Introduction:

There are currently four FDA-approved poly (ADP-ribose) polymerase inhibitors (PARPi), such as Olaparib, for treating metastatic castration-resistant prostate cancer (mCRPC) harboring specific DNA repair defects. 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 (LNCaP, LNCaP ShPrKD1, C4-2, and C4-2 PrKD1) to assess viability following Olaparib and/or Compound-10 (Comp-10), a selective PrKD1 inhibitor, treatment. Subcellular fractionation, immunoprecipitation, in silico modeling, and Western blotting were utilized to evaluate protein expression, interaction, and localization.

Results:

PrKD1-overexpressing C4-2 cells exhibited significantly increased sensitivity to growth inhibition by Olaparib at concentrations ranging from 2.5–50.0 μM. Conversely, downregulation of PrKD1 in LNCaP cells conferred resistance to Olaparib, suggesting a role for PrKD1 in modulating PARPi sensitivity in prostate cancer cells (Figure 1). Unexpectedly, biochemical inhibition of PrKD1 by a novel PrKD1 selective inhibitor, Comp-10, also increased sensitivity to Olaparib. Co-immunoprecipitation experiments demonstrated that PrKD1 and PARP1 are present in the same immunocomplex. Furthermore, stable transfection of PrKD1 in C4-2 cells increased membrane localization of PARP1 (Figure 2). In silico molecular modeling identified multiple potential interaction sites between PARP1 and PrKD1 across several protein domains, which may facilitate their co-translocation to the membrane.

Conclusion:

Our study has discovered PrKD1 as a novel modulator of prostate cancer cell sensitivity to the PARPi Olaparib. Co-targeting PrKD1 using small-molecular inhibitors, such as PrKD1 selective Comp-10, may enhance Olaparib efficacy at lower doses and support combination strategies to improve PARPi tolerability and therapeutic index. The presence of PARP1 at the membrane is a novel finding. In addition to targeting nuclear PARP1, this finding introduces the potential to target membranous PARP1 via monoclonal antibodies or small molecules for theranostics purposes.

Funding: N/A