Killing cells by targeting mitosis

Abstract

Cell cycle deregulation is a common feature of human cancer. Tumor cells accumulate mutations that result in unscheduled proliferation, genomic instability and chromosomal instability. Several therapeutic strategies have been proposed for targeting the cell division cycle in cancer. Whereas inhibiting the initial phases of the cell cycle is likely to generate viable quiescent cells, targeting mitosis offers several possibilities for killing cancer cells. Microtubule poisons have proved efficacy in the clinic against a broad range of malignancies, and novel targeted strategies are now evaluating the inhibition of critical activities, such as cyclin-dependent kinase 1, Aurora or Polo kinases or spindle kinesins. Abrogation of the mitotic checkpoint or targeting the energetic or proteotoxic stress of aneuploid or chromosomally instable cells may also provide further benefits by inducing lethal levels of instability. Although cancer cells may display different responses to these treatments, recent data suggest that targeting mitotic exit by inhibiting the anaphase-promoting complex generates metaphase cells that invariably die in mitosis. As the efficacy of cell-cycle targeting approaches has been limited so far, further understanding of the molecular pathways modulating mitotic cell death will be required to move forward these new proposals to the clinic.

Figure 1

Targeting mitosis for cancer therapy. Cdk1 is a master protein kinase required for several processes during mitotic entry and its inhibition results in G2 arrest. This may require the cooperation of Mastl to inhibit phosphatases, although the relevance of this protein in cancer remains mostly unexplored. Centrosome duplication and separation, and the formation of a bipolar spindle are also required for normal chromosome segregation. Kinases such as Aurora A and Plk1 and kinesins such as Eg5 are critical regulators of these processes and their inhibition results in arrest during prometaphase (PM). This arrest is mediated by the SAC, whose activity depends on several effectors such as Mad2, BubR1, Mps1 or Aurora B, a kinase essential during the error correction mechanism that monitors the proper attachment of microtubules to kinetochores. Abrogation of this checkpoint provokes abnormal chromosome segregation and chromosome instability. APC/C along with its cofactor Cdc20 is required for anaphase (A) onset by targeting critical mitotic regulators for ubiquitin-dependent degradation. Inhibition of this E3-ubiquitin ligase leads to metaphase (M) blockade due to the stabilization of cyclin B1 and mitotic cell death. C, cytokinesis; P, prophase; T, telophase

Figure 2

Models for targeting mitosis as an anticancer strategy. Treatment with mitotic entry inhibitors or spindle poisons leads to G2 or prometaphase arrest that eventually may end up in apoptosis during interphase or mitosis. However, cells may escape entering into a new cell cycle. Depending of the status of genes such as p53, pRb or p38, these cells may arrest before entering S phase, die or continue proliferating. Targeting mitotic checkpoint regulators, such as BubR1, Mps1 or Mad2, leads to severe levels of aneuploidy. Cells containing numerical chromosome aberrations can similarly either arrest in the subsequent G1 phase, progress through the cell cycle or undergo cell death. Inhibiting mitotic exit (e.g., by targeting APC/C–Cdc20) provokes a permanent metaphase arrest by preventing cyclin B1 degradation, thus irreversibly leading to mitotic cell death

Figure 3

Mitotic cell death is governed by Cdk1 activity. During prolonged mitotic arrest (e.g., by using microtubule poisons, AurA or Plk1 inhibitors or inhibiting the APC/C), Cdk1 activity regulates the stability of antiapoptotic effectors. Cdk1-dependent phosphorylation of Mcl-1 allows its phosphorylation by other kinases, such as JNK, p38 and CKII. Highly phosphorylated forms of Mcl-1 are recognized and ubiquitinated by the E3-ubiquitin ligase SCF, leading to its degradation by the proteasome 26S. On the other hand, phosphorylation of Bcl-X_L and Bcl-2 by Cdk1 drives their conversion to an inactive form. Thus, degradation of Mcl-1 and inactivation of Bcl-X_L and Bcl-2 lead to the loss of balance between pro- or antiapoptotic signals, resulting in mitotic cell death. Cdk1 may also prevent mitotic cell death by phosphorylating and inhibiting caspases (not shown), and the regulation of its pro- and antiapoptotic functions is not well understood (see main text)