Relative Contributions of CYP1A2 and CYP2E1 to the Bioactivation and Clearance of 4-Aminobiphenyl in Adult Mice

Abstract

4-Aminobiphenyl (ABP), a prototypical aromatic amine carcinogen in rodents and humans, requires bioactivation to manifest its toxic effects. A traditional model of ABP bioactivation, based on in vitro enzyme kinetic evidence, had postulated initial N-hydroxylation by the cytochrome P450 isoform CYP1A2. This is followed by phase 2 O-conjugation and hydrolysis to form a reactive nitrenium ion that covalently binds to DNA and produces tumor-initiating mutations. However, Cyp1a2(2/2) mice still possess significant liver ABP N-hydroxylation activity, DNA damage, and incidence of ABP-induced liver tumors, and in vivo induction of CYP1A2 paradoxically reduces levels of ABP-induced DNA damage. Competing ABP detoxification pathways can include N-acetylation by arylamine N-acetyltransferase 1 (NAT1) and/or NAT2; however, wild-type and Nat1/2(2/2) mice have similar in vivo ABP clearance rates. Together, these studies suggest the existence of novel ABP bioactivating and clearance/detoxification enzymes. In the present study, we detected similar reductions in V max for ABP N-hydroxylation by liver microsomes from Cyp1a2(2/2) and Cyp2e1(2/2) mice when compared with wild-type mice. In addition, recombinant mouse CYP1A2 and CYP2E1 were both able to N-hydroxylate ABP in mouse hepatoma cells. However, the in vivo clearance of ABP was significantly reduced in Cyp1a2(2/2) but not in Cyp2e1(2/2) mice. Our results support a significant role for CYP2E1 as a novel ABP N-oxidizing enzyme in adult mice, and suggest a more important contribution of CYP1A2 to the in vivo plasma clearance and thus detoxification of ABP.

Canadian Content in the Pages ofDrug Metabolism and Disposition: A Comprehensive Historical Analysis

“…The group of Denis Grant generated and characterized novel mouse models with genetic manipulations of N-acetyltransferase (NAT) enzymes: double Nat1/2-null mice were used to characterize the in vitro and in vivo metabolism of arylamine procarcinogens (Sugamori et al, 2006); Nat3-null mice were used to show minimal roles for this enzyme in the N-acetylation of arylamines (Sugamori et al, 2007); and human NAT2-transgenic mice on the double Nat1/2-null background show liver-selective expression of the human transgene (Sugamori et al, 2011). The Grant group also discovered that CYP2E1 is a novel 4-aminobiphenyl N-oxidizing enzyme in mice and that CYP1A2 plays a more important role in detoxification and clearance (Wang et al, 2015). Allan Okey, my postdoctoral supervisor in the early-1990s, later reported the AHRdependent induction of mouse hepatic FMO2 and FMO3 mRNAs by 2,3,7,8-tetrachlorodibenzop-dioxin (TCDD) (Celius et al, 2008).…”

The role of cytochrome P450 enzymes in carcinogen activation and detoxication: an in vivo–in vitro paradox

“…Metabolic activation of N -hydroxy-4-ABP is by esterification mediated by conjugation enzymes such sulfotransferases or N -acetyltransferases (53,58). An in-vitro study utilizing Hepa1c1c7 cells transfected with mouse Cyp1a2 and Cyp2e1 expression plasmids demonstrated the ability of recombinant mouse Cyp1a2 and Cyp2e1 to N -hydroxylate 4-ABP in living cells to produce increased levels of N -hydroxy-4-ABP compared with wild-type cells (59). Enzyme kinetic studies carried out with liver microsomes from Cyp1a2(−/− ) and Cyp2e1(−/− ) mice showed a significant contribution from Cyp2e1 toward 4-ABP N -hydroxylation in male mouse microsomes and a contribution from both Cyp2e1 and Cyp1a2 in female mouse microsomes (59).…”

Mutagenicity of N‐hydroxy‐4‐aminobiphenyl in human TP53 knock‐in (Hupki) mouse embryo fibroblasts

“…4‐ABP is N ‐hydroxylated to N ‐hydroxy‐4‐ABP ( N ‐OH‐4‐ABP) by cytochrome P450 (CYP) 1A2 in the liver as well as by CYP1A1, CYP1B1, and CYP2A13 in extrahepatic tissues (Butler et al ., 1989; Shimada et al ., 1996; Nakajima et al ., 2006). Newer studies have further suggested an involvement of CYP2E1 in the hydroxylation of 4‐ABP (Wang et al ., 2015a; Wang et al ., 2015b). N ‐OH‐4‐ABP can be further metabolized by N ‐acetyltransferases and sulfotransferases to unstable esters, which will undergo heterolytic cleavage and form an electrophilic nitrenium ion that can bind to DNA (Turesky and Le Marchand, 2011; Wang et al ., 2019).…”