Involvement of hepatocyte nuclear factor-4 in the expression of the growth hormone receptor 1A messenger ribonucleic acid in bovine liver.

The GH receptor 1A mRNA (GHR 1A mRNA) is one of the major GHR mRNA variants that differ in the 5'-untranslated region. The GHR 1A mRNA is unique because it is exclusively expressed in liver. The objective of the present study was to understand the mechanism for the liver-specific expression of the GHR 1A mRNA in the bovine. Twenty-six kilobases of 5'-flanking region of the bovine GHR gene was cloned and sequenced. The first exon (exon 1A) that corresponded to the 5'-untranslated region of the GHR 1A mRNA was 15,250 bp upstream from exon 2 in the GHR gene. The major transcription start site for the GHR 1A mRNA was 19 bp downstream from a putative TATA box. Transient transfection analyses of the 5'-flanking region of exon 1A in liver cell lines vs. nonliver cell lines did not reveal a positively regulatory region responsible for the liver-specific expression of the GHR 1A mRNA perhaps because the liver cell lines do not recapitulate the in vivo hepatic environment. A putative regulatory region was then found by deoxyribonuclease I footprinting analyses of the proximal 5'-flanking region of exon 1A with nuclear extracts from bovine liver tissue. This regulatory region contained a putative binding site for the liver-enriched transcription factor hepatocyte nuclear factor-4 (HNF-4). Binding of HNF-4 in bovine liver to this putative HNF-4 binding site was confirmed by electrophoretic mobility shift assays. Overexpression of HNF-4 enhanced the transcriptional activity of the 5'-proximal region of exon 1A in various cell lines. Mutation of the HNF-4 binding site abolished the transactivation. In addition, the HNF-4 mRNA was found to be primarily expressed in liver and absent in most nonhepatic tissues in the bovine. Collectively, these observations suggest that the liver-enriched transcription factor HNF-4 plays a role in the expression of GHR 1A mRNA in bovine liver.

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