Rapid Destruction of Human Cdc25A in Response to DNA Damage

TLDR

To protect genome integrity and ensure survival, eukaryotic cells exposed to genotoxic stress cease proliferating to provide time for DNA repair. Upon UV or ionizing radiation, human cells trigger rapid, ubiquitin‑ and proteasome‑dependent degradation of Cdc25A via Chk1 activation, while p53 remains uninvolved, and sustained inhibitory tyrosine phosphorylation of Cdk2 blocks S‑phase entry. Overexpression of Cdc25A bypasses the checkpoint, causing increased DNA damage and reduced survival, indicating that its specific degradation is essential for the DNA damage checkpoint and that its overexpression may promote tumorigenesis.

Abstract

To protect genome integrity and ensure survival, eukaryotic cells exposed to genotoxic stress cease proliferating to provide time for DNA repair. Human cells responded to ultraviolet light or ionizing radiation by rapid, ubiquitin- and proteasome-dependent protein degradation of Cdc25A, a phosphatase that is required for progression from G 1 to S phase of the cell cycle. This response involved activated Chk1 protein kinase but not the p53 pathway, and the persisting inhibitory tyrosine phosphorylation of Cdk2 blocked entry into S phase and DNA replication. Overexpression of Cdc25A bypassed this mechanism, leading to enhanced DNA damage and decreased cell survival. These results identify specific degradation of Cdc25A as part of the DNA damage checkpoint mechanism and suggest how Cdc25A overexpression in human cancers might contribute to tumorigenesis.