Alterations of intrarenal adrenomedullin and its receptor system in heart failure rats

Regulation of RAMP expression in diseases

Receptor-activity modifying proteins (RAMPs) belong to a single family of transmembrane proteins. RAMPs determine ligand specificity of G-protein coupled receptors; calcitonin receptor and the calcitonin-receptor like receptor (CLR). To date, three members of RAMP family (RAMP-1, -2, -3) have been identified. The co-expression of RAMP-1 with CLR constitutes the calcitonin gene related peptide receptor whereas the association of the RAMP-2 or RAMP-3 with CLR forms the adrenomedullin (AM) receptor. Alterations in signaling and subcellular distribution of G-protein coupled receptors can be responsible for the regulation of many disease conditions. These changes may be mediated by the different isoforms of RAMPs associated with such receptors. In this chapter, we describe the differential responses associated with upregulation of RAMPs in disease conditions. For instance, the upregulation of all three RAMP isoforms contributes to the cardioprotective effects of the CLR/RAMP ligands. On the other hand, strong evidence exists for the involvement of AM in various cancers and that its action is mediated by the upregulation of RAMP isoforms, RAMP-2 and -3. Though limited, a few studies have been reported on the differential response associated with the upregulation of RAMP in other disease conditions such as sepsis, liver cirrhosis, glomerulonephritis, Type 1 diabetes and Parkinson's disease. Thus, the regulation of RAMP expression is involved in the pathophysiology associated with various diseases.

Pathophysiologic and therapeutic implications of adrenomedullin in cardiovascular disorders

Adrenomedullin (AM) is a vasodilator peptide that originally isolated from pheochromocytoma tissue. However, the mRNA is expressed in the normal adrenal gland, heart, kidney and blood vessels. The human AM gene is located in the short arm of chromosome 11 and is composed of 4 exons. There are 2 single nucleotide polymorphisms in introns 1 and 3, and the 3'-end of the AM gene is flanked by a microsatellite marker of cytosine-adenine repeats that is associated with an increased risk of developing hypertension and diabetic nephropathy. AM gene expression is promoted by various stimuli, including inflammation, hypoxia, oxidative stress, mechanical stress and activation of the renin-angiotensin and sympathetic nervous systems. The AM gene promoter region possessed binding site for several transcription factors, including nuclear factor for interleukin-6 expression (NF-IL6) and activator protein 2 (AP-2). Further, plasma AM levels are increased in patients with various cardiovascular diseases, including hypertension, heart failure and renal failure. These findings suggest that AM plays a role in the development of or response to cardiovascular disease. Indeed, experimental and clinical studies have demonstrated that systemic infusion of AM may have a therapeutic effect on myocardial infarction, heart failure and renal failure. Further, vasopeptidase inhibitors which augment the bioactivity of endogenous AM may benefit patients with hypertension and arteriosclerosis. Finally, the angiogenic and cytoprotective properties of AM may have utility in revascularization and infarcted myocardium and ischemic limbs. Because of the potential clinical benefits of AM, indications for use and optimal dosing strategies should be established.

Upregulation of intracardiac adrenomedullin and its receptor system in rats with volume overload-induced cardiac hypertrophy

Specific adrenomedullin receptors have been identified as calcitonin receptor-like receptor (CRLR)/receptor activity-modifying proteins (RAMP2 and RAMP3) complexes. Although we have demonstrated that adrenomedullin is increased in volume overload-induced cardiac hypertrophy, it remains unknown whether the adrenomedullin receptor is altered or not. This study sought to investigate the significance of intracardiac adrenomedullin and its receptor system in volume overload-induced cardiac hypertrophy. Left ventricular adrenomedullin levels were higher in aortocaval shunt (ACS) rats than in controls (+58%). The left ventricular gene expressions of adrenomedullin, CRLR, RAMP2 and RAMP3 were increased (+27%, +76%, +108% and +131%, respectively) and the left ventricular collagen gene expressions were also increased (type I: +138%, type III: +87%). The left ventricular adrenomedullin level correlated with the gene expression of type III collagen (R=0.42). These results suggest that intracardiac adrenomedullin and its receptor system are upregulated and may participate in the regulation of cardiac remodeling in volume overload-induced cardiac hypertrophy.

The clinical relevance of adrenomedullin: a promising profile?

Adrenomedullin (AM) is a peptide that possesses potentially beneficial properties. Since the initial discovery of the peptide by Kitamura et al. in 1993, the literature has been awash with reports describing its novel mechanisms of action and huge potential as a therapeutic target. Strong evidence now exists that AM is able to act as an autocrine, paracrine, or endocrine mediator in a number of biologically significant functions, including the endothelial regulation of blood pressure, protection against organ damage in sepsis or hypoxia, and the control of blood volume through the regulation of thirst. Its early promise as a potential mediator/modulator of disease was not, however, entirely as a result of the discovery of physiological functions but due more to the observation of increasing levels measured in plasma in direct correlation with disease progression. In health, AM circulates at low picomolar concentrations in plasma in 2 forms, a mature 52-amino acid peptide and an immature 53-amino acid peptide. Plasma levels of AM have now been shown to be increased in a number of pathological states, including congestive heart failure, sepsis, essential hypertension, acute myocardial infarction, and renal impairment. These earliest associations have been further supplemented with evidence of a role for AM in other pathologies including, most intriguingly, cancer. In this review, we offer a timely review of our current knowledge on AM and give a detailed account of the putative role of AM in those clinical areas in which the best therapeutic opportunities might exist.

Production and autocrine/paracrine effects of endogenous insulin-like growth factor-1 in rat cardiac fibroblasts

Insulin-like growth factor (IGF)-1 appears to play an important role in cardiac hypertrophy or remodeling. However, the role of endogenous IGF-1 in the growth of cardiac myocytes and fibroblasts remains unclear. This study investigated the major site of the production of cardiac IGF-1 and the local effects of endogenous IGF-1 secreted from cardiac cells. A significant expression of IGF-1 mRNA was found in cultured neonatal and adult rat cardiac fibroblasts, but not in myocytes. In addition, an in vivo examination by in situ hybridization histochemical analyses demonstrated the IGF-1 transcripts in the interstitial fibrotic tissue of the ventricle. Time-dependent secretion of IGF-1 protein was also observed in cultured cardiac fibroblasts. An antibody against IGF-1 decreased collagen synthesis in cardiac fibroblasts under basal conditions. Fibroblast-conditioned medium, as well as exogenous IGF-1, increased protein synthesis in cardiac myocytes, and this increase was inhibited by antibodies against IGF-1 and IGF-1 receptor, IGF binding protein-3, and IGF-1 receptor antagonist. These observations suggest that IGF-1 is produced and released mainly from cardiac fibroblasts and that endogenous IGF-1 promotes collagen synthesis by cardiac fibroblasts and hypertrophy of myocytes as an autocrine and a paracrine factor. Cardiac IGF-1 may function as an endogenous modulator of cardiac hypertrophy or remodeling.

Alteration of renal adrenomedullin and its receptor system in the severely hypertensive rat: effect of diuretic

OBJECTIVE

We investigated the pathophysiological role of the renal adrenomedullin (AM) system, including the ligand, receptor, and amidating activity, in severe hypertensive rats.

METHOD

We studied three groups: control Wistar Kyoto rats (WKY), spontaneously hypertensive stroke-prone rats (SHR-SP), and diuretic-treated SHR-SP. We measured AM-mature, active form, and AM-total (active form+inactive form) in plasma and renal tissues, and mRNA levels of AM and AM receptor system components such as calcitonin receptor-like receptor (CRLR), receptor activity-modifying protein (RAMP) 2, and RAMP3 in renal tissues.

RESULTS

SHR-SP had higher blood pressure, plasma neurohumoral factors, and lower renal function than WKY. SHR-SP had higher AM-mature and AM-total levels in plasma and renal tissues than WKY. Although the plasma AM-mature/AM-total ratio was similar in the two groups, AM-mature/AM-total ratio in renal tissues was higher in SHR-SP than in WKY. In addition, mRNA levels of AM in the renal cortex and medulla and the mRNA levels of CRLR, RAMP2, and RAMP3 in the renal cortex were higher in SHR-SP than in WKY. Chronic diuretic treatment decreased blood pressure and improved kidney function and neurohumoral factors, with reductions in plasma and renal AM system.

CONCLUSION

Upregulation of circulating and renal AM system may modulate pathophysiology in SHR-SP.

Adrenomedullin receptors: pharmacological features and possible pathophysiological roles

Three receptor activity modifying proteins (RAMPs) chaperone calcitonin-like receptor (CLR) to the cell surface. RAMP2 enables CLR to form an adrenomedullin (AM)-specific receptor that is sensitive to AM-(22-52) (AM(1) receptor). RAMP3 enables CLR to form an AM receptor sensitive to both calcitonin gene-related peptide (CGRP)-(8-37) and AM-(22-52) (AM(2) receptor), though rat and mouse AM(2) receptors show a clear preference for CGRP alpha-(8-37) over AM-(22-52). RAMP1 enables CRL to form the CGRP-(8-37)-sensitive CGRP(1) receptor, which can also be activated by higher concentrations of AM. Here we review the available information on the pharmacological features and possible pathophysiological roles of the aforementioned AM receptors.

Fasudil, a Rho-kinase inhibitor, attenuates glomerulosclerosis in Dahl salt-sensitive rats

OBJECTIVE

The present study was designed to clarify whether the Rho-Rho-kinase pathway is involved in the process of hypertensive glomerulosclerosis and to assess the therapeutic effect of fasudil, a specific Rho-kinase inhibitor.

METHOD AND RESULTS

Dahl salt-sensitive rats (DS) and Dahl salt-resistant rats (DR) were fed a high-salt diet at 6 weeks of age. Fasudil (30 mg/kg per day) was administered for 7 weeks to DS starting at the age of 11 weeks. After 7 weeks, untreated DS were characterized by decreased kidney function, increased proteinuria, abnormal morphological findings, increased adrenomedullin and atrial natriuretic peptide (ANP) levels, and increased renal messenger RNA expression of RhoB, Rho-kinasealpha, Rho-kinasebeta, collagen I and collagen III, and transforming growth factor-beta (TGF-beta) in the renal cortex compared with DR. Chronic fasudil treatment significantly improved renal function (serum creatinine, -26%; blood urea nitrogen, -41%; creatinine clearance, +42%), proteinuria (-24%) and histological findings (glomerular injury score, -49%; afferent arteriolar injury score, -17%) without changing blood pressure compared with untreated DS. Interestingly, long-term fasudil treatment decreased the plasma adrenomedullin (-25%) and ANP (-49%), but did not change the plasma renin or aldosterone. Furthermore, fasudil significantly decreased the messenger RNA expression of TGF-beta (-20%), collagen I (-23%), and collagen III (-24%) in the renal cortex. However, there were still significant differences in the aforementioned parameters between DR and fasudil-treated DS.

CONCLUSION

These results suggest that the Rho-Rho-kinase pathway may be partly responsible for the pathogenesis of hypertensive glomerulosclerosis independently of blood pressure in DS, and that chronic inhibition of the Rho-Rho-kinase pathway may be a new strategy for treating hypertensive nephrosclerosis.

Mycophenolate mofetil prevents autoimmune glomerulonephritis and alterations of intrarenal adrenomedullin in rats

We studied the effects of mycophenolate mofetil, a specific inhibitor of inosine monophosphate dehydrogenase, on the mercuric chloride induced autoimmune glomerulonephritis in Brown Norway rats and also on the renal contents of adrenomedullin. In the rats with autoimmune glomerulonephritis, plasma and renal tissue adrenomedullin levels were increased significantly. Coadministration of mycophenolate mofetil resulted in prevention of autoimmune glomerulonephritis and also in maintaining of plasma and renal tissue adrenomedullin levels at control levels. Adrenomedullin mRNA expressions in the renal cortex were also higher in the rats with autoimmune glomerulonephritis. Significant positive correlations were found between renal cortical adrenomedullin levels and urinary Na+ and N-acetyl-beta-D-glucosaminidase excretion. A significant negative correlation between renal cortical adrenomedullin levels and creatinine clearance was also found. These results suggest that mycophenolate mofetil suppresses the renal damage in rats with autoimmune glomerulonephritis and renal adrenomedullin may participate in the pathophysiology of autoimmune glomerulonephritis.

Altered gene expression of adrenomedullin and its receptor system and molecular forms of tissue adrenomedullin in left ventricular hypertrophy induced by malignant hypertension

To investigate the pathophysiological role of adrenomedullin (AM) in left ventricular hypertrophy (LVH) in hypertension, we measured the plasma level, left ventricle (LV) tissue level, and mRNA abundance of AM and the mRNA abundance of the AM receptor system in the LV. We also analyzed the molecular forms of AM in the plasma and LV tissue and investigated the relationships between AM and the degree of LVH. We studied the following three groups: control Wistar Kyoto rats (WKY), control spontaneously hypertensive rats (SHR), and deoxycorticosterone acetate (DOCA)-salt SHR (D-SHR). We measured AM-mature, active form, and AM-total (active form+inactive form) in plasma and the LV by a newly developed immunoradiometric assay. Gene expression of AM was measured by Northern blot analysis and gene expression of AM receptor system components, such as calcitonin receptor-like receptor (CRLR), receptor activity modifying protein 2 (RAMP2), and RAMP3 was measured by the reverse transcription polymerase chain reaction method. After 3 weeks of DOCA treatment, D-SHR was characterized by higher blood pressure, LV weight, and plasma atrial natriuretic peptide levels compared with those in the other two groups. Plasma AM-mature and AM-total levels were significantly higher in D-SHR than in the other two groups, whereas there were no significant differences in the AM-mature/AM-total ratio among the three groups. On the other hand, LV tissue AM-mature and AM-total levels were also significantly higher in D-SHR than in the other two groups, and the AM-mature/AM-total ratio was significantly higher in LV tissues than in plasma. Furthermore, the LV tissue AM-mature/AM-total ratio was significantly higher in D-SHR compared with the other two groups. The LV tissue AM-mature/AM-total ratio was significantly correlated with LV weight/body weight (r=0.92, p<0.001). The gene expression levels of AM, CRLR, RAMP2, and RAMP3 in the LV were significantly higher in D-SHR than in the other two groups. These results suggest that the AM amidating enzyme activity, ligand, and receptor system are all upregulated in the LV hypertrophy in this malignant hypertensive rat model. Considering that AM serves as a local antihypertrophic autocrine and/or paracrine factor, the induction of AM system observed here may modulate the pathophysiology of LV hypertrophy in certain forms of malignant hypertension.

Cardioprotective effect of adrenomedullin in heart failure

Many neurohumoral factors participate in the pathophysiology of heart failure, and adrenomedullin (AM) may be involved in their derangement. This work reviews the accumulating evidence in support of a compensatory role of AM in heart failure, and describes the possible mechanisms of this role. It has been established that plasma AM levels are increased in patients with heart failure in proportion to the severity of the disease. Furthermore, recent studies suggest that plasma AM level is an independent prognostic indicator of heart failure. Thus, AM may be not only a biochemical marker for evaluating the severity of heart failure, but also a prognostic indicator of this syndrome. In patients with heart failure, AM production is increased not only in the plasma, but also in the heart. AM secretion from the failing human heart is also increased, but this increase is small and responds slowly to the stimulus. This phenomenon may be explained by the fact that AM is secreted via a constitutive pathway and that AM is an autocrine and/or a paracrine factor in the heart. An experiment using cultured myocytes suggested that cytokines and mechanical stress are important stimuli for AM production in the heart. Regarding the action of AM in the heart, recent studies have suggested that AM exerts an inotropic action both in vitro and in vivo. AM also attenuates cardiac hypertrophy in myocytes and inhibits proliferation and collagen production in cardiac fibroblasts. These results suggest that AM may be an antifibrotic, antihypertrophic, and positive inotropic factor in the failing and hypertrophied heart. Because AM has many cardiorenal actions, AM administration may be useful for the treatment of heart failure. Indeed, acute administration of AM has been shown to improve the hemodynamics, renal function, and hormonal parameters in patients with heart failure. Moreover, recent studies have shown that AM gene therapy or long-term AM infusion significantly improved cardiac hypertrophy and fibrosis, and prolonged the survival time in an animal model of hypertension and heart failure. In conclusion, these findings suggest that AM plays a compensatory role in the pathophysiology of heart failure and that administration of AM may be a new and promising approach for the treatment of patients with this syndrome.

Adrenomedullin in heart failure

Patients with heart failure have frequently been reported to show elevated levels of plasma adrenomedullin. These levels generally correlate with severity of hemodynamic dysfunction and also with neurohormonal indices which are activated according to the severity of heart failure. Furthermore, adrenomedullin gene expression in the heart and kidney is increased in experimental and clinical heart failure. A small number of studies have examined the responses to infusion of adrenomedullin in experimental and clinical heart failure. These studies have generally shown that infusion of adrenomedullin has beneficial hemodynamic effects and promotes maintenance or improvement in renal function, although most of these trials were of short duration. The available data suggest that adrenomedullin in the heart, kidney and plasma is increased in heart failure, possibly to counter the activation or actions of vasoconstricting and sodium-retaining hormone systems. An improved understanding of the role of adrenomedullin in heart failure might lead to the development of therapeutic agents acting through adrenomedullin receptors.

Novel regulation of adrenomedullin receptor by PDGF: role of receptor activity modifying protein-3

Receptor activity modifying protein-3 (RAMP-3) has been shown to complex with the calcitonin receptor-like receptor, establishing a functional receptor for adrenomedullin (AM). AM exhibits potent antiproliferative and antimigratory effects on rat mesangial cells (RMCs). In this study we investigated the effect of platelet-derived growth factor (PDGF) on RAMP-3 expression in RMCs. We show here that PDGF-BB stimulates RAMP-3 mRNA expression in a concentration-dependent manner. Pretreatment with actinomycin-D and alpha-amanitin demonstrates that this effect is independent of new RNA synthesis. Furthermore, PDGF increased the half-life of RAMP-3 mRNA from 66.5 to 331.6 min. Using selective inhibitors, our results also indicate that the increase in RAMP-3 mRNA is mitogen-activated protein kinase (MAPK) kinase (MEK)/MAPK and p38 MAPK dependent. PDGF also caused a corresponding elevation in membrane-associated RAMP-3 protein. Associated with this increase, PDGF pretreatment led to a significantly higher AM-mediated adenylate cyclase activity, suggesting a functional consequence for the PDGF-induced increase in RAMP-3 expression. Taken together, these data identify PDGF-dependent regulation of RAMP-3 expression as a possible mechanism for modulating the responsiveness of the mesangial cell to AM.

Role of adrenomedullin and its receptor system in renal pathophysiology

Adrenomedullin (AM), a potent vasorelaxing, natriuretic and cell growth-modulating peptide, is thought to act as an autocrine/paracrine regulator in renal glomeruli and tubules. AM receptors comprise the calcitonin receptor-like receptor (CRLR) and a family of receptor-activity-modifying proteins (RAMPs 1-3); however, the pathophysiological role of AM and its receptor system in the kidney remains to be clarified. We examined the regulation of their expression in a rat model of renal injury and found that RAMP1, RAMP2 and CRLR expressions were markedly upregulated upon induction of fibrosis during obstructive nephropathy. Since AM exerts potent antiproliferative effects in various cell types, upregulation of the AM receptor system may play important roles in modulating the progression of renal diseases.

Adrenomedullin and diabetes mellitus

Adrenomedullin (AM) is a novel 52 amino acid peptide hormone, originally isolated from human pheochromocytoma. AM acts as a local autocrine and/or paracrine vasoactive hormone and has vasodilator and blood pressure lowering properties. AM as a vasodilative molecule protects the vascular wall but its exact role is still uncertain. AM is considered to play an important endocrine role in various tissues in maintaining electrolyte and fluid homeostasis. Its plasma concentration in healthy conditions is low. In hypertension, chronic renal failure and congestive heart failure its plasma concentration increases in a parallel manner with the severity of the disease. It is assumed that this peptide plays an important role in physiological and pathological conditions compensating the effects of vasoconstrictive molecules. Investigations have proven that in diabetic angiopathies the levels and production of vasoconstrictive factors and AM are increased, while other relaxing substances such as nitric oxide (NO) are decreased. It is still uncertain whether the increased release of AM is a compensatory mechanism or a coincidental event. Although the precise role of AM in the pathogenesis of diabetic complications is still to be elucidated, the altered concentration of AM in diabetes could indicate a certain interaction between AM induction and vascular function. Hence, the induction of vascular AM can be a new target of therapeutic approach to diabetic complications.