Role of nitric oxide and peroxynitrite in gut barrier failure

Bacterial translocation (BT) may be a normal physiologic process that is important for mucosal antigen sampling in the gut. However, physiologic insults such as endotoxemia, hemorrhagic shock, or necrotizing enterocolitis (NEC) may lead to pathologic BT and thus contribute to the pathogenesis of nosocomial infection. The mechanism may involve accelerated enterocyte apoptosis at the intestinal villus apex resulting, at least transiently, in a "bare area" at the villus tip where bacteria can attach and traverse the epithelium. Evidence suggests that sustained upregulation of the inducible isoform of nitric oxide synthase (NOS-2) co-localizes with enterocyte apoptosis and immunoreactivity to 3-nitrotyrosine, the footprint of peroxynitrite (ONOO-), a potent oxidant formed by the reaction of nitric oxide (NO) with superoxide. We propose that the bare area at the villus apex is caused by apoptosis of enterocytes that have migrated from the base of the crypts to the villus apex and are shed into the intestinal lumen. These bare areas, and thus the degree of BT, may be the result of an imbalance between enterocyte proliferation and apoptosis. We postulate that normal enterocyte apoptosis is mediated by the caspase cascade, whereas enterocyte proliferation and differentiation in the crypt may be regulated by tyrosine kinase-dependent signaling pathways. Both of these cellular pathways may be influenced by overproduction of NO and its metabolite ONOO-. Therefore, sustained NO production and ONOO- formation occurring in inflammatory states may differentially accelerate apoptosis in the villus apex and/or inhibit proliferation at the base of the crypts resulting in expanded extrusion zones at the villus tip and accelerated BT.