INITIAL LEVELS OF AZOXYMETHANE-INDUCED DNA METHYL ADDUCTS ARE NOT PREDICTIVE OF TUMOR SUSCEPTIBILITY IN INBRED MICE

Inbred mice vary in susceptibility to colon carcinogens such as 1,2-di methylhydrazine (DMH). Differential susceptibility may depend, in part , on formation of promutagenic DNA methyl adducts within target coloni c mucosa. The present study was undertaken to evaluate the extent of D NA adduct formation in susceptible (SWR) and resistant (AKR) mice acut ely exposed to the colon carcinogen azoxymethane (AOM), a direct metab olite of DMH. In the first experiment, 8-week-old SWR and AKR mice wer e treated ip with 20 mg/kg AOM and sacrificed 6 h later. DNA was isola ted from distal colon and liver, and O-6-methylguanine (O-6-MeGua) add uct levels were assessed by immunoslot blot (ISB) analysis, using a mo nospecific antibody raised against O-6-methyldeoxyguanosine. HPLC-fluo rescence detection was also used to quantitate O-6-MeGua and 7-methylg uanine (7-MeGua), and to generate standard curves, At 6 h, both O-6-Me Gua and 7-MeGua were significantly higher (2- to 3-fold, p < 0.05) in AKR colon, while an opposite pattern was found in liver. In Experiment 2, mice were injected with AOM (20 mg/kg) and euthanized 12 and 48 h later. At 12 h, O-6-MeGua levels were higher in colons (1.4-fold) of S WR mice. Forty-eight hours after treatment, however, adduct levels in colon were markedly (5-fold) reduced in SWR but were unchanged from 12 h in AKR. To further compare activation of AOM in both strains, colon microsomes were incubated with AOM and calf thymus DNA. Comparable le vels of O-6-MeGua were detected by ISB, demonstrating equivalent metab olic capacity in both SWR and AKR mice. These studies suggest that dif ferential susceptibility to AOM-induced colon carcinogenesis is not ba sed on initial target tissue DNA alkylation and unlikely to depend on differential metabolic capacity. (C) 1998 Academic Press.