Regulatory Xenobiotic Responsive Elements in the Distal 5′-Flanking Region of the Mouse Cyp1a2 Gene Required for Transcriptional Activation by 3-Methylcholanthrene and 2,3,7,8-Tetrachlorodibenzo-p-dioxin

We examined the xenobiotic responsive element (XRE) responsible for induction of the mouse Cyp1a2 gene by 3-methylcholanthrene (3MC) and 2,3,7,8-tetrachlorodibenzo- p -dioxin (TCDD) using a reporter gene assay in mouse hepatocytes in primary culture. Although, the 5′-flanking region up to −9.5 kilobase pairs did not show a significant increase in transcriptional activity after treatment with 3MC or TCDD, a further distal 5′-flanking region from −13,958 to −12,520 containing 12 putative XREs (5′-GCGTG-3′) demonstrated distinctive transcriptional activity after treatment with 3MC or TCDD. When a mutation was introduced into XRE14 at −12,972, the activation was decreased, and concurrent mutations in XRE14, XRE13, and XRE15 completely abolished it. However, mutations in XRE13, XRE15, XRE16, or XRE17 did not affect the inducible transcriptional activation of the mouse Cyp1a2 gene. These results suggest that XRE14 is important and that XRE13 at −12,897 and/or XRE15 at −13,061 are cooperative to the inducible transcriptional activation of the mouse Cyp1a2 gene by ligands of the aryl hydrocarbon receptor.