Regulation of the human bilirubin UDP-glucuronosyltransferase gene

Abstract

The human UGT1 gene is a single copy gene consisting of four common exons and more than 13 variable exons which span more than 200 kb of the human genome. A single variable exon is spliced to the four common exons to form the mRNA for synthesis of a single UDP-glucuronosyltransferase (UGT) isoenzyme. Treatment of humans or hepatoma cell lines with drugs such as phenobarbital causes the induction of hepatic bilirubin UGT by increased transcription from the UGT1 gene. The upstream region of UGT1∗1 (bilirubin UGT) was sequenced and found to contain consensus sequences for several transcriptional regulatory elements including a ‘BARBIE box’. An unusual ‘TATA’ promoter sequence A(TA) 6TAA was also observed. The 5′ region flanking the UGT1∗1 exon when cloned into reporter constructs and transfected into four cell lines was capable of promoting reporter gene expression, but not when transfected into monkey kidney cell fibroblasts (COS-7 cells) indicating a cell specific expression. Sequential deletion of the 5′ flanking region in the plasmid constructs did not cause any significant reduction in reporter expression. Treatment of cells transfected with these plasmid constructs with drugs did not cause a significant increase in reporter expression except with retinoic acid plus WY 14643. Introduction of an additional two base pairs (TA) into the ‘TATA’ box of the 5′ gene sequence (as observed in Gilbert's patients) did not significantly change reporter expression levels. The regulation of the biliruibin UGT gene by drugs is not yet understood and it will be important to identify additional genetic elements possibly further than −2 kb upstream of the UGT1∗1 coding region, which regulate the expression of this gene.