Selective inhibition of methylbenzylnitrosamine-induced formation of esophageal O6-methylguanine by dietary ellagic acid in rats.

Ellagic acid is a naturally occurring plant phenol which has been shown to reduce the incidence of a number of carcinogen-induced tumors including methylbenzylnitrosamine (MBN)-induced esophageal carcinoma in the rat. The postulated mechanism of MBN-induced esophageal carcinogenesis is through oxidation of MBN to form benzaldehyde and an activated metabolite which methylates DNA forming a variety of methylated DNA adducts including O6-methylguanine (O6-mGua) and 7-methylguanine (m7Gua). O6-mGua adducts have been shown to induce DNA mutations which can lead to cancer, while m7Gua adducts do not appear to be related to tumor induction. In this study, we examined whether the decreased incidence of MBN-induced esophageal carcinoma observed with dietary ellagic acid was associated with a decrease in the in vivo and in vitro formation of MBN-induced DNA adducts and whether this reduction was specific to O6-mGua or due to a reduction in total methylation. Weanling male Sprague-Dawley rats were fed a nutritionally complete diet with and without the addition of 0.4 g of ellagic acid per kg of diet. This dose of dietary ellagic acid has previously been shown to reduce the incidence of MBN-induced esophageal carcinoma by 30 to 50%. After 3 wk on the diets, rats were given injections of a single dose of MBN (2.0 mg/kg of body weight i.p.) and sacrificed 1 h after injection. Dietary ellagic acid significantly reduced the MBN-induced in vivo formation of esophageal O6-mGua, without significantly reducing the formation of esophageal m7Gua. Examination of this effect in an in vitro methylation assay demonstrated that dietary ellagic acid did not reduce the ability of esophageal microsomes to methylate purified calf thymus DNA; however, pretreatment of the calf thymus DNA with ellagic acid selectively reduced the MBN-induced formation of O6-mGua by microsomes from both ellagic acid-fed and control animals without altering the in vitro formation of m7Gua. These results suggest that ellagic acid bound to DNA selectively blocks methylation of the O6-position of guanine without inhibiting the activation of MBN or the ability of MBN to methylate DNA.