Chemoprevention of 4-nitroquinoline 1-oxide-induced oral carcinogenesis by citrus auraptene in rats

The modifying effects of citrus auraptene given during the initiation and post-initiation phases of oral carcinogenesis initiated with 4-nitroquinoline 1-oxide (4-NQO) were investigated in male F344 rats. At 6 weeks of age, animals were divided into experimental and control groups, and fed the diets containing 100 ppm or 500 ppm auraptene. At 7 weeks of age, all animals except those treated with auraptene alone and control groups were given 4-NQO (20 ppm) in the drinking water for 8 weeks to induce tongue carcinoma. Starting 7 days before the 4-NQO exposure, groups of animals were fed the diets containing auraptene (100 and 500 ppm) for 10 weeks and then switched to the basal diet. Starting 1 week after the cessation of 4-NQO exposure, the groups given 4-NQO and a basal diet were switched to the diets mixed with auraptene (100 and 500 ppm), and maintained on these diets for 22 weeks. The other groups consisted of rats fed auraptene alone (500 ppm) or untreated rats. All rats were necropsied at the termination of the study (week 32). The incidences of tongue lesions (neoplasms and preneoplasms), polyamine levels in the tongue tissue and cell proliferation activity estimated by 5-bromodeoxyuridine (BrdU)-labelling index were compared among the groups. In addition, the activities of gluthathione S-transferase (GST) and quinone reductase (QR) in liver and tongue of rats gavaged various doses of auraptene (0, 200, 400 and 800 mg/kg body wt) for 5 days were assayed. Feeding of auraptene at both doses during the initiation phase caused a significant reduction in the frequency of tongue carcinoma (100 ppm auraptene, 91% reduction, P < 0.001; 500 ppm auraptene, 63% reduction, P < 0.05). When fed auraptene after 4-NQO exposure, the frequency of tongue carcinoma was also decreased (100 ppm auraptene, 100% reduction, P < 0.001; 500 ppm auraptene, 74% reduction, P < 0.01). The incidences of tongue severe dysplasia in these groups were significantly smaller than those in carcinogen controls (P < 0.05). There were no pathological alterations in rats treated with 500 ppm auraptene alone or those in an untreated control group. Dietary administration of auraptene significantly decreased BrdU-labelling index and polyamine concentrations in the oral mucosa (P < 0.05). In addition, auraptene administration significantly increased the activities of GST and QR in the liver and tongue. Although dose-dependent effect was not found, citrus auraptene is effective in inhibiting the development of oral neoplasms induced by 4-NQO. Thus, suppression by the initiation-feeding of auraptene might relate to elevation in the phase II enzymes GST and QR of the liver and tongue, and inhibition occurring during the post-initiation might be related to suppression of increased cell proliferation caused by 4-NQO in the oral mucosa.