Cell Cycle–Dependent Antagonistic Interactions between Paclitaxel and γ-Radiation in Combination Therapy

Abstract

Purpose: The promising clinical activity of paclitaxel, a naturally occurring antimicrotubule agent, has promoted considerable interest in combining this drug with radiation therapy, but it remains unclear whether such a combination would increase the therapeutic efficacy. This study is to assess the potential interactions between paclitaxel and ␥-radiation against human tumor cells in vitro.Experimental Design: Paclitaxel and ␥-radiation were administered in three different sequences designated as preradiated, co-radiated, and post-radiated to BCap37 (human breast cancer cell line) and KB (human epidermoid carcinoma cell line) cells. The cytotoxic interactions between and mutual influences of these two agents on their antitumor activities were analyzed by a series of assays including cytotoxic, morphological, and biochemical examinations.Results: The combination of paclitaxel and ␥-radiation did not produce a synergistic or additive effect. Instead, the overall in vitro cytotoxicity of these combinations was much lower than that of paclitaxel treatment alone. DNA fragmentation and flow cytometric assays showed that the addition of ␥-radiation interfered with paclitaxel-induced apoptosis. Further analyses indicated that the addition of ␥-radiation resulted in a transient or prolonged cell cycle arrest at G 2 phase, which likely prevented the cytotoxic effects of paclitaxel on both mitotic arrest and apoptosis. In addition, biochemical examinations revealed that ␥-radiation inhibited paclitaxel-induced IB␣ degradation and bcl-2 phosphorylation and increased the protein levels of cyclin B1 and inhibitory phosphorylation of p34 cdc2 . Conclusions: Our results suggest that ␥-radiation might specifically block the cell cycle at G 2 phase, which in turn prevents the cytotoxic effects of paclitaxel on both mitotic arrest and apoptosis. Therefore, it eventually results in a cell cycle-dependent antagonistic effect on the antitumor activity of paclitaxel. This finding may be relevant to the clinical application of combination therapy with paclitaxel and radiation.

Estrogen Receptor α Mediates Breast Cancer Cell Resistance to Paclitaxel through Inhibition of Apoptotic Cell Death

“…The human beast cancer BCap37 cell line has been used as a wellcharacterized cell model in our previous studies for the evaluation of taxane-containing combination therapy (19,20,(30)(31)(32). This ERÀ cell line is very sensitive to paclitaxel and docetaxel in both mitotic arrest and apoptosis.…”

“…Approximately 0.5 Â 10 5 to 1 Â 10 5 cells were plated onto microscope slides using the Cytospin 3 cell preparation system. As described previously (30), slides were air dried and fixed in absolute methanol before Giemsa staining. Slides from three independent experiments were examined and photographed using bright-field microscopy.…”

Potent antitumor activity of 10-methoxy-9-nitrocamptothecin

“…It is widely believed that the CDK1/cyclin B1 complex plays a critical role during the G 2 phase. During the G 2 phase, CDK1 forms a heterodimeric complex with cyclin B1 and remains inactive, the CDK1/cyclin B1 complex could be activated by CDK7 (33). The up-regulation process of CDK1 and cyclin B seem to be necessary to block the cell at G 2 -M phase (34).…”

Cell-cycle dependent antagonistic interactions between paclitaxel and carboplatin in combination therapy

“…19,20 In addition, our recent studies indicated that IkBa/NFkb signaling pathway might play a critical role in the mediation of paclitaxel-induced apoptotic cell death. 21,22 We performed Western blotting of BCap37 cell extracts to examine the potential altered expression of a number of regulatory proteins that could contribute to the activity of paclitaxel in inducing cell cycle arrest and apoptosis.…”