Peroxisome proliferator-activated receptor-alpha activators regulate genes governing lipoprotein metabolism, vascular inflammation and atherosclerosis

Abstract

The peroxisome proliferator-activated receptors (PPARs) [alpha, delta (beta) and gamma] form a subfamily of the nuclear receptor gene family. All PPARs are, albeit to different extents, activated by fatty acids and derivatives; PPAR-alpha binds the hypolipidemic fibrates whereas antidiabetic glitazones are ligands for PPAR-gamma. PPAR-alpha activation mediates pleiotropic effects such as stimulation of lipid oxidation, alteration in lipoprotein metabolism and inhibition of vascular inflammation. PPAR-alpha activators increase hepatic uptake and the esterification of free fatty acids by stimulating the fatty acid transport protein and acyl-CoA synthetase expression. In skeletal muscle and heart, PPAR-alpha increases mitochondrial free fatty acid uptake and the resulting free fatty acid oxidation through stimulating the muscle-type carnitine palmitoyltransferase-I. The effect of fibrates on the metabolism of triglyceride-rich lipoproteins is due to a PPAR-alpha dependent stimulation of lipoprotein lipase and an inhibition of apolipoprotein C-III expressions, whereas the increase in plasma HDL cholesterol depends on an overexpression of apolipoprotein A-I and apolipoprotein A-II. PPARs are also expressed in atherosclerotic lesions. PPAR-alpha is present in endothelial and smooth muscle cells, monocytes and monocyte-derived macrophages. It inhibits inducible nitric oxide synthase in macrophages and prevents the IL-1-induced expression of IL-6 and cyclooxygenase-2, as well as thrombin-induced endothelin-1 expression, as a result of a negative transcriptional regulation of the nuclear factor-kappa B and activator protein-1 signalling pathways. PPAR activation also induces apoptosis in human monocyte-derived macrophages most likely through inhibition of nuclear factor-kappa B activity. Therefore, the pleiotropic effects of PPAR-alpha activators on the plasma lipid profile and vascular wall inflammation certainly participate in the inhibition of atherosclerosis development observed in angiographically documented intervention trials with fibrates.