Abstract 4607: Mediator kinase module as a transducer of oncogenic Wnt/beta-catenin signaling

Abstract

Abstract Dysregulation of Wnt/β-catenin signaling promotes colorectal cancer (CRC) and other types of malignancies through unprogrammed changes in gene transcription that drive tumorigenesis. CDK8 is a CRC oncoprotein whose amplification-dependent overexpression identifies a significant subset of CRC patients with poor prognosis. CDK8 kinase activity, along with Cyclin C (CycC), drives tumorigenesis by stimulating β-catenin transcriptional activity. Thus, inhibition of CDK8 kinase function offers a promising therapeutic approach for CDK8-overexpressing CRCs. CycC-CDK8, along with MED12 and MED13, compose a four-subunit “kinase” module within Mediator, a conserved multiprotein interface between gene-specific transcription factors and RNA Polymerase II. We have previously identified a network of physical and functional interactions within the Mediator kinase module critical for oncogenic Wnt/β-catenin signaling. Mechanistically, β-catenin binds directly to MED12 in Mediator, thus activating CycC-dependent CDK8 kinase activity and β-catenin transcriptional activity. More specifically, MED12-dependent CDK8 activation occurs through a direct interaction involving the MED12 N-terminus and a phylogenetically conserved surface groove on CycC. Here, we demonstrate that mutagenic disruption of the MED12/CycC interface in CRC cell lines leads to uncoupling of CycC-CDK8 to MED12 and core Mediator, and concomitant loss of Mediator-associated CDK8 activity. Transcriptome analysis of cells lacking CDK8 kinase function revealed downregulation of Wnt/β-catenin signaling and other oncogenic pathways, consequently impairing CRC cell proliferation and clonogenicity. Our studies therefore identify the MED12/CycC interface as a critical transducer of oncogenic Wnt/β-catenin signaling. By validating MED12/CycC as a potential therapeutic target, our findings have significant implications in current methods of treating CRC as they pave a way for development of novel, targeted inhibitors of Wnt/β-catenin signaling to treat colorectal and other CDK8-driven driven malignancies. Supported by MH085320-05 (TGB), RP140435 (TGB) and T32DE14318 (ADC). Citation Format: Alison Clark, Marieke Oldenbroek, Yao Wang, Jason Spaeth, Fangjian Gao, Victor X. Jin, Thomas Boyer. Mediator kinase module as a transducer of oncogenic Wnt/beta-catenin signaling. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4607.