MECHANISMS OF TAXOL-INDUCED CELL-DEATH ARE CONCENTRATION-DEPENDENT

Although the ability of Taxol to stabilize cellular microtubules is we ll accepted, the mechanisms by which Taxol induces growth arrest and c ell death remain unclear. Recent evidence indicates that Taxol alters specific intracellular signal transduction events, such as the activat ion of Raf-1 kinase, that may be essential for drug-induced apoptosis, To determine whether Raf-1 kinase activation occurs at different conc entrations of Taxol and in response to disruption of the normal microt ubule cytoskeleton, A549 cells were treated with different concentrati ons of Taxol after which Raf-1 activation and the microtubule cytoskel eton were analyzed, Raf-1 activation was observed at Taxol concentrati ons of 9 nhl and greater. However, disruption of the normal microtubul e cytoskeleton was seen at lower Taxol concentrations (1-7 nM), indica ting that this process begins in the absence of Raf-1 activation. Raf- 1 activation correlated with the induction of a G(2)-M block. Depletio n of Raf-1 resulted in the accumulation of cells in the G(2)-M phase o f the cell cycle, suggesting that Raf-1 may play an important role in the passage through mitosis, Supporting this idea, Raf-1 was activated in mitotic cells. Low concentrations of Taxol induced cell death in t he absence of Raf-1 activation, indicating that Taxol-induced cell dea th is not dependent on Raf-1 activation. At concentrations of drug low er than the critical concentration required for Raf-1 activation, p53 and p21(WAF-1) were induced independently of Raf-l. These studies sugg est that Taxol-mediated cell death may result from two different mecha nisms. At low Taxol concentrations (<9 nM), cell death may occur after an aberrant mitosis by a Raf-1 independent pathway, whereas at higher Taxol concentrations (greater than or equal to 9 nM) cell death may b e the result of a terminal mitotic arrest occurring by a Raf-1-depende nt pathway.