Regulation of cytochrome P-450 (CYP) 1B1 in mouse Hepa-1 variant cell lines: A possible role for aryl hydrocarbon receptor nuclear translocator (ARNT) as a suppressor of CYP1B1 gene expression.

Cytochrome P-450 (CYP) 1B1 expression in mouse hepatoma (Hepa-1) wild-type (WT) cells was compared with responses in Hepa-1 variants LA1 and LA2, which, respectively, exhibit low aryl hydrocarbon receptor (AhR) level and defective AhR nuclear translocator (ARNT) protein. 10T1/2 mouse embryo fibroblasts express predominantly CYP1B1 and at a 100 times higher level than in Hepa-1 cells, whereas they express about 300-fold lower CYP1A1 than Hepa-1 cells. The expression of CYP1B1 in WT and LA1 variant, although at a much lower level, follows that of CYP1A1, reflecting their common regulation through the AhR. The LA2 (ARNT-defective) cells showed a major difference between CYP1B1 and CYP1A1 expression. Although CYP1A1 mRNA levels in LA2 were extremely low and unresponsive to 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD), basal CYP1B1 mRNA and protein were expressed at levels similar to those seen in TCDD-induced WT. The elevated basal CYP1B1 mRNA in LA2 cells decreased by 50% after transient transfection of ARNT cDNA, in parallel with substantial restoration of CYP1A1 induction. This implicates ARNT as a suppressor of CYP1B1 basal expression in Hepa cells. In transient CYP1B1-luciferase constructs in LA2 cells, ARNT shows stimulatory effects in the enhancer region but an inhibitory effect on the proximal promoter. Two CYP1B1 enhancer elements [xenobiotic-responsive element (XRE) 1/2 and XRE4] formed TCDD-unresponsive complexes of similar mobility to TCDD-stimulated AhR-ARNT complex with XRE5. However, because these two complexes were formed to the same extent in LA2 as in WT cells, they cannot be due to ARNT or contribute to ARNT-regulated suppression.