One of the highlights of age-related changes of cellular metabolism is the
accumulation of oxidized proteins. The aging process on a cellular level ca
n be treated either as the ongoing proliferation until a certain number of
cell divisions is reached (the Hayflick Limit) or as the aging of nondividi
ng cells, that is, the age-related changes in cells without proliferation.
The present investigation was undertaken to reveal the changes in protein t
urnover, proteasome activity, and protein oxidation status during prolifera
tive senescence. We were able to demonstrate that the activity of the cytos
olic proteasomal system declines dramatically during the proliferative sene
scence of human MRC-5 fibroblasts. Regardless of the loss in activity, it c
ould be demonstrated that there are no changes in the transcription and tra
nslation of proteasomal subunits. This decline in proteasome activity was a
ccompanied by an increased concentration of oxidized proteins. Cells at hig
her proliferation stages were no longer able to respond with increased degr
adation of endogenous [S-35]-Met-radiolabeled proteins after hydrogen perox
ide- or quinone-induced oxidative stress. It could be demonstrated that oxi
dized proteins in senescent human MRC-5 fibroblasts are not as quickly remo
ved as they are in young cells. Therefore, our study demonstrates that the
accumulation of oxidized proteins and decline in protein turnover and activ
ity of the proteasomal system are not only a process of postmitotic aging b
ut also occur during proliferative senescence and result in an increased ha
lf-life of oxidized proteins. (C) 2000 Elsevier Science Inc.