IGF1 Growth Hormone Response Element Undergoes Developmentally Regulated Demethylation Independent of STA5b

Abstract

The insulin‐like growth factor 1 (IGF1)/growth hormone (GH) axis, a major factor in growth, development and metabolism, is significantly affected by intrauterine growth retardation. Some of these alterations may lead to permanent pathological programming of the IGF axis. Prenatally, IGF1 signaling is GH independent, while postnatally, it is partly or fully GH dependent. GH regulates IGF1 via binding of the intermediate transcription factor, STAT5b, to a GH response element (GHRE) in the hepatic IGF1 gene. During adolescence, the rodent hepatic GHRE converts from closed to open chromatin and remains open throughout life. However it is not known when epigenetic events first begin to occur that prepare the chromatin for opening. CpG methylation and demethylation is often associated with closed and open chromatin respectively. We therefore sought to follow changes in DNA methyaltion in the GHRE as a possible indicator of transition to open chromatin. The GHRE was subjected to Na‐bisulfite sequencing at various time points in mouse development in wild‐type and STA5b knockout mice. The GHRE was also fused to a CG‐free reporter to test the effect of methylation on enhancer activity. The GHRE undergoes 3 stages of gradual DNA demethylation declining from a high of 80% methylation in embryonic stem (ES) cells to <30% by 5 weeks of life and remains hypomethyated for 2 yrs. Demethylation of the mouse GHRE is independent of STAT5b and is required for GHRE function in vitro.