Our previous study has shown that dietary administration of protocatechuic acid (PCA) acts as potential chemopreventive agent in inhibiting diethylnitrosamine-induced liver carcinogenesis in male F344 rats. The present study was designed to determine the modifying effect of PCA on azoxymethane (AOM)-induced colon carcinogenesis in male F344 rats and the effect on intermediate biomarkers, i.e., colonic mucosal ornithine decarboxylase activity and colonic epithelial proliferation, which can be used as effective predictors of colon cancer. Staring at 6 weeks of age, groups of animals were fed the basal diet and experimental diet containing PCA at dose levels of 250, 500, and 1000 ppm. At 7 weeks of age, all animals except the PCA alone group (1000 ppm) and untreated controls were given s.c. injections of AOM at a dose level of 15 mg/kg body weight/week for 3 weeks. PCA at 3 doses was fed during the initiation phase (before 1 week, during, and after 1 week of AOM exposure) or postinitiation phase (for 28 weeks starting 1 week after the last injection of AOM). All animals were then killed at 32 weeks after the start and colonic tumor incidence and multiplicity were determined. Animals intended for cell proliferation study were given injections of bromodeoxyuridine/5-fluoro-2′-deoxyurdine (1 ml/100 g body weight) 1 h prior to be killing. The rate of colonic cell proliferation in the distal portion was assessed by immunohistochemistry using antibromodeoxyuridine and by counting silverstained nucleolar organizer regions protein. The colonic mucosal ornithine decarboxylase activity was also measured at the termination. The results indicate that dietary PCA administration at 500 and 1000 ppm during the initiation or postinitiation phase significantly inhibited intestinal carcinogenesis induced by AOM as revealed by the reduction of tumor incidence and multiplicity. The data also demonstrate that PCA at 500 ppm and 1000 ppm significantly inhibited bromodeoxyuridine labeling index and also silver-stained nucleolar organizer regions protein number at three doses when animals were fed PCA at the initiation or postinitiation stage. Also, feeding of PCA at 1000 ppm during the initiation and postinitiation phase exerted a pronounced inhibitory effect on the colonic ornithine decarboxylase levels. PCA feeding did not cause any toxicity. These results demonstrate that PCA is a possible new chemopreventive agent for colon carcinogenesis through the suppression of manifestation of intermediate biomarkers induced by AOM, although the precise mechanisms of PCA-induced inhibition during the initiation and postinitiation phases remain to be elucidated.
Supported by Grant-in Aids for Cancer Research from the Ministry of Health and Welfare of Japan.