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

The effect of PPAR isoform (de)activation on the lipid composition in full-thickness skin models

Human skin equivalents (HSEs) are 3D-cultured human skin models that mimic many aspects of native human skin (NHS). Although HSEs resemble NHS very closely, the barrier located in the stratum corneum (SC) is impaired. This is caused by an altered lipid composition in the SC of HSEs compared with NHS. One of the most pronounced changes in this lipid composition is a high level of monounsaturation. One key enzyme in this change is stearoyl-CoA desaturase-1 (SCD1), which catalyses the monounsaturation of lipids. In order to normalize the lipid composition, we aimed to target a group of nuclear receptors that are important regulators in the lipid synthesis. This group of receptors are known as the peroxisome proliferating activating receptors (PPARs). By (de)activating each isoform (PPAR-α, PPAR-δ and PPAR-γ), the PPAR isoforms may have normalizing effects on the lipid composition. In addition, another PPAR-α agonist Wy14643 was included as this supplement demonstrated normalizing effects in the lipid composition in a more recent study. After PPAR (ant)agonists supplementation, the mRNA of downstream targets, lipid synthesis genes and lipid composition were investigated. The PPAR downstream targets were activated, indicating that the supplements reached the keratinocytes to trigger their effect. However, minimal impact was observed on the lipid composition after PPAR isoform (de) activation. Only the highest concentration Wy14643 resulted in strong, but negative effects on CER composition. Although the novel tested modifications did not result in an improvement, more insight is gained on the nuclear receptors PPARs and their effects on the lipid barrier in full-thickness skin models.

Omega-3 Versus Omega-6 Polyunsaturated Fatty Acids in the Prevention and Treatment of Inflammatory Skin Diseases

Omega-3 (ω-3) and omega-6 (ω-6) polyunsaturated fatty acids (PUFAs) are nowadays desirable components of oils with special dietary and functional properties. Their therapeutic and health-promoting effects have already been established in various chronic inflammatory and autoimmune diseases through various mechanisms, including modifications in cell membrane lipid composition, gene expression, cellular metabolism, and signal transduction. The application of ω-3 and ω-6 PUFAs in most common skin diseases has been examined in numerous studies, but their results and conclusions were mostly opposing and inconclusive. It seems that combined ω-6, gamma-linolenic acid (GLA), and ω-3 long-chain PUFAs supplementation exhibits the highest potential in diminishing inflammatory processes, which could be beneficial for the management of inflammatory skin diseases, such as atopic dermatitis, psoriasis, and acne. Due to significant population and individually-based genetic variations that impact PUFAs metabolism and associated metabolites, gene expression, and subsequent inflammatory responses, at this point, we could not recommend strict dietary and supplementation strategies for disease prevention and treatment that will be appropriate for all. Well-balanced nutrition and additional anti-inflammatory PUFA-based supplementation should be encouraged in a targeted manner for individuals in need to provide better management of skin diseases but, most importantly, to maintain and improve overall skin health.

Regulatory mechanisms mediated by peroxisome proliferator-activated receptor-β/δ in skin cancer

Considerable progress has been made during the past 20 years towards elucidating the role of peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) in skin cancer. In 1999, the original notion that PPARβ/δ was involved with epithelial cell function was postulated based on a correlation between PPARβ/δ expression and the induction of messenger RNAs encoding proteins that mediate terminal differentiation in keratinocytes. Subsequent studies definitively revealed that PPARβ/δ could induce terminal differentiation and inhibit proliferation of keratinocytes. Molecular mechanisms have since been discovered to explain how this nuclear receptor can be targeted for preventing and treating skin cancer. This includes the regulation of terminal differentiation, mitotic signaling, endoplasmic reticulum stress, and cellular senescence. Interestingly, the effects of activating PPARβ/δ can preferentially target keratinocytes with genetic mutations associated with skin cancer. This review provides the history and current understanding of how PPARβ/δ can be targeted for both nonmelanoma skin cancer and melanoma and postulates how future approaches that modulate PPARβ/δ signaling may be developed for the prevention and treatment of these diseases.

Pulmonary Administration of GW0742, a High-Affinity Peroxisome Proliferator-Activated Receptor Agonist, Repairs Collapsed Alveoli in an Elastase-Induced Mouse Model of Emphysema

Pulmonary emphysema is a disease in which lung alveoli are irreversibly damaged, thus compromising lung function. Our previous study revealed that all-trans-retinoic acid (ATRA) induces the differentiation of human lung alveolar epithelial type 2 progenitor cells and repairs the alveoli of emphysema model mice. ATRA also reportedly has the ability to activate peroxisome proliferator-activated receptor (PPAR) β/δ. A selective PPARβ/δ ligand has been reported to induce the differentiation of human keratinocytes during wound repair. Here, we demonstrate that treatment using a high-affinity PPARβ/δ agonist, GW0742, reverses the lung tissue damage induced by elastase in emphysema-model mice and improves respiratory function. Mice treated with elastase, which collapsed their alveoli, were then treated with either 10% dimethyl sulfoxide (DMSO) in saline (control group) or GW0742 (1.0 mg/kg twice a week) by pulmonary administration. Treatment with GW0742 for 2 weeks increased the in vivo expression of surfactant proteins A and D, which are known alveolar type II epithelial cell markers. GW0742 treatment also shortened the average distance between alveolar walls in the lungs of emphysema model mice, compared with a control group treated with 10% DMSO in saline. Treatment with GW0742 for 3 weeks also improved tissue elastance (cm H2O/mL), as well as the ratio of the forced expiratory volume in the first 0.05 s to the forced vital capacity (FEV 0.05/FVC). In each of these experiments, GW0742 treatment reversed the damage caused by elastase. In conclusion, PPARβ/δ agonists are potential therapeutic agents for pulmonary emphysema.

Picrasidine N Is a Subtype-Selective PPARβ/δ Agonist

Recently, growing evidence of the pivotal roles of peroxisome proliferator-activated receptor (PPAR) β/δ in various physiological functions, including lipid homeostasis, cancer, and inflammation, has raised interest in this receptor. In this study, the naturally occurring dimeric alkaloid picrasidine N (1) from Picrasma quassioides was identified as a novel PPARβ/δ agonist from a library consisting of plant extracts and natural compounds using a mammalian one-hybrid assay, and this compound was characterized. Compound 1 activated PPARβ/δ but did not activate or slightly activated PPARα and PPARγ. Furthermore, a peroxisome proliferator response element-driven luciferase reporter gene assay demonstrated that 1 enhanced PPARβ/δ transcriptional activity. Moreover, 1 selectively induced mRNA expression of ANGPTL4, which is a PPAR target gene. This observation is quite different from previously identified synthetic PPARβ/δ agonists, which can induce the expression of not only ANGPTL4 but also other PPAR target genes, such as ADRP, PDK4, and CPT-1. These results demonstrate that 1 is a potent subtype-selective and gene-selective PPARβ/δ agonist, suggesting its potential as a lead compound for further drug development. This compound would also be a useful chemical tool for elucidating the mechanism of PPARβ/δ-regulated specific gene expression and the biological significance of PPARβ/δ.

Targeting Peroxisome Proliferator-Activated Receptor-β/δ (PPARβ/δ) for Cancer Chemoprevention

The role of peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) in cancer remains contentious due in large part to divergent publications indicating opposing effects in different rodent and human cell culture models. During the past 10 years, some facts regarding PPARβ/δ in cancer have become clearer, while others remain uncertain. For example, it is now well accepted that (1) expression of PPARβ/δ is relatively lower in most human tumors as compared to the corresponding non-transformed tissue, (2) PPARβ/δ promotes terminal differentiation, and (3) PPARβ/δ inhibits pro-inflammatory signaling in multiple in vivo models. However, whether PPARβ/δ is suitable to target with natural and/or synthetic agonists or antagonists for cancer chemoprevention is hindered because of the uncertainty in the mechanism of action and role in carcinogenesis. Recent findings that shed new insight into the possibility of targeting this nuclear receptor to improve human health will be discussed.

Inhibition of FAAH and activation of PPAR: new approaches to the treatment of cognitive dysfunction and drug addiction

Enhancing the effects of endogenously-released cannabinoid ligands in the brain might provide therapeutic effects more safely and effectively than administering drugs that act directly at the cannabinoid receptor. Inhibitors of fatty acid amide hydrolase (FAAH) prevent the breakdown of endogenous ligands for cannabinoid receptors and peroxisome proliferator-activated receptors (PPAR), prolonging and enhancing the effects of these ligands when they are naturally released. This review considers recent research on the effects of FAAH inhibitors and PPAR activators in animal models of addiction and cognition (specifically learning and memory). These studies show that FAAH inhibitors can produce potentially therapeutic effects, some through cannabinoid receptors and some through PPAR. These effects include enhancing certain forms of learning, counteracting the rewarding effects of nicotine and alcohol, relieving symptoms of withdrawal from cannabis and other drugs, and protecting against relapse-like reinstatement of drug self-administration. Since FAAH inhibition might have a wide range of therapeutic actions but might also share some of the adverse effects of cannabis, it is noteworthy that at least one FAAH-inhibiting drug (URB597) has been found to have potentially beneficial effects but no indication of liability for abuse or dependence. Although these areas of research are new, the preliminary evidence indicates that they might lead to improved therapeutic interventions and a better understanding of the brain mechanisms underlying addiction and memory.

Dissecting the role of peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) in colon, breast, and lung carcinogenesis

Peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) is a promising drug target since its agonists increase serum high-density lipoprotein; decrease low-density lipoprotein, triglycerides, and insulin associated with metabolic syndrome; improve insulin sensitivity; and decrease high fat diet-induced obesity. PPARβ/δ agonists also promote terminal differentiation and elicit anti-inflammatory activities in many cell types. However, it remains to be determined whether PPARβ/δ agonists can be developed as therapeutics because there are reports showing either pro- or anti-carcinogenic effects of PPARβ/δ in cancer models. This review examines studies reporting the role of PPARβ/δ in colon, breast, and lung cancers. The prevailing evidence would suggest that targeting PPARβ/δ is not only safe but could have anti-carcinogenic protective effects.

GW0742, a high-affinity PPAR-δ agonist, mediates protection in an organotypic model of spinal cord damage

STUDY DESIGN

Experimental study of spinal cord injury (SCI) using an organotypic slice culture.

OBJECTIVE

To clarify the protective mechanism of PPAR-δ agonist GW0742 in the injured spinal cord using an in vitro model.

SUMMARY OF BACKGROUND DATA

In vivo data suggest that ligands of the δ isoform have activity in a number of disease models that are partly driven by the inflammatory response. Moreover, reports from in vivo studies using models of ischemia reperfusion and Parkinson disease also have shown neuroprotection conferred by PPAR-δ. The biological role and function of PPAR-δ remains relatively unclear.

METHODS

Spinal cord from 6-week-old mice was cut into transverse slices of 400-μm thickness to generate the organotypic slice cultures. The slices were injured using a weight dropped onto the center of the slice. PPAR-δ agonist was applied at 10 μM at 1 hour before injury.

RESULTS

Our study shows that GW0742 incubation (10 μM) at 1 hour before transverse lesion significantly reduced (1) p38 mitogen-activated protein kinase (MAPK), (2) c-Jun N-terminal kinase (JNK/SAP kinase), (3) NF-κB activation, (4) loss of neurotrophic factors (BDNF, GDNF), (5) COX-2 expression, and (6) cell death.

CONCLUSION

GW0742 reduces the cellular and molecular changes occurring in SCI by targeting different downstream pathways modulating PPAR-δ receptors.

Role of PPARs and Retinoid X Receptors in the Regulation of Lung Maturation and Development

Understanding lung development has significant importance to public health because of the fact that interruptions in the normal developmental processes can have prominent effects on childhood and adult lung health. It is widely appreciated that the retinoic acid (RA) pathway plays an important role in lung development. Additionally, PPARs are believed to partner with receptors of this pathway and therefore could be considered extensions of retinoic acid function, including during lung development. This review will begin by introducing the relationship between the retinoic acid pathway and PPARs followed by an overview of lung development stages and regulation to conclude with details on PPARs and the retinoic acid pathway as they may relate to lung development.

Healing fats of the skin: the structural and immunologic roles of the omega-6 and omega-3 fatty acids

Linoleic acid (18:2omega6) and alpha-linolenic acid (18:3omega3) represent the parent fats of the two main classes of polyunsaturated fatty acids: the omega-6 (n-6) and the omega-3 (n-3) fatty acids, respectively. Linoleic acid and alpha-linolenic acid both give rise to other long-chain fatty acid derivatives, including gamma-linolenic acid and arachidonic acid (omega-6 fatty acids) and docosahexaenoic acid and eicosapentaenoic acid (omega-3 fatty acids). These fatty acids are showing promise as safe adjunctive treatments for many skin disorders, including atopic dermatitis, psoriasis, acne vulgaris, systemic lupus erythematosus, nonmelanoma skin cancer, and melanoma. Their roles are diverse and include maintenance of the stratum corneum permeability barrier, maturation and differentiation of the stratum corneum, formation and secretion of lamellar bodies, inhibition of proinflammatory eicosanoids, elevation of the sunburn threshold, inhibition of proinflammatory cytokines (tumor necrosis factor-alpha, interferon-gamma, and interleukin-12), inhibition of lipoxygenase, promotion of wound healing, and promotion of apoptosis in malignant cells, including melanoma. They fulfill these functions independently and through the modulation of peroxisome proliferator-activated receptors and Toll-like receptors.

GW0742, a high-affinity PPAR -beta/delta agonist, inhibits acute lung injury in mice

Several lines of evidence suggest a biological role for peroxisome proliferator-activated receptor (PPAR) beta/delta in the pathogenesis of a number of diseases. The aim of this study was to investigate the effects of a high-affinity PPAR-beta/delta agonist, GW0742, in a mouse model of carrageenan (CAR)-induced pleurisy. Injection of CAR into the pleural cavity of mice elicited an acute inflammatory response characterized by accumulation of fluid containing a large number of neutrophils (polymorphonuclear leukocytes) in the pleural cavity, infiltration of polymorphonuclear leukocytes in lung tissues and subsequent lipid peroxidation, and increased production of nitrite/nitrate, TNF-alpha, and IL-1beta. Furthermore, CAR induced lung apoptosis (Bax and Bcl-2 expression), and nitrotyrosine formation was determined by immunohistochemical analysis of lung tissues. Administration of GW0742 (0.3 mg/kg, i.p. bolus) 30 min before and 30 min after a challenge with CAR caused a reduction in all the parameters of inflammation measured. Thus, based on these findings, we propose that a PPAR-beta/delta agonist such as GW0742 may be useful in the treatment of various inflammatory diseases.

Peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta) acts as regulator of metabolism linked to multiple cellular functions

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors. They function as ligand activated transcription factors. They exist in three isoforms, PPARalpha, PPARbeta (formerly PPARdelta), and PPARgamma. For all PPARs lipids are endogenous ligands, linking them directly to metabolism. PPARs form heterodimers with retinoic X receptors, and, upon ligand binding, modulate gene expression of downstream target genes dependent on the presence of co-repressors or co-activators. This results in cell-type specific complex regulations of proliferation, differentiation and cell survival. Specific synthetic agonists for all PPARs are available. PPARalpha and PPARgamma agonists are already in clinical use for the treatment of hyperlipidemia and type 2 diabetes, respectively. More recently, PPARbeta activation came into focus as an interesting novel approach for the treatment of metabolic syndrome and associated cardiovascular diseases. Although the initial notion was that PPARbeta is expressed ubiquitously, more recently extensive investigations have been performed demonstrating high PPARbeta expression in a variety of tissues, e.g. skin, skeletal muscle, adipose tissue, inflammatory cells, heart, and various types of cancer. In addition, in vitro and in vivo studies using specific PPARbeta agonists, tissue-specific over-expression or knockout mouse models have demonstrated a variety of functions of PPARbeta in adipose tissue, muscle, skin, inflammation, and cancer. We will focus here on functions of PPARbeta in adipose tissue, skeletal muscle, heart, angiogenesis and cancer related to modifications in metabolism and the identified underlying molecular mechanisms.

Sorting out the functional role(s) of peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) in cell proliferation and cancer

Peroxisome proliferator-activated receptor-beta/delta (PPARbeta/delta) has many beneficial physiological functions ranging from enhancing fatty acid catabolism, improving insulin sensitivity, inhibiting inflammation and increasing oxidative myofibers allowing for improved athletic performance. Thus, given the potential for targeting PPARbeta/delta for the prevention and/or treatment of diseases including diabetes, dyslipidemias, metabolic syndrome and cancer, it is critical to clarify the functional role of PPARbeta/delta in cell proliferation and associated disorders such as cancer. However, there is considerable controversy whether PPARbeta/delta stimulates or inhibits cell proliferation. This review summarizes the literature describing the influence of PPARbeta/delta on cell proliferation, with an emphasis toward dissecting the data that give rise to opposing hypotheses. Suggestions are offered to standardize measurements associated with these studies so that interlaboratory comparisons can be accurately assessed.

Activation of peroxisome proliferator-activated receptor beta/delta induces lung cancer growth via peroxisome proliferator-activated receptor coactivator gamma-1alpha

We previously demonstrated that a selective agonist of peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta), GW501516, stimulated human non-small cell lung carcinoma (NSCLC) growth, partly through inhibition of phosphatase and tensin homolog deleted on chromosome 10 expression. Here, we show that GW501516 also decreases the phosphorylation of AMP-activated protein kinase alpha (AMPKalpha), a major regulator of energy metabolism. This was mediated through specific activation of PPARbeta/delta, as a PPARbeta/delta small interfering RNA inhibited the effect. However, AMPKalpha did not mediate the growth-promoting effects of GW501516, as silencing of AMPKalpha did not inhibit GW501516-induced cell proliferation. Instead, we found that GW501516 stimulated peroxisome proliferator-activated receptor coactivator gamma (PGC)-1alpha, which activated the phosphatidylinositol 3 kinase (PI3-K)/Akt mitogenic pathway. An inhibitor of PI3-K, LY294002, had no effect on PGC-1alpha, consistent with PGC-1alpha being upstream of PI3-K/Akt. Of note, an activator of AMPKalpha, 5-amino-4-imidazole carboxamide riboside, inhibited the growth-promoting effects of GW501516, suggesting that although AMPKalpha is not responsible for the mitogenic effects of GW501516, its activation can oppose these events. This study unveils a novel mechanism by which GW501516 and activation of PPARbeta/delta stimulate human lung carcinoma cell proliferation, and suggests that activation of AMPKalpha may oppose this effect.

Role of peroxisome-proliferator-activated receptor beta/delta (PPARbeta/delta) in gastrointestinal tract function and disease

PPARbeta/delta (peroxisome-proliferator-activated receptor beta/delta) is one of three PPARs in the nuclear hormone receptor superfamily that are collectively involved in the control of lipid homoeostasis among other functions. PPARbeta/delta not only acts as a ligand-activated transcription factor, but also affects signal transduction by interacting with other transcription factors such as NF-kappaB (nuclear factor kappaB). Constitutive expression of PPARbeta/delta in the gastrointestinal tract is very high compared with other tissues and its potential physiological roles in this tissue include homoeostatic regulation of intestinal cell proliferation/differentiation and modulation of inflammation associated with inflammatory bowel disease and colon cancer. Analysis of mouse epithelial cells in the intestine and colon has clearly demonstrated that ligand activation of PPARbeta/delta induces terminal differentiation. The PPARbeta/delta target genes mediating this effect are currently unknown. Emerging evidence suggests that PPARbeta/delta can suppress inflammatory bowel disease through PPARbeta/delta-dependent and ligand-independent down-regulation of inflammatory signalling. However, the role of PPARbeta/delta in colon carcinogenesis remains controversial, as conflicting evidence suggests that ligand activation of PPARbeta/delta can either potentiate or attenuate this disease. In the present review, we summarize the role of PPARbeta/delta in gastrointestinal physiology and disease with an emphasis on findings in experimental models using both high-affinity ligands and null-mouse models.

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.

Ligand activation of peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta) attenuates carbon tetrachloride hepatotoxicity by downregulating proinflammatory gene expression

Peroxisome proliferator-activated receptor (PPAR) beta/delta-null mice exhibit exacerbated hepatotoxicity in response to administration of carbon tetrachloride (CCl(4)). To determine whether ligand activation of the receptor protects against chemical toxicity in the liver, wild-type and PPARbeta/delta-null mice were administered CCl(4) with or without coadministration of the highly specific PPARbeta/delta ligand GW0742. Biomarkers of liver toxicity, including serum alanine aminotransferase (ALT) and hepatic tumor necrosis factor (TNF) alpha mRNA, were significantly higher in CCl(4)-treated PPARbeta/delta-null mice compared to wild-type mice. Hepatic expression of TNF-like weak inducer of apoptosis receptor (TWEAKr) and S100 calcium-binding protein A6 (S100A6/calcyclin), genes involved in nuclear factor kappa B signaling, was higher in the CCl(4)-treated PPARbeta/delta-null mice compared to wild-type mice. GW0742 treatment resulted in reduced serum ALT concentration and lower expression of CCl(4)-induced TNF-alpha, S100A6, monocyte chemoattractant protein-1 (MCP1), and TWEAKr in wild-type mice, and these effects were not observed in PPARbeta/delta-null mice. Expression of TNF-alpha was higher in PPARbeta/delta-null primary hepatocytes in response to interleukin-1beta treatment compared to wild-type hepatocytes, but GW0742 did not significantly modulate TNF-alpha expression in hepatocytes from either genotype. While PPARbeta/delta-null hepatic stellate exhibited higher rates of proliferation compared to wild-type cells, GW0742 did not affect alpha-smooth muscle actin expression in these cells. Combined, these findings demonstrate that ligand activation of PPARbeta/delta protects against chemically induced hepatotoxicity by downregulating expression of proinflammatory genes. Hepatocytes and hepatic stellate cells do not appear to directly mediate the inhibitory effects of ligand activation of PPARbeta/delta in liver, suggesting the involvement of paracrine and autocrine events mediated by hepatic cells.

Peroxisome proliferator-activated receptors (PPARs) and the human skin: importance of PPARs in skin physiology and dermatologic diseases

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor superfamily that regulate lipid, glucose, and amino acid metabolism. More recently, PPARs and corresponding ligands have been shown in skin and other organs to regulate important cellular functions, including cell proliferation and differentiation, as well as inflammatory responses. These new functions identify PPARs and corresponding ligands as potential targets for the treatment of various skin diseases and other disorders. It has been shown that in inflammatory skin disorders, including hyperproliferative psoriatic epidermis and the skin of patients with atopic dermatitis, the expression of both PPARalpha and PPARgamma is decreased. This observation suggests the possibility that PPARalpha and PPARgamma activators, or compounds that positively regulate PPAR gene expression, may represent novel NSAIDs for the topical or systemic treatment of common inflammatory skin diseases such as atopic dermatitis, psoriasis, and allergic contact dermatitis. Moreover, recent findings indicate that PPAR-signaling pathways may act as a promising therapeutic target for the treatment of hyperproliferative skin diseases including skin malignancies. Studies in non-diabetic patients suggest that oral thiazolidinediones, which are synthetic ligands of PPARgamma, not only exert an antidiabetic effect but also may be beneficial for moderate chronic plaque psoriasis by suppressing proliferation and inducing differentiation of keratinocytes; furthermore, they may even induce cell growth arrest, apoptosis, and terminal differentiation in various human malignant tumors. It has been reported that PPARalpha immunoreactivity is reduced in human keratinocytes of squamous cell carcinoma (SCC) and actinic keratosis (AK), while PPARdelta appears to be upregulated. Additionally, the microvessel density is significantly higher in AK and SCC that express high levels of PPARdelta. PPARdelta has been demonstrated to have an anti-apoptotic role and to maintain survival and differentiation of epithelial cells, whereas PPARalpha and PPARgamma activators induce differentiation and inhibit proliferation and regulate apoptosis. In melanoma, the growth inhibitory effect of PPARgamma activation is independent of apoptosis and seems to occur primarily through induction of cell cycle arrest in the G1 phase of the cell cycle or induction of re-differentiation. PPARalpha activation causes inhibition of migration of melanoma cells and anchorage-independent growth, whereas primary tumor growth remains unaltered. In clinical trials of gemfibrozil, a PPARalpha ligand, significantly fewer patients treated with this lipid-lowering drug were diagnosed with melanoma as compared to those in the control group. In conclusion, an increasing body of evidence indicates that PPAR signaling pathways may represent interesting therapeutic targets for a broad variety of skin disorders, including inflammatory skin diseases such as psoriasis and atopic dermatitis, and skin malignancies.

Germ cell nuclear factor directly represses the transcription of peroxisome proliferator-activated receptor delta gene

Germ cell nuclear factor (GCNF) is a transcription factor that can repress gene transcription and plays an important role during spermatogenesis. Peroxisome proliferator-activated receptor delta (PPARdelta) is a nuclear hormone receptor belonging to the steroid receptor superfamily. It can activate the expression of many genes, including those involved in lipid metabolism. In this report, we showed that GCNF specifically interacts with PPARdelta promoter. Overexpression of GCNF in African green monkey SV40-transformed kidney fibroblast COS7 cells and mouse embryo fibroblast NIH 3T3 cells represses the activity of PPARdelta promoter. The mutation of GCNF response element in PPARdelta promoter relieves the repression in NIH 3T3 cells and mouse testis. Moreover, we showed that GCNF in nuclear extracts of mouse testis is able to bind to PPARdelta promoter directly. We also found that GCNF and PPARdelta mRNA were expressed with different patterns in mouse testis by in situ hybridization. These results suggested that GCNF might be a negative regulator of PPARdelta gene expression through its direct interaction with PPARdelta promoter in mouse testis.

In vivo activation of peroxisome proliferator-activated receptor-delta protects the heart from ischemia/reperfusion injury in Zucker fatty rats

Peroxisome proliferator-activated receptor (PPAR)-delta is a transcription factor that belongs to the PPAR family. PPAR-delta is abundantly expressed in the heart, and its role in the heart is largely unknown. We tested whether pharmacological activation of PPAR-delta protects the heart from ischemia/reperfusion (I/R) injury in male Zucker fatty rats, a rodent model of obesity and dyslipidemia. A highly selective PPAR-delta agonist, [4-[[[2-[3-fluoro-4-(trifluoromethyl)phenyl]-4-methyl-5-thiazolyl]methyl] thio]-2-methylphenoxy]acetic acid (GW0742), was administered for 7 days at 10 mg/kg/day (p.o., once a day). Ischemic injury was produced by occlusion of the left anterior descending artery for 30 min followed by reperfusion for up to 24 h. Treatment with GW0742 reduced serum levels of cardiac troponin-I and infarct size by 63% (p < 0.01) and 32% (p < 0.01), respectively, and improved left ventricular function. Treatment with GW0742 up-regulated gene expression involved in cardiac fatty acid oxidation, increased fat use in the heart, and reduced serum levels of free fatty acids. The enhanced cardiac expression of interleukin (IL)-6, IL-8, intercellular adhesion molecule-1, and monocyte chemoattractant protein-1 induced by I/R were significantly attenuated by GW0742. Treatment with GW0742 also reduced apoptotic cardiomyocytes by 34% and cardiac caspase-3 activity by 61% (both p < 0.01 versus vehicle). GW0742 differentially regulated Bcl family members, favoring cell survival, and attenuated I/R-induced cardiac mitochondrial damage. In addition, GW0742 treatment augmented the cardiac Akt signaling pathway, as reflected by enhanced phospho-3-phosphoinositide-dependent kinase-1 and p-Akt. The results indicate that activation of PPAR-delta protected the heart from I/R injury in Zucker fatty rats, and multiple mechanisms including amelioration of lipotoxicity, anti-inflammation, and up-regulation of prosurvival signaling contribute together to the cardioprotection.

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.

YY1 repressing peroxisome proliferator-activated receptor delta promoter

Peroxisome proliferator-activated receptors delta (PPARdelta) is a nuclear hormone receptor belonging to the steroid receptor superfamily and is molecular targets for drugs to treat hypertriglyceridemia and type 2 diabetes. Yin Yang 1 (YY1) is a transcription factor that can repress or activate transcription of the genes with which it interacts. In this report, we show that YY1 specifically interacts with the PPARdelta promoter. Overexpression of YY1 in Hela and NIH 3T3 cells repressed the activity of the PPARdelta promoter, while the PPARdelta promoter activity was enhanced when YY1 was knocked down by siRNA YY1. We also show that YY1 in nuclear extracts was able to bind the PPARdelta promoter directly. These results suggest that YY1 might be a negative regulator of PPARdelta gene expression through its direct interaction with the PPARdelta promoter.

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.

Present concepts and future outlook: function of peroxisome proliferator-activated receptors (PPARs) for pathogenesis, progression, and therapy of cancer

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor superfamily of transcriptional regulators that regulate lipid, glucose, and amino acid metabolism. In recent studies it also has been shown that these receptors are implicated in tumor progression, cellular differentiation, and apoptosis and modulation of their function is therefore considered as a potential target for cancer prevention and treatment. PPAR ligands and other agents influencing PPAR signalling pathways have been shown to reveal chemopreventive potential by mediating tumor suppressive activities in a variety of human cancers and could represent a potential novel strategy to inhibit tumor carcinogenesis and progression. This review summarizes the currently available data on the roles of PPARs in relation to the processes of cell differentiation and carcinogenesis as well as their role as promising future therapeutic targets.

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.

n-3 and n-6 polyunsaturated fatty acids induce the expression of COX-2 via PPARgamma activation in human keratinocyte HaCaT cells

Polyunsaturated fatty acids (PUFA) n-3 inhibit inflammation, in vivo and in vitro in keratinocytes. We examined in HaCaT keratinocyte cell line whether eicosapentaenoic acid (EPA) a n-3 PUFA, gamma-linoleic acid (GLA) a n-6 PUFA, and arachidic acid a saturated fatty acid, modulate expression of cyclooxygenase-2 (COX-2), an enzyme pivotal to skin inflammation and reparation. We demonstrate that only treatment of HaCaT with GLA and EPA or a PPARgamma ligand (roziglitazone), induced COX-2 expression (protein and mRNA). Moreover stimulation of COX-2 promoter activity was increased by those PUFAs or rosiglitazone. The inhibitory effects of GW9662 and T0070907 (PPARgamma antagonists), on COX-2 expression and on stimulation of COX-2 promoter activity by EPA and GLA suggest that PPARgamma is implicated in COX-2 induction. Finally, PLA2 inhibitor methyl arachidonyl fluorophosphonate blocked the PUFA effects on COX-2 induction, promoter activity and arachidonic acid mobilization suggesting involvement of AA metabolites in PPAR activation. These findings demonstrate that n-3 and n-6 PUFA increased PPARgamma activity is necessary for the COX-2 induction in HaCaT human keratinocyte cells. Given the anti-inflammatory properties of EPA, we suggest that induction of COX-2 in keratinocytes may be important in the anti-inflammatory and protective mechanism of action of PUFAs n-3 or n-6.

Peroxisome proliferator-activated receptors as transcriptional nodal points and therapeutic targets

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors involved in the transcriptional regulation of key metabolic pathways such as lipid metabolism, adipogenesis, and insulin sensitivity. More recent work implicates all 3 PPAR isotypes (alpha, gamma, and delta, also known as beta or beta/delta) in inflammatory and atherosclerotic pathways. Because these nuclear receptors are activated by extracellular signals and control multiple gene targets, PPARs can be seen as nodes that control multiple inputs and outputs involved in energy balance, providing insight into how metabolism and the vasculature may be integrated. The ongoing clinical use of fibrates, which activate PPARalpha, and thiazolidinediones, which activate PPARgamma, establishes these receptors as viable drug targets, whereas considerable in vitro animal model and human surrogate marker studies suggest that PPAR activation may limit inflammation and atherosclerosis. Together, these various observations have stimulated intense interest in PPARs as therapeutic targets and led to large-scale cardiovascular end-point trials with PPAR agonists. The first of these studies has generated mixed results that require careful review, especially in anticipation of additional clinical trial data and ongoing attempts to develop novel PPAR modulators. Such analysis of the existing PPAR data, the appropriate use of currently approved PPAR agonists, and continued progress in PPAR therapeutics will be predicated on a better understanding of PPAR biology.

Signatures of positive selection in genes associated with human skin pigmentation as revealed from analyses of single nucleotide polymorphisms

Phenotypic variation between human populations in skin pigmentation correlates with latitude at the continental level. A large number of hypotheses involving genetic adaptation have been proposed to explain human variation in skin colour, but only limited genetic evidence for positive selection has been presented. To shed light on the evolutionary genetic history of human variation in skin colour we inspected 118 genes associated with skin pigmentation in the Perlegen dataset, studying single nucleotide polymorphisms (SNPs), and analyzed 55 genes in detail. We identified eight genes that are associated with the melanin pathway (SLC45A2, OCA2, TYRP1, DCT, KITLG, EGFR, DRD2 and PPARD) and presented significant differences in genetic variation between Europeans, Africans and Asians. In six of these genes we detected, by means of the EHH test, variability patterns that are compatible with the hypothesis of local positive selection in Europeans (OCA2, TYRP1 and KITLG) and in Asians (OCA2, DCT, KITLG, EGFR and DRD2), whereas signals were scarce in Africans (DCT, EGFR and DRD2). Furthermore, a statistically significant correlation between genotypic variation in four pigmentation candidate genes and phenotypic variation of skin colour in 51 worldwide human populations was revealed. Overall, our data also suggest that light skin colour is the derived state and is of independent origin in Europeans and Asians, whereas dark skin color seems of unique origin, reflecting the ancestral state in humans.

Neuroprotective Efficacy of the Peroxisome Proliferator-Activated Receptor δ-Selective Agonists in Vitro and in Vivo

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor superfamily and function as ligand-modulated transcription factors that regulate gene expression in many important biological processes. The PPARdelta subtype has the highest expression in the brain and is postulated to play a major role in neuronal cell function; however, the precise physiological roles of this receptor remain to be elucidated. Herein, we show that the high-affinity PPARdelta agonists L-165041 [4-[3-(4-acetyl-3-hydroxy-2-propylphenoxy)-propoxyl]phenoxy]-acetic acid] and GW501516 [2-methyl4-((4-methyl-2-(4-trifluoromethylphenyl)-1,3-triazol-5-yl)-methylsulfanyl)phenoxy acetic acid] protect against cytotoxin-induced SH-SY5Y cell injury in vitro and both ischemic brain injury and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxicity in vivo. In the SH-SY5Y studies, treatment with L-165041 or GW501516 significantly and concentration-dependently attenuated cell death following thapsigargin, 1-methyl-4-phenylpyridinium, or staurosporine exposure, with the extent of damage correlated with the level of caspase-3 inhibition. In the transient (90 min) middle cerebral artery occlusion model of ischemic brain injury in rats, i.c.v. infusion of L-165041 or GW501516 significantly attenuated the ischemic brain damage measured 24 h after reperfusion. Moreover, the PPARdelta agonists also significantly attenuated MPTP-induced depletion of striatal dopamine and related metabolite contents in mouse brain. These results demonstrate that subtype-selective PPARdelta agonists possess antiapoptotic properties in vitro, which may underlie their potential neuroprotective potential in in vivo experimental models of cerebral ischemia and Parkinson's disease (PD). These findings suggest that PPARdelta agonists could be useful tools for understanding the role of PPARdelta in other neurodegenerative disorders, as well as attractive therapeutic candidates for stroke and neurodegenerative diseases such as PD.

Genetic and pharmacological evidence that a retinoic acid cannot be the RXR-activating ligand in mouse epidermis keratinocytes

Using genetic and pharmacological approaches, we demonstrate that both RARgamma/RXRalpha heterodimers involved in repression events, as well as PPARbeta(delta)/RXRalpha heterodimers involved in activation events, are cell-autonomously required in suprabasal keratinocytes for the generation of lamellar granules (LG), the organelles instrumental to the formation of the skin permeability barrier. In activating PPARbeta(delta)/RXRalpha heterodimers, RXRalpha is transcriptionally active as its AF-2 activation function is required and can be inhibited by an RXR-selective antagonist. Within repressing RARgamma/RXRalpha heterodimers, induction of the transcriptional activity of RXRalpha is subordinated to the addition of an agonistic ligand for RARgamma. Thus, the ligand that possibly binds and activates RXRalpha heterodimerized with PPARbeta(delta) cannot be a retinoic acid, as it would also bind RARgamma and relieve the RARgamma-mediated repression, thereby yielding abnormal LGs. Our data also demonstrate for the first time that subordination of RXR transcriptional activity to that of its RAR partner plays a crucial role in vivo, because it allows RXRs to act concomitantly, within the same cell, as heterodimerization partners for repression, as well as for activation events in which they are transcriptionally active.

Potential therapeutic role of peroxisome proliferator activated receptor-gamma agonists in psoriasis

Psoriasis is a chronic inflammatory condition affecting 1 - 3% of the world's population. Despite the availability of several agents, therapeutic options remain limited. With a better understanding of the pathophysiology of psoriasis, several potential therapeutic targets have been identified. Peroxisome proliferator activated receptors have been shown to play a role in cutaneous homeostasis. This review focuses on the potential therapeutic role of peroxisome proliferator activated receptor-gamma agonists in psoriasis and the possibility for the future prospects.

PPARbeta/delta selectively induces differentiation and inhibits cell proliferation

Peroxisome proliferator-activated receptor (PPAR) beta-null mice exhibit exacerbated epithelial cell proliferation and enhanced sensitivity to skin carcinogenesis, suggesting that ligand activation of PPARbeta will inhibit keratinocyte proliferation. By using of a highly specific ligand (GW0742) and the PPARbeta-null mouse model, activation of PPARbeta was found to selectively induce keratinocyte terminal differentiation and inhibit keratinocyte proliferation. Additionally, GW0742 was found to be anti-inflammatory due to inhibition of myeloperoxidase activity, independent of PPARbeta. These data suggest that ligand activation of PPARbeta could be a novel approach to selectively induce differentiation and inhibit cell proliferation, thus representing a new molecular target for the treatment of skin disorders resulting from altered cell proliferation such as psoriasis and cancer.

Differential Effects of n-3 and n-6 Fatty Acids on Prostaglandin F2 Production by Bovine Endometrial Cells

Recent studies have implicated n-3 polyunsaturated fatty acids in the reduction of eicosanoid production in the bovine uterus. The objective of this study was to determine whether the effect of eicosapentaenoic acid (EPA; C(20:5), n-3) on PGF2alpha production by bovine endometrial (BEND) cells is influenced by the quantity of linoleic acid (C(18:2), n-6) in the incubation medium. Confluent BEND cells were incubated in the absence (control) or presence of 100 microM of EPA for 24 h. After incubation, cells were rinsed and then stimulated with phorbol 12,13-dibutyrate (PDBu; 100 ng/mL) for 6 h. Additional sets of culture dishes were treated with a combination of EPA and increasing n-6/n-3 fatty acid ratios for 24 h and then challenged with PDBu for 6 h. The PDBu stimulated PGF2alpha secretion and upregulated steady-state concentrations of prostaglandin endoperoxide synthase-2 and peroxisome proliferator-activated receptor delta mRNA within 6 h. Preincubation of BEND cells with EPA for 24 h decreased PGF2alpha response to phorbol ester, but had no detectable effects on prostaglandin endoperoxide synthase-2 or peroxisome proliferator-activated receptor delta mRNA abundance in PDBu-stimulated BEND cells. The inhibitory effect of EPA on PGF2alpha production was reverted in BEND cells treated with an increasing n-6-to-n-3 fatty acid ratio. Findings indicate that the net inhibition of endometrial PGF2alpha bioynthesis by n-3 fatty acids may vary depending on the ratio of n-6 to n-3 fatty acids in the uterus.

Increased expression of transglutaminase-1 and PPARgamma after vitamin E treatment in human keratinocytes

In skin, vitamin E acts as the predominant lipophilic antioxidant with a protective function against irradiation and oxidative stress. In addition to that, vitamin E can also modulate signal transduction and gene expression. To study whether the four natural tocopherol analogues (alpha-, beta-, gamma-, delta-tocopherol) can influence transcriptional activity by modulating the activity of nuclear receptors, a human keratinocytes cell line (NCTC 2544) was transfected with plasmids containing the luciferase reporter gene under control by direct repeat elements (DR1-DR4), representing binding sites for four different classes of nuclear receptors. In this model, the tocopherols positively modulated only the reporter construct containing a consensus element for peroxisome proliferator-activated receptors (PPARs). The induction was strongest with gamma-tocopherol and was most likely the direct consequence of stimulation of PPARgamma protein expression in keratinocytes. Vitamin E treatment also led to increased expression of a known PPARgamma target gene involved in terminal keratinocytes differentiation, the transglutaminase-1.

Pilot trial: Pioglitazone versus placebo in patients with plaque psoriasis (the P6)

BACKGROUND

Disordered differentiation and hyperproliferation of keratinocytes with inflammation are the hallmarks of psoriasis. Ligand activation of peroxisome proliferator receptor-gamma (a class of nuclear receptors) by thiazolidinediones can normalize the histologic features of psoriasis.

METHOD

In a 10-week, double-blind, randomized, placebo-controlled, parallel-group study, 70 patients with moderate to severe psoriasis received one of the following treatments: pioglitazone 15 mg, pioglitazone 30 mg or placebo. Efficacy was evaluated by observing the change in the psoriasis area and severity index (PASI) after 10 weeks of treatment.

RESULTS

There was a dose-dependent improvement in psoriasis. Median PASI scores at the end of 10 weeks were significantly reduced in the pioglitazone treatment groups as compared to the placebo-treated group. The psoriasis lesions cleared in more than 40% of patients treated with pioglitazone as compared to 12.5% of those with placebo. The percentage reduction in mean PASI scores was 21.6%, 41.1% and 47.5% in the placebo, pioglitazone 15 mg, and 30 mg groups, respectively. No serious adverse events were detected.

CONCLUSION

This is the first report from a controlled trial demonstrating that pioglitazone could be considered as an efficacious and safe agent for the treatment of plaque psoriasis. The optimum dose and duration of pioglitazone therapy remain to be determined.

Isoform specific changes in PPAR alpha and beta in colon and breast cancer with differentiation

To investigate the role of peroxisome proliferator-activated receptors (PPARs) alpha and beta in the differentiation of colon cancer cells, we differentiated HT-29 cells using sodium butyrate (NaB) and culturing post-confluence and assessed differentiation using the marker intestinal alkaline phosphatase. While PPARalpha levels only changed with culturing post confluence, PPARbeta levels increased independent of the method of differentiation. To explore further the differences induced by NaB, we assessed changes in both PPAR isoforms in MCF-7 breast cancer cells cultured in the presence of NaB over 48h. Again a very different expression pattern was observed with PPARalpha increasing after 4h and remaining elevated, while PPARbeta increased transiently. Our studies suggest that the expression of PPARs is dependent upon both the method of differentiation and on time. Moreover, these studies show that changes in PPARalpha levels are not required for the differentiation of colon cancer cell lines, whereas changes in PPARbeta are more closely associated with differentiation.

The role of peroxisome proliferator-activated receptor-beta/delta in epithelial cell growth and differentiation

The physiological and pharmacological roles of peroxisome proliferator-activated receptor-beta (PPARbeta-also referred to as PPARdelta) are just beginning to emerge. It has recently become clear that PPARbeta has a function in epithelial tissues, but controversy exists due to inconsistencies in the literature. There is strong evidence that ligand activation of PPARbeta can induce terminal differentiation of keratinocytes, with a concomitant inhibition of cell proliferation. However, the role of PPARbeta in keratinocyte-specific apoptosis is less clear. Additionally, the role of PPARbeta in colonic epithelium remains unclear due to conflicting evidence suggesting that ligand activation of PPARbeta can potentiate, as well as attenuate, intestinal cancer. Recent studies revealed that ligand activation of PPARbeta can induce fatty acid catabolism in skeletal muscle and is associated with improved insulin sensitivity, attenuated weight gain and elevated HDL levels thus demonstrating promising potential for targeting PPARbeta for treating obesity, dyslipidemias and type 2 diabetes. Therefore, it becomes critical to determine the safety of PPARbeta ligands. This review focuses on recent literature describing the role of PPARbeta in epithelial tissues and highlights critical discrepancies that need to be resolved for a more comprehensive understanding of how this receptor modulates epithelial homeostasis.

Peroxisome proliferator-activated receptors in squamous cell carcinoma and its precursors

BACKGROUND

Peroxisome proliferator-activated receptors (PPARs) mediate several functions that are of interest in carcinogenesis. Although PPARalpha, PPARbeta, and PPARgamma are expressed in multiple human, their expression has not been investigated in non-melanoma skin cancer.

METHODS

We performed a retrospective paired immunohistochemical analysis of normal skin, actinic keratosis (AK), and squamous cell carcinoma (SCC) among 35 individuals. Specimens were considered PPAR immunoreactive when 1% or more of the tumor cells showed clear evidence of immunostaining. Cyclooxygenase-2 (COX-2) expression, the fraction of proliferating endothelial cells, and microvessel density were also evaluated in these samples.

RESULTS

PPARalpha immunoreactivity was significantly less likely to occur in SCC and AK than in normal skin of each individual. In contrast to PPARalpha, PPARbeta appeared to be upregulated in (pre)malignant skin lesions. For each individual, the likelihood that normal skin, AK, or SCC was immunoreactive against PPARgamma was comparable. COX-2 immunopositivity was significantly associated with PPARbeta and PPARgamma immunoreactivity. No statistical differences were noted for the angiogenesis parameters and PPARalpha, PPARbeta, or PPARgamma expression, except that the microvessel density was significantly higher among PPARbeta-immunoreactive SCCs compared to that among immunonegative SCCs.

CONCLUSION

Although further research is warranted, these results suggest that PPAR ligands such as fibrates and thiazolidinediones may have chemoprophylactic properties in skin carcinogenesis.

Functions of peroxisome proliferator-activated receptors (PPAR) in skin homeostasis

The peroxisome proliferator-activated receptors (PPAR) are ligand-activated transcription factors that belong to the nuclear hormone receptor family. Three isotypes (PPAR alpha, PPAR beta or delta, and PPAR gamma) with distinct tissue distributions and cellular functions have been found in vertebrates. All three PPAR isotypes are expressed in rodent and human skin. They were initially investigated for a possible function in the establishment of the permeability barrier in skin because of their known function in lipid metabolism in other cell types. In vitro studies using specific PPAR agonists and in vivo gene disruption approaches in mice indeed suggest an important contribution of PPAR alpha in the formation of the epidermal barrier and in sebocyte differentiation. The PPAR gamma isotype plays a role in stimulating sebocyte development and lipogenesis, but does not appear to contribute to epidermal tissue differentiation. The third isotype, PPAR beta, regulates the late stages of sebaceous cell differentiation, and is the most effective isotype in stimulating lipid production in these cells, both in rodents and in humans. In addition, PPAR beta activation has pro-differentiating effects in keratinocytes under normal and inflammatory conditions. Finally, preliminary studies also point to a potential role of PPAR in hair follicle growth and in melanocyte differentiation. By their diverse biological effects on cell proliferation and differentiation in the skin, PPAR agonists or antagonists may offer interesting opportunities for the treatment of various skin disorders characterized by inflammation, cell hyperproliferation, and aberrant differentiation.

Peroxisome proliferator-activated receptor delta suppresses 11beta-hydroxysteroid dehydrogenase type 2 gene expression in human placental trophoblast cells

Accumulating evidence suggests that the human placental enzyme 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2) plays a key role in fetal development by controlling fetal exposure to maternal glucocorticoids. Recently, the nuclear peroxisome proliferator-activated receptor delta (PPAR delta) has been found to be the most abundantly expressed PPAR subtype in the human placenta, but its function in this organ is unknown. Given that PPAR delta-null mice exhibited placental defects and consequent intrauterine growth restriction, the present study was undertaken to examine the hypothesis that PPAR delta regulates human placental function in part by targeting 11beta-HSD2. Using cultured human trophoblast cells as a model system, we demonstrated that 1) the putative PPAR delta agonist carbaprostacyclin (cPGI2) reduced 11beta-HSD2 activity as well as 11beta-HSD2 expression at both protein and mRNA levels; 2) GW610742 (a selective PPAR delta agonist) mimicked the effect of cPGI2, whereas indomethacin (a known ligand for PPARalpha and PPAR gamma) had no effect; 3) the cPGI2-induced down-regulation of 11beta-HSD2 mRNA did not require de novo protein synthesis; 4) cPGI2 suppressed HSD11B2 promoter activity, but did not alter the half-life of 11beta-HSD2 mRNA; and 5) the inhibitory effect of cPGI2 on HSD11B2 promoter activity was abrogated in trophoblast cells cotransfected with a dominant negative PPAR delta mutant. Taken together, these findings suggest that activation of PPAR delta down-regulates HSD11B2 gene expression in human trophoblast cells, and that this effect is mediated primarily at the transcriptional level. Thus, the present study reveals 11beta-HSD2 as an additional target for PPAR delta and identifies a molecular mechanism by which this nuclear receptor may regulate human placental function.

Effects of Bovine Somatotropin on Uterine Genes Related to the Prostaglandin Cascade in Lactating Dairy Cows

Multiparous Holstein cows, averaging 80 d in milk, were used to examine the effect of exogenous bovine somatotropin (bST) on uterine expression of estrogen receptor alpha (ERalpha), prostaglandin endoperoxide synthase-2 (PGHS-2), and peroxisome proliferator-activated receptor delta (PPARdelta). About 12 h before expected ovulation in a synchronization protocol, cows were assigned to receive bST (500 mg, n = 11) or serve as untreated controls (n = 10). Cows that ovulated (n = 9 bST, 8 control) were divided within treatment to be killed on d 3 or 7 postovulation. Samples of intercaruncular endometrial tissue from uterine horns ipsilateral to the corpus luteum were collected and stored at -80 degrees C for subsequent mRNA analyses. Endometrial concentrations of ERalpha and PGHS-2 mRNA transcripts were greater on d 7 than on d 3 of the estrous cycle, but did not differ between treatments. Compared with untreated cows, short-term bST treatment decreased PGHS-2 protein expression at d 7 of the estrous cycle. Concentration of PPARdelta mRNA transcript in the uterus decreased between d 3 and 7 of the estrous cycle and was negatively correlated with ERalpha and PGHS-2 mRNA concentrations. Short-term administration of bST to lactating dairy cows had minimal effects on uterine genes encoding ERalpha, PGHS-2, and PPARdelta at d 3 and 7 of the estrous cycle but there may be an inverse relationship between PPARdelta and uterine expression of ERalpha and PGHS-2 genes.

Peroxisome proliferator-activated receptor-beta/delta inhibits epidermal cell proliferation by down-regulation of kinase activity

Recent work has shown that peroxisome proliferator-activated receptor beta (PPARbeta) attenuates cell proliferation and skin carcinogenesis, and this is due in part to regulation of ubiquitin C expression. In these studies, the role of PPARbeta in modulating ubiquitin-dependent protein kinase Calpha (PKCalpha) levels and phosphorylation signaling pathways was evaluated. Intracellular phosphorylation analysis showed that phosphorylated PKCalpha and other kinases were lower in wild-type mouse skin treated with 12-O-tetradecanoylphorbol-13-acetate (TPA) as compared with PPARbeta-null mouse skin. No differences in expression levels of other PKC isoforms present in skin were observed. Lower ubiquitination of PKCalpha was found in TPA-treated PPARbeta-null skin as compared with wild-type, and inhibition of ubiquitin-dependent proteasome degradation prevented TPA-induced down-regulation of PKCalpha. The activity of PKCalpha and downstream signaling kinases is enhanced, and expression of cyclooxygenase-2 (COX-2) is significantly greater, in PPARbeta-null mouse skin in response to TPA compared with wild-type mouse skin. Inhibition of PKCalpha or COX-2 reduced cell proliferation in TPA-treated PPARbeta-null keratinocytes in a dose-dependent manner, whereas it only slightly influenced cell proliferation in wild-type keratinocytes. Combined, these studies provide strong evidence that PPARbeta attenuates cell proliferation by modulating PKCalpha/Raf1/MEK/ERK activity that may be due in part to reduced ubiquitin-dependent turnover of PKCalpha.

Peroxisome Proliferator-Activated Receptors and their Ligands

Glucocorticoids have remained one of the most frequently used classes of drugs for the treatment of skin diseases since their introduction more than 50 years ago. As a result of the discovery of new members of the nuclear hormone receptor (NR) superfamily, alternative therapeutic interventions that target retinoid and vitamin D receptors have been developed. Peroxisome proliferator-activated receptors (PPARs) comprise another important NR subfamily, consisting of three different isotypes: PPARalpha, PPARdelta (PPARbeta) and PPARgamma. These NRs are activated by a variety of natural and synthetic ligands such as fatty acids, eicosanoids, and antidiabetic and antihyperlipidaemic agents. While these receptors are established as regulators of gene expression in lipid and glucose homeostasis, evidence is now accumulating that PPARs also play a crucial role in cutaneous biology. Results from in vitro and in vivo studies have indicated the involvement of PPARs in epidermal maturation, proliferation and differentiation, as well as in immune and inflammatory responses, carcinogenesis, hyperpigmentation and skin wound healing. Furthermore, treatment of psoriatic patients with PPARgamma activators (thiazolidinediones) has been shown to induce beneficial effects. However, the effects of PPAR ligands should be carefully evaluated to determine whether they are in fact mediated via PPAR-dependent mechanisms. Nonetheless, PPARs seem to have significant potential as therapeutic targets in skin inflammatory disorders.

Critical role of p63 in the development of a normal esophageal and tracheobronchial epithelium

The trachea and esophagus originate from the foregut endoderm during early embryonic development. Their epithelia undergo a series of changes involving the differentiation of stem cells into unique cell types and ultimately forming the mature epithelia. In this study, we monitored the expression of p63 in the esophagus and the trachea during development and examined in detail morphogenesis in p63(-/-) mice. At embryonic day 15.5 (E15.5), the esophageal and tracheobronchial epithelia contain two to three layers of cells; however, only the progenitor cells express p63. These progenitor cells differentiate first into ciliated cells (p63(-)/beta-tubulin IV(+)) and after birth into mature basal cells (p63(+)/K14(+)/K5(+)/BS-I-B4(+)). In the adult pseudostratified, columnar tracheal epithelium, K14(+)/K5(+)/BS-I-B4(+) basal cells stain most intensely for p63, whereas ciliated and mucosecretory cells are negative. In stratified squamous esophageal epithelium and during squamous metaplasia in the trachea, cells in the basal layer stain strongest for p63, whereas p63 staining declines progressively in transient amplifying and squamous differentiated cells. Generally, p63 expression is restricted to human squamous cell carcinomas, and adenocarcinomas and Barrett's metaplasia do not stain for p63. Examination of morphogenesis in newborn p63(-/-) mice showed an abnormal persistence of ciliated cells in the esophagus. Significantly, in both tissues, lack of p63 expression results in the development of a highly ordered, columnar ciliated epithelium deficient in basal cells. These observations indicate that p63 plays a critical role in the development of normal esophageal and tracheobronchial epithelia and appears to control the commitment of early stem cells into basal cell progeny and the maintenance of basal cells.

Expression and localization of peroxisome proliferator-activated receptors and nuclear factor kappaB in normal and lesional psoriatic skin

Abnormal epidermal proliferation and differentiation characterize the inflammatory skin disease psoriasis. Here we demonstrate that expression of PPARdelta mRNA and protein is markedly upregulated in psoriatic lesions and that lipoxygenase products accumulating in psoriatic lesions are potent activators of PPARdelta. The expression levels of NF-kappaB p50 and p65 were not significantly altered in lesional compared with nonlesional psoriatic skin. In the basal layer of normal epidermis both p50 and p65 were sequestered in the cytoplasm, whereas p50, but not p65, localized to nuclei in the suprabasal layers, and this distribution was maintained in lesional psoriatic skin. In normal human keratinocytes PPAR agonists neither impaired IL-1beta-induced translocation of p65 nor IL-1beta-induced NF-kappaB DNA binding. We show that PPARdelta physically interacts with the N-terminal Rel homology domain of p65. Irrespective of the presence of agonists none of the PPAR subtypes decreased p65-mediated transactivation in keratinocytes. In contrast p65, but not p50, was a potent repressor of PPAR-mediated transactivation. The p65-dependent repression of PPARdelta- but not PPARalpha- or PPARgamma-mediated transactivation was partially relieved by forced expression of the coactivators p300 or CBP. We suggest that deficient NF-kappaB activation in chronic psoriatic plaques permitting unabated PPARdelta-mediated transactivation contributes to the pathologic phenotype of psoriasis.

Immunolocalization of peroxisome proliferator-activated receptors and retinoid x receptors in the adult rat CNS

Peroxisome proliferator-activated and retinoid X receptors (PPARs and RXRs) are transcription factors belonging to the steroid hormone receptor superfamily. Upon activation by their ligands, PPARs and RXRs bind to their target genes as heterodimers. Ligands of these receptors include lipophylic molecules, such as retinoids, fatty acids and eicosanoids, the importance of which in the metabolism and functioning of the nervous tissue is well documented. The immunohistochemical distribution of PPARs and RXRs in the CNS of the adult rat was studied by means of a sensitive biotinyl-tyramide method. All PPAR (alpha, beta/delta and gamma) and RXR (alpha, beta and gamma) isotypes were detected and found to exhibit specific patterns of localization in the different areas of the brain and spinal cord. The presence of the nuclear receptors was observed in both neuronal and glial cells. While PPAR beta/delta and RXR beta showed a widespread distribution, alpha and gamma isotypes exhibited a more restricted pattern of expression. The frontal cortex, basal ganglia, reticular formation, some cranial nerve nuclei, deep cerebellar nuclei, and cerebellar Golgi cells appeared rather rich in all studied receptors. Based on our data, we suggest that in the adult CNS, PPARs and RXRs, besides playing roles common to many other tissues, may have specific functions in regulating the expression of genes involved in neurotransmission, and therefore play roles in complex processes, such as aging, neurodegeneration, learning and memory.

Peroxisome proliferator-activated receptor delta expression and regulation in mouse uterus during embryo implantation and decidualization

The aim of this study was to examine the expression and regulation of peroxisome proliferator-activated receptor (PPAR) PPARdelta gene in mouse uterus during early pregnancy by in situ hybridization and immunohistochemistry. PPARdelta expression under pseudopregnancy, delayed implantation, hormonal treatment, and artificial decidualization was also investigated. There was a very low level of PPARdelta expression on days 1-4 of pregnancy. On day 5 when embryo implanted, PPARdelta expression was exclusively observed in the subluminal stroma surrounding the implanting blastocyst. No corresponding signals were seen in the uterus on day 5 of pregnancy. There was no detectable PPARdelta signal under delayed implantation. Once delayed implantation was terminated by estrogen treatment and embryo implanted, a strong level of PPARdelta expression was induced in the subluminal stroma surrounding the implanting blastocyst. Estrogen treatment induced a moderate level of PPARdelta expression in the glandular epithelium, while progesterone treatment had no effects in the ovariectomized mice. A strong level of PPARdelta expression was seen in the decidua on days 6-8 of pregnancy. PPARdelta expression was also induced under artificial decidualization. These data suggest that PPARdelta expression at implantation sites require the presence of an active blastocyst and may play an essential role for blastocyst implantation.