3T3-L1 adipocyte glucose transporter (HepG2 class): sequence and regulation of protein and mRNA expression by insulin, differentiation, and glucose starvation

Dictyostelium differentiation-inducing factor-1 induces glucose transporter 1 translocation and promotes glucose uptake in mammalian cells

The differentiation-inducing factor-1 (DIF-1) is a signal molecule that induces stalk cell formation in the cellular slime mold Dictyostelium discoideum, while DIF-1 and its analogs have been shown to possess antiproliferative activity in vitro in mammalian tumor cells. In the present study, we investigated the effects of DIF-1 and its analogs on normal (nontransformed) mammalian cells. Without affecting the cell morphology and cell number, DIF-1 at micromolar levels dose-dependently promoted the glucose uptake in confluent 3T3-L1 fibroblasts, which was not inhibited with wortmannin or LY294002 (inhibitors for phosphatidylinositol 3-kinase). DIF-1 affected neither the expression level of glucose transporter 1 nor the activities of four key enzymes involved in glucose metabolism, such as hexokinase, fluctose 6-phosphate kinase, pyruvate kinase, and glucose 6-phosphate dehydrogenase. Most importantly, stimulation with DIF-1 was found to induce the translocation of glucose transporter 1 from intracellular vesicles to the plasma membranes in the cells. In differentiated 3T3-L1 adipocytes, DIF-1 induced the translocation of glucose trasporter 1 (but not of glucose transporter 4) and promoted glucose uptake, which was not inhibited with wortmannin. These results indicate that DIF-1 induces glucose transporter 1 translocation and thereby promotes glucose uptake, at least in part, via a inhibitors for phosphatidylinositol 3-kinase/Akt-independent pathway in mammalian cells. Furthermore, analogs of DIF-1 that possess stronger antitumor activity than DIF-1 were less effective in promoting glucose consumption, suggesting that the mechanism of the action of DIF-1 for stimulating glucose uptake should be different from that for suppressing tumor cell growth.

Acute activation of glucose uptake by glucose deprivation in L929 fibroblast cells

Glucose is a very important energy source for a wide variety of cells, and the ability of cells to respond to changes in glucose availability or other cell stresses is of critical importance. Many mammalian cells respond to acute stress by increasing the V(max) of transport through GLUT1; the most ubiquitously expressed glucose transporter isoform. This study investigated the acute response of glucose uptake to glucose deprivation in L929 fibroblast cells--a cell line that expresses only the GLUT1 transporter. Results indicated that glucose deprivation of only a minute activated glucose uptake 10-fold and reached a maximum of 20-fold within 10 min. The activation was dose dependent and only partially muted by addition of up to 20mM pyruvate as an alternate energy source. In contrast to the kinetics of acute metabolic stress, glucose deprivation decreased the K(m) of transport, but did not alter the V(max). Maximal activation of glucose transport by glucose deprivation was completely additive to activation of transport by methylene blue--a stimulant that increased the V(max) of transport without a change in the K(m). Glucose-deprived activation of glucose transport was not inhibited by wortmannin or herbimycin A, but was completely inhibited by phenylarsine oxide. Altogether, the data indicate that L929 fibroblast cells respond quickly and robustly to the cell stress of glucose deprivation and methylene blue treatment by two distinct activation pathways.

Nonmetabolizable glucose compounds impart cryotolerance to primary rat hepatocytes

We herein report a novel method for the cryopreservation of hepatocytes using a non-metabolizable glucose derivative in an attempt to mimic the natural cryoprotective adaptations observed in freeze-tolerant frogs. Primary rat hepatocytes were loaded with 3-O-methyl glucose (3OMG) through endogenous glucose transporters without evident toxicity. The 3OMG-loaded hepatocytes were then frozen in a controlled rate freezer down to -80 degrees C and stored in liquid nitrogen at -196 degrees C. Hepatocytes cryopreserved with a relatively small amount of intracellular 3OMG (<0.2 M) showed high post-thaw viability and maintained long-term hepatospecific functions, including synthesis, metabolism, and detoxification. Metabolite uptake and secretion rates were also largely preserved in the cryopreserved hepatocytes. This is the first study to demonstrate the use of the non-metabolizable glucose derivative 3OMG in hepatocyte cryopreservation.

GLUT1CBP(TIP2/GIPC1) interactions with GLUT1 and myosin VI: evidence supporting an adapter function for GLUT1CBP

We identified a novel interaction between myosin VI and the GLUT1 transporter binding protein GLUT1CBP(GIPC1) and first proposed that as an adapter molecule it might function to couple vesicle-bound proteins to myosin VI movement. This study refines the model by identifying two myosin VI binding domains in the GIPC1 C terminus, assigning respective oligomerization and myosin VI binding functions to separate N- and C-terminal domains, and defining a central region in the myosin VI tail that binds GIPC1. Data further supporting the model demonstrate that 1) myosin VI and GIPC1 interactions do not require a mediating protein; 2) the myosin VI binding domain in GIPC1 is necessary for intracellular interactions of GIPC1 with myosin VI and recruitment of overexpressed myosin VI to membrane structures, but not for the association of GIPC1 with such structures; 3) GIPC1/myosin VI complexes coordinately move within cellular extensions of the cell in an actin-dependent and microtubule-independent manner; and 4) blocking either GIPC1 interactions with myosin VI or GLUT1 interactions with GIPC1 disrupts normal GLUT1 trafficking in polarized epithelial cells, leading to a reduction in the level of GLUT1 in the plasma membrane and concomitant accumulation in internal membrane structures.

Glucose deprivation enhances targeting of GLUT1 to lipid rafts in 3T3-L1 adipocytes

Glucose deprivation dramatically increases glucose transport activity in 3T3-L1 adipocytes without changing the concentration of GLUT1 in the plasma membrane (PM). Recent data suggest that subcompartments within the PM, specifically lipid rafts, may sequester selected proteins and alter their activity. To evaluate this possibility, we examined the distribution of GLUT1 in Triton X-100-soluble and -insoluble fractions. Our data show that 77% of the GLUT1 pool in PMs isolated from control 3T3-L1 adipocytes was extracted by 0.2% Triton X-100. After glucose deprivation for 12 h, only 56% of GLUT1 was extracted by detergent. In contrast, there was a twofold increase in the GLUT1 content of the detergent-resistant fraction. To evaluate whether GLUT1 interacts with a specific protein within lipid rafts, we focused on stomatin, recently shown to interact with and inhibit GLUT1 activity. Stomatin is distributed about equally between the PM and the biosynthetic compartments, and its expression is not affected by glucose deprivation. Nearly 90% of the PM pool of stomatin is in detergent-resistant lipid rafts. In normal 3T3-L1 adipocytes, we were unable to demonstrate an interaction between GLUT1 and stomatin in coimmunoprecipitation experiments. However, in stomatin-overexpressing cells, there was clear coprecipitation of stomatin with GLUT1 antibodies. Glucose deprivation increased this interaction threefold, which may reflect the increase of GLUT1 in lipid rafts. Despite this, there was little change in transport activity in glucose-deprived, stomatin-overexpressing cells vs. that in control cells. Thus GLUT1 interacts with stomatin in lipid rafts, but this interaction per se does not alter transport activity. Rather, stomatin may serve as an anchor for GLUT1 in lipid rafts, the environment of which favors activation.

Sphingomyelin/cholesterol ratio: an important determinant of glucose transport mediated by GLUT-1 in 3T3-L1 preadipocytes

Sphingomyelin pathway has been linked with insulin signaling through insulin-dependent GLUT-4 glucose transporter, but a relationship between sphingomyelin and the GLUT-1 transporter responsible for the basal (insulin-independent) glucose transport has not been clearly established. As GLUT-1 is mainly distributed to the cell surface, we explored the effects of changes in membrane sphingomyelin content on glucose transport through GLUT-1. The addition of exogenous sphingomyelin or glutathione (an inhibitor of endogenous sphingomyelinase) to the culture medium increased membrane sphingomyelin and cholesterol contents. Basal glucose uptake was enhanced and positively correlated to sphingomyelin (SM), cholesterol (CL) and SM/CL ratio. The exposure of 3T3-L1 preadipocytes to sphingomyelinase (SMase) significantly increased basal glucose uptake, membrane fluidity and decreased membrane sphingomyelin and cholesterol contents 60 min after SMase addition. There was no significant change in the abundance of GLUT-1 at the cell surface. The membrane sphingomyelin and cholesterol contents, fluidity and basal glucose transport returned to baseline levels within 2 h. The basal glucose uptake was negatively correlated with cholesterol contents and positively with SM/CL ratio. The SM/CL ratio might represent an important parameter controlling basal glucose uptake and a mechanism by which insulin resistance might be induced.

Evaluation of 2-deoxy-D-glucose as a chemotherapeutic agent: mechanism of cell death

Nutrient deprivation has been shown to cause cancer cell death. To exploit nutrient deprivation as anti-cancer therapy, we investigated the effects of the anti-metabolite 2-deoxy-D-glucose on breast cancer cells in vitro. This compound has been shown to inhibit glucose metabolism. Treatment of human breast cancer cell lines with 2-deoxy-D-glucose results in cessation of cell growth in a dose dependent manner. Cell viability as measured by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide conversion assay and clonogenic survival are decreased with 2-deoxy-D-glucose treatment indicating that 2-deoxy-D-glucose causes breast cancer cell death. The cell death induced by 2-deoxy-D-glucose was found to be due to apoptosis as demonstrated by induction of caspase 3 activity and cleavage of poly (ADP-ribose) polymerase. Breast cancer cells treated with 2-deoxy-D-glucose express higher levels of Glut1 transporter protein as measured by Western blot analysis and have increased glucose uptake compared to non-treated breast cancer cells. From these results we conclude that 2-deoxy-D-glucose treatment causes death in human breast cancer cell lines by the activation of the apoptotic pathway. Our data suggest that breast cancer cells treated with 2-deoxy-D-glucose accelerate their own demise by initially expressing high levels of glucose transporter protein, which allows increased uptake of 2-deoxy-D-glucose, and subsequent induction of cell death. These data support the targeting of glucose metabolism as a site for chemotherapeutic intervention by agents such as 2-deoxy-D-glucose.

Glycogen phosphorylase is activated in response to glucose deprivation but is not responsible for enhanced glucose transport activity in 3T3-L1 adipocytes

We have previously shown that glucose deprivation activates glucose transport in a time- and protein synthesis-dependent fashion in 3T3-L1 adipocytes, a mouse cell line. Coincident with this is loss of glycogen. Because glycogen phosphorylase (GP) is responsible for glycogen degradation, we have examined its regulation to determine the relationship between transport activation and glycogen turnover. We first cloned the adipose GP cDNA and found sequence similarity to rat and human liver GP. Because the mouse liver GP cDNA sequence was unavailable, we cloned this cDNA as well and showed 100% identity between mouse adipose and liver sequences. A 3.1 kb transcript was readily observed in total RNA isolated from mouse liver or adipose by Northern blot analysis but, surprisingly, not in either total or poly(A) selected RNA from 3T3-L1 adipocytes. To evaluate regulation in 3T3-L1 adipocytes, we amplified GP mRNA from total RNA using multiplex, semi-quantitative PCR but found that expression did not change in response to deprivation. GP protein levels did not change either. However, endogenous GP activity from glucose-deprived cells was significantly elevated relative to controls, due to an increase in the phosphorylated form of GP (GPa). Finally, we overexpressed GP to determine its direct influence on the glucose transport system. These results were negative, which suggests that the nutrient control of glucose transport and GP occurs independently despite kinetic similarities in transport activation and glycogen turnover.

A new gene trap construct enriching for insertion events near the 5' end of genes

The gene trap approach is based on the integration of a gene trap vector into the genome. This can be done either by electroporation of a plasmid construct or by infection with a viral vector. Commonly used viral gene trap vectors have been shown to select for integrations near the 5' end of genes. To date, no plasmid vector with a similar tendency has been reported. In this paper we describe a new plasmid vector, pKC199beta geo. This vector contained a short splice acceptor fragment from the Hoxc9 gene, a full length lacZ gene, including an ATG, and a reduced activity, mutant neomycin phosphotransferase gene as a selectable marker. This vector enriched the population of trapped genes in our gene trap screen for insertion events in the 5' end of genes. In the two cases examined the beta-galactosidase activity pattern accurately reflected the endogenous promotor activity.

Glucose deprivation does not affect GLUT1 targeting in 3T3-L1 adipocytes

We have previously demonstrated that glucose deprivation alters the glycosylation of the GLUT1 glucose transporter in 3T3-L1 adipocytes. Many aberrantly glycosylated proteins are retained in the endoplasmic reticulum by interaction with chaperones. Herein, we use three independent procedures to show that GLUT1 is targeted to the plasma membrane, despite alterations in glycosylation. While earlier experiments revealed that plasma membrane targeting of aglyco GLUT 1 transporter was significantly reduced, our data show for the first time that altered glycosylation provides sufficient information to drive appropriate trafficking.

Expression of rab GTP-binding proteins during oligodendrocyte differentiation in culture

Oligodendrocytes (OLs) synthesize and transport vast amounts of proteins and lipids from the cell body to the morphologically and biochemically distinct domains of the myelin membrane. From our prediction that regulators of vesicular transport should be up-regulated at the time of myelin production, we hypothesized that the up-regulated and unidentified small GTPases found by Huber et al. [1994a] may be Rab proteins. We have analyzed the mRNA expression of rabs in OLs, and have detected rabs 10, 11b, 18, 24, 26, and 28 in addition to rabs that were found previously. Our data show that among the Rabs so far detected during differentiation, only Rabs 5a and 8a exhibited up-regulation in addition to the previously published Rab3a (Madison et al. [1999], J. Neurochem. 72:988-998). We discuss the limited extent of up-regulation of rabs in the context of the presumed necessity for an increase in Rab activity during myelin assembly.

High glucose concentrations inhibit glucose phosphorylation, but not glucose transport, in human endothelial cells

Glucose uptake is autoregulated in a variety of cell types and it is thought that glucose transport is the major step that is subjected to control by sugar availability. Here, we examined the effect of high glucose concentrations on the rate of glucose uptake by human ECV-304 umbilical vein-derived endothelial cells. A rise in the glucose concentration in the medium led a dose-dependent decrease in the rate of 2-deoxyglucose uptake. The effect of high glucose was independent of protein synthesis and the time-course analysis indicated that it was relatively slow. The effect was not due to inhibition of glucose transport since neither the expression nor the subcellular distribution of the major glucose transporter GLUT1, nor the rate of 3-O-methylglucose uptake was affected. The total in vitro assayed hexokinase activity and the expression of hexokinase-I were similar in cells treated or not with high concentrations of glucose. In contrast, exposure of cells to a high glucose concentration caused a marked decrease in phosphorylated 2-deoxyglucose/free 2-deoxyglucose ratio. This suggests the existence of alterations in the rate of in vivo glucose phosphorylation in response to high glucose. In summary, we conclude that ECV304 human endothelial cells reduce glucose utilization in response to enhanced levels of glucose in the medium by inhibiting the rate of glucose phosphorylation, rather than by blocking glucose transport. This suggests a novel metabolic effect of high glucose on cellular glucose utilization.

Reduced glucose consumption in the curly tail mouse does not initiate the pathogenesis leading to spinal neural tube defects

At embryonic stages of neural tube closure, the mouse embryo exhibits a high rate of glycolysis with glucose as the main energy source. In the curly tail mouse, often used as model system for study of human neural tube defects, a delay in closure of the posterior neuropore (PNP) is proposed to be indirectly caused by a proliferation defect in the caudal region. Because glucose is important for proliferation, we tested glucose uptake in curly tail and control embryos, and in a BALB/c-curly tail recombinant strain. The structure and expression of Glut-1, a glucose transporter molecule that is abundantly present during those embryonic stages and that has been mapped in the region of the major curly tail gene, were also studied; however, no strain differences could be demonstrated. Glucose uptake was determined by measuring glucose depletion from the medium in long-term embryo cultures that encompassed the stages of PNP closure and by measuring accumulation of 3H-deoxyglucose in short-term cultures at the stages of early and final PNP closure. Both approaches indicated a reduced glucose uptake by curly tail and recombinant embryos. Surprisingly, the uptake per cell appeared normal, accompanied by a significantly lower DNA content of the mutant embryos. Therefore, it is unlikely that reduced cell proliferation is caused by a reduction in glucose supply during the pathogenesis of the defects in curly tail embryos. The reduced DNA content as well as the reduced glucose uptake per embryo are likely downstream effects of the aberrant proliferation pattern.

Development of insulin resistance in 3T3-L1 adipocytes

Insulin resistance is a manifestation of both diabetes mellitus and obesity. However, the mechanism is still not clearly identified. Herein, we describe a procedure that allows us to evaluate the development of insulin resistance in 3T3-L1 adipocytes. Under these conditions, we show that the concentration of insulin required for 50% desensitization of glucose transport activity is 100 pM; maximal desensitization could be achieved with 1 nM. This demonstrates for the first time that 3T3-L1 adipocytes develop insulin resistance in response to physiologically relevant concentrations of insulin. Glucose (or glucosamine), in addition to insulin, was required to establish desensitization. The expression of GLUT4 protein decreased by 50% with exposure to 10 nM insulin. The dose-dependent loss of GLUT4 was similar to the dose dependence for insulin-resistant transport activity. Translocation in the presence of acute insulin was apparent, but the extent of recruitment directly reflected the decrease in GLUT4 protein. GLUT4 mRNA also declined, but the ED50 was approximately 5 nM. Together, these data suggest that the loss of GLUT4 protein likely underlies the cause of desensitization. However, the loss of GLUT4 protein did not correlate with the loss in GLUT4 mRNA suggesting post-translational control of GLUT4 expression.

Differential regulation of GRP78 and GLUT1 expression in 3T3-L1 adipocytes

We tested the hypothesis that the constitutive glucose transporter (GLUT1) in 3T3-L1 adipocytes belongs to the family of glucose-regulated proteins which are transcriptionally regulated by glucose deprivation. Using cDNA probes for both GRP78 (BiP) and GLUT1, we show that the level of GRP78 mRNA increased by 15-fold within 24 h of glucose deprivation with little change in GLUT1 mRNA. The elevated GRP78 mRNA in turn led to a time-dependent increase in GRP78 protein. While glucose deprivation did not alter the expression of the normal glycoform of GLUT1, a lower molecular weight glycoform accumulated with extended deprivation. Mannose and fructose, but not galactose, prevented the induction of GRP78 and accumulation of the abnormal GLUT1. Because GRP78 acts as a chaperone in other cell systems, we also sought evidence to support this activity in 3T3-L1 adipocytes. Using the technique of co-immunoprecipitation, we demonstrate that GRP78 bound several proteins unique to the glucose-deprived state. No deprivation-specific proteins could be detected in association with GLUT1. These data lead us to conclude that GLUT1 does not display characteristics of the glucose-regulated proteins, at least in 3T3-L1 adipocytes, a widely used model for differentiation, hormone action, and nutrient control. However, the mechanisms for activating traditional members of this family appear intact.

Translocation of GLUT1 does not account for elevated glucose transport in glucose-deprived 3T3-L1 adipocytes

Glucose deprivation increases the rate of glucose transport in 3T3-L1 adipocytes in a protein synthesis-dependent fashion. To determine if translocation of either GLUT1 or GLUT4 is responsible for this phenomenon, we adapted existing fractionation procedures toward isolating 3T3-L1 adipocyte membranes. By Western blot analysis of equal protein, GLUT1 was distributed between plasma membranes, high density "microsomal" membranes, and low density "microsomal" membranes isolated from control cells. GLUT4 comigrated with high density and low density membranes. Glucose deprivation for 12 h did not alter the distribution of either GLUT1 or GLUT4, despite an 8-10-fold increase in glucose transport activity in intact cells. Importantly, increased transport activity was retained in plasma membrane vesicles isolated from glucose-deprived cells. These data show for the first time that the increase in transport activity associated with glucose deprivation does not result from the translocation of either of the glucose transporters known to exist in 3T3-L1 adipocytes. As GLUT4 is excluded from the plasma membrane, these data provide evidence for activation of GLUT1.

C-terminal mutations that alter the turnover number for 3-O-methylglucose transport by GLUT1 and GLUT4

Turnover numbers for 3-O-methylglucose transport by the homologous glucose transporters GLUT1 and GLUT4 were compared to those for truncated and chimeric transporters expressed in Xenopus oocytes to assess potential regulatory properties of the C-terminal domain. The ability of high intracellular sugar concentrations to increase the turnover number for sugar entry ("accelerated exchange") by GLUT1 and not by GLUT4 was maintained in oocytes. Replacing the GLUT1 C terminus with that of GLUT4 stimulated turnover 1.6-fold, but abolished accelerated exchange. Thus, the GLUT1 C terminus permits accelerated exchange by GLUT1, but in doing so must interact with other GLUT1 specific sequences since the GLUT4ctrm1 chimera did not exhibit this kinetic property. Removal of 38 C-terminal amino acids from GLUT4 reduced its turnover number by 40%, whereas removing only 20 residues or replacing its C terminus with that of GLUT1 increased its turnover number 3.5-3.9 fold. Therefore, using mechanisms independent of those which alter transporter targeting to the plasma membrane, C-terminal mutations in either GLUT1 or GLUT4 can activate transport normally restricted by the native C-terminal domain. These results implicate the C termini as targets of physiological factors, which through covalent modification or direct binding might alter C-terminal interactions to regulate intrinsic GLUT1 and GLUT4 transporter activity.

Glucose transporter gene expression: regulation of transcription and mRNA stability

The facilitated diffusion of D-glucose across the plasma membrane is carried out by a set of stereospecific transport proteins known as the glucose transporters. These integral membrane proteins are members of a gene family where tissue-specific expression of one or more members will determine in part the net rate of glucose entry into the cell. The regulation of glucose transporter gene expression is a critical feature of cellular homeostasis, as defects in specific transporter expression can lead to profound alterations in cellular physiology. In this review, we provide a brief descriptive background on the family of glucose transporters and examine in depth the regulation of the two transporters expressed in adipose tissue, GLUTI, a basal growth-related transporter and GLUT4, the insulin-responsive glucose transporter.

Nutrient control of GLUT1 processing and turnover in 3T3-L1 adipocytes

Metabolic labeling and immunoprecipitation were used to analyze the glucose-dependent regulation of GLUT1 synthesis, processing, and turnover in a murine adipocyte cell line. Metabolically labeled GLUT1 from control cells migrated as a 46-kDa protein, while GLUT1 from cells deprived of glucose for more than 12 h migrated as a 37-kDa protein. On the basis of tunicamycin sensitivity, both GLUT1 species arose from a common protein migrating at 36 kDa. In addition, the rate of synthesis of GLUT1 in control and glucose-deprived cells was similar. In short pulse-chase experiments, we distinguished two species arising from the core GLUT1 protein in control cells; an intermediate and the mature 46-kDa species. In contrast, only one glycoform, the 37-kDa species, arose from the core protein in glucose-deprived cells, which was not further processed in either the presence or absence of glucose. Although 12-18 h of glucose deprivation were required to affect GLUT1 glycosylation, glucose-deprived cells quickly recovered the ability to correctly glycosylate GLUT1 upon the readdition of glucose (t1/2 < 1 h). GLUT1 in control adipocytes exhibited a half-life of approximately 14 h, while that in glucose-deprived adipocytes was greater than 50 h. This effect was readily reversed upon the readdition of glucose. In total, these data show that glucose deprivation alters both the processing (glycosylation) and turnover (degradation) of GLUT1. These results are discussed in light of transport function.

Use of hexose transport mutants to examine the expression and properties of the rat myoblast GLUT 1 transport process

Rat L6 myoblasts were recently shown to possess the GLUT 1, 3 and 4 transporters, and not the GLUT 2 isoform [1]. This investigation examined the expression and properties of the GLUT 1 isoform. GLUT 1 transcript level was significantly reduced in cells grown at high densities and during myogenic differentiation. A comparison of the GLUT 1 and 4 transcript levels in myogenesis-competent and impaired cells revealed an inverse relationship between these two isoforms. This relationship was confirmed by studies using two independent spontaneous GLUT 3- GLUT 4- mutants, M1 and M3. These mutants possessed very high level of the GLUT 1 isoform, but negligible amount of the GLUT 3 and 4 isoforms. GLUT 1 expression was also subject to positive regulation. Glucose starvation was found to increase not only the levels of the GLUT 1 transcript and transporter, but also the intrinsic activity of the GLUT 1 transporter. Studies with M1 and M3 mutants revealed that the GLUT 1 transporter was not functional in glucose-grown cells, even though it was present at a very high level in the plasma membrane. This transporter became functional when cells were starved for glucose. The functional GLUT 1 transporter had an apparent Km value of around 0.9 mM, and was sensitive to cytochalasin B, phloretin, phlorizin and pCMBS.

Cloning of glucose transporter-3 (GLUT3) cDNA from rat brain

Using PCR, glucose transporter-3 (GLUT3) was cloned in its entirety from rat brain. The similarity between human, murine and rodent glucose transporter-3 proteins exceeds 90%. Such extensive sequence conservation suggests that essentially all regions of the protein, including the transmembrane domains, are functionally important. This clone can be used to study GLUT3 expression in the rat by Northern analysis or in situ hybridization, or the sequence can be used to generate peptide antibodies. Finally, the availability of a probe for glucose transporter-3 in the rat allows studies of the regulation of GLUT3 gene expression in neurons under a variety of physiological and pathophysiological conditions.

Difference in allelic expression of genes probably associated with tumor progression in murine fibrosarcomas and cell lines

Allelic expression was examined by single-strand conformation polymorphism analysis in murine fibrosarcomas from inter-subspecific F1 mice between C57BL/6 and MSM. Ten genes encoding p53, mdm2, E-cadherin, 72 kD metalloproteinase and its inhibitor (Timp2), thymidine kinase and four glucose transporters (Gluts) were examined. These genes were chosen because of their probable association with tumor development and progression. In some of the tumors and cell lines, p53, E-cadherin and Glut3 genes showed remarkable differences in allelic expression, one allele being poorly expressed. The allele-specificity persisted in nine cell lines obtained by repeated transplantations from one tumor. These results suggest that expression of some genes is allele-specific in tumor cells and the pattern of specificity is stable. Such a decrease or a loss of expression in one of the alleles may be functionally equivalent to the loss of heterozygosity of the gene, and therefore this may confer malignant properties on tumor cells. It is also suggested that differential expression of two alleles is a common event in tumor cells.

Apoptosis is regulated by the rate of glucose transport in an interleukin 3 dependent cell line

In the absence of a survival stimulus, the interleukin 3 (IL-3)-dependent IC.DP cell line undergoes a process termed programmed cell death or apoptosis. Survival can be induced by IL-3, which can also stimulate proliferation of IC.DP cells. IC.DP cells have been stably transfected with the p160v-abl protein tyrosine kinase, activation of the kinase at the permissive temperature permits cell survival in the absence of IL-3 by suppression of apoptosis, although the growth factor is still required for proliferation. Both IL-3 and activation of the v-ABL tyrosine kinase stimulated glucose transport, which may in part be due to a translocation of transporters to the cell surface. Inhibition of glucose uptake markedly increased the rate of apoptosis in these cells, an effect that could be reversed by the provision of alternative energy sources such as glutamine. Growth factor- or oncogene-mediated increases in glucose uptake may therefore represent an important regulatory point in the suppression of apoptosis.

Pancreatic beta cells cultured from individual preneoplastic foci in a multistage tumorigenesis pathway: a potentially general technique for isolating physiologically representative cell lines

Culturing and comparing the discrete stages of tumorigenesis provide a route to defining important components of the cancer phenotype and, in addition, present the opportunity to establish cell cultures more representative of normal cells than the ultimate malignant cancer cells. Herein we report that preneoplastic foci in one multistep tumorigenesis pathway can be cultured in vitro and show that they preserve distinctive characteristics of the normal cells from which they arose, pancreatic beta cells. In the RIP1-Tag2 line of transgenic mice, which express the simian virus 40 T antigen in insulin-producing beta cells, pancreatic islets develop into vascularized tumors in a multistage pathway. We established conditions for reproducible derivation of beta-cell lines from individual hyperplastic islets that have not yet developed into solid tumors. Most of these cell lines, designated beta HC, release insulin at physiological concentrations of glucose. In contrast to tumor-derived lines (beta TC), which are not properly regulated, the ability of the beta HC lines to respond correctly to glucose correlated with maintenance of normally depressed levels of low-Km hexokinases. Glutamic acid decarboxylase (GAD), an early autoantigen in type I diabetes, was detected in most of the beta HC lines. The relative levels of the two forms of this enzyme (GAD65 and GAD67) varied significantly between the different cell lines, suggesting independent regulation. Class I major histocompatibility complex antigens were detected on the beta HC cells, and the levels of surface major histocompatibility complex expression correlated with their capacity to serve as targets in a cytotoxic T-cell killing assay. The beta HC lines will be of value for studies of beta-cell physiology, autoantigenicity, and tumor development. This work suggests the possibility of culturing preneoplastic stages of other cancers, both to address the mechanisms of transformation and to provide a source of cells that maintain important qualities of their normal progenitors.

Pancreatic beta cells cultured from individual preneoplastic foci in a multistage tumorigenesis pathway: a potentially general technique for isolating physiologically representative cell lines

Culturing and comparing the discrete stages of tumorigenesis provide a route to defining important components of the cancer phenotype and, in addition, present the opportunity to establish cell cultures more representative of normal cells than the ultimate malignant cancer cells. Herein we report that preneoplastic foci in one multistep tumorigenesis pathway can be cultured in vitro and show that they preserve distinctive characteristics of the normal cells from which they arose, pancreatic beta cells. In the RIP1-Tag2 line of transgenic mice, which express the simian virus 40 T antigen in insulin-producing beta cells, pancreatic islets develop into vascularized tumors in a multistage pathway. We established conditions for reproducible derivation of beta-cell lines from individual hyperplastic islets that have not yet developed into solid tumors. Most of these cell lines, designated beta HC, release insulin at physiological concentrations of glucose. In contrast to tumor-derived lines (beta TC), which are not properly regulated, the ability of the beta HC lines to respond correctly to glucose correlated with maintenance of normally depressed levels of low-Km hexokinases. Glutamic acid decarboxylase (GAD), an early autoantigen in type I diabetes, was detected in most of the beta HC lines. The relative levels of the two forms of this enzyme (GAD65 and GAD67) varied significantly between the different cell lines, suggesting independent regulation. Class I major histocompatibility complex antigens were detected on the beta HC cells, and the levels of surface major histocompatibility complex expression correlated with their capacity to serve as targets in a cytotoxic T-cell killing assay. The beta HC lines will be of value for studies of beta-cell physiology, autoantigenicity, and tumor development. This work suggests the possibility of culturing preneoplastic stages of other cancers, both to address the mechanisms of transformation and to provide a source of cells that maintain important qualities of their normal progenitors.

Antidiabetic agent pioglitazone enhances adipocyte differentiation of 3T3-F442A cells

Adipocytes play an important role in normal physiology as a major site for systemic energy homeostasis. In disorders such as diabetes, adipocyte function is markedly altered. In this study, we investigated the effect of pioglitazone, a novel antidiabetic agent known to lower plasma glucose in animal models of diabetes mellitus, on cellular differentiation and expression of adipose-specific genes. Treatment of confluent 3T3-F442A preadipocyte cultures for 7 days with pioglitazone (Pio; 1 microM) and insulin (Ins; 0.17 microM) resulted in > 95% cell differentiation into lipid-accumulating adipocytes in comparison with 60-80% cell differentiation by treatment with either agent alone. Analysis of triglyceride accumulation showed increases of triglyceride content over time above untreated preadipocytes by treatment of the cells with Ins, Pio, and especially with Ins + Pio. Basal glucose transport, as measured by cellular uptake of 2-deoxy-D-[14C]glucose, was likewise enhanced in a time-dependent manner by treatment of preadipocytes with Ins, Pio, or Ins + Pio, such that a synergistic effect resulted from the combined treatment with both agents. It was further determined that RNA transcript abundance for genes encoding glucose transporters GLUT-1 and GLUT-4, as well as the adipose-specific genes encoding adipsin and aP2, were increased by the Ins, Pio, or Ins + Pio treatment. Taken together, these findings indicate that pioglitazone is a potent adipogenic agent. By promoting differentiation, this agent may move cells into a state active for glucose uptake, storage, and metabolism.

Glucose transporter gene expression in rat conceptus during early organogenesis and exposure to insulin-induced hypoglycemic serum

We investigated the glucose transporter gene and protein expression during early organogenesis in the rat and in rat embryos cultured with hypoglycemic serum. Erythrocyte-type glucose transporter (GLUT-1) mRNA was expressed at a high level in embryos; peak levels were reached at days 10.5-11.5 and decreased as gestational age increased. In contrast, the insulin regulatable glucose transporter (GLUT-4) mRNA was not detected. The levels of GLUT-1 protein determined by Western blot analysis increased in parallel with expression of the glucose transporter (GLUT-1) gene and peak levels were observed on days 10.5 and 11.5, which correspond to the main periods of neural tube formation. Immunohistochemical staining of the embryo on day 10.5 showed that GLUT-1 protein was abundantly located in the tissue of neural tube. When embryos were cultured from day 9.5 to day 10.5 with insulin-induced hypoglycemic serum containing 2-3 mM glucose an increased frequency of anterior neural tube defects was observed in association with a significant reduction of the glycolytic flux. Increased levels of GLUT-1 mRNA and protein were not observed during the culture with hypoglycemic serum compared with the levels in embryos cultured in normal serum. Addition of insulin to normal serum (500 microU/ml) did not affect the GLUT-1 mRNA and protein levels. GLUT-1 mRNA and protein are strongly expressed in the embryo during early organogenesis, especially in the tissues of the neural tube, and the expression of the glucose transporter did not increase in response to prolonged glycopenia. This may account for the vulnerability of embryogenesis to hypoglycemia during these critical developmental periods.

Expression of Na(+)-coupled sugar transport in HT-29 cells: modulation by glucose

The human colon carcinoma cell line HT-29 adapted to grow in absence of glucose exhibits a typical enterocytic differentiation. In contrast, cells grown in glucose always remain undifferentiated. To investigate whether differentiated HT-29 cells express a Na(+)-dependent sugar transporter, isotopic tracer flux measurements of a non-metabolizable sugar analogue methyl alpha-D-glucoside (AMG) were undertaken. AMG accumulation in confluent monolayer of differentiated HT-29 cells was inhibited by replacement of sodium, phlorizin, phloretin, and glucose. Kinetic studies demonstrate the presence of only one Na(+)-dependent phlorizin-sensitive sugar transporter in differentiated HT-29 cells. Undifferentiated HT-29 cells cultured in the presence of glucose did not show a Na(+)-dependent AMG accumulation. As previously demonstrated for other markers of the enterocytic differentiation, this transporter has a growth-related expression. Moreover, it shares similar properties with the Na(+)-dependent glucose transport in the human fetal small intestine and colon. To demonstrate that the expression of the Na(+)-dependent sugar cotransporter can be modulated by glucose, differentiated HT-29 cells grown in glucose-free medium were switched to 25 mM glucose. In that condition the Na(+)-dependent AMG uptake was almost abolished. However, when these cells were switched back to glucose-free medium, the Na(+)-dependent AMG uptake was restored, although at a lower level. These experiments show that differentiated HT-29 cells are a good cellular model to study the regulation of the Na(+)-dependent sugar transporter.