Critical role of prostate apoptosis response-4 in determining the sensitivity of pancreatic cancer cells to small-molecule inhibitor-induced apoptosis

Role of prostate apoptosis response-4 (PAR-4) has been well described in prostate cancer. However, its significance in other cancers has not been fully elucidated. For the current study, we selected four pancreatic cancer cell lines (BxPC-3, Colo-357, L3.6pl, and HPAC) that showed differential endogenous expression of PAR-4. We found that nonpeptidic small-molecule inhibitors (SMI) of Bcl-2 family proteins (apogossypolone and TW-37; 250 nmol/L and 1 μmol/L, respectively) could induce PAR-4-dependent inhibition of cell growth and induction of apoptosis. Sensitivity to apoptosis was directly related to the expression levels of PAR-4 (R = 0.92 and R² = 0.95). Conversely, small interfering RNA against PAR-4 blocked apoptosis, confirming that PAR-4 is a key player in the apoptotic process. PAR-4 nuclear localization is considered a prerequisite for cells to undergo apoptosis, and we found that the treatment of Colo-357 and L3.6pl cells with 250 nmol/L SMI leads to nuclear localization of PAR-4 as confirmed by 4′,6-diamidino-2-phenylindole staining. In combination studies with gemcitabine, pretreatment with SMI leads to sensitization of Colo-357 cells to the growth-inhibitory and apoptotic action of a therapeutic drug, gemcitabine. In an in vivo setting, the maximum tolerated dose of TW-37 in xenograft of severe combined immunodeficient mice (40 mg/kg for three i.v. injections) led to significant tumor inhibition. Our results suggest that the observed antitumor activity of SMIs is mediated through a novel pathway involving induction of PAR-4. To our knowledge, this is the first study reporting SMI-mediated apoptosis involving PAR-4 in pancreatic cancer.