Endothelin B receptor-like immunoreactivity in podocytes of the rat kidney

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.

Endothelin-a receptor antagonism after renal angioplasty enhances renal recovery in renovascular disease

Percutaneous transluminal renal angioplasty/stenting (PTRAS) is frequently used to treat renal artery stenosis and renovascular disease (RVD); however, renal function is restored in less than one half of the cases. This study was designed to test a novel intervention that could refine PTRAS and enhance renal recovery in RVD. Renal function was quantified in pigs after 6 weeks of chronic RVD (induced by unilateral renal artery stenosis), established renal damage, and hypertension. Pigs with RVD then underwent PTRAS and were randomized into three groups: placebo (RVD+PTRAS), chronic endothelin-A receptor (ET-A) blockade (RVD+PTRAS+ET-A), and chronic dual ET-A/B blockade (RVD+PTRAS+ET-A/B) for 4 weeks. Renal function was again evaluated after treatments, and then, ex vivo studies were performed on the stented kidney. PTRAS resolved renal stenosis, attenuated hypertension, and improved renal function but did not resolve renal microvascular rarefaction, remodeling, or renal fibrosis. ET-A blocker therapy after PTRAS significantly improved hypertension, microvascular rarefaction, and renal injury and led to greater recovery of renal function. Conversely, combined ET-A/B blockade therapy blunted the therapeutic effects of PTRAS alone or PTRAS followed by ET-A blockade. These data suggest that ET-A receptor blockade therapy could serve as a coadjuvant intervention to enhance the outcomes of PTRAS in RVD. These results also suggest that ET-B receptors are important for renal function in RVD and may contribute to recovery after PTRAS. Using clinically available compounds and techniques, our results could contribute to both refinement and design of new therapeutic strategies in chronic RVD.

Direct action of endothelin-1 on podocytes promotes diabetic glomerulosclerosis

The endothelin system has emerged as a novel target for the treatment of diabetic nephropathy. Endothelin-1 promotes mesangial cell proliferation and sclerosis. However, no direct pathogenic effect of endothelin-1 on podocytes has been shown in vivo and endothelin-1 signaling in podocytes has not been investigated. This study investigated endothelin effects in podocytes during experimental diabetic nephropathy. Stimulation of primary mouse podocytes with endothelin-1 elicited rapid calcium transients mediated by endothelin type A receptors (ETARs) and endothelin type B receptors (ETBRs). We then generated mice with a podocyte-specific double deletion of ETAR and ETBR (NPHS2-Cre×Ednra(lox/lox)×Ednrb(lox/lox) [Pod-ETRKO]). In vitro, treatment with endothelin-1 increased total β-catenin and phospho-NF-κB expression in wild-type glomeruli, but this effect was attenuated in Pod-ETRKO glomeruli. After streptozotocin injection to induce diabetes, wild-type mice developed mild diabetic nephropathy with microalbuminuria, mesangial matrix expansion, glomerular basement membrane thickening, and podocyte loss, whereas Pod-ETRKO mice presented less albuminuria and were completely protected from glomerulosclerosis and podocyte loss, even when uninephrectomized. Moreover, glomeruli from normal and diabetic Pod-ETRKO mice expressed substantially less total β-catenin and phospho-NF-κB compared with glomeruli from counterpart wild-type mice. This evidence suggests that endothelin-1 drives development of glomerulosclerosis and podocyte loss through direct activation of endothelin receptors and NF-κB and β-catenin pathways in podocytes. Notably, both the expression and function of the ETBR subtype were found to be important. Furthermore, these results indicate that activation of the endothelin-1 pathways selectively in podocytes mediates pathophysiologic crosstalk that influences mesangial architecture and sclerosis.

Endothelin-1 and the kidney--beyond BP

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

Endothelin and endothelin receptors in the renal and cardiovascular systems

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

Endothelin 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.

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.

Cerebral reactive oxygen species assessed by electron spin resonance spectroscopy in the initial stage of ischemia-reperfusion are not associated with hypothermic neuroprotection

Using an in vivo L-band electron spin resonance (ESR) system, we determined changes in reactive oxygen species (ROS) levels during the early stage (within 60 minutes) of global cerebral ischemia-reperfusion (IR) under normothermic and hypothermic conditions in rats. To confirm the neuroprotective role of hypothermia in this IR model, we immunohistochemically evaluated the levels of active caspase-3 in the hippocampal CA1 sector. ROS levels increased within the first 15 minutes following IR under both normothermic and hypothermic conditions; however, the ROS levels did not differ significantly between normothermic and hypothermic conditions. In the later periods of IR, there were no significant changes in ROS levels for either normothermic or hypothermic conditions relative to the control. As expected, normothermia increased the number of active caspase-3 immunoreactive nuclei in the IR model. However, this induction was prevented by hypothermia. These results suggest that the neuroprotective role of hypothermia does not correlate with the early ROS-induced oxidative stress following IR as measured by ESR.

Endothelin type A and B receptors in the control of afferent and efferent arterioles in mice

BACKGROUND

Endothelin 1 contributes to renal blood flow control and pathogenesis of kidney diseases. The differential effects, however, of endothelin 1 (ET-1) on afferent (AA) and efferent arterioles (EA) remain to be established.

METHODS

We investigated endothelin type A and B receptor (ETA-R, ETB-R) functions in the control of AA and EA. Arterioles of ETB-R deficient, rescued mice [ETB(-/-)] and wild types [ETB(+/+)] were microperfused.

RESULTS

ET-1 constricted AA stronger than EA in ETB(-/-) and ETB(+/+) mice. Results in AA: ET-1 induced similar constrictions in ETB(-/-) and ETB(+/+) mice. BQ-123 (ETA-R antagonist) inhibited this response in both groups. ALA-ET-1 and IRL1620 (ETB-R agonists) had no effect on arteriolar diameter. L-NAME did neither affect basal diameters nor ET-1 responses. Results in EA: ET-1 constricted EA stronger in ETB(+/+) compared to ETB(-/-). BQ-123 inhibited the constriction completely only in ETB(-/-). ALA-ET-1 and IRL1620 constricted only arterioles of ETB(+/+) mice. L-NAME decreased basal diameter in ETB(+/+), but not in ETB(-/-) mice and increased the ET-1 response similarly in both groups. The L-NAME actions indicate a contribution of ETB-R in basal nitric oxide (NO) release in EA and suggest dilatory action of ETA-R in EA.

CONCLUSIONS

ETA-R mediates vasoconstriction in AA and contributes to vasoconstriction in EA in this mouse model. ETB-R has no effect in AA but mediates basal NO release and constriction in EA. The stronger effect of ET-1 on AA supports observations of decreased glomerular filtration rate to ET-1 and indicates a potential contribution of ET-1 to the pathogenesis of kidney diseases.

Endothelin A receptor-like immunoreactivity on the basal infoldings of rat renal tubules and collecting ducts

We investigated the distribution of endothelin A (ET(A)) receptor-like immunoreactivity in the rat kidney using affinity-purified antibodies against amino acid residues 403-417 of the rat ET(A) receptor modified by the multiple antigen peptide complex system. Western blot analysis using the affinity-purified anti-ET(A) antibody detected bands of approximately 47.3 and 64.5 kDa in the rat kidney. By light microscopy, ET(A) receptor-like immunoreactivity was seen in the basal side of the renal tubules and collecting ducts. The most intense immunoreactivity was present in the distal renal tubules and inner medullary collecting ducts. In addition to the basal infoldings, immunoreactive puncta were scattered in the epithelial cells of the renal tubules and collecting ducts. Specimens prepared using the pre-embedding method were examined by electron microscopy, and some immunopositive signals were seen on the basal infodings of the renal tubules and collecting ducts. The lengths of immunopositive cytoplasmic membrane were far longer in the distal tubules and inner medullary collecting ducts than in the proximal tubules and outer medullary collecting ducts. Immunopositive signals were also sometimes observed in the thick portion of Henle's loop, but never in the thin portion. We have not previously detected immunopositive signals on the renal vascular systems with the antibody used here. These results suggest that endothelin acts on the basal infoldings through the ET(A) receptor, particularly in the distal tubules and inner medullary collecting ducts, although involvement of the ET(B) receptor cannot be excluded.

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.

Shigatoxin-induced endothelin-1 expression in cultured podocytes autocrinally mediates actin remodeling

Shigatoxin (Stx) is the offending agent of post-diarrheal hemolytic uremic syndrome, characterized by glomerular ischemic changes preceding microvascular thrombosis. Because podocytes are highly sensitive to Stx cytotoxicity and represent a source of vasoactive molecules, we studied whether Stx-2 modulated the production of endothelin-1 (ET-1), taken as candidate mediator of podocyte dysfunction. Stx-2 enhanced ET-1 mRNA and protein expression via activation of nuclear factor kappaB (NF-kappaB) and activator protein-1 (Ap-1) to the extent that transfection with the dominant-negative mutant of IkappaB-kinase 2 or with Ap-1 decoy oligodeoxynucleotides reduced ET-1 mRNA levels. We propose a role for p38 and p42/44 mitogen-activated protein kinases (MAPKs) in mediating NF-kappaB-dependent gene transcription induced by Stx-2, based on data that Stx-2 phosphorylated p38 and p42/44 MAPKs and that MAPK inhibitors reduced transcription of NF-kappaB promoter/luciferase reporter gene construct induced by Stx-2. Stx-2 caused F-actin redistribution and intercellular gaps via production of ET-1 acting on ETA receptor, because cytoskeleton changes were prevented by ETA receptor blockade. Exogenous ET-1 induced cytoskeleton rearrangement and intercellular gaps via phosphatidylinositol-3 kinase and Rho-kinase pathway and increased protein permeability across the podocyte monolayer. These data suggest that the podocyte is a target of Stx, a novel stimulus for the synthesis of ET-1, which may control cytoskeleton remodeling and glomerular permeability in an autocrine fashion.

In response to protein load podocytes reorganize cytoskeleton and modulate endothelin-1 gene: implication for permselective dysfunction of chronic nephropathies

Effacement of podocyte foot processes occurs in many proteinuric nephropathies and is accompanied by rearrangement of the actin cytoskeleton. Here, we studied whether protein overload affects intracellular pathways, leading to cytoskeletal architecture changes and ultimately to podocyte dysfunction. Mouse podocytes bound and endocytosed both albumin and IgG via receptor-specific mechanisms. Protein overload caused redistribution of F-actin fibers instrumental to up-regulation of the prepro-endothelin (ET)-1 gene and production of the corresponding peptide. Increased DNA-binding activity for nuclear factor (NF)-kappaB and Ap-1 nuclear proteins was measured in nuclear extracts of podocytes exposed to excess proteins. Both Y27632, which inhibits Rho kinase-dependent stress fiber formation, and jasplakinolide, an F-actin stabilizer, decreased NF-kappaB and Ap-1 activity and reduced ET-1 expression. This suggested a role for the cytoskeleton, through activated Rho, in the regulation of the ET-1 peptide. Focal adhesion kinase (FAK), an integrin-associated nonreceptor tyrosine kinase, was phosphorylated by albumin treatment via Rho kinase-triggered actin reorganization. FAK activation led to NF-kappaB- and Ap-1-dependent ET-1 expression. These data suggest that reorganization of the actin cytoskeletal network in response to protein load is implicated in modulation of the ET-1 gene via Rho kinase-dependent FAK activation of NF-kappaB and Ap-1 in differentiated podocytes. Increased ET-1 generation might alter glomerular permselectivity and amplify the noxious effect of protein overload on dysfunctional podocytes.