Mediator Kinase Phosphorylation of STAT1 S727 Promotes Growth of Neoplasms With JAK-STAT Activation

Abstract

Constitutive JAK-STAT signaling drives the proliferation of most myeloproliferative neoplasms (MPN) and a subset of acute myeloid leukemia (AML), but persistence emerges with chronic exposure to JAK inhibitors. MPN and post-MPN AML are dependent on tyrosine phosphorylation of STATs, but the role of serine STAT1 phosphorylation remains unclear. We previously demonstrated that Mediator kinase inhibitor cortistatin A (CA) reduced proliferation of JAK2-mutant AML in vitro and in vivo and also suppressed CDK8-dependent phosphorylation of STAT1 at serine 727. Here we report that phosphorylation of STAT1 S727 promotes the proliferation of AML cells with JAK-STAT pathway activation. Inhibition of serine phosphorylation by CA promotes growth arrest and differentiation, inhibits colony formation in MPN patient samples and reduces allele burden in MPN mouse models. These results reveal that STAT1 pS727 regulates growth and differentiation in JAK-STAT activated neoplasms and suggest that Mediator kinase inhibition represents a therapeutic strategy to regulate JAK-STAT signaling.

Regulatory functions of the Mediator kinases CDK8 and CDK19

“…Based upon these observations, a reasonable expectation is that disruption of CDK8 and/or CDK19 function would markedly impact global gene expression patterns. Contrary to these expectations, knockdown of CDK8 or CDK19 protein levels [1,17] or inhibition of CDK8 and CDK19 kinase activity [46,48,49] does not globally affect steady-state mRNA levels; rather, only subsets of genes are affected that vary with context (e.g. hypoxia) or cell type.…”

“…Sequence-specific, DNA-binding TFs represent a major class of proteins that are targeted by CDK8 and CDK19 [62], and the cell-type and context-specific functions for Mediator kinases could reflect CDK8/19-dependent TF phosphorylation. In a few well-studied cases, CDK8 and/or CDK19-dependent TF phosphorylation has been shown to alter TF activity [45,46,99]. Future experiments will no doubt provide additional insights, but it is notable that even two-fold changes in the level of lineage-specifying TFs can alter cell state and induce differentiation [131,132].…”

“…A growing number of studies directly link CDK8 and CDK8 module subunits to specific types of cancer (reviewed in [121]) and the development of selective inhibitors of CDK8 and CDK19 has helped establish Mediator kinases as therapeutic targets [48,49,150]. Studies that have used a variety of methods to disrupt CDK8 and/or CDK19 function have generally shown that kinase inhibition does not markedly affect normal cellular function [46,48,73]. Instead, Mediator kinase activity appears more critical for transcriptional "reprogramming" in response to developmental or environmental cues (see above).…”

“…Super-enhancers represent clusters of enhancers that form interconnected hubs with other enhancer and promoter sequences to support high levels of gene expression [44]. The Mediator kinases CDK8 and CDK19 regulate TF function through phosphorylation [45,46]; in addition, Mediator appears to be required for expression of most, if not all, pol II transcripts in mammalian genomes, and ChIP-Seq experiments indicate that the CDK8 module co-localizes with Mediator genome-wide [2,3], including at superenhancers [41,47,48]. Based upon these observations, a reasonable expectation is that disruption of CDK8 and/or CDK19 function would markedly impact global gene expression patterns.…”

“…CDK8 occupies enhancer elements genome-wide [48], and CDK8 (and/or CDK19) phosphorylates TFs that bind enhancer elements [62]. TF phosphorylation by Mediator kinases has been shown to alter TF activity [45,46]; TF activity, in turn, correlates with expression of bidirectional, enhancer-associated eRNAs [54,55,57]. The expression of eRNAs also correlates with formation of enhancer-promoter loops [63], and may function, at least in part, through direct interactions with the CDK8-Mediator complex [64].…”