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      Partial inhibition of Cdk1 in G 2 phase overrides the SAC and decouples mitotic events

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          Abstract

          Entry and progression through mitosis has traditionally been linked directly to the activity of cyclin-dependent kinase 1 (Cdk1). In this study we utilized low doses of the Cdk1-specific inhibitor, RO3306 from early G 2 phase onwards. Addition of low doses of RO3306 in G 2 phase induced minor chromosome congression and segregation defects. In contrast, mild doses of RO3306 during G 2 phase resulted in cells entering an aberrant mitosis, with cells fragmenting centrosomes and failing to fully disassemble the nuclear envelope. Cells often underwent cytokinesis and metaphase simultaneously, despite the presence of an active spindle assembly checkpoint, which prevented degradation of cyclin B1 and securin, resulting in the random partitioning of whole chromosomes. This highly aberrant mitosis produced a significant increase in the proportion of viable polyploid cells present up to 3 days post-treatment. Furthermore, cells treated with medium doses of RO3306 were only able to reach the threshold of Cdk1 substrate phosphorylation required to initiate nuclear envelope breakdown, but failed to reach the levels of phosphorylation required to correctly complete pro-metaphase. Treatment with low doses of Okadaic acid, which primarily inhibits PP2A, rescued the mitotic defects and increased the number of cells that completed a normal mitosis. This supports the current model that PP2A is the primary phosphatase that counterbalances the activity of Cdk1 during mitosis. Taken together these results show that continuous and subtle disruption of Cdk1 activity from G 2 phase onwards has deleterious consequences on mitotic progression by disrupting the balance between Cdk1 and PP2A.

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          Most cited references42

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          Global analysis of Cdk1 substrate phosphorylation sites provides insights into evolution.

          To explore the mechanisms and evolution of cell-cycle control, we analyzed the position and conservation of large numbers of phosphorylation sites for the cyclin-dependent kinase Cdk1 in the budding yeast Saccharomyces cerevisiae. We combined specific chemical inhibition of Cdk1 with quantitative mass spectrometry to identify the positions of 547 phosphorylation sites on 308 Cdk1 substrates in vivo. Comparisons of these substrates with orthologs throughout the ascomycete lineage revealed that the position of most phosphorylation sites is not conserved in evolution; instead, clusters of sites shift position in rapidly evolving disordered regions. We propose that the regulation of protein function by phosphorylation often depends on simple nonspecific mechanisms that disrupt or enhance protein-protein interactions. The gain or loss of phosphorylation sites in rapidly evolving regions could facilitate the evolution of kinase-signaling circuits.
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            Selective small-molecule inhibitor reveals critical mitotic functions of human CDK1.

            CDK1 is a nonredundant cyclin-dependent kinase (CDK) with an essential role in mitosis, but its multiple functions still are poorly understood at a molecular level. Here we identify a selective small-molecule inhibitor of CDK1 that reversibly arrests human cells at the G(2)/M border of the cell cycle and allows for effective cell synchronization in early mitosis. Inhibition of CDK1 during cell division revealed that its activity is necessary and sufficient for maintaining the mitotic state of the cells, preventing replication origin licensing and premature cytokinesis. Although CDK1 inhibition for up to 24 h is well tolerated, longer exposure to the inhibitor induces apoptosis in tumor cells, suggesting that selective CDK1 inhibitors may have utility in cancer therapy.
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              Loss of human Greatwall results in G2 arrest and multiple mitotic defects due to deregulation of the cyclin B-Cdc2/PP2A balance.

              Here we show that the functional human ortholog of Greatwall protein kinase (Gwl) is the microtubule-associated serine/threonine kinase-like protein, MAST-L. This kinase promotes mitotic entry and maintenance in human cells by inhibiting protein phosphatase 2A (PP2A), a phosphatase that dephosphorylates cyclin B-Cdc2 substrates. The complete depletion of Gwl by siRNA arrests human cells in G2. When the levels of this kinase are only partially depleted, however, cells enter into mitosis with multiple defects and fail to inactivate the spindle assembly checkpoint (SAC). The ability of cells to remain arrested in mitosis by the SAC appears to be directly proportional to the amount of Gwl remaining. Thus, when Gwl is only slightly reduced, cells arrest at prometaphase. More complete depletion correlates with the premature dephosphorylation of cyclin B-Cdc2 substrates, inactivation of the SAC, and subsequent exit from mitosis with severe cytokinesis defects. These phenotypes appear to be mediated by PP2A, as they could be rescued by either a double Gwl/PP2A knockdown or by the inhibition of this phosphatase with okadaic acid. These results suggest that the balance between cyclin B-Cdc2 and PP2A must be tightly regulated for correct mitotic entry and exit and that Gwl is crucial for mediating this regulation in somatic human cells.
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                Author and article information

                Journal
                Cell Cycle
                Cell Cycle
                CC
                Cell Cycle
                Landes Bioscience
                1538-4101
                1551-4005
                01 May 2014
                06 March 2014
                : 13
                : 9
                : 1400-1412
                Affiliations
                [1 ]The Kinghorn Cancer Centre; Garvan Institute of Medical Research; Sydney, New South Wales, Australia
                [2 ]St. Vincent’s Clinical School; Faculty of Medicine; UNSW; Sydney, New South Wales, Australia
                [3 ]Universités Montpellier 2 et 1; Centre de Recherche de Biochimie Macromoléculaire; CNRS UMR 5237; Montpellier, France
                Author notes
                [* ]Correspondence to: Andrew Burgess, Email: a.burgess@ 123456garvan.org.au
                Article
                2013CC5465R 28401
                10.4161/cc.28401
                4050138
                24626186
                46eeaff0-decb-4cc0-9376-a8487c3936ae
                Copyright © 2014 Landes Bioscience

                This is an open-access article licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License. The article may be redistributed, reproduced, and reused for non-commercial purposes, provided the original source is properly cited.

                History
                : 02 December 2013
                : 14 February 2014
                : 02 March 2014
                Categories
                Report

                Cell biology
                cdk1,mitosis,sac,pp2a,cyclin b1,ro3306
                Cell biology
                cdk1, mitosis, sac, pp2a, cyclin b1, ro3306

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