Induction of mouse thymocyte apoptosis by inhibitors of tyrosine kinases is associated with dephosphorylation of nuclear proteins.

Incubation of mouse thymocytes with the protein tyrosine kinase inhibitors herbimycin A and methyl-2,5-dihydroxycinnamate induced a decreased and altered profile of nuclear phosphotyrosine proteins in parallel with an increase in internucleosomal DNA fragmentation and cell death dose-dependently. No change in the profile of cytoplasmic phosphotyrosine proteins was observed. DNA fragmentation was dependent on the synthesis of RNA and protein, suggesting that the inhibition of tyrosine phosphorylation of the nuclear proteins induces apoptosis. DNA fragmentation was enhanced by simultaneous incubation with phorbol esters capable of activating protein kinase C. Genistein, another inhibitor of protein tyrosine kinase, induced DNA fragmentation more rapidly than herbimycin A, but there was no predominant alteration of phosphotyrosine proteins in early incubation, suggesting that genistein may induce apoptosis by a mechanism other than direct inhibition of protein tyrosinekinase activity.