Interleukin-6 (IL-6) is a pleiotropic cytokine, whose plasma levels are elevated in inflammatory diseases such as atherosclerosis. We have previously reported that peroxisome proliferator-activated receptor ␣

Peroxisome proliferator-activated receptors (PPARs) are key players in lipid and glucose metabolism and are implicated in metabolic disorders predisposing to atherosclerosis, such as dyslipidaemia and diabetes. Whereas PPARgamma promotes lipid storage by regulating adipocyte differentiation, PPARalpha stimulates the beta-oxidative degradation of fatty acids. PPARalpha-deficient mice show a prolonged response to inflammatory stimuli, suggesting that PPARalpha is also a modulator of inflammation. Hypolipidaemic fibrate drugs are PPARalpha ligands that inhibit the progressive formation of atherosclerotic lesions, which involves chronic inflammatory processes, even in the absence of their atherogenic lipoprotein-lowering effect. Here we show that PPARalpha is expressed in human aortic smooth-muscle cells, which participate in plaque formation and post-angioplasty re-stenosis. In these smooth-muscle cells, we find that PPARalpha ligands, and not PPARgamma ligands, inhibit interleukin-1-induced production of interleukin-6 and prostaglandin and expression of cyclooxygenase-2. This inhibition of cyclooxygenase-2 induction occurs transcriptionally as a result of PPARalpha repression of NF-kappaB signalling. In hyperlipidaemic patients, fenofibrate treatment decreases the plasma concentrations of interleukin-6, fibrinogen and C-reactive protein. We conclude that activators of PPARalpha inhibit the inflammatory response of aortic smooth-muscle cells and decrease the concentration of plasma acute-phase proteins, indicating that PPARalpha in the vascular wall may influence the process of atherosclerosis and re-stenosis.

Peroxisome proliferator-activated receptors (PPARs) have been implicated in metabolic diseases, such as obesity, diabetes, and atherosclerosis, due to their activity in liver and adipose tissue on genes involved in lipid and glucose homeostasis. Here, we show that the PPAR␣ and PPAR␥ forms are expressed in differentiated human monocyte-derived macrophages, which participate in inflammation control and atherosclerotic plaque formation. Whereas PPAR␣ is already present in undifferentiated monocytes, PPAR␥ expression is induced upon differentiation into macrophages. Immunocytochemistry analysis demonstrates that PPAR␣ resides constitutively in the cytoplasm, whereas PPAR␥ is predominantly nuclear localized. Transient transfection experiments indicate that PPAR␣ and PPAR␥ are transcriptionally active after ligand stimulation. Ligand activation of PPAR␥, but not of PPAR␣, results in apoptosis induction of unactivated differentiated macrophages as measured by the TUNEL assay and the appearance of the active proteolytic subunits of the cell death protease caspase-3. However, both PPAR␣ and PPAR␥ ligands induce apoptosis of macrophages activated with tumor necrosis factor ␣/interferon ␥. Finally, PPAR␥ inhibits the transcriptional activity of the NFB p65/RelA subunit, suggesting that PPAR activators induce macrophage apoptosis by negatively interfering with the anti-apoptotic NFB signaling pathway. These data demonstrate a novel function of PPAR in human macrophages with likely consequences in inflammation and atherosclerosis.

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors belonging to the nuclear receptor superfamily. PPAR is highly expressed in liver, skeletal muscle, kidney, heart and the vascular wall. PPAR is predominantly detected in adipose tissue, intestine and macrophages. PPARs are activated by fattyacid derivatives and pharmacological agents such as fibrates and glitazones which are specific for PPAR and PPAR respectively. PPARs regulate lipid and lipoprotein metabolism, glucose homeostasis, cell proliferation and differentiation, and apoptosis. PPAR controls intra-and extracellular lipid metabolisms whereas PPAR triggers adipocyte differentiation and promotes lipid storage. In addition, PPARs also modulate the inflammatory response. PPAR activators have been shown to exert antiinflammatory activities in various cell types by inhibiting the expression of proinflammatory genes such as cytokines, metalloproteases and acute-phase proteins. PPARs negatively regulate the transcription of inflammatory response genes by antagonizing the AP-1, nuclear factor-B (NF-B), signal transducer and activator of transcription and nuclear factor of activated T-cells signalling pathways and by stimulating the catabolism of proinflammatory eicosanoids. These recent findings indicate a modulatory role for PPARs in inflammation with potential therapeutical applications in chronic inflammatory diseases.

Endothelin-1 (ET-1), a 21-amino acid vasoactive peptide mainly produced by vascular endothelial cells, is involved in the regulation of vascular tone and smooth muscle cell proliferation. Peroxisome proliferator-activated receptors (PPARs), key players in lipid and glucose metabolism, have been implicated in metabolic disorders that are predisposing to atherosclerosis. Because of the potential role of ET-1 in vascular disorders such as hypertension and atherosclerosis, we investigated the regulation of ET-1 expression by PPAR activators. Western blot and reverse transcription-polymerase chain reaction analyses demonstrated that both PPARalpha and PPARgamma are expressed in human coronary artery endothelial cells as well as in endothelial cell lines such as HMEC-1 and ECV304. In bovine aortic endothelial cells and HMEC-1 cells, both PPARalpha and PPARgamma ligands inhibited thrombin-induced ET-1 secretion, whereas basal ET-1 secretion was only slightly suppressed. Reverse transcription-polymerase chain reaction experiments showed that this inhibition of ET-1 production occurs at the gene expression level. Using transient transfection assays, we demonstrated that PPARs downregulate thrombin-activated transcription of the human ET-1 promoter. Transactivation studies with c-Jun and c-Fos expression plasmids indicated that PPARs negatively interfere with the activator protein-1 signaling pathway, which mediates thrombin activation of ET-1 gene transcription. Furthermore, electrophoretic mobility shift assays demonstrated that PPAR activators reduce the thrombin-stimulated binding activity of bovine aortic endothelial cell nuclear extracts as well as c-Jun binding to an activator protein-1 consensus site. Taken together, these data indicate that (1) both PPARalpha and PPARgamma are expressed in human vascular endothelial cells and (2) PPAR activators inhibit thrombin-induced ET-1 biosynthesis, indicating a novel role for PPARs in vascular endothelial function.

Chronic inflammation is a hallmark of degenerative diseases such as atherosclerosis. Peroxisome proliferator-activated receptors (PPARs) are transcription factors belonging to the nuclear receptor superfamily, which are expressed in the cells of the atherosclerosic lesion. PPAR␣ ligands have been reported to exert antiinflammatory activities in different cell types by antagonizing the transcriptional activity of NF-B. In the present study, the influence of PPAR␣ activators on the NF-B signaling pathway was investigated. Our results show that fibrates, synthetic PPAR␣ activators, induced the expression of the inhibitory protein IB␣ in human aortic smooth muscle cells as well as in primary human hepatocytes, whereas neither IB-kinase activity nor the degradation rate of IB␣ were affected. Using PPAR␣-null mice, we demonstrated that fibrates induced IB␣ in liver in vivo and that this action required PPAR␣. Furthermore, fibrate treatment induced IB␣ protein expression in the cytoplasm and also enhanced IL-1␤-induced accumulation of IB␣ protein in the nucleus. These actions of fibrates on IB␣ expression were accompanied by a decrease in NF-B DNA binding activity as demonstrated by electrophoretic mobility shift assays. Taken together, these data provide an additional molecular mechanism for the anti-inflammatory activity of PPAR␣ agonists and reinforce their potential use in the treatment of inflammatory diseases.

Retinoid-related orphan receptor α (RORα) (NR1F1) is a member of the nuclear receptor superfamily whose biological functions are largely unknown. Since staggerer mice, which carry a deletion in the RORα gene, suffer from immune abnormalities, we generated an adenovirus encoding RORα1 to investigate its potential role in control of the inflammatory response. We demonstrated that RORα is expressed in human primary smooth-muscle cells and that ectopic expression of RORα1 inhibits TNFα-induced IL-6, IL-8 and COX-2 expression in these cells. RORα1 negatively interferes with the NF-κB signalling pathway by reducing p65 translocation as demonstrated by western blotting, immunostaining and electrophoretic mobility shift assays. This action of RORα1 on NF-κB is associated with the induction of IκBα, the major inhibitory protein of the NF-κB signalling pathway, whose expression was found to be transcriptionally upregulated by RORα1 via a ROR response element in the IκBα promoter. Taken together, these data identify RORα1 as a potential target in the treatment of chronic inflammatory diseases, including atherosclerosis and rheumatoid arthritis.

Peroxisome proliferator‐activated receptor γ  (PPARγ ), a member of the nuclear receptor superfamily, has recently been described as a modulator of macrophage functions and as an inhibitor of T cell proliferation. Here, we investigated the role of PPARγ  in dendritic cells (DC), the most potent antigen‐presenting cells. We showed that PPARγ  is highly expressed in immature human monocyte‐derived DC (MDDC) and that it may affect the immunostimulatory function of MDDC stimulated with lipopolysaccharide (LPS) or via CD40 ligand (CD40L). We found that the synthetic PPARγ  agonist rosiglitazone (as well as pioglitazone and troglitazone) significantly increases on LPS‐ and CD40L‐activated MDDC, the surface expression of CD36 (by 184% and 104%, respectively) and CD86 (by 54% and 48%), whereas it reduces the synthesis of CD80 (by 42% and 42%). Moreover, activation of PPARγ  resulted in a dramatic decreased secretion of the Th1‐promoting factor IL‐12 in LPS‐ and CD40L‐stimulated cells (by 47% and 62%), while the production of IL‐1β , TNF‐α , IL‐6 and IL‐10 was unaffected. Finally, PPARγ  ligands down‐modulate the synthesis of IFN‐γ ‐inducible protein‐10 (recently termed as CXCL10) and RANTES (CCL5), both chemokines involved in the recruitment of Th1 lymphocytes (by 49% and 30%), but not the levels of the Th2 cell‐attracting chemokines,macrophage‐derived chemokine (CCL22) and thymus and activation regulated chemokine (CCL17), in mature MDDC. Taken together, our data suggest that activation of PPARγ  in human DC may have an impact in the orientation of primary and secondary immune responses by favoring type 2 responses.