Protein Kinase D1 Translocates PARP1 to the Membrane and Is Associated With Increased Sensitivity to Cell Viability Inhibition by PARP1 Inhibitor Olaparib.

[BACKGROUND] There are four FDA-approved poly (ADP-ribose) polymerase inhibitors (PARPi) for treating metastatic castration-resistant prostate cancer. However, dose-limiting toxicities may reduce efficacy when treatment is de-escalated. Identifying modulators of PARPi sensitivity could enable combination strategies that enhance efficacy while minimizing toxicity. This study investigates whether Protein kinase D1 (PrKD1), recently discovered to modulate DNA repair, influences sensitivity to PARP inhibition.

[METHODS] We used prostate cancer cell lines with altered PrKD1 expression to assess viability following olaparib and/or Compound-10, a selective PrKD1 inhibitor, treatment. Subcellular fractionation, immunoprecipitation, in silico modeling and Western blotting of lysates of cells and tumor samples harvested from patient-derived xenograft tumor engrafted mice treated with Compound-10 were used to evaluate protein expression, interaction, and localization.

[RESULTS] PrKD1-overexpressing C4-2 cells exhibited significantly increased sensitivity to growth inhibition by olaparib. Downregulation of PrKD1 in LNCaP cells conferred resistance. Biochemical inhibition of PrKD1 by Compound-10 also enhanced sensitivity. Co-immunoprecipitation experiments demonstrated that PrKD1 and PARP1 are present in the same immunocomplex, and PrKD1 transfection in C4-2 cells increased PARP1 membrane localization. Treatment of prostate cancer PDX models with Compound-10 increased PARP1 expression. In silico molecular modeling identified a site adjacent to the PARP1 WGR domain potentially binding to multiple PrKD1 domains.

[CONCLUSION] Our study identifies PrKD1 as a novel modulator of sensitivity to olaparib. Co-targeting PrKD1 using small molecular inhibitors may enhance olaparib efficacy at lower doses and improve PARPi tolerability and therapeutic index. The demonstration of PARP1 at the membrane is novel, introducing the possibility of targeting membranous PARP1 for theranostic applications.