Evaluation of CDK9 Inhibition by Dinaciclib in Combination with Apoptosis Modulating izTRAIL for the Treatment of Colorectal Cancer

Treatment options for colorectal cancer (CRC), especially in advanced stages are still insufficient. There, the discovery of Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) was a bright spot. However, most cancers show resistance toward apoptotic signals. Cyclin-dependent kinase 9 (CDK9) plays a crucial role in cell cycle progression in most tissues. We recently demonstrated the role of CDK9 in mediating TRAIL resistance. In this work, we investigated the role of CDK9 in colorectal cancer. Immunohistochemical analysis of CDK9 expression in cancer and normal tissues of CRC specimens was performed. The effect of selective CDK9 inhibition in combination with TRAIL on CRC cells was analyzed via cell viability, colony formation, and induction of apoptosis by flow cytometry. The mechanism of action was conducted via western blotting. We now have confirmed overexpression of CDK9 in cancer tissues, with low expression associated with poorer survival in a subset of CRC patients. In-vitro, CDK9 inhibition could strongly promote TRAIL-induced cell death in TRAIL-resistant CRC cells. Mechanistically, CDK9 inhibition induced apoptosis by downregulation of antiapoptotic proteins, myeloid leukemia cell differentiation protein 1 (Mcl-1) and FLICE-inhibitory protein (c-FLIP). Overall, we identified CDK9 as a prognostic marker and combined CDK9 inhibition and TRAIL as a novel and promising therapeutic approaches for colorectal cancer.