The facilitative glucose transporter, GLUT4, mediates insulin-stimulated glucose uptake in adipocytes and muscles, and the participation of GLUT4 in the pathogenesis of various clinical conditions associated with obesity, visceral fat accumulation and insulin resistance has been proposed. Glucose uptake by some members of the GLUT family, mainly GLUT1, is inhibited by flavonoids, the natural polyphenols present in fruits, vegetables and wine. Therefore it is of interest to establish if these polyphenolic compounds present in the diet, known to be effective antioxidants but also endowed with several other biological activities such as protein-tyrosine kinase inhibition, interfere with GLUT4 function. In the present study, we show that three flavonoids, quercetin, myricetin and catechin-gallate, inhibit the uptake of methylglucose by adipocytes over the concentration range of 10-100 microM. These three flavonoids show a competitive pattern of inhibition, with K(i)=16, 33.5 and 90 microM respectively. In contrast, neither catechin nor gallic acid inhibit methylglucose uptake. To obtain a better understanding of the interaction among GLUT4 and flavonoids, we have derived a GLUT4 three-dimensional molecular comparative model, using structural co-ordinates from a GLUT3 comparative model and a mechanosensitive ion channel [PDB (Protein Data Bank) code 1MSL] solved by X-ray diffraction. On the whole, the experimental evidence and computer simulation data favour a transport inhibition mechanism in which flavonoids and GLUT4 interact directly, rather than by a mechanism related to protein-tyrosine kinase and insulin signalling inhibition. Furthermore, the results suggest that GLUT transporters are involved in flavonoid incorporation into cells.

Despite the physiologic importance of vitamin E, in particular its alpha-tocopherol (alpha-T) isoform, the molecular mechanisms involved in the cellular uptake of this antioxidant from plasma lipoproteins have not been well-defined. Recent studies have suggested that selective lipid uptake, rather than endocytosis, is important for alpha-T delivery to cells. Here we show that the scavenger receptor class B type I (SR-BI), which mediates cellular selective cholesteryl ester uptake from lipoproteins, facilitates efficient transfer of alpha-T from HDL to cultured cells. In SR-BI-deficient mutant mice, relative to wild-type control animals, there was a significant increase in plasma alpha-T levels (1.1- to 1.4-fold higher) that was mostly due to the elevated alpha-T content of their abnormally large plasma HDL-like particles. This increase in plasma alpha-T in SR-BI knockout mice was accompanied by a significant decrease (65-80%) in the alpha-T concentrations in bile and several tissues including ovary, testis, lung and brain. SR-BI deficiency did not alter the alpha-T concentrations of the liver, spleen, kidney or white fat. These data show that SR-BI plays an important role in transferring alpha-T from plasma lipoproteins to specific tissues. Also, in the case of the liver as was previously shown for SR-BI-dependent hepatic cholesterol transport, SR-BI-mediated uptake of alpha-T was primarily coupled to biliary excretion rather than to tissue accumulation. Defective tissue uptake of lipoprotein alpha-T in SR-BI-deficient mice may contribute to the reproductive and cardiovascular pathologies exhibited by these animals.

The facilitative hexose transporter GLUT1 is a multifunctional protein that transports hexoses and dehydroascorbic acid, the oxidized form of vitamin C, and interacts with several molecules structurally unrelated to the transported substrates. Here we analyzed in detail the interaction of GLUT1 with a group of tyrosine kinase inhibitors that include natural products of the family of flavones and isoflavones and synthetic compounds such as the tyrphostins. These compounds inhibited, in a dose-dependent manner, the transport of hexoses and dehydroascorbic acid in human myeloid HL-60 cells, in transfected Chinese hamster ovary cells overexpressing GLUT1, and in normal human erythrocytes, and blocked the glucose-displaceable binding of cytochalasin B to GLUT1 in erythrocyte ghosts. Kinetic analysis of transport data indicated that only tyrosine kinase inhibitors with specificity for ATP binding sites inhibited the transport activity of GLUT1 in a competitive manner. In contrast, those inhibitors that are competitive with tyrosine but not with ATP failed to inhibit hexose uptake or did so in a noncompetitive manner. These results, together with recent evidence demonstrating that GLUT1 is a nucleotide binding protein, support the concept that the inhibitory effect on transport is related to the direct interaction of the inhibitors with GLUT1. We conclude that predicted nucleotide-binding motifs present in GLUT1 are important for the interaction of the tyrosine kinase inhibitors with the transporter and may participate directly in the binding transport of substrates by GLUT1.