Growth hormone regulates the expression of hepatocyte nuclear factor-3 gamma and other liver-enriched transcription factors in the bovine liver

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Abstract
Growth hormone (GH) regulates the expression of many genes in the liver, and for some genes this regulation may be mediated through liver-enriched transcription factors (LETFs). As part of the long-term goal to investigate the role of LETFs in GH regulation of gene expression in the liver, in this study we determined the effect of GH administration on the expression of 10 LETFs, including hepatocyte nuclear factor (HNF)-1α, HNF-1β, HNF-3α, HNF-3β, HNF-3γ, HNF-4α, HNF-6, CCAAT/enhancer-binding protein (C/EBP) α, C/EBPβ, and albumin D-element binding protein (DBP) in the bovine liver. Eighteen non-lactating and non-pregnant Angus cows were assigned randomly to three groups (n=6 per group) and each cow received a single intramuscular injection of 500 mg slow-release recombinant bovine GH. Liver biopsy samples were taken from group 1 cows 6 h after GH administration, from group 2 cows 24 h after GH administration, and from group 3 cows 1 week after GH administration. Liver biopsies were also collected from group 3 cows 1 day before GH administration, serving as pre-GH controls. The LETF mRNAs in these liver samples were quantified using ribonuclease protection assays with probes generated from bovine LETF cDNAs cloned by standard reverse transcription–polymerase chain reaction. The levels of HNF-3γ and HNF-6 mRNAs were higher (P< 0.05) in the cows 24 h and 1 week after GH administration than in the untreated cows or the cows 6 h after GH administration. The levels of HNF-4α mRNA were higher (P< 0.05) in the cows 1 week after GH administration than in the other three groups of cows. The levels of C/EBPα mRNA were higher (P< 0.05) in the cows 24 h after GH administration than in the untreated cows or the cows 6 h after GH administration. The levels of HNF-3α mRNA were higher (P< 0.05) in the cows 6 h after GH administration but were lower (P< 0.05) in the cows 24 h or 1 week after GH administration compared with those in the untreated cows. The levels of DBP mRNA were higher (P< 0.05) in the cows 6 h after GH administration but were lower (P< 0.05) in the cows 24 h after GH administration compared with those in the untreated cows. The levels of HNF-1α, HNF-3α, and C/EBPβ mRNAs were not different (P>0.05) between groups. The expression of HNF-1β mRNA was not detectable. Thus, the expression of six LETFs including HNF-3γ , HNF-3β, HNF-4α, HNF-6, C/EBPα, and DBP mRNAs in the bovine liver is regulated by GH, and these six LETFs may play a role in mediating GH regulation of gene expression in the liver. Among the 10 LETFs, the response of HNF-3γ to GH is most significant. Cloning and sequencing the promoter region of this gene revealed multiple putative binding elements for signal transducers and activators of transcription 5 (STAT5), suggesting that GH regulation of HNF-3γ expression in the liver may be mediated through direct binding of STAT5 to the HNF-3γ promoter.

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