NIK is involved in nucleosomal regulation by enhancing histone H3 phosphorylation by IKKα

The exact physiological role of NF-kappa B-inducing kinase (NIK) in the NF-kappa B activation pathway has not been defined, although it is an upstream kinase of IKK alpha. Recent studies have indicated that IKK alpha is a nucleosomal modifier of NF-kappa B signaling. We hypothesized that NIK generates a proximal signal that contributes to IKK alpha modification of nucleosomal structure through phosphorylation of histone H3 and enhancement of target gene expression. By using a chromatin immunoprecipitation assay, our data show that endogenous IKK alpha is recruited to the promoter site of several NF-kappa B-dependent genes in macrophages. Our data show that immunoreactive NIK is rapidly recruited to nuclear compartment in macrophages in response to treatment with endotoxin where it augments phosphorylation of histone H3 by inducing phosphorylation and kinase activity of IKK alpha. A small interfering RNA knockdown of NIK markedly reduces phosphorylation of histone H3 in endotoxin treated macrophages. These data, together, demonstrate a novel role for NIK as a histone H3 modifier, through an accessory pathway from NIK to IKK alpha, that could play an important role in the endotoxin response through modification of nucleosomal structure.