Réjane Paumelle

Acute Antiinflammatory Properties of Statins Involve Peroxisome Proliferator–Activated Receptor-α via Inhibition of the Protein Kinase C Signaling Pathway

Statins are inhibitors of 3-hydroxy-3-methylglutaryl–coenzyme A (HMG-CoA) reductase used in the prevention of cardiovascular disease (CVD). In addition to their cholesterol-lowering activities, statins exert pleiotropic antiinflammatory effects, which might contribute to their beneficial effects not only on CVD but also on lipid-unrelated immune and inflammatory diseases, such as rheumatoid arthritis, asthma, stroke, and transplant rejection. However, the molecular mechanisms involved in these antiinflammatory properties of statins are unresolved. Here we show that the peroxisome proliferator–activated receptor (PPAR) α mediates antiinflammatory effects of simvastatin in vivo in models of acute inflammation. The inhibitory effects of statins on lipopolysaccharide-induced inflammatory response genes were abolished in PPARα-deficient macrophages and neutrophils. Moreover, simvastatin inhibited PPARα phosphorylation by lipopolysaccharide-activated protein kinase C (PKC) α. A constitutive active form of PKCα inhibited nuclear factor &kgr;B transrepression by PPARα whereas simvastatin enhanced transrepression activity of wild-type PPARα, but not of PPARα mutated in its PKC phosphorylation sites. These data indicate that the acute antiinflammatory effect of simvastatin occurs via PPARα by a mechanism involving inhibition of PKCα inactivation of PPARα transrepression activity.

Hepatocyte growth factor/scatter factor activates the ETS1 transcription factor by a RAS-RAF-MEK-ERK signaling pathway

Hepatocyte growth factor/scatter factor (HGF/SF) induces scattering and morphogenesis of epithelial cells through the activation of the MET tyrosine kinase receptor. Although the activated MET receptor recruits a number of signaling proteins, little is known of the downstream signaling pathways activated by HGF/SF. In this study, we wished to examine the signaling pathway leading to activation of the ETS1 transcription factor. Using in vitro and in vivo kinase assays, we found that HGF/SF activates the ERK1 MAP kinase, leading to the phosphorylation of the threonine 38 residue of ETS1 within a putative MAP kinase phosphorylation site (PLLT38P). This threonine residue was neither phosphorylated by JNK1, nor by p38 MAP kinases and was required for the induction of transcriptional activity of ETS1 by HGF/SF. Using kinase and transcription assays, we further demonstrated that phosphorylation and activation of ETS1 occurs downstream of a RAS-RAF-MEK-ERK pathway. The functional involvement of this pathway in HGF/SF action was demonstrated using U0126, a pharmacological inhibitor of MEK, which blocked phosphorylation and activation of ETS1, RAS-dependent transcriptional responses, cell scattering and morphogenesis. These data demonstrated that ETS1 is a downstream target of HGF/SF acting through a RAS-RAF-MEK-ERK pathway and provides a signaling pathway leading to the regulation of gene expression by HGF/SF.

Peroxisome Proliferator–Activated Receptor α Induces NADPH Oxidase Activity in Macrophages, Leading to the Generation of LDL with PPAR-α Activation Properties

Peroxisome proliferator–activated receptors (PPARs) are nuclear receptors controlling lipid and glucose metabolism as well as inflammation. PPARs are expressed in macrophages, cells that also generate reactive oxygen species (ROS). In this study, we investigated whether PPARs regulate ROS production in macrophages. Different PPAR-&agr;, but not PPAR-&ggr; agonists, increased the production of ROS (H2O2 and O2&OV0254;) in human and murine macrophages. PPAR-&agr; activation did not induce cellular toxicity, but significantly decreased intracellular glutathione levels. The increase in ROS production was not attributable to inherent prooxidant effects of the PPAR-&agr; agonists tested, but was mediated by PPAR-&agr;, because the effects were lost in bone marrow–derived macrophages from PPAR-&agr;−/− mice. The PPAR-&agr;–induced increase in ROS was attributable to the induction of NADPH oxidase, because (1) preincubation with the NADPH oxidase inhibitor diphenyleneiodinium prevented the increase in ROS production; (2) PPAR-&agr; agonists increased O2&OV0254; production measured by superoxide dismutase–inhibitable cytochrome c reduction; (3) PPAR-&agr; agonists induced mRNA levels of the NADPH oxidase subunits p47phox, p67phox, and gp91phox and membrane p47phox protein levels; and (4) induction of ROS production was abolished in p47phox−/− and gp91phox−/− macrophages. Finally, induction of NADPH oxidase by PPAR-&agr; agonists resulted in the formation of oxidized LDL metabolites that exert PPAR-&agr;–independent proinflammatory and PPAR-&agr;–dependent decrease of lipopolysaccharide-induced inducible nitric oxide synthase expression in macrophages. These data identify a novel mechanism of autogeneration of endogenous PPAR-&agr; ligands via stimulation of NADPH oxidase activity.

Peroxisome Proliferator–Activated Receptor α Improves Pancreatic Adaptation to Insulin Resistance in Obese Mice and Reduces Lipotoxicity in Human Islets

Peroxisome proliferator–activated receptor (PPAR) α is a transcription factor controlling lipid and glucose homeostasis. PPARα-deficient (−/−) mice are protected from high-fat diet–induced insulin resistance. However, the impact of PPARα in the pathophysiological setting of obesity-related insulin resistance is unknown. Therefore, PPARα−/− mice in an obese (ob/ob) background were generated. PPARα deficiency did not influence the growth curves of the obese mice but surprisingly resulted in a severe, age-dependent hyperglycemia. PPARα deficiency did not aggravate peripheral insulin resistance. By contrast, PPARα−/− ob/ob mice developed pancreatic β-cell dysfunction characterized by reduced mean islet area and decreased insulin secretion in response to glucose in vitro and in vivo. In primary human pancreatic islets, PPARα agonist treatment prevented fatty acid–induced impairment of glucose-stimulated insulin secretion, apoptosis, and triglyceride accumulation. These results indicate that PPARα improves the adaptative response of the pancreatic β-cell to pathological conditions. PPARα could thus represent a promising target in the prevention of type 2 diabetes.

Rimonabant, a Selective Cannabinoid CB1 Receptor Antagonist, Inhibits Atherosclerosis in LDL Receptor–Deficient Mice

Objective—The objective of this study was to determine whether the potent selective cannabinoid receptor-1 antagonist rimonabant has antiatherosclerotic properties. Methods and Results—Rimonabant (50 mg/kg/d in the diet) significantly reduced food intake (from 3.35±.04 to 2.80±0.03 g/d), weight gain (from 14.6±0.7 g to −0.6±0.3 g), serum total cholesterol (from 8.39±0.54 to 5.32±0.18 g/L), and atherosclerotic lesion development in the aorta (from 1.7±0.22 to 0.21±0.037 mm2) and aortic sinus (from 101 000±7800 to 27 000±2900 &mgr;m2) of LDLR−/− mice fed a Western-type diet for 3 months. Rimonabant also reduced plasma levels of the proinflammatory cytokines MCP-1 and IL12 by 85% (P<0.05) and 76% (P<0.05), respectively. Pair-fed animals had reduced weight gain (6.2±0.6 g gain), but developed atherosclerotic lesions which were as large as those of untreated animals, showing that the antiatherosclerotic effect of rimonabant is not related to reduced food intake. Interestingly, rimonabant at a lower dose (30 mg/kg/d in the diet) reduced atherosclerosis development in the aortic sinus (from 121 000±20 000 to 62 000±11 000 &mgr;m2, 49% reduction, P<0.05), without affecting serum total cholesterol (7.8±0.7 g/L versus 8.1±1.3 g/L in the control group). Rimonabant decreased lipopolysaccharide (LPS)- and IL1&bgr;-induced proinflammatory gene expression in mouse peritoneal macrophages in vitro as well as thioglycollate-induced recruitment of macrophages in vivo (10 mg/kg, po bolus). Conclusions—These results show that rimonabant has antiatherosclerotic effects in LDLR−/− mice. These effects are partly unrelated to serum cholesterol modulation and could be related to an antiinflammatory effect.

PPARα blocks glucocorticoid receptor α-mediated transactivation but cooperates with the activated glucocorticoid receptor α for transrepression on NF-κB

Glucocorticoid receptor α (GRα) and peroxisome proliferator-activated receptor α (PPARα) are transcription factors with clinically important immune-modulating properties. Either receptor can inhibit cytokine gene expression, mainly through interference with nuclear factor κB (NF-κB)-driven gene expression. The present work aimed to investigate a functional cross-talk between PPARα- and GRα-mediated signaling pathways. Simultaneous activation of PPARα and GRα dose-dependently enhances transrepression of NF-κB-driven gene expression and additively represses cytokine production. In sharp contrast and quite unexpectedly, PPARα agonists inhibit the expression of classical glucocorticoid response element (GRE)-driven genes in a PPARα-dependent manner, as demonstrated by experiments using PPARα wild-type and knockout mice. The underlying mechanism for this transcriptional antagonism relies on a PPARα-mediated interference with the recruitment of GRα, and concomitantly of RNA polymerase II, to GRE-driven gene promoters. Finally, the biological relevance of this phenomenon is underscored by the observation that treatment with the PPARα agonist fenofibrate prevents glucocorticoid-induced hyperinsulinemia of mice fed a high-fat diet. Taken together, PPARα negatively interferes with GRE-mediated GRα activity while potentiating its antiinflammatory effects, thus providing a rationale for combination therapy in chronic inflammatory disorders.

PPARα gene expression correlates with severity and histological treatment response in patients with non-alcoholic steatohepatitis

BACKGROUND & AIMS Peroxisome proliferator-activated receptors (PPARs) have been implicated in non-alcoholic steatohepatitis (NASH) pathogenesis, mainly based on animal data. Gene expression data in NASH patients are scarce. We studied liver PPARα, β/δ, and γ expression in a large cohort of obese patients assessed for presence of NAFLD at baseline and 1 year follow-up. METHODS Patients presented to the obesity clinic underwent a hepatic work-up. If NAFLD was suspected, liver biopsy was performed. Gene expression was studied by mRNA quantification. Patients were reassessed after 1 year. RESULTS 125 patients were consecutively included in the study, of which 85 patients had paired liver biopsy taken at 1 year of follow-up. Liver PPARα expression negatively correlated with the presence of NASH (p=0.001) and with severity of steatosis (p=0.003), ballooning (p=0.001), NASH activity score (p=0.008) and fibrosis (p=0.003). PPARα expression was positively correlated to adiponectin (R(2)=0.345, p=0.010) and inversely correlated to visceral fat (R(2)=-0.343, p<0.001), HOMA IR (R(2)=-0.411, p<0.001) and CK18 (R(2)=-0.233, p=0.012). Liver PPARβ/δ and PPARγ expression did not correlate with any histological feature nor with glucose metabolism or serum lipids. At 1 year, correlation of PPARα expression with liver histology was confirmed. In longitudinal analysis, an increase in expression of PPARα and its target genes was significantly associated with histological improvement (p=0.008). CONCLUSION Human liver PPARα gene expression negatively correlates with NASH severity, visceral adiposity and insulin resistance and positively with adiponectin. Histological improvement is associated with an increase in expression of PPARα and its target genes. These data might suggest that PPARα is a potential therapeutic target in NASH.

Phosphorylation of Farnesoid X Receptor by Protein Kinase C Promotes Its Transcriptional Activity

The farnesoid X receptor (FXR, NR1H4) belongs to the nuclear receptor superfamily and is activated by bile acids such as chenodeoxycholic acid, or synthetic ligands such as GW4064. FXR is implicated in the regulation of bile acid, lipid, and carbohydrate metabolism. Posttranslational modifications regulating its activity have not been investigated yet. Here, we demonstrate that calcium-dependent protein kinase C (PKC) inhibition impairs ligand-mediated regulation of FXR target genes. Moreover, in a transactivation assay, we show that FXR transcriptional activity is modulated by PKC. Furthermore, phorbol 12-myristate 13-acetate , a PKC activator, induces the phosphorylation of endogenous FXR in HepG2 cells and PKCalpha phosphorylates in vitro FXR in its DNA-binding domain on S135 and S154. Mutation of S135 and S154 to alanine residues reduces in cell FXR phosphorylation. In contrast to wild-type FXR, mutant FXRS135AS154A displays an impaired PKCalpha-induced transactivation and a decreased ligand-dependent FXR transactivation. Finally, phosphorylation of FXR by PKC promotes the recruitment of peroxisomal proliferator-activated receptor gamma coactivator 1alpha. In conclusion, these findings show that the phosphorylation of FXR induced by PKCalpha directly modulates the ability of agonists to activate FXR.

Cross-talk Between Statins and PPARα in Cardiovascular Diseases: Clinical Evidence and Basic Mechanisms

Although a change in lifestyle is the first choice in controlling cardiovascular risk, lipid-lowering drugs are effective in normalizing different forms of atherogenic dyslipidemia. Although statins are a class of drugs which primarily lower low-density lipoprotein cholesterol, fibrates decrease triglycerides, normalize the low-density lipoprotein cholesterol profile, and increase high-density lipoprotein cholesterol. As lipids are important determinants for cardiovascular diseases, these drugs reduce cardiovascular morbidity. However, a number of recent studies indicate that, in addition to their lipid-normalizing activities, statins and fibrates exhibit pleiotropic actions, such as inhibit inflammation, improve endothelial function, suppress the production of reactive oxygen species, etc. Statins are competitive inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A reductase, the rate-limiting enzyme of cholesterol synthesis, whereas fibrates are activators of the nuclear receptor peroxisome proliferator-activated receptor alpha (PPARalpha). The similarity between the pleiotropic effects of statins and fibrates is remarkable and suggests a mechanistic link between these two classes of drugs. Here we discuss recent data on the cross-talk between statins and PPARalpha agonists and the mechanisms behind these actions.

The RXR Agonist Bexarotene Improves Cholesterol Homeostasis and Inhibits Atherosclerosis Progression in a Mouse Model of Mixed Dyslipidemia

Objective—The activity of the antitumoral agent bexarotene (Targretin, Bexarotene) depends on its binding to the nuclear retinoid-X receptor (RXR) and subsequent transcriptional regulation of target genes. Through RXR activation, bexarotene may modulate numerous metabolic pathways involved in atherosclerosis. Here, we investigated the effect of bexarotene on atherosclerosis progression in a dyslipidemic murine model, the human apolipoprotein E2 knockin mouse, that develops essentially macrophage-laden lesions. Methods and Results—Atherosclerotic lesions together with different metabolic pathways involved in atherosclerosis were investigated in mice treated or not with bexarotene. Bexarotene protects from atherosclerosis development in mice, at least in part by improving the circulating cholesterol distribution profile likely via a marked decrease of dietary cholesterol absorption caused by modulation of intestinal expression of genes recently identified as major players in this process, Niemann-Pick-C1-Like1 (NPC1L1) and CD13. This atheroprotection appears despite a strong hypertriglyceridemia. Moreover, bexarotene treatment only modestly modulates inflammatory gene expression in the vascular wall, but markedly enhanced the capacity of macrophages to efflux cellular lipids. Conclusion—These data provide evidence of a favorable pharmacological effect of bexarotene on atherosclerosis despite the induction of hypertriglyceridemia, likely via a beneficial action on intestinal absorption and macrophage efflux.