Chemopreventive effects of dietary D,L-alpha-difluoromethylornithine, an ornithine decarboxylase inhibitor, on initiation and postinitiation stages of diethylnitrosamine-induced rat hepatocarcinogenesis.

The modifying effects of dietary administration of D,L-alpha-difluoromethylornithine (DFMO) during initiation or postinitiation phase on the hepatocarcinogenesis initiated by diethylnitrosamine (DEN) were investigated in male F344 rats. A total of 129 animals were divided into seven groups. Groups 1-5 were given the drinking water containing 40 ppm DEN for 5 weeks, starting at 7 weeks of age. Groups 2 and 3 were fed the diets mixed with 500 and 1000 ppm DFMO, respectively, for 7 weeks, starting at 6 weeks of age. Groups 4 and 5 were given the diets containing 500 and 1000 ppm DFMO, respectively, starting 1 week after DEN exposure and maintained on these diets until the end of the study (Week 32). Rats in group 6 were fed the DFMO diet (1000 ppm) alone during the experiment. Group 7 served as an untreated control. At the end of the study, the incidences of liver cell foci (resistant iron accumulation or positive for glutathione S-transferase placental form) and hepatocellular neoplasms along with polyamine levels in the liver were measured. Also, morphometric analysis of silver-stained nucleolar organizer regions proteins as cell proliferation activity in liver cells was performed. The mean incidences and areas of foci in rats given DEN and DFMO in groups 2-5 were significantly lower than those of group 1 (P < 0.01). The frequencies of liver cell tumors in group 3 (50%), 4 (24%), and 5 (45%) were significantly reduced compared to that of group 1 (100%) (P < 0.01). The multiplicities of neoplasms in group 2 (1.15/rat), 3 (0.65/rat), 4 (0.35/rat), and 5 (0.95/rat) were significantly smaller than that of group 1 (3.34/rat) (P < 0.001). Although the polyamine levels of liver tissues among the groups showed no clear differences among the groups, the number and area of silver-stained nucleolar organizer regions proteins/nucleus in rats given DEN and DFMO (groups 2-5) were significantly lower than those of group 1. These results indicate that the feeding of DFMO during the initiation or postinitiation stage clearly inhibited DEN-induced rat hepatocarcinogenesis and that such inhibition may be due to alteration in cell proliferation activity caused by DFMO.