Defining the epigenetic actions of growth hormone: acute chromatin changes accompany GH-activated gene transcription.

Many of the long-term physiological effects of GH require hormone-mediated changes in gene expression. The transcription factor signal transducer and activator of transcription 5b (Stat5b) plays a critical role in the actions of GH on growth and metabolism by regulating a large number of GH-dependent genes by incompletely understood mechanisms. Here we have assessed the impact of GH-initiated and Stat5b-mediated signaling on the chromatin landscape of hormone-regulated genes in the liver of pituitary-deficient young adult male rats. In the absence of GH there was minimal ongoing transcription at the Socs2, Cish, Igfals, and Spi 2.1 promoters, minimal occupancy of Stat5b at proximal promoter sites, and relatively closed chromatin, as evidenced by low levels of core histone acetylation. In contrast, transcriptionally silent Igf1 promoter 1 appeared poised to be activated, based on binding of coactivators p300 and Med1/Trap220, high levels of histone acetylation, and the presence of RNA polymerase II. GH treatment led to a 8- to 20-fold rise in transcriptional activity of all five genes within 30-60 min and was accompanied by binding of Stat5b to the proximal Socs2, Cish, Igfals, and Spi 2.1 promoters and to seven distal Igf1 Stat5b elements, by enhanced histone acetylation at all five promoters, by recruitment of RNA polymerase II to the Socs2, Cish, Igfals, and Spi 2.1 promoters, and by loss of the transcriptional repressor Bcl6 from Socs2, Cish, and Igfals Stat5b sites, but not from two Igf1 Stat5b domains. We conclude that GH actions induce rapid and dramatic changes in hepatic chromatin at target promoters and propose that the chromatin signature of Igf1 differs from other GH-and Stat5b-dependent genes.