Peroxisome proliferator-activated receptors in the cardiovascular system

Peroxisome proliferator-activated receptor (PPAR)s are a family of three nuclear hormone receptors, PPARalpha, -delta, and -gamma, which are members of the steriod receptor superfamily. The first member of the family (PPARalpha) was originally discovered as the mediator by which a number of xenobiotic drugs cause peroxisome proliferation in the liver. Defined functions for all these receptors, until recently, mainly concerned their ability to regulate energy balance, with PPARalpha being involved in beta-oxidation pathways, and PPARgamma in the differentiation of adipocytes. Little is known about the functions of PPARdelta, though it is the most ubiquitously expressed. Since their discovery, PPARs have been shown to be expressed in monocytes/macrophages, the heart, vascular smooth muscle cells, endothelial cells, and in atherosclerotic lesions. Furthermore, PPARs can be activated by a vast number of compounds including synthetic drugs, of the clofibrate, and anti-diabetic thiazoldinedione classes, polyunsaturated fatty acids, and a number of eicosanoids, including prostaglandins, lipoxygenase products, and oxidized low density lipoprotein. This review will aim to introduce the field of PPAR nuclear hormone receptors, and discuss the discovery and actions of PPARs in the cardiovascular system, as well as the source of potential ligands.     

Evidence for peroxisome proliferator-activated receptor (PPAR)alpha-independent peroxisome proliferation: effects of PPARgamma/delta-specific agonists in PPARalpha-null mice

Peroxisome proliferators are a diverse group of compounds that cause hepatic hypertrophy and hyperplasia, increase peroxisome number, and on chronic high-dose administration, lead to rodent liver tumorigenesis. Various lines of evidence have led to the conclusion that these agents induce their pleiotropic effects exclusively via agonism of peroxisome proliferator-activated receptor (PPAR)alpha, a member of the steroid receptor superfamily involved in the regulation of fatty acid metabolism. Recently, agonists of two other members of this receptor family have been identified. PPARgamma is predominantly expressed in adipocytes where it mediates differentiation; PPARdelta is a widely expressed orphan receptor with yet unresolved physiologic functions. In the course of characterizing newer PPAR ligands, we noted that highly selective PPARgamma agonists or dual PPARgamma/PPARdelta agonists, lacking apparent murine PPARalpha agonist activity, cause peroxisome proliferation in CD-1 mice. We therefore made use of PPARalpha knockout mice to investigate whether these effects resulted from agonism of PPARalpha by these agents at very high dose levels or whether PPARgamma (or PPARdelta) agonism alone can result in peroxisome proliferation. We report here that several parameters linked to the hepatic peroxisome proliferation response in mice that were seen with these agents resulted from PPARalpha-independent effects.     

Role of PPAR- gamma agonist thiazolidinediones in treatment of pre-diabetic and diabetic individuals: a cardiovascular perspective

The peroxisome proliferator-activated receptors (PPARs) are nuclear fatty acid receptors, which contain a type II zinc finger DNA binding motif and a hydrophobic ligand binding pocket. These receptors are thought to play an important role in metabolic diseases such as obesity, insulin resistance, and coronary artery disease. Three subtypes of PPAR receptors have been described: PPARalpha, PPARdelta/beta, and PPARgamma. PPARalpha is found in the liver, muscle, kidney, and heart. In the liver, its role is to up-regulate genes involved in fatty acid uptake (beta-oxidation and omega-oxidation). PPARdelta/beta is involved in fatty acid oxidation in muscle. PPARgamma has high expression in fat, low expression in the liver, and very low expression in the muscle. The thiazolidinediones (TZD) are synthetic ligands of PPARgamma. By activating a number of genes in tissues, PPARgamma increases glucose and lipid uptake, increases glucose oxidation, decreases free fatty acid concentration, and decreases insulin resistance. There is a sound rationale for the use of TZDs in patients with type 2 diabetes mellitus and promising preliminary data in patients with patients with pre-diabetes. In patients with type 2 diabetes, thiazolidinediones had been shown to decrease mean HbA(1c)by 1.5% and lower HbA(1c) to less than 7% in 30% of patients. Decreased muscle insulin resistance primarily mediates the glucose lowering effect. In addition, there are several nonhypoglycemic effects of TZDs which may be beneficial to both diabetics and patients with pre-diabetes. These include effects on lipid metabolism, blood pressure, endothelial function, atherosclerotic plaque, coagulation, and albuminuria. In a pilot study, we recently demonstrated that insulin sensitizers such as thiazolidinediones appear to be associated with better clinical outcomes compared to insulin providers in diabetic patients presenting with acute coronary syndromes. In another study, we showed that the prediabetic state is a marker for worse prognosis in patients with acute coronary syndromes. In this article, we review the existing literature on the effectiveness of PPAR-gamma agonists in patients with either overt diabetes or a prediabetic state.     

Cardiovascular disease and PPARdelta: targeting the risk factors

Metabolism, in part, is regulated by the peroxisome proliferator-activated receptors (PPARs). The PPARs act as nutritional lipid sensors and three mammalian PPAR subtypes designated PPARalpha (NR1C1), PPARgamma (NR1C3) and PPARdelta (NR1C2) have been identified. This subgroup of nuclear hormone receptors binds DNA and controls gene expression at the nexus of pathways that regulate lipid and glucose homeostasis, energy storage and expenditure in an organ-specific manner. Recent evidence has demonstrated activation of PPARdelta in the major mass peripheral tissue (ie, adipose and skeletal muscle). It enhances glucose tolerance, insulin-stimulated glucose disposal, lipid catabolism, energy expenditure, cholesterol efflux and oxygen consumption. These effects positively influence the blood-lipid profile. Furthermore, PPARdelta activation produces a predominant type I/slow twitch/oxidative muscle fiber phenotype that leads to increased endurance, insulin sensitivity and resistance to obesity. PPARdelta has rapidly emerged as a potential target in the battle against dyslipidemia, insulin insensitivity, type II diabetes and obesity, with therapeutic efficacy in the treatment of cardiovascular disease risk factors. GW-501516 is currently undergoing phase II safety and efficacy trials in human volunteers for the treatment of dyslipidemia. The outcome of these clinical trials are eagerly awaited against a background of conflicting reports about cancer risks in genetically predisposed animal models. This review focuses on the potential pharmacological utility of selective PPARdelta agonists in the context of risk factors associated with metabolic and cardiovascular disease.     

Synthesis and identification of [1,2,4]thiadiazole derivatives as a new series of potent and orally active dual agonists of peroxisome proliferator-activated receptors alpha and delta

Cardiovascular disease is the most common cause of morbidity and mortality in developed nations. To effectively target dyslipidemia to reduce the risk of cardiovascular disease, it may be beneficial to activate the peroxisome proliferator-activated receptors (PPARs) PPARalpha and PPARdelta simultaneously through a single molecule. Replacement of the methylthiazole of 5 (the PPARdelta selective agonist) with [1,2,4]thiadiazole gave compound 13, which unexpectedly displayed submicromolar potency as a partial agonist at PPARalpha in addition to the high potency at PPARdelta. Optimization of 13 led to the identification of 24 as a potent and selective PPARalpha/delta dual agonist. Compound 24 and its close analogs represent a new series of PPARalpha/delta dual agonists. The high potency, significant gene induction, excellent PK profiles, and good in vivo efficacies in three animal models may render compound 24 as a valuable pharmacological tool in elucidating the complex roles of PPARalpha/delta dual agonists and as a potential treatment of the metabolic syndrome.     

Screening of herbal extracts for activation of the human peroxisome proliferator-activated receptor

The peroxisome proliferator-activated receptors play a pivotal role in metazoan lipid and glucose homeostasis. Synthetic activators of PPARalpha (fibrates) and PPARgamma (glitazones) are therefore widely used for treatment of dislipidemia and diabetes, respectively. There is growing evidence for herbal compounds to influence nuclear receptor signalling e.g. the PPARs. We recently reported carnosic acid and carnosol, both being diterpenes found in the labiate herbs sage and rosemary, to be activators of PPARgamma. The subsequent screening of a variety of ethanolic extracts, obtained from traditionally used herbs, for PPAR activation, led to an exceptionally high hit rate. Among 52 extracts nearly the half significantly activated PPARgamma and 14 activated PPARalpha in addition, whereas three of them were pan-PPAR activators, which also activated PPARdelta. The most active extracts, for which a concentration dependent effect could be shown, were the extracts of Alisma plantago aquatica (ze xie/european waterplantain), Catharanthus roseus (madagascar periwinkle), Acorus calamus (sweet calamus), Euphorbia balsamifera (balsam spurge), Jatropha curcas (barbados nut), Origanum majorana (marjoram), Zea mays (corn silk), Capsicum frutescens (chilli) and Urtica dioica (stinging nettle). The results of the present study provide a possible rationale for the traditional use of many herbs as antidiabetics.     

Teratogenic phthalate esters and metabolites activate the nuclear receptors PPARs and induce differentiation of F9 cells

Industrial plasticizers such as phthalates can induce peroxisome proliferation. Some phthalates such as di-2-ethyl-hexyl-phthalate (DEHP) and its metabolites mono-2-ethyl-hexyl-phthalate and 2-ethyl-hexanoic acid are also known teratogens. Recently, we introduced two in vitro test systems consisting of F9 teratocarcinoma cell differentiation and activation of peroxisome proliferator-activated receptor (PPAR)-ligand-binding domain in Chinese hamster ovary-reporter cells for the detection of teratogenic compounds related to the antiepileptic drug valproic acid. We now applied these methods to the class of phthalate esters and their metabolites by testing 2 diphthalate esters and 19 monophthalate esters in vitro. In the F9 cell assay only five compounds, mono-2-ethyl-hexyl-phthalate, mono-1-methyl-heptyl-phthalate, mono-benzyl-phthalate, benzyl-butyl-phthalate, and 2-ethyl-hexanoic acid were found to induce F9 cell differentiation. The other test compounds were not able to induce differentiation of F9 cells. Three compounds (mono-methyl-phthalate, mono-ethyl-phthalate, and mono-2,2-dimethyl-1-phenyl-propyl-phthalate, and phthalic acid di-methyl-ester were found not to interact with any PPARs. All other phthalate esters activated PPARs. Most compounds activated PPARalpha and PPARgamma. Interestingly PPARgamma in most cases was activated stronger than PPARalpha. Only the five test compounds, mono-2-ethyl-hexyl-phthatate, mono-1-methyl-heptyl-phthalate, mono-benzyl-phthalate, benzyl-butyl-phthalate, and 2-ethyl-hexanoic acid activated PPARdelta and interacted with a specific PPARdelta-response element. These are the same compounds that induced F9 cell differentiation and three of them are known teratogenic compounds. It is concluded that phthatate esters are acting like hormones by activating PPARs. The combination of F9 cell differentiation assay and PPARdelta activation assay detected possible teratogenic phthalate-ester and derivatives. Therefore the test systems seem useful for a screening test system in the early development of new plasticizers.     

Insulin resistance and PPAR insulin sensitizers

Drugs that reverse insulin resistance are of importance as insulin resistance is frequently associated with type 2 diabetes. The three peroxisome proliferator-activated receptors (PPARs) PPARalpha, PPAR90 and PPARgamma are essential for the actions of the many insulin sensitizers. PPARalpha activation enhances free fatty acid oxidation and potentiates anti-inflammatory effects, while PPARgamma is essential for normal adipocyte differentiation and proliferation, as well as fatty acid uptake and storage. Thiazolidinediones (TZDs) are selective ligands of PPARgamma and act as insulin sensitizers. TZDs also suppress free fatty acids via the inhibition of lipolysis in adipose tissue. Insulin sensitizers currently under development include partial PPARgamma agonists and antagonists, and dual PPARalpha/PPARgamma agonists. Given that TZDs show anti-inflammatory, anti-oxidant and antiprocoagulant properties in addition to their insulin sensitizing and antilipotoxic properties, a case may be made for initiating TZD therapy early in the treatment of type 2 diabetes, particularly in those patients at risk of cardiovascular disease. TZDs may also be an important therapeutic option in the treatment of metabolic syndrome.     

Mono(2-ethylhexyl)phthalate and mono-n-butyl phthalate activation of peroxisome proliferator activated-receptors alpha and gamma in breast

The phthalates di(2-ethylhexyl)phthalate (DEHP) and di-n-butyl phthalate (DBP) are environmental contaminants with significant human exposures. Both compounds are known reproductive toxins in rodents and DEHP also induces rodent hepatocarcinogenesis in a process believed to be mediated via the peroxisome proliferator-activated receptor alpha (PPARalpha). DEHP and DBP are metabolised to their respective monoesters, mono-(2-ethylhexyl)phthalate (MEHP) and mono-n-butyl phthalate (MBP), which are the active metabolites. MEHP also activates another member of the PPAR subfamily, PPARgamma. The effects of PPARalpha and PPARgamma activation in human breast cells appears to be opposing; PPARalpha activators in breast cells cause an increase in proliferation, while PPARgamma activation in breast cells is associated with differentiation and an inhibition of cell proliferation. Further to this the activation of the PPARs is cell and ligand specific, suggesting the importance of examining the effect of MEHP and MBP on the activation of PPARalpha, PPARbeta and PPARgamma in human breast. We used the common model of human breast cancer MCF-7 and examined the ability of MEHP and MBP to activate human PPARs in this system. The ability of MBP and MEHP to block PPAR responses was also assessed. We found that both human PPARalpha and PPARgamma were activated by MEHP whereas MEHP could not activate PPARbeta. MBP was unable to activate any PPAR isoforms in this breast model, despite being a weak peroxisome proliferator in liver, although MBP was an antagonist for both PPARgamma and PPARbeta. Our results suggest that the toxicological consequences of MEHP in the breast could be complex given the opposing effects of PPARalpha and PPARgamma in human breast cells.