Paclitaxel and irradiation induce apoptosis in squamous cell carcinoma cell lines in an additive way

Paclitaxel (Taxol) is a new antimicrotubule plant product, which not only stabilizes the microtubules by inhibiting their disassembly but also promotes the assembly of microtubules. This causes a block in the G2-M phase of the cell cycle known to be a radiosensitive phase of the cycle. Previously we demonstrated that 10 nM paclitaxel accumulated the cells of laryngeal carcinoma cell lines in the G2-M phase as measured by flow cytometry. Time-lapse videomicroscopy demonstrated that the cells died morphologically by apoptosis after a premitotic block. Agarose gel electrophoresis showed the DNA-laddering typical of apoptosis. To investigate the effects of irradiation and paclitaxel separately and concomitantly, we used five newly established laryngeal carcinoma cell lines. The effects were recorded with time-lapse videomicroscopy over 96 hours on controls, cells irradiated with 2 Gy, cells exposed to 1 nM or 5 nM paclitaxel and cells irradiated with 2 Gy after incubating in 1 nM or 5 nM paclitaxel for 24 hours. Spontaneous apoptoses were seen in all cell lines tested. The 2 Gy irradiation dose induced a propagated apoptotic response in two of these cell lines. In all cell lines paclitaxel induced a premitotic block only in some cells at the tested concentrations and these cells died morphologically by apoptosis, whereas irradiation and paclitaxel concomitantly caused an additive inhibition in mitotic activity and caused an additive apoptotic response. An additive effect of paclitaxel and radiation was seen with doses readily achievable in clinical treatment. This additive effect seems to be due to other mechanisms of action than the premitotic block.