TAXOL INDUCTION OF P21(WAF1) AND P53 REQUIRES C-RAF-1

Taxol stabilizes microtubules, prevents tubulin depolymerization, and promotes tubulin bundling and is one of the most effective drugs for t he treatment of metastatic breast and ovarian cancer. Although its int eraction with tubulin has been well characterized, the mechanism by wh ich taxol induces growth arrest and cytoxicity is not well understood. Herein, me show that taxol induced dose- and time-dependent accumulat ion of the cyclin inhibitor p21(WAF1) in both p53 wild-type and p53-nu ll cells, although the degree of induction was greater in cells expres sing wild-type p53. In MCM cells, wild-type p53 protein was also induc ed after taxol treatment, and this induction was mediated primarily by increased protein stability. Taxol induced both p21(WAF1) and wild-ty pe p53 optimally in MCF7 cells after 20-24-h exposure with an EC(50)(3 ) of 5 nM. In p53-null PC3M cells, p21(WAF1) was similarly induced aft er 24-h exposure to taxol. Coincident with these biochemical effects, taxol altered the electrophoretic mobility of c-raf-1 and stimulated m itogen activated protein kinase. Previous depletion of c-raf-1 inhibit ed both the p21(WAF1)- and p53-inducing properties of taxol, as well a s the activation of MAP kinase. These data suggest that induction of p 21(WAF1) by taxol requires c-raf-1 activity, but that it is not strict ly dependent on wild-type p53. Furthermore, the ability of taxol to ba th induce wild-type p53 in MCF7 cells and activate MAP kinase is also dependent on c-raf-1 expression.