Abstract 2899: Mitophagy imparts enzalutamide resistance in prostate cancer

Purpose: Anti-androgens are widely used in androgen deprivation therapy (ADT), a standard-of-care for patients with recurrent prostate cancer (PCa). Unfortunately, most patients ultimately develop resistance to ADT and progress to castrate-resistant prostate cancer (CRPC). Recently, two agents that block androgen receptor activation [abiraterone acetate (Zytiga, J&J) and enzalutamide (Xtandi, Medivation/Astellas)] have been approved for CRPC therapy. While some patients respond to these therapies, many fail. We investigated if sequestration of metabolically aberrant mitochondria in the autophagosomes (mitophagy) imparts anti-androgen resistance. Method: We studied the effects of anti-androgen enzalutamide on the autophagy of androgen-dependent LNCaP and -independent C4-2 cells. Autophagy was monitored by cellular fluorescence in cells treated with monodansylcadavarine (MDC). Cellular fluorescence due to Mitosox dye oxidation was used to identify mitochondria producing high superoxide (O2-). Mitophagy of O2- producing mitochondria was monitored using fluorescence resonance energy transfer (FRET) between MDC and Mitosox in a 96-well plate based high throughput (HTS) assay and by visualization of FRET images and quantitation of FRET image intensities using a Nikon A1 fluorescence confocal microscope and associated software. Results: Our data show that in low androgen media, the degree of autophagy is less in C4-2 cells than in LNCaP cells. Enzalutamide treatment induces autophagy in both cell lines, but the increase in autophagy is more pronounced in the androgen-independent C4-2 than in the -dependent LNCaP cells. FRET data from both HTS and fluorescence microscopy show that while enzalutamide increases mitophagy of O2- producing mitochondria in C4-2 cells in a dose dependent manner, it decreases after an initial increase in LNCaP cells. Mitophagy of such mitochondria has also been observed by FRET based fluorescence microscopy in live circulating tumor cells (CTCs) isolated from blood sample of a patient undergoing enzalutamide therapy. Conclusion: Our data demonstrate that PCa cells resistant to enzalutamide show high degree of mitophagy of O2- mitochondria in autophagosomes. If this effect correlates with CTCs from blood collected from CRPC patients undergoing enzalutamide therapy, it can become a clinically useful method of identifying patients who are most likely to benefit from enzalutamide treatment. Citation Format: Hirak S. Basu, Cynthia L. Schrieber, Jamie M. Sperger, Maryanne Naundorf, Ashley M. Weichman, Farideh Mehraein-Ghomi, Dawn R. Church, Joshua M. Lang, George Wilding. Mitophagy imparts enzalutamide resistance in prostate cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2899. doi:10.1158/1538-7445.AM2015-2899