Dynamic protein complexes regulate NF-kappaB signaling

NF-kappaB is a major regulator of the first-line defense against invading pathogens, antigen-specific adaptive immune responses or chemical stress. Stimulation either by extracellular ligands (e.g., inflammatory cytokines, microbial pathogens, peptide antigens) or by intracellular Stressors (e.g., genotoxic drugs) initiates signal-specific pathways that all converge at the IkappaB kinase (IKK) complex, the gatekeeper for NF-kappaB activation. During recent years, considerable progress has been made in understanding the function of NF-kappaB in the regulation of cell growth, survival and apoptosis. In this review, we will focus on the regulation of large signaling complexes on the route to NF-kappaB. Recently published data demonstrate that the assembly, maintenance and activity of the IKK complex determine downstream activation of NF-kappaB. In addition, dynamic complexes upstream of IKK are formed in response to tumor necrosis factor (TNF), antigenic peptides or DNA-damaging agents. Clustering of signaling adaptors promotes the association and activation of ubiquitin ligases that trigger the conjugation of regulatory ubiquitin to target proteins. Ubiquitination serves as a platform to recruit the IKK complex and potentially other protein kinases to trigger IKK activation. These findings support a concept whereby protein complex assembly induces regulatory ubiquitination, which in turn recruits and activates protein kinases. Notably, the great interest in a detailed description of the mechanisms that regulate NF-kappaB activity stems from many observations that link dysregulated NF-kappaB signaling with the onset or progression of various diseases, including cancer, chronic inflammation, cardiovascular disorders and neurodegenerative diseases. Thus, the formation of large signaling clusters and regulatory ubiquitin chains represents promising targets for pharmacological intervention to modulate NF-kappaB signal transduction in disease.