Short‐Term Regulation of NHE3 by EGF and Protein Kinase C but Not Protein Kinase A Involves Vesicle Trafficking in Epithelial Cells and Fibroblasts

Abstract

NHE3 is an intestinal epithelial isoform Na+/H+ exchanger that is present in the brush border of small intestinal, colonic, and gallbladder Na+‐absorbing epithelial cells. NHE3 is acutely up‐ and downregulated in response to some G protein‐linked receptors, tyrosine kinase receptors, and protein kinases when studied in intact ileum, when stably expressed in PS120 fibroblasts, and in the few studies reported in the human colon cancer cell line Caco‐2. In most cases this is due to changes in Vmax of NHE3, although in response to cAMP and squalamine there are also changes in the K′ (H+)i of the exchanger. The mechanism of the Vmax regulation as shown by cell surface biotinylation and confocal microscopy in Caco‐2 cells and biotinylation in PS120 cells involves changes in the amount of NHE3 on the plasma membrane. In addition, in some cases there are also changes in turnover number of the exchanger. In some cases, the change in amount of NHE3 in the plasma membrane is associated with a change in the amount of plasma membrane. A combination of biochemical studies and transport/inhibitor studies in intact ileum and Caco‐2 cells demonstrated that the increase in brush border Na+/H+ exchange caused by acute exposure to EGF was mediated by PI 3‐kinase. PI 3‐kinase was also involved in FGF stimulation of NHE3 expressed in fibroblasts. Thus, NHE3 is another example of a transport protein that is acutely regulated in part by changing the amount of the transporter on the plasma membrane by a process that appears to involve vesicle trafficking and also to involve changes in turnover number.

Induction of Na+/K+/2Cl− cotransporter expression mediates chronic potentiation of intestinal epithelial Cl− secretion by EGF

“…The role of the EGFR as a critical regulator of intestinal epithelial ion and fluid transport is gradually emerging. Acutely, EGFR ligands inhibit Cl Ϫ secretion and promote Na ϩ and nutrient absorption (3,16,22,29,36,40,42,44). On the other hand, in addition to its long-term effects in promoting agonist-induced secretory responses described here, EGF also upregulates epithelial absorptive function by increasing the expression of transport proteins involved in Na ϩ absorption, namely, Na ϩ /H ϩ exhanger isoform 2 (43).…”

Rapid activation of Na+/H+exchange by EPEC is PKC mediated

“…NHE3 is regulated primarily by changing the amount of protein expressed at the cell surface. The level of cell surface protein is determined through fusion of recycling endosomes containing NHE3 or through the stimulation of apical recycling removing NHE3 from the surface (1,12,23). This process is mediated in part via PKC␣ interaction with E3KARP (NHERF2), a cytoskeletal anchor (34).…”

βPix Up-regulates Na+/H+ Exchanger 3 through a Shank2-mediated Protein-Protein Interaction

“…NHE3 is expressed on the plasma membranes of many gastrointestinal organs and contributes to the maintenance of intracellular pH and volume, transcellular absorption of NaCl and NaHCO 3 , and fluid balance as well as regulation of systemic pH (31). NHE3 is both rapidly stimulated and inhibited as part of normal digestive physiology, and it contributes to multiple pathophysiological states when it is down-regulated for a prolonged period (7,(31)(32)(33). PDZ-based adapter molecules are important mediators of NHE3 regulation, participating in apical targeting, surface retention, and the acute control of NHE3 activity in epithelial cells (7).…”