alpha-Difluoromethylornithine induction of apoptosis: A mechanism which reverses pre-established cell proliferation and cancer initiation in esophageal carcinogenesis in zinc-deficient rats

alpha -Difluoromethylornithine (DFMO) is an irreversible inhibitor of ornit hine decarboxylase, the first enzyme in polyamine synthesis. Previous work showed simultaneous administration of DFMO and a zinc-deficient (ZD) diet t o weanling rats from the beginning inhibited the onset of zinc-deficiency-i nduced esophageal cell proliferation by activating apoptosis and reduced th e incidence of N-nitrosomethylbenzylamine (NMBA)-induced esophageal cancer. Because esophageal cancer initiation by NMBA is very rapid in ZD rats, thi s study determined whether DFMO is effective in preventing esophageal carci nogenesis when administered after the establishment of a carcinogenic envir onment. Weanling rats were given a ZD diet for 5 weeks to establish sustain ed increased esophageal cell proliferation and then an intragastric dose of NMBA, Thereafter, 20 rats were switched to DFMO-containing water while nin e control ZD animals remained on deionized water; all of the animals contin ued on the ZD diet, Esophagi were collected 15 weeks later. The upper porti on was processed for immunohistochemical analysis of cell proliferation, ap optosis, and expression of related genes, and the lower was processed for p olyamine content. DFMO substantially reduces the levels of esophageal putre scine and spermidine and esophageal tumor incidence from 89 to 10% in ZD ra ts. Importantly, DFMO-treated ZD esophagi display increased rate of apoptos is accompanied by intense bar expression and greatly reduced cell prolifera tion by proliferating cell nuclear antigen expression. In addition, the p16 (ink-1a)/retinoblastoma control at G(1) to S, deregulated in ZD esophagi, i s restored after DFMO treatment, These results demonstrate that DFMO, a hig hly effective chemopreventive agent in esophageal carcinogenesis, reverses and counteracts esophageal cell proliferation/cancer initiation in ZD anima ls by way of stimulating apoptosis.