The cyclin-dependent kinase inhibitor p21(Cip1) is up-regulated in response to mitogenic stimulation in various cells. PPARgamma ligands troglitazone (TRO, 10 microm) and rosiglitazone (RSG, 10 microm) attenuated the induction of p21(Cip1) protein by platelet-derived growth factor (PDGF) and insulin without affecting cognate mRNA levels in rat aortic smooth muscle cells (RASMC). The protein kinase Cdelta (PKCdelta) inhibitor rottlerin also blocked the induction of p21(Cip1) protein, whereas the conventional PKC isotype inhibitor Gö 6976 had no effect. Kinetic studies using the protein synthesis inhibitor cycloheximide showed that TRO, RSG, and rottlerin shortened the half-life of p21(Cip1) protein. TRO, RSG, and rottlerin inhibited PDGF-induced expression of p21(Cip1), but they did not affect insulin-induced expression of p21(Cip1). Both ligands inhibited PKCdelta enzymatic activity in PDGF-stimulated RASMC but not in insulin-stimulated cells. Adenovirus-mediated overexpression of PKCdelta rescued the down-regulation of p21(Cip1) expression both by TRO and RSG in PDGF-treated RASMC. These data suggested that the PKCdelta pathway plays a critical role in PDGF-induced expression of p21(Cip1) in RASMC and may be the potential target for PPARgamma ligand effects. Src kinase-dependent tyrosine phosphorylation of PKCdelta was decreased substantially by TRO and RSG. Tyrosine phosphorylation and activation of c-Src in response to PDGF were unaffected by either PPARgamma ligand. Protein-tyrosine-phosphatase inhibitors sodium orthovanadate and dephostatin prevented PPARgamma ligand effects on PKCdelta tyrosine phosphorylation and enzymatic activity. Both inhibitors also reversed PPARgamma ligand effects on p21(Cip1) expression in PDGF-treated RASMC. PPARgamma ligands enhanced protein-tyrosine-phosphatase activity in RASMC, which may be the mechanism for decreased PKCdelta tyrosine phosphorylation and activity. PPARgamma ligands regulate p21(Cip1) at a post-translational level by blocking PKCdelta signaling and accelerating p21(Cip1) turnover.