RETINOBLASTOMA PROTEIN-DEPENDENT GROWTH SIGNAL CONFLICT AND CASPASE ACTIVITY ARE REQUIRED FOR PROTEIN-KINASE C-SIGNALED APOPTOSIS OF PROSTATE EPITHELIAL-CELLS

dBoth protein kinase C and the retinoblastoma tumor suppressor protein have been linked to the regulation of cell growth and bell death, sug gesting the differential roles these factors play in mediating cell fa te, In some cells, protein kinase C-induced activation of the retinobl astoma protein results in G(1) arrest, However, inducible overexpressi on and activation of the protein kinase C alpha isozyme or the additio n of 12-O-tetradecanoylphorbol-13-acetate in the prostate epithelial c ell line, LNCaP, resulted in apoptosis preceded by induction of p21 an d dephosphorylation of the retinoblastoma protein, Consistent with a r ole for the retinoblastoma growth suppressor protein in protein kinase C-induced apoptosis, DU145 cells, which do not express functional ret inoblastoma protein or LNCaP cells, which have been transfected with t he retinoblastoma inhibitor, E1a, were resistant to apoptosis, LNCaP a poptosis was initiated by a unique conflict between the growth-suppres sive activity of the retinoblastoma protein and growth-promoting mitog enic signals, Thus, when this conflict was prevented by serum depletio n, apoptosis was suppressed, The caspase family of cysteine proteases is believed to encompass the execution machinery of mammalian apoptosi s, and addition of the cell-permeable caspase inhibitor, Z-Val-Ala-Asp -fluoromethylketone, afforded nearly total protection from protein kin ase C-signaled apoptosis, This protection correlated with the total lo ss of caspase activity as measured by the proteolytic cleavage of nucl ear poly(ADP-ribose) polymerase, On the basis of these results, we pro pose that protein kinase C regulates a novel cell death pathway that i s initiated by a cellular conflict between retinoblastoma growth-suppr essive signals and serum mitogenic signals in proliferating prostate e pithelial cells and that is executed by the caspase family of cysteine proteases.