Functional characterization of the human facilitative glucose transporter 12 (GLUT12) by electrophysiological methods

Abstract

GLUT12 is a member of the facilitative family of glucose transporters. The goal of this study was to characterize the functional properties of GLUT12, expressed in Xenopus laevis oocytes, using radiotracer and electrophysiological methods. Our results showed that GLUT12 is a facilitative sugar transporter with substrate selectivity: d-glucose ≥ α-methyl-d-glucopyranoside (α-MG) > 2-deoxy-d-glucose(2-DOG) > d-fructose = d-galactose. α-MG is a characteristic substrate of the Na+/glucose (SGLT) family and has not been shown to be a substrate of any of the GLUTs. In the absence of sugar, 22Na+ was transported through GLUT12 at a higher rate (40%) than noninjected oocytes, indicating that there is a Na+ leak through GLUT12. Genistein, an inhibitor of GLUT1, also inhibited sugar uptake by GLUT12. Glucose uptake was increased by the PKA activator 8-bromoadenosine 3′,5′-cyclic monophosphate (8-Br-cAMP) but not by the PKC activator phorbol-12-myristate-13-acetate (PMA). In high K+ concentrations, glucose uptake was blocked. Addition of glucose to the external solution induced an inward current with a reversal potential of approximately −15 mV and was blocked by Cl− channel blockers, indicating the current was carried by Cl− ions. The sugar-activated Cl− currents were unaffected by genistein. In high external K+ concentrations, sugar-activated Cl− currents were also blocked, indicating that GLUT12 activity is voltage dependent. Furthermore, glucose-induced current was increased by the PKA activator 8-Br-cAMP but not by the PKC activator PMA. These new features of GLUT12 are very different from those described for other GLUTs, indicating that GLUT12 must have a specific physiological role within glucose homeostasis, still to be discovered.