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.

Antimicrotubule Vinca alkaloids, such as vinblastine and vincristine, interfere with the dynamics of microtubules and have shown significant cell killing activity in a variety of tumor cells through induction of apoptosis. The mechanism by which Vinca alkaloids induce apoptosis is not entirely clear. In this study, we found that glucocorticoids inhibit Vinca alkaloid-induced apoptosis without affecting G2-M arrest in human breast cancer BCap37 cells and human epidermoid tumor KB cells, suggesting that Vinca alkaloid-induced apoptosis may occur via a pathway independent of cell cycle arrest. Further analyses indicated that Vinca alkaloids cause significant degradation of IκBα, which in turn results in nuclear factor-κB (NF-κB) activation. Transfection of antisense IκBα in BCap37 cells sensitizes Vinca alkaloid-induced apoptosis. Moreover, in vitro kinase assays show that the activity of IκB kinase (IKK) was activated by Vinca alkaloids and was not affected by glucocorticoids. Stable transfection of dominant-negative deletional mutant IκBα, which is insensitive to IKK-mediated phosphorylation and degradation, resulted in the inhibition of Vinca alkaloid-induced NF-κB activation and reduced sensitivity of tumor cells to Vinca alkaloid-induced apoptosis. These findings suggest that the NF-κB/IκB signaling pathway may contribute to the mediation of Vinca alkaloid-induced apoptosis in human tumor cells.

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