An endothelin-1 mediated autocrine growth loop involved in human renal tubular regeneration

Co-localization of the sodium-glucose co-transporter-2 channel (SGLT-2) with endothelin ETA and ETB receptors in human cardiorenal tissue

Endothelin (ET) receptor antagonists are being investigated in combination with sodium-glucose co-transporter-2 inhibitors (SGLT-2i). These drugs primarily inhibit the SGLT-2 transporter that, in humans, is thought to be mainly restricted to the renal proximal convoluted tubule, resulting in increased glucose excretion favouring improved glycaemic control and diuresis. This action reduces fluid retention with ET receptor antagonists. Studies have suggested SGLT-2 may also be expressed in cardiomyocytes of human heart. To understand the potential of combining the two classes of drugs, our aim was to compare the distribution of ET receptor sub-types in human kidney, with SGLT-2. Secondly, using the same experimental conditions, we determined if SGLT-2 expression could be detected in human heart and whether the transporter co-localised with ET receptors.

METHODS

Immunocytochemistry localised SGLT-2, ETA and ETB receptors in sections of histologically normal kidney, left ventricle from patients undergoing heart transplantation or controls. Primary antisera were visualised using fluorescent microscopy. Image analysis was used to measure intensity compared with background in adjacent control sections.

RESULTS

As expected, SGLT-2 localised to epithelial cells of the proximal convoluted tubules, and co-localised with both ET receptor sub-types. Similarly, ETA receptors predominated in cardiomyocytes; low (compared with kidney but above background) positive staining was also detected for SGLT-2.

DISCUSSION

Whether low levels of SGLT-2 have a (patho)physiological role in cardiomyocytes is not known but results suggest the effect of direct blockade of sodium (and glucose) influx via SGLT-2 inhibition in cardiomyocytes should be explored, with potential for additive effects with ETA antagonists.

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.

Modulation of polycystic kidney disease by G-protein coupled receptors and cyclic AMP signaling

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a systemic disorder associated with polycystic liver disease (PLD) and other extrarenal manifestations, the most common monogenic cause of end-stage kidney disease, and a major burden for public health. Many studies have shown that alterations in G-protein and cAMP signaling play a central role in its pathogenesis. As for many other diseases (35% of all approved drugs target G-protein coupled receptors (GPCRs) or proteins functioning upstream or downstream from GPCRs), treatments targeting GPCR have shown effectiveness in slowing the rate of progression of ADPKD. Tolvaptan, a vasopressin V2 receptor antagonist is the first drug approved by regulatory agencies to treat rapidly progressive ADPKD. Long-acting somatostatin analogs have also been effective in slowing the rates of growth of polycystic kidneys and liver. Although no treatment has so far been able to prevent the development or stop the progression of the disease, these encouraging advances point to G-protein and cAMP signaling as a promising avenue of investigation that may lead to more effective and safe treatments. This will require a better understanding of the relevant GPCRs, G-proteins, cAMP effectors, and of the enzymes and A-kinase anchoring proteins controlling the compartmentalization of cAMP signaling. The purpose of this review is to provide an overview of general GPCR signaling; the function of polycystin-1 (PC1) as a putative atypical adhesion GPCR (aGPCR); the roles of PC1, polycystin-2 (PC2) and the PC1-PC2 complex in the regulation of calcium and cAMP signaling; the cross-talk of calcium and cAMP signaling in PKD; and GPCRs, adenylyl cyclases, cyclic nucleotide phosphodiesterases, and protein kinase A as therapeutic targets in ADPKD.

Renal Autocrine and Paracrine Signaling: A Story of Self-protection

Autocrine and paracrine signaling in the kidney adds an extra level of diversity and complexity to renal physiology. The extensive scientific production on the topic precludes easy understanding of the fundamental purpose of the vast number of molecules and systems that influence the renal function. This systematic review provides the broader pen strokes for a collected image of renal paracrine signaling. First, we recapitulate the essence of each paracrine system one by one. Thereafter the single components are merged into an overarching physiological concept. The presented survey shows that despite the diversity in the web of paracrine factors, the collected effect on renal function may not be complicated after all. In essence, paracrine activation provides an intelligent system that perceives minor perturbations and reacts with a coordinated and integrated tissue response that relieves the work load from the renal epithelia and favors diuresis and natriuresis. We suggest that the overall function of paracrine signaling is reno-protection and argue that renal paracrine signaling and self-regulation are two sides of the same coin. Thus local paracrine signaling is an intrinsic function of the kidney, and the overall renal effect of changes in blood pressure, volume load, and systemic hormones will always be tinted by its paracrine status.

MicroRNAs as markers to monitor endothelin-1 signalling and potential treatment in renal disease: Carcinoma - proteinuric damage - toxicity

This review highlights new developments in miRNA as diagnostic and surveillance tools in diseases damaging the renal proximal tubule mediated by endothelin in the field of renal carcinoma, proteinuric kidney disease and tubulotoxicity. A new mechanism in the miRNA regulation of proteins leads to the binding of the miRNA directly to the DNA with premature transcriptional termination and hence the formation of truncated protein isoforms (Mxi2, Vim3). These isoforms are mediated through miRNA15a or miRNA 498, respectively. ET-1 can activate a cytoplasmic complex consisting of NF-κB p65, MAPK p38α, and PKCα. Consequently, PKCα does not transmigrate into the nucleus, which leads to the loss of suppression of a primiRNA15a, maturation of this miRNA in the cytoplasm, tubular secretion and detectability in the urine. This mechanism has been shown in renal cell carcinoma and in proteinuric disease as a biomarker for the activation of the signalling pathway. Similarly, ET-1 induced miRNA 498 transmigrates into the nucleus to form the truncated protein Vim3, which is a biomarker for the benign renal cell tumour, oncocytoma. In tubulotoxicity, ET-1 induced miRNa133a down-regulating multiple-drug-resistant related protein-2, relevant for proteinuric and cisplatin/cyclosporine A toxicity. Current advantages and limitations of miRNAs as urinary biomarkers are discussed.

Drug repurposing in kidney disease

Drug repurposing, is the re-tasking of known medications for new clinical indications. Advantages, compared to de novo drug development, include reduced cost and time to market plus the added benefit of a known pharmacokinetic and safety profiles. Suitable drug candidates are identified through serendipitous observations, data mining, or increased understanding of disease mechanisms. This review highlights drugs suited for repurposing in kidney disease. The main cause of mortality in patients with chronic kidney disease is cardiovascular disease. Hence, we have included CV endpoints for the drugs. This review begins with candidates in acute kidney injury: vasodilators levosimendan and vitamin D, followed by candidates in CKD, with particular focus on diabetic kidney disease, autosomal dominant polycystic kidney disease, and focal segmental glomerulosclerosis. Examples include glucose-lowering drugs (sodium glucose co-transporter 2 inhibitors, glucagon-like peptide 1 agonists, and metformin), which have mechanistic potential for cardiac and/or renal protection beyond glucose lowering, with broader applicability to the nondiabetic population; xanthine oxidase inhibitors (allopurinol, febuxostat), selective endothelin receptor A antagonist (atrasentan), Janus kinase inhibitor (baricitinib), selective costimulation modulator (abatacept), pentoxyfylline, and the DNA demethylating agent/vasodilator (hydralazine).

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.

Endothelin Blockade in Diabetic Kidney Disease

Diabetic kidney disease (DKD) remains the most common cause of chronic kidney disease and multiple therapeutic agents, primarily targeted at the renin-angiotensin system, have been assessed. Their only partial effectiveness in slowing down progression to end-stage renal disease, points out an evident need for additional effective therapies. In the context of diabetes, endothelin-1 (ET-1) has been implicated in vasoconstriction, renal injury, mesangial proliferation, glomerulosclerosis, fibrosis and inflammation, largely through activation of its endothelin A (ET_A) receptor. Therefore, endothelin receptor antagonists have been proposed as potential drug targets. In experimental models of DKD, endothelin receptor antagonists have been described to improve renal injury and fibrosis, whereas clinical trials in DKD patients have shown an antiproteinuric effect. Currently, its renoprotective effect in a long-time clinical trial is being tested. This review focuses on the localization of endothelin receptors (ET_A and ET_B) within the kidney, as well as the ET-1 functions through them. In addition, we summarize the therapeutic benefit of endothelin receptor antagonists in experimental and human studies and the adverse effects that have been described.

ET-1 Induced Downregulation of MRP2 via miRNA 133a - A Marker for Tubular Nephrotoxicity?

BACKGROUND

Multiple drug resistance (MDR), known from treating malignant tumors with chemotherapy, increases the efflux of reabsorbed reagents in tumor cells. This mechanism has been reported in the renal proximal tubule and may prevent therapeutic tubular protection in proteinuria. Since endothelin-1 (ET-1), a major component in the urine of proteinuric patients, stimulates proximal tubules, its influence on MDR was analyzed with emphasis on the multidrug resistance-associated protein 2 (MRP2), a prominent transporter in the human proximal tubule and microRNA (miRNA) 133a.

METHODS

ET-1 stimulated, cultured human renal proximal tubule cells (RPTECs), were analyzed via Western blot for the expression of MRP2 and via qRT-PCR for miRNA 133a. For direct interaction between the miRNA 133a and the 3'UTR of MRP2, an immunoprecipitation was performed using FITC-labelled miRNA 133a as capture, followed by MRP2 PCR analysis and Sanger sequencing. Murine Adriamycin nephropathic model and human proteinuric samples showed high levels of miRNA 133a but low levels of MRP2. The increasing miRNA 133a levels were detectable in urine samples of humans and animals.

RESULTS

ET-1 activates the miRNA 133a, which can bind to the 3'UTR of MRP2 and is therefore responsible for the detectable decrease of MRP2.

CONCLUSION

This is the first report to analyze the correlation between ET-1-induced miRNA 133a overexpression in proteinuria resulting in MRP2 downregulation, which is a contributing factor for renal cytotoxicity. The detection of the miRNA 133a in urine samples can be possibly used as a monitor for cytotoxicity.

Structure and function of the thin limbs of the loop of Henle

The thin limbs of the loop of Henle, which comprise the intermediate segment, connect the proximal tubule to the distal tubule and lie entirely within the renal medulla. The descending thin limb consists of at least two or three morphologically and functionally distinct subsegments and participates in transepithelial transport of NaCl, urea, and water. Only one functionally distinct segment is recognized for the ascending thin limb, which carries out transepithelial transport of NaCl and urea in the reabsorptive and/or secretory directions. Membrane transporters involved with passive transcellular Cl, urea, and water fluxes have been characterized for thin limbs; however, these pathways do not account for all transepithelial fluid and solute fluxes that have been measured in vivo. The paracellular pathway has been proposed to play an important role in transepithelial Na and urea fluxes in defined thin-limb subsegments. As the transport pathways become clearer, the overall function of the thin limbs is becoming better understood. Primary and secondary signaling pathways and protein-protein interactions are increasingly recognized as important modulators of thin-limb cell function and cell metabolism. These functions must be investigated under diverse extracellular conditions, particularly for those cells of the deep inner medulla that function in an environment of wide variation in hyperosmolality. Transgenic mouse models of several key water and solute transport proteins have provided significant insights into thin-limb function. An understanding of the overall architecture of the medulla, including juxtapositions of thin limbs with collecting ducts, thick ascending limbs, and vasa recta, is essential for understanding the role of the kidney in maintaining Na and water homeostasis, and for understanding the urine concentrating mechanism.

ETS-dependent p16INK4a and p21waf1/cip1 gene expression upon endothelin-1 stimulation in malignant versus and non-malignant proximal tubule cells

AIM

Cellular senescence, leading to cell death through prevention of regular cell renewal, is associated with the upregulation of the tumor suppressor gene p16(INK4a). While this mechanism has been described as leading to progressive nephron loss, p16(INK4a) upregulation in renal cell carcinoma has been linked to a disease-specific improved patient survival rate. While in both conditions endothelin-1 is also upregulated, the signaling pathway connecting ET-1 to p16(INK4a) has not been characterized until this study.

MAIN METHODS

Cell culture, qRT-PCR, Western Blot, immunoprecipitation (IP), proximity ligation assay (PLA), and non-radioactive electrophoretic mobility shift assay (EMSA).

KEY FINDINGS

In malignant renal proximal tumor cells (Caki-1), an activation of p16(INK4a) and p21(waf1/cip1) was observed. An increased expression of E-26 transformation-specific (ETS) transcription factors was detectable. Using specific antibodies, a complex formation between ETS1 and extracellular signal-regulated kinase-2 (ERK2) was shown. A further complex partner was Mxi2. EMSA with supershift analysis for ETS1 and Mxi2 indicated the involvement of both factors in the protein-DNA interaction. After specifically blocking the endothelin receptors, ETS1 expression was significantly downregulated. However, the endothelin B receptor dependent downregulation was stronger than that of the A receptor. In contrast, primary proximal tubule cells showed a nuclear decrease after ET-1 stimulation. This indicates that other ETS members may be involved in the observed p16(INK4a) upregulation (as described in the literature).

SIGNIFICANCE

ETS1, ERK2 and Mxi2 are important complex partners initiating increased p16(INK4a) and p21w(af1/cip1) activation in renal tumor cells.

Physiology of endothelin and the kidney

Since its discovery in 1988 as an endothelial cell-derived peptide that exerts the most potent vasoconstriction of any known endogenous compound, endothelin (ET) has emerged as an important regulator of renal physiology and pathophysiology. This review focuses on how the ET system impacts renal function in health; it is apparent that ET regulates multiple aspects of kidney function. These include modulation of glomerular filtration rate and renal blood flow, control of renin release, and regulation of transport of sodium, water, protons, and bicarbonate. These effects are exerted through ET interactions with almost every cell type in the kidney, including mesangial cells, podocytes, endothelium, vascular smooth muscle, every section of the nephron, and renal nerves. In addition, while not the subject of the current review, ET can also indirectly affect renal function through modulation of extrarenal systems, including the vasculature, nervous system, adrenal gland, circulating hormones, and the heart. As will become apparent, these pleiotropic effects of ET are of fundamental physiologic importance in the control of renal function in health. In addition, to help put these effects into perspective, we will also discuss, albeit to a relatively limited extent, how alterations in the ET system can contribute to hypertension and kidney disease.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Low NaCl intake elevates renal medullary endothelin-1 and endothelin A (ETA) receptor mRNA but not the sensitivity of renal Na+ excretion to ETA receptor blockade in rats

AIMS

This study was performed to investigate the effects of NaCl intake on renal mRNA expression of pre-pro-endothelin-1 (ET-1), endothelin A (ET(A)) and endothelin B (ET(B)) receptors as well as on renal ET-1 content in rats. We further tested for NaCl intake-dependent differences in the contribution of the ET system to renal sodium handling.

METHODS

Male Sprague-Dawley rats with telemetric devices were randomized to 0.15%, 0.60% and 1.80% NaCl diets with or without losartan. Renal sodium balance and arterial pressure were monitored. Renal blood flow and fractional sodium excretion (FENa) were measured in response to acute infusion of ET(A) and ET(B) blockers into the inner stripe of the outer renal medulla.

RESULTS

Medullary pre-pro-ET-1, ET(A) and ET(B) receptor mRNA was 50%, 81% and 33% higher in rats on 0.15% vs. 1.80% NaCl. Losartan reduced medullary gene expression in rats on 0.15% NaCl. Medullary ET-1 content was 983 +/- 88 and 479 +/- 42 ng mg(-1) protein in rats on 0.15% and 1.80% NaCl (P < 0.001). Chronic ET(A) receptor blocker treatment reduced arterial pressure by 8-10 mmHg in rats on 0.15% vs. 1.80% NaCl without affecting renal sodium balances. Acute medullary ET(A) or ET(B) receptor blockade did not alter medullary blood flow and FENa in animals on either diet.

CONCLUSION

In rats renal medullary ET-1 content and mRNA expression of three ET system components are inversely related to NaCl intake. Higher expression levels on low NaCl intake are AT(1) receptor dependent but are not associated with increased sensitivity of renal sodium handling to ET(A) receptor blockade.

Endothelin-1 induces NF-kappaB via two independent pathways in human renal tubular epithelial cells

BACKGROUND

Endothelin-1 (ET-1) is a major transcriptional activator of renal proximal tubule cells acting in an autocrine and paracrine manner. In animal studies, ET-1 has been implicated in progressive renal interstitial fibrosis by promoting gene expression, possibly via the inflammatory NF-kappaB signal pathway. While ET-1-dependent mechanisms of signal transduction have been studied mainly in tumor cell lines, we analyzed the mechanism of ET-1-induced, NF-kappaB-mediated target gene activation in proximal tubule cells.

METHODS

Human renal proximal tubule cells were stimulated with ET-1 and gene expression analyzed by protein microarray, Western blot, non-radioactive electromobility shift assay, and quantitative real-time polymerase chain reaction.

RESULTS

Activation of NF-kappaB occurs only via an ET-1-specific type A receptor (not type B as in animals). Induction can be blocked by bosentan, and endothelin-A but not endothelin-B receptor-specific antagonists. Protein microarray screening shows activation of two independent cascades (via the endothelin-A receptor, or via diacylglycerol) leading to NF-kappaB induction. The independent induction is also reflected by target gene expression such as the vascular cell adhesion molecule-1, interleukin-6, and fractalkine at different time points.

CONCLUSION

Thus prohibiting ET-1-mediated gene transcription necessitates blocking of NF-kappaB and diacylglycerol signal transduction in proximal tubule cells.

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.

Intrarenal oxygenation in chronic renal failure

In chronic renal failure (CRF), renal impairment correlates with tubulointerstitial fibrosis characterized by inflammation, interstitial expansion with accumulation of extracellular matrix (ECM), tubular atrophy and vascular obliteration. Tubulointerstitial injury subsequent to glomerular sclerosis may be induced by proteinuria, leakage of glomerular filtrate or injury to the post-glomerular peritubular capillaries (hypoxia). In vivo data in animal models suggest that CRF is associated with hypoxia, with the decline in renal Po2 preceding ECM accumulation. Chronic renal failure is characterized by loss of microvascular profiles but, in the absence of microvascular obliteration, hypoxia can occur by a variety of complementary mechanisms, including anaemia, decreased capillary flow, increased vasoconstriction, increased metabolic demand and increased diffusion distances due to ECM deposition. Hypoxia regulates a wide array of genes, including many fibrogenic factors. Hypoxia-inducible factors (HIF) are the major, but not the sole, transcriptional regulators in the hypoxic response. In CRF, hypoxia may play a role in the sustained inflammatory response. In vitro studies in tubulointerstitial cells suggest that hypoxia can induce profibrogenic changes in proximal tubular epithelial cells and interstitial fibroblasts consistent with changes observed in CRF in vivo. The effect of hypoxia on renal microvascular cells warrants investigation. Hypoxia may play a role in the recruitment, retention and differentiation of circulating progenitor cells to the kidney contributing to the disease process and may also affect intrinsic stem cell populations. Chronic hypoxia in CRF fails to induce a sustained angiogenic response. Therapeutic manipulation of the hypoxic response may be of benefit in slowing progression of CRF. Potential therapies include correction of anaemia, inhibition of the renin-angiotensin system, administration of exogenous pro-angiogenic factors to protect the microvasculature, activation of HIF and hypoxia-mediated targeting of engineered progenitor cells.

Diabetic Endothelin B Receptor–Deficient Rats Develop Severe Hypertension and Progressive Renal Failure

The endothelin (ET) system has been implicated in the pathogenesis of diabetic nephropathy. The role of the ET-B receptor (ETBR) is still unclear. The effect of ETBR deficiency on the progression of diabetic nephropathy in a streptozotocin model was analyzed in four groups: (1) Homozygous ETBR-deficient (ETBRd) diabetic rats, (2) ETBRd rats, (3) diabetic controls, and (4) wild-type controls. BP and kidney function were measured for 10 wk, followed by biochemical and histologic analysis of the kidneys. The study demonstrates that ETBRd diabetic rats on a normal-sodium diet develop severe hypertension, albuminuria, and a mild reduction of creatinine clearance. The strong BP rise seems not to be caused by activation of the renin-angiotensin-aldosterone system or by suppression of the nitric oxide system. Elevated plasma ET-1, possibly reflecting a reduced ETBR-dependent clearance, seems to cause the severe hypertension via the ETA receptor. The results do not support the hypothesis that a reduction of ETBR activity inhibits the progression of diabetic nephropathy. The study demonstrates for the first time that the combination of diabetes and ETBR deficiency causes severe low-renin hypertension with progressive renal failure.

Association between endothelin-1 and collagen deposition in db/db diabetic mouse kidneys

Endothelin-1 has been implicated in diabetic kidney injury, but there are few firm data establishing the temporal and spatial expression of kidney endothelin-1 in diabetes. We performed an immunohistochemical and histopathological analysis to determine endothelin-1 peptide expression in the kidneys of diabetic db/db mice and non-diabetic db/m controls. Diabetic mice were studied at 8 weeks, before histological damage is evident, and again at 16 weeks, when significant glomerular injury has occurred. Urinary endothelin-1 was 6.2- and 3.6-fold higher in 8- and 16-week diabetic mice compared to age-matched controls (P<0.01 db/db vs. db/m). Compared to non-diabetic kidneys, immunoreactive endothelin-1 was first elevated 2.5-fold (P=0.02) in the tubulointerstitial compartment at 8-week and remained high (3.8-fold, P<0.01) at 16 weeks. In contrast, glomerular endothelin-1 was elevated 3.2-fold (P=0.03) only in 16-week diabetic mice. Glomerular and tubulointerstitial endothelin-1 were unchanged in 8- and 16-week non-diabetic mice. Elevated endothelin-1 in diabetic mice associated temporally and spatially with collagen deposition, especially in the tubulointerstitial compartment. The localization of kidney endothelin-1 is consistent with a role for this peptide in renal fibrogenesis. These results also highlight the potential role of ET-1 in the pathogenesis of early tubulointerstitial changes in diabetes.

Src homology 2-containing phosphotyrosine phosphatase regulates endothelin-1-induced epidermal growth factor receptor transactivation in rat renal tubular cell NRK-52E

Epidermal growth factor (EGF) and endothelin-1 (ET-1) have been shown to be involved in proliferation and autoregeneration of renal tubular cells. This study aims to investigate the regulatory mechanism of ET-1-mediated EGF receptor (EGFR) transactivation in rat renal tubular cells (NRK-52E). Exposure of NRK-52E cells to ET-1 was found to stimulate the phosphorylation of EGFR and induce reactive oxygen species (ROS) generation. Both NAD(P)H oxidase inhibitor, diphenyliodonium (DPI) and ROS scavenger N-acetylcysteine (NAC), inhibited EGFR transactivation and extracellular signal-regulated kinase (ERK) phosphorylation caused by ET-1. In contrast, blockade of EGFR by AG1478 inhibited the phosphorylation of ERK but not ROS generation following ET-1 exposure. We found that the catalytic cysteine of Src homology 2-containing phosphotyrosine phosphatase (SHP-2) was transiently oxidized by ET-1 treatment in a modified malachite green phosphatase assay. In EGFR co-immunoprecipitation, SHP-2 was also found to interact with EGFR following ET-1 treatment. In SHP-2 knockdown NRK-52E cells, ET-1-induced EGFR transactivation was dramatically elevated and not influenced by NAC. However, GM6001 (an MMP inhibitor) and heparin binding (HB)-EGF neutralizing antibody suppressed this elevation. Our data suggest that ROS-mediated oxidation of SHP-2 is essential for HB-EGF-mediated EGFR transactivation in ET-1 signaling pathway in NRK-52E cells.

Altered AT 1 Receptor Regulation of ETB Receptors in Renal Proximal Tubule Cells of Spontaneously Hypertensive Rats

The renin-angiotensin and endothelin systems regulate blood pressure, in part, by affecting renal tubular sodium transport. In rodents, ETB receptors decrease proximal tubular reabsorption, whereas AT 1 receptors produce the opposite effect. We hypothesize that ETB and AT 1 receptors interact at the receptor level, and that the interaction is altered in spontaneously hypertensive rats (SHRs). In immortalized renal proximal tubule (RPT) cells from Wistar-Kyoto (WKY) rats, angiotensin II, via AT 1 receptors, increased ETB receptor protein in a time- and concentration-dependent manner. In contrast, in SHR RPT cells, angiotensin II (10 −8 M/24 hours) had no effect on ETB receptor protein. AT 1 /ETB receptors colocalized and co-immunoprecipitated in both rat strains but long-term angiotensin II (10 −8 M/24 hours) treatment increased AT 1 /ETB co-immunoprecipitation in WKY but not in SHR cells. Short-term angiotensin II (10 −8 M/15 minutes) treatment decreased ETB receptor phosphorylation in both WKY and SHR cells, and increased ETB receptors in RPT cell surface membranes of RPT cells in WKY but not SHRs. Basal cell surface membrane ETB receptor expression was also higher in WKY than in SHRs. We conclude that AT 1 receptors regulate ETB receptors by receptor interaction and modulation of receptor expression. The altered AT 1 receptor regulation of ETB receptors in SHRs may play a role in the pathogenesis of hypertension.

Histochemical distribution of endothelin-converting enzyme subtypes in Podarcis sicula (Squamata, Lacertidae) tissues

The distribution of endothelin-converting enzyme (ECE) in the lizard Podarcis sicula was investigated immunohistochemically using antibodies against endothelin-converting enzyme ECE-1 and endothelin-converting enzyme ECE-2 homologues. In all the tissues examined, immunoreactivity for both antibodies was found, although the distribution and degree of expression varied. Strong immunoreactivity was found in the endothelial cells and chromaffin tissue for both enzymes, whereas other tissues such as nervous tissue, renal tissue and hepatocytes display distinct patterns. Current knowledge does not allow correlation of these distribution patterns to specific functions but the data suggest that, in reptiles as in mammals, ECE is probably involved in physiological functions such as paracrine activity through endothelins and/or other substrates.

Tempol Protects against Ischemic Acute Renal Failure by Inhibiting Renal Noradrenaline Overflow and Endothelin-1 Overproduction

The effects of tempol, a superoxide dismutase mimetic, on ischemia/reperfusion-induced acute renal failure (ARF), noradrenaline (NA) overflow and endothelin-1 (ET-1) overproduction in rats were examined. Ischemic ARF was induced by occlusion of the left renal artery and vein for 45 min followed by reperfusion, 2 weeks after contralateral nephrectomy. Renal functional parameters such as blood urea nitrogen, plasma creatinine concentration, and fractional excretion of sodium, NA concentrations in renal venous plasma, and renal ET-1 contents were determined. Renal function in ARF rats markedly decreased at 1 d after reperfusion. Pre-ischemic treatment with tempol (10, 100 mg/kg, i.v.) dose-dependently attenuated the ischemia/reperfusion-induced renal dysfunction. Histopathological examination of the kidney of ARF rats revealed severe renal damages, such as tubular necrosis, proteinaceous casts in tubuli and medullary congestion, which were also significantly suppressed by the tempol treatment. There was a significant increase in NA concentrations in renal venous plasma after the ischemia/reperfusion, and this increase was markedly suppressed by the treatment with tempol. In addition, tempol treatment significantly attenuated the increment of ET-1 content in the kidney exposed to the ischemia/reperfusion. These findings suggest that tempol improves the post-ischemic renal injury by inhibiting the neural activity of renal sympathetic nerve and ET-1 overproduction.

HIF-1alpha expression follows microvascular loss in advanced murine adriamycin nephrosis

Cellular hypoxia has been proposed as a major factor in the pathogenesis of chronic renal injury, yet to date there has been no direct evidence to support its importance. Therefore, we examined cortical hypoxia in an animal model of chronic renal injury (murine adriamycin nephrosis; AN) by assessing nuclear localization of the oxygen-dependent alpha-subunit of hypoxia-inducible factor-1 (HIF-1alpha) in animals 7, 14, and 28 days after adriamycin. Results were assessed in conjunction with quantitation of the cortical microvasculature (by CD34 immunostaining) and cortical expression of VEGF. Cortical apoptosis was also examined by terminal deoxynucleotidyl transferase dUTP nick-end labeling staining. A dramatic and significant increase in nuclear localization of HIF-1alpha was seen 28 days after adriamycin in the context of severe glomerular and tubulointerstitial damage. Areas of nuclear HIF-1alpha staining did not colocalize with areas of cellular apoptosis. AN was also associated with a significant attenuation of the peritubular capillaries that was significant at 14 and 28 days after adriamycin. Cortical VEGF expression fell in a stepwise manner from day 7 until day 28 after adriamycin. In conclusion, these data are consistent with a significant increase in cellular hypoxia occurring in the advanced stages of murine AN. Increased cortical hypoxia was preceded by significant reductions in both the number of peritubular capillaries (i.e., oxygen supply) and the angiogenic cytokine VEGF. Apart from providing the first direct evidence for cellular hypoxia in a model of chronic renal disease, these results suggest that a primary dysregulation of angiogenesis may be the cause of increased hypoxia in this model.

A novel and selective Na+/Ca2+ exchange inhibitor, SEA0400, improves ischemia/reperfusion-induced renal injury

We evaluated the effects of SEA0400 (2-[4-[(2,5-difluorophenyl)methoxy]phenoxy]-5-ethoxyaniline), a novel and selective Na+/Ca2+ exchange inhibitor, on ischemic acute renal failure. Ischemic acute renal failure in rats was induced by clamping the left renal artery and vein for 45 min followed by reperfusion, 2 weeks after the contralateral nephrectomy. SEA0400 administration (0.3, 1 and 3 mg/kg, i.v.) before ischemia dose-dependently attenuated the ischemia/reperfusion-induced renal dysfunction and histological damage such as tubular necrosis. SEA0400 pretreatment at the higher dose suppressed the increment of renal endothelin-1 content after reperfusion. The ischemia/reperfusion-induced renal dysfunction was also overcome by post-ischemia treatment with SEA0400 at 3 mg/kg, i.v. In in vitro study, SEA0400 (0.2 and 1 microM) protected cultured porcine tubular cells (LLC-PK1) from hypoxia/reoxygenation-induced cell injury. These findings support the view that Ca2+ overload via the reverse mode of Na+/Ca2+ exchange, followed by endothelin-1 overproduction, plays an important role in the pathogenesis of ischemia/reperfusion-induced renal injury. The possibility exists that a selective Na+/Ca2+ exchange inhibitor such as SEA0400 is useful as effective therapeutic agent against ischemic acute renal failure in humans.

Endothelin a receptor blockade and endothelin B receptor blockade improve hypokalemic nephropathy by different mechanisms

Hypokalemia causes renal tubulointerstitial injury with an elevation in renal endothelin-1 (ET-1). It was hypothesized that hypokalemic tubulointerstitial injury is ameliorated by the blockade of ET-A receptors (ETA), whereas ET-B receptor (ETB) antagonism may exacerbate the injury, because ETB is thought to mediate vasodilation. Rats were fed a K(+)-deficient diet alone (LC) or with an ETA-selective antagonist ABT-627 (LA) or an ETB-selective antagonist A-192621 (LB) for 8 wk. Control rats were on a normal K(+) diet alone or with the ETA-selective or ETB-selective antagonists. The severity of hypokalemia was not significantly different among LA, LB, and LC. LC developed tubulointerstitial injury with an elevation of renal preproET-1 mRNA level. There was an increase in tubular osteopontin expression, macrophage infiltration, collagen accumulation, and tubular cell hyperplasia. ETA blockade significantly ameliorated all parameters for renal injury in the cortex without suppressing local ET-1 and ETA expression. By contrast, ETB blockade significantly reduced local ET-1 and ETA expression and improved the injury to a similar extent in the cortex. In the medulla, ETA or ETB blockade only partially blocked renal injury. ETA blockade did not affect BP in normokalemic or hypokalemic rats. ETB blockade induced a BP elevation with a decrease in urinary Na(+) excretion in normokalemic but not in hypokalemic rats. These results indicate that ET-1 can mediate hypokalemic renal injury in two different ways: by directly stimulating ETA and by locally promoting endogenous ET-1 production via ETB. Thus, ETA as well as ETB blockade may be renoprotective in hypokalemic nephropathy.

Attenuation of ischemia/reperfusion-induced renal injury in mice deficient in Na+/Ca2+ exchanger

Using Na+/Ca2+ exchanger (NCX1)-deficient mice, the pathophysiological role of Ca2+ overload via the reverse mode of NCX1 in ischemia/reperfusion-induced renal injury was investigated. Because NCX1(-/-) homozygous mice die of heart failure before birth, we used NCX1(+/-) heterozygous mice. NCX1 protein in the kidney of heterozygous mice decreased to about half of that of wild-type mice. Expression of NCX1 protein in the tubular epithelial cells and Ca2+ influx via NCX1 in renal tubules were markedly attenuated in the heterozygous mice. Ischemia/reperfusion-induced renal dysfunction in heterozygous mice was significantly attenuated compared with cases in wild-type mice. Histological renal damage such as tubular necrosis and proteinaceous casts in tubuli in heterozygous mice were much less than that in wild-type mice. Ca2+ deposition in necrotic tubular epithelium was observed more markedly in wild-type than in heterozygous mice. Increases in renal endothelin-1 content were greater in wild-type than in heterozygous mice, and this reflected the difference in immunohistochemical endothelin-1 localization in necrotic tubular epithelium. When the preischemic treatment with KB-R7943 was performed, the renal functional parameters of both NCX1(+/+) and NCX1(+/-) acute renal failure mice were improved to the same level. These findings strongly support the view that Ca2+ overload via the reverse mode of Na+/Ca2+ exchange, followed by renal endothelin-1 overproduction, plays an important role in the pathogenesis of ischemia/reperfusion-induced renal injury.

Role of endothelin B receptor in the pathogenesis of ischemic acute renal failure

This study evaluated the role of endothelin B (ET ) receptor-mediated action in the development and maintenance of ischemic acute renal failure (ARF), using the spotting-lethal ( ) rat that carries a naturally occurring deletion in the ET receptor gene. Because homozygous ( ) rats die shortly after birth due to congenital distal intestinal aganglionosis, genetic rescue of rats from the developmental defect using a dopamine-beta-hydroxylase (DbetaH)-ET transgene was performed to produce ET -deficient adult rats. Rescued homozygous (DbetaH-ET ) and wild-type (DbetaH-ET +/+) rats were subjected to ischemic ARF by clamping the renal pedicle for 45 min followed by reperfusion. At 24 h after the reperfusion, renal glomerular dysfunction and histologic damage, such as proteinaceous casts in tubuli, were markedly and observed equally in homozygous and wild-type groups, and these renal injuries gradually recovered. However, when the ischemia/reperfusion-induced renal injury was examined 7 days after the reperfusion, the recovery in homozygous ARF rats was obviously delayed compared with the wild-type animals. Two of the eight homozygous ARF rats died within 3 days after the reperfusion. Increment of renal endothelin-1 content after the ischemia/reperfusion was more marked in homozygous than in wild-type rats. Thus, ET receptor-mediated actions do not play an important role in the development of ischemic ARF but may be involved in the recovery process from ischemia/reperfusion-induced renal injury.

Renal protective effect of YM598, a selective endothelin ET(A) receptor antagonist, against diabetic nephropathy in OLETF rats

We have investigated the effect of potassium (E)-N-[6-methoxy-5-(2-methoxyphenoxy)-2-(pyrimidin-2-yl) pyrimidin-4-yl]-2-phenylenthenesulfonamidate (YM598), a selective endothelin ET(A) receptor antagonist, on renal function in Otsuka Long-Evans Tokushima Fatty (OLETF) rats, an animal model of type II diabetes. YM598 (0.1 or 1 mg kg(-1)), enalapril (5 mg kg(-1)), an angiotensin-converting enzyme inhibitor, or vehicle was administered once daily by gastric gavage to 22-week-old male Otsuka Long-Evans Tokushima Fatty rats for 32 weeks. Enalapril but not YM598 mildly lowered blood pressure in the diabetic rats. YM598 blunted the development of albuminuria in a dose-dependent manner. High dose of YM598 reduced albuminuria comparable to enalapril. Urinary endothelin-1 excretion was greater in the diabetic than in the control rats, and was not substantially influenced by the agents. These data suggest that endothelin is involved in the progression of diabetic nephropathy in Otsuka Long-Evans Tokushima Fatty rats, and an endothelin ET(A) receptor antagonist may be useful for the treatment of diabetic nephropathy.

Role of endothelin-1 in lung disease

Endothelin-1 (ET-1) is a 21 amino acid peptide with diverse biological activity that has been implicated in numerous diseases. ET-1 is a potent mitogen regulator of smooth muscle tone, and inflammatory mediator that may play a key role in diseases of the airways, pulmonary circulation, and inflammatory lung diseases, both acute and chronic. This review will focus on the biology of ET-1 and its role in lung disease.

Renal endothelin-1 and endothelin receptor type B expression in glomerular diseases with proteinuria

The endothelin (ET) system has been studied extensively in experimental models of progressive chronic renal disease, but there is limited information regarding the ET system in renal patients. First, the expression of human ET-1, as well as ET receptor type A (ET-R(A)) and ET-R(B), was studied in 26 renal biopsies from patients with different renal diseases. Gene expression was assessed by quantitative reverse transcription-PCR. Second, ET-1 and ET-R(B) protein expression and localization were examined, by immunohistochemical analyses, among a homogeneous cohort of 16 patients with IgA nephropathy and different degrees of proteinuria. ET-R(B) mRNA expression was threefold higher among patients with higher-grade proteinuria [> or =2 g/24 h, n = 10; OD ratio (ODR), i.e., wild-type/mutant mRNA ratio, 1.81 +/- 0.3], compared with patients with lower-grade proteinuria (<2 g/24 h, n = 8; ODR, 0.63 +/- 0.1; P < 0.01) or control subjects (n = 9; ODR, 0.57 +/- 0.1; P < 0.01). ET-1 gene expression was significantly higher among patients with higher-grade proteinuria, compared with patients with lower-grade proteinuria (P < 0.01) or control subjects (P < 0.05). ET-R(A) mRNA expression was not different among the groups. Patients with higher-grade proteinuria who were receiving angiotensin-converting enzyme inhibitors exhibited significantly (P < 0.05) lower ET-1 and ET-R(B) mRNA expression, which was comparable to that of control subjects. By using immunohistochemical analyses, an association between proteinuria and expression of ET-1 and ET-R(B) in proximal tubular epithelial cells and of ET-1 in glomeruli was confirmed in the separate cohort of patients with IgA nephropathy. It is concluded that the increased ET-R(B) and ET-1 mRNA and protein expression observed in animal models of renal disease is also demonstrable among patients with renal disease and high-grade proteinuria.

Endothelin-1 synthesis and endothelin B receptor expression in human coronary artery smooth muscle cells and monocyte-derived macrophages is up-regulated by low density lipoproteins

Endothelin-1 (ET-1) is a potent vasoconstrictive peptide exerting its effects predominantly by paracrine and autocrine mechanisms. ET-1 acts as a mitogen and co-mitogen on vascular smooth muscle cells, and accumulating evidence suggests that ET-1 is involved in the pathogenesis of atherosclerosis. Deposition of low density lipoproteins (LDL) in the vessel wall is known to play a crucial role in the development of atherosclerotic lesions. In the present study, we have investigated the effect of native LDL (nLDL) and oxidatively modified LDL (oxLDL) on ET-1 synthesis and endothelin receptor expression in cultured human coronary artery smooth muscle cells and human monocyte-derived macrophages. Native LDL and oxLDL induced a significant stimulation of ET-1 release and ET-1 mRNA expression in human coronary artery smooth muscle cells and monocyte-derived macrophages. Antibodies against the scavenger receptor CD36 significantly reduced the oxLDL-induced stimulation of ET-1 synthesis. The antioxidants trolox and probucol did not significantly inhibit the LDL-induced rise of ET-1 release. Endothelin B receptor expression was up-regulated in both cell types after incubation with nLDL and oxLDL. In coronary smooth muscle cells, endothelin A receptor expression was slightly increased by LDL, whereas endothelin A receptor was not detectable in monocyte-derived macrophages. Coronary artery smooth muscle cells secreted a more than 150-fold higher amount of immunoreactive ET-1 into the cell culture medium than monocyte-derived macrophages. In summary, the present data, demonstrating a LDL-induced up-regulation of the endothelin system in coronary smooth muscle cells and in monocyte-derived macrophages, provide further support for a pathophysiological role of endothelin in coronary atherosclerosis and suggest that ET-1 might be involved in mediating the atherogenic effects of LDL.

Endothelin synthesis and receptors in porcine kidney

As insufficient information on the endothelin (ET) system in the porcine kidney is available at present, we investigated renal ET-1 synthesis and ET receptors in this species. Because ET specifically affects renal and glomerular haemodynamics and distal tubular reabsorption, we studied ET-1 synthesis in isolated glomeruli and in inner medullary collecting duct (IMCD) cells and preproET-1 mRNA in renal cortex, isolated glomeruli and papillary tissue. In addition, we characterized density and properties of ET receptors in membranes from isolated glomeruli and papillary tissue. In contrast to isolated IMCD cells, which synthesized 120 +/- 11 fmol h(-1) mg-1 protein of ET-1, no such synthesis was found with isolated glomeruli in our assay system. Nevertheless, with RT-PCR preproET(-1) mRNA was clearly present in renal cortex and glomeruli as well as in papillary tissue. Glomerular membranes were found to have ET receptors with Bmax of 1.6 +/- 0.2 pmol mg-1 protein and Kd of 311 +/- 33 pmol L(-1). Using BQ-123 (10-5 M), a specific blocker of ETA receptors, we found that 58% of total receptors are ETA receptors. Thus, presumably 42% are ETB receptors (Bmax 0.7 +/- 0.1 pmol mg-1 protein; Kd 429 +/- 110 pmol L(-1)). Bosentan (10-5 M), an ETA- and ETB-receptor antagonist, blocked all ET receptors in glomerular membranes. Papillary membranes showed ET receptors with Bmax of 2.1 +/- 0.2 pmol mg-1 protein and Kd of 137 +/- 11 pmol L(-1). In the presence of BQ-123 (10-5 M) we found that all receptors are ETB receptors (Bmax 2.3 +/- 0.4 pmol mg-1 protein; Kd 162 +/- 25 pmol L(-1)). Bosentan (10-5 M) again blocked all ET receptors in papillary membranes, thus confirming our previous finding that IMCD cells possess high-affinity ETB receptors mediating the diuretic effects of ET. Thus, in the porcine kidney the ET system may act in an autocrine/paracrine manner at the glomerular as well as at the IMCD level.

KB-R7943, a selective Na+/Ca2+ exchange inhibitor, protects against ischemic acute renal failure in mice by inhibiting renal endothelin-1 overproduction

We investigated whether the preischemic or postischemic treatment with KB-R7943, a novel and selective Na+/Ca2+ exchange inhibitor, has renal protective effects in mice with ischemic acute renal failure (ARF). Ischemic ARF was induced by clamping the left renal pedicle for 45 min followed by reperfusion, 2 weeks after contralateral nephrectomy. Renal function was markedly diminished 24 h after reperfusion. Preischemic treatment with KB-R7943 attenuated the ARF-induced renal dysfunction. The ischemia/reperfusion-induced renal dysfunction was also overcome by postischemic treatment with KB-R7943. Histopathologic examination of the kidneys of ARF mice revealed severe renal damage such as tubular necrosis, proteinaceous casts in tubuli, and medullary congestion. Histologically evident damage and Ca2+ deposition in necrotic tubular epithelium were improved by preischemic treatment with KB-R7943. In addition, preischemic treatment with KB-R7943 significantly suppressed the increment of endothelin-1 (ET-1) content in the kidney at 2, 6, and 24 h after reperfusion. These findings suggest that Ca2+ overload via the reverse mode of Na+/Ca2+ exchange, followed by renal ET-1 overproduction, plays an important role in the pathogenesis of the ischemia/reperfusion-induced ARF. KB-R7943 may prove to be an effective therapeutic agent for cases of ischemic ARF in humans.

Hypoxia promotes fibrogenesis in human renal fibroblasts

BACKGROUND

The mechanisms underlying progressive renal fibrosis are unknown, but the common association of fibrosis and microvascular loss suggests that hypoxia per se may be a fibrogenic stimulus.

METHODS

To determine whether human renal fibroblasts (HRFs), the primary matrix-producing cells in the tubulointerstitium, possess oxygen-sensitive responses relevant to fibrogenesis, cells were exposed to 1% O2 in vitro.

RESULTS

Hypoxia simultaneously stimulated extracellular matrix synthesis and suppressed turnover with increased production of collagen alpha1(I) (Coll-I), decreased expression of collagenase, and increased tissue inhibitor of metalloproteinase (TIMP)-1. These effects are time dependent, require new RNA and protein synthesis, and are specific to hypoxia. The changes in Coll-I and TIMP-1 gene expression involve a heme-protein O2 sensor and protein kinase- and tyrosine kinase-mediated signaling. Although hypoxia induced transforming growth factor-beta1 (TGF-beta1), neutralizing anti-TGF-beta1-antibody did not block hypoxia-induced Coll-I and TIMP-1 mRNA expression. Furthermore, hypoxic-cell conditioned-medium had no effect on the expression of these mRNAs in naive fibroblasts, suggesting direct effects on gene transcription. Transient transfections identified a hypoxia response element (HRE) in the TIMP-1 promoter and demonstrated HIF-1-dependent promoter activation by decreased ambient pO2.

CONCLUSIONS

These data suggest that hypoxia co-ordinately up-regulates matrix production and decreases turnover in renal fibroblasts. The results support a role for hypoxia in the pathogenesis of fibrosis and provide evidence for novel, direct hypoxic effects on the expression of genes involved in fibrogenesis.

Renal nerves and endothelins interaction in the control of renal excretory function in conscious Long-Evans rats

The role of renal nerves and endothelins, acting at ET(A) receptors, in the regulation of renal excretory function was investigated in male Long-Evans rats. Catheters were placed in the femoral vein for fluid and drug infusion, in the femoral artery for blood pressure recording as well as in the bladder for urine collection. Infusion of 16.4 nmol/kg/min of the ET(A) receptor antagonist BQ-123 for 50 min was performed in freely moving, intact and renal denervated rats. As a result of BQ- 123 infusion, urine flow rate diminished (P < 0.02) and Uosm increased (P < 0.05) in the intact rats, but not in the renal denervated rats. Bilateral renal denervation itself as well as ET(A) receptor inhibition in both intact and renal denervated rats did not change the mean arterial pressure, heart rate, or the excretion of sodium, potassium and chloride. The data obtained suggest an interrelationship between renal nerves and endothelin-A receptors in the regulation of renal water excretion.

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.

Improvement of renal dysfunction in rats with chronic heart failure after myocardial infarction by treatment with the endothelin A receptor antagonist, LU 135252

OBJECTIVE

To investigate the role of an activated endothelin system in the renal dysfunction observed in chronic heart failure after myocardial infarction.

METHODS

In rats with heart failure after myocardial infarction and in sham-operated animals (Sham), we investigated the effect on renal function of long-term oral treatment with the selective endothelin A (ETA) receptor antagonist, LU 135252 (30 mg/kg per day; groups MI/LU and Sham/LU) or placebo (groups MI/P, Sham/P). Only animals with extensive myocardial infarction (at least 46% of the left ventricle) were included in the study. Infarct size was matched between groups MI/P and MI/LU. Endogenous creatinine clearance, fractional sodium excretion, and plasma and urinary concentrations of endothelin were determined 12 weeks after myocardial infarction.

RESULTS

Endogenous creatinine clearance was significantly lower in group MI/P than in group Sham/P (MI/P: 0.64 +/- 0.05, Sham/P: 0.81 +/- 0.04 ml/min per 100 g body weight; P= 0.01 (means +/- SEM)). Treatment with LU 135252 completely prevented the decline in creatinine clearance in rats with chronic myocardial infarction (MI/LU: 0.98 +/- 0.21; Sham/LU: 0.83 +/- 0.10). Fractional sodium and protein excretion did not differ among the four groups. Group MI/P had a marked increase in plasma endothelin concentrations, which was not affected by treatment with LU 135252. Urinary endothelin excretion was significantly lower in group MI/P than in group Sham/P. In the treatment groups, no difference could be observed between animals that had suffered myocardial infarction and the sham-operated group, although LU 135252 markedly increased the urinary excretion of endothelin.

CONCLUSION

Our data demonstrate a restoration of impaired renal function in chronic ischaemic heart failure by treatment with the selective ETA receptor antagonist, LU 135252. These results offer a promising therapeutic option for the treatment of renal insufficiency in patients with chronic heart failure.

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.

Protective effect of SM-19712, a novel and potent endothelin converting enzyme inhibitor, on ischemic acute renal failure in rats

Effects of SM-19712 (4-chloro-N-[[(4-cyano-3-methyl- 1-1-phenyl- 1H-pyrazol-5-yl)amino]carbonyl] benzenesulfonamide, monosodium salt), a novel endothelin converting enzyme (ECE) inhibitor, on ischemic acute renal failure (ARF) in rats were examined in comparison with those of phosphoramidon, a conventional ECE inhibitor. ARF was induced by occlusion of the left renal artery and vein for 45 min followed by reperfusion, 2 weeks after contralateral nephrectomy. Renal function in ARF rats markedly decreased at 24 h after reperfusion. Intravenous bolus injection of SM-19712 (3, 10, 30 mg/kg) prior to the occlusion attenuated dose-dependently the ischemia/reperfusion-induced renal dysfunction. Histopathological examination of the kidney of ARF rats revealed severe renal damages such as tubular necrosis, proteinaceous casts in tubuli and medullary congestion, all of which were dose-dependently attenuated by SM-19712. Protective effects of phosphoramidon (10 mg/kg) on ARF-induced functional and tissue damages were less potent than that of the same dose of SM-19712. Endothelin-1 (ET-1) content in the kidney after the ischemia/reperfusion was significantly increased, being the maximum level at 6 h after reperfusion, and this elevation was completely suppressed by the higher dose of SM-19712. Our findings support the view that renal ET-1 plays an important role in the development of ischemia/reperfusion-induced renal injury. SM-19712 may be useful in the treatment of ischemic ARF.

Selective ET(A) receptor antagonism with ABT-627 attenuates all renal effects of endothelin in humans

++Endothelin (ET-1) acts as a potent vasoconstrictor in the human kidney, and this vasoconstriction could contribute to the ischemia seen in acute renal failure. In animal studies, the vasoactive properties of ET-1 are known to be ET(A) receptor-and/or ET(B) receptor-mediated; however, the receptor subtype involved in the human kidney remains to be defined. In a phase I, single-center, double-blind, randomized, three-period, crossover design, the effects of orally administered ABT-627, a selective ET(A) receptor antagonist, on renal hemodynamics during ET-1 infusion were evaluated. Two doses of ABT-627 (5 and 20 mg) were compared with placebo and nifedipine. For each dose level of ABT-627, a cohort of nine subjects was studied. A para-aminohippuric acid/inulin clearance test was performed once at the end of each 7-d treatment period. Infusion of ET-1 significantly decreased effective renal plasma flow, GFR, sodium excretion, and urine flow. Pretreatment with 20 mg of ABT-627 significantly decreased mean arterial pressure. In contrast, 7 d of treatment with both doses of ABT-627 did not affect baseline renal parameters. However, because mean arterial pressure decreased, a tendency toward a reduction of renal vascular resistance could indeed be demonstrated. Compared with placebo, both doses of ABT-627 were equally effective in blocking all renal effects caused by ET-1 infusion. In the model of exogenous ET-1 infusion, ABT-627 had a tendency to prevent ET-1-induced renal changes more effectively compared with nifedipine. The contribution of endogenous ET-1 and the ET(A) receptor in maintaining basal renal vascular tone in the human kidney is small. In addition, compared with placebo, selective ET(A) receptor antagonism with both doses of ABT-627 completely prevented all renal changes caused by ET-1 infusion.

Osteopontin mediates hypoxia-induced proliferation of cultured mesangial cells: role of PKC and p38 MAPK

BACKGROUND

We previously reported that hypoxia induces the proliferation of cultured mesangial cells mediated by the stimulation of intracellular calcium and the activation of protein kinase C (PKC). In the present study, we examined the roles of mesangial cell specific growth factors (platelet-derived growth factor and endothelin-1) and osteopontin (OPN) in hypoxia-induced proliferation of mesangial cells. In addition, we determined the effect of hypoxia on p38 mitogen-activated protein (MAP) kinase activity and the roles of both PKC and p38 MAP kinase in hypoxia-induced alterations in OPN and mesangial cell growth.

METHODS

Quiescent cultures of mesangial cells were exposed to hypoxia (3% O2) or normoxia (18% O2) in a serum-free medium, and [3H]-thymidine incorporation, OPN protein and mRNA expression, and p38 MAP kinase activity were assessed.

RESULTS

Hypoxic-conditioned medium mimicked the effect of hypoxia on thymidine incorporation, suggesting the release of diffusable growth promoting factor(s) by hypoxia. Neither anti-endothelin-1 nor anti-platelet-derived growth factor-neutralizing antibodies had an effect on increased thymidine incorporation induced by hypoxia. However, blocking the effects of OPN either with anti-OPN antibody or its beta3 integrin receptor antibody completely prevented the hypoxia-induced increase in thymidine incorporation. Hypoxia also stimulated OPN protein and mRNA levels. Hypoxia caused an acute activation of p38 MAP kinase, which was inhibited by both verapamil and an inhibitor of PKC (calph C). PKC inhibitor and an inhibitor of p38 MAP kinase (SB203580) reduced the hypoxia-induced stimulation of both OPN and cell growth.

CONCLUSIONS

These studies provide, to our knowledge, the first evidence demonstrating the role of OPN in hypoxia-induced proliferation of mesangial cells. In addition, hypoxia causes an activation of p38 MAP kinase in a calcium- and PKC-dependent manner, and the activation of PKC and p38 MAP kinase appears to be involved in the stimulation of both OPN and mesangial cell proliferation induced by hypoxia.

Pharmacokinetics and pharmacodynamic effects of ABT-627, an oral ETA selective endothelin antagonist, in humans

AIMS

Endothelins (ETs) may play a role in the pathogenesis of a variety of cardiovascular diseases. The present study was designed to investigate the pharmacokinetic and pharmacodynamic effects of the orally active ETA selective receptor antagonist ABT-627 in healthy humans.

METHODS

Healthy volunteers were included in two studies with cross-over design. Subjects received single or multiple dose (an 8 day period) administration of oralABT-627 or matched placebo, in a dose range of 0.2-40 mg. The pharmacokinetics of ABT-627 were described and its effects on systemic haemodynamics under resting conditions and on forearm vasoconstriction in response to ET-1 were assessed.

RESULTS

ABT-627 was generally well tolerated in both studies, with transient headache being the most reported adverse event (in 62% vs 4% during placebo, P < 0.05, for Study 1 and in 42% vs 60%, P = 0.2, for Study 2). ABT-627 was rapidly absorbed, reaching maximum plasma levels at approximately 1 h post dose. Single dose ABT-627, at a dose of 20 and 40 mg, inhibited ET-1 induced forearm vasoconstriction at 8 h post dose. Eight days ABT-627 treatment, at a dose level of 5 mg and above, also effectively blocked forearm vasoconstriction to ET-1. ABT-627 caused a significant reduction in peripheral resistance as compared with placebo (16 +/- 1 vs 19 +/- 1, 18 +/- 2 vs 23 +/- 3, 15 +/- 1 vs 17 +/- 1 AU at 1, 5, 20 mg in Study 2) with only a mild decrease in blood pressure (79 +/- 2 vs 84 +/- 3, 80 +/- 4 vs 90 +/- 5, 75 +/- 3 vs 79 +/- 1 at 1, 5, 20 mg in Study 2). ABT-627 caused a moderate dose-dependent increase in circulating immunoreactive ET levels (a maximal increase of 50% over baseline at the 20 mg dose level).

CONCLUSIONS

The oral ETA receptor blocker ABT-627 is well tolerated, rapidly absorbed, effectively blocks ET-1 induced vasoconstriction and causes a decrease in total peripheral resistance and mean arterial pressure. Our data suggest that ABT-627 may be a valuable tool in treatment of cardiovascular disease.

Neural reorganisation in porcine vein grafts: a potential role for endothelin-1

Vein grafts are associated with adventitial remodelling which may influence innervation of the graft. Since there is also evidence that endothelin-1 (ET-1) plays a role in the adventitial remodelling process, we investigated neural distribution in porcine vein grafts 1 and 6 months after implantation, as well as the localisation of immunoreactive ET-1 and its receptors in the same tissues. Saphenous vein-carotid artery interposition grafting was performed in Landrace pigs. One and 6 months after surgery, vein grafts and ungrafted saphenous veins were excised; neural tissue and ET-1 were identified by immunocytochemistry and ET receptors were identified using in vitro autoradiography. In ungrafted saphenous veins, abundant perivascular nerves were located in the outer region of the media with only a few paravascular nerves in the adventitia. In vein grafts at 1 month after implantation, there was almost complete depletion of perivascular nerves in the media. In contrast, in the neoadventitia, there was an emphatic appearance of large paravascular nerve bundles and a marked increase in small paravascular nerves. These changes were more pronounced at 6 months after surgery, although the principal changes had occurred within 1 month. Immunoreactive ET-1 (index of ET-1 content) was associated with paravascular nerve bundles, appearing as a dark, dense ring at the perineurium. Furthermore, within the nerve bundles, positive ET-1 immunoreactivity was associated with positive alpha-actin staining, indicating that ET-1 is associated with (neural) microvessels. Also, dense 125I-labelled BQ3020 (ET(B)-selective) binding to nerve bundles was observed, indicating the presence of ET(B) receptor subtypes. ET(A) receptor subtypes were not detected in neural tissue. These data demonstrate neural reorganisation in vein grafts and indicate that ET-1 content and binding may play a role in this process. The functional consequences of these changes on neointima formation, a major cause of vein graft failure, remain to be determined.

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.