Pulmonary arterial hypertension is linked to insulin resistance and reversed by peroxisome proliferator-activated receptor-gamma activation

Peroxisome proliferator-activated receptor gamma (PPARγ) regulates thrombospondin-1 and Nox4 expression in hypoxia-induced human pulmonary artery smooth muscle cell proliferation

Transforming growth factor-β1 (TGF- β1) and thrombospondin-1 (TSP-1) are hypoxia-responsive mitogens that promote vascular smooth muscle cell (SMC) proliferation, a critical event in the pathogenesis of pulmonary hypertension (PH). We previously demonstrated that hypoxia-induced human pulmonary artery smooth muscle (HPASMC) cell proliferation and expression of the NADPH oxidase subunit, Nox4, were attenuated by the peroxisome proliferator-activated receptor γ (PPARγ) agonist, rosiglitazone. The current study examines the hypothesis that rosiglitazone regulates Nox4 expression and HPASMC proliferation by attenuating TSP-1 signaling. Selected HPASMC were exposed to normoxic or hypoxic (1% O₂) environments or TSP-1 (0-1 μg/ ml) for 72 hours ± administration of rosiglitazone (10 μM). Cellular proliferation, Nox4, TSP-1, and TGF-β1 expression and reactive oxygen species generation were measured. Mice exposed to hypoxia (10% O₂) for three weeks were treated with rosiglitazone (10 mg/kg/day) for the final 10 days, and lung TSP-1 expression was examined. Hypoxia increased TSP-1 and TGF-β1 expression and HPASMC proliferation, and neutralizing antibodies to TSP-1 or TGF-β1 attenuated proliferation. Rosiglitazone attenuated hypoxia-induced HPASMC proliferation and increases in mouse lung and HPASMC TSP-1 expression, but failed to reduce increases in TGF-β1 expression or Nox4 expression and activity caused by direct TSP-1 stimulation. Transfecting HPASMC with siRNA to Nox4 attenuated hypoxia- or TSP-1-stimulated HPASMC proliferation. These findings provide novel evidence that TSP-1-mediated Nox4 expression plays a critical role in hypoxia-induced HPASMC proliferation. PPARγ activation with exogenous ligands attenuates TSP-1 expression to reduce Nox4 expression. These results clarify mechanisms of hypoxia-induced SMC proliferation and suggest additional pathways by which PPARγ agonists may regulate critical steps in the pathobiology of PH.

A process-based review of mouse models of pulmonary hypertension

Genetically modified mouse models have unparalleled power to determine the mechanisms behind different processes involved in the molecular and physiologic etiology of various classes of human pulmonary hypertension (PH). Processes known to be involved in PH for which there are extensive mouse models available include the following: (1) Regulation of vascular tone through secreted vasoactive factors; (2) regulation of vascular tone through potassium and calcium channels; (3) regulation of vascular remodeling through alteration in metabolic processes, either through alteration in substrate usage or through circulating factors; (4) spontaneous vascular remodeling either before or after development of elevated pulmonary pressures; and (5) models in which changes in tone and remodeling are primarily driven by inflammation. PH development in mice is of necessity faster and with different physiologic ramifications than found in human disease, and so mice make poor models of natural history of PH. However, transgenic mouse models are a perfect tool for studying the processes involved in pulmonary vascular function and disease, and can effectively be used to test interventions designed against particular molecular pathways and processes involved in disease.

Critical role for the advanced glycation end-products receptor in pulmonary arterial hypertension etiology

Background

Pulmonary arterial hypertension (PAH) is a vasculopathy characterized by enhanced pulmonary artery smooth muscle cell (PASMC) proliferation and suppressed apoptosis. This results in both increase in pulmonary arterial pressure and pulmonary vascular resistance. Recent studies have shown the implication of the signal transducer and activator of transcription 3 (STAT3)/bone morphogenetic protein receptor 2 (BMPR2)/peroxisome proliferator‐activated receptor gamma (PPARγ) in PAH. STAT3 activation induces BMPR2 downregulation, decreasing PPARγ, which both contribute to the proproliferative and antiapoptotic phenotype seen in PAH. In chondrocytes, activation of this axis has been attributed to the advanced glycation end‐products receptor (RAGE). As RAGE is one of the most upregulated proteins in PAH patients' lungs and a strong STAT3 activator, we hypothesized that by activating STAT3, RAGE induces BMPR2 and PPARγ downregulation, promoting PAH‐PASMC proliferation and resistance to apoptosis.

Methods and Results

In vitro, using PASMCs isolated from PAH and healthy patients, we demonstrated that RAGE is overexpressed in PAH‐PASMC (6‐fold increase), thus inducing STAT3 activation (from 10% to 40% positive cells) and decrease in BMPR2 and PPARγ levels (>50% decrease). Pharmacological activation of RAGE in control cells by S100A4 recapitulates the PAH phenotype (increasing RAGE by 6‐fold, thus activating STAT3 and decreasing BMPR2 and PPARγ). In both conditions, this phenotype is totally reversed on RAGE inhibition. In vivo, RAGE inhibition in monocrotaline‐ and Sugen‐induced PAH demonstrates therapeutic effects characterized by PA pressure and right ventricular hypertrophy decrease (control rats have an mPAP around 15 mm Hg, PAH rats have an mPAP >40 mm Hg, and with RAGE inhibition, mPAP decreases to 20 and 28 mm Hg, respectively, in MCT and Sugen models). This was associated with significant improvement in lung perfusion and vascular remodeling due to decrease in proliferation (>50% decrease) and BMPR2/PPARγ axis restoration (increased by ≥60%).

Conclusion

We have demonstrated the implications of RAGE in PAH etiology. Thus, RAGE constitutes a new attractive therapeutic target for PAH.

Molecular pathogenesis of pulmonary arterial hypertension

Recent clinical and experimental studies are redefining the cellular and molecular bases of pulmonary arterial hypertension (PAH). The genetic abnormalities first identified in association with the idiopathic form of PAH--together with a vast increase in our understanding of cell signaling, cell transformation, and cell-cell interactions; gene expression; microRNA processing; and mitochondrial and ion channel function--have helped explain the abnormal response of vascular cells to injury. Experimental and clinical studies now converge on the intersection and interactions between a genetic predisposition involving the BMPR2 signaling pathway and an impaired metabolic and chronic inflammatory state in the vessel wall. These deranged processes culminate in an exuberant proliferative response that occludes the pulmonary arterial (PA) lumen and obliterates the most distal intraacinar vessels. Here, we describe emerging therapies based on preclinical studies that address these converging pathways.

Liraglutide increases FGF-21 activity and insulin sensitivity in high fat diet and adiponectin knockdown induced insulin resistance

Background

Liraglutide is a glucagon-like peptide-1 analogue that stimulates insulin secretion and improves β-cell function. However, it is not clear whether liraglutide achieves its glucose lowering effect only by its known effects or whether other as yet unknown mechanisms are involved. The aim of this study was to examine the effects of liraglutide on Fibroblast growth factor-21 (FGF-21) activity in High-fat diet (HFD) fed ApoE^−/− mice with adiponectin (Acrp30) knockdown.

Method

HFD-fed ApoE^−/− mice were treated with adenovirus vectors expressing shAcrp30 to produce insulin resistance. Hyperinsulinemic-euglycemic clamp studies were performed to evaluate insulin sensitivity of the mouse model. QRT-PCR and Western blot were used to measure the mRNA and protein expression of the target genes.

Results

The combination of HFD, ApoE deficiency, and hypoadiponectinemia resulted in an additive effect on insulin resistance. FGF-21 mRNA expressions in both liver and adipose tissues were significantly increased while FGF-21 receptor 1 (FGFR-1) and β-Klotho mRNA levels in adipose tissue, as well as FGFR-1-3 and β-Klotho mRNA levels in liver were significantly decreased in this model. Liraglutide treatment markedly improved insulin resistance and increased FGF-21 expression in liver and FGFR-3 in adipose tissue, restored β-Klotho mRNA expression in adipose tissue as well as FGFR-1-3, β-Klotho levels and phosphorylation of FGFR1 up to the levels observed in control mice in liver. Liraglutide treatment also further increased FGF-21 proteins in liver and plasma. In addition, as shown by hyperinsulinemic-euglycemic clamp, liraglutide treatment also markedly improved glucose metabolism and insulin sensitivity in these animals.

Conclusion

These findings demonstrate an additive effect of HFD, ApoE deficiency, and adiponectin knockdown on insulin resistance and unveil that the regulation of glucose metabolism and insulin sensitivity by liraglutide may be partly mediated via increased FGF-21 and its receptors action.

Peroxisome proliferator-activated receptor-γ ameliorates pulmonary arterial hypertension by inhibiting 5-hydroxytryptamine 2B receptor

An elevated plasma level of 5-hydroxytryptamine (5-HT) or upregulation of 5-HT receptor signaling or both is implicated in vascular contraction and remodeling in pulmonary arterial hypertension (PAH). Recently, peroxisome proliferator-activated receptor-γ (PPARγ) agonists have been shown to ameliorate PAH. However, their effects on the 5-HT-induced contraction of pulmonary arteries remain unknown. Here, we examined the role of PPARγ in inhibiting 5-HT2B receptor (5-HT2BR) to ameliorate PAH. Pulmonary arteries from PAH rats induced by monocrotaline or chronic hypoxia showed an enhanced vasoconstriction in response to BW723C86, a specific agonist for 5-HT2BR. Expression of 5-HT2BR was also increased in pulmonary arteries from the PAH rats, accompanied by vascular remodeling and right ventricular hypertrophy. Treatment with the PPARγ agonist rosiglitazone in vivo reversed the expression and the vasocontractive effect of 5-HT2BR as well as the thickening of pulmonary arteries. In pulmonary artery smooth muscle cells, 5-HT induced the gene expression of 5-HT2BR, which was inhibited by rosiglitazone, pioglitazone, or adenovirus-mediated overexpression of constitutively activated PPARγ. The pharmacological effect of PPARγ was through the suppression of the 5-HT-induced activator protein-1 activity. These results demonstrated the beneficial effect of PPARγ on 5-HT2BR-mediated vasocontraction, providing a new mechanism for the potential use of PPARγ agonists in PAH.

Inhibition of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) reverses experimental pulmonary hypertension

Genetic deletion of TRAIL or antibody blockade prevents the development of pulmonary arterial hypertension and can reverse vascular remodeling in established disease.

Pulmonary arterial hypertension (PAH) is a life-threatening disease characterized by the progressive narrowing and occlusion of small pulmonary arteries. Current therapies fail to fully reverse this vascular remodeling. Identifying key pathways in disease pathogenesis is therefore required for the development of new-targeted therapeutics. We have previously reported tumor necrosis factor–related apoptosis-inducing ligand (TRAIL) immunoreactivity within pulmonary vascular lesions from patients with idiopathic PAH and animal models. Because TRAIL can induce both endothelial cell apoptosis and smooth muscle cell proliferation in the systemic circulation, we hypothesized that TRAIL is an important mediator in the pathogenesis of PAH. We demonstrate for the first time that TRAIL is a potent stimulus for pulmonary vascular remodeling in human cells and rodent models. Furthermore, antibody blockade or genetic deletion of TRAIL prevents the development of PAH in three independent rodent models. Finally, anti-TRAIL antibody treatment of rodents with established PAH reverses pulmonary vascular remodeling by reducing proliferation and inducing apoptosis, improves hemodynamic indices, and significantly increases survival. These preclinical investigations are the first to demonstrate the importance of TRAIL in PAH pathogenesis and highlight its potential as a novel therapeutic target to direct future translational therapies.

Does the small tyrosine kinase inhibitor Imatinib mesylate counteract diabetes by affecting pancreatic islet amyloidosis and fibrosis?

INTRODUCTION

The small tyrosine kinase inhibitor Imatinib Mesylate (Gleevec) protects against diabetes, but it is not known how.

AREAS COVERED

It has been suggested that islet amyloid and fibrotic deposits promote beta-cell failure and death, leading to Type-2 diabetes. As Imatinib is known to possess anti-fibrotic/amyloid properties, in for example systemic sclerosis and mouse models for Alzheimer's disease, the present review will discuss the possibility that Imatinib acts, at least in part, by ameliorating islet hyalinization and its consequences in the pathogenesis of Type-2 diabetes.

EXPERT OPINION

A better understanding of how Imatinib counteracts Type-2 diabetes will possibly help to clarify the pathogenic role of islet amyloid and fibrosis, and hopefully lead to improved treatment of the disease.

Today's and tomorrow's imaging and circulating biomarkers for pulmonary arterial hypertension

The pathobiology of pulmonary arterial hypertension (PAH) involves a remodeling process in distal pulmonary arteries, as well as vasoconstriction and in situ thrombosis, leading to an increase in pulmonary vascular resistance, right heart failure and death. Its etiology may be idiopathic, but PAH is also frequently associated with underlying conditions such as connective tissue diseases. During the past decade, more than welcome novel therapies have been developed and are in development, including those increasingly targeting the remodeling process. These therapeutic options modestly increase the patients' long-term survival, now approaching 60% at 5 years. However, non-invasive tools for confirming PAH diagnosis, and assessing disease severity and response to therapy, are tragically lacking and would help to select the best treatment. After exclusion of other causes of pulmonary hypertension, a final diagnosis still relies on right heart catheterization, an invasive technique which cannot be repeated as often as an optimal follow-up might require. Similarly, other techniques and biomarkers used for assessing disease severity and response to treatment generally lack specificity and have significant limitations. In this review, imaging as well as current and future circulating biomarkers for diagnosis, prognosis, and follow-up are discussed.

A potential role for insulin resistance in experimental pulmonary hypertension

Patients with pulmonary arterial hypertension have increased prevalence of insulin resistance. We aimed to determine whether metabolic defects are associated with bone morphogenic protein receptor type 2 (Bmpr2) mutations in mice, and whether these may contribute to pulmonary vascular disease development. Metabolic phenotyping was performed on transgenic mice with inducible expression of Bmpr2 mutation, R899X. Phenotypic penetrance in Bmpr2(R899X) was assessed in a high-fat diet model of insulin resistance. Alterations in glucocorticoid responses were assessed in murine pulmonary microvascular endothelial cells and Bmpr2(R899X) mice treated with dexamethasone. Compared to controls, Bmpr2(R899X) mice showed increased weight gain and demonstrated insulin resistance as assessed by the homeostatic model assessment insulin resistance (1.0 ± 0.4 versus 2.2 ± 1.8) and by fat accumulation in skeletal muscle and decreased oxygen consumption. Bmpr2(R899X) mice fed a high-fat diet had strong increases in pulmonary hypertension penetrance (seven out of 11 versus three out of 11). In cell culture and in vivo experiments, Bmpr2 mutation resulted in a combination of constitutive glucocorticoid receptor activation and insensitivity. Insulin resistance is present as an early feature of Bmpr2 mutation in mice. Exacerbated insulin resistance through high-fat diet worsened pulmonary phenotype, implying a possible causal role in disease. Impaired glucocorticoid responses may contribute to metabolic defects.

Mesenchymal stem cell-mediated reversal of bronchopulmonary dysplasia and associated pulmonary hypertension

Clinical trials have failed to demonstrate an effective preventative or therapeutic strategy for bronchopulmonary dysplasia (BPD), a multifactorial chronic lung disease in preterm infants frequently complicated by pulmonary hypertension (PH). Mesenchymal stem cells (MSCs) and their secreted components have been shown to prevent BPD and pulmonary fibrosis in rodent models. We hypothesized that treatment with conditioned media (CM) from cultured mouse bone marrow-derived MSCs could reverse hyperoxia-induced BPD and PH. Newborn mice were exposed to hyperoxia (FiO₂=0.75) for two weeks, were then treated with one intravenous dose of CM from either MSCs or primary mouse lung fibroblasts (MLFs), and placed in room air for two to four weeks. Histological analysis of lungs harvested at four weeks of age was performed to determine the degree of alveolar injury, blood vessel number, and vascular remodeling. At age six weeks, pulmonary artery pressure (PA acceleration time) and right ventricular hypertrophy (RVH; RV wall thickness) were assessed by echocardiography, and pulmonary function tests were conducted. When compared to MLF-CM, a single dose of MSC-CM-treatment (1) reversed the hyperoxia-induced parenchymal fibrosis and peripheral PA devascularization (pruning), (2) partially reversed alveolar injury, (3) normalized lung function (airway resistance, dynamic lung compliance), (4) fully reversed the moderate PH and RVH, and (5) attenuated peripheral PA muscularization associated with hyperoxia-induced BPD. Reversal of key features of hyperoxia-induced BPD and its long-term adverse effects on lung function can be achieved by a single intravenous dose of MSC-CM, thereby pointing toward a new therapeutic intervention for chronic lung diseases.

Obesity and pulmonary arterial hypertension: Is adiponectin the molecular link between these conditions?

Pulmonary arterial hypertension (PAH) is a condition of unknown etiology whose pathological features include increased vascular resistance, perivascular inflammatory cell infiltration and pulmonary arteriolar remodeling. Although risk factors for PAH are poorly defined, recent studies indicate that obesity may be an important risk factor for this condition. The mechanisms leading to this association are largely unknown, but bioactive mediators secreted from adipose tissue have been implicated in this process. One of the most important mediators released from adipose tissue is the adipokine adiponectin. Adiponectin is highly abundant in the circulation of lean healthy individuals, and possesses well-described metabolic and antiinflammatory actions. Levels of adiponectin decrease with increasing body mass, and low levels are directly linked to the development of PAH in mice. Moreover, overexpression of adiponectin has been shown to protect mice from developing PAH in response to inflammation and hypoxia. Based on the findings from these studies, it is suggested that the effects of adiponectin are mediated, in part, through its antiinflammatory and antiproliferative properties. In this review, we discuss the emerging evidence demonstrating a role for adiponectin in lung vascular homeostasis and discuss how deficiency in this adipocyte-derived hormone might explain the recent association between obesity and PAH.

Fenfluramine-induced gene dysregulation in human pulmonary artery smooth muscle and endothelial cells

Fenfluramine is prescribed either alone or in combination with phentermine as part of Fen-Phen, an anti-obesity medication. Fenfluramine was withdrawn from the US market in 1997 due to reports of heart valvular disease, pulmonary arterial hypertension, and cardiac fibrosis. Particularly, idiopathic pulmonary arterial hypertension (IPAH), previously referred to as primary pulmonary hypertension (PPH), was found to be associated with the use of Fen-Phen, fenfluramine, and fenfluramine derivatives. The underlying mechanism of fenfluramine-associated pulmonary hypertension is still largely unknown. We reasoned that investigating drug-induced gene dysregulation would enhance our understanding of the fenfluramine-associated pathogenic mechanism of IPAH. Whole-genome gene expression profiles in fenfluramine-treated human pulmonary artery smooth muscle (PASMC) and endothelial (PAEC) cells (isolated from normal subjects) were compared with baseline expression in untreated cells. Fenfluramine treatment caused dysregulation in a substantial number of genes involved in a variety of pathways and biological processes. In addition to several common pathways and biological processes such as "MAPK signaling pathway," "inflammation response," and "calcium signaling pathway" shared between both cell types, pathways and biological processes such as "blood circulation," "muscle system process," and "immune response" were enriched among the dysregulated genes in PASMC. Pathways and biological processes such as those related to cell cycle, however, were enriched among the dysregulated genes in PAEC, indicating that fenfluramine could affect unique pathways (or differentially) in different types of pulmonary artery cells. While awaiting validation in a larger cohort, these results strongly suggested that fenfluramine could induce significant dysregulation of genes in multiple biological processes and pathways critical for normal pulmonary vascular functions and structure. The transcriptional and posttranscriptional changes in these genes may, therefore, contribute to the pathogenesis of fenfluramine-associated IPAH.

Metabolomic analysis of bone morphogenetic protein receptor type 2 mutations in human pulmonary endothelium reveals widespread metabolic reprogramming

Pulmonary arterial hypertension (PAH) is a progressive and fatal disease of the lung vasculature for which the molecular etiologies are unclear. Specific metabolic alterations have been identified in animal models and in PAH patients, though existing data focus mainly on abnormalities of glucose homeostasis. We hypothesized that analysis of the entire metabolome in PAH would reveal multiple other metabolic changes relevant to disease pathogenesis and possible treatment. Layered transcriptomic and metabolomic analyses of human pulmonary microvascular endothelial cells (hPMVEC) expressing two different disease-causing mutations in the bone morphogenetic protein receptor type 2 (BMPR2) confirmed previously described increases in aerobic glycolysis but also uncovered significant upregulation of the pentose phosphate pathway, increases in nucleotide salvage and polyamine biosynthesis pathways, decreases in carnitine and fatty acid oxidation pathways, and major impairment of the tricarboxylic acid (TCA) cycle and failure of anaplerosis. As a proof of principle, we focused on the TCA cycle, predicting that isocitrate dehydrogenase (IDH) activity would be altered in PAH, and then demonstrating increased IDH activity not only in cultured hPMVEC expressing mutant BMPR2 but also in the serum of PAH patients. These results suggest that widespread metabolic changes are an important part of PAH pathogenesis, and that simultaneous identification and targeting of the multiple involved pathways may be a more fruitful therapeutic approach than targeting of any one individual pathway.

The Nox4 inhibitor GKT137831 attenuates hypoxia-induced pulmonary vascular cell proliferation

Increased NADP reduced (NADPH) oxidase 4 (Nox4) and reduced expression of the nuclear hormone receptor peroxisome proliferator-activated receptor γ (PPARγ) contribute to hypoxia-induced pulmonary hypertension (PH). To examine the role of Nox4 activity in pulmonary vascular cell proliferation and PH, the current study used a novel Nox4 inhibitor, GKT137831, in hypoxia-exposed human pulmonary artery endothelial or smooth muscle cells (HPAECs or HPASMCs) in vitro and in hypoxia-treated mice in vivo. HPAECs or HPASMCs were exposed to normoxia or hypoxia (1% O(2)) for 72 hours with or without GKT137831. Cell proliferation and Nox4, PPARγ, and transforming growth factor (TGF)β1 expression were measured. C57Bl/6 mice were exposed to normoxia or hypoxia (10% O(2)) for 3 weeks with or without GKT137831 treatment during the final 10 days of exposure. Lung PPARγ and TGF-β1 expression, right ventricular hypertrophy (RVH), right ventricular systolic pressure (RVSP), and pulmonary vascular remodeling were measured. GKT137831 attenuated hypoxia-induced H(2)O(2) release, proliferation, and TGF-β1 expression and blunted reductions in PPARγ in HPAECs and HPASMCs in vitro. In vivo GKT137831 inhibited hypoxia-induced increases in TGF-β1 and reductions in PPARγ expression and attenuated RVH and pulmonary artery wall thickness but not increases in RVSP or muscularization of small arterioles. This study shows that Nox4 plays a critical role in modulating proliferative responses of pulmonary vascular wall cells. Targeting Nox4 with GKT137831 provides a novel strategy to attenuate hypoxia-induced alterations in pulmonary vascular wall cells that contribute to vascular remodeling and RVH, key features involved in PH pathogenesis.

Adipokines and the cardiovascular system: mechanisms mediating health and disease

This review focuses on the role of adipokines in the maintenance of a healthy cardiovascular system, and the mechanisms by which these factors mediate the development of cardiovascular disease in obesity. Adipocytes are the major cell type comprising the adipose tissue. These cells secrete numerous factors, termed adipokines, into the blood, including adiponectin, leptin, resistin, chemerin, omentin, vaspin, and visfatin. Adipose tissue is a highly vascularised endocrine organ, and different adipose depots have distinct adipokine secretion profiles, which are altered with obesity. The ability of many adipokines to stimulate angiogenesis is crucial for adipose tissue expansion; however, excessive blood vessel growth is deleterious. As well, some adipokines induce inflammation, which promotes cardiovascular disease progression. We discuss how these 7 aforementioned adipokines act upon the various cardiovascular cell types (endothelial progenitor cells, endothelial cells, vascular smooth muscle cells, pericytes, cardiomyocytes, and cardiac fibroblasts), the direct effects of these actions, and their overall impact on the cardiovascular system. These were chosen, as these adipokines are secreted predominantly from adipocytes and have known effects on cardiovascular cells.

Fat, fire and muscle--the role of adiponectin in pulmonary vascular inflammation and remodeling

Pulmonary hypertension is a life-threatening condition that results from a heterogeneous group of diseases, many of which demonstrate characteristic pathologic changes of pulmonary vascular inflammation and remodeling. Recent clinical studies indicate obesity to be a risk factor for the development of pulmonary hypertension; however, the mechanisms leading to this association are unknown. Adipocytes secrete multiple bioactive mediators that can influence inflammation and tissue remodeling, suggesting that adipose tissue may directly influence the pathogenesis of pulmonary hypertension. One of these mediators is adiponectin, a protein with a wide range of metabolic, anti-inflammatory, and anti-proliferative activities. Paradoxically, adiponectin is present in high concentration in the serum of lean healthy individuals, but decreases in obesity. Studies suggest that relative adiponectin-deficiency may contribute to the development of inflammatory diseases in obesity, and recent animal studies implicate adiponectin in the pathogenesis of pulmonary hypertension. Most notably, experimental studies show that adiponectin can reduce lung vascular remodeling in response to inflammation and hypoxia. Moreover, mice deficient in adiponectin develop a spontaneous lung vascular phenotype characterized by age-dependent increases in peri-vascular inflammatory cells and elevated pulmonary artery pressures. Emerging evidence indicates adiponectin's effects are mediated through anti-inflammatory and anti-proliferative actions on cells in the lung. This review aims to synthesize the existing data related to adiponectin's effects on the pulmonary vasculature and to discuss how changes in adiponectin levels might contribute to the development of pulmonary hypertension.

Microparticles and acute lung injury

The pathophysiology of acute lung injury (ALI) and its most severe form, acute respiratory distress syndrome (ARDS), is characterized by increased vascular and epithelial permeability, hypercoagulation and hypofibrinolysis, inflammation, and immune modulation. These detrimental changes are orchestrated by cross talk between a complex network of cells, mediators, and signaling pathways. A rapidly growing number of studies have reported the appearance of distinct populations of microparticles (MPs) in both the vascular and alveolar compartments in animal models of ALI/ARDS or respective patient populations, where they may serve as diagnostic and prognostic biomarkers. MPs are small cytosolic vesicles with an intact lipid bilayer that can be released by a variety of vascular, parenchymal, or blood cells and that contain membrane and cytosolic proteins, organelles, lipids, and RNA supplied from and characteristic for their respective parental cells. Owing to this endowment, MPs can effectively interact with other cell types via fusion, receptor-mediated interaction, uptake, or mediator release, thereby acting as intrinsic stimulators, modulators, or even attenuators in a variety of disease processes. This review summarizes current knowledge on the formation and potential functional role of different MPs in inflammatory diseases with a specific focus on ALI/ARDS. ALI has been associated with the formation of MPs from such diverse cellular origins as platelets, neutrophils, monocytes, lymphocytes, red blood cells, and endothelial and epithelial cells. Because of their considerable heterogeneity in terms of origin and functional properties, MPs may contribute via both harmful and beneficial effects to the characteristic pathological features of ALI/ARDS. A better understanding of the formation, function, and relevance of MPs may give rise to new promising therapeutic strategies to modulate coagulation, inflammation, endothelial function, and permeability either through removal or inhibition of "detrimental" MPs or through administration or stimulation of "favorable" MPs.

Emerging PPARγ-Independent Role of PPARγ Ligands in Lung Diseases

Peroxisome proliferator activated receptor (PPAR)-γ is a nuclear hormone receptor that is activated by multiple agonists including thiazolidinediones, prostaglandins, and synthetic oleanolic acids. Many PPARγ ligands are under investigation as potential therapies for human diseases. These ligands modulate multiple cellular pathways via both PPARγ-dependent and PPARγ-independent mechanisms. Here, we review the role of PPARγ and PPARγ ligands in lung disease, with emphasis on PPARγ-independent effects. PPARγ ligands show great promise in moderating lung inflammation, as antiproliferative agents in combination to enhance standard chemotherapy in lung cancer and as treatments for pulmonary fibrosis, a progressive fatal disease with no effective therapy. Some of these effects occur when PPARγ is pharmaceutically antagonized or genetically PPARγ and are thus independent of classical PPARγ-dependent transcriptional control. Many PPARγ ligands demonstrate direct binding to transcription factors and other proteins, altering their function and contributing to PPARγ-independent inhibition of disease phenotypes. These PPARγ-independent mechanisms are of significant interest because they suggest new therapeutic uses for currently approved drugs and because they can be used as probes to identify novel proteins and pathways involved in the pathogenesis or treatment of disease, which can then be targeted for further investigation and drug development.

Estrogen paradox in pulmonary hypertension: current controversies and future perspectives

Although the incidence of pulmonary hypertension is higher in female patients, numerous experimental studies have demonstrated better outcome in female animals, exacerbation of the disease after ovariectomy, and a strong protective effect of estrogen: a phenomenon known as the "estrogen paradox" of pulmonary hypertension. On the other hand, some clinical studies have indirectly linked estrogen to increased risk of portopulmonary hypertension, whereas others implicate increased estrogen metabolism and high levels of certain estrogen metabolites in promoting pulmonary vascular remodeling in familial pulmonary arterial hypertension. In this review we investigate the estrogen paradox through highlighting the differential receptor-mediated effects of estrogen. Although estrogen and estrogen receptor-based therapies have shown promise in rescuing preexisting pulmonary hypertension in animals, their role is yet to be defined in humans.

Rodent models of pulmonary hypertension: harmonisation with the world health organisation's categorisation of human PH

The WHO classification of pulmonary hypertension (PH) recognises five distinct groups, all sharing a mean, resting, pulmonary artery pressure (PAP) > 25 mmHg. The aetiology of PH varies by group (1-pulmonary vascular disease, 2-high left heart filling pressures, 3-hypoxia, 4-unresolved pulmonary embolism and 5-miscellaneous). Inclusion in a group reflects shared histological, haemodynamic and pathophysiological features and has therapeutic implications. Advantages of using rodent models to understand the pathophysiology of human PH and to test experimental therapies include the economy, safety and mechanistic certainty they provide. As rodent models are meant to reflect human PH, they should be categorised by a parallel PH classification and limitations in achieving this ideal recognised. Challenges with rodent models include: accurate phenotypic characterisation (haemodynamics, histology and imaging), species and strain variations in the natural history of PH, and poor fidelity to the relevant human PH group. Rat models of group 1 PH include: monocrotaline (± pneumonectomy), chronic hypoxia + SU-5416 (a VEGF receptor inhibitor) and the fawn-hooded rat (FHR). Mouse models of group 1 PH include: transgenic mice overexpressing the serotonin transporter or dominant-negative mutants of bone morphogenetic protein receptor-2. Group 1 PH is also created by infecting S100A4/Mts1 mice with γ-herpesvirus. The histological features of group 1 PH, but not PH itself, are induced by exposure to Schistosoma mansoni or Stachybotrys chartarum. Group 3 PH is modelled by exposure of rats or mice to chronic hypoxia. Rodent models of groups 2, 4 and 5 PH are needed. Comprehensive haemodynamic, histological and molecular phenotyping, coupled with categorisation into WHO PH groups, enhances the utility of rodent models.

Different patterns of pulmonary vascular disease induced by type 1 diabetes and moderate hypoxia in rats

Although type 1 and type 2 diabetes are strongly associated with systemic cardiovascular morbidity, the relationship with pulmonary vascular disease had been almost disregarded until recent epidemiological data revealed that diabetes might be a risk factor for pulmonary hypertension. Recent experimental studies suggest that diabetes induces changes in lung function insufficient to elevate pulmonary pressure. The aim of this study was to assess the effects of diabetes on the sensitivity to other risk factors for pulmonary hypertension. We therefore analysed the effects of the combination of diabetes with exposure to moderate hypoxia on classical markers of pulmonary hypertension. Control (saline-treated) and diabetic (70 mg kg(-1) streptozotocin-treated) male Wistar-Kyoto rats were followed for 4 weeks and exposed to normoxia or moderate normobaric hypoxia (14%) for another 2 weeks. Hypoxia, but not diabetes, strongly reduced voltage-gated potassium currents, whereas diabetes, but not hypoxia, induced pulmonary artery endothelial dysfunction. Both factors independently induced pulmonary vascular remodelling and downregulated the lung bone morphogenetic protein receptor type 2. However, diabetes, but not hypoxia, induced pulmonary infiltration of macrophages, which was markedly increased when both factors were combined. Diabetes plus hypoxia induced a modest increase in diastolic and mean pulmonary artery pressure and right ventricular weight, while each of the two factors alone had no significant effect. The pattern of changes in markers of pulmonary hypertension was different for moderate hypoxia and diabetes, with no synergic effect except for macrophage recruitment, and the combination of both factors was required to induce a moderate elevation in pulmonary arterial pressure.

Obesity and pulmonary hypertension: a review of pathophysiologic mechanisms

Pulmonary hypertension (PH) is a potentially life-threatening condition arising from a wide variety of pathophysiologic mechanisms. Effective treatment requires a systematic diagnostic approach to identify all reversible mechanisms. Many of these mechanisms are relevant to those afflicted with obesity. The unique mechanisms of PH in the obese include obstructive sleep apnea, obesity hypoventilation syndrome, anorexigen use, cardiomyopathy of obesity, and pulmonary thromboembolic disease. Novel mechanisms of PH in the obese include endothelial dysfunction and hyperuricemia. A wide range of effective therapies exist to mitigate the disability of PH in the obese.

Genetically manipulated mouse models of lung disease: potential and pitfalls

Gene targeting in mice (transgenic and knockout) has provided investigators with an unparalleled armamentarium in recent decades to dissect the cellular and molecular basis of critical pathophysiological states. Fruitful information has been derived from studies using these genetically engineered mice with significant impact on our understanding, not only of specific biological processes spanning cell proliferation to cell death, but also of critical molecular events involved in the pathogenesis of human disease. This review will focus on the use of gene-targeted mice to study various models of lung disease including airways diseases such as asthma and chronic obstructive pulmonary disease, and parenchymal lung diseases including idiopathic pulmonary fibrosis, pulmonary hypertension, pneumonia, and acute lung injury. We will attempt to review the current technological approaches of generating gene-targeted mice and the enormous dataset derived from these studies, providing a template for lung investigators.

17β-estradiol protects the lung against acute injury: possible mediation by vasoactive intestinal polypeptide

Beyond their classical role as a class of female sex hormones, estrogens (e.g. 17β-estradiol) exert important biological actions, both protective and undesirable. We have investigated the ability of estradiol to protect the lung in three models of acute injury induced by 1) oxidant stress due to the herbicide paraquat; 2) excitotoxicity, caused by glutamate agonist N-methyl-d-aspartate; and 3) acute alveolar anoxia. We also assessed the role of estrogen receptors (ER) ERα and ERβ and the neuropeptide vasoactive intestinal peptide (VIP) in mediating this protection. Isolated guinea pig or rat lungs were perfused in situ at constant flow and mechanically ventilated. The onset and severity of lung injury were monitored by increases in pulmonary arterial and airway pressures, wet/dry lung weight ratio, and bronchoalveolar lavage fluid protein content. Estradiol was infused into the pulmonary circulation, beginning 10 min before induction of injury and continued for 60-90 min. Lung injury was marked by significant increases in the above measurements, with paraquat producing the most severe, and excitotoxicity the least severe, injury. Estradiol significantly attenuated the injury in each model. Both ER were constitutively expressed and immunohistochemically demonstrable in normal lung, and their selective agonists reduced anoxic injury, the only model in which they were tested. As it protected against injury, estradiol rapidly and significantly stimulated VIP mRNA expression in rat lung. Estradiol attenuated acute lung injury in three experimental models while stimulating VIP gene expression, a known mechanism of lung protection. The up-regulated VIP expression could have partially mediated the protection by estrogen.

Hypoxia induces downregulation of PPAR-γ in isolated pulmonary arterial smooth muscle cells and in rat lung via transforming growth factor-β signaling

Chronic hypoxia activates transforming growth factor-β (TGF-β) signaling and leads to pulmonary vascular remodeling. Pharmacological activation of peroxisome proliferator-activated receptor-γ (PPAR-γ) has been shown to prevent hypoxia-induced pulmonary hypertension and vascular remodeling in rodent models, suggesting a vasoprotective effect of PPAR-γ under chronic hypoxic stress. This study tested the hypothesis that there is a functional interaction between TGF-β/Smad signaling pathway and PPAR-γ in isolated pulmonary artery small muscle cells (PASMCs) under hypoxic stress. We observed that chronic hypoxia led to a dramatic decrease of PPAR-γ protein expression in whole lung homogenates (rat and mouse) and hypertrophied pulmonary arteries and isolated PASMCs. Using a transgenic model of mouse with inducible overexpression of a dominant-negative mutant of TGF-β receptor type II, we demonstrated that disruption of TGF-β pathway significantly attenuated chronic hypoxia-induced downregulation of PPAR-γ in lung. Similarly, in isolated rat PASMCs, antagonism of TGF-β signaling with either a neutralizing antibody to TGF-β or the selective TGF-β receptor type I inhibitor SB431542 effectively attenuated hypoxia-induced PPAR-γ downregulation. Furthermore, we have demonstrated that TGF-β1 treatment suppressed PPAR-γ expression in PASMCs under normoxia condition. Chromatin immunoprecipitation analysis showed that TGF-β1 treatment significantly increased binding of Smad2/3, Smad4, and the transcriptional corepressor histone deacetylase 1 to the PPAR-γ promoter in PASMCs. Conversely, treatment with the PPAR-γ agonist rosiglitazone attenuated TGF-β1-induced extracellular matrix molecule expression and growth factor in PASMCs. These data provide strong evidence that activation of TGF-β/Smad signaling, via transcriptional suppression of PPAR-γ expression, mediates chronic hypoxia-induced downregulation of PPAR-γ expression in lung.

The PPARγ ligand rosiglitazone attenuates hypoxia-induced endothelin signaling in vitro and in vivo

Peroxisome proliferator-activated receptor (PPAR) γ activation attenuates hypoxia-induced pulmonary hypertension (PH) in mice. The current study examined the hypothesis that PPARγ attenuates hypoxia-induced endothelin-1 (ET-1) signaling to mediate these therapeutic effects. To test this hypothesis, human pulmonary artery endothelial cells (HPAECs) were exposed to normoxia or hypoxia (1% O(2)) for 72 h and treated with or without the PPARγ ligand rosiglitazone (RSG, 10 μM) during the final 24 h of exposure. HPAEC proliferation was measured with MTT assays or cell counting, and mRNA and protein levels of ET-1 signaling components were determined. To explore the role of hypoxia-activated transcription factors, selected HPAECs were treated with inhibitors of hypoxia-inducible factor (HIF)-1α (chetomin) or nuclear factor (NF)-κB (caffeic acid phenethyl ester, CAPE). In parallel studies, male C57BL/6 mice were exposed to normoxia (21% O(2)) or hypoxia (10% O(2)) for 3 wk with or without gavage with RSG (10 mg·kg(-1)·day(-1)) for the final 10 days of exposure. Hypoxia increased ET-1, endothelin-converting enzyme-1, and endothelin receptor A and B levels in mouse lung and in HPAECs and increased HPAEC proliferation. Treatment with RSG attenuated hypoxia-induced activation of HIF-1α, NF-κB activation, and ET-1 signaling pathway components. Similarly, treatment with chetomin or CAPE prevented hypoxia-induced increases in HPAEC ET-1 mRNA and protein levels. These findings indicate that PPARγ activation attenuates a program of hypoxia-induced ET-1 signaling by inhibiting activation of hypoxia-responsive transcription factors. Targeting PPARγ represents a novel therapeutic strategy to inhibit enhanced ET-1 signaling in PH pathogenesis.

Eosinophils are necessary for pulmonary arterial remodeling in a mouse model of eosinophilic inflammation-induced pulmonary hypertension

There is increasing evidence that inflammation plays a pivotal role in the pathogenesis of some forms of pulmonary hypertension (PH). We recently demonstrated that deficiency of adiponectin (APN) in a mouse model of PH induced by eosinophilic inflammation increases pulmonary arterial remodeling, pulmonary pressures, and the accumulation of eosinophils in the lung. Based on these data, we hypothesized that APN deficiency exacerbates PH indirectly by increasing eosinophil recruitment. Herein, we examined the role of eosinophils in the development of inflammation-induced PH. Elimination of eosinophils in APN-deficient mice by treatment with anti-interleukin-5 antibody attenuated pulmonary arterial muscularization and PH. In addition, we observed that transgenic mice that are devoid of eosinophils also do not develop pulmonary arterial muscularization in eosinophilic inflammation-induced PH. To investigate the mechanism by which APN deficiency increased eosinophil accumulation in response to an allergic inflammatory stimulus, we measured expression levels of the eosinophil-specific chemokines in alveolar macrophages isolated from the lungs of mice with eosinophilic inflammation-induced PH. In these experiments, the levels of CCL11 and CCL24 were higher in macrophages isolated from APN-deficient mice than in macrophages from wild-type mice. Finally, we demonstrate that the extracts of eosinophil granules promoted the proliferation of pulmonary arterial smooth muscle cells in vitro. These data suggest that APN deficiency may exacerbate PH, in part, by increasing eosinophil recruitment into the lung and that eosinophils could play an important role in the pathogenesis of inflammation-induced PH. These results may have implications for the pathogenesis and treatment of PH caused by vascular inflammation.

Dehydroepiandrosterone inhibits the Src/STAT3 constitutive activation in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is an obstructive vasculopathy characterized by enhanced pulmonary artery smooth muscle cell (PASMC) proliferation and suppressed apoptosis. This phenotype is sustained by the activation of the Src/signal transducer and activator of transcription 3 (STAT3) axis, maintained by a positive feedback loop involving miR-204 and followed by an aberrant expression/activation of its downstream targets such as Pim1 and nuclear factor of activated T-cells (NFATc2). Dehydroepiandrosterone (DHEA) is a steroid hormone shown to reverse vascular remodeling in systemic vessels. Since STAT3 has been described as modulated by DHEA, we hypothesized that DHEA reverses human pulmonary hypertension by inhibiting Src/STAT3 constitutive activation. Using PASMCs isolated from patients with PAH (n = 3), we demonstrated that DHEA decreases both Src and STAT3 activation (Western blot and nuclear translocation assay), resulting in a significant reduction of Pim1, NFATc2 expression/activation (quantitative RT-PCR and Western blot), as well as Survivin and upregulation of bone morphogenetic protein receptor 2 (BMPR2) and miR-204. Src/STAT3 axis inhibition by DHEA is associated with 1) mitochondrial membrane potential (tetramethylrhodamine methyl-ester perchlorate; n = 150; P < 0.05) depolarization increasing apoptosis by 25% (terminal deoxynucleotidyl transferase dUTP-mediated nick-end labeling; n = 150; P < 0.05); and 2) decreased intracellular Ca(2+) concentration (fluo-3 AM; n = 150; P < 0.05) and proliferation by 30% (PCNA). Finally, in vivo similarly to STAT3 inhibition DHEA improves experimental PAH (monocrotaline rats) by decreasing mean PA pressure and right ventricle hypertrophy. These effects were associated with the inhibition of Src, STAT3, Pim1, NFATc2, and Survivin and the upregulation of BMPR2 and miR-204. We demonstrated that DHEA reverses pulmonary hypertension in part by inhibiting the Src/STAT3.

Paigen diet-fed apolipoprotein E knockout mice develop severe pulmonary hypertension in an interleukin-1-dependent manner

Inflammatory mechanisms are proposed to play a significant role in the pathogenesis of pulmonary arterial hypertension (PAH). Previous studies have described PAH in fat-fed apolipoprotein E knockout (ApoE(-/-)) mice. We have reported that signaling in interleukin-1-receptor-knockout (IL-1R1(-/-)) mice leads to a reduction in diet-induced systemic atherosclerosis. We subsequently hypothesized that double-null (ApoE(-/-)/IL-1R1(-/-)) mice would show a reduced PAH phenotype compared with that of ApoE(-/-) mice. Male IL-1R1(-/-), ApoE(-/-), and ApoE(-/-)/IL-1R1(-/-) mice were fed regular chow or a high-fat diet (Paigen diet) for 8 weeks before phenotyping for PAH. No abnormal phenotype was observed in the IL-1R1(-/-) mice. Fat-fed ApoE(-/-) mice developed significantly increased right ventricular systolic pressure and substantial pulmonary vascular remodeling. Surprisingly, ApoE(-/-)/IL-1R1(-/-) mice showed an even more severe PAH phenotype. Further molecular investigation revealed the expression of a putative, alternatively primed IL-1R1 transcript expressed within the lungs but not aorta of ApoE(-/-)/IL-1R1(-/-) mice. Treatment of ApoE(-/-) and ApoE(-/-)/IL-1R1(-/-) mice with IL-1-receptor antagonist prevented progression of the PAH phenotype in both strains. Blocking IL-1 signaling may have beneficial effects in treating PAH, and alternative IL-1-receptor signaling in the lung may be important in driving PAH pathogenesis.

Design and validation of an endothelial progenitor cell capture chip and its application in patients with pulmonary arterial hypertension

The number of circulating endothelial progenitor cells (EPCs) inversely correlates with cardiovascular risk and clinical outcome, and thus has been proposed as a valuable biomarker for risk assessment, disease progression, and response to therapy. However, current strategies for isolation of these rare cells are limited to complex, laborious approaches. The goal of this study was the design and validation of a disposable microfluidic platform capable of selectively capturing and enumerating EPCs directly from human whole blood in healthy and diseased subjects, eliminating sample preprocessing. We then applied the "EPC capture chip" clinically and determined EPC numbers in blood from patients with pulmonary arterial hypertension (PAH). Blood was collected in tubes and injected into polymeric microfluidic chips containing microcolumns pre-coated with anti-CD34 antibody. Captured cells were immunofluorescently stained for the expression of stem and endothelial antigens, identified and counted. The EPC capture chip was validated with conventional flow cytometry counts (r = 0.83). The inter- and intra-day reliability of the microfluidic devices was confirmed at different time points in triplicates over 1-5 months. In a cohort of 43 patients with three forms of PAH (idiopathic/heritable, drug-induced, and connective tissue disease), EPC numbers are ≈50% lower in PAH subjects vs. matched controls and inversely related to two potential disease modifiers: body mass index and postmenopausal status. The EPC capture chip (5 × 30 × 0.05 mm(3)) requires only 200 μL of human blood and has the strong potential to serve as a rapid bedside test for the screening and monitoring of patients with PAH and other proliferative cardiovascular, pulmonary, malignant, and neurodegenerative diseases.

Estrogen rescues preexisting severe pulmonary hypertension in rats

RATIONALE

Pulmonary hypertension (PH) is characterized by progressive increase in pulmonary artery pressure leading to right ventricular (RV) hypertrophy, RV failure, and death. Current treatments only temporarily reduce severity of the disease, and an ideal therapy is still lacking.

OBJECTIVES

Estrogen pretreatment has been shown to attenuate development of PH. Because PH is not often diagnosed early, we examined if estrogen can rescue preexisting advanced PH.

METHODS

PH was induced in male rats with monocrotaline (60 mg/kg). At Day 21, rats were either treated with 17-β estradiol or estrogen (E2, 42.5 μg/kg/d), estrogen receptor-β agonist (diarylpropionitrile, 850 μg/kg/d), or estrogen receptor α-agonist (4,4',4"-[4-Propyl-(1H)-pyrazole-1,3,5-triyl] trisphenol, 850 μg/kg/d) for 10 days or left untreated to develop RV failure. Serial echocardiography, cardiac catheterization, immunohistochemistry, Western blot, and real-time polymerase chain reaction were performed.

MEASUREMENTS AND MAIN RESULTS

Estrogen therapy prevented progression of PH to RV failure and restored lung and RV structure and function. This restoration was maintained even after removal of estrogen at Day 30, resulting in 100% survival at Day 42. Estradiol treatment restored the loss of blood vessels in the lungs and RV. In the presence of angiogenesis inhibitor TNP-470 (30 mg/kg) or estrogen receptor-β antagonist (PHTPP, 850 μg/kg/d), estrogen failed to rescue PH. Estrogen receptor-β selective agonist was as effective as estrogen in rescuing PH.

CONCLUSIONS

Estrogen rescues preexisting severe PH in rats by restoring lung and RV structure and function that are maintained even after removal of estrogen. Estrogen-induced rescue of PH is associated with stimulation of cardiopulmonary neoangiogenesis, suppression of inflammation, fibrosis, and RV hypertrophy. Furthermore, estrogen rescue is likely mediated through estrogen receptor-β.

Mechanisms of disease: pulmonary arterial hypertension

In the past decade or so, our understanding of pulmonary arterial hypertension has undergone a paradigm shift. In this article, Dr Schermuly and colleagues discuss the known molecular mechanisms of the pathogenesis of this disease, and highlight the molecular technologies that are currently being used to further our understanding of these disease processes.

Our understanding of, and approach to, pulmonary arterial hypertension has undergone a paradigm shift in the past decade. Once a condition thought to be dominated by increased vasoconstrictor tone and thrombosis, pulmonary arterial hypertension is now seen as a vasculopathy in which structural changes driven by excessive vascular cell growth and inflammation, with recruitment and infiltration of circulating cells, play a major role. Perturbations of a number of molecular mechanisms have been described, including pathways involving growth factors, cytokines, metabolic signaling, elastases, and proteases, that may underlie the pathogenesis of the disease. Elucidating their contribution to the pathophysiology of pulmonary arterial hypertension could offer new drug targets. The role of progenitor cells in vascular repair is also under active investigation. The right ventricular response to increased pressure load is recognized as critical to survival and the molecular mechanisms involved are attracting increasing interest. The challenge now is to integrate this new knowledge and explore how it can be used to categorize patients by molecular phenotype and tailor treatment more effectively.

Pulmonary hypertension is a progressive disease of various origins, which has a poor prognosis and affects, in its different forms, more than 100 million people worldwide

Pulmonary arterial hypertension (PAH) is now considered to be a vasculopathy in which structural changes driven by excessive vascular cell growth and inflammation have a major role

A number of proproliferative signaling pathways involving growth factors, cytokines, metabolic signaling, and elastases and proteases have been identified in the pathophysiology of PAH

Clinical studies with tyrosine kinase inhibitors, serotonin antagonists, and soluble guanylate cyclase stimulators are underway in patients with PAH

The benefits of progenitor cells for vascular repair in PAH are under active investigation

The right ventricular response to increased pressure load is recognized as critical to survival in patients with PAH, and strategies for preserving myocardial function are increasingly attracting interest

Cytokine abnormalities in the etiology of the cardiometabolic syndrome

The cardiometabolic syndrome comprises a cluster of risk factors, including abdominal obesity, dyslipidemia, hypertension, insulin resistance/glucose intolerance, and proteinuria. This syndrome is due, in part, to the accumulation of visceral fat, which promotes synthesis of proinflammatory adipokines resulting in a visceral adipose tissue-specific increase in reactive oxygen species derived from NADPH oxidase. Adipose tissue oxidative stress results in the development of systemic oxidative stress and inflammation, which further lead to development of metabolic dyslipidemia, impaired glucose metabolism, renal disease, and hypertension. Importantly, visceral-not subcutaneous-fat is the significant source of the circulating adipokines that promote these systemic abnormalities. Chronic low-grade inflammation develops within adipose tissue because of the additional infiltration and accumulation of inflammatory macrophages. There is evidence that lifestyle changes, bariatric surgery, and/or administration of insulin-sensitizing, anti-inflammatory, or antihypertensive drugs that address the risk factors promoting the cardiometabolic syndrome act, in part, by promoting an anti-inflammatory adipokine profile in visceral fat.

Pediatric pulmonary arterial hypertension: current and emerging therapeutic options

INTRODUCTION

Pulmonary arterial hypertension (PAH) is a rare disease in neonates, infants and children that is associated with significant morbidity and mortality. An adequate understanding of the controlling pathophysiologic mechanisms is lacking and although mortality has decreased as therapeutic options have increased over the past several decades, outcomes remain unacceptable.

AREAS COVERED

This review summarizes the currently available therapies for neonates, infants and children with PAH and describes emerging therapies in the context of what is known about the underlying pathophysiology of the disease.

EXPERT OPINION

All of the currently approved PAH therapies impact one of three endothelial-based pathways: nitric oxide-guanosine-3'-5'cyclic monophosphate, prostacyclin or endothelin-1. The beneficial effects of these agents may relate to their impact on pulmonary vascular tone, and/or their antiproliferative and antithrombotic properties. Fundamental advances in PAH therapy are likely to relate to: i) a better understanding of PAH subpopulations, allowing for therapies to be better tailored to individual patients and pathophysiologic processes; and ii) therapies that promote the regression of advanced structural remodeling.

New perspectives for the treatment of pulmonary hypertension

Pulmonary hypertension (PH) is a debilitating disease with a poor prognosis. Therapeutic options remain limited despite the introduction of prostacyclin analogues, endothelin receptor antagonists and phosphodiesterase 5 inhibitors within the last 15 years; these interventions address predominantly the endothelial and vascular dysfunctionS associated with the condition, but simply delay progression of the disease rather than offer a cure. In an attempt to improve efficacy, emerging approaches have focused on targeting the pro-proliferative phenotype that underpins the pulmonary vascular remodelling in the lung and contributes to the impaired circulation and right heart failure. Many novel targets have been investigated and validated in animal models of PH, including modulation of guanylate cyclases, phosphodiesterases, tyrosine kinases, Rho kinase, bone morphogenetic proteins signalling, 5-HT, peroxisome proliferator activator receptors and ion channels. In addition, there is hope that combinations of such treatments, harnessing and optimizing vasodilator and anti-proliferative properties, will provide a further, possibly synergistic, increase in efficacy; therapies directed at the right heart may also offer an additional benefit. This overview highlights current therapeutic options, promising new therapies, and provides the rationale for a combination approach to treat the disease.

Type 1 diabetes-induced hyper-responsiveness to 5-hydroxytryptamine in rat pulmonary arteries via oxidative stress and induction of cyclooxygenase-2

Recent epidemiological data suggest that diabetes is a risk factor for pulmonary arterial hypertension. The aim of the present study was to analyze the link between type 1 diabetes and pulmonary arterial dysfunction in rats. Male Sprague-Dawley rats were randomly divided into a control group (saline) and a diabetic group (70 mg/kg streptozotocin). After 6 weeks, diabetic animals showed a down-regulation of the lung bone morphogenetic protein receptor type 2, up-regulation of 5-hydroxytryptamine (5-HT) 2A receptors and cyclooxygenase-2 (COX-2) proteins as measured by Western blot analysis, and increased contractile responses to 5-HT in isolated intrapulmonary arteries. The hyper-responsiveness to 5-HT was endothelium-independent and unaffected by inhibition of nitric-oxide synthase but prevented by indomethacin, the selective COX-2 inhibitor N-[2-(cyclohexyloxyl)-4-nitrophenyl]-methane sulfonamide (NS-398), superoxide dismutase, and the NADPH oxidase inhibitor apocynin or chronic treatment with insulin. However, diabetic rats at 6 weeks did not develop elevated right ventricular pressure or pulmonary artery muscularization, whereas a longer exposure (4 months) to diabetes induced a modest, but significant, increase in right ventricular systolic pressure. In conclusion, type 1 diabetes mellitus in rats induces a number of changes in lung protein expression and pulmonary vascular reactivity characteristic of clinical and experimental pulmonary arterial hypertension but insufficient to elevate pulmonary pressure. Our results further strengthen the link between diabetes and pulmonary arterial hypertension.

Pulmonary arterial dysfunction in insulin resistant obese Zucker rats

BACKGROUND

Insulin resistance and obesity are strongly associated with systemic cardiovascular diseases. Recent reports have also suggested a link between insulin resistance with pulmonary arterial hypertension. The aim of this study was to analyze pulmonary vascular function in the insulin resistant obese Zucker rat.

METHODS

Large and small pulmonary arteries from obese Zucker rat and their lean counterparts were mounted for isometric tension recording. mRNA and protein expression was measured by RT-PCR or Western blot, respectively. KV currents were recorded in isolated pulmonary artery smooth muscle cells using the patch clamp technique.

RESULTS

Right ventricular wall thickness was similar in obese and lean Zucker rats. Lung BMPR2, KV1.5 and 5-HT2A receptor mRNA and protein expression and KV current density were also similar in the two rat strains. In conductance and resistance pulmonary arteries, the similar relaxant responses to acetylcholine and nitroprusside and unchanged lung eNOS expression revealed a preserved endothelial function. However, in resistance (but not in conductance) pulmonary arteries from obese rats a reduced response to several vasoconstrictor agents (hypoxia, phenylephrine and 5-HT) was observed. The hyporesponsiveness to vasoconstrictors was reversed by L-NAME and prevented by the iNOS inhibitor 1400W.

CONCLUSIONS

In contrast to rat models of type 1 diabetes or other mice models of insulin resistance, the obese Zucker rats did not show any of the characteristic features of pulmonary hypertension but rather a reduced vasoconstrictor response which could be prevented by inhibition of iNOS.

Unrecognized glucose intolerance is common in pulmonary arterial hypertension

BACKGROUND

Animal and human data suggest insulin resistance is common in pulmonary arterial hypertension (PAH). Although routine assessment of insulin resistance is difficult, hemoglobin A(1c) (HbA(1c)) is a sensitive test to detect diabetes mellitus (DM) and those at high risk for DM. We aimed to define the prevalence of elevated HbA(1c) in PAH patients and to correlate HbA(1c) levels with functional assessment.

METHODS

HbA(1c) was measured in 41 PAH patients without a diagnosis of DM, along with demographic, functional, and hemodynamic data. Using published criteria, HbA(1c) ≤ 5.9% defined normal, 6.0% to 6.4% was glucose intolerance, and ≥ 6.5% was DM.

RESULTS

Twenty-three patients (56%) had HbA(1c) ≥ 6.0%, and 6 (15%) had unrecognized DM (HbA(1c) ≥ 6.5%). Age and body mass index were similar in patients with HbA(1c) ≥ 6.0% vs < 6.0%. There was a trend towards lower mean 6-minute walk distance in patients with elevated HbA(1c) (331.0 ± 126.6 vs 413.6 ± 74.9 meters, p = 0.07). The 6-month event-free survival was not significantly different in patients with elevated HbA(1c).

CONCLUSIONS

Unrecognized glucose intolerance as assessed by HbA(1c) is common in PAH. Further studies are needed to discern if glucose or insulin dysregulation mediates PAH pathogenesis or is secondary to advanced PAH.

BMP promotes motility and represses growth of smooth muscle cells by activation of tandem Wnt pathways

We present a novel cell-signaling paradigm in which bone morphogenetic protein 2 (BMP-2) consecutively and interdependently activates the wingless (Wnt)-β-catenin (βC) and Wnt-planar cell polarity (PCP) signaling pathways to facilitate vascular smooth muscle motility while simultaneously suppressing growth. We show that BMP-2, in a phospho-Akt-dependent manner, induces βC transcriptional activity to produce fibronectin, which then activates integrin-linked kinase 1 (ILK-1) via α4-integrins. ILK-1 then induces the Wnt-PCP pathway by binding a proline-rich motif in disheveled (Dvl) and consequently activating RhoA-Rac1-mediated motility. Transfection of a Dvl mutant that binds βC without activating RhoA-Rac1 not only prevents BMP-2-mediated vascular smooth muscle cell motility but promotes proliferation in association with persistent βC activity. Interfering with the Dvl-dependent Wnt-PCP activation in a murine stented aortic graft injury model promotes extensive neointima formation, as shown by optical coherence tomography and histopathology. We speculate that, in response to injury, factors that subvert BMP-2-mediated tandem activation of Wnt-βC and Wnt-PCP pathways contribute to obliterative vascular disease in both the systemic and pulmonary circulations.

Serotonin transporter, sex, and hypoxia: microarray analysis in the pulmonary arteries of mice identifies genes with relevance to human PAH

Pulmonary arterial hypertension (PAH) is up to threefold more prevalent in women than men. Female mice overexpressing the serotonin transporter (SERT; SERT+ mice) exhibit PAH and exaggerated hypoxia-induced PAH, whereas male SERT+ mice remain unaffected. To further investigate these sex differences, microarray analysis was performed in the pulmonary arteries of normoxic and chronically hypoxic female and male SERT+ mice. Quantitative RT-PCR analysis was employed for validation of the microarray data. In relevant groups, immunoblotting was performed for genes of interest (CEBPβ, CYP1B1, and FOS). To translate clinical relevance to our findings, CEBPβ, CYP1B1, and FOS mRNA and protein expression was assessed in pulmonary artery smooth muscle cells (PASMCs) derived from idiopathic PAH (IPAH) patients and controls. In female SERT+ mice, multiple pathways with relevance to PAH were altered. This was also observed in chronically hypoxic female SERT+ mice. We selected 10 genes of interest for qRT-PCR analysis (FOS, CEBPβ, CYP1B1, MYL3, HAMP2, LTF, PLN, NPPA, UCP1, and C1S), and 100% concordance was reported. Protein expression of three selected genes, CEBPβ, CYP1B1, FOS, was also upregulated in female SERT+ mice. Serotonin and 17β-estradiol increased CEBPβ, CYP1B1, and FOS protein expression in PASMCs. In addition, CEBPβ, CYP1B1, and FOS mRNA and protein expression was also increased in PASMCs derived from IPAH patients. Here, we have identified a number of genes that may predispose female SERT+ mice to PAH, and these findings may also be relevant to human PAH.

Resistant hypertension in the high-risk metabolic patient

The metabolic syndrome is a constellation of metabolic and vascular abnormalities that include insulin resistance with compensatory hyperinsulinemia, central or visceral obesity, hypertension, dyslipidemia, microalbuminuria, and oxidative stress as well as prothrombotic and inflammatory abnormalities that contribute to a hypercoagulable state and systemic endothelial dysfunction. Visceral adipose tissue is now known to secrete into the circulation a number of protein and nonprotein factors that regulate glucose metabolism in traditional insulin-sensitive tissue as well as nontraditional insulin-sensitive tissue including cardiovascular tissue. Collectively, this constellation of factors that lead to metabolic dysregulation contributes to a substantial risk for adverse cardiovascular and renal outcomes. The development of a particularly resistant form of hypertension in these individuals can be attributed to a number of factors including vasoconstriction from increased sympathetic activation, proinflammatory cytokines, and inappropriate activation of the renin-angiotensin-aldosterone system. The management of hypertension in such patients can be challenging and generally requires nonpharmacologic as well as pharmacologic interventions.

Adiponectin provides cardiovascular protection in metabolic syndrome

Adipose tissue plays a central role in the pathogenesis of metabolic syndrome. Adiponectin (APN) is a bioactive adipocytokine secreted from adipocytes. Low plasma APN levels (hypoadiponectinemia) are observed among obese individuals and in those with related disorders such as diabetes, hypertension, and dyslipidemia. APN ameliorates such disorders. Hypoadiponectinemia is also associated with major cardiovascular diseases including atherosclerosis and cardiac hypertrophy. Accumulating evidence indicates that APN directly interacts with cardiovascular tissue and prevents cardiovascular pathology. Increasing plasma APN or enhancing APN signal transduction may be an ideal strategy to prevent and treat the cardiovascular diseases associated with metabolic syndrome. However, further studies are required to uncover the precise biological actions of APN.

Pulmonary vascular disease related to hemodynamic stress in the pulmonary circulation

Hemodynamic stress in the pulmonary vessel is directly linked to the development of vascular remodeling and dysfunction, ultimately leading to pulmonary hypertension. Recently, some advances have been made in our molecular understanding of the exogenous upstream stimuli that initiate hemodynamic pertubations as well as the downstream vasoactive effectors that control these responses. However, much still remains unknown regarding how these complex signaling pathways connect in order to result in these characteristic pathophysiological changes. This chapter will describe our current understanding of and needed areas of research into the clinical, physiological, and molecular changes associated with pressure/volume overload in the pulmonary circulation.

The serotonin transporter, gender, and 17β oestradiol in the development of pulmonary arterial hypertension

AIMS

Idiopathic and familial forms of pulmonary arterial hypertension (PAH) predominantly affect females through an unknown mechanism. Activity of the serotonin transporter (SERT) may modulate the development of PAH, and mice overexpressing SERT (SERT+ mice) develop PAH and severe hypoxia-induced PAH. In the central nervous system, oestrogens influence activity of the serotonin system. Therefore, we examined the influence of gender on the development of PAH in SERT+ mice and how this is modulated by female hormones.

METHODS AND RESULTS

PAH was assessed via measurement of right ventricular systolic pressure (RVSP), pulmonary vascular remodelling (PVR), and right ventricular hypertrophy. Male SERT+ mice did not develop PAH. Female SERT+ mice demonstrated increased RVSP and PVR and this was abolished by ovariectomy. Following exposure to hypoxia, SERT+ mice exhibited severe PAH and this was also attenuated by ovariectomy. Chronic administration of 17β oestradiol re-established the PAH phenotype in ovariectomized, normoxic, and hypoxic SERT+ mice. 17β oestradiol also up-regulated tryptophan hydroxylase-1 (TPH1), 5-hydroytryptamine(1B) (5-HT(1B)) receptor, and SERT expression in human pulmonary arterial smooth muscle cells (hPASMCs). 17β oestradiol stimulated hPASMC proliferation and this was inhibited by both the TPH inhibitor para-chlorophenylalanine and the 5-HT(1B) receptor antagonist SB224289.

CONCLUSION

17β oestradiol is critical to the development of PAH and severe hypoxia-induced PAH in female SERT+ mice. In hPASMCs, 17β oestradiol-induced proliferation is dependant on de novo serotonin synthesis and stimulation of the 5-HT(1B) receptor. These interactions between the serotonin system and 17β oestradiol may contribute to the increased risk of PAH associated with female gender.

Adiponectin decreases pulmonary arterial remodeling in murine models of pulmonary hypertension

Remodeling of the pulmonary arteries is a common feature among the heterogeneous disorders that cause pulmonary hypertension. In these disorders, the remodeled pulmonary arteries often demonstrate inflammation and an accumulation of pulmonary artery smooth muscle cells (PASMCs) within the vessels. Adipose tissue secretes multiple bioactive mediators (adipokines) that can influence both inflammation and remodeling, suggesting that adipokines may contribute to the development of pulmonary hypertension. We recently reported on a model of pulmonary hypertension induced by vascular inflammation, in which a deficiency of the adipokine adiponectin (APN) was associated with the extensive proliferation of PASMCs and increased pulmonary artery pressures. Based on these data, we hypothesize that APN can suppress pulmonary hypertension by directly inhibiting the proliferation of PASMCs. Here, we tested the effects of APN overexpression on pulmonary arterial remodeling by using APN-overexpressing mice in a model of pulmonary hypertension induced by inflammation. Consistent with our hypothesis, mice that overexpressed APN manfiested reduced pulmonary hypertension and remodeling compared with wild-type mice, despite developing similar levels of pulmonary vascular inflammation in the model. The overexpression of APN was also protective in a hypoxic model of pulmonary hypertension. Furthermore, APN suppressed the proliferation of PASMCs, and reduced the activity of the serum response factor-serum response element pathway, which is a critical signaling pathway for smooth muscle cell proliferation. Overall, these data suggest that APN can regulate pulmonary hypertension and pulmonary arterial remodeling through its direct effects on PASMCs. Hence, the activation of APN-like activity in the pulmonary vasculature may be beneficial in pulmonary hypertension.

New therapies for pulmonary arterial hypertension: an update on current bench to bedside translation

IMPORTANCE OF THE FIELD

Treatments of pulmonary arterial hypertension (PAH) that have so far proven efficacious are all based on the restoration of endothelium control of pulmonary vascular tone and structure, by administration of prostacyclins, endothelin receptor antagonists and phosphodiesterase-5 inhibitors. However, results remain unsatisfactory, with persistent high mortality, insufficient clinical improvement and no convincing report of any reversal of the disease process.

AREAS COVERED IN THIS REVIEW

New antiproliferative approaches that aim to actively limit pulmonary vascular remodeling are being sought. Several such treatments have shown promise in experimental models and in preliminary clinical studies. Noteworthy among these are dichloroacetate, survivin antagonists, nuclear factor of activated T-cell inhibitors, PPAR-gamma agonists, tyrosine kinase inhibitors, Rho-kinase inhibitors, statins, vasoactive intestinal peptide, soluble guanylate cyclase stimulators/activators, adrenomedullin, elastase inhibitors, serotonin reuptake inhibitors, anti-inflammatory agents, and bone marrow-derived progenitor cells.

WHAT THE READER WILL GAIN

Update on various strategies targeting proliferative, inflammatory and regenerating processes currently under evaluation in patients with PAH.

TAKE HOME MESSAGE

In spite of favorable results in experimental models, none of these strategies has achieved the ultimate goal of curing PAH. Further developments will depend on progress made in our pathobiological understanding of the disease and carefully designed randomized, controlled trials.