Modifying effects of a flavonoid morin on azoxymethane-induced large boweltumorigenesis in rats

The modifying effect of dietary exposure to a flavonoid morin during the in itiation and post-initiation phases of azoxymethane (AOM)-initiated colorec tal carcinogenesis was investigated in male F344 rats. A total of 55 animal s were initiated with AOM by weekly s.c. injections of 15 mg/kg body wt for 3 weeks to induce colorectal neoplasms. Rats were fed a diet containing 50 0 p.p.m. morin for 5 ('initiation feeding') or 28 ('post-initiation feeding ') weeks. Other groups contained rats treated with morin alone (500 p.p.m. in diet) and untreated rats. At the end of the study (32 weeks), the incide nce of adenocarcinoma in the large intestine of rats initiated with AOM tog ether with (43%) or followed by (29%) a diet containing morin was smaller t han that of rats given AOM alone (75%), A significant difference was found between 'post-initiation feeding' and untreated groups (P = 0.023). Althoug h both 'initiation feeding' and 'post-initiation feeding' of morin reduced polyamine levels in colorectal mucosa and blood, 'post-initiation feeding' of morin significantly decreased the proliferating cell nuclear antigen-pos itive index in aberrant crypt foci, 'Post-initiation feeding' of morin sign ificantly elevated glutathione S-transferase and quinone reductase activiti es in the liver and large bowel, but 'initiation feeding' caused a signific ant elevation of these enzymes activities only in the large bowel, These re sults indicate that morin could exert a weak chemopreventive effect on larg e bowel tumorigenesis induced by AOM when fed during the post-initiation ph ase.