hSMG-1 and ATM sequentially and independently regulate the G1 checkpoint during oxidative stress

Area of Science:

• Cell Biology
• Molecular Biology
• Genetics

Background:

• Genotoxic stress triggers phosphatidylinositol 3-kinase-like kinases (PIKKs) to regulate DNA damage responses.
• Previous studies identified ATM as a key PIKK in p53 phosphorylation during hyperoxia, but its early role was unclear.

Purpose of the Study:

• To elucidate the roles of hSMG-1 and ATM in p53 phosphorylation and the G(1) cell cycle checkpoint during prolonged oxidative stress.
• To investigate the regulation of p21 stability and its impact on the G(1) checkpoint under hyperoxic conditions.

Main Methods:

• Utilized siRNA to deplete hSMG-1 and ATM in cells.
• Assessed p53 phosphorylation and abundance via Western blotting.
• Quantified p21 levels and monitored the G(1) cell cycle checkpoint.
• Investigated p21 degradation using wortmannin, a PIKK inhibitor.

Main Results:

• hSMG-1 mediates rapid, early p53 phosphorylation at Ser15, with ATM contributing to sustained phosphorylation.
• Depletion of hSMG-1 or ATM reduced p53 phosphorylation but did not abolish the G(1) checkpoint.
• Hyperoxia induced wortmannin-sensitive degradation of p21, which was reversed by hSMG-1 or ATM depletion, restoring checkpoint function.

Conclusions:

• hSMG-1 is a proximal regulator of DNA damage signaling, initiating p53 phosphorylation.
• The G(1) checkpoint is tightly controlled during oxidative stress through PIKK-dependent regulation of p21 synthesis and degradation.