No abstract

I kappa B-alpha inhibits transcription factor NF-kappa B by retaining it in the cytoplasm. Various stimuli, typically those associated with stress or pathogens, rapidly inactivate I kappa B-alpha. This liberates NF-kappa B to translocate to the nucleus and initiate transcription of genes important for the defense of the organism. Activation of NF-kappa B correlates with phosphorylation of I kappa B-alpha and requires the proteolysis of this inhibitor. When either serine-32 or serine-36 of I kappa B-alpha was mutated, the protein did not undergo signal-induced phosphorylation or degradation, and NF-kappa B could not be activated. These results suggest that phosphorylation at one or both of these residues is critical for activation of NF-kappa B.

NF-B is a family of related, dimeric transcription factors that are readily activated in cells by signals associated with stress or pathogens. These factors are critical to host defense, as demonstrated previously with mice deficient in individual subunits of NF-B. We have generated mice deficient in both the p50 and p52 subunits of NF-B to reveal critical functions that may be shared by these two highly homologous proteins. We now demonstrate that unlike the respective single knockout mice, the p50/p52 double knockout mice fail to generate mature osteoclasts and B cells, apparently because of defects that track with these lineages in adoptive transfer experiments. Furthermore, these mice present markedly impaired thymic and splenic architectures and impaired macrophage functions. The blocks in osteoclast and B-cell maturation were unexpected. Lack of mature osteoclasts caused severe osteopetrosis, a family of diseases characterized by impaired osteoclastic bone resorption. These findings now establish critical roles for NF-B in development and expand its repertoire of roles in the physiology of differentiated hematopoietic cells.

In addition to coordinating immune and inflammatory responses, NF-kappaB/Rel transcription factors control cell survival. Normally, NF-kappaB dimers are sequestered in the cytoplasm by binding to inhibitory IkappaB proteins, and can be activated rapidly by signals that induce the sequential phosphorylation and proteolysis of IkappaBs. Activation of NF-kappaB antagonizes apoptosis or programmed cell death by numerous triggers, including the ligand engagement of 'death receptors' such as tumour-necrosis factor (TNF) receptor. The anti-apoptotic activity of NF-kappaB is also crucial to oncogenesis and to chemo- and radio-resistance in cancer. Cytoprotection by NF-kappaB involves the activation of pro-survival genes; however, its basis remains poorly understood. Here we report that NF-kappaB complexes downregulate the c-Jun amino-terminal kinase (JNK) cascade, thus establishing a link between the NF-kappaB and the JNK pathways. This link involves the transcriptional upregulation of gadd45beta/myd118 (ref. 4), which downregulates JNK signalling induced by the TNF receptor (TNF-R). This NF-kappaB-dependent inhibition of the JNK pathway is central to the control of cell death. Our findings define a protective mechanism that is mediated by NF-kappaB complexes and establish a role for the persistent activation of JNK in the apoptotic response to TNF-alpha.