Pulmonary vascular disease in bronchopulmonary dysplasia: pulmonary hypertension and beyond

Arginase inhibition prevents bleomycin-induced pulmonary hypertension, vascular remodeling, and collagen deposition in neonatal rat lungs

Arginase is an enzyme that limits substrate L-arginine bioavailability for the production of nitric oxide by the nitric oxide synthases and produces L-ornithine, which is a precursor for collagen formation and tissue remodeling. We studied the pulmonary vascular effects of arginase inhibition in an established model of repeated systemic bleomycin sulfate administration in neonatal rats that results in pulmonary hypertension and lung injury mimicking the characteristics typical of bronchopulmonary dysplasia. We report that arginase expression is increased in the lungs of bleomycin-exposed neonatal rats and that treatment with the arginase inhibitor amino-2-borono-6-hexanoic acid prevented the bleomycin-induced development of pulmonary hypertension and deposition of collagen. Arginase inhibition resulted in increased L-arginine and L-arginine bioavailability and increased pulmonary nitric oxide production. Arginase inhibition also normalized the expression of inducible nitric oxide synthase, and reduced bleomycin-induced nitrative stress while having no effect on bleomycin-induced inflammation. Our data suggest that arginase is a promising target for therapeutic interventions in neonates aimed at preventing lung vascular remodeling and pulmonary hypertension.

Advances in paediatric pulmonary vascular disease associated with bronchopulmonary dysplasia

Pulmonary hypertension (PH) is a common finding in infants with bronchopulmonary dysplasia (BPD). The aim of this review is to describe recent advances in the diagnosis and treatment of PH and discuss whether they will benefit infants and children with BPD related PH. Echocardiography remains the mainstay of diagnosis but has limitations, further developments in diagnostic techniques and identification of biomarkers are required. There are many potential therapies for PH associated with BPD. Inhaled nitric oxide has been shown to improve short term outcomes only. Sidenafil in resource limited settings was shown in three randomized trials to significantly reduce mortality. The efficacy of other therapies including prostacyclin, PDE3 inhibitors and endothelin receptor blockers has only been reported in case reports or case series. Randomized controlled trials with long term follow up are required to appropriately assess the efficacy of therapies aimed at improving the outcome of children with PH.

Pulmonary artery smooth muscle cell endothelin-1 expression modulates the pulmonary vascular response to chronic hypoxia

Endothelin-1 (ET-1) increases pulmonary vascular tone through direct effects on pulmonary artery smooth muscle cells (PASMC) via membrane-bound ET-1 receptors. Circulating ET-1 contributes to vascular remodeling by promoting SMC proliferation and migration and inhibiting SMC apoptosis. Although endothelial cells (EC) are the primary source of ET-1, whether ET-1 produced by SMC modulates pulmonary vascular tone is unknown. Using transgenic mice created by crossbreeding SM22α-Cre mice with ET-1(flox/flox) mice to selectively delete ET-1 in SMC, we tested the hypothesis that PASMC ET-1 gene expression modulates the pulmonary vascular response to hypoxia. ET-1 gene deletion and selective activity of SM22α promoter-driven Cre recombinase were confirmed. Functional assays were performed under normoxic (21% O2) or hypoxic (5% O2) conditions using murine PASMC obtained from ET-1(+/+) and ET-1(-/-) mic and in human PASMC (hPASMC) after silencing of ET-1 using siRNA. Under baseline conditions, there was no difference in right ventricular systolic pressure (RVSP) between SM22α-ET-1(-/-) and SM22α-ET-1(+/+) (control) littermates. After exposure to hypoxia (10% O2, 21-24 days), RVSP was and vascular remodeling were less in SM22α-ET-1(-/-) mice compared with control littermates (P < 0.01). Loss of ET-1 decreased PASMC proliferation and migration and increased apoptosis under normoxic and hypoxic conditions. Exposure to selective ET-1 receptor antagonists had no effect on either the hypoxia-induced hPASMC proliferative or migratory response. SMC-specific ET-1 deletion attenuates hypoxia-induced increases in pulmonary vascular tone and structural remodeling. The observation that loss of ET-1 inhibited SMC proliferation, survival, and migration represents evidence that ET-1 derived from SMC plays a previously undescribed role in modulating the response of the pulmonary circulation to hypoxia. Thus PASMC ET-1 may modulate vascular tone independently of ET-1 produced by EC.

Critical role of vascular endothelial growth factor secreted by mesenchymal stem cells in hyperoxic lung injury

Intratracheal transplantation of human umbilical cord blood (UCB)-derived mesenchymal stem cells (MSCs) protects against neonatal hyperoxic lung injury by a paracrine rather than a regenerative mechanism. However, the role of paracrine factors produced by the MSCs, such as vascular endothelial growth factor (VEGF), has not been delineated. This study examined whether VEGF secreted by MSCs plays a pivotal role in protecting against neonatal hyperoxic lung injury. VEGF was knocked down in human UCB-derived MSCs by transfection with small interfering RNA specific for human VEGF. The in vitro effects of MSCs with or without VEGF knockdown or neutralizing antibody were evaluated in a rat lung epithelial (L2) cell line challenged with H2O2. To confirm these results in vivo, newborn Sprague-Dawley rats were exposed to hyperoxia (90% O2) for 14 days. MSCs (1 × 10(5) cells) with or without VEGF knockdown were administered intratracheally at postnatal Day 5. Lungs were serially harvested for biochemical and histologic analyses. VEGF knockdown and antibody abolished the in vitro benefits of MSCs on H2O2-induced cell death and the up-regulation of inflammatory cytokines in L2 cells. VEGF knockdown also abolished the in vivo protective effects of MSCs in hyperoxic lung injury, such as the attenuation of impaired alveolarization and angiogenesis, reduction in the number of terminal deoxynucleotidyl transferase dUTP nick end labeling-positive and ED-1-positive cells, and down-regulation of proinflammatory cytokine levels. Our data indicate that VEGF secreted by transplanted MSCs is one of the critical paracrine factors that play seminal roles in attenuating hyperoxic lung injuries in neonatal rats.

Progress in the diagnosis and management of pulmonary hypertension in children

PURPOSE OF REVIEW

Pulmonary hypertension is a complex disease that extends beyond merely elevated pulmonary blood pressures and right ventricular dysfunction. Its multiple causes and ever-expanding diagnostic tools and therapeutic approaches make it a heterogeneous disease with widely variable clinical sequelae. There are still many unanswered questions that challenge our understanding of this disease.

RECENT FINDINGS

The study of pulmonary hypertension in the pediatric patient is as robust as ever, with the creation and inclusion of pediatric-specific disease characteristics in the most recent WHO classification system, improved understanding of the pathophysiology of pulmonary hypertension in pediatric diseases such as bronchopulmonary dysplasia, and increasingly expanding diagnostic tools and management possibilities. Although the use of pulmonary hypertension therapies in children previously often relied on expert opinion and inferences from studies involving adults, pediatric-targeted research is becoming more widely supported and pursued, and has even come under recent debate, which at the very least stimulates further collaboration and discussion.

SUMMARY

This review will highlight the changes in the pulmonary hypertension classification system, briefly explore pulmonary hypertension in bronchopulmonary dysplasia, and provide updates on the diagnostic and management tools used by experts in the field.

L-citrulline provides a novel strategy for treating chronic pulmonary hypertension in newborn infants

Effective therapies are urgently needed for infants with forms of pulmonary hypertension that develop or persist beyond the first week of life. The L-arginine nitric oxide (NO) precursor, L-citrulline, improves NO signalling and ameliorates pulmonary hypertension in newborn animals. In vitro studies demonstrate that manipulating L-citrulline transport alters NO production.

CONCLUSION

Strategies that increase the supply and transport of L-citrulline merit pursuit as novel approaches to managing infants with chronic, progressive pulmonary hypertension.

Inhibition of β-catenin signaling improves alveolarization and reduces pulmonary hypertension in experimental bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is the most common and serious chronic lung disease of preterm infants. The development of pulmonary hypertension (PH) significantly increases the mortality and morbidity of this disease. β-Catenin signaling plays an important role in tissue development and remodeling. Aberrant β-catenin signaling is associated with clinical and experiment models of BPD. To test the hypothesis that inhibition of β-catenin signaling is beneficial in promoting alveolar and vascular development and preventing PH in experimental BPD, we examined the effects of ICG001, a newly developed pharmacological inhibitor of β-catenin, in preventing hyperoxia-induced BPD in neonatal rats. Newborn rat pups were randomized at postnatal day (P)2 to room air (RA) + DMSO (placebo), RA + ICG001, 90% FiO2 (O2) + DMSO, or O2 + ICG001. ICG001 (10 mg/kg) or DMSO was given by daily intraperitoneal injection for 14 days during continuous exposure to RA or hyperoxia. Primary human pulmonary arterial smooth muscle cells (PASMCs) were cultured in RA or hyperoxia (95% O2) in the presence of DMSO or ICG001 for 24 to 72 hours. Treatment with ICG001 significantly increased alveolarization and reduced pulmonary vascular remodeling and PH during hyperoxia. Furthermore, administering ICG001 decreased PASMC proliferation and expression of extracellular matrix remodeling molecules in vitro under hyperoxia. Finally, these structural, cellular, and molecular effects of ICG001 were associated with down-regulation of multiple β-catenin target genes. These data indicate that β-catenin signaling mediates hyperoxia-induced alveolar impairment and PH in neonatal animals. Targeting β-catenin may provide a novel strategy to alleviate BPD in preterm infants.

Disrupted lung development and bronchopulmonary dysplasia: opportunities for lung repair and regeneration

PURPOSE OF REVIEW

Advances in medical therapy have increased survival of extremely premature infants and changed the pathology of bronchopulmonary dysplasia (BPD) from one of acute lung injury to a disease of disrupted lung development. With this evolution, new questions emerge regarding the molecular mechanisms that control postnatal lung development, the effect of early disruptions of postnatal lung development on long-term lung function, and the existence of endogenous mechanisms that permit lung regeneration after injury.

RECENT FINDINGS

Recent data demonstrate that a significant component of alveolarization, the final stage of lung development, occurs postnatally. Further, clinical and experimental studies demonstrate that premature birth disrupts alveolarization, decreasing the gas exchange surface area of the lung and causing BPD. BPD is associated with significant short-term morbidity, and new longitudinal, clinical data demonstrate that survivors of BPD have long-standing deficits in lung function and may be at risk for the development of additional lung disease as adults. Unfortunately, current care is mainly supportive with few effective therapies that prevent or treat established BPD. These studies underscore the need to further elucidate the mechanisms that direct postnatal lung growth and develop innovative strategies to stimulate lung regeneration.

SUMMARY

Despite significant improvements in the care and survival of extremely premature infants, BPD remains a major clinical problem. Although efforts should remain focused on the prevention of preterm labor and BPD, novel research aimed at promoting postnatal alveolarization offers a unique opportunity to develop effective strategies to treat established BPD.

Anti-inflammatory actions of endogenous and exogenous interleukin-10 versus glucocorticoids on macrophage functions of the newly born

OBJECTIVE

To determine whether specific macrophage immune functions of the newly born are insensitive to the actions of therapeutic levels of dexamethasone (DEX), previously measured in infants with bronchopulmonary dysplasia (BPD), compared with betamethasone (BETA) and exogenous or endogenous interleukin-10 (IL-10).

STUDY DESIGN

Macrophages were differentiated from cord blood monocytes (N=18). A serial dose-response (around 10(-8 )M), in vitro study was used to examine the effect of DEX, BETA and IL-10, on proinflammatory (PI) cytokine release, phagocytosis and respiratory burst.

RESULT

Exogenous IL-10 (10(-8 )M) significantly (P<0.05) inhibited the endotoxin-stimulated release of IL-6, IL-8 and tumor necrosis factor by 63 to 82% with no significant effect by DEX and BETA. There was no inhibition by these three agents at 10(-8 )M on phagocytosis and respiratory burst. Inhibition of endogenous IL-10 with a monoclonal antibody significantly increased endotoxin-stimulated cytokine release by at least fourfold.

CONCLUSION

Macrophages were relatively insensitive to therapeutic levels of DEX and BETA with regard to PI cytokine release. This study provides rationale for translational and preclinical research using airway instillation of IL-10 for the treatment of BPD.

Existence, functional impairment, and lung repair potential of endothelial colony-forming cells in oxygen-induced arrested alveolar growth

BACKGROUND

Bronchopulmonary dysplasia and emphysema are life-threatening diseases resulting from impaired alveolar development or alveolar destruction. Both conditions lack effective therapies. Angiogenic growth factors promote alveolar growth and contribute to alveolar maintenance. Endothelial colony-forming cells (ECFCs) represent a subset of circulating and resident endothelial cells capable of self-renewal and de novo vessel formation. We hypothesized that resident ECFCs exist in the developing lung, that they are impaired during arrested alveolar growth in experimental bronchopulmonary dysplasia, and that exogenous ECFCs restore disrupted alveolar growth.

METHODS AND RESULTS

Human fetal and neonatal rat lungs contain ECFCs with robust proliferative potential, secondary colony formation on replating, and de novo blood vessel formation in vivo when transplanted into immunodeficient mice. In contrast, human fetal lung ECFCs exposed to hyperoxia in vitro and neonatal rat ECFCs isolated from hyperoxic alveolar growth-arrested rat lungs mimicking bronchopulmonary dysplasia proliferated less, showed decreased clonogenic capacity, and formed fewer capillary-like networks. Intrajugular administration of human cord blood-derived ECFCs after established arrested alveolar growth restored lung function, alveolar and lung vascular growth, and attenuated pulmonary hypertension. Lung ECFC colony- and capillary-like network-forming capabilities were also restored. Low ECFC engraftment and the protective effect of cell-free ECFC-derived conditioned media suggest a paracrine effect. Long-term (10 months) assessment of ECFC therapy showed no adverse effects with persistent improvement in lung structure, exercise capacity, and pulmonary hypertension.

CONCLUSIONS

Impaired ECFC function may contribute to arrested alveolar growth. Cord blood-derived ECFC therapy may offer new therapeutic options for lung diseases characterized by alveolar damage.

Exogenous hydrogen sulfide (H2S) protects alveolar growth in experimental O2-induced neonatal lung injury

Background

Bronchopulmonary dysplasia (BPD), the chronic lung disease of prematurity, remains a major health problem. BPD is characterized by impaired alveolar development and complicated by pulmonary hypertension (PHT). Currently there is no specific treatment for BPD. Hydrogen sulfide (H₂S), carbon monoxide and nitric oxide (NO), belong to a class of endogenously synthesized gaseous molecules referred to as gasotransmitters. While inhaled NO is already used for the treatment of neonatal PHT and currently tested for the prevention of BPD, H₂S has until recently been regarded exclusively as a toxic gas. Recent evidence suggests that endogenous H₂S exerts beneficial biological effects, including cytoprotection and vasodilatation. We hypothesized that H₂S preserves normal alveolar development and prevents PHT in experimental BPD.

Methods

We took advantage of a recently described slow-releasing H₂S donor, GYY4137 (morpholin-4-ium-4-methoxyphenyl(morpholino) phosphinodithioate) to study its lung protective potential in vitro and in vivo.

Results

In vitro, GYY4137 promoted capillary-like network formation, viability and reduced reactive oxygen species in hyperoxia-exposed human pulmonary artery endothelial cells. GYY4137 also protected mitochondrial function in alveolar epithelial cells. In vivo, GYY4137 preserved and restored normal alveolar growth in rat pups exposed from birth for 2 weeks to hyperoxia. GYY4137 also attenuated PHT as determined by improved pulmonary arterial acceleration time on echo-Doppler, pulmonary artery remodeling and right ventricular hypertrophy. GYY4137 also prevented pulmonary artery smooth muscle cell proliferation.

Conclusions

H₂S protects from impaired alveolar growth and PHT in experimental O₂-induced lung injury. H₂S warrants further investigation as a new therapeutic target for alveolar damage and PHT.

Update on PPHN: mechanisms and treatment

Persistent pulmonary hypertension of the newborn (PPHN) is a syndrome of failed circulatory adaptation at birth, seen in about 2/1000 live born infants. While it is mostly seen in term and near-term infants, it can be recognized in some premature infants with respiratory distress or bronchopulmonary dysplasia. Most commonly, PPHN is secondary to delayed or impaired relaxation of the pulmonary vasculature associated with diverse neonatal pulmonary pathologies, such as meconium aspiration syndrome, congenital diaphragmatic hernia, and respiratory distress syndrome. Gentle ventilation strategies, lung recruitment, inhaled nitric oxide, and surfactant therapy have improved outcome and reduced the need for extracorporeal membrane oxygenation (ECMO) in PPHN. Newer modalities of treatment discussed in this article include systemic and inhaled vasodilators like sildenafil, prostaglandin E1, prostacyclin, and endothelin antagonists. With prompt recognition/treatment and early referral to ECMO centers, the mortality rate for PPHN has significantly decreased. However, the risk of potential neurodevelopmental impairment warrants close follow-up after discharge for infants with PPHN.

Pulmonary Hypertension in Preterm Infants with Bronchopulmonary Dysplasia

Bronchopulmonary dysplasia (BPD), the chronic lung disease of prematurity, is a significant contributor to perinatal morbidity and mortality. Premature birth disrupts pulmonary vascular growth and initiates a cascade of events that result in impaired gas exchange, abnormal vasoreactivity, and pulmonary vascular remodeling that may ultimately lead to pulmonary hypertension (PH). Even infants who appear to have mild BPD suffer from varying degrees of pulmonary vascular disease (PVD). Although recent studies have enhanced our understanding of the pathobiology of PVD and PH in BPD, much remains unknown with respect to how PH should be properly defined, as well as the most accurate methods for the diagnosis and treatment of PH in infants with BPD. This article will provide neonatologists and primary care providers, as well as pediatric cardiologists and pulmonologists, with a review of the pathophysiology of PH in preterm infants with BPD and a summary of current clinical recommendations for managing PH in this population.

Bronchopulmonary dysplasia: clinical perspective

Since Northway's original description of BPD almost 45 years ago, the clinical presentation of BPD has evolved into a disease process, which mostly involves extremely premature infants. This new form of BPD is the result of multiple antenatal and postnatal factors that can cause injury to the developing lung leading to altered alveolar and vascular development. Over the years, there has been considerable increase in knowledge of factors that contribute to the development of BPD. This has led to different strategies for prevention as well as management of BPD. Some of these strategies have been successful and have withstood the test of clinical trials, such as vitamin A supplementation, post-natal steroids, caffeine, and volume targeted ventilation. The evidence for other interventions has been weak or negative. With better understanding of the complex and multifactorial pathogenesis of BPD, it is quite clear that any single therapy is very unlikely to eliminate this problem unless it reduces prematurity. Further development in prevention and treatment of BPD will likely need a multi-pronged strategy with novel therapeutic agents acting at various stages of the disease process.

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

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

Recent advances in late lung development and the pathogenesis of bronchopulmonary dysplasia

In contrast to early lung development, a process exemplified by the branching of the developing airways, the later development of the immature lung remains very poorly understood. A key event in late lung development is secondary septation, in which secondary septa arise from primary septa, creating a greater number of alveoli of a smaller size, which dramatically expands the surface area over which gas exchange can take place. Secondary septation, together with architectural changes to the vascular structure of the lung that minimize the distance between the inspired air and the blood, are the objectives of late lung development. The process of late lung development is disturbed in bronchopulmonary dysplasia (BPD), a disease of prematurely born infants in which the structural development of the alveoli is blunted as a consequence of inflammation, volutrauma, and oxygen toxicity. This review aims to highlight notable recent developments in our understanding of late lung development and the pathogenesis of BPD.

Diagnosis and treatment of pulmonary hypertension in infancy

Normal pulmonary vascular development in infancy requires maintenance of low pulmonary vascular resistance after birth, and is necessary for normal lung function and growth. The developing lung is subject to multiple genetic, pathological and/or environmental influences that can adversely affect lung adaptation, development, and growth, leading to pulmonary hypertension. New classifications of pulmonary hypertension are beginning to account for these diverse phenotypes, and or pulmonary hypertension in infants due to PPHN, congenital diaphragmatic hernia, and bronchopulmonary dysplasia (BPD). The most effective pharmacotherapeutic strategies for infants with PPHN are directed at selective reduction of PVR, and take advantage of a rapidly advancing understanding of the altered signaling pathways in the remodeled vasculature.

Chronic morbidities of premature newborns

The most important chronic morbidities of premature newborns, deeply influencing quality of life, are retinopathy of prematurity, bronchopulmonary dysplasia, intraventricular hemorrhage and periventricular leukomalacia. Since the rate of premature birth has not decreased in recent years in Hungary, and treatments of these end stage disorders are extremely difficult, prevention gains tremendous significance. Effective prevention is based on detailed knowledge of the pathophysiological mechanisms of these special diseases having multifactorial nature sharing several common risk factors, and one is the pathological angiogenesis. This sensitive system is affected by several stress situations which are the consequences of prematurity leading to abnormal vascular growth. After birth, relative hyperoxia, compared to intrauterine life, and decreasing concentrations of vascular growth factors result in vascular injury, moreover, may cause vessel apoptosis. The consequence of this phenomenon is the activation of hypoxia responsible genes resulting in robust pathological neovascularization and organ damage during the later phase. Saving normal angiogenesis and inhibiting reactive neovascularization may lead to better quality of life in these premature infants. Orv. Hetil., 2013, 154, 1498–1511.