Immunosuppression by Glucocorticoids: Inhibition of NF-κB Activity Through Induction of IκB SynthesisGlucocorticoids are among the most potent anti-inflammatory and immunosuppressive agents. They inhibit synthesis of almost all known cytokines and of several cell surface molecules required for immune function, but the mechanism underlying this activity has been unclear. Here it is shown that glucocorticoids are potent inhibitors of nuclear factor kappa B (NF-kappa B) activation in mice and cultured cells. This inhibition is mediated by induction of the I kappa B alpha inhibitory protein, which traps activated NF-kappa B in inactive cytoplasmic complexes. Because NF-kappa B activates many immunoregulatory genes in response to pro-inflammatory stimuli, the inhibition of its activity can be a major component of the anti-inflammatory activity of glucocorticoids.
Ultraviolet Light and Osmotic Stress: Activation of the JNK Cascade Through Multiple Growth Factor and Cytokine ReceptorsExposure of mammalian cells to ultraviolet (UV) light or high osmolarity strongly activates the c-Jun amino-terminal protein kinase (JNK) cascade, causing induction of many target genes. Exposure to UV light or osmotic shock induced clustering and internalization of cell surface receptors for epidermal growth factor (EGF), tumor necrosis factor (TNF), and interleukin-1 (IL-1). Activation of the EGF and TNF receptors was also detected biochemically. Whereas activation of each receptor alone resulted in modest activation of JNK, coadministration of EGF, IL-1, and TNF resulted in a strong synergistic response equal to that caused by exposure to osmotic shock or UV light. Inhibition of clustering or receptor down-regulation attenuated both the osmotic shock and UV responses. Physical stresses may perturb the cell surface or alter receptor conformation, thereby subverting signaling pathways normally used by growth factors and cytokines.
Mapping of the Inducible IκB Phosphorylation Sites That Signal Its Ubiquitination and Degradation†Joseph A. DiDonato, Frank Mercurio, Caridad Rosette et al.|Molecular and Cellular Biology|1996 Extracellular stimuli that activate the transcription factor NF-kappaB cause rapid phosphorylation of the IkappaBalpha inhibitor, which retains NF-kappaB in the cytoplasm of nonstimulated cells. Phosphorylation of IkappaBalpha is followed by its rapid degradation, the inhibition of which prevents NF-kappaB activation. To determine the relationship between these events, we mapped the inducible phosphorylation sites of IkappaBalpha. We found that two residues, serines 32 and 36, were phosphorylated in response to either tumor necrosis factor, interleukin-1, or phorbol ester. Substitution of either serine blocks or slows down induction of IkappaBalpha degradation. Substitutions of the homologous sites in IkappaBbeta, serines 19 and 23, also prevent inducible IkappaBbeta degradation. We suggest that activation of a single IkappaB kinas e or closely related IkappaB kinases is the first cr itical step in NF-kappaB activation. Once phosphorylated, IkappaB is ubiquitinated. Unlike wild-type IkappaBalpha, the phosphorylation-defective mutants do not undergo inducible polyubiquitination. As substitution of a conserved lysine residue slows down the ubiquitination and degradation of IkappaBalpha without affecting its phosphorylation, polyubiquitination is required for inducible IkappaB degradation.
NF-kappa B activation by ultraviolet light not dependent on a nuclear signalExposure of mammalian cells to radiation triggers the ultraviolet (UV) response, which includes activation of activator protein-1 (AP-1) and nuclear factor kappa B (NF-kappa B). This was postulated to occur by induction of a nuclear signaling cascade by damaged DNA. Recently, induction of AP-1 by UV was shown to be mediated by a pathway involving Src tyrosine kinases and the Ha-Ras small guanosine triphosphate-binding protein, proteins located at the plasma membrane. It is demonstrated here that the same pathway mediates induction of NF-kappa B by UV. Because inactive NF-kappa B is stored in the cytosol, analysis of its activation directly tests the involvement of a nuclear-initiated signaling cascade. Enucleated cells are fully responsive to UV both in NF-kappa B induction and in activation of another key signaling event. Therefore, the UV response does not require a signal generated in the nucleus and is likely to be initiated at or near the plasma membrane.
p105 and p98 precursor proteins play an active role in NF-kappa B-mediated signal transduction.The Rel/NF-kappa B family of transcription factors is composed of two distinct subgroups, proteins that undergo proteolytic processing and contain SWI6/ankyrin repeats in their carboxyl termini (p105, p98), and those without such repeats that do not require processing (p65, c-Rel, RelB, and Dorsal). We demonstrate that the p105 and p98 precursors share functional properties with the I kappa B proteins, which also contain SWI6/ankyrin repeats. Both p105 and p98 were found to form stable complexes with other Rel/NF-kappa B family members, including p65 and c-Rel. Association with the precursors is sufficient for cytoplasmic retention of either p65 or c-Rel, both of which are otherwise nuclear. These complexes undergo stimulus-responsive processing to produce active p50/c-Rel and p55/c-Rel complexes. These observations suggest a second pathway leading to NF-kappa B induction, in which processing of the precursors rather than phosphorylation of I kappa B plays a major role.