alpha-difluoromethylornithine inhibits N-nitrosomethylbenzylamine-induced esophageal carcinogenesis in zinc-deficient rats: Effects on esophageal cell proliferation and apoptosis

Sustained, increased cell proliferation induced by dietary nine deficiency in rats plays a critical role in esophageal carcinogenesis. It is the deter mining factor that converts an otherwise nontumorigenic dose of N-nitrosome thylbenzylamine (NMBA) into a highly tumorigenic one. We studied whether th e increased esophageal cell proliferation and susceptibility to NMBA-induce d carcinogenesis induced by zinc deficiency can be inhibited by alpha-diflu oromethylornithine (DFMO), an enzyme-activated, irreversible inhibitor of o rnithine decarboxylase (the first enzyme in polyamine synthesis). Weanling rats were divided into four groups: Zn+/ DFMO-, Zn+/DFMO+, Zn-/DFMO-, and Z n-/DFMO+. They were fed ad libitum either a zinc-sufficient (Zn+, 75 ppm zi nc) or a zinc-deficient (Zn-, 4 ppm zinc) diet and given either deionized w ater (DPMO-) or 1% DFMO in deionized water (DFMO+). After 5 weeks, 5-19 ani mals from each group were sacrificed after in vivo 5-bromo-5'-deoxyuridine labeling to detect cells in S phase. The remaining animals in each group we re given a single intragastric dose of NMBA at 2 mg/kg and sacrificed 12 we eks later for tumor incidence analysis. At week 5, DFMO treatment greatly d ecreased (by 48-82%) the levels of putrescine and spermidine in rat esophag us, colon, and Liver, irrespective of dietary zinc intake. The increased es ophageal cell proliferation induced by dietary zinc deficiency, as measured by the labeling index, the number of labeled cells, and the total number o f cells, was substantially reduced by DFMO, This was accompanied by an incr ease in the rate of apoptosis, In addition, the expression of bar protein, an apoptosis accelerator, was markedly stronger in esophagi from Zn-/DPMOanimals that showed increased apoptosis, whereas increased expression of bc l-2, an inhibitor of apoptosis, was only seen in the highly proliferative, zinc-deficient esophagus (Zn-/ DFMO-). At week 12 after NMBA dosing, DFMO r educed the incidence of esophageal tumors from 80 to 4% in zinc-deficient r ats. Our data showed that DPMO effectively inhibited the increased esophage al cell proliferation induced by dietary zinc deficiency and reduced the in cidence of esophageal tumors induced by a single dose of NMBA in zinc-defic ient animals. Our results also indicate a role for increased apoptosis in t he mechanism(s) whereby DFMO brings about the inhibition of cell proliferat ion and tumor induction. These findings support a role for DPMO as a chemop reventive agent.