phosphatase.
      ,
      • Gurung R.
      • Tan A.
      • Ooms L.M.
      • McGrath M.J.
      • Huysmans R.D.
      • Munday A.D.
      • Prescott M.
      • Whisstock J.C.
      • Mitchell C.A.
      Identification of a novel domain in two mammalian inositol-polyphosphate 5-phosphatases that mediates membrane ruffle localization. The inositol 5-phosphatase skip localizes to the endoplasmic reticulum and translocates to membrane ruffles following epidermal growth factor stimulation.
      ), and similarly the 5-position of PtdIns(4,5)P2 (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ,
      • Ijuin T.
      • Mochizuki Y.
      • Fukami K.
      • Funaki M.
      • Asano T.
      • Takenawa T.
      Identification and characterization of a novel inositol polyphosphate 5-phosphatase.
      ,
      • Gurung R.
      • Tan A.
      • Ooms L.M.
      • McGrath M.J.
      • Huysmans R.D.
      • Munday A.D.
      • Prescott M.
      • Whisstock J.C.
      • Mitchell C.A.
      Identification of a novel domain in two mammalian inositol-polyphosphate 5-phosphatases that mediates membrane ruffle localization. The inositol 5-phosphatase skip localizes to the endoplasmic reticulum and translocates to membrane ruffles following epidermal growth factor stimulation.
      ). INPP5K has further been reported to dephosphorylate Ins(1,4,5)P3 and Ins(1,3,4,5)P4 in vitro in some studies (
      • Ijuin T.
      • Mochizuki Y.
      • Fukami K.
      • Funaki M.
      • Asano T.
      • Takenawa T.
      Identification and characterization of a novel inositol polyphosphate 5-phosphatase.
      ), but not others (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ,
      • Gurung R.
      • Tan A.
      • Ooms L.M.
      • McGrath M.J.
      • Huysmans R.D.
      • Munday A.D.
      • Prescott M.
      • Whisstock J.C.
      • Mitchell C.A.
      Identification of a novel domain in two mammalian inositol-polyphosphate 5-phosphatases that mediates membrane ruffle localization. The inositol 5-phosphatase skip localizes to the endoplasmic reticulum and translocates to membrane ruffles following epidermal growth factor stimulation.
      ). PtdIns(3,5)P2 is a very poor substrate for INPP5K (
      • Schmid A.C.
      • Wise H.M.
      • Mitchell C.A.
      • Nussbaum R.
      • Woscholski R.
      Type II phosphoinositide 5-phosphatases have unique sensitivities towards fatty acid composition and head group phosphorylation.
      ,
      • Gurung R.
      • Tan A.
      • Ooms L.M.
      • McGrath M.J.
      • Huysmans R.D.
      • Munday A.D.
      • Prescott M.
      • Whisstock J.C.
      • Mitchell C.A.
      Identification of a novel domain in two mammalian inositol-polyphosphate 5-phosphatases that mediates membrane ruffle localization. The inositol 5-phosphatase skip localizes to the endoplasmic reticulum and translocates to membrane ruffles following epidermal growth factor stimulation.
      ). INPP5K has been shown to regulate PtdIns(3,4,5)P3 levels in response to growth factors (
      • Ijuin T.
      • Takenawa T.
      SKIP negatively regulates insulin-induced GLUT4 translocation and membrane ruffle formation.
      ,
      • Ijuin T.
      • Yu Y.E.
      • Mizutani K.
      • Pao A.
      • Tateya S.
      • Tamori Y.
      • Bradley A.
      • Takenawa T.
      Increased insulin action in SKIP heterozygous knockout mice.
      ), suggesting that the physiological substrate for INPP5K is PtdIns(3,4,5)P3. In keeping with this, decreased expression of INPP5K in C2C12 myoblast cells has been reported to elevate AKT phosphorylation in response to insulin (
      • Ijuin T.
      • Mochizuki Y.
      • Fukami K.
      • Funaki M.
      • Asano T.
      • Takenawa T.
      Identification and characterization of a novel inositol polyphosphate 5-phosphatase.
      ). INPP5K expression is decreased in lung carcinoma (
      • Stearman R.S.
      • Dwyer-Nield L.
      • Zerbe L.
      • Blaine S.A.
      • Chan Z.
      • Bunn P.A.J.
      • Johnson G.L.
      • Hirsch F.R.
      • Merrick D.T.
      • Franklin W.A.
      • et al.
      Analysis of orthologous gene expression between human pulmonary adenocarcinoma and a carcinogen-induced murine model.
      ) and increased in renal cancer (
      • Jones J.
      • Otu H.
      • Spentzos D.
      • Kolia S.
      • Inan M.
      • Beecken W.D.
      • Fellbaum C.
      • Gu X.
      • Joseph M.
      • Pantuck A.J.
      • et al.
      Gene signatures of progression and metastasis in renal cell cancer.
      ). This contrasting data suggests that it is unlikely that INPP5K plays a role in malignancy and, further, there are no reports that INPP5K can function as a tumor suppressor.
      SHIP1/INPP5D and SHIP2/INPPL1 are SH2-containing type III inositol phosphatases (
      • Damen J.E.
      • Liu L.
      • Rosten P.
      • Humphries R.K.
      • Jefferson A.B.
      • Majerus P.W.
      • Krystal G.
      The 145-kDa protein induced to associate with Shc by multiple cytokines is an inositol tetraphosphate and phosphatidylinositol 3,4,5-triphosphate 5-phosphatase.