Differentiation-associated expression of ceramidase isoforms in cultured keratinocytes and epidermis

Tumor suppressor ACER1 correlates with prognosis and Immune Infiltration in head and neck squamous cell carcinoma

Head and neck squamous cell carcinoma (HNSCC) is notorious for poor prognoses, and effective biomarkers are urgently needed for early diagnosis of HNSCC patients. We investigate the role of alkaline ceramidase 1 (ACER1) and its relationship with immune infiltration in HNSCC. The differential expression and clinical prognostic significance of ACER1 in HNSCC patients are explored using bioinformatics methods and verified in human HNSCC samples. Genetic mutation, DNA methylation and drug sensitivity linked with ACER1 are examined. The potential biological function of ACER1 co-expression genes is assessed, and a series of functional assays are performed on ACER1in vitro. The results comprehensively reveal a relationship between ACER1 and immune infiltration in HNSCC patients. ACER1 expression is significantly downregulated in HNSCC tissues and closely correlated with better prognoses for HNSCC patients, and this prognostic significance is determined by distinct clinical characteristics. Genetic alteration and promoter hypomethylation of ACER1 are involved in progression of HNSCC, and ACER1 expression is significantly related to several drug sensitivities. Functional analysis shows that ACER1 co-expression genes are mainly enriched in the sphingolipid signaling pathway associated with inhibition of tumorigenesis, leading to better prognoses for HNSCC patients. In vitro, ACER1 overexpression inhibits proliferation and migration, induces apoptosis, and promotes adhesion of Fadu and SCC9 cells. In addition, high ACER1 expression is closely linked with infiltration levels of immune cells, and strongly associated with biomarkers of immune cells in HNSCC, suggesting the important role of ACER1 in regulating tumor immunity in HNSCC patients. In summary, ACER1 may be a useful indicator for diagnosis and prognosis, and may regulate immune infiltration in HNSCC patients, thus promising targeted immunotherapy for HNSCC.

The manganese transporter SLC39A8 links alkaline ceramidase 1 to inflammatory bowel disease

The metal ion transporter SLC39A8 is associated with physiological traits and diseases, including blood manganese (Mn) levels and inflammatory bowel diseases (IBD). The mechanisms by which SLC39A8 controls Mn homeostasis and epithelial integrity remain elusive. Here, we generate Slc39a8 intestinal epithelial cell-specific-knockout (Slc39a8-IEC KO) mice, which display markedly decreased Mn levels in blood and most organs. Radiotracer studies reveal impaired intestinal absorption of dietary Mn in Slc39a8-IEC KO mice. SLC39A8 is localized to the apical membrane and mediates 54Mn uptake in intestinal organoid monolayer cultures. Unbiased transcriptomic analysis identifies alkaline ceramidase 1 (ACER1), a key enzyme in sphingolipid metabolism, as a potential therapeutic target for SLC39A8-associated IBDs. Importantly, treatment with an ACER1 inhibitor attenuates colitis in Slc39a8-IEC KO mice by remedying barrier dysfunction. Our results highlight the essential roles of SLC39A8 in intestinal Mn absorption and epithelial integrity and offer a therapeutic target for IBD associated with impaired Mn homeostasis.

Effect of the gut microbiota on the expression of genes that are important for maintaining skin function: Analysis using aged mice

The gut microbiota changes greatly at the onset of disease, and the importance of intestinal bacteria has been highlighted. The gut microbiota also changes greatly with aging. Aging causes skin dryness, but it is not known how changes in the gut microbiota with aging affects the expression of genes that are important for maintaining skin function. In this study, we investigated how age-related changes in gut microbiota affect the expression of genes that regulate skin function. The gut microbiotas from young mice and aged mice were transplanted into germ-free mice (fecal microbiota transplantation [FMT]). These recipient mice were designated FMT-young mice and FMT-old mice respectively, and the expression levels of genes important for maintaining skin function were analyzed. The dermal water content was significantly lower in old mice than that in young mice, indicating dry skin. The gut microbiota significantly differed between old mice and young mice. The water channel aquaporin-3 (Aqp3) expression level in the skin of FMT-old mice was significantly higher than that in FMT-young mice. In addition, among the genes that play an important role in maintaining skin function, the expression levels of those encoding ceramide-degrading enzyme, ceramide synthase, hyaluronic acid-degrading enzyme, and Type I collagen were also significantly higher in FMT-old mice than in FMT-young mice. It was revealed that the gut microbiota, which changes with age, regulates the expression levels of genes related to skin function, including AQP3.

Transcriptomic landscape of early hair follicle and epidermal development

Morphogenesis of ectodermal organs, such as hair, tooth, and mammary gland, starts with the formation of local epithelial thickenings, or placodes, but it remains to be determined how distinct cell types and differentiation programs are established during ontogeny. Here, we use bulk and single-cell transcriptomics and pseudotime modeling to address these questions in developing hair follicles and epidermis and produce a comprehensive transcriptomic profile of cellular populations in the hair placode and interplacodal epithelium. We report previously unknown cell populations and marker genes, including early suprabasal and genuine interfollicular basal markers, and propose the identity of suprabasal progenitors. By uncovering four different hair placode cell populations organized in three spatially distinct areas, with fine gene expression gradients between them, we posit early biases in cell fate establishment. This work is accompanied by a readily accessible online tool to stimulate further research on skin appendages and their progenitors.

Exogenous Ceramide Serves as a Precursor to Endogenous Ceramide Synthesis and as a Modulator of Keratinocyte Differentiation

Since ceramide is a key epidermal barrier constituent and its deficiency causes barrier-compromised skin, several molecular types of ceramides are formulated in commercial topical agents to improve barrier function. Topical ceramide localizes on the skin surface and in the stratum corneum, but certain amounts of ceramide penetrate the stratum granulosum, becoming precursors to endogenous ceramide synthesis following molecular modification. Moreover, exogenous ceramide as a lipid mediator could modulate keratinocyte proliferation/differentiation. We here investigated the biological roles of exogenous NP (non-hydroxy ceramide containing 4-hydroxy dihydrosphingosine) and NDS (non-hydroxy ceramide containing dihydrosphingosine), both widely used as topical ceramide agents, in differentiated-cultured human keratinocytes. NDS, but not NP, becomes a precursor for diverse ceramide species that are required for a vital permeability barrier. Loricrin (late differentiation marker) production is increased in keratinocytes treated with both NDS and NP vs. control, while bigger increases in involucrin (an early differentiation marker) synthesis were observed in keratinocytes treated with NDS vs. NP and control. NDS increases levels of a key antimicrobial peptide (an innate immune component), cathelicidin antimicrobial peptide (CAMP/LL-37), that is upregulated by a ceramide metabolite, sphingosine-1-phosphate. Our studies demonstrate that NDS could be a multi-potent ceramide species, forming heterogenous ceramide molecules and a lipid mediator to enhance differentiation and innate immunity.

The Effect of Vitamin D3 and Silver Nanoparticles on HaCaT Cell Viability

Vitamin D3, known to regulate bone homeostasis, has recently been shown to have many pleiotropic effects in different tissues and organs due to the presence of its receptor in a wide range of cells. Our previous study demonstrated that vitamin D3 was able to increase the wound healing respect to the control sample, 24 h after cutting, without however leading to a complete repair. The aim of the study was to combine vitamin D3 with silver nanoparticles to possibly enable a faster reparative effect. The results showed that this association was capable of inducing a complete wound healing only after 18 h. Moreover, a treatment of vitamin D3 + silver nanoparticles yielded a small percentage of keratinocytes vimentin-positive, suggesting the possibility that the treatment was responsible for epithelial to mesenchymal transition of the cells, facilitating wound healing repair. Since vitamin D3 acts via sphingolipid metabolism, we studied the expression of gene encoding for the metabolic enzymes and protein level. We found an increase in neutral sphingomyelinase without involvement of neutral ceramidase or sphingosine kinase2. In support, an increase in ceramide level was identified by Ultrafast Liquid Chromatography–Tandem Mass Spectrometry, suggesting a possible involvement of ceramides in wound healing process.

Ceramides in Skin Health and Disease: An Update

Ceramides are a class of sphingolipid that is the backbone structure for all sphingolipids, such as glycosphingolipids and phosphosphingolipids. While being a minor constituent of cellular membranes, ceramides are the major lipid component (along with cholesterol, free fatty acid, and other minor components) of the intercellular spaces of stratum corneum that forms the epidermal permeability barrier. These stratum corneum ceramides consist of unique heterogenous molecular species that have only been identified in terrestrial mammals. Alterations of ceramide molecular profiles are characterized in skin diseases associated with compromised permeability barrier functions, such as atopic dermatitis, psoriasis and xerosis. In addition, hereditary abnormalities of some ichthyoses are associated with an epidermal unique ceramide species, omega-O-acylceramide. Ceramides also serve as lipid modulators to regulate cellular functions, including cell cycle arrest, differentiation, and apoptosis, and it has been demonstrated that changes in ceramide metabolism also cause certain diseases. In addition, ceramide metabolites, sphingoid bases, sphingoid base-1-phosphate and ceramide-1-phosphate are also lipid mediators that regulate cellular functions. In this review article, we describe diverse physiological and pathological roles of ceramides and their metabolites in epidermal permeability barrier function, epidermal cell proliferation and differentiation, immunity, and cutaneous diseases. Finally, we summarize the utilization of ceramides as therapy to treat cutaneous disease.

The Pathogenic and Therapeutic Implications of Ceramide Abnormalities in Atopic Dermatitis

Ceramides play an essential role in forming a permeability barrier in the skin. Atopic dermatitis (AD) is a common chronic skin disease associated with skin barrier dysfunction and immunological abnormalities. In patients with AD, the amount and composition of ceramides in the stratum corneum are altered. This suggests that ceramide abnormalities are involved in the pathogenesis of AD. The mechanism underlying lipid abnormalities in AD has not yet been fully elucidated, but the involvement of Th2 and Th1 cytokines is implicated. Ceramide-dominant emollients have beneficial effects on skin barrier function; thus, they have been approved as an adjunctive barrier repair agent for AD. This review summarizes the current understanding of the mechanisms of ceramide abnormalities in AD. Furthermore, the potential therapeutic approaches for correcting ceramide abnormalities in AD are discussed.

Lipid Metabolic Events Underlying the Formation of the Corneocyte Lipid Envelope

Cornified cells of the stratum corneum have a monolayer of an unusual lipid covalently attached to the outer surface. This is referred to as the corneocyte lipid envelope (CLE). It consists of a monolayer of ω-hydroxyceramides covalently attached to the outer surface of the cornified envelope. The CLE is essential for proper barrier function of the skin and is derived from linoleate-rich acylglucosylceramides synthesized in the viable epidermis. Biosynthesis of acylglucosylceramide and its conversion to the cornified envelope is complex. Acylglucosylceramide in the bounding membrane of the lamellar granule is the precursor of the CLE. The acylglucosylceramide in the limiting membrane of the lamellar granule may be oriented with the glucosyl moiety on the inside. Conversion of the acylglucosylceramide to the CLE requires removal of the glucose by action of a glucocerebrosidase. The ester-linked fatty acid may be removed by an as yet unidentified esterase, and the resulting ω-hydroxyceramide may become ester linked to the outer surface of the cornified envelope through action of transglutaminase 1. Prior to removal of ester-linked fatty acids, linoleate is oxidized to an epoxy alcohol through action of 2 lipoxygenases. This can be further oxidized to an epoxy-enone, which can spontaneously attach to the cornified envelope through Schiff's base formation. Mutations of genes coding for enzymes involved in biosynthesis of the CLE result in ichthyosis, often accompanied by neurologic dysfunction. The CLE is recognized as essential for barrier function of skin, but many questions about details of this essentiality remain. What are the relative roles of the 2 mechanisms of lipid attachment? What is the orientation of acylglucosylceramide in the bounding membrane of lamellar granules? Some evidence supports a role for CLE as a scaffold upon which intercellular lamellae unfold, but other evidence does not support this role. There is also controversial evidence for a role in stratum corneum cohesion. Evidence is presented to suggest that covalently bound ω-hydroxyceramides serve as a reservoir for free sphingosine that can serve in communicating with the viable epidermis and act as a potent broad-acting antimicrobial at the skin surface. Many questions remain.

Alkaline ceramidase family: The first two decades

Ceramidases are a group of enzymes that catalyze the hydrolysis of ceramide, dihydroceramide, and phytoceramide into sphingosine (SPH), dihydrosphingosine (DHS), and phytosphingosine (PHS), respectively, along with a free fatty acid. Ceramidases are classified into the acid, neutral, and alkaline ceramidase subtypes according to the pH optima for their catalytic activity. YPC1 and YDC1 were the first alkaline ceramidase genes to be identified and cloned from the yeast Saccharomyces cerevisiae two decades ago. Subsequently, alkaline ceramidase genes were identified from other species, including one Drosophila melanogaster ACER gene (Dacer), one Arabidopsis thaliana ACER gene (AtACER), three Mus musculus ACER genes (Acer1, Acer2, and Acer3), and three Homo sapiens ACER genes (ACER1, ACER2, and ACER3). The protein products of these genes constitute a large protein family, termed the alkaline ceramidase (ACER) family. All the biochemically characterized members of the ACER family are integral membrane proteins with seven transmembrane segments in the Golgi complex or endoplasmic reticulum, and they each have unique substrate specificity. An increasing number of studies suggest that the ACER family has diverse roles in regulating sphingolipid metabolism and biological processes. Here we discuss the discovery of the ACER family, the biochemical properties, structures, and catalytic mechanisms of its members, and its role in regulating sphingolipid metabolism and biological processes in yeast, insects, plants, and mammals.

Elusive Roles of the Different Ceramidases in Human Health, Pathophysiology, and Tissue Regeneration

Ceramide and sphingosine are important interconvertible sphingolipid metabolites which govern various signaling pathways related to different aspects of cell survival and senescence. The conversion of ceramide into sphingosine is mediated by ceramidases. Altogether, five human ceramidases—named acid ceramidase, neutral ceramidase, alkaline ceramidase 1, alkaline ceramidase 2, and alkaline ceramidase 3—have been identified as having maximal activities in acidic, neutral, and alkaline environments, respectively. All five ceramidases have received increased attention for their implications in various diseases, including cancer, Alzheimer’s disease, and Farber disease. Furthermore, the potential anti-inflammatory and anti-apoptotic effects of ceramidases in host cells exposed to pathogenic bacteria and viruses have also been demonstrated. While ceramidases have been a subject of study in recent decades, our knowledge of their pathophysiology remains limited. Thus, this review provides a critical evaluation and interpretive analysis of existing literature on the role of acid, neutral, and alkaline ceramidases in relation to human health and various diseases, including cancer, neurodegenerative diseases, and infectious diseases. In addition, the essential impact of ceramidases on tissue regeneration, as well as their usefulness in enzyme replacement therapy, is also discussed.

Inhibition of sphingosine 1-phosphate lyase activates human keratinocyte differentiation and attenuates psoriasis in mice

Sphingosine 1-phosphate (S1P) lyase is an intracellular enzyme that catalyzes the irreversible degradation of S1P and has been suggested as a therapeutic target for the treatment of psoriasis vulgaris. Because S1P induces differentiation of keratinocytes, we examined whether modulation of S1P lyase and altered intracellular S1P levels regulate proliferation and differentiation of human neonatal epidermal keratinocyte (HEKn) cells. To identify the physiological functions of S1P lyase in skin, we inhibited S1P lyase in HEKn cells with an S1P lyase-specific inhibitor (SLI) and with S1P lyase 1 (SGPL1)-specific siRNA (siSGPL1). In HEKn cells, pharmacological treatment with the SLI caused G1 arrest by upregulation of p21 and p27 and induced keratin 1, an early differentiation marker. Similarly, genetic suppression by siSGPL1 arrested the cell cycle at the G1 phase and activated differentiation. In addition, enzyme suppression by siSGPL1 upregulated keratin 1 and differentiation markers including involucrin and loricrin. When hyperproliferation of HEKn cells was induced by interleukin (IL)-17 and IL-22, pharmacologic inhibition of S1P lyase by SLI decreased proliferation and activated differentiation of HEKn cells simultaneously. In addition, SLI administration ameliorated imiquimod-induced psoriatic symptoms including erythema, scaling, and epidermal thickness in vivo. We thus demonstrated that S1P lyase inhibition reduces cell proliferation and induces keratinocyte differentiation, and that inhibition may attenuate psoriasiform changes. Collectively, these findings suggest that S1P lyase is a modulating factor for proliferation and differentiation, and support its potential as a therapeutic target for psoriasis in human keratinocytes.

Single-Cell Transcriptomics Reveals Spatial and Temporal Turnover of Keratinocyte Differentiation Regulators

Keratinocyte differentiation requires intricately coordinated spatiotemporal expression changes that specify epidermis structure and function. This article utilizes single-cell RNA-seq data from 22,338 human foreskin keratinocytes to reconstruct the transcriptional regulation of skin development and homeostasis genes, organizing them by differentiation stage and also into transcription factor (TF)–associated modules. We identify groups of TFs characterized by coordinate expression changes during progression from the undifferentiated basal to the differentiated state and show that these TFs also have concordant differential predicted binding enrichment in the super-enhancers previously reported to turn over between the two states. The identified TFs form a core subset of the regulators controlling gene modules essential for basal and differentiated keratinocyte functions, supporting their nomination as master coordinators of keratinocyte differentiation. Experimental depletion of the TFs ZBED2 and ETV4, both predicted to promote the basal state, induces differentiation. Furthermore, our single-cell RNA expression analysis reveals preferential expression of antioxidant genes in the basal state, suggesting keratinocytes actively suppress reactive oxygen species to maintain the undifferentiated state. Overall, our work demonstrates diverse computational methods to advance our understanding of dynamic gene regulation in development.

Endogenous acid ceramidase protects epithelial cells from Porphyromonas gingivalis-induced inflammation in vitro

Ceramidases are a group of enzymes that degrade pro-inflammatory ceramide by cleaving a fatty acid to form anti-inflammatory sphingosine lipid. Thus far, acid, neutral and alkaline ceramidase isozymes have been described. However, the expression patterns of ceramidase isoforms as well as their role in periodontal disease pathogenesis remain unknown. In this study, expression patterns of ceramidase isoforms were quantified by real-time PCR and immunohistochemistry in gingival samples of patients with periodontitis and healthy subjects, as well as in EpiGingivalTM-3D culture and OBA-9 gingival epithelial cells both of which were stimulated with or without the presence of live Porphyromonas gingivalis (ATCC 33277 strain). A significantly lower level of acid ceramidase expression was detected in gingival tissues from periodontal patients compared to those from healthy subjects. In addition, acid-ceramidase expression in EpiGingival™ 3D culture and OBA-9 cells was suppressed by stimulation with P. gingivalis in vitro. No significant fluctuation was detected for neutral or alkaline ceramidases in either gingival samples or cell cultures. Next, to elucidate the role of acid ceramidase in P. gingivalis-induced inflammation in vitro, OBA-9 cells were transduced with adenoviral vector expressing the human acid ceramidase (Ad-ASAH1) gene or control adenoviral vector (Ad-control). In response to stimulation with P. gingivalis, ASAH1-over-expressing OBA-9 cells showed significantly lower mRNA expressions of caspase-3 as well as the percentage of Annexin V-positive cells, when compared with OBA-9 cells transduced with Ad-control vector. Furthermore, in response to stimulation with P. gingivalis, ASAH1-over-expressing OBA-9 cells produced less TNF-α, IL-6, and IL1β pro-inflammatory cytokines than observed in OBA-9 cells transduced with Ad-control vector. Collectively, our data show the novel discovery of anti-inflammatory and anti-apoptotic effects of acid ceramidase in host cells exposed to periodontal bacteria, and the attenuation of the expression of host-protective acid ceramidase in periodontal lesions.

Alkaline Ceramidase 1 Protects Mice from Premature Hair Loss by Maintaining the Homeostasis of Hair Follicle Stem Cells

Ceramides and their metabolites are important for the homeostasis of the epidermis, but much remains unknown about the roles of specific pathways of ceramide metabolism in skin biology. With a mouse model deficient in the alkaline ceramidase (Acer1) gene, we demonstrate that ACER1 plays a key role in the homeostasis of the epidermis and its appendages by controlling the metabolism of ceramides. Loss of Acer1 elevated the levels of various ceramides and sphingoid bases in the skin and caused progressive hair loss in mice. Mechanistic studies revealed that loss of Acer1 widened follicular infundibulum and caused progressive loss of hair follicle stem cells (HFSCs) due to reduced survival and stemness. These results suggest that ACER1 plays a key role in maintaining the homeostasis of HFSCs, and thereby the hair follicle structure and function, by regulating the metabolism of ceramides in the epidermis.

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Acer1 is a skin-specific ceramidase that controls the catabolism of ceramides

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Acer1 plays a key role in the homeostasis of the epidermis and its appendages

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Acer1^−/− mice suffer from progressive alopecia

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Loss of Acer1 progressively depletes the population of hair follicle stem cells

Alkaline ceramidase 2 and its bioactive product sphingosine are novel regulators of the DNA damage response

Human cells respond to DNA damage by elevating sphingosine, a bioactive sphingolipid that induces programmed cell death (PCD) in response to various forms of stress, but its regulation and role in the DNA damage response remain obscure. Herein we demonstrate that DNA damage increases sphingosine levels in tumor cells by upregulating alkaline ceramidase 2 (ACER2) and that the upregulation of the ACER2/sphingosine pathway induces PCD in response to DNA damage by increasing the production of reactive oxygen species (ROS). Treatment with the DNA damaging agent doxorubicin increased both ACER2 expression and sphingosine levels in HCT116 cells in a dose-dependent manner. ACER2 overexpression increased sphingosine in HeLa cells whereas knocking down ACER2 inhibited the doxorubicin-induced increase in sphingosine in HCT116 cells, suggesting that DNA damage elevates sphingosine by upregulating ACER2. Knocking down ACER2 inhibited an increase in the apoptotic and necrotic cell population and the cleavage of poly ADP ribose polymerase (PARP) in HCT116 cells in response to doxorubicin as well as doxorubicin-induced release of lactate dehydrogenase (LDH) from these cells. Similar to treatment with doxorubicin, ACER2 overexpression induced an increase in the apoptotic and necrotic cell population and PARP cleavage in HeLa cells and LDH release from cells, suggesting that ACER2 upregulation mediates PCD in response to DNA damage through sphingosine. Mechanistic studies demonstrated that the upregulation of the ACER2/sphingosine pathway induces PCD by increasing ROS levels. Taken together, these results suggest that the ACER2/sphingosine pathway mediates PCD in response to DNA damage through ROS production.

Alkaline ceramidases: Biochemical properties, biological function, and role in liver cancer

Alkaline ceramidases (ACERs) are a class of ceramidases (CDase), and three types including ACER1, ACER2, and ACER3 have been identified. ACERs can catalyze the hydrolysis of ceramide (Cer) to generate sphingosine (SPH), and SPH is further phosphorylated to produce sphingosine-1-phosphate (S1P). Cer, SPH, and S1P are several important bioactive metabolites of sphingolipids. ACERs regulate the balance of Cer, SPH and S1P, and thus mediate cell proliferation, differentiation, survival, apoptosis, and tumor initiation and development. This article reviews the biochemical properties and biological function of ACER and its role in liver cancer.

Alkaline ceramidase 1 is essential for mammalian skin homeostasis and regulating whole-body energy expenditure

The epidermis is the outermost layer of skin that acts as a barrier to protect the body from the external environment and to control water and heat loss. This barrier function is established through the multistage differentiation of keratinocytes and the presence of bioactive sphingolipids such as ceramides, the levels of which are tightly regulated by a balance of ceramide synthase and ceramidase activities. Here we reveal the essential role of alkaline ceramidase 1 (Acer1) in the skin. Acer1-deficient (Acer1(-/-) ) mice showed elevated levels of ceramide in the skin, aberrant hair shaft cuticle formation and cyclic alopecia. We demonstrate that Acer1 is specifically expressed in differentiated interfollicular epidermis, infundibulum and sebaceous glands and consequently Acer1(-/-) mice have significant alterations in infundibulum and sebaceous gland architecture. Acer1(-/-) skin also shows perturbed hair follicle stem cell compartments. These alterations result in Acer1(-/-) mice showing increased transepidermal water loss and a hypermetabolism phenotype with associated reduction of fat content with age. We conclude that Acer1 is indispensable for mammalian skin homeostasis and whole-body energy homeostasis. © 2016 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Inhibitors of Ceramidases

The topic of ceramidases has experienced an enormous boost during the last few years. Ceramidases catalyze the degradation of ceramide to sphingosine and fatty acids. Ceramide is not only the central hub of sphingolipid biosynthesis and degradation, it is also a key molecule in sphingolipid signaling, promoting differentiation or apoptosis. Acid ceramidase inhibition sensitizes certain types of cancer to chemo- and radio-therapy and this is suggestive of a role of acid ceramidase inhibitors as chemo-sensitizers which can act synergistically with chemo-therapeutic drugs. In this review, we summarize the development of ceramide analogues as first-generation ceramidase inhibitors together with data on their activity in cells and disease models. Furthermore, we describe the recent developments that have led to highly potent second-generation ceramidase inhibitors that act at nanomolar concentrations. In the third part, various assays of ceramidases are described and their relevance for accurately measuring ceramidase activities and for the development of novel inhibitors is highlighted. Besides potential clinical implications, the recent improvements in ceramidase inhibition and assaying may help to better understand the mechanisms of ceramide biology.

Mechanisms of abnormal lamellar body secretion and the dysfunctional skin barrier in patients with atopic dermatitis

I review how diverse inherited and acquired abnormalities in epidermal structural and enzymatic proteins converge to produce defective permeability barrier function and antimicrobial defense in patients with atopic dermatitis (AD). Although best known are mutations in filaggrin (FLG), mutations in other member of the fused S-100 family of proteins (ie, hornerin [hrn] and filaggrin 2 [flg-2]); the cornified envelope precursor (ie, SPRR3); mattrin, which is encoded by TMEM79 and regulates the assembly of lamellar bodies; SPINK5, which encodes the serine protease inhibitor lymphoepithelial Kazal-type trypsin inhibitor type 1; and the fatty acid transporter fatty acid transport protein 4 have all been linked to AD. Yet these abnormalities often only predispose to AD; additional acquired stressors that further compromise barrier function, such as psychological stress, low ambient humidity, or high-pH surfactants, often are required to trigger disease. T(H)2 cytokines can also compromise barrier function by downregulating expression of multiple epidermal structural proteins, lipid synthetic enzymes, and antimicrobial peptides. All of these inherited and acquired abnormalities converge on the lamellar body secretory system, producing abnormalities in lipid composition, secretion, and/or extracellular lamellar membrane organization, as well as antimicrobial defense. Finally, I briefly review therapeutic options that address this new pathogenic paradigm.

The dietary ingredient, genistein, stimulates cathelicidin antimicrobial peptide expression through a novel S1P-dependent mechanism

We recently discovered that a signaling lipid, sphingosine-1-phosphate (S1P), generated by sphingosine kinase 1, regulates a major epidermal antimicrobial peptide's [cathelicidin antimicrobial peptide (CAMP)] expression via an NF-κB→C/EBPα-dependent pathway, independent of vitamin D receptor (VDR) in epithelial cells. Activation of estrogen receptors (ERs) by either estrogens or phytoestrogens also is known to stimulate S1P production, but it is unknown whether ER activation increases CAMP production. We investigated whether a phytoestrogen, genistein, simulates CAMP expression in keratinocytes, a model of epithelial cells, by either a S1P-dependent mechanism(s) or the alternate VDR-regulated pathway. Exogenous genistein, as well as an ER-β ligand, WAY-200070, increased CAMP mRNA and protein expression in cultured human keratinocytes, while ER-β antagonist, ICI182780, attenuated the expected genistein- and WAY-200070-induced increase in CAMP mRNA/protein expression. Genistein treatment increased acidic and alkaline ceramidase expression and cellular S1P levels in parallel with increased S1P lyase inhibition, accounting for increased CAMP production. In contrast, siRNA against VDR did not alter genistein-mediated up-regulation of CAMP. Taken together, genistein induces CAMP production via an ER-β→S1P→NF-κB→C/EBPα- rather than a VDR-dependent mechanism, illuminating a new role for estrogens in the regulation of epithelial innate immunity and pointing to potential additional benefits of dietary genistein in enhancing cutaneous antimicrobial defense.

Pseudomonas-derived ceramidase induces production of inflammatory mediators from human keratinocytes via sphingosine-1-phosphate

Ceramide is important for water retention and permeability barrier functions in the stratum corneum, and plays a key role in the pathogenesis of atopic dermatitis (AD). A Pseudomonas aeruginosa-derived neutral ceramidase (PaCDase) isolated from a patient with AD was shown to effectively degrade ceramide in the presence of Staphylococcus aureus-derived lipids or neutral detergents. However, the effect of ceramide metabolites on the functions of differentiating keratinocytes is poorly understood. We found that the ceramide metabolite sphingosine-1-phosphate (S1P) stimulated the production of inflammatory mediators such as TNF-α and IL-8 from three-dimensionally cultured human primary keratinocytes (termed "3D keratinocytes"), which form a stratum corneum. PaCDase alone did not affect TNF-α gene expression in 3D keratinocytes. In the presence of the detergent Triton X-100, which damages stratum corneum structure, PaCDase, but not heat-inactivated PaCDase or PaCDase-inactive mutant, induced the production of TNF-α, endothelin-1, and IL-8, indicating that this production was dependent on ceramidase activity. Among various ceramide metabolites, sphingosine and S1P enhanced the gene expression of TNF-α, endothelin-1, and IL-8. The PaCDase-enhanced expression of these genes was inhibited by a sphingosine kinase inhibitor and by an S1P receptor antagonist VPC 23019. The TNF-α-binding antibody infliximab suppressed the PaCDase-induced upregulation of IL-8, but not TNF-α, mRNA. PaCDase induced NF-κB p65 phosphorylation. The NF-κB inhibitor curcumin significantly inhibited PaCDase-induced expression of IL-8 and endothelin-1. VPC 23019 and infliximab inhibited PaCDase-induced NF-κB p65 phosphorylation and reduction in the protein level of the NF-κB inhibitor IκBα. Collectively, these findings suggest that (i) 3D keratinocytes produce S1P from sphingosine, which is produced through the hydrolysis of ceramide by PaCDase, (ii) S1P induces the production of TNF-α via S1P receptors, and (iii) released TNF-α stimulates the production of inflammatory mediators such as IL-8.

Formation and functions of the corneocyte lipid envelope (CLE)

Corneocytes in mammalian stratum corneum are surrounded by a monolayer of covalently bound ω-OH-ceramides that form the corneocyte (-bound) lipid envelope (CLE). We review here the structure, composition, and possible functions of this structure, with insights provided by inherited and acquired disorders of lipid metabolism. 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.

Ceramide signaling in mammalian epidermis

Ceramide, the backbone structure of all sphingolipids, as well as a minor component of cellular membranes, has a unique role in the skin, by forming the epidermal permeability barrier at the extracellular domains of the outermost layer of the skin, the stratum corneum, which is required for terrestrial mammalian survival. In contrast to the role of ceramide in forming the permeability barrier, the signaling roles of ceramide and its metabolites have not yet been recognized. Ceramide and/or its metabolites regulate proliferation, differentiation, and apoptosis in epidermal keratinocytes. Recent studies have further demonstrated that a ceramide metabolite, sphingosine-1-phosphate, modulates innate immune function. Ceramide has already been applied to therapeutic approaches for treatment of eczema associated with attenuated epidermal permeability barrier function. Pharmacological modulation of ceramide and its metabolites' signaling can also be applied to cutaneous disease prevention and therapy. The author here describes the signaling roles of ceramide and its metabolites in mammalian cells and tissues, including the epidermis. 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.

Ceramide synthesis in the epidermis

The epidermis and in particular its outermost layer the stratum corneum provides terrestrial vertebrates with a pivotal defensive barrier against water loss, xenobiotics and harmful pathogens. A vital demand for this epidermal permeability barrier is the lipid-enriched lamellar matrix that embeds the enucleated corneocytes. Ceramides are the major components of these highly ordered intercellular lamellar structures, in which linoleic acid- and protein-esterified ceramides are crucial for structuring and maintaining skin barrier integrity. In this review, we describe the fascinating diversity of epidermal ceramides including 1-O-acylceramides. We focus on epidermal ceramide biosynthesis emphasizing its metabolic and topological requirements and discuss enzymes that may be involved in α- and ω-hydroxylation. Finally, we turn to epidermal ceramide regulation, highlighting transcription factors and liposensors recently described to play crucial roles in modulating skin lipid metabolism and epidermal barrier homeostasis. 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.

Altered sphingoid base profiles predict compromised membrane structure and permeability in atopic dermatitis

BACKGROUND

Ceramide hydrolysis by ceramidase in the stratum corneum (SC) yields both sphingoid bases and free fatty acids (FFA). While FFA are key constituents of the lamellar bilayers that mediate the epidermal permeability barrier, whether sphingoid bases influence permeability barrier homeostasis remains unknown. Pertinently, alterations of lipid profile, including ceramide and ceramidase activities occur in atopic dermatitis (AD).

OBJECT

We investigated alterations in sphingoid base levels and/or profiles (sphingosine to sphinganine ratio) in the SC of normal vs. AD mice, a model that faithfully replicates human AD, and then whether altered sphingoid base levels and/or profiles influence(s) membrane stability and/or structures.

METHODS

Unilamellar vesicles (LV), incorporating the three major SC lipids (ceramides/FFA/cholesterol) and different ratios of sphingosine/sphinganine, encapsulating carboxyfluorescein, were used as the model of SC lipids. Membrane stability was measured as release of carboxyfluorescein. Thermal analysis of LV was conducted by differential scanning calorimetry (DSC).

RESULTS

LV containing AD levels of sphingosine/sphinganine (AD-LV) displayed altered membrane permeability vs. normal-LV. DSC analyses revealed decreases in orthorhombic structures that form tightly packed lamellar structures in AD-LV.

CONCLUSION

Sphingoid base composition influences lamellar membrane architecture in SC, suggesting that altered sphingoid base profiles could contribute to the barrier abnormality in AD.

Ceramides stimulate caspase-14 expression in human keratinocytes

Caspase-14 is an enzyme that is expressed predominantly in cornifying epithelia and catalyses the degradation of profilaggrin. Additionally, caspase-14 plays an important role in the terminal differentiation of keratinocytes. However, how caspase-14 expression is regulated remains largely unknown. Here we demonstrate that ceramides (C(2) -Cer and C(6) -Cer), but not other sphingolipids (C(8) -glucosylceramides, sphinganine, sphingosine-1-phosphate or ceramide-1-phosphate), increase caspase-14 expression (mRNA and protein) in cultured human keratinocytes in a dose- and time-dependent manner. Inhibitors of glucosylceramide synthase and ceramidase increase endogenous ceramide levels and also increase caspase-14 expression, indicating an important regulatory role for ceramides and suggesting that the conversion of ceramides to other metabolites is not required. The increase in caspase-14 expression induced by ceramides is first seen at 16 h and requires new protein synthesis, suggesting that the ceramide-induced increase is likely an indirect effect. Furthermore, ceramides increase caspase-14 gene expression primarily by increasing transcription. Blocking de novo synthesis of ceramides does not affect caspase-14 expression, suggesting that basal expression is not dependent on ceramide levels. These studies show that ceramides, an important structural lipid, stimulate caspase-14 expression providing a mechanism for coordinately regulating the formation of lipid lamellar membranes with the formation of corneocytes.

Cannabinoid receptors 1 and 2 oppositely regulate epidermal permeability barrier status and differentiation

Cannabinoid receptors (CBR) 1 and 2 have been implicated in keratinocyte differentiation/proliferation. How CB receptors affect epidermal permeability barrier and stratum corneum structure and function remains unclear. Permeability barrier abrogation was induced by sequential tape-stripping of the SC and assessed in both CB1R and CB2R knockout (-/-) mice in comparison with wild-type (+/+) littermates. Absence of CB1R delays permeability barrier recovery, while the latter was found to be accelerated in CB2R -/- mice. While increased lamellar body (LB) secretion is observed in CB2R -/- mice accounting for the enhanced recovery, CB1R -/- animals display strong alterations in lipid bilayer structures. Markers for epidermal differentiation (i.e. filaggrin, loricrin and involucrin) and terminal differentiation (i.e. TUNEL assay and caspase-14 activation) were respectively decreased and increased in CB1R and CB2R -/- mice. Surprisingly, CB1R agonist treatment of human cultured keratinocytes increases mRNA of p21 and cytokeratin 1 and 10 and decreases cyclin D1 but protein levels remained unchanged. Such paradox could partially be explained by the increase in non-phosphorylated-4E-BP1, an inhibitor of mRNA translation, following CB1R agonist treatment. Altogether, these observations put forward the importance and the complexity of cannabinoid signalling for the regulation of permeability barrier and epidermal differentiation.

Cellular changes that accompany shedding of human corneocytes

Corneocyte desquamation has been ascribed to either: 1) proteolytic degradation of corneodesmosomes (CD); 2) disorganization of extracellular lamellar bilayers; and/or 3) ‘swell-shrinkage-slough’ (SSS) from hydration/dehydration. To address the cellular basis for normal exfoliation, we compared changes in lamellar bilayer architecture and CD structure in DSquame® strips from the 1st vs. 5th stripping (‘outer’ vs. ‘mid’-stratum corneum [SC], respectively) from 9 normal adult forearms. Strippings were either processed for standard EM or for ruthenium (Ru-V)- or osmium-tetroxide (Os-V) vapor fixation, followed by immediate epoxy embedment, an artifact-free protocol that to our knowledge is previously unreported. CDs are largely intact in the mid-SC, but replaced by electron-dense (hydrophilic) clefts (lacunae) that expand laterally, splitting lamellar arrays in the outer SC. Some undegraded DSG1/DSC1 redistribute uniformly into corneocyte envelopes (CEs) in the outer SC (shown by proteomics, Z-stack confocal imaging and immunoEM). CEs then thicken, likely facilitating exfoliation by increasing corneocyte rigidity. In vapor-fixed images, hydration only altered the volume of the extracellular compartment, expanding lacunae further separating membrane arrays. During dehydration, air replaced water, maintaining the expanded extracellular compartment. Hydration also provoked degradation of membranes by activating contiguous acidic ceramidase activity. Together, these studies identify several parallel mechanisms that orchestrate exfoliation from the surface of normal human skin.

Analysis of ceramide metabolites in differentiating epidermal keratinocytes treated with calcium or vitamin C

Ceramides (Cer) comprise the major constituent of sphingolipids in the epidermis and are known to play diverse roles in the outermost layers of the skin including water retention and provision of a physical barrier. In addition, they can be hydrolyzed into free sphingoid bases such as C₁₈ sphingosine (SO) and C₁₈ sphinganine (SA) or can be further metabolized to C₁₈ So-1-phosphate (S1P) and C₁₈ Sa-1-phosphate (Sa1P) in keratinocytes. The significance of ceramide metabolites emerged from studies reporting altered levels of SO and SA in skin disorders and the role of S1P and Sa1P as signaling lipids. However, the overall metabolism of sphingoid bases and their phosphates during keratinocyte differentiation remains not fully understood. Therefore, in this study, we analyzed these Cer metabolites in the process of keratinocyte differentiation. Three distinct keratinocyte differentiation stages were prepared using 0.07 mM calcium (Ca²⁺) (proliferation stage), 1.2 mM Ca²⁺ (early differentiation stage) in serum-free medium, or serum-containing medium with vitamin C (50 µL/mL) (late differentiation stage). Serum-containing medium was also used to determine whether vitamin C increases the concentrations of sphingoid bases and their phosphates. The production of sphingoid bases and their phosphates after hydrolysis by alkaline phosphatase was determined using high-performance liquid chromatography. Compared to cells treated with 0.07 mM Ca²⁺, levels of SO, SA, S1P, and SA1P were not altered after treatment with 1.2 mM Ca²⁺. However, in keratinocytes cultured in serum-containing medium with vitamin C, levels of SO, SA, S1P, and SA1P were dramatically higher than those in 0.07- and 1.2-mM Ca²⁺-treated cells; however, compared to serum-containing medium alone, vitamin C did not significantly enhance their production. Taken together, we demonstrate that late differentiation induced by vitamin C and serum was accompanied by dramatic increases in the concentration of sphingoid bases and their phosphates, although vitamin C alone had no effect on their production.

Cholinergic regulation of keratinocyte innate immunity and permeability barrier integrity: new perspectives in epidermal immunity and disease

Several cutaneous inflammatory diseases and their clinical phenotypes are recapitulated in animal models of skin disease. However, the identification of shared pathways for disease progression is limited by the ability to delineate the complex biochemical processes fundamental for development of the disease. Identifying common signaling pathways that contribute to cutaneous inflammation and immune function will facilitate better scientific and therapeutic strategies to span a variety of inflammatory skin diseases. Aberrant antimicrobial peptide (AMP) expression and activity is one mechanism behind the development and severity of several inflammatory skin diseases and directly influences the susceptibility of skin to microbial infections. Our studies have recently exposed a newly identified pathway for negative regulation of AMPs in the skin by the cholinergic anti-inflammatory pathway via acetylcholine (ACh). The role of ACh in AMP regulation of immune and permeability barrier function in keratinocytes is reviewed, and the importance for a better comprehension of cutaneous disease progression by cholinergic signaling is discussed.

Regulation of cathelicidin antimicrobial peptide expression by an endoplasmic reticulum (ER) stress signaling, vitamin D receptor-independent pathway

Vitamin D receptor (VDR)-dependent mechanisms regulate human cathelicidin antimicrobial peptide (CAMP)/LL-37 in various cell types, but CAMP expression also increases after external perturbations (such as infection, injuries, UV irradiation, and permeability barrier disruption) in parallel with induction of endoplasmic reticulum (ER) stress. We demonstrate that CAMP mRNA and protein expression increase in epithelial cells (human primary keratinocytes, HaCaT keratinocytes, and HeLa cells), but not in myeloid (U937 and HL-60) cells, following ER stress generated by two mechanistically different, pharmacological stressors, thapsigargin or tunicamycin. The mechanism for increased CAMP following exposure to ER stress involves NF-κB activation leading to CCAAT/enhancer-binding protein α (C/EBPα) activation via MAP kinase-mediated phosphorylation. Furthermore, both increased CAMP secretion and its proteolytic processing to LL-37 are required for antimicrobial activities occur following ER stress. In addition, topical thapsigargin also increases production of the murine homologue of CAMP in mouse epidermis. Finally and paradoxically, ER stress instead suppresses the 1,25(OH)(2) vitamin D(3)-induced activation of VDR, but blockade of VDR activity does not alter ER stress-induced CAMP up-regulation. Hence, ER stress increases CAMP expression via NF-κB-C/EBPα activation, independent of VDR, illuminating a novel VDR-independent role for ER stress in stimulating innate immunity.

Transcriptional regulation of the human neutral ceramidase gene

Ceramidases play a critical role in generating sphingosine-1-phosphate by hydrolyzing ceramide into sphingosine, a substrate for sphingosine kinase. In order to elucidate its transcriptional regulation, we identify here a putative promoter region in the 5'-UTR of the human neutral CDase (nCDase) gene. Using human genomic DNA, we cloned a 3000 bp region upstream of the translational start site of the nCDase gene. Luciferase reporter analyses demonstrated that this 3000 bp region had promoter activity, with the strongest induction occurring within the first 200 bp. Computational analysis revealed the 200 bp essential promoter region contained several well-characterized promoter elements, lacked a conical TATA box, but did contain a reverse oriented CCAAT box, a feature common to housekeeping genes. Electrophoretic mobility shift assays demonstrated that the identified candidate transcriptional response elements (TRE) bind their respective transcription factors, including NF-Y, AP-2, Oct-1, and GATA. Mutagenic analyses of the TRE revealed that these sites regulated promoter activity and mutating an individual site decreased promoter reporter activity by up to 50%. Together, our findings suggest that regulation of nCDase expression involves coordinated TATA-less transcriptional activity.

A hypothesis concerning a potential involvement of ceramide in apoptosis and acantholysis induced by pemphigus autoantibodies

Autoimmune diseases affect more than 50 million Americans, resulting in significant healthcare costs. Most autoimmune diseases occur sporadically; however, endemic pemphigus foliaceus (EPF) is an autoimmune skin disease localized to specific geographic loci. EPF, and the related diseases pemphigus vulgaris (PV) and pemphigus foliaceus (PF), are characterized by skin lesions and autoantibodies to molecules found on epidermal keratinocytes. A variant of EPF in patients from El Bagre, Colombia, South America, has recently been reported to be distinct from previously described loci in Brazil and Tunisia epidemiologically and immunologically. As in PF and EPF, El Bagre EPF patients exhibit autoantibodies towards desmoglein-1, a cell adhesion molecule critical for maintaining epidermal integrity. An association of El Bagre EPF with sun exposure has been detected, and ultraviolet irradiation also exacerbates symptoms in PV, PF and EPF. Our hypothesis is that: (1) the autoantibodies generate pathology through an alteration in ceramide metabolism in targeted keratinocytes, resulting in apoptosis and/or cell death and acantholysis, but only when the cell's ability to metabolize ceramide is exceeded, and (2) apoptosis in response to this altered ceramide metabolism is initiated and/or exacerbated by other agents that increase ceramide levels, such as cytokines, ultraviolet irradiation, and senescence.

Hydrolytic pathway protects against ceramide-induced apoptosis in keratinocytes exposed to UVB

Although ceramides (Cers) are key constituents of the epidermal permeability barrier, they also function as apoptogenic signals for UVB irradiation-induced apoptosis in epidermal keratinocytes. As epidermis is continuously exposed to UV irradiation, we hypothesized that Cer hydrolysis protects keratinocytes from UVB-induced apoptosis by attenuating Cer levels. Both low-dose UVB (L-UVB) (< 35 mJ cm(-2)) and high-dose UVB (H-UVB) (> or = 45 mJ cm(-2)) irradiation inhibited DNA synthesis in cultured human keratinocytes, but apoptosis occurred only after H-UVB. Whereas Cer production increased after both L- and H-UVB, it normalized only in L-UVB-exposed keratinocytes, but remained elevated after H-UVB. Both acidic ceramidase (aCDase) and neutral ceramidase (nCDase) activities declined after L- and H-UVB, but returned to normal only in L-UVB cells, with decreased CDase activities or mRNA or protein levels being sustained in H-UVB cells. Inhibition of CDase using either a CDase inhibitor, N-oleoylethanolamine, or small interfering RNA (siRNA) (either to a- and/or n-CDase(s)) sensitized keratinocytes to L-UVB-induced apoptosis in parallel with further Cer accumulation. Blockade of sphingosine kinase 1 (SPHK1) (but not SPHK2) by siRNA also increased apoptosis in L-UVB keratinocytes, revealing that conversion of sphingosine to sphingosine-1-phosphate (S1P) further protects keratinocytes from UVB-induced cell death. Thus, Cer → sphingosine → S1Pmetabolic conversion protects against UVB-induced, Cer-mediated apoptosis in keratinocytes, but excessive UVB overwhelms this mechanism, thereby leading to keratinocyte apoptosis.

Control of metabolism and signaling of simple bioactive sphingolipids: Implications in disease

Simple bioactive sphingolipids include ceramide, sphingosine and their phosphorylated forms sphingosine 1-phosphate and ceramide 1-phosphate. These molecules are crucial regulators of cell functions. In particular, they play important roles in the regulation of angiogenesis, apoptosis, cell proliferation, differentiation, migration, and inflammation. Decoding the mechanisms by which these cellular functions are regulated requires detailed understanding of the signaling pathways that are implicated in these processes. Most importantly, the development of inhibitors of the enzymes involved in their metabolism may be crucial for establishing new therapeutic strategies for treatment of disease.

Ceramide stimulates ABCA12 expression via peroxisome proliferator-activated receptor {delta} in human keratinocytes

ABCA12 (ATP binding cassette transporter, family 12) is a cellular membrane transporter that facilitates the delivery of glucosylceramides to epidermal lamellar bodies in keratinocytes, a process that is critical for permeability barrier formation. Following secretion of lamellar bodies into the stratum corneum, glucosylceramides are metabolized to ceramides, which comprise approximately 50% of the lipid in stratum corneum. Gene mutations of ABCA12 underlie harlequin ichthyosis, a devastating skin disorder characterized by abnormal lamellar bodies and a severe barrier abnormality. Recently we reported that peroxisome proliferator-activated receptor (PPAR) and liver X receptor activators increase ABCA12 expression in human keratinocytes. Here we demonstrate that ceramide (C(2)-Cer and C(6)-Cer), but not C(8)-glucosylceramides, sphingosine, or ceramide 1-phosphate, increases ABCA12 mRNA expression in a dose- and time-dependent manner. Inhibitors of glucosylceramide synthase, sphingomyelin synthase, and ceramidase and small interfering RNA knockdown of human alkaline ceramidase, which all increase endogenous ceramide levels, also increased ABCA12 mRNA levels. Moreover, simultaneous treatment with C(6)-Cer and each of these same inhibitors additively increased ABCA12 expression, indicating that ceramide is an important inducer of ABCA12 expression and that the conversion of ceramide to other sphingolipids or metabolites is not required. Finally, both exogenous and endogenous ceramides preferentially stimulate PPARdelta expression (but not other PPARs or liver X receptors), whereas PPARdelta knockdown by siRNA transfection specifically diminished the ceramide-induced increase in ABCA12 mRNA levels, indicating that PPARdelta is a mediator of the ceramide effect. Together, these results show that ceramide, an important lipid component of epidermis, up-regulates ABCA12 expression via the PPARdelta-mediated signaling pathway, providing a substrate-driven, feed-forward mechanism for regulating this key lipid transporter.

Niemann-Pick disease type C1 is a sphingosine storage disease that causes deregulation of lysosomal calcium

Niemann-Pick type C1 (NPC1) disease is a neurodegenerative lysosomal storage disorder caused by mutations in the acidic compartment (which we define as the late endosome and the lysosome) protein, NPC1. The function of NPC1 is unknown, but when it is dysfunctional, sphingosine, glycosphingolipids, sphingomyelin and cholesterol accumulate. We have found that NPC1-mutant cells have a large reduction in the acidic compartment calcium store compared to wild-type cells. Chelating luminal endocytic calcium in normal cells with high-affinity Rhod-dextran induced an NPC disease cellular phenotype. In a drug-induced NPC disease cellular model, sphingosine storage in the acidic compartment led to calcium depletion in these organelles, which then resulted in cholesterol, sphingomyelin and glycosphingolipid storage in these compartments. Sphingosine storage is therefore an initiating factor in NPC1 disease pathogenesis that causes altered calcium homeostasis, leading to the secondary storage of sphingolipids and cholesterol. This unique calcium phenotype represents a new target for therapeutic intervention, as elevation of cytosolic calcium with curcumin normalized NPC1 disease cellular phenotypes and prolonged survival of the NPC1 mouse.

Ceramidases: regulators of cellular responses mediated by ceramide, sphingosine, and sphingosine-1-phosphate

Ceramidases catalyze hydrolysis of ceramides to generate sphingosine (SPH), which is phosphorylated to form sphingosine-1-phosphate (S1P). Ceramide, SPH, and S1P are bioactive lipids that mediate cell proliferation, differentiation, apoptosis, adhesion, and migration. Presently, 5 human ceramidases encoded by 5 distinct genes have been cloned: acid ceramidase (AC), neutral ceramidase (NC), alkaline ceramidase 1 (ACER1), alkaline ceramidase 2 (ACER2), and alkaline ceramidase 3 (ACER3). Each human ceramidase has a mouse counterpart. AC, NC, and ACER1-3 have maximal activities in acidic, neutral, and alkaline environments, respectively. ACER1-3 have similar protein sequences but no homology to AC and NC. AC and NC also have distinct protein sequences. The human AC (hAC) was implicated in Farber disease, and hAC may be important for cell survival. The mouse AC (mAC) is needed for early embryo survival. NC is protective against inflammatory cytokines, and the mouse NC (mNC) is required for the catabolism of ceramides in the digestive tract. ACER1 is critical in mediating cell differentiation by controlling the generation of SPH and S1P and that ACER2's role in cell proliferation and survival depends on its expression or the cell type in which it is found. Here, we discuss the role of each ceramidase in regulating cellular responses mediated by ceramides, SPH, and S1P.

Decreased ceramide transport protein (CERT) function alters sphingomyelin production following UVB irradiation

Increased cellular ceramide accounts in part for UVB irradiation-induced apoptosis in cultured human keratinocytes with concurrent increased glucosylceramide but not sphingomyelin generation in these cells. Given that conversion of ceramide to non-apoptotic metabolites such as sphingomyelin and glucosylceramide protects cells from ceramide-induced apoptosis, we hypothesized that failed up-regulation of sphingomyelin generation contributes to ceramide accumulation following UVB irradiation. Because both sphingomyelin synthase and glucosylceramide synthase activities were significantly decreased in UVB-irradiated keratinocytes, we investigated whether alteration(s) in the function of ceramide transport protein (or CERT) required for sphingomyelin synthesis occur(s) in UVB-irradiated cells. Fluorescently labeled N-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-pentanoyl)-d-erythro-sphingosine (C(5)-DMB-ceramide) relocation to the Golgi was diminished after irradiation, consistent with decreased CERT function, whereas the CERT inhibitor N-(3-hydroxy-1-hydroxymethyl-3-phenylpropyl)dodecanamide (1R,3R isomer) (HPA-12) produced an equivalent effect. UVB irradiation also induced the rapid formation of a stable CERT homotrimer complex in keratinocytes as determined by Western immunoblot and mass spectrometry analyses, a finding replicated in HeLa, HEK293T, and HaCaT cells and in murine epidermis. Ceramide binding activity was decreased in recombinant CERT proteins containing the UVB-induced homotrimer. The middle region domain of the CERT protein was required for the homotrimer formation, whereas neither the pleckstrin homology (Golgi-binding) nor the START (ceramide-binding) domains were involved. Finally like UVB-treated keratinocytes, HPA-12 blockade of CERT function increased keratinocyte apoptosis, decreased sphingomyelin synthesis, and led to accumulation of ceramide. Thus, UVB-induced CERT homotrimer formation accounts, at least in part, for apoptosis and failed up-regulation of sphingomyelin synthesis following UVB irradiation, revealing that inactive CERT can attenuate a key metabolic protective mechanism against ceramide-induced apoptosis in keratinocytes.

Epidermal ceramidase activity regulates epidermal desquamation via stratum corneum acidification

The acidic pH of the outer surface of the mammalian skin plays several important roles in the epidermal barrier function. The 2 endogenous pathways that are currently known to elicit this acidic pH are the generation of free fatty acids from phospholipids and the exchange of protons for sodium ions by non-energy-dependent sodium-proton exchangers. In this study, we propose a third endogenous pathway, i.e. epidermal ceramidase activity, generating free fatty acids from ceramides. By topical application of N-oleylethanolamine, a well-known ceramidase inhibitor, we could demonstrate a significant increase in the stratum corneum pH and a corresponding decrease in the epidermal free fatty acid content. Moreover, we could show that the resulting change in the apparent skin pH also provoked a delay in early barrier recovery and an increased epidermal desquamation, corresponding to earlier observations made for the already known endogenous mechanisms.

The skin barrier as an innate immune element

Since life in a terrestrial environment threatens mammals continuously with desiccation, the structural, cellular, biochemical, and regulatory mechanisms that sustain permeability barrier homeostasis have justifiably comprised a major thrust of prior and recent research on epidermal barrier function. Yet, the epidermis mediates a broad set of protective 'barrier' functions that includes defense against pathogen challenges. Permeability and antimicrobial function are both co-regulated and interdependent, overlapping through the dual activities of their lipid/protein constituents. Most of the defensive (barrier) functions of the epidermis localize to the stratum corneum (SC), which limits pathogen colonization through its low water content, acidic pH, resident (normal) microflora, and surface-deposited antimicrobial lipids (1 degree free fatty acid). These various barrier functions are largely mediated by either the corneocyte or the extracellular matrix, and it is both the localization and the organization of secreted hydrophobic lipids into characteristic lamellar bilayers that is critical not only for permeability barrier function, but also for antimicrobial function through its contribution to the maintenance of SC integrity. Low constitutive levels of antimicrobial peptides under basal conditions emphasize the key role of epithelial structure in antimicrobial defense. But antimicrobial peptide synthesis and delivery to the SC interstices accelerates after external insults to the barrier.

Upregulation of the human alkaline ceramidase 1 and acid ceramidase mediates calcium-induced differentiation of epidermal keratinocytes

Extracellular calcium (Ca2+(o)) potently induces the growth arrest and differentiation of human epidermal keratinocytes (HEKs). We report that Ca2+(o) markedly upregulates the human alkaline ceramidase 1 (haCER1) in HEKs; and its upregulation mediates the Ca2+(o)-induced growth arrest and differentiation of HEKs. haCER1 is the human ortholog of mouse alkaline ceramidase 1 that we previously identified. haCER1 catalyzed the hydrolysis of very long-chain ceramides to generate sphingosine (SPH). This in vitro activity required Ca2+. Ectopic expression of haCER1 in HEKs decreased the levels of D-e-C(24:1)-ceramide and D-e-C(24:0)-ceramide but elevated the levels of both SPH and its phosphate (S1P), whereas RNA interference-mediated knockdown of haCER1 caused the opposite effects on the levels of these sphingolipids in HEKs. Similar to haCER1 overexpression, Ca2+(o) increased the levels of SPH and S1P, and this was attenuated by haCER1 knockdown. haCER1 knockdown also inhibited the Ca2+(o)-induced growth arrest of HEKs and the Ca2+(o)-induced expression of keratin 1 and involucrin in HEKs. In addition, the acid ceramidase (AC) was also upregulated by Ca2+(o); and its knockdown attenuated the Ca2+(o)-induced expression of keratin 1 and involucrin in HEKs. These results strongly suggest that upregulation of haCER1 and AC mediates the Ca2+(o)-induced growth arrest and differentiation of HEKs by generating SPH and S1P.

Kinetic Characteristics of Acidic and Alkaline Ceramidase in Human Epidermis

It has recently become evident that at least five ceramidase (CDase) isoforms are present in human epidermis, and that specifically acidic CDase (aCDase) and alkaline CDase (alkCDase) activities increase during keratinocyte differentiation, and thus might play a pivotal role(s) in permeability barrier function. Prior to investigating their possible roles in the epidermal barrier function, it is necessary to characterize basic kinetic parameters for these enzymes, as well as to determine the effects of the established CDase inhibitors and their activities. In this study, assays for both aCDase and alkCDase activities in fully differentiated human epidermis were optimized using a radiolabeled substrate. These studies revealed that aCDase activity is substantially higher than alkCDase activity, and that both isoenzymes are inhibited by a CDase inhibitor N-oleylethanolamine. These findings were also confirmed using an in situ enzyme assay.

Fatty acid 2-hydroxylase, encoded by FA2H, accounts for differentiation-associated increase in 2-OH ceramides during keratinocyte differentiation

Ceramides in mammalian stratum corneum comprise a heterogeneous mixture of molecular species that subserve the epidermal permeability barrier, an essential function for survival in a terrestrial environment. In addition to a variation of sphingol species, hydroxylation of the amide-linked fatty acids contributes to the diversity of epidermal ceramides. Fatty acid 2-hydroxylase, encoded by the gene FA2H, the mammalian homologue of FAH1 in yeast, catalyzes the synthesis of 2-hydroxy fatty acid-containing sphingolipids. We assessed here whether FA2H accounts for 2-hydroxyceramide/2-hydroxyglucosylceramide synthesis in epidermis. Reverse transcription-PCR and Western immunoblots demonstrated that FA2H is expressed in cultured human keratinocytes and human epidermis, with FA2H expression and fatty acid 2-hydroxylase activity increased with differentiation. FA2H-siRNA suppressed 2-hydroxylase activity and decreased 2-hydroxyceramide/2-hydroxyglucosylceramide levels, demonstrating that FA2H accounts for synthesis of these sphingolipids in keratinocytes. Whereas FA2H expression and 2-hydroxy free fatty acid production increased early in keratinocyte differentiation, production of 2-hydroxyceramides/2-hydroxyglucosylceramides with longer chain amide-linked fatty acids (> or =C24) increased later. Keratinocytes transduced with FA2H-siRNA contained abnormal epidermal lamellar bodies and did not form the normal extracellular lamellar membranes required for the epidermal permeability barrier. These results reveal that 1) differentiation-dependent up-regulation of ceramide synthesis and fatty acid elongation is accompanied by up-regulation of FA2H; 2) 2-hydroxylation of fatty acid by FA2H occurs prior to generation of ceramides/glucosylceramides; and 3) 2-hydroxyceramides/2-hydroxyglucosylceramides are required for epidermal lamellar membrane formation. Thus, late differentiation-linked increases in FA2H expression are essential for epidermal permeability barrier homeostasis.

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.