Nitric oxide regulates synthesis of gene products involved in keratinocyte differentiation and ceramide metabolism

L-Arginine Enhances Oral Keratinocyte Proliferation under High-Glucose Conditions via Upregulation of CYP1A1, SKP2, and SRSF5

High glucose inhibits oral keratinocyte proliferation. Diabetes can lead to delayed oral wound healing and periodontal disease. L-Arginine, one of the most versatile amino acids, plays an important role in wound healing, organ maturation, and development. In this study, L-Arginine was found to enhance oral keratinocyte proliferation under high-glucose conditions. RNA sequencing analysis discovered a significant number of genes differentially upregulated following L-Arginine treatment under high-glucose conditions. Cytochrome P450 family 1 subfamily A member 1 (CYP1A1) was the most significantly upregulated gene at 24 and 48 h after L-Arginine treatment. Gene Ontology enrichment analysis found that cell proliferation- and mitosis-related biological processes, such as mitotic nuclear division, mRNA processing, and positive regulation of cell cycle processes, were significantly upregulated. Pathway enrichment analysis found that S-phase kinase-associated protein 2 (SKP2) and serine- and arginine-rich splicing factor 5 (SRSF5) were the top upregulated genes in cell cycle and spliceosome pathways, respectively. Indirect immunofluorescent cytochemistry confirmed increased protein levels of CYP1A1, SKP2, and SRSF5 after L-Arginine treatment. Knockdown of CYP1A1, SKP2, and SRSF5 abolished the enhanced proliferative effect of L-Arginine on oral keratinocytes under high-glucose conditions. In conclusion, L-Arginine enhances oral keratinocyte proliferation under high-glucose conditions via upregulation of CYP1A1, SKP2, and SRSF5, suggesting that supplemental L-Arginine in oral care products may be beneficial for oral tissue repair and regeneration.

ZFP750 affects the cutaneous barrier through regulating lipid metabolism

An essential function of the epidermis is to provide a physical barrier that prevents the loss of water. Essential mediators of this barrier function include ceramides, cholesterol, and very long chain fatty acids, and their alteration causes human pathologies, including psoriasis and atopic dermatitis. A frameshift mutation in the human ZNF750 gene, which encodes a zinc finger transcription factor, has been shown to cause a seborrhea-like dermatitis. Here, we show that genetic deletion of the mouse homolog ZFP750 results in loss of epidermal barrier function, which is associated with a substantial reduction of ceramides, nonpolar lipids. The alteration of epidermal lipid homeostasis is directly linked to the transcriptional activity of ZFP750. ZFP750 directly and/or indirectly regulates the expression of crucial enzymes primarily involved in the biosynthesis of ceramides. Overall, our study identifies the transcription factor ZFP750 as a master regulator epidermal homeostasis through lipid biosynthesis and thus contributing to our understanding of the pathogenesis of several human skin diseases.

Role of nitric oxide in regulating epidermal permeability barrier function

Nitric oxide (NO), a free radical molecule synthesized by nitric oxide synthases (NOS), regulates multiple cellular functions in a variety of cell types. These NOS, including endothelial NOS (eNOS), inducible NOS (iNOS) and neural NOS (nNOS), are expressed in keratinocytes. Expression levels of both iNOS and nNOS decrease with ageing, and insufficient NO has been linked to the development of a number of disorders such as diabetes and hypertension, and to the severity of atherosclerosis. Conversely, excessive NO levels can induce cellular oxidative stress, but physiological levels of NO are required to maintain the normal functioning of cells, including keratinocytes. NO also regulates cutaneous functions, including epidermal permeability barrier homeostasis and wound healing, through its stimulation of keratinocyte proliferation, differentiation and lipid metabolism. Topical applications of a diverse group of agents which generate nitric oxide (called NO donors) such as S-nitroso-N-acetyl-D,L-penicillamine (SNAP) can delay permeability barrier recovery in barrier-disrupted skin, but iNOS is still required for epidermal permeability barrier homeostasis. This review summarizes the regulatory role that NO plays in epidermal permeability barrier functions and the underlying mechanisms involved.

3D model of harlequin ichthyosis reveals inflammatory therapeutic targets

The biology of harlequin ichthyosis (HI), a devastating skin disorder caused by loss-of-function mutations in the gene ABCA12, is poorly understood, and to date, no satisfactory treatment has been developed. We sought to investigate pathomechanisms of HI that could lead to the identification of new treatments for improving patients' quality of life. In this study, RNA-Seq and functional assays were performed to define the effects of loss of ABCA12 using HI patient skin samples and an engineered CRISPR/Cas9 ABCA12 KO cell line. The HI living skin equivalent (3D model) recapitulated the HI skin phenotype. The cytokines IL-36α and IL-36γ were upregulated in HI skin, whereas the innate immune inhibitor IL-37 was strongly downregulated. We also identified STAT1 and its downstream target inducible nitric oxide synthase (NOS2) as being upregulated in the in vitro HI 3D model and HI patient skin samples. Inhibition of NOS2 using the inhibitor 1400W or the JAK inhibitor tofacitinib dramatically improved the in vitro HI phenotype by restoring the lipid barrier in the HI 3D model. Our study has identified dysregulated pathways in HI skin that are feasible therapeutic targets.

Regulation of de novo sphingolipid biosynthesis by the ORMDL proteins and sphingosine kinase-1

Sphingolipids are a diverse set of structurally and metabolically related lipids that have numerous functions in cell structure and signaling. The regulation of these lipids is critical for normal cell function and disregulation has been implicated in pathophysiological conditions such as cancer and inflammation. Here we examine control of the initiating, and rate limiting, enzyme in sphingolipid biosynthesis, serine palmitoyltransferase (SPT). We find that de novo synthesis of sphingolipid is stimulated by a number of cancer chemotherapeutics, suggesting that this may be an important aspect of their cytotoxic effects. The three ORMDL proteins are membrane proteins of the endoplasmic reticulum related to the yeast Orm proteins, which have been shown to be homeostatic regulators of SPT. We find that the ORMDL proteins are also negative regulators of SPT that transmit cellular levels of sphingolipids to SPT. The three isoforms have redundant functions in this system. The sphingosine kinases (sphingosine kinase-1 and -2) phosphorylate both sphingosine, which is released from ceramide, but also dihydrosphingosine, which is in the de novo biosynthetic pathway. We therefore examined the role of the sphingosine kinases in controlling de novo ceramide biosynthesis and find that sphingosine kinase-1 does indeed act as a negative regulator of this pathway. This establishes that sphingosine kinase, in addition to producing sphingosine-1-phosphate as a signaling molecule, also consumes dihydrosphingosine to regulate ceramide synthesis. Our studies demonstrate that there are multiple mechanisms of regulation of SPT and suggest that these regulators are important mediators of cell stress responses.

The role of sphingolipid metabolism in cutaneous permeability barrier formation

The epidermal permeability barrier of mammalian skin is localized in the stratum corneum. Corneocytes are embedded in an extracellular, highly ordered lipid matrix of hydrophobic lipids consisting of about 50% ceramides, 25% cholesterol and 15% long and very long chain fatty acids. The most important lipids for the epidermal barrier are ceramides. The scaffold of the lipid matrix is built of acylceramides, containing ω-hydroxylated very long chain fatty acids, acylated at the ω-position with linoleic acid. After glucosylation of the acylceramides at Golgi membranes and secretion, the linoleic acid residues are replaced by glutamate residues originating from proteins exposed on the surface of corneocytes. Removal of their glucosyl residues generates a hydrophobic surface on the corneocytes used as a template for the formation of extracellular lipid layers of the water permeability barrier. Misregulation or defects in the formation of extracellular ceramide structures disturb barrier function. Important anabolic steps are the synthesis of ultra long chain fatty acids, their ω-hydroxylation, and formation of ultra long chain ceramides and glucosylceramides. The main probarrier precursor lipids, glucosylceramides and sphingomyelins, are packed in lamellar bodies together with hydrolytic enzymes such as glucosylceramide-β-glucosidase and acid sphingomyelinase and secreted into the intercelullar space between the stratum corneum and stratum granulosum. Inherited defects in the extracellular hydrolytic processing of the probarrier acylglucosylceramides impair epidermal barrier formation and cause fatal diseases: such as prosaposin deficiency resulting in lack of lysosomal lipid binding and transfer proteins, or the symptomatic clinical picture of the "collodion baby" in the absence of glucocerebrosidase. 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.

Off-target function of the Sonic hedgehog inhibitor cyclopamine in mediating apoptosis via nitric oxide-dependent neutral sphingomyelinase 2/ceramide induction

Sonic hedgehog (SHh) signaling is important in the pathogenesis of various human cancers, such as medulloblastomas, and it has been identified as a valid target for anticancer therapeutics. The SHh inhibitor cyclopamine induces apoptosis. The bioactive sphingolipid ceramide mediates cell death in response to various chemotherapeutic agents; however, ceramide's roles/mechanisms in cyclopamine-induced apoptosis are unknown. Here, we report that cyclopamine mediates ceramide generation selectively via induction of neutral sphingomyelin phosphodiesterase 3, SMPD3 (nSMase2) in Daoy human medulloblastoma cells. Importantly, short interfering RNA-mediated knockdown of nSMase2 prevented cyclopamine-induced ceramide generation and protected Daoy cells from drug-induced apoptosis. Accordingly, ectopic wild-type N-SMase2 caused cell death, compared with controls, which express the catalytically inactive N-SMase2 mutant. Interestingly, knockdown of smoothened (Smo), a target protein for cyclopamine, or Gli1, a downstream signaling transcription factor of Smo, did not affect nSMase2. Mechanistically, our data showed that cyclopamine induced nSMase2 and cell death selectively via increased nitric oxide (NO) generation by neuronal-nitric oxide synthase (n-NOS) induction, in Daoy medulloblastoma, and multiple other human cancer cell lines. Knockdown of n-NOS prevented nSMase2 induction and cell death in response to cyclopamine. Accordingly, N-SMase2 activity-deficient skin fibroblasts isolated from homozygous fro/fro (fragilitas ossium) mice exhibited resistance to NO-induced cell death. Thus, our data suggest a novel off-target function of cyclopamine in inducing apoptosis, at least in part, by n-NOS/NO-dependent induction of N-SMase2/ceramide axis, independent of Smo/Gli inhibition.

Dihydroceramide desaturase inhibition by a cyclopropanated dihydroceramide analog in cultured keratinocytes

Most mammalian sphingolipids contain a 4,5-(E)-double bond. We report on the chemical synthesis of a dihydroceramide derivative that prevents the introduction of the double bond into sphingolipids. Minimal alteration of the parent structure by formally replacing the hydrogen atoms in the 5- and in the 6-position of the sphinganine backbone by a methylene group leads to an inhibitor of dihydroceramide desaturase in cultured cells. In the presence of 10–50 μM of compound (1), levels of biosynthetically formed dihydroceramide and—surprisingly—also of phytoceramide are elevated at the expense of ceramide. The cells respond to the lack of unsaturated sphingolipids by an elevation of mRNAs of enzymes required for sphingosine formation. At the same time, the analysis of proliferation and differentiation markers indicates that the sphingolipid double bond is required to keep the cells in a differentiated state.

Structure and functions of keratin proteins in simple, stratified, keratinized and cornified epithelia

Historically, the term 'keratin' stood for all of the proteins extracted from skin modifications, such as horns, claws and hooves. Subsequently, it was realized that this keratin is actually a mixture of keratins, keratin filament-associated proteins and other proteins, such as enzymes. Keratins were then defined as certain filament-forming proteins with specific physicochemical properties and extracted from the cornified layer of the epidermis, whereas those filament-forming proteins that were extracted from the living layers of the epidermis were grouped as 'prekeratins' or 'cytokeratins'. Currently, the term 'keratin' covers all intermediate filament-forming proteins with specific physicochemical properties and produced in any vertebrate epithelia. Similarly, the nomenclature of epithelia as cornified, keratinized or non-keratinized is based historically on the notion that only the epidermis of skin modifications such as horns, claws and hooves is cornified, that the non-modified epidermis is a keratinized stratified epithelium, and that all other stratified and non-stratified epithelia are non-keratinized epithelia. At this point in time, the concepts of keratins and of keratinized or cornified epithelia need clarification and revision concerning the structure and function of keratin and keratin filaments in various epithelia of different species, as well as of keratin genes and their modifications, in view of recent research, such as the sequencing of keratin proteins and their genes, cell culture, transfection of epithelial cells, immunohistochemistry and immunoblotting. Recently, new functions of keratins and keratin filaments in cell signaling and intracellular vesicle transport have been discovered. It is currently understood that all stratified epithelia are keratinized and that some of these keratinized stratified epithelia cornify by forming a Stratum corneum. The processes of keratinization and cornification in skin modifications are different especially with respect to the keratins that are produced. Future research in keratins will provide a better understanding of the processes of keratinization and cornification of stratified epithelia, including those of skin modifications, of the adaptability of epithelia in general, of skin diseases, and of the changes in structure and function of epithelia in the course of evolution. This review focuses on keratins and keratin filaments in mammalian tissue but keratins in the tissues of some other vertebrates are also considered.

Optimization of submerged keratinocyte cultures for the synthesis of barrier ceramides

Epidermal differentiation results in the formation of the extracellular lipid barrier in the stratum corneum, which mainly consists of ceramides, free fatty acids, and cholesterol. Differentiating keratinocytes of the stratum granulosum synthesize a series of complex long-chain ceramides and glucosylceramides with different chain lengths and hydroxylation patterns at intracellular membranes of the secretory pathway. Formation of complex extracellular ceramides parallels the transition of keratinocytes from the stratum granulosum to the stratum corneum, where their precursors, complex glucosylceramides and sphingomyelin, are secreted and exposed to extracellular lysosomal lipid hydrolases. Submerged cultures used so far showed a reduced ceramide content compared to the native epidermis or the air-exposed, organotypic culture system. In order to investigate the sphingolipid metabolism during keratinocyte differentiation, we optimized a simple cell culture system to generate the major barrier sphingolipids. This optimized model is based on the chemically well-defined serum-free MCDB medium. At low calcium ion concentrations (0.1mM), keratinocytes proliferate and synthesize mainly Cer(NS) and a small amount of Cer(NP). Supplementation of the MCDB cell culture medium with calcium ions (1.1mM) and 10 microM linoleic acid triggered differentiation of keratinocytes and synthesis of a complex pattern of free and covalently bound ceramides as found in native epidermis or air-exposed organotypic cultures, though at a reduced level. The mRNA levels of the differentiation markers keratin 10 and profilaggrin increased, as well as those of ceramide glucosyltransferase and glucosylceramide-beta-glucosidase. The described culture system was thus suitable for biochemical studies of the sphingolipid metabolism during keratinocyte differentiation. The addition of serum or vitamin A to the medium resulted in a decrease in ceramide and glucosylceramide content. Lowering the medium pH to 6, while maintained cell viability, led to an increase in the processing of probarrier lipids glucosylceramide and sphingomyelin to free ceramides and protein-bound ceramide Cer(OS).

Topical Application of Neuronal Nitric Oxide Synthase Inhibitor Accelerates Cutaneous Barrier Recovery and Prevents Epidermal Hyperplasia Induced by Barrier Disruption

The effect of nitric oxide (NO) on skin barrier recovery rate was evaluated in hairless mouse. Topical application of an NO synthase (NOS) inhibitor and a neuronal nitric oxide synthase (nNOS) inhibitor accelerated the barrier recovery after tape stripping, whereas application of an inducible NOS (iNOS) inhibitor had no effect. After tape stripping, the barrier recovery in nNOS-/- mice was significantly faster than in wild type. Topical application of the NO donor S-nitroso-N-acetyl-D,L-penicillamine (SNAP) delayed the barrier recovery in hairless mice. Immediately after barrier disruption on skin organ culture, NO release from the skin was significantly increased. The increase was blocked by nNOS inhibitor, but not by iNOS inhibitor. Topical application of the guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) accelerated the barrier recovery, whereas SIN-1 chloride, a guanylyl cyclase activator, delayed the barrier recovery. In cultured human keratinocytes, SNAP increased the intracellular calcium concentration. The increase was blocked by ODQ, but not by the calcium channel-blocker nifedipine. In calcium-free medium, SNAP increased the intracellular calcium concentration. Topical application of both nNOS inhibitor and ODQ also reduced the epidermal hyperplasia induced by barrier disruption under low environmental humidity. These results suggest that NO plays an important signaling role in cutaneous barrier homeostasis and in epidermal hyperplasia induced by barrier disruption.

Inducible nitric oxide has protective effect on fumonisin B1 hepatotoxicity in mice via modulation of sphingosine kinase

Fumonisin B(1), a mycotoxin, is an inhibitor of ceramide synthase causing marked dysregulation of sphingolipid metabolism in cells. This mycotoxin causes accumulation of free sphingoid bases (sphingosine and dihydrosphingosine or sphinganine) and their metabolites, important messengers involved in signal transduction leading to either cell survival or death. Free sphingoid bases are known apoptotic molecules whereas sphingosine 1-phosphate is protective. We previously reported that fumonisin B(1) caused sphingosine kinase (SPHK) induction along with the increase of serine palmitoyltransferase (SPT). Fumonisin B(1) also increased inducible nitric oxide synthase (iNOS) expression. In the current study we employed a mouse strain with the targeted deletion of iNOS gene (Nos-KO) to evaluate the role of nitric oxide (NO) on fumonisin B(1)-induced hepatotoxicity. The Nos-KO mice exhibited increased hepatotoxicity after subacute fumonisin B(1) exposure compared to their wild type counterparts, the liver regeneration was lower in Nos-KO compared to that in the WT mice. Increased hepatotoxicity in Nos-KO was not related to the extent of free sphingoid base accumulation after fumonisin B(1) treatment; however, it was accompanied by a lack of fumonisin B(1)-induced SPHK induction. The fumonisin B(1)-induced SPT was unaffected by lack of iNOS gene. Deletion of iNOS gene did not prevent fumonisin B(1)-dependent induction of inflammatory cytokines, namely tumor necrosis factor alpha, interferon gamma and interleukin-12. The lack of fumonisin B(1)-induced SPHK induction in Nos-KO was supported by a similar effect on phosphorylated metabolites of sphingoid bases; the equilibrium between sphingoid bases and their phosphates is maintained by SPHK. We therefore conclude that iNOS induction produced by fumonisin B(1) modulates SPHK activity; the lack of iNOS prevents generation of sphingosine 1-phosphate and deprives cells from its protective effects.