In Vivo Stimulation of IκB Phosphorylation Is Not Sufficient To Activate NF-κB

Abstract

NF-kappa B is a major inducible transcription factor in many immune and inflammatory reactions. Its activation involves the dissociation of the inhibitory subunit I kappa B from cytoplasmic NF-kappa B/Rel complexes, following which the Rel proteins are translocated to the nucleus, where they bind to DNA and activate transcription. Phosphorylation of I kappa B in cell-free experiments results in its inactivation and release from the Rel complex, but in vivo NF-kappa B activation is associated with I kappa B degradation. In vivo phosphorylation of I kappa B alpha was demonstrated in several recent studies, but its role is unknown. Our study shows that the T-cell activation results in rapid phosphorylation of I kappa B alpha and that this event is a physiological one, dependent on appropriate lymphocyte costimulation. Inducible I kappa B alpha phosphorylation was abolished by several distinct NF-kappa B blocking reagents, suggesting that it plays an essential role in the activation process. However, the in vivo induction of I kappa B alpha phosphorylation did not cause the inhibitory subunit to dissociate from the Rel complex. We identified several protease inhibitors which allow phosphorylation of I kappa B alpha but prevent its degradation upon cell stimulation, presumably through inhibition of the cytoplasmic proteasome. In the presence of these inhibitors, phosphorylated I kappa B alpha remained bound to the Rel complex in the cytoplasm for an extended period of time, whereas NF-kappa B activation was abolished. It appears that activation of NF-kappa B requires degradation of I kappa B alpha while it is a part of the Rel cytoplasmic complex, with inducible phosphorylation of the inhibitory subunit influencing the rate of degradation.