Expression of the α7 isoform of hepatocyte nuclear factor (HNF) 4 is activated by HNF6/OC-2 and HNF1 and repressed by HNF4α1 in the liver

Abstract

The hepatocyte nuclear factor (HNF) 4α gene possesses two promoters, proximal P1 and distal P2, whose use results in HNF4α1 and HNF4α7 transcripts, respectively. Both isoforms are expressed in the embryonic liver, whereas HNF4α1 is almost exclusively in the adult liver. A 516-bp fragment, encompassing a DNase I-hypersensitive site associated with P2 activity that is still retained in adult liver, contains functional HNF1 and HNF6 binding sites and confers full promoter activity in transient transfections. We demonstrate a critical role of the Onecut factors in P2 regulation using site-directed mutagenesis and embryos doubly deficient for HNF6 and OC-2 that show reduced hepatic HNF4α7 transcript levels. Transient transgenesis showed that a 4-kb promoter region is sufficient to drive expression of a reporter gene in the stomach, intestine, and pancreas, but not the liver, for which additional activating sequences may be required. Quantitative PCR analysis revealed that throughout liver development HNF4α7 transcripts are lower than those of HNF4α1. HNF4α1 represses P2 activity in transfection assays and as deduced from an increase in P2-derived transcript levels in recombinant mice in which HNF4α1 has been deleted and replaced by HNF4α7. We conclude that although HNF6/OC-2 and perhaps HNF1 activate the P2 promoter in the embryo, increasing HNF4α1 expression throughout development causes a switch to essentially exclusive P1 promoter activity in the adult liver.