Modulation of angiotensin II receptor expression during development and regression of hypoxic pulmonary hypertension

Cell type-specific expression of angiotensin receptors in the human lung with implications for health, aging, and chronic disease

The renin-angiotensin system is a highly characterized integrative pathway in mammalian homeostasis whose clinical spectrum has been expanded to lung disorders such as chronic obstructive pulmonary disease (COPD)-emphysema, idiopathic pulmonary fibrosis (IPF), and COVID pathogenesis. Despite this widespread interest, specific localization of this receptor family in the mammalian lung is limited, partially due to the imprecision of available antibody reagents. In this study, we establish the expression pattern of the two predominant angiotensin receptors in the human lung, AGTR1 and AGTR2, using complementary and comprehensive bulk and single-cell RNA-sequence datasets that are publicly available. We show these two receptors have distinct localization patterns and developmental trajectories in the human lung, pericytes for AGTR1 and a subtype of alveolar epithelial type 2 cells for AGTR2. In the context of disease, we further pinpoint AGTR2 localization to the COPD-associated subpopulation of alveolar epithelial type 2 (AT2B) and AGTR1 localization to fibroblasts, where their expression is upregulated in individuals with COPD, but not in individuals with IPF. Finally, we examine the genetic variation of the angiotensin receptors, finding AGTR2 associated with lung phenotype (i.e., cystic fibrosis) via rs1403543. Together, our findings provide a critical foundation for delineating this pathway's role in lung homeostasis and constructing rational approaches for targeting specific lung disorders.

Catch your breath: The protective role of the angiotensin AT2 receptor for the treatment of idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive interstitial lung disease whereby excessive deposition of extracellular matrix proteins (ECM) ultimately leads to respiratory failure. While there have been advances in pharmacotherapies for pulmonary fibrosis, IPF remains an incurable and irreversible disease. There remains an unmet clinical need for treatments that reverse fibrosis, or at the very least have a more tolerable side effect profile than currently available treatments. Transforming growth factor β1(TGFβ1) is considered the main driver of fibrosis in IPF. However, as our understanding of the role of the pulmonary renin-angiotensin system (PRAS) in the pathogenesis of IPF increases, it is becoming clear that targeting angiotensin receptors represents a potential novel treatment strategy for IPF - in particular, via activation of the anti-fibrotic angiotensin type 2 receptor (AT2R). This review describes the current understanding of the pathophysiology of IPF and the mediators implicated in its pathogenesis; focusing on TGFβ1, angiotensin II and related peptides in the PRAS and their contribution to fibrotic processes in the lung. Preclinical and clinical assessment of currently available AT2R agonists and the development of novel, highly selective ligands for this receptor will also be described, with a focus on compound 21, currently in clinical trials for IPF. Collectively, this review provides evidence of the potential of AT2R as a novel therapeutic target for IPF.

Genomic adaptation of Ethiopian indigenous cattle to high altitude

The mountainous areas of Ethiopia represent one of the most extreme environmental challenges in Africa faced by humans and other inhabitants. Selection for high-altitude adaptation is expected to have imprinted the genomes of livestock living in these areas. Here we assess the genomic signatures of positive selection for high altitude adaptation in three cattle populations from the Ethiopian mountainous areas (Semien, Choke, and Bale mountains) compared to three Ethiopian lowland cattle populations (Afar, Ogaden, and Boran), using whole-genome resequencing and three genome scan approaches for signature of selection (iHS, XP-CLR, and PBS). We identified several candidate selection signature regions and several high-altitude adaptation genes. These include genes such as ITPR2, MB, and ARNT previously reported in the human population inhabiting the Ethiopian highlands. Furthermore, we present evidence of strong selection and high divergence between Ethiopian high- and low-altitude cattle populations at three new candidate genes (CLCA2, SLC26A2, and CBFA2T3), putatively linked to high-altitude adaptation in cattle. Our findings provide possible examples of convergent selection between cattle and humans as well as unique African cattle signature to the challenges of living in the Ethiopian mountainous regions.

Effects of high altitude on renal physiology and kidney diseases

The hypobaric and hypoxic conditions of high-altitude areas exert adverse effects on the respiratory, circulatory and nervous systems. The kidneys have an abundant blood supply (20%–25% of cardiac output) and high blood flow; thus, they are susceptible to the effects of hypoxia. However, the effects of acute and chronic exposure to high altitudes on renal physiology and pathology are not fully understood. Moreover, few studies have investigated the impact of high-altitude exposure on patients with chronic kidney disease or acute kidney injury. In this review, we summarized changes in renal physiology and renal pathology due to high-altitude exposure as well as the impact of high-altitude exposure on existing kidney diseases, with the aim of informing the prevention and treatment of kidney diseases at high altitudes.

The Angiotensin AT2 Receptor: From a Binding Site to a Novel Therapeutic Target

Discovered more than 30 years ago, the angiotensin AT2 receptor (AT2R) has evolved from a binding site with unknown function to a firmly established major effector within the protective arm of the renin-angiotensin system (RAS) and a target for new drugs in development. The AT2R represents an endogenous protective mechanism that can be manipulated in the majority of preclinical models to alleviate lung, renal, cardiovascular, metabolic, cutaneous, and neural diseases as well as cancer. This article is a comprehensive review summarizing our current knowledge of the AT2R, from its discovery to its position within the RAS and its overall functions. This is followed by an in-depth look at the characteristics of the AT2R, including its structure, intracellular signaling, homo- and heterodimerization, and expression. AT2R-selective ligands, from endogenous peptides to synthetic peptides and nonpeptide molecules that are used as research tools, are discussed. Finally, we summarize the known physiological roles of the AT2R and its abundant protective effects in multiple experimental disease models and expound on AT2R ligands that are undergoing development for clinical use. The present review highlights the controversial aspects and gaps in our knowledge of this receptor and illuminates future perspectives for AT2R research. SIGNIFICANCE STATEMENT: The angiotensin AT2 receptor (AT2R) is now regarded as a fully functional and important component of the renin-angiotensin system, with the potential of exerting protective actions in a variety of diseases. This review provides an in-depth view of the AT2R, which has progressed from being an enigma to becoming a therapeutic target.

Angiotensin-(1-9) in hypertension

Angiotensin-(1-9) [Ang-(1-9)] is a peptide of the non-canonical renin-angiotensin system (RAS) synthesized from angiotensin I by the monopeptidase angiotensin-converting enzyme type 2 (ACE2). Using osmotic minipumps, infusion of Ang-(1-9) consistently reduces blood pressure in several rat hypertension models. In these animals, hypertension-induced end-organ damage is also decreased. Several pieces of evidence suggest that Ang-(1-9) is the endogenous ligand that binds and activates the type-2 angiotensin II receptor (AT2R). Activation of AT2R triggers different tissue-specific signaling pathways. This phenomenon could be explained by the ability of AT2R to form different heterodimers with other G protein-coupled receptors. Because of the antihypertensive and protective effects of AT2R activation by Ang-(1-9), associated with a short half-life of RAS peptides, several synthetic AT2R agonists have been synthesized and assayed. Some of them, particularly CGP42112, C21 and novokinin, have demonstrated antihypertensive properties. Only two synthetic AT2R agonists, C21 and LP2-3, have been tested in clinical trials, but none of them like an antihypertensive. Therefore, Ang-(1-9) is a promising antihypertensive drug that reduces hypertension-induced end-organ damage. However, further research is required to translate this finding successfully to the clinic.

Defective Uteroplacental Vascular Remodeling in Preeclampsia: Key Molecular Factors Leading to Long Term Cardiovascular Disease

Preeclampsia is a complex hypertensive disorder in pregnancy which can be lethal and is responsible for more than 70,000 maternal deaths worldwide every year. Besides the higher risk of unfavorable obstetric outcomes in women with preeclampsia, another crucial aspect that needs to be considered is the association between preeclampsia and the postpartum cardiovascular health of the mother. Currently, preeclampsia is classified as one of the major risk factors of cardiovascular disease (CVD) in women, which doubles the risk of venous thromboembolic events, stroke, and ischemic heart disease. In order to comprehend the pathophysiology behind the linkage between preeclampsia and the development of postpartum CVD, a thorough understanding of the abnormal uteroplacental vascular remodeling in preeclampsia is essential. Therefore, this review aims to summarize the current knowledge of the defective process of spiral artery remodeling in preeclampsia and how the resulting placental damage leads to excessive angiogenic imbalance and systemic inflammation in long term CVD. Key molecular factors in the pathway-including novel findings of microRNAs-will be discussed with suggestions of future management strategies of preventing CVD in women with a history of preeclampsia.

Comprehensive identification of signaling pathways for idiopathic pulmonary arterial hypertension

Whole exome sequencing (WES) was used in the research of familial pulmonary arterial hypertension (FPAH). CAV1 and KCNK3 were found as two novel candidate genes of FPAH. However, few pathogenic genes were identified in idiopathic pulmonary arterial hypertension (IPAH). We conducted WES in 20 unrelated IPAH patients who did not carry the known PAH-pathogenic variants among BMPR2, CAV1, KCNK3, SMAD9, ALK1, and ENG. We found a total of 4,950 variants in 3,534 genes, including 4,444 single-nucleotide polymorphisms and 506 insertions/deletions (InDels). Through the comprehensive and multilevel analysis, we disclosed several novel signaling cascades significantly connected to IPAH, including variants related to cadherin signaling pathway, dilated cardiomyopathy, glucose metabolism, immune response, mucin-type O-glycosylation, phospholipase C (PLC)-activating G protein-coupled receptor (GPCR) signaling pathway, vascular contraction and generation, and voltage-dependent Ca²⁺ channels. We also conducted validation studies in five mutant genes related to PLC-activating GPCR signaling pathway potentially involved in intracellular calcium regulation through Sanger sequencing for mutation accuracy, qRT-PCR for mRNA stability, immunofluorescence for subcellular localization, Western blotting for protein level, Fura-2 imaging for intracellular calcium, and proliferation analysis for cell function. The validation experiments showed that those variants in CCR5 and C3AR1 significantly increased the rise of intracellular calcium and the variant in CCR5 profoundly enhanced proliferative capacity of human pulmonary artery smooth muscle cells. Thus, our study suggests that multiple genetically affected signaling pathways take effect together to cause the formation of IPAH and the development of right heart failure and may further provide new therapy targets or putative clues for the present treatments such as limited therapeutic effectiveness of Ca²⁺ channel blockers.

Structural alterations and markers of endothelial activation in pulmonary and bronchial arteries in fatal asthma

Background

There is interest in better understanding vessel pathology in asthma, given the findings of loss of peripheral vasculature associated with disease severity by imaging and altered markers of endothelial activation. To date, vascular changes in asthma have been described mainly at the submucosal capillary level of the bronchial microcirculation, with sparse information available on the pathology of bronchial and pulmonary arteries. The aim of this study was to describe structural and endothelial activation markers in bronchial arteries (BAs) and pulmonary arteries (PAs) of asthma patients who died during a fatal asthma attack.

Methods

Autopsy lung tissue was obtained from 21 smoking and non-smoking patients who died of an asthma attack and nine non-smoking control patients. Verhoeff–Masson trichrome staining was used to analyse the structure of arteries. Using immuno-histochemistry and image analyses, we quantified extracellular matrix (ECM) components (collagen I, collagen III, versican, tenascin, fibronectin, elastic fibres), adhesion molecules [vascular cell adhesion molecule 1 (VCAM-1) and intercellular adhesion molecule 1 (ICAM-1)] and markers of vascular tone/dysfunction [endothelin-1 (ET-1) and angiotensin II type 2 receptor (AT2)] in PAs and BAs.

Results

There were no significant differences in ECM components, ICAM-1, ET-1 or AT2 between asthma patients and controls. Smoking asthma patients presented with decreased content of collagen III in both BA (p = 0.046) and PA (p = 0.010) walls compared to non-smoking asthma patients. Asthma patients had increased VCAM-1 content in the BA wall (p = 0.026) but not in the PA wall.

Conclusion

Our data suggest that the mechanisms linking asthma and arterial functional abnormalities might involve systemic rather than local mediators. Loss of collagen III in the PA was observed in smoking asthma patients, and this was compatible with the degradative environment induced by cigarette smoking. Our data also reinforce the idea that the mechanisms of leukocyte efflux via adhesion molecules differ between bronchial and pulmonary circulation, which might be relevant to understanding and treating the distal lung in asthma.

Pulmonary Arterial Hypertension Due to NPR-C Mutation: A Novel Paradigm for Normal and Pathologic Remodeling?

Idiopathic Pulmonary Arterial Hypertension (IPAH) is a deadly and disabling disease characterized by severe vascular remodeling of small pulmonary vessels by fibroblasts, myofibroblasts and vascular smooth muscle cell proliferation. Recent studies suggest that the Natriuretic Peptide Clearance Receptor (NPR-C) signaling pathways may play a crucial role in the development of IPAH. Reduced expression or function of NPR-C signaling in pulmonary artery smooth muscle cells may contribute to the pulmonary vascular remodeling, which is characteristic of this disease. The likely mechanisms may involve an impaired interaction between NPR-C, specific growth factors and other signal transduction pathways including but not limited to Gqα/mitogen-activated protein kinase (MAPK)/PI3K and AKT signaling. The resulting failure of growth suppression in pulmonary artery smooth muscle cells provides critical clues to the cellular pathobiology of IPAH. The reciprocal regulation of NPR-C signaling in models of tissue remodeling may thus provide new insights to our understanding of IPAH.

A pilot investigation into the effects of acute normobaric hypoxia, high altitude exposure and exercise on serum angiotensin-converting enzyme, aldosterone and cortisol

Introduction:

Aldosterone decreases at high altitude (HA) but the effect of hypoxia on angiotensin-converting enzyme (ACE), a key step in the renin-angiotensin-aldosterone system, is unclear.

Methods:

We investigated the effects of exercise and acute normobaric hypoxia (NH, ~11.0% FiO₂) on nine participants and six controls undertaking the same exercise at sea level (SL). NH exposure lasted 5 hours with 90 minutes of submaximal treadmill walking. Blood samples for aldosterone, ACE and cortisol were taken throughout exposure and at rest during a trek to HA (5140 m) in eight separate participants.

Results:

There was no difference in cortisol or aldosterone between groups pre-exercise. Aldosterone rose with exercise to a greater extent at SL than in NH (post-exercise: 700 ± 325 versus 335 ± 238 pmol/L, mean ± SD, p = 0.044). Conversely, cortisol rose to a greater extent in NH (post-exercise: 734 ± 165 versus 344 ± 159 nmol/L, mean ± SD, p = 0.001). There were no differences in ACE activity. During the trek to HA, resting aldosterone and cortisol reduced with no change in ACE.

Conclusions:

Acute NH subdues the exercise-associated rise in aldosteroe but stimulates cortisol, whereas prolonged exposure at HA reduces both resting aldosterone and cortisol. As ACE activity was unchanged in both environments, this is not the mechanism underlying the fall in aldosterone.

Further investigation of a quantitative trait locus for ascites on chromosome 9 in broiler chicken lines

Previously, we reported a genome wide association study (GWAS) that had shown association of a region between 11.8 and 13.6 Mbp on chromosome 9 with ascites phenotype in broilers. We had used microsatellite loci to demonstrate an association of particular genotypes for this region with ascites in experimental ascites lines and commercial broiler breeder lines. We identified two potential candidate genes, AGTR1 and UTS2D, within that chromosomal region for mediating the quantitative effect. We have now extended our analysis using SNPs for these genes to assess association with resistance or susceptibility to ascites in these same broiler lines. Surprisingly, in contrast to our previous GWAS and microsatellite data for this region, we find no association of the SNP genotypes or haplotypes in the region suggesting that the two genes might have limited association with the disease phenotype.

Pulmonary artery denervation improves pulmonary arterial hypertension induced right ventricular dysfunction by modulating the local renin-angiotensin-aldosterone system

Background

Pulmonary arterial hypertension (PAH) is commonly accompanied with the activation of the renin-angiotensin-aldosterone system (RAAS). Renal sympathetic denervation (RSD) reduces PAH partly through the inhibition of RAAS. Analogically, we hypothesized that pulmonary artery denervation (PADN) could reverse PAH and PAH-induced right ventricular (RV) dysfunction by downregulating the local RAAS activity.

Methods

Twenty-five beagle dogs were randomized into two groups: control group (intra-atrial injection of N-dimethylacetamide, 3 mg/kg, n = 6) and test group (intra-atrial injection of dehydrogenized-monocrotaline, 3 mg/kg, n = 19). Eight weeks later, dogs in the test group with mean pulmonary arterial pressure (mPAP) ≥25 mmHg (n = 16) were reassigned into the sham (n = 8) and PADN groups (n = 8) by chance. After another 6 weeks, the hemodynamics, pulmonary tissue morphology and the local RAAS expression in lung and right heart tissue were measured.

Results

PADN reduced the mPAP (25.94 ± 3.67 mmHg vs 33.72 ± 5.76 mmHg, P < 0.05) and the percentage of medial wall thickness (%MWT) (31.0 ± 2.6 % vs 37.9 ± 2.8 %, P < 0.05) compared with the sham group. PADN attenuated RV dysfunction, marked with reduced atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and ratio of right ventricular to left ventricular plus septum weight [RV/(LV + S)]. Moreover, the local RAAS expression was activated in PAH dogs while inhibited after PADN.

Conclusions

PADN improves hemodynamics and relieves RV dysfunction in dogs with PAH, which can be associated with the downregulating RAAS activity in local tissue.

Bioactive factors in uteroplacental and systemic circulation link placental ischemia to generalized vascular dysfunction in hypertensive pregnancy and preeclampsia

Preeclampsia is a pregnancy-associated disorder characterized by hypertension, and could lead to maternal and fetal morbidity and mortality; however, the pathophysiological mechanisms involved are unclear. Predisposing demographic, genetic and environmental risk factors could cause localized abnormalities in uteroplacental cytoactive factors such as integrins, matrix metalloproteinases, cytokines and major histocompatibility complex molecules leading to decreased vascular remodeling, uteroplacental vasoconstriction, trophoblast cells apoptosis, and abnormal development of the placenta. Defective placentation and decreased trophoblast invasion of the myometrium cause reduction in uteroplacental perfusion pressure (RUPP) and placental ischemia/hypoxia, an important event in preeclampsia. RUPP could stimulate the release of circulating bioactive factors such as the anti-angiogenic factors soluble fms-like tyrosine kinase-1 and soluble endoglin that cause imbalance with the pro-angiogenic factors vascular endothelial growth factor and placental growth factor, or cause the release of inflammatory cytokines, reactive oxygen species, hypoxia-induced factor-1 and AT1 angiotensin receptor agonistic autoantibodies. The circulating bioactive factors target endothelial cells causing generalized endotheliosis, endothelial dysfunction, decreased vasodilators such as nitric oxide and prostacyclin and increased vasoconstrictors such as endothelin-1 and thromboxane A2, leading to increased vasoconstriction. The bioactive factors also stimulate the mechanisms of VSM contraction including Ca(2+), protein kinase C, and Rho-kinase and induce extracellular matrix remodeling leading to further vasoconstriction and hypertension. While therapeutic options are currently limited, understanding the underlying mechanisms could help design new interventions for management of preeclampsia.

Molecular and clinical analysis of TRPC6 and AGTR1 genes in patients with pulmonary arterial hypertension

BACKGROUND

Pulmonary arterial hypertension (PAH) is a rare and progressive vascular disorder characterized by increased pulmonary vascular resistance and right heart failure. The aim of this study was to analyze 5'UTR region in canonical transient receptor potential isoform 6 (TRPC6) and 3'UTR region in Angiotensin II type I receptor (AGTR1) genes in patients with idiopathic and associated PAH. Correlation among mutations and clinical and functional parameters was further analyzed.

METHODS

Analysis of TRPC6 and AGTR1 genes was performed by polymerase chain reaction (PCR) and direct sequencing. We used a non-parametric test to determine if significant differences were found between the groups studied and chi-square test to compare clinical and hemodynamic variables among genotypes.

RESULTS

Fifty five patients and fifty two controls were included in this study. We found statistically significant differences for c.1-361A > T (p = 0.0077), c.1-254C > G (p < 0.0001) and c.1-218C > T (p = 0.0021) in TRPC6 gene and c.1166A > C (p < 0.001) in AGTR1 gene, between patients and controls. Idiopathic PAH patients (IPAH) and controls presented significant differences for all 3 TRPC6 polymorphisms (p = 0.020), (p = 0.002) and (p = 0.008) respectively, and also showed differences for AGTR1 gene (p < 0.001). In associated PAH (APAH) patients we found statistical differences for c.1-254C > G (p < 0.001) and c.1-218C > T (p = 0.001) in TRPC6 gene and c.1166A > C (p = 0.001) in AGTR1 gene. Several clinical and hemodynamic parameters showed significant differences between carriers and non-carriers of these single nucleotide polymorphisms (SNPs). Nineteen patients were carriers of all 3 SNPs in TRPC6 gene and presented a more severe phenotype with differences in mean pulmonary arterial pressure (p = 0.016), systolic pulmonary arterial pressure (p = 0.040), cardiac index (p < 0.001) and 6 minute walking test (p = 0.049). 16 of these patients harbored the SNP in AGTR1 gene. These patients showed differences in age at diagnosis (p = 0.049), mean pulmonary arterial pressure (p = 0.033), cardiac index (p = 0.002) and 6 minute walking test (p = 0.039).

CONCLUSIONS

PAH is a rare disease with pulmonary vascular remodeling caused in part by a heterogeneous constellation of genetic arrangements. This study seems to suggest that c.1-361A > T, c.1-254C > G and c.1-218C > T polymorphisms in TRPC6 gene and c.1166A > C polymorphism in AGTR1 could have a role in the development of this disease.

Determination of PKC isoform-specific protein expression in pulmonary arteries of rats with chronic hypoxia-induced pulmonary hypertension

Background

Evidence indicates that protein kinase C (PKC) plays a pivotal role in hypoxia-induced pulmonary hypertension (PH), but PKC isoform-specific protein expression in pulmonary arteries and their involvement in hypoxia-induced PH are unclear.

Material/Methods

Male SD rats (200–250 g) were exposed to normobaric hypoxia (10% oxygen) for 1, 3, 7, 14 and 21 d (days) to induce PH. PKC isoform-specific membrane translocation and protein expression in pulmonary arteries were determined by using Western blot and immunostaining.

Results

We found that only 6 isoforms of conventional PKC (cPKC) α, βI and βII, and novel PKC (nPKC) δ, ɛ and η were detected in pulmonary arteries of rats by Western blot. Hypoxic exposure (1–21 d) could induce rat PH with right ventricle (RV) hypertrophy and vascular remodeling. The cPKCβII membrane translocation at 3–7 d and protein levels of cPKCα at 3–14 d, βI and βII at 1–21 d decreased, while the nPKCδ membrane translocation at 3–21 d and protein levels at 3–14 d after hypoxic exposure in pulmonary arteries increased significantly when compared with that of the normoxia control group (p<0.05 vs. 0 d, n=6 per group). In addition, the down-regulation of cPKCα, βI and βII, and up-regulation of nPKCδ protein expressions at 14 d after hypoxia were further confirmed by immunostaining.

Conclusions

This study is the first systematic analysis of PKC isoform-specific membrane translocation and protein expression in pulmonary arteries, suggesting that the changes in membrane translocation and protein expression of cPKCα, βI, βII and nPKCδ are involved in the development of hypoxia-induced rat PH.

Targeting RhoA/ROCK pathway in pulmonary arterial hypertension

INTRODUCTION

Pulmonary arterial hypertension (PAH) is a rare disease with a complex pathogenesis. It is often associated with an increased vascular resistance, whilst in the more advanced stages there is a remodelling of the vascular walls. PAH has an intricate involvement of various signaling pathways, including the ras homolog family member A (RhoA)-Rho kinase (ROCK) axis. Currently, available therapies are not always able to significantly slow PAH progression. Therefore, newer approaches are needed.

AREAS COVERED

In this review, areas covered include the role of the RhoA/ROCK in PAH pathogenesis and the plausibility of its therapeutic targeting. Furthermore, various inhibitory compounds are discussed, including Fasudil and SB-772077-B.

EXPERT OPINION

Currently, specific RhoA/ROCK inhibition is the most promising therapeutic approach for PAH. Research has shown that it suppresses both the components of this axis and the upstream upregulating mediators. An inhaled RhoA/ROCK inhibitor may be a successful future therapy; however, further clinical trials are needed to support this approach.

Angiotensin II modulates VEGF-driven angiogenesis by opposing effects of type 1 and type 2 receptor stimulation in the microvascular endothelium

Vascular endothelial growth factor (VEGF) is a main stimulator of pathological vessel formation. Nevertheless, increasing evidence suggests that Angiotensin II (Ang II) can play an augmentory role in this process. We thus analyzed the contribution of the two Ang II receptor types, AT(1)R and AT(2)R, in a mouse model of VEGF-driven angiogenesis, i.e. oxygen-induced proliferative retinopathy. Application of the AT(1)R antagonist telmisartan but not the AT(2)R antagonist PD123,319 largely attenuated the pathological response. A direct effect of Ang II on endothelial cells (EC) was analyzed by assessing angiogenic responses in primary bovine retinal and immortalized rat microvascular EC. Selective stimulation of the AT(1)R by Ang II in the presence of PD123,319 revealed a pro-angiogenic activity which further increased VEGF-driven EC sprouting and migration. In contrast, selective stimulation of the AT(2)R by either CGP42112A or Ang II in the presence of telmisartan inhibited the VEGF-driven angiogenic response. Using specific inhibitors (pertussis toxin, RGS proteins, kinase inhibitors) we identified G(12/13) and G(i) dependent signaling pathways as the mediators of the AT(1)R-induced angiogenesis and the AT(2)R-induced inhibition, respectively. As AT(1)R and AT(2)R stimulation displays opposing effects on the activity of the monomeric GTPase RhoA and pro-angiogenic responses to Ang II and VEGF requires activation of Rho-dependent kinase (ROCK), we conclude that the opposing effects of the Ang II receptors on VEGF-driven angiogenesis converge on the regulation of activity of RhoA-ROCK-dependent EC migration.

Angiotensin II type 1 receptor blockade partially attenuates hypoxia-induced pulmonary hypertension in newborn piglets: relationship with the nitrergic system

The objective of this study was to observe possible interactions between the renin-angiotensin and nitrergic systems in chronic hypoxia-induced pulmonary hypertension in newborn piglets. Thirteen chronically instrumented newborn piglets (6.3 ± 0.9 days; 2369 ± 491 g) were randomly assigned to receive saline (placebo, P) or the AT₁ receptor (AT₁-R) blocker L-158,809 (L) during 6 days of hypoxia (FiO₂ = 0.12). During hypoxia, pulmonary arterial pressure (Ppa; P < 0.0001), pulmonary vascular resistance (PVR; P < 0.02) and the pulmonary to systemic vascular resistance ratio (PVR/SVR; P < 0.05) were significantly attenuated in the L (N = 7) group compared to the P group (N = 6). Western blot analysis of lung proteins showed a significant decrease of endothelial NOS (eNOS) in both P and L animals, and of AT₁-R in P animals during hypoxia compared to normoxic animals (C group, N = 5; P < 0.01 for all groups). AT₁-R tended to decrease in L animals. Inducible NOS (iNOS) did not differ among P, L, and C animals and iNOS immunohistochemical staining in macrophages was significantly more intense in L than in P animals (P < 0.01). The vascular endothelium showed moderate or strong eNOS and AT₁-R staining. Macrophages and pneumocytes showed moderate or strong iNOS and AT₁-R staining, but C animals showed weak iNOS and AT₁-R staining. Macrophages of L and P animals showed moderate and weak AT₂-R staining, respectively, but the endothelium of all groups only showed weak staining. In conclusion, pulmonary hypertension induced by chronic hypoxia in newborn piglets is partially attenuated by AT₁-R blockade. We suggest that AT₁-R blockade might act through AT₂-R and/or Mas receptors and the nitrergic system in the lungs of hypoxemic newborn piglets.

Role of serum renin and aldosterone in animal adaptation to high-altitude conditions

Adaptation to high-altitude conditions in rats with experimental renal failure is associated with shifts in the rennin-angiotensin-aldosterone system, which manifested in different serum levels of renin and aldosterone in response to water and salt loads depending on the stage of the compensation processes.

Risk factors and mediators of the vascular dysfunction associated with hypertension in pregnancy

Normal pregnancy is associated with significant hemodynamic changes and vasodilation in the uterine and systemic circulation in order to meet the metabolic demands of the mother and developing fetus. Hypertension in pregnancy (HTN-Preg) and preeclampsia (PE) are major complications and life-threatening conditions to both the mother and fetus. PE is precipitated by various genetic, dietary and environmental factors. Although the initiating events of PE are unclear, inadequate invasion of cytotrophoblasts into the uterine artery is thought to reduce uteroplacental perfusion pressure and lead to placental ischemia/hypoxia. Placental hypoxia induces the release of biologically active factors such as growth factor inhibitors, anti-angiogenic proteins, inflammatory cytokines, reactive oxygen species, hypoxia-inducible factors, and antibodies to vascular angiotensin II receptor. These bioactive factors affect the production/activity of various vascular mediators in the endothelium, smooth muscle and extracellular matrix, leading to severe vasoconstriction and HTN. As an endothelial cell disorder, PE is associated with decreased vasodilator mediators such as nitric oxide, prostacyclin and hyperpolarizing factor and increased vasoconstrictor mediators such as endothelin, angiotensin II and thromboxane A(2). PE also involves enhanced mechanisms of vascular smooth muscle contraction including intracellular free Ca(2+) concentration ([Ca(2+)](i)), and [Ca(2+)](i) sensitization pathways such as protein kinase C, Rho-kinase and mitogen-activated protein kinase. Changes in extracellular matrix composition and matrix metalloproteases activity also promote vascular remodeling and further vasoconstriction in the uterine and systemic circulation. Characterization of the predisposing risk factors, the biologically active factors, and the vascular mediators associated with PE holds the promise for early detection, and should help design specific genetic and pharmacological tools for the management of the vascular dysfunction associated with HTN-Preg.

Circulating and Vascular Bioactive Factors during Hypertension in Pregnancy

Normal pregnancy is associated with significant vascular remodeling in the uterine and systemic circulation in order to meet the metabolic demands of the mother and developing fetus. The pregnancy-associated vascular changes are largely due to alterations in the amount/activity of vascular mediators released from the endothelium, vascular smooth muscle and extracellular matrix. The endothelium releases vasodilator substances such as nitric oxide, prostacyclin and hyperpolarizing factor as well as vasoconstrictor factors such as endothelin, angiotensin II and thromboxane A(2). Vascular smooth muscle contraction is mediated by intracellular free Ca(2+) concentration ([Ca(2+)](i)), and [Ca(2+)](i) sensitization pathways such as protein kinase C, Rho-kinase and mitogen-activated protein kinase. Extracellular matrix and vascular remodeling are regulated by matrix metalloproteases. Hypertension in pregnancy and preeclampsia are major complications and life threatening conditions to both the mother and fetus, precipitated by various genetic, dietary and environmental factors. The initiating mechanism of preeclampsia and hypertension in pregnancy is unclear; however, most studies have implicated inadequate invasion of cytotrophoblasts into the uterine artery, leading to reduction in the uteroplacental perfusion pressure and placental ischemia/hypoxia. This placental hypoxic state is thought to induce the release of several circulating bioactive factors such as growth factor inhibitors, anti-angiogenic proteins, inflammatory cytokines, reactive oxygen species, hypoxia-inducible factors, and vascular receptor antibodies. Increases in the plasma levels and vascular content of these factors during pregnancy could cause an imbalance in the vascular mediators released from the endothelium, smooth muscle and extracellular matrix, and lead to severe vasoconstriction and hypertension. This review will discuss the interactions between the various circulating bioactive factors and the vascular mediators released during hypertension in pregnancy, and provide an insight into the current and future approaches in the management of preeclampsia.

Losartan attenuates chronic cigarette smoke exposure-induced pulmonary arterial hypertension in rats: possible involvement of angiotensin-converting enzyme-2

Chronic cigarette smoking induces pulmonary arterial hypertension (PAH) by largely unknown mechanisms. Renin–angiotensin system (RAS) is known to function in the development of PAH. Losartan, a specific angiotensin II receptor antagonist, is a well-known antihypertensive drug with a potential role in regulating angiotensin-converting enzyme-2 (ACE2), a recently found regulator of RAS. To determine the effect of losartan on smoke-induced PAH and its possible mechanism, rats were daily exposed to cigarette smoke for 6 months in the absence and in the presence of losartan. Elevated right ventricular systolic pressure (RVSP), thickened wall of pulmonary arteries with apparent medial hypertrophy along with increased angiotensin II (Ang II) and decreased ACE2 levels were observed in smoke-exposed-only rats. Losartan administration ameliorated pulmonary vascular remodeling, inhibited the smoke-induced RVSP and Ang II elevation and partially reversed the ACE2 decrease in rat lungs. In cultured primary pulmonary artery smooth muscle cells (PASMCs) from 3- and 6-month smoke-exposed rats, ACE2 levels were significantly lower than in those from the control rats. Moreover, PASMCs from 6-month exposed rats proliferated more rapidly than those from 3-month exposed or control rats, and cells grew even more rapidly in the presence of DX600, an ACE2 inhibitor. Consistent with the in vivo study, in vitro losartan pretreatment also inhibited cigarette smoke extract (CSE)-induced cell proliferation and ACE2 reduction in rat PASMCs. The results suggest that losartan may be therapeutically useful in the chronic smoking-induced pulmonary vascular remodeling and PAH and ACE2 may be involved as part of its mechanism. Our study might provide insight into the development of new therapeutic interventions for PAH smokers.

Not just angiotensinases: new roles for the angiotensin-converting enzymes

The renin-angiotensin system (RAS) is a critical regulator of blood pressure and fluid homeostasis. Angiotensin II, the primary bioactive peptide of the RAS, is generated from angiotensin I by angiotensin-converting enzyme (ACE). A homologue of ACE, ACE2, is able to convert angiotensin II to a peptide with opposing effects, angiotensin-(1-7). It is proposed that disturbance of the balance of ACE and ACE2 expression and/or function is important in pathologies in which angiotensin II plays a role. These include cardiovascular and renal disease, lung injury and liver fibrosis. The critical roles of ACE and ACE2 in regulating angiotensin II levels have traditionally focussed attention on their activities as angiotensinases. Recent discoveries, however, have illuminated the roles of these enzymes and of the ACE2 homologue, collectrin, in intracellular trafficking and signalling. This paper reviews the key literature regarding both the catalytic and non-catalytic roles of the angiotensin-converting enzyme gene family.

Increased expression of endothelin ET(B) and angiotensin AT(1) receptors in peripheral resistance arteries of patients with suspected acute coronary syndrome

Patients who experience chest pain, in which ischemic heart disease has been ruled out, still have an increased risk of future ischemic cardiac events and premature death, possibly due to subclinical endothelial dysfunction. A feature of endothelial dysfunction is an increased expression of arterial vasoconstrictor endothelin (ET) and angiotensin (AT) receptors. Our aim was to investigate if the arterial expressions of these receptors are changed in patients with suspected but ruled out acute coronary syndrome (ACS). Small subcutaneous arteries (diameter of 100 microm) were surgically removed in an abdominal biopsy from 12 patients suspicious of ACS (susp ACS), admitted to the medical telemetry unit for chest pain. The vessels were analyzed for their receptor protein expression by quantitative immunohistochemistry using specific antibodies directed against ET(A), ET(B), AT(1), and AT(2) receptors. The control group (controls) consisted of eight healthy volunteers matched for age and sex with no previous cardiac illness or medication. The susp ACS group had an increased expression of ET(B) (by 94%) and AT(1) (by 34%) receptors in the smooth muscle cells of resistance arteries as compared to the control group. There were no significant differences in AT(2) and ET(A) receptor expression between the groups. The results indicate that the expression of arterial smooth muscle ET(B) and AT(1) receptors are increased in patients with suspected but ruled out ACS. These receptor changes could be important in the regulation of coronary tone and in the development of atherosclerosis, and may be related to increased cardiovascular risk.

Protective role of the antidiabetic drug metformin against chronic experimental pulmonary hypertension

BACKGROUND AND PURPOSE

Pulmonary arterial hypertension (PAH) is associated with increased contraction and proliferation of pulmonary vascular smooth muscle cells. The anti-diabetic drug metformin has been shown to have relaxant and anti-proliferation properties. We thus examined the effect of metformin in PAH.

EXPERIMENTAL APPROACH

Metformin effects were analysed in hypoxia- and monocrotaline-induced PAH in rats. Ex vivo and in vitro analyses were performed in lungs, pulmonary artery rings and cells.

KEY RESULTS

In hypoxia- and monocrotaline-induced PAH, the changes in mean pulmonary arterial pressure and right heart hypertrophy were nearly normalized by metformin treatment (100 mg.kg(-1).day(-1)). Pulmonary arterial remodelling occurring in both experimental models of PAH was also inhibited by metformin treatment. In rats with monocrotaline-induced PAH, treatment with metformin significantly increased survival. Metformin increased endothelial nitric oxide synthase phosphorylation and decreased Rho kinase activity in pulmonary artery from rats with PAH. These effects are associated with an improvement of carbachol-induced relaxation and reduction of phenylephrine-induced contraction of pulmonary artery. In addition, metformin inhibited mitogen-activated protein kinase activation and strongly reduced pulmonary arterial cell proliferation during PAH. In vitro, metformin directly inhibited pulmonary artery smooth muscle cell growth.

CONCLUSIONS AND IMPLICATIONS

Metformin protected against PAH, regardless of the initiating stimulus. This protective effect may be related to its anti-remodelling property involving improvement of endothelial function, vasodilatory and anti-proliferative actions. As metformin is currently prescribed to treat diabetic patients, assessment of its use as a therapy against PAH in humans should be easier.

Increased angiotensin II AT1 receptor mRNA and binding in spleen and lung of AT2 receptor gene disrupted mice

To clarify the relationship between Angiotensin II AT(1) and AT(2) receptors, we studied AT(1) receptor mRNA and binding expression in tissues from AT(2) receptor gene disrupted (AT(2)(-/-)) female mice, where AT(2) receptors are not expressed in vivo, using in situ hybridization and quantitative autoradiography. Wild type mice expressed AT(1A) receptor mRNA and AT(1) receptor binding in lung parenchyma, the spleen, predominantly in the red pulp, and in liver parenchyma. In wild type mice, lung AT(2) receptors were expressed in lung bronchial epithelium and smooth muscle, and were not present in the lung parenchyma, the spleen or the liver. This indicates that AT(1) and AT(2) receptors were not expressed in the same cells. In AT(2)(-/-) mice, we found higher AT(1A) receptor mRNA and AT(1) receptor binding in lung parenchyma and in the red pulp of the spleen, but not in the liver, when compared to littermate wild type controls. Our results suggest that impaired AT(2) receptor function upregulates AT(1) receptor transcription and expression in a tissue-specific manner and in cells not expressing AT(2) receptors. AT(1) upregulation explains the increased sensitivity to Angiotensin II characteristic of the AT(2)(-/-) phenotype, consistent with enhanced AT(1) receptor activation in a number of tissues.

Increased expression of vascular endothelin type B and angiotensin type 1 receptors in patients with ischemic heart disease

Background

Endothelin-1 and angiotensin II are strong vasoconstrictors. Patients with ischemic heart disease have elevated plasma levels of endothelin-1 and angiotensin II and show increased vascular tone. The aim of the present study was to examine the endothelin and angiotensin II receptor expression in subcutaneous arteries from patients with different degrees of ischemic heart disease.

Methods

Subcutaneous arteries were obtained, by biopsy from the abdomen, from patients undergoing coronary artery bypass graft (CABG) surgery because of ischemic heart disease (n = 15), patients with angina pectoris without established myocardial infarction (n = 15) and matched cardiovascular healthy controls (n = 15). Endothelin type A (ET_A) and type B (ET_B), and angiotensin type 1 (AT₁) and type 2 (AT₂) receptors expression and function were examined using immunohistochemistry, Western blot and in vitro pharmacology.

Results

ET_A and, to a lesser extent, ET_B receptor staining was observed in the healthy vascular smooth muscle cells. The level of ET_B receptor expression was higher in patients undergoing CABG surgery (250% ± 23%; P < 0.05) and in the patients with angina pectoris (199% ± 6%; P < 0.05), than in the healthy controls (100% ± 28%). The data was confirmed by Western blotting. Arteries from CABG patients showed increased vasoconstriction upon administration of the selective ET_B receptor agonist sarafotoxin S6c, compared to healthy controls (P < 0.05). No such difference was found for the ET_A receptors. AT₁ and, to a lesser extent, AT₂ receptor immunostaining was seen in the vascular smooth muscle cells. The level of AT₁ receptor expression was higher in both the angina pectoris (128% ± 25%; P < 0.05) and in the CABG patients (203% ± 41%; P < 0.05), as compared to the healthy controls (100% ± 25%). The increased AT₁ receptor expression was confirmed by Western blotting. Myograph experiment did however not show any change in vasoconstriction to angiotensin II in CABG patients compared to healthy controls (P = n.s).

Conclusion

The results demonstrate, for the first time, upregulation of ET_B and AT₁ receptors in vascular smooth muscle cells in ischemic heart disease. These receptors may play a role in the pathophysiology of ischemic heart disease and could provide important targets for pharmaceutical interventions.

Prevention of pulmonary hypertension by Angiotensin-converting enzyme 2 gene transfer

In spite of recent advancements in the treatment of pulmonary hypertension, successful control has yet to be accomplished. The abundant presence of angiotensin-converting enzyme 2 (ACE2) in the lungs and its impressive effect in the prevention of acute lung injury led us to test the hypothesis that pulmonary overexpression of this enzyme could produce beneficial outcomes against pulmonary hypertension. Monocrotaline (MCT) treatment of mice for 8 weeks resulted in significant increases in right ventricular systolic pressure, right ventricle:left ventricle plus septal weight ratio, and muscularization of pulmonary vessels. Administration of a lentiviral vector containing ACE2, 7 days before MCT treatment prevented the increases in right ventricular systolic pressure (control: 25+/-1 mm Hg; MCT: 44+/-5 mm Hg; MCT+ACE2: 26+/-1 mm Hg; n=6; P<0.05) and right ventricle:left ventricle plus septal weight ratio (control: 0.25+/-0.01; MCT: 0.31+/-0.01; MCT+ACE2: 0.26+/-0.01; n=8; P<0.05). A significant attenuation in muscularization of pulmonary vessels induced by MCT was also observed in animals overexpressing ACE2. These beneficial effects were associated with an increase in the angiotensin II type 2 receptor:angiotensin II type 1 receptor mRNA ratio. Also, pulmonary hypertension-induced increases in proinflammatory cytokines were significantly attenuated by lentiviral vector-containing ACE2 treatment. Furthermore, ACE2 gene transfer in mice after 6 weeks of MCT treatment resulted in a significant reversal of right ventricular systolic pressure. These observations demonstrate that ACE2 overexpression prevents and reverses right ventricular systolic pressure and associated pathophysiology in MCT-induced pulmonary hypertension by a mechanism involving a shift from the vasoconstrictive, proliferative, and fibrotic axes to the vasoprotective axis of the renin-angiotensin system and inhibition of proinflammatory cytokines.

Role of HIF-1alpha in the regulation ACE and ACE2 expression in hypoxic human pulmonary artery smooth muscle cells

Angiotensin-converting enzyme (ACE) enhances the proliferation and migration of pulmonary artery smooth muscle cells (PASMCs), which contribute to the pathogenesis of hypoxic pulmonary hypertension (HPH). Previous reports have demonstrated that hypoxia upregulates ACE expression, but the underlying mechanism is unknown. Here, we found that ACE is persistently upregulated in PASMCs on the transcriptional level during hypoxia. Hypoxia-inducible factor 1alpha (HIF-1alpha), a key transcription factor activated during hypoxia, was able to upregulate ACE protein expression under normoxia, whereas knockdown of HIF-1alpha expression in PASMCs inhibited hypoxia-induced ACE upregulation. Furthermore, HIF-1alpha can bind and transactivate the ACE promoter directly. Therefore, we report that ACE is a novel target of HIF-1alpha. Recently, a homolog of ACE, ACE2, was reported to counterbalance the function of ACE. In contrast to ACE, we found that ACE2 mRNA and protein levels increased during the early stages of hypoxia and decreased to near-baseline levels at the later stages after HIF-1alpha accumulation. Thus HIF-1alpha inhibited ACE2 expression, and the accumulated ANG II catalyzed by ACE is a key mediator in the downregulation of ACE2 by HIF-1alpha. Moreover, a reduction of ACE2 expression in PASMCs by RNA interference was accompanied by significantly enhanced proliferation and migration during hypoxia. We conclude that ACE is directly regulated by HIF-1alpha, whereas ACE2 is regulated in a bidirectional way during hypoxia and may play a protective role during the development of HPH. In sum, these findings contribute to the understanding of the pathogenesis of HPH.

Time-dependent phenotypic and contractile changes of pulmonary artery in chronic hypoxia-induced pulmonary hypertension

Phenotypic and contractile changes in pulmonary arterial smooth muscle cells (PASMCs) were examined in rats with pulmonary hypertension induced by hypoxia. Exposure to hypoxia induced pulmonary hypertension within 1-4 weeks. Staining with BrdU revealed that proliferative activities of PASMCs peaked at 1 week of hypoxic exposure, and then moderate proliferative activity was maintained for the next 2-4 weeks. The beta-actin/alpha-actin ratio also increased at 1-2 weeks of exposure to hypoxia. Absolute contractility of the pulmonary arterial ring continuously decreased during hypoxia, whereas the basal active tonus of the pulmonary artery increased at 1-3 weeks. Nicardipine, the ETA-receptor antagonis, CI-1034 and the rho-kinase inhibitor Y27632 partially inhibited the elevated active tonus. Endothelin-1 content in the pulmonary hypertensive lung was continuously increased during exposure to hypoxia. In conclusion, the hypoxia-induced proliferative activity of PASMCs comprised a transient phase followed by a sustained phase. The change in PASMCs from a contractile to a synthetic phenotype also correlated with proliferative activity, which subsequently decreased PASMC contractility. The continuous production of endothelin-1 upon hypoxic exposure might contribute to the increased basal tonus of the pulmonary arterial wall, which might subsequently increase pulmonic arterial pressure, resulting in accelerated pulmonary hypertension.

Specific single chain variable fragment (ScFv) antibodies to angiotensin II AT(2) receptor: evaluation of the angiotensin II receptor expression in normal and tumor-bearing mouse lung

To gain insight into the mechanism by which angiotensin II type 2 receptor (AT(2)) regulates carcinogen-induced lung tumorigenesis, we have newly developed anti-AT(2) single chain variable fragment (ScFv) antibodies using a rodent phage-displayed recombinant antibody library with various peptide fragments of the receptor protein, and investigated the expression of the AT(2) receptor protein. The specificity of the antibodies was verified using AT(2) over-expressing COS-7 cells and AT(2) naturally expressing PC12W cells. In control wild type mouse lung, a stronger immunoreactivity was observed in bronchial epithelial cells. A moderate immunoreactivity was detected in pulmonary vascular walls and vascular endothelial cells. In the lungs possessing tobacco-specific nitrosamine (NNK)-induced tumors, significantly increased AT(2) and AT(1 )immunostaining was observed in adenomatous lesions. These data suggest that the increase in both receptors' expression in the alveolar epithelial cells may be accompanied with the onset of NNK-induced tumorigenesis and hence play important roles in lung tumorigenesis.

Oxidative stress contributes to pulmonary hypertension in the transgenic (mRen2)27 rat

The transgenic (mRen2)27 (Ren2) rat overexpresses mouse renin in extrarenal tissues, causing increased local synthesis of ANG II, oxidative stress, and hypertension. However, little is known about the role of oxidative stress induced by the tissue renin-angiotensin system (RAS) as a contributing factor in pulmonary hypertension (PH). Using male Ren2 rats, we test the hypothesis that lung tissue RAS overexpression and resultant oxidative stress contribute to PH and pulmonary vascular remodeling. Mean arterial pressure (MAP), right ventricular systolic pressure (RVSP), and wall thickness of small pulmonary arteries (PA), as well as intrapulmonary NADPH oxidase activity and subunit protein expression and reactive oxygen species (ROS), were compared in age-matched Ren2 and Sprague-Dawley (SD) rats pretreated with the SOD/catalase mimetic tempol for 21 days. In placebo-treated Ren2 rats, MAP and RVSP, as well as intrapulmonary NADPH oxidase activity and subunits (Nox2, p22phox, and Rac-1) and ROS, were elevated compared with placebo-treated SD rats (P < 0.05). Tempol decreased RVSP (P < 0.05), but not MAP, in Ren2 rats. Tempol also reduced intrapulmonary NADPH oxidase activity, Nox2, p22phox, and Rac-1 protein expression, and ROS in Ren2 rats (P < 0.05). Compared with SD rats, the cross-sectional surface area of small PA was 38% greater (P < 0.001) and luminal surface area was 54% less (P < 0.001) in Ren2 rats. Wall surface area was reduced and luminal area was increased in tempol-treated SD and Ren2 rats compared with untreated controls (P < 0.05). Collectively, the results of this investigation support a seminal role for enhanced tissue RAS/oxidative stress as factors in development of PH and pulmonary vascular remodeling.

The role of angiotensin II receptor-1 blockade in the hypoxic pulmonary vasoconstriction response in newborn piglets

BACKGROUND

Angiotensin-converting enzyme activity is increased in newborn infants with respiratory distress syndrome and in animals with alveolar hypoxia.

OBJECTIVE

To test whether angiotensin II (Ang II) mediates the pulmonary vasoconstriction induced by acute hypoxia in newborn piglets.

METHODS

Eight unanesthetized chronically instrumented newborn piglets (mean +/- SEM; age 6.6 +/- 0.6 days; weight 2,181 +/- 174 g) were randomly assigned to receive a saline solution or the Ang II type 1 receptor (AT(1)) antagonist, losartan, in a crossover study design, with an interval of at least 48 h between the first and second study. Pulmonary artery (Ppa), wedge, systemic arterial (Psa) and right atrial pressures, cardiac output (CO), pulmonary (PVR) and systemic (SVR) vascular resistances, and arterial blood gases were obtained in room air, before and during the saline or losartan infusion (6 mg/kg followed by 3 mg/kg/h), and during 6 h of hypoxia (FiO(2) = 0.11) and saline or losartan infusion. Data were analyzed by repeated measures analysis of variance.

RESULTS

The pulmonary vasoconstriction induced by acute hypoxia was significantly attenuated during losartan infusion, while Psa, SVR, CO, pH, PaCO(2), PaO(2) and base excess did not differ between groups. During room air, Ppa, PVR, Psa, SVR and CO values were not modified by saline or losartan infusion.

CONCLUSION

These data suggest that the pulmonary vasoconstriction induced by acute hypoxia in newborn piglets is partially mediated by Ang II, acting via AT(1).

NFATc3 mediates chronic hypoxia-induced pulmonary arterial remodeling with alpha-actin up-regulation

Physiological responses to chronic hypoxia include polycythemia, pulmonary arterial remodeling, and vasoconstriction. Chronic hypoxia causes pulmonary arterial hypertension leading to right ventricular hypertrophy and heart failure. During pulmonary hypertension, pulmonary arteries exhibit increased expression of smooth muscle-alpha-actin and -myosin heavy chain. NFATc3 (nuclear factor of activated T cells isoform c3), which is aCa(2+)-dependent transcription factor, has been recently linked to smooth muscle phenotypic maintenance through the regulation of the expression of alpha-actin. The aim of this study was to determine if: (a) NFATc3 is expressed in murine pulmonary arteries, (b) hypoxia induces NFAT activation, (c) NFATc3 mediates the up-regulation of alpha-actin during chronic hypoxia, and (d) NFATc3 is involved in chronic hypoxia-induced pulmonary vascular remodeling. NFATc3 transcript and protein were found in pulmonary arteries. NFAT-luciferase reporter mice were exposed to normoxia (630 torr) or hypoxia (380 torr) for 2, 7, or 21 days. Exposure to hypoxia elicited a significant increase in luciferase activity and pulmonary arterial smooth muscle nuclear NFATc3 localization, demonstrating NFAT activation. Hypoxia induced up-regulation of alpha-actin and was prevented by the calcineurin/NFAT inhibitor, cyclosporin A (25 mg/kg/day s.c.). In addition, NFATc3 knock-out mice did not showed increased alpha-actin levels and arterial wall thickness after hypoxia. These results strongly suggest that NFATc3 plays a role in the chronic hypoxia-induced vascular changes that underlie pulmonary hypertension.

Time course of hypoxia-induced lung injury in rats

We investigated the effects of normobaric hypoxia on rat lungs and hypothesized that the hypoxic exposure would induce lung injury with pulmonary edema and inflammation ensued by development of fibrosis. Rats were exposed to 10% O(2) in nitrogen over 6-168h. We analyzed cardiovascular function and pulmonary changes, lung histology and mRNA expression of extracellular matrix (ECM) molecules in the lung. Significant hemodynamic changes occurred after 168h of hypoxic exposure. Moderate pulmonary edema appeared after 8h and peaked after 16h of hypoxia. It was accompanied by inflammation, fibrosis and vascular hypertrophy. mRNA expression of transforming growth factor-beta2 and -beta3 was up-regulated in lung tissue after 8h of hypoxia. After 8-16h, mRNA expression of collagen types I and III and of other ECM molecules was significantly elevated and increased further with longer exposure to hypoxia. The time course of hypoxia-induced pulmonary injury resembled that previously observed after continuous norepinephrine infusion in rats.

Angiotensin II Induces Angiogenesis in the Hypoxic Adult Mouse Heart In Vitro Through an AT 2 –B2 Receptor Pathway

Angiotensin II is a vasoactive peptide that may affect vascularization of the ischemic heart via angiogenesis. In this study we aimed at studying the mechanisms underlying the angiogenic effects of angiotensin II under hypoxia in the mouse heart in vitro. Endothelial sprout formation from pieces of mouse hearts was assessed under normoxia (21% O 2 ) and hypoxia (1% O 2 ) during a 7-day period of in vitro culture. Only under hypoxia did angiotensin II dose-dependently induce endothelial sprout formation, peaking at 10 −7 mol/L of angiotensin II. Angiotensin II type 1 (AT 1 ) receptor blockade by losartan did not affect angiotensin II–induced sprouting in wild-type mice. Conversely, the angiotensin II type 2 (AT 2 ) receptor antagonist PD 123319 blocked this response. In hearts from AT 1 −/− mice, angiotensin II–elicited sprouting was preserved but blocked again by AT 2 receptor antagonism. In contrast, no angiotensin II–induced sprouting was found in preparations from hearts of AT 2 −/− mice. Angiotensin II–mediated angiogenesis was also abolished by a specific inhibitor of the B2 kinin receptor in both wild-type and AT 1 −/− mice. Furthermore, angiotensin II failed to induce endothelial sprout formation in hearts from B2 −/− mice. Finally, NO inhibition completely blunted sprouting in hearts from wild-type mice, whereas NO donors could restore sprouting in AT 2 −/− and B2 −/− hearts. This in vitro study suggests the obligatory role of hypoxia in the angiogenic effect of angiotensin II in the mouse heart via the AT 2 receptor through a mechanism that involves bradykinin, its B2 receptor, and NO as a downstream effector.

Differential effects of losartan and candesartan on vasoconstrictor responses in the ratThis paper is one of a selection of papers published in this Special Issue, entitled The Cellular and Molecular Basis of Cardiovascular Dysfunction, Dhalla 70th Birthday Tribute

Losartan has been reported to have inhibitory effects on thromboxane (TP) receptor-mediated responses. In the present study, the effects of 2 nonpeptide angiotensin II (AT1) receptor antagonists, losartan and candesartan, on responses to angiotensin II, the thromboxane A2 mimic, U46619, and norepinephrine were investigated and compared in the pulmonary and systemic vascular beds of the intact-chest rat. In this study, intravenous injections of angiotensin II, U46619, and norepinephrine produced dose-related increases in pulmonary and systemic arterial pressure. Losartan and candesartan, in the doses studied, decreased or abolished responses to angiotensin II. Losartan, but not candesartan, and only in a higher dose, produced small, but statistically significant, reductions in pressor responses to U46619 and to norepinephrine in the pulmonary and systemic vascular beds. Furthermore, losartan significantly reduced arachidonic acid-induced platelet aggregation, whereas candesartan had no effect. Pressor responses to angiotensin II were not changed by thromboxane and alpha-adrenergic receptor antagonists, or by cyclooxygenase and NO synthase inhibitors. These results show that losartan and candesartan are potent selective AT1 receptor antagonists in the pulmonary and systemic vascular beds and that losartan can attenuate thromboxane and alpha-adrenergic responses when administered at a high dose, whereas candesartan in the highest dose studied had no effect on responses to U46619 or to norepinephrine. The present data show that the effects of losartan and candesartan on vasoconstrictor responses are different and that pulmonary and systemic pressor responses to angiotensin II are not modulated or mediated by the release of cyclooxygenase products, activation of TP receptors, or the release of NO in the anesthetized rat.

Effects of olmesartan, an AT1 receptor antagonist, on hypoxia-induced activation of ERK1/2 and pro-inflammatory signals in the mouse lung

The present study aimed to investigate the effects of olmesartan, an antagonist for angiotensin II receptor type 1(AT1), on the activation of extracellular signal-regulated kinases (ERK)1/2, tissue remodeling, and pro-inflammatory signals in the right ventricle and lung of mice during the early phase of hypobaric hypoxia. Phosphorylation of ERK1/2 in both tissue types in response to hypoxia peaked at 1-3 days, and declined rapidly in the right ventricle, whereas in the lung it was sustained for at least 8 days. Upregulation of angiotensinogen mRNA was observed in the hypoxic lung at 4-9 days, but not in the hypoxic right ventricle and pulmonary artery. Olmesartan inhibited the hypoxia-induced phosphorylation of ERK1/2 in the lung, but not in the right ventricle. Neither right ventricular hypertrophy nor the thickening of the intrapulmonary arterial wall was ameliorated by olmesartan. However, this drug inhibited the expression of the mRNA for angiotensinogen and several pro-inflammatory factors, including interleukin-6 and inducible nitric oxide synthase in the hypoxic lung. These results suggest that olmesartan blocks a potential positive feedback loop of the angiotensin II-AT1 receptor system, which may lead to attenuate pro-inflammatory signals in the mouse lung, that are associated with hypoxic pulmonary hypertension, without inducing any appreciable effects on the compensatory cardiopulmonary hypertrophy at an early phase of exposure to a hypobaric hypoxic environment.

Angiotensin-converting enzyme 2 in lung diseases

The renin-angiotensin system (RAS) plays a key role in maintaining blood pressure homeostasis, as well as fluid and salt balance. Angiotensin II, a key effector peptide of the system, causes vasoconstriction and exerts multiple biological functions. Angiotensin-converting enzyme (ACE) plays a central role in generating angiotensin II from angiotensin I, and capillary blood vessels in the lung are one of the major sites of ACE expression and angiotensin II production in the human body. The RAS has been implicated in the pathogenesis of pulmonary hypertension and pulmonary fibrosis, both commonly seen in chronic lung diseases such as chronic obstructive lung disease. Recent studies indicate that the RAS also plays a critical role in acute lung diseases, especially acute respiratory distress syndrome (ARDS). ACE2, a close homologue of ACE, functions as a negative regulator of the angiotensin system and was identified as a key receptor for SARS (severe acute respiratory syndrome) coronavirus infections. In the lung, ACE2 protects against acute lung injury in several animal models of ARDS. Thus, the RAS appears to play a critical role in the pathogenesis of acute lung injury. Indeed, increasing ACE2 activity might be a novel approach for the treatment of acute lung failure in several diseases.

Pilot study of losartan for pulmonary hypertension in chronic obstructive pulmonary disease

BACKGROUND

Morbidity in COPD results from a combination of factors including hypoxia-induced pulmonary hypertension, in part due to pulmonary vascular remodelling. Animal studies suggest a role of angiotensin II and acute studies in man concur. Whether chronic angiotensin-II blockade is beneficial is unknown. We studied the effects of an angiotensin-II antagonist losartan, on haemodynamic variables, exercise capacity and symptoms.

METHODS

This was a double-blind, randomized, parallel group, placebo- controlled study of 48 weeks duration. Forty patients with COPD and pulmonary hypertension (Tran tricuspid pressure gradient (TTPG) = 30 mmHg) were randomised to losartan 50 mg or placebo. Changes in TTPG were assessed at 3, 6 and 12 months.

RESULTS

There was a trend for TTPG to increase in the placebo group (baseline 43.4 versus 48.4 mmHg at endpoint) and stay constant in the losartan group (baseline 42.8 versus 43.6 mmHg). More patients in the losartan group (50%) than in the placebo group (22%) showed a clinically meaningful reduction in TTPG at any timepoint; these effects seemed more marked in patients with higher baseline TTPG. There were no clear improvements in exercise capacity or symptoms.

CONCLUSION

In this 12-month pilot study, losartan 50 mg had no statistically significant beneficial effect on TTPG, exercise capacity or symptoms in pulmonary hypertension secondary to obstructive disease. A sub-group of patients with higher TTPG may benefit.

Prevention of pulmonary vascular remodeling and of decreased BMPR-2 expression by losartan therapy in shunt-induced pulmonary hypertension

The renin-ANG system has been reported to be overexpressed in pulmonary arterial hypertension (PAH). We investigated the effects of ANG receptor-1 blockade by losartan on hemodynamics and signaling molecules in a piglet overflow model of early PAH. Twenty-six 3-wk-old piglets were randomized to placebo or losartan therapy (1 mg.kg(-1).day(-1)) after anastomosis of the inominate to the main pulmonary artery or after a sham operation. Three months later, the animals underwent a hemodynamic evaluation, followed by pulmonary tissue sampling for morphometry, immunohistochemistry, and real-time quantitative-PCR. Chronic systemic-to-pulmonary shunting increased the pulmonary vascular resistance from 2.5 +/- 0.2 to 6.2 +/- 0.3 mmHg.l(-1).min.m(-2) and arteriolar medial thickness from 13.6 to 25.4%. These changes were associated with increased expressions of ANG II and its type 1 (AT1) and type 2 (AT2) receptors, endothelin-1 (ET-1) and its type B receptor (ETB), and angiopoietin-1, together with decreased expressions of bone morphogeneic protein receptor-1A and -2 (BMPR-1A and BMPR-2, respectively) and unchanged expression of the receptor tyrosine kinase with immunoglobulin and EGF homology domains-2 (Tie 2). Pretreatment with losartan decreased shunt-induced pulmonary vascular resistance and medial thickness by 51% and 35%, respectively. Losartan therapy was associated with persistent overexpressions of ANG II, AT2, ET-1, ETB, and angiopoietin-1 and with a return to normal of the BMPR-2 expression. These results suggest that ANG II contributes to left-to-right, shunt-induced PAH.

Hypoxia-driven proliferation of human pulmonary artery fibroblasts: cross-talk between HIF-1alpha and an autocrine angiotensin system

Pulmonary artery adventitial fibroblasts (FBPA) may play a central role in lung vascular remodeling under conditions of hypoxia and inflammation, the result being pulmonary hypertension and cor pulmonale. In cultured human FBPA, both angiotensin II (Ang II) and hypoxia promoted cell cycle progression and cell proliferation and suppressed apoptosis. These effects were further enhanced when both stimuli were applied simultaneously. Hypoxia elevated the expression of hypoxia-inducible factor 1alpha (HIF-1alpha) and increased the expression of genes regulated by the hypoxia-responsive element (HRE). Up-regulation of both angiotensin-converting enzyme (ACE) and Ang II receptor type 1 (AT1) was also observed. Exogenous Ang II further increased HIF/HRE-dependent signaling in FBPA, whereas suppression of the autocrine ACE-Ang II-AT1 loop with inhibitors of ACE, AT1, and phosphatidylinositol 3-kinase (PI3K) reduced the proliferative response to both hypoxia and exogenous Ang II. Overexpression of HIF-1alpha by transient transfection caused the same proliferative effect and up-regulation of AT1 expression that were observed under hypoxic conditions. In contrast, small interfering RNA targeting HIF-1alpha inhibited hypoxia-induced ACE and AT1 expression. Our studies indicate that the ACE-Ang II-AT1 system serves as a positive feedback loop and fosters FBPA proliferation under hypoxic conditions, with the PI3K-HIF-HRE axis as the central effector pathway. This pathway may thus facilitate vascular remodeling under hypoxic conditions.

Cancer, inflammation and the AT1 and AT2 receptors

The critical role of inappropriate inflammation is becoming accepted in many diseases that affect man, including cardiovascular diseases, inflammatory and autoimmune disorders, neurodegenerative conditions, infection and cancer.

This review proposes that cancer up-regulates the angiotensin II type 1 (AT1) receptor through systemic oxidative stress and hypoxia mechanisms, thereby triggering chronic inflammatory processes to remodel surrounding tissue and subdue the immune system. Based on current literature and clinical studies on angiotensin receptor inhibitors, the paper concludes that blockade of the AT1 receptor in synergy with cancer vaccines and anti-inflammatory agents should offer a therapy to regress most, if not all, solid tumours.

With regard to cancer being a systemic disease, an examination of supporting evidence for a systemic role of AT1 in relationship to inflammation in disease and injury is presented as a logical progression. The evidence suggests that regulation of the mutually antagonistic angiotensin II receptors (AT1 and AT2) is an essential process in the management of inflammation and wound recovery, and that it is an imbalance in the expression of these receptors that leads to disease.

In consideration of cancer induced immune suppression, it is further postulated that the inflammation associated with bacterial and viral infections, is also an evolved means of immune suppression by these pathogens and that the damage caused, although incidental, leads to the symptoms of disease and, in some cases, death.

It is anticipated that manipulation of the angiotensin system with existing anti-hypertensive drugs could provide a new approach to the treatment of many of the diseases that afflict mankind.

Angiotensin II stimulates nitric oxide production in pulmonary artery endothelium via the type 2 receptor

We previously reported that angiotensin II stimulates an increase in nitric oxide production in pulmonary artery endothelial cells. The aims of this study were to determine which receptor subtype mediates the angiotensin II-dependent increase in nitric oxide production and to investigate the roles of the angiotensin type 1 and type 2 receptors in modulating angiotensin II-dependent vasoconstriction in pulmonary arteries. Pulmonary artery endothelial cells express both angiotensin II type 1 and type 2 receptors as assessed by RT-PCR, Western blot analysis, and flow cytometry. Treatment of the endothelial cells with PD-123319, a type 2 receptor antagonist, prevented the angiotensin II-dependent increase in nitric oxide synthase mRNA, protein levels, and nitric oxide production. In contrast, the type 1 receptor antagonist losartan enhanced nitric oxide synthase mRNA levels, protein expression, and nitric oxide production. Pretreatment of the endothelial cells with either PD-123319 or an anti-angiotensin II antibody prevented this losartan enhancement of nitric oxide production. Angiotensin II-dependent enhanced hypoxic contractions in pulmonary arteries were blocked by the type 1 receptor antagonist candesartan; however, PD-123319 enhanced hypoxic contractions in angiotensin II-treated endothelium-intact vessels. These data demonstrate that angiotensin II stimulates an increase in nitric oxide synthase mRNA, protein expression, and nitric oxide production via the type 2 receptor, whereas signaling via the type 1 receptor negatively regulates nitric oxide production in the pulmonary endothelium. This endothelial, type 2 receptor-dependent increase in nitric oxide may serve to counterbalance the angiotensin II-dependent vasoconstriction in smooth muscle cells, ultimately regulating pulmonary vascular tone.

Endogenous angiotensin II in the regulation of hypoxic pulmonary vasoconstriction in anaesthetized dogs

Introduction

The role played by several vasoactive mediators that are synthesized and released by the pulmonary vascular endothelium in the regulation of hypoxic pulmonary vasoconstriction (HPV) remains unclear. As a potent vasoconstrictor, angiotensin II could be involved. We tested the hypothesis that angiotensin-converting enzyme inhibition by enalaprilat and type 1 angiotensin II receptor blockade by candesartan would inhibit HPV.

Methods

HPV was evaluated in anaesthetized dogs, with an intact pulmonary circulation, by examining the increase in the Ppa–Ppao gradient (mean pulmonary artery pressure minus occluded pulmonary artery pressure) that occurred in response to hypoxia (inspiratory oxygen fraction of 0.1) at constant pulmonary blood flow. Plasma renin activity and angiotensin II immunoreactivity were measured to determine whether activation or inhibition of the renin–angiotensin system was present.

Results

Administration of enalaprilat and candesartan did not affect the Ppa–Ppao gradient at baseline or during hypoxia. Plasma renin activity and angiotensin II immunoreactivity increased during hypoxia, and subsequent measurements were consistent with effective angiotensin-converting enzyme inhibition after administration of enalaprilat, and with angiotensin receptor blockade after administration of candesartan.

Conclusion

These results suggest that, although the renin–angiotensin system was activated in hypoxia, angiotensin II is not normally involved in mediating acute HPV.

Translation of the human angiotensin II type 1 receptor mRNA is mediated by a highly efficient internal ribosome entry site

Activation of the angiotensin II type 1 receptor (AT1R) is closely involved in the pathogenesis of cardiovascular disease. The human AT1R (hAT1R) mRNA splice variants have long 5'-untranslated regions (5'-UTRs) ranging from 272 to 414 bp that have the potential to form stable secondary structures. In this study, we show that the 5'-UTR of hAT(1)R mRNAs contains an internal ribosome entry site (IRES) located within the first 40 bp of the proximal end of exon 1. Experiments utilizing the hAT1R 5'-UTR as a molecular decoy demonstrate a reduction in IRES activity of approximately 50%. This inhibition is most efficient for the hAT1R IRES suggesting that a defined set of trans-factors are required to initiate translation through this cis-element. Translation initiation from the hAT1R IRES appears to be physiologically relevant since IRES activity was maintained during serum starvation, a cellular stress known to inhibit cap-dependent translation. These results suggest that cap-independent translation initiation by internal ribosome entry may represent an important mechanism for the regulation of hAT1R expression.

Regulation of the angiotensin-converting enzyme activity by a time-course hypoxia in the carotid body

Chronic hypoxia activates a local angiotensin-generating system in the carotid body. Here, we test the hypothesis that the activity of the critical enzyme for this system, angiotensin-converting enzyme (ACE), in the carotid body is subject to regulation by a time-course hypoxia. Results from the carotid body assays showed that ACE activity was markedly increased under the hypoxic stress of 7-, 14-, 21-, and 28-day exposures. The changes in ACE activity of 7-day (15.00 vs. 30.95 x 10(-5) nmol.microg(-1).min(-1)), 14-day (8.73 vs. 30.25 x 10(-5) nmol.microg(-1).min(-1)), and 21-day (11.41 vs. 31.83 x 10(-5) nmol.microg(-1).min(-1)) hypoxia treatments were enhanced significantly. However, ACE activity in 28-day (13.18 vs. 24.53 x 10(-5) nmol.microg(-1).min(-1)) hypoxia treatment was observed to increase insignificantly when compared with results in the respective control groups. Captopril inhibited all rises in ACE activity in both the control and experimental groups. Results clearly indicate an activation of the enzymatic activity of ACE, the critical enzyme for determining the conversion of angiotensin I into the physiologically active angiotensin II, by chronic hypoxia in the carotid body. An increase in the ACE activity may increase the local production of angiotensin II in the carotid body and thus its agonist action at the AT1 receptor. This may be important in the modulation of cardiopulmonary adaptation in the hypoxic ventilatory response as well as for electrolyte and water homeostasis during chronic hypoxia.

Angiotensin II type 2 receptor inhibits vascular endothelial growth factor-induced migration and in vitro tube formation of human endothelial cells

Endothelial cell migration and tube formation in response to vascular endothelial growth factor (VEGF) play an important role in the process of angiogenesis. Recent data indicate that angiotensin type 2 (AT2) receptor stimulation is antiangiogenic. Therefore, we studied the effect of angiotensin II (Ang II) on VEGF-induced migration and in vitro tube formation of human endothelial cells. Ang II inhibited VEGF-induced migration of EA.hy926 cells, human coronary artery (HCA) and human dermal microvascular (HDM) endothelial cells (ECs) as well as tube formation by HDMECs. The AT2 receptor antagonist PD123,319 but not the AT1 receptor antagonist losartan blocked the inhibitory effect of Ang II. The inhibitory effect of Ang II on VEGF-induced migration of endothelial cells was mimicked by the specific AT2 receptor agonist CGP-42112A. The phosphorylation of Akt and its downstream effector endothelial NO synthase (eNOS) is pivotal to VEGF-induced angiogenesis. We therefore investigated the effect of Ang II on VEGF-induced Akt and eNOS phosphorylation. Ang II diminished the VEGF-induced phosphorylation of Akt and eNOS in endothelial cells, whereas the autophosphorylation of VEGF receptors was unaffected. CGP-42112A again mimicked and PD123,319 but not losartan blocked the inhibitory effect of Ang II. Treatment of endothelial cells with pertussis toxin (PTX) totally abolished the AT2 receptor-mediated inhibition of VEGF-induced endothelial cell migration and blocked the inhibition of Akt and eNOS phosphorylation. In conclusion, this study indicates that AT2 receptor stimulation inhibits VEGF-induced endothelial cell migration and tube formation via activation of a PTX-sensitive G protein. These findings may explain the reported antiangiogenic properties of the AT2 receptor.