Abstract
Na+-dependent glucose reabsorption in the renal proximal tubule is dynamically regulated by changes in blood glucose levels. There is, however, a disparity in reports studying the relationship between hyperglycemia and Na+-glucose-linked transporter (SGLT) function and expression. Similarly, manipulation of the glucose content in growth media of cultured renal cells has been shown to influence SGLT activity. In this investigation, SGLT activity was significantly lower in proximal tubule LLC-PK1 cells cultured in medium containing 17.5 than 5 mM glucose. α-Methyl d-glucopyranoside (AMG) transport kinetics showed reduced apparent Vmax and Km in cells grown in 17.5 mM glucose. SGLT2 was identified as the isoform responsible for glucose transport, and protein expression analyses showed decreased apical membrane localization of SGLT2 in cells grown in 17.5 mM glucose, explaining the reduced activity. Multiple signaling pathways have been implicated in regulation of SGLT activity and trafficking. Elevated media glucose decreased intracellular cAMP and PKA activation, leading to decreased SGLT2 trafficking into the plasma membrane, which was reversed after treatment with 1 µM forskolin. The effects of media glucose on SGLT activity were found to be dependent on p38 MAPK activation due to PKA-mediated signaling. Glucose-modulated AMG uptake is reversible and was associated with altered SGLT2 membrane trafficking and cAMP alterations. In summary, elevated glucose concentrations in culture medium decrease SGLT activity in LLC-PK1 cells by reducing membrane trafficking of SGLT2 via decreasing intracellular cAMP, resulting in a lowered PKA-dependent phosphorylation of p38 MAPK.