Lung EC-SOD overexpression attenuates hypoxic induction of Egr-1 and chronic hypoxic pulmonary vascular remodeling

Use of Electron Paramagnetic Resonance (EPR) to Evaluate Redox Status in a Preclinical Model of Acute Lung Injury

PURPOSE

Patients with hyper- vs. hypo-inflammatory subphenotypes of acute respiratory distress syndrome (ARDS) exhibit different clinical outcomes. Inflammation increases the production of reactive oxygen species (ROS) and increased ROS contributes to the severity of illness. Our long-term goal is to develop electron paramagnetic resonance (EPR) imaging of lungs in vivo to precisely measure superoxide production in ARDS in real time. As a first step, this requires the development of in vivo EPR methods for quantifying superoxide generation in the lung during injury, and testing if such superoxide measurements can differentiate between susceptible and protected mouse strains.

PROCEDURES

In WT mice, mice lacking total body extracellular superoxide dismutase (EC-SOD) (KO), or mice overexpressing lung EC-SOD (Tg), lung injury was induced with intraperitoneal (IP) lipopolysaccharide (LPS) (10 mg/kg). At 24 h after LPS treatment, mice were injected with the cyclic hydroxylamines 1-hydroxy-3-carboxy-2,2,5,5-tetramethylpyrrolidine hydrochloride (CPH) or 4-acetoxymethoxycarbonyl-1-hydroxy-2,2,5,5-tetramethylpyrrolidine-3-carboxylic acid (DCP-AM-H) probes to detect, respectively, cellular and mitochondrial ROS - specifically superoxide. Several probe delivery strategies were tested. Lung tissue was collected up to one hour after probe administration and assayed by EPR.

RESULTS

As measured by X-band EPR, cellular and mitochondrial superoxide increased in the lungs of LPS-treated mice compared to control. Lung cellular superoxide was increased in EC-SOD KO mice and decreased in EC-SOD Tg mice compared to WT. We also validated an intratracheal (IT) delivery method, which enhanced the lung signal for both spin probes compared to IP administration.

CONCLUSIONS

We have developed protocols for delivering EPR spin probes in vivo, allowing detection of cellular and mitochondrial superoxide in lung injury by EPR. Superoxide measurements by EPR could differentiate mice with and without lung injury, as well as mouse strains with different disease susceptibilities. We expect these protocols to capture real-time superoxide production and enable evaluation of lung EPR imaging as a potential clinical tool for subphenotyping ARDS patients based on redox status.

Several lines of antioxidant defense against oxidative stress: antioxidant enzymes, nanomaterials with multiple enzyme-mimicking activities, and low-molecular-weight antioxidants

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are well recognized for playing a dual role, since they can be either deleterious or beneficial to biological systems. An imbalance between ROS production and elimination is termed oxidative stress, a critical factor and common denominator of many chronic diseases such as cancer, cardiovascular diseases, metabolic diseases, neurological disorders (Alzheimer's and Parkinson's diseases), and other disorders. To counteract the harmful effects of ROS, organisms have evolved a complex, three-line antioxidant defense system. The first-line defense mechanism is the most efficient and involves antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx). This line of defense plays an irreplaceable role in the dismutation of superoxide radicals (O2•-) and hydrogen peroxide (H2O2). The removal of superoxide radicals by SOD prevents the formation of the much more damaging peroxynitrite ONOO- (O2•-  + NO• → ONOO-) and maintains the physiologically relevant level of nitric oxide (NO•), an important molecule in neurotransmission, inflammation, and vasodilation. The second-line antioxidant defense pathway involves exogenous diet-derived small-molecule antioxidants. The third-line antioxidant defense is ensured by the repair or removal of oxidized proteins and other biomolecules by a variety of enzyme systems. This review briefly discusses the endogenous (mitochondria, NADPH, xanthine oxidase (XO), Fenton reaction) and exogenous (e.g., smoking, radiation, drugs, pollution) sources of ROS (superoxide radical, hydrogen peroxide, hydroxyl radical, peroxyl radical, hypochlorous acid, peroxynitrite). Attention has been given to the first-line antioxidant defense system provided by SOD, CAT, and GPx. The chemical and molecular mechanisms of antioxidant enzymes, enzyme-related diseases (cancer, cardiovascular, lung, metabolic, and neurological diseases), and the role of enzymes (e.g., GPx4) in cellular processes such as ferroptosis are discussed. Potential therapeutic applications of enzyme mimics and recent progress in metal-based (copper, iron, cobalt, molybdenum, cerium) and nonmetal (carbon)-based nanomaterials with enzyme-like activities (nanozymes) are also discussed. Moreover, attention has been given to the mechanisms of action of low-molecular-weight antioxidants (vitamin C (ascorbate), vitamin E (alpha-tocopherol), carotenoids (e.g., β-carotene, lycopene, lutein), flavonoids (e.g., quercetin, anthocyanins, epicatechin), and glutathione (GSH)), the activation of transcription factors such as Nrf2, and the protection against chronic diseases. Given that there is a discrepancy between preclinical and clinical studies, approaches that may result in greater pharmacological and clinical success of low-molecular-weight antioxidant therapies are also subject to discussion.

Unleashing the Potential of Nrf2: A Novel Therapeutic Target for Pulmonary Vascular Remodeling

Pulmonary vascular remodeling, characterized by the thickening of all three layers of the blood vessel wall, plays a central role in the pathogenesis of pulmonary hypertension (PH). Despite the approval of several drugs for PH treatment, their long-term therapeutic effect remains unsatisfactory, as they mainly focus on vasodilation rather than addressing vascular remodeling. Therefore, there is an urgent need for novel therapeutic targets in the treatment of PH. Nuclear factor erythroid 2-related factor 2 (Nrf2) is a vital transcription factor that regulates endogenous antioxidant defense and emerges as a novel regulator of pulmonary vascular remodeling. Growing evidence has suggested an involvement of Nrf2 and its downstream transcriptional target in the process of pulmonary vascular remodeling. Pharmacologically targeting Nrf2 has demonstrated beneficial effects in various diseases, and several Nrf2 inducers are currently undergoing clinical trials. However, the exact potential and mechanism of Nrf2 as a therapeutic target in PH remain unknown. Thus, this review article aims to comprehensively explore the role and mechanism of Nrf2 in pulmonary vascular remodeling associated with PH. Additionally, we provide a summary of Nrf2 inducers that have shown therapeutic potential in addressing the underlying vascular remodeling processes in PH. Although Nrf2-related therapies hold great promise, further research is necessary before their clinical implementation can be fully realized.

Smooth muscle Acid-sensing ion channel 1a as a therapeutic target to reverse hypoxic pulmonary hypertension

Acid-sensing ion channel 1a (ASIC1a) is a voltage-independent, non-selective cation channel that conducts both Na⁺ and Ca²⁺. Activation of ASIC1a elicits plasma membrane depolarization and stimulates intracellular Ca²⁺-dependent signaling pathways in multiple cell types, including vascular smooth muscle (SM) and endothelial cells (ECs). Previous studies have shown that increases in pulmonary vascular resistance accompanying chronic hypoxia (CH)-induced pulmonary hypertension requires ASIC1a to elicit enhanced pulmonary vasoconstriction and vascular remodeling. Both SM and EC dysfunction drive these processes; however, the involvement of ASIC1a within these different cell types is unknown. Using the Cre-LoxP system to generate cell-type-specific Asic1a knockout mice, we tested the hypothesis that SM-Asic1a contributes to CH-induced pulmonary hypertension and vascular remodeling, whereas EC-Asic1a opposes the development of CH-induced pulmonary hypertension. The severity of pulmonary hypertension was not altered in mice with specific deletion of EC-Asic1a (Tek^Cre-Asic1a^fl/fl). However, similar to global Asic1a knockout (Asic1a^−/-) mice, mice with specific deletion of SM-Asic1a (MHC^CreER-Asic1a^fl/fl) were protected from the development of CH-induced pulmonary hypertension and right heart hypertrophy. Furthermore, pulmonary hypertension was reversed when deletion of SM-Asic1a was initiated in conditional MHC^CreER-Asic1a^fl/fl mice with established pulmonary hypertension. CH-induced vascular remodeling was also significantly attenuated in pulmonary arteries from MHC^CreER-Asic1a^fl/fl mice. These findings were additionally supported by decreased CH-induced proliferation and migration of pulmonary arterial smooth muscle cells (PASMCs) from Asic1a^−/- mice. Together these data demonstrate that SM-, but not EC-Asic1a contributes to CH-induced pulmonary hypertension and vascular remodeling. Furthermore, these studies provide evidence for the therapeutic potential of ASIC1a inhibition to reverse pulmonary hypertension.

The interplay between oxidative stress and autophagy in chronic obstructive pulmonary disease

Autophagy is a highly conserved process that is indispensable for cell survival, embryonic development, and tissue homeostasis. Activation of autophagy protects cells against oxidative stress and is a major adaptive response to injury. When autophagy is dysregulated by factors such as smoking, environmental insults and aging, it can lead to enhanced formation of aggressors and production of reactive oxygen species (ROS), resulting in oxidative stress and oxidative damage to cells. ROS activates autophagy, which in turn promotes cell adaptation and reduces oxidative damage by degrading and circulating damaged macromolecules and dysfunctional cell organelles. The cellular response triggered by oxidative stress includes changes in signaling pathways that ultimately regulate autophagy. Chronic obstructive pulmonary disease (COPD) is the most common lung disease among the elderly worldwide, with a high mortality rate. As an induced response to oxidative stress, autophagy plays an important role in the pathogenesis of COPD. This review discusses the regulation of oxidative stress and autophagy in COPD, and aims to provide new avenues for future research on target-specific treatments for COPD.

Extracellular superoxide dismutase (EC-SOD) R213G variant reduces mitochondrial ROS and preserves mitochondrial function in bleomycin-induced lung injury: EC-SOD R213G variant and intracellular redox regulation

Extracellular superoxide dismutase (EC-SOD) is highly expressed in the lung and vasculature. A common human single nucleotide polymorphism (SNP) in the matrix binding region of EC-SOD leads to a single amino acid substitution, R213G, and alters EC-SOD tissue binding affinity. The change in tissue binding affinity redistributes EC-SOD from tissue to extracellular fluids. Mice (R213G mice) expressing a knock-in of this EC-SOD SNP exhibit elevated plasma and reduced lung EC-SOD content and activity and are protected against bleomycin-induced lung injury and inflammation. It is unknown how the redistribution of EC-SOD alters site-specific redox-regulated molecules relevant for protection. In this study, we tested the hypothesis that the change in the local EC-SOD content would influence not only the extracellular redox microenvironment where EC-SOD is localized but also protect the intracellular redox status of the lung. Mice were treated with bleomycin and harvested 7 days post-treatment. Superoxide levels, measured by electron paramagnetic resonance (EPR), were lower in plasma and Bronchoalveolar lavage fluid (BALF) cells in R213G mice compared to wild-type (WT) mice, while lung cellular superoxide levels in R213G mice were not elevated post-bleomycin compared to WT mice despite low lung EC-SOD levels. Lung glutathione redox potential (EhGSSG), determined by HPLC and fluorescence, was more oxidized in WT compared to R213G mice. In R213G mice, lung mitochondrial oxidative stress was reduced shown by mitochondrial superoxide level measured by EPR in lung and the resistance to bleomycin-induced cardiolipin oxidation. Bleomycin treatment suppressed mitochondrial respiration in WT mice. Mitochondrial function was impaired at baseline in R213G mice but did not exhibit further suppression in respiration post-bleomycin. Collectively, the results indicate that R213G variant preserves intracellular redox state and protects mitochondrial function in the setting of bleomycin-induced inflammation.

Vascular Remodeling in Pulmonary Arterial Hypertension: The Potential Involvement of Innate and Adaptive Immunity

Pulmonary arterial hypertension (PAH) is a severe disease with high morbidity and mortality. Current therapies are mainly focused on vasodilative agents to improve prognosis. However, recent literature has shown the important interaction between immune cells and stromal vascular cells in the pathogenic modifications of the pulmonary vasculature. The immunological pathogenesis of PAH is known as a complex interplay between immune cells and vascular stromal cells, via direct contacts and/or their production of extra-cellular/diffusible factors such as cytokines, chemokines, and growth factors. These include, the B-cell—mast-cell axis, endothelium mediated fibroblast activation and subsequent M2 macrophage polarization, anti-endothelial cell antibodies and the versatile role of IL-6 on vascular cells. This review aims to outline the major pathophysiological changes in vascular cells caused by immunological mechanisms, leading to vascular remodeling, increased pulmonary vascular resistance and eventually PAH. Considering the underlying immunological mechanisms, these mechanisms may be key to halt progression of disease.

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.

Basement Membrane Extracellular Matrix Proteins in Pulmonary Vascular and Right Ventricular Remodeling in Pulmonary Hypertension

The extracellular matrix (ECM), a highly organized network of structural and non-structural proteins, plays a pivotal role in cellular and tissue homeostasis. Changes in the ECM are critical for normal tissue repair, while dysregulation contributes to aberrant tissue remodeling. Pulmonary arterial hypertension (PAH) is a severe disorder of the pulmonary vasculature characterized by pathologic remodeling of the pulmonary vasculature and right ventricle (RV), increased production and deposition of structural and non-structural proteins, and altered expression of ECM growth factors and proteases. Furthermore, ECM remodeling plays a significant role in disease progression as several dynamic changes in its composition, quantity, and organization are documented in both humans and animal models of disease. These ECM changes impact upon vascular cell biology and affect proliferation of resident cells. Further, ECM components determine the tissue architecture of the pulmonary and myocardial vasculature as well as the myocardium itself, and provide mechanical stability crucial for tissue homeostasis. However, little is known about the basement membrane (BM), a specialized, self-assembled conglomerate of ECM proteins, during remodeling. In the vasculature, the BM is in close physical association with the vascular endothelium and smooth muscle cells. While in the myocardium, each cardiomyocyte is enclosed by a BM that serves as the interface between cardiomyocytes and the surrounding interstitial matrix. In this review, we provide a brief overview on the current state of knowledge of the BM and its ECM composition and their impact on pulmonary vascular remodeling and RV dysfunction and failure in PAH.

Alteration of Extracellular Superoxide Dismutase in Idiopathic Pulmonary Arterial Hypertension

Background: Superoxide dismutases (SODs) are an important family of antioxidant enzymes that modulate reactive oxygen species levels. It is largely unknown which SOD isoform(s) change in vivo in idiopathic pulmonary arterial hypertension (IPAH) patients.

Methods: A total of 133 consecutive adult IPAH patients who underwent bone morphogenetic protein receptor type 2 (BMPR2) genetic counseling were enrolled in this prospective study. The plasma activities of three subtypes of SOD [copper–zinc (Cu/Zn-SOD), manganese (Mn-SOD), and extracellular SOD (Ec-SOD)] were examined.

Results: The activities of SODs were significantly lower in IPAH patients than in healthy subjects. However, only Ec-SOD activity in BMPR2 mutation patients was significantly decreased compared to those in patients without a mutation. The reduced Ec-SOD activity was markedly associated with mean pulmonary arterial pressure, pulmonary vascular resistance (PVR), and 6-min walking distance (6MWD). The reduction of Mn-SOD activity was only associated with 6MWD. There was no association between Cu/Zn-SOD and hemodynamics. Patients with a lower Ec-SOD level had a worse survival compared to those with a higher baseline. The reduced Ec-SOD activity and the raised PVR increased the mortality risk.

Conclusions: Ec-SOD was correlated with BMPR2 mutation, hemodynamic dysfunction, and poor outcomes. Circulating Ec-SOD could be a potentially vital antioxidant enzyme in the pathogenesis of IPAH.

Potential role of cartilage oligomeric matrix protein in the modulation of pulmonary arterial smooth muscle superoxide by hypoxia

Changes in reactive oxygen species and extracellular matrix seem to participate in pulmonary hypertension development. Because we recently reported evidence for chronic hypoxia decreasing expression of cartilage oligomeric matrix protein (COMP) and evidence for this controlling loss of pulmonary arterial smooth muscle bone morphogenetic protein receptor-2 (BMPR2) and contractile phenotype proteins, we examined if changes in superoxide metabolism could be an important factor in a bovine pulmonary artery (BPA), organoid cultured under hypoxia for 48 h model. Hypoxia (3% O2) caused a depletion of COMP in BPA, but not in bovine coronary arteries. Knockdown of COMP by small-interfering RNA (siRNA) increased BPA levels of mitochondrial and extra-mitochondrial superoxide detected by MitoSOX and dihydroethidium (DHE) HPLC products. COMP siRNA-treated BPA showed reduced levels of SOD2 and SOD3 and increased levels of NADPH oxidases NOX2 and NOX4. Hypoxia increased BPA levels of MitoSOX-detected superoxide and caused changes in NOX2 and SOD2 expression similar to COMP siRNA, and exogenous COMP (0.5 μM) prevented the effects of hypoxia. In the presence of COMP, BMPR2 siRNA-treated BPA showed increases in superoxide detected by MitoSOX and depletion of SOD2. Superoxide scavengers (0.5 μM TEMPO or mitoTEMPO) maintained the expression of contractile phenotype proteins calponin and SM22α decreased by 48 h hypoxia (1% O2). Adenoviral delivery of BMPR2 to rat pulmonary artery smooth muscle cells prevented the depletion of calponin and SM22α by COMP siRNA. Thus, COMP regulation of BMPR2 appears to have an important role in controlling hypoxia-elicited changes in BPA superoxide and its potential regulation of contractile phenotype proteins.

Redox Biology of Peroxisome Proliferator-Activated Receptor-γ in Pulmonary Hypertension

Significance: Peroxisome proliferator-activated receptor-gamma (PPARγ) maintains pulmonary vascular health through coordination of antioxidant defense systems, inflammation, and cellular metabolism. Insufficient PPARγ contributes to pulmonary hypertension (PH) pathogenesis, whereas therapeutic restoration of PPARγ activity attenuates PH in preclinical models. Recent Advances: Numerous studies in the past decade have elucidated the complex mechanisms by which PPARγ in the pulmonary vasculature and right ventricle (RV) protects against PH. The scope of PPARγ-interconnected pathways continues to expand and includes induction of antioxidant genes, transrepression of inflammatory signaling, regulation of mitochondrial biogenesis and bioenergetic integrity, control of cell cycle and proliferation, and regulation of vascular tone through interactions with nitric oxide and endogenous vasoactive molecules. Furthermore, PPARγ interacts with an extensive regulatory network of transcription factors and microRNAs leading to broad impact on cell signaling. Critical Issues: Abundant evidence suggests that targeting PPARγ exerts diverse salutary effects in PH and represents a novel and potentially translatable therapeutic strategy. However, progress has been slowed by an incomplete understanding of how specific PPARγ pathways are critically disrupted across PH disease subtypes and lack of optimal pharmacological ligands. Future Directions: Recent studies indicate that ligand-induced post-translational modifications of the PPARγ receptor differentially induce therapeutic benefits versus adverse side effects of PPARγ receptor activation. Strategies to selectively target PPARγ activity in diseased cells of pulmonary circulation and RV, coupled with development of ligands designed to specifically regulate post-translational PPARγ modifications, may unlock the full therapeutic potential of this versatile master transcriptional and metabolic regulator in PH.

Redox Regulation of Ion Channels and Receptors in Pulmonary Hypertension

Significance: Pulmonary hypertension (PH) is characterized by elevated vascular resistance due to vasoconstriction and remodeling of the normally low-pressure pulmonary vasculature. Redox stress contributes to the pathophysiology of this disease by altering the regulation and activity of membrane receptors, K+ channels, and intracellular Ca2+ homeostasis. Recent Advances: Antioxidant therapies have had limited success in treating PH, leading to a growing appreciation that reductive stress, in addition to oxidative stress, plays a role in metabolic and cell signaling dysfunction in pulmonary vascular cells. Reactive oxygen species generation from mitochondria and NADPH oxidases has substantial effects on K+ conductance and membrane potential, and both receptor-operated and store-operated Ca2+ entry. Critical Issues: Some specific redox changes resulting from oxidation, S-nitrosylation, and S-glutathionylation are known to modulate membrane receptor and ion channel activity in PH. However, many sites of regulation that have been elucidated in nonpulmonary cell types have not been tested in the pulmonary vasculature, and context-specific molecular mechanisms are lacking. Future Directions: Here, we review what is known about redox regulation of membrane receptors and ion channels in PH. Further investigation of the mechanisms involved is needed to better understand the etiology of PH and develop better targeted treatment strategies.

Dynamic and diverse changes in the functional properties of vascular smooth muscle cells in pulmonary hypertension

Pulmonary hypertension (PH) is the end result of interaction between pulmonary vascular tone and a complex series of cellular and molecular events termed 'vascular remodelling'. The remodelling process, which can involve the entirety of pulmonary arterial vasculature, almost universally involves medial thickening, driven by increased numbers and hypertrophy of its principal cellular constituent, smooth muscle cells (SMCs). It is noted, however that SMCs comprise heterogeneous populations of cells, which can exhibit markedly different proliferative, inflammatory, and extracellular matrix production changes during remodelling. We further consider that these functional changes in SMCs of different phenotype and their role in PH are dynamic and may undergo significant changes over time (which we will refer to as cellular plasticity); no single property can account for the complexity of the contribution of SMC to pulmonary vascular remodelling. Thus, the approaches used to pharmacologically manipulate PH by targeting the SMC phenotype(s) must take into account processes that underlie dominant phenotypes that drive the disease. We present evidence for time- and location-specific changes in SMC proliferation in various animal models of PH; we highlight the transient nature (rather than continuous) of SMC proliferation, emphasizing that the heterogenic SMC populations that reside in different locations along the pulmonary vascular tree exhibit distinct responses to the stresses associated with the development of PH. We also consider that cells that have often been termed 'SMCs' may arise from many origins, including endothelial cells, fibroblasts and resident or circulating progenitors, and thus may contribute via distinct signalling pathways to the remodelling process. Ultimately, PH is characterized by long-lived, apoptosis-resistant SMC. In line with this key pathogenic characteristic, we address the acquisition of a pro-inflammatory phenotype by SMC that is essential to the development of PH. We present evidence that metabolic alterations akin to those observed in cancer cells (cytoplasmic and mitochondrial) directly contribute to the phenotype of the SM and SM-like cells involved in PH. Finally, we raise the possibility that SMCs transition from a proliferative to a senescent, pro-inflammatory and metabolically active phenotype over time.

Oxidation of PKGIα mediates an endogenous adaptation to pulmonary hypertension

Chronic hypoxia causes pulmonary hypertension (PH), vascular remodeling, right ventricular (RV) hypertrophy, and cardiac failure. Protein kinase G Iα (PKGIα) is susceptible to oxidation, forming an interprotein disulfide homodimer associated with kinase targeting involved in vasodilation. Here we report increased disulfide PKGIα in pulmonary arteries from mice with hypoxic PH or lungs from patients with pulmonary arterial hypertension. This oxidation is likely caused by oxidants derived from NADPH oxidase-4, superoxide dismutase 3, and cystathionine γ-lyase, enzymes that were concomitantly increased in these samples. Indeed, products that may arise from these enzymes, including hydrogen peroxide, glutathione disulfide, and protein-bound persulfides, were increased in the plasma of hypoxic mice. Furthermore, low-molecular-weight hydropersulfides, which can serve as "superreductants" were attenuated in hypoxic tissues, consistent with systemic oxidative stress and the oxidation of PKGIα observed. Inhibiting cystathionine γ-lyase resulted in decreased hypoxia-induced disulfide PKGIα and more severe PH phenotype in wild-type mice, but not in Cys42Ser PKGIα knock-in (KI) mice that are resistant to oxidation. In addition, KI mice also developed potentiated PH during hypoxia alone. Thus, oxidation of PKGIα is an adaptive mechanism that limits PH, a concept further supported by polysulfide treatment abrogating hypoxia-induced RV hypertrophy in wild-type, but not in the KI, mice. Unbiased transcriptomic analysis of hypoxic lungs before structural remodeling identified up-regulation of endothelial-to-mesenchymal transition pathways in the KI compared with wild-type mice. Thus, disulfide PKGIα is an intrinsic adaptive mechanism that attenuates PH progression not only by promoting vasodilation but also by limiting maladaptive growth and fibrosis signaling.

Role of Gender in Regulation of Redox Homeostasis in Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is one of the diseases with a well-established gender dimorphism. The prevalence of PAH is increased in females with a ratio of 4:1, while poor survival prognosis is associated with the male gender. Nevertheless, the specific contribution of gender in disease development and progression is unclear due to the complex nature of the PAH. Oxidative and nitrosative stresses are important contributors in PAH pathogenesis; however, the role of gender in redox homeostasis has been understudied. This review is aimed to overview the possible sex-specific mechanisms responsible for the regulation of the balance between oxidants and antioxidants in relation to PAH pathobiology.

Role of SOD3 in silica-related lung fibrosis and pulmonary vascular remodeling

BACKGROUND

Work-place exposure to silica dust may lead to progressive lung inflammation culminating in the development of silicosis, an irreversible condition that can be complicated by onset of pulmonary hypertension (PH). The molecular mechanisms leading to the development of PH and lung fibrosis in response to silica are not well understood. Oxidant/antioxidant imbalance in the lung may promote fibroproliferation and vascular smooth muscle proliferation, ultimately leading to the development of PH. Herein, we analyze the development of PH and lung fibrosis in mice deficient in extracellular superoxide dismutase (SOD3), an enzyme with anti-oxidant activity.

METHODS

PH and silicosis were induced in wild-type and Sod3-/- mice through intratracheal injection of crystalline silica at dose 0.4 g/kg. Pulmonary hypertension and lung fibrosis were characterized by changes in right ventricular systolic pressure (RVSP) and collagen deposition 28 days following silica injections. Vascular remodeling was analyzed using immunohistochemistry and morphometric analysis. The expression of genes were analyzed using qRT-PCR and Western blot.

RESULTS

C57BL6 mice exposed to silica showed attenuated expression of Sod3 in the lung suggesting a protective role for Sod3. Consistent with this, Sod3-/- mice developed more severe fibrotic inflammatory nodules with increased collagen deposition. Furthermore, the expression of genes involved in tissue remodeling (Timp1), fibrotic lesion formation (Fsp1) and inflammatory response (Mcp1) were significantly elevated in Sod3-/- mice compared to Sod3+/+ mice treated with silica. Infiltration of neutrophils and activated macrophages into affected lung was significantly higher in Sod3 deficient mice. In addition, silica produced more profound effects on elevation of RVSP in Sod3-/- compared to wild-type littermate. Increase in RVSP was concomitant with hypertrophy of pulmonary arteries located in silicotic nodules of both mouse strains, however, vascular remodeling in unaffected areas of lung was detected only in Sod3-/- mice.

CONCLUSIONS

Our data suggest that Sod3 and extracellular oxidative stress may play an important role in the development of pneumoconiosis and pulmonary vascular remodeling following exposure to environmental and occupational silica.

Mechanisms contributing to persistently activated cell phenotypes in pulmonary hypertension

Chronic pulmonary hypertension (PH) is characterized by the accumulation of persistently activated cell types in the pulmonary vessel exhibiting aberrant expression of genes involved in apoptosis resistance, proliferation, inflammation and extracellular matrix (ECM) remodelling. Current therapies for PH, focusing on vasodilatation, do not normalize these activated phenotypes. Furthermore, current approaches to define additional therapeutic targets have focused on determining the initiating signals and their downstream effectors that are important in PH onset and development. Although these approaches have produced a large number of compelling PH treatment targets, many promising human drugs have failed in PH clinical trials. Herein, we propose that one contributing factor to these failures is that processes important in PH development may not be good treatment targets in the established phase of chronic PH. We hypothesize that this is due to alterations of chromatin structure in PH cells, resulting in functional differences between the same factor or pathway in normal or early PH cells versus cells in chronic PH. We propose that the high expression of genes involved in the persistently activated phenotype of PH vascular cells is perpetuated by an open chromatin structure and multiple transcription factors (TFs) via the recruitment of high levels of epigenetic regulators including the histone acetylases P300/CBP, histone acetylation readers including BRDs, the Mediator complex and the positive transcription elongation factor (Abstract figure). Thus, determining how gene expression is controlled by examining chromatin structure, TFs and epigenetic regulators associated with aberrantly expressed genes in pulmonary vascular cells in chronic PH, may uncover new PH therapeutic targets.

Mitochondrial metabolism in pulmonary hypertension: beyond mountains there are mountains

Pulmonary hypertension (PH) is a heterogeneous and fatal disease of the lung vasculature, where metabolic and mitochondrial dysfunction may drive pathogenesis. Similar to the Warburg effect in cancer, a shift from mitochondrial oxidation to glycolysis occurs in diseased pulmonary vessels and the right ventricle. However, appreciation of metabolic events in PH beyond the Warburg effect is only just emerging. This Review discusses molecular, translational, and clinical concepts centered on the mitochondria and highlights promising, controversial, and challenging areas of investigation. If we can move beyond the "mountains" of obstacles in this field and elucidate these fundamental tenets of pulmonary vascular metabolism, such work has the potential to usher in much-needed diagnostic and therapeutic approaches for the mitochondrial and metabolic management of PH.

R213G polymorphism in SOD3 protects against bleomycin-induced inflammation and attenuates induction of proinflammatory pathways

Extracellular superoxide dismutase (EC-SOD), one of three mammalian SOD isoforms, is the sole extracellular enzymatic defense against superoxide. A known human single nucleotide polymorphism (SNP) in the matrix-binding domain of EC-SOD characterized by an arginine-to-glycine substitution at position 213 (R213G) redistributes EC-SOD from the matrix into extracellular fluids. We previously reported that knock-in mice harboring the human R213G SNP (R213G mice) exhibited enhanced resolution of inflammation with subsequent protection against fibrosis following bleomycin treatment compared with wild-type (WT) littermates. Herein we set out to determine the underlying pathways with RNA-Seq analysis of WT and R213G lungs 7 days post-PBS and bleomycin. RNA-Seq analysis uncovered significant differential gene expression changes induced in WT and R213G strains in response to bleomycin. Ingenuity Pathways Analysis was used to predict differentially regulated up- and downstream processes based on transcriptional changes. Most prominent was the induction of inflammatory and immune responses in WT mice, which were suppressed in the R213G mice. Specifically, PKC signaling in T lymphocytes, IL-6, and NFΚB signaling were opposed in WT mice when compared with R213G. Several upstream regulators such as IFNγ, IRF3, and IKBKG were implicated in the divergent responses between WT and R213G mice. Our data suggest that the redistributed EC-SOD due to the R213G SNP attenuates the dysregulated inflammatory responses observed in WT mice. We speculate that redistributed EC-SOD protects against dysregulated alveolar inflammation via reprogramming of recruited immune cells toward a proresolving state.

NOX4 expression and distal arteriolar remodeling correlate with pulmonary hypertension in COPD

Background

Pulmonary hypertension (PH) in chronic obstructive pulmonary disease (COPD) is suggested as the consequence of emphysematous destruction of vascular bed and hypoxia of pulmonary microenvironment, mechanisms underpinning its pathogenesis however remain elusive. The dysregulated expression of nicotinamide adenine dinucleotide phosphate (NADPH)-oxidases and superoxide generation by pulmonary vasculatures have significant implications in the hypoxia-induced PH.

Methods

In this study, the involvement of NADPH oxidase subunit 4 (NOX4) in pulmonary arteriolar remodeling of PH in COPD was investigated by ascertaining the morphological alteration of pulmonary arteries and pulmonary blood flow using cardiac magnetic resonance imaging (cMRI), and the expression and correlation of NOX4 with pulmonary vascular remodeling and pulmonary functions in COPD lungs.

Results

Results demonstrated that an augmented expression of NOX4 was correlated with the increased volume of pulmonary vascular wall in COPD lung. While the volume of distal pulmonary arteries was inversely correlated with pulmonary functions, despite it was positively associated with the main pulmonary artery distensibility, right ventricular myocardial mass end-systolic and right ventricular myocardial mass end-diastolic in COPD. In addition, an increased malondialdehyde and a decreased superoxide dismutase were observed in sera of COPD patients. Mechanistically, the abundance of NOX4 and production of reactive oxygen species (ROS) in pulmonary artery smooth muscle cells could be dynamically induced by transforming growth factor-beta (TGF-β), which in turn led pulmonary arteriolar remodeling in COPD lungs.

Conclusion

These results suggest that the NOX4-derived ROS production may play a key role in the development of PH in COPD by promoting distal pulmonary vascular remodeling.

Pulmonary Arterial Hypertension Affects the Rat Gut Microbiome

We have analysed whether pulmonary arterial hypertension (PAH) alters the rat faecal microbiota. Wistar rats were injected with the VEGF receptor antagonist SU5416 (20 mg/kg s.c.) and followed for 2 weeks kept in hypoxia (10% O₂, PAH) or injected with vehicle and kept in normoxia (controls). Faecal samples were obtained and microbiome composition was determined by 16S rRNA gene sequencing and bioinformatic analysis. No effect of PAH on the global microbiome was found (α- or β-diversity). However, PAH-exposed rats showed gut dysbiosis as indicated by a taxonomy-based analysis. Specifically, PAH rats had a three-fold increase in Firmicutes-to-Bacteroidetes ratio. Within the Firmicutes phylum, there were no large changes in the relative abundance of the bacterial families in PAH. Among Bacteroidetes, all families were less abundant in PAH. A clear separation was observed between the control and PAH clusters based on short chain fatty acid producing bacterial genera. Moreover, acetate was reduced in the serum of PAH rats. In conclusion, faecal microbiota composition is altered as a result of PAH. This misbalanced bacterial ecosystem might in turn play a pathophysiological role in PAH by altering the immunologic, hormonal and metabolic homeostasis.

Egr-1 mediates leptin-induced PPARγ reduction and proliferation of pulmonary artery smooth muscle cells

Loss of peroxisome proliferator-activated receptor γ (PPARγ) has been found to contribute to pulmonary artery smooth muscle cell (PASMC) proliferation and pulmonary arterial remodeling therefore the development of pulmonary hypertension (PH). Yet, the molecular mechanisms underlying PPARγ reduction in PASMC remain poorly understood. Here, we demonstrated that leptin dose- and time-dependently inducued PPARγ down-regulation and proliferation of primary cultured rat PASMC, this was accompanied with the activation of extracellular regulated kinase1/2 (ERK1/2) signaling pathway and subsequent induction of early growth response-1 (Egr-1) expression. The presence of MEK inhibitors U0126 or PD98059, or prior silencing Egr-1 with small interfering RNA suppressed leptin-induced PPARγ reduction. In addition, activation of PPARγ by pioglitazone or targeting ERK1/2/Egr-1 suppressed leptin-induced PASMC proliferation. Taken together, our study indicates that ERK1/2 signaling pathway-mediated leptin-induced PPARγ reduction and PASMC proliferation through up-regulation of Egr-1 and suggests that targeting leptin/ERK1/2/Egr-1 pathway might have potential value in ameliorating vascular remodeling and benefit PH.

Murine Models of Heart Failure with Preserved Ejection Fraction: a "Fishing Expedition"

Heart failure with preserved ejection fraction (HFpEF) is characterized by signs and symptoms of HF in the presence of a normal left ventricular (LV) ejection fraction (EF). Despite accounting for up to 50% of all clinical presentations of HF, the mechanisms implicated in HFpEF are poorly understood, thus precluding effective therapy. The pathophysiological heterogeneity in the HFpEF phenotype also contributes to this disease and likely to the absence of evidence-based therapies. Limited access to human samples and imperfect animal models that completely recapitulate the human HFpEF phenotype have impeded our understanding of the mechanistic underpinnings that exist in this disease. Aging and comorbidities such as atrial fibrillation, hypertension, diabetes and obesity, pulmonary hypertension and renal dysfunction are highly associated with HFpEF. Yet, the relationship and contribution between them remains ill-defined. This review discusses some of the distinctive clinical features of HFpEF in association with these comorbidities and highlights the advantages and disadvantage of commonly used murine models, used to study the HFpEF phenotype.

Role of early growth response 1 in arteriogenesis: Impact on vascular cell proliferation and leukocyte recruitment in vivo

Based on previous findings that early growth response 1 (Egr-1) participates in leukocyte recruitment and cell proliferation in vitro, this study was designed to investigate its mode of action during arteriogenesis in vivo. In a model of peripheral arteriogenesis, Egr-1 was significantly upregulated in growing collaterals of wild-type (WT) mice, both on mRNA and protein level. Egr-1−/− mice demonstrated delayed arteriogenesis after femoral artery ligation. They further showed increased levels of monocytes and granulocytes in the circulation, but reduced levels in adductor muscles under baseline conditions. After femoral artery ligation, elevated numbers of macrophages were detected in the perivascular zone of collaterals in Egr-1−/− mice and mRNA of leukocyte recruitment mediators was upregulated. Other Egr family members (Egr-2 to -4) were significantly upregulated only in Egr-1−/− mice, suggesting a mechanism of counterbalancing Egr-1 deficiency. Moreover, splicing factor-1, downregulated in WT mice after femoral artery ligation in the process of increased vascular cell proliferation, was upregulated in Egr-1−/− mice. αSM-actin on the other hand, significantly downregulated in WT mice, showed no differential expression in Egr-1−/− mice. While cell cycle regulator cyclin E and cdc20 were upregulated in Egr-1−/− mice, cyclin D1 expression decreased below the detection limit in collaterals, and the proliferation marker ki67 was not differentially expressed. In conclusion, compensation for deficiency in Egr-1 function in leukocyte recruitment can presumably be mediated by other transcription factors; however, Egr-1 is indispensable for effective vascular cell cycle progression in arteriogenesis.

Contribution of reactive oxygen species to the pathogenesis of pulmonary arterial hypertension

Pulmonary arterial hypertension is associated with a decreased antioxidant capacity. However, neither the contribution of reactive oxygen species to pulmonary vasoconstrictor sensitivity, nor the therapeutic efficacy of antioxidant strategies in this setting are known. We hypothesized that reactive oxygen species play a central role in mediating both vasoconstrictor and arterial remodeling components of severe pulmonary arterial hypertension. We examined the effect of the chemical antioxidant, TEMPOL, on right ventricular systolic pressure, vascular remodeling, and enhanced vasoconstrictor reactivity in both chronic hypoxia and hypoxia/SU5416 rat models of pulmonary hypertension. SU5416 is a vascular endothelial growth factor receptor antagonist and the combination of chronic hypoxia/SU5416 produces a model of severe pulmonary arterial hypertension with vascular plexiform lesions/fibrosis that is not present with chronic hypoxia alone. The major findings from this study are: 1) compared to hypoxia alone, hypoxia/SU5416 exposure caused more severe pulmonary hypertension, right ventricular hypertrophy, adventitial lesion formation, and greater vasoconstrictor sensitivity through a superoxide and Rho kinase-dependent Ca2+ sensitization mechanism. 2) Chronic hypoxia increased medial muscularization and superoxide levels, however there was no effect of SU5416 to augment these responses. 3) Treatment with TEMPOL decreased right ventricular systolic pressure in both hypoxia and hypoxia/SU5416 groups. 4) This effect of TEMPOL was associated with normalization of vasoconstrictor responses, but not arterial remodeling. Rather, medial hypertrophy and adventitial fibrotic lesion formation were more pronounced following chronic TEMPOL treatment in hypoxia/SU5416 rats. Our findings support a major role for reactive oxygen species in mediating enhanced vasoconstrictor reactivity and pulmonary hypertension in both chronic hypoxia and hypoxia/SU5416 rat models, despite a paradoxical effect of antioxidant therapy to exacerbate arterial remodeling in animals with severe pulmonary arterial hypertension in the hypoxia/SU5416 model.

Superoxide Dismutase 3 R213G Single-Nucleotide Polymorphism Blocks Murine Bleomycin-Induced Fibrosis and Promotes Resolution of Inflammation

Loss of extracellular superoxide dismutase 3 (SOD3) contributes to inflammatory and fibrotic lung diseases. The human SOD3 R213G polymorphism decreases matrix binding, redistributing SOD3 from the lung to extracellular fluids, and protects against LPS-induced alveolar inflammation. We used R213G mice expressing a naturally occurring single-nucleotide polymorphism, rs1799895, within the heparin-binding domain of SOD3, which results in an amino acid substitution at position 213 to test the hypothesis that the redistribution of SOD3 into the extracellular fluids would impart protection against bleomycin-induced lung fibrosis and secondary pulmonary hypertension (PH). In R213G mice, SOD3 content and activity was increased in extracellular fluids and decreased in lung at baseline, with greater increases in bronchoalveolar lavage fluid (BALF) SOD3 compared with wild-type mice 3 days after bleomycin. R213G mice developed less fibrosis based on pulmonary mechanics, fibrosis scoring, collagen quantification, and gene expression at 21 days, and less PH by right ventricular systolic pressure and pulmonary arteriole medial wall thickening at 28 days. In wild-type mice, macrophages, lymphocytes, neutrophils, proinflammatory cytokines, and protein increased in BALF on Day 7 and/or 21. In R213G mice, total BALF cell counts increased on Day 7 but resolved by 21 days. At 1 or 3 days, BALF pro- and antiinflammatory cytokines and BALF protein were higher in R213G mice, resolving by 21 days. We conclude that the redistribution of SOD3 as a result of the R213G single-nucleotide polymorphism protects mice from bleomycin-induced fibrosis and secondary PH by improved resolution of alveolar inflammation.

Altered Redox Balance in the Development of Chronic Hypoxia-induced Pulmonary Hypertension

Normally, the pulmonary circulation is maintained in a low-pressure, low-resistance state with little resting tone. Pulmonary arteries are thin-walled and rely heavily on pulmonary arterial distension and recruitment for reducing pulmonary vascular resistance when cardiac output is elevated. Under pathophysiological conditions, however, active vasoconstriction and vascular remodeling lead to enhanced pulmonary vascular resistance and subsequent pulmonary hypertension (PH). Chronic hypoxia is a critical pathological factor associated with the development of PH resulting from airway obstruction (COPD, sleep apnea), diffusion impairment (interstitial lung disease), developmental lung abnormalities, or high altitude exposure (World Health Organization [WHO]; Group III). The rise in pulmonary vascular resistance increases right heart afterload causing right ventricular hypertrophy that can ultimately lead to right heart failure in patients with chronic lung disease. PH is typically characterized by diminished paracrine release of vasodilators, antimitogenic factors, and antithrombotic factors (e.g., nitric oxide and protacyclin) and enhanced production of vasoconstrictors and mitogenic factors (e.g., reactive oxygen species and endothelin-1) from the endothelium and lung parenchyma. In addition, phenotypic changes to pulmonary arterial smooth muscle cells (PASMC), including alterations in Ca²⁺ homeostasis, Ca²⁺ sensitivity, and activation of transcription factors are thought to play prominent roles in the development of both vasoconstrictor and arterial remodeling components of hypoxia-associated PH. These changes in PASMC function are briefly reviewed in Sect. 1 and the influence of altered reactive oxygen species homeostasis on PASMC function discussed in Sects. 2-4.

Redox Regulation of the Superoxide Dismutases SOD3 and SOD2 in the Pulmonary Circulation

When evaluating the role of redox-regulating signaling in pulmonary vascular diseases, it is intriguing to consider the modulation of key antioxidant enzymes like superoxide dismutase (SOD) because SOD isoforms are regulated by redox reactions, and, in turn, modulate downstream redox sensitive processes. The emerging field of redox biology is built upon understanding the regulation and consequences of tightly controlled and specific reduction-oxidation reactions that are critical for diverse cellular processes including cell signaling. Of relevance, both the site of production of specific reactive oxygen and nitrogen species and the site of the antioxidant defenses are highly compartmentalized within the cell. For example, superoxide is generated during oxidative phosphorylation in the mitochondria as well as by a number of enzymatic sources within the cytosol and at the cell membrane. In the pulmonary circulation, these sources include the mitochondrial electron transport chain, NADPH oxidases (NOX1-4, Duox1,2), nitric oxide synthases, and xanthine oxidase; this important topic has been thoroughly reviewed recently [1]. In parallel with these different cellular sites of superoxide production, the three SOD isoforms are also specifically localized to the cytosol (SOD1), mitochondria (SOD2) or extracellular compartment (SOD3). This chapter focuses on the role of redox mechanisms regulating SOD2 and SOD3, with an emphasis on these processes in the setting of pulmonary hypertension.

Update on novel targets and potential treatment avenues in pulmonary hypertension

Pulmonary hypertension (PH) is a condition marked by a combination of constriction and remodeling within the pulmonary vasculature. It remains a disease without a cure, as current treatments were developed with a focus on vasodilatory properties but do not reverse the remodeling component. Numerous recent advances have been made in the understanding of cellular processes that drive pathologic remodeling in each layer of the vessel wall as well as the accompanying maladaptive changes in the right ventricle. In particular, the past few years have yielded much improved insight into the pathways that contribute to altered metabolism, mitochondrial function, and reactive oxygen species signaling and how these pathways promote the proproliferative, promigratory, and antiapoptotic phenotype of the vasculature during PH. Additionally, there have been significant advances in numerous other pathways linked to PH pathogenesis, such as sex hormones and perivascular inflammation. Novel insights into cellular pathology have suggested new avenues for the development of both biomarkers and therapies that will hopefully bring us closer to the elusive goal: a therapy leading to reversal of disease.

Histone deacetylation contributes to low extracellular superoxide dismutase expression in human idiopathic pulmonary arterial hypertension

Epigenetic mechanisms, including DNA methylation and histone acetylation, regulate gene expression in idiopathic pulmonary arterial hypertension (IPAH). These mechanisms can modulate expression of extracellular superoxide dismutase (SOD3 or EC-SOD), a key vascular antioxidant enzyme, and loss of vascular SOD3 worsens outcomes in animal models of pulmonary arterial hypertension. We hypothesized that SOD3 gene expression is decreased in patients with IPAH due to aberrant DNA methylation and/or histone deacetylation. We used lung tissue and pulmonary artery smooth muscle cells (PASMC) from subjects with IPAH at transplantation and from failed donors (FD). Lung SOD3 mRNA expression and activity was decreased in IPAH vs. FD. In contrast, mitochondrial SOD (Mn-SOD or SOD2) protein expression was unchanged and intracellular SOD activity was unchanged. Using bisulfite sequencing in genomic lung or PASMC DNA, we found the methylation status of the SOD3 promoter was similar between FD and IPAH. Furthermore, treatment with 5-aza-2'-deoxycytidine did not increase PASMC SOD3 mRNA, suggesting DNA methylation was not responsible for PASMC SOD3 expression. Though total histone deacetylase (HDAC) activity, histone acetyltransferase (HAT) activity, acetylated histones, and acetylated SP1 were similar between IPAH and FD, treatment with two selective class I HDAC inhibitors increased SOD3 only in IPAH PASMC. Class I HDAC3 siRNA also increased SOD3 expression. Trichostatin A, a pan-HDAC inhibitor, decreased proliferation in IPAH, but not in FD PASMC. These data indicate that histone deacetylation, specifically via class I HDAC3, decreases SOD3 expression in PASMC and HDAC inhibitors may protect IPAH in part by increasing PASMC SOD3 expression.

Aberrant Pulmonary Vascular Growth and Remodeling in Bronchopulmonary Dysplasia

In contrast to many other organs, a significant portion of lung development occurs after birth during alveolarization, thus rendering the lung highly susceptible to injuries that may disrupt this developmental process. Premature birth heightens this susceptibility, with many premature infants developing the chronic lung disease, bronchopulmonary dysplasia (BPD), a disease characterized by arrested alveolarization. Over the past decade, tremendous progress has been made in the elucidation of mechanisms that promote postnatal lung development, including extensive data suggesting that impaired pulmonary angiogenesis contributes to the pathogenesis of BPD. Moreover, in addition to impaired vascular growth, patients with BPD also frequently demonstrate alterations in pulmonary vascular remodeling and tone, increasing the risk for persistent hypoxemia and the development of pulmonary hypertension. In this review, an overview of normal lung development will be presented, and the pathologic features of arrested development observed in BPD will be described, with a specific emphasis on the pulmonary vascular abnormalities. Key pathways that promote normal pulmonary vascular development will be reviewed, and the experimental and clinical evidence demonstrating alterations of these essential pathways in BPD summarized.

Activation of Calpain-2 by Mediators in Pulmonary Vascular Remodeling of Pulmonary Arterial Hypertension

Calpain mediates collagen synthesis and cell proliferation and plays an important role in pulmonary vascular remodeling in pulmonary arterial hypertension (PAH). In the present study, we investigated whether and how calpain is activated by PAH mediators in pulmonary artery smooth muscle cells (PASMCs). These data show that smooth muscle-specific knockout of calpain attenuated and knockout of calpastatin potentiated pulmonary vascular remodeling and pulmonary hypertension. Treatment of PASMCs with the PAH mediators platelet-derived growth factor (PDGF), serotonin, H2O2, endothelin-1, and IL-6 caused significant increases in calpain activity, cell proliferation, and collagen-I protein level without changes in protein levels of calpain-1 and -2. The calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxymethyl ester) (BAPTA/AM) did not affect calpain activation, but the extracellular signal-regulated kinase (ERK) 1/2 inhibitor PD98059 and knocking down of calpain-2 prevented calpain activation in PAH mediator-treated PASMCs. Mass spectrometry data showed that the phosphorylation of calpain-2 at serine (Ser) 50 was increased and the phosphorylation of calpain-2 at Ser369 was decreased in PDGF-treated PASMCs. The PDGF-induced increase in Ser50 phosphorylation of calpain-2 was prevented by PD98059, whereas dephosphorylation of calpain-2 at Ser369 was blocked by the protein phosphatase 2A inhibitor fostriecin. Furthermore, smooth muscle of pulmonary arteries in PAH animal models and patients with PAH showed higher levels of phospho-Ser50-calpain-2 (P-Ser50) and lower levels of phospho-Ser369-calpain-2 (P-Ser369). These data support that calpain modulates pulmonary vascular remodeling in PAH. PAH mediator-induced activation of calpain is caused by ERK1/2-dependent phosphorylation of calpain-2 at Ser50 and protein phosphatase 2A-dependent dephosphorylation of calpain-2 at Ser369 in pulmonary vascular remodeling of PAH.

Paradoxical Roles of Antioxidant Enzymes: Basic Mechanisms and Health Implications

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are generated from aerobic metabolism, as a result of accidental electron leakage as well as regulated enzymatic processes. Because ROS/RNS can induce oxidative injury and act in redox signaling, enzymes metabolizing them will inherently promote either health or disease, depending on the physiological context. It is thus misleading to consider conventionally called antioxidant enzymes to be largely, if not exclusively, health protective. Because such a notion is nonetheless common, we herein attempt to rationalize why this simplistic view should be avoided. First we give an updated summary of physiological phenotypes triggered in mouse models of overexpression or knockout of major antioxidant enzymes. Subsequently, we focus on a series of striking cases that demonstrate "paradoxical" outcomes, i.e., increased fitness upon deletion of antioxidant enzymes or disease triggered by their overexpression. We elaborate mechanisms by which these phenotypes are mediated via chemical, biological, and metabolic interactions of the antioxidant enzymes with their substrates, downstream events, and cellular context. Furthermore, we propose that novel treatments of antioxidant enzyme-related human diseases may be enabled by deliberate targeting of dual roles of the pertaining enzymes. We also discuss the potential of "antioxidant" nutrients and phytochemicals, via regulating the expression or function of antioxidant enzymes, in preventing, treating, or aggravating chronic diseases. We conclude that "paradoxical" roles of antioxidant enzymes in physiology, health, and disease derive from sophisticated molecular mechanisms of redox biology and metabolic homeostasis. Simply viewing antioxidant enzymes as always being beneficial is not only conceptually misleading but also clinically hazardous if such notions underpin medical treatment protocols based on modulation of redox pathways.

Bone morphogenetic protein receptor type II deficiency and increased inflammatory cytokine production. A gateway to pulmonary arterial hypertension

RATIONALE

Mutations in bone morphogenetic protein receptor type II (BMPR-II) underlie most cases of heritable pulmonary arterial hypertension (PAH). However, disease penetrance is only 20-30%, suggesting a requirement for additional triggers. Inflammation is emerging as a key disease-related factor in PAH, but to date there is no clear mechanism linking BMPR-II deficiency and inflammation.

OBJECTIVES

To establish a direct link between BMPR-II deficiency, a consequentially heightened inflammatory response, and development of PAH.

METHODS

We used pulmonary artery smooth muscle cells from Bmpr2(+/-) mice and patients with BMPR2 mutations and compared them with wild-type controls. For the in vivo model, we used mice heterozygous for a null allele in Bmpr2 (Bmpr2(+/-)) and wild-type littermates.

MEASUREMENTS AND MAIN RESULTS

Acute exposure to LPS increased lung and circulating IL-6 and KC (IL-8 analog) levels in Bmpr2(+/-) mice to a greater extent than in wild-type controls. Similarly, pulmonary artery smooth muscle cells from Bmpr2(+/-) mice and patients with BMPR2 mutations produced higher levels of IL-6 and KC/IL-8 after lipopolysaccharide stimulation compared with controls. BMPR-II deficiency in mouse and human pulmonary artery smooth muscle cells was associated with increased phospho-STAT3 and loss of extracellular superoxide dismutase. Chronic lipopolysaccharide administration caused pulmonary hypertension in Bmpr2(+/-) mice but not in wild-type littermates. Coadministration of tempol, a superoxide dismutase mimetic, ameliorated the exaggerated inflammatory response and prevented development of PAH.

CONCLUSIONS

This study demonstrates that BMPR-II deficiency promotes an exaggerated inflammatory response in vitro and in vivo, which can instigate development of pulmonary hypertension.

N-n-butyl Haloperidol Iodide Protects against Hypoxia/Reoxygenation Injury in Cardiac Microvascular Endothelial Cells by Regulating the ROS/MAPK/Egr-1 Pathway

Endothelium dysfunction induced by reactive oxygen species (ROS) is an important initial event at the onset of myocardial ischemia/reperfusion in which the Egr-1 transcription factor often serves as a master switch for various damage pathways following reperfusion injury. We hypothesized that an intracellular ROS/MAPK/Egr-1 signaling pathway is activated in cardiac microvascular endothelial cells (CMECs) following hypoxia/reoxygenation (H/R). ROS generation, by either H/R or the ROS donor xanthine oxidase-hypoxanthine (XO/HX) activated all three MAPKs (ERK1/2, JNK, p38), and induced Egr-1 expression and Egr-1 DNA-binding activity in CMECs, whereas ROS scavengers (EDA and NAC) had the opposite effect following H/R. Inhibitors of all three MAPKs individually inhibited induction of Egr-1 expression by H/R in CMECs. Moreover, N-n-butyl haloperidol (F₂), previously shown to protect cardiomyocytes subjected to I/R, dose-dependently downregulated H/R-induced ROS generation, MAPK activation, and Egr-1 expression and activity in CMECs, whereas XO/HX and MAPK activators (EGF, anisomycin) antagonized the effects of F₂. Inhibition of the ROS/MAPK/Egr-1 signaling pathway, by either F₂, NAC, or inhibition of MAPK, increased CMEC viability and the GSH/GSSG ratio, and decreased Egr-1 nuclear translocation. These results show that the ROS/MAPK/Egr-1 signaling pathway mediates H/R injury in CMECs, and F₂ blocks this pathway to protect against H/R injury and further alleviate myocardial I/R injury.

Lack of EC-SOD worsens alveolar and vascular development in a neonatal mouse model of bleomycin-induced bronchopulmonary dysplasia and pulmonary hypertension

BACKGROUND

Pulmonary hypertension (PH) worsens clinical outcomes in former preterm infants with bronchopulmonary dysplasia (BPD). Oxidant stress disrupts alveolar and vascular development in models of BPD. Bleomycin causes oxidative stress and induces BPD and PAH in neonatal rats. Disruption in the vascular endothelial growth factor (VEGF) and nitric oxide signaling pathways contributes to BPD. We hypothesized that loss of EC-SOD would worsen PAH associated with BPD in a neonatal mouse model of bleomycin-induced BPD by disrupting the VEGF/NO signaling pathway.

METHODS

Neonatal wild-type mice (WT), and mice lacking EC-SOD (EC-SOD KO) received intraperitoneal bleomycin (2 units/kg) or phosphate-buffered saline (PBS) three times weekly and were evaluated at weeks 3 or 4.

RESULTS

Lack of EC-SOD impaired alveolar development and resulted in PH (elevated right ventricular systolic pressures, right ventricular hypertrophy (RVH)), decreased vessel density, and increased small vessel muscularization. Exposure to bleomycin further impaired alveolar development, worsened RVH and vascular remodeling. Lack of EC-SOD and bleomycin treatment decreased lung total and phosphorylated VEGFR2 and eNOS protein expression.

CONCLUSION

EC-SOD is critical in preserving normal lung development and loss of EC-SOD results in disrupted alveolar development, PAH and vascular remodeling at baseline, which is further worsened with bleomycin and associated with decreased activation of VEGFR2.

Effect of N-n-butyl haloperidol iodide on ROS/JNK/Egr-1 signaling in H9c2 cells after hypoxia/reoxygenation

Reactive oxygen species (ROS)-induced oxidative stress in cells is an important pathophysiological process during myocardial ischemia/reperfusion (I/R) injury, and the transcription factor Egr-1 is a master switch for various damage pathways during reperfusion injury. An in vitro model of myocardial I/R injury and H9c2 cardiomyoblast cells hypoxia/reoxygenation (H/R) was used to assess whether there is abnormal intracellular ROS/JNK/Egr-1 signaling. We also assessed whether N-n-butyl haloperidol (F2), which exerts protective effects during myocardial I/R injury, can modulate this pathway. H/R induced ROS generation, JNK activation, and increased the expression of Egr-1 protein in H9c2 cells. The ROS scavengers edaravone (EDA) and N-acetyl-L-cysteine (NAC) reduced ROS level, downregulated JNK activation, and Egr-1 expression in H9c2 cells after H/R. The JNK inhibitor SP600125 inhibited Egr-1 overexpression in H9c2 cells caused by H/R. F2 could downregulate H/R-induced ROS level, JNK activation, and Egr-1 expression in H9c2 cells in a dose-dependent manner. The ROS donor hypoxanthine-xanthine oxidase (XO/HX) and the JNK activator ANISO antagonized the effects of F2. Therefore, H/R activates ROS/Egr-1 signaling pathway in H9c2 cells, and JNK activation plays an important role in this pathway. F2 regulates H/R-induced ROS/JNK/Egr-1 signaling, which might be an important mechanism by which it antagonizes myocardial I/R injury.

Altered pulmonary artery endothelial-smooth muscle cell interactions in experimental congenital diaphragmatic hernia

BACKGROUND

Pulmonary hypertension (PH) secondary to vascular remodeling contributes to poor outcomes in congenital diaphragmatic hernia (CDH), however mechanisms responsible are unknown. We hypothesized that pulmonary artery endothelial cell (PAEC) dysfunction contributes to smooth muscle cell (SMC) hyperplasia in experimental CDH.

METHODS

PAEC and SMC were isolated from fetal sheep with experimental CDH and controls. SMC growth was assessed alone and with SOD plus catalase and during coculture with control or CDH PAEC with and without ET-1 siRNA transfection. ET-1 protein was measured in PAEC and SMC lysates and supernatant. ROS production was measured in normal and CDH PAECs with and without ET-1 siRNA. PAEC growth and tube formation were measured with SOD plus catalase.

RESULTS

CDH SMC growth was decreased and increased with coculture with CDH PAEC more than control PAEC. Treatment of CDH PAEC with SOD plus catalase or ET-1 siRNA prevented the increase in SMC growth seen with coculture. ET-1 protein was increased in CDH PAEC and SMC. ROS production was increased in CDH PAEC and decreased with ET-1 SiRNA. SOD plus catalase restored CDH PAEC growth and tube formation.

CONCLUSION

PAEC dysfunction in experimental CDH increases SMC proliferation via ET-1 induced ROS production by PAEC.

Extracellular superoxide dismutase regulates the expression of small gtpase regulatory proteins GEFs, GAPs, and GDI

Extracellular superoxide dismutase (SOD3), which catalyzes the dismutation of superoxide anions to hydrogen peroxide at the cell membranes, regulates the cellular growth in a dose-dependent manner. This enzyme induces primary cell proliferation and immortalization at low expression levels whereas it activates cancer barrier signaling through the p53-p21 pathway at high expression levels, causing growth arrest, senescence, and apoptosis. Because previous reports suggested that the SOD3–induced reduction in the rates of cellular growth and migration also occurred in the absence of functional p53 signaling, in the current study we investigated the SOD3-induced growth-suppressive mechanisms in anaplastic thyroid cancer cells. Based on our data, the robust over-expression of SOD3 increased the level of phosphorylation of the EGFR, ERBB2, RYK, ALK, FLT3, and EPHA10 receptor tyrosine kinases with the consequent downstream activation of the SRC, FYN, YES, HCK, and LYN kinases. However, pull-down experiments focusing on the small GTPase RAS, RAC, CDC42, and RHO revealed a reduced level of growth and migration signal transduction, such as the lack of stimulation of the mitogen pathway, in the SOD3 over-expressing cells, which was confirmed by MEK1/2 and ERK1/2 Western blotting analysis. Interestingly, the mRNA expression analyses indicated that SOD3 regulated the expression of guanine nucleotide-exchange factors (RHO GEF16, RAL GEF RGL1), GTPase-activating proteins (ARFGAP ADAP2, RAS GAP RASAL1, RGS4), and a Rho guanine nucleotide-disassociation inhibitor (RHO GDI 2) in a dose dependent manner, thus controlling signaling through the small G protein GTPases. Therefore, our current data may suggest the occurrence of dose-dependent SOD3–driven control of the GTP loading of small G proteins indicating a novel growth regulatory mechanism of this enzyme.

Selective depletion of vascular EC-SOD augments chronic hypoxic pulmonary hypertension

Excess superoxide has been implicated in pulmonary hypertension (PH). We previously found lung overexpression of the antioxidant extracellular superoxide dismutase (EC-SOD) attenuates PH and pulmonary artery (PA) remodeling. Although comprising a small fraction of total SOD activity in most tissues, EC-SOD is abundant in arteries. We hypothesize that the selective loss of vascular EC-SOD promotes hypoxia-induced PH through redox-sensitive signaling pathways. EC-SOD(loxp/loxp) × Tg(cre/SMMHC) mice (SMC EC-SOD KO) received tamoxifen to conditionally deplete smooth muscle cell (SMC)-derived EC-SOD. Mice were exposed to hypobaric hypoxia for 35 days, and PH was assessed by right ventricular systolic pressure measurements and right ventricle hypertrophy. Vascular remodeling was evaluated by morphometric analysis and two-photon microscopy for collagen. We examined cGMP content and soluble guanylate cyclase expression and activity in lung, lung phosphodiesterase 5 (PDE5) expression and activity, and expression of endothelial nitric oxide synthase and GTP cyclohydrolase-1 (GTPCH-1), the rate-limiting enzyme in tetrahydrobiopterin synthesis. Knockout of SMC EC-SOD selectively decreased PA EC-SOD without altering total lung EC-SOD. PH and vascular remodeling induced by chronic hypoxia was augmented in SMC EC-SOD KO. Depletion of SMC EC-SOD did not impact content or activity of lung soluble guanylate cyclase or PDE5, yet it blunted the hypoxia-induced increase in cGMP. Although total eNOS was not altered, active eNOS and GTPCH-1 decreased with hypoxia only in SMC EC-SOD KO. We conclude that the localized loss of PA EC-SOD augments chronic hypoxic PH. In addition to oxidative inactivation of NO, deletion of EC-SOD seems to reduce eNOS activity, further compromising pulmonary vascular function.

Role of reactive oxygen species in neonatal pulmonary vascular disease

SIGNIFICANCE

Abnormal lung development in the perinatal period can result in severe neonatal complications, including persistent pulmonary hypertension (PH) of the newborn and bronchopulmonary dysplasia. Reactive oxygen species (ROS) play a substantive role in the development of PH associated with these diseases. ROS impair the normal pulmonary artery (PA) relaxation in response to vasodilators, and ROS are also implicated in pulmonary arterial remodeling, both of which can increase the severity of PH.

RECENT ADVANCES

PA ROS levels are elevated when endogenous ROS-generating enzymes are activated and/or when endogenous ROS scavengers are inactivated. Animal models have provided valuable insights into ROS generators and scavengers that are dysregulated in different forms of neonatal PH, thus identifying potential therapeutic targets.

CRITICAL ISSUES

General antioxidant therapy has proved ineffective in reversing PH, suggesting that it is necessary to target specific signaling pathways for successful therapy.

FUTURE DIRECTIONS

Development of novel selective pharmacologic inhibitors along with nonantioxidant therapies may improve the treatment outcomes of patients with PH, while further investigation of the underlying mechanisms may enable earlier detection of the disease.

A common polymorphism in extracellular superoxide dismutase affects cardiopulmonary disease risk by altering protein distribution

BACKGROUND

The enzyme extracellular superoxide dismutase (EC-SOD; SOD3) is a major antioxidant defense in lung and vasculature. A nonsynonomous single-nucleotide polymorphism in EC-SOD (rs1799895) leads to an arginine to glycine amino acid substitution at position 213 (R213G) in the heparin-binding domain. In recent human genetic association studies, this single-nucleotide polymorphism attenuates the risk of lung disease, yet paradoxically increases the risk of cardiovascular disease.

METHODS AND RESULTS

Capitalizing on the complete sequence homology between human and mouse in the heparin-binding domain, we created an analogous R213G single-nucleotide polymorphism knockin mouse. The R213G single-nucleotide polymorphism did not change enzyme activity, but shifted the distribution of EC-SOD from lung and vascular tissue to extracellular fluid (eg, bronchoalveolar lavage fluid and plasma). This shift reduces susceptibility to lung disease (lipopolysaccharide-induced lung injury) and increases susceptibility to cardiopulmonary disease (chronic hypoxic pulmonary hypertension).

CONCLUSIONS

We conclude that EC-SOD provides optimal protection when localized to the compartment subjected to extracellular oxidative stress: thus, the redistribution of EC-SOD from the lung and pulmonary circulation to the extracellular fluids is beneficial in alveolar lung disease but detrimental in pulmonary vascular disease. These findings account for the discrepant risk associated with R213G in humans with lung diseases compared with cardiovascular diseases.

Chronic hypoxia limits H2O2-induced inhibition of ASIC1-dependent store-operated calcium entry in pulmonary arterial smooth muscle

Our laboratory shows that acid-sensing ion channel 1 (ASIC1) contributes to the development of hypoxic pulmonary hypertension by augmenting store-operated Ca(2+) entry (SOCE) that is associated with enhanced agonist-induced vasoconstriction and arterial remodeling. However, this enhanced Ca(2+) influx following chronic hypoxia (CH) is not dependent on an increased ASIC1 protein expression in pulmonary arterial smooth muscle cells (PASMC). It is well documented that hypoxic pulmonary hypertension is associated with changes in redox potential and reactive oxygen species homeostasis. ASIC1 is a redox-sensitive channel showing increased activity in response to reducing agents, representing an alternative mechanism of regulation. We hypothesize that the enhanced SOCE following CH results from removal of an inhibitory effect of hydrogen peroxide (H2O2) on ASIC1. We found that CH increased PASMC superoxide (O2 (·-)) and decreased rat pulmonary arterial H2O2 levels. This decrease in H2O2 is a result of decreased Cu/Zn superoxide dismutase expression and activity, as well as increased glutathione peroxidase (GPx) expression and activity following CH. Whereas H2O2 inhibited ASIC1-dependent SOCE in PASMC from control and CH animals, addition of catalase augmented ASIC1-mediated SOCE in PASMC from control rats but had no further effect in PASMC from CH rats. These data suggest that, under control conditions, H2O2 inhibits ASIC1-dependent SOCE. Furthermore, H2O2 levels are decreased following CH as a result of diminished dismutation of O2 (·-) and increased H2O2 catalysis through GPx-1, leading to augmented ASIC1-dependent SOCE.

Inhibition of Egr1 expression underlies the anti-mitogenic effects of cAMP in vascular smooth muscle cells

AIMS

Cyclic AMP inhibits vascular smooth muscle cell (VSMC) proliferation which is important in the aetiology of numerous vascular diseases. The anti-mitogenic properties of cAMP in VSMC are dependent on activation of protein kinase A (PKA) and exchange protein activated by cAMP (EPAC), but the mechanisms are unclear.

METHODS AND RESULTS

Selective agonists of PKA and EPAC synergistically inhibited Egr1 expression, which was essential for VSMC proliferation. Forskolin, adenosine, A2B receptor agonist BAY60-6583 and Cicaprost also inhibited Egr1 expression in VSMC but not in endothelial cells. Inhibition of Egr1 by cAMP was independent of cAMP response element binding protein (CREB) activity but dependent on inhibition of serum response element (SRE) activity. SRF binding to the Egr1 promoter was not modulated by cAMP stimulation. However, Egr1 expression was dependent on the SRF co-factors Elk1 and 4 but independent of MAL. Inhibition of SRE-dependent Egr1 expression was due to synergistic inhibition of Rac1 activity by PKA and EPAC, resulting in rapid cytoskeleton remodelling and nuclear export of ERK1/2. This was associated with de-phosphorylation of the SRF co-factor Elk1.

CONCLUSION

cAMP inhibits VSMC proliferation by rapidly inhibiting Egr1 expression. This occurs, at least in part, via inhibition of Rac1 activity leading to rapid actin-cytoskeleton remodelling, nuclear export of ERK1/2, impaired Elk1-phosphorylation and inhibition of SRE activity. This identifies one of the earliest mechanisms underlying the anti-mitogenic effects of cAMP in VSMC but not in endothelial cells, making it an attractive target for selective inhibition of VSMC proliferation.

Exposure of mice to chronic hypoxia attenuates pulmonary arterial contractile responses to acute hypoxia by increases in extracellular hydrogen peroxide

Exposing mice to a chronic hypoxic treatment (10% oxygen, 21 days) that promotes pulmonary hypertension was observed to attenuate the pulmonary vasoconstriction response to acute hypoxia (HPV) both in vivo and in isolated pulmonary arteries. Since catalase restored the HPV response in isolated arteries, it appeared to be attenuated by extracellular hydrogen peroxide. Chronic hypoxia promoted the detection of elevated lung superoxide, extracellular peroxide, extracellular SOD expression, and protein kinase G (PKG) activation [based on PKG dimerization and vasodilator-stimulated phosphoprotein (VASP) phosphorylation], suggesting increased generation of extracellular peroxide and PKG activation may contribute to the suppression of HPV. Aorta from mice exposed to 21 days of hypoxia also showed evidence for extracellular hydrogen peroxide, suppressing the relaxation response to acute hypoxia. Peroxide appeared to partially suppress contractions to phenylephrine used in the study of in vitro hypoxic responses. Treatment of mice with the heme precursor δ-aminolevulinic acid (ALA; 50 mg·kg(-1)·day(-1)) during exposure to chronic hypoxia was examined as a pulmonary hypertension therapy because it could potentially activate beneficial cGMP-mediated effects through promoting a prolonged protoporphyrin IX (PpIX)-elicited activation of soluble guanylate cyclase. ALA attenuated pulmonary hypertension, increases in both superoxide and peroxide, and the suppression of in vitro and in vivo HPV responses. ALA generated prolonged detectible increases in PpIX and PKG-associated phosphorylation of VASP, suggesting PKG activation may contribute to suppression of pulmonary hypertension and prevention of alterations in extracellular peroxide that appear to be attenuating HPV responses caused by chronic hypoxia.