Suppression of colon carcinogenesis by bioactive compounds in grapefruit

This study evaluated the hypothesis that untreated and irradiated grapefruit as well as the isolated citrus compounds naringin and limonin would protect against azoxymethane (AOM)-induced aberrant crypt foci (ACF) by suppressing proliferation and elevating apoptosis through anti-inflammatory activities. Male Sprague-Dawley rats (n = 100) were provided one of five diets: control (without added grapefruit components), untreated or irradiated (300 Gy, ¹³⁷Cs) grapefruit pulp powder (13.7 g/kg), naringin (200 mg/kg) or limonin (200 mg/kg). Rats were injected with saline or AOM (15 mg/kg) during the third and fourth week and colons were resected (6 weeks post second injection) for evaluation of ACF, proliferation, apoptosis, and cyclooxygenase (COX)-2 and inducible nitric oxide synthase (iNOS) protein levels. Experimental diets had no effect on the variables measured in saline-injected rats. However, in AOM-injected rats, the experimental diets suppressed (P ≤ 0.02) aberrant crypt and high multiplicity ACF (HMACF; P ≤ 0.01) formation and the proliferative index (P ≤ 0.02) compared with the control diet. Only untreated grapefruit and limonin suppressed (P ≤ 0.04) HMACF/cm and expansion (P ≤ 0.008) of the proliferative zone that occurred in the AOM-injected rats consuming the control diet. All diets elevated (P ≤ 0.05) the apoptotic index in AOM-injected rats, compared with the control diet; however, the greatest enhancement was seen with untreated grapefruit and limonin. Untreated grapefruit and limonin diets suppressed elevation of both iNOS (P ≤ 0.003) and COX-2 (P ≤ 0.032) levels observed in AOM-injected rats consuming the control diet. Although irradiated grapefruit and naringin suppressed iNOS levels in AOM-injected rats, no effect was observed with respect to COX-2 levels. Thus, lower levels of iNOS and COX-2 are associated with suppression of proliferation and upregulation of apoptosis, which may have contributed to a decrease in the number of HMACF in rats provided with untreated grapefruit and limonin. These results suggest that consumption of grapefruit or limonin may help to suppress colon cancer development.