Glucocorticoid Receptor Activation of the IκBα Promoter within Chromatin

Abstract

The glucocorticoid receptor (GR) is a ligand-activated transcription factor that induces expression of many genes. The GR has been useful for understanding how chromatin structure regulates steroid-induced transcription in model systems. However, the effect of glucocorticoids on chromatin structure has been examined on few endogenous mammalian promoters. We investigated the effect of glucocorticoids on the in vivo chromatin structure of the glucocorticoid-responsive IκBα gene promoter, the inhibitor of the ubiquitous transcription factor, nuclear factor kappa B (NFκB). Glucocorticoids inhibit NFκB activity in some tissues by elevating the levels of IκBα. We found that glucocorticoids activated the IκBα promoter in human T47D/A1-2 cells containing the GR. We then investigated the chromatin structure of the IκBα promoter in the absence and presence of glucocorticoids with the use of micrococcal nuclease, restriction enzyme, and deoxyribonuclease (DNaseI) analyses. In untreated cells, the promoter assembles into regularly positioned nucleosomes, and glucocorticoid treatment did not alter nucleosomal position. Restriction enzyme accessiblity studies indicated that the IκBα promoter is assembled as phased nucleosomes that adopt an “open” chromatin architecture in the absence of hormone. However, glucocorticoids may be required for transcription factor binding, because DNaseI footprinting studies suggested that regulatory factors bind to the promoter upon glucocorticoid treatment.