PARylation and Prostate Cancer – Effects of PARP Inhibition on the Androgen Receptor

Poly adenosine diphosphate ribose polymerase (PARP), is a class of enzymes that post-translationally modify proteins with the PAR tag. PARP enzymes are thought to participate in multiple cellular functions, including in DNA repair pathways. PARP inhibitors are being developed as a potential treatment option for different types of cancer, including ovarian, breast, and prostate. In prostate cancer, previous studies have shown the efficacy of PARP inhibitors like olaparib to kill castration-resistant prostate cancer (CRPC) cells that have DNA repair defects (Langelier, Riccio, and Pascal 7762-7775). Olaparib is approved for the treatment of ovarian and breast cancers harboring BRAC1/2 mutations. However, whether PARP inhibition may have efficacy in CRPC through additional mechanisms not directly related to DNA repair defects is not fully understood. Initial experiments in our lab have demonstrated the ability of olaparib to curtail tumor growth in prostate cancer xenografts in vivo. Immunohistochemistry of the androgen receptor (AR) on these AR-positive xenografts have shown lower rates of AR nuclear localization in olaparib treatment groups. We hypothesized that difference in nuclear localization is due to the differences in the AR’s protein binding partners or interactome when exposed to olaparib. We found that depriving androgens from androgen dependent prostate cancer cell lines cause an increase in global PARylation levels, likely owing to an increase in PARP activity. We also demonstrate that the AR itself may be PARylated. An immunoprecipitation of the AR followed by a mass spectrometry assay allowed us to identify the AR’s binding partners. In CRPC, we see that in cells treated with olaparib, AR does not interact with the nuclear pore complex, or other nuclear transport factors like importins and exportins. A lack of the PAR tag in olaparib treated cells could be the reason for this differential interaction profile. We conclude that the removal of PAR tags from the AR in CRPC prevents its nuclear import and hinders the AR’s functional role as a transcription factor. These studies and results provide support for a hypothesis that PARP inhibition may have anti-cancer activity in AR-positive CRPC through modulation of AR nuclear localization and signaling