Enzymatic inactivation of major circulating forms of atrial and brain natriuretic peptides

B-Type Natriuretic Peptide (BNP) Revisited—Is BNP Still a Biomarker for Heart Failure in the Angiotensin Receptor/Neprilysin Inhibitor Era?

Simple Summary

Active BNP-32, less active proBNP-108, and inactive N-terminal proBNP-76 all circulate in the blood. The circulating protease neprilysin has lower substrate specificity for BNP than ANP, while proBNP and N-terminal proBNP are not degraded by neprilysin. Currently available BNP immunoassays react with both mature BNP and proBNP; therefore, measured plasma BNP is mature BNP + proBNP. Because ARNI administration increases mature BNP, measured plasma BNP initially increases with ARNI administration by the amount of the increase in mature BNP. Later, ARNI administration reduces myocardial wall stress, and the resultant reduction in BNP production more than offsets the increase of mature BNP due to inhibition of degradation by neprilysin, resulting in lower plasma BNP levels. In the ARNI era, BNP remains a useful biomarker for heart failure, though mild increases early during ARNI administration should be taken into consideration.

Abstract

Myocardial wall stress, cytokines, hormones, and ischemia all stimulate B-type (or brain) natriuretic peptide (BNP) gene expression. Within the myocardium, ProBNP-108, a BNP precursor, undergoes glycosylation, after which a portion is cleaved by furin into mature BNP-32 and N-terminal proBNP-76, depending on the glycosylation status. As a result, active BNP, less active proBNP, and inactive N-terminal proBNP all circulate in the blood. There are three major pathways for BNP clearance: (1) cellular internalization via natriuretic peptide receptor (NPR)-A and NPR-C; (2) degradation by proteases in the blood, including neprilysin, dipeptidyl-peptidase-IV, insulin degrading enzyme, etc.; and (3) excretion in the urine. Because neprilysin has lower substrate specificity for BNP than atrial natriuretic peptide (ANP), the increase in plasma BNP after angiotensin receptor neprilysin inhibitor (ARNI) administration is much smaller than the increase in plasma ANP. Currently available BNP immunoassays react with both mature BNP and proBNP. Therefore, BNP measured with an immunoassay is mature BNP + proBNP. ARNI administration increases mature BNP but not proBNP, as the latter is not degraded by neprilysin. Consequently, measured plasma BNP initially increases with ARNI administration by the amount of the increase in mature BNP. Later, ARNI reduces myocardial wall stress, and the resultant reduction in BNP production more than offsets the increase in mature BNP mediated by inhibiting degradation by neprilysin, which lowers plasma BNP levels. These results suggest that even in the ARNI era, BNP can be used for diagnosis and assessment of the pathophysiology and prognosis of heart failure, though the mild increases early during ARNI administration should be taken into consideration.

Potential pitfalls when interpreting plasma BNP levels in heart failure practice

B-type (or brain) natriuretic peptide (BNP) is synthesized in cardiac myocytes and released constitutively into the circulation. Pressure/volume overload, neurohumoral factors, cytokines, and ischemia enhance BNP gene expression, and then precursor proBNP is produced. It has been thought that proBNP is cleaved into active BNP molecule and inactive marker molecule NT-proBNP intracellularly by processing enzyme furin, and they are released into the circulation. However, recent studies have shown that considerable amount of uncleaved proBNP circulates in the blood. The commercially available BNP assay kits consist of two antibodies that sandwich the BNP molecule. Therefore, if proBNP is present, BNP assay kit cross-reacts to proBNP and measures it as BNP. Therefore, it should be noted that the current BNP value is proBNP plus BNP. BNP and NT-proBNP have been established as a biomarker for heart failure patients presenting dyspnea. But many pitfalls are present for interpreting the BNP value. For example, the presence of renal dysfunction, age, female sex, atrial fibrillation, inflammation, hyperthyroidism, use of sacubitril/valsartan, and macro-proBNPemia overestimate BNP value, whereas the presence of obesity, immediately after acute coronary syndrome onset, and pericardial effusion underestimate BNP value. In the management for heart failure patients, BNP plays an important role. Therefore, clinicians should note the pitfall of interpretation of BNP and we describe the mechanism involved.

A review on B-type natriuretic peptide monitoring: assays and biosensors

Since its discovery in 1988, B-type natriuretic peptide (BNP) has been recognized as a powerful cardiovascular biomarker for a number of disease states, specifically heart failure. Concurrent with such a discovery, much effort has been allocated to the precise monitoring of physiological BNP levels. Thus, it can be used to guide the therapy of heart failure and determine the patient's stage of disease. Thus, we discuss in this article BNP as a potent biomarker. Subsequently, we will review the progress of biosensing devices as they could be applied to monitor BNP levels as assays, benchtop biosensors and implantable biosensors. The analytical characteristics of commercially available BNP assays are presented. Still emerging as a field, we define four obstacles that present opportunity for the future development of implantable biosensor: foreign body response, sensor renewability, sensitivity and selectivity.

Biochemistry, Therapeutics, and Biomarker Implications of Neprilysin in Cardiorenal Disease

BACKGROUND

Neprilysin (NEP) is a membrane-bound neutral endopeptidase that degrades a variety of bioactive peptides. The substrates include natriuretic peptides (NPs), which are important regulating mediators for cardiovascular and renal biology. Inhibition of NEP activity and exogenous NP administration thus have emerged as potential therapeutic strategies for treating cardiorenal diseases. More recently, B-type natriuretic peptide (BNP) or N-terminal-proBNP (NT-proBNP), 3'-5' cyclic guanosine monophosphate (cGMP), and soluble NEP as biomarkers have also been investigated in heart failure (HF) trials and their predictive value are beginning to be recognized.

CONTENT

The biological functions of NEP and NPs are discussed. Enhancing NPs through NEP inhibition combined with renin-angiotensin-aldosterone system (RAAS) antagonism has proved to be successful in HF treatment, although future surveillance studies will be required. Direct NP enhancement through peptide delivery may have fewer potentially hazardous effects compared to NEP inhibition. Strategies of combined inhibition on NEP with other cardiorenal pathophysiological pathways are promising. Finally, monitoring BNP/NT-proBNP/cGMP concentrations during NEP inhibition treatment may provide supplemental benefits to conventional biomarkers, and the identification of soluble NEP as a novel biomarker for HF needs further investigation.

SUMMARY

In this review, the biology of NEP is summarized, with a focus on NP regulation. The degradation of NPs by NEP provides the rationale for NEP inhibition as a strategy for cardiorenal disease treatment. We also describe the current therapeutic strategies of NEP inhibition and NP therapeutics in cardiorenal diseases. Moreover, the discovery of its circulating form, soluble NEP, as a biomarker is also discussed.

Comparison of the diagnostic performance of three natriuretic peptides in hemodialysis patients: which is the appropriate biomarker?

BACKGROUND

Plasma concentrations of natriuretic peptides are often elevated in chronic hemodialysis patients and difficult to interpret due to accumulation, high incidence of cardiac disease and changes in volume status. Mid-regional pro-ANP is a newly developed assay whereas BNP and its fragment NT-pro-BNP are available for a longer time. In this cross-sectional study, we compared the plasma concentration of MR-pro-ANP, BNP and NT-pro-BNP in stable ambulatory hemodialysis patients (n = 239) and investigated their associations with clinical factors such as residual diuresis, cardiac status and interdialytic weight gain and with mortality.

METHODS AND RESULTS

In all patients enrolled, the plasma concentration of all natriuretic peptides were largely elevated with a median concentration of 337 pg/ml (interquartile range 146-684) for BNP, 4435 pg/ml (1687-16228) for NT-proBNP and 907 pmol/L (650-1298) for MR-pro-ANP. Plasma concentration of all natriuretic peptides correlated independently with age, degree of systolic dysfunction and negatively with residual diuresis. Dependency on residual renal clearance was strongest for the fragments MR-pro-ANP and NT-pro-BNP. The plasma concentration of all natriuretic peptides was associated with mortality within 2 years of follow-up. Receiver-operated curves revealed a low sensitivity (32-45%), but high specificity for all natriuretic peptides (85-93%) resulting in a high negative predictive (82-87%). Best cut-off values obtained from were 18 611 pg/ml for NT-pro-BNP, 958 pg/ml for BNP and 1684 pmol/L for MR-pro-ANP.

CONCLUSIONS

In hemodialysis patients, the fragments NTproBNP and MR-pro-ANP are largely elevated compared to BNP which is explained by accumulation. The prognostic performance of MR-pro-ANP is similar to that of NT-pro-BNP or BNP.

Comparison of the cardiac markers B-type natriuretic peptide and N-terminal pro B-type natriuretic peptide

B-type natriuretic peptide and N-terminal pro B-type natriuretic peptide have emerged as powerful biomarkers for heart failure and other cardiovascular conditions. B-type natriuretic peptide and N-terminal pro B-type natriuretic peptide are synthesized on the basis of myocardial stress and hypertrophy and are detectable in serum by several commercially available assays. Although both markers display wide similarities in their predictive values for acute and chronic heart failure, important differences exist regarding cutoff values and influence of noncardiac variables. The similarities and differences between B-type natriuretic peptide and N-terminal pro B-type natriuretic peptide will be reviewed and illustrated in detail regarding preanalytics and analytics, predictive properties for acute and chronic heart failure and prognosis as well as the influence of noncardiac parameters.

Gene expression analysis in LLC-PK1 renal tubular cells by atrial natriuretic peptide (ANP): correlation of homologous human genes with renal response

We used human DNA microarray to explore the differential gene expression profiling of atrial natriuretic peptide (ANP)-stimulated renal tubular epithelial kidney cells (LLC-PK1) in order to understand the biological effect of ANP on renal kidney cell's response. Gene expression profiling revealed 807 differentially expressed genes, consisting of 483 up-regulated and 324 down-regulated genes. The bioinformatics tool was used to gain a better understanding of differentially expressed genes in porcine genome homologous with human genome and to search the gene ontology and category classification, such as cellular component, molecular function and biological process. Four up-regulated genes of ATP1B1, H3F3A, ITGB1 and RHO that were typically validated by real-time quantitative PCR (RT-qPCR) analysis serve important roles in the alleviation of renal hypertrophy as well as other related effects. Therefore, the human array can be used for gene expression analysis in pig kidney cells and we believe that our findings of differentially expressed genes served as genetic markers and biological functions can lead to a better understanding of ANP action on the renal protective system and may be used for further therapeutic application.

Des-serine-proline brain natriuretic peptide 3–32 in cardiorenal regulation

Brain natriuretic peptide (BNP 1–32) plays an important physiologic role in cardiorenal homeostasis. Recently, it has been reported that BNP 1–32 is rapidly cleaved by the ubiquitous enzyme dipeptidyl peptidase IV to BNP 3–32, which lacks the two NH2-terminal amino acids of BNP 1–32. The bioactivity of BNP 3–32 in cardiorenal regulation is unknown. We hypothesized that BNP 3–32 has reduced vasodilating and natriuretic bioactivity compared with BNP 1–32 in vivo. Synthetic human BNP 3–32 and BNP 1–32 were administered to eight anesthetized normal canines. After baseline measurements, BNP 1–32 at 30 ng·kg−1·min−1was administered, followed by a washout, a postinfusion clearance, and a clearance with an equimolar dose of BNP 3–32. In four studies, the sequence of BNP 1–32 and BNP 3–32 infusion was reversed. Peptides were compared by analyzing the changes from the respective preinfusion clearance to the respective infusion clearance. * P < 0.05 between peptides. BNP 3–32, unlike BNP 1–32, did not decrease mean arterial pressure (0 ± 1 vs. −7 ± 2* mmHg, respectively) and did not increase renal blood flow (+12 ± 10 vs. +52 ± 10* ml/min). Effects on heart rate and cardiac output were similar. Urinary sodium excretion increased 128 ± 18 μeq/min with BNP 3–32 and 338 ± 40* μeq/min with BNP 1–32. Urine flow increased 1.1 ± 0.2 ml/min with BNP 3–32 and 2.8 ± 0.4* ml/min with BNP 1–32. Plasma BNP immunoreactivity was lower with BNP 3–32, suggesting accelerated degradation. In this study, BNP 3–32 showed reduced natriuresis and diuresis and a lack of vasodilating actions compared with BNP 1–32.

The use of B-type natriuretic peptide to assess volume status in patients with end-stage renal disease

BACKGROUND

Although B-type natriuretic peptide (BNP) levels correlate with volume overload in congestive heart failure, its usefulness in patients with renal dysfunction has been questioned. A simple test to estimate volume overload and assist in the management of dry weight in hemodialysis (HD) patients would be useful.

METHODS

Thirty-nine patients--aged 64 +/- 2 years (mean +/- SEM), male-female ratio of 37:2--undergoing HD thrice weekly for at least 30 days were studied. Samples were collected at the start and end of each of 3 consecutive HD sessions. Pre- and postsession weights and blood pressures were recorded. Left ventricular ejection fractions were obtained from echocardiograms performed within 1 year of enrollment. The first session was the dialysis session after a 72-hour interdialytic period, whereas the second and third sessions were after a 48-hour period. Plasma volume changes were measured in a subset of 13 patients.

RESULTS

Pre- and postdialysis BNP levels for each of the 3 sessions were 434 and 343 pg/mL, 347 and 231 pg/mL, and 249 and 202 pg/mL, respectively. The values for body weights were 82.6 +/- 3.6 and 78.6 +/- 3.5 kg, 81.5 +/- 3.6 and 78.2 +/- 3.5 kg, and 81.5 +/- 3.46 and 78.3 +/- 3.5 kg, respectively. The values of mean systolic blood pressures were 150 +/- 4 and 134 +/- 3 mm Hg, 142 +/- 4 and 134 +/- 4 mm Hg, and 142 +/- 4 and 131 +/- 4 mm Hg, respectively. The values for mean diastolic blood pressures were 81 +/- 2.5 and 70 +/- 2.4 mm Hg, 74 +/- 2.4 and 72.1 +/- 2.2 mm Hg, and 76 +/- 2.9 and 72 +/- 2.9 mm Hg, respectively. There was no correlation between changes in intradialytic BNP values and other measured parameters. Plasma volume changed minimally during dialysis.

CONCLUSIONS

Values of BNP are elevated in patients with end-stage renal disease and decline after each dialysis session. Over the course of a week, BNP levels gradually declined irrespective of changes in weight or blood pressure. The lack of correlation between changes in BNP and changes in measured clinical parameters is partly explained by a lack of a significant change in plasma volume. The highest BNP values were seen in patients with systolic dysfunction.

Dipeptidyl-peptidase IV converts intact B-type natriuretic peptide into its des-SerPro form

BACKGROUND

Analysis of plasma B-type natriuretic peptide (BNP) has suggested the in vivo formation of a truncated form, BNP (3-32), also called des-SerPro-BNP. The objectives of this study were to investigate (a) whether BNP and other natriuretic peptides are truncated by dipeptidyl-peptidase IV (DPP IV/CD26; EC 3.4.14.5) and (b) whether this truncation affects the susceptibility to cleavage by neutral endopeptidase (NEP; EC 3.4.24.11).

METHODS

Human BNP (1-32), A-type natriuretic peptide 1-28 (ANP 1-28), and related peptides were incubated with purified DPP IV and with human plasma. In addition, BNP (1-32), BNP (3-32), and ANP (1-28) were subjected to hydrolysis by NEP. Cleavage products were analyzed by mass spectrometry.

RESULTS

BNP (1-32) was cleaved by purified DPP IV with a specificity constant of 0.37 x 10(6) L.mol(-1).s(-1). The DPP IV activity in EDTA-plasma was able to truncate BNP (1-32) ex vivo. Addition of Vildagliptin, a specific DPP IV inhibitor, prevented this truncation in a concentration-dependent manner. Under in vitro circumstances in which ANP was hydrolyzed extensively, BNP (1-32) and BNP (3-32) were very resistant to NEP-mediated cleavage.

CONCLUSIONS

DPP IV cleaves BNP (1-32) with an efficiency higher than or comparable to several known in vivo substrates of the enzyme. Even after loss of the amino-terminal dipeptide, BNP remains highly resistant to cleavage by NEP.

B-type natriuretic peptide and renal disease

B-type natriuretic peptide (BNP) is a cardiac neurohormone which has a principal effect on the kidney to signal both natriuresis and diuresis. Both BNP and renal function are prognostic indicators of survival in patients with congestive heart failure (CHF). However, the relationships between BNP, renal function, and CHF as an emergency diagnosis, are not completely understood. The correlation between BNP and estimated glomerular filtration rate (eGFR) is approximately r = -0.20. At an eGFR < 60 ml/min/1.73 m2, the optimum cutpoint for BNP to diagnose CHF rises to approximately 200 pg/ml. At this cutpoint the area under the receiver operating characteristic curve is 0.81, indicating that BNP is of diagnostic value in this group. Importantly, the precursor molecule N-terminal proBNP has a stronger correlation with eGFR of approximately -0.60, and is influenced by the age-related decline in renal function above the lower bounds of normal of < 60 ml/min/1.73 m2. Because BNP is a principal messenger from the heart to the kidneys, and because it is influenced by renal filtering function, parenchymal mass, and tubular function, BNP can be leveraged in assisting in the diagnosis and management of combined heart and renal failure.

Synthesis and in vitro analysis of atrial natriuretic peptide–albumin conjugates

Atrial natriuretic peptide (ANP) is a clinically useful anti-hypertensive hormone. Maleimide derivatives of ANP have been synthesized and conjugated to cysteine-34 of human serum albumin. The conjugates were analyzed to assess their stability, receptor binding affinity and ability to stimulate guanylyl-cyclase activity in rat lung fibroblasts.

B-type natriuretic peptide and renal function in the diagnosis of heart failure: an analysis from the Breathing Not Properly Multinational Study

BACKGROUND

Both B-type natriuretic peptide (BNP) and renal function are prognostic indicators of survival in patients with congestive heart failure (CHF). However, relationships between BNP, renal function, and heart failure as an emergency diagnosis are unknown.

METHODS

The Breathing Not Properly Multinational Study was a prospectively designed diagnostic test evaluation study conducted in seven centers. Of 1,586 participants who presented with acute dyspnea, 1,452 patients (91.6%) had both BNP level and baseline estimated glomerular filtration rate (eGFR) available. Patients with an eGFR less than 15 mL/min/1.73 m2 and those on dialysis therapy were excluded. The final diagnosis was adjudicated by two independent cardiologists who were blinded to BNP results.

RESULTS

The final diagnosis was CHF in 715 patients (49.2%). Raw and log-log transformed correlations between BNP and eGFR values were r = -0.19 and r = -0.17 for those with CHF and r = -0.20 and r = -0.31 for those without CHF (both P < 0.0001 for r not equal 0). Mean BNP levels were 561.6 pg/mL (162.3 fmol/mL), 647.5 pg/mL (187.1 fmol/mL), 745.6 pg/mL (215.5 fmol/mL), and 850.7 pg/mL (245.8 fmol/mL) for those with CHF and 85.4 pg/mL (24.7 fmol/mL), 131.7 pg/mL (38.1 fmol/mL), 297.2 pg/mL (85.9 fmol/mL), and 285.0 pg/mL (82.3 fmol/mL) for those without CHF in eGFR categories of 90 or greater, 89 to 60, 59 to 30, and less than 30 mL/min/1.73 m2, respectively. The area under the receiver operating characteristic curve and optimum cut points for BNP were 0.91 and 70.7 pg/mL (20.4 fmol/mL), 0.90 and 104.3 pg/mL (30.1 fmol/mL), 0.81 and 201.2 pg/mL (58.1 fmol/mL), and 0.86 and 225.0 pg/mL (65.0 fmol/mL) for the eGFR categories of 90 or greater, 89 to 60, 59 to 30, and less than 30 mL/min/1.73 m2, respectively.

CONCLUSION

Renal function correlates weakly with BNP and influences the optimal cut point for BNP, particularly in those with an eGFR less than 60 mL/min/1.73 m2.

Cyclic GMP signaling in podocytes

Natriuretic peptides (NP), together with nitric oxide (NO) are powerful relaxing factors acting via a common second messenger, cyclic GMP (cGMP). Together with other vasoactive modulators, these vasorelaxing factors play an essential role in regulating the function of kidney glomeruli. The presence of NP receptors in podocytes has been well documented. Recently, also mRNA for soluble guanylate cyclase, the NO receptor, has been shown in these cells. Stimulation of podocytes with atrial natriuretic peptide (ANP), C-type natriuretic peptide (CNP), and NO donors results in considerable upregulation of cellular cGMP synthesis. The podocyte foot processes contain a highly organized network of microfilaments adhering to the glomerular basement membrane (GBM). Changes in podocyte cytoskeleton accompanied by detachment of the cells from the GBM are closely associated with many glomerulopathies. The contractile apparatus in the podocyte foot processes seems to be an obvious target for the cyclic GMP signaling cascade. However, little is known about implications of the cGMP synthesis in these cells. We briefly review the current art regarding generation and modulation of cyclic GMP levels in podocytes. We discuss also the possible targets for this secondary messenger as well as its functional role in podocytes.

Effect of neutral endopeptidase inhibitor on endogenous atrial natriuretic peptide as a paracrine factor in cultured cardiac fibroblasts

1. Cardiac remodelling is a fundamental response to hypertension, myocardial infarction and chronic heart failure, and involves cardiac fibroblast proliferation and production of extracellular matrix components such as collagen. The present study was performed to examine the role of endogenous atrial natriuretic peptide (ANP) as a possible paracrine factor for cardiac fibroblasts, and to examine the effects of three neutral endopeptidase (NEP) inhibitors, thiorphan, phosphoramidon and ONO-BB-039-02 (ONO-BB) on endogenous ANP-induced changes in collagen synthesis by cultured neonatal rat cardiac fibroblasts. 2. Each NEP inhibitor singly had no significant effect on collagen synthesis by cardiac fibroblasts, except for maximum concentration (10(-3) M) of thiorphan. 3. Exogenous ANP inhibited collagen synthesis in a concentration-dependent manner (10(-8) - 10(-6) M). Thiorphan (10(-4) and 10(-3) M) and phosphoramidon (10(-5) and 10(-4) M) enhanced the ANP (10(-7) M)-induced decrease in collagen synthesis. ONO-BB (10(-5) and 10(-4) M) slightly enhanced the ANP-induced decrease in collagen synthesis. 4. Myocyte-conditioned medium (MC-CM), as well as exogenous ANP, inhibited collagen synthesis dose-dependently. The decrease in collagen synthesis at 100% MC-CM was augmented by thiorphan (10(-3) M), phosphoramidon (10(-4) M) and ONO-BB (10(-4) M). 5. HS-142-1, a natriuretic peptide receptor antagonist, significantly reduced the MC-CM plus thiorphan- and MC-CM plus ONO-BB-induced decrease in collagen synthesis, by 92 and 62%, respectively and showed a tendency to attenuate the MC-CM plus phosphoramidon-induced decrease in collagen synthesis by 40%. 6. Our observations suggested that endogenous ANP released from cardiomyocytes inhibited collagen synthesis as a paracrine factor and that NEP inhibitors enhanced the activity of this peptide in cardiac fibroblasts.