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

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 γ-radiation against human tumor cells in vitro.

Experimental Design: Paclitaxel and γ-radiation were administered in three different sequences designated as pre-radiated, 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 γ-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 γ-radiation interfered with paclitaxel-induced apoptosis. Further analyses indicated that the addition of γ-radiation resulted in a transient or prolonged cell cycle arrest at G2 phase, which likely prevented the cytotoxic effects of paclitaxel on both mitotic arrest and apoptosis. In addition, biochemical examinations revealed that γ-radiation inhibited paclitaxel-induced IκBα degradation and bcl-2 phosphorylation and increased the protein levels of cyclin B1 and inhibitory phosphorylation of p34cdc2.

Conclusions: Our results suggest that γ-radiation might specifically block the cell cycle at G2 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.

Taxol Induces Apoptosis in Cortical Neurons by a Mechanism Independent of Bcl-2 Phosphorylation

Bcl-2, an antiapoptotic protein, protects cells against many but not all forms of apoptosis. For example, Bcl-2 does not protect non-neuronal cells against taxol, a microtubule-stabilizing agent. The underlying mechanism for the ineffectiveness of Bcl-2 against taxol has been the subject of intense interest. Data from non-neuronal cells indicate that taxol-induced apoptosis requires activation of N-terminal c-Jun protein kinase (JNK) that phosphorylates and inactivates Bcl-2. This suggests the interesting possibility that the apoptotic activity of JNK may be caused by phosphorylation of Bcl-2 and inhibition of the antiapoptotic activity of Bcl-2. Here we report that taxol induces apoptosis in cortical neurons but by a mechanism significantly different from that in non-neuronal cells. In contrast to human embryonic kidney 293 cells, expression of wild-type Bcl-2 in cortical neurons protected against taxol-induced apoptosis, and taxol did not induce Bcl-2 phosphorylation. Furthermore, cortical neurons express high basal JNK activity, and taxol did not stimulate total JNK activity. However, taxol activated a subpool of JNK in the nucleus and stimulated c-Jun phosphorylation. JNK inhibition or expression of a dominant-negative c-Jun abrogated taxol-induced apoptosis in cortical neurons, suggesting a role for JNK and JNK-mediated transcription in taxol-stimulated apoptosis. Furthermore, taxol-induced apoptosis in cortical neurons required inhibition of phosphatidylinositol 3-kinase signaling. These data suggest that taxol induces apoptosis in neurons by a mechanism quite distinct from that of non-neuronal cell lines and emphasize the importance of elucidating apoptotic mechanisms specific for neurons in the CNS.

Total synthesis of taxol

TAXOL1–4, a substance originally isolated from the Pacific yew tree (Taxus brevifolia) more than two decades ago, has recently been approved for the clinical treatment of cancer patients. Hailed as having provided one of the most significant advances in cancer therapy5, this molecule exerts its anticancer activity by inhibiting mitosis through enhancement of the polymerization of tubulin and consequent stabilization of microtubules6. The scarcity of taxol and the ecological impact of harvesting it have prompted extensive searches for alternative sources including semisynthesis, cellular culture production and chemical synthesis2,3. The latter has been attempted for almost two decades, but these attempts have been thwarted by the magnitude of the synthetic challenge. Here we report the total synthesis of taxol by a convergent strategy, which opens a chemical pathway for the production of both the natural product itself and a variety of designed taxoids.

Taxol induces internucleosomal DNA fragmentation associated with programmed cell death in human myeloid leukemia cells

The present results demonstrate that the exposure of human myeloid leukemia HL-60 and KG-1 cells to clinically achievable concentrations of taxol produced internucleosomal DNA fragmentation of approximately 200 base-pair multiples, and the morphologic changes characteristic of cells undergoing programmed cell death (PCD) or apoptosis.Taxol-induced PCD was associated with a marked inhibition of suspension culture growth and clonogenic survival of HL-60 cells. In addition, taxol treatment decreased BCL-2 oncogene expression, which is known to block PCD. The exposure to taxol moderately decreased c-myc expression, but did not induce c-jun expression--which has been previously noted for a variety of DNA interactive, antileukemic drugs. These findings indicate that taxol may induce leukemic cell death partly by the alternative but gene-directed and active mechanism of PCD.