Cytoplasmic PARP‐1 promotes pancreatic cancer tumorigenesis and resistance

The poly(ADP‐ribose) polymerases (PARP) play important roles in repairing damaged DNA during intrinsic cell death. We recently linked PARP‐1 to death receptor (DR)‐activated extrinsic apoptosis, the present studies sought to elucidate the function of cytoplasmic PARP‐1 in pancreatic cancer tumorigenesis and therapy. Using human normal and pancreatic cancer tissues, we analyzed the prevalence of cytoplasmic PARP‐1 expression. In normal human pancreatic tissues, PARP‐1 expression was present in the nucleus; however, cytoplasmic PARP‐1 expression was identified in pancreatic cancers. Therefore, cytoplasmic PARP‐1 mutants were generated by site‐direct mutagenesis, to determine a causative effect of cytoplasmic PARP‐1 on pancreatic cancer tumorigenesis and sensitivity to therapy with TRA‐8, a humanized DR5 antibody. PARP‐1 cytoplasmic mutants rendered TRA‐8 sensitive pancreatic cancer cells, BxPc‐3 and MiaPaCa‐2, more resistant to TRA‐8‐induced apoptosis; whereas wild‐type PARP‐1, localizing mainly in the nucleus, had no effects. Additionally, cytoplasmic PARP‐1, but not wild‐type PARP‐1, increased resistance of BxPc‐3 cells to TRA‐8 therapy in a mouse xenograft model in vivo . Inhibition of PARP enzymatic activity attenuated cytoplasmic PARP‐1‐mediated TRA‐8 resistance. Furthermore, increased cytoplasmic PARP‐1, but not wild‐type PARP‐1, was recruited into the TRA‐8‐activated death‐inducing signaling complex and associated with increased and sustained activation of Src‐mediated survival signals. In contrast, PARP‐1 knockdown inhibited Src activation. Taken together, we have identified a novel function and mechanism underlying cytoplasmic PARP‐1, distinct from nuclear PARP‐1, in regulating DR5‐activated apoptosis. Our studies support an innovative application of available PARP inhibitors or new cytoplasmic PARP‐1 antagonists to enhance TRAIL therapy for TRAIL‐resistant pancreatic cancers.