Abstract LB-243: Protein kinase D1 induces cell cycle arrest independent from check point kinases
Authors: Nickkholgh Bita , Sittadjody Sivanandane , Rothberg Michael B. , Balaji KC 
Published: August 2018

Introduction: Prostate cancer (PCa), one of the most common cancers among men, initially manifests as an androgen-dependent malignancy. PCa eventually and inevitably progresses into hormone refractory or castration-resistant condition. Relapse of castration-resistant prostate cancer (CRPC) is life threatening, and responsible for the majority of mortality among PCa patients. During the transition from the hormone naive disease into CRPC, there is an up regulation of cell cycle markers and a decline in the expression of tumor suppressors like protein kinase D1 (PrKD1). The role of PrKD1 as a tumor suppressor has been identified in regulating various cellular events including the inhibition of the mitogenic pathways induced by growth factors and androgens. However, the potential role of PrKD1 as a cell cycle regulator has not been explored in solid cancers including prostate. Therefore, the present study has been designed to investigate the role of PrKD1 as a cell cycle regulator in PCa cells.

Methods: LNCaP cells with high expression of PrKD1 and its aggressive derivative C4-2 cells with low expression of PrKD1 were used in this study. To evaluate the direct effect of PrKD1, C4-2 cells were stably transfected with PrKD1 and used alongside the two other cell lines. Cells were cultured in the presence and absence of Bryostatin 1 (Bryo-1, a PrKD1 activator) for 30 hours and exposed to 10 Gray gamma radiation (IR) to induce DNA damage. After the treatment period, cell cycle was assessed using flow cytometry and the phosphorylation of Cdc25 phophatases were analyzed by western blot. All experiments were repeated after inhibiting check point kinase 1 and 2 (CHEK1 and CHEK2) with siRNA to evaluate the independency of PrKD1 action on cell cycle.

Results: LNCaP cells underwent cell cycle arrest in both Bryo-1 and IR group, whereas non-transfected C4-2 cells responded only to IR, not to Bryo-1. However, C4-2 cells when transfected with PrKD1 were arrested at G1/S checkpoint in response to Bryo-1. Down regulation of CHEK1 and CHEK2 with SiRNA in C4-2 cells did not cause any significant cell cycle arrest in response to DNA damage due to IR. Over expression of PrKD1 in these cells rescued the effect of siRNA against CHEK1 and CHEK2. Western blot analysis showed that activation of PrKD1 with Bryo-1 causes phophorylation of Cdc25 C phophatase.

Conclusion: Our study showed that PrKD1 phosphorylates CDC25C and induces cell cycle arrest in G1/S phase in response to DNA damage. The results suggest that PrKD1, like the two other Serine Threonine kinases (CHEK1 and CHEK2), mediates cell cycle arrest in response to DNA damage.

Citation Format: Bita Nickkholgh, Sivanandane Sittadjody, Michael B. Rothberg, KC Balaji. Protein kinase D1 induces cell cycle arrest independent from check point kinases [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-243.