Natriuretic Peptides: Historical Background

The discovery of natriuretic peptides (NPs) changed the classical paradigm of the heart as solely a pump and developed the novel concept of the heart as an endocrine organ. The current and potential applications of NPs in clinical practice and medical research are endless. Currently, NPs remain the “gold standard” for the diagnosis and prognosis of heart failure (HF) and the evaluation of HF treatment efficacy. The NP story began in the 1950s when Gauer and his coworkers reported that distension of the left atrium after the expansion of an intra-atrial balloon resulted in a prompt diuresis linking this physiologic effect to the changes in circulating blood volume. Concurrently, Kisch first described specific dense homogenous granules in atria using a new electron microscopy technique, and Jamieson and Palade revealed the secretory nature of these granules. Later, Poche found that the number of granules depends on water intake. Indeed, Marie and colleagues confirmed that salt and water intake increases the numbers of granules in the atrial cardiomyocytes. In cross-circulation experiments in canines, De Wardner described the humoral nature of a substance with natriuretic properties. Although the presence of the “specific atrial granules” and their secretory phenotype were confirmed, the function of these storage granules remained a mystery.

In 1981 de Bold et al. performed the groundbreaking experiment that showed that the injection of atrial homogenates causes rapid renal sodium and water excretion and the reduction of blood pressure. Interestingly, the manuscript describing for the first time natriuretic, diuretic, and vasodilatory properties of atrial natriuretic peptide (ANP) was initially rejected by the journal to which the study was submitted. Following this landmark study several individual groups purified, sequenced, and synthesized ANP. Therefore ANP became the first peptide hormone isolated from the heart ( Fig. 9.1 ) . B-type NP (BNP), originally named brain-type natriuretic peptide, was first isolated from porcine brain tissues in 1988. Further studies revealed that BNP is also synthesized by both atrial and ventricular cardiomyocytes and like ANP is responsive to mechanical stretch. The discovery of ANP and BNP is considered a fundamental advance in the field of cardiovascular biology and has tremendously impacted HF treatment and diagnostics. Urodilatin (URO) is a molecular form of ANP derived from the ANP prohormone proANP and processed in the kidney, resulting in a more renal-specific NP. C-type NP (CNP), a third peptide in NP family, was first extracted from porcine brain and then from endothelial cells. CNP, like ANP, URO, and BNP, has a similar but distinct 17–amino acid disulfide bridge ring and is synthesized in vascular endothelium. Finally, the concept of the heart as an endocrine organ was importantly solidified by the landmark work of Murad’s group, which identified 3′,5′-cyclic guanosine monophosphate (cGMP) as the second messenger of ANP, and seminal studies identified particulate guanylyl cyclase receptors (pGC-A and pGC-B) as the targets of ANP, URO, BNP, and CNP.

Natriuretic peptides.

Atrial natriuretic peptide (ANP) , B-type natriuretic peptide (BNP) , urodilatin, and Dendroaspis natriuretic peptide (DNP) bind to guanylyl cyclase A (GC-A) . C-type natriuretic peptide (CNP) binds to guanylyl cyclase B (GC-B) , ANP, BNP, CNP, and urodilatin are found in humans; DNP is found in the venom of the green mamba snake.

The therapeutic effects of NPs have been widely applied in patients with acute decompensated heart failure (ADHF) (see Chapter 36 ). Synthetic ANP (Carperitide) and synthetic BNP (Nesiritide) have been approved in several countries for the treatment of HF. Carperitide was approved for the clinical management of ADHF in Japan in 1995. Nesiritide was considered as a first line therapeutic agent for ADHF. Initial clinical trials revealed that Nesiritide significantly led to beneficial hemodynamic and natriuretic effects, reduced pulmonary capillary wedge pressure, and increased cardiac output. The Natrecor Study Group has also confirmed the beneficial hemodynamic effects of Nesiritide in patients with ADHF. Moreover, Nesiritide was associated with significantly lower mortality than dobutamine in the PRECEDENT (Prospective, Randomized Evaluation of Cardiac Ectopy with Dobutamine or Natrecor Therapy) study, caused a faster and greater improvement in pulmonary capillary wedge pressure compared with intravenous nitroglycerin. Due to these results of the clinical trials, Nesiritide was approved for the treatment of ADHF in 2001 by the US Food and Drug Administration (FDA) and marketed under the trade name Natrecor. However, meta-analysis of Natrecor clinical trials by Sackner-Bernstein raised concerns about Nesiritide-associated mortality and renal dysfunction in patients with ADHF. These reports significantly contributed to a decline in both prescriptions and Nesiritide sales. The controversy between defenders of nesiritide, including its manufacturer, Johnson & Johnson, and their opponents was finally resolved by the Acute Study of Clinical Effectiveness of Nesiritide in Decompensated Heart Failure (ASCEND-HF) trial. The trial included 7141 patients with ADHF who received continuous intravenous infusion of Nesiritide or placebo. Results showed that there were no significant differences in the incidence of death or renal injury. On the other side, the trial also showed no significant differences in the end point of dyspnea. In addition, standard doses of Nesiritide caused symptomatic and asymptomatic hypotension. Finally, Nesiritide was not recommended for routine treatment of ADHF. This information was important for further development of chimeric NP designs specifically for the treatment and prevention of HF.

General Overview of Natriuretic Peptide System

The pharmacologic actions of NPs are based on interaction between specific molecular amino acid sequences in naturally occurring NPs and respective receptors (see Fig. 9.1 and Table 9.1 ). NPs all possess a similar but distinct 17–amino acid ring formed by an intramolecular disulfide bridge. This ring structure is essential for exerting biologic activity. ANP is synthesized and stored within atrial cardiomyocytes as a 151–amino acid preproANP peptide which is cleaved to generate 126–amino acid proANP. ANP is stored in atrial cardiomyocytes in granules as proANP. Studies have established that the atrium is the major site of ANP synthesis in which the level of ANP mRNA expression is 100-fold higher than in the ventricles. ANP is synthesized and stored within atrial cardiomyocytes as a 151–amino acid preproANP peptide which is cleaved to generate 126–amino acid proANP. When atrial myocytes are stretched, proANP is cleaved by corin, a myocardium-specific type II transmembrane protease, into the nonbiologically active N-terminus of pro-ANP and the biologically active C-terminal 28–amino acid ANP. Corin is also involved in BNP processing, which is initially synthesized as 134-amino acid preproBNP. After processing by corin or intracellular Golgi-localized protease furin, a low level of proBNP can be stored along with ANP in the atrial-specific granules. In contrast to ANP, BNP is predominantly secreted by ventricular myocardium and is constitutively released. Both ANP and BNP are secreted in response to myocardial stretch induced by volume or hemodynamic overload. Upon release into the circulation, proBNP is converted to a 76–amino acid inactive N-terminal fragment of proBNP and biologically active C-terminal 32–amino acid BNP. The lack of ANP in genetically modified mice may lead to chronic hypertension, cardiac dilation, hypertrophy, fibrosis, and HF. Mice lacking BNP do not develop cardiac hypertrophy or hypertension but display cardiac fibrosis that leads to ventricular stiffness, altered chamber compliance, and contractile dysfunction. In the cardiovascular system, CNP can be produced in the myocardium but at low levels, and highly expressed in vascular endothelium. PreproCNP is a 126–amino acid peptide, which is cleaved by a signal peptidase to form 103–amino acid proCNP. The latter is cleaved by furin to produce the biologically active 22–amino acid CNP and a larger 53–amino acid inactive fragment. Defects in CNP production lead to endothelial dysfunction, hypertension, atherogenesis, and aneurysm formation, and reduced ability of CNP to activate pGC-B leads to hypertension, tachycardia, and impaired left ventricular systolic function. Plasma CNP is also elevated in patients with HF, although its level is lower compared with ANP and BNP. Due to increased expression levels of pGC-B in HF, CNP may exert cardioprotective action against myocardial injury. Less information is known about the processing of URO.

As introduced previously, NPs act via transmembrane GC-coupled receptors. The receptors pGC-A and pGC-B are composed of an extracellular domain, which binds endogenous ligands, a single transmembrane region, a kinase homology domain, and intracellular catalytic domain with GC activity ( Fig. 9.2 ) . The pGC-A receptor mediates the physiologic action of ANP and BNP by generating the intracellular secondary messenger cGMP, which acts on cGMP-dependent protein kinase, or protein kinase G (PKG), cGMP-gated ion channels, and cGMP-regulated phosphodiesterases (PDEs). Studies have established that pGC-A is mostly expressed in kidneys, vascular smooth muscle, endothelium, heart, and adrenals, as well as adipocytes. The pGC-B receptor is predominantly expressed in the brain, kidney, heart, lung, and bone.

Natriuretic peptide, receptor, and biologic targets: Particulate (pGCA) guanylyl cyclase signaling pathways. Natriuretic peptides bind to GC-A and/or GC-B (membrane-bound pGCs) and activate the signaling pathways of (GC) cyclic guanosine monophosphate (cGMP) . Once the intracellular concentration of cGMP increases, cGMP-gated cation channels, cGMP-dependent protein kinases, and phosphodiesterases generate important biologic responses in different tissues. ANP , Atrial natriuretic peptide; BNP , B-type natriuretic peptide; CD-NP , CD natriuretic peptide; CNP , C-type natriuretic peptide; CU-NP , CU natriuretic peptide; DNP , Dendroaspis natriuretic peptide; GC-A , particulate guanylyl cyclase A; GC-B , particulate guanylyl cyclase B; GTP , guanosine triphosphate; NPR, natriuretic peptide receptor; PDEs , phosphodiesterases; PKGs , cGMP-dependent protein kinases; URO , Urodilatin.

NPR-C mediates the clearance of NPs through an internalization and degradation process. NPR-C is expressed in several tissues, including kidneys, endothelium, heart, lungs, and adrenals. Although NPR-C is mainly known as a receptor involving in NP clearance from the circulation by receptor-mediated endocytosis, key studies have reported NPR-C–mediated inhibition of endothelial and vascular smooth muscle cell (VSMC) proliferation. Moreover, NPR-C may be involved in modulating coronary endothelial cell permeability and may be a target of CNP in modulating vascular tone. In addition to proteolytic inactivation and NPR-C–mediated clearance, enzymatic pathways clear the NPs. Specifically, NPs can be cleaved by the zinc metalloprotease insulin-degrading enzyme and by the membrane-bound zinc-dependent enzyme neutral endopeptidase or neprilysin (NEP), which plays a critical role both in regulating NP levels and also serving as a therapeutic target.

From a physiologic perspective, a growing concept is that the NP/pGC/cGMP system plays an important role in the long-term regulation of sodium and water balance and blood pressure homeostasis. An additional new role for the NP system is in metabolic regulation. In key genetic epidemiology studies, genetic variants of the ANP and BNP genes in which circulating ANP or BNP may be elevated, the phenotype is one of lower blood pressure, reduced risk for hypertension, and protection from obesity and metabolic syndrome. From studies in genetically altered mice and physiologic and pharmacologic studies in animals and humans, the key biologic properties of NPs are summarized in Table 9.2 and include inhibition of myocardial hypertrophy, organ fibrosis, maintenance of the endothelial barrier, vasorelaxation, natriuresis including an increase in glomerular filtration rate (GFR) and a decrease in proximal tubule reabsorption, suppression of aldosterone, and lipolysis.

Natriuretic Peptide System in Heart Failure: Pathophysiology, Diagnostics, and Treatment

HF in advanced stages is characterized by renal retention of sodium and water together with systemic and renal vasoconstriction and myocardial remodeling. The disease involves the prolonged activation of the sympathetic nervous system (SNS) (see Chapters 6 and 13 ) and renin-angiotensin-aldosterone system (RAAS) (see Chapter 5 ) that together contribute to the worsening of HF. The NP/pGC/cGMP system plays a counterregulatory role to all of these deleterious hallmarks of chronic HF.

Although the NP system mediates beneficial actions in HF, especially in the early stages of HF, it is important to recognize that an “NP resistance” or “NP paradox” exists in which, despite elevation of circulating NPs in HF, there is overt HF with salt and water retention, activation of RAAS and SNS, and progressive myocardial failure and remodeling. This “resistance” or “paradox” in part is a result of abnormal molecular forms of circulating NPs, as well as increased activity of NEP and NPR-C, which may contribute to decreased efficacy of NPs. Importantly, reductions of NP convertases affect proNP processing and contribute to HF progression by reducing the processing of proNPs to biologically active NPs. Ichiki and colleagues found in a canine model of HF that the level of corin gene and protein expression decreased in HF. Tripathi et al. showed in a mouse model of cardiomyopathy that cardiac corin gene expression was reduced in experimental HF and remained low during HF progression, whereas cardiac ANP and BNP gene expression and plasma levels increased only at the terminal stage of HF. In human HF, Dong and colleagues confirmed that plasma corin levels were significantly reduced in HF. Similarly, Ibebuogu and associates revealed that patients with HF have significantly low levels of plasma corin and higher levels of plasma uncleaved proANP. This implies that impaired cleavage of proANP and reduced effectiveness of corin may contribute to the progression of HF. In addition, Zhou et al. suggested that low level of corin might be associated with higher risk of major adverse cardiovascular events, HF progression, and poor prognosis. Interestingly, the authors also stated that plasma corin levels are significantly reduced in female patients. This may indicate the presence of sex differences in the pathogenesis of HF as it relates to NPs. This study also suggested that the patients with higher corin-mediated processing of NP precursors might have better outcomes. It should also be noted that proBNP, also due to reduced processing by corin, is the predominant molecular form of immunoreactive BNP detected by clinical assays for BNP. Importantly, Huntley et al. recently tested the bioactivity of proBNP in activating the pGC-A receptor in human kidney cells. They reported that proBNP was markedly reduced in its potency to activate pGC-A and the second messenger cGMP.

Additional mechanisms for impaired NP bioactivity in HF also include furin-mediated processing of NPs and NP glycosylation. Nagai-Okatani and Minamino were the first who reported the mechanism of O-glycosylation of human proBNP. This study demonstrated that the upregulation of mucin-type O-glycosylation may contribute to the increase in sialylated O-glycan at Thr71, close to the processing site of proBNP, resulting in the inhibition of the binding of furin and the conversion of proBNP to bioactive BNP.

It is now recognized that patients with HF may have higher concentration of circulating soluble NEP. NEP is an integral type II, membrane-bound, zinc-dependent endopeptidase. NEP is found in high concentrations in kidneys and also in brain, lung, endothelial, and blood cells. NEP cleaves and degrades ANP, BNP, and CNP, as well as bradykinin, substance P, adrenomedullin, and vasoactive intestinal peptide. The enzyme has an ectodomain, presenting the catalytic site, a transmembrane domain, and a short intracellular domain. NEP cleaves peptides with a molecular weight at 3000 Da or less and attacks peptides at the amino side of hydrophobic amino acids, with a preference for Phe or Leu, releasing di or tripeptides from the carboxy-terminus of peptides. Thus enhanced NEP activity increases enzymatic clearance of the NPs, thus limiting their beneficial actions. From a biologic prospective, the clearance and degradation of NPs by NEP has led to the concept that the inhibition of NEP may enhance the activity of NPs and result in better regulation of sodium and water balance in HF. Previously, Martin and coworkers demonstrated that chronic oral administration of the NEP inhibitor (NEPi) Candoxatril prolongs the compensated phase of experimental early chronic HF. The treatment with NEPi led to enhanced renal actions of NPs, prevented sodium retention, and suppressed the activation of aldosterone. However, the inhibition of NEP alone not only increases NP levels, but also raises the levels of circulating vasopressors, angiotensin II and endothelin I, that weaken the beneficial effect of the NPs. As a consequence, NEPi alone had little effect in patients with HF. Therefore NEPi together with blockers of RAAS can potentially improve the treatment efficacy and prognosis of HF (see also Chapter 37 ) .

Omapatrilat (OMA) was the first drug in this group that was extensively studied in several randomized clinical trials. Specifically, OMA was a single small molecule that could inhibit NEP but also angiotensin-converting enzyme (ACE), becoming a dual NEP/ACE inhibitor (NEPi/ACEi). Inhibition of Metalloprotease by Omapatrilat in a Randomized Exercise and Symptoms Study in Heart Failure (IMPRESS) showed an improved hemodynamic profile with OMA in patients with HF with reduced ejection fraction (HFrEF) when compared with ACEi, lisinopril, alone. The Omapatrilat Versus Enalapril Randomized Trial of Utility in Reducing Events (OVERTURE) trial showed no significant difference between OMA and enalapril in reducing the primary end point of combined all-cause mortality and hospitalizations in patients with New York Heart Association (NYHA) class II to IV HF, although post hoc analysis revealed a strong signal for improvement with OMA. However, treatment with OMA caused a higher incidence of angioedema (0.8%) compared with the enalapril group (0.5%).

Although both ACEi and angiotensin receptor blockers (ARBs) are considered first-line drugs in HF, ARBs do not inhibit the degradation of kinins, therefore reducing the risk of angioedema. It was considered logical to develop the next OMA-like drug based on the combination of ARBs and NEPi, a class that is now called ARNI. The novel drug Entresto (LCZ696), a combination of the ARB valsartan and the NEPi sacubitril, has been first compared with valsartan alone in the Prospective comparison of ARNI with ARB on Management of Heart Failure with Preserved Ejection Fraction (PARAMOUNT) trial in patients with HF with preserved ejection fraction (HFpEF) (see Chapter 3 9 ). The treatment with LCZ696 caused decreases in left atrial size and volume, as well as in more significant decline in N-terminal proBNP (NT-proBNP). The Prospective Comparison of LCZ696 with ARB Global Outcome in HF with Preserved Ejection Fraction (PARAGON-HF) trial is currently underway and aims to evaluate the effect of LCZ696 compared with valsartan in the reduction of cardiovascular death and hospitalizations in patients with HFpEF. In the landmark study Prospective Comparison of ARNI with ACEI to Determine Impact on Global Mortality and Morbidity in Heart Failure (PARADIGM-HF) led by McMurray and associates in collaboration with scientists at Novartis, LCZ696 was compared with enalapril in patients with HFrEF. The study was stopped early because of significant reduction in HF hospitalization and a significant survival rate in the ARNI group (see Chapter 37 ). Although there was a nonsignificant trend for an increase for symptomatic hypotension and angioedema, this did not lead to serious cases or drug discontinuation.

Although increased circulating NPs may have biologic significance as a compensatory mechanism in HF to oppose the deleterious RAAS and SNS pathways, plasma levels of BNP and NT-proBNP are powerful biomarkers for prognosis and risk stratification of HF, in addition to aiding in HF diagnosis. The levels of BNP and NT-proBNP are strongly correlated to clinical status and survival rate of patients. Moreover, BNP and NT-proBNP may play a role in the medical management of HF both in guiding therapy and assessing the efficacy of therapy. This is discussed in greater detail in Chapter 33 , which focuses on biomarkers and precision medicine.

The fact that the NP system counteracts the adverse effects of RAAS and other neurohumoral systems in HF as a key compensatory mechanism together with possible impaired bioavailability, as discussed previously, provides a powerful rationale for the development of NP-based HF therapy. Increasing the activity of NP or reducing their degradation may achieve a more favorable balance between RAAS/SNS and the NP system and therefore mediate beneficial actions on cardiorenal function and structure positively impacting outcomes, as was seen in the PARADIGM-HF trial. Furthermore, the use of long-term NP therapy has also had positive outcomes in chronic HF. Specifically, Chen et al. reported that 8 weeks of twice-daily administration of subcutaneously administered BNP to patients with NYHA class III HF resulted in symptom improvement, reduced plasma renin activity, and improved myocardial structure and function with preservation of renal function. Most recently, novel peptide-based NP therapies have emerged that go beyond the properties of the naturally occurring NPs and possess greater resistance to degradation by NEP. This is discussed next.

Novel Designer Natriuretic Peptides in the Therapy Of Heart Failure

Advances in peptide engineering have resulted in the design, synthesis, and investigation of chimeric designer NPs. It is increasingly recognized that peptide therapeutics permit a more targeted approaches through well-characterized receptors and molecular pathways, thus avoiding off-target actions associated with small molecules. Peptides also possess larger surface area than small molecules, which may optimize receptor activation. However, a limitation to peptide therapies is rapid degradation such as NEP, therefore limiting the bioavailability of peptides as compared with small molecules.

Breakthroughs in peptide engineering have accelerated peptide therapeutics in disease areas other than HF and include novel glucagon-like peptide (GLP)-1 receptor activators for diabetes; in human immunodeficiency virus (HIV), peptides target novel molecular markers; and more recently in cardiovascular (CV) disease we have seen the use of peptides such as seralaxin. Indeed, insights into peptide and receptor biology may result in peptide modifications resulting in innovative peptide analogues with enhanced activity and attractive properties.

Cenderitide is the most clinically advanced designer NP. Cenderitide (CD-NP) peptide fuses the 15–amino acid C-terminus of Dendroaspis natriuretic peptide (DNP), a pGC-A activator, isolated from the venom of the green mamba to the 22–amino acid human C-type NP, the latter a pGC-B activator ( Fig. 9.3 ). Thus Cenderitide is the only known NP that uniquely binds to both NP receptors (pGC-A and pGC-B) and consequently activates the second messenger cGMP through two separate and complementary receptor signaling pathways. Moreover, Cenderitide is more resistant to NEP-mediated degradation due to the longer C-terminus. A goal in the engineering of Cenderitide was to develop a less hypotensive NP as CNP/pGC-B has minimal action on blood pressure while lacking renal and RAAS-modulating action together with natriuretic and RAAS-suppressing actions of DNP/pGC-A activation. Such a dual receptor targeting property is also supported by the report by Ichiki et al. that myocardial production of CNP is reduced in human HF, whereas expression of pGC-B is preserved. Thus such design and properties of Cenderitide may lead to enhanced cardiovascular and renal effects without causing profound hypotension, which is the common side effect of recombinant ANP and BNP, because the avoidance of profound hypotension is the main concern in treating HF. Importantly, Lee and associates demonstrated in healthy volunteers that Cenderitide activates cGMP, increases renal sodium excretion, and inhibits RAAS without clinically significant hypotension. Kawakami et al. reported that, in a study of safety and tolerability, acute intravenous infusion of Cenderitide in subjects with stable HF and reduced EF was safe and well tolerated. Without hypotension, the drug also activated plasma and urinary cGMP.

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related

Related posts:

1. Alterations in Cardiac Metabolism in Heart Failure
2. Heart Failure as a Consequence of Viral and Nonviral Myocarditis
3. Biomarkers and Precision Medicine in Heart Failure
4. Cardio-Oncology and Heart Failure

Stay updated, free articles. Join our Telegram channel

Join