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* Institute of Pathology,
Clinics of Physical Medicine and Rehabilitation, and
Institute of Medical Immunology, Charité, Humboldt University, Berlin, Germany; and
§ Ethel Percy Andrus Gerontology Center and Division of Molecular Biology, the University of Southern California, Los Angeles, California 90089-0191, USA
1Correspondence: Ethel Percy Andrus Gerontology Center, University of Southern California, 3715 McClintock Ave., Room 306, Los Angeles, CA 90089-0191, USA. E-mail: kelvin{at}usc.edu
We have studied the effects of hyperoxia and of cell loading with artificial lipofuscin or ceroid pigment on the postmitotic aging of human lung fibroblast cell cultures. Normobaric hyperoxia (40% oxygen) caused an irreversible senescence-like growth arrest after about 4 wk and shortened postmitotic life span from 11/2 years down to 3 months. During the first 8 wk of hyperoxia-induced aging, overall protein degradation (breakdown of [35S]methionine metabolically radiolabeled cell proteins) increased somewhat, but by 12 wk and thereafter overall proteolysis was significantly depressed. In contrast, protein synthesis rates were unaffected by 12 wk of hyperoxia. Lysosomal cathepsin-specific activity (using the fluorogenic substrate z-FR-MCA) and cytoplasmic proteasome-specific activity (measured with suc-LLVY-MCA) both declined by 80% or more over 12 wk. Hyperoxia also caused a remarkable increase in lipofuscin/ceroid formation and accumulation over 12 wk, as judged by both fluorescence measurements and FACscan methods. To test whether the association between lipofuscin/ceroid accumulation and decreased proteolysis might be causal, we next exposed cells to lipofuscin/ceroid loading under normoxic conditions. Lipofuscin/ceroid-loaded cells indeed exhibited a gradual decrease in overall protein degradation over 4 wk of treatment, whereas protein synthesis was unaffected. Proteasome specific activity decreased by 25% over this period, which is important since proteasome is normally responsible for degrading oxidized cell proteins. In contrast, an apparent increase in lysosomal cathepsin activity was actually caused by a large increase in the number of lysosomes per cell. To test whether lipofuscin/ceroid could in fact directly inhibit proteasome activity, thus causing oxidized proteins to accumulate, we incubated purified proteasome with lipofuscin/ceroid preparations in vitro. We found that proteasome is directly inhibited by lipofuscin/ceroid. Our results indicate that an accumulation of oxidized proteins (and lipids) such as lipofuscin/ceroid may actually cause further increases in damage accumulation during aging by inhibiting the proteasome.Sitte, N., Huber, M., Grune, T., Ladhoff, A., Doecke, W.-D., von Zglinicki, T., Davies, K. J. A. Proteasome inhibition by lipofuscin/ceroid during postmitotic aging of fibroblasts.
Key Words: ceroid pigment protein turnover lysosome oxidative stress free radicals
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