Protein oxidation and degradation during cellular senescence of human BJ fibroblasts: part I--effects of proliferative senescence.

Oxidized and cross-linked proteins tend to accumulate in aging cells. Declining activity of proteolytic enzymes, particularly the proteasome, has been proposed as a possible explanation for this phenomenon, and direct inhibition of the proteasome by oxidized and cross-linked proteins has been demonstrated in vitro. We have further examined this hypothesis during both proliferative senescence (this paper) and postmitotic senescence (see the accompanying paper, ref 1 ) of human BJ fibroblasts. During proliferative senescence, we found a marked decline in all proteasome activities (trypsin-like activity, chymotrypsin-like activity, and peptidyl-glutamyl-hydrolyzing activity) and in lysosomal cathepsin activity. Despite the loss of proteasome activity, there was no concomitant change in cellular levels of actual proteasome protein (immunoassays) or in the steady-state levels of mRNAs for essential proteasome subunits. The decline in proteasome activities and lysosomal cathepsin activities was accompanied by dramatic increases in the accumulation of oxidized and cross-linked proteins. Furthermore, as proliferation stage increased, cells exhibited a decreasing ability to degrade the oxidatively damaged proteins generated by an acute, experimentally applied oxidative stress. Thus, oxidized and cross-linked proteins accumulated rapidly in cells of higher proliferation stages. Our data are consistent with the hypothesis 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 attempt to deal with the accumulating oxidized and cross-linked proteins, but declining lysosomal cathepsin activity apparently limits their effectiveness. This hypothesis, which may explain the progressive intracellular accumulation of oxidized and cross-linked proteins in aging, is further explored during postmitotic senescence in the accompanying paper (1).