Keratinocyte differentiation is stimulated by activators of the nuclear hormone receptor PPARalpha

Implementation of fatty acid carriers to skin irritation and the epidermal barrier

Acute perturbations are followed by barrier repair and enhanced lipid synthesis, as well as cellular fatty acid trafficking, yet irritation of the skin may be induced by repeat disturbance of barrier function. Recently, new insights in cellular fatty acid transport and metabolism have evolved with respect to skin irritation and barrier disturbances: (1) Employing sodium dodecyl sulfate, skin irritation is accompanied by the induction of an epidermal (E) cytosolic fatty acid binding protein (FABP) associated with enhanced barrier repair. Whether E-FABP contributes to the water barrier function in normal skin remains to be elucidated; (2) Cutaneous inflammation, as it occurs in irritant contact dermatitis, can be reduced by peroxisome proliferating activated receptor (PPAR) agonists, such as linoleic acid, with clinical effects comparable to that of glucocorticoids; (3) PPARalpha agonists accelerate barrier recovery and enhance lamellar body synthesis, neutral lipid synthesis, in particular that of ceramides and cholesterol; (4) PPARalpha agonists increase the minimal erythema dose in UVB-irradiated human skin. This review provides a brief overview of the current understanding of mammalian fatty acid (FA) metabolism with respect to epidermal barrier abrogation and repair, including new insights into cellular FA transport and metabolism.

15-Lipoxygenase metabolism of 2-arachidonylglycerol. Generation of a peroxisome proliferator-activated receptor alpha agonist

The recent demonstrations that cyclooxygenase-2 and leukocyte-type 12-lipoxygenase (LOX) efficiently oxygenate 2-arachidonylglycerol (2-AG) prompted an investigation into related oxygenases capable of metabolizing this endogenous cannabinoid receptor ligand. We evaluated the ability of six LOXs to catalyze the hydroperoxidation of 2-AG. Soybean 15-LOX, rabbit reticulocyte 15-LOX, human 15-LOX-1, and human 15-LOX-2 oxygenate 2-AG, providing 15(S)-hydroperoxyeicosatetraenoic acid glyceryl ester. In contrast, potato and human 5-LOXs do not efficiently metabolize this endocannabinoid. Among a series of structurally related arachidonyl esters, arachidonylglycerols serve as the preferred substrates for 15-LOXs. Steady-state kinetic analysis demonstrates that both 15-LOX-1 and 15-LOX-2 oxygenate 2-AG comparably or preferably to arachidonic acid. Furthermore, 2-AG treatment of COS-7 cells transiently transfected with human 15-LOX expression vectors or normal human epidermal keratinocytes results in the production and extracellular release of 15-hydroxyeicosatetraenoic acid glyceryl ester (15-HETE-G), establishing that lipoxygenase metabolism of 2-AG occurs in an eukaryotic cellular environment. Investigations into the potential biological actions of 15-HETE-G indicate that this lipid, in contrast to its free-acid counterpart, acts as a peroxisome proliferator-activated receptor alpha agonist. The results demonstrate that 15-LOXs are capable of acting on 2-AG to provide 15-HETE-G and elucidate a potential role for endocannabinoid oxygenation in the generation of peroxisome proliferator-activated receptor alpha agonists.

PPARgamma-mediated antineoplastic effect of NSAID sulindac on human oral squamous carcinoma cells

There is strong evidence that nonsteroidal antiinflammatory drug (NSAID) sulindac may exert a significant antineoplastic effect. The purpose of our study was to explore the effects of sulindac on human oral squamous cell carcinoma (SCCa) cells and to elucidate the underlying molecular mechanisms. The changes that sulindac treatment induced on growth, apoptosis and cell cycle distribution of human oral SCCa cell lines were assessed by cell growth and flow cytometry experiments. Utilizing quantitative RT-PCR and immunocytochemistry, we determined the effect of sulindac treatment on mRNA and protein expression of different sulindac's targets. Also, PPARgamma expression was selectively targeted by antisense oligonucleotide treatment. Both sulfide and sulfone metabolites of sulindac, which differ in the ability to cause COX-2 inhibition, induced a significant dose- and time-dependent cell growth reduction accompanied by increase in apoptosis without concomitant cell cycle arrest. Sulindac treatment also caused upregulation of the protein and mRNA expression levels of COX-2 and PPARs. Treatment with antisense PPARgamma oligonucleotides abolished sulindac's growth inhibitory effect. Our results are consistent with a significant growth inhibitory effect of NSAID sulindac on human oral SCCa cells, which is mediated, at least partially, through induction of apoptosis. We suggest that upregulation of PPARgamma expression and activation may be, at least partially, responsible for sulindac's antiproliferative effect.

Peroxisome proliferators activate growth regulatory pathways largely via peroxisome proliferator-activated receptor alpha-independent mechanisms

Peroxisome proliferators (PPs) induce liver tumors in rodents through an unknown mechanism requiring PP-activated receptor (PPAR) alpha. Since PPs possess growth modulatory activities that may be important to their hepatocarcinogenicity, we aimed at dissociating the activation of growth signaling pathways from the PPARalpha-mediated response induced by PPs in cultured rat primary hepatocytes. Pretreatment with the differentiation-promoting agent dimethylsulfoxide (DMSO) increased PPARalpha mRNA/protein and enhanced the expression of PPARalpha-regulated genes [fatty acyl Co-A oxidase (FACO), cytochrome P450 4A1 (CYP4A1)] induced by PPs. In contrast, DMSO reduced the expression of immediate early genes (IEG) expression (c-myc, c-jun, c-fos, junB, egr-1) and inhibited mitogen-activated protein kinase (MEK) kinase/extracellular signal-regulated kinases (ERKs) and p38 phosphorylation. Furthermore, the inhibitors Tyrphostin and PD98059 dowregulated IEG/ERKs induction and slightly enhanced the FACO/CYP4A1 response induced by the PP WY-14,643. The stimulation of signal transduction pathways by PPs can be dissociated from PPARalpha activation, thus suggesting that PPs could activate growth regulatory pathways largely via PPARalpha-independent mechanisms.

Chemoprevention of human skin cancer

The incidence of skin cancer has been rising in recent years with significant effects on public health. Primary prevention has proven inadequate in impacting the incidence of skin cancer, thus stimulating the development of chemopreventive strategies. The majority of skin cancer chemoprevention studies focus on occurrence of new nonmelanoma skin cancers (NMSC) in individuals with a previous NMSC, or on reduction in the number of premalignant skin lesions such as actinic keratoses (AK). Dysplastic nevi, a likely precursor of melanoma, are also potential targets for chemoprevention strategies. Premalignant lesions are especially attractive as endpoints since they are more common than frank cancer, resulting in reduced sample size, length, and cost of clinical trials. Development of new agents that affect the pathogenesis of skin cancer will be discussed, from elucidation of molecular targets to implementation of trials designed to determine the effects of chemopreventive interventions on human skin cancer.

Synergistic effect of 4-hydroxynonenal and PPAR ligands in controlling human leukemic cell growth and differentiation

Peroxisome proliferator-activated receptors play an important role in the differentiation of different cell lines. In this study we demonstrate that PPAR-alpha ligands (clofibrate and ciprofibrate) and PPAR-gamma ligands (troglitazone and 15d-prostaglandin J2) inhibit growth and induce monocytic differentiation in HL-60 cells, whereas only PPAR-gamma ligands inhibit growth of U937 cells. Differentiation was demonstrated by the analysis of surface antigen expression CD11b and CD14, and by the characteristic morphological changes. PPAR-gamma ligands are more effective than PPAR-alpha ligands in the inhibition of cell growth and in the induction of differentiation. The physiological product of lipid peroxidation, 4-hydroxynonenal (HNE), which alone induces granulocytic-like differentiation of HL-60 cells, potentiates the monocytic differentiation induced by ciprofibrate, troglitazone, and 15d-prostaglandin J2. The same HNE treatment significantly inhibits U937 cell growth and potentiates the inhibition of cell growth in PPAR-gamma ligand-treated cells. However, HNE does not induce a significant number of CD14-positive U937 cells. HNE causes a great increase of PPAR-gamma expression in both HL-60 and U937 cells, whereas it does not modify the PPAR-alpha expression. This observation may account for the high synergistic effect displayed by HNE and PPAR-gamma ligands in the inhibition of cell growth and differentiation induction. These results represent the first evidence of the involvement of a product of lipid peroxidation in the modulation of PPAR ligand activity and suggest a relationship between HNE and PPAR ligand pathways in leukemic cell growth and differentiation.

Green tea polyphenol stimulates a Ras, MEKK1, MEK3, and p38 cascade to increase activator protein 1 factor-dependent involucrin gene expression in normal human keratinocytes

(-)-Epigallocatechin-3-gallate (EGCG) is an important bioactive constituent of green tea that efficiently reduces epidermal cancer cell proliferation. This inhibition is associated with a reduction in activator protein 1 (AP1) transcription factor level and activity. However, its effects on AP1 function in normal epidermal cells have not been extensively explored. Our present studies show that EGCG regulates normal keratinocyte function. To understand the mechanism of action, we examined the effects of EGCG on AP1 factor activity, MAPK signal transduction, and expression of the AP1 factor-regulated human involucrin (hINV) gene. EGCG increases hINV promoter activity in a concentration-dependent manner that requires the presence of an intact hINV promoter AP1 factor binding site. This response appears to be physiologic, as endogenous hINV gene expression is also increased. Fra-1, Fra-2, FosB, JunB, JunD, c-Jun, and c-Fos levels are increased by EGCG treatment, as is AP1 factor binding to hINV promoter AP1 site. Gel mobility shift studies show that this complex contains Fra-1 and JunD. Signal transduction analysis indicates that the EGCG response requires Ras, MEKK1, MEK3, and p38 kinases. Kinase assays and inhibitor studies suggest that p38delta is the p38 isoform responsible for the regulation. These changes are also associated with a cessation of cell proliferation and enhanced cornified envelope formation. These studies show that in normal human keratinocytes EGCG markedly increases, via a MAPK signaling mechanism, AP1 factor-associated responses.

Peroxisome proliferator-activated receptors and skin development

PPARs are nuclear hormone receptors. PPAR subtypes (alpha, gamma, delta, the latter a xPPARbeta homologue) were initially investigated in skin because of their known role in regulating lipid metabolism. Studies adding specific PPAR ligand activators to cultured skin or skin cells are compatible with the concepts that PPARalpha activation mediates early lipogenic steps common to the function of both skin epidermal cells (keratinocytes) and sebaceous cells (sebocytes), PPARgamma activation plays a unique role in stimulating sebocyte lipogenesis, and PPARdelta activation may contribute to lipid biosynthesis in both sebocytes and keratinocytes under certain circumstances. Epidermal keratinocytes appear to express small amounts of PPARalpha and PPARdelta mRNA and a trace of PPARgamma mRNA which is up-regulated with differentiation. Sebocytes express all subtypes; PPARgamma gene expression excedes that in epidermis. The emerging data on PPAR protein expression suggests that epidermis normally expresses predominantly PPARalpha, while sebocytes express more PPARgamma than PPARalpha. These expression patterns may change during hyperplasia, differentiation and inflammation. Gene disruption studies in mice are compatible with a contribution of PPARalpha to skin barrier function, suggest that PPARgamma is necessary for sebocyte differentiation, and indicate that PPARdelta can ameliorate inflammatory responses in skin. PPARs appear to play a role in keratinocyte synthesis of the lipids that they export to the intercellular space to form the skin permeability barrier. They also appear to be important for sebocyte formation of the intracellular fused lipid droplets that constitute the holocrine secretion of the sebaceous gland. In addition, they may play roles in keratinocyte growth and differentiation and the inhibition of skin inflammation by diverse mechanisms not necessarily related to fat metabolism.

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

Liver X receptor-alpha and -beta are members of the nuclear hormone receptor superfamily that heterodimerize with retinoid X receptor and are activated by oxysterols. In recent studies we found that treatment of cultured human keratinocytes with oxysterolstimulated differentiation, as demonstrated by increased expression of involucrin and transglutaminase, and inhibited proliferation. The aims of this study were to determine: (i) whether oxysterols applied topically to the skin of mice induce differentiation in normal epidermis; (ii) whether this effect is mediated via liver X receptor-alpha and/or liver X receptor-beta; and (iii) whether oxysterols normalize epidermal morphology in an animal model of epidermal hyperplasia. Topical treatment of normal hairless mice with 22(R)-hydroxycholesterol or 24(S),25-epoxycholesterol resulted in a decrease in epidermal thickness and a decrease in keratinocyte proliferation assayed by proliferating cell nuclear antigen staining. Moreover, oxysterol treatment increased the levels of involucrin, loricrin, and profilaggrin protein and mRNA in the epidermis, indicating that oxysterols stimulate epidermal differentiation. Additionally, topical oxysterol pretreatment improved permeability barrier homeostasis. Whereas liver X receptor-alpha-/- mice revealed no alterations in epidermal differentiation, the epidermis was thinner in liver X receptor-beta-/- mice than in wild-type mice, with a reduced number of proliferating cell nuclear antigen positive cells and a modest reduction in the expression of differentiation markers. Topical oxysterol treatment induced differentiation in liver X receptor-alpha-/- mice whereas in liver X receptor-beta-/- mice there was no increase in the expression of differentiation markers. Whereas both liver X receptor-alpha and liver X receptor-beta are expressed in cultured human keratinocytes and in fetal rat skin, only liver X receptor-beta was observed on northern blotting in adult mouse epidermis. Finally, treatment of hyperproliferative epidermis with oxysterols restored epidermal homeostasis. These studies demonstrate that epidermal differentiation is regulated by liver X receptor-beta and that oxysterols, acting via liver X receptor-beta, can induce differentiation and inhibit proliferation in vivo. The ability of oxysterols to reverse epidermal hyperplasia suggests that these agents could be beneficial for the treatment of skin disorders associated with hyperproliferation and/or altered differentiation.

Topical peroxisome proliferator activated receptor-alpha activators reduce inflammation in irritant and allergic contact dermatitis models

Activators of peroxisome proliferator activated receptor-alpha, a nuclear hormone receptor that heterodimerizes with retinoid X receptor, stimulate epidermal differentiation and inhibit proliferation. Here we determined the anti-inflammatory effects of peroxisome proliferator activated receptor-alpha agonists in models of irritant and allergic contact dermatitis produced in mouse ears by topical treatment with 12-O-tetradecanoylphorbol-13-acetate and oxazalone, respectively. As expected, 12-O-tetradecanoylphorbol-13-acetate treatment resulted in a marked increase in the thickness and weight of the ears and provoked an inflammatory cell infiltrate in the dermis. Topical treatment with three different peroxisome proliferator activated receptor-alpha agonists, clofibrate, WY 14643, or linoleic acid, 45 min and 4 h after 12-O-tetradecanoylphorbol-13-acetate application, resulted in a marked decrease in ear thickness and weight and a reduction in the number of inflammatory cells in the dermis. The reduction in inflammation by these peroxisome proliferator activated receptor-alpha agonists was of similar magnitude to that seen with a potent topical glucocorticoid, clobetasol. In contrast, stearic acid, a free fatty acid that does not activate peroxisome proliferator activated receptor-alpha, had no effect on the 12-O-tetradecanoylphorbol-13-acetate-induced inflammation. Moreover, clofibrate did not significantly alter ear thickness following 12-O-tetradecanoylphorbol-13-acetate treatment in peroxisome proliferator activated receptor-alpha-/- mice, indicating that the anti-inflammatory effect is mediated by peroxisome proliferator activated receptor-alpha. As tumor necrosis factor-alpha and interleukin-1alpha are major mediators of cutaneous inflammation we next used immunohistochemistry to determine whether the peroxisome proliferator activated receptor-alpha agonists reduce the levels of these cytokines in 12-O-tetradecanoylphorbol-13-acetate-treated skin. 12-O-tetradecanoylphorbol-13-acetate treatment resulted in an increase in tumor necrosis factor and interleukin-1alpha staining in the epidermis that was reduced by clofibrate treatment. Finally, clofibrate treatment also reduced ear thickness and weight in oxazalone-induced allergic dermatitis, a change that was accompanied by a reduction in inflammatory cells in the dermis and a decrease in tumor necrosis factor-alpha and interleukin-1alpha levels in the oxazalone-treated epidermis. These studies demonstrate that topically applied peroxisome proliferator activated receptor-alpha agonists possess receptor mediated, anti-inflammatory activity in both irritant and allergic contact dermatitis animal models. The anti-inflammatory properties of peroxisome proliferator activated receptor-alpha agonists, coupled with their anti-proliferative and pro-differentiating effects, suggest that they could be beneficial for the treatment of a variety of cutaneous diseases.

Sebaceous epithelial cell differentiation requires cyclic adenosine monophosphate generation

The cyclic adenosine monophosphate (cAMP) generator choleratoxin is known to promote the growth of sebaceous epithelial cells (sebocytes) in monolayer culture in classical serum-containing media. Now that sebocytes can be grown in serum-free medium, we have examined whether choleratoxin or other cAMP generators are required for differentiation of rat preputial sebocytes in response to specific ligand activators of peroxisome proliferator-activated receptors (PPARs). Unexpectedly, choleratoxin reduced sebocyte proliferation. However, sebocyte differentiation in response to specific PPARalpha and PPARgamma agonists required a cAMP generator such as choleratoxin, and this response was suppressed by a protein kinase A inhibitor. In contrast, the stable prostacyclin analog, carbaprostacyclin (cPGI2), a PPARalpha,delta agonist that also generates cAMP, stimulated differentiation independently of choleratoxin. Furthermore, unlike the selective PPARalpha and PPARgamma agonists, cPGI2 stimulated both sebocyte DNA synthesis and proliferation. These data are compatible with the evidence that prostacyclin has the additional effect of generating cAMP. In addition, we addressed the possibility that choleratoxin may act as a surrogate for beta-adrenergic catecholamines in generating cAMP. In contrast with choleratoxin, both alpha- and beta-adrenergic catecholamines stimulated sebocyte growth and interfered with the choleratoxin effect on differentiation. These data suggest ligand-dependent, complex interactions between cAMP and the other signal transduction pathways involved in sebocyte growth and development.

Deletion of RAR carboxyl terminus reveals promoter- and receptor-specific AF-1 effects

Retinoic acid receptors (RARs) are transcription factors with both amino-terminal ligand-independent and carboxyl-terminal ligand-dependent activation functions (AF-1 and AF-2, respectively). RAR-dependent gene activation in keratinocytes was investigated via expression of varied RARalpha and RARgamma carboxyl terminal truncation mutants lacking the AF-2 domain. Overexpression of the AF-1 domain of RARalpha or RARgamma was sufficient to decrease transcriptional activation of retinoid-dependent genes in keratinocytes. Conversely, expression of the same constructs was associated with an increase in expression of endogenous and synthetic reporter genes otherwise negatively regulated by RARs. These effects on transcription driven by some but not all retinoid-sensitive promoters tested could be alleviated by mutation of a serine phosphorylation site in the A/B domain. These results further support the promoter-specificity previously attributed to the RAR AF-1 region and functionally define a particular amino acid residue likely to contribute to the regulation of RARs and other proteins in the transcription complex.

Critical roles of PPAR beta/delta in keratinocyte response to inflammation

The immediate response to skin injury is the release of inflammatory signals. It is shown here, by use of cultures of primary keratinocytes from wild-type and PPAR beta/delta(-/-) mice, that such signals including TNF-alpha and IFN-gamma, induce keratinocyte differentiation. This cytokine-dependent cell differentiation pathway requires up-regulation of the PPAR beta/delta gene via the stress-associated kinase cascade, which targets an AP-1 site in the PPAR beta/delta promoter. In addition, the pro-inflammatory cytokines also initiate the production of endogenous PPAR beta/delta ligands, which are essential for PPAR beta/delta activation and action. Activated PPAR beta/delta regulates the expression of genes associated with apoptosis resulting in an increased resistance of cultured keratinocytes to cell death. This effect is also observed in vivo during wound healing after an injury, as shown in dorsal skin of PPAR beta/delta(+/+) and PPAR beta/delta(+/-) mice.

Activators of peroxisome proliferator-activated receptors protect human skin from ultraviolet-B-light-induced inflammation

Peroxisome proliferator-activated receptors (PPAR) are members of a nuclear receptor superfamily, which were initially described in the context of fatty acid degradation and adipocyte differentiation. In this study we tested the hypothesis that peroxisome proliferator-activated receptor activation also controls inflammation. In an in vitro model with human keratinocytes inflammation was mimicked by irradiation with ultraviolet B light (150 mJ per cm(2)). Activators for PPAR-alpha (WY-14,643, clofibrate) were shown to reverse ultraviolet-B-light-mediated expression of inflammatory cytokines (interleukin-6, interleukin-8). An activator preferentially for PPAR-beta (bezafibrate) did not show prominent effects on interleukin-6 and interleukin-8 expression. The anti-inflammatory action of WY-14,643 on skin cells was further demonstrated by in vivo testings in which topically applied WY-14,643 markedly increased the minimal erythema dose in ultraviolet-B-irradiated skin. Additionally, it was shown that ultraviolet B irradiation led to a decrease of all three peroxisome proliferator-activated receptor subsets at the mRNA level. Also transactivation of peroxisome proliferator response element was attenuated by ultraviolet B irradiation. The downregulation of peroxisome proliferator-activated receptors by ultraviolet B irradiation provides a possible mechanism that leads to exaggerated and prolonged inflammation. This work suggests the possibility of PPAR-alpha activators as novel nonsteroidal anti-inflammatory drugs in the topical treatment of common inflammatory skin diseases such as atopic dermatitis, psoriasis, and photodermatitis.

Impaired skin wound healing in peroxisome proliferator-activated receptor (PPAR)alpha and PPARbeta mutant mice

We show here that the alpha, beta, and gamma isotypes of peroxisome proliferator-activated receptor (PPAR) are expressed in the mouse epidermis during fetal development and that they disappear progressively from the interfollicular epithelium after birth. Interestingly, PPARalpha and beta expression is reactivated in the adult epidermis after various stimuli, resulting in keratinocyte proliferation and differentiation such as tetradecanoylphorbol acetate topical application, hair plucking, or skin wound healing. Using PPARalpha, beta, and gamma mutant mice, we demonstrate that PPARalpha and beta are important for the rapid epithelialization of a skin wound and that each of them plays a specific role in this process. PPARalpha is mainly involved in the early inflammation phase of the healing, whereas PPARbeta is implicated in the control of keratinocyte proliferation. In addition and very interestingly, PPARbeta mutant primary keratinocytes show impaired adhesion and migration properties. Thus, the findings presented here reveal unpredicted roles for PPARalpha and beta in adult mouse epidermal repair.

Modulation of keratinocyte gene expression and differentiation by PPAR-selective ligands and tetradecylthioacetic acid

Peroxisome proliferator-activated receptors (PPARs) are pleiotropic regulators of growth and differentiation of many cell types. We have performed a comprehensive analysis of the expression of PPARs, transcriptional cofactors, and marker genes during differentiation of normal human keratinocytes using a combination of reverse transcriptase polymerase chain reaction, Northern and Western blotting, and immunohistochemistry. PPARdelta was the predominant PPAR subtype in human keratinocytes and highly expressed in basal cells and suprabasal cells. Induction of PPARalpha and PPARgamma expression was linked to differentiation, and accordingly, expression of PPARalpha and PPARgamma was in essence confined to suprabasal cells. Differentiation was not accompanied by significant changes in the expression of the coactivators CREB-binding protein, p300, steroid receptor coactivator 1, or the corepressors nuclear receptor corepressor and silence mediator for retinoid and thyroid hormone receptors. We critically evaluated the effects of selective PPAR ligands and a synthetic fatty acid analog, tetradecylthioacetic acid. Tetradecylthioacetic acid activated all human PPAR subtypes in the ranking order PPARdelta >> PPARalpha > PPARgamma. All selective PPAR ligands marginally induced transglutaminase-1 expression with the PPARdelta-selective ligand L165041 being the most potent. The PPARalpha- and PPARgamma-selective ligands Wy14643 and BRL49653 had negligible effect on involucrin expression, whereas a dose-dependent induction was observed with L165041. Simultaneous addition of L165041 and BRL49653 synergistically induced strong involucrin expression. Additionally, L165041 potently induced CD36 mRNA expression. Administration of tetradecylthioacetic acid resulted in a dramatic decrease in proliferation and a robust upregulation of the expression of involucrin and transglutaminase. Our results indicate that tetradecylthioacetic acid may affect keratinocyte gene expression and differentiation via PPAR-dependent and PPAR-independent pathways, and that the latter play an important role.

Epithelial structural proteins of the skin and oral cavity: function in health and disease

Epithelial tissues function to protect the organism from physical, chemical, and microbial damage and are essential for survival. To perform this role, epithelial keratinocytes undergo a well-defined differentiation program that results in the expression of structural proteins which maintain the integrity of epithelial tissues and function as a protective barrier. This review focuses on structural proteins of the epidermis and oral mucosa. Keratin proteins comprise the predominant cytoskeletal component of these epithelia. Keratin filaments are attached to the plasma membrane via desmosomes, and together these structural components form a three-dimensional array within the cytoplasm of epithelial cells and tissues. Desmosomes contain two types of transmembrane proteins, the desmogleins and desmocollins, that are members of the cadherin family. The desmosomal cadherins are linked to the keratin cytoskeleton via several cytoplasmic plaque proteins, including desmoplakin and plakoglobin (gamma-catenin). Epidermal and oral keratinocytes express additional differentiation markers, including filaggrin and trichohyalin, that associate with the keratin cytoskeleton during terminal differentiation, and proteins such as loricrin, small proline-rich proteins, and involucrin, that are cross-linked into the cornified envelope by transglutaminase enzymes. The importance of these cellular structures is highlighted by the large numbers of genetic and acquired (autoimmune) human disorders that involve mutations in, or autoantibodies to, keratins and desmosomal and cornified envelope proteins. While much progress has been made in the identification of the structural proteins and enzymes involved in epithelial differentiation, regulation of this process is less clear. Both calcium and retinoids influence epithelial differentiation by altering the transcription of target genes and by regulating activity of enzymes critical in epithelial differentiation, such as transglutaminases, proteinases, and protein kinases. These studies have furthered our understanding of how epithelial tissue and cell integrity is maintained and provide a basis for the future treatment of skin and oral disorders by gene therapy and other novel therapeutics.

RXR-alpha ablation in skin keratinocytes results in alopecia and epidermal alterations

RXR-alpha is the most abundant of the three retinoid X receptors (RXRs) in the epidermis. In this study, we have used Cre-mediated recombination to selectively disrupt the mouse gene for RXR-alpha in epidermal and hair follicle keratinocytes. We show that RXR-alpha is apparently dispensable for prenatal epidermal development, while it is involved in postnatal skin maturation. After the first hair pelage, mutant mice develop a progressive alopecia, histologically characterised by the destruction of hair follicle architecture and the formation of utriculi and dermal cysts in adult mice. Our results demonstrate that RXR-alpha plays a key role in anagen initiation during the hair follicle cycle. In addition, RXR-alpha ablation results in epidermal interfollicular hyperplasia with keratinocyte hyperproliferation and aberrant terminal differentiation, accompanied by an inflammatory reaction of the skin. Our data not only provide genetic evidence that RXR-alpha/VDR heterodimers play a major role in controlling hair cycling, but also suggest that additional signalling pathways mediated by RXR-alpha heterodimerised with other nuclear receptors are involved in postnatal hair follicle growth, and homeostasis of proliferation/differentiation of epidermal keratinocytes and of the skin's immune system.

Activators of peroxisome proliferator-activated receptor-alpha partially inhibit mouse skin tumor promotion

Several recent reports have suggested that peroxisome proliferator-activated receptors (PPARs) may be involved in the development of neoplasias in different tissue types. The present study was undertaken to determine whether PPARs play a role in skin physiology and tumorigenesis. In an initiation-promotion study, SENCAR mice treated topically with the PPARalpha ligands conjugated linoleic acid and 4-chloro-6-(2,3-xylidino)-2-pyrimidinylthioacetic acid (Wy-14643) exhibited an approximately 30% lower skin tumor yield compared with mice treated with vehicle. The PPARgamma and PPARdelta activators troglitazone and bezafibrate, respectively, exerted little, if any, inhibitory activity. PPARalpha was detected in normal and hyperplastic skin and in papillomas and carcinomas by immunohistochemistry. In addition, PPARalpha, PPARdelta/PPARbeta, and PPARgamma protein levels were analyzed by immunoblotting in normal epidermis and papillomas. Surprisingly, the levels of all three isoforms were increased significantly in tumors as opposed to normal epidermis. In primary keratinocyte cultures, protein levels of PPARalpha and, to a lesser extent, PPARgamma were markedly increased when the cells were induced to differentiate with high-calcium (0.12 mM) conditions. In addition, we observed that Wy-14643 enhanced transcriptional activity of a peroxisome proliferator-response element-driven promoter in a mouse keratinocyte cell line. These results demonstrate that keratinocytes express functional PPARalpha, that PPARalpha may play a role in differentiation, and that ligands for PPARalpha are moderately protective against skin tumor promotion. We conclude that selective PPARalpha ligands may exert their protective role against skin tumor promotion by ligand activation of PPARalpha.

Keratinocyte differentiation in hyperproliferative epidermis: topical application of PPARalpha activators restores tissue homeostasis

We recently showed that topically applied PPARalpha activators promote epidermal differentiation in intact adult mouse skin. In this study we determined the effect of clofibrate and Wy-14,643, activators of PPARalpha, on hyperproliferative epidermis in hairless mice, induced either by repeated barrier abrogation (subacute model) or by essential fatty acid deficiency (chronic model). The hyperproliferative epidermis was characterized by an increased number of proliferating cells expressing proliferating cell nuclear antigen. Topical treatment with PPARalpha activators resulted in a substantial decrease in epidermal hyperplasia in both the subacute and chronic models of hyperproliferation. Following topical treatment, proliferating cell nuclear antigen-expressing cells were restricted to the basal layer, similar to normal epidermis. In hyperproliferative epidermis there was decreased expression of involucrin, profilaggrin-filaggrin, and loricrin as assayed by in situ hybridization and immunohistochemistry. Following topical treatment with PPAR activators staining for these mRNAs and proteins increased towards normal levels. Finally, topically applied clofibrate also increased apoptosis. This study demonstrates that topical PPAR activators have profound effects on epidermal gene expression in hyperproliferative skin disorders. Treatment with PPARalpha activators normalizes cell proliferation and promotes epidermal differentiation, correcting the cutaneous pathology. This study identifies PPARalpha activators as potential skin therapeutic agents.

Stimulation of PPARalpha promotes epidermal keratinocyte differentiation in vivo

Our recent studies have demonstrated that PPARalpha activators stimulate differentiation and inhibit proliferation in cultured human keratinocytes and accelerate epidermal development and permeability barrier formation in fetal rat skin explants. As the role of PPARalpha activation in adult epidermis is not known, the aim of this study was to determine if topically applied PPARalpha ligands regulate keratinocyte differentiation in murine epidermis. Topical treatment with PPARalpha activators resulted in decreased epidermal thickness. Expression of structural proteins of the upper spinous/granular layers (involucrin, profilaggrin-filaggrin, loricrin) increased following topical treatment with PPARalpha activators. Furthermore, topically applied PPARalpha activators also increased apoptosis, decreased cell proliferation, and accelerated recovery of barrier function following acute barrier abrogation. Experiments with PPARalpha-/- knockout mice showed that these effects are specifically mediated via PPARalpha. Compared with the epidermis of PPARalpha+/+ mice, involucrin, profilaggrin-filaggrin, and loricrin expression were slightly decreased in PPARalpha-/- mice. Moreover, topical clofibrate treatment did not increase epidermal differentiation in PPARalpha-/- mice. Furthermore, in cultured human keratinocytes we have demonstrated that PPARalpha activators induce an increase in involucrin mRNA levels. We have also shown that this increase in gene expression requires an intact AP-1 response element at -2117 to -2111 bp. Thus, stimulation of PPARalpha stimulates keratinocyte/epidermal differentiation and inhibits proliferation.

Farnesol stimulates differentiation in epidermal keratinocytes via PPARalpha

The isoprenoids farnesol and juvenile hormone III (JH), metabolites of the cholesterol biosynthetic pathway, have been shown to stimulate fetal epidermal development in rodents. In this study we determined whether this effect might be attributed to a direct induction of keratinocytes differentiation and examined the mechanisms responsible for these effects. Rates of cornified envelope formation, a marker of keratinocyte terminal differentiation, as well as protein and mRNA levels of two proteins required for cornified envelope formation, involucrin (INV) and transglutaminase, increased 2- to 3-fold in normal human keratinocytes (NHK) treated with either farnesol or JH, even at low calcium concentrations (0.03 mM), which otherwise inhibit differentiation. In contrast, neither cholesterol nor mevalonate affected INV or transglutaminase mRNA levels. Effects of farnesol and JH on INV and transglutaminase mRNA levels were additive with high calcium concentrations (1.2 mM) that independently stimulate keratinocyte differentiation. In contrast, keratinocyte DNA synthesis was inhibited by these compounds. Both farnesol and JH stimulated INV and transglutaminase promoter activity, suggesting regulation at the transcriptional level. A series of truncation and deletion experiments revealed a farnesol-responsive region (-2452 to -1880 base pairs (bp)) in the INV gene. This region contained an AP-1 site. A single base pair mutation of the AP-1 site at -2116 to -2110 bp abolished farnesol responsiveness, identical to effects by peroxisome proliferator-activated receptor (PPARalpha) activators. Farnesoid X-activated receptor mRNA was not detected in NHK, but farnesol treatment increased activities of both a PPAR response element and PPARalpha mRNA levels in NHK. Furthermore, the increase in PPRE activity by farnesol was dependent upon PPARalpha in CV-1 cells. Finally, topical applications of farnesol increased mRNA and protein levels of the differentiation-specific genes, profilaggrin and loricrin, determined by immunohistochemistry and in situ hybridization, in wild-type but not in PPARalpha-/- murine epidermis. These findings suggest a novel role for selected isoprenoid cholesterol intermediates in the regulation of differentiation-specific gene transcription and a convergence of PPARalpha with the cholesterol synthetic pathway.

Peroxisome proliferator-activated receptor-alpha enhances lipid metabolism in a skin equivalent model

Peroxisome proliferator-activated receptors are involved in certain cell types such as adipocytes and hepatocytes, in the control of several pathways of lipid synthesis or catabolism by regulating the gene expression level of key lipid metabolizing enzymes. As the epidermis exhibits an extensive lipid metabolism necessary for the establishment of the barrier function, we have examined the role of peroxisome proliferator-activated receptor-alpha activation in this process. Living skin equivalents were treated with Wy 14,643, a selective peroxisome proliferator- activated receptor-alpha ligand, which enhanced greatly the synthesis of membrane coating granules, the organelles specialized in the processing of stratum corneum lipids. Also, the overall stratum corneum neutral lipid content assessed by Oil red O staining was increased. A detailed analysis of the lipid species present in the reconstructed epidermis showed that peroxisome proliferator-activated receptor-alpha activation increased the synthesis of ceramides and cholesterol derivatives, thought to be essential structural components of the permeability barrier. A synergistic effect was observed on lipid synthesis when peroxisome proliferator-activated receptor-alpha and retinoid X receptor were simultaneously activated by selective ligands. Furthermore, activation of peroxisome proliferator-activated receptor-alpha led to increased mRNA expression of several key enzymes of ceramide and cholesterol metabolism. An increase of serine-palmitoyl transferase and of beta-glucocerebrosidase enzymatic activity was also demonstrated. Altogether, these results show that peroxisome proliferator-activated receptor-alpha is a key transcription factor involved in the control of the epidermal lipid barrier.

Oxysterols induce differentiation in human keratinocytes and increase Ap-1-dependent involucrin transcription

Ligands and activators of the nuclear hormone receptor superfamily are important in the regulation of epidermal development and differentiation. Previously, we showed that naturally occurring fatty acids, as well as synthetic ligands for the peroxisome proliferator-activated receptor, induce keratinocyte differentiation in vitro. Here we asked whether oxysterols, another class of lipids formed de novo in the epidermis and that activate liver X-activated receptor, regulate keratinocyte differentiation. mRNA and protein levels of involucrin and transglutaminase 1, markers of differentiation, increased 2- to 3-fold in normal human keratinocytes incubated in the presence of 25- or 22R-hydroxycholesterol in low calcium. In high calcium, which alone induces differentiation, mRNA levels were further increased by oxysterols. Rates of cornified envelope formation, an indicator of terminal differentiation, also increased 2-fold with oxysterol treatment. In contrast, the rate of DNA synthesis was inhibited approximately 50% by oxysterols. Transcriptional regulation was assessed in keratinocytes transfected with either transglutaminase 1 or involucrin promoter-luciferase constructs. 22R-hydroxycholesterol increased transglutaminase 1 and involucrin promoter activity 2- to 3-fold. Either deletion of the -2452 bp to -1880 bp region of the involucrin promoter, or mutation of the AP-1 site within this region, abolished oxysterol responsiveness. Moreover, increased AP-1 DNA binding was observed in oxysterol-treated keratinocytes by gel shift analyses. Finally, we demonstrated the presence of liver X-activated receptor alpha and beta mRNAs, and showed that oxysterols stimulate a liver X-activated receptor response element transfected into keratinocytes. These data suggest that oxysterols induce keratinocyte differentiation, in part through increased AP-1-dependent transcription of the involucrin gene, an effect that may be mediated by liver X-activated receptor.

The flavonoid apigenin suppresses vitamin D receptor expression and vitamin D responsiveness in normal human keratinocytes

Apigenin, a flavonoid with chemopreventive properties, induces cellular growth arrest, with concomitant inhibition of intracellular signaling cascades and decreased proto-oncogene expression. We report that apigenin potently inhibited vitamin D receptor (VDR) mRNA and protein expression in human keratinocytes without changes in VDR mRNA half-life. Concurrently, downregulation of retinoid X receptor alpha, a dramatic loss of c-myc mRNA, and upregulation of p21(WAF1) took place. Furthermore, a nearly complete suppression of vitamin D responsiveness was observed as estimated by induction of 24-hydroxylase mRNA. The apigenin effect on VDR expression was shared by some other (quercetine and fisetine) but not all tested flavonoids. Interestingly, the apigenin-mediated VDR suppression was counteracted by the NFkappaB inhibitors sodium salicylate and caffeic acid phenethyl ester. The presented results propose suppression of nuclear receptor levels as a novel mechanism whereby flavonoids exert their pleiotropic effects. This study may also contribute to the understanding of the regulation of VDR expression in epidermal keratinocytes.

Peroxisome proliferator-activated receptors: three isotypes for a multitude of functions

The peroxisome proliferator-activated receptors (PPARs) are fatty acid and eicosanoid inducible nuclear receptors, which occur in three different isotypes. Upon activator binding, they modulate the expression of various target genes implicated in several important physiological pathways. During the past few years, the identification of both PPAR ligands, natural and synthetic, and PPAR targets and their associated functions has been one of the most important achievements in the field. It underscores the potential therapeutic application of PPAR-specific compounds on the one side, and the crucial biological roles of endogenous PPAR ligands on the other.

Fetal epidermal differentiation and barrier development In vivo is accelerated by nuclear hormone receptor activators

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.

Peroxisome proliferator-activated receptors: a critical link among fatty acids, gene expression and carcinogenesis

It has been known for many years that long-chain fatty acids derived from endogenous metabolism and/or nutrition can act as second messengers and regulators of cell signaling pathways. For example, fatty acids regulate the activity of protein kinase C (PKC) in a mechanism distinct from activation by diacylglycerol. Like PKC activators such as phorbol esters, essential fatty acids activate PKC and in doing so modulate the activity of growth factor receptors such as epidermal growth factor receptor (EGFR). Unsaturated fatty acids can inhibit GTPase activating protein, thereby quenching signals from p21-ras. These studies have shown that fatty acids can influence numerous signaling pathways and that these small lipophilic substances may be ancient second messengers. Fatty acids are also known modulators of the carcinogenic process, showing distinct tissue-specific pro- or anticancer effects. However, the reason for such a dichotomous effect on cellular processes has not been adequately described. In this article, the inclusion of a steroid hormone receptor-signaling pathway in mediating fatty acids' effects will be summarized. This signaling molecule has been deemed the peroxisome proliferator-activated receptor (PPAR) and has been extensively examined in regard to its response to xenobiotic, fatty acid-like chemicals (peroxisome proliferators, PP). PP, like fatty acids, activate PPAR and modulate tissue-specific responses. The goal of this review is to describe a potential role for PPAR in mediating the effects of fatty acids on gene expression, cell growth, differentiation and apoptosis.

Correlation between expression of peroxisome proliferator-activated receptor beta and squamous differentiation in epidermal and tracheobronchial epithelial cells

Previously, several members of the nuclear receptor superfamily have been implicated in the regulation of epidermal differentiation. In this study, we analyze the expression of members of the PPAR nuclear receptor subfamily in relation to the process of squamous differentiation in normal human epidermal keratinocytes (NHEK), human tracheobronchial epithelial (HBE) cells and the epidermis in vivo. Our results demonstrate that induction of differentiation in NHEK by either treatment with the phorbol ester phorbol 12-myristate-13-acetate (PMA), suspension culture or confluence greatly enhances the expression of PPARbeta mRNA. Likewise, topical treatment of mouse skin with PMA results in increased PPARbeta mRNA expression in the epidermis. In addition, the induction of squamous differentiation in HBE cells was also associated with an upregulation of PPARbeta mRNA expression. Finally, in situ hybridization analysis localized PPARbeta mRNA to the suprabasal layers of normal human skin. Our results demonstrate that the expression of PPARbeta is associated with squamous differentiation suggesting a regulatory role for this receptor in the control of specific genes during this differentiation process.

Ligand selectivity of the peroxisome proliferator-activated receptor alpha

Peroxisome proliferator-activated receptors (PPAR alpha, beta, and gamma) are nuclear hormone receptors that play critical roles in regulating lipid metabolism. It is well established that PPARs are the targets for the hypolipidemic synthetic compounds known as peroxisome proliferators, and it has been proposed that various long-chain fatty acids and metabolites of arachidonic acid serve as the physiological ligands that activate these receptors in vivo. However, a persistent problem is that reported values of the equilibrium dissociation constants (Kds) of complexes of PPARs with these ligands are in the micromolar range, at least an order of magnitude higher than the physiological concentrations of the ligands. Thus, the identity of the endogenous ligands for PPAR remains unclear. Here we report on a fluorescence-based method for investigating the interactions of PPAR with ligands. It is shown that the synthetic fluorescent long-chain fatty acid trans-parinaric acid binds to PPARalpha with high affinity and can be used as a probe to monitor protein-ligand interactions by the receptor. Measurements of Kds characterizing the interactions of PPARalpha with various ligands revealed that PPARalpha interacts with unsaturated C:18 fatty acids, with arachidonic acid, and with the leukotriene LTB4 with affinities in the nanomolar range. These data demonstrate the utility of the optical method in examining the ligand-selectivity of PPARs, and resolve a long-standing uncertainty in understanding how the activities of these receptors are regulated in vivo.