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

Abstract

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.

• Authors: M J Ciesielski, R A Fenstermaker
• Journal: Journal of neuro-oncology (J Neurooncol) Vol. 41 Issue 3 Pg. 223-34 (Feb 1999) ISSN: 0167-594X [Print] United States
• PMID: 10359142 (Publication Type: Journal Article, Research Support, U.S. Gov't, P.H.S.)

Chemical References

• Tumor Suppressor Protein p53
• Phosphotyrosine
• Etoposide
• Camptothecin

Topics

• Brain Neoplasms
• Camptothecin (toxicity)
• Cell Survival (drug effects)
• Colonic Neoplasms
• DNA Damage
• Drug Synergism
• Etoposide (toxicity)
• Glioblastoma
• Glioma
• Humans
• Mitotic Index (drug effects)
• Phosphotyrosine (metabolism)
• Tumor Cells, Cultured
• Tumor Suppressor Protein p53 (analysis)