Differential regulation of Na⁺/H⁺exchange isoform activities by enteropathogenicE. coliin human intestinal epithelial cells

Abstract

Enteropathogenic Escherichia coli (EPEC) is an important human intestinal foodborne pathogen associated with diarrhea, especially in infants and young children. Although EPEC produces characteristic attaching and effacing lesions and loss of microvilli, the pathophysiology of EPEC-associated diarrhea, particularly during early infection, remains elusive. The present studies were designed to examine the direct effects of EPEC infection on intestinal absorption via Na(+)/H(+) exchanger (NHE) isoforms. Caco-2 cells were infected with EPEC strain E2348/69 or nonpathogenic E. coli HB101 for a period of 60 to 120 min. Total NHE activity was significantly increased at 60 min, reaching approximately threefold increase after 90 min of EPEC infection. Similar findings were seen in HT-29 cells and T84 cells indicating that the response was not cell-line specific. Most surprising was the differential regulation of NHE2 and NHE3 by EPEC. Marked activation of NHE2 (300%) occurred, whereas significant inhibition ( approximately 50%) of NHE3 activity was induced. The activity of basolateral isoform NHE1 was also significantly increased in response to EPEC infection. Mutations that disrupted the type III secretion system (TTSS) ablated the effect of EPEC on the activity of both NHE2 and NHE3. These results suggest that EPEC, through a TTSS-dependent mechanism, exerts differential effects on NHE isoform activity in intestinal epithelial cells. Additionally, NHEs do not appear to play any role in EPEC-mediated inflammation, because the NHE inhibitors amiloride and 5-(N-ethyl-N-isopropyl)amiloride did not prevent EPEC-mediated IkappaBalpha degradation.

Na-H Exchanger Isoform-2 (NHE2) Mediates Butyrate-dependent Na+ Absorption in Dextran Sulfate Sodium (DSS)-induced Colitis

“…EPEC that causes diarrhea in newborns and infants has been shown to differentially regulate NHE2 and NHE3 in human colon cell lines (CaCo2, HT29, and T84) (37). Thus, an EPEC effector protein EspF inhibits NHE3 activity, whereas EPEC-mediated protein kinase C (PKC) regulates the up-regulation of NHE2 function (39,40).…”

“…Although NHE2 function is increased in cell lines, diarrhea is present in EPEC-infected patients. Thus, Hecht et al (37) have concluded that increased NHE2 may not compensate for the fluid losses caused by defective NHE3 function during EPEC infection. In recent studies, however, we have shown that butyrate stimulates NHE2-mediated Na ϩ absorption, whereas NHE3 is inhibited by dibutyryl-cAMP in colon and cholera toxin in ileum (17,26).…”

Reduced colonic microbial diversity is associated with colitis in NHE3-deficient mice

“…This intestinal transport mechanism, mediated primarily by NHE3, plays a critical role in the intestinal sodium and water absorption, regulation of intracellular pH, as well as the acidity of the apical microenvironment. NHE3 expression and/or activity is inhibited by proinflammatory cytokines (3,19) and enteropathogenic bacteria (9), whereas commensal Lactobacillus acidophilus upregulates intestinal NHE3 expression and function (24). Inhibition of NHE3 expression and/or activity has been described in several experimental models of colitis, including IL-2 Ϫ/Ϫ mice (3), IL-10 Ϫ/Ϫ mice (17), and in dextran sodium sulfate (DSS)-and trinitrobenzene sulfonic acid-induced colitis (27).…”

Tumor necrosis factor-α downregulates intestinal NHE8 expression by reducing basal promoter activity

“…The reduction of intestinal NaCl absorption in IBDs (33,36) and in chronic enteritis (44) suggested that decreased Na ϩ /H ϩ exchange activity in the inflamed intestinal mucosa may impair electrolyte absorption and contribute to diarrhea since NHEs are important players in intestinal NaCl absorption. Currently, only NHE3 inhibition has been reported in enteropathogenic Escherichia coli-infected Caco-2 cells (20) and in a interleukin-2-deficient mouse model of colonic inflammation (4), but the involvement of proinflammatory mediators with other NHE isoforms is poorly understood.…”