Inhibition of poly(ADP-ribose) polymerase activity is insufficient to induce tetraploidy

Poly(ADP-ribose) polymerase (PARP) knockout mice are resistant to murine mo dels of human diseases such as cerebral and myocardial ischemia, traumatic brain injury, diabetes, Parkinsonism, endotoxic shock and arthritis, implic ating PARP in the pathogenesis of these diseases. Potent selective PARP inh ibitors are therefore being evaluated as novel therapeutic agents in the tr eatment of these diseases. Inhibition or depletion of PARP, however, increa ses genomic instability in cells exposed to genotoxic agents. We recently d emonstrated the presence of a genomically unstable tetraploid population in PARP(-/-)fibroblasts and its loss after stable transfection with PARP cDNA . To elucidate whether the genomic instability is attributable to PARP defi ciency or lack of PARP activity, we investigated the effects of PARP inhibi tion on development of tetraploidy. Immortalized wild-type and PARP(-/-) fi broblasts were exposed for 3 weeks to 20 muM GPI 6150 (1,11b-dihydro-[2H]be nzopyrano[4,3,2-de]isoquinolin-3-one), a novel small molecule specific comp etitive inhibitor of PARP (Ki = 60 nM) and one of the most potent PARP inhi bitors to date (1C(50) = 0.15 muM). Although GPI 6150 initially decreased c ell growth in wild-type cells, there was no effect on cell growth or viabil ity after 24 h, GPI 6150 inhibited endogenous PARP activity in wild-type ce lls by similar to 91%, to about the residual levels in PARP(-/-)cells. Flow cytometric analysis of unsynchronized wild-type cells exposed for 3 weeks to GPI 6150 did not induce the development of tetraploidy, suggesting that, aside from its catalytic function, PARP may play other essential roles in the maintenance of genomic stability.