Potentiation of CD95L-induced apoptosis of human malignant glioma cells by topotecan involves inhibition of RNA synthesis but not changes in CD95 or CD95L protein expression.

Topotecan is a novel topoisomerase I inhibitor that may have a role in the adjuvant chemotherapy of several solid tumors, including malignant glioma. Here, we have characterized the time- and concentration-dependent toxicity of topotecan in four human malignant glioma cell lines, LN-18, LN-229, LN-308 and T98G. High micromolar concentrations of topotecan, which are unlikely to be achieved in plasma in human patients in vivo, were cytotoxic within 48 hr, induced DNA fragmentation, did not induce major cell cycle changes, failed to consistently alter BCL-2 or BAX protein levels but inhibited RNA synthesis and induced cleavable DNA/topoisomerase I complex formation. Prolonged exposure for 72 hr to high nanomolar to low micromolar concentrations of topotecan augmented p21 protein levels and induced G2/M arrest but failed to consistently alter BCL-2 and BAX protein levels, did not induce significant DNA/topoisomerase I complex formation and did not inhibit RNA synthesis. Neither short-term nor long-term topotecan toxicity was blocked by ectopic expression of bcl-2 or wild-type p53. Transfer of a mutant p53 gene enhanced topotecan sensitivity in wild-type p53 LN-229 but not mutant p53 LN-18 cells. CD95 ligand (CD95L)-induced apoptosis was synergistically enhanced by short-term/high concentration but not long-term/low concentration exposure to topotecan, suggesting that topotecan sensitizes human malignant glioma cells to CD95L-induced apoptosis via inhibition of RNA synthesis. These data suggest that topotecan needs to be administered in high concentrations, such as an intratumoral polymer, to limit glioma cell growth in synergy with CD95L in vivo.