Molecular effects of paclitaxel: myths and reality (a critical review)

Recent studies on paclitaxel (Taxol), a microtubule-stabilizing agent and effective anti-cancer drug, have identified numerous cellular and molecular effects, such as induction of cytokines and tumor-suppressor genes, indirect cytotoxicity due to secretion of tumor necrosis factor, vast activation of signal-transduction pathways and selective activity against cells lacking functional p53. Some of these results, including the immediate activation of signaling pathways and gene expression, have been observed only with paclitaxel concentrations 1,000-fold higher than those required for mitotic arrest and apoptosis. The effects of loss of p53 on paclitaxel cytotoxicity depend on cell type (normal murine fibroblasts vs. human cancer cells) and duration of exposure to paclitaxel; p53 status marginally affects paclitaxel sensitivity in human cancer. Although the biochemistry of mitosis and meiosis has been studied independently of research on the mechanism of action of anti-cancer drugs, it eventually provided insight into the effects of paclitaxel. For example, serine protein phosphorylation, which occurs during mitotic arrest or meiosis, explains paclitaxel-induced hyperphosphorylation of Bcl-2 and Bcl-xL. Although some observations are disputed, such mitotic arrest correlates with paclitaxel cytotoxicity, while there is currently no evidence that any paclitaxel effect at clinically relevant concentrations is independent of its tubulin-binding properties. Thus, paclitaxel exerts two types of effect: mitotic arrest with coincidental serine protein phosphorylation and cytotoxicity at clinically relevant concentrations as well as immediate activation of tyrosine kinase pathways and activation of gene expression at much higher concentrations.