Hallmarks of mitotic catastrophe
The hallmarks of mitotic catastrophe were first recognised in a series of papers published by Spear and Glucksmann during the 1930s and 1940s, where exponentially-growing cells were subjected to radiation treatment (GlǬcksmann et al., 1939; Spear et al., 1941). Cells were found to undergo cell death at the first mitotic transition after radiation exposure, and these cells exhibited severe numerical and structural karyotypic abnormalities (Vakifahmetoglu et al., 2008).
Mitotic catastrophe has been comprehensively described by the Kroemer group as a mechanism that senses mitotic failure and responds to it by driving the cell to an irreversible fate, be it apoptosis, necrosis or senescence (Vitale et al., 2011). Therefore mitotic catastrophe can be seen as an oncosuppressive mechanism for diverting potentially aneuploid cells into an antiproliferative fate (Vitale et al., 2011). Mitotic catastrophe results in cell commitment to one of three possible fates (Vitale et al., 2011).
Firstly, cells may experience death without exiting mitosis (mitotic death). Secondly, cells may progress through aberrant mitosis into a subsequent G1 phase (mitotic slippage), where they undergo either immediate (Tao et al., 2005) or delayed (Roninson et al., 2001) cell death. Finally, cells may exit mitosis and become senescent (Roninson, 2003). All three outcomes present attractive opportunities for therapeutic intervention (Manchado et al., 2012).
Proteolysis of cyclin B during mitotic catastrophe
Mitotic slippage can occur in the absence of SAC satisfaction, and bypass the inactivation of APC via alternative proteolysis of its substrate cyclin B1 (Brito et al., 2006). Proteolysis of cyclin B leads to the premature inactivation of the mitotic kinase Cdk1, and subsequent mitotic slippage, with tetraploid cells proceeding directly to a further G1 phase. Mitotic catastrophe is induced by perturbations of the SAC. MTAs bind directly to tubulin and disrupt spindle formation. Furthermore, depletion of the SAC checkpoint protein Chk1, a kinase which regulates mitotic entry, can drive cells towards mitotic catastrophe by inducing the premature activation of Cdk1 (Niida et al., 2005).
Duration of mitotic arrest
Reports have indicated that cell commitment to mitotic catastrophe is highly dependent on the duration of mitotic arrest. A report from the Taylor group determined that 15 hr was the cutoff point for the treatment of HeLa cells with MTAs, beyond which cell commitment to mitotic catastrophe was irreversible (Bekier et al., 2009). It has been proposed that this duration effect is directly due to the prolonged activation of Cdk1 (Manchado et al., 2012).
Pro-longed mitotic arrest (D-mitosis)
Cdk1 is the main kinase active during both typical mitosis and prolonged mitotic arrest, which has been termed D-mitosis (Brito et al., 2006) – and it is thought that the Cdk1-dependent hyperphosphorylation of Bcl-2/Bcl-XL during prolonged mitotic arrest acts as a molecular switch from anti-apoptotic to pro-apoptotic Cdk1 signalling (Terrano et al., 2010). Furthermore, the Cdk1-dependent phosphorylation of the anti-apoptotic Bcl-2 member Mcl1 leads to its degradation during prolonged mitotic arrest (Harley et al., 2010; Manchado et al., 2012). Contemporaneous work from several groups indicates that Mcl1 degradation is carried out by either the APC/C (Harley et al., 2010) or the related ubiquitination complex SCF (Wertz et al., 2011), via their respective adaptor proteins Cdc20 and Fbw7.
Ubiquitination during mitotic catastrophe
It is currently unclear whether these results are contradictory, or represent an example of a cooperative mechanism between the two ubiquitin ligase complexes, which has previously been described (Fasanaro et al., 2010). It is likely however, given that Cdk1 is such a prolific kinase during mitosis, that Cdk1-dependent phosphorylation of numerous substrates is important in the induction of cell death due to prolonged mitotic arrest.
The Nomenclature Committee on Cell Death (NCCD) provides unified criteria for the definition of cell death modalities through morphological and biochemical hallmarks (Galluzzi et al., 2012). According to current recommendations of the NCCD published in 2011, mitotic catastrophe would not constitute a pure cell death executioner pathway, but rather an oncosuppressive mechanism that is (i) initiated by perturbation of the mitotic machinery, (ii) occurs during mitosis, (iii) is concomitant with mitotic arrest, and (iv) ultimately results in either cell death or senescence.
Caspase activation during mitotic catastrophe
Caspase activation has been known to be involved in mitotic cell death, particularly the involvement of caspase-2 during mitotic catastrophe (Castedo et al., 2004), and the involvement of caspases-3 and -8 during MTA-induced apoptosis (Oyaizu et al., 1999). In support of this, the phosphorylation of caspase-2 and caspase-9 during mitosis has been demonstrated to be cytoprotective against the initiation of apoptosis (Allan et al., 2007; Andersen et al., 2009).
The activation of caspases is concomitant with the release of mitochondrial apoptogenic factors such as cytochrome C (Castedo et al., 2004), and the DNAses AIF1 and endonuclease G (Kroemer et al., 2005; Niikura et al., 2010), indicating that mitotic cell death in these instances is occurring through a classical apoptotic mechanism. However, recent studies have revealed that the activation of caspases is not an absolute requirement for the initiation of programmed cell death during mitosis, with the release of mitochondrial apoptogens inducing a cell death effect in the absence of caspase activation (van Loo et al., 2001; Cande et al., 2004).
Caspase independent mitotic death
This new pathway has been termed caspase-independent mitotic death (CIMD). The activation of spindle checkpoint proteins has been shown to be an integral event in the initiation of CIMD (Kitagawa et al., 2008), with involvement of both Bub1/Bub3 (Niikura et al., 2010), and the mitotic kinase Plk1 (Lin et al., 2011). Contradictory reports have shown that CIMD is either a p53-dependent process (Lin et al., 2011), or may function via the p53 homolog p73 in the absence of p53 (Niikura et al., 2010). In the balance of probability, CIMD like all other cell death modalities exhibits a degree of heterogeneity, and the precise signalling pathways depend on the proteomic profile of the individual cell line/tissue type.
Targeting mitotic catastrophe
The characterisation of the pathways of mitotic catastrophe is currently the subject of intense experimental effort, due to the potential therapeutic benefits of this avenue of research. Many current chemotherapeutics, such as doxorubicin, are used at concentrations which non-selectively induce apoptosis at any stage of the cell cycle, but may be more effectively implemented at lower concentrations which selectively induce mitotic catastrophe, reducing off-target chemotherapeutic effects (Eom et al., 2005). Furthermore, the inherent karyotypic abnormality of most tumours makes these cell types especially vulnerable to cell death induced by mitotic catastrophe (Vitale et al., 2011).
Cell death strategies
Several novel strategies for inducing mitotic catastrophe are currently under investigation. Kinesin spindle protein (KSP) is responsible for the correct bipolar spindle orientation during mitosis, and inhibition of this motor protein leads to mitotic catastrophe and monoastral mitoses (Kapoor et al., 2000), from which the KSP inhibitor monastrol is named (Maliga et al., 2002).
Mitotic spindle formation
The chromosomal passenger complex (CPC) is a multiprotein complex required for microtubule stabilization and mitotic spindle formation (Sampath et al., 2004). As such, specific small-molecule chemical inhibition of components of this complex presents an attractive target for the induction of mitotic catastrophe. Aurora B kinase is an enzymatic component of the CPC, and Aurora B kinase inhibitors including AZD1152 and VX-680 are currently in clinical trials as putative therapies for various hematopoietic malignancies (Dar et al., 2010). Survivin is a structural component of the CPC (Giodini et al., 2002), and has been investigated as a therapeutic target. However, so far the only survivin inhibitor to reach Phase II clinical trial, YM155, has failed to demonstrate therapeutic efficacy (Altieri, 2012).
Degradation of Mcl-1 during apoptosis
It has been proposed that cell death initiation following prolonged mitotic arrest may be due the degradation of Mcl-1, allowing for the release of the proapoptotic BH3-only proteins, Bim and Noxa, and subsequent apoptosome formation due to the slow net dephosphorylation of caspase-9, which may lead to the activation of cell death [Harley et al., 2009]. However, exactly how upstream activation of cell death occurs following prolonged mitotic arrest resulting in mitotic catastrophe remains unresolved.