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.