Pertussis toxin attenuates atrial natriuretic factor-mediated inhibition of adenylate cyclase. Involvement of inhibitory guanine nucleotide regulatory protein

Role of Gi proteins in the regulation of blood pressure and vascular remodeling

Heterotrimeric guanine nucleotide regulatory proteins (G-proteins) through the activation of several signaling mechanisms including adenylyl cyclase/cAMP and phospholipase C (PLC)/phosphatidyl inositol (PI) turnover. regulate a variety of cellular functions, including vascular reactivity, proliferation and hypertrophy of VSMC. Activity of adenylyl cyclase is regulated by two G proteins, stimulatory (Gsα) and inhibitory (Giα). Gsα stimulates adenylyl cyclase activity and increases the levels of cAMP, whereas Giα inhibits the activity of adenylyl cyclase and results in the reduction of cAMP levels. Abnormalities in Giα protein expression and associated adenylyl cyclase\cAMP levels result in the impaired cellular functions and contribute to various pathological states including hypertension. The expression of Giα proteins is enhanced in various tissues including heart, kidney, aorta and vascular smooth muscle cells (VSMC) from genetic (spontaneously hypertensive rats (SHR)) and experimentally - induced hypertensive rats and contribute to the pathogenesis of hypertension. In addition, the enhanced expression of Giα proteins exhibited by VSMC from SHR is also implicated in the hyperproliferation and hypertrophy, the two key players contributing to vascular remodelling in hypertension. The enhanced levels of endogenous vasoactive peptides including angiotensin II (Ang II), endothelin-1 (ET-1) and growth factors contribute to the overexpression of Giα proteins in VSMC from SHR. In addition, enhanced oxidative stress, activation of c-Src, growth factor receptor transactivation and MAP kinase/PI3kinase signaling also contribute to the augmented expression of Giα proteins in VSMC from SHR. This review summarizes the role of Giα proteins, and the underlying molecular mechanisms implicated in the regulation of high blood pressure and vascular remodelling.

Downregulation of natriuretic peptide receptor-C in vascular smooth muscle cells from spontaneously hypertensive rats contributes to vascular remodeling

Hypertension is associated with vascular remodeling due to hyperproliferation and hypertrophy of vascular smooth muscle cells (VSMC). VSMC from several animal models of hypertensive rats including spontaneously hypertensive rats (SHR) exhibit hyperproliferation, hypertrophy and decreased expression of natriuretic peptide receptor-C (NPR-C). In addition, angiotensin II (Ang II) and growth factors that promotes vascular remodeling have also been shown to attenuate the expression of NPR-C in VSMC. The present study investigates the relationship between the decreased expression of NPR-C and vascular remodeling in SHR and the underlying molecular mechanisms. Aortic VSMC from SHR and their control Wistar Kyoto (WKY) rats were transfected with cDNA of NPR-C and used for the vascular remodeling studies. Transfection of VSMC with cDNA of NPR-C augmented the expression of NPR-C in both VSMC from SHR and WKY rats and resulted in the attenuation of hyperproliferation and hypertrophy of VSMC from SHR. The overexpression of NPR-C also resulted in the attenuation of increased expression of epidermal growth factor receptor (EGFR), platelet derived growth factor receptor (PDGFR), cell cycle proteins, cyclin D1, cyclin-dependent kinase 4 (Cdk4), phospho-retinoblastoma (pRb) and Giα-2 proteins, all these signaling molecules implicated in the hyperproliferation/hypertrophy of VSMC from SHR. In summary, these results indicate that augmenting the decreased expression of NPR-C in VSMC from SHR improves vascular remodeling by attenuating hyperproliferation and hypertrophy through decreasing the overexpression of several signaling molecules. It may be suggested that NPR-C plays a vasculoprotective role and that the downregulation of NPR-C contributes to the vascular remodeling in SHR.

A compendium of G-protein-coupled receptors and cyclic nucleotide regulation of adipose tissue metabolism and energy expenditure

With the ever-increasing burden of obesity and Type 2 diabetes, it is generally acknowledged that there remains a need for developing new therapeutics. One potential mechanism to combat obesity is to raise energy expenditure via increasing the amount of uncoupled respiration from the mitochondria-rich brown and beige adipocytes. With the recent appreciation of thermogenic adipocytes in humans, much effort is being made to elucidate the signaling pathways that regulate the browning of adipose tissue. In this review, we focus on the ligand-receptor signaling pathways that influence the cyclic nucleotides, cAMP and cGMP, in adipocytes. We chose to focus on G-protein-coupled receptor (GPCR), guanylyl cyclase and phosphodiesterase regulation of adipocytes because they are the targets of a large proportion of all currently available therapeutics. Furthermore, there is a large overlap in their signaling pathways, as signaling events that raise cAMP or cGMP generally increase adipocyte lipolysis and cause changes that are commonly referred to as browning: increasing mitochondrial biogenesis, uncoupling protein 1 (UCP1) expression and respiration.

A Differential Hypofunctionality of Gαi Proteins Occurs in Adolescent Idiopathic Scoliosis and Correlates with the Risk of Disease Progression

Adolescent idiopathic scoliosis is the most prevalent spine deformity and the molecular mechanisms underlying its pathophysiology remain poorly understood. We have previously found a differential impairment of melatonin receptor signaling in AIS osteoblasts allowing the classification of patients into three biological endophenotypes or functional groups (FG1, FG2 and FG3). Here, we provide evidence that the defect characterizing each endophenotype lies at the level of Gαi proteins leading to a systemic and generalized differential impairment of Gi-coupled receptor signaling. The three Gαi isoforms exhibited a selective serine phosphorylation patterns for each AIS endophenotype resulting in a differential reduction in Gαi protein activity as determined by cellular dielectric spectroscopy and small interfering RNA methods. We found that one endophenotype (FG2) with phosphorylated Gαi₁ and Gαi₂ was consistently associated with a significantly high risk of spinal deformity progression when compared to the other two endophenotypes (FG1 and FG3). We further demonstrated that each endophenotype is conserved among affected family members. This study expands our understanding of the mechanism underlying the Gi-coupled receptor signaling dysfunction occurring in AIS and provides the first evidence for its hereditary nature. Collectively, our findings offers a new perspective on Gαi hypofunctionality in a human disease by revealing specific serine phosphorylation signatures of Gαi isoforms that may facilitate the identification of AIS patients at risk of spinal deformity progression.

Natriuretic peptide receptor-C-mediated attenuation of vascular smooth muscle cell hypertrophy involves Gqα/PLCβ1 proteins and ROS-associated signaling

Hypertension is associated with vascular remodeling due to hyperproliferation and hypertrophy of vascular smooth muscle cells (VSMC). Recently, we showed the implication of enhanced expression of Gqα and PLCβ1 proteins in hypertrophy of VSMCs from 16‐week‐old spontaneously hypertensive rats (SHR). The aim of this study was to investigate whether C‐ANP_4‐23, a natriuretic peptide receptor‐C (NPR‐C) ligand that was shown to inhibit vasoactive peptide‐induced enhanced protein synthesis in A10 VSMC could also attenuate hypertrophy of VSMC isolated from rat model of cardiac hypertrophy and to further explore the possible involvement of Gqα/PLCβ1 proteins and ROS‐mediated signaling in this effect. The protein synthesis and cell volume, markers of hypertrophy were significantly enhanced in VSMC from 16‐week‐old SHR compared with age‐matched WKY rats and C‐ANP_4‐23 treatment attenuated both to WKY levels. In addition, C‐ANP_4‐23 treatment also attenuated the enhanced expression of AT1 receptor, Gqα, PLCβ1, Nox4, and p47^phox proteins, the enhanced activation of EGFR, PDGFR, IGF‐1R, enhanced phosphorylation of ERK1/2/AKT and c‐Src in VSMC from SHR. Furthermore, the enhanced levels of superoxide anion and NADPH oxidase activity exhibited by VSMC from SHR were also attenuated to control levels by C‐ANP_4‐23 treatment. These results indicate that C‐ANP_4‐23 via the activation of NPR‐C attenuates VSMC hypertrophy through decreasing the overexpression of Gqα/PLCβ1 proteins, enhanced oxidative stress, increased activation of growth factor receptors, and enhanced phosphorylation of MAPK/AKT signaling pathways. Thus, it can be suggested that C‐ANP_4‐23 may be used as a therapeutic agent for the treatment of vascular complications associated with hypertension and atherosclerosis.

Contribution of oxidative stress and growth factor receptor transactivation in natriuretic peptide receptor C-mediated attenuation of hyperproliferation of vascular smooth muscle cells from SHR

Earlier studies have shown the implication of growth factor receptor activation in angiotensin II (Ang II)-induced hyperproliferation of aortic VSMC as well as in hyperproliferation of VSMC from spontaneously hypertensive rats (SHR). We previously showed that NPR-C specific agonist C-ANP_4-23 attenuates the hyperproliferation of VSMC from SHR through the inhibition of MAP kinase, Giα protein signaling and overexpression of cell cycle proteins. The aim of the present study was to investigate if C-ANP_4-23- mediated attenuation of hyperproliferation of VSMC from SHR also involves growth factor receptor activation and upstream signaling molecules. For this study, C-ANP _4–23 (10 nmole/kg body weight) was injected intraperitoneally into 2 week-old prehypertensive SHR and Wistar Kyoto (WKY) rats twice per week for 6 weeks. The blood pressure in SHR was significantly attenuated by C-ANP_4-23 treatment. In addition, C-ANP_4-23 treatment also attenuated the hyperproliferation of VSMC from SHR as well as the enhanced phosphorylation of EGF-R, PDGF-R, IGF-R and c-Src. Furthermore, the enhanced levels of superoxide anion, NADPH oxidase activity, and enhanced expression of Nox4,Nox1,Nox2 and P47^phox in SHR compared to WKY rats was also significantly attenuated by C-ANP_4-23 treatment. In addition, N-acetyl cysteine (NAC), a scavenger of O₂^-, inhibitors of growth factor receptors and of c-Src, all inhibited the overexpression of cell cycle proteins cyclin D1 and cdk4 in VSMC from SHR. These results suggest that in vivo treatment of SHR with C-ANP_4-23 inhibits the enhanced oxidative stress, c-Src and EGF-R, PDGF-R, IGF-R activation which through the inhibition of overexpression of cell cycle proteins result in the attenuation of hyperproliferation of VSMC.

The biological impact of blood pressure-associated genetic variants in the natriuretic peptide receptor C gene on human vascular smooth muscle

Elevated blood pressure (BP) is a major global risk factor for cardiovascular disease. Genome-wide association studies have identified several genetic variants at the NPR3 locus associated with BP, but the functional impact of these variants remains to be determined. Here we confirmed, by a genome-wide association study within UK Biobank, the existence of two independent BP-related signals within NPR3 locus. Using human primary vascular smooth muscle cells (VSMCs) and endothelial cells (ECs) from different individuals, we found that the BP-elevating alleles within one linkage disequilibrium block identified by the sentinel variant rs1173771 was associated with lower endogenous NPR3 mRNA and protein levels in VSMCs, together with reduced levels in open chromatin and nuclear protein binding. The BP-elevating alleles also increased VSMC proliferation, angiotensin II-induced calcium flux and cell contraction. However, an analogous genotype-dependent association was not observed in vascular ECs. Our study identifies novel, putative mechanisms for BP-associated variants at the NPR3 locus to elevate BP, further strengthening the case for targeting NPR-C as a therapeutic approach for hypertension and cardiovascular disease prevention.

A natriuretic peptides clearance receptor's agonist reduces pulmonary artery pressures and enhances cardiac performance in preclinical models: New hope for patients with pulmonary hypertension due to left ventricular heart failure

BACKGROUND

In patients with left ventricular heart failure (HF), the development of pulmonary hypertension (PH) is common and represents a strong predictor of death. Despite recent advances in the pathophysiological understanding there is as yet no prospect of cure of this deadly clinical entity and the majority of patients continue to progress to right ventricular failure and die. Furthermore, there is no single medical treatment currently approved for PH related to HF. There is, therefore an urgent unmet need to identify novel pharmacological agents that will prevent the progressive increased or reverse the elevated pulmonary arterial pressures while enhancing cardiac performance in HF.

METHOD AND RESULTS

We here reported, for the first time, using a pressure-loop (P-V) conductance catheter system, that a specific natriuretic peptides clearance receptors' agonist, the ring-deleted atrial natriuretic peptide analogue, cANF4-23 (cANF) reduces pulmonary artery pressures. Strikingly, the administration of the cANF in these mice decreased the RVSP by 50% (n=5, F 25.687, DF 14, p<0.001) and heart rate (HR) by 11% (n=5, F 25.69, DF 14, p<0.001) as well as enhancing cardiac performance including left ventricular contractility in mice. Most strikingly, mice lacking NPR-C were much more susceptible to develop HF, indicating that NPR-C is a critical protective receptor in the heart.

CONCLUSION

Natriuretic peptides clearance receptors' agonists may, therefore represent a novel and attractive therapeutic strategy for PH related to HF, and ultimately improves the life expectancy and quality for millions of people around the planet.

Knockdown of Inhibitory Guanine Nucleotide Binding Protein Giα-2 by Antisense Oligodeoxynucleotides Attenuates the Development of Hypertension and Tachycardia in Spontaneously Hypertensive Rats

BACKGROUND

We previously showed that the levels of both Giα-2 and Giα-3 proteins were augmented in spontaneously hypertensive rats (SHRs) before the onset of hypertension. In addition, intraperitoneal injection of pertussis toxin, which inactivates both Giα proteins, prevented the development of hypertension in SHRs. The aim of the present study was to determine the specific contributions of Giα-2 and Giα-3 proteins to the development of hypertension.

METHODS AND RESULTS

Antisense oligodeoxynucleotide of Giα-2 and Giα-3 encapsulated in PEG/DOTAP/DOPE cationic liposomes were administrated intravenously into 3-week-old prehypertensive SHRs and Wistar Kyoto rats, whereas the control Wistar Kyoto rats and SHRs received PBS, empty liposomes, or sense. The knockdown of Giα-2 but not Giα-3 protein attenuated tachycardia and prevented the development of hypertension up to age 6 weeks; thereafter, blood pressure started increasing and reached the same level as that of untreated SHRs at 9 weeks. Furthermore, Giα-2 and Giα-3 antisense oligodeoxynucleotide treatments significantly decreased the enhanced levels of Giα-2 and Giα-3 proteins, respectively, and enhanced levels of superoxide anion and NADPH oxidase activity in heart, aorta, and kidney and hyperproliferation of vascular smooth muscle cells from SHRs aged 6 weeks. In addition, antisense oligodeoxynucleotide treatment with Giα-2 but not Giα-3 restored enhanced inhibition of adenylyl cyclase by oxotremorine to WKY levels.

CONCLUSIONS

These results suggested that the enhanced expression of Giα-2 but not Giα-3 protein plays an important role in the pathogenesis of hypertension and tachycardia in SHRs.

An analogue of atrial natriuretic peptide (C-ANP4-23) modulates glucose metabolism in human differentiated adipocytes

The present study was undertaken to investigate the effects of C-atrial natriuretic peptide (C-ANP4-23) in human adipose-derived stem cells differentiated into adipocytes over 10 days (1 μM for 4 h). The intracellular cAMP, cGMP and protein kinase A levels were determined by ELISA and gene and protein expression were determined by qRT-PCR and Western blot, respectively, in the presence or absence of C-ANP4-23. The levels of lipolysis and glucose uptake were also determined. C-ANP4-23 treatment significantly increased the intracellular cAMP levels and the gene expression of glucose transporter type 4 (GLUT4) and protein kinase, AMP-activated, alpha 1 catalytic subunit (AMPK). Western blot showed a significant increase in GLUT4 and phosphor-AMPKα levels. Importantly, the adenylate cyclase inhibitor SQ22536 abolished these effects. Additionally, C-ANP4-23 increased glucose uptake by 2-fold. Our results show that C-ANP4-23 enhances glucose metabolism and might contribute to the development of new peptide-based therapies for metabolic diseases.

Atrial natriuretic peptide and regulation of vascular function in hypertension and heart failure: implications for novel therapeutic strategies

Atrial natriuretic peptide (ANP) plays a pivotal role in modulation of vascular function and it is also involved in the pathophysiology of several cardiovascular diseases. We provide an updated overview of the current appraisal of ANP vascular effects in both animal models and humans. We describe the physiological implications of ANP vasomodulatory properties as well as the involvement of ANP, through its control of vascular function, in hypertension and heart failure. The principal molecular mechanisms underlying regulation of vascular tone, that is natriuretic peptide receptor type A/cyclic guanylate monophosphate, natriuretic peptide receptor type C, nitric oxide system, are discussed. We review the literature on therapeutic implications of ANP in hypertension and heart failure, examining the potential use of ANP analogues, neutral endopeptidase (NEP) inhibitors, ACE/NEP inhibitors, angiotensin receptor blocker (ARB)/NEP inhibitors, the new dual endothelin-converting enzyme (ECE)/NEP inhibitors and ANP-based gene therapy. The data discussed support the role of ANP in different pathological conditions through its vasomodulatory properties. They also indicate that ANP may represent an optimal therapeutic agent in cardiovascular diseases.

Natriuretic peptide receptor-C attenuates hypertension in spontaneously hypertensive rats: role of nitroxidative stress and Gi proteins

C-Atrial natriuretic peptide (ANP)4-23, a ring deleted analog of ANP that specifically interacts with natriuretic peptide receptor-C (NPR-C), has been shown to decrease the enhanced expression of Giα proteins implicated in the pathogenesis of hypertension. In the present study, we investigated whether in vivo treatment of spontaneously hypertensive rats (SHRs) with C-ANP4-23 could attenuate the development of high blood pressure (BP) and explored the underlying mechanisms responsible for this response. Intraperitoneal injection of C-ANP4-23 at the concentration of 2 or 10 nmol/kg body weight to prehypertensive SHRs attenuated the development of high BP, and at 8 weeks it was decreased by ≈20 and 50 mm Hg, respectively; however, this treatment did not affect BP in Wistar-Kyoto rats. C-ANP4-23 treatment of adult SHRs for 2 weeks also attenuated high BP, heart rate, and restored the impaired vasorelaxation toward control levels. In addition, the enhanced levels of superoxide anion (O2(-)), peroxynitrite, NADPH oxidase activity, and the enhanced expression of Giα proteins, NOX4, p47(phox), nitrotyrosine, and decreased levels of endothelial nitric oxide synthase (eNOS or NOS3) and NO in SHRs were attenuated by C-ANP4-23 treatment; however, the altered levels of NPR-A/NPR-C were not affected by this treatment. In conclusion, these results indicate that NPR-C activation by C-ANP4-23 attenuates the development of high BP in SHRs through the inhibition of enhanced levels of Giα proteins and nitroxidative stress and not through eNOS/cGMP pathway and suggest that NPR-C ligand may have the potential to be used as therapeutic agent in the treatment of cardiovascular complications including hypertension.

Natriuretic peptide receptor-C agonist attenuates the expression of cell cycle proteins and proliferation of vascular smooth muscle cells from spontaneously hypertensive rats: role of Gi proteins and MAPkinase/PI3kinase signaling

Vascular smooth muscle cells (VSMC) from spontaneously hypertensive rats (SHR) exhibit hyperproliferation and overexpression of cell cycle proteins. We earlier showed that small peptide fragments of cytoplasmic domain of natriuretic receptor-C (NPR-C) attenuate vasoactive peptide-induced hyperproliferation of VSMC. The present study investigated if C-ANP_4–23, a specific agonist of NPR-C, could attanuate the hyperproliferation of VSMC from SHR by inhibiting the overexpression of cell cycle proteins and examine the underlying signaling pathways contributing to this inhibition. The proliferation of VSMC was determined by [³H] thymidine incorporation and the expression of proteins was determined by Western blotting. The hyperproliferation of VSMC from SHR and overexpression of cyclin D1,cyclin A, cyclin E, cyclin-dependent kinase 2 (cdk2), phosphorylated retinoblastoma protein (pRb), Giα proteins and enhanced phosphorylation of ERK1/2 and AKT exhibited by VSMC from SHR were attenuated by C-ANP_4–23 to control levels. In addition, in vivo treatment of SHR with C-ANP_4–23 also attenuated the enhanced proliferation of VSMC. Furthemore, PD98059, wortmannin and pertussis toxin, the inhibitors of MAP kinase, PI3kinase and Giα proteins respectively, also attenuated the hyperproliferation of VSMC from SHR and overexpression of cell cycle proteins to control levels. These results indicate that NPR-C activation by C-ANP_4–23 attenuates the enhanced levels of cell cycle proteins through the inhibition of enhanced expression of Giα proteins and enhanced activation of MAPkinase/PI3kinase and results in the attenuation of hyperproliferation of VSMC from SHR. It may be suggested that C-ANP_4–23 could be used as a therapeutic agent in the treatment of vascular complications associated with hypertension, atherosclerosis and restenosis.

Knockdown of natriuretic peptide receptor-A enhances receptor C expression and signalling in vascular smooth muscle cells

AIMS

Natriuretic peptide receptor-A (NPR-A) knockout mice exhibited an increased blood pressure that may also be attributed to the up-regulation of NPR-C and associated signalling; however, the interaction between the two receptors has not been investigated. In the present study, we investigated the effect of knockdown of NPR-A using NPR-A antisense (AS) on the expression of NPR-C and adenylyl cyclase (AC) signalling in A10 vascular smooth muscle cells (VSMC).

METHODS AND RESULTS

The receptor and G protein expression was determined by western blotting, and AC activity was determined by measuring [(32)P]cAMP formation from [α-(32)P]ATP. Treatment of A10 VSMC with NPR-A AS decreased NPR-A and enhanced NPR-C expression without altering the levels of angiotensin II AT1 and muscarinic M2 receptors. In addition, siRNA-NPR-A also resulted in the up-regulation of NPR-C. The re-expression of NPR-A in AS-treated cells reversed the enhanced expression of NPR-C to control levels. In addition, NPR-C-, AT1, and M2 receptor-mediated inhibition of AC and Giα protein expression was enhanced in AS-treated cells, whereas NPR-A-mediated cyclic GMP (cGMP) formation and Gsα-mediated stimulation of AC were significantly reduced. Pertussis toxin treatment attenuated the AS-induced enhanced inhibition of AC to control levels. Furthermore, the enhanced levels of NPR-C and Giα proteins were reversed to control levels by 8-bromo-cGMP (8Br-cGMP) and PD98059, an MEK inhibitor. In addition, 8Br-cGMP also attenuated AS-induced enhanced ERK1/2 phosphorylation to control levels.

CONCLUSION

These results demonstrate that knockdown of NPR-A up-regulates the expression of NPR-C, Giα proteins, and NPR-C-linked AC signalling and suggests a cross-talk between NPR-A and NPR-C.

Transactivation of epidermal growth factor receptor by enhanced levels of endogenous angiotensin II contributes to the overexpression of Giα proteins in vascular smooth muscle cells from SHR

We earlier showed that the increased expression of Gi proteins exhibited by vascular smooth muscle cells (VSMC) from spontaneously hypertensive rats (SHR) was attributed to the enhanced levels of endogenous endothelin. Since the levels of angiotensin II (Ang II) are also enhanced in VSMC from SHR, the present study was undertaken to examine the role of enhanced levels of endogenous Ang II in the overexpression of Giα proteins in VSMC from SHR and to further explore the underlying mechanisms responsible for this increase. The enhanced expression of Giα-2 and Giα-3 proteins in VSMC from SHR compared to WKY was attenuated by the captopril, losartan and AG1478, inhibitors of angiotensin converting enzyme, AT(1) receptor and epidermal growth factor receptor (EGFR) respectively as well as by the siRNAs of AT1, cSrc and EGFR. The enhanced inhibition of forskolin-stimulated adenylyl cyclase activity by low concentrations of GTPγS (receptor-independent functions) and of inhibitory responses of hormones on adenylyl cyclase activity (receptor-dependent functions) in VSMC from SHR was also attenuated by losartan. Furthermore, the enhanced phosphorylation of EGFR in VSMC from SHR was also restored to control levels by captopril, losartan, PP2, a c-Src inhibitor and N-acetyl-L-cysteine (NAC), superoxide anion (O(2)(-)) scavenger, whereas enhanced ERK1/2 phosphorylation was attenuated by captopril and losartan. Furthermore, NAC also restored the enhanced phosphorylation of c-Src in SHR to control levels. These results suggest that the enhanced levels of endogenous Ang II in VSMC from SHR, transactivate EGFR, which through MAP kinase signaling, enhance the expression of Giα proteins and associated adenylyl cyclase signaling.

Enhanced levels of endogenous endothelin-1 contribute to the over expression of Giα protein in vascular smooth muscle cells from SHR: Role of growth factor receptor activation

We earlier showed that vascular smooth muscle cells (VSMC) from spontaneously hypertensive rats (SHR) exhibit increased expression of Gi proteins. Since the levels of endothelin-1 (ET-1) are enhanced in VSMC from SHR, we undertook the present study to examine the implication of endogenous ET-1 and the underlying mechanisms in the enhanced expression of Giα proteins in VSMC from SHR. The enhanced expression of Giα-2 and Giα-3 proteins in VSMC from SHR was inhibited by ET(A) and ET(B) receptor antagonists, BQ123 and BQ788 respectively. In addition, these antagonists also attenuated the enhanced inhibition of forskolin-stimulated adenylyl cyclase activity by low concentrations of GTPγS and by inhibitory hormones in VSMC from SHR compared to WKY. Furthermore, AG1295, AG1024 and PP2, inhibitors of platelet derived growth factor receptor (PDGFR), insulin-like growth factor 1 receptor (IGF-1R) and c-Src respectively, inhibited the enhanced expression of Giα protein and the enhanced phosphorylation of PDGFR and IGF-1R in VSMC from SHR to WKY levels. In addition, NAD(P)H oxidase inhibitor DPI and N-acetylcysteine (NAC), a scavenger of superoxide anion (O₂⁻) also inhibited the enhanced phosphorylation of PDGFR and IGF-1R and c-Src in VSMC from SHR to control levels. Furthermore, the augmented phosphorylation of ERK1/2 in VSMC from SHR was attenuated by BQ123 and BQ788, growth factor receptors inhibitors and PP2. These results suggest that the enhanced levels of endogenous ET-1 in VSMC from SHR increase oxidative stress, which through c-Src-mediated activation of growth factor receptors and associated MAP kinase signaling, contribute to the enhanced expression of Giα proteins.

cANF causes endothelial cell hyperpolarization by activation of chloride channels

OBJECTIVES

Natriuretic peptides bind with natriuretic peptide receptor (NPR)-C, which can alter cellular function through its interaction with the G(i) protein complex. NPR-C has been found to mediate the activation of K(+) channels and non-selective cation channels in vascular smooth muscle and cardiac fibroblast cells, respectively. However, the electrophysiological effect of NPR-C activation on endothelial cells (EC) has not been previously examined. In this study we sought to elucidate the effect of cANF(4-23), a selective NPR-C ligand, on EC membrane potential (E(m)).

METHODS/RESULTS

Changes in EC E(m) was measured through non-invasive fluorescence imaging. EC were preincubated in the potentiometric dye, DiBAC(4)(3) and subsequently exposed to cANF(4-23), in the presence of selective inhibitors of ion-channels or second messengers. NPR-C expression in rat lung microvascular endothelial cells was assessed by RT-PCR. cANF(4-23) induced a sustained decrease in EC cellular fluorescence, indicating endothelial cell hyperpolarization. The cANF-induced hyperpolarization could not be attenuated by TEA, barium, ouabain or by the reduction of extracellular Ca(2+). Further, the cANF-induced hyperpolarization was insensitive to inhibition of G(i) and protein kinase G (PKG), downstream messengers of NPRs. However, the Cl(-) channel inhibitors, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, niflumic acid, and hypertonic saline attenuated the cANF-induced hyperpolarization. Perforated patch clamp recordings confirmed the cANF-induced current was carried by Cl(-) and could be inhibited by niflumic acid. RT-PCR confirmed expression of NPR-C in vascular smooth muscle cells but not in EC.

CONCLUSIONS

cANF causes hyperpolarization that is most likely mediated via activation of Cl(-) channels by a PKG and G(i) independent mechanism.

Central role of guanylyl cyclase in natriuretic peptide signaling in hypertension and metabolic syndrome

Studied for nearly 30 years for its ability to control many parameters, such as vascular smooth muscle cell relaxation, heart fibrosis, and kidney function, the natriuretic peptide (NP) system is now considered to be a key element in several other major metabolic pathways. After stimulation by NPs, natriuretic peptide receptors (NPR) convert GTP to the second messenger cGMP. In addition to its vasodilatory effects and natriuretic and diuretic functions, cGMP has been positively associated with fat cell function, apoptosis, and NPR expression/activity modulation. The NP system is also closely linked to metabolic syndrome (MetS) progression and obesity control. A new era is now on its way targeting the NP system to not only treat high blood pressure, but to also assist in the fight against the obesity pandemic. Here, we summarize recent data on the role of NPs in hypertension and MetS.

C-type natriuretic peptide receptor expression in pancreatic alpha cells

Atrial natriuretic peptide (ANP), brain type natriuretic peptide (BNP) and C-type natriuretic peptide (CNP) comprise a family of natriuretic peptides that mediate their biological effects through three natriuretic peptide receptor subtypes, NPR-A (ANP, BNP), NPR-B (CNP) and NPR-C (ANP, BNP, CNP). Several reports have provided evidence for the expression of ANP and specific binding sites for ANP in the pancreas. The purpose of this study was to identify the ANP receptor subtype and to localize its expression to a specific cell type in the human pancreas. NPR-C immunoreactivity, but neither ANP nor NPR-A, was detected in human islets by immunofluorescent staining. No immunostaining was observed in the exocrine pancreas or ductal structures. Double-staining revealed that NPR-C was expressed mainly in the glucagon-containing alpha cells. NPR-C mRNA and protein were detected in isolated human islets by RT-PCR and Western blot analysis, respectively. NPR-C expression was also detected by immunofluorescent staining in glucagonoma but not in insulinoma. ANP, as well as BNP and CNP, stimulated glucagon secretion from perifused human islets (1,111 +/- 55% vs. basal [7.3 fmol/min]; P < 0.001). This response was mimicked by cANP(4-23), a selective agonist of NPR-C. In conclusion, the NPR-C receptor is expressed in normal and neoplastic human alpha cells. These findings suggest a role for natriuretic peptides in the regulation of glucagon secretion from human alpha cells.

Natriuretic peptides as regulatory mediators of secretory activity in the digestive system

Atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) are members of the natriuretic peptide family best known for their role in blood pressure regulation. However, in recent years all the natriuretic peptides and their receptors have been described in the gastrointestinal tract, digestive glands and central nervous system, as well as implicated in the regulation of digestive gland functions. The current review highlights the regulatory role of ANP and CNP in pancreatic and other digestive secretions. ANP and CNP stimulate basal as well as induced pancreatic secretion and modify bicarbonate and chloride secretions. Whereas ANP and CNP exert effects directly on pancreatic cells, CNP also acts through a vago-vagal reflex. At high doses both peptides attenuate pancreatic secretion induced by high doses of secretin through the PLC/PKC pathway. With regards to other digestive secretions, ANP and CNP decrease bile secretion in the rat. ANP does not induce salivation by itself but enhances stimulated salivary secretion and modifies salivary composition in rat parotid as well as submandibular glands. In rat pancreatic, hepatic, parotid and submandibular tissues, the NPR-C receptor mediates mostly peripheral responses whereas NPR-A and NPR-B receptors, which are coupled to guanylate cyclase, likely mediate the central response. In addition, ANP modulates gastric acid secretion via a vagal-dependent mechanism. In the intestine, ANP and CNP decrease water and sodium chloride absorption through an increase in cGMP levels. Overall, these findings indicate that ANP and CNP are members of the large group of regulatory peptides affecting digestive secretions.

NPRA-mediated suppression of AngII-induced ROS production contribute to the antiproliferative effects of B-type natriuretic peptide in VSMC

Excessive proliferation of vascular smooth cells (VSMCs) plays a critical role in the pathogenesis of diverse vascular disorders, and inhibition of VSMCs proliferation has been proved to be beneficial to these diseases. In this study, we investigated the antiproliferative effect of B-type natriuretic peptide (BNP), a natriuretic peptide with potent antioxidant capacity, on rat aortic VSMCs, and the possible mechanisms involved. The results indicate that BNP potently inhibited AngiotensinII (AngII)-induced VSMCs proliferation, as evaluated by [(3)H]-thymidine incorporation assay. Consistently, BNP significantly decreased AngII-induced intracellular reactive oxygen species (ROS) and NAD(P)H oxidase activity. 8-Br-cGMP, a cGMP analog, mimicked these effects. To confirm its mechanism, siRNA of natriuretic peptide receptor-A(NRPA) strategy technology was used to block cGMP production in VSMCs, and siNPRA attenuated the inhibitory effects of BNP in VSMCs. Taken together, these results indicate that BNP was capable of inhibiting VSMCs proliferation by NPRA/cGMP pathway, which might be associated with the suppression of ROS production. These results might be related, at least partly, to the anti-oxidant property of BNP.

Regulation of steroidogenesis by atrial natriuretic peptide (ANP) in the rat testis: differential involvement of GC-A and C receptors

Previous studies have established a stimulatory effect of natriuretic peptides (NP) on testosterone production in mouse Leydig cells as intense as that of LH. Chronic administration of ANP in mice, on the other side, reduced testosterone levels. So, the understanding of the role of ANP on testicular steroidogenesis has been impaired by discrepant findings. The aim of the present study was to clarify the physiological role of ANP in the rat testis steroidogenesis using a model that preserves the interactions between testis cells and a medium devoid of any circulating factors that could interfere with testosterone production. First, ANP was immunolocalized in the interstitial compartment of the rat testis, mainly in Leydig cells. We also determined the presence of ANP and both GC-A (guanylyl cyclase A) and C receptors by real-time PCR in testis. Perfusion in vitro of testis with ANP (1 and 3x10(-7)M) stimulated testosterone production in a time- and dose-dependent manner. On the other side, testosterone secretion induced by LH was blunted by ANP. Similar effect was obtained using the specific C receptor ligand, cANF, indicating the involvement of C receptor in such response. In conclusion, ANP stimulated testosterone production in the rat testis perfused in vitro but decreased testosterone production LH-induced, effect that seems to involve C receptor. To this extent, our results suggest the existence of a local and complex peptidergic system in the rat testis, involving ANP and its receptors that could importantly modulate the androgen biosynthesis.

Role of oxidative stress in high glucose-induced decreased expression of Gialpha proteins and adenylyl cyclase signaling in vascular smooth muscle cells

We have recently shown that aorta from streptozotocin (STZ)-induced diabetic rats and A10 vascular smooth muscle cells (VSMCs) exposed to high glucose exhibited decreased levels of inhibitory guanine nucleotide regulatory protein (Gi)alpha proteins. In the present studies, we investigated the implication of oxidative stress in the hyperglycemia/diabetes-induced decreased expression of the Gialpha protein and adenylyl cyclase signaling in VSMCs by using antioxidants. The levels of Gialpha proteins were significantly decreased in A10 VSMCs exposed to high glucose and in aortic VSMCs from STZ-diabetic rats compared with control cells and were restored to control levels by antioxidants. In addition, (111)Mn-tetralis(benzoic acid porphyrin) and uric acid, scavengers of peroxynitrite, and NG-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthase but not catalase, also restored the high glucose-induced decreased expression of Gialpha proteins to the control levels in A10 VSMCs. Furthermore, the enhanced production of superoxide anion (O2-) and increased activity of NADPH oxidase in these cells were also restored to control levels by diphenyleneiodonium, an inhibitor of NADPH oxidase. In addition, the diminished inhibition of adenylyl cyclase activity by inhibitory hormones and forskolin-stimulated adenylyl cyclase activity by low concentrations of GTPgammaS as well as the enhanced stimulation of adenylyl cyclase by stimulatory agonists in hyperglycemic cells were restored to control levels by antioxidant treatments. These results suggest that high glucose-induced decreased levels of Gialpha proteins and associated signaling in A10 VSMCs may be attributed to the enhanced oxidative stress due to augmented levels of peroxynitrite and not to H2O2.

Cyclic GMP modulates the expression of Gi protein and adenylyl cyclase signaling in vascular smooth muscle cells

We have recently shown that the nitric oxide (NO) donor, SNAP, decreased the expression of Gialpha proteins and associated functions in vascular smooth muscle cells. Because NO stimulates soluble guanylyl cyclase and increases the levels of guanosine 3\',5\'-cyclic monophosphate (cGMP), the present studies were undertaken to investigate whether cGMP can also modulate the expression of Gi proteins and associated adenylyl cyclase signaling. A10 vascular smooth muscle cells (VSMCs) and primary cultured cells from aorta of Sprague Dawley rats were used for these studies. The cells were treated with 8-bromoguanosine 3\',5\'-cyclic monophosphate (8BrcGMP) for 24 h and the expression of Gialpha proteins was determined by immunobloting techniques. Adenylyl cyclase activity was determined by measuring [32P]cAMP formation for [alpha-32P]ATP. Treatment of cells with 8-BrcGMP (0.5 mM) decreased the expression of Gialpha-2 and Gialpha-3 by about 30-45%, which was restored towards control levels by KT5823, an inhibitor of protein kinase G. On the other hand, the levels of Gsalpha protein were not altered by this treatment. The decreased expression of Gialpha proteins by 8Br-cGMP treatment was reflected in decreased Gi functions. For example, the inhibition of forskolin (FSK)-stimulated adenylyl cyclase activity by low concentrations of GTPgammaS (receptor-independent Gi functions) was significantly decreased by 8Br-cGMP treatment. In addition, exposure of the cells to 8Br-cGMP also resulted in the attenuation of angiotensin (Ang) II- and C-ANP4-23 (a ring-deleted analog of atrial natriuretic peptide [ANP])-mediated inhibition of adenylyl cyclase activity (receptor-dependant functions of Gi). On the other hand, Gsalpha-mediated stimulations of adenylyl cyclase by GTPgammaS, isoproterenol and FSK were significantly augmented in 8Br-cGMP-treated cells. These results indicate that 8Br-cGMP decreased the expression of Gialpha proteins and associated functions in VSMCs. From these studies, it can be suggested that 8Br-cGMP-induced decreased levels of Gi proteins and resultant increased levels of cAMP may be an additional mechanism through which cGMP regulates vascular tone and thereby blood pressure.

Peroxynitrite inhibits the expression of G(i)alpha protein and adenylyl cyclase signaling in vascular smooth muscle cells

We previously showed that S-nitroso-N-acetylpenicillamine, a nitric oxide donor, decreased the levels and functions of G(i)alpha proteins by formation of peroxynitrite (ONOO(-)) in vascular smooth muscle cells (VSMC). The present studies were undertaken to investigate whether ONOO(-) can modulate the expression of G(i)alpha protein and associated adenylyl cyclase signaling in VSMC. Treatment of A-10 and aortic VSMC with ONOO(-) for 24 h decreased the expression of G(i)alpha-2 and G(i)alpha-3, but not G(s)alpha, protein in a concentration-dependent manner; expression was restored toward control levels by (111)Mn-tetralis(benzoic acid porphyrin) and uric acid, but not by 1H[1,2,4]oxadiazole[4,3-a]quinoxaline-1-one (ODQ) and KT-5823. cGMP levels were increased by approximately 50% and 150% by 0.1 and 0.5 mM ONOO(-), respectively, and attenuated toward control levels by ODQ. In addition, 0.5 mM ONOO(-) attenuated the inhibition of adenylyl cyclase by ANG II and C-type atrial natriuretic peptide (C-ANP(4-23)), as well as the inhibition of forskolin-stimulated adenylyl cyclase activity by GTPgammaS, whereas, the G(s)-mediated stimulations were augmented. In addition, 0.5 mM ONOO(-) decreased phosphorylation of ERK1/2 and p38 MAP kinase and enhanced JNK phosphorylation but did not affect AKT1/3 phosphorylation. These results suggest that ONOO(-) decreased the expression of G(i) proteins and associated functions in VSMC through a cGMP-independent mechanism and may involve the MAP kinase signaling pathway.

Natriuretic peptide C receptor signalling in the heart and vasculature

Natriuretic peptides (NPs), including atrial, brain and C-type natriuretic peptides (ANP, BNP and CNP), bind two classes of cell surface receptors: the guanylyl cyclase-linked A and B receptors (NPR-A and NPR-B) and the C receptor (NPR-C). The biological effects of NPs have been mainly attributed to changes in intracellular cGMP following their binding to NPR-A and NPR-B. NPR-C does not include a guanylyl cyclase domain. It has been denoted as a clearance receptor and is thought to bind and internalize NPs for ultimate degradation. However, a substantial body of biochemical work has demonstrated the ability of NPR-C to couple to inhibitory G proteins (Gi) and cause inhibition of adenylyl cyclase and activation of phospholipase-C. Recently, novel physiological effects of NPs, mediated specifically by NPR-C, have been discovered in the heart and vasculature. We have described the ability of CNP, acting via NPR-C, to selectively inhibit L-type calcium currents in atrial and ventricular myocytes, as well as in pacemaker cells (sinoatrial node myocytes). In contrast, our studies of the electrophysiological effects of CNP on cardiac fibroblasts demonstrated an NPR-C-Gi-phospholipase-C-dependent activation of a non-selective cation current mediated by transient receptor potential (TRP) channels. It is also known that CNP and BNP have important anti-proliferative effects in cardiac fibroblasts that appear to involve NPR-C. In the mammalian resistance vessels, including mesenteric and coronary arteries, CNP has been found to function as an NPR-C-dependent endothelium-derived hyperpolarizing factor that regulates local blood flow and systemic blood pressure by hyperpolarizing smooth muscle cells. In this review we highlight the role of NPR-C in mediating these NP effects in myocytes and fibroblasts from the heart as well as in vascular smooth muscle cells.

Nitric oxide modulates Gi-protein expression and adenylyl cyclase signaling in vascular smooth muscle cells

We have previously shown that treatment of rats with the nitric oxide (NO) synthase inhibitor N6-nitro-L-arginine methyl ester for 4 weeks resulted in the augmentation of blood pressure and enhanced levels of Gialpha proteins. The present studies were undertaken to investigate if NO can modulate the expression of Gi proteins and associated adenylyl cyclase signaling. A10 vascular smooth muscle cells (VSMC) and primary cultured cells from aorta of Sprague-Dawley rats were used for these studies. The cells were treated with S-nitroso-N-acetylpenicillamine (SNAP) or sodium nitroprusside (SNP) for 24 h and the expression of Gialpha proteins was determined by immunobloting techniques. Adenylyl cyclase activity was determined by measuring [32P]cAMP formation for [alpha-32P]ATP. Treatment of cells with SNAP (100 microM) or SNP (0.5 mM) decreased the expression of Gialpha-2 and Gialpha-3 by about 25-40% without affecting the levels of Gsalpha proteins. The decreased expression of Gialpha proteins was reflected in decreased Gi functions (receptor-independent and -dependent) as demonstrated by decreased or attenuated forskolin-stimulated adenylyl cyclase activity by GTPgammaS and inhibition of adenylyl cyclase activity by angiotensin II and C-ANP4-23, a ring-deleted analog of atrial natriuretic peptide (ANP) that specifically interacts with natriuretic peptide receptor-C (NPR-C) in SNAP-treated cells. The SNAP-induced decreased expression of Gialpha-2 and Gialpha-3 proteins was not blocked by 1H[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one, an inhibitor of soluble guanylyl cyclase, or KT5823, an inhibitor of protein kinase G, but was restored toward control levels by uric acid, a scavenger of peroxynitrite and Mn(111)tetralis (benzoic acid porphyrin) MnTBAP, a peroxynitrite scavenger and a superoxide dismutase mimetic agent that inhibits the production of peroxynitrite, suggesting that NO-mediated decreased expression of Gialpha protein was cGMP-independent and may be attributed to increased levels of peroxynitrite. In addition, Gsalpha-mediated stimulation of adenylyl cyclase by GTPgammaS, isoproterenol, and forskolin was significantly augmented in SNAP-treated cells. These results indicate that NO decreased the expression of Gialpha protein and associated functions in VSMC by cGMP-independent mechanisms. From these studies, it can be suggested that NO-induced decreased levels of Gi proteins and resultant increased levels of cAMP may be an additional mechanism through which NO regulates blood pressure.

Small cytoplasmic domain peptides of natriuretic peptide receptor-C attenuate cell proliferation through Gialpha protein/MAP kinase/PI3-kinase/AKT pathways

The present studies were undertaken to investigate the effect of C-atrial natriuretic peptide (ANP)(4-23) and several peptide fragments containing 12 amino acids from different regions of the cytoplasmic domain of natriuretic peptide receptor (NPR)-C on cell proliferation in the absence or presence of angiotensin (ANG) II, endothelin (ET)-1, and arginine vasopressin (AVP) in A-10 vascular smooth muscle cells (VSMC). The peptide fragments used have either complete G(i) activator sequences K(461)-H(472) (peptide 1) and H(481)-H(492) (peptide 3) or partial G(i) activator sequences R(469)-K(480) (peptide 2) and I(465)-H(472) (peptide Y) with truncated COOH or NH(2) terminus, respectively. The other peptide used had no structural specificity (Q(473)-K(480), peptide X) or was the scrambled peptide control for peptide 1 (peptide Z). ANG II, ET-1 and AVP significantly stimulated DNA synthesis in these cells as determined by [(3)H]thymidine incorporation that was inhibited by peptides 1, 2, and 3 and not by peptides X, Y, and Z in a concentration-dependent manner, with an apparent K(i) between 1 and 10 nM. In addition, C-ANP(4-23), which interacts with NPR-C, also inhibited DNA synthesis stimulated by vasoactive peptides; however, the inhibition elicited by C-ANP(4-23) was not additive with the inhibition elicited by peptide 1. On the other hand, basal DNA synthesis in these cells was not inhibited by C-ANP(4-23) or the peptide fragments. Furthermore, vasoactive peptide-induced stimulation of DNA synthesis was inhibited by PD-98059 and wortmannin, and this inhibition was potentiated by peptide 1. In addition, peptide 1 also inhibited vasoactive peptide-induced phosphorylation of ERK1/2 and AKT and enhanced expression of G(i)alpha proteins. These data suggest that C-ANP(4-23) and small peptide fragments containing 12 amino acids irrespective of the region of the cytoplasmic domain of NPR-C inhibit proliferative responses of vasoactive peptides through G(i)alpha protein and MAP kinase/phosphatidylinositol 3-kinase/AKT pathways.

Natriuretic peptides, their receptors, and cyclic guanosine monophosphate-dependent signaling functions

Natriuretic peptides are a family of structurally related but genetically distinct hormones/paracrine factors that regulate blood volume, blood pressure, ventricular hypertrophy, pulmonary hypertension, fat metabolism, and long bone growth. The mammalian members are atrial natriuretic peptide, B-type natriuretic peptide, C-type natriuretic peptide, and possibly osteocrin/musclin. Three single membrane-spanning natriuretic peptide receptors (NPRs) have been identified. Two, NPR-A/GC-A/NPR1 and NPR-B/GC-B/NPR2, are transmembrane guanylyl cyclases, enzymes that catalyze the synthesis of cGMP. One, NPR-C/NPR3, lacks intrinsic enzymatic activity and controls the local concentrations of natriuretic peptides through constitutive receptor-mediated internalization and degradation. Single allele-inactivating mutations in the promoter of human NPR-A are associated with hypertension and heart failure, whereas homozygous inactivating mutations in human NPR-B cause a form of short-limbed dwarfism known as acromesomelic dysplasia type Maroteaux. The physiological effects of natriuretic peptides are elicited through three classes of cGMP binding proteins: cGMP-dependent protein kinases, cGMP-regulated phosphodiesterases, and cyclic nucleotide-gated ion channels. In this comprehensive review, the structure, function, regulation, and biological consequences of natriuretic peptides and their associated signaling proteins are described.

Biology of natriuretic peptides and their receptors

Increasing evidence suggests that natriuretic peptides (NPs) play diverse roles in mammals, including renal hemodynamics, neuroendocrine, and cardiovascular functions. Collectively, NPs are classified as hypotensive hormones; the main actions of NPs are implicated in eliciting natriuretic, diuretic, steroidogenic, antiproliferative, and vasorelaxant effects, important factors in the control of body fluid volume and blood pressure homeostasis. One of the principal loci involved in the regulatory actions of NPs is their cognate plasma membrane receptor molecules, which are activated by binding with specific NPs. Interaction of NPs with their receptors plays a central role in physiology and pathophysiology of hypertension and cardiovascular disorders. Gaining insight into the intricacies of NPs-specific receptor signaling pathways is of pivotal importance for understanding both hormone-receptor biology and the disease states arising from abnormal hormone receptor interplay. During the last decade there has been a surge in interest in NP receptors; consequently, a wealth of information has emerged concerning molecular structure and function, signaling mechanisms, and use of transgenics and gene-targeted mouse models. The objective of this present review is to summarize and document the previous findings and recent discoveries in the field of the natriuretic peptide hormone family and receptor systems with emphasis on the structure-function relationship, signaling mechanisms, and the physiological and pathophysiological significance in health and disease.

Natriuretic peptide receptor-C signaling and regulation

The natriuretic peptides (NP) are a family of three polypeptide hormones termed atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP). ANP regulates a variety of physiological parameters by interacting with its receptors present on the plasma membrane. These are of three subtypes NPR-A, NPR-B, and NPR-C. NPR-A and NPR-B are guanylyl cyclase receptors, whereas NPR-C is non-guanylyl cyclase receptor and is coupled to adenylyl cyclase inhibition or phospholipase C activation through inhibitory guanine nucleotide regulatory protein (Gi). ANP, BNP, CNP, as well as C-ANP(4-23), a ring deleted peptide that specifically interacts with NPR-C receptor inhibit adenylyl cyclase activity through Gi protein. Unlike other G-protein-coupled receptors, NPR-C receptors have a single transmembrane domain and a short cytoplasmic domain of 37 amino acids, which has a structural specificity like those of other single transmembrane domain receptors. A 37 amino acid cytoplasmic peptide is sufficient to inhibit adenylyl cyclase activity with an apparent Ki similar to that of ANP(99-126) or C-ANP(4-23). In addition, C-ANP(4-23) also stimulates phosphatidyl inositol (PI) turnover in vascular smooth muscle cells (VSMC) which is attenuated by dbcAMP and cAMP-stimulatory agonists, suggesting that NPR-C receptor-mediated inhibition of adenylyl cyclase and resultant decreased levels of cAMP may be responsible for NPR-C-mediated stimulation of PI turnover. Furthermore, the activation of NPR-C receptor by C-ANP(4-23) and CNP inhibits the mitogen-activated protein kinase activity stimulated by endothelin-3, platelet-derived growth factor, phorbol-12 myristate 13-acetate, suggesting that NPR-C receptor might also be coupled to other signal transduction system or that there may be an interaction of the NPR-C receptor and some other signaling pathways. In this review article, NPR-C receptor coupling to different signaling pathways and their regulation will be discussed.

C-type natriuretic peptide inhibits L-type Ca2+ current in rat magnocellular neurosecretory cells by activating the NPR-C receptor

Magnocellular neurosecretory cells (MNCs), of the paraventricular and supraoptic nuclei of the hypothalamus, secrete the hormones vasopressin and oxytocin. As a result, they have an essential role in fundamental physiological responses including regulation of blood volume and fluid homeostasis. C-type natriuretic peptide (CNP) is present at high levels in the hypothalamus. Although CNP is known to decrease hormone secretion from MNCs, no studies have examined the role of the natriuretic peptide C receptor (NPR-C) in these neurons. In this study, whole cell recordings from acutely isolated MNCs, and MNCs in a coronal slice preparation, show that CNP (2 x 10(-8) M) and the selective NPR-C agonist, cANF (2 x 10(-8) M), significantly inhibit L-type Ca2+ current (I(Ca(L))) by approximately 50%. This effect on I(Ca(L)) is mimicked by dialyzing a G(i)-activator peptide (10(-7) M) into these cells, implicating a role for the inhibitory G protein, G(i). These NPR-C-mediated effects were specific to I(Ca(L)). T-type Ca2+ channels were unaffected by CNP. Current-clamp experiments revealed the ability of CNP, acting via the NPR-C receptor, to decrease (approximately 25%) the number of action potentials elicited during a 500 ms depolarizing stimulus. Analysis of action potential duration revealed that CNP and cANF significantly decreased 50% repolarization time (APD50) in MNCs. In summary, our findings show that CNP has a potent and selective inhibitory effect on I(Ca(L)) and on excitability in MNCs that is mediated by the NPR-C receptor. These data represent the first electrophysiological evidence of a functional role for the NPR-C receptor in the mammalian hypothalamus.

Single Transmembrane Spanning Heterotrimeric G Protein-Coupled Receptors and Their Signaling Cascades

Heptahelical of serpentine receptors such as the adrenergic receptors are well known to mediate their actions via heterotrimeric GTP-binding proteins. Likewise, receptors that traverse the cell membrane once have been shown to mediate their biological actions by activating several different mechanisms including stimulation of their intrinsic tyrosine kinase activities or the kinase activities of other proteins. Some of these single transmembrane receptors have an intrinsic guanylyl cyclase activity and can stimulate the cyclic GMP second messenger system; however, over the last few years, several studies have shown the involvement of heterotrimeric GTP-binding proteins in mediating signals that eventually culminate in the biological actions of single transmembrane spanning receptors and proteins. These receptors include the receptor tyrosine kinases that mediate the actions of growth factors such as epidermal growth factor, insulin, insulin-like growth factor as well as receptors for atrial natiuretic hormone or the zona pellucida protein (ZP3) and integrins. In this review, the significance of the coupling of the single transmembrane spanning receptors to G proteins has been highlighted by providing several examples of the concept that signaling via these receptors may involve the activation of multiple signaling cascades.

Water deprivation enhances the inhibitory effect of natriuretic peptides on cAMP synthesis in rat renal glomeruli

This study investigates the effect of water deprivation on the expression of atrial natiruretic peptide (ANP)(1-28) binding sites in rat kidney. Water deprivation increased the B(max) of glomerular binding sites for ANP(1-28) and C-type natriuretic peptide (CNP)(1-22) without modifying their affinity, an effect that was prevented in the presence of C-atrial natriuretic factor (C-ANF), suggesting that natriuretic peptide receptor-C (NPR-C) binding sites might be enhanced. Our results indicate that ANP(1-28), CNP(1-22), and C-ANF inhibit cAMP synthesis directly stimulated by forskolin or by the physiological agonists histamine and 5-hydroxytryptamine. The inhibitory effect was found to be significantly greater in water-deprived rats than in controls. Our observations suggest that this effect must be attributed to the 67-kDa NPR-C-like protein, because the 67- and 77-kDa NPR-C-like proteins show high and low affinities for CNP(1-22), respectively, and the enhanced inhibitory effect of CNP on cAMP generation in water-deprived rats was detected at subnanomolar concentrations. In addition, using affinity cross-linking studies we have observed that water deprivation increases the expression of the 67-kDa NPR-C-like protein, and HS-142, which binds to NPR-A and the 77-kDa NPR-C-like but not the 67-kDa protein, reduced ligand internalization without affecting cAMP inhibition by ANP(1-28). Finally, we have found that ligand binding to the 67-kDa NPR-C-like protein is reduced by GTPgammaS, suggesting that this receptor is associated with a G protein in renal glomeruli. The enhanced inhibitory role of natriuretic peptides on cAMP synthesis induced by water deprivation may influence glomerular function in the rat kidney.

Atrial natriuretic peptide-C receptor-induced attenuation of adenylyl cyclase signaling activates phosphatidylinositol turnover in A10 vascular smooth muscle cells

Atrial natriuretic peptide (ANP)-C receptor activation has been shown to inhibit adenylyl cyclase (AC) activity as well as to stimulate phospholipase C (PLC) signaling pathways. The present studies were undertaken to investigate whether ANP-C receptor-mediated decreased cAMP levels contribute to the activation of PLC signaling. C-ANP(4-23) [des(Gln(18),Ser(19), Glu(20),Leu(21),Gly(22))ANP(4-23)-NH(2)], a ring-deleted peptide of ANP that interacts specifically with ANP-C receptor, stimulated inositol 1,4,5-tris-phosphate (IP(3)) production (PLC activity) in A10 vascular smooth muscle cells in a concentration- and time-dependent manner. The maximal stimulation observed was about 75% at 2 h of treatment, with an apparent EC(50) of about 20 to 30 nM. Pertussis toxin treatment of the cells completely abolished the C-ANP(4-23)-mediated stimulation of IP(3) production. Forskolin (FSK), a stimulator of adenylyl cyclase, dibutyryl cAMP (db cAMP), and isoproterenol (ISO), a beta-adrenergic agonist that stimulates adenylyl cyclase activity and cAMP levels, inhibited IP(3) production by about 35, 30, and 50%, respectively, whereas dideoxyadenosine (DDA), an inhibitor of adenylyl cyclase activity, and oxotremorine stimulated IP(3) production by about 90 and 80%, respectively, in these cells, suggesting a functional interaction between these two signaling pathways. Treatment of the cells with antisense oligonucleotide of ANP-C receptor that attenuated ANP-C receptor-mediated inhibition of adenylyl cyclase resulted in a complete attenuation of C-ANP(4-23)-induced stimulation of IP(3) formation, whereas FSK, db cAMP, and ISO-mediated decrease and oxotremorine and endothelin-1 (ET-1)-induced increase in IP(3) production was not affected by this treatment. Furthermore, C-ANP(4-23)-induced increase in IP(3) formation was significantly potentiated by DDA and inhibited by FSK and db cAMP, whereas ET-1-induced increase in IP(3) production was not affected by FSK. In addition, N-[2-(4-bromocinnamylamino)ethyl]-5-isoquinoline (H-89), an inhibitor of protein kinase A, completely abolished C-ANP(4-23) and not ET-1-induced stimulation of IP(3) production. These results indicate that ANP-C receptor activation by C-ANP(4-23) and resulting decrease in cAMP levels may be responsible for the activation of phosphatidylinositol (PI) turnover signaling, suggesting a cross-talk between ANP-C receptor-mediated adenylyl cyclase and PLC signaling pathways.

Redox modulation of Gi protein expression and adenylyl cyclase signaling: role of nitric oxide

Nitric oxide (NO) has been shown to regulate a variety of physiological functions, including vascular tone. The inhibition of NO synthase by N(omega)-nitro-L-arginine methyl ester (L-NAME) has been reported to increase arterial blood pressure. The present studies were undertaken to investigate if the increased blood pressure by L-NAME is associated with enhanced expression of Gi proteins, implicated in the pathogenesis of hypertension. L-NAME was administered orally into Sprague-Dawley rats for a period of 4 weeks. Control rats were given plain tap water only. The systolic blood pressure was enhanced in L-NAME-treated rats as compared with control rats; however, the heart-to-body weight ratio was not different in the two groups. The levels of Gialpha-2 and Gialpha-3 proteins and their mRNA as determined by western and northern blotting, respectively, were significantly augmented in hearts from L-NAME-treated rats, whereas the levels of Gsalpha and Gbeta were unaltered. In addition, the effect of low concentrations of GTPgammaS on forskolin-stimulated adenylyl cyclase activity (receptor-independent functions of Gialpha) was significantly enhanced, whereas the receptor-dependent inhibitions of adenylyl cyclase were completely attenuated in L-NAME-treated rats. Whereas cholera toxin-mediated stimulation of adenylyl cyclase was unaltered in both group of rats, the stimulatory effects of some agonists on adenylyl cyclase activity were diminished in L-NAME-treated rats. These results suggest the implication of NO in the modulation of Gi protein expression and associated adenylyl cyclase signaling.

Inhibition of L-type Ca2+ current by C-type natriuretic peptide in bullfrog atrial myocytes: an NPR-C-mediated effect

Single atrial myocytes were isolated from the bullfrog heart and studied under current and voltage clamp conditions to determine the electrophysiological effects of the C-type natriuretic peptide (CNP). CNP (10(-8) M) significantly shortened the action potential and reduced its peak amplitude after the application of isoproteronol (10(-7) M). In voltage clamp studies, CNP inhibited isoproteronol-stimulated L-type Ca2+ current (ICa) without any significant effect on the inward rectifier K+ current. The effects of cANF (10(-8) M), a selective agonist of the natriuretic peptide C receptor (NPR-C), were very similar to those of CNP. Moreover, HS-142-1, an antagonist of the guanylyl cyclase-linked NPR-A and NPR-B receptors did not alter the inhibitory effect of CNP on ICa. Inclusion of cAMP in the recording pipette to stimulate ICa at a point downstream from adenylyl cyclase increased ICa, but this effect was not inhibited by cANF. These results provide the first demonstration that CNP can inhibit ICa after binding to NPR-C, and suggest that this inhibition involves a decrease in adenylyl cyclase activity, which leads to reduced intracellular levels of cAMP.

Modulation of ANP-C receptor signaling by arginine-vasopressin in A-10 vascular smooth muscle cells: role of protein kinase C

We have previously shown that pretreatment of A-10 vascular smooth muscle cells (VSMC) with angiotensin II (Ang II) attenuated atrial natriuretic peptide receptor-C (ANP-C)-mediated inhibition of adenylyl cyclase without altering [125I]ANP binding. In the present studies, we have investigated the modulation of ANP-C receptor signaling by arginine-vasopressin (AVP). Pretreatment of A-10 VSMC with AVP for 24h resulted in a reduction in ANP receptor binding activity by about 50% (B(max); control cells, 22.9+/-2.5 fmol/mg protein, AVP-treated cells, 11.4+/-1.2 fmol/mg protein). In addition, the expression of ANP-C receptor as determined by immunoblotting was also decreased by about 50% by AVP treatment, which was prevented by GF109203X, an inhibitor of protein kinase C (PKC). The decreased expression of ANP-C receptor was reflected in an attenuation of ANP-C receptor-mediated inhibition of adenylyl cyclase. C-ANP(4-23) [des(Gln(18),Ser(19),Gln(20),Leu(21),Gly(22))ANP(4-23)-NH(2)], a ring deleted peptide of ANP that interacts specifically with ANP-C receptor, inhibited adenylyl cyclase activity by about 30% in control cells, which was completely attenuated in AVP-treated cells. This attenuated inhibition was significantly restored by GF 109203X. In addition, AVP treatment augmented the levels of Gialpha-2 and Gialpha-3 proteins; however, the Gi functions were completely attenuated. The increased expression of Gialpha proteins induced by AVP was inhibited by GF109203X as well as by actinomycin D treatments. In addition, AVP treatment also enhanced the expression of Gsalpha protein and Gsalpha-mediated stimulation of adenylyl cyclase by GTPgammaS, N-ethylcarboxamide adenosine (NECA), and forskolin (FSK), whereas the levels of Gbeta were not altered by AVP treatment. These results indicate that AVP-induced PKC signaling may be responsible for the down-regulation of ANP-C receptor that results in the attenuation of C-ANP(4-23)-mediated inhibition of adenylyl cyclase activity, and suggest a cross-talk between vasopressin V(1) and ANP-C receptor-mediated signaling pathways.

Angiotensin-II-induced enhanced expression of Gi proteins is attenuated by losartan in A10 vascular smooth muscle cells: role of AT1 receptors

We have previously shown that treatment of A10 vascular smooth muscle cells (VSMCs) with angiotensin II (Ang II) enhanced the expression of inhibitory guanine nucleotide regulatory proteins (Gi alpha2 and Gi alpha3). In the present studies, we have investigated the role of type 1 angiotensin receptors (AT1) in the Ang-II-induced enhanced expression of Gi alpha proteins and their functions in A10 SMCs. Ang II enhanced the levels of Gi alpha2 and Gi alpha3 proteins and their mRNA, as determined by Western and Northern blot analysis, respectively; losartan treatment attenuated the enhanced expression of Gi alpha2 and Gi alpha3 proteins and their mRNA in a concentration-dependent manner. In addition, the inhibition of adenylyl cyclase induced by Ang II and des(Glu18,Ser19,Glu20,Leu21,Gly22)ANP(4-23)-NH2 (C-ANP(4-23)), which was attenuated by Ang-II treatment, was partially restored by losartan treatment. Similarly, losartan was also able to restore the Ang-II-induced stimulatory responses of isoproterenol and N-ethylcarboxamide adenosine (NECA) on adenylyl cyclase activity. These results suggest a role for AT1 receptors in Ang-II-evoked increases in Gi alpha protein expression and Gs-mediated stimulation in VSMCs.

Beta-adrenoceptor control of G protein function in the neonate: determinant of desensitization or sensitization

Neonatal beta-adrenoceptors (beta-ARs) are resistant to agonist-induced desensitization. We examined the functioning of G(i) and G(s) after repeated administration of beta-AR agonists to newborn rats. Isoproterenol (beta(1)/beta(2) agonist) obtunded G(i) function in the heart but not the liver; in contrast, terbutaline, a beta(2)-selective agonist, enhanced G(i) function. Isoproterenol, but not terbutaline, increased membrane-associated G((s)alpha), which would enhance receptor function. In addition, isoproterenol increased and terbutaline maintained the proportion of the short-splice (S) variant of G((s)alpha) in the membrane fraction; G((s)alpha)S is functionally more active than the long-splice variant. Either isoproterenol or terbutaline treatment increased G((s)alpha) in the cytosolic fraction, a characteristic usually associated with desensitization in the adult. Decreased G(i) activity, coupled with increased membrane-associated G((s)alpha) concentrations and maintenance or increases in membrane G((s)alpha)S, provide strong evidence that unique effects on G protein function underlie the ability of the immature organism to sustain beta-AR cell signaling in the face of excessive or prolonged stimulation; these mechanisms also contribute to tissue selectivity of the effects of beta-agonists with divergent potencies toward different beta-AR subtypes.

Alterations in g-protein-linked signal transduction in vascular smooth muscle in diabetes

The present studies were undertaken to determine the levels of stimulatory and inhibitory guanine nucleotide regulatory proteins (Gs and Gi respectively) and their relationship with adenylyl cyclase activity in aorta from 5-day streptozotocin-induced diabetic (STZ) rats. The levels of Gi alpha-2 as determined by immunoblotting techniques using AS/7 antibody were significantly decreased by about 60% in STZ as compared to control rats, whereas the levels of Gs alpha were not altered. In addition, the stimulatory effect of cholera toxin (CT) on GTP-sensitive adenylyl cyclase was not different in STZ as compared to control rats. On the other hand, the stimulatory effects of GTPgammaS, isoproterenol, glucagon, forskolin (FSK) and sodium fluoride on adenylyl cyclase were enhanced in STZ-rats. Furthermore, GTPgammaS inhibited FSK-stimulated adenylyl cyclase activity in a concentration-dependent manner (receptor independent functions of Gi) in control rats which was almost completely abolished in STZ rats. In addition, receptor-mediated inhibition of adenylyl cyclase by angiotensin II (AII), oxotremorine and atrial natriuretic peptide (ANP) was attenuated in STZ rats. These results suggest that the decreased expression of Gi alpha, but not of Gs alpha, may be responsible for the observed altered responsiveness of adenylyl cyclase to hormonal stimulation and inhibition in STZ-rats. It may thus be suggested that the decreased Gi activity may be one of the possible mechanisms responsible for the impaired vascular functions in diabetes.

Inactivation of enhanced expression of G(i) proteins by pertussis toxin attenuates the development of high blood pressure in spontaneously hypertensive rats

We have previously shown that the enhanced expression of G(i) proteins in spontaneously hypertensive rats (SHR) that precedes the development of high blood pressure may be one of the contributing factors in the pathogenesis of hypertension. In the present study, we demonstrate that the inactivation of G(i) proteins by intraperitoneal injection of pertussis toxin (PT, 1.5 micro g/100 g body wt) into 2-week-old prehypertensive SHR prevented the development of hypertension up to 4 weeks and that, thereafter, it started to increase and reached the same level found in untreated SHR after 6 weeks. A second injection of PT after 4 weeks delayed the increase in blood pressure for another week. The PT-induced decrease in blood pressure in 6-week-old SHR was associated with a decreased level of G(i)alpha-2 and G(i)alpha-3 proteins in the heart, as determined by in vitro ADP ribosylation and immunoblotting. The decreased level of G(i) proteins was reflected in decreased G(i) functions. Furthermore, an augmentation of blood pressure to the same level in PT-treated SHR as found in untreated SHR was associated with enhanced expression and function of G(i). These results indicate that the inactivation of G(i) proteins by PT treatment in prehypertensive SHR attenuates the development of hypertension and suggest that the enhanced levels of G(i) proteins that result in the decreased levels of cAMP and associated impaired cellular functions may be contributing factors in the pathogenesis of hypertension in SHR.

Modulation of ANP-C receptor signaling by endothelin-1 in A-10 smooth muscle cells

We have previously shown that pretreatment of A-10 smooth muscle cells (SMC) with angiotensin II (Ang II) attenuated atrial natriuretic peptide (ANP) receptor-C (ANP-C)-mediated inhibition of adenylyl cyclase without altering (125)I-ANP binding. In the present studies, we have investigated the modulation of ANP-C receptor signaling by endothelin-1 (ET-1). Pretreatment of A-10 SMC with ET-1 for 24 h attenuated the expression of ANP-C receptor by about 60% as determined by immunoblotting which was reflected in attenuation of ANP-C-receptor-mediated inhibition of adenylyl cyclase. C-ANP(4-23) [des(Gln(18),Ser(19),Gln(20),Leu(21),Gly(22))ANP(4-23)-NH(2)], a ring-deleted peptide of ANP that interacts specifically with ANP-C receptor, inhibited adenylyl cyclase activity in a concentration-dependent manner with an apparent K(i) of about 1 nM in control cells. The maximal inhibition observed was about 30% which was almost completely attenuated in ET-1-treated cells. In addition, Ang II- and oxotremorine-mediated inhibitions of adenylyl cyclase were also attenuated by ET-1 treatment; however, the expression of Gialpha-2 and Gialpha-3 proteins and not of Gsalpha and Gbeta proteins was augmented by such treatment. The increased expression of Gialpha-2 and Gialpha-3 proteins by ET-1 treatment was inhibited by actinomycin D treatment (RNA synthesis inhibitor). On the other hand, the Gsalpha-mediated effects of some agonists on adenylyl cyclase activity were significantly decreased by ET-1 treatment. These results suggest that ET-1-induced downregulation of ANP-C receptor and not the overexpression of Gi proteins may be responsible for the attenuation of C-ANP(4-23)-mediated inhibition of adenylyl cyclase activity. From these studies it may be suggested that the downregulation of ANP-C receptors by increased levels of endothelin in vivo may be one of the possible mechanisms for the pathophysiology of hypertension.

Relaxation and modulation of cyclic AMP production in response to atrial natriuretic peptides in guinea pig tracheal smooth muscle

Relaxation and modulation of cyclic AMP production in response to atrial natriuretic peptides were investigated in epithelium-denuded guinea pig tracheal rings, treated with indomethacin (5 microM) and phosphoramidon (1 microM) and contracted with histamine (3 microM). Atrial natriuretic peptide (ANP) was a more potent relaxant than C-type natriuretic peptide whereas ANP-(4-23) was inactive suggesting the involvement of ANP(A) receptors in the relaxant effect of ANP. ODQ (1H-[1,2,4]oxadiazolo[4,3-A]quinoxalin-1-one, 10 microM), a selective inhibitor of soluble guanylyl cyclase, markedly inhibited the relaxant response to sodium nitroprusside. The relaxant response to ANP was not altered by ODQ demonstrating the involvement of particulate guanylyl cyclase. ANP-induced relaxations, as well as sodium nitroprusside-induced relaxations, were similarly potentiated by rolipram (4-(3-(cyclopentyloxy)-4-methoxyphenyl)pyrrolidin-2-one, 3 microM), a type IV phosphodiesterase inhibitor, and by zaprinast (2-(2-propyloxyphenyl)-8-azapurin-6-one, 10 microM), a type V phosphodiesterase inhibitor. ANP-mediated response was unaffected by glibenclamide (10 microM), a selective blocker of ATP-sensitive K(+) channels, and by apamin (1 microM), a selective blocker of small-conductance Ca(2+)-activated K(+) channels. Iberiotoxin (100 nM) extensively prevented the relaxant effect of ANP suggesting the activation of large-conductance Ca(2+)-activated K(+) channels. In addition, ANP (10 nM) and ANP-(4-23) (100 nM) significantly reduced forskolin (1 microM)-stimulated cAMP accumulation suggesting, for the first time, the presence of functional ANP(C) receptors in guinea pig airway smooth muscle. However, relaxations to forskolin and to isoproterenol were not altered in the presence of ANP-(4-23) or ANP demonstrating that the inhibitory effect of ANP-(4-23) and ANP on adenylyl cyclase was not sufficient to alter the functional response induced by these two activators of adenylyl cyclase.