Comparison of the tumor-initiating activity of 7,12-dimethylbenz[a]anthracene and benzo[a]pyrene in female SENCAR and CD-1 mice

Abstract

The derivation of mice resistant and susceptible to skin tumorigenesis using the initiation-promotion regimen is described. Dose-response relationships for tumor-initiating activities of 7,12-dimethylbenz[a]-anthracene (DMBA) and benzo[a]pyrene (BP) in the susceptible line (SENCAR) are presented. A single topical dose of either 0.1, 1.0, 10 or 100 nmol DMBA, followed one week later by twice weekly applications of 8.5 nmol 12–0-tetradecanoylphorbol-13-acetate (TPA) for 19 weeks, produced 0, 3.3, 4.9 and 23.1 papillomas per mouse, respectively. Single topical initiating doses of either 50, 100 or 200 nmol BP produced 1.7, 3.8 or 7.8 papillomas per mouse, respectively, after 28 weeks of promotion with 8.5 nmol TPA. SENCAR mice were compared with CD-1 mice for the initiating activity of DMBA and BP. Initiating doses of 0.1, 1.0, 10 and 100 nmol DMBA produced 0.6, 3.8, 7.0 and 24 papillomas per mouse, respectively, in SENCAR mice and in CD-1 mice produced 0, 0.2, 3.0 and 5.6 papillomas per mouse, respectively, after 25 weeks of promotion with TPA. With BP as the initiator, 10, 50, 100 and 200 nmol doses produced 0.9, 1.6, 3.8 and 8.3 papillomas per mouse, respectively, in SENCAR mice and in CD-1 mice produced 0.1, 0.7,1.8 and 3.8 papillomas per mouse, respectively, after 25 weeks of promotion with TPA. SENCAR mice were compared with CD-1 mice for possible differences in the oxidative metabolism of DMBA using epidermal homogenates as the enzyme source. Basal levels of monooxygenase activity toward DMBA were similar in both mouse stocks. Epidermal monooxygenase activities following pre-treatment with inducers including DMBA, 3-methylcholanthrene, dibenz[a,c]anthracene, Aroclor 1254 and 2,3,7,8-tetrachlorodibenzo-p-dioxin, also were quite similar in both mouse stocks. High-pressure liquid chromatographic profiles of ethyl acetate/ acetone (2:1) extractable metabolites revealed a close similarity in the patterns as well as the rates of formation of specific metabolites. Metabolites of DMBA tentatively identified based on cochromato-graphy with purified reference standards included phenols, 12-hydroxymethyl-7-methylbenz[a]anthracene, 7-hydroxymethyl-12-methylbenz[a]antnracene, 7,12-dihydroxymethylbenz[a]anthracene, (±)-trans-8,9-dihydro-8,9-dihydroxy-7,12-dimethylbenz[a]anthracene and (±)-trans-5,6-dihydro-5,6-dihydroxy-7,12-dimethylbenz[a]anthracene. The results suggested that differences in oxidative metabolism of DMBA were not responsible for the differences in sensitivity to tumor-initiation between SENCAR and CD-1 mice.