Peroxisome proliferator-activated receptor gamma (PPARgamma) is a member of the nuclear receptor superfamily that is activated by binding certain fatty acids, eicosanoids, and insulin-sensitizing thiazolidinediones (TZD). The TZD troglitazone (TRO) inhibits vascular smooth muscle cell proliferation and migration both in vitro and in vivo. The precise mechanism of its antiproliferative activity, however, has not been elucidated. We report here that PPARgamma ligands inhibit rat aortic vascular smooth muscle cell proliferation by blocking the events critical for G(1) --> S progression. Flow cytometry demonstrated that both TRO and another TZD, rosiglitazone, prevented G(1) --> S progression induced by platelet-derived growth factor and insulin. Movement of cells from G(1) --> S was also inhibited by the non-TZD, natural PPARgamma ligand 15-deoxy-(12,14)Delta prostaglandin J(2) (15d-PGJ(2)), and the mitogen-activated protein kinase pathway inhibitor PD98059. Inhibition of G(1) --> S exit by these compounds was accompanied by a substantial blockade of retinoblastoma protein phosphorylation. TRO and rosiglitazone attenuated both the mitogen-induced degradation of p27(kip1) and the mitogenic induction of p21(cip1). 15d-PGJ(2) and PD98059 inhibited both the degradation of p27(kip1) and the induction of cyclin D1 in response to mitogens. These effects resulted in the inhibition of mitogenic stimulation of cyclin-dependent kinases activated by cyclins D1 and E. These data demonstrate that PPARgamma ligands are antiproliferative drugs that act by modulating cyclin-dependent kinase inhibitors; they may provide a new therapeutic approach for proliferative vascular diseases.