Glucocorticoid Receptor Mediated CRH Gene Repression

The hypothalamic pituitary adrenal (HPA) axis is an important neuroendocrine system which mediates the mammalian stress response. Dysregulation of the HPA axis, often due to failed glucocorticoid receptor (GR) mediated negative feedback, is tightly associated with many psychiatric disorders including depression. Glucocorticoids attenuate the activated HPA axis partly by suppressing Corticotropin releasing hormone (CRH) gene (crh) expression. Though regulation of crh is a critical component of the HPA axis, the molecular mechanisms are poorly understood. In this study, we sought to investigate the molecular mechanism by which GR regulates crh expression. The results indicate that Histone deacetylase (HDAC) 1 and methyl CpG binding protein 2 (MeCP2) interact with GR forming a putative complex and that this interaction is GR-ligand, Dexamethasone (Dex), dependent. Furthermore, results indicate that DNA methyltransferase (DnMT) 3b also interacts with GR and this interaction is Dex dependent. Next we tested the role of MeCP2 and DNA methylation in GR mediated crh repression. The results suggest that MeCP2 is necessary for maintenance of basal crh levels. Decreased levels of MeCP2 are associated with the increased crh expression and Dex fails to repress crh in the absence of MeCP2. Then we examined the role of DNA methylation in crh regulation. The data suggest that Dex increases promoter methylation at specific sites, and that inhibition of methylation at these sites by 5-Aza-2-deoxycytidine (5-AzaDC) is associated with increased expression of crh. The results also suggest that inhibition of DNA methylation abrogates Dex mediated repression of crh. In fact, Dex activates crh in the face of reduced promoter methylation. While 5-AzaDC and Dex do not alter the protein levels of GR and MeCP2, inhibition of DNA methylation decreases the ability of GR and MeCP2 to occupy the crh promoter. Taken together, the data indicate that Dex mediated repression of crh is mediated through the formation of a putative complex and requires MeCP2 and site specific promoter methylation. The findings from this study add novel aspects to the molecular mechanism of GR mediated crh repression. This will lead to better treatment and management of depression which affects nearly 10% of the US population.