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

Fatty acid transport protein 4 is required for incorporation of saturated ultralong-chain fatty acids into epidermal ceramides and monoacylglycerols

Fatty acid transport protein 4 (FATP4) is an acyl-CoA synthetase that is required for normal permeability barrier in mammalian skin. FATP4 (SLC27A4) mutations cause ichthyosis prematurity syndrome, a nonlethal disorder. In contrast, Fatp4-/- mice die neonatally from a defective barrier. Here we used electron microscopy and lipidomics to characterize defects in Fatp4-/- mice. Mutants showed lamellar body, corneocyte lipid envelope, and cornified envelope abnormalities. Lipidomics identified two lipids previously speculated to be present in mouse epidermis, sphingosine β-hydroxyceramide and monoacylglycerol; mutants displayed decreased proportions of these and the two ceramide classes that carry ultralong-chain, amide-linked fatty acids (FAs) thought to be critical for barrier function, unbound ω-O-acylceramide and bound ω-hydroxyceramide, the latter constituting the major component of the corneocyte lipid envelope. Other abnormalities included elevated amounts of sphingosine α-hydroxyceramide, phytosphingosine non-hydroxyceramide, and 1-O-acylceramide. Acyl chain length alterations in ceramides also suggested roles for FATP4 in esterifying saturated non-hydroxy and β-hydroxy FAs with at least 25 carbons and saturated or unsaturated ω-hydroxy FAs with at least 30 carbons to CoA. Our lipidomic analysis is the most thorough such study of the Fatp4-/- mouse skin barrier to date, providing information about how FATP4 can contribute to barrier function by regulating fatty acyl moieties in various barrier lipids.

Phytosphingosine Increases Biosynthesis of Phytoceramide by Uniquely Stimulating the Expression of Dihydroceramide C4-desaturase (DES2) in Cultured Human Keratinocytes

Ceramide NP is known to be the most abundant class of 12 ceramide (CER) families that form a permeability barrier in the human skin barrier. However, not many studies have been reported on the regulation of the biosynthesis of ceramide NP. Recently, it has been reported that phytosphingosine (PHS) treatment in the cultured keratinocytes (KC) notably increased the content of ceramide NP. However, the mechanism behind the PHS-induced enhancement of ceramide NP has not been elucidated. In this study, we investigated the effects of PHS on the expression of several essential genes for the biosynthesis of CER. Also, we determined the molecular mechanism behind the unique enhancement of ceramide NP upon treatment of PHS in the cultured KC. The expressions of all of the three genes (SPT, ceramide synthase 3 [CERS3], and ELOVL4) and their respective proteins were markedly increased in PHS-treated KC. In addition, the expression of the dihydroceramide C4-desaturase (DES2) responsible for conversion of dihydroceramide into ceramide NP was uniquely enhanced only by PHS treatment. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis revealed that more than 20-fold increase of ceramide NP by PHS was observed while no significant enhancement of ceramide NS and NDS was observed. This study demonstrates that PHS plays a fundamental role in strengthening the epidermal permeability barrier by stimulating the overall processes of biosynthesis of all classes of CER in epidermis. The dramatic increase of ceramide NP upon PHS treatment seemed to be the outcome of transformation of dihydroceramide and/or ceramide NS by C4-hydroxylase activity.

Role of Ceramides in Drug Delivery

Ceramides belong to the sphingolipid group of lipids, which serve as both intracellular and intercellular messengers and as regulatory molecules that play essential roles in signal transduction, inflammation, angiogenesis, and metabolic disorders such as diabetes, neurodegenerative diseases, and cancer cell degeneration. Ceramides also play an important structural role in cell membranes by increasing their rigidity, creating micro-domains (rafts and caveolae), and altering membrane permeability; all these events are involved in the cell signaling. Ceramides constitute approximately half of the lipid composition in the human skin contributing to barrier function as well as epidermal signaling as they affect both proliferation and apoptosis of keratinocytes. Incorporation of ceramides in topical preparations as functional lipids appears to alter skin barrier functions. Ceramides also appear to enhance the bioavailability of drugs by acting as lipid delivery systems. They appear to regulate the ocular inflammation signaling, and external ceramides have shown relief in the anterior and posterior eye disorders. Ceramides play a structural role in liposome formulations and enhance the cellular uptake of amphiphilic drugs, such as chemotherapies. This review presents an overview of the various biological functions of ceramides, and their utility in topical, oral, ocular, and chemotherapeutic drug delivery.

Lysosome Alterations in the Human Epithelial Cell Line HaCaT and Skin Specimens: Relevance to Psoriasis

Despite the constantly updated knowledge regarding the alterations occurring in the cells of patients with psoriasis, the status and the role of the lysosome, a control center of cell metabolism, remain to be elucidated. The architecture of the epidermis is largely regulated by the action of lysosomes, possibly activating signaling pathways in the cellular crosstalk of keratinocytes-epidermal cells-with infiltrating immune cells. Thus, in the present study, lysosome alterations were examined in vitro and in situ using a two-dimensional (2D) keratinocyte model of HaCaT cells with "psoriasis-like" inflammation and skin specimens, respectively. Specific fluorescence and immunohistochemical staining showed an augmented level of acidic organelles in response to keratinocyte activation (mimicking a psoriatic condition while maintaining the membrane integrity of these structures) as compared with the control, similar to that seen in skin samples taken from patients. Interestingly, patients with the most pronounced PASI (Psoriasis Area and Severity Index), BSA (Body Surface Area), and DLQI (Dermatology Life Quality Index) scores suffered a high incidence of positive lysosomal-associated membrane protein 1 (LAMP1) expression. Moreover, it was found that the gene deregulation pattern was comparable in lesioned (PP) and non-lesioned (PN) patient-derived skin tissue, which may indicate that these alterations occur prior to the onset of the characteristic phenotype of the disease. Changes in the activity of genes encoding the microphthalmia family (MiT family) of transcription factors and mammalian target of rapamycin complex 1 (MTORC1) were also observed in the in vitro psoriasis model, indicating that the biogenesis pathway of this arm is inhibited. Interestingly, in contrast to the keratinocytes of HaCaT with "psoriasis-like" inflammation, LAMP1 was up-regulated in both PP and PN skin, which can be a potential sign of an alternative mechanism of lysosome formation. Defining the molecular profile of psoriasis in the context of "the awesome lysosome" is not only interesting, but also desired; therefore, it is believed that this paper will serve to encourage other researchers to conduct further studies on this subject.

Aging of the skin barrier

The skin barrier is mainly present in the stratum corneum (SC), composed of corneocytes surrounded by intercellular lipid lamellae, and attached by corneodesmosome. The tight junction attached to the lateral walls of keratinocytes in the upper part of the stratum granulosum is also included in the skin barrier. During aging, the following structures and functions of the skin barrier are changed or disturbed: (1) skin barrier structure, (2) permeability barrier function, (3) epidermal calcium gradient, (4) epidermal lipid synthesis and SC lipid processing, (5) cytokine production and response after insults, (6) SC acidity, (7) SC hydration, and (8) antimicrobial barrier. Patients with diabetes also show changes in the skin barrier similar to those in aged skin, and the characteristics of the skin barrier are very similar. Understanding the pathogenic mechanisms of the skin barrier in aging will permit us to develop therapeutic strategies for aged or diabetic skin.

Pathophysiology of atopic dermatitis: Clinical implications

Atopic dermatitis (AD) is the most common chronic inflammatory skin disease. Genetic predisposition, epidermal barrier disruption, and dysregulation of the immune system are some of the critical components of AD. An impaired skin barrier may be the initial step in the development of the atopic march as well as AD, which leads to further skin inflammation and allergic sensitization. Type 2 cytokines as well as interleukin 17 and interleukin 22 contribute to skin barrier dysfunction and the development of AD. New insights into the pathophysiology of AD have focused on epidermal lipid profiles, neuroimmune interactions, and microbial dysbiosis. Newer therapeutic strategies focus on improving skin barrier function and targeting polarized immune pathways found in AD. Further understanding of AD pathophysiology will allow us to achieve a more precision medicine approach to the prevention and the treatment of AD.

Reconstructed Human Epidermis Predicts Barrier-Improving Effects of Lactococcus lactis Emulsion in Humans

BACKGROUND/AIMS

The skin provides protection against chemical, physical, and biological stressors, yet the skin morphology changes over the course of life. These changes might affect the skin barrier function and facilitate the onset of age-related diseases. Since orally applied lactic acid bacteria ameliorate signs of aged and atopic skin, we investigated the effects of a topically applied Lactococcus lactis emulsion.

METHODS

In a blinded, randomized, vehicle-controlled trial, we studied topical Lactococcus effects both in vitro and in 20 healthy female volunteers. Commercially available reconstructed human epidermis (RHE) was treated for 4 days (once daily) and volar forearms were treated for 30 days (twice daily).

RESULTS

Lactococcus formulations improve the skin barrier in RHE as shown by increased filaggrin and human β-defensin-2 expression as well as by the 23% declined mean apparent permeability coefficients for caffeine. A reduction of 18% in transepidermal water loss confirms this effect in humans. Moreover, Lactococcus emulsion optimized skin hydration and surface pH. Skin irritation was not detected.

CONCLUSIONS

Lactococcus emulsion improved the skin barrier function with good biocompatibility. Moreover, our study exemplifies the translational predictive capacity of testing on RHE with respect to Lactococcus emulsion.

A unique structural distribution pattern discovered for the cerebrosides from starfish Asterias amurensis

Cerebroside is an important family of the mono-glycosylated ceramides involved in the larger family of glycosphingolipid and sulfatide. Cerebroside is synthesized from ceramide by the transfer of glucose from UDP-glucose, and degraded back to ceramide, which plays an important role at the epidermis protecting interior of the body as a barrier. Because cerebroside is regarded as the source molecule of ceramide and is amphiphilic in nature, cerebroside is considered valuable as the ingredient of cosmetic lotion. Various sources can be considered as raw material of cerebrosides. Starfish is considered as one of such potent source. However, the structure of the ceramide part of cerebroside is not fully investigated. Therefore, the individual structures of cerebroside molecules need to be identified including sphingosine and fatty acyl group composition to assess the potential of the molecule. We investigated and determined the structures of cerebrosides in starfish Asterias amurensis using LC-MS, GC-MS, tandem mass spectrometry (MS/MS), and ¹H NMR. We also discovered a characteristic structure distribution that was divided into three major groups: 1) a group composed of a relatively long sphingosine (C22) and a short length of fatty acyl group (less than C16), 2) a group composed of a typical C18 sphingosine and long fatty acyl groups (greater than C23), and 3) a group composed of C18 sphingosine and fatty acyl groups with their length less than C18. The calculated Log P values of cerebrosides ranging from 9 to 11 covered about 80% of the molecules that were in the range of those used in cosmetics, thus showing the potential usefulness of starfish Asterias amurensis as a source of raw material for cerebrosides.

Reduction of stratum corneum ceramides in Neu-Laxova syndrome caused by phosphoglycerate dehydrogenase deficiency

Neu-Laxova syndrome (NLS) is a very rare autosomal recessive congenital disorder characterized by disturbed development of the central nervous system and the skin and caused by mutations in any of the three genes involved in de novo l-serine biosynthesis: PHGDH, PSAT1, and PSPH l-Serine is essential for the biosynthesis of phosphatidylserine and sphingolipids. The extracellular lipid of the stratum corneum, of which sphingolipid constitutes a significant part, plays a primary role in skin barrier function. Here, we describe a Japanese NLS pedigree with a previously unreported nonsense mutation in PHGDH and a unique inversion of chromosome 1. In addition, the levels of 11 major ceramide classes in the tape-stripped stratum corneum of the NLS patient's skin were assessed by LC/MS. Notably, lower amounts of ceramides of all classes were found in the patient's stratum corneum than in those of controls. This is the first report to demonstrate the reduction of ceramides in the stratum corneum of an NLS patient due to PHGDH mutations. The clinical findings and a detailed analysis of ceramides from the stratum corneum in the family extend the spectrum of clinical anomalies and give us a clue to the pathomechanisms of ichthyosis in NLS patients with phosphoglycerate dehydrogenase deficiency.

Topically applied fatty acids are elongated before incorporation in the stratum corneum lipid matrix in compromised skin

In several skin diseases, both the lipid composition and organization in the stratum corneum (SC) are altered which contributes to the impaired skin barrier function in patients. One of the approaches for skin barrier repair is treatment with topical formulations to normalize SC lipid composition and organization. Vernix caseosa (VC), a white cheesy cream on the skin during gestational delivery, has shown to enhance skin barrier repair. In this study, we examined how a fatty acid (FA) containing formulation mimicking the lipid composition of VC interacts with the lipid matrix in the SC. The formulation was applied on ex vivo human skin after SC removal. Subsequently, the ex vivo human skin generated SC during culture. The effect of FA containing formulations on the lipid organization and composition in the regenerated SC was analysed by Fourier transform infrared (FTIR) spectroscopy and liquid chromatography mass spectroscopy (LC/MS), respectively. FTIR results demonstrate that the FAs are intercalated in the lipid matrix of the regenerated SC and partition in the same lattice with the endogenous SC lipids, thereby enhancing the fraction of lipids forming an orthorhombic (very dense) packing in the SC. LC/MS data show that the topically applied FAs are elongated before intercalation in the lipid matrix and are thus involved in the lipid biosynthesis in the skin.

Epidermal permeability barrier function and sphingolipid content in the skin of sphingomyelin synthase 2 deficient mice

Sphingomyelin synthase (SMS) is an enzyme that generates sphingomyelin (SM) from ceramide (CER) and phosphatidylcholine. SM in the epidermis is a precursor of CER, an important lipid for epidermal permeability barrier function. However, the physiological role of SMS in skin is unclear. To uncover the function of SMS in skin, we investigated sphingolipid metabolism enzyme activity in skin, SM content in the epidermis, CER content in the stratum corneum (SC) and transepidermal water loss (TEWL) as an indicator of barrier function in SMS2-knockout (KO) mice. The activities of sphingolipid metabolism enzymes in skin homogenates were measured using a fluorescently labelled substrate. Enzymatic reaction products were detected by high-performance liquid chromatography (HPLC). Lipids in the epidermis or SC were extracted and quantified by high-performance thin layer chromatography (HPTLC). TEWL was measured using a Tewameter TM300. In SMS2-KO mice, SMS activity in skin homogenates, epidermal SM content and SC CER content were significantly decreased relative to wild-type (WT) mice. The TEWL of SMS2-KO mice was significantly increased compared to WT mice. Our data indicate that SMS2 generates SM in the epidermis and contributes to epidermal permeability barrier function and will support understanding of SM-related metabolic disorders.

Abnormal Sphingolipid World in Inflammation Specific for Lysosomal Storage Diseases and Skin Disorders

Research in recent years has shown that sphingolipids are essential signalling molecules for the proper biological and structural functioning of cells. Long-term studies on the metabolism of sphingolipids have provided evidence for their role in the pathogenesis of a number of diseases. As many inflammatory diseases, such as lysosomal storage disorders and some dermatologic diseases, including psoriasis, atopic dermatitis and ichthyoses, are associated with the altered composition and metabolism of sphingolipids, more studies precisely determining the responsibilities of these compounds for disease states are required to develop novel pharmacological treatment opportunities. It is worth emphasizing that knowledge from the study of inflammatory metabolic diseases and especially the possibility of their treatment may lead to insight into related metabolic pathways, including those involved in the formation of the epidermal barrier and providing new approaches towards workable therapies.

Ceramides in the skin barrier

The skin barrier, which is essential for human survival on dry land, is located in the uppermost skin layer, the stratum corneum. The stratum corneum consists of corneocytes surrounded by multilamellar lipid membranes that prevent excessive water loss from the body and entrance of undesired substances from the environment. To ensure this protective function, the composition and organization of the lipid membranes is highly specialized. The major skin barrier lipids are ceramides, fatty acids and cholesterol in an approximately equimolar ratio. With hundreds of molecular species of ceramide, skin barrier lipids are a highly complex mixture that complicate the investigation of its behaviour. In this minireview, the structures of the major skin barrier lipids, formation of the stratum corneum lipid membranes and their molecular organization are described.

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.

Characterization of the Growth of Chlamydia trachomatis in In Vitro-Generated Stratified Epithelium

Chlamydia infection targets the mucosal epithelium, where squamous and columnar epithelia can be found. Research on Chlamydia-epithelia interaction has predominantly focused on columnar epithelia, with very little known on how Chlamydia interacts with the squamous epithelium. The stratification and differentiation processes found in the squamous epithelium might influence chlamydial growth and infection dissemination. For this reason, three-dimensional (3D) organotypic stratified squamous epithelial cultures were adapted to mimic the stratified squamous epithelium and chlamydial infection was characterized. Chlamydia trachomatis infection in monolayers and 3D cultures were monitored by immunofluorescence and transmission electron microscopy to evaluate inclusion growth and chlamydial interconversion between elementary and reticulate body. We observed that the stratified epithelium varied in susceptibility to C. trachomatis serovars L2 and D infection. The undifferentiated basal cells were susceptible to infection by both serovars, while the terminally differentiated upper layers were resistant. The differentiating suprabasal cells exhibited different susceptibilities to serovars L2 and D, with the latter unable to establish a successful infection in this layer. Mature elementary body-containing inclusions were much more prevalent in these permissive basal layers, while the uppermost differentiated layers consistently harbored very few reticulate bodies with no elementary bodies, indicative of severely limited bacterial replication and development. For serovar D, the differentiation state of the host cell was a determining factor, as calcium-induced differentiation of cells in a monolayer negatively affected growth of this serovar, in contrast to serovar L2. The apparent completion of the developmental cycle in the basal layers of the 3D cultures correlated with the greater degree of dissemination within and the level of disruption of the stratified epithelium. Our studies indicate that the squamous epithelium is a suboptimal environment for growth, and thus potentially contributing to the protection of the lower genital tract from infection. The relatively more fastidious serovar D exhibited more limited growth than the faster-growing and more invasive L2 strain. However, if given access to the more hospitable basal cell layer, both strains were able to produce mature inclusions, replicate, and complete their developmental cycle.

Novel phytoceramides containing fatty acids of diverse chain lengths are better than a single C18-ceramide N-stearoyl phytosphingosine to improve the physiological properties of human stratum corneum

Ceramides in the human stratum corneum (SC) are a mixture of diverse N-acylated fatty acids (FAs) with different chain lengths. C24 is the major class of FAs of ceramides. However, there are also other classes of ceramides with diverse chain lengths of FAs, and these lengths generally range from C16 to C26. This study aimed to prepare several types of phytoceramide containing diverse chain lengths of N-acylated FAs and compare them with C18-ceramide N-stearoyl phytosphingosine (NP) in terms of their effects on the physiological properties of the SC. We chose natural oils, such as horse fat oil, shea butter, sunflower oil, and a mixture of macadamia nut, shea butter, moringa, and meadowfoam seed oil, as sources of FAs and phytosphingosine as a sphingoid backbone to synthesize diverse phytoceramides. Each phytoceramide exhibited a distinctive formation of the lamellar structure, and their FA profiles were similar to those of their respective natural oil. The skin barrier properties, as analyzed in human skin, clearly demonstrated that all the phytoceramides improved the recovery rate of the damaged SC and enhanced hydration better than C18-ceramide NP did. In conclusion, natural oil-derived phytoceramides could represent a novel class of ceramides for cosmetic applications in the development of an ideal skin barrier moisturizer.

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.

Altered expression of epidermal lipid bio-synthesis enzymes in atopic dermatitis skin is accompanied by changes in stratum corneum lipid composition

BACKGROUND

The barrier dysfunction in atopic dermatitis (AD) skin correlates with stratum corneum (SC) lipid abnormalities including reduction of global lipid content, shorter ceramide (CER) as well as free fatty acid (FFA) chain length and altered CER subclass levels. However, the underlying cause of these changes in lipid composition has not been fully investigated.

AIM

We investigated whether the expression of CER and FFA biosynthesis enzymes are altered in AD skin compared with control skin and determine whether changes in enzyme expression can be related with changes in lipid composition.

METHODS

In AD patients and controls the expression of enzymes involved in the biosynthesis of FFAs and CERs was analyzed in relation to the SC lipid composition. These enzymes include stearoyl CoA desaturase (SCD), elongase 1 (ELOVL1) and ELOVL6 involved in FFA synthesis and β-glucocerebrosidase (GBA), acid-sphingomyelinase (aSmase), ceramide synthase 3 (CerS3) involved in CER synthesis. In TH2 treated human skin equivalents (AD HSEs) mimicking lesional AD skin, the mRNA expression of these enzymes was investigated.

RESULTS

The results reveal an altered expression of SCD and ELOVL1 in AD lesional skin. This was accompanied by functional changes displayed by increased unsaturated FFAs (SCD) and reduced FFA C22-C28 (ELOVL1) in AD lesional skin. The expression of GBA, aSmase and CerS3 were also altered in lesional skin. The CER composition in AD lesional skin showed corresponding changes such as increased CER AS and NS (aSmase) and decreased esterified ω-hydroxy CERs (CerS3). In support of the results from AD skin, the AD HSEs showed reduced mRNA ELOVL1, GBA and a Smase levels.

CONCLUSION

This study shows that alterations in the expression of key enzymes involved in SC lipid synthesis contribute to changes in the lipid composition in AD skin and inflammation may influence expression of these enzymes.

Potential Applications of Phyto-Derived Ceramides in Improving Epidermal Barrier Function

The outer most layer of the skin, the stratum corneum, consists of corneocytes which are coated by a cornified envelope and embedded in a lipid matrix of ordered lamellar structure. It is responsible for the skin barrier function. Ceramides (CERs) are the backbone of the intercellular lipid membranes. Skin diseases such as atopic dermatitis and psoriasis and aged skin are characterized by dysfunctional skin barrier and dryness which are associated with reduced levels of CERs. Previously, the effectiveness of supplementation of synthetic and animal-based CERs in replenishing the depleted natural skin CERs and restoring the skin barrier function have been investigated. Recently, however, the barrier function improving effect of plant-derived CERs has attracted much attention. Phyto-derived CERs (phytoCERs) are preferable due to their assumed higher safety as they are mostly isolated from dietary sources. The beneficial effects of phytoCER-based oral dietary supplements for skin hydration and skin barrier reinforcement have been indicated in several studies involving animal models as well as human subjects. Ingestible dietary supplements containing phytoCERs are also widely available on the market. Nonetheless, little effort has been made to investigate the potential cosmetic applications of topically administered phytoCERs. Therefore, summarizing the foregoing investigations and identifying the gap in the scientific data on plant-derived CERs intended for skin-health benefits are of paramount importance. In this review, an attempt is made to synthesize the information available in the literature regarding the effects of phytoCER-based oral dietary supplements on skin hydration and barrier function with the underlying mechanisms.

An integrative metabolomics and transcriptomics study to identify metabolic alterations in aged skin of humans in vivo

Background

Aging human skin undergoes significant morphological and functional changes such as wrinkle formation, reduced wound healing capacity, and altered epidermal barrier function. Besides known age-related alterations like DNA-methylation changes, metabolic adaptations have been recently linked to impaired skin function in elder humans. Understanding of these metabolic adaptations in aged skin is of special interest to devise topical treatments that potentially reverse or alleviate age-dependent skin deterioration and the occurrence of skin disorders.

Results

We investigated the global metabolic adaptions in human skin during aging with a combined transcriptomic and metabolomic approach applied to epidermal tissue samples of young and old human volunteers. Our analysis confirmed known age-dependent metabolic alterations, e.g. reduction of coenzyme Q10 levels, and also revealed novel age effects that are seemingly important for skin maintenance. Integration of donor-matched transcriptome and metabolome data highlighted transcriptionally-driven alterations of metabolism during aging such as altered activity in upper glycolysis and glycerolipid biosynthesis or decreased protein and polyamine biosynthesis. Together, we identified several age-dependent metabolic alterations that might affect cellular signaling, epidermal barrier function, and skin structure and morphology.

Conclusions

Our study provides a global resource on the metabolic adaptations and its transcriptional regulation during aging of human skin. Thus, it represents a first step towards an understanding of the impact of metabolism on impaired skin function in aged humans and therefore will potentially lead to improved treatments of age related skin disorders.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3547-3) contains supplementary material, which is available to authorized users.

Phytosphingosine, sphingosine and dihydrosphingosine ceramides in model skin lipid membranes: permeability and biophysics

Ceramides based on phytosphingosine, sphingosine and dihydrosphingosine are essential constituents of the skin lipid barrier that protects the body from excessive water loss. The roles of the individual ceramide subclasses in regulating skin permeability and the reasons for C4-hydroxylation of these sphingolipids are not completely understood. We investigated the chain length-dependent effects of dihydroceramides, sphingosine ceramides (with C4-unsaturation) and phytoceramides (with C4-hydroxyl) on the permeability, lipid organization and thermotropic behavior of model stratum corneum lipid membranes composed of ceramide/lignoceric acid/cholesterol/cholesteryl sulfate. Phytoceramides with very long C24 acyl chains increased the permeability of the model lipid membranes compared to dihydroceramides or sphingosine ceramides with the same chain lengths. Either unsaturation or C4-hydroxylation of dihydroceramides induced chain length-dependent increases in membrane permeability. Infrared spectroscopy showed that C4-hydroxylation of the sphingoid base decreased the relative ratio of orthorhombic chain packing in the membrane and lowered the miscibility of C24 phytoceramide with lignoceric acid. The phase separation in phytoceramide membranes was confirmed by X-ray diffraction. In contrast, phytoceramides formed strong hydrogen bonds and highly thermostable domains. Thus, the large heterogeneity in ceramide structures and in their aggregation mechanisms may confer resistance towards the heterogeneous external stressors that are constantly faced by the skin barrier.

Application of Topical Acids Improves Atopic Dermatitis in Murine Model by Enhancement of Skin Barrier Functions Regardless of the Origin of Acids

BACKGROUND

The acidic pH of the stratum corneum (SC) is important for epidermal permeability barrier homeostasis. Acidification of the skin surface has been suggested as a therapeutic strategy for skin disorders such as atopic dermatitis (AD).

OBJECTIVE

We performed an animal study to evaluate the usefulness of acidification of SC for inhibition of AD lesions and to find out if the therapeutic effect of vinegar is attributable to its herbal contents, rather than its acidity.

METHODS

Five groups of six oxazolone-treated (Ox)-AD mice were treated for three weeks with creams of different acidity: vehicle cream alone (pH 5.5), neutralized vinegar cream (pH 7.4), pH 5.0 vinegar cream, pH 3.5 vinegar cream, and pH 3.5 hydrogen chloride (HCl) cream. Also, we have compared two groups of Ox-AD mice treated with pH 5.5 vehicle cream or pH 5.5 vinegar cream.

RESULTS

Ox-AD mice treated with acidic creams exhibited fewer AD-like lesions, had significantly lower eczema scores, decreased basal by transepidermal water loss (TEWL), and increased SC hydration compared to the groups given only vehicle and neutral cream. There was no significant difference between the acidic vinegar and HCl groups. Between the groups treated with vehicle and pH 5.5 vinegar cream, there was no difference in eczema score, basal TEWL and SC hydration.

CONCLUSION

Application of topical acids, regardless of their source materials, inhibits the development of AD lesions by maintenance of skin surface pH and skin barrier function in murine model.

Serine biosynthesis and transport defects

l-serine is a non-essential amino acid that is biosynthesized via the enzymes phosphoglycerate dehydrogenase (PGDH), phosphoserine aminotransferase (PSAT), and phosphoserine phosphatase (PSP). Besides its role in protein synthesis, l-serine is a potent neurotrophic factor and a precursor of a number of essential compounds including phosphatidylserine, sphingomyelin, glycine, and d-serine. Serine biosynthesis defects result from impairments of PGDH, PSAT, or PSP leading to systemic serine deficiency. Serine biosynthesis defects present in a broad phenotypic spectrum that includes, at the severe end, Neu-Laxova syndrome, a lethal multiple congenital anomaly disease, intermediately, infantile serine biosynthesis defects with severe neurological manifestations and growth deficiency, and at the mild end, the childhood disease with intellectual disability. A serine transport defect resulting from deficiency of the ASCT1, the main transporter for serine in the central nervous system, has been recently described in children with neurological manifestations that overlap with those observed in serine biosynthesis defects. l-serine therapy may be beneficial in preventing or ameliorating symptoms in serine biosynthesis and transport defects, if started before neurological damage occurs. Herein, we review serine metabolism and transport, the clinical, biochemical, and molecular aspects of serine biosynthesis and transport defects, the mechanisms of these diseases, and the potential role of serine therapy.

On the phenotypic spectrum of serine biosynthesis defects

L-serine is a non-essential amino acid that is de novo synthesized via the enzymes phosphoglycerate dehydrogenase (PGDH), phosphoserine aminotransferase (PSAT), and phosphoserine phosphatase (PSP). Besides its role in protein synthesis, L-serine is a precursor of a number of important compounds. Serine biosynthesis defects result from deficiencies in PGDH, PSAT, or PSP and have a broad phenotypic spectrum ranging from Neu-Laxova syndrome, a lethal multiple congenital anomaly disease at the severe end to a childhood disease with intellectual disability at the mild end, with infantile growth deficiency, and severe neurological manifestations as an intermediate phenotype. In this report, we present three subjects with serine biosynthesis effects. The first was a stillbirth with Neu-Laxova syndrome and a homozygous mutation in PHGDH. The second was a neonate with growth deficiency, microcephaly, ichthyotic skin lesions, seizures, contractures, hypertonia, distinctive facial features, and a homozygous mutation in PSAT1. The third subject was an infant with growth deficiency, microcephaly, ichthyotic skin lesions, anemia, hypertonia, distinctive facial features, low serine and glycine in plasma and CSF, and a novel homozygous mutation in PHGDH gene. Herein, we also review previous reports of serine biosynthesis defects and mutations in the PHGDH, PSAT1, and PSPH genes, discuss the variability in the phenotypes associated with serine biosynthesis defects, and elaborate on the vital roles of serine and the potential consequences of its deficiency.

Identification of candidate genes in atopic dermatitis based on bioinformatic methods

BACKGROUND

Atopic dermatitis (AD) is a chronic or relapsing inflammatory disorder of the skin that frequently precedes asthma and allergic disorders. This study aimed to identify candidate genes related to AD using bioinformatic methods.

METHODS

The microarray data of GSE32924, including 12 nonlesional AD (ANL) and 13 lesional AD (AL) skin samples obtained from 14 patients with AD as well as eight other normal human skin samples, was downloaded from the Gene Expression Omnibus database. Differentially expressed genes (DEGs) were screened in ANL and AL skin samples compared with normal controls, followed by gene ontology (GO) and pathway enrichment analyses. Furthermore, the selected overlapping DEGs were analyzed to identify co-expressed genes, and a co-expression network was established. GeneCodis database was selected for functional annotation of the differentially co-expressed genes and a regulatory network was constructed.

RESULTS

Compared with normal controls, 438 DEGs were identified in ANL skin samples and were mainly enriched in two pathways and the GO terms associated with epidermis development. Besides, 779 DEGs were identified in AL skin samples and were mainly enriched in four pathways and GO terms associated with immune response. Stimulated by retinoic acid 13 (STRA13), presenilin enhancer gamma secretase subunit (PSENEN), and nucleosome assembly protein 1-like 2 (NAP1L2) were significantly enriched by integration analysis of the co-expression and regulatory network.

CONCLUSION

Genes STRA13, PSENEN, and NAP1L2 were presumed to play critical roles in AD and they may serve as potential targets for the treatment of AD.

The role of epidermal sphingolipids in dermatologic diseases

Sphingolipids, a group of lipids containing the sphingoid base, have both structural and biological functions in human epidermis. Ceramides, as a part of extracellular lipids in the stratum corneum, are important elements of the skin barrier and are involved in the prevention of transepidermal water loss. In addition, ceramides regulate such processes as proliferation, differentiation and apoptosis of keratinocytes. Another important sphingolipid, sphingosine-1-phosphate (S1P), inhibits proliferation and induces differentiation of keratinocytes. A recent clinical study of the efficacy and safety of ponesimod (a selective modulator of the S1P receptor 1) suggested that sphingolipid metabolism may become a new target for the pharmacological treatment of psoriasis. The role of sphingolipids in some dermatologic diseases, including psoriasis, atopic dermatitis and ichthyoses was summarized in this article.

Producing human ceramide-NS by metabolic engineering using yeast Saccharomyces cerevisiae

Ceramide is one of the most important intercellular components responsible for the barrier and moisture retention functions of the skin. Because of the risks involved with using products of animal origin and the low productivity of plants, the availability of ceramides is currently limited. In this study, we successfully developed a system that produces sphingosine-containing human ceramide-NS in the yeast Saccharomyces cerevisiae by eliminating the genes for yeast sphingolipid hydroxylases (encoded by SUR2 and SCS7) and introducing the gene for a human sphingolipid desaturase (encoded by DES1). The inactivation of the ceramidase gene YDC1, overexpression of the inositol phosphosphingolipid phospholipase C gene ISC1, and endoplasmic reticulum localization of the DES1 gene product resulted in enhanced production of ceramide-NS. The engineered yeast strains can serve as hosts not only for providing a sustainable source of ceramide-NS but also for developing further systems to produce sphingosine-containing sphingolipids.

Scalable Synthesis of Human Ultralong Chain Ceramides

Ceramides with ultralong chains (≥30 carbons), also known as acylceramides, play a critical role in the survival of mammals on dry land. An efficient and scalable synthesis of four major classes of ultralong human skin ceramides is reported. The key approach involves the use of a succinimidyl ester that acts as a protective group, helps overcome the extremely low solubility, and simultaneously activates the fatty acid for its clean and high-yielding attachment to a sphingoid base.

Circulating levels of sphingosine-1-phosphate are elevated in severe, but not mild psoriasis and are unresponsive to anti-TNF-α treatment

Sphingolipids are bioactive molecules with a putative role in inflammation. Alterations in sphingolipids, in particular ceramides, have been consistently observed in psoriatic skin. Herein, we quantified the circulating sphingolipid profile in individuals with mild or severe psoriasis as well as healthy controls. In addition, the effects of anti-TNF-α treatment were determined. Levels of sphingoid bases, including sphingosine-1-phosphate (S1P), increased in severe (P < 0.001; n = 32), but not in mild (n = 32), psoriasis relative to healthy controls (n = 32). These alterations were not reversed in severe patients (n = 16) after anti-TNF-α treatment despite significant improvement in psoriasis lesions. Circulating levels of sphingomyelins and ceramides shifted in a fatty acid chain length-dependent manner. These alterations were also observed in psoriasis skin lesions and were associated with changes in mRNA levels of ceramide synthases. The lack of S1P response to treatment may have pathobiological implications due to its close relation to the vascular and immune systems. In particular, increased levels of sphingolipids and especially S1P in severe psoriasis patients requiring biological treatment may potentially be associated with cardiovascular comorbidities. The fact that shifts in S1P levels were not ameliorated by anti-TNF-α treatment, despite improvements in the skin lesions, further supports targeting S1P receptors as therapy for severe psoriasis.

Mitochondrial and lipogenic effects of vitamin D on differentiating and proliferating human keratinocytes

Even in cells that are resistant to the differentiating effects of vitamin D, the activated vitamin D receptor (VDR) can downregulate the mitochondrial respiratory chain and sustain cell growth through enhancing the activity of biosynthetic pathways. The aim of this study was to investigate whether vitamin D is effective also in modulating mitochondria and biosynthetic metabolism of differentiating cells. We compared the effect of vitamin D on two cellular models: the primary human keratinocytes, differentiating and sensitive to the genomic action of VDR, and the human keratinocyte cell line HaCaT, characterized by a rapid growth and resistance to vitamin D. We analysed the nuclear translocation and features of VDR, the effects of vitamin D on mitochondrial transcription and the consequences on lipid biosynthetic fate. We found that the negative modulation of respiratory chain is a general mechanism of action of vitamin D, but at high doses, the HaCaT cells became resistant to mitochondrial effects by upregulating the catabolic enzyme CYP24 hydroxylase. In differentiating keratinocytes, vitamin D treatment promoted intracellular lipid deposition, likewise the inhibitor of respiratory chain stigmatellin, whereas in proliferating HaCaT, this biosynthetic pathway was not inducible by the hormone. By linking the results on respiratory chain and lipid accumulation, we conclude that vitamin D, by suppressing respiratory chain transcription in all keratinocytes, is able to support both the proliferation and the specialized metabolism of differentiating cells. Through mitochondrial control, vitamin D can have an essential role in all the metabolic phenotypes occurring in healthy and diseased skin.

Epidermal Hydration Is Improved by Enhanced Ceramide Metabolism in Aged C57BL/6J Mice After Dietary Supplementation of Royal Jelly

Epidermal hydration is maintained by the epidermal lipid barrier, of which ceramide (Cer) is the major constituent. We examined the dietary effect of royal jelly (RJ) on epidermal hydration in aged mice. Altered Cer metabolism was further determined by measuring epidermal levels of individual Cer, glucosylceramide (GC), and sphingomyelin (SM) species, and of Cer-metabolizing enzymes. Aged C57BL/6J mice were fed a control diet (group AGED) or diets with 1% RJ harvested from two different areas (groups AGED+RJ1:AGED + RJ2) for 16 weeks. Aged C57BL/6J mice with no dietary intervention (the control group: group C) represented the onset of aging. In group AGED, epidermal levels of hydration, Cer1/2/5/6/7, GC-A/B/C/D, SM1/2/3, and β-glucocerebrosidase (GCase) protein, an enzyme of GC hydrolysis for Cer generation, were lower than in group C; these levels, as well as those of Cer3/4 and acidic sphingomyelinase (aSMase) protein, an enzyme of SM hydrolysis for Cer generation, were higher in group AGED + RJ1 than in group AGED. Despite increases in GC-B, SM1/2/3, and serine palmitoyltransferase2 protein, an enzyme of de novo Cer synthesis, in group AGED + RJ2 to levels higher than in group AGED, epidermal levels of hydration, Cer1-7, GC-A/C/D, GCase, and aSMase proteins were similar in these two groups. Expression of GCase and aSMase mRNAs, and of Cer synthase3 and ceramidase proteins, enzymes of de novo Cer synthesis and degradation, did not differ among groups. Dietary RJ1 improved epidermal hydration by enhancing Cer metabolism with increased levels of all Cer, GC, and SM species, and of GCase and aSMase proteins.

Probing the role of the ceramide acyl chain length and sphingosine unsaturation in model skin barrier lipid mixtures by (2)H solid-state NMR spectroscopy

We investigated equimolar mixtures of ceramides with lignoceric acid and cholesterol as models for the human stratum corneum by differential scanning calorimetry and (2)H solid-state NMR spectroscopy. Our reference system consisted of lignoceroyl sphingosine (Cer[NS24]), which represents one of the ceramides in the human stratum corneum. Furthermore, the effect of ceramide acyl chain truncation to 16 carbons as in Cer[NS16] and the loss of the C4 trans double bond as in dihydroceramide Cer[NDS24] were studied. Fully relaxed (2)H NMR spectra were acquired for each deuterated component of each mixture separately, allowing the quantitative determination of the individual lipid phases. At skin temperature, the reference system containing Cer[NS24] is characterized by large portions of each component of the mixture in a crystalline phase, which largely restricts the permeability of the skin lipid barrier. The loss of the C4 trans double bond in Cer[NDS24] leads to the replacement of more than 25% of the crystalline phase by an isotropic phase of the dihydroceramide that shows the importance of dihydroceramide desaturation in the formation of the skin lipid barrier. The truncated Cer[NS16] is mostly found in the gel phase at skin temperature, which may explain its negative effect on the transepidermal water loss in atopic dermatitis patients. These significant alterations in the phase behavior of all lipids are further reflected at elevated temperatures. The molecular insights of our study may help us to understand the importance of the structural parameters of ceramides in healthy and compromised skin barriers.

Emerging biology of sphingosine-1-phosphate: its role in pathogenesis and therapy

Membrane sphingolipids are metabolized to sphingosine-1-phosphate (S1P), a bioactive lipid mediator that regulates many processes in vertebrate development, physiology, and pathology. Once exported out of cells by cell-specific transporters, chaperone-bound S1P is spatially compartmentalized in the circulatory system. Extracellular S1P interacts with five GPCRs that are widely expressed and transduce intracellular signals to regulate cellular behavior, such as migration, adhesion, survival, and proliferation. While many organ systems are affected, S1P signaling is essential for vascular development, neurogenesis, and lymphocyte trafficking. Recently, a pharmacological S1P receptor antagonist has won approval to control autoimmune neuroinflammation in multiple sclerosis. The availability of pharmacological tools as well as mouse genetic models has revealed several physiological actions of S1P and begun to shed light on its pathological roles. The unique mode of signaling of this lysophospholipid mediator is providing novel opportunities for therapeutic intervention, with possibilities to target not only GPCRs but also transporters, metabolic enzymes, and chaperones.

Simplifying complexity: genetically resculpting glycosphingolipid synthesis pathways in mice to reveal function

Glycosphingolipids (GSLs) are a group of plasma-membrane lipids notable for their extremely diverse glycan head groups. The metabolic pathways for GSLs, including the identity of the biosynthetic enzymes needed for synthesis of their glycans, are now well understood. Many of their cellular functions, which include plasma-membrane organization, regulation of cell signaling, endocytosis, and serving as binding sites for pathogens and endogenous receptors, have also been established. However, an understanding of their functions in vivo had been lagging. Studies employing genetic manipulations of the GSL synthesis pathways in mice have been used to systematically reduce the large numbers and complexity of GSL glycan structures, allowing the in vivo functions of GSLs to be revealed from analysis of the resulting phenotypes. Findings from these studies have produced a clearer picture of the role of GSLs in mammalian physiology, which is the topic of this review.

Skin diseases associated with the depletion of stratum corneum lipids and stratum corneum lipid substitution therapy

The skin is the largest organ of the body, whose main function is to protect the body against the loss of physiologically important components as well as harmful environmental insults. From the inside to the outside, the skin comprises three major structural layers: the hypodermis, the dermis and the epidermis. The epidermis contains four different sublayers, the stratum corneum (SC), stratum granulosum, stratum spinosum and stratum basale, where the barrier function of the skin mainly lies in the outermost layer of the epidermis, the SC. The SC contains corneocytes that are embedded in a lipid matrix existing in the form of lipid bilayers. The lipid bilayers are formed mainly from ceramides, free fatty acids and cholesterol, constitute the only continuous pathway across the SC and are responsible for the barrier function of the skin. However, the depletion or disturbance of SC lipids in the SC leads to a perturbation of the barrier function of the skin, and, conversely, several skin diseases such as psoriasis and atopic dermatitis are associated with the depletion of these SC lipids. Therefore, it is of paramount importance to understand the interrelationship between the depletion of SC lipids and skin diseases as well as factors that affect the composition and organization of SC lipids in order to assess the potential benefit of a direct replacement of the missing SC lipids as a means of treating affected, aged or diseased skin.