Research Papers: Chromosome:
Mcl-1 dynamics influence mitotic slippage and death in mitosis
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Olivia Sloss1, Caroline Topham1,2, Maria Diez1,3, Stephen Taylor1
1Faculty of Life Sciences, University of Manchester, Manchester M13 9PT, United Kingdom
2Present Address: School of Environment & Life Sciences, Cockcroft Building, University of Salford, Salford M5 4WT, United Kingdom
3Present Address: School of Medicine, University of Nottingham, City Hospital, Nottingham NG5 1PB, United Kingdom
Stephen Taylor, e-mail: [email protected]
Keywords: Chromosome Section, spindle assembly checkpoint, taxol, APC/C-Cdc20, FBW7, Bcl-xL
Received: July 06, 2015 Accepted: December 17, 2015 Published: January 12, 2016
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-in-mitosis. 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.
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