NRH:quinone oxidoreductase 2 and NAD(P)H:quinone oxidoreductase 1 protect tumor suppressor p53 against 20s proteasomal degradation leading to stabilization and activation of p53.

Tumor suppressor p53 is either lost or mutated in several types of cancer. MDM2 interaction with p53 results in ubiquitination and 26S proteasomal degradation of p53. Chronic DNA damage leads to inactivation of MDM2, stabilization of p53, and apoptotic cell death. Here, we present a novel MDM2/ubiquitination-independent mechanism of stabilization and transient activation of p53. The present studies show that 20S proteasomes degrade p53. The 20S degradation of p53 was observed in ubiquitin-efficient and -deficient cells, indicating that this pathway of degradation did not require ubiquitination of p53. The cytosolic quinone oxidoreductases [NRH:quinone oxidoreductase 2 (NQO2) and NAD(P)H:quinone oxidoreductase 1 (NQO1)] interacted with p53 and protected p53 against 20S proteasomal degradation. Further studies revealed that acute exposure to radiation or chemical leads to induction of NQO1 and NQO2 that stabilizes and transiently activates p53 and downstream genes. These results suggest that stress-induced NQO1 and NQO2 transiently stabilize p53, which leads to protection against adverse effects of stressors.