Prostate cancer is the second leading cause of cancer death in American men. It has been recognized that prostate cancer progression results from inhibition of apoptotic mechanisms rather than from uncontrolled cell proliferation. In addition to the loss of wild-type tumor suppressor and pro-apoptotic functions of p53 or PTEN, elevated expression of anti-apoptotic proteins including Bcl-2, Bcl-xL, and activated Akt or NF-kappaB have been noted in human prostate cancer specimens or cell lines. Toward the goal of identifying molecules that can potentially antagonize or override the anti-apoptotic mechanisms and prevent progression of the disease, we performed a differential screen for genes whose expression was upregulated when androgen-independent prostate cancer cell cultures were induced to undergo apoptosis. One such gene induced exclusively during apoptosis of cancer cell lines and of the rat ventral prostate after castration was prostate apoptosis response-4 (par-4). Ectopic expression of par-4 induces apoptosis in prostate cancer cells that show aberrantly elevated NF-kappaB activity. Apoptosis by par-4 occurs in the absence of wild type p53 or PTEN, and in the presence of Bcl-2 or Bcl-xL protein. Apoptosis by par-4 is not restricted to cell culture paradigms; solid tumors resulting from implants of prostate cancer cells in nude mice undergo apoptosis and remarkable reduction in tumor volume upon adenoviral expression of par-4. These findings indicate that Par-4 expression is sufficient on its own, in the absence of another genotoxic insult, to induce apoptosis in prostate cancer cells. The proposed study will: (1) determine whether inhibition of NF-kappaB transcription activity is functionally essential for apoptosis by Par-4; determine the mechanism of NF-kappaB inhibition by Par-4; and functionally characterize the downstream targets of NF-kappaB that are regulated by Par-4; (2) identify the domains of Par-4 that induce NF-kappaB inhibition and apoptosis; and (3) identify the pro-death pathway activated by Par-4. The studies will provide an insight into the molecular determinants of apoptosis in prostate cancer cells. Because Par-4 on its own does not induce apoptosis in normal or non-transformed cells, these studies may provide the groundwork to develop Par-4 or its active domains as candidate molecules for therapy of prostate cancer.