Signal transduction pathways of taxanes-induced apoptosis

Docetaxel (Taxotere) is a member of the taxane class of anticancer agents to reach clinical use. This semisynthetic analog of paclitaxel (Taxol) is one of the newer potent anti-neoplastic agents now undergoing extensive laboratory and clinical investigations. Several studies indicate that antimicrotubule agents are potent promoters of apoptosis in cancer cells. Cytotoxic mechanisms of antimitotic taxoids are not yet fully understood, but it has been demonstrated that docetaxel increases tubulin polymerisation, promotes microtubule assembly and also inhibits tubulin depolymerisation. Disruption of microtubules results also in the induction of tumor suppressor gene p53 and inhibitor of cyclin-dependent kinases and activation/inactivation of several protein kinases. As a consequence cells are arrested in the G2-M phase of the cell cycle, after which they may either undergo cell death by apoptosis or necrosis or overcome the G2-M stop and continue in the division cycle (often toward a post-mitotic cell death) depending on the tumor cell type. Nevertheless, how docetaxel induces apoptotic cell death or caspases activation is not yet defined. One may assume that taxanes are able to induce the phosphorylation of Bcl-X(L)/Bcl-2 members and thus inactivate their anti-apoptotic capacities. The down-regulation of Bcl-2 and/or the upregulation of p53 and p21/WAF-1 are certainly one of the important modes of apoptosis induction by taxanes. The aim of this framework is to summarize the effects of microtubuline targeting agents on apoptotic signal transduction and new molecular pathways. Finally, we will also discuss the potential therapeutic interest in the association of docetaxel and ionizing radiation.