CDK8 fine-tunes IL-6 transcriptional activities by limiting STAT3 resident time at the gene loci

Abstract

Cytokines are highly pleiotropic ligands that critically contribute to a balanced immune response. We have an incomplete understanding of how cytokines elicit their functional pleiotropy, which has limited their therapeutic use. Here, using Interleukin-6 (IL-6) as a model system, we have performed detailed phosphoproteomic and transcriptomic studies in human Th-1 cells to address the molecular bases defining cytokine functional pleiotropy. We have identified CDK8 as a new negative regulator of STAT3 transcriptional activities that contributes to the diversification of IL-6 responses. We found that CDK8 is a major regulator of the IL-6 phosphoproteome and interacts with STAT3 in the nucleus upon IL-6 stimulation. Inhibition of CDK8 activity, using specific small molecules inhibitors, reduced the IL-6-induced phosphoproteome by 23% in Th-1 cells, including STAT3 Ser727 phosphorylation. This, in turn, resulted in retention of tyrosine phosphorylated STAT3 in the nucleus, which increased the binding of STAT3 to target DNA sites in the genome with a concomitant increase in STAT3 mediated transcriptional activity. Importantly, inhibition of CDK8 activity under Th-17 polarizing conditions resulted in an enhancement of Th-17 differentiation. Our results support a model where CDK8 regulates STAT3 transcriptional processivity via modulation of its gene loci resident time, critically contributing to tuning STAT3 mediated responses.