The epidermis as metabolically active tissue: regulation of lipid synthesis by the barrier function

Numerous investigations have shown that the lipids of the horny layer play an important role in the epidermal barrier function. These lipids consist of sphingolipids, cholesterol, and free fatty acids in nearly equimolar proportions. If the barrier function is disturbed--i.e. in case of lipid extraction or a diet deficient of essential fatty acids--we find an increased synthesis of free fatty acids, cholesterol, and non-saponifiable lipides in the epidermis. Covering the skin with a Latex wrap prevents an increased lipid synthesis. The synthesis of cholesterol depending on the barrier function is regulated by the enzyme HMG CoA reductase. The regulation process involves both the quantity and the activity (phosphorylation) of the enzyme. Acute disruption of the permeability barrier results in an increased synthesis of cholesterol in the lower epidermis, whereas in case of chronic barrier disorders, the specific increase takes place in the upper dermis. A reduction of the cholesterol synthesis by the HMG CoA reductase inhibitor Lovastatin leads to a disturbed permeability barrier and epidermal hyperplasia.     

Regulation of the epidermal permeability barrier by lipids and hyperproliferation]

The stratum corneum, the permeability barrier between the internal and external milieu, is composed of protein-enriched cells and lipid-enriched intercellular domains. Lipid synthesis is localized in the keratinocytes. The lamellar bodies located in the keratinocytes secrete lipids (sphingolipids, free fatty acids and cholesterol) into in the intercellular spaces of the stratum corneum. A disturbance of barrier function results in an increase in the synthesis of free fatty acids, non-saponified lipids and cholesterol in all nucleated layers of the epidermis. Cholesterol synthesis is regulated by the enzyme HMG-CoA reductase. After acute disturbance of barrier function by acetone treatment the increase in cholesterol synthesis occurs mainly in the lower epidermis (stratum basale/stratum spinosum), while after chronic disturbance by a diet deficient in essential fatty acids the increase shifts to the upper epidermis (stratum granulosum). After barrier disturbance not only lipid but also DNA synthesis is stimulated. Stimulation of DNA synthesis leading to epidermal hyperplasia may be a second mechanism by which the epidermis tries to correct defects in barrier function. Artificial barrier repair with latex occlusion prevents an increase in lipid and in DNA synthesis. Chronic barrier impairment by topical application of lovastatin, an inhibitor of cholesterol synthesis, or by a diet deficient in essential fatty acid also leads to an increase in lipid and DNA synthesis and to epidermal hyperplasia. Epidermal lipid and DNA synthesis in essential fatty acid deficiency is independent of prostaglandin E2, but depends on n-6-unsaturated fatty acids such as linoleic and columbinic acid.     

Cutaneous lipid synthesis during late fetal development in the rat

Lipid synthesis in fetal skin may be important both for the development of a mature epidermal permeability barrier and for growth. In these studies, we measured cutaneous cholesterol, sphingolipid and fatty acid synthesis during the critical period of epidermal barrier development in fetal rats to determine whether barrier function influences synthetic rates. In addition, the activities of HMG CoA reductase, serine palmitoyl transferase and acetyl coenzyme A carboxylase were evaluated. In whole skin, synthesis of cholesterol, ceramide, sphingomyelin and fatty acid decreased from day 17 to day 21 of gestation, as did the activity of HMG CoA reductase, serine palmitoyl transferase and acetyl coenzyme A carboxylase. In both the epidermis and dermis, a decrease in cholesterol, ceramide, sphingomyelin and fatty acid synthesis was measured over days 19–21 of gestation. Epidermal HMG CoA reductase activity also decreased over this same time period. In summary, epidermal and dermal synthetic rates and enzyme activity were highest early in gestation when the barrier was least competent and decreased as competence was achieved. Since other studies with mature animals have revealed that epidermal synthetic rates and enzyme activity are highest when barrier disruption is maximal, enhanced epidermal lipid synthesis precedes the estabilishment of a competent barrier in both fetal and mature rodents.     

Epidermal HMG CoA reductase activity in essential fatty acid deficiency: barrier requirements rather than eicosanoid generation regulate cholesterol synthesis.

We showed previously that the activity of 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase, the rate-limiting enzyme of cholesterol biosynthesis, increases after both barrier disruption with organic solvents and in essential fatty acid deficiency (EFAD). Here, we treated EFAD hairless mice with linoleic acid, columbinic acid (C18: 3, n-6, trans; not metabolizable to known regulatory eicosanoids), prostaglandin E2 (PGE2), or latex occlusion, and determined transepidermal water loss (TEWL), epidermal protein content, and epidermal HMG CoA reductase activity. Increased TEWL rates in EFAD were accompanied by increased HMG CoA reductase activity (+130%, n = 6, p less than 0.01) and protein content (+69%; n = 6, p less than 0.025). Artificial restoration of the barrier by occlusion reduced the increase in enzyme activity and protein content toward normal, but barrier function, measured immediately after removal of the latex wrap, deteriorated further (TEWL: two-fold greater than EFAD unoccluded; p less than 0.01). Topical applications of either linoleate or columbinate (but not PGE2), normalized barrier function, HMG CoA reductase activity, and protein content. These results show that a) barrier function modulates HMG CoA reductase activity; b) reduction of cholesterol synthesis with occlusion results in a further deterioration in barrier function, suggesting that increased synthesis is a protective homeostatic response; and c) the barrier abnormality reflects a requirement for specific fatty acids for the barrier rather than resulting from epidermal hyperplasia or decreased prostaglandin generation.     

Structural basis for the barrier abnormality following inhibition of HMG CoA reductase in murine epidermis.

Recent studies have shown that increased epidermal 3-hydroxy-3-methyl-glutaryl coenzyme A (HMG CoA) reductase activity is crucial for the barrier recovery response that follows solvent-induced barrier perturbation. Upregulation of this enzyme leads to increased cholesterologenesis, formation and secretion of cholesterol-enriched lamellar bodies, and barrier repair. Topical lovastatin-induced inhibition of HMG CoA reductase activity both delays the acute barrier-repair response, as well as leading to a chronic barrier abnormality when applied repeatedly to intact skin. Presently, we assessed the effects of repeated topical applications of two different specific inhibitors of HMG CoA reductase on barrier function, the lamellar body-secretory system, and stratum corneum intercellular domains, with functional and morphologic parameters. Once-daily applications of lovastatin or fluindostatin (XU62-320; Sandoz) for 4-8 d to intact hairless mouse epidermis produced a progressive abnormality in barrier function (transepidermal water loss greater than 2.0-5.0 in treated versus less than 0.25 mg/cm2/h for weakly active analogues or vehicle controls). The barrier defect was preceded by alterations in lamellar body internal structure and a partial failure of lamellar body secretion into the stratum corneum interstices, further confirmed by enzyme cytochemistry. Moreover, the deposition of abnormal lamellar body contents resulted in the formation of clefts in the intercellular spaces at the stratum granulosum-stratum corneum interface, resulting in increased permeability through these domains shown by lanthanum perfusion. Applications of irritants, even when producing a barrier abnormality, did not alter the lamellar body secretory system. Co-applications of cholesterol with the inhibitors reversed both the barrier abnormality and the abnormalities in the lamellar body secretory system that occur with the inhibitor alone. Finally, membrane bilayer structures in the mid-to-outer stratum corneum of inhibitor-treated specimens appeared normal, but the intercellular domains displayed enormously expanded lacunae. However, because similar dilatations also occurred in vehicle-treated samples, they can be attributed to the vehicle alone. These studies provide further evidence that the inhibitor-induced defect in barrier function a) is initiated by inhibition of HMG CoA reductase; b) can be attributed to defects in both lamellar body structure and deposition with resultant abnormalities in intercellular membrane domains in the lower stratum corneum; and c) is further enhanced by permissive effects of the vehicle on the permeability of the outer stratum corneum.     

The lovastatin-treated rodent: a new model of barrier disruption and epidermal hyperplasia

Recent studies have linked epidermal cholesterol synthesis with maintenance of the permeability barrier. To assess directly the importance of cholesterol synthesis, we applied lovastatin, a potent inhibitor of cholesterol synthesis, to hairless mouse skin. Transepidermal water loss (TEWL) began to increase after four to six daily applications. Co-application of cholesterol blocked the expected increase in TEWL, demonstrating the importance of cholesterol for development of the lesion. The histology of lovastatin-treated skin revealed epidermal hyperplasia, accompanied by accelerated DNA synthesis. Whereas cholesterol synthesis initially was reduced in lovastatin-treated epidermis, with further treatment cholesterol synthesis normalized, while fatty acid synthesis accelerated greatly. Although the total free sterol content of lovastatin-treated epidermis remained normal, the fatty acid content increased coincident with barrier disruption. Finally, morphologic abnormalities of both lamellar body structure and their deposited, intercellular contents occurred coincident with the emerging biochemical abnormalities. Thus, the abnormal barrier function in this model can be ascribed to an initial inhibition of epidermal sterol synthesis followed by an alteration in cholesterol and fatty acid synthesis, leading to an imbalance in stratum corneum lipid composition and abnormal membrane bilayer structure.     

Localization of epidermal sphingolipid synthesis and serine palmitoyl transferase activity: alterations imposed by permeability barrier requirements

Sphingolipids, the predominant lipid species in mammalian stratum corneum play, a central role in permeability barrier homeostatis. Prior studies have shown that the epidermis synthesizes abundant sphingolipids, a process regulated by barrier requirements, and that inhibition of sphingolipid synthesis interferes with barrier homeostasis. To investigate further the relationship between epidermal sphingolipid metabolism and barrier function, we localized sphingolipid synthetic activity in murine epidermis under basal conditions, and following acute (acetone treatment) or chronic (essential fatty acid deficiency, EFAD) barrier perturbation, using dithiothreitol and/or the staphylococcal epidermolytic toxin to isolate the lower from the outer epidermis. Under basal conditions, both the activity of serine palmitoyl transferase (SPT), the rate-limiting enzyme of sphingolipid synthesis, and the rates of³H-H₂O incorporation into sphingolipids were nearly equivalent in the lower and the outer epidermis. Following acute barrier perturbation, SPT activity increased significantly in both the lower (35%;P < 0.05) and the outer epidermal layers (60%;P < 0.01). The rates of³H-H₂O incorporation into each major sphingolipid family, including ceramides, glucosylceramides and sphingomyelin, increased significantly in both the lower and the outer epidermis of treated flanks after acute barrier disruption. Finally, SPT activity was modestly elevated (20%;P < 0.01) in the lower but not in the outer epidermis of EFAD animals. These studies demonstrate the ability of both lower and outer epidermal cells to generate sphingolipids, and that permeability barrier homeostatic mechanisms appear to differentially regulate SPT acitivity and sphingolipid synthesis in the lower and the outer epidermis in response to acute and chronic barrier perturbation. Moreover, intraepidermal sites of sphingolipid synthesis displayed distinctive differences in the localization and alterations of cholesterologenesis in response to equivalent barrier perturbations.     

Induction of selected lipid metabolic enzymes and differentiation-linked structural proteins by air exposure in fetal rat skin explants

The epidermal permeability barrier of premature infants matures rapidly following birth. Previous studies suggest that air exposure could contribute to this acceleration, because: (i) development of a structurally and functionally mature barrier accelerates when fetal rat skin explants are incubated at an air-medium interface, and (ii) occlusion with a water-impermeable membrane prevents this acceleration. To investigate further the effects of air exposure on epidermal barrier ontogenesis, we compared the activities of several key enzymes of lipid metabolism and gene expression of protein markers of epidermal differentiation in fetal rat skin explants grown immersed versus air exposed. The rate-limiting enzymes of cholesterol (HMG CoA reductase) and ceramide (serine palmitoyl transferase) synthesis were not affected. In contrast, the normal developmental increases in activities of glucosylceramide synthase and cholesterol sulfotransferase, responsible for the synthesis of glucosylceramides and cholesterol sulfate, respectively, were accelerated further by air exposure. Additionally, two enzymes required for the final stages of barrier maturation and essential for normal stratum corneum function, beta-glucocerebrosidase, which converts glucosylceramide to ceramide, and steroid sulfatase, which desulfates cholesterol sulfate, also increased with air exposure. Furthermore, filaggrin and loricrin mRNA levels, and filaggrin, loricrin, and involucrin protein levels all increased with air exposure. Finally, occlusion with a water-impermeable membrane prevented both the air-exposure-induced increase in lipid enzyme activity, and the expression of loricrin, filaggrin, and involucrin. Thus, air exposure stimulates selected lipid metabolic enzymes and the gene expression of key structural proteins in fetal epidermis, providing a biochemical basis for air-induced acceleration of permeability barrier maturation in premature infants.     

Nicotinamide increases biosynthesis of ceramides as well as other stratum corneum lipids to improve the epidermal permeability barrier

BACKGROUND: Stratum corneum lipids, particularly ceramides, are important components of the epidermal permeability barrier that are decreased in atopic dermatitis and aged skin. OBJECTIVES: We investigated the effects of nicotinamide, one of the B vitamins, on biosynthesis of sphingolipids, including ceramides and other stratum corneum lipids, in cultured normal human keratinocytes, and on the epidermal permeability barrier in vivo. METHODS: The rate of sphingolipid biosynthesis was measured by the incorporation of [14C]-serine into sphingolipids. RESULTS: When the cells were incubated with 1-30 micromol L-1 nicotinamide for 6 days, the rate of ceramide biosynthesis was increased dose-dependently by 4.1-5. 5-fold on the sixth day compared with control. Nicotinamide also increased the synthesis of glucosylceramide (7.4-fold) and sphingomyelin (3.1-fold) in the same concentration range effective for ceramide synthesis. Furthermore, the activity of serine palmitoyltransferase (SPT), the rate-limiting enzyme in sphingolipid synthesis, was increased in nicotinamide-treated cells. Nicotinamide increased the levels of human LCB1 and LCB2 mRNA, both of which encode subunits of SPT. This suggested that the increase in SPT activity was due to an increase in SPT mRNA. Nicotinamide increased not only ceramide synthesis but also free fatty acid (2.3-fold) and cholesterol synthesis (1.5-fold). Topical application of nicotinamide increased ceramide and free fatty acid levels in the stratum corneum, and decreased transepidermal water loss in dry skin. CONCLUSIONS: Nicotinamide improved the permeability barrier by stimulating de novo synthesis of ceramides, with upregulation of SPT and other intercellular lipids.     

Topical mevalonic acid stimulates de novo cholesterol synthesis and epidermal permeability barrier homeostasis in aged mice

Extracellular lipids of the stratum corneum, which are composed of cholesterol, fatty acid, and ceramides, are essential for the epidermal permeability barrier function. With damage to the barrier, a decreased capacity for epidermal lipid biosynthesis in aged epidermis results in an impaired repair response. Mevalonic acid is an intermediate after the rate-limiting step in cholesterol biosynthesis, which is catalyzed by 3-hydroxy-3-methylglutaryl coenzyme A reductase. In the present study, we investigated the effect of topical mevalonic acid on the murine epidermal permeability barrier function, comparing it with that of cholesterol. Topical treatment with acetone caused linear increases in transepidermal water loss, in proportion to the number of treatments more rapidly in aged mice than in young mice. Administration of mevalonic acid on aged murine epidermis enhanced its resistance against damage and the recovery rate of barrier function from acute barrier disruption. In contrast, although cholesterol also had the same effect, it required a much higher amount than mevalonic acid. In young mice, neither mevalonic acid nor cholesterol had any effect on resistance against acetone damage nor the recovery rate from acetone damage. In the skin of mice topically administered with mevalonic acid, stimulation of cholesterol synthesis and 3-hydroxy-3-methylglutaryl coenzyme A reductase activity were both observed, whereas none was seen with stimulation by equimolar cholesterol. These data indicate that a topical application of mevalonic acid enhances barrier recovery in aged mice, which is accompanied by not only acceleration of cholesterol synthesis from mevalonic acid but also stimulation of the whole cholesterol biosynthesis.     

Ceramides and skin function

Ceramides are the major lipid constituent of lamellar sheets present in the intercellular spaces of the stratum corneum. These lamellar sheets are thought to provide the barrier property of the epidermis. It is generally accepted that the intercellular lipid domain is composed of approximately equimolar concentrations of free fatty acids, cholesterol, and ceramides. Ceramides are a structurally heterogeneous and complex group of sphingolipids containing derivatives of sphingosine bases in amide linkage with a variety of fatty acids. Differences in chain length, type and extent of hydroxylation, saturation etc. are responsible for the heterogeneity of the epidermal sphingolipids. It is well known that ceramides play an essential role in structuring and maintaining the water permeability barrier function of the skin. In conjunction with the other stratum corneum lipids, they form ordered structures. An essential factor is the physical state of the lipid chains in the nonpolar regions of the bilayers. The stratum corneum intercellular lipid lamellae, the aliphatic chains in the ceramides and the fatty acids are mostly straight long-chain saturated compounds with a high melting point and a small polar head group. This means that at physiological temperatures, the lipid chains are mostly in a solid crystalline or gel state, which exhibits low lateral diffusional properties and is less permeable than the state of liquid crystalline membranes, which are present at higher temperatures. The link between skin disorders and changes in barrier lipid composition, especially in ceramides, is difficult to prove because of the many variables involved. However, most skin disorders that have a diminished barrier function present a decrease in total ceramide content with some differences in the ceramide pattern. Formulations containing lipids identical to those in skin and, in particular, some ceramide supplementation could improve disturbed skin conditions. Incomplete lipid mixtures yield abnormal lamellar body contents, and disorder intercellular lamellae, whereas complete lipid mixtures result in normal lamellar bodies and intercellular bilayers. The utilization of physiological lipids according to these parameters have potential as new forms of topical therapy for dermatoses. An alternative strategy to improving barrier function by topical application of the various mature lipid species is to enhance the natural lipid-synthetic capability of the epidermis through the topical delivery of lipid precursors.     

Acid and neutral sphingomyelinase, ceramide synthase, and acid ceramidase activities in cutaneous aging

In aged skin, decreased levels of stratum corneum ceramides have been described. Epidermal ceramides are generated by sphingomyelin hydrolysis or synthesis from sphingosin and fatty acids and are degraded by ceramidase. We recently showed that epidermal acid sphingomyelinase (A-SMase) generates ceramides with structural function in the stratum corneum lipid bilayers, which provide for the permeability barrier function of the skin. Here, we examined the activities of epidermal A-SMase, ceramide synthase, and ceramidase in chronologically aged versus young hairless mouse skin. We found reduced A-SMase and ceramide synthase activities in the epidermis of aged mice. However, studies on enzyme localization revealed unchanged, ongoing high A-SMase activity in the outer epidermis, which correlated with reported normal barrier function found in aged skin under basal conditions. Reduced A-SMase and ceramide synthase activity was noted in the inner epidermis, correlating with reduced capacity for permeability barrier repair in aging. Ceramidase activity was not age dependent. In summary, we found reduced activities of ceramide-generating SMase and ceramide synthase in the inner epidermis of aged skin, explaining its reduced capacity in barrier repair. In contrast, A-SMase activity in the outer epidermis was unchanged, indicating that this enzyme is crucially involved in basal permeability barrier homeostasis.     

Short-term glucocorticoid treatment compromises both permeability barrier homeostasis and stratum corneum integrity: inhibition of epidermal lipid synthesis accounts for functional abnormalities

Prolonged exposure of human epidermis to excess endogenous or exogenous glucocorticoids can result in well-recognized cutaneous abnormalities. Here, we determined whether short-term glucocorticoid treatment would also display adverse effects, specifically on two key epidermal functions, permeability barrier homeostasis and stratum corneum integrity and cohesion, and the basis for such changes. In humans 3 d of treatment with a potent, commonly employed topical glucocorticoid (clobetasol), applied topically, produced a deterioration in barrier homeostasis, characterized by delayed barrier recovery and abnormal stratum corneum integrity (rate of barrier disruption with tape strippings) and stratum corneum cohesion (microg protein removed per stripping). Short-term systemic and topical glucocorticoid produced similar functional defects in mice, where the basis for these abnormalities was explored further. Both the production and secretion of lamellar bodies were profoundly decreased in topical glucocorticoid-treated mice resulting in decreased extracellular lamellar bilayers. These structural changes, in turn, were attributable to a profound global inhibition of lipid synthesis, demonstrated both in epidermis and in cultured human keratinocytes. The basis for the abnormality in stratum corneum integrity and cohesion was a diminution in the density of corneodesmosomes in the lower stratum corneum. We next performed topical replacement studies to determine whether lipid deficiency accounts for the glucocorticoid-induced functional abnormalities. The abnormalities in both permeability barrier homeostasis and stratum corneum integrity were corrected by topical applications of an equimolar distribution of free fatty acids, cholesterol, and ceramides, indicating that glucocorticoid-induced inhibition of epidermal lipid synthesis accounts for the derangements in both cutaneous barrier function and stratum corneum integrity/cohesion. These studies indicate that even short-term exposure to potent glucocorticosteroids can exert profound negative effects on cutaneous structure and function. Finally, topical replenishment with epidermal physiologic lipids could represent a potential method to reduce the adverse cutaneous effects of both topical glucocorticoid treatment and Cushing's syndrome.     

Omega-O-acylceramide, a lipid essential for mammalian survival

The prevention of water loss through the skin is critical for terrestrial mammalian species. This function is served by the epidermal permeability barrier, which resides primarily in the extracellular domains of the stratum corneum, the outermost layer of skin, and its highly ordered lamellar membranes composed primarily of free fatty acids, cholesterol, and ceramides (Cer). The dominant lipids in these lamellae are Cer, which comprise a heterogeneous group of chemically distinct species. One particular subfamily of Cer, which is unique to the outer layers of the epidermis of terrestrial mammals, is omega (omega)-O-acylCer (or acylCer). Myriad evidence suggests that these acylCer play critical roles in barrier function. The formation of these epidermal acylCer requires several metabolic steps, including synthesis of very long chain fatty acids, omega-hydroxylation of the fatty acids, and esterification at the omega-hydroxy group with primarily linoleic acid. The authors previously demonstrated that a cytochrome P-450-type enzyme is involved in omega-hydroxylation during acylCer generation and that inhibition of omega-hydroxylation leads to a barrier abnormality in murine epidermis. More recently, we discovered that lack of normal elongation of very long chain fatty acid (or ELOVL) 4 function in mutant ELOVL4 knock-in mice causes acylCer deficiency associated with abnormal barrier formation and neonatal lethality. These results indicate not only that acylCer are critical lipid components for mammalian survival, but also that keratinocytes deploy a complex metabolic pathway leading to the formation of these unique Cer.     

Preparation of model membranes for skin permeability studies using stratum corneum lipids

Liposomes were prepared from stratum corneum lipids consisting of epidermal ceramides (55% by weight), cholesterol (25%), free fatty acids (15%), and cholesteryl sulfate (5%). Multiple lamellae were formed by air-drying the liposomal suspensions on hydrophilic filter disks, and water permeabilities through these filter-supported lamellae were measured using a diffusion cell. Ultrastructure of the lipid lamellae was characterized by scanning and thin-section electron microscopy. Water flux data and ultrastructure of the model membranes are discussed in relation to stratum corneum, the horny layer of the epidermis that constitutes the epidermal barrier.     

Stratum corneum lipid function.

The stratum corneum contains a complex mixture of polar and nonpolar lipids in its intercellular spaces. These lipids, present in form of multiple lamellae, have been investigated for their role in providing the epidermal barrier to transcutaneous water loss, the selective barrier from the inside to the outside of the organism and partly the process of physiological desquamation. The composition of these lipids varies from species to species, with the body region and the degree of keratinocyte differentiation. The most undifferentiated layers of the epidermis contain typical membrane lipids, phospholipids, while more differentiated layers contain ceramides, cholesterol and free fatty acids. Essential fatty acids are essential for the maintenance of the lamellar structures and epidermal barrier function. Epidermal linoleic and arachidonic acids derive from exogenous sources. Only recently attempts have been made to elucidate the timing and regulation of epidermal fatty acid metabolism. Keratinocytes do not express a low molecular weight fatty acid binding protein like other cells active in lipid metabolism, but may employ alternative ways in fatty acid uptake and metabolism.     

Adiponectin Attenuates the Inflammation in Atopic Dermatitis-Like Reconstructed Human Epidermis

BackgroundAtopic dermatitis (AD) is a chronic disorder, with a vicious cycle of repetitive inflammation and deterioration of the epidermal barrier function. Adiponectin, an adipokine, has anti-inflammatory effects on various metabolic and inflammatory disorders. Recently, its level was found to be reduced in serum and tissue samples from AD patients.ObjectiveWe aimed to investigate the effects of adiponectin on epidermal inflammation and barrier structures in AD skin.MethodsA three-dimensional in vitro epidermal equivalent model mimicking AD was obtained by adding an inflammatory substance cocktail to normal human epidermal equivalents (HEEs). The expression of epidermal differentiation markers, primary inflammatory mediators, and lipid biosynthetic enzymes was compared between adiponectintreated AD-HEEs, untreated control AD-HEEs, and normal HEEs.ResultsAdiponectin co-treatment 1) inhibited the increase in mRNA expression of major inflammatory mediators (carbonic anhydrase II, neuron-specific NEL-like protein 2, thymic stromal lymphopoietin, interleukin-8, tumor necrosis factor-alpha, and human beta-defensin-2) from keratinocytes in AD-inflammatory HEEs, 2) enhanced the expression of lipid biosynthetic enzymes (fatty acid synthase, HMG CoA reductase, and serine-palmitoyl transferase), and 3) promoted the expression of differentiation factors, especially filaggrin. We also found that the expression of adiponectin receptor-1 and -2 decreased in the epidermis of chronic AD lesion.ConclusionActivation of the adiponectin pathway is expected to enhance epidermal differentiation and barrier function as well as attenuate inflammatory response to AD as a therapeutic approach.     

Human epidermal glucosylceramides are major precursors of stratum corneum ceramides

Ceramides are the major component of the stratum corneum, accounting for 30%-40% of stratum corneum lipids by weight, and are composed of at least seven molecular groups (designated ceramides 1-7). Stratum corneum ceramides, together with cholesterol and fatty acids, form extracellular lamellae that are responsible for the epidermal permeability barrier. Previous studies indicated that beta-glucocerebrosidase- and sphingomyelinase-dependent ceramide production from glucosylceramides and sphingomyelins, respectively, is important for epidermal permeability barrier homeostasis. A recent study indicated that sphingomyelins are precursors of two stratum corneum ceramide molecular groups (ceramides 2 and 5). In this study, we have examined the role of glucosylceramides in the generation of each of the seven stratum corneum ceramide molecular groups. First, the structures of various glucosylceramide species in human epidermis were determined by gas chromatography-mass spectrometry, fast atom bombardment-mass spectrometry, and nuclear magnetic resonance. The results indicate that total epidermal glucosylceramides are composed of six distinct molecular groups, glucosylceramides 1-6. Glucosylceramide 1 contains sphingenine and nonhydroxy fatty acids, glucosylceramide 2, phytosphingosine and nonhydroxy fatty acids, glucosylceramide 3, phytosphingosine with one double bond and nonhydroxy fatty acids, glucosylceramide 4, sphingenine and alpha-hydroxy fatty acids, glucosylceramide 5, phytosphingosine and alpha-hydroxy fatty acids, and glucosylceramide 6, phytosphingosine with one double bond and alpha-hydroxy fatty acids. The nonhydroxy fatty acids typically have 16-24-carbon-length chains, whereas alpha-hydroxy fatty acids are limited to 24-, 25-, and 26-carbon chains. The sphingosine bases are C18 or C20 chains. Next, acylglucosylceramides and glucosylceramides were treated with beta-glucocerebrosidase and the ceramides released were compared with stratum corneum ceramides. Ceramide moieties of acylglucosylceramides and glucosylceramides 1, 2, 4-6 correspond to stratum corneum ceramides 1-7. These results, together with those of our previous reports characterizing epidermal sphingomyelins, indicate that all ceramide species, including omega-hydroxy fatty-acid-containing ceramides, are derived from glucosylceramides, and fractions of ceramides 2 and 5 are from sphingomyelins. Furthermore, structural analysis of glucosylceramides revealed that human epidermal glycosphingolipids display a unique lipid profile that is rich in very long chain hydroxylated (alpha- and omega-hydroxy) fatty acids and phytosphingosine.