Synergistic cytotoxicity, apoptosis and protein-linked DNA breakage by etoposide and camptothecin in human U87 glioma cells: dependence on tyrosine phosphorylation.
In this study, simultaneous administration of certain inhibitors of topoisomerase I and topoisomerase II produced synergistic cytotoxicity in a series of human glioma cell lines. Camptothecin (CPT) and etoposide (VP-16) produced combination indices (CI) <1.0 in all glioma cell lines tested, including those that were relatively resistant to the two topoisomerase inhibitors individually. In contrast, CPT and VP-16 produced additive cytotoxicity in HT-29 and SW-620 colon carcinoma cell lines. To explore the molecular basis for synergy in glioma cells, we focused on one glioma cell line (U87) in which even sub-cytotoxic doses of CPT potentiated the action of VP-16. Except for genistein (a topo II agent with tyrosine kinase inhibitory function), all topo II inhibitors tested (doxorubicin, ellipticine, and m-AMSA) were synergistic with CPT. While CPT and VP-16 produced cytotoxicity and protein-linked DNA breaks (PLDB) that were supra-additive in U87 glioma cells, CPT and genistein produced additive results. Pretreatment of U87 cells with the tyrosine kinase inhibitor tyrphostin-A23 or the tyrosine phosphatase activator O-phospho-L-tyrosine (OPLT) reduced combination PLDB from synergistic to additive levels, but had no effect on the formation of PLDB induced by either CPT or VP-16 alone. CPT and VP-16 also produced a synergistic accumulation of sub-G0 (apoptotic) cells which was blocked by tyrphostin-A23. No significant increase in topoisomerase protein levels could be detected in response to combination treatment. Thus, synergistic effects between topoisomerase I and topoisomerase II inhibitors in U87 glioma cells may depend upon phosphorylation of cellular proteins other than the topoisomerases themselves.
Ciesielski
M J
MJ
Department of Neurosurgery, Roswell Park Cancer Institute, Buffalo, New York 14263, USA.
Fenstermaker
R A
RA
D001932:Brain Neoplasms; D002166:Camptothecin / Q000633:toxicity*; D002470:Cell Survival / Q000187:drug effects*; D003110:Colonic Neoplasms; D004249:DNA Damage; D004357:Drug Synergism; D005047:Etoposide / Q000633:toxicity*; D005909:Glioblastoma; D005910:Glioma; D006801:Humans; D008940:Mitotic Index / Q000187:drug effects; D019000:Phosphotyrosine / Q000378:metabolism*; D014407:Tumor Cells, Cultured; D016159:Tumor Suppressor Protein p53 / Q000032:analysis
D016159:Tumor Suppressor Protein p53; D019000:Phosphotyrosine; D005047:Etoposide; D002166:Camptothecin