Genetic polymorphisms at the human liver/islet glucose transporter (GLUT2) gene locus in Caucasian and West Indian subjects with type 2 (non-insulin-dependent) diabetes mellitus.

The liver/islet glucose transporter (GLUT2) is mainly expressed in the hepatocytes of the liver and the beta-cells of the pancreatic islets and a defect in this transporter could lead to diabetic phenotypes, such as relative hypoinsulinaemia and reduced uptake and metabolism of glucose in the liver. DNA from unrelated individuals was digested with the restriction endonucleases Bgl-I and Taq-I followed by blotting and hybridisation with a 32P-labelled GLUT2 cDNA which revealed three restriction fragment length polymorphisms (RFLPs) (B1, T1 and T2) in both Caucasian and West Indian populations. Linkage analysis between these variant sites demonstrated that the alleles of these polymorphisms were in strong linkage disequilibrium. Disease association of genetic variants at the GLUT2 locus with type 2 diabetes was examined with these RFLPs in both Caucasian (n = 54) and West Indian (n = 46) populations with type 2 diabetes. There were no significant differences in the frequency of alleles, genotypes or haplotypes between diabetic patients and non-diabetic controls. However, there were significant differences in the allele frequencies of all these three polymorphisms between Caucasian and West Indian populations.     

Polymorphisms at the GLUT1 (HepG2) and GLUT4 (muscle/adipocyte) glucose transporter genes and non-insulin-dependent diabetes mellitus (NIDDM)

Summary In order to determine the possible contribution of the GLUT1 (HepG2) glucose transporter gene to the inheritance of non-insulin-dependent diabetes mellitus (NIDDM), two restriction fragment length polymorphisms (RFLPs) and the related haplotypes at this locus were studied in 48 Italian diabetic patients and 58 normal subjects. Genotype frequencies for the XbaI polymorphism were significantly different between patients and controls (XbaI: ² = 9.80, df= 2, P < 0.0079). A significant difference was also found in the allele frequencies between NIDDM patients and controls (² =9.39, df = 1, P < 0.0022), whereas no differences were found for the StuI RFLP. No linkage disequilibrium was detected between the XbaI and StuI RFLPs in this sample. The analysis of the four haplotype frequencies (X1S1, X1S2, X2S1, X2S2) revealed a significant difference between diabetic patients and controls (² = 14.26, df =3, P < 0.002). By comparing single haplotype frequencies, a significant difference between the two groups was found for the X1S1 and X2S2 haplotypes. A two-allele RFLP at the GLUT4 (muscle/adipocyte) glucose transporter gene, detected with the restriction enzyme KpnI, was also examined; no differences were found between patients and controls for this RFLP. The finding of an association between polymorphic markers at the GLUT1 transporter and NIDDM suggests that this locus may contribute to the inherited susceptibility to the disease in this Italian population.     

Characterization and partial purification of liver glucose transporter GLUT2

GLUT2, the major facilitative glucose transporter isoform expressed in hepatocytes, pancreatic beta-cells, and absorptive epithelial cells, is unique not only with its low affinity and broad substrate specificity as a glucose transporter, but also with its implied function as a glucose-sensor. As a first essential step toward structural and biochemical elucidation of these unique, GLUT2 functions, we describe here the differential solubilization and DEAE-column chromatography of rat hepatocyte GLUT2 protein and its reconstitution into liposomes. The reconstituted GLUT2 bound cytochalasin B in a saturable manner with an apparent dissociation constant (K(d)) of 2.3 x 10(-6) M and a total binding capacity (B(T)) of 8.1 nmol per mg protein. The binding was completely abolished by 2% mercury chloride, but not affected by cytochalasin E. Significantly, the binding was also not affected by 500 mM D-glucose or 3-O-methyl D-glucose (3OMG). The purified GLUT2 catalyzed mercury chloride-sensitive 3OMG uptake, and cytochalasin B inhibited this 3OMG uptake. The inhibition was dose-dependent with respect to cytochalasin B, but was independent of 3OMG concentrations. These findings demonstrate that our solubilized GLUT2 reconstituted in liposomes is at least 60% pure and functional, and that GLUT2 is indeed unique in that its cytochalasin B binding is not affected by its substrate (D-glucose) binding. Our partially purified GLUT2 reconstituted in vesicles will be useful in biochemical and structural elucidation of GLUT2 as a glucose transporter and as a possible glucose sensor.     

Localization of erythrocyte/HepG2-type glucose transporter (GLUT1) in human placental villi

Summary The syncytiotrophoblast covering the surface of the placental villi contains the machinery for the transfer of specific substances between maternal and fetal blood, and also serves as a barrier. Existence of a facilitated-diffusion transporter for glucose in the syncytiotrophoblast has been suggested. Using antibodies to erythrocyte/HepG2-type glucose transporter (GLUT1), one isoform of the facilitated-diffusion glucose transporters, we detected a 50 kD protein in human placenta at term. By use of immunohistochemistry, GLUT1 was found to be abundant in both the syncytiotrophoblast and cytotrophoblast. Endothelial cells of the fetal capillaries also showed positive staining for GLUT1. Electron-microscopic examination revealed that GLUT1 was concentrated at both the microvillous apical plasma membrane and the infolded basal plasma membrane of the syncytiotrophoblast. Plasma membrane of the cytotrophoblast was also positive for GLUT1. GLUT1 at the apical plasma membrane of the syncytiotrophoblast may function for the entry of glucose into its cytoplasm, while GLUT1 at the basal plasma membrane may be essential for the exit of glucose from the cytoplasm into the stroma of the placental villi. Thus, GLUT1 at the plasma membranes of syncytiotrophoblast and endothelial cells may play an important role in the transport of glucose across the placental barrier.     

Mapping the human liver/islet glucose transporter (GLUT2) gene within a genetic linkage map of chromosome 3q using a (CA)n dinucleotide repeat polymorphism and characterization of the polymorphism in three racial groups.

The human liver/islet glucose transporter (GLUT2), a candidate gene for diabetes, has been incorporated into a genetic linkage map for chromosome 3q using a (CA)n dinucleotide repeat polymorphism adjacent to the 3'-end of exon 4a. We have found a total of nine alleles ranging in length from 153 to 169 nucleotides in three racial groups and have determined the precise structure of the variable region for four of the alleles by DNA sequencing. Five alleles were found to be common to the American Black, Caucasian, and Pima Indian racial groups studied. One allele (169 bp) was unique to American Blacks, and another rare allele (153 bp) was found only in the Caucasian population studied. Observed heterozygosity of the polymorphism in the Caucasian (CEPH) reference pedigree collection is 60%, for American Blacks 71%, and for Pima Indians 53%. An independent study recently identified the same dinucleotide repeat and found six alleles in a Caucasian population (Froguel et al., 1991), a result that we confirm; however, our sequencing data indicate a different molecular structure for the polymorphism for some of the alleles. We have constructed a new genetic linkage map of chromosome 3q uniquely placing the GLUT2 gene between flanking markers D3S26 and D3S43. The genetic map consists of 23 loci (25 RFLPs and 2 (CA)n dinucleotide repeat markers) with 14 markers uniquely localized with odds of at least 1000:1. Three genes (FTHL4, TF, GLUT2) are integrated into the map, which spans a sex-average distance of 147.3 cM, 103.8 cM in males and 227.0 cM in females.(ABSTRACT TRUNCATED AT 250 WORDS)