Effect of inhibition of aldose reductase on glucose flux, diacylglycerol formation, protein kinase C, and phospholipase A2 activation

Research Progress on Mitochondrial Dysfunction in Diabetic Retinopathy

Diabetic Retinopathy (DR) is one of the most important microvascular complications of diabetes mellitus, which can lead to blindness in severe cases. Mitochondria are energy-producing organelles in eukaryotic cells, which participate in metabolism and signal transduction, and regulate cell growth, differentiation, aging, and death. Metabolic changes of retinal cells and epigenetic changes of mitochondria-related genes under high glucose can lead to mitochondrial dysfunction and induce mitochondrial pathway apoptosis. In addition, mitophagy and mitochondrial dynamics also change adaptively. These mechanisms may be related to the occurrence and progression of DR, and also provide valuable clues for the prevention and treatment of DR. This article reviews the mechanism of DR induced by mitochondrial dysfunction, and the prospects for related treatment.

The role of inflammation and neurodegeneration in diabetic macular edema

The pathogenesis of diabetic macular edema (DME) is complex. Persistently high blood glucose activates multiple cellular pathways and induces inflammation, oxidation stress, and vascular dysfunction. Retinal ganglion cells, macroglial and microglial cells, endothelial cells, pericytes, and retinal pigment epithelium cells are involved. Neurodegeneration, characterized by dysfunction or apoptotic loss of retinal neurons, occurs early and independently from the vascular alterations. Despite the increasing knowledge on the pathways involved in DME, only limited therapeutic strategies are available. Besides antiangiogenic drugs and intravitreal corticosteroids, alternative therapeutic options tackling inflammation, oxidative stress, and neurodegeneration have been considered, but none of them has been currently approved.

Perturbed Biochemical Pathways and Associated Oxidative Stress Lead to Vascular Dysfunctions in Diabetic Retinopathy

Diabetic retinopathy (DR) is a vascular insult that accompanies the hyperglycemic state. Retinal vasculature holds a pivotal role in maintaining the integrity of the retina, and any alteration to retinal vasculature affects retinal functions. The blood retinal barrier, a prerequisite to vision acuity, is most susceptible to damage during the progression of DR. This is a consequence of impaired biochemical pathways such as the polyol, advanced end glycation products (AGE), hexosamine, protein kinase C (PKC), and tissue renin-angiotensin system (RAS) pathways. Moreover, the role of histone modification and altered miRNA expression is also emerging as a major contributor. Epigenetic changes create a link between altered protein function and redox status of retinal cells, creating a state of metabolic memory. Although various biochemical pathways underlie the etiology of DR, the major insult to the retina is due to oxidative stress, a unifying factor of altered biochemical pathways. This review primarily focuses on the critical biochemical pathways altered in DR leading to vascular dysfunctions and discusses antioxidants as plausible treatment strategies.

Nrf2, a novel molecular target to reduce type 1 diabetes associated secondary complications: The basic considerations

Oxidative stress and inflammation are the mediators of diabetes and related secondary complications. Oxidative stress arises because of the excessive production of reactive oxygen species and diminished antioxidant production due to impaired Nrf2 activation, the master regulator of endogenous antioxidant. It has been established from various animal models that the transcription factor Nrf2 provides cytoprotection, ameliorates oxidative stress, inflammation and delays the progression of diabetes and its associated complications. Whereas, deletion of the transcription factor Nrf2 amplifies tissue level pathogenic alterations. In addition, Nrf2 also regulates the expression of numerous cellular defensive genes and protects against oxidative stress-mediated injuries in diabetes. The present review provides an overview on the role of Nrf2 in type 1 diabetes and explores if it could be a potential target for the treatment of diabetes and related complications. Further, the rationality of different agent's intervention has been discussed to mitigate organ damages induced by diabetes.

Diabetic nephropathy: Is there a role for oxidative stress?

Oxidative stress has been implicated in the pathophysiology of diabetic nephropathy. Studies in experimental animal models of diabetes strongly implicate oxidant species as a major determinant in the pathophysiology of diabetic kidney disease. The translation, in the clinical setting, of these concepts have been quite disappointing, and new theories have challenged the concepts that oxidative stress per se plays a role in the pathophysiology of diabetic kidney disease. The concept of mitochondrial hormesis has been introduced to explain this apparent disconnect. Hormesis is intended as any cellular process that exhibits a biphasic response to exposure to increasing amounts of a substance or condition: specifically, in diabetic kidney disease, oxidant species may represent, at determined concentration, an essential and potentially protective factor. It could be postulated that excessive production or inhibition of oxidant species formation might result in an adverse phenotype. This review discusses the evidence underlying these two apparent contradicting concepts, with the aim to propose and speculate on potential mechanisms underlying the role of oxidant species in the pathophysiology of diabetic nephropathy and possibly open future more efficient therapies to be tested in the clinical settings.

Renoprotective Effects of Aldose Reductase Inhibitor Epalrestat against High Glucose-Induced Cellular Injury

Diabetic nephropathy (DN) is the leading cause of end stage renal disease worldwide. Increased glucose flux into the aldose reductase (AR) pathway during diabetes was reported to exert deleterious effects on the kidney. The objective of this study was to investigate the renoprotective effects of AR inhibition in high glucose milieu in vitro. Rat renal tubular (NRK-52E) cells were exposed to high glucose (30 mM) or normal glucose (5 mM) media for 24 to 48 hours with or without the AR inhibitor epalrestat (1 μM) and assessed for changes in Akt and ERK1/2 signaling, AR expression (using western blotting), and alterations in mitochondrial membrane potential (using JC-1 staining), cell viability (using MTT assay), and cell cycle. Exposure of NRK-52E cells to high glucose media caused acute activation of Akt and ERK pathways and depolarization of mitochondrial membrane at 24 hours. Prolonged high glucose exposure (for 48 hours) induced AR expression and G1 cell cycle arrest and decreased cell viability (84% compared to control) in NRK-52E cells. Coincubation of cells with epalrestat prevented the signaling changes and renal cell injury induced by high glucose. Thus, AR inhibition represents a potential therapeutic strategy to prevent DN.

Cardiac mitochondrial dysfunction during hyperglycemia--the role of oxidative stress and p66Shc signaling

Diabetes mellitus is a chronic disease caused by a deficiency in the production of insulin and/or by the effects of insulin resistance. Insulin deficiency leads to hyperglycemia which is the major initiator of diabetic cardiovascular complications escalating with time and driven by many complex biochemical and molecular processes. Four hypotheses, which propose mechanisms of diabetes-associated pathophysiology, are currently considered. Cardiovascular impairment may be caused by an increase in polyol pathway flux, by intracellular advanced glycation end-products formation or increased flux through the hexosamine pathway. The latter of these mechanisms involves activation of the protein kinase C. Cellular and mitochondrial metabolism alterations observed in the course of diabetes are partially associated with an excessive production of reactive oxygen species (ROS). Among many processes and factors involved in ROS production, the 66 kDa isoform of the growth factor adaptor shc (p66Shc protein) is of particular interest. This protein plays a key role in the control of mitochondria-dependent oxidative balance thus it involvement in diabetic complications and other oxidative stress based pathologies is recently intensively studied. In this review we summarize the current understanding of hyperglycemia induced cardiac mitochondrial dysfunction with an emphasis on the oxidative stress and p66Shc protein. This article is part of a Directed Issue entitled: Bioenergetic dysfunction, adaptation and therapy.

Beneficial effects of Trigonella foenum graecum and sodium orthovanadate on metabolic parameters in experimental diabetes

Oxidative stress in diabetic tissues is accompanied by high-level of free radicals with simultaneously declined antioxidant enzymes status leading to cell membrane damage. The present study was carried out to observe the effect of sodium orthovanadate (SOV) and Trigonella foenum graecum seed powder (TSP) administration on blood glucose and insulin levels, antioxidant enzymes, lipid peroxidation, pyruvate kinase, lactate dehydrogenase and protein kinase C in heart, muscle and brain of the alloxan-induced diabetic rats to see whether the treatment with SOV and TSP was capable of reversing the diabetic effects. Diabetes was induced by administration of alloxan monohydrate (15 mg/100 g body weight), and rats were treated with 2 IU insulin, 0.6 mg/ml SOV, 5% TSP in the diet and a combination of 0.2 mg/ml SOV and 5% TSP separately for 21 days. Blood glucose levels increased markedly in diabetic rats, animals treated with a combined dose of SOV and TSP had glucose levels almost comparable with controls, similar results were obtained in the activities of pyruvate kinase, lactate dehydrogenase, antioxidant enzymes and protein kinase C in diabetic animals. Our results showed that lower doses of SOV (0.2 mg/ml) could be used in combination with TSP to effectively reverse diabetic alterations in experimental diabetes.

Oxidative stress in preretinopathic diabetes subjects and antioxidants

BACKGROUND

This study assessed the effect of a systemic oral treatment with antioxidants (AOs) in preretinopathic diabetes (PRD) patients, through the evaluation of oxidative stress in plasma and changes in the full-field electroretinogram (ERG).

METHODS

Thirty-two PRD subjects with good metabolic control were recruited. Patients were randomized in two groups, one of which received oral AO treatment with α-lipoic acid at 400 mg/day in association with genistein and vitamins, whereas the other group received a placebo. Free radicals and the AO barrier were evaluated in plasma with the Free Radical Analytical System 4 instrument (H&D srl, Parma, Italy), and the same day the electrophysiological response was measured by ERG. These analyses were performed at enrollment and after 30 days of treatment.

RESULTS

Statistically significant increases of plasma AO levels and ERG oscillatory potential values were observed in the group treated with AO, but not in the control group.

CONCLUSIONS

Results of this preliminary study suggest that an oral treatment with AOs in PRD subjects may have a protective effect on retinal cells, as detected by ERG analysis, through the strengthening of the plasma AO barrier.

Effects of glutamine supplementation on oxidative stress-related gene expression and antioxidant properties in rats with streptozotocin-induced type 2 diabetes

There are close links among hyperglycaemia, oxidative stress and diabetic complications. Glutamine (GLN) is an amino acid with immunomodulatory properties. The present study investigated the effect of dietary GLN on oxidative stress-relative gene expressions and tissue oxidative damage in diabetes. There were one normal control (NC) and two diabetic groups in the present study. Diabetes was induced by an intraperitoneal injection of nicotinamide followed by streptozotocin (STZ). Rats in the NC group were fed a regular chow diet. In the two diabetic groups, one group (diabetes mellitus, DM) was fed a common semi-purified diet while the other group received a diet in which part of the casein was replaced by GLN (DM-GLN). GLN provided 25% of total amino acid N. The experimental groups were fed the respective diets for 8 weeks, and then the rats were killed for further analysis. The results showed that blood thioredoxin-interacting protein (Txnip) mRNA expression in the diabetic groups was higher than that in the NC group. Compared with the DM group, the DM-GLN group had lower glutamine fructose-6-phosphate transaminase 1, a receptor of advanced glycation end products, and Txnip gene expressions in blood mononuclear cells. The total antioxidant capacity was lower and antioxidant enzyme activities were altered by the diabetic condition. GLN supplementation increased antioxidant capacity and normalised antioxidant enzyme activities. Also, the renal nitrotyrosine level and Txnip mRNA expression were lower when GLN was administered. These results suggest that dietary GLN supplementation decreases oxidative stress-related gene expression, increases the antioxidant potential and may consequently attenuate renal oxidative damage in rats with STZ-induced diabetes.

Genetic deficiency of aldose reductase counteracts the development of diabetic nephropathy in C57BL/6 mice

AIMS/HYPOTHESIS

The aim of the study was to investigate the effects of genetic deficiency of aldose reductase in mice on the development of key endpoints of diabetic nephropathy.

METHODS

A line of Ar (also known as Akr1b3)-knockout (KO) mice, a line of Ar-bitransgenic mice and control C57BL/6 mice were used in the study. The KO and bitransgenic mice were deficient for Ar in the renal glomeruli and all other tissues, with the exception of, in the bitransgenic mice, a human AR cDNA knockin-transgene that directed collecting-tubule epithelial-cell-specific AR expression. Diabetes was induced in 8-week-old male mice with streptozotocin. Mice were further maintained for 17 weeks then killed. A number of serum and urinary variables were determined for these 25-week-old mice. Periodic acid-Schiff staining, western blots, immunohistochemistry and protein kinase C (PKC) activity assays were performed for histological analyses, and to determine the levels of collagen IV and TGF-β1 and PKC activities in renal cortical tissues.

RESULTS

Diabetes-induced extracellular matrix accumulation and collagen IV overproduction were completely prevented in diabetic Ar-KO and bitransgenic mice. Ar deficiency also completely or partially prevented diabetes-induced activation of renal cortical PKC, TGF-β1 and glomerular hypertrophy. Loss of Ar results in a 43% reduction in urine albumin excretion in the diabetic Ar-KO mice and a 48% reduction in the diabetic bitransgenic mice (p < 0.01).

CONCLUSIONS/INTERPRETATION

Genetic deficiency of Ar significantly ameliorated development of key endpoints linked with early diabetic nephropathy in vivo. Robust and specific inhibition of aldose reductase might be an effective strategy for the prevention and treatment of diabetic nephropathy.

Dietary hyperglycemia, glycemic index and metabolic retinal diseases

The glycemic index (GI) indicates how fast blood glucose is raised after consuming a carbohydrate-containing food. Human metabolic studies indicate that GI is related to patho-physiological responses after meals. Compared with a low-GI meal, a high-GI meal is characterized with hyperglycemia during the early postprandial stage (0-2h) and a compensatory hyperlipidemia associated with counter-regulatory hormone responses during late postprandial stage (4-6h). Over the past three decades, several human health disorders have been related to GI. The strongest relationship suggests that consuming low-GI foods prevents diabetic complications. Diabetic retinopathy (DR) is a complication of diabetes. In this aspect, GI appears to be useful as a practical guideline to help diabetic people choose foods. Abundant epidemiological evidence also indicates positive associations between GI and risk for type 2 diabetes, cardiovascular disease, and more recently, age-related macular degeneration (AMD) in people without diabetes. Although data from randomized controlled intervention trials are scanty, these observations are strongly supported by evolving molecular mechanisms which explain the pathogenesis of hyperglycemia. This wide range of evidence implies that dietary hyperglycemia is etiologically related to human aging and diseases, including DR and AMD. In this context, these diseases can be considered as metabolic retinal diseases. Molecular theories that explain hyperglycemic pathogenesis involve a mitochondria-associated pathway and four glycolysis-associated pathways, including advanced glycation end products formation, protein kinase C activation, polyol pathway, and hexosamine pathway. While the four glycolysis-associated pathways appear to be universal for both normoxic and hypoxic conditions, the mitochondria-associated mechanism appears to be most relevant to the hyperglycemic, normoxic pathogenesis. For diseases that affect tissues with highly active metabolism and that frequently face challenge from low oxygen tension, such as retina in which metabolism is determined by both glucose and oxygen homeostases, these theories appear to be insufficient. Several lines of evidence indicate that the retina is particularly vulnerable when hypoxia coincides with hyperglycemia. We propose a novel hyperglycemic, hypoxia-inducible factor (HIF) pathway, to complement the current theories regarding hyperglycemic pathogenesis. HIF is a transcription complex that responds to decrease oxygen in the cellular environment. In addition to playing a significant role in the regulation of glucose metabolism, under hyperglycemia HIF has been shown to increase the expression of HIF-inducible genes, such as vascular endothelial growth factor (VEGF) leading to angiogenesis. To this extent, we suggest that HIF can also be described as a hyperglycemia-inducible factor. In summary, while management of dietary GI appears to be an effective intervention for the prevention of metabolic diseases, specifically AMD and DR, more interventional data is needed to evaluate the efficacy of GI management. There is an urgent need to develop reliable biomarkers of exposure, surrogate endpoints, as well as susceptibility for GI. These insights would also be helpful in deciphering the detailed hyperglycemia-related biochemical mechanisms for the development of new therapeutic agents.

Aldose reductase and cardiovascular diseases, creating human-like diabetic complications in an experimental model

Hyperglycemia and reduced insulin actions affect many biological processes. One theory is that aberrant metabolism of glucose via several pathways including the polyol pathway causes cellular toxicity. Aldose reductase (AR) is a multifunctional enzyme that reduces aldehydes. Under diabetic conditions AR converts glucose into sorbitol, which is then converted to fructose. This article reviews the biology and pathobiology of AR actions. AR expression varies considerably among species. In humans and rats, the higher level of AR expression is associated with toxicity. Flux via AR is increased by ischemia and its inhibition during ischemia reperfusion reduces injury. However, similar pharmacological effects are not observed in mice unless they express a human AR transgene. This is because mice have much lower levels of AR expression, probably insufficient to generate toxic byproducts. Human AR expression in LDL receptor knockout mice exacerbates vascular disease, but only under diabetic conditions. In contrast, a recent report suggests that genetic ablation of AR increased atherosclerosis and increased hydroxynonenal in arteries. It was hypothesized that AR knockout prevented reduction of toxic aldehydes. Like many in vivo effects found in genetically manipulated animals, interpretation requires the reproduction of human-like physiology. For AR, this will require tissue specific expression of AR in sites and at levels that approximate those in humans.

Aldose reductase inhibitor ameliorates renal vascular endothelial growth factor expression in streptozotocin-induced diabetic rats

PURPOSE

The vascular endothelial growth factor (VEGF) expression of podocyte is one of the well-known major factors in development of diabetic nephropathy. In this study, we investigated the effects of aldose reductase inhibitor, fidarestat on diabetic nephropathy, and renal VEGF expression in a type 1 diabetic rat model.

MATERIALS AND METHODS

Twenty four Sprague-Dawley male rats which were performed intraperitoneal injection of streptozotocin and normal six rats were divided into four groups including a normal control group, untreated diabetic control group, aldose reductase (AR) inhibitor (fidarestat, 16 mg kg(-1) day(-1)) treated diabetic group, and angiotensin receptor blocker (losartan, 20 mg kg(-1) day(-1)) treated diabetic group. We checked body weights and blood glucose levels monthly and measured urine albumin-creatinine ratio (ACR) at 8 and 32 weeks. We extracted the kidney to examine the renal morphology and VEGF expressions.

RESULTS

The ACR decreased in fidarestat and losartan treated diabetic rat groups than in untreated diabetic group (24.79 +/- 11.12, 16.11 +/- 9.95, and 84.85 +/- 91.19, p < 0.05). The renal VEGF messenger RNA (mRNA) and protein expression were significantly decreased in the fidarestat and losartan treated diabetic rat groups than in the diabetic control group.

CONCLUSION

We suggested that aldose reductase inhibitor may have preventive effect on diabetic nephropathy by reducing renal VEGF overexpression.

Polyol pathway and RAGE: a central metabolic and signaling axis in diabetic complications

There are multiple metabolic and molecular consequences of hyperglycemia. This review will focus on the roles of the polyol pathway and the receptor for advanced glycation end products (RAGE) in the pathogenesis of diabetic complications. The lead enzyme of the polyol pathway, aldose reductase, transduces maladaptive effects of hyperglycemia by multiple mechanisms, at least in part via the generation of the products of nonenzymatic glycation of proteins, the advanced glycation end products (AGEs). Furthermore, seminal shifts in metabolic flux in the intracellular space stimulated by aldose reductase action activate signal transduction pathways, which alter gene expression and change cellular phenotype. Among the ligands of the multi-ligand receptor RAGE are the AGEs. AGE-RAGE stimulation mediates vascular and target cell dysfunction. The intersection and interdependence of the polyol pathway-RAGE connection suggest that targeting this axis may provide benefit in reducing the complications of diabetes.

Aldose reductase regulates hepatic peroxisome proliferator-activated receptor alpha phosphorylation and activity to impact lipid homeostasis

Aldose reductase (AR) is implicated in the development of a number of diabetic complications, but the underlying mechanisms remain to be fully elucidated. We performed this study to determine whether and how AR might influence hepatic peroxisome proliferator-activated receptor alpha (PPARalpha) activity and lipid metabolism. Our results in mouse hepatocyte AML12 cells show that AR overexpression caused strong suppression of PPARalpha/delta activity (74%, p < 0.001) together with significant down-regulation of mRNA expression for acetyl-CoA oxidase and carnitine palmitoyltransferase-1. These suppressive effects were attenuated by the selective AR inhibitor zopolrestat. Furthermore, AR overexpression greatly increased the levels of phosphorylated PPARalpha and ERK1/2. Moreover, AR-induced suppression of PPARalpha activity was attenuated by treatment with an inhibitor for ERK1/2 but not that for phosphoinositide 3-kinase, p38, or JNK. Importantly, similar effects were observed for cells exposed to 25 mm glucose. In streptozotocin-diabetic mice, AR inhibitor treatment or genetic deficiency of AR resulted in significant dephosphorylation of both PPARalpha and ERK1/2. With the dephosphorylation of PPARalpha, hepatic acetyl-CoA oxidase and apolipoprotein C-III mRNA expression was greatly affected and that was associated with substantial reductions in blood triglyceride and nonesterified fatty acid levels. These data indicate that AR plays an important role in the regulation of hepatic PPARalpha phosphorylation and activity and lipid homeostasis. A significant portion of the AR-induced modulation is achieved through ERK1/2 signaling.

Strategies for blocking angiogenesis in diabetic retinopathy: from basic science to clinical practice

Proliferative diabetic retinopathy (PDR) demands both more effective and less expensive biologically based treatments. Our understanding of the pathophysiology of the disease is increasing as new biochemical pathways are identified. Most reports emphasize proangiogenic stimuli, with the natural inhibitory elements receiving little attention. There are two therapeutic strategies for blocking retinal angiogenesis in PDR: systemic drug administration (protein kinase C inhibitors and somatostatin analogs) or local therapies (anti-vascular endothelial growth factor strategies, anti-inflammatory agents, gene therapy and stem cell therapy). This review mainly focuses on the role of local therapies, especially intravitreous delivery, in the management of PDR. The potential for adverse effect are also discussed. The availability of these new strategies or the combination of them will not only be beneficial in treating PDR but may also result in a shift towards treating earlier stages of diabetic retinopathy, thus easing the burden of this devastating disease.

Contribution of polyol pathway to arteriolar dysfunction in hyperglycemia. Role of oxidative stress, reduced NO, and enhanced PGH(2)/TXA(2) mediation

Hyperglycemia increases glucose metabolism via the polyol pathway, which results in elevations of intracellular sorbitol concentration. Thus we hypothesized that elevated level of sorbitol contributes to the development of hyperglycemia-induced dysfunction of microvessels. In isolated, pressurized (80 mmHg) rat gracilis muscle arterioles (approximately 150 microm), high glucose treatment (25 mM) induced reduction in flow-dependent dilation (from maximum of 39 +/- 2% to 15 +/- 1%), which was significantly mitigated by an aldose reductase inhibitor, zopolrestat (maximum 27 +/- 2%). Increasing doses of sorbitol (10(-10)-10(-4) M) elicited dose-dependent constrictions (maximum 22 +/- 3%), which were abolished by endothelium removal, a prostaglandin H(2)/thromboxane A(2) (PGH(2)/TXA(2)) receptor (TP) antagonist SQ-29548, or superoxide dismutase (SOD) plus catalase (CAT). Incubation of arterioles with sorbitol (10(-7) M) reduced flow-dependent dilations (from maximum of 39 +/- 2% to 20 +/- 1.5%), which was not further affected by inhibition of nitric oxide synthase by N(omega)-nitro-l-arginine methyl ester but was prevented by SOD plus CAT and mitigated by SQ-29548. Nitric oxide donor sodium nitroprusside-induced (10(-9)-10(-6) M) dilations were also decreased in a SQ-29548 and SOD plus CAT-reversible manner, whereas adenosine dilations were not affected by sorbitol exposure. Sorbitol significantly increased arterial superoxide production detected by lucigenin-enhanced chemiluminescence, which was inhibited by SOD plus CAT. Sorbitol treatment also increased arterial formation of 3-nitrotyrosine. We suggest that hyperglycemia by elevating intracellular sorbitol induces oxidative stress, which interferes with nitric oxide bioavailability and promotes PGH(2)/TXA(2) release, both of which affect regulation of vasomotor responses of arterioles. Thus increased activity of the polyol pathway may contribute to the development of microvascular dysfunction in diabetes mellitus.

Inhibitory effects of astragaloside IV on diabetic peripheral neuropathy in rats

Astragaloside IV (AGS-IV), a new glycoside of cycloartane-type triterpene isolated from the root of Astragalus membranaceus (Fisch.) Bunge, has been used experimentally for its potent immune-stimulating, anti-inflammatory, and antioxidative actions. A recent study has shown AGS-IV to be an aldose-reductase inhibitor and a free-radical scavenger. This study examined the effects of AGS-IV on motor nerve conduction velocity (MNCV), tailflick threshold temperature, biochemical indexes, and the histology of the sural nerve after diabetes was induced in rats with 75 mg/kg streptozotocin (STZ). AGS-IV (3, 6, 12 mg/kg, twice a day) was administered by oral gavage for 12 weeks after diabetes was induced. Compared with control (nondiabetic) rats, obvious changes in physiological behaviors and a significant reduction in sciatic MNCV in diabetic rats were observed after 12 weeks of STZ administration. Morphological analysis showed that AGS-IV suppressed a decrease in myelinated fiber area, an increase in myelinated fiber density, and an increase in segmental demyelination in diabetic rats. The protective mechanism of AGS-IV involved a decrease in declining blood glucose concentration and HbA1C levels, and an increase in plasma insulin levels. AGS-IV increased the activity of glutathione peroxidase in nerves, depressed the activation of aldose reductase in erythrocytes, and decreased the accumulation of advanced glycation end products in both nerves and erythrocytes. Moreover, AGS-IV elevated Na+,K+-ATPase activity in both the nerves and erythrocytes of diabetic rats. These results indicate that AGS-IV exerts protective effects against the progression of peripheral neuropathy in STZ-induced diabetes in rats through several interrelated mechanisms.

Aldose reductase inhibition counteracts nitrosative stress and poly(ADP-ribose) polymerase activation in diabetic rat kidney and high-glucose-exposed human mesangial cells

Both increased aldose reductase (AR) activity and oxidative/nitrosative stress have been implicated in the pathogenesis of diabetic nephropathy, but the relation between the two factors remains a subject of debate. This study evaluated the effects of AR inhibition on nitrosative stress and poly(ADP-ribose) polymerase (PARP) activation in diabetic rat kidney and high-glucose-exposed human mesangial cells. In animal experiments, control (C) and streptozotocin-diabetic (D) rats were treated with/without the AR inhibitor fidarestat (F, 16 mg kg(-1) day(-1)) for 6 weeks starting from induction of diabetes. Glucose, sorbitol, and fructose concentrations were significantly increased in the renal cortex of D vs C (p < 0.01 for all three comparisons), and sorbitol pathway intermediate, but not glucose, accumulation, was completely prevented in D + F. F at least partially prevented diabetes-induced increase in kidney weight as well as nitrotyrosine (NT, a marker of peroxynitrite-induced injury and nitrosative stress), and poly(ADP-ribose) (a marker of PARP activation) accumulation, assessed by both immunohistochemistry and Western blot analysis, in glomerular and tubular compartments of the renal cortex. In vitro studies revealed the presence of both AR and PARP-1 in human mesangial cells, and none of these two variables were affected by high glucose or F treatment. Nitrosylated and poly(ADP-ribosyl)ated proteins (Western blot analysis) accumulated in cells cultured in 30 mM D-glucose (vs 5.55 mM glucose, p < 0.01), but not in cells cultured in 30 mM L-glucose or 30 mM D-glucose plus 10 microM F. AR inhibition counteracts nitrosative stress and PARP activation in the diabetic renal cortex and high-glucose-exposed human mesangial cells. These findings reveal new beneficial properties of the AR inhibitor F and provide the rationale for detailed studies of F on diabetic nephropathy.

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

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

Oxidative stress and stress signaling: menace of diabetic cardiomyopathy

Cardiovascular disease is the most common cause of death in the diabetic population and is currently one of the leading causes of death in the United States and other industrialized countries. The health care expenses associated with cardiovascular disease are staggering, reaching more than 350 billion dollars in 2003. The risk factors for cardiovascular disease include high fat/cholesterol levels, alcoholism, smoking, genetics, environmental factors and hypertension, which are commonly used to gauge an individual's risk of cardiovascular disease and to track their progress during therapy. Most recently, these factors have become important in the early prevention of cardiovascular diseases. Oxidative stress, the imbalance between reactive oxygen species production and breakdown by endogenous antioxidants, has been implicated in the onset and progression of cardiovascular diseases such as congestive heart failure and diabetes-associated heart dysfunction (diabetic cardiomyopathy). Antioxidant therapy has shown promise in preventing the development of diabetic heart complications. This review focuses on recent advances in oxidative stress theory and antioxidant therapy in diabetic cardiomyopathy, with an emphasis on the stress signaling pathways hypothesized to be involved. Many of these stress signaling pathways lead to activation of reactive oxygen species, major players in the development and progression of diabetic cardiomyopathy.

Central role for aldose reductase pathway in myocardial ischemic injury

Aldose reductase (AR), a member of the aldo-keto reductase family, has been implicated in the development of vascular and neurological complications of diabetes. Recently, we demonstrated that aldose reductase is a component of myocardial ischemic injury and that inhibitors of this enzyme protect rat hearts from ischemia-reperfusion injury. To rigorously test the effect of aldose reductase on myocardial ischemia-reperfusion injury, we used transgenic mice broadly overexpressing human aldose reductase (ARTg) driven by the major histocompatibility complex I promoter. Hearts from these ARTg or littermate mice (WT) (n=6 in each group) were isolated, perfused under normoxic conditions, then subjected to 50 min of severe low flow ischemia followed by 60 min of reperfusion. Creatine kinase (CK) release (a marker of ischemic injury) was measured during reperfusion; left ventricular developed pressure (LVDP), end diastolic pressure (EDP), and ATP were measured throughout the protocol. CK release was significantly greater in ARTg mice compared with the WT mice. LVDP recovery was significantly reduced in ARTg mice compared with the WT mice. Furthermore, ATP content was higher in WT mice compared with ARTg mice during ischemia and reperfusion. Infarct size measured by staining techniques and myocardial damage evaluated histologically were also significantly worse in ARTg mice hearts than in controls. Pharmacological inhibition of aldose reductase significantly reduced ischemic injury and improved functional recovery in ARTg mice. These data strongly support key roles for AR in ischemic injury and impairment of functional and metabolic recovery after ischemia. We propose that interventions targeting AR may provide a novel adjunctive approach to protect ischemic myocardium.

Aldose reductase inhibition counteracts oxidative-nitrosative stress and poly(ADP-ribose) polymerase activation in tissue sites for diabetes complications

This study evaluated the effects of aldose reductase inhibition on diabetes-induced oxidative-nitrosative stress and poly(ADP-ribose) polymerase (PARP) activation. In animal experiments, control and streptozotocin-induced diabetic rats were treated with or without the aldose reductase inhibitor (ARI) fidarestat (16 mg . kg(-1) . day(-1)) for 6 weeks starting from induction of diabetes. Sorbitol pathway intermediate, but not glucose, accumulation in sciatic nerve and retina was completely prevented in diabetic rats treated with fidarestat. Sciatic motor nerve conduction velocity, hindlimb digital sensory nerve conduction velocity, and sciatic nerve concentrations of two major nonenzymatic antioxidants, glutathione and ascorbate, were reduced in diabetic versus control rats, and these changes were prevented in diabetic rats treated with fidarestat. Fidarestat prevented the diabetes-induced increase in nitrotyrosine (a marker of peroxynitrite-induced injury) and poly(ADP-ribose) immunoreactivities in sciatic nerve and retina. Fidarestat counteracted increased superoxide formation in aorta and epineurial vessels and in in vitro studies using hyperglycemia-exposed endothelial cells, and the DCF test/flow cytometry confirmed the endothelial origin of this phenomenon. Fidarestat did not cause direct inhibition of PARP activity in a cell-free system containing PARP and NAD(+) but did counteract high-glucose-induced PARP activation in Schwann cells. In conclusion, aldose reductase inhibition counteracts diabetes-induced nitrosative stress and PARP activation in sciatic nerve and retina. These findings reveal the new beneficial properties of fidarestat, thus further justifying the ongoing clinical trials of this specific, potent, and low-toxic ARI.

Interaction between the polyol pathway and non-enzymatic glycation on aortic smooth muscle cell migration and monocyte adhesion

We investigated for the interaction between the polyol pathway and enhanced non-enzymatic glycation, both implicated in the pathogenesis of diabetic atherosclerosis, in the activation of aortic smooth muscle cell (SMC) function. Mouse aortas and primary cultures of SMCs from wildtype (WT) mice and transgenic (TG) mice expressing human aldose reductase (AR) were studied regarding changes in AR activity, and SMC gene activation, migration and monocyte adhesion, in response to advanced glycation end-product modified BSA (AGE-BSA). Results showed that AGE-BSA increased AR activity in both WT and TG aortas, with greater increments (p < 0.05) in TG aortas which, basally, had elevated AR activity (2.8 fold of WT). These increments were attenuated by zopolrestat, an AR inhibitor. Similar AGE-induced increments in AR activity were observed in primary cultures of aortic SMCs from WT and TG mice (60% and 100%, respectively, P < 0.01). Such increments were accompanied by increases in intercellular adhesion molecule-1 (ICAM-1) and monocyte chemoattractant protein-1 (MCP-1) mRNA levels (both P < 0.05), activation of membrane-associated PKC-beta1 (P < 0.05) as well as increased SMC migration and Tamm-Horsfall protein (THP)-1 monocyte adhesion to SMCs (both p < 0.01), with all changes being significantly greater in TG SMCs (P < 0.05) and suppressible by either zopolrestat or transfection with an AR antisense oligonucleotide. Our findings suggest that the effects of AGEs on SMC activation, migration and monocyte adhesion are mediated partly through the polyol pathway and, possibly, PKC activation. The greater AGE-induced changes in the TG SMCs have provided further support for the dependency of such changes on polyol pathway hyperactivity.

From hyperglycemia to diabetic kidney disease: the role of metabolic, hemodynamic, intracellular factors and growth factors/cytokines

At present, diabetic kidney disease affects about 15-25% of type 1 and 30-40% of type 2 diabetic patients. Several decades of extensive research has elucidated various pathways to be implicated in the development of diabetic kidney disease. This review focuses on the metabolic factors beyond blood glucose that are involved in the pathogenesis of diabetic kidney disease, i.e., advanced glycation end-products and the aldose reductase system. Furthermore, the contribution of hemodynamic factors, the renin-angiotensin system, the endothelin system, and the nitric oxide system, as well as the prominent role of the intracellular signaling molecule protein kinase C are discussed. Finally, the respective roles of TGF-beta, GH and IGFs, vascular endothelial growth factor, and platelet-derived growth factor are covered. The complex interplay between these different pathways will be highlighted. A brief introduction to each system and description of its expression in the normal kidney is followed by in vitro, experimental, and clinical evidence addressing the role of the system in diabetic kidney disease. Finally, well-known and potential therapeutic strategies targeting each system are discussed, ending with an overall conclusion.

Normalization of lens protein kinase Cgamma in galactosemic dogs by a novel aldose reductase inhibitor

The purpose of this study was to determine the effects of a novel aldose reductase inhibitor on lens protein kinase Cgamma (PKCgamma) levels in galactosemic dogs. Six-month old Beagles (12 total; 6 male and 6 female) were made galactosemic by feeding a diet of 40% galactose for 6 weeks. Three dogs per group were fed either control, normal diet, 40% galactose diet, 40% galactose diet with aldose reductase inhibitor at 100 mg/kg body weight per day given orally, or a control diet with aldose reductase inhibitor alone (1-H,7-H-5alpha,6,8,9-tetrahydro-1-oxopyran[4,3-beta](1) benzopyran, referred to herein as HAR-1). Lenses were removed and analyzed for toxicity by pathological examination. Lens polyol concentrations were determined by GC/MS. PKCgamma levels were determined by Western blot and by reverse transcriptase-polymerase chain reaction (RT-PCR). No toxicity was observed from the aldose reductase inhibitor when given at 100 mg/kg body weight per day for 6 weeks. Galactosemic dogs showed deterioration of lens cells. Deterioration included vacuole formation in the lens, cell lysis, and loss of cell nuclei. Galactosemic dogs given the HAR-1 appeared identical to control dogs. Polyol concentrations in the lenses were reduced by 50% in dogs fed the 40% galactose diet with the aldose reductase inhibitor, HAR-1. PKCgamma protein levels were reduced in the galactosemic dog lenses, but synthesis of PKCgamma was not affected, as measured by RT-PCR. The PKCgamma protein levels were similar to controls in dogs given the aldose reductase inhibitor, HAR-1, even when polyol concentrations remained 50% elevated above control levels. HAR-1, when given to control dogs, caused a reduction in the synthesis of PKCgamma mRNA but not in total PKCgamma protein levels. This study demonstrates the use of a novel aldose reductase inhibitor to control changes in PKCgamma in dog lens, a PKC that is known to control gap junction activity.

A mammalian protein homologous to fructosamine-3-kinase is a ketosamine-3-kinase acting on psicosamines and ribulosamines but not on fructosamines

Fructosamine-3-kinase (FN3K) is an enzyme that appears to be responsible for the removal of fructosamines from proteins. In this study, we report the sequence of human and mouse cDNAs encoding proteins sharing 65% sequence identity with FN3K. The genes encoding FN3K and FN3K-related protein (FN3K-RP) are present next to each other on human chromosome 17q25, and they both have a similar 6-exon structure. Northern blots of mouse tissues RNAs indicate a high level of expression of both genes in bone marrow, brain, kidneys, and spleen. Human FN3K-RP was transfected in human embryonic kidney (HEK) cells, and the expressed protein was partially purified by chromatography on Blue Sepharose. Unlike FN3K, FN3K-RP did not phosphorylate fructoselysine, 1-deoxy-1-morpholino-fructose, or lysozyme glycated with glucose. In a more systematic screening for potential substrates for FN3K-RP, we found, however, that both enzymes phosphorylated ketosamines with a D-configuration in C3 (psicoselysine, 1-deoxy-1-morpholino-psicose, 1-deoxy-1-morpholino-ribulose, lysozyme glycated with allose-the C3 epimer of glucose, or with ribose). Tandem mass spectrometry and nuclear magnetic resonance analysis of the product of phosphorylation of 1-deoxy-1-morpholino-psicose by FN3K-RP indicated that this enzyme phosphorylates the third carbon of the sugar moiety. These results indicate that FN3K-RP is a ketosamine-3-kinase (ketosamine-3-kinase 2). This enzyme presumably plays a role in freeing proteins from ribulosamines or psicosamines, which might arise in a several step process, from the reaction of amines with glucose and/or glycolytic intermediates. This role is shared by fructosamine-3-kinase (ketosamine-3-kinase 1), which has, in addition, the unique capacity to phosphorylate fructosamines.

Glucose-mediated induction of TGF-beta 1 and MCP-1 in mesothelial cells in vitro is osmolality and polyol pathway dependent

BACKGROUND

Glucose is converted to sorbitol and then to fructose via the polyol pathway that has been implicated in the pathogenesis of organ damage. The contribution of the polyol pathway to mesothelial cell activation has, however, not been fully determined.

METHODS

The effect of increasing glucose concentrations on transforming growth factor-beta 1 (TGF-beta 1) and monocyte chemoattractant protein-1 (MCP-1) secretion by human peritoneal mesothelial cells (HPMC) was examined. The importance of the polyol pathway was identified by its specific inhibition with an aldose reductase inhibitor.

RESULTS

Incubation of HPMC with 5 to 100 mmol/L glucose resulted in an induction of aldose reductase mRNA and intracellular sorbitol accumulation accompanied by the induction of TGF-beta 1 and MCP-1 mRNA expression and protein secretion. Mannitol at the same concentrations also induced aldose reductase, TGF-beta 1 and MCP-1 mRNA and protein expression but at a lower level than glucose. Sorbinil dose-dependently reduced both intracellular sorbitol levels (79.8% reduction of 60 mmol/L D-glucose induced intracellular sorbitol with 100 micromol/L sorbinil (N = 3, P < 0.01) and glucose-induced TGF-beta 1 and MCP-1 secretion. Mannitol induced TGF-beta 1 and MCP-1 secretion was not reduced by sorbinil. The addition of 15 to 40 mmol/L sodium lactate, either alone or in the presence of D-glucose enhanced TGF-beta 1 and MCP-1 secretion, which was inhibited by sorbinil. In contrast, sodium pyruvate appeared to antagonize D-glucose-induced TGF-beta 1 and MCP-1 secretion.

CONCLUSION

These data suggest that the polyol pathway and osmolality contribute to the regulation of HPMC function by glucose. Control of polyol pathway activation might reduce glucose-mediated damage to the peritoneal membrane and promote its long-term survival.

High glucose down-regulates angiotensin II binding via the PKC-MAPK-cPLA2 signal cascade in renal proximal tubule cells

BACKGROUND

It has been reported that renal renin-angiotensin system contributes to the development of diabetic nephropathy. However, the mechanism of angiotensin II receptor regulation in diabetic condition has not been elucidated.

METHODS

The effects of high glucose on [(3)H]-arachidonic acid (AA) release and angiotensin II (Ang II) binding and its related signal pathway were examined in primary cultured rabbit renal proximal tubule cells (PTCs).

RESULTS

High glucose down-regulated (125)I-Ang II binding from 12 hours and this response was sustained over 48 hours. Thus, the treatment of 25 mmol/L glucose for 48 hours was used for this study. High glucose-induced down-regulation of (125)I-Ang II binding was reversed by the removal of extracellular glucose, suggesting a role for glucose specificity. The high glucose-induced down-regulation of (125)I-Ang II binding was blocked by mepacrine, AACOCF3, phospholipase A2 inhibitors, indomethacin, ibuprofen, and cyclooxygenase inhibitors. Indeed, high glucose significantly increased prostaglandin E2 synthesis. In addition, the high glucose-induced AA release was blocked by PD 98059, a p44/42 mitogen-activated protein kinase (MAPK) inhibitor. PD 98059 also prevented the down-regulation of (125)I-Ang II binding by high glucose, suggesting a role for p44/42 MAPK. Indeed, high glucose significantly increased p44/42 MAPK activity after the 15-minute time point. Protein kinase C (PKC) inhibitor blocked high glucose-induced activation of p44/42 MAPK, increase of the [(3)H]-AA release, and down-regulation of 125I-Ang II binding. W-7 and KN-62 also blocked the high glucose-induced increase of [(3)H]-AA release and down-regulation of (125)I-Ang II binding. However, phospholipase A2 inhibitor did not block high glucose-induced activation of p44/42 MAPK.

CONCLUSION

High glucose down-regulates (125)I-Ang II binding via the PKC-MAPK-cPLA2 signal pathway.

Aldose reductase activation is a key component of myocardial response to ischemia

Aldose reductase, a member of the aldo-keto reductase family, has been implicated in the development of vascular and neurological complications in diabetes. Despite recent studies from our laboratory demonstrating protection of ischemic hearts by an aldose reductase inhibitor, the presence and influence of aldose reductase in cardiac tissue remain unknown. Our goal in this study was to isolate and characterize the kinetic properties of cardiac aldose reductase, as well as to study the impact of flux via this enzyme on glucose metabolism and contractile function in hearts subjected to ischemia-reperfusion. Results demonstrate that ischemia increases myocardial aldose reductase activity and that these increases are, in part, due to activation by nitric oxide. The kinetic parameter of cardiac aldose reductase (Kcat) was significantly higher in ischemic tissues. Aldose reductase inhibition increased glycolysis and glucose oxidation. Aldose reductase inhibited hearts, when subjected to ischemia/reperfusion, exhibited less ischemic injury and was associated with lower lactate/pyruvate ratios (a measure of cytosolic NADH/NAD+), greater tissue content of adenosine triphosphate, and improved cardiac function. These findings indicate that aldose reductase is a component of ischemic injury and that pharmacological inhibitors of aldose reductase present a novel adjunctive approach for protecting ischemic hearts.

Biochemistry and molecular cell biology of diabetic complications

Diabetes-specific microvascular disease is a leading cause of blindness, renal failure and nerve damage, and diabetes-accelerated atherosclerosis leads to increased risk of myocardial infarction, stroke and limb amputation. Four main molecular mechanisms have been implicated in glucose-mediated vascular damage. All seem to reflect a single hyperglycaemia-induced process of overproduction of superoxide by the mitochondrial electron-transport chain. This integrating paradigm provides a new conceptual framework for future research and drug discovery.

Prostaglandylinositol cyclic phosphate (cPIP): a novel second messenger of insulin action. Comparative analysis of two kinds of "insulin mediators"

Insulin induces a broad spectrum of effects over a wide time interval. It also stimulates the phosphorylation of some cellular proteins, while decreasing the state of phosphorylation of others. These observations indicate the presence of different, but not necessarily mutually exclusive, pathways of insulin action. One well-known pathway represents a phosphorylation cascade initiated by the tyrosine kinase activity of the insulin receptor followed by involvement of different MAP-kinases. Another pathway suggests the existence of low molecular weight insulin mediators whose synthesis and/or release is initiated by insulin. Comparable analysis of two kinds of insulin mediators, namely inositolphosphoglycans and prostaglandylinositol cyclic phosphate (cPIP), has been carried out. It has been shown that the expression of a number of enzymes, such as phospholipase A(2), phospholipase C, cyclo-oxygenase and IRS-1-like enzyme, could regulate the biosynthesis of cPIP in both normal and diabetes-related conditions. Data on the activity of a key enzyme of cPIP biosynthesis termed cPIP synthase (IRS-1-like enzyme) in various monkey tissues before and twice during an euglycemic hyperinsulinemic clamp have been presented. It has been concluded that in vivo insulin increases cPIP synthase activity in both liver and subcutaneous adipose tissue of lean normal monkeys. It has been also suggested that abnormal production of cPIP could be related to several pathologies including glucocorticoid-induced insulin resistance and diabetic embryopathy. Further studies on cPIP and other types of insulin mediators are necessary to aid our understanding of insulin action.

Aldose reductase and the role of the polyol pathway in diabetic nephropathy

BACKGROUND; In diabetic renal complications, hyperglycemia may cause damage at a cellular level in both glomerular and tubular locations, often preceding overt dysfunction. Our previous work has implicated aldose reductase in a pathway whereby aldose reductase-induced use of nicotinamide adenine dinucleotide phosphate (reduced form) (NADPH) drives the pentose phosphate pathway, which culminates in a protein kinase C-induced increase in glomerular prostaglandin production and loss of mesangial cell contractility as a possible cause of hyperfiltration and glomerular dysfunction in diabetes. In this model, aldose reductase inhibition in vitro redresses all aspects of the pathway proposed to lead to hyperfiltration; aldose reductase inhibition in vivo gives only a partial amelioration over the short-term or is without effect in the longer term on microalbuminuria, which follows glomerular and tubular dysfunction. In diabetes, hyperglycemia-induced renal polyol pathway activity does not occur in isolation but instead in tandem with oxidative changes and the production of reactive dicarbonyls and alpha,beta-unsaturated aldehydes. Aldose reductase may detoxify these compounds. We investigated this aspect in a transgenic rat model with human aldose reductase cDNA under the control of the cytomegalovirus promoter with tubular expression of transgene.

METHODS

Tubules (S3 region-enriched) from transgenic and control animals were prepared, exposed to oxidative stress, and analyzed to determine the cellular protein dicarbonyl content.

RESULTS

In tubules from transgenic animals, oxidative stress-induced dicarbonyls were significantly reduced, an effect not seen when an aldose reductase inhibitor was present.

CONCLUSIONS

Aldose reductase may both exacerbate and alleviate the production of metabolites that lead to hyperglycemia-induced cellular impairment, with the balance determining the extent of dysfunction.

Deranged transcriptional regulation of cell-volume-sensitive kinase hSGK in diabetic nephropathy

Transforming growth factor beta (TGF-beta) has been shown to participate in the pathophysiology of diabetic complications. As shown most recently, TGF-beta stimulates the expression of a distinct serine/threonine kinase (hSGK) which had previously been cloned as an early gene transcriptionally regulated by cell volume alterations. The present study was performed to elucidate transcription and function of hSGK in diabetic nephropathy. As shown by Northern blotting, an increase of extracellular glucose concentration increased hSGK mRNA levels in cultured cells, an effect qualitatively mimicked by osmotic cell shrinkage or treatment with TGF-beta (2 microgram/liter), phorbol 12,13-didecanoate (1 microM), or the Ca(2+) ionophore ionomycin (1 microM) and blunted by high concentrations of nifedipine (10 and 100 microM). In situ hybridization revealed that hSGK transcription was markedly enhanced in diabetic nephropathy, with particularly high expression in mesangial cells, interstitial cells, and cells in thick ascending limbs of Henle's loop and distal tubules. According to voltage clamp and tracer flux studies in Xenopus oocytes expressing the renal epithelial Na(+) channel ENaC or the mouse thick ascending limb Na(+),K(+),2Cl(-) cotransporter BSC-1, coexpression with hSGK stimulated ENaC and BSC-1 11-fold and 6-fold, respectively, effects reversed by kinase inhibitors staurosporine (1 microM) and chelerythrine (1 microM) and not elicited by inactive hSGK. In conclusion, excessive extracellular glucose concentrations enhance hSGK transcription, which in turn stimulates renal tubular Na(+) transport. These observations disclose an additional element in the pathophysiology of diabetic nephropathy.

Molecular mechanism(s) of insulin action on the expression of the angiotensinogen gene in kidney proximal tubular cells

To investigate the molecular mechanism(s) of insulin action on the expression of the angiotensinogen (ANG) gene in kidney proximal tubular cells, we constructed a fusion gene, pOGH (hANG N-1064/+27), containing the 5'-flanking regulatory sequence of the human ANG gene fused with the human growth hormone (hGH) gene as a reporter and stably integrated the fusion gene into the opossum kidney (OK) cell genomes. The level of expression of pOGH (hANG N-1064/+27) was quantified by the amount of immunoreactive hGH secreted into the medium. The addition of a high level of D(+)-glucose (25 mM) or phorbol 12-myristate 13-acetate (PMA, 10(-7) M) stimulated the expression of the fusion gene in OK cells. The stimulatory effect of glucose (25 mM) was blocked by insulin and tolrestat (an inhibitor of aldose reductase). Tolrestat also inhibited the increase of cellular DAG and PKC activity stimulated by 25 mM glucose. While insulin did not affect the cellular DAG and PKC activity, it did block the stimulatory effect of high glucose (25 mM) and PMA on the expression of the fusion gene. Finally, PD98059 (an inhibitor of mitogen-activated protein kinase kinase (MEK)) enhanced the stimulatory effect of high levels of glucose and blocked the inhibitory effect of insulin on the expression of the fusion gene as well as on the phosphorylation of MEK and mitogen-activated protein kinase (MAPK). In contrast, Wortmannin (an inhibitor of phosphatidylinositol-3-kinase) did not block the inhibitory effect of insulin on the ANG gene expression. These studies demonstrate that the action of insulin, blocking the stimulatory effect of a high level of D(+)-glucose (25 mM) on the ANG gene expression is mediated, at least in part, via the 5'-flanking region of the ANG gene and MAPK signal transduction pathway.

Aldose reductase inhibitors: therapeutic implications for diabetic complications

The 'late complications' of diabetes mellitus, i.e., nephropathy, neuropathy and retinopathy are firmly rooted in inadequate control of blood glucose: hyperglycaemia. Hyperglycaemia causes elevated cytosolic glucose and/or rates of glucose metabolism, i.e., 'hyperglysolia,' within cells of vulnerable tissues. Although the molecular basis for the pathogenic effects of hyperglysolia remains to be proven, substantial evidence points to a key role for increased glucose metabolism through a cytosolic enzyme, aldose reductase (AR). Recent human genetic and biochemical data link polymorphisms of the AR gene (technically called the AR2 gene) and elevated tissue levels of AR with strongly altered risks for diabetic complications. Despite several genetic reports failing to confirm such an association, there are now ten concordant reports from five continents that certain polymorphisms of the AR gene are associated with an ~ 3- to 20-fold higher risk for diabetic complications. Moreover, in US and European diabetic study populations the principle allele of the AR gene associated with elevated disease risk, the Z-2 allele, correlates with an ~ 2- to 3-fold increase in AR expression. These results, together with recent clinical, experimental and pharmacological data, provide powerful new support for the rationale for research and development of aldose reductase inhibitors (ARIs) targeted at slowing the progression of diabetic complications. Although past clinical trials of ARIs have been disappointing, this has stemmed from overly optimistic expectations, inadequate trial designs and lack of pharmacological robustness and/or acceptable systemic toleration of the agents tested. However, a more realistic and encouraging perspective for therapeutic expectations for ARIs has arisen from recent data revealing that the seemingly modest short-term effects of intensified glycaemic control and of pancreatic transplantation are followed by substantial long-term benefits on diabetic complications. In addition, robust inhibition of AR in human nerve has recently yielded dose-dependent efficacy on nerve structure and function. Thus, the quest for well-tolerated, potent ARIs continues to be a worthy and more urgent objective than ever before.

Increased levels of aldose reductase in peripheral mononuclear cells from type 2 diabetic patients with microangiopathy

Aldose reductase (AR) protein was measured in peripheral mononuclear cells (PMCs) from 55 patients with type 2 diabetes by a two-site ELISA using anti-human AR monoclonal antibody. AR levels did not correlate with age, duration of diabetes, and HbAlc. Furthermore, no significant differences were found in AR levels between the patients and healthy subjects. Thirty seven patients had at least one of diabetic microangiopathy; retinopathy, neuropathy, or nephropathy. AR levels were significantly higher in the patients with microangiopathy than in those without it (52.3 +/- 15.7 vs. 43.0 +/- 15.2 ng/10(6) cells, P < 0.05). The patients with neuropathy had significantly higher AR levels than those without neuropathy (53.7 +/- 15.8 vs. 42.7 +/- 14.3 ng/l0(6) cells, P < 0.05). The same result applied to the patients with retinopathy (54.5 + 15.4 vs. 44.6 +/- 15.3 ng/10(6) cells, P < 0.05). The AR levels in the patients with nephropathy tended to give a higher value than those without it. However, there were no significant differences between the two (53.9 +/- 3.6 vs. 46.4 +/- 2.6 ng/10(6) cells, NS). These results indicate that AR levels in PMCs from type 2 diabetic patients are associated with the presence of microangiopathy. The measurement of AR proteins in PMCs with this ELISA system is a useful tool for the clinical study of diabetic complications, and would increase our understanding of the pathogenesis of the disease.

TGF-beta 1 stimulates glucose uptake by enhancing GLUT1 expression in mesangial cells

BACKGROUND

An increase in the expression of transforming growth factor-beta 1 (TGF-beta 1) has been proposed to play an important role in the excessive production of extracellular matrix (ECM) proteins seen in diabetes. Because the linkage between glucose metabolism and ECM protein production was found in mesangial cells overexpressed with the brain-type glucose transporter (GLUT1), we hypothesized that TGF-beta 1 could affect glucose metabolism.

METHODS

To prove this hypothesis, we examined the effect of TGF-beta 1 on glucose uptake, the first step of glucose metabolism, in mesangial cells. 2-Deoxy-D-glucose (2DOG) uptake and the expression of GLUT1 were measured in mesangial cells exposed to various concentrations of TGF-beta 1. The kinetic constants were determined using 2DOG and 3-O-methyl-D-glucose (3OMG). The effect of anti-TGF-beta neutralizing antibody on 2DOG uptake and GLUT1 mRNA was also examined in mesangial cells cultured under high-glucose (22.2 mM) conditions for 72 hours.

RESULTS

TGF-beta 1 stimulated 2DOG uptake in mesangial cells by approximately 2.5-fold in a dose- (1.25 ng/ml maximum) and time-dependent manner, with a peak stimulation at nine hours. The increase in 2DOG uptake by TGF-beta 1 was completely abolished by the addition of 1 microgram/ml cycloheximide, and kinetic analysis of 2DOG or 3OMG uptake revealed an increase in Vmax by TGF-beta 1. Furthermore, TGF-beta 1 enhanced the expression of GLUT1 mRNA from one hour, followed by an enhancement of the expression of GLUT1 protein at nine hours. Finally, 2DOG uptake was significantly enhanced in cells cultured under high-glucose (22.2 mM) conditions as compared with that in cells under normal glucose (5.6 mM) conditions, and this increase in 2DOG uptake in cells under high-glucose conditions was inhibited by the addition of anti-TGF-beta neutralizing antibody.

CONCLUSIONS

TGF-beta 1 stimulates glucose uptake by enhancing the expression of GLUT1 in mesangial cells, which leads to the acceleration of intracellular metabolic abnormalities in diabetes.

Neutrophils in galactose-fed dogs: suppressed apoptosis and increased adhesion to retinal capillary endothelial cells

Dogs fed a diet containing 30% galactose develop diabetes-like retinal capillary changes. As retinal capillary occlusion is commonly observed in diabetic retinopathy, neutrophil apoptosis and the interaction of neutrophils with retinal capillary endothelial cells were investigated. Neutrophils were isolated with Ficoll-Hypaque centrifugation from dogs fed a 30% galactose diet and dogs fed a normal, control diet containing 30% non-nutrient filler. Apoptosis of neutrophils was microscopically examined after incubation at 37 degrees C for 3 hours with either 100 U/mL tumor necrosis factor alpha (TNF-alpha), 2 microg/mL cycloheximide or 50 ng/mL phorbol 12-myristate 13-acetate (PMA). Neutrophil adhesion to dog retinal capillary endothelial cells was examined by counting the cells attached to the surface of endothelial cells after the incubation in the presence of either 100 U/mL TNF-alpha or 5 microg/mL lipopolysaccharides (LPS) at 37 degrees C for 3 hours. With all three stimulants TNF-alpha, cycloheximide and PMA, the rate of apoptosis was significantly lower for neutrophils isolated from galactose-fed dogs compared to control dogs fed a normal diet. Preincubation of neutrophils from control dogs in medium containing 30% galactose for 3 hours did not affect the rate of apoptosis. Neutrophil adhesion to retinal capillary endothelial cells induced by incubation in the presence of either 100 U/mL TNF-alpha or 5 microg/ml LPS was significantly higher with neutrophils isolated from galactose-fed dogs than those from control dogs. The data indicate that long-term galactose feeding is essential with development of various neutrophil dysfunctions. These neutrophil changes may contribute to the development of retinal microangiopathy associated with diabetes and galactosemia.