Glucose transporters in the transepithelial transport of glucose

Glucose transporters are integral membrane proteins that mediate the transport of glucose and structurally-related substances across the cellular membranes. Two families of glucose transporter have been identified: the facilitated-diffusion glucose transporter family (GLUT family), and the NA(+)-dependent glucose transporter one (SGLT family). These transporters play a pivotal role in the transfer of glucose across the epithelial cell layers that separate distinct compartments in the mammalian body. In the small intestine, a Na(+)-dependent glucose transporter, SGLT1, is localized at the apical plasma membrane of the absorptive epithelial cells, whereas a facilitated-diffusion glucose transporter, GLUT2, is at the basolateral membrane of the cells. Similar localization is seen in the kidney proximal tubules in the reabsorption of glucose. For the absorption of fructose in the small intestine, fructose transporter GLUT5 is localized at the apical membrane. The expressed GLUT5 in polarized cultured cells is targeted to the apical membrane, showing that the GLUT5 molecule itself has sufficient information to determine its cellular localization. In the blood-tissue barriers, such as the blood-brain barrier, blood-ocular barrier, and placental barrier, either endothelial or epithelial cell layers constitute the barrier. GLUT1 is abundant at the plasma membrane of these barrier cells, and plays a crucial role in the specific transfer of glucose across the barrier. When the barrier is composed of a two-cell layer, gap junctions connecting them could serve as intercellular channels for glucose transfer in addition to GLUT1. Proper localization of glucose transporters and gap junctions is a prerequisite for the successful transepithelial transport of sugars.