Induction of the rat Cu/Zn superoxide dismutase gene through the peroxisome proliferator-responsive element by arachidonic acid

Role of Oxidative Stress in Peroxisome Proliferator-Mediated Carcinogenesis

In this review, the evidence about the role of oxidative stress in the induction of hepatocellular carcinomas by peroxisome proliferators is examined. The activation of PPAR-alpha by peroxisome proliferators in rats and mice may produce oxidative stress, due to the induction of enzymes like fatty acyl coenzyme A (CoA) oxidase (AOX) and cytochrome P-450 4A1. The effect of peroxisome proliferators on the antioxidant defense system is reviewed, as is the effect on endpoints resulting from oxidative stress that may be important in carcinogenesis, such as lipid peroxidation, oxidative DNA damage, and transcription factor activation. Peroxisome proliferators clearly inhibit several enzymes in the antioxidant defense system, but studies examining effects on lipid peroxidation and oxidative DNA damage are conflicting. There is a profound species difference in the induction of hepatocellular carcinomas by peroxisome proliferators, with rats and mice being sensitive, whereas species such as nonhuman primates and guinea pigs are not susceptible to the effects of peroxisome proliferators. The possible role of oxidative stress in these species differences is also reviewed. Overall, peroxisome proliferators produce changes in oxidative stress, but whether these changes are important in the carcinogenic process is not clear at this time.

PPARα Agonist-Induced Rodent Tumors: Modes of Action and Human Relevance

Widely varied chemicals--including certain herbicides, plasticizers, drugs, and natural products--induce peroxisome proliferation in rodent liver and other tissues. This phenomenon is characterized by increases in the volume density and fatty acid oxidation of these organelles, which contain hydrogen peroxide and fatty acid oxidation systems important in lipid metabolism. Research showing that some peroxisome proliferating chemicals are nongenotoxic animal carcinogens stimulated interest in developing mode of action (MOA) information to understand and explain the human relevance of animal tumors associated with these chemicals. Studies have demonstrated that a nuclear hormone receptor implicated in energy homeostasis, designated peroxisome proliferator-activated receptor alpha (PPARalpha), is an obligatory factor in peroxisome proliferation in rodent hepatocytes. This report provides an in-depth analysis of the state of the science on several topics critical to evaluating the relationship between the MOA for PPARalpha agonists and the human relevance of related animal tumors. Topics include a review of existing tumor bioassay data, data from animal and human sources relating to the MOA for PPARalpha agonists in several different tissues, and case studies on the potential human relevance of the animal MOA data. The summary of existing bioassay data discloses substantial species differences in response to peroxisome proliferators in vivo, with rodents more responsive than primates. Among the rat and mouse strains tested, both males and females develop tumors in response to exposure to a wide range of chemicals including DEHP and other phthalates, chlorinated paraffins, chlorinated solvents such as trichloroethylene and perchloroethylene, and certain pesticides and hypolipidemic pharmaceuticals. MOA data from three different rodent tissues--rat and mouse liver, rat pancreas, and rat testis--lead to several different postulated MOAs, some beginning with PPARalpha activation as a causal first step. For example, studies in rodent liver identified seven "key events," including three "causal events"--activation of PPARalpha, perturbation of cell proliferation and apoptosis, and selective clonal expansion--and a series of associative events involving peroxisome proliferation, hepatocyte oxidative stress, and Kupffer-cell-mediated events. Similar in-depth analysis for rat Leydig-cell tumors (LCTs) posits one MOA that begins with PPARalpha activation in the liver, but two possible pathways, one secondary to liver induction and the other direct inhibition of testicular testosterone biosynthesis. For this tumor, both proposed pathways involve changes in the metabolism and quantity of related hormones and hormone precursors. Key events in the postulated MOA for the third tumor type, pancreatic acinar-cell tumors (PACTs) in rats, also begin with PPARalpha activation in the liver, followed by changes in bile synthesis and composition. Using the new human relevance framework (HRF) (see companion article), case studies involving PPARalpha-related tumors in each of these three tissues produced a range of outcomes, depending partly on the quality and quantity of MOA data available from laboratory animals and related information from human data sources.

Endothelium-specific interference with peroxisome proliferator activated receptor gamma causes cerebral vascular dysfunction in response to a high-fat diet

The ligand-activated transcription factor peroxisome proliferator activated receptor gamma (PPARgamma) is expressed in vascular endothelium where it exerts anti-inflammatory and antioxidant effects. However, its role in regulating vascular function remains undefined. We examined endothelial function in transgenic mice expressing dominant-negative mutants of PPARgamma under the control of an endothelial-specific promoter to test the hypothesis that endothelial PPARgamma plays a protective role in the vasculature. Under baseline conditions, responses to the endothelium-dependent agonist acetylcholine were not affected in either aorta or the basilar artery in vitro. In response to feeding a high-fat diet for 12 weeks, acetylcholine produced dilation that was markedly impaired in the basilar artery of mice expressing dominant-negative mutants, but not in mice expressing wild-type PPARgamma controlled by the same promoter. Unlike basilar artery, 12 weeks of a high-fat diet was not sufficient to cause endothelial dysfunction in the aorta of mice expressing dominant-negative PPARgamma, although aortic dysfunction became evident after 25 weeks. The responses to acetylcholine in basilar artery were restored to normal after treatment with a scavenger of superoxide. Baseline blood pressure was only slightly elevated in the transgenic mice, but the pressor response to angiotensin II was augmented. Thus, interference with PPARgamma in the endothelium produces endothelial dysfunction in the cerebral circulation through a mechanism involving oxidative stress. Consistent with its role as a fatty acid sensor, these findings provide genetic evidence that endothelial PPARgamma plays a critical role in protecting blood vessels in response to a high-fat diet.

Adiponectin protects against angiotensin II-induced cardiac fibrosis through activation of PPAR-alpha

OBJECTIVE

Adiponectin is recognized as an antidiabetic, antiatherosclerotic, and anti-inflammatory protein derived from adipocytes. However, the role of adiponectin in cardiac fibrosis remains uncertain. We herein explore the effects of adiponectin on cardiac fibrosis induced by angiotensin II (Ang II).

METHODS AND RESULTS

Wild-type (WT), adiponectin knockout (Adipo-KO), and PPAR-alpha knockout (PPAR-alpha-KO) mice were infused with Ang II at 1.2 mg/kg/d. Severe cardiac fibrosis and left ventricular dysfunction were observed in Ang II-infused Adipo-KO mice compared to WT mice. Adenovirus-mediated adiponectin treatment improved the above phenotypes and the dysregulation of reactive oxygen species (ROS)-related mRNAs in Adipo-KO mice, whereas such amelioration was not observed in PPAR-alpha-KO mice despite adiponectin accumulation in heart tissue. In cultured cardiac fibroblasts, adiponectin improved the reduction of AMP-activated protein kinase (AMPK) activity and elevation of extracellular signal-regulated kinase 1/2 (ERK1/2) activity induced by Ang II. Adiponectin significantly enhanced PPAR-alpha activity, whereas the adiponectin-dependent PPAR-alpha activation was diminished by Compound C, an inhibitor of AMPK.

CONCLUSIONS

The present study suggests that adiponectin protects against Ang II-induced cardiac fibrosis possibly through AMPK-dependent PPAR-alpha activation.

The integration of lipid-sensing and anti-inflammatory effects: how the PPARs play a role in metabolic balance

The peroxisomal proliferating-activated receptors (PPARs) are lipid-sensing transcription factors that have a role in embryonic development, but are primarily known for modulating energy metabolism, lipid storage, and transport, as well as inflammation and wound healing. Currently, there is no consensus as to the overall combined function of PPARs and why they evolved. We hypothesize that the PPARs had to evolve to integrate lipid storage and burning with the ability to reduce oxidative stress, as energy storage is essential for survival and resistance to injury/infection, but the latter increases oxidative stress and may reduce median survival (functional longevity). In a sense, PPARs may be an evolutionary solution to something we call the 'hypoxia-lipid' conundrum, where the ability to store and burn fat is essential for survival, but is a 'double-edged sword', as fats are potentially highly toxic. Ways in which PPARs may reduce oxidative stress involve modulation of mitochondrial uncoupling protein (UCP) expression (thus reducing reactive oxygen species, ROS), optimising forkhead box class O factor (FOXO) activity (by improving whole body insulin sensitivity) and suppressing NFkB (at the transcriptional level). In light of this, we therefore postulate that inflammation-induced PPAR downregulation engenders many of the signs and symptoms of the metabolic syndrome, which shares many features with the acute phase response (APR) and is the opposite of the phenotype associated with calorie restriction and high FOXO activity. In genetically susceptible individuals (displaying the naturally mildly insulin resistant 'thrifty genotype'), suboptimal PPAR activity may follow an exaggerated but natural adipose tissue-related inflammatory signal induced by excessive calories and reduced physical activity, which normally couples energy storage with the ability to mount an immune response. This is further worsened when pancreatic decompensation occurs, resulting in gluco-oxidative stress and lipotoxicity, increased inflammatory insulin resistance and oxidative stress. Reactivating PPARs may restore a metabolic balance and help to adapt the phenotype to a modern lifestyle.

Expression of heme oxygenase-1 in human vascular cells is regulated by peroxisome proliferator-activated receptors

OBJECTIVE

Activation of peroxisome proliferator-activated receptors (PPARs) by lipid-lowering fibrates and insulin-sensitizing thiazolidinediones inhibits vascular inflammation, atherosclerosis, and restenosis. Here we investigate if the vasculoprotective and anti-inflammatory enzyme heme oxygenase-1 (HO-1) is regulated by PPAR ligands in vascular cells.

METHODS AND RESULTS

We show that treatment of human vascular endothelial and smooth muscle cells with PPAR ligands leads to expression of HO-1. Analysis of the human HO-1 promoter in transient transfection experiments together with mutational analysis and gel shift assays revealed a direct transcriptional regulation of HO-1 by PPARalpha and PPARgamma via 2 PPAR responsive elements. We demonstrate that a clinically relevant polymorphism within the HO-1 promoter critically influences its transcriptional activation by both PPAR isoforms. Moreover, inhibition of HO-1 enzymatic activity reversed PPAR ligand-mediated inhibition of cell proliferation and expression of cyclooxygenase-2 in vascular smooth muscle cells.

CONCLUSION

We demonstrate that HO-1 expression is transcriptionally regulated by PPARalpha and PPARgamma, indicating a mechanism of anti-inflammatory and antiproliferative action of PPAR ligands via upregulation of HO-1. Identification of HO-1 as a target gene for PPARs provides new strategies for therapy of cardiovascular diseases and a rationale for the use of PPAR ligands in the treatment of other chronic inflammatory diseases.

Mechanisms underlying the chronic pioglitazone treatment-induced improvement in the impaired endothelium-dependent relaxation seen in aortas from diabetic rats

The objectives of this study were to determine the effects of chronic treatment with pioglitazone, a peroxisome proliferator-activated receptor gamma agonist, on the impaired endothelium-dependent relaxation seen in aortas from established streptozotocin (STZ)-induced diabetic rats, and to identify some of the molecular mechanisms involved. Starting at 8 weeks of diabetes, pioglitazone (10 mg/kg) was administered to STZ-induced diabetic rats for 4 weeks. In untreated STZ rats (vs age-matched control rats): (1) ACh-induced relaxation, cGMP accumulation, phosphorylation of the cGMP-dependent protein kinase substrate vasodilator-stimulated phosphoprotein at Ser-239 [an established biochemical end-point of nitric oxide (NO)/cGMP signaling], and Cu/Zn-superoxide dismutase (SOD) expression and SOD activity were all reduced; (2) aortic superoxide generation, nitrotyrosine expression, and NAD(P)H oxidase activity were increased; (3) plasma endothelin-1 (ET-1) and aortic c-Jun (AP-1 component) protein expressions were increased. Pioglitazone treatment markedly corrected the above abnormalities. Collectively, these results suggest that pioglitazone treatment improves endothelium-dependent relaxation by reducing oxidative stress via increased SOD activity, decreased NAD(P)H oxidase activity, and a decreased ET-1 level, and that this decreased ET-1 level may be attributable to an inhibition of the AP-1 signaling pathway.

The PPARgamma ligand, rosiglitazone, reduces vascular oxidative stress and NADPH oxidase expression in diabetic mice

Oxidative stress plays an important role in diabetic vascular dysfunction. The sources and regulation of reactive oxygen species production in diabetic vasculature continue to be defined. Because peroxisome proliferator-activated receptor gamma (PPARgamma) ligands reduced superoxide anion (O(2)(-.)) generation in vascular endothelial cells in vitro by reducing NADPH oxidase and increasing Cu/Zn superoxide dismutase (SOD) expression, the current study examined the effect of PPARgamma ligands on vascular NADPH oxidase and O(2)(-.) generation in vivo. Lean control (db(+)/db(-)) and obese, diabetic, leptin receptor-deficient (db(-)/db(-)) mice were treated with either vehicle or rosiglitazone (3 mg/kg/day) by gavage for 7-days. Compared to controls, db(-)/db(-) mice weighed more and had metabolic derangements that were not corrected by treatment with rosiglitazone for 1-week. Aortic O(2)(-.) generation and mRNA levels of the NADPH oxidase subunits, Nox-1, Nox-2, and Nox-4 as well as Nox-4 protein expression were elevated in db(-)/db(-) compared to db(+)/db(-) mice, whereas aortic Cu/Zn SOD protein and PPARgamma mRNA levels were reduced in db(-)/db(-) mice. Treatment with rosiglitazone for 1-week significantly reduced aortic O(2)(-.) production and the expression of Nox-1, 2, and 4 but failed to increase Cu/Zn SOD or PPARgamma in aortic tissue from db(-)/db(-) mice. These data demonstrate that the vascular expression of Nox-1, 2, and 4 subunits of NADPH oxidase is increased in db(-)/db(-) mice and that short-term treatment with the PPARgamma agonist, rosiglitazone, has the potential to rapidly suppress vascular NADPH oxidase expression and O(2)(-.) production through mechanisms that do not appear to depend on correction of diabetic metabolic derangements.

Lipid-lowering drugs in the MPTP mouse model of Parkinson's disease: fenofibrate has a neuroprotective effect, whereas bezafibrate and HMG-CoA reductase inhibitors do not

We tested the ability of simvastatin, atorvastatin, fenofibrate and bezafibrate (two synthetic peroxisome proliferator-activated receptor-alpha (PPAR-alpha) agonists) to prevent dopaminergic cell death in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease. Tyrosine hydroxylase (TH) immunochemistry was performed 8 days after acute MPTP intoxication. When orally administered for the week prior to intoxication and a week thereafter, fenofibrate prevented the MPTP-induced dopaminergic cell loss in the substantia nigra pars compacta (SNpc) and attenuated the loss of tyrosine hydroxylase immunoreactivity in the striatum. The dosage of 1-methyl-4-phenyl pyridinium (MPP+) in the striatum by high-performance liquid chromatography indicated that fenofibrate did not affect MPTP metabolism. Bezafibrate had no effect and, strikingly, simvastatin and atorvastatin had a negative effect. We also demonstrated the presence of PPAR-alpha in the dopaminergic neurons of the murine substantia nigra. Our data suggest that PPAR-alpha activation by fenofibrate could have a neuroprotective effect in PD through inhibition of inflammation, oxidative stress and/or apoptosis.

Peroxisomes and oxidative stress

The discovery of the colocalization of catalase with H2O2-generating oxidases in peroxisomes was the first indication of their involvement in the metabolism of oxygen metabolites. In past decades it has been revealed that peroxisomes participate not only in the generation of reactive oxygen species (ROS) with grave consequences for cell fate such as malignant degeneration but also in cell rescue from the damaging effects of such radicals. In this review the role of peroxisomes in a variety of physiological and pathological processes involving ROS mainly in animal cells is presented. At the outset the enzymes generating and scavenging H2O2 and other oxygen metabolites are reviewed. The exposure of cultured cells to UV light and different oxidizing agents induces peroxisome proliferation with formation of tubular peroxisomes and apparent upregulation of PEX genes. Significant reduction of peroxisomal volume density and several of their enzymes is observed in inflammatory processes such as infections, ischemia-reperfusion injury and hepatic allograft rejection. The latter response is related to the suppressive effects of TNFalpha on peroxisomal function and on PPARalpha. Their massive proliferation induced by a variety of xenobiotics and the subsequent tumor formation in rodents is evidently due to an imbalance in the formation and scavenging of ROS, and is mediated by PPARalpha. In PEX5-/- mice with the absence of functional peroxisomes severe abnormalities of mitochondria in different organs are observed which resemble closely those in respiratory chain disorders associated with oxidative stress. Interestingly, no evidence of oxidative damage to proteins or lipids, nor of increased peroxide production has been found in that mouse model. In this respect the role of PPARalpha, which is highly activated in those mice, in prevention of oxidative stress deserves further investigation.

Peroxisome Proliferator–Activated Receptor α Protects against Glomerulonephritis Induced by Long-Term Exposure to the Plasticizer Di-(2-Ethylhexyl)Phthalate

Safety concerns about di-(2-ethylhexyl)phthalate (DEHP), a plasticizer and a probable endocrine disruptor, have attracted considerable public attention, but there are few studies about long-term exposure to DEHP. DEHP toxicity is thought to involve peroxisome proliferator-activated receptor alpha (PPARalpha), but this contention remains controversial. For investigation of the long-term toxicity of DEHP and determination of whether PPARalpha mediates toxicity, wild-type and PPARalpha-null mice were fed a diet that contained 0.05 or 0.01% DEHP for 22 mo. PPARalpha-null mice that were exposed to DEHP exhibited prominent immune complex glomerulonephritis, most likely related to elevated glomerular oxidative stress. Elevated NADPH oxidase, low antioxidant enzymes, and absence of the PPARalpha-dependent anti-inflammatory effects that normally antagonize the NFkappaB signaling pathway accompanied the glomerulonephritis in PPARalpha-null mice. The results reported here indicate that PPARalpha protects against the nephrotoxic effects of long-term exposure to DEHP.

Neuroprotective effects of arachidonic acid against oxidative stress on rat hippocampal slices

Arachidonic acid (AA), 5,8,11,14-eicosateraenoic acid is abundant, active and necessary in the human body. In the present study, we reported the neuroprotective effects and mechanism of arachidonic acid on hippocampal slices insulted by glutamate, NaN(3) or H(2)O(2)in vitro. Different types of models of brain injury in vitro were developed by 1mM glutamate, 10mM NaN(3) or 2mM H(2)O(2). After 30 min of preincubation with arachidonic acid or linoleic acid, hippocampal slices were subjected to glutamate, NaN(3) or H(2)O(2), then the tissue activities were evaluated by using the 2,3,5-triphenyltetrazolium chloride method. Endogenous antioxidant enzymes activities (SOD, GSH-PX and catalase) in hippocampal slices were evaluated during the course of incubation. MK886 (5 microM; a noncompetitive inhibitor of proliferator-activated receptor [PPAR]alpha), BADGE (bisphenol A diglycidyl ether; 100 microM; an antagonist of PPARgamma) and cycloheximide (CHX; 30 microM; an inhibitor of protein synthesis) were tested for their effects on the neuroprotection afforded by arachidonic acid. Population spikes were recorded in randomly selected hippocapal slices. Arachidonic acid (1-10 microM) dose dependently protected hippocampal slices from glutamate and H(2)O(2) injury (P<0.01), and arachidonic acid (10 microM) can significantly improve the activities of Cu/Zn-SOD in hippocampal slices after 1h incubation. In addition, 10 microM arachidonic acid significantly increased the activity of Mn-SOD and catalase, and decreased the activities of Cu/Zn-SOD to control value after 3h incubation. These secondary changes of SOD during incubation can be reversed by indomethacine (10 microM; a nonspecific cyclooxygenase inhibitor) or AA 861 (20 microM; a 5-lipoxygenase inhibitor). Its neuroprotective effect was completely abolished by BADGE and CHX. These observations reveal that arachidonic acid can defense against oxidative stress by boosting the internal antioxidant system of hippocampal slices. Its neuroprotective effect may be mainly mediated by the activation of PPARgamma and synthesis of new protein in tissue.

Peroxisome proliferator-activated receptor gamma promotes lymphocyte survival through its actions on cellular metabolic activities

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a metabolic regulator that plays an important role in sensitizing tissues to the action of insulin and in normalizing serum glucose and free fatty acids in type 2 diabetic patients. The receptor has also been implicated in the modulation of inflammatory responses, and ligands of PPARgamma have been found to induce apoptosis in lymphocytes. However, apoptosis induction may not depend on the receptor, because high doses of PPARgamma agonists are required for this process. Using cells containing or lacking PPARgamma, we reported previously that PPARgamma attenuates apoptosis induced by cytokine withdrawal in a murine lymphocytic cell line via a receptor-dependent mechanism. PPARgamma exerts this effect by enhancing the ability of cells to maintain their mitochondrial membrane potential during cytokine deprivation. In this report, we demonstrate that activation of PPARgamma also protects cells from serum starvation-induced apoptosis in human T lymphoma cell lines. Furthermore, we show that the survival effect of PPARgamma is mediated through its actions on cellular metabolic activities. In cytokine-deprived cells, PPARgamma attenuates the decline in ATP level and suppresses accumulation of reactive oxygen species (ROS). Moreover, PPARgamma regulates ROS through its coordinated transcriptional control of proteins and enzymes involved in ROS scavenging, including uncoupling protein 2, catalase, and copper zinc superoxide dismutase. Our studies identify cell survival promotion as a novel activity of PPARgamma and suggest that PPARgamma may modulate cytokine withdrawal-induced activated T cell death.

Thioredoxin-mediated negative autoregulation of peroxisome proliferator-activated receptor alpha transcriptional activity

PPARalpha, a member of the nuclear receptor superfamily, and thioredoxin, a critical redox-regulator in cells, were found to form a negative feedback loop, which autoregulates transcriptional activity of PPARalpha. Thioredoxin was identified as a target gene of PPARalpha. Activation of PPARalpha leads to increase of thioredoxin expression as well as its translocation from cytoplasm to nucleus, whereas ectopic overexpression of thioredoxin in the nucleus dramatically inhibited both constitutive and ligand-dependent PPARalpha activation. As PPARalpha-target genes, the expression of muscle carnitine palmitoyltransferase I, medium chain acyl CoA dehydrogenase, and apolipoprotein A-I were significantly down-regulated by nucleus-targeted thioredoxin at transcriptional or protein level. The suppression of PPARalpha transcriptional activity by Trx could be enhanced by overexpression of thioredoxin reductase or knockdown of thioredoxin-interacting protein, but abrogated by mutating the redox-active sites of thioredoxin. Mammalian one-hybrid assays showed that thioredoxin inhibited PPARalpha activity by modulating its AF-1 transactivation domain. It was also demonstrated by electrophoretic mobility-shift assay that thioredoxin inhibited the binding of PPARalpha to the PPAR-response element. Together, it is speculated that the reported negative-feedback loop may be essential for maintaining the homeostasis of PPARalpha activity.

All-trans retinoic acid induces free radical generation and modulate antioxidant enzyme activities in rat sertoli cells

In this work we investigated the effects of retinoic acid (RA) in Sertoli cells. Sertoli cells isolated from 15-day-old Wistar rats were previously cultured for 48 h and then treated with RA for 24 h. RA at high doses (1-10 microM) increased TBARS levels and induced a decrease in cell viability. At low doses (0.1-100 nM) RA did not increase TBARS level. RA also did not increase cell death at these doses. In order to investigate changes in antioxidant defenses we measured the CAT, SOD and GPx activities in Sertoli cells treated with RA. Compared to control, RA increased around 200% SOD activity in all doses tested (0.1-100 nM); GPx activity was increased 407.49, 208.98 and 243.88% (0.1, 1 and 10 nM, respectively); CAT activity was increased 127% with RA 1 nM. To clarify if RA induces ROS production per se, we performed experiments in vitro using 2-deoxyribose as specific substrate of oxidative degradation by *OH radical as well as TRAP assay. RA at 10 microM increased 2-deoxyribose degradation, suggesting that some of the RA-induced effects are mediated via *OH formation. Furthermore, the total reactive antioxidant potential (TRAP) of the RA was determined. At low concentrations RA has induced no redox activity. Conversely, higher concentration of RA (1-10 microM) increased chemiluminescence. The chemiluminescence produced was directly proportional to radical generation. We provide, for the first time, evidence for a free radical generation by RA. Our results demonstrated that RA plays an important role in Sertoli cells and these effects appear to be mediated by ROS.

Pretreatment with peroxysome proliferator-activated receptor alpha agonist fenofibrate protects endothelium in rabbit Escherichia coli endotoxin-induced shock

OBJECTIVE

To investigate the effects of fenofibrate, an activator of peroxysome proliferator activated receptor (PPAR) alpha, on vascular endothelium and on hemostasis in a rabbit endotoxic shock model.

DESIGN AND SETTING

Prospective laboratory study in a university laboratory.

SUBJECTS

36 male New Zealand rabbits weighing 2.5-3 kg.

INTERVENTIONS

We determined in vitro vascular reactivity, endothelium CD31-platelet/endothelial cell adhesion molecule (PECAM) 1 immunohistochemistry, plasma coagulation factors, and monocyte tissue factor expression 5 days after onset of endotoxic shock (0.5 mg/kg intravenous bolus, Escherichia coli lipopolysaccharide) with or without treatment by fenofibrate (mixed in the chow at a concentration of 0.5%) for 15 days before lipopolysaccharide injection and 5 days afterward.

MEASUREMENTS AND RESULTS

Metabolic acidosis and coagulation activation confirmed presence of shock. Fenofibrate decreased monocyte tissue factor expression. It improved endothelial-dependent relaxation at 5 days (Emax=68.2+/-3.3%, vs. 44.2+/-2.5% in the non-treated group). Endotoxin-induced deendothelialization was significantly decreased by fenofibrate at 5 days (8.5+/-1.3% vs. 19.2+/-3.1% in the nontreated group) .

CONCLUSIONS

These data indicate for the first time that fenofibrate, an activator of PPAR-alpha, inhibits monocyte tissue factor expression and protects against endothelial dysfunction and histological injury in endotoxin-induced shock.

Transcriptional regulation of lipid metabolism by fatty acids: a key determinant of pancreatic beta-cell function

BACKGROUND: Optimal pancreatic beta-cell function is essential for the regulation of glucose homeostasis in both humans and animals and its impairment leads to the development of diabetes. Type 2 diabetes is a polygenic disease aggravated by environmental factors such as low physical activity or a hypercaloric high-fat diet. RESULTS: Free fatty acids represent an important factor linking excess fat mass to type 2 diabetes. Several studies have shown that chronically elevated free fatty acids have a negative effect on beta-cell function leading to elevated insulin secretion basally but with an impaired response to glucose. The transcription factors PPARalpha, PPARgamma and SREBP-1c respond to changing fat concentrations in tissues, thereby coordinating the genomic response to altered metabolic conditions to promote either fat storage or catabolism. These transcription factors have been identified in beta-cells and it appears that each may exert influence on beta-cell function in health and disease. CONCLUSION: The role of the PPARs and SREBP-1c as potential mediators of lipotoxicity is an emerging area of interest.

Peroxisome proliferator-activated receptor-gamma ligands regulate endothelial membrane superoxide production

Recently, we demonstrated that the peroxisome proliferator-activated receptor-gamma (PPAR-gamma) ligands, either 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2) or ciglitazone, increased endothelial nitric oxide (.NO) release without altering endothelial nitric oxide synthase (eNOS) expression (4). However, the precise molecular mechanisms of PPAR-gamma-stimulated endothelial.NO release remain to be defined. Superoxide anion radical (O2-.) combines with .NO to decrease.NO bioavailability. NADPH oxidase, which produces O2-., and Cu/Zn-superoxide dismutase (Cu/Zn-SOD), which degrades O2-., thereby contribute to regulation of endothelial cell.NO metabolism. Therefore, we examined the ability of PPAR-gamma ligands to modulate endothelial O2-. metabolism through alterations in the expression and activity of NADPH oxidase or Cu/Zn-SOD. Treatment with 10 microM 15d-PGJ2 or ciglitazone for 24 h decreased human umbilical vein endothelial cell (HUVEC) membrane NADPH-dependent O2-. production detected with electron spin resonance spectroscopy. Treatment with 15d-PGJ2 or ciglitazone also reduced relative mRNA levels of the NADPH oxidase subunits, nox-1, gp91phox (nox-2), and nox-4, as measured using real-time PCR analysis. Concordantly, Western blot analysis demonstrated that 15d-PGJ2 or ciglitazone decreased nox-2 and nox-4 protein expression. PPAR-gamma ligands also stimulated both activity and expression of Cu/Zn-SOD in HUVEC. These data suggest that in addition to any direct effects on endothelial.NO production, PPAR-gamma ligands enhance endothelial.NO bioavailability, in part by altering endothelial O2-. metabolism through suppression of NADPH oxidase and induction of Cu/Zn-SOD. These findings further elucidate the molecular mechanisms by which PPAR-gamma ligands directly alter vascular endothelial function.

Peroxisome proliferator-activated receptor alpha protects against alcohol-induced liver damage

The mechanisms underlying alcoholic liver disease are not completely understood, but lipid accumulation seems to be central to the cause of this disease. The peroxisome proliferator-activated receptor alpha (PPARalpha) plays an important role in the control of lipid homeostasis, metabolism of bioactive molecules, and modulation of inflammatory responses. To investigate the roles of PPARalpha in alcoholic liver injury, wild-type and PPARalpha-null mice were continuously fed a diet containing 4% ethanol, and liver injury was analyzed. PPARalpha-null mice fed ethanol exhibited marked hepatomegaly, hepatic inflammation, cell toxicity, fibrosis, apoptosis, and mitochondrial swelling. Some of these hepatic abnormalities were consistent with those of patients with alcoholic liver injury and were not found in wild-type mice. Next, the molecular mechanisms of ethanol-induced liver injury in PPARalpha-null mice were investigated, and changes related to ethanol and acetaldehyde metabolism, oxidative stress, inflammation, hepatocyte proliferation, fibrosis, and mitochondrial permeability transition activation occurred specifically in PPARalpha-null mice as compared with wild-type mice. In conclusion, these studies suggest a protective role for PPARalpha in alcoholic liver disease. Humans may be more susceptible to liver toxicity induced by ethanol as PPARalpha expression in human liver is considerably lower compared to that of rodents.

Peroxisome proliferator-activated receptor alpha protects against alcohol-induced liver damage

The mechanisms underlying alcoholic liver disease are not completely understood, but lipid accumulation seems to be central to the cause of this disease. The peroxisome proliferator-activated receptor alpha (PPARalpha) plays an important role in the control of lipid homeostasis, metabolism of bioactive molecules, and modulation of inflammatory responses. To investigate the roles of PPARalpha in alcoholic liver injury, wild-type and PPARalpha-null mice were continuously fed a diet containing 4% ethanol, and liver injury was analyzed. PPARalpha-null mice fed ethanol exhibited marked hepatomegaly, hepatic inflammation, cell toxicity, fibrosis, apoptosis, and mitochondrial swelling. Some of these hepatic abnormalities were consistent with those of patients with alcoholic liver injury and were not found in wild-type mice. Next, the molecular mechanisms of ethanol-induced liver injury in PPARalpha-null mice were investigated, and changes related to ethanol and acetaldehyde metabolism, oxidative stress, inflammation, hepatocyte proliferation, fibrosis, and mitochondrial permeability transition activation occurred specifically in PPARalpha-null mice as compared with wild-type mice. In conclusion, these studies suggest a protective role for PPARalpha in alcoholic liver disease. Humans may be more susceptible to liver toxicity induced by ethanol as PPARalpha expression in human liver is considerably lower compared to that of rodents.

Transforming growth factor-beta (TGF-beta) activates cytosolic phospholipase A2alpha (cPLA2alpha)-mediated prostaglandin E2 (PGE)2/EP1 and peroxisome proliferator-activated receptor-gamma (PPAR-gamma)/Smad signaling pathways in human liver cancer cells. A novel mechanism for subversion of TGF-beta-induced mitoinhibition

Transforming growth factor-beta (TGF-beta) potently inhibits the growth of human epithelial cells. However, neoplastic epithelial cells become resistant to TGF-beta-mediated mitoinhibition, and the mechanisms for this alteration during tumorigenesis are not fully understood. This study was designed to determine whether there is an association between the cytosolic phospholipase A2alpha (cPLA2alpha)-controlled eicosanoid metabolism and the growth response to TGF-beta in human liver cancer cells. TGF-beta treatment induced simultaneous Smad-mediated gene transcription and phosphorylation of cPLA2alpha. Whereas Smad activation inhibited tumor cell growth, phosphorylation of cPLA2 alpha promoted growth and counteracted Smad-mediated mitoinhibition. TGF-beta1 failed to prevent the growth of cells with high basal expression of cPLA2alpha, but inhibition of cPLA2 alpha, cyclooxygenase-2 (COX-2), or EP1 receptor restored mitoinhibition by TGF-beta1 in these cells. These results suggest that resistance of tumor cells to TGF-beta-mediated mitoinhibition involves activation of cPLA2alpha/COX-2/EP1 signaling. Furthermore, the TGF-beta1-induced Smad transcriptional activity and mitoinhibition were blocked by overexpression of cPLA2alpha or peroxisome proliferator-activated receptor-gamma (PPAR-gamma) but enhanced by depletion of cPLA2alpha or PPAR-gamma. These findings, along with the observations that cPLA2alpha activates PPAR-gamma and that PPAR-gamma binds Smad3, illustrate novel cPLA2alpha/COX-2/EP1 and cPLA2alpha/PPAR-gamma/Smad signaling pathways that counteract the mitoinhibition by TGF-beta in human cancer cells.

Retinoic acid-mediated phospholipase A2 signaling in the nucleus

Retinoic acid modulates a wide variety of biological processes including proliferation, differentiation, and apoptosis. It interacts with specific receptors in the nucleus, the retinoic acid receptors (RARs). The molecular mechanism by which retinoic acid mediates cellular differentiation and growth suppression in neural cells remains unknown. However, retinoic acid-induced release of arachidonic acid and its metabolites may play an important role in cell proliferation, differentiation, and apoptosis. In brain tissue, arachidonic acid is mainly released by the action of phospholipase A2 (PLA2) and phospholipase C (PLC)/diacylglycerol lipase pathways. We have used the model of differentiation in LA-N-1 cells induced by retinoic acid. The treatment of LA-N-1 cells with retinoic acid produces an increase in phospholipase A2 activity in the nuclear fraction. The pan retinoic acid receptor antagonist, BMS493, can prevent this increase in phospholipase A2 activity. This suggests that retinoic acid-induced stimulation of phospholipase A2 activity is a retinoic acid receptor-mediated process. LA-N-1 cell nuclei also have phospholipase C and phospholipase D (PLD) activities that are stimulated by retinoic acid. Selective phospholipase C and phospholipase D inhibitors block the stimulation of phospholipase C and phospholipase D activities. Thus, both direct and indirect mechanisms of arachidonic acid release exist in LA-N-1 cell nuclei. Arachidonic acid and its metabolites markedly affect the neurite outgrowth and neurotransmitter release in cells of neuronal and glial origin. We propose that retinoic acid receptors coupled with phospholipases A2, C and D in the nuclear membrane play an important role in the redistribution of arachidonic acid in neuronal and non-nuclear neuronal membranes during differentiation and growth suppression. Abnormal retinoid metabolism may be involved in the downstream transcriptional regulation of phospholipase A2-mediated signal transduction in schizophrenia and Alzheimer disease (AD). The development of new retinoid analogs with diminished toxicity that can cross the blood-brain barrier without harm and can normalize phospholipase A2-mediated signaling will be important in developing pharmacological interventions for these neurological disorders.

Protection cellulaire par activation des récepteurs nucléaires PPAR

Apart from their effects on lipid and glucose metabolism, the nuclear receptors PPARs (peroxysome proliferator-activated receptors) could also be involved in cellular protection. Indeed, an increasing body of literature provides arguments in favour of a protective role of PPAR alpha (fenofibrate, gemfibrozil) and PPAR gamma (ciglitazone, pioglitazone, rosiglitazone, troglitazone) agonists, particularly in myocardial or cerebral ischaemia as well as in neurodegenerative diseases. Such cellular protection could be the result of the modulation, at a molecular level, of inflammation pathways, oxidative stress and apoptosis. If these experimental results are confirmed by appropriate clinical trials, pharmacological modulation of the nuclear receptor PPARs, as well as the development of highly selective and more effective PPAR agonists, could become an important challenge in the field of cellular protection.

Peroxisome proliferator-activated receptor-alpha activation as a mechanism of preventive neuroprotection induced by chronic fenofibrate treatment

The treatment of ischemic strokes is limited to the prevention of cerebrovascular risk factors and to the modulation of the coagulation cascade during the acute phase. A new therapeutic strategy could be to preventively protect the brain against noxious biological reactions induced by cerebral ischemia such as oxidative stress and inflammation to minimize their neurological consequences. Here, we show that a peroxisome proliferator-activated receptor (PPAR-alpha) activator, fenofibrate, protects against cerebral injury by anti-oxidant and anti-inflammatory mechanisms. A 14 d preventive treatment with fenofibrate reduces susceptibility to stroke in apolipoprotein E-deficient mice as well as decreases cerebral infarct volume in C57BL/6 wild-type mice. The neuroprotective effect of fenofibrate is completely absent in PPAR-alpha-deficient mice, suggesting that PPAR-alpha activation is involved as a mechanism of the protection against cerebral injury. Furthermore, this neuroprotective effect appears independently of any improvement in plasma lipids or glycemia and is associated with (1) an improvement in middle cerebral artery sensitivity to endothelium-dependent relaxation unrelated to an increase in nitric oxide synthase (NOS) type III expression, (2) a decrease in cerebral oxidative stress depending on the increase in numerous antioxidant enzyme activities, and (3) the prevention of ischemia-induced expression of vascular cell adhesion molecule-1 and intercellular adhesion molecule-1 in cerebral vessels without any change in NOS II expression. These data demonstrate that PPAR-alpha could be a new pharmacological target to preventively reduce the deleterious neurological consequences of stroke in mice and suggest that PPAR-alpha activators could preventively decrease the severity of stroke in humans.

[Antilipemics: from prevention to protection]

Statins and fibrates have been demonstrated to prevent both cardiovascular events and stroke. While this preventive effect was initially thought to be related to their lipid-lowering effects, in particular hypocholesterolaemic effect, analysis of primary and secondary prevention trials suggest that these preventive effects could be partly independent of their effects on lipid disorders. The pleiotropic effects, such as vascular, anti-inflammatory or anti-oxidants effects, were described for both the statins and fibrates. In addition to the preventive effects, these pleiotropic effects could partially explain the decrease in myocardial or cerebral ischemia consequences in experimental models. These cellular protective effects may have a therapeutic interest to decrease severity of stroke or coronary acute syndrome. They could also explain the drugs' lipid-lowering preventive effects independent of the treatment of lipid disorders. Beyond vascular pathologies, the pleiotropic effects of lipid-lowering drugs could explain their potentially beneficial effect in different diseases, such as dementia or cancer.

Absence of retinoids can induce motoneuron disease in the adult rat and a retinoid defect is present in motoneuron disease patients

We generated retinoid-deficient adult rats by the removal of retinoids from their diet. We show that their motoneurons undergo neurodegeneration and that there is an accumulation of neurofilaments and an increase in astrocytosis, which is associated with motoneuron disease. These effects are mediated through the retinoic acid receptor alpha. The same receptor deficit is found in motoneurons from patients suffering from spontaneous amyotrophic lateral sclerosis. Furthermore, we show that there is a loss of expression of the retinaldehyde dehydrogenase enzyme II in motoneurons. Therefore, we propose that a defect in the retinoid signalling pathway is in part be responsible for some types of motoneuron disease.

The transcriptional activation of the human copper/zinc superoxide dismutase gene by 2,3,7,8-tetrachlorodibenzo-p-dioxin through two different regulator sites, the antioxidant responsive element and xenobiotic responsive element

Cu/Zn superoxide dismutase (SOD1) catalyzes the dismutation of superoxide radicals produced during biological oxidations and environmental stress. The most toxic dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), induces SOD1 in human liver cells. Deletion analyses showed that the promoter region between -400 and -239 was responsible for the induction, in which two different characteristic regulatory elements, the antioxidant responsive element (ARE) and xenobiotic responsive element (XRE), are located. When the cells transfected with the plasmid containing those two cis-elements, the transactivation of SOD1 promoter was about 4-fold by TCDD, whereas mutation either on the ARE or XRE elevated the promoter activity by about 2-fold. Functional analyses of these two elements by deletion, mutation in the natural context, heterologous promoter assay, and gel mobility shift assay supported the notion that the activation of the SOD1 promoter was induced by TCDD through these two regulatory elements ARE and XRE. These results alongside our previous data indicate that the induction of SOD1 in response to TCDD is mediated by either Nrf2 protein or Ah receptor protein through ARE and XRE, respectively. These results also imply that the SOD1 can be induced by dioxin either in combination with or independently of these two regulatory elements to effectively defend cells from oxidative stress.

85-kDa cytosolic phospholipase A2 mediates peroxisome proliferator-activated receptor gamma activation in human lung epithelial cells

The 85-kDa cytosolic phospholipase A(2) (cPLA(2)) plays an important role in the control of arachidonic acid metabolism. This study was designed to investigate the possible contributions of cPLA(2) and group IIA secretory phospholipase A(2) (sPLA(2)) in the regulation of peroxisome proliferator-activated receptor (PPAR)-mediated gene transcription in human airway epithelial cells. Primary normal human bronchial epithelial cells and human lung epithelial cell lines BEAS 2B, A549, and NCI-H292 all express PPARgamma and -beta. Overexpression of cPLA(2) in BEAS 2B cells and primary bronchial epithelial cells resulted in a significant increase of PPARgamma-mediated reporter activity. In contrast, overexpression of group IIA sPLA(2) had no effect on PPARgamma activation. The PPARgamma activity in A549 cells was significantly inhibited by the cPLA(2) inhibitor arachidonyltrifluoromethyl ketone but not by the sPLA(2) inhibitor LY311727 and the iPLA(2) inhibitor HELSS. Activation of cPLA(2) by the calcium ionophore, induced a dose-dependent increase of PPAR activity in normal human bronchial epithelial cells and in the A549 cells. Electrophoretic mobility shift assays show that the binding between PPAR isolated from A549 cells and peroxisome proliferator response element (PPRE) is enhanced by but partially blocked by the cPLA(2) inhibitors arachidonyltrifluoromethyl ketone and methyl arachidonyl fluorophosphate. Finally, NS 398, a COX-2 inhibitor, partially blocked the effect on PPAR activity and binding to the PPRE suggesting involvement of COX-2 metabolites in PPRE activation. The above results demonstrate a novel function of cPLA(2) in the control of PPARgamma activation in human lung epithelial cells.

Superoxide dismutase multigene family: a comparison of the CuZn-SOD (SOD1), Mn-SOD (SOD2), and EC-SOD (SOD3) gene structures, evolution, and expression

Superoxide dismutases are an ubiquitous family of enzymes that function to efficiently catalyze the dismutation of superoxide anions. Three unique and highly compartmentalized mammalian superoxide dismutases have been biochemically and molecularly characterized to date. SOD1, or CuZn-SOD (EC 1.15.1.1), was the first enzyme to be characterized and is a copper and zinc-containing homodimer that is found almost exclusively in intracellular cytoplasmic spaces. SOD2, or Mn-SOD (EC 1.15.1.1), exists as a tetramer and is initially synthesized containing a leader peptide, which targets this manganese-containing enzyme exclusively to the mitochondrial spaces. SOD3, or EC-SOD (EC 1.15.1.1), is the most recently characterized SOD, exists as a copper and zinc-containing tetramer, and is synthesized containing a signal peptide that directs this enzyme exclusively to extracellular spaces. What role(s) these SODs play in both normal and disease states is only slowly beginning to be understood. A molecular understanding of each of these genes has proven useful toward the deciphering of their biological roles. For example, a variety of single amino acid mutations in SOD1 have been linked to familial amyotrophic lateral sclerosis. Knocking out the SOD2 gene in mice results in a lethal cardiomyopathy. A single amino acid mutation in human SOD3 is associated with 10 to 30-fold increases in serum SOD3 levels. As more information is obtained, further insights will be gained.

85-kDa cPLA(2) plays a critical role in PPAR-mediated gene transcription in human hepatoma cells

In an effort to understand the role of key eicosanoid-forming enzymes in the activation of peroxisome proliferator-activated receptor (PPAR), this study was designed to evaluate the possible contributions of cytosolic phospholipase A(2) (cPLA(2)) and group IIA secretory phospholipase A(2) (sPLA(2)) in the regulation of PPAR-mediated gene transcription in a human hepatoma cell line (HepG2). The HepG2 cells express both PPAR-alpha and -gamma but not PPAR-beta. Overexpression of cPLA(2), but not group IIA sPLA(2) in the HepG2 cells, caused a significantly increased PPAR-alpha/gamma-mediated reporter activity. Antisense inhibition of cPLA(2) resulted in a significantly decreased PPAR-alpha/gamma activity. The PPAR-alpha/gamma-induced gene transcription in the HepG2 cells was inhibited by the cPLA(2) inhibitors methyl arachidonyl fluorophosphonate and arachidonyltrifluoromethyl ketone, but not by the sPLA(2) inhibitor LY311727. The expression of PPAR-alpha-mediated endogenous gene apolipoprotein A-II was increased in cells with overexpression of cPLA(2), decreased in cells with antisense inhibition of cPLA(2), but unaltered in cells with overexpression of group IIA sPLA(2). The above results demonstrated an important role of cPLA(2), but not group IIA sPLA(2) in the control of PPAR activation. The cPLA(2)-mediated PPAR activation was likely mediated by arachidonic acid and prostaglandin E(2). This study reveals a novel intracellular function of cPLA(2) in PPAR activation in HepG2 cells. The cPLA(2) thus may represent a potential therapeutic target for the control of PPAR-related liver and metabolic disorders such as obesity, lipid metabolic disorders, diabetes mellitus, and atherosclerosis.

Retinoic acid reduces apoptosis and oxidative stress by preservation of SOD protein level

Retinoic acid (RA) has already been shown to exert antiapoptotic and antioxidative activity in various cells. In this study, we determined the effect of RA on the mRNA and protein levels of the Cu-,Zn-superoxide dismutase (SOD-1) and Mn-superoxide dismutase (SOD-2) during staurosporine-induced apoptosis in primary cultures from neonatal rat hippocampus. Exposure to staurosporine (300 nM, 24 h) increased the percentage of apoptotic neurons to 62% compared with 18% in controls. We determined an increase in the reactive oxygen species (ROS) content from 4 up to 48 h after the induction of the injury. Treatment with staurosporine did not significantly change the mRNA levels of SOD-1 and SOD-2. However, the SOD-1 and SOD-2 protein levels markedly decreased 24 and 48 h after the addition of staurosporine. Compared with staurosporine-exposed controls, RA (10 nM)-treated cultures showed a significant increase in neuronal survival, a reduced neuronal ROS content, and enhanced protein levels of SOD-1 and SOD-2 24 and 48 h after the start of the exposure to staurosporine. The results suggest that RA reduced staurosporine-induced oxidative stress and apoptosis by preventing the decrease in the protein levels of SOD-1 and SOD-2, and thus supported the antioxidant defense system.

Monitoring expression of genes involved in drug metabolism and toxicology using DNA microarrays

Oligonucleotide DNA microarrays were investigated for utility in measuring global expression profiles of drug metabolism genes. This study was performed to investigate the feasibility of using microarray technology to minimize the long, expensive process of testing drug candidates for safety in animals. In an evaluation of hybridization specificity, microarray technology from Affymetrix distinguished genes up to a threshold of approximately 90% DNA identity. Oligonucleotides representing human cytochrome P-450 gene CYP3A5 showed heterologous hybridization to CYP3A4 and CYP3A7 RNAs. These genes could be clearly distinguished by selecting a subset of oligonucleotides that hybridized selectively to CYP3A5. Further validation of the technology was performed by measuring gene expression profiles in livers of rats treated with vehicle, 3-methylcholanthrene (3MC), phenobarbital, dexamethasone, or clofibrate and by confirming data for six genes using quantitative RT-PCR. Responses of drug metabolism genes, including CYPs, epoxide hydrolases (EHs), UDP-glucuronosyl transferases (UGTs), glutathione sulfotransferases (GSTs), sulfotransferases (STs), drug transporter genes, and peroxisomal genes, to these well-studied compounds agreed well with, and extended, published observations. Additional gene regulatory responses were noted that characterize metabolic effects or stress responses to these compounds. Thus microarray technology can provide a facile overview of gene expression responses relevant to drug metabolism and toxicology.

Control of cell proliferation via transduction of sPLA(2)-I activity and possible PPAR activation at the nuclear level

Pancreatic phospholipase A2 (PLA(2)-I) stimulates U(III) cells proliferation, a rat uterine cell line, after binding to membrane receptors, internalization and translocation. Here, we demonstrate that during these steps of internalization, PLA(2)-I retains its hydrolytic activity and thus could exert its proliferative effect via nuclear phospholipids hydrolysis. Since fatty acids and eicosanoids released by such activity are known to be ligands of PPAR, we study the expression of these nuclear receptors and demonstrate that, in the experimental conditions where PLA(2)-I stimulates U(III) cells proliferation, PLA(2)-I also regulates PPAR expression indicating a possible mechanism of its proliferative effect.