TAXOL MEDIATED AUGMENTATION OF CD95 LIGAND-INDUCED APOPTOSIS OF HUMAN-MALIGNANT GLIOMA-CELLS - ASSOCIATION WITH BCL-2 PHOSPHORYLATION BUT NEITHER ACTIVATION OF P53 NOR G(2) M CELL-CYCLE ARREST/

The anti-tumour alkaloid taxol shows strong cytotoxic and antiprolifer ative activity in two human malignant glioma cell lines, T98G and LN-2 29. CD95 (Fas/APO-1) ligand is a novel cytotoxic cytokine of the tumou r necrosis factor (TNF) family that exerts prominent antiglioma activi ty. At clinically relevant taxol concentrations of 5-100 nM, taxol and CD95 ligand showed significant synergistic cytotoxicity and growth in hibition. High concentrations of taxol induced G(2)/M cell cycle arres t in both cell lines. The synergy of taxol and CD95 ligand was indepen dent of cell cycle effects of taxol as synergy was achieved at much lo wer taxol concentrations than G(2)/M arrest and as cell cycle effects of taxol were unaffected by co-exposure to CD95 ligand. Similarly, hig h concentrations of taxol were required to induce p53 activity in the p53 wild-type cell line LN-229. This effect was not modulated by CD95 ligand, suggesting that synergy is also independent of p53 activation. However, taxol induced a mobility shift of the bcl-2 protein on immun oblot analysis, indicative of bcl-2 phosphorylation. Bcl-2 phosphoryla tion on serine was confirmed by immunoprecipitation and phosphoserine immunoblot analysis. Considering (1) that phosphorylation of bcl-2 int erferes with its heterodimerization with bar and (2) the inhibition of CD95-mediated apoptosis by bcl-2, we propose that taxol sensitizes ma lignant glioma cells to CD95 ligand by increasing the functional bax/b cl-2 rheostat in favour of bar and thus cell death.