Hyperglycemia and intestinal and renal glucose transport: implications for diabetic renal injury

The SGLT2 inhibitor dapagliflozin improves kidney function in glycogen storage disease XI

Glycogen storage disease XI, also known as Fanconi-Bickel syndrome (FBS), is a rare autosomal recessive disorder caused by mutations in the SLC2A2 gene that encodes the glucose-facilitated transporter type 2 (GLUT2). Patients develop a life-threatening renal proximal tubule dysfunction for which no treatment is available apart from electrolyte replacement. To investigate the renal pathogenesis of FBS, SLC2A2 expression was ablated in mouse kidney and HK-2 proximal tubule cells. GLUT2Pax8Cre+ mice developed time-dependent glycogen accumulation in proximal tubule cells and recapitulated the renal Fanconi phenotype seen in patients. In vitro suppression of GLUT2 impaired lysosomal autophagy as shown by transcriptomic and biochemical analysis. However, this effect was reversed by exposure to a low glucose concentration, suggesting that GLUT2 facilitates the homeostasis of key cellular pathways in proximal tubule cells by preventing glucose toxicity. To investigate whether targeting proximal tubule glucose influx can limit glycogen accumulation and correct symptoms in vivo, we treated mice with the selective SGLT2 inhibitor dapagliflozin. Dapagliflozin reduced glycogen accumulation and improved metabolic acidosis and phosphaturia in the animals by normalizing the expression of Napi2a and NHE3 transporters. In addition, in a patient with FBS, dapagliflozin was safe, improved serum potassium and phosphate concentrations, and reduced glycogen content in urinary shed cells. Overall, this study provides proof of concept for dapagliflozin as a potentially suitable therapy for FBS.

Therapeutic activity of sarpogrelate and dopamine D2 receptor agonists on cardiovascular and renal systems in rats with alloxan-induced diabetes

BACKGROUND

Dopamine D2 receptor agonists, bromocriptine and cabergoline, are notable medications in the treatment of Parkinsonism, hyperprolactinemia, and hyperglycemia. An affiliation was found between the initiation of myocardial injury ailment and long term treatment with dopamine D2 agonist drugs identified with the partial activation of 5-hydroxytryptamine receptor 2 A (5-HT2A). The investigation aimed to examine the activity of sarpogrelate (a 5-HT2A receptor blocker) in reducing myocardial injury prompted by extended haul utilisation of D2 receptor agonists in rats with alloxan-induced diabetes.

METHODS

Both bromocriptine and cabergoline were managed independently and combined with sarpogrelate for about a month in diabetic nephropathy rats. Both tail-cuff blood pressure and the BGL were recorded weekly. For all animals, the kidney hypertrophy index, serum creatinine, blood urea nitrogen, alanine transaminase, and aspartate transaminase levels were measured after one month of treatment. The severity of the cardiac injury was assessed by the estimation of lactate dehydrogenase-1 (LDH-1), cardiac troponin I, and tumor necrosis factor alpha 1 (TNF1). The triphenyltetrazolium chloride (TTC) staining method was used to determine the experimental myocardial infarction (MI) size.

RESULTS

Bromocriptine and cabergoline created a significant reduction in BGL, BP, and kidney hypertrophy index in diabetic nephropathy rats. Administration of bromocriptine and cabergoline, alone, or in combination with sarpogrelate fundamentally diminished the blood concentrations of alkaline phosphatase (ALP), Aspartate aminotransferase (AST), urea, and creatinine. Bromocriptine and cabergoline alone showed a noteworthy increase in the LDH-1, Troponin I, and TNF1 levels in the serum (p < 0.05). Paradoxically, utilising bromocriptine or cabergoline with sarpogrelate treatment altogether decreased the levels of the myocardial biomarkers in the serum. A mix of bromocriptine or cabergoline with sarpogrelate diminished the level of the myocardial infarct size in the heart assessed through the TTC staining method.

CONCLUSIONS

The examination demonstrated that the combined use of sarpogrelate with bromocriptine or cabergoline decreased the potential adverse effects of these two drugs on the myocardial tissues.

Nrf2 a molecular therapeutic target for Astaxanthin

Astaxanthin (ATX) is a red pigment carotenoid present in shrimp, salmon, crab, and asteroidean. Several studies have corroborated the anti-oxidant efficacy of ATX. In addition, ATX has anti-inflammatory, anti-apoptotic and anti-proliferative properties. In the present review, we discuss the role of Nrf2 in mediating the anti-cancer, anti-aging, neuroprotective, lung-protective, skin-protective, cardioprotective, hepatoprotective, anti-diabetic and muscloprotective effects of ATX.

Sinapic Acid Ameliorates the Progression of Streptozotocin (STZ)-Induced Diabetic Nephropathy in Rats via NRF2/HO-1 Mediated Pathways

Diabetic neuropathy (DN) is a complicated inauspicious outcome of diabetes, like other abnormalities of diabetes the cause of DN is still vague and it may be the result of various pathological conditions leading up to end-stage renal failure. The present study examines the efficacy of sinapic acid (SA) in streptozotocin (STZ)-induced DN nephropathy and the linked pathway. Twenty-four rats were equally divided randomly into four categories: Normal control (NC), STZ, STZ + SA 20 mg/kg bw, and STZ + SA 40 mg/kg bw. After 8 weeks they were evaluated for ratio of renal index, the fasting blood glucose (FBG), blood urea nitrogen (BUN), 24 h urea protein, serum creatinine (SCr), reduced glutathione peroxidase (GPx), superoxide dismutase (SOD), lipid peroxidation (MDA), tumor necrosis factor α (TNFα), interleukin (IL)-6, as well as lipid profile total cholesterol (TC), total triglycerides (TG), very low density lipoprotein (VLDL), low density lipoprotein (LDL), and high density lipoprotein (HDL) levels. Additionally, histomorphology and ultrastructure of the kidneys were also assessed. Protein expression levels of transforming growth factor-β1 (TGF-β1), nuclear factor erythroid 2-related factor 2 (Nrf2), heme oxygenase-1 (HO-1), IκBα protein (IkBα), anti-apoptotic protein BCl2, nuclear factor kappa B (NF-kB), and Bax were examined. We observed that SA 20 mg/kg bw and 40 mg/kg bw pretreatment significantly and dose-dependently upregulated the protein expression of HO-1, Nrf2, IKBα, and Bcl-2 but downregulated the protein expression of NF-κB, proposing that the nephroprotective mechanism of SA is due to its antioxidant and anti-inflammatory activity; SA prevents the release of cytokines and inflammatory markers (TNFα and IL-6), upregulates antioxidant defense enzymes, and reduces lipid peroxidation, as well as nitric oxide, and anti-apoptotic activity, which may be influenced by the regulation of TNF-α, IL-6, Bcl-2, NF-kB, and BaX via the Nrf2/HO-1 pathway in STZ induced DN. Thus, our results suggest that SA ameliorates the development of STZ-induced DN in rats via NRF2/HO-1 mediated pathways. Further comprehensive studies are required for complete elucidation of the fundamental mechanisms.

Increased circulating cytokine levels in African American women with obesity and elevated HbA1c

PURPOSE

Obesity has emerged as one of the biggest health crisis and is the leading cause of death and disabilities around the world. BMI trends suggest that majority of the increase in T2D is resulting from the increased prevalence of obesity. In fact, 85.2% of people with T2D are overweight or obese. The highest prevalence for obesity is seen in non-Hispanic, African American women (56.6%). T2D is classified as an inflammatory disease because of elevated, circulating pro-inflammatory cytokines and acute-phase inflammatory proteins. This study was designed to determine how high HbA1c and serum glucose correlate with circulatory cytokine levels in obese, African American women.

METHODS

We investigated cytokine/chemokine serum levels using a multiplex assay. Then we used Pairwise Pearson Correlation Test to determine the relationship between clinical metabolic parameters and cytokine/chemokine serum levels.

RESULTS

The results indicated that participants with elevated HbA1c exhibited an up regulation of IL-3, IL-4, IL-7, TNF-α, IFN-α2 and CX3CL1 serum levels compared to participants with normal HbA1c. These cytokines were also correlated with several clinical metabolic parameters.

CONCLUSIONS

The results suggest that IL-3, IL-4, IL-7, TNF-α, IFN-α2 and CX3CL1 serum levels may contribute to the development and onset of type 2 diabetes.

Activation of Nrf2 signaling by natural products-can it alleviate diabetes?

Type 2 diabetes mellitus (DM) has reached pandemic proportions and effective prevention strategies are wanted. Its onset is accompanied by cellular distress, the nuclear factor erythroid 2-related factor 2 (Nrf2) is a transcription factor boosting cytoprotective responses, and many phytochemicals activate Nrf2 signaling. Thus, Nrf2 activation by natural products could presumably alleviate DM. We summarize function, regulation and exogenous activation of Nrf2, as well as diabetes-linked and Nrf2-susceptible forms of cellular stress. The reported amelioration of insulin resistance, β-cell dysfunction and diabetic complications by activated Nrf2 as well as the status quo of Nrf2 in precision medicine for DM are reviewed.

Astaxanthin Promotes Nrf2/ARE Signaling to Alleviate Renal Fibronectin and Collagen IV Accumulation in Diabetic Rats

Astaxanthin (AST), a natural keto-carotenoid classified as a xanthophyll, is well known for its antioxidant properties. AST can ameliorate the pathological characteristics of diabetic nephropathy (DN). However, the underlying mechanisms remain to be explored. This study was aimed at exploring whether AST exerts a protective effect on DN via activating nuclear factor erythroid 2-related factor 2- (Nrf2-) antioxidative response element (ARE) signaling. Streptozotocin-induced diabetic rats were treated with AST for 12 weeks. We found that AST treatment ameliorated renal morphological injury. Reduced fibronectin and collagen IV protein expression were found in the kidneys of diabetic rats. Furthermore, AST promoted the nuclear translocation of Nrf2 and increased its downstream protein heme oxygenase-1 and superoxide dismutase 1 expression. AST also increased the activity of SOD and decreased malondialdehyde generation in the serum of diabetic rats. These results suggest that the renoprotective effect of AST on DN partly depends on Nrf2-ARE signaling. The antioxidative stress effect of AST is responsible for the activation of Nrf2-ARE signaling in DN.

Connexin43 regulates high glucose-induced expression of fibronectin, ICAM-1 and TGF-β1 via Nrf2/ARE pathway in glomerular mesangial cells

Nrf2/ARE signaling pathway is a crucial cellular defense system to cope with oxidative stress, which is adaptively activated, in diabetic condition that is not efficient enough to resist the oxidative stress provoked by hyperglycemia. We have previously demonstrated that Connexin43 (Cx43) attenuates renal fibrosis through c-Src. However, the underlying mechanisms need to be further clarified. It has been reported that Cx43 possesses the ability of anti-oxidative. The current study aimed to determine if Cx43 exerts protective effects on renal fibrosis in diabetes via activation of Nrf2/ARE pathway and explore the underlying molecular mechanisms. The following findings were observed: (1) Cx43 expression decreased and c-Src activity increased in kidneys of diabetic animals; (2) Over-expressed Cx43 in high glucose treated GMCs inhibited protein levels of FN, ICAM-1 and TGF-β1; (3) Nrf2/ARE signaling adaptively responded to high glucose treatment in GMCs; (4) Cx43 reduced ROS generation by boost Nrf2/ARE signaling under high glucose condition; (5) Inhibition of c-Src activity promoted nucleus accumulation of Nrf2; (6) Over-expressed Cx43 inhibited c-Src activity and the interaction between c-Src and Nrf2 in GMCs cultured in high glucose. Thus we propose that Cx43 might enhance the activation of Nrf2/ARE pathway by means of inhibiting c-Src activity to hinder the nuclear export of Nrf2, and then reduce expression of FN, ICAM-1 and TGF-β1, ultimately attenuating renal fibrosis in diabetes.

Glucose transporters: cellular links to hyperglycemia in insulin resistance and diabetes

Abnormal expression and/or function of mammalian hexose transporters contribute to the hallmark hyperglycemia of diabetes. Due to different roles in glucose handling, various organ systems possess specific transporters that may be affected during the diabetic state. Diabetes has been associated with higher rates of intestinal glucose transport, paralleled by increased expression of both active and facilitative transporters and a shift in the location of transporters within the enterocyte, events that occur independent of intestinal hyperplasia and hyperglycemia. Peripheral tissues also exhibit deregulated glucose transport in the diabetic state, most notably defective translocation of transporters to the plasma membrane and reduced capacity to clear glucose from the bloodstream. Expression of renal active and facilitative glucose transporters increases as a result of diabetes, leading to elevated rates of glucose reabsorption. However, this may be a natural response designed to combat elevated blood glucose concentrations and not necessarily a direct effect of insulin deficiency. Functional foods and nutraceuticals, by modulation of glucose transporter activity, represent a potential dietary tool to aid in the management of hyperglycemia and diabetes.

Therapeutic potential of digitoflavone on diabetic nephropathy: nuclear factor erythroid 2-related factor 2-dependent anti-oxidant and anti-inflammatory effect

Nuclear factor erythroid 2-related factor 2 (Nrf2) has emerged as a therapeutic target in many diseases, because it can induce antioxidant enzymes and other cytoprotective enzymes. Moreover, some Nrf2 activators have strong anti-inflammatory activities. Oxidative stress and inflammation are major components involved in the pathology of diabetic nephropathy. In the present study, we evaluated the Nrf2-dependent anti-oxidative and anti-inflammatory effects of digitoflavone in streptozotocin-induced diabetic nephropathy. The molecular mechanisms of digitoflavone were investigated in vitro using SV40-transformed mouse mesangial cells (SV40-Mes13). For the in vivo experiment, diabetes was induced in Nrf2+/+ and Nrf2-/- mice by STZ injection, and digitoflavone was administered 2 weeks after the STZ injection. Digitoflavone induced Nrf2 activation and decreased oxidative damage, inflammation, TGF-β1 expression, extracellular matrix protein expression, and mesangial cell hyperplasia in SV40-Mes13 cells. Digitoflavone-treated Nrf2+/+ mice, but not Nrf2-/- mice, showed attenuated common metabolic disorder symptoms, improved renal performance, minimized pathological alterations, and decreased oxidative damage, inflammatory gene expression, inflammatory cell infiltration, TGF-β1 expression, and extracellular matrix protein expression. Our results show that the anti-oxidative and anti-inflammatory effects of digitoflavone are mediated by Nrf2 activation and that digitoflavone can be used therapeutically to improve metabolic disorders and relieve renal damage induced by diabetes.

Inflammation in diabetic kidney disease

Diabetes mellitus entails significant health problems worldwide. The pathogenesis of diabetes is multifactorial, resulting from interactions of both genetic and environmental factors that trigger a complex network of pathophysiological events, with metabolic and hemodynamic alterations. In this context, inflammation has emerged as a key pathophysiology mechanism. New pathogenic pathways will provide targets for prevention or future treatments. This review will focus on the implications of inflammation in diabetes mellitus, with special attention to inflammatory cytokines.

Resveratrol attenuates early diabetic nephropathy by down-regulating glutathione s-transferases Mu in diabetic rats

Diabetic nephropathy (DN) is the major cause of end-stage renal disease. Resveratrol has been shown to ameliorate hyperglycemia in diabetic rats. However, the effects of resveratrol on DN remain unknown. The aim of the present study is to investigate the effects of resveratrol on early-stage DN. Diabetes was induced by streptozotocin injection in male Wistar rats. The diabetic rats were treated with resveratrol at a dose of 20 mg/kg body weight for 8 weeks. Plasma glucose, creatinine, kidney/body weight ratio, and 24-h urinary protein were determined. The renal pathological changes were examined with periodic acid Schiff staining, and renal mesangial cells were cultured in high glucose concentrations with indicated concentrations of resveratrol (2.5, 5.0, and 10.0 μmol/L). The proliferation of mesangial cells was evaluated by methylthiazoletetrazolium assay. Expressions of glutathione S-transferases Mu (GSTM) and nuclear factor erythroid 2-related factor 2 (Nrf2) were detected by western blot, and apoptosis was analyzed using a flow cytometer. Resveratrol reduced plasma glucose, creatinine, and urinary protein excretion, and attenuated renal hypertrophy. Moreover, resveratrol also reduced the expression of GSTM in diabetic rats. In vitro, resveratrol inhibited the proliferation of mesangial cells caused by high glucose and down-regulated GSTM and Nrf2 expressions in a dose-dependent manner. These findings suggest that resveratrol help prevent the progression of DN. The renoprotection by resveratrol is in part mediated through the inhibition of high glucose-induced rat mesangial cell proliferation and downregulation of GSTM expression.

Nuclear factor (erythroid-derived 2)-like 2 (NFE2L2) is a novel therapeutic target for diabetic complications

Diabetes is a leading cause of death and disability. In 2004, 3.4 million people worldwide died of symptoms relating to high blood sugar. Diabetic complications are caused by organ damage resulting from long-term exposure to high blood sugar, and include diseases such as heart failure, kidney failure, vision loss and neuropathy. The transcription factor nuclear factor (erythroid-derived 2)-like 2 (NFE2L2, also known as NRF2) is an important component of the intracellular antioxidant machinery and a target for treatment of diabetic complications. This article reviews the role of NFE2L2 in diabetic complications with a focus on diabetic nephropathy, cardiomyopathy, neuropathy and retinopathy. Activation of NFE2L2 protects against oxidative stress in vitro and in vivo, and represents an important target for prophylaxis and treatment of diabetic complications. NFE2L2 has potential clinical applications for diabetic patients in the near future.

Upregulation of ACE2-ANG-(1-7)-Mas axis in jejunal enterocytes of type 1 diabetic rats: implications for glucose transport

The inhibitory effects of the angiotensin-converting enzyme (ACE)-ANG II-angiotensin type 1 (AT₁) receptor axis on jejunal glucose uptake and the reduced expression of this system in type 1 diabetes mellitus (T1DM) have been documented previously. The ACE2-ANG-(1-7)-Mas receptor axis is thought to oppose the actions of the ACE-ANG II-AT₁ receptor axis in heart, liver, and kidney. However, the possible involvement of the ACE2-ANG-(1-7)-Mas receptor system on enhanced jejunal glucose transport in T1DM has yet to be determined. Rat everted jejunum and Caco-2 cells were used to determine the effects of ANG-(1-7) on glucose uptake and to study the ACE2-ANG-(1-7)-Mas receptor signaling pathway. Expression of target gene and protein in jejunal enterocytes and human Caco-2 cells were quantified using real-time PCR and Western blotting. T1DM increased jejunal protein and mRNA expression of ACE2 (by 59 and 173%, respectively) and Mas receptor (by 55 and 100%, respectively) in jejunum. One millimolar ANG-(1-7) reduced glucose uptake in jejunum and Caco-2 cells by 30.6 and 30.3%, respectively, effects that were abolished following addition of 1 μM A-779 (a Mas receptor blocker) or 1 μM GF-109203X (protein kinase C inhibitor) to incubation buffer for jejunum or Caco-2 cells, respectively. Finally, intravenous treatment of animals with ANG-(1-7) significantly improved oral glucose tolerance in T1DM but not control animals. In conclusion, enhanced activity of the ACE2-ANG-(1-7)-Mas receptor axis in jejunal enterocytes is likely to moderate the T1DM-induced increase in jejunal glucose uptake resulting from downregulation of the ACE-ANG II-AT₁ receptor axis. Therefore, altered activity of both ACE and ACE2 systems during diabetes will determine the overall rate of glucose transport across the jejunal epithelium.

The protective role of Nrf2 in streptozotocin-induced diabetic nephropathy

OBJECTIVE

Diabetic nephropathy is one of the major causes of renal failure, which is accompanied by the production of reactive oxygen species (ROS). Nrf2 is the primary transcription factor that controls the antioxidant response essential for maintaining cellular redox homeostasis. Here, we report our findings demonstrating a protective role of Nrf2 against diabetic nephropathy.

RESEARCH DESIGN AND METHODS

We explore the protective role of Nrf2 against diabetic nephropathy using human kidney biopsy tissues from diabetic nephropathy patients, a streptozotocin-induced diabetic nephropathy model in Nrf2(-/-) mice, and cultured human mesangial cells.

RESULTS

The glomeruli of human diabetic nephropathy patients were under oxidative stress and had elevated Nrf2 levels. In the animal study, Nrf2 was demonstrated to be crucial in ameliorating streptozotocin-induced renal damage. This is evident by Nrf2(-/-) mice having higher ROS production and suffering from greater oxidative DNA damage and renal injury compared with Nrf2(+/+) mice. Mechanistic studies in both in vivo and in vitro systems showed that the Nrf2-mediated protection against diabetic nephropathy is, at least, partially through inhibition of transforming growth factor-beta1 (TGF-beta1) and reduction of extracellular matrix production. In human renal mesangial cells, high glucose induced ROS production and activated expression of Nrf2 and its downstream genes. Furthermore, activation or overexpression of Nrf2 inhibited the promoter activity of TGF-beta1 in a dose-dependent manner, whereas knockdown of Nrf2 by siRNA enhanced TGF-beta1 transcription and fibronectin production.

CONCLUSIONS

This work clearly indicates a protective role of Nrf2 in diabetic nephropathy, suggesting that dietary or therapeutic activation of Nrf2 could be used as a strategy to prevent or slow down the progression of diabetic nephropathy.

Dietary fructose causes tubulointerstitial injury in the normal rat kidney

Recent studies suggest that the metabolic syndrome is associated with renal disease. We previously reported that a high-fructose diet, but not a high-glucose diet, can induce metabolic syndrome and accelerate chronic renal disease in rats. We now examined the effects of a high-fructose diet on normal rat kidneys. Three groups of Sprague-Dawley rats were pair fed a special diet containing 60% fructose, 60% glucose, or control standard rat chow for 6 wk, and then histological studies were performed. The effect of fructose to induce cell proliferation in cultured proximal tubular cells was also performed. Fructose diet, but not glucose diet, significantly increased kidney weight by 6 wk. The primary finding was tubular hyperplasia and proliferation involving all segments of the proximal tubules while glomerular changes were not observed. This is the same site where the fructose transporters (GLUT2 and -5) as well as the key enzyme in fructose metabolism (ketohexokinase) were expressed. Consistently, fructose also induced proliferation of rat proximal tubular cells in culture. In vivo, tubular proliferation was also associated with focal tubular injury, with type III collagen deposition in the interstitium, an increase in alpha-smooth muscle actin positive myofibroblasts, and an increase in macrophage infiltration. In conclusion, a high-fructose diet induces cell proliferation and hyperplasia in proximal tubules, perhaps via a direct metabolic effect. The effect is independent of total energy intake and is associated with focal tubulointerstitial injury. These studies may provide a mechanism by which metabolic syndrome causes renal disease.

GLUT2 protein at the rat proximal tubule brush border membrane correlates with protein kinase C (PKC)-betal and plasma glucose concentration

AIMS/HYPOTHESIS

GLUT2 is the main renal glucose transporter upregulated by hyperglycaemia, when it becomes detectable at the brush border membrane (BBM). Since glucose-induced protein kinase C (PKC) activation in the kidney is linked to diabetic nephropathy, we investigated the effect of glycaemic status on the protein levels of PKC isoforms alpha, betaI, betaII, delta and epsilon in the proximal tubule, as well as the relationship between them and changes in GLUT2 production at the BBM.

METHODS

Plasma glucose concentrations were modulated in rats by treatment with nicotinamide 15 min prior to induction of diabetes with streptozotocin. Levels of GLUT2 protein and PKC isoforms in BBM were measured by western blotting. Additionally, the role of calcium signalling and PKC activation on facilitative glucose transport was examined by measuring glucose uptake in BBM vesicles prepared from proximal tubules that had been incubated either with thapsigargin, which increases cytosolic calcium, or with the PKC activator phorbol 12-myristate,13-acetate (PMA).

RESULTS

Thapsigargin and PMA enhanced GLUT-mediated glucose uptake, but had no effect on sodium-dependent glucose transport. Diabetes significantly increased the protein levels of GLUT2 and PKC-betaI at the BBM. Levels of GLUT2 and PKC-betaI correlated positively with plasma glucose concentration. Diabetes had no effect on BBM levels of alpha, betaII, delta or epsilon isoforms of PKC.

CONCLUSIONS/INTERPRETATION

Enhanced GLUT2-mediated glucose transport across the proximal tubule BBM during diabetic hyperglycaemia is closely associated with increased PKC-betaI. Thus, altered levels of GLUT2 and PKC-betaI proteins in the BBM may be important factors in the pathogenic processes underlying diabetic renal injury.

Glucose transporters in human renal proximal tubular cells isolated from the urine of patients with non-insulin-dependent diabetes

The bulk of glucose that is filtered by the renal glomerulus is reabsorbed by the glucose transporters of the proximal convoluted tubular epithelium. However, it has been difficult to investigate this in diseases such as type 2 diabetes because of the inability to isolate primary renal cells from patients without a renal biopsy. We report here a method for the immunomagnetic isolation and novel primary culture of human exfoliated proximal tubular epithelial cells (HEPTECs) from fresh urine. The primary isolates are highly enriched and differentiated and express characteristic proximal tubular phenotypic markers. They continue to express the proximal tubular markers CD13/aminopeptidase-N, sodium glucose cotransporter (SGLT) 2, and alkaline phosphatase through up to six subsequent subcultures in a similar way to human proximal cells isolated from renal biopsies. In a hyperglycemic environment, HEPTECs isolated from patients with type 2 diabetes expressed significantly more SGLT2 and the facilitative glucose transporter GLUT2 than cells from healthy individuals. We also demonstrated a markedly increased renal glucose uptake in HEPTECs isolated from patients with type 2 diabetes compared with healthy control subjects. Our findings indicate for the first time in a human cellular model that increased renal glucose transporter expression and activity is associated with type 2 diabetes.

Different modes of sodium-D-glucose cotransporter-mediated D-glucose uptake regulation in Caco-2 cells

We recently reported that a considerable amount of the sodium-d-glucose cotransporter SGLT1 present in Caco-2 cells, a model for human enterocytes, is located in intracellular compartments attached to microtubules. A similar distribution pattern was also observed in enterocytes in thin sections from human jejunum, highlighting the validity of the Caco-2 cell model. Fluorescent surface labeling of live Caco-2 cells revealed that the intracellular compartments containing SGLT1 were accessible by endocytosis. To elucidate the role of endosomal SGLT1 in the regulation of sodium-dependent d-glucose uptake into enterocytes, we compared SGLT1-mediated D-glucose uptake into Caco-2 cells with the subcellular distribution of SGLT1 after challenging the cells with different stimuli. Incubation (90 min) of Caco-2 cells with mastoparan (50 microM), a drug that enhances apical endocytosis, shifted a large amount of SGLT1 from the apical membrane to intracellular sites and significantly reduced sodium-dependent alpha-[(14)C]methyl-D-glucose uptake (-60%). We also investigated the effect of altered extracellular D-glucose levels. Cells preincubated (1 h) with d-glucose-free medium exhibited significantly higher sodium-dependent alpha-[(14)C]methyl-D-glucose uptake (+45%) than did cells preincubated with high d-glucose medium (100 mM, 1 h). Interestingly, regulation of SGLT1-mediated d-glucose uptake into Caco-2 cells by extracellular D-glucose levels occurred without redistribution of cellular SGLT1. These data suggest that, pharmacologically, d-glucose uptake can be regulated by a shift of SGLT1 between the plasma membrane and the endosomal pool; however, regulation by the physiological substrate d-glucose can be explained only by an alternative mechanism.

Diabetes increases facilitative glucose uptake and GLUT2 expression at the rat proximal tubule brush border membrane

The mechanism of renal glucose transport involves the reabsorption of filtered glucose from the proximal tubule lumen across the brush border membrane (BBM) via a sodium-dependent transporter, SGLT, and exit across the basolateral membrane via facilitative, GLUT-mediated, transport. The aim of the present study was to determine the effect of streptozotocin-induced diabetes on BBM glucose transport. We found that diabetes increased facilitative glucose transport at the BBM by 67.5 % (P < 0.05)--an effect that was abolished by overnight fasting. Western blotting and immunohistochemistry demonstrated GLUT2 expression at the BBM during diabetes, but the protein was undetectable at the BBM of control animals or diabetic animals that had been fasted overnight. Our findings indicate that streptozotocin-induced diabetes causes the insertion of GLUT2 into the BBM and this may provide a low affinity/high capacity route of entry into proximal tubule cells during hyperglycaemia.

VEGF expression in hypoxia and hyperglycemia: reciprocal effect on branching angiogenesis in epithelial-endothelial co-cultures

Vascular endothelial growth factor (VEGF), an angiogenic factor for endothelial cells, is produced by glomerular and tubular epithelia. Using immunoelectron microscopy, VEGF expression by podocytes (GEC) and the proximal tubular epithelium of rat kidney was confirmed. To elucidate the mechanisms of VEGF production and its physiologic consequences, studies were performed in cultured GEC and proximal tubular epithelial cells (RPTEC). Both GEC and RPTEC expressed VEGF-120 and 164 mRNA, as detected by quantitative RT-PCR. Hypoxia resulted in an increase in mRNA abundance, more robust in RPTEC than in GEC, and an increase in VEGF expression by 1.9- and 1.6-fold, respectively. 30 mM D-glucose, but not 30 mM L-glucose, resulted in the elevation of VEGF mRNA in RPTEC, but not in GEC, although both cell types showed a comparable modest increase in VEGF expression. Combined treatment (hypoxia and 30 mM D-glucose) resulted in an increase of VEGF mRNA only in RPTEC; however, an enhanced protein expression was detectable in both cell types. To investigate the role of VEGF in branching angiogenesis, "sandwich" co-cultures were applied with endothelial cells and capillary tube formation was compared under the above conditions. Both RPTEC and GEC induced VEGF-dependent capillary tube formation by co-cultured endothelial cells and in both cell types hypoxia further augmented angiogenesis. In contrast, 30 mM D-glucose suppressed angiogenesis in co-cultures with both cell types despite increased mRNA for VEGF receptors 1 and 2. This study shows (1) that VEGF produced by GEC and RPTEC is necessary for branching angiogenesis and (2) that hypoxic environment stimulates VEGF production by both epithelial cell types and augments branching angiogenesis, whereas (3) hyperglycemic microenvironment, although also stimulatory for VEGF production, fails to augment angiogenesis.

Glucose transporter (GLUT1) allele (XbaI-) associated with nephropathy in non-insulin-dependent diabetes mellitus

BACKGROUND

Although multiple factors contribute to the initiation and progression of diabetic nephropathy (DN), hyperglycemia and genetic predisposition are two major components implicated in the development of DN. Several pieces of experimental evidence suggest that glucose transporter (GLUT1) activity is an important modulator for the cell hypertrophy and extracellular matrix formation of glomerular mesangial cells.

METHODS

To evaluate the role of the GLUT1 gene mutation in the development of DN in Chinese patients with non-insulin-dependent diabetes mellitus (NIDDM), the polymorphic XbaI site of GLUT1 gene was analyzed by polymerase chain reaction in 124 normal subjects and 131 patients with NIDDM, among whom 64 were complicated with DN. DN was defined as persistent albuminuria with or without impaired renal function with no known cause of renal disease other than diabetes.

RESULTS

The frequencies of XbaI (+/-) genotype (75 vs. 44%, P < 0.01) and XbaI (-) allele (44 vs. 29%, P < 0.05) were significantly higher in NIDDM patients with DN than those without nephropathy. There were no significant differences for GLUT1 genotype and allele frequency between NIDDM patients without nephropathy and normal subjects. The presence of the XbaI (-) allele appeared to have a strong association with the development of DN. The odds ratio was 1.915, and the 95% confidence interval was 1.044 to 3.514. In addition, no strong association was found between GLUT1 gene polymorphism and retinopathy in NIDDM patients.

CONCLUSION

Our results indicate that the XbaI (-) allele of the GLUT1 gene might be a genetic marker of NIDDM with DN, and this genetic susceptibility is independent of its retinopathy in Chinese subjects.