Mediator kinase module and human tumorigenesis

Abstract

Mediator is a conserved multi-subunit signal processor through which regulatory informatiosn conveyed by gene-specific transcription factors is transduced to RNA Polymerase II (Pol II). In humans, MED13, MED12, CDK8 and Cyclin C (CycC) comprise a four-subunit "kinase" module that exists in variable association with a 26-subunit Mediator core. Genetic and biochemical studies have established the Mediator kinase module as a major ingress of developmental and oncogenic signaling through Mediator, and much of its function in signal-dependent gene regulation derives from its resident CDK8 kinase activity. For example, CDK8-targeted substrate phosphorylation impacts transcription factor half-life, Pol II activity and chromatin chemistry and functional status. Recent structural and biochemical studies have revealed a precise network of physical and functional subunit interactions required for proper kinase module activity. Accordingly, pathologic change in this activity through altered expression or mutation of constituent kinase module subunits can have profound consequences for altered signaling and tumor formation. Herein, we review the structural organization, biological function and oncogenic potential of the Mediator kinase module. We focus principally on tumor-associated alterations in kinase module subunits for which mechanistic relationships as opposed to strictly correlative associations are established. These considerations point to an emerging picture of the Mediator kinase module as an oncogenic unit, one in which pathogenic activation/deactivation through component change drives tumor formation through perturbation of signal-dependent gene regulation. It follows that therapeutic strategies to combat CDK8-driven tumors will involve targeted modulation of CDK8 activity or pharmacologic manipulation of dysregulated CDK8-dependent signaling pathways.

Keywords: Cancer; Mediator; RNA polymerase II transcription; development; signal transduction.

Figure 1

Mediator is a central integrator and processor of Pol II transcription. Mediator transduces regulatory information conveyed by signal-activated transcription factors to effect changes in gene expression programs that control diverse biological processes, including development, differentiation and homeostasis. To achieve this, Mediator functions at multiple steps in the Pol II transcription process. Structurally, it cooperates with cohesion to promote long-range chromatin interactions (via looping) by bridging enhancer-bound activators with the Pol II machinery. Functionally, it facilitates pre-initiation complex formation through chromatin reconfiguration and Pol II recruitment onto core promoters, regulates early initiation events linked to Pol II initiation and promoter escape and regulates Pol II pausing and/or pause release. Mediator also participates in co-transcriptional RNA processing (data not shown). (see colour version of this figure at www.informahealthcare.com/bmg).

Figure 2

Modular organization of human Mediator. The Mediator core (~1.5 mDa; 26 subunits) is comprised of head (11 subunits), middle (9 subunits) and tail (6 subunits). Also shown is the dissociable kinase module (~0.5 mDa; 4 subunits). The Mediator head module is the primary interface for Pol II, whereas transcription factors bind principally to the tail and kinase modules. (see colour version of this figure at www.informahealthcare.com/bmg).

Figure 3

Mediator kinase module subunit organization. (A) Hierarchical subunit organization within the kinase module. (B) Structure of H. sapiens CycC–CDK8 (Schneider et al., 2011) (Protein Data Bank accession number 3RGF). Cyclin C, blue; CDK8, gray. Targeted residues that lie within (W177, N181, D182 and Y238) and outside (W6 and E98) the groove are rendered yellow (Turunen et al., 2014). The MED12-binding site on CycC corresponds to the CycC surface groove as indicated. CDK8 surface residues potentially available for interaction with MED12 in the ternary MED12–CycC–CDK8 complex are rendered in red. (C) Representation of kinase module paralog interactions. Black lines represent permissive interactions and red lines represent mutual exclusivity. (see color version of this figure at www.informahealthcare.com/bmg).

Figure 4

Biological functions of the Mediator kinase module. Wheel diagram depicting biological functions in which the Mediator kinase module subunits are implicated. (see colour version of this figure at www.informahealthcare.com/bmg).

Figure 5

CDK8 regulates oncogenic Wnt/β-catenin signaling through two different mechanisms. 13q12-encoded CDK8, overexpressed through amplification-dependent means in a colorectal cancer subset, controls Wnt/β-catenin signaling directly as a β-catenin co-activator and indirectly as a suppressor of the β-catenin inhibitor E2F1. Both of these CDK8 regulatory functions require its kinase activity and both promote oncogenic Wnt/β-catenin signaling (Firestein et al., 2008; Morris et al., 2008). (see colour version of this figure at www.informahealthcare.com/bmg).

Figure 6

A conserved MacroH2A-CDK8 core regulatory axis controls melanoma and breast cancer progression. Left: During melanoma progression, mH2A expression levels decrease, leading to chromatin decondensation and upregulation of CDK8, which drives tumorigenesis as part of the Mediator kinase module (Kapoor et al., 2010). Right: In breast cancer, Skp2, through suppression of mH2A1, promotes CDK8 expression to control G2/M phase progression, ploidy and tumorigenesis through modulation of p27 levels. Mechanistically, CDK8 phosphorylates p27, which primes the latter for Skp2-dependent degradation. Thus, Skp2 and CDK8, both components of a common signaling axis, cooperate in the turnover of p27, yet another component of the same axis, thereby providing a positive feedback loop to amplify their regulatory influence over p27 and breast cancer progression (Xu et al., 2015). (see colour version of this figure at www.informahealthcare.com/bmg).

Figure 7

CycC is a haploinsufficient tumor suppressor that antagonizes oncogenic Notch1. Notch1 is a binary cell-fate determinant. Signal-regulated intramembrane proteolysis of Notch1 produces a transcriptionally active Notch intracellular domain (ICN1) that traffics to the nucleus and regulates gene expression programs involved in proliferation, differentiation and apoptosis. Left: In CycC-proficient (CycC^+/+) cells, CycC–CDK8/19 function as physiological suppressors of ICN1-dependent gene expression by phosphorylating and thereby priming ICN1 for degradation via the ubiquitin-proteosome pathway (Li et al., 2014b). Right: In CycC-deficient (CycC^+/− or CycC^−/−) cells, oncogenic ICN1 escapes proteolytic destruction resulting hyperactivated signaling that drives tumorigenesis (Li et al., 2014b). Note that proteolytic turnover of promoter-bound ICN1 shown here is based on prior findings (Fryer et al., 2004). The location of ICN1 turnover as described by Li et al. (2014b) has not been described. (see colour version of this figure at www.informahealthcare.com/bmg).

Figure 8

Tumor-associated exon 1/2 mutations in MED12 disrupt Mediator-associated CDK activity. Schematic diagram of CycC, CDK8, MED13 and MED12, with chromosomal location of each indicated. Pathogenic mutations and their approximate locations in MED12 are colored and annotated in the legend. The CycC/CDK8 binding interface on MED12, corresponding to its N-terminal 100 amino acids, is disrupted by exon 1/2 mutations, leading to loss of Mediator-associated CDK activity. Shown here is the predicted disposition of Mediator in myometrium and MED12 WT UL compared to MED12 exon 1/2 mutant UL (Turunen et al., 2014; Kampjarvi et al., 2014). FG, Lujan, Ohdo and prostate cancer associated mutations generally cluster in the middle region of MED12 and are therefore positionally distinct from exon 1/2 mutations. Nonetheless they may confer a similar defect (indicated by the dashed line), as FG and Lujan mutant MED12/Mediator complexes are known to suffer a gene-specific defect in the recruitment of Cyc-CDK8 (Zhou et al., 2012). (see colour version of this figure at www.informahealthcare.com/bmg).

Figure 9

Inhibitors and microRNAs that target the kinase module. Compounds and microRNAs (miR) that have been established to inhibit (blunted line) or activate (arrow) CDK8 are shown. Notably, miR-107 has been shown to both inhibit and promote CDK8 expression in a cancer cell-type specific manner. The question marks denote CDK8 activity-dependent subunit interfaces (CycC–CDK8 and MED12–CycC interfaces) that can putatively be targeted for CDK8 kinase inactivation. (see colour version of this figure at www.informahealthcare.com/bmg).

Figure 10

Mediator kinase module as an oncogenic unit. A dominant theme to emerge from studies of the kinase module in human cancers is loss or gain of kinase activity, which appears to function in a context-dependent manner to either promote or suppress tumorigenesis. Mechanistically, kinase activity change could derive from oncogenic alteration in any one of the kinase module subunits, particularly MED12, CycC and CDK8, all of which contribute directly to kinase activity. In this respect, the kinase module operates much like an oncogenic unit, one in which pathogenic activation/deactivation through component change drives tumor formation through perturbation of signal-dependent gene regulation. The large arrow represents CDK8 activity increase (green) or decrease (red). The specific cancer settings in which these changes occur are listed in the green or red boxes above or below the arrow. Some established CDK8 substrates are listed to the right of the arrow, and those identified to be effectors of oncogenic signaling are rendered in color (green: substrate phosphorylation suppresses tumorigenesis; red: substrate phosphorylation promotes tumorigenesis; black: impact of phosphorylation on tumorigenesis not yet established). (see color version of this figure at www.informahealthcare.com/bmg).