Taxol-induced mitotic block triggers rapid onset of a p53-independent apoptotic pathway.
BACKGROUND
At therapeutic concentrations, the antineoplastic agent taxol selectively perturbs mitotic spindle microtubules. Taxol has recently been shown to induce apoptosis, similar to the mechanism of cell death 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 pathway.


METHODS
The kinetics and IC50 of mitotic (M) block, aberrant mitosis, and cytotoxicity following taxol treatment were analyzed in human cell lines as well as normal mouse embryo fibroblasts (MEFs) and MEFs derived from p53-null mice. Apoptosis was followed by DNA gel electrophoresis and by in situ DNA end-labeling (TUNEL).


RESULTS
Taxol induced two forms of cell cycle arrest: either directly in early M at prophase or, for those cells progressing through aberrant mitosis, arrest in G1 as multimininucleated cells. TUNEL labeling revealed that DNA nicking occurred within 30 min of the arrest in prophase. In contrast, G1-arrested, multimininucleated cells became TUNEL positive only after several days. In the subset of cells that became blocked directly in prophase, both wt p53-expressing and p53-null MEFs responded similarly to taxol, showing rapid onset of DNA nicking and apoptosis. However, p53-null MEFs progressing through aberrant mitosis failed to arrest in the subsequent G1 phase or to become TUNEL positive, and remained viable.


CONCLUSIONS
Taxol induces two forms of cell cycle arrest, which in turn induce two independent apoptotic pathways. Arrest in prophase induces rapid onset of a p53-independent pathway, whereas G1-block and the resulting slow (3-5 days) apoptotic pathway are p53 dependent.
Department of Pharmacology, Merck Research Laboratories, West Point, Pennsylvania, USA.
C M
CM
Woods
J
J
Zhu
P A
PA
McQueney
D
D
Bollag
E
E
Lazarides
D000818:Animals; D000972:Antineoplastic Agents, Phytogenic / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects*; D002452:Cell Count / Q000187:drug effects; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism / Q000648:ultrastructure; D002478:Cells, Cultured; D005047:Etoposide / Q000494:pharmacology; D005455:Fluorescent Antibody Technique; D016195:G2 Phase / Q000187:drug effects; D006367:HeLa Cells; D006801:Humans; D007700:Kinetics; D051379:Mice; D008938:Mitosis / Q000187:drug effects; D017239:Paclitaxel / Q000494:pharmacology*; D010641:Phenotype; D011418:Prophase / Q000187:drug effects; D051381:Rats; D014404:Tubulin / Q000276:immunology / Q000378:metabolism / Q000648:ultrastructure; D016159:Tumor Suppressor Protein p53 / Q000502:physiology*
D000972:Antineoplastic Agents, Phytogenic; D014404:Tubulin; D016159:Tumor Suppressor Protein p53; D005047:Etoposide; D017239:Paclitaxel