Modulation of parameters of glucocorticoid receptor-mediated gene repression

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What little is known about the mechanism(s) by which the EC50 and partial agonist activity are determined derives from our studies of GR-regulated gene induction (reviewed in Simons Jr., 2003, TIPS, 24, 253-259; Simons Jr., 2006, Current Topics in Medicinal Chemistry, 6, 271-285). However, the most commonly prescribed clinical use of glucocorticoids is in their capacity to repress gene induction, such as in the treatment of lymphomas by causing cell death and in the suppression of inflammatory responses. Furthermore, the mechanism of GR-regulated induction and repression is often different, with induction proceeding via GRs bound directly to DNA sequences called hormone response elements (HREs) while repression often involves GRs indirectly bound to DNA through some other DNA-bound factor, such as AP-1 or NF-kB. Finally, the EC50 of GR repression of gene expression is usually 10-fold lower than that for gene induction. Thus, at least some of the mechanistic details for GR-regulated induction and repression are different. Our studies of GR-regulated gene induction at physiological levels of steroid have documented that the EC50 and partial agonist activity for gene induction can be significantly altered simply by varying the concentration of a variety of transcription factors, such as the receptor itself, p160 coactivators, corepressors, Sur2, GMEB-1 and GMEB-2, and STAMP. As gene repression accounts for about half of all of the GR-mediated responses, it is clearly important to determine whether these various factors can similarly modulate the EC50 and partial agonist activity of GR-regulated repression. The approach of this study was to determine whether several factors (i.e., Ubc9, SRC-1, TIF2, and STAMP) known to modulate the EC50 and partial agonist activity of GR-regulated gene induction would be similarly active in gene repression by GRs. Ubc9 is a human homolog of the E2 ubiquitin-conjugating enzymes of yeast that can transfer a ubiquitin-like molecule, called small ubiquitin-like modifier-1 (SUMO-1), to proteins in vertebrate cells. However, Ubc9 also displays non-enzymatic effects on a variety of cellular actions including GR-mediated transcription (Kaul et al., 2002, J. Biol. Chem., 277, 12541-12549; Cho et al., 2005, Mol. Endo., 19, 290-311). SRC-1 and TIF2 are two well-know p160 coactivators for gene induction by GRs. STAMP is a novel protein that was cloned by its ability to augment the modulatory activity of the coactivator TIF2 in both GR-mediated induction and GR-regulated repression of target genes (He and Simons Jr., 2007, Mol. Cell. Biol., 27, 1467-1485). We find that transiently transfected Ubc9, SRC-1, and TIF2 cause modulation of the EC50 and partial agonist activity of GR-regulated repression of a co-transfected reporter (AP1Luc) by the endogenous GRs of U2OS.rGR cells, which are human osteosarcoma cells containing stably transfected GRs. We conclude that at least some of the molecular mechanisms responsible for altering the EC50 and partial agonist activity in gene induction by GRs are shared in gene repression. On going studies with the modulatory activity of STAMP TIF2 with exogenous and endogenous genes will determine the generality of our current conclusions. These studies demonstrate that two important transcriptional properties (EC50 and partial agonist activity) are similarly modified by several factors for both of the major actions of glucocorticoid steroids: gene induction and gene repression. These modulatory factors permit a continuum of responses and constitute new therapeutic targets for differential control of gene expression by steroid hormones during development, differentiation, homeostasis, and endocrine therapies. These combined findings contribute to our long-term goal of defining the action of steroid hormones at a molecular level and of understanding their role in human physiology.