α-difluoromethylornithine blocks thymocyte apoptosis via a reduction in tyrosine phosphorylation

The effect of α-difluoromethylornithine on cell apoptosis was investigated. Freshly isolated mouse thymocytes were cultured in the medium alone or with dexamethasone, and apoptotic cell death was monitored after 6 h. A correlation was seen between cell apoptosis and a reduction in the polyamine levels of thymocytes. Addition of exogenous polyamines decreased the levels of apoptosis induced spontaneously in the culture medium or by dexamethasone. However, addition of α-difluoromethylornithine, an irreversible inhibitor of ornithine decarboxylase, to the cultures did not enhance apoptosis but rather caused inhibition of thymocyte apoptosis. Analysis of the mechanism of α-difluoromethylornithine-mediated inhibition of apoptosis indicated that α-difluoromethylornithine treatment blocked protein tyrosine phosphorylation, which was elevated drastically during the first hour of thymocyte cultivation. Treatment with the phosphotyrosine phosphatase inhibitor phenylarsine oxide reversed this inhibitory effect of α-difluoromethylornithine on apoptotic cell death. Our results provide an alternative mechanism for α-difluoromethylornithine showing the inhibition of apoptosis via reduction of protein tyrosine phosphorylation.