Short-term regulation of NHE3 by EGF and protein kinase C but not protein kinase A involves vesicle trafficking in epithelial cells and fibroblasts

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 kinoses 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 V_max 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 V_max regulation as shown by cell surface blotinylation 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.