Peroxisome proliferator-activated receptor beta expression in human breast epithelial cell lines of tumorigenic and non-tumorigenic origin

Peroxisome proliferator-activated receptor beta (PPARbeta) is a member of the nuclear hormone receptor superfamily and is a ligand activated transcription factor. although the precise genes that it regulates and its physiological and pathophysiological role remain unclear. In view of the association of PPARbeta with colon cancer and increased mRNA levels of PPARbeta in colon tumours we sought in this study to examine the expression of PPARbeta in human breast epithelial cells of tumorigenic (MCF-7 and MDA-MB-231) and non-tumorigenic origin (MCF-10A). Using quantitative RT-PCR we measured PPARbeta mRNA levels in MCF-7. MDA-MB-231 and MCF-10A cells at various stages in culture. After serum-deprivation, MDA-MB-231 and MCF-10A cells had a 4.2- and 3.8-fold statistically greater expression of PPARbeta compared with MCF-7 cells. The tumorigenic cell lines also exhibited a significantly greater level of PPARbeta mRNA after serum deprivation compared with subconfluence whereas such an effect was not observed in non-tumorigenic MCF-10A cells. The expression of PPARbeta was inducible upon exposure to the PPARbeta ligand bezafibrate. Our results suggest that unlike colon cancer. PPARbeta overexpression is not an inherent property of breast cancer cell lines. However, the dynamic changes in PPARbeta mRNA expression and the ability of PPARbeta in the MCF-7 cells to respond to ligand indicates that PPARbeta may play a role in mammary gland carcinogenesis through activation of downstream genes via endogenous fatty acid ligands or exogenous agonists. (C) 2002 Elsevier Science Ltd. All rights reserved.

The peroxisome proliferator-activated receptor beta/delta agonist, GW501516, regulates the expression of genes involved in lipid catabolism and energy uncoupling in skeletal muscle cells

Lipid homeostasis is controlled by the peroxisome proliferator-activated receptors (PPARalpha, -beta/delta, and -gamma) that function as fatty acid-dependent DNA-binding proteins that regulate lipid metabolism. In vitro and in vivo genetic and pharmacological studies have demonstrated PPARalpha regulates lipid catabolism. In contrast, PPARgamma regulates the conflicting process of lipid storage. However, relatively little is known about PPARbeta/delta in the context of target tissues, target genes, lipid homeostasis, and functional overlap with PPARalpha and -gamma. PPARbeta/delta, a very low-density lipoprotein sensor, is abundantly expressed in skeletal muscle, a major mass peripheral tissue that accounts for approximately 40% of total body weight. Skeletal muscle is a metabolically active tissue, and a primary site of glucose metabolism, fatty acid oxidation, and cholesterol efflux. Consequently, it has a significant role in insulin sensitivity, the blood-lipid profile, and lipid homeostasis. Surprisingly, the role of PPARbeta/delta in skeletal muscle has not been investigated. We utilize selective PPARalpha, -beta/delta, -gamma, and liver X receptor agonists in skeletal muscle cells to understand the functional role of PPARbeta/delta, and the complementary and/or contrasting roles of PPARs in this major mass peripheral tissue. Activation of PPARbeta/delta by GW501516 in skeletal muscle cells induces the expression of genes involved in preferential lipid utilization, beta-oxidation, cholesterol efflux, and energy uncoupling. Furthermore, we show that treatment of muscle cells with GW501516 increases apolipoprotein-A1 specific efflux of intracellular cholesterol, thus identifying this tissue as an important target of PPARbeta/delta agonists. Interestingly, fenofibrate induces genes involved in fructose uptake, and glycogen formation. In contrast, rosiglitazone-mediated activation of PPARgamma induces gene expression associated with glucose uptake, fatty acid synthesis, and lipid storage. Furthermore, we show that the PPAR-dependent reporter in the muscle carnitine palmitoyltransferase-1 promoter is directly regulated by PPARbeta/delta, and not PPARalpha in skeletal muscle cells in a PPARgamma coactivator-1-dependent manner. This study demonstrates that PPARs have distinct roles in skeletal muscle cells with respect to the regulation of lipid, carbohydrate, and energy homeostasis. Moreover, we surmise that PPARgamma/delta agonists would increase fatty acid catabolism, cholesterol efflux, and energy expenditure in muscle, and speculate selective activators of PPARbeta/delta may have therapeutic utility in the treatment of hyperlipidemia, atherosclerosis, and obesity.

Genetic- or transforming growth factor-beta 1-induced changes in epidermal peroxisome proliferator-activated receptor beta/delta expression dictate wound repair kinetics.

Advances in wound care are of great importance in clinical injury management. In this respect, the nuclear receptor peroxisome proliferator-activated receptor (PPAR)beta/delta occupies a unique position at the intersection of diverse inflammatory or anti-inflammatory signals that influence wound repair. This study shows how changes in PPARbeta/delta expression have a profound effect on wound healing. Using two different in vivo models based on topical application of recombinant transforming growth factor (TGF)-beta1 and ablation of the Smad3 gene, we show that prolonged expression and activity of PPARbeta/delta accelerate wound closure. The results reveal a dual role of TGF-beta1 as a chemoattractant of inflammatory cells and repressor of inflammation-induced PPARbeta/delta expression. Also, they provide insight into the so far reported paradoxical effects of the application of exogenous TGF-beta1 at wound sites.

The pollutant diethylhexyl phthalate regulates hepatic energy metabolism via species-specific PPARalpha-dependent mechanisms.

Background: The modulation of energetic homeostasis by pollutants has recently emerged as a potential contributor to the onset of metabolic disorders. Diethylhexyl phthalate (DEHP) is a widely used industrial plasticizer to which humans are widely exposed. Phthalates can activate the three peroxisome proliferatoractivated receptor (PPAR) isotypes on cellular models and induce peroxisome proliferation in rodents.Objectives: In this study, we aimed to evaluate the systemic and metabolic consequences of DEHP exposure that have remained so far unexplored and to characterize the underlying molecular mechanisms of action.Methods: As a proof of concept and mechanism, genetically engineered mouse models of PPARs were exposed to high doses of DEHP, followed by metabolic and molecular analyses.Results: DEHP-treated mice were protected from diet-induced obesity via PPARalpha-dependent activation of hepatic fatty acid catabolism, whereas the activity of neither PPARbeta nor PPARgamma was affected. However, the lean phenotype observed in response to DEHP in wild-type mice was surprisingly abolished in PPARalpha-humanized mice. These species differences are associated with a different pattern of coregulator recruitment.Conclusion: These results demonstrate that DEHP exerts species-specific metabolic actions that rely to a large extent on PPARalpha signaling and highlight the metabolic importance of the species-specific activation of PPARalpha by xenobiotic compounds. Editor's SummaryDiethylhexyl phthalate (DEHP) is an industrial plasticizer used in cosmetics, medical devices, food packaging, and other applications. Evidence that DEHP metabolites can activate peroxisome proliferatoractivated receptors (PPARs) involved in fatty acid oxidation (PPARalpha and PPARbeta) and adiposite function and insulin resistance (PPARgamma) has raised concerns about potential effects of DEHP on metabolic homeostasis. In rodents, PPARalpha activation also induces hepatic peroxisome proliferation, but this response to PPARalpha activation is not observed in humans. Feige et al. (p. 234) evaluated systemic and metabolic consequences of high-dose oral DEHP in combination with a high-fat diet in wild-type mice and genetically engineered mouse PPAR models. The authors report that mice exposed to DEHP gained less weight than controls, without modifying their feeding behavior; they also exhibited lower triglyceride levels, smaller adipocytes, and improved glucose tolerance compared with controls. These effects, which were observed in mice fed both high-fat and standard diets, appeared to be mediated by PPARalpha-dependent activation of hepatic fatty acid catabolism without apparent involvement of PPARbeta or PPARgamma. However, mouse models that expressed human (versus mouse) PPARalpha tended to gain more weight on a high-fat diet than their DHEP-unexposed counterparts. The authors conclude that findings support species-specific metabolic effects of DEHP mediated by PPARalpha activation.

Peroxisome proliferator-activated receptors mediate host cell proinflammatory responses to Pseudomonas aeruginosa autoinducer.

The pathogenic bacterium Pseudomonas aeruginosa utilizes the 3-oxododecanoyl homoserine lactone (3OC(12)-HSL) autoinducer as a signaling molecule to coordinate the expression of virulence genes through quorum sensing. 3OC(12)-HSL also affects responses in host cells, including the upregulation of genes encoding inflammatory cytokines. This proinflammatory response may exacerbate underlying disease during P. aeruginosa infections. The specific mechanism(s) through which 3OC(12)-HSL influences host responses is unclear, and no mammalian receptors for 3OC(12)-HSL have been identified to date. Here, we report that 3OC(12)-HSL increases mRNA levels for a common panel of proinflammatory genes in murine fibroblasts and human lung epithelial cells. To identify putative 3OC(12)-HSL receptors, we examined the expression patterns of a panel of nuclear hormone receptors in these two cell lines and determined that both peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta) and PPARgamma were expressed. 3OC(12)-HSL functioned as an agonist of PPARbeta/delta transcriptional activity and an antagonist of PPARgamma transcriptional activity and inhibited the DNA binding ability of PPARgamma. The proinflammatory effect of 3OC(12)-HSL in lung epithelial cells was blocked by the PPARgamma agonist rosiglitazone, suggesting that 3OC(12)-HSL and rosiglitazone are mutually antagonistic negative and positive regulators of PPARgamma activity, respectively. These data identify PPARbeta/delta and PPARgamma as putative mammalian 3OC(12)-HSL receptors and suggest that PPARgamma agonists may be employed as anti-inflammatory therapeutics for P. aeruginosa infections.

Role of prostacyclin versus peroxisome proliferator-activated receptor beta receptors in prostacyclin sensing by lung fibroblasts.

Prostacyclin and its mimetics are used therapeutically for the treatment of pulmonary hypertension. These drugs act via cell surface prostacyclin receptors (IP receptors); however, some of them can also activate the nuclear receptor peroxisome proliferator-activated receptor beta (PPARbeta). We examined the possibility that PPARbeta is a therapeutic target for the treatment of pulmonary hypertension. Using the newly approved (for pulmonary hypertension) prostacyclin mimetic treprostinil sodium, reporter gene assays for PPARbeta activation and measurement of lung fibroblast proliferation were analyzed. Treprostinil sodium was found to activate PPARbeta in reporter gene assays and to inhibit proliferation of human lung fibroblasts at concentrations consistent with an effect on PPARs but not on IP receptors. The effects of treprostinil sodium on human lung cell proliferation are mimicked by those of the highly selective PPARbeta ligand GW0742. There are no receptor antagonists for PPARbeta or for IP receptors, but by using lung fibroblasts cultured from mice lacking PPARbeta (PPARbeta-/-) or IP (IP-/-), we demonstrate that the antiproliferative effects of treprostinil sodium are mediated by PPARbeta and not IP in lung fibroblasts. These observations suggest that some of the local, longer-term benefits of treprostinil sodium on reducing the remodeling associated with pulmonary hypertension may be mediated by PPARbeta. This study is the first to identify PPARbeta as a potential therapeutic target for the treatment of pulmonary hypertension, which is important because orally active PPARbeta ligands have been developed for the treatment of dyslipidemia.

Peroxisome proliferator-activated receptor-beta as a target for wound healing drugs.

Healing of cutaneous wounds, which is crucial for survival after an injury, proceeds via a well-tuned pattern of events including inflammation, re-epithelialisation, and matrix and tissue remodelling. These events are regulated spatio-temporally by a variety of growth factors and cytokines. The inflammation that immediately follows injury increases the expression of peroxisome proliferator-activated receptor (PPAR)-beta in the wound edge keratinocytes and triggers the production of endogenous PPARbeta ligands that activate the newly produced receptor. This elevated PPARbeta activity results in increased resistance of the keratinocytes to the apoptotic signals released during wounding, allowing faster re-epithelialisation. The authors speculate that, in parallel, ligand activation of PPARbeta in infiltrated macrophages attenuates the inflammatory response, which also promotes repair. Thus, current understanding of the roles of PPARbeta in different cell types implicated in tissue repair has revealed an intriguing intercellular cross-talk that coordinates, spatially and temporally, inflammation, keratinocyte survival, proliferation and migration, which are all essential for efficient wound repair. These novel insights into the orchestrating roles of PPARbeta during wound healing may be helpful in the development of drugs for acute and chronic wound disorders.

Be fit or be sick: peroxisome proliferator-activated receptors are down the road.

Investigating metabolism by unveiling the functions of the nuclear receptors peroxisome proliferator-activated receptors (PPARs) in the numerous intricate pathways ensuring energy homeostasis and fitness has been extremely rewarding. Major lines of research were initially determined by the first-characterized crucial roles of PPARalpha in fatty oxidation and of PPARgamma in adipocyte differentiation and lipid storage. Today, the molecular bases of the functional links between glucose, lipid, and protein metabolism, under the important but nonexclusive control of PPARalpha and PPARgamma, are starting to be uncovered. In addition, in the last couple of years evidence has been provided for an important role of PPARbeta (delta) in lipid metabolism. Inevitably, such actors of metabolic homeostasis are implicated in the physiopathology of complex metabolic disorders, such as those constituting the metabolic syndrome, resulting in atherosclerosis and cardiovascular diseases. This review presents a summary of the recent findings on their dual involvement in health and disease.

Peroxisome proliferator-activated receptors (PPARs): from metabolic control to epidermal wound healing.

Peroxisome proliferator-activated receptors control many cellular and metabolic processes. They are transcription factors belonging to the family of ligand-inducible nuclear receptors. Three isotypes called PPARalpha, PPARbeta/delta and PPARgamma have been identified in lower vertebrates and mammals. They display differential tissue distribution and each of the three isotypes fulfills specific functions. PPARalpha and PPARgamma control energy homoeostasis and inflammatory responses. Their activity can be modulated by drugs such as the hypolipidaemic fibrates and the insulin sensitising thiazolidinediones (pioglitazone and rosiglitazone). Thus, these receptors are involved in the control of chronic diseases such as diabetes, obesity, and atherosclerosis. Little is known about the main function of PPARbeta, but it has been implicated in embryo implantation, tumorigenesis in the colon, reverse cholesterol transport, and recently in skin wound healing. Here, we present recent developments in the PPAR field with particular emphasis on both the function of PPARs in lipid metabolism and energy homoeostasis (PPARalpha and PPARgamma), and their role in epidermal maturation and skin wound repair (PPARalpha and PPARbeta).

Peroxisome proliferator-activated receptor beta/delta exerts a strong protection from ischemic acute renal failure.

Ischemic acute renal failure is characterized by damages to the proximal straight tubule in the outer medulla. Lesions include loss of polarity, shedding into the tubule lumen, and eventually necrotic or apoptotic death of epithelial cells. It was recently shown that peroxisome proliferator-activated receptor beta/delta (PPARbeta/delta) increases keratinocyte survival after an inflammatory reaction. Therefore, whether PPARbeta/delta could contribute also to the control of tubular epithelium death after renal ischemia/reperfusion was tested. It was found that PPARbeta/delta+/- and PPARbeta/delta-/- mutant mice exhibited much greater kidney dysfunction and injury than wild-type counterparts after a 30-min renal ischemia followed by a 36-h reperfusion. Conversely, wild-type mice that were given the specific PPARbeta/delta ligand L-165041 before renal ischemia were completely protected against renal dysfunction, as indicated by the lack of rise in serum creatinine and fractional excretion of Na+. This protective effect was accompanied by a significant reduction in medullary necrosis, apoptosis, and inflammation. On the basis of in vitro studies, PPARbeta/delta ligands seem to exert their role by activating the antiapoptotic Akt signaling pathway and, unexpectedly, by increasing the spreading of tubular epithelial cells, thus limiting potentially their shedding and anoikis. These results point to PPARbeta/delta as a remarkable new target for preconditioning strategies.

Peroxisome proliferator-activated receptors (PPARs) in skin health, repair and disease.

Peroxisome proliferator-activated receptors, PPARalpha, PPARbeta/delta and PPARgamma, are fatty acid activated transcription factors that belong to the nuclear hormone receptor family. While they are best known as transcriptional regulators of lipid and glucose metabolism, evidence has also accumulated for their importance in skin homeostasis. The three PPAR isotypes are expressed in rodent and human skin. Various cell culture and in vivo approaches suggest that PPARalpha contributes to fetal skin development, to epidermal barrier maturation and to sebocyte activity. PPARbeta/delta regulates sebocyte differentiation, promotes hair follicle growth and has pro-differentiating effects in keratinocytes in normal and inflammatory conditions. In contrast, the role of PPARgamma appears to be rather minor in keratinocytes, whereas its activity is required for sebaceous gland differentiation. Importantly, PPARalpha and beta/delta are instrumental in skin repair after an injury, each of them playing specific roles. Due to their collective diverse functions in skin biology, PPARs represent a major research target for the understanding and treatment of many skin diseases, such as benign epidermal tumors, papillomas, acne vulgaris and psoriasis.

PPAR expression and function during vertebrate development.

The peroxisome proliferator activated receptors (PPARs) are ligand activated receptors which belong to the nuclear hormone receptor family. As with other members of this superfamily, it is thought that the ability of PPAR to bind to a ligand was acquired during metazoan evolution. Three different PPAR isotypes (PPARalpha, PPARbeta, also called 6, and PPARgamma) have been identified in various species. Upon binding to an activator, these receptors stimulate the expression of target genes implicated in important metabolic pathways. The present article is a review of PPAR expression and involvement in some aspects of Xenopus laevis and rodent embryonic development. PPARalpha and beta are ubiquitously expressed in Xenopus early embryos but become more tissue restricted later in development. In rodents, PPARalpha, PPARbeta and PPARgamma show specific time- and tissue-dependent patterns of expression during fetal development and in the adult animals. PPARs are implicated in several aspects of tissue differentiation and rodent development, such as differentiation of the adipose tissue, brain, placenta and skin. Particular attention is given to studies undertaken by us and others on the implication of PPARalpha and beta in rodent epidermal differentiation.

Peroxisome proliferator-activated receptor beta regulates acyl-CoA synthetase 2 in reaggregated rat brain cell cultures.

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors that regulate the expression of many genes involved in lipid metabolism. The biological roles of PPARalpha and PPARgamma are relatively well understood, but little is known about the function of PPARbeta. To address this question, and because PPARbeta is expressed to a high level in the developing brain, we used reaggregated brain cell cultures prepared from dissociated fetal rat telencephalon as experimental model. In these primary cultures, the fetal cells initially form random aggregates, which progressively acquire a tissue-specific pattern resembling that of the brain. PPARs are differentially expressed in these aggregates, with PPARbeta being the prevalent isotype. PPARalpha is present at a very low level, and PPARgamma is absent. Cell type-specific expression analyses revealed that PPARbeta is ubiquitous and most abundant in some neurons, whereas PPARalpha is predominantly astrocytic. We chose acyl-CoA synthetases (ACSs) 1, 2, and 3 as potential target genes of PPARbeta and first analyzed their temporal and cell type-specific pattern. This analysis indicated that ACS2 and PPARbeta mRNAs have overlapping expression patterns, thus designating the ACS2 gene as a putative target of PPARbeta. Using a selective PPARbeta activator, we found that the ACS2 gene is transcriptionally regulated by PPARbeta, demonstrating a role for PPARbeta in brain lipid metabolism.

Differential regulation of vascular endothelial growth factor expression by peroxisome proliferator-activated receptors in bladder cancer cells.

The growth of any solid tumor depends on angiogenesis. Vascular endothelial growth factor (VEGF) plays a prominent role in vesical tumor angiogenesis regulation. Previous studies have shown that the peroxisome proliferator-activated receptor gamma (PPARgamma) was involved in the angiogenesis process. Here, we report for the first time that in two different human bladder cancer cell lines, RT4 (derived from grade I tumor) and T24 (derived from grade III tumor), VEGF (mRNA and protein) is differentially up-regulated by the three PPAR isotypes. Its expression is increased by PPARalpha, beta, and gamma in RT4 cells and only by PPARbeta in T24 cells via a transcriptional activation of the VEGF promoter through an indirect mechanism. This effect is potentiated by an RXR (retinoid-X-receptor), selective retinoid LG10068 providing support for a PPAR agonist-specific action on VEGF expression. While investigating the downstream signaling pathways involved in PPAR agonist-mediated up-regulation of VEGF, we found that only the MEK inhibitor PD98059 reduced PPAR ligand-induced expression of VEGF. These data contribute to a better understanding of the mechanisms by which PPARs regulate VEGF expression. They may lead to a new therapeutic approach to human bladder cancer in which excessive angiogenesis is a negative prognostic factor.

Peroxisome proliferator-activated receptor beta/delta as a therapeutic target for metabolic diseases.

The peroxisome proliferator-activated receptor (PPAR) family comprises three distinct isotypes: PPARalpha, PPARbeta/delta and PPARgamma. PPARs are nuclear hormone receptors that mediate the effects of fatty acids and their derivatives at the transcriptional level. Until recently, the characterisation of the important role of PPARalpha in fatty acid oxidation and of PPARgamma in lipid storage contrasted with the sparse information concerning PPARbeta/delta. However, evidence is now emerging for a role of PPARbeta/delta in tissue repair and energy homeostasis. Experiments with tissue-specific overexpression of PPARbeta/delta or treatment of mice with selective PPARbeta/delta agonists demonstrated that activation of PPARbeta/delta in vivo increases lipid catabolism in skeletal muscle, heart and adipose tissue and improves the serum lipid profile and insulin sensitivity in several animal models. PPARbeta/delta activation also prevents the development of obesity and improves cholesterol homeostasis in obesity-prone mouse models. These new insights into PPARbeta/delta functions suggest that targeting PPARbeta/delta may be helpful for treating disorders associated with the metabolic syndrome. Although these perspectives are promising, several independent and contradictory reports raise concerns about the safety of PPARbeta/delta ligands with respect to tumourigenic activity in the gut. Thus, it appears that further exploration of PPARbeta/delta functions is necessary to better define its potential as a therapeutic target.