Advertisement

Hematopoiesis is regulated by cholesterol efflux pathways and lipid rafts: connections with cardiovascular diseases

Thematic Review Series: Biology of Lipid Rafts
  • Pooranee K. Morgan
    Affiliations
    Division of Immunometabolism,Baker Heart and Diabetes Institute, Melbourne, Australia

    School of Life Sciences,La Trobe University, Bundoora, Australia
    Search for articles by this author
  • Longhou Fang
    Correspondence
    To whom correspondence should be addressed
    Affiliations
    Center for Cardiovascular Regeneration,Houston Methodist, Houston, TX
    Search for articles by this author
  • Author Footnotes
    1 G. I. Lancaster and A. J. Murphy contributed equally to this work.
    Graeme I. Lancaster
    Footnotes
    1 G. I. Lancaster and A. J. Murphy contributed equally to this work.
    Affiliations
    Division of Immunometabolism,Baker Heart and Diabetes Institute, Melbourne, Australia
    Search for articles by this author
  • Author Footnotes
    1 G. I. Lancaster and A. J. Murphy contributed equally to this work.
    Andrew J. Murphy
    Footnotes
    1 G. I. Lancaster and A. J. Murphy contributed equally to this work.
    Affiliations
    Division of Immunometabolism,Baker Heart and Diabetes Institute, Melbourne, Australia

    School of Life Sciences,La Trobe University, Bundoora, Australia
    Search for articles by this author
  • Author Footnotes
    1 G. I. Lancaster and A. J. Murphy contributed equally to this work.
Open AccessPublished:August 30, 2019DOI:https://doi.org/10.1194/jlr.TR119000267
      Lipid rafts are highly ordered regions of the plasma membrane that are enriched in cholesterol and sphingolipids and play important roles in many cells. In hematopoietic stem and progenitor cells (HSPCs), lipid rafts house receptors critical for normal hematopoiesis. Lipid rafts also can bind and sequester kinases that induce negative feedback pathways to limit proliferative cytokine receptor cycling back to the cell membrane. Modulation of lipid rafts occurs through an array of mechanisms, with optimal cholesterol efflux one of the major regulators. As such, cholesterol homeostasis also regulates hematopoiesis. Increased lipid raft content, which occurs in response to changes in cholesterol efflux in the membrane, can result in prolonged receptor occupancy in the cell membrane and enhanced signaling. In addition, certain diseases, like diabetes, may contribute to lipid raft formation and affect cholesterol retention in rafts. In this review, we explore the role of lipid raft-related mechanisms in hematopoiesis and CVD (specifically, atherosclerosis) and discuss how defective cholesterol efflux pathways in HSPCs contribute to expansion of lipid rafts, thereby promoting myelopoiesis and thrombopoiesis. We also discuss the utility of cholesterol acceptors in contributing to lipid raft regulation and disruption, and highlight the potential to manipulate these pathways for therapeutic gain in CVD as well as other disorders with aberrant hematopoiesis.

      Abbreviations:

      AIBP
      apoA-I binding protein
      CBS
      common β subunit
      c-CBL
      castias B-lineage lymphoma
      CMP
      common myeloid progenitor
      CSF
      colony stimulating factor
      EC
      endothelial cell
      GM-CSF
      granulocyte-macrophage-colony stimulating factor
      GMP
      granulocyte macrophage progenitor
      HEC
      hemogenic endothelial cell
      HSC
      hematopoietic stem cell
      HSPC
      hematopoietic stem and progenitor cell
      IL
      interleukin
      MEP
      megakaryocytic-erythroid progenitor
      TGF
      transforming growth factor
      TLR
      toll-like receptor
      WTD
      Western type diet
      Lipid rafts are specialized regions of organization within the plasma membrane. Lipid rafts are enriched in cholesterol and sphingolipids, and the structural rigidity of these lipids underlies lipid raft formation. As a consequence of their specific composition, certain proteins, in particular transmembrane receptors, are more likely to concentrate within the ordered environment that lipid rafts provide. Moreover, upon stimulation, lipid rafts are capable of undergoing alterations to favor protein dimerization, phosphorylation, or cross-linking of receptor modifications to trigger intracellular downstream signaling (
      • Simons K.
      • Toomre D.
      Lipid rafts and signal transduction.
      ). Given cholesterol's critical importance in raft formation, changes in cholesterol efflux dramatically alter lipid raft abundance and the signaling pathways downstream of receptors contained within lipid rafts. Additionally, exogenous fatty acid production in settings such as obesity and diabetes can also contribute to increased lipid raft abundance to promote inflammation (
      • Wei X.
      • Song H.
      • Yin L.
      • Rizzo M.G.
      • Sidhu R.
      • Covey D.F.
      • Ory D.S.
      • Semenkovich C.F.
      Fatty acid synthesis configures the plasma membrane for inflammation in diabetes.
      ). Alterations in cholesterol efflux pathways can induce profound effects in proliferative cells, particularly those of the hematopoietic system. Hematopoiesis describes the production of mature blood cells and accumulating evidence shows that enhanced hematopoiesis occurs as a result of cellular lipid raft accumulation meditated by alterations to cholesterol efflux pathways (
      • Yvan-Charvet L.
      • Pagler T.
      • Gautier E.L.
      • Avagyan S.
      • Siry R.L.
      • Han S.
      • Welch C.L.
      • Wang N.
      • Randolph G.J.
      • Snoeck H.W.
      • et al.
      ATP-binding cassette transporters and HDL suppress hematopoietic stem cell proliferation.
      ,
      • Murphy A.J.
      • Akhtari M.
      • Tolani S.
      • Pagler T.
      • Bijl N.
      • Kuo C.L.
      • Wang M.
      • Sanson M.
      • Abramowicz S.
      • Welch C.
      • et al.
      ApoE regulates hematopoietic stem cell proliferation, monocytosis, and monocyte accumulation in atherosclerotic lesions in mice.
      ). This review highlights the correlation between defective cholesterol efflux pathways and lipid raft formation on disordered hematopoiesis in relation to CVDs.

      OVERVIEW OF HEMATOPOIESIS WITH A FOCUS ON THE MYELOID BRANCH

      Hematopoiesis is the process by which all mature blood cells are produced. Hematopoiesis occurs primarily in the bone marrow and proceeds in a hierarchical manner, with hematopoietic stem cells (HSCs) being at the apex of the hematopoietic tree. HSCs have the capability to self-renew or differentiate into all the different lineages of the immune system in addition to platelets and red blood cells. HSCs are subdivided into three major subpopulations: long-term HSCs, short-term HSCs, or multipotent progenitors (which can be further defined) (
      • Morrison S.J.
      • Wandycz A.M.
      • Hemmati H.D.
      • Wright D.E.
      • Weissman I.L.
      Identification of a lineage of multipotent hematopoietic progenitors.
      ,
      • Morrison S.J.
      • Weissman I.L.
      The long-term repopulating subset of hematopoietic stem cells is deterministic and isolatable by phenotype.
      ,
      • Grinenko T.
      • Eugster A.
      • Thielecke L.
      • Ramasz B.
      • Krüger A.
      • Dietz S.
      • Glauche I.
      • Gerbaulet A.
      • von Bonin M.
      • Basak O.
      • et al.
      Hematopoietic stem cells can differentiate into restricted myeloid progenitors before cell division in mice.
      ). These subpopulations of HSCs differ in their self-renewal and differentiation potential/capabilities, which can be immunophenotyped (
      • Al-Sharea A.
      • Lee M.K.S.
      • Purton L.E.
      • Hawkins E.D.
      • Murphy A.J.
      The haematopoietic stem cell niche: a new player in cardiovascular disease?.
      ). True HSCs predominantly exist in a nonreplicative and quiescent state; however, in the presence of growth factors or hematological stressors, HSCs enter the cell cycle and form lineage-distinct progenitor cells. These progenitor cells then mature into specific lineage cells that later populate the circulation and tissues.
      Myelopoiesis is a subdivision of hematopoiesis that specifically relates to the development of myeloid cells, for example, monocytes and neutrophils. During myelopoiesis, HSCs mature to give rise first to common myeloid progenitors (CMPs) that in turn further mature into lineage-committed megakaryocytic-erythroid progenitors (MEPs) or granulocyte macrophage progenitors (GMPs) (
      • Akashi K.
      • Traver D.
      • Miyamoto T.
      • Weissman I.L.
      A clonogenic common myeloid progenitor that gives rise to all myeloid lineages.
      ). While MEPs differentiate into megakaryocytes to produce platelets or erythroid progenitors to produce red blood cells, GMPs ultimately give rise to lineage-committed mature myeloid cells (
      • Akashi K.
      • Traver D.
      • Miyamoto T.
      • Weissman I.L.
      A clonogenic common myeloid progenitor that gives rise to all myeloid lineages.
      ,
      • Kondo M.
      • Weissman I.L.
      • Akashi K.
      Identification of clonogenic common lymphoid progenitors in mouse bone marrow.
      ). Myelopoiesis can also occur in sites other than the bone marrow in a process known as extramedullary hematopoiesis (
      • Robbins C.S.
      • Chudnovskiy A.
      • Rauch P.J.
      • Figueiredo J-L.
      • Iwamoto Y.
      • Gorbatov R.
      • Etzrodt M.
      • Weber G.F.
      • Ueno T.
      • van Rooijen N.
      • et al.
      Extramedullary hematopoiesis generates Ly-6C(high) monocytes that infiltrate atherosclerotic lesions.
      ). The initiating steps of extramedullary hematopoiesis involve the mobilization of hematopoietic stem and progenitor cells (HSPCs) from the bone marrow into the circulation where they populate tissues (e.g., liver and spleen) and proliferate/differentiate into lineage-specific cells, eventually establishing a primary site of hematopoiesis (
      • Robbins C.S.
      • Chudnovskiy A.
      • Rauch P.J.
      • Figueiredo J-L.
      • Iwamoto Y.
      • Gorbatov R.
      • Etzrodt M.
      • Weber G.F.
      • Ueno T.
      • van Rooijen N.
      • et al.
      Extramedullary hematopoiesis generates Ly-6C(high) monocytes that infiltrate atherosclerotic lesions.
      ,
      • Inra C.N.
      • Zhou B.O.
      • Acar M.
      • Murphy M.M.
      • Richardson J.
      • Zhao Z.
      • Morrison S.J.
      A perisinusoidal niche for extramedullary haematopoiesis in the spleen.
      ). Extramedullary hematopoiesis is particularly important in CVDs, such as myocardial infarction and atherosclerosis (
      • Robbins C.S.
      • Chudnovskiy A.
      • Rauch P.J.
      • Figueiredo J-L.
      • Iwamoto Y.
      • Gorbatov R.
      • Etzrodt M.
      • Weber G.F.
      • Ueno T.
      • van Rooijen N.
      • et al.
      Extramedullary hematopoiesis generates Ly-6C(high) monocytes that infiltrate atherosclerotic lesions.
      ,
      • Hill D.A.
      • Swanson P.E.
      Myocardial extramedullary hematopoiesis: a clinicopathologic study.
      ,
      • Dutta P.
      • Courties G.
      • Wei Y.
      • Leuschner F.
      • Gorbatov R.
      • Robbins C.S.
      • Iwamoto Y.
      • Thompson B.
      • Carlson A.L.
      • Heidt T.
      • et al.
      Myocardial infarction accelerates atherosclerosis.
      ), where it exacerbates the development of atherosclerosis.

      MYELOPOIESIS IS ENHANCED IN CVDs

      Atherosclerosis is a major underlying cause of CVD. Atherosclerosis is driven by the deposition of cholesterol-rich lipoproteins in the arterial wall and the consequent recruitment of myeloid cells, notably monocytes, which differentiate into macrophages once resident within the artery. Epidemiological studies have demonstrated that leukocytosis is an independent risk factor for CVD (
      • Friedman G.D.
      • Klatsky A.L.
      • Siegelaub A.B.
      The leukocyte count as a predictor of myocardial infarction.
      ,
      • Olivares R.
      • Ducimetière P.
      • Claude J.R.
      Monocyte count: a risk factor for coronary heart disease?.
      ,
      • Sweetnam P.M.
      • Thomas H.F.
      • Yarnell J.W.
      • Baker I.A.
      • Elwood P.C.
      Total and differential leukocyte counts as predictors of ischemic heart disease: the Caerphilly and Speedwell studies.
      ,
      • Lee C.D.
      • Folsom A.R.
      • Nieto F.J.
      • Chambless L.E.
      • Shahar E.
      • Wolfe D.A.
      White blood cell count and incidence of coronary heart disease and ischemic stroke and mortality from cardiovascular disease in African-American and White men and women: atherosclerosis risk in communities study.
      ). Importantly, pioneering work over the last decade has revealed that disordered hematopoiesis, specifically enhanced myelopoiesis, is central to the increased monocyte and neutrophil numbers in CVD (
      • Yvan-Charvet L.
      • Pagler T.
      • Gautier E.L.
      • Avagyan S.
      • Siry R.L.
      • Han S.
      • Welch C.L.
      • Wang N.
      • Randolph G.J.
      • Snoeck H.W.
      • et al.
      ATP-binding cassette transporters and HDL suppress hematopoietic stem cell proliferation.
      ,
      • Murphy A.J.
      • Akhtari M.
      • Tolani S.
      • Pagler T.
      • Bijl N.
      • Kuo C.L.
      • Wang M.
      • Sanson M.
      • Abramowicz S.
      • Welch C.
      • et al.
      ApoE regulates hematopoietic stem cell proliferation, monocytosis, and monocyte accumulation in atherosclerotic lesions in mice.
      ,
      • Dragoljevic D.
      • Kraakman M.J.
      • Nagareddy P.R.
      • Ngo D.
      • Shihata W.
      • Kammoun H.L.
      • Whillas A.
      • Lee M.K.S.
      • Al-Sharea A.
      • Pernes G.
      • et al.
      Defective cholesterol metabolism in haematopoietic stem cells promotes monocyte-driven atherosclerosis in rheumatoid arthritis.
      ,
      • Drechsler M.
      • Megens R.T.
      • van Zandvoort M.
      • Weber C.
      • Soehnlein O.
      Hyperlipidemia-triggered neutrophilia promotes early atherosclerosis.
      ,
      • Swirski F.K.
      • Libby P.
      • Aikawa E.
      • Alcaide P.
      • Luscinskas F.W.
      • Weissleder R.
      • Pittet M.J.
      Ly-6Chi monocytes dominate hypercholesterolemia-associated monocytosis and give rise to macrophages in atheromata.
      ,
      • Tacke F.
      • Alvarez D.
      • Kaplan T.J.
      • Jakubzick C.
      • Spanbroek R.
      • Llodra J.
      • Garin A.
      • Liu J.
      • Mack M.
      • van Rooijen N.
      • et al.
      Monocyte subsets differentially employ CCR2, CCR5, and CX3CR1 to accumulate within atherosclerotic plaques.
      ,
      • Tolani S.
      • Pagler T.A.
      • Murphy A.J.
      • Bochem A.E.
      • Abramowicz S.
      • Welch C.
      • Nagareddy P.R.
      • Holleran S.
      • Hovingh G.K.
      • Kuivenhoven J.A.
      • et al.
      Hypercholesterolemia and reduced HDL-C promote hematopoietic stem cell proliferation and monocytosis: studies in mice and FH children.
      ,

      Deleted in proof.

      ,
      • Murphy A.J.
      • Tall A.R.
      Disordered haematopoiesis and athero-thrombosis.
      ). This increased supply of circulating myeloid cells exacerbates the formation of atherosclerotic lesions. In Apoe−/− or Ldlr−/− atherosclerotic mice, deficiency in macrophage colony-stimulating factor, a key myeloid cell growth factor, reduced monocyte numbers and resulted in smaller atherosclerotic plaques (
      • Smith J.D.
      • Trogan E.
      • Ginsberg M.
      • Grigaux C.
      • Tian J.
      • Miyata M.
      Decreased atherosclerosis in mice deficient in both macrophage colony-stimulating factor (op) and apolipoprotein E.
      ,
      • Rajavashisth T.
      • Qiao J.H.
      • Tripathi S.
      • Tripathi J.
      • Mishra N.
      • Hua M.
      • Wang X.P.
      • Loussararian A.
      • Clinton S.
      • Libby P.
      • et al.
      Heterozygous osteopetrotic (op) mutation reduces atherosclerosis in LDL receptor- deficient mice.
      ,
      • Combadière C.
      • Potteaux S.
      • Rodero M.
      • Simon T.
      • Pezard A.
      • Esposito B.
      • Merval R.
      • Proudfoot A.
      • Tedgui A.
      • Mallat Z.
      Combined inhibition of CCL2, CX3CR1, and CCR5 abrogates Ly6C(hi) and Ly6C(lo) monocytosis and almost abolishes atherosclerosis in hypercholesterolemic mice.
      ). There is a broad consensus that hypercholesteremia enhances atherogenesis by increasing Ly6-Chi monocyte entry into atherosclerotic lesions (
      • Murphy A.J.
      • Akhtari M.
      • Tolani S.
      • Pagler T.
      • Bijl N.
      • Kuo C.L.
      • Wang M.
      • Sanson M.
      • Abramowicz S.
      • Welch C.
      • et al.
      ApoE regulates hematopoietic stem cell proliferation, monocytosis, and monocyte accumulation in atherosclerotic lesions in mice.
      ,
      • Tacke F.
      • Alvarez D.
      • Kaplan T.J.
      • Jakubzick C.
      • Spanbroek R.
      • Llodra J.
      • Garin A.
      • Liu J.
      • Mack M.
      • van Rooijen N.
      • et al.
      Monocyte subsets differentially employ CCR2, CCR5, and CX3CR1 to accumulate within atherosclerotic plaques.
      ,
      • Potteaux S.
      • Gautier E.L.
      • Hutchison S.B.
      • van Rooijen N.
      • Rader D.J.
      • Thomas M.J.
      • Sorci-Thomas M.G.
      • Randolph G.J.
      Suppressed monocyte recruitment drives macrophage removal from atherosclerotic plaques of Apoe-/- mice during disease regression.
      ,

      Deleted in proof.

      ,
      • Rahman K.
      • Vengrenyuk Y.
      • Ramsey S.A.
      • Vila N.R.
      • Girgis N.M.
      • Liu J.
      • Gusarova V.
      • Gromada J.
      • Weinstock A.
      • Moore K.J.
      • et al.
      Inflammatory Ly6Chi monocytes and their conversion to M2 macrophages drive atherosclerosis regression.
      ). Interestingly, monocytes generated in the spleen appear to preferentially migrate to the atherosclerotic plaque (
      • Robbins C.S.
      • Chudnovskiy A.
      • Rauch P.J.
      • Figueiredo J-L.
      • Iwamoto Y.
      • Gorbatov R.
      • Etzrodt M.
      • Weber G.F.
      • Ueno T.
      • van Rooijen N.
      • et al.
      Extramedullary hematopoiesis generates Ly-6C(high) monocytes that infiltrate atherosclerotic lesions.
      ). This association was also witnessed in other inflammatory diseases that cause accelerated atherogenesis, such as diabetes and rheumatoid arthritis (
      • Dragoljevic D.
      • Kraakman M.J.
      • Nagareddy P.R.
      • Ngo D.
      • Shihata W.
      • Kammoun H.L.
      • Whillas A.
      • Lee M.K.S.
      • Al-Sharea A.
      • Pernes G.
      • et al.
      Defective cholesterol metabolism in haematopoietic stem cells promotes monocyte-driven atherosclerosis in rheumatoid arthritis.
      ,
      • Nagareddy P.R.
      • Murphy A.J.
      • Stirzaker R.A.
      • Hu Y.
      • Yu S.
      • Miller R.G.
      • Ramkhelawon B.
      • Distel E.
      • Westerterp M.
      • Huang L.S.
      • et al.
      Hyperglycemia promotes myelopoiesis and impairs the resolution of atherosclerosis.
      ). Murine models of human diseases have been pivotal in investigating the molecular processes that underlie the development and progression of CVD. Cytokines meditating intracellular and extracellular pathways have been well-described in many CVD studies, and among their various functions are myeloid cell recruitment, activation, and proliferation.

      CYTOKINES INFLUENCE MYELOID CELLS TO PROLIFERATE AND DIFFERENTIATE

      By virtue of their diverse range of functions, cytokines are involved in numerous aspects of CVD. In the early stages of atherosclerosis, interferon (IFN)-γ and transforming growth factor (TGF)-β reorganize the cytoskeleton of the extracellular matrix to change the morphology of endothelial cells (ECs), which increases plaque permeability and accumulation of immune cells (
      • Libby P.
      Inflammation in atherosclerosis.
      ). Moreover, colony stimulating factors (CSFs) and inflammatory cytokines can promote myelopoiesis and inflammatory activation of myeloid cells, respectively, enhancing atherosclerotic plaque progression (
      • Fatkhullina A.R.
      • Peshkova I.O.
      • Koltsova E.K.
      The role of cytokines in the development of atherosclerosis.
      ). For example, macrophage-CSF deficiency reduced atherosclerotic lesion size in both diet-induced hypercholesteremia and Apoe−/− atherosclerotic mouse models (
      • Smith J.D.
      • Trogan E.
      • Ginsberg M.
      • Grigaux C.
      • Tian J.
      • Miyata M.
      Decreased atherosclerosis in mice deficient in both macrophage colony-stimulating factor (op) and apolipoprotein E.
      ,
      • Rajavashisth T.
      • Qiao J.H.
      • Tripathi S.
      • Tripathi J.
      • Mishra N.
      • Hua M.
      • Wang X.P.
      • Loussararian A.
      • Clinton S.
      • Libby P.
      • et al.
      Heterozygous osteopetrotic (op) mutation reduces atherosclerosis in LDL receptor- deficient mice.
      ). In addition to atherosclerosis, following a myocardial infarction, sympathetic activity initiates extramedullary hematopoiesis via the canonical inflammatory cytokine, interleukin (IL)-1β, which drives HSCs toward myelopoiesis (
      • Sager H.B.
      • Heidt T.
      • Hulsmans M.
      • Dutta P.
      • Courties G.
      • Sebas M.
      • Wojtkiewicz G.R.
      • Tricot B.
      • Iwamoto Y.
      • Sun Y.
      • et al.
      Targeting interleukin-1beta reduces leukocyte production after acute myocardial infarction.
      ,
      • Pietras E.M.
      • Mirantes-Barbeito C.
      • Fong S.
      • Loeffler D.
      • Kovtonyuk L.V.
      • Zhang S.
      • Lakshminarasimhan R.
      • Chin C.P.
      • Techner J-M.
      • Will B.
      • et al.
      Chronic interleukin-1 exposure drives haematopoietic stem cells towards precocious myeloid differentiation at the expense of self-renewal.
      ,
      • Nagareddy P.R.
      • Kraakman M.
      • Masters S.L.
      • Stirzaker R.A.
      • Gorman D.J.
      • Grant R.W.
      • Dragoljevic D.
      • Hong E.S.
      • Abdel-Latif A.
      • Smyth S.S.
      • et al.
      Adipose tissue macrophages promote myelopoiesis and monocytosis in obesity.
      ). Finally, we have recently shown that rheumatoid arthritis, an auto-inflammatory condition in which numerous cytokines are chronically elevated, is associated with disordered hematopoiesis and enhanced myelopoiesis through altered cholesterol metabolism, ultimately exacerbating atherosclerosis (
      • Dragoljevic D.
      • Kraakman M.J.
      • Nagareddy P.R.
      • Ngo D.
      • Shihata W.
      • Kammoun H.L.
      • Whillas A.
      • Lee M.K.S.
      • Al-Sharea A.
      • Pernes G.
      • et al.
      Defective cholesterol metabolism in haematopoietic stem cells promotes monocyte-driven atherosclerosis in rheumatoid arthritis.
      ).

      CYTOKINE RECEPTORS LOCALIZED IN LIPID RAFTS ENHANCE MYELOPOIESIS

      Lipid rafts are highly dynamic assemblies of proteins, cholesterol, and sphingolipids found within the bilayer of cellular membranes. An important function of lipid rafts is serving as a platform for cytokine and pattern recognition receptors [e.g., toll-like receptors (TLRs) and growth factor receptors]. The embedding of these receptors within lipid rafts facilitates the activation and, consequently, efficient downstream signaling. For instance, the TGF-β1 receptor is localized in lipid rafts of HSCs. Expansion and clustering of lipid rafts following a high-fat diet was reported to enhance dysregulation of the TGF-β1 receptor in lipid rafts (
      • Hermetet F.
      • Buffière A.
      • Aznague A.
      • Pais de Barros J-P.
      • Bastie J-N.
      • Delva L.
      • Quéré R.
      High-fat diet disturbs lipid raft/TGF-β signaling-mediated maintenance of hematopoietic stem cells in mouse bone marrow.
      ), stimulating downregulation of TGF-β1-induced HSC quiescence and, subsequently, promotion of hematopoiesis. Importantly, receptors for granulocyte-macrophage-CSF (GM-CSF) and IL-3, key effectors of myelopoiesis, are located within lipids rafts. GM-CSF meditates myeloid lineage specification of CMPs to monocytes and neutrophils, while IL-3 promotes proliferation and differentiation (
      • Testa U.
      • Fossati C.
      • Samoggia P.
      • Masciulli R.
      • Mariani G.
      • Hassan H.J.
      • Sposi N.M.
      • Guerriero R.
      • Rosato V.
      • Gabbianelli M.
      • et al.
      Expression of growth factor receptors in unilineage differentiation culture of purified hematopoietic progenitors.
      ,
      • Metcalf D.
      Hematopoietic cytokines.
      ). GM-CSF and IL-3 bind to a heterodimeric receptor complex consisting of a cytokine-specific α subunit and a common β subunit (CBS), which is shared by the receptors for GM-CSF, IL-3, and IL-5. Both α and β receptor subunits for GM-CSF and IL-3 are highly expressed on HSPCs, with the CBS in particular playing a critical role in hematopoiesis. CBS-deficient hematopoietic cells transplanted into Apoe−/− recipient mice displayed a decrease in peripheral neutrophil and monocyte counts (
      • Wang M.
      • Subramanian M.
      • Abramowicz S.
      • Murphy A.J.
      • Gonen A.
      • Witztum J.
      • Welch C.
      • Tabas I.
      • Westerterp M.
      • Tall A.R.
      Interleukin-3/granulocyte macrophage colony-stimulating factor receptor promotes stem cell expansion, monocytosis, and atheroma macrophage burden in mice with hematopoietic ApoE deficiency.
      ). Further investigation also revealed that depletion of CBS decreased the macrophage content of atherosclerotic lesions in Apoe−/− mice (
      • Wang M.
      • Subramanian M.
      • Abramowicz S.
      • Murphy A.J.
      • Gonen A.
      • Witztum J.
      • Welch C.
      • Tabas I.
      • Westerterp M.
      • Tall A.R.
      Interleukin-3/granulocyte macrophage colony-stimulating factor receptor promotes stem cell expansion, monocytosis, and atheroma macrophage burden in mice with hematopoietic ApoE deficiency.
      ), suggesting that lipid raft-located CBS is essential for hypercholesterolemia-driven myelopoiesis in both the spleen and bone marrow.

      DEFECTIVE CELLULAR CHOLESTEROL EFFLUX EXPANDS LIPID RAFTS TO PROMOTE MYELOPOIESIS

      It is well-established that cellular cholesterol homeostasis is an important factor in the regulation of hematopoiesis; specifically, increasing membrane cholesterol levels favors HSPC proliferation and mobilization, which increases the risk of atherosclerosis (
      • Swirski F.K.
      • Libby P.
      • Aikawa E.
      • Alcaide P.
      • Luscinskas F.W.
      • Weissleder R.
      • Pittet M.J.
      Ly-6Chi monocytes dominate hypercholesterolemia-associated monocytosis and give rise to macrophages in atheromata.
      ,
      • Westerterp M.
      • Gourion-Arsiquaud S.
      • Murphy A.J.
      • Shih A.
      • Cremers S.
      • Levine R.L.
      • Tall A.R.
      • Yvan-Charvet L.
      Regulation of hematopoietic stem and progenitor cell mobilization by cholesterol efflux pathways.
      ). Specifically, we have previously shown that either Apoe−/− or Ldlr−/− mice fed a Western type diet (WTD) had a significant increase in their populations of bone marrow HSPCs, CMPs, and GMPs, leading to increased blood leukocytes (
      • Murphy A.J.
      • Akhtari M.
      • Tolani S.
      • Pagler T.
      • Bijl N.
      • Kuo C.L.
      • Wang M.
      • Sanson M.
      • Abramowicz S.
      • Welch C.
      • et al.
      ApoE regulates hematopoietic stem cell proliferation, monocytosis, and monocyte accumulation in atherosclerotic lesions in mice.
      ).
      The abundance and functionality of lipid rafts is influenced by changes in cholesterol levels and also by changes in sphingolipid content. For example, in macrophages, cellular cholesterol depletion via extrinsic agents such as β-cyclodextrin disrupted lipid raft formation and stability (
      • Zimmer S.
      • Grebe A.
      • Bakke S.S.
      • Bode N.
      • Halvorsen B.
      • Ulas T.
      • Skjelland M.
      • De Nardo D.
      • Labzin L.I.
      • Kerksiek A.
      • et al.
      Cyclodextrin promotes atherosclerosis regression via macrophage reprogramming.
      ,
      • Han J.
      • Hajjar D.P.
      • Tauras J.M.
      • Nicholson A.C.
      Cellular cholesterol regulates expression of the macrophage type B scavenger receptor, CD36.
      ). Alternatively, increase in lipid rafts has inflammatory consequences (
      • Chakraborty D.
      • Banerjee S.
      • Sen A.
      • Banerjee K.K.
      • Das P.
      • Roy S.
      Leishmania donovani affects antigen presentation of macrophage by disrupting lipid rafts.
      ,
      • Nguyen D.H.
      • Taub D.
      Cholesterol is essential for macrophage inflammatory protein 1 beta binding and conformational integrity of CC chemokine receptor 5.
      ). In vivo, cellular cholesterol and lipid raft content are highly modulated by cellular cholesterol efflux pathways (
      • Shadan S.
      • James P.S.
      • Howes E.A.
      • Jones R.
      Cholesterol efflux alters lipid raft stability and distribution during capacitation of boar spermatozoa.
      ), several of which exist within hematopoietic cells (
      • Yvan-Charvet L.
      • Pagler T.
      • Gautier E.L.
      • Avagyan S.
      • Siry R.L.
      • Han S.
      • Welch C.L.
      • Wang N.
      • Randolph G.J.
      • Snoeck H.W.
      • et al.
      ATP-binding cassette transporters and HDL suppress hematopoietic stem cell proliferation.
      ,
      • Landry Y.D.
      • Denis M.
      • Nandi S.
      • Bell S.
      • Vaughan A.M.
      • Zha X.
      ATP-binding cassette transporter A1 expression disrupts raft membrane microdomains through its ATPase-related functions.
      ,
      • Koseki M.
      • Hirano K.
      • Masuda D.
      • Ikegami C.
      • Tanaka M.
      • Ota A.
      • Sandoval J.C.
      • Nakagawa-Toyama Y.
      • Sato S.B.
      • Kobayashi T.
      • et al.
      Increased lipid rafts and accelerated lipopolysaccharide-induced tumor necrosis factor-alpha secretion in Abca1-deficient macrophages.
      ). In particular, the active transport of cholesterol by ABC transporters that facilitate cholesterol efflux from the cytoplasm of the cell to exogenous apolipoproteins within lipoproteins are major regulators of cellular cholesterol and lipid raft content. With the strong association between CVD and impaired cholesterol efflux pathways, ABC transporters located in lipid rafts have been shown to play a critical role in cellular cholesterol efflux pathways in HSPCs (
      • Yvan-Charvet L.
      • Pagler T.
      • Gautier E.L.
      • Avagyan S.
      • Siry R.L.
      • Han S.
      • Welch C.L.
      • Wang N.
      • Randolph G.J.
      • Snoeck H.W.
      • et al.
      ATP-binding cassette transporters and HDL suppress hematopoietic stem cell proliferation.
      ,
      • Murphy A.J.
      • Bijl N.
      • Yvan-Charvet L.
      • Welch C.B.
      • Bhagwat N.
      • Reheman A.
      • Wang Y.
      • Shaw J.A.
      • Levine R.L.
      • Ni H.
      • et al.
      Cholesterol efflux in megakaryocyte progenitors suppresses platelet production and thrombocytosis.
      ). Genetic analysis of bone marrow HSPCs revealed high expression of Abca1, Abcg1, and Apoe, which was enhanced in the different progenitor cell subsets and HSPCs with the in vivo administration of LXR agonists (
      • Murphy A.J.
      • Akhtari M.
      • Tolani S.
      • Pagler T.
      • Bijl N.
      • Kuo C.L.
      • Wang M.
      • Sanson M.
      • Abramowicz S.
      • Welch C.
      • et al.
      ApoE regulates hematopoietic stem cell proliferation, monocytosis, and monocyte accumulation in atherosclerotic lesions in mice.
      ). Importantly, genetic deficiencies or loss of function of ABC transporters increases the formation of lipid rafts within HSPCs, leading to a myeloproliferative phenotype Fig. 1 (
      • Yvan-Charvet L.
      • Pagler T.
      • Gautier E.L.
      • Avagyan S.
      • Siry R.L.
      • Han S.
      • Welch C.L.
      • Wang N.
      • Randolph G.J.
      • Snoeck H.W.
      • et al.
      ATP-binding cassette transporters and HDL suppress hematopoietic stem cell proliferation.
      ). We have previously shown that defective cholesterol efflux via deletion of ABC transporters in HSPCs and/or their downstream myeloid cells through a number of pathways enhances inflammation in a number of tissues, while also causing HSPC proliferation, mobilization, and extramedullary hematopoiesis (
      • Yvan-Charvet L.
      • Pagler T.
      • Gautier E.L.
      • Avagyan S.
      • Siry R.L.
      • Han S.
      • Welch C.L.
      • Wang N.
      • Randolph G.J.
      • Snoeck H.W.
      • et al.
      ATP-binding cassette transporters and HDL suppress hematopoietic stem cell proliferation.
      ,
      • Westerterp M.
      • Gourion-Arsiquaud S.
      • Murphy A.J.
      • Shih A.
      • Cremers S.
      • Levine R.L.
      • Tall A.R.
      • Yvan-Charvet L.
      Regulation of hematopoietic stem and progenitor cell mobilization by cholesterol efflux pathways.
      ,
      • Westerterp M.
      • Bochem A.E.
      • Yvan-Charvet L.
      • Murphy A.J.
      • Wang N.
      • Tall A.R.
      ATP-binding cassette transporters, atherosclerosis, and inflammation.
      ,
      • Yvan-Charvet L.
      • Welch C.
      • Pagler T.A.
      • Ranalletta M.
      • Lamkanfi M.
      • Han S.
      • Ishibashi M.
      • Li R.
      • Wang N.
      • Tall A.R.
      Increased inflammatory gene expression in ABC transporter-deficient macrophages: free cholesterol accumulation, increased signaling via toll-like receptors, and neutrophil infiltration of atherosclerotic lesions.
      ). In Ldlr−/− mice, bone marrow deletion of Abcg4 in MEPs resulted in prominent platelet production and progression of atherosclerosis and thrombosis (
      • Murphy A.J.
      • Bijl N.
      • Yvan-Charvet L.
      • Welch C.B.
      • Bhagwat N.
      • Reheman A.
      • Wang Y.
      • Shaw J.A.
      • Levine R.L.
      • Ni H.
      • et al.
      Cholesterol efflux in megakaryocyte progenitors suppresses platelet production and thrombocytosis.
      ). Likewise, Abca1 and Abcg1 double knockout in macrophages or dendritic cells resulted in granulocyte-colony stimulating factor-dependent HSPC mobilization, which also likely contributes to accelerated atherogenesis (
      • Al-Sharea A.
      • Lee M.K.S.
      • Purton L.E.
      • Hawkins E.D.
      • Murphy A.J.
      The haematopoietic stem cell niche: a new player in cardiovascular disease?.
      ,
      • Westerterp M.
      • Gourion-Arsiquaud S.
      • Murphy A.J.
      • Shih A.
      • Cremers S.
      • Levine R.L.
      • Tall A.R.
      • Yvan-Charvet L.
      Regulation of hematopoietic stem and progenitor cell mobilization by cholesterol efflux pathways.
      ). Given the importance of ABC transporters in HSPC maintenance, the following sections highlight the role of lipid raft-resident ABC transporters on myelopoiesis and CVDs.
      Figure thumbnail gr2
      Fig. 1Defective cholesterol efflux via the loss of ABC transporters increases lipid raft formation to trigger myelopoiesis. A: Co-deletion of ABCA1 and ABCG1 in HSC and myeloid progenitors results in defective cholesterol efflux and increase in lipid rafts of the plasma membrane. This increases CBS expression on lipid rafts and amplifies GM-CSF/IL-3-induced myelopoiesis and monocyte and neutrophil production. B: Depletion of trans-Golgi-localized ABCG4 in megakaryocyte progenitors (MkP) results in defective cholesterol efflux and expansion of membrane lipid rafts. This causes increased expression of the TPO receptor, c-MPL, on MkPs, thus amplifying TPO-mediated megakaryopoiesis and platelet formation. In addition, the formation of lipid rafts indirectly diminishes Lyn-kinase activity, thus downregulating the phosphorylation of c-CBL and the degradation of c-MPL, to further enhance TPO-induced myelopoiesis.
      Figure thumbnail gr1

      ABC TRANSPORTERS AND THEIR ROLE IN LIPID RAFT-MEDITATED MYELOPOIESIS

      ABCA1

      Early studies identified the functions of ABCA1 in cell processes such as apoptosis and inflammation (
      • Hamon Y.
      • Luciani M.F.
      • Becq F.
      • Verrier B.
      • Rubartelli A.
      • Chimini G.
      Interleukin-1beta secretion is impaired by inhibitors of the ATP binding cassette transporter, ABC1.
      ,
      • Becq F.
      • Hamon Y.
      • Bajetto A.
      • Gola M.
      • Verrier B.
      • Chimini G.
      ABC1, an ATP binding cassette transporter required for phagocytosis of apoptotic cells, generates a regulated anion flux after expression in Xenopus laevis oocytes.
      ). A major advancement in the understanding of ABCA1 function was as a consequence of elucidating the basis of Tangier disease, a rare inherited disorder. In humans, mutations in the Abca1 gene result in familial HDL deficiency in which individuals have low HDL levels and apoA-I (
      • Brooks-Wilson A.
      • Marcil M.
      • Clee S.M.
      • Zhang L.H.
      • Roomp K.
      • van Dam M.
      • Yu L.
      • Brewer C.
      • Collins J.A.
      • Molhuizen H.O.
      • et al.
      Mutations in ABC1 in Tangier disease and familial high-density lipoprotein deficiency.
      ,
      • Rust S.
      • Rosier M.
      • Funke H.
      • Real J.
      • Amoura Z.
      • Piette J.C.
      • Deleuze J.F.
      • Brewer H.B.
      • Duverger N.
      • Denefle P.
      • et al.
      Tangier disease is caused by mutations in the gene encoding ATP-binding cassette transporter 1.
      ,
      • Bodzioch M.
      • Orso E.
      • Klucken J.
      • Langmann T.
      • Bottcher A.
      • Diederich W.
      • Drobnik W.
      • Barlage S.
      • Buchler C.
      • Porsch-Ozcurumez M.
      • et al.
      The gene encoding ATP-binding cassette transporter 1 is mutated in Tangier disease.
      ). The discovery of the mechanism behind Tangier disease revealed the critical role played by ABCA1 in HDL synthesis via the lipidation of apoA-I (
      • Lawn R.M.
      • Wade D.P.
      • Garvin M.R.
      • Wang X.
      • Schwartz K.
      • Porter J.G.
      • Seilhamer J.J.
      • Vaughan A.M.
      • Oram J.F.
      The Tangier disease gene product ABC1 controls the cellular apolipoprotein-mediated lipid removal pathway.
      ). In addition to low HDL and apoA-I levels, patients were characterized with elevated cholesterol accumulation in macrophages and tissues, which made patients at a moderately high risk of CVD (
      • Marcil M.
      • Brooks-Wilson A.
      • Clee S.M.
      • Roomp K.
      • Zhang L.H.
      • Yu L.
      • Collins J.A.
      • van Dam M.
      • Molhuizen H.O.
      • Loubster O.
      • et al.
      Mutations in the ABC1 gene in familial HDL deficiency with defective cholesterol efflux.
      ). A similar phenotype has been replicated in mice deficient in ABCA1 (
      • McNeish J.
      • Aiello R.J.
      • Guyot D.
      • Turi T.
      • Gabel C.
      • Aldinger C.
      • Hoppe K.L.
      • Roach M.L.
      • Royer L.J.
      • de Wet J.
      • et al.
      High density lipoprotein deficiency and foam cell accumulation in mice with targeted disruption of ATP-binding cassette transporter-1.
      ,
      • Christiansen-Weber T.A.
      • Voland J.R.
      • Wu Y.
      • Ngo K.
      • Roland B.L.
      • Nguyen S.
      • Peterson P.A.
      • Fung-Leung W.P.
      Functional loss of ABCA1 in mice causes severe placental malformation, aberrant lipid distribution, and kidney glomerulonephritis as well as high-density lipoprotein cholesterol deficiency.
      ). Conversely, the overexpression of ABCA1 increased plasma HDL and apoA-I expression in mice (
      • Vaisman B.L.
      • Lambert G.
      • Amar M.
      • Joyce C.
      • Ito T.
      • Shamburek R.D.
      • Cain W.J.
      • Fruchart-Najib J.
      • Neufeld E.D.
      • Remaley A.T.
      • et al.
      ABCA1 overexpression leads to hyperalphalipoproteinemia and increased biliary cholesterol excretion in transgenic mice.
      ). A significant pathological finding was identified by van Eck et al. (
      • van Eck M.
      • Bos I.S.
      • Kaminski W.E.
      • Orso E.
      • Rothe G.
      • Twisk J.
      • Bottcher A.
      • Van Amersfoort E.S.
      • Christiansen-Weber T.A.
      • Fung-Leung W.P.
      • et al.
      Leukocyte ABCA1 controls susceptibility to atherosclerosis and macrophage recruitment into tissues.
      ), where hematopoietic deletion of Abca1 in Ldlr−/− mice resulted in significantly advanced atherosclerotic lesions and infiltration of macrophages in major organs like the liver and spleen. Conversely, transgenic overexpression of ABCA1 specifically in macrophages led to a 65% reduction in aortic atherosclerosis compared with WT mice (
      • Joyce C.W.
      • Amar M.J.
      • Lambert G.
      • Vaisman B.L.
      • Paigen B.
      • Najib-Fruchart J.
      • Hoyt Jr., R.F.
      • Neufeld E.D.
      • Remaley A.T.
      • Fredrickson D.S.
      • et al.
      The ATP binding cassette transporter A1 (ABCA1) modulates the development of aortic atherosclerosis in C57BL/6 and apoE-knockout mice.
      ). An anti-inflammatory role of ABCA1 meditated by lipid rafts was identified in many studies (
      • Koseki M.
      • Hirano K.
      • Masuda D.
      • Ikegami C.
      • Tanaka M.
      • Ota A.
      • Sandoval J.C.
      • Nakagawa-Toyama Y.
      • Sato S.B.
      • Kobayashi T.
      • et al.
      Increased lipid rafts and accelerated lipopolysaccharide-induced tumor necrosis factor-alpha secretion in Abca1-deficient macrophages.
      ,
      • Zhu X.
      • Lee J.Y.
      • Timmins J.M.
      • Brown J.M.
      • Boudyguina E.
      • Mulya A.
      • Gebre A.K.
      • Willingham M.C.
      • Hiltbold E.M.
      • Mishra N.
      • et al.
      Increased cellular free cholesterol in macrophage-specific Abca1 knock-out mice enhances pro-inflammatory response of macrophages.
      ,
      • Murphy A.J.
      • Woollard K.J.
      • Suhartoyo A.
      • Stirzaker R.A.
      • Shaw J.
      • Sviridov D.
      • Chin-Dusting J.P.
      Neutrophil activation is attenuated by high-density lipoprotein and apolipoprotein A-I in in vitro and in vivo models of inflammation.
      ,
      • Murphy A.J.
      • Woollard K.J.
      • Hoang A.
      • Mukhamedova N.
      • Stirzaker R.A.
      • McCormick S.P.
      • Remaley A.T.
      • Sviridov D.
      • Chin-Dusting J.
      High-density lipoprotein reduces the human monocyte inflammatory response.
      ). Specifically, the loss of ABCA1 in macrophages impaired cholesterol efflux, resulting in the deposition of cholesterol to lipid rafts. These effects led to an increased release of inflammatory cytokines, such as TNFα, following treatment with LPS. Interestingly, another study showed that the increase in TNFα production induced ABCA1 expression through the NF-κB signaling pathway as a feedback mechanism to disperse lipid raft formation (
      • Gerbod-Giannone M.C.
      • Li Y.
      • Holleboom A.
      • Han S.
      • Hsu L.C.
      • Tabas I.
      • Tall A.R.
      TNFalpha induces ABCA1 through NF-kappaB in macrophages and in phagocytes ingesting apoptotic cells.
      ).

      ABCG1

      ABCG1 is a critical mediator of cholesterol efflux to mature HDL. Several human and murine studies demonstrate that, unlike ABCA1, which can efflux cholesterol to lipid-poor HDL and lipid-free apoA-I, ABCG1 promotes cholesterol efflux solely to mature HDL (
      • Wang N.
      • Lan D.
      • Chen W.
      • Matsuura F.
      • Tall A.R.
      ATP-binding cassette transporters G1 and G4 mediate cellular cholesterol efflux to high-density lipoproteins.
      ,
      • Heinecke J.W.
      Small HDL promotes cholesterol efflux by the ABCA1 pathway in macrophages: implications for therapies targeted to HDL.
      ). As anticipated, a high-cholesterol diet and targeted disruption of ABCG1 led to cellular lipid accumulation in tissue macrophages, while ABCG1 overexpression protected against lipid deposition (
      • Kennedy M.A.
      • Barrera G.C.
      • Nakamura K.
      • Baldan A.
      • Tarr P.
      • Fishbein M.C.
      • Frank J.
      • Francone O.L.
      • Edwards P.A.
      ABCG1 has a critical role in mediating cholesterol efflux to HDL and preventing cellular lipid accumulation.
      ). Paradoxically, with the exception of one study (
      • Out R.
      • Hoekstra M.
      • Hildebrand R.B.
      • Kruit J.K.
      • Meurs I.
      • Li Z.
      • Kuipers F.
      • Van Berkel T.J.
      • Van Eck M.
      Macrophage ABCG1 deletion disrupts lipid homeostasis in alveolar macrophages and moderately influences atherosclerotic lesion development in LDL receptor-deficient mice.
      ), transplantation of Abcg1-deficient bone marrow into either Apoe−/− or Ldlr−/− mice reduced atherosclerotic aortic lesions (
      • Baldán A.
      • Pei L.
      • Lee R.
      • Tarr P.
      • Tangirala R.K.
      • Weinstein M.M.
      • Frank J.
      • Li A.C.
      • Tontonoz P.
      • Edwards P.A.
      Impaired development of atherosclerosis in hyperlipidemic Ldlr-/- and ApoE-/- mice transplanted with Abcg1-/- bone marrow.
      ,
      • Ranalletta M.
      • Wang N.
      • Han S.
      • Yvan-Charvet L.
      • Welch C.
      • Tall A.R.
      Decreased atherosclerosis in low-density lipoprotein receptor knockout mice transplanted with Abcg1−/− bone marrow.
      ). However, this decreased anti-atherogenic role was attributed to increased macrophage apoptosis due to oxidized LDL ingestion (
      • Baldán A.
      • Pei L.
      • Lee R.
      • Tarr P.
      • Tangirala R.K.
      • Weinstein M.M.
      • Frank J.
      • Li A.C.
      • Tontonoz P.
      • Edwards P.A.
      Impaired development of atherosclerosis in hyperlipidemic Ldlr-/- and ApoE-/- mice transplanted with Abcg1-/- bone marrow.
      ) or compensatory upregulation of ABCA1 and increased ApoE secretion in Abcg1−/− macrophages (
      • Ranalletta M.
      • Wang N.
      • Han S.
      • Yvan-Charvet L.
      • Welch C.
      • Tall A.R.
      Decreased atherosclerosis in low-density lipoprotein receptor knockout mice transplanted with Abcg1−/− bone marrow.
      ). Gelissen et al. (
      • Gelissen I.C.
      • Harris M.
      • Rye K.A.
      • Quinn C.
      • Brown A.J.
      • Kockx M.
      • Cartland S.
      • Packianathan M.
      • Kritharides L.
      • Jessup W.
      ABCA1 and ABCG1 synergize to mediate cholesterol export to apoA-I.
      ) proposed a synergistic role of ABC transporters, where ABCG1 promotes cholesterol efflux to phospholipid-rich nascent HDL particles generated by the lipidation of apoA-I via the ABCA1 transporter. Convincingly, Yvan-Charvet et al. (
      • Yvan-Charvet L.
      • Ranalletta M.
      • Wang N.
      • Han S.
      • Terasaka N.
      • Li R.
      • Welch C.
      • Tall A.R.
      Combined deficiency of ABCA1 and ABCG1 promotes foam cell accumulation and accelerates atherosclerosis in mice.
      ) discovered that hematopoietic co-deletion of both ABCA1 and ABCG1 resulted in an exacerbated pathology including foam cell infiltration of the myocardium, with larger lesion sizes in the proximal aorta. Further investigation revealed that Abca1−/−Abcg1−/− macrophages stimulated with oxidized LDL undergo apoptosis and express inflammatory genes (
      • Yvan-Charvet L.
      • Ranalletta M.
      • Wang N.
      • Han S.
      • Terasaka N.
      • Li R.
      • Welch C.
      • Tall A.R.
      Combined deficiency of ABCA1 and ABCG1 promotes foam cell accumulation and accelerates atherosclerosis in mice.
      ). In addition to increased lipid raft formation, many studies have reported that Abca1−/−Abcg1−/− mice present with marked leukocytosis accompanied with infiltration and accumulation of inflammatory cell infiltrates in various organs (
      • Yvan-Charvet L.
      • Pagler T.
      • Gautier E.L.
      • Avagyan S.
      • Siry R.L.
      • Han S.
      • Welch C.L.
      • Wang N.
      • Randolph G.J.
      • Snoeck H.W.
      • et al.
      ATP-binding cassette transporters and HDL suppress hematopoietic stem cell proliferation.
      ,
      • Yvan-Charvet L.
      • Welch C.
      • Pagler T.A.
      • Ranalletta M.
      • Lamkanfi M.
      • Han S.
      • Ishibashi M.
      • Li R.
      • Wang N.
      • Tall A.R.
      Increased inflammatory gene expression in ABC transporter-deficient macrophages: free cholesterol accumulation, increased signaling via toll-like receptors, and neutrophil infiltration of atherosclerotic lesions.
      ,
      • Yvan-Charvet L.
      • Ranalletta M.
      • Wang N.
      • Han S.
      • Terasaka N.
      • Li R.
      • Welch C.
      • Tall A.R.
      Combined deficiency of ABCA1 and ABCG1 promotes foam cell accumulation and accelerates atherosclerosis in mice.
      ,
      • Out R.
      • Hoekstra M.
      • Habets K.
      • Meurs I.
      • de Waard V.
      • Hildebrand R.B.
      • Wang Y.
      • Chimini G.
      • Kuiper J.
      • Van Berkel T.J.
      • et al.
      Combined deletion of macrophage ABCA1 and ABCG1 leads to massive lipid accumulation in tissue macrophages and distinct atherosclerosis at relatively low plasma cholesterol levels.
      ,
      • Out R.
      • Jessup W.
      • Le Goff W.
      • Hoekstra M.
      • Gelissen I.C.
      • Zhao Y.
      • Kritharides L.
      • Chimini G.
      • Kuiper J.
      • Chapman M.J.
      • et al.
      Coexistence of foam cells and hypocholesterolemia in mice lacking the ABC transporters A1 and G1.
      ). This inflammatory phenotype is driven via the hematopoietic compartment as bone marrow restricted deletion of Abca1 and/or Abcg1 resulted in accelerated hematopoiesis. (
      • Yvan-Charvet L.
      • Pagler T.
      • Gautier E.L.
      • Avagyan S.
      • Siry R.L.
      • Han S.
      • Welch C.L.
      • Wang N.
      • Randolph G.J.
      • Snoeck H.W.
      • et al.
      ATP-binding cassette transporters and HDL suppress hematopoietic stem cell proliferation.
      ). Combined Abca1 and Abcg1 deficiency led to an increase in membrane cholesterol, which increased the cellular lipid raft content of the HSPCs within the bone marrow (
      • Yvan-Charvet L.
      • Pagler T.
      • Gautier E.L.
      • Avagyan S.
      • Siry R.L.
      • Han S.
      • Welch C.L.
      • Wang N.
      • Randolph G.J.
      • Snoeck H.W.
      • et al.
      ATP-binding cassette transporters and HDL suppress hematopoietic stem cell proliferation.
      ). Importantly, this increase in membrane cholesterol content resulted in a secondary increase in the expression of the CBS on HSPCs, amplifying the effects of GM-CSF and IL-3, notably the activation of the signaling proteins, ERK1/2, and signal transducer and activator of transcription (STAT)5. Cumulatively, these effects lead to HSPC proliferation, enhanced myelopoiesis, and accelerated atherosclerosis (Fig. 1A). The importance of membrane cholesterol, and by extension lipid rafts, in this process is further highlighted by the fact that HSPCs from Abca1−/−Abcg1−/− mice that simultaneously harbor a transgene expressing apoA-I exhibit reduced CBS expression (
      • Yvan-Charvet L.
      • Pagler T.
      • Gautier E.L.
      • Avagyan S.
      • Siry R.L.
      • Han S.
      • Welch C.L.
      • Wang N.
      • Randolph G.J.
      • Snoeck H.W.
      • et al.
      ATP-binding cassette transporters and HDL suppress hematopoietic stem cell proliferation.
      ). As mentioned, Wang et al. (
      • Wang M.
      • Subramanian M.
      • Abramowicz S.
      • Murphy A.J.
      • Gonen A.
      • Witztum J.
      • Welch C.
      • Tabas I.
      • Westerterp M.
      • Tall A.R.
      Interleukin-3/granulocyte macrophage colony-stimulating factor receptor promotes stem cell expansion, monocytosis, and atheroma macrophage burden in mice with hematopoietic ApoE deficiency.
      ) formally confirmed the requirement of the CBS of the IL-3/GM-CSF receptor in HSPCs through genetic studies. Additionally, Westerterp et al. (
      • Westerterp M.
      • Gourion-Arsiquaud S.
      • Murphy A.J.
      • Shih A.
      • Cremers S.
      • Levine R.L.
      • Tall A.R.
      • Yvan-Charvet L.
      Regulation of hematopoietic stem and progenitor cell mobilization by cholesterol efflux pathways.
      ) reported an enhancement in granulocyte-colony stimulating factor-dependent HSPC mobilization and extramedullary hematopoiesis in spleen and liver of Abca1−/−Abcg1−/− mice, underlining the importance of ABCA1 and ABCG1 in the regulation of cellular cholesterol levels and myelopoiesis.

      ABCG4

      ABCG4 is closely related to ABCG1 and was discovered by Wang et al. (
      • Wang N.
      • Lan D.
      • Chen W.
      • Matsuura F.
      • Tall A.R.
      ATP-binding cassette transporters G1 and G4 mediate cellular cholesterol efflux to high-density lipoproteins.
      ) to mediate cholesterol efflux to HDL. However, unlike ABCA1 and ABCG1, ABCG4 is not expressed in macrophage foam cells and does not regulate cholesterol efflux in macrophages (
      • Wang N.
      • Ranalletta M.
      • Matsuura F.
      • Peng F.
      • Tall A.R.
      LXR-induced redistribution of ABCG1 to plasma membrane in macrophages enhances cholesterol mass efflux to HDL.
      ,
      • Demeester N.
      • Castro G.
      • Desrumaux C.
      • De Geitere C.
      • Fruchart J.C.
      • Santens P.
      • Mulleners E.
      • Engelborghs S.
      • De Deyn P.P.
      • Vandekerckhove J.
      • et al.
      Characterization and functional studies of lipoproteins, lipid transfer proteins, and lecithin:cholesterol acyltransferase in CSF of normal individuals and patients with Alzheimer's disease.
      ). We discovered that ABCG4 is selectively expressed in megakaryocyte progenitors and is localized in the trans-Golgi (
      • Murphy A.J.
      • Bijl N.
      • Yvan-Charvet L.
      • Welch C.B.
      • Bhagwat N.
      • Reheman A.
      • Wang Y.
      • Shaw J.A.
      • Levine R.L.
      • Ni H.
      • et al.
      Cholesterol efflux in megakaryocyte progenitors suppresses platelet production and thrombocytosis.
      ). Bone marrow Abcg4 deficiency resulted in defective cholesterol efflux to HDL and increased cholesterol content. Importantly, the accumulation of cholesterol was accompanied by an increase in the surface expression of the thrombopoietin receptor (c-MPL) on megakaryocyte progenitors, amplifying the effects of TPO-induced proliferation of megakaryocytes and differentiation to platelets, thus accelerating atherosclerosis and thrombosis. Notably, we highlighted that the role of ABCG4 on hematopoiesis potentially acts in a Lck/Yes novel (LYN) kinase-dependent fashion with a strong association to E3 ligase castias B-lineage lymphoma (c-CBL). Loss of ABCG4 leads to elevated membrane cholesterol content due to impaired efflux, which decreases LYN kinase activity and its downstream regulatory effects on c-CBL. As a consequence, the negative feedback of phosphorylated c-CBL on c-MPL expression is diminished and there is an increased expression of c-MPL, sensitivity to TPO, and activation of ERK1/2, AKT, and STAT5. These effects lead to enhanced megakaryocyte progenitor proliferation and platelet differentiation and increased atherosclerosis and thrombosis (Fig. 1B). Further highlighting the importance of ABCG4, a single or double dose of HDL infusion reduced megakaryocyte progenitors and platelet counts in WT mice but not in Abcg4−/− mice, showing a strong dependence of HDL on ABCG4 for cholesterol efflux in megakaryocyte progenitors. Interestingly, this is reflected in patients with peripheral vascular disease, where a single dose of rHDL significantly decreased platelet levels as compared with placebo (
      • Murphy A.J.
      • Bijl N.
      • Yvan-Charvet L.
      • Welch C.B.
      • Bhagwat N.
      • Reheman A.
      • Wang Y.
      • Shaw J.A.
      • Levine R.L.
      • Ni H.
      • et al.
      Cholesterol efflux in megakaryocyte progenitors suppresses platelet production and thrombocytosis.
      ). Cumulatively, these findings emphasize a close relationship between ABCG4-regulated cholesterol content (potentially increasing lipid rafts) in megakaryocyte progenitors and disordered thrombopoiesis and atherosclerosis.

      AIBP: A NEW PLAYER IN CHOLESTEROL-MEDIATED HEMATOPOIESIS AND INFLAMMATION

      Several current therapeutic interventions aim at promoting cholesterol efflux via HDL mimetics or by increasing the expression of ABC transporters via LXR agonists (
      • Rigamonti E.
      • Helin L.
      • Lestavel S.
      • Mutka A.L.
      • Lepore M.
      • Fontaine C.
      • Bouhlel M.A.
      • Bultel S.
      • Fruchart J.C.
      • Ikonen E.
      • et al.
      Liver X receptor activation controls intracellular cholesterol trafficking and esterification in human macrophages.
      ,
      • Peng D.
      • Hiipakka R.A.
      • Xie J.T.
      • Dai Q.
      • Kokontis J.M.
      • Reardon C.A.
      • Getz G.S.
      • Liao S.
      A novel potent synthetic steroidal liver X receptor agonist lowers plasma cholesterol and triglycerides and reduces atherosclerosis in LDLR(-/-) mice.
      ,
      • Carballo-Jane E.
      • Chen Z.
      • O'Neill E.
      • Wang J.
      • Burton C.
      • Chang C.H.
      • Chen X.
      • Eveland S.
      • Frantz-Wattley B.
      • Gagen K.
      • et al.
      ApoA-I mimetic peptides promote pre-beta HDL formation in vivo causing remodeling of HDL and triglyceride accumulation at higher dose.
      ). These approaches are designed to improve cholesterol homeostasis, consequently reducing lipid raft content by promoting cholesterol efflux from target cells (
      • Peng D.
      • Hiipakka R.A.
      • Xie J.T.
      • Dai Q.
      • Kokontis J.M.
      • Reardon C.A.
      • Getz G.S.
      • Liao S.
      A novel potent synthetic steroidal liver X receptor agonist lowers plasma cholesterol and triglycerides and reduces atherosclerosis in LDLR(-/-) mice.
      ,
      • Carballo-Jane E.
      • Chen Z.
      • O'Neill E.
      • Wang J.
      • Burton C.
      • Chang C.H.
      • Chen X.
      • Eveland S.
      • Frantz-Wattley B.
      • Gagen K.
      • et al.
      ApoA-I mimetic peptides promote pre-beta HDL formation in vivo causing remodeling of HDL and triglyceride accumulation at higher dose.
      ). However, the feasibility of these interventions to date has been limited in clinical studies, as these methods ultimately require cell-specific and organ-selective cholesterol efflux to avoid side effects (for example, LXR agonists that do not cause de novo lipogenesis). Recent studies suggest that the non-cell autonomous acting protein, apoA-I binding protein (AIBP), is important in the regulation of cholesterol efflux in hematopoiesis and has a potential therapeutic function (
      • Gu Q.
      • Yang X.
      • Lv J.
      • Zhang J.
      • Xia B.
      • Kim J.D.
      • Wang R.
      • Xiong F.
      • Meng S.
      • Clements T.P.
      • et al.
      AIBP-mediated cholesterol efflux instructs hematopoietic stem and progenitor cell fate.
      ). AIBP is a secreted protein that is physically associated with apoA-I (
      • Ritter M.
      • Buechler C.
      • Boettcher A.
      • Barlage S.
      • Schmitz-Madry A.
      • Orso E.
      • Bared S.M.
      • Schmiedeknecht G.
      • Baehr C.H.
      • Fricker G.
      • et al.
      Cloning and characterization of a novel apolipoprotein A-I binding protein, AI-BP, secreted by cells of the kidney proximal tubules in response to HDL or ApoA-I.
      ). AIBP binds to both apoA-I and HDL, thereby augmenting cholesterol efflux and disrupting lipid rafts in ECs and macrophages (
      • Choi S.H.
      • Wallace A.M.
      • Schneider D.A.
      • Burg E.
      • Kim J.
      • Alekseeva E.
      • Ubags N.D.
      • Cool C.D.
      • Fang L.
      • Suratt B.T.
      • et al.
      AIBP augments cholesterol efflux from alveolar macrophages to surfactant and reduces acute lung inflammation.
      ,
      • Schneider D.A.
      • Choi S.H.
      • Agatisa-Boyle C.
      • Zhu L.
      • Kim J.
      • Pattison J.
      • Sears D.D.
      • Gordts P.
      • Fang L.
      • Miller Y.I.
      AIBP protects against metabolic abnormalities and atherosclerosis.
      ,
      • Zhang M.
      • Li L.
      • Xie W.
      • Wu J.F.
      • Yao F.
      • Tan Y.L.
      • Xia X.D.
      • Liu X.Y.
      • Liu D.
      • Lan G.
      • et al.
      Apolipoprotein A-1 binding protein promotes macrophage cholesterol efflux by facilitating apolipoprotein A-1 binding to ABCA1 and preventing ABCA1 degradation.
      ,
      • Woller S.A.
      • Choi S.H.
      • An E.J.
      • Low H.
      • Schneider D.A.
      • Ramachandran R.
      • Kim J.
      • Bae Y.S.
      • Sviridov D.
      • Corr M.
      • et al.
      Inhibition of neuroinflammation by AIBP: spinal effects upon facilitated pain states.
      ,
      • Fang L.
      • Choi S.H.
      • Baek J.S.
      • Liu C.
      • Almazan F.
      • Ulrich F.
      • Wiesner P.
      • Taleb A.
      • Deer E.
      • Pattison J.
      • et al.
      Control of angiogenesis by AIBP-mediated cholesterol efflux.
      ). Apoa1bp−/− mice on a WTD exhibited a greater abundance of inflammatory macrophages with exacerbated hyperlipidemia and atherosclerosis. Furthermore, in what appears to be a negative feedback pathway, AIBP binds to activated TLR4 dimers localized within lipid rafts. This promotes the recruitment of HDL/apoA-I and removal of cholesterol from lipid rafts leading to the disassociation of the active TLR4 dimer and, hence, an inhibition of TLR4 signaling and reduced inflammation (
      • Woller S.A.
      • Choi S.H.
      • An E.J.
      • Low H.
      • Schneider D.A.
      • Ramachandran R.
      • Kim J.
      • Bae Y.S.
      • Sviridov D.
      • Corr M.
      • et al.
      Inhibition of neuroinflammation by AIBP: spinal effects upon facilitated pain states.
      ). Recently, AIBP has been shown to play a role in regulating hematopoiesis via two mechanisms: i) regulation of the hemogenic endothelium; and ii) regulation of the HSPCs themselves (
      • Gu Q.
      • Yang X.
      • Lv J.
      • Zhang J.
      • Xia B.
      • Kim J.D.
      • Wang R.
      • Xiong F.
      • Meng S.
      • Clements T.P.
      • et al.
      AIBP-mediated cholesterol efflux instructs hematopoietic stem and progenitor cell fate.
      ). Aibp2 (the zebrafish functional paralog of AIBP) transgenic overexpressing zebrafish had increased expression of Srebf2, the gene encoding Srebp2. Srebp2 is the master regulator of cholesterol synthesis in cells, and the increased expression of Srebf2 would indicate that AIBP is a regulator of Srebp2 activity (likely by cholesterol removal), promoting cholesterol synthesis. Because AIBP augments cholesterol efflux via HDL, cotreatment with HDL and AIBP in ECs revealed a dose-dependent increase of Srebp2 activity (
      • Gu Q.
      • Yang X.
      • Lv J.
      • Zhang J.
      • Xia B.
      • Kim J.D.
      • Wang R.
      • Xiong F.
      • Meng S.
      • Clements T.P.
      • et al.
      AIBP-mediated cholesterol efflux instructs hematopoietic stem and progenitor cell fate.
      ). The HDL-AIBP induction of Srebp2 is likely a response to depleted cellular cholesterol, in keeping with Srebp2's well-defined role (
      • Yokoyama C.
      • Wang X.
      • Briggs M.R.
      • Admon A.
      • Wu J.
      • Hua X.
      • Goldstein J.L.
      • Brown M.S.
      SREBP-1, a basic-helix-loop-helix-leucine zipper protein that controls transcription of the low density lipoprotein receptor gene.
      ). In cholesterol-rich environments, Srebp2 is downregulated, preventing cholesterol synthesis (e.g., Hmgcr expression) and uptake (e.g., Ldlr expression) to avoid lipotoxicity.
      Interestingly, while classical Srebp2 target binding motifs (i.e., Srebf2, Hmgcr, Ldlr) were confirmed in hemogenic ECs (HECs), Notch was also shown to be occupied by Srepb2. A comparative RNA-seq analysis of ECs, HECs, HSCs, and progenitors with lymphoid potential from E10.5 mouse embryos also revealed that Notch and some Notch target genes (e.g., Hey1 and Hey2) were enriched in HECs as they transitioned to HSCs (
      • Solaimani Kartalaei P.
      • Yamada-Inagawa T.
      • Vink C.S.
      • de Pater E.
      • van der Linden R.
      • Marks-Bluth J.
      • van der Sloot A.
      • van den Hout M.
      • Yokomizo T.
      • van Schaick-Solerno M.L.
      • et al.
      Whole-transcriptome analysis of endothelial to hematopoietic stem cell transition reveals a requirement for Gpr56 in HSC generation.
      ). Interestingly, aside from Scap, Srebp2 cholesterol metabolism target genes were not strongly represented in this data set. Considering that there is remarkable enrichment of the Notch signaling pathway in Srebp2-regulated genes, it is possible that the role of Srebp2 is diverted to Notch signaling during endothelial to hematopoietic transition, a process that perhaps does not rely on cholesterol synthesis.
      The role of Srebp2 in hypercholesterolemia has been explored in Ldlr−/− mice fed a WTD. Consistent with our previous finding (
      • Murphy A.J.
      • Akhtari M.
      • Tolani S.
      • Pagler T.
      • Bijl N.
      • Kuo C.L.
      • Wang M.
      • Sanson M.
      • Abramowicz S.
      • Welch C.
      • et al.
      ApoE regulates hematopoietic stem cell proliferation, monocytosis, and monocyte accumulation in atherosclerotic lesions in mice.
      ), WTD-fed Ldlr−/− mice had an expansion of bone marrow HSPCs, and blocking Srebp2 prevented HSPC expansion (
      • Gu Q.
      • Yang X.
      • Lv J.
      • Zhang J.
      • Xia B.
      • Kim J.D.
      • Wang R.
      • Xiong F.
      • Meng S.
      • Clements T.P.
      • et al.
      AIBP-mediated cholesterol efflux instructs hematopoietic stem and progenitor cell fate.
      ). While these studies provide a clue that Srebp2 transactivation of Notch is associated with HSPC expansion, HSPC-specific inhibition of Srebp2 in Ldlr−/− mice is required to confirm this. Importantly, this effect of Srebp2 may also involve its classical cholesterol synthesis mechanism, as we have previously shown that HSPCs use the LDLR to uptake cholesterol in order to proliferate (
      • Tolani S.
      • Pagler T.A.
      • Murphy A.J.
      • Bochem A.E.
      • Abramowicz S.
      • Welch C.
      • Nagareddy P.R.
      • Holleran S.
      • Hovingh G.K.
      • Kuivenhoven J.A.
      • et al.
      Hypercholesterolemia and reduced HDL-C promote hematopoietic stem cell proliferation and monocytosis: studies in mice and FH children.
      ). In the hypercholesterolemic milieu, it is likely that in response to the inflammatory environment in WTD-fed Ldlr−/− mice, intrinsic cholesterol synthesis is dysregulated in the Ldlr-deficient HSPCs. In humans with elevated LDL cholesterol, circulating HSPCs have increased protein levels of Srebp2 and Notch. While LDL levels are correlated with circulating HSPCs, no correlation was observed with the white blood cell population. However, subjects in this study were either healthy or had only mild hypercholesterolemia, which may not necessarily lead to the typically observed increase in mature myeloid cells that occurs in familial hypercholesterolemia patients, who have 2- to 3-fold higher LDL-C levels (
      • Tolani S.
      • Pagler T.A.
      • Murphy A.J.
      • Bochem A.E.
      • Abramowicz S.
      • Welch C.
      • Nagareddy P.R.
      • Holleran S.
      • Hovingh G.K.
      • Kuivenhoven J.A.
      • et al.
      Hypercholesterolemia and reduced HDL-C promote hematopoietic stem cell proliferation and monocytosis: studies in mice and FH children.
      ,
      • Ma X.
      • Feng Y.
      Hypercholesterolemia tunes hematopoietic stem/progenitor cells for inflammation and atherosclerosis.
      ). It appears paradoxical that high LDL-C content activates Srebp2 in circulating HSPCs. However, oxidative stress and oxidized phospholipids, both of which are likely abundant in 4 month WTD-fed Ldlr−/− mice, may contribute to Srebp2 activation, as has been reported previously (
      • Chen Z.
      • Wen L.
      • Martin M.
      • Hsu C.Y.
      • Fang L.
      • Lin F.M.
      • Lin T.Y.
      • Geary M.J.
      • Geary G.G.
      • Zhao Y.
      • et al.
      Oxidative stress activates endothelial innate immunity via sterol regulatory element binding protein 2 (SREBP2) transactivation of microRNA-92a.
      ,
      • Yeh M.
      • Cole A.L.
      • Choi J.
      • Liu Y.
      • Tulchinsky D.
      • Qiao J.H.
      • Fishbein M.C.
      • Dooley A.N.
      • Hovnanian T.
      • Mouilleseaux K.
      • et al.
      Role for sterol regulatory element-binding protein in activation of endothelial cells by phospholipid oxidation products.
      ). Another possibility is that endoplasmic reticulum stress can trigger noncanonical caspase2-dependent Srebp2 activation (
      • Kim J.Y.
      • Garcia-Carbonell R.
      • Yamachika S.
      • Zhao P.
      • Dhar D.
      • Loomba R.
      • Kaufman R.J.
      • Saltiel A.R.
      • Karin M.
      ER stress drives lipogenesis and steatohepatitis via caspase-2 activation of S1P.
      ). All of these speculations warrant further investigation to explore the underlying mechanism of hypercholesterolemia regulation of Srebp2 activity. In the context of lipid raft disruption, it would also be important to determine whether modulation of lipid rafts by alternative methods also alters Notch. Nonetheless, these studies define a novel mechanism for hematopoiesis in which disruption of lipid rafts by AIBP causes SCAP-mediated Srebp2 activation, which regulates Notch signaling Fig. 2. This pathway was characterized in detail during developmental HSPC emergence largely in zebrafish, but requires further understanding in hypercholesterolemia-induced HSPC expansion, where cell cholesterol accumulation and increased lipid raft content have been found to contribute to hyperproliferation and enhanced myelopoiesis (
      • Yvan-Charvet L.
      • Pagler T.
      • Gautier E.L.
      • Avagyan S.
      • Siry R.L.
      • Han S.
      • Welch C.L.
      • Wang N.
      • Randolph G.J.
      • Snoeck H.W.
      • et al.
      ATP-binding cassette transporters and HDL suppress hematopoietic stem cell proliferation.
      ,
      • Murphy A.J.
      • Akhtari M.
      • Tolani S.
      • Pagler T.
      • Bijl N.
      • Kuo C.L.
      • Wang M.
      • Sanson M.
      • Abramowicz S.
      • Welch C.
      • et al.
      ApoE regulates hematopoietic stem cell proliferation, monocytosis, and monocyte accumulation in atherosclerotic lesions in mice.
      ,
      • Murphy A.J.
      • Dragoljevic D.
      • Tall A.R.
      Cholesterol efflux pathways regulate myelopoiesis: a potential link to altered macrophage function in atherosclerosis.
      ).
      Figure thumbnail gr3
      Fig. 2AIBP regulation of hematopoiesis in zebrafish. A: Transverse view of the zebrafish trunk. B: AIBP (brown) is secreted from the sclerotome and acts on ECs in a paracrine fashion. C: AIBP accelerates cholesterol efflux from the hemogenic endothelium (floor of dorsal aorta), reducing cellular cholesterol levels. This promotes the activation of Srebp2, which in turn transactivates Notch contribution to HSPC emergence. SP, spinal cord; N, notochord; DA, dorsal aorta; SRE, sterol responsive element.

      CONCLUSIONS

      Defective cholesterol efflux promotes the accumulation of membrane cholesterol and an increase in lipid rafts. The lipid composition of lipid rafts enables the preferential localization of cytokine receptors to these microdomains on the plasma membrane. The deficiency of ABC transporters and the consequent reduced ability to efflux cholesterol leads to an increase in lipid rafts. As a consequence, the expression of lipid raft-localized cytokine receptors (e.g., CBS and c-MPL) that favors myeloid skewing is increased, leading to the progression of atherosclerosis. Current treatments to lower plasma LDL-C using statins largely achieve this via shutting down cholesterol synthesis in the hepatocytes causing the induction of the LDLR to clear plasma lipoproteins. This balance of cellular cholesterol metabolism occurs in most cells and, thus, statins likely promote cholesterol uptake in HSPCs in this way. Therefore, direct modulators of lipid rafts in combination with statins may be more effective in restoring normal hematopoiesis driven by hypercholesterolemia and the associated inflammation. AIBP is capable of disrupting lipid raft integrity. Studies in macrophages and microglial cells indicate that AIBP binds TLR4 and disrupts the lipid rafts where TLR4 resides (
      • Choi S.H.
      • Wallace A.M.
      • Schneider D.A.
      • Burg E.
      • Kim J.
      • Alekseeva E.
      • Ubags N.D.
      • Cool C.D.
      • Fang L.
      • Suratt B.T.
      • et al.
      AIBP augments cholesterol efflux from alveolar macrophages to surfactant and reduces acute lung inflammation.
      ,
      • Woller S.A.
      • Choi S.H.
      • An E.J.
      • Low H.
      • Schneider D.A.
      • Ramachandran R.
      • Kim J.
      • Bae Y.S.
      • Sviridov D.
      • Corr M.
      • et al.
      Inhibition of neuroinflammation by AIBP: spinal effects upon facilitated pain states.
      ,
      • Zhang M.
      • Zhao G.J.
      • Yin K.
      • Xia X.D.
      • Gong D.
      • Zhao Z.W.
      • Chen L.Y.
      • Zheng X.L.
      • Tang X.E.
      • Tang C.K.
      Apolipoprotein A-1 binding protein inhibits inflammatory signaling pathways by binding to apolipoprotein A-1 in THP-1 macrophages.
      ). It remains to be determined whether the same mechanism acts in HSPCs. Further studies should address the hematopoietic- and niche-specific deletion of AIBP and its role on Srebp2 and Notch signaling in HSPCs. Nonetheless, cholesterol metabolism is critically important in hematopoiesis, and manipulating this in a targeted manner will likely provide beneficial effects in the setting of CVD and other disorders with aberrant hematopoiesis.

      REFERENCES

        • Simons K.
        • Toomre D.
        Lipid rafts and signal transduction.
        Nat. Rev. Mol. Cell Biol. 2000; 1: 31-39
        • Wei X.
        • Song H.
        • Yin L.
        • Rizzo M.G.
        • Sidhu R.
        • Covey D.F.
        • Ory D.S.
        • Semenkovich C.F.
        Fatty acid synthesis configures the plasma membrane for inflammation in diabetes.
        Nature. 2016; 539: 294-298
        • Yvan-Charvet L.
        • Pagler T.
        • Gautier E.L.
        • Avagyan S.
        • Siry R.L.
        • Han S.
        • Welch C.L.
        • Wang N.
        • Randolph G.J.
        • Snoeck H.W.
        • et al.
        ATP-binding cassette transporters and HDL suppress hematopoietic stem cell proliferation.
        Science. 2010; 328: 1689-1693
        • Murphy A.J.
        • Akhtari M.
        • Tolani S.
        • Pagler T.
        • Bijl N.
        • Kuo C.L.
        • Wang M.
        • Sanson M.
        • Abramowicz S.
        • Welch C.
        • et al.
        ApoE regulates hematopoietic stem cell proliferation, monocytosis, and monocyte accumulation in atherosclerotic lesions in mice.
        J. Clin. Invest. 2011; 121: 4138-4149
        • Morrison S.J.
        • Wandycz A.M.
        • Hemmati H.D.
        • Wright D.E.
        • Weissman I.L.
        Identification of a lineage of multipotent hematopoietic progenitors.
        Development. 1997; 124: 1929-1939
        • Morrison S.J.
        • Weissman I.L.
        The long-term repopulating subset of hematopoietic stem cells is deterministic and isolatable by phenotype.
        Immunity. 1994; 1: 661-673
        • Grinenko T.
        • Eugster A.
        • Thielecke L.
        • Ramasz B.
        • Krüger A.
        • Dietz S.
        • Glauche I.
        • Gerbaulet A.
        • von Bonin M.
        • Basak O.
        • et al.
        Hematopoietic stem cells can differentiate into restricted myeloid progenitors before cell division in mice.
        Nat. Commun. 2018; 9: 1898
        • Al-Sharea A.
        • Lee M.K.S.
        • Purton L.E.
        • Hawkins E.D.
        • Murphy A.J.
        The haematopoietic stem cell niche: a new player in cardiovascular disease?.
        Cardiovasc. Res. 2019; 115: 277-291
        • Akashi K.
        • Traver D.
        • Miyamoto T.
        • Weissman I.L.
        A clonogenic common myeloid progenitor that gives rise to all myeloid lineages.
        Nature. 2000; 404: 193-197
        • Kondo M.
        • Weissman I.L.
        • Akashi K.
        Identification of clonogenic common lymphoid progenitors in mouse bone marrow.
        Cell. 1997; 91: 661-672
        • Robbins C.S.
        • Chudnovskiy A.
        • Rauch P.J.
        • Figueiredo J-L.
        • Iwamoto Y.
        • Gorbatov R.
        • Etzrodt M.
        • Weber G.F.
        • Ueno T.
        • van Rooijen N.
        • et al.
        Extramedullary hematopoiesis generates Ly-6C(high) monocytes that infiltrate atherosclerotic lesions.
        Circulation. 2012; 125: 364-374
        • Inra C.N.
        • Zhou B.O.
        • Acar M.
        • Murphy M.M.
        • Richardson J.
        • Zhao Z.
        • Morrison S.J.
        A perisinusoidal niche for extramedullary haematopoiesis in the spleen.
        Nature. 2015; 527: 466-471
        • Hill D.A.
        • Swanson P.E.
        Myocardial extramedullary hematopoiesis: a clinicopathologic study.
        Mod. Pathol. 2000; 13: 779-787
        • Dutta P.
        • Courties G.
        • Wei Y.
        • Leuschner F.
        • Gorbatov R.
        • Robbins C.S.
        • Iwamoto Y.
        • Thompson B.
        • Carlson A.L.
        • Heidt T.
        • et al.
        Myocardial infarction accelerates atherosclerosis.
        Nature. 2012; 487: 325-329
        • Friedman G.D.
        • Klatsky A.L.
        • Siegelaub A.B.
        The leukocyte count as a predictor of myocardial infarction.
        N. Engl. J. Med. 1974; 290: 1275-1278
        • Olivares R.
        • Ducimetière P.
        • Claude J.R.
        Monocyte count: a risk factor for coronary heart disease?.
        Am. J. Epidemiol. 1993; 137: 49-53
        • Sweetnam P.M.
        • Thomas H.F.
        • Yarnell J.W.
        • Baker I.A.
        • Elwood P.C.
        Total and differential leukocyte counts as predictors of ischemic heart disease: the Caerphilly and Speedwell studies.
        Am. J. Epidemiol. 1997; 145: 416-421
        • Lee C.D.
        • Folsom A.R.
        • Nieto F.J.
        • Chambless L.E.
        • Shahar E.
        • Wolfe D.A.
        White blood cell count and incidence of coronary heart disease and ischemic stroke and mortality from cardiovascular disease in African-American and White men and women: atherosclerosis risk in communities study.
        Am. J. Epidemiol. 2001; 154: 758-764
        • Dragoljevic D.
        • Kraakman M.J.
        • Nagareddy P.R.
        • Ngo D.
        • Shihata W.
        • Kammoun H.L.
        • Whillas A.
        • Lee M.K.S.
        • Al-Sharea A.
        • Pernes G.
        • et al.
        Defective cholesterol metabolism in haematopoietic stem cells promotes monocyte-driven atherosclerosis in rheumatoid arthritis.
        Eur. Heart J. 2018; 39: 2158-2167
        • Drechsler M.
        • Megens R.T.
        • van Zandvoort M.
        • Weber C.
        • Soehnlein O.
        Hyperlipidemia-triggered neutrophilia promotes early atherosclerosis.
        Circulation. 2010; 122: 1837-1845
        • Swirski F.K.
        • Libby P.
        • Aikawa E.
        • Alcaide P.
        • Luscinskas F.W.
        • Weissleder R.
        • Pittet M.J.
        Ly-6Chi monocytes dominate hypercholesterolemia-associated monocytosis and give rise to macrophages in atheromata.
        J. Clin. Invest. 2007; 117: 195-205
        • Tacke F.
        • Alvarez D.
        • Kaplan T.J.
        • Jakubzick C.
        • Spanbroek R.
        • Llodra J.
        • Garin A.
        • Liu J.
        • Mack M.
        • van Rooijen N.
        • et al.
        Monocyte subsets differentially employ CCR2, CCR5, and CX3CR1 to accumulate within atherosclerotic plaques.
        J. Clin. Invest. 2007; 117: 185-194
        • Tolani S.
        • Pagler T.A.
        • Murphy A.J.
        • Bochem A.E.
        • Abramowicz S.
        • Welch C.
        • Nagareddy P.R.
        • Holleran S.
        • Hovingh G.K.
        • Kuivenhoven J.A.
        • et al.
        Hypercholesterolemia and reduced HDL-C promote hematopoietic stem cell proliferation and monocytosis: studies in mice and FH children.
        Atherosclerosis. 2013; 229: 79-85
      1. Deleted in proof.

        • Murphy A.J.
        • Tall A.R.
        Disordered haematopoiesis and athero-thrombosis.
        Eur. Heart J. 2016; 37: 1113-1121
        • Smith J.D.
        • Trogan E.
        • Ginsberg M.
        • Grigaux C.
        • Tian J.
        • Miyata M.
        Decreased atherosclerosis in mice deficient in both macrophage colony-stimulating factor (op) and apolipoprotein E.
        Proc. Natl. Acad. Sci. USA. 1995; 92: 8264-8268
        • Rajavashisth T.
        • Qiao J.H.
        • Tripathi S.
        • Tripathi J.
        • Mishra N.
        • Hua M.
        • Wang X.P.
        • Loussararian A.
        • Clinton S.
        • Libby P.
        • et al.
        Heterozygous osteopetrotic (op) mutation reduces atherosclerosis in LDL receptor- deficient mice.
        J. Clin. Invest. 1998; 101: 2702-2710
        • Combadière C.
        • Potteaux S.
        • Rodero M.
        • Simon T.
        • Pezard A.
        • Esposito B.
        • Merval R.
        • Proudfoot A.
        • Tedgui A.
        • Mallat Z.
        Combined inhibition of CCL2, CX3CR1, and CCR5 abrogates Ly6C(hi) and Ly6C(lo) monocytosis and almost abolishes atherosclerosis in hypercholesterolemic mice.
        Circulation. 2008; 117: 1649-1657
        • Potteaux S.
        • Gautier E.L.
        • Hutchison S.B.
        • van Rooijen N.
        • Rader D.J.
        • Thomas M.J.
        • Sorci-Thomas M.G.
        • Randolph G.J.
        Suppressed monocyte recruitment drives macrophage removal from atherosclerotic plaques of Apoe-/- mice during disease regression.
        J. Clin. Invest. 2011; 121: 2025-2036
      2. Deleted in proof.

        • Rahman K.
        • Vengrenyuk Y.
        • Ramsey S.A.
        • Vila N.R.
        • Girgis N.M.
        • Liu J.
        • Gusarova V.
        • Gromada J.
        • Weinstock A.
        • Moore K.J.
        • et al.
        Inflammatory Ly6Chi monocytes and their conversion to M2 macrophages drive atherosclerosis regression.
        J. Clin. Invest. 2017; 127: 2904-2915
        • Nagareddy P.R.
        • Murphy A.J.
        • Stirzaker R.A.
        • Hu Y.
        • Yu S.
        • Miller R.G.
        • Ramkhelawon B.
        • Distel E.
        • Westerterp M.
        • Huang L.S.
        • et al.
        Hyperglycemia promotes myelopoiesis and impairs the resolution of atherosclerosis.
        Cell Metab. 2013; 17: 695-708
        • Libby P.
        Inflammation in atherosclerosis.
        Nature. 2002; 420: 868-874
        • Fatkhullina A.R.
        • Peshkova I.O.
        • Koltsova E.K.
        The role of cytokines in the development of atherosclerosis.
        Biochemistry (Mosc.). 2016; 81: 1358-1370
        • Sager H.B.
        • Heidt T.
        • Hulsmans M.
        • Dutta P.
        • Courties G.
        • Sebas M.
        • Wojtkiewicz G.R.
        • Tricot B.
        • Iwamoto Y.
        • Sun Y.
        • et al.
        Targeting interleukin-1beta reduces leukocyte production after acute myocardial infarction.
        Circulation. 2015; 132: 1880-1890
        • Pietras E.M.
        • Mirantes-Barbeito C.
        • Fong S.
        • Loeffler D.
        • Kovtonyuk L.V.
        • Zhang S.
        • Lakshminarasimhan R.
        • Chin C.P.
        • Techner J-M.
        • Will B.
        • et al.
        Chronic interleukin-1 exposure drives haematopoietic stem cells towards precocious myeloid differentiation at the expense of self-renewal.
        Nat. Cell Biol. 2016; 18: 607
        • Nagareddy P.R.
        • Kraakman M.
        • Masters S.L.
        • Stirzaker R.A.
        • Gorman D.J.
        • Grant R.W.
        • Dragoljevic D.
        • Hong E.S.
        • Abdel-Latif A.
        • Smyth S.S.
        • et al.
        Adipose tissue macrophages promote myelopoiesis and monocytosis in obesity.
        Cell Metab. 2014; 19: 821-835
        • Hermetet F.
        • Buffière A.
        • Aznague A.
        • Pais de Barros J-P.
        • Bastie J-N.
        • Delva L.
        • Quéré R.
        High-fat diet disturbs lipid raft/TGF-β signaling-mediated maintenance of hematopoietic stem cells in mouse bone marrow.
        Nat. Commun. 2019; 10: 523
        • Testa U.
        • Fossati C.
        • Samoggia P.
        • Masciulli R.
        • Mariani G.
        • Hassan H.J.
        • Sposi N.M.
        • Guerriero R.
        • Rosato V.
        • Gabbianelli M.
        • et al.
        Expression of growth factor receptors in unilineage differentiation culture of purified hematopoietic progenitors.
        Blood. 1996; 88: 3391-3406
        • Metcalf D.
        Hematopoietic cytokines.
        Blood. 2008; 111: 485-491
        • Wang M.
        • Subramanian M.
        • Abramowicz S.
        • Murphy A.J.
        • Gonen A.
        • Witztum J.
        • Welch C.
        • Tabas I.
        • Westerterp M.
        • Tall A.R.
        Interleukin-3/granulocyte macrophage colony-stimulating factor receptor promotes stem cell expansion, monocytosis, and atheroma macrophage burden in mice with hematopoietic ApoE deficiency.
        Arterioscler. Thromb. Vasc. Biol. 2014; 34: 976-984
        • Westerterp M.
        • Gourion-Arsiquaud S.
        • Murphy A.J.
        • Shih A.
        • Cremers S.
        • Levine R.L.
        • Tall A.R.
        • Yvan-Charvet L.
        Regulation of hematopoietic stem and progenitor cell mobilization by cholesterol efflux pathways.
        Cell Stem Cell. 2012; 11: 195-206
        • Zimmer S.
        • Grebe A.
        • Bakke S.S.
        • Bode N.
        • Halvorsen B.
        • Ulas T.
        • Skjelland M.
        • De Nardo D.
        • Labzin L.I.
        • Kerksiek A.
        • et al.
        Cyclodextrin promotes atherosclerosis regression via macrophage reprogramming.
        Sci. Transl. Med. 2016; 8: 333ra50
        • Han J.
        • Hajjar D.P.
        • Tauras J.M.
        • Nicholson A.C.
        Cellular cholesterol regulates expression of the macrophage type B scavenger receptor, CD36.
        J. Lipid Res. 1999; 40: 830-838
        • Chakraborty D.
        • Banerjee S.
        • Sen A.
        • Banerjee K.K.
        • Das P.
        • Roy S.
        Leishmania donovani affects antigen presentation of macrophage by disrupting lipid rafts.
        J. Immunol. 2005; 175: 3214-3224
        • Nguyen D.H.
        • Taub D.
        Cholesterol is essential for macrophage inflammatory protein 1 beta binding and conformational integrity of CC chemokine receptor 5.
        Blood. 2002; 99: 4298-4306
        • Shadan S.
        • James P.S.
        • Howes E.A.
        • Jones R.
        Cholesterol efflux alters lipid raft stability and distribution during capacitation of boar spermatozoa.
        Biol. Reprod. 2004; 71: 253-265
        • Landry Y.D.
        • Denis M.
        • Nandi S.
        • Bell S.
        • Vaughan A.M.
        • Zha X.
        ATP-binding cassette transporter A1 expression disrupts raft membrane microdomains through its ATPase-related functions.
        J. Biol. Chem. 2006; 281: 36091-36101
        • Koseki M.
        • Hirano K.
        • Masuda D.
        • Ikegami C.
        • Tanaka M.
        • Ota A.
        • Sandoval J.C.
        • Nakagawa-Toyama Y.
        • Sato S.B.
        • Kobayashi T.
        • et al.
        Increased lipid rafts and accelerated lipopolysaccharide-induced tumor necrosis factor-alpha secretion in Abca1-deficient macrophages.
        J. Lipid Res. 2007; 48: 299-306
        • Murphy A.J.
        • Bijl N.
        • Yvan-Charvet L.
        • Welch C.B.
        • Bhagwat N.
        • Reheman A.
        • Wang Y.
        • Shaw J.A.
        • Levine R.L.
        • Ni H.
        • et al.
        Cholesterol efflux in megakaryocyte progenitors suppresses platelet production and thrombocytosis.
        Nat. Med. 2013; 19: 586-594
        • Westerterp M.
        • Bochem A.E.
        • Yvan-Charvet L.
        • Murphy A.J.
        • Wang N.
        • Tall A.R.
        ATP-binding cassette transporters, atherosclerosis, and inflammation.
        Circ. Res. 2014; 114: 157-170
        • Yvan-Charvet L.
        • Welch C.
        • Pagler T.A.
        • Ranalletta M.
        • Lamkanfi M.
        • Han S.
        • Ishibashi M.
        • Li R.
        • Wang N.
        • Tall A.R.
        Increased inflammatory gene expression in ABC transporter-deficient macrophages: free cholesterol accumulation, increased signaling via toll-like receptors, and neutrophil infiltration of atherosclerotic lesions.
        Circulation. 2008; 118: 1837-1847
        • Hamon Y.
        • Luciani M.F.
        • Becq F.
        • Verrier B.
        • Rubartelli A.
        • Chimini G.
        Interleukin-1beta secretion is impaired by inhibitors of the ATP binding cassette transporter, ABC1.
        Blood. 1997; 90: 2911-2915
        • Becq F.
        • Hamon Y.
        • Bajetto A.
        • Gola M.
        • Verrier B.
        • Chimini G.
        ABC1, an ATP binding cassette transporter required for phagocytosis of apoptotic cells, generates a regulated anion flux after expression in Xenopus laevis oocytes.
        J. Biol. Chem. 1997; 272: 2695-2699
        • Brooks-Wilson A.
        • Marcil M.
        • Clee S.M.
        • Zhang L.H.
        • Roomp K.
        • van Dam M.
        • Yu L.
        • Brewer C.
        • Collins J.A.
        • Molhuizen H.O.
        • et al.
        Mutations in ABC1 in Tangier disease and familial high-density lipoprotein deficiency.
        Nat. Genet. 1999; 22: 336-345
        • Rust S.
        • Rosier M.
        • Funke H.
        • Real J.
        • Amoura Z.
        • Piette J.C.
        • Deleuze J.F.
        • Brewer H.B.
        • Duverger N.
        • Denefle P.
        • et al.
        Tangier disease is caused by mutations in the gene encoding ATP-binding cassette transporter 1.
        Nat. Genet. 1999; 22: 352-355
        • Bodzioch M.
        • Orso E.
        • Klucken J.
        • Langmann T.
        • Bottcher A.
        • Diederich W.
        • Drobnik W.
        • Barlage S.
        • Buchler C.
        • Porsch-Ozcurumez M.
        • et al.
        The gene encoding ATP-binding cassette transporter 1 is mutated in Tangier disease.
        Nat. Genet. 1999; 22: 347-351
        • Lawn R.M.
        • Wade D.P.
        • Garvin M.R.
        • Wang X.
        • Schwartz K.
        • Porter J.G.
        • Seilhamer J.J.
        • Vaughan A.M.
        • Oram J.F.
        The Tangier disease gene product ABC1 controls the cellular apolipoprotein-mediated lipid removal pathway.
        J. Clin. Invest. 1999; 104: R25-R31
        • Marcil M.
        • Brooks-Wilson A.
        • Clee S.M.
        • Roomp K.
        • Zhang L.H.
        • Yu L.
        • Collins J.A.
        • van Dam M.
        • Molhuizen H.O.
        • Loubster O.
        • et al.
        Mutations in the ABC1 gene in familial HDL deficiency with defective cholesterol efflux.
        Lancet. 1999; 354: 1341-1346
        • McNeish J.
        • Aiello R.J.
        • Guyot D.
        • Turi T.
        • Gabel C.
        • Aldinger C.
        • Hoppe K.L.
        • Roach M.L.
        • Royer L.J.
        • de Wet J.
        • et al.
        High density lipoprotein deficiency and foam cell accumulation in mice with targeted disruption of ATP-binding cassette transporter-1.
        Proc. Natl. Acad. Sci. USA. 2000; 97: 4245-4250
        • Christiansen-Weber T.A.
        • Voland J.R.
        • Wu Y.
        • Ngo K.
        • Roland B.L.
        • Nguyen S.
        • Peterson P.A.
        • Fung-Leung W.P.
        Functional loss of ABCA1 in mice causes severe placental malformation, aberrant lipid distribution, and kidney glomerulonephritis as well as high-density lipoprotein cholesterol deficiency.
        Am. J. Pathol. 2000; 157: 1017-1029
        • Vaisman B.L.
        • Lambert G.
        • Amar M.
        • Joyce C.
        • Ito T.
        • Shamburek R.D.
        • Cain W.J.
        • Fruchart-Najib J.
        • Neufeld E.D.
        • Remaley A.T.
        • et al.
        ABCA1 overexpression leads to hyperalphalipoproteinemia and increased biliary cholesterol excretion in transgenic mice.
        J. Clin. Invest. 2001; 108: 303-309
        • van Eck M.
        • Bos I.S.
        • Kaminski W.E.
        • Orso E.
        • Rothe G.
        • Twisk J.
        • Bottcher A.
        • Van Amersfoort E.S.
        • Christiansen-Weber T.A.
        • Fung-Leung W.P.
        • et al.
        Leukocyte ABCA1 controls susceptibility to atherosclerosis and macrophage recruitment into tissues.
        Proc. Natl. Acad. Sci. USA. 2002; 99: 6298-6303
        • Joyce C.W.
        • Amar M.J.
        • Lambert G.
        • Vaisman B.L.
        • Paigen B.
        • Najib-Fruchart J.
        • Hoyt Jr., R.F.
        • Neufeld E.D.
        • Remaley A.T.
        • Fredrickson D.S.
        • et al.
        The ATP binding cassette transporter A1 (ABCA1) modulates the development of aortic atherosclerosis in C57BL/6 and apoE-knockout mice.
        Proc. Natl. Acad. Sci. USA. 2002; 99: 407-412
        • Zhu X.
        • Lee J.Y.
        • Timmins J.M.
        • Brown J.M.
        • Boudyguina E.
        • Mulya A.
        • Gebre A.K.
        • Willingham M.C.
        • Hiltbold E.M.
        • Mishra N.
        • et al.
        Increased cellular free cholesterol in macrophage-specific Abca1 knock-out mice enhances pro-inflammatory response of macrophages.
        J. Biol. Chem. 2008; 283: 22930-22941
        • Murphy A.J.
        • Woollard K.J.
        • Suhartoyo A.
        • Stirzaker R.A.
        • Shaw J.
        • Sviridov D.
        • Chin-Dusting J.P.
        Neutrophil activation is attenuated by high-density lipoprotein and apolipoprotein A-I in in vitro and in vivo models of inflammation.
        Arterioscler. Thromb. Vasc. Biol. 2011; 31: 1333-1341
        • Murphy A.J.
        • Woollard K.J.
        • Hoang A.
        • Mukhamedova N.
        • Stirzaker R.A.
        • McCormick S.P.
        • Remaley A.T.
        • Sviridov D.
        • Chin-Dusting J.
        High-density lipoprotein reduces the human monocyte inflammatory response.
        Arterioscler. Thromb. Vasc. Biol. 2008; 28: 2071-2077
        • Gerbod-Giannone M.C.
        • Li Y.
        • Holleboom A.
        • Han S.
        • Hsu L.C.
        • Tabas I.
        • Tall A.R.
        TNFalpha induces ABCA1 through NF-kappaB in macrophages and in phagocytes ingesting apoptotic cells.
        Proc. Natl. Acad. Sci. USA. 2006; 103: 3112-3117
        • Wang N.
        • Lan D.
        • Chen W.
        • Matsuura F.
        • Tall A.R.
        ATP-binding cassette transporters G1 and G4 mediate cellular cholesterol efflux to high-density lipoproteins.
        Proc. Natl. Acad. Sci. USA. 2004; 101: 9774-9779
        • Heinecke J.W.
        Small HDL promotes cholesterol efflux by the ABCA1 pathway in macrophages: implications for therapies targeted to HDL.
        Circ. Res. 2015; 116: 1101-1103
        • Kennedy M.A.
        • Barrera G.C.
        • Nakamura K.
        • Baldan A.
        • Tarr P.
        • Fishbein M.C.
        • Frank J.
        • Francone O.L.
        • Edwards P.A.
        ABCG1 has a critical role in mediating cholesterol efflux to HDL and preventing cellular lipid accumulation.
        Cell Metab. 2005; 1: 121-131
        • Out R.
        • Hoekstra M.
        • Hildebrand R.B.
        • Kruit J.K.
        • Meurs I.
        • Li Z.
        • Kuipers F.
        • Van Berkel T.J.
        • Van Eck M.
        Macrophage ABCG1 deletion disrupts lipid homeostasis in alveolar macrophages and moderately influences atherosclerotic lesion development in LDL receptor-deficient mice.
        Arterioscler. Thromb. Vasc. Biol. 2006; 26: 2295-2300
        • Baldán A.
        • Pei L.
        • Lee R.
        • Tarr P.
        • Tangirala R.K.
        • Weinstein M.M.
        • Frank J.
        • Li A.C.
        • Tontonoz P.
        • Edwards P.A.
        Impaired development of atherosclerosis in hyperlipidemic Ldlr-/- and ApoE-/- mice transplanted with Abcg1-/- bone marrow.
        Arterioscler. Thromb. Vasc. Biol. 2006; 26: 2301-2307
        • Ranalletta M.
        • Wang N.
        • Han S.
        • Yvan-Charvet L.
        • Welch C.
        • Tall A.R.
        Decreased atherosclerosis in low-density lipoprotein receptor knockout mice transplanted with Abcg1−/− bone marrow.
        Arterioscler. Thromb. Vasc. Biol. 2006; 26: 2308-2315
        • Gelissen I.C.
        • Harris M.
        • Rye K.A.
        • Quinn C.
        • Brown A.J.
        • Kockx M.
        • Cartland S.
        • Packianathan M.
        • Kritharides L.
        • Jessup W.
        ABCA1 and ABCG1 synergize to mediate cholesterol export to apoA-I.
        Arterioscler. Thromb. Vasc. Biol. 2006; 26: 534-540
        • Yvan-Charvet L.
        • Ranalletta M.
        • Wang N.
        • Han S.
        • Terasaka N.
        • Li R.
        • Welch C.
        • Tall A.R.
        Combined deficiency of ABCA1 and ABCG1 promotes foam cell accumulation and accelerates atherosclerosis in mice.
        J. Clin. Invest. 2007; 117: 3900-3908
        • Out R.
        • Hoekstra M.
        • Habets K.
        • Meurs I.
        • de Waard V.
        • Hildebrand R.B.
        • Wang Y.
        • Chimini G.
        • Kuiper J.
        • Van Berkel T.J.
        • et al.
        Combined deletion of macrophage ABCA1 and ABCG1 leads to massive lipid accumulation in tissue macrophages and distinct atherosclerosis at relatively low plasma cholesterol levels.
        Arterioscler. Thromb. Vasc. Biol. 2008; 28: 258-264
        • Out R.
        • Jessup W.
        • Le Goff W.
        • Hoekstra M.
        • Gelissen I.C.
        • Zhao Y.
        • Kritharides L.
        • Chimini G.
        • Kuiper J.
        • Chapman M.J.
        • et al.
        Coexistence of foam cells and hypocholesterolemia in mice lacking the ABC transporters A1 and G1.
        Circ. Res. 2008; 102: 113-120
        • Wang N.
        • Ranalletta M.
        • Matsuura F.
        • Peng F.
        • Tall A.R.
        LXR-induced redistribution of ABCG1 to plasma membrane in macrophages enhances cholesterol mass efflux to HDL.
        Arterioscler. Thromb. Vasc. Biol. 2006; 26: 1310-1316
        • Demeester N.
        • Castro G.
        • Desrumaux C.
        • De Geitere C.
        • Fruchart J.C.
        • Santens P.
        • Mulleners E.
        • Engelborghs S.
        • De Deyn P.P.
        • Vandekerckhove J.
        • et al.
        Characterization and functional studies of lipoproteins, lipid transfer proteins, and lecithin:cholesterol acyltransferase in CSF of normal individuals and patients with Alzheimer's disease.
        J. Lipid Res. 2000; 41: 963-974
        • Rigamonti E.
        • Helin L.
        • Lestavel S.
        • Mutka A.L.
        • Lepore M.
        • Fontaine C.
        • Bouhlel M.A.
        • Bultel S.
        • Fruchart J.C.
        • Ikonen E.
        • et al.
        Liver X receptor activation controls intracellular cholesterol trafficking and esterification in human macrophages.
        Circ. Res. 2005; 97: 682-689
        • Peng D.
        • Hiipakka R.A.
        • Xie J.T.
        • Dai Q.
        • Kokontis J.M.
        • Reardon C.A.
        • Getz G.S.
        • Liao S.
        A novel potent synthetic steroidal liver X receptor agonist lowers plasma cholesterol and triglycerides and reduces atherosclerosis in LDLR(-/-) mice.
        Br. J. Pharmacol. 2011; 162: 1792-1804
        • Carballo-Jane E.
        • Chen Z.
        • O'Neill E.
        • Wang J.
        • Burton C.
        • Chang C.H.
        • Chen X.
        • Eveland S.
        • Frantz-Wattley B.
        • Gagen K.
        • et al.
        ApoA-I mimetic peptides promote pre-beta HDL formation in vivo causing remodeling of HDL and triglyceride accumulation at higher dose.
        Bioorg. Med. Chem. 2010; 18: 8669-8678
        • Gu Q.
        • Yang X.
        • Lv J.
        • Zhang J.
        • Xia B.
        • Kim J.D.
        • Wang R.
        • Xiong F.
        • Meng S.
        • Clements T.P.
        • et al.
        AIBP-mediated cholesterol efflux instructs hematopoietic stem and progenitor cell fate.
        Science. 2019; 363: 1085-1088
        • Ritter M.
        • Buechler C.
        • Boettcher A.
        • Barlage S.
        • Schmitz-Madry A.
        • Orso E.
        • Bared S.M.
        • Schmiedeknecht G.
        • Baehr C.H.
        • Fricker G.
        • et al.
        Cloning and characterization of a novel apolipoprotein A-I binding protein, AI-BP, secreted by cells of the kidney proximal tubules in response to HDL or ApoA-I.
        Genomics. 2002; 79: 693-702
        • Choi S.H.
        • Wallace A.M.
        • Schneider D.A.
        • Burg E.
        • Kim J.
        • Alekseeva E.
        • Ubags N.D.
        • Cool C.D.
        • Fang L.
        • Suratt B.T.
        • et al.
        AIBP augments cholesterol efflux from alveolar macrophages to surfactant and reduces acute lung inflammation.
        J. Clin. Invest. 2018; 3: 120519
        • Schneider D.A.
        • Choi S.H.
        • Agatisa-Boyle C.
        • Zhu L.
        • Kim J.
        • Pattison J.
        • Sears D.D.
        • Gordts P.
        • Fang L.
        • Miller Y.I.
        AIBP protects against metabolic abnormalities and atherosclerosis.
        J. Lipid Res. 2018; 59: 854-863
        • Zhang M.
        • Li L.
        • Xie W.
        • Wu J.F.
        • Yao F.
        • Tan Y.L.
        • Xia X.D.
        • Liu X.Y.
        • Liu D.
        • Lan G.
        • et al.
        Apolipoprotein A-1 binding protein promotes macrophage cholesterol efflux by facilitating apolipoprotein A-1 binding to ABCA1 and preventing ABCA1 degradation.
        Atherosclerosis. 2016; 248: 149-159
        • Woller S.A.
        • Choi S.H.
        • An E.J.
        • Low H.
        • Schneider D.A.
        • Ramachandran R.
        • Kim J.
        • Bae Y.S.
        • Sviridov D.
        • Corr M.
        • et al.
        Inhibition of neuroinflammation by AIBP: spinal effects upon facilitated pain states.
        Cell Reports. 2018; 23: 2667-2677
        • Fang L.
        • Choi S.H.
        • Baek J.S.
        • Liu C.
        • Almazan F.
        • Ulrich F.
        • Wiesner P.
        • Taleb A.
        • Deer E.
        • Pattison J.
        • et al.
        Control of angiogenesis by AIBP-mediated cholesterol efflux.
        Nature. 2013; 498: 118-122
        • Yokoyama C.
        • Wang X.
        • Briggs M.R.
        • Admon A.
        • Wu J.
        • Hua X.
        • Goldstein J.L.
        • Brown M.S.
        SREBP-1, a basic-helix-loop-helix-leucine zipper protein that controls transcription of the low density lipoprotein receptor gene.
        Cell. 1993; 75: 187-197
        • Solaimani Kartalaei P.
        • Yamada-Inagawa T.
        • Vink C.S.
        • de Pater E.
        • van der Linden R.
        • Marks-Bluth J.
        • van der Sloot A.
        • van den Hout M.
        • Yokomizo T.
        • van Schaick-Solerno M.L.
        • et al.
        Whole-transcriptome analysis of endothelial to hematopoietic stem cell transition reveals a requirement for Gpr56 in HSC generation.
        J. Exp. Med. 2015; 212: 93-106
        • Ma X.
        • Feng Y.
        Hypercholesterolemia tunes hematopoietic stem/progenitor cells for inflammation and atherosclerosis.
        Int. J. Mol. Sci. 2016; 17: E1162
        • Chen Z.
        • Wen L.
        • Martin M.
        • Hsu C.Y.
        • Fang L.
        • Lin F.M.
        • Lin T.Y.
        • Geary M.J.
        • Geary G.G.
        • Zhao Y.
        • et al.
        Oxidative stress activates endothelial innate immunity via sterol regulatory element binding protein 2 (SREBP2) transactivation of microRNA-92a.
        Circulation. 2015; 131: 805-814
        • Yeh M.
        • Cole A.L.
        • Choi J.
        • Liu Y.
        • Tulchinsky D.
        • Qiao J.H.
        • Fishbein M.C.
        • Dooley A.N.
        • Hovnanian T.
        • Mouilleseaux K.
        • et al.
        Role for sterol regulatory element-binding protein in activation of endothelial cells by phospholipid oxidation products.
        Circ. Res. 2004; 95: 780-788
        • Kim J.Y.
        • Garcia-Carbonell R.
        • Yamachika S.
        • Zhao P.
        • Dhar D.
        • Loomba R.
        • Kaufman R.J.
        • Saltiel A.R.
        • Karin M.
        ER stress drives lipogenesis and steatohepatitis via caspase-2 activation of S1P.
        Cell. 2018; 175: 133-145.e15
        • Murphy A.J.
        • Dragoljevic D.
        • Tall A.R.
        Cholesterol efflux pathways regulate myelopoiesis: a potential link to altered macrophage function in atherosclerosis.
        Front. Immunol. 2014; 5: 490
        • Zhang M.
        • Zhao G.J.
        • Yin K.
        • Xia X.D.
        • Gong D.
        • Zhao Z.W.
        • Chen L.Y.
        • Zheng X.L.
        • Tang X.E.
        • Tang C.K.
        Apolipoprotein A-1 binding protein inhibits inflammatory signaling pathways by binding to apolipoprotein A-1 in THP-1 macrophages.
        Circ. J. 2018; 82: 1396-1404