Regulation Of Epidermal Differentiation Research Articles

The Adhesion G-Protein-Coupled Receptor GPR115/ADGRF4 Regulates Epidermal Differentiation and Associates with Cytoskeletal KRT1.

Among the 33 human adhesion G-protein-coupled receptors (aGPCRs), a unique subfamily of GPCRs, only ADGRF4, encoding GPR115, shows an obvious skin-dominated transcriptomic profile, but its expression and function in skin is largely unknown. Here, we report that GPR115 is present in a small subset of basal and in most suprabasal, noncornified keratinocytes of the stratified epidermis, supporting epidermal transcriptomic data. In psoriatic skin, characterized by hyperproliferation and delayed differentiation, the expression of GPR115 and KRT1/10, the fundamental suprabasal keratin dimer, is delayed. The deletion of ADGRF4 in HaCaT keratinocytes grown in an organotypic mode abrogates KRT1 and reduces keratinocyte stratification, indicating a role of GPR115 in epidermal differentiation. Unexpectedly, endogenous GPR115, which is not glycosylated and is likely not proteolytically processed, localizes intracellularly along KRT1/10-positive keratin filaments in a regular pattern. Our data demonstrate a hitherto unknown function of GPR115 in the regulation of epidermal differentiation and KRT1.

Loss-of-function variants in KLF4 underlie autosomal dominant palmoplantar keratoderma

PurposePalmoplantar keratodermas (PPKs) form a group of disorders characterized by thickening of palm and sole skin. Over the past 2 decades, many types of inherited PPKs have been found to result from abnormal expression, processing, or function of adhesion proteins. MethodsWe used exome and direct sequencing to detect causative pathogenic variants. Functional analysis of these variants was conducted using reverse transcription quantitative polymerase chain reaction, immunofluorescence confocal microscopy, immunoblotting, a promoter reporter assay, and chromatin immunoprecipitation. ResultsWe identified 2 heterozygous variants (c.1226A>G and c.633_634dupGT) in KLF4 in 3 individuals from 2 different unrelated families affected by a dominant form of PPK. Immunofluorescence staining for a number of functional markers revealed reduced epidermal DSG1 expression in patients harboring heterozygous KLF4 variants. Accordingly, human keratinocytes either transfected with constructs expressing these variants or downregulated for KLF4 displayed reduced DSG1 expression, which in turn has previously been found to be associated with PPK. A chromatin immunoprecipitation assay confirmed direct binding of KLF4 to the DSG1 promoter region. The ability of mutant KLF4 to transactivate the DSG1 promoter was significantly decreased when compared with wild-type KLF4. ConclusionLoss-of-function variants in KLF4 cause a novel form of dominant PPK and show its importance in the regulation of epidermal differentiation.

KLF3 Mediates Epidermal Differentiation through the Epigenomic Writer CBP.

SummaryImpairments in the differentiation process can lead to skin diseases that can afflict ∼20% of the population. Thus, it is of utmost importance to understand the factors that promote the differentiation process. Here we identify the transcription factor KLF3 as a regulator of epidermal differentiation. Knockdown of KLF3 results in reduced differentiation gene expression and increased cell cycle gene expression. Over half of KLF3's genomic binding sites occur at active enhancers. KLF3 binds to active enhancers proximal to differentiation genes that are dependent upon KLF3 for expression. KLF3's genomic binding sites also highly overlaps with CBP, a histone acetyltransferase necessary for activating enhancers. Depletion of KLF3 causes reduced CBP localization at enhancers proximal to differentiation gene clusters, which leads to loss of enhancer activation but not priming. Our results suggest that KLF3 is necessary to recruit CBP to activate enhancers and drive epidermal differentiation gene expression.

Rab25 Deficiency Perturbs Epidermal Differentiation and Skin Barrier Function in Mice.

Rab25, a member of the Rab11 small GTPase family, is central to achieving cellular polarity in epithelial tissues. Rab25 is highly expressed in epithelial cells of various tissues including breast, vagina, cervix, the gastrointestinal tract, and skin. Rab25 plays key roles in tumorigenesis, mainly by regulating epithelial differentiation and proliferation. However, its role in skin physiology is relatively unknown. In this study, we demonstrated that Rab25 knock-out (KO) mice show a skin barrier dysfunction with high trans-epidermal water loss and low cutaneous hydration. To examine this observation, we investigated the histology and epidermal differentiation markers of the skin in Rab25 KO mice. Rab25 KO increased cell proliferation at the basal layer of epidermis, whereas the supra-basal layer remained unaffected. Ceramide, which is a critical lipid component for skin barrier function, was not altered by Rab25 KO in its distribution or amount, as determined by immunohistochemistry. Notably, levels of epidermal differentiation markers, including loricrin, involucrin, and keratins (5, 14, 1, and 10) increased prominently in Rab25 KO mice. In line with this, depletion of Rab25 with single hairpin RNA increased the expression of differentiation markers in a human keratinocyte cell line, HaCaT. Transcriptomic analysis of the skin revealed increased expression of genes associated with skin development, epidermal development, and keratinocyte differentiation in Rab25 KO mice. Collectively, these results suggested that Rab25 is involved in the regulation of epidermal differentiation and proliferation.

261 Nucleocytoplasmic shuttling of iRHOM2 and its regulation of epidermal differentiation

261 Nucleocytoplasmic shuttling of iRHOM2 and its regulation of epidermal differentiation

157 Uncharacterized protein REDI is required for epidermal differentiation and suppression of cancer invasion

Cutaneous squamous cell carcinoma (SCC) often demonstrates loss of epidermal differentiation that is regulated by an ever-increasing number of genes. REDI (Regulator of Epidermal Differentiation and Invasion) is a previously uncharacterized protein that we show controls skin differentiation and functions as a tumor suppressor in SCC. We identified REDI in an RNAseq-based screen for uncharacterized open reading frames with opposite patterns of expression in human SCC and keratinocyte differentiation. REDI is necessary and sufficient for epidermal differentiation; CRISPR/Cas9-mediated depletion of REDI in organotypic human epidermal tissue attenuates differentiation, while enforcing REDI expression enhances it. Proximity-dependent biotin identification was performed in organotypic skin tissue to distill REDI’s interactors and highlighted calcyclin binding protein (CACYBP), a protein not previously implicated in epidermal differentiation. Depletion of CACYBP phenocopies REDI loss and reduces differentiation in human skin tissue. Whole transcriptome analysis confirms that REDI and CACYBP co-control a significant subset of genes involved in keratinization, skin differentiation and development. Given REDI downregulation in SCC and its deletion in 21 TCGA cancer types, we evaluated the impact of REDI gain- and loss-of-function in organotypic human skin tissue programmed to transform into invasive epidermal neoplasia. REDI deletion enhances the extent and depth of invasion of Ras-transformed keratinocytes across the basement membrane, while enforced expression of REDI profoundly blocks early tumor progression. In each case, expression of REDI and the proto-oncogene c-RET are negatively correlated. Efforts are underway to confirm whether pharmacologic inhibition of c-RET offsets the impacts of REDI deletion on neoplastic invasion. Drug repurposing to identify a REDI mimic highlights HDAC inhibitors and inhibitors of PI3K/Akt as potential therapeutic candidates.

The Type IV Collagenase Inhibitor, BiPS, Alters Keratin Expression in Sulfur Mustard Exposed Mouse Skin

Sulfur mustard (SM) is a potent chemical vesicant targeting many tissues including skin. The SM induced cutaneous toxicity is associated with edema, epidermal erosions, inflammation, and prolonged wound repair. Earlier studies have shown that overexpression of matrix metalloproteinase 9 (MMP9) degrades extracellular matrix proteins and leads to the separation of epidermis from dermis in skin exposed to SM. We have found that MMP9, a type IV collagenase, disrupts collagen IV (Col IV) a major component of the basement membrane (BM) zone in SM induced blisters. Col IV in the BM network has recently been reported to be a regulator of epidermal differentiation and keratin expression during wound repair. The type IV collagen (Col IV) BM network may regulate epidermal differentiation and keratin expression during development and wound repair. In this present study, we evaluated the MMP9 inhibitor BiPS [N‐hydroxy‐3‐phenyl‐2‐(4‐phenylbenzenesulfonamido) propanamide] in an epidermal wound healing model following SM exposure. Time course (24, 72, and 168 hrs post‐SM exposure) and compound evaluation studies (BiPS as a single pretreatment) were performed in a mouse ear vesicant model (MEVM) using histopathological changes, degree of hyperplasia and edema, and expression of type IV collagen, MMP9, keratin 10 (K10, differentiation marker), and keratin 6 (K6, wound‐inducible marker) for comparison. MMP9 mRNA and protein expression increased with time. Dual immunofluorescence studies using Col IV α1 and MMP9 antibodies showed BM zone breakdown of Col IV and apparent MMP9 expression in the epidermis and the adjacent dermal matrix of the SM injured skin. Dual immunofluorescent studies using K6 and K10 showed strong K6 and weak K10 expression in the hyperplastic epidermis. In the compound evaluation study, the BiPS pretreatment group demonstrated significant reduction in edema with 25% reduction of epidermal thickness, and improvement in overall tissue structure by 72 hr post‐SM exposure. mRNA expression of MMP9 was significantly downregulated in the BiPS treated group by 168 hr post exposure. Immunofluorescent studies showed that significant reduction of MMP9 expression in the skin and a continuous expression pattern of Col IV; similar to the unexposed control skin. Dual immunofluorescent studies show reduced K6 and increased K10 expression in hyperplastic epidermis when compared to SM treated samples. These results suggest that BiPS treatment may effectively improve BM integrity by the inhibition of MMP9 in SM exposed skin. This may in turn modulate the epidermal keratin expression and promote keratinocyte differentiation for wound repair. Targeting MMP9 for the restoration of BM may be an effective strategy for countering SM‐induced cutaneous injury.Support or Funding InformationGrant funding supported by ES005022, T32ES007148, and NIAMS U54AR055073.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

697 Epidermal tissue differentiation requires the uncharacterized protein RED1

The human genome encodes >4,000 uncharacterized open reading frames (ORFs) with undiscovered biological functions. To identify potentially novel mediators of epidermal differentiation from this candidate pool, RNA-Seq of primary human keratinocytes during calcium-induced differentiation was used to screen for uncharacterized ORFs whose transcript levels are increased during this process. The top up-regulated ORF encodes a 50 kD protein that we have termed Regulator of Epidermal Differentiation (RED) 1. An orthogonal screen also identified RED1 as highly down-regulated in primary human squamous cell carcinomas compared to patient-matched normal skin. As malignant neoplasms frequently demonstrate loss of differentiation, we hypothesized that RED1 is required for epidermal differentiation. To test this possibility, we undertook gain- and loss-of-function experiments in organotypic human epidermal tissue. Enforced expression of RED1 accelerated differentiation in regenerated human skin while CRISPR/Cas9-mediated depletion attenuated this process. In each case, whole transcriptome profiling to assess the global impact of altering RED1 expression identified gene groups that control skin differentiation, development, and function. Proximity-dependent biotin ligation assays in organotypic human epidermal tissue identified validated RED1 interactors including envoplakin (EVPL) and periplakin (PPL), genes known to be essential to skin differentiation, as well as genes not previously implicated in skin homeostasis, such as calcyclin binding protein (CACYBP). Gene set enrichment analysis suggests RED1 and its interacting proteins co-control distinct subsets of genes that induce epidermal differentiation and drive neoplastic transformation. Efforts are underway to study RED1s impacts on epidermal neoplasia. RED1 therefore represents a new regulator of epidermal differentiation with potential roles in skin carcinogenesis.

Commensal microbiota modulate gene expression in the skin

BackgroundThe skin harbors complex communities of resident microorganisms, yet little is known of their physiological roles and the molecular mechanisms that mediate cutaneous host-microbe interactions. Here, we profiled skin transcriptomes of mice reared in the presence and absence of microbiota to elucidate the range of pathways and functions modulated in the skin by the microbiota.ResultsA total of 2820 genes were differentially regulated in response to microbial colonization and were enriched in gene ontology (GO) terms related to the host-immune response and epidermal differentiation. Innate immune response genes and genes involved in cytokine activity were generally upregulated in response to microbiota and included genes encoding toll-like receptors, antimicrobial peptides, the complement cascade, and genes involved in IL-1 family cytokine signaling and homing of T cells. Our results also reveal a role for the microbiota in modulating epidermal differentiation and development, with differential expression of genes in the epidermal differentiation complex (EDC). Genes with correlated co-expression patterns were enriched in binding sites for the transcription factors Klf4, AP-1, and SP-1, all implicated as regulators of epidermal differentiation. Finally, we identified transcriptional signatures of microbial regulation common to both the skin and the gastrointestinal tract.ConclusionsWith this foundational approach, we establish a critical resource for understanding the genome-wide implications of microbially mediated gene expression in the skin and emphasize prospective ways in which the microbiome contributes to skin health and disease.

Image-based transcription factor siRNA screen reveals novel regulators of epidermal differentiation

Image-based transcription factor siRNA screen reveals novel regulators of epidermal differentiation

PDK1 Is a Regulator of Epidermal Differentiation that Activates and Organizes Asymmetric Cell Division.

Asymmetric cell division (ACD) in a perpendicular orientation promotes cell differentiation and organizes the stratified epithelium. However, the upstream cues regulating ACD have not been identified. Here, we report that phosphoinositide-dependent kinase 1 (PDK1) plays a critical role in establishing ACD in the epithelium. Production of phosphatidyl inositol triphosphate (PIP3) is localized to the apical side of basal cells. Asymmetric recruitment of atypical protein kinase C (aPKC) and partitioning defective (PAR) 3 is impaired in PDK1 conditional knockout (CKO) epidermis. PDK1(CKO) keratinocytes do not undergo calcium-induced activation of aPKC or IGF1-induced activation of AKT and fail to differentiate. PDK1(CKO) epidermis shows decreased expression of Notch, a downstream effector of ACD, and restoration of Notch rescues defective expression of differentiation-induced Notch targets invitro. Wetherefore propose that PDK1 signaling regulatesthe basal-to-suprabasal switch in developing epidermis by acting as both an activator and organizer of ACD and the Notch-dependent differentiation program.

Skin Metabolite, Farnesyl Pyrophosphate, Regulates Epidermal Response to Inflammation, Oxidative Stress, and Migration

Skin produces cholesterol and a wide array of sterols and non-sterol mevalonate metabolites, including isoprenoid derivative farnesyl pyrophosphate (FPP). To characterize FPP action in epidermis, we generated transcriptional profiles of primary human keratinocytes treated with zaragozic acid (ZGA), a squalene synthase inhibitor that blocks conversion of FPP to squalene resulting in endogenous accumulation of FPP. The elevated levels of intracellular FPP resulted in regulation of epidermal differentiation and adherens junction signaling, insulin growth factor (IGF) signaling, oxidative stress response and interferon (IFN) signaling. Immunosuppressive properties of FPP were evidenced by STAT-1 downregulation and prominent suppression of its nuclear translocation by IFNγ. Furthermore, FPP profoundly downregulated genes involved in epidermal differentiation of keratinocytes in vitro and in human skin ex vivo. Elevated levels of FPP resulted in induction of cytoprotective transcriptional factor Nrf2 and its target genes. We have previously shown that FPP functions as ligand for the glucocorticoid receptor (GR), one of the major regulator of epidermal homeostasis. Comparative microarray analyses show significant but not complete overlap between FPP and glucocorticoid regulated genes, suggesting that FPP may have wider transcriptional impact. This was further supported by co-transfection and chromatin immunoprecipitation experiments where we show that upon binding to GR, FPP recruits β-catenin and, unlike glucocorticoids, recruits co-repressor GRIP1 to suppress keratin 6 gene. These findings have many clinical implications related to epidermal lipid metabolism, response to glucocorticoid therapy as well as pleiotropic effects of cholesterol lowering therapeutics, statins. J. Cell. Physiol. 231: 2452-2463, 2016. © 2016 Wiley Periodicals, Inc.

Notch1 Signaling Regulates Wound Healing via Changing the Characteristics of Epidermal Stem Cells

During wound healing and reconstruction, epidermal stem cells (ESCs) migrate to the wound site and activate to repair the damaged epithelium. Moreover, there exist complicate signaling pathways to regulate wound regeneration including Notch signaling. The Notch signaling pathway is a regulator of epidermal differentiation, which may be an important mediator of wound regeneration that participates in various processes, from the development of the dermis to the formation of skin appendages. Here, we show that Notch signaling pathways are upregulated by Jagged1 in ESCs and stem cell characteristics of ESCs change when Notch1 signaling varies. By administration of siRNAJagged1 knockdown ESCs in wounds, we observe that the suppression of Jagged1 down regulate expression of Notch signaling and resulted in poor-quality wound healing. Connecting Notch1 pathways activity to ESCs response to wound repairing may develop a new therapeutic strategy for delayed healing.

Highly Rapid and Efficient Conversion of Human Fibroblasts to Keratinocyte-Like Cells

Cell fate commitment during development is achieved through the expression of lineage specific transcription factors. Recent studies have suggested that expression of combinations of these lineage specific transcription factors can convert adult somatic cells from one type to another. Here we report that the combination of p63, a master regulator of epidermal development and differentiation, and KLF4, a regulator of epidermal differentiation is sufficient to convert dermal fibroblasts to a keratinocyte phenotype. Induced keratinocytes expressed keratinocyte specific proteins and had a transcriptome similar to keratinocytes. Reprogramming to a keratinocyte phenotype was rapid and efficient with a vast majority of cells morphologically resembling and expressing keratinocyte specific genes within a week of p63 and KLF4 transduction. Furthermore, p63 and KLF4 are capable of inducing a keratinocyte phenotype even in a cancerous cell line highlighting their importance for epidermal specification. The robustness of the conversion process also allows the use of this as a model system to study the mechanisms of reprogramming.

The Retinoid-Related Orphan Receptor RORα Promotes Keratinocyte Differentiation via FOXN1

RORα is a retinoid-related orphan nuclear receptor that regulates inflammation, lipid metabolism, and cellular differentiation of several non-epithelial tissues. In spite of its high expression in skin epithelium, its functions in this tissue remain unclear. Using gain- and loss-of-function approaches to alter RORα gene expression in human keratinocytes (HKCs), we have found that this transcription factor functions as a regulator of epidermal differentiation. Among the 4 RORα isoforms, RORα4 is prominently expressed by keratinocytes in a manner that increases with differentiation. In contrast, RORα levels are significantly lower in skin squamous cell carcinoma tumors (SCCs) and cell lines. Increasing the levels of RORα4 in HKCs enhanced the expression of structural proteins associated with early and late differentiation, as well as genes involved in lipid barrier formation. Gene silencing of RORα impaired the ability of keratinocytes to differentiate in an in vivo epidermal cyst model. The pro-differentiation function of RORα is mediated at least in part by FOXN1, a well-known pro-differentiation transcription factor that we establish as a novel direct target of RORα in keratinocytes. Our results point to RORα as a novel node in the keratinocyte differentiation network and further suggest that the identification of RORα ligands may prove useful for treating skin disorders that are associated with abnormal keratinocyte differentiation, including cancer.

CYLD regulates keratinocyte differentiation and skin cancer progression in humans

CYLD is a gene mutated in familial cylindromatosis and related diseases, leading to the development of skin appendages tumors. Although the deubiquitinase CYLD is a skin tumor suppressor, its role in skin physiology is unknown. Using skin organotypic cultures as experimental model to mimic human skin, we have found that CYLD acts as a regulator of epidermal differentiation in humans through the JNK signaling pathway. We have determined the requirement of CYLD for the maintenance of epidermal polarity, keratinocyte differentiation and apoptosis. We show that CYLD overexpression increases keratinocyte differentiation while CYLD loss of function impairs epidermal differentiation. In addition, we describe the important role of CYLD in the control of human non-melanoma skin cancer progression. Our results show the reversion of the malignancy of human squamous cell carcinomas that express increased levels of CYLD, while its functional inhibition enhances the aggressiveness of these tumors which progress toward spindle cell carcinomas. We have found that the mechanisms through which CYLD regulates skin cancer progression include the control of tumor differentiation, angiogenesis and cell survival. These findings of the role of CYLD in human skin cancer prognosis make our results relevant from a therapeutic point of view, and open new avenues for exploring novel cancer therapies.

Hoxb13 Up-regulates Transglutaminase Activity and Drives Terminal Differentiation in an Epidermal Organotypic Model

Hox genes act to differentiate and pattern embryonic structures by promoting the proliferation of specific cell types. An exception is Hoxb13, which functions as a proapoptotic and antiproliferative protein during development of the caudal spinal cord and tail vertebrae and has also been implicated in adult cutaneous wound repair. The adult epidermis, which expresses several Hox genes including Hoxb13, is continually renewed in a program of growth arrest, differentiation, and a specialized form of apoptosis (cornification). Yet little is known about the function(s) of these genes in skin. Based on its role during embryogenesis, Hoxb13 is an attractive candidate to be involved in the regulation of epidermal differentiation. Here, we demonstrate that Hoxb13 overexpression in an adult organotypic epidermal model recapitulates actions of Hoxb13 reported in embryonic development. Epidermal cell proliferation is decreased, apoptosis increased, and excessive terminal differentiation observed, as characterized by enhanced transglutaminase activity and excessive cornified envelope formation. Overexpression of Hoxb13 also produces abnormal phenotypes in the epidermal tissue that resemble certain pathological features of dysplastic skin diseases. Our results suggest that Hoxb13 functions to promote epidermal differentiation, a critical process for skin regeneration and for the maintenance of normal barrier function.

Peroxisome proliferator-activated receptor and farnesoid X receptor ligands differentially regulate sebaceous differentiation in human sebaceous gland organ cultures in vitro

Nuclear hormone receptors are important in the regulation of epidermal differentiation and have been implicated in lipid metabolism. In particular, there is evidence suggesting that the activation of peroxisome proliferator-activated receptors (PPARs) is an important factor in the regulation of sebocyte lipogenesis. To determine the role of PPARs, farnesoid X receptor (FXR) and other orphan nuclear hormone receptors in sebaceous gland function in vitro by investigating the biochemical effects of appropriate ligands, and by establishing the RNA and protein expression patterns of a number of nuclear receptors in sebaceous glands ex vivo. Human chest sebaceous glands were maintained in vitro as freshly isolated and as 7-day cultured whole organs. We then studied the effects of appropriate ligands on the glandular rates of lipogenesis and DNA synthesis, as well as determining the mRNA (reverse transcription-polymerase chain reaction) and protein expression patterns (immunohistochemistry/immunoblotting) of the nuclear hormone receptors of interest. PPAR ligands, but not FXR ligands, inhibited sebaceous lipogenesis, in particular the PPARalpha ligands LY 171883 and WY 14643, and the PPARgamma ligands BRL 49653 and 15-deoxy-Delta-12,14-prostaglandin J(2). We detected RNA expression of PPARalpha, PPARbeta, PPARgamma, retinoid X receptor alpha, liver X receptor alpha (LXRalpha) and pregnane X receptor but not FXR in freshly isolated and 7-day maintained sebaceous glands. PPARalpha, PPARbeta, PPARgamma and LXRalpha protein were detected in nuclear extracts of sebaceous glands. We conclude that activation of nuclear hormone receptors, in particular activation of PPARalpha and PPARgamma, can regulate lipogenesis in human sebaceous glands. As suppression of sebum secretion is associated with reduced acne activity, the nuclear hormone receptors involved may open new avenues in the development of novel acne treatments.

The Novel Murine Ca2+-binding Protein, Scarf, Is Differentially Expressed during Epidermal Differentiation

Calcium (Ca2+) signaling-dependent systems, such as the epidermal differentiation process, must effectively respond to variations in Ca2+ concentration. Members of the Ca2+-binding proteins play a central function in the transduction of Ca2+ signals, exerting their roles through a Ca2+-dependent interaction with their target proteins, spatially and temporally. By performing a suppression subtractive hybridization screen we identified a novel mouse gene, Scarf (skin calmodulin-related factor), which has homology to calmodulin (CaM)-like Ca2+-binding protein genes and is exclusively expressed in differentiating keratinocytes in the epidermis. The Scarf open reading frame encodes a 148-amino acid protein that contains four conserved EF-hand motifs (predicted to be Ca2+-binding domains) and has homology to mouse CaM, human CaM-like protein, hClp, and human CaM-like skin protein, hClsp. The functionality of Scarf EF-hand domains was assayed with a radioactive Ca2+-binding method. By Southern blot and computational genome sequence analysis, a highly related gene, Scarf2, was found 15 kb downstream of Scarf on mouse chromosome 13. The functional Scarf Ca2+-binding domains suggest a role in the regulation of epidermal differentiation through the control of Ca2+-mediated signaling.

Fetal Epidermal Differentiation and Barrier Development In Vivo is Accelerated by Nuclear Hormone Receptor Activators1

Nuclear receptors which interact with the retinoid X receptor are involved in the regulation of epidermal differentiation and development. We have recently shown that activators of the peroxisome proliferator-activated receptor and of the farnesoid X-activated receptor accelerate epidermal barrier maturation in fetal rat skin in vitro. In this study we asked whether cutaneous development in utero was affected by peroxisome proliferator-activated receptor or farnesoid X-activated receptor activators, or by an activator of another retinoid X receptor partner, liver X receptor. Activators of the peroxisome proliferator-activated receptor (clofibrate or linoleic acid), farnesoid X-activated receptor (farnesol or juvenile hormone III), or liver X receptor (22R-hydroxycholesterol), were injected into the amniotic fluid of fetal rats on gestational day 17. Fetal epidermal barrier function and morphology was assessed on day 19. Whereas vehicle-treated fetal rats displayed no measurable barrier (transepidermal water loss > 10 mg per cm2 per h), a measurable barrier was induced by the intra-amniotic administration of all activators tested (transepidermal water loss range 4.0-8.5 mg per cm2 per h). By light microscopy, control pups lacked a well-defined stratum corneum, whereas a distinct stratum corneum and a thickened stratum granulosum were present in treated pups. By electron microscopy, the extracellular spaces of the stratum corneum in control pups revealed a paucity of mature lamellar unit structures, whereas these structures filled the stratum corneum interstices in treated pups. Additionally, protein and mRNA levels of loricrin and filaggrin, two structural proteins of stratum corneum, were increased in treated epidermis, as were the activities of two lipid catabolic enzymes critical to stratum corneum function, beta-glucocerebrosidase and steroid sulfatase. Finally, peroxisome proliferator-activated receptor-alpha and -delta and liver X receptor-alpha and -beta mRNAs were detected in fetal epidermis by reverse transcriptase-polymerase chain reaction and northern analyses. The presence of these receptors and the ability of their activators to stimulate epidermal barrier and stratum corneum development suggest a physiologic role for peroxisome proliferator-activated receptor and liver X receptor and their endogenous ligands in the regulation of cutaneous development.