G9a functions as a molecular scaffold for assembly of transcriptional coactivators on a subset of Glucocorticoid Receptor target genes

Abstract

Histone H3 lysine-9 methyltransferase G9a/EHMT2/KMT1C is a key corepressor of gene expression. However, activation of a limited number of genes by G9a (independent of its catalytic activity) has also been observed, although the precise molecular mechanisms are unknown. By using RNAi in combination with gene expression microarray analysis, we found that G9a functions as a positive and a negative transcriptional coregulator for discrete subsets of genes that are regulated by the hormone-activated Glucocorticoid Receptor (GR). G9a was recruited to GR-binding sites (but not to the gene body) of its target genes and interacted with GR, suggesting recruitment of G9a by GR. In contrast to its corepressor function, positive regulation of gene expression by G9a involved G9a-mediated enhanced recruitment of coactivators CARM1 and p300 to GR target genes. Further supporting a role for G9a as a molecular scaffold for its coactivator function, the G9a-specific methyltransferase inhibitor UNC0646 did not affect G9a coactivator function but selectively decreased G9a corepressor function for endogenous target genes. Overall, G9a functioned as a coactivator for hormone-activated genes and as a corepressor in support of hormone-induced gene repression, suggesting that the positive or negative actions of G9a are determined by the gene-specific regulatory environment and chromatin architecture. These findings indicate distinct mechanisms of G9a coactivator vs. corepressor functions in transcriptional regulation and provide insight into the molecular mechanisms of G9a coactivator function. Our results also suggest a physiological role of G9a in fine tuning the set of genes that respond to glucocorticoids.

Hic-5 is a transcription coregulator that acts before and/or after glucocorticoid receptor genome occupancy in a gene-selective manner

“…Second, Hic-5 depletion had dramatic effects on Dexregulated transcription of some genes, in which Hic-5 served as an all-or-none switch, so that Dex-induced transcriptional regulation by GR was completely dependent on the presence of Hic-5 for some genes and on the absence of Hic-5 for other genes. In contrast, depletion studies with many other coregulators have indicated that they serve as modulators rather than as all-or-none determinants of steroid hormone-regulated gene expression (2,(36)(37)(38)(39). Third, Hic-5 was able to function as a coactivator and as a corepressor.…”

“…Current evidence suggests that scores of coregulators cooperate in the transcriptional regulation of each gene, with each coregulator contributing a specific molecular function, e.g., remodeling of chromatin or transcription complex assembly/disassembly (1). Moreover, coregulators act gene specifically, using different activation and repression domains to perform different actions on different genes, as specified by the bound GR (2)(3)(4)(5)(6). Protein allostery and the combinatorial nature of transcriptional regulation are responsible for the gene specificity of receptor and coregulator function (7,8), but the specific steps facilitated by a coregulator in a given context are mostly unknown.…”

The expanding role of the Ehmt2/G9a complex in neurodevelopment

“…45 Ehmt2 has been shown to be essential for activation of several genes critical to early development including p21 46 and b-Globin. 47 Unlike transcriptional repression, Ehmt2's co-activator role does not require either the SET or Ank repeat domains, 6,46,48 suggesting that this coactivator role is methylation-independent. Furthermore, Ehmt2 is dependent upon a different pool of transcription factors, such as Runx2 49 and Nfe2, 47 to recruit it to sites of transcriptional activation.…”

“…Furthermore, Ehmt2 is dependent upon a different pool of transcription factors, such as Runx2 49 and Nfe2, 47 to recruit it to sites of transcriptional activation. Once recruited Ehmt2 in turn functions as a scaffold protein to enhance recruitment of further co-activators including Carm1, 45 p300, 48 the mediator complex 14 and DNA Pol II. 47 In contrast, Ehmt1 has also been recently implicated in transcriptional activation during adipogenesis independent of Ehmt2.…”

Human EHMT2/G9a activates p53 through methylation-independent mechanism

“…In this respect, G9a was recently reported as a transcriptional coactivator for Glucocorticoid Receptor (GR), by forming a molecular scaffold to recruit and facilitate binding of other co-activating proteins including GRIP1, CARM1 and p300 32,34 . Interestingly, the underlying mechanism of G9a function as a positive regulator is methylation-independent, because UNC0646 (G9a-specific methyltransferase inhibitor) can not abolish this feature 34 , 30 .…”