Drugging Transcription: Exploring Roles of CDK8 From Cancer to Immunity

Abstract

Kinases are cellular enzymes, integral in control of cellular growth and development. Kinases are frequently mutated or dysregulated in various disease states, such a as cancer, neurological diseases, and immune disorders. The cyclin dependent kinase (CDK) family is a large family of kinases named for their members’ close association with obligate regulatory binding partners known as cyclins. CDKs can be grouped primarily into CDKs involved in regulation of the cell cycle, or regulation of transcription. CDK8 is one of the transcriptional CDKs, part of the eukaryotic transcriptional co-activator, Mediator. CDK8 has distinct structural roles as part of Mediator, as deletion of CDK8 or its submodule leads to impairment of transcription and is embryonically lethal. CDK8 has been implicated in playing oncogenic roles in numerous cancers, including Wnt/ beta-catenin dependent colorectal cancer and gastric adenocarcinoma. However, this link emerged through genetic studies via silencing of CDK8 with siRNA, supporting the importance of structural presence of CDK8, but not confirming the importance of CDK8 kinase activity. My initial studies suggest that while CDK8 knock down may be anti-proliferative, CDK8 kinase inhibition is not. CDK8 inhibition had no anti-proliferative impact on the subset colorectal cells previously reported as sensitive to CDK8 siRNA. However, a clear role for CDK8 kinase activity in regulation of select transcriptional networks is present in the literature, wherein CDK8 regulates gene expression through direct phosphorylation of transcription factors, including STAT1, SMADs, E2F1, HIF1a, p53, serum response network, among others. Likewise, I observed CDK8 inhibition affected a subset of genes controlled by IFN activation, wherein CDK8 phosphorylates STAT1 on a regulatory serine, S727, in its transactivation domain. This supports a divergence in the role CDK8 plays structurally versus catalytically. While steric presence of CDK8 with Mediator is necessary for maintaining basal transcription, the kinase activity is important when the cell needs to respond to stimuli. In this vein, I explored areas where responses to external cellular stimuli were critical, alighting on immune activation. We utilized a small molecule screen in mouse primary bone marrow derived dendritic cells (BMDCs), identifying a previously unappreciated role for CDK8 inhibition in innate immune activation. Therein, inhibition of CDK8 leads to an enhancement of production of the anti-inflammatory cytokine IL-10, while reducing production of pro-inflammatory cytokines such as TNF and IL-6 in activated myeloid cells This lead to the conclusion CDK8 inhibition promotes tolerance in the innate immune compartment. Interestingly, the mechanistic underpinnings of this result occur through CDK8 driven phosphorylation of the transcription factor c-Jun, on a regulatory Serine, Ser243. While this finding suggested a critical role for CDK8 in innate immunity, it does also bring to question the therapeutic value of CDK8 inhibition as an oncogenic target, where increased tolerance and IL-10 production may be detrimental. From there I endeavored to determine if this tolerogenic effect was restricted to the innate immune compartment, or if CDK8 played any role in the adaptive immunity. To explore this I looked at the impact of CDK8 inhibition on polarization of CD4+ T helper cells towards pro-inflammatory (TH1/TH17) or anti-inflammatory (Treg) lineages. Interestingly, CDK8 inhibition selectively promoted differentiation of naïve CD62L+/CD4+ T helper cells to anti-inflammatory FOXP3+/CD4+ Tregs, which were functional in both in vitro and in vivo models. While more work is left to fully elucidate CDK8’s mechanistic role in CD4+ T helper cell differentiation, this work further supports a general role for CDK8 in control of inflammation and tolerance in both the innate and adaptive immune compartments.