Poly(ADP-ribose) polymerase (PARP) knockout mice are resistant to murine models of human diseases such as cerebral and
myocardial ischemia,
traumatic brain injury, diabetes,
Parkinsonism, endotoxic
shock and
arthritis, implicating PARP in the pathogenesis of these diseases. Potent selective
PARP inhibitors are therefore being evaluated as novel therapeutic agents in the treatment of these diseases. Inhibition or depletion of PARP, however, increases
genomic instability in cells exposed to genotoxic agents. We recently demonstrated 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 deficiency or lack of PARP activity, we investigated the effects of PARP inhibition on development of
tetraploidy. Immortalized wild-type and PARP(-/-) fibroblasts were exposed for 3 weeks to 20 microM
GPI 6150 (1,11b-dihydro-[2H:]benzopyrano[4,3,2-de]isoquinolin-3-one), a novel small molecule specific competitive inhibitor of PARP (K(i) = 60 nM) and one of the most potent
PARP inhibitors to date (IC(50) = 0.15 microM). Although
GPI 6150 initially decreased cell growth in wild-type cells, there was no effect on cell growth or viability after 24 h.
GPI 6150 inhibited endogenous PARP activity in wild-type cells by approximately 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.