Inhibition of the transcription of CYP1A1 gene by the upstream stimulatory factor 1 in rabbits. Competitive binding of USF1 with AhR.Arnt complex.

A xenobiotic-responsive element (XRE)-binding factor(s) other than the AhR.Arnt complex was found to inhibit the transcription of CYP1A1 gene in the liver from adult rabbits, known to be nonresponsive to CYP1A1 inducers. The constitutive factor(s) in liver nuclear extracts bound to the core sequence of XRE. The binding was eliminated by the presence of an excess amount of the AhR.Arnt complex synthesized in vitro. To identify the constitutive factor(s), a sequence similar to rabbit XRE was sought. It was found that the sequence of rabbit XRE overlapped with that of the upstream stimulatory factor 1 (USF1)-binding site in the mouse metallothionein I promoter. In fact, a super shift assay using a specific antibody against human USF1 indicated that USF1 was capable of binding to rabbit XRE. Additionally, the AhR.Arnt-mediated activation of XRE-TK/Luc reporter gene in RK13 cells was blocked by the transfection with a USF1 expression vector with the amounts of the expression vector transfected. These results indicate that the XRE of the rabbit CYP1A1 gene is recognized by the basic helix-loop-helix proteins to regulate the expression of CYP1A1 in both an agonistic (AhR.Arnt) and an antagonistic (USF1) manner.