Growth hormone stimulates transcription of the fibroblast growth factor 21 gene in the liver through the signal transducer and activator of transcription 5.

Fibroblast growth factor 21 (FGF21) is a recently discovered metabolic regulator. Interestingly, FGF21 is also known to inhibit Janus kinase 2 (JAK2)-signal transducer and activator of transcription 5 (STAT5) signaling from the GH receptor in the liver, where FGF21 mRNA is predominantly expressed. In this study, we tested the hypothesis that FGF21 gene expression in the liver is controlled by GH through STAT5. We found that GH injection to cattle increased FGF21 mRNA expression in the liver. Mapped by a 5'-rapid amplification of cDNA ends assay, transcription of the FGF21 gene in the bovine liver was mainly initiated from a nucleotide 24 bp downstream of a TATA box. The bovine FGF21 promoter contains three putative STAT5-binding sites. EMSA confirmed the ability of them to bind to liver STAT5 protein from GH-injected cattle. Chromatin immunoprecipitation assays demonstrated that GH administration increased the binding of STAT5 to the FGF21 promoter in the liver. Cotransfection analyses showed that GH induced reporter gene expression from the FGF21 promoter in a STAT5-dependent manner. GH also stimulated FGF21 mRNA expression in cultured mouse hepatocytes. These data together indicate that GH directly stimulates FGF21 gene transcription in the liver, at least in part, through STAT5. This finding, together with the fact that FGF21 inhibits GH-induced JAK2-STAT5 signaling in the liver, suggests a novel negative feedback loop that prevents excessive JAK2-STAT5 signaling from the GH receptor in the liver.

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.

Reduced serum insulin-like growth factor (IGF) I is associated with reduced liver IGF-I mRNA and liver growth hormone receptor mRNA in food-deprived cattle.

Nutritional deprivation decreases blood insulin-like growth factor I (IGF-I) concentrations in a variety of species. In this study, we explored the underlying mechanism by determining the effects of food deprivation on the levels of total IGF-I mRNA and total growth hormone receptor (GHR) mRNA, as well as the levels of individual IGF-I mRNA variants and GHR mRNA variants in the liver of steers. Food deprivation for nearly 3 d decreased the levels of serum IGF-I by 63% (P < 0.01), and this decrease was associated with a 75% decrease (P < 0.01) in total IGF-I mRNA in the liver. The food deprivation-induced decrease in liver total IGF-I mRNA was associated with an equivalent decrease in the levels of both class 1 and class 2 IGF-I mRNA. In addition to IGF-I mRNA, food deprivation also decreased the levels of total GHR mRNA in the liver (P < 0.05), and this decrease was associated with a decrease in the liver expression of GHR mRNA variants 1C3 (P < 0.05) and 1A (P = 0.08). Food deprivation did not affect the levels of two other major GHR mRNA variants, 1B and 1C2, in the liver. These results demonstrate that the food deprivation-induced decrease in circulating IGF-I in steers is associated with a coordinate decrease in the expression of different IGF-I mRNA variants and a specific decrease in the expression of GHR mRNA variants 1C3 and 1A in the liver.