Potential use of glutathione for the prevention and treatment of diabetic neuropathy in the streptozotocin-induced diabetic rat

Combination therapy is it in the future for successfully treating peripheral diabetic neuropathy?

In 2022, the Center for Disease Control and Prevention reported that 11.3% of the United States population, 37.3 million people, had diabetes and 38% of the population had prediabetes. A large American study conducted in 2021 and supported by many other studies, concluded that about 47% of diabetes patients have peripheral neuropathy and that diabetic neuropathy was present in 7.5% of patients at the time of diabetes diagnosis. In subjects deemed to be pre-diabetes and impaired glucose tolerance there was a wide range of prevalence estimates (interquartile range (IQR): 6%-34%), but most studies (72%) reported a prevalence of peripheral neuropathy ≥10%. There is no recognized treatment for diabetic peripheral neuropathy (DPN) other than good blood glucose control. Good glycemic control slows progression of DPN in patients with type 1 diabetes but for patients with type 2 diabetes it is less effective. With obesity and type 2 diabetes at epidemic levels the need of a treatment for DPN could not be more important. In this article I will first present background information on the "primary" mechanisms shown from pre-clinical studies to contribute to DPN and then discuss mono- and combination therapies that have demonstrated efficacy in animal studies and may have success when translated to human subjects. I like to compare the challenge of finding an effective treatment for DPN to the ongoing work being done to treat hypertension. Combination therapy is the recognized approach used to normalize blood pressure often requiring two, three or more drugs in addition to lifestyle modification to achieve the desired outcome. Hypertension, like DPN, is a progressive disease caused by multiple mechanisms. Therefore, it seems likely as well as logical that combination therapy combined with lifestyle adjustments will be required to successfully treat DPN.

Constipation in DM are associated with both poor glycemic control and diabetic complications: Current status and future directions

Constipation is a major complications of diabetes mellitus. With the accelerating prevalence of diabetes worldwide and an aging population, there is considerable research interest regarding the altered function and structure of the gastrointestinal tract in diabetic patients. Despite current advances in hyperglycemic treatment strategies, the specific pathogenesis of diabetic constipation remains unknown. Patients with constipation, may be reluctant to eat regularly, which may worsen glycemic control and thus worsen symptoms associated with underlying diabetic bowel disease. This paper presents a review of the complex relationship between diabetes and constipation, exploring the morphological alterations and biomechanical remodeling associated with intestinal motility dysfunction, as well as alterations in intestinal neurons, cellular signaling pathways, and oxidative stress. Further studies focusing on new targets that may play a role in the pathogenesis of diabetic constipation may, provide new ideas for the development of novel therapies to treat or even prevent diabetic constipation.

Detection of Reactive Oxygen Species in Human Neutrophils Under Various Conditions of Exposure to Galectin

Reactive oxygen species (ROS) have been extensively studied in biology in the past years. This class of molecules can be derived from endogenous sources (e.g., phagocytic cells as neutrophils, eosinophils, monocytes, macrophages, and organelles as mitochondria and peroxisomes) and participate in physiological and pathological conditions. The beneficial and harmful effects of ROS depend on redox regulation, which establishes the balance between their production and the activity of antioxidant systems to prevent oxidative stress in vivo. Neutrophils are the immune effectors most well depicted with an intense oxidative burst in response to tissue inflammation. Several proteins and members of the galectin family are involved in this fine modulation of ROS production by neutrophils. Interestingly, studies have indicated that Galectin-1 (Gal-1) can up- or downregulate ROS production by neutrophils even when exposed to N-formyl-Met-Leu-Phe (fMLP) or Phorbol Myristate Acetate (PMA), both of which are potent neutrophil stimulants that trigger high levels of ROS production. Similarly, Galectin-3 (Gal-3) induces ROS in neutrophils from a sterile or nonsterile inflammatory environment, possibly creating a negative loop that could control ROS production. Besides, superoxide production is also induced by Galectin-8 (Gal-8) and Galectin-9 (Gal-9) in neutrophils but in a different manner. We describe herein the luminol and lucigenin-dependent chemiluminescence technique by using a luminometer as a method of assessment to measure ROS production by human neutrophils isolated and exposed to purified human recombinant Gal-1. The protocol described herein could be applied for the investigation of the role of other galectins in the modulation of ROS production by neutrophils.

Effect of mitoquinone (Mito-Q) on neuropathic endpoints in an obese and type 2 diabetic rat model

This study sought to determine whether the addition of mitoquinone (Mito-Q) in the diet is an effective treatment for peripheral neuropathy in animal models of diet-induced obesity (pre-diabetes) and type 2 diabetes. Unlike other anti-oxidative stress compounds investigated as a treatment for peripheral neuropathy, Mito-Q specifically targets mitochondria. Although mito-Q has been shown to reduce oxidative stress generated by mitochondria there have been no studies performed of the effect of Mito-Q on peripheral neuropathy induced by diet-induced obesity or type 2 diabetes. Diet-induced obese (12 weeks after high fat diet) or type 2 diabetic rats (12 weeks of high fat diet and 4 weeks after the onset of hyperglycemia) were treated via the diet with Mito-Q (0.93 g/kg diet) for 12 weeks. Afterwards, glucose utilization, vascular reactivity of epineurial arterioles to acetylcholine and peripheral neuropathy related endpoints were examined. The addition of Mito-Q to the diets of obese and diabetic rats improved motor and/or sensory nerve conduction velocity, cornea and intraepidermal nerve fibre density, cornea sensitivity and thermal nociception. Surprisingly, treating obese and diabetic rats with Mito-Q did not improve glucose utilization or vascular reactivity by epineurial arterioles to acetylcholine. These studies imply that mitochondrial dysfunction contributes to peripheral neuropathy in animal models of pre-diabetes and late-stage type 2 diabetes. However, improvement in peripheral neuropathy following treatment with Mito-Q was not associated with improvement in glucose utilization or vascular reactivity of epineurial arterioles to acetylcholine.

Effect of treatment with vitamin D plus calcium on oxidative stress in streptozotocin-induced diabetic rats

Background

In diabetes mellitus, uncontrolled hyperglycemia has been reported to induce oxidative stress, which may lead to health complications. Vitamin D, however, acts as a non-enzymatic antioxidant to protect cells against oxidative stress and damage.

Objective

To investigate the antioxidative effect of vitamin D combined with calcium in streptozotocin (STZ)-induced diabetic rats.

Methods

Rats were divided into four groups (ten rats in each group). The first group (control) received a normal diet and water. The second group, including STZ-induced diabetic rats (diabetic controls), received a normal diet and water. The third group, also including STZ-induced diabetic rats, received vitamin D (2000 IU/day) with calcium (500 mg/kg/day) orally for 28 consecutive days. The fourth group consisted of STZ-induced diabetic rats that received insulin treatment for 28 consecutive days. Activities of superoxide dismutase (SOD), glutathione peroxidase (GPO) and catalase were measured in the liver tissues. The level of malonaldehyde (MDA) was measured in the plasma.

Results

Diabetic rats showed a significant decrease in the activities of SOD, GPO and catalase compared to normal rats. Oral administration of vitamin D with calcium to diabetic rats caused a significant increase in the activities of SOD, GPO and catalase compared with the untreated group. Furthermore, the plasma level of MDA was significantly elevated in diabetic rats compared to normal rats. Diabetic rats treated with vitamin D and calcium had a significantly reduced level of MDA, suggesting that vitamin D with calcium played a vital role in the protection of tissues from damage by free radicals.

Conclusion

Oral supplementation with vitamin D and calcium may be a useful treatment for diabetic patients to reduce/prevent the pathological complications of diabetes.

Diabetes and the Small Intestine

OPINION STATEMENT

Diabetes mellitus (DM) and its associated complications are becoming increasingly prevalent. Gastrointestinal symptoms associated with diabetes is known as diabetic enteropathy (DE) and may manifest as either diarrhea, fecal incontinence, constipation, dyspepsia, nausea, and vomiting or a combination of symptoms. The long-held belief that vagal autonomic neuropathy is the primary cause of DE has recently been challenged by newer theories of disease development. Specifically, hyperglycemia and the resulting oxidative stress on neural networks, including the nitrergic neurons and interstitial cells of Cajal (ICC), are now believed to play a central role in the development of DE. DE occurs in the majority of patients with diabetes; however, tools for early diagnosis and targeted therapy to counter the detrimental and potentially irreversible effects on the small bowel are lacking. Delay in diagnosis is further compounded by the fact that DE symptoms overlap with those of gastroparesis or can be confused with side effects from diabetes medications. Still, early recognition of the presence of DE is essential to mitigating symptoms and preventing further progression of complications including dysmotility and malabsorption. Current diagnostic modalities include manometry, wireless motility capsule (SmartPill™), and scintigraphy; however, these are not regularly utilized in clinical practice due to limited availability. Several medications are available for symptom relief in DE patients including rifaximin for small intestinal bacterial overgrowth (SIBO) and somatostatin analogues for diarrhea. While rodent models on stem cell therapy and alteration of the microbiome are promising, there is still a great need for further research on the pathologic underpinnings and development of novel treatment modalities for DE.

Prophylactic L-arginine and ibuprofen delay the development of tactile allodynia and suppress spinal miR-155 in a rat model of diabetic neuropathy

Diabetic neuropathy (DN) is a common complication of diabetes mellitus that is hardly reversible at the late stages. Since treatment of neuropathic pain is predominantly symptomatic, a prophylactic measure would be useful. Both ibuprofen and L-arginine exert antiallodynic effects on chronic constriction injury (CCI)-induced cold allodynia. Furthermore, ibuprofen is effective in CCI-induced mechanical allodynia. The aim of the study was to assess the antiallodynic effect of prophylactic ibuprofen and L-arginine in streptozotocin-induced DN in rats and to further investigate the role of spinal miR-155 and nitric oxide (NO) in this effect. Tactile allodynia was assessed weekly by von Frey filaments. Oral daily administration of ibuprofen, L-arginine and their combination, for 4 weeks starting 1 week after streptozotocin injection (ie, before the development of tactile allodynia), resulted in a significant decrease of tactile allodynia compared with the control diabetic group. This was evident in the fifth week of the experiment. The 3 treatments prevented the decrease in muscle fiber diameter and epidermal thickness, seen in the control diabetic group. Furthermore, ibuprofen, L-arginine and their combination prevented the increase in the spinal NO level and miRNA-155, seen in the control diabetic group. In conclusion, both ibuprofen and L-arginine delayed the development of behavioral and histologic changes of DN, with concomitant suppression of spinal miR-155 and NO level. L-arginine being tolerable may be useful prophylactically in diabetic patients.

Vascular Impairment of Epineurial Arterioles of the Sciatic Nerve: Implications for Diabetic Peripheral Neuropathy

This article reviews the impact of diabetes and its treatment on vascular function with a focus on the reactivity of epineurial arterioles, blood vessels that provide circulation to the sciatic nerve. Another focus is the relationship between the dysregulation of neurovascular function and diabetic peripheral neuropathy. Diabetic peripheral neuropathy is a debilitating disorder that occurs in more than 50 percent of patients with diabetes. The etiology involves metabolic, vascular, and immunologic pathways besides neurohormonal growth factor deficiency and extracellular matrix remodeling. In the light of this complex etiology, an effective treatment for diabetic peripheral neuropathy has not yet been identified. Current opinion postulates that any effective treatment for diabetic peripheral neuropathy will require a combination of life style and therapeutic interventions. However, a more comprehensive understanding of the factors contributing to neurovascular and neural dysfunction in diabetes is needed before such a treatment strategy can be developed. After reading this review, the reader should have gained insight into the complex regulation of vascular function and blood flow to the sciatic nerve, and the impact of diabetes on numerous elements of vascular reactivity of epineurial arterioles of the sciatic nerve.

Association of chronic diabetes and hypertension in sural nerve morphometry: an experimental study

BACKGROUND

Prospective studies have shown incidence rates of hypertension in diabetes mellitus to be three times that of subjects without diabetes mellitus. The reverse also applies, with the incidence of diabetes two to three times higher in patients with hypertension. Despite this common clinical association, the contribution of each isolated entity in the development of a neuropathy is still not well understood. The aims of the present study were to investigate the presence of peripheral neuropathy in spontaneously hypertensive rats (SHR) and SHR with chronically induced diabetes, using a morphological and morphometric study of the sural nerves.

METHODS

Female SHR and normotensive Wistar rats (WR), 8 weeks old, received a single intravenous injection of streptozotocin (STZ) through the tail vein. Controls from both strains received vehicle. Twelve weeks after the injection, sural nerves were dissected and prepared for light microscopy. Morphometry of sural nerve fascicles and myelinated fibers was performed with the aid of computer software.

RESULTS

The sural nerve myelinated fibers were highly affected by experimental diabetes in normotensive rats, causing mainly the reduction of the fiber size. Hypertensive rats showed characteristics of small fiber neuropathy and a severe reduction of the number and density or Schwann cells. The association between diabetes and hypertension caused an increase on the average size of the myelinated fibers, pointing to a small fiber loss, associated to axonal atrophy.

CONCLUSIONS

Our study gives morphological support to the existence of a neuropathy due to hypertension, which is among one of the most common risk factors for diabetic neuropathy. The association between the two neuropathies showed to be a complex alteration, involving and including both, large and small fibers neuropathy. Hypertension caused, indeed, an exacerbation of the alterations already observed in experimental models of diabetic neuropathy.

Mechanisms of diabetic neuropathy: Schwann cells

As ensheathing and secretory cells, Schwann cells are a ubiquitous and vital component of the endoneurial microenvironment of peripheral nerves. The interdependence of axons and their ensheathing Schwann cells predisposes each to the impact of injury in the other. Further, the dependence of the blood-nerve interface on trophic support from Schwann cells during development, adulthood, and after injury suggests these glial cells promote the structural and functional integrity of nerve trunks. Here, the developmental origin, injury-induced changes, and mature myelinating and nonmyelinating phenotypes of Schwann cells are reviewed prior to a description of nerve fiber pathology and consideration of pathogenic mechanisms in human and experimental diabetic neuropathy. A fundamental role for aldose-reductase-containing Schwann cells in the pathogenesis of diabetic neuropathy, as well as the interrelationship of pathogenic mechanisms, is indicated by the sensitivity of hyperglycemia-induced biochemical alterations, such as polyol pathway flux, formation of reactive oxygen species, generation of advanced glycosylation end products (AGEs) and deficient neurotrophic support, to blocking polyol pathway flux.

Role of oxidative stress and Ca²⁺ signaling on molecular pathways of neuropathic pain in diabetes: focus on TRP channels

Diabetes mellitus, a debilitating chronic disease, affects ~100 million people. Peripheral neuropathy is one of the most common early complications of diabetes in ~66 % of these patients. Altered Ca(2+) handling and Ca(2+) signaling were detected in a huge variety of preparations isolated from animals with experimentally induced type 1 and 2 diabetes as well as patients suffering from the disease. We reviewed the role of Ca(2+) signaling through cation channels and oxidative stress on diabetic neuropathic pain in sensory neurons. The pathogenesis of diabetic neuropathy involves the polyol pathway, advanced glycation end products, oxidative stress, protein kinase C activation, neurotrophism, and hypoxia. Experimental studies with respect to oxidative stress and Ca(2+) signaling, inhibitor roles of antioxidants in diabetic neuropathic pain are also summarized in the review. We hypothesize that deficits in insulin, triggers alterations of sensory neurone phenotype that are critical for the development of abnormal Ca(2+) homeostasis and oxidative stress and associated mitochondrial dysfunction. The transient receptor potential channels are a large family of proteins with six main subfamilies. The sheer number of different TRPs with distinct functions supports the statement that these channels are involved in a wide range of processes ranging in diabetic neuropathic pain and it seems that the TRPC, TRPM and TRPV groups are mostly responsible from diabetic neuropathic pain. In conclusion, the accumulating evidence implicating Ca(2+) dysregulation and over production of oxidative stress products in diabetic neuropathic pains, along with recent advances in understanding of genetic variations in cation channels such as TRP channels, makes modulation of neuronal Ca(2+) handling an increasingly viable approach for therapeutic interventions against the painful and degenerative aspects of many diabetic neuropathies.

Taurine reduces nitrosative stress and nitric oxide synthase expression in high glucose-exposed human Schwann cells

The role of taurine in regulating glucose-induced nitrosative stress has been examined in human Schwann cells, a model for understanding the pathogenesis of diabetic neuropathy. Exposure to high glucose increased nitrated proteins (1.56 fold p<0.05), inducible nitric oxide synthase (iNOS) and neuronal NOS (nNOS) mRNA expression (1.55 fold and 2.2 fold respectively, p<0.05 both), phospho-p38 MAPK (1.32 fold, p<0.05) abundance and decreased Schwann cell growth (11±2%, p<0.05). Taurine supplementation prevented high-glucose induced iNOS and nNOS mRNA upregulation, reduced nitrated proteins and phospho-p38 MAPK (56±11% and 45±18% (p<0.05 both) respectively) and restored Schwann cell growth to control levels. High glucose and taurine treatment alone reduced phospho-p42/44 MAPK and phospho-AKT to below detectable levels. Treatment of human Schwann cells with donors of nitric oxide and peroxynitrite reduced taurine transporter (TauT) expression (by 35±5% and 29±7% respectively p<0.05 both) as well as the maximum velocity of taurine uptake (TauT Vmax). NOS inhibition prevented glucose-mediated TauT mRNA downregulation, and restored TauT Vmax. These data demonstrate an important role for taurine in the prevention of nitrosative stress in human Schwann cells, which may have important implications for the development and treatment of diabetic neuropathy.

Efeito do extrato aquoso de alecrim (Rosmarinus officinalis L.) sobre o estresse oxidativo em ratos diabéticos

OBJETIVO: Avaliar o efeito do extrato aquoso de alecrim sobre o estresse oxidativo em ratos diabéticos. MÉTODOS: O extrato aquoso de alecrim foi obtido por método sequencial. Os fenólicos totais foram determinados pelo método de Folin Ciocateau e a atividade antioxidante in vitro foi determinada através de três métodos: β-caroteno/ácido linoleico, varredura do radical 2,2 Difenil-1-Picril-hidrazil e oxigen radical absorbance capacity. Ratos Wistar machos foram distribuídos em 5 grupos: controle, diabético, e três grupos de animais diabéticos tratados com extrato aquoso de alecrim em concentrações diferentes: 25, 50 ou 100mg/kg por via oral durante 30 dias. O diabetes foi induzido por estreptozotocina e, no final do experimento, foi coletado sangue para avaliar o percentual de hemoglobina glicada e os tecidos hepático e cerebral para determinação das enzimas antioxidantes: superóxido dismutase, catalase, glutationa peroxidase e glutationa redutase. RESULTADOS: Constatou-se que o extrato aquoso de alecrim apresentou altos teores de compostos fenólicos totais e expressiva atividade antioxidante in vitro nos três métodos de avaliação. O extrato aquoso de alecrim na concentração de 50mg/kg diminuiu o percentual de hemoglobina glicada e aumentou a atividade das enzimas catalase e glutationa peroxidase no fígado, e da superóxido dismutase no cérebro de ratos diabéticos. No entanto, não foi observado efeito dose-resposta nas demais concentrações analisadas. CONCLUSÃO: O extrato aquoso de alecrim apresenta significativa capacidade antioxidante in vitro, atribuída à presença de compostos fenólicos em sua composição. E, quando administrado em ratos na concentração de 50mg/kg, demonstrou-se eficiente na atenuação do estresse oxidativo presente no diabetes experimental.

Oxidative stress and dysregulation of the taurine transporter in high-glucose-exposed human Schwann cells: implications for pathogenesis of diabetic neuropathy

In human Schwann cells, the role of taurine in regulating glucose-induced changes in antioxidant defense systems has been examined. Treatment with high glucose for 7 days induced reactive oxygen species, increased 4-hydroxynoneal adducts (20 +/- 5%, P < 0.05) and poly(ADP-ribosyl)ated proteins (40 +/- 13%, P < 0.05). Increases in these markers of oxidative stress were reversed by simultaneous incubation in 0.25 mM taurine. Both high glucose and taurine independently increased superoxide dismutase and catalase activity and decreased glutathione levels, but their effects were not additive. Glucose reduced taurine transporter (TauT) mRNA and protein in a dose-dependent manner with maximal decreases of 66 +/- 6 and 63 +/- 12%, respectively (P < 0.05 both). The V(max) for taurine uptake was decreased in 30 mM glucose from 61 +/- 5 to 42 +/- 3 pmol x min(-1) x mg protein(-1) (P < 0.001). Glucose-induced TauT downregulation could be reversed by inhibition of aldose reductase, a pathway that depletes NADPH and increases osmotic stress and protein glycation. TauT protein was increased more than threefold, and the V(max) for taurine uptake doubled (P < 0.05 both) by prooxidants. TauT downregulation was reversed both by treatment with the antioxidant alpha-lipoic acid, which increased TauT mRNA by 60% and V(max) by 50% (P < 0.05 both), and by the aldose reductase inhibitor sorbinil, which increased TauT mRNA 380% and V(max) by 98% (P < 0.01 both). These data highlight the potential therapeutic benefits of taurine supplementation in diabetic complications and provide mechanisms whereby taurine restoration could be achieved in order to prevent or reverse diabetic complications.

Elevated lipid peroxidation and DNA oxidation in nerve from diabetic rats: effects of aldose reductase inhibition, insulin, and neurotrophic factors

We investigated the effect of treatment with an aldose reductase inhibitor, insulin, or select neurotrophic factors on the generation of oxidative damage in peripheral nerve. Rats were either treated with streptozotocin to induce insulin-deficient diabetes or fed with a diet containing 40% d-galactose to promote hexose metabolism by aldose reductase. Initial time course studies showed that lipid peroxidation and DNA oxidation were significantly elevated in sciatic nerve after 1 week or 2 weeks of streptozotocin-induced diabetes, respectively, and that both remained elevated after 12 weeks of diabetes. The increase in nerve lipid peroxidation was completely prevented or reversed by treatment with the aldose reductase inhibitor, ICI 222155, or by insulin, but not by the neurotrophic factors, prosaptide TX14(A) or neurotrophin-3. The increase in nerve DNA oxidation was significantly prevented by insulin treatment. In contrast, up to 16 weeks of galactose feeding did not alter nerve lipid peroxidation or protein oxidation, despite evidence of ongoing nerve conduction deficits. These observations demonstrate that nerve oxidative damage develops early after the onset of insulin-deficient diabetes and that it is not induced by increased hexose metabolism by aldose reductase per se, but rather is a downstream consequence of flux through this enzyme. Furthermore, the beneficial effect of prosaptide TX14(A) and neurotrophin-3 on nerve function and structure in diabetic rats is not due to amelioration of increased lipid peroxidation.

The therapeutic potential of gene transfer for the treatment of peripheral neuropathies

Peripheral neuropathy is a common medical problem with numerous aetiologies. Unfortunately, for the majority of cases there is no available medical solution for the underlying cause, and the only option is to try to treat the resulting symptoms. Treatment options exist when neuropathy results in positive symptoms such as pain, but there is a significant lack of treatments for negative symptoms such as numbness and weakness. Systemic application of growth factor peptides has shown promise in protecting nerves from neuropathic insults in preclinical animal studies, but translation into human trials has been problematic and disappointing. Significant advancements have been made in the past few years in utilising gene therapy approaches to treat peripheral neuropathy by expressing neuroprotective gene products either systemically or in specific nervous tissues. For example, plasmids expressing vascular endothelial growth factor injected into muscle, or herpes-simplex-virus-based vectors expressing neurotrophin gene products delivered to dorsal root ganglion neurons, have been used to protect peripheral nerve function in animal models of diabetes-associated peripheral neuropathy. Many published studies support the feasibility of this approach, although several questions still need to be addressed as gene therapy to treat peripheral neuropathy moves out of the laboratory and into the clinic.

Alterations in somatostatin cells and biochemical parameters following zinc supplementation in gastrointestinal tissue of streptozotocin-induced diabetic rats

Chronic hyperglycemia in diabetes is a major causative factor of free radical generation which further leads to many secondary diabetic complications via the damage to cellular proteins, membrane lipids, and nucleic acids. Zinc is an essential trace element in all living systems and plays a structural role in many proteins and enzymes. Somatostatin is known to have inhibitory effects on various gastrointestinal functions. Therefore, we determined somatostatin protein production and secretion levels, and biochemical and light microscopical changes following zinc supplementation in the gastrointestinal tract of streptozotocin (STZ)-diabetic rats. The animals were divided into four groups: Group I: control (untreated) animals; Group II: control animals given zinc sulfate; Group III: diabetic animals; and Group IV: diabetic animals given zinc sulfate. Zinc sulfate was given to the animals by gavage at a daily dose of 100 mg/kg body weight for 60 days. Diabetes was induced by intraperitoneal (i.p.) injection of STZ in a single dose of 65 mg/kg. For histological studies, stomach and duodenum tissues were fixed in Bouin solution and sections stained with Masson’s trichrome and Periodic-Acid-Schiff. Tissue homogenates were used for protein, lipid peroxidation (LPO), glutathione (GSH), and nonenzymatic glycosylation (NEG) analyses. Zinc supplementation to the STZ-diabetic rats revealed the protective effect of zinc on these parameters. Zinc supplementation may contribute to prevent at least some complications of diabetes mellitus.

Regulation of the human taurine transporter by oxidative stress in retinal pigment epithelial cells stably transformed to overexpress aldose reductase

In diabetes, overexpression of aldose reductase (AR) and consequent glucose-induced impairment of antioxidant defense systems may predispose to oxidative stress and the development of diabetic complications, but the mechanisms are poorly understood. Taurine (2-aminoethanesulfonic acid) functions as an antioxidant, osmolyte, and calcium modulator such that its intracellular depletion could promote cytotoxicity in diabetes. The relationships of oxidative stress and basal AR gene expression to Na+-taurine cotransporter (TT) gene expression, protein abundance, and TT activity were therefore explored in low AR-expressing human retinal pigment epithelial (RPE) 47 cells and RPE 47 cells stably transformed to overexpress AR (RPE 75). Changes in TT gene expression were determined using a 4.6-kb TT promoter-luciferase fusion gene. Compared with RPE 47 cells, in high AR-expressing RPE 75 cells, TT promoter activity was decreased by 46%, which was prevented by an AR inhibitor. TT promoter activity increased up to 900% by prooxidant exposure, which was associated with increased TT peptide abundance and taurine transport. However, induction of TT promoter activity by oxidative stress was attenuated in high AR-expressing cells and partially corrected by AR inhibitor. Finally, exposure of RPE 75 cells to high glucose increased oxidative stress, but down-regulated TT expression. These studies demonstrate for the first time that the TT is regulated by oxidative stress and that overexpression of AR and high glucose impair this response. Abnormal expression of AR may therefore impair antioxidant defense, which may determine tissue susceptibility to chronic diabetic complications.

Taurine replacement attenuates hyperalgesia and abnormal calcium signaling in sensory neurons of STZ-D rats

The etiology of painful diabetic neuropathy is poorly understood, but may result from neuronal hyperexcitability secondary to alterations of Ca2+ signaling in sensory neurons. The naturally occurring amino acid taurine functions as an osmolyte, antioxidant, Ca2+ modulator, inhibitory neurotransmitter, and analgesic such that its depletion in diabetes may predispose one to neuronal hyperexcitability and pain. This study reports the effects of taurine replacement on hyperalgesia and sensory neuron Ca2+ homeostasis in streptozotocin-diabetic (STZ-D) rats. Nondiabetic and STZ-D rats were treated with a 2% taurine-supplemented diet for 6-12 wk. Thermal hyperalgesia and mechanical allodynia were determined by measuring hindpaw withdrawal latency to radiant heat and the withdrawal threshold to the von Frey anesthesiometer. Intracellular Ca2+ signaling was explored in neurons from L4-L6 dorsal root ganglia (DRG), using fura 2 fluorescence. Taurine replacement of diabetic rats attenuated deficits of nerve conduction and prevented reductions of mechanical and thermal withdrawal threshold and latency, respectively. In small DRG sensory neurons from diabetic rats, recovery of intracellular Ca2+ concentration ([Ca2+]i) in response to KCl was slowed and 73% corrected by taurine. The amplitudes of caffeine and ATP-induced [Ca2+]i transients were decreased by 47 and 27% (P < 0.05), respectively, in diabetic rat DRG sensory neurons and corrected by 74 and 93% (P < 0.05), respectively, by taurine replacement. These data indicate that taurine is important in the regulation of neuronal Ca2+ signaling and that taurine deficiency may predispose one to nerve hyperexcitability and pain, complicating diabetes.

Dysregulation of hepatic superoxide dismutase, catalase and glutathione peroxidase in diabetes: response to insulin and antioxidant therapies

Recent evidence suggests that impaired antioxidant status is involved in oxidative stress associated with diabetes. The main antioxidant enzymes include superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPX). The aim of the present investigation was to evaluate the activities and protein expression of these antioxidant enzymes in streptozotocin-induced diabetes. Furthermore, the effects of insulin and antioxidant therapy alone and in combination were studied. Male Sprague-Dawley rats were rendered diabetic by streptozotocin administration and randomly assigned to untreated, insulin-treated, antioxidant (vitamin E and C)-treated and insulin plus antioxidant-treated groups. Normal rats fed either a regular diet or the antioxidant (vitamin E and C)-rich diet served as controls. The animals were observed for 4 weeks. Diabetic animals showed marked weight loss, decreased activities of Cu Zn SOD and CAT and normal GPX activity. Additionally, the expression of all antioxidant enzyme proteins was decreased in the diabetic rats compared to the untreated controls. Insulin therapy prevented weight loss and normalized the activities and protein expression of all antioxidant enzymes. Antioxidant therapy in the diabetic rats normalized Cu Zn SOD and GPX protein expression. Combined therapy with insulin and antioxidants normalized all measured antioxidant enzyme protein expression and activities. Thus diabetes-associated reductions in antioxidant enzymes can be ameliorated by insulin and/or antioxidant therapy.

C-peptide corrects endoneurial blood flow but not oxidative stress in type 1 BB/Wor rats

Oxidative stress and neurovascular dysfunction have emerged as contributing factors to the development of experimental diabetic neuropathy (EDN) in streptozotocin-diabetic rodents. Additionally, depletion of C-peptide has been implicated in the pathogenesis of EDN, but the mechanisms of these effects have not been fully characterized. The aims of this study were therefore to explore the effects of diabetes on neurovascular dysfunction and indexes of nerve oxidative stress in type 1 bio-breeding Worcester (BB/Wor) rats and type 2 BB Zucker-derived (ZDR)/Wor rats and to determine the effects of C-peptide replacement in the former. Motor and sensory nerve conduction velocities (NCVs), hindlimb thermal thresholds, endoneurial blood flow, and indicators of oxidative stress were evaluated in nondiabetic control rats, BB/Wor rats, BB/Wor rats with rat II C-peptide replacement (75 nmol C-peptide.kg body wt(-1).day(-1)) for 2 mo, and diabetes duration-matched BBZDR/Wor rats. Endoneurial perfusion was decreased and oxidative stress increased in type 1 BB/Wor rats. C-peptide prevented NCV and neurovascular deficits and attenuated thermal hyperalgesia. Inhibition of nitric oxide (NO) synthase, but not cyclooxygenase, reversed the C-peptide-mediated effects on NCV and nerve blood flow. Indexes of oxidative stress were unaffected by C-peptide. In type 2 BBZDR/Wor rats, neurovascular deficits and increased oxidative stress were unaccompanied by sensory NCV slowing or hyperalgesia. Therefore, nerve oxidative stress is increased and endoneurial perfusion decreased in type 1 BB/Wor and type 2 BBZDR/Wor rats. NO and neurovascular mechanisms, but not oxidative stress, appear to contribute to the effects of C-peptide in type 1 EDN. Sensory nerve deficits are not an inevitable consequence of increased oxidative stress and decreased nerve perfusion in a type 2 diabetic rodent model.

Clinical experience with thioctacid (thioctic acid) in the treatment of distal symmetric polyneuropathy in Korean diabetic patients

OBJECTIVE

This open-label study was performed to evaluate the efficacy and safety of oral treatment with the antioxidant alpha-lipoic acid (ALA, thioctic acid) in Korean diabetic patients with distal symmetric polyneuropathy (DSP).

SUBJECTS AND METHODS

Thioctic acid was administered orally using 600 mg once daily for 8 weeks in 61 diabetic patients with symptomatic polyneuropathy. Neuropathic symptoms (pain, burning sensation, paresthesia, and numbness) were scored at baseline as well as at 4 and 8 weeks following treatment. In addition, neurological assessment was carried out before and after 8 weeks of treatment, and an overall evaluation was performed at the end of treatment. The primary endpoint was the response rate after 8 weeks of treatment, defined as an improvement in the Total Symptom Score (TSS) of > or =30%.

RESULTS

Efficacy was evaluated in 38 patients who had completed the study according to the protocol. Safety was evaluated in all 61 patients who had taken the study medication. Fasting blood glucose and HbA(1)c did not change during the study. The response rate after 8 weeks was 71.4%. At 4 weeks, the response rate was 47.4%. The TSS significantly decreased at 4 weeks, which decreased further at 8 weeks (P<.05). All the individual scores for neuropathic symptoms (pain, burning sensation, paresthesia, and numbness) were also significantly reduced at 4 weeks and further decreased at 8 weeks (P<.05). The duration of diabetes, severity and duration of diabetic polyneuropathy, and all the other demographic and metabolic parameters did not demonstrate an effect on the response rate. The parameters of neurological assessment (ankle reflexes, pin-prick test, 10-g monofilament test) and quantitative sensory tests (vibration, warm and cold sensation) were not influenced by 8 weeks of treatment with 600 mg of oral thioctic acid per day. Overall efficacy rated as "good/fair" was 86.8% by the physician and 76.3% by the patients at the end of an 8-week treatment period. Eleven episodes (18.0%) of adverse events (possibly, probably, definitely related) were reported in seven patients (11.5%).

CONCLUSION

These findings indicate that oral treatment with thioctic acid at a dose of 600 mg/day for 8 weeks improved symptoms of polyneuropathy in Korean diabetic patients without causing serious adverse events.

Increased nitric oxide levels as an early sign of premature aging in diabetes

The levels of different reactive species, especially those of nitric oxide and peroxynitrite, were determined in streptozotocin-induced diabetic rat tissues, before the development of histopathological damages. Significantly higher steady state free radical concentrations were found in the liver 3 weeks after the onset of diabetes compared to age-matched control groups. Increased nitric oxide levels in diabetic vasculature and kidney decreased the production of detectable reactive oxygen species. High peroxynitrite generation suggested the onset of processes characteristic to premature aging of the endothelium. According to the histopathological results, there were no signs of late complications in the tissues up to 7 weeks after induction of diabetes. These results support the idea that oxidative stress is increased at a very early stage of diabetes and, in particular, that high levels of nitric oxide and peroxynitrite could play a decisive role in the development of late complications in the diabetic vasculature and kidney.

Acute glucose deprivation leads to apoptosis in a cell model of acute diabetic neuropathy

OBJECTIVE

Our aims were to better understand the mechanisms underlying peripheral neuropathy with diabetes mellitus and to test the hypothesis that acute lowering of glucose levels induces apoptosis in hypoxic neurons.

METHODS

We used rat dissociated dorsal root ganglion (DRG) neurons incubated in a medium high in glucose concentration (700 mg%) and room air (PO2 150 torr). After 5 days, DRG neurons were placed in hypoxic conditions (PO2 7.6 torr) with a normal-glucose (100 mg%) or high-glucose (700 mg%) medium containing 3 or 100 ng/mL of nerve growth factor. Acute lowering of glucose levels under hypoxic conditions led to apoptosis of DRG neurons. Apoptosis was demonstrated by bis-benzimide staining for nuclear fragmentation, electron microscopy, DNA laddering, and TUNEL staining. Caspase 3 immunocytochemistry and inhibition of neuronal death by the caspase inhibitor z-VAD-fmk (100 microM) confirmed that death was apoptotic.

RESULTS

Hypoxia-induced death was decreased when DRG neurons were maintained in high-glucose medium, suggesting that high levels of substrate protected against hypoxia. Apoptosis was completely prevented by increasing the concentration of nerve growth factor from 3 to 100 ng/mL and was partially prevented by the addition of the antioxidant alpha-lipoic acid (500 microM).

CONCLUSIONS

This model provides a novel means for studying the pathogenesis and treatment of early stages of diabetic neuropathy.

The sensory symptoms of diabetic polyneuropathy are improved with alpha-lipoic acid: the SYDNEY trial

OBJECTIVE

Because alpha-lipoic acid (ALA), a potent antioxidant, prevents or improves nerve conduction attributes, endoneurial blood flow, and nerve (Na(+) K(+) ATPase activity in experimental diabetes and in humans and may improve positive neuropathic sensory symptoms, in this report we further assess the safety and efficacy of ALA on the Total Symptom Score (TSS), a measure of positive neuropathic sensory symptoms.

RESEARCH DESIGN AND METHODS

Metabolically stable diabetic patients with symptomatic (stage 2) diabetic sensorimotor polyneuropathy (DSPN) were randomized to a parallel, double-blind study of ALA (600 mg) (n = 60) or placebo (n = 60) infused daily intravenously for 5 days/week for 14 treatments. The primary end point was change of the sum score of daily assessments of severity and duration of TSS. Secondary end points were sum scores of neuropathy signs (NIS), symptoms (NSC), attributes of nerve conduction, quantitative sensation tests (QSTs), and an autonomic test.

RESULTS

At randomization, the groups were not significantly different by the criteria of metabolic control or neuropathic end points. After 14 treatments, the TSS of the ALA group had improved from baseline by an average of 5.7 points and the placebo group by an average of 1.8 points (P < 0.001). Statistically significant improvement from baseline of the ALA, as compared with the placebo group, was also found for each item of the TSS (lancinating and burning pain, asleep numbness and prickling), NIS, one attribute of nerve conduction, and global assessment of efficacy.

CONCLUSIONS

Intravenous racemic ALA, a potent antioxidant, rapidly and to a significant and meaningful degree, improved such positive neuropathic sensory symptoms as pain and several other neuropathic end points. This improvement of symptoms was attributed to improved nerve pathophysiology, not to increased nerve fiber degeneration. Because of its safety profile and its effect on positive neuropathic sensory symptoms and other neuropathic end points, this drug appears to be a useful ancillary treatment for the symptoms of diabetic polyneuropathy.

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.

Transition metals and polyol pathway in the development of diabetic neuropathy in rats

BACKGROUND

The transition metal-catalyzed reaction is a major source of oxygen free radicals, which play an important role in vascular dysfunction leading to ischemia in diabetic tissues. The inhibition of polyol pathway hyperactivity has been reported to ameliorate neurovascular abnormalities in diabetic rats and has been proposed to improve the oxygen free radical scavenging capacity. The present study was conducted to compare the effect of a transition metal chelating agent, trientine (TRI), on diabetic neuropathy with that of an aldose reductase inhibitor, NZ-314 (NZ).

METHODS

Diabetic rats were divided into three groups: (1). untreated, (2). TRI-treated, and (3). NZ-treated. TRI (20 mg/kg) or NZ (100 mg/kg) was administered by gavage or chow containing NZ, respectively, for 8 weeks. Motor nerve conduction velocity (MNCV), coefficient of variation of the R - R interval on electrocardiogram (CVr-r), sciatic nerve blood flow (SNBF), platelet aggregation activities, and serum concentrations of malondialdehyde were measured.

RESULTS

Untreated diabetic rats showed delayed MNCV, decreased CV(R-R), and reduced SNBF compared to normal rats. TRI or NZ completely prevented these deficits. Platelet hyperaggregation activities in diabetic rats were prevented by NZ, but not by TRI. Increased concentrations of malondialdehyde in diabetic rats were partially but significantly ameliorated by either TRI or NZ.

CONCLUSIONS

These observations suggest that increased free radical formation through the transition metal-catalyzed reaction plays an important role in the development of diabetic neuropathy and that the preventive effect of an aldose reductase inhibitor on diabetic neuropathy may also be mediated by decreasing oxygen free radicals.

Neurovascular disease, antioxidants and glycation in diabetes

People with diabetes are ten to fifteen times more likely to have a lower limb amputation (LLA) than non-diabetic individuals. Fifteen percent of people with diabetes will develop a foot ulcer during their lifetime, the rate of major amputation amongst diabetic individuals continues to rise, foot problems remain the commonest reason for diabetes-related hospitalisation and recurrence rates in patients with previous foot ulcers are 50% or more. Hyperglycaemia-induced oxidative stress has been shown to result in decreased nerve conduction velocity, and decreased endoneural blood flow-both precursors for neuropathy. Vitamin antioxidants have been shown to be effective therapy in experimental models in reducing free radical species and inhibiting the oxidative process in diabetes subjects. Little work has been published, however, regarding the dietary use of antioxidants from foods, and their specific effects on neurovascular disease and glycation within the diabetes population. Aetiological and prevention studies with dietary antioxidants from foods aimed at the complex nature of foot problems in diabetes are needed.

Altered arachidonic acid biosynthesis and antioxidant protection mechanisms in Schwann cells grown in elevated glucose

In cultured Schwann cells, elevated glucose induces alterations in arachidonic acid metabolism that cause a decrease in the content of glycerophospholipid arachidonoyl-containing molecular species (ACMS). This could result from decreased de novo arachidonic acid biosynthesis, or increased arachidonic acid release from phospholipids. Incorporation of radioactive 8,11,14-eicosatrienoic acid into ACMS was lower for cells grown in 30 mm versus 5 mm glucose, consistent with a decrease in delta5 desaturase activity. However, neither basal arachidonic acid release from prelabeled cells nor stimulated generation of arachidonic acid in the presence of the reacylation inhibitor, thimerosal, the phosphotyrosine phosphatase inhibitor, bipyridyl peroxovanadium, or both together, were altered by varying the glucose concentrations, indicating that arachidonic acid turnover did not contribute to ACMS depletion. Free cytosolic NAD+ /NADH decreased, whereas NADP+ /NADPH remained unchanged for cells grown in elevated glucose, implying that decreased desaturase activity is a result of metabolic changes other than cofactor availability. Schwann cells in elevated glucose were susceptible to oxidative stress, as shown by increased malondialdehyde, depleted glutathione levels, and reduced cytosolic superoxide dismutase activity. Glutathione-altering compounds had no effect on ACMS levels, in contrast to N -acetylcysteine and alpha-lipoic acid, which partly corrected ACMS depletion in phosphatidylcholine. These findings suggest that in the Schwann cell cultures, a high glucose level elicits oxidative stress and weakens antioxidant protection mechanisms which could decrease arachidonic acid biosynthesis and that this deficit can be partly corrected by treatment with exogenous antioxidants.

Oxidative stress and diabetic neuropathy: pathophysiological mechanisms and treatment perspectives

Increased oxidative stress is a mechanism that probably plays a major role in the development of diabetic complications, including peripheral neuropathy. This review summarises recent data from in vitro and in vivo studies that have been performed both to understand this aspect of the pathophysiology of diabetic neuropathy and to develop therapeutic modalities for its prevention or treatment. Extensive animal studies have demonstrated that oxidative stress may be a final common pathway in the development of diabetic neuropathy, and that antioxidants can prevent or reverse hyperglycaemia-induced nerve dysfunction. Most probably, the effects of antioxidants are mediated by correction of nutritive blood flow, although direct effects on endoneurial oxidative state are not excluded. In a limited number of clinical studies, antioxidant drugs including alpha-lipoic acid and vitamin E were found to reduce neuropathic symptoms or to correct nerve conduction velocity. These data are promising, and additional larger studies with alpha-lipoic acid are currently being performed.

Lipoic acid confers protection against oxidative injury in non-neuronal and neuronal tissue

In the past decade or so, a convincing link between oxidative stress and degenerative conditions has been made and with the knowledge that oxidatiye changes may actually trigger deterioration in cell function, a great deal of energy has focussed on identifying agents which may have possible therapeutic value in combating oxidative changes. One agent which has received attention, because of its powerful antioxidative effects, particularly in neuronal tissue, is lipoic acid.

An aldose reductase inhibitor reverses early diabetes-induced changes in peripheral nerve function, metabolism, and antioxidative defense

Aldose reductase inhibitors (ARIs) prevent peripheral nerve dysfunction and morphological abnormalities in diabetic animal models. However, some experimental intervention studies and clinical trials of ARIs on diabetic neuropathy appeared disappointing because of either 1) their inadequate design and, in particular, insufficient correction of the sorbitol pathway activity or 2) the inability to reverse established functional and metabolic deficits of diabetic neuropathy by AR inhibition in general. We evaluated whether diabetes-induced changes in nerve function, metabolism, and antioxidative defense are corrected by the dose of ARI (sorbinil, 65 mg/kg/d in the diet), resulting in complete inhibition of increased sorbitol pathway activity. The groups included control rats and streptozotocin-diabetic rats treated with/without ARI for 2 weeks after 4 weeks of untreated diabetes. ARI treatment corrected diabetes-induced nerve functional changes; that is, decrease in endoneurial nutritive blood flow, motor and sensory nerve conduction velocities, and metabolic abnormalities (i.e., mitochondrial and cytosolic NAD+/NADH redox imbalances and energy deficiency). ARI restored nerve concentrations of two major non-enzymatic antioxidants, reduced glutathione (GSH) and ascorbate, and completely arrested diabetes-induced lipid peroxidation. In conclusion, treatment with adequate doses of ARIs (that is, doses that completely inhibit increased sorbitol pathway activity) is an effective approach for reversal of, at least, early diabetic neuropathy.

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.

A role for mitogen-activated protein kinases in the etiology of diabetic neuropathy

The onset of diabetic neuropathy, a complication of diabetes mellitus, has been linked to poor glycemic control. We tested the hypothesis that the mitogen-activated protein kinases (MAPK) form transducers for the damaging effects of high glucose. In cultures of adult rat sensory neurons, high glucose activated JNK and p38 MAPK but did not result in cell damage. However, oxidative stress activated ERK and p38 MAPKs and resulted in cellular damage. In the dorsal root ganglia of streptozotocin-induced diabetic rats (a model of type I diabetes), ERK and p38 were activated at 8 wk duration, followed by activation of JNK at 12 wk duration. We report activation of JNK and increases in total levels of p38 and JNK in sural nerve of type I and II diabetic patients. These data implicate MAPKs in the etiology of diabetic neuropathy both via direct effects of glucose and via glucose-induced oxidative stress.

Glutathione and alpha-lipoate in diabetic rats: nerve function, blood flow and oxidative state

BACKGROUND

Increased oxidative stress is considered to be a causal factor in the development of diabetic complications, among which peripheral neuropathy. The pathophysiology of nerve dysfunction in diabetes has been explained both by reduced endoneurial microcirculation and alterations in endoneurial metabolism. It is unclear whether antioxidants primarily improve nerve blood flow or normalise systemic or endoneurial oxidative metabolism. Therefore, we evaluated the effects of the antioxidants glutathione and alpha-lipoic acid on both nerve microcirculation and the antioxidative capacity and lipid peroxidation in experimentally diabetic rats.

MATERIALS AND METHODS

Streptozotocin-diabetic rats were treated with different doses of alpha-lipoic acid, reduced glutathione or placebo, and were compared with nondiabetic controls. We measured systemic and endoneurial antioxidants, malondialdehyde and whole blood hydrogen peroxide. Furthermore, we evaluated sciatic and tibial motor and sensory nerve conduction velocity, caudal nerve conduction velocity, and assessed sciatic nerve blood flow and vascular resistance by Laser-Doppler flowmetry.

RESULTS

We observed a rise in erythrocyte glutathione by 27 % (P < 0.05), and a trend towards decreased plasma malondialdehyde in alpha-lipoic acid, but not in glutathione-treated animals in comparison with the placebo group. Simultaneously, sciatic nerve blood flow and vascular resistance were improved by daily alpha-lipoic acid administration by 38% (P < 0.05). Peripheral nerve conduction velocity and endoneurial glutathione were not significantly influenced by antioxidant treatment.

CONCLUSIONS

Only minor beneficial effects of alpha-lipoic acid on nerve blood flow and oxidative state occur at the given doses; these effects were insufficient to improve nerve conduction deficits.

Depletion of taurine in experimental diabetic neuropathy: implications for nerve metabolic, vascular, and functional deficits

In diabetes, increased oxidative stress, disruption of signal transduction pathways, and endothelial dysfunction have been critically implicated in the pathogenesis of experimental diabetic neuropathy (EDN). The development of nerve conduction slowing in diabetes is accompanied by depletion of the beta-amino acid taurine. Since taurine functions as an antioxidant, calcium modulator, and vasodilator, taurine depletion may provide a pathogenetic link between nerve metabolic, vascular, and functional deficits complicating diabetes. The mechanism(s) of nerve taurine depletion, the localization of critical taurine deficits, and its pathophysiological significance in EDN are however unknown. This study explored the pathophysiological effects of selective nerve taurine replacement in streptozotocin-diabetic (STZ-D) rats. A polyclonal human taurine transporter (TT) antibody was also generated in order to determine potential loci of critical taurine depletion. Two weeks of STZ-D reduced sciatic motor nerve conduction velocity (NCV) by 23% (P < 0.01), decreased composite nerve blood flow by 38% (P < 0.01), and reduced nerve taurine content by 29% (P < 0.05). In STZ-D rats, a 1% taurine diet corrected nerve taurine depletion, prevented motor NCV slowing, and partially attenuated composite nerve blood flow deficits. After 6 weeks of STZ-D, a 1% taurine diet ameliorated motor NCV slowing and endoneurial nutritive blood flow deficits, prevented digital sensory NCV slowing, and reduced ouabain-sensitive nerve (Na,K)-ATPase activity. Immunohistochemical studies localized taurine and the TT to the vascular endothelium and Schwann cells of the sciatic nerve. In conclusion, taurine depletion in the vascular endothelium and Schwann cells of the sciatic nerve may contribute to the neurovascular and metabolic deficits in EDN.

Effect of alpha-tocopherol supplementation on the ultrastructural abnormalities of peripheral nerves in experimental diabetes

Ultrastructural observations were made on myelinated fibers in the tibial nerves in order to investigate the beneficial effects of alpha-tocopherol administration in streptozotocin-diabetic rats. Male Wistar rats, aged 12 weeks and weighing between 250 g to 300 g were studied. Six onset control rats were used to obtain the baseline parameters for this strain and age. Further 3 groups--untreated diabetic animals, diabetic animals treated with alpha-tocopherol, and age-matched controls--were studied over a 3-month period. In the diabetic animal, administration of alpha-tocopherol resulted in a significant increase (p < 0.05) in total plasma vitamin E levels when compared with other groups. Myelinated fiber cross-sectional area (p < 0.05), axonal area (p < 0.01) and myelin sheath area (p < 0.05) were significantly less in the tibial nerve of diabetic animals than in age-matched controls, but not different from those of onset controls. In the alpha-tocopherol treated diabetic animals, the values for these parameters were intermediate without showing significant difference when compared with age-matched controls and untreated diabetics. The "g" ratio (axon to fiber area) did not differ between any experimental groups. The number of large myelinated fibers were less in the untreated diabetic animals, but in the alpha-tocopherol-treated diabetics, the values were significantly higher (p < 0.05) than with untreated diabetics and were similar to those of age-matched controls. In conclusion, this ultrastructural study reiterated the fact that structural abnormalities of myelinated fibers occur in experimental diabetes and that alpha-tocopherol administration may be useful in preventing the development of these abnormalities.

No association of glutathione S-transferase M1 gene polymorphism with diabetic nephropathy in Japanese type 2 diabetic patients

Oxidative stress possibly contributes to the development of diabetic nephropathy. Therefore, the levels of endogenous antioxidants may be one of determinants of the susceptibility to diabetic nephropathy. Glutathione S-transferases (GSTs) can work as one of endogenous antioxidants to protect cells from oxidative stress. The M1 member of GST mu class (GSTM1) is polymorphic and only expressed in 55-60% of Caucasians because of the homozygous deletion of the gene (null genotype). Recent studies have provided evidence that the GSTM1 null genotype, i.e. lack of the GSTM1 activity, is associated with an increased susceptibility to lung cancer and colorectal cancer. The present study was conducted to determine whether the genetic polymorphism influences the development of diabetic nephropathy. We examined 105 patients with diabetic nephropathy and 69 patients without diabetic nephropathy in Japanese type 2 diabetic patients with proliferative diabetic retinopathy. GSTM1 genotyping was performed by polymerase chain reaction. The two patient groups were well matched with regard to age, body mass index and HbAlc. GSTM1 null genotype was observed in 48.6% of patients with nephropathy versus 55.1% of patients without nephropathy. The frequency of GSTM1 null genotype was not significantly higher in the patient group with nephropathy than in the patient group without nephropathy. This study is the first to investigate the association of GSTM1 gene polymorphism with the development of diabetic nephropathy. The present results suggest that GSTM1 null genotype does not contribute to the development of diabetic nephropathy in Japanese type 2 diabetic patients.

Pyridoxine and pyridoxamine inhibits superoxide radicals and prevents lipid peroxidation, protein glycosylation, and (Na+ + K+)-ATPase activity reduction in high glucose-treated human erythrocytes

Vitamin B(6) (pyridoxine) supplementation has been found beneficial in preventing diabetic neuropathy and retinopathy, and the glycosylation of proteins. Oxygen radicals and oxidative damage have been implicated in the cellular dysfunction and complications of diabetes. This study was undertaken to test the hypothesis that pyridoxine (P) and pyridoxamine (PM) inhibit superoxide radical production, reduce lipid peroxidation and glycosylation, and increase the (Na+ + K+)-ATPase activity in high glucose-exposed red blood cells (RBC). Superoxide radical production was assessed by the reduction of cytochrome C by glucose in the presence and absence of P or PM in a cell-free buffered solution. To examine cellular effects, washed normal human RBC were treated with control and high glucose concentrations with and without P or PM. Both P and PM significantly lowered lipid peroxidation and glycated hemoglobin (HbA(1)) formation in high glucose-exposed RBC. P and PM significantly prevented the reduction in (Na+ + K+)-ATPase activity in high glucose-treated RBC. Thus, P or PM can inhibit oxygen radical production, which in turn prevents the lipid peroxidation, protein glycosylation, and (Na+ + K+)-ATPase activity reduction induced by the hyperglycemia. This study describes a new biochemical mechanism by which P or PM supplementation may delay or inhibit the development of complications in diabetes.

Effects of dietary supplementation of alpha-lipoic acid on early glomerular injury in diabetes mellitus

Antioxidants, in particular vitamin E (VE), have been reported to protect against diabetic renal injury. alpha-Lipoic acid (LA) has been found to attenuate diabetic peripheral neuropathy, but its effects on nephropathy have not been examined. In the present study, parameters of glomerular injury were examined in streptozotocin diabetic rats after 2 mo on unsupplemented diets and in diabetic rats that received the lowest daily dose of dietary LA (30 mg/kg body wt), VE (100 IU/kg body wt), or vitamin C (VC; 1 g/kg body wt), which detectably increased the renal cortical content of each antioxidant. Blood glucose values did not differ among the diabetic groups. At 2 mo, inulin clearance, urinary albumin excretion, fractional albumin clearance, glomerular volume, and glomerular content of immunoreactive transforming growth factor-beta (TGF-beta) and collagen alpha1 (IV) all were significantly increased in unsupplemented D compared with age-matched nondiabetic controls. With the exception of inulin clearance, LA prevented or significantly attenuated the increase in all of these glomerular parameters in D, as well as the increases in renal tubular cell TGF-beta seen in D. At the dose used, VE reduced inulin clearance in D to control levels but failed to alter any of the other indices of glomerular injury or to suppress renal tubular cell TGF-beta in D. VC suppressed urinary albumin excretion, fractional albumin clearance, and glomerular volume but not glomerular or tubular TGF-beta or glomerular collagen alpha1 (IV) content. LA but not VE or VC significantly increased renal cortical glutathione content in D. These data indicate that LA is effective in the prevention of early diabetic glomerular injury and suggest that this agent may have advantages over high doses of either VE or VC.

Lipoic acid decreases lipid peroxidation and protein glycosylation and increases (Na(+) + K(+))- and Ca(++)-ATPase activities in high glucose-treated human erythrocytes

Lipoic acid supplementation has been found to be beneficial in preventing neurovascular abnormalities in diabetic neuropathy. Insufficient (Na(+) + K(+))-ATPase activity has been suggested as a contributing factor in the development of diabetic neuropathy. This study was undertaken to test the hypothesis that lipoic acid reduces lipid peroxidation and glycosylation and can increase the (Na(+) + K(+))- and Ca(++)-ATPase activities in high glucose-exposed red blood cells (RBC). Washed normal human RBC were treated with normal (6 mM) and high glucose concentrations (45 mM) with 0-0.2 mM lipoic acid (mixture of S and R sterioisomers) in a shaking water bath at 37 degrees C for 24 h. There was a significant stimulation of glucose consumption by RBC in the presence of lipoic acid both in normal and high glucose-treated RBC. Lipoic acid significantly lowered the level of glycated hemoglobin (GHb) and lipid peroxidation in RBC exposed to high glucose concentrations. High glucose treatment significantly lowered the activities of (Na(+) + K(+))- and Ca(++)-ATPases of RBC membranes. Lipoic acid addition significantly blocked the reduction in activities of (Na(+) + K(+))- and Ca(++)-ATPases in high glucose- treated RBC. There were no differences in lipid peroxidation, GHb and (Na(+) + K(+))- and Ca(++)-ATPase activity levels in normal glucose-treated RBC with and without lipoic acid. Thus, lipoic acid can lower lipid peroxidation and protein glycosylation, and increase (Na(+) + K(+))- and Ca(++)-ATPase activities in high-glucose exposed RBC, which provides a potential mechanism by which lipoic acid may delay or inhibit the development of neuropathy in diabetes.

Antioxidant status in delayed healing type of wounds

This investigation studied the contribution of antioxidants in delaying healing in excision cutaneous wounds (8 mm) in diabetic, aged and immunocompromised animals. Skin levels of catalase, glutathione (GSH), ascorbic acid (AA) and vitamin E in streptozotocin-induced diabetic rat were lower as compared to nondiabetics. The 7-d wound tissue of diabetic rats showed an increased vitamin E level along with depleted GSH content. In aged rats (18 months old), higher levels of skin superoxide dismutase (SOD), glutathione peroxidase (Gpx) and thiobarbituric acid reactive substances (TBARS) and lower levels of catalase and GSH were found as compared to their values in young rats (3-4 months old). The levels of SOD, GPx, catalase, AA, GSH and vitamin E in 7-d wound tissue of aged rats were significantly lower in comparison to those in young rats. However, TBARS were elevated in these wound tissues. The non-wounded skin of immunocompromised (athymic) mice showed lower levels of SOD, catalase, and TBARS and higher GSH and GPx levels in comparison to those present in normal mouse skin. Surprisingly, the analysis of 7-d wound tissue showed higher levels of SOD, catalase, GPx, and GSH and lower TBARS level in athymic mice compared to the wound tissue of normal mice. Thus low levels of antioxidants accompanied by raised levels of markers of free radical damage play a significant role in delaying wound healing in aged rats. In diabetic rats reduced glutathione levels may have a contributory role in delaying the healing process. However, in immunocompromised mice the antioxidant status following injury showed an adapted response.

High rat food vitamin E content improves nerve function in streptozotocin-diabetic rats

Antioxidants can improve nerve dysfunction in hyperglycaemic rats. We evaluated whether the standard supplementation of rat food with vitamin E (normally added for preservation purposes) or high-dose vitamin E treatment improves nerve conduction in maturing streptozotocin-diabetic rats, a model widely used to study diabetic neuropathy. Hyperglycaemic rats received food containing 25 mg/kg (non-supplemented), 70 mg/kg (standard food) or 12 g/kg (high-dose) vitamin E. Non-diabetic controls received non-supplemented food. Sciatic and tibial sensory and motor nerve conduction velocity were decreased in all diabetic animals. In comparison with standard feeding, the non-supplemented diabetic rats showed lower plasma vitamin E levels but no significant change in nerve conduction. High-dose treatment prevented nerve dysfunction by 50%, and led to attenuated endoneurial lipid peroxidation (measured as malondialdehyde). We conclude that high doses of vitamin E, but not standard vitamin E supplementation of rat food partially prevent nerve dysfunction in young adult streptozotocin-diabetic rats.