Involvement of Hepatocyte Nuclear Factor 1 in the Regulation of the UDP-Glucuronosyltransferase 1A7 (UGT1A7) Gene in Rat Hepatocytes

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Abstract
UDP-glucuronosyltransferase 1A7 (UGT1A7) is a major UGT contributing to the glucuronidation of xenobiotic phenols in rats. Its expression in rat liver is tightly regulated, with low constitutive and high inducible expression in response to aryl hydrocarbon receptor ligands and oltipraz. Previously, we reported the absence of 3-methylcholanthrene- or oltipraz-responsive elements in the 1.6-kbp region flanking the UGT1A7 promoter. However, potential binding sites were noted for several liver-enriched transcription factors. Here we show that deletion of the hepatic nuclear factor (HNF)3, HNF4, and CCAAT-enhancer binding protein-like binding sites had no effect on the expression of a UGT1A7 reporter plasmid, p(−965/+56)1A7-Luc, in primary rat hepatocytes. The full activity of the promoter was contained in the region between bases −157 and +76. Two sites of binding by rat liver nuclear proteins were detected in this region by DNase footprinting. PR-1 corresponded to the HNF1-like binding site between bases −52 and −38, whereas PR-2 was located between −30 to −6. Gel retardation studies supported the presence of HNF1α in the PR-1 DNA-liver nuclear protein complex. Mutation of PR-1 inhibited binding in the gel shift assay, prevented activation by overexpressed HNF1 in human embryonic kidney cells, and reduced by >80% the maximal luciferase activities expressed from basal and 3-methylcholanthrene-responsive UGT1A7 gene reporter constructs in primary rat hepatocytes. These data provide evidence for an important stimulatory role of HNF1 in promoting UGT1A7 gene expression in rat liver.