UC Berkeley

Glucocorticoids increase hepatic gluconeogenesis during fasting and the times of stress and have many beneficial pharmacological functions such as reducing inflammation. Chronic glucocorticoid exposure through continued stress or pharmacological interventions can have significant side effects such as insulin resistance and hyperglycemia. Glucocorticoids relay their function through its intracellular glucocorticoid receptor (GR). Upon binding to glucocorticoids, GR binds to genomic glucocorticoid response elements to regulate transcription of GR primary target genes. To regulate the rate of transcription, GR needs to recruit transcriptional coregulators. Ehmt2 is a GR coregulator that participates in the regulation of a subset of GR target genes in cell culture studies. However, its role in metabolic functions of glucocorticoids in vivo is unknown. Notably, glucocorticoids have been shown to increase hepatic production of sphingosine-1-phosphate (S1P), a signaling molecule that acts through its membrane G protein coupled receptors, S1P receptors (S1PRs). The aim of this dissertation work is to explore the role of Ehmt2 and hepatic S1P signaling in the regulation of glucose homeostasis.

Chapter 1 reviews efforts to characterize the role of extracellular S1P-induced signaling in chronic glucocorticoid exposure enhanced hepatic gluconeogenesis. By using antagonists specific to S1PR1-3 which are highly expressed in the liver, I identified that S1PR2 plays a role in chronic glucocorticoid treatment induced glucose intolerance. S1PR2 knockdown in mouse liver resulted in decreased hepatic gluconeogenesis, improved glucose tolerance, and reduced recruitment of GR to the genomic regions of key gluconeogenic genes. RNA sequencing revealed RAR-related orphan receptor c (Rorc) as a downstream effector of S1PR2 signaling whose expression in glucocorticoid-treated mouse liver was further reduced by S1PR2 knockdown. Through in vitro and in vivo studies, it was determined that Rorc plays an important role in chronic glucocorticoid treatment promoted hepatic gluconeogenesis. Rorc augments GR recruitment to the GREs of gluconeogenic genes, which elevates their transcription.

Chapter 2 covers the investigation of the physiological functions of the GR coregulator, EHMT2. EHMT2 is a histone methyltransferase and catalyzes the mono and dimethylation of histone 3 lysine 9, repressive epigenetic marks. This is the reason that EHMT2 can serve as a transcriptional corepressor. EHMT2, however, also can serve as a coactivator for GR. I create EHMT2 mutant mice that lacks coactivation function of EHMT2 with its corepression function remains intact. These approaches discover novel physiological function of EHMT2 in glucocorticoid-regulated insulin sensitivity.