Evidence that erythroid-type glucose transporter intrinsic activity is modulated by cadmium treatment of mouse 3T3-L1 cells

The endocrine disruptor cadmium: a new player in the pathophysiology of metabolic diseases

Cadmium (Cd), a highly toxic heavy metal, is found in soil, environment and contaminated water and food. Moreover, Cd is used in various industrial activities, such as electroplating, batteries production, fertilizers, while an important non-occupational source is represented by cigarette smoking, as Cd deposits in tobacco leaves. Since many years it is clear a strong correlation between Cd body accumulation and incidence of many diseases. Indeed, acute exposure to Cd can cause inflammation and affect many organs such as kidneys and liver. Furthermore, the attention has focused on its activity as environmental pollutant and endocrine disruptor able to interfere with metabolic and energy balance of living beings. Both in vitro and in vivo experiments have demonstrated that the Cd-exposure is related to metabolic diseases such as obesity, diabetes and osteoporosis even if human studies are still controversial. Recent data show that Cd-exposure is associated with atherosclerosis, hypertension and endothelial damage that are responsible for cardiovascular diseases. Due to the large environmental diffusion of Cd, in this review, we summarize the current knowledge concerning the role of Cd in the incidence of metabolic and cardiovascular diseases.

Emerging Links between Cadmium Exposure and Insulin Resistance: Human, Animal, and Cell Study Data

Recent research has helped clarify the role of cadmium (Cd) in various pathological states. We have demonstrated Cd involvement in pancreatic cancer, as well as the bioaccumulation of Cd in the pancreas. Bioaccumulation and increased toxicity suggest that Cd may also be involved in other pancreas-mediated diseases, like diabetes. Cd falls into the category of "hyperglycemic" metals, i.e., metals that increase blood glucose levels, which could be due to increased gluconeogenesis, damage to β-cells leading to reduced insulin production, or insulin resistance at target tissue resulting in a lack of glucose uptake. This review addresses the current evidence for the role of Cd, leading to insulin resistance from human, animal, and in vitro studies. Available data have shown that Cd may affect normal insulin function through multiple pathways. There is evidence that Cd exposure results in the perturbation of the enzymes and modulatory proteins involved in insulin signal transduction at the target tissue and mutations of the insulin receptor. Cd, through well-described mechanisms of oxidative stress, inflammation, and mitochondrial damage, may also alter insulin production in β-cells. More work is necessary to elucidate the mechanisms associated with Cd-mediated insulin resistance.

Differences in cytosolic glucose dynamics in astrocytes and adipocytes measured by FRET-based nanosensors

The cellular response to fluctuations in blood glucose levels consists of integrative regulation of cell glucose uptake and glucose utilization in the cytosol, resulting in altered levels of glucose in the cytosol. Cytosolic glucose is difficult to be measured in the intact tissue, however recently methods have become available that allow measurements of glucose in single living cells with fluorescence resonance energy transfer (FRET) based protein sensors. By studying the dynamics of cytosolic glucose levels in different experimental settings, we can gain insights into the properties of plasma membrane permeability to glucose and glucose utilization in the cytosol, and how these processes are modulated by different environmental conditions, agents and enzymes. In this review, we compare the cytosolic regulation of glucose in adipocytes and astrocytes - two important regulators of energy balance and glucose homeostasis in whole body and brain, respectively, with particular emphasis on the data obtained with FRET based protein sensors as well as other biochemical and molecular approaches.

Urinary cadmium concentrations and metabolic syndrome in U.S. adults: The National Health and Nutrition Examination Survey 2001-2014

BACKGROUND

Low to moderate acute cadmium exposure has been associated with increased risk of chronic diseases, such as cardiovascular and kidney disease. Little is known about the association between urinary cadmium levels-an indicator of longer-term exposure-and metabolic syndrome (MetS).

METHODS

We analysed data from 3982 participants aged 20-<80 years of the National Health and Nutrition Examination Survey 2001-2014. Urinary cadmium levels were measured and adjusted for creatinine using spot urine samples. Cadmium levels were evaluated in quintiles (Q). MetS was defined by National Cholesterol Education Program's Adult Treatment Panel III report criteria. Prevalence odds ratios (OR) and 95% confidence intervals (CI) were calculated using multivariable logistic regression accounting for complex survey design, while adjusting for potential confounders and stratifying by sex and smoking status.

RESULTS

In the overall study population, there was a marginal inverse association between urinary cadmium and MetS (adj. OR for Q5 versus Q1: 0.7; 95% CI: 0.5-1.0). Sex stratified models were similar. When examining individual components of MetS, participants with higher levels of urinary cadmium had decreased odds of abdominal obesity (adj. OR for Q5 versus Q1 0.4; 95% CI: 0.3-0.6), but increased odds for low HDL (adj. OR for Q5 versus Q1 2.1; 95% CI: 1.4-3.1). Among current smokers, higher urinary cadmium was associated with increased odds of MetS, hypertension, and low HDL even after accounting for serum cotinine-a marker of smoking intensity.

CONCLUSIONS

Higher levels of urinary cadmium, a marker of long term exposure, were not associated with an increased risk of MetS in the overall study population. However, higher urine cadmium was associated with altered MetS components. Current smokers were the most vulnerable group, with higher long-term cadmium exposure being associated with increased risk of MetS, low HDL, and hypertension.

The role of cadmium in obesity and diabetes

Multiple studies have shown an association between environmental exposure to hazardous chemicals including toxic metals and obesity, diabetes, and metabolic syndrome. At the same time, the existing data on the impact of cadmium exposure on obesity and diabetes are contradictory. Therefore, the aim of the present work was to review the impact of cadmium exposure and status on the risk and potential etiologic mechanisms of obesity and diabetes. In addition, since an effect of cadmium exposure on incidence of diabetes mellitus and insulin resistance was suggested by several epidemiologic studies, we carried out a meta-analysis of all studies assessing risk of prevalence and incidence of diabetes. By comparing the highest versus the lowest cadmium exposure category, we found a high risk of diabetes incidence (odds ratio=1.38, 95% confidence interval 1.12-1.71), which was higher for studies using urine as exposure assessment. On the converse, results of epidemiologic studies linking cadmium exposure and overweight or obesity are far less consistent and even conflicting, also depending on differences in exposure levels and the specific marker of exposure (blood, urine, hair, nails). In turn, laboratory studies demonstrated that cadmium adversely affects adipose tissue physiopathology through several mechanisms, thus contributing to increased insulin resistance and enhancing diabetes. However, intimate biological mechanisms linking Cd exposure with obesity and diabetes are still to be adequately investigated.

A PPARγ-Bnip3 Axis Couples Adipose Mitochondrial Fusion-Fission Balance to Systemic Insulin Sensitivity

Aberrant mitochondrial fission plays a pivotal role in the pathogenesis of skeletal muscle insulin resistance. However, fusion-fission dynamics are physiologically regulated by inherent tissue-specific and nutrient-sensitive processes that may have distinct or even opposing effects with respect to insulin sensitivity. Based on a combination of mouse population genetics and functional in vitro assays, we describe here a regulatory circuit in which peroxisome proliferator-activated receptor γ (PPARγ), the adipocyte master regulator and receptor for the thiazolidinedione class of antidiabetic drugs, controls mitochondrial network fragmentation through transcriptional induction of Bnip3. Short hairpin RNA-mediated knockdown of Bnip3 in cultured adipocytes shifts the balance toward mitochondrial elongation, leading to compromised respiratory capacity, heightened fatty acid β-oxidation-associated mitochondrial reactive oxygen species generation, insulin resistance, and reduced triacylglycerol storage. Notably, the selective fission/Drp1 inhibitor Mdivi-1 mimics the effects of Bnip3 knockdown on adipose mitochondrial bioenergetics and glucose disposal. We further show that Bnip3 is reciprocally regulated in white and brown fat depots of diet-induced obesity and leptin-deficient ob/ob mouse models. Finally, Bnip3(-/-) mice trade reduced adiposity for increased liver steatosis and develop aggravated systemic insulin resistance in response to high-fat feeding. Together, our data outline Bnip3 as a key effector of PPARγ-mediated adipose mitochondrial network fragmentation, improving insulin sensitivity and limiting oxidative stress.

Macrophage embedded fibrin gels: an in vitro platform for assessing inflammation effects on implantable glucose sensors

The erroneous and unpredictable behavior of percutaneous glucose sensors just days following implantation has limited their clinical utility for diabetes management. Recent research has implicated the presence of adherent inflammatory cells as the key mitigating factor limiting sensor functionality in this period of days post-implantation. Here we present a novel in vitro platform to mimic the cell-embedded provisional matrix that forms adjacent to the sensor immediately after implantation for the focused investigation of the effects of early stage tissue response on sensor function. This biomimetic surrogate is formed by imbibing fibrin-based gels with physiological densities of inflammatory RAW 264.7 macrophages. When surrounding functional sensors, macrophage-embedded fibrin gels contribute to sensor signal declines that are similar in both shape and magnitude to those observed in previous whole blood and small animal studies. Signal decline in the presence of gels is both metabolically-mediated and sensitive to cell type and activation. Computational modeling of the experimental setup is also presented to validate the design by showing that the cellular glucose uptake parameters necessary to achieve such experimental declines align well with literature values. Together, these data suggest this in vitro provisional matrix surrogate may serve as an effective screening tool for testing the biocompatibility of future glucose sensor designs.

Cadmium exposure in relation to insulin production, insulin sensitivity and type 2 diabetes: a cross-sectional and prospective study in women

BACKGROUND

Cadmium is a wide-spread pollutant. Observational studies suggest associations between cadmium and prevalence of type 2 diabetes. Experimental studies indicate that cadmium may cause impaired insulin production.

OBJECTIVES

To examine whether cadmium exposure is associated with impaired glucose tolerance and type 2 diabetes and impaired pancreatic beta-cell function.

METHODS

Oral glucose tolerance tests were used in a screening examination of 64-year old women (n=2595) to identify all with diabetes, impaired (IGT) and normal (NGT) glucose tolerance. Random samples of women with type 2 diabetes (n=215), IGT (n=207) and NGT (n=194) were recruited in a cross-sectional examination including measurement of pancreatic insulin production (acute insulin response) and insulin sensitivity (homeostasis model assessment). Cadmium concentrations were measured in blood and urine. A follow-up examination was performed after 5.4 years with assessment of new cases with diabetes or impaired glucose tolerance.

RESULTS

At baseline, neither blood nor urinary cadmium concentrations showed any statistically significant differences between women with type 2 diabetes, IGT or NGT. The prospective analysis included 68 cases with incident diabetes, 58 with IGT and 118 women with NGT and no associations with cadmium levels at baseline were observed. Blood and urinary cadmium at baseline were not associated with insulin production, blood glucose, HbA1c, or changes in HbA1c during follow-up.

CONCLUSIONS

This is the first study of cadmium and diabetes with detailed data on pancreatic beta-cell function, insulin sensitivity and glucose tolerance. Cadmium exposure was not associated with increased risk of type 2 diabetes or IGT.

Differential effects of cobalt and mercury on lipid metabolism in the white adipose tissue of high-fat diet-induced obesity mice

Metals and metalloid species are involved in homeostasis in energy systems such as glucose metabolism. Enlarged adipocytes are one of the most important causes of obesity-associated diseases. In this study, we studied the possibility that various metals, namely, CoCl(2), HgCl(2), NaAsO(2) and MnCl(2) pose risk to or have beneficial effects on white adipose tissue (WAT). Exposure to the four metals resulted in decreases in WAT weight and the size of enlarged adipocytes in mice fed a high-fat diet (HFD) without changes in liver weight, suggesting that the size and function of adipocytes are sensitive to metals. Repeated administration of CoCl(2) significantly increased serum leptin, adiponectin and high-density lipoprotein (HDL) cholesterol levels and normalized glucose level and adipose cell size in mice fed HFD. In contrast, HgCl(2) treatment significantly decreased serum leptin level with the down-regulation of leptin mRNA expression in WAT and a reduction in adipocyte size. Next, we tried to investigate possible factors that affect adipocyte size. Repeated exposure to HgCl(2) significantly decreased the expression levels of factors upon the regulation of energy such as the PPARα and PPARγ mRNA expression levels in adipocytes, whereas CoCl(2) had little effect on those genes expressions compared with that in the case of the mice fed HFD with a vehicle. In addition, repeated administration of CoCl(2) enhanced AMPK activation in a dose-dependent manner in the liver, skeletal muscle and WAT; HgCl(2) treatment also enhanced AMPK activation in the liver. Thus, both Co and Hg reduced WAT weight and the size of enlarged adipocytes, possibly mediated by AMKP activation in the mice fed HFD. However, inorganic cobalt may have a preventive role in obesity-related diseases through increased leptin, adiponectin and HDL-cholesterol levels, whereas inorganic mercury may accelerate the development of such diseases. These results may lead to the development of new approaches to establishing the role of metals in adipose tissue of obesity-related diseases.

Cadmium and transport of ions and substances across cell membranes and epithelia

Toxic metals such as cadmium (Cd(2+)) pose serious risks to human health. However, even though the importance of Cd(2+) as environmental health hazards is now widely appreciated, the specific mechanisms by which it produces its adverse effects have yet to be fully elucidated. Cd(2+) is known to enter cells, it binds and interacts with a multitude of molecules, it may indirectly induce oxidative stress and interfere with gene expression and repair of DNA. It also interacts with transport across cell membranes and epithelia and may therefore disturb the cell's homeostasis and function. Interaction with epithelial transport, especially in the kidney and the liver, may have serious consequences in general health. A lot of research still needs to be done to understand the exact way in which Cd(2+) interferes with these transport phenomena. It is not always clear whether Cd(2+) has primary or secondary effects on cell membrane transport. In the present review we try to summarize the work that has been done up to now and to critically discuss the relevance of the experimental work in vitro with respect to the in vivo situation.

Cadmium reduces adipocyte size and expression levels of adiponectin and Peg1/Mest in adipose tissue

Adipose tissue dysfunction has been associated with diabetogenic effects. The effects of repeated Cd exposure on adipocytes remain largely unknown. We administered Cd at doses of 0, 5, 10, and 20 micromol/kgbw sc for 2 weeks (3.5 times/week) to mice and assessed the possible alteration of epididymal white adipose tissue (WAT), including histological difference, adipocyte differentiation and functional capacity. Whereas hepatic weight did not differ between the control and Cd-exposed groups, WAT weight, as well as adipose cell mass, significantly decreased in a dose-dependent manner in Cd-treated mice. The Cd concentration in WAT significantly increased in Cd-treated groups after 2 weeks of exposure. Next, we examined the effects of Cd on adipocyte differentiation and hypertrophy. Cd exposure significantly decreased the paternally expressed gene 1/Mesoderm-specific transcript mRNA expression levels. Both peroxisome proliferator-activated receptor gamma2 and CCAAT/enhancer-binding protein alpha mRNA expression levels in WAT tended to decrease in the Cd-treated groups. Next, we determined the effects of Cd exposure on the mRNA expression levels of adipose-derived hormones, such as adiponectin and resistin. The adiponectin mRNA expression level in WAT decreased after both 6h and 2 weeks of exposure to a high dose of Cd, and the reduction in resistin mRNA expression levels was observed after 2 weeks of exposure. These results suggest that Cd exposure causes abnormal adipocyte differentiation, expansion, and function, which might lead to development of insulin resistance, hypertension, and cardiovascular disease.

Evaluation of status of toxic metals in biological samples of diabetes mellitus patients

There is accumulating evidence that the metabolism of several trace elements is altered in diabetes mellitus and that these nutrients might have specific roles in the pathogenesis and progress of this disease. The aim of present study was to compare the level of toxic elements, lead (Pb), cadmium (Cd), and arsenic (As) in biological samples (whole blood, urine, and scalp hair) of patients having diabetes mellitus type-2 age ranged (31-60) (n=238), with those of age matched non-diabetics (ND) as control subjects (n=196), of both genders. The concentrations of elements were measured by means of an atomic absorption spectrophotometer after microwave-assisted acid digestion. The validity and accuracy was checked by conventional wet acid digestion method and using certified reference materials. The overall recoveries of all elements were found in the range of 98.1-99.4% of certified values. The results of this study showed that the mean values of Pb, Cd and, As were significantly higher in scalp hair samples of smoker and non-smoker diabetic patients as compared to control subjects (p<0.001). The concentration of understudy toxic metals was also high in blood and urine samples of DM patient but difference was more significant in smoker DM patients. These results are consistent with those obtained in other studies, confirming that toxic metals may play a role in the development of diabetes mellitus.

Effect of huanglian jiedu decoction ) on glucose transporter 4 expression in adipose and skeletal muscle tissues of insulin resistant rats

OBJECTIVE

To investigate the effects of Huanglian Jiedu Decoction (HLJDD) on glucose transporter 4 (GLUT4) protein expressions in insulin-resistant murine target tissues.

METHODS

The experimental male Wistar rats were established into insulin resistant models by injecting streptozotocin (STZ 30 mg/kg) via caudal vein and feeding them with high fat high caloric diet, and randomly divided into the model group, the aspirin group and the HLJDD group. Besides, a normal group was set up for control. Changes of body weight (BW), levels of serum fasting blood glucose (FBG), serum fasting insulin (FINS) and oral glucose tolerance test (OGTT) were routinely determined. The expression of GLUT4 protein in adipose and skeletal muscle tissues before and after insulin stimulation was determined with Western blot.

RESULTS

In the HLJDD group after treatment, BW and FBG got decreased, OGTT improved, and the expression and translocation of GLUT4 protein elevated obviously, either before or after insulin stimulation, as compared with those in the model group, showing significant differences respectively.

CONCLUSION

The mechanism of improving insulin resistance by HLJDD is probably associated with its effect in elevating GLUT4 protein expression and translocation in adipose and skeletal muscle tissues of insulin resistant rats.

Effects of mercuric chloride on glucose transport in 3T3-L1 adipocytes

Mercury, as well as the other Group IIB metals, stimulates glucose transport in adipocytes. Here we characterize the action of mercury on adipocyte glucose transport and examine several potential mechanisms of action. Mercury exposure causes a modest (compared to insulin) 1.8-fold increase in glucose transport. This glucose transport corresponds with an increase in GLUT 1, but not GLUT 4 glucose transporters. Phosphorylation of p38 kinase and c-Jun N-terminal kinase (JNK) were examined as possible mediators of mercury induced GLUT 1 levels. Phosphorylation of p38 kinase, but not JNK, increased with mercury exposure. Activation of p38 and an increase in glucose transport corresponding to an increase in GLUT 1 are indicative the induction of a stress response, which can contribute to the induction of insulin resistance in adipocytes. However, inhibition of p38 by the p38 inhibitor SB203580 did not prevent mercury-mediated glucose uptake. While the magnitude of the action of mercury is modest, its effects were sustained over many days of exposure and impacted subsequent insulin-mediated glucose transport. Pre-treatment with HgCl2 decreased insulin-mediated glucose transport 1.3-fold suggesting that exposure to mercury may contribute to pathologies associated with glucose homeostasis.

Mercuric chloride acting through Mg stimulates protein synthesis in Xenopus oocytes

In Xenopus laevis oocytes, addition of HgCl2 (Hg) to the medium rapidly stimulated incorporation of [35S]methionine (MET) into protein, increasing incorporation up to five-fold over control values. The action of inorganic mercury persisted after removal of Hg. Microinjection of HgCl2 into oocytes maintained in buffer also increased MET incorporation. However, no such stimulation was found when Hg was microinjected into oocytes maintained under oil, suggesting that Hg action was dependent on a cell-medium interaction. Removing medium Mg2+ decreased insulin- and Hg-stimulated methionine incorporation. Increasing medium Mg2+ from 1 mM to 10 mM increased Hg-stimulated methionine incorporation twofold. Hypotonic swelling of oocytes stimulated hexose transport but inhibited protein synthesis. Together these data indicate that inorganic mercury activates translation in the oocyte through an Mg-dependent mechanism, possibly increased Mg2+ influx.

Parallel stimulation of glucose and Mg(2+) accumulation by insulin in rat hearts and cardiac ventricular myocytes

The stimulation of beta-adrenoceptors in cardiac cells results in a rapid loss of cellular Mg(2+). Because insulin physiologically counteracts several of the cellular effects mediated by the activation of beta-adrenoceptors and the elevation of cytosolic cAMP levels, we investigated whether insulin administration could prevent Mg(2+) mobilization from rat hearts and ventricular myocytes. Rat hearts were perfused in a retrograde Langendorff system, and the changes in extracellular Mg(2+) were measured by atomic absorbance spectrophotometry. Pretreatment of the hearts with 6 nmol/L insulin completely prevented the Mg(2+) extrusion induced by the beta-adrenergic agonist isoproterenol. Furthermore, the administration of insulin per se induced an accumulation of Mg(2+) by the heart. This accumulation was small but detectable in the presence of 25 to 35 micromol/L [Mg(2+)](o) and increased in proportion to [Mg(2+)](o). Insulin-mediated Mg(2+) accumulation was not observed in hearts perfused with a medium devoid of glucose or with a medium containing the inhibitors of glucose transport, cytochalasin B and phloretin. Insulin-stimulated [(3)H]2-deoxyglucose accumulation was measured in collagenase-dispersed cardiac ventricular myocytes in the presence of varying levels of [Mg(2+)](o). Glucose transport was not observed below 25 micromol/L [Mg(2+)](o), and it also increased in proportion to [Mg(2+)](o). Taken together, these results indicate the presence of a major uptake of Mg(2+) into cardiac cells that is stimulated by insulin and may require the insulin-induced operation of a glucose transporter. Hence, extracellular and/or intracellular Mg(2+) may modulate glucose transport and/or utilization.

Multiple effects of mercuric chloride on hexose transport in Xenopus oocytes

HgCl(2) had both stimulatory and inhibitory effects on [(3)H]2-deoxyglucose (DG) uptake in Xenopus laevis oocytes. The Hg dose response was complex, with 0.1-10 microM Hg increasing total DG uptake, 30-50 microM Hg inhibiting, and concentrations >100 microM increasing uptake. Analyses of the effects of Hg on DG transport kinetics and cell membrane permeability indicated that low concentrations of Hg stimulated mediated uptake, intermediate concentrations inhibited mediated uptake, but high Hg concentrations increased non-mediated uptake. 10 microM Hg increased the apparent V(max) for DG uptake, but caused little or no change in apparent K(m). Phenylarsine oxide prevented the increase in DG uptake by 10 microM Hg, suggesting that the increase was due to transporter recruitment. Microinjecting low doses of HgCl(2) into the cell increased mediated DG uptake. Higher intracellular doses of Hg increased both mediated and non-mediated DG uptake. Both insulin and Hg cause cell swelling in isotonic media and, for insulin, this swelling has been linked to the mechanism of hormone action. Osmotically swelling Xenopus oocytes stimulated DG transport 2-5-fold and this increase was due to an increased apparent V(max). Exposing cells to 10 microM Hg or 140 nM insulin both increased cellular water content by 18% and increased hexose transport 2-4-fold. These data indicate that low concentrations of Hg and insulin affect hexose transport in a similar manner and that for both an increase cellular water content could be an early event in signaling the increase in hexose transport.

An inhibitor of p38 mitogen-activated protein kinase prevents insulin-stimulated glucose transport but not glucose transporter translocation in 3T3-L1 adipocytes and L6 myotubes

The precise mechanisms underlying insulin-stimulated glucose transport still require investigation. Here we assessed the effect of SB203580, an inhibitor of the p38 MAP kinase family, on insulin-stimulated glucose transport in 3T3-L1 adipocytes and L6 myotubes. We found that SB203580, but not its inactive analogue (SB202474), prevented insulin-stimulated glucose transport in both cell types with an IC50 similar to that for inhibition of p38 MAP kinase (0.6 microM). Basal glucose uptake was not affected. Moreover, SB203580 added only during the transport assay did not inhibit basal or insulin-stimulated transport. SB203580 did not inhibit insulin-stimulated translocation of the glucose transporters GLUT1 or GLUT4 in 3T3-L1 adipocytes as assessed by immunoblotting of subcellular fractions or by immunofluorescence of membrane lawns. L6 muscle cells expressing GLUT4 tagged on an extracellular domain with a Myc epitope (GLUT4myc) were used to assess the functional insertion of GLUT4 into the plasma membrane. SB203580 did not affect the insulin-induced gain in GLUT4myc exposure at the cell surface but largely reduced the stimulation of glucose uptake. SB203580 had no effect on insulin-dependent insulin receptor substrate-1 phosphorylation, association of the p85 subunit of phosphatidylinositol 3-kinase with insulin receptor substrate-1, nor on phosphatidylinositol 3-kinase, Akt1, Akt2, or Akt3 activities in 3T3-L1 adipocytes. In conclusion, in the presence of SB203580, insulin caused normal translocation and cell surface membrane insertion of glucose transporters without stimulating glucose transport. We propose that insulin stimulates two independent signals contributing to stimulation of glucose transport: phosphatidylinositol 3-kinase leads to glucose transporter translocation and a pathway involving p38 MAP kinase leads to activation of the recruited glucose transporter at the membrane.

The role of the GLUT 4 transporter in regulating rat myoblast glucose transport processes

Previous studies revealed an inverse relationship between GLUT 1 and GLUT 4 expression in rat myoblasts [L. Xia, Z. Lu, T.C.Y. Lo, J. Biol. Chem., 268 (1993) 23258-23266]. It was not clear whether these were coincidental or causal occurrences. To examine the regulatory roles of the GLUT 4 isoform, rat L6 myoblasts were transfected with full length GLUT 4 cDNAs (2.5 kb) in the sense or antisense orientation. L6 myoblasts transfected with the GLUT 4 sense cDNA (L6/G4S transfectants) possessed much elevated levels of both endogenous GLUT 4 transcripts (1.4 kb and 2.8 kb). Transport and immunofluorescence studies showed that this GLUT 4 sense cDNA was responsible for a functional GLUT 4 transporter. L6 cells transfected with the GLUT 4 antisense cDNA (L6/G4A transfectants) possessed only 6% of the L6 level in day 6 cultures. These antisense transfectants were essentially devoid of any functional GLUT 4 transporter. The activation of transcription of the endogenous GLUT 4 gene in L6/G4S myoblasts suggested auto-regulation of GLUT 4 expression. GLUT 3 expression and activity were not altered in both sense and antisense GLUT 4 transfectants. More interestingly, GLUT 1 expression was reduced in L6/G4S myoblasts, whereas it was elevated in L6/G4A myoblasts. This was the first direct evidence indicating GLUT 4 might play an important role in suppressing GLUT 1 expression.

Acute Regulation of Glucose Transport After Activation of Human Peripheral Blood Neutrophils by Phorbol Myristate Acetate, fMLP, and Granulocyte-Macrophage Colony-Stimulating Factor

Activation of human peripheral blood neutrophils by pathogens or by phorbol myristate acetate (PMA), fMLP, or myeloid growth factors generates a respiratory burst in which superoxide production plays an important role in killing invading microorganisms. Although the increased energy demands of activated neutrophils would be expected to be associated with increased glucose uptake and utilization, previous studies have shown that PMA inhibits 2-deoxyglucose (2-DOG) uptake. In this study, we show that PMA activation of neutrophils, isolated by methods not involving hypotonic lysis, increases the rate of 2-DOG uptake and results in a 1.6-fold to 2.1-fold increase in transporter affinity for glucose without changing Vmax. Increased transporter affinity in response to PMA was also observed with 3-O-methyglucose, which is not phosphorylated, and inclusion of glucose in the activation medium further increased respiratory burst activity. Increased 2-DOG uptake and increased transporter affinity for glucose were also observed with the peptide activator, fMLP, and with granulocyte-macrophage colony-stimulating factor (GM-CSF). The protein kinase C (PKC) inhibitor, calphostin C, and the tyrosine kinase inhibitor, genistein, inhibited both PMA- and fMLP-stimulated 2-DOG uptake. In contrast, genistein inhibited fMLP-induced superoxide production, but had little effect on the PMA-induced response, while staurosporine differentially inhibited PMA-induced superoxide production. These results show that neutrophil activation involves increased glucose transport and intrinsic activation of glucose transporter molecules. Both tyrosine kinases and PKC are implicated in the activation process.

Acute Regulation of Glucose Transport After Activation of Human Peripheral Blood Neutrophils by Phorbol Myristate Acetate, fMLP, and Granulocyte-Macrophage Colony-Stimulating Factor

Activation of human peripheral blood neutrophils by pathogens or by phorbol myristate acetate (PMA), fMLP, or myeloid growth factors generates a respiratory burst in which superoxide production plays an important role in killing invading microorganisms. Although the increased energy demands of activated neutrophils would be expected to be associated with increased glucose uptake and utilization, previous studies have shown that PMA inhibits 2-deoxyglucose (2-DOG) uptake. In this study, we show that PMA activation of neutrophils, isolated by methods not involving hypotonic lysis, increases the rate of 2-DOG uptake and results in a 1.6-fold to 2.1-fold increase in transporter affinity for glucose without changing Vmax. Increased transporter affinity in response to PMA was also observed with 3-O-methyglucose, which is not phosphorylated, and inclusion of glucose in the activation medium further increased respiratory burst activity. Increased 2-DOG uptake and increased transporter affinity for glucose were also observed with the peptide activator, fMLP, and with granulocyte-macrophage colony-stimulating factor (GM-CSF). The protein kinase C (PKC) inhibitor, calphostin C, and the tyrosine kinase inhibitor, genistein, inhibited both PMA- and fMLP-stimulated 2-DOG uptake. In contrast, genistein inhibited fMLP-induced superoxide production, but had little effect on the PMA-induced response, while staurosporine differentially inhibited PMA-induced superoxide production. These results show that neutrophil activation involves increased glucose transport and intrinsic activation of glucose transporter molecules. Both tyrosine kinases and PKC are implicated in the activation process.

The hemopoietic growth factor, interleukin-3, promotes glucose transport by increasing the specific activity and maintaining the affinity for glucose of plasma membrane glucose transporters

Most mammalian cells rely on an external supply of glucose for survival, proliferation, and function. Glucose enters cells through specific transporter molecules at the plasma membrane by a facilitative process that does not expend energy. Regulation of glucose transport into cells is thought to occur largely through transporter expression at the cell surface, but the extent to which the intrinsic properties of glucose transporters are regulated is at present controversial. Using a bone marrow-derived cell line that responds to the hemopoietic growth factor, interleukin-3 (IL-3), we investigated IL-3 regulation of glucose transport. IL-3 significantly increased 2-deoxyglucose (2-DOG) uptake within 1 h (26 +/- 8.0%, n = 11) with a maximum 73% increase after 6 h. Withdrawal of IL-3 resulted in decreased uptake within 1 h and this continued to decline to 43% of initial uptake by 16 h. To determine whether these changes in 2-DOG uptake were associated with corresponding changes in glucose transporter expression, subtype-specific antisera against Glut-1 and Glut-3 were used. Little change in membrane expression of these transporters was observed prior to 16 h. Fractionation of cell membranes on Nycodenz gradients showed that the majority of each transporter subtype was associated with the plasma membrane (63-93%) and that transporter distribution did not change markedly in response to addition or withdrawal of IL-3. These results demonstrate that IL-3 regulates glucose uptake by modulating the intrinsic transporting ability of glucose transporters. Decreased transporter affinity for 2-DOG and 3-O-methylglucose was observed following IL-3 withdrawal. Similar affinity changes were observed with 2-DOG following exposure of IL-3-stimulated cells to the protein kinase inhibitors, genistein and staurosporine. In contrast, the tyrosine phosphatase inhibitor, vanadate, acted like IL-3 to increase transporter affinity for glucose. Together these results demonstrate that IL-3 acts to maintain the intrinsic transport properties of glucose transporters without markedly affecting their expression or translocation.

Overexpression of Rad inhibits glucose uptake in cultured muscle and fat cells

Rad is a Ras-like GTPase that was isolated by subtraction cloning of human muscle and shown to have increased expression in some individuals with Type II diabetes. To ascertain the potential role of Rad in insulin-mediated signaling, we have overexpressed Rad in myocyte and adipocyte cell lines. Expression of Rad resulted in a 50-90% reduction in insulin-stimulated 2-deoxyglucose glucose uptake in C2C12 murine myotubes, L6 rat myotubes, and 3T3-L1 adipocytes and a 25% reduction in 3-O-methylglucose uptake in 3T3-L1 adipocytes. This occurred despite unaltered levels of glucose transporter expression, with no detectable change in Glut4 translocation and with no alteration in insulin receptor or substrate phosphorylation or phosphatidylinositol 3-kinase activity. These data indicate that Rad is a negative regulator of glucose uptake and that this effect may be due to a decrease in the intrinsic activity of the transporter molecules, rather than an effect on the translocation of Glut4.

Evidence that modulation of glucose transporter intrinsic activity is the mechanism involved in the allose-mediated depression of hexose transport in mammalian cells

In serum starved V79 Chinese hamster lung fibroblast cells, replacement of D-glucose with D-allose resulted in a significant 38 +/- 18% (P < 0.05) reduction of 2-deoxy-D-glucose (2-DG) transport. Similarly, in a respiration-deficient mutant cell line (V79-G14), which has elevated 2-DG transport activity, D-allose reduced 2-DG transport by 59 +/- 18% (P < 0.05). [3H]D-allose uptake by V79 cells occurred slowly and was not inhibited by cytochalasin B, suggesting diffusion as the mode of D-allose entry. Western blot analysis using a rabbit polyclonal antibody to the human erythrocyte glucose transporter (GT) demonstrated that, in both cell lines, GT content and GT subcellular distribution were not significantly different in D-glucose vs. D-allose-treated cells. delta-Antibody, which has been shown to bind to exofacial epitopes of the GT (Harrison et al., 1990, J. Biol. Chem., 265:5793-5801), did not demonstrate any differences in surface binding to D-glucose vs. D-allose-treated intact V79 cells. D-allose treatment of 3T3 fibroblasts resulted in a similar decrease (72%) of 2-DG transport, however D-allose had no apparent effect on basal sugar transport in 3T3 adipocytes. These results suggest that D-allose reduces sugar transport through a modulation of the intrinsic activity of the GT, and that D-allose may act in a tissue-specific manner.

Early changes in glucose metabolism in the cerebrum of senescence accelerated mouse: involvement of glucose transporter

The rate of 6-[14C]D-glucose oxidation in cerebral cells of SAMP8, a substrain of senescence accelerated mouse, was investigated in vitro. The production of 14CO2 in dissociated intact brain cells prepared from the cerebrum of 4-8-week-old SAMP8 was higher than that of age-matched SAMR2 as a control mouse, while no difference between these two strains was observed in the production of 14CO2 in the cerebral homogenates. These results indicated that the increased metabolism of glucose in SAMP8 might be associated with the glucose transport system across the cell membrane. Therefore, 2-deoxy-D-glucose (2-DG) uptake into the brain cells and cytochalasin B (CB) binding to cerebral crude membranes were examined. Both the 2-DG uptake and the CB binding in SAMP8 were much greater than in SAMR2. Furthermore, the increased CB binding in SAMP8 was seen only in the cerebral cortex of 4- to 8-week-old mice, and neither in other regions of the cerebrum nor in other aged mice (2-week- and 40- to 48-week-old mice). These results suggest that the transient overproduction of the glucose transporter protein in the cerebral cortex is involved in the increased glucose metabolism in 4- to 8-week-old SAMP8.

Complex regulation of simple sugar transport in insulin-responsive cells

Facilitated sugar entry into mammalian cells is catalysed by multiple isoforms of the glucose transporter and regulated by hormonal stimuli, nutritional status and oncogenesis. A large reserve of latent glucose transport capacity must be maintained by muscle and adipose cells that are sensitive to insulin, the primary activator of sugar uptake after feeding. Intracellular sequestration of sugar transporters accounts for a large part of this latent capacity, but new findings suggest that there is also reversible suppression of intrinsic catalytic activity of those glucose transporters residing at the cell surface. The mechanism of this suppression appears to be occlusion or disruption of the exofacial sugar-binding sites on the glucose-transporter proteins.