Gene expression, processing, and secretion of natriuretic peptides: physiologic and diagnostic implications

From Beats to Metabolism: the Heart at the Core of Interorgan Metabolic Cross Talk

The heart, once considered a mere blood pump, is now recognized as a multifunctional metabolic and endocrine organ. Its function is tightly regulated by various metabolic processes, at the same time it serves as an endocrine organ, secreting bioactive molecules that impact systemic metabolism. In recent years, research has shed light on the intricate interplay between the heart and other metabolic organs, such as adipose tissue, liver, and skeletal muscle. The metabolic flexibility of the heart and its ability to switch between different energy substrates play a crucial role in maintaining cardiac function and overall metabolic homeostasis. Gaining a comprehensive understanding of how metabolic disorders disrupt cardiac metabolism is crucial, as it plays a pivotal role in the development and progression of cardiac diseases. The emerging understanding of the heart as a metabolic and endocrine organ highlights its essential contribution to whole body metabolic regulation and offers new insights into the pathogenesis of metabolic diseases, such as obesity, diabetes, and cardiovascular disorders. In this review, we provide an in-depth exploration of the heart's metabolic and endocrine functions, emphasizing its role in systemic metabolism and the interplay between the heart and other metabolic organs. Furthermore, emerging evidence suggests a correlation between heart disease and other conditions such as aging and cancer, indicating that the metabolic dysfunction observed in these conditions may share common underlying mechanisms. By unraveling the complex mechanisms underlying cardiac metabolism, we aim to contribute to the development of novel therapeutic strategies for metabolic diseases and improve overall cardiovascular health.

Regulation of cardiomyocyte intracellular trafficking and signal transduction by protein palmitoylation

Despite the well-established functions of protein palmitoylation in fundamental cellular processes, the roles of this reversible post-translational lipid modification in cardiomyocyte biology remain poorly studied. Palmitoylation is catalyzed by a family of 23 zinc finger and Asp-His-His-Cys domain-containing S-acyltransferases (zDHHC enzymes) and removed by select thioesterases of the lysophospholipase and α/β-hydroxylase domain (ABHD)-containing families of serine hydrolases. Recently, studies utilizing genetic manipulation of zDHHC enzymes in cardiomyocytes have begun to unveil essential functions for these enzymes in regulating cardiac development, homeostasis, and pathogenesis. Palmitoylation co-ordinates cardiac electrophysiology through direct modulation of ion channels and transporters to impact their trafficking or gating properties as well as indirectly through modification of regulators of channels, transporters, and calcium handling machinery. Not surprisingly, palmitoylation has roles in orchestrating the intracellular trafficking of proteins in cardiomyocytes, but also dynamically fine-tunes cardiomyocyte exocytosis and natriuretic peptide secretion. Palmitoylation has emerged as a potent regulator of intracellular signaling in cardiomyocytes, with recent studies uncovering palmitoylation-dependent regulation of small GTPases through direct modification and sarcolemmal targeting of the small GTPases themselves or by modification of regulators of the GTPase cycle. In addition to dynamic control of G protein signaling, cytosolic DNA is sensed and transduced into an inflammatory transcriptional output through palmitoylation-dependent activation of the cGAS-STING pathway, which has been targeted pharmacologically in preclinical models of heart disease. Further research is needed to fully understand the complex regulatory mechanisms governed by protein palmitoylation in cardiomyocytes and potential emerging therapeutic targets.

Sibjotang Protects against Cardiac Hypertrophy In Vitro and In Vivo

Cardiac hypertrophy is developed by various diseases such as myocardial infarction, valve diseases, hypertension, and aortic stenosis. Sibjotang (, Shizaotang, SJT), a classic formula in Korean traditional medicine, has been shown to modulate the equilibrium of body fluids and blood pressure. This research study sought to explore the impact and underlying process of Sibjotang on cardiotoxicity induced by DOX in H9c2 cells. In vitro, H9c2 cells were induced by DOX (1 μM) in the presence or absence of SJT (1-5 μg/mL) and incubated for 24 h. In vivo, SJT was administrated to isoproterenol (ISO)-induced cardiac hypertrophy mice (n = 8) at 100 mg/kg/day concentrations. Immunofluorescence staining revealed that SJT mitigated the enlargement of H9c2 cells caused by DOX in a dose-dependent way. Using SJT as a pretreatment notably suppressed the rise in cardiac hypertrophic marker levels induced by DOX. SJT inhibited the DOX-induced ERK1/2 and p38 MAPK signaling pathways. In addition, SJT significantly decreased the expression of the hypertrophy-associated transcription factor GATA binding factor 4 (GATA 4) induced by DOX. SJT also decreased hypertrophy-associated calcineurin and NFAT protein levels. Pretreatment with SJT significantly attenuated DOX-induced apoptosis-associated proteins such as Bax, caspase-3, and caspase-9 without affecting cell viability. In addition, the results of the in vivo study indicated that SJT significantly reduced the left ventricle/body weight ratio level. Administration of SJT reduced the expression of hypertrophy markers, such as ANP and BNP. These results suggest that SJT attenuates cardiac hypertrophy and heart failure induced by DOX or ISO through the inhibition of the calcineurin/NFAT/GATA4 pathway. Therefore, SJT may be a potential treatment for the prevention and treatment of cardiac hypertrophy that leads to heart failure.

Is Marfan Syndrome Associated with Primary Structural Changes in the Left Atrium?

Marfan syndrome (MFS) is an autosomal-dominant multisystem connective tissue disorder that is based on mutations in the FBN1 gene and variably affects different organs, including the heart. In this study, we investigated cardiac function with a focus on the left atrium (LA) in a relatively large cohort of patients with MFS. After screening of 1165 patients that had been examined in our center between 2016 and 2020, 231 adult MFS patients with and without aortic operation were included in our study and compared to a healthy control group (n = 106). Cardiac function was assessed by transthoracic echocardiography and NT-proBNP was used as a secretory marker. Most (94.8%) of the patients received genetic testing. Left ventricular function was within normal ranges and not impaired. Interestingly, we found that LA size and secretory activity were increased in MFS patients, despite normal left ventricular filling pressures. This finding was even more pronounced in MFS patients with prior aortic surgery. A correlation between LA size or NT-proBNP levels and the type of pathogenic FBN1 variant could not be identified. Right ventricular function and right atrial size were increased only in MFS patients that had undergone aortic surgery. In conclusion, these findings suggest that MFS leads to structural changes in the LA that are not solely resulting from left ventricular dysfunction, but probably can be considered a primary pathology of MFS.

Cardiac natriuretic peptides

Investigations into the mixed muscle-secretory phenotype of cardiomyocytes from the atrial appendages of the heart led to the discovery that these cells produce, in a regulated manner, two polypeptide hormones - the natriuretic peptides - referred to as atrial natriuretic factor or atrial natriuretic peptide (ANP) and brain or B-type natriuretic peptide (BNP), thereby demonstrating an endocrine function for the heart. Studies on the gene encoding ANP (NPPA) initiated the field of modern research into gene regulation in the cardiovascular system. Additionally, ANP and BNP were found to be the natural ligands for cell membrane-bound guanylyl cyclase receptors that mediate the effects of natriuretic peptides through the generation of intracellular cGMP, which interacts with specific enzymes and ion channels. Natriuretic peptides have many physiological actions and participate in numerous pathophysiological processes. Important clinical entities associated with natriuretic peptide research include heart failure, obesity and systemic hypertension. Plasma levels of natriuretic peptides have proven to be powerful diagnostic and prognostic biomarkers of heart disease. Development of pharmacological agents that are based on natriuretic peptides is an area of active research, with vast potential benefits for the treatment of cardiovascular disease.

Procholecystokinin expression and processing in cardiac myocytes

The mammalian heart is by now an established endocrine organ whose myocytes in a regulated manner release atrial and ventricular natriuretic peptides (ANP and BNP). But like other hormone-producing cells in classic endocrine organs, the cardiac myocytes also express genes of additional peptide hormones. One such hormone gene is that of the well-known pleiotropic gut-brain peptide system, cholecystokinin (CCK), which is expressed at mRNA and protein levels in both atrial and ventricular cardiac myocytes. The posttranslational processing of proCCK in the myocytes, however, deviates substantially from that of other CCK-producing cells. Hence, the predominant cardiac proCCK product is devoid of the N-terminal 1-24 fragment, and besides O-sulfated at three C-terminal tyrosyl residues (Y₇₆, Y₉₀, and Y₉₂). Moreover, carboxyamidated CCK peptides are present only in very low trace amounts (≤0.1%) in comparison with the truncated and triple-sulfated proCCK fragment. The present review first summarizes present knowledge about the wide-spread expression of the CCK system in mammals, and then discusses the possible function and biomarker role of the specific cardiac proCCK variant. The review concludes that the many unsettled questions about the specific cardiac expression cascade as well as the functional and diagnostic roles of cardiac CCK are worth pursuing.

Serum Natriuretic Peptides as Differential Biomarkers Allowing for the Distinction between Physiologic and Pathologic Left Ventricular Hypertrophy

Given the proven utility of natriuretic peptides as serum biomarkers of cardiovascular maladaptation and dysfunction in humans and the high cross-species sequence conservation of atrial natriuretic peptides, natriuretic peptides have the potential to serve as translational biomarkers for the identification of cardiotoxic compounds during multiple phases of drug development. This work evaluated and compared the response of N-terminal proatrial natriuretic peptide (NT-proANP) and N-terminal probrain natriuretic peptide (NT-proBNP) in rats during exercise-induced and drug-induced increases in cardiac mass after chronic swimming or daily oral dosing with a peroxisome proliferator-activated receptor γ agonist. Male Sprague-Dawley rats aged 8 to 10 weeks were assigned to control, active control, swimming, or drug-induced cardiac hypertrophy groups. While the relative heart weights from both the swimming and drug-induced cardiac hypertrophy groups were increased 15% after 28 days of dosing, the serum NT-proANP and NT-proBNP values were only increased in association with cardiac hypertrophy caused by compound administration. Serum natriuretic peptide concentrations did not change in response to adaptive physiologic cardiac hypertrophy induced by a 28-day swimming protocol. These data support the use of natriuretic peptides as fluid biomarkers for the distinction between physiological and drug-induced cardiac hypertrophy.

INTERACTING DISCIPLINES: Cardiac natriuretic peptides and obesity: perspectives from an endocrinologist and a cardiologist

Since their discovery in 1981, the cardiac natriuretic peptides (cNP) atrial natriuretic peptide (also referred to as atrial natriuretic factor) and brain natriuretic peptide have been well characterised in terms of their renal and cardiovascular actions. In addition, it has been shown that cNP plasma levels are strong predictors of cardiovascular events and mortality in populations with no apparent heart disease as well as in patients with established cardiac pathology. cNP secretion from the heart is increased by humoral and mechanical stimuli. The clinical significance of cNP plasma levels has been shown to differ in obese and non-obese subjects. Recent lines of evidence suggest important metabolic effects of the cNP system, which has been shown to activate lipolysis, enhance lipid oxidation and mitochondrial respiration. Clinically, these properties lead to browning of white adipose tissue and to increased muscular oxidative capacity. In human association studies in patients without heart disease higher cNP concentrations were observed in lean, insulin-sensitive subjects. Highly elevated cNP levels are generally observed in patients with systolic heart failure or high blood pressure, while obese and type-2 diabetics display reduced cNP levels. Together, these observations suggest that the cNP system plays a role in the pathophysiology of metabolic vascular disease. Understanding this role should help define novel principles in the treatment of cardiometabolic disease.

Protective effect of rhBNP on intestinal injury in the canine models of sepsis

Sepsis is the leading cause of death in the intensive care units worldwide. Proinflammatory cytokines such as TNF (tumor necrosis factor)-α and IL (interleukin)-6 mediate the pathogenesis of septic shock characterized by hemodynamic instability and end-stage multi-organ functional failure. Brain natriuretic peptide (BNP) has been used as a diagnostic and prognostic biomarker in the cardiovascular disorders. Most recently, plasma level of BNP has also been used to predict outcomes of critical illnesses including sepsis. We have recently reported that human recombinant BNP (rhBNP) could protect lungs from acute proinflammatory injury in response to LPS-injection. In the current study, using LPS (lipopolysaccharide)-induced canine sepsis models, we further investigated the effect of rhBNP on intestinal injury and its potential mechanisms. We have found that rhBNP (5μg or 10μg/kg weight) could significantly reduce intestinal tissue damage in response to LPS-injection in the dog sepsis models through down-regulating proinflammatory cytokines TNF-α and IL-6 (5-10 fold decrease compared to LPS-injection only group) by a mechanism of suppressing IκB phosphorylation and NF-κB expression. These findings suggest that BNP protect intestinal tissues from endotoxin-induced hyper-inflammatory injury and thus, may be used as therapeutic agents for sepsis.

The heart as an endocrine organ

The concept of the heart as an endocrine organ arises from the observation that the atrial cardiomyocytes in the mammalian heart display a phenotype that is partly that of endocrine cells. Investigations carried out between 1971 and 1983 characterised, by virtue of its natriuretic properties, a polypeptide referred to atrial natriuretic factor (ANF). Another polypeptide isolated from brain in 1988, brain natriuretic peptide (BNP), was subsequently characterised as a second hormone produced by the mammalian heart atria. These peptides were associated with the maintenance of extracellular fluid volume and blood pressure. Later work demonstrated a plethora of other properties for ANF and BNP, now designated cardiac natriuretic peptides (cNPs). In addition to the cNPs, other polypeptide hormones are expressed in the heart that likely act upon the myocardium in a paracrine or autocrine fashion. These include the C-type natriuretic peptide, adrenomedullin, proadrenomedullin N-terminal peptide and endothelin-1. Expression and secretion of ANF and BNP are increased in various cardiovascular pathologies and their levels in blood are used in the diagnosis and prognosis of cardiovascular disease. In addition, therapeutic uses for these peptides or related substances have been found. In all, the discovery of the endocrine heart provided a shift from the classical functional paradigm of the heart that regarded this organ solely as a blood pump to one that regards this organ as self-regulating its workload humorally and that also influences the function of several other organs that control cardiovascular function.

Myocardial expression level of neural cell adhesion molecule correlates with reduced left ventricular function in human cardiomyopathy

BACKGROUND

Recently, we screened for cardiac genes induced by metabolic stress and identified neural cell adhesion molecule (NCAM) as a candidate. This study aimed to clarify the expression pattern of NCAM in human cardiomyopathy.

METHODS AND RESULTS

A total of 64 cardiac tissue samples of patients with dilated cardiomyopathy were dichotomized according to the immunohistochemically determined signal intensity of NCAM staining (NCAM-high and NCAM-low groups). Clinical and hemodynamic data of the patients were compared between the 2 groups. Fibrosis area, left ventricular end-diastolic volume index, and left ventricular diastolic pressure were greater in the NCAM-high group (22.8% versus 11.6%, P<0.05; 130.3±57.6 versus 104.8±31.7 mL/m(2), P<0.05; 14.3±8.0 versus 8.8±4.7 mm Hg, P<0.005; respectively). Incidence of cardiac death and admission for worsening heart failure was higher in the NCAM-high group during a follow-up of 6.3 years (log-rank P<0.05). Another 18 tissue samples were analyzed to determine the relationships between expression level of NCAM and major metabolic genes as well as hemodynamic parameters. The mRNA level of NCAM correlated with the serum (r=0.58; P=0.01) and mRNA levels (r=0.61; P=0.008) of brain-derived natriuretic peptides. It was also correlated with the mRNA levels of proliferator-activated receptor-γ coactivator-1 α (r=0.69; P=0.002) and the nuclear respiratory factor 1 (r=0.74; P<0.001).

CONCLUSIONS

Expression of NCAM was associated with worsening hemodynamic parameters and major metabolic genes. Together with our previous findings, these data support the involvement of NCAM in left ventricular remodeling, revealing new insights into the pathophysiology of heart failure.

Brain natriuretic Peptide production and secretion in inflammation

Gene expression and secretion of the cardiac polypeptide hormones atrial natriuretic factor (ANF) and brain natriuretic peptide (BNP) are simultaneously upregulated in various cardiac disorders such as congestive heart failure, ischemic heart disease, and hypertensive heart disease, in which hemodynamic or neuroendocrine changes are key components in the progression of disease. However, during acute cardiac allograft rejection, plasma BNP levels are increased but not those of ANF. Successful treatment of the rejection episode decreases the elevated plasma BNP to prerejection values suggesting that substances related to inflammation may selectively influence BNP gene expression. Indeed, cytokines such as TNFα and IL-1β selectively stimulate cardiac BNP at the transcriptional and translational levels in cardiomyocyte cultures without affecting ANF. This selective BNP increase is seen in vivo, in addition to acute cardiac allograft rejection, in several circumstances where inflammation significantly contributes to the pathogenesis of disease such as in sepsis and in acute myocarditis.

Uncoordinated regulation of atrial natriuretic factor and brain natriuretic peptide in lipopolysaccharide-treated rats

We investigated the expression and secretion of the natriuretic peptides (NPs) ANF and BNP in lipopolysaccharide (LPS)-induced sepsis and its association with cytokines and other biologically active substances. LPS treatment increased plasma levels of ANF and BNP. The latter increase was larger than the increase in plasma ANF. LPS also increased cardiac content and gene expression of BNP but not of ANF. LPS treatment significantly increased gene expression cytokines, chemokines and proteases, which significantly correlated with BNP gene expression. SB203580, a p38 MAP kinase inhibitor, inhibited the elevation of BNP in plasma. The present work suggests that during inflammation, BNP gene expression and secretion is uniquely related to changes in gene expression in the absence of hemodynamic changes and hence differentiates ANF and BNP as biomarkers of cardiac disease.

Endogenous vascular synthesis of B-type and C-type natriuretic peptides in the rainbow trout

In mammals, natriuretic peptides (NPs) lower blood pressure, reduce blood volume and broadly inhibit cardiovascular remodeling. NPs are often referred to as cardiac hormones, though they also have integral roles in regulating vascular tone, endothelial remodeling and inhibiting vascular smooth muscle cell hypertrophy. Two NPs [atrial (ANP) and C-type (CNP)] have been identified as endogenous constituents in the vasculature of mammals, though such a phenomenon has not previously been described in fishes. Here we describe the endogenous production of B-type NP (BNP) and CNP in multiple blood vessels of the rainbow trout. Western blot analysis showed pro-BNP and pro-CNP production in the efferent branchial artery, celiacomesenteric artery, ventral aorta and anterior cardinal vein. The detection of pro-BNP and pro-CNP was also supported by MALDI-TOF mass spectrometry analysis of NP-enriched tissue extracts. Although vascular pro-peptide levels of BNP and CNP were quantitatively quite comparable to those found in reference tissues (the atrium for BNP and brain for CNP), mRNA levels of these NPs in the vasculature were greatly reduced as determined by quantitative PCR. When the evolutionarily conserved vascular NP (CNP) was infused into un-anesthetized trout, it reduced central venous pressure and mean circulatory filling pressure. CNP also decreased cardiac output via a reduction in preload. The presence of endogenous NP production in the trout vasculature and potent in vivo hypotensive effects further support the numerous functional similarities between teleost and mammalian NP systems.

Thirty years of research on atrial natriuretic factor: historical background and emerging concepts

The discovery of the natriuretic properties of atrial muscle extracts pointed to the existence of an endocrine function of the heart that is now known to be mediated by the polypeptide hormones atrial natriuretic factor (ANF) and brain natriuretic peptide (BNP). On the basis of such a finding, approximately 27 000 publications to date have described a wide variety of biological properties of the heart hormones as well as their application as therapeutic agents and biomarkers of cardiac disease. Stimulation of secretion of ANF and BNP from the atria is mediated through mechanisms involving G proteins of the G(q) or G(o) types. We showed that the latter type underlies the transduction of muscle stretch into stimulated secretion and that it is more highly abundant in atria than in ventricles. The Gα(o)()-1 subunit appears to play a key role in the biogenesis of atrial granules and in the intracellular targeting of their contents. Protein interaction studies using a yeast two-hybrid approach showed interactions between Gα(o)()-1, proANF, and the intermediate conductance, calcium-activated K(+) channel SK4. Pharmacological inhibition of this channel decreases ANF secretion. Unpublished studies using in vitro knockdowns suggest interdependency in granule protein expression levels. These studies suggest previously unknown mechanisms of intracellular targeting and secretion control of the heart hormones that may find an application in the therapeutic manipulation of circulating ANF and BNP.

Clinical implications of defective B-type natriuretic peptide

Our understanding of the natriuretic peptide system continues to evolve rapidly. B-type natriuretic peptide (BNP), originally thought to be a simple volume-regulating hormone that is produced in response to cardiac stretch, has been shown to also play important roles in modulating bronchodilation, endothelial function, and cardiac remodeling. Recent data demonstrate that elevated levels of BNP in patients with heart failure do not represent a simple ratcheting up of normal production in response to increased stimulus. Instead, we now know that chronic stimulation of BNP synthesis induces a reversion to fetal gene expression, resulting in production of high molecular weight forms of BNP that are functionally deficient. Standard point-of-care BNP assays are immunoassays that will detect any molecule containing the target epitopes. Consequently, these assays cannot distinguish between defective, high molecular weight forms of BNP and normal, physiologically active BNP. In 2 separate evaluations, mass spectroscopy detected little, if any, normal BNP in patients with heart failure, despite the appearance of high circulating levels of immunoreactive BNP (iBNP) using commercial assays. Therefore, these commercial assays should be considered to be only an indication of myocardial stress. They do not measure physiologic BNP activity. This accounts for the "BNP paradox," namely, that administration of exogenous recombinant human BNP (rhBNP, nesiritide) has substantial clinical and hemodynamic impact in the presence of high levels of circulating iBNP using commercial assays. In addition to its short-term hemodynamic impact, rhBNP may have other important effects in this setting, and further investigation is warranted.

Dysregulation of pulsatility in aging IV. Pulsatile signaling and cardiovascular aging: functions and regulation of natriuretic peptide signaling

Atrial and brain natriuretic peptides (ANP and BNP) are cardiac hormones connecting heart and kidney and playing a key role in hydromineral and hemodynamic homeostasis. In contrast with the renin-angiotensin-aldosterone system, circulating ANP and BNP are not temporally related with rapid eye movement (REM)-nonREM sleep cycles, autonomic activity, or blood pressure. Cardiac natriuretic peptides are secreted in a pulsatile manner, with short periods of 20-48min, in normal as well as in pathological conditions. The frequency of pulses seems to be unchanged with aging, whereas the absolute amplitude of the oscillations seems to increase, most likely as a result of an increase in the plasma hormone levels observed in elderly. Enhanced cardiac secretion and decreased degradation partly explain the higher ANP and BNP concentrations observed in elderly. Despite higher levels, the natriuretic system seems to loose efficiency at the renal site in elderly. This more probably relies on reduced target organ responsivity and not on deeply altered hormone secretion. Here we review the impact of aging on the renal effects of the natriuretic peptides, and point out the lack of knowledge on the precise interactions between the ultradian rhythms of the systems involved in salt and water balance in elderly. Additional studies focusing on potential age-induced alterations of the intracellular signaling pathway are now needed.