Progression of nephropathy in spontaneous diabetic rats is prevented by OPB-9195, a novel inhibitor of advanced glycation

The dynamic roles of advanced glycation end products

Advanced glycation end products (AGEs) are a heterogeneous group of potentially harmful molecules that can form as a result of a non-enzymatic reaction between reducing sugars and proteins, lipids, or nucleic acids. The total body pool of AGEs reflects endogenously produced AGEs as well as exogeneous AGEs that come from sources such as diet and the environment. Engagement of AGEs with their cellular receptor, the receptor for advanced glycation end products (RAGE), which is expressed on the surface of various cell types, converts a brief pulse of cellular activation to sustained cellular dysfunction and tissue destruction. The AGEs/RAGE interaction triggers a cascade of intracellular signaling pathways such as mitogen-activated protein kinase/extracellular signal-regulated kinase, phosphoinositide 3-kinases, transforming growth factor beta, c-Jun N-terminal kinases (JNK), and nuclear factor kappa B, which leads to the production of pro-inflammatory cytokines, chemokines, adhesion molecules, and oxidative stress. All these events contribute to the progression of several chronic diseases. This chapter will provide a comprehensive understanding of the dynamic roles of AGEs in health and disease which is crucial to develop interventions that prevent and mitigate the deleterious effects of AGEs accumulation.

Non-enzymatic glycation and diabetic kidney disease

Chronic diabetes leads to various complications including diabetic kidney disease (DKD). DKD is a major microvascular complication and the leading cause of morbidity and mortality in diabetic patients. Varying degrees of proteinuria and reduced glomerular filtration rate are the cardinal clinical manifestations of DKD that eventually progress into end-stage renal disease. Histopathologically, DKD is characterized by renal hypertrophy, mesangial expansion, podocyte injury, glomerulosclerosis, and tubulointerstitial fibrosis, ultimately leading to renal replacement therapy. Amongst the many mechanisms, hyperglycemia contributes to the pathogenesis of DKD via a mechanism known as non-enzymatic glycation (NEG). NEG is the irreversible conjugation of reducing sugars onto a free amino group of proteins by a series of events, resulting in the formation of initial Schiff's base and an Amadori product and to a variety of advanced glycation end products (AGEs). AGEs interact with cognate receptors and evoke aberrant signaling cascades that execute adverse events such as oxidative stress, inflammation, phenotypic switch, complement activation, and cell death in different kidney cells. Elevated levels of AGEs and their receptors were associated with clinical and morphological manifestations of DKD. In this chapter, we discussed the mechanism of AGEs accumulation, AGEs-induced cellular and molecular events in the kidney and their impact on the pathogenesis of DKD. We have also reflected upon the possible options to curtail the AGEs accumulation and approaches to prevent AGEs mediated adverse renal outcomes.

Contribution of animal models to diabetes research: Its history, significance, and translation to humans

Diabetes mellitus is still expanding globally and is epidemic in developing countries. The combat of this plague has caused enormous economic and social burdens related to a lowered quality of life in people with diabetes. Despite recent significant improvements of life expectancy in patients with diabetes, there is still a need for efforts to elucidate the complexities and mechanisms of the disease processes to overcome this difficult disorder. To this end, the use of appropriate animal models in diabetes studies is invaluable for translation to humans and for the development of effective treatment. In this review, a variety of animal models of diabetes with spontaneous onset in particular will be introduced and discussed for their implication in diabetes research.

Advanced glycation end products: Key mediator and therapeutic target of cardiovascular complications in diabetes

The incidence of type 2 diabetes mellitus is growing in epidemic proportions and has become one of the most critical public health concerns. Cardiovascular complications associated with diabetes are the leading cause of morbidity and mortality. The cardiovascular diseases that accompany diabetes include angina, myocardial infarction, stroke, peripheral artery disease, and congestive heart failure. Among the various risk factors generated secondary to hyperglycemic situations, advanced glycation end products (AGEs) are one of the important targets for future diagnosis and prevention of diabetes. In the last decade, AGEs have drawn a lot of attention due to their involvement in diabetic patho-physiology. AGEs can be derived exogenously and endogenously through various pathways. These are a non-homogeneous, chemically diverse group of compounds formed non-enzymatically by condensation between carbonyl groups of reducing sugars and free amino groups of protein, lipids, and nucleic acid. AGEs mediate their pathological effects at the cellular and extracellular levels by multiple pathways. At the cellular level, they activate signaling cascades via the receptor for AGEs and initiate a complex series of intracellular signaling resulting in reactive oxygen species generation, inflammation, cellular proliferation, and fibrosis that may possibly exacerbate the damaging effects on cardiac functions in diabetics. AGEs also cause covalent modifications and cross-linking of serum and extracellular matrix proteins; altering their structure, stability, and functions. Early diagnosis of diabetes may prevent its progression to complications and decrease its associated comorbidities. In the present review, we recapitulate the role of AGEs as a crucial mediator of hyperglycemia-mediated detrimental effects in diabetes-associated complications. Furthermore, this review presents an overview of future perspectives for new therapeutic interventions to ameliorate cardiovascular complications in diabetes.

Diabetic Kidney Disease: From Pathogenesis to Novel Treatment Possibilities

One of the microvascular complications of diabetes is diabetic kidney disease (DKD), often leading to end stage renal disease (ESRD) in which patients require costly dialysis or transplantation. The silent onset and irreversible progression of DKD are characterized by a steady decline of the estimated glomerular filtration rate, with or without concomitant albuminuria. The diabetic milieu allows the complex pathophysiology of DKD to enter a vicious cycle by inducing the synthesis of excessive amounts of reactive oxygen species (ROS) causing oxidative stress, inflammation, and fibrosis. As no cure is available, intensive research is required to develop novel treatments possibilities. This chapter provides an overview of the important pathomechanisms identified in diabetic kidney disease, the currently established therapies, as well as recently developed novel therapeutic strategies in DKD.

AGE/RAGE signaling-mediated endoplasmic reticulum stress and future prospects in non-coding RNA therapeutics for diabetic nephropathy

Disturbance of endoplasmic reticulum (ER) homeostasis triggered by the accumulation of unfolded proteins and advanced glycation end-products (AGEs) plays a major role in pathophysiology of diabetic nephropathy. Activation of receptor for AGEs (RAGE) stimulates NADPH oxidase-mediated reactive oxygen species (ROS) production, leading to ER stress, inflammation, glomerular hypertrophy, podocyte injury, and renal fibrosis. A growing body of evidence indicates that non-coding RNAs (ncRNAs) could rescue ER stress and renal inflammation by the epigenetic modification. This review summarizes ncRNA regulation in AGE/RAGE signaling-mediated ER stress, and discusses the opportunities and challenges of ncRNA-loaded extracellular vesicle therapy in diabetic nephropathy.

Galactose-Induced Skin Aging: The Role of Oxidative Stress

Skin aging has been associated with a higher dietary intake of carbohydrates, particularly glucose and galactose. In fact, the carbohydrates are capable of damaging the skin's vital components through nonenzymatic glycation, the covalent attachment of sugar to a protein, and subsequent production of advanced glycation end products (AGEs). This review is focused on the role of D-galactose in the development of skin aging and its relation to oxidative stress. The interest in this problem was dictated by recent findings that used in vitro and in vivo models. The review highlights the recent advances in the underlying molecular mechanisms of D-galactose-mediated cell senescence and cytotoxicity. We have also proposed the possible impact of galactosemia on skin aging and its clinical relevance. The understanding of molecular mechanisms of skin aging mediated by D-galactose can help dermatologists optimize methods for prevention and treatment of skin senescence and aging-related skin diseases.

Glucose and Blood Pressure-Dependent Pathways-The Progression of Diabetic Kidney Disease

The major clinical associations with the progression of diabetic kidney disease (DKD) are glycemic control and systemic hypertension. Recent studies have continued to emphasize vasoactive hormone pathways including aldosterone and endothelin which suggest a key role for vasoconstrictor pathways in promoting renal damage in diabetes. The role of glucose per se remains difficult to define in DKD but appears to involve key intermediates including reactive oxygen species (ROS) and dicarbonyls such as methylglyoxal which activate intracellular pathways to promote fibrosis and inflammation in the kidney. Recent studies have identified a novel molecular interaction between hemodynamic and metabolic pathways which could lead to new treatments for DKD. This should lead to a further improvement in the outlook of DKD building on positive results from RAAS blockade and more recently newer classes of glucose-lowering agents such as SGLT2 inhibitors and GLP1 receptor agonists.

Targeting the receptor for advanced glycation end products (RAGE) in type 1 diabetes

Type 1 diabetes (T1D) is one of the most common chronic diseases manifesting in early life, with the prevalence increasing worldwide at a rate of approximately 3% per annum. The prolonged hyperglycaemia characteristic of T1D upregulates the receptor for advanced glycation end products (RAGE) and accelerates the formation of RAGE ligands, including advanced glycation end products, high-mobility group protein B1, S100 calcium-binding proteins, and amyloid-beta. Interestingly, changes in the expression of RAGE and these ligands are evident in patients before the onset of T1D. RAGE signals via various proinflammatory cascades, resulting in the production of reactive oxygen species and cytokines. A large number of proinflammatory ligands that can signal via RAGE have been implicated in several chronic diseases, including T1D. Therefore, it is unsurprising that RAGE has become a potential therapeutic target for the treatment and prevention of disease. In this review, we will explore how RAGE might be targeted to prevent the development of T1D.

Carbonyl Stress Reduction in Peritoneal Dialysis Fluid: Development of a Novel High-Affinity Adsorption Bead

Objective

Heat sterilization of glucose peritoneal dialysis (PD) fluid generates reactive carbonyl compounds (RCOs), which have been implicated in the formation of advanced glycation end products (AGEs) on peritoneal proteins, with an attendant deterioration of peritoneal permeability in PD patients. To reduce their levels in PD fluid, we had previously devised beads coupled with RCO-trapping agents. The hazards linked to the diffusion of RCO-trapping compounds in the systemic circulation are avoided. Hydrazine-epoxy beads proved the most effective. Still, the amount needed to trap all RCOs remained relatively large.

Methods

We developed a novel agent linking a powerful RCO-trapping AGE inhibitor, pyrazolinone-polyethyleneimine, with cellulose beads (PPCBs). We tested its effectiveness to lower RCOs and AGE formation.

Results

Mixed with glucose PD fluid, PPCBs markedly lowered RCOs (α–dicarbonyls and aldehydes) and inhibited the generation of pentosidine, an AGE, to levels similar to those of filter-sterilized PD fluid. Their effectiveness is more than one order of magnitude above those of previously developed beads. The PPCBs markedly improved PD fluid biocompatibility. Incubation of 1 L commercial glucose PD fluid at 25°C for 24 hours with 10 or 30 g of wet PPCBs reduced RCO content by 75% – 90% and 100% respectively, without altering the pH or glucose and electrolyte contents of the PD fluid.

Conclusions

We developed a high-affinity adsorption bead to reduce the toxic RCO content and AGE formation potential (carbonyl stress) of PD fluid.

Glycation-induced modification of tissue-specific ECM proteins: A pathophysiological mechanism in degenerative diseases

BACKGROUND

Glycation driven generation of advanced glycation end products (AGEs) and their patho-physiological role in human degenerative diseases has remained one of the thrust areas in the mainstream of disease biology. Glycation of extracellular matrix (ECM) proteins have deleterious effect on the mechanical and functional properties of tissues. Owing to the adverse pathophysiological concerns of glycation, there is a need to decipher the underlying mechanisms.

SCOPE OF REVIEW

AGE-modified ECM proteins affect the cell in the vicinity by altering protein structure-function, matrix-matrix or matrix-cell interaction and by activating signalling pathway through receptor for AGE. This review is intended for addressing the AGE-induced modification of tissue-specific ECM proteins and its implication in the pathogenesis of various organ-specific human ailments.

MAJOR CONCLUSIONS

The glycation affects the canonical cell behaviour due to alteration in the interaction of glycated ECM with receptors like integrins and discodin domain, and the signalling cues generated subsequently affect the downstream signalling pathways. Consequently, the variation of structural and functional properties of tissues due to matrix glycation helps in the initiation or progression of the disease condition.

GENERAL SIGNIFICANCE

This review offers comprehensive knowledge about the remodelling of glycation induced ECM and tissue-specific pathological concerns. As glycation of ECM affects the normal tissues and cell behaviour, the scientific discourse may also provide cues for developing candidate drugs that may help in attenuating the adverse effects of AGEs and perhaps open a research window of tailoring novel strategies for the management of glycation induced human degenerative diseases.

Glycation of Plant Proteins: Regulatory Roles and Interplay with Sugar Signalling?

Glycation can be defined as an array of non-enzymatic post-translational modifications of proteins formed by their interaction with reducing carbohydrates and carbonyl products of their degradation. Initial steps of this process rely on reducing sugars and result in the formation of early glycation products-Amadori and Heyns compounds via Schiff base intermediates, whereas their oxidative degradation or reactions of proteins with α-dicarbonyl compounds yield a heterogeneous group of advanced glycation end products (AGEs). These compounds accompany thermal processing of protein-containing foods and are known to impact on ageing, pathogenesis of diabetes mellitus and Alzheimer's disease in mammals. Surprisingly, despite high tissue carbohydrate contents, glycation of plant proteins was addressed only recently and its physiological role in plants is still not understood. Therefore, here we summarize and critically discuss the first steps done in the field of plant protein glycation during the last decade. We consider the main features of plant glycated proteome and discuss them in the context of characteristic metabolic background. Further, we address the possible role of protein glycation in plants and consider its probable contribution to protein degradation, methylglyoxal and sugar signalling, as well as interplay with antioxidant defense.

FPS-ZM1 and valsartan combination protects better against glomerular filtration barrier damage in streptozotocin-induced diabetic rats

Despite the effectiveness of renin-angiotensin blockade in retarding diabetic nephropathy progression, a considerable number of patients still develop end-stage renal disease. The present investigation aims to evaluate the protective potential of FPS-ZM1, a selective inhibitor of receptor for advanced glycation end products (RAGE), alone and in combination with valsartan, an angiotensin receptor blocker, against glomerular injury parameters in streptozotocin-induced diabetic rats. FPS-ZM1 at 1 mg/kg (i.p.), valsartan at 100 mg/kg (p.o.), and their combination were administered for 4 weeks, starting 2 months after diabetes induction in rats. Tests for kidney function, glomerular filtration barrier, and podocyte slit diaphragm integrities were performed. Combined FPS-ZM1/valsartan attenuated diabetes-induced elevations in renal levels of RAGE and phosphorylated NF-κB p65 subunit. It ameliorated glomerular injury due to diabetes by increasing glomerular nephrin and synaptopodin expressions, mitigating renal integrin-linked kinase (ILK) levels, and lowering urinary albumin, collagen type IV, and podocin excretions. FPS-ZM1 also improved renal function as demonstrated by decreasing levels of serum cystatin C. Additionally, the combination also alleviated indices of renal inflammation as revealed by decreased renal monocyte chemoattractant protein 1 (MCP-1) and chemokine (C-X-C motif) ligand 12 (CXCL12) expressions, F4/80-positive macrophages, glomerular TUNEL-positive cells, and urinary alpha-1-acid glycoprotein (AGP) levels. These findings underline the benefits of FPS-ZM1 added to valsartan in alleviating renal glomerular injury evoked by diabetes in streptozotocin rats and suggest FPS-ZM1 as a new potential adjunct to the conventional renin-angiotensin blockade.

AGE-RAGE axis blockade in diabetic nephropathy: Current status and future directions

Diabetic nephropathy is one of the most frequent micro-vascular complications both in type 1 and type 2 diabetic patients and is the leading cause of end-stage renal disease worldwide. Although disparate mechanisms give rise to the development of diabetic nephropathy, prevailing evidence accentuates that hyperglycemia-associated generation of advanced glycation end products (AGEs) plays a central role in the disease pathophysiology. Engagement of the receptor for AGE (RAGE) with its ligands provokes oxidative stress and chronic inflammation in renal tissues, ending up with losses in kidney function. Moreover, RAGE activation evokes the activation of different intracellular signaling pathways like PI3K/Akt, MAPK/ERK, and NF-κB; and therefore, its blockade seems to be an attractive therapeutic target in these group of patients. By recognizing the contribution of AGE-RAGE axis to the pathogenesis of diabetic nephropathy, agents that block AGEs formation have been at the heart of investigations for several years, yielding encouraging improvements in experimental models of diabetic nephropathy. Even so, recent studies have evaluated the effects of specific RAGE inhibition with FPS-ZM1 and RAGE-aptamers as novel therapeutic strategies. Despite all these promising outcomes in experimental models of diabetic nephropathy, no thorough clinical trial have ever examined the end results of AGE-RAGE axis blockade in patients of diabetic nephropathy. As most of the AGE lowering or RAGE inhibiting compounds have emerged to be non-toxic, devising novel clinical trials appears to be inevitable. Here, the current potential treatment options for diabetic nephropathy by AGE-RAGE inhibitory modalities have been reviewed.

Kidney, heart and brain: three organs targeted by ageing and glycation

Advanced glycation end-product (AGE) is the generic term for a heterogeneous group of derivatives arising from a non-enzymatic reaction between reducing sugars and proteins. In recent years, evidence has accumulated that incriminates AGEs in pathogenic processes associated with both chronic hyperglycaemia and age-related diseases. Regardless of their exogenous or endogenous origin, the accumulation of AGEs and their derivatives could promote accelerated ageing by leading to protein modifications and activating several inflammatory signalling pathways via AGE-specific receptors. However, it remains to be demonstrated whether preventing the accumulation of AGEs and their effects is an important therapeutic option for successful ageing. The present review gives an overview of the current knowledge on the pathogenic role of AGEs by focusing on three AGE target organs: kidney, heart and brain. For each of these organs we concentrate on an age-related disease, each of which is a major public health issue: chronic kidney disease, heart dysfunction and neurodegenerative diseases. Even though strong connections have been highlighted between glycation and age-related pathogenesis, causal links still need to be validated. In each case, we report evidence and uncertainties suggested by animal or epidemiological studies on the possible link between pathogenesis and glycation in a chronic hyperglycaemic state, in the absence of diabetes, and with exogenous AGEs alone. Finally, we present some promising anti-AGE strategies that are currently being studied.

Combination of Neuroprotective and Regenerative Agents for AGE-Induced Retinal Degeneration: In Vitro Study

To determine the most effective combination of neuroprotective and regenerative agents for cultured retinal neurons from advanced glycation end products- (AGEs-) induced degeneration, retinal explants of 7 adult Sprague-Dawley rats were three-dimensionally cultured in collagen gel and incubated in serum-free media and in 7 media; namely, AGEs, AGEs + 100 μM citicoline, AGEs + 10 ng/mL NT-4, AGEs + 100 μM TUDCA, AGEs + 100 μM citicoline + TUDCA (doublet), and AGEs + 100 μM citicoline + TUDCA + 10 ng/mL NT-4 (triplet) were examined. The number of regenerating neurites was counted after 7 days of culture, followed by performing TUNEL and DAPI staining. The ratio of TUNEL-positive cells to the number of DAPI-stained nuclei was calculated. Immunohistochemical examinations for the active form of caspase-9 and JNK were performed. All of the neuroprotectants increased the number of neurites and decreased the number of TUNEL-positive cells. However, the number of neurites was significantly higher, and the number of TUNEL-positive cells and caspase-9- and JNK-immunopositive cells was fewer in the retinas incubated with the combined three agents. Combination solutions containing citicoline, TUDCA, and NT-4 should be considered for neuroprotective and regenerative therapy for AGE-related retinal degeneration.

The effects of advanced glycation end products (AGEs) on dermal wound healing and scar formation: a systematic review

Introduction:

With ageing, the skin gradually loses its youthful appearance and functions like wound healing and scar formation. The pathophysiological theory of Advanced Glycation End products (AGEs) has gained traction during the last decade. This review aims to document the influence of AGEs on the mechanical and physiologic properties of the skin, how they affect dermal wound healing and scar formation in high-AGE populations like elderly patients and diabetics, and potential therapeutic strategies.

Methods:

This systematic literature study involved a structured search in Pubmed and Web of Science with qualitative analysis of 14 articles after a three-staged selection process with the use of in- and exclusion criteria.

Results:

Overall, AGEs cause shortened, thinned, and disorganized collagen fibrils, consequently reducing elasticity and skin/scar thickness with increased contraction and delayed wound closure. Documented therapeutic strategies include dietary AGE restriction, sRAGE decoy receptors, aminoguanidine, RAGE-blocking antibodies, targeted therapy, thymosin β4, anti-oxidant agents and gold nanoparticles, ethyl pyruvate, Gal-3 manipulation and metformin.

Discussion:

With lack of evidence concerning scars, no definitive conclusions can yet be made about the role of AGEs on possible appearance or function of scar tissue. However, all results suggest that scars tend to be more rigid and contractile with persistent redness and reduced tendency towards hypertrophy as AGEs accumulate.

Conclusion:

Abundant evidence supports the pathologic role of AGEs in ageing and dermal wound healing and the effectiveness of possible therapeutic agents. More research is required to conclude its role in scar formation and scar therapy.

Our skin is the body’s first line of defense. It is the barrier that protects us from chemical and biological threats such as viruses, bacteria or corrosive liquids. It is the sensor that allows us to detect physical threats like extreme temperatures, pressure and pain. And when these preventative measures fail, the skin has yet another property: the ability to heal.

Skin changes visibly with age, most notably with the appearance of wrinkles. However, there is more to ageing than meets the eye; invisible alterations cause the decline of various functions of the skin, such as wound healing and scar formation. An array of non-conclusive research has been done in this field. One theory that has gained traction during the last decade is the Advanced Glycation End products (AGEs) theory. The theory states that AGEs play an important role in skin aging, wound healing and the effectiveness of different therapeutic options. Their presence supposedly indicates a diminished ability for wound healing and scar formation.

AGEs are proteins to which sugar molecule is bound. The sugar molecule inhibits the original protein from functioning properly. As skin contains many proteins like collagen, the formation of these AGEs could be a viable explanation for the diminished functioning with ageing. In this review, we investigated whether the accumulation of AGEs affects wound healing and scar formation.

Normal scar formation results in a thin scar. However, it may happen that scarring results in thick, large, painful and itchy scars. We investigated whether people with a high AGE content in their skin, like diabetics and elderly, have difficulties forming aesthetically pleasing scars. Secondly, we investigated which therapies reduce the AGE content and, if so, whether these therapies can improve wound healing and scarring. This literature study involved research in scientific databases with qualitative analysis of 14 articles after a three-staged selection process with the use of set criteria.

We found the different ways in which AGEs affect skin properties and wound healing. Collagen, one of the most important proteins in the skin, is affected by these AGEs. Once a sugar binds to it, the collagen strings becomes thinner and shorter, and the different collagen proteins cross-link with each other in an unstructured way. The result of these alterations is a reduced elasticity, i.e. the skin becomes stiffer. The scar will be thinner and the time for wounds to close is longer. We also found strategies to diminish the AGE content, including dietary AGE restriction and Metformin, a drug used in diabetes.

We can conclude that there is proof of AGEs playing an important role in skin ageing, wound healing and the effectiveness of different therapeutic options. However, more research is required to conclude the exact role of AGEs in scar formation and scar therapy.

Effect of glycation inhibitors on aging and age-related diseases

Vast evidence supports the view that glycation of proteins is one of the main factors contributing to aging and is an important element of etiopathology of age-related diseases, especially type 2 diabetes mellitus, cataract and neurodegenerative diseases. Counteracting glycation can therefore be a means of increasing both the lifespan and healthspan. In this review, accumulation of glycation products during aging is presented, pathophysiological effects of glycation are discussed and ways of attenuation of the effects of glycation are described, concentrating on prevention of glycation. The effects of glycation and glycation inhibitors on the course of selected age-related diseases, such as Alzheimer's disease, Parkinson's disease and cataract are also reviewed.

Targeting advanced glycation with pharmaceutical agents: where are we now?

Advanced glycation end products (AGEs) are the final products of the Maillard reaction, a complex process that has been studied by food chemists for a century. Over the past 30 years, the biological significance of advanced glycation has also been discovered. There is mounting evidence that advanced glycation plays a homeostatic role within the body and that food-related Maillard products, intermediates such as reactive α-dicarbonyl compounds and AGEs, may influence this process. It remains to be understood, at what point AGEs and their intermediates become pathogenic and contribute to the pathogenesis of chronic diseases that inflict current society. Diabetes and its complications have been a major focus of AGE biology due to the abundance of excess sugar and α-dicarbonyls in this family of diseases. While further temporal information is required, a number of pharmacological agents that inhibit components of the advanced glycation pathway have already showed promising results in preclinical models. These therapies appear to have a wide range of mechanistic actions to reduce AGE load. Some of these agents including Alagebrium, have translated successfully to clinical trials, while others such as aminoguanidine, have had undesirable side-effect profiles. This review will discuss different pharmacological agents that have been used to reduce AGE burden in preclinical models of disease with a focus on diabetes and its complications, compare outcomes of those therapies that have reached clinical trials, and provide further rationale for the use of inhibitors of the glycation pathway in chronic diseases.

Altered Expression of NF- κ B and SP1 after Exposure to Advanced Glycation End-Products and Effects of Neurotrophic Factors in AGEs Exposed Rat Retinas

To determine the effect of advanced glycation end-products (AGEs) on neurite regeneration, and also to determine the regenerative effects of different neurotrophic factors (NTFs) on rat retinal explants, the retinas of SD rats were cultured in three-dimensional collagen gels and incubated in 6 types of media: (1) serum-free control culture media; (2) 100 μg/mL AGEs-BSA media; (3) AGEs-BSA + 100 ng/mL neurotrophin-4 (NT-4) media; (4) AGEs-BSA + 100 ng/mL hepatocyte growth factor media; (5) AGEs-BSA + 100 ng/mL glial cell line-derived neurotrophic factor media; or (6) AGEs-BSA + 100 µM tauroursodeoxycholic acid media. After 7 days, the number of regenerating neurites was counted. The explants were immunostained for nuclear factor-κB (NF-κB) and specificity protein 1 (SP1). Statistical analyses were performed by one-way ANOVA. In retinas incubated with AGEs, the numbers of neurites were fewer than in control. All of the NTFs increased the number of neurites, and the increase was more significant in the NT-4 group. The number of NF-κB and SP1 immunopositive cells was higher in retinas exposed to AGEs than in control. All of the NTFs decreased the number of NF-κB immunopositive cells but did not significantly affect SP1 expression. These results demonstrate the potential of the NTFs as axoprotectants in AGEs exposed retinal neurons.

Recent development of plant products with anti-glycation activity: a review

This review article summarizes the plant natural products that inhibit glycation at different stages leading to the AGEs formation.

Topiramate Protects Pericytes from Glucotoxicity: Role for Mitochondrial CA VA in Cerebromicrovascular Disease in Diabetes

Hyperglycemia in diabetes mellitus causes oxidative stress and pericyte depletion from the microvasculature of the brain thus leading to the Blood-Brain Barrier (BBB) disruption. The compromised BBB exposes the brain to circulating substances, resulting in neurotoxicity and neuronal cell death. The decline in pericyte numbers in diabetic mouse brain and pericyte apoptosis in high glucose cultures are caused by excess superoxide produced during enhanced respiration (mitochondrial oxidative metabolism of glucose). Superoxide is precursor to all Reactive Oxygen Species (ROS) which, in turn, cause oxidative stress. The rate of respiration and thus the ROS production is regulated by mitochondrial carbonic anhydrases (mCA) VA and VB, the two isoforms expressed in the mitochondria. Inhibition of both mCA: decreases the oxidative stress and restores the pericyte numbers in diabetic brain; and reduces high glucose-induced respiration, ROS, oxidative stress, and apoptosis in cultured brain pericytes. However, the individual role of the two isoforms has not been established. To investigate the contribution of mCA VA in ROS production and apoptosis, a mCA VA overexpressing brain pericyte cell line was engineered. These cells were exposed to high glucose and analyzed for the changes in ROS and apoptosis. Overexpression of mCA VA significantly increased pericyte ROS and apoptosis. Inhibition of mCA VA with topiramate prevented increases both in glucose-induced ROS and pericyte death. These results demonstrate, for the first time, that mCA VA regulates the rate of pericyte respiration. These findings identify mCA VA as a novel and specific therapeutic target to protect the cerebromicrovascular bed in diabetes.

DW1029M, a novel botanical drug candidate, inhibits advanced glycation end-product formation, rat lens aldose reductase activity, and TGF-β1 signaling

DW1029M is a botanical extract consisting of Morus bark and Puerariae radix, produced by Dong-Wha Pharmaceutical, for nephroprotective drug development; it has been in phase II clinical trials in Korea. In our mechanistic investigations, we found that DW1029M inhibits advanced glycation end products (AGEs), rat lens aldose reductase (RLAR), and transforming growth factor (TGF)-β1 signaling, all of which are implicated in diabetic complications such as diabetic nephropathy and diabetic retinopathy. DW1029M inhibits AGE formation via Fe(2+) chelation. The extract contains 13 active constituents that inhibit AGE formation, 8 active constituents that inhibit RLAR activity, and 1 inhibitor of TGF-β1 signaling. Our results suggest DW1029M protects against diabetic nephropathy via blockade of AGE formation, RLAR activity, and TGF-β1 signaling.

The pathobiology of diabetic vascular complications--cardiovascular and kidney disease

With the increasing incidence of obesity and type 2 diabetes, it is predicted that more than half of Americans will have diabetes or pre-diabetes by 2020. Diabetic patients develop vascular complications at a much faster rate in comparison to non-diabetic individuals, and cardiovascular risk is increased up to tenfold. With the increasing incidence of diabetes across the world, the development of vascular complications will become an increasing medical burden. Diabetic vascular complications affect the micro- and macro-vasculature leading to kidney disease often requiring dialysis and transplantation or cardiovascular disease increasing the risk for myocardial infarction, stroke and amputations as well as leading to premature mortality. It has been suggested that many complex pathways contribute to the pathobiology of diabetic complications including hyperglycaemia itself, the production of advanced glycation end products (AGEs) and interaction with the receptors for AGEs such as the receptor for advanced glycation end products (RAGE), as well as the activation of vasoactive systems such as the renin-angiotensin aldosterone system (RAAS) and the endothelin system. More recently, it has been hypothesised that reactive oxygen species derived from NAD(P)H oxidases (Nox) may represent a common downstream mediator of vascular injury in diabetes. Current standard treatment of care includes the optimization of blood glucose and blood pressure usually including inhibitors of the renin-angiotensin system. Although these interventions are able to delay progression, they fail to prevent the development of complications. Thus, there is an urgent medical need to identify novel targets in diabetic vascular complications which may include the blockade of Nox-derived ROS formation, as well as blockade of AGE formation and inhibitors of RAGE activation. These strategies may provide superior protection against the deleterious effects of diabetes on the vasculature.

Advanced glycation end products (AGE) and diabetes: cause, effect, or both?

Despite new and effective drug therapies, insulin resistance (IR), type 2 diabetes mellitus (T2D) and its complications remain major medical challenges. It is accepted that IR, often associated with over-nutrition and obesity, results from chronically elevated oxidant stress (OS) and chronic inflammation. Less acknowledged is that a major cause for this inflammation is excessive consumption of advanced glycation end products (AGEs) with the standard western diet. AGEs, which were largely thought as oxidative derivatives resulting from diabetic hyperglycemia, are increasingly seen as a potential risk for islet β-cell injury, peripheral IR and diabetes. Here we discuss the relationships between exogenous AGEs, chronic inflammation, IR, and T2D. We propose that under chronic exogenous oxidant AGE pressure the depletion of innate defense mechanisms is an important factor, which raises susceptibility to inflammation, IR, T2D and its complications. Finally we review evidence on dietary AGE restriction as a nonpharmacologic intervention, which effectively lowers AGEs, restores innate defenses and improves IR, thus, offering new perspectives on diabetes etiology and therapy.

High glucose-induced mitochondrial respiration and reactive oxygen species in mouse cerebral pericytes is reversed by pharmacological inhibition of mitochondrial carbonic anhydrases: Implications for cerebral microvascular disease in diabetes

Hyperglycemia-induced oxidative stress leads to diabetes-associated damage to the microvasculature of the brain. Pericytes in close proximity to endothelial cells in the brain microvessels are vital to the integrity of the blood-brain barrier and are especially susceptible to oxidative stress. According to our recently published results, streptozotocin-diabetic mouse brain exhibits oxidative stress and loose pericytes by twelve weeks of diabetes, and cerebral pericytes cultured in high glucose media suffer intracellular oxidative stress and apoptosis. Oxidative stress in diabetes is hypothesized to be caused by reactive oxygen species (ROS) produced during hyperglycemia-induced enhanced oxidative metabolism of glucose (respiration). To test this hypothesis, we investigated the effect of high glucose on respiration rate and ROS production in mouse cerebral pericytes. Previously, we showed that pharmacological inhibition of mitochondrial carbonic anhydrases protects the brain from oxidative stress and pericyte loss. The high glucose-induced intracellular oxidative stress and apoptosis of pericytes in culture were also reversed by inhibition of mitochondrial carbonic anhydrases. Therefore, we extended our current study to determine the effect of these inhibitors on high glucose-induced increases in pericyte respiration and ROS. We now report that both the respiration and ROS are significantly increased in pericytes challenged with high glucose. Furthermore, inhibition of mitochondrial carbonic anhydrases significantly slowed down both the rate of respiration and ROS production. These data provide new evidence that pharmacological inhibitors of mitochondrial carbonic anhydrases, already in clinical use, may prove beneficial in protecting the brain from oxidative stress caused by ROS produced as a consequence of hyperglycemia-induced enhanced respiration.

Pathogenesis and novel treatment from the mouse model of type 2 diabetic nephropathy

Diabetic nephropathy (DN) is the leading cause of end-stage kidney disease worldwide. However, current treatments remain suboptimal. Many factors, such as genetic and nongenetic promoters, hypertension, hyperglycemia, the accumulation of advanced glycation end products (AGEs), dyslipidemia, and albuminuria/proteinuria itself, influence the progression of this disease. It is important to determine the molecular mechanisms and treatment of this disease. The development of diabetes results in the formation of AGEs, oxidative stress, and the activation of the renin-angiotensin-aldosterone system (RAAS) within the kidney, which promotes progressive inflammation and fibrosis, leading to DN and declining renal function. A number of novel therapies have also been tested in the experimental diabetic model, including exercise, inhibitors of the RAAS (angiotensin type 1 receptor blockers (ARB), angiotensin-converting enzyme (ACE) inhibitors), inhibitors of AGE (pyridoxamine), peroxisome proliferator-activated receptor (PPAR) γ agonists (pioglitazone), inhibitors of lipid accumulation (statins and eicosapentaenoic acid (EPA)), and the vitamin D analogues. This review summarizes the advances in knowledge gained from our studies and therapeutic interventions that may prevent this disease.

Diabetes, insulin-mediated glucose metabolism and Sertoli/blood-testis barrier function

Blood testis barrier (BTB) is one of the tightest blood-barriers controlling the entry of substances into the intratubular fluid. Diabetes Mellitus (DM) is an epidemic metabolic disease concurrent with falling fertility rates, which provokes severe detrimental BTB alterations. It induces testicular alterations, disrupting the metabolic cooperation between the cellular constituents of BTB, with dramatic consequences on sperm quality and fertility. As Sertoli cells are involved in the regulation of spermatogenesis, providing nutritional support for germ cells, any metabolic alteration in these cells derived from DM may be responsible for spermatogenesis disruption, playing a crucial role in fertility/subfertility associated with this pathology. These cells have a glucose sensing machinery that reacts to hormonal fluctuations and several mechanisms to counteract hyper/hypoglycemic events. The role of DM on Sertoli/BTB glucose metabolism dynamics and the metabolic molecular mechanisms through which DM and insulin deregulation alter its functioning, affecting male reproductive potential will be discussed.

Mechanisms of diabetic complications

It is increasingly apparent that not only is a cure for the current worldwide diabetes epidemic required, but also for its major complications, affecting both small and large blood vessels. These complications occur in the majority of individuals with both type 1 and type 2 diabetes. Among the most prevalent microvascular complications are kidney disease, blindness, and amputations, with current therapies only slowing disease progression. Impaired kidney function, exhibited as a reduced glomerular filtration rate, is also a major risk factor for macrovascular complications, such as heart attacks and strokes. There have been a large number of new therapies tested in clinical trials for diabetic complications, with, in general, rather disappointing results. Indeed, it remains to be fully defined as to which pathways in diabetic complications are essentially protective rather than pathological, in terms of their effects on the underlying disease process. Furthermore, seemingly independent pathways are also showing significant interactions with each other to exacerbate pathology. Interestingly, some of these pathways may not only play key roles in complications but also in the development of diabetes per se. This review aims to comprehensively discuss the well validated, as well as putative mechanisms involved in the development of diabetic complications. In addition, new fields of research, which warrant further investigation as potential therapeutic targets of the future, will be highlighted.

Rehmannia glutinosa suppresses inflammatory responses elicited by advanced glycation end products

Fresh rhizome of Rehmannia glutinosa Libosch. (Saeng-jihwang in Korean: SJH) has been prescribed for the treatment of diabetes-associated complications. The purpose of the present study is to investigate the underlying mechanisms of the efficacy of SJH in diabetes-related complications. Decoction was obtained after boiling SJH in water and subsequent lyophilization. The cellular toxicity of SJH was determined by MTT assay. The antioxidant activity of SJH was measured by DPPH and DCFH-DA assays. The effects of SJH on inflammatory responses elicited by AGEs were assessed by western blotting and semi-quantitative RT-PCR analyses. The water extract of SJH had a high free radical scavenging activity in vitro and decreased the level of intracellular ROS in THP-1 cells treated with AGEs. SJH suppressed the expression of pro-inflammatory genes, including TNF-α, MCP-1, IP-10, COX-2, and iNOS; the activation of NF-κB; and the expression of RAGE, a receptor for AGEs, where the expressions of which were induced by AGEs. These results suggest the possibility that SJH can be an alternative therapeutics for diabetes-associated diseases.

Advanced glycation urinary protein-bound biomarkers and severity of diabetic nephropathy in man

BACKGROUND/AIMS

The formation of advanced glycation end products (AGEs) is accelerated in patients with diabetic nephropathy. The aim of this study was to ascertain if the urinary excretion of proteins modified by advanced glycation can be used as biomarkers for albuminuria in individuals with type 1 or type 2 diabetes.

METHODS

Community-based patients with type 1 (n = 68) or type 2 diabetes (n = 216) attending a diabetes clinic of a tertiary referral hospital were classified as having normoalbuminuria (Normo, albumin excretion rate (AER) <20 μg/min), microalbuminuria (Micro, AER 20-200 μg/min) or macroalbuminuria (Macro, AER ≥200 μg/min). Serum and urine AGE-modified proteins were measured.

RESULTS

In patients with both type 1 diabetes and type 2 diabetes, there was a clear association between the degree of albuminuria and urinary AGE-modified proteins (p < 0.0001). Exclusive to patients with type 1 diabetes, urinary excretion of the AGE carboxymethyllysine correlated with AER, whereas patients with type 2 diabetes and macroalbuminuria had an increase in urinary methylglyoxal, an AGE intermediate. These changes were independent of isotopic glomerular filtration rate levels. Serum concentrations of AGEs or soluble receptor for AGEs were not consistently associated with albuminuria in either type 1 or type 2 diabetes.

CONCLUSIONS

Urinary excretion of proteins modified by AGEs may be useful biomarkers of albuminuria in individuals with type 1 and type 2 diabetes, warranting prospective investigation in larger diabetic cohorts.

The Nε-(carboxymethyl)lysine-RAGE axis: putative implications for the pathogenesis of obesity-related complications

Obesity is an important contributor to the burden of insulin resistance, Type 2 diabetes and cardiovascular disease. An important mechanism by which excess adiposity causes obesity-associated complications is the dysregulated production and secretion of biologically active molecules derived from adipocytes. These adipokines affect the vascular wall and contribute to the development of insulin resistance and Type 2 diabetes. However, factors that cause an increased production of pro-inflammatory adipokines, while decreasing anti-inflammatory adipokines, have not been fully clarified. Owing to local conditions in adipose tissue, that is, increased fatty acids, hypoxia and oxidative stress, we speculate that an increased formation of the major advanced lipoxidation end product, N^ε-(carboxymethyl)lysine (CML), may play a role. CML-adducts in proteins are major ligands for the receptor for advanced glycation end products (RAGE). The consequence of RAGE activation by CML is the activation of important signaling inflammatory pathways. The putative role of CML-modified proteins in obesity is addressed in this article. The identification of this pathway may provide an important strategy for novel therapeutic approaches against obesity-associated complications.

Early- and advanced non-enzymatic glycation in diabetic vascular complications: the search for therapeutics

Cardiovascular disease is a common complication of diabetes and the leading cause of death among people with diabetes. Because of the huge premature morbidity and mortality associated with diabetes, prevention of vascular complications is a key issue. Although the exact mechanism by which vascular damage occurs in diabetes in not fully understood, numerous studies support the hypothesis of a causal relationship of non-enzymatic glycation with vascular complications. In this review, data which point to an important role of Amadori-modified glycated proteins and advanced glycation endproducts in vascular disease are surveyed. Because of the potential role of early- and advanced non-enzymatic glycation in vascular complications, we also described recent developments of pharmacological inhibitors that inhibit the formation of these glycated products or the biological consequences of glycation and thereby retard the development of vascular complications in diabetes.

Emerging drugs for diabetic neuropathy

IMPORTANCE OF THE FIELD

Diabetic neuropathy (DN) is a very common and disabling diabetes-related complication. DN is associated with significant morbidity and mortality. Diabetic peripheral neuropathy (DPN) can be painful in the earlier stages of the disease before becoming painless. Most of the currently available therapies are symptomatic (focusing on pain relief) rather than disease-modifying. With the exception of good glycemic control, there is currently no effective treatment to slow the progression of or reverse DPN.

AREAS COVERED IN THIS REVIEW

In this article, we review the epidemiology, pathogenesis, currently available and future treatments for DPN, and the potential development issues/challenges related to such new therapies. Literature search was performed using PubMed, Medline and Pharmaprojects from 1950 onwards. Search terms include a combination of terms such as diabetic neuropathy, pathogenesis, pathophysiology, mechanisms, treatment, therapy, oxidative/nitrosative stress, anti-oxidants, serotonin, nitrotyrosine, protein kinase C, aldose reductase, sodium channels, taurine, lipoic acid and poly (ADP-ribose) polymerase.

WHAT THE READER WILL GAIN

The reader will gain an overview of the epidemiology, clinical features and risk factors of DN. In addition, the reader will have a better understanding of the mechanisms that underpin the development of DPN and their relationships to the current and future therapies. The reader will also develop an insight into the limitations of the current approach to DPN treatment and the potential avenues for future research.

TAKE HOME MESSAGE

DN is a very common and disabling complication that currently has no effective treatments other than diabetes control. The pathogenesis of DPN is complex and multi-factorial. Several disease-modifying and symptomatic treatments are currently under development. Oxidative and nitrosative stress have been identified as key pathogenic factors in the development of DPN and new treatments target these pathways and/or their downstream consequences. Gene therapy and growth factors have also emerged as potential new therapies that target particular cellular compartments as opposed to being delivered systemically. The recognition of the difficulty in reversing established DN has focused efforts on slowing its progression.

Advanced glycation end-products and the kidney

BACKGROUND

Advanced glycation end-products (AGEs) are increased in situations with hyperglycemia and oxidative stress such as diabetes mellitus. They are products of nonenzymatic glycation and oxidation of proteins and lipids. The kidney plays an important role in clearance and metabolism of AGEs.

METHODS

Medline and other relevant databases were searched. In addition, key review articles were scanned for relevant original publication. Finally, original data from our research group were also included.

RESULTS

Kidney podocytes and endothelial cells express specific receptors for AGEs. Their activation leads to multiple pathophysiological effects including hypertrophy with cell cycle arrest and apoptosis, altered migration, and generation of proinflammatory cytokines. AGEs have been primarily implicated in the pathophysiology of diabetic nephropathy and diabetic microvascular complications. AGEs are also involved in other primary renal diseases as well as in the development and progression of atherosclerosis. However, serum or plasma concentrations of AGEs do not correlate well with cardiovascular events in patients with chronic kidney disease (CKD). This is likely due to the fact that serum concentrations failed to correlate with AGEs deposited in target tissues. Several inhibitors of the AGE-RAGE axis are currently tested for various indications.

CONCLUSION

AGEs and their receptors are involved in the pathogenesis of vascular and kidney disease. The role of circulating AGEs as biomarkers for cardiovascular risk estimation is questionable. Whether putative inhibitors of AGEs will get the maturity for its therapeutic use in the future remains open.

Neurodegenerative diseases of the retina and potential for protection and recovery

Recent advances in our understanding of the mechanisms in the cascade of events resulting in retinal cell death in ocular pathologies like glaucoma, diabetic retinopathy and age-related macular degeneration led to the common descriptive term of neurodegenerative diseases of the retina. The final common pathophysiologic pathway of these diseases includes a particular form of metabolic stress, resulting in an insufficient supply of nutrients to the respective target structures (optic nerve head, retina). During metabolic stress, glutamate is released initiating the death of neurones containing ionotropic glutamate (N-methyl-D-aspartat, NMDA) receptors present on ganglion cells and a specific type of amacrine cells. Experimental studies demonstrate that several drugs reduce or prevent the death of retinal neurones deficient of nutrients. These agents generally block NMDA receptors to prevent the action of glutamate or halt the subsequent pathophysiologic cycle resulting in cell death. The major causes for cell death following activation of NMDA receptors are the influx of calcium and sodium into cells, the generation of free radicals linked to the formation of advanced glycation endproducts (AGEs) and/or advanced lipoxidation endproducts (ALEs) as well as defects in the mitochondrial respiratory chain. Substances preventing these cytotoxic events are considered to be potentially neuroprotective.

Potential new therapeutic agents for diabetic kidney disease

Diabetic nephropathy is the leading cause of end-stage renal disease, and both the incidence and prevalence of diabetic nephropathy continue to increase. Currently, various treatment regimens and combinations of therapies provide only partial renoprotection. It is obvious that new approaches are desperately needed to retard the progression of diabetic nephropathy. Recently, a number of new agents have been described that have the potential to delay the progression of diabetic kidney disease and minimize the growing burden of end-stage renal disease. These include inhibitors and breakers of advanced glycation end products, receptor antagonists for advanced glycation end products, protein kinase C inhibitors, NADPH (reduced nicotinamide adenine dinucleotide phosphate) oxidase inhibitors, glycosaminoglycans, endothelin receptor antagonists, antifibrotic agents, and growth factor inhibitors. This review addresses these promising new therapeutic agents for delaying the progression of diabetic kidney disease.

Association analysis of ADPRT1, AKR1B1, RAGE, GFPT2 and PAI-1 gene polymorphisms with chronic renal insufficiency among Asian Indians with type-2 diabetes

BACKGROUND

To determine association of nine single nucleotide polymorphisms (SNPs) in ADP ribosyltransferase-1 (ADPRT1), aldo-keto reductase family 1 member B1 (AKR1B1), receptor for advanced glycation end-products (RAGE), glutamine:fructose-6-phosphate amidotransferase-2 (GFPT2), and plasminogen activator inhibitor-1 (PAI-1) genes with chronic renal insufficiency (CRI) among Asian Indians with type 2 diabetes; and to identify epistatic interactionss between genes from the present study and those from renin-angiotensin-aldosterone system (RAAS), and chemokine-cytokine, dopaminergic and oxidative stress pathways (previously investigated using the same sample set).

METHODS

Type 2 diabetes subjects with CRI (serum creatinine > or =3.0 mg/dl) constituted the cases (n = 196), and ethnicity and age matched individuals with diabetes for a duration of > or = 10 years, normal renal functions and normoalbuminuria recruited as controls (n = 225). Allelic and genotypic constitution of 10 polymorphisms (SNPs) from five genes namely--ADPRT1, AKR1B1, RAGE, GFPT2 and PAI-1 with diabetic CRI was investigated. The genetic associations were evaluated by computation of odds ratio and 95% confidence interval. Multiple logistic regression analysis was carried out to correlate various clinical parameters with genotypes, and to study epistatic interactions between SNPs in different genes.

RESULTS

Single nucleotide polymorphisms -429 T>C in RAGE and rs7725 C>T SNP in 3' UTR in GFPT2 gene showed a trend towards association with diabetic CRI. Investigation using miRBase statistical tool revealed that rs7725 in GFPT2 was a perfect target for predicted miRNA (hsa miR-378) suggesting the presence of the variant 'T' allele may result in an upregulation of GFPT2 contributing to diabetic renal complication. Epistatic interaction between SNPs in transforming growth factor TGF-beta1 (investigated using the same sample set and reported elsewhere) and GFPT2 genotype was observed.

CONCLUSIONS

Association of SNPs in RAGE and GFPT2 suggest that the genes involved in modulation of oxidative pathway could be major contributor to diabetic chronic renal insufficiency. In addition, GFPT2 mediated overproduction of TGF-beta1 leading to endothelial expansion and thereby CRI seems likely, suggested by our observation of a significant interaction between GFPT2 with TGF-beta1 genes. Further, identification of predicted miRNA targets spanning the associated SNP in GFPT2 implicates the rs7725 SNP in transcriptional regulation of the gene, and suggests GFPT2 could be a relevant target for pharmacological intervention. Larger replication studies are needed to confirm these observations.

[Oxidative stress: a review on metabolic signaling in type 1 diabetes]

Diabetic complications appear to be multifactorial process. The biochemical and pathological mechanisms are associated with chronic hyperglycemia of diabetes and the increased oxidative stress which has been postulated to play a central role in these disorders. Accumulating evidence suggests that oxidative cell injury caused by free radicals contributes to the development of type 1 diabetes (DM1) complications and decreased efficiency of antioxidant defenses (both enzymatic and nonenzymatic) seems to correlate with the severity of pathological tissue changes in DM1. In this review, we report as oxidative stress may exert deleterious effects in diabetes, as well as address current strategies in study to down-regulating vascular injury.

Methylglyoxal contributes to the development of insulin resistance and salt sensitivity in Sprague-Dawley rats

OBJECTIVES

Methylglyoxal, a metabolite of the glycolysis pathway, may play an important role in the development of diabetes and hypertension, but the exact mechanism has not been fully elucidated. The present study was designed to investigate whether methylglyoxal could directly induce insulin resistance and salt sensitivity in Sprague-Dawley rats.

METHODS

Rats were allocated to four groups: control (normal drinking water), 1% methylglyoxal in drinking water, 1% methylglyoxal plus N-acetyl cysteine (NAC) (800 mg/kg per day), a methylglyoxal scavenger, or TM2002 (100 mg/kg per day), an advanced glycation endproducts (AGEs) inhibitor. After 4-week treatment insulin resistance was evaluated by an euglycemic hyperinsulinemic glucose clamp technique. In another set of rats, either a high-salt diet (4%) alone, standard rat chow with 1% methylglyoxal in drinking water or high-salt diet plus methylglyoxal was given for 4 weeks. Immunohistochemistry was performed to measure nitrotyrosine and methylglyoxal-induced AGEs, N-carboxyethyl-lysine (CEL) in the kidney.

RESULTS

Four-week treatment with NAC or TM2002 completely improved methylglyoxal-induced insulin resistance. Co-administration of methylglyoxal and high-salt diet significantly increased systolic blood pressure, urinary albumin excretion, urinary thiobarbituric acid-reactive substances excretion and the renal nitrotyrosine expression in the kidney (markers of oxidative stress) compared with methylglyoxal or high-salt diet alone. Renal CEL was significantly increased in methylglyoxal-treated rats compared with nonmethylglyoxal-treated rats.

CONCLUSION

These results indicate that methylglyoxal-induced insulin resistance and salt sensitivity at least in part by increasing oxidative stress and/or AGEs formation in Sprague-Dawley rats. The present study provides further evidence for methylglyoxal as one of the causative factors in the pathogenesis of insulin resistance and salt-sensitive hypertension.

A Novel Sartan Derivative With Very Low Angiotensin II Type 1 Receptor Affinity Protects the Kidney in Type 2 Diabetic Rats

Background— Antihypertensive angiotensin II receptor blockers (ARBs) protect the kidney, at least in part, independently of blood pressure lowering. Still, the extent to which blood pressure lowering is related to renoprotection remains unclear.

Methods and Results— 139 newly synthesized ARB-derivatives were assayed for inhibition of advanced glycation (AGEs). The 9 most powerful compounds were then tested for transition metal chelation, angiotensin II type 1 receptor (AT1R) affinity, and pharmacokinetic parameters. R-147176 was eventually selected as it strongly inhibits advanced glycation but is 6700 times less effective than olmesartan in AT1R binding. It is orally bioavailable and toxicologically safe. Despite a minimal blood pressure lowering effect, it provides significant renoprotection in 3 experimental rat models with renal injury, ie, obese, hypertensive, type 2 diabetic rats (SHR/NDmcr-cp), normotensive type 2 diabetic rats (Zucker diabetic fatty), and remnant kidney rats.

Conclusion— R-147176 retains renal protective properties despite a minimal blood pressure–lowering effect. Clearly, the renal benefits of ARBs do not necessarily depend on blood pressure lowering and AT1R affinity, but rather on the inhibition of AGEs and oxidative stress inherent to their chemical structure. R-147176 opens new avenues in the treatment of cardiovascular and kidney diseases.

Advanced glycation endproducts induce a proliferative response in vascular smooth muscle cells via altered calcium signaling

Advanced glycation endproducts (AGEs) are proteins that accumulate in the plasma of diabetics as a result of increased glucose concentrations and are closely linked with vascular disease. The mechanisms involved are still not clear. The aim of this study was to investigate whether AGE-induced changes in calcium (Ca2+) homeostasis could contribute to these mechanisms. Cultured porcine coronary artery vascular smooth muscle (VSM) cells were preincubated with glycated albumin for 96 h. The sphingosine 1-phosphate (S1P)-induced intracellular Ca2+ increase, although not increased in amplitude, was significantly prolonged in cells preincubated with glycated albumin. Intracellular Ca2+ imaging and electrophysiological recording of ion channel currents following release of caged Ca2+ indicated that this prolonged Ca2+ rise occurred predominantly via changes in Ca2+-induced Ca2+ release. Preincubation with glycated albumin also resulted in a threefold increase in expression of the receptor for AGE. As a consequence of the prolonged intracellular Ca2+ rise following preincubation with glycated albumin, the S1P-induced activation of the Ca2+-dependent phosphatase, calcineurin (CaN) was increased. This resulted in increased S1P-induced activation of the Ca2+-dependent transcription factor, nuclear factor of activated T cells (NFATc). BrdU incorporation in VSM cells was increased in cells preincubated with glycated albumin and was inhibited by the CaN inhibitor, cyclosporin A. In conclusion, AGE can induce VSM proliferation via a prolonged agonist-induced Ca2+ increase leading to increased activation of CaN and subsequently NFATc. This mechanism may contribute to pathogenesis of vascular disease in diabetes mellitus.

Inhibitors of advanced glycation end products (AGEs): potential utility for the treatment of cardiovascular disease

Accelerated atherosclerosis and microvascular complications are the leading causes of coronary heart disease, stroke, blindness, and end-stage renal failure, which could account for disabilities and high mortality rates in patients with diabetes. Recent clinical studies have substantiated the concept of "hyperglycemic memory" in the pathogenesis of cardiovascular disease (CVD) in diabetes. Indeed, the Diabetes Control and Complications Trial-Epidemiology of Diabetes Interventions and Complications (DCCT-EDIC) Research, has revealed that intensive therapy during the DCCT reduces the risk of cardiovascular events by about 50% in type 1 diabetic patients 11 years after the end of the trial. Among various biochemical pathways activated under diabetic conditions, the process of formation and accumulation of advanced glycation end products (AGEs) and their mode of action are most compatible with the theory "hyperglycemic memory." Further, there is a growing body of evidence that AGEs play an important role in CVD in diabetes. These observations suggest that the inhibition of AGEs formation may be a promising target for therapeutic intervention in diabetic vascular complications. Therefore, in this article, we review several agents with inhibitory effects on AGEs formation and their therapeutic implications in CVD in diabetes.