Dysregulated bone morphogenetic protein signaling in monocrotaline-induced pulmonary arterial hypertension

The flavonoid quercetin reverses pulmonary hypertension in rats

Quercetin is a dietary flavonoid which exerts vasodilator, antiplatelet and antiproliferative effects and reduces blood pressure, oxidative status and end-organ damage in humans and animal models of systemic hypertension. We hypothesized that oral quercetin treatment might be protective in a rat model of pulmonary arterial hypertension. Three weeks after injection of monocrotaline, quercetin (10 mg/kg/d per os) or vehicle was administered for 10 days to adult Wistar rats. Quercetin significantly reduced mortality. In surviving animals, quercetin decreased pulmonary arterial pressure, right ventricular hypertrophy and muscularization of small pulmonary arteries. Classic biomarkers of pulmonary arterial hypertension such as the downregulated expression of lung BMPR2, Kv1.5, Kv2.1, upregulated survivin, endothelial dysfunction and hyperresponsiveness to 5-HT were unaffected by quercetin. Quercetin significantly restored the decrease in Kv currents, the upregulation of 5-HT2A receptors and reduced the Akt and S6 phosphorylation. In vitro, quercetin induced pulmonary artery vasodilator effects, inhibited pulmonary artery smooth muscle cell proliferation and induced apoptosis. In conclusion, quercetin is partially protective in this rat model of PAH. It delayed mortality by lowering PAP, RVH and vascular remodeling. Quercetin exerted effective vasodilator effects in isolated PA, inhibited cell proliferation and induced apoptosis in PASMCs. These effects were associated with decreased 5-HT2A receptor expression and Akt and S6 phosphorylation and partially restored Kv currents. Therefore, quercetin could be useful in the treatment of PAH.

Id proteins in the vasculature: from molecular biology to cardiopulmonary medicine

The inhibitors of differentiation (Id) proteins belong to the helix-loop-helix group of transcription factors and regulate cell differentiation and proliferation. Recent studies have reported that Id proteins play important roles in cardiogenesis and formation of the vasculature. We have also demonstrated that heritable pulmonary arterial hypertension (HPAH) patients have dysregulated Id gene expression in pulmonary artery smooth muscle cells. The interaction between bone morphogenetic proteins and other growth factors or cytokines regulates Id gene expression, which impacts on pulmonary vascular cell differentiation and proliferation. Exploration of the roles of Id proteins in vascular remodelling that occurs in PAH and atherosclerosis might provide new insights into the molecular basis of these diseases. In addition, current progress in identification of the interactors of Id proteins will further the understanding of the function of Ids in vascular cells and enable the identification of novel targets for therapy in PAH and other cardiovascular diseases.

Loss of bone morphogenetic protein receptor 2 is associated with abnormal DNA repair in pulmonary arterial hypertension

Occlusive vasculopathy with intimal hyperplasia and plexogenic arteriopathy are severe histopathological changes characteristic of pulmonary arterial hypertension (PAH). Although a phenotypic switch in pulmonary endothelial cells (ECs) has been suggested to play a critical role in the formation of occlusive lesions, the pathobiology of this process is poorly understood. The goal of this study was to identify novel molecular mechanisms associated with EC dysfunction and PAH-associated bone morphogenetic protein receptor 2 (BMPR2) deficiency during PAH pathogenesis. A bioinfomatics approach, patient samples, and in vitro experiments were used. By combining a metaanalysis of human idiopathic PAH (iPAH)-associated gene-expression microarrays and a unique gene expression-profiling technique in rat endothelium, our bioinformatics approach revealed a PAH-associated dysregulation of genes involving chromatin organization, DNA metabolism, and repair. Our hypothesis that altered DNA repair and loss of genomic stability play a role in PAH was supported by in vitro assays where pulmonary ECs from patients with iPAH and BMPR2-deficient ECs were highly susceptible to DNA damage. Furthermore, we showed that BMPR2 expression is tightly linked to DNA damage control because excessive DNA damage leads to rapid down-regulation of BMPR2 expression. Moreover, we identified breast cancer 1 (BRCA1) as a novel target for BMPR2 signaling and a novel modulator of pulmonary EC homeostasis. We show here that BMPR2 signaling plays a critical role in the regulation of genomic integrity in pulmonary ECs via genes such as BRCA1. We propose that iPAH-associated EC dysfunction and genomic instability are mediated through BMPR2 deficiency-associated loss of DNA damage control.

BMP4 protects rat pulmonary arterial smooth muscle cells from apoptosis by PI3K/AKT/Smad1/5/8 signaling

Bone morphogenetic protein-4 (BMP4), a member of the transforming growth factor β (TGF-β) family of growth factors, is activated and increased under hypoxic conditions, which plays an important role in the progression of pulmonary arterial hypertension (PAH). Previous studies have shown that BMP4 is involved in the regulation of proliferation, differentiation, migration and apoptosis of various cell types. However, the precise mechanisms involved in the regulation of pulmonary artery smooth muscle cells (PASMCs) in PAH are still incompletely understood. It has been reported that AKT is a critical regulator of cell survival and vascular remodeling. Therefore, there may be crosstalk between BMP4 anti-apoptotic processes and PI3K/AKT survival effect in rat PASMCs. To test this hypothesis, we performed confocal, cell viability measurement, mitochondrial potential, real-time polymerase chain reaction (PCR), and Western blot analysis to determine the role of BMP4 on cell survival and apoptosis. We found that hypoxia up-regulated the expression of BMP4. BMP4 promoted cell survival, reduced mitochondrial depolarization, and increased the expression of Bcl-2 and procaspase-3 in PASMCs under serum-deprived condition. These effects were reversed by PI3K/AKT inhibitors (LY294002 and wortmannin). Thus, these findings indicate that BMP4 protects PASMCs from apoptosis at least in part, mediated via the PI3K/AKT pathway.

Treatment with anti-gremlin 1 antibody ameliorates chronic hypoxia/SU5416-induced pulmonary arterial hypertension in mice

The expression of the bone morphogenetic protein antagonist, Gremlin 1, was recently shown to be increased in the lungs of pulmonary arterial hypertension patients, and in response to hypoxia. Gremlin 1 released from the vascular endothelium may inhibit endogenous bone morphogenetic protein signaling and contribute to the development of pulmonary arterial hypertension. Here, we investigate the impact of Gremlin 1 inhibition in disease after exposure to chronic hypoxia/SU5416 in mice. We investigated the effects of an anti-Gremlin 1 monoclonal antibody in the chronic hypoxia/SU5416 murine model of pulmonary arterial hypertension. Chronic hypoxic/SU5416 exposure of mice induced upregulation of Gremlin 1 mRNA in lung and right ventricle tissue compared with normoxic controls. Prophylactic treatment with an anti-Gremlin 1 neutralizing mAb reduced the hypoxic/SU5416-dependent increase in pulmonary vascular remodeling and right ventricular hypertrophy. Importantly, therapeutic treatment with an anti-Gremlin 1 antibody also reduced pulmonary vascular remodeling and right ventricular hypertrophy indicating a role for Gremlin 1 in the progression of the disease. We conclude that Gremlin 1 plays a role in the development and progression of pulmonary arterial hypertension in the murine hypoxia/SU5416 model, and that Gremlin 1 is a potential therapeutic target for pulmonary arterial hypertension.

Nur77 suppresses pulmonary artery smooth muscle cell proliferation through inhibition of the STAT3/Pim-1/NFAT pathway

The orphan nuclear receptor 4A (NR4A) family plays critical roles in the regulation of cell proliferation, differentiation, and survival in the cardiovascular system. However, the molecular mechanisms underlying the regulation of NR4A receptor expression and its role in pulmonary artery smooth muscle cell (PASMC) function remain unclear. Here, we investigated whether the NR4A family regulates PASMC proliferation, and if so, which mechanisms are involved. By using quantitative real-time RT-PCR, we showed that the orphan nuclear receptor Nur77 was the most abundant member of NR4A family expressed in rat PASMCs, as compared with the two other members, NOR-1 and Nurr1. In rat PASMCs, expression of Nur77 was robustly induced in response to several pathologic stimuli of pulmonary arterial hypertension (PAH), such as hypoxia, 5-hydroxytryptamine (5-HT), platelet-derived growth factor, and endothelin-1. Importantly, Nur77 was also significantly increased in lungs of rats with monocrotaline-induced PAH. Furthermore, we demonstrated that 5-HT markedly up-regulated Nur77 expression through the mitogen-activated protein kinases/extracellular signal-regulated kinase 1/2 pathway. Overexpression of Nur77 inhibited 5-HT-induced PASMC proliferation, as well as the expression of cyclin D1 and proliferating cell nuclear antigen. Mechanistically, we demonstrated that Nur77 specifically interacts with signal transducer and activator of transcription 3, thus inhibiting its phosphorylation and expression of its target genes, such as Pim-1, nuclear factor of activated T cells c2, and survivin in PASMCs. These results indicate that Nur77 is a novel negative-feedback regulator of PASMC proliferation through inhibition of the signal transducer and activator of transcription 3/Pim-1/nuclear factor of activated T cells axis. Modulation of Nur77 activity may potentially represent a novel therapeutic strategy for the treatment of PAH.

Inflammatory cytokines in pulmonary hypertension

Pulmonary hypertension is an “umbrella term” used for a spectrum of entities resulting in an elevation of the pulmonary arterial pressure. Clinical symptoms include dyspnea and fatigue which in the absence of adequate therapeutic intervention may lead to progressive right heart failure and death. The pathogenesis of pulmonary hypertension is characterized by three major processes including vasoconstriction, vascular remodeling and microthrombotic events. In addition accumulating evidence point to a cytokine driven inflammatory process as a major contributor to the development of pulmonary hypertension.

This review summarizes the latest clinical and experimental developments in inflammation associated with pulmonary hypertension with special focus on Interleukin-6, and its role in vascular remodeling in pulmonary hypertension.

Systemic hydrogen sulfide administration partially restores normal alveolarization in an experimental animal model of bronchopulmonary dysplasia

Arrested alveolarization is the pathological hallmark of bronchopulmonary dysplasia (BPD), a complication of premature birth. Here, the impact of systemic application of hydrogen sulfide (H2S) on postnatal alveolarization was assessed in a mouse BPD model. Exposure of newborn mice to 85% O2 for 10 days reduced the total lung alveoli number by 56% and increased alveolar septal wall thickness by 29%, as assessed by state-of-the-art stereological analysis. Systemic application of H2S via the slow-release H2S donor GYY4137 for 10 days resulted in pronounced improvement in lung alveolarization in pups breathing 85% O2, compared with vehicle-treated littermates. Although without impact on lung oxidative status, systemic H2S blunted leukocyte infiltration into alveolar air spaces provoked by hyperoxia, and restored normal lung interleukin 10 levels that were otherwise depressed by 85% O2. Treatment of primary mouse alveolar type II (ATII) cells with the rapid-release H2S donor NaHS had no impact on cell viability; however, NaHS promoted ATII cell migration. Although exposure of ATII cells to 85% O2 caused dramatic changes in mRNA expression, exposure to either GYY4137 or NaHS had no impact on ATII cell mRNA expression, as assessed by microarray, suggesting that the effects observed were independent of changes in gene expression. The impact of NaHS on ATII cell migration was attenuated by glibenclamide, implicating ion channels, and was accompanied by activation of Akt, hinting at two possible mechanisms of H2S action. These data support further investigation of H2S as a candidate interventional strategy to limit the arrested alveolarization associated with BPD.

Aberrant chloride intracellular channel 4 expression contributes to endothelial dysfunction in pulmonary arterial hypertension

BACKGROUND

Chloride intracellular channel 4 (CLIC4) is highly expressed in the endothelium of remodeled pulmonary vessels and plexiform lesions of patients with pulmonary arterial hypertension. CLIC4 regulates vasculogenesis through endothelial tube formation. Aberrant CLIC4 expression may contribute to the vascular pathology of pulmonary arterial hypertension.

METHODS AND RESULTS

CLIC4 protein expression was increased in plasma and blood-derived endothelial cells from patients with idiopathic pulmonary arterial hypertension and in the pulmonary vascular endothelium of 3 rat models of pulmonary hypertension. CLIC4 gene deletion markedly attenuated the development of chronic hypoxia-induced pulmonary hypertension in mice. Adenoviral overexpression of CLIC4 in cultured human pulmonary artery endothelial cells compromised pulmonary endothelial barrier function and enhanced their survival and angiogenic capacity, whereas CLIC4 shRNA had an inhibitory effect. Similarly, inhibition of CLIC4 expression in blood-derived endothelial cells from patients with idiopathic pulmonary arterial hypertension attenuated the abnormal angiogenic behavior that characterizes these cells. The mechanism of CLIC4 effects involves p65-mediated activation of nuclear factor-κB, followed by stabilization of hypoxia-inducible factor-1α and increased downstream production of vascular endothelial growth factor and endothelin-1.

CONCLUSION

Increased CLIC4 expression is an early manifestation and mediator of endothelial dysfunction in pulmonary hypertension.

Transforming growth factor-β(1) represses bone morphogenetic protein-mediated Smad signaling in pulmonary artery smooth muscle cells via Smad3

Previous studies of pulmonary arterial hypertension (PAH) have implicated excessive transforming growth factor (TGF)-β1 signaling and reduced bone morphogenetic protein (BMP) signaling in the disease pathogenesis. Reduced BMP signaling in pulmonary artery smooth muscle cells (PASMCs) from patients with heritable PAH is a consequence of germline mutations in the BMP type II receptor (BMPR-II). We sought to establish whether the TGF-β1 and BMP4 pathways interact in PASMCs, and if this is altered in cells with BMPR-II mutations. Control PASMCs or from patients with PAH harboring BMPR-II mutations were treated with BMP4, TGF-β1, or cotreated with both ligands. Signaling was assessed by examination of Smad phosphorylation, luciferase reporters, and the transcription of BMP4 or TGF-β1-responsive genes. TGF-β1 attenuated BMP4-mediated inhibitors of differentiation 1/2 induction and abolished the response in BMPR-II mutant PASMCs, whereas BMP4 did not alter TGF-β1-mediated transcription. Activin-like kinase 5 inhibition blocked this effect, whereas cycloheximide or pharmacological inhibitors of TGF-β-activated kinase 1, extracellular signal-regulated kinase 1/2, or p38 mitogen-activated protein kinase were ineffective. BMP4 and TGF-β1 cotreatment did not alter the activation or nuclear translocation of their respective Smad signaling proteins. Small interfering RNA for Smad3, but not Smad2, Smad6, or Smad7, reversed the inhibition by TGF-β1. In addition, TGF-β-activated kinase 1 inhibition blocked Smad3 phosphorylation, implying that C-terminal Smad3 phosphorylation is not required for the inhibition of BMP4 signaling by TGF-β1. TGF-β1 reduces BMP4 signaling in PASMCs, a response that is exacerbated on the background of reduced BMP responsiveness due to BMPR-II mutations. These data provide a rationale for therapeutic inhibition of TGF-β1 signaling in PAH.

Glucocorticoids recruit Tgfbr3 and Smad1 to shift transforming growth factor-β signaling from the Tgfbr1/Smad2/3 axis to the Acvrl1/Smad1 axis in lung fibroblasts

Glucocorticoids represent the mainstay therapy for many lung diseases, providing outstanding management of asthma but performing surprisingly poorly in patients with acute respiratory distress syndrome, chronic obstructive pulmonary disease, lung fibrosis, and blunted lung development associated with bronchopulmonary dysplasia in preterm infants. TGF-β is a pathogenic mediator of all four of these diseases, prompting us to explore glucocorticoid/TGF-β signaling cross-talk. Glucocorticoids, including dexamethasone, methylprednisolone, budesonide, and fluticasone, potentiated TGF-β signaling by the Acvrl1/Smad1/5/8 signaling axis and blunted signaling by the Tgfbr1/Smad2/3 axis in NIH/3T3 cells, as well as primary lung fibroblasts, smooth muscle cells, and endothelial cells. Dexamethasone drove expression of the accessory type III TGF-β receptor Tgfbr3, also called betaglycan. Tgfbr3 was demonstrated to be a "switch" that blunted Tgfbr1/Smad2/3 and potentiated Acvrl1/Smad1 signaling in lung fibroblasts. The Acvrl1/Smad1 axis, which was stimulated by dexamethasone, was active in lung fibroblasts and antagonized Tgfbr1/Smad2/3 signaling. Dexamethasone acted synergistically with TGF-β to drive differentiation of primary lung fibroblasts to myofibroblasts, revealed by acquisition of smooth muscle actin and smooth muscle myosin, which are exclusively Smad1-dependent processes in fibroblasts. Administration of dexamethasone to live mice recapitulated these observations and revealed a lung-specific impact of dexamethasone on lung Tgfbr3 expression and phospho-Smad1 levels in vivo. These data point to an interesting and hitherto unknown impact of glucocorticoids on TGF-β signaling in lung fibroblasts and other constituent cell types of the lung that may be relevant to lung physiology, as well as lung pathophysiology, in terms of drug/disease interactions.

MicroRNA expression profile of pulmonary artery smooth muscle cells and the effect of let-7d in chronic thromboembolic pulmonary hypertension

Abstract Chronic thromboembolic pulmonary hypertension (CTEPH) is a life-threatening condition characterized by single or recurrent pulmonary thromboemboli, which promote pulmonary vascular remodeling. MicroRNA (miRNA), is a small, noncoding RNA that is involved in multiple cell processes and functions and may participate in the pathogenesis of CTEPH. Our aims were to identify the miRNA expression signature in pulmonary artery smooth muscle cells (PASMCs) of CTEPH patients and to study the role of let-7d in CTEPH pathogenesis. The miRNA expression profile was analyzed by microarray in PASMCs of CTEPH and control patients. Differentially expressed miRNAs were selectively validated by stem-loop quantitative real-time reverse-transcription polymerase chain reaction (qRT-PCR). The role of let-7d was identified by in silico analysis, and its effect on the proliferation of PASMCs was measured by methyl thiazolyl tetrazolium (MTT). Student's unpaired t test, the Fisher exact test, and the χ(2) test were used for statistical analysis. Eighteen miRNAs were differentially expressed in PASMCs from CTEPH patients, including 12 upregulated miRNAs and 6 downregulated miRNAs; among the latter, let-7d decreased 0.58-fold in CTEPH patients, as validated by qRT-PCR. It was found that let-7d could inhibit the proliferation of PASMCs through upregulation of p21. In conclusion, PASMCs in CTEPH patients have an aberrant miRNA profile and reduced let-7d, which could promote PASMC proliferation and may be involved in the pathogenesis of CTEPH.

Deregulation of the lysyl hydroxylase matrix cross-linking system in experimental and clinical bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is a common and serious complication of premature birth, characterized by a pronounced arrest of alveolar development. The underlying pathophysiological mechanisms are poorly understood although perturbations to the maturation and remodeling of the extracellular matrix (ECM) are emerging as candidate disease pathomechanisms. In this study, the expression and regulation of three members of the lysyl hydroxylase family of ECM remodeling enzymes (Plod1, Plod2, and Plod3) in clinical BPD, as well as in an experimental animal model of BPD, were addressed. All three enzymes were localized to the septal walls in developing mouse lungs, with Plod1 also expressed in the vessel walls of the developing lung and Plod3 expressed uniquely at the base of developing septa. The expression of plod1, plod2, and plod3 was upregulated in the lungs of mouse pups exposed to 85% O2, an experimental animal model of BPD. Transforming growth factor (TGF)-β increased plod2 mRNA levels and activated the plod2 promoter in vitro in lung epithelial cells and in lung fibroblasts. Using in vivo neutralization of TGF-β signaling in the experimental animal model of BPD, TGF-β was identified as the regulator of aberrant plod2 expression. PLOD2 mRNA expression was also elevated in human neonates who died with BPD or at risk for BPD, compared with neonates matched for gestational age at birth or chronological age at death. These data point to potential roles for lysyl hydroxylases in normal lung development, as well as in perturbed late lung development associated with BPD.

BMP4 increases canonical transient receptor potential protein expression by activating p38 MAPK and ERK1/2 signaling pathways in pulmonary arterial smooth muscle cells

Abnormal bone morphogenetic protein (BMP) signaling has been implicated in the pathogenesis of pulmonary hypertension. We previously found that BMP4 elevated basal intracellular Ca(2+) ([Ca(2+)]i) concentrations in distal pulmonary arterial smooth muscle cells (PASMCs), attributable in large part to enhanced store-operated Ca(2+) entry through store-operated Ca(2+) channels (SOCCs). Moreover, BMP4 up-regulated the expression of canonical transient receptor potential (TRPC) proteins thought to compose SOCCs. The present study investigated the signaling pathways through which BMP4 regulates TRPC expression and basal [Ca(2+)]i in distal PASMCs. Real-time quantitative PCR was used for the measurement of mRNA, Western blotting was used for the measurement of protein, and fluorescent microscopic for [Ca(2+)]i was used to determine the involvement of p38 and extracellular regulated kinase (ERK)-1/2 mitogen-activated protein kinase (MAPK) signaling in BMP4-induced TRPC expression and the elevation of [Ca(2+)]i in PASMCs. We found that the treatment of BMP4 led to the activation of both p38 MAPK and ERK1/2 in rat distal PASMCs. The induction of TRPC1, TRPC4, and TRPC6 expression, and the increases of [Ca(2+)]i caused by BMP4 in distal PASMCs, were inhibited by treatment with either SB203580 (10 μM), the selective inhibitor for p38 activation, or the specific p38 small interfering RNA (siRNA). Similarly, those responses induced by BMP4 were also abolished by treatment with PD98059 (5 μM), the selective inhibitor of ERK1/2, or by the knockdown of ERK1/2 using its specific siRNA. These results indicate that BMP4 participates in the regulation of Ca(2+) signaling in PASMCs by modulating TRPC channel expression via activating p38 and ERK1/2 MAPK pathways.

FK506 activates BMPR2, rescues endothelial dysfunction, and reverses pulmonary hypertension

Dysfunctional bone morphogenetic protein receptor-2 (BMPR2) signaling is implicated in the pathogenesis of pulmonary arterial hypertension (PAH). We used a transcriptional high-throughput luciferase reporter assay to screen 3,756 FDA-approved drugs and bioactive compounds for induction of BMPR2 signaling. The best response was achieved with FK506 (tacrolimus), via a dual mechanism of action as a calcineurin inhibitor that also binds FK-binding protein-12 (FKBP12), a repressor of BMP signaling. FK506 released FKBP12 from type I receptors activin receptor-like kinase 1 (ALK1), ALK2, and ALK3 and activated downstream SMAD1/5 and MAPK signaling and ID1 gene regulation in a manner superior to the calcineurin inhibitor cyclosporine and the FKBP12 ligand rapamycin. In pulmonary artery endothelial cells (ECs) from patients with idiopathic PAH, low-dose FK506 reversed dysfunctional BMPR2 signaling. In mice with conditional Bmpr2 deletion in ECs, low-dose FK506 prevented exaggerated chronic hypoxic PAH associated with induction of EC targets of BMP signaling, such as apelin. Low-dose FK506 also reversed severe PAH in rats with medial hypertrophy following monocrotaline and in rats with neointima formation following VEGF receptor blockade and chronic hypoxia. Our studies indicate that low-dose FK506 could be useful in the treatment of PAH.

Impact of dietary iron restriction on the development of monocrotaline-induced pulmonary vascular remodeling and right ventricular failure in rats

Pulmonary hypertension (PH) is characterized by pulmonary vascular remodeling leading to right ventricular (RV) failure. Recently, iron deficiency is reported to be prevalent in patients with PH. However, the mechanism by which iron deficiency occurs in patients with PH remains unknown. Here, we investigated the effects of dietary iron restriction on the development of monocrotaline-induced pulmonary vascular remodeling and the involved mechanisms. Male Sprague-Dawley rats were subcutaneously injected with monocrotaline (60mg/kg). Afterwards, monocrotaline-injected rats were randomly divided into two groups and were given a normal diet (n=6) or an iron-restricted diet (n=6) for 4weeks. Saline-injected rats given a normal diet were served as controls (n=6). Monocrotaline-injected rats showed pulmonary vascular remodeling, increased RV pressure, RV hypertrophy, and decreased RV ejection fraction, followed by RV failure after 4weeks. In contrast, iron restriction attenuated the development of pulmonary vascular remodeling and RV failure. Of interest, expression of cellular iron transport protein, transferrin receptor 1 was increased in the pulmonary remodeled artery and the failing right ventricle of monocrotaline-injected rats, as compared with the controls. Moreover, a key regulator of iron homeostasis, hepcidin gene expression was increased in the failing right ventricle of monocrotaline-injected rats. Iron restriction attenuated the development of monocrotaline-induced pulmonary vascular remodeling and RV failure. Cellular iron transport might be involved in the pathophysiology of PH and PH induced RV failure.

The transforming growth factor-β-bone morphogenetic protein type signalling pathway in pulmonary vascular homeostasis and disease

Germ-line mutations in the bone morphogenetic protein type II receptor (BMPR2; BMPR-II) gene, a transforming growth factor-β (TGFβ) receptor superfamily member, cause the majority of cases of heritable pulmonary arterial hypertension (PAH). Pulmonary arterial hypertension is a subset of pulmonary hypertension (PH) disorders, which also encompass hypoxia-related lung diseases. Bone morphogenetic proteins (BMPs), via BMPR-II, activate the canonical Smad1/5/9 pathway, whereas TGFβs (TGFβ1-3) activate the Smad2/3 pathway via the ALK5 receptor. Dysregulated TGFβ1 signalling is pathogenic in fibrotic diseases. We compared two rat PH models, monocrotaline-induced PAH (MCT-PAH) and chronic normobaric hypoxia (fractional inspired O2 10%), to address whether BMPR-II loss is common to PH and permits pathogenic TGFβ1 signalling. Both models exhibited reduced lung BMPR-II expression, but increased TGFβ1 signalling and decreased BMP signalling were observed only in MCT-PAH. Furthermore, a pharmacological ALK5 inhibitor prevented disease progression in the MCT-PAH model, but not in hypoxia. In vitro studies using human pulmonary artery smooth muscle cells showed that TGFβ1 directly inhibits BMP-Smad signalling. In conclusion, BMPR-II loss is common to the hypoxic and MCT-PAH models, but systemic ALK5 inhibition is effective only in the MCT model, highlighting a specific role for TGFβ1 in vascular remodelling in MCT-PAH, potentially via direct inhibition of BMP signalling.

A molecular mechanism for therapeutic effects of cGMP-elevating agents in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a progressive, usually fatal disease with abnormal vascular remodeling. Pulmonary artery smooth muscle cells (PASMCs) from PAH patients are hyperproliferative and apoptosis-resistant and demonstrate decreased signaling in response to bone morphogenetic proteins (BMPs). Cyclic GMP-elevating agents are beneficial in PAH, but their mechanism(s) of action are incompletely understood. Here we show that BMP signaling via Smad1/5/8 requires cGMP-dependent protein kinase isotype I (PKGI) to maintain PASMCs in a differentiated, low proliferative state. BMP cooperation with cGMP/PKGI was crucial for transcription of contractile genes and suppression of pro-proliferative and anti-apoptotic genes. Lungs from mice with low or absent PKGI (Prkg1(+/-) and Prkg1(-/-) mice) exhibited impaired BMP signaling, decreased contractile gene expression, and abnormal vascular remodeling. Conversely, cGMP stimulation of PKGI restored defective BMP signaling in rats with hypoxia-induced PAH, consistent with cGMP-elevating agents reversing vascular remodeling in this PAH model. Our results provide a mechanism for the therapeutic effects of cGMP-elevating agents in PAH and suggest that combining them with BMP mimetics may provide a novel, disease-modifying approach to PAH therapy.

The genetics of pulmonary arterial hypertension in the post-BMPR2 era

Pulmonary arterial hypertension (PAH) is a rapidly progressive and fatal disease for which there is an ever-expanding body of genetic and related pathophysiological information on disease pathogenesis. The most common single culprit gene known is BMPR2, and animal models of the disease in several forms exist. There is a wealth of genetic data regarding modifiers of disease expression, penetrance, and severity. Despite the rapid accumulation of data in the last decade, a complete picture of the molecular pathogenesis of PAH leading to novel therapies is lacking. In this review, we attempt to summarize the current understanding of PAH from the genetic perspective. The most recent PAH demographics are discussed. Heritable PAH in the post-BMPR2 era is examined in detail as the most robust model of PAH genetics in both animal models and human pedigrees. Important downstream molecular pathways and modifiers of disease expression are reviewed in light of what is known about PAH pathogenesis. Current and emerging therapies are examined in light of genetic data. The role of genetic testing in PAH in the post-BMPR2 era is discussed. Finally, directions for future investigations that ideally will fulfill the promise of novel therapeutic or preventive strategies are discussed.

AntagomiR directed against miR-20a restores functional BMPR2 signalling and prevents vascular remodelling in hypoxia-induced pulmonary hypertension

AIMS

Dysregulation of the bone morphogenetic protein receptor type 2 (BMPR2) is a hallmark feature that has been described in several forms of pulmonary hypertension. We recently identified the microRNA miR-20a within a highly conserved pathway as a regulator of the expression of BMPR2. To address the pathophysiological relevance of this pathway in vivo, we employed antagomiR-20a and investigated whether specific inhibition of miR-20a could restore functional levels of BMPR2 and, in turn, might prevent pulmonary arterial vascular remodelling.

METHODS AND RESULTS

For specific inhibition of miR-20a, cholesterol-modified RNA oligonucleotides (antagomiR-20a) were synthesized. The experiments in mice were performed by using the hypoxia-induced mouse model for pulmonary hypertension and animal tissues were analysed for right ventricular hypertrophy and pulmonary arterial vascular remodelling. Treatment with antagomiR-20a enhanced the expression levels of BMPR2 in lung tissues; moreover, antagomiR-20a significantly reduced wall thickness and luminal occlusion of small pulmonary arteries and reduced right ventricular hypertrophy. To assess BMPR2 signalling and proliferation, we performed in vitro experiments with human pulmonary arterial smooth muscle cells (HPASMCs). Transfection of HPASMCs with antagomiR-20a resulted in activation of downstream targets of BMPR2 showing increased activation of Id-1 and Id-2. Proliferation of HPASMCs was found to be reduced upon transfection with antagomiR-20a.

CONCLUSION

This is the first report showing that miR-20a can be specifically targeted in an in vivo model for pulmonary hypertension. Our data emphasize that treatment with antagomiR-20a restores functional levels of BMPR2 in pulmonary arteries and prevents the development of vascular remodelling.

Gremlin plays a key role in the pathogenesis of pulmonary hypertension

BACKGROUND

Pulmonary hypertension occurs in chronic hypoxic lung diseases, significantly worsening morbidity and mortality. The important role of altered bone morphogenetic protein (BMP) signaling in pulmonary hypertension was first suspected after the identification of heterozygous BMP receptor mutations as the underlying defect in the rare heritable form of pulmonary arterial hypertension. Subsequently, it was demonstrated that BMP signaling was also reduced in common forms of pulmonary hypertension, including hypoxic pulmonary hypertension; however, the mechanism of this reduction has not previously been elucidated.

METHODS AND RESULTS

Expression of 2 BMP antagonists, gremlin 1 and gremlin 2, was higher in the lung than in other organs, and gremlin 1 was further increased in the walls of small intrapulmonary vessels of mice during the development of hypoxic pulmonary hypertension. Hypoxia stimulated gremlin secretion from human pulmonary microvascular endothelial cells in vitro, which inhibited endothelial BMP signaling and BMP-stimulated endothelial repair. Haplodeficiency of gremlin 1 augmented BMP signaling in the hypoxic mouse lung and reduced pulmonary vascular resistance by attenuating vascular remodeling. Furthermore, gremlin was increased in the walls of small intrapulmonary vessels in idiopathic pulmonary arterial hypertension and the rare heritable form of pulmonary arterial hypertension in a distribution suggesting endothelial localization.

CONCLUSIONS

These findings demonstrate a central role for increased gremlin in hypoxia-induced pulmonary vascular remodeling and the increased pulmonary vascular resistance in hypoxic pulmonary hypertension. High levels of basal gremlin expression in the lung may account for the unique vulnerability of the pulmonary circulation to heterozygous mutations of BMP type 2 receptor in pulmonary arterial hypertension.

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.

Potential Interventions Against BMPR2-Related Pulmonary Hypertension

For more than 60 years, researchers have sought to understand the molecular basis of idiopathic pulmonary arterial hypertension (PAH). Recognition of the heritable form of the disease led to the creation of patient registries in the 1980s and 1990s, and discovery of BMPR2 as the cause of roughly 80% of heritable PAH in 2000. With discovery of the disease gene came opportunity for intervention, with focus on 2 alternative approaches. First, it may be possible to correct the effects of BMPR2 mutation directly through interventions targeted at correction of trafficking defects, increasing expression of the unmutated allele, and correction of splicing defects. Second, therapeutic interventions are being targeted at the signaling consequences of BMPR2 mutation. In particular, therapies targeting cytoskeletal and metabolic defects caused by BMPR2 mutation are currently in trials, or will be ready for human trials in the near future. Translation of these findings into therapies is the culmination of decades of research, and holds great promise for treatment of the underlying molecular bases of disease.

Inhibition of microRNA-17 improves lung and heart function in experimental pulmonary hypertension

RATIONALE

MicroRNAs (miRs) control various cellular processes in tissue homeostasis and disease by regulating gene expression on the posttranscriptional level. Recently, it was demonstrated that the expression of miR-21 and members of the miR-17-92 cluster was significantly altered in experimental pulmonary hypertension (PH).

OBJECTIVES

To evaluate the therapeutic efficacy and antiremodeling potential of miR inhibitors in the pathogenesis of PH.

METHODS

We first tested the effects of miR inhibitors (antagomirs), which were specifically designed to block miR-17 (A-17), miR-21 (A-21), and miR-92a (A-92a) in chronic hypoxia-induced PH in mice and A-17 in monocrotaline-induced PH in rats. Moreover, biological function of miR-17 was analyzed in cultured pulmonary artery smooth muscle cells.

MEASUREMENTS AND MAIN RESULTS

In the PH mouse model, A-17 and A-21 reduced right ventricular systolic pressure, and all antagomirs decreased pulmonary arterial muscularization. However, only A-17 reduced hypoxia-induced right ventricular hypertrophy and improved pulmonary artery acceleration time. In the monocrotaline-induced PH rat model, A-17 treatment significantly decreased right ventricular systolic pressure and total pulmonary vascular resistance index, increased pulmonary artery acceleration time, normalized cardiac output, and decreased pulmonary vascular remodeling. Among the tested miR-17 targets, the cyclin-dependent kinase inhibitor 1A (p21) was up-regulated in lungs undergoing A-17 treatment. Likewise, in human pulmonary artery smooth muscle cells, A-17 increased p21. Overexpression of miR-17 significantly reduced p21 expression and increased proliferation of smooth muscle cells.

CONCLUSIONS

Our data demonstrate that A-17 improves heart and lung function in experimental PH by interfering with lung vascular and right ventricular remodeling. The beneficial effects may be related to the up-regulation of p21. Thus, inhibition of miR-17 may represent a novel therapeutic concept to ameliorate disease state in PH.

Altered MicroRNA processing in heritable pulmonary arterial hypertension: an important role for Smad-8

RATIONALE

Heritable pulmonary arterial hypertension (HPAH) is primarily caused by mutations of the bone morphogenetic protein (BMP) type-II receptor (BMPR2). Recent identification of mutations in the downstream mediator Smad-8 (gene, SMAD9) was surprising, because loss of Smad-8 function in canonical BMP signaling is largely compensated by Smad-1 and -5. We therefore hypothesized that noncanonical pathways may play an important role in PAH.

OBJECTIVES

To determine whether HPAH mutations disrupt noncanonical Smad-mediated microRNA (miR) processing.

METHODS

Expression of miR-21, miR-27a, and miR-100 was studied in pulmonary artery endothelial (PAEC) and pulmonary artery smooth muscle cells (PASMC) from explant lungs of patients with PAH.

MEASUREMENTS AND MAIN RESULTS

SMAD9 mutation completely abrogated miR induction, whereas canonical signaling was only reduced by one-third. miR-21 levels actually decreased, suggesting that residual canonical signaling uses up or degrades existing miR-21. BMPR2 mutations also led to loss of miR induction in two of three cases. HPAH cells proliferated faster than other PAH or controls. miR-21 and miR-27a each showed antiproliferative effects in PAEC and PASMC, and PAEC growth rate after BMP treatment correlated strongly with miR-21 fold-change. Overexpression of SMAD9 corrected miR processing and reversed the hyperproliferative phenotype.

CONCLUSIONS

HPAH-associated mutations engender a primary defect in noncanonical miR processing, whereas canonical BMP signaling is partially maintained. Smad-8 is essential for this miR pathway and its loss was not complemented by Smad-1 and -5; this may represent the first nonredundant role for Smad-8. Induction of miR-21 and miR-27a may be a critical component of BMP-induced growth suppression, loss of which likely contributes to vascular cell proliferation in HPAH.

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.

Involvement of BMPR2 in the protective effect of fluoxetine against monocrotaline-induced endothelial apoptosis in rats

Mutations in bone morphogenetic protein (BMP) receptor II (BMPR2) are associated with the apoptosis of the pulmonary artery endothelial cells and the loss of the pulmonary small vessels. The present study was designed to investigate the involvement of BMPR2 in the protective effect of fluoxetine against monocrotaline (MCT)-induced endothelial apoptosis in rats. Models of pulmonary arterial hypertension in rats were established by a single intraperitoneal injection of MCT (60 mg/kg). Fluoxetine (2 and 10 mg/kg) was intragastrically administered once a day. After 21 days, MCT caused pulmonary hypertension, right ventricular hypertrophy, and pulmonary vascular remodeling and significantly reduced the BMPR2 expression in lungs and pulmonary arteries. Fluoxetine dose-dependently inhibited MCT-induced pulmonary arterial hypertension and effectively protected the lungs against MCT-induced endothelial apoptosis, reduction in the number of alveolar sacs, and loss of the pulmonary small vessels. Fluoxetine reversed the expression of cyclic guanosine 3',5'-monophosphate-dependent kinase І, BMPR2, phospho-Smad1, β-catenin, and reduced the expression of caspase 3 in rat lungs. These findings suggest that BMPR2 is probably involved in the protective effect of fluoxetine against MCT-induced endothelial apoptosis in rats.

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.

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.

Dexamethasone reverses monocrotaline-induced pulmonary arterial hypertension in rats

Pulmonary arterial hypertension (PAH) is associated with dysregulated bone morphogenetic protein receptor (BMPR)-II signaling and pulmonary vascular inflammation. We evaluated the effects of dexamethasone on monocrotaline (MCT)-induced PAH in rats for potential reversal of PAH at late time-points. Saline-treated control, MCT-exposed, MCT-exposed and dexamethasone-treated rats (5 mg·kg⁻¹·day⁻¹, 1.25 mg·kg⁻¹ and 2.5 mg·kg⁻¹·48 h⁻¹) were evaluated at day 28 and day 35 following MCT for haemodynamic parameters, right ventricular hypertrophy, morphometry, immunohistochemistry, and IL6 and BMPR2 expression. Dexamethasone improved haemodynamics and pulmonary vascular remodelling, preventing PAH development at early (day 1-14 and 1-28) and reversing PAH at late (day 14-28 and 21-35) time-points following MCT, as well as improving survival in MCT-exposed rats compared with controls. Both MCT-induced pulmonary IL6 overexpression and interleukin (IL)-6-expressing adventitial inflammatory cell infiltration were reduced with dexamethasone. This was associated with pulmonary BMPR2 downregulation following MCT, which was increased with dexamethasone, in whole lung and control pulmonary artery smooth muscle cells. Dexamethasone also reduced proliferation of rat pulmonary artery smooth muscle cells in vitro. Experimental PAH can be prevented and reversed by dexamethasone, and survival is improved. In this model, mechanisms may involve reduction of IL-6-expressing inflammatory cells, restoration of pulmonary BMPR2 expression and reduced proliferation of vascular smooth muscle cells.

Obesity hypoventilation syndrome: a state-of-the-art review

Obesity hyoventilation syndrome (OHS) is defined as the triad of obesity, daytime hypoventilation, and sleep-disordered breathing in the absence of an alternative neuromuscular, mechanical or metabolic explanation for hypoventilation. During the last 3 decades the prevalence of extreme obesity has markedly increased in the United States and other countries. With such a global epidemic of obesity, the prevalence of OHS is bound to increase. Patients with OHS have a lower quality of life, with increased healthcare expenses, and are at higher risk of developing pulmonary hypertension and early mortality, compared to eucapnic patients with sleep-disordered breathing. OHS often remains undiagnosed until late in the course of the disease. Early recognition is important, as these patients have significant morbidity and mortality. Effective treatment can lead to significant improvement in patient outcomes, underscoring the importance of early diagnosis. This review will include disease definition and epidemiology, clinical characteristics of the syndrome, pathophysiology, and morbidity and mortality associated with it. Lastly, treatment modalities will be discussed in detail.

Sildenafil added to sitaxsentan in overcirculation-induced pulmonary arterial hypertension

Experimental left-to-right shunt-induced pulmonary arterial hypertension (PAH) can be partially prevented by the endothelin-A receptor blocker sitaxsentan or by the phosphodiesterase-5 inhibitor sildenafil. We hypothesized that the combined administration of these drugs would completely prevent shunt-induced PAH, arguing in favor of a major role of endothelial dysfunction in the initiation of the disease. Twenty-four 3-wk-old piglets were randomized to a sham operation or to placebo, sitaxsentan therapy, or sitaxsentan combined with sildenafil after the anastomosis of the left subclavian artery to the pulmonary arterial trunk. Three months later, the animals underwent a hemodynamic evaluation, followed by pulmonary tissue sampling for morphometry and quantitative real-time PCR for endothelin-1, angiopoietin-1, and bone morphogenetic protein receptor (BMPR) signaling molecules. Three months of left-to-right shunting induced an increase in pulmonary vascular resistance (PVR) and medial thickness, an overexpression of endothelin-1, and angiopoietin-1 and decreased expressions of BMPR-2 and BMPR-1A. Sitaxsentan partially prevented a shunt-induced increase in PVR, medial thickness, and associated biological disturbances. Sildenafil combined with sitaxsentan normalized PVR, medial thickness, and the expression of endothelin-1. However, the expression of angiopoietin-1 remained increased, and the expressions of BMPR-1A and BMPR-2 were incompletely returned to normal. The coupling of right ventricular end-systolic to arterial elastances was maintained in all circumstances. Sitaxsentan combined with sildenafil prevents shunt-induced PAH more effectively than sitaxsentan alone, suggesting a major role for the targeted signaling pathways in the initiation of the disease. Sitaxsentan alone or combined with sildenafil did not affect right ventricular function.

ID family protein expression and regulation in hypoxic pulmonary hypertension

Bone morphogenetic protein (BMP) signaling has been linked to the development of pulmonary hypertension (PH). Inhibitors of differentiation (ID) proteins (ID1-4) are a family of basic helix-loop-helix transcription factors that are downstream targets of the BMP signaling pathway, but the role that ID proteins play in the development of PH is unknown. To address this, we evaluated pulmonary expression of ID proteins in a mouse model of hypoxia-induced PH. There is selective induction of ID1 and ID3 expression in hypoxic pulmonary vascular smooth muscle cells (VSMCs) in vivo, and ID1 and ID3 expression are increased by hypoxia in cultured pulmonary VSMCs in a BMP-dependent fashion. ID4 protein is barely detectable in the mouse lung, and while ID2 is induced in hypoxic peripheral VSMCs in vivo, it is not increased by hypoxia or BMP signaling in cultured pulmonary VSMCs. In addition, the PH response to chronic hypoxia is indistinguishable between wild type and Id1 null mice. This is associated with a compensatory increase in ID3 but not ID2 expression in pulmonary VSMCs of Id1 null mice. These findings indicate that ID1 is dispensable for mounting a normal pulmonary vascular response to hypoxia, but suggest that ID3 may compensate for loss of ID1 expression in pulmonary VSMCs. Taken together, these findings indicate that ID1 and ID3 expression are regulated in a BMP-dependent fashion in hypoxic pulmonary VSMCs, and that ID1 and ID3 may play a cooperative role in regulating BMP-dependent VSMC responses to chronic hypoxia.

Bone morphogenetic protein 4 enhances canonical transient receptor potential expression, store-operated Ca2+ entry, and basal [Ca2+]i in rat distal pulmonary arterial smooth muscle cells

Recent advances have identified an important role of bone morphogenetic protein 4 (BMP4) in pulmonary vascular remodeling, yet the underlying mechanisms remain largely unexplored. We have previously found that Ca(2+) influx through store-operated calcium channels (SOCC), which are mainly thought to be composed of canonical transient receptor potential (TRPC) proteins, likely contribute to the pathogenic development of chronic hypoxic pulmonary hypertension. In this study, we investigated the effect of BMP4 on expression of TRPC and store-operated Ca(2+) entry (SOCE) in pulmonary arterial smooth muscle cells (PASMCs). Real-time quantitative PCR and Western blotting revealed that treatment with BMP4 (50 ng/ml, 60 h) increased TRPC1, TRPC4, and TRPC6 mRNA and protein expression in growth-arrested rat distal PASMCs. Moreover, in comparison to vehicle control, cells treated with BMP4 also exhibited enhanced SOCE, and elevated basal intracellular calcium concentration ([Ca(2+)](i)) as determined by fluorescent microscopy using the Ca(2+) indicator Fura-2 AM. Perfusing cells with Ca(2+)-free Krebs-Ringer bicarbonate solution (KRBS) or KRBS containing SOCC antagonists SKF-96365 or NiCl(2) attenuated the increases in basal [Ca(2+)](i) caused by BMP4. Specific knockdown of BMP4 by small interference RNA significantly decreased the mRNA and protein expression of TRPC1, TRPC4, and TRPC6 and reduced SOCE and basal [Ca(2+)](i) in serum-stimulated PASMCs. We conclude that BMP4 regulates calcium signaling in PASMCs likely via upregulation of TRPC expression, leading to enhanced SOCE and basal [Ca(2+)](i) in PASMCs, and by this mechanism contributes to pulmonary vascular remodeling during pulmonary arterial hypertension.

Inherited human diseases of heterotopic bone formation

Human disorders of hereditary and nonhereditary heterotopic ossification are conditions in which osteogenesis occurs outside of the skeleton, within soft tissues of the body. The resulting extraskeletal bone is normal. The aberration lies within the mechanisms that regulate cell-fate determination, directing the inappropriate formation of cartilage or bone, or both, in tissues such as skeletal muscle and adipose tissue. Specific gene mutations have been identified in two rare inherited disorders that are clinically characterized by extensive and progressive extraskeletal bone formation-fibrodysplasia ossificans progressiva and progressive osseous heteroplasia. In fibrodysplasia ossificans progressiva, activating mutations in activin receptor type-1, a bone morphogenetic protein type I receptor, induce heterotopic endochondral ossification, which results in the development of a functional bone organ system that includes skeletal-like bone and bone marrow. In progressive osseous heteroplasia, the heterotopic ossification leads to the formation of mainly intramembranous bone tissue in response to inactivating mutations in the GNAS gene. Patients with these diseases variably show malformation of normal skeletal elements, identifying the causative genes and their associated signaling pathways as key mediators of skeletal development in addition to regulating cell-fate decisions by adult stem cells.

Targeting the vasoprotective axis of the renin-angiotensin system: a novel strategic approach to pulmonary hypertensive therapy

A decade has passed since the discovery of angiotensin-converting enzyme 2 (ACE2), a component of the ACE2-angiotensin (Ang)-(1-7)-Mas counterregulatory axis of the renin angiotensin system (RAS). ACE2 is considered an endogenous regulator of the vasoconstrictive, proliferative, fibrotic, and proinflammatory effects of the ACE-Ang II-angiotensin II type 1 receptor (AT(1)R) axis. Both animal and clinical studies have emerged to define a role for ACE2 in pulmonary arterial hypertension (PAH). There is scientific evidence supporting the concept that ACE2 maintains the RAS balance and plays a protective role in PAH. The activation of pulmonary ACE2 could influence the pathogenesis of PAH and serve as a novel therapeutic target in PAH. Current therapeutic strategies and interventions have limited success, and PAH remains a fatal disease. Thus, more research that establishes the novel therapeutic potential and defines the mechanism of the ACE2-Ang-(1-7)-Mas counterregulatory axis in PAH is needed.

Genetics of pulmonary arterial hypertension: do the molecular findings have translational value?

Pulmonary arterial hypertension (PAH) is usually a devastating condition with a poor prognosis. Nearly 10 years ago, the underlying molecular basis of heritable PAH was elucidated with the identification of mutations in the gene encoding the bone morphogenetic protein type II receptor (BMPR-II). This discovery is now beginning to suggest novel approaches to therapy in heritable PAH. Moreover, recent studies provide evidence that dysfunction of the BMPR-II pathway is a feature of non-familial forms of PAH, broadening the scope for intervention in this pathway.

Gene delivery of cytochrome p450 epoxygenase ameliorates monocrotaline-induced pulmonary artery hypertension in rats

Pulmonary arterial hypertension (PAH) is a life-threatening disease that leads to progressive pulmonary hypertension, right heart failure, and death. Endothelial dysfunction and inflammation were implicated in the pathogenesis of PAH. Epoxyeicosatrienoic acids (EETs), products of the cytochrome P450 epoxygenase metabolism of arachidonic acid, are potent vasodilators that possess anti-inflammatory and other protective properties in endothelial cells. We investigated whether gene delivery with the human cytochrome P450 epoxygenase 2J2 (CYP2J2) ameliorates monocrotaline (MCT)-induced pulmonary hypertension in rats. Significant pulmonary hypertension developed 3 weeks after the administration of MCT, but gene therapy with CYP2J2 significantly attenuated the development of pulmonary hypertension and pulmonary vascular remodeling, without causing changes in systemic arterial pressure or heart rate. These effects were associated with increased pulmonary endothelial NO synthase (eNOS) expression and its activity, inhibition of inflammation in the lungs, and transforming growth factor (TGF)-β/type II bone morphogenetic protein receptor (BMPRII)-drosophila mothers against decapentaplegic proteins (Smads) signaling. Collectively, these data suggest that gene therapy with CYP2J2 may have potential as a novel therapeutic approach to this progressive and oftentimes lethal disorder.

Lysyl oxidase activity is dysregulated during impaired alveolarization of mouse and human lungs

RATIONALE

Disordered extracellular matrix production is a feature of bronchopulmonary dysplasia (BPD). The basis of this phenomenon is not understood.

OBJECTIVES

To assess lysyl oxidase expression and activity in the injured developing lungs of newborn mice and of prematurely born infants with BPD or at risk for BPD.

METHODS

Pulmonary lysyl oxidase and elastin gene and protein expression were assessed in newborn mice breathing 21 or 85% oxygen, in patients who died with BPD or were at risk for BPD, and in control patients. Signaling by transforming growth factor (TGF-beta) was preemptively blocked in mice exposed to hyperoxia using TGF-beta-neutralizing antibodies. Lysyl oxidase promoter activity was assessed using plasmids containing the lox or loxl1 promoters fused upstream of the firefly luciferase gene.

MEASUREMENTS AND MAIN RESULTS

mRNA and protein levels and activity of lysyl oxidases (Lox, LoxL1, LoxL2) were elevated in the oxygen-injured lungs of newborn mice and infants with BPD or at risk for BPD. In oxygen-injured mouse lungs, increased TGF-beta signaling drove aberrant lox, but not loxl1 or loxl2, expression. Lox expression was also increased in oxygen-injured fibroblasts and pulmonary artery smooth muscle cells.

CONCLUSIONS

Lysyl oxidase expression and activity are dysregulated in BPD in injured developing mouse lungs and in prematurely born infants. In developing mouse lungs, aberrant TGF-beta signaling dysregulated lysyl oxidase expression. These data support the postulate that excessive stabilization of the extracellular matrix by excessive lysyl oxidase activity might impede the normal matrix remodeling that is required for pulmonary alveolarization and thereby contribute to the pathological pulmonary features of BPD.

Resveratrol prevents monocrotaline-induced pulmonary hypertension in rats

Proliferation of pulmonary arterial smooth muscle cells, endothelial dysfunction, oxidative stress, and inflammation promotes the development of pulmonary hypertension. Resveratrol is a polyphenolic compound that exerts antioxidant and anti-inflammatory protective effects in the systemic circulation, but its effects on pulmonary arteries remain poorly defined. The present study was undertaken to investigate the efficacy of resveratrol to prevent pulmonary hypertension. Rats injected with monocrotaline progressively developed pulmonary hypertension. Resveratrol treatment (25 mg/kg per day, PO, from day 1 postmonocrotaline) attenuated right ventricular systolic pressure and pulmonary arterial remodeling, decreased expression of inflammatory cytokines (tumor necrosis factor-alpha, interleukin 1beta, interleukin 6, and platelet-derived growth factor-alpha/beta), and limited leukocyte infiltration in the lung. Resveratrol also inhibited proliferation of pulmonary arterial smooth muscle cells. Treatment of rats with resveratrol increased expression of endothelial NO synthase, decreased oxidative stress, and improved endothelial function in small pulmonary arteries. Pulmonary hypertension was associated with an upregulation of NAD(P)H oxidase in small pulmonary arteries, which was significantly attenuated by resveratrol treatment. Our studies show that resveratrol exerts anti-inflammatory, antioxidant, and antiproliferative effects in the pulmonary arteries, which may contribute to the prevention of pulmonary hypertension.

Hemodynamic forces, vascular oxidative stress, and regulation of BMP-2/4 expression

Changes in the hemodynamic environment (e.g., hypertension, disturbed-flow conditions) are known to promote atherogenesis by inducing proinflammatory phenotypic alterations in endothelial and smooth muscle cells; however, the mechanisms underlying mechanosensitive induction of inflammatory gene expression are not completely understood. Bone morphogenetic protein-2 and -4 (BMP-2/4) are TGF-beta superfamily cytokines that are expressed by both endothelial and smooth muscle cells and regulate a number of cellular processes involved in atherogenesis, including vascular calcification and endothelial activation. This review considers how hemodynamic forces regulate BMP-2/4 expression and explores the role of mechanosensitive generation of reactive oxygen species by NAD(P)H oxidases in the control of BMP signaling.

Interleukin-6 modulates the expression of the bone morphogenic protein receptor type II through a novel STAT3-microRNA cluster 17/92 pathway

Dysregulated expression of bone morphogenetic protein receptor type II (BMPR2) is a pathogenetic hallmark of pulmonary hypertension. Downregulation of BMPR2 protein but not mRNA has been observed in multiple animal models mimicking the disease, indicating a posttranscriptional mechanism of regulation. Because microRNAs (miRNAs) regulate gene expression mainly through inhibition of target gene translation, we hypothesized that miRNAs may play a role in the modulation of BMPR2. Performing a computational algorithm on the BMPR2 gene, several miRNAs encoded by the miRNA cluster 17/92 (miR-17/92) were retrieved as potential regulators. Ectopic overexpression of miR-17/92 resulted in a strong reduction of the BMPR2 protein, and a reporter gene system showed that BMPR2 is directly targeted by miR-17-5p and miR-20a. By stimulation experiments, we found that the miR-17/92 cluster is modulated by interleukin (IL)-6, a cytokine involved in the pathogenesis of pulmonary hypertension. Because IL-6 signaling is mainly mediated by STAT3 (signal transducer and activator of transcription 3), the expression of STAT3 was knocked down by small interfering RNA, which abolished the IL-6-mediated expression of miR-17/92. Consistent with these data, we found a highly conserved STAT3-binding site in the promoter region of the miR-17/92 gene (C13orf25). Promoter studies confirmed that IL-6 enhances transcription of C13orf25 through this distinct region. Finally, we showed that persistent activation of STAT3 leads to repressed protein expression of BMPR2. Taken together, we describe here a novel STAT3-miR-17/92-BMPR2 pathway, thus providing a mechanistic explanation for the loss of BMPR2 in the development of pulmonary hypertension.

Bone morphogenetic protein signalling in heritable versus idiopathic pulmonary hypertension

Mutations in the gene encoding bone morphogenetic protein (BMP) receptor type 2 (BMPR-2) have been reported in pulmonary arterial hypertension (PAH), but their functional relevance remains incompletely understood. BMP receptor expression was evaluated in human lungs and in cultured pulmonary artery smooth muscle cells (PASMCs) isolated from 19 idiopathic PAH patients and nine heritable PAH patients with demonstrated BMPR-2 mutations. BMP4-treated PASMCs were assessed for Smad and p38 mitogen-activated protein kinase (MAPK) signalling associated with mitosis and apoptosis. Lung tissue and PASMCs from heritable PAH patients presented with decreased BMPR-2 expression and variable increases in BMPR-1A and BMPR-1B expression, while a less important decreased BMPR-2 expression was observed in PASMCs from idiopathic PAH patients. Heritable PAH PASMCs showed no increased phosphorylation of Smad1/5/8 in the presence of BMP4, which actually activated the p38MAPK pathway. Individual responses varied from one mutation to another. PASMCs from PAH patients presented with an in vitro proliferative pattern, which could be inhibited by BMP4 in idiopathic PAH but not in heritable PAH. PASMCs from idiopathic PAH and more so from heritable PAH presented an inhibition of BMP4-induced apoptosis. Most heterogeneous BMPR-2 mutations are associated with defective Smad signalling compensated for by an activation of p38MAPK signalling, accounting for PASMC proliferation and deficient apoptosis.

Altered bone morphogenetic protein and transforming growth factor-beta signaling in rat models of pulmonary hypertension: potential for activin receptor-like kinase-5 inhibition in prevention and progression of disease

BACKGROUND

Recent genetic studies have highlighted the role of the bone morphogenetic protein (BMP)/transforming growth factor (TGF)-beta signaling pathways in the pathogenesis of familial pulmonary arterial hypertension (PAH). It remains unclear whether alterations in these pathways contribute to other forms of pulmonary hypertension and to what extent these changes can be exploited for therapeutic intervention.

METHODS AND RESULTS

We studied BMP/TGF-beta signaling in 2 rat models of PAH due to chronic hypoxia and monocrotaline. In both models, there was a significant reduction in lung BMP type IA receptor and BMP type II receptor mRNA expression, although these changes were more pronounced in the monocrotaline model. This was accompanied by a reduction in lung levels of phospho-Smad1/5 and Id (inhibitor of DNA binding) gene expression in the monocrotaline model. In contrast, we observed increased TGF-beta activity, again more marked in the monocrotaline model, as evidenced by increased phospho-Smad2/3 and increased expression of TGF-beta-regulated genes. Immunohistochemistry revealed increased TGF-beta(1) expression in pulmonary artery smooth muscle cells and macrophages surrounding remodeled pulmonary arteries in monocrotaline rats. Inhibition of activin receptor-like kinase-5 signaling in vivo with the selective small-molecule inhibitor IN-1233 prevented PAH, right ventricular hypertrophy, and vascular remodeling after monocrotaline injection and inhibited the progression of established PAH in this model. No significant effect was observed in hypoxic PAH. In vitro studies confirmed that TGF-beta stimulated migration of distal rat pulmonary artery smooth muscle cells and that this effect was inhibited by IN-1233.

CONCLUSIONS

Disruption of BMP/TGF-beta signaling is more pronounced in the monocrotaline model of PAH than in the chronic hypoxia model. Increased TGF-beta activity is associated with greater macrophage recruitment with monocrotaline treatment. Inhibition of TGF-beta signaling via activin receptor-like kinase-5 prevents development and progression of PAH in the monocrotaline model and may involve inhibition of pulmonary artery smooth muscle cell migration.