The protein family of glucose transport facilitators: It's not only about glucose after all.

The protein family of facilitative glucose transporters comprises 14 isoforms that share common structural features such as 12 transmembrane domains, N- and C-termini facing the cytoplasm of the cell, and a N-glycosylation side either within the first or fifth extracellular loop. Based on their sequence homology, three classes can be distinguished: class I includes GLUT1-4 and GLUT14, class II the "odd transporters" GLUT5, 7, 9, 11, and class III the "even transporters" GLUT6, 8, 10, 12 and the proton driven myoinositol transporter HMIT (or GLUT13). With the cloning and characterization of the more recent class II and III isoforms, it became apparent that despite their structural similarities, the different isoforms not only show a distinct tissue-specific expression pattern but also show distinct characteristics such as alternative splicing, specific (sub)cellular localization, and affinities for a spectrum of substrates. This review summarizes the current understanding of the physiological role for the various transport facilitators based on human genetically inherited disorders or single-nucleotide polymorphisms and knockout mice models. The emphasis of the review will be on the potential functional role of the more recent isoforms.

Dexamethasone sensitizes the neonatal intestine to fructose induction of intestinal fructose transporter (Slc2A5) function.

The recent dramatic increase in fructose consumption is tightly correlated with an equally dramatic surge in the incidence of type 2 diabetes and obesity in children, but little is known about dietary fructose metabolism and absorption in neonates. The expression of the rat intestinal fructose transporter GLUT5 [Slc2A5, a member of the glucose transporter family (GLUT)] can be specifically induced by its substrate fructose, but only after weaning begins at 14 d of age. In suckling rats younger than 14 d old, dietary fructose cannot enhance GLUT5 expression. The aim of this study was to identify the mechanisms allowing fructose to stimulate GLUT5 during weaning. After intestines were perfused with fructose or glucose (control), using microarray hybridization we showed that of 5K genes analyzed in 10-d-old pups, only 13 were fructose responsive. Previous work found approximately 50 fructose-responsive genes in 20-d-old pups. To identify fructose-responsive genes whose expression also changed with age, intestines of 10- and 20-d-old littermate pups perfused with fructose were compared by microarray. Intestines of 10- and 20-d-old pups perfused with glucose were used to segregate age- but not fructose-responsive genes. About 28 genes were up- and 22 down-regulated in 20- relative to 10-d-old pups, under conditions of fructose perfusion, and many were found, by cluster analysis, to be regulated by corticosterone. When dexamethasone was injected into suckling pups before fructose perfusion, the expression of GLUT5 but not that of the sodium glucose cotransporter (SGLT) 1 and of GLUT2, as well as the uptake of fructose but not of glucose increased dramatically. Thus, dexamethasone, which allows dietary fructose to precociously stimulate intestinal fructose absorption, can mimic the effect of age and modify developmental timing mechanisms regulating GLUT5.

A highly conserved hydrophobic motif in the exofacial vestibule of fructose transporting SLC2A proteins acts as a critical determinant of their substrate selectivity.

The substrate specificity of the facilitated hexose transporter, GLUT, family, (gene SLC2A) is highly varied. Some appear to be able to translocate both glucose and fructose, while the ability to handle 2-deoxyglucose and galactose does not necessarily correlate with the other two hexoses. It has become generally accepted that a central substrate binding/translocation site determines which hexoses can be transported. However, a recent study showed that a single point mutation of a hydrophobic residue in GLUTs 2, 5 & 7 removed their ability to transport fructose without affecting the kinetics of glucose permeation. This residue is in the 7th transmembrane helix, facing the aqueous pore and lies close to the opening of the exofacial vestibule. This study expands these observations to include the other class II GLUTs (9 & 11) and shows that a three amino acid motif (NXI/NXV) appears to be critical in determining if fructose can access the translocation mechanism. GLUT11 can also transport fructose, but it has the motif DSV at the same position, which appears to function in the same manner as NXI and when all three residues are replaced with NAV fructose transport lost. These results are discussed in relation to possible roles for hydrophobic residues lining the aqueous pore at the opening of the exofacial vestibule. Finally, the possibility that the translocation binding site may not be the sole determinant of substrate specificity for these proteins is examined.

Prestin-prestin and prestin-GLUT5 interactions in HEK293T cells.

The remarkable hearing sensitivity and frequency selectivity in mammals is attributed to cochlear amplifier in the outer hair cells (OHCs). Prestin, a membrane protein in the lateral wall of OHC plasma membrane, is required for OHC electromotility and cochlear amplifier. In addition, GLUT5, a fructose transporter, is reported to be abundant in the plasma membrane of the OHC lateral wall and has been originally proposed as the OHC motor protein. Here we provide evidence of interactions between prestin/prestin and prestin/GLUT5 in transiently transfected HEK293T cells. We used a combination of techniques: (1) membrane colocalization by confocal microscopy, (2) fluorescence resonance energy transfer (FRET) by fluorescence activated cell sorting (FACS), (3) FRET by acceptor photobleaching, (4) FRET by fluorescence lifetime imaging (FRET-FLIM), and (5) coimmunoprecipitation. Our results suggest that homomeric and heteromeric prestin interactions occur in native OHCs to facilitate its electromotile function and that GLUT5 interacts with prestin for its elusive function.

Inhibition of the intestinal glucose transporter GLUT2 by flavonoids.

We tested whether the dominant intestinal sugar transporter GLUT2 was inhibited by intestinal luminal compounds that are inefficiently absorbed and naturally present in foods. Because of their abundance in fruits and vegetables, flavonoids were selected as model compounds. Robust inhibition of glucose and fructose transport by GLUT2 expressed in Xenopus laevis oocytes was produced by the flavonols myricetin, fisetin, the widely consumed flavonoid quercetin, and its glucoside precursor isoquercitrin [corrected]. IC50s for quercetin, myricetin, and isoquercitirin [corrected]were approximately 200- to 1000-fold less than glucose or fructose concentrations, and noncompetitive inhibition was observed. The two other major intestinal sugar transporters, GLUT5 and SGLT1, were unaffected by flavonoids. Sugar transport by GLUT2 overexpressed in pituitary cells and naturally present in Caco-2E intestinal cells was similarly inhibited by quercetin. GLUT2 was detected on the apical side of Caco-2E cells, indicating that GLUT2 was in the correct orientation to be inhibited by luminal compounds. Quercetin itself was not transported by the three major intestinal glucose transporters. Because the flavonoid quercetin, a food component with an excellent pharmacology safety profile, might act as a potent luminal inhibitor of sugar absorption independent of its own transport, flavonols show promise as new pharmacologic agents in the obesity epidemic.

Glucagon-like peptide-2 induces a specific pattern of adaptation in remnant jejunum.

Glucagon-like peptide-2 (GLP-2) is an enteroendocrine hormone which is uniquely trophic for the intestine; a physiological role in regulating nutrient absorptive capacity is becoming apparent. GLP-2, independent of enteral feeding, stimulates a classical pattern of intestinal adaptation in terminal ileum following resection. Herein we investigate the effects of GLP-2 on the jejunal remant using a rat model of short bowel syndrome (SBS). Juvenile 250- to 275-g SD rats underwent 80% distal small bowel resection, leaving 20 cm of proximal jejunum and venous catheterization. Animals were maintained with total parenteral nutrition (TPN) or TPN+10 microg/kg/hr GLP-2 (n=8 per group). After 7 days, intestinal permeability was assessed by urinary recovery of gavaged carbohydrate probes. Animals were euthanized, and the intestines taken for analysis of morphology, crypt cell proliferation, apoptosis, and expression of SGLT-1 and GLUT-5 transport proteins. GLP-2 treatment reduced intestinal permeability and increased in vivo glucose absorption, small intestinal weight, surface area, villus height, crypt depth, and microvillus height. Intestinal mucosal DNA and protein content per unit length of the small bowel were increased (P < 0.05 for all comparisons). However, in contrast to previous studies examining GLP-2's effects on remnant ileum, the jejunal crypt apoptotic index was increased in GLP-2-treated animals, with no increase in SGLT-1 or GLUT 5 expression. These results show that exogenous GLP-2 treatment of animals with jejunal remnant reduces intestinal permeability, increases glucose absorption, and stimulates morphological features of intestinal adaptation including increased micovillus height and surface area. However, the pattern of changes seen is different from that in remnant ileum. This suggests that GLP-2's effects are specific to different regions of the bowel. Nonetheless, remnant jejunum is responsive to GLP-2 in the absence of enteral nutrition. Further studies are warranted to establish the mechanisms of action and therapeutic potential of GLP-2 in modulating nutrient absorptive capacity.