ABSTRACT:We have previously shown that cadmium, a metal that alters cellular redox status, induces CYP2A5 expression in nuclear factor (erythroid-derived 2)-like 2 wild-type (Nrf2 ϩ/ϩ ) mice but not in the knockout (Nrf2 ؊/؊ ) mice. In the present studies, the potential role The mouse CYP2A5 and its human ortholog CYP2A6 mediate the phase I metabolism of a diverse range of toxic compounds, including nitrosamines and aflatoxins (Su and Ding, 2004). These enzymes are predominantly expressed in hepatocytes but are also present in extrahepatic tissue, particularly in the nasal mucosa (Su and Ding, 2004). CYP2A5 enzyme is the major catalyst of coumarin 7-hydroxylation in mouse liver (Lang et al., 1989). Its regulation is complex and unique among other major cytochromes P450 (P450s). It can be induced by structurally unrelated compounds and also by several chemicals and pathophysiological conditions that usually repress other P450 isoforms. The Cyp2a5 gene is induced by classical inducers, such as phenobarbital (Wood and Conney, 1974), and by various hepatotoxic agents, including pyrazole, carbon tetrachloride, and metals (Seubert et al., 2002;Abu-Bakar et al., 2004;Su and Ding, 2004). Elevated CYP2A5 protein was also observed in spontaneous, transplanted, or chemically induced mouse hepatomas (Su and Ding, 2004).Depending on the inducer, the activation of hepatic CYP2A5 can be achieved both by transcriptional and post-transcriptional mechanisms. Transcriptional induction of Cyp2a5 by 2,3,7,8-tetrachlorodibenzo-pdioxin is mediated by the binding of a ligand-activated aryl hydrocarbon receptor (AHR)/aryl hydrocarbon receptor nuclear translocator (ARNT) complex to the xenobiotic response element (XRE) site at the Cyp2a5 distal promoter (Arpiainen et al., 2005). Pyrazole, a hepatotoxin, induces CYP2A5 by a post-transcriptional mechanism involving binding of heterogenous nuclear ribonucleoprotein A1 to the 3Ј-untranslated region of CYP2A5 mRNA, with subsequent stabilization of the mRNA (Glisovic et al., 2003).However, given the structural diversity of the inducers, it is possible that induction of CYP2A5 is not directly related to the nature of the inducing agents, but instead may be an indirect consequence of a

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Mouse cytochrome P450 2A5 (CYP2A5) is upregulated in various pathophysiological liver diseases and induced by structurally variable hepatotoxic chemicals. A putative common feature for all of these conditions is altered cellular redox status. Nuclear factor erythroid 2-like 2 (Nrf2) is a transcription factor that is post-translationally regulated by oxidative stress and controls the transcription of numerous protective target genes. In the present study, we have extensively characterized the regulation of Cyp2a5 by Nrf2 and compared it to a well-characterized target gene Hmox1. The treatment of mouse primary hepatocytes with lead chloride, methylmercury chloride, or phenethyl isothiocyanate all leads to nuclear accumulation of Nrf2. Both CYP2A5 and HMOX1 were induced by all three compounds; however, HMOX1 responded more rapidly and transiently as compared to CYP2A5. Experiments in Nrf2(-/-) primary hepatocytes showed that Nrf2 is crucial for CYP2A5 induction but not for elevation of HMOX1. Both CYP2A5 and HMOX1 were upregulated by Nrf2 overexpression and downregulated by Keap1 or Bach1 overexpression. However, in all cases, CYP2A5 responded much more potently. Results in Nrf2-deficient animals showed that CYP2A5 expression is significantly attenuated in the absence of Nrf2, while expression of HMOX1 was unaffected. Therefore, Cyp2a5 joins the group of genes constitutively regulated by Nrf2. Our current results unequivocally show that expression of CYP2A5 is tightly controlled by Nrf2 in liver. Nrf2 is needed for constitutive expression of CYP2A5, and CYP2A5 is also sensitively upregulated by an increased level of Nrf2 protein. Therefore, CYP2A5 upregulation could be a useful indicator for hepatic activation of the Nrf2 pathway.