Signal transduction in the development of pulmonary arterial hypertension

Increasing pulmonary artery pulsatile flow improves hypoxic pulmonary hypertension in piglets

Pulmonary arterial hypertension (PAH) is a disease affecting distal pulmonary arteries (PA). These arteries are deformed, leading to right ventricular failure. Current treatments are limited. Physiologically, pulsatile blood flow is detrimental to the vasculature. In response to sustained pulsatile stress, vessels release nitric oxide (NO) to induce vasodilation for self-protection. Based on this observation, this study developed a protocol to assess whether an artificial pulmonary pulsatile blood flow could induce an NO-dependent decrease in pulmonary artery pressure. One group of piglets was exposed to chronic hypoxia for 3 weeks and compared to a control group of piglets. Once a week, the piglets underwent echocardiography to assess PAH severity. At the end of hypoxia exposure, the piglets were subjected to a pulsatile protocol using a pulsatile catheter. After being anesthetized and prepared for surgery, the jugular vein of the piglet was isolated and the catheter was introduced through the right atrium, the right ventricle and the pulmonary artery, under radioscopic control. Pulmonary artery pressure (PAP) was measured before (T0), immediately after (T1) and 30 min after (T2) the pulsatile protocol. It was demonstrated that this pulsatile protocol is a safe and efficient method of inducing a significant reduction in mean PAP via an NO-dependent mechanism. These data open up new avenues for the clinical management of PAH.

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.

Skeletal muscle proteomic signature and metabolic impairment in pulmonary hypertension

Exercise limitation comes from a close interaction between cardiovascular and skeletal muscle impairments. To better understand the implication of possible peripheral oxidative metabolism dysfunction, we studied the proteomic signature of skeletal muscle in pulmonary arterial hypertension (PAH). Eight idiopathic PAH patients and eight matched healthy sedentary subjects were evaluated for exercise capacity, skeletal muscle proteomic profile, metabolism, and mitochondrial function. Skeletal muscle proteins were extracted, and fractioned peptides were tagged using an iTRAQ protocol. Proteomic analyses have documented a total of 9 downregulated proteins in PAH skeletal muscles and 10 upregulated proteins compared to healthy subjects. Most of the downregulated proteins were related to mitochondrial structure and function. Focusing on skeletal muscle metabolism and mitochondrial health, PAH patients presented a decreased expression of oxidative enzymes (pyruvate dehydrogenase, p < 0.01) and an increased expression of glycolytic enzymes (lactate dehydrogenase activity, p < 0.05). These findings were supported by abnormal mitochondrial morphology on electronic microscopy, lower citrate synthase activity (p < 0.01) and lower expression of the transcription factor A of the mitochondria (p < 0.05), confirming a more glycolytic metabolism in PAH skeletal muscles. We provide evidences that impaired mitochondrial and metabolic functions found in the lungs and the right ventricle are also present in skeletal muscles of patients.

KEY MESSAGE

• Proteomic and metabolic analysis show abnormal oxidative metabolism in PAH skeletal muscle. • EM of PAH patients reveals abnormal mitochondrial structure and distribution. • Abnormal mitochondrial health and function contribute to exercise impairments of PAH. • PAH may be considered a vascular affliction of heart and lungs with major impact on peripheral muscles.

MicroRNAs in pulmonary arterial hypertension: pathogenesis, diagnosis and treatment

Pulmonary arterial hypertension (PAH) is a severe and increasingly prevalent disease, manifested by the maladaptation of pulmonary vasculature, which consequently leads to right heart failure and possibly even death. The development of PAH is characterized by specific functional as well as structural changes, primarily associated with the aberrant function of the pulmonary artery endothelial cells, smooth muscle cells, and vascular fibroblasts. MicroRNAs constitute a class of small ≈22-nucleotides-long non-coding RNAs that post-transcriptionally regulate gene expression and that may lead to significant cell proteome changes. While the involvement of miRNAs in the development of various diseases--especially cancer--has been reported, numerous miRNAs have also been associated with PAH onset, progression, or treatment responsiveness. This review focuses on the role of microRNAs in the development of PAH as well as on their potential use as biomarkers and therapeutic tools in both experimental PAH models and in humans. Special attention is given to the roles of miR-21, miR-27a, the miR-17-92 cluster, miR-124, miR-138, the miR-143/145 cluster, miR-150, miR-190, miR-204, miR-206, miR-210, miR-328, and the miR-424/503 cluster, specifically with the objective of providing greater insight into the pervasive roles of miRNAs in the pathogenesis of this deadly condition.

Adaptation and remodelling of the pulmonary circulation in pulmonary hypertension

Pulmonary arterial hypertension (PAH) is characterized by remodelling of pulmonary arteries caused by a proliferation/apoptosis imbalance within the vascular wall. This pathological phenotype seems to be triggered by different environmental stress and injury events such as increased inflammation, DNA damage, and epigenetic deregulation. It appears that one of the first hit to occur is endothelial cells (ECs) injury and apoptosis, which leads to paracrine signalling to other ECs, pulmonary artery smooth muscle cells (PASMCs), and fibroblasts. These signals promote a phenotypic change of surviving ECs by disturbing different signalling pathways leading to sustained vasoconstriction, proproliferative and antiapoptotic phenotype, deregulated angiogenesis, and formation of plexiform lesions. EC signalling also recruits proinflammatory cells, leading to pulmonary infiltration of lymphocytes, macrophages, and dendritic cells, sustaining the inflammatory environment and autoimmune response. Finally, EC signalling promotes proliferative and antiapoptotic PAH-PASMC phenotypes, which acquire migratory capacities, resulting in increased vascular wall thickness and muscularization of small pulmonary arterioles. Adaptation and remodelling of pulmonary circulation also involves epigenetic components, such as microRNA deregulation, DNA methylation, and histone modification. This review will focus on the different cellular and epigenetic aspects including EC stress response, molecular mechanisms contributing to PAH-PASMC and PAEC proliferation and resistance to apoptosis, as well as epigenetic control involved in adaptation and remodelling of the pulmonary circulation in PAH.

Systems-level regulation of microRNA networks by miR-130/301 promotes pulmonary hypertension

Development of the vascular disease pulmonary hypertension (PH) involves disparate molecular pathways that span multiple cell types. MicroRNAs (miRNAs) may coordinately regulate PH progression, but the integrative functions of miRNAs in this process have been challenging to define with conventional approaches. Here, analysis of the molecular network architecture specific to PH predicted that the miR-130/301 family is a master regulator of cellular proliferation in PH via regulation of subordinate miRNA pathways with unexpected connections to one another. In validation of this model, diseased pulmonary vessels and plasma from mammalian models and human PH subjects exhibited upregulation of miR-130/301 expression. Evaluation of pulmonary arterial endothelial cells and smooth muscle cells revealed that miR-130/301 targeted PPARγ with distinct consequences. In endothelial cells, miR-130/301 modulated apelin-miR-424/503-FGF2 signaling, while in smooth muscle cells, miR-130/301 modulated STAT3-miR-204 signaling to promote PH-associated phenotypes. In murine models, induction of miR-130/301 promoted pathogenic PH-associated effects, while miR-130/301 inhibition prevented PH pathogenesis. Together, these results provide insight into the systems-level regulation of miRNA-disease gene networks in PH with broad implications for miRNA-based therapeutics in this disease. Furthermore, these findings provide critical validation for the evolving application of network theory to the discovery of the miRNA-based origins of PH and other diseases.

Role of epigenetics in pulmonary hypertension

A significant amount of research has been conducted to examine the pathologic processes and epigenetic mechanisms contributing to peripheral hypertension. However, few studies have been carried out to understand the vascular remodeling behind pulmonary hypertension (PH), including peripheral artery muscularization, medial hypertrophy and neointima formation in proximal arteries, and plexiform lesion formation. Similarly, research examining some of the epigenetic principles that may contribute to this vascular remodeling, such as DNA methylation and histone modification, is minimal. The understanding of these principles may be the key to developing new and more effective treatments for PH. The purpose of this review is to summarize epigenetic research conducted in the field of hypertension that could possibly be used to understand the epigenetics of PH. Possible future therapies that could be pursued using information from these studies include selective histone deacetylase inhibitors and targeted DNA methyltransferases. Both of these could potentially be used to silence proproliferative or antiapoptotic genes that lead to decreased smooth muscle cell proliferation. Epigenetics may provide a glimmer of hope for the eventual improved treatment of this highly morbid and debilitating disease.

Upregulated copper transporters in hypoxia-induced pulmonary hypertension

Pulmonary vascular remodeling and increased arterial wall stiffness are two major causes for the elevated pulmonary vascular resistance and pulmonary arterial pressure in patients and animals with pulmonary hypertension. Cellular copper (Cu) plays an important role in angiogenesis and extracellular matrix remodeling; increased Cu in vascular smooth muscle cells has been demonstrated to be associated with atherosclerosis and hypertension in animal experiments. In this study, we show that the Cu-uptake transporter 1, CTR1, and the Cu-efflux pump, ATP7A, were both upregulated in the lung tissues and pulmonary arteries of mice with hypoxia-induced pulmonary hypertension. Hypoxia also significantly increased expression and activity of lysyl oxidase (LOX), a Cu-dependent enzyme that causes crosslinks of collagen and elastin in the extracellular matrix. In vitro experiments show that exposure to hypoxia or treatment with cobalt (CoCl2) also increased protein expression of CTR1, ATP7A, and LOX in pulmonary arterial smooth muscle cells (PASMC). In PASMC exposed to hypoxia or treated with CoCl2, we also confirmed that the Cu transport is increased using 64Cu uptake assays. Furthermore, hypoxia increased both cell migration and proliferation in a Cu-dependent manner. Downregulation of hypoxia-inducible factor 1α (HIF-1α) with siRNA significantly attenuated hypoxia-mediated upregulation of CTR1 mRNA. In summary, the data from this study indicate that increased Cu transportation due to upregulated CTR1 and ATP7A in pulmonary arteries and PASMC contributes to the development of hypoxia-induced pulmonary hypertension. The increased Cu uptake and elevated ATP7A also facilitate the increase in LOX activity and thus the increase in crosslink of extracellular matrix, and eventually leading to the increase in pulmonary arterial stiffness.

Hypoxic response contributes to altered gene expression and precapillary pulmonary hypertension in patients with sickle cell disease

BACKGROUND

We postulated that the hypoxic response in sickle cell disease (SCD) contributes to altered gene expression and pulmonary hypertension, a complication associated with early mortality.

METHODS AND RESULTS

To identify genes regulated by the hypoxic response and not other effects of chronic anemia, we compared expression variation in peripheral blood mononuclear cells from 13 subjects with SCD with hemoglobin SS genotype and 15 subjects with Chuvash polycythemia (VHL(R200W) homozygotes with constitutive upregulation of hypoxia-inducible factors in the absence of anemia or hypoxia). At a 5% false discovery rate, 1040 genes exhibited >1.15-fold change in both conditions; 297 were upregulated and 743 downregulated including MAPK8 encoding a mitogen-activated protein kinase important for apoptosis, T-cell differentiation, and inflammatory responses. Association mapping with a focus on local regulatory polymorphisms in 61 patients with SCD identified expression quantitative trait loci for 103 of these hypoxia response genes. In a University of Illinois SCD cohort, the A allele of a MAPK8 expression quantitative trait locus, rs10857560, was associated with precapillary pulmonary hypertension defined as mean pulmonary artery pressure ≥25 mm Hg and pulmonary capillary wedge pressure ≤15 mm Hg at right heart catheterization (allele frequency, 0.66; odds ratio, 13.8; n=238). This association was confirmed in an independent Walk-Treatment of Pulmonary Hypertension and Sickle Cell Disease With Sildenafil Therapy cohort (allele frequency, 0.65; odds ratio, 11.3; n=519). The homozygous AA genotype of rs10857560 was associated with decreased MAPK8 expression and present in all 14 of the identified precapillary pulmonary hypertension cases among the combined 757 patients.

CONCLUSIONS

Our study demonstrates a prominent hypoxic transcription component in SCD and a MAPK8 expression quantitative trait locus associated with precapillary pulmonary hypertension.

Animal Models of Pulmonary Hypertension: Matching Disease Mechanisms to Etiology of the Human Disease

Recently a great deal of progress has been made in our understanding of pulmonary hypertension (PH). Research from the past 30 years has resulted in newer treatments that provide symptomatic improvements and delayed disease progression. Unfortunately, the cure for patients with this lethal syndrome remains stubbornly elusive. With the relative explosion of scientific literature regarding PH, confusion has arisen regarding animal models of the disease and their correlation to the human condition. This short review uniquely focuses on the clear and present need to better correlate mechanistic insights from existing and emerging animal models of PH to specific etiologies and histopathologies of human PH. A better understanding of the pathologic processes in various animal models and how they relate to the human disease should accelerate the development of newer and more efficacious therapies.

Role for DNA damage signaling in pulmonary arterial hypertension

BACKGROUND

Pulmonary arterial hypertension (PAH) is associated with sustained inflammation known to promote DNA damage. Despite these unfavorable environmental conditions, PAH pulmonary arterial smooth muscle cells (PASMCs) exhibit, in contrast to healthy PASMCs, a pro-proliferative and anti-apoptotic phenotype, sustained in time by the activation of miR-204, nuclear factor of activated T cells, and hypoxia-inducible factor 1-α. We hypothesized that PAH-PASMCs have increased the activation of poly(ADP-ribose) polymerase-1 (PARP-1), a critical enzyme implicated in DNA repair, allowing proliferation despite the presence of DNA-damaging insults, eventually leading to PAH.

METHODS AND RESULTS

Human PAH distal pulmonary arteries and cultured PAH-PASMCs exhibit increased DNA damage markers (53BP1 and γ-H2AX) and an overexpression of PARP-1 (immunoblot and activity assay), in comparison with healthy tissues/cells. Healthy PASMCs treated with a clinically relevant dose of tumor necrosis factor-α harbored a similar phenotype, suggesting that inflammation induces DNA damage and PARP-1 activation in PAH. We also showed that PARP-1 activation accounts for miR-204 downregulation (quantitative reverse transcription polymerase chain reaction) and the subsequent activation of the transcription factors nuclear factor of activated T cells and hypoxia-inducible factor 1-α in PAH-PASMCs, previously shown to be critical for PAH in several models. These effects resulted in PASMC proliferation (Ki67, proliferating cell nuclear antigen, and WST1 assays) and resistance to apoptosis (terminal deoxynucleotidyl transferase dUTP nick end labeling and Annexin V assays). In vivo, the clinically available PARP inhibitor ABT-888 reversed PAH in 2 experimental rat models (Sugen/hypoxia and monocrotaline).

CONCLUSIONS

These results show for the first time that the DNA damage/PARP-1 signaling pathway is important for PAH development and provide a new therapeutic target for this deadly disease with high translational potential.