Circulation Journal
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Natriuretic Peptides and Cardiometabolic Health
Deepak K. GuptaThomas J. Wang
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Article ID: CJ-15-0589

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Abstract

Natriuretic peptides are cardiac-derived hormones with a range of protective functions, including natriuresis, diuresis, vasodilation, lusitropy, lipolysis, weight loss, and improved insulin sensitivity. Their actions are mediated through membrane-bound guanylyl cyclases that lead to production of the intracellular second-messenger cyclic guanosine monophosphate. A growing body of evidence demonstrates that genetic and acquired deficiencies of the natriuretic peptide system can promote hypertension, cardiac hypertrophy, obesity, diabetes mellitus, the metabolic syndrome, and heart failure. Clinically, natriuretic peptides are robust diagnostic and prognostic markers, and augmenting natriuretic peptides is a target for therapeutic strategies in cardiometabolic disease. This review will summarize current understanding and highlight novel aspects of natriuretic peptide biology.

Natriuretic Peptides (NPs) and Their Receptors

The NPs are a family of cardiac-derived hormones that have pleiotropic cardiometabolic protective effects.1 Three NPs, atrial (ANP), B-type (BNP), and C-type (CNP), have been described, with ANP being the first, identified by de Bold et al and sequenced by Kangawa, Matsuo, and colleagues.27 In humans, these peptides are encoded by the NPPA (NP precursor A) and NPPB genes located in tandem on chromosome 1, and NPPC on chromosome 2.810 Mechanical stretch of cardiomyocytes and/or stimulation by endothelin, angiotensin II, the sympathetic nervous system, vasopressin, hypoxia, cold, or exercise induces the transcription factor GATA to bind the NP promoters.3,11 The NP precursor genes are transcribed and translated into preprohormones that undergo post-translational processing and cleavage into biologically active carboxy-terminal and inactive amino-terminal fragments by the serine proteases corin and/or furin (Figure 1).12 ANP is predominantly synthesized, stored in preformed granules, and released from atrial cardiomyocytes; BNP is produced in atrial and ventricular cardiomyocytes; and CNP is largely derived from vascular endothelial cells and neurons.1315 The bioactive carboxy-terminal NPs have relatively short half-lives in the circulation, while the inactive amino-terminal fragments are more stable with longer half-lives.16

Figure 1.

The post-translational processing of natriuretic peptides (NPs). NP protein sequences and post-translational processing cleavage and degradation sites. ANP, atrial NP; BNP, B-type NP; CNP, C-type NP; NEP, neprilysin; DPPIV, dipeptidyl peptidase IV; IDE, insulin degrading enzyme. Reproduced with permission from Volpe M, et al.12

The NPs exert their actions by binding guanylyl cyclase receptors A (GC-A for ANP and BNP) and B (GC-B for CNP), which are transmembrane proteins that catalyze the conversion of intracellular guanosine triphosphate into cyclic guanosine monophosphate (cGMP), which then increases intracellular protein kinase G (PKG) (Figure 2).17 The NP receptor C (NPR-C) functions predominantly as a clearance receptor for all 3 NPs, but also exerts effects on inhibitory G-proteins and adenylyl cyclase with activation of phospholipase C.18 Receptors for the NPs are not only present on cardiomyocytes and fibroblasts, but also the kidneys, vascular and gastrointestinal smooth muscle, adrenal glands, brain, pancreas, adipocytes, chondrocytes, platelets, and the liver, suggesting that NPs have biologic actions beyond natriuresis. In addition to clearance through NPR-C, NPs are inactivated by neutral endopeptidases located within renal tubular cells and the vasculature, as well as insulin-degrading enzyme and dipeptidyl peptidase-IV, and may also be passively excreted in the urine (Figure 1).12

Figure 2.

The natriuretic peptide (NP) system responds to hemodynamic and metabolic stimuli through activation of guanylyl cyclase receptors resulting in cardiometabolic protective effects. Cardiomyocytes and endothelial cells are stimulated to release NPs, which bind receptors with guanylyl cyclase activity. This activation leads to increased intracellular cGMP with beneficial downstream effects mediated through protein kinase G (PKG). Phosphodiesterase (PDE) inactivates cGMP. CNP also inhibits adenylate cyclase to reduce cAMP levels through the NP receptor-C. ANP, atrial NP; BNP, B-type NP; CNP, C-type NP; NO, nitric oxide; SNS, sympathetic nervous system. Reproduced with permission from Volpe M, et al.12

Biologic Effects of NPs: Experimental Evidence

Genetic Models

Experimental evidence supports the broad range of cardiovascular and metabolic actions of the NPs. Transgenic overexpression or knockout mouse models for each of the NPs and their receptors provide consistent evidence of these hormones’ protective cardiometabolic effects (Table 1).11 Overexpression of NPPA, NPPB, and GC-A leads to blood pressure lowering and protection against salt-sensitive hypertension.1922 Knockout of NPR-C yields a similar phenotype of lower blood pressure.23 Mice overexpressing the BNP gene (NPPB) are also resistant to obesity and demonstrate lower glucose and insulin concentrations compared with wild-type mice, a finding attributed to increased skeletal muscle mitochondrial content and fatty acid oxidation.2426 In contrast, NPPA, NPPB, and GC-A knockout mice exhibit hypertension, salt-sensitivity, cardiac hypertrophy, cardiac fibrosis, and susceptibility to heart failure, as well as obesity.2737 Alterations in the corin protein (corresponding to known human genetic variants) that lead to reduced cleavage of the NP prohormone into the active peptide also result in salt-sensitive hypertension and cardiac hypertrophy.38,39

Table 1. Summary of the Phenotypes Associated With Genetic Manipulation of the Natriuretic Peptide System in Animals
Gene disruption Phenotype/physiology
ANP overexpression Hypotension, decrease in hypoxic HT, normal salt excretion, increased H2O intake and
excretion
ANP knockout (Nppa−/−) HT, BP-independent right and LVH, impaired Na and Cl excretion
BNP overexpression Hypotension, skeletal overgrowth, resistance to immune-mediated renal injury
BNP knockout (Nppb−/−) Load-dependent ventricular fibrotic lesions, no hypertrophy, no HT
CNP knockout (Nppc−/−) Dwarfism, early death
CNP overexpression (chondrocyte targeted) Rescue of dwarfism phenotype
NPR-A (GC-A) overexpression Hypotension, protection against salt-sensitive HT
NPR-A (GC-A) knockout (Npr1−/−) Salt-resistant HT, BP-independent ventricular hypertrophy, increase in sudden death,
enhanced NHE-1 activity, increased susceptibility to HF
NPR-A targeted knockout
 Cardiomyocyte Hypertrophy, increase in hypertrophy markers, hypotension
 Smooth muscle Loss of ANP response, volume dependent HT
 Vascular endothelium Arterial HT and cardiac hypertrophy, increased plasma volume
NPR-B (GC-B) knockout (Npr2−/−) Dwarfism, neuronal disorders, female infertility
NPR-B (GC-B) dominant-negative
overexpression (rat)
BP-independent cardiac hypertrophy, increased congestive HF, elevated heart rate
NPR-C knockout (Npr3−/−) Hypotension, bone overgrowth, reduced blood volume

ANP, atrial natriuretic peptide; BNP, B-type natriuretic peptide; BP, blood pressure; CNP, C-type natriuretic peptide; HF, heart failure; HT, hypertension; LVH, left ventricular hypertrophy. Reproduced with permission from Gardner DG, et al.11

Non-Genetic Experiments

In vitro experiments and in vivo data highlight the role of the NPs in cardiovascular and metabolic physiology. Animals exposed to infusion of ANP or BNP have lower blood pressure, not only through increased natriuresis and diuresis, but also through arterial and venodilation, increased vascular permeability (shifting volume from the intracellular to extracellular space), and direct suppression of the renin-angiotensin-aldosterone and sympathetic nervous systems.3,40,41 CNP administration induces marked venodilation.3 The natriuretic and diuretic effects are caused by (1) enhanced glomerular filtration through simultaneous dilation of afferent arterioles and constriction of efferent arterioles and (2) direct effects on renal tubular cells through antagonism of angiotensin II and vasopressin.4244 The vasodilatory effects of ANP and BNP are also mediated centrally in the brainstem through decreased sympathetic outflow.41,45,46

ANP inhibits the growth of cardiac fibroblasts and can induce cardiomyocyte apoptosis.4749 Similar to ANP, CNP is a potent inhibitor of cardiac fibroblasts and exerts antifibrotic effects,50 which may be in part mediated by PKG-dependent phosphorylation of Smad3, resulting in less nuclear translocation when stimulated by transforming growth factor-β.51 Through p38 MAPK, NPs also exhibit anti-mitogenic properties, with some indication of anti-neoplastic potential through reduction of inflammation and cell adhesion processes as well.52,53

The p38 MAPK pathway may also modulate the effect of NPs on the induction of brown adipose tissue.54 Further supporting a role for the NPs in the control of energy homeostasis, exposure of cultured adipocytes to physiologic doses of ANP and/or BNP promote cGMP-dependent activation of hormone sensitive-lipase, leading to lipolysis.55,56

Biologic Effects of NPs: Clinical Evidence

Genetic Variants

The biologic importance of the NP system is supported by the finding that NPPA is highly conserved across species.57 Nevertheless, genetic variants in the NPs, their receptors, and activating proteases have been identified in humans and their associations with cardiometabolic phenotypes described.57 The results of these genetic variation studies in humans parallel the evidence from animal models regarding the role of the NP system.

A number of variants in the promoter, coding, intronic, and 3’ untranslated region of NPPA have been characterized (Table 2).57 Candidate gene studies in Japanese and Italian individuals have associated a C-664G variant with lower circulating ANP, hypertension, and left ventricular hypertrophy.5860 There are mixed data regarding another missense variant, rs5063, which results in a valine to methionine substitution and has been linked to lower blood pressure among Chinese individuals and participants in the Women’s Genome Health study, although this was not observed among Japanese individuals.58,6163 In other populations, the rs5063 variant was associated with an increased risk of hypertension or stroke.6466 Interestingly, the rs5065 (2238 T>C) variant in exon 3 has been associated with a decreased risk of hypertension,67 but higher risk of myocardial infarction and stroke, which may be mediated through altered NPR-C activation and resultant endothelial dysfunction.6872 Nonetheless, many of the aforementioned candidate genes have not been reliably reproduced in large-scale population genetic studies nor in meta-analyses of genome-wide association studies (GWAS).

Table 2. Summary of NPPA Genetic Variants in Humans and Their Associated Clinical Phenotypes
NPPA variant HT LVH CV acute events AF MetS HF
−664C>G G allele more frequent
in young subjects with
HT; C allele more
frequent in Japanese
subjects with HT
G allele
associated with
increased LVH
in HT
No association of
either allele with
stroke and AMI
No association of
either allele with AF
in high risk Italian
patients
rs5063
(664G>A)
A allele associated
with lower DBP; A
allele associated with
BP progression; no
association in
Japanese patients
A allele associated
with increased risk of
stroke; common
allele associated with
higher risk of acute
events
A allele associated
with increased risk of
lone AF in Chinese
patients; no
association with AF
in North American
subjects
HpaII Variant allele
associated with
increased risk of HT;
common allele
associated with
increased risk of HT
Variant allele
associated with
increased risk of
stroke
rs5065
(2238T>C)
C allele associated
with a decreased risk
of HT
  C allele associated
with an increased
risk of stroke, AMI
and MACE; no
association with
stroke; no association
with CV events; C
allele associated with
a greater response to
diuretic in HT
No association of C
allele with AF in both
North American and
Italian patients
  Allele variant
associated with
ANP plasma
levels in NYHA
class III–IV
rs5068 Allele variant
associated with a
decreased risk of HT
Allele variant
associated with
decreased
occurrence of
LVH
Allele variant
associated with
a decreased
risk of the MetS
No association
of allele variant
with HF

AF, atrial fibrillation; AMI, acute myocardial infarction; CV, cardiovascular; DBP, diastolic BP; MACE, major adverse cardiovascular events; MetS, metabolic syndrome. Other abbreviations as in Table 1. Reproduced with permission from Rubattu S, et al.57 See original publication for detailed references.

The most statistically robust findings to date have derived from studies with larger sample sizes. For instance, from a meta-analysis of data from the Framingham Heart Study, the Malmo Diet and Cancer Study, and the Finrisk study, the rs5068 A/G variant in the 3’ untranslated region of NPPA is associated with higher circulating ANP levels at a genome-wide level of significance (among carriers of the minor allele, G, P=8×10−70). The G allele has been associated with lower blood pressure, less hypertension, and less ventricular hypertrophy.73,74 Additional studies demonstrate that the rs5068 A/G variant relates to a favorable metabolic profile as evidenced by lower body mass index, smaller waist circumference, higher high-density lipoprotein and lower C-reactive protein levels, as well as less susceptibility to heart failure.75,76 Recently, Arora et al elucidated the molecular mechanism by which the rs5068 variant influenced ANP production. The variant is in the non-coding 3’ UTR of NPPA, a region that is targeted by micro-RNAs. ANP expression was modulated through negative regulation by a specific microRNA, miR-425, which binds to the site of rs5068. Thus, individuals with the AG allele combination are resistant to miR-425, and therefore have higher circulating ANP levels and less hypertension compared with AA homozygote individuals.77

Genetic variants in other NP and related genes have also been described. The rs198388 (presence of A allele) and 198389 (presence of the C allele) variants in NPPB are associated with lower blood pressure, improved left ventricular diastolic function, reduced left ventricular remodeling, and lower risk of diabetes mellitus.73,7881 A functional deletion mutation in the 5’ flanking region of the NP receptor GC-A gene reduces transcription and is associated with hypertension and ventricular hypertrophy among Japanese individuals.82 In GWAS, the NPR-C variants are associated with hypertension in Caucasian and Asian individuals.80,81 Less is known about variants in NPPC and GC-B.57 Missense variants in CORIN (555T>I and 568Q>P), which encodes a serine protease that cleaves natriuretic prohormones into the active carboxy- and inactive amino-terminal peptides, has been found in approximately 9% of African-Americans and is associated with a greater risk for hypertension and cardiac hypertrophy.83,84

Physiologic Studies

The beneficial cardiovascular effects of NPs have been demonstrated through infusions of ANP, BNP, and CNP. All 3 NPs induce vasodilation, with ANP and BNP also lowering blood pressure.85,86 Infusion of ANP, BNP, or CNP may also limit post-acute myocardial infarction adverse cardiac modeling.8789 In the setting of heart failure, ANP and BNP infusions decrease pulmonary capillary wedge pressure and systemic vascular resistance, leading to increased stroke volume.9094

NPs not only influence myocardial structure and function, but also exert positive influences on the vasculature. Cultured endothelial cells exposed to ANP or CNP demonstrated reduced expression of adhesion molecules (MCP-1 and P-selectin), which are needed for leukocyte infiltration into atherosclerotic plaques.95,96 CNP also inhibits coronary vascular smooth muscle proliferation in models of atherosclerosis,9799 reduces platelet leukocyte aggregation, and limits thrombus formation through reduction in PAI-1, perhaps through NPR-C.100102

The beneficial metabolic effects of NPs have also been demonstrated in humans. Infusion of ANP at physiologic levels induced lipid mobilization from subcutaneous adipose tissue, with a concomitant increase in lipid oxidation by skeletal muscle.25,26,103 BNP has been demonstrated to lower glucose levels,104 and both ANP and BNP converted white to brown fat through mitochondrial uncoupling protein-1 and p38 MAPK.54 It has also been suggested that exercise-induced lipolysis may be mediated through ANP.105

Epidemiologic Associations

The cross-sectional associations between circulating NP levels and cardiovascular and metabolic disease have been examined in epidemiologic studies. An inverse relationship has been demonstrated between plasma NP levels and body mass index.106,107 Similarly, low levels of NT-proBNP and NT-proANP have been found in individuals with the metabolic syndrome and/or left ventricular hypertrophy.108111 The association of NPs with endothelial function has also been demonstrated in the Framingham Heart Study.112 Congruent with the animal studies, low NP levels have been associated with the development of diabetes mellitus.113,114

NPs as Biomarkers

Although experimental, population genetic, and NP infusion studies demonstrate inverse associations between NP levels and cardiometabolic disease, clinical studies of NPs as prognostic biomarkers typically yield positive associations between circulating NP levels and adverse cardiovascular outcomes.16 This apparent paradox is attributable to the fact that NPs are counter-regulatory hormones that are released in response to cardiac stress. In population studies, higher NP levels, even within what might be considered a “normal” range, are commonly seen in the setting of subclinical cardiovascular disease. Thus, the elevated NP levels observed in clinical biomarker studies reflect normal physiologic responses to elevated cardiac wall stress.

For example, among individuals without prevalent cardiovascular disease in the Framingham Offspring Study and in Copenhagen, higher NP levels were positively and significantly associated with cardiovascular mortality115 or major adverse cardiovascular events,116 respectively. Among persons with stable coronary artery disease or acute coronary syndromes, higher NPs were significantly and positively associated with greater risk for recurrent cardiovascular events and/or death.117121 Similarly, higher NP levels are associated with worse outcomes among individuals with heart failure.122125 However, further evidence for the beneficial effects of NPs, even in the setting of subclinical or overt cardiovascular disease, comes from therapeutic trials in which augmentation of NPs led to favorable cardiovascular effects.

NPs as a Therapeutic Target

Most strategies for the prevention and treatment of cardiovascular disease have been directed at blocking the deleterious effects of the renin-angiotensin-aldosterone axis and sympathetic nervous system.12 Given that hypertension, obesity, and insulin resistance are major risk factors for the development of cardiovascular disease and that NPs guard against the development and progression of these disorders, NPs are attractive therapeutic targets (Figure 3).

Figure 3.

Natriuretic peptides (NPs) as novel therapeutic targets in cardiometabolic disease. NPs exert beneficial effects on cardiac and vascular function directly and through inhibition of the renin-angiotensin-aldosterone axis (RAAS) and sympathetic nervous systems (SNS), making them ideal and novel targets for cardiovascular protection. BP, blood pressure; CAD, coronary artery disease; CO, cardiac output; HF, heart failure; HT, hypertension; SVR, systemic vascular resistance. Reproduced with permission from Volpe M, et al.12

Therapeutic approaches have included intravenous infusions of recombinant ANP or BNP, oral inhibitors of neutral endopeptidases, and synthetic or “designer” NP analogs.3 Intravenous infusions of ANP and BNP have been tested in clinical trials for hypertension126 and heart failure, with favorable hemodynamic effects, but no clear benefit on long-term clinical outcomes.94,127 Furthermore, oral formulations of ANP and BNP are not stable, currently limiting their therapeutic application in chronic disease.

An alternative strategy to direct supplementation of NPs is to limit the breakdown of endogenous NPs. Inhibitors of neutral endopeptidases are stable when given orally. The first to be tested in hypertension was candoxatril; however, this agent was not of substantial benefit because of simultaneous vasoconstriction due to increases in endothelin-1 and angiotensin II.128,129 Subsequently, combined inhibition of neutral endopeptidases and angiotensin-converting enzyme (ACE) with omapatrilat was evaluated in hypertensive and heart failure patients in the OCTAVE, OVERTURE, and IMPRESS trials. Blood pressure was lower in omapatrilat-treated patients compared with those treated with enalapril alone; however, omapatrilat was associated with a higher frequency of angioedema and symptomatic hypotension, without demonstration of superior efficacy, thereby preventing approval for clinical use.130132 More recently, neutral endopeptidase inhibition has been combined with angiotensin-receptor blockade to avoid the angioedema seen with ACE inhibition. The angiotensin-receptor and neprilysin inhibitor LCZ696 is efficacious for lowering blood pressure among patients with essential hypertension, without increased angioedema compared with valsartan alone.133 In a phase II study of heart failure with preserved ejection fraction patients (PARAMOUNT), LCZ696 was superior to valsartan alone in reducing NT-proBNP levels over 12 weeks of follow up.134 Recently, LCZ696 was demonstrated to be superior to enalapril for reducing cardiovascular death and heart failure hospitalizations in patients with heart failure and reduced ejection fraction (PARADIGM-HF),135 lending further support for the therapeutic benefit of NPs.

“Designer” or synthetic NPs that are more stable than native NPs have been developed. For example, the ANP analog carperitide promotes vasodilation, natriuresis, and inhibition of the renin-angiotensin-aldosterone axis and is approved in Japan for treatment of acute decompensated heart failure.136 Another analog, M-ANP, which is more resistant to neutral endopeptidase degradation than native ANP, has been designed and has favorable antihypertensive effects.137 A novel chimeric molecule, CD-NP, has been engineered by combining the 15 amino acid carboxy-terminus of dendrapsis NP with CNP, resulting in a protein that is able to activate both GC-A and GC-B. This chimeric peptide demonstrates potent natriuresis and diuresis, as well as antifibrotic and antiproliferative properties.138

Summary

NPs are cardiac-derived hormones and the principal counter-regulatory system guarding against salt-retention, volume expansion, cardiac stress, and remodeling. They also modulate energy metabolism, lipolysis, weight loss, and insulin sensitivity. Low NP activity can be associated with increased risk for hypertension, obesity, and diabetes mellitus, conditions that are increasing in prevalence and are major risk factors for cardiovascular disease. Consequently, NPs are attractive targets for new therapeutic approaches to cardiometabolic disease.

Acknowledgments

Funding for this study was from National Heart, Lung, and Blood Institute grants K12 HL109019 and R01-HL-102780, and the National Center for Advancing Translational Sciences of the National Institutes of Health award number UL1TR000445.

References
 
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