Growth hormone-activated STAT5 may indirectly stimulate IGF-I gene transcription through HNF-3{gamma}.
IGF-I is abundantly expressed in the liver under the stimulation of GH. We showed previously that expression of hepatocyte nuclear factor (HNF)-3gamma, a liver-enriched transcription factor, was strongly stimulated by GH in bovine liver. In this study, we determined whether GH-increased HNF-3gamma might contribute to GH stimulation of IGF-I gene expression in bovine liver and the underlying mechanism. A sequence analysis of the bovine IGF-I promoter revealed three putative HNF-3 binding sites, which all appear to be conserved in mammals. Chromatin immunoprecipitation assays showed that GH injection increased binding of HNF-3gamma to the IGF-I promoter in bovine liver. Gel-shift assays indicated that one of the three putative HNF-3 binding sites, HNF-3 binding site 1, bound to the HNF-3gamma protein from bovine liver with high affinity. Cotransfection analyses demonstrated that this HNF-3 binding site was essential for the transcriptional response of the IGF-I promoter to HNF-3gamma in CHO cells and to GH in primary mouse hepatocytes. Using similar approaches, we found that GH increased binding of the signal transducer and activator of transcription 5 (STAT5) to the HNF-3gamma promoter in bovine liver, that this binding occurred at a conserved STAT5 binding site, and that this STAT5 binding site was necessary for the HNF-3gamma promoter to respond to GH. Taken together, these results suggest that in addition to direct action, GH-activated STAT5 may also indirectly stimulate IGF-I gene transcription in the liver by directly enhancing the expression of the HNF-3gamma gene.
Eleswarapu
Satyanarayana
S
Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg, 24061, USA.
Ge
Xiaomei
X
Wang
Ying
Y
Yu
Jie
J
Jiang
Honglin
H
D000818:Animals; D001665:Binding Sites; D016466:CHO Cells; D002417:Cattle; D002478:Cells, Cultured; D047369:Chromatin Immunoprecipitation; D006224:Cricetinae; D003412:Cricetulus; D024202:Electrophoretic Mobility Shift Assay; D005260:Female; D005786:Gene Expression Regulation; D013006:Growth Hormone / Q000494:pharmacology*; D051543:Hepatocyte Nuclear Factor 3-gamma / Q000378:metabolism*; D007334:Insulin-Like Growth Factor I / Q000235:genetics*; D051379:Mice; D016133:Polymerase Chain Reaction; D011401:Promoter Regions, Genetic / Q000235:genetics; D011485:Protein Binding / Q000187:drug effects; D050799:STAT5 Transcription Factor / Q000378:metabolism*
D050799:STAT5 Transcription Factor; D051543:Hepatocyte Nuclear Factor 3-gamma; D007334:Insulin-Like Growth Factor I; D013006:Growth Hormone