Half-lives of plasma membrane Na+/H+ exchangers NHE1-3: plasma membrane NHE2 has a rapid rate of degradation

The Na+/H+ exchangers NHE2 and NHE3 are involved in epithelial Na+ and HCO3 - absorption. To increase insights into the functions of NHE2 vs. NHE3, we compared their cellular processing with each other and with the housekeepin g isoform NHE1. Using biotinylated exchanger, we determined that the half-l ife of plasma membrane NHE2 was short (3 h) compared with that of NHE1 (24 h) and NHE3 (14 h) in both PS120 fibroblasts and Caco-2 cells. NHE2 transpo rt and plasma membrane levels were reduced by 3 h of Brefeldin A treatment, whereas NHE1 was unaffected. NHE2 was degraded by the lysosomes but not pr oteosomes, as demonstrated by increasing levels of endocytosed NHE2 protein after inhibition of the lysosomes, but not with proteosome inhibition. Unl ike that of NHE3, basal NHE2 transport activity was not affected by phospha tidylinositol 3-kinase inhibition and did not appear to be localized in the juxtanuclear recycling endosome. Therefore, for NHE2, protein degradation and/or protein synthesis probably play important roles in its basal and reg ulated states. These results suggest fundamental differences in the cellula r processing and trafficking of NHE2 and NHE3. These differences may underl ie the specialized roles that these exchangers play in epithelial cells.