Abstract 785: ETS-mediated expression of miR-24 regulates Top1 levels and resistance of prostate cancer cell lines to camptothecin
Authors: Kayali Samer , Roche Emmanuel , Montaudon Danièle , Pourquier Philippe , Houédé Nadine 
Published: May 2015

Abstract
Human nuclear topoisomerase I (Top1) is a crucial enzyme involved in the removal of DNA supercoiling during replication, transcription and chromosome segregation. These roles rely on its capability to introduce reversible single-strand breaks in duplex DNA. Top1 is the unique target of camptothecin (CPT) derivatives such as Irinotecan and Topotecan that are approved for the treatment of colon, ovarian and lung cancers. These inhibitors stabilize covalent Top1-DNA complexes resulting in cytotoxic double-strand breaks by collision of stabilized complexes with advancing replication forks. The mechanisms of resistance to CPT derivatives are complex and not fully understood yet, though it is known that the amount of Top1 can directly affect tumor cell response to these agents. Advanced forms of prostate cancers (PCa) that usually become resistant to castration are treated with chemotherapy using taxane-based regimen with disappointing response rates. Unfortunately, they also display an intrinsic resistance to many other cytotoxic agents including CPT derivatives. In this study, we identified one resistance mechanism linked to the regulation of Top1 levels via the overexpression of transcription factors from the ETS family (mainly ERG and ETV1) that is detected in 50-80% of PCa. We first showed that siRNA-mediated transient repression of ERG or ETV1 in ERG-overexpressing VCaP cells and in ETV1-overexpressing LNCaP cells, respectively, resulted in increased cell sensitivity to CPT, confirming the role of these two ETS transcription factors in PCa cell response to Top1 inhibitors. Using the CHiP-Seq database from UCSC, we could identify potential ETS binding domains in the promoter region of a cluster of miRNAs containing miR-24, which expression level across the NCI-60 cell line panel was found to be negatively correlated with the sensitivity to CPT derivatives. We then evaluated the modulation of miR-24 expression on cell sensitivity to CPT. We found that overexpression of miR-24 reduced sensitivity of LNCaP cells to CPT, whereas overexpression of an anti-miR-24 had the opposite effect, confirming previously published bioinformatics predictions. Interestingly, miR-24 overexpression in LNCaP cells was accompanied with a reduction in Top1 protein levels whereas transfection with anti-miR-24 resulted in increased Top1 levels. Together, our preliminary results provide the first functional evidences that Top1 levels and subsequent resistance of PCa cell lines to CPT are, at least in part, regulated via ETS-mediated expression of miR-24. They further represent a rational basis for the clinical development of miR-24 inhibitors for the sensitization of CPT-resistant tumors such as advanced PCa.
Citation Format: Samer Kayali, Emmanuel Roche, Danièle Montaudon, Philippe Pourquier, Nadine Houédé. ETS-mediated expression of miR-24 regulates Top1 levels and resistance of prostate cancer cell lines to 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 785. doi:10.1158/1538-7445.AM2014-785