Rescue of monocrotaline-induced pulmonary arterial hypertension using bone marrow-derived endothelial-like progenitor cells: efficacy of combined cell and eNOS gene therapy in established disease

Stem cells in pulmonary hypertension: Current understanding and future challenges

Stem cells possess the unique ability to develop into different cell types within the body. Researchers are exploring the use of different types of stem cells to potentially repair damaged blood vessels, reduce inflammation, and improve overall vascular function, all of which are crucial factors in pulmonary hypertension (PH). While it is important to acknowledge that further clinical studies and trials are necessary to fully understand the efficacy and safety of stem cell therapy for PH, ongoing research and initial findings present promising avenues for potentially developing new treatments or therapeutic strategies for PH.

Stem cell therapy in pulmonary hypertension: current practice and future opportunities

Pulmonary hypertension (PH) is a progressive disease characterised by elevated pulmonary arterial pressure and right-sided heart failure. While conventional drug therapies, including prostacyclin analogues, endothelin receptor antagonists and phosphodiesterase type 5 inhibitors, have been shown to improve the haemodynamic abnormalities of patients with PH, the 5-year mortality rate remains high. Thus, novel therapies are urgently required to prolong the survival of patients with PH. Stem cell therapies, including mesenchymal stem cells, endothelial progenitor cells and induced pluripotent stem cells, have shown therapeutic potential for the treatment of PH and clinical trials on stem cell therapies for PH are ongoing. This review aims to present the latest preclinical achievements of stem cell therapies, focusing on the therapeutic effects of clinical trials and discussing the challenges and future perspectives of large-scale applications.

PDK1 upregulates PINK1-mediated pulmonary endothelial cell mitophagy during hypoxia-induced pulmonary vascular remodeling

BACKGROUND

Hypoxic pulmonary hypertension (HPH) is a complication of lung diseases with pulmonary vascular remodeling, although the underlying molecular mechanisms have not been fully elucidated. This study investigated the underlying molecular events by using a rat HPH model and primary pulmonary microvascular endothelial cells (PMVECs).

METHODS AND RESULTS

This study first established a rat HPH model and cultured PMVECs for transmission electron microscopic analysis and manipulation of 3-phosphoinositide-dependent protein kinase 1 (PDK1) or phosphatase and tensin homolog-induced kinase 1 (PINK1) expression in vitro. After that, the cell viability was assessed and the expression of different proteins was assayed using cell viability and western blot assays, respectively. Reactive oxygen species production, apoptosis, NLR family pyrin domain containing 3 (NLRP3) expression, and the levels of interleukin (IL)-1β, IL-6, and IL-8 were also assessed, while the interaction of PDK1 and PINK1 was determined using co-immunoprecipitation/western blot assays. Hypoxia induced mitophagy in the PMVECs and upregulated PINK1/Parkin expression, whereas knockdown of PINK1 expression under hypoxic conditions inhibited cell proliferation but induced endothelial cell apoptosis in vitro, decreased reactive oxygen species production and NLRP3 expression, and reduced the levels of inflammatory factors in PMVECs. However, hypoxia induced PDK1 expression, whereas knockdown of PDK1 downregulated PINK1 expression. Furthermore, treatment of the model rats with the PDK1 inhibitor dichloroacetate (DCA) was able to decrease PINK1 expression. In addition, the PDK1 and PINK1 proteins could interact with each other in the mitochondria of PMVECs to regulate the cell viability.

CONCLUSIONS

This study revealed that PDK1 induced PMVEC proliferation but inhibited their apoptosis to participate in pulmonary vascular remodeling, ultimately leading to HPH through regulation of PINK1-mediated mitophagy signaling. Therefore, PINK1 is a novel therapeutic target for the control of HPH.

Endothelial progenitor cells derived from embryonic stem cells prevent alveolar simplification in a murine model of bronchopulmonary dysplasia

Introduction: Vascular remodeling and compromised alveolar development are hallmarks of chronic pulmonary diseases such as bronchopulmonary dysplasia (BPD). Despite advances in neonatal healthcare the number of BPD cases worldwide continues to increase. One approach to overcoming the premature arrest in lung development seen in BPD is to stimulate neonatal angiogenesis via delivery and engraftment of endothelial progenitor cells (EPCs). One such population is resident to the pulmonary microvasculature and expresses both FOXF1 and c-KIT. Previous studies have shown that c-KIT⁺FOXF1⁺ EPCs are highly sensitive to elevated levels of oxygen (hyperoxia) and are decreased in premature infants with BPD and hyperoxia-induced BPD mouse models. We hypothesize that restoring EPCs through transplantation of c-KIT⁺FOXF1⁺ EPCs derived in vitro from pluripotent embryonic stem cells (ESCs), will stimulate neonatal angiogenesis and alveolarization in mice with hyperoxia-induced lung injury. Methods: Utilizing a novel ESC line with a FOXF1:GFP reporter, we generated ESC-derived c-KIT⁺FOXF1⁺ EPCs in vitro. Using a second ESC line which contains FOXF1:GFP and tdTomato transgenes, we differentiated ESCs towards c-KIT⁺FOXF1⁺ EPCs and tracked them in vivo after injection into the neonatal circulation of hyperoxia-injured mice. After a recovery period in room air conditions, we analyzed c-KIT⁺FOXF1⁺ EPC engraftment and quantified the number of resident and circulating endothelial cells, the size of alveolar spaces, and the capillary density after EPC transplantations. Results and conclusion: Herein, we demonstrate that addition of BMP9 to the directed endothelial differentiation protocol results in very efficient generation of c-KIT⁺FOXF1⁺ EPCs from pluripotent ESCs. ESC-derived c-KIT⁺FOXF1⁺ EPCs effectively engraft into the pulmonary microvasculature of hyperoxia-injured mice, promote vascular remodeling in alveoli, increase the number of resident and circulating endothelial cells, and improve alveolarization. Altogether, these results provide a proof-of-principle that cell therapy with ESC-derived c-KIT⁺FOXF1⁺ EPCs can prevent alveolar simplification in a hyperoxia-induced BPD mouse model.

Emerging biologics for the treatment of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a rare pulmonary vascular disorder, wherein mean systemic arterial pressure (mPAP) becomes abnormally high because of aberrant changes in various proliferative and inflammatory signalling pathways of pulmonary arterial cells. Currently used anti-PAH drugs chiefly target the vasodilatory and vasoconstrictive pathways. However, an imbalance between bone morphogenetic protein receptor type II (BMPRII) and transforming growth factor beta (TGF-β) pathways is also implicated in PAH predisposition and pathogenesis. Compared to currently used PAH drugs, various biologics have shown promise as PAH therapeutics that elicit their therapeutic actions akin to endogenous proteins. Biologics that have thus far been explored as PAH therapeutics include monoclonal antibodies, recombinant proteins, engineered cells, and nucleic acids. Because of their similarity with naturally occurring proteins and high binding affinity, biologics are more potent and effective and produce fewer side effects when compared with small molecule drugs. However, biologics also suffer from the limitations of producing immunogenic adverse effects. This review describes various emerging and promising biologics targeting the proliferative/apoptotic and vasodilatory pathways involved in PAH pathogenesis. Here, we have discussed sotatercept, a TGF-β ligand trap, which is reported to reverse vascular remodelling and reduce PVR with an improved 6-minute walk distance (6-MWDT). We also elaborated on other biologics including BMP9 ligand and anti-gremlin1 antibody, anti-OPG antibody, and getagozumab monoclonal antibody and cell-based therapies. Overall, recent literature suggests that biologics hold excellent promise as a safe and effective alternative to currently used PAH therapeutics.

Targeting extracellular vesicle delivery to the lungs by microgel encapsulation

Extracellular vesicles (EVs) secreted by stem and progenitor cells have significant potential as cell-free 'cellular' therapeutics. Yet, small EVs (<200 nm) are rapidly cleared after systemic administration, mainly by the liver, presenting challenges targeting EVs to a specific organ or tissue. Microencapsulation using natural nano-porous hydrogels (microgels) has been shown to enhance engraftment and increase the survival of transplanted cells. We sought to encapsulate EVs within microgels to target their delivery to the lung by virtue of their size-based retention within the pulmonary microcirculation. Mesenchymal stromal cell (MSC) derived EVs were labelled with the lipophilic dye (DiR) and encapsulated within agarose-gelatin microgels. Endothelial cells and bone marrow derived macrophages were able to take up EVs encapsulated in microgels in vitro, but less efficiently than the uptake of free EVs. Following intrajugular administration, microgel encapsulated EVs were selectively retained within the lungs for 72h, while free EVs were rapidly cleared by the liver. Furthermore, microgel-loaded EVs demonstrated greater uptake by lung cells, in particular CD45+ immune cells, as assessed by flow cytometry compared to free EVs. Microencapsulation of EVs may be a novel tool for enhancing the targeted delivery of EVs for future therapeutic applications.

Endothelial progenitor cell-derived exosomes inhibit pulmonary artery smooth muscle cell in vitro proliferation and resistance to apoptosis by modulating the Mitofusin-2 and Ras-Raf-ERK1/2 signaling pathway

Pulmonary arterial hypertension (PAH) mainly occurs as a result of abnormal proliferation and apoptosis resistance of pulmonary artery smooth muscle cells (PASMCs). Endothelial progenitor cell (EPC)-derived exosomes (Exos) (EPC-Exos) relieve PAH. However, there is still insufficient knowledge of whether EPC-Exos contribute to the pathological process of PAH, especially for PASMC repair. This study aimed to determine the effects of EPC-Exos on the proliferation, migration, and apoptosis of PASMCs and explore the possible underlying molecular mechanisms through bioinformatics analysis and in vitro testing. Bioinformatics analysis showed that the Ras signaling pathway and Exos were crucial in PAH. The PAH differential microRNAs (miRNAs) and miRNAs identified in EPC-Exos were intersected to obtain miR-21-5p. A target gene prediction program predicted mitofusin-2 (Mfn2) as a potential target of miR-21-5p. Cellular experiments demonstrated that EPC-Exos attenuated the viability, proliferation, migration, and apoptosis resistance of PASMCs under hypoxia. Mechanistically, EPC-Exos significantly upregulated Mfn2 expression and attenuated Ras-Raf-ERK1/2 signaling pathway activity. In conclusion, EPC-Exos suppress cell viability, proliferation, and migration and promote apoptosis in PASMCs under hypoxic conditions. It is possible to transport miR-21-5p to improve the expression of Mfn2 and inhibit the Ras-Raf-ERK1/2 signaling pathway directly or by targeting the expression of Mfn2. EPC-Exos are a potential therapeutic candidate for the treatment of PAH.

Novel Molecular Mechanisms Involved in the Medical Treatment of Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a severe condition with a high mortality rate despite advances in diagnostic and therapeutic strategies. In recent years, significant scientific progress has been made in the understanding of the underlying pathobiological mechanisms. Since current available treatments mainly target pulmonary vasodilation, but lack an effect on the pathological changes that develop in the pulmonary vasculature, there is need to develop novel therapeutic compounds aimed at antagonizing the pulmonary vascular remodeling. This review presents the main molecular mechanisms involved in the pathobiology of PAH, discusses the new molecular compounds currently being developed for the medical treatment of PAH and assesses their potential future role in the therapeutic algorithms of PAH.

What is the need and why is it time for innovative models for understanding lung repair and regeneration?

Advances in tissue engineering continue at a rapid pace and have provided novel methodologies and insights into normal cell and tissue homeostasis, disease pathogenesis, and new potential therapeutic strategies. The evolution of new techniques has particularly invigorated the field and span a range from novel organ and organoid technologies to increasingly sophisticated imaging modalities. This is particularly relevant for the field of lung biology and diseases as many lung diseases, including chronic obstructive pulmonary disease (COPD) and idiopathic fibrosis (IPF), among others, remain incurable with significant morbidity and mortality. Advances in lung regenerative medicine and engineering also offer new potential avenues for critical illnesses such as the acute respiratory distress syndrome (ARDS) which also continue to have significant morbidity and mortality. In this review, an overview of lung regenerative medicine with focus on current status of both structural and functional repair will be presented. This will serve as a platform for surveying innovative models and techniques for study, highlighting the need and timeliness for these approaches.

Radiation-induced cardiac side-effects: The lung as target for interacting damage and intervention

Radiotherapy is part of the treatment for many thoracic cancers. During this treatment heart and lung tissue can often receive considerable doses of radiation. Doses to the heart can potentially lead to cardiac effects such as pericarditis and myocardial fibrosis. Common side effects after lung irradiation are pneumonitis and pulmonary fibrosis. It has also been shown that lung irradiation has effects on cardiac function. In a rat model lung irradiation caused remodeling of the pulmonary vasculature increasing resistance of the pulmonary vascular bed, leading to enhanced pulmonary artery pressure, right ventricle hypertrophy and reduced right ventricle performance. Even more pronounced effects are observed when both, lung and heart are irradiated.

The effects observed after lung irradiation show striking similarities with symptoms of pulmonary arterial hypertension. In particular, the vascular remodeling in lung tissue seems to have similar underlying features. Here, we discuss the similarities and differences of vascular remodeling observed after thoracic irradiation compared to those in pulmonary arterial hypertension patients and research models. We will also assess how this knowledge of similarities could potentially be translated into interventions which would be beneficial for patients treated for thoracic tumors, where dose to lung tissue is often unavoidable.

Endothelial Senescence: A New Age in Pulmonary Hypertension

Pulmonary hypertension is an enigmatic, deleterious disease driven by multiple heterogeneous causes with a burgeoning proportion of older patients with complex, chronic comorbidities without adequate treatment options. The underlying endothelial pathophenotypes that direct vasoconstriction and panvascular remodeling remain both controversial and incompletely defined. This review discusses emerging concepts centered on endothelial senescence in pulmonary vascular disease. This principle proposes a more heterogeneous, dynamic pulmonary endothelium in disease; it provides a potentially unifying feature of endothelial dysfunction in pulmonary hypertension irrespective of cause; and it supports a clinically relevant link between aging and pulmonary hypertension like other chronic illnesses. Thus, taking cues from studies on aging and age-related diseases, we present possible opportunities and barriers to diagnostic and therapeutic targeting of senescence in pulmonary hypertension.

Model difference in the effect of cilostazol on the development of experimental pulmonary hypertension in rats

BACKGROUND

Preventing pulmonary vascular remodeling is a key strategy for pulmonary hypertension (PH). Causes of PH include pulmonary vasoconstriction and inflammation. This study aimed to determine whether cilostazol (CLZ), a phosphodiesterase-3 inhibitor, prevents monocrotaline (MCT)- and chronic hypoxia (CH)-induced PH development in rats.

METHODS

Fifty-one male Sprague-Dawley rats were fed rat chow with (0.3% CLZ) or without CLZ for 21 days after a single injection of MCT (60 mg/kg) or saline. Forty-eight rats were fed rat chow with and without CLZ for 14 days under ambient or hypobaric (air at 380 mmHg) CH exposure. The mean pulmonary artery pressure (mPAP), the right ventricle weight-to-left ventricle + septum weight ratio (RV/LV + S), percentages of muscularized peripheral pulmonary arteries (%Muscularization) and medial wall thickness of small muscular arteries (%MWT) were assessed. Levels of the endothelial nitric oxide synthase (eNOS), phosphorylated eNOS (peNOS), AKT, pAKT and IκB proteins in lung tissue were measured using Western blotting. Monocyte chemotactic protein (MCP)-1 mRNA in lung tissue was also assessed.

RESULTS

mPAP [35.1 ± 1.7 mmHg (MCT) (n = 9) vs. 16.6 ± 0.7 (control) (n = 9) (P < 0.05); 29.1 ± 1.5 mmHg (CH) (n = 10) vs. 17.5 ± 0.5 (control) (n = 10) (P < 0.05)], RV/LV + S [0.40 ± 0.01 (MCT) (n = 18) vs. 0.24 ± 0.01 (control) (n = 10) (P < 0.05); 0.41 ± 0.03 (CH) (n = 13) vs. 0.27 ± 0.06 (control) (n = 10) (P < 0.05)], and %Muscularization and %MWT were increased by MCT injection and CH exposure. CLZ significantly attenuated these changes in the MCT model [mPAP 25.1 ± 1.1 mmHg (n = 11) (P < 0.05), RV/LV + S 0.30 ± 0.01 (n = 14) (P < 0.05)]. In contrast, these CLZ effects were not observed in the CH model. Lung eNOS protein expression was unchanged in the MCT model and increased in the CH model. Lung protein expression of AKT, phosphorylated AKT, and IκB was downregulated by MCT, which was attenuated by CLZ; the CH model did not change these proteins. Lung MCP-1 mRNA levels were increased in MCT rats but not CH rats.

CONCLUSIONS

We found model differences in the effect of CLZ on PH development. CLZ might exert a preventive effect on PH development in an inflammatory PH model but not in a vascular structural change model of PH preceded by vasoconstriction. Thus, the preventive effect of CLZ on PH development might depend on the PH etiology.

"NOVEL MECHANISMS TARGETED BY DRUG TRIALS IN PULMONARY ARTERIAL HYPERTENSION"

Pulmonary arterial hypertension (PAH) is a rare disease associated with abnormally elevated pulmonary pressures and right heart failure resulting in high morbidity and mortality. While PAH prognosis has improved with the introduction of pulmonary vasodilators, disease progression remains a major problem. Given that available therapies are inadequate for preventing small vessel loss and obstruction, there is an active interest in identifying drugs capable of targeting angiogenesis and mechanisms involved in regulation of cell growth and fibrosis. Among the mechanisms linked to PAH pathogenesis, recent preclinical studies have identified promising compounds that are currently being tested in clinical trials. These drugs target seven of the major mechanisms associated with PAH pathogenesis: BMP signaling, tyrosine kinase receptors, estrogen metabolism, extracellular matrix, angiogenesis, epigenetics, and serotonin metabolism. In this review, we will discuss the preclinical studies that led to prioritization of these mechanisms and will discuss recently completed and ongoing phase 2/3 trials using novel interventions such as sotatercept, anastrozole, rodatristat ethyl, tyrosine kinase inhibitors, and endothelial progenitor cells among others. We anticipate that the next generation of compounds will build upon the success of the current standard of care and improve clinical outcomes and quality of life of patients afflicted with PAH.

Endothelial cells in the pathogenesis of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a devastating disease that involves pulmonary vasoconstriction, small vessel obliteration, large vessel thickening and obstruction, and development of plexiform lesions. PAH vasculopathy leads to progressive increases in pulmonary vascular resistance, right heart failure and, ultimately, premature death. Besides other cell types that are known to be involved in PAH pathogenesis (e.g. smooth muscle cells, fibroblasts and leukocytes), recent studies have demonstrated that endothelial cells (ECs) have a crucial role in the initiation and progression of PAH. The EC-specific role in PAH is multi-faceted and affects numerous pathophysiological processes, including vasoconstriction, inflammation, coagulation, metabolism and oxidative/nitrative stress, as well as cell viability, growth and differentiation. In this review, we describe how EC dysfunction and cell signalling regulate the pathogenesis of PAH. We also highlight areas of research that warrant attention in future studies, and discuss potential molecular signalling pathways in ECs that could be targeted therapeutically in the prevention and treatment of PAH.

Therapeutic Potential of Endothelial Progenitor Cells in Pulmonary Diseases

Compromised alveolar development and pulmonary vascular remodeling are hallmarks of pediatric lung diseases such as bronchopulmonary dysplasia (BPD) and alveolar capillary dysplasia with misalignment of pulmonary veins (ACDMPV). Although advances in surfactant therapy, corticosteroids, and anti-inflammatory drugs have improved clinical management of preterm infants, still those who suffer with severe vascular complications lack viable treatment options. Paucity of the alveolar capillary network in ACDMPV causes respiratory distress and leads to mortality in a vast majority of ACDMPV infants. The discovery of endothelial progenitor cells (EPCs) in 1997 brought forth the paradigm of postnatal vasculogenesis and hope for promoting vascularization in fragile patient populations, such as those with BPD and ACDMPV. The identification of diverse EPC populations, both hematopoietic and nonhematopoietic in origin, provided a need to identify progenitor cell selective markers which are linked to progenitor properties needed to develop cell-based therapies. Focusing to the future potential of EPCs for regenerative medicine, this review will discuss various aspects of EPC biology, beginning with the identification of hematopoietic, nonhematopoietic, and tissue-resident EPC populations. We will review knowledge related to cell surface markers, signature gene expression, key transcriptional regulators, and will explore the translational potential of EPCs for cell-based therapy for BPD and ACDMPV. The ability to produce pulmonary EPCs from patient-derived induced pluripotent stem cells (iPSCs) in vitro, holds promise for restoring vascular growth and function in the lungs of patients with pediatric pulmonary disorders.

Pediatric Pulmonary Hypertension: Definitions, Mechanisms, Diagnosis, and Treatment

Pediatric pulmonary hypertension (PPH) is a multifactorial disease with diverse etiologies and presenting features. Pulmonary hypertension (PH), defined as elevated pulmonary artery pressure, is the presenting feature for several pulmonary vascular diseases. It is often a hidden component of other lung diseases, such as cystic fibrosis and bronchopulmonary dysplasia. Alterations in lung development and genetic conditions are an important contributor to pediatric pulmonary hypertensive disease, which is a distinct entity from adult PH. Many of the causes of pediatric PH have prenatal onset with altered lung development due to maternal and fetal conditions. Since lung growth is altered in several conditions that lead to PPH, therapy for PPH includes both pulmonary vasodilators and strategies to restore lung growth. These strategies include optimal alveolar recruitment, maintaining physiologic blood gas tension, nutritional support, and addressing contributing factors, such as airway disease and gastroesophageal reflux. The outcome for infants and children with PH is highly variable and largely dependent on the underlying cause. The best outcomes are for neonates with persistent pulmonary hypertension (PPHN) and reversible lung diseases, while some genetic conditions such as alveolar capillary dysplasia are lethal. © 2021 American Physiological Society. Compr Physiol 11:2135-2190, 2021.

Progenitor/Stem Cells in Vascular Remodeling during Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is characterized by an important occlusive vascular remodeling with the production of new endothelial cells, smooth muscle cells, myofibroblasts, and fibroblasts. Identifying the cellular processes leading to vascular proliferation and dysfunction is a major goal in order to decipher the mechanisms leading to PAH development. In addition to in situ proliferation of vascular cells, studies from the past 20 years have unveiled the role of circulating and resident vascular in pulmonary vascular remodeling. This review aims at summarizing the current knowledge on the different progenitor and stem cells that have been shown to participate in pulmonary vascular lesions and on the pathways regulating their recruitment during PAH. Finally, this review also addresses the therapeutic potential of circulating endothelial progenitor cells and mesenchymal stem cells.

ACE2 gene transfer ameliorates vasoreparative dysfunction in CD34+ cells derived from diabetic older adults

Diabetes increases the risk for ischemic vascular diseases, which is further elevated in older adults. Bone marrow-derived hematopoietic CD34+ stem/progenitor cells have the potential of revascularization; however, diabetes attenuates vasoreparative functions. Angiotensin-converting enzyme 2 (ACE2) is the vasoprotective enzyme of renin-angiotensin system in contrast with the canonical angiotensin-converting enzyme (ACE). The present study tested the hypothesis that diabetic dysfunction is associated with ACE2/ACE imbalance in hematopoietic stem/progenitor cells (HSPCs) and that increasing ACE2 expression would restore reparative functions. Blood samples from male and female diabetic (n=71) or nondiabetic (n=62) individuals were obtained and CD34+ cells were enumerated by flow cytometry. ACE and ACE2 enzyme activities were determined in cell lysates. Lentiviral (LV) approach was used to increase the expression of soluble ACE2 protein. Cells from diabetic older adults (DB) or nondiabetic individuals (Control) were evaluated for their ability to stimulate revascularization in a mouse model of hindlimb ischemia (HLI). DB cells attenuated the recovery of blood flow to ischemic areas in nondiabetic mice compared with that observed with Control cells. Administration of DB cells modified with LV-ACE2 resulted in complete restoration of blood flow. HLI in diabetic mice resulted in poor recovery with amputations, which was not reversed by either Control or DB cells. LV-ACE2 modification of Control or DB cells resulted in blood flow recovery in diabetic mice. In vitro treatment with Ang-(1-7) modified paracrine profile in diabetic CD34+ cells. The present study suggests that vasoreparative dysfunction in CD34+ cells from diabetic older adults is associated with ACE2/ACE imbalance and that increased ACE2 expression enhances the revascularization potential.

Genetic Delivery and Gene Therapy in Pulmonary Hypertension

Pulmonary hypertension (PH) is a progressive complex fatal disease of multiple etiologies. Hyperproliferation and resistance to apoptosis of vascular cells of intimal, medial, and adventitial layers of pulmonary vessels trigger excessive pulmonary vascular remodeling and vasoconstriction in the course of pulmonary arterial hypertension (PAH), a subgroup of PH. Multiple gene mutation/s or dysregulated gene expression contribute to the pathogenesis of PAH by endorsing the proliferation and promoting the resistance to apoptosis of pulmonary vascular cells. Given the vital role of these cells in PAH progression, the development of safe and efficient-gene therapeutic approaches that lead to restoration or down-regulation of gene expression, generally involved in the etiology of the disease is the need of the hour. Currently, none of the FDA-approved drugs provides a cure against PH, hence innovative tools may offer a novel treatment paradigm for this progressive and lethal disorder by silencing pathological genes, expressing therapeutic proteins, or through gene-editing applications. Here, we review the effectiveness and limitations of the presently available gene therapy approaches for PH. We provide a brief survey of commonly existing and currently applicable gene transfer methods for pulmonary vascular cells in vitro and describe some more recent developments for gene delivery existing in the field of PH in vivo.

Inflammation in Pulmonary Arterial Hypertension

Pulmonary artery hypertension (PAH) is a devastating cardiopulmonary disease characterized by vascular remodeling and obliteration of the precapillary pulmonary arterioles. Alterations in the structure and function of pulmonary vessels result in the resistance of blood flow and can progress to right-sided heart failure, causing significant morbidity and mortality. There are several types of PAH, and the disease can be familial or secondary to an underlying medical condition such as a connective tissue disorder or infection. Regardless of the cause, the exact pathophysiology and cellular interactions responsible for disease development and progression are largely unknown.There is significant evidence to suggest altered immune and vascular cells directly participate in disease progression. Inflammation has long been hypothesized to play a vital role in the development of PAH, as an altered or skewed immune response favoring a proinflammatory environment that can lead to the infiltration of cells such as lymphocytes, macrophages, and neutrophils. Current treatment strategies focus on the dilation of partially occluded vessels; however, such techniques have not resulted in an effective strategy to reverse or prevent vascular remodeling. Therefore, current studies in human and animal models have attempted to understand the underlying pathophysiology of pulmonary hypertension (PH), specifically focusing on the inflammatory cascade predisposing patients to disease so that better therapeutic targets can be developed to potentially reverse or prevent disease progression.The purpose of this chapter is to provide a comprehensive review of the expanding literature on the inflammatory process that participates in PH development while highlighting important and current studies in both animal and human models. While our primary focus will be on cells found in the adaptive and innate immune system, we will review all potential causes of PAH, including cells of the endothelium, pulmonary lymphatics, and genetic mutations predisposing patients. In addition, we will discuss current therapeutic options while highlighting potential future treatments and the questions that still remain unanswered.

Stem/Progenitor Cells and Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a progressive disease characterized by endothelial dysfunction and vascular remodeling. Despite significant advancement in our understanding of the pathogenesis of PAH in recent years, treatment options for PAH are limited and their prognosis remains poor. PAH is now seen as a severe pulmonary arterial vasculopathy with structural changes driven by excessive vascular proliferation and inflammation. Perturbations of a number of cellular and molecular mechanisms have been described, including pathways involving growth factors, cytokines, metabolic signaling, elastases, and proteases, underscoring the complexity of the disease pathogenesis. Interestingly, emerging evidence suggests that stem/progenitor cells may have an impact on disease development and therapy. In preclinical studies, stem/progenitor cells displayed an ability to promote endothelial repair of dysfunctional arteries and induce neovascularization. The stem cell-based therapy for PAH are now under active investigation. This review article will briefly summarize the updates in the research field, with a special focus on the contribution of stem/progenitor cells to lesion formation via influencing vascular cell functions and highlight the potential clinical application of stem/progenitor cell therapy to PAH.

Current and emerging therapeutic approaches to pulmonary hypertension

Pulmonary arterial hypertension (PAH) is a progressive and fatal lung disease of multifactorial etiology. Most of the available drugs and FDA-approved therapies for treating pulmonary hypertension attempt to overcome the imbalance between vasoactive and vasodilator mediators, and restore the endothelial cell function. Traditional medications for treating PAH include the prostacyclin analogs and receptor agonists, phosphodiesterase 5 inhibitors, endothelin-receptor antagonists, and cGMP activators. While the current FDA-approved drugs showed improvements in quality of life and hemodynamic parameters, they have shown only very limited beneficial effects on survival and disease progression. None of them offers a cure against PAH, and the median survival rate remains less than three years from diagnosis. Extensive research efforts have led to the emergence of innovative therapeutic approaches in the area of PAH. In this review, we provide an overview of the current FDA-approved therapies in PAH and discuss the associated clinical trials and reported-side effects. As recent studies have led to the emergence of innovative therapeutic approaches in the area of PAH, we also focus on the latest promising therapies in preclinical studies such as stem cell-based therapies, gene transfer, and epigenetic therapies.

Translating Basic Research into Safe and Effective Cell-based Treatments for Respiratory Diseases

Respiratory diseases, such as chronic obstructive pulmonary disease and pulmonary fibrosis, result in severely impaired quality of life and impose significant burdens on healthcare systems worldwide. Current disease management involves pharmacologic interventions, oxygen administration, reduction of infections, and lung transplantation in advanced disease stages. An increasing understanding of mechanisms of respiratory epithelial and pulmonary vascular endothelial maintenance and repair and the underlying stem/progenitor cell populations, including but not limited to airway basal cells and type II alveolar epithelial cells, has opened the possibility of cell replacement-based regenerative approaches for treatment of lung diseases. Further potential for personalized therapies, including in vitro drug screening, has been underscored by the recent derivation of various lung epithelial, endothelial, and immune cell types from human induced pluripotent stem cells. In parallel, immunomodulatory treatments using allogeneic or autologous mesenchymal stromal cells have shown a good safety profile in clinical investigations for acute inflammatory conditions, such as acute respiratory distress syndrome and septic shock. However, as yet, no cell-based therapy has been shown to be both safe and effective for any lung disease. Despite the investigational status of cell-based interventions for lung diseases, businesses that market unproven, unlicensed and potentially harmful cell-based interventions for respiratory diseases have proliferated in the United States and worldwide. The current status of various cell-based regenerative approaches for lung disease as well as the effect of the regulatory environment on clinical translation of such approaches are presented and critically discussed in this review.

Evidence of Accumulated Endothelial Progenitor Cells in the Lungs of Rats with Pulmonary Arterial Hypertension by 89Zr-oxine PET Imaging

Endothelial progenitor cells (EPCs) play a major role in regulating pulmonary vascular remodeling during pulmonary arterial hypertension (PAH) development. Several preclinical and clinical trials of EPCs transplantation have been performed for the treatment of PAH. However, there is no reliable method to monitor real-time cell trafficking and quantify transplanted EPCs. Here in this paper we isolated EPCs from human peripheral blood, identified their functional integrity, and efficiently labeled the EPCs with ⁸⁹Zr-oxine and DiO. Labeled EPCs were injected into the tail vein of normal and PAH rats to be tracked in vivo. From the microPET/CT images, we found EPCs were distributed primarily in the lung at 1 h and then migrated to the liver and spleen. We could observe the 3,3′ dioctadecyloxacarbocyanine perchlorate (DiO)-labeled EPCs binding in the pulmonary vasculature by CellVizio confocal. The result of quantitative analysis revealed significantly higher accumulation of EPCs in the lungs of PAH rats than in those of healthy rats. The distribution and higher accumulation of EPCs in the lungs of PAH rats could help to evaluate the safety and provide evidence of effectiveness of EPC therapy.

A non-selective endothelin receptor antagonist bosentan modulates kinetics of bone marrow-derived cells in ameliorating pulmonary hypertension in mice

The aim of this study was to investigate whether a dual endothelin receptor antagonist bosentan modulates the kinetics of bone marrow-derived stem cells in inhibiting the development of pulmonary hypertension. Bone marrow chimeric mice, transplanted with enhanced green fluorescent protein (eGFP)-positive bone marrow mononuclear cells, were exposed to hypobaric hypoxia or kept in the ambient air, and were daily treated with bosentan sodium salt or saline for 21 days. After the treatment period, right ventricular pressure was measured and pulmonary vascular morphometry was conducted. Incorporation of bone marrow-derived cells was analyzed by immunohistochemistry. Gene expression and protein level in the lung tissue were evaluated by quantitative real-time PCR and western blotting, respectively. The results showed that, in hypoxic mice, right ventricular pressure and the percentage of muscularized vessel were increased and pulmonary vascular density was decreased, each of which was reversed by bosentan. Bone marrow-derived endothelial cells and macrophages in lungs were increased by hypoxia. Bosentan promoted bone marrow-derived endothelial cell incorporation but inhibited macrophage infiltration into lungs. Quantitative real-time PCR analysis revealed that interleukin 6, stromal cell-derived factor-1, and monocyte chemoattractant protein-1 were upregulated by hypoxia, in which interleukin 6 and monocyte chemoattractant protein-1 were downregulated and stromal cell-derived factor-1 was upregulated by bosentan. Protein level of endothelial nitric oxide synthase (eNOS) in the whole lung was significantly upregulated by hypoxia, which was further upregulated by bosentan. Bosentan modulated kinetics of bone marrow-derived ECs and macrophages and related gene expression in lungs in ameliorating pulmonary hypertension in mice. Altered kinetics of bone marrow-derived stem cells may be a novel mechanism of the endothelin receptor blockade in vivo and confer a new understanding of the therapeutic basis for pulmonary hypertension.

Drug Treatment of Pulmonary Hypertension in Children

Pulmonary arterial hypertension (PAH) is a rare disease in infants and children that is associated with significant morbidity and mortality. The disease is characterized by progressive pulmonary vascular functional and structural changes resulting in increased pulmonary vascular resistance and eventual right heart failure and death. In many pediatric patients, PAH is idiopathic or associated with congenital heart disease and rarely is associated with other conditions such as connective tissue or thromboembolic disease. PAH associated with developmental lung diseases such as bronchopulmonary dysplasia or congenital diaphragmatic hernia is increasingly more recognized in infants and children. Although treatment of the underlying disease and reversal of advanced structural changes have not yet been achieved with current therapy, quality of life and survival have improved significantly. Targeted pulmonary vasodilator therapies, including endothelin receptor antagonists, prostacyclin analogs, and phosphodiesterase type 5 inhibitors have resulted in hemodynamic and functional improvement in children. The management of pediatric PAH remains challenging as treatment decisions depend largely on results from evidence-based adult studies and the clinical experience of pediatric experts. This article reviews the current drug therapies and their use in the management of PAH in children.

Schistosomiasis-associated pulmonary arterial hypertension: a systematic review

Schistosomiasis-associated pulmonary arterial hypertension (Sch-PAH) is a life-threatening complication of chronic hepatosplenic schistosomiasis. It is suggested to be the leading cause of pulmonary arterial hypertension (PAH) worldwide. However, pathophysiological data on Sch-PAH are scarce. We examined the hypothesis that there are pronounced similarities in pathophysiology, haemodynamics, and survival of Sch-PAH and idiopathic PAH (iPAH).

This systematic review and meta-analysis was registered in the PROSPERO database (identifier CRD42018104066). A systematic search and review of the literature was performed according to PRISMA guidelines for studies published between 01 January 1990 and 29 June 2018.

For Sch-PAH, 18 studies evaluating pathophysiological mechanisms, eight studies on haemodynamics (n=277), and three studies on survival (n=191) were identified. 16 clinical registries reporting data on haemodynamics and survival including a total of 5792 patients with iPAH were included for comparison. Proinflammatory molecular pathways are involved in both Sch-PAH and iPAH. The transforming growth factor (TGF)-β signalling pathway is upregulated in Sch-PAH and iPAH. While there was no difference in mean pulmonary artery pressure (54±17 mmHg versus 55±15 mmHg, p=0.29), cardiac output (4.4±1.3 L·min−1versus 4.1±1.4 L·min−1, p=0.046), and cardiac index (2.6±0.7 L·min−1·m−2versus 2.3±0.8 L·min−1·m−2, p<0.001) were significantly higher in Sch-PAH compared to iPAH, resulting in a lower pulmonary vascular resistance in Sch-PAH (10±6 Woods units versus 13±7 Woods units, p<0.001). 1- and 3-year survival were significantly better in the Sch-PAH group (p<0.001).

Sch-PAH and iPAH share common pathophysiological mechanisms related to inflammation and the TGF-β signalling pathway. Patients with Sch-PAH show a significantly better haemodynamic profile and survival than patients with iPAH.

Involvement of miR-200b-PKCα signalling in pulmonary hypertension in cor pulmonale model

The right ventricle (RV) enlargement and pulmonary fibrosis are involved in cor pulmonale. The role of miR-200b in cor pulmonale is less well understood. This study was designed to evaluate the regulatory roles of miR-200b in cor pulmonale. Cor pulmonary mouse model was built via monocrotaline injection of monocrotaline (MCT). The expression of miR-200b in the lungs, RV and left ventricle (LV) are using real-time polymerase chain reaction. The transthoracic echocardiography was employed to determine the effects of miR-200b mimics and Gö6976 injection on MCT mice. The protein levels of protein kinase C α (PKCα), collagen, and fibronectin in the lung, RV, and LV in the mice with and without miR-200b mimics and Gö6976 injection were evaluated using western blot. The expression of miR-200b decreased in MCT mice, while there was no difference in LV. Both the miR-200b mimics and Gö6976 injection reversed the muscularization in the pulmonary artery, reversed RV hypertrophy, reduced RV systolic pressure, wall thickness and pulmonary fibrosis. The injection of miR-200b can reduce the PKCα expression in the lung, RV, and LV. This study confirmed the down-regulation of miR-200b in cor pulmonale. The reverse effects of miR-200b in the present study may provide a potential tool for cor pulmonary treatment.

Targeted Delivery of Sildenafil for Inhibiting Pulmonary Vascular Remodeling

Pulmonary arterial hypertension is a fatal lung disease caused by the progressive remodeling of small pulmonary arteries (PAs). Sildenafil can prevent the remodeling of PAs, but conventional sildenafil formulations have shown limited treatment efficacy for their poor accumulation in PAs. Here, glucuronic acid (GlcA)-modified liposomes (GlcA-Lips) were developed to improve the delivery of sildenafil to aberrant over-proliferative PA smooth muscle cells via targeting the GLUT-1 (glucose transport-1), and, therefore, inhibiting the remodeling of PAs in a monocrotaline-induced PA hypertension model. GlcA-Lips encapsulating sildenafil (GlcA-sildenafil-Lips) had a size of 90 nm and a pH-sensitive drug release pattern. Immunostaining assay indicated the overexpression of GLUT-1 in PA smooth muscle cells. Cellular uptake studies showed a 1-fold increase of GlcA-Lips uptake by PA smooth muscle cells and pharmacokinetics and biodistribution experiments indicated longer blood circulation time of GlcA-Lips and increased ability to target PAs by 1-fold after 8 hours administration. Two-week treatment indicated GlcA-sildenafil-Lips significantly inhibited the remodeling of PAs, with a 32% reduction in the PA pressure, a 41% decrease in the medial thickening, and a 44% reduction of the right ventricle cardiomyocyte hypertrophy, and improved survival rate. Immunohistochemical analysis showed enhanced expression of caspase-3, after administration of GlcA-sildenafil-Lips, and reduced expression of P-ERK1/2 (phosphorylated ERK1/2) and HK-2 (hexokinase-2), and increased level of eNOS (endothelial nitric oxide synthase) and cyclic GMP (cGMP). In conclusion, targeted delivery of sildenafil to PA smooth muscle cells with GlcA-Lips could effectively inhibit the remodeling of PAs in the monocrotaline-induced PA hypertension.

Blood outgrowth endothelial cells overexpressing eNOS mitigate pulmonary hypertension in rats: a unique carrier cell enabling autologous cell-based gene therapy

We have investigated a unique cell type, blood outgrowth endothelial cells (BOEC), as a cell-based gene therapy approach to pulmonary hypertension. BOEC are bona fide endothelial cells, obtained from peripheral blood, that can be expanded to vast numbers, and are amenable to both cryopreservation and genetic modification. We established primary cultures of rat BOEC and genetically altered them to over-express human eNOS plus green fluorescent protein (rBOEC/eNOS) or to express GFP only (rBOEC/GFP). We gave monocrotaline to rats on day 0, and they developed severe pulmonary hypertension. As a Prevention model, we infused saline or rBOEC/GFP or rBOEC/eNOS on day 3, and then examined endpoints on day 24. The rBOEC/eNOS recipients developed elevated NOx (serum and lung) and less severe: elevation of right ventricular systolic pressure (RVSP), right ventricular hypertrophy, and pulmonary arteriolar muscularization and loss of alveolar density. As an Intervention model, we waited until day 21 to give the test infusions, and we examined endpoints on day 35. The rBOEC/eNOS recipients again developed elevated NOx and manifested the same improvements. Indeed, rBOEC/eNOS infusion not only prevented worsening of RVSP but also partially reversed established arteriolar muscularization. These data suggest that BOEC may be useful as a carrier cell for genetic strategies targeting pulmonary hypertension. Their properties render BOEC amenable to preclinical and scale-up studies, available for autologous therapies, and tolerant of modification and storage for potential future use in patients at risk for PAH, eg, as defined by genetics or medical condition.

Combination therapy improves vascular volume in female rats with pulmonary hypertension

Pulmonary arterial hypertension (PAH) is a female predominant disease in which progressive vascular remodeling and vasoconstriction result in right ventricular (RV) failure and death. Most PAH patients utilize multiple therapies. In contrast, the majority of preclinical therapeutic studies are performed in male rats with a single novel drug often markedly reversing disease in the model. We sought to differentiate single drug therapy from combination therapy in female rats with severe disease. One week after left pneumonectomy, we induced PH in young female Sprague-Dawley rats with an injection of monocrotaline (45 mg/kg). Female rats were then randomized to receive combination therapy (ambrisentan plus tadalafil), ambrisentan monotherapy, tadalafil monotherapy, or vehicle. We measured RV size and function on two serial echocardiograms during the development of disease. We measured RV systolic pressure (RVSP) invasively at day 28 after monocrotaline before analyzing the vascular volume with microcomputed tomography (microCT) of the right middle lobe. RVSP was significantly lower in female rats treated with combination therapy, and combination therapy resulted in increased small vessel volume density measured by microCT compared with untreated rats. Combination-treated rats had the smallest RV end-diastolic diameter on echocardiogram as compared with the other groups. In summary, we report a female model of pulmonary hypertension that can distinguish between one and two drug therapies; this model may facilitate better preclinical drug testing for novel compounds.

The Search for Disease-Modifying Therapies in Pulmonary Hypertension

Pulmonary hypertension (PH) and its severe subtype pulmonary arterial hypertension (PAH) encompass a set of multifactorial diseases defined by sustained elevation of pulmonary arterial pressure and pulmonary vascular resistance leading to right ventricular failure and subsequent death. Pulmonary hypertension is characterized by vascular remodeling in association with smooth muscle cell proliferation of the arterioles, medial thickening, and plexiform lesion formation. Despite our recent advances in understanding its pathogenesis and related therapeutic discoveries, PH still remains a progressive disease without a cure. Nevertheless, development of drugs that specifically target molecular pathways involved in disease pathogenesis has led to improvement in life quality and clinical outcomes in patients with PAH. There are presently more than 12 Food and Drug Administration-approved vasodilator drugs in the United States for the treatment of PAH; however, mortality with contemporary therapies remains high. More recently, there have been exuberant efforts to develop new pharmacologic therapies that target the fundamental origins of PH and thus could represent disease-modifying opportunities. This review aims to summarize recent developments on key signaling pathways and molecular targets that drive PH disease progression, with emphasis on new therapeutic options under development.

Regenerative cell therapy for pulmonary arterial hypertension in animal models: a systematic review

Background

Pulmonary arterial hypertension (PAH) is a rare disease characterized by widespread loss of the pulmonary microcirculation and elevated pulmonary arterial pressures leading to pathological right ventricular remodeling and ultimately right heart failure. Regenerative cell therapies could potentially restore the effective lung microcirculation and provide a curative therapy for PAH. The objective of this systematic review was to compare the efficacy of regenerative cell therapies in preclinical models of PAH.

Methods

A systematic search strategy was developed and executed. We included preclinical animal studies using regenerative cell therapy in experimental models of PAH. Primary outcomes were right ventricular systolic pressure (RVSP) and mean pulmonary arterial pressure (mPAP). The secondary outcome was right ventricle/left ventricle + septum weight ratio (RV/LV+S). Pooled effect sizes were undertaken using random effects inverse variance models. Risk of bias and publication bias were assessed.

Results

The systematic search yielded 1285 studies, of which 44 met eligibility criteria. Treatment with regenerative cell therapy was associated with decreased RVSP (SMD − 2.10; 95% CI − 2.59 to − 1.60), mPAP (SMD − 2.16; 95% CI − 2.97 to − 1.35), and RV/LV+S (SMD − 1.31, 95% CI − 1.64 to − 0.97). Subgroup analysis demonstrated that cell modification resulted in greater reduction in RVSP. The effects on RVSP and mPAP remained statistically significant even after adjustment for publication bias. The majority of studies had an unclear risk of bias.

Conclusions

Preclinical studies of regenerative cell therapy demonstrated efficacy in animal models of PAH; however, future studies should consider incorporating design elements to reduce the risk of bias.

Systematic review registration

Suen CM, Zhai A, Lalu MM, Welsh C, Levac BM, Fergusson D, McIntyre L and Stewart DJ. Efficacy and safety of regenerative cell therapy for pulmonary arterial hypertension in animal models: a preclinical systematic review protocol. Syst Rev. 2016;5:89.

Trial registration

CAMARADES-NC3Rs Preclinical Systematic Review & Meta-analysis Facility (SyRF). http://syrf.org.uk/protocols/. Syst Rev 5:89, 2016

Electronic supplementary material

The online version of this article (10.1186/s13287-019-1172-6) contains supplementary material, which is available to authorized users.

New and Emerging Therapies for Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a pulmonary vasculopathy that causes right ventricular dysfunction and exercise limitation and progresses to death. New findings from translational studies have suggested alternative pathways for treatment. These avenues include sex hormones, genetic abnormalities and DNA damage, elastase inhibition, metabolic dysfunction, cellular therapies, and anti-inflammatory approaches. Both novel and repurposed compounds with rationale from preclinical experimental models and human cells are now in clinical trials in patients with PAH. Findings from these studies will elucidate the pathobiology of PAH and may result in clinically important improvements in outcome.

Paving the Road for Mesenchymal Stem Cell-Derived Exosome Therapy in Bronchopulmonary Dysplasia and Pulmonary Hypertension

Bronchopulmonary dysplasia (BPD) is a chronic neonatal lung disease characterized by inflammation and arrest of alveolarization. Its common sequela, pulmonary hypertension (PH), presents with elevated pulmonary vascular resistance associated with remodeling of the pulmonary arterioles. Despite notable advancements in neonatal medicine, there is a severe lack of curative treatments to help manage the progressive nature of these diseases. Numerous studies in preclinical models of BPD and PH have demonstrated that therapies based on mesenchymal stem/stromal cells (MSCs) can resolve pulmonary inflammation and ameliorate the severity of disease. Recent evidence suggests that novel, cell-free approaches based on MSC-derived exosomes (MEx) might represent a compelling therapeutic alternative offering major advantages over treatments based on MSC transplantation. Here, we will discuss the development of MSC-based therapies, stressing the centrality of paracrine action as the actual vector of MSC therapeutic functionality, focusing on MEx. We will briefly present our current understanding of the biogenesis and secretion of MEx, and discuss potential mechanisms by which they afford such beneficial effects, including immunomodulation and restoration of homeostasis in diseased states. We will also review ongoing clinical trials using MSCs as treatment for BPD that pave the way for bringing cell-free, MEx-based therapeutics from the bench to the NICU setting.

Myeloid angiogenic cells exhibit impaired migration, reduced expression of endothelial markers, and increased apoptosis in idiopathic pulmonary arterial hypertension 1

Idiopathic pulmonary arterial hypertension (IPAH) is a rare and devastating condition. There is no known cure for IPAH, and current treatment options are not always effective. Autologous myeloid angiogenic cells (MACs) have been explored as a novel therapy for IPAH, but preliminary data from clinical trials show limited beneficial effects. A complete understanding of IPAH MAC function remains elusive. This study was designed to comprehensively compare cell function between IPAH MACs and healthy control MACs. MACs were procured through the culture of peripheral blood mononuclear cells in endothelial selective medium for 7 days. Compared with healthy MACs, IPAH MACs exhibited (1) significantly lower levels of endothelial markers as shown by fluorescence microscopy; (2) a markedly higher rate of apoptosis under both normal culture condition and serum starvation as shown by the TUNEL assay; (3) significantly decreased migration towards vascular endothelial growth factor as shown by a modified Boyden chamber migration assay; and (4) similar vascular endothelial growth factor and endothelial nitric oxide synthase mRNA levels as shown by reverse transcription quantitative PCR. In conclusion, various aspects of IPAH MAC function are impaired. To achieve greater therapeutic benefits, pharmacologic and (or) genetic manipulations to improve IPAH MAC function, particularly to promote cell survival and migration, are warranted.

Human Mesenchymal Stem Cell Therapy Reverses Su5416/Hypoxia-Induced Pulmonary Arterial Hypertension in Mice

Aims: Pulmonary arterial hypertension (PAH) is a disease characterized by an increase in pulmonary vascular resistance and right ventricular (RV) failure. We aimed to determine the effects of human mesenchymal stem cell (hMSC) therapy in a SU5416/hypoxia (SuH) mice model of PAH. Methods and Results: C57BL/6 mice (20-25 g) were exposure to 4 weeks of hypoxia combined vascular endothelial growth factor receptor antagonism (20 mg/kg SU5416; weekly s.c. injections; PAH mice). Control mice were housed in room air. Following 2 weeks of SuH exposure, we injected 5 × 10⁵ hMSCs cells suspended in 50 μL of vehicle (0.6 U/mL DNaseI in PBS) through intravenous injection in the caudal vein. PAH mice were treated only with vehicle. Ratio between pulmonary artery acceleration time and RV ejection time (PAAT/RVET), measure by echocardiography, was significantly reduced in the PAH mice, compared with controls, and therapy with hMSCs normalized this. Significant muscularization of the PA was observed in the PAH mice and hMSC reduced the number of fully muscularized vessels. RV free wall thickness was higher in PAH animals than in the controls, and a single injection of hMSCs reversed RV hypertrophy. Levels of markers of exacerbated apoptosis, tissue inflammation and damage, cell proliferation and oxidative stress were significantly greater in both lungs and RV tissues from PAH group, compared to controls. hMSC injection in PAH animals normalized the expression of these molecules which are involved with PAH and RV dysfunction development and the state of chronicity. Conclusion: These results indicate that hMSCs therapy represents a novel strategy for the treatment of PAH in the future.

Blood Outgrowth and Proliferation of Endothelial Colony Forming Cells are Related to Markers of Disease Severity in Patients with Pulmonary Arterial Hypertension

In pulmonary arterial hypertension (PAH), lung-angioproliferation leads to increased pulmonary vascular resistance, while simultaneous myocardial microvessel loss contributes to right ventricular (RV) failure. Endothelial colony forming cells (ECFC) are highly proliferative, angiogenic cells that may contribute to either pulmonary vascular obstruction or to RV microvascular adaptation. We hypothesize ECFC phenotypes (outgrowth, proliferation, tube formation) are related to markers of disease severity in a prospective cohort-study of 33 PAH and 30 healthy subjects. ECFC were transplanted in pulmonary trunk banded rats with RV failure. The presence of ECFC outgrowth in PAH patients was associated with low RV ejection fraction, low central venous saturation and a shorter time to clinical worsening (5.4 months (0.6–29.2) vs. 36.5 months (7.4–63.4), p = 0.032). Functionally, PAH ECFC had higher proliferative rates compared to control in vitro, although inter-patient variability was high. ECFC proliferation was inversely related to RV end diastolic volume (R² = 0.39, p = 0.018), but not pulmonary vascular resistance. Tube formation-ability was similar among donors. Normal and highly proliferative PAH ECFC were transplanted in pulmonary trunk banded rats. While no effect on hemodynamic measurements was observed, RV vascular density was restored. In conclusion, we found that ECFC outgrowth associates with high clinical severity in PAH, suggesting recruitment. Transplantation of highly proliferative ECFC restored myocardial vascular density in pulmonary trunk banded rats, while RV functional improvements were not observed.

A pro-con debate: current controversies in PAH pathogenesis at the American Thoracic Society International Conference in 2017

The following review summarizes the pro-con debate about current controversies regarding the pathogenesis of pulmonary arterial hypertension (PAH) that took place at the American Thoracic Society Conference in May 2017. Leaders in the field of PAH research discussed the importance of the immune system, the role of hemodynamic stress and endothelial apoptosis, as well as bone morphogenetic protein receptor-2 signaling in PAH pathogenesis. Whereas this summary does not intend to resolve obvious conflicts in opinion, we hope that the presented arguments entice further discussions and draw a new generation of enthusiastic researchers into this vibrant field of science to bridge existing gaps for a better understanding and therapy of this fatal disease.

Bone Marrow-Derived Endothelial Progenitor Cells Contribute to Monocrotaline-Induced Pulmonary Arterial Hypertension in Rats via Inhibition of Store-Operated Ca2+ Channels

Purpose

This study aimed to explore whether bone marrow- (BM-) derived endothelial progenitor cells (EPCs) contributing to monocrotaline- (MCT-) induced pulmonary arterial hypertension (PAH) in rats via modulating store-operated Ca²⁺ channels (SOC).

Methods

Sprague Dawley (SD) rats were assigned into MCT group (n = 30) and control group (n = 20). Rats in MCT group were subcutaneously administered with 60 mg/kg MCT solution, and rats in control group were injected with equal amount of vehicle. After 3 weeks of treatment, right ventricular systolic pressure (RVSP) and right ventricular hypertrophy index (RVHI) of two groups were measured, and BM-derived EPCs were isolated. Immunochemistry identification and vasculogenesis detection of EPCs were then performed. [Ca²⁺]_cyt measurement was performed to detect store-operated calcium entry (SOCE) in two groups, followed by determination of Orai and canonical transient receptor potential (TRPC) channels expression.

Results

After 3 weeks of treatment, there were significant increases in RVSP and RVHI in MCT group compared with control group, indicating that MCT successfully induced PAH in rats. Moreover, the SOCE ([Ca²⁺]_cyt rise) in BM-derived EPCs of MCT group was lower than that of control group. Furthermore, the expression levels of Orai3, TRPC1, TRPC3, and TRPC6 in BM-derived EPCs were decreased in MCT group in comparison with control group.

Conclusions

The SOC activities were inhibited in BM-derived EPCs of MCT-treated rats. These results may be associated with the depressed expression of Orai3, TRPC1, TRPC3, and TRPC6, which are major mediators of SOC.

Myeloid-Derived Suppressor Cells and Pulmonary Hypertension

Myeloid–derived suppressor cells (MDSCs) comprised a heterogeneous subset of bone marrow–derived myeloid cells, best studied in cancer research, that are increasingly implicated in the pathogenesis of pulmonary vascular remodeling and the development of pulmonary hypertension. Stem cell transplantation represents one extreme interventional strategy for ablating the myeloid compartment but poses a number of translational challenges. There remains an outstanding need for additional therapeutic targets to impact MDSC function, including the potential to alter interactions with innate and adaptive immune subsets, or alternatively, alter trafficking receptors, metabolic pathways, and transcription factor signaling with readily available and safe drugs. In this review, we summarize the current literature on the role of myeloid cells in the development of pulmonary hypertension, first in pulmonary circulation changes associated with myelodysplastic syndromes, and then by examining intrinsic myeloid cell changes that contribute to disease progression in pulmonary hypertension. We then outline several tractable targets and pathways relevant to pulmonary hypertension via MDSC regulation. Identifying these MDSC-regulated effectors is part of an ongoing effort to impact the field of pulmonary hypertension research through identification of myeloid compartment-specific therapeutic applications in the treatment of pulmonary vasculopathies.

Stem cell therapy targeting the right ventricle in pulmonary arterial hypertension: is it a potential avenue of therapy?

Pulmonary arterial hypertension (PAH) is an incurable disease characterized by an increase in pulmonary arterial pressure due to pathological changes to the pulmonary vascular bed. As a result, the right ventricle (RV) is subject to an increased afterload and undergoes multiple changes, including a decrease in capillary density. All of these dysfunctions lead to RV failure. A number of studies have shown that RV function is one of the main prognostic factors for PAH patients. Many stem cell therapies targeting the left ventricle are currently undergoing development. The promising results observed in animal models have led to clinical trials that have shown an improvement of cardiac function. In contrast to left heart disease, stem cell therapy applied to the RV has remained poorly studied, even though it too may provide a therapeutic benefit. In this review, we discuss stem cell therapy as a treatment for RV failure in PAH. We provide an overview of the results of preclinical and clinical studies for RV cell therapies. Although a large number of studies have targeted the pulmonary circulation rather than the RV directly, there are nonetheless encouraging results in the literature that indicate that cell therapies may have a direct beneficial effect on RV function. This cell therapy strategy may therefore hold great promise and warrants further studies in PAH patients.

Molecular pathogenesis and current pathology of pulmonary hypertension

Following its initial description over a century ago, pulmonary arterial hypertension (PAH) continues to challenge researchers committed to understanding its pathobiology and finding a cure. The last two decades have seen major developments in our understanding of the genetics and molecular basis of PAH that drive cells within the pulmonary vascular wall to produce obstructive vascular lesions; presently, the field of PAH research has taken numerous approaches to dissect the complex amalgam of genetic, molecular and inflammatory pathways that interact to initiate and drive disease progression. In this review, we discuss the current understanding of PAH pathology and the role that genetic factors and environmental influences share in the development of vascular lesions and abnormal cell function. We also discuss how animal models can assist in elucidating gene function and the study of novel therapeutics, while at the same time addressing the limitations of the most commonly used rodent models. Novel experimental approaches based on application of next generation sequencing, bioinformatics and epigenetics research are also discussed as these are now being actively used to facilitate the discovery of novel gene mutations and mechanisms that regulate gene expression in PAH. Finally, we touch on recent discoveries concerning the role of inflammation and immunity in PAH pathobiology and how they are being targeted with immunomodulatory agents. We conclude that the field of PAH research is actively expanding and the major challenge in the coming years is to develop a unified theory that incorporates genetic and mechanistic data to address viable areas for disease modifying drugs that can target key processes that regulate the evolution of vascular pathology of PAH.

Low dose monocrotaline causes a selective pulmonary vascular lesion in male and female pneumonectomized rats

Purpose/Aim: Low doses (30-80 mg/kg) of monocrotaline are commonly used to create experimental models of pulmonary hypertension in rats. At these doses, monocrotaline causes pulmonary endothelial apoptosis and acute lung injury which ultimately results in pulmonary vascular disease. Higher doses of monocrotaline (300 mg/kg) are known to create severe liver injury, but previous investigations with lower doses have not reported histology in other organs to determine whether the vascular injury with monocrotaline is pulmonary-selective or generalized.

MATERIALS AND METHODS

We therefore sought to determine whether monocrotaline caused extra-pulmonary injury at doses commonly used in pulmonary hypertension studies. We performed left pneumonectomy on young male and female rats before administering 50-60 mg/kg monocrotaline 7 days later. We monitored serum chemistry and urine dipsticks during the first 3 weeks while the animals developed pulmonary hypertension. After 3 weeks, we sacrificed animals and stained the lungs and highly vascular visceral organs (kidney, liver, and spleen) for elastin to evaluate the degree of vascular injury and remodeling.

RESULTS

We did not observe proteinuria or significant transaminitis over the 3 weeks following monocrotaline. As previously published, monocrotaline caused severe pulmonary vascular disease with neointimal lesions and medial hypertrophy. We did not identify significant large or small arterial damage in the kidneys, liver, or spleen. Two external veterinary pathologists did not identify histopathology in the kidneys, liver, or spleen of these rats.

CONCLUSIONS

We conclude that 50-60 mg/kg of monocrotaline causes a selective pulmonary vascular lesion and that male and female rats have little non-pulmonary damage over 3 weeks at these doses of monocrotaline.

Endothelial nitric oxide synthase overexpressing human early outgrowth cells inhibit coronary artery smooth muscle cell migration through paracrine functions

Cells mobilized from the bone marrow can contribute to endothelial regeneration and repair. Nevertheless, cardiovascular diseases are associated with diminished numbers and function of these cells, attenuating their healing potential. Gene transfer of endothelial nitric oxide synthase (eNOS) can restore the activity of circulating cells. Furthermore, estrogen accelerates the reendothelialization capacity of early outgrowth cells (EOCs). We hypothesized that overexpressing eNOS alone or in combination with estrogen stimulation in EOCs would potentiate the beneficial effects of these cells in regulating smooth muscle cell (SMC) function. Native human EOCs did not have any effect on human coronary artery SMC (hCASMC) proliferation or migration. Transfecting EOCs with a human eNOS plasmid and/or stimulating with 17β-estradiol (E₂) increased NO production 3-fold and enhanced EOC survival. Moreover, in co-culture studies, eNOS overexpressing or E2-stimulated EOCs reduced hCASMC migration (by 23% and 56% respectively), vs. control EOCs. These effects do not implicate ERK1/2 or focal adhesion kinases. Nevertheless, NOS-EOCs had no effect on hCASMC proliferation. These results suggest that overexpressing or activating eNOS in EOCs increases their survival and enhances their capacity to regulate SMC migration through paracrine effects. These data elucidate how eNOS overexpression or activation in EOCs can prevent vascular remodeling.

Endothelial Cell Metabolism

Endothelial cells (ECs) are more than inert blood vessel lining material. Instead, they are active players in the formation of new blood vessels (angiogenesis) both in health and (life-threatening) diseases. Recently, a new concept arose by which EC metabolism drives angiogenesis in parallel to well-established angiogenic growth factors (e.g., vascular endothelial growth factor). 6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3-driven glycolysis generates energy to sustain competitive behavior of the ECs at the tip of a growing vessel sprout, whereas carnitine palmitoyltransferase 1a-controlled fatty acid oxidation regulates nucleotide synthesis and proliferation of ECs in the stalk of the sprout. To maintain vascular homeostasis, ECs rely on an intricate metabolic wiring characterized by intracellular compartmentalization, use metabolites for epigenetic regulation of EC subtype differentiation, crosstalk through metabolite release with other cell types, and exhibit EC subtype-specific metabolic traits. Importantly, maladaptation of EC metabolism contributes to vascular disorders, through EC dysfunction or excess angiogenesis, and presents new opportunities for anti-angiogenic strategies. Here we provide a comprehensive overview of established as well as newly uncovered aspects of EC metabolism.

Intratracheal Administration of Autologous Bone Marrow-Derived Cells Ameliorates Monocrotaline-Induced Pulmonary Vessel Remodeling and Lung Inflammation in Rats

PURPOSE

Inflammation is a feature of lung injury and plays a critical role in pulmonary vascular remodeling. Bone marrow-derived cells (BMCs) have anti-inflammatory properties and favor macrophage differentiation into an alternatively activated regulatory M2 profile. We investigated the effect of autologous BMCs on monocrotaline-induced pulmonary vessel remodeling and lung inflammation in rats, by direct administration into lungs via the airway.

METHODS

BMCs were isolated and plastic-adherent cells were cultured for 3 weeks. 1 week following monocrotaline (60 mg/kg) treatment, fluorescently labeled autologous BMCs (1 × 10⁶ cells) or vehicle were administered intratracheally to male Sprague-Dawley rats. 4 weeks following monocrotaline treatment, lung pathology was evaluated.

RESULTS

Monocrotaline increased pulmonary vessel wall thickness, perivascular infiltration, alveolar septal thickening, and inflammatory cell infiltration including T lymphocytes and monocytes/macrophages in alveolar areas, and also increased mRNA expression of inflammatory-related cytokines including IL-10 in the lung. Intratracheal administration of autologous BMCs prevented pulmonary vessel wall thickening and perivascular infiltration, and increased CD163-positive M2-like macrophages in perivascular areas. BMC administration inhibited the thickening of alveolar septa and reduced monocrotaline-induced inflammatory cell infiltration in lung parenchyma compared with monocrotaline-vehicle-treated-rats. Furthermore, BMCs administration increased expression of CD163-positive cells in perivascular areas and maintained the increased mRNA expression of IL-10.

CONCLUSIONS

Intratracheal administration of autologous BMCs prevented monocrotaline-induced pulmonary vessel remodeling and lung inflammation, at least in part, through induction of alternatively activated macrophages and regulation of the local lung environment toward resolving inflammation.