Ubiquitin pools, ubiquitin mRNA levels, and ubiquitin-mediated proteolysis in aging human fibroblasts

Senescent cells have less free ubiquitin and more ubiquitin-protein conjugates than do young cells. The ubiquitin-protein conjugates are heterogeneous in size but contain prominent bands at 106, 55, and 22 kDa. The age-related increase in ubiquitin-protein conjugates applies primarily to the 55-kDa band, while the 106-kDa and 22-kDa conjugates change little with age. Ubiquitin mRNA levels do not change with age, and the ability of cells to degrade two proteins that are good substrates for ubiquitin-mediated proteolysis is unaltered by aging. These results indicate that an increase in ubiquitin-protein conjugates does not necessarily reflect alterations in ubiquitin-mediated proteolysis. Furthermore, an overactive pathway of ubiquitin-mediated proteolysis does not appear to contribute to the proliferative arrest in senescent cells.