A new role for Cdks in the DNA damage response

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Jun 14, 2010 - cells with the Cdk inhibitors roscovitine and RO3306 during the DNA damage- induced arrest completely abolishes the ability of these cells to ...
Cell Cycle Feature

Cell Cycle Feature

Cell Cycle 9:15, 2915-2916; August 1, 2010; © 2010 Landes Bioscience

A new role for Cdks in the DNA damage response Mónica Alvarez-Fernández and René H. Medema* Department of Medical Oncology and Cancer Genomics Centre; University Medical Center; Utrecht, The Netherlands

In response to DNA damage, cells activate checkpoints that block cell cycle progression to allow repair of the damage. Once the lesion is repaired, the DNA damage checkpoint is silenced and cells are able to re-enter the cell cycle, a process that is called checkpoint recovery. In G2, the ultimate targets of the DNA damage checkpoint are Cyclin A-Cdk1/2 and Cyclin B-Cdk1 complexes, which are required for mitotic entry.1 Activation of the effector kinases Chk1 and Chk2 downstream of ATM/ATR leads to activation of the Cdk-inhibitory kinases Wee1/Myt1, and to degradation or inhibition of the CDC25 family of phosphatases that normally activate Cdks.1 Moreover, maintenance of the G2 arrest depends on the activation of the tumour suppressor p53, which directly represses the expression of G2 /M genes and induces the expression of p21waf1, an inhibitor of several Cyclin-Cdk complexes.2,3 Paradoxically, although Cdks are the ultimate target of the checkpoint and their inhibition is thought to be essential to establish a cell cycle arrest, we have found that cells need to retain a certain level of Cdk activity during a DNA damage-induced G2 arrest in order to recover.4 In our study we show that treatment of cells with the Cdk inhibitors roscovitine and RO3306 during the DNA damageinduced arrest completely abolishes the ability of these cells to enter mitosis after removal of the inhibitor and inactivation of the checkpoint.4 Thus, while it is clearly established that an active checkpoint results in inhibition of Cdk activity, we find that further inhibition of Cdk activity compromises the capacity of the cells to recover from the arrest. This is probably due to insufficient levels of mitotic inducers, such as Cyclin B and

Figure 1. Cdk activity is required to maintain the reversibility of the cell cycle arrest induced by the DNA damage checkpoint. DNA damage-arrested cells need to retain sufficient Cdk activity to maintain FoxM1-dependent transcription of G2 genes, such as Cyclin B and Plk1. In this way, cells are able to re-enter the cell cycle after checkpoint inactivation (green line). But if Cdk activity drops below a certain threshold (dashed line), cells lose the expression of G2 genes and, therefore, are unable to recover even if the checkpoint is silenced (red line).

Plk1, which are clearly diminished in the damaged cells treated with the inhibitor.4 In an unperturbed cell cycle, expression of these pro-mitotic genes is induced by G2 transcription factors, which are activated in S and G2 phase by Cdk activity.5 One of those factors is the forkhead protein FoxM1, which is essential for proper mitotic progression.6 Interestingly, we have found that FoxM1 remains active during a DNA damage-induced arrest, despite the fact that its transcriptional activity depends on phosphorylation by Cyclin A-Cdk complexes.5 Indeed, we could demonstrate that the activity of FoxM1 in DNA damage-arrested cells

continues to depend on Cyclin A-Cdk activity, consistent with the notion that a (low) residual level of Cyclin-Cdk activity is maintained during the arrest. In fact, we find that FoxM1 is required for checkpoint recovery and expression of a constitutively active form of FoxM1 can partially restore expression of Cyclin B and Plk1 and the ability to recover when Cdk activity is fully inhibited during the arrest.4 This indicates that during the DNA damage-induced arrest, cells need to retain sufficient levels of Cdk activity to sustain FoxM1-dependent transcription of G2 /M genes, like Cyclin B and Plk1. In such a way, cells can maintain a G2-state

*Correspondence to: René H. Medema; Email: [email protected] Submitted: 06/14/10; Accepted: 06/16/10 Previously published online: www.landesbioscience.com/journals/cc/article/12700 DOI: 10.4161/cc.9.15.12700 Comment on: Alvarez-Fernández M, et al. EMBO Rep 2010; 11:452–8. www.landesbioscience.com

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throughout the arrest and are “ready to go” after eventual repair of the damage (Fig. 1). Our study demonstrates that Cdks play an active role in the DNA damage checkpoint and this is in agreement with previous reports that show a role for Cdk activity in other aspects of the DNA damage response. In budding yeast, Cdk activity is required for end resection of DNA double strand breaks and homologous recombination.7 In mammalian cells, treatment with the Cdk inhibitor roscovitine also inhibits resection of the breaks and compromises Chk1 activation.8 More recently, it has been found that Cdk1 is required for efficient phosphorylation of ATM/ATR substrates and recruitment

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of BRCA1 to the DNA damage foci.9 All these studies clearly indicated that Cdk activity is essential to efficiently turn on the checkpoint and to activate the proper repair pathways. But all of this could be explained by an initial contribution of Cdk activity to the DNA damage response, after which it could be completely shut down. However, our study shows that this is not the case, and that Cdk activity has to persist at a minimal level to maintain the reversibility of the arrest. This shows that Cdks are not only terminal targets of the checkpoint but they are also actively engaged in the DNA damage response at different levels. Therefore, cells need to fine-tune Cdk activity during the DNA damage response in such a way that it is

Cell Cycle

low enough to block cell cycle progression, but sufficiently high to ensure proper checkpoint signaling, DNA repair and reversibility of the arrest (Fig. 1). This suggests that Cdk inhibitors could be employed to enhance the anti-proliferative effects of genotoxic therapies, a prospect that will require further study. References 1. 2. 3. 4. 5. 6. 7. 8. 9.

Kastan MB, et al. Nature 2004; 432:316-23. Bunz F, et al. Science 1998; 282:1497-501. Spurgers KB, et al. J Biol Chem 2006; 281:25134-42. Alvarez-Fernandez M, et al. EMBO Rep 2010; 11:452-8. Lindqvist A, et al. J Cell Biol 2009; 185:193-202. Laoukili J, et al. Nat Cell Biol 2005; 7:126-36. Ira G, et al. Nature 2004; 431:1011-7. Jazayeri A, et al. Nat Cell Biol 2006; 8:37-45. Johnson N, et al. Mol Cell 2009; 35:327-39.

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