Synergistic action of tiazofurin and genistein on growth inhibition and differentiation of K-562 human leukemic cells

Tiazofurin (2-beta-D-ribofuranosylthiazole-4-carboxamide, NSC 286193), an oncolytic drug, inhibits IMP DH (inosine 5'-monophosphate dehydrogenase, EC 1.1.1.205), reduces signal transduction activity and IP2 (inositol 1,4,5-trisphosphate) concentration and arrests the cell cycle chiefly in S phase. Genistein (4',5,7-trihydroxyisoflavone), an inhibitor of PIP kinase (1-phosphatidylinositol 4-phosphate 5-kinase, EC 2.7.1.68), tyrosine kinase and topoisomerase-II, induces arrest in G(2) and/or early M phase in most carcinoma cells. Both drugs, as single agents, induce differentiation. Since tiazofurin and genistein attack different enzymic targets and arrest the cell cycle at different phases and they each induce differentiation, we tested the hypothesis that tiazofurin might be synergistic with genistein in inducing differentiation. Human leukemic K-562 cells were grown in suspension culture and were seeded in 24-well culture plates. In growth inhibition assays for tiazofurin and genistein IC(50)s (drug concentration that inhibits 50% of cell proliferation) were 7 and 37 mu M, respectively. For tiazofurin and genistein the concentrations of drug that induce differentiation in 50% of the cells were 35 and 45 mu M, respectively. Various combinations of these two drugs were tested. Since tiazofurin decreased GTP concentration in cells by 50% at 12 hr after administration, genistein (10 to 30 mu M) was added 12 hr after tiazofurin (5 to 15 mu M). Synergistic action on differentiation was obtained from all tiazofurin and genistein combinations and in most combinations on growth inhibition. The percent of differentiating cells induced by genistein (10 mu M) and tiazofurin (10 mu M) as single agents increased 1.1- and 2.8-fold, respectively, of the control values. The two drugs together caused 5.9-fold elevation in inducing differentiation. Similar action was observed on inhibition of proliferation.