Peroxisome proliferator-activated receptors (PPARs): Nuclear receptors at the crossroads between lipid metabolism and inflammation
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Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors belonging to the nuclear receptor family. PPARs function as regulators of lipid and lipoprotein metabolism and glucose homeostasis and influence cellular proliferation, differentiation and apoptosis. PPARα is highly expressed in tissues such as liver, muscle, kidney and heart, where it stimulates the β-oxidative degradation of fatty acids. PPARγ is predominantly expressed in intestine and adipose tissue. PPARγ triggers adipocyte differentiation and promotes lipid storage. The hypolipidemic fibrates and the antidiabetic glitazones are synthetic ligands for PPARα and PPARγ, respectively. Furthermore, fatty acids and eicosanoids are natural PPAR ligands: PPARα is activated by leukotriene B4, whereas prostaglandin J2 is a PPARγ ligand. These observations suggested a potential role for PPARs not only in metabolic but also in inflammation control. The first evidence for a role of PPARα in inflammation control came from the demonstration that PPARα deficient mice display a prolonged response to inflammatory stimuli. It was suggested that PPARα deficiency results in a reduced β-oxidative degradation of these inflammatory fatty acid derivatives. More recently, PPAR activators were shown to inhibit the activation of inflammatory response genes (such as IL-2, IL-6, IL-8, TNFα and metalloproteases) by negatively interfering with the NF-κB, STAT and AP-1 signalling pathways. PPAR activators exert these anti-inflammatory activities in different immunological and vascular wall cell types such as monocyte/macrophages, endothelial, epithelial and smooth muscle cells in which PPARs are expressed. These recent findings indicate a modulatory role for PPARs in the control of the inflammatory response with potential therapeutic applications in inflammation-related diseases, such as atherosclerosis and inflammatory bowel disease.
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