Mechanism of SGLT1-Dependent Translocation of GLUT2 to the Apical Membrane of Enterocytes

Abstract

Introduction: Traditional models of intestinal glucose absorption confine glucose transporter 2 (GLUT2) to the basolateral membrane. However, considerable evidence suggests that GLUT2 is translocated rapidly to the apical membrane when the enterocyte is exposed to high luminal glucose concentrations. We hypothesized that GLUT2 translocates to the apical membrane of enterocytes by a protein kinase C (PKC) signaling mechanism dependent on activity of the hexose transporter, sodium-glucose co-transporter 1 (SGLT1) and the cellular cytostructure. Methods: Transporter-mediated glucose uptake was studied in Lewis rat jejunum using everted sleeves under 7 conditions: Control, SGLT1 inhibition (phlorizin 0.2 mM), GLUT2 inhibition (phloretin 1 mM), both SGLT1 and GLUT2 inhibition, PKC inhibition (calphostin 500 nM, chelerythrine 50 uM), and disruption of cellular cytostructure (nocodazole 10 μM). Each condition was tested in iso-osmotic solutions of 1, 20, or 50 mM glucose for 1 or 5 min incubations (n=6 rats each). Results: Phlorizin inhibited glucose uptake nearly completely in 1, 20, and 50 mM glucose at both 1 and 5 minutes (p≤0.006 each). Phloretin partially inhibited glucose uptake (p≤0.01 each) in all concentrations and time. Phloretin and phlorizin together almost totally inhibited uptake (p=0.004 each). Calphostin and chelerythrine did not inhibit transporter-mediated glucose uptake except in 1 mM glucose at 5 min (p≤0.05 each). Nocodazole inhibited uptake only in 1 mM glucose at 1 and 5 min (p≤0.01 each) (see figure). Conclusion: Inhibition of SGLT1 led to near complete cessation of transporter-mediated glucose uptake. GLUT2 inhibition led to partial inhibition of glucose uptake suggesting constitutive expression of GLUT2 in the apical membrane. Disruption of PKC signaling or cell cytoskeletal integrity partially inhibited carrier-mediated glucose uptake only in 1 mM glucose, suggesting a non-specific effect rather than inhibition of GLUT2 translocation. In this model, it does not appear that GLUT2 is translocated to the apical membrane on the cellular cytostructure in response to PKC signaling from high luminal concentrations of glucose.