Although peroxisome proliferator-activated receptor (PPAR) is widely expressed in many tissues, the role of PPAR is poorly understood. In this study, we report that PPAR was up-regulated in vascular smooth muscle cells (VSMC) during vascular lesion formation. By using Northern blot analysis, we demonstrated that PPAR was increased by 3-4-fold in VSMC treated with platelet-derived growth factor (PDGF) (20ng/ml). In addition, PDGF-induced PPAR mRNA expression neither needs de novo protein synthesis nor affects the stability of PPAR mRNA in VSMC. Preincubation of VSMC with phosphatidylinositol 3-kinase inhibitor (LY294002, 50mol/liter) or infection of VSMC with an adenovirus carrying the gene for a dominant negative form of Akt abrogated PDGF-induced PPAR mRNA expression, suggesting that phosphatidylinositol 3-kinase/Akt signaling pathway is involved in the regulation of PDGFinduced PPAR mRNA expression in VSMC. To explore the role of PPAR in VSMC, we generated rat vascular smooth muscle cells (A7r5) stably overexpressing PPAR and the control green fluorescent protein. Overexpression of PPAR in VSMC increased post-confluent cell proliferation by increasing the cyclin A and CDK2 as well as decreasing p57kip2. Taken together, the results suggest that PPAR plays an important role in the pathology of diseases associated with VSMC proliferation, such as primary atherosclerosis and restenosis.