Topical treatment with thiazolidinediones, activators of peroxisome proliferator-activated receptor-?, normalizes epidermal homeostasis in a murine hyperproliferative disease model

Ligand activation of peroxisome proliferator-activated receptor-beta/delta inhibits cell proliferation in human HaCaT keratinocytes

Although there is strong evidence that ligand activation of peroxisome proliferator-activated receptor (PPAR)-beta/delta induces terminal differentiation and attenuates cell growth, some studies suggest that PPARbeta/delta actually enhances cell proliferation. For example, it was suggested recently that retinoic acid (RA) is a ligand for PPARbeta/delta and potentiates cell proliferation by activating PPARbeta/delta. The present study examined the effect of ligand activation of PPARbeta/delta on cell proliferation, cell cycle kinetics, and target gene expression in human HaCaT keratinocytes using two highly specific PPARbeta/delta ligands [4-[[[2-[3-fluoro-4-(trifluoromethyl)phenyl]-4-methyl-5-thiazolyl]methyl]thio]-2-methylphenoxy acetic acid (GW0742) and 2-methyl-4-((4-methyl-2-(4-trifluoromethylphenyl)-1,3-thiazol-5-yl)-methylsulfanyl)phenoxy-acetic acid (GW501516)] and RA. Both PPARbeta/delta ligands and RA inhibited cell proliferation of HaCaT keratinocytes. GW0742 and GW501516 increased expression of known PPARbeta/delta target genes, whereas RA did not; RA increased the expression of known retinoic acid receptor/retinoid X receptor target genes, whereas GW0742 did not affect these genes. GW0742, GW501516, and RA did not modulate the expression of 3-phosphoinositide-dependent protein kinase or alter protein kinase B phosphorylation. GW0742 and RA increased annexin V staining as quantitatively determined by flow cytometry. The effects of GW0742 and RA were also examined in wild-type and PPARbeta/delta-null primary mouse keratinocytes to determine the specific role of PPARbeta/delta in modulating cell growth. Although inhibition of keratinocyte proliferation by GW0742 was PPARbeta/delta-dependent, inhibition of cell proliferation by RA occurred in both genotypes. Results from these studies demonstrate that ligand activation of PPARbeta/delta inhibits keratinocyte proliferation through PPARbeta/delta-dependent mechanisms. In contrast, the observed inhibition of cell proliferation in mouse and human keratinocytes by RA is mediated by PPARbeta/delta-independent mechanisms and is inconsistent with the notion that RA potentiates cell proliferation by activating PPARbeta/delta.

The many faces of PPARgamma: anti-inflammatory by any means?

The peroxisome proliferator-activated receptor gamma (PPARgamma) is a member of the nuclear receptor superfamily, a group of transcription factors that regulate expression of their target genes upon ligand binding. As endogenous ligands, oxidized fatty acids and prostanoids can bind to and activate the receptor. Natural and synthetic PPARgamma activators have been studied extensively in many inflammatory settings and in most instances they have been shown to be anti-inflammatory. In this review we give an overview of the different molecular mechanisms how PPARgamma and its agonists exert their anti-inflammatory effects both at the cellular level and the level of the organism. The action of PPARgamma in acute and chronic inflammatory diseases and disease models will be presented.

Chinese herbal medicine (Tuhuai extract) exhibits topical anti-proliferative and anti-inflammatory activity in murine disease models

While psoriasis is one of the most common skin disorders in humans, effective, safe and inexpensive treatments are still largely unavailable. Chinese herbal medicine (CHM) has been used for centuries for treating psoriasis and several reports claim that systemic administration of one such CHM, Tuhuai, mainly composed of flos sophorae, smilax glabra roxb and licorice, is effective in psoriasis. However, the mechanisms by which this CHM improves psoriasis are not yet clear. Two universal features of psoriasis are epidermal hyperplasia and inflammation. Moreover, drugs that specifically inhibit epidermal hyperplasia and/or inflammation are widely used to treat psoriasis. Here, we investigated whether topical applications of Tuhuai extract exhibit anti-proliferative and anti-inflammatory activities in two murine models of inflammatory dermatoses. To assess Tuhuai's potential anti-proliferative effect, we disrupted epidermal barrier function twice-daily for 4 days in normal hairless mice followed by topical applications of either 1% Tuhuai extract or Vehicle to both flanks immediately after each barrier perturbation. Changes in epidermal proliferation and apoptosis were evaluated by immunohistochemistry and TUNEL staining. To assess the anti-inflammatory effects of Tuhuai, both irritant (phorbol ester) and acute allergic contact dermatitis (oxazolone) models were used. Whereas topical Tuhuai extract did not alter epidermal proliferation or induce irritation in normal skin, it both reduced epidermal hyperplasia in the epidermal hyperproliferative model, and reduced inflammation in both irritant and allergic contact dermatitis models. As topical Tuhuai extract exhibits anti-proliferative and anti-inflammatory properties in a variety of human models of inflammatory dermatoses, Tuhuai could provide an effective, relatively safe and inexpensive therapeutic alternative for the treatment of inflammatory dermatoses, including psoriasis.

Oxidized glycerophosphocholines as biologically active mediators for ultraviolet radiation-mediated effects

Ultraviolet light radiation (UVR) has profound effects upon human skin. Yet, the exact targets for UVR are unclear. Inasmuch as UVR is a known pro-oxidative stressor, one potential target for UVR could be oxidatively modified glycerophosphocholines (GPC). Importantly, recent studies demonstrate that these oxidized GPCs (ox-GPC) are potent agonists for the platelet-activating factor receptor and peroxisome proliferator-activated receptor gamma. This review discusses these new biologically active lipids and their down-stream receptor targets that provide a unique system of biosensors for detecting and responding to UVR photo-oxidation.

Toll-like receptor 4 mediates cross-talk between peroxisome proliferator-activated receptor gamma and nuclear factor-kappaB in macrophages

The peroxisome proliferator-activated receptor gamma (PPARgamma) is expressed in macrophages and plays an important role in suppressing the inflammatory response. Lipopolysaccharides (LPS), which activate Toll-like receptor 4 (TLR4), reduced PPARgamma expression and function in peritoneal macrophages and macrophage cell lines. Moreover, pretreatment with the synthetic PPARgamma ligand, rosiglitazone did not prevent LPS-mediated downregulation of PPARgamma. Inhibition of PPARgamma expression was not blocked by cycloheximide, indicating that de novo protein synthesis is not required for LPS-mediated suppression of PPARgamma. Destabilization of PPARgamma messenger RNA (mRNA) was not observed in LPS-stimulated macrophages, suggesting that LPS regulates the synthesis of PPARgamma mRNA. LPS had no effect on PPARgamma expression in macrophages from TLR4 knockout mice, whereas LPS inhibited PPARgamma expression in cells that had been reconstituted to express functional TLR4. Targeting the TLR4 pathway with inhibitors of MEK1/2, p38, JNK and AP-1 had no effect on PPARgamma downregulation by LPS. However, inhibitors that target NEMO, IkappaB and NF-kappaB abolished LPS-mediated downregulation of PPARgamma in LPS-stimulated macrophages. Our data indicate that activation of TLR4 inhibits PPARgamma mRNA synthesis by an NF-kappaB-dependent mechanism. Low-density genomic profiling of macrophage-specific PPARgamma knockout cells indicated that PPARgamma suppresses inflammation under basal conditions, and that loss of PPARgamma expression is sufficient to induce a proinflammatory state. Our data reveal a regulatory feedback loop in which PPARgamma represses NF-kappaB-mediated inflammatory signalling in unstimulated macrophages; however, upon activation of TLR4, NF-kappaB drives down PPARgamma expression and thereby obviates any potential anti-inflammatory effects of PPARgamma in LPS-stimulated macrophages.

K6PC-5, a sphingosine kinase activator, induces anti-aging effects in intrinsically aged skin through intracellular Ca2+ signaling

BACKGROUND

Sphingosine-1-phosphate (S1P), a bioactive sphingolipid metabolite, regulates multiple cellular responses such as Ca(2+) signaling, growth, survival, and differentiation. Because sphingosine kinase (SK) is the enzyme directly responsible for the production of S1P, many factors have been identified that regulate its activity and subsequent S1P levels. To date, there are no reports to demonstrate a chemically induced, direct activation of SK.

OBJECTIVE

Here we have studied the effects of K6PC-5 as a newly synthesized SK activator on fibroblast proliferation in both human fibroblasts and aged mouse skin. To demonstrate that K6PC-5 has S1P-mediated action mechanism in fibroblasts, we have measured SK-dependent intracellular Ca(2+) signaling.

METHODS

Fibroblasts were cultured primarily from human foreskin and were used to study the effect of K6PC-5 and S1P on intracellular Ca(2+) signaling and fibroblast proliferation. Changes in intracellular Ca(2+) were detected by fluorescence with fura-2/AM. To study skin anti-aging effects of K6PC-5, we used intrinsically aged hairless mice (56 weeks old).

RESULTS

K6PC-5 promoted fibroblast proliferation and procollagen production in human fibroblasts significantly. K6PC-5 induced intracellular Ca(2+) concentration ([Ca(2+)](i)) oscillations in human fibroblasts. Both dimethylsphingosine and dihydroxysphingosine, SK inhibitors, and the transfection of SK1-siRNA blocked the K6PC-5-induced increases in [Ca(2+)](i), an effect independent of the classical PLC/IP(3)-mediated pathway. The K6PC-5-induced [Ca(2+)](i) oscillations were dependent on thapsigargin-sensitive Ca(2+) stores and Ca(2+) entry. Topical application of K6PC-5 for 2 weeks to intrinsically aged hairless mice enhanced fibroblast proliferation, collagen production, and eventually increased dermal thickness (10%). K6PC-5 also promoted specific epidermal differentiation marker proteins, including involucrin, loricrin, filaggrin, and keratin 5, without any alterations on epidermal barrier function.

CONCLUSION

These results suggest that K6PC-5 acts to regulate fibroblast proliferation through intracellular S1P production, and can further promote keratinocyte differentiation. We anticipate that the regulation of S1P levels may represent a novel approach for the treatment of skin disorders, including skin aging.

Systemic PPARgamma ligation inhibits allergic immune response in the skin

We have shown previously that specific ligands of the peroxisome proliferator-activated receptor-gamma (PPARgamma) inhibit the systemic allergic immune response. The objective of this study was to investigate the impact of PPARgamma-ligand treatment on the local allergic immune response. We established a murine model exhibiting clinical and histological features of AD-like skin lesions with high reproducibility. In this model, the PPARgamma ligand was applied in an either preventive or therapeutic manner via systemic and local routes. The affected skin areas were assessed by standardized skin score, histological analyses, and immunohistochemical examinations. Our data show that systemic application of PPARgamma ligand by a preventive protocol led to significantly reduced onset of eczematous skin lesions. This was confirmed by histology, showing decreased skin thickness accompanied by significantly reduced infiltrations of CD4+ and CD8+ lymphocytes but also mast cells. Additionally, early allergen-specific IgE and IgG1 responses were reduced (day 21/35), whereas IgG2a levels remained unchanged. In conclusion, our results demonstrate that PPARgamma-ligand treatment inhibits not only systemic allergic immune response, but also local allergen-mediated dermatitis. Our findings point to therapeutic strategies, including a PPARgamma-ligand-based treatment.

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.

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.

Deficiency of PPARbeta/delta in the epidermis results in defective cutaneous permeability barrier homeostasis and increased inflammation

In cultured human keratinocytes or murine epidermis, peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta) (NR1C2) activators (1) stimulate keratinocyte differentiation; (2) decrease keratinocyte proliferation; (3) accelerate permeability barrier repair; (4) increase epidermal lipid synthesis; and (5) reduce cutaneous inflammation. Since these results suggest that PPARbeta/delta could play an important role in cutaneous homeostasis, we assessed here the skin phenotype of mice deficient in PPARbeta/delta. Gross cutaneous abnormalities were not evident, and both stratum corneum (SC) skin hydration and surface pH were normal. However, the epidermis was thickened and proliferating cell nuclear antigen (PCNA) staining was increased, indicating increased cell proliferation. No change in apoptosis was observed but the expression of differentiation markers, such as filaggrin, involucrin, and loricrin, was slightly increased in PPARbeta/delta(-/-) mice. Although basal permeability barrier function was normal, PPARbeta/delta knockout (KO) mice show a significant delay in barrier recovery rates following acute barrier disruption by either acetone treatment or tape-stripping. Delayed barrier recovery correlated with decreased production and secretion of lamellar bodies (LBs), and with reduced numbers of extracellular lamellar membranes in the SC. Finally, PPARbeta/delta KO mice displayed increased inflammation in response to 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment. Together, these results further demonstrate that PPARbeta/delta in the epidermis: (1) is required for permeability barrier homeostasis; (2) regulates keratinocyte proliferation; and (3) modulates cutaneous inflammation.

Characterization of a hapten-induced, murine model with multiple features of atopic dermatitis: structural, immunologic, and biochemical changes following single versus multiple oxazolone challenges

Atopic dermatitis (AD) is a chronic dermatosis bearing clinical, histological, and immunologic similarities to chronic allergic contact dermatitis (ACD). AD shows a Th2 cell-dominant inflammatory infiltrate, elevated serum IgE levels, a permeability barrier abnormality, and Staphylococcus aureus colonization. Repeated hapten challenges reportedly produce a Th2-like hypersensitivity reaction (Th2-like HR). Here, 9-10 challenges with oxazolone (Ox) to hairless mice also produced a chronic Th2-like HR. Permeability barrier function and expression of differentiation proteins, filaggrin, loricrin, and involucrin, became abnormal. CRTH-positive Th2-dominant inflammatory infiltrate, with increased IL-4 expression, and a large increase in serum IgE levels were observed. The barrier abnormality was associated with decreased stratum corneum (SC) ceramide content and impaired lamellar body secretion, resulting in abnormal lamellar membranes, as in human AD. Furthermore, as in human AD, epidermal serine protease activity in SC increased and expression of two lamellar body-derived antimicrobial peptides, CRAMP and mBD3, declined after Ox challenges, paralleling the decrease of their human homologues in AD. Thus, multiple Ox challenges to normal murine skin produce a chronic Th2-like HR, with multiple features of human AD. Because of its reproducibility, predictability, and low cost, this model could prove useful for evaluating both pathogenic mechanisms and potential therapies for AD.

STAT5a/PPARγ Pathway Regulates Involucrin Expression in Keratinocyte Differentiation

Signal transducers and activators of transcription (STATs) are critical to growth factor-mediated intracellular signal transduction. We observed the rapid expression and activation of STAT5a during keratinocyte differentiation induced by suspension culture. STAT5a expression preceded that of involucrin, an important molecule in the terminal differentiation of keratinocytes. To determine whether STAT5a regulated involucrin expression, we expressed a dominant-negative (dn) STAT5a that blocks the dimerization of STAT5 and inhibits its nuclear translocation. We found that dn-STAT5a inhibited involucrin expression in keratinocytes. Given that STAT5 regulates adipogenesis via activating the peroxisome proliferator-activated receptor (PPAR) gamma signal, we hypothesized that STAT5a regulated involucrin expression in the same manner. To test this hypothesis, we examined the expression and transactivation of PPARgamma in a suspension culture of keratinocytes. Suspension culture induced PPARgamma expression and triggered PPARgamma transactivation rapidly and dn-STAT5a downregulated this induction and suppressed PPARgamma transactivation. Furthermore, preincubation with the PPARgamma/retinoid X-receptor inhibitor HX-531 or the introduction of a dn-PPARgamma prevented the activation of involucrin promoter and inhibited its induction. This report provides early evidence of a major role for STAT5a in the differentiation of keratinocytes, where it contributes to involucrin expression by activating the PPARgamma signal.

PPAR-gamma agonist, ciglitazone, increases pigmentation and migration of human melanocytes

Peroxisome proliferator-activated receptors (PPARs) play an important role in cellular responses. It was reported that three subtypes of PPAR are expressed in human melanocytes. In this study, we investigated the effects of the PPAR-gamma agonist, ciglitazone, on pigmentation and migration of human melanocytes. Ciglitazone stimulated the melanin content of cells and cultured skin. This increase in pigmentation was due to the stimulation of tyrosinase activity and expression of tyrosinase and microphthalmia-associated transcription factor protein of the melanocytes. Migration was increased after ciglitazone treatment, which was observed by the Boyden chamber checkerboard analysis and a scratch motility assay. These results suggest the regulatory role of PPAR-gamma in pigmentation and migration of human melanocytes.

Thiazolidinediones: potential as therapeutics for psoriasis and perhaps other hyperproliferative skin disease

The thiazolidinediones constitute a family of synthetic compounds that act as high-affinity ligands for persoxisome proliferator-activated receptor-gamma (PPAR-gamma), a member of the nuclear hormone receptor family. Although originally developed to facilitate glucose control in patients with Type 2 diabetes, a number of studies showed that these agents effectively inhibited epithelial cell proliferation and tissue inflammation. Many of the initial cell growth inhibition studies were conducted with malignant epithelial cells from various sites; however, in addition to malignant epithelial cells, other studies showed that rapidly proliferating epidermal keratinoctyes in culture were also sensitive to the growth-inhibiting action of these moieties. Additional studies subsequently demonstrated that some patients with plaque psoriasis responded to treatment with one or another member of the thiazolidinedione family. Due to the potential therapeutic benefit of these compounds in diseases such as psoriasis, studies have been conducted to elucidate mechanisms by which growth inhibition is achieved. Interference with a number of growth-influencing signalling pathways has been demonstrated. Of interest, some of the growth-inhibiting effects are seen under conditions in which PPAR-gamma activation may not be responsible for the activity. Based on therapeutic potential, additional ongoing studies are aimed at developing novel thiazolidinediones that may have better efficacy than the currently available agents. Other studies are aimed at identifying optimal ways to use these agents in the treatment of hyperplastic skin diseases such as psoriasis.