Paclitaxel is a potential anti-cancer agent for several malignancies including ovary, breast, and head and neck cancers. This study investigated the kinetics of paclitaxel-induced cell cycle perturbation in two human nasopharyngeal carcinoma (NPC) cell lines, NPC-TWO1 and NPC-TWO4. NPC cells treated with higher concentrations (0.1 or 1 mu M) of paclitaxel showed obvious G(2)/M arrest and then converted to a cell population with reduced DNA content, which was detected as a sub-G(2) peak in the flow cytometric histographs. If a low concentration (5 nM) of paclitaxel was used instead, transient G(2)/M arrest was observed in NPC cells, which subsequently converted to a sub-G(1) form during the treatment period. Internucleosomal fragmentation and chromatin condensation were detectable in these sub-G(1) and sub-G(2) cells, suggesting that persistent or transient G(2)/M arrest is a prerequisite step for apoptosis elicited by varying doses of paclitaxel. The levels of cyclins A, B1, Dl, E, CDK 1. (CDC 2), CDK 2 and proliferating cell nuclear antigen (PCNA) were unchanged in NPC cells following treatment with any concentration of paclitaxel; however, apoptosis-related cyclin B1-associated CDC 2 kinase was highly activated by paclitaxel even at concentrations as low as 5 nM, which is consistent with the finding that low-dose paclitaxel is also able to induce apoptosis in NPC cells. Activation of cyclin B1-associated CDC 2 kinase seems to be an important G(2)/M event required for paclitaxel-induced apoptosis, and this activation of cyclin B1/CDC 2 kinase could be attributed to the increased activity of CDK 7 kinase.
