A simple blood test could identify seemingly-healthy people with a high hidden risk of heart disease thanks to a world-first discovery by University of Otago, Christchurch researchers.
Researchers from the University’s Christchurch Heart Institute studied the blood samples and cardiology scans of 665 healthy young and middle-aged people with no previous heart conditions.
They found people with high levels of a hormone in the blood, called C-type Natriuretic Peptide (CNP), were significantly more likely to have stiffening of the arteries, reduced pumping action of the heart, higher fat levels in the blood and liver, and reduced kidney function – all signs of increased risk of heart disease.
The discovery could one day enable doctors to identify those people whose lives could be saved from a future heart attack by interventions such as drugs or lifestyle changes.
The study is the first to describe a link between the blood hormone CNP and inflammation across a range of tissues including arteries and the heart. The results were recently published in the prestigious Peptides journal.
Lead researcher Dr. Tim Prickett says CNP seems to protect arteries from hardening and blocking. This means it is working hard and present in higher levels in those with potentially poor, and undetected, cardiovascular health.
“We examined two quite different groups of healthy people – one group age 28 years, the other age 50 years – both without history of heart or kidney disease. High levels of CNP in both age groups were found in people who had stiffer arteries, reduced pumping action of the heart, higher fat levels in the blood and liver, and reduced kidney function.”
Dr. Prickett says inflamed and blocked arteries can cause numerous physical problems including scarring and stiffness and damage to organs such as the heart, liver and kidneys. “We found that CNP in the blood stream reflects an increased production of CNP in these tissues, as part of a protective response to inflammation.”
He says the finding that CNP acts to protect the body is key to helping save lives through early detection of serious conditions such as atherosclerosis, which can lead to heart attack or stroke.
This is one of a number of discoveries by the Christchurch Heart Institute over the past 25 years.
The research group has discovered and developed blood tests for heart disease diagnosis and treatment, some of which are used in hospitals and emergency departments in New Zealand and around the globe.
Role of natriuretic peptide in cardiovascular homeostasis
Atrial natriuretic peptide (ANP) was first isolated from cardiac tissues [1,2]. Other natriuretic peptides, brain or B-type natriuretic peptide (BNP) and C-type natriuretic peptide (CNP), were isolated from porcine brain tissues by the same Japanese investigators [3,4].
Later studies found that ANP and BNP are mainly synthesized in cardiac tissue: ANP in the atrium and BNP in the ventricle. CNP is mainly expressed in the central nervous system, although recent studies indicate that expression of CNP in the endothelium , macrophages , and cardiac fibroblasts  are higher than that in the brain.
Subsequently, three major natriuretic peptide receptors were cloned [8–10]. Natriuretic peptide receptor-A (NPR-A) and natriuretic peptide receptor-B (NPR-B) are guanylyl cyclase-linked, and they utilize cyclic guanosine monophosphate (cGMP) as the intracellular messenger.
Both ANP and BNP bind preferentially to NPR-A, whereas CNP preferentially binds to NPR-B. All three natriuretic peptides bind to the third receptor, known as natriuretic peptide receptor-C (NPR-C). NPR-C is not linked to guanylyl cyclase, but appears to act to clear the natriuretic peptides from the circulation (Fig. 1).
Thus, the natriuretic peptide system consists of three ligands and three receptors. These peptides cause effects such as diuresis, natriuresis, vasodilation, and inhibition of aldosterone synthesis and renin secretion as a circulating hormone, and thereby play an important role in regulating blood pressure and blood volume (Table 1).
The intensity of actions differs among the three peptides. ANP and BNP are each produced within the heart and secreted in response to stretching of muscles that typifies an increase in blood volume.
The release of ANP and BNP from the heart has the most immediate biologic effect of increasing electrolyte and water excretion in the kidney by functionally antagonizing the “salt-sparing” role of the renin–angiotensin–aldosterone system.
However, ANP and BNP also regulate the permeability of the systemic vasculature, cellular growth, cellular proliferation, and, as shown more recently, cardiac hypertrophy.
Accumulating evidence suggests that the three natriuretic peptides act not only as circulating hormones, but also as autocrine and/or paracrine factors.
In this review, we focus on the recent advances in our understanding of the natriuretic peptides as a cardioprotective peptide.
Pharmacological Action of Natriuretic Peptides
|1. Renal Action|
|Dilatation of afferent arteriole and constriction of efferent arteriole|
|Relaxation of mesangial cells|
|Inhibition of renin secretion|
|Inhibition of aldosterone synthesis|
|4. Cell Growth, Proliferation|
|Inhibition of proliferation in vascular smooth muscle cells, mesangial|
|cells, cardiac fibroblasts,|
|Inhibition of hypertrophy in cardiac myocytes|
|6. Central Nervous System|
|Inhibition of drinking|
|Inhibition of salt|
|Inhibition of vasopressin secretion|
|Inhibition of ACTH secretion|
ACTH: adrenocorticotropic hormone.
Natriuretic peptides in the modulation of cardiac hypertrophy
Apart from acting as circulating hormones, ANP and BNP act locally at the sites of their synthesis. This has been suggested by the observation that in addition to being produced in the heart, these peptides are produced by many other tissues but in amounts far too low to induce endocrine effects .
However, whether the heart itself represents a site of action for natriuretic peptides was controversial. Early experiments using isolated heart preparations failed to demonstrate direct effects of ANP on cardiac performance [12,13].
Moreover, ANP had no direct effect on cardiac performance in patients with heart failure or hypertension using simultaneous left ventricular micromanometer pressure and radionuclide volume measurement techniques [14,15].
These findings appear to be in accordance with autoradiographic data suggesting that binding of natriuretic peptides in the heart is confined to the endocardium and absent in cardiac myocytes [16–18].
Subsequently, however, several experiments showed that administration of ANP altered the physiological functions of isolated myocytes, supporting the view that natriuretic peptides exist on heart muscle cells [19,20].
Furthermore, Lin et al.  showed the presence of transcripts for NPR-A as well as NPR-B and NPR-C directly by using the RT-PCR technique after single cell collections. cGMP generation in purified myocytes was stimulated only by ANP and BNP, which specifically bind to NPR-A, whereas CNP (an NPR-B agonist) was ineffective, suggesting that rat ventricular myocytes produce predominantly NPR-A.
The role of NPR-A in the myocyte has also been addressed using the recent advances of gene technology. Thus NPR-A deficiency in mice leads to marked cardiac hypertrophy , suggesting that the endogenous ANP-NPR-A system may have an inhibitory role in the regulation of cardiac cell growth.
Calderone et al.  investigated the effect of exogenously administered ANP on the cardiac cell hypertrophy induced by norepinephrine. ANP, as well as nitric oxide (NO) donor and 8-bromo-cGMP, decreased the norepinephrine-stimulated incorporation of [3H] leucine in ventricular myocytes.
They also showed that ANP inhibited an increase by the Ca2+ channel agonist BAY K8644 of norepinephrine-stimulated incorporation of [3H] leucine in myocytes. These findings indicate that ANP and NO can attenuate the effects of norepinephrine on the growth of cardiac myocytes via a cGMP-mediated inhibition of norepinephrine-stimulated Ca2+ influx, and raise the possibility that endogenous ANP suppressively regulates the development of cardiac myocyte hypertrophy.
To test the hypothesis that endogenous ANP inhibits the cardiac myocyte hypertrophy, we investigated the effects of a specific antagonist of natriuretic peptide receptors, HS-142-1, on the expression of fetal-type contractile protein genes as well as the protein synthesis in cultured cardiac myocytes . HS-142-1 increases the basal and phenylephrine-stimulated protein syntheses in a concentration-dependent manner.
This antagonist also induced a significant increase in the size of myocytes. In addition, the expression levels of the genes coding for skeletal-actin, beta-myosin heavy chain, and ANP, markers of hypertrophy were partially elevated by treatment with HS-142-1 under nonstimulated or phenylephrine-stimulated conditions.
Zaprinast, a cGMP analogue and an inhibitor for a cGMP-specific phosphodiesterase, suppressed the basal and PE-stimulated protein syntheses. These observations indicate that endogenous ANP inhibits cardiac myocyte hypertrophy under basal as well as phenylephrine-stimulated conditions, probably through a cGMP-dependent process.
Thus, ANP may play a role as an autocrine factor in the regulation of cardiac myocyte growth. Consistent with these observations, chronic treatment with either enalapril, furosemide, hydralazine, or losartan were all effective in reducing and maintaining BP at normal levels without affecting heart weight/body weight in NPR-A knockout mice .
In addition, transverse aortic constriction (TAC) resulted in a 15-fold increase in ANP expression, a 55% increase in left ventricular weight/body weight (LV/BW), dilatation of the LV, and a significant decline in cardiac function in NPR-A knockout mice.
In contrast, banded wild-type mice showed only a three-fold increase in ANP expression, an 11% increase in LV/BW, a 0.2 mm decrease in LV end diastolic dimension, and no change in fractional shortening.
These results suggest that the NPR-A system has direct antihypertrophic actions in the heart, independent of its role in BP control. Thus, the NPR-A system modulates the cardiac response to hypertrophic stimuli, such as TAC.
To examine the effect of overproduction of NPR-A in the cardiac myocytes, Kishimoto et al.  produced transgenic mice that specifically express NPR-A in the heart with myosin heavy chain promoter. They crossed NPR-A null mice and wild-type mice with the NPR-A transgenic mice.
Cardiac myocyte size was larger (approximately 20%) in NPR-A null mice than in wild-type mice at baseline. However, overexpression of the NPR-A gene in the heart reduced cardiac myocyte size in both wild type and null mice.
Coincident with the reduction in myocyte size, ANP was reduced significantly at both mRNA and peptide levels by the overexpression of NPR-A. This reduction was independent of the genotype of amimals.
Thus, cardiac overexpression of NPR-A reduced cardiomyocyte size and ventricular ANP expression in either wild-type or NPR-A null background, suggesting again a role of NPR-A/cGMP signaling pathway in the regulation of cardiac myocyte hypertrophy and ANP mRNA expression.
These findings were further confirmed by the studies in cardiomyocyte-restricted NPR-A deletion in mice . In these mice, the NPR-A gene was selectively deleted in cardiomyocytes by homologous loxP/Cre-mediated recombination.
The mice exhibited mild cardiac hypertrophy, marked increase in mRNA expression of cardiac hypertrophy markers such as ANP (5-fold), skeletal-actin (1.7-fold), and alpha-myosin heavy chain (2-fold), and increase in circulating ANP levels.
Their blood pressure was 7–10 mm Hg below normal, probably because of the elevated systemic levels and endocrine actions of ANP. Furthermore, cardiac hypertrophic responses to aortic constriction were enhanced and accompanied by marked deterioration of cardiac function.
The phenotype of these mice provides definitive evidence that an antihypertrophic regulatory circuit within cardiac myocytes directly antagonizes the hypertrophic growth response.
Thus, ANP, BNP/NPR-A/cGMP system plays an important role in modulating the molecular program of cardiac hypertrophy as an antihypertrophic factor (Fig. 2).
Multiple signaling pathways agonize and antagonize hypertrophic growth of cardiac myocytes. In the NPR-A deficient mice, a previous study showed that ligand induced cGMP elevation, and the following activation of cGMP-dependent protein kinase type I (PKG I) negatively regulate cardiac myocyte hypertrophy via inhibition of the calcineurin-nuclear factor of activated T cells (NFAT) signaling pathway .
Consistent with this finding, a recent study suggests that local ANP/NPR-A/cyclic GMP signaling counter-regulates the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK)- and calcineurin/NFAT-dependent pathways of cardiac myocyte growth in hypertensive eNOS (−/−) mice .
The action of ANP and BNP, through the NPR-A, stimulates the production of cGMP and PKG, which in turn function to antagonize hypertrophic growth within the cardiac myocyte itself.
A recent study by Tokudome et al. also showed the interaction of natriuretic peptide with hypertrophic signaling, such as endothelin-1 , by demonstrating that increased cGMP production by CNP inhibits endothelin-1-mediated Ca2+ influx, calmodulin-dependent protein kinase II activation, ERK phosphorylation, and the activation of transcription factors of GATA-4 and myocyte enhancer factor-2 in cultured cardiac myocytes.
CNP also inhibits endothelin-1 secretion from cardiac nonmyocytes. The same group of investigators very recently demonstrated that calcineurin activity, nuclear translocation of NFAT and modulatory calcineurin-interacting protein 1 gene expressions were increased in the hearts of NPR-A knock out mice compared with wild-type mice .
Blockade of calcineurin activation by FK506 significantly decreased the increased heart weight, cardiomyocyte size, collagen volume fraction, and mRNA levels of ANP, BNP, collagen, and fibronectin in NPR-A knock out mice.
Electrophoretic mobility shift assays showed that GATA4 DNA-binding activity was increased in NPR-A knock out mice, and this increase was inhibited by calcineurin blockade. These results suggest that the natriuretic peptides/cGMP system protects the heart from excessive cardiac remodeling by inhibiting the calcineurin-NFAT pathway .
Thus, natriuretic peptide/GC-A/cGMP signaling negatively regulates cardiac hypertrophy by inhibiting the variety of hypertrophic cellular signaling.
Natriuretic peptides in heart failure
As we described above, ANP, BNP, and CNP have actions of diuresis, natriuresis, vasodilation, inhibition of aldosterone synthesis and renin secretion as circulating hormones, and play an important role in regulating blood pressure and blood volume (Table 1), although there are different intensity for their actions among the three peptides.
Since ANP and BNP predominantly bind to NPR-A, whereas CNP predominantly binds to NPR-B, the physiological effects of ANP and BNP on the vasculature and kidney are stronger than those of CNP. In heart failure, it is well known that plasma levels of ANP and BNP are increased and considered to compensate heart failure [53,54].
In fact, a natriuretic peptide receptor antagonist, HS-142-1, significantly decreases urinary sodium excretion in an animal model of heart failure without changing hemodynamics, suggesting that the natriuretic peptide system exclusively compensates for heart failure through its natriuretic and diuretic action [55,56].
Furthermore, HS-142-1 increases plasma renin and aldosterone levels in an animal model of severe pacing-induced heart failure, suggesting an inhibitory role of the NPR-A system in renin and aldosterone secretion in severe heart failure .
Recently, we have investigated the contribution of the NPR-A/cGMP system in the development of heart failure using mice lacking NPR-A . Volume-overload heart failure was produced by aortocaval fistula in mice that were wild-type (+/+), heterozygous (+/−), and homozygous null (−/−) for the NPR-A gene. NPR-A (−/−) mice with aortocaval fistula had higher left ventricular end-diastolic pressure, left and right ventricular weights, lung weight, and left ventricular dimension, as well as lower fractional shortening and urinary sodium and cGMP excretion than did (+/+) mice with aortocaval fistula.
In addition, ventricular mRNA expression of natriuretic peptides and ß-myosin heavy chain was increased markedly only in (−/−) mice. Increase in the plasma levels of ANP, renin, and aldosterone was greater in (−/−) mice than in (+/+) mice.
But the levels of cGMP in response to aortocaval fistula were the same in two groups. These results provide genetic evidence that NPR-A signaling protects against heart failure induced by volume overload in mice.
link reference : https://academic.oup.com/cardiovascres/article/69/2/318/283233
More information: Timothy CR Prickett et al. Circulating products of C-type natriuretic peptide and links with organ function in health and disease, Peptides (2020). DOI: 10.1016/j.peptides.2020.170363