During liver injury, hepatic stellate cells (HSC) acquire a myofibroblast-like phenotype associated with reduction of lipid droplets, increased collagen synthesis, and proliferation. Peroxisome proliferator-activated receptor γ (PPARγ) regulates adipocyte differentiation and controls gene transcription in response to various activators including prostanoids and antidiabetic thiazolidinediones. We explored whether the presence of PPARγ and its transcriptional activity were involved in control of HSC proliferation in vitro. PPARγ ligands, 15-deoxy-▵1214 prostaglandin J2 (15d-PGJ2 ) and ciglitizone, significantly decrease platelet-derived growth factor (PDGF)–induced proliferation in activated human HSC and inhibit α smooth muscle actin (α-SMA) expression during HSC transdifferentiation. Treatment with 9-cis retinoic acid (9-cisRA) and LG268, ligands of the heterodimerization partner retinoic X receptor (RXR), had a negligible effect in PDGF–treated cells but caused a further reduction of proliferation when used in combination with ciglitizone. Transfection experiments with a reporter gene consisting of 3 copies of a PPAR response element (peroxisome proliferator response element [PPRE]3 -tk-luciferase) showed a progressive reduction of PPAR transcriptional activity during plastic-induced HSC transdifferentiation. Cotransfection with human PPARγ expression vector restored the PPRE3 -tk-luciferase reporter expression and the increased level of the receptor in activated HSC-inhibited cell proliferation in a dose-dependent manner. Incubation of human PPARγ-cotransfected HSC with PDGF strongly inhibited luciferase activity and this effect was blocked by the inhibition of the mitogen-activated protein (MAP) kinase signal cascade. Our results indicate that depression of PPARγ expression and activity is involved in HSC proliferation and that the PPARγ ligand-mediated activation exerts a previously unrecognized inhibition of PDGF-induced mitogenesis in activated human HSC.