HIF inactivation of p53 in Ovarian Cancer Can be Reversed by Topotecan, Restoring Cisplatin and Paclitaxel Sensitivity

Ovarian cancer (OC) growth under hypoxic conditions results in hypoxia inducible factor-1α (HIF-1α) stabilization. HIF-1α is an adverse prognostic factor that may contribute to worse outcomes via its capacity to bind to p53, potentially blocking p53-mediated apoptosis. We determined whether HIF-1α-p53 binding occurred in hypoxic OC cell lines, and if this blocked p53 transcriptional activity. Topotecan [TPT], used in the treatment of OC, inhibits HIF-1α translation via a topoisomerase-1 (TOPO1)-dependent mechanism. We examined if TPT knockdown of HIF-1α restored p53 transcriptional function. TPT effects on HIF-1α and p53-related transcriptional targets were assessed by PCR. Associations between TPT effects and TOPO1 expression levels were examined by western blots and knock-down by siRNA. RNA-binding protein immunoprecipitation (RIP) was used to assess if TOPO1 was resident on HIF1α mRNA. We determined if sub-lethal doses of TPT, used to knockdown HIF-1α, reversed hypoxia-related cisplatin and paclitaxel resistance (XTT assay). Flow cytometry was used to assess HIF-1α-mediated upregulation of ABCB1 and ABCB5 efflux pump expression. We found that HIF-1α binding to, and inhibition of, p53 transcriptional activity in hypoxic OC cells was associated with drug resistance. TPT-mediated down-regulation of HIF-1α in hypoxic cells required TOPO1 resident on HIF-1α mRNA, restored p53 transcriptional activity, down-regulated ABCB1/ABCB5 cell surface expression, and reversed hypoxia-related cisplatin and paclitaxel resistance. Implications: TPT-mediated reduction of HIF-1α accumulation in hypoxic OC cell lines restores p53 tumor suppressor function, offering a novel approach to reverse chemoresistance. Further clinical investigation is warranted.