Tissue ascorbic acid and polyol pathway metabolism in experimental diabetes

Attenuation of diabetic nephropathy by dietary fenugreek (Trigonella foenum-graecum) seeds and onion (Allium cepa) via suppression of glucose transporters and renin-angiotensin system

OBJECTIVES

The aim of this study was to determine the effects of dietary fenugreek (Trigonella foenum-graecum) seeds and onion on the hyperglycemia-stimulated glucose transporters and activation of renin-angiotensin system-mediated cascade of events leading to renal lesions in diabetic animals.

METHODS

The mechanistic aspects of nephroprotective influence of dietary fenugreek seeds (10%) and onion (3%) on diabetic renal lesions was investigated in streptozotocin diabetic rats. Renal damage was assessed by measuring proteinuria, enzymuria, expression of glucose transporters, renin-angiotensin system, and activities of polyol pathway enzymes.

RESULTS

Diabetes resulted in an upregulation of glucose transporters in kidney tissue, which was countered by these dietary interventions. The upregulation of renal angiotensin-converting enzyme and its receptor was also countered by these dietary interventions. Dietary fenugreek and onion significantly reduced metabolites of polyol pathway, nitric oxide, and N-acetyl-β-d-glucosaminidase activity. Markers of podocyte damage in kidney (nephrin, podocin, and podocalyxin) and their urinary excretion were normalized along with downregulation of the expression of kidney injury molecule-1 by these dietary interventions. Dietary fenugreek and onion effectively countered the diabetes-induced structural abnormalities of renal tissue.

CONCLUSION

Feeding fiber-rich fenugreek seeds and sulfur compounds-rich onion produced a blockade in glucose translocation and renin-angiotensin system in the early stage of diabetic nephropathy. This involved a downregulation of the expression of polyol pathway enzymes, partial restoration of the podocyte damage, revival of renal architecture and functional abnormality. The present study also suggested that these two dietary interventions offer a higher renoprotective influence when consumed together.

Antioxidative effect of flavonoid naringenin in the lenses of type 1 diabetic rats

Oxidative stress arising during diabetes may lead to cataract formation. Thus, in order to prevent oxidative stress development, antioxidants could be considered helpful agents. Naringenin, a flavonoid with a well-documented antioxidative activity, can be found in many plant-derived products, especially citrus fruits. The aim of the study was to examine the effect of naringenin on oxidative stress markers in the lenses of type 1 diabetic rats. The study was conducted on 3-month-old male Wistar rats with streptozotocin-induced type 1 diabetes. The rats were treated orally with naringenin at the doses of 50 and 100 mg/kg for 4 weeks. In the lenses obtained from the animals, enzymatic and non-enzymatic parameters connected with oxidative stress were measured. The enzymatic parameters included superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase and glucose-6-phosphate dehydrogenase activity. For non-enzymatic parameters, the total thiol groups, reduced and oxidized glutathione, protein carbonyl groups, advanced oxidation protein products, malondialdehyde and vitamin C level were assayed. Oral administration of naringenin counteracted most of the unfavorable changes induced by diabetes, including reduction of elevated antioxidative enzymes activity and amelioration of oxidative damage in proteins and lipids. Naringenin administered orally reduces oxidative stress markers in the lenses of type 1 diabetic rats.

The beneficial effect of long-term supplementation of vitamin C on renal mitochondrial disturbances in streptozotocin-induced diabetic rats

Background: Oxidative stress induces renal dysfunction in diabetes, in which renal mitochondrial disturbance was implicated. Vitamin C (VC) supplementation may ameliorate the renal dysfunction in diabetics. However, it is not clear whether VC supplementation is effective for renal mitochondrial disturbances in diabetes.

Objective: Investigate whether long-term continuous VC supplementation could ameliorate the renal mitochondrial disturbances in streptozotocin (STZ)-induced diabetic rats.

Methods: Thirty-five male Sprague-Dawley rats were used, and diabetes was induced by an injection of STZ. The rats were divided into three groups: control rats (CON), STZ-induced diabetic rats (STZ), and diabetic rats supplemented by vitamin C (STZ-VC). The CON and STZ rats were given tap water, while STZ-VC rats received VC (1 g/L) every day for eight, 24 and 52 weeks. The kidney was isolated and homogenized. Oxygen comsumption (Vo2) was measured in mitochondria homogenate using an oxygen consumption monitor. Based on Vo2 tracings, the respiration control index (RCI) and P/O ratio (= ADP/ O ratio) were measured at week 8, 24 and 52.

Results: At week eight, using either glutamate plus malate (for site I) or succinate (for site II) as substrates, both RCI and P/O ratio were not significantly different among three groups. The P/O ratio in STZ and STZ-VC rats increased from eight to 52 weeks after VC supplementation. At week 24, the P/O ratio at site II was normalized in STZ-VC rat. The increased P/O ratio (only site I) and the increased RCI (only site II) of STZ-VC rats were slower than those of STZ rats.

Conclusion: Short-term VC supplementation might not influence the renal mitochondrial activity. The long-term VC supplementation could ameliorate the mitochondrial disturbances induced in STZ-induced diabetic rats.

Low Red Blood Cell Vitamin C Concentrations Induce Red Blood Cell Fragility: A Link to Diabetes Via Glucose, Glucose Transporters, and Dehydroascorbic Acid

Strategies to prevent diabetic microvascular angiopathy focus on the vascular endothelium. Because red blood cells (RBCs) are less deformable in diabetes, we explored an original concept linking decreased RBC deformability to RBC ascorbate and hyperglycemia. We characterized ascorbate concentrations from human and mouse RBCs and plasma, and showed an inverse relationship between RBC ascorbate concentrations and deformability, measured by osmotic fragility. RBCs from ascorbate deficient mice were osmotically sensitive, appeared as spherocytes, and had decreased β-spectrin. These aberrancies reversed with ascorbate repletion in vivo. Under physiologic conditions, only ascorbate's oxidation product dehydroascorbic acid (DHA), a substrate for facilitated glucose transporters, was transported into mouse and human RBCs, with immediate intracellular reduction to ascorbate. In vitro, glucose inhibited entry of physiologic concentrations of dehydroascorbic acid into mouse and human RBCs. In vivo, plasma glucose concentrations in normal and diabetic mice and humans were inversely related to respective RBC ascorbate concentrations, as was osmotic fragility. Human RBC β-spectrin declined as diabetes worsened. Taken together, hyperglycemia in diabetes produced lower RBC ascorbate with increased RBC rigidity, a candidate to drive microvascular angiopathy. Because glucose transporter expression, DHA transport, and its inhibition by glucose differed for mouse versus human RBCs, human experimentation is indicated.

Polyol profile as an early diagnostic and prognostic marker in natural product chemoprevention of hepatocellular carcinoma in diabetic rats

AIM

Diabetes mellitus (DM) is a risk factor for hepatocellular carcinoma (HCC). It directs glucose to sorbitol and fructose in polyol pathway (PP). To pursue contribution of PP in hepatocarcinogenesis.

METHODS

We utilized ascorbic acid (AA) and diallyl sulfide (DAS) in experimental DM and HCC against control. HCC was induced by diethyl nitrosamine (DENA, one intraperitoneal (IP) dose 125 mg/kg), DM, by streptozotocin (STZ, IP dose 65 mg/kg). AA was given as 7.4 g/kg/d, I.P., DAS 200mg/kg/d, orally. All animals were killed after 10 weeks.

RESULTS

DENA elevated serum AFP, erythrocyte sorbitol (ES), neoplastic changes in liver, lowered blood glucose, increased hepatocyte aldose reductase (AR) and sorbitol dehydrogenase (SDH), significantly alleviated by DAS/AA combination. DM elevated ES activating AR, inhibiting SDH, improved by DAS and AA.

CONCLUSION

Co-induction of DM and HCC increased liver tissue lesion, serum AFP, ES, liver AR and SDH. Co-administration of DAS/AA reduced ES, AR without changing SDH. DAS/AA co-therapy lowered ES by depressing AR without affecting SDH, meaning that AR is activated by cancer and DM in different ways. PP is early marker for HCC detection and response to chemoprevention. DAS/AA combination is promising cost effective chemopreventive and anti-diabetic combination.

Sodium-dependent vitamin C transporter 2 (SVCT2) is necessary for the uptake of L-ascorbic acid into Schwann cells

Ascorbic acid has been shown to be an essential component for in vitro myelination and to improve the clinical and pathological phenotype of a mouse model of Charcot-Marie-tooth disease 1A. The mechanism of ascorbic acid uptake into peripheral nerves, however, has not been addressed so far. Hence, we studied the expression and activity of sodium-dependent vitamin C transporters 1 and 2 (SVCT1 and 2) in the peripheral nervous system. Using immunohistochemistry, immunoblotting, and reverse transcription PCR, we could show that SVCT1 and 2 were differentially expressed in myelinated peripheral nerve fibers and Schwann cell (SC) cultures. SVCT1 was expressed at very low levels confined to the axons, whereas SVCT2 was highly expressed both in the axons and in the SCs. SVCT2 was localized particularly in SC compartments of uncompacted myelin. Uptake assays using (14)C-labeled ascorbic acid showed transport of ascorbic acid into SC cultures. Ascorbic acid transport was dependent on the concentration of sodium, magnesium, and calcium in the extracellular medium. Treatment with the flavonoid phloretin, a known inhibitor of SVCT1 and 2, and specific RNA interference with SVCT2 caused significant reductions in ascorbic acid uptake into SCs. Phloretin-inhibited uptake of ascorbic acid was further shown in freshly dissected, cell-culture-naïve rat sciatic nerves. These results provide evidence for the first time that uptake of ascorbic acid in the peripheral nervous system is crucially dependent on the expression and activity of SVCT2.

Myenteric neurons and intestinal mucosa of diabetic rats after ascorbic acid supplementation

AIM: To investigate the effect of ascorbic acid (AA) dietary supplementation on myenteric neurons and epithelial cell proliferation of the jejunum of adult rats with chronic diabetes mellitus.

METHODS: Thirty rats at 90 d of age were divided into three groups: Non-diabetic, diabetic and diabetic treated with AA (DA) (1 g/L). After 120 d of treatment with AA the animals were killed. The myenteric neurons were stained for myosin-V and analyzed quantitatively in an area of 11.2 mm²/animal. We further measured the cellular area of 500 neurons per group. We also determined the metaphasic index (MI) of the jejunum mucosa layer of about 2500 cells in the intestinal crypts, as well as the dimensions of 30 villi and 30 crypts/animal. The data area was analyzed using the Olympus BX40 microscope.

RESULTS: There was an increase of 14% in the neuronal density (792.6 ± 46.52 vs 680.6 ± 30.27) and 4.4% in the cellular area (303.4 ± 5.19 vs 291.1 ± 6.0) respectively of the diabetic group treated with AA when compared to control diabetic animals. There were no significant differences in MI parameters, villi height or crypt depths among the groups.

CONCLUSION: Supplementation with AA in the diabetic animal promoted moderate neuroprotection. There was no observation of alteration of the cellular proliferation of the jejunum mucosa layer of rats with chronic diabetes mellitus with or without supplementation with AA.

Assessment of NADPH-diaphorase stained myenteric neurons of the jejunum of diabetic rats supplemented with ascorbic acid

The relation between hyperglycemia and diabetic neuropathy has already been demonstrated in some studies. Among the theories proposed for its etiology the oxidative stress stands out. The performance of nitric oxide as a link between the metabolic and vascular neuropathogenic factors that triggers the diabetic neuropathy has already been put forward. This study aimed to assess the quantification and measurements of the cell body profile area (CBPA) of NADPH-diaphorase reactive (NADPH-dp) myenteric neurons of the jejunum of diabetic rats (induced by streptozotocin) supplemented with Ascorbic Acid (AA). These changes in the myenteric neurons seem to be related to the gastrointestinal disturbances observed in diabetes mellitus (DM). Twenty male Wistar rats (Rattus norvegicus) were distributed in 4 groups (n=5): controls (C), control supplemented (CS), diabetic (D), and diabetic suplemented (DS). DM was induced by estreptozotocin (50mg/kg body wt). One week after the induction and confirmation of the DM (glycemia exam), animals of the groups CS and DS received 50mg of AA three times a week by gavage. After 90 days of experiment, the animals were anesthetized with lethal thiopental dose (40mg/kg) and the collected jejunum processed for the histochemistry NADPH-diaphorase technique. Whole-mount preparations were obtained for quantitative and morphometric analysis of the myenteric neurons. A quantity of jejunum neurons in the Group D (96±7.5) was not different (P>0.05) from Group DS (116±8.08), C (92±9.7), and CS (81±5.4), but in Group DS the quantity was higher (P<0.05) than in Group C and CS. The CBPA of neurons from Group D (189.50±2.68µm²) and DS (195.92±3.75µm²) were lower (P<0.05) than from Group C (225.13±4.37µm²) and CS (210.23±3.15µm²). The streptozotocin-induced DM did not change the jejunum-ileum area, the jejunum myenteric plexus space organization and the density of NADPH-dp neurons. The 50g AA-supplementation, three times a week, during 90 days, did not decrease hyperglycemia; however, it had a neuroprotective effect on the myenteric neurons, minimizing the increase on the CBPA of NADPH-dp neurons and increasing the amount of NADPD-dp neurons.

High-fructose feeding of streptozotocin-diabetic rats is associated with increased cataract formation and increased oxidative stress in the kidney

We examined the effects of high-fructose (FR) feeding on the development of diabetic complications in the lens and the kidney of streptozotocin (STZ)-diabetic rats. Male Wistar Furth rats were treated with one of two doses of STZ (HIGH STZ, 55 mg/kg body weight; MOD STZ, 35 mg/kg body weight) or vehicle alone (SHAM) and were then assigned to a control (CNTL) or 400 g FR/kg diet for 12 weeks. At the end of the study, body weight, plasma glucose and insulin concentrations differed among STZ groups (HIGH v. MOD v. SHAM, P<0·001) but did not differ due to diet. Plasma FR concentrations were significantly higher in FR-fed v. CNTL-fed groups (P<0·0001) and in HIGH-STZ groups v. MOD-STZ and SHAM groups (P<0·0004 and P<0·0001 respectively). Focal length variability of the lens, a quantitative measure of cataract formation, was increased in the HIGH STZ, FR group compared with the HIGH STZ, CNTL group (P<0·01). The concentration of H2O2 in kidney microsomes was significantly higher in HIGH STZ, FR rats v. HIGH STZ, CNTL rats (P<0·01). Microalbuminuria was not observed in any of the groups examined, and there was no evidence of extensive histological damage in the kidney from any rats. Under conditions of severe hyperglycaemia, high FR intake promotes the development of cataracts in the lens of the eye, and results in increased concentrations of substances indicative of oxidative stress in the kidney. Although FR has been suggested as a carbohydrate source for diabetics, a high FR diet coupled with hyperglycaemia produces effects that may promote some of the complications associated with diabetes.

Increased sorbitol pathway activity generates oxidative stress in tissue sites for diabetic complications

Chronic diabetic complications, in particular, nephropathy, peripheral and autonomic neuropathy, "diabetic foot," retinopathy, and cardiovascular disease, remain the major cause of morbidity and mortality in patients with diabetes mellitus. Growing evidence indicates that both increased activity of the sorbitol pathway of glucose metabolism and enhanced oxidative stress are the leading factors in the pathogenesis of diabetic complications. The relation between the two mechanisms remains the area of controversy. One group has reported that increased sorbitol pathway activity has a protective rather than detrimental role in complication-prone tissues because the pathway detoxifies toxic lipid peroxidation products. Others put forward a so-called "unifying hypothesis" suggesting that activation of several major pathways implicated in diabetic complications (e.g., sorbitol pathway) occurs due to increased production of superoxide anion radicals in mitochondria and resulting poly(ADP-ribose) polymerase activation. This review (a) presents findings supporting a key role for the sorbitol pathway in oxidative stress and oxidative stress-initiated downstream mechanisms of diabetic complications, and (b) summarizes experimental evidence against a detoxifying role of the sorbitol pathway, as well as the "unifying concept."

Effects of pravastatin on progression of glucose intolerance and cardiovascular remodeling in a type II diabetes model

OBJECTIVES

We examined the effects of early treatment with a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor pravastatin on the progression of glucose intolerance and cardiovascular remodeling in a model of spontaneously developing type II diabetes mellitus (DM), the Otsuka Long-Evans Tokushima Fatty (OLETF) rats.

BACKGROUND

Clinical trials showed that pravastatin prevented new-onset DM in hypercholesterolemic patients, and that it was effective in prevention of cardiovascular events in diabetics.

METHODS

The OLETF rats were treated with pravastatin (100 mg/kg/day) from 5 weeks of age and compared with age-matched untreated OLETF rats and normal Long-Evans Tokushima Otsuka (LETO) rats on serial oral glucose tolerance tests (OGTT) and Doppler echocardiography and on histopathological/biochemical analyses of the heart at 30 weeks.

RESULTS

The OGTT revealed that 40% and 89% of untreated OLETF rats were diabetic at 20 and 30 weeks, respectively, but 0% and only 30%, respectively, were diabetic in the treated OLETF. Left ventricular diastolic function was found impaired from 20 weeks in untreated OLETF but remained normal in the treated-OLETF. The wall-to-lumen ratio and perivascular fibrosis of coronary arteries were increased in untreated-OLETF but were limited in the treated-OLETF at 30 weeks. Moreover, cardiac expressions of a fibrogenic growth factor, transforming growth factor-beta1 (TGF-beta1), and a proinflammatory chemokine, monocyte chemoattractant protein-1 (MCP-1), were increased in untreated-OLETF. However, in the treated-OLETF, overexpressions of TGF-beta1 and MCP-1 were attenuated, which was associated with overexpression of endothelial nitric oxide synthase (eNOS) (2.5-fold of control LETO).

CONCLUSIONS

Early pravastatin treatment prevented cardiovascular remodeling in the spontaneous DM model by retarding the progression of glucose intolerance, overexpressing cardiac eNOS, and inhibiting overexpressions of fibrogenic/proinflammatory cytokines.

Effect of ascorbic acid on airway responsiveness in ovalbumin sensitized guinea pigs

OBJECTIVE

The most important pathological feature of asthma is airway inflammation, which results in airway hyper-responsiveness. We hypothesized that excessive oxidation is likely to contribute to airway inflammation in asthma. The aim of this study was to evaluate the effects of both acute exposure and a 30-day administration of ascorbic acid (AA), which has an antioxidant effect, on airway responsiveness in sensitized guinea pigs.

METHODOLOGY

Guinea pigs sensitized to ovalbumin (OA), were given drinking water without AA (group 2) or with AA (group 3). The responses of tracheal chains of control animals (group 1) and both groups of sensitized guinea pigs (n = 10, for all groups) to cumulative concentrations of methacholine were measured, and the effective concentrations of methacholine causing 50% of maximum response (EC50 M) were obtained. The response of tracheal chains to 0.1% OA, relative to contraction obtained with 10 micro mol/L methacholine, was also measured. The tracheal responses to methacholine and OA were measured on tissues both incubated and not incubated with AA.

RESULTS

The tracheal responses of group 2 tissues were significantly greater than those of groups 1 and 3 to both OA and methacholine (P < 0.05). There were no significant differences in tracheal responses to OA and methacholine between groups 1 and 3. Acute incubation of tissues caused a reduction of tracheal response to methacholine in all groups, but this was only significantly differ-ent for group 3 (P < 0.05). Acute incubation of tissues did not change tracheal response to OA significantly.

CONCLUSION

These results showed that although short-term administration of AA had no major effect on tracheal responsiveness among sensitized animals, 30-day administration of AA could lead to a decrease in airway responsiveness of sensitized guinea pigs to both OA and methacholine.

Effect of the new thiazolidinedione-pioglitazone on the development of oxidative stress in liver and kidney of diabetic rabbits

Impaired homeostasis under diabetic conditions is connected with the increased production of free radicals and deficiency of antioxidative systems. The aim of this study was to analyze the effect of new oral antidiabetic drug-pioglitazone on activity of antioxidant factors and lipid peroxidation in vivo. The liver and kidney of alloxan-induced diabetic rabbits were examined after 4 and 8 weeks of treatment. After 4 weeks of diabetes the superoxide dismutase (Cu,Zn-SOD) activity in the liver was diminished while the catalase (CAT) activity and the level of ascorbic acid (AA) were elevated in comparison with the control group. Pioglitazone treatment during 4 weeks decreased the catalase activity in relation to the control diabetic animals. After 8 weeks of diabetes the CAT activity in the liver was elevated in comparison with the control group. Pioglitazone treatment during 8 weeks decreased the CAT activity and the level of lipid peroxidation products (LPO), and increased the Cu,Zn-SOD activity in relation to control diabetic animals. After 4 weeks of diabetes in the kidney the Cu,Zn-SOD activity and the level of ascorbic acid (AA) were diminished while the CAT activity and the LPO level were elevated in comparison with the control group. Pioglitazone treatment during 4 weeks increased the AA and decreased the LPO levels in relation to non-treated diabetic animals. After 8 weeks of disease the Cu,Zn-SOD activity in the kidney was diminished in comparison with the control group. Pioglitazone during 8 weeks decreased the LPO level in relation to non-treated diabetic animals. This study shows that diabetic animals undergo an important oxidative stress, which is partially corrected by pioglitazone treatment.

Vitamin C metabolomic mapping in experimental diabetes with 6-deoxy-6-fluoro-ascorbic acid and high resolution 19F-nuclear magnetic resonance spectroscopy

Metabolomic mapping is an emerging discipline geared at providing information on a large number of metabolites as a complement to genomics and proteomics. Here we have probed ascorbic acid homeostasis and degradation in diabetes using 6-deoxy-6-fluoro ascorbic acid (F-ASA) and 750 MHz (19)F-nuclear magnetic resonance (NMR) spectroscopy with proton decoupling In vitro, Cu(2+)-mediated degradation of F-ASA revealed the formation of 4 major stable degradation products at 24 hours. However, when normal or diabetics rats were injected with F-ASA intraperitoneally (IP) for 4 days, up to 20 fluorine-labeled compounds were observed in the urine. Their composition resembled, in part, metal catalyzed degradation of F-ASA and was not explained by spontaneous degradation in the urine. Diabetes led to a dramatic increase in urinary F-ASA loss and a relative decrease in most other urinary F-compounds. Diabetes tilted F-ASA homeostasis toward oxidation in liver (P <.01), kidney (P <.01), spleen (P <.01), and plasma (P <.01), but tended to decrease oxidation in brain, adrenal glands, and heart. Surprisingly, however, besides the major oxidation product fluoro-dehydroascorbic acid (F-DHA), no F-ASA advanced catabolites were detected in tissues at 5 micromol/L sensitivity. These findings not only confirm the key role of the kidney in diabetes-mediated loss of ascorbic acid, but demonstrate that only selected tissues are prone to increased oxidation in diabetes. While the structure of most degradation products needs to be established, the method illustrates the power of high resolution (19)F-NMR spectroscopy for the mapping of complex metabolomic pathways in disease states.

How does glucose generate oxidative stress in peripheral nerve?

Diabetes-associated oxidative stress is clearly manifest in peripheral nerve, dorsal root, and sympathetic ganglia of the peripheral nervous system and endothelial cells and is implicated in nerve blood flow and conduction deficits, impaired neurotrophic support, changes in signal transduction and metabolism, and morphological abnormalities characteristic of peripheral diabetic neuropathy (diabetic peripheral neuropathy). Hyperglycemia has a key role in oxidative stress in diabetic nerve, whereas the contribution of other factors, such as endoneurial hypoxia, transition metal imbalance, and hyperlipidemia, has not been rigorously proven. It has been suggested that oxidative stress, particularly mitochondrial superoxide production, is responsible for sorbitol pathway hyperactivity, nonenzymatic glycation/glycooxidation, and activation of protein kinase C. However, this concept is not supported by in vivo studies demonstrating the lack of any inhibition of the sorbitol pathway activity in peripheral nerve, retina, and lens by antioxidants, including potent superoxide scavengers. Its has been also hypothesized that aldose reductase (AR) detoxifies lipid peroxidation products, and therefore, the enzyme inhibition in diabetes is detrimental rather than benefical. However, the role for AR in lipid peroxdation product metabolism has never been demonstrated in vivo, and the effects of aldose reductase inhibitors and antioxidants on diabetic peripheral neuropathy are unidirectional, i.e., both classes of agents prevent and correct functional, metabolic, neurotrophic, and morphological changes in diabetic nerve. Growing evidence indicates that AR has a key role in oxidative stress in the peripheral nerve and contributes to superoxide production by the vascular endothelium. The potential mechanisms of this phenonmenon are discussed.

Early oxidative stress in the diabetic kidney: effect of DL-alpha-lipoic acid

Oxidative stress is implicated in the pathogenesis of diabetic nephropathy. The attempts to identify early markers of diabetes-induced renal oxidative injury resulted in contradictory findings. We characterized early oxidative stress in renal cortex of diabetic rats, and evaluated whether it can be prevented by the potent antioxidant, DL-alpha-lipoic acid. The experiments were performed on control rats and streptozotocin-diabetic rats treated with/without DL-alpha-lipoic acid (100 mg/kg i.p., for 3 weeks from induction of diabetes). Malondialdehyde plus 4-hydroxyalkenal concentration was increased in diabetic rats vs. controls (p <.01) and this increase was partially prevented by DL-alpha-lipoic acid. F(2) isoprostane concentrations (measured by GCMS) expressed per either mg protein or arachidonic acid content were not different in control and diabetic rats but were decreased several-fold with DL-alpha-lipoic acid treatment. Both GSH and ascorbate (AA) levels were decreased and GSSG/GSH and dehydroascorbate/AA ratios increased in diabetic rats vs. controls (p <.01 for all comparisons), and these changes were completely or partially (AA) prevented by DL-alpha-lipoic acid. Superoxide dismutase, glutathione peroxidase, glutathione reductase, glutathione transferase, and NADH oxidase, but not catalase, were upregulated in diabetic rats vs. controls, and these activities, except glutathione peroxidase, were decreased by DL-alpha-lipoic acid. In conclusion, enhanced oxidative stress is present in rat renal cortex in early diabetes, and is prevented by DL-alpha-lipoic acid.

Are diabetic neuropathy, retinopathy and nephropathy caused by hyperglycemic exclusion of dehydroascorbate uptake by glucose transporters?

Vitamin C exists in two major forms. The charged form, ascorbic acid (AA), is taken up into cells via sodium-dependent facilitated transport. The uncharged form, dehydroascorbate (DHA), enters cells via glucose transporters (GLUT) and is then converted back to AA within these cells. Cell types such as certain endothelial and epithelial cells as well as neurons that are particularly prone to damage during diabetes tend to be those that appear to be dependent on GLUT transport of DHA rather than sodium-dependent AA uptake. We hypothesize that diabetic neuropathies, nephropathies and retinopathies develop in part by exclusion of DHA uptake by GLUT transporters when blood glucose levels rise above normal. AA plays a central role in the antioxidant defense system. Exclusion of DHA from cells by hyperglycemia would deprive the cells of the central antioxidant, worsening the hyperglycemia-induced oxidative stress level. Moreover, AA participates in many cellular oxidation-reduction reactions including hydroxylation of polypeptide lysine and proline residues and dopamine that are required for collagen production and metabolism and storage of catecholamines in neurons. Increase in the oxidative stress level and metabolic perturbations can be expected in any tissue or cell type that relies exclusively or mainly on GLUT for co-transport of glucose and DHA including neurons, epithelial cells, and vascular tissues. On the other hand, since DHA represents a significant proportion of total serum ascorbate, by increasing total plasma ascorbate concentrations during hyperglycemia, it should be possible to correct the increase in the oxidative stress level and metabolic perturbations, thereby sparing diabetic patients many of their complications.

Differences in antioxidant status in skeletal muscle tissue in experimental diabetes

BACKGROUND

It has been suggested that oxidative stress may play an important role in pathogenesis of diabetic complications. The present study was designed to evaluate the oxidative stress-related parameters in alloxan (A)-induced long-term diabetes in rabbits.

METHODS

After 3, 6 and 12 weeks of diabetes, activities of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), glutathione reductase (GSSG-R) and concentrations of ascorbic acid (AA) and free sulfhydryl compounds (SH) were measured in skeletal muscle of diabetic rabbits and the normal control subjects. The products of lipid peroxidation (MDA) were also estimated.

RESULTS

In our tests, the muscle SOD activity, SH and AA concentrations were significantly reduced. CAT activity increased significantly at all time intervals. GSH-Px activity decreased after 3 weeks and then remained at the control level. GSSG-R activity decreased progressively at 3rd and 6th week and then significantly increased. MDA level increased initially, dropped below baseline after 6 weeks and then remained at the level of the control group.

CONCLUSIONS

The changes observed in the present experiment suggest a significant imbalance in antioxidative system in the skeletal muscle of rabbits with alloxan-induced diabetes. Such study may lead to therapeutic approaches for limiting the damage from oxidation reactions and preventing the diabetic complications.

Taurine counteracts oxidative stress and nerve growth factor deficit in early experimental diabetic neuropathy

Oxidative stress has a key role in the pathogenesis of diabetic complications. We have previously reported that taurine (T), which is known to counteract oxidative stress in tissues (lens, kidney, retina) of diabetic rats, attenuates nerve blood flow and conduction deficits in early experimental diabetic neuropathy (EDN). The purpose of this study was to evaluate whether dietary T supplementation counteracts oxidative stress and the nerve growth factor (NGF) deficit in the diabetic peripheral nerve. The experiments were performed in control rats and streptozotocin-diabetic rats fed standard or 1% T-supplemented diets for 6 weeks. All measurements were performed in the sciatic nerve. Malondialdehyde (MDA) plus 4-hydroxyalkenals (4-HA) were quantified with N-methyl-2-phenylindole. GSH, GSSG, dehydroascorbate (DHAA), and ascorbate (AA) were assayed spectrofluorometrically, T by reverse-phase HPLC, and NGF by ELISA. MDA plus 4-HA concentration (mean +/- SEM) was increased in diabetic rats (0.127 +/- 0.006 vs 0.053 +/- 0.003 micromol/g in controls, P < 0.01), and this increase was partially prevented by T (0.096 +/- 0.004, P < 0.01 vs untreated diabetic group). GSH levels were similarly decreased in diabetic rats treated with or without taurine vs controls. GSSG levels were similar in control and diabetic rats but were lower in diabetic rats treated with T (P < 0.05 vs controls). AA levels were decreased in diabetic rats (0.133 +/- 0.015 vs 0.219 +/- 0.023 micromol/g in controls, P < 0.05), and this deficit was prevented by T. DHAA/AA ratio was increased in diabetic rats vs controls (P < 0.05), and this increase was prevented by T. T levels were decreased in diabetic rats (2.7 +/- 0.16 vs 3.8 +/- 0.1 micromol/g in controls, P < 0.05) and were repleted by T supplementation (4.2 +/- 0.3). NGF levels were decreased in diabetic rats (2.35 +/- 0.20 vs 3.57 +/- 0.20 ng/g in controls, P < 0.01), and this decrease was attenuated by T treatment (3.16 +/- 0.28, P < 0.05 vs diabetic group). In conclusion, T counteracts oxidative stress and the NGF deficit in early EDN. Antioxidant effects of T in peripheral nerve are, at least in part, mediated through the ascorbate system of antioxidative defense. The findings are consistent with the important role for oxidative stress in impaired neurotrophic support in EDN.

Quercetin prevents glutathione depletion induced by dehydroascorbic acid in rabbit red blood cells

Exposure of rabbit red blood cells to dehydroascorbic acid (DHA) caused a significant decline in glutathione content which was largely prevented by quercetin, whereas it was insensitive to various antioxidants, iron chelators or scavengers of reactive oxygen species. This response was not mediated by chemical reduction of either extracellular DHA or intracellular glutathione disulfide. In addition, the flavonoid did not affect the uptake of DHA or its reduction to ascorbic acid. Rather, quercetin appeared to specifically stimulate downstream events promoting GSH formation.

Dehydroascorbic acid transport by GLUT4 in Xenopus oocytes and isolated rat adipocytes

Dehydroascorbic acid (DHA), the first stable oxidation product of vitamin C, was transported by GLUT1 and GLUT3 in Xenopus laevis oocytes with transport rates similar to that of 2-deoxyglucose (2-DG), but due to inherent difficulties with GLUT4 expression in oocytes it was uncertain whether GLUT4 transported DHA (Rumsey, S. C. , Kwon, O., Xu, G. W., Burant, C. F., Simpson, I., and Levine, M. (1997) J. Biol. Chem. 272, 18982-18989). We therefore studied DHA and 2-DG transport in rat adipocytes, which express GLUT4. Without insulin, rat adipocytes transported 2-DG 2-3-fold faster than DHA. Preincubation with insulin (0.67 micrometer) increased transport of each substrate similarly: 7-10-fold for 2-DG and 6-8-fold for DHA. Because intracellular reduction of DHA in adipocytes was complete before and after insulin stimulation, increased transport of DHA was not explained by increased internal reduction of DHA to ascorbate. To determine apparent transport kinetics of GLUT4 for DHA, GLUT4 expression in Xenopus oocytes was reexamined. Preincubation of oocytes for >4 h with insulin (1 micrometer) augmented GLUT4 transport of 2-DG and DHA by up to 5-fold. Transport of both substrates was inhibited by cytochalasin B and displayed saturable kinetics. GLUT4 had a higher apparent transport affinity (K(m) of 0.98 versus 5.2 mm) and lower maximal transport rate (V(max) of 66 versus 880 pmol/oocyte/10 min) for DHA compared with 2-DG. The lower transport rate for DHA could not be explained by binding differences at the outer membrane face, as shown by inhibition with ethylidene glucose, or by transporter trans-activation and therefore was probably due to substrate-specific differences in transporter/substrate translocation or release. These novel data indicate that the insulin-sensitive transporter GLUT4 transports DHA in both rat adipocytes and Xenopus oocytes. Alterations of this mechanism in diabetes could have clinical implications for ascorbate utilization.