NF-κB inhibition enhances peroxynitrite-induced enterocyte apoptosis

Background. Sustained overproduction of nitric oxide and peroxynitrite (ONOO^-) in conditions such as necrotizing enterocolitis and inflammatory bowel disease may promote gut barrier failure by inducing enterocyte apoptosis. NF-κB is upregulated in the gut during inflammation and, in addition to its proinflammatory effects, may upregulate protective or anti-apoptotic factors such as inhibitor of apoptosis proteins (IAPs). We have previously demonstrated that NF-κB inhibition increases cytokine-induced enterocyte apoptosis; however, the effect of NF-κB on ONOO^--induced enterocyte apoptosis is unknown. Materials and methods. Rat intestinal epithelial cells (IEC-6) were transfected with the adenoviral vector AdIκB or AdlacZ. AdIκB contains a mutated form of IκB which functions as a superrepressor of NF-κB. Cells were then treated with 50 μM ONOO^- or decomposed ONOO^-. Apoptosis was then determined by flow cytometry with annexin V-FITC and propidium iodide staining. Caspase activation and IAP, Bcl-2, Bad, and Bax expression were examined using Western blot analysis, and NF-κB activation was determined via electrophoretic mobility shift assay (EMSA). Results. Inhibition of NF-κB with AdIκB significantly enhanced ONOO^--induced apoptosis in IEC-6 cells. ONOO^- treatment did not activate NF-κB in IEC-6 cells as determined by EMSA. There was no difference in IAP, Bcl-2, Bad, and Bax expression between nontransfected, AdlacZ-transfected, and AdIκB-transfected cells. Baseline procaspase 3 activation was increased in AdIκB-transfected cells. Conclusions. NF-κB inhibition enhances ONOO^--induced enterocyte apoptosis, suggesting that NF-κB upregulates a protective factor. This protective factor does not appear to be an IAP or Bcl-2 family member and may be expressed constitutively, since ONOO^- did not activate NF-κB over baseline levels of activation.