Synergistic cytotoxicity, apoptosis and protein-linked DNA breakage by etoposide and camptothecin in human U87 glioma cells : dependence on tyrosine phosphorylation

• Published in: Journal of neuro-oncology Vol. 41; no. 3; pp. 223 - 234
• Main Authors: CIESIELSKI, M. J, FENSTERMAKER, R. A
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer 01-02-1999; Springer Nature B.V
• Subjects: Antineoplastic agents; Biological and medical sciences; Brain Neoplasms; Camptothecin - toxicity; Cell Survival - drug effects; Chemotherapy; Colonic Neoplasms; DNA Damage; Drug Synergism; Etoposide - toxicity; Glioblastoma; Glioma; Humans; Medical sciences; Mitotic Index - drug effects; Pharmacology. Drug treatments; Phosphotyrosine - metabolism; Tumor Cells, Cultured; Tumor Suppressor Protein p53 - analysis; Antineoplastic agent; Human; Tyrosine; DNA topoisomerase (ATP-hydrolysing); Nervous system diseases; Drug combination; Phosphorylation; Synergist; Enzyme; Etoposide; Enzyme inhibitor; Cytotoxicity; Podophyllotoxine derivatives; Alkaloid; Isomerases; Cell line; Glioma; Central nervous system disease; Tumor; Mechanism of action; Antimitotic
• ISSN: 0167-594X; 1573-7373
• DOI: 10.1023/A:1006129119460

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.