The modifying effects of two natural products, curcumin and hesperidin, given during the initiation and postinitiation phases of oral carcinogenesis initiated with 4-nitroquinoline 1-oxide (4-NQO) were investigated in male F344 rats and compared with that of β-carotene. At 6 weeks of age, rats were divided into experimental and control groups and fed the diet containing β-carotene, hesperidin, or curcumin at a dose of 0.5 g/kg diet (500 ppm). At 7 weeks of age, all animals except those treated with each test chemical alone and control groups were given 4-NQO (20 ppm) in the drinking water for 8 weeks to induce oral cancer. Seven days after the 4-NQO exposure, groups of animals fed the diets containing test chemicals were switched to the basal diet and continued on this diet until the end of the study. Starting 1 week after the stop of 4-NQO exposure, the groups given 4-NQO and a basal diet were switched to the diets containing β-carotene, hesperidin, and curcumin and maintained on these diets for 22 weeks. The other groups consisted of rats given 500 ppm β-carotene, hesperidin, or curcumin alone or untreated rats. All animals were necropsied at the termination of the experiment (week 32). The incidences of tongue neoplasms and preneoplastic lesions, polyamine levels in the tongue tissue, and cell proliferation activity estimated by bromodeoxyuridine-labeling index and by morphometric analysis of silver-stained nucleolar organizer region proteins were compared among the groups. Feeding of curcumin and β-carotene during the initiation and postinitiation phases and hesperidin at the initiation stage caused a significant reduction in the frequency of tongue carcinoma (41–91% reduction, P < 0.05) and the order of chemopreventive efficacy was curcumin > β-carotene > hesperidin. The incidences of oral preneoplasia in rats fed the diets mixed with these compounds were also decreased (P < 0.05). There were no such lesions in rats treated with test compounds alone or those in an untreated control group. Dietary administration of these compounds significantly decreased the labeling index of bromodeoxyuridine and the number and area of silver-stained nucleolar organizer region proteins per cell nucleus that are proliferation biomarkers, of the tongue squamous epithelium (P < 0.05). In addition, polyamine levels in the oral mucosa were lowered in rats treated with 4-NQO and three test compounds when compared to those give 4-NQO alone (P < 0.05). These results indicated that natural compounds curcumin and hesperidin inhibited rat oral carcinogenesis initiated with 4-NQO (relatively weak in hesperidin) as did β-carotene, and such inhibition might be related to suppression of cell proliferation.
This work was supported in part by Grant 05671568 from the Ministry of Health and Welfare of Japan, a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan, and a 1993 grant from the Sagawa Foundation for Promotion of Cancer Research in Japan.