Role of p53 in the Protection Against MG132-Induced Apoptosis in a Cell State-Dependent Manner

Abstract

Proteasome inhibition is an important strategy for treating cancer. While proteasome inhibitors are effective at inducing apoptosis in certain rapidly proliferating cancer cells, a fraction of cells enters quiescence and survives. The mechanism by which quiescent cells become resistant to proteasome inhibitors remains to be elucidated. The tumor suppressor protein p53 plays an essential role in establishing and maintaining quiescence in human cells. It is an important regulator of several cell survival mechanisms, such as cell cycle arrest, autophagy, and activation of detoxifying enzymes. Here, we investigate the role of p53 in establishing resistance against proteasome inhibitors in proliferating and quiescent cells. We found that MG132 (a known proteasome inhibitor) treatment induces the accumulation of p53 and its downstream target p21 more in quiescent than in proliferating primary human fibroblasts. shRNA-mediated down-regulation of p53 sensitizes both proliferating and quiescent fibroblasts to MG132-mediated apoptosis and cell death, suggesting that p53 plays a protective role in all cell states. Additional experiments demonstrated that proliferating fibroblasts are susceptible to inhibition of cytoplasmic p53 activity, whereas quiescent fibroblasts are sensitive to inhibition of nuclear p53 in the short-term and cytoplasmic p53 in the long-term. These findings suggest that proliferating and quiescent cells depend on different p53-induced survival mechanisms to be protected from proteasome inhibition. Autophagy, an alternative degradation pathway regulated by p53, is induced in both cell states upon MG132 treatment and may contribute to both cell survival and cell death. Identification of p53- mediated regulatory mechanisms in response to proteasome inhibition will help develop new strategies to increase the effectiveness of proteasome inhibitors in clinical therapy.