Regulation of human pituitary growth hormone gene (hGH1) expression by energy homeostasis
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Human (h) growth hormone (GH) levels decline rapidly in response to excess caloric intake before there is any evidence of obesity. In this thesis, the mechanism underlying this response was examined by manipulating levels of caloric intake and physical activity at the levels of gene expression and chromosomal structure. Transgenic mice containing the intact hGH locus were used as a model system. Briefly, the findings are: (I) High caloric intake (high fat diet) for three days resulted in hyperinsulinemia and a decrease in both hGH synthesis and secretion. (II) Incorporation of physical activity (swim) muted the effects of excess caloric intake on insulin levels as well as hGH production. (III) Human GH RNA accumulation was shown for the first time to be negatively regulated by insulin in pituitary cultures, and (IV) an enhancer box (E-box) DNA element was implicated in this response. (V) Induction of the E-box associated transcription factor HIF-1α with insulin significantly decreased hGH RNA levels, and was accompanied by recruitment of HIF-1α to the hGH gene (hGH) promoter in situ. (VI) Both a reduction in HIF-1α synthesis and HIF-1 DNA binding blunted the negative effect of insulin on hGH RNA levels. (VII) The hGH response to insulin was associated with a decrease in histone H3/H4 hyperacetylation in the proximal hGH promoter region. The same pattern of chromatin remodelling was observed in pituitary cells in vivo in response to excess caloric intake. (IX) Increased recruitment of nuclear receptor co-repressor and decreased association of RNA polymerase II were also observed. Collectively, these effects are consistent with reduced hGH promoter function. (X) This reduction by excess caloric intake was also consistent with changes in the three dimensional-structure of the hGH locus including detected loss of physical interaction between hGH enhancer and promoter regions. (XI) By contrast, physical activity combined with the high caloric intake preserved the chromosomal structure of the hGH locus. These observations are discussed in relation to a physiological requirement for rapid control of hGH levels in response to energy homeostasis, as well as the molecular basis governing this process.