NAD(P) H:quinone Oxidoreductase 1 (NQO1) Competes with 20S Proteasome for Binding with C/EBPα Leading to Its Stabilization and Protection against Radiation-induced Myeloproliferative Disease

NAD(P)H:quinone oxidoreductase 1 (NQO1) is a flavoprotein that protects cells against radiation and chemical-induced oxidative stress. Disruption of NQO1 gene in mice leads to increased susceptibility to myeloproliferative disease. In this report, we demonstrate that NQO1 controls the stability of myeloid differentiation factor C/EBP alpha against 20S proteasomal degradation during radiation exposure stress. Co-immunoprecipitation studies showed that NQO1, C/EBP alpha, and 20S all interacted with each other. C/EBP alpha interaction with 20S led to the degradation of C/EBP alpha. NQO1 in presence of its cofactor NADH protected C/EBP alpha against 20S degradation. Deletion and site-directed mutagenesis demonstrated that NQO1 and 20S competed for the same binding region (268)SGAGAG-KAKKSV(279) in C/EBP alpha. Mutagenesis studies also revealed that NQO1Y127/Y129 required for NADH binding is essential for NQO1 stabilization of C/EBP alpha. Exposure of mice and HL-60 cells to 3 Grays of alpha-radiation led to increased NQO1 that stabilized C/EBP alpha against 20S proteasomal degradation. This mechanism of NQO1 regulation of C/EBP alpha may provide protection to bone marrow against adverse effects of radiation exposure. The studies have significance for human individuals carrying hetero- or homozygous NQO1P187S mutation and are deficient or lack NQO1 protein.