DNA-PKcs: Novel Roles and Therapeutic Targeting in Prostate Cancer

Prostate cancer (PCa) is the most frequently diagnosed non-cutaneous malignancy and the second leading cause of cancer-related deaths in men in the Unites States. While early-stage disease is treatable, the advanced stage of disease termed castration resistant prostate cancer (CRPC) remains fatal. The androgen receptor (AR) has been identified as a driver of the disease at all stages, thus first line of therapy is directed toward control of AR activity and AR-mediated processes. Nevertheless, AR-directed therapies are not durable. Significant work has been done to identify AR partners and targets that promote proliferation and metastasis, in order to provide therapeutic targeting downstream of AR. DNA-dependent protein kinase catalytic subunit (DNA-PKcs, referred as DNA-PK herein) is a pleiotropic protein well known for its role in DNA repair through non-homologous end joining (NHEJ). AR was found to promote DNA repair through regulation of DNA-PK expression and activity upon DNA damage, while activated DNA-PK transcriptionally regulates pro-metastatic gene networks to promote aggressive disease. Furthermore, DNA-PK has been identified as the most deregulated kinase in men with metastatic CRPC and can independently predict for poor outcome in patients with high-risk disease. Consequently, DNA-PK has been nominated as a therapeutic target and DNA-PK inhibitors are in clinical development. While DNA-PK is well known for its role in NHEJ, delineating the molecular mechanisms of DNA-PK pro-tumorigenic functions beyond DNA repair will allow for improved management and treatment of CRPC patients. The studies herein utilized unbiased proteomic and metabolomic approaches to identify a novel role of DNA-PK in regulation of glycolysis through interaction, phosphorylation, and modulation of enzymatic activity of glycolysis protein partners ALDOA and PKM2. The effects of DNA-PK in metabolic regulation were tested in in vitro, in vivo, and patient tumor samples. These studies also suggest a new therapeutic strategy by combining DNA-PK and glycolysis inhibitors for enhanced anti-proliferative effects in CRPC. Moreover, since DNA-PK inhibitors are being investigated in the clinic, the translational capacity of a clinically actionable dual DNA-PK/TORK (mammalian target of rapamycin kinase, mTOR kinase) inhibitor was tested in CRPC models. The global molecular impact of this treatment was assessed using unbiased transcriptomic analysis. The transcriptomic analysis identified novel DNA-PK-specific functions in transcriptional regulation of cancer-relevant pathways including oxidative phosphorylation, immune response and epithelial-mesenchymal (EMT) transition. Moreover, AR-axis upregulation was induced upon treatment, but was mitigated through combination of DNA-PK inhibitor with standard of care AR antagonist, Enzalutamide. Currently, combination of DNA-PK inhibition and Enzalutamide is being evaluated in a Phase II trial. Together, the findings herein have identified novel roles of DNA-PK in metabolic regulation through glycolysis and transcriptional regulation of cancer relevant pathways, expanding the knowledge in the field and identifying novel therapeutic strategies for treatment of CRPC