Down-regulation of adenylate cyclase coupled to adrenomedullin receptor in vascular smooth muscle cells

Protection of angiotensin II-induced vascular hypertrophy in vascular smooth muscle-targeted receptor activity-modifying protein 2 transgenic mice

The vasodilator and vascular regulatory peptide adrenomedullin (AM), a member of the calcitonin gene-related peptide family of peptides, is predicted to play a pivotal protective role in cardiovascular dysfunction. The principle AM (AM1) receptor is composed of a G protein-linked calcitonin receptor-like receptor and a receptor activity-modifying protein (receptor activity-modifying protein 2). There is little knowledge of the receptors via which AM acts in diseases. Using smooth muscle-targeted receptor activity-modifying protein 2 transgenic mice with increased vascular density of functional AM1 receptors, we demonstrate that receptor activity-modifying protein 2 transgenic mice are not protected against angiotensin II-induced hypertension or cardiac hypertrophy. However, vascular hypertrophy, together with vascular cell adhesion molecule 1 and monocyte chemotactic protein 1 expression, is significantly reduced in the aortic walls of transgenic mice, as determined by histological techniques. This indicates that the AM1 vascular smooth muscle receptor can mediate local protection in vivo. This is supported by proliferation studies in cultured smooth muscle cells. By comparison, levels of hypotension and inflammation in a shock model were similar to those in wild-type mice. Thus, a role of the AM1 receptor in the vasoactive component could not be detected, and evidence is provided to show that the hypotensive response to AM is subject to desensitization in vivo. The finding that the vascular smooth muscle AM1 receptor acts at a local level to protect against hypertension-induced vascular hypertrophy and inflammation provides evidence that targeting this receptor may be a beneficial therapeutic approach.

Protein kinase C is a common component of CGRP receptor desensitization induced by distinct agonists

The calcitonin gene-related peptide (CGRP) is a neuropeptide involved in vasodilation and other physiological functions throughout the body. The receptor for CGRP has been cloned and well studied, but the mechanism of CGRP receptor desensitization has not been fully elucidated. In the present study, we evaluated the kinetics for agonist-mediated desensitization of the adenylate cyclase response in human neuroblastoma SK-N-MC cells. Distinct CGRP receptor agonists were used, including alpha and beta isoforms of CGRP, the linearized derivative cys(Et)2,7 alphaCGRP, adrenomedullin, and adrenomedullin 2. betaCGRP was 4-600 times more potent at desensitizing the cAMP production as compared to the other receptor-activating ligands, and all of the desensitization effects were blocked by a CGRP receptor antagonist. Although the different agonists vary in their ability to induce functional desensitization, a pretreatment/washout sequence with each peptide was able to reduce the activation potency of the other members of the calcitonin/CGRP peptide family. Next we tested whether the desensitizing effects of the distinct peptides involve protein kinase C (PKC) or protein kinase A (PKA). A PKC inhibitor, Ro 31-8220, concentration-dependently reduced the desensitization induced by the 5 CGRP receptor agonists, while having little effect on their desensitization potencies. PKA inhibitors KT-5720 and H-89, on the other hand, showed little effect on the induced level of desensitization. The findings indicate that functional desensitization is produced by distinct peptides acting through the active site of CGRP receptors, and involves the activation of PKC as a common component necessary to achieve maximal desensitization of receptor signaling.

Gs and Gi coupling of adrenomedullin in adult rat ventricular myocytes

Adrenomedullin (ADM) acts as an autocrine or a paracrine factor in the regulation of cardiac function. The intracellular mechanisms involved in the direct effect of ADM on adult rat ventricular myocytes (ARVMs) are still to be elucidated. In ARVMs from normal rats, ADM produced an initial (<30 min) increase in cell shortening and Ca2+ transients and a marked decrease in both on prolonged incubation (>1 h). Both effects were sensitive to ADM antagonist ADM-(22–52). Treatment with SQ-22536, an inhibitor of adenylate cyclase, blocked the positive inotropic effect of ADM and potentiated its negative inotropic effect. The negative inotropic effect was sensitive to inhibition by pertussis toxin (PTX), an inhibitor of Gi proteins and KT-5720, an inhibitor of PKA. The observations suggest a switch from Gs-coupled to PTX-sensitive, PKA-dependent Gi coupling by ADM in ARVMs. The ADM-mediated Gi-signaling system involves cAMP-dependent pathways because SQ-22536 further increased the negative inotropic actions of ADM. Also, because ADM is overproduced by ARVMs in our rat model of septic shock, ARVMs from LPS-treated rats were subjected to treatment with ADM-(22–52) and PTX. The decrease in cell shortening and Ca2+ transients in LPS-treated ARVMs could be reversed back with ADM-(22–52) and PTX. This indicates that ADM plays a role in mediating the negative inotropic effect in LPS-treated ARVM through the activation of Gi signaling. This study delineates the intracellular pathways involved in ADM-mediated direct inotropic effects on ARVMs and also suggests a role of ADM in sepsis.

Adrenomedullin is an autocrine/paracrine growth factor for rat vascular smooth muscle cells

Adrenomedullin is a potent vasodilator peptide secreted by vascular endothelial and smooth muscle cells. Adrenomedullin stimulates the proliferation of quiescent rat vascular smooth muscle cells (VSMCs) via p42/p44 ERK/MAP kinase activation. Recently, receptor-activity-modifying proteins (RAMPs) have been shown to transport calcitonin-receptor-like-receptor (CRLR) to the cell surface to present either as CGRP receptor or adrenomedullin receptor. We investigated whether adrenomedullin acts as an autocrine/paracrine growth factor for cultured rat VSMCs and whether coexpressions of RAMP isoform and CRLR may mediate p42/p44 ERK/MAP kinase activation by adrenomedullin. Adrenomedullin dose-dependently stimulated the proliferation of quiescent rat VSMCs, and this effect was inhibited by an adrenomedullin receptor antagonist, a MAP kinase kinase inhibitor and phosphatidylinositol 3-kinase inhibitors. Addition of either CGRP(8-37) or anti-adrenomedullin antibody to exponentially growing rat VSMCs inhibited the serum-induced cell proliferation, suggesting its role as an autocrine/paracrine growth factor. Cotransfection of RAMP2 or RAMP3 with CRLR into rat VSMCs potentiated activation of cAMP activity, but not of p42/p44 ERK/MAP kinase activity in response to adrenomedullin. Our results suggest that adrenomedullin is an autocrine/paracrine growth factor for rat VSMCs via p42/p44 ERK/MAP kinase and phosphatidylinositol 3-kinase pathways and that it is not mediated by human RAMP-CRLR receptors.

Desensitisation of adrenomedullin and CGRP receptors

Adrenomedullin (AM), a potent vasoactive peptide, is elevated in certain disease states such as sepsis. Its role as a physiologically relevant peptide has been confirmed with the advent of the homozygous lethal AM peptide knockout mouse. So far, there have been few and conflicting studies which examine the regulatory role of AM at the receptor level. In this article, we discuss the few studies that have been presented on the desensitisation of AM receptors and also present novel data on the desensitisation of endogenous AM receptors in Rat-2 fibroblasts.

Coexpression of calcitonin receptor-like receptor and receptor activity-modifying protein 2 or 3 mediates the antimigratory effect of adrenomedullin

Three isoforms of the receptor activity-modifying protein (RAMP) are thought to transport the calcitonin receptor-like receptor (CRLR) to the plasma membrane to function as calcitonin gene-related peptide or adrenomedullin receptors, but their role remains largely unknown. We investigated whether coexpression of RAMP and CRLR are involved in the regulation of cell migration using a monolayer-wounding protocol. Quantification of gene transcripts revealed expression of all RAMP isoforms and CRLR in cultured rat vascular smooth muscle cells (VSMCs), RAMP2 and RAMP3 in rat endothelial cells, and RAMP1 in rat fibroblasts. CRLR expression was minimal in endothelial cells and fibroblasts. Adrenomedullin potently suppressed the migration of VSMCs, whereas calcitonin gene-related peptide did not suppress migration in any cell type. The antimigratory effect of adrenomedullin on VSMCs was potentiated by transfecting CRLR cDNA. Cotransfection of RAMP2 or RAMP3 with CRLR into VSMCs resulted in a slower migratory rate, and this effect was enhanced by adrenomedullin. Migration of fibroblasts was also suppressed after cotransfection of RAMP2 or RAMP3 with CRLR. cAMP agonists had no effect on VSMC migration, and a cAMP antagonist failed to abrogate the antimigratory effect of adrenomedullin. Thus, coexpression of CRLR and RAMP2 or RAMP3 mediates the inhibitory effect of adrenomedullin on cell migration, independent of cAMP-dependent signaling pathways.

Adrenomedullin: a cardiac depressant factor in septic shock

Despite intensive research, septic shock is still the most common cause of death in surgical intensive care, and its incidence keeps increasing. No curative treatment is yet available. The critical aspect of septic shock is the refractory hypotension that develops during its late phase which leads to a progressive deterioration of cell and organ functions, and in most instances, death. During septic shock, following the overproduction of cytokines, many factors such as nitric oxide and adrenomedullin (ADM) are produced in abnormally large quantities, but our understanding of their contribution to the pathophysiology of sepsis is limited. Here we show that adrenomedullin (22-52), an adrenomedullin receptor antagonist, improves the contractility of myocytes isolated from lipopolysaccharide (LPS)-treated rats, whereas in normal myocytes, adrenomedullin, acting through an adrenomedullin (22-52) sensitive receptor, decreases their contractility. In addition, adrenomedullin antiserum and inducible nitric oxide (NO) synthase inhibitor improve the survival of LPS-treated rats. The data indicate that adrenomedullin is a cardiac depressant factor, which along with NO precipitates ventricular failure during septic shock.

Andrenomedullin and cardiovascular responses in sepsis

The typical cardiovascular response to polymicrobial sepsis is characterized by an early, hyperdynamic phase followed by a late, hypodynamic phase. Although the factors and/or mediators responsible for producing the transition from the hyperdynamic to the hypodynamic stage are not fully understood, recent studies have suggested that adrenomedullin (AM), a potent vasodilatory peptide, appears to play an important role in initiating the hyperdynamic response following the onset of sepsis. In addition, the reduced vascular responsiveness to AM may result in the transition from the early, hyperdynamic phase to the late, hypodynamic phase of sepsis. It is possible that changes in newly reported AM receptors calcitonin receptor-like receptor (CRLR) and receptor activity modifying protein-2 or -3 (RAMP2, RAMP3) as well as AM binding protein-1 (AMBP-1) may also play distinct roles in the biphasic cardiovascular response observed during sepsis. Although it remains unknown whether AM gene delivery or a chronic increase in vascular AM production in transgenic animals attenuates the development of hypodynamic sepsis and septic shock, it has been shown that modulation of AM vascular responsiveness with pharmacologic agents reduces sepsis-induced mortality. It has been recently demonstrated that AMBP-1 enhances AM's physiologic effects and plasma levels of AMBP-1 decrease following infections. We therefore propose that downregulation of AMBP-1 and the reduced AM receptor responsiveness are crucial factors responsible for the transition from the hyperdynamic phase to the hypodynamic phase of sepsis.

Cellular and molecular actions of adrenomedullin in glomerular mesangial cells

Adrenomedullin (AM), a potent vasodilatory and hypotensive peptide produces several biological outcomes in glomerular mesangial cells. Mesangial cells are important in the pathogenesis of glomerulonephritis, and therefore the actions of AM on mesangial cells have important clinical and therapeutic implications. This minireview describes the various actions of AM on mesangial cell function and the signal transduction mechanisms involved. As in other systems, most actions of AM can be explained by increase in cAMP levels in the cell, although a few exceptions remain. The fact that most data obtained to date has been in culture, the physiological significance of the actions of AM in mesangial cells is discussed.

Adrenomedullin receptors: molecular identity and function

Since its discovery in 1993 adrenomedullin (AM) has been the subject over 600 published articles. This multifunctional peptide has powerful vasodilator actions and recent evidence from AM gene-deleted mice suggest that AM plays an essential role in vascular development. However the lack of valid AM receptor clones and non-peptide receptor ligands has considerably slowed research progress on this important peptide. In this review we have focused on the proposition that the calcitonin receptor-like receptor (CRLR) is a receptor both for AM and the related vasoactive peptide calcitonin gene-related peptide (CGRP). The receptor activity modifying proteins (RAMPs) that are essential for defining CRLR pharmacology will also be discussed. We will describe how AM receptors have been reported to signal and be regulated and to consider whether further receptors for AM beyond CRLR/RAMP combinations might exist.

Association of plasma adrenomedullin with carotid atherosclerosis in chronic ischemic stroke

Adrenomedullin is a potent vasodilator peptide exerting anti-atherosclerotic actions in vitro. We investigated the impact of the severity of atherosclerosis on plasma mature-adrenomedullin (m-AM) levels in 38 patients with chronic ischemic stroke. The variables of carotid artery atherosclerosis assessed using ultrasound measurement, blood pressure, and risk factors were related to m-AM levels. Severe atherosclerosis was associated with a further elevation of the increased m-AM level in patients with high systolic blood pressure. Even in patients with fewer risk factors, the presence of severe atherosclerosis was associated with an increased m-AM level. Thus, atherosclerosis elevates m-AM independent of the blood pressure level or presence of risk factors.

The role of adrenomedullin in producing differential hemodynamic responses during sepsis

Although the hemodynamic response to polymicrobial sepsis is characterized by an early, hyperdynamic phase followed by a late, hypodynamic phase, the factors responsible for producing the transition from the hyperdynamic to the hypodynamic stage are not fully understood. The failure to recognize or prevent this transition may lead to progressive deteriorations in cell and organ functions and ultimately result in multiple organ failure. Despite the fact that several vasoactive mediators (i.e., nitric oxide, prostacyclin, calcitonin gene-related peptide) have been implicated in producing cardiovascular alterations during sepsis, recent studies have indicated that adrenomedullin (AM), a novel vasodilatory peptide, plays an important role in initiating the hyperdynamic response during the early stage of polymicrobial sepsis. In addition, the reduced vascular responsiveness appears to be responsible for producing the transition from the early, hyperdynamic phase to the late, hypodynamic phase of sepsis. Moreover, modulation of AM vascular responsiveness reduces sepsis-induced mortality. In this review the physiological effects of AM, mechanisms of its action, and regulation of its production under various pathophysiological conditions will be discussed. Furthermore, the role of AM in producing the biphasic hemodynamic responses observed during polymicrobial sepsis and approaches for pharmacologically modulating vascular responsiveness and hemodynamic stability under such conditions will be described.

Desensitization and resensitization of adrenomedullin-sensitive receptor in rat mesangial cells

Adrenomedullin is a potent adenylate cyclase activator and a vasodilatory peptide, that has anti-proliferative and apoptotic effects in rat mesangial cells. The present study was designed to determine the mechanisms of desensitization and resensitization of adrenomedullin-sensitive receptor in mesangial cells. Adrenomedullin caused a rapid desensitization of cAMP response evident within 5 min that was almost complete by 1 h of treatment. Pretreatment of cells with forskolin, that activates protein kinase-A by direct activation of adenylate cyclase, also caused adrenomedullin receptor desensitization. In addition, H89 [¿N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide, hydrochloride¿], a potent protein kinase-A inhibitor inhibited adrenomedullin-induced desensitization of cAMP response. Adrenomedullin also caused desensitization of isoproterenol- and epinephrine-mediated cAMP accumulation. Furthermore, adrenomedullin induced cross-desensitization of endothelin-stimulated inositol phosphate accumulation. The attenuated cAMP response of adrenomedullin was restored to original levels within 2 h of agonist removal. This resensitization response was blocked by treatment with okadaic acid, a protein phosphatase (protein phosphatase-1/protein phosphatase-2A) inhibitor, during the 2 h resensitization period, indicating that protein phosphatase-1/protein phosphatase-2A may be involved in the resensitization of the adrenomedullin-sensitive receptor. We demonstrate for the first time in rat mesangial cells that the adrenomedullin-sensitive receptor undergoes heterologous desensitization and resensitization, and that it likely involves protein kinase-A and protein phosphatase-1/protein phosphatase-2A, respectively.

Adrenomedullin, a multifunctional regulatory peptide

Since the discovery of adrenomedullin in 1993 several hundred papers have been published regarding the regulation of its secretion and the multiplicity of its actions. It has been shown to be an almost ubiquitous peptide, with the number of tissues and cell types synthesizing adrenomedullin far exceeding those that do not. In Section II of this paper we give a comprehensive review both of tissues and cell lines secreting adrenomedullin and of the mechanisms regulating gene expression. The data on circulating adrenomedullin, obtained with the various assays available, are also reviewed, and the disease states in which plasma adrenomedullin is elevated are listed. In Section III the pharmacology and biochemistry of adrenomedullin binding sites, both specific sites and calcitonin gene-related peptide (CGRP) receptors, are discussed. In particular, the putative adrenomedullin receptor clones and signal transduction pathways are described. In Section IV the various actions of adrenomedullin are discussed: its actions on cellular growth, the cardiovascular system, the central nervous system, and the endocrine system are all considered. Finally, in Section V, we consider some unresolved issues and propose future areas for research.

Adrenomedullin receptor antagonism by calcitonin gene-related peptide(8-37) inhibits carotid artery neointimal hyperplasia after balloon injury

Intimal injury by angioplasty results in a series of changes, including smooth muscle cell hyperplasia, that lead to vascular restenosis. Adrenomedullin, a potent vasodilator peptide, has natriuretic effects, and its plasma concentration is elevated in cardiovascular diseases. Adrenomedullin is secreted by endothelial and vascular smooth muscle cells, but its role in neointimal hyperplasia after balloon injury has not been previously described. We investigated the role of endogenous adrenomedullin in neointimal hyperplasia using an in vivo rat model of postinjury vascular restenosis. In the injured rats, bromodeoxyuridine-labeled nuclei in the media of untreated common carotid arteries were increased 2 days after injury, which were suppressed by in vivo treatment with the adrenomedullin receptor antagonist calcitonin gene-related peptide (CGRP)(8-37). Inhibition of neointimal hyperplasia by CGRP(8-37) was distinct at 7 and 14 days, whereas CGRP(1-37) had no effect. The expression of adrenomedullin in the media of both untreated and treated common carotid arteries was elevated at 2 days and further enhanced in hyperplastic intima of untreated common carotid arteries at 7 days. Our findings suggest a novel role for endogenous adrenomedullin in balloon injury-induced restenosis and indicate that CGRP(8-37) may be useful for the prevention of vascular restenosis.

cAMP-induced cytoskeleton rearrangement increases calcium transients through the enhancement of capacitative calcium entry

In this report we investigated the correlation between cell morphology and regulation of cytosolic calcium homeostasis. Type I astrocytes were differentiated to stellate process-bearing cells by a 100-min exposure to cAMP. Differentiation of cortical astrocytes increased the magnitude and duration of calcium transients elicited by phospholipase C-activating agents as measured by single cell Fura-2-based imaging. Calcium imaging showed differences in the spatial pattern of the response. In both differentiated and the control cells, the response originated in the periphery and gradually extended into the center of the cell. However, the elevation of cytosolic calcium concentration ([Ca(2+)](i)) was particularly evident within the processes and adjacent to the inner cell membrane of the differentiated astrocytes. In addition, differentiation significantly prolonged the duration of the [Ca(2+)](i) elevation. Potentiation of the calcium transients was mimicked by forskolin-induced differentiation and abolished by a specific protein kinase-A blocker. Conversely, the enhancement of the calcium transients was not mimicked by brief exposure to cAMP not causing morphological differentiation, and in PC12 cells that did not undergo morphological changes after 100 min of cAMP treatment. Impairing cAMP-induced cytoskeleton re-organization, by means of cytochalasin D and nocodazole, prevented the potentiation of the calcium transients in cAMP-treated astrocytes. Phospholipase C activity and sensitivity to inositol (1,4,5)-trisphosphate were not involved in the enhancement of the calcium responses. Also, potentiation of the calcium transients was dependent on extracellular calcium. Calcium storage and thapsigargin-depletable intracellular calcium reservoirs were analogously not increased in differentiated astrocytes. Rearrangement of the cell shape also caused a condensation of the endoplasmic reticulum and altered the spatial relationship between the endoplasmic reticulum and the cell membrane. In conclusion, morphological rearrangements of type I astrocytes increase the magnitude and the duration of agonist-induced calcium transients via enhancement of capacitative calcium entry and is associated with a spatial reorganization of the relationship between cell membrane and the endoplasmic reticulum structures.

Reduction in vascular responsiveness to adrenomedullin during sepsis

BACKGROUND

Although sepsis is characterized by an early, hyperdynamic phase followed by a late, hypodynamic phase, the mechanism responsible for the transition from the hyperdynamic to the hypodynamic state remains unknown. Since recent studies have shown that adrenomedullin (ADM), a novel potent vasodilatory peptide, is upregulated during sepsis, the aim of this study was to determine whether the reduced vascular responsiveness to ADM is associated with the transition from the hyperdynamic phase to the hypodynamic phase of sepsis.

MATERIALS AND METHODS

Adult male Sprague-Dawley rats were subjected to sepsis by cecal ligation and puncture (CLP). At 5 and 10 h (i.e., the hyperdynamic phase of sepsis) or 20 h (the hypodynamic phase) after CLP, the thoracic aorta or small intestine was harvested and preconstricted with norepinephrine. Adrenomedullin (10(-7) M) was applied and the percentage of ADM-induced vascular relaxation in the aortic ring and isolated small intestine was determined.

RESULTS

The responsiveness to ADM in the thoracic aorta was not altered at 5-10 h, but decreased significantly at 20 h after CLP. Although ADM-induced relaxation in resistance blood vessels of the small intestine did not change at 5 h, it decreased markedly at 10 and 20 h after the onset of sepsis.

CONCLUSIONS

Since the transition from hyperdynamic to hypodynamic sepsis takes place between 10 and 20 h after CLP, it is likely that reduced vascular responsiveness to ADM may be responsible for such an event during the course of polymicrobial sepsis. In view of this, maintenance of vascular ADM responsiveness by pharmacologic agents appears to be a novel approach for preventing or delaying the occurrence of hypodynamic sepsis and septic shock.