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Paclitaxel sensitivity correlates with p53 status and DNA fragmentation, but not G2/M accumulation
E Rakovitch1, W Mellado, E J Hall
1Center for Radiological Research, College of Physicians and Surgeons, Columbia University, New York, NY 10032, USA.
Purpose:
The antitumor agent paclitaxel (Taxol) has been shown to arrest cells in mitosis through microtubule stabilization and to induce apoptosis. The tumor suppressor gene p53 is implicated in the regulation of cell cycle checkpoints and can mediate apoptotic cell death. Although initial studies demonstrated that various DNA-damaging agents can induce p53, more recent studies have also shown p53 induction following nonDNA-damaging agents, including paclitaxel. We investigated the influence of p53 abrogation on paclitaxel-induced cell kill and correlated the extent of mitotic arrest and DNA fragmentation by paclitaxel with the drug's cytotoxic effect.
Materials And Methods:
The parental human colorectal carcinoma cell line (RKO) with wild-type p53 alleles, and two transfected RKO cell lines with inactivated p53 (RKO.p53.13 with transfected mutant p53 and RC 10.3 with HPV-16-derived E6 gene) were exposed to graded doses of paclitaxel (1-100 nM) for 24-h intervals. The functional status of p53 in cells was assessed by thymidine and BrdU incorporation following exposure to ionizing radiation (4 Gy). Reproductive integrity following paclitaxel treatment was assessed by clonogenic assay. Immunolabeling and microscopic evaluation were used to assess mitotic accumulation and micronucleation. Apoptosis was assayed using DNA fragmentation analyses.
Results:
A 4-fold increase in paclitaxel sensitivity was observed among RKO cells deficient in p53 function compared with wild-type RKO cells (IC 50: 4 nM, 1 nM, 1nM for RKO, RKO.p53.13, RC 10.3, respectively). The increased cytotoxic effect in RKO cells with inactive p53 correlated with an increased propensity towards micronucleation and DNA fragmentation following paclitaxel treatment. However, no significant difference in peak mitotic accumulation was observed among RKO cells with functional or abrogated p53.
Conclusions:
RKO cells lacking functional p53 demonstrate significantly enhanced sensitivity to paclitaxel compared with that of wild-type RKO cells. This response corresponded with increased micronucleation and DNA fragmentation in cells deficient in p53 function. Although previous published reports of enhanced paclitaxel sensitivity in p53-deficient cells correlated this finding with increased G2/M arrest, we did not observe any significant correlation between paclitaxel-induced cell kill and the degree of mitotic arrest. Our data suggest that apoptosis is the predominant mechanism of paclitaxel cytotoxicity in RKO cells and is likely mediated by a p53-independent process.
Insights
The tumor suppressor p53 (a gene) does not affect paclitaxel (Taxol) sensitivity in colorectal cancer cells. Paclitaxel induces apoptosis, a cell death process, in a p53-independent manner, enhancing its cytotoxic effect.
Area of Science:
- Oncology
- Molecular Biology
- Cell Biology
Background:
- Paclitaxel (Taxol) is an antitumor agent that induces apoptosis and mitotic arrest.
- The tumor suppressor gene p53 regulates cell cycle checkpoints and apoptosis.
- p53 can be induced by DNA-damaging and non-DNA-damaging agents like paclitaxel.
Purpose of the Study:
- To investigate the influence of p53 abrogation on paclitaxel-induced cell kill.
- To correlate mitotic arrest and DNA fragmentation with paclitaxel's cytotoxic effect.
- To determine the role of p53 in paclitaxel's mechanism of action.
Main Methods:
- Utilized human colorectal carcinoma cell lines (RKO) with wild-type and inactivated p53.
- Exposed cells to paclitaxel (1-100 nM) and assessed cell viability via clonogenic assay.
- Evaluated mitotic accumulation, micronucleation, and DNA fragmentation to analyze apoptosis.
Main Results:
- Cells lacking functional p53 showed a 4-fold increase in paclitaxel sensitivity (IC50: 1 nM) compared to wild-type cells (IC50: 4 nM).
- Increased cytotoxicity in p53-deficient cells correlated with higher rates of micronucleation and DNA fragmentation.
- No significant difference in peak mitotic arrest was observed between p53-proficient and p53-deficient cells.
Conclusions:
- p53-deficient RKO cells exhibit significantly enhanced sensitivity to paclitaxel.
- This enhanced sensitivity is linked to increased micronucleation and DNA fragmentation.
- Apoptosis, mediated by a p53-independent pathway, is the primary mechanism of paclitaxel cytotoxicity in these cells.