The ET(A) receptor blocker LU 135252 prevents chronic transplant nephropathy in the "Fisher to Lewis" model

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: 30 Years From Discovery to Therapy

Discovered in 1987 as a potent endothelial cell-derived vasoconstrictor peptide, endothelin-1 (ET-1), the predominant member of the endothelin peptide family, is now recognized as a multifunctional peptide with cytokine-like activity contributing to almost all aspects of physiology and cell function. More than 30 000 scientific articles on endothelin were published over the past 3 decades, leading to the development and subsequent regulatory approval of a new class of therapeutics-the endothelin receptor antagonists (ERAs). This article reviews the history of the discovery of endothelin and its role in genetics, physiology, and disease. Here, we summarize the main clinical trials using ERAs and discuss the role of endothelin in cardiovascular diseases such as arterial hypertension, preecclampsia, coronary atherosclerosis, myocardial infarction in the absence of obstructive coronary artery disease (MINOCA) caused by spontaneous coronary artery dissection (SCAD), Takotsubo syndrome, and heart failure. We also discuss how endothelins contributes to diabetic kidney disease and focal segmental glomerulosclerosis, pulmonary arterial hypertension, as well as cancer, immune disorders, and allograft rejection (which all involve ET_A autoantibodies), and neurological diseases. The application of ERAs, dual endothelin receptor/angiotensin receptor antagonists (DARAs), selective ET_B agonists, novel biologics such as receptor-targeting antibodies, or immunization against ET_A receptors holds the potential to slow the progression or even reverse chronic noncommunicable diseases. Future clinical studies will show whether targeting endothelin receptors can prevent or reduce disability from disease and improve clinical outcome, quality of life, and survival in patients.

Endothelin-1 levels and renal function in newborns of various gestational ages

BACKGROUND

Renal failure is common in the NICU; Acute Kidney Injury (AKI) occurs in 8-24% of admissions. Although AKI is preventable with early diagnosis, no reliable AKI biomarkers exist. Endothelin-1 (ET-1) has been implicated in renal pathogenesis, and elevated urinary ET-1 (uET-1) levels may correlate with progression of renal dysfunction. The study objectives were to determine whether uET-1 levels correlate with renal function parameters and/or fetal growth restriction, and if uET-1 is a potential neonatal AKI biomarker.

METHODS

Sixty-three neonates were enrolled and divided into gestational age (GA) groups by weeks: 1) (24-30 6/7; n = 24); 2) (31-36 6/7; n = 26); and 3) (37-42; n = 13). Additional preterm subgroups for fetal growth restriction analysis included: 1) Appropriate for GA (AGA; n = 40), and 2) Small for GA (SGA; n = 10). ET-1 levels, measured using enzyme linked immunosorbent assay, were collected at birth (cord blood) and 24 h ( ± 4) of life (blood/urine).

RESULTS

No correlation was found between uET-1 and blood plasma levels at birth (r = 0.15; p > 0.05) or 24 h (r = 0.17; p > 0.05). uET-1 negatively correlated with GA (r = -0.44; p < 0.001) and GFR (r = -0.34; p < 0.01). uET-1 levels did not correlate with creatinine (r = 0.13; p > 0.05), BUN (r = 0.19; p > 0.05), BUN/Cr ratio (r = 0.15; p > 0.05), or urinary output (r = 0.12; p > 0.05). In fetal growth restriction subgroup analyses: uET-1 levels negatively correlated with GFR in the PT-AGA subgroup (r = -0.38; p = 0.017), but not with PT-SGA (r = 0.01; p > 0.05).

CONCLUSION

Plasma and uET-1 levels did not correlate; therefore, renal ET-1 excretion may reflect renal ET-1 production. uET-1 levels correlated negatively with GA and GFR. uET-1 may be a marker of impaired neonatal circulatory regulation and consequent renal injury.

The pathophysiology of endothelin in complications after solid organ transplantation: a potential novel therapeutic role for endothelin receptor antagonists

Although short-term allograft survival after solid organ transplantation has improved during the past two decades, improvement in long-term graft survival has been less pronounced. Common complications after transplantation include chronic allograft rejection, nephrotoxicity from calcineurin inhibitors (CNIs), and systemic hypertension, which all impact posttransplantation morbidity and mortality. Endothelin (ET)-1, a potent endogenous vasoconstrictor, inducer of fibrosis, and vascular smooth muscle cell proliferation, may play a key role in both the development of CNI-induced nephrotoxicity and endothelial vasculopathy in chronic allograft rejection. ET-1 levels increase after isograft implantation, and ET-1 plays a key role in CNI-induced renal vasoconstriction, sodium retention, and hypertension. Preclinical studies have demonstrated that endothelin receptor antagonists (ERAs) can reduce or prevent CNI-induced hypertension after renal transplantation. In addition, ERAs can ameliorate CNI-induced renal vasoconstriction and improve proteinuria and preserve renal function in animal models of renal transplantation. ET-1 may also play a significant role in cardiac allograft vasculopathy, and in animal models, ERAs improve pulmonary function and ischemic-reperfusion injury in lung transplantation and hepatic function and structure in liver transplantation. Emerging pharmacokinetic data suggest that the selective ERA ambrisentan may be used safely in conjunction with the most commonly used immunosuppressive agents tacrolimus and mycophenolate, albeit with appropriate dose adjustment. The weight of available evidence pointing toward a potential beneficial role of ERAs in ameliorating common complications after solid organ transplantation must be balanced with potential toxicities of ERAs but suggests that a randomized clinical trial of ERAs in transplant patients is warranted.

Endothelin and the podocyte

In the past decade, research has advanced our understanding how endothelin contributes to proteinuria and glomerulosclerosis. Data from pre-clinical and clinical studies now provide evidence that proteinuric diseases such as focal segmental glomerulosclerosis and diabetic nephropathy as well as hypertension nephropathy are sensitive to treatment with endothelin receptor antagonists (ERAs). Like blockade of the renin-angiotensin system, ERA treatment-under certain conditions-may even cause disease regression, effects that could be achieved on top of renin-angiotensin-aldosterone system blockade, suggesting independent therapeutic mechanisms by which ERAs convey nephroprotection. Beneficial effects of ERAs on podocyte function, which is essential to maintain the glomerular filtration barrier, have been identified as one of the key mechanisms by which inhibition of the endothelin ETA receptor ameliorates renal structure and function. In this article, we will review pre-clinical studies demonstrating a causal role for endothelin in proteinuric chronic kidney disease (with a particular focus on functional and structural integrity of podocytes in vitro and in vivo). We will also review the evidence suggesting a therapeutic benefit of ERA treatment on the functional integrity of podocytes in humans.

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: 20 years from discovery to therapy

Since its identification as an endothelial cell-derived vasoconstrictor peptide in 1988, endothelin-1, the predominant member of the endothelin peptide family, has received considerable interest in basic medical science and in clinical medicine, which is reflected by more than 20 000 scientific publications on endothelin research in the past 20 years. The story of endothelin is unique as the gene sequences of endothelin receptors and the first receptor antagonists became available within only 4 years of the identification of the peptide sequence. The first clinical study in patients with congestive heart failure was published only 3 years thereafter. Yet, despite convincing experimental evidence of a pathogenetic role for endothelin in development, cell function, and disease, many initial clinical studies on endothelin antagonism were negative. In many of these studies, study designs or patient selection were inadequate. Today, for diseases such as pulmonary hypertension, endothelin antagonist treatment has become reality in clinical medicine, and ongoing clinical studies are evaluating additional indications, such as renal disease and cancer. Twenty years after the discovery of endothelin, its inhibitors have finally arrived in the clinical arena and are now providing us with new options to treat disease and prolong the lives of patients. Possible future indications include resistant arterial hypertension, proteinuric renal disease, cancer, and connective tissue diseases.

Deletion of transient receptor potential vanilloid type 1 receptors exaggerates renal damage in deoxycorticosterone acetate-salt hypertension

To determine whether the transient receptor potential vanilloid type 1 (TRPV1) channel provides protection against hypertension-induced renal damage, hypertension was induced by uninephrectomy and by giving deoxycorticosterone acetate (DOCA)-salt in wild-type (WT) and TRPV1-null mutant (TRPV1-/-) mice. Mean arterial pressure, as determined by radiotelemetry, increased significantly and reached the peak 7 days after DOCA-salt treatment in both WT and TRPV1-/- mice. There was no difference in mean arterial pressure between the 2 strains at the baseline or at the peak that lasted for 4 treatment weeks. DOCA-salt treatment in both WT and TRPV1-/- mice led to increased urinary excretion of albumin and 8-isoprostane, glomerulosclerosis, renal cortical tubulointerstitial injury, tubulointerstitial fibrosis, increased number of tubular proliferating cell nuclear antigen-positive cells, and renal monocyte/macrophage infiltration, all of which were much more severe in DOCA-salt-treated TRPV1-/- compared with DOCA-salt-treated WT mice. Renal TRPV1 protein expression, but not the renal anandamide content, was elevated in DOCA-salt-treated WT compared with vehicle-treated WT mice. Renal anandamide levels were markedly elevated in DOCA-salt-treated TRPV1-/- but not in vehicle-treated TRPV1-/- mice. Thus, our data show that ablation of the TRPV1 gene exacerbates renal damage induced by DOCA-salt hypertension, indicating that TRPV1 may constitute a protective mechanism against end-organ damage induced by hypertension.

The Effect of FK778 on the Progression of Chronic Allograft Nephropathy in a Rat Model

BACKGROUND

Chronic allograft nephropathy (CAN) still belongs to the leading causes of graft loss over the long term. The leflunomide derivative FK778 is a novel immunosuppressant with improved pharmacokinetic properties that effectively prolonged graft survival in several transplantation models. In the present study, we investigated the effects of FK778 at different phases after transplantation on the progression of CAN.

METHODS

Fisher 344 kidneys were orthotopically transplanted into Lewis recipients. Recipients were treated with FK778 (5 mg/kg/day) over different time periods (early: days 0-10 only, continuous: day 0 to week 24, or late: weeks 16-24 only posttransplantation). Proteinuria was measured every 4 weeks, whereas grafts were harvested at 24 weeks posttransplantation for morphological and immunohistochemical analysis as well as transforming growth factor-beta and platelet derived growth factor-B chain expression.

RESULTS

Continuous treatment with FK778 ameliorated the progression of CAN, whereas late treatment reduced proteinuria and resulted in a similar grade of CAN as compared to animals with continuous treatment. In contrast, FK778 given only during the early phase after transplantation had no effect on the progression of CAN as compared to controls.

CONCLUSION

In summary, FK778 is a potent immunosuppressive drug that can delay the progression of CAN, even when given at later stages after transplantation.

Endothelin B receptor blockade accelerates disease progression in a murine model of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common genetic disease that causes kidney failure and accounts for 10% of all patients who are on renal replacement therapy. However, the marked phenotypic variation between patients who carry the same PKD1 or PKD2 mutation suggests that nonallelic factors may have a greater influence on the cystic phenotype. Endothelin-1 (ET-1) transgenic mice have been reported to develop profound renal cystic disease and interstitial fibrosis without hypertension. The hypothesis that ET-1 acts as a modifying factor for cystic disease progression was tested in an orthologous mouse model of ADPKD (Pkd2(WS25/-)). Four experimental groups (n = 8 to 11) were treated from 5 to 16 wk of age with the highly selective orally active receptor antagonists ABT-627 (ETA) and A-192621 (ETB) singly or in combination. Unexpected, ETB blockade led to accelerated cystic kidney disease. Of significance, this was associated with a reduction in urine volume and sodium excretion and increases in urine osmolarity and renal cAMP and ET-1 concentrations. The deleterious effect of chronic ETB blockade was neutralized by simultaneous ETA blockade. ETA blockade alone resulted in a significant increase in tubular cell proliferation but did not alter the cystic phenotype. It is concluded that the balance between ETA and ETB signaling is critical for maintaining tubular structure and function in the cystic kidney. These results implicate ET, acting via vasopressin-dependent and independent pathways, as a major modifying factor for cystic disease progression in human ADPKD.

Recipient age and risk of chronic allograft nephropathy in primary deceased donor kidney transplant

Single center and registry data studies have had conflicting results regarding the impact of recipient age on chronic allograft nephropathy (CAN). We tested the hypothesis that advanced recipient age is a risk factor for graft failure due to CAN. All patients who underwent primary deceased donor kidney transplant between January 1, 1995 and December 31, 2000 recorded in the United Network of Organ Sharing (UNOS) database were analyzed for the occurrence of death censored graft loss and by two different definitions of graft loss due to CAN. Kaplan-Meier analysis based on the recipient age, and Cox proportional hazard regression was used to estimate the independent effect of recipient age on the three endpoints of interest. For all endpoints, after age of 9 years, the risk of graft loss declined with each successive decade increase in age. This pattern of risk was similar for both Caucasian and African-American recipients, although for any given age the risk of graft loss was always higher in African-American recipients. Analysis of UNOS data does not support the hypothesis that advanced recipient age is a risk factor for CAN.

Comprehensive immunohistological analysis of the endothelin system in human kidney grafts

BACKGROUND

In experimental models of renal transplantation, upregulation of the endothelin (ET) system and amelioration of renal injury by ET-receptor blockers have been documented. In contrast, little information is available on the expression of the ET system in human kidney allografts. It was the purpose of the present study to analyse by immunohistology the expression of ET-1 as well as of the two ET receptors (ET-RA and ET-RB) in the different cells and compartments of kidney grafts and control kidneys.

METHODS

Fifty-five graft biopsies were taken from 55 kidney allograft recipients (mean age: 32+/-2.8 years) who were all on a calcineurin inhibitor. The indication for biopsies was delayed graft function or suspected rejection. The underlying diagnoses were acute allograft rejection (n = 14), chronic allograft nephropathy (n = 14), cyclosporin A (CSA) toxicity (n = 10), post-operative acute tubular necrosis (ATN) (n = 11) and recurrent primary disease (n = 6). As control, tissues of non-grafted kidneys with ATN (mean age: 35+/-24 years), of primary glomerulonephritis (mean age: 69+/-10 years) and of non-tumour-bearing parts of eight tumour nephrectomy specimens (mean age: 67+/-5 years) were assessed. The biopsies were scored using the 1997 Banff criteria. Expression of ET-1, ET-RA and ET-RB as well as of vascular endothelial growth factor was evaluated by immunohistochemistry and a semi-quantitative scoring system. Interstitial infiltrating cells were characterized using antibodies against T cells, B cells and macrophages (CD3, CD20 and CD68).

RESULTS

Control cases showed only faint expression of ET-1 in glomeruli (in podocytes and endothelial cells), whereas marked expression was seen in distal, but less in proximal tubular cells. The interstitium was completely negative. ET-1 expression was seen in vascular endothelial cells (VEC) and vascular smooth muscle cells (VSMC). Only faint expression of ET-RA and ET-RB was found in glomeruli and tubuli (distal more than proximal). Marked ET-RA and ET-RB expression was seen in VEC and VSMC. In all transplanted kidneys, irrespective of the underlying diagnosis, expression of ET-1, ET-RA and ET-RB was markedly higher compared with control kidneys. ET-1 was strikingly upregulated in glomeruli and tubuli, but surprisingly not in the vasculature of grafts with CSA toxicity. Expression of ET-RB was markedly increased in CSA toxicity in glomeruli, tubuli and vessels. In grafts with ATN and acute rejection, pronounced expression of ET-RA was noted. There was a strong correlation between proteinuria and expression of ET-1 in glomeruli and proximal tubuli and of ET-RB in proximal tubuli.

CONCLUSIONS

The above data in human kidney allograft biopsies are consistent with an important role of the ET system in different types of renal allograft damage. This finding extends and clarifies the somewhat contradictory results in animal models.

The potential of endothelin antagonism as a therapeutic approach

Endothelin (ET) is a pivotal physiological regulator of blood pressure through its effects on blood vessels, heart, lung and kidneys, and the ET system can be overactive in disorders such as pulmonary hypertension, heart failure and renal disease. Such observations stimulated interest among scientists and pharmaceutical companies that have set up high-throughput screens to search for antagonists of ET receptors. The emerging compounds have been tested in animals with exciting results, leading to great hope that such inhibitors could be translated into human drugs with desirable therapeutic activities and few side effects. This review will describe the most relevant results obtained in experimental animals in a wide variety of disease conditions and focus on the data of selected compounds that have been employed in clinical trials.

Function and survival of dendritic cells depend on endothelin-1 and endothelin receptor autocrine loops

The biologic effects of endothelin-1 (ET-1) are not limited to its potent vasoconstricting activity. The endothelin receptors, ETA and ETB, have differential tissue and functional distributions. Here we showed that dendritic cells (DCs), the major antigen-presenting cells in the adaptive limb of the immune system, produce large amounts of ET-1 and significantly increase the expression of endothelin receptors upon maturation. Selective blockade of the ETA receptor significantly reduced expression of the mature DC marker CD83, decreased the production of the immunostimulatory cytokine interleukin-12, down-regulated DC ability to stimulate T cells, and promoted DC apoptosis. Selective ETB receptor blockade, on the other hand, resulted in increased expression of CD83 and improved DC survival. Therefore, ET-1/ETA/ETB autocrine/paracrine loops on DCs appear to be essential for the normal maturation and function of human DCs, presenting a unique target for immunomodulatory therapies.

Renal damage is not improved by blockade of endothelin receptors in primary renin-dependent hypertension

OBJECTIVE

Secondary activation of the renin-angiotensin system plays a major role in the progression of chronic nephropathies, and blockade of endothelin (ET) receptors has been shown to confer nephroprotection in experimental models of proteinuric renal disease. We tested the nephroprotective potential of selective endothelin A receptor (ETA) and non-selective ETA and endothelin B (ETA/B) receptor blockade in the TGR(mRen2)27 transgenic rat model with renin-dependent hypertension (Ren2).

DESIGN

Ren2 animals were treated between 10 and 30 weeks of age with the selective ETA receptor antagonist darusentan (Ren2-ETA) and the ETA/B receptor antagonist Lu420627 (Ren2-ETA/B), and compared with transgene negative Sprague-Dawley (SD) controls. Since the elevated systolic blood pressure in Ren2 was not affected in either Ren2-ETA or Ren2-ETA/ETB, an additional Ren-2 group was treated with a non-antihypertensive dose of the angiotensin II type 1 receptor blocker eprosartan (Ren2-AT1).

RESULTS

During the 20-week observation period 35% of untreated Ren2, 30% of Ren2-ETA/B, 50% of Ren2-ETA, and 83% of Ren2-AT1 animals survived compared with 100% of SD rats. Renal endothelin-1 mRNA expression and proteinuria (4.1-fold) were significantly elevated in Ren2 compared with SD rats (P < 0.05, respectively). Proteinuria was normalized to SD control levels in Ren2-AT1 (P < 0.05) but increased further in Ren2-ETA (7.7-fold) and Ren2-ETA/B (15-fold) (P < 0.05, respectively). Glomerulosclerosis, tubulointerstitial damage and renal osteopontin mRNA expression were reduced in Ren2-AT1 (P < 0.05, respectively) but remained unchanged or increased further in Ren2-ETA and Ren2-ETA/B compared with Ren2.

CONCLUSION

ET receptor blockade fails to improve renal damage and mortality in primary renin-dependent hypertension.

Endothelin-1 in chronic renal failure and hypertension

Investigation into the role of endothelin-1 (ET-1) in renal function has revealed two major direct actions leading to the control of extracellular volume and blood pressure. These are the regulation of renal hemodynamics and glomerular filtration rate and the modulation of sodium and water excretion. In the rat remnant kidney model of chronic renal failure, ET-1 production is increased in blood vessels and renal tissues. These changes are related to an increase in preproET-1 expression and correlate with the rise in blood pressure, the development of cardiovascular hypertrophy, and the degree of renal insufficiency and injury. Selective ETA receptor blockade prevents the progression of hypertension and the vascular and renal damage, supporting a role for ET-1 in chronic renal failure progression. The increase in ET-1 production can be associated with other local mediators, including angiotensin II, transforming growth factor-beta1 and nitric oxide, the local production of which is also altered in chronic renal failure. In human patients with essential hypertension, atherosclerosis, and nephrosclerosis, plasma ET-1 levels are increased compared with patients with uncomplicated essential hypertension. Similarly, plasma ET-1 concentrations are markedly increased in patients with end-stage renal disease undergoing dialysis, and this correlates with blood pressure, suggesting that ET-1 may contribute to hypertension in these patients. The treatment of anemia in patients with renal failure with human recombinant erythropoietin increases blood pressure by accentuating the underlying endothelial dysfunction and the elevated vascular ET-1 production. Overall, these results support a role for ET-1 in hypertension and the end-organ damage associated with chronic renal failure. ETA receptor blockade may then represent a potential target for the management of hypertension and cardiovascular and renal protection.

Influence of acute selective endothelin-receptor-A blockade on renal hemodynamics in a rat model of chronic allograft rejection

We have recently demonstrated up-regulation of renal endothelin (ET) synthesis in a rat model of chronic renal allograft rejection. Treatment with a selective ET-A receptor antagonist improved survival and reduced functional and morphological kidney damage. However, the underlying mechanisms have not yet been elucidated, as ET exhibits both hemodynamic and inflammatory properties. Therefore, in the present study we investigated acute hemodynamic effects of the selective ET-A receptor antagonist LU 302146 (LU) on chronic renal allograft rejection in rats. Experiments were performed in the Fisher-to-Lewis model of chronic renal allograft rejection. Lewis-to-Lewis isografts served as controls. After 2, 12, and 24 weeks, hemodynamic measurements were performed on anesthetized animals. Measurement of mean arterial pressure (MAP) was performed via a catheter in the femoral artery. Renal blood flow (RBF) was measured by an ultrasonic flow probe placed around the renal transplant artery. Medulla blood flow (MBF) and cortex blood flow (CBF) were determined with laser Doppler probes. Hemodynamic response upon intravenous bolus injection of LU (50 mg/kg) was investigated. The application of LU was followed by a decline in MAP that reached statistical significance only in isografts (ISOs) after 12 weeks and allografts (ALLOs) after 24 weeks. RBF slightly decreased in all groups; however, without reaching statistical significance. MBF showed a small increase in ALLO12 and ALLO24 whereas CBF slightly decreased in all groups. Acute ET-A receptor blockade does not induce important hemodynamic effects in kidneys undergoing chronic rejection. The lack of response to ET-A receptor blockade suggests that the beneficial effect of ET receptor antagonists in this model is likely to be due to improvement of renal morphology.

Impact of renal transplantation on small vessel reactivity

BACKGROUND

The function of large arteries is altered after renal transplantation. Whether transplantation also induces agonist-dependent functional changes in small arterial renal and extrarenal vessels has not yet been studied.

METHODS

Chronic rejection was induced by grafting Lewis rats with kidneys from Fischer rats (FL). Rats that underwent transplantation were bilaterally nephrectomized. Rats that underwent syngeneic transplantation, uninephrectomized rats, uninephrectomized rats with denervated kidneys or with kidneys made ischemic, and native rats served as controls. All animals were treated with cyclosporine for 10 days. Eighteen weeks after surgery, the reactivity of small arteries (220-270 microm) was tested by myography.

RESULTS

Weight gain, glomerular filtration rate, and arterial pressure were similar in all groups, whereas proteinuria was elevated in FL. Only kidneys from FL showed glomerular lesions, tubular atrophy, and vasculopathy. Responsiveness of coronary, mesenteric, and femoral resistance vessels to both constrictor and dilator agonists was similar in transplanted and nontransplanted animals. Resistance vessels obtained from both allogeneically and syngeneically transplanted kidneys were more sensitive to norepinephrine, phenylephrine, angiotensin II, and vasopressin than renal vessels from weight-matched controls. Vasodilation in response to acetylcholine and sodium nitroprusside was mitigated in transplanted versus nontransplanted kidneys.

CONCLUSIONS

In rat renal transplantation, renal resistance vessel responsiveness to constrictor or dilator stimuli is altered. Extrarenal small vessel function is not affected. The changes in function of renal resistance vessels are not explained by reduction of nephron mass, denervation, ischemia, or chronic rejection.

ETA receptor blockade induces tubular cell proliferation and cyst growth in rats with polycystic kidney disease

Tissue concentrations of ET-1 are markedly elevated in the kidneys of Han:Sprague-Dawley (Han:SPRD) rats, a model of human autosomal dominant polycystic kidney disease (ADPKD). This study analyzed whether disease progression might be attenuated by endothelin receptor antagonists. Heterozygous Han:SPRD rats received an ETA receptor antagonist (LU 135252), a combined ETA/ETB receptor antagonist (LU 224332), or placebo for 4 mo. Glomerulosclerosis, protein excretion, and GFR remained unchanged, whereas interstitial fibrosis was enhanced by both compounds. BP was not reduced by both compounds in Han:SPRD rats. Renal blood flow (RBF) decreased in ADPKD rats treated with the ETA receptor antagonist. Long-term ETA receptor blockade furthermore increased markedly the number of renal cysts (ADPKD rats, 390 +/- 119 [cysts/kidney section +/- SD]; LU 135252-treated APKD rats, 1084 +/- 314; P < 0.001), cyst surface area (ADPKD rats, 7.97 +/- 2.04 [% of total section surface +/- SD]; LU 135252-treated ADPKD rats, 33.83 +/- 10.03; P < 0.001), and cell proliferation of tubular cells (ADPKD rats, 42.2 +/- 17.3 [BrdU-positive cells/1000 cells]; LU 135252-treated ADPKD rats, 339.4 +/- 286.9; P < 0.001). The additional blockade of the ETB receptor attenuated these effects in Han:SPRD rats. Both endothelin receptor antagonists had no effect on BP, protein excretion, GFR, and kidney morphology in Sprague-Dawley rats without renal cysts. It is concluded that ETA receptor blockade enhances tubular cell proliferation, cyst number, and size and reduces RBF in Han:SPRD rats. This is of major clinical impact because endothelin receptor antagonists are upcoming clinically used drugs.

New therapeutics that antagonize endothelin: promises and frustrations

The discovery of endothelin--a highly potent endogenous vasoconstrictor - in 1988 has led to considerable efforts to develop antagonists of endothelin receptors that could have therapeutic potential in disorders including hypertension, heart failure and renal diseases. However, in general, the results of trials in humans have not mirrored the highly promising effects in animal disease models. Here, we discuss preclinical and clinical results with endothelin antagonists, and consider possible approaches to fully realizing the potential of endothelin antagonism.

Inhibition of endothelin-1 improves survival and vasculopathy in rat cardiac transplants treated with cyclosporine

BACKGROUND

In animal models, endothelin-1 (ET-1) blockade attenuates transplant vasculopathy and chronic allograft dysfunction even in the absence of cyclosporine (CsA). As CsA has side effects and ET-1 antagonism alone has significant benefits, we postulated that allograft survival could be significantly improved by combining an endothelin-converting enzyme inhibitor with low-dose CsA.

METHODS

Survival of Lewis to Fisher 344 rat heterotopic cardiac allografts was determined in untreated animals and compared with those treated with high-dose CsA (62 mg/kg i.m. on day 2), low-dose CsA (25 mg/kg), an endothelin-converting enzyme inhibitor, phosphoramidon (PA, 10 mg/kg/day), or low-dose CsA + PA.

RESULTS

Untreated allografts had a median survival of 16 days compared with 20 days for low-dose CsA. Grafts treated with PA survived for 28 days, and combination of PA and low-dose CsA improved median survival to 47 days (P<0.01). Median survival with combination therapy was similar to that for high-dose CsA (42 days). To explore mechanisms underlying the benefits of combination therapy, cardiac allografts treated as above (n=4 each group) were explanted at 20 d and analyzed for parenchymal rejection, neointimal vasculopathy, myocardial fibrosis, and macrophage infiltration. Low-dose CsA alone but not PA improved parenchymal rejection; in contrast, PA alone but not low-dose CsA improved vasculopathy. Both parenchymal rejection and vasculopathy were improved by combination therapy with low-dose CsA and PA. Unlike CsA, inhibition of ET-1 biosynthesis significantly reduced myocardial fibrosis in allografts.

CONCLUSIONS

These results suggest that the combination of low-dose CsA and endothelin-converting enzyme inhibition may prove useful to improve long-term graft survival while minimizing potential side effects of CsA.

The therapeutic potential of endothelin receptor antagonists in cardiovascular disease

Endothelin (ET)-1, a 21-amino acid peptide, is the predominant isoform of the endothelin peptide family. ET-1 is ubiquitously expressed and stimulates vasoconstriction and cell proliferation. Enzymes such as endothelin converting enzymes (ECE), chymases, and non-ECE metalloproteinases contribute to the synthesis of ET-1, which is regulated in an autocrine fashion in vascular and nonvascular cells. Endothelin ET(A) receptors mediate vasoconstriction and cell proliferation, whereas ET(B) receptors are involved in the clearance of ET-1, inhibition of endothelial apoptosis, release of nitric oxide and prostacyclin, and inhibition of ECE-1 expression. Most cardiovascular diseases, such as arterial hypertension, atherosclerosis, restenosis, heart failure, idiopathic cardiomyopathy, pulmonary hypertension, and renal failure are associated with local activation of the endothelin system. Experimental studies and first clinical trials suggest that ET-1 is importantly involved in the functional and structural changes in the cardiovascular system, and that many of the actions of ET-1 are mediated through pressure-independent mechanisms. Endothelin antagonists promise to be successful as a new class of drugs for the treatment of cardiovascular diseases.

Research on renal endothelin in proteinuric nephropathies dictates novel strategies to prevent progression

Proteinuria is one of the major risk factors for renal disease progression in patients with chronic nephropathies. Studies in disease models have helped to delineate mechanisms leading to renal structural damage as a result of persistent dysfunction of the glomerular barrier to proteins, even when the primary immune or non-immune insult to the kidney has ceased. From these preclinical studies, a role for endothelin in proteinuric chronic renal diseases has been suggested, thus providing the rationale for novel therapeutic approaches with endothelin receptor antagonists to maximize renoprotection so far achieved with blockade of the renin-angiotensin system by angiotensin-converting enzyme inhibition or angiotensin II receptor antagonism. Trials are needed to explore this potential area of clinical interest.

ETA receptor blockade induces fibrosis of the clipped kidney in two-kidney-one-clip renovascular hypertensive rats

BACKGROUND

In two kidney-one clip renovascular hypertension (2K1C), blood flow is reduced in the clipped kidney leading to ischaemia. The non-clipped kidney is characterized by increased shear stress. Circulating Ang II is elevated. All these factors are stimuli of the paracrine renal endothelin system. Indeed, we demonstrated an activation of the renal endothelin system in the 2K1C rat model.

METHODS

We analysed the effects of chronic treatment with the ETA receptor antagonist BQ-123 on blood pressure, heart rate, plasma renin activity, and on the progression of glomerulosclerosis, interstitial fibrosis and vascular remodeling in the clipped and non-clipped kidney.

RESULTS

Long-term treatment with BQ-123 led to a fibrotic atrophy of the clipped kidney characterized by a significantly reduced weight of the clipped kidney compared to the clipped kidney of the placebo-treated group. Computer-aided image analysis revealed a markedly enhanced interstitial fibrosis of these clipped kidneys after long-term ETA blockade. The effects of ETA receptor antagonists on the non-clipped kidney were less pronounced. Neither blood pressure nor plasma renin activity were significantly altered by BQ-123 treatment.

CONCLUSIONS

The present study indicates that long-term blockade of the activated endothelin system in the clipped kidney of rats with renovascular hypertension using an ETA receptor antagonist led to a fibrotic atrophy of the clipped kidney.

Endothelins and endothelin receptor antagonists: therapeutic considerations for a novel class of cardiovascular drugs

The 21-amino acid peptide endothelin-1 (ET-1) is the predominant isoform of the endothelin peptide family, which includes ET-2, ET-3, and ET-4. It exerts various biological effects, including vasoconstriction and the stimulation of cell proliferation in tissues both within and outside of the cardiovascular system. ET-1 is synthesized by endothelin-converting enzymes (ECE), chymases, and non-ECE metalloproteases; it is regulated in an autocrine fashion in vascular and nonvascular cells. ET-1 acts through the activation of G(i)-protein-coupled receptors. ET(A) receptors mediate vasoconstriction and cell proliferation, whereas ET(B) receptors are important for the clearance of ET-1, endothelial cell survival, the release of nitric oxide and prostacyclin, and the inhibition of ECE-1. ET is activated in hypertension, atherosclerosis, restenosis, heart failure, idiopathic cardiomyopathy, and renal failure. Tissue concentrations more reliably reflect the activation of the ET system because increased vascular ET-1 levels occur in the absence of changes in plasma. Experimental studies using molecular and pharmacological inhibition of the ET system and the first clinical trials have demonstrated that ET-1 takes part in normal cardiovascular homeostasis. Thus, ET-1 plays a major role in the functional and structural changes observed in arterial and pulmonary hypertension, glomerulosclerosis, atherosclerosis, and heart failure, mainly through pressure-independent mechanisms. ET antagonists are promising new agents in the treatment of cardiovascular diseases.

Treatment with a combined endothelin A/B-receptor antagonist does not prevent chronic renal allograft rejection in rats

A markedly increased expression of endothelin (ET)-1 has been observed in renal allografts with chronic rejection, one of the most common causes of kidney graft loss. In this study we investigated the effect of treatment with a combined ET-A/B-receptor antagonist on the course of chronic renal allograft rejection. Experiments were performed in the Fisher-to-Lewis rat model of chronic rejection. Lewis-to-Lewis isografts and uninephrectomized Lewis rats served as controls. Animals were treated with either the oral combined ET-A/B-receptor antagonist LU224332 (20 mg/kg/day) or vehicle. Animal survival, blood pressure, creatinine clearance, proteinuria, and urinary ET excretion were investigated for 24 weeks. Kidneys were removed for light-microscopic evaluation and immunohistochemical assessment of cell-surface markers. Treatment with LU224332 did not improve survival after 24 weeks (0.47 vs. 0.38; p > 0.05 by log-rank test), nor did it have an influence on blood pressure, creatinine clearance, or proteinuria. Combined ET-A/B-receptor blockade was associated with a reduction of expression of cell-surface markers for macrophages (EDI), T-cells (R73), and major histocompatibility complex (MHC) II (F17-23-2), but did not lead to an improvement of histologic changes of chronic allograft rejection. Our data show that blocking both ET-A- and -B receptors, in opposition to a previously published beneficial effect of selective ET-A blockade, does not prevent the progression of chronic renal allograft rejection and does not prolong survival in this model. Functional integrity of the ET-B receptor therefore seems to play an important role in the nephroprotection provided by selective ET-A-receptor antagonists in chronic renal allograft nephropathy.

Elevated endothelin-1 in tubular epithelium is associated with renal allograft rejection

Vascular endothelin-1 (ET-1) levels are elevated in patients with renal allograft rejection, and the mitogenic and pressor actions of ET-1 might contribute to transplant vasculopathy, posttransplantation hypertension, and ischemia-reperfusion injury. In contrast, relatively little is known about tubular expression of ET-1 in acute or chronic rejection of renal allografts. We sought to determine whether tubular ET-1 levels were altered in patients with acute or chronic renal allograft rejection. Immunohistochemical analysis of tubular ET-1 was performed in renal biopsy specimens from 18 patients with acute rejection, 7 patients with chronic rejection, and 5 normal kidneys excised for localized neoplasm. The diagnosis of acute or chronic rejection in each patient was verified and graded using the Banff schema. Renal tubular epithelium from patients with allograft rejection had markedly elevated staining for ET-1 compared with normal kidneys. Tubular ET-1 levels were elevated in 18 of 18 patients with acute rejection and 5 of 7 patients with chronic rejection. Tubular ET-1 staining was graded from 0 to +3 as follows: normal kidneys, 1.2 +/- 0.2; acute rejection, 2.3 +/- 0.4 (P < 0.01); and chronic rejection, 2.2 +/- 0.5 (P < 0.01). ET-1 staining was prominent in both proximal and distal tubules, and we observed abundant ET-1 secretion from proximal tubular epithelium in culture. Moreover, ET-1 activated the c-fos immediate early gene promoter in proximal tubular cells transfected with a c-fos luciferase reporter. We conclude that elevated tubular ET-1 levels are associated with acute and chronic rejection of renal allografts. Our results also suggest distinct pathophysiological roles for the tubular and vascular ET-1 systems in renal allograft rejection.

The renoprotective potential of endothelin receptor antagonists

The endothelin system has been identified as having a substantial role in renal failure, both acute and chronic. Beside its well characterised haemodynamic effects, its mitogenic and pro-fibrotic properties have gained increased interest in the pathophysiology of chronic renal failure. This review outlines the role of endothelin in the pathogenesis of various renal diseases with a special focus on the potential of blocking this system with endothelin receptor antagonists. So far, most data were derived from animal models, but they provide strong evidence that endothelin receptor antagonists may represent a powerful therapeutic strategy in ameliorating the course of acute and chronic renal failure.

Regulatory peptides and their antagonists in nephropathies

Many peptides influence renal function and structure in physiological and pathophysiological situations. Bioactive peptides that regulate renal function and structure encompass various substances including vasopeptides, growth factors, cytokines and peptide hormones. We highlight some novel concepts indicating that the vasoactive peptides angiotensin II and endothelin-1 play a major role in the progression of renal disease. These effects may be amplified by reduced concentration of counteracting natriuretic peptides. In addition, recent evidence suggests that peptides such as leptin, previously not considered to exert any renal effects, may be involved in renal pathophysiology under certain conditions. One of the most imperative tasks in nephrology is to develop innovative strategies to slow the progression of chronic renal disease. Interference with the renal action of bioactive peptides will certainly be part of this strategy.

Counteracting progression of renal disease: A look into the future

It is the very nature of research, including medical research, that truly novel results are not predictable. Nevertheless, some predictions concerning the understanding and management of progression of renal disease are plausible. It is very likely that in the future, exploding health budgets will force authorities and nephrologists to more effectively apply existing knowledge in this field to patients with early renal disease, particularly diabetics. We hope that this optimistic note is justified, although experience admittedly indicates that it is very much against human nature to behave rationally. With the powerful methodological tools available today, it is safe to predict that insight into the mechanisms underlying progression will also increase. Although pharmacological blockade of the renin-angiotensin system has been one of the great success stories of the past two decades, in many patients, progression is seen despite administration of angiotensin-converting enzyme (ACE) inhibitors and angiotensin receptor blockers. Fortunately, additional targets for intervention, such as endothelin-1, inflammatory cascades, chemokines, etc., are on the horizon. A particularly fertile target for prevention of progression will be kidney grafts, since it has become increasingly clear that factors unrelated to allo-immunity play an important role in chronic allograft nephropathy.

Vasoactive substances in renal transplantation

During and after transplantation the kidney experiences a variety of insults that result in functional impairment and structural damage. These changes are mediated or influenced by hormones, cytokines, enzymes and growth factors, which are excreted by endothelial, graft parenchymal as well as by graft infiltrating cells. This review evaluates the pathophysiological role of vasoactive substances (for example, the vasoconstrictors angiotensin II and endothelin, as well as vasodilators such as nitric oxide, adrenomedullin and atrial natriuretic peptide) in kidney transplantation and summarizes recent reports that indicate that targeting vasoactive substances may represent effective therapeutic strategies for the achievement of long-term allograft survival.

Endothelial dysfunction and atherosclerosis: endothelin receptor antagonists as novel therapeutics

Atherosclerosis, a chronic systemic disease of the vasculature with an inflammatory component, is the primary cause of cardiovascular morbidity and mortality in industrialized countries. It is associated with the impairment of endothelium-dependent relaxation in the coronary, systemic circulation due to decreased bioavailability of nitric oxide, and increased release oxygen-derived free radicals, thus promoting vasoconstriction, leukocyte adhesion, thrombosis, inflammation, and cell proliferation. Expression of endothelin (ET)-1, a 21-amino acid peptide and major isoform of the endothelin peptide family, is produced by endothelial, vascular smooth muscle cells, and macrophages and acts through Gi-protein-coupled ET(A) and ET(B) receptors. Endothelin-1 increases in hypercholesterolemia and atherosclerosis in humans and experimental animals. This paper reviews current experimental and clinical evidence for the involvement of ET-1 in atherogenesis. Furthermore, the effects of ET receptor blockade on experimental hypercholesterolemia and atherosclerosis will be discussed. As chronic endothelin blockade inhibits fatty streak formation and improves vascular function in experimental hypercholesterolemia, hypertension, and heart failure, and as it restores nitric oxide (NO)-mediated endothelial function and reduces atheroma formation in animals with atherosclerosis, endothelin receptor blockade may therefore offer a novel approach for the treatment of atherosclerosis and its vascular complications.

Endothelin antagonists for hypertension and renal disease

The endothelin system has been implicated in the pathogenesis of arterial hypertension and renal disorders. Endothelin-1, the predominant isoform of the endothelin peptide family, regulates vasoconstriction and cell proliferation in tissues both within and outside the cardiovascular system through activation of Gi-protein-coupled ET(A) and ET(B) receptors. Endothelin synthesis is regulated through autocrine mechanisms by endothelin converting enzymes, chymases, and non-endothelin converting enzyme metalloproteases. In-vitro experiments have demonstrated that endothelin-1 stimulates growth in vascular smooth muscle and in the kidney. Recent studies indicate that endothelin mRNA and protein are also increased in vivo in the kidney and vasculature in hypertension and renal disease. Studies using molecular or pharmacological inhibition of the endothelin system demonstrate that endothelin-1 contributes to the functional and structural changes associated with arterial hypertension and glomerulosclerosis, and that these effects are only in part dependent on blood pressure. These experimental studies and first clinical trials suggest that endothelin antagonists may offer therapeutic potential to reduce end-organ damage in diseases associated with vascular remodeling and renal injury.