Glucocorticoid-induced DNA demethylation and gene memory during development

Glucocorticoid hormones were found to regulate DNA demethylation within a k ey enhancer of the rat liver-specific tyrosine aminotransferase (Tat) gene, Genomic footprinting analysis shows that the glucocorticoid receptor uses local DNA demethylation as one of several steps to recruit transcription fa ctors in hepatoma tells. Demethylation occurs within 2-3 days following rap id (<1 h) chromatin remodeling and recruitment of a first transcription fac tor, HNF-3, Upon demethylation, two additional transcription factors are re cruited when chromatin is remodeled. In contrast to chromatin remodeling, t he demethylation is stable following hormone withdrawal. As a stronger subs equent glucocorticoid response is observed, demethylation appears to provid e memory of the first stimulation, During development, this demethylation o ccurs before birth, at a stage where the Tat gene is not yet inducible, and it could thus prepare the enhancer for subsequent stimulation by hypoglyce mia at birth, In vitro cultures of fetal hepatocytes recapitulate the regul ation analyzed in hepatoma cells. Therefore, demethylation appears to contr ibute to the fine-tuning of the enhancer and to the memorization of a regul atory event during development.