RAF-1 BCL-2 PHOSPHORYLATION - A STEP FROM MICROTUBULE DAMAGE TO CELL-DEATH/

Recent studies have shown that paclitaxel leads to activation of Raf-l kinase and hare suggested that this activation is essential for bcl-2 phosphorylation and apoptosis, Ins the present study, we demonstrate that, in addition to paclitaxel, other agents that interact with tibul in and microtubules also induce Raf-1/bcl-2 phosphorylation, whereas D NA-damaging drugs, antimetabolites, and alkylating agents do not, Acti vation of Raf-l kinase by paclitaxel is linked to tubulin polymerizati on; the effect is blunted in paclitaxel-resistant cells, the tubulin o f which does not polymerize following the addition of paclitaxel, In c ontrast. vincristine and vinblastine, drugs to which the paclitaxel-re sisiant cells retain sensitivity were able to bring shout Raf-1 phosph orylation. The requirement for disruption of microtubules in this sign aling cascade was strengthened further using paclitaxel analogues by d emonstrating a correlation between tubulin polymerization, RaF-1/bcl-2 phosphorylation, and cytotoxicity. Inhibition of RNA or protein synth esis prevents Raf-l activation anti bcl-2 phosphorylation, suggesting that an intermediate protein(s) acts upstream of Raf-1 in this microtu bule damage-activating pathway. A model is proposed that envisions a p athway of Raf-l activation and bcl-2 phosphorylation following disrupt ion of microtubular architecture, serving a role similar to p53 induct ion following DNA damage.