Regulation of glucose transporter SGLT1 by ubiquitin ligase Nedd4-2 and kinases SGK1, SGK3, and PKB

Objective: Serum- and glucocorticoid-inducible kinase 1 (SGK1) inhibits the ubiquitin ligase neuronal cell expressed developmentally downregulated 4-2 (Nedd4-2), which retards the retrieval of the epithelial Na+ channel ENaC. Accordingly, SGK1 enhances ENaC abundance in the cell membrane. The significance of this effect is shown by an association of an E8CC/CT;16CC polymorphism in the SGK1 gene with increased blood pressure. However, strong expression of SGK1 in enterocytes not expressing ENaC points to further functions of SGK1. This study was performed to test for regulation of Na+-coupled glucose transporter 1 (SGLT1) by Nedd4-2, SGK1, and/or the related kinases SGK3 and PKB. Additional studies searched for an association of the SGK1 gene with BMI. Research Methods and Procedures: mRNA encoding SGLT1, wild-type Nedd4-2, inactive (C938S)Nedd4-2, wild type SGK1, constitutively active (S422D)SGK1 or inactive (K127N)SGK1, wild-type SGK3, and constitutively active (PKB)-P-T308DS473D or inactive (PKB)-P-T308AS473A were injected into Xenopus oocytes, and glucose transport was quantified from glucose-induced current (I-glc). BMI was determined in individuals with or without the E8CC/CT;16CC polymorphism. Results: I-glc was significantly decreased by coexpression of Nedd4-2 but not of (C938S)Nedd4-2. Coexpression of SGK1, (S422D)SGK1, SGK3, or (PKB)-P-T308DS473D, but not of (K127N)SGK1 or (PKB)-P-T308AS473A, enhanced 191, and reversed the effect of Nedd4-2. SGK1 and SGK3 phosphorylated Nedd4-2. Deletion of the SGK/PKB phosphorylation sites in Nedd4-2 blunted the kinase effects. BMI was significantly (p < 0.008) greater in individuals with the E8CC/CT;16CC polymorphism than in individuals without. Discussion: Overactivity of SGK1 may lead not only to excessive ENaC activity and hypertension but also to enhanced SGLT1 activity and obesity.