Density-dependent variations in the lipid content and metabolism of cultured human keratinocytes

Lipin-1 expression is critical for keratinocyte differentiation

Lipin-1 is an Mg(2+)-dependent phosphatidate phosphatase that facilitates the dephosphorylation of phosphatidic acid to generate diacylglycerol. Little is known about the expression and function of lipin-1 in normal human epidermal keratinocytes (NHEKs). Here, we demonstrate that lipin-1 is present in basal and spinous layers of the normal human epidermis, and lipin-1 expression is gradually downregulated during NHEK differentiation. Interestingly, lipin-1 knockdown (KD) inhibited keratinocyte differentiation and caused G1 arrest by upregulating p21 expression. Cell cycle arrest by p21 is required for commitment of keratinocytes to differentiation, but must be downregulated for the progress of keratinocyte differentiation. Therefore, reduced keratinocyte differentiation results from sustained upregulation of p21 by lipin-1 KD. Lipin-1 KD also decreased the phosphorylation/activation of protein kinase C (PKC)α, whereas lipin-1 overexpression increased PKCα phosphorylation. Treatment with PKCα inhibitors, like lipin-1 KD, stimulated p21 expression, while lipin-1 overexpression reduced p21 expression, implicating PKCα in lipin-1-induced regulation of p21 expression. Taken together, these results suggest that lipin-1-mediated downregulation of p21 is critical for the progress of keratinocyte differentiation after the initial commitment of keratinocytes to differentiation induced by p21, and that PKCα is involved in p21 expression regulation by lipin-1.

EpCAM Induction Functionally Links to the Wnt-Enhanced Cell Proliferation in Human Keratinocytes

Accelerating proliferation of primary keratinocytes benefits skin autografts for severely burned patients. Wnt signal, a conserved pathway controlling cell cycle and morphogenesis in embryo, also involves in cell proliferation and tumorigenesis in adult tissues. Here the effects of Wnt signal on the growth of human interfollicular keratinocytes were investigated. We demonstrated that recombinant Wnt3a significantly promoted the growth of primary keratinocytes at a low cell density. A well-characterized GSK-3b inhibitor, BIO, activated the Wnt signals and also enhanced the colony formation of keratinocytes dose dependently. Gene expression profile of the BIO-treated keratinocytes revealed the linkage of BIO with cell mitosis and indicated that epithelial cell adhesion molecule (EpCAM), a Wnt target gene, was significantly upregulated. Compared to the sorted EpCAM^- keratinocytes, the EpCAM⁺ cells showed a higher proliferation rate and efficacy of colony formation. Inhibiting the EpCAM expression by shRNA attenuated the proliferation effect of BIO and the growth advantage of the EpCAM⁺ keratinocytes. These evidences emphasize the positive roles of canonical Wnt and EpCAM on the regulation of cell growth and self-renewal of human keratinocytes.

Role of cholesterol sulfate in epidermal structure and function: lessons from X-linked ichthyosis

X-linked ichthyosis is a relatively common syndromic form of ichthyosis most often due to deletions in the gene encoding the microsomal enzyme, steroid sulfatase, located on the short area of the X chromosome. Syndromic features are mild or unapparent unless contiguous genes are affected. In normal epidermis, cholesterol sulfate is generated by cholesterol sulfotransferase (SULT2B1b), but desulfated in the outer epidermis, together forming a 'cholesterol sulfate cycle' that potently regulates epidermal differentiation, barrier function and desquamation. In XLI, cholesterol sulfate levels my exceed 10% of total lipid mass (≈1% of total weight). Multiple cellular and biochemical processes contribute to the pathogenesis of the barrier abnormality and scaling phenotype in XLI. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Pathogenesis of permeability barrier abnormalities in the ichthyoses: inherited disorders of lipid metabolism

Many of the ichthyoses are associated with inherited disorders of lipid metabolism. These disorders have provided unique models to dissect physiologic processes in normal epidermis and the pathophysiology of more common scaling conditions. In most of these disorders, a permeability barrier abnormality "drives" pathophysiology through stimulation of epidermal hyperplasia. Among primary abnormalities of nonpolar lipid metabolism, triglyceride accumulation in neutral lipid storage disease as a result of a lipase mutation provokes a barrier abnormality via lamellar/nonlamellar phase separation within the extracellular matrix of the stratum corneum (SC). Similar mechanisms account for the barrier abnormalities (and subsequent ichthyosis) in inherited disorders of polar lipid metabolism. For example, in recessive X-linked ichthyosis (RXLI), cholesterol sulfate (CSO(4)) accumulation also produces a permeability barrier defect through lamellar/nonlamellar phase separation. However, in RXLI, the desquamation abnormality is in part attributable to the plurifunctional roles of CSO(4) as a regulator of both epidermal differentiation and corneodesmosome degradation. Phase separation also occurs in type II Gaucher disease (GD; from accumulation of glucosylceramides as a result of to beta-glucocerebrosidase deficiency). Finally, failure to assemble both lipids and desquamatory enzymes into nascent epidermal lamellar bodies (LBs) accounts for both the permeability barrier and desquamation abnormalities in Harlequin ichthyosis (HI). The barrier abnormality provokes the clinical phenotype in these disorders not only by stimulating epidermal proliferation, but also by inducing inflammation.

Further optimization of culture method for rat keratinocytes: titration of glucose and sodium chloride

The present study further improved the serum-free method of culturing rat keratinocytes. To obtain the best growth of rat keratinocytes, we modified our previous serum-free medium (MCDB153 based medium), particularly the amounts of glucose and sodium chloride (NaCl). Titration experiments showed the optimal concentration to be 0.8 mM for glucose and 100 mM for NaCl. This modification eliminated the requirement for albumin, which had been essential for colony formation when our previous medium was used. Titration of glucose and NaCl, followed by adjustment of essential amino acids and growth factors, produced a new formulation. More satisfactory and better growth was achieved with the new medium than with the previous medium. Accumulation of monoalkyldiacylglycerol (MADAG) was consistently noted in this study, representing the unusual lipid profile. A tendency toward normalization was, however, noted with the neutral lipid profile of keratinocytes cultivated in the new medium: lower production of MADAG was obtained with the new formulation, rather than the previous one.

Accumulation of 1-o-alkyl-2,3-diacylglycerols in cultured rat keratinocytes

The present study was undertaken to identify the chemical structure of neutral lipid accumulated in cultured rat keratinocytes and to address their metabolism. Neutral lipid of similar mobility with alkyldiacylglycerol was isolated from cultured rat keratinocytes by thin layer chromatography. The long-chain diols derived from the neutral lipids were identified as 1-alkylglycerol based on the mass spectra of their nicotinylidene derivatives. Thus these neutral lipids were identified as 1-o-alkyl-2,3-diacylglycerols (ADAG). Addition of rat serum elevated the level of ADAG with increasing trend of linoleic acid concentration in this fraction. [14C]Acetate added to the confluent plates was incorporated into alkyl- and acyl-chains of ADAG with incubation in 24 h, and remained un-metabolized up to 72 h. This, however, is not the case for the label incorporation into phospholipid and triacylglycerol. Radioactivities of these two lipid fractions appeared to reach the maximum in 24 h, and thereafter decreased to 72 h with a similar decay curve. Incorporation of [14C]acetate into phospholipid and ADAG was significantly depressed, and that into triacylglycerol and free cholesterol was increased by the supplementation of the medium with rat serum. In concomitance with the accumulation of ADAG, the concentration of ethanolamine-plasmalogen increased in the cultured keratinocytes. The results of the present study first showed the elevated level of ether lipid synthesis in the proliferating primary culture of rat keratinocytes.

Triglyceride metabolism in human keratinocytes cultured at the air-liquid interface

Although epidermis reconstructed in vitro histologically demonstrates the presence of fully differentiated tissue with cornified strata, it does not synthesize or release epidermal barrier lipids in the same proportions as does native skin, causing the barrier function to be impaired. Lipids, the content of which deviates the most, include triglycerides that are present in high amounts and stored as lipid droplets. Our recent studies have revealed that a high triglyceride content may be a reflection of a high synthetic rate and a low turnover. Therefore, the present study was undertaken to examine whether the triglyceride accumulation in the air-exposed cultures may be a result of insufficient supplementation of cells with oxygen, an excessive supplementation of cells with glucose, dysregulation of lipogenesis, or an impaired catabolism of triglycerides caused either by insufficient activity of triglyceride lipase and/or accumulation of free fatty acids due to insufficient activity of beta-oxidase. When keratinocytes were cultured at the air-liquid interface in medium containing a standard glucose concentration, both the lactate and triglyceride production was high. Lowering glucose content in the medium resulted in a decrease in both lactate production and triglyceride synthesis. However, even when grown at a low glucose concentration the triglyceride content remained higher than found in vivo and synthesized triglycerides were stored in the cells as a stable pool, suggesting that the catabolism of triglycerides was impaired. Since both lipase and beta-oxidase were found to be active in cultured keratinocytes, another factor or other factors are probably implicated in the regulation of triglyceride metabolism.

Normalization of essential-fatty-acid-deficient keratinocytes requires palmitic acid

Cultured adult human keratinocytes show accelerated growth rates in medium that is essential fatty acid deficient. The cells also show decreased amounts of the essential fatty acids 18:2, 20:3, and 20:4 and contain increased amounts of the monounsaturated fatty acids 16:1 and 18:1. These lower levels of polyunsaturated fatty acids were only partially restored by supplementing the medium with 18:2 and 20:4 fatty acid. The addition of the non-essential fatty acid 16:0 (5 microM), along with the essential fatty acids, resulted in the successful normalization of the major fatty acids in the deficient keratinocytes. Normalized cells showed a constant total fatty acid/mg of protein in the phospholipid fraction, as the total cell fatty acid content per cell increased with augmenting fatty acid supplementation. Supplementation of the medium with 16:0 and essential fatty acids decreased the growth and passage potential of the cells. Use of 18:1 in lieu of 18:2 fatty acid yielded essential-fatty-acid-deficient keratinocyte growth values. Likewise the least supplemented medium (5 microM 18:2 + 5 microM 16:0) also gave the accelerated cell growth rates. This study shows that manipulation of the essential fatty acid levels, if accompanied by 5 microM 16:0 in the growth medium, alters the growth properties of adult human primary keratinocytes.

Expression of differentiation markers in human adult keratinocytes cultured in submerged conditions

A number of studies have shown that human keratinocytes cultured in submerged conditions with non-delipidized serum do not express the major differentiation markers, i.e. 67 kDa keratin, ceramides, and lanosterol. However, they were mostly performed with neonatal or juvenile keratinocytes after a few passages, and not all the markers were analyzed in parallel. In this study, we compared the expression of several differentiation markers in preconfluent and postconfluent adult breast keratinocytes in primary and secondary cultures before and after cryopreservation. When primary cultures reached confluence, the 67 kDa keratin was synthesized, transglutaminase activity was increased, and, although overall lipid synthesis dropped, both lanosterol and free fatty acids contents were augmented. The same pattern was observed in postconfluent subcultures at Passage 2; however decreased overall lipid synthesis was more pronounced. Cryopreservation of keratinocytes just after isolation or after a few days in culture did not result in the loss of expression of these specific epidermic markers. Thus, adult breast keratinocytes in postconfluent submerged cultures represent an in vitro model that possesses various features of the normal epidermis, even after cryopreservation.

Evidence for a novel keratinocyte fatty acid uptake mechanism with preference for linoleic acid: comparison of oleic and linoleic acid uptake by cultured human keratinocytes, fibroblasts and a human hepatoma cell line

Keratinocytes require the essential fatty acid (FA), linoleic acid (LA), for the synthesis of stratum corneum membrane lipids. A plasma membrane-FA binding protein (PM-FABP), is postulated to mediate cellular FA-uptake in hepatocytes and several other tissues, but the mechanism whereby exogenous FA are taken up by keratinocytes has not been investigated. This study examines the uptake of LA and oleic acid (non-essential) in cultured human keratinocytes, in comparison to dermal fibroblasts and the human hepatoma cell line, HepG2. As previously reported for hepatocytes, FA-uptake in keratinocytes was curvilinear, with an initial (30 s) rapid cellular influx. The initial uptake component was temperature dependent, exhibited saturable kinetics and was significantly inhibited by pretreatment with trypsin. In contrast, fibroblast FA-uptake lacked an initial rapid uptake component, was relatively temperature insensitive, and was not inhibited by trypsin. Keratinocytes differed from both hepatocytes and fibroblasts by more rapid uptake of LA in comparison to oleic acid during the initial influx phase. Moreover, FA-uptake in keratinocytes was not inhibited by preincubation with a anti-rat liver PM-FABP antibody. These data provide evidence for a PM-FA transporter in keratinocytes that is distinct from the hepatic PM-FABP. The apparent preference of the putative keratinocyte FA transporter for LA may function to ensure epidermal capture of sufficient LA for barrier lipid synthesis.

Lipid composition and synthesis of HaCaT cells, an immortalized human keratinocyte line, in comparison with normal human adult keratinocytes

Cultured keratinocytes are frequently employed for studies of epidermal lipid metabolism. Interpretation of experimental data may be complicated by donor to donor variability, the relatively short culture lifetime and variations between passages, problems that are not encountered to the same extent with immortalized cell lines. The present study was undertaken to compare the lipid composition and synthesis of normal human adult keratinocytes (NHAK) with HaCaT cells, a long-lived, spontaneously immortalized human keratinocyte line, in relation to proliferation and differentiation. No differences between the two cell types were observed: a) in total lipid content; b) in the distribution of major lipid classes during growth at 50%, 75% and 100% confluence; c) in cultures grown at 0.6 mM calcium, at which differentiation is retarded, or at 1.6 mM calcium, at which some differentiation takes place; d) in the incorporation of [14C] acetate into cellular lipids at confluence, or e) in the fatty acid composition of major cellular lipid classes. At 100% confluence NHAK and HaCaT cells differ in their cholesterol metabolism. At all stages of growth, cholesterol synthesis in HaCaT cells is more LDL-dependent than in NHAK. Furthermore, NHAK become less LDL-dependent at confluence whereas HaCaT cells do not. HaCaT cells also revealed a significantly larger fraction of phosphatidyl-ethanolamine, -serine and -inositol at 0.6 mM calcium concentration than NHAK. These findings suggest that HaCaT cells do not differentiate as well as NHAK in vitro and may therefore serve as a model for the study of lipid metabolism in cells defective in terminal differentiation.

Fatty acid and lipid composition in vitro and in vivo of rat epidermis

1. Fatty acid and lipid compositions of cultured rat keratinocytes were compared with those of intact epidermis prepared from newborn and adult rats. 2. The uniqueness of the lipid profile of cultured cell manifested itself in the accumulation of a novel lipid which co-migrated with monoalkyldiacylglyceride on thin-layer chromatography. 3. Concentration of the branched-chain fatty acid was specifically high in the cholesterol ester fraction of the intact cell, and was decreased by cultivation under submerged conditions.

High-affinity fatty acid-binding activity in epidermis and cultured keratinocytes is attributable to high-molecular-weight and not low-molecular-weight fatty acid-binding proteins

Fatty acid-binding proteins (FABPs) are abundant low-molecular-weight cytosolic proteins in tissues involved in fatty acid (FA) metabolism. Because epidermis is also an active lipogenic tissue, we examined cytosols from murine and porcine epidermis and cultured human keratinocytes and fibroblasts for FABPs. High-affinity FA-binding activity was present in both epidermis and differentiated keratinocytes, whereas no high-affinity FA-binding activity was found in cultured human fibroblasts or undifferentiated keratinocytes. By column chromatography, a single binding peak was identified in the high (90-100 kDa)-molecular-weight range and no binding activity was evident in the low (14-15 kDa)-molecular-weight range, where conventional FABPs elute. Moreover, rabbit anti-rat heart FABP, anti-rat intestine FABP, and anti-rat liver FABP antisera did not identify proteins in the 14-15-kDa range in murine epidermal cytosol by Western immunoblots, whereas the anti-rat-heart antibody recognized a protein of approximately 32 kDa. Isoelectric focusing of differentiated keratinocyte cytosol demonstrated a single FA-binding peak having a pI of approximately 4.0. Analysis of this binding peak by SDS-PAGE revealed peptides of approximately 66 and 38 kDa. These findings suggest the possibility that the FA-binding protein in keratinocyte cytosol normally exists as a heterodimer. Western immunoblots of both differentiated keratinocyte cytosol and keratinocyte-conditional media stained with a rabbit anti-human serum albumin antibody identified a protein of approximately 67 kDa, but the electrofocused fraction did not react with this antibody. Thus, epidermis and differentiated keratinocytes possess high-affinity cytosolic FA-binding activity that cannot be ascribed either to conventional low-molecular-weight FABPs or to albumin.

In vitro model of essential fatty acid deficiency

The polyunsaturated fatty acids linoleic acid (18:2, n-6) and arachidonic acid (20:4, n-6) are essential for normal skin function and structure, both as eicosanoid precursors and as components of lipids forming cell membranes. Adult human keratinocytes grow optimally in serum-free medium (MCDB 153) that contains no fatty acids. These keratinocytes expand rapidly and produce normal epidermis upon in vivo grafting. Analysis of lipid extracts of epidermis and of cultured keratinocytes was done to determine the fatty acid composition of cells grown in essential fatty acid (EFA)-deficient medium. Gas chromatography and high-performance liquid chromatography analyses were done of the fatty acids in the entire cell and in a thin-layer chromatography separated fraction containing those lipids that form cellular membranes. Comparison of snap-frozen epidermis and epidermal basal cell suspensions to passage 1 to 4 cultures shows that the cells are in an extreme essential fatty acid-deficient state by the first passage. The amount of the saturated fatty acids 16:0, 18:0, and 14:0 is unchanged by culture. The polyunsaturated fatty acids are found to be significantly decreased, the cells balancing their lack with a significant increase in the relative abundance of the monounsaturated fatty acids, 18:1 and 16:1. Greater than 85-90% of the fatty acids was found in lipids associated with membranes and no unusual fatty acids were detected. Because the serum-free medium is fatty acid free and the cells cannot synthesize essential fatty acids, the rapid division of the cells results in the predominance of an extreme EFA-deficient cell type. The essential fatty acid-deficient keratinocyte is an excellent adult, normal epidermal cell model that can be used to study EFA deficiency and the effect of the eicosanoid and fatty acids on cell function and structure.

LDL receptors in keratinocytes

The presence of sufficient amounts of cholesterol in the epidermis is necessary for proper functioning of plasma membranes in the viable epidermal cell layers and also for the barrier quality of lipid intercellular bilayers of the stratum corneum. Cholesterol can be generated by local epidermal synthesis, or imported from the circulation as low-density lipoprotein (LDL), which is internalized by the cells by receptor-mediated endocytosis. Because of the complex structure of the skin, a model consisting of cultured human keratinocytes has been used to study in detail the regulation of epidermal sterologenesis in relation to keratinocyte differentiation. Experimental modulation of the differentiation of normal human keratinocytes has been achieved by varying extracellular calcium concentration or by comparison of a number of human squamous carcinoma cell lines and normal keratinocytes. These studies have clearly demonstrated a reciprocal correlation between the ability of cells to differentiate and LDL receptor activity. Regulation of LDL receptor expression has been found to occur at the DNA, mRNA, and protein levels, depending on the cell line studied. In normal but not malignant keratinocytes, the induction of keratinocyte differentiation was associated not only with a decrease of functional LDL receptors but also with changes in their cellular distribution. This conclusion is drawn from the observations that only in normal human keratinocytes, cultured at physiologic calcium concentrations, high levels of intracellular, cytoskeleton-associated receptors were found. Differentiation-related modulations of the LDL-receptor expression and of the cellular LDL-receptor distribution found in cultured keratinocytes were in agreement with observations made in the epidermis in situ.

Distribution of low-density lipoprotein receptors and apolipoprotein B on normal and on reconstructed human epidermis

Immunoelectronmicroscopy was employed to investigate the ultrastructural localization of the low density lipoprotein (LDL) receptor and the protein moiety of LDL (apo B) on normal human epidermis in situ and reconstructed human epidermis in vitro. For this purpose ultrathin sections of Lowicryl K4M embedded material were incubated with monoclonal antibodies against the LDL receptor and apo B followed by a second antibody conjugated to 15 nm colloidal gold. Examination of the sections revealed a similar distribution of the LDL receptor and apo B in normal human epidermis and in reconstructed epidermis. In both systems the amount of LDL receptors decreases during keratinocyte differentiation. In contrast, apo B molecules are more abundantly expressed in the upper layers of the stratum spinosum and the stratum granulosum. The great similarities in the distribution of the LDL receptor and apo B between the in vitro reconstructed epidermis and its in vivo counterpart provide additional proof that reconstructed epidermis is an excellent tool to investigate proliferation and differentiation processes of keratinocytes in vitro.

Characterization of human sebaceous cells in vitro

Human sebaceous cells, isolated from adult human skin, were cultured on either bovine type I collagen or mitomycin-C-treated 3T3 fibroblasts. Sebaceous cells, termed "sebocytes", were determined to be epithelial in nature by positive staining with monoclonal antikeratin antibodies BG2 and BG12. However, sebocyte colonies were also negative for keratins found in differentiated cells of keratinocyte colonies, as defined by monoclonal antikeratin antibodies CC2 and CC6. Sebocytes did not produce cornified envelopes in vitro and could only be induced to produce small quantities (less than 5%) of envelopes with a calcium ionophore. Sebocyte growth characteristics in a variety of serum, dexamethasone, and hydrocortisone combinations were significantly different from those of human facial keratinocytes. Sebocytes also displayed a growth curve and plating efficiency that were different from those of keratinocytes. Large lipid droplets within growing sebocytes could be visualized with oil red o staining. Additionally, squalene and wax/cholesterol esters were made by sebocytes in vitro in greater amounts than by facial keratinocytes, as determined by thin-layer chromatography of organic extracts of 3H2O-labeled sebocytes. Sebocytes synthesized greater quantities of lipid, on a per-cell and protein basis, than did keratinocytes.

Neutral lipid storage disease: a possible functional defect in phospholipid- linked triacylglycerol metabolism

Neutral lipid storage disease (NLSD) (Chanarin-Dorfman Syndrome) is an autosomal recessive disorder of multisystem triacylglycerol (TAG) storage. Previous work has pointed to a defect in intracellular TAG metabolism. In the studies reported here, the lipid metabolism of three lines of NLSD fibroblasts were compared to normal skin fibroblasts. When pulsed with [3H]oleic acid, the earliest observed abnormality in NLSD cell lines was increased incorporation into phosphatidylethanolamine, followed by accumulation of radiolabel in TAG. Activities of several glycerolipid synthetic enzymes were comparable in NLSD and normal fibroblast lines, excluding oversynthesis of glycerolipid. The proportion of plasmalogen and neutral ether lipid synthesized was normal and alkylglycerols did not accumulate, excluding a defect in ether lipid metabolism. Activities of both acid lipase and Mn2(+)-sensitive lipase within the particulate fractions of NLSD and normal fibroblasts were comparable. These studies are most consistent with functional deficiency of a TAG lipase with activity against a pool of TAG that are normally utilized for phospholipid biosynthesis.

The use of cultured epithelial and endothelial cells for drug transport and metabolism studies

In an effort to develop novel strategies for delivery of drug candidates arising from rational drug design and recombinant DNA technology, pharmaceutical scientists have begun to employ the techniques of cell culture to study drug transport and metabolism at specific biological barriers. This review describes some of the general factors that should be considered in developing a cell culture model for transport studies and metabolism studies. In addition, we review in detail the recent progress that has been made in establishing, validating, and using cell cultures of epithelial barriers (e.g., cells that constitute the intestinal, rectal, buccal, sublingual, nasal, and ophthalmic mucosa as well as the epidermis of the skin) and the endothelial barriers (e.g., brain microvessel endothelial cells).

Differentiation of cultured human keratinocytes: effect of culture conditions on lipid composition of normal vs. malignant cells

Differentiation in keratinocytes can be experimentally modulated by changing the culture conditions. When cultured under conventional, submerged conditions, the extent of cellular differentiation is reduced in the presence of low calcium medium and is enhanced in medium containing physiologic calcium concentrations. Moreover, cultures grown at the air-medium interface or on a dermal substrate, or both, differentiate even further. Herein we report the effect of culture conditions on lipid composition in normal human keratinocytes and three squamous carcinoma cell (SCC) lines that vary in their capacity to differentiate as assessed by cornified envelope formation. Under submerged conditions, the total phospholipid content was lower, triglyceride content higher, and phospholipid:neutral lipid ratio lower in direct correlation to the degree of differentiation in these cultures. When grown at the air-medium interface on de-epidermized dermis, evidence of further morphologic differentiation was found only for well-differentiated SCC cells and normal keratinocytes. Similarly, the phospholipid content remained high in poorly differentiated SCC cells and it decreased modestly in well-differentiated SCC cells and markedly in normal keratinocytes. In all cell lines the triglyceride content was increased and cholesterol content decreased when compared to parallel submerged cultures, but these differences were most pronounced in well-differentiated cell lines. Acylceramides and acylglucosylceramides were found only in normal keratinocytes and only under the most differentiation-enhancing conditions. These studies demonstrate differentiation-related changes in the lipid content of both normal and neoplastic keratinocytes.

Fatty acid metabolism in human keratinocytes cultivated at an air-medium interface

Stratum corneum lipids, which provide the mammalian permeability barrier, display a distinctive fatty acid profile with a predominance of long chain, saturated fatty acids. In addition, linoleic acid (18:2) is present in substantial quantities, implying that it is an important structural component. To investigate selectivity of fatty acid incorporation into epidermal lipids, we examined the metabolism of exogenous fatty acids in cultured human keratinocytes, grown at the air-medium interface to enhance differentiation. Keratinocytes were pulsed with [3H] oleic, [14C] stearic, [14C] palmitic, or [14C] linoleic acids; lipids were extracted and fractionated by thin layer chromatography. All fatty acids were taken up and incorporated into complex lipids in a dose-dependent manner that was linear over the first 60 min. These fatty acids were incorporated predominantly into phospholipids and triacylglycerols; their incorporation could be rank ordered: linoleic greater than oleic greater than or equal to palmitic greater than stearic acid. Less than 2% of each fatty acid taken up by keratinocytes was oxidized to CO2; therefore, these differences in utilization cannot be ascribed to differences in rates of beta-oxidation. In pulse-chase studies fatty acids incorporated initially into triacylglycerols, subsequently chased into phospholipids. [14C]Palmitic acid and [14C] acetate were incorporated into sphingolipids more efficiently than the other fatty acids studied. These studies demonstrate that 1) keratinocytes have the ability to incorporate exogenous fatty acids preferentially into complex lipids; 2) triacylglycerols provide a pool of fatty acids for phospholipid synthesis; and 3) palmitate and de novo synthesized fatty acid are preferably utilized for sphingolipid synthesis.