Abstract 3115: PARP overactivation predicts the susceptibility of human cancer cells to apoptosis induction by PARP inhibitors

Abstract Chemical inhibitors of poly(ADP)ribose (PAR) polymerases (PARP) have generated raising expectations in cancer treatment, either as monotherapeutic agents for the therapy of tumors that are deficient in DNA repair by homologous recombination, or combined with DNA-damaging compounds. Results from early clinical trials suggest that PARP inhibitors are well tolerated, and would be particularly useful for BRCA1- and BRCA2-mutated tumors. However, as these tumors constitute a minor fraction of all cancers, the main challenge is to extend the efficacy of PARP inhibitors to BRCA-proficient tumors, calling for the need of reliable biomarkers that predict the efficiency of PARP inhibitor-based therapy. Here, we report the effects of two chemically distinct PARP inhibitors, the phenanthridinone PJ34 and the 4-methoxy-carbazole derivate CEP8983, on non-small cell lung cancer (NSCLC) cells exhibiting increased PARP activity. We found that PAR polymers are a predictive marker of the sensitivity of NSCLC cells to PARP inhibitors. We were able to confirm these findings in mesothelioma, cervical cancer and ovarian cancer cell lines that are characterized by constitutively increased expression of PARP (as monitored by immunoblotting with a PARP1-specific antibody) as well as enhanced PARP enzymatic activity (as detected by immunoblotting with an antibody that recognizes poly(ADP)ribosylated proteins). Upon treatment with PARP inhibitors, cells with hyperactivated PARP exhibited signs of DNA damage (as assessed by immunofluorescence microscopy with an antibody that recognizes the phosphorylated form of histone 2AX) and underwent cell death via the intrinsic pathway of apoptosis (as monitored by immunofluorescence microscopy with antibodies for the detection of cytochrome c and active caspase-3). The chemosensitizing or cell death-inducing effects of PJ34 and CEP8983 were mimicked by PARP1- and PARP2-specific siRNAs, suggesting that they are indeed mediated by PARP inhibition. Moreover, PARP inhibitors efficiently reduced the growth of NSCLC xenografts exhibiting increased PARP activity (as evaluated by immunohistochemistry for poly(ADP)ribosylated proteins on paraffin embedded xenografts). Altogether, our results indicate that PARP overactivation reflects the susceptibility of NSCLC cells to apoptosis induction by PARP inhibitors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3115. doi:1538-7445.AM2012-3115