Mutations in bone morphogenetic protein type II receptor cause dysregulation of Id gene expression in pulmonary artery smooth muscle cells: implications for familial pulmonary arterial hypertension

An emerging class of new therapeutics targeting TGF, Activin, and BMP ligands in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is an often fatal condition, the primary pathology of which involves loss of pulmonary vascular perfusion due to progressive aberrant vessel remodeling. The reduced capacity of the pulmonary circulation places increasing strain on the right ventricle of the heart, leading to death by heart failure. Currently, licensed therapies are primarily vasodilators, which have increased the median post-diagnosis life expectancy from 2.8 to 7 years. Although this represents a substantial improvement, the search continues for transformative therapeutics that reverse established disease. The genetics of human PAH heavily implicates reduced endothelial bone morphogenetic protein (BMP) signaling as a causal role for the disease pathobiology. Recent approaches have focused on directly enhancing BMP signaling or removing the inhibitory influence of pathways that repress BMP signaling. In this critical commentary, we review the evidence underpinning the development of two approaches: BMP-based agonists and inhibition of activin/GDF signaling. We also address the key considerations and questions that remain regarding these approaches.

SB202190 Predicts BRAF-Activating Mutations in Primary Colorectal Cancer Organoids via Erk1-2 Modulation

The p38 inhibitor SB202190 is a necessary component of the medium used for normal colorectal mucosa cultures. Sato et al. suggested that the primary activity of SB202190 may be EGFR signaling stabilization, causing an increased phosphorylation of Erk1-2 sustaining organoid proliferation. However, the growth of some colorectal cancer (CRC)-derived organoid cultures is inhibited by this molecule via an unknown mechanism. We biochemically investigated SB202190 activity on a collection of 25 primary human CRC organoids, evaluating EGFR, Akt and Erk1-2 activation using Western blot. We found that Erk1-2 phosphorylation was induced by SB202190 in 20 organoid cultures and inhibited in 5 organoid cultures. A next-generation sequencing (NGS) analysis revealed that the inhibition of p-Erk1-2 signaling corresponded to the cultures with BRAF mutations (with four different hits, one being undescribed), while p-Erk1-2 induction was apparently unrelated to other mutations involving the EGFR pathway (Her2, KRAS and NRAS). We found that SB202190 mirrored the biochemical activity of the BRAF inhibitor Dabrafenib, known to induce the paradoxical activation of p-Erk1-2 signaling in BRAF wild-type cells. SB202190 was a more effective inhibitor of BRAF-mutated organoid growth in the long term than the specific BRAF inhibitors Dabrafenib and PLX8394. Overall, SB202190 can predict BRAF-activating mutations in patient-derived organoids, as well as allowing for the identification of new BRAF variants, preceding and enforcing NGS data.

Therapeutic Approaches for Treating Pulmonary Arterial Hypertension by Correcting Imbalanced TGF-β Superfamily Signaling

Pulmonary arterial hypertension (PAH) is a rare disease characterized by high blood pressure in the pulmonary circulation driven by pathological remodeling of distal pulmonary arteries, leading typically to death by right ventricular failure. Available treatments improve physical activity and slow disease progression, but they act primarily as vasodilators and have limited effects on the biological cause of the disease—the uncontrolled proliferation of vascular endothelial and smooth muscle cells. Imbalanced signaling by the transforming growth factor-β (TGF-β) superfamily contributes extensively to dysregulated vascular cell proliferation in PAH, with overactive pro-proliferative SMAD2/3 signaling occurring alongside deficient anti-proliferative SMAD1/5/8 signaling. We review the TGF-β superfamily mechanisms underlying PAH pathogenesis, superfamily interactions with inflammation and mechanobiological forces, and therapeutic strategies under development that aim to restore SMAD signaling balance in the diseased pulmonary arterial vessels. These strategies could potentially reverse pulmonary arterial remodeling in PAH by targeting causative mechanisms and therefore hold significant promise for the PAH patient population.

Therapy for Pulmonary Arterial Hypertension: Glance on Nitric Oxide Pathway

Pulmonary arterial hypertension (PAH) is a severe disease with a resultant increase of the mean pulmonary arterial pressure, right ventricular hypertrophy and eventual death. Research in recent years has produced various therapeutic options for its clinical management but the high mortality even under treatment remains a big challenge attributed to the complex pathophysiology. Studies from clinical and non-clinical experiments have revealed that the nitric oxide (NO) pathway is one of the key pathways underlying the pathophysiology of PAH. Many of the essential drugs used in the management of PAH act on this pathway highlighting its significant role in PAH. Meanwhile, several novel compounds targeting on NO pathway exhibits great potential to become future therapy medications. Furthermore, the NO pathway is found to interact with other crucial pathways. Understanding such interactions could be helpful in the discovery of new drug that provide better clinical outcomes.

Therapeutic Challenges and Emerging Treatment Targets for Pulmonary Hypertension in Left Heart Disease

Pulmonary hypertension (PH) attributable to left heart disease (LHD) is believed to be the most common form of PH and is strongly associated with increased mortality and morbidity in this patient population. Specific therapies for PH‐LHD have not yet been identified and the use of pulmonary artery hypertension‐targeted therapies in PH‐LHD are not recommended. Endothelin receptor antagonists, phosphodiesterase‐5 inhibitors, guanylate cyclase stimulators, and prostacyclins have all been studied in PH‐LHD with conflicting results. Understanding the mechanisms underlying PH‐LHD could potentially provide novel therapeutic targets. Fibrosis, oxidative stress, and metabolic syndrome have been proposed as pathophysiological components of PH‐LHD. Genetic associations have also been identified, offering additional mechanisms with biological plausibility. This review summarizes the evidence and challenges for treatment of PH‐LHD and focuses on underlying mechanisms on the horizon that could develop into potential therapeutic targets for this disease.

Role of bone morphogenic protein-4 in gestational diabetes mellitus-related hypertension

Hyperglycaemia stimulates the synthesis and release of bone morphogenetic protein-4 (BMP-4) in vascular endothelial cells, which further induces peroxide production and inflammatory responses, leading to vascular endothelial dysfunction. However, the role of BMP-4 in gestational diabetes mellitus (GDM)-related vascular endothelial dysfunction remains unclear. In the present study, the hypothesis that the overexpression of BMP-4 would induce GDM-related hypertension by impairing vascular endothelial function was evaluated. An animal model of GDM was established in Sprague-Dawley (SD) rats. Based on blood pressure, rats were divided into control, GDM and GDM + hypertension (HT) groups. The expression levels of BMP-4, cyclooxygenase-2 (COX-2), nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 1 (NOX-1) and vascular cell adhesion molecule 1 (VCAM-1) in the endothelium of the abdominal aorta of rats in each group were determined via immunohistochemistry and western blotting. Pregnant SD rats were divided into four groups, separately infused with BMP-4, BMP-4 + noggin, noggin or vehicle by osmotic pumps, and blood pressure and vasorelaxation were examined. Immunohistochemistry indicated that the expression levels of the four proteins were lower in the control group than in the GDM and GDM + HT groups. The positive expression rate of VCAM-1 was significantly lower in the control group than in the GDM and GDM+HT groups, and the differences were statistically significant (χ²=17.325, P<0.05; χ²=10.080, P<0.05). Western blotting revealed that the expression level of the COX-2 protein exhibited a sequential increase in the three groups. The expression level of COX-2 in the control and GDM groups was significantly lower than that in the GDM+HT group (3.358±1.286; P<0.05 and P<0.05, respectively). The expression level of VCAM-1 protein in the three groups also exhibited a significant sequential increase (F=31.732; P≤0.001). The expression level of VCAM-1 in the control and GDM groups was significantly lower than that in the GDM+HT group (2.698±0.223; P≤0.001 and P≤0.001, respectively). Infusion of BMP-4 increased systolic blood pressure (from 82 to 112 mmHg) and impaired vasorelaxation in pregnant SD rats after 2 weeks. Co-treatment with noggin completely blocked BMP-4-induced effects. Thus, the BMP-4/NOX-1/COX-2 signalling pathway may be involved in GDM-related hypertension, but VCAM-1 may be substantially associated with GDM-related hypertension. Furthermore, overexpression of BMP-4 could lead to hypertension by impairing endothelial function in pregnancy.

Approaches to treat pulmonary arterial hypertension by targeting bmpr2 - from cell membrane to nucleus

Pulmonary arterial hypertension (PAH) is estimated to affect between 10-50 people per million worldwide. The lack of cure and devastating nature of the disease means that treatment is crucial to arrest rapid clinical worsening. Current therapies are limited by their focus on inhibiting residual vasoconstriction rather than targeting key regulators of the cellular pathology. Potential disease-modifying therapies may come from research directed towards causal pathways involved in the cellular and molecular mechanisms of disease. It is widely acknowledged, that targeting reduced expression of the critical bone morphogenetic protein type-2 receptor (BMPR2) and its associated signalling pathways is a compelling therapeutic avenue to explore. In this review we highlight the advances that have been made in understanding this pathway and the therapeutics that are being tested in clinical trials and the clinic to treat PAH.

Restoring BMP4 expression in vascular endothelial progenitors ameliorates maternal diabetes-induced apoptosis and neural tube defects

During mouse embryonic development, vasculogenesis initially occurs in the yolk sac, preceding neurulation. Our previous studies have demonstrated that maternal diabetes induces embryonic vasculopathy at early embryonic developmental stage by suppressing the expression of vascular growth factors including BMP4 (bone morphogenetic protein 4). This study aimed to determine whether restoring diabetes-inhibited BMP4 expression in Flk-1+ progenitors effectively prevented maternal diabetes-induced embryonic vasculopathy and NTDs. Transgenic (Tg) BMP4 expression in the vascular endothelial growth factor receptor 2 (Flk-1)-positive (Flk-1+) progenitors was achieved by crossing a Floxed BMP4 Tg mouse line with the Flk-1-Cre mouse line. Non-BMP4 Tg and BMP4 Tg embryos were harvested at E8.5 to assess the expression of BMP4, markers of endoplasmic reticulum stress, and expression of the Id genes, direct targets of BMP4; and the presence of cleaved caspase 3 and 8, apoptosis, and Smad signaling. BMP4 Tg overexpression neutralized its down-regulation by maternal diabetes in E8.5 embryos. Maternal diabetes-induced Flk-1+ progenitor apoptosis, impairment of blood island formation, and reduction of Flk-1+ progenitor number and blood vessel density, which were reversed by BMP4 Tg expression. BMP4 Tg expression in Flk-1+ progenitors blocked maternal diabetes-induced vasculopathy in early stage embryos (E7.5-E8.5) and consequently led to amelioration of maternal diabetes-induced neural tube defects (NTDs) at E10.5. BMP4 Tg expression inhibited maternal diabetes-induced endoplasmic reticulum stress and caspase cascade activation in the developing neuroepithelium, and reduced neuroepithelial cell apoptosis. BMP4 Tg expression re-activated Smad1/5/8 phosphorylation and reversed maternal diabetes-suppressed Smad4 expression. BMP4 Tg expression restored Id1 and Smad6 expression inhibited by maternal diabetes. In vitro, recombinant BMP4 protein blocked high glucose-induced Flk-1+ progenitor apoptosis and NTDs. These data demonstrate that BMP4 down-regulation in Flk-1+ progenitors are responsible for diabetes-induced yolk sac vasculopathy, and that restoring BMP4 expression prevents vasculopathy and rescues neuroepithelial cells from cellular organelle stress, leading to NTD reduction.

The regulation of IGFBP3 by BMP2 has a role in human endometrial remodeling

In mammals, bone morphogenetic protein 2 (BMP2) is a critical regulator of endometrial decidualization and early implantation. Insulin-like growth factor-binding protein 3 (IGFBP3) is highly expressed in the endometrium and at the maternal-fetal interface in multiple species, including humans. BMP2-induced IGFBP3 signaling has been confirmed to have a role in trophoblast cell invasion; however, the involvement of this signaling pathway in endometrial remodeling remains poorly understood. To determine the roles of BMP2 in regulating IGFBP3 expression during the transformation of endometrial stromal cells, we employed immortalized human endometrial stromal cells (HESCs) and primary human decidual stromal cells (HDSCs) as study models. We showed that BMP2 significantly increased the expression of IGFBP3 in a dose- and time-dependent manner in both HESCs and primary HDSCs. Additionally, the BMP2-induced upregulation of IGFBP3 is mediated by the inhibitor of DNA-binding 1 (ID1), and knockdown of ALK3 completely abolished BMP2-induced upregulation of ID1. Moreover, BMP2 increased the expression of matrix metalloproteinases 2 (MMP2) and promoted cell migration in HESCs and primary HDSCs. Knockdown of either IGFBP3 or ID1 significantly suppressed the basal and the BMP2-induced increase in MMP2 expression as well as the cell migration in both cell models. These data demonstrated that BMP2 upregulated the expression of ID1, which in turn induced the expression of IGFBP3, and these BMP2-induced cell activities were most likely mediated by the ALK3 type I receptor. The increased expression of IGFBP3 promoted the MMP2 expression and cell migration in both HESCs and HDSCs. These findings deepen our understanding of a newly identified mechanism by which BMP2 and IGFBP3 regulate endometrial remodeling in humans, which provides insight into potential therapies for endometrium-related diseases and pregnancy-induced complications.

4PBA Restores Signaling of a Cysteine-substituted Mutant BMPR2 Receptor Found in Patients with Pulmonary Arterial Hypertension

Mutations in the gene encoding BMPR2 (bone morphogenetic protein type 2 receptor) are the major cause of heritable pulmonary arterial hypertension (PAH). Point mutations in the BMPR2 ligand-binding domain involving cysteine residues (such as C118W) are causative of PAH and predicted to cause protein misfolding. Using heterologous overexpression systems, we showed previously that these mutations lead to retention of BMPR2 in the endoplasmic reticulum but are partially rescued by chemical chaperones. Here, we sought to determine whether the chemical chaperone 4-phenylbutyrate (4PBA) restores BMPR2 signaling in primary cells and in a knockin mouse harboring a C118W mutation. First, we confirmed dysfunctional BMP signaling in dermal fibroblasts isolated from a family with PAH segregating the BMPR2 C118W mutation. After BMP4 treatment, the induction of downstream signaling targets (Smad1/5, ID1 [inhibitor of DNA binding 1], and ID2) was significantly reduced in C118W mutant cells. Treatment with 4PBA significantly rescued Smad1/5, ID1, and ID2 expression. Pulmonary artery smooth muscle cells isolated from the lungs of heterozygous mice harboring the Bmpr2 C118W mutation exhibited significantly increased proliferation. In the presence of 4PBA, hyperproliferation was dramatically reduced. Furthermore, in vivo, 4PBA treatment of Bmpr2 C118W mice partially rescued Bmpr2 expression, restored downstream signaling, and improved vascular remodeling. These findings demonstrate in primary cells and in a knockin mouse that the repurposed small-molecule chemical chaperone 4PBA might be a promising precision medicine approach to treat PAH in patients with specific subtypes of BMPR2 mutation involving cysteine substitutions in the ligand-binding domain.

Functional Impact of Human Genetic Variants of COL18A1/Endostatin on Pulmonary Endothelium

Pulmonary arterial hypertension (PAH) is an incurable disease characterized by disordered and dysfunctional angiogenesis leading to small-vessel loss and an obliterative vasculopathy. The pathogenesis of PAH is not fully understood, but multiple studies have demonstrated links between elevated angiostatic factors, disease severity, and adverse clinical outcomes. ES (endostatin), one such circulating angiostatic peptide, is the cleavage product of the proteoglycan COL18A1 (collagen α1[XVIII] chain). Elevated serum ES is associated with increased mortality and disease severity in PAH. A nonsynonymous variant of ES (aspartic acid-to-asparagine substitution at amino acid 104; p.D104N) is associated with differences in PAH survival. Although COL18A1/ES expression is markedly increased in remodeled pulmonary vessels in PAH, the impact of ES on pulmonary endothelial cell (PEC) biology and molecular contributions to PAH severity remain undetermined. In the present study, we characterized the effects of exogenous ES on human PEC biology and signaling. We demonstrated that ES inhibits PEC migration, proliferation, and cell survival, with significant differences between human variants, indicating that they are functional genetic variants. ES promotes proteasome-mediated degradation of the transcriptional repressor ID1, increasing expression and release of TSP-1 (thrombospondin 1). ES inhibits PEC migration via an ID1/TSP-1/CD36-dependent pathway, in contrast to proliferation and apoptosis, which require both CD36 and CD47. Collectively, the data implicate ES as a novel negative regulator of ID1 and an upstream propagator of an angiostatic signal cascade converging on CD36 and CD47, providing insight into the cellular and molecular effects of a functional genetic variant linked to altered outcomes in PAH.

Transcriptome-wide analysis associates ID2 expression with combined pre- and post-capillary pulmonary hypertension

Heart failure with preserved ejection fraction (HFpEF) patients who develop pulmonary hypertension (PH) have an increased risk of death, with combined pre- and post-capillary PH (CpcPH) having the highest risk. However, the mechanism behind PH development in HFpEF is poorly understood. We aimed to identify transcriptomic associations with PH development in HFpEF. Blood was collected from 30 HFpEF patients: 10 without PH, 10 with isolated post-capillary PH, and 10 with CpcPH. Gene expression measurements were completed using transcriptome-wide RNA sequencing. Gene expression differences were compared using a quasi-likelihood method adjusting for age, sex, race, and smoking-status. Biological pathways were compared using global gene expression differences. A replication in 34 additional heart failure patients and a validation in lung tissue from a representative mouse model were completed using quantitative PCR. Six differentially expressed genes were identified when comparing transcriptomics between subjects with CpcPH and those without PH. When tested in additional subjects, only the association with ID2 replicated. Consistent with clinical findings, Id2 expression was also upregulated in mice with HFpEF and PH. Pathway analysis identified proliferative and mitochondrial pathways associated with CpcPH. Thus, these patients may possess systemic pathophysiological differences similar to those observed in pulmonary arterial hypertension patients.

Prevention of progression of pulmonary hypertension by the Nur77 agonist 6-mercaptopurine: role of BMP signalling

Pulmonary arterial hypertension (PAH) is a progressive fatal disease characterised by abnormal remodelling of pulmonary vessels, leading to increased vascular resistance and right ventricle failure. This abnormal vascular remodelling is associated with endothelial cell dysfunction, increased proliferation of smooth muscle cells, inflammation and impaired bone morphogenetic protein (BMP) signalling. Orphan nuclear receptor Nur77 is a key regulator of proliferation and inflammation in vascular cells, but its role in impaired BMP signalling and vascular remodelling in PAH is unknown.

We hypothesised that activation of Nur77 by 6-mercaptopurine (6-MP) would improve PAH by inhibiting endothelial cell dysfunction and vascular remodelling.

Nur77 expression is decreased in cultured pulmonary microvascular endothelial cells (MVECs) and lungs of PAH patients. Nur77 significantly increased BMP signalling and strongly decreased proliferation and inflammation in MVECs. In addition, conditioned medium from PAH MVECs overexpressing Nur77 inhibited the growth of healthy smooth muscle cells. Pharmacological activation of Nur77 by 6-MP markedly restored MVEC function by normalising proliferation, inflammation and BMP signalling. Finally, 6-MP prevented and reversed abnormal vascular remodelling and right ventricle hypertrophy in the Sugen/hypoxia rat model of severe angioproliferative PAH.

Our data demonstrate that Nur77 is a critical modulator in PAH by inhibiting vascular remodelling and increasing BMP signalling, and activation of Nur77 could be a promising option for the treatment of PAH.

DAN plays important compensatory roles in systemic-to-pulmonary shunt associated pulmonary arterial hypertension

AIM

Proteins mainly expressed in normal lungs and significantly changed in lungs exposed to systemic-to-pulmonary shunts might be promising targets for pulmonary arterial hypertension induced by congenital heart diseases (PAH/CHD). This study aimed to investigate the potential role of differential screening-selected gene aberrative in neuroblastoma (DAN) in PAH/CHD.

METHODS

PAH was surgically induced by the combined surgery (right pulmonary artery ligation and left cervical systemic-to-pulmonary shunt) in Sprague-Dawley (SD) rats. Exogenous DAN was supplemented by osmotic minipumps.

RESULTS

Firstly, DAN was significantly decreased in patients with severe PAH/CHD and negatively correlated with pulmonary hemodynamic indices derived from right cardiac catheterization. Secondly, pulmonary hypertensive status and apparent pulmonary vasculopathies of PAH/CHD were surgically reproduced in SD rats. Real time-PCR and Western blot analysis revealed that DAN mRNA and protein levels decreased in lungs exposed to systemic-to-pulmonary shunts, and immunofluorescence staining found that DAN was highly expressed in pulmonary arteries of normal lungs but seldom detected in severely remodelling pulmonary arteries, furthermore, plasma levels of DAN in shunted-rats manifested a time-depended decrease and negatively correlated with pulmonary hemodynamic indices. Thirdly, DAN specially reversed the anti-proliferative and pro-apoptotic effects of bone morphogenetic protein 2/4 (BMP2/4) on pulmonary arterial smooth muscle cells via BMP2/4-BMPR2-Smad1/5/8-Id1 signalling pathway. Furthermore, continuous supplementation of exogenous DAN protein increased the extent of shunt-associated PAH.

CONCLUSION

Compensatory decrease of DAN in hypertensive lungs may retard the deterioration of shunt-associated PAH, at least in part, by antagonizing BMP signalling pathway. Furthermore, DAN might be a potential biomarker for PAH/CHD.

miR-27a promotes endothelial-mesenchymal transition in hypoxia-induced pulmonary arterial hypertension by suppressing BMP signaling

AIM

Growing evidence suggests that endothelial-mesenchymal transition (EndMT) play key roles in pulmonary arterial remodeling during pulmonary arterial hypertension (PAH), but the underlying mechanisms have yet to be fully understood. miR-27a has been shown to promote proliferation of pulmonary arterial cells during PAH, but its role in EndMT remains unexplored. This study was designed to investigate the role and underlying mechanism of miR-27a in EndMT during PAH.

MAIN METHODS

Rats were exposed in hypoxia (10% O₂) for 3 weeks to induce PAH, and human pulmonary artery endothelial cells (HPAECs) were exposed in hypoxia (1% O₂) for 48 h to induce EndMT. Immunohistochemistry, in situ hybridization, immunofluorescence, real-time PCR and Western blot were conducted to detect the expressions of RNAs and proteins, and luciferase assay was used to verify the putative binding site of miR-27a.

KEY FINDINGS

We found that hypoxia up-regulated miR-27a in the tunica intima of rat pulmonary arteries and HPAECs, and that inhibition of miR-27a suppressed hypoxia-induced EndMT. Furthermore, elevated expression of miR-27a suppressed bone morphogenetic protein (BMP) signaling by targeting Smad5, thereby lessening Id2-mediated repression of the 2 critical mediators of EndMT (Snail and Twist).

SIGNIFICANCE

Our data unveiled a novel role of miR-27a in EndMT during hypoxia-induced PAH. Thus, targeting of miR-27a-related pathway may be therapeutically harnessed to treat PAH.

Collagen type II suppresses articular chondrocyte hypertrophy and osteoarthritis progression by promoting integrin β1-SMAD1 interaction

Hypertrophic differentiation is not only the terminal process of endochondral ossification in the growth plate but is also an important pathological change in osteoarthritic cartilage. Collagen type II (COL2A1) was previously considered to be only a structural component of the cartilage matrix, but recently, it has been revealed to be an extracellular signaling molecule that can significantly suppress chondrocyte hypertrophy. However, the mechanisms by which COL2A1 regulates hypertrophic differentiation remain unclear. In our study, a Col2a1 p.Gly1170Ser mutant mouse model was constructed, and Col2a1 loss was demonstrated in homozygotes. Loss of Col2a1 was found to accelerate chondrocyte hypertrophy through the bone morphogenetic protein (BMP)-SMAD1 pathway. Upon interacting with COL2A1, integrin β1 (ITGB1), the major receptor for COL2A1, competed with BMP receptors for binding to SMAD1 and then inhibited SMAD1 activation and nuclear import. COL2A1 could also activate ITGB1-induced ERK1/2 phosphorylation and, through ERK1/2-SMAD1 interaction, it further repressed SMAD1 activation, thus inhibiting BMP-SMAD1-mediated chondrocyte hypertrophy. Moreover, COL2A1 expression was downregulated, while chondrocyte hypertrophic markers and BMP-SMAD1 signaling activity were upregulated in degenerative human articular cartilage. Our study reveals novel mechanisms for the inhibition of chondrocyte hypertrophy by COL2A1 and suggests that the degradation and decrease in COL2A1 might initiate and promote osteoarthritis progression.

A signaling feedback loop that contributes to cartilage degeneration may offer a fruitful target for the treatment of osteoarthritis. During the early stages of this disorder, cartilage-forming chondrocytes undergo a process of expansion known as hypertrophy, after which they die and are replaced by calcium. Researchers led by Peiqiang Su and Dongsheng Huang of Sun Yat-sen University have demonstrated that COL2A1, an important structural protein, represents an important safeguard against hypertrophy. COL2A1 helps maintain chondrocytes in their normal, healthy state, but Su and Huang showed that signaling factors produced during cartilage repair can reduce COL2A1 levels. This in turn accelerates hypertrophy, promoting further depletion of COL2A1 and ultimately leading to full-blown osteoarthritis. Drugs that break this cycle and preserve COL2A1 could thus help protect endangered joints before the damage becomes severe.

Berberine attenuates hypoxia-induced pulmonary arterial hypertension via bone morphogenetic protein and transforming growth factor-β signaling

Hypoxia-induced excessive pulmonary artery smooth muscle cell (PASMC) proliferation plays an important role in the pathology of pulmonary arterial hypertension (PAH). Berberine (BBR) is reported as an effective antiproliferative properties applied in clinical. However, the effect of BBR on PAH remains unclear. In the present study, we elucidated the protective effects of BBR against abnormal PASMC proliferation and vascular remodeling in chronic hypoxia-induced hearts. Furthermore, the potential mechanisms of BBR were investigated. For this purpose, C57/BL6 mice were exposed to chronic hypoxia for 4 weeks to mimic severe PAH. Hemodynamic and pulmonary pathomorphology data showed that chronic hypoxia significantly increased the right ventricular systolic pressure (RVSP), the right ventricle/left ventricle plus septum RV/(LV + S) weight ratio, and the median width of pulmonary arterioles. BBR attenuated the elevations in RVSP and RV/(LV + S) and mitigated pulmonary vascular structure remodeling. BBR also suppressed the hypoxia-induced increases in the expression of proliferating cell nuclear antigen (PCNA) and of α-smooth muscle actin. Furthermore, administration of BBR significantly increased the expression of bone morphogenetic protein type II receptor (BMPR-II) and its downstream molecules P-smad1/5 and decreased the expression of transforming growth factor-β (TGF-β) and its downstream molecules P-smad2/3. Moreover, peroxisome proliferator-activated receptor γ expression was significantly decreased in the hypoxia group, and this decrease was reversed by BBR treatment. Our study demonstrated that the protective effect of BBR against hypoxia-induced PAH in a mouse model may be achieved through altered BMPR-II and TGF-β signaling.

Hematopoietic stem cell transplantation alters susceptibility to pulmonary hypertension in Bmpr2-deficient mice

Increasing evidence suggests that patients with pulmonary arterial hypertension (PAH) demonstrate abnormalities in the bone marrow (BM) and hematopoietic progenitor cells. In addition, PAH is associated with myeloproliferative diseases. We have previously demonstrated that low-dose lipopolysaccharide (LPS) is a potent stimulus for the development of PAH in the context of a genetic PAH mouse model of BMPR2 dysfunction. We hypothesized that the hematopoietic progenitor cells might be driving disease in this model. To test this hypothesis, we performed adoptive transfer of BM between wild-type (Ctrl) and heterozygous Bmpr2 null (Mut) mice. Sixteen weeks after BM reconstitution, mice were exposed to low-dose chronic LPS (0.5 mg/kg three times a week for six weeks). Mice underwent right heart catheterization and tissues were removed for histology. After chronic LPS dosing, Ctrl mice in receipt of Mut BM developed PAH, whereas Mut mice receiving Ctrl BM were protected from PAH. BM histology demonstrated an increase in megakaryocytes and there was an increase in circulating platelets in Ctrl mice receiving Mut BM. These findings demonstrate that the hematopoietic stem cell compartment is involved in the susceptibility to PAH in the Mut mouse. The results raise the possibility that hematopoietic stem cell transplantation might be a potential treatment strategy in genetic forms of PAH.

TGF-β and BMPR2 Signaling in PAH: Two Black Sheep in One Family

Knowledge pertaining to the involvement of transforming growth factor β (TGF-β) and bone morphogenetic protein (BMP) signaling in pulmonary arterial hypertension (PAH) is continuously increasing. There is a growing understanding of the function of individual components involved in the pathway, but a clear synthesis of how these interact in PAH is currently lacking. Most of the focus has been on signaling downstream of BMPR2, but it is imperative to include the role of TGF-β signaling in PAH. This review gives a state of the art overview of disturbed signaling through the receptors of the TGF-β family with respect to vascular remodeling and cardiac effects as observed in PAH. Recent (pre)-clinical studies in which these two pathways were targeted will be discussed with an extended view on cardiovascular research fields outside of PAH, indicating novel future perspectives.

Clinical significance linked to functional defects in bone morphogenetic protein type 2 receptor, BMPR2

Bone morphogenetic protein type 2 receptor (BMPR2) is one of the transforming growth factor-β (TGF-β) superfamily receptors, performing diverse roles during embryonic development, vasculogenesis, and osteogenesis. Human BMPR2 consists of 1,038 amino acids, and contains functionally conserved extracellular, transmembrane, kinase, and C-terminal cytoplasmic domains. Bone morphogenetic proteins (BMPs) engage the tetrameric complex, composed of BMPR2 and its corresponding type 1 receptors, which initiates SMAD proteins-mediated signal transduction leading to the expression of target genes implicated in the development or differentiation of the embryo, organs and bones. In particular, genetic alterations of BMPR2 gene are associated with several clinical disorders, including representative pulmonary arterial hypertension, cancers, and metabolic diseases, thus demonstrating the physiological importance of BMPR2. In this mini review, we summarize recent findings regarding the molecular basis of BMPR2 functions in BMP signaling, and the versatile roles of BMPR2. In addition, various aspects of experimentally validated pathogenic mutations of BMPR2 and the linked human diseases will also be discussed, which are important in clinical settings for diagnostics and treatment.

Baicalin attenuates monocrotaline-induced pulmonary hypertension through bone morphogenetic protein signaling pathway

Baicalin, a flavonoid compound extracted from roots of Scutellaria baicalensis Georgi (huang qin), it has been shown to effectively attenuates pulmonary hypertension (PH), however, the potential mechanism remains unexplored. In this study, we investigated the potential mechanism of baicalin on monocrotaline (MCT)-induced PH in rats. The results showed that baicalin attenuated lung damage in PH rat model through inhibiting the pulmonary arterial smooth muscle cell proliferation and induction of cells apoptosis. Furthermore, we demonstrated that baicalin inhibition the expression of nuclear factor-κB (NF-κB) p65 and bone morphogenetic protein (BMP) antagonists gremlin-1, but increased the expression of inhibitor of NF-κB (I-κBα), BMPR2, BMP-4, BMP-9 and Smad1/5/8. Additionally, baicalin suppression endothelial-to-mesenchymal transition in PH lung tissue. Collectively, we confirmed that baicalin via inhibition of NF-κB signaling to further activation of BMP signaling to have a therapeutic effect on PH and providing a promising therapeutic strategy for PH.

TNFα drives pulmonary arterial hypertension by suppressing the BMP type-II receptor and altering NOTCH signalling

Heterozygous germ-line mutations in the bone morphogenetic protein type-II receptor (BMPR-II) gene underlie heritable pulmonary arterial hypertension (HPAH). Although inflammation promotes PAH, the mechanisms by which inflammation and BMPR-II dysfunction conspire to cause disease remain unknown. Here we identify that tumour necrosis factor-α (TNFα) selectively reduces BMPR-II transcription and mediates post-translational BMPR-II cleavage via the sheddases, ADAM10 and ADAM17 in pulmonary artery smooth muscle cells (PASMCs). TNFα-mediated suppression of BMPR-II subverts BMP signalling, leading to BMP6-mediated PASMC proliferation via preferential activation of an ALK2/ACTR-IIA signalling axis. Furthermore, TNFα, via SRC family kinases, increases pro-proliferative NOTCH2 signalling in HPAH PASMCs with reduced BMPR-II expression. We confirm this signalling switch in rodent models of PAH and demonstrate that anti-TNFα immunotherapy reverses disease progression, restoring normal BMP/NOTCH signalling. Collectively, these findings identify mechanisms by which BMP and TNFα signalling contribute to disease, and suggest a tractable approach for therapeutic intervention in PAH.

Reduced BMP receptor II signalling underlies pulmonary arterial hypertension (PAH). Here, Hurst et al. show that TNFα subverts BMP signalling by increasing BMP6 expression and signalling via an alternative BMP receptor, ALK2, in pulmonary artery smooth muscle cells to drive abnormal proliferation and PAH.

Salvianolic acid A attenuates vascular remodeling in a pulmonary arterial hypertension rat model

AIM

The current therapeutic approaches have a limited effect on the dysregulated pulmonary vascular remodeling, which is characteristic of pulmonary arterial hypertension (PAH). In this study we examined whether salvianolic acid A (SAA) extracted from the traditional Chinese medicine 'Dan Shen' attenuated vascular remodeling in a PAH rat model, and elucidated the underlying mechanisms.

METHODS

PAH was induced in rats by injecting a single dose of monocrotaline (MCT 60 mg/kg, sc). The rats were orally treated with either SAA (0.3, 1, 3 mg·kg(-1)·d(-1)) or a positive control bosentan (30 mg·kg(-1)·d(-1)) for 4 weeks. Echocardiography and hemodynamic measurements were performed on d 28. Then the hearts and lungs were harvested, the organ indices and pulmonary artery wall thickness were calculated, and biochemical and histochemical analysis were conducted. The levels of apoptotic and signaling proteins in the lungs were measured using immunoblotting.

RESULTS

Treatment with SAA or bosentan effectively ameliorated MCT-induced pulmonary artery remodeling, pulmonary hemodynamic abnormalities and the subsequent increases of right ventricular systolic pressure (RVSP). Furthermore, the treatments significantly attenuated MCT-induced hypertrophic damage of myocardium, parenchymal injury and collagen deposition in the lungs. Moreover, the treatments attenuated MCT-induced apoptosis and fibrosis in the lungs. The treatments partially restored MCT-induced reductions of bone morphogenetic protein type II receptor (BMPRII) and phosphorylated Smad1/5 in the lungs.

CONCLUSION

SAA ameliorates the pulmonary arterial remodeling in MCT-induced PAH rats most likely via activating the BMPRII-Smad pathway and inhibiting apoptosis. Thus, SAA may have therapeutic potential for the patients at high risk of PAH.

MicroRNA-140-5p and SMURF1 regulate pulmonary arterial hypertension

Loss of the growth-suppressive effects of bone morphogenetic protein (BMP) signaling has been demonstrated to promote pulmonary arterial endothelial cell dysfunction and induce pulmonary arterial smooth muscle cell (PASMC) proliferation, leading to the development of pulmonary arterial hypertension (PAH). MicroRNAs (miRs) mediate higher order regulation of cellular function through coordinated modulation of mRNA targets; however, miR expression is altered by disease development and drug therapy. Here, we examined treatment-naive patients and experimental models of PAH and identified a reduction in the levels of miR-140-5p. Inhibition of miR-140-5p promoted PASMC proliferation and migration in vitro. In rat models of PAH, nebulized delivery of miR-140-5p mimic prevented the development of PAH and attenuated the progression of established PAH. Network and pathway analysis identified SMAD-specific E3 ubiquitin protein ligase 1 (SMURF1) as a key miR-140-5p target and regulator of BMP signaling. Evaluation of human tissue revealed that SMURF1 is increased in patients with PAH. miR-140-5p mimic or SMURF1 knockdown in PASMCs altered BMP signaling, further supporting these factors as regulators of BMP signaling. Finally, Smurf1 deletion protected mice from PAH, demonstrating a critical role in disease development. Together, these studies identify both miR-140-5p and SMURF1 as key regulators of disease pathology and as potential therapeutic targets for the treatment of PAH.

Targeting BMP signalling in cardiovascular disease and anaemia

Bone morphogenetic proteins (BMPs) and their receptors, known to be essential regulators of embryonic patterning and organogenesis, are also critical for the regulation of cardiovascular structure and function. In addition to their contributions to syndromic disorders including heart and vascular development, BMP signalling is increasingly recognized for its influence on endocrine-like functions in postnatal cardiovascular and metabolic homeostasis. In this Review, we discuss several critical and novel aspects of BMP signalling in cardiovascular health and disease, which highlight the cell-specific and context-specific nature of BMP signalling. Based on advancing knowledge of the physiological roles and regulation of BMP signalling, we indicate opportunities for therapeutic intervention in a range of cardiovascular conditions including atherosclerosis and pulmonary arterial hypertension, as well as for anaemia of inflammation. Depending on the context and the repertoire of ligands and receptors involved in specific disease processes, the selective inhibition or enhancement of signalling via particular BMP ligands (such as in atherosclerosis and pulmonary arterial hypertension, respectively) might be beneficial. The development of selective small molecule antagonists of BMP receptors, and the identification of ligands selective for BMP receptor complexes expressed in the vasculature provide the most immediate opportunities for new therapies.

The promise of recombinant BMP ligands and other approaches targeting BMPR-II in the treatment of pulmonary arterial hypertension

Human genetic discoveries offer a powerful method to implicate pathways of major importance to disease pathobiology and hence provide targets for pharmacological intervention. The genetics of pulmonary arterial hypertension (PAH) strongly implicates loss-of-function of the bone morphogenetic protein type II receptor (BMPR-II) signalling pathway and moreover implicates the endothelial cell as a central cell type involved in disease initiation. We and others have described several approaches to restore BMPR-II function in genetic and non-genetic forms of PAH. Of these, supplementation of endothelial BMP9/10 signalling with exogenous recombinant ligand has been shown to hold considerable promise as a novel large molecule biopharmaceutical therapy. Here, we describe the mechanism of action and discuss potential additional effects of BMP ligand therapy.

Selective enhancement of endothelial BMPR-II with BMP9 reverses pulmonary arterial hypertension

Genetic evidence implicates the loss of bone morphogenetic protein type II receptor (BMPR-II) signaling in the endothelium as an initiating factor in pulmonary arterial hypertension (PAH). However, selective targeting of this signaling pathway using BMP ligands has not yet been explored as a therapeutic strategy. Here, we identify BMP9 as the preferred ligand for preventing apoptosis and enhancing monolayer integrity in both pulmonary arterial endothelial cells and blood outgrowth endothelial cells from subjects with PAH who bear mutations in the gene encoding BMPR-II, BMPR2. Mice bearing a heterozygous knock-in allele of a human BMPR2 mutation, R899X, which we generated as an animal model of PAH caused by BMPR-II deficiency, spontaneously developed PAH. Administration of BMP9 reversed established PAH in these mice, as well as in two other experimental PAH models, in which PAH develops in response to either monocrotaline or VEGF receptor inhibition combined with chronic hypoxia. These results demonstrate the promise of direct enhancement of endothelial BMP signaling as a new therapeutic strategy for PAH.

Sex affects bone morphogenetic protein type II receptor signaling in pulmonary artery smooth muscle cells

RATIONALE

Major pulmonary arterial hypertension (PAH) registries report a greater incidence of PAH in women; mutations in the bone morphogenic protein type II receptor (BMPR-II) occur in approximately 80% of patients with heritable PAH (hPAH).

OBJECTIVES

We addressed the hypothesis that women may be predisposed to PAH due to normally reduced basal BMPR-II signaling in human pulmonary artery smooth muscle cells (hPASMCs).

METHODS

We examined the BMPR-II signaling pathway in hPASMCs derived from men and women with no underlying cardiovascular disease (non-PAH hPASMCs). We also determined the development of pulmonary hypertension in male and female mice deficient in Smad1.

MEASUREMENTS AND MAIN RESULTS

Platelet-derived growth factor, estrogen, and serotonin induced proliferation only in non-PAH female hPASMCs. Female non-PAH hPASMCs exhibited reduced messenger RNA and protein expression of BMPR-II, the signaling intermediary Smad1, and the downstream genes, inhibitors of DNA binding proteins, Id1 and Id3. Induction of phospho-Smad1/5/8 and Id protein by BMP4 was also reduced in female hPASMCs. BMP4 induced proliferation in female, but not male, hPASMCs. However, small interfering RNA silencing of Smad1 invoked proliferative responses to BMP4 in male hPASMCs. In male hPASMCs, estrogen decreased messenger RNA and protein expression of Id genes. The estrogen metabolite 4-hydroxyestradiol decreased phospho-Smad1/5/8 and Id expression in female hPASMCs while increasing these in males commensurate with a decreased proliferative effect in male hPASMCs. Female Smad1(+/-) mice developed pulmonary hypertension (reversed by ovariectomy).

CONCLUSIONS

We conclude that estrogen-driven suppression of BMPR-II signaling in non-PAH hPASMCs derived from women contributes to a pro-proliferative phenotype in hPASMCs that may predispose women to PAH.

Defective cellular trafficking of the bone morphogenetic protein receptor type II by mutations underlying familial pulmonary arterial hypertension

Familial pulmonary arterial hypertension (FPAH) is a relatively rare but fatal disorder characterized by elevated arterial pressure caused by abnormal proliferation of endothelial cells of the arteries, which eventually leads to heart failure and death. FPAH is inherited as an autosomal dominant trait and is caused by heterozygous mutations in the BMPR2 gene encoding the bone morphogenetic protein type II receptor (BMPR2). BMPR2 belongs to the TGF β/BMP super-family of receptors involved in a signal transduction cascade via the SMAD signaling pathway. The BMPR2 polypeptide is composed of 1038 amino acids and consists of a ligand binding domain, a kinase domain and a cytoplasmic tail. To investigate the cellular and functional consequence of BMPR2 mutations, C-terminally FLAG-tagged constructs of eighteen pathogenic BMPR2 missense mutants were generated by site directed mutagenesis and expressed in HeLa and HEK-293T cell lines. The subcellular localizations of the mutant proteins were investigated using immunostaining and confocal microscopy. Post-translational modifications of the proteins were analyzed by Endoglycosidase H deglycosylation assay. Our results indicated that mutations in the ligand binding domain affecting highly conserved cysteine residues resulted in retention of the mutant proteins in the endoplasmic reticulum (ER), as evident from their co-localization with the ER resident protein calnexin. The kinase domain mutants showed both ER and plasma membrane (PM) distributions, while the cytoplasmic tail domain variants were localized exclusively to the PM. The subcellular localizations of the mutants were further confirmed by their characteristic glycosylation profiles. In conclusion, our results indicate that ER quality control (ERQC) is involved in the pathological mechanism of several BMPR2 receptor missense mutations causing FPAH, which can be explored as a potential therapeutic target in the future.

The mechanistic basis of prostacyclin and its stable analogues in pulmonary arterial hypertension: Role of membrane versus nuclear receptors

Pulmonary arterial hypertension (PAH) is a progressive disease of distal pulmonary arteries in which patients suffer from elevated pulmonary arterial pressure, extensive vascular remodelling and right ventricular failure. To date prostacyclin (PGI2) therapy remains the most efficacious treatment for PAH and is the only approved monotherapy to have a positive impact on long-term survival. A key thing to note is that improvement exceeds that predicted from vasodilator testing strongly suggesting that additional mechanisms contribute to the therapeutic benefit of prostacyclins in PAH. Given these agents have potent antiproliferative, anti-inflammatory and endothelial regenerating properties suggests therapeutic benefit might result from a slowing, stabilization or even some reversal of vascular remodelling in vivo. This review discusses evidence that the pharmacology of each prostacyclin (IP) receptor agonist so far developed is distinct, with non-IP receptor targets clearly contributing to the therapeutic and side effect profile of PGI2 (EP3), iloprost (EP1), treprostinil (EP2, DP1) along with a family of nuclear receptors known as peroxisome proliferator-activated receptors (PPARs), to which PGI2 and some analogues directly bind. These targets are functionally expressed to varying degrees in arteries, veins, platelets, fibroblasts and inflammatory cells and are likely to be involved in the biological actions of prostacylins. Recently, a highly selective IP agonist, selexipag has been developed for PAH. This agent should prove useful in distinguishing IP from other prostanoid receptors or PPAR binding effects in human tissue. It remains to be determined whether selectivity for the IP receptor gives rise to a superior or inferior clinical benefit in PAH.

Hypoxia inducible factor-1-dependent up-regulation of BMP4 mediates hypoxia-induced increase of TRPC expression in PASMCs

AIMS

Previously we demonstrated that both hypoxia inducible factor-1 (HIF-1) and bone morphogenetic protein-4 (BMP4) up-regulate transient receptor potential canonical (TRPC) 1 and TRPC6, resulting in increased basal intracellular Ca(2+) concentration ([Ca(2+)]i) in pulmonary arterial smooth muscle cells (PASMCs), driving development of chronic hypoxia (CH)-induced pulmonary hypertension (CHPH). This study aims to determine whether HIF-1 regulates BMP4, and whether BMP4 mediates TRPC and basal [Ca(2+)]i increases in hypoxic PASMCs.

METHODS AND RESULTS

The level of BMP4 mature protein was increased for ∼183% in distal pulmonary arterial smooth muscle (PA) from CH (10% O2 for 21 days; CH) exposed rats, and 143% in PASMCs cultured under prolonged hypoxia (4% O2 for 60 h). In rat PASMCs, HIF-1α overexpression up-regulated, whereas HIF-1α knockdown under hypoxia decreased BMP4 expression; site-mutation identified two functional HIF-1-binding sites in Bmp4 gene promoter; noggin or BMP4 siRNA treatment blocked hypoxia-induced increases of TRPC1 and TRPC6 expression and basal [Ca(2+)]i. Likewise, in mice, exposure to CH increased BMP4 expression in distal PA for ∼80%, which was absent in HIF-1α heterozygous mutant mice. Comparing with wild-type littermates, BMP4 heterozygous mutant mice exposed to CH displayed lower BMP4 and TRPC levels in PA, decreased basal [Ca(2+)]i in PASMCs, and attenuated CHPH. In human PASMCs, HIF-1α knockdown attenuated hypoxia-induced BMP4 expression and knockdown of either HIF-1α or BMP4 abolished hypoxia-induced TRPC expression and basal [Ca(2+)]i.

CONCLUSIONS

BMP4 acts downstream of HIF-1 and mediates hypoxia-induced up-regulation of TRPC, leading to increased basal [Ca(2+)]i in PASMCs, promoting CHPH pathogenesis.

Sex-dependent influence of endogenous estrogen in pulmonary hypertension

RATIONALE

The incidence of pulmonary arterial hypertension is greater in women, suggesting estrogens may play a role in the disease pathogenesis. Experimentally, in males, exogenously administered estrogen can protect against pulmonary hypertension (PH). However, in models that display female susceptibility, estrogens may play a causative role.

OBJECTIVES

To clarify the influence of endogenous estrogen and sex in PH and assess the therapeutic potential of a clinically available aromatase inhibitor.

METHODS

We interrogated the effect of reduced endogenous estrogen in males and females using the aromatase inhibitor, anastrozole, in two models of PH: the hypoxic mouse and Sugen 5416/hypoxic rat. We also determined the effects of sex on pulmonary expression of aromatase in these models and in lungs from patients with pulmonary arterial hypertension.

MEASUREMENTS AND MAIN RESULTS

Anastrozole attenuated PH in both models studied, but only in females. To verify this effect was caused by reduced estrogenic activity we confirmed that in hypoxic mice inhibition of estrogen receptor α also has a therapeutic effect specifically in females. Female rodent lung displays increased aromatase and decreased bone morphogenetic protein receptor 2 and Id1 expression compared with male. Anastrozole treatment reversed the impaired bone morphogenetic protein receptor 2 pathway in females. Increased aromatase expression was also detected in female human pulmonary artery smooth muscle cells compared with male.

CONCLUSIONS

The unique phenotype of female pulmonary arteries facilitates the therapeutic effects of anastrozole in experimental PH confirming a role for endogenous estrogen in the disease pathogenesis in females and suggests aromatase inhibitors may have therapeutic potential.

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.

Pulmonary venoocclusive disease in childhood

BACKGROUND

Pulmonary venoocclusive disease (PVOD) is a rare lung disease, diagnosed in 5% to 10% of patients with pulmonary hypertension (PH). The incidence, prevalence, and etiology of PVOD in children are not well defined. The mortality remains high, related, at least partly, to the limited treatment options.

METHODS

This retrospective analysis (1985-2011) summarizes symptoms, associated factors, treatment, and outcomes of nine pediatric patients (five girls, four boys) with histologic confirmation of PVOD.

RESULTS

PH was diagnosed at a mean age of 13.5 years (range, 8-16 years), followed by the definitive diagnosis of PVOD at a mean age of 14.3 years (range, 10-16 years). Symptoms such as decreased exercise tolerance (n = 6) and/or shortness of breath (n = 9) preceded the diagnosis by 21 months on average; the mean survival time after diagnosis was 14 months (range, 0-47 months). CT scans of the lungs showed typical radiologic features. Treatment included supplemental home oxygen (n = 5), diuretics (n = 9), warfarin (n = 4), and pulmonary vasodilators (n = 4). Four children were listed for lung transplantation, and three have undergone transplantation. Eight patients died, including two after lung transplantation. One patient with lung transplant survived with good quality of life.

CONCLUSIONS

PVOD is an important differential diagnosis for pediatric patients with PH. CT scanning is a valuable tool to image lung abnormalities; the definitive diagnosis can only be made by examination of lung biopsy specimens, which subjects the patient to additional risk. Early listing for lung transplantation is essential, as the mean survival time is only 14 months.

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.

Endothelial dysfunction in diabetes and hypertension: cross talk in RAS, BMP4, and ROS-dependent COX-2-derived prostanoids

Vascular endothelium regulates cardiovascular function, and endothelial dysfunction is the key initiator for arteriosclerosis and thrombosis and their complications. The endothelium is a dynamic interface that responds to various stimuli and synthesizes and liberates vasoactive molecules such as nitric oxide, prostaglandins, hyperpolarizing factor, and endothelin. Risk factors such as hypertension, hypercholesterolemia, smoking, and hyperglycemia impair the ability of the endothelium to respond to physical or chemical stimulation appropriately, and increased oxidative stress is believed to be a major culprit. This brief article reviews the interplay among several oxidative stress regulators in the vascular wall and highlights therapeutic relevance through deeper understanding of the interplay between the renin-angiotensin system, nicotinamide adenine dinucleotide phosphate, reduced oxidase, bone morphogenic protein 4, and cyclooxygenase 2-derived prostaglandins as a concerted pathogenic cascade in inducing and maintaining endothelial dysfunction in hypertension and diabetes.

Inhibition of overactive transforming growth factor-β signaling by prostacyclin analogs in pulmonary arterial hypertension

The heterozygous loss of function mutations in the Type II bone morphogenetic protein receptor (BMPR-II), a member of the transforming growth factor (TGF-β) receptor family, underlies the majority of familial cases of pulmonary arterial hypertension (PAH). The TGF-β1 pathway is activated in PAH, and inhibitors of TGF-β1 signaling prevent the development and progression of PAH in experimental models. However, the effects of currently used therapies on the TGF-β pathway remain unknown. Prostacyclin analogs comprise the first line of treatment for clinical PAH. We hypothesized that these agents effectively decrease the activity of the TGF-β1 pathway. Beraprost sodium (BPS), a prostacyclin analog, selectively inhibits proliferation in a dose-dependent manner in murine primary pulmonary arterial smooth muscle cells (PASMCs) harboring a pathogenic BMPR2 nonsense mutation in both the presence and absence of TGF-β1 stimulation. Our study demonstrates that this agent inhibits TGF-β1-induced SMAD-dependent and SMAD-independent signaling via a protein kinase A-dependent pathway by reducing the phosphorylation of SMADs 2 and 3 and p38 mitogen-activated protein kinase proteins. Finally, in a monocrotaline-induced rat model of PAH, which is associated with increased TGF-β signaling, this study confirms that treprostinil, a stable prostacyclin analog, inhibits the TGF-β pathway by reducing SMAD3 phosphorylation. Taken together, these data suggest that prostacyclin analogs inhibit dysregulated TGF-β signaling in vitro and in vivo, and reduce BMPR-II-mediated proliferation defects in mutant mice PASMCs.

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.

Id proteins are critical downstream effectors of BMP signaling in human pulmonary arterial smooth muscle cells

Bone morphogenetic protein type II receptor (BMPR-II) mutations are responsible for over 70% of cases of heritable pulmonary arterial hypertension (PAH). Loss of BMP signaling promotes pulmonary vascular remodeling via modulation of pulmonary artery smooth muscle cell (PASMC) proliferation. Id proteins (Id1-4) are major downstream transcriptional targets of BMP signaling. However, the impact of BMPR-II mutation on the expression of the range of Id proteins and the contribution of individual Id proteins to abnormal PASMC function remain unclear. Human PASMCs were used to determine the expression of Id proteins (Id1-4) by real-time PCR and immunoblotting. The BMP responses in control cells were compared with PASMCs harboring BMPR-II mutations and cells in which BMPR-II was knocked down by siRNA transfection. Id3 expression in pulmonary vessels was also investigated in BMPR-II mutant mice and in patients with heritable PAH. BMP4 and BMP6, but not BMP9, induced mRNA expression of Id1, Id2, and Id3. The BMP-stimulated induction of Id1 and Id3 was markedly reduced in BMPR-II mutant PASMCs and in control PASMCs following siRNA silencing of BMPR-II. Pulmonary arteries in BMPR-II mutant mice and patients with heritable PAH demonstrated reduced levels of Id3 compared with control subjects. Lentiviral overexpression of Id3 reduced cell cycle progression and inhibited proliferation of PASMCs. Lipopolysaccharide further reduced Id3 expression in mutant PASMCs. In conclusion, Id proteins, and particularly Id1 and Id3, are critical downstream effectors of BMP signaling in PASMCs. Loss of BMPR-II function reduces the induction of Id genes in PASMCs, Id1, and Id3 regulate the proliferation of PASMCs via cell cycle inhibition, an effect that may be exacerbated by inflammatory stimuli.

The lysosomal inhibitor, chloroquine, increases cell surface BMPR-II levels and restores BMP9 signalling in endothelial cells harbouring BMPR-II mutations

Pulmonary arterial hypertension (PAH) is characterized by dysregulated pulmonary artery endothelial cell (PAEC) proliferation, apoptosis and permeability. Loss-of-function mutations in the bone morphogenetic protein receptor type-II (BMPR-II) are the most common cause of heritable PAH, usually resulting in haploinsufficiency. We previously showed that BMPR-II expression is regulated via a lysosomal degradative pathway. Here, we show that the antimalarial drug, chloroquine, markedly increased cell surface expression of BMPR-II protein independent of transcription in PAECs. Inhibition of protein synthesis experiments revealed a rapid turnover of cell surface BMPR-II, which was inhibited by chloroquine treatment. Chloroquine enhanced PAEC expression of BMPR-II following siRNA knockdown of the BMPR-II transcript. Using blood outgrowth endothelial cells (BOECs), we confirmed that signalling in response to the endothelial BMPR-II ligand, BMP9, is compromised in BOECs from patients harbouring BMPR-II mutations, and in BMPR-II mutant PAECs. Chloroquine significantly increased gene expression of BMP9-BMPR-II signalling targets Id1, miR21 and miR27a in both mutant BMPR-II PAECs and BOECs. These findings provide support for the restoration of cell surface BMPR-II with agents such as chloroquine as a potential therapeutic approach for heritable PAH.

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.

Vascular repair and regeneration as a therapeutic target for pulmonary arterial hypertension

The last decade has seen substantial changes in our understanding of the pathobiology of pulmonary arterial hypertension (PAH), a severe and devastating disease without curative treatment. It is now accepted that injury to the endothelial cells of the pulmonary arteries is central for the subsequent development of lumen-obliterative lung vascular lesions. A variety of circulating and lung-resident progenitor and stem cells likely contribute to vascular integrity, and evidence for the presence of cells expressing stem and progenitor cell markers is found inside and in the immediate vicinity of pulmonary vascular lesions in PAH. The currently available vasodilator therapies mainly target enhanced vasoconstriction in the lung circulation and help to maintain or improve right ventricular function, but do not treat pulmonary vascular remodeling, the underlying cause of the disease. Vascular gene therapy and cell therapy with progenitor and stem cells is a progressing field in the context of the development of novel treatment options for PAH, but the majority of the studies are currently performed at the level of preclinical studies in animal models. The current review provides an overview of the current knowledge on cell- and gene therapy-based approaches for vascular repair and regeneration in PAH.

Chloroquine prevents progression of experimental pulmonary hypertension via inhibition of autophagy and lysosomal bone morphogenetic protein type II receptor degradation

RATIONALE

Pulmonary arterial hypertension (PAH) is characterized by excessive proliferation and apoptosis resistance in pulmonary artery smooth muscle cells (PASMCs).

OBJECTIVE

We reasoned that chloroquine, based on its ability to inhibit autophagy and block lysosomal degradation of the bone morphogenetic protein type II receptor (BMPR-II), might exert beneficial effects in this disease.

METHODS AND RESULTS

PAH was induced in male Sprague-Dawley rats by administering monocrotaline. The induction of PAH was associated with changes in lung expression of LC3B-II, ATG5, and p62, consistent with increased autophagy, and decreased BMPR-II protein expression. Administration of chloroquine prevented the development of PAH, right ventricular hypertrophy, and vascular remodelling after monocrotaline, and prevented progression of established PAH in this model. Similar results were obtained with hydroxychloroquine. Chloroquine treatment increased whole lung and PASMC p62 protein levels consistent with inhibition of autophagy, and increased levels of BMPR-II protein. Chloroquine inhibited proliferation and increased apoptosis of PASMCs in vivo. In cultured rat PASMCs we confirmed that chloroquine both inhibited autophagy pathways and increased expression of BMPR-II protein via lysosomal inhibition. Consistent with the in vivo findings, chloroquine inhibited the proliferation and stimulated apoptosis of rat PASMCs in vitro, with no effect on endothelial cell proliferation or survival. Moreover, direct inhibition of autophagy pathways by ATG5 small interfering RNA knockdown inhibited proliferation of rat PASMCs.

CONCLUSIONS

Chloroquine and hydroxychloroquine exert beneficial effects in experimental PAH. The mechanism of action includes inhibition of autophagy pathways and inhibition of lysosomal degradation of BMPR-II.

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.

Sildenafil potentiates bone morphogenetic protein signaling in pulmonary arterial smooth muscle cells and in experimental pulmonary hypertension

OBJECTIVE

Mutations in the bone morphogenetic protein type II receptor (BMPR-II) are responsible for the majority of cases of heritable pulmonary arterial hypertension (PAH), and BMPR-II deficiency contributes to idiopathic and experimental forms of PAH. Sildenafil, a potent type-5 nucleotide-dependent phosphodiesterase inhibitor, is an established treatment for PAH, but whether sildenafil affects bone morphogenetic protein (BMP) signaling in the pulmonary circulation remains unknown.

METHODS AND RESULTS

Studies were undertaken in human pulmonary arterial smooth muscle cells (PASMCs) and in vivo in the monocrotaline rat model of PAH. In PASMCs, sildenafil enhanced BMP4-induced phosphorylation of Smad1/5, Smad nuclear localization, and Inhibitor of DNA binding protein 1 gene and protein expression. This effect was mimicked by 8-bromo-cyclic GMP. Pharmacological inhibition or small interfering RNA knockdown of cyclic GMP-dependent protein kinase I inhibited the effect of sildenafil on BMP signaling. In functional studies, we observed that sildenafil potentiated the antiproliferative effects of BMP4 on PASMC proliferation. Furthermore, sildenafil restored the antiproliferative response to BMP4 in PASMCs harboring mutations in BMPR-II. In the monocrotaline rat model of PAH, which is characterized by BMPR-II deficiency, sildenafil prevented the development of pulmonary hypertension and vascular remodeling, and partly restored Smad1/5 phosphorylation and Inhibitor of DNA binding protein 1 gene expression in vivo in monocrotaline exposed rat lungs.

CONCLUSIONS

Sildenafil enhances canonical BMP signaling via cyclic GMP and cyclic GMP-dependent protein kinase I in vitro and in vivo, and partly restores deficient BMP signaling in BMPR-II mutant PASMCs. Our findings demonstrate a novel mechanism of action of sildenafil in the treatment of PAH and suggest that targeting BMP signaling may be beneficial in this disease.