The postreceptor signaling pathway(s) that mediates the effects of transforming growth factor-beta1 (TGF-beta1) is incompletely understood. The present study investigated the involvement of protein kinase C (PKC) in the growth-inhibitory action of TGF-beta1 in PC3, a human prostate cancer cell line. PKC alpha, the only conventional PKC isoform detected in PC3 cells, appeared to be constitutively active based on its presence in both Triton-soluble membrane fraction and cytosol. However, levels of membrane-associated PKC alpha were decreased by a growth-inhibitory dose of TGF-beta1. The response to TGF-beta1 was rapid (within 5 min), time dependent, isoform specific, and occurred without apparent changes in levels of total PKC alpha protein. TGF-beta1 also decreased the levels of membrane-associated PKC activity coincident with its inhibitory effect on PKC alpha's membrane association. Inhibition of PKC activity appeared to be associated with growth inhibition in PC3 cells, because chelerythrine (a specific PKC inhibitor) likewise decreased cell proliferation. Taken together, our data suggest that inhibition of PKC activity, at least in part due to inactivation of PKC alpha, is an early event associated with TGF-beta1 postreceptor signaling that might mediate suppression of cell proliferation.