Differential effects of CYP2E1 status on the metabolic activation of the colon carcinogens azoxymethane and methylazoxymethanol

Methylazoxymethanol (MAM) and its chemical and metabolic precursor, azoxyme thane (AOM), both strong colon carcinogens in rodents, can be metabolically activated by CYP2E1 in vitro. Using CYP2E1-null mice, we found that CYP2E1 deficiency differentially affects the activation of AOM and MAM, as reflec ted in DNA guanine alkylation in the colon and in the formation of colonic aberrant crypt foci (ACF). Male and female inbred 129/SV wild-type (WT) and CYP2E1-null (null) mice were treated with 189 mu mol/kg of either AOM or m ethylazoxymethyl acetate (MAMAc), and 7-methylguanine (7-MeG) and O-6-methy lguanine (O-6-MeG) were measured in the DNAs of various organs. The levels of O-6-MeG (as pmol/nmol guanine) in the liver, colon, kidney, and lung of male null mice treated with AOM were 87, 48, 70, and 43% lower, respectivel y, than in AOM-treated WT mice. In null mice treated with MAMAc, the DNA O- 6-MeG levels were lower by 38% in the liver but were higher by 368,146, and 194% in the colon, kidney, and lung, respectively, compared with the same organs of WT mice treated in the same way. Determination of ACF revealed th at although AOM-induced ACF formation was significantly lower in the null g roup than in the WT group, MAMAc-induced ACF formation was significantly hi gher in the null group than in the WT group. These results demonstrate an i mportant role for CYP2E1 in the in vivo activation of AOM and MAM and sugge st that agents that modify CYP2E1 activity at the tumor initiation stage mi ght either enhance or inhibit colon carcinogenesis, depending on whether AO M or MAMAc is used as the carcinogen. The mechanism of this effect is discu ssed.