NF-κB regulates the stability and activity of p73 by inducing its proteolytic degradation through a ubiquitin-dependent proteasome pathway

Nuclear factor kappa B (NF-kappa B), which exists as heterodimeric complexes composed of p50 and p65, has been shown to play an important role in cell survival processes. In the present study, we found for the first time that NF-kappa B has an ability to induce the ubiquitin-dependent proteasomal degradation of proapoptotic p73 alpha. The activation of NF-kappa B in tumor necrosis factor a (TNF-alpha)-stimulated H1299 cells resulted in a significant reduction in the amounts of the endogenous p73 alpha. Consistent with these results, TNF-alpha-mediated down-regulation of p73 alpha was observed in wild-type (WT) mouse embryonic fibroblasts (MEFs) but not in p65-deficient MEFs. Ectopic expression of NF-kappa B decreased a half-life of p73a by increasing its ubiquitination levels, and thereby inhibiting the transcriptional activity as well as proapoptotic function of p73a, whereas NF-kappa B had undetectable effects on p53. Immunoprecipitation experiments demonstrated that, under our experimental conditions, NF-kappa B does not bind to p73a in mammalian cultured cells. In contrast to WT p65, the COOH-terminal deletion mutant of p65 (p65 Delta C) failed to reduce the expression levels of p73a, suggesting that NF-kappa B-mediated proteolytic degradation of p73a requires the transcriptional activity of NF-kappa B. Taken together, our present results imply that NF-kappa B-mediated degradation of proapoptotic p73 is a novel inhibitory mechanism of p73 that regulates cell survival and death.