Role of the GLUT 2 glucose transporter in the response of the L-type pyruvate kinase gene to glucose in liver-derived cells

The effect of carbohydrates with different levels of digestibility on energy metabolism in vivo under hypobaric hypoxic conditions

Current strategies for improving energy supply in hypobaric hypoxic environments are limited. Therefore, this study investigates the effects of four carbohydrates with different levels of digestibility on energy metabolism in vivo in hypobaric hypoxic environments. First, we characterized the four types of carbohydrates. Subsequently, reverse transcription quantitative polymerase chain reaction (RT-qPCR) was used to characterize the expression of GLUT1, GLUT2, and SGLT1 in the glucose transport pathway in vivo. In addition, the effects of different levels of carbohydrate digestibility on energy expenditure were evaluated in vivo. The results showed that pre-gelatinized corn starch significantly increased GLUT1 gene expression in the hypobaric hypoxic conditions (1.58 times, compared to normobaric normoxic). In addition, pre-gelatinized corn starch increased energy expenditure in the hypobaric hypoxic conditions and performed better in terms of glycogen accumulation and glucose transport. Therefore, pre-gelatinized corn starch administration may be a promising strategy for long-term energy supplementation in hypobaric hypoxic.

Glucose transporter GLUT1 expression is important for oriental river prawn (Macrobrachium nipponense) hemocyte adaptation to hypoxic conditions

Crustaceans have an open vascular system in which hemocytes freely circulate in hemolymph. Hemocytes are rich in hemocyanin, a specific oxygen-transport protein in crustaceans; therefore, understanding the response of hemocytes to hypoxia is crucial. Although hemocytes take up glucose during hypoxia, the molecular mechanism of glucose uptake in crustaceans remains unclear. Herein, we identified two highly conserved glucose transporters (GLUT1 and GLUT2) in Macrobrachium nipponense (oriental river prawn) and analyzed their tissue-specific expression patterns. Our immunofluorescence assays showed that GLUT1 and GLUT2 are located on the cell membrane, with a strong GLUT1 signal in primary hemocytes under hypoxia. We found that during acute hypoxia, hypoxia-inducible factor-1α-related metabolic alterations result in decreased mitochondrial cytochrome c oxidase activity, implying a classic glycolytic mechanism. As a proof of concept, we replicated these findings in insect S2 cells. Acute hypoxia significantly induced hypoxia-inducible factor-1α, GLUT1, and pyruvate dehydrogenase kinase isozyme 1 expression in primary hemocytes, and hypoxia-induced increases in glucose uptake and lactate secretion were observed. GLUT1 knockdown induced intracellular reactive oxygen species generation and apoptosis in vitro and in vivo, resulting in increased prawn mortality and more apoptotic cells in their brains, implying a vital function of GLUT1 in hypoxia adaptation. Taken together, our results suggest a close relationship between hypoxia-mediated glycolysis and GLUT1 in hemocytes. These results demonstrated that in crustaceans, adaptation to hypoxia involves glucose metabolic plasticity.

Dietary fructose and high salt in young male Sprague Dawley rats induces salt-sensitive changes in renal function in later life

Dietary fructose and salt are associated with hypertension and renal disease. Dietary input during critical postnatal periods may impact pathophysiology in maturity. The highest consumption of fructose occurs during adolescence. We hypothesized that a diet high in fructose with or without high salt in young male Sprague Dawley rats will lead to salt-sensitive hypertension, albuminuria, and decreased renal function in maturity. Four groups were studied from age 5 weeks: 20% glucose + 0.4% salt (GCS-GCS) or 20% fructose + 4% salt throughout (FHS-FHS). Two groups received 20% fructose + 0.4% salt or 20% fructose + 4% salt for 3 weeks (Phase I) followed by 20% glucose + 0.4% salt (Phase II). In Phase III (age 13-15 weeks), these two groups were challenged with 20% glucose + 4% salt, (FCS-GHS) and (FHS-GHS), respectively. Each group fed fructose in Phase I exhibited significantly higher MAP than GCS-GCS in Phase III. Net sodium balance, unadjusted, or adjusted for caloric intake and urine flow rate, and cumulative sodium balance were positive in FHS during Phase I and were significantly higher in FCS-GHS, FHS-GHS, and FHS-FHS vs GCS-GCS during Phase III. All three groups fed fructose during Phase I displayed significantly elevated albuminuria. GFR was significantly lower in FHS-FHS vs GCS-GCS at maturity. Qualitative histology showed mesangial expansion and hypercellularity in FHS-FHS rats. Thus, fructose ingestion during a critical period in rats, analogous to human preadolescence and adolescence, results in salt-sensitive hypertension and albuminuria in maturity. Prolonged dietary fructose and salt ingestion lead to a decline in renal function with evidence suggestive of mesangial hypercellularity.

Silencing of CD133 inhibits GLUT1-mediated glucose transport through downregulation of the HER3/Akt/mTOR pathway in colon cancer

Cluster of differentiation 133 (CD133) is a transmembrane glycoprotein that has been reported as a marker of cancer stem cells or cancer-initiating cells in various cancers. However, its contribution to tumorigenesis and differentiation remains to be elucidated. To determine the role of CD133 in colon cancer, we silenced CD133 in human colon cancer cells. Silencing of CD133 results in decreased cell proliferation, survival, migration, invasion, and glucose transport. These effects are mediated by downregulation of the human epidermal growth factor receptor 3 (HER3)/Akt/mTOR signaling pathway, culminating in reduced expression of the glucose transporter GLUT1. We also confirm that the cellular phenotypes of CD133-silenced cells are mediated by GLUT1 downregulation. We conclude that CD133 is a potential tumor initiator that positively regulates GLUT1 expression through modulation of HER3/Akt/mTOR signaling.

High Basolateral Glucose Increases Sodium-Glucose Cotransporter 2 and Reduces Sirtuin-1 in Renal Tubules through Glucose Transporter-2 Detection

Under diabetic conditions, sodium-glucose cotransporter 2 (SGLT2) for glucose uptake in proximal tubules (PTs) increases, whereas NAD⁺-dependent protein deacetylase silent mating type information regulation 2 homolog 1 (Sirtuin-1; SIRT1) for PT survival decreases. Therefore, we hypothesized that increased glucose influx by SGLT2 reduces SIRT1 expression. To test this hypothesis, db/db mice with diabetes and high-glucose (HG)-cultured porcine PT LLC-PK1 cells in a two-chamber system were treated with the SGLT2 inhibitor canagliflozin. We also examined SIRT1 and SGLT2 expression in human kidney biopsies. In db/db mice, SGLT2 expression increased with concomitant decreases in SIRT1, but was inhibited by canagliflozin. For determination of the polarity of SGLT2 and SIRT1 expression, LLC-PK1 cells were seeded into Transwell chambers (pore size, 0.4 µm; Becton Dickinson, Oxford, UK). HG medium was added to either or to both of the upper and lower chambers, which corresponded to the apical and basolateral sides of the cells, respectively. In this system, the lower chamber with HG showed increased SGLT2 and decreased SIRT1 expression. Canagliflozin reversed HG-induced SIRT1 downregulation. Gene silencing and inhibitors for glucose transporter 2 (GLUT2) blocked HG-induced SGLT2 expression upregulation. Gene silencing for the hepatic nuclear factor-1α (HNF-1α), whose nuclear translocation was enhanced by HG, blocked HG-induced SGLT2 expression upregulation. Similarly, gene silencing for importin-α1, a chaperone protein bound to GLUT2, blocked HG-induced HNF-1α nuclear translocation and SGLT2 expression upregulation. In human kidney, SIRT1 immunostaining was negatively correlated with SGLT2 immunostaining. Thus, under diabetic conditions, SIRT1 expression in PTs was downregulated by an increase in SGLT2 expression, which was stimulated by basolateral HG through activation of the GLUT2/importin-α1/HNF-1α pathway.

4-Methylcatechol stimulates apoptosis and reduces insulin secretion by decreasing betacellulin and inhibin beta-A in INS-1 beta-cells

Insulinoma INS-1 cell line is a pancreatic beta cell tumor which is characterized with high insulin content and secretion in response to increasing glucose levels. 4-Methylcatechol (4-MC) is a metabolite of quercetin, which is known as a potential drug for inhibition of tumorigenesis. The aim of this study was to determine the applying doses of 4-methylcatechol (4-MC) for triggening cell death and decreasing the cell function of rat insulinoma INS-1 beta cells. The rate of apoptosis and the amount of insulin in the cell and the secretions were determined by the ELISA method. Betacellulin (BTC) and inhibin beta-A amounts in both the cell and the glucose induced secretion were investigated by Western blotting. Furthermore, BTC, Inhibin beta-A, Ins1, Ins2, and GLUT2 gene expression levels were determined by the by the real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR) method. We noted a significant decrease in cell viability, while an increase in apoptotic cell death by 4-MC treatment. It caused a decrease in the secretion of BTC, expressions of both BTC and inhibin beta-A. We showed a decrease in the expressions of Ins1 and GLUT2, while there is no alteration in the level of insulin protein. Insulin secretion levels increased in INS-1 cells given 4-MC by basal glucose concentration while they did not response to high concentration of glucose, which indicates that 4-MC disrupts the functionality of INS-1 cells. These results revealed that 4-MC induces apoptosis and decreases insulin secretion by reducing BTC and inhibin beta-A in insulinoma INS-1 cells. Thus, 4-MC may be offered as a potential molecule for treatment of insulinoma.

Cooperation of Secondary Transporters and Sensor Kinases in Transmembrane Signalling: The DctA/DcuS and DcuB/DcuS Sensor Complexes of Escherichia coli

Many membrane-bound sensor kinases require accessory proteins for function. The review describes functional control of membrane-bound sensors by transporters. The C4-dicarboxylate sensor kinase DcuS requires the aerobic or anaerobic C4-dicarboxylate transporters DctA or DcuB, respectively, for function and forms DctA/DcuS or DcuB/DcuS sensor complexes. Free DcuS is in the permanent (ligand independent) ON state. The DctA/DcuS and DcuB/DcuS complexes, on the other hand, control expression in response to C4-dicarboxylates. In DctA/DcuS, helix 8b of DctA and the PASC domain of DcuS are involved in interaction. The stimulus is perceived by the extracytoplasmic sensor domain (PASP) of DcuS. The signal is transmitted across the membrane by a piston-type movement of TM2 of DcuS which appears to be pulled (by analogy to the homologous citrate sensor CitA) by compaction of PASP after C4-dicarboxylate binding. In the cytoplasm, the signal is perceived by the PASC domain of DcuS. PASC inhibits together with DctA the kinase domain of DcuS which is released after C4-dicarboxylate binding. DcuS exhibits two modes for regulating expression of target genes. At higher C4-dicarboxylate levels, DcuS is part of the DctA/DcuS complex and in the C4-dicarboxylate-responsive form which stimulates expression of target genes in response to the concentration of the C4-dicarboxylates (catabolic use of C4-dicarboxylates, mode I regulation). At limiting C4-dicarboxylate concentrations (≤0.05mM), expression of DctA drops and free DcuS appears. Free DcuS is in the permanent ON state (mode II regulation) and stimulates low level (C4-dicarboxylate independent) DctA synthesis for DctA/DcuS complex formation and anabolic C4-dicarboxylate uptake.

Relative contributions of L-FABP, SCP-2/SCP-x, or both to hepatic biliary phenotype of female mice

Both sterol carrier protein-2/sterol carrier protein-x (SCP-2/SCP-x) and liver fatty acid binding protein (L-FABP) have been proposed to function in hepatobiliary bile acid metabolism/accumulation. To begin to address this issue, the impact of ablating L-FABP (LKO) or SCP-2/SCP-x (DKO) individually or both together (TKO) was examined in female mice. Biliary bile acid levels were decreased in LKO, DKO, and TKO mice; however, hepatic bile acid concentration was decreased in LKO mice only. In contrast, biliary phospholipid level was decreased only in TKO mice, while biliary cholesterol levels were unaltered regardless of phenotype. The loss of either or both genes increased hepatic expression of the major bile acid synthetic enzymes (CYP7A1 and/or CYP27A1). Loss of L-FABP and/or SCP-2/SCP-x genes significantly altered the molecular composition of biliary bile acids, but not the proportion of conjugated/unconjugated bile acids or overall bile acid hydrophobicity index. These data suggested that L-FABP was more important in hepatic retention of bile acids, while SCP-2/SCP-x more broadly affected biliary bile acid and phospholipid levels.

Effect of reduced heifer nutrition during in utero and post-weaning development on glucose and acetate kinetics

Energetic efficiency was evaluated in composite bred heifers born from dams receiving 1·8 or 1·2 kg/d winter supplementation for approximately 80 d before parturition. Heifers were then developed post-weaning and randomly assigned to heifer development treatments of either control (100 %; ad libitum; n 8/year) or restricted (80 %; fed 80 % of supplementation fed to controls adjusted to a common body weight: n 8/year) in a 2-year study. A glucose tolerance test (GTT) and acetate irreversible loss test (AILT) were administered to heifers at the termination of a 140 d development period when the heifers were approximately 403 d of age and consumed a silage-based diet, and again at 940 d of age when pregnant with their second calf and grazing dormant forage. No differences were measured (P>0·08) for dam winter nutrition or heifer development treatment for baseline serum metabolites or measures in either the GTT or the AILT. However, changes in baseline serum concentrations (P>0·05) were different between metabolic challenges, which occurred at different stages of development. No difference in acetate disappearance (P = 0·18) and half-life (P = 0·66) was measured between the two metabolic challenges. A trend for glucose half-life to be shorter in heifers born from dams receiving in utero winter treatments that supplied 1·2 kg/d of winter supplementation was observed (P = 0·083). Heifers developed with lower total DM intake during a 140 d development period had similar glucose and acetate incorporation rates as ad libitum-fed heifers when evaluated at two different production stages.

Exendin-4 regulates GLUT2 expression via the CaMKK/CaMKIV pathway in a pancreatic β-cell line

The GLUT2 glucose transporter plays an important role in glucose-induced insulin secretion in pancreatic β-cells by catalyzing the uptake of glucose into the cell. In this study, we investigated whether exendin-4, a long-acting agonist of glucagon-like peptide-1, mediates stimulatory effects on GLUT2 gene expression through the Ca²+/calmodulin (CaM)-dependent protein kinase IV (CaMKIV) cascade. GLUT2 expression was examined by real-time polymerase chain reaction, Western blot analysis, and a reporter gene assay in rat insulin-secreting INS-1 cells incubated with exendin-4. An increased expression level of GLUT2 protein was noted in response to increasing concentrations of exendin-4, with maximal induction at 10 nmol/L. Real-time polymerase chain reaction analysis similarly revealed a significant increase in the amount of GLUT2 messenger RNA by 10 nmol/L exendin-4. Exendin-4 also stimulated GLUT2 promoter activity in response to increasing exendin-4 concentrations, but failed to do so in the presence of STO-609, a CaMKK inhibitor. We also investigated the effect of the constitutively active form of CaMKK (CaMKKc) on GLUT2 promoter activity. The result is consistent with the observations that CaMKKc/CaMKIV enhanced or up-regulated GLUT2 promoter activity in INS-1 cells. Furthermore, exendin-4 induction of GLUT2 protein expression was significantly suppressed in the cells knocking down the CaMKIV. In summary, activation of the CaMKK/CaMKIV cascade might be required for exendin-4-induced GLUT2 gene transcription, indicating that exendin-4 plays an important role in insulin secretion in pancreatic β-cells.

GLUT2 mutations, translocation, and receptor function in diet sugar managing

Cloned 20 years ago, GLUT2 is a facilitative glucose transporter in the liver, pancreas, intestine, kidney, and brain. It ensures large bidirectional fluxes of glucose in and out the cell due to its low affinity and high capacity. It also transports other dietary sugars, such as fructose and galactose, within the range of physiological concentrations. Sugars and hormones regulate its gene expression. The contribution of GLUT2 to human metabolic diseases previously appeared modest. However, in the past decade, three major features of the GLUT2 protein have been revealed. First, GLUT2 mutations cause the severe but rare Fanconi-Bickel syndrome, mainly characterized by glycogenosis. Recently, a GLUT2 polymorphism has been associated with preferences for sugary food. Second, the GLUT2 location at the cell surface is regulated; this governs cellular activities dependent on glucose in the intestine and possibly those in the liver and pancreas. For instance, GLUT2 translocation from an intracellular pool to the apical membrane after a sugar meal transiently increases sugar uptake by enterocytes (reviewed in 32). Third, GLUT2 functions as a membrane receptor of sugar. Independently of glucose metabolism, GLUT2 detects the presence of extracellular sugar and transduces a signal to modulate cell functions, including beta-cell insulin secretion, renal reabsorption, and intestinal absorption according to the sugar environment. These recent developments are examined here in heath and metabolic disease, highlighting various unanswered questions.

The early steps of glucose signalling in yeast

In the presence of glucose, yeast undergoes an important remodelling of its metabolism. There are changes in the concentration of intracellular metabolites and in the stability of proteins and mRNAs; modifications occur in the activity of enzymes as well as in the rate of transcription of a large number of genes, some of the genes being induced while others are repressed. Diverse combinations of input signals are required for glucose regulation of gene expression and of other cellular processes. This review focuses on the early elements in glucose signalling and discusses their relevance for the regulation of specific processes. Glucose sensing involves the plasma membrane proteins Snf3, Rgt2 and Gpr1 and the glucose-phosphorylating enzyme Hxk2, as well as other regulatory elements whose functions are still incompletely understood. The similarities and differences in the way in which yeasts and mammalian cells respond to glucose are also examined. It is shown that in Saccharomyces cerevisiae, sensing systems for other nutrients share some of the characteristics of the glucose-sensing pathways.

The role of GLUT2 in dietary sugar handling

GLUT2 is a facilitative glucose transporter located in the plasma membrane of the liver, pancreatic, intestinal, kidney cells as well as in the portal and the hypothalamus areas. Due to its low affinity and high capacity, GLUT2 transports dietary sugars, glucose, fructose and galactose in a large range of physiological concentrations, displaying large bidirectional fluxes in and out the cells. This review focuses on the roles of GLUT2. The first identified function of GLUT2 is its capacity to fuel metabolism and to provide metabolites stimulating the transcription of glucose sensitive genes. Recently, two other functions of GLUT2 are uncovered. First, the insertion of GLUT2 into the apical membrane of enterocytes induces the acute regulation of intestinal sugar absorption after a meal. Second, the GLUT2 protein itself initiates a protein signalling pathway triggering a glucose signal from the plasma membrane to the transcription machinery.

A karyopherin alpha2 nuclear transport pathway is regulated by glucose in hepatic and pancreatic cells

We studied the role of the karyopherin alpha2 nuclear import carrier (also known as importin alpha2) in glucose signaling. In mhAT3F hepatoma cells, GFP-karyopherin alpha2 accumulated massively in the cytoplasm within minutes of glucose extracellular addition and returned to the nucleus after glucose removal. In contrast, GFP-karyopherin alpha1 distribution was unaffected regardless of glucose concentration. Glucose increased GFP-karyopherin alpha2 nuclear efflux by a factor 80 and its shuttling by a factor 4. These glucose-induced movements were not due to glycolytic ATP production. The mechanism involved was leptomycin B-insensitive, but phosphatase- and energy-dependent. HepG2 and COS-7 cells displayed no glucose-induced GFP-karyopherin alpha2 movements. In pancreatic MIN-6 cells, the glucose-induced movements of karyopherin alpha2 and the stimulation of glucose-induced gene transcription were simultaneously lost between passages 28 and 33. Thus, extracellular glucose regulates a nuclear transport pathway by increasing the nuclear efflux and shuttling of karyopherin alpha2 in cells in which glucose can stimulate the transcription of sugar-responsive genes.

Connection of transport and sensing by UhpC, the sensor for external glucose-6-phosphate in Escherichia coli

UhpC is a membrane-bound sensor protein in Escherichia coli required for recognizing external glucose-6-phosphate (Glc6P) and induction of the transport protein UhpT. Recently, it was shown that UhpC is also able to transport Glc6P. In this study we investigated whether these transport and sensing activities are obligatorily coupled in UhpC. We expressed a His-UhpC protein in a UhpC-deficient E. coli strain and verified that this construct does not alter the basic biochemical properties of the Glc6P sensor system. The effects of arginine replacements, mutations of the central loop, and introduction of a salt bridge in UhpC on transport and sensing were compared. The exchanges R46C, R266C and R149C moderately affected transport by UhpC but strongly decreased the sensing ability. This suggested that the affinity for Glc6P as a transported substrate is uncoupled in UhpC from its affinity for Glc6P as an inducer. Four of the 11 arginine mutants showed a constitutive phenotype but had near wild-type transport activity suggesting that Glc6P can be transported by a molecule locked in the inducing conformation. Introduction of an intrahelical salt bridge increased the transport activity of UhpC but abolished sensing. Three conserved residues from the central loop were mutated and although none of these showed transport, one exhibited increased affinity for sensing. Taken together, these data show that transport by UhpC is not required for sensing, that conserved arginine residues are important for sensing and not for transport, and that residues located in the central hydrophilic loop are critical for transport and for sensing.

Karyopherin alpha2: a control step of glucose-sensitive gene expression in hepatic cells

Glucose is required for an efficient expression of the glucose transporter GLUT2 and other genes. We have shown previously that the intracytoplasmic loop of GLUT2 can divert a signal, resulting in the stimulation of glucose-sensitive gene transcription. In the present study, by interaction with the GLUT2 loop, we have cloned the rat karyopherin alpha2, a receptor involved in nuclear import. The specificity of the binding was restricted to GLUT2, and not GLUT1 or GLUT4, and to karyopherin alpha2, not alpha1. When rendered irreversible by a cross-linking agent, this transitory interaction was detected in vivo in hepatocytes. A role for karyopherin alpha2 in the transcription of two glucose-sensitive genes was investigated by transfection of native and inactive green fluorescent protein-karyopherin alpha2 in GLUT2-expressing hepatoma cells. The amount of inactive karyopherin alpha2 receptor reduced, in a dose-dependent manner, the GLUT2 and liver pyruvate kinase mRNA levels by competition with endogenous active receptor. In contrast, the overexpression of karyopherin alpha2 did not significantly stimulate GLUT2 and liver pyruvate kinase mRNA accumulation in green fluorescent protein-sorted cells. The present study suggests that, in concert with glucose metabolism, karyopherin alpha2 transmits a signal to the nucleus to regulate glucose-sensitive gene expression. The transitory tethering of karyopherin alpha2 to GLUT2 at the plasma membrane might indicate that the receptor can load the cargo to be imported locally.

Monosaccharide/proton symporter AtSTP1 plays a major role in uptake and response of Arabidopsis seeds and seedlings to sugars

The aim of this study was to investigate the in vivo properties and function of the high-affinity monosaccharide/proton symporter AtSTP1 of Arabidopsis. We isolated an Atstp1 knock-out mutant and found that this plant grows and develops normally. The AtSTP1 gene is expressed in germinating seeds and seedlings, with AtSTP1 activity found mainly in the seedling root. The rate of uptake of [(14)C]-3-O-methylglucose and [(14)C]-D-glucose is 60% less in Atstp1 seedlings than in the wild type, showing that AtSTP1 is the major monosaccharide transporter in Arabidopsis seedlings. Transport of D-galactose and D-mannose is also up to 60% less in Atstp1 seedlings compared to wild type, but transport of D-fructose, L-arabinose and sucrose is not reduced. Germination of Atstp1 seed shows reduced sensitivity to D-mannose, demonstrating that AtSTP1 is active before germination. Atstp1 seedlings grow effectively on concentrations of D-galactose that inhibit wild-type growth, even at up to 100 mM D-galactose, indicating that active transport by AtSTP1 plays a major role at very high concentrations of exogenous sugar. These findings provide insight into the physiological function of AtSTP1 and clearly establish its importance in the uptake of extracellular sugars by the embryo and in seedlings.

Transgenic reexpression of GLUT1 or GLUT2 in pancreatic beta cells rescues GLUT2-null mice from early death and restores normal glucose-stimulated insulin secretion

GLUT2-null mice are hyperglycemic, hypoinsulinemic, hyperglucagonemic, and glycosuric and die within the first 3 weeks of life. Their endocrine pancreas shows a loss of first phase glucose-stimulated insulin secretion (GSIS) and inverse alpha to beta cell ratio. Here we show that reexpression by transgenesis of either GLUT1 or GLUT2 in the pancreatic beta cells of these mice allowed mouse survival and breeding. The rescued mice had normal-fed glycemia but fasted hypoglycemia, glycosuria, and an elevated glucagon to insulin ratio. Glucose tolerance was, however, normal. In vivo, insulin secretion assessed following hyperglycemic clamps was normal. In vitro, islet perifusion studies revealed that first phase of insulin secretion was restored as well by GLUT1 or GLUT2, and this was accompanied by normalization of the glucose utilization rate. The ratio of pancreatic insulin to glucagon and volume densities of alpha to beta cells were, however, not corrected. These data demonstrate that 1) reexpression of GLUT1 or GLUT2 in beta cells is sufficient to rescue GLUT2-null mice from lethality, 2) GLUT1 as well as GLUT2 can restore normal GSIS, 3) restoration of GSIS does not correct the abnormal composition of the endocrine pancreas. Thus, normal GSIS does not depend on transporter affinity but on the rate of uptake at stimulatory glucose concentrations.

Regulation of cyclooxygenase 2 expression in hepatocytes by CCAAT/enhancer-binding proteins

BACKGROUND & AIMS

Expression of cyclooxygenase (COX)-2 in response to lipopolysaccharide (LPS) challenge has been analyzed in cultured fetal, neonatal, and adult hepatocytes and in hepatoma cell lines.

METHODS

To study the mechanisms of LPS-dependent expression of COX-2 in these cells, the activity of the COX-2 promoter and the levels of CCAAT/enhancer-binding proteins (C/EBPs) were determined.

RESULTS

COX-2 was induced in fetal hepatocytes, but this response declined rapidly after birth. This loss of inducibility of COX-2 paralleled the expression of C/EBP-alpha in neonatal hepatocytes. Transfection of fetal and adult hepatocytes with sequences corresponding to the 5'-flanking region of the rat COX-2 gene confirmed the absence of promoter activity in adult hepatocytes. Moreover, transient expression of C/EBP-alpha, but not C/EBP-delta, in the hepatoma cell line AT3F cells abolished the COX-2 promoter activity. Prolonged culture of adult hepatocytes restored the induction of COX-2 after complete disappearance of C/EBP-alpha.

CONCLUSIONS

These results suggest that the presence of high levels of C/EBP-alpha is involved in the impairment of COX-2 expression in adult hepatocytes challenged with proinflammatory stimuli.

Liver hyperplasia and paradoxical regulation of glycogen metabolism and glucose-sensitive gene expression in GLUT2-null hepatocytes. Further evidence for the existence of a membrane-based glucose release pathway

We investigated the impact of GLUT2 gene inactivation on the regulation of hepatic glucose metabolism during the fed to fast transition. In control and GLUT2-null mice, fasting was accompanied by a approximately 10-fold increase in plasma glucagon to insulin ratio, a similar activation of liver glycogen phosphorylase and inhibition of glycogen synthase and the same elevation in phosphoenolpyruvate carboxykinase and glucose-6-phosphatase mRNAs. In GLUT2-null mice, mobilization of glycogen stores was, however, strongly impaired. This was correlated with glucose-6-phosphate (G6P) levels, which remained at the fed values, indicating an important allosteric stimulation of glycogen synthase by G6P. These G6P levels were also accompanied by a paradoxical elevation of the mRNAs for L-pyruvate kinase. Re-expression of GLUT2 in liver corrected the abnormal regulation of glycogen and L-pyruvate kinase gene expression. Interestingly, GLUT2-null livers were hyperplasic, as revealed by a 40% increase in liver mass and 30% increase in liver DNA content. Together, these data indicate that in the absence of GLUT2, the G6P levels cannot decrease during a fasting period. This may be due to neosynthesized glucose entering the cytosol, being unable to diffuse into the extracellular space, and being phosphorylated back to G6P. Because hepatic glucose production is nevertheless quantitatively normal, glucose produced in the endoplasmic reticulum may also be exported out of the cell through an alternative, membrane traffic-based pathway, as previously reported (Guillam, M.-T., Burcelin, R., and Thorens, B. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 12317-12321). Therefore, in fasting, GLUT2 is not required for quantitative normal glucose output but is necessary to equilibrate cytosolic glucose with the extracellular space. In the absence of this equilibration, the control of hepatic glucose metabolism by G6P is dominant over that by plasma hormone concentrations.

The large intracytoplasmic loop of the glucose transporter GLUT2 is involved in glucose signaling in hepatic cells

The hypothesis that the glucose transporter GLUT2 can function as a protein mediating transcriptional glucose signaling was addressed. To divert the putative interacting proteins from a glucose signaling pathway, two intracytoplasmic domains of GLUT2, the C terminus and the large loop located between transmembrane domains 6 and 7, were transfected into mhAT3F hepatoma cells. Glucose-induced accumulation of two hepatic gene mRNAs (GLUT2 and L-pyruvate kinase) was specifically inhibited in cells transfected with the GLUT2 loop and not with the GLUT2 C terminus. The dual effects of glucose were dissociated in cells expressing the GLUT2 loop; in fact a normal glucose metabolism into glycogen occurred concomitantly with the inhibition of the glucose-induced transcription. This inhibition by the GLUT2 loop could be due to competitive binding of a protein that normally interacts with endogenous GLUT2. In addition, the GLUT2 loop, tagged with green fluorescent protein (GFP), was located within the nucleus, whereas the GFP and GFP-GLUT2 C-terminal proteins remained in the cytoplasm. In living cells, a fraction (50%) of the expressed GFP-GLUT2 loop translocated rapidly from the cytoplasm to the nucleus in response to high glucose concentration and conversely in the absence of glucose. We conclude that, via protein interactions with its large loop, GLUT2 may transduce a glucose signal from the plasma membrane to the nucleus.

Immortalized kidney cells derived from transgenic mice harboring L-type pyruvate kinase and vimentin promoters

Targeted oncogenesis in transgenic mice, where an oncogene is placed under the control of the regulatory sequences of a cell-specific gene, has been used to derive several new types of differentiated nonepithelial and epithelial cell lines. This review summarizes the properties of cell lines derived from proximal, distal and collecting duct cells. The cells were obtained from kidneys of transgenic mice harboring the 5' regulatory sequences of the L-type pyruvate kinase or vimentin genes controlling the expression of either the large T and little t antigens or the temperature-sensitive large T antigen.

Regulation of glucose production by the liver

Glucose is an essential nutrient for the human body. It is the major energy source for many cells, which depend on the bloodstream for a steady supply. Blood glucose levels, therefore, are carefully maintained. The liver plays a central role in this process by balancing the uptake and storage of glucose via glycogenesis and the release of glucose via glycogenolysis and gluconeogenesis. The several substrate cycles in the major metabolic pathways of the liver play key roles in the regulation of glucose production. In this review, we focus on the short- and long-term regulation glucose-6-phosphatase and its substrate cycle counter-part, glucokinase. The substrate cycle enzyme glucose-6-phosphatase catalyzes the terminal step in both the gluconeogenic and glycogenolytic pathways and is opposed by the glycolytic enzyme glucokinase. In addition, we include the regulation of GLUT 2, which facilitates the final step in the transport of glucose out of the liver and into the bloodstream.

Dominant-negative suppression of HNF-1alpha function results in defective insulin gene transcription and impaired metabolism-secretion coupling in a pancreatic beta-cell line

Mutations in the hepatocyte nuclear factor-1alpha (HNF-1alpha) have been linked to subtype 3 of maturity-onset diabetes of the young (MODY3), which is characterized by a primary defect in insulin secretion. The role of HNF-1alpha in the regulation of pancreatic beta-cell function was investigated. Gene manipulation allowed graded overexpression of HNF-1alpha and controlled dominant-negative suppression of HNF-1alpha function in insulinoma INS-1 cells. We show that HNF-1alpha is essential for insulin gene transcription, as demonstrated by a pronounced decrease in insulin mRNA expression and in insulin promoter activity under dominant-negative conditions. The expression of genes involved in glucose transport and metabolism including glucose transporter-2 and L-type pyruvate kinase is also regulated by HNF-1alpha. Loss of HNF-1alpha function leads to severe defects in insulin secretory responses to glucose and leucine, resulting from impaired glucose utilization and mitochondrial oxidation. The nutrient-evoked ATP production and subsequent changes in plasma membrane potential and intracellular Ca2+ were diminished by suppression of HNF-1alpha function. These results suggest that HNF-1alpha function is essential for maintaining insulin storage and nutrient-evoked release. The defective mitochondrial oxidation of metabolic substrates causes impaired insulin secretion, indicating a molecular basis for the diabetic phenotype of MODY3 patients.

CCAAT/enhancer binding protein regulates the promoter activity of the rat GLUT2 glucose transporter gene in liver cells

The liver-specific expression of the GLUT2 glucose transporter gene is suppressed in cultured hepatoma cell lines as well as in hepatocytes in primary culture. To understand the underlying mechanism involved in this process, we analysed the rat GLUT2 promoter region. A DNase I footprinting assay with rat liver nuclear extract revealed eight protected regions within a -500 bp region of the GLUT2 promoter (sites A to H). Three of these sites (B, F and H) were occupied by transcription factors that are considerably enriched in liver cells compared with spleen or kidney. The proteins binding to these sites were investigated by a combination of DNase I footprinting assay and electrophoretic mobility-shift assay with the addition of specific oligonucleotide competitors and specific antibody against known transcription factors. As a result it was revealed that hepatocyte nuclear factor 3 binds to site B (-120 to -70), and CCAAT/enhancer binding protein alpha (C/EBPalpha) and C/EBPbeta bind to site F (-375 to -356) and site H (-500 to -471). The binding of C/EBP to sites F and H was markedly decreased within 4 h when liver cells were subjected to primary culture, suggesting that C/EBP might be responsible for the decreased expression of GLUT2 in this process. In contrast, Western blot analysis revealed that C/EBPalpha began to decrease after 1 h of hepatocyte culture, and C/EBPbeta was not changed significantly throughout the culture period, suggesting that C/EBP could be regulated at the transcriptional level as well as the post-translational level when hepatocytes were put in culture. To confirm the role of C/EBP in the regulation of GLUT2 promoter activity, sites F and H were ligated to a chloramphenicol acetyltransferase (CAT) reporter gene and co-transfected with a C/EBP expression vector into HepG2 cells. The co-expression of C/EBPalpha and C/EBPbeta resulted in 9.1-fold and 3. 8-fold increases of CAT activities in the site F-CAT and site H-CAT constructs respectively. These results indicate that C/EBPalpha and C/EBPbeta regulate the promoter activity of the GLUT2 gene and might be responsible for the down-regulation of the GLUT2 gene when hepatocytes are subjected to primary culture.

An unusual high-Km hexokinase is expressed in the mhAT3F hepatoma cell line

In most hepatoma cells, the high-Km GLUT2/glucokinase proteins are replaced by the ubiquitous low-Km GLUT1/hexokinase type I proteins. In the mhAT3F hepatoma cells, the stimulatory effect of glucose on gene expression and glycogen accumulation was not maximal at 5 mmol/liter glucose. This response to high glucose is observed in mhAT3F cells, where GLUT2 was expressed, but not glucokinase (assessed by Northern blotting and reverse transcription-polymerase chain reaction). A low-Km hexokinase activity (19.6 +/- 3.8 milliunits/mg of protein) was present, but a high-Km (40 mmol/liter) hexokinase activity (13.9 +/- 2.5 milliunits/mg) was also detected in mhAT3F cells. The high-Km hexokinase activity was dependent on both ATP (or PPi) and glucose in the assay and was recovered in a 10-50-kDa fraction after filtration. A 30-kDa protein was detected using an anti-glucokinase antibody and localized by confocal microscopy at the same sites as glucokinase in hepatocytes. In FAO cells, the high-Km hexokinase activity and 30-kDa protein were not found. We conclude that a high-Km hexokinase activity is present in mhAT3F cells. This might explain why the effects of glucose on gene expression were not maximal at a glucose concentration of 5 mmol/liter. A 30-kDa protein identified using an anti-glucokinase antibody may be responsible for this activity present in mhAT3F cells.

The 5'-AMP-activated protein kinase inhibits the transcriptional stimulation by glucose in liver cells, acting through the glucose response complex

5-Amino-4-imidazolecarboxamide riboside (AICAR) is known to stimulate rat liver 5'-AMP-activated protein kinase (AMPK). AMPK is the mammalian homologue of Snf1p in yeast, involved in derepression of glucose-repressed genes. We used AICAR to test if AMPK could also play a role in the regulation of glucose-dependent genes in mammalian cells. At a concentration which induces phosphorylation-dependent inactivation of HMG-CoA reductase, AICAR blocked glucose activation of three glucose responsive genes, namely L-type pyruvate kinase (L-PK), Spot 14 and fatty acid synthase genes in primary cultured hepatocytes, but was without any action on glucose phosphorylation to glucose 6-phosphate and on expression of PEPCK, albumin and beta-actin genes. AICAR was also found to inhibit activation of the L-PK gene promoter by glucose in transiently transfected hepatoma cells. Therefore our results suggest that AMPK is probably involved in the glucose signal pathway regulating gene expression in the liver.