Spindle checkpoint function and cellular sensitivity to antimitotic drugs.

AntimitoticDrugsDisrupt theNormal Function of the Mitotic Spindle and Cause Spindle Checkpoint ^MediatedMitotic Arrest Microtubule inhibitors such as Vinka alkaloids (e.g., vinblastine and vincristine) and Taxans (e.g., taxol/paclitaxel and docetaxel) are potent therapeutic drugs for cancer treatment. The cellular consequences of taxol/paclitaxel treatments over a wide range of doses (1 nmol/L–100 Amol/L) have been reported. Overall, the main target of clinically relevant, low-to-moderate doses of taxol (5–200 nmol/L) is the mitotic spindle (1). In response to treatment with moderate doses of microtubule inhibitor, cells in culture and cells in tumor xenografts in nude mice exhibit mitotic arrest and/or apoptosis (2, 3). Other modes of cell death (e.g., lytic death) have been reported in mice treated with 30 mg/kg of i.v. docetaxel (4). However, the complexities of the cellular microenvironment in whole animals make the assessment of the mode of cell death in living animals somewhat more difficult. Recently, other mitosis-targeting drugs have been developed (e.g., inhibitors of mitotic kinesin motors such as KSP-IA, monastrol, and HR22C16; refs. 5–7). These potential therapeutic agents join the microtubule inhibitors in the general category of antimitotic drugs. Different types of antimitotic drugs are thought to impose different types of stress on kinetochores, which are considered to be a central origin for the generation of a signaling pathway called the mitotic spindle checkpoint or simply the spindle checkpoint. These different types of stress may activate different signaling cascades through the kinetochores, or may emphasize particular branches of checkpoint signaling [discussed in a review by Pinsky and Biggins (8)]. Some observations support this view. For example, microtubule-stabilizing drugs (e.g., taxol, epothilone B) lead A549 cells to aneuploidy and/or apoptosis (cells with sub-G1 DNA content) more efficiently than microtubule-destabilizing drugs (e.g., vinblastine, colchicine, nocodazole; ref. 9). Although aspects of signaling may differ among the major classes of antimitotic drugs, these drugs eventually lead cells to temporary or permanent mitotic arrest due to inhibition of the mitotic spindle or the proteins that regulate it. Mitotic arrest induced by antimitotic drugs through the spindle checkpoint. The spindle checkpoint detects loss or impairment of functional connections between kinetochores and spindle microtubules during mitosis and disseminates signals that inhibit the anaphase-promoting complex/ cyclosome. The anaphase-promoting complex/cyclosome is a ubiquitin ligase complex that catalyzes polyubiquitylation of a variety of targets including securin and mitotic cyclins during mitosis. Although the anaphase-promoting complex/cyclosome functions throughout the cell cycle with different activator subunits (Cdh1 and Cdc20), the major phenotype of the inhibition is mitotic delay/arrest. The inhibition of the anaphase-promoting complex/cyclosome causes high cyclin levels, sustained cyclin-dependent kinase 1 (cdk1) activity, and prolonged mitotic arrest. The mitotic arrest allows time for correction of chromosome connections to the spindle (10–12). Studies with cultured cells have suggested that the spindle checkpoint–mediated mitotic arrest may be a requirement for the subsequent cell death induced by the antimitotic drugs (5, 13). The main scope of this review is to discuss the mitotic/ postmitotic events and molecular signaling involved in treatments with antimitotic drugs, using information derived primarily from research with cultured cells. We will describe the events that occur during and after mitotic arrest, clarify the notion of cellular sensitivity to mitotic arrest, and discuss the correlation between spindle checkpoint function and antimitotic drug–mediated cell death. These are factors which are important in explaining the molecular basis for the efficacy of antimitotic drugs. Other cell biological effects induced by microtubule inhibitors during interphase prior to mitotic arrest may also be relevant to the induction of cell death. However, these Mol Cancer Ther 2006;5(12):2963–9