Abstract

The UGT1A3-1A5 genes are a highly related UDP-glucuronosyltransferase (UGT) cluster exhibiting high levels of coding and regulatory region homology. However, the ensuing proteins have both differing substrate specificities and differing expression patterns. The expression profile of each enzyme also varies considerably from one individual to the next. Differences in UGT expression have been predicted to contribute to an individual's response to pharmaceuticals and to predisposition toward cancer in the event of carcinogen exposure. Therefore, it is desirable to elucidate the mechanisms that drive the transcription of UGT genes and identify the factors responsible for their variable expression. To this end, we have isolated the UGT1A3, UGT1A4, and UGT1A5 proximal promoters and begun to investigate the regulatory elements necessary for activity in vitro. We have established that the nucleotide sequence upstream of the UGT1A5 exon 1 is an ineffective promoter, correlating with the lack of substantial expression of this UGT in human tissues. In contrast, the UGT1A3 and UGT1A4 proximal promoters are both highly active in hepatic and colonic cell lines, with maximal activity being encoded by the proximal 500 base pairs. However, the UGT1A3 and UGT1A4 promoters exhibit low activity in the human embryonic kidney cell line HEK293, unless coexpressed with hepatocyte nuclear factor (HNF) 1α. Furthermore, mutation of the consensus-like HNF1-binding site in the UGT1A3 promoter abolishes promoter function in all cell types. This study suggests an important role for HNF1α in the transcriptional regulation of the human UGT1A3 and UGT1A4 genes.