Effects of endothelin receptor antagonists on the progression of diabetic nephropathy

Pathomechanisms of Diabetic Kidney Disease

The worldwide occurrence of diabetic kidney disease (DKD) is swiftly rising, primarily attributed to the growing population of individuals affected by type 2 diabetes. This surge has been transformed into a substantial global concern, placing additional strain on healthcare systems already grappling with significant demands. The pathogenesis of DKD is intricate, originating with hyperglycemia, which triggers various mechanisms and pathways: metabolic, hemodynamic, inflammatory, and fibrotic which ultimately lead to renal damage. Within each pathway, several mediators contribute to the development of renal structural and functional changes. Some of these mediators, such as inflammatory cytokines, reactive oxygen species, and transforming growth factor β are shared among the different pathways, leading to significant overlap and interaction between them. While current treatment options for DKD have shown advancement over previous strategies, their effectiveness remains somewhat constrained as patients still experience residual risk of disease progression. Therefore, a comprehensive grasp of the molecular mechanisms underlying the onset and progression of DKD is imperative for the continued creation of novel and groundbreaking therapies for this condition. In this review, we discuss the current achievements in fundamental research, with a particular emphasis on individual factors and recent developments in DKD treatment.

Endothelin receptor antagonists in kidney protection for diabetic kidney disease and beyond?

The burden of chronic kidney disease is increasing worldwide, largely due to the increasing global prevalence of diabetes mellitus and hypertension. While renin angiotensin system inhibitors and sodium-glucose cotransporter two inhibitors are the management cornerstone for reducing kidney and cardiovascular complications in patients with diabetic and non-diabetic kidney disease (DKD), they are partially effective and further treatments are needed to prevent the progression to kidney failure. Endothelin receptor antagonism represent a potential additional therapeutic option due to its beneficial effect on pathophysiological processes involved in progressive kidney disease including proteinuria, which are independently associated with progression of kidney disease. This review discusses the biological mechanisms of endothelin receptor antagonists (ERA) in kidney protection, the efficacy and safety of ERA in randomised controlled trials reporting on kidney outcomes, and its potential future use in both diabetic and non-DKDs.

Endothelin receptors in renal interstitial cells do not contribute to the development of fibrosis during experimental kidney disease

Renal interstitial fibrosis is characterized by the development of myofibroblasts, originating from resident renal and immigrating cells. Myofibroblast formation and extracellular matrix production during kidney damage are triggered by various factors. Among these, endothelins have been discussed as potential modulators of renal fibrosis. Utilizing mouse models of adenine nephropathy (AN) and unilateral ureter occlusion (UUO), this study aimed to investigate the contribution of endothelin signaling in stromal mesenchymal resident renal interstitial cells. We found in controls that adenine feeding and UUO caused marked upregulations of endothelin-1 (ET-1) gene expression in endothelial and in tubular cells and a strong upregulation of ET_A-receptor (ET_A-R) gene expression in interstitial and mesangial cells, while the gene expression of ET_B-receptor (ET_B-R) did not change. Conditional deletion of ET_A-R and ET_B-R gene expression in the FoxD1 stromal cell compartment which includes interstitial cells significantly reduced renal ET_A-R gene expression and moderately lowered renal ET_B-R gene expression. ET receptor (ET-R) deletion exerted no apparent effects on kidney development nor on kidney function. Adenine feeding and UUO led to similar increases in profibrotic and proinflammatory gene expression in control as well as in ET_A^flflET_B^flfl FoxD1^Cre+ mice (ET-Ko). In summary, our findings suggest that adenine feeding and UUO activate endothelin signaling in interstitial cells which is due to upregulated ET_A-R expression and enhanced renal ET-1 production Our data also suggest that the activation of endothelin signaling in interstitial cells has less impact for the development of experimentally induced fibrosis.

Endothelin-targeted new treatments for proteinuric and inflammatory glomerular diseases: focus on the added value to anti-renin-angiotensin system inhibition

Chronic kidney disease (CKD) is the main cause of end-stage renal disease worldwide arising as a frequent complication of diabetes, obesity, and hypertension. Current therapeutic options, mainly based of inhibition of the renin-angiotensin system (RAS), provide imperfect renoprotection if started at an advanced phase of the disease, and treatments that show or even reverse the progression of CKD are needed. The endothelin (ET) system contributes to the normal renal physiology; however, robust evidence suggests a key role of ET-1 and its cognate receptors, in the progression of CKD. The effectiveness of ET receptor antagonists in ameliorating renal hemodynamics and fibrosis has been largely demonstrated in different experimental models. A significant antiproteinuric effect of ET receptor antagonists has been found in diabetic and non-diabetic CKD patients even on top of RAS blockade, and emerging evidence from ongoing clinical trials highlights their beneficial effects on a wide range of kidney disorders.

Update on diagnosis, pathophysiology, and management of diabetic kidney disease

Diabetic kidney disease (DKD) is a chronic complication of diabetes mellitus which may eventually lead to end-stage kidney disease (ESKD). Despite improvements in glycaemic control and blood pressure management with renin-angiotensin-aldosterone system (RAAS) blockade, the current therapy cannot completely halt DKD progression to ESKD in some patients. DKD is a heterogeneous disease entity in terms of its clinical manifestations, histopathology and the rate of progression, which makes it difficult to develop effective therapeutics. It was formerly considered that albuminuria preceded kidney function decline in DKD, but recent epidemiological studies revealed that a distinct group of patients presented kidney dysfunction without developing albuminuria. Other comorbidities, such as hypertension, obesity and gout, also affect the clinical course of DKD. The pathophysiology of DKD is complex and multifactorial, involving both metabolic and haemodynamic factors. These induce activation of intracellular signalling pathways, oxidative stress, hypoxia, dysregulated autophagy and epigenetic changes, which result in kidney inflammation and fibrosis. Recently, two groups of antidiabetic drugs, sodium-glucose cotransporter 2 (SGLT2) inhibitors and glucagon-like peptide-1 (GLP-1) receptor agonists, were demonstrated to provide renoprotection on top of their glucose-lowering effects. Several other therapeutic agents are also being developed and evaluated in clinical trials.

Endothelin receptor antagonists for the treatment of diabetic nephropathy: A meta-analysis and systematic review

BACKGROUND

Diabetic nephropathy (DN) is the main cause of chronic kidney disease and end-stage renal disease worldwide. Although available clinical trials have shown that endothelin receptor (ER) antagonists may be a novel and beneficial drug for DN, no consistent conclusions regarding their sufficient effectiveness and safety for patients with DN have been presented.

AIM

To assess the effectiveness and safety of ER antagonists among patients with DN.

METHODS

The EMBASE, PubMed, MEDLINE, Cochrane, and ClinicalTrials.gov databases were searched without any language restrictions. Relative risks with 95% confidence intervals (CIs) for dichotomous data and mean differences or standardized mean difference with 95%CIs for continuous data were calculated using Review Manager 5.3 software. Publication bias was assessed using Egger's test with Stata/SE software.

RESULTS

We enrolled seven studies with six data sets and 5271 participants. The ER antagonists group showed a significantly greater reduction in albuminuria and more patients with 40% reduction in urinary albumin-to-creatinine ratio than the control group (P < 0.0001 and P = 0.02, respectively). Subgroup analysis for reductions in estimated glomerular filtration rate (eGFR) showed that for the middle-dosage subgroup, the ER antagonists group exhibited lower eGFR reduction than the control group (P < 0.00001; mean difference, 0.70 95%CI: 0.66, 0.74). Moreover, significant reductions in systolic and diastolic blood pressure were observed in the invention group.

CONCLUSION

ER blockades combined with angiotensin converting enzyme inhibitor /angiotensin II type 1 receptor blockers may be an effective treatment to lower blood pressure and reduce proteinuria in DN with declined eGFR. However, attention should be given to adverse events, including cardiac failure, anemia, and hypoglycemia, as well as serious adverse events.

Beneficial effect on podocyte number in experimental diabetic nephropathy resulting from combined atrasentan and RAAS inhibition therapy

Podocyte loss and proteinuria are both key features of human diabetic nephropathy (DN). The leptin-deficient BTBR mouse strain with the ob/ob mutation develops progressive weight gain, type 2 diabetes, and diabetic nephropathy that has many features of advanced human DN, including increased mesangial matrix, mesangiolysis, podocyte loss, and proteinuria. Selective antagonism of the endothelin-1 type A receptor (ET_AR) by atrasentan treatment in combination with renin-angiotensin-aldosterone system inhibition with losartan has been shown to have the therapeutic benefit of lowering proteinuria in patients with DN, but the underlying mechanism for this benefit is not well understood. Using a similar therapeutic approach in diabetic BTBR ob/ob mice, this treatment regimen significantly increased glomerular podocyte number compared with diabetic BTBR ob/ob controls and suggested that parietal epithelial cells were a source for podocyte restoration. Atrasentan treatment alone also increased podocyte number but to a lesser degree. Mice treated with atrasentan demonstrated a reduction in proteinuria, matching the functional improvement reported in humans. This is a first demonstration that treatment with the highly selective ET_AR antagonist atrasentan can lead to restoration of the diminished podocyte number characteristic of DN in humans and thereby underlies the reduction in proteinuria in patients with diabetes undergoing similar treatment. The benefit of ET_AR antagonism in DN extended to a decrease in mesangial matrix as measured by a reduction in accumulations of collagen type IV in both the atrasentan and atrasentan + losartan-treated groups compared with untreated controls.

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.

Diabetic nephropathy: newer therapeutic perspectives

Diabetic nephropathy (DN is a dreaded consequence of diabetes mellitus, accounting for about 40% of end-stage renal disease (ESRD). It is responsible for significant morbidity and mortality, both directly by causing ESRD and indirectly by increasing cardiovascular risk. Extensive research in this field has thrown light on multiple pathways that can be pharmacologically targeted, to control or reverse the process of DN. Glomerulocentric approach of DN still continues to produce favourable results as evidenced by the recent data on SGLT-2 (sodium glucose co-transporter type 2) inhibitors. Beyond the glomerular mechanisms, numerous novel pathways have been discovered in the last decade. Some of these pathways target inflammatory and oxidative damage, while the others target more specific mechanisms such as AGE-RAGE (advanced glycation end products-receptors for advanced glycation end products), ASK (apoptotic signal-regulating kinase), and endothelin-associated pathways. As a result of the research, a handful of clinically relevant drugs have made it to the human trials which have been elucidated in the following review, bearing in the mind that there are many more to come over the next few years. Ongoing research is expected to inform the clinicians regarding the use of the newer drugs in DN. Abbreviations: USFDA: Unites States Food and Drug Administration; SGLT-2: Sodium glucose transporter type 2; GLP-1: Glucagon-like peptide-1; DDP-4: Dipeptidyl peptidase-4; UACR: urinary albumin creatinine ratio; eGFR: Estimated glomerular filtration rate; CKD: Chronic kidney disease; DN: Diabetic nephropathy; TGF: Tubuloglomerular feedback; RAAS: Renin angiotensin aldosterone system; T1DM: Type 1 diabetes mellitus; T2DM: Type 2 diabetes mellitus; RCT: Randomized controlled trial; AGE-RAGE: Advanced glycation end products-receptors for advanced glycation end products; ASK-1: Apoptotic signal-regulating kinase-1; Nrf-2: Nuclear 1 factor [erythroid derived-2]-related factor 2; ml/min/1.73m²: Millilitre/minute/1.73 square meters of body surface area; ~: Approximately.

Cross-Talk Between Insulin Signaling and G Protein-Coupled Receptors

Diabetes is a major risk factor for the development of heart failure. One of the hallmarks of diabetes is insulin resistance associated with hyperinsulinemia. The literature shows that insulin and adrenergic signaling is intimately linked to each other; however, whether and how insulin may modulate cardiac adrenergic signaling and cardiac function remains unknown. Notably, recent studies have revealed that insulin receptor and β2 adrenergic receptor (β2AR) forms a membrane complex in animal hearts, bringing together the direct contact between 2 receptor signaling systems, and forming an integrated and dynamic network. Moreover, insulin can drive cardiac adrenergic desensitization via protein kinase A and G protein-receptor kinases phosphorylation of the β2AR, which compromises adrenergic regulation of cardiac contractile function. In this review, we will explore the current state of knowledge linking insulin and G protein-coupled receptor signaling, especially β-adrenergic receptor signaling in the heart, with emphasis on molecular insights regarding its role in diabetic cardiomyopathy.

Endothelin A receptor antagonists in diabetic kidney disease

PURPOSE OF REVIEW

Despite optimal therapy of diabetic nephropathy with agents blocking the renin-angiotensin-aldosterone system, the residual risk of nephropathy progression to end-stage renal disease (ESRD) remains high. The purpose of this review is to discuss the potential role of endothelin antagonism as a therapeutic tool to reduce residual proteinuria and delay kidney injury progression among patients with diabetic nephropathy.

RECENT FINDINGS

Preclinical studies have shown that endothelin receptor antagonists (ERAs) exert proteinuria lowering and nephroprotective actions in experimental models of diabetic nephropathy. ERAs reduce proteinuria in phase 2 trials that included therapy with renin-angiotensin-aldosterone system blockers. Safety of these agents and protection from ESRD needs to be demonstrated in phase 3 trials. Excess risk of fluid retention and heart failure risk remains.

SUMMARY

The hypothesis that the antiproteinuric effect of endothelin antagonism may be translated into a slower progression of diabetic nephropathy to ESRD is investigated in ongoing randomized trials assessing 'hard' renal endpoints. ERAs may represent a promising tool toward renoprotection in diabetic nephropathy by individualizing therapy and mitigating the risk of heart failure, if these trials are positive.

Atrasentan for the treatment of diabetic nephropathy

INTRODUCTION

Endothelin-1 (ET-1) is the most potent vasoconstrictor, and is involved in the renal regulation of salt and water homeostasis. When produced in excess in the kidney, ET-1 promotes proteinuria and tubulointerstitial injury. There is great interest in the clinical use of endothelin receptor antagonists (ERAs) in chronic kidney disease (CKD), mainly in diabetic nephropathy (DN). Areas covered: Physiopathological actions of ET-1 on the kidney. Both dual ETAR/ET_BR (bosentan) or ET_AR specific endothelin antagonists (avosentan and atrasentan, among others), which have progressed to early clinical development, with particular emphasis on atrasentan. Expert opinion: Different phase I and II clinical trials with ERAs in DN, mostly with atrasentan, have shown that these drugs have a marked anti-proteinuric effect on residual proteinuria when administered as add-on therapy in addition to ACEi or ARAII treatment. In the past few years, a series of randomized controlled trials investigating new approaches to DN have provided negative or inconclusive data, or even were terminated due to safety concerns or lack of efficacy. Therefore, we eagerly but cautiously await the results of the ongoing SONAR trial with atrasentan in more than 4,000 patients including assessment of renal and cardiovascular hard-end points (estimated primary completion date, July 2018).

Protein Z and Endothelin-1 genetic polymorphisms in pediatric Egyptian sickle cell disease patients

BACKGROUND

Sickle cell disease (SCD) is a monogenic disease associated with multisystem morbidity. Vasculopathy caused by delicate imbalance between coagulation and endothelial systems plays a pivotal role in disease course. As Protein Z and Endothelin-1 genetic polymorphisms may increase the thrombotic risk, the aim of the current work was to verify the possible impact of Protein Z (PROZ G79A) and Endothelin-1 (EDN1 G5665T) polymorphisms on the clinic-laboratory features of the SCD in a cohort of Egyptian pediatric patients.

METHODS

Genotyping of Protein Z G79A and Endothelin-1 G5665T was carried out by polymerase chain reaction-restricted fragment length polymorphism (PCR-RFLP) assay for 100 SCD patients and 100 controls.

RESULTS

Protein -Z G79A polymorphism was not associated with vascular complications in the studied SCD patients. Endothelin-1 G5665T polymorphism was associated with pulmonary dysfunction (pulmonary artery hypertension and acute chest syndrome) and severe vaso-occlusive crises (VOC).

CONCLUSION

Endothelin-1 G5665T polymorphism could be considered as a molecular predictor for pulmonary dysfunction and severe VOC in SCD. Further researches with larger cohorts are recommended to understand the pathophysiology of SCD and to explain the inter-patients' variability of disease severity.

The next generation of therapeutics for chronic kidney disease

Chronic kidney disease (CKD) represents a leading cause of death in the United States. There is no cure for this disease, with current treatment strategies relying on blood pressure control through blockade of the renin-angiotensin system. Such approaches only delay the development of end-stage kidney disease and can be associated with serious side effects. Recent identification of several novel mechanisms contributing to CKD development - including vascular changes, loss of podocytes and renal epithelial cells, matrix deposition, inflammation and metabolic dysregulation - has revealed new potential therapeutic approaches for CKD. This Review assesses emerging strategies and agents for CKD treatment, highlighting the associated challenges in their clinical development.

Endothelin and tubulointerstitial renal disease

All components of the endothelin (ET) system are present in renal tubular cells. In this review, we summarize current knowledge about ET and the most common tubular diseases: acute kidney injury (AKI) and polycystic kidney disease. AKI originally was called acute tubular necrosis, pointing to the most prominent morphologic findings. Similarly, cysts in polycystic kidney disease, and especially in autosomal-dominant polycystic kidney disease, are of tubular origin. Preclinical studies have indicated that the ET system and particularly ETA receptors are involved in the pathogenesis of ischemia-reperfusion injury, although these findings have not been translated to clinical studies. The ET system also has been implicated in radiocontrast-dye-induced AKI, however, ET-receptor blockade in a large human study was not successful. The ET system is activated in sepsis models of AKI; the effectiveness of ET blocking agents in preclinical studies is variable depending on the model and the ET-receptor antagonist used. Numerous studies have shown that the ET system plays an important role in the complex pathophysiology associated with cyst formation and disease progression in polycystic kidney disease. However, results from selective targeting of ET-receptor subtypes in animal models of polycystic kidney disease have proved disappointing and do not support clinical trials. These studies have shown that a critical balance between ETA and ETB receptor action is necessary to maintain structure and function in the cystic kidney. In summary, ETs have been implicated in the pathogenesis of several renal tubulointerstitial diseases, however, experimental animal findings have not yet led to use of ET blockers in human beings.

Diabetic nephropathy: What does the future hold?

The consensus management of diabetic nephropathy (DN) in 2015 involves good control of glycaemia, dyslipidaemia and blood pressure (BP). Blockade of the renin-angiotensin-aldosterone system using angiotensin-converting enzyme inhibitors, angiotensin-2 receptor blockers or mineralocorticoid inhibitors are key therapeutic approaches, shown to be beneficial once overt nephropathy is manifest, as either, or both, of albuminuria and loss of glomerular filtration rate. Some significant additional clinical benefits in slowing the progression of DN was reported from the Remission clinic experience, where simultaneous intensive control of BP, tight glycaemic control, weight loss, exercise and smoking cessation were prioritised in the management of DN. This has not proved possible to translate to more conventional clinical settings. This review briefly looks over the history and limitations of current therapy from landmark papers and expert reviews, and following an extensive PubMed search identifies the most promising clinical biomarkers (both established and proposed). Many challenges need to be addressed urgently as in order to obtain novel therapies in the clinic; we also need to examine what we mean by remission, stability and progression of DN in the modern era.

Endothelin type A receptor inhibition normalises intrarenal hypoxia in rats used as a model of type 1 diabetes by improving oxygen delivery

AIMS/HYPOTHESIS

Intrarenal tissue hypoxia, secondary to increased oxygen consumption, has been suggested as a unifying mechanism for the development of diabetic nephropathy. Increased endothelin-1 signalling via the endothelin type A receptor (ETA-R) has been shown to contribute to the development of chronic kidney disease, but its role in kidney oxygen homeostasis is presently unknown.

METHODS

The effects of acute ETA-R inhibition (8 nmol/l BQ-123 for 30-40 min directly into the left renal artery) on kidney function and oxygen metabolism were investigated in normoglycaemic control and insulinopenic male Sprague Dawley rats (55 mg/kg streptozotocin intravenously 2 weeks before the main experiment) used as a model of type 1 diabetes.

RESULTS

Local inhibition of ETA-R in the left kidney did not affect BP in either the control or the diabetic rats. As previously reported, diabetic rats displayed increased kidney oxygen consumption resulting in tissue hypoxia in both the kidney cortex and medulla. The inhibition of ETA-Rs restored normal kidney tissue oxygen availability in the diabetic kidney by increasing renal blood flow, but did not affect oxygen consumption. Furthermore, ETA-R inhibition reduced the diabetes-induced glomerular hyperfiltration and increased the urinary sodium excretion. Kidney function in normoglycaemic control rats was largely unaffected by BQ-123 treatment, although it also increased renal blood flow and urinary sodium excretion in these animals.

CONCLUSIONS/INTERPRETATION

Acutely reduced intrarenal ETA-R signalling results in significantly improved oxygen availability in the diabetic kidney secondary to elevated renal perfusion. Thus, the beneficial effects of ETA-R inhibition on kidney function in diabetes may be due to improved intrarenal oxygen homeostasis.

Endothelin-1 gene polymorphisms and diabetic kidney disease in patients with type 2 diabetes mellitus

BACKGROUND AND AIMS

Diabetic kidney disease (DKD) is the leading cause of end stage renal disease worldwide and is associated with increased cardiovascular mortality. The endothelin system has been implicated in the pathogenesis of arterial hypertension and renal dysfunction. In the present study, the association of DKD with polymorphisms in ET-1 (EDN1) and ETRA (EDNRA) genes was analyzed in patients with type 2 diabetes mellitus (T2DM).

METHODS

A case-control study was conducted in 548 white T2DM patients. Patients with proteinuria or on dialysis were considered cases and patients with normoalbuminuria were considered controls. Two polymorphisms in the EDN1 gene (rs1800541 and rs57072783) and five in EDNRA gene (rs6842241; rs4835083; rs4639051; rs5333 and rs5343) were genotyped and haplotype analyses were performed.

RESULTS

The presence of rs57072783 T allele (TT/TG vs. GG) or rs1800541 G allele (GG/GT vs. TT) protected against DKD (OR = 0.69, 95 % CI 0.48-0.99, P = 0.049; and OR = 0.60, 95 % CI 0.41-0.88, P = 0.009, respectively). However in multivariate analyses, only the rs1800541 G allele remained independently associated with DKD (P = 0.046).

CONCLUSIONS

The present study shows that ET-1 could be involved in the pathogenesis of DKD in patients with T2DM.

Endothelin antagonists for diabetic and non-diabetic chronic kidney disease

Numerous pre-clinical studies have implicated endothelin-1 in the pathogenesis of diabetic and non-diabetic chronic kidney disease (CKD). Renal endothelin-1 production is almost universally increased in kidney disease. The pathologic effects of endothelin-1, including vasoconstriction, proteinuria, inflammation, cellular injury and fibrosis, are likely mediated by the endothelin A (ETA) receptor. ETA antagonism alone, and/or combined ETA/B blockade, reduces CKD progression. Based on the strong pre-clinical data, several clinical trials using ETA antagonists were conducted. Small trials involving acute intravenous endothelin receptor blockade suggest that ETA, but not ETB, blockade exerts protective renal and vascular effects in CKD patients. A large phase 3 trial (ASCEND) examined the effects of avosentan, an endothelin receptor antagonist, on renal disease progression in diabetic nephropathy. Proteinuria was reduced after 3-6 months of treatment. However the study was terminated due to increased morbidity and mortality associated with avosentan-induced fluid retention. Several phase 2 trials using avosentan at lower doses than in ASCEND, atrasentan or sitaxsentan (the latter two being highly ETA-selective) showed reductions in proteinuria on top of renin-angiotensin system blockade. Infrequent and clinically insignificant fluid retention was observed at the most effective doses. Additional trials using ETA blockers are ongoing or being planned in patients with diabetic nephropathy or focal segmental glomerulosclerosis. Moving forward, such studies must be conducted with careful patient selection and attention to dosing in order to minimize adverse side effects. Nonetheless, there is cause for optimism that this class of agents will ultimately prove to be effective for the treatment of CKD.

Linking the beneficial effects of current therapeutic approaches in diabetes to the vascular endothelin system

The rising epidemic of diabetes worldwide is of significant concern. Although the ultimate objective is to prevent the development and find a cure for the disease, prevention and treatment of diabetic complications is very important. Vascular complications in diabetes, or diabetic vasculopathy, include macro- and microvascular dysfunction and represent the principal cause of morbidity and mortality in diabetic patients. Endothelial dysfunction plays a pivotal role in the development and progression of diabetic vasculopathy. Endothelin-1 (ET-1), an endothelial cell-derived peptide, is a potent vasoconstrictor with mitogenic, pro-oxidative and pro-inflammatory properties that are particularly relevant to the pathophysiology of diabetic vasculopathy. Overproduction of ET-1 is reported in patients and animal models of diabetes and the functional effects of ET-1 and its receptors are also greatly altered in diabetic conditions. The current therapeutic approaches in diabetes include glucose lowering, sensitization to insulin, reduction of fatty acids and vasculoprotective therapies. However, whether and how these therapeutic approaches affect the ET-1 system remain poorly understood. Accordingly, in the present review, we will focus on experimental and clinical evidence that indicates a role for ET-1 in diabetic vasculopathy and on the effects of current therapeutic approaches in diabetes on the vascular ET-1 system.

Endothelin-1 and the kidney--beyond BP

Since its discovery over 20 years ago endothelin-1 (ET-1) has been implicated in a number of physiological and pathophysiological processes. Its role in the development and progression of chronic kidney disease (CKD) is well established and is an area of ongoing intense research. There are now available a number of ET receptor antagonists many of which have been used in trials with CKD patients and shown to reduce BP and proteinuria. However, ET-1 has a number of BP-independent effects. Importantly, and in relation to the kidney, ET-1 has clear roles to play in cell proliferation, podocyte dysfunction, inflammation and fibrosis, and arguably, these actions of ET-1 may be more significant in the progression of CKD than its prohypertensive actions. This review will focus on the potential role of ET-1 in renal disease with an emphasis on its BP-independent actions.

Emerging drugs for managing kidney disease in patients with diabetes

INTRODUCTION

The need for new approaches to manage the increasing numbers of patients with diabetes and their burden of complications is urgent. Of these, chronic kidney disease imposes some of the highest costs, both in dollars and in terms of human suffering. In individuals with diabetes, the presence and severity of kidney disease adversely affects their well-being, contributes to disease morbidity and increases their risk of a premature death.

AREAS COVERED

To collect information for the strategies previously or currently under investigation for managing kidney disease in patients with diabetes, a literature search was performed through the search engines PubMed and ClinicalTrials.gov.

EXPERT OPINION

Despite advancing knowledge on the pathogenesis of diabetic kidney disease, and promising effects in experimental models, at present there are no new drugs that come close to providing the solutions we desire for our patients. Even when used in combination with standard care, renal complications are at best only modestly reduced, at the considerable expense of additional pill burden and exposure to serious off-target effects. Some of the most exciting advances over the last decade, including thiazolidinediones, direct renin inhibitors, endothelin antagonists and most recently bardoxolone methyl have all fallen at this last hurdle. Better targeted ('smarter') drugs appear to be the best hope for renoprotective therapy.

Endothelin and endothelin receptors in the renal and cardiovascular systems

Endothelin-1 (ET-1) is a multifunctional hormone which regulates the physiology of the cardiovascular and renal systems. ET-1 modulates cardiac contractility, systemic and renal vascular resistance, salt and water renal reabsorption, and glomerular function. ET-1 is responsible for a variety of cellular events: contraction, proliferation, apoptosis, etc. These effects take place after the activation of the two endothelin receptors ET(A) and ET(B), which are present - among others - on cardiomyocytes, fibroblasts, smooth muscle and endothelial cells, glomerular and tubular cells of the kidney. The complex and numerous intracellular pathways, which can be contradictory in term of functional response depending on the receptor type, cell type and physiological situation, are described in this review. Many diseases share an enhanced ET-1 expression as part of the pathophysiology. However, the use of endothelin blockers is currently restricted to pulmonary arterial hypertension, and more recently to digital ulcer. The complexity of the endothelin system does not facilitate the translation of the molecular knowledge to clinical applications. Endothelin antagonists can prevent disease development but secondary undesirable effects limit their usage. Nevertheless, the increasing understanding of the effects of ET-1 on the cardiac and renal physiology maintains the endothelin system as a promising therapeutic target.

Endothelin-1 and endothelin a receptor immunoreactivity is increased in patients with diabetic nephropathy

BACKGROUND

Endothelin-1 (ET-1) is associated with progression of renal disease, acting as a vasoconstrictor and growth factor for mesangial cells. ET-1 and endothelin A receptor (ET-RA) might have a role in the development of diabetic nephropathy (DN). The aims of this study were to determine ET-1 and ET-RA expressions in patients with DN and to correlate these expressions with renal function and proteinuria.

MATERIALS AND METHODS

This is a cross-sectional study comprising 13 patients with type 2 diabetes mellitus and DN, 10 patients with proteinuric IgA nephropathy, and 13 samples of normal kidney from tumor nephrectomies. Demographic and selected data were collected from medical charts. The distribution and intensity of ET-1 and ET-RA immunostaining in renal biopsies were determined by immunohistochemistry and these correlated with the estimated glomerular filtration rate (eGFR) and proteinuria.

RESULTS

Patients with DN and IgA nephropathy on biopsy had markedly increased staining for ET-1 in endothelial cells of glomerular and peritubular capillaries when compared with controls (p < 0.001). ET-RA staining was also more intense and more diffuse in DN and IgA nephropathy than in controls (p = 0.019) and was restricted to tubular epithelial cells. A positive correlation was observed between ET-1 expression and proteinuria (r = 0.634, p = 0.027), but both ET-1 and ET-RA expressions did not correlate with eGFR.

CONCLUSION

In this preliminary report, the higher expressions of ET-1 and ET-RA found in both DN and IgA nephropathy suggest a potential role for the endothelin system in DN as well as in other nondiabetic glomerular diseases.

TRC120038, a Novel Dual AT(1)/ET(A) Receptor Blocker for Control of Hypertension, Diabetic Nephropathy, and Cardiomyopathy in ob-ZSF1 Rats

In hypertensive subjects, angiotensin II and endothelin participate in a manner involving closely interwoven pathways in increasing blood pressure (BP) and inducing end organ damage. The primary objective of this study was to determine the effect of TRC120038, a novel dual AT₁/ET_A receptor blocker on BP, in obese Zucker spontaneously hypertensive fatty rats (ob-ZSF1), an animal model of moderate hypertension, diabetes with progressive renal and cardiac dysfunction. Ob-ZSF1 rats loaded with 0.5% salt were treated with TRC120038 (11.8 mg/kg bid.) or candesartan cilexetil (0.3 mg/kg od.) or vehicle control. Blood pressure (by radio-telemetry) and renal functional markers were monitored throughout the study. Cardiac function was assessed terminally by pressure volume catheter. Markers for renal dysfunction were measured and changes were evaluated histopathologically. TRC120038 showed greater fall in both systolic and diastolic BP in comparison to candesartan at its maximum antihypertensive dose. TRC120038 also reduced the severity of renal dysfunction and preserved cardiac function in ob-ZSF1 rat.

Distinct actions of endothelin A-selective versus combined endothelin A/B receptor antagonists in early diabetic kidney disease

Selective endothelin A (ET(A)) and combined ET(A) and ET(B) receptor antagonists are being investigated for use in treating diabetic nephropathy. However, the receptor-specific mechanisms responsible for producing the potential benefits have not been discerned. Thus, we determined the actions of ET(A) and ET(B) receptors on measures of glomerular function and renal inflammation in the early stages of diabetic renal injury in rats through the use of selective and combined antagonists. Six weeks after streptozotocin (STZ)-induced hyperglycemia, rats were given 2R-(4-methoxyphenyl)-4S-(1,3-benzodioxol-5-yl)-1-(N,N-di(n-butyl)aminocarbonyl-methyl)-pyrrolidine-3R-carboxylic acid (ABT-627) (5 mg/kg/day), a selective ET(A) antagonist; (2R,3R,4S)-4-(benzo[d][1,3]dioxol-5-yl)-2-(3-fluoro-4-methoxyphenyl)-1-(2-(N-propylpentylsulfonamido)ethyl)pyrrolidine-3-carboxylic acid hydrochloride (A-182086) (10 mg/kg/day), a combined ET(A/B) antagonist; or vehicle for 1 week. Sham controls received STZ vehicle (saline). Hyperglycemia led to significant proteinuria, increased glomerular permeability to albumin (P(alb)), nephrinuria, and an increase in total matrix metalloprotease (MMP) and transforming growth factor-β1 (TGF-β1) activities in glomeruli. Plasma and glomerular soluble intercellular adhesion molecule-1 (sICAM-1) and monocyte chemoattractant protein-1 (MCP-1) were elevated after 7 weeks of hyperglycemia. Daily administration of both ABT-627 and A-182086 for 1 week significantly attenuated proteinuria, the increase in P(alb), nephrinuria, and total MMP and TGF-β1 activity. However, glomerular sICAM-1 and MCP-1 expression was attenuated with ABT-627, but not A-182086, treatment. In summary, both selective ET(A) and combined ET(A/B) antagonists reduced proteinuria and glomerular permeability and restored glomerular filtration barrier component integrity, but only ET(A)-selective blockade had anti-inflammatory and antifibrotic effects. We conclude that selective ET(A) antagonists are more likely to be preferred for the treatment of diabetic kidney disease.

Endothelin receptor A-specific stimulation of glomerular inflammation and injury in a streptozotocin-induced rat model of diabetes

AIMS/HYPOTHESIS

Activation of endothelin receptor-A (ET(A)) increases glomerular permeability to albumin (P(alb)) and elevates pro-inflammatory markers in hyperglycaemic rats.

METHODS

Male Sprague-Dawley rats were given streptozotocin (n = 32) or saline (sham; n = 32). Half of the animals in each group received the ET(A)-selective antagonist, ABT-627 (atrasentan; orally), beginning immediately after hyperglycaemia was confirmed. Glomeruli were isolated by sieving techniques and P(alb) determined from the change in glomerular volume induced by oncotic gradients of albumin. Glomerular nephrin levels were assessed by immunofluorescence, whereas urinary nephrin was measured by immunoassay.

RESULTS

At 3 and 6 weeks after streptozotocin injection, proteinuria was significantly increased compared with sham controls and significantly reduced by ABT-627 treatment. P(alb) was also increased at 3 and 6 weeks post-streptozotocin. ABT-627 had no effect on P(alb) or protein excretion in sham control rats. In glomeruli isolated from hyperglycaemic rats, incubation with BQ-123, a selective ET(A) antagonist, reduced P(alb), whereas BQ-788, a selective endothelin receptor-B antagonist had no effect (n = 6 rats per group, 5-8 glomeruli per rat). Glomerular and plasma content of soluble intercellular adhesion molecule-1 and monocyte chemoattractant protein-1 were significantly increased 6 weeks after streptozotocin (ELISA). ABT-627 attenuated these increases. After 6 weeks of hyperglycaemia, glomerular nephrin content was decreased with a concurrent increase in urinary nephrin excretion. ABT-627 prevented glomerular nephrin loss in hyperglycaemic rats (n = 5-8 rats per group; eight groups).

CONCLUSIONS/INTERPRETATION

These observations support the hypothesis that endothelin-1, via the ET(A) receptor, directly increases P(alb), possibly via nephrin loss, as well as early inflammation in the hyperglycaemic rat.

Endothelin-1 increases collagen accumulation in renal mesangial cells by stimulating a chemokine and cytokine autocrine signaling loop

Endothelin-1 (ET-1), a potent vasoconstrictor, has been implicated in the pathogenesis of collagen accumulation, extracellular matrix remodeling, and renal and cardiac fibrosis in diabetes. However, the mechanism by which ET-1 promotes collagen accumulation remains unclear. Here, we analyzed the gene expression profile of ET-1-stimulated mesangial cells to identify determinants of collagen accumulation. In human mesangial cells (a microvascular pericyte that secretes excess collagen in diabetic glomerulosclerosis), ET-1 increased mRNA and protein for MCP-1 (macrophage chemoattractant protein-1) and IL-6. ET-1-induced MCP-1 and IL-6 mRNAs and proteins were blocked by an ET(A) (but not ET(B)) receptor antagonist. ET-1/ET(A) receptor signaling evoked a 7.4-fold increase in collagen accumulation. Exogenous addition of either recombinant MCP-1 or IL-6 increased collagen accumulation by 3.5-fold. Co-stimulation with both MCP-1 and IL-6 did not elevate collagen accumulation further. Neither an MCP-1-neutralizing antibody nor an MCP-1 receptor antagonist inhibited ET-1-induced collagen accumulation. Similarly, neutralizing antibodies against IL-6 or the gp130 subunit of the IL-6 receptor did not attenuate ET-1-induced collagen accumulation. However, co-incubation with MCP-1- and IL-6-neutralizing antibodies inhibited ET-1-induced collagen accumulation by 52%, suggesting a robust autocrine loop wherein MCP-1 and IL-6 are redundant. Taken together, these results demonstrate that an autocrine signaling loop involving MCP-1 and IL-6 contributes to ET-1-induced collagen accumulation.

Endothelin-1 enhances superoxide and prostaglandin E2 production of isolated diabetic glomeruli

Endothelin-1 (ET-1) plays an important role in the pathogenic mechanism of diabetic nephropathy. However, the regulatory effects of ET-1 on superoxide and prostaglandin E2 (PGE2) in diabetic glomeruli are unclear. The aim of this study was to determine whether ET-1 exerts a differential effect on the production of superoxide and PGE2 in diabetic glomeruli. The regulatory effects of indomethacin, insulin, dexamethasone, and heparin were also investigated. Freshly isolated glomeruli were obtained from normal and streptozotocin-induced diabetic rats for 1 week (DM1W), 1 month (DM1M), and 3 months (DM3M), respectively. Our results showed that the basal superoxide production of isolated glomeruli was significantly higher in DM1M and DM3M than in the normal rats (p < 0.01). ET-1 stimulated superoxide production in normal, DM1W and DM1M glomeruli (p < 0.01) but not in DM3M rats. The basal production of PGE2 in isolated glomeruli did not differ between diabetic and normal rats. ET-1 also stimulated PGE2 production in diabetic rats (p < 0.05). Pretreatment with indomethacin further enhanced ET-1-stimulated superoxide production in all groups of diabetic rats (p < 0.05), while the ET-1-stimulated PGE2 production was attenuated by indomethacin. Insulin, dexamethasone and heparin had no additional effects on ET-1-mediated superoxide and PGE2 production. In conclusion, basal glomerular production of superoxide but not PGE2 was increased in the diabetic glomeruli. ET-1 further stimulated production of both superoxide and PGE2. Indomethacin could enhance ET-1-stimulated superoxide production while attenuating PGE2 production.

Endothelin-1 increases glomerular permeability and inflammation independent of blood pressure in the rat

Endothelin (ET) 1 is a potent vasoactive peptide implicated in the pathogenesis of hypertension and renal disease. The aim of the current study was to test the hypotheses that ET-1 increases albumin permeability of glomeruli isolated from normal rats and that chronic ET-1 infusion will increase glomerular permeability and inflammation independent of blood pressure. Glomerular permeability to albumin was determined from the change in glomerular volume induced by exposing isolated glomeruli to oncotic gradients. Incubation of glomeruli taken from normal rats with ET-1 at a concentration that did not produce direct glomerular contraction (1 nmol/L) significantly increased glomerular permeability to albumin, reaching a maximum after 4 hours. Chronic ET-1 infusion for 2 weeks in Sprague-Dawley rats significantly increased glomerular permeability to albumin and nephrin excretion rate, effects that were attenuated in rats given an ET(A) receptor antagonist (ABT-627, 5 mg/kg per day). Urinary protein and albumin excretion and mean arterial pressure (telemetry) were not changed by ET-1 infusion. Acute incubation of glomeruli isolated from ET-1-infused rats with the selective ET(A) antagonist significantly reduced glomerular permeability to albumin, an effect not observed with acute treatment with a selective ET(B) antagonist. Chronic ET-1 infusion increased glomerular and plasma soluble intercellular adhesion molecule 1 and monocyte chemoattractant protein 1 and elevated the number of macrophages and lymphocytes in renal cortices (ED-1 and CD3-positive staining, respectively). These effects were all attenuated in rats given an ET(A) selective antagonist. These data support the hypothesis that ET-1 directly increases glomerular permeability to albumin and renal inflammation via ET(A) receptor activation independent of changes in arterial pressure.

Therapeutic potential of endothelin receptor antagonists for chronic proteinuric renal disease in humans

Diabetes and arterial hypertension continue to be the main causes of chronic renal failure in 2010, with a rising prevalence in part due to the worldwide obesity epidemic. Proteinuria is a main feature of chronic renal disease and mediated by defects in the glomerular filtration barrier and is as a good predictor of cardiovascular events. Indeed, chronic renal disease due to glomerulosclerosis is one of the important risk factors for the development of coronary artery disease and stroke. Glomerulosclerosis develops in response to inflammatory activation and increased growth factor production. Preclinical and first preliminary clinical studies provide strong evidence that endogenous endothelin-1 (ET-1), a 21-amino-acid peptide with strong growth-promoting and vasoconstricting properties, plays a central role in the pathogenesis of proteinuria and glomerulosclerosis via activation of its ET(A) subtype receptor involving podocyte injury. These studies have not only shown that endothelin participates in the disease processes of hypertension and glomerulosclerosis but also that features of chronic renal disease such as proteinuria and glomerulosclerosis are reversible processes. Remarkably, the protective effects of endothelin receptors antagonists (ERAs) are present even on top of concomitant treatments with inhibitors of the renin-angiotensin system. This review discusses current evidence for a role of endothelin for proteinuric renal disease and podocyte injury in diabetes and arterial hypertension and reviews the current status of endothelin receptor antagonists as a potential new treatment option in renal medicine.

Avosentan for overt diabetic nephropathy

In the short term, the endothelin antagonist avosentan reduces proteinuria, but whether this translates to protection from progressive loss of renal function is unknown. We examined the effects of avosentan on progression of overt diabetic nephropathy in a multicenter, multinational, double-blind, placebo-controlled trial. We randomly assigned 1392 participants with type 2 diabetes to oral avosentan (25 or 50 mg) or placebo in addition to continued angiotensin-converting enzyme inhibition and/or angiotensin receptor blockade. The composite primary outcome was the time to doubling of serum creatinine, ESRD, or death. Secondary outcomes included changes in albumin-to-creatinine ratio (ACR) and cardiovascular outcomes. We terminated the trial prematurely after a median follow-up of 4 months (maximum 16 months) because of an excess of cardiovascular events with avosentan. We did not detect a difference in the frequency of the primary outcome between groups. Avosentan significantly reduced ACR: In patients who were treated with avosentan 25 mg/d, 50 mg/d, and placebo, the median reduction in ACR was 44.3, 49.3, and 9.7%, respectively. Adverse events led to discontinuation of trial medication significantly more often for avosentan than for placebo (19.6 and 18.2 versus 11.5% for placebo), dominated by fluid overload and congestive heart failure; death occurred in 21 (4.6%; P = 0.225), 17 (3.6%; P = 0.194), and 12 (2.6%), respectively. In conclusion, avosentan reduces albuminuria when added to standard treatment in people with type 2 diabetes and overt nephropathy but induces significant fluid overload and congestive heart failure.

The endothelin receptor antagonist avosentan ameliorates nephropathy and atherosclerosis in diabetic apolipoprotein E knockout mice

AIMS/HYPOTHESIS

There is convincing evidence that the endothelin system contributes to diabetic nephropathy and cardiovascular disease. This study aimed to assess the effects of the non-peptidergic endothelin receptor A (ETA) antagonist avosentan in a mouse model of accelerated diabetic nephropathy and atherosclerosis in comparison with the ACE inhibitor, quinapril.

METHODS

Apolipoprotein E (Apoe) knockout (KO) mice (n = 20 per group, five groups) were randomised to the following groups: non-diabetic controls and streptozotocin-induced diabetic animals gavaged daily for 20 weeks with placebo, avosentan (high dose: 30 mg/kg, or low dose: 10 mg/kg) or quinapril (given in drinking water, 30 mg/kg).

RESULTS

BP was unchanged by avosentan treatment but decreased with quinapril treatment. Diabetes-associated albuminuria was significantly attenuated by high-dose avosentan after 10 and 20 weeks of treatment. Diabetic animals showed a decreased creatinine clearance, which was normalised by avosentan treatment. In diabetic mice, high-dose avosentan treatment significantly attenuated the glomerulosclerosis index, mesangial matrix accumulation, glomerular accumulation of the matrix proteins collagen IV, and renal expression of genes encoding connective tissue growth factor, vascular endothelial growth factor, transforming growth factor beta and nuclear factor kappaB (p65 subunit). Furthermore, high-dose avosentan treatment was also associated with reduced expression of the genes for ETA, ETB and angiotensin receptor 1. The renoprotective effects of avosentan were comparable or superior to those observed with quinapril. High-dose avosentan also significantly attenuated diabetes-associated aortic atherosclerosis in Apoe KO mice and reduced macrophage infiltration and aortic nitrotyrosine expression.

CONCLUSIONS/INTERPRETATION

This study demonstrates that ETA blockade with avosentan may provide an alternate therapeutic strategy for the treatment of diabetic micro- and macrovascular complications.

Endothelin receptor antagonists as antihypertensives: the next frontier

The endothelin system is a pivotal player along the continuum of cardiovascular disease. There is convincing evidence that the system not only exerts a potent pressor effect but also promotes end-organ damage independent from blood pressure changes. The role of endothelin receptor antagonists (ERAs) in the treatment of hypertension is rapidly evolving. Recent studies demonstrate a formidable antihypertensive effect. Utility of ERAs is likely to be greatest in patients with resistant hypertension. Beyond blood pressure lowering, ERAs exert several properties that may confer additional protection, including effects on endothelial function, atherosclerosis, arterial stiffening, renal function and proteinuria. The full potential of this class will only be realized when the results of ongoing and future studies in hypertension, heart failure and other forms of cardiovascular disease are completed.

[Endothelin system function in diabetic nephropathy]

Diabetic Nephropathy (DN) is a major chronic complication of diabetes mellitus (DM), and one of the main causes of new cases for dialysis, being associated with increasing mortality. The main risk factors for DN are hypertension, hyperglycemia, dyslipidemia, and genetic susceptibility. The renin-angiotensin system (RAS) plays an important role in genesis and progression of DN and there is evidence of an interrelationship between this system and the endothelins. Endothelins are powerful vasoconstrictor peptides and act as modulators of vasomotor tone, cell proliferation, and hormone production. These peptides act through two types of receptors (ET-A and ET-B) and are expressed on endothelial cells and vascular smooth-muscle cells. Activation of this receptor in renal cells leads to a complex signaling cascade resulting in stimulation of mesangial cell hypertrophy, proliferation, contraction, and extracellular matrix accumulation. These hemodynamic renal alterations are associated with the onset and progress of renal disease in DM. Elevated endothelin-1 (ET-1) levels have been reported in patients with DM. There is evidence suggesting that an increase in the production of ET-1 leads to glomerular damage. The use of ET receptor antagonists has been reported as renoprotective, correcting the early hemodynamic abnormalities in experimental DM, reinforcing the importance of this system in DN.

Endothelin receptor selectivity in chronic renal failure

Chronic kidney diseases are increasing worldwide at an alarming rate, and they are emerging as a major public health problem. Treatments that slow the progression of chronic kidney disease are needed. Endothelin-1 (ET-1) is a potent vasoconstrictor with proinflammatory, mitogenic and profibrotic effects that is closely involved in both normal renal physiology and pathology. Increasing evidence suggests that ET-1 and its cognate receptors are involved in a variety of progressive renal disorders to the extent that renal ET-1 expression correlates with disease severity and renal function impairment. Endothelin receptor antagonists have been used in renoprotection studies owing to their capacity of improving renal hemodynamics and reducing proteinuria. Whether selective ET(A) or non-selective ET(A)/ET(B) receptor antagonists are preferable is still a matter of debate. As angiotensin II blockers are not invariably effective in retarding disease progression when treatment is started late in the course of the disease, it is foreseeable that an ET-1 antagonist in addition to angiotensin-converting enzyme inhibitors could represent a combined treatment for progressive nephropathies. The focus of this review is to examine the role endothelin-1 plays in kidney diseases and to determine the ideal setting for antagonizing its biological activity in chronic nephropathies.

Do resident renal mast cells play a role in the pathogenesis of diabetic nephropathy?

Diabetic nephropathy is associated with high morbidity and mortality and the prevalence of this disease is continuously increasing world wide. Though, the major risk factors like hyperglycemia and hypertension play a pivotal role in the pathogenesis of diabetic nephropathy, the etiology of this insidious disorder is not well understood. Mast cells are pluripotent bone marrow derived cells that play a key role in inflammation. Degranulation of mast cells releases various mediators including inflammatory cytokines, endothelins, growth factors, and proteolytic enzymes. Infiltration of mast cells has been noted to occur in renal diseases. In addition, the renal density of mast cells is significantly increased in diabetic patients with nephropathy. It remains unclear whether resident renal mast cells derived mediators play a role in the pathogenesis of diabetic nephropathy. Recent studies suggest the involvement of renal mast cell infiltration and degranulation in diabetic nephropathy. The present review focuses on the role of resident renal mast cells in diabetic nephropathy.

Avosentan reduces albumin excretion in diabetics with macroalbuminuria

Despite the first-line use of angiotensin-converting enzyme inhibitors (ACEIs) and angiotensin receptor blockers (ARBs), there is still a large need to improve the prevention and progression of diabetic nephropathy and its associated cardiovascular events. Endothelin antagonists have shown anti-inflammatory, antifibrotic, and antiproteinuric effects in experimental studies. This study was a randomized, placebo-controlled, double-blind, parallel-design, dosage-range study of the effect of the endothelin-A antagonist avosentan (SPP301) on urinary albumin excretion rate (UAER) in patients with diabetic nephropathy. We randomly assigned 286 patients with diabetic nephropathy, macroalbuminuria (UAER 0.2 to 5.6 mg/min), and BP <180/110 mmHg to 12 wk of avosentan (5, 10, 25, and 50 mg) or placebo, in addition to standard ACEI/ARB therapy. Relative to baseline, all avosentan dosages decreased mean relative UAER (-16.3 to -29.9%) compared with placebo (35.5%). Median relative UAER decreased with all avosentan dosages (-28.7 to -44.8%) compared with placebo (12.1%). Creatinine clearance and BP were unchanged at 12 wk. The main adverse events were peripheral edema (12%), mainly with high (>/=25 mg) dosages of avosentan; significant increases in liver enzymes did not occur. Twenty-one (7.3%) patients experienced adverse events that led to withdrawal from study medication. In summary, the endothelin-A antagonist avosentan given in addition to standard ACEI/ARB treatment decreases UAER in patients with diabetic nephropathy and macroalbuminuria.

Experimental models for nephropathy

Nephropathy is a leading cause of morbidity and mortality and its prevalence is continuously increasing in industrialised nations. Nephropathy is characterised to varying degrees by nodular glomerulosclerosis, glomerular basement membrane thickness and mesangial expansion, leading to a decline in glomerular filtration rate, persistent elevated albuminuria, elevated arterial blood pressure and fluid retention. Hyperglycaemia, hyperlipidaemia and hypertension are considered to be the major risk factors implicated in the progression of nephropathy.Various signalling systems, such as vasoconstrictor peptides, inflammatory mediators, growth factors and adhesion molecules, are involved in the pathogenesis of nephropathy.At present, no promising therapy is available to treat patients with nephropathy due to lack of understanding of signalling culprits involved in the pathogenesis of nephropathy. Animal models are being developed to better understand the disease pathogenesis and develop drugs for nephropathy. In the present review, we have discussed various animal models for nephropathy, which may open vistas for developing new drugs to treat nephropathy.

Fibrosis in diabetes complications: pathogenic mechanisms and circulating and urinary markers

Diabetes mellitus is characterized by a lack of insulin causing elevated blood glucose, often with associated insulin resistance. Over time, especially in genetically susceptible individuals, such chronic hyperglycemia can cause tissue injury. One pathological response to tissue injury is the development of fibrosis, which involves predominant extracellular matrix (ECM) accumulation. The main factors that regulate ECM in diabetes are thought to be pro-sclerotic cytokines and protease/anti-protease systems. This review will examine the key markers and regulators of tissue fibrosis in diabetes and whether their levels in biological fluids may have clinical utility.

Reversal of proteinuric renal disease and the emerging role of endothelin

Proteinuria is a major long-term clinical consequence of diabetes and hypertension, conditions that lead to progressive loss of functional renal tissue and, ultimately, end-stage renal disease. Proteinuria is also a strong predictor of cardiovascular events. Convincing preclinical and clinical evidence exists that proteinuria and the underlying glomerulosclerosis are reversible processes. This Review outlines the mechanisms involved in the development of glomerulosclerosis--particularly those responsible for podocyte injury--with an emphasis on the potential capacity of endothelin receptor blockade to reverse this process. There is strong evidence that endothelin-1, a peptide with growth-promoting and vasoconstricting properties, has a central role in the pathogenesis of proteinuria and glomerulosclerosis, which is mediated via activation of the ET(A) receptor. Several antiproteinuric drugs, including angiotensin-converting-enzyme inhibitors, angiotensin receptor antagonists, statins and certain calcium channel blockers, inhibit the formation of endothelin-1. Preclinical studies have demonstrated that endothelin receptor antagonists can reverse proteinuric renal disease and glomerulosclerosis, and preliminary studies in humans with renal disease have shown that these drugs have remarkable antiproteinuric effects that are additive to those of standard antiproteinuric therapy. Additional clinical studies are needed.