Lipopolysaccharide-induced activation of NF-κB non-canonical pathway requires BCL10 serine 138 and NIK phosphorylations.

Abstract Background and Aims: B-cell lymphoma / leukemia (BCL)-10 and reactive oxygen species mediate two pathways of NF-κB (RelA) activation by lipopolysaccharide (LPS) in human colonic epithelial cells. The pathway for LPS activation of RelB by the non-canonical pathway (RelB) in non-myeloid cells was not yet reported, but important for understanding the range of potential microbial LPS-induced effects in inflammatory bowel disease. Methods: Experiments were performed in human colonic epithelial cells and in mouse embryonic fibroblasts deficient in components of the IkappaB kinase (IKK) signalosome, in order to detect mediators of the non-canonical pathway of NFκB activation, including nuclear RelB and p52 and phospho- and total NF-κB inducing kinase (NIK). BCL10 was silenced by siRNA and effects of mutations of specific phosphorylation sites of BCL10 (Ser138Gly and Ser218Gly) were determined. Results: By the non-canonical pathway, LPS exposure increased nuclear RelB and p52, and phospho-NIK, with no change in total NIK. Phosphorylation of BCL10 Serine 138 was required for NIK phosphorylation, since mutation of this residue eliminated the increases in phospho-NIK and nuclear RelB and p52. Mutations of either Serine 138 or Serine 218 reduced RelA, p50, and phospho-IκBα of the canonical pathway. Effects of LPS stimulation and BCL10 silencing on NIK phosphorylation were demonstrated in confocal images. Conclusions: LPS-induces activation of both canonical and non-canonical pathways of NFκB in human colonic epithelial cells, and the non-canonical pathway requires phosphorylations of BCL10 (Serine 138) and NIK. These findings demonstrate the important role of BCL10 in mediating LPS-induced inflammation in human colonic epithelial cells and may open new avenues for therapeutic interventions.