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

The effect of alpha-difluoromethylornithine on cell apoptosis was investiga ted. Freshly isolated mouse thymocytes were cultured in the medium alone or with dexamethasone, and apoptotic cell death was monitored after 6 h. A co rrelation 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 dexamethaso ne. However, addition of alpha-difluoromethylornithine, an irreversible inh ibitor of ornithine decarboxylase, to the cultures did not enhance apoptosi s but rather caused inhibition of thymocyte apoptosis. Analysis of the mech anism of alpha-difluoromethylornithine-mediated inhibition of apoptosis ind icated that alpha-difluoromethylornithine treatment blocked protein tyrosin e phosphorylation, which was elevated drastically during the first hour of thymocyte cultivation. Treatment with the phosphotyrosine phosphatase inhib itor phenylarsine oxide reversed this inhibitory effect of alpha-difluorome thylornithine on apoptotic cell death. Our results provide an alternative m echansim for alpha-difluoromethylornithine showing the inhibition of apopto sis via reduction of protein tyrosine phosphorylation.