Roles of ZO-1, occludin, and actin in oxidant-induced barrier disruption

Abstract

Oxidants such as monochloramine (NH2Cl) decrease epithelial barrier function by disrupting perijunctional actin and possibly affecting the distribution of tight junctional proteins. These effects can, in theory, disturb cell polarization and affect critical membrane proteins by compromising molecular fence function of the tight junctions. To examine these possibilities, we investigated the actions of NH2Cl on the distribution, function, and integrity of barrier-associated membrane, cytoskeletal, and adaptor proteins in human colonic Caco-2 epithelial monolayers. NH2Cl causes a time-dependent decrease in both detergent-insoluble and -soluble zonula occludens (ZO)-1 abundance, more rapidly in the former. Decreases in occludin levels in the detergent-insoluble fraction were observed soon after the fall of ZO-1 levels. The actin depolymerizer cytochalasin D resulted in a decreased transepithelial resistance (TER) more quickly than NH2Cl but caused a more modest and slower reduction in ZO-1 levels and in occludin redistribution. No changes in the cellular distribution of claudin-1, claudin-5, or ZO-2 were observed after NH2Cl. However, in subsequent studies, the immunofluorescent cellular staining pattern of all these proteins was altered by NH2Cl. The actin-stabilizing agent phalloidin did not prevent NH2Cl-induced decreases in TER or increases of apical to basolateral flux of the paracellular permeability marker mannitol. However, it partially blocked changes in ZO-1 and occludin distribution. Tight junctional fence function was also compromised by NH2Cl, observed as a redistribution of the α-subunit of basolateral Na+-K+-ATPase to the apical membrane, an effect not found with the apical membrane protein Na+/H+exchanger isoform 3. In conclusion, oxidants not only disrupt perijunctional actin but also cause redistribution of tight junctional proteins, resulting in compromised intestinal epithelial barrier and fence function. These effects are likely to contribute to the development of malabsorption and dysfunction associated with mucosal inflammation of the digestive tract.