FoxA1 and glucocorticoid receptor crosstalk via histone H4K16 acetylation at a hormone regulated enhancer.

The forkhead transcription factor FoxA1 participates in many gene regulatory events with steroid hormone receptors, one example being the integrated mouse mammary tumor virus (MMTV) promoter. Its enhancer harbors several FoxA1 binding sites. FoxA1 promotes glucocorticoid receptor (GR)-DNA binding and transcription. Here we analyze the regulatory capacity of GR, FoxA1 and hormone in quantitative terms when reconstituted with the MMTV enhancer in Xenopus oocytes. By titrating each component we demonstrate that FoxA1 is required for hormone induction at low GR concentration and that FoxA1 is a potent enhancer of GR-induced transcription. Conversely, specific DNA binding of FoxA1 at low intranuclear concentration is highly responsive to minute levels of hormone-activated GR while increased FoxA1 concentration results in constitutive binding. When bound to DNA, FoxA1 induces DNase I hypersensitivity, this is accompanied by increased acetylation, specifically at histone H4K16. Expression of FoxA1 deletion mutants demonstrated its DNA binding domain to be sufficient for DNA binding in vivo. The C-terminal and N-terminal domains both contribute to chromatin remodeling while the latter is more important for GR mediated transcription. Thus FoxA1 is primarily responsible for the chromatin presetting while GR supports chromatin presetting at low hormone concentration and transcriptional induction at high hormone concentration.