Glucocorticoids (GC) are powerful metabolic hormones with anti-inflammatory effects. Exogenous GC are in wide therapeutic use but often cardio-metabolic side effects are limiting and resistance may develop. GC act via a nuclear hormone receptor (GR), which is a ligand activated transcription factor. GR binds DNA at sites distant from target gene promoters such as enhancers. Macrophages are ubiquitous innate immune cells that are major targets of GC and have roles in normal development and homeostasis. GC also antagonise the major macrophage growth factor, Csf1.
We have previously generated genome wide expression and GR-DNA binding data in primary mouse and human macrophages responding to 100 nM dexamethasone. We selected a locus with a highly conserved GC response and GR binding pattern, Fkbp5, for further study in mouse bone marrow derived macrophages. Fkbp5 is a GR co-chaperone that is strongly induced by GC 2 h after simulation. Using DNA fluorescence in-situ hybridization at the Fkbp5 locus there was rapid (<5 min) and persistent (>5 days) chromatin decondensation after treatment with dexamethasone. By contrast a locus with similar expression kinetics, but no local GR binding, Tmod1, decondensed in parallel with its expression response. Decondensation at Tmod1 was abolished by using α-amanitin to block transcription, but the decondensation at Fkbp5 was not prevented.
In summary in primary macrophages GC induced a rapid, transcription independent, persistent change to higher order chromatin structure, at a locus involved in feedback control of the GC response in both mice and humans. It is not clear how the chromatin decondensation we measured fits with current models of enhancer activity, which require enhancers to come closer to promoters. Further, long range GR-DNA driven chromatin dynamics may be a novel mechanism involved in transcriptional regulation by GC with potential consequent immunological and metabolic effects.