The Na+/glucose cotransporters: from genes to therapy

Glucose enters eukaryotic cells via two types of membrane-associated carrier proteins, the Na+/glucose cotransporters (SGLT) and the facilitative glucose transporters (GLUT). The SGLT family consists of six members. Among them, the SGLT1 and SGLT2 proteins, encoded by the solute carrier genes SLC5A1 and SLC5A2, respectively, are believed to be the most important ones and have been extensively explored in studies focusing on glucose fluxes under both physiological and pathological conditions. This review considers the regulation of the expression of the SGLT promoted by protein kinases and transcription factors, as well as the alterations determined by diets of different compositions and by pathologies such as diabetes. It also considers congenital defects of sugar metabolism caused by aberrant expression of the SGLT1 in glucose-galactose malabsorption and the SGLT2 in familial renal glycosuria. Finally, it covers some pharmacological compounds that are being currently studied focusing on the interest of controlling glycemia by antagonizing SGLT in renal and intestinal tissues.

Na+-glucose transporter-2 messenger ribonucleic acid expression in kidney of diabetic rats correlates with glycemic levels: Involvement of hepatocyte nuclear factor-1 alpha expression and activity

Mutations in Na+-glucose transporters (SGLT)-2 and hepatocyte nuclear factor (HNF)-1 alpha genes have been related to renal glycosuria and maturity-onset diabetes of the young 3, respectively. However, the expression of these genes have not been investigated in type 1 and type 2 diabetes. Here in kidney of diabetic rats, we tested the hypotheses that SGLT2 mRNA expression is altered; HNF-1 alpha is involved in this regulation; and glycemic homeostasis is a related mechanism. The in vivo binding of HNF-1 alpha into the SGLT2 promoter region in renal cortex was confirmed by chromatin immunoprecipitation assay. SGLT2 and HNF-1 alpha mRNA expression (by Northern and RT-PCR analysis) and HNF-1 binding activity of nuclear proteins (by EMSA) were investigated in diabetic rats and treated or not with insulin or phlorizin (an inhibitor of SGLT2). Results showed that diabetes increases SGLT2 and HNF-1 alpha mRNA expression (similar to 50%) and binding of nuclear proteins to a HNF-1 consensus motif (similar to 100%). Six days of insulin or phlorizin treatment restores these parameters to nondiabetic-rat levels. Moreover, both treatments similarly reduced glycemia, despite the differences in plasma insulin and urinary glucose concentrations, highlighting the plasma glucose levels as involved in the observed modulations. This study shows that SGLT2 mRNA expression and HNF-1 alpha expression and activity correlate positively in kidney of diabetic rats. It also shows that diabetes-induced changes are reversed by lowering glycemia, independently of insulinemia. Our demonstration that HNF-1 alpha binds DNA that encodes SGLT2 supports the hypothesis that HNF-1 alpha, as a modulator of SGLT2 expression, may be involved in diabetic kidney disease.

Glucose-Induced Regulation of NHEs Activity and SGLTs Expression Involves the PKA Signaling Pathway

The effect of glucose on the intracellular pH (pH(i)) recovery rate (dpH(i)/dt) and Na(+)-glucose transporter (SGLT) localization was investigated in HEK-293 cells, a cell line that expresses endogenous NHE1, NHE3, SGLT1, and SGLT2 proteins. The activity of the Na(+)/H(+) exchangers (NHEs) was evaluated by using fluorescence microscopy. The total and membrane protein expression levels were analyzed by immunoblotting. In cells cultivated in 5 mM glucose, the pH(i) recovery rate was 0.169 +/- A 0.020 (n = 6). This value did not change in response to the acute presence of glucose at 2 or 10 mM, but decreased with 25 mM glucose, an effect that was not observed with 25 mM mannitol. Conversely, the chronic effect of high glucose (25 mM) increased the pH(i) recovery rate (similar to 40%, P < 0.05), without changes in the total levels of NHE1, NHE3, or SGLT1 expression, but increasing the total cellular (similar to 50%, P < 0.05) and the plasma membrane (similar to 100%, P < 0.01) content of SGLT2. Treatment with H-89 (10(-6) M) prevented the stimulatory effect of chronic glucose treatment on the pH(i) recovery rate and SGLT2 expression in the plasma membrane. Our results indicate that the effect of chronic treatment with a high glucose concentration is associated with increased NHEs activity and plasma membrane expression of SGLT2 in a protein kinase A-dependent way. The present results reveal mechanisms of glucotoxicity and may contribute to understanding the diabetes-induced damage of this renal epithelial cell.