ROCK Mediates Phorbol Ester-induced Apoptosis in Prostate Cancer Cells via p21Cip1 Up-regulation and JNK

It is established that androgen-dependent prostate cancer cells undergo apoptosis upon treatment with phorbol esters and related analogs, an effect primarily mediated by PKCδ. Treatment of LNCaP prostate cancer cells with phorbol 12-myristate 13-acetate (PMA) causes a strong and sustained activation of RhoA and its downstream effector ROCK (Rho kinase) as well as the formation of stress fibers. These effects are impaired in cells subjected to PKCδ RNA interference depletion. Functional studies revealed that expression of a dominant negative RhoA mutant or treatment with the ROCK inhibitor Y-27632 inhibits the apoptotic effect of PMA in LNCaP cells. Remarkably, the cytoskeleton inhibitors cytochalasin B and blebbistatin blocked not only PMA-induced apoptosis but also the activation of JNK, a mediator of the cell death effect by the phorbol ester. In addition, we found that up-regulation of the cell cycle inhibitor p21Cip1 is required for PMA-induced apoptosis and that inhibitors of ROCK or the cytoskeleton organization prevent p21Cip1 induction. Real time PCR analysis and reporter gene assay revealed that PMA induces p21Cip1 transcriptionally in a ROCK- and cytoskeleton-dependent manner. p21Cip1 promoter analysis revealed that PMA induction is dependent on Sp1 elements in the p21Cip1 promoter but independent of p53. Taken together, our studies implicate ROCK-mediated up-regulation of p21Cip1 and the cytoskeleton in PKCδ-dependent apoptosis in prostate cancer cells.