Characterization of the rat uridine diphosphate glucuronosyltransferase 1A6 gene: Transcriptional control of inducer- and tissue-specific expression

Rat UDP-glucuronosyltransferase 1A6 (UGT1A6), a detoxifying enzyme with activity towards simple planar phenolic compounds, exhibits a complex pattern of tissue and inducer-specific expression. UGT1A6 mRNA levels are increased significantly following exposure to the chemopreventive agent oltipraz and polycyclic aromatic hydrocarbons (PAH). The goal of these studies was to determine the mechanism mediating constitutive and oltipraz-inducible UGT1A6 expression.; A new UGT1A6 mRNA (class 2) was identified and found to contribute to the expression of UGT1A6. This alternative transcript arises from the use of a different promoter “UGT1A6P2” and differs from the class 1 transcript in the 5′-untranslated region sequence. Both transcripts have identical coding sequences and are therefore predicted to encode identical proteins. The class 2 mRNA was highly expressed in liver, kidney, and gastrointestinal tract relative to other extrahepatic tissues. Both classes were elevated in liver by PAHs, an environmental contaminant, and oltipraz, a chemopreventive agent.; We show that oltipraz, or possibly one of its metabolites, possesses the capacity to activate the aryl hydrocarbon receptor (AHR) pathway which contributes to P1 activation. Luciferase reporter assays indicate that an intact xenobiotic-response element (XRE) located at −134/−129 of UGT1A6P1 is required to confer oltipraz-inducibility. AHR complex binding to the UGT1A6P1 XRE site was confirmed using electrophoretic mobility shift assays. These results provide the first direct evidence that oltipraz may induce biotransformation enzymes via an AHR-based mechanism and suggest that oltipraz transcriptionally activates both phase 1 and phase 2 enzymes.; A hepatic nuclear factor-1 (HNF1) binding site located in UGT1A6P2 was characterized. Specific binding by nuclear extract from rat liver and well differentiated human hepatoma cells (HepG2), but not the underdifferentiated mouse hepatoma cell line (Hepa1), was demonstrated. The functionality of this site was supported by (1) the reduced expression of a UGT1A6P2 reporter when the HNF1 site was mutated and (2) the enhanced expression from UGT1A6P2 when either HNF1α or HNF1β were overexpressed. These data suggest that transcription from UGT1A6P2 contributes to the tissue-specific expression of rat UGT1A6 and provide a possible basis for the high expression of the class 2 transcript observed in liver and other HNF1-enriched tissues.