High levels of proinflammatory cytokines are linked to pathogenesis of diarrhea in inflammatory bowel disease (IBD). Na+ absorption is compromised in IBD. The studies were designed to determine the effect of tumor necrosis factor-α (TNF-α) on the expression and activity of NHE2, a Na+/H+ exchanger (NHE) that is involved in transepithelial Na+ absorption in intestinal epithelial cells.
NHE2 regulation was examined in TNF-α-treated C2BBe1 cells by reverse-transcription polymerase chain reaction (RT-PCR), reporter gene assays, and Western blot analysis. NHE isoform activities were measured as ethyl-isopropyl-amiloride- and HOE694-sensitive 22Na-uptake. In vitro and in vivo protein-DNA interactions were assessed by gel mobility shift assays and chromatin immunoprecipitation studies.
TNF-α treatment of C2BBe1 cells led to repression of NHE2 promoter activity, mRNA, and protein levels; and inhibited both NHE2 and NHE3 mediated 22Na-uptake. 5′-deletion analysis of the NHE2 promoter-reporter constructs identified basepair −621 to −471 as the TNF-α-responsive region (TNF-RE). TNF-α activated NF-κB subunits, p50 and p65, and their DNA-binding to a putative NF-κB motif within TNF-RE. Mutations in the NF-κB motif abolished NF-κB-DNA interactions and abrogated TNF-α-induced repression. Ectopic overexpression of NF-κB resulted in repression of NHE2 expression. Two functionally distinct inhibitors of NF-κB blocked the inhibitory effect of TNF-α.
The human NHE2 isoform is a direct target of transcription factor NF-κB. TNF-α-mediated activation of NF-κB decreases the expression and activity of NHE2 in the intestinal epithelial cell line, C2BBe1. These findings implicate NF-κB in the modulation of Na+ absorption during intestinal inflammatory conditions such as IBD where a high level of TNF-α is detected. (Inflamm Bowel Dis 2011;)