EFFECTS OF PROTEIN-TYROSINE KINASE INHIBITORS WITH DIFFERENT MODES OFACTION ON TOPOISOMERASE ACTIVITY AND DEATH OF IL-2-DEPENDENT CTLL-2 CELLS

We studied the effects of protein tyrosine kinase inhibitors with diff erent modes of action on topoisomerase activity and cell death in CTLL -2 cells, whose growth is IL-2-dependent. The Flavonoids genistein, bi ochanin A, and apigenin inhibited topoisomerase II to the same extent as etoposide, a specific inhibitor of the enzyme. Methyl 2,5-dihydroxy cinnamate (2,5-MeC) also inhibited topoisomerase II, but was less pote nt than genistein. Herbimycin A and staurosporine did not inhibit topo isomerase II. None of the inhibitors of protein tyrosine kinases exami ned inhibited topoisomerase I activity. All the inhibitors induced cel l death with internucleosomal DNA fragmentation in the presence of IL- 2. Genistein, biochanin A, and apigenin induced DNA fragmentation and cell death early in the incubation period and did not alter the profil es of phosphotyrosine proteins in either the lysate or pelleted fracti ons, indicating that the early cell death was induced by the inhibitio n of topoisomerase II activity rather than by the inhibition of protei n tyrosine kinase activity. 2,5-MeC similarly induced early cell death and DNA fragmentation, but to a lesser extent than genistein presumab ly due to the inhibition of topoisomerase II activity. Herbimycin A in duced a slow increase in DNA fragmentation and cell death, accompanied by a decrease in phosphotyrosine proteins in the pelleted fraction, s uggesting that the inhibition of protein tyrosine phosphorylation, pre sumably of the nuclear proteins, is related to cell death and DNA frag mentation. Staurosporine-induced DNA fragmentation appeared to be due to mechanism(s) other than the inhibition of topoisomerases and protei n tyrosine kinases, since it neither altered the profiles of phosphoty rosine proteins nor inhibited topoisomerase activity.