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Biomedical subjects

G I Bell

Publications and source records attributed to G I Bell.

At least 253 records · Page 14Linked to original sources
PubMed1990

Facilitative glucose transporters: an expanding family.

The uptake of glucose into most eukaryotic cells is accomplished by a carrier-mediated transport system, facilitative diffusion, which transports glucose down its chemical gradient in a stereospecific manner. Recent studies have shown that facilitative transport of glucose across the plasma membrane is mediated by a family of structurally related proteins. This review summarizes the structural and functional features of the family of facilitative glucose transporters.

Amino Acid Sequence↗
PubMed1990

Macula densa cells of mouse kidney do not synthesize epidermal growth factor precursor mRNA.

Epidermal growth factor (EGF) has previously been found in cells of the thick ascending limb of Henle's loop (TALH) and the first part of the distal convoluted tubule (DCT) of mouse kidney. To define the cellular localization of the mRNA that encodes the protein precursor of EGF, we used high-resolution in situ hybridization. EGF precursor cDNA was hybridized to both paraffin-embedded and frozen sections of mouse kidney. EGF precursor mRNA was detected in cells comprising the TALH and the first part of the DCT. In contrast, tubular cells in the macula densa did not express this specific mRNA. The results indicate that expression of the EGF precursor gene is restricted to specific types of cells in the distal nephron.

Animals↗
PubMed1990

Tumor hypoglycemia: relationship to high molecular weight insulin-like growth factor-II.

The mechanism of tumor-associated hypoglycemia was examined in 11 patients with hepatocellular carcinoma, 6 of whom presented with severe hypoglycemia and 5 in whom plasma glucose was persistently normal. Serum insulin levels in the hypoglycemic patients were low. Although total serum insulin-like growth factor II (IGF-II) levels in both groups of tumor patients were lower than normal, tumor tissue from hypoglycemic patients contained levels of IGF-II mRNA that were 10-20-fold higher than those present in normal liver. IGF-II immunoreactivity consisted in all cases of a mixture of both higher molecular weight forms and material having the character of IGF-II itself. The former comprised a greater proportion of total IGF-II, in patients with hypoglycemia. Studies to characterize the interactions of IGF-II with serum proteins showed that (a) the radiolabeled peptide bound to an approximately 40,000-D protein in sera from both hypoglycemic patients and normal subjects, (b) sera from hypoglycemic patients and normal subjects had similar capacity to bind the radiolabeled peptide, and (c) the apparent affinities of serum binding proteins for IGF-II were the same for both hypoglycemic patients and normal subjects. Whereas, acid extracted, tumor-derived IGF-II immunoreactive peptides with low or intermediate molecular weights bound to serum proteins in a manner indistinguishable from that of IGF-II itself, the highest molecular weight IGF-II immunoreactive peptide exhibited negligible ability to compete for radiolabeled ligand binding to serum proteins. The low affinity of serum binding proteins for this component suggests that high molecular weight IGF-II immunoreactivity might circulate free and be available for interaction with cell-surface receptors.

Adolescent↗
PubMed1990

Identification of a ligand-binding region of the human insulin receptor encoded by the second exon of the gene.

Structure-function studies of the insulin molecule indicate that an insulin B chain domain comprising residues 22-26 is involved both in binding to the insulin receptor (INSR) and in insulin dimer formation, suggesting that this domain might also interact with a structure resembling the insulin dimer interface in the INSR. Expression of a mutant INSR cDNA with a deletion of the region corresponding to exon 2 of the INSR gene produces a protein devoid of insulin-binding activity, although the mutant protein is processed appropriately to alpha- and beta-subunits, suggesting that the insulin-binding domain is encoded at least in part by exon 2. Within this region of the INSR molecule, the sequence 83-103 fulfills the structural criteria for a dimer interface. Studies of mutant INSRs with substitutions for phenylalanine 88 or 89 show that the presence of phenylalanine at position 89 is essential for full binding affinity.

Amino Acid Sequence↗
PubMed1990

Regulation of glucose transporter messenger RNA levels in rat adipose tissue by insulin.

Analysis of glucose transporter mRNA levels in adipose tissue from streptozotocin (STZ)-induced diabetic rats demonstrated a specific decrease (10-fold) in adipose tissue GLUT-4 mRNA with no significant effect on GLUT-1 mRNA levels. Treatment of STZ-diabetic rats with twice daily injections of insulin for 1-3 days resulted in a 16-fold increase in the relative amount of GLUT-4 mRNA to levels approximately 2-fold greater than those in control animals. However, after 7 days of insulin therapy the amount of GLUT-4 mRNA decreased approximately 2-fold back to the levels in the control animals. Normalization of the STZ-induced serum hyperglycemia by phlorizin treatment, which inhibits renal tubular reabsorption of glucose, had no effect on GLUT-4 mRNA in the absence of insulin. Similar to STZ-diabetes, fasting for 48 h also reduced adipose GLUT-4 mRNA levels. Parenteral administration of insulin with glucose over 7.5 h, but not glucose alone, increased the levels of the GLUT-4 mRNA 3- to 4-fold. These studies demonstrate that the relative glycemic state does not influence GLUT-4 glucose transporter mRNA expression in vivo and strongly suggests that insulin is a major factor regulating the levels of GLUT-4 mRNA in adipose tissue.

Adipose Tissue↗
PubMed1990

Human insulin-receptor gene.

The human insulin-receptor (hINSR) gene spans a region of greater than 120,000 base pairs (bp) on the short arm of chromosome 19. It is comprised of 22 exons or coding regions that vary in size from 36 to greater than 2500 bp. To a large degree, the introns appear to divide the hINSR gene into segments that encode structural and/or functional elements of the hINSR protein. The exon-intron organization of the hINSR gene provides a clue to the evolutionary history of this gene and suggests that it is a mosaic constructed from protein-coding regions recruited from other genes. Eight mutations in the hINSR gene that result in expression of structurally abnormal proteins have been described. These mutations are associated with insulin resistance and provide insight into the role of the hINSR gene in the development of diabetes mellitus.

Amino Acid Sequence↗
PubMed1990

Molecular biology of mammalian glucose transporters.

The oxidation of glucose represents a major source of metabolic energy for mammalian cells. However, because the plasma membrane is impermeable to polar molecules such as glucose, the cellular uptake of this important nutrient is accomplished by membrane-associated carrier proteins that bind and transfer it across the lipid bilayer. Two classes of glucose carriers have been described in mammalian cells: the Na(+)-glucose cotransporter and the facilitative glucose transporter. The Na(+)-glucose cotransporter transports glucose against its concentration gradient by coupling its uptake with the uptake of Na+ that is being transported down its concentration gradient. Facilitative glucose carriers accelerate the transport of glucose down its concentration gradient by facilitative diffusion, a form of passive transport. cDNAs have been isolated from human tissues encoding a Na(+)-glucose-cotransporter protein and five functional facilitative glucose-transporter isoforms. The Na(+)-glucose cotransporter is expressed by absorptive epithelial cells of the small intestine and is involved in the dietary uptake of glucose. The same or a related protein may be responsible for the reabsorption of glucose by the kidney. Facilitative glucose carriers are expressed by most if not all cells. The facilitative glucose-transporter isoforms have distinct tissue distributions and biochemical properties and contribute to the precise disposal of glucose under varying physiological conditions. The GLUT1 (erythrocyte) and GLUT3 (brain) facilitative glucose-transporter isoforms may be responsible for basal or constitutive glucose uptake. The GLUT2 (liver) isoform mediates the bidirectional transport of glucose by the hepatocyte and is responsible, at least in part, for the movement of glucose out of absorptive epithelial cells into the circulation in the small intestine and kidney. This isoform may also comprise part of the glucose-sensing mechanism of the insulin-producing beta-cell. The subcellular localization of the GLUT4 (muscle/fat) isoform changes in response to insulin, and this isoform is responsible for most of the insulin-stimulated uptake of glucose that occurs in muscle and adipose tissue. The GLUT5 (small intestine) facilitative glucose-transporter isoform is expressed at highest levels in the small intestine and may be involved in the transcellular transport of glucose by absorptive epithelial cells. The exon-intron organizations of the human GLUT1, GLUT2, and GLUT4 genes have been determined. In addition, the chromosomal locations of the genes encoding the Na(+)-dependent and facilitative glucose carriers have been determined. Restriction-fragment-length polymorphisms have also been identified at several of these loci.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acid Sequence↗
PubMed1990

Human insulin-receptor gene. Partial sequence and amplification of exons by polymerase chain reaction.

The partial sequence of the human insulin-receptor (hINSR) gene is presented. Using the gene sequence as a guide, we selected pairs of oligonucleotide primers from sites in the introns that flank each exon. These primers allowed each of the 22 exons of the hINSR gene to be amplified in vitro by the polymerase chain reaction. The sequences of the gene and oligonucleotide primers will facilitate studies of genetic variation in the hINSR gene and thereby increase our understanding of the role of this gene in the development of insulin-resistant states and glucose intolerance.

Amino Acid Sequence↗
PubMed1990

Human fibroblasts express the insulin-responsive glucose transporter (GLUT4).

The transport of glucose across the plasma membrane of nonepithelial cells is mediated by a family of facilitative glucose transporters. One glucose transporter is insulin-responsive (GLUT4) and is found in muscle, heart, and fat cells, differentiated cells which are difficult to maintain and study in culture. Cultured dermal fibroblasts, on the other hand, also are insulin-responsive, can be easily maintained in culture, and retain the genetic complement of the donor. In this paper, we evaluate RNA isolated from cultured human fibroblasts for the expression of four different glucose transporters. Northern blot analysis indicated that human fibroblasts expressed the erythrocyte (GLUT1), the fetal skeletal muscle (GLUT3), and the insulin-responsive (GLUT4) glucose transporters, but not the liver glucose transporter (GLUT2). To confirm the presence of GLUT4 mRNA, cDNA was synthesized from human fibroblast RNA, amplified using primers specific for GLUT4 by the polymerase chain reaction, and sequenced. The sequence was identical to that of GLUT4 cDNA. These data indicate that cultured human fibroblasts express at least three genetically distinct facilitative glucose transporters.

Base Sequence↗
PubMed1990

Transformation of NIH3T3 fibroblasts by an expression vector for the human epidermal growth factor precursor.

Epidermal growth factor (EGF) and transforming growth factor alpha (TGF alpha) bind to a common cell surface receptor that mediates their diverse biological activities. NIH3T3 fibroblasts transfected with either full-length EGF precursor (preproEGF) or proTGF alpha cDNA displayed distinct patterns of growth in culture. PreproEGF induced focal transformation, and transfectants grew in a chemically defined medium (CDM) at low cell density in the absence of added EGF. In contrast, TGF alpha failed to cause focal transformation, and transfectants grew in CDM in the absence of added growth factors only when seeded at high cell density. The 53 amino acid EGF portion of the preproEGF translation product was essential for its effects. These results indicate that constitutive expression of preproEGF is sufficient to establish autocrine growth of NIH3T3 expressing low levels of EGF receptors. At high cell density, where paracrine as well as autocrine effects of these growth factors would be evident, TGF alpha transfectants displayed at least as high or higher levels of EGF receptor (EGFR) tyrosine phosphorylation than preproEGF transfectants. Since quantitative levels of ligand expression did not account for differences in their transforming properties, preproEGF must be more efficient than proTGF alpha in binding and/or activating EGF receptors in an autocrine manner.

Cell Transformation, Neoplastic↗
PubMed1989

Regulation of glucose transporter messenger RNA in insulin-deficient states.

Recent studies have indicated that a family of structurally related proteins with distinct but overlapping tissue distributions are responsible for facilitative glucose transport in mammalian tissues. Insulin primarily stimulates glucose transport by inducing the redistribution of a unique glucose transporter protein from an intracellular pool to the plasma membrane. This 509-amino-acid integral membrane protein, termed GLUT-4, is the main insulin-responsive glucose transporter in adipose and muscle tissues. We have observed a dramatic decrease (tenfold) in the steady-state levels of GLUT-4 messenger RNA in adipose tissue from fasted rats or rats made insulin deficient with streptozotocin. Insulin treatment of the streptozotocin-diabetic rats or refeeding the fasted animals causes a rapid recovery of the GLUT-4 mRNA to levels significantly above those observed in untreated control animals. By contrast, the levels of the erythrocyte/HepG2/rat brain-type glucose transporter mRNA remain essentially unchanged under these conditions. These data suggest that the in vivo expression of GLUT-4 mRNA in rat adipose tissue is regulated by insulin.

Animals↗
PubMed1989

Exon-intron organization, expression, and chromosomal localization of the human motilin gene.

The human motilin gene has been isolated and characterized. The gene spans about 9 kilobase pairs (kb) and the 0.7 kb motilin mRNA is encoded by five exons. The 22-amino-acid motilin sequence is encoded by exons 2 and 3. The human motilin gene was mapped to the p21.2----p21.3 region of chromosome 6 by hybridization of the cloned cDNA to DNAs from a panel of reduced human-mouse somatic cell hybrids and by in situ hybridization to human prometaphase chromosomes. RNA blotting using RNA prepared from various regions of the human gastrointestinal tract revealed high levels of motilin mRNA in duodenum and lower levels in the antrum of the stomach; motilin mRNA could not be detected by this procedure in the esophagus, cardia of the stomach, descending colon or gallbladder.

Amino Acid Sequence↗
PubMed1989

Cloning and characterization of the major insulin-responsive glucose transporter expressed in human skeletal muscle and other insulin-responsive tissues.

Complementary DNA clones encoding a facilitative glucose transporter-like protein have been isolated from human small intestine and muscle cDNA libraries. This 509-amino acid protein has 65.3, 54.3, and 57.5% identity with the previously described human erythrocyte/HepG2, liver, and fetal muscle glucose transporter/transporter-like proteins, respectively. RNA blotting studies indicate that transcripts encoding this protein are very abundant in adult human skeletal muscle and subcutaneous fat. The adult skeletal muscle glucose transporter-like protein was expressed in vitro by cDNA-directed transcription and cell-free translation of the synthetic mRNA. The in vitro-synthesized protein reacted with a monoclonal antibody, 1F8, which recognizes the insulin-regulatable glucose transporter expressed in rat skeletal muscle, heart, and adipocytes. In contrast, in vitro-synthesized erythrocyte/HepG2 and fetal muscle glucose transporters did not react with 1F8. The high levels in adult skeletal muscle and subcutaneous fat of mRNA encoding the adult skeletal muscle glucose transporter and its specific reactivity with monoclonal antibody 1F8 suggest that this protein is the major insulin-regulatable glucose transporter expressed in skeletal muscle and other insulin-responsive tissues.

Adipose Tissue↗
PubMed1989

Alternative splicing of human insulin receptor messenger RNA.

The polymerase chain reaction has been used to examine alternative splicing of human insulin receptor (hINSR) mRNA. Alternative splicing of a 36 base pair exon, exon 11, generates hINSR transcripts encoding receptor isoforms which differ in sequence at the C-terminal end of the insulin-binding alpha-subunit. This process appears to be tissue-specific and, in addition, may be developmentally regulated.

Adult↗