Peroxisome proliferator-activated receptors in atherosclerosis and inflammation--an update

The role of C-peptide in diabetes and its complications: an updated review

Worldwide, diabetes and its complications have seriously affected people's quality of life and become a serious public health problem. C-peptide is not only an indicator of pancreatic β-cell function, but also a biologically active peptide that can bind to cell membrane surface signaling molecules and activate downstream signaling pathways to play antioxidant, anti-apoptotic and inflammatory roles, or regulate cellular transcription through internalization. It is complex how C-peptide is related to diabetic complications. Both deficiencies and overproduction can lead to complications, but their mechanisms of action may be different. C-peptide replacement therapy has shown beneficial effects on diabetic complications in animal models when C-peptide is deficient, but results from clinical trials have been unsatisfactory. The complex pattern of the relationship between C-peptide and diabetic chronic complications has not yet been fully understood. Future basic and clinical studies of C-peptide replacement therapies will need to focus on baseline levels of C-peptide in addition to more attention also needs to be paid to post-treatment C-peptide levels to explore the optimal range of fasting C-peptide and postprandial C-peptide maintenance.

Protective effects of bezafibrate against elaidic acid-induced accumulation of lipid droplets in monocytic cells

Some factors related to diet, such as trans fatty acids (TFA), are known to be involved in the progression of atherosclerosis in humans. Thus, the aim of our study was (i) to evaluate the effects of three dietary free fatty acids (FFA) (elaidic (EA), oleic (OA) and palmitic acid (PA)) on U937 human monocytes, and (ii) to study the eventual benefits of bezafibrate (BZF), a pan-agonist for PPAR isoforms (α, γ and δ) in U937 cells treated with FFA. Morphologic and functional changes were investigated by microscopic and flow cytometric methods. Cellular lipid content, lipid droplets and FA composition were identified and studied. All analyses were also realized in association with or without BZF. Contrary to OA and PA, EA slightly induced both propidium iodide-positive cells and mitochondrial depolarization. In addition, in contrast to OA and PA, EA induced only a slight increase in superoxide anion production. However, EA and OA promoted cytoplasmic lipid droplets accumulation. Only EA and OA significantly increased CD36 expression. It is noteworthy that BZF had a more or less pronounced protective effect against EA-, OA- and PA-induced side effects: BZF attenuated the impaired cell viability and inflammatory response, decreased superoxide anion production and prevented the accumulation of neutral and polar lipids. The effects were less pronounced with OA and PA than with EA. Altogether, our data revealed a benefit of BZF on the side effects induced especially with EA. It may thus be of interest in preventing the early stages of atherosclerotic plaque formation.

Dihydrocapsaicin Attenuates Plaque Formation through a PPARγ/LXRα Pathway in apoE(-/-) Mice Fed a High-Fat/High-Cholesterol Diet

Aims

Atherosclerosis is a chronic inflammatory disease and represents the major cause of cardiovascular morbidity and mortality. There is evidence that dihydrocapsaicin (DHC) can exert multiple pharmacological and physiological effects. Here, we explored the effect of DHC in atherosclerotic plaque progression in apoE^−/− mice fed a high-fat/high-cholesterol diet.

Methods and Results

apoE^−/− mice were randomly divided into two groups and fed a high-fat/high-cholesterol diet with or without DHC for 12 weeks. We demonstrated that cellular cholesterol content was significantly decreased while apoA1-mediated cholesterol efflux was significantly increased following treatment with DHC in THP-1 macrophage-derived foam cells. We also observed that plasma levels of TG, LDL-C, VLDL-C, IL-1β, IL-6, TNF-α and CRP were markedly decreased while plasma levels of apoA1 and HDL-C were significantly increased, and consistent with this, atherosclerotic lesion development was significantly inhibited by DHC treatment of apoE^−/− mice fed a high-fat/high-cholesterol diet. Moreover, treatment with both LXRα siRNA and PPARγ siRNA made the up-regulation of DHC on ABCA1, ABCG1, ABCG5, SR-B1, NPC1, CD36, LDLR, HMGCR, apoA1 and apoE expression notably abolished while made the down-regulation of DHC on SRA1 expression markedly compensated. And treatment with PPARγ siRNA made the DHC-induced up-regulation of LXRα expression notably abolished while treatment with LXRα siRNA had no effect on DHC-induced PPARγ expression.

Conclusion

These observations provide direct evidence that DHC can significantly decrease atherosclerotic plaque formation involving in a PPARγ/LXRα pathway and thus DHC may represent a promising candidate for a therapeutic agent for the treatment or prevention of atherosclerosis.

The involvement of the CD40-CD40L pathway in activated platelet-induced changes in HUVEC COX-2 and PPARα expression

We aim to determine the extent of the CD40-CD40L pathway involvement in activated platelet-induced changes in human umbilical endothelial cells (HUVECs). Activated platelets were co-incubated with HUVECs in the presence or absence of CD40LmAb. HUVECs were also directly stimulated with rhCD40L. HUVEC endothelial cyclooxygenase 2 (COX-2) and peroxisome proliferator-activated receptor alpha (PPARα) expression was then assessed. To estimate COX-2 activity, PGE2 concentration was determined. PPARα activity was assessed using a nuclear factor activity kit. Co-incubation with activated platelets increased HUVEC COX-2 and PPARα mRNA expression (P < 0.01). The addition of CD40L mAb significantly attenuated these increases in mRNA and protein (both P < 0.01). Direct stimulation by rhCD40L increased HUVEC COX-2 mRNA and protein (P < 0.05) but did not significantly change the expression of PPARα mRNA and protein. CD40LmAb significantly decreased (P < 0.05) and rhCD40L significantly (P < 0.01) increased COX-2 enzymatic activity, but had almost no effects on PPARα binding activity. Activated platelets may increase HUVEC COX-2 expression and activity partly through the CD40-CD40L pathway.

Modulation of nutrient sensing nuclear hormone receptors promotes weight loss through appetite suppression in mice

AIM

Peroxisome proliferator activated receptors (PPARs) are nuclear receptors involved in glucose and lipid metabolism. Three isoforms of PPARs have been identified with different tissue distribution and biological functions. Although the pharmacology of each receptor is well studied, the physiological effect of simultaneous activation of PPARalpha, gamma and delta is only starting to emerge. We sought to determine the biological effects of a novel PPAR pan activator and elucidate the physiological mechanisms involved.

METHODS

Ob/ob, diet-induced obese (DIO) or PPARalpha knockout mice were administered a novel agonist that activates all PPARs to various degrees to determine the effect on body weight, body composition, food intake and energy expenditure. In addition, serum parameters including glucose, insulin, triglycerides and ketone bodies as well as tissue acylcarnitine were evaluated. The effect of the novel agonist on liver and skeletal muscle histopathology was also studied.

RESULTS

We report that simultaneous activation of all PPARs resulted in substantial weight loss in ob/ob and DIO mice. Consistent with known PPAR pharmacology, we observed that agonist treatment increased lipid oxidation, although appetite suppression was mainly responsible for the weight loss. Agonist-induced weight loss was completely absent in PPARalpha knockout mice suggesting that PPARalpha pharmacology was the major contributor to weight regulation in mice.

CONCLUSIONS

Our work provides evidence that simultaneous activation of PPARalpha, gamma and delta decreases body weight by regulating appetite. These effects of the pan agonist were completely absent in PPARalpha knockout mice, suggesting that PPARalpha pharmacology was the major contributor to weight loss.

Effects of ciglitazone and troglitazone on the proliferation of human stomach cancer cells

AIM: To determine the cytological and molecular effects of peroxisome proliferation-activated receptor (PPAR)-γ and PPAR-γ agonists on stomach cancer cells.

METHODS: To determine the proliferation-suppressive effects of troglitazone and ciglitazone, SNU-216 and SNU-668 stomach cancer cells were plated in media containing 40 &mgr;mol/L troglitazone and ciglitazone at a density of 1 × 10⁴ cells/well. After 3, 5 and 7 d, the cells were counted with a hemocytometer. To assess the appearance of PPAR-γ, a reverse-transcription polymerase chain reaction analysis was performed. On day 7, Western blotting was used to determine the effects of troglitazone and ciglitazone on the expression of p21 and phosphorylated-ERK (pERK) genes. Flow cytometry analysis was used to determine which portion of the cell cycle was delayed when troglitazone was used to suppress cell proliferation. In order to clarify the mechanism underlying the activity of troglitazone, microarray analysis was conducted.

RESULTS: PPAR-γ was manifested in both SNU-216 and SNU-668 cells. Ciglitazone and troglitazone suppressed cell growth, and troglitazone was a stronger suppressor of stomach cancer cells than ciglitazone, an inducer of cell cycle arrest in the G1 phase. SNU-668 cells were also determined to be more sensitive to ciglitazone and troglitazone than SNU-216 cells. When troglitazone and ciglitazone were administered to stomach cancer cells, levels of p21 expression were increased, but ERK phosphorylation levels were reduced. When GW9662, an antagonist of PPAR-γ, was applied in conjunction with ciglitazone and troglitazone, the cell growth suppression effect was unaffected. The gene transcription program revealed a variety of alterations as the consequence of troglitazone treatment, and multiple troglitazone-associated pathways were detected. The genes whose expression was increased by troglitazone treatment were associated with cell development, differentiation, signal transmission between cells, and cell adhesion, and were also associated with reductions in cell proliferation, the cell cycle, nuclear metabolism, and phosphorylation.

CONCLUSION: Troglitazone and ciglitazone suppress the proliferation of stomach cancer cells via a PPAR-γ-independent pathway.

PPARdelta Agonism for the Treatment of Obesity and Associated Disorders: Challenges and Opportunities

The prevalence of obesity in the USA and worldwide has reached epidemic proportions during the last two decades. Drugs currently available for the treatment of obesity provide no more than 5% placebo-adjusted weight loss and are associated with undesirable side effects. Peroxisome proliferator-activated receptor (PPAR) modulators offer potential benefits for the treatment of obesity and its associated complications but their development has been complicated by biological, technical, and regulatory challenges. Despite significant challenges, PPAR modulators are attractive targets for the treatment of obesity and could offer a viable alternative to the millions of patients who fail to lose weight following rigorous dieting and exercise protocols. In addition, PPAR modulators have the potential-added benefit of ameliorating the associated comorbidities.

Anti-inflammatory actions of perfluorooctanoic acid and peroxisome proliferator-activated receptors (PPAR) alpha and gamma in experimental acute pancreatitis

Perfluorooctanoic acid (PFOA) and agonists of peroxisome proliferator-activated receptors (PPAR) alpha and gamma were investigated for potential anti-inflammatory effects in cerulein-induced acute pancreatitis in rats. PFOA significantly reduced both leukocyte accumulation and prostanoid synthesis. The PPAR-alpha agonist clofibrate had no effect on leukocyte activation but significantly inhibited prostanoid synthesis whereas the PPAR-gamma agonist rosiglitazone significantly reduced leukocyte activation but did not affect synthesis of prostaglandins in the pancreas. Neither PFOA, nor clofibrate or rosiglitazone had an effect on the formation of the inflammatory edema or elevated levels of lipase activity in the blood serum. In summary, PFOA attenuates the accumulation of activated leukocytes and reduces the synthesis of prostanoids in the pancreas during cerulein-induced acute pancreatitis. An activation of PPAR-alpha causes inhibition of prostanoid synthesis while activation of PPAR-gamma inhibits leukocyte activation.

Peroxisome proliferator-activated receptor gamma: a novel target for cancer therapeutics?

Peroxisome proliferator-activated receptors are ligand-activated intracellular transcription factors that have been implicated in important biological processes such as inflammation, tissue remodeling and atherosclerosis. Emerging information also implicates peroxisome proliferator-activated receptors in oncogenesis. Peroxisome proliferator-activated receptor gamma, the best studied of the peroxisome proliferator-activated receptors, modulates the proliferation and apoptosis of many cancer cell types, and it is expressed in many human tumors including lung, breast, colon, prostate and bladder. Natural and synthetic agents capable of activating peroxisome proliferator-activated receptor gamma have been found to inhibit cancer cell growth in vitro and in animal models. These agents, however, are not specific and both peroxisome proliferator-activated receptor gamma-dependent and peroxisome proliferator-activated receptor gamma-independent pathways involved in their effects have been identified. Together, these studies, coupled with a few clinical trials, suggest that peroxisome proliferator-activated receptor gamma is a novel target for the development of new and effective anticancer therapies.

Identifying leukocyte gene expression patterns associated with plasma lipid levels in human subjects

Plasma lipid levels have been known to be risk factors for atherosclerosis for decades, and in recent years it has become accepted that inflammation is a crucial event in the pathogenesis of atherosclerosis. In this study, we investigated the relationship between plasma lipids and leukocytes by profiling and analyzing leukocyte gene expression in response to plasma lipid levels. We discovered several interesting patterns of leukocyte gene expression: (1) the expression of a number of immune response- and inflammation-related genes are correlated with plasma lipid levels; (2) genes involved in lipid metabolism and in the electron transport chain were positively correlated with triglycerides and low-density lipoprotein cholesterol (LDL) levels, and negatively correlated with high-density lipoprotein cholesterol (HDL) levels; (3) genes involved in platelet activation were negatively correlated with HDL levels; (4) transcription factors regulating lipogenesis-related genes were correlated with plasma lipid levels; (5) a number of genes correlated with plasma lipid levels were found to be located in the regions of known quantitative trait loci (QTLs) associated with hyperlipemia. Our findings suggest that leukocytes respond to changing plasma lipid levels by regulating a network of genes, including genes involved in immune response, and lipid and fatty acid metabolism.

Molecular aspects of atherogenesis: new insights and unsolved questions

The development of atherosclerotic disease results from the interaction between environment and genetic make up. A key factor in atherogenesis is the oxidative modification of lipids, which is involved in the recruitment of mononuclear leukocytes to the arterial intima--a process regulated by several groups of adhesion molecules and cytokines. Activated leukocytes, as well as endothelial mitochondria, can produce reactive oxygen species (ROS) that are associated with endothelial dysfunction, a cause of reduced nitric oxide (NO) bioactivity and further ROS production. Peroxisome proliferator-activated receptors (PPAR) and liver X receptors (LXR) are nuclear receptors significantly involved in the control of lipid metabolism, inflammation and insulin sensitivity. Also, an emerging role has been suggested for G protein coupled receptors and for the small Ras and Rho GTPases in the regulation of the expression of endothelial NO synthase (eNOS) and of tissue factor, which are involved in thrombus formation and modulation of vascular tone. Further, the interactions among eNOS, cholesterol, oxidated LDL and caveola membranes are probably involved in some molecular changes observed in vascular diseases. Despite the relevance of oxidative processes in atherogenesis, anti-oxidants have failed to significantly improve atherosclerosis (ATS) prevention, while statins have proved to be the most successful drugs.

Postprandial activation of hemostatic factors: role of dietary fatty acids

Intake of dietary fat is an important determinant of the plasma concentration of triacylglycerol-rich lipoproteins, and the degree of alimentary lipemia is reported to have effects on hemostatic status including platelet function. Although association between the amount of dietary fat intake, lipemic response and certain cardiovascular disease (CVD) risk factors (VIIa and PAI-1) has been reported, the significance of the fatty acid composition of ingested fat for the postprandial lipid concentrations and the hemostatic factors is still unclear. Accumulating evidence suggests a relationship between dietary fatty acids and emerging hemostatic CVD risk factors, although much of this evidence is incomplete or conflicting. In order to improve our knowledge in this area, sufficient sample size in future studies are required to take into account of the genetic variation (gene polymorphisms for VII, PAI-1), sex, physical activity, stage of life factors, and sufficient duration to account for adaptation for definitive conclusions.

Bezafibrate suppresses rat antiglomerular basement membrane crescentic glomerulonephritis

BACKGROUND

The immunoregulatory activity of ligands for peroxisome proliferator-activated receptors (PPARs) has been recently paid attention. The regulatory effect of bezafibrate (BZF), a ligand for PPARalpha on glomerulonephritis was investigated using a rat anti-glomerular basement membrane (GBM) glomerulonephritis model.

METHODS

The effect on development of anti-GBM glomerulonephritis was examined by treatment with BZF from day -7 to day 7 after intravenous injection of rabbit anti-GBM serum into Wistar Kyoto (WKY) rats. The therapeutic efficacy after onset of the glomerulonephritis was also checked by treatment with BZF from day 3 to 7. On day 7, the condition was evaluated histologically. The expression of a tissue injury molecule, macrophage metalloesterase (MME), was measured by Northern blot analysis. The suppressive effect on immune cells was assessed by proliferation assay with mitogen-stimulated rat spleen cells.

RESULTS

Histopathologic changes induced by anti-GBM in rats treated with BZF (day -7 to day 7) were markedly suppressed in a dose-dependent fashion. Infiltration of ED-1+ macrophages in glomeruli, proteinuria, and mRNA expression of MME in kidneys were diminished in parallel with histologic improvement. Moreover, the disease activity was also attenuated even by the treatment after onset of the glomerulonephritis (day 3 to 7). The mitogen-induced proliferation of spleen cells was down-regulated at concentrations of BZF, which were equivalent to those in sera of BZF-treated rats.

CONCLUSION

BZF markedly suppresses the activity of rat anti-GBM crescentic glomerulonephritis. Fibrates might serve as a therapeutic option for crescentic glomerulonephritis.

Therapeutic effects of troglitazone in experimental chronic pancreatitis in mice

Peroxisome proliferator-activated receptor (PPAR)-gamma controls growth, differentiation, and inflammation. PPAR-gamma agonists exert anti-inflammatory effects in vitro and inhibit the activation of pancreas stellate cells, implicated in the formation and progression of fibrosis. We determined the influence of troglitazone, a ligand for PPAR-gamma, on pancreatic damage and fibrosis in experimental chronic pancreatitis. Mice received six hourly intraperitoneal injections with 50 microg/kg of cerulein or saline, three times a week for 6 weeks. One week after the last injection all mice were sacrificed. Untreated mice were compared with mice treated with troglitazone either during weeks 1 to 6 or weeks 4 to 6. All mice that received cerulein injections displayed histopathological signs of chronic pancreatitis at week 7. Troglitazone treatment improved all markers for severity of pancreatitis. Moreover, early and postponed troglitazone treatments were equally effective in diminishing intrapancreatic fibrosis as quantified by Sirius red staining, hydroxyproline content, and laminin staining as well as the increased number of pancreatic stellate cells and pancreas levels of transforming growth factor-beta. Thus, troglitazone attenuated pancreatic damage and inflammation in experimental chronic pancreatitis and remained beneficial in a therapeutic setting when given after initial damage had been established.

Biochemical and morphological effects of K-111, a peroxisome proliferator-activated receptor (PPAR)alpha activator, in non-human primates

K-111 has been characterized as a potent peroxisome proliferator-activated receptor (PPAR)alpha activator. Antidiabetic potency and amelioration of disturbed lipid metabolism were demonstrated in rodents, which were accompanied by elevations of peroxisomal enzymes and liver weight. To examine the possible therapeutic application of K-111 we have now assessed its efficacy in non-human primates with high transferability to humans. For this purpose obese, hypertriglyceridaemic, hyperinsulinaemic prediabetic rhesus monkeys were dosed sequentially with 0, 1, 3 and 10mg/kg per day orally over a period of 4 weeks each. In addition, the effect of K-111 on the peroxisome compartment was analyzed in cynomolgus monkeys using liver samples obtained following a 13-week oral toxicity study. In prediabetic monkeys, the reduction of hyperinsulinaemia and improvement of insulin-stimulated glucose uptake rate indicated amelioration of insulin resistance. These effects were nearly maximal at a dose of 3mg/kg per day, while triglycerides and body weight were lowered significantly in a dose-dependent manner. This reduction of body weight contrasts sharply with the adipogenic response observed with thiazolidinediones, another family of insulin-sensitizing agents. In young cynomolgus monkeys at a dosage of 5mg/kg per day and more, K-111 induced an up to three-fold increase in lipid beta-oxidation enzymes with an 1.5- to 2-fold increase in peroxisome volume density. This moderate increase in peroxisomal activity by K-111 in monkeys is consistent with its role as an PPARalpha activator and corresponds to the observations with fibrates in other low responder mammalian species. The increase in beta-oxidation may explain, at least in part, the lipid modulating effect as well as the antidiabetic potency of K-111. This pharmacological profile makes K-111 a highly promising drug candidate for clinical applications in the treatment of type 2 diabetes, dyslipidaemia, obesity and the metabolic syndrome.

Peroxisome proliferator-activated receptor γ C161T polymorphisms and survival of Japanese patients with immunoglobulin A nephropathy

Peroxisome proliferator-activated receptor gamma (PPAR gamma) plays an important role in lipid metabolism, insulin sensitivity, atherogenesis, and immune regulation. A genetic polymorphism (C161T) at exon 6 of PPAR gamma gene (PPARG) was reported to be associated with the onset of coronary artery disease. However, there has been no report of an association with renal disease. Genomic DNAs were isolated from 225 Japanese patients with histologically confirmed immunoglobulin A nephropathy (IgAN). The PPARG C161T genotype was determined by polymerase chain reaction-restriction fragment length polymorphism. The association of the polymorphism with renal prognosis in IgAN patients was analyzed using the Kaplan-Meier method and Cox proportional hazard regression model. The PPARG polymorphism was not associated with the renal survival rate. However, when patients were stratified into those either with or without hypertension at the time of diagnosis, the renal survival of the CT/TT genotypes was significantly better in those without hypertension than those with the CC genotype. We report that the PPARG C161T polymorphism is associated with the survival of IgAN patients without hypertension. The T allele of the polymorphism might have a protective effect on the progression of IgAN.

Effects of dibromoacetonitrile on rats following 13-week drinking water exposure

The subchronic toxicity of dibromoacetonitrile (DBAN), a disinfection by-product in drinking water, was studied in the rat. Male (180+/-18 g) and female (152+/-9 g) Sprague-Dawley rats (10 animals per group) were fed DBAN in organic-free distilled water at concentrations of 0.1, 1, 10 and 100 ppm for 13 weeks. Control rats received organic-free distilled water only. Water intakes in the highest dose males and females were reduced by 25 and 32% as compared to the controls, respectively (P<0.05), with no significant reductions in food consumption and body weight gain. The organ to body weight ratio was significantly increased in the highest-dose males and females for kidneys but not for the brain, liver, spleen, thymus and testicles. In the males, decreases were detected in serum uric acid levels at 1 and 100 ppm, and in urinary uric acid at 10 and 100 ppm. Decreased serum protein was detected in the highest-dose males and decreased serum LDH was found in the highest-dose females. Both the white blood cell and lymphocyte counts were significantly elevated in the highest-dose females. A significant increase in hepatic catalase activity was observed only in males starting at 1 ppm, and increased palmitoyl-CoA oxidase (PCO) activity was found in males and females of the highest dose group. In the males, decreased thiobarbituric acid reactive substance (TBARS) level was detected in the liver at 1.0 and 100 ppm groups, while increased TBARS was found in the serum at 100 ppm DBAN. No treatment-related changes were detected in the activities of hepatic benzyloxyresorufin O-dealkylase (BROD), pentoxyresorufin O-dealkylase (PROD) and ethoxresorufin O-deethylase (EROD), and in hepatic UDP-glucuronosyltransferase (UDPGT) and glutathione S-transferases (GST). Although DBAN is a potent inhibitor of hepatic aldehyde dehydrogenase (ALDH) and GST in vitro, there was no evidence of suppression of these enzymes in the treated animals. Mild histological changes were detected in animals receiving the highest dose, consisting of collapsed angularity, increased epithelial height in the thyroid of both sexes, and cytoplasmic vacuolation and nuclear vesiculation in the thyroid of females, increased myeloid to erythroid ratio in the bone marrow of both sexes, and cytoplasmic inclusions in the proximal tubules of male kidneys. In summary, treatment effects occurred predominantly at 100 ppm and included in both sexes: increased kidney weights, histological changes in the thyroid and bone marrow, and increased peroxisomal enzyme activities; and in males: decreased serum and urinary uric acid levels, and indication of oxidative stress. The no-observed-adverse-effect level (NOAEL) was therefore judged to be 10 ppm, equivalent to 1.11 and 1.21 mg/kg/day in the males and females, respectively.

Peroxisome proliferator-activated receptors: are they involved in atherosclerosis progression?

Peroxisome proliferator-activated receptors (PPAR) are nuclear receptors present in several organs and cell types. They are subdivided into PPAR alpha, PPAR gamma and PPAR delta (or beta). PPAR alpha and gamma are the two main categories of these receptors, which are both characterized by their ability to influence lipid metabolism, glucose homeostasis, cell proliferation, differentiation and apoptosis, as well as the inflammatory response, by transcriptional activation of target genes. PPAR alpha are activated by fatty acids, eicosanoids and fibrates, while PPAR gamma activators include arachidonic acid metabolites, oxidized low density lipoprotein and thiazolidinediones. Atherosclerosis is now considered a chronic inflammatory condition. Thus, PPAR activation appears a promising approach to favorably affect atherosclerosis development through both metabolic and anti-inflammatory effects. However, the clinical data in favor of an anti-atherosclerotic action of PPAR agonists are still scanty, and some experimental data would even indicate possible pro-atherogenic effects, or a lack of effect in the female sex. New controlled clinical studies will provide the information necessary to understand the true significance and usefulness of PPAR alpha, gamma and delta activators in the control of atherosclerotic disease.

Neuroprotective effects of PPARgamma agonists against oxidative insults in HT-22 cells

Peroxisome proliferator-activated receptors (PPARs) are involved in regulating many metabolic and inflammatory processes. The present study explores the role of PPAR ligands in protecting neuronal cultures from toxic insults. For that purpose, we used WY14643 [4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio acetic acid] as a PPARalpha agonist, L-165041 and L-783483 as PPARbeta ligands, and 15-deoxy-Delta(12,14)-PGJ2 (15d-PGJ2), troglitazone, and ciglitazone for PPARgamma. Experiments were performed using HT-22, an immortalized mouse hippocampal cell line, and SK-N-SH, a human neuroblastoma cell line. Cell viability against glutamate, hydrogen peroxide (H(2)O(2)), and serum deprivation insults was determined using a calcein acetoxymethyl (AM) assay. Of the compounds tested, only 15d-PGJ2 and troglitazone showed a dose-dependent neuroprotection from glutamate and H(2)O(2) insults in HT-22 cells. None of the PPAR agonists was protective in SK-N-SH cells. A minimum of 4-6 h preincubation with 15d-PGJ2 was required to achieve significant neuroprotection. On the other hand, troglitazone was protective even when administered simultaneously with glutamate, or for up to 8 h postglutamate insult. To investigate whether the neuroprotective effects are mediated through PPARgamma, we first determined through Western blotting that HT-22 and SK-N-SH cells express PPARgamma. However, the neuroprotective effects of those compounds are unlikely to be mediated through the PPARgamma for two reasons: (1) various concentrations of another PPARgamma agonist (ciglitazone) were not neuroprotective; (2) by itself, PPAR exhibits a low affinity for DNA, and high-affinity binding requires heterodimerization with RXR, the 9-cis-retinoic acid receptor; administering 9-cis-retinoic acid in conjunction with 15d-PGJ2 did not alter the neuroprotective effects of the latter. Our results demonstrate neuroprotective effects of 15d-PGJ2 and troglitazone that are likely independent of PPARgamma.

Metabolic and additional vascular effects of thiazolidinediones

Several cardiovascular risk factors (dyslipidaemia, hypertension, glucose intolerance, hypercoagulability, obesity, hyperinsulinaemia and low-grade inflammation) cluster in the insulin resistance syndrome. Treatment of these individual risk factors reduces cardiovascular complications. However, targeting the underlying pathophysiological mechanisms of the insulin resistance syndrome is a more rational treatment strategy to further improve cardiovascular outcome. Our understanding of the so-called cardiovascular dysmetabolic syndrome has been improved by the discovery of nuclear peroxisome proliferator-activated receptors (PPARs). PPARs are ligand-activated transcription factors belonging to the nuclear receptor superfamily. As transcription factors, PPARs regulate the expression of numerous genes and affect glycaemic control, lipid metabolism, vascular tone and inflammation. Activation of the subtype PPAR-gamma improves insulin sensitivity. Expression of PPAR-gamma is present in several cell types involved in the process of atherosclerosis. Thus, modulation of PPAR-gamma activity is an interesting therapeutic approach to reduce cardiovascular events. Thiazolidinediones are PPAR-gamma agonists and constitute a new class of pharmacological agents for the treatment of type 2 (non-insulin-dependent) diabetes mellitus. Two such compounds are currently available for clinical use: rosiglitazone and pioglitazone. Thiazolidinediones improve insulin sensitivity and glycaemic control in patients with type 2 diabetes. In addition, improvement in endothelial function, a decrease in inflammatory conditions, a decrease in plasma levels of free fatty acids and lower blood pressure have been observed, which may have important beneficial effects on the vasculature. Several questions remain to be answered about PPAR-gamma agonists, particularly with respect to the role of PPAR-gamma in vascular pathophysiology. More needs to be known about the adverse effects of thiazolidinediones, such as hepatotoxicity, increased low-density lipoprotein cholesterol levels and increased oedema. The paradox of adipocyte differentiation with weight gain concurring with the insulin-sensitising effect of thiazolidinediones is not completely understood. The decrease in blood pressure induced by thiazolidinedione treatment seems incompatible with an increase in the plasma volume, and the discrepancy between the stimulation of the expression of CD36 and the antiatherogenic effects of the thiazolidinediones also needs further explanation. Long-term clinical trials of thiazolidinediones with cardiovascular endpoints are currently in progress. In conclusion, studying the effects of thiazolidinediones may shed more light on the mechanisms involved in the insulin resistance syndrome. Furthermore, thiazolidinediones could have specific, direct effects on processes involved in the development of vascular abnormalities.

Vascular effects of GI262570X (PPAR-gamma agonist) in the brown adipose tissue of Han Wistar rats: a review of 1-month, 13-week, 27-week and 2-year oral toxicity studies

We describe and discuss microscopic findings in the brown adipose tissue (BAT) blood vessels of Han Wistar rats treated with GI262570X, a peroxisome proliferator-activated receptor-gamma agonist (PPAR-gamma agonist) by oral gavage for 28 days, 13 weeks, 27 weeks, and 2 years. Review of these studies revealed a consistent vascular change, consisting of multifocal fatty infiltration in the BAT of treated rats. A similar vascular change was not seen in other vessels or organs. Microscopically, fatty infiltration was characterized primarily by round, clear vacuoles within the tunica media and/or tunica adventitia of small and medium-sized arteries and arterioles. Occasionally, these vacuoles had peripherally located nuclei and morphologically resembled adipocytes, suggesting a well-characterized PPAR effect (ie, differentiation of stem cells or preadipocytes into mature adipocytes). However, administration of GI262570X up to 2 years failed to induce more severe or progressive lesions in the blood vessels of rat BAT and, in particular, did not result in induction of any atherosclerotic-like lesions or foam cell infiltration. At the longer exposure, there was an apparent reduction of severity and/or incidence, indicating a possible adaptive response. These results suggest that the possibility of generating atherosclerotic-like lesions through prolonged treatment of GI262570X (PPAR-gamma agonist) is highly unlikely in rats.

Peroxisome proliferator-activated receptor ligands as antiatherogenic agents: panacea or another Pandora's box?

Peroxisome proliferator activated receptors (PPARs) are members of the nuclear receptor super family that modulate gene expression upon ligand activation. They are 3 major subtypes of PPARs: alpha, delta (also called beta), and gamma. PPAR-gamma is widely expressed in the cardiovascular system and is involved in the regulation of tissue inflammation and smooth muscle cell growth pathways as well as in lipoprotein metabolism and coagulation cascades. PPAR-gamma ligands of (e.g., rosigitazone and pioglitazone) have been shown to exert antiatherogenic effects both in vitro and in vivo. PPAR-alpha ligands (e.g., clofibrate and benzofibrate) modulate lipoprotein metabolism, and affect inflammation and coagulation cascade. These effects may be helpful in resolving the dilemma arising from studies that showed significant mortality and morbidity benefits of fibrates in the face of minimal changes in HDL-cholesterol levels. The role of PPAR-delta in atherogenesis remains largely unknown, although it appears that PPAR-delta activation affects lipoprotein metabolism. PPAR ligands appear to be promising agents in limiting atherosclerosis; however, large-scale clinical trials are required to assess their safety and efficacy before they can be added to the clinicians' arsenal of antiatherosclerotic agents.

Peroxisome proliferator-activated receptors in endothelial cell biology

New insights into the endothelium as a dynamic, interactive organ have generated increased interest in endothelial cell transcriptional regulation. Peroxisomal proliferator-activated receptors (PPARs), as ligand-activated nuclear receptors expressed in endothelial cells, represent one important pathway that likely influences vascular responses both directly and indirectly by altering gene expression. PPAR ligands such as fibrates (PPAR-alpha) and insulin-sensitizing thiazolidinediones (PPAR-gamma) are in clinical use and may alter the process of atherosclerosis. The present review highlights the emerging evidence for PPAR-alpha and PPAR-gamma expression in the vasculature, as well as their potential roles in endothelial cell biology.