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Recovery from DNA damage is critical for cell survival. However, serious damage cannot be repaired, leading to cell death for prevention of abnormal cell growth. Previously, we demonstrated that 4N-DNA accumulates via the initiation of an abnormal interphase without cytokinesis and that re-replication occurs during a prolonged recovery period in the presence of severe DNA damage in mitotic cells. Mitotic phosphorylated Plk1 is typically degraded during mitotic exit. However, Plk1 has unusually found to be dephosphorylated in mitotic slippage without cytokinesis during recovery from mitotic DNA damage. Here, we investigated how Plk1 dephosphorylation is established during recovery from mitotic DNA damage. Mitotic DNA damage activated ATM and Chk1/2 and repressed Cdk1 and Greatwall protein kinase, followed by PP2A activation through the dissociation of ENSA and PP2A-B55. Interaction between Plk1 and PP2A-B55α or PP2A-B55δ was strongly induced during recovery from mitotic DNA damage. Moreover, the depletion of PP2A-B55α and/or PP2A-B55δ by siRNA transfection led to the recovery of Plk1 phosphorylation and progression of the cell cycle into the G 1 phase. Therefore, to adapt to severe DNA damage, the activated Greatwall/ENSA signaling pathway was repressed by ATM/Chk1/2, even in mitotic cells. Activation of the PP2A-B55 holoenzyme complex induced the dephosphorylation of Plk1 and Cdk1, and finally, mitotic slippage occurred without normal chromosome segregation and cytokinesis.

Polo-like kinase-1 (Plk1) belongs to a family of serine-threonine kinases and plays a critical role in mitotic progression. Plk1 involves in the initiation of mitosis, centrosome maturation, bipolar spindle formation, and cytokinesis, well-reported as traditional functions of Plk1. In this review, we discuss the role of Plk1 during DNA damage response beyond the functions in mitotsis. When DNA is damaged in cells under various stress conditions, the checkpoint mechanism is activated to allow cells to have enough time for repair. When damage is repaired, cells progress continuously their division, which is called checkpoint recovery. If damage is too severe to repair, cells undergo apoptotic pathway. If damage is not completely repaired, cells undergo a process called checkpoint adaptation, and resume cell division cycle with damaged DNA. Plk1 targets and regulates many key factors in the process of damage response, and we deal with these subjects in this review.

Cdc20 is an activator of the anaphase-promoting complex (APC/C), and APC/C(Cdc20) is essential for metaphase-anaphase transition. To allow progression beyond mitosis, Cdc20 is degraded through KEN-box-dependent APC/C(Cdh1) activity. Mammalian Cdc20 contains the CRY box, a second APC/C(Cdh1)-dependent degron, but the molecular mechanism in degradation process remains undefined. Polo-like kinase-1 (Plk1) is an essential mitotic kinase regulating various targets in kinetochore, centrosome, and midbody for proper mitotic progression. Plk1 directly bound to Cdc20 and phosphorylates it on serine-170 located in CRY-box. Whereas wild-type Cdc20 was degraded according to progress cell cycle beyond mitosis, the phosphorylation-defective mutant, which serine-170 was changed into alanine, was not destroyed in early G1 phase. The phosphorylation on serine-170 by Plk1 was important for ubiquitination and Cdh1-dependent proteolysis. However, this modification by Plk1 on CRY box had no effect on the subcellular localization of Cdc20 and the formation of APC/C-inhibitory checkpoint complexes under spindle assembly checkpoint. This mechanism will be the first finding of inhibitory phosphorylation related to Cdc20 instability., (Copyright © 2013 Elsevier Inc. All rights reserved.)

Polo-like kinase-1 (Plk1) is phosphorylated on Thr210 for activation during mitosis. Here, we investigated the question of which kinase(s) is the specific upstream kinase of mitotic Plk1. Upstream kinases of Plk1 were purified from mitotic cell extracts through column chromatography procedures, and identified by mass spectrometry. Candidates for Plk1 kinase included p21-activated kinase, aurora A, and mammalian Ste20-like kinases. Immunoprecipitates of these proteins from mitotic cell extracts phosphorylated Plk1 on Thr210. Even if the activity of Aurora A was blocked with a specific inhibitor, Plk1 phosphorylation still occurred, suggesting that function of Plk1 could be controlled by these kinases for proper mitotic progression, as well as by Aurora A in very late G2 phase for the beginning of mitosis., (Copyright © 2010 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.)