Oxysterol stimulation of epidermal differentiation is mediated by liver X receptor-beta in murine epidermis

Nuclear receptors for epidermal lipid barrier: Advances in mechanisms and applications

The skin plays an essential role in preventing the entry of external environmental threats and the loss of internal substances, depending on the epidermal permeability barrier. Nuclear receptors (NRs), present in various tissues and organs including full-thickness skin, have been demonstrated to exert significant effects on the epidermal lipid barrier. Formation of the lipid lamellar membrane and the normal proliferation and differentiation of keratinocytes (KCs) are crucial for the development of the epidermal permeability barrier and is regulated by specific NRs such as PPAR, LXR, VDR, RAR/RXR, AHR, PXR and FXR. These receptors play a key role in regulating KC differentiation and the entire process of epidermal lipid synthesis, processing and secretion. Lipids derived from sebaceous glands are influenced by NRs as well and participate in regulation of the epidermal lipid barrier. Furthermore, intricate interplay exists between these receptors. Disturbance of barrier function leads to a range of diseases, including psoriasis, atopic dermatitis and acne. Targeting these NRs with agonists or antagonists modulate pathways involved in lipid synthesis and cell differentiation, suggesting potential therapeutic approaches for dermatosis associated with barrier damage. This review focuses on the regulatory role of NRs in the maintenance and processing of the epidermal lipid barrier through their effects on skin lipid synthesis and KC differentiation, providing novel insights for drug targets to facilitate precision medicine strategies.

Liver X receptors and skeleton: Current state-of-knowledge

The liver X receptors (LXR) is a nuclear receptor that acts as a prominent regulator of lipid homeostasis and inflammatory response. Its therapeutic effectiveness against various diseases like Alzheimer's disease and atherosclerosis has been investigated in detail. Emerging pieces of evidence now reveal that LXR is also a crucial modulator of bone remodeling. However, the molecular mechanisms underlying the pharmacological actions of LXR on the skeleton and its role in osteoporosis are poorly understood. Therefore, in the current review, we highlight LXR and its actions through different molecular pathways modulating skeletal homeostasis. The studies described in this review propound that LXR in association with estrogen, PTH, PPARγ, RXR hedgehog, and canonical Wnt signaling regulates osteoclastogenesis and bone resorption. It regulates RANKL-induced expression of c-Fos, NFATc1, and NF-κB involved in osteoclast differentiation. Additionally, several studies suggest suppression of RANKL-induced osteoclast differentiation by synthetic LXR ligands. Given the significance of modulation of LXR in various physiological and pathological settings, our findings indicate that therapeutic targeting of LXR might potentially prevent or treat osteoporosis and improve bone quality.

Genetic polymorphism of liver X receptor gene in vitiligo: Does it have an association?

BACKGROUND

Vitiligo is an acquired depigmentation of the skin and the mucous membranes, exhibited as white macules and patches due to selective loss of melanocytes. Etiological theories of vitiligo include genetic, immunological, neurohormonal, cytotoxic, biochemical, oxidative stress, and newer theories of melanocytorrhagy and diminished melanocytes survival. It has been revealed that liver X receptor alpha gene is expressed in skin tissue such as sebaceous glands, hair follicle, keratinocytes, and fibroblasts and is linked to various skin disorders as acne vulgaris and psoriasis.

AIM OF THE STUDY

To evaluate the association between liver X receptor-α gene polymorphism (rs11039155 and rs2279238) and vitiligo and whether they are related to disease activity and severity or not.

SUBJECTS AND METHODS

50 vitiligo patients and 20 age- and sex-matched apparently healthy controls were enrolled. All the included subjects were genotyped using polymerase chain reaction-restriction fragment length polymorphism analysis technique for (-6G/A) and (+1257C/T) SNPs.

RESULTS

Significant statistical difference between cases and controls regarding genotype and allele frequencies for -6G/A polymorphism with predominance of AA genotype (OR: 5.1, 95% CI: 1.6-15.9) and A allele (OR: 5.3, 95% CI: 1.6-15.9) in cases and also for +1257C/T polymorphism with predominance of TT genotype OR: 9.2 (95% CI: 1.4-82.9) and T allele OR: 3.4 (95% CI: 1.4-8.1) in vitiligo cases. No significant relationship between -6G/A genotypes nor +1257C/T genotypes and disease activity and severity.

CONCLUSION

The study showed significant association between Liver X receptor gene polymorphisms (-6G/A, +1257 C/T) and development of vitiligo in Egyptian patients. However, it failed to show any relation with disease activity nor severity.

miR-203 promotes HaCaT cell overproliferation through targeting LXR-α and PPAR-γ

Psoriasis is an immune-mediated chronic inflammatory skin disease. Keratinocyte hyperproliferation has been regarded as a significant event in psoriasis pathogenesis. Considering the vital role of miRNA-mediated mRNA repression in psoriasis pathogenesis, in the present study, we attempted to investigate the mechanism of keratinocyte overproliferation from the point of miRNA-mRNA regulation. Both online microarray expression profiles and experimental results indicated that the expression of LXR-α and PPAR-γ was downregulated in psoriasis lesion skin. LXR-α or PPAR-γ overexpression alone was sufficient to inhibit keratinocyte proliferation, decrease KRT5 and KRT14 protein levels and increase KRT1 and KRT10 protein levels. miR-203 negatively regulated LXR-α and PPAR-γ expression through direct targeting. miR-203 inhibition exerted the opposite effects to LXR-α or PPAR-γ overexpression on HaCaT cells. More importantly, LXR-α or PPAR-γ overexpression could markedly remarkably attenuate the effects of miR-203 overexpression in keratinocytes, indicating that miR-203 promotes keratinocyte proliferation by targeting LXR-α and PPAR-γ. In conclusion, the miR-203-LXR-α/PPAR-γ axis modulates the proliferation of keratinocytes and might be a novel target for psoriasis treatment, which needs further in vivo investigation.

Xenobiotic Receptors and Their Mates in Atopic Dermatitis

Atopic dermatitis (AD) is the most common inflammatory skin disease worldwide. It is a chronic, relapsing and pruritic skin disorder which results from epidermal barrier abnormalities and immune dysregulation, both modulated by environmental factors. AD is strongly associated with asthma and allergic rhinitis in the so-called 'atopic march.' Xenobiotic receptors and their mates are ligand-activated transcription factors expressed in the skin where they control cellular detoxification pathways. Moreover, they regulate the expression of genes in pathways involved in AD in epithelial cells and immune cells. Activation or overexpression of xenobiotic receptors in the skin can be deleterious or beneficial, depending on context, ligand and activation duration. Moreover, their impact on skin might be amplified by crosstalk among xenobiotic receptors and their mates. Because they are activated by a broad range of endogenous molecules, drugs and pollutants owing to their promiscuous ligand affinity, they have recently crystalized the attention of researchers, including in dermatology and especially in the AD field. This review examines the putative roles of these receptors in AD by critically evaluating the conditions under which the proteins and their ligands have been studied. This information should provide new insights into AD pathogenesis and ways to develop new therapeutic interventions.

Re-appraisal of Keratinocytes' Role in Vitiligo Pathogenesis

Background

Vitiligo is a common pigmentary disorder. Studies on its pathogenesis extensively investigated melanocytes' abnormalities and few studies searched for keratinocytes' role in disease development. Liver X receptor-α (LXR-α) is a member of nuclear hormone receptors that acts as a transcription factor. Its target genes are the main regulators of melanocyte functions.

Aim

The aim of this study is to investigate keratinocytes' role in vitiligo pathogenesis through immunohistochemical expression of LXR-α in lesional, perilesional, and distant nonlesional vitiligo skin.

Materials and Methods

This case-control study was carried out on 44 participants. These included 24 patients with vitiligo and 20 age- and sex-matched normal individuals as a control group. Biopsies, from cases, were taken from lesional, perilesional, and distant nonlesional areas. Evaluation was done using immunohistochemical technique.

Results

Keratinocyte LXR-α expression was upregulated in the lesional and perilesional skin (follicular and interfollicular epidermis) compared with control skin (P <0.001 for all). There was significant association between higher histoscore (H-score) in lesional epidermis (P <0.001) and in hair follicle (P =0.001) and the presence of angiogenesis. There was significant association between higher H-score in lesional epidermis and suprabasal vacuolization (P =0.02). No significant association was found between H-score or expression percentage and clinical data of selected cases.

Conclusion

LXR-α upregulation is associated with keratinocyte damage in vitiligo lesional skin that leads to decreased keratinocyte-derived mediators and growth factors supporting the growth and/or melanization of surrounding melanocytes. Therefore, melanocyte function and survival are affected.

Effect of short-term liver X receptor activation on epidermal barrier features in mild to moderate atopic dermatitis: A randomized controlled trial

BACKGROUND

Liver X receptors (LXRs) are involved in maintaining epidermal barrier and suppressing inflammatory responses in model systems. The LXR agonist VTP-38543 showed promising results in improving barrier function and inflammatory responses in model systems.

OBJECTIVE

To assess the safety, tolerability, cellular and molecular changes, and clinical efficacy of the topical VTP-38543 in adults with mild to moderate atopic dermatitis (AD).

METHODS

A total of 104 ambulatory patients with mild to moderate AD were enrolled in this randomized, double-blind, vehicle-controlled trial between December 2015 and September 2016. VTP-38543 cream in 3 concentrations (0.05%, 0.15%, and 1.0%) or placebo was applied twice daily for 28 days. Pretreatment and posttreatment skin biopsy specimens were obtained from a subset of 33 patients. Changes in SCORing of Atopic Dermatitis, Eczema Area and Severity Index, Investigator's Global Assessment, and tissue biomarkers (by real-time polymerase chain reaction and immunostaining) were evaluated.

RESULTS

Topical VTP-38543 was safe and well tolerated. VTP-38543 significantly increased messenger RNA (mRNA) expression of epidermal barrier differentiation (loricrin and filaggrin, P = .02) and lipid (adenosine triphosphate-binding cassette subfamily G member 1 and sterol regulatory element binding protein 1c, P < .01) measures and reduced epidermal hyperplasia markers (thickness, keratin 16 mRNA). VTP-38543 nonsignificantly suppressed cellular infiltrates and down-regulated mRNA expression of several T_H17/T_H22-related (phosphatidylinositol 3, S100 calcium-binding protein A12) and innate immunity (interleukin 6) markers.

CONCLUSION

Topical VTP-38543 is safe and well tolerated. Its application led to improvement in barrier differentiation and lipids. Longer-term studies are needed to clarify whether a barrier-based approach can induce meaningful suppression of immune abnormalities.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT02655679.

Recent development of signaling pathways inhibitors of melanogenesis

Human skin, eye and hair color rely on the production of melanin, depending on its quantity, quality, and distribution, Melanin plays a monumental role in protecting the skin against the harmful effect of ultraviolet radiation and oxidative stress from various environmental pollutants. However, an excessive production of melanin causes serious dermatological problems such as freckles, solar lentigo (age spots), melasma, as well as cancer. Hence, the regulation of melanin production is important for controlling the hyper-pigmentation. Melanogenesis, a biosynthetic pathway to produce melanin pigment in melanocyte, involves a series of intricate enzymatic and chemical catalyzed reactions. Several extrinsic factors include ultraviolet radiation and chemical drugs, and intrinsic factors include molecules secreted by surrounding keratinocytes or melanocytes, and fibroblasts, all of which regulate melanogenesis. This article reviews recent advances in the development of melanogenesis inhibitors that directly/indirectly target melanogenesis-related signaling pathways. Efforts have been made to provide a description of the mechanism of action of inhibitors on various melanogenesis signaling pathways.

Immunohistochemical Expression of Cyclo-oxygenase 2 and Liver X Receptor-α in Acne Vulgaris

Introduction

Acne Vulgaris (AV) is a common inflammatory disease of pilosebaceous units. Liver X Receptor-α (LXR-α) is a ligand activated transcription factor. It controls transcription of genes involved in lipid and fatty acid synthesis. Cyclo-oxygenase 2 (COX2) is a rate limiting enzyme in prostaglandin synthesis. It plays important role in inflammation.

Aim

To evaluate the immunohistochemical expression of LXR-α and COX2 in acne vulgaris skin biopsies to explore their possible pathogenic role in this disease.

Materials and Methods

Sixty five subjects were included (45 cases with AV and 20 age and gender-matched healthy controls). Skin biopsies were taken from lesional and perilesional skin of cases and from site-matched areas of control subjects. The evaluation of LXR-α and COX2 was done using immunohistochemical technique. Data were collected, tabulated and statistically analysed using a personal computer with "(SPSS) version 11" program. Chi-square test was used to study the association between qualitative variables. Mann-Whitney test was used for comparison between quantitative variables. Student's t-test was used for comparison between two groups having quantitative variables. Spearman's coefficient was used to study the correlation between two different variables. Differences were considered statistically significant with p<0.05.

Results

COX2 was upregulated in lesional skin compared with peilesional and control skin both in epidermis and pilosebaceous units (p<0.001 for all). Higher epidermal COX2% was significantly associated with papulopustular acne (p=0.009) and higher acne score (p=0.018). Higher pilosebaceous units COX2% was significantly associated with papulopustular acne (p=0.04). LXR-α was upregulated in lesional skin compared with peilesional and control skin both in epidermis and pilosebaceous units (p<0.001 for all). Higher LXR-α % in epidermis and pilosebaceous units was significantly associated with papulopustular acne (p=0.01 for both) and higher acne score (p=0.03 for both). Significant positive correlation was detected between COX2% and LXR-α % in epidermis (p=0.001, r=0.87) and pilosebaceous units (p=0.001, r=0.65).

Conclusion

Both LXR-α and COX-2 play a role in the pathogenesis of acne vulgaris through their effects on cellular proliferation, inflammation and lipid synthesis. Research for new therapeutic modalities based on their inhibition is needed. More understanding of the interaction between LXR-α, COX2 and acne lesions may lead to effective interference, possibly directed toward specific cell types or steps within inflammatory pathways.

The trisaccharide raffinose modulates epidermal differentiation through activation of liver X receptor

The epidermal barrier function requires optimal keratinocyte differentiation and epidermal lipid synthesis. Liver X receptor (LXR) α and β, are important transcriptional regulators of the epidermal gene expression. Here, we show that raffinose, a ubiquitously present trisaccharide in plants, activated the transcriptional activity of LXRα/β, which led to the induction of genes required for keratinocyte differentiation such as involucrin and filaggrin, and genes involved in lipid metabolism and transport including SCD1 and ABCA1 in both HaCaT and normal human epidermal keratinocytes. Raffinose induced the expression of JunD and Fra1, and their DNA binding in the AP1 motif in the promoters of involucrin and loricrin. Interestingly, LXR bound the AP1 motif upon raffinose treatment, and conversely, JunD and Fra1 bound the LXR response element in promoters of LXR target genes, which indicates the presence of a postive cross-talk between LXR and AP1 in the regualtion of these genes. Finally, the effect of raffinose in epidermal barrier function was confirmed by applying raffinose in an ointment formulation to the skin of hairless mice. These findings suggest that raffinose could be examined as an ingredient in functional cosmetics and therapeutic agents for the treatment of cutaneous disorders associated with abnormal epidermal barrier function.

Role of the liver X receptors in skin physiology: Putative pharmacological targets in human diseases

Liver X receptors (LXRs) are members of the nuclear receptor superfamily that have been shown to regulate various physiological functions such as lipid metabolism and cholesterol homeostasis. Concordant reports have elicited the possibility to target them to cure many human diseases including arteriosclerosis, cancer, arthritis, and diabetes. The high relevance of modulating LXR activities to treat numerous skin diseases, mainly those with exacerbated inflammation processes, contrasts with the lack of approved therapeutic use. This review makes an assessment to sum up the findings regarding the physiological roles of LXRs in skin and help progress towards the therapeutic and safe management of their activities. It focuses on the possible pharmacological targeting of LXRs to cure or prevent selected skin diseases.

Liver X receptor β increases aquaporin 2 protein level via a posttranscriptional mechanism in renal collecting ducts

Liver X receptors (LXRs) including LXRα and LXRβ are nuclear receptor transcription factors and play an important role in lipid and glucose metabolism. It has been previously reported that mice lacking LXRβ but not LXRα develop a severe urine concentrating defect, likely via a central mechanism. Here we provide evidence that LXRβ regulates water homeostasis through increasing aquaporin 2 (AQP2) protein levels in renal collecting ducts. LXRβ^-/- mice exhibited a reduced response to desmopressin (dDAVP) stimulation, suggesting that the diabetes insipidus phenotype is of both central and nephrogenic origin. AQP2 protein abundance in the renal inner medulla was significantly reduced in LXRβ^-/- mice but with little change in AQP2 mRNA levels. In vitro studies showed that AQP2 protein levels were elevated upon LXR agonist treatment in both primary cultured mouse inner medullary duct cells (mIMCD) and the mIMCD3 cell line with stably expressed AQP2. In addition, LXR agonists including TO901317 and GW3965 failed to induce AQP2 gene transcription but diminished its protein ubiquitination in primary cultured mIMCD cells, thereby inhibiting its degradation. Moreover, LXR activation-induced AQP2 protein expression was abolished by the protease inhibitor MG132 and the ubiquitination-deficient AQP2 (K270R). Taken together, the present study demonstrates that activation of LXRβ increases AQP2 protein levels in the renal collecting ducts via a posttranscriptional mechanism. As such, LXRβ represents a key regulator of body water homeostasis.

Nuclear receptor function in skin health and disease: therapeutic opportunities in the orphan and adopted receptor classes

The skin forms a vital barrier between an organism's external environment, providing protection from pathogens and numerous physical and chemical threats. Moreover, the intact barrier is essential to prevent water and electrolyte loss without which terrestrial life could not be maintained. Accordingly, acute disruption of the skin through physical or chemical trauma needs to be repaired timely and efficiently as sustained skin pathologies ranging from mild irritations and inflammation through to malignancy impact considerably on morbidity and mortality. The Nuclear Hormone Receptor Family of transcriptional regulators has proven to be highly valuable targets for addressing a range of pathologies, including metabolic syndrome and cancer. Indeed members of the classic endocrine sub-group, such as the glucocorticoid, retinoid, and Vitamin D receptors, represent mainstay treatment strategies for numerous inflammatory skin disorders, though side effects from prolonged use are common. Emerging evidence has now highlighted important functional roles for nuclear receptors belonging to the adopted and orphan subgroups in skin physiology and patho-physiology. This review will focus on these subgroups and explore the current evidence that suggests these nuclear receptor hold great promise as future stand-alone or complementary drug targets in treating common skin diseases and maintaining skin homeostasis.

Hataedock Treatment Has Preventive Therapeutic Effects in Atopic Dermatitis-Induced NC/Nga Mice under High-Fat Diet Conditions

This study investigated the preventive therapeutic effects of Hataedock (HTD) treatment on inflammatory regulation and skin protection in AD-induced NC/Nga mice under high-fat diet conditions. Before inducing AD, the extract of Coptidis Rhizoma and Glycyrrhiza uralensis was administered orally to the 3-week-old mice. After that, AD-like skin lesions were induced by applying DNFB. All groups except the control group were fed a high-fat diet freely. We identified the effects of HTD on morphological changes, cytokine release and the induction of apoptosis through histochemistry, immunohistochemistry, and TUNEL assay. HTD downregulated the levels of IL-4 and PKC but increased the levels of LXR. HTD also suppressed the mast cell degranulation and release of MMP-9, Substance P. The levels of TNF-α, p-IκB, iNOS, and COX-2 were also decreased. The upregulation of inflammatory cell's apoptosis is confirmed by our results as increase of apoptotic body and cleaved caspase-3 and decrease of Bcl-2. HTD also reduced edema, angiogenesis, and skin lesion inflammation. Our results indicate HTD suppresses various inflammatory response on AD-induced mice with obesity through the regulation of Th2 differentiation and the protection of lipid barrier. Therefore, HTD could be used as an alternative and preventive therapeutic approach in the management of AD.

Emodin isolated from Polygoni Multiflori Ramulus inhibits melanogenesis through the liver X receptor-mediated pathway

Melanogenesis is a physiological process that results in the synthesis of melanin pigments, which play a crucial protective role against skin photocarcinogenesis. We investigated the effects of a Polygoni Multiflori Ramulus extract on melanogenesis and isolated emodin from Polygoni Multiflori as an active compound. In addition, the possible mechanisms of action were examined. We found that emodin inhibited both melanin content and tyrosinase activity concentration and time dependently. Tyrosinase, tyrosinase-related protein (TRP)-1, and TRP-2 mRNA levels decreased following emodin treatment. However, while the mRNA levels of microphthalmia-associated transcription factor (MITF) were not affected by emodin, emodin reduced MITF protein levels. Furthermore, expression of the liver X-receptor (LXR) α gene, but not the LXR β gene was upregulated by emodin. Moreover, emodin regulated melanogenesis by promoting degradation of the MITF protein by upregulating the LXR α gene. The emodin effects on MITF was found to be mediated by phosphorylation of p42/44 MAPK. Taken together, these findings indicate that the inhibition of melanogenesis by emodin occurs through reduced MITF protein expression, which is mediated by upregulation of the LXR α gene and suggest that emodin may be useful as a hyperpigmentation inhibitor.

On the role of skin in the regulation of local and systemic steroidogenic activities

The mammalian skin is a heterogeneous organ/tissue covering our body, showing regional variations and endowed with neuroendocrine activities. The latter is represented by its ability to produce and respond to neurotransmitters, neuropeptides, hormones and neurohormones, of which expression and phenotypic activities can be modified by ultraviolet radiation, chemical and physical factors, as well as by cytokines. The neuroendocrine contribution to the responses of skin to stress is served, in part, by local synthesis of all elements of the hypothalamo-pituitary-adrenal axis. Skin with subcutis can also be classified as a steroidogenic tissue because it expresses the enzyme, CYP11A1, which initiates steroid synthesis by converting cholesterol to pregnenolone, as in other steroidogenic tissues. Pregnenolone, or steroidal precursors from the circulation, are further transformed in the skin to corticosteroids or sex hormones. Furthermore, in the skin CYP11A1 acts on 7-dehydrocholesterol with production of 7-dehydropregnolone, which can be further metabolized to other Δ7steroids, which after exposure to UVB undergo photochemical transformation to vitamin D like compounds with a short side chain. Vitamin D and lumisterol, produced in the skin after exposure to UVB, are also metabolized by CYP11A1 to several hydroxyderivatives. Vitamin D hydroxyderivatives generated by action of CYP11A1 are biologically active and are subject to further hydroxylations by CYP27B1, CYP27A1 and CP24A. Establishment of which intermediates are produced in the epidermis in vivo and whether they circulate on the systemic level represent a future research challenge. In summary, skin is a neuroendocrine organ endowed with steroid/secosteroidogenic activities.

Cutaneous glucocorticosteroidogenesis: securing local homeostasis and the skin integrity

Human skin has the ability to synthesize glucocorticoids de novo from cholesterol or from steroid intermediates of systemic origin. By interacting with glucocorticoid receptors, they regulate skin immune functions as well as functions and phenotype of the epidermal, dermal and adnexal compartments. Most of the biochemical (enzyme and transporter activities) and regulatory (neuropeptides mediated activation of cAMP and protein kinase A dependent pathways) principles of steroidogenesis in the skin are similar to those operating in classical steroidogenic organs. However, there are also significant differences determined by the close proximity of synthesis and action (even within the same cells) allowing para-, auto- or intracrine modes of regulation. We also propose that ultraviolet light B (UVB) can regulate the availability of 7-dehydrocholesterol for transformation to cholesterol with its further metabolism to steroids, oxysterols or ∆7 steroids, because of its transformation to vitamin D3. In addition, UVB can rearrange locally produced ∆7 steroids to the corresponding secosteroids with a short- or no-side chain. Thus, different mechanisms of regulation occur in the skin that can be either stochastic or structuralized. We propose that local glucocorticosteroidogenic systems and their regulators, in concert with cognate receptors operate to stabilize skin homeostasis and prevent or attenuate skin pathology.

Active ingredients against human epidermal aging

The decisive role of the epidermis in maintaining body homeostasis prompted studies to evaluate the changes in epidermal structure and functionality over the lifetime. This development, along with the identification of molecular mechanisms of epidermal signaling, maintenance, and differentiation, points to a need for new therapeutic alternatives to treat and prevent skin aging. In addition to recovering age- and sun-compromised functions, proper treatment of the epidermis has important esthetic implications. This study reviews active ingredients capable of counteracting symptoms of epidermal aging, organized according to the regulation of specific age-affected epidermal functions: (1) several compounds, other than retinoids and derivatives, act on the proliferation and differentiation of keratinocytes, supporting the protective barrier against mechanical and chemical insults; (2) natural lipidic compounds, as well as glycerol and urea, are described as agents for maintaining water-ion balance; (3) regulation of immunological pathogen defense can be reinforced by natural extracts and compounds, such as resveratrol; and (4) antioxidant exogenous sources enriched with flavonoids and vitamin C, for example, improve solar radiation protection and epidermal antioxidant activity. The main objective is to provide a functional classification of active ingredients as regulatory elements of epidermal homeostasis, with potential cosmetic and/or dermatological applications.

Role of PPAR, LXR, and PXR in epidermal homeostasis and inflammation

Epidermal lipid synthesis and metabolism are regulated by nuclear hormone receptors (NHR) and in turn epidermal lipid metabolites can serve as ligands to NHR. NHR form a large superfamily of receptors modulating gene transcription through DNA binding. A subgroup of these receptors is ligand-activated and heterodimerizes with the retinoid X receptor including peroxisome proliferator-activated receptor (PPAR), liver X receptor (LXR) and pregnane X receptor (PXR). Several isotypes of these receptors exist, all of which are expressed in skin. In keratinocytes, ligand activation of PPARs and LXRs stimulates differentiation, induces lipid accumulation, and accelerates epidermal barrier regeneration. In the cutaneous immune system, ligand activation of all three receptors, PPAR, LXR, and PXR, has inhibitory properties, partially mediated by downregulation of the NF-kappaB pathway. PXR also has antifibrotic effects in the skin correlating with TGF-beta inhibition. In summary, ligands of PPAR, LXR and PXR exert beneficial therapeutic effects in skin disease and represent promising targets for future therapeutic approaches in dermatology. 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.

Topical apigenin improves epidermal permeability barrier homoeostasis in normal murine skin by divergent mechanisms

The beneficial effects of certain herbal medicines on cutaneous function have been appreciated for centuries. Among these agents, chrysanthemum extract, apigenin, has been used for skin care, particularly in China, for millennia. However, the underlying mechanisms by which apigenin benefits the skin are not known. In this study, we first determined whether topical apigenin positively influences permeability barrier homoeostasis, and then the basis thereof. Hairless mice were treated topically with either 0.1% apigenin or vehicle alone twice daily for 9 days. At the end of the treatments, permeability barrier function was assessed with either an electrolytic water analyzer or a Tewameter. Our results show that topical apigenin significantly enhanced permeability barrier homoeostasis after tape stripping, although basal permeability barrier function remained unchanged. Improved barrier function correlated with enhanced filaggrin expression and lamellar body production, which was paralleled by elevated mRNA levels for the epidermal ABCA12. The mRNA levels for key lipid synthetic enzymes also were upregulated by apigenin. Finally, both cathelicidin-related peptide and mouse beta-defensin 3 immunostaining were increased by apigenin. We conclude that topical apigenin improves epidermal permeability barrier function by stimulating epidermal differentiation, lipid synthesis and secretion, as well as cutaneous antimicrobial peptide production. Apigenin could be useful for the prevention and treatment of skin disorders characterized by permeability barrier dysfunction, associated with reduced filaggrin levels and impaired antimicrobial defenses, such as atopic dermatitis.

Targeting liver X receptors in inflammation

INTRODUCTION

The two oxysterol receptors, 'liver X receptors (LXRs)' LXRα and LXRβ, are amongst the emerging newer drug targets within the nuclear receptor family and targeting LXRs represents novel strategies needed for prevention and treatment of diseases where current therapeutics is inadequate.

AREAS COVERED

This review discusses the current understanding of LXR biology with an emphasis on the molecular aspects of LXR signalling establishing their potential as drug targets. Recent advances of their transcriptional mechanisms in inflammatory pathways and their physiological roles in inflammation and immunity are described.

EXPERT OPINION

The new discoveries of LXR-regulated inflammatory pathways have ignited new promises for LXRs as drug targets. The broad physiological roles of LXRs involve a high risk of unwanted side effects. Recent insights into LXR biology of the brain indicate a highly important role in neuronal development and a clinical trial testing an LXR agonist reported adverse neurological side effects. This suggests that drug development must focus on limiting the range of LXR signalling - possibly achieved through subtype, tissue specific, promoter specific or pathway specific activation of LXRs where a successful candidate drug must be carefully studied for its effect in the central nervous system.

Nuclear hormone receptor functions in keratinocyte and melanocyte homeostasis, epidermal carcinogenesis and melanomagenesis

Skin homeostasis is maintained, in part, through regulation of gene expression orchestrated by type II nuclear hormone receptors in a cell and context specific manner. This group of transcriptional regulators is implicated in various cellular processes including epidermal proliferation, differentiation, permeability barrier formation, follicular cycling and inflammatory responses. Endogenous ligands for the receptors regulate actions during skin development and maintenance of tissue homeostasis. Type II nuclear receptor signaling is also important for cellular crosstalk between multiple cell types in the skin. Overall, these nuclear receptors are critical players in keratinocyte and melanocyte biology and present targets for cutaneous disease management.

Liver X receptor activation inhibits melanogenesis through the acceleration of ERK-mediated MITF degradation

Liver X receptors (LXRs) are nuclear receptors that act as ligand-activated transcription factors regulating lipid metabolism and inflammation. In the skin, activation of LXRs stimulates differentiation of keratinocytes and augments lipid synthesis in sebocytes. However, the function of LXRs in melanocytes remains largely unknown. We investigated whether LXR activation would affect melanogenesis. In human primary melanocytes, MNT-1, and B16 melanoma cells, TO901317, a synthetic LXR ligand, inhibited melanogenesis. Small interfering RNA (siRNA) experiments revealed the dominant role of LXRβ in TO901317-mediated antimelanogenesis. Enzymatic activities of tyrosinase were unaffected, but the expression of tyrosinase, tyrosinase-related protein-1 (TRP-1), and TRP-2 was suppressed by TO901317. Expressions of microphthalmia-associated transcription factor (MITF), a master transcriptional regulator of melanogenesis, and cAMP-responsive element-binding activation were not affected. It is noteworthy that the degradation of MITF was accelerated by TO901317. Extracellular signal-regulated kinase (ERK) contributed to TO901317-induced antimelanogenesis, which was evidenced by recovery of melanogenesis with ERK inhibitor. Other LXR ligands, 22(R)-hydroxycholesterol (22(R)HC) and GW3965, also activated ERK and suppressed melanogenesis. The intermediary role of Ras was confirmed in TO901317-induced ERK phosphorylation. Finally, antimelanogenic effects of TO901317 were confirmed in vivo in UVB-tanning model in brown guinea pigs, providing a previously unreported line of evidence that LXRs may be important targets for antimelanogenesis.

Selective liver X receptor modulators (SLiMs): what use in human health?

Liver X receptors (LXR) are members of the nuclear receptor family. As activated transcription factors, their putative association with human diseases makes them promising pharmacological targets because of the large potential to develop ligands. LXR are mainly considered as intracellular cholesterol "sensors" whose activation leads to decreased plasma cholesterol. They also modulate numerous physiological functions: fatty acid synthesis and metabolism, glucose homeostasis, steroidogenesis, immunity, and neurological homeostasis. LXR-deficiency in mouse results in several phenotypes mimicking pathological conditions in humans. This review will be focused on the various natural and synthetic LXR agonists and antagonists. Putative clinical targets including atherosclerosis, diabetes, Alzheimer's disease, skin disorders, and cancer will be covered.

Expression of epidermal CAMP changes in parallel with permeability barrier status

Two critical defensive functions of the outer epidermis, the permeability barrier and antimicrobial defense, share certain structural and biochemical features. Moreover, 3antimicrobial peptides (AMP); i.e., mouse beta-defensin 3 (mBD3), mouse cathelicidin protein (mCAMP), and the neuroendocrine peptide, catestatin, all localize to the outer epidermis, and both mBD3 and mCAMP are secreted from epidermal lamellar bodies with other organelle contents that subserve the permeability barrier. These 3 AMP are up-regulated in response to acute permeability barrier disruption, while conversely, mCAMP−/− mice (unable to combatgram-positive pathogens) also display abnormal barrier homeostasis. To determine further whether these two functions are co-regulated, we investigated changes in immunostaining for these 3 AMP in skin samples in which permeability barrier function in mice had been either compromised or enhanced. Compromised or enhanced barrier function correlated with reduced or enhanced immunohistochemical expression of mCAMP, respectively, but conversely with Cst expression likely due to the role of this AMP as an endogenous inhibitor of cathelicidin expression. mBD3 expression correlated with experimental barrier perturbations, but poorly with developmental changes in barrier function. These studies show that changes in cathelicidin and Cst expression parallel changes in permeability barrier status, with a less clear relationship with mBD3 expression.

Liver X Receptor: an oxysterol sensor and a major player in the control of lipogenesis

De novo fatty acid biosynthesis is also called lipogenesis. It is a metabolic pathway that provides the cells with fatty acids required for major cellular processes such as energy storage, membrane structures and lipid signaling. In this article we will review the role of the Liver X Receptors (LXRs), nuclear receptors that sense oxysterols, in the transcriptional regulation of genes involved in lipogenesis.

PPARgamma activators stimulate aquaporin 3 expression in keratinocytes/epidermis

Aquaporin 3 (AQP3), a member of the aquaglyceroporin family, which transports water and glycerol, is robustly expressed in epidermis and plays an important role in stratum corneum hydration, permeability barrier function and wound healing. PPAR and LXR activation regulates the expression of many proteins in the epidermis and thereby can affect epidermal function. Here, we report that PPARgamma activators markedly stimulate AQP3 mRNA expression in both undifferentiated and differentiated cultured human keratinocytes (CHKs). The increase in AQP3 mRNA by PPARgamma activator occurs in a dose- and time-dependent fashion. Increased AQP3 mRNA levels are accompanied by an increase in AQP3 protein in undifferentiated keratinocytes and a significant increase in glycerol uptake. Activation of LXR, RAR and RXR also increases AQP3 mRNA levels in undifferentiated and differentiated CHKs, but to a lesser extent. PPARdelta activation stimulates AQP3 expression in undifferentiated CHKs but decreases expression in differentiated CHKs. In contrast, PPARalpha activators do not alter AQP3 expression. AQP9 and AQP10, other members of aquaglyceroporin family, are less abundantly expressed in CHKs, and their expression levels are not significantly altered by treatment with LXR, PPAR, RAR or RXR activators. Finally, when topically applied, the PPARgamma activator, ciglitazone, induces AQP3 but not AQP9 gene expression in mouse epidermis. Our data demonstrate that PPAR and LXR activators stimulate AQP3 expression, providing an additional mechanism by which PPAR and LXR activators regulate epidermal function.

The mechanisms by which lipids coordinately regulate the formation of the protein and lipid domains of the stratum corneum: Role of fatty acids, oxysterols, cholesterol sulfate and ceramides as signaling molecules

The formation of a permeability barrier between the external environment and the host is essential for survival. To provide this barrier keratinocytes undergo a complex pathway of differentiation, which culminates in keratinocyte cornification and the formation of extracellular lipid enriched lamellar membranes in the stratum corneum. The mechanisms that coordinately regulate the parallel formation of the corneocytes and lamellar membranes are unknown. The extracellular lamellar membranes are derived from the exocytosis of lamellar bodies and to synthesize lamellar bodies the keratinocyte must have abundant quantities of cholesterol, fatty acids and ceramides. These lipids could serve as signaling molecules and thereby coordinately regulate the formation of the stratum corneum. Fatty acids activate PPARs and studies have shown that PPAR activation stimulates keratinocyte differentiation. Cholesterol is converted to oxysterols that activate LXR and studies have shown that LXR activation also stimulates keratinocyte differentation. Additionally, PPAR and LXR activation also facilitates the formation of the lipid enriched lamellar membranes. Ceramides, via a number of mechanisms also stimulate keratinocyte differentiation. Recently, studies have shown that ceramides by increasing PPAR delta also increase the expression of ABCA12, which would facilitate the formation of lamellar bodies. Finally, keratinocytes accumulate a large quantity of cholesterol sulfate, which plays a key role in regulating desquamation. Cholesterol sulfate has also been shown to stimulate keratinocyte differentiation. Thus, cholesterol, cholesterol sulfate, fatty acids and ceramides all stimulate keratinocyte differentiation and thereby could coordinately regulate the formation of the stratum corneum.

Liver X receptor-retinoid X receptor (LXR-RXR) heterodimer cistrome reveals coordination of LXR and AP1 signaling in keratinocytes

Liver X receptors (LXRs) play a critical role in regulating lipid synthesis and transport in numerous tissues. In the skin, activation of LXR induces keratinocyte differentiation and improves epidermal permeability barrier homeostasis. To elucidate the mechanism of LXR action in skin, we mapped its cistrome by identifying LXRβ-RXRα binding sites using ChIP-on-chip in normal human epidermal keratinocytes (NHEKs). The cistrome was integrated with transcription data to obtain a global view of LXR action in keratinocyte biology. Here, we identify 2035 LXRβ-RXRα binding sites containing 4794 LXR response elements in NHEKs and show the presence of consensus heterodimer active regions in genes involved in keratinocyte lipid transport/synthesis and terminal differentiation. Bioinformatics analysis of the cistrome revealed an enrichment of AP1 cis-regulatory motifs in the vicinity of the LXRβ-RXRα binding sites. Importantly, we have demonstrated a direct interaction between LXR and Jun/Fos, indicating that the cooperation between LXR and AP1 may orchestrate keratinocyte differentiation. Finally, we corroborated these results by genome-wide mapping of the c-Fos and c-Jun cistromes in NHEKs, demonstrating that 77% of all the LXRβ-RXRα binding regions show the presence of AP1 motifs at adjacent locations. Our findings provide new insight into the mechanism of LXR action in keratinocyte differentiation, lipid production and barrier formation, further strengthening the validation of LXR as a potential therapeutic target for skin disorders including skin aging, psoriasis, and atopic dermatitis.

Activation of LXRα induces lipogenesis in HaCaT cells

The oxysterol nuclear receptors, LXRα (liver X receptor α; NR1H3) and LXRβ (NR1H2), coordinately regulate the expression of genes involved in lipid metabolism, anti-inflammation, and cholesterol transport. Previous studies have demonstrated that ligands of LXRα are important in the maintenance of the normal epidermal barrier function and keratinocyte differentiation. In this study, we examined whether LXRα and its ligands regulate lipid synthesis in HaCaT cells, a spontaneously transformed human keratinocyte cell line. When HaCaT cells were treated with the LXRα ligand TO901317, lipid droplets accumulated in the majority of cells, which were stained by Oil Red O. A luciferase reporter construct containing the LXR response element was activated about fourfold in HaCaT cells by TO901317 treatment, suggesting that LXR has a role in lipid synthesis in these cells. The expression of LXRα target genes, such as those encoding sterol regulatory binding protein and fatty acid synthase, were induced time dependently by TO901317, as measured by RT-PCR and western blotting. The expression of PPAR-α, -β, and -γ which regulate lipid metabolism, was also increased by TO901317 treatment. In contrast, TO901317 reduced the lipopolysaccharide-induced expression of cyclooxygenase 2 and inducible nitric oxide synthase in HaCaT cells. These results indicate that LXRα activation leads to lipogenesis in keratinocytes, which may enhance the epidermal barrier function of the skin.

Lipid disturbances in psoriasis: an update

Psoriasis is a common disease with the population prevalence ranging from 2% to 3%. Its prevalence in the population is affected by genetic, environmental, viral, infectious, immunological, biochemical, endocrinological, and psychological factors, as well as alcohol and drug abuse. In the recent years, psoriasis has been recognised as a systemic disease associated with numerous multiorgan abnormalities and complications. Dyslipidemia is one of comorbidities in psoriatic patients. Lipid metabolism studies in psoriasis have been started at the beginning of the 20th century and are concentrated on skin surface lipids, stratum corneum lipids and epidermal phospholipids, serum lipids, dermal low-density lipoproteins in the psoriatic skin, lipid metabolism, oxidative stress and correlations between inflammatory parameters, lipid parameters and clinical symptoms of the disease. On the basis of the literature data, psoriasis can be described as an immunometabolic disease.

Liver X receptor β: maintenance of epidermal expression in intrinsic and extrinsic skin aging

Aging in human skin is the composite of time-dependent intrinsic aging plus photoaging induced by chronic exposure to ultraviolet radiation. Nuclear hormone receptors coordinate diverse processes including metabolic homeostasis. Liver X receptor β (LXRβ) is a close human homologue of daf-12, a regulator of nematode longevity. LXRβ is positively regulated by sirtuin-1 and resveratrol, while LXRβ-null mice show transcriptional profiles similar to those seen in aged human skin. In these studies, we examined LXRβ expression in aged and photoaged human skin. Volunteers were recruited to assess intrinsic aging and photoaging. Epidermal LXRβ mRNA was examined by in situ hybridization while protein was identified by immunofluorescence. No significant changes were observed in either LXRβ mRNA or protein expression between young and aged volunteers (mRNA p = 0.90; protein p = 0.26). Similarly, LXRβ protein expression was unaltered in photoaged skin (p = 0.75). Our data therefore suggest that, while not playing a major role in skin aging, robust cutaneous expression implies a fundamental role for LXRβ in epidermal biology.

PPARδ activation promotes stratum corneum formation and epidermal permeability barrier development during late gestation

The goal of epidermal ontogenesis is to form a stratum corneum (SC), which is required for post-natal permeability barrier function. The regulation of epidermal ontogenesis is poorly understood, but nuclear hormone receptors have been shown to have an important function. As peroxisome proliferator-activated receptor-delta (PPARdelta) is very abundant in fetal epidermis and PPARdelta activation stimulates differentiation and permeability barrier formation in adults, we hypothesized that PPARdelta might regulate epidermal ontogenesis. Treatment of fetal rat explants with the PPARdelta ligand, GW 610742X, accelerates permeability barrier development, evidenced by a decrease in transepidermal water loss and an enhanced outside-in barrier function, attributable to the presence of more mature lamellar membranes in the SC and enhanced expression of loricrin and involucrin. Similarly, the intra-amniotic administration of GW 610742X also accelerates the formation of the SC and permeability barrier development. Finally, in PPARdelta-deficient mice the formation of the SC and the expression of differentiation-related proteins were delayed on days 16.5 and 17.5 of gestation. However, at later stages (day 18.5 and after birth), there were no differences between wild-type- and PPARdelta-deficient mice, indicating only a transient delay in epidermal ontogenesis. These studies show that PPARdelta has a role in SC formation and permeability barrier development.

Activators of PPARs and LXR decrease the adverse effects of exogenous glucocorticoids on the epidermis

While glucocorticoids (GC) exert beneficial effects (anti-inflammatory), they also have adverse effects on the epidermis including decreased epidermal differentiation, decreased keratinocyte proliferation, and decreased cutaneous permeability barrier homeostasis. Thus, the purpose of this study was to develop strategies to prevent these GC toxicities using simultaneous topical treatments in clobetasol-treated mice. While a triple-lipid mixture of stratum corneum lipids (ceramide, free fatty acid and cholesterol) was previously shown to reverse the GC-induced abnormality in cutaneous barrier function [J Invest Dermatol, 120 (2003) 456], this lipid mixture did not prevent the GC-induced abnormalities in either keratinocyte proliferation or differentiation. As activators of PPARalpha, beta/delta, gamma and LXR, regulate keratinocyte proliferation and differentiation and improve permeability barrier homeostasis, we next assessed the effects of these activators during concurrent GC treatment. Co-application of either ciglitazone (PPARgamma activator), clofibrate (PPARalpha activator) or 22R (OH) cholesterol (LXR activator) with clobetasol prevented the decrease in involucrin, filaggrin and loricrin expression. By contrast, a PPARbeta/delta activator (GW501516) normalized only the expression of involucrin and filaggrin but not loricrin. Moreover, topical application of PPARalpha, beta/delta or LXR activators partially prevented the decrease in keratinocyte proliferation in GC-treated murine skin, as measured using PCNA, while no effect was seen after co-treatment with PPARgamma activators. Finally, PPARgamma and PPARbeta/delta activators but not PPARalpha and LXR activators improved permeability barrier homeostasis in GC-treated mice. Together, these studies demonstrate that PPAR and LXR activators can prevent several of the adverse effects of topical GC on the epidermis.

Minireview: liver X receptor beta: emerging roles in physiology and diseases

Liver X receptors, LXRalpha and LXRbeta, are nuclear receptors belonging to the large family of transcription factors. After activation by oxysterols, LXRs play a central role in the control of lipid and carbohydrate metabolism as well as inflammation. The role of LXRalpha has been extensively studied, particularly in the liver and macrophages. In the liver it prevents cholesterol accumulation by increasing bile acid synthesis and secretion into the bile through ATP-binding cassette G5/G8 transporters, whereas in macrophages it increases cholesterol reverse transport. The function of LXRbeta is still under investigation with most of the current knowledge coming from the study of phenotypes of LXRbeta-/- mice. With these mice new emerging roles for LXRbeta have been demonstrated in the pathogenesis of diseases such as amyotrophic lateral sclerosis and chronic pancreatitis. The present review will focus on the abnormalities described so far in LXRbeta-/- mice and the insight gained into the possible roles of LXRbeta in human diseases.

Thematic review series: skin lipids. Peroxisome proliferator-activated receptors and liver X receptors in epidermal biology

The epidermis is a very active site of lipid metabolism, and all peroxisome proliferator-activated receptor (PPAR) and liver X receptor (LXR) isoforms are expressed in the epidermis. Activation of PPARalpha, -beta/delta, or -gamma or LXRs stimulates keratinocyte differentiation. Additionally, activation of these receptors also improves permeability barrier homeostasis by a number of mechanisms, including stimulating epidermal lipid synthesis, increasing lamellar body formation and secretion, and increasing the activity of enzymes required for the extracellular processing of lipids in the stratum corneum, leading to the formation of lamellar membranes that mediate permeability barrier function. The stimulation of keratinocyte differentiation and permeability barrier formation also occurs during fetal development, resulting in accelerated epidermal development. PPAR and LXR activation regulates keratinocyte proliferation and apoptosis, and studies have shown that these receptors play a role in cutaneous carcinogenesis. Lastly, PPAR and LXR activation is anti-inflammatory, reducing inflammation in animal models of allergic and irritant contact dermatitis. Because of their broad profile of beneficial effects on skin homeostasis, PPAR and LXR have great potential to serve as drug targets for common skin diseases such as psoriasis, atopic dermatitis, and skin cancer.

PPAR and LXR Activators Regulate ABCA12 Expression in Human Keratinocytes

ATP-binding cassette (ABC) transporter, family 12 (ABCA12), a member of the ABC superfamily, facilitates the delivery of lipids to lamellar bodies (LB) in keratinocytes, which is critical for permeability barrier function. Recently, gene mutations of ABCA12 were found to underlie Harlequin ichthyosis and lamellar ichthyosis, two devastating skin disorders. Previously we and others have demonstrated that peroxisome proliferators-activated receptors (PPARs) and liver X receptor (LXR) activation improved epidermal permeability barrier homeostasis by stimulating keratinocyte differentiation, lipid synthesis, and increasing LB formation/secretion. Here we report that both PPAR-gamma and -beta/delta activators markedly stimulate ABCA12 mRNA expression in cultured human keratinocyte (CHK) in a dose- and time-dependent manner. Increased ABCA12 mRNA levels are accompanied by an increase in ABCA12 protein, suggesting biological importance of this upregulation. LXR activators also increase ABCA12 mRNA levels in CHK, but to a lesser extent. In contrast, activators of PPAR-alpha, RAR, RXR, or vitamin D receptor did not alter ABCA12 expression. Two major ABCA12 alternative transcripts and their corresponding proteins are also expressed and upregulated by PPAR or LXR activator in both undifferentiated and differentiated CHK. Together, our data demonstrate that PPAR and LXR activators increase ABCA12 expression, providing an additional mechanism by which PPAR and LXR activators promote epidermal permeability barrier homeostasis.

Characterization of liver X receptor expression and function in human skin and the pilosebaceous unit

The nuclear receptors liver X receptor alpha (LXRalpha) and liver X-receptor beta (LXRbeta) have a well documented role in cholesterol homeostasis and lipid metabolism within tissues and cells including the liver, small intestine and macrophages. In keratinocytes, LXRs have been shown to up-regulate differentiation in vitro via increased transcription of proteins of the AP1 complex and to down-regulate proliferation in vivo. In this study, we provide a detailed description of the location and possible role of LXRs within human skin and its associated glands and appendages. Using RT-PCR, Western blotting and immunohistochemistry, we have demonstrated expression of LXRalpha and LXRbeta mRNA and proteins in whole human skin as well as within a range of primary and immortalized human cell lines derived from human skin, hair follicle and sebaceous glands. Furthermore, we have shown that synthetic LXR specific agonists GW683965 and TO901317 significantly inhibit cell proliferation in primary epidermal keratinocytes, immortalized N/TERT keratinocytes and the immortalized SZ95 sebocyte line, and significantly increase lipogenesis in SZ95 sebocytes. In addition, we showed that the synthetic agonist TO901317 significantly reduced hair growth, in vitro.