Octamer transcription factor-1 enhances hepatic nuclear factor-1α- mediated activation of the human UDP glucuronosyltransferase 2B7 promoter

The human UDP glucuronosyltransferase, UGT2B7, is expressed in the liver and gastrointestinal tract, where it catalyzes the glucuronidation of steroids and bile acids. In this study, the UGT2B7 gene was isolated and its proximal promoter was analyzed. The UGT2B7 gene consists of 6 exons and extends over 16 kilobases (kb). It does not contain a canonical TATA box but has a region (-2 to -40) adjacent to the transcription start site that binds nuclear proteins. This region contains a consensus hepatic nuclear factor-1α (HNF1α)-binding site and an overlapping AT-rich segment. Varying lengths of the UGT2B7 gene promoter, with and without these sites, were fused to the firefly luciferase reporter gene and transfected into HepG2 cells. UGT2B7 promoter activity with the HNF1/AT-rich element was stimulated by cotransfection with HNF1α. Additional activation was observed when HNF1α and octamer transcription factor-1 (Oct-1) were cotransfected simultaneously. However, Oct-1 alone did not stimulate promoter activity and did not bind to the promoter in the absence of HNF1α. Deletion of the HNF1/AT-rich region, or mutations in this region, abolished UGT2B7 gene promoter activity and prevented HNF1α-mediated increases in promoter activity. The presence of HNF1α and octamer transcription factor-1 (Oct-1) in the protein complex that bound to the HNF1/AT-rich region was demonstrated by gel shift analyses with antibodies specific to HNF1α and Oct-1 protein. These results strongly suggest that the liver-enriched factor HNF1α binds to, and activates, the UGT2B7 gene promoter and that the ubiquitous transcription factor, Oct-1, enhances this activation by directly interacting with HNF1α. This interaction between HNF1α and Oct-1 may fine-tune UGT2B7 expression.