Ionizing radiation activates the ATM kinase throughout the cell cycle

Area of Science:

• Cellular biology
• Molecular oncology
• DNA damage response

Background:

• The ATM (ataxia-telangiectasia mutated) protein kinase is a key regulator of cellular responses to DNA damage, particularly from ionizing radiation (IR).
• ATM dysfunction leads to cell cycle checkpoint failures in G1, S, and G2 phases.
• ATM acts upstream of p53, phosphorylating it at serine 15 in response to IR.

Purpose of the Study:

• To investigate the cell cycle stage-specificity of ATM activation and p53Ser15 phosphorylation.
• To determine if DNA replication-dependent strand breaks are required for ATM activation.
• To explore the role of p53 in ATM-mediated cell cycle checkpoints.

Main Methods:

• Utilized a normal lymphoblastoid cell line (GM536).
• Exposed cells to ionizing radiation (IR) at different cell cycle phases.
• Assessed ATM kinase activity and p53Ser15 phosphorylation levels.

Main Results:

• Ionizing radiation enhanced ATM kinase activity across all cell cycle phases (G1, S, and G2).
• Enhanced ATM activity was observed immediately post-IR, without changes in ATM protein levels.
• ATM activation by IR does not depend on DNA replication-dependent strand breaks.
• p53 protein is not essential for IR-induced S and G2-phase checkpoints, suggesting ATM has phase-specific targets.

Conclusions:

• The ATM kinase is crucial for the immediate cellular response to DNA damage induced by IR in all cell cycle phases.
• ATM activation is independent of DNA replication status and p53 in response to IR.
• ATM likely targets different proteins in distinct cell cycle phases to execute its functions.