Cytokine-dependent transcriptional down-regulation of epithelial sodium channel in ulcerative colitis

Abstract

Background & Aims : The main limiting factor for sodium absorption in distal colon is the amiloride-sensitive epithelial sodium channel (ENaC). This study aimed to characterize mechanisms involved in the dysregulation of ENaC expression in ulcerative colitis (UC). Methods : Epithelial preparations from surgically removed inflamed and control sigmoid colons were used. Active electrogenic Na + transport (J Na) was determined after 8-hour aldosterone stimulation in Ussing-chambers (corrected for the altered epithelial/subepithelial resistance ratio). Subsequently, ENaC α-, β-, and γ-subunits were analyzed immunohistochemically and in Western and Northern blots (corrected for the inflammatory increase in subepithelial protein content). To study gene regulation, the promoters of β- and γ-ENaC were analyzed in reporter gene assays. Results : In controls, aldosterone stimulated J Na and induced ENaC β- and γ-subunit expression, whereas this response was virtually abolished in UC. Preservation of surface epithelium in UC was indicated by unchanged ENaC α-subunit expression, which points also against a mere immaturity or epithelial cell loss. Inhibition of electrogenic sodium transport as well as β- and γ-ENaC mRNA expression could be mimicked in control colon by in vitro preexposure for 8 hours to tumor necrosis factor α and interferon γ. Promoter analysis revealed that down-regulation of β- and γ-ENaC gene expression was primarily induced by tumor necrosis factor α. Conclusions : We conclude that, in UC, elevated proinflammatory cytokines selectively impair β- and γ-ENaC expression, which contributes to diarrhea by reducing colonic sodium absorption.