Enhanced radiosensitization of enzalutamide via schedule dependent administration to androgen-sensitive prostate cancer cells

The androgen receptor (AR) signaling axis plays a critical role in the development, function and homeostasis of the prostate. The classical action of AR is to regulate gene transcriptional processes via AR nuclear translocation, binding to androgen response elements on target genes and recruitment of, or crosstalk with, transcription factors. Prostate cancer initiation and progression is also uniquely dependent on AR. Androgen deprivation therapy remains the standard of care for treatment of advanced prostate cancer. Despite an initial favorable response, almost all patients invariably progress to a more aggressive, castrate-resistant phenotype. Considerable evidence now supports the concept that development of castrate-resistant prostate cancer (CRPC) is causally related to continued transactivation of AR. Understanding the critical events and complexities of AR signaling in the progression to CRPC is essential in developing successful future therapies. This review provides a synopsis of AR structure and signaling in prostate cancer progression, with a special focus on recent findings on the role of AR in CRPC. Clinical implications of these findings and potential directions for future research are also outlined.

Akt phosphorylation has previously been described to be involved in mediating DNA damage repair through the nonhomologous end-joining (NHEJ) repair pathway. Yet the mechanism how Akt stimulates DNA-protein kinase catalytic subunit (DNA-PKcs)-dependent DNA double-strand break (DNA-DSB) repair has not been described so far. In the present study, we investigated the mechanism by which Akt can interact with DNA-PKcs and promote its function during the NHEJ repair process. The results obtained indicate a prominent role of Akt, especially Akt1 in the regulation of NHEJ mechanism for DNA-DSB repair. As shown by pull-down assay of DNA-PKcs, Akt1 through its C-terminal domain interacts with DNA-PKcs. After exposure of cells to ionizing radiation (IR), Akt1 and DNA-PKcs form a functional complex in a first initiating step of DNA-DSB repair. Thereafter, Akt plays a pivotal role in the recruitment of AKT1/DNA-PKcs complex to DNA duplex ends marked by Ku dimers. Moreover, in the formed complex, Akt1 promotes DNA-PKcs kinase activity, which is the necessary step for progression of DNA-DSB repair. Akt1-dependent DNA-PKcs kinase activity stimulates autophosphorylation of DNA-PKcs at S2056 that is needed for efficient DNA-DSB repair and the release of DNA-PKcs from the damage site. Thus, targeting of Akt results in radiosensitization of DNA-PKcs and Ku80 expressing, but not of cells deficient for, either of these proteins. The data showed indicate for the first time that Akt through an immediate complex formation with DNA-PKcs can stimulate the accumulation of DNA-PKcs at DNA-DSBs and promote DNA-PKcs activity for efficient NHEJ DNA-DSB repair.

Upregulation of constitutively-active androgen receptor splice variants (AR-Vs) has been implicated in AR-driven tumor progression in castration-resistant prostate cancer. To date, functional studies of AR-Vs have been focused mainly on their ability to regulate gene expression independent of the full-length AR (AR-FL). Here, we showed that AR-V7 and ARv567es, two major AR-Vs, both facilitated AR-FL nuclear localization in the absence of androgen and mitigated the ability of the antiandrogen enzalutamide to inhibit AR-FL nuclear trafficking. AR-V bound to the promoter of its specific target without AR-FL, but co-occupied the promoter of canonical AR target with AR-FL in a mutually-dependent manner. AR-V expression attenuated both androgen and enzalutamide modulation of AR-FL activity/cell growth, and mitigated the in vivo antitumor efficacy of enzalutamide. Furthermore, ARv567es levels were upregulated in xenograft tumors that had acquired enzalutamide resistance. Collectively, this study highlights a dual function of AR-Vs in mediating castration resistance. In addition to trans-activating target genes independent of AR-FL, AR-Vs can serve as a “rheostat” to control the degree of response of AR-FL to androgen-directed therapy via activating AR-FL in an androgen-independent manner. The findings shed new insights into the mechanisms of AR-V-mediated castration resistance and have significant therapeutic implications.