Guanylyl cyclase/natriuretic peptide receptor-A gene disruption causes increased adrenal angiotensin II and aldosterone levels

Triiodo-L-thyronine (T3) downregulates Npr1 gene (coding for natriuretic peptide receptor-A) transcription in H9c2 cells: involvement of β-AR-ROS signaling

INTRODUCTION

Natriuretic peptide receptor-A (NPR-A) signaling system is considered as an intrinsic productive mechanism of the heart that opposes abnormal cardiac remodeling and hypertrophic growth. NPR-A is coded by Npr1 gene, and its expression is downregulated in the hypertrophied heart.

AIM

We sought to examine the levels of Npr1 gene transcription in triiodo-L-thyronine (T3) treated hypertrophied cardiomyocyte (H9c2) cells, in vitro, and also the involvement of β-adrenergic receptor (β-AR) - Reactive oxygen species (ROS) signaling system in the down-regulation of Npr1 transcription also studied.

MAIN METHODS

Anti-hypertrophic Npr1 gene transcription was monitored in control and T3-treated (dose and time dependent) H9c2 cells, using a real time PCR method. Further, cell size, intracellular cGMP, ROS, hypertrophy markers (ANP, BNP, α-sk, α-MHC and β-MHC), β-AR, and protein kinase cGMP-dependent 1 (PKG-I) genes expression were also determined. The intracellular cGMP and ROS levels were determined by ELISA and DCF dye method, respectively. In addition, to neutralize T3 mediated ROS generation, H9c2 cells were treated with T3 in the presence and absence of antioxidants [curcumin (CU) or N-acetyl-L-cysteine (NAC)].

RESULTS

A dose dependent (10 pM, 100 pM, 1 nM and 10 nM) and time dependent (12 h, 24 h and 48 h) down-regulation of Npr1 gene transcription (20, 39, 60, and 74% respectively; 18, 55, and 85%, respectively) were observed in T3-treated H9c2 cells as compared with control cells. Immunofluorescence analysis also revealed that a marked down regulation of NPR- A protein in T3-treated cells as compared with control cells. Further, a parallel downregulation of cGMP and PKG-I (2.4 fold) were noticed in the T3-treated cells. In contrast, a time dependent increased expression of β-AR (60, 72, and 80% respectively) and ROS (26, 48, and 74%, respectively) levels were noticed in T3-treated H9c2 cells as compared with control cells. Interestingly, antioxidants, CU or NAC co-treated T3 cells displayed a significant reduction in ROS (69 and 81%, respectively) generation and to increased Npr1 gene transcription (81 and 88%, respectively) as compared with T3 alone treated cells.

CONCLUSION

Our result suggest that down regulation of Npr1 gene transcription is critically involved in T3- induced hypertrophic growth in H9c2 cells, and identifies the cross-talk between T3-β-AR-ROS and NPR-A signaling.

Natriuretic Peptide Receptors (NPRs) as a Potential Target for the Treatment of Heart Failure

PURPOSE OF REVIEW

Heart failure is defined as a complex clinical syndrome that results from any structural or functional impairment of ventricular filling or ejection of blood. The natriuretic peptide is known to exert its biological action on the kidney, heart, blood vessels, renin-angiotensin system, autonomous nervous system, and central nervous system. The natriuretic peptide-natriuretic receptor system plays an important role in the regulation of blood pressure and body fluid volume through its pleiotropic effects.

RECENT FINDINGS

The clinical and animal studies suggest that natriuretic peptide-natriuretic receptors are important targets for the treatment of heart failure and other cardiovascular diseases. Even though attempts targeting natriuretic peptide receptors are underway for heart failure treatment, they seem insufficient despite the receptor systems' potential. This review summarizes natriuretic peptide-natriuretic receptor system's physiological actions and potential target for the treatment of heart failure. Natriuretic peptides play multiple roles in different parts of the body, almost all of the activities related to this receptor system appear to have the potential to be harnessed to treat heart failure or symptoms associated with heart failure.

Guanylyl cyclase/natriuretic peptide receptor-A: Identification, molecular characterization, and physiological genomics

The natriuretic peptides (NPs) hormone family, which consists mainly of atrial, brain, and C-type NPs (ANP, BNP, and CNP), play diverse roles in mammalian species, ranging from renal, cardiac, endocrine, neural, and vascular hemodynamics to metabolic regulations, immune responsiveness, and energy distributions. Over the last four decades, new data has transpired regarding the biochemical and molecular compositions, signaling mechanisms, and physiological and pathophysiological functions of NPs and their receptors. NPs are incremented mainly in eliciting natriuretic, diuretic, endocrine, vasodilatory, and neurological activities, along with antiproliferative, antimitogenic, antiinflammatory, and antifibrotic responses. The main locus responsible in the biological and physiological regulatory actions of NPs (ANP and BNP) is the plasma membrane guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), a member of the growing multi-limbed GC family of receptors. Advances in this field have provided tremendous insights into the critical role of Npr1 (encoding GC-A/NPRA) in the reduction of fluid volume and blood pressure homeostasis, protection against renal and cardiac remodeling, and moderation and mediation of neurological disorders. The generation and use of genetically engineered animals, including gene-targeted (gene-knockout and gene-duplication) and transgenic mutant mouse models has revealed and clarified the varied roles and pleiotropic functions of GC-A/NPRA in vivo in intact animals. This review provides a chronological development of the biochemical, molecular, physiological, and pathophysiological functions of GC-A/NPRA, including signaling pathways, genomics, and gene regulation in both normal and disease states.

Genetic Disruption of Guanylyl Cyclase/Natriuretic Peptide Receptor-A Triggers Differential Cardiac Fibrosis and Disorders in Male and Female Mutant Mice: Role of TGF-β1/SMAD Signaling Pathway

The global targeted disruption of the natriuretic peptide receptor-A (NPRA) gene (Npr1) in mice provokes hypertension and cardiovascular dysfunction. The objective of this study was to determine the mechanisms regulating the development of cardiac fibrosis and dysfunction in Npr1 mutant mice. Npr1 knockout (Npr1^-/-, 0-copy), heterozygous (Npr1^+/-, 1-copy), and wild-type (Npr1^+/+, 2-copy) mice were treated with the transforming growth factor (TGF)-β1 receptor (TGF-β1R) antagonist GW788388 (2 µg/g body weight/day; ip) for 28 days. Hearts were isolated and used for real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR), Western blot, and immunohistochemical analyses. The Npr1^-/- (0-copy) mice showed a 6-fold induction of cardiac fibrosis and dysfunction with markedly induced expressions of collagen-1α (3.8-fold), monocyte chemoattractant protein (3.7-fold), connective tissue growth factor (CTGF, 5.3-fold), α-smooth muscle actin (α-SMA, 6.1-fold), TGF-βRI (4.3-fold), TGF-βRII (4.7-fold), and phosphorylated small mothers against decapentaplegic (pSMAD) proteins, including pSMAD-2 (3.2-fold) and pSMAD-3 (3.7-fold), compared with wild-type mice. The expressions of phosphorylated extracellular-regulated kinase ERK1/2 (pERK1/2), matrix metalloproteinases-2, -9, (MMP-2, -9), and proliferating cell nuclear antigen (PCNA) were also significantly upregulated in Npr1 0-copy mice. The treatment of mutant mice with GW788388 significantly blocked the expression of fibrotic markers, SMAD proteins, MMPs, and PCNA compared with the vehicle-treated control mice. The treatment with GW788388 significantly prevented cardiac dysfunctions in a sex-dependent manner in Npr1 0-copy and 1-copy mutant mice. The results suggest that the development of cardiac fibrosis and dysfunction in mutant mice is predominantly regulated through the TGF-β1-mediated SMAD-dependent pathway.

Natriuretic peptide pathways in heart failure: further therapeutic possibilities

The discovery of the heart as an endocrine organ resulted in a remarkable recognition of the natriuretic peptide system (NPS). Specifically, research has established the production of atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) from the heart, which exert pleiotropic cardiovascular, endocrine, renal, and metabolic actions via the particulate guanylyl cyclase A receptor (GC-A) and the second messenger, cGMP. C-type natriuretic peptide (CNP) is produced in the endothelium and kidney and mediates important protective auto/paracrine actions via GC-B and cGMP. These actions, in part, participate in the efficacy of sacubitril/valsartan in heart failure (HF) due to the augmentation of the NPS. Here, we will review important insights into the biology of the NPS, the role of precision medicine, and focus on the phenotypes of human genetic variants of ANP and BNP in the general population and the relevance to HF. We will also provide an update of the existence of NP deficiency states, including in HF, which provide the rationale for further therapeutics for the NPS. Finally, we will review the field of peptide engineering and the development of novel designer NPs for the treatment of HF. Notably, the recent discovery of a first-in-class small molecule GC-A enhancer, which is orally deliverable, will be highlighted. These innovative designer NPs and small molecule possess enhanced and novel properties for the treatment of HF and cardiovascular diseases.

Molecular Signaling Mechanisms and Function of Natriuretic Peptide Receptor-A in the Pathophysiology of Cardiovascular Homeostasis

The discovery of atrial, brain, and C-type natriuretic peptides (ANP, BNP, and CNP) and their cognate receptors has greatly increased our knowledge of the control of hypertension and cardiovascular homeostasis. ANP and BNP are potent endogenous hypotensive hormones that elicit natriuretic, diuretic, vasorelaxant, antihypertrophic, antiproliferative, and antiinflammatory effects, largely directed toward the reduction of blood pressure (BP) and cardiovascular diseases (CVDs). The principal receptor involved in the regulatory actions of ANP and BNP is guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), which produces the intracellular second messenger cGMP. Cellular, biochemical, molecular, genetic, and clinical studies have facilitated understanding of the functional roles of natriuretic peptides (NPs), as well as the functions of their receptors, and signaling mechanisms in CVDs. Transgenic and gene-targeting (gene-knockout and gene-duplication) strategies have produced genetically altered novel mouse models and have advanced our knowledge of the importance of NPs and their receptors at physiological and pathophysiological levels in both normal and disease states. The current review describes the past and recent research on the cellular, molecular, genetic mechanisms and functional roles of the ANP-BNP/NPRA system in the physiology and pathophysiology of cardiovascular homeostasis as well as clinical and diagnostic markers of cardiac disorders and heart failure. However, the therapeutic potentials of NPs and their receptors for the diagnosis and treatment of cardiovascular diseases, including hypertension, heart failure, and stroke have just begun to be expanded. More in-depth investigations are needed in this field to extend the therapeutic use of NPs and their receptors to treat and prevent CVDs.

Protection of kidney function and tissue integrity by pharmacologic use of natriuretic peptides and neprilysin inhibitors

With variable potencies atrial-, brain-type and c-type natriuretic peptides (NP)s, best documented for ANP and its analogues, promote sodium and water excretion, renal blood flow, lipolysis, lower blood pressure, and suppress renin and aldosterone secretion through interaction predominantly with cGMP-coupled NPR-A receptor. Infusion of especially ANP and its analogues up to 50 ng/kg/min in patients with high risk of acute kidney injury (cardiac vascular bypass surgery, intraabdominal surgery, direct kidney surgery) protects kidney function (GFR, plasma flow, medullary flow, albuminuria, renal replacement therapy, tissue injury) at short term and also long term and likely additively with the diuretic furosemide. This documents a pharmacologic potential for the pathway. Neprilysin (NEP, neutral endopeptidase) degrades NPs, in particular ANP, and angiotensin II. The drug LCZ696, a mixture of the neprilysin inhibitor sacubitril and the ANGII-AT1 receptor blocker valsartan, was FDA approved in 2015 and marketed as Entresto®. In preclinical studies of kidney injury, LCZ696 and NPs lowered plasma creatinine, countered hypoxia and oxidative stress, suppressed proinflammatory cytokines, and inhibited fibrosis. Few randomized clinical studies exist and were designed with primary cardiac outcomes. The studies showed that LCZ696/entresto stabilized and improved glomerular filtration rate in patients with chronic kidney disease. LCZ696 is safe to use concerning kidney function and stabilizes or increases GFR. In perspective, combined AT1 and neprilysin inhibition is a promising approach for long-term renal protection in addition to AT1 receptor blockers in acute kidney injury and chronic kidney disease.

Genetic Ablation and Guanylyl Cyclase/Natriuretic Peptide Receptor-A: Impact on the Pathophysiology of Cardiovascular Dysfunction

Mice bearing targeted gene mutations that affect the functions of natriuretic peptides (NPs) and natriuretic peptide receptors (NPRs) have contributed important information on the pathogenesis of hypertension, kidney disease, and cardiovascular dysfunction. Studies of mice having both complete gene disruption and tissue-specific gene ablation have contributed to our understanding of hypertension and cardiovascular disorders. These phenomena are consistent with an oligogenic inheritance in which interactions among a few alleles may account for genetic susceptibility to hypertension, renal insufficiency, and congestive heart failure. In addition to gene knockouts conferring increased risks of hypertension, kidney disorders, and cardiovascular dysfunction, studies of gene duplications have identified mutations that protect against high blood pressure and cardiovascular events, thus generating the notion that certain alleles can confer resistance to hypertension and heart disease. This review focuses on the intriguing phenotypes of Npr1 gene disruption and gene duplication in mice, with emphasis on hypertension and cardiovascular events using mouse models carrying Npr1 gene knockout and/or gene duplication. It also describes how Npr1 gene targeting in mice has contributed to our knowledge of the roles of NPs and NPRs in dose-dependently regulating hypertension and cardiovascular events.

Genetic disruption of Npr1 depletes regulatory T cells and provokes high levels of proinflammatory cytokines and fibrosis in the kidneys of female mutant mice

The present study was designed to determine the effects of gene knockout of guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA) on immunogenic responses affecting kidney function and blood pressure (BP) in Npr1 (coding for GC-A/NPRA)-null mutant mice. We used female Npr1 gene-disrupted (Npr1^-/-, 0 copy), heterozygous (Npr1^+/-, 1 copy), wild-type (Npr1^+/+, 2 copy), and gene-duplicated (Npr1^++/++, 4 copy) mice. Expression levels of Toll-like receptor (TLR)2/TLR4 mRNA were increased 4- to 5-fold in 1-copy mice and 6- to 10-fold in 0-copy mice; protein levels were increased 2.5- to 3-fold in 1-copy mice and 4- to 5-fold in 0-copy mice. Expression of proinflammatory cytokines and BP was significantly elevated in 1-copy and 0-copy mice compared with 2-copy and 4-copy mice. In addition, 0-copy and 1-copy mice exhibited drastic reductions in regulatory T cells (Tregs). After rapamycin treatment, Tregs were increased by 17% (P < 0.001) in 0-copy mice and 8% (P < 0.001) in 1-copy mice. Renal mRNA and protein levels of TLR2 and TLR4 were decreased by 70% in 0-copy mice and 50% in 1-copy mice. There were significantly higher levels of Tregs and very low levels of TLR2/TLR4 expression in 4-copy mice (P < 0.001). These findings indicate that the disruption of Npr1 in female mice triggers renal immunogenic pathways, which transactivate the expression of proinflammatory cytokines and renal fibrosis with elevated BP in mutant animals. The data suggest that rapamycin treatment attenuates proinflammatory cytokine expression, dramatically increases anti-inflammatory cytokines, and substantially reduces BP and renal fibrosis in mutant animals.

Genetic disruption of guanylyl cyclase/natriuretic peptide receptor-A upregulates renal (pro) renin receptor expression in Npr1 null mutant mice

The objective of the present study was to determine whether targeted-disruption of Npr1 gene (encoding for guanylyl cyclase/natriuretic peptide receptor-A; GC-A/NPRA) upregulates pro(renin) receptor (P)RR expression and leads to the activation of MAPKs in Npr1 gene-knockout mice. The Npr1 homozygous (Npr1^-/-; 0-copy), heterozygous (Npr1^+/-; 1-copy), wild-type (Npr1^+/+; 2-copy), and gene-duplicated (Npr1^++/++; 4-copy) mice were utilized. To identify the canonical pathway of (P)RR, we administered ACE-1 inhibitor (captopril), AT1R blocker (losartan), and MAPKs inhibitors (U0126 and SB203580) to all Npr1 mice genotypes. The renal expression of (P)RR mRNA was increased by 3-fold in 0-copy mice and 2-fold in 1-copy mice compared with 2-copy mice, which was also associated with significantly increased expression of ACE-1 and AT1R mRNA levels. Similarly, the phosphorylation of MAPKs (Erk1/2 and p-p38) was enhanced by 3.5-fold and 3.2-fold, respectively, in 0-copy mice with significant increases in 1-copy mice compared with 2-copy mice. The kidney and plasma levels of proinflammatory cytokines were significantly elevated in 0-copy and 1-copy mice. Treatment with captopril and losartan did not alter the expression of (P)RR in any of the Npr1 mice genotypes. Interestingly, losartan significantly reduced the phosphorylation of Erk1/2 and p38 in Npr1 mice. The present results suggest that the ablation of Npr1 upregulates (P)RR, MAPKs (Erk1/2 and p38), and proinflammatory cytokines in 0-copy and 1-copy mice. In contrast, the duplication of Npr1 exhibits the anti-inflammatory and antihypertensive effects by reducing the activation of MAPKs and inhibiting the expression levels of RAAS components and proinflammatory cytokines.

The Biology of Normal Zona Glomerulosa and Aldosterone-Producing Adenoma: Pathological Implications

The identification of several germline and somatic ion channel mutations in aldosterone-producing adenomas (APAs) and detection of cell clusters that can be responsible for excess aldosterone production, as well as the isolation of autoantibodies activating the angiotensin II type 1 receptor, have rapidly advanced the understanding of the biology of primary aldosteronism (PA), particularly that of APA. Hence, the main purpose of this review is to discuss how discoveries of the last decade could affect histopathology analysis and clinical practice. The structural remodeling through development and aging of the human adrenal cortex, particularly of the zona glomerulosa, and the complex regulation of aldosterone, with emphasis on the concepts of zonation and channelopathies, will be addressed. Finally, the diagnostic workup for PA and its subtyping to optimize treatment are reviewed.

Molecular and genetic aspects of guanylyl cyclase natriuretic peptide receptor-A in regulation of blood pressure and renal function

Natriuretic peptides (NPs) exert diverse effects on several biological and physiological systems, such as kidney function, neural and endocrine signaling, energy metabolism, and cardiovascular function, playing pivotal roles in the regulation of blood pressure (BP) and cardiac and vascular homeostasis. NPs are collectively known as anti-hypertensive hormones and their main functions are directed toward eliciting natriuretic/diuretic, vasorelaxant, anti-proliferative, anti-inflammatory, and anti-hypertrophic effects, thereby, regulating the fluid volume, BP, and renal and cardiovascular conditions. Interactions of NPs with their cognate receptors display a central role in all aspects of cellular, biochemical, and molecular mechanisms that govern physiology and pathophysiology of BP and cardiovascular events. Among the NPs atrial and brain natriuretic peptides (ANP and BNP) activate guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA) and initiate intracellular signaling. The genetic disruption of Npr1 (encoding GC-A/NPRA) in mice exhibits high BP and hypertensive heart disease that is seen in untreated hypertensive subjects, including high BP and heart failure. There has been a surge of interest in the NPs and their receptors and a wealth of information have emerged in the last four decades, including molecular structure, signaling mechanisms, altered phenotypic characterization of transgenic and gene-targeted animal models, and genetic analyses in humans. The major goal of the present review is to emphasize and summarize the critical findings and recent discoveries regarding the molecular and genetic regulation of NPs, physiological metabolic functions, and the signaling of receptor GC-A/NPRA with emphasis on the BP regulation and renal and cardiovascular disorders.

Guanylyl cyclase/natriuretic peptide receptor-A signaling antagonizes phosphoinositide hydrolysis, Ca(2+) release, and activation of protein kinase C

Thus far, three related natriuretic peptides (NPs) and three distinct sub-types of cognate NP receptors have been identified and characterized based on the specific ligand binding affinities, guanylyl cyclase activity, and generation of intracellular cGMP. Atrial and brain natriuretic peptides (ANP and BNP) specifically bind and activate guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), and C-type natriuretic peptide (CNP) shows specificity to activate guanylyl cyclase/natriuretic peptide receptor-B (GC-B/NPRB). All three NPs bind to natriuretic peptide receptor-C (NPRC), which is also known as clearance or silent receptor. The NPRA is considered the principal biologically active receptor of NP family; however, the molecular signaling mechanisms of NP receptors are not well understood. The activation of NPRA and NPRB produces the intracellular second messenger cGMP, which serves as the major signaling molecule of all three NPs. The activation of NPRB in response to CNP also produces the intracellular cGMP; however, at lower magnitude than that of NPRA, which is activated by ANP and BNP. In addition to enhanced accumulation of intracellular cGMP in response to all three NPs, the levels of cAMP, Ca²⁺ and inositol triphosphate (IP₃) have also been reported to be altered in different cells and tissue types. Interestingly, ANP has been found to lower the concentrations of cAMP, Ca²⁺, and IP₃; however, NPRC has been proposed to increase the levels of these metabolic signaling molecules. The mechanistic studies of decreased and/or increased levels of cAMP, Ca²⁺, and IP₃ in response to NPs and their receptors have not yet been clearly established. This review focuses on the signaling mechanisms of ANP/NPRA and their biological effects involving an increased level of intracellular accumulation of cGMP and a decreased level of cAMP, Ca²⁺, and IP₃ in different cells and tissue systems.

Atrial natriuretic factor receptor guanylate cyclase, ANF-RGC, transduces two independent signals, ANF and Ca(2+)

Atrial natriuretic factor receptor guanylate cyclase (ANF-RGC), was the first discovered member of the mammalian membrane guanylate cyclase family. The hallmark feature of the family is that a single protein contains both the site for recognition of the regulatory signal and the ability to transduce it into the production of the second messenger, cyclic GMP. For over two decades, the family has been classified into two subfamilies, the hormone receptor subfamily with ANF-RGC being its paramount member, and the Ca²⁺ modulated subfamily, which includes the rod outer segment guanylate cyclases, ROS-GC1 and 2, and the olfactory neuroepithelial guanylate cyclase. ANF-RGC is the receptor and the signal transducer of the most hypotensive hormones, ANF– and B-type natriuretic peptide (BNP). After binding these hormones at the extracellular domain it, at its intracellular domain, signals activation of the C-terminal catalytic module and accelerates the production of cyclic GMP. Cyclic GMP then serves the second messenger role in biological responses of ANF and BNP such as natriuresis, diuresis, vasorelaxation, and anti-proliferation. Very recently another modus operandus for ANF-RGC was revealed. Its crux is that ANF-RGC activity is also regulated by Ca²⁺. The Ca²⁺ sensor neurocalcin d mediates this signaling mechanism. Strikingly, the Ca²⁺ and ANF signaling mechanisms employ separate structural motifs of ANF-RGC in modulating its core catalytic domain in accelerating the production of cyclic GMP. In this review the biochemistry and physiology of these mechanisms with emphasis on cardiovascular regulation will be discussed.

Interactive roles of NPR1 gene-dosage and salt diets on cardiac angiotensin II, aldosterone and pro-inflammatory cytokines levels in mutant mice

OBJECTIVE

The objective of the present study was to elucidate the interactive roles of guanylyl cyclase/natriuretic peptide receptor-A (NPRA) gene (Npr1) and salt diets on cardiac angiotensin II (ANG II), aldosterone and pro-inflammatory cytokines levels in Npr1 gene-targeted (1-copy, 2-copy, 3-copy, 4-copy) mice.

METHODS

Npr1 genotypes included 1-copy gene-disrupted heterozygous (+/-), 2-copy wild-type (+/+), 3-copy gene-duplicated heterozygous (++/+) and 4-copy gene-duplicated homozygous (++/++) mice. Animals were fed low, normal and high-salt diets. Plasma and cardiac levels of ANG II, aldosterone and pro-inflammatory cytokines were determined.

RESULTS

With a high-salt diet, cardiac ANG II levels were increased (+) in 1-copy mice (13.7 ± 2.8 fmol/mg protein, 111%) compared with 2-copy mice (6.5 ± 0.6), but decreased (-) in 4-copy (4.0 ± 0.5, 38%) mice. Cardiac aldosterone levels were increased (+) in 1-copy mice (80 ± 4 fmol/mg protein, 79%) compared with 2-copy mice (38 ± 3). Plasma tumour necrosis factor alpha was increased (+) in 1-copy mice (30.27 ± 2.32 pg/ml, 38%), compared with 2-copy mice (19.36 ± 2.49, 24%), but decreased (-) in 3-copy (11.59 ± 1.51, 12%) and 4-copy (7.13 ± 0.52, 22%) mice. Plasma interleukin (IL)-6 and IL-1α levels were also significantly increased (+) in 1-copy compared with 2-copy mice but decreased (-) in 3-copy and 4-copy mice.

CONCLUSION

These results demonstrate that a high-salt diet aggravates cardiac ANG II, aldosterone and pro-inflammatory cytokine levels in Npr1 gene-disrupted 1-copy mice, whereas, in Npr1 gene-duplicated (3-copy and 4-copy) mice, high salt did not render such elevation, suggesting the potential roles of Npr1 against salt loading.

The ANF-RGC gene motif (669)WTAPELL(675) is vital for blood pressure regulation: biochemical mechanism

ANF-RGC is the prototype membrane guanylate cyclase, both the receptor and the signal transducer of the hormones ANF and BNP. After binding them at the extracellular domain, it, at its intracellular domain, signals activation of the C-terminal catalytic module and accelerates production of the second messenger, cyclic GMP. This, in turn, controls the physiological processes of blood pressure, cardiovascular function, fluid secretion, and others: metabolic syndrome, obesity, and apoptosis. The biochemical mechanism by which this single molecule controls these diverse processes, explicitly blood pressure regulation, is the subject of this study. In line with the concept that the structural modules of ANF-RGC are designed to respond to more than one yet distinctive signals, the study demonstrates the construction of a novel ANF-RGC-In-gene-(669)WTAPELL(675) mouse model. Through this model, the study establishes that (669)WTAPELL(675) is a vital ANF signal transducer motif of the guanylate cyclase. Its striking physiological features linked with their biochemistry are the following. (1) It controls the hormonally dependent cyclic GMP production in the kidney and the adrenal gland. Its deletion causes (2) hypertension and (3) cardiac hypertrophy. (4) These mice show higher levels of the plasma aldosterone. For the first time, a mere seven-amino acid-encoded motif of the mouse gene has been directly linked with the physiological control of blood pressure regulation, a detailed biochemistry of this linkage has been established, and a model for this linkage has been described.

Emerging Roles of Natriuretic Peptides and their Receptors in Pathophysiology of Hypertension and Cardiovascular Regulation

Thus far, three related natriuretic peptides (NPs) and three distinct receptors have been identified, which have advanced our knowledge towards understanding the control of high blood pressure, hypertension, and cardiovascular disorders to a great extent. Biochemical and molecular studies have been advanced to examine receptor function and signaling mechanisms and the role of second messenger cGMP in pathophysiology of hypertension, renal hemodynamics, and cardiovascular functions. The development of gene-knockout and gene-duplication mouse models along with transgenic mice have provided a framework for understanding the importance of the antagonistic actions of natriuretic peptides receptor in cardiovascular events at the molecular level. Now, NPs are considered as circulating markers of congestive heart failure, however, their therapeutic potential for the treatment of cardiovascular diseases such as hypertension, renal insufficiency, cardiac hypertrophy, congestive heart failure, and stroke has just begun to unfold. Indeed, the alternative avenues of investigations in this important are need to be undertaken, as we are at the initial stage of the molecular therapeutic and pharmacogenomic implications.

Interaction between the heme oxygenase system and aldosterone in hypertension

The chronic intraperitoneal administration of the heme oxygenase inducer, hemin (15 mg/kg daily), for three weeks reduced blood pressure in adult spontaneously hypertensive rats (SHR) from 210.1±1.03 mmHg to 127±0.9 mmHg (n=10, P<0.01) but had no effect on age-matched normotensive Wistar-Kyoto or Sprague-Dawley strains. The antihypertensive effect of hemin was accompanied by reduced expression of aldosterone synthase messenger RNA and depleted levels of plasma aldosterone (675.7±121.6 pg/mL versus 365.7±37 pg/mL; n=4, P<0.05).Because aldosterone is known to stimulate phospholipase C (PLC), the effect of hemin on PLC was examined. Hemin abated PLC activity (29.6±1.5 nmol/min/mL versus 3.1±0.9 nmol/min/mL; n=5, P<0.01) and this was accompanied by depleted levels of intracellular calcium (551±46 nM versus 103.2±6.3 nM; n=4, P<0.01) in the aorta of SHR. In contrast, enhanced heme oxygenase activity and elevated cyclic GMP levels (17.74±0.08 pmol/mg versus 30.4±2.3 pmol/mg protein; n=6, P<0.01) were detected in hemin-treated SHR. Additionally, hemin therapy also suppressed inflammatory and oxidative insults by significantly reducing nuclear factor kappa B messenger RNA expression while enhancing the total antioxidant capacity (0.22±0.02 Trolox equivalent antioxidant capacity (TEA C)/mg protein versus 0.60±0.04 TEA C/mg protein; n=4, P<0.01).The concomitant depletion of aldosterone, PLC activity, intracellular calcium and the corresponding decline of inflammatory and oxidative insults may account for the antihypertensive effects of hemin.

Guanylyl cyclase / atrial natriuretic peptide receptor-A: role in the pathophysiology of cardiovascular regulation

Atrial natriuretic factor (ANF), also known as atrial natriuretic peptide (ANP), is an endogenous and potent hypotensive hormone that elicits natriuretic, diuretic, vasorelaxant, and anti-proliferative effects, which are important in the control of blood pressure and cardiovascular events. One principal locus involved in the regulatory action of ANP and brain natriuretic peptide (BNP) is guanylyl cyclase / natriuretic peptide receptor-A (GC-A/NPRA). Studies on ANP, BNP, and their receptor, GC-A/NPRA, have greatly increased our knowledge of the control of hypertension and cardiovascular disorders. Cellular, biochemical, and molecular studies have helped to delineate the receptor function and signaling mechanisms of NPRA. Gene-targeted and transgenic mouse models have advanced our understanding of the importance of ANP, BNP, and GC-A/NPRA in disease states at the molecular level. Importantly, ANP and BNP are used as critical markers of cardiac events; however, their therapeutic potentials for the diagnosis and treatment of hypertension, heart failure, and stroke have just begun to be realized. We are now just at the initial stage of molecular therapeutics and pharmacogenomic advancement of the natriuretic peptides. More investigations should be undertaken and ongoing ones be extended in this important field.

The functional genomics of guanylyl cyclase/natriuretic peptide receptor-A: perspectives and paradigms

The cardiac hormones atrial natriuretic peptide and B-type natriuretic peptide (brain natriuretic peptide) activate guanylyl cyclase (GC)-A/natriuretic peptide receptor-A (NPRA) and produce the second messenger cGMP. GC-A/NPRA is a member of the growing family of GC receptors. The recent biochemical, molecular and genomic studies on GC-A/NPRA have provided important insights into the regulation and functional activity of this receptor protein, with a particular emphasis on cardiac and renal protective roles in hypertension and cardiovascular disease states. The progress in this field of research has significantly strengthened and advanced our knowledge about the critical roles of Npr1 (coding for GC-A/NPRA) in the control of fluid volume, blood pressure, cardiac remodeling, and other physiological functions and pathological states. Overall, this review attempts to provide insights and to delineate the current concepts in the field of functional genomics and signaling of GC-A/NPRA in hypertension and cardiovascular disease states at the molecular level.

Transcriptome profiling and network analysis of genetically hypertensive mice identifies potential pharmacological targets of hypertension

Hypertension is a condition with major cardiovascular and renal complications, affecting nearly a billion patients worldwide. Few validated gene targets are available for pharmacological intervention, so there is a need to identify new biological pathways regulating blood pressure and containing novel targets for treatment. The genetically hypertensive “blood pressure high” (BPH), normotensive “blood pressure normal” (BPN), and hypotensive “blood pressure low” (BPL) inbred mouse strains are an ideal system to study differences in gene expression patterns that may represent such biological pathways. We profiled gene expression in liver, heart, kidney, and aorta from BPH, BPN, and BPL mice and determined which biological processes are enriched in observed organ-specific signatures. As a result, we identified multiple biological pathways linked to blood pressure phenotype that could serve as a source of candidate genes causal for hypertension. To distinguish in the kidney signature genes whose differential expression pattern may cause changes in blood pressure from those genes whose differential expression pattern results from changes in blood pressure, we integrated phenotype-associated genes into Genetic Bayesian networks. The integration of data from gene expression profiling and genetics networks is a valuable approach to identify novel potential targets for the pharmacological treatment of hypertension.

Regulation of cardiac angiotensin-converting enzyme and angiotensin AT1 receptor gene expression in Npr1 gene-disrupted mice

1. Understanding of the regulatory mechanisms of gene expression in the control of blood pressure and fluid volume is a key issue in cardiovascular medicine. Guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA) signalling antagonizes the physiological and pathophysiological effects mediated by the renin-angiotensin-aldosterone system (RAAS) in the regulation of cardiovascular homeostasis. 2. The targeted-disruption of the Npr1 gene (coding for GC-A/PRA) leads to activation of the cardiac RAAS involved in the hypertrophic remodelling process, which influences cardiac size, expression of pro-inflammatory cytokine genes and the behaviour of various hypertrophy marker genes. The Npr1 gene-knockout (Npr1(-/-)) mice exhibit 35-40 mmHg higher systolic blood pressure and a significantly greater heart weight to bodyweight ratio than wild-type (Npr1(+/+)) mice. 3. The expression of both angiotensin-converting enzyme (ACE) and angiotensin II AT(1a) receptors are significantly increased in hearts from Npr1(-/-) mice compared with hearts from Npr1(+/+) mice. In parallel, the expression of interleukin-6 and tumour necrosis factor-alpha is also markedly increased in hearts from Npr1(-/-) mice. 4. These findings indicate that disruption of NPRA/cGMP signalling leads to augmented expression of the cardiac RAAS in conjunction with pro-inflammatory cytokines in Npr1-null mutant mice, which promotes the development of cardiac hypertrophy and remodelling.

ANP-mediated inhibition of distal nephron fractional sodium reabsorption in wild-type and mice overexpressing natriuretic peptide receptor

Atrial natriuretic peptide (ANP) elicits natriuresis; however, the relative contributions of proximal and distal nephron segments to the overall ANP-induced natriuresis have remained uncertain. This study was performed to characterize the effects of ANP on distal nephron sodium reabsorption determined after blockade of the two major distal nephron sodium transporters with amiloride (5 microg/g body wt) plus bendroflumethiazide (12 microg/g body wt) in male anesthetized C57/BL6 and natriuretic peptide receptor-A gene (Npr1) targeted four-copy mice. The lower dose of ANP (0.1 ng x g body wt(-1) x min(-1), n = 6) increased distal sodium delivery (DSD, 2.4 +/- 0.4 vs. 1.6 +/- 0.2 mueq/min, P < 0.05) but did not change fractional reabsorption of DSD compared with control (86.3 +/- 2.0 vs. 83.9 +/- 3.6%, P > 0.05), thus limiting the magnitude of the natriuresis. In contrast, the higher dose (0.2 ng x g body wt(-1) x min(-1), n = 6) increased DSD (2.8 +/- 0.3 mueq/min, P < 0.01) and also decreased fractional reabsorption of DSD (67.4 +/- 4.5%, P < 0.01), which markedly augmented the natriuresis. In Npr1 gene-duplicated four-copy mice (n = 6), the lower dose of ANP increased urinary sodium excretion (0.6 +/- 0.1 vs. 0.3 +/- 0.1 mueq/min, P < 0.05) and decreased fractional reabsorption of DSD compared with control (72.2 +/- 3.4%, P < 0.05) at similar mean arterial pressures (91 +/- 6 vs. 92 +/- 3 mmHg, P > 0.05). These results provide in vivo evidence that ANP-mediated increases in DSD alone exert modest effects on sodium excretion and that inhibition of fractional reabsorption of distal sodium delivery is requisite for the augmented natriuresis in response to the higher dose of ANP or in Npr1 gene-duplicated mice.

The heme oxygenase system abates hyperglycemia in Zucker diabetic fatty rats by potentiating insulin-sensitizing pathways

Emerging evidence indicates that aldosterone causes oxidative stress by stimulating proinflammatory/oxidative mediators, including nuclear factor-kappaB, activating protein (AP-1), and c-Jun N-terminal kinase. Thus, in insulin-resistant type 2 diabetes (T2D), oxidative stress generated by hyperglycemia and aldosterone would potentiate the oxidative destruction of tissue and important regulators of glucose metabolism like adiponectin and insulin. Although heme oxygenase (HO)-1 is cytoprotective, its effects on T2D have not been fully characterized. Here we report an enduring antidiabetic effect of the HO inducer, hemin, on Zucker diabetic-fatty rat (ZDF), a model of insulin-resistant T2D. Chronically applied hemin to ZDF reduced and maintained significantly low fasting and postprandial hyperglycemia for 4 months after therapy. The antidiabetic effect was accompanied by enhanced HO activity, catalase, cyclic GMP, bilirubin, ferritin, total antioxidant capacity, and insulin. In contrast, reduced aldosterone alongside markers/mediators of oxidative stress, including 8-isoprostane, c-Jun N-terminal kinase, nuclear factor-kappaB, AP-1, and AP-2 were observed. Interestingly, in hemin-treated ZDF, inhibitory proteins of insulin-signaling, such as glycogen synthase kinase-3 and protein-tyrosine phosphatase-1B were reduced, whereas agents that promote insulin signaling including adiponectin, cAMP, AMP-activated protein kinase, aldolase-B, and glucose transporter-4 (GLUT4), were robustly increased. Correspondingly, hemin improved ip glucose tolerance, reduced insulin intolerance, and lowered insulin resistance (homeostasis model assessment of insulin resistance), and the inability of insulin to enhance GLUT4 was overturned. These results suggest that the suppression of hyperglycemia and aldosterone-induced oxidative stress alongside the potentiation of insulin-sensitizing pathways may account for the 4-month enduring antidiabetic effect. The synergistic interaction between the HO system, aldolase-B, adiponectin, AMP-activated protein kinase, and GLUT4 may be explored for novel strategies against postprandial/fasting hyperglycemia and insulin-resistant T2D.

Upregulating the heme oxygenase system suppresses left ventricular hypertrophy in adult spontaneously hypertensive rats for 3 months

BACKGROUND

Aldosterone and phospholipase C (PLC) stimulate nuclear factor-kappaB (NF-kappaB) and activating-protein (AP-1), causing fibrosis and hypertrophy. Besides harboring binding sites for NF-kappaB and AP-1, heme oxygenase (HO-1) generates cytoprotective products, including bilirubin and ferritin. The multifaceted interaction between HO-1 and aldosterone-PLC profibrotic axis in cardiac hypertrophy of spontaneously hypertensive rats (SHR) was studied.

METHODS AND RESULTS

HO-1 was induced with hemin or blocked with chromium mesoporphyrin (CrMP). The study groups included: (A) controls (SHR, WKY, and SD), (B) SHR+hemin, (C) SHR+hemin+CrMP, (D) SHR+CrMP, and (E) SHR+vehicle. Histological and morphological/morphometrical, quantitative reverse transcription-polymerase chain reaction, Western blot, enzyme immunoassay, and spectrophotometric assays were used to assess the effect of the HO system on cardiac hypertrophy. Hemin therapy evoked a 3-month enduring cardioprotection in adult SHR by lowering blood pressure, and reducing left-to-right ventricular ratio, left ventricular wall-thickness, and left ventricle-to-body-weight ratio, whereas CrMP exacerbated cardiac fibrosis/hypertrophy. The cardioprotection was accompanied by reduced aldosterone, PLC, inositol-triphosphate, NF-kappaB, AP-1, heme, and 8-isoprostane, a marker of oxidative stress, whereas HO-1, HO activity, cGMP, bilirubin, ferritin, superoxide dismutase, and the total antioxidant capacity were increased. Correspondingly, extracellular matrix/remodeling proteins such as fibronectin, collagen-1, collagen-IV, alongside cardiac histopathological lesions including fibrosis, scarring, muscular-hypertrophy, coronary-arteriolar thickening, and interstitial/perivascular collagen deposition were attenuated.

CONCLUSIONS

Our study unveils sustained cardioprotection by hemin that may have clinical relevance.