Constitutive nuclear expression of the IκB kinase complex and its activation in human neutrophils

A singular feature of human neutrophils is that they constitutively express substantial amounts of NF-kappa B/Rel proteins and I kappa B-alpha in the nucleus. In this study,. we show that in these cells, I kappa B kinase a (IKK alpha), IKK beta, and IKK gamma also partially localize to the nucleus, whereas IKK-related kinases (IKK is an element of, TANK-binding kinase-1) are strictly cytoplasmic, and the NF-kappa B-inducing kinase is strictly nuclear. Following neutrophil activation, IKK beta and IKK gamma become transiently phosphorylated in both the cytoplasm and nucleus, whereas IKKa transiently vanishes from both compartments in what appears to be an IKK beta-dependent process. These responses are paralleled by the degradation of I kappa B-alpha, and by the phosphorylation of Re1A on serine 536, in both compartments. Although both proteins can be IKK substrates,inhibition of IKK prevented I kappa B-alpha phosphorylation, while that of RelA was mostly unaffected. Finally, we provide evidence that the nuclear IKK isoforms (alpha, beta, gamma) associate with chromatin following neutrophil activation, which suggests a potential role in gene regulation. This is the first study to document IKK activation and the phosphorylation of NF-kappa B/Re1 proteins in primary neutrophils. More importantly, our findings unveil a hitherto unsuspected mode of activation for the IKK/I kappa B signaling cascade within the cell nucleus.