Carcinogenesis by methylbenzylnitrosamine near the squamocolumnar junction and methylamylnitrosamine metabolism in the mouse forestomach

Abstract

We repeated and extended a 1973 study by Sander and Schweinsberg on forestomach tumorigenesis in mice by methylbenzylnitrosamine (MBZN). Groups of 80 adult CD-1 mice of both sexes received 96 mg/kg of MBZN subdivided into 24 doses of 4 mg/kg, 12 doses of 8 mg/kg or 6 doses of 16 mg/kg (groups 1-3, respectively). The mice were injected i.p. twice weekly with MBZN in 30% dimethylsulfoxide and 6-8 mice/group were killed every 4 weeks up to 40 weeks. Ten untreated control mice did not develop forestomach tumors. Forestomach papillomas occurred in 35-53% of the treated mice, with the highest incidence and shortest latency (mostly <24 weeks) in group 3. Squamous carcinomas of the forestomach were found in 31% of group 1 and 4-6% of groups 2 and 3. Ninety-two percent of the carcinomas and 94% of the papillomas in the 8-mm wide forestomach occurred < or = 1 mm from the squamocolumnar junction (SCJ) with the glandular stomach. This is interesting in view of the rising incidence of human adenocarcinoma near the gastroesophageal SCJ. Methyl-n-amylnitrosamine (MNAN) yields 2-, 3- and 4-hydroxy-MNAN (HO-MNAN) in a 1:3:2 ratio when incubated with rodent tissues for which MNAN is carcinogenic. This metabolism may be due to cytochrome P450 isoform believed responsible for MNAN and, probably, MBZN activation. When freshly excised mouse forestomach and esophagus were incubated for 2 h with 23 microM MNAN, total HO-MNAN yields were 0.79 +/- 0.05 and 1.81 +/-0.08 nmol/100 mg tissue per h (mean +/- SE), respectively, with about 1:3:2 ratios between 2-,3- and 4-HO-MNAN. This compares with published mean HO-MNAN yields in nmol/100 mg per h of 1.2 for rat esophagus (where MNAN and MBZN are strongly carcinogenic) and <0.1 for rat forestomach. These findings may explain why MNAN and MBZN induce forestomach tumors in mice but not in rats and why MNAN induces esophageal tumors in mice, but does not explain why MBZN given i.p. fails to induce esophageal tumors in mice. Three sections of the mouse forestomach (closest-to to furthest-from the SCG) showed total HO-MNAN yields from MNAN of 0.61+/-0.05, 0.38+/-0.03 and 0.48+/-0.02 nmol/100 mg per h (mean +/-SE), respectively. This may help explain why the MBZN-induced forestomach tumors were non-centrated near the SCJ.