An autoregulatory loop controlling CYP1A1 gene expression: Role of H2O2 and NFI

Cytochrome P450 1A1 (CYP1A1), like many monooxygenases, can produce reactiv e oxygen species during its catalytic cycle, Apart from the well-characteri zed xenobiotic-elicited induction, the regulatory mechanisms involved in th e control of the steady-state activity of CYP1A1 have not been elucidated. We show here that reactive oxygen species generated from the activity of CY P1A1 limit the levels of induced CYP1A1 mRNAs. The mechanism involves the r epression of the CYP1A1 gene promoter activity in a negative-feedback autor egulatory loop. Indeed, increasing the CYP1A1 activity by transfecting CYP1 A1 expression vectors into hepatoma cells elicited an oxidative stress and led to the repression of a reporter gene driven by the CYP1A1 gene promoter . This negative autoregulation is abolished by ellipticine (an inhibitor of CYP1A1) and by catalase (which catalyzes H2O2 catabolism), thus implying t hat H2O2 is an intermediate. Down-regulation is also abolished by the mutat ion of the proximal nuclear factor I (NFI) site in the promoter. The transa ctivating domain of NFI/CTF was found to act in synergy with the arylhydroc arbon receptor pathway during the induction of CYP1A1 by 2,3,7,8-tetrachlor o-p-dibenzodioxin. Using an NFI/CTF-Gal4 fusion, we show that NFI/CTF trans activating function is decreased by a high activity of CYP1A1. This regulat ion is also abolished by catalase or ellipticine. Consistently, the transac tivating function of NFI/CTF is repressed in cells treated with H2O2, a nov el finding indicating that the transactivating domain of a transcription fa ctor can be targeted by oxidative stress. In conclusion, an autoregulatory loop leads to the fine tuning of the CYP1A1 gene expression through the dow n-regulation of NFI activity by CYP1A1-based H2O2 production. This mechanis m allows a limitation of the potentially toxic CYP1A1 activity within the c ell.