The 11S Proteasome Subunit PSME3 Is a Positive Feedforward Regulator of NF-κB and Important for Host Defense against Bacterial Pathogens

The NF-kappa B pathway plays important roles in immune responses. Although its regulation has been extensively studied, here, we report an unknown feedforward mechanism for the regulation of this pathway by Toll-like receptor (TLR) ligands in macrophages. During bacterial infections, TLR ligands upregulate the expression of the 11S proteasome subunit PSME3 via NF-kappa B-mediated transcription in macrophages. PSME3, in turn, enhances the transcriptional activity of NF-kappa B by directly binding to and destabilizing KLF2, a negative regulator of NF-kappa B transcriptional activity. Consistent with this positive role of PSME3 in NF-kappa B regulation and importance of the NF-kappa B pathway in host defense against bacterial infections, the lack of PSME3 in hematopoietic cells renders the hosts more susceptible to bacterial infections, accompanied by increased bacterial burdens in host tissues. Thus, this study identifies a substrate for PSME3 and elucidates a proteolysis-dependent, but ubiquitin-independent, mechanism for NF-kappa B regulation that is important for host defense and innate immunity.