Background: Emerging evidence suggests that there is an urgent need for new strategies to combat prostate cancer (PC) that is unresponsive to therapy. In recent years major emphasis has been placed on the role of exosomes (Exo) in cancers. Exosomes are membrane-bound vesicles produced by all cell types under physiologic and pathological states. The objective of the present study was to identify via high throughput library screens compounds that selectively inhibit Exo biogenesis and release by PC cells and further examine if they chemosensitize castration-resistant PC (CRPC) cells to Enzalutamide (ENZ), an FDA-approved second generation antiandrogen drug. The ultimate goal is to incorporate functionally validated compounds for advanced PC therapy. Manumycin-A (MA), a natural, well-tolerated microbial metabolite and a potent experimental tumoricide, has been identified as one of the lead compounds in the initial screens.

Results: A time-course, dose-dependent analysis revealed that MA up to 250 nM is not toxic to C4-2B or RWPE1 cells. The MA-mediated inhibition of Exo secretion was evident by decreased expression of Exo markers (ALIX, TG6101, and tetraspanins; CD9, CD63, CD81) as well as early (Rab5) and late (Hrs) endosome markers in C4-2B cells, with no effect observed in RWPE1 cells. Interestingly, the MA-mediated inhibition of Exo biogenesis was coupled with inhibition of Ras activation. MEK selective inhibitor (U0126), but not the p38 MAPK (SB203580) or JNK (SP600125) inhibitor, suppressed the expression of Exo markers, indicating that Ras-MEK pathway is involved in MA's mediated inhibition of exosome biogenesis and secretion. Subsequent analysis revealed that MA inhibited p-cRaf, pMEK and pERK in C4-2B cells, but not in RWPE1 cells)–further attesting to direct role of Ras/Raf/ERK1/2 pathway in mediating MA inhibition of Exo biogenesis and secretion by C4-2B cells. Treatment of C4-2B cells with ENZ and MA combination further inhibited cell survival than either drug alone, suggesting that MA may chemosensitize CRPC cells to ENZ.

Conclusions: Our findings suggest that at low, non-cytotoxic concentrations, Manumycin is a suitable candidate to suppress exosome biogenesis and secretion and enhances sensitivity of CRPC cells to ENZ cytotoxicity. Further preclinical analysis is required to examine its in vivo therapeutic efficacy of MA against CRPC when administrated in combination with antiandrogen drugs.

Citation Format: Hogyoung Kim, Amrita Datta, Madhu Lal-Nag, Adedoyin Johnson, Ahmed Moustafa, Debasis Mondal, Marc Ferrer, Asim B. Abdel-Mageed. Manumycin-A suppresses exosome biogenesis and chemosensitizes CRPC cells to enzalutamide through inhibition of Ras/Raf/ERK1/2 signaling pathway. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1882.