Glucose transporters: structure, function and consequences of deficiency

Energy balance and metabolic changes with sodium-glucose co-transporter 2 inhibition

Sodium-glucose co-transporter 2 (SGLT2) inhibitors are the latest addition to the class of oral glucose-lowering drugs. They have been rapidly adopted into clinical practice because of therapeutic advantages, including weight loss and reduction in blood pressure, in addition to glycaemic benefits and a low intrinsic risk of hypoglycaemia. Although there are extensive data on the clinical effects of SGLT2 inhibition, the metabolic effects of inhibiting renal glucose reabsorption have not been fully described. Recent studies have identified compensatory metabolic effects, such as an increase in endogenous glucose production, and have also shown an increase in glucagon secretion during SGLT2 inhibition. In addition, there is a discrepancy between the expected and observed weight loss found in clinical studies on SGLT2 inhibitors, probably as a result of changes in energy balance with this treatment approach. SGLT2 inhibition is likely to have intriguing effects on whole body metabolism which have not been fully elucidated, and which, if explained, might help optimize the use of this new class of medicines.

Empagliflozin: a sodium-glucose cotransporter 2 inhibitor for treatment of type 2 diabetes

PURPOSE

The pharmacology, pharmacokinetics, pharmacodynamics, clinical efficacy, adverse effects, dosage and administration, and drug-drug interactions of empagliflozin are reviewed.

SUMMARY

Empagliflozin is a direct inhibitor of sodium-glucose cotransporter 2 (SGLT2), which acts to lower the renal threshold and increase urinary glucose excretion. SGLT2 is found in the proximal tubules of the kidneys and reabsorbs about 90% of the filtered glucose. Because the mechanism of action of empagliflozin is not insulin dependent or insulin sensitive, it may be used in patients at different stages of diabetes with nonfunctional or impaired pancreatic β cells. Furthermore, empagliflozin can be used with other antidiabetic drugs due to its lack of any additive hypoglycemic effects. Long-term efficacy studies revealed significant reductions with empagliflozin in glycosylated hemoglobin (HbA1c) values at week 78 compared with placebo. Secondary endpoints in clinical trials showed improvements in lowering blood pressure and reductions in body weight. The risk:benefit ratio must be assessed for empagliflozin as the safety profile includes an increase in urinary and genital infections.

CONCLUSION

Empagliflozin has shown efficacy in lowering HbA1c and blood glucose levels both as monotherapy and as an add-on to existing therapy. Despite the drug's promising outlook, empagliflozin also leads to common but serious adverse events not seen with other classes of antihyperglycemic agents. Considering the current data on its efficacy and its safety profile, empagliflozin can be used as a second- or third-line agent in treating diabetes.

Pharmacotherapy of type 2 diabetes mellitus: an update on drug-drug interactions

The incidence of type 2 diabetes mellitus is increasing rapidly, as are the associated co-morbidities. Consequently, it has become necessary for a diabetic patient to take multiple medications at the same time to delay progression of the disease. This can put patients at an increased risk of moderate to severe drug interactions, which may threaten patients' life or may deteriorate the quality of their life. Hence, managing drug-drug interactions is the cornerstone of anti-diabetic therapy. Most of the clinically important drug-drug interactions of anti-diabetic agents are related to their metabolic pathways, but drugs that compete for renal excretion or impair renal status can also play an important role. In this review, we have examined the clinical implications and underlying mechanisms of drugs that are likely to alter the pharmacologic response of or cause adverse events with antidiabetic drugs, and we have outlined safe and efficacious treatment modalities.

Polymorphisms in sweet taste genes (TAS1R2 and GLUT2), sweet liking, and dental caries prevalence in an adult Italian population

The aim of the study was to assess the relationship between sweet taste genes and dental caries prevalence in a large sample of adults. In addition, the association between sweet liking and sugar intake with dental caries was investigated. Caries was measured by the decayed, missing, filled teeth (DMFT) index in 647 Caucasian subjects (285 males and 362 females, aged 18-65 years), coming from six villages in northeastern Italy. Sweet liking was assessed using a 9-point scale, and the mean of the liking given by each individual to specific sweet food and beverages was used to create a sweet liking score. Simple sugar consumption was estimated by a dietary history interview, considering both added sugars and sugar present naturally in foods. Our study confirmed that polymorphisms in TAS1R2 and GLUT2 genes are related to DMFT index. In particular, GG homozygous individuals for rs3935570 in TAS1R2 gene (p value = 0.0117) and GG homozygous individuals for rs1499821 in GLUT2 gene (p value = 0.0273) showed higher DMFT levels compared to both heterozygous and homozygous for the alternative allele. Furthermore, while the relationship sugar intake-DMFT did not achieve statistical significance (p value = 0.075), a significant association was identified between sweet liking and DMFT (p value = 0.004), independent of other variables. Our study showed that sweet taste genetic factors contribute to caries prevalence and highlighted the role of sweet liking as a predictor of caries risk. Therefore, these results may open new perspectives for individual risk identification and implementation of target preventive strategies, such as identifying high-risk patients before caries development.

Dose-ranging efficacy and safety study of ertugliflozin, a sodium-glucose co-transporter 2 inhibitor, in patients with type 2 diabetes on a background of metformin

AIM

To investigate the efficacy and safety of ertugliflozin, in a phase II dose-ranging study, in patients with type 2 diabetes mellitus (T2DM) inadequately controlled on metformin.

METHODS

A total of 328 patients [mean T2DM duration, 6.3 years; mean glycated haemoglobin (HbA1c), 8.1%] were randomized to once-daily ertugliflozin (1, 5, 10, 25 mg), sitagliptin (100 mg) or placebo, for 12 weeks. The primary efficacy endpoint was change from baseline to week 12 in HbA1c concentration and the secondary efficacy endpoints were changes from baseline to week 12 in body weight, fasting plasma glucose (FPG) and systolic/diastolic blood pressure (SBP/DBP). Safety and tolerability were also monitored.

RESULTS

Ertugliflozin (1-25 mg/day) produced significant reductions in HbA1c concentration [placebo-corrected least-squares mean (LSM) -0.45% (1 mg) to -0.72% (25 mg); p ≤ 0.002, similar to sitagliptin (-0.76%; p = 0.0001)], FPG (LSM -1.17 to -1.90 mmol/l; p < 0.0001) and body weight (-1.15 to -2.15%; p < 0.0001). The LSM SBP decreased by -3.4 to -4.0 mmHg from baseline with ertugliflozin 5-25 mg/day. No reductions in body weight or blood pressure were observed with sitagliptin. After randomization, 2.7% of patients (9/328) withdrew because of adverse events (AEs); the frequency of AEs was evenly distributed across groups. No dose-related increase in AE frequency occurred with ertugliflozin. Hypoglycaemia was reported in 5 (1.5%) randomized participants (all in the ertugliflozin group). The frequency of urinary tract infection was 3.2% for ertugliflozin (pooled across groups), 1.8% for sitagliptin, 7.4% for placebo, and the frequency of genital fungal infections was 3.7% for ertugliflozin (pooled) versus 1.9% for placebo.

CONCLUSION

Ertugliflozin (1-25 mg/day) improved glycaemic control, body weight and blood pressure in patients with T2DM suboptimally controlled on metformin, and was well tolerated.

Mechanisms of expression and translocation of major fission yeast glucose transporters regulated by CaMKK/phosphatases, nuclear shuttling, and TOR

Glucose transporters play a pivotal role in glucose homeostasis. The fission yeast high-affinity glucose transporter Ght5 is regulated with regard to transcription and localization via CaMKK and TOR pathways. These results clarify the evolutionarily conserved mechanisms underlying glucose homeostasis that prevent hyperglycemia in humans.

Hexose transporters are required for cellular glucose uptake; thus they play a pivotal role in glucose homeostasis in multicellular organisms. Using fission yeast, we explored hexose transporter regulation in response to extracellular glucose concentrations. The high-affinity transporter Ght5 is regulated with regard to transcription and localization, much like the human GLUT transporters, which are implicated in diabetes. When restricted to a glucose concentration equivalent to that of human blood, the fission yeast transcriptional regulator Scr1, which represses Ght5 transcription in the presence of high glucose, is displaced from the nucleus. Its displacement is dependent on Ca²⁺/calmodulin-dependent kinase kinase, Ssp1, and Sds23 inhibition of PP2A/PP6-like protein phosphatases. Newly synthesized Ght5 locates preferentially at the cell tips with the aid of the target of rapamycin (TOR) complex 2 signaling. These results clarify the evolutionarily conserved molecular mechanisms underlying glucose homeostasis, which are essential for preventing hyperglycemia in humans.

Antidiabetic Effect of Schisandrae Chinensis Fructus Involves Inhibition of the Sodium Glucose Cotransporter

Preclinical Research Schisandrae Chinensis Fructus (SCF), the fruit of Schisandra chinensis (Turcz.) Baill. (family Schisandraceae) is traditionally used as a tonic and antidiabetic agent in Asia. In this study, SCF was investigated for its effects on sodium glucose cotransporters 1 and 2 (SGLT 1 and 2) expressed in a COS-7 cell line for its specificity in inhibiting SGLT2, which is a novel mechanism to screen for potential antidiabetic agents. Using a bioassay-guided fractionation, we then tried to isolate and identify the active fraction(s)/component(s). The ethanol extract of SCF at a concentration of 1 mg/mL significantly inhibited 89% of SGLT1 and 73% of SGLT2 activities in a [¹⁴ C]-α-methyl-d-glucopyranoside ([¹⁴ C]-AMG) uptake assay. Fractionation of the ethanol extract yielded nine fractions, of which F8, at a concentration of 1 mg/mL, was specific in inhibiting SGLT 2 (42% inhibition, P < 0.001), without inhibiting SGLT 1. Using LC/MS-MS, three compounds, deoxyschisandrin, schisandrin B (γ-schisandrin) and schisandrin were identified in F8 and their amounts quantified. However, subsequent evaluation in the [¹⁴ C]-AMG uptake assay showed that these three compounds failed to inhibit SGLT 2 activity indicating that the SGLT active component(s) from SCF have yet to be identified. Drug Dev Res 76 : 1-8, 2015.

Sodium-glucose cotransport inhibitors: mechanisms, metabolic effects and implications for the treatment of diabetic patients with chronic kidney disease

Remarkable progress has been achieved in the field of diabetes with the development of incretin analogues, dipeptidyl peptidase IV inhibitors and novel insulin analogues; nevertheless, there is an unmet need for additional therapeutic options. Individualization of HbA1c target levels is a recent progress within the field. Approximately 50% of diabetics do not reach a previously aspired treatment goal of glycosylated HbA1 levels below 7% and often face a vicious circle with accelerated weight gain. Current antidiabetic therapeutics mainly target the decline in insulin secretion and ameliorate insulin resistance. In this regard a new generation of drugs, denoted gliflozines, that specifically interfere with sodium-glucose cotransporters (SGLT)-2 and exhibit a favourable impact on glucose metabolism in patients with type 2 diabetes are emerging as hopeful avenues. The resultant negative energy balance caused by glucosuria results in long-term weight losses, significantly reduced HbA1c levels approximating 0.5-1.0% and may in addition exert beneficial effects on blood pressure, reactive oxygen products and inflammatory mediators. Recent studies indicate improvement in β-cell glucose sensitivity and insulin sensitivity in patients treated with gliflozines, a decrease in tissue glucose disposal and interestingly an increase in endogenous glucose production. The list of side effects observed under SGLT2 inhibition includes increased rates of genitourinary infections, balanitis, vulvovaginitis, hypotensive episodes and acute deterioration of kidney function. Main questions towards the safety profile are still unanswered given that long-term clinical outcome data with SGLT2 inhibition are lacking and the cardiovascular safety profile is under scrutiny in large trials. Thus, the successful development of selective SGLT2 inhibitors for therapeutic use in diabetics has a huge potential to meet patients' needs. However, it awaits quick results from clinical trials with meaningful clinical endpoints.

Effects of ursolic acid derivatives on Caco-2 cells and their alleviating role in streptozocin-induced type 2 diabetic rats

In this study, the effect and mechanism of a series of ursolic acid (UA) derivatives on glucose uptake were investigated in a Caco-2 cells model. Their effect on hyperglycemia, hyperlipidemia and oxidative stress were also demonstrated in streptozocin (STZ)-induced diabetic rats. 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxy-glucose (2-NBDG) was used as a fluorescein in Caco-2 cells model to screen UA derivatives by glucose uptake and expression of glucose transporter protein (SGLT-1, GLUT-2). Moreover, STZ-induced diabetic rats were administered with these derivatives for 4 weeks of treatment. The fasting blood glucose (FBG), insulin levels, biochemical parameters, lipid levels, and oxidative stress markers were finally evaluated. The results of this study indicated that compounds 10 and 11 significantly inhibited 2-NBDG uptake under both Na+-dependent and Na+-independent conditions by decreasing SGLT-1 and GLUT-2 expression in the Caco-2 cells model. Further in vivo studies revealed that compound 10 significantly reduced hyperglycemia by increasing levels of serum insulin, total protein, and albumin, while the fasting blood glucose, body weight and food intake were restored much closer to those of normal rats. Compounds 10 and 11 showed hypolipidemic activity by decreasing the total amounts of cholesterol (TC) and triglycerides (TG). Furthermore, compound 10 showed antioxidant potential which was confirmed by elevation of glutathione (GSH) and superoxide dismutase (SOD) and reduction of malondialdehyde (MDA) levels in the liver and kidney of diabetic rats. It was concluded that compound 10 caused an apparent inhibition of intestinal glucose uptake in Caco-2 cells and hypoglycemia, hypolipidemia and augmented oxidative stress in STZ-induced diabetic rats. Thus, compound 10 could be developed as a potentially complementary therapeutic or prophylactic agent for diabetics mellitus and its complications.

Comparative cation dependency of sugar transport by crustacean hepatopancreas and intestine

Glucose is transported in crustacean hepatopancreas and intestine by Na(+)-dependent co-transport, while Na(+)-dependent D-fructose influx has only been described for the hepatopancreas. It is still unclear if the two sugars are independently transported by two distinct cation-dependent co-transporter carrier systems. In this study, lobster (Homarus americanus) hepatopancreas brush border membrane vesicles (BBMV) were used to characterize, in detail, the cation-dependency of both D-[(3)H]-glucose and D-[(3)H]-fructose influxes, while in vitro perfused intestines were employed to determine the nature of cation-dependent sugar transport across this organ. Over the sodium concentration range of 0-100 mM, both [(3)H]-glucose and [(3)H]-fructose influxes (0.1 mM; 1 min uptakes) by hepatopancreatic BBMV were hyperbolic functions of [Na(+)]. [(3)H]-glucose and [(3)H]-fructose influxes by hepatopancreatic BBMV over a potassium concentration range of 15-100 mM were hyperbolic functions of [K(+)]. Both sugars displayed significant (p<0.01) Na(+)/K(+)-dependent and cation-independent uptake processes. Transepithelial 25 µM [(3)H]-glucose and [(3)H]-fructose fluxes across lobster intestine over luminal sodium and potassium concentration ranges of 0-50 mM and 5-100 mM, respectively, were hyperbolic functions of luminal [Na(+)] and [K(+)]. As with hepatopancreatic sugar transport, transepithelial intestinal sugar transport exhibited both significant (p<0.01) Na(+)/K(+)-dependent and cation-independent processes. Results suggest that both D-glucose and D-fructose are transported by a single SGLT-type carrier in each organ with sodium being the "preferred", high affinity, cation for both sugars in the hepatopancreas, and potassium being the "preferred", high affinity, cation for both sugars in the intestine.

SGLT2 inhibitors in the treatment of type 2 diabetes

The kidney plays an important role in glucose homeostasis via its production, utilization, and, most importantly, reabsorption of glucose from glomerular filtrate which is largely mediated via the sodium glucose co-transporter 2 (SGLT2). Pharmacological inhibition of SGLT2 increases urinary glucose excretion and decreases plasma glucose levels in an insulin-independent manner. Agents that inhibit SGLT2 represent a novel class of drugs, which has recently become available for treatment of type 2 diabetes. This article summarizes the rationale for use of these agents and reviews available clinical data on their efficacy, safety, and risks/benefits.

Antidiabetic activity of zinc oxide and silver nanoparticles on streptozotocin-induced diabetic rats

The use of nanoparticles in medicine is an attractive proposition. In the present study, zinc oxide and silver nanoparticles were evaluated for their antidiabetic activity. Fifty male albino rats with weight 120 ± 20 and age 6 months were used. Animals were grouped as follows: control; did not receive any type of treatment, diabetic; received a single intraperitoneal dose of streptozotocin (100 mg/kg), diabetic + zinc oxide nanoparticles (ZnONPs), received single daily oral dose of 10 mg/kg ZnONPs in suspension, diabetic + silver nanoparticles (SNPs); received a single daily oral dose of SNP of 10 mg/kg in suspension and diabetic + insulin; received a single subcutaneous dose of 0.6 units/50 g body weight. Zinc oxide and silver nanoparticles induce a significant reduced blood glucose, higher serum insulin, higher glucokinase activity higher expression level of insulin, insulin receptor, GLUT-2 and glucokinase genes in diabetic rats treated with zinc oxide, silver nanoparticles and insulin. In conclusion, zinc oxide and sliver nanoparticles act as potent antidiabetic agents.

Chronic stress modulates regional cerebral glucose transporter expression in an age-specific and sexually-dimorphic manner

Facilitative glucose transporters (GLUT) mediate glucose uptake across the blood-brain-barrier into neurons and glia. Deficits in specific cerebral GLUT isoforms are linked to developmental and neurological dysfunction, but less is known about the range of variation in cerebral GLUT expression in normal conditions and the effects of environmental influences on cerebral GLUT expression. Knowing that puberty is a time of increased cerebral plasticity, metabolic demand, and shifts in hormonal balance for males and females, we first assessed gene expression of five GLUT subtypes in four brain regions in male and female adolescent and adult Wistar rats. The data indicated that sex differences in GLUT expression were most profound in the hypothalamus, and the transition from adolescence to adulthood had the most profound effect on GLUT expression in the hippocampus. Next, given the substantial energetic demands during adolescence and prior demonstrations of the adverse effects of adolescent stress, we determined the extent to which chronic stress altered GLUT expression in males and females in both adolescence and adulthood. Chronic stress significantly altered cerebral GLUT expression in males and females throughout both developmental stages but in a sexually dimorphic and brain region-specific manner. Collectively, our data demonstrate that cerebral GLUTs are expressed differentially based on brain region, sex, age, and stress exposure. These results suggest that developmental and environmental factors influence GLUT expression in multiple brain regions. Given the importance of appropriate metabolic balance within the brain, further assessment of the functional implications of life stage and environmentally-induced changes in GLUTs are warranted.

Molecular mechanisms of [18F]fluorodeoxyglucose accumulation in liver cancer

To elucidate the molecular mechanisms underlying the insufficient sensitivity in the detection of hepatocellular carcinoma (HCC) by [18F] 2-fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET), the characteristics of glucose metabolism-related protein expression in HCC were examined in liver metastasis from colorectal cancer (Meta). Thirty-four patients (14 Meta and 20 HCC) who underwent FDG-PET and hepatectomy were studied. The relationships between the maximum standardized uptake value (SUV) in tumors and the mRNA expression of glucose metabolism-related proteins [hexokinase (HK), glucose transporter 1 (GLUT1), and glucose-6-phosphatase (G6Pase)] and proliferating cell nuclear antigen (PCNA) were examined in snap-frozen specimens with quantitative PCR. Tumor detection rates were lower in HCC (15/20) compared to Meta (13/14) patients. HK and GLUT1 expression was lower and G6Pase expression was higher in HCC compared to Meta. In particular, GLUT1 overexpression was 92-fold in Meta and 11-fold in HCC compared to the surrounding liver. The SUV correlated with GLUT1 and PCNA expression in HCC, but not Meta patients. Of note, four cases of poorly differentiated (P/D) HCC compared to moderately differentiated (M/D) HCC produced completely different results for FDG uptake (SUV, 14.4 vs. 4.0) and mRNA expression (G6Pase expression, 0.007 vs. 1.5). Variations in the expression of glucose metabolism-related enzymes between HCC and Meta patients are attributed to origin or degree of differentiation. Low FDG uptake in M/D HCC reflected low GLUT1 and high G6Pase expression, while high FDG accumulation in P/D HCC could reflect increased GLUT1 and decreased G6Pase expression. These results may explain why M/D HCC is not detected as sensitively by FDG-PET.

Physiology of cholangiocytes

Cholangiocytes are epithelial cells that line the intra- and extrahepatic ducts of the biliary tree. The main physiologic function of cholangiocytes is modification of hepatocyte-derived bile, an intricate process regulated by hormones, peptides, nucleotides, neurotransmitters, and other molecules through intracellular signaling pathways and cascades. The mechanisms and regulation of bile modification are reviewed herein.

The clinical efficacy and safety of sodium glucose cotransporter-2 inhibitors in adults with type 2 diabetes mellitus

The use of currently available antihyperglycemic agents can be limited by contraindications; cost; renal and hepatic dosage adjustments; dosing schedules; and adverse effects such as gastrointestinal upset, weight gain, and hypoglycemia. These limitations have led the pharmaceutical industry to identify and pursue alternative therapies. Sodium glucose cotransporter-2 (SGLT-2) inhibitors belong to a new class of diabetes drugs and have a novel mechanism of action. These agents are unique in that they increase glucose excretion, independent of insulin secretion, by inhibiting the renal reabsorption of glucose, inducing glycosuria. To summarize the current evidence for SGLT-2 inhibitor therapy, we reviewed abstracts and published data from human trials evaluating the efficacy and safety of dapagliflozin, canagliflozin, and empagliflozin through February 2013. Data from these trials suggest that SGLT-2 inhibitors are able to lower hemoglobin A1c and fasting blood glucose when used as either monotherapy or combination therapy. Cardiometabolic benefits included a reduction in systolic blood pressure, reduction in triglycerides, and weight loss of up to 3 kg. Common and serious adverse effects including infections, cancer, and pollakiuria were identified and reviewed. Although these agents have generally demonstrated efficacy, the adverse effects associated with dapagliflozin have caused a delay in its regulatory approval. Continued research in this area will determine the risk:benefit ratio of SGLT-2 inhibitor therapy.

SGLT inhibitors: a novel target for diabetes

Inhibiting sodium–glucose co-transporters (SGLT1/SGLT2), which have a key role in the absorption of glucose in the kidney and/or GI tract has been proposed as a novel therapeutic strategy for diabetes. Thus, screening and patenting of chemical compounds for SGLT1/SGLT2 gets more importance in the development of new drugs in diabetes. Several companies are developing SGLT inhibitors, some of which are now in various stages of clinical development. Some molecules in the pipeline, including dapagliflozin, canagliflozin, ASP1941, BI10773, LX4211, RG7201 and TS071, are at various stages of drug development. This patent review presents the overall progress carried out in the development of SGLT inhibitors over the last decade with the active participation of various pharmaceutical companies. This class of drug is anticipated to have a large impact on diabetes field and predicting to attain a blockbuster status.

SLC2A2 mutations can cause neonatal diabetes, suggesting GLUT2 may have a role in human insulin secretion

AIMS

The gene SLC2A2 encodes GLUT2, which is found predominantly in pancreas, liver, kidney and intestine. In mice, GLUT2 is the major glucose transporter into pancreatic beta cells, and biallelic Slc2a2 inactivation causes lethal neonatal diabetes. The role of GLUT2 in human beta cells is controversial, and biallelic SLC2A2 mutations cause Fanconi-Bickel syndrome (FBS), with diabetes rarely reported. We investigated the potential role of GLUT2 in the neonatal period by testing whether SLC2A2 mutations can present with neonatal diabetes before the clinical features of FBS appear.

METHODS

We studied SLC2A2 in patients with transient neonatal diabetes mellitus (TNDM; n = 25) or permanent neonatal diabetes mellitus (PNDM; n = 79) in whom we had excluded the common genetic causes of neonatal diabetes, using a combined approach of sequencing and homozygosity mapping.

RESULTS

Of 104 patients, five (5%) were found to have homozygous SLC2A2 mutations, including four novel mutations (S203R, M376R, c.963+1G>A, F114LfsX16). Four out of five patients with SLC2A2 mutations presented with isolated diabetes and later developed features of FBS. Four out of five patients had TNDM (16% of our TNDM cohort of unknown aetiology). One patient with PNDM remains on insulin at 28 months.

CONCLUSIONS

SLC2A2 mutations are an autosomal recessive cause of neonatal diabetes that should be considered in consanguineous families or those with TNDM, after excluding common causes, even in the absence of features of FBS. The finding that patients with homozygous SLC2A2 mutations can have neonatal diabetes supports a role for GLUT2 in the human beta cell.

Lipid composition of cell membranes and its relevance in type 2 diabetes mellitus

Identifying the causative relationship between the fatty acid composition of cell membranes and type 2 diabetes mellitus fundamentally contributes to the understanding of the basic pathophysiological mechanisms of the disease. Important outcomes of the reviewed studies appear to support the hypotheses that the flexibility of a membrane determined by the ratio of (poly)unsaturated to saturated fatty acyl chains of its phospholipids influences the effectiveness of glucose transport by insulin-independent glucose transporters (GLUTs) and the insulin-dependent GLUT4, and from the prediabetic stage on a shift from unsaturated towards saturated fatty acyl chains of membrane phospholipids directly induces a decrease in glucose effectiveness and insulin sensitivity. In addition, it has become evident that a concomitant increase in stiffness of both plasma and erythrocyte membranes may decrease the microcirculatory flow, leading ultimately to tissue hypoxia, insufficient tissue nutrition, and diabetes-specific microvascular pathology. As to the etiology of type 2 diabetes mellitus, a revised hypothesis that attempts to accommodate the reviewed findings is presented.

Weight loss induced by chronic dapagliflozin treatment is attenuated by compensatory hyperphagia in diet-induced obese (DIO) rats

Dapagliflozin is a potent and selective sodium glucose cotransporter-2 (SGLT2) inhibitor which promotes urinary glucose excretion and induces weight loss. Since metabolic compensation can offset a negative energy balance, we explored the potential for a compensatory physiological response to the weight loss induced by dapagliflozin. Dapagliflozin was administered (0.5-5 mpk; p.o.) to diet-induced obese (DIO) rats with or without ad libitum access to food for 38 days. Along with inducing urinary glucose excretion, chronic administration of dapagliflozin dose-dependently increased food and water intake relative to vehicle-treated controls. Despite this, it reduced body weight by 4% (relative to controls) at the highest dose. The degree of weight loss was increased by an additional 9% if hyperphagia was prevented by restricting food intake to that of vehicle controls. Neither oxygen consumption (vO2) or the respiratory exchange ratio (RER) were altered by dapagliflozin treatment alone. Animals treated with dapagliflozin and pair-fed to vehicle controls (5 mpk PF-V) showed a reduction in RER and an elevation in nonfasting β-hydroxybutyrate (BHBA) relative to ad libitum-fed 5 mpk counterparts. Fasting BHBA was elevated in the 1 mpk, 5 mpk, and 5 mpk PF-V groups. Serum glucose was reduced in the fasted, but not the unfasted state. Insulin was reduced in the non-fasted state. These data suggest that in rodents, the persistent urinary glucose excretion induced by dapagliflozin was accompanied by compensatory hyperphagia, which attenuated the weight loss induced by SGLT2 inhibition. Therefore, it is possible that dapagliflozin-induced weight loss could be enhanced with dietary intervention.

The HMGA1-IGF-I/IGFBP system: a novel pathway for modulating glucose uptake

We previously showed that loss of the high mobility group A1 (HMGA1) protein expression, induced in mice by disrupting the Hmga1 gene, considerably decreased insulin receptor expression in the major target tissues of insulin action, causing a type 2-like diabetic phenotype, in which, however, glucose intolerance was paradoxically associated with increased peripheral insulin sensitivity. Insulin hypersensitivity despite impairment of insulin action supports the existence of molecular adaptation mechanisms promoting glucose disposal via insulin-independent processes. Herein, we provide support for these compensatory pathways/circuits of glucose uptake in vivo, the activation of which under certain adverse metabolic conditions may protect against hyperglycemia. Using chromatin immunoprecipitation combined with protein-protein interaction studies of nuclear proteins in vivo, and transient transcription assays in living cells, we show that HMGA1 is required for gene activation of the IGF-binding proteins 1 (IGFBP1) and 3 (IGFBP3), two major members of the IGF-binding protein superfamily. Furthermore, by using positron emission tomography with (18)F-labeled 2-fluoro-2-deoxy-d-glucose, in combination with the euglycemic clamp with IGF-I, we demonstrated that IGF-I's bioactivity was increased in Hmga1-knockout mice, in which both skeletal muscle Glut4 protein expression and glucose uptake were enhanced compared with wild-type littermates. We propose that, by affecting the expression of both IGFBP protein species, HMGA1 can serve as a modulator of IGF-I activity, thus representing an important novel mediator of glucose disposal.

Transepithelial D-glucose and D-fructose transport across the American lobster, Homarus americanus, intestine

Transepithelial transport of dietary D-glucose and d-fructose was examined in the lobster Homarus americanus intestine using D-[(3)H]glucose and D-[(3)H]fructose. Lobster intestines were mounted in a perfusion chamber to determine transepithelial mucosal to serosal (MS) and serosal to mucosal (SM) transport mechanisms of glucose and fructose. Both MS glucose and fructose transport, as functions of luminal sugar concentration, increased in a hyperbolic manner, suggesting the presence of mucosal transport proteins. Phloridizin inhibited the MS flux of glucose, but not that of fructose, suggesting the presence of a sodium-dependent (SGLT1)-like glucose co-transporter. Immunohistochemical analysis, using a goat anti-rabbit GLUT5 polyclonal antibody, revealed the localization of a brush border GLUT5-like fructose transport protein. MS fructose transport was decreased in the presence of mucosal phloretin in warm spring/summer animals, but the same effect was not observed in cold autumn/winter animals, suggesting a seasonal regulation of sugar transporters. Mucosal phloretin had no effect on MS glucose transport. Both SM glucose and SM fructose transport were decreased in the presence of increasing concentrations of serosal phloretin, providing evidence for the presence of a shared serosal GLUT2 transport protein for the two sugars. The transport of d-glucose and d-fructose across lobster intestine is similar to sugar uptake in mammalian intestine, suggesting evolutionarily conserved absorption processes for these solutes.

Kidney: its impact on glucose homeostasis and hormonal regulation

According to current textbook wisdom the liver is the exclusive site of glucose production in humans in the postabsorptive state. Although animal and in vitro studies have documented that the kidney is capable of gluconeogenesis, glucose production by the human kidney has been regarded as negligible. This knowledge is based on net balance measurements across the kidney. Recent studies combining isotopic and balance techniques have demonstrated that the human kidney is involved in the regulation of glucose homeostasis by making glucose via gluconeogenesis, taking up glucose from the circulation, and by reabsorbing glucose from the glomerular filtrate. The human liver and kidneys release approximately equal amounts of glucose via gluconeogenesis in the postabsorptive state. In the postprandial state, although overall endogenous glucose release decreases substantially, renal gluconeogenesis actually increases by approximately 2-fold. Following meal ingestion, glucose utilization by the kidney increases. Increased glucose uptake into the kidney may be implicated in diabetic nephropathy. Normally each day, ∼ 180 g of glucose is filtered by the kidneys; almost all of this is reabsorbed by means of sodium glucose cotransporter 2 (SGLT2), expressed in the proximal tubules. However, the capacity of SGLT2 to reabsorb glucose from the renal tubules is finite and when plasma glucose concentrations exceed a threshold, glucose begins to appear in the urine. Renal glucose release is stimulated by epinephrine and is inhibited by insulin. Handling of glucose by the kidney is altered in type 2 diabetes mellitus (T2DM): renal gluconeogenesis and renal glucose uptake are increased in both the postabsorptive and postprandial states, and renal glucose reabsorption is also increased Since renal glucose release is almost exclusively due to gluconeogenesis, it seems that the kidney is as important gluconeogenic organ as the liver. The most important renal gluconeogenic precursors appear to be lactae glutamine and glycerol.

Role of sodium-glucose cotransporter 2 (SGLT 2) inhibitors in the treatment of type 2 diabetes

Hyperglycemia plays an important role in the pathogenesis of type 2 diabetes mellitus, i.e., glucotoxicity, and it also is the major risk factor for microvascular complications. Thus, effective glycemic control will not only reduce the incidence of microvascular complications but also correct some of the metabolic abnormalities that contribute to the progression of the disease. Achieving durable tight glycemic control is challenging because of progressive β-cell failure and is hampered by increased frequency of side effects, e.g., hypoglycemia and weight gain. Most recently, inhibitors of the renal sodium-glucose cotransporter have been developed to produce glucosuria and reduce the plasma glucose concentration. These oral antidiabetic agents have the potential to improve glycemic control while avoiding hypoglycemia, to correct the glucotoxicity, and to promote weight loss. In this review, we will summarize the available data concerning the mechanism of action, efficacy, and safety of this novel antidiabetic therapeutic approach.

Hypoxic stress and cancer: imaging the axis of evil in tumor metastasis

Tumors emerge as a result of the sequential acquisition of genetic, epigenetic and somatic alterations promoting cell proliferation and survival. The maintenance and expansion of tumor cells rely on their ability to adapt to changes in their microenvironment, together with the acquisition of the ability to remodel their surroundings. Tumor cells interact with two types of interconnected microenvironments: the metabolic cell autonomous microenvironment and the nonautonomous cellular-molecular microenvironment comprising interactions between tumor cells and the surrounding stroma. Hypoxia is a central player in cancer progression, affecting not only tumor cell autonomous functions, such as cell division and invasion, resistance to therapy and genetic instability, but also nonautonomous processes, such as angiogenesis, lymphangiogenesis and inflammation, all contributing to metastasis. Closely related microenvironmental stressors affecting cancer progression include, in addition to hypoxia, elevated interstitial pressure and oxidative stress. Noninvasive imaging offers multiple means to monitor the tumor microenvironment and its consequences, and can thus assist in the understanding of the biological basis of hypoxia and microenvironmental stress in cancer progression, and in the development of strategies to monitor therapies targeted at stress-induced tumor progression.

SGLT2 mediates glucose reabsorption in the early proximal tubule

Mutations in the gene encoding for the Na(+)-glucose co-transporter SGLT2 (SLC5A2) associate with familial renal glucosuria, but the role of SGLT2 in the kidney is incompletely understood. Here, we determined the localization of SGLT2 in the mouse kidney and generated and characterized SGLT2-deficient mice. In wild-type (WT) mice, immunohistochemistry localized SGLT2 to the brush border membrane of the early proximal tubule. Sglt2(-/-) mice had glucosuria, polyuria, and increased food and fluid intake without differences in plasma glucose concentrations, GFR, or urinary excretion of other proximal tubular substrates (including amino acids) compared with WT mice. SGLT2 deficiency did not associate with volume depletion, suggested by similar body weight, BP, and hematocrit; however, plasma renin concentrations were modestly higher and plasma aldosterone levels were lower in Sglt2(-/-) mice. Whole-kidney clearance studies showed that fractional glucose reabsorption was significantly lower in Sglt2(-/-) mice compared with WT mice and varied in Sglt2(-/-) mice between 10 and 60%, inversely with the amount of filtered glucose. Free-flow micropuncture revealed that for early proximal collections, 78 ± 6% of the filtered glucose was reabsorbed in WT mice compared with no reabsorption in Sglt2(-/-) mice. For late proximal collections, fractional glucose reabsorption was 93 ± 1% in WT and 21 ± 6% in Sglt2(-/-) mice, respectively. These results demonstrate that SGLT2 mediates glucose reabsorption in the early proximal tubule and most of the glucose reabsorption by the kidney, overall. This mouse model mimics and explains the glucosuric phenotype of individuals carrying SLC5A2 mutations.

SGLT2 inhibition--a novel strategy for diabetes treatment

Inhibiting sodium-glucose co-transporters (SGLTs), which have a key role in the reabsorption of glucose in the kidney, has been proposed as a novel therapeutic strategy for diabetes. Genetic mutations in the kidney-specific SGLT2 isoform that result in benign renal glycosuria, as well as preclinical and clinical studies with SGLT2 inhibitors in type 2 diabetes, support the potential of this approach. These investigations indicate that elevating renal glucose excretion by suppressing SGLT2 can reduce plasma glucose levels, as well as decrease weight. Although data from ongoing Phase III trials of these agents are needed to more fully assess safety, results suggest that the beneficial effects of SGLT2 inhibition might be achieved without exerting significant side effects--an advantage over many current diabetes medications. This article discusses the role of SGLT2 in glucose homeostasis and the evidence available so far on the therapeutic potential of blocking these transporters in the treatment of diabetes.

Fanconi syndrome in lymphoma patients: report of the first case series

BACKGROUND

Fanconi syndrome (FS) is a generalized transport defect in the proximal renal tubule leading to renal losses of phosphate, calcium, uric acid, bicarbonates as well as glucose, amino acids and other organic compounds. It is caused by inherited or acquired disorders including low mass or high mass multiple myeloma.

OBJECTIVES

To report the first case series of patients with lymphoma and FS.

DESIGN, SETTING, PARTICIPANTS, AND MEASUREMENTS

Patients with lymphoma and FS were identified in the nephrology department of two teaching hospitals in Paris, France and Ghent, Belgium. FS was defined by the presence of at least three out of the four following criteria: hypophosphataemia, metabolic acidosis, normoglycaemic glucosuria and hypokalaemia. Patients files were reviewed and relevant data were collected.

RESULTS

Eight patients with lymphoma and FS were identified. In six patients, the lymphoma was of the acute T cell leukaemia/lymphoma (ATLL) type, related to human T cell lymphotropic virus 1 (HTLV1) infection. In all patients, FS was severe requiring supplementation. A kidney biopsy performed in a patient with post-transplantation primary renal lymphoma disclosed intense proximal tubule infiltration by lymphomatous cells. In one patient with ATLL, FS features regressed following the successful treatment of lymphoma.

CONCLUSION

Patients with lymphoma require careful monitoring for features of FS; lymphoma should also be added to the spectrum of disorders associated to FS. Prospective studies are needed to ascertain the implication of HTLV1 in the genesis of FS.

Role of the kidney in normal glucose homeostasis and in the hyperglycaemia of diabetes mellitus: therapeutic implications

Considerable data have accumulated over the past 20 years, indicating that the human kidney is involved in the regulation of glucose via gluconeogenesis, taking up glucose from the circulation, and by reabsorbing glucose from the glomerular filtrate. In light of the development of glucose-lowering drugs involving inhibition of renal glucose reabsorption, this review summarizes these data. Medline was searched from 1989 to present using the terms 'renal gluconeogenesis', 'renal glucose utilization', 'diabetes mellitus' and 'glucose transporters'. The human liver and kidneys release approximately equal amounts of glucose via gluconeogenesis in the post-absorptive state. In the postprandial state, although overall endogenous glucose release decreases substantially, renal gluconeogenesis increases by approximately twofold. Glucose utilization by the kidneys after an overnight fast accounts for approximately 10% of glucose utilized by the body. Following a meal, glucose utilization by the kidney increases. Normally each day, approximately 180 g of glucose is filtered by the kidneys; almost all of this is reabsorbed by means of sodium-glucose co-transporter 2 (SGLT2), expressed in the proximal tubules. However, the capacity of SGLT2 to reabsorb glucose from the renal tubules is finite and, when plasma glucose concentrations exceed a threshold, glucose appears in the urine. Handling of glucose by the kidney is altered in Type 2 diabetes mellitus (T2DM): renal gluconeogenesis and renal glucose uptake are increased in both the post-absorptive and postprandial states, and renal glucose reabsorption is increased. Specific SGLT2 inhibitors are being developed as a novel means of controlling hyperglycaemia in T2DM.

Familial renal glucosuria and SGLT2: from a mendelian trait to a therapeutic target

Four members of two glucose transporter families, SGLT1, SGLT2, GLUT1, and GLUT2, are differentially expressed in the kidney, and three of them have been shown to be necessary for normal glucose resorption from the glomerular filtrate. Mutations in SGLT1 are associated with glucose-galactose malabsorption, SGLT2 with familial renal glucosuria (FRG), and GLUT2 with Fanconi-Bickel syndrome. Patients with FRG have decreased renal tubular resorption of glucose from the urine in the absence of hyperglycemia and any other signs of tubular dysfunction. Glucosuria in these patients can range from <1 to >150 g/1.73 m(2) per d. The majority of patients do not seem to develop significant clinical problems over time, and further description of specific disease sequelae in these individuals is reviewed. SGLT2, a critical transporter in tubular glucose resorption, is located in the S1 segment of the proximal tubule, and, as such, recent attention has been given to SGLT2 inhibitors and their utility in patients with type 2 diabetes, who might benefit from the glucose-lowering effect of such compounds. A natural analogy is made of SGLT2 inhibition to observations with inactivating mutations of SGLT2 in patients with FRG, the hereditary condition that results in benign glucosuria. This review provides an overview of renal glucose transport physiology, FRG and its clinical course, and the potential of SGLT2 inhibition as a therapeutic target in type 2 diabetes.

A study of dapagliflozin in patients with type 2 diabetes receiving high doses of insulin plus insulin sensitizers: applicability of a novel insulin-independent treatment

OBJECTIVE

To determine whether dapagliflozin, which selectively inhibits renal glucose reabsorption, lowers hyperglycemia in patients with type 2 diabetes that is poorly controlled with high insulin doses plus oral antidiabetic agents (OADs).

RESEARCH DESIGN AND METHODS

This was a randomized, double-blind, three-arm parallel-group, placebo-controlled, 26-center trial (U.S. and Canada). Based on data from an insulin dose-adjustment setting cohort (n = 4), patients in the treatment cohort (n = 71) were randomly assigned 1:1:1 to placebo, 10 mg dapagliflozin, or 20 mg dapagliflozin, plus OAD(s) and 50% of their daily insulin dose. The primary outcome was change from baseline in A1C at week 12 (dapagliflozin vs. placebo, last observation carried forward [LOCF]).

RESULTS

At week 12 (LOCF), the 10- and 20-mg dapagliflozin groups demonstrated -0.70 and -0.78% mean differences in A1C change from baseline versus placebo. In both dapagliflozin groups, 65.2% of patients achieved a decrease from baseline in A1C > or =0.5% versus 15.8% in the placebo group. Mean changes from baseline in fasting plasma glucose (FPG) were +17.8, +2.4, and -9.6 mg/dl (placebo, 10 mg dapagliflozin, and 20 mg dapagliflozin, respectively). Postprandial glucose (PPG) reductions with dapagliflozin also showed dose dependence. Mean changes in total body weight were -1.9, -4.5, and -4.3 kg (placebo, 10 mg dapagliflozin, and 20 mg dapagliflozin). Overall, adverse events were balanced across all groups, although more genital infections occurred in the 20-mg dapagliflozin group than in the placebo group.

CONCLUSIONS

In patients receiving high insulin doses plus insulin sensitizers who had their baseline insulin reduced by 50%, dapagliflozin decreased A1C, produced better FPG and PPG levels, and lowered weight more than placebo.

The glucose transporter 2 undergoes plasma membrane endocytosis and lysosomal degradation in a secretagogue-dependent manner

In beta-cells of the pancreas, the glucose transporter (GLUT)-2 facilitative glucose transporter protein is localized to the plasma membrane and functions as part of the glucose sensing mechanism for the stimulation of insulin secretion. We observed that expressed GLUT2 protein in the cultured Min6B1 cell line undergoes enhanced endocytosis at high extracellular glucose concentrations that stimulate insulin secretion. Moreover, the internalized GLUT2 protein undergoes rapid degradation induced by chronic high-glucose or arginine stimulation but does not undergo plasma membrane recycling or accumulation in any microscopically apparent intracellular membrane compartment. The rapid degradation of GLUT2 was prevented by lysosomal inhibition (chloroquine) concomitant with the accumulation of GLUT2 in endomembrane structures. In contrast, neither endocytosis nor the lack of internal membrane localized GLUT2 remained completely unaffected by proteosomal inhibition (lactacystin) or an heat shock protein-90 inhibitor (geldanamycin). Moreover, the endocytosis and degradation of GLUT2 was specific for beta-cells because expression of GLUT2 in 3T3L1 adipocytes remained cell surface localized and did not display a rapid rate of degradation. Together, these data demonstrate that hyperglycemia directly affects beta-cell function and activates a trafficking pathway that results in the rapid endocytosis and degradation of the cell surface GLUT2 glucose transporter.

On the free energy that drove primordial anabolism

A key problem in understanding the origin of life is to explain the mechanism(s) that led to the spontaneous assembly of molecular building blocks that ultimately resulted in the appearance of macromolecular structures as they are known in modern biochemistry today. An indispensable thermodynamic prerequisite for such a primordial anabolism is the mechanistic coupling to processes that supplied the free energy required. Here I review different sources of free energy and discuss the potential of each form having been involved in the very first anabolic reactions that were fundamental to increase molecular complexity and thus were essential for life.

Glucose control by the kidney: an emerging target in diabetes

The full significance of the kidney's role in glucose homeostasis is now well recognized. For example, it is now known that renal gluconeogenesis contributes substantially to total-body glucose release in the postabsorptive state. The kidney contributes to glucose homeostasis by filtering and reabsorbing glucose. Under normal circumstances, glucose filtered by glomeruli is completely reabsorbed, but glucosuria may occur under conditions of hyperglycemia or reduced reabsorptive capacity. The sodium-glucose cotransporter SGLT2 (encoded by the SLC5A2 gene), which is expressed almost exclusively in proximal tubules, mediates approximately 90% of active renal glucose reabsorption. This transporter can be blocked by SGLT2 inhibitors, a class of compound that may prove effective in managing type 2 diabetes. The glucosuria induced by these compounds has a naturally occurring parallel in familial renal glucosuria (FRG), a condition in which SGLT2 mutations reduce renal reabsorptive capacity. Interestingly, the chronic glucosuria of patients with FRG does not appear to be associated with other pathological changes, and patients with FRG are mostly asymptomatic. This suggests that glucosuria is not intrinsically detrimental. Selective SGLT2 inhibitors are currently in clinical trials.

From victim to ally: the kidney as an emerging target for the treatment of diabetes mellitus

BACKGROUND

Elevated glucose blood levels are the key criteria for diagnosing diabetes mellitus (DM). Hyperglycaemia contributes to the pathophysiology associated with DM, including microvascular and possibly macrovascular disease. In spite of a wide range of pharmacological options available to reduce hyperglycaemia in DM, epidemiological studies suggest that glucose levels remain high in a substantial proportion of patients. This supports the need for additional strategies for the treatment of hyperglycaemia.

SCOPE

This review focuses on the role of the kidney in glucose reabsorption and explores inhibition of renal glucose reabsorption as a novel approach to treat type 2 DM. A literature search to August 2008 using PubMed was used to compile data for review. Abstracts and presentations from the American Diabetes Association and the European Association for the Study of Diabetes, the American Society of Nephrology, and the International Society of Nephrology Annual Meetings were also searched for relevant studies.

FINDINGS

Glucose filtered by the kidney is normally reabsorbed into the proximal renal tubule. Data from animal models suggest that approximately 90% of this reabsorption occurs through the sodium-coupled glucose cotransporter (SGLT) 2, which is a protein expressed almost exclusively in the proximal tubule of the kidney. Inhibition of SGLT2, and thus inhibition of renal glucose reabsorption, has the potential to reduce hyperglycaemia in patients with DM. Patients with familial renal glucosuria, a genetic disorder of SGLT2, do not appear to have adverse clinical consequences related to impaired renal reabsorption of glucose, which suggests that SGLT2 might be both an effective and safe treatment target for hyperglycaemia. In animal models of DM, pharmaceutical inhibition of SGLT2 reduces hyperglycaemia, and may improve insulin resistance.

CONCLUSION

Reduction of renal glucose reabsorption is a novel approach to DM treatment that potentially provides improvements in glucose lowering. Various SGLT2 inhibitors are currently in development in human trials.

Dapagliflozin, a novel SGLT2 inhibitor, induces dose-dependent glucosuria in healthy subjects

Dapagliflozin selectively inhibits renal glucose reabsorption by inhibiting sodium-glucose cotransporter-2 (SGLT2). It was developed as an insulin-independent treatment approach for type 2 diabetes mellitus (T2DM). The safety, tolerability, pharmacokinetics, and pharmacodynamics of the drug were evaluated in single-ascending-dose (SAD; 2.5-500 mg) and multiple-ascending-dose (MAD; 2.5-100 mg daily for 14 days) studies in healthy subjects. Dapagliflozin exhibited dose-proportional plasma concentrations with a half-life of approximately 17 h. The amount of glucosuria was also dose-dependent. Cumulative amounts of glucose excreted on day 1, relating to doses from 2.5-100 mg (MAD), ranged from 18 to 62 g; day 14 values were comparable to day 1 values, with no apparent changes in glycemic parameters. Doses of approximately 20-50 mg provided close-to-maximal SGLT2 inhibition for at least 24 h. Dapagliflozin demonstrates pharmacokinetic (PK) characteristics and dose-dependent glucosuria that are sustained over 24 h, which indicates that it is suitable for administration in once-daily doses and suggests that further investigation of its efficacy in T2DM patients is warranted.

Extrahepatic hepatitis C virus after transplantation: diabetes and renal dysfunction

1. Insulin resistance is associated with hepatitis C virus infection and plays a role in the progression of hepatitis C virus-related liver disease and fibrosis. 2. Treating insulin resistance and achieving glycemic control will be important for improving post-liver transplant morbidity and mortality: control of the hepatitis C virus will help to accomplish this. 3. The main renal complication of hepatitis C virus is membranoproliferative glomerulonephritis, and this occurs most commonly in the setting of mixed cryoglobulinemia.

Hypoxia, HIF1 and glucose metabolism in the solid tumour

It has been known for many years that cellular metabolism within the solid tumour is markedly different from that of the corresponding normal tissue. The transcription factor hypoxia-inducible factor 1 (HIF1) has been implicated in regulating many of the genes that are responsible for the metabolic difference. However, it remains unclear how this 'aerobic glycolysis', originally described by Otto Warburg, offers tumour cells a growth advantage. As discussed in this Perspective, new data suggests that this metabolic switch may provide a benefit to the tumour not by increasing glycolysis but by decreasing mitochondrial activity.

Resistin and insulin resistance in hepatocytes: resistin disturbs glycogen metabolism at the protein level

Resistin has been considered to link obesity with type 2 diabetes. Liver glycogen metabolism plays an essential role in maintaining glucose homeostasis, we investigated the effect of resistin on liver glycogen metabolism and attempted to identify its role in initiating insulin resistance and type 2 diabetes. Primary culture of rat hepatocytes was treated by resistin and insulin. Glycogen content was determined by the anthrone-reagent method. Real-time PCR, Western blot and enzymatic activity assay were used to detect key enzymes and genes involved in glucose metabolism. Hepatocytes exposed to resistin, but only in the presence of insulin, show a decrease in insulin-stimulated glycogen content. Decreased insulin receptor expression and GS activity and elevated GP activity was observed after the treatment of hepatocytes with resistin. No significant changes in the expression of the genes for these proteins were observed. These results strongly suggest that resistin effects glycogen metabolism at the protein level, and resistin is highly associated with insulin resistance and type 2 diabetes and is a candidate for the prevention and treatment of type 2 diabetes. Our results should lead to the development of novel strategies for the treatment of type 2 diabetes.

Genetic variant in the glucose transporter type 2 is associated with higher intakes of sugars in two distinct populations

Glucose sensing in the brain has been proposed to be involved in regulating food intake, but the mechanism is not known. Glucose transporter type 2 (GLUT2)-null mice fail to control their food intake in response to glucose, suggesting a potential role for this transporter as a glucose sensor in the brain. Here we show that individuals with a genetic variation in GLUT2 (Thr110Ile) have a higher daily intake of sugars in two distinct populations. In the first population, compared with individuals with the Thr/Thr genotype, carriers of the Ile allele had a significantly higher intake of sugars as assessed from 3-day food records administered on two separate visits (visit 1: 112 +/- 9 vs. 86 +/- 4 g/day, P = 0.01; visit 2: 111 +/- 8 vs. 82 +/- 4 g/day, P = 0.003), demonstrating within-population reproducibility. In a second population, carriers of the Ile allele also reported consuming a significantly greater intake of sugars (131 +/- 5 vs. 115 +/- 3 g/day, P = 0.007) over a 1-mo period as measured from a food frequency questionnaire. GLUT2 genotypes were not associated with fat, protein, or alcohol intake in either population. These observations were consistent across older and younger adults as well as among subjects with early Type 2 diabetes and healthy individuals. Taken together, our findings show that a genetic variation in GLUT2 is associated with habitual consumption of sugars, suggesting an underlying glucose-sensing mechanism that regulates food intake.

Apical and basolateral localisation of GLUT2 transporters in human lung epithelial cells

Glucose concentrations of normal human airway surface liquid are ~12.5 times lower than blood glucose concentrations indicating that glucose uptake by epithelial cells may play a role in maintaining lung glucose homeostasis. We have therefore investigated potential glucose uptake mechanisms in non-polarised and polarised H441 human airway epithelial cells and bronchial biopsies. We detected mRNA and protein for glucose transporter type 2 (GLUT2) and glucose transporter type 4 (GLUT4) in non-polarised cells but GLUT4 was not detected in the plasma membrane. In polarised cells, GLUT2 protein was detected in both apical and basolateral membranes. Furthermore, GLUT2 protein was localised to epithelial cells of human bronchial mucosa biopsies. In non-polarised H441 cells, uptake of d-glucose and deoxyglucose was similar. Uptake of both was inhibited by phloretin indicating that glucose uptake was via GLUT-mediated transport. Phloretin-sensitive transport remained the predominant route for glucose uptake across apical and basolateral membranes of polarised cells and was maximal at 5–10 mM glucose. We could not conclusively demonstrate sodium/glucose transporter-mediated transport in non-polarised or polarised cells. Our study provides the first evidence that glucose transport in human airway epithelial cells in vitro and in vivo utilises GLUT2 transporters. We speculate that these transporters could contribute to glucose uptake/homeostasis in the human airway.