Transcriptional down-regulation of poly(ADP-ribose) polymerase gene expression by E1A binding to pRb proteins protects murine keratinocytes from radiation-induced apoptosis

Adenovirus E1A confers enhanced cell sensitivity to radiation and drug-indu ced DNA damage by a mechanism involving the binding to cellular proteins. M utant analysis in E1A-transfected murine keratinocytes demonstrates that in creased sensitivity to DNA damage requires at least E1A binding to the p300 /CREB-binding protein (CBP) transcriptional coactivators and to pRb family members, indicating that this biological activity of E1A is the result of t he concomitant perturbation of different cell pathways. Here we show that i n the same cells E1A binding to members of the retinoblastoma protein famil y induces transcriptional down-regulation of the poly(ADP-ribose) polymeras e (PARP) gene, coding for a NAD-dependent enzyme stimulated by DNA breaks, Inhibition of PARP expression is accompanied by a decrement of gamma-irradi ation-induced apoptosis, which is overridden by reconstitution of wild type levels of PARP. Hence, E1A effects on PARP transcription are central deter minant of the apoptotic sensitivity of E1A-expressing keratinocytes, Conver sely, E1A binding to only p300/CBP results in an increase in PARP enzyme ac tivity and consequently in cell death susceptibility to irradiation, which is effectively counteracted by the PARP chemical inhibitor 3-aminobenzamide . Therefore, our results identify in the E1A-mediated effects on PARP expre ssion and activity a key molecular event involved in E1A-induced cell sensi tization to genotoxic stress.