Involvement of STAT5 (signal transducer and activator of transcription 5) and HNF-4 (hepatocyte nuclear factor 4) in the transcriptional control of the hnf6 gene by growth hormone

HNF-6 is a tissue-restricted transcription factor that participates in the regulation of several genes in liver. We reported earlier that in adult rat s, HNF-6 mRNA concentration in liver drops to almost undetectable levels af ter hypophysectomy and returns to normal after 1 week of GH treatment. We n ow show that this results from a rapid effect of GH, and we characterize it s molecular mechanism. In hypophysectomized rats, HNF-6 mRNAs increased wit hin 1 h after a single injection of GH. The same GH-dependent induction was reproduced on isolated hepatocytes. To determine whether GH regulates hnf6 expression at the gene level, we studied its promoter. DNA binding experim ents showed that 1) the transcription factors STATE (signal transducer and activator of transcription 5) and HNF-4 (hepatocyte nuclear factor 4) bind to sites located around -110 and -650, respectively; and 2) STATE binding i s induced and HNF-4 binding affinity is increased in liver within 1 h after GH injection to hypophysectomized rats. Using transfection experiments and site-directed mutagenesis, we found that STATE and HNF-4 stimulated transc ription of an hnf6 gene promoter-reporter construct. Furthermore, GH stimul ated transcription of this construct in cells that express GH receptors. Co nsistent with our earlier finding that HNF-6 stimulates the hnf4 and hnf3 b eta gene promoters, GH treatment of hypophysectomized rats increased the li ver concentration of HNF-4 and HNF-3 beta mRNAs, Together, these data demon strate that GH stimulates transcription of the hnf6 gene by a mechanism inv olving STATE and HNF-4. They show that HNF-6 participates not only as an ef fector, but also as a target, to the regulatory network of liver transcript ion factors, and that several members of this network are GH regulated.