Abstract 3813: Role of the ERG transcription factor in the resistance of prostate cancer cells to the topoisomerase I inhibitor camptothecin

Abstract

Abstract Prostate cancer (PCa) is a major public health issue as it is the second cause of death among cancers in industrialized countries, and its incidence is rising. Surgery is the main option for the curative treatment of localized forms of PCa and recurrences are managed with androgen deprivation strategies. In the case of advanced metastatic PCa and when tumors become resistant to castration (mCRPC), chemotherapy with taxanes is initiated. However, response rates remain low with a restricted number of alternatives due to the intrinsic resistance of CRPC to a wide range of cytotoxic agents. Advanced PCa are particularly resistant to topoisomerase I (Top1) inhibitors from the camptothecin (CPT) family, but the mechanistic bases of this resistance have not been investigated in details. Top1 is a nuclear enzyme that induces transient single strand breaks in duplex DNA to ensure the removal of torsional constraints associated with crucial DNA transactions such as replication, transcription, chromosome segregation or DNA recombination. We have previously shown that interaction of Top1 with DNA-PKcs, a kinase involved in non-homologous end-joining, could regulate the cellular response to CPT independently of DNA repair. This was further confirmed by the fact that NU7441, a specific inhibitor of the kinase activity of DNA-PKcs, had no effect on Hela cell sensitivity to CPT. Recently, it was shown that, in PCa cells, DNA-PKcs could interact with ERG, a transcription factor of the ETS family. As advanced PCa are often characterized by a TMPRSS2-ERG fusion leading to wild-type ERG overexpression, we investigated whether ERG could play a role in the resistance of PCa cells to CPT by regulating Top1/DNA-PKcs interaction. We showed that VCaP cells harboring the TMPRSS2-ERG fusion resulting in a 6-fold increase of ERG protein as compared to ERG fusion-negative (control) DU145 or PC3 cells were >100-fold more resistant to CPT. RWPE-1 stably overexpressing low levels of wild-type ERG (2-fold increase) did not show a major change in resistance to CPT as compared to control RWPE-1 cells stably expressing lacZ. Using two specific siRNA, we also showed that transient downregulation of ERG drastically reduced the sensitivity of VCaP cells to CPT, this effect being dependent on the efficacy of ERG repression. Interestingly, this effect was not due to a change in Top1 levels, suggesting that ERG downregulation may affect Top1-DNA-PKcs interaction. We detected this interaction in VCaP cells by immunoprecipitation in the presence of ethidium bromide, indicating that it was independent of DNA and it remains to be seen whether it is disrupted following ERG repression. Our preliminary findings provide evidences for a new mechanism by which ERG and its interaction with DNA-PKcs could explain, at least in part, the intrinsic resistance of PCa to Top1 inhibitors by the regulation of Top1-DNA-PKcs interaction. Citation Format: Emmanuel Roche, Danièle Montaudon, Samer Kayali, Nadine Houédé, Philippe Pourquier. Role of the ERG transcription factor in the resistance of prostate cancer cells to the topoisomerase I inhibitor camptothecin. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3813. doi:10.1158/1538-7445.AM2014-3813