Paclitaxel Induces Primary and Postmitotic G1 Arrest in Human Arterial Smooth Muscle Cells

Paclitaxel (PTX), a microtubule-active drug, causes mitotic arrest leading to apoptosis in certain tumor cell lines. Here we investigated the effects of PTX on human arterial smooth muscle cell (SMC) cells. In SMC, PTX caused both (a) primary arrest in G1 and (b) post-mitotic arrest in G1. Post-mitotic cells were multinucleated (MN) with either 2C (near-diploid) or 4C (tetraploid) DNA content. At PTX concentrations above12 ng/ml, MN cells had 4C DNA content consistent with the lack of cytokinesis during abortive mitosis. Treatment with 6-12 ng/ml PTX yielded MN cells with 2C DNA content. Finally, 1-6 ng/ml of PTX, the lowest concentrations that affected cell proliferation, caused G1 arrest without multinucleation. It is important that PTX did not cause apoptosis in SMC. The absence of apoptosis could be explained by mitotic exit and G1 arrest as well as by low constitutive levels of caspase expression and by p53 and p21 induction. Thus, following transient mitotic arrest, SMC exit mitosis to form MN cells. These post-mitotic cells were subsequently arrested in G1 but maintained normal elongated morphology and were viable for at least 21 days. We conclude that in SMC PTX causes post-mitotic cell cycle arrest rather than cell death.