Death Receptor Signaling in Pancreatic Cancer: Mechanisms and Therapeutic Targets

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Pancreatic cancer is the fourth leading cause of cancer death in the United States, and remains an important health problem for our veterans. Since 1975, the 5-year survival rate for pancreatic cancer patients has only improved from 2% to 6%. We and others have demonstrated that modulating death receptor-activated extrinsic apoptosis pathways provides novel avenues for cancer therapy. Recombinant ligands or agonistic antibodies for death receptor 4 or 5 (DR4/5), including DR5 agonistic antibody TRA-8, have been shown to induce apoptosis of several types of cancers in preclinical studies and are currently being tested in several phase I-III clinical trials for solid tumors, including pancreatic tumors. However, resistance of pancreatic cancer to TRA-8, and other DR ligands or agonistic antibodies, are major hurdles for effective therapy. Over the past 8 years, we have been seeking effective reagents that overcome the resistance of cancer cells to DR-induced apoptosis. We have demonstrated that modulating the components in the Death-Inducing Signaling Complex (DISC) determines the downstream survival and apoptosis signals conveyed by the DRs, and thus the sensitivity of cancer cells to DR-induced apoptosis. Recently, we identified a new component of the DR5-associated DISC, poly(ADP-ribose) polymerase 1 (PARP1). PARP1 expression levels correlated with the resistance of pancreatic cancer cells to TRA-8-induced apoptosis. Inhibition of PARP1 sensitized resistant pancreatic cancer cells to TRA-8-induced apoptosis in vitro; and enhanced the efficacy of TRA-8 therapy of resistant pancreatic tumors in vivo. In contrast, overexpression of PARP1 in TRA-8-sensitive cells rendered them more resistant to TRA-8. PARP1 is known to function mainly in the nucleus for DNA repair. In human pancreatic cancer tissues and cells, we identified cytoplasmic localization of PARP1. We demonstrated that PARP1 regulated DR5-induced apoptotic signals through novel mechanisms, by promoting poly(ADP-ribose) (pADPr) modification of caspase-8 that inhibited caspase-8 activation. On the other hand, PARP1 inhibition decreased DISC-recruitment of calmodulin (CaM) and Src, which we previously reported to mediate death receptor-induced survival pathways in pancreatic cancer cells. Therefore, we hypothesize that inhibition of PARP1 promotes death receptor-induced apoptotic signals and inhibits survival signals, thus enhancing the sensitivity of pancreatic cancers to TRA-8 therapy. The proposed studies will characterize the function of PARP1 in 1) regulating death receptor-mediated apoptotic signals; 2) regulating death receptor- mediated survival signals; and 3) TRA-8 therapy in mouse models of pancreatic cancer. The present studies will define the novel function and molecular mechanisms underlying PARP1 regulation of death receptor- mediated apoptotic and survival signaling pathways, which will not only elucidate the role of PARP1 in the fundamental extrinsic apoptotic machinery, but also provide novel molecular insights facilitating further studies to translate these findings into therapeutic strategies for patient care for our veterans.