Abstract 5409: Asymmetric interdependency among the CDK8 module subunits

Abstract The CDK8 kinase module (CKM) of the transcriptional cofactor Mediator complex plays a pivotal role in regulating RNA polymerase II-dependent transcription. Dysregulation of the CKM has been implicated in various diseases, as CDK8 overexpression promotes tumorigenesis in colorectal cancer and melanoma, CKM subunits are mutated or amplified in other types of cancers and cardiovascular diseases. Understanding the in vivo functions and regulatory mechanisms of the CKM and Mediator complexes is essential for uncovering their contributions to tumorigenesis. Previous studies showed that the loss of CKM subunits in Drosophila resulted in distinct phenotypes, particularly in the eyes and wings. In this study, we observed that loss of Med13 and Med12 caused more significant reductions in multiple biological functions compared to the effects observed with CDK8 and CycC. Depleting individual CKM subunits revealed that Med13 has the strongest influence on cell growth and cell proliferation, likely due to differences in the rate of nascent protein synthesis. This hierarchy is followed by Med12 and then the CDK8-CycC complex, suggesting a differential impact of these CKM subunits at the cellular level. Immunoblot analyses further revealed that the stability of CDK8 and Med12 depends on the presence of Med13 in Drosophila. These findings support the hypothesis that CKM subunits are asymmetrically interdependent. Furthermore, knockdown of CKM subunits via lentiviral transduction in human LnCaP cells confirmed this asymmetric interdependency in mammalian systems. ​Taken together, our results suggest the stability and asymmetrical dependence of CKM subunits play a critical role in regulating nascent protein synthesis, cellular growth, and proliferation, with Med13 serving a critical role within the CKM module. Citation Format: Tzu-Hao Liu, Xiao Li, Fu-Ning Hsu, Mengmeng Liu, Rajitha-Udakara-Sampath Hemba-Waduge, Jun-Yuan Ji. Asymmetric interdependency among the CDK8 module subunits [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 5409.