Molecular physiology of glucose transporters.

Molecular cloning of cDNA encoding the human erythrocyte facilitated-diffusion glucose transporter (GT) has elucidated its structure and has permitted a careful study of its tissue distribution and of its involvement in processes such as insulin-stimulated glucose uptake by adipose cells or transformation-induced increase in glucose metabolism. An important outcome of these studies was the discovery that additional isoforms of this transporter were expressed in a tissue-specific manner; these comprise a family of structurally and functionally related molecules. Their tissue distribution, differences in kinetic properties, and differential regulation by ambient glucose and insulin levels suggest that they play specific roles in the control of glucose homeostasis. Herein, we will discuss the structure of three members of the GT family: erythroid/brain GT, liver GT, and adipose cell/muscle GT. In the light of their tissue-specific expression, kinetic parameters, and susceptibility to insulin action, we discuss their possible specific functions.