Permeability barrier requirements regulate epidermal beta-glucocerebrosidase

Epidermal Biophysical Characteristics in Patients with Vitiligo and the Association with Thyroid Auto-immunity and Itch

Background

Vitiligo is characterized by depigmentation due to melanocyte destruction. Itch is an under-recognized symptom; its pathophysiology is unclear.

Aims

To compare epidermal biophysical characteristics of the vitiligous skin and normal skin and to determine the association with thyroid auto-immunity and itch.

Methods

A cross-sectional study involving vitiligo patients was conducted. Hydration, pH, and trans-epidermal water loss (TEWL) at the vitiligous skin and normal adjacent skin were measured. The Vitiligo Disease Activity Score (VIDA) and Vitiligo Area Scoring Index (VASI) were assessed. Itch severity and thyroid auto-antibodies were determined.

Results

Thirty-nine (62.9%) females and 23 (37.1%) males participated. Twenty-six (41.9%) had stable vitiligo, and 36 (58.1%) had active disease with a median VASI was 0.8 (2.2). Hydration was lower [93 (83) to 125.5 (111) vs 104 (73) to 156 (100), P < 0.01] and TEWL [7.13 (6.18) to 8.86 (6.93) vs 5.54 (5.90) to 6.88 (6.37), P < 0.01] was higher at the vitiligous skin compared to the normal skin. A non-significant higher pH trend was observed in the vitiligous skin. Thyroid antibody was detected in 19.7% patients. There were no significant differences in biophysical characteristics between patients with and without thyroid antibodies, with hydration of 88 (159) to 129.5 (120) vs 91.5 (81) to 116 (101) and TEWL of 7.08 (2.03) to 9.97 (6.38) vs 7.65 (7.54) to 8.22 (6.52). Itch was reported by 14 (22.6%). Patients with itch had lower hydration and higher TEWL but were not significantly different from patients without itch.

Conclusions

The vitiligous skin has reduced hydration and increased TEWL, suggesting a defective epidermal barrier. Thyroid antibody positivity was not associated with biophysical characteristics or itch. Itch was not associated with hydration, pH, and TEWL. An impaired epidermal barrier and itch need to be addressed in vitiligo management.

Horse-Derived Ceramide Accentuates Glucosylceramide Synthase and Ceramide Synthase 3 by Activating PPARβ/δ and/or PPARγ to Stimulate Ceramide Synthesis

Horse-derived ceramide (HC), which contains galactosylceramides as its main component, significantly improves skin symptoms when applied topically to patients with atopic dermatitis. We speculated that efficacy resulted from the amelioration of epidermal ceramide metabolism, and we characterized those effects using reconstructed human epidermal equivalents. Lipid analysis, RT-PCR and Western blotting revealed that HC significantly increased the total ceramide content of the stratum corneum (SC), accompanied by significantly increased gene and/or protein expression levels of ceramide synthase (CERS) 3, fatty acid elongase (ELOVL) 4, glucosylceramide synthase (GCS), β-glucocerebrosidase, sphingomyelin synthase and acid sphingomyelinase. Mechanistic analyses using cultures of primary human keratinocytes revealed the marked stimulatory effects of HC on the mRNA expression levels of CERS3, ELOVL4 and GCS under high calcium-derived differentiation conditions. Signaling analyses demonstrated that an antagonist of PPARβ/δ significantly abrogated the HC-stimulated mRNA expression levels of GCS, CERS3 and ELOVL4. GW9662, an antagonist of PPARγ, significantly abolished the HC-up-regulated mRNA expression levels of GCS and ELOVL4, but not of CERS3. These findings suggest that HC has the distinct potential to accentuate the expression of GCS, CERS3 and ELOVL4 via the activation of PPARβ/δ and/or PPARγ to accelerate ceramide synthesis in the SC.

Children with atopic dermatitis show increased activity of β- glucocerebrosidase and stratum corneum levels of glucosylcholesterol that are strongly related to local cytokine milieu

Background

Atopic dermatitis (AD) is characterized by immune dysregulations and an impaired skin barrier, including abnormalities in lipid organization. In the stratum corneum (SC), β‐glucocerebrosidase (GBA) mediates transformation of glucosylceramide (GlcCER) into ceramide (CER) and cholesterol into glucosylcholesterol (GlcChol). Alteration in GBA activity might contribute to skin barrier defects in AD.

Objectives

To investigate GBA activity in the SC of children with AD before and after topical corticosteroid therapy and to compare it with healthy controls; to determine SC levels of GlcCER‐ and CER‐containing hydroxysphingosine base (GlcCER[H] and CER[H], respectively) and GlcChol; and to relate them to disease severity, skin barrier function and the local cytokine milieu.

Methods

Lipid markers and cytokines of innate, T helper 1 and T helper 2 immunity were determined in SC collected from healthy children and from clinically unaffected skin of children with AD, before and after 6 weeks of therapy with topical corticosteroids. AD severity was assessed by Scoring Atopic Dermatitis and skin barrier function by transepidermal water loss (TEWL).

Results

Baseline GBA activity and GlcChol levels were increased in children with AD but declined after therapy. CER[H] levels and the CER[H] to GlcCER[H] ratio were increased in AD. GBA activity and GlcChol correlated with TEWL and levels of multiple cytokines, especially interleukin‐1α and interleukin‐18. GlcChol was strongly associated with disease severity.

Conclusions

We show increased GBA activity and levels of GlcChol in AD. Our data suggest an important role of inflammation in disturbed lipid processing. GBA activity or GlcChol might be useful biomarkers in the monitoring of therapeutic responses in AD.

What is already known about this topic?

Patients with atopic dermatitis (AD) have a reduced skin barrier, mainly caused by altered lipid organization.

The mechanisms underlying these lipid anomalies are not fully understood but likely reflect both genetic abnormalities in AD skin and the local cutaneous inflammatory environment.

What does this study add?

We show increased activity of the ceramide‐generating enzyme β‐glucocerebrosidase in AD. Activity of this enzyme was correlated with the local cytokine milieu and declined after local corticosteroid therapy.

We show that glucosylcholesterol levels in the stratum corneum are increased in AD.

The function of glucosylcholesterol and the physiological consequences of increased levels are not clear yet; however, its levels were strongly correlated with skin barrier function: high transepidermal water loss strongly correlated with high levels of glucosylcholesterol.

What is the translational message?

Correction of cutaneous inflammation largely restores alterations in lipid metabolism in the stratum corneum of infants with AD.

Decrease of ceramides with long-chain fatty acids in psoriasis: Possible inhibitory effect of interferon gamma on chain elongation

Reportedly, decreases in fatty acid (FA) chain length of ceramide (CER) are associated with interferon-γ (IFN-γ), which shows increased expression in psoriasis. However, the underlying mechanism of this association remains unclear. Therefore, in this study, we aimed to clarify this association between FA chain length of CER, IFN-γ, and the major transcriptional factors involving psoriasis. CER profiling according to FA chain length and class was performed in murine epidermis (n = 10 BALB/c mice topically treated with imiquimod, n = 10 controls) and human stratum corneum (SC) (n = 12 psoriasis, n = 11 controls). The expression of lipid synthetic enzymes, including elongases (ELOVLs), in murine epidermis was also measured using RT-PCR. Furthermore, the association of IFN-γ with various enzymes and transcription factors involved in the generation of long-chain CERs was also investigated using in vitro keratinocyte. A significant decrease in the percentage of long-chain CERs was observed in psoriasis-like murine epidermis and human psoriatic SC. Additionally, the expression levels of ELOVL1, ELOVL4, and ceramide synthase3 (CerS3) were significantly decreased in psoriasis-like murine epidermis and IFN-γ-treated keratinocyte. There was also a significant decrease in the expression of transcriptional factors, including peroxisome proliferator-activated receptor (PPAR), in IFN-γ treated keratinocyte. Thus, it could be suggested that IFN-γ may regulate ELOVL and CerS levels by down-regulating the transcriptional factors. Additionally, given the possible involvement of PPARs or liver X receptor agonist in the CER elongation process, they may serve as potential therapeutic agents for lengthening the CER FAs in psoriasis.

Cutting Edge of the Pathogenesis of Atopic Dermatitis: Sphingomyelin Deacylase, the Enzyme Involved in Its Ceramide Deficiency, Plays a Pivotal Role

Atopic dermatitis (AD) is characterized clinically by severe dry skin and functionally by both a cutaneous barrier disruption and an impaired water-holding capacity in the stratum corneum (SC) even in the nonlesional skin. The combination of the disrupted barrier and water-holding functions in nonlesional skin is closely linked to the disease severity of AD, which suggests that the barrier abnormality as well as the water deficiency are elicited as a result of the induced dermatitis and subsequently trigger the recurrence of dermatitis. These functional abnormalities of the SC are mainly attributable to significantly decreased levels of total ceramides and the altered ceramide profile in the SC. Clinical studies using a synthetic pseudo-ceramide (pCer) that can function as a natural ceramide have indicated the superior clinical efficacy of pCer and, more importantly, have shown that the ceramide deficiency rather than changes in the ceramide profile in the SC of AD patients plays a central role in the pathogenesis of AD. Clinical studies of infants with AD have shown that the barrier disruption due to the ceramide deficiency is not inherent and is essentially dependent on postinflammatory events in those infants. Consistently, the recovery of trans-epidermal water loss after tape-stripping occurs at a significantly slower rate only at 1 day post-tape-stripping in AD skin compared with healthy control (HC) skin. This resembles the recovery pattern observed in Niemann-Pick disease, which is caused by an acid sphingomyelinase (aSMase) deficiency. Further, comparison of ceramide levels in the SC between before and after tape-stripping revealed that whereas ceramide levels in HC skin are significantly upregulated at 4 days post-tape-stripping, their ceramide levels remain substantially unchanged at 4 days post-tape-stripping. Taken together, the sum of these findings strongly suggests that an impaired homeostasis of a ceramide-generating process may be associated with these abnormalities. We have discovered a novel enzyme, sphingomyelin (SM) deacylase, which cleaves the N-acyl linkage of SM and glucosylceramide (GCer). The activity of SM deacylase is significantly increased in AD lesional epidermis as well as in the involved and uninvolved SC of AD skin, but not in the skin of patients with contact dermatitis or chronic eczema, compared with HC skin. SM deacylase competes with aSMase and β-glucocerebrosidase (BGCase) to hydrolyze their common substrates, SM and GCer, to yield their lysoforms sphingosylphosphorylcholine (SPC) and glucosylsphingosine (GSP), respectively, instead of ceramide. Consistently, those reaction products (SPC and GSP) accumulate to a greater extent in the involved and uninvolved SC of AD skin compared with chronic eczema or contact dermatitis skin as well as HC skin. Successive chromatographies were used to purify SM deacylase to homogeneity with a single band of ≈43 kDa and with an enrichment of >14,000-fold. Analysis of a protein spot with SM deacylase activity separated by 2D-SDS-PAGE using MALDI-TOF MS/MS allowed its amino acid sequence to be determined and to identify it as the β-subunit of acid ceramidase (aCDase), an enzyme consisting of α- and β-subunits linked by amino-bonds and a single S-S bond. Western blotting of samples treated with 2-mercaptoethanol revealed that whereas recombinant human aCDase was recognized by antibodies to the α-subunit at ≈56 and ≈13 kDa and the β-subunit at ≈43 kDa, the purified SM deacylase was detectable only by the antibody to the β-subunit at ≈43 kDa. Breaking the S-S bond of recombinant human aCDase with dithiothreitol elicited the activity of SM deacylase with an apparent size of ≈40 kDa upon gel chromatography in contrast to aCDase activity with an apparent size of ≈50 kDa in untreated recombinant human aCDase. These results provide new insights into the essential role of SM deacylase as the β-subunit aCDase that causes the ceramide deficiency in AD skin.

Sphingomyelin Deacylase, the Enzyme Involved in the Pathogenesis of Atopic Dermatitis, Is Identical to the β-Subunit of Acid Ceramidase

A ceramide deficiency in the stratum corneum (SC) is an essential etiologic factor for the dry and barrier-disrupted skin of patients with atopic dermatitis (AD). Previously, we reported that sphingomyelin (SM) deacylase, which hydrolyzes SM and glucosylceramide at the acyl site to yield their lysoforms sphingosylphosphorylcholine (SPC) and glucosylsphingosine, respectively, instead of ceramide and/or acylceramide, is over-expressed in AD skin and results in a ceramide deficiency. Although the enzymatic properties of SM deacylase have been clarified, the enzyme itself remains unidentified. In this study, we purified and characterized SM deacylase from rat skin. The activities of SM deacylase and acid ceramidase (aCDase) were measured using SM and ceramide as substrates by tandem mass spectrometry by monitoring the production of SPC and sphingosine, respectively. Levels of SM deacylase activity from various rat organs were higher in the order of skin > lung > heart. By successive chromatography using Phenyl-5PW, Rotofor, SP-Sepharose, Superdex 200 and Shodex RP18-415, SM deacylase was purified to homogeneity with a single band of an apparent molecular mass of 43 kDa with an enrichment of > 14,000-fold. Analysis by MALDI-TOF MS/MS using a protein spot with SM deacylase activity separated by 2D-SDS-PAGE allowed its amino acid sequence to be determined and identified as the β-subunit of aCDase, which consists of α- and β-subunits linked by amino bonds and a single S-S bond. Western blotting of samples treated with 2-mercaptoethanol revealed that, whereas recombinant human aCDase was recognized by antibodies to the α-subunit at ~56 kDa and ~13 kDa and the β-subunit at ~43 kDa, the purified SM deacylase was detectable only by the antibody to the β-subunit at ~43 kDa. Breaking the S-S bond of recombinant human aCDase with dithiothreitol elicited the activity of SM deacylase with ~40 kDa upon gel chromatography. These results provide new insights into the essential role of SM deacylase expressed as an aCDase-degrading β-subunit that evokes the ceramide deficiency in AD skin.

Glucocerebrosidase: Functions in and Beyond the Lysosome

Glucocerebrosidase (GCase) is a retaining β-glucosidase with acid pH optimum metabolizing the glycosphingolipid glucosylceramide (GlcCer) to ceramide and glucose. Inherited deficiency of GCase causes the lysosomal storage disorder named Gaucher disease (GD). In GCase-deficient GD patients the accumulation of GlcCer in lysosomes of tissue macrophages is prominent. Based on the above, the key function of GCase as lysosomal hydrolase is well recognized, however it has become apparent that GCase fulfills in the human body at least one other key function beyond lysosomes. Crucially, GCase generates ceramides from GlcCer molecules in the outer part of the skin, a process essential for optimal skin barrier property and survival. This review covers the functions of GCase in and beyond lysosomes and also pays attention to the increasing insight in hitherto unexpected catalytic versatility of the enzyme.

Skin of atopic dermatitis patients shows disturbed β-glucocerebrosidase and acid sphingomyelinase activity that relates to changes in stratum corneum lipid composition

Patients with Atopic Dermatitis (AD) suffer from inflamed skin and skin barrier defects. Proper formation of the outermost part of the skin, the stratum corneum (SC), is crucial for the skin barrier function. In this study we analyzed the localization and activity of lipid enzymes β-glucocerebrosidase (GBA) and acid sphingomyelinase (ASM) in the skin of AD patients and controls. Localization of both the expression and activity of GBA and ASM in the epidermis of AD patients was altered, particularly at lesional skin sites. These changes aligned with the altered SC lipid composition. More specifically, abnormal localization of GBA and ASM related to an increase in specific ceramide subclasses [AS] and [NS]. Moreover we related the localization of the enzymes to the amounts of SC ceramide subclasses and free fatty acids (FFAs). We report a correlation between altered localization of active GBA and ASM and a disturbed SC lipid composition. Localization of antimicrobial peptide beta-defensin-3 (HBD-3) and AD biomarker Thymus and Activation Regulated Chemokine (TARC) also appeared to be diverging in AD skin compared to control. This research highlights the relation between correct localization of expressed and active lipid enzymes and a normal SC lipid composition for a proper skin barrier.

Treatment with Synthetic Pseudoceramide Improves Atopic Skin, Switching the Ceramide Profile to a Healthy Skin Phenotype

Little is known about the pathophysiological linkages between altered ceramide profiles in the stratum corneum (SC) of patients with atopic dermatitis and their impaired skin barrier and water-holding functions. We studied those characteristics following topical treatment with a designed synthetic pseudoceramide (pCer) and analyzed that pathophysiological linkage by microanalyzing ceramides using normal phase liquid chromatography-electrospray ionization mass spectrometry. Four weeks of treatment with pCer significantly reduced skin symptoms, accompanied by significant decreases in transepidermal water loss and increases in water content. In the SC ceramide profiles, ceramides containing nonhydroxy fatty acids and 6-hydroxysphingosines (Cer[NH]) and ceramides containing nonhydroxy fatty acids and phytosphingosines (Cer[NP]) increased, whereas ceramides containing nonhydroxy fatty acids and sphingosines (Cer[NS]) and ceramides containing a-hydroxy fatty acids and sphingosines (Cer[AS]) decreased, with larger alkyl chain lengths in Cer[NS], distinctly representing a switch from an atopic dermatitis to a healthy skin phenotype. The level of pCer that penetrated into the SC was significantly correlated with the SC water content but not with transepidermal water loss. The level and the average carbon chain length of Cer[NS] were closely correlated with the pCer level in the SC. These findings indicate that the penetrated pCer contributes to shift the ceramide profile from an atopic dermatitis to a healthy skin phenotype. Taken together, the observed clinical efficacy of treatment with pCer provides a deep insight into the pathogenesis of atopic dermatitis as a ceramide-deficient disease.

Combination of acid β-glucosidase mutation and Saposin C deficiency in mice reveals Gba1 mutation dependent and tissue-specific disease phenotype

Gaucher disease is caused by mutations in GBA1 encoding acid β-glucosidase (GCase). Saposin C enhances GCase activity and protects GCase from intracellular proteolysis. Structure simulations indicated that the mutant GCases, N370S (0 S), V394L (4L) and D409V(9V)/H(9H), had altered function. To investigate the in vivo function of Gba1 mutants, mouse models were generated by backcrossing the above homozygous mutant GCase mice into Saposin C deficient (C*) mice. Without saposin C, the mutant GCase activities in the resultant mouse tissues were reduced by ~50% compared with those in the presence of Saposin C. In contrast to 9H and 4L mice that have normal histology and life span, the 9H;C* and 4L;C* mice had shorter life spans. 9H;C* mice developed significant visceral glucosylceramide (GC) and glucosylsphingosine (GS) accumulation (GC»GS) and storage macrophages, but lesser GC in the brain, compared to 4L;C* mice that presents with a severe neuronopathic phenotype and accumulated GC and GS primarily in the brain. Unlike 9V mice that developed normally for over a year, 9V;C* pups had a lethal skin defect as did 0S;C* mice resembled that of 0S mice. These variant Gaucher disease mouse models presented a mutation specific phenotype and underscored the in vivo role of Saposin C in the modulation of Gaucher disease.

Identification of Rice Koji Extract Components that Increase β-Glucocerebrosidase Levels in Human Epidermal Keratinocytes

Rice miso contains many ingredients derived from rice koji and has been a valuable source of nutrition since ancient times. We found that the consumption of rice miso led to improvements in the moisture content of cheek stratum corneum, skin viscoelasticity, and skin texture. Further, rice miso extract was found to increase the mRNA expression and activity of β-glucocerebrosidase (β-GCase), an enzyme involved in ceramide synthesis in the stratum corneum, in cultures. In this study, we identified the lipid-derived components of rice koji that increase the β-GCase activity in cultured human epidermal keratinocytes. The methanol fraction of rice koji extract induced an increase in the mRNA expression and activity of β-GCase in keratinocytes. The active fraction of rice koji was found to contain phosphatidic acid (PA) and lysophosphatidic acid (LPA). The total PA concentration in rice koji was 973.9 ng/mg dry weight, which was 17.5 times higher than that in steamed rice. Among the molecular species, PA_18:2/18:2 was the most frequently found. The total LPA concentration in rice koji was 29.6 ng/mg dry weight, and 2-LPA_18:2 was the most frequently found LPA. Since PA and LPA increase the mRNA expression and activity of β-GCase in keratinocytes, they are thought to be the active ingredients in rice koji that increase the β-GCase levels in human epidermal keratinocytes.

In situ visualization of glucocerebrosidase in human skin tissue: zymography versus activity-based probe labeling

Epidermal β-glucocerebrosidase (GBA1), an acid β-glucosidase normally located in lysosomes, converts (glucosyl)ceramides into ceramides, which is crucial to generate an optimal barrier function of the outermost skin layer, the stratum corneum (SC). Here we report on two developed in situ methods to localize active GBA in human epidermis: i) an optimized zymography method that is less labor intensive and visualizes enzymatic activity with higher resolution than currently reported methods using either substrate 4-methylumbelliferyl-β-D-glucopyranoside or resorufin-β-D-glucopyranoside; and ii) a novel technique to visualize active GBA1 molecules by their specific labeling with a fluorescent activity-based probe (ABP), MDW941. The latter method pro-ved to be more robust and sensitive, provided higher resolution microscopic images, and was less prone to sample preparation effects. Moreover, in contrast to the zymography substrates that react with various β-glucosidases, MDW941 specifically labeled GBA1. We demonstrate that active GBA1 in the epidermis is primarily located in the extracellular lipid matrix at the interface of the viable epidermis and the lower layers of the SC. With ABP-labeling, we observed reduced GBA1 activity in 3D-cultured skin models when supplemented with the reversible inhibitor, isofagomine, irrespective of GBA expression. This inhibition affected the SC ceramide composition: MS analysis revealed an inhibitor-dependent increase in the glucosylceramide:ceramide ratio.

Permeability Barrier and Microstructure of Skin Lipid Membrane Models of Impaired Glucosylceramide Processing

Ceramide (Cer) release from glucosylceramides (GlcCer) is critical for the formation of the skin permeability barrier. Changes in β-glucocerebrosidase (GlcCer'ase) activity lead to diminished Cer, GlcCer accumulation and structural defects in SC lipid lamellae; however, the molecular basis for this impairment is not clear. We investigated impaired GlcCer-to-Cer processing in human Cer membranes to determine the physicochemical properties responsible for the barrier defects. Minor impairment (5-25%) of the Cer generation from GlcCer decreased the permeability of the model membrane to four markers and altered the membrane microstructure (studied by X-ray powder diffraction and infrared spectroscopy), in agreement with the effects of topical GlcCer in human skin. At these concentrations, the accumulation of GlcCer was a stronger contributor to this disturbance than the lack of human Cer. However, replacement of 50-100% human Cer by GlcCer led to the formation of a new lamellar phase and the maintenance of a rather good barrier to the four studied permeability markers. These findings suggest that the major cause of the impaired water permeability barrier in complete GlcCer'ase deficiency is not the accumulation of free GlcCer but other factors, possibly the retention of GlcCer bound in the corneocyte lipid envelope.

Upregulation of gene expression levels of ceramide metabolic enzymes after application of sphingomyelin-based liposomes to a three-dimensional cultured human epidermis model

BACKGROUND/AIMS

We have previously reported that the application of sphingomyelin-based liposomes (SM-L) to a three-dimensional cultured skin model increase the content of ceramides NS, NP, AS and AP. However, the mechanism responsible for these increased ceramide levels was not identified.

METHODS

SM-L and sphingomyelinase (SMase) were combined and incubated at 37 °C for 24 h. SM-L were also applied to three-dimensional cultured skin for 24 h and quantification of SMase and β-glucocerebrosidase (β-GCase) mRNA expression levels performed using real-time PCR. Additionally, three dimensional cultured skin was incubated with SM-L and the β-GCase inhibitor conduritol B epoxide (CBE) and the ceramide content determined by high performance thin layer chromatography.

RESULTS

We observed generation of ceramide NS after reaction of SM-L and SMase. However, the other ceramide classes were not detected. Notably, SMase and β-GCase mRNA expression levels were significantly increased in cells of the skin model following application of SM-L. The levels of ceramides NS, NP, AS and AP were decreased by treatment with CBE. However, only ceramide NS was significantly increased by treatment with CBE and SM-L in combination.

CONCLUSION

These findings indicate that application of SM-L to cultured skin upregulates the expression of SMase and β-GCase and increases ceramide content.

Reevaluation of the non-lesional dry skin in atopic dermatitis by acute barrier disruption: an abnormal permeability barrier homeostasis with defective processing to generate ceramide

Atopic dermatitis is characterized by disruption of the cutaneous barrier due to reduced ceramide levels even in non-lesional dry skin. Following further acute barrier disruption by repeated tape strippings, we re-characterized the non-lesional dry skin of subjects with atopic dermatitis, which shows significantly reduced levels of barrier function and ceramide but not of beta-glucocerebrosidase activity. For the first time, we report an abnormal trans-epidermal water loss homeostasis in which delayed recovery kinetics of trans-epidermal water loss occurred on the first day during the 4 days after acute barrier disruption compared with healthy control skin. Interestingly, whereas the higher ceramide level in the stratum corneum of healthy control skin was further significantly up-regulated at 4 days post-tape stripping, the lower ceramide level in the stratum corneum of subjects with atopic dermatitis was not significantly changed. In a parallel study, whereas beta-glucocerebrosidase activity at 4 days post-tape stripping was significantly up-regulated in healthy control skin compared with before tape stripping, the level of that activity remained substantially unchanged in atopic dermatitis. These findings indicate that subjects with atopic dermatitis have a defect in sphingolipid-metabolic processing that generates ceramide in the interface between the stratum corneum and the epidermis. The results also support the notion that the continued disruption of barrier function in atopic dermatitis non-lesional skin is associated with the impaired homeostasis of a ceramide-generating process, which underscores an atopy-specific inflammation-triggered ceramide deficiency that is distinct from other types of dermatitis.

Role of lipids in the formation and maintenance of the cutaneous permeability barrier

The major function of the skin is to form a barrier between the internal milieu and the hostile external environment. A permeability barrier that prevents the loss of water and electrolytes is essential for life on land. The permeability barrier is mediated primarily by lipid enriched lamellar membranes that are localized to the extracellular spaces of the stratum corneum. These lipid enriched membranes have a unique structure and contain approximately 50% ceramides, 25% cholesterol, and 15% free fatty acids with very little phospholipid. Lamellar bodies, which are formed during the differentiation of keratinocytes, play a key role in delivering the lipids from the stratum granulosum cells into the extracellular spaces of the stratum corneum. Lamellar bodies contain predominantly glucosylceramides, phospholipids, and cholesterol and following the exocytosis of lamellar lipids into the extracellular space of the stratum corneum these precursor lipids are converted by beta glucocerebrosidase and phospholipases into the ceramides and fatty acids, which comprise the lamellar membranes. The lipids required for lamellar body formation are derived from de novo synthesis by keratinocytes and from extra-cutaneous sources. The lipid synthetic pathways and the regulation of these pathways are described in this review. In addition, the pathways for the uptake of extra-cutaneous lipids into keratinocytes are discussed. 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.

Caffeic acid induces keratinocyte differentiation by activation of PPAR-α

Objectives

Peroxisome proliferator-activated receptors (PPAR)-α plays an important role in epidermal differentiation and barrier recovery, and topical treatment with PPAR-α agonists restores epidermal homeostasis in essential fatty acid deficiency and permeability barrier in skin disruptions. Therefore, we performed structure-based pharmacophore screening to search for a novel PPAR-α agonist. Caffeic acid was ultimately selected and evaluated for its effects on keratinocyte differentiation and epidermal permeability barrier.

Methods

The transactivation activity of PPAR-responsive element (PPRE) and cornified envelope (CE) formation were assayed. Also, immunoblot analysis and anti-oxidant activity were investigated on caffeic acid.

Key findings

Caffeic acid increases the transactivation activity of PPRE and CE formation in keratinocytes. In addition, caffeic acid promotes the expression of genes and proteins related to CE formation such as involucrin and transglutaminase-1. Additionally, anti-oxidant activity were improved by caffeic acid.

Conclusions

Caffeic acid can promote keratinocyte differentiation and restore skin barrier homeostasis and is suggested to be an appropriate skin therapeutic agent for improving epidermal permeability barrier function.

Gromwell (Lithospermum erythrorhizon) supplementation enhances epidermal levels of ceramides, glucosylceramides, β-glucocerebrosidase, and acidic sphingomyelinase in NC/Nga mice

We have previously reported that dietary gromwell (Lithospermum erythrorhizon; LE) prevents the development of atopic dermatitis (AD) with increased epidermal levels of total ceramide (Cer), the major lipid maintaining epidermal barrier. In this study, we investigated whether the increased level of total Cer induced by dietary LE would be related to the altered metabolism of glucosylceramide (GlcCer) and sphingomyelin (SM), two major precursor lipids in Cer generation. NC/Nga mice, an animal model of AD, were fed a control diet (group CA: atopic control) or a diet with 70% ethanol LE extracts (1% in diet; group LE) for 10 weeks. Individual species of Cer, GlcCer, and SM were analyzed by high-performance thin layer chromatography. In the epidermis of group CA, total Cer (including Cer2 and Cer5-7) and total GlcCer (including GlcCer-B/C/D) were significantly reduced; these levels in group LE were increased to levels similar to the normal control group of BALB/c mice (group C). In addition, protein expressions and activities of β-glucocerebrosidase (β-GlcCer'ase) and acidic sphingomyelinase (aSMase), enzymes for GlcCer or SM hydrolysis, respectively, were increased in group LE. However, alterations of Cer1, Cer3/4, GlcCer-A, and all SM species (including SM1-3) were not significant among groups C, CA, and LE. Dietary gromwell increases GlcCer-B/C/D, and further enhances the generation of Cer2 and Cer5-7 with high protein expressions and activities of β-GlcCer'ase and aSMase.

Activation of TLR3 in keratinocytes increases expression of genes involved in formation of the epidermis, lipid accumulation, and epidermal organelles

Injury to the skin, and the subsequent release of non-coding double-stranded RNA from necrotic keratinocytes, has been identified as an endogenous activator of Toll-like receptor 3 (TLR3). Since changes in keratinocyte growth and differentiation follow injury, we hypothesized that TLR3 might trigger some elements of the barrier repair program in keratinocytes. Double-stranded RNA was observed to induce TLR3-dependent increases in human keratinocyte mRNA abundance for ABCA12 (ATP-binding cassette, sub-family A, member 12), glucocerebrosidase, acid sphingomyelinase, and transglutaminase 1. Additionally, treatment with double-stranded RNA resulted in increases in sphingomyelin and morphologic changes including increased epidermal lipid staining by oil-red O and TLR3-dependent increases in lamellar bodies and keratohyalin granules. These observations show that double-stranded RNA can stimulate some events in keratinocytes that are important for skin barrier repair and maintenance.

Isolation and identification of human hair ceramides

Synopsis The analysis of the lipids extracted from human hair by silica-gel-column liquid chromatography permitted an isolation of the fraction enriched in free ceramides. These were identified by GC/MS and are related to ceramide classes 2 and 5 in which the long-chain base was predominantly sphinganine. This differs from that observed in the free ceramides found within the stratum corenum.

The lipid alterations in the stratum corneum of dogs with atopic dermatitis are alleviated by topical application of a sphingolipid-containing emulsion

BACKGROUND

Atopic dermatitis (AD) results from an altered skin barrier associated with defects in the lipid composition of the skin. Dogs with AD present similar clinical symptoms to humans, and may be a useful model for investigations into AD.

AIM

To analyse the changes occurring in the lipids of the stratum corneum (SC) of dogs with AE after 3 weeks of topical treatment with an emulsion containing ceramides, free fatty acids (FFAs) and cholesterol (skin lipid complex; SLC).

METHODS

Nonlesional SC was collected by tape stripping from control and treated areas. Free and protein-bound lipids were purified, and the various classes were isolated by column chromatography, analysed by thin-layer chromatography and assayed.

RESULTS

Ceramides, FFA and cholesterol were all found to be lower in the skin of untreated dogs with AD than in normal dogs, and the topical treatment resulted in significantly increased values for ceramides. Conversely, only trace amounts of glucosylceramides were present in normal SC, but a high concentration (27 μg per mg protein) was detected in canine atopic SC, which disappeared after treatment with SLC. There was a heterogeneous distribution of all of the lipids in the different layers of canine atopic SC, which was more pronounced for protein-bound than for free lipids. Following topical treatment, the protein-bound lipid content normalized.

CONCLUSIONS

Topical treatment with SLC resulted in a significant improvement of the lipid biosynthesis of keratinocytes in atopic dogs, thereby potentially enabling the formation of a tighter epidermal barrier.

Bridging microscopes: 3D correlative light and scanning electron microscopy of complex biological structures

The rationale of correlative light and electron microscopy (CLEM) is to collect data on different information levels--ideally from an identical area on the same sample--with the aim of combining datasets at different levels of resolution to achieve a more holistic view of the hierarchical structural organization of cells and tissues. Modern three-dimensional (3D) imaging techniques in light and electron microscopy opened up new possibilities to expand morphological studies into the third dimension at the nanometer scale and over various volume dimensions. Here, we present two alternative approaches to correlate 3D light microscopy (LM) data with scanning electron microscopy (SEM) volume data. An adapted sample preparation method based on high-pressure freezing for structure preservation, followed by freeze-substitution for multimodal en-bloc imaging or serial-section imaging is described. The advantages and potential applications are exemplarily shown on various biological samples, such as cells, individual organisms, human tissue, as well as plant tissue. The two CLEM approaches presented here are per se not mutually exclusive, but have their distinct advantages. Confocal laser scanning microscopy (CLSM) and focused ion beam-SEM (FIB-SEM) is most suitable for targeted 3D correlation of small volumes, whereas serial-section LM and SEM imaging has its strength in large-area or -volume screening and correlation. The second method can be combined with immunocytochemical methods. Both methods, however, have the potential to extract statistically relevant data of structural details for systems biology.

Fat in the skin: Triacylglycerol metabolism in keratinocytes and its role in the development of neutral lipid storage disease

Keratinocyte differentiation is essential for skin development and the formation of the skin permeability barrier. This process involves an orchestrated remodeling of lipids. The cleavage of precursor lipids from lamellar bodies by β-glucocerebrosidase, sphingomyelinase, phospholipases and sterol sulfatase generates ceramides, non-esterified fatty acids and cholesterol for the lipid-containing extracellular matrix, the lamellar membranes in the stratum corneum. The importance of triacylglycerol (TAG) hydrolysis for the formation of a functional permeability barrier was only recently appreciated. Mice with defects in TAG synthesis (acyl-CoA:diacylglycerol acyltransferase-2-knock-out) or TAG catabolism (comparative gene identification-58, -CGI-58-knock-out) develop severe permeability barrier defects and die soon after birth because of desiccation. In humans, mutations in the CGI-58 gene also cause (non-lethal) neutral lipid storage disease with ichthyosis. As a result of defective TAG synthesis or catabolism, humans and mice lack ω-(O)-acylceramides, which are essential lipid precursors for the formation of the corneocyte lipid envelope. This structure plays an important role in linking the lipid-enriched lamellar membranes to highly cross-linked corneocyte proteins. This review focuses on the current knowledge of biochemical mechanisms that are essential for epidermal neutral lipid metabolism and the formation of a functional skin permeability barrier.

The role of fatty acid desaturases in epidermal metabolism

The lipid composition of skin is important to a variety of functions served by this organ. Therefore, skin expresses multiple enzymes that synthesize and metabolize lipids. An important class of lipid metabolism enzymes expressed in skin is the lipid desaturases. Various isoforms of stearoyl-CoA desaturase, a delta-9 desaturase, as well as a delta-6 desaturase alter the lipid composition of the skin, thereby affecting skin barrier homeostasis and consequently, whole body energy balance. This review will focus on the role of fatty acid desaturases in maintaining epidermal metabolism.

Abnormal nonstoring capillary endothelium: a novel feature of Gaucher disease. Ultrastructural study of dermal capillaries

Ultrastructural study of skin biopsies in two cases of Gaucher disease (GD) patients (types II and III) revealed hitherto unknown alteration of the blood capillary endothelial cells (ECs) featured by hypertrophy and numerous subplasmalemmal microvesicles underneath both the apical and basal membranes. There was also prominent apical membrane folding with formation of filiform and large cytoplasmic projections, with occasional transcapillary cytoplasmic bridges. Similar, though less frequently expressed, changes were manifested at the basal membrane by numerous cytoplasmic projections into the subendothelial space. Regressive changes with EC breakdown were rare. Lysosomal storage was always absent. Besides EC hypertrophy, there was also increased EC density in the capillary lumen, leading to pronounced changes in capillary architecture with loose or incomplete EC anchoring. There were also signs of EC sprouting. Some pericytes displayed an increase in size and number of cytoplasmic processes, which often extended into distant pericapillary regions. The spectrum of changes suggests that a significant positive growth effect on EC occurs in GD. The putative mechanisms triggered by GBA1 deficiency leading to EC involvement are discussed. The authors are well aware of the fact the results, based on a nontraditional type of bioptic samples, are preliminary, but they are worth following, as further ultrastructural and functional studies of blood endothelium in GD may open a novel field in molecular cell pathophysiology of the disorder: endothelial dysfunction.

Acute acidification of stratum corneum membrane domains using polyhydroxyl acids improves lipid processing and inhibits degradation of corneodesmosomes

Neutralization of the normally acidic stratum corneum (SC) has deleterious consequences for permeability barrier homeostasis and SC integrity/cohesion attributable to serine proteases (SPs) activation leading to deactivation/degradation of lipid-processing enzymes and corneodesmosomes (CD). As an elevated pH compromises SC structure and function, we asked here whether SC hyperacidification would improve the structure and function. We lowered the pH of mouse SC using two polyhydroxyl acids (PHA), lactobionic acid (LBA), or gluconolactone (GL). Applications of the PHA reduced the pH at all levels of SC of hairless mouse, with further selective acidification of SC membrane domains, as shown by fluorescence lifetime imaging. Hyperacidification improved permeability barrier homeostasis, attributable to increased activities of two key membrane-localized, ceramide-generating hydrolytic enzymes (beta-glucocerebrosidase and acidic sphingomyelinase), which correlated with accelerated extracellular maturation of SC lamellar membranes. Hyperacidification generated "supernormal" SC integrity/cohesion, attributable to an SP-dependent decreased degradation of desmoglein-1 (DSG1) and the induction of DSG3 expression in lower SC. As SC hyperacidification improves the structure and function, even of normal epidermis, these studies lay the groundwork for an assessment of the potential utility of SC acidification as a therapeutic strategy for inflammatory dermatoses, characterized by abnormalities in barrier function, cohesion, and surface pH.

Skin barrier disruption by acetone: observations in a hairless mouse skin model

To disrupt the barrier function of the skin, different in vivo methods have been established, e.g., by acetone wiping or tape-stripping. In this study, the acetone-induced barrier disruption of hairless mice was investigated in order to establish a reliable model to study beneficial, long-term effects on barrier recovery after topical application. For both treatments (i.e., acetone treatment and tape-stripping) the transepidermal water loss directly after disruption and the subsequent barrier recovery profile were similar. Histological assessment showed significant lower number of corneocyte layers in acetone-treated and tape-stripped skin compared to untreated skin, while there was no statistical difference between the two treatments. Lipid analysis of acetone-treated skin revealed that only small fraction of lipids were extracted consisting of predominantly nonpolar lipids. Importantly, the ratio of the barrier lipids, i.e., cholesterol, free fatty acids and ceramides, remained similar between control and acetone-treated skin. This reflects the undisrupted lipid organization, as determined by small-angle X-ray diffraction measurements: the long-periodicity lamellar phase was still present after acetone treatment. Our results contradict earlier studies which reported no mechanical stratum corneum removal, a substantial extraction of lipids and disruption in lipid organization. In conclusion, our studies demonstrate that barrier disruption due to acetone treatment is mainly due to removal of corneocytes.

Lipids and skin barrier function--a clinical perspective

The stratum corneum (SC) protects us from dehydration and external dangers. Much is known about the morphology of the SC and penetration of drugs through it, but the data are mainly derived from in vitro and animal experiments. In contrast, only a few studies have the human SC lipids as their focus and in particular, the role of barrier function in the pathogenesis of skin disease and its subsequent treatment protocols. The 3 major lipids in the SC of importance are ceramides, free fatty acids, and cholesterol. Human studies comparing levels of the major SC lipids in patients with atopic dermatitis and healthy controls have suggested a possible role for ceramide 1 and to some extent ceramide 3 in the pathogenesis of the disease. Therapies used in diseases involving barrier disruption have been sparely investigated from a lipid perspective. It has been suggested that ultraviolet light as a treatment increases the amount of all 3 major SC lipids, while topical glucocorticoids may lead to a decrease. Such effects may influence the clinical outcome of treatment in diseases with impaired barrier function. We have, therefore, conducted a review of the literature on SC lipids from a clinical perspective. It may be concluded that the number of human studies is very limited, and in the perspective of how important diseases of impaired barrier function are in dermatology, further research is needed.

Co-regulation and interdependence of the mammalian epidermal permeability and antimicrobial barriers

Human epidermis elaborates two small cationic, highly hydrophobic antimicrobial peptides (AMP), beta-defensin 2 (hBD2), and the carboxypeptide cleavage product of human cathelicidin (hCAP18), LL-37, which are co-packaged along with lipids within epidermal lamellar bodies (LBs) before their secretion. Because of their colocalization, we hypothesized that AMP and barrier lipid production could be coregulated by altered permeability barrier requirements. mRNA and immunostainable protein levels for mBD3 and cathelin-related antimicrobial peptide (CRAMP) (murine homologues of hBD2 and LL-37, respectively) increase 1-8 hours after acute permeability barrier disruption and normalize by 24 hours, kinetics that mirror the lipid metabolic response to permeability barrier disruption. Artificial permeability barrier restoration, which inhibits the lipid-synthetic response leading to barrier recovery, blocks the increase in AMP mRNA/protein expression, further evidence that AMP expression is linked to permeability barrier function. Conversely, LB-derived AMPs are also important for permeability barrier homeostasis. Despite an apparent increase in mBD3 protein, CRAMP-/- mice delayed permeability barrier recovery, attributable to defective LB contents and abnormalities in the structure of the lamellar membranes that regulate permeability barrier function. These studies demonstrate that (1) the permeability and antimicrobial barriers are coordinately regulated by permeability barrier requirements and (2) CRAMP is required for permeability barrier homeostasis.

Acute modulations in permeability barrier function regulate epidermal cornification: role of caspase-14 and the protease-activated receptor type 2

Stratum corneum comprises corneocytes, derived from outer stratum granulosum during terminal differentiation, embedded in a lipid-enriched extracellular matrix, secreted from epidermal lamellar bodies. Permeability barrier insults stimulate rapid secretion of preformed lamellar bodies from the outer stratum granulosum, regulated through modulations in ionic gradients and serine protease (SP)/protease-activated receptor type 2 (PAR2) signaling. Because corneocytes are also required for barrier function, we hypothesized that corneocyte formation could also be regulated by barrier function. Barrier abrogation by two unrelated methods initiated a wave of cornification, assessed as TdT-mediated dUTP nick end-labeling-positive cells in stratum granulosum and newly cornified cells by electron microscopy. Because cornification was blocked by occlusion, corneocytes formed specifically in response to barrier, rather than injury or cell replacement, requirements. SP inhibitors and hyperacidification (which decreases SP activity) blocked cornification after barrier disruption. Similarly, cornification was delayed in PAR2(-/-) mice. Although classical markers of apoptosis [poly(ADP-ribose)polymerase and caspase (Casp)-3] remained unchanged, barrier disruption activated Casp-14. Moreover, the pan-Casp inhibitor Z-VAD-FMK delayed cornification, and corneocytes were structurally aberrant in Casp14(-/-) mice. Thus, permeability barrier requirements coordinately drive both the generation of the stratum corneum lipid-enriched extracellular matrix and the transformation of granular cells into corneocytes, in an SP- and Casp-14-dependent manner, signaled by PAR2.

Postnatal requirement of the epithelial sodium channel for maintenance of epidermal barrier function

In skin, the physiological consequence of an epithelial sodium channel (ENaC) deficiency is not obvious directly at birth. Nevertheless, within hours after birth, mice deficient for the alpha-subunit of the highly amiloride-sensitive epithelial sodium channel (alphaENaC/Scnn1a) suffer from a significant increased dehydration. This is characterized by a loss of body weight (by 6% in 6 h) and an increased transepidermal water loss, which is accompanied by a higher skin surface pH in 1-day-old pups. Although early and late differentiation markers, as well as tight junction protein distribution and function, seem unaffected, deficiency of alphaENaC severely disturbs the stratum corneum lipid composition with decreased ceramide and cholesterol levels, and increased pro-barrier lipids, whereas covalently bound lipids are drastically reduced. Ultrastructural analysis revealed morphological changes in the formation of intercellular lamellar lipids and the lamellar body secretion. Extracellular formation of the lamellar lipids proved to be abnormal in the knockouts. In conclusion, ENaC deficiency results in progressive dehydration and, consequently, weight loss due to severe impairment of lipid formation and secretion. Our data demonstrate that ENaC expression is required for the postnatal maintenance of the epidermal barrier function but not for its generation.

Thematic review series: skin lipids. The role of epidermal lipids in cutaneous permeability barrier homeostasis

The permeability barrier is required for terrestrial life and is localized to the stratum corneum, where extracellular lipid membranes inhibit water movement. The lipids that constitute the extracellular matrix have a unique composition and are 50% ceramides, 25% cholesterol, and 15% free fatty acids. Essential fatty acid deficiency results in abnormalities in stratum corneum structure function. The lipids are delivered to the extracellular space by the secretion of lamellar bodies, which contain phospholipids, glucosylceramides, sphingomyelin, cholesterol, and enzymes. In the extracellular space, the lamellar body lipids are metabolized by enzymes to the lipids that form the lamellar membranes. The lipids contained in the lamellar bodies are derived from both epidermal lipid synthesis and extracutaneous sources. Inhibition of cholesterol, fatty acid, ceramide, or glucosylceramide synthesis adversely affects lamellar body formation, thereby impairing barrier homeostasis. Studies have further shown that the elongation and desaturation of fatty acids is also required for barrier homeostasis. The mechanisms that mediate the uptake of extracutaneous lipids by the epidermis are unknown, but keratinocytes express LDL and scavenger receptor class B type 1, fatty acid transport proteins, and CD36. Topical application of physiologic lipids can improve permeability barrier homeostasis and has been useful in the treatment of cutaneous disorders.

Targeted ablation of Arnt in mouse epidermis results in profound defects in desquamation and epidermal barrier function

The molecular mechanisms of skin adaptation to the environmental stress are poorly understood. The aryl hydrocarbon receptor nuclear translocator (Arnt) lies at the intersection of several crucial adaptive pathways. Nevertheless, its role in adaptation of the skin to environmental stress has just begun to be unraveled. Here we show that Arnt is expressed in human and mouse skin in a developmentally dependent manner. Targeted K14-driven deletion of Arnt in the mouse epidermis resulted in early postnatal death, associated with a failure of epidermal barrier function. Gene expression profiling of Arnt-null mouse epidermis revealed upregulation of genes of the epidermal differentiation complex on mouse chromosome 3, including S100a genes (S100a8, S100a9, S100a10) and genes coding for small proline-rich proteins (Sprr1a, Sprr2i, Sprr2j, Sprrl1). HPTLC analysis showed significant accumulation of Cer[NS] and Cer[NH] ceramide species in Arnt-null epidermis, suggesting alterations in lipid metabolism. Continuous retention of corneosomes in Arnt-null epidermis that resulted in an abnormally dense corny layer and impaired desquamation was associated with upregulation of Slpi, an inhibitor of stratum corneum chymotryptic enzyme (SCCE) that plays a key role in corneosome degradation. The functional defects in Arnt-null mouse epidermis underscore the crucial role of Arnt in the maintenance of epidermal homeostasis, especially during the perinatal transition to the ex utero environment.

Epidermal sphingolipids: Metabolism, function, and roles in skin disorders

Mammalian epidermis produces and delivers large quantities of glucosylceramide and sphingomyelin precursors to stratum corneum extracellular domains, where they are hydrolyzed to corresponding ceramide species. This cycle of lipid precursor formation and subsequent hydrolysis represents a mechanism that protects the epidermis against potentially harmful effects of ceramide accumulation within nucleated cell layers. Prominent skin disorders, such as psoriasis and atopic dermatitis, have diminished epidermal ceramide levels, reflecting altered sphingolipid metabolism, that may contribute to disease severity/progression. Enzymatic processes in the hydrolysis of glucosylceramide and sphingomyelin, and the roles of sphingolipids in skin diseases, are the focus of this review.

Rottlerin, a Specific Inhibitor of Protein Kinase C-delta, Impedes Barrier Repair Response by Increasing Intracellular Free Calcium

Several signals have been suggested in maintaining skin barrier homeostasis, but epidermal calcium ions are currently thought to be a main signaling factor. It is not clear, however, exactly how an intracellular calcium level decreases in response to the loss of an extracellular calcium gradient. In this study, we investigated the effects of several broad-type and isozyme-specific protein kinase C (PKC) inhibitors on epidermal permeability barrier recovery. Topical application of chelerythrine chloride, a broad-type PKC inhibitor, and rottlerin, a PKCdelta-specific inhibitor, significantly impeded the barrier recovery rate at 3 and 6 hours after barrier disruption. A significant decrease in the number and secretion of lamellar bodies was also observed at the inhibitor-treated site. Calcium ion-capture cytochemistry showed that the epidermal calcium gradient was rapidly reformed in inhibitor-treated skin, though recovery of the corresponding barrier function was not observed. In cultured keratinocytes treated with either inhibitor, increased intracellular calcium did not return to the baseline concentration after extracellular calcium decreased. These results suggest that PKC inhibitors, especially a PKCdelta-specific inhibitor, delay barrier recovery by affecting the intracellular calcium concentration after a loss of the extracellular calcium gradient. Furthermore, PKCdelta is important in controlling a decrease in intracellular calcium concentration.

[Disturbances of antimicrobial lipids in atopic dermatitis]

Patients with atopic dermatitis exhibit an increased susceptibility to cutaneous infections, especially to pathological colonization with superantigen-secreting Staphylococcus aureus. Recent attention has been focused on antimicrobial peptides, especially on cathelicidin and human beta-defensin-2, which are under-expressed in atopic skin. Antimicrobial lipids from the stratum corneum are also major contributors to cutaneous antimicrobial defense. Current aspects of biochemistry and function of antimicrobial lipids in atopic dermatitis are reviewed in detail. The major classes of stratum corneum lipids with antimicrobial activity are free fatty acids, glucosylceramides, and free sphingosines. Diminished levels of free sphingosines in the stratum corneum have recently been detected in atopic dermatitis and have been associated with the pathological colonization of atopic skin with Staphylococcus aureus. The superantigen staphylococcal enterotoxin B has been shown to reduce the suppressive effect of regulatory T cells on T-cell proliferation, thus augmenting T-cell activation in patients with atopic dermatitis. The killing of superantigen-secreting bacterial strains with topically applied antimicrobial lipids offers new antiseptic and immunomodulatory options for the treatment and secondary prevention of atopic dermatitis.

Role of ceramides in barrier function of healthy and diseased skin

Stratum corneum intercellular lipids play an important role in the regulation of skin water barrier homeostasis and water-holding capacity. Modification of intercellular lipid organization and composition may impair these properties. Patients with skin diseases such as atopic dermatitis, psoriasis, contact dermatitis, and some genetic disorders have diminished skin barrier function. Lipid composition in diseased skin is characterized by decreased levels of ceramide and altered ceramide profiles. To clarify mechanisms underlying ceramides as a causative factor of skin disease, investigators have examined the activity of enzymes in the stratum corneum on ceramide production and degradation. The activities of ceramidase, sphingomyelin deacylase, and glucosylceramide deacylase are increased in epidermal atopic dermatitis. Investigators have also compared the expression levels of sphingolipid activator protein in the epidermis of normal and diseased skin. A decreased level of prosaposin has been identified in both atopic dermatitis and psoriasis. These results indicate that decreased ceramide level is a major etiologic factor in skin diseases. Hence, topical skin lipid supplementation may provide opportunities for controlling ceramide deficiency and improving skin condition.