The hexosamine pathway regulates the plasminogen activator inhibitor-1 gene promoter and Sp1 transcriptional activation through protein kinase C-beta I and -delta

Pathogenesis of Distal Symmetrical Polyneuropathy in Diabetes

Distal symmetrical polyneuropathy (DSPN) is a serious complication of diabetes associated with significant disability and mortality. Although more than 50% of people with diabetes develop DSPN, its pathogenesis is still relatively unknown. This lack of understanding has limited the development of novel disease-modifying therapies and left the reasons for failed therapies uncertain, which is critical given that current management strategies often fail to achieve long-term efficacy. In this article, the pathogenesis of DSPN is reviewed, covering pathogenic changes in the peripheral nervous system, microvasculature and central nervous system (CNS). Furthermore, the successes and limitations of current therapies are discussed, and potential therapeutic targets are proposed. Recent findings on its pathogenesis have called the definition of DSPN into question and transformed the disease model, paving the way for new research prospects.

Liposome-encapsulated glycyrrhizin alleviates hyperglycemia and glycation-induced iron-catalyzed oxidative reactions in streptozotocin-induced diabetic rats

Glycyrrhizin, a bioactive constituent of Glycyrrhiza glabra has been reported to ameliorate diabetes. Here, the effects of liposome-encapsulated glycyrrhizin on STZ-induced diabetes and associated oxidative stress were investigated. Wistar rats were grouped as control (NC, received placebo), diabetic (DC, STZ-induced), diabetic treated with free glycyrrhizin (DTG, 3 i.v. doses, 1.6 mg/0.5 ml), empty liposomes (DTl, 3 i.v. doses), and liposome-encapsulated glycyrrhizin (DTbd, 3 i.v. doses, 1.6 mg/0.5 ml). Serum glucose, insulin, intraperitoneal glucose tolerance test and glycohemoglobin were estimated. Free iron and iron-mediated oxidative stress were examined. Histological examinations of the kidney and liver were performed. Liposomal-glycyrrhizin treatment caused significant improvement of hyperglycemia (DC vs. DTbd p < .05), glucose intolerance (DC vs. DTG p < .01 and DC vs. DTbd p < .05), insulin (DC vs. DTG p < .1, DTbd vs. DC p < .05 and DTbd vs. DTG p < .1) and glycohemoglobin (DC vs. DTG p < .1 and DC vs. DTbd p < .05) levels in the DTbd group. Alleviation of free iron release (DC vs. DTbd p < .05), lipid peroxidation (DC + H₂O₂ vs. DTbd + H₂O₂ p < .05), deoxyribose (DC + H₂O₂ vs. DTbd + H₂O₂, p < .05), and DNA degradation occurred in the DTbd group. The abnormalities of the kidney and liver were abolished in the DTbd group. The inhibitory effects were more pronounced compared to free glycyrrhizin. Liposome-encapsulated glycyrrhizin treatment caused inhibition of diabetic complications through its antioxidant effects and can be exploited for effective treatment of diabetes.

GYY4137 Regulates Extracellular Matrix Turnover in the Diabetic Kidney by Modulating Retinoid X Receptor Signaling

Diabetic kidney is associated with an accumulation of extracellular matrix (ECM) leading to renal fibrosis. Dysregulation of retinoic acid metabolism involving retinoic acid receptors (RARs) and retinoid X receptors (RXRs) has been shown to play a crucial role in diabetic nephropathy (DN). Furthermore, RARs and peroxisome proliferator-activated receptor γ (PPARγ) are known to control the RXR-mediated transcriptional regulation of several target genes involved in DN. Recently, RAR and RXR have been shown to upregulate plasminogen activator inhibitor-1 (PAI-1), a major player involved in ECM accumulation and renal fibrosis during DN. Interestingly, hydrogen sulfide (H₂S) has been shown to ameliorate adverse renal remodeling in DN. We investigated the role of RXR signaling in the ECM turnover in diabetic kidney, and whether H₂S can mitigate ECM accumulation by modulating PPAR/RAR-mediated RXR signaling. We used wild-type (C57BL/6J), diabetic (C57BL/6-Ins2^Akita/J) mice and mouse mesangial cells (MCs) as experimental models. GYY4137 was used as a H₂S donor. Results showed that in diabetic kidney, the expression of PPARγ was decreased, whereas upregulations of RXRα, RXRβ, and RARγ1 expression were observed. The changes were associated with elevated PAI-1, MMP-9 and MMP-13. In addition, the expressions of collagen IV, fibronectin and laminin were increased, whereas elastin expression was decreased in the diabetic kidney. Excessive collagen deposition was observed predominantly in the peri-glomerular and glomerular regions of the diabetic kidney. Immunohistochemical localization revealed elevated expression of fibronectin and laminin in the glomeruli of the diabetic kidney. GYY4137 reversed the pathological changes. Similar results were observed in in vitro experiments. In conclusion, our data suggest that RXR signaling plays a significant role in ECM turnover, and GYY4137 modulates PPAR/RAR-mediated RXR signaling to ameliorate PAI-1-dependent adverse ECM turnover in DN.

NFAT5 directs hyperosmotic stress-induced fibrin deposition and macrophage infiltration via PAI-1 in endothelium

Although stress can significantly promote atherosclerosis, the underlying mechanisms are still not completely understood. Here we successfully unveiled that high salt-induced nuclear factor of activated T cells 5 (NFAT5) control the endothelial-dependent fibrinolytic activity and the inflammatory adhesion-related molecules expression through regulation of plasminogen activator inhibitor-1 (PAI-1). We first observed that high salt diets instigated the expression of NFAT5 and PAI-1 in the endothelium which brought about the fibrin deposition and macrophage infiltration in the atherosclerotic arteries of ApoE^-/- mice. Overexpression of NFAT5 increased PAI-1-mediated antifibrinolytic activity and activated inflammatory adhesion-related genes in endothelial cells. Knockdown of NFAT5 by siRNA inhibited the expression of PAI-1, antifibrinolytic and adhesive molecules. Moreover, chromatin immunoprecipitation assay demonstrated that high salt intake significantly promoted the binding of NFAT5 to PAI-1 promoter (TGGAATTATTT) in endothelial cells. Our study identified that NFAT5 has great potential to activate the PAI-1-mediated fibrinolytic dysfunction and inflammatory cell adhesion, thus promoting high salt-induced atherosclerosis disease.

The role of diabetes in the onset and development of endothelial dysfunction

The vascular endothelium performs many functions. It is a key regulator of vascular homeostasis, maintains a balance between vasodilation and vasoconstriction, inhibition and stimulation of smooth muscle cell migration and proliferation, fibrinolysis and thrombosis, and is involved to regulation of platelet adhesion and aggregation. Endothelial dysfunction (ED) plays the critical role in pathogenesis of diabetes mellitus (DM) vascular complications. The purpose of this review was to consider the mechanisms leading to the occurrence of ED in DM. The paper discusses current literature data concerning the role of hyperglycemia, oxidative stress, advanced glycation end products in endothelial alteration. A separate section is devoted to the particularities of the functioning of the antioxidant system and their significance in the development of ED in DM. The analysis of the literature allows to conclude that pathological activation of glucose utilization pathways causes damage of endothelial cells, which is accompanied by disorders of all their basic functions. Metabolic disorders in DM cause a pronounced imbalance of free radical processes and antioxidant defense, accompanied by oxidative stress of endotheliocytes, which contributes to the progression of ED and the development of vascular complications. Many aspects of multicomponent regulatory reactions in the pathogenesis of the development of ED in DM have not been sufficiently studied.

Phytochemicals: Target-Based Therapeutic Strategies for Diabetic Retinopathy

Background: A variety of causative factors are involved in the initiation of diabetic retinopathy (DR). Current antidiabetic therapies are expensive and not easily accessible by the public. Furthermore, the use of multiple synthetic drugs leads to severe side effects, which worsen the diabetic patient’s condition. Medicinal plants and their derived phytochemicals are considered safe and effective treatment and their consumption can reduce the DR risk. In this article, we discuss a variety of medicinal plants, and their noteworthy bio-active constituents, that will be utilized as target based therapeutic strategies for DR. Methods: A broad-spectrum study was conducted using published English works in various electronic databases including Science Direct, PubMed, Scopus, and Google Scholar. Results: Targeting the multiple pathological factors including ROS, AGEs formation, hexosamine flux, PARP, PKC, and MAPK activation through variety of bioactive constituents in medicinal plants, diabetes progression can be delayed with improved loss of vision. Conclusions: Data reveals that traditional herbs and their prominent bioactive components control and normalize pathological cellular factors involved in DR progression. Therefore, studies should be carried out to explore the protective retinopathy effects of medicinal plants using experimental animal and humans models.

Oxidative toxicity in diabetes and Alzheimer's disease: mechanisms behind ROS/ RNS generation

Reactive oxidative species (ROS) toxicity remains an undisputed cause and link between Alzheimer’s disease (AD) and Type-2 Diabetes Mellitus (T2DM). Patients with both AD and T2DM have damaged, oxidized DNA, RNA, protein and lipid products that can be used as possible disease progression markers. Although the oxidative stress has been anticipated as a main cause in promoting both AD and T2DM, multiple pathways could be involved in ROS production. The focus of this review is to summarize the mechanisms involved in ROS production and their possible association with AD and T2DM pathogenesis and progression. We have also highlighted the role of current treatments that can be linked with reduced oxidative stress and damage in AD and T2DM.

Damaging effects of hyperglycemia on cardiovascular function: spotlight on glucose metabolic pathways

The incidence of cardiovascular complications associated with hyperglycemia is a growing global health problem. This review discusses the link between hyperglycemia and cardiovascular diseases onset, focusing on the role of recently emerging downstream mediators, namely, oxidative stress and glucose metabolic pathway perturbations. The role of hyperglycemia-mediated activation of nonoxidative glucose pathways (NOGPs) [i.e., the polyol pathway, hexosamine biosynthetic pathway, advanced glycation end products (AGEs), and protein kinase C] in this process is extensively reviewed. The proposal is made that there is a unique interplay between NOGPs and a downstream convergence of detrimental effects that especially affect cardiac endothelial cells, thereby contributing to contractile dysfunction. In this process the AGE pathway emerges as a crucial mediator of hyperglycemia-mediated detrimental effects. In addition, a vicious metabolic cycle is established whereby hyperglycemia-induced NOGPs further fuel their own activation by generating even more oxidative stress, thereby exacerbating damaging effects on cardiac function. Thus NOGP inhibition, and particularly that of the AGE pathway, emerges as a novel therapeutic intervention for the treatment of cardiovascular complications such as acute myocardial infarction in the presence hyperglycemia.

Diabetic macular edema: new concepts in patho-physiology and treatment

Diabetic macular edema (DME), a serious eye complication caused primarily by hyperglycemia, is one of the major causes of blindness. DME, which is characterized by cystic retinal thickening or lipid deposition, is prone to relapse after successful treatment. DME is a complex pathological process caused by multiple factors, including breakdown of the inner and outer blood-retinal barriers, oxidative stress, and elevated levels of vascular endothelial growth factor which have been demonstrated in both preclinical and clinical studies. Starling's law theory explains many of the features of DME. Early detection and treatment of DME can prevent vision loss. Current effective interventions for DME include treatment of systemic risk factors, such as elevated blood glucose, blood pressure and dyslipidemia. Ophthalmic treatments include laser photocoagulation, surgery and intraocular pharmacotherapy. New drugs, which are given by intraocular injection, have emerged in recent years to become first line treatment for DME that affects the central macula with loss of vision. Laser photocoagulation is still the gold standard of treatment for DME which does not involve the central macular. This review outlines these new treatments with particular emphasis on the optimal timing of how they are given.

O-GlcNAcylation and Inflammation: A Vast Territory to Explore

O-GlcNAcylation is a reversible post-translational modification that regulates the activities of cytosolic and nuclear proteins according to glucose availability. This modification appears to participate in several hyperglycemia-associated complications. An important feature of metabolic diseases such as diabetes and obesity is the presence of a low-grade chronic inflammation that causes numerous complications. Hyperglycemia associated with the metabolic syndrome is known to promote inflammatory processes through different mechanisms including oxidative stress and abnormally elevated protein O-GlcNAcylation. However, the role of O-GlcNAcylation on inflammation remains contradictory. O-GlcNAcylation associated with hyperglycemia has been shown to increase nuclear factor κB (NFκB) transcriptional activity through different mechanisms. This could contribute in inflammation-associated diabetic complications. However, in other conditions such as acute vascular injury, O-linked N-acetyl glucosamine (O-GlcNAc) also exerts anti-inflammatory effects via inhibition of the NFκB pathway, suggesting a complex regulation of inflammation by O-GlcNAc. Moreover, whereas macrophages and monocytes exposed to high glucose for a long-term period developed a pro-inflammatory phenotype, the impact of O-GlcNAcylation in these cells remains unclear. A future challenge will be to clearly establish the role of O-GlcNAcylation in pro- and anti-inflammatory functions in macrophages.

Computational and experimental analysis identified 6-diazo-5-oxonorleucine as a potential agent for treating infection by Plasmodium falciparum

Plasmodium falciparum (PF) is the most severe malaria parasite. It is developing resistance quickly to existing drugs making it indispensable to discover new drugs. Effective drugs have been discovered targeting metabolic enzymes of the parasite. In order to predict new drug targets, computational methods can be used employing database information of metabolism. Using this data, we performed recently a computational network analysis of metabolism of PF. We analyzed the topology of the network to find reactions which are sensitive against perturbations, i.e., when a single enzyme is blocked by drugs. We now used a refined network comprising also the host enzymes which led to a refined set of the five targets glutamyl-tRNA (gln) amidotransferase, hydroxyethylthiazole kinase, deoxyribose-phophate aldolase, pseudouridylate synthase, and deoxyhypusine synthase. It was shown elsewhere that glutamyl-tRNA (gln) amidotransferase of other microorganisms can be inhibited by 6-diazo-5-oxonorleucine. Performing a half maximal inhibitory concentration (IC50) assay, we showed, that 6-diazo-5-oxonorleucine is also severely affecting viability of PF in blood plasma of the human host. We confirmed this by an in vivo study observing Plasmodium berghei infected mice.

NGF upregulates the plasminogen activation inhibitor-1 in neurons via the calcineurin/NFAT pathway and the Down syndrome-related proteins DYRK1A and RCAN1 attenuate this effect

BACKGROUND

Plasminogen activator inhibitor 1 (PAI-1) is a key regulator of the plasminogen activation system. Although several lines of evidence support a significant role of PAI-1 in the brain, the regulation of its expression in neurons is poorly understood. In the present study we tested the hypothesis that NGF induces the upregulation of PAI-1 via the calcineurin/nuclear factor of activated T cells (NFAT) pathway and analysed whether the overexpression of the Down syndrome-related proteins DYRK1A and RCAN1 modulated the effect of NGF on PAI-1 expression.

RESULTS

NGF upregulated PAI-1 mRNA levels in primary mouse hippocampal neurons cultured for 3 days in vitro and in the rat pheochromocytoma cell line PC12. Reporter gene assays revealed that NGF activated the calcineurin/NFAT pathway in PC12 cells. Induction of PAI-1 by NGF was sensitive to the calcineurin inhibitor FK506 and the specific inhibition of NFAT activation by the cell permeable VIVIT peptide. Activation of calcineurin/NFAT signalling through other stimuli resulted in a much weaker induction of PAI-1 expression, suggesting that other NGF-induced pathways are involved in PAI-1 upregulation. Overexpression of either DYRK1A or RCAN1 negatively regulated NFAT-dependent transcriptional activity and reduced the upregulation of PAI-1 levels by NGF.

CONCLUSION

The present results show that the calcineurin/NFAT pathway mediates the upregulation of PAI-1 by NGF. The negative effect of DYRK1A and RCAN1 overexpression on NGF signal transduction in neural cells may contribute to the altered neurodevelopment and brain function in Down syndrome.

Emerging therapy for diabetic neuropathy: cell therapy targeting vessels and nerves

Diabetic neuropathy (DN), the most common complication of diabetes, frequently leads to foot ulcers and may progress to limb amputations. Despite continuous increase in incidence, there is no clinical therapy to effectively treat DN. Pathogenetically, DN is characterized by reduced vascularity in peripheral nerves and deficiency in angiogenic and neurotrophic factors. We will briefly review the pathogenetic mechanism of DN and address the effects and the mechanisms of cell therapies for DN. To reverse the changes of DN, studies have attempted to deliver neurotrophic or angiogenic factors for treatment in the form of protein or gene therapy; however, the effects turned out to be very modest if not ineffective. Recent studies have demonstrated that bone marrow (BM)-derived cells such as mononuclear cells or endothelial progenitor cells (EPCs) can effectively treat various cardiovascular diseases through their paracrine effects. As BM-derived cells include multiple angiogenic and neurotrophic cytokines, these cells were used for treating experimental DN and found to reverse manifestations of DN. Particularly, EPCs were shown to exert favorable therapeutic effects through enhanced neural neovascularization and neuro-protective effects. These findings clearly indicate that DN is a complex disorder with pathogenetic involvement of both vascular and neural components. Studies have shown that cell therapies targeting both vascular and neural elements are shown to be advantageous in treating 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.

Preventive effects of North American ginseng (Panax quinquefolium) on diabetic nephropathy

PURPOSE

Ginseng has been used as an herbal medicine and nutritional supplement in East Asia for thousands of years and gained popularity in the west because of its various pharmacological properties. Panax ginseng (Asian ginseng) and Panax quinquefolium (North American ginseng) both are reported to possess antihyperglycemic properties. The aim of the present study is to evaluate the preventive effects of North American ginseng on diabetic nephropathy (DN) and the underlying mechanisms of such effects.

METHODS

Models of both type 1 (C57BL/6 mice with STZ-induced diabetes) and type 2 diabetes (db/db mice) and age- and sex-matched controls were examined. Alcoholic ginseng root (200mg/kgbodywt, daily oral gavage) extract was administered to the diabetic mice (type 1 and type 2) for two or four months in order to evaluate its effects on DN.

RESULTS

Dysmetabolic state in the diabetic mice was significantly improved by ginseng treatment. In the kidneys of diabetic animals, ginseng significantly prevented oxidative stress and reduced the NF-κB (p65) levels. Diabetes-induced up-regulations of ECM proteins and vasoactive factors in the kidneys were significantly diminished by ginseng administration. Furthermore, albuminuria and mesangial expansion in the diabetic mice were prevented by ginseng therapy.

CONCLUSION

North American ginseng has preventive effects on DN and it works through a combination of mechanisms such as antihyperglycemic and antioxidant activities.

Cell oxidant stress delivery and cell dysfunction onset in type 2 diabetes

Most known pathways of diabetic complications involve oxidative stress. The mitochondria electron transport chain is a significant source of reactive oxygen species (ROS) in insulin secretory cells, insulin peripheral sensitive cells and endothelial cells. Elevated intracellular glucose level induces tricarboxylic acid cycle electron donor overproduction and mitochondrial proton gradient increase leading to an increase in electron transporter lifetime. Subsequently, the electrons leaked combine with respiratory oxygen (O(2)) resulting in superoxide anion ((•)O(2)(-)) production. Advanced glycation end products derive ROS via interaction with their receptors. Elevated diacylglycerol and ROS activate the protein kinase C pathway which, in turn, activates NADPH oxidases. A vicious circle of pathway derived ROS installs. Pathologic pathways induced ROS are activated and persistent though glycemia returns to normal due to hyperglycemia memory. Endothelial nitric oxide synthase may produce both superoxide anion ((•)O(2)(-)) and nitric oxide (NO) leading to peroxynitrite ((•)ONOO(-)) generation. Homocysteine is also implicated in oxidative stress pathogenesis. In this paper we have highlighted the pathologic mechanisms of ROS on atherosclerosis, renal dysfunction, retina dysfunction and nerve dysfunction in type 2 diabetes. Cell oxidant stress delivery have pivotal role in cell dysfunction onset and progression of angiopathies but an early introduction of good glycemic control may protect cells more efficiently than antioxidants.

O-linked β-N-acetylglucosamine supports p38 MAPK activation by high glucose in glomerular mesangial cells

Hyperglycemia augments flux through the hexosamine biosynthetic pathway and subsequent O-linkage of single β-N-acetyl-d-glucosamine moieties to serine and threonine residues on cytoplasmic and nuclear proteins (O-GlcNAcylation). Perturbations in this posttranslational modification have been proposed to promote glomerular matrix accumulation in diabetic nephropathy, but clear evidence and mechanism are lacking. We tested the hypothesis that O-GlcNAcylation enhances profibrotic signaling in rat mesangial cells. An adenovirus expressing shRNA directed against O-GlcNAc transferase (OGT) markedly reduced basal and high-glucose-stimulated O-GlcNAcylation. Interestingly, O-GlcNAc depletion prevented high-glucose-induced p38 mitogen-activated protein kinase (MAPK) and c-Jun NH(2)-terminal kinase phosphorylation. Downstream of p38, O-GlcNAc controlled the expression of plasminogen activator inhibitor-1, fibronectin, and transforming growth factor-β, important factors in matrix accumulation in diabetic nephropathy. Treating mesangial cells with thiamet-G, a highly selective inhibitor of O-GlcNAc-specific hexosaminidase (O-GlcNAcase), increased O-GlcNAcylation and p38 phosphorylation. The high-glucose-stimulated kinase activity of apoptosis signal-regulating kinase 1 (ASK1), an upstream MAPK kinase kinase for p38 that is negatively regulated by Akt, was inhibited by OGT shRNA. Akt Thr(308) and Ser(473) phosphorylation were enhanced following OGT shRNA expression in high-glucose-exposed mesangial cells, but high-glucose-induced p38 phosphorylation was not attenuated by OGT shRNA in cells pretreated with the phosphatidylinositol 3-kinase inhibitor LY-294002. OGT shRNA also reduced high-glucose-stimulated reactive oxygen species (ROS) formation. In contrast, diminished O-GlcNAcylation caused elevated ERK phosphorylation and PKCδ membrane translocation. Thus, O-GlcNAcylation is coupled to profibrotic p38 MAPK signaling by high glucose in part through Akt and possibly through ROS.

Expression profiles of podocytes exposed to high glucose reveal new insights into early diabetic glomerulopathy

Podocyte injury has been suggested to have a pivotal role in the pathogenesis of diabetic glomerulopathy. To glean insights into molecular mechanisms underlying diabetic podocyte injury, we generated temporal global gene transcript profiles of podocytes exposed to high glucose for a time interval of 1 or 2 weeks using microarrays. A number of genes were altered at both 1 and 2 weeks of glucose exposure compared with controls grown under normal glucose. These included extracellular matrix modulators, cell cycle regulators, extracellular transduction signals and membrane transport proteins. Novel genes that were altered at both 1 and 2 weeks of high-glucose exposure included neutrophil gelatinase-associated lipocalin (LCN2 or NGAL, decreased by 3.2-fold at 1 week and by 7.2-fold at 2 weeks), endothelial lipase (EL, increased by 3.6-fold at 1 week and 3.9-fold at 2 week) and UDP-glucuronosyltransferase 8 (UGT8, increased by 3.9-fold at 1 week and 5.0-fold at 2 weeks). To further validate these results, we used real-time PCR from independent podocyte cultures, immunohistochemistry in renal biopsies and immunoblotting on urine specimens from diabetic patients. A more detailed time course revealed changes in LCN2 and EL mRNA levels as early as 6 hours and in UGT8 mRNA level at 12 hours post high-glucose exposure. EL immunohistochemistry on human tissues showed markedly increased expression in glomeruli, and immunoblotting readily detected EL in a subset of urine samples from diabetic nephropathy patients. In addition to previously implicated roles of these genes in ischemic or oxidative stress, our results further support their importance in hyperglycemic podocyte stress and possibly diabetic glomerulopathy pathogenesis and diagnosis in humans.

Stress-induced expression of the p75 neurotrophin receptor is regulated by O-GlcNAcylation of the Sp1 transcription factor

Injury-induced expression of p75 neurotrophin receptor (p75NTR) in the CNS induces neuronal apoptosis and prevents neuronal regrowth. The mechanisms regulating injury-induced p75NTR expression are poorly characterized but previous studies have established that reductions in extracellular osmolarity which mimic cytotoxic edema induce p75NTR gene expression through pathways that activate the Sp1 transcription factor. In this report, we examined how extracellular osmolarity converges on Sp1 to regulate p75NTR expression. We report that levels of O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT), the enzyme that mediates O-linked attachment of GlcNAc, are reduced by extracellular hypo-osmolarity and that global levels of protein O-GlcNAcylation and of Sp1 show a corresponding decline. We demonstrate that chemical and RNAi-based treatments that reduce cellular O-GlcNAcylation facilitate p75NTR induction by hypo-osmolarity, directly linking protein O-GlcNAcylation to p75NTR induction. To determine if Sp1 O-GlcNAc content regulates p75NTR expression, we replaced endogenous Sp1 with a Sp1 mutated at O-GlcNAc target residues. This O-GlcNAc-deficient form of Sp1-enhanced p75NTR expression, demonstrating that O-GlcNAcylation of Sp1 negatively regulates p75NTR expression. We conclude that a stress-induced decline in the O-GlcNAc content of Sp1 drives expression of p75NTR.

Protein kinase C upregulates intercellular adhesion molecule-1 and leukocyte-endothelium interactions in hyperglycemia via activation of endothelial expressed calpain

OBJECTIVE

We tested the hypothesis of a role for the calcium-dependent protease calpain in the endothelial dysfunction induced by hyperglycemic activation of protein kinase C (PKC).

METHODS AND RESULTS

Chronic hyperglycemia with insulin deficiency (type 1 diabetes) was induced in rats by streptozotocin. Total PKC and calpain activities, along with activity and expression level of the 2 endothelial-expressed calpains isoforms, μ- and m-calpain, were measured in vascular tissue homogenates by enzymatic assays and Western blot analysis, respectively. Intravital microscopy was used to measure and correlate leukocyte-endothelium interactions with calpain activity in the microcirculation. Expression levels and endothelial localization of the inflammatory adhesion molecule intercellular adhesion molecule-1 were studied by Western blot analysis and immunofluorescence, respectively. The mechanistic role of hyperglycemia alone in the process of PKC-induced calpain activation and actions was also investigated. We found that in the type 1 diabetic vasculature, PKC selectively upregulates the activity of the μ-calpain isoform. Mechanistic studies confirmed a role for hyperglycemia and PKCβ in this process. The functional implications of PKC-induced calpain activation were upregulation of endothelial expressed intercellular adhesion molecule-1 and leukocyte-endothelium interactions.

CONCLUSIONS

Our results uncover the role of μ-calpain in the endothelial dysfunction of PKC. Calpain may represent a novel molecular target for the treatment of PKC-associated diabetic vascular disease.

O-GlcNAc modification, insulin signaling and diabetic complications

O-GlcNAc glycosylation (O-GlcNAcylation) corresponds to the addition of N-acetylglucosamine on serine and threonine residues of cytosolic and nuclear proteins. O-GlcNAcylation is a dynamic post-translational modification, analogous to phosphorylation, that regulates the stability, the activity or the subcellular localisation of target proteins. This reversible modification depends on the availability of glucose and therefore constitutes a powerful mechanism by which cellular activities are regulated according to the nutritional environment of the cell. O-GlcNAcylation has been implicated in important human pathologies including Alzheimer disease and type-2 diabetes. Only two enzymes, OGT and O-GlcNAcase, control the O-GlcNAc level on proteins. Therefore, O-GlcNAcylations cannot organize in signaling cascades as observed for phosphorylations. O-GlcNAcylations should rather be considered as a "rheostat" that controls the intensity of the signals traveling through different pathways according to the nutritional status of the cell. Thus, OGT attenuates insulin signal by O-GlcNAcylation of proteins involved in proximal and distal steps in the PI-3 kinase signaling pathway. This negative feedback may be exacerbated when cells are chronically exposed to elevated glucose concentrations and could thereby contribute to alterations in insulin signaling observed in diabetic patients. O-GlcNAcylation also appears to contribute to the deleterious effects of hyperglycaemia on excessive glucose production by the liver and deterioration of β-cell pancreatic function, resulting in worsening of hyperglycaemia (glucotoxicity). Moreover, O-GlcNAcylations directly participate in several diabetic complications. O-GlcNAcylation of eNOS in endothelial cells have been involved in micro- and macrovascular complications. In addition, O-GlcNAcylations activate the expression of profibrotic and antifibrinolytic factors, contributing to vascular and renal dysfunctions.

Transforming growth factor-β-inducible early response gene 1 is a novel substrate for atypical protein kinase Cs

The protein kinase C (PKC) family of serine/threonine kinases consists of ten different isoforms grouped into three subfamilies, denoted classical, novel and atypical PKCs (aPKCs). The aPKCs, PKCι/λ and PKCζ serve important roles during development and in processes subverted in cancer such as cell and tissue polarity, cell proliferation, differentiation and apoptosis. In an effort to identify novel interaction partners for aPKCs, we performed a yeast two-hybrid screen with the regulatory domain of PKCι/λ as bait and identified the Krüppel-like factors family protein TIEG1 as a putative interaction partner for PKCι/λ. We confirmed the interaction of both aPKCs with TIEG1 in vitro and in cells, and found that both aPKCs phosphorylate the DNA-binding domain of TIEG1 on two critical residues. Interestingly, the aPKC-mediated phosphorylation of TIEG1 affected its DNA-binding activity, subnuclear localization and transactivation potential.

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.

O-linked N-acetylglucosaminylation of Sp1 inhibits the human immunodeficiency virus type 1 promoter

Human immunodeficiency virus type 1 (HIV-1) gene expression and replication are regulated by the promoter/enhancer located in the U3 region of the proviral 5' long terminal repeat (LTR). The binding of cellular transcription factors to specific regulatory sites in the 5' LTR is a key event in the replication cycle of HIV-1. Since transcriptional activity is regulated by the posttranslational modification of transcription factors with the monosaccharide O-linked N-acetyl-D-glucosamine (O-GlcNAc), we evaluated whether increased O-GlcNAcylation affects HIV-1 transcription. In the present study we demonstrate that treatment of HIV-1-infected lymphocytes with the O-GlcNAcylation-enhancing agent glucosamine (GlcN) repressed viral transcription in a dose-dependent manner. Overexpression of O-GlcNAc transferase (OGT), the sole known enzyme catalyzing the addition of O-GlcNAc to proteins, specifically inhibited the activity of the HIV-1 LTR promoter in different T-cell lines and in primary CD4(+) T lymphocytes. Inhibition of HIV-1 LTR activity in infected T cells was most efficient (>95%) when OGT was recombinantly overexpressed prior to infection. O-GlcNAcylation of the transcription factor Sp1 and the presence of Sp1-binding sites in the LTR were found to be crucial for this inhibitory effect. From this study, we conclude that O-GlcNAcylation of Sp1 inhibits the activity of the HIV-1 LTR promoter. Modulation of Sp1 O-GlcNAcylation may play a role in the regulation of HIV-1 latency and activation and links viral replication to the glucose metabolism of the host cell. Hence, the establishment of a metabolic treatment might supplement the repertoire of antiretroviral therapies against AIDS.

Protein O-GlcNAcylation: a new signaling paradigm for the cardiovascular system

The posttranslational modification of serine and threonine residues of nuclear and cytoplasmic proteins by the O-linked attachment of the monosaccharide beta-N-acetylglucosamine (O-GlcNAc) is a highly dynamic and ubiquitous protein modification. Protein O-GlcNAcylation is rapidly emerging as a key regulator of critical biological processes including nuclear transport, translation and transcription, signal transduction, cytoskeletal reorganization, proteasomal degradation, and apoptosis. Increased levels of O-GlcNAc have been implicated as a pathogenic contributor to glucose toxicity and insulin resistance, which are both major hallmarks of diabetes mellitus and diabetes-related cardiovascular complications. Conversely, there is a growing body of data demonstrating that the acute activation of O-GlcNAc levels is an endogenous stress response designed to enhance cell survival. Reports on the effect of altered O-GlcNAc levels on the heart and cardiovascular system have been growing rapidly over the past few years and have implicated a role for O-GlcNAc in contributing to the adverse effects of diabetes on cardiovascular function as well as mediating the response to ischemic injury. Here, we summarize our present understanding of protein O-GlcNAcylation and its effect on the regulation of cardiovascular function. We examine the pathways regulating protein O-GlcNAcylation and discuss, in more detail, our understanding of the role of O-GlcNAc in both mediating the adverse effects of diabetes as well as its role in mediating cellular protective mechanisms in the cardiovascular system. In addition, we also explore the parallels between O-GlcNAc signaling and redox signaling, as an alternative paradigm for understanding the role of O-GlcNAcylation in regulating cell function.

Enhanced activation of a "nutrient-sensing" pathway with age contributes to insulin resistance

Calorie restriction improves life span whereas nutrient excess leads to obesity and unfavorable metabolic consequences, supporting the role for a cellular "nutrient sensor" in aging. Hexosamine biosynthetic pathway (HBP) is a candidate nutrient-sensing pathway. We hypothesized that altered nutrient sensing (by HBP) with age may provide a link among aging, nutrient flux, and insulin resistance. Using a hyperinsulinemic clamp in young rats, we show that experimental activation of HBP, through the systemic infusion of glucosamine, induced severe insulin resistance (36% decline in peripheral insulin action; P<0.05), increased adipose tissue gene expression of fat-derived peptides (PAI-1 by 4-fold, angiotensinogen 3-fold, leptin 2-fold, resistin 4-fold, and adiponectin 4-fold; P<0.01 compared with young saline-infused), and enhanced glycosylation of transcription factors, thus mimicking a physiological and biological phenotype of aging. We further demonstrate a greater activation of nutrient-sensing HBP with age in both old ad libitum-fed and calorie-restricted rats. Interestingly, old calorie-restricted animals rapidly develop insulin resistance when exposed to glucosamine, despite their "young" phenotype. These results suggest that altered nutrient sensing by HBP with age may be the link among nutrients, insulin resistance, and age-related diabetes.

Chronic hyperglicemia and nitric oxide bioavailability play a pivotal role in pro-atherogenic vascular modifications

Diabetes is associated with accelerated atherosclerosis and macrovascular complications are a major cause of morbidity and mortality in this disease. Although our understanding of vascular pathology has lately greatly improved, the mechanism(s) underlying enhanced atherosclerosis in diabetes remain unclear. Endothelial cell dysfunction is emerging as a key component in the pathophysiology of cardiovascular abnormalities associated with diabetes. Although it has been established that endothelium plays a critical role in overall homeostasis of the vessels, vascular smooth muscle cells (vSMC) in the arterial intima have a relevant part in the development of atherosclerosis in diabetes. However, high glucose induced alterations in vSMC behaviour are not fully characterized. Several studies have reported that impaired nitric oxide (NO) synthesis and/or actions are often present in diabetes and endothelial dysfunction. Furthermore, although endothelial cells are by far the main site of vascular NO synthesis, vSMC do express nitric oxyde synthases (NOSs) and NO synthesis in vSMC might be important in vessel's function. Although it is known that vSMC contribute to vascular pathology in diabetes by their change from a quiescent state to an activated proliferative and migratory phenotype (termed phenotypic modulation), whether this altered phenotypic modulation might also involve alterations in the nitrergic systems is still controversial. Our recent data indicate that, in vivo, chronic hyperglycemia might induce an increased number of vSMC proliferative clones which persist in culture and are associated with increased eNOS expression and activity. However, upregulation of eNOS and increased NO synthesis occur in the presence of a marked concomitant increase of O(2-) production. Since NO bioavailabilty might not be increased in high glucose stimulated vSMC, it is tempting to hypothesize that the proliferative phenotype observed in cells from diabetic rats is associated with a redox imbalance responsible quenching and/or trapping of NO, with the consequent loss of its biological activity. This might provide new insight on the mechanisms responsible for accelerated atherosclerosis in diabetes.

Collagen I induction by high glucose levels is mediated by epidermal growth factor receptor and phosphoinositide 3-kinase/Akt signalling in mesangial cells

AIMS/HYPOTHESIS

Glomerular matrix accumulation is a hallmark of diabetic nephropathy. Recent data have linked the serine/threonine kinase protein kinase B (Akt) to matrix modulation. Here, we studied its role in high glucose-induced collagen elaboration by mesangial cells.

METHODS

Primary rat mesangial cells were treated with high glucose levels (30 mmol/l) or mannitol as osmotic control. Western blots, northern blots, ELISA and immunohistochemistry were used for assessment. Diabetes was induced in rats by streptozotocin.

RESULTS

Phosphorylated Akt at S473 (pAktS473), corresponding to Akt activation, was seen in diabetic glomeruli. In mesangial cells, high glucose levels induced pAktS473 by 20 min. This was sustained to 72 h, while mannitol had no effect. Akt activation by kinase assay and phosphorylation on threonine 308 was also observed. Phosphoinositide 3-kinase (PI3K) inhibitors LY294002 (20 micromol/l) and wortmannin (100 nmol/l) prevented pAktS473. Collagen IA1 transcript and collagen I protein upregulation by high glucose levels were inhibited by PI3K blockade, as was collagen I secretion into the medium (ELISA). Dominant-negative Akt overexpression also inhibited high glucose-induced collagen IA1 transcript and collagen I protein production. Since signalling through the epidermal growth factor receptor (EGFR) can activate PI3K-Akt, we studied its activation by high glucose levels. EGFR was correspondingly activated by 10 min; mannitol had no effect. EGFR activation was also seen in glomeruli from diabetic rats and co-localised with collagen IA1 in diabetic glomeruli. Specific EGFR inhibition (AG1478, 5 micromol/l or dominant-negative EGFR) blocked high glucose-induced pAktS473, phosphorylation on threonine 308 and activation of the EGFR downstream target p44 extracellular signal-regulated kinase (Erk) mitogen-activated protein kinase. Finally, EGFR inhibition also blocked high glucose-induced collagen I upregulation at transcriptional and protein levels.

CONCLUSIONS/INTERPRETATION

We conclude that EGFR-PI3K-Akt signalling mediates high glucose-induced collagen I upregulation in mesangial cells and that this pathway is activated in diabetic glomeruli. Targeting its components may provide a new therapeutic approach to diabetic kidney disease.

Effects of long-term elevated glucose on collagen formation by mesangial cells

Glomerulosclerosis is one of the complications of diabetes that occurs after many years of uncontrolled hyperglycemia. Mesangial cells (MCs) exposed to high glucose (HG) for short periods have shown that transforming growth factor-beta (TGF-beta) and activated diacylglycerol-dependent protein kinase C (PKC) mediate increased collagen formation. Our study examined collagen formation by MCs exposed to HG for 8 weeks. Exposure to HG in overnight culture resulted in the activation of all PKC isoforms. In contrast, 8-week exposure to HG resulted in the persistent activation of PKC-delta, did not change PKC-alpha or -beta activity, and decreased PKC-epsilon activity while increasing collagen I and IV gene and protein expression. Collagen IV accumulation was reversed by specific PKC-delta inhibition. Collagen IV gene expression was completely normalized by TGF-beta neutralization; however, this was associated with plasminogen activator inhibitor-1 (PAI-1) overexpression and a modest reduction in collagen protein. Our studies suggest that prolonged exposure to HG results in PKC-delta-driven collagen accumulation by MCs mediated by PAI-1 but independent of TGF-beta.

Diabetes associated cell stress and dysfunction: role of mitochondrial and non-mitochondrial ROS production and activity

It is now widely accepted, given the current weight of experimental evidence, that reactive oxygen species (ROS) contribute to cell and tissue dysfunction and damage caused by glucolipotoxicity in diabetes. The source of ROS in the insulin secreting pancreatic beta-cells and in the cells which are targets for insulin action has been considered to be the mitochondrial electron transport chain. While this source is undoubtably important, we provide additional information and evidence for NADPH oxidase-dependent generation of ROS both in pancreatic beta-cells and in insulin sensitive cells. While mitochondrial ROS generation may be important for regulation of mitochondrial uncoupling protein (UCP) activity and thus disruption of cellular energy metabolism, the NADPH oxidase associated ROS may alter parameters of signal transduction, insulin secretion, insulin action and cell proliferation or cell death. Thus NADPH oxidase may be a useful target for intervention strategies based on reversing the negative impact of glucolipotoxicity in diabetes.

Hyperglycemia in acute ischemic stroke: a vascular perspective

Admission hyperglycemia complicates approximately one-third of acute ischemic strokes and is associated with a worse clinical outcome. Both human and animal studies have showed that hyperglycemia is particularly detrimental in ischemia/reperfusion. Decreased reperfusion blood flow has been observed after middle cerebral artery occlusion in acutely hyperglycemic animals, suggesting the vasculature as an important site of hyperglycemic reperfusion injury. This paper reviews biochemical and molecular pathways in the vasculature that are rapidly affected by hyperglycemia and concludes that these changes result in a pro-vasoconstrictive, pro-thrombotic and pro-inflammatory phenotype that renders the vasculature vulnerable to reperfusion injury. Understanding these pathways should lead to the development of rational therapies that reduce hyperglycemic reperfusion injury and thus improve outcome in this large subset of acute ischemic stroke patients.

Different metabolic correlations of thrombin-activatable fibrinolysis inhibitor and plasminogen activator inhibitor-1 in non-obese type 2 diabetic patients

We investigated the plasma levels of thrombin-activatable fibrinolysis inhibitor (TAFI), plasminogen activator inhibitor-1 (PAI-1) and their relation with clinical and metabolic parameters in non-obese type 2 diabetic patients. The plasma levels of TAFI and PAI-1 were evaluated in 47 non-obese type 2 diabetic patients and 31 normal subjects. The intra-abdominal visceral and subcutaneous fat areas were measured by computed tomography (CT). The degree of insulin resistance was evaluated by the euglycemic-hyperinsulinemic clamp technique using artificial pancreas. The plasma levels of TAFI (169.0+/-108.8% versus 103.7+/-52.3%; p<0.001, mean+/-S.D.) and PAI-1 (82.7+/-54.5ng/ml versus 52.9+/-51.7ng/ml; p<0.05) were significantly higher in non-obese type 2 diabetic patients than in normal subjects. Univariate analysis showed that the plasma TAFI levels are significantly and inversely correlated with the glucose infusion rate (GIR) (r=-0.42, p<0.005) in all diabetic patients. Moreover, the plasma levels of TAFI were significantly correlated with fasting plasma glucose levels (r=0.47, p<0.001) and HbA(1c) (r=0.38, p<0.005) in all subjects. The plasma levels of PAI-1 were significantly and proportionally correlated with the visceral fat area (r=0.42, p<0.005) and body mass index (r=0.33, p<0.05). There was no significant correlation between plasma levels of TAFI and PAI-1 (r=0.04). These results show that the plasma levels of TAFI and PAI-1 differently correlate with insulin resistance and visceral fat accumulation, suggesting that different factors are implicated in the plasma elevation of TAFI and PAI-1 in non-obese type 2 diabetes mellitus.

Cell signaling, the essential role of O-GlcNAc!

An increasing body of evidence points to a central regulatory role for glucose in mediating cellular processes and expands the role of glucose well beyond its traditional role(s) in energy metabolism. Recently, it has been recognized that one downstream effector produced from glucose is UDP-GlcNAc. Levels of UDP-GlcNAc, and the subsequent addition of O-linked beta-N-acetylglucosamine (O-GlcNAc) to Ser/Thr residues, is involved in regulating nuclear and cytoplasmic proteins in a manner analogous to protein phosphorylation. O-GlcNAc protein modification is essential for life in mammalian cells, highlighting the importance of this simple post-translational modification in basic cellular regulation. Recent research has highlighted key roles for O-GlcNAc serving as a nutrient sensor in regulating insulin signaling, the cell cycle, and calcium handling, as well as the cellular stress response.

Glucose induces increases in levels of the transcriptional repressor Id2 via the hexosamine pathway

Changes in glucose levels are known to directly alter gene expression. A number of previous studies have found that these effects are in part mediated by modulating the levels and the activity of transcription factors. We have investigated an alternative mechanism by which glucose might regulate gene expression by modulating levels of a transcriptional repressor. We have focused on Id2, which is a protein that indirectly regulates gene expression by sequestering certain transcription factors and preventing them from forming functional dimers. Id2 targets include the class A basic helix-loop-helix transcription factors and the sterol regulatory element-binding protein (SREBP)-1. We demonstrate that increases in glucose levels cause a rapid increase in levels of Id2 in J774.2 macrophages, and a number of lines of evidence indicate that this is via the hexosamine pathway because 1) the effect of glucose requires glutamine; 2) the effect of glucose is mimicked by low levels of glucosamine; 3) the effect of glucose is inhibited by azaserine, an inhibitor of glutamine:fructose-6-phosphate amidotransferase (GFAT); and 4) adenoviral mediated overexpression of GFAT increases levels of Id2. We go on to show that increases in Id2 can have functional effects on metabolic genes, because Id2 blocked the SREBP-1-induced induction of hormone-sensitive lipase (HSL) promoter activity, whereas Id2 alone does not modulate activity of the HSL promoter. In summary, these studies define a new mechanism by which glucose uses the hexosamine pathway to regulate gene expression by increasing levels of a transcriptional repressor.

Diabetes and mitochondrial function: role of hyperglycemia and oxidative stress

Hyperglycemia resulting from uncontrolled glucose regulation is widely recognized as the causal link between diabetes and diabetic complications. Four major molecular mechanisms have been implicated in hyperglycemia-induced tissue damage: activation of protein kinase C (PKC) isoforms via de novo synthesis of the lipid second messenger diacylglycerol (DAG), increased hexosamine pathway flux, increased advanced glycation end product (AGE) formation, and increased polyol pathway flux. Hyperglycemia-induced overproduction of superoxide is the causal link between high glucose and the pathways responsible for hyperglycemic damage. In fact, diabetes is typically accompanied by increased production of free radicals and/or impaired antioxidant defense capabilities, indicating a central contribution for reactive oxygen species (ROS) in the onset, progression, and pathological consequences of diabetes. Besides oxidative stress, a growing body of evidence has demonstrated a link between various disturbances in mitochondrial functioning and type 2 diabetes. Mutations in mitochondrial DNA (mtDNA) and decreases in mtDNA copy number have been linked to the pathogenesis of type 2 diabetes. The study of the relationship of mtDNA to type 2 diabetes has revealed the influence of the mitochondria on nuclear-encoded glucose transporters, glucose-stimulated insulin secretion, and nuclear-encoded uncoupling proteins (UCPs) in beta-cell glucose toxicity. This review focuses on a range of mitochondrial factors important in the pathogenesis of diabetes. We review the published literature regarding the direct effects of hyperglycemia on mitochondrial function and suggest the possibility of regulation of mitochondrial function at a transcriptional level in response to hyperglycemia. The main goal of this review is to include a fresh consideration of pathways involved in hyperglycemia-induced diabetic complications.

Plasminogen activator inhibitor-1 and diabetic nephropathy

Diabetic nephropathy is characterized by excessive accumulation of extracellular matrix (ECM) in the kidney. Decreased ECM degradation as well as increased ECM synthesis plays an important role in ECM remodeling that favours tissue fibrosis. Plasminogen activator (PA)/plasmin/PA inhibitor (PAI) system is involved in ECM degradation and PAI-1 plays a critical role in ECM remodeling in the kidney. Normal human kidneys do not express PAI-1 but PAI-1 is overexpressed in pathologic conditions associated with renal fibrosis including diabetic nephropathy. Reactive oxygen species mediate PAI-1 up-regulation in renal cells cultured under high glucose, hypoxia, and TGF-beta1. Recent studies utilizing PAI-1 deficient mice suggest that PAI-1 induce ECM deposition in diabetic kidney through increased ECM synthesis by TGF-beta1 up-regulation as well as through decreased ECM degradation by suppression of plasmin and MMP-2 activity.

CONVERGENCE OF GLUCOSE- AND FATTY ACID-INDUCED ABNORMAL MYOCARDIAL EXCITATION-CONTRACTION COUPLING AND INSULIN SIGNALLING

1. Myocardial insulin resistance and abnormal Ca(2+) regulation are hallmarks of hypertrophic and diabetic hearts, but deprivation of energetic substrates does not tell the whole story. Is there a link between the aetiology of these dysfunctions? 2. Diabetic cardiomyopathy is defined as phenotypic changes in the heart muscle cell independent of associated coronary vascular disease. The cellular consequences of diabetes on excitation-contraction (E-C) coupling and insulin signalling are presented in various models of diabetes in order to set the stage for exploring the pathogenesis of heart disease. 3. Excess glucose or fatty acids can lead to augmented flux through the hexosamine biosynthesis pathway (HBP). The formation of uridine 5 cent-diphosphate-hexosamines has been shown to be involved in abnormal E-C coupling and myocardial insulin resistance. 4. There is growing evidence that O-linked glycosylation (downstream of HBP) may regulate the function of cytosolic and nuclear proteins in a dynamic manner, similar to phosphorylation and perhaps involving reciprocal or synergistic modification of serine/threonine sites. 5. This review focuses on the question of whether there is a role for HBP and dynamic O-linked glycosylation in the development of myocardial insulin resistance and abnormal E-C coupling. The emerging concept that O-linked glycosylation is a regulatory, post-translational modification of cytosolic/nuclear proteins that interacts with phosphorylation in the heart is explored.

Posttranslational, reversible O-glycosylation is stimulated by high glucose and mediates plasminogen activator inhibitor-1 gene expression and Sp1 transcriptional activity in glomerular mesangial cells

Metabolic flux through the hexosamine biosynthetic pathway (HBP) is increased in the presence of high glucose (HG) and potentially stimulates the expression of genes associated with the development of diabetic nephropathy. A number of synthetic processes are coupled to the HBP, including enzymatic intracellular O-glycosylation (O-GlcNAcylation), the addition of single O-linked N-acetylglucosamine monosaccharides to serine or threonine residues. Despite much data linking flow through the HBP and gene expression, the exact contribution of O-GlcNAcylation to HG-stimulated gene expression remains unclear. In glomerular mesangial cells, HG-stimulated plasminogen activator inhibitor-1 (PAI-1) gene expression requires the HBP and the transcription factor, Sp1. In this study, the specific role of O-GlcNAcylation in HG-induced PAI-1 expression was tested by limiting this modification with a dominant-negative O-linked N-acetylglucosamine transferase, by overexpression of neutral beta-N-acetylglucosaminidase, and by knockdown of O-linked beta-N-acetylglucosamine transferase expression by RNA interference. Decreasing O-GlcNAcylation by these means inhibited the ability of HG to increase endogenous PAI-1 mRNA and protein levels, the activity of a PAI-1 promoter-luciferase reporter gene, and Sp1 transcriptional activation. Conversely, treatment with the beta-N-acetylglucosaminidase inhibitor, O-(2-acetamido-2-deoxy-d-glucopyranosylidene)amino-N-phenylcarbamate, in the presence of normal glucose increased Sp1 O-GlcNAcylation and PAI-1 mRNA and protein levels. These findings demonstrate for the first time that among the pathways served by the HBP, O-GlcNAcylation, is obligatory for HG-induced PAI-1 gene expression and Sp1 transcriptional activation in mesangial cells.

Lipoxin A4 modifies platelet-derived growth factor-induced pro-fibrotic gene expression in human renal mesangial cells

Lipoxins (LXs), endogenously produced eicosanoids, possess potent anti-inflammatory, pro-resolution bioactivities. We investigated the potential of LXA(4) (1 to 10 nmol/L) to modify the effects of platelet-derived growth factor (PDGF)-induced gene expression in human renal mesangial cells (hMCs). Using oligonucleotide microarray analysis we profiled pro-fibrotic cytokines and matrix-associated genes induced in response to PDGF. LXA(4) modulated the expression of many PDGF-induced genes, including transforming growth factor-beta1, fibronectin, thrombospondin, matrix metalloproteinase 1, and several collagens. Analysis of both transcript and protein levels confirmed these findings. Because the activated glomerulus is frequently a source of injurious mediators that contribute to tubulointerstitial damage, we investigated the effect of hMC-secreted products on the integrity of renal proximal tubular epithelial cells using an in vitro model of progressive renal disease. Cell supernatant from PDGF-stimulated hMCs caused morphological and genetic changes in proximal tubular epithelial cells, consistent with a pro-fibrotic phenotype. Interestingly, supernatant from cells pre-exposed to LXA(4) and PDGF did not induce these effects. These results suggest a novel role for LXA(4) as a potent modulator of matrix accumulation and pro-fibrotic change and suggest a potential protective role in progressive renal disease.

Glucosamine induces cell-cycle arrest and hypertrophy of mesangial cells: implication of gangliosides

Alterations in proliferation and hypertrophy of renal mesangial cells are typical features of diabetic nephropathy. The HP (hexosamine pathway) has been proposed as a biochemical hypothesis to explain microvascular alterations due to diabetic nephropathy; however, involvement of HP in the regulation of mesangial cell growth or hypertrophy has been poorly studied. Although gangliosides are known to regulate cell proliferation, their potential role in mesangial cell-growth perturbations has hardly been explored. In the present study, we investigated the effects of the HP activation, mimicked by GlcN (glucosamine) treatment, on mesangial cell growth and hypertrophy and the potential implication of gangliosides in these processes. Our results indicate that GlcN induced hypertrophy of mesangial cells, as measured by an increase in the protein/cell ratio, and it caused cell-cycle arrest by an increase in the expression of cyclin-dependent kinase inhibitor p21(Waf1/Cip1). Furthermore, GlcN treatment resulted in a massive increase in the levels of gangliosides G(M2) and G(M1). Treatment of cells with exogenous G(M2) and G(M1) reproduced the effects of 0.5 mM GlcN on p21(Waf1/Cip1) expression, cell-cycle arrest and hypertrophy, suggesting that gangliosides G(M2) and G(M1) are probably involved in mediating GlcN effects. These results document a new role of the HP in the regulation of mesangial cell growth and hypertrophy. They also suggest a potential new mechanism of action of the HP through modulation of ganglioside levels.

Acute glucosamine-induced insulin resistance in muscle in vivo is associated with impaired capillary recruitment

AIMS/HYPOTHESIS

Glucose toxicity and glucosamine-induced insulin resistance have been attributed to products of glucosamine metabolism. In addition, endothelial cell nitric oxide synthase is inhibited by glucosamine. Since insulin has endothelial nitric-oxide-dependent vasodilatory effects in muscle, we hypothesise that glucosamine-induced insulin resistance in muscle in vivo is associated with impaired vascular responses including capillary recruitment.

MATERIALS AND METHODS

Glucosamine (6.48 mg kg(-1) min(-1) for 3 h) was infused with or without insulin (10 mU kg(-1) min(-1)) into anaesthetised rats under euglycaemic conditions.

RESULTS

Glucosamine infusion alone increased blood glucosamine (1.9+/-0.1 mmol/l) and glucose (5.4+/-0.2 to 7.7+/-0.3 mmol/l) (p<0.05) but not insulin. Glucosamine induced both hepatic and muscle insulin resistance as evident from measures of glucose appearance and disposal as well as hind-leg glucose uptake, which was inhibited by approx. 50% (p<0.05). Insulin-mediated increases in femoral arterial blood flow and capillary recruitment were completely blocked by glucosamine.

CONCLUSION/INTERPRETATION

Glucosamine mediates a major impairment of insulin action in muscle vasculature associated with the insulin resistance of muscle. Further studies will be required to assess whether the impaired capillary recruitment contributes to insulin resistance.

Reactive oxygen species amplify glucose signalling in renal cells cultured under high glucose and in diabetic kidney

Diabetic nephropathy is characterized by excessive accumulation of extracellular matrix (ECM) in the kidney. Reactive oxygen species (ROS) play a central role in the ECM synthesis and degradation in the glomeruli and tubulointerstitium leading to renal fibrosis. High glucose (HG) induces cellular ROS through protein kinase C (PKC)-dependent activation of NADPH oxidase and through mitochondrial metabolism. ROS thus generated activate signal transduction cascade (PKC, mitogen-activated protein kinases, and janus kinase/signal transducers and activators of transcription) and transcription factors (nuclear factor-kappaB, activated protein-1, and specificity protein-1), up-regulate transforming growth factor-beta1 (TGF-beta1), angiotensin II (Ang II), monocyte chemoattractant protein-1 (MCP-1), and plasminogen activator inhibitor-1 (PAI-1) gene and protein expression, and promote formation of advanced glycation end-products (AGE). PKC, TGF-beta1, Ang II, and AGE also induce cellular ROS and signal through ROS leading to enhanced ECM synthesis. NF-kappaB-MCP-1 pathway is activated by ROS and promotes monocyte recruitment and profibrotic process in the kidney. HG- and TGF-beta1-induced PAI-1 up-regulation is mediated by ROS and contribute to ECM accumulation via suppression of plasmin ativity. TGF-beta1-induced myofibroblast transformation of renal tubular epithelial cells (epithelial-mesenchymal transition) is also mediated by ROS and contribute to tubulointerstitial fibrosis. In summary, ROS transduce and amplify glucose signalling in renal cells under high glucose environment and play a critical role in excessive ECM deposition in the diabetic kidney. A better understanding of ROS production and removal will allow more effective therapeutic strategies in diabetic renal and other vascular complications.

Reactive oxygen species mediate high glucose-induced plasminogen activator inhibitor-1 up-regulation in mesangial cells and in diabetic kidney

BACKGROUND

Plasminogen activator inhibitor-1 (PAI-1) plays an important role in remodeling of extracellular matrix (ECM) in the glomeruli. PAI-1 is up-regulated by high glucose and is overexpressed in diabetic kidney. Since reactive oxygen species (ROS) mediate ECM accumulation in diabetic glomeruli and was recently found to mediate transforming growth factor-beta1 (TGF-beta1)-induced PAI-1 up-regulation in glomerular mesangial cells, we examined the role of ROS in high glucose-induced PAI-1 expression in cultured glomerular mesangial cells and in streptozotocin-induced diabetic rat glomeruli.

METHODS

Growth arrested and synchronized primary rat mesangial cells were treated with different concentrations of glucose in the presence or absence of N-acetylcysteine (NAC) or trolox, or after cellular reduced form of glutathione (GSH) depleted with DL-buthionine-(S,R)-sulfoximine (BSO). Taurine was administered to diabetic rats from 2 days to 4 weeks after streptozotocin injection. Urinary protein excretion, glomerular volume, and fractional mesangial area were measured as markers of renal injury and lipid peroxide (LPO) as an oxidative stress marker. PAI-1 mRNA expression was measured by Northern blot analysis in mesangial cells and reverse transcription-polymerase chain reaction (RT-PCR) in glomeruli, PAI-1 protein by Western blot analysis and enzyme-linked immunosorbent assay (ELISA), and plasmin activity by fluorometry.

RESULTS

High glucose significantly increased PAI-1 mRNA and protein expression and decreased plasmin activity in mesangial cells. Equimolar concentrations of l-glucose or mannitol did not affect PAI-1 expression. BSO pretreatment significantly increased basal PAI-1 expression and amplified the response to high glucose. NAC effectively inhibited high glucose-induced, but not basal, PAI-1 expression. Reduced plasmin activity in mesangial cells by high glucose was rescued by antioxidants. Anti-TGF-beta antibody inhibited both high glucose- and H(2)O(2)-induced PAI-1 up-regulation. Taurine significantly reduced plasma LPO, glomerular PAI-1 expression, glomerular volume, fractional mesangial area, and proteinuria in streptozotocin-induced diabetic rats.

CONCLUSION

These results demonstrate that ROS mediate high glucose-induced up-regulation of PAI-1 expression in cultured mesangial cells and in diabetic glomeruli. Since both high glucose and TGF-beta1 induce cellular ROS and ROS mediate both high glucose- and TGF-beta1-induced PAI-1, ROS appear to amplify TGF-beta1 signaling in high glucose-induced PAI-1 up-regulation. Antioxidants can prevent accumulation of ECM protein in diabetic glomeruli partly by abrogating up-regulation of PAI-1 and suppression of plasmin activity.

Sp-family of transcription factors regulates human SHIP2 gene expression

We have characterized the regulation of human SH-2 containing inositol 5'-phosphatase 2 (SHIP2) gene expression. First, the transcription initiation sites and the sequence of the 5' upstream region of human SHIP2 gene were elucidated. Next, the minimal promoter of the human SHIP2 gene was identified by reporter gene assays in HL60 cells and differentiated human subcutaneous white adipocytes. An Sp1 element proximal to the transcription initiation site was indispensable for full promoter activity and bound specifically by Sp1 and Sp3 proteins. These findings suggest that human SHIP2 gene expression, like other housekeeping genes, is controlled by the Sp-family of transcription factors.

Glucosamine induces rapid desensitization of glucose transport in isolated adipocytes by increasing GlcN-6-P levels

We have examined the hypothesis that glucosamine (GlcN) can rapidly induce insulin resistance through an allosteric mechanism. When insulin-treated adipocytes were exposed to 2mM GlcN, glucose uptake was rapidly reduced by approximately 60% with a T(1/2) of 2 min. We also observed an increase in intracellular GlcN-6-P (at 5 min) from undetectable levels to approximately 260 nmol/g. Continued GlcN treatment resulted in additional accumulation of GlcN-6-P (>1200 nmol/g at 2h), but caused no further decrease in glucose uptake. Although the acute inhibitory action of GlcN could be completely reversed by removing extracellular GlcN, a slow and progressive decrease in insulin-stimulated glucose transport was observed with longer treatment times (T(1/2) of 45 min, 62% loss by 5h). From these data, we conclude that: (1) GlcN elevates intracellular GlcN-6-P levels within minutes, resulting in desensitization of the glucose transport system through allosteric inhibition of hexokinase; (2) prolonged treatment elevates GlcN-6-P to levels that cannot be effectively lowered by cell washing; and (3) residual levels of GlcN-6-P continue to allosterically inhibit glucose uptake, resulting in a slower rate of desensitization that is temporally similar to glucose-induced desensitization, but mechanistically different.

De novo synthesis of diacylglycerol in endothelium may mediate the association between PAI-1 and the insulin resistance syndrome

Increased free fatty acid flux, giving rise to increased de novo synthesis of diacylglycerol (DAG) and activation of protein kinase C (PKC) in vascular endothelium, may be largely responsible for the endotheliopathy and increased vascular risk associated with insulin resistance syndrome. This mechanism may also mediate, in large part, the increase in plasminogen activator inhibitor-1 (PAI-1) observed in this syndrome. PKC activation promotes transcription of PAI-1 in endothelial cells and other tissues, apparently by boosting the activity of Sp1 transcription factors that bind to the PAI-1 promoter. Plasma PAI-1 correlates inversely with the ability of insulin infusion to suppress free fatty acid levels. Moreover, infusion of triglycerides with heparin - inducing a marked increase in free fatty acids - has been shown to induce a rapid increase in plasma PAI-1. Alternatively, hyperinsulinemia and hypertriglyceridemia have been suggested as mediators of PAI-1 excess in insulin resistance, inasmuch as insulin and VLDL can stimulate PAI-1 production in cell cultures. However, plasma PAI-1 tends to decline in response to hyperinsulinemic clamps and insulin treatment of type 2 diabetes, and gemfibrozil treatment of hypertriglyceridemia does not decrease PAI-1 - suggesting that elevations of insulin or triglycerides are not likely to mediate PAI-1 excess in vivo. Hypertrophied adipose mass can secrete PAI-1, and is likely to contribute to the plasma PAI-1 pool in obese insulin-resistant subjects, but current evidence suggests that this is not likely to be the primary source of the elevated plasma PAI-1 in insulin resistance syndrome. Plasma PAI-1 can be decreased in insulin resistant subjects by improving adipocyte insulin sensitivity (with weight loss and thiazolidinediones), by consuming a very-low-fat diet that minimizes postprandial free fatty acid flux, and by administering activators of AMP-activated kinase (e.g., metformin), which can be expected to lessen tissue DAG synthesis.