Distinction of three types of D-glucose transport systems in animal cells

Key genes differential expressions and pathway involved in salt and water-deprivation stresses for renal cortex in camel

Background

Camels possess the characteristics of salt- and drought-resistances, due to the long-time adaption to the living environment in desert. The camel resistance research on transcriptome is rare and deficient, especially reabsorption in renal cortex. Non-coding RNAs are normally considered as the RNA molecules that are not translated into proteins, their current roles remain mostly in regulation of information flux from DNA to protein, further on normal life activities and diseases. In order to reveal the mysterious veil of the post-transcriptional regulation of ncRNAs in renal cortex for the first time as far as we know, we designed and carried out the experiment of salt stress and water-deprivation stress in camel.

Results

By means of RNA-seq in renal cortex of Alxa Bactrian Camel (Camelus bactrianus), we identified certain significantly differential RNAs, including 4 novel lncRNAs, 11 miRNAs and 13 mRNAs under salt stress, 0 lncRNAs, 18 miRNAs and 14 mRNAs under water-deprivation stress. By data analysis, the response pathway of post-transcriptional regulation concerning salt and water-deprivation stresses was put forward, involving preventing sodium from entering the cell, purifying of water and compensating neutral amino acids by miR-193b, miR-542-5p interaction with SLC6A19 mRNA.

Conclusion

Based on the resistance-related lncRNAs, miRNAs, and mRNAs, we proposed the post-transcriptional regulation pathway to explain how camels respond to salt and water-deprivation stresses in the ncRNAs regulation level of renal cortex for the first time, thus hoping to provide a theoretical basis for therapy of disease that is similar to high blood pressure in humans.

Electronic supplementary material

The online version of this article (10.1186/s12867-019-0129-8) contains supplementary material, which is available to authorized users.

Biology of kidney cells: ontogeny-recapitulating phylogeny

Biology of kidney cells can be used as a model for further understanding of ontogeny-recapitulating phylogeny. The common and species-specific structural and functional relationship between blood capillaries and the environment via a filtration barrier of nephrons is a biological phenomenon resulting from renal cell memory acquired through evolution. Genetically programmed development, a subsequent series of gene expression, and inductive interactions played a key role in differentiation and maintenance of specific activities of kidneys in birds and mammals. Various environmental factors may alter kidney development and specific activities at the levels of gene expression, repression, or derepression, and defensive mechanisms involved in reaction to risk factors are developed. Autoimmunity and cancerogenesis are closely dependent on a variety of environmental agents, such as antigens originating from infections with some viruses and toxins, or irradiation, advanced industrialization, and progress of civilization. As a result of gene mutation, delation, rearrangement, and/or susceptibility to different agents, renal cell memory is altered. Instead of cell-specific activities, the abilities for regeneration, and other genetically programmed activities, the genesis of kidney diseases are common. Balkan endemic nephropathy, as regional disease, is an important example of the role, of environmental agents, at the level of genes. Research programs on molecular genetics will contribute to our efforts both to prevent infections and to elucidate the genesis, diagnosis, prognosis, prevention, and therapy of kidney diseases.

Glucose transporter GLUT3 in the rat placental barrier: a possible machinery for the transplacental transfer of glucose

Glucose transfer across the placental barrier is crucial for fetal development. To investigate the role of glucose transporter isoforms in the transplacental transfer of glucose, we investigated the localization of glucose transporters GLUT1 and GLUT3 immunohistochemically in the rat placenta. In the labyrinth, the site of maternofetal exchange of substances, both GLUT1 and GLUT3 were present, whereas only GLUT1 was detected in the junctional region. In the labyrinthine wall, which lies between maternal and fetal circulations, GLUT3 exhibited polarized localization; i.e. it was present at the plasma membranes of the maternal blood side in the syncytiotrophoblast layers. GLUT1 was concentrated at plasma membranes of the maternal and fetal blood sides of syncytiotrophoblast layers. The asymmetric distribution of GLUT3 across the placental barrier may suggest asymmetric transfer of glucose, which would be beneficial to provide a stable milieu for fetal development.

Transport of glucose across the blood-tissue barriers

In specialized parts of the body, free exchange of substances between blood and tissue cells is hindered by the presence of a barrier cell layer(s). Specialized milieu of the compartments provided by these "blood-tissue barriers" seems to be important for specific functions of the tissue cells guarded by the barriers. In blood-tissue barriers, such as the blood-brain barrier, blood-cerebrospinal fluid barrier, blood-nerve barrier, blood-retinal barrier, blood-aqueous barrier, blood-perilymph barrier, and placental barrier, endothelial or epithelial cells sealed by tight junctions, or a syncytial cell layer(s), serve as a structural basis of the barrier. A selective transport system localized in the cells of the barrier provides substances needed by the cells inside the barrier. GLUT1, an isoform of facilitated-diffusion glucose transporters, is abundant in cells of the barrier. GLUT1 is concentrated at the critical plasma membranes of cells of the barriers and thereby constitutes the major machinery for the transport of glucose across these barriers where transport occurs by a transcellular mechanism. In the barrier composed of double-epithelial layers, such as the epithelium of the ciliary body in the case of the blood-aqueous barrier, gap junctions appear to play an important role in addition to GLUT1 for the transfer of glucose across the barrier.

Expression of the rat GLUT1 glucose transporter in the yeast Saccharomyces cerevisiae

We expressed the rat GLUT1 facilitative glucose transporter in the yeast Saccharomyces cerevisiae with the use of a galactose-inducible expression system. Confocal immunofluorescence microscopy indicated that a majority of this protein is retained in an intracellular structure that probably corresponds to endoplasmic reticulum. Yeast cells expressing GLUT1 exhibited little increase in glucose-transport activity. We prepared a crude membrane fraction from these cells and made liposomes with this fraction using the freeze-thaw/sonication method. In this reconstituted system, D-glucose-transport activity was observed with a Km for D-glucose of 3.4 +/- 0.2 mM (mean +/- S.E.M.) and was inhibited by cytochalasin B (IC50= 0.44 +/- 0.03 microM), HgCl2 (IC50)= 3.5 +/- 0.5 microM), phloretin (IC50= 49 +/- 12 microM) and phloridzin (IC50= 355 +/- 67 microM). To compare these properties with native GLUT1 we made reconstituted liposomes with a membrane fraction prepared from human erythrocytes, in which the Km of D-glucose transport and ICs of these inhibitors were approximately equal to those obtained with GLUT1 made by yeast. When the relative amounts of GLUT1 in the crude membrane fractions were measured by quantitative immunoblotting, the specific activity of the yeast-made GLUT1 was 110% of erythrocyte GLUT1, indicating that GLUT1 expressed in yeast is fully active in glucose transport.

Immunolocalization of glucose transporter GLUT1 in the rat placental barrier: possible role of GLUT1 and the gap junction in the transport of glucose across the placental barrier

GLUT1 is an isoform of facilitated-diffusion glucose transporters and has been shown to be abundant in cells of blood-tissue barriers. Using antibodies against GLUT1, we investigated the immunohistochemical localization of GLUT1 in the rat placenta. Rat placenta is of the hemotrichorial type. Three cell layers (from the maternal blood side inward) cytotrophoblast and syncytiotrophoblasts I and II, lie between the maternal and fetal bloodstreams. GLUT1 was abundant along the invaginating plasma membrane facing the cytotrophoblast and the syncytiotrophoblast I. Also, the infolded basal plasma membrane of the syncytiotrophoblast II was rich in GLUT1. Apposing plasma membranes of syncytiotrophoblasts I and II, however, had only a small amount of GLUT1. Numerous gap junctions were seen between syncytiotrophoblasts I and II. Taking into account the localization of GLUT1 and the gap junctions, we suggest a possible major transport route of glucose across the placental barrier, as follows: glucose in the maternal blood passes freely through pores of the cytotrophoblast. Glucose is then transported into the cytoplasm of the syncytiotrophoblast I via GLUT1. Glucose enters the syncytiotrophoblast II through the gap junctions. Finally glucose leaves the syncytiotrophoblast II via GLUT1 and enters the fetal blood through pores of the endothelial cells.

Alteration by transforming growth factor-beta 1 of asparagine-linked sugar chains in glucose transporter protein in Swiss 3T3 cells

GLUT1 protein in Swiss 3T3 cells is a 55-kDa glycoprotein with an N-linked oligosaccharide chain. We previously showed that the 65-kDa GLUT1 protein with modulated glycosylation was induced by transforming growth factor-beta 1 (TGF-beta 1) in Swiss 3T3 cells. To further investigate the altered structures of these sugar chains, the membrane glycoproteins solubilized with Triton X-100 were fractionated by lectin-affinity chromatography. The 55-kDa GLUT1 in control and TGF-beta 1-treated cells showed partial binding to Datura stramonium agglutinin (DSA), whereas the 65-kDa GLUT1 exclusively bound to DSA- and wheat germ agglutinin (WGA)-agarose. The 65-kDa GLUT1 in TGF-beta 1-treated cells was sensitive to endo-beta-galactosidase, which cleaves unsubstituted polylactosamine chains. While the 55-kDa GLUT1 in control 3T3 cells was similarly digested by endo-beta-galactosidase, that in TGF-beta 1-treated cells was resistant to this enzyme. These results suggest that the N-linked oligosaccharides of GLUT1 in Swiss 3T3 cells were altered by TGF-beta 1 to forms with more branched and/or repeated polylactosamines as well as with some substitution in the polylactosamines, resulting in a larger GLUT1 molecule. These GLUT1 proteins were exclusively located at the plasma membrane and served as a glucose transporter. However, the affinity to 2-deoxyglucose was significantly increased by TGF-beta 1, associated with the altered glycosylation of GLUT1 protein.

Modulation of the synthesis and glycosylation of the glucose transporter protein by transforming growth factor-beta 1 in Swiss 3T3 fibroblasts

Transforming growth factor-beta 1 (TGF-beta 1) stimulated growth and glucose uptake in Swiss mouse fibroblasts. DNA synthesis was increased 2-3-fold after 48 h incubation of growing 3T3 cells with TGF-beta 1 in calf serum-containing medium. Glucose transport activity in the cells was increased within 3 h after addition of TGF-beta 1 and this stimulation continued during incubation for 48 h. TGF-beta 1 also increased the levels of a brain type-glucose transporter (GLUT1) mRNA and the GLUT1 protein (55 kDa) in the membranes, consistent with the increase in glucose uptake. Furthermore, a longer exposure of TGF-beta 1 for 24-48 h induced a marked increase in the 65 kDa GLUT1 in 3T3 cell membranes. Other growth factors such as epidermal growth factor, fibroblast growth factor, transforming growth factor-alpha, and insulin did not elevate glucose uptake and the levels of 55 and 65 kDa GLUT1 proteins. Adding tunicamycin or deoxymannojirimycin to the TGF-beta 1-treated and untreated cells caused these 55 and 65 kDa glucose transporters to migrate as one band at 40-43 kDa. In addition, treating membrane proteins with glycopeptidase F, which removes N-linked oligosaccharides, also generated a glucose transporter of 40 kDa, suggesting that the 55 and 65 kDa GLUT1 proteins have a similar or identical core polypeptide but with different N-linked oligosaccharides. These results indicate that TGF-beta 1 modulates the synthesis of GLUT1 protein as well as its glycosylation in Swiss 3T3 cells, and that these changes may contribute to the control of cell proliferation by TGF-beta 1.

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

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.

Erythrocyte/HepG2-type glucose transporter is concentrated in cells of blood-tissue barriers

In search of possible diverse roles of glucose transporters (GT's), we examined whether any GT's are present in blood-tissue barriers where selective flow of glucose from blood to tissue cells is critically important. We found in rat that the erythrocyte/HepG2-type GT is localized in all the limiting plasma membranes known to serve as blood-tissue barriers, whether the barriers are endothelial type (brain, iris, inner retina, peripheral nerve) or epithelial type (choroid plexus, ciliary body, outer retina, peripheral nerve, placenta), except for plasma membranes in testis and thymus where no appreciable amount of the GT was found. The erythrocyte/HepG2-type GT may play a vital role for the entry of glucose into these firmly guarded tissues.

Isolation and characterization of Chinese hamster ovary cell mutants defective in glucose transport

Cultured Chinese hamster ovary (CHO) cells possess an insulin-sensitive facilitated diffusion system for glucose transport. Mutant clones of CHO cells defective in glucose transport were obtained by repeating the selection procedure, which involved mutagenesis with ethyl methanesulfonate, radiation suicide with tritiated 2-deoxy-D-glucose, the polyester replica technique and in situ autoradiographic assaying for glucose accumulation. On the first selection, we obtained mutants exhibiting about half the glucose uptake activity of parental CHO-K1 cells and half the amount of a glucose transporter, the amount of which was determined by immunoblotting with an antibody to the human erythrocyte glucose transporter. The second selection, starting from one of the mutants obtained in the first-step selection, yielded a strain, GTS-31, in which both glucose uptake activity and the quantity of the glucose transporter were 10-20% of the levels in CHO-K1 cells, whereas the responsiveness of glucose transport to insulin, and the activities of leucine uptake and several glycolytic enzymes remained unchanged. GTS-31 cells grew slower than CHO-K1 cells at both 33 and 40 degrees C, and in a medium containing a low concentration of glucose (0.1 mM), the mutant cells lost the ability to form colonies. All the three spontaneous GTS-31 cell revertants, which were isolated by growing the mutant cells in medium containing 0.1 mM glucose, exhibited about half the glucose uptake activity and about half the amount of glucose transporter, as compared to in CHO-K1 cells, these characteristics being similar to those of the first-step mutant. These results indicate that the decrease in glucose uptake activity in strain GTS-31 is due to a mutation which induces a reduction in the amount of the glucose transporter, providing genetic evidence that the glucose transporter functions as a major route for glucose entry into CHO-K1 cells.

Insulin receptor and altered glucose transport in a monensin-resistant mutant of Chinese hamster ovary cell

A monensin-resistant mutant Monr-31, derived from Chinese hamster ovary (CHO) cell line, has been shown to have a reduced number of insulin receptors and a reduction in glucose uptake in response to insulin. We have further investigated the possibility that altered glucose uptake in Monr-31 cells is related to an alteration in the activity of the insulin receptor. Uptake of glucosamine, 2-deoxy-D-glucose, and 3-O-methyl-D-glucose in Monr-31 cells was one-half to one-third that of CHO cells. The cellular content of the glucose transporter in Monr-31 was reduced to about one-third that of CHO as assayed by use of an antiglucose transporter antibody. After transfection with the human insulin receptor cDNA, we obtained clones CIR-0 from CHO, and MIR-2 and MIR-15 from Monr-31; CIR-0 expressed a tenfold higher level of the insulin-binding activity than did CHO, and MIR-2 and MIR-15 expressed a 20-fold higher level than did Monr-31. Glucose uptake in both CHO and CIR-0 was significantly enhanced by exogenous insulin, but not in Monr-31, MIR-2, and MIR-15. The beta-subunits of insulin receptor in CHO, CIR-0, Monr-31, and MIR-2 were similarly phosphorylated. The decreased glucose transport activity in Monr-31 cells is discussed in relation to the absence or presence of insulin receptor expression.

Developmentally regulated 75-kilodalton protein expressed in LLC-PK1 cultures is a component of the renal Na+/glucose cotransport system

Na+/D-glucose symport is a secondary active glucose transport mechanism expressed only in kidney proximal tubule and in small intestine. A monoclonal antibody that recognized the Na+/glucose symporter of pig renal brush border membranes also recognized a 75-kD protein in apical membranes isolated from highly differentiated LLC-PK1 cultures, an epithelial cell line of pig renal proximal tubule origin. The 75-kD antigen was enriched from solubilized LLC-PK1 apical membranes by means of high-pressure liquid chromatography. The symporter antigen became apparent on the apical membrane surface after the development of a confluent monolayer in correlation with the expression of transport activity. Long-term treatment of cultures with the differentiation inducer hexamethylene bisacetamide was accompanied by a dramatically increased expression of the symporter antigen as detected quantitatively by Western blot analysis and qualitatively by immunofluorescence staining. The number of symporter-positive cells was dramatically increased after inducer treatment as predicted for differentiation-regulated expression. These results identify a 75-kD protein as a component of a developmentally regulated renal Na+/glucose symporter expressed in cell culture.

Evidence for expression of the facilitated glucose transporter in rat hepatocytes

The eukaryotic facilitated glucose transporter (GT) is expressed by many cell types, with the notable exception of hepatocytes; however, GT is expressed by several hepatoma cell lines, including the well-differentiated lines Fao, Hep3B, and HepG2. We report on studies carried out to determine the aspect(s) of the transformed phenotype that might be responsible for activating GT expression. Using RNA blot analysis with probes derived from rat GT cDNA, we found that GT was expressed by rat hepatocytes under two conditions (i) in vitro, when isolated hepatocytes were placed in cell culture, and (ii) in vivo, when rats were subjected to starvation for greater than or equal to 2 days. However, GT expression was not an obligatory feature of hepatomas, since two primary hepatocellular carcinomas did not express any GT mRNA. GT expression in hepatocytes was reduced by addition of dimethyl sulfoxide or sodium butyrate to the culture medium. Since these reagents are known to promote differentiation in some cell culture systems, their effect on hepatocytes may be to maintain the GT repression normally observed in vivo. Inclusion or exclusion in the culture medium of several other agents that enhance hepatocyte viability (serum, insulin, corticosteroids, epidermal growth factor, or triiodothyronine) did not affect GT expression. It is unclear whether the two conditions that led to GT expression in hepatocytes are related by a common signaling mechanism. Possibly, both cases involve a "stress" response: in vivo, a normal physiological response to starvation; in vitro, a response to a major alteration in the cellular environment.

Cytochalasin B-sensitive, sodium ion-dependent glucose transport in intestinal microvillous membrane

It was found that sodium ion-dependent glucose uptake by microvillous membrane (MVM) vesicles was partially inhibited by cytochalasin B with a half-maximum inhibition at ca. 10 microM. The MVM was photolabeled with [3]cytochalasin B. The Kd value and the maximum number of binding sites for cytochalasin B were ca. 8 microM and 70 pmol/mg protein, respectively. SDS-PAGE of the photolabeled MVM revealed 2 binding components. One was 86 K in Mr and the other 42 K. The binding of cytochalasin B to the 86 K component was affected neither by cytochalasin E nor by the presence of 0.5 M NaCl, but was depressed in the presence of 2-deoxy-D-glucose or phlorizin, which had no effect on the labeling of the 42 K component. These and other data suggested that the 86 K component might be responsible for a cytochalasin B-sensitive glucose transport in intestinal epithelial MVM.

Sodium butyrate increases glucose transporter expression in LLC-PK1 cells

The effect of sodium butyrate on the expression of the facilitated glucose transporter (GT) was investigated in the pig kidney cell line LLC-PK1. When cells were treated with butyrate, GT mRNA expression was remarkably enhanced with a maximal effect at 5 mM. Levels of GT mRNA were increased at 1 day after butyrate treatment and continued to increase for at least 4 days; however, acetate and propionate did not affect GT mRNA levels significantly. The induction of GT mRNA by butyrate was accompanied by an increase in GT function. The expression of GT mRNA decreased in HepG2, HT-29, and COS cells by treatment with butyrate for 1 day. Interestingly, glucose deprivation of LLC-PK1 cells reduced the induction of GT mRNA by butyrate, although starvation itself slightly enhanced steady-state GT mRNA levels. Therefore, expression of GT in LLC-PK1 cells is strongly induced by butyrate by a pathway that apparently depends on the presence of glucose in culture medium.

Constitutive glucose transport in rat Leydig cells and pyruvate support of steroidogenesis

To facilitate the interpretation of in vitro experiments that use pharmacological agents, such as cytochalasin B, which inhibits glucose transport, an alternative to glucose as an energy source for rat Leydig cells was sought. Pyruvate was superior to glucose as an energy source for the support of steroidogenesis. The concentration of pyruvate required to support half-maximum androgen production was lower than that for glucose (60 +/- 8 versus 478 +/- 87 microM, respectively), and pyruvate supported a higher rate of maximum LH-stimulated androgen production. The latter result suggested that glucose availability can be rate limiting for steroidogenesis in in vitro experiments. The acute regulation of glucose transport was therefore investigated using 2-Deoxy-D-[2,6(3)H]glucose. Uptake of 2-Deoxy-D-[2,6(3)H]glucose by Leydig cells was not affected by insulin, LH, or glucose deprivation, suggesting that it is a constitutive process in these cells.

Entry of Shigella flexneri into HeLa cells: evidence for directed phagocytosis involving actin polymerization and myosin accumulation

The enteroinvasive bacterium Shigella flexneri expresses a plasmid-mediated capacity to penetrate into nonphagocytic cells. By using 7-nitrobenz-2-oxa-1,3-diazole-phallacidin (NBD-phallacidin), a fluorescent dye which specifically stains microfilaments, we observed condensations of filamentous actin underneath the plasma membrane of HeLa cells which interacted with the invasive isolate M90T. As demonstrated by indirect immunofluorescence with the antimyosin monoclonal antibody CC-212, myosin accumulated at the same sites. The entry process could be synchronized by using strain SC301, a pIL22 transformant of M90T. pIL22, a recombinant plasmid encoding the Escherichia coli afimbrial adhesin AFA I, rendered shigellae highly adherent to HeLa cells. Using such a system, we demonstrated that the occurrence of bacterial penetration and the appearance of structures brightly stained by NBD-phallacidin were simultaneous events. Such microfilamentous structures resulted from de novo polymerization of the monomeric actin pool in a DNase I inhibition assay, as shown by measurement of the monomeric versus total actin content of infected HeLa cells. These data provide direct evidence that the penetration of S. flexneri into HeLa cells occurs through a mechanism similar to phagocytosis by professional phagocytes.

Identification of proximal tubular transport functions in the established kidney cell line, OK

OK cells, derived from an American opossum kidney, were analyzed for proximal tubular transport functions. In monolayers, L-glutamate, L-proline, L-alanine, and alpha-methyl-glucopyranoside (alpha-methyl D-glucoside) were accumulated through Na+-dependent and Na+-independent transport pathways. D-Glucose and inorganic sulfate were accumulated equally well in the presence or absence of Na+. Influx of inorganic phosphate was only observed in the presence of Na+. Na+/alpha-methyl D-glucoside uptake was preferentially inhibited by phlorizin and D-glucose uptake by cytochalasin B. An amiloride-sensitive Na+-transport was also identified. In isolated apical vesicles (enriched 8-fold in gamma-glutamyltransferase), L-glutamate, L-proline, L-alanine, alpha-methyl D-glucoside and inorganic phosphate transport were stimulated by an inwardly directed Na+-gradient as compared to an inwardly directed K+-gradient. L-Glutamate transport required additionally intravesicular K+. D-Glucose transport was similar in the presence of a Na+- and a K+-gradient. Na+/alpha-methyl D-glucoside uptake was inhibited by phlorizin whereas cytochalasin B had no effect on Na+/D-glucose transport. An amiloride-sensitive Na+/H+ exchange mechanism was also found in the apical vesicle preparation. It is concluded that the apical membrane of OK cells contains Na+-coupled transport systems for amino acids, hexoses, protons and inorganic phosphate. D-Glucose appears a poor substrate for the Na+/hexose transport system.

Expression of size-selected mRNA encoding the intestinal Na/glucose cotransporter in Xenopus laevis oocytes

The expression of the rabbit intestinal brushborder Na/glucose cotransporter has been studied in Xenopus oocytes. Poly(A)+ RNA isolated from the intestinal mucosa was injected into oocytes, and the expression of the transporter in the oocyte plasma membrane was assayed by measuring the Na-dependent phlorizin-sensitive uptake of methyl alpha-D-[14C]glucopyranoside (MeGlc). Expression of the glucose carrier was detected 3-7 days after mRNA injection, and the rate of glucose transport was proportional to the amount of mRNA injected. mRNA (50 ng) increased the maximum velocity (Vmax) of MeGlc uptake by as much as 10-fold over background. The total mRNA was fractionated by preparative agarose gel electrophoresis and each fraction was assayed for its ability to induce transport activity. The mRNA encoding the Na/glucose cotransporter was found in a single fraction of approximately 2.3 kilobases (kb), which contained 3% of the total mRNA. A similar mRNA fraction (2.0-2.6 kb) isolated from colon did not induce expression of this transporter. In vitro translation of the fractionated intestinal mRNA showed enhanced synthesis of two protein bands at 57 and 63 kDa. The mRNA encoding the cotransporter is smaller (2.3 kb) than that (2.6-2.9 kb) encoding the 55-kDa facilitated glucose carrier in human hepatoma cells and rat brain.

Cloning and characterization of a cDNA encoding the rat brain glucose-transporter protein

Antibody raised against the human erythrocyte glucose transporter identified a recombinant lambda gt11 bacteriophage in a cDNA library prepared from immunoselected polysomal RNA from adult rat brain. The cDNA predicts a 492-amino acid protein that demonstrates 97.6% identity to the human hepatoma hexose carrier. The tissue distribution of the transporter mRNA is identical to that of immunologically identifiable protein and transport activity, except in liver in which high levels of transport are associated with little or no transporter mRNA or protein. As assayed by blot-hybridization analysis, mRNA from insulin-responsive and nonresponsive tissues are indistinguishable. These data suggest that a genetically unrelated protein is responsible for hexose transport in normal liver.

Active transport of myo-inositol in rat pancreatic islets

myo-Inositol transport by isolated pancreatic islets was measured with a dual isotope technique. Uptake was saturable with a half-maximal response at approx. 75 microM. With 50 microM-inositol, uptake was linear for at least 2 h during which time the free intracellular concentration rose to double that of the incubation medium. Inositol transport is therefore active and probably energized by electrogenic co-transport of Na+ down its concentration gradient as uptake was inhibited by ouabain, Na+ removal or depolarizing K+ concentrations. Inositol transport was abolished by cytochalasin B which binds to hexose carriers, but not by carbamoylcholine or Li+ which respectively stimulate or inhibit phosphoinositide turnover. Uptake of inositol was not affected by 3-O-methylglucose or L-glucose (both 100 mM) nor by physiological concentrations of D-glucose. The results suggest that most intracellular inositol in pancreatic islets would be derived from the extracellular medium. Since the transport mechanism is distinct from that of glucose, inositol uptake would not be inhibited during periods of hyperglycaemia.