Background and aims: Expression of inducible nitric oxide synthase (iNOS) is greatly upregulated in the colonic mucosa of patients with collagenous and ulcerative colitis. As the transcription factor nuclear factor κB (NFκB) is a major inducer of iNOS gene expression, we compared activation and transcriptional activity of NFκB in colonic mucosal biopsies from these patients.

Patients: Eight patients with collagenous colitis, six with relapsing ulcerative colitis, and eight with uninflamed bowel were studied.

Methods: NFκB DNA binding activity was assessed by electrophoretic mobility shift assay and inhibitor of NFκB (IκB) kinase (IKK) activity by immunocomplex kinase assay. In vivo recruitment of NFκB to the iNOS promoter was determined by chromatin immunoprecipitation analysis and transcriptional activity by NFκB gene expression profiling arrays. Cells showing NFκB activation were identified by immunohistochemistry.

Results: In collagenous and ulcerative colitis, as opposed to uninflamed bowel, IKKβ activity and strong NFκB DNA binding gave rise to activation of identical NFκB subunits and recruitment of transcriptionally active p65 to the iNOS promoter. In collagenous colitis, activated NFκB was observed only in epithelial cells while up to 10% of lamina propria macrophages showed activation in ulcerative colitis.

Conclusions: In collagenous and ulcerative colitis, colonic mucosal NFκB is activated and recruited to the iNOS promoter in vivo via an IKKβ mediated pathway. As collagenous colitis is not associated with tissue injury, these data challenge the prevailing view that activation of NFκB per se mediates tissue injury. Our results suggest that downstream inflammatory reactions leading to tissue damage originate in lamina propria immune cells, as increased NFκB activity in collagenous colitis was localised solely in epithelial cells, but present also in macrophages in ulcerative colitis.