Senescent human fibroblasts have elevated Ku86 proteolytic cleavage activity.

Area of Science:

• Molecular biology
• Cellular senescence
• Protein biochemistry

Background:

• Ku protein (Ku70/Ku86) is crucial for DNA double-strand break repair.
• Cellular senescence is a state of irreversible growth arrest.
• Changes in DNA repair proteins during senescence are not fully understood.

Purpose of the Study:

• To investigate the presence and activity of a protease that cleaves Ku86 in human diploid fibroblasts (HDFs).
• To determine if Ku86 cleavage occurs during replicative senescence and affects DNA-end binding activity.

Main Methods:

• In vitro proteolytic assays using HDF cell extracts.
• Analysis of Ku86 subunit cleavage products via molecular mass.
• Inclusion of protease inhibitors (leupeptin) during extraction.
• Assessment of Ku70, Ku86 levels, and DNA-end binding activity in senescent cells.

Main Results:

• A protease activity cleaving Ku86 into 69 and 17 kDa fragments was identified in HDFs.
• This cleavage activity was elevated in late-passaged and senescent cells.
• In vitro, the 69 kDa fragment appeared to bind DNA ends with Ku70.
• However, no 69 kDa variant was detected in cells, and Ku86 levels and DNA-end binding remained unchanged during senescence when protease activity was inhibited.

Conclusions:

• Human diploid fibroblasts contain a protease that can cleave Ku86, but this cleavage is an artifact of in vitro extraction and not representative of cellular conditions.
• Failure to inhibit this protease during sample preparation can lead to inaccurate assessments of Ku86 expression and function.
• Replicative senescence does not alter the levels of Ku86, Ku70, or DNA-end binding activity in HDFs.