p38 and Chk1 kinases: different conductors for the G2/M checkpoint symphony

doi:10.1016/S0959-437X(01)00270-2

Current Opinion in Genetics & Development

Volume 12, Issue 1, 1 February 2002, Pages 92-97

https://doi.org/10.1016/S0959-437X(01)00270-2 Get rights and content

Abstract

The mechanism controlling G₂/M checkpoint activation after DNA damage was thought to be mediated primarily by nuclear Chk1/Chk2 kinases. Recent evidence indicates that this checkpoint is more complex, involving at least two different biochemical systems that target the Cdc25B and Cdc25C phosphatases. Following genotoxic stress, different kinases integrate signaling from the damaged DNA and other damaged cellular components to regulate Cdc25 inactivation. Our current model for G₂/M checkpoint activation after genotoxic stress is discussed emphasizing the roles for Chk1 and p38 kinases in checkpoint regulation.

Introduction

For protection from a variety of different types of stress, eukaryotic cells have developed a system of checkpoints that delay progression to the next phase of the cell cycle. A critical evolutionarily-conserved checkpoint exists at the boundary between the G₂ phase of the cell cycle and mitosis. Considering the wide variety of agents that can trigger a G₂/M checkpoint, its regulation is complex and many of its components are still being defined. In addition, there is significant redundancy in the control of this checkpoint allowing for responses to a broad spectrum of genotoxic and non-genotoxic stresses.

All checkpoint mechanisms can be classified into two general groups: transcription dependent and transcription independent. The p53 tumor-suppressor protein is a major transcription factor that is involved in the regulation of G₂/M checkpoint activation after stress by suppressing (cyclin B1, Cdc2 or Cdc25C) or transactivating (Gadd45a, 14-3-3σ) specific genes (more details in review [1]). In addition, p53-independent transcriptional mechanisms also suppress expression of key cell-cycle genes such as cyclin B1 [2]. Transcription-dependent control generally requires several hours for full effectiveness and can contribute to the maintenance of a G₂ delay after stress. Activation of the checkpoint is a very rapid event, however, highlighting the dominant role of transcription-independent mechanisms in checkpoint regulation.

During normal cell-cycle progression, initiation of mitosis is triggered by a complex process of activation of the cyclin-dependent protein kinase Cdc2. Prior to mitosis, the Cdc2–cyclin B1 complex is held in the cytoplasm in an inactive state by Cdc2 phosphorylation at Thr14 and Tyr 15. Transcription-independent regulation of the G₂/M checkpoint involves several stress-inducible signal-transduction pathways regulating Cdc2 phosphorylation at inhibitory sites after stress. Dephosphorylation of these sites in mammals is regulated by two Cdc25 phosphatases, Cdc25B and Cdc25C. Different kinases are required to regulate the activity of Cdc25B and Cdc25C phosphatases after DNA damage 3•., 4•.. Along with the previously characterized functions of the Chk1 kinase, we discuss the role of the p38 kinase in regulation of G₂/M checkpoint activation.

Section snippets

Mechanism for G₂/M checkpoint control after genotoxic stress

Cdc2 kinase is the major regulator or ‘engine’ that drives the G₂→M transition. Cdc2 is phosphorylated at inhibitory sites during the G₂ phase of the cell cycle, and previous studies have shown that a G₂ arrest caused by DNA-damaging agents involves ‘stabilization’ of these phos-phorylation sites in yeast and Aspergillus 5., 6.. In mammals, conditional expression of the inhibitory sites mutant, Cdc2AF, in HeLa cells reduces but does not completely abolish a G₂ delay after DNA damage [7]. These

A model for G₂/M checkpoint activation after genotoxic stress

On the basis of the data discussed here, a simplified model can be proposed for the mechanism of G₂/M checkpoint activation after stress. Different types of stress induce several independent pathways. The first group of pathways is responsible for checkpoint initiation and regulates mostly Cdc25B phosphorylation in the cytoplasm. One example of this group is the p38 MAPK pathway that induces G₂ delay at early time points after UV irradiation through the serine 309 phosphorylation of Cdc25B [3•].

Conclusions

We are just beginning to realize the complexity of the stress-induced G₂/M checkpoint and the exact role that different Cdc25 phosphatases play in controlling mitotic entry in mammalian cells. Recent findings showing the importance of p38 and Chk1 kinases in the regulation of Cdc25B and Cdc25C phosphorylation at inhibitory sites have revealed an additional layer of complexity in when and how the components that regulate these phosphorylations can interact with each other. Multiple pathways are

References and recommended reading

Papers of particular interest, published within the annual period of review,have been highlighted as:

• of special interest
•• of outstanding interest

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Reviewp38 and Chk1 kinases: different conductors for the G2/M checkpoint symphony

Abstract

Introduction

Section snippets

Mechanism for G2/M checkpoint control after genotoxic stress

A model for G2/M checkpoint activation after genotoxic stress

Conclusions

References and recommended reading

J Biol Chem

J Biol Chem

Curr Biol

J Biol Chem

Curr Biol

J Biol Chem

Biochem Biophys Res Commun

Immunopharmacology

Regulation of the G2/M transition by p53

Oncogene

Initiation of a G2/M checkpoint after ultraviolet radiation requires p38 kinase

Nature

Cdc2 tyrosine phosphorylation is required for the DNA damage checkpoint in fission yeast

Genes Dev

The G2/M DNA damage checkpoint inhibits mitosis through Tyr15 phosphorylation of p34cdc2 in Aspergillus nidulans

EMBO J

Role of inhibitory CDC2 phosphorylation in radiation-induced G2 arrest in human cells

J Cell Biol

Nuclear localization of cyclin B1 controls mitotic entry after DNA damage

J Cell Biol

Nuclear export of cyclin B1 and its possible role in the DNA damage-induced G2 checkpoint

EMBO J

Inhibition of ATM and ATR kinase activities by the radiosensitizing agent, caffeine

Cancer Res

Aberrant cell cycle checkpoint function and early embryonic death in Chk1−/− mice

Genes Dev

Review
p38 and Chk1 kinases: different conductors for the G₂/M checkpoint symphony

Mechanism for G₂/M checkpoint control after genotoxic stress

A model for G₂/M checkpoint activation after genotoxic stress

Aberrant cell cycle checkpoint function and early embryonic death in Chk1^−/− mice