Many of the growth-promoting effects of GH are mediated by insulin-like growth factor-I (IGF-I), a highly conserved, 70-residue basic peptide. Previous studies have demonstrated that GH rapidly stimulates IGF-I expression in vivo, and our laboratory has identified a GH-regulated alteration in chromatin configuration, manifested as a hormonally induced deoxyribonuclease-I (DNase-I)-hypersensitive site in the second IGF-I intron. In the current study, we have used in vivo DNase-I footprinting to map this hormonally responsive chromatin domain to an approximately 350-nucleotide region and have identified DNA-protein interactions within the hypersensitive site by in vitro gel mobility shift experiments and DNase-I footprinting studies. DNA-protein binding was localized to two adjacent segments of 32 and 48 nucleotides. In 1 of these regions, protein-DNA contacts were also detected in vivo on guanine residues by dimethylsulfate footprinting. DNA-binding activity was present in GH-deficient rats, but was not modified by hormone treatment. Our results define a rapid and reversible genomic alteration in response to GH in a GH-regulated gene and delineate a target within chromatin for GH action.
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