Natriuretic peptides: novel therapeutic targets in heart failure

Natriuretic Peptide Receptor-C Protects Against Angiotensin II-Mediated Sinoatrial Node Disease in Mice

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SAN disease is prevalent in hypertension and heart failure and can be induced by chronic Ang II treatment in mice.

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Ang II caused SAN disease in mice in association with impaired electrical conduction, reduction in the hyperpolarization-activated current (I_f) in SAN myocytes, and increased SAN fibrosis.

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Ang II-induced SAN disease was worsened in mice lacking NPR-C in association with enhanced SAN fibrosis.

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Mice co-treated with Ang II and an NPR-C agonist (cANF) were protected from SAN disease.

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NPR-C may represent a new target to protect against Ang II-induced SAN disease.

Sinoatrial node (SAN) disease mechanisms are poorly understood, and therapeutic options are limited. Natriuretic peptide(s) (NP) are cardioprotective hormones whose effects can be mediated partly by the NP receptor C (NPR-C). We investigated the role of NPR-C in angiotensin II (Ang II)-mediated SAN disease in mice. Ang II caused SAN disease due to impaired electrical activity in SAN myocytes and increased SAN fibrosis. Strikingly, Ang II treatment in NPR-C^−/− mice worsened SAN disease, whereas co-treatment of wild-type mice with Ang II and a selective NPR-C agonist (cANF) prevented SAN dysfunction. NPR-C may represent a new target to protect against the development of Ang II-induced SAN disease.

Impaired sinoatrial node function and increased susceptibility to atrial fibrillation in mice lacking natriuretic peptide receptor C

Natriuretic peptides (NPs) are critical regulators of the cardiovascular system that are currently viewed as possible therapeutic targets for the treatment of heart disease. Recent work demonstrates potent NP effects on cardiac electrophysiology, including in the sinoatrial node (SAN) and atria. NPs elicit their effects via three NP receptors (NPR-A, NPR-B and NPR-C). Among these receptors, NPR-C is poorly understood. Accordingly, the goal of this study was to determine the effects of NPR-C ablation on cardiac structure and arrhythmogenesis. Cardiac structure and function were assessed in wild-type (NPR-C(+/+)) and NPR-C knockout (NPR-C(-/-)) mice using echocardiography, intracardiac programmed stimulation, patch clamping, high-resolution optical mapping, quantitative polymerase chain reaction and histology. These studies demonstrate that NPR-C(-/-) mice display SAN dysfunction, as indicated by a prolongation (30%) of corrected SAN recovery time, as well as an increased susceptibility to atrial fibrillation (6% in NPR-C(+/+) vs. 47% in NPR-C(-/-)). There were no differences in SAN or atrial action potential morphology in NPR-C(-/-) mice; however, increased atrial arrhythmogenesis in NPR-C(-/-) mice was associated with reductions in SAN (20%) and atrial (15%) conduction velocity, as well as increases in expression and deposition of collagen in the atrial myocardium. No differences were seen in ventricular arrhythmogenesis or fibrosis in NPR-C(-/-) mice. This study demonstrates that loss of NPR-C results in SAN dysfunction and increased susceptibility to atrial arrhythmias in association with structural remodelling and fibrosis in the atrial myocardium. These findings indicate a critical protective role for NPR-C in the heart.

Cardiac hormones for the treatment of cancer

Four cardiac hormones, namely atrial natriuretic peptide, vessel dilator, kaliuretic peptide, and long-acting natriuretic peptide, reduce up to 97% of all cancer cells in vitro. These four cardiac hormones eliminate up to 86% of human small-cell lung carcinomas, two-thirds of human breast cancers, and up to 80% of human pancreatic adenocarcinomas growing in athymic mice. Their anticancer mechanisms of action, after binding to specific receptors on cancer cells, include targeting the rat sarcoma-bound GTP (RAS) (95% inhibition)-mitogen-activated protein kinase kinase 1/2 (MEK 1/2) (98% inhibition)-extracellular signal-related kinase 1/2 (ERK 1/2) (96% inhibition) cascade in cancer cells. They also inhibit MAPK9, i.e. c-Jun N-terminal kinase 2. They are dual inhibitors of vascular endothelial growth factor (VEGF) and its VEGFR2 receptor (up to 89%). One of the downstream targets of VEGF is β-catenin, which they reduce up to 88%. The WNT pathway is inhibited up to 68% and secreted frizzled-related protein 3 decreased up to 84% by the four cardiac hormones. AKT, a serine/threonine protein kinase, is reduced up to 64% by the cardiac hormones. STAT3, a final 'switch' that activates gene expression that leads to malignancy, is decreased by up to 88% by the cardiac hormones. STAT3 is specifically decreased as they do not affect STAT1. There is a cross-talk between the RAS-MEK 1/2-ERK 1/2 kinase cascade, VEGF, β-catenin, WNT, JNK, and STAT pathways and each of these pathways is inhibited by the cardiac hormones.

VAMP-1, VAMP-2, and syntaxin-4 regulate ANP release from cardiac myocytes

ANP is a peptide released by cardiac myocytes that regulates blood pressure and natriuresis. However, the molecular mechanisms controlling ANP release from cardiac myocytes are not defined. We now identify three components of the exocytic machinery that regulate ANP release from atrial myocytes. We found that cardiac myocytes express N-ethylmaleimide sensitive factor (NSF), soluble NSF attachment protein (α-SNAP), and SNAP receptors (SNAREs). Additionally we found that specific SNARE molecules, VAMP-1 and VAMP-2, both co-sediment and co-localize with ANP. Also, one SNARE molecule, syntaxin-4, partially co-sediments and partially co-localizes with ANP. Furthermore, these three SNAREs, syntaxin-4 and VAMP-1 and VAMP-2, form a SNARE complex inside cardiac myocytes. Finally, knockdown of VAMP-1, VAMP-2, or syntaxin-4 blocks regulated release of ANP. In contrast, silencing of VAMP-3 did not have an effect on ANP release. Our data suggest that three specific SNAREs regulate cardiac myocyte exocytosis of ANP. Pathways that modify the exocytic machinery may influence natriuresis and blood pressure.

Insights into natriuretic peptides in heart failure: an update

Natriuretic peptides (NPs) secreted by the heart in response to volume overload are pleiotropic molecules with vasodilating, diuretic, natriuretic, antiproliferative, and antifibrotic actions. Functioning of the NP system is altered in congestive heart failure (CHF), suggesting that support of the NP system might be beneficial in treatment of acute and chronic CHF. Several approaches alone or in combination with other pharmacologic therapies have been shown to enhance function of the NP system: direct administration of native and designer NPs, inhibition of degradation of NPs and their second messenger (cyclic guanosine monophosphate ), and stimulation of cGMP generation. Despite increasing numbers of studies using NPs in therapy of acute and chronic CHF, several controversies regarding safety, efficacy, and dosing of NPs need to be addressed. Moreover, further research is warranted to identify the stages and etiologies of CHF that may profit from NP therapy.

Chronic actions of a novel oral B-type natriuretic peptide conjugate in normal dogs and acute actions in angiotensin II-mediated hypertension

BACKGROUND

We previously reported the feasibility of an acute, orally delivered, newly developed, conjugated form of human B-type natriuretic peptide (hBNP) in normal animals. The objective of the present study was to extend our findings and to define the chronic actions of an advanced oral conjugated hBNP (hBNP-054) administered for 6 days on sodium excretion and blood pressure. We also sought to establish the ability of this new conjugate to acutely activate cGMP and to reduce blood pressure in an experimental model of angiotensin II (ANG II) -mediated hypertension.

METHODS AND RESULTS

First, we developed additional novel conjugated forms of oral hBNP that were superior to our previously reported hBNP-021 in reducing blood pressure in 6 normal dogs. We then tested the new conjugate, hBNP-054, chronically in 2 normal dogs to assess its biological actions as a blood pressure-lowering agent and as a natriuretic factor. Second, we investigated the effects of acute oral hBNP-054 or vehicle in 6 dogs that received continuous infusion of ANG II to induce hypertension. After baseline determination of mean blood pressure (MAP) and blood collection for plasma hBNP and cGMP, all dogs received continuous ANG II infusion (20 ng . kg(-1) . min(-1), 1 mL/min) for 4 hours. After 30 minutes of ANG II, dogs received oral hBNP-054 (400 microg/kg) or vehicle in a random crossover fashion with a 1-week interval between dosing. Blood sampling and MAP measurements were repeated 30 minutes after ANG II administration and 10, 30, 60, 120, 180, and 240 minutes after oral administration of hBNP-054 or vehicle. In the chronic study in normal dogs, oral hBNP-054 effectively reduced MAP for 6 days and induced a significant increase in 24-hour sodium excretion. hBNP was not present in the plasma at baseline in any dogs, and it was not detected at any time in the vehicle group. However, hBNP was detected throughout the duration of the study after oral hBNP-054, with a peak concentration at 30 minutes of 1060+/-818 pg/mL. In the acute study, after ANG II administration, plasma cGMP was not activated after vehicle, whereas it was significantly increased after oral hBNP-054 (P=0.01 between the 2 groups). Importantly, MAP was significantly increased after ANG II throughout the acute study protocol. However, although no changes occurred in MAP after vehicle administration, oral hBNP-054 reduced MAP for >2 hours (from 138+/-1 mm Hg after ANG II to 124+/-2 mm Hg at 30 minutes, 124+/-2 mm Hg at 1 hour, and 130+/-5 mm Hg at 2 hours after oral hBNP-054; P<0.001).

CONCLUSIONS

This study reports for the first time that a novel conjugated oral hBNP possesses blood pressure-lowering and natriuretic actions over a 6-day period in normal dogs. Furthermore, hBNP-054 activates cGMP and reduces MAP in a model of acute hypertension. These findings advance the concept that orally administered chronic BNP is a potential therapeutic strategy for cardiovascular diseases such as hypertension.

Apoptosis signal-regulating kinase 1 attenuates atrial natriuretic peptide secretion

Atrial natriuretic peptide (ANP) is an endogenous peptide hormone that is synthesized and secreted by the myocardium in health and disease. Although the bioactivity of this molecule has been studied extensively, cellular mechanisms governing its processing and secretion are not fully understood. Through a yeast two-hybrid screen of a cDNA library made from tissue of a failing human heart, we have discovered that the precursor of ANP, natriuretic peptide precursor (NPPA), physically interacts with the N-terminus of apoptosis signal-regulating kinase 1 (ASK1), a kinase believed to be involved in the pathogenesis of heart failure. We demonstrated that NPPA is a substrate of ASK1 in an in vitro kinase assay. Indirect immunofluorescence microscopy shows that, when expressed in Hela cells, ASK1 and NPPA exhibit distinct, but overlapping, staining patterns, suggesting partial colocalization in cells. Additionally, coexpressing wild-type ASK1 with NPPA in Hela cells led to reduced levels of NPPA in the culture medium, suggesting that ASK1 negatively impacts NPPA processing and/or secretion. This negative effect was less pronounced when a dominant-negative allele of ASK1 with deficient kinase activity was coexpressed with NPPA. Because both ASK1 and ANP are associated with pathologic cardiac hypertrophy, their interaction may have pathophysiological and therapeutic relevance.

Small-molecule therapies for cardiac hypertrophy: moving beneath the cell surface

Pathological stress from cardiovascular disease stimulates hypertrophy of heart cells, which increases the risk of cardiac morbidity and mortality. Recent evidence has indicated that inhibiting such hypertrophy could be beneficial, encouraging drug discovery and development efforts for agents that could achieve this goal. Most existing therapies that have antihypertrophic effects target outside-in signalling in cardiac cells, but their effectiveness seems limited, and so attention has recently turned to the potential of targeting intracellular signalling pathways. Here, we focus on new developments with small-molecule inhibitors of cardiac hypertrophy, summarizing both agents that have been in or are poised for clinical testing, and pathways that offer further promising potential therapeutic targets.