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 mechansim for α‐difluoromethylornithine showing the inhibition of apoptosis via reduction of protein tyrosine phosphorylation.