Mcl-1 dynamics influence mitotic slippage and death in mitosis

Abstract

Microtubule-binding drugs such as taxol are frontline treatments for a variety of cancers but exactly how they yield patient benefit is unclear. In cell culture, inhibiting microtubule dynamics prevents spindle assembly, leading to mitotic arrest followed by either apoptosis in mitosis or slippage, whereby a cell returns to interphase without dividing. Myeloid cell leukaemia-1 (Mcl-1), a pro-survival member of the Bcl-2 family central to the intrinsic apoptosis pathway, is degraded during a prolonged mitotic arrest and may therefore act as a mitotic death timer. Consistently, we show that blocking proteasome-mediated degradation inhibits taxol-induced mitotic apoptosis in a Mcl-1-dependent manner. However, this degradation does not require the activity of either APC/C-Cdc20, FBW7 or MULE, three separate E3 ubiquitin ligases implicated in targeting Mcl-1 for degradation. This therefore challenges the notion that Mcl-1 undergoes regulated degradation during mitosis. We also show that Mcl-1 is continuously synthesized during mitosis and that blocking protein synthesis accelerates taxol induced death-inmitosis. Modulating Mcl-1 levels also influences slippage; overexpressing Mcl-1 extends the time from mitotic entry to mitotic exit in the presence of taxol, while inhibiting Mcl-1 accelerates it. We suggest that Mcl-1 competes with Cyclin B1 for binding to components of the proteolysis machinery, thereby slowing down the slow degradation of Cyclin B1 responsible for slippage. Thus, modulating Mcl-1 dynamics influences both death-in-mitosis and slippage. However, because mitotic degradation of Mcl-1 appears not to be under the control of an E3 ligase, we suggest that the notion of network crosstalk is used with caution.

Belinostat and vincristine demonstrate mutually synergistic cytotoxicity associated with mitotic arrest and inhibition of polyploidy in a preclinical model of aggressive diffuse large B cell lymphoma

“…15 MCL-1 is involved in regulation of both death in mitosis and death after mitotic slippage. [50][51][52] Mitotic degradation of MCL-1 can increase the probability of death in mitosis when MYC or NOXA is downregulated. 50,52 It can also hasten slippage during mitotic arrest caused by inhibitors of mitosis and increase the probability of apoptosis after slippage.…”

“…50,52 It can also hasten slippage during mitotic arrest caused by inhibitors of mitosis and increase the probability of apoptosis after slippage. 51 While it is unclear if apoptosis induced by the belinostat-vincristine combination occurs during mitosis or after mitotic slippage, the concomitant downregulation of MCL-1 and upregulation of BIM could contribute to either.…”

Mitosis and mitochondrial priming for apoptosis

“…Mcl-1 has a particularly short half-life, due to proteasomal degradation. This instability of Mcl-1, compared to other multi-domain Bcl-2 proteins, is exploited in mitotic cells to allow the loss of its anti-apoptotic activity over time (Harley et al, 2010;Wertz et al, 2011;Sloss et al, 2016). However, the mechanism controlling Mcl-1 degradation during mitosis is not as clear as that for cyclin B.…”

“…The E3 ligase Mcl-1 ubiquitin ligase E3 (MULE) was shown to be important in Mcl-1 stability (Zhong et al, 2005), CDK1-mediated phosphorylation of Mcl-1 was shown to promote its degradation mediated by APC/C (Harley et al, 2010) and FBW-7/SCF, which targets Mcl-1 for ubiquitin-mediated degradation in interphase, was also shown to play a significant role in mitotic cells (Inuzuka et al, 2011;Wertz et al, 2011). However, a recent study has suggested that in colon carcinoma cell lines, none of these mechanisms has a dominant effect on promoting Mcl-1 loss during mitotic arrest (Sloss et al, 2016). Despite this lack of consensus as to the mechanism, it is clear that stabilisation of Mcl-1 in mitosis has a significant impact in delaying onset of DiM (Tunquist et al, 2010).…”

“…Despite this lack of consensus as to the mechanism, it is clear that stabilisation of Mcl-1 in mitosis has a significant impact in delaying onset of DiM (Tunquist et al, 2010). In a recent study, inhibiting the loss of Mcl-1 in RKO cells that were blocked in mitosis with a degradation-resistant Cdc-20 resulted in a significant increase in time to DiM (Sloss et al, 2016). If Mcl-1 was knocked down under the same conditions, there was a striking synchronisation of apoptosis as cells entered mitosis, an effect not observed if Bcl-XL was depleted instead.…”