Early changes in lipid peroxidation and antioxidative defense in diabetic rat retina: effect of DL-alpha-lipoic acid

The central role of glutathione in the pathophysiology of human diseases

Reduced glutathione (L-gamma-glutamyl-L-cysteinyl-glycine, GSH) is the prevalent low-molecular-weight thiol in mammalian cells. It is formed in a two-step enzymatic process including, first, the formation of gamma-glutamylcysteine from glutamate and cysteine, by the activity of the gamma-glutamylcysteine synthetase; and second, the formation of GSH by the activity of GSH synthetase which uses gamma-glutamylcysteine and glycine as substrates. While its synthesis and metabolism occur intracellularly, its catabolism occurs extracellularly by a series of enzymatic and plasma membrane transport steps. Glutathione metabolism and transport participates in many cellular reactions including: antioxidant defense of the cell, drug detoxification and cell signaling (involved in the regulation of gene expression, apoptosis and cell proliferation). Alterations in its concentration have also been demonstrated to be a common feature of many pathological conditions including diabetes, cancer, AIDS, neurodegenerative and liver diseases. Additionally, GSH catabolism has been recently reported to modulate redox-sensitive components of signal transduction cascades. In this manuscript, we review the current state of knowledge on the role of GSH in the pathogenesis of human diseases with the aim to underscore its relevance in translational research for future therapeutic treatment design.

Mechanisms for oxidative stress in diabetic cardiovascular disease

Obesity, metabolic syndrome, and diabetes are increasingly prevalent in Western society, and they markedly increase the risk for atherosclerotic vascular disease, the major cause of death in diabetics. Although recent evidence suggests a causal role for oxidative stress in insulin resistance, diabetes, and atherosclerosis, there is considerable controversy regarding its nature, magnitude, and underlying mechanisms. Glucose promotes glycoxidation reactions in vitro, and products of glycoxidation and lipoxidation are elevated in plasma and tissue from humans suffering from diabetes, but the exact relationships between hyperglycemia and oxidative stress are poorly understood. This review focuses on molecular mechanisms of increased oxidative stress in diabetes, the relationship of oxidant production to hyperglycemia, and the potential interaction of reactive carbonyls and lipids in oxidant generation. Using highly sensitive and specific gas chromatography-mass spectrometry, molecular signatures of specific oxidation pathways were identified in tissues of diabetic humans and animals. These studies support the hypothesis that unique reactive intermediates generated in localized microenvironments of vulnerable tissues promote diabetic damage. Therapies interrupting these reactive pathways in vascular tissue might help prevent cardiovascular disease in this high-risk population.

Glutathione depletion induces differential apoptosis in cells of mouse retina, in vivo

Oxidative stress affects numerous intracellular macromolecules, and may result in cell death unless precisely regulated. Unregulated oxidative stress can be controlled by various cellular defense mechanisms such as glutathione (GSH) which can critically counteract the damaging effects of oxidative stress in mammalian cells. We determined the effects of unregulated oxidative stress induced by GSH depletion on cells in mouse retina. Mice were intraperitoneally injected with buthionine sulphoximine (BSO) at 1.5 g/kg. After 0, 1, 4, and 7 days of BSO administration, retinas were excised and sections were subjected to GSH assay and terminal uridine deoxynucleotidyl nick end labeling (TUNEL) analysis. After 4 days of BSO administration, the number of TUNEL positive cells was significantly increased. However, after 7 days, TUNEL positive cells returned to the basal level. The retinal region most affected by the BSO treatment appeared to be the outer nuclear layer where the photoreceptor cells reside. Different from cells in other regions, retinal cells in the inner nuclear layer increased in their apoptosis even after the first day of BSO injection, and the increase was further potentiated after 4 days. Taken together, our studies suggested that GSH depletion may cause unregulated oxidative stress to the cells in the retina and indeed increased cell death in the retina. The cells in the inner nuclear layer seemed to be affected earlier than the cells in other layers of the retina. The GSH level in the retina may be a crucial therapeutic target in preventing blindness.

Differential expression of heat shock protein mRNAs under in vivo glutathione depletion in the mouse retina

Heat shock proteins (HSPs) are highly conserved proteins playing a protective role under deleterious conditions caused by a wide variety of pathophysiological, including environmental stresses. Glutathione (GSH) is known to play a critical role in the cellular defense against unregulated oxidative stress in mammalian cells including neurons. We previously demonstrated that GSH depletion induced cell death in the retina, but the mechanism(s) of cellular protection were not clear. Unregulated oxidative stress was induced by depletion of intracellular GSH by systematic administration of buthionine sulphoximine (BSO), an inhibitor of gamma-glutamylcysteine synthetase. After 0, 1, 4 and 7 days of BSO administration, we examined expression of both large and small HSP mRNAs (hsp90alpha, hsp90beta, hsp70, hsp60 and hsp25) in oxidative-stressed mouse retina. Of large HSPs, only hsp70 expression was significantly decreased from 1 day after BSO injection, whereas expression of other large hsps was not changed on day 1. Expression of hsp60 decreased on 4 days, whereas expression of hsp90 decreased on 7 days after BSO administration. Different from large HSPs, a small HSP, hsp25 increased its expression to a great extent from 1 day after BSO administration. Taken together, our results show that unregulated oxidative stress could induce differential expression of HSPs, which, in turn, may play distinct roles in the cellular defense. Targeting HSPs, therefore, may provide novel tools for treatment of retinal degenerative diseases such as glaucoma, retinopathy or age-related macular degeneration.

Influence of diabetes on critical flaps based on the vascular axis of a sensory nerve

Delayed sural flap based on the vascular axis of the sural nerve has been advocated for coverage of diabetic foot ulcers. In this study we compared the survival of neurovenous and standard inferior epigastric island flaps in diabetic and non-diabetic rats. VEGF concentrations and electrolyte balance of the flaps were also investigated during elevation and on the fifth day to explore the possible mechanisms that influence ischaemia of the flap during the hyperglycaemic state. There were no statistical differences in area surviving between diabetic and control rats for either flap. The VEGF concentrations were also similar in the two flaps in the two groups during elevation. On the fifth day, VEGF concentrations had decreased significantly in all of the flaps. Electrolyte balance paralleled VEGF concentrations. We conclude that flaps based on the vascular axis of a superficial nerve deserve further experimental and clinical attention as potential options for reconstruction of ulcers on diabetic feet.

Early diabetes-induced biochemical changes in the retina: comparison of rat and mouse models

AIMS/HYPOTHESIS

Recently, various transgenic and knock-out mouse models have become available for studying the pathogenesis of diabetic retinopathy. At the same time, diabetes-induced retinal changes in the wild-type mice remain poorly characterised. The present study compared retinal biochemical changes in rats and mice with similar (6-week) durations of streptozotocin-induced diabetes.

MATERIALS AND METHODS

The experiments were performed on Wistar rats and C57Bl6/J mice. Retinal glucose, sorbitol, fructose, lactate, pyruvate, glutamate, alpha-ketoglutarate and ammonia were measured spectrofluorometrically by enzymatic methods. Vascular endothelial growth factor (VEGF) protein was assessed by ELISA, and poly(ADP-ribosyl)ation by immunohistochemistry and western blot analysis. Free mitochondrial and cytosolic NAD(+)/NADH ratios were calculated from the glutamate and lactate dehydrogenase systems.

RESULTS

Retinal glucose concentrations were similarly increased in diabetic rats and mice, vs controls. Diabetic rats manifested approximately 26- and 5-fold accumulation of retinal sorbitol and fructose, respectively, whereas elevation of both metabolites in diabetic mice was quite modest. Correspondingly, diabetic rats had (1) increased retinal malondialdehyde plus 4-hydroxyalkenal concentrations, (2) reduced superoxide dismutase (SOD), glutathione peroxidase, glutathione reductase and glutathione transferase activities, (3) slightly increased poly(ADP-ribose) immunoreactivity and poly(ADP-ribosyl)ated protein abundance, and (4) VEGF protein overexpression. Diabetic mice lacked these changes. SOD activity was 21-fold higher in murine than in rat retinas (the difference increased to 54-fold under diabetic conditions), whereas other antioxidative enzyme activities were 3- to 10-fold lower. With the exception of catalase, the key antioxidant defence enzyme activities were increased, rather than reduced, in diabetic mice. Diabetic rats had decreased free mitochondrial and cytosolic NAD(+)/NADH ratios, consistent with retinal hypoxia, whereas both ratios remained in the normal range in diabetic mice.

CONCLUSIONS/INTERPRETATION

Mice with short-term streptozotocin-induced diabetes lack many biochemical changes that are clearly manifest in the retina of streptozotocin-diabetic rats. This should be considered when selecting animal models for studying early retinal pathology associated with diabetes.

Parameters of oxidative stress in saliva from diabetic and parenteral drug addict patients

BACKGROUND

Oxidative stress constitutes the basis for many diseases and it may account for the severity of systemic and oral disease complications. The aim of this study was to assess whether saliva may be used to detect the body's oxidative stress level.

METHODS

Oxidative stress was determined in saliva from 14 diabetic patients and 10 heroin addicts; two different pathologic conditions related to free radical damage, and 21 healthy control subjects were included in the study. Glutathione peroxidase (GPx) and reductase (GRd) activities, and glutathione (GSH) and glutathione disulfide (GSSG) levels were analyzed in the saliva of all individuals. Other variables including salivary volume and the oral status were also analyzed.

RESULTS

Diabetic patients had GPx and GRd activities of 39.98 +/- 1.61 and 6.19 +/- 0.61 nmol/min/mg prot, respectively. These values were significantly higher (P < 0.001) than those obtained in control saliva (27.51 +/- 0.86 and 3.44 +/- 0.25 nmol/min/mg prot, respectively). Drug addicts showed significantly (P < 0.001) lower salivary GPx and GRd activities than controls. Both group of patients had significantly lower levels of GSH and higher of GSSG than controls (P < 0.001).

CONCLUSIONS

Changes in the antioxidant enzymes and glutathione levels in saliva from two different pathologic situations as those here studied suggest that this biologic fluid may be suitable for determining the prognosis and evolution of these diseases and its oral manifestations.

Thioredoxin interacting protein is increased in sensory neurons in experimental diabetes

Diabetic neuropathy is a major complication of diabetes and has multifactoral aetiology. The exact cause of damage is unknown although high glucose and oxidative stress are known to contribute significantly. In order to identify molecular targets of the disease and possibly new therapeutic targets, we previously examined the effect of diabetes on dorsal root ganglia (DRG) neurons using Affymetrix gene chip arrays. A number of individual genes and groups of genes were found to be dysregulated; the most significant of these was thioredoxin interacting protein (Txnip). This gene was found to have increased expression in DRG from diabetic rats with all durations of diabetes examined, including those that preceded the onset of functional changes such as decreased nerve conduction velocity. Increased Txnip expression therefore represents an early change in diabetic neuropathy that could, at least in part, be responsible for causing the initial functional deficits. This study confirmed the changes in Txnip expression at the mRNA and protein levels and identified the cell types responsible for the change. Furthermore we investigated the mechanism of diabetes-induced Txnip gene induction. Neither the antioxidant dexlipotam (R-lipoic acid) nor the p38 MAP kinase inhibitor SB239063 could prevent increases in Txnip expression despite reducing oxidative stress. However, treatment of rats with insulin prevented diabetes-induced increases in Txnip gene expression. These results indicate another mechanism by which diabetes may cause oxidative damage in peripheral nerve, and may represent a novel target for therapeutic intervention.

Identification of changes in gene expression in dorsal root ganglia in diabetic neuropathy: correlation with functional deficits

This study aimed to correlate the onset of functional deficits in diabetic neuropathy with changes in gene expression in rat dorsal root ganglia (DRG). After 1, 4, or 8 weeks of streptozotocin-induced diabetes, sensory and motor nerve conduction velocities (NCV) were measured as an indicator of neuropathy and changes in gene expression were measured using Affymetrix oligonucleotide microarrays. No significant changes in NCV were found after 1 week of diabetes, but after 4 and 8 weeks, there was a significant reduction in both sensory and motor NCV. Global gene expression changes in diabetic rat DRG were evident from principal component analysis of microarray data after 1, 4, and 8 weeks. Expression changes in individual genes were relatively small in line with a gradual degenerative neuropathy indirectly resulting from diabetes. Sets of differentially expressed genes have been identified and quantitative reverse transcriptase-polymerase chain reaction has been used to confirm the microarray data for several genes. Gene ontology overrepresentation analysis was performed on the microarray data to identify biologic processes altered in diabetic DRG. The genes identified in this study may be responsible for causing the functional deficits and suggest pathways/processes that require further investigation as possible targets for therapeutic intervention.

Diabetic neuropathy and oxidative stress

This review will focus on the impact of hyperglycemia-induced oxidative stress in the development of diabetes-related neural dysfunction. Oxidative stress occurs when the balance between the production of reactive oxygen species (ROS) and the ability of cells or tissues to detoxify the free radicals produced during metabolic activity is tilted in the favor of the former. Although hyperglycemia plays a key role in inducing oxidative stress in the diabetic nerve, the contribution of other factors, such as endoneurial hypoxia, transition metal imbalances, and hyperlipidemia have been also suggested. The possible sources for the overproduction of ROS in diabetes are widespread and include enzymatic pathways, auto-oxidation of glucose, and mitochondrial superoxide production. Increase in oxidative stress has clearly been shown to contribute to the pathology of neural and vascular dysfunction in diabetes. Potential therapies for preventing increased oxidative stress in diabetic nerve dysfunction will be discussed.

Effects of Low-Carbohydrate Diet and Pycnogenol®Treatment on Retinal Antioxidant Enzymes in Normal and Diabetic Rats

Because chronic hyperglycemia of uncontrolled diabetes mellitus may lead to increased reactive oxygen species and decreased enzymatic antioxidant defenses responsible for pathological processes in diabetic retinopathy, this study examined the hypothesis that a low-carbohydrate, high-fat diet, either alone or in combination with Pinus maritima can reduce hyperglycemia, restoring a more balanced, oxidative condition. Normal and streptozotocininduced diabetic rats were fed either a regular or low-carbohydrate diet for 30 or 90 d. In addition, normal and diabetic rats on the chronic (90-d) low-carbohydrate diet were treated with daily intraperitoneal Pinus maritima doses (10 mg/kg) for 14 consecutive days. Retinas were fractionated to assay activities of glutathione peroxidase, glutathione reductase, and gamma-glutamyl transferase. After 30 d, the low-carbohydrate diet reduced glycemic parameters and normalized aspartate aminotransferase activity in diabetic animals, suggesting less organ damage. No differences were observed between males and females in any measured glycemic parameters. Whereas all diabetic control animals developed cataracts bilaterally, no treated diabetic animals developed cataracts. There were no deleterious effects on retinal antioxidant defenses with either a 30-d or chronic low-carbohydrate diet. When diet was combined with Pinus maritima treatment, both retinal glutathione peroxidase and glutathione reductase activities increased, suggesting that a low-carbohydrate diet plus Pinus maritima may be an effective antioxidant and antihyperglycemic therapy, reducing the risk of diabetic retinopathy and cataract formation.

Peripheral nerve dysfunction in experimental diabetes is mediated by cyclooxygenase-2 and oxidative stress

Glucose-mediated oxidative stress and alterations in cyclooxygenase (COX) pathway activity with secondary deficits of endoneurial perfusion have been implicated in the pathogenesis of experimental diabetic neuropathy (EDN). We have previously reported that activation of the COX-2 pathway is an important mediator of neurochemical and neurovascular defects in EDN in a rat model. Considering that chemical COX inhibition may exert other pharmacological effects in addition to inhibition of COX activity, the aim of this study was to explore the role of COX-2 in experimental diabetic neuropathy, using a COX-2 knockout mouse model. Here we provide evidence that COX-2 inactivation had a protective effect against diabetes-induced motor and sensory nerve conduction slowing and impaired nerve antioxidative defense that were clearly manifest in the wild-type (COX-2(+/+)) diabetic mice. These preliminary data support the role of the activation of the COX-2 pathway in mediating sensory and motor nerve conduction velocity deficits in EDN. These findings also suggest that the COX-2 pathway seems to be an important modulator of oxidative stress in EDN.

Cell culture modeling to test therapies against hyperglycemia-mediated oxidative stress and injury

The concept that oxidative stress is a key mediator of nerve injury in diabetes has led us to design therapies that target oxidative stress mechanisms. Using an in vitro model of glucose-treated dorsal root ganglion (DRG) neurons in culture, we can examine both free radical generation, using fluorimetric probes for reactive oxygen species, and cell death via the TUNEL assay. The cell culture system is scaled down to a 96-well plate format, and so is well suited to high-throughput screening. In the present study, we test the ability of three drugs, nicotinamide, allopurinol, and alpha-lipoic acid, alone and in combination to prevent DRG neuron oxidative stress and cell death. This combination of drugs is currently in clinical trial in type 1 diabetic patients. We demonstrate independent effects on oxidative stress and neuronal survival for the three drugs, and neuronal protection using the three drugs in combination. The data strengthen the rationale for the current clinical trial. In addition, we describe an effective tool for rapid preclinical testing of novel therapies against diabetic neuropathy.

Mechanisms of antioxidant and pro-oxidant effects of alpha-lipoic acid in the diabetic and nondiabetic kidney

BACKGROUND

alpha-Lipoic acid is a potent antioxidant that improves renal function in diabetes by lowering glycemia, however, the mechanisms by which alpha-lipoic acid exerts its antioxidant effects are not completely understood.

METHODS

Metabolic parameters, renal function, and morphology, nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity and subunit expression were analyzed in nondiabetic and streptozotocin-induced diabetic rats fed normal rat chow (control) with or without alpha-lipoic acid (30 mg/kg body weight) for 12 weeks.

RESULTS

Blood glucose was increased with diabetes (nondiabetic + control 89 +/- 3 mg/dL and diabetic + control 336 +/- 28 mg/dL) and was similar with alpha-lipoic acid treatment (diabetic +alpha-lipoic acid 351 +/- 14 mg/dL). In contrast, alpha-lipoic acid attenuated albuminuria (nondiabetic + control 8.9 +/- 1.3 mg/day; diabetic + control 28.1 +/- 4.6 mg/day; and diabetic +alpha-lipoic acid 17.8 +/- 1.2 mg/day) associated with diabetes. Similarly, alpha-lipoic acid attenuated glomerulosclerosis (nondiabetic + control 0.22 +/- 0.01; diabetic + control 0.55 +/- 0.04; diabetic +alpha-lipoic acid 0.36 +/- 0.03), tubulointerstitial fibrosis (nondiabetic + control 0.42 +/- 0.18; diabetic + control 1.52 +/- 0.05; diabetic +alpha-lipoic acid 1.10 +/- 0.05), superoxide anion (O(.-) (2)) generation (nondiabetic +control 15.8 +/- 1.7; diabetic +control 87.1 +/- 3.5; diabetic +alpha-lipoic acid 25.5 +/- 3.3 RLU/mg protein), and urine 8-isoprostane (8-iso) excretion (nondiabetic + control 7.4 +/- 1.4; diabetic + control 26.0 +/- 4.5; diabetic +alpha-lipoic acid 19.6 +/- 5.6 ng/day) associated with diabetes. alpha-Lipoic acid also reduced kidney expression of NADPH oxidase subunits p22phox and p47phox. Surprisingly, alpha-lipoic acid appears to cause pro-oxidant effects in nondiabetic animals, resulting in increased albuminuria (nondiabetic +alpha-lipoic acid 14.2 +/- 1.2 mg/day), increase in plasma creatinine levels (nondiabetic + control 59 +/- 6; diabetic + control 68 +/- 6; nondiabetic +alpha-lipoic acid 86 +/- 9; diabetic +alpha-lipoic acid 69 +/- 7 mumol/L), exacerbated glomerulosclerosis and tubulointerstitial fibrosis, increased O(.-) (2) generation, up-regulated p22phox and p47phox expression and increased 8-iso excretion.

CONCLUSION

We conclude that alpha-lipoic acid improves albuminuria and pathology in diabetes by reducing oxidative stress, while in healthy animals, alpha-lipoic acid may act as a pro-oxidant, contributing to renal dysfunction.

M40403 prevents myocardial injury induced by acute hyperglycaemia in perfused rat heart

M40403 is a low-molecular-weight, synthetic manganese-containing biscyclohexylpyridine superoxide dismutase mimetic (SODm) that removes superoxide anions (O(2)(-)) without interfering with other reactive species known to be involved in cardiovascular alterations (e.g. nitric oxide [NO] and peroxynitrite [ONOO(-)]). As such, M40403 represents an important pharmacological tool to dissect the roles of O(2)(-) in functional and biochemical cardiovascular alterations induced by perfusion of high glucose concentrations into the heart. Perfusion of a high glucose concentration of glucose into the heart elicited important cardiovascular alterations characterized by QT interval prolongation, increase in coronary perfusion pressure (CPP), lipid peroxidation, decrease in MnSOD activity and DNA damage. All parameters of cardiovascular alteration were attenuated by M40403 (1-10 mg/l). Furthermore, perfusion of a high of glucose concentration induced a significant formation of nitrotyrosine as well as an activation of poly(adenosine diphosphate [ADP]-ribose) synthetase (PARS), as determined by immunohistochemical analysis of heart tissue. The extent of staining for nitrotyrosine and PARS was reduced by M40403. These results clearly indicate that O(2)(-) plays a critical role in the development of the functional and biochemical cardiovascular alterations induced by perfusion of a high concentration of glucose into the heart. Therefore, synthetic enzymes of SOD, such as M40403, offer a novel therapeutic approach for the management of various cardiovascular diseases where these radicals have been postulated to play a role.

Effects of antioxidant treatment on normal and diabetic rat retinal enzyme activities

Diabetes mellitus is characterized by hyperglycemia and, in chronic disease, by microvascular pathologies, especially in the kidney, peripheral nerve, and eye. Although hyperglycemia can be controlled with insulin and/or antihyperglycemic medications, diabetic retinopathy continues to be the leading cause of blindness in the United States. Because increased oxidative stress may be a cause of retinopathy, this study examined the hypothesis that administration of exogenous antioxidants can restore a more balanced oxidative condition. Normal and 30-day streptozotocin-induced diabetic Sprague-Dawley rats received daily intraperitoneal doses (10 mg/kg) of beta-carotene, alpha-lipoic, and Pycnogenol individually or in combinations for 14 days, after which retinae were dissected and fractionated for the assay of activities of glutathione reductase, glutathione peroxidase, gamma-glutamyl transferase, and superoxide dismutase. In normal rats, treatment with antioxidant combinations led to a decrease in gamma-glutamyl transferase activity; beta-carotene plus pycnogenol treatment decreased the activity of both glutathione-related enzymes. Decreased retinal gamma-glutamyl transferase activity of diabetic rats was normalized by the administration of pycnogenol alone or in combination with beta-carotene. In diabetic rats, retinal glutathione reductase activity increased after treatment with beta-carotene alone or with pycnogenol. Treatment with pycnogenol and alpha-lipoic acid alone or in combination decreased the activity of glutathione peroxidase, while this activity was increased after treatment with a combination of all antioxidants. Elevated activity of superoxide dismutase in diabetic retina was normalized by treatment with alpha-lipoic acid and with pycnogenol and beta-carotene in combination, but not with all three together. Antioxidants can access the retina and, once there, can alter antioxidant enzyme activities. In both normal and diabetic rats, combinations of antioxidants have different effects on retinal antioxidant enzyme activities than do individual antioxidants.

Oxidative stress and the use of antioxidants in diabetes: linking basic science to clinical practice

Cardiovascular complications, characterized by endothelial dysfunction and accelerated atherosclerosis, are the leading cause of morbidity and mortality associated with diabetes. There is growing evidence that excess generation of highly reactive free radicals, largely due to hyperglycemia, causes oxidative stress, which further exacerbates the development and progression of diabetes and its complications. Overproduction and/or insufficient removal of these free radicals result in vascular dysfunction, damage to cellular proteins, membrane lipids and nucleic acids. Despite overwhelming evidence on the damaging consequences of oxidative stress and its role in experimental diabetes, large scale clinical trials with classic antioxidants failed to demonstrate any benefit for diabetic patients. As our understanding of the mechanisms of free radical generation evolves, it is becoming clear that rather than merely scavenging reactive radicals, a more comprehensive approach aimed at preventing the generation of these reactive species as well as scavenging may prove more beneficial. Therefore, new strategies with classic as well as new antioxidants should be implemented in the treatment of diabetes.

Subconjunctivally administered celecoxib-PLGA microparticles sustain retinal drug levels and alleviate diabetes-induced oxidative stress in a rat model

We have previously reported that repeated oral doses of celecoxib, a selective cyclooxygenase-2 (COX-2) inhibitor, reduced diabetes-induced retinal vascular endothelial growth factor (VEGF) expression [Ayalasomayajula, S.P., Kompella, U.B., 2003. Celecoxib, a selective cyclooxygenase-2 inhibitor, inhibits retinal vascular endothelial growth factor expression and vascular leakage in a streptozotocin-induced diabetic rat model. Eur J Pharmacol 458, 283-289] and that retinal celecoxib delivery can be improved by several-fold following subconjunctival administration [Ayalasomayajula, S.P., Kompella, U.B., 2004. Retinal delivery of celecoxib is several-fold higher following subconjunctival administration compared to systemic administration. Pharm Res 21, 1797-1804]. The objective of the current study was to determine whether polymeric microparticles of celecoxib sustain retinal drug levels following subconjunctival administration and alleviate diabetes-induced oxidative stress in a streptozotocin-induced diabetic rat model. Biodegradable poly (lactide-co-glycolide) (PLGA; 85:15) microparticles of celecoxib were prepared using solvent evaporation method and characterized for their size, morphology, encapsulation efficiencies, and in vitro release. The celecoxib-PLGA microparticles or solution containing 75 microg of celecoxib was administered subconjunctivally to one eye (ipsilateral) of Sprague Dawley rats and drug levels in the retina, vitreous, lens, and cornea of ipsilateral and contralateral eyes were determined on 1, 7, and 14 days using high-performance liquid chromatography (HPLC). The effect of subconjunctivally administered celecoxib-PLGA microparticles on oxidative stress in day 14 diabetic rat retinas was determined by measuring the retinal glutathione (reduced (GSH) and oxidized (GSSG)), thiobarbituric acid reactive substances, and 4-hydroxynonenal levels using spectrofluorometric and colorimetric methods. Solvent evaporation method produced spherical celecoxib-PLGA microparticles with mean diameters of 3.9+/-0.6 microm and 68.5% loading efficiency. These microparticles sustained celecoxib release during the 49-day in vitro release study. Subconjunctivally administered celecoxib-PLGA microparticles sustained retinal and other ocular tissue drug levels during the 14-day study in rats. No detectable celecoxib levels were observed in the contralateral eye. The celecoxib-PLGA microparticles significantly inhibited the diabetes-induced increases in thiobarbituric acid reactive substance (P=0.012) and 4-hydroxynonenal levels (P=0.029). The particles also inhibited the GSH depletion and the increase in GSSH/GSH ratio associated with diabetes but the effects were not statistically significant (P=0.12). Thus, following subconjunctival administration, celecoxib-PLGA microparticles sustained retinal celecoxib delivery and inhibited diabetes-induced retinal oxidative damage, indicating their potential usefulness in treating diabetes-induced retinal abnormalities.

The protective effect of alpha-lipoic acid against oxidative damage in rabbit conjunctiva and cornea exposed to ultraviolet radiation

PURPOSE

The purpose of this study was to determine the protective effect of alpha-lipoic acid against oxidative damage in rabbit conjunctiva and cornea exposed to ultraviolet radiation.

METHODS

20 rabbits weighing 2,500- 3,000 g were used, and we divided them into 4 groups with 5 randomly selected rabbits. The rabbits were exposed to 2 J/cm(2)/h of ultraviolet A radiation (UVA) in the range of 320-405 nm for 12 h per day within 90 days. The control group did not undergo any procedure, the UVA group was only exposed to UVA radiation. The PUVA group was treated with 8-methoxypsoralen and UVA. The alpha-lipoic acid group was administered 8-methoxypsoralen + UVA + alpha-lipoic acid. At the end of 90 days, the rabbits were killed by decapitation, and the eyes were enucleated. Both eyes of each rabbit were used for biochemical evaluation. Conjunctival and corneal free malondialdehyde (MDA), glutathione peroxidase (GSH-PX) and superoxide dismutase (SOD) levels were compared among the groups.

RESULTS

Conjunctival free MDA levels were lower in the alpha-lipoic acid group compared with the UVA and PUVA groups (p < 0.05, p < 0.001, respectively). Both conjunctival SOD levels (p < 0.05, p < 0.01, respectively) and conjunctival GSH-PX levels (p < 0.01, p < 0.001, respectively) were higher in the alpha-lipoic acid group compared with other groups. Corneal free MDA levels were lower in the alpha-lipoic acid group compared with the UVA and PUVA groups (p < 0.01, p < 0.001, respectively). Both corneal SOD levels (p < 0.01, p < 0.01, respectively) and corneal GSH-PX levels (p < 0.01, p < 0.01, respectively) were higher in the alpha-lipoic acid group compared with the other groups.

CONCLUSION

alpha-Lipoic acid which is considered as potent antioxidant protects the eye from the damaging effect of ultraviolet exposure.

Diabetes-induced changes in glucose synthesis, intracellular glutathione status and hydroxyl free radical generation in rabbit kidney-cortex tubules

Diabetes-induced changes in glucose formation, intracellular and mitochondrial glutathione redox states as well as hydroxyl free radicals (HFR) generation have been investigated in rabbit kidney-cortex tubules. In contrast to renal tubules of control animals, diabetes-evoked increase in glucose formation in the presence of either aspartate+glycerol+octanoate or malate as gluconeogenic precursors (for about 50%) was accompanied by a diminished intracellular glutathione reduced form (GSH)/glutathione oxidised one (GSSG) ratio by about 30-40%, while the mitochondrial GSH/GSSG ratio was not altered. However, a relationship between the rate of gluconeogenesis and the intracellular glutathione redox state was maintained in renal tubules of both control and diabetic rabbits, as concluded from measurements in the presence of various gluconeogenic precursors. Moreover, diabetes resulted in both elevation of the glutathione reductase activity in rabbit kidney-cortex and acceleration of renal HFR generation (by about 2-fold). On the addition of melatonin, the hormone exhibiting antioxidative properties, the control values of HFR production were restored, suggesting that this compound might be beneficial during diabetes therapy. In view of the data, it seems likely that diabetes-induced increase in HFR formation in renal tubules might be responsible for a diminished intracellular glutathione redox state despite elevated glutathione reductase activity and accelerated rate of gluconeogenesis, providing glucose-6-phosphate for NADPH generation via pentose phosphate pathway.

Effect of long-term administration of alpha-lipoic acid on retinal capillary cell death and the development of retinopathy in diabetic rats

Oxidative stress is increased in the retina in diabetes, and it is considered to play an important role in the development of retinopathy. alpha-Lipoic acid, a thiol antioxidant, has been shown to have beneficial effects on polyneuropathy and on the parameters of oxidative stress in various tissues, including nerve, kidney, and retina. The purpose of this study was to examine the effect of alpha-lipoic acid on retinal capillary cell apoptosis and the development of pathology in diabetes. Retina was used from streptozotocin-induced diabetic rats receiving diets supplemented with or without alpha-lipoic acid (400 mg/kg) for 11 months of diabetes. Capillary cell apoptosis (by terminal transferase-mediated dUTP nick-end labeling) and formation of acellular capillaries were investigated in the trypsin-digested retinal microvessels. The effect of alpha-lipoic acid administration on retinal 8-hydroxy-2'deoxyguanosine (8-OHdG) and nitrotyrosine levels was determined by enzyme-linked immunosorbent assay. alpha-Lipoic acid administration for the entire duration of diabetes inhibited capillary cell apoptosis and the number of acellular capillaries in the retina, despite similar severity of hyperglycemia in the two diabetic groups (with and without alpha-lipoic acid). Retinal 8-OHdG and nitrotyrosine levels were increased by over twofold and 70%, respectively, in diabetes, and alpha-lipoic acid administration inhibited these increases. Our results demonstrate that the long-term administration of alpha-lipoic acid has beneficial effects on the development of diabetic retinopathy via inhibition of accumulation of oxidatively modified DNA and nitrotyrosine in the retina. alpha-Lipoic acid supplementation represents an achievable adjunct therapy to help prevent vision loss in diabetic patients.

Oxidative stress in the pathogenesis of diabetic neuropathy

Oxidative stress results from a cell or tissue failing to detoxify the free radicals that are produced during metabolic activity. Diabetes is characterized by chronic hyperglycemia that produces dysregulation of cellular metabolism. This review explores the concept that diabetes overloads glucose metabolic pathways, resulting in excess free radical production and oxidative stress. Evidence is presented to support the idea that both chronic and acute hyperglycemia cause oxidative stress in the peripheral nervous system that can promote the development of diabetic neuropathy. Proteins that are damaged by oxidative stress have decreased biological activity leading to loss of energy metabolism, cell signaling, transport, and, ultimately, to cell death. Examination of the data from animal and cell culture models of diabetes, as well as clinical trials of antioxidants, strongly implicates hyperglycemia-induced oxidative stress in diabetic neuropathy. We conclude that striving for superior antioxidative therapies remains essential for the prevention of neuropathy in diabetic patients.

Impairment of conjunctival glutathione secretion and ion transport by oxidative stress in an adenovirus type 5 ocular infection model of pigmented rabbits

Conjunctival epithelial cells of pigmented rabbits secrete reduced glutathione (GSH) into the apical (mucosal) fluid. The aim of the current study was to determine the effect of oxidative stress resulting from viral infection and that of GSH supplementation on redox status, GSH, and ion transport in freshly excised conjunctival tissues and epithelial cell layers in primary culture (RCEC) of adenovirus type 5 (Ad5)-infected rabbits. Lipid peroxidation (LPO) products, nitric oxide (NO), and expression of nitric oxide synthase (NOS2) were quantitated as a function of time after viral inoculation. Unidirectional fluxes of [3H]GSH and changes in short-circuit current (Isc) from mucosal supplementation of Ad5-inoculated conjunctival tissues with GSH and glutathione monoethyl ester (GSH-MEE) were also measured. Ad5 inoculation significantly decreased conjunctival GSH level by 19, 45, 48, and 50% at 8, 24, 48, and 72 h postinfection, respectively. LPO product and NO levels increased significantly (2- and 100-fold, respectively) above that of uninfected controls on Day 3 post-Ad5 inoculation, and co-treatment with GSH-MEE and tocopherol succinate abolished this effect. NO levels showed a progressive increase post-Ad5 inoculation, reaching 0.22 +/- 0.06, 8.12 +/- 0.91, and 2.05 +/- 0.65 microM on Days 1, 3, and 5, respectively, and the highest level was observed on the day of maximal viral replication (Day 3). A very significant induction of the expression of NOS2 on Days 1, 3, and 5 post-Ad5 inoculation was observed. Uninfected control conjunctival tissues displayed a net serosal-to-mucosal GSH flux (Jsm), where the mucosal-to-serosal flux (Jms) was approximately 14 pmol h(-1) cm(-2) and the Jsm was approximately 22 pmol h(-1) cm(-2). In Ad5-inoculated rabbits similar GSH flux was observed in both the sm and ms directions, and the net GSH flux was negligible. Isc and potential difference (PD) across conjunctival tissues of Ad5-inoculated rabbits decreased by > or = 50% compared with control, while the transepithelial electrical resistance (TEER) remained unchanged. Mucosal, but not serosal, superfusion of GSH or GSH-MEE in Ad5-inoculated conjunctival tissues increased the Isc by up to 40% in approximately 100 min. Our results show that net secretion of GSH across rabbit conjunctiva is totally blocked after Ad5 inoculation and active ion transport rate decreased by approximately 50%. Decreased net GSH secretion into mucosal fluid after Ad5 infection may have resulted from a decreased intracellular GSH pool due to oxyradical-induced changes in redox status and lower active ion transport. Mucosal treatment of Ad5-infected conjunctival tissues with pharmacological levels of GSH appears to transstimulate mucosal GSH secretion and restore active ion transport activity, suggesting a potentially useful therapeutic regimen for ocular infections.

Reduction of retinal albumin leakage by the antioxidant calcium dobesilate in streptozotocin-diabetic rats

Calcium dobesilate stabilizes blood-retinal barrier in patients with diabetic retinopathy and possesses antioxidant properties in the retinas of rats with streptozotocin-induced diabetes, exposed ex vivo to ischemia-reperfusion. Here we investigated the action of calcium dobesilate on retinal albumin leakage in streptozotocin-diabetic rats, together with relevant in vivo retinal antioxidant and permeability markers, i.e., carboxymethyl-lysine-advanced glycation end product (CML-AGE) formation and vascular endothelial cell growth factor (VEGF) overexpression. Twenty days after streptozotocin administration, diabetic rats were treated for 10 days with calcium dobesilate (100 mg/kg/day per os) or vehicle. Retinal albumin leakage, CML-AGE formation, and VEGF overexpression were evaluated by immunohistochemistry of frozen eye sections. Diabetic rats exhibited dramatic increases in: (i) retinal albumin leakage (31% of positive vessels vs. 0.2% in nondiabetic rats, P<0.008), (ii) CML-AGE retinal occurrence (40+/-3% vs. undetectable positive vessels), and (iii) retinal VEGF protein expression (14.6+/-1.1 vs. 3.5+/-0.5 VEGF-positive spots/field, P<10(-4)). Calcium dobesilate significantly reduced: (i) retinal albumin leakage (by 70%, P<0.008), (ii) retinal CML-AGEs contents (by 62%, P<0.008), and (iii) retinal VEGF expression (by 69.4%, P<0.008). In conclusion, calcium dobesilate orally given to diabetic rats markedly reduced retinal hyperpermeability, CML-AGE contents, and VEGF overexpression. These results strongly suggest that calcium dobesilate stabilizes blood-retinal barrier in diabetic retinopathy via an in situ antioxidant action. Further studies in patients are required to confirm such view.

The retina: oxidative stress and diabetes

A prominent and early feature of the retinopathy of diabetes mellitus is a diffuse increase in vascular permeability. As the disease develops, the development of frank macular oedema may result in vision loss. That reactive oxygen species production is likely to be elevated in the retina, and that certain regions of the retina are enriched in substrates for lipid peroxidation, may create an environment susceptible to oxidative damage. This may be more so in the diabetic retina, where hyperglycaemia may lead to elevated oxidant production by a number of mechanisms, including the production of oxidants by vascular endothelium and leukocytes. There is substantial evidence from animal and clinical studies for both impaired antioxidant defences and increased oxidative damage in the retinae of diabetic subjects that have been, in the case of animal studies, reversible with antioxidant supplementation. Whether oxidative damage has a causative role in the pathology of diabetic retinopathy, and thus whether antioxidants can prevent or correct any retinal damage, has not been established, nor has the specific nature of any damaging species been characterised.

Diabetes, oxidative stress, and antioxidants: a review

Increasing evidence in both experimental and clinical studies suggests that oxidative stress plays a major role in the pathogenesis of both types of diabetes mellitus. Free radicals are formed disproportionately in diabetes by glucose oxidation, nonenzymatic glycation of proteins, and the subsequent oxidative degradation of glycated proteins. Abnormally high levels of free radicals and the simultaneous decline of antioxidant defense mechanisms can lead to damage of cellular organelles and enzymes, increased lipid peroxidation, and development of insulin resistance. These consequences of oxidative stress can promote the development of complications of diabetes mellitus. Changes in oxidative stress biomarkers, including superoxide dismutase, catalase, glutathione reductase, glutathione peroxidase, glutathione levels, vitamins, lipid peroxidation, nitrite concentration, nonenzymatic glycosylated proteins, and hyperglycemia in diabetes, and their consequences, are discussed in this review. In vivo studies of the effects of various conventional and alternative drugs on these biomarkers are surveyed. There is a need to continue to explore the relationship between free radicals, diabetes, and its complications, and to elucidate the mechanisms by which increased oxidative stress accelerates the development of diabetic complications, in an effort to expand treatment options.

Effects of alpha-lipoic acid on biomarkers of oxidative stress in streptozotocin-induced diabetic rats

Increased oxidative stress and impaired antioxidant defense mechanisms are important factors in the pathogenesis and progression of diabetes mellitus and other oxidant-related diseases. This study was designed to determine whether alpha-lipoic acid, which has been shown to have substantial antioxidant properties, when administered (10 mg/kg ip) once daily for 14 days to normal and diabetic female Sprague-Dawley rats would prevent diabetes-induced changes in biomarkers of oxidative stress in liver, kidney and heart. Serum glucose concentrations, aspartate aminotransferase activity, and glycated hemoglobin levels, which were increased in diabetes, were not significantly altered by alpha-lipoic acid treatment. Normal rats treated with a high dose of alpha-lipoic acid (50 mg/kg) survived but diabetic rats on similar treatment died during the course of the experiment. The activity of glutathione peroxidase was increased in livers of normal rats treated with alpha-lipoic acid, but decreased in diabetic rats after alpha-lipoic acid treatment. Hepatic catalase activity was decreased in both normal and diabetic rats after alpha-lipoic acid treatment. Concentrations of reduced glutathione and glutathione disulfide in liver were increased after alpha-lipoic acid treatment of normal rats, but were not altered in diabetics. In kidney, glutathione peroxidase activity was elevated in diabetic rats, and in both normal and diabetic animals after alpha-lipoic acid treatment. Superoxide dismutase activity in heart was decreased in diabetic rats but normalized after treatment with alpha-lipoic acid; other cardiac enzyme activities were not influenced by either diabetes or antioxidant treatment. These results suggest that after 14 days of treatment with an appropriate pharmacological dose, alpha-lipoic acid may reduce oxidative stress in STZ-induced diabetic rats, perhaps by modulating the thiol status of the cells.

NAD(P)H oxidase activation by angiotensin II is dependent on p42/44 ERK-MAPK pathway activation in rat's vascular smooth muscle cells

OBJECTIVE

To determine whether the activation of nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase and the increase of superoxide anion production by angiotensin II is dependent upon the activation of the ERK-MAPK pathway.

METHODS

Hypertension was induced in Sprague-Dawley rats by infusing angiotensin II (200 ng/kg per min) through osmotic pumps for 12 days. The effects of treatments including an angiotensin II type 1 (AT(1)) blocker losartan (20 mg/kg per day), a tyrosine kinase inhibitor genistein (1.6 microg/kg per min), a specific ERK-MAPK inhibitor, PD98059 (2 mg/kg per day) and an antioxidant alpha-lipoic acid (500 mg/kg of chow) were evaluated during angiotensin infusion. The aortic superoxide anion production, the ERK-MAPK pathway activity and the systolic blood pressure (SBP), were measured following those treatments.

RESULTS

Increases in the concentration of the superoxide anion (1622 to 3719 cpm), in NAD(P)H activity (107%) and in the ERK-MAPK activity (3.6-fold) in the aorta as well as a rise in the arterial pressure (136 to 184 mmHg) were observed 12 days after initiating the treatments (P < 0.05). When the angiotensin-treated rats were treated either with losartan, genistein, PD98059 or alpha-lipoic acid, increases in superoxide anion production, in NAD(P)H oxidase activity, in ERK-MAPK activity and in blood pressure were attenuated. A correlation between the superoxide anion production and the ERK-MAPK activity was also observed.

CONCLUSIONS

The present study suggests that the NAD(P)H-dependent increase of the superoxide anion production in the vascular tissue following a treatment with angiotensin II is dependent on the activation of the ERK-MAPK pathway.

Aldose reductase inhibitor fidarestat prevents retinal oxidative stress and vascular endothelial growth factor overexpression in streptozotocin-diabetic rats

The study addressed the role for aldose reductase (AR) in 1) retinal oxidative stress and vascular endothelial growth factor (VEGF) overexpression in early diabetes, and 2) high glucose-induced oxidative stress in retinal endothelial cells. In vivo experiments were performed on control rats and diabetic rats treated with or without low or high dose of the AR inhibitor (ARI) fidarestat (2 or 16 mg. kg(-1). day(-1)). In vitro studies were performed on bovine retinal endothelial cells (BREC) cultured in either 5 or 30 mmol/l glucose with or without 1 micro mol/l fidarestat. Intracellular reactive oxygen species were assessed using the 5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate (H(2)DCFDA) probe and flow cytometry. Both low and high doses of fidarestat (i.e., the doses that partially and completely inhibited sorbitol pathway hyperactivity) arrested diabetes-induced retinal lipid peroxidation. This was achieved due to upregulation of the key antioxidative defense enzyme activities rather than changes in reduced glutathione, oxidized glutathione, ascorbate and dehydroascorbate concentrations, and the glutathione and ascorbate redox states. Diabetes-associated 2.1-fold VEGF protein overexpression (enzyme-linked immunosorbent assay; ELISA) was dose-dependently prevented by fidarestat, whereas total VEGF mRNA and VEGF-164 mRNA (RT-PCR) abundance were not affected by either diabetes or the ARI. In BREC, fidarestat corrected hyperglycemia-induced increase in H(2)DCFDA fluorescence but not oxidative stress caused by three different pro-oxidants in normoglycemic conditions. In conclusion, increased AR activity contributes to retinal oxidative stress and VEGF protein overexpression in early diabetes. The findings justify the rationale for evaluation of fidarestat on diabetic retinopathy.

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.

Alpha-lipoic acid protects the retina against ischemia-reperfusion

The aim of this study was to examine whether the antioxidant alpha-lipoic acid protects retinal neurons from ischemia-reperfusion injury. Rats were injected intraperitoneally with either vehicle or alpha-lipoic acid (100 mg/kg) once daily for 11 days. On the third day, ischemia was delivered to the rat retina by raising the intraocular pressure above systolic blood pressure for 45 min. The electroretinogram was measured prior to ischemia and 5 days after reperfusion. Rats were killed 5 or 8 days after reperfusion and the retinas were processed for immunohistochemistry and for determination of mRNA levels by RT-PCR. Ischemia-reperfusion caused a significant reduction of the a- and b-wave amplitudes of the electroretinogram, a decrease in nitric oxide synthase and Thy-1 immunoreactivities, a decrease of retinal ganglion cell-specific mRNAs and an increase in bFGF and CNTF mRNA levels. All of these changes were clearly counteracted by alpha-lipoic acid. Moreover, in mixed rat retinal cultures, alpha-lipoic acid partially counteracted the loss of GABA-immunoreactive neurons induced by anoxia. The results of the study demonstrate that alpha-lipoic acid provides protection to the retina as a whole, and to ganglion cells in particular, from ischemia-reperfusion injuries. alpha-Lipoic acid also displayed negligible affinity for voltage-dependent sodium and calcium channels.

The AGE inhibitor pyridoxamine inhibits development of retinopathy in experimental diabetes

We examined the ability of pyridoxamine (PM), an inhibitor of formation of advanced glycation end products (AGEs) and lipoxidation end products (ALEs), to protect against diabetes-induced retinal vascular lesions. The effects of PM were compared with the antioxidants vitamin E (VE) and R-alpha-lipoic acid (LA) in streptozotocin-induced diabetic rats. Animals were given either PM (1 g/l drinking water), VE (2,000 IU/kg diet), or LA (0.05%/kg diet). After 29 weeks of diabetes, retinas were examined for pathogenic changes, alterations in extracellular matrix (ECM) gene expression, and accumulation of the immunoreactive AGE/ALE N( epsilon )-(carboxymethyl)lysine (CML). Acellular capillaries were increased more than threefold, accompanied by significant upregulation of laminin immunoreactivity in the retinal microvasculature. Diabetes also increased mRNA expression for fibronectin (2-fold), collagen IV (1.6-fold), and laminin beta chain (2.6-fold) in untreated diabetic rats compared with nondiabetic rats. PM treatment protected against capillary drop-out and limited laminin protein upregulation and ECM mRNA expression and the increase in CML in the retinal vasculature. VE and LA failed to protect against retinal capillary closure and had inconsistent effects on diabetes-related upregulation of ECM mRNAs. These results indicate that the AGE/ALE inhibitor PM protected against a range of pathological changes in the diabetic retina and may be useful for treating diabetic retinopathy.

Induction of vascular endothelial growth factor by 4-hydroxynonenal and its prevention by glutathione precursors in retinal pigment epithelial cells

Although 4-hydroxynonenal, a highly reactive lipid peroxidation product, is implicated in several age-related disorders such as Alzheimer's and Parkinson's diseases, its role in age-related macular degeneration is not known. The purpose of this study was to determine whether 4-hydroxynonenal increases vascular endothelial growth factor (VEGF) expression in human retinal pigment epithelial cells (ARPE-19), a source of VEGF in choroidal neovascularization observed in age-related macular degeneration. In addition, it was the purpose of this study to assess whether glutathione (GSH) and GSH precursors can inhibit the effects of 4-hydroxynonenal. At 1 micro M, 4-hydroxynonenal did not alter cell viability, but elevated VEGF secretion and mRNA expression by 35% (p<0.05) and 1.9-fold (p<0.05), respectively. However, at concentrations 5 microM and above, 4-hydroxynonenal reduced VEGF secretion as well as cell viability. At 1 and 10 microM, 4-hydroxynonenal did not induce apoptosis in ARPE-19 cells. 4-Hydroxynonenal (1 microM) reduced intracellular GSH by 25% (p<0.05) and increased oxidative stress by 50% (p<0.05). GSH precursor pretreatment for 1 h, which increased intracellular GSH levels by 50% (p<0.05), as well as GSH co-treatment, inhibited the VEGF-inductive and cytotoxic effects of 4-hydroxynonenal. Thus, 4-hydroxynonenal (1 microM) induces VEGF expression and secretion in ARPE-19 cells. This effect is likely due to GSH depletion and an associated increase in intracellular oxidative stress, resulting in increased VEGF mRNA levels. 4-Hydroxynonenal-mediated VEGF secretion as well as cytotoxicity can be reversed with GSH precursor pretreatment or GSH co-treatment.

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.

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.

Cigarette smoke and its formaldehyde component inhibit bradykinin-induced calcium increase in pig aortic endothelial cells

Bradykinin-induced increase in the intracellular concentration of free calcium evokes an activation of the endothelial nitric oxide synthase (eNOS) enzyme, producing nitric oxide (NO). Cigarette smoke inhibits the eNOS-NO-cGMP signaling pathway. The pathomechanism of this deleterious effect of smoke on NO production is unknown. The aim of this study was to investigate the effect of gas phase smoke trapped in a buffer (smoke buffer, SB) on the bradykinin-induced calcium increase in cultured endothelial cells. FURA-2-AM was used to detect bradykinin-induced calcium increase. A sensitive, fluorescent method using O-phthaldialdehyde was used for the determination of intracellular reduced glutathione (GSH) and protein-thiol levels. SB caused a time- and concentration-dependent inhibition of bradykinin-induced calcium increase. Formaldehyde, a component of SB, inhibited bradykinin-induced calcium increase in concentrations characteristic for SB. SB decreased both the intracellular GSH (0.22 +/- 0.06 vs. 2.23 +/- 0.32 mumol/g protein, SB vs. control, p < .001) and protein-thiol levels (4.98 +/- 0.54 vs. 7.31 +/- 0.97 microEqu GSH/g protein, SB vs. control, p < .05) in the endothelial cells. Intracellular GSH and protein-thiol levels were not changed by 80 microM formaldehyde. GSH (4 mM) prevented the effect of SB (p < .001) and formaldehyde (p < .05) on the bradykinin-induced calcium increase. Our data support the premise that SB inhibits bradykinin-induced calcium increase. This inhibition is partially due to protein-thiol oxidation but may also be caused by the formaldehyde content of SB, which inhibits calcium increase in a protein-thiol-independent manner.

Alpha-lipoic acid attenuates hyperglycemia and prevents glomerular mesangial matrix expansion in diabetes

Previous studies demonstrated that 2 mo of dietary supplementation with alpha-lipoic acid (LA) prevented early glomerular injury in non-insulin-treated streptozotocin diabetic rats (D). The present study examined the effects of chronic LA supplementation (30 mg/kg body wt per d) on nephropathy in D after 7 mo of diabetes. Compared with control rats, D developed increased urinary excretion of albumin and transforming growth factor beta, renal insufficiency, glomerular mesangial matrix expansion, and glomerulosclerosis in association with depletion of glutathione and accumulation of malondialdehyde in renal cortex. LA prevented or ameliorated all of these changes in D. Because chronic LA supplementation also attenuated hyperglycemia in D after 3 mo, its effects on renal injury were compared with treatment of rats with sufficient insulin to maintain a level of glycemic control for the entire 7-mo period (D-INS) equivalent to that observed with LA during the final 4 mo. Despite superior longitudinal glycemic control in D-INS, urinary excretion of albumin and transforming growth factor beta, glomerular mesangial matrix expansion, the extent of glomerulosclerosis, and renal cortical malondialdehyde content were all significantly greater, whereas cortical glutathione content was lower than corresponding values in D given LA. Thus, the renoprotective effects of LA in D were not attributable to improved glycemic control alone but also likely reflected its antioxidant activity. The combined antioxidant and hypoglycemic actions of LA both may contribute to its utility in preventing renal injury and other complications of diabetes.

Evidence that interleukin-1beta and reactive oxygen species production play a pivotal role in stress-induced impairment of LTP in the rat dentate gyrus

Long-term potentiation (LTP) in both area CA1 and the dentate gyrus is attenuated by stress and the evidence is consistent with the view that this is a consequence of increased activation of glucocorticoid receptors, in the hippocampus, following the stress-induced increase in circulating corticosterone. It has been shown that expression of the pro-inflammatory cytokine, interleukin-1beta (IL-1beta), is increased in hippocampus in response to stress; this finding together with the observation that IL-1beta exerts an inhibitory effect on LTP, suggests that IL-1beta may play a key role in mediating this inhibitory effect of stress on LTP. In this study, we explore this possibility and report that stress is also associated with increased reactive oxygen species production. The evidence presented supports the view that this is secondary to the stress-induced increase in IL-1beta concentration, as IL-1beta increased activity of superoxide dismutase and increased reactive oxygen species accumulation in hippocampus in vitro. We report that the inhibitory effect of stress on LTP is mimicked by H2O2, which increases reactive oxygen species accumulation, and by IL-1beta, the effect of which is overcome by the antioxidant, phenylarsine oxide. The hypothesis that the stress-induced increase in reactive oxygen species production may underlie the suppression of LTP is further supported by the finding that the effect of stress is abrogated by dietary manipulation with antioxidant vitamins E and C.

Effect of M40403 treatment of diabetic rats on endoneurial blood flow, motor nerve conduction velocity and vascular function of epineurial arterioles of the sciatic nerve

1. To further explore the effect of antioxidants in preventing diabetes-induced vascular and neural dysfunction we treated streptozotocin-induced diabetic rats daily with subcutaneous injections of 10 mg kg(-1) of M40403 (n=11) and compared the results obtained from 17 control rats and 14 untreated diabetic rats. M40403 is a manganese(II) complex with a bis(cyclo-hexylpyridine)-substituted macrocyclic ligand that was designed to be a selective functional mimetic of superoxide dismutase. Thus, M40403 provides a useful tool to evaluate the roles of superoxide in disease states. 2. Treatment with M40403 significantly improved diabetes-induced decrease in endoneurial blood flow, acetylcholine-mediated vascular relaxation in arterioles that provide circulation to the region of the sciatic nerve, and motor nerve conduction velocity (P<0.05). M40403 treatment also reduced the appearance of superoxide in the aorta and epineurial vessels and peroxynitrite in epineurial vessels. Treating diabetic rats with M40403 reduced the diabetes-induced increase in thiobarbituric acid reactive substances in serum but did not prevent the decrease in lens glutathione level. Treating diabetic rats with M40403 did not improve sciatic nerve Na(+)/K(+) ATPase activity or the sorbitol, fructose or myo-inositol content of the sciatic nerve. 3. These studies provide additional evidence that diabetes-induced oxidative stress and the generation of superoxide and perhaps peroxynitrite may be partially responsible for the development of diabetic vascular and neural complications.

Therapeutic potential of anti-oxidants and diabetic retinopathy

Retinopathy, a severely disabling complication of diabetes mellitus, is today the leading cause of acquired blindness among young adults in developed countries. Good glycaemic control can attenuate the development of diabetic retinopathy but such metabolic control is often difficult to achieve and maintain and additional therapies need to be identified by which retinopathy can be prevented or arrested. Hyperglycaemia plays a critical role in the development and progression of retinopathy, but the mechanism by which hyperglycaemia results in the development of retinopathy is not clear. Oxidative stress is increased in the retina in diabetes. The possible sources of increased oxidative stress might include increased generation of free radicals or impaired anti-oxidant defence system. Dietary supplementation with anti-oxidants in animal models of diabetic retinopathy inhibits retinal metabolic abnormalities and retinal histopathology, suggesting that oxidative stress is associated with the development of retinopathy. The mechanism by which anti-oxidants inhibit retinopathy in diabetes warrants further investigation, but animal studies show that increasing the diversity of anti-oxidants provides significantly more protection than using any single anti-oxidant. Thus, supplementation with anti-oxidants represents an achievable adjunct therapy to help preserve vision in diabetic patients.

Effect of antioxidant treatment of streptozotocin-induced diabetic rats on endoneurial blood flow, motor nerve conduction velocity, and vascular reactivity of epineurial arterioles of the sciatic nerve

We have shown that diabetes-induced reduction in endoneurial blood flow (EBF) and impaired endothelium-dependent vascular relaxation precede slowing of motor nerve conduction velocity (MNCV) and decreased sciatic nerve Na(+)/K(+) ATPase activity. Furthermore, vascular dysfunction was accompanied by an accumulation of superoxide in arterioles that provide circulation to the sciatic nerve. In the present study, we examined the effect that treatment of streptozotocin-induced diabetic rats with antioxidants has on vascular and neural function. Diabetic rats were treated with 0.5% alpha-lipoic acid as a diet supplement or with hydroxyethyl starch deferoxamine (HES-DFO) by weekly intravenous injections at a dose of 75 mg/kg. The treatments significantly improved diabetes-induced decrease in EBF, acetylcholine-mediated vascular relaxation in arterioles that provide circulation to the region of the sciatic nerve, and MNCV. The treatments also reduced the production of superoxide by the aorta and superoxide and peroxynitrite by arterioles that provide circulation to the region of the sciatic nerve. Treating diabetic rats with alpha-lipoic acid prevented the diabetes-induced increase in thiobarbituric acid-reactive substances in serum and significantly improved lens glutathione levels. In contrast, treating diabetic rats with HES-DFO did not prevent diabetes-induced changes of either of these markers of oxidative stress. Diabetes-induced increase in sciatic nerve conjugated diene levels was not improved by treatment with either alpha-lipoic acid or HES-DFO. Treating diabetic rats with alpha-lipoic acid but not HES-DFO partially improved sciatic nerve Na(+)/K(+) ATPase activity and myo-inositol content. The increase in sciatic nerve sorbitol levels in diabetic rats was unchanged by either treatment. These studies suggest that diabetes-induced oxidative stress and the generation of superoxide may be partially responsible for the development of diabetic vascular and neural complications.