N. Sitte et al., Protein oxidation and degradation during cellular senescence of human BJ fibroblasts: part I - effects of proliferative senescence, FASEB J, 14(15), 2000, pp. 2495-2502
Oxidized and cross-linked proteins tend to accumulate in aging cells. Decli
ning activity of proteolytic enzymes, particularly the proteasome, has been
proposed as a possible explanation for this phenomenon, and direct inhibit
ion of the proteasome by oxidized and cross-linked proteins has been demons
trated in vitro. We have further examined this hypothesis during both proli
ferative senescence (this paper) and postmitotic senescence (see the accomp
anying paper, ref 1) of human BJ fibroblasts. During proliferative senescen
ce, we found a marked decline in all proteasome activities (trypsin-like ac
tivity, chymotrypsin-like activity, and peptidyl-glutamyl-hydrolyzing activ
ity) and in lysosomal cathepsin activity. Despite the loss of proteasome ac
tivity, there was no concomitant change in cellular levels of actual protea
some protein (immunoassays) or in the steady-state levels of mRNAs for esse
ntial proteasome subunits. The decline in proteasome activities and lysosom
al cathepsin activities was accompanied by dramatic increases in the accumu
lation of oxidized and cross-linked proteins. Furthermore, as proliferation
stage increased, cells exhibited a decreasing ability to degrade the oxida
tively damaged proteins generated by an acute, experimentally applied oxida
tive stress. Thus, oxidized and cross-linked proteins accumulated rapidly i
n cells of higher proliferation stages. Our data are consistent with the hy
pothesis that proteasome is progressively inhibited by small accumulations
of oxidized and cross-linked proteins during proliferative senescence until
late proliferation stages, when so much proteasome activity has been lost
that oxidized proteins accumulate at ever-increasing rates. Lysosomes attem
pt to deal with the accumulating oxidized and cross-linked proteins, but de
clining lysosomal cathepsin activity apparently limits their effectiveness.
This hypothesis, which may explain the progressive intracellular accumulat
ion of oxidized and cross-linked proteins in aging, is further explored dur
ing postmitotic senescence in the accompanying paper (1).