TAXOL-INDUCED MITOTIC BLOCK TRIGGERS RAPID ONSET OF A P53-INDEPENDENTAPOPTOTIC PATHWAY

Background: At therapeutic concentrations, the antineoplastic agent ta xol selectively perturbs mitotic spindle microtubules. Taxol has recen tly been shown to induce apoptosis, similar to the mechanism of cell d eath induced by other antineoplastic agents. However, taxol has shown efficacy against drug-refractory cancers, raising the possibility that this pharmacological agent may trigger an alternative apoptotic pathw ay. Materials and Methods: The kinetics and IC50 of mitotic (M) block aberrant mitosis, and cytotoxicity following taxol treatment were anal yzed in human cell lines as well as normal mouse embryo fibroblasts (M EFs) and MEFs derived from p53-null mice. Apoptosis was followed by DN A gel electrophoresis and by in situ DNA end-labeling (TUNEL). Results : Taxol induced two forms of cell cycle arrest: either directly in ear ly M at prophase or, for those cells progressing through aberrant mito sis, arrest in G(1) as multimininucleated cells. TUNEL labeling reveal ed that DNA nicking occurred within 30 min of the arrest in prophase. In contrast, G(1)-arrested, multimininucleated cells became TUNEL posi tive only after several days. In the subset of cells that became block ed directly in prophase, both wt p53-expressing and p53-null MEFs resp onded similarly to taxol, showing rapid onset of DNA nicking and apopt osis. However, p53-null MEFs progressing through aberrant mitosis fail ed to arrest in the subsequent G(1) phase or to become TUNEL positive, and remained viable. Conclusions: Taxol induces two forms of cell cyc le arrest which in turn induce two independent apoptotic pathways. Arr est in prophase induces rapid onset of a p53-independent pathway where as G(1)-block and the resulting slow (3-5 days) apoptotic pathway are p53 dependent.