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Molecular Nutrition
GLUT in the plasma membrane are one of the most important cellular nutrient transporters, as glucose plays a central role in cellular metabolism. Besides acting as shuttles in different tissues, a growing body of evidence suggests that GLUT participate in various physiological and metabolic functions in human beings and animals. For instance, GLUT Na+/glucose co-transporter 1 (SGLT1)-mediated glucose uptake increases anti-apoptotic proteins of the small intestine and protects enterocytes from apoptosis and barrier defects( 1 ). Obesity contributes to type 2 diabetes with the down-regulated expressions of GLUT1 and GLUT4 in skeletal muscle( 2 , 3 ). Therefore, the knowledge of understanding the regulation of GLUT contributes to our understanding of whole-body glucose homoeostasis and human metabolic diseases.
In human beings and animals, highly specialised GLUT are expressed in various tissues. In muscle and adipose tissues, GLUT1 and GLUT4 are responsible for the majority of whole-body glucose metabolism( 4 ). GLUT1 is largely responsible for basal glucose transport, as >40 % of this transporter isoform is present on the cell surface in the absence of insulin( 5 ). However, the majority of GLUT4 is localised in intracellular tubulovesicular structures clustered in the cytoplasm and is translocated to the plasma membrane for its functions( 6 , 7 ). Different from the system in muscle, SGLT1 and GLUT2 mediate glucose absorption and utilisation in the small intestine. Glucose and, to a lesser extent, galactose are transported from the lumen of the intestine into enterocytes by SGLT1( 8 , 9 ), whereas fructose is transported by a facilitated-diffusion GLUT5( 10 - 12 ). After being transported in the cells, glucose and fructose are passively transported from enterocytes into the systemic system by another facilitated-diffusion GLUT such as GLUT2( 8 , 13 ). Some studies have shown that different concentrations of dietary carbohydrate and artificial sweeteners up-regulate the intestinal GLUT expression in piglets( 14 , 15 ), which indicates that other dietary nutrients can also regulate intestinal GLUT expression.
In addition to providing fuel and essential nutrients, circulating substrates derived from food, including amino acids, have specific direct and indirect functions to activate receptors and signalling pathways, and then regulate whole-body nutritional homoeostasis( 16 )....