Thursday, April 25, 2024
4/25/2024
Chronic Glucocorticoids (GC) exposure has long been associated with metabolic disorders, including insulin resistance. GC convey their signals through an intracellular glucocorticoid receptor (GR), a transcription factor that binds to genomic glucocorticoid response elements and recruits specific transcriptional coregulators to modulate the transcription of its target genes. Ehmt2 (a.k.a. G9a) is a GR coregulator that acts as a coactivator or corepressor. Our preliminary studies found that reducing Ehmt2 in liver exacerbated GC-induced insulin resistance. Intriguingly, this is caused by the coactivation but corepression function of Ehmt2, because mice carrying a mutation at the lysine 182 automethylation site of Ehmt2 (Ehmt2K182R/K182R mice) which abolishes the coactivation but not corepression function of Ehmt2, also had exacerbated GC-induced insulin resistance. RNA sequencing experiment identified Dusp4 (a.k.a. MKP-2) as an Ehmt2 coactivation dependent GR-activated gene, which when overexpressed in liver, attenuated GC-induced insulin resistance. Thus, we hypothesize a novel GR-Ehmt2-Dusp4 axis that counteracts GR-induced insulin resistant genes to control the extent of insulin resistance. In Aim 1, we will examine the effect of losing the Ehmt2 coactivation on the ability of GC to modulate insulin signaling. Hyperinsulinemic-euglycemic clamp will be used to examine Ehmt2’s role in regulating hepatic glucose production and peripheral glucose utilization. We will also test whether increasing Ehmt2 coactivation capacity by elevating its automethylation reverses GC-induced insulin resistance. In Aim 2, we will reduce hepatic Dusp4 expression to further validate its role in GC-induced insulin resistance. We found that MAP kinase ERK1/2 activity was increased in GC-treated Ehmt2K182R/K182R mouse liver. As Dusp4 is a MAP Kinase phosphatase, we will test whether Dusp4 is responsible for GC-induced ERK1/2 in Ehmt2K182R/K182R mouse liver and whether ERK1/2 is a target of Dusp4 in the regulation of GC-induced insulin resistance. Notably, Ehmt2 forms a heterodimer with Ehmt1 (a.k.a. GLP) and automethylation of Ehmt2 and Ehmt1 create a docking site for a transcriptional coactivator Cbx3 (a.k.a. HP1g). Our preliminary study found that reducing Ehmt1 expression had worsened GC-induced insulin resistance (similar to hepatic Ehmt2 knockdown and Ehmt2K182R/K182R mice). In Aim 3, we will examine whether automethylation of Ehmt1 results in similar phenotypes with hmt2K182R/K182R mice. We will also investigate whether hepatic Cbx3 knockdown exacerbates GC-induced insulin resistance. Finally, we will analyze the mechanism underlying Dusp4 gene transcription by GR and Ehmt2-Ehmt1-Cbx3 coactivator complex. The dogma for GC-induced insulin resistance is: GR activates genes promoting insulin resistance and represses genes promoting insulin sensitivity. In this proposal, we provide a revolutionary concept in which the extent of GC-induced insulin resistance is controlled by the balance of GR-activated genes that promoting insulin sensitivity or insulin resistance. Exploring the mechanism underlying this novel GR-Ehmt2-Dusp4 axis shall significantly impact on our understanding of the metabolic functions of GC.
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