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Journal of Lipid Research, Vol. 46, 2448-2457, November 2005
Copyright © 2005 by American Society for Biochemistry and Molecular Biology

Expression and regulation of 1-acyl-sn-glycerol- 3-phosphate acyltransferases in the epidermis

Biao Lu, Yan J. Jiang, Mao Q. Man, Barbara Brown, Peter M. Elias and Kenneth R. Feingold¹

Dermatology and Medicine Services, Veterans Administration Medical Center and University of California School of Medicine, San Francisco, CA 94121

Published, JLR Papers in Press, September 8, 2005. DOI 10.1194/jlr.M500258-JLR200

¹ To whom correspondence should be addressed. e-mail: kfngld{at}itsa.ucsf.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Phospholipids are a major class of lipids in epidermis, where they serve as a source of free fatty acids that are important for the maintenance of epidermal permeability barrier function. The phospholipid biosynthetic enzyme, 1-acyl-sn-glycerol-3-phosphate acyltransferase (AGPAT), catalyzes the acylation of lysophosphatidic acid to form phosphatidic acid, the major precursor of all glycerolipids. We identified an expression pattern of AGPAT isoforms that is unique to epidermis, with relatively high constitutive expression of mouse AGPAT (mAGPAT) 3, 4, and 5 but low constitutive expression of mAGPAT 1 and 2. Localization studies indicate that all five isoforms of AGPAT were expressed in all nucleated layers of epidermis. Furthermore, rat AGPAT 2 and 5 mRNAs increased in parallel with both an increase in enzyme activity and permeability barrier formation late in rat epidermal development. Moreover, after two methods of acute permeability barrier disruption, mAGPAT 1, 2, and 3 mRNA levels increased rapidly and were sustained for at least 24 h. In parallel with the increase in mRNA levels, an increase in AGPAT activity also occurred.

Because upregulation of mAGPAT mRNAs after tape-stripping could be partially reversed by artificial barrier restoration by occlusion, these studies suggest that an increase in the expression of AGPATs is linked to barrier requirements.

Supplementary key words lamellar body • phospholipids • epidermal permeability barrier

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
The principal function of the epidermis is to provide a protective barrier against transcutaneous water loss (1, 2). This permeability barrier is localized to the stratum corneum, where three key lipids, cholesterol and ceramides as well as free fatty acids, form the extracellular lamellar membrane structures that mediate permeability barrier function (3, 4). In mammals, these lipid-enriched extracellular lamellar membranes derive from the exocytosis of lamellar bodies from the outmost stratum granulosum cells (1, 2). To form lamellar bodies, three precursor lipids, phospholipids along with cholesterol and glucosylceramides, are synthesized within the stratum spinosum and stratum granulosum cells (1, 2). After lamellar body secretion, phospholipids are further hydrolyzed in the extracellular domains of the stratum corneum to release hydrophobic essential and nonessential fatty acids that are required for the formation of a competent permeability barrier (5). Although phospholipids appear to be essential for epidermal permeability barrier function, little is known about their synthesis in normal epidermis or about the regulation of the enzymes of phospholipid synthesis.

1-Acyl-sn-glycerol-3-phosphate acyltransferase (AGPAT; EC 2.3.1.51), also known as lysophosphatidic acid acyltransferase, is the key enzyme of phospholipid and triglyceride biosynthesis. Several lines of evidence suggest that AGPAT should play a key role in the synthesis of phospholipids/triglycerides required for permeability barrier homeostasis. First, AGPAT catalyzes the further acylation of lysophospholipids to phosphatidic acid, the major precursor of all phospholipids/triglycerides. Second, some AGPAT isoforms are endoplasmic reticulum transmembrane proteins, so these enzyme isoforms would be situated at the correct location to produce phospholipids for lamellar body formation (6, 7). Third, AGPAT functions as an acyltransferase that transfers an acyl moiety onto the sn-2 position of the glycerol backbone. In most tissues, this position is largely occupied by a high prevalence of unsaturated fatty acids. Although the unsaturated, essential free fatty acid linoleic acid has been shown to be necessary for normal permeability barrier formation (8, 9), there is evidence that phospholipid-derived nonessential free fatty acids are also required for the barrier (5, 10). Finally, triglycerides, although not packaged in membrane in lamellar bodies, provide N-acyl fatty acids to sphingolipids, the most abundant barrier lipids (11, 12). Because of its apparent importance, a tight control of this acylation reaction is expected under normal physiological conditions.

In mammals, multiple AGPAT isoforms have been identified (five in mice and six in humans), each encoded by a distinct gene (13–16). These isoforms display tissue-specific variation in expression and activity as well as different substrate preferences (7, 13, 15–17), which suggests tissue-specific functions (7, 17, 18). However, the biological significance of the existence of these variations in isoform expression remains largely unknown. Understanding the expression profile of these enzymes in epidermis, where there is a tissue-specific requirement for phospholipid synthesis for the barrier, could increase our understanding of how AGPAT participates in permeability barrier homeostasis and perhaps other epidermal functions. The aims of the present study are as follows: 1) to determine which AGPAT isoforms are expressed in the epidermis and their localization; 2) to determine the regulation of each isoform during fetal epidermal barrier development; and 3) to assess the regulation of isoform expression in relation to epidermal permeability barrier homeostasis.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Materials
Female hairless mice (hl/hl), 8–12 weeks old, and timed pregnant Sprague-Dawley rats (plug date = day 0) were purchased from Simonsen Laboratories (Gilroy, CA). Acetone was purchased from Fisher Scientific (Fairlane, NJ). 5,5'-Dithiobis(2-nitrobenzoic acid), oleoyl--lysophosphatidic acid, and oleoyl CoA were purchased from Sigma (St. Louis, MO).

Experimental protocols
Epidermal barrier was disrupted by either tape-stripping or acetone treatment, as described previously (19–22). Impaired barrier function was monitored before and immediately after disruption by measurement of transepidermal water loss using an electrolytic water analyzer (MEECO, Warrington, PA). In some experiments, animals were occluded with a tightly fitted water vapor-impermeable latex membrane to artificially restore barrier function (20, 21, 23). To prepare epidermis from hairless mouse, flank skin was excised and epidermis was separated from dermis by incubating in 10 mM EDTA in Ca²⁺- and Mg²⁺-free phosphate-buffered saline (pH 7.4) at 37°C for 45 min. In another group of mice, the full-thickness skin was incubated in 10 mM DTT in PBS for 40 min at 37°C, and epidermal fractions (lower and upper layers) were prepared (24, 25). The separation occurs above the basal layer; thus, the lower layer comprises stratum basale and the upper layer comprises stratum granulosum, stratum spinosum, and stratum corneum. Epidermal samples were snap-frozen in liquid nitrogen and stored at –80°C until total RNA and protein extraction. The preparation of fetal rat epidermis was described previously (26, 27). Briefly, full-thickness skin was removed from fetus, and fetal epidermis was prepared using the protocol described above but with a shorter incubation time (30–35 min).

Isolation of total RNA and semiquantitative PCR
Total RNA was extracted from murine tissues and epidermis using TRIzol reagent according to the manufacturer's protocol (Invitrogen). The purity and the yield of isolated RNA were determined by monitoring the absorbance at optical densities of 260 and 280 nm. The integrity of the RNA was confirmed by performing denaturing gel electrophoresis on the isolated RNA samples. An aliquot of 1 µg of total RNA was reverse-transcribed with 20 ng of random hexamer (RT-for-PCR kit; Clontech, San Diego, CA) at 42°C for 1 h. The cDNA for each mouse AGPAT (mAGPAT) 1, 2, 3, 4, and 5 was amplified with a pair of specific primers. The primers were designed using Vector NTI Suite software for InforMax (version 7; Oxford, UK). For comparison purposes, all primer sets designed are located in different exons to avoid amplifying genomic DNA, and homologous full-length coding regions are amplified. The primer sets designed for mouse are also homologous to those of rats. All primers were synthesized by Qiagen (Alameda, CA). Each pair of primers, the primer sites, the lengths of predicted PCR products, and the GenBank accession number for each primer were reported previously (7). Primer sequences are as follows: mAGPAT 1, 5'-ACC AGA ATG GAG CTG TGG CC-3' (upper), 5'-CGC TCC CCC AGG CTT CTT CA-3' (lower); mAGPAT 2, 5'-CGC CGT CGG GGC TGG GGT GC-3' (upper), 5'-CTG GGC TGG CAA GAC CCC AG-3' (lower); mAGPAT 3, 5'-TGT TCT CAG TGA AGG ACC GT-3' (upper), 5'-CTT AAG CTC TTG GTT GCC AT-3' (lower); mAGPAT 4, 5'-GAT TTA TCT CTT GAG AAT CCC CAC ACC-3' (upper), 5'-GTC CGT TTG TTT CCG TTT GTT GTC-3' (lower); mAGPAT 5, 5'-AGA GGA TGC TGC TGT CCC T-3' (upper), 5'-AAC AAA CCA CAG GCA GCC-3' (lower); mGPAT, 5'-TCC CCG TCT TCA GTA CCT TG-3' (upper), 5'-AAA ATG GCT GTG CAA AAT CC-3' (lower); internal control 36B4, 5'-GCG ACC TGG AAG TCC AAC TAC-3' (upper), 5'-ATC TGC TGC ATC TGC TTG G-3' (lower). All PCRs were carried out using an Eppendorf Mastercycler (96-well model). The validation of the quantitative measurement for each mAGPAT mRNA by RT-PCR was established previously (7). To ensure that the semiquantitative nature of the RT-PCR was valid using cDNA from skin epidermis, we performed the validation experiments using cDNA from mouse epidermis and confirmed that the method was accurate and reproducible. Briefly, under the RT-PCR conditions used, the levels of the PCR products were dependent on the amount of templates used in the reaction. At least an 8- to 10-fold difference of cDNA concentration of each mAGPAT was detected.

Preparation of whole cell extracts and AGPAT enzyme activity assay
Whole cell extracts were prepared by a brief 30 s homogenization (Kinematica, Kriens-Luzern, Switzerland) followed by three rounds of sonification (10 s/round) on ice with the Sonifier (Cell Disruptor 350; Branson Sonic Power Co.) in a buffer containing 100 mM Tris-HCl (pH 7.4), 3 mM MgCl₂, and 1 unit of proteinase inhibitors (proteinase inhibitor cocktail; Sigma). Subsequently, the cell lysate was centrifuged at 3,000 rpm for 10 min to remove cell debris, and the resulting supernatant was saved for protein and enzyme assay. The protein concentration of whole cell extract was determined by Bio-Rad Protein Assay (Bio-Rad Laboratories, Inc., Hercules, CA) using BSA as a standard. AGPAT enzyme activity was determined spectrophotometrically using 50 µM oleoylglycerol-phosphate and 40 µM oleoyl-CoA as described (28, 29).

Statistical analysis
The results are expressed as means ± SEM. Statistical significance between two experimental groups was determined using Student's t-test. A value of P < 0.05 was considered significant.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Epidermis displays a unique AGPAT expression profile
In murine tissues, AGPAT exists in at least five isoforms. Our initial studies have focused on determining the pattern and relative expression levels of AGPAT isoforms in murine epidermis. To compare the relative abundance of each isoform, similar sets of primers were designed and the same epidermal cDNAs were used for all isoform amplification. The specific amplification and semiquantitative measurement for each mAGPAT mRNA have been established (7) and confirmed in this study using cDNA from mouse epidermis (data not shown). As shown in Fig. 1A, the expression pattern of mAGPATs in epidermis was unique. Both mAGPAT 1 and 2 mRNAs were low in murine epidermis, compared with the high expression levels of these isoforms in other tissues, including brain, heart, lung, liver, kidney, and spleen. In contrast, relatively high levels of expression for mAGPAT 3, 4, and 5 were observed in the epidermis compared with other tissues (Fig. 1A). These studies show that the expression profile of mAGPAT isoforms differs in epidermis compared with other murine tissues.

View larger version (53K): [in this window] [in a new window]   Fig. 1. mRNA expression profile of mouse 1-acyl-sn-glycerol-3-phosphate acyltransferase (mAGPAT) isoforms in murine tissues. Mouse organ and epidermal samples were prepared. Total RNA was extracted and RT-PCR was performed as described in Materials and Methods. A: Representative gel image of PCR products of each mouse AGPAT (mAGPAT) isoform and the internal control 36B4 gene. NC, negative control. B: Representative gel image of PCR products of each mAGPAT isoform from several selected tissues (n = 4).

Epidermal AGPAT isoforms are expressed in all nucleated layers
The epidermis generates a broad range of lipid species (30, 31). Although significant amounts of lipids are synthesized in the basal layer, lipid synthesis continues in all of the nucleated layers of the epidermis (32). To determine the localization of mAGPAT isoforms, we next measured mAGPAT mRNA levels in the lower versus upper layers of epidermis. Using the DTT method (24, 25), we separated fractions of lower (stratum basale) and upper (stratum spinosum, stratum granulosum, and stratum corneum) epidermis. To obtain sufficient signals for mAGPAT 1 and 2, PCR conditions were optimized and 32 cycles, instead of 25, were used for PCR amplification. As shown in Fig. 2A, mAGPAT 1 and 2 exhibited higher expression levels in the lower than in the upper epidermis. However, there was no significant difference in mAGPAT 1 and 2 mRNA levels in lower versus upper epidermis when the results were normalized to an internal control (i.e., 36B4) (Fig. 2B). Furthermore, AGPAT 3, 4, and 5 also displayed no differences in mRNA levels in lower versus upper epidermis. These results show that AGPAT mRNAs are expressed throughout the nucleated layers of murine epidermis.

View larger version (42K): [in this window] [in a new window]   Fig. 2. mAGPAT expression in the lower and upper layers of murine epidermis. The lower and upper layers of epidermis were prepared as described in Materials and Methods. Total RNA was extracted and RT-PCR was performed. A: Representative gel image of PCR products for each isoform and the internal control 36B4. B: Densitometry values normalized with 36B4 for each mAGPAT isoform. Data are presented as means ± SEM (n = 4).

View larger version (45K): [in this window] [in a new window]   Fig. 3. Rat AGPAT (rAGPAT) expression during fetal skin barrier development. Rat fetal epidermis was prepared as described in Materials and Methods. Total RNA was extracted and RT-PCR was performed. A: Representative gel image of PCR products for each rAGPAT isoform and the internal control 36B4. NC, negative control. B: Densitometry values normalized with 36B4 for each isoform. Data are presented as means ± SEM (n = 2). Note that day 17 and 18 samples were pooled from three to five fetal epidermises.

View larger version (12K): [in this window] [in a new window]   Fig. 4. AGPAT activity in fetal rat epidermis. Rat fetal skin epidermis was prepared as described in Materials and Methods. AGPAT enzyme activities were determined using epidermal extracts from day 17, 19, and 21 samples. Data are presented as means ± SEM (n = 3). Note that day 17 and 18 samples were pooled from three to five fetal skin epidermises.

View larger version (60K): [in this window] [in a new window]   Fig. 5. Time course of mAGPAT expression in murine epidermis after acute barrier disruption. Mouse skin was tape-stripped or unstripped (for each side of the back skin from the same mouse; the unstripped side serves as a control) as described in Materials and Methods. Skin samples were collected at the indicated time points, and epidermis was prepared. Total RNA was extracted and RT-PCR was performed. A: Representative gel image of PCR products for each mAGPAT isoform and 36B4. B: Densitometry values normalized with 36B4 for each isoform. Data are presented as means ± SEM (n = 4). * P < 0.05.

View larger version (46K): [in this window] [in a new window]   Fig. 6. mAGPAT expression in murine epidermis after acetone application. Mouse skin was treated with acetone or PBS (for each side of the back skin from the same mouse; the PBS side serves as a control) as described in Materials and Methods. After 1 h, skin samples were collected and epidermis was prepared. Total RNA was then extracted and RT-PCR was performed. A: Representative gel image of PCR products for each isoform and 36B4. B: Densitometry values normalized with 36B4 for each isoform. Data are presented as means ± SEM (n = 5). * P < 0.05.

View larger version (43K): [in this window] [in a new window]   Fig. 7. mAGPAT expression in the lower and upper layers of epidermis after tape-stripping. Mouse skin was tape-stripped, and after 3 h the lower and upper layers of epidermis were prepared as described in Materials and Methods. Total RNA was extracted and RT-PCR was performed. A: Representative gel image of PCR products for each mAGPAT isoform and 36B4. B: Densitometry values normalized with 36B4 for each isoform. Data are presented as means ± SEM (n = 5). * P < 0.05.

View larger version (51K): [in this window] [in a new window]   Fig. 8. mAGPAT expression in murine epidermis after tape-stripping. Mouse skin was tape-stripped, tape-stripped plus occlusion, or unstripped (for each side of the back skin from the same mouse; the unstripped side serves as a control) as described in Materials and Methods. After 3 h, epidermis was prepared. Total RNA was extracted and RT-PCR was performed. A: Representative gel image of PCR products for each mAGPAT isoform and 36B4. NC, negative control. B: Densitometry values normalized with 36B4 for each isoform. Data are presented as means ± SEM (n = 4). * P < 0.05.

View larger version (10K): [in this window] [in a new window]   Fig. 9. mAGPAT enzyme activity in epidermis after tape-stripping. Mouse skin was tape-stripped or unstripped (for each side of the back skin from the same mouse; the unstripped side serves as a control), and epidermis was prepared after 3 h. Whole cell extracts from epidermis were prepared, and total AGPAT enzyme activities were assayed as described in Materials and Methods. Data are presented as means ± SEM (n = 5). * P < 0.05.

View larger version (52K): [in this window] [in a new window]   Fig. 10. Expression of mitochondrial sn-glycerol-3-phosphate acyltransferase (GPAT) in mouse epidermis before and after tape-stripping. Mouse skin was either tape-stripped or unstripped (for each side of the back skin from the same mouse; the unstripped side serves as a control) as described in Materials and Methods. After 3 h, skin samples were collected and epidermis was prepared. Total RNA was then extracted and RT-PCR was performed. Upper panel: Representative gel image of PCR products of mitochondrial GPAT and 36B4 obtained from RT-PCR. Lower panel: Densitometry values normalized with 36B4 for mitochondrial GPAT.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

The stratum corneum lipid mixture that mediates permeability barrier function originates from the exocytosis of lamellar bodies. Lamellar bodies are specialized membrane-bound secretory organelles that contain abundant phospholipids. To provide sufficient substrates for phospholipid synthesis, keratinocytes first must synthesize nonessential fatty acids as well as take up essential fatty acids from the circulation (8–10). Epidermis is thus a highly active lipid synthetic tissue, which generates abundant free fatty acids destined not only for barrier but also for membrane biogenesis. Because of the potential detergent action of free fatty acids, the intracellular fatty acids are quickly assembled into complex, nontoxic species, such as phospholipids and triglycerides, by the action of phospholipid acyltransferases.

In mammalian cells, two major phospholipid acyltransferases catalyze glycerolipid biosynthesis (i.e., GPAT and AGPAT). GPAT mediates the acylation of glycerol-3-phosphate at the sn-1 position, which is the first committed step in glycerolipid biosynthesis, whereas AGPAT mediates the second acylation reaction by catalyzing the transfer of another acyl chain to the sn-2 position on the glycerol backbone (38, 39). Both GPAT and AGPAT have multiple enzyme isoforms, which localize to endoplasmic reticulum and mitochondrial membranes (6, 40, 41). Lamellar body lipids are derived mainly from endoplasmic reticulum; thus, microsomal GPAT/AGPAT isoforms are more important for lamellar body formation. Because microsomal GPAT has not yet been cloned, studies on the regulation of microsomal GPAT are not yet possible. However, mitochondrial GPAT has been cloned, and its expression did not change after barrier disruption (Fig. 10). Thus, the microsomal acyltransferases, but not mitochondrial acyltransferases, are likely modulated after barrier disruption.

One of the major aims of the present study was to determine the pattern of expression and the localization of AGPAT isoforms in murine epidermis. Although a high mRNA level of mAGPAT 1 has been demonstrated in most organs analyzed (brain, heart, lung, liver, and kidney), very low levels of constitutive expression were detected in murine epidermis. Similarly, mAGPAT 2 exhibited high basal levels of expression in most other organs, but its constitutive expression was low in epidermis. Consistent with previous observations that the level of AGPAT 3 is variably expressed in extracutaneous organs (7), its expression in epidermis was moderate. Interestingly, AGPAT 4 exhibited a relatively high constitutive expression in epidermis, as this isoform was barely detectable in several other organs, such as heart, liver, kidney, and spleen, and AGPAT 5 had the highest constitutive expression compared with those of other isoforms. To further define the localization of AGPAT isoforms in different layers of epidermis, we examined the mRNA levels of AGPAT isoforms in the lower and upper layers of epidermis. Under basal conditions, no significant difference was found for any of those isoforms in the lower versus upper layers of epidermis, suggesting that phospholipid biosynthesis may be a continuous process that occurs in all of the nucleated layers of epidermis. The data demonstrated low constitutive expression for mAGPAT isoforms 1 and 2 but high constitutive expression for mAGPAT isoforms 3, 4, and 5. This unique profile of AGPAT expression in epidermis indicates that a tissue-specific composition of these enzymes might be required for proper epidermal function.

Although multiple enzymes have been designated as AGPATs, the functional role of these isoforms has not yet been firmly established. According to sequence identities, these five isoforms can be further divided into three subgroups: group I (AGPAT 1 and 2); group II (AGPAT 3 and 4); and group III (AGPAT 5) (7). It appears that group I enzymes confer relatively higher acyltransferase activities than enzymes in either group II or group III (7). However, data from this study show that both group II and group III enzymes display high basal expression in adult murine epidermis. One possible explanation is that enzymes in groups II and III may be involved in maintaining basal barrier function when phospholipid synthesis is not subject to increased demand. In support of this notion, group I enzymes appear to be upregulated during fetal epidermal ontogenesis (AGPAT 2) and during barrier repair (AGPAT 1 and 2). In these conditions, AGPAT 1 and/or AGPAT 2 are upregulated in parallel with a burst of barrier lipid biosynthesis. Yet, the question remains why mammalian tissues express multiple isoforms of an enzyme that catalyzes the same biochemical reaction. Presumably, different enzyme isoforms could localize to discrete subcellular compartments, or they could exhibit different substrate preferences (i.e., they could incorporate different fatty acid species into phospholipids). Finally, these isoforms could provide regulatory mechanisms in response to certain requirements, such as cell repair, growth, proliferation, and/or lipid synthesis (42, 43). These assumptions are supported by the fact that many other enzymes in lipid metabolism also exist as duplicate/multiple isoforms with discrete localization, function, and regulation (6, 43, 44).

Among AGPAT isoforms, the largest body of information is known for AGPAT 1 and 2. mAGPAT 1 shares 44% sequence identity with AGPAT 2 and much less homology with the three other isoforms (<11% sequence identity). Several studies have suggested that AGPAT 1 is expressed ubiquitously in most tissues and hence functions as a housekeeping gene (13, 14). In contrast, the expression of AGPAT 2 is more tissue-specific; it is expressed at high levels in liver, heart, and adipose tissue, at intermediate levels in kidney, lung, and spleen, at very low levels in epidermis, and is almost undetectable in brain. Linkage studies have revealed that AGPAT 2 is critical for triglyceride biosynthesis (18). Mutations in human AGPAT 2 have been shown to be one of the causes of congenital generalized lipodystrophy by inhibiting triacylglycerol synthesis in adipocytes (18, 45). Of note, triglycerides are a major component of lipids in epidermis and constitute 12% and 24% of total weight in the basal/spinous and granular layers, respectively (38, 39). Recently, additional roles for AGPAT 2 have emerged. AGPAT 2, but not AGPAT 1, may play a role in providing proliferative/survival signals in normal and malignant cells, presumably by generating phosphatidic acid, an important intracellular secondary messenger (42, 46, 47). We speculate that AGPAT 2 might perform these two functions in skin epidermis. For example, AGPAT 2 has a low constitutive expression but is induced during fetal epidermal development or after epidermal barrier disruption (Figs. 3, 5). In these two situations, the upregulation of AGPAT 2 parallels a burst of both cell proliferation and barrier lipid synthesis.

In summary, we have identified a unique expression pattern of AGPAT isoforms in murine epidermis, with both constitutive (isoforms 3, 4, and 5) and/or inducible (isoforms 1, 2, 3, and 5) forms present. The temporal regulation of AGPATs during normal epidermal development and during barrier repair suggests that an increase in epidermal phospholipid synthesis may play a role in maintaining epidermal permeability barrier homeostasis.

Manuscript received June 20, 2005 and in revised form July 28, 2005.

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