Abstract 4098: Topotecan-mediated inhibition of HIF1α/ARV7 heterodimers in 22RV1 prostate cancer cells prevents ARV7 nuclear entry, reversing enzalutamide resistance

Background: Although enzalutamide (ENZ) is an effective treatment for prostate cancer (PC), development of ENZ resistance is inevitable. One mechanism for resistance is upregulation of ARV7, a splice variant of the full-length androgen receptor (FLAR) that lacks the androgen binding and hinge region domains. Nuclear translocation of FLAR requires the hinge region for coupling to tubulin or importin. ARV7 gains nuclear entry independent of the canonical pathway. FLAR has been reported to bind to Hypoxia Inducible Factor-1α (HIF1α) under normoxic conditions in the presence of dihydrotestosterone (DHT). We hypothesized that ARV7 may also heterodimerize with HIF1α in the cytoplasm, enabling nuclear localization and transcriptional activity of both transcription factors. We reported that topotecan (TPT) blocks HIF1α mRNA translation by targeting topoisomerase I (TOPO1) resident on HIF1α mRNA. We determined if ARV7 was bound to HIF1α in ENZ resistant cells and if inhibition of HIF1α translation by topotecan (TPT) could block ARV7 nuclear localization, thereby resensitizing ARV7 expressing cells to ENZ. We also evaluated if TOPO1 resided on FLAR/ARV7 mRNA, providing an additional target for TPT. Methods: 22Rv1 ENZ resistant PC cells that express FLAR and ARV7 protein were treated with the hypoxia mimetic cobalt chloride to stabilize HIF1α. Cytoplasmic and nuclear extracts from 22Rv1 cells were used for co-immunoprecipitation (Co-IP) using anit-HIF1α antibody. The effect of DHT, ENZ, and TPT on HIF1α and AR/ARV7 expression, and interactions between Topo-1 and HIF1α and AR mRNA were determined by western blot and RNA-IP. The effects of ENZ and TPT on 22Rv1 cell viability were evaluated using the XTT viability assay. Results: Topo-I RNA IP demonstrated its linkage to HIF-1α, FLAR and ARV7 mRNA. Co-IP of 22Rv1 whole cell lysates and nuclear extracts with HIF1α-specific antibodies yielded both full length AR and ARV7 protein bands on western blots under normoxic conditions in the presence of DHT and under hypoxic conditions. TPT and ENZ treatment of 22Rv1 cells under hypoxic conditions resulted in significantly reduced HIF1α, FLAR and ARV7 nuclear localization. TPT had an IC50 of 20 nM in normoxia and 91 nM in hypoxia. ENZ had an IC50 of 24.4 uM in normoxia and 388.7 uM in hypoxia. Dose-response curves for TPT / ENZ combinations at concentrations achieved clinically demonstrated synergistic activity, with combination index values of 0.54 under normoxic and 0.19 under hypoxic conditions. Conclusions: Our data indicate that Topo I resides on HIF-1α, FLAR and ARV7 mRNA, providing a potential target for TPT-mediated inhibition of their translation. Clinically achievable TPT concentrations caused HIF1α knockdown in hypoxic 22Rv1 cells in association with diminished nuclear localization of ARV7. 22Rv1 cells showed a synergistic dose-response to the combination of TPT and enzalutamide. These findings support exploration of TPT in combination with ENZ for treatment of PC. Citation Format: John P. Fruehauf, Diana C. Gomez, Leslie Spitalny, Jai-Hyun Kim. Topotecan-mediated inhibition of HIF1α/ARV7 heterodimers in 22RV1 prostate cancer cells prevents ARV7 nuclear entry, reversing enzalutamide resistance [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4098.