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Poly(ADP-ribose) polymerase (PARP) inhibitors enhance DNA topoisomerase I (topo I) poison-induced cytotoxicity and antitumor activity in vitro and in vivo, but the mechanism has not been defined. We investigated the role of PARP-1 in the response to topo I poisons using PARP-1-/- and PARP-1+/+ mouse embryonic fibroblasts and the potent PARP-1 inhibitor, AG14361 (Ki 3-fold sensitization of PARP-1+/+ cells to topotecan compared with a <1.4-fold sensitization in PARP-1-/- cells. In human leukemia K562 cells, AG14361 caused a 2-fold sensitization to camptothecin-induced cytotoxicity. AG14361 did not affect the cellular activity of topo I as determined by measurement of cleavable complexes and topo I relaxation activity, showing that sensitization was not due to topo I activation. In contrast, repair of DNA following camptothecin removal, normally very rapid, was significantly retarded by AG14361, resulting in a 62% inhibition of repair 10 minutes after camptothecin removal. This led to a 20% increase in the net accumulation of camptothecin-induced DNA strand break levels in cells coexposed to AG14361 for 16 hours. We investigated the DNA repair mechanism involved using a panel of DNA repair-deficient Chinese hamster ovary cells. AG14361 significantly potentiated camptothecin-mediated cytotoxicity in all cells, except the base excision repair-deficient EM9 cells. Therefore, the most likely mechanism for the potentiation of topo I poison-mediated cytotoxicity by AG14361 is via PARP-1-dependent base excision repair.
Smith et al. (Thu,) studied this question.