Modulation of N-methyl-N-nitrosourea-induced crypt restricted metallothionein immunopositivity in mouse colon by a non-genotoxic diet-related chemical

Red meat consumption is associated with endogenous metabolic generation of mutagenic N-nitroso compounds (NOC) and may be implicated in causation of colorectal cancer. Assessment of a biologically relevant dose of NOCs is hampered by imperfect understanding of NOC interactions with other dietary components. This study tests the hypothesis that NOC effects upon mutational biomarkers in mouse colon may be modulated by a non-genotoxic diet-related compound. N-methyl-N-nitrosourea (MNU) and undegraded λ carrageenan (λCgN) were selected as test chemicals, representing a NOC and a non-genotoxic agent, respectively. Study end-points included (i) DNA adduct formation and (ii) metallothionein (MT) crypt restricted immunopositivity indices (MTCRII) which are considered representative of crypt stem cell mutations. Frequency and size of MT immunopositive foci as well as total number of MT immunopositive crypts were assessed. Biologically effective doses of MNU and λCgN were determined in model validation studies and the agents were then tested alone and in combination. Continuous λCgN treatment for 10 weeks induced significantly greater colonic mucosal injury than a drinking water control. In combined treatment regimens, λCgN treatment did not significantly affect MNU-induced DNA adduct formation. However, combinations of λCgN with MNU significantly increased MTCRII in excess of those induced by MNU alone. Recurrent or continuous λCgN regimens had greater interactive effects with MNU upon MTCRII than short-term λCgN treatment. This study has shown that exposure to a non-genotoxic diet-related compound (λCgN) modulates the effective NOC dosimetry for induction of MT crypt restricted immunopositivity.