Potentiated response to adrenomedullin in myocardia and aortas in spontaneously hypertensive rat

Loss of receptor activity-modifying protein 3 exacerbates cardiac hypertrophy and transition to heart failure in a sex-dependent manner

Sex differences exist in the hypertrophic response, cardiac remodeling, and transition to heart failure of hypertensive patients, and while some of these differences are likely influenced by estrogen, the genetic pathways downstream of estrogen that impact on cardioprotection have yet to be fully elucidated. We have previously shown that the cardioprotective effects of adrenomedullin (AM), an emerging clinical biomarker for cardiovascular disease severity, vary with sex in mouse models. AM signaling during cardiovascular stress is strongly modulated by receptor activity-modifying protein 3 (RAMP3) via its interaction with the G protein-coupled receptor calcitonin receptor-like receptor (CLR). Like AM, RAMP3 expression is potently regulated by estrogen, and so we sought to determine the consequences of genetic Ramp3 loss on cardiac adaptation to chronic hypertension, with a particular focus on characterizing potential sex differences. We generated and bred RAMP3(-/-) mice to RenTgMK mice that consistently display severe angiotensin II-mediated CV disease and compared CV disease progression in RenTgMK to that of RenTgMK:RAMP3(-/-) offspring. As expected, RAMP3 gene expression was higher in cardiovascular tissues of RenTgMK mice and more strongly up-regulated in female RenTgMK mice relative to wildtype controls. RAMP3 loss did not affect the development of hypertension or the presence and severity of perivascular and interstitial fibrosis in the left ventricle (LV). However, echocardiography revealed that while RenTgMK mice developed concentric cardiac hypertrophy with sustained systolic function, male RenTgMK:RAMP3(-/-) mice showed evidence of LV chamber dilatation and depressed systolic function, suggestive of cardiac decompensation. Consistent with these measures of heart failure, male RenTgMK:RAMP3(-/-) mice had increased cardiac apoptosis and elevated activation of Akt. These phenotypes were not present in female RenTgMK:RAMP3(-/-) mice. Collectively, these data demonstrate a sex-dependant, cardioprotective role of RAMP3 in the setting of chronic hypertension.

Shared and separate functions of the RAMP-based adrenomedullin receptors

Adrenomedullin (AM) is a novel hypotensive peptide that exerts a variety of strongly protective effects against multiorgan damage. AM-specific receptors were first identified as heterodimers composed of calcitonin-receptor-like receptor (CLR), a G protein coupled receptor, and one of two receptor activity-modifying proteins (RAMP2 or RAMP3), which are accessory proteins containing a single transmembrane domain. RAMPs are required for the surface delivery of CLR and the determination of its phenotype. CLR/RAMP2 (AM₁ receptor) is more highly AM-specific than CLR/RAMP3 (AM₂ receptor). Although there have been no reports showing differences in intracellular signaling via the two AM receptors, in vitro studies have shed light on their distinct trafficking and functionality. In addition, the tissue distributions of RAMP2 and RAMP3 differ, and their gene expression is differentially altered under pathophysiological conditions, which is suggestive of the separate roles played by AM₁ and AM₂ receptors in vivo. Both AM and the AM₁ receptor, but not the AM₂ receptor, are crucial for the development of the fetal cardiovascular system and are able to effectively protect against various vascular diseases. However, AM₂ receptors reportedly play an important role in maintaining a normal body weight in old age and may be involved in immune function. In this review article, we focus on the shared and separate functions of the AM receptor subtypes and also discuss the potential for related drug discovery. In addition, we mention their possible function as receptors for AM2 (or intermedin), an AM-related peptide whose biological functions are similar to those of AM.

Adrenomedullin ameliorates the development of atherosclerosis in apoE-/- mice

Adrenomedullin (ADM) is a multifunctional peptide regulating cardiovascular homeostasis. We studied the role of ADM in the pathogenesis of atherosclerosis by investigating changes in ADM and its receptors - calcitonin receptor-like receptor (CRLR) and receptor activity-modifying proteins (RAMPs) - in aorta of apoE-/- mice and the effect of exogenous ADM administration. ApoE-/- mice were fed an atherogenic diet for 4 weeks, and apoE-/-+ADM mice were additionally given subcutaneous injections of ADM, 300ng/kg/h, for 4 weeks. ApoE-/- mice fed an atherogenic diet showed hyperlipidemia, a large plaque area and increased vessel wall thickness. The mRNA expression and protein level of ADM/ADM receptors were increased in the aorta, compared with C57BL/6J mice. The elevated mRNA level of CRLR and RAMPs correlated positively with ADM mRNA level. Radioimmunoassay revealed a higher plasma and aorta ADM content, by 61.6% and 285% (both P<0.01), respectively, in apoE-/- mice than that in C57BL/6J mice. Exogenous ADM significantly ameliorated dyslipidemia in apoE-/- mice. ADM-treated mice showed fewer aortic plaques, decreased plaque area, by 76% (P<0.01), and reduced ratio of plaque area to luminal area, by 65% (P<0.01), and ultrasonography revealed significantly reduced intima-media thickness of the ascending branch and abdominal aorta. The results suggest that atherosclerotic apoE-/- mice fed an atherogenic diet showed upregulated endogenous ADM and its receptors, and exogenous ADM treatment ameliorated the dyslipidemia and vascular atherosclerotic lesions. ADM/ADM receptors might be an important protective system against atherosclerosis and could become a new target of prevention and therapy for atherosclerosis.

Receptor activity-modifying protein 1 increases baroreflex sensitivity and attenuates Angiotensin-induced hypertension

Calcitonin gene-related peptide (CGRP) is a powerful vasodilator that interacts with the autonomic nervous system. A subunit of the CGRP receptor complex, receptor activity-modifying protein 1 (RAMP1), is required for trafficking of the receptor to the cell surface and high-affinity binding to CGRP. We hypothesized that upregulation of RAMP1 would favorably enhance autonomic regulation and attenuate hypertension. Blood pressure, heart rate, and locomotor activity were measured by radiotelemetry in transgenic mice with ubiquitous expression of human RAMP1 (hRAMP1) and littermate controls. Compared with control mice, hRAMP1 mice exhibited similar mean arterial pressure, a lower mean heart rate, increased heart rate variability, reduced blood pressure variability, and increased baroreflex sensitivity (2.83+/-0.20 versus 1.49+/-0.10 ms/mm Hg in controls; P<0.05). In control mice, infusion of angiotensin II (Ang-II) increased mean arterial pressure from 118+/-2 mm Hg to 153+/-4 and 174+/-6 mm Hg after 7 and 14 days of infusion, respectively (P<0.05). In contrast, Ang-II hypertension was markedly attenuated in hRAMP1 mice with corresponding values of mean arterial pressure of 111+/-2, 119+/-2, and 132+/-3 mm Hg. Ang-II induced decreases in baroreflex sensitivity and heart rate variability, and increases in blood pressure variability observed in control mice were also abrogated or reversed in hRAMP1 mice (P<0.05). Moreover, during the Ang-II infusion, the pressor response to the CGRP receptor antagonist CGRP(8-37) was significantly greater (P<0.05) in hRAMP1 mice (+30+/-2 mm Hg) than in control mice (+19+/-2 mm Hg), confirming a significantly greater antihypertensive action of endogenous CGRP in hRAMP1 mice. We conclude that RAMP1 overexpression attenuates Ang-II-induced hypertension and induces a protective change in cardiovascular autonomic regulation.

Intermedin 1-53 inhibits rat cardiac fibroblast activation induced by angiotensin II

Intermedin (IMD) is a novel peptide related to calcitonin gene-related peptide (CGRP) and adrenomedullin (ADM). Proteolytic processing of a larger precursor of IMD yields a biologically active C-terminal fragment IMD(1-53). We aimed to observe the cardioprotective antifibrotic effects of IMD(1-53) and its mechanism. Radioimmunoassay and Western blot analysis was used to determine IMD content in angiotensin II (AngII)-treated rat cardiac fibroblasts (CFs). Real-time PCR was used to measure mRNA levels of IMD and the IMD receptor components calcitonin receptor-like receptor (CRLR) and receptor activity modifying protein (RAMP) 1, 2 and 3. AngII was a powerful stimulator of CF activation. It decreased the production and secretion of IMD and increased the mRNA levels of the IMD receptor components CRLR, RAMP2 and RAMP3, but not IMD and RAMP1. Moreover, IMD(1-53) (10(-8) or 10(-7) mol/l) exerted a 25% and 45% respective inhibition in [(3)H]-thymidine incorporation and 16% and 36% respective inhibition in [(3)H]-proline incorporation in rat CFs incubated with AngII, and the actions of IMD(1-53) could be blocked by CGRP(8-37) and ADM(22-52). Immunofluorescence and Western blot analysis revealed that IMD(1-53) inhibited the increase of alpha-SMA in CFs induced by AngII, and the above effects of IMD(1-53) were similar to or more potent than those of an equivalent dose of ADM. Otherwise, IMD(1-53) resulted in dose-dependent increases of cAMP production in CFs, and co-incubated with H89 blocked the inhibition effect of IMD(1-53) on AngII-induced [(3)H]-thymidine, [(3)H]-proline incorporation and alpha-SMA expression. Collectively, these results show that IMD and its receptor components could be involved in an onset of cardiac fibrosis, and like ADM, IMD(1-53) exerts an antifibrotic effect in CFs, and the effect can be mediated by cAMP-PKA pathway and implicated with the ADM and CGRP receptors.

Apelin activates L-arginine/nitric oxide synthase/nitric oxide pathway in rat aortas

Apelin was recently found to be an inotropic polypeptide in isolated rat hearts, and intravenous injection of apelin can induce a transient decrease in blood pressure. To illustrate the mechanism of apelin-induced vasodilation, we observed the in vitro effects of apelin on the L-arginine (L-Arg)/nitric oxide (NO) pathway in the incubated, isolated rat aorta. Apelin stimulated vascular NO(2)(-) product and NOS activation in a concentration- and time-dependent manner. Compared with no apelin treatment, incubation with apelin (10(-9), 10(-8), and 10(-7)mol/L) increased NO(2)(-) product by 33%, 46%, and 69% (all p<0.01), respectively, and Ca(2+)-dependent constitutive NOS (cNOS) activity by 200%, 460%, and 550% (all p<0.01), respectively. However, Ca(2+)-independent NOS (iNOS) activity was not significantly altered (p>0.05). Apelin incubation (10(-9), 10(-8), and 10(-7)mol/L) increased L-Arg uptake by 130%, 180%, and 240% (all p<0.01), respectively. The mRNA level of cationic amino acid transporters, CAT-1 and CAT-2B, in rat aortic tissues treated with 10(-7)mol/L apelin was increased by 110% and 128%, respectively (both p<0.01). Incubation with 10(-7)mol/L apelin elevated eNOS mRNA and protein levels, by 53% (p<0.05) and 319% (p<0.01), respectively. Collectively, these results demonstrate that apelin directly activated the vascular L-Arg/NOS/NO pathway, which could be one of the important mechanisms of apelin-regulated vascular function.

The myocardial response to adrenomedullin involves increased cAMP generation as well as augmented Akt phosphorylation

In this work we aimed to observe (1) the changes in adrenomedullin (AM) and its receptor system - calcitonin receptor-like receptor (CRLR) and receptor activity modifying proteins (RAMPs) - in myocardial ischemic injury and (2) the response of injuried myocardia to AM and the phosphorylation of Akt to illustrate the protective mechanism of AM in ischemic myocardia. Male SD rats were subcutaneously injected with isoproterenol (ISO) to induce myocardial ischemia. The mRNA levels of AM, CRLR, RAMP1, RAMP2 and RAMP3 were determined by RT-PCR. Protein levels of Akt, phosphor-Akt, CRLR, RAMP1, RAMP2 and RAMP3 were assayed by Western blot. Results showed that, compared with that of the controls, ISO-treated rats showed lower cardiac function and myocardial injury. The mRNA relative amount of AM, CRLR, RAMP1, RAMP2 and RAMP3 in the myocardia of ISO-treated rats was increased. The elevated mRNA levels of CRLR, RAMP1, RAMP2 and RAMP3 were positively correlated with AM content in injured myocardia. The protein levels of CRLR, RAMP1, RAMP2 and RAMP3 in injured myocardia were increased compared with that of control myocardia. AM-stimulated cAMP generation in myocardia was elevated in the ISO group, and was antagonized by AM(22-52) and CGRP(8-37). Western blot analyses revealed that AM significantly enhanced Akt phosphorylation in injured myocardia, which was blocked by pretreatment with AM(22-52) or CGRP(8-37). Ischemia-injured myocardia hyper-expressed AM and its receptors - CRLR, RAMP1, RAMP2 and RAMP3 - and the response of ischemic myocardia to AM was potentiated, and the level of Akt phosphorylation was also increased, which suggests that changes in cardiac AM/AM receptor might play an important role in the pathogenesis of myocardial ischemic injury.