Critical enhancer region to which AhR/ARNT and Sp1 bind in the human CYP1B1 gene

Cytochrome P450 (CYP) 1B1 is known to be induced by polycyclic aromatic hydrocarbons including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The constitutive and TCDD-inducible transcriptional expression of human CYP1B1 is known to be cell-specific. In order to identify the cis-elements that cell-specifically regulate the constitutive and TCDD-inducible transcription of CYP1B1, we constructed luciferase reporter plasmids containing a series of deletions of the XRE core sequence in the 5'-flanking region of the human CYP1B1 gene. Luciferase assays were performed with MCF-7 (breast carcinoma), HepG2 (hepatocellular carcinoma), LS-180 (colon carcinoma), and OMC-3 (ovarian carcinoma) cells. Although there were large differences in the relative luciferase activity and inducibility between these four cell lines, the contribution of each reporter construct was similar. Constitutive expression increased with the regulatory elements that are present at -910 to -852 and -1652 to -1243. Potential enhancer elements for TCDD-induction were located from -1022 to -852 including three XREs, XRE3 at -853, XRE4 at -940, and XRE5 at -989. Gel shift analyses revealed binding of the AhR/ARNT heterodimer to XRE2 at -834, XRE3 at -853, XRE6 at -1024, and XRE7 at -1490. In addition, the binding of a nuclear transcriptional factor, Sp1, near XRE2 and XRE8 was observed. It was suggested that mutual interaction of XRE2 and XRE3 is important for transcriptional regulation, and that the Sp1 binding to the Sp1-like motif (-824) enhances both the constitutive and inducible transcriptional activities of the human CYP1B1 gene.