The Differential Role of SGK3 from SGK1 and SGK2 in Activation of NHE3 by Glucocorticoid

Abstract

NHE3 is the major Na+/H+ transporter located in the apical membrane of epithelial cells in the intestine and renal proximal tubule. We have previously shown that SGK1 is involved in activation of NHE3 by dexamethasone (DEX). In this study, we examined whether SGK2 and SGK3 are also involved in DEX stimulation of NHE3 as both SGK2 and SGK3 are expressed in the small intestine. We used PS120/NHE3V and Caco-2BBe/hNHE3V cells as the models. Overexpression of either SGK2 or SGK3 in PS120/NHE3V cells caused an increase in NHE3 activity by potentiating exocytosis of NHE3 to the plasma membrane. Importantly, in contrast to SGK1 and SGK2, SGK3-activated NHE3 occurred as early as 15 min in both PS120 and Caco2-BBe cells after DEX treatment. Similarly, kinase assay showed that SGK3 but not SGK1 or SGK2 activity was clearly increased at 15 min after DEX. Immunofluorescence staining showed that SGK3 is located in endosomes, whereas SGK1 and SGK2 are ubiquitously distributed throughout the cell. Interestingly, a mutation of SGK3 (R90A) caused disruption of its endosomal localization and delays of NHE3 activation. In addition, we showed that activation of SGK3 and SGK3-mediated NHE3 activation by DEX are dependent on glucocorticoid receptor (GR) and PI3K. In conclusion, the endosomal location of SGK3 makes it distinct from SGK1 and SGK2 by mediating an early response of NHE3 to DEX in a GR-PI3K dependent manner.