Animal models of pulmonary arterial hypertension: the hope for etiological discovery and pharmacological cure

Hypoxia-induced mitogenic factor (HIMF/FIZZ1/RELMalpha) increases lung inflammation and activates pulmonary microvascular endothelial cells via an IL-4-dependent mechanism

Hypoxia-induced mitogenic factor (HIMF), also known as found in inflammatory zone 1 and resistin-like molecule α, belongs to a novel class of cysteine-rich secreted proteins. It exhibits mitogenic and chemotactic properties during pulmonary hypertension-associated vascular remodeling, as well as fibrogenic properties during pulmonary fibrosis. HIMF expression in the lung was reported to be regulated by Th2 cytokines (IL-4 and IL-13) via the transcription factor STAT6 pathway in a bleomycin-induced pulmonary fibrosis model. However, in this study, we found that in the hypoxia-induced pulmonary hypertension model, lung HIMF expression is increased in IL-4 and STAT6 knockout (KO) mice to the same degree as in wild-type (WT) mice, suggesting that induction of HIMF expression does not require Th2 regulation in this model. We also found that HIMF-induced proliferative activity, hypertrophy, collagen, and extracellular matrix deposition in the pulmonary arteries are significantly less in IL-4 KO mice than in WT mice. In addition, HIMF-induced production of angiogenic factors/chemokines, such as vascular endothelial growth factor, MCP-1, and stromal-derived factor-1, in the lung resident cells, as well as macrophage infiltration, were significantly suppressed in the lungs of IL-4 KO mice. We also show that IL-4 was significantly increased in the lungs of HIMF-treated WT mice. Our in vitro studies using pulmonary microvascular endothelial cells revealed that HIMF stimulated cell proliferation, vascular endothelial growth factor expression, and MCP-1 production in a manner that is dependent on the IL-4/IL-4Rα system. These findings suggest that IL-4 signaling may play a significant role in HIMF-induced lung inflammation and vascular remodeling.

Regulation of the Pulmonary Circulation in the Fetus and Newborn

During the development of the pulmonary vasculature in the fetus, many structural and functional changes occur to prepare the lung for the transition to air breathing. The development of the pulmonary circulation is genetically controlled by an array of mitogenic factors in a temporo-spatial order. With advancing gestation, pulmonary vessels acquire increased vasoreactivity. The fetal pulmonary vasculature is exposed to a low oxygen tension environment that promotes high intrinsic myogenic tone and high vasocontractility. At birth, a dramatic reduction in pulmonary arterial pressure and resistance occurs with an increase in oxygen tension and blood flow. The striking hemodynamic differences in the pulmonary circulation of the fetus and newborn are regulated by various factors and vasoactive agents. Among them, nitric oxide, endothelin-1, and prostaglandin I2 are mainly derived from endothelial cells and exert their effects via cGMP, cAMP, and Rho kinase signaling pathways. Alterations in these signaling pathways may lead to vascular remodeling, high vasocontractility, and persistent pulmonary hypertension of the newborn.

Hypoxic upregulation of arginase II in human lung endothelial cells

Activated arginase has been implicated in many diseases including cancer, immune cell dysfunction, infections, and vascular disease. Enhanced arginase activity has been reported in lungs of patients with pulmonary artery hypertension. We used hypoxia as a model for pulmonary hypertension and studied the effect of exposure to hypoxia on arginase activity in human lung microvascular endothelial cells (HMVEC). Hypoxia induces upregulation of arginase activity as well as mRNA and protein levels of arginase II (Arg II), the only arginase isoform we were able to identify in HMVEC. In endothelial cells, arginase shares and competes for the substrate l-arginine with nitric oxide (NO) synthase (NOS). Through regulation of substrate availability for NOS, arginase is able to modulate NO production. To evaluate the role of Arg II in regulation of NO production under hypoxia, we compared NO output (RFL-6 reporter assay) in cells with normal and silenced Arg II. Exposure to hypoxia led to an increase in NO levels produced by HMVEC. Inhibition of Arg II by specific small interfering RNA or by the pharmacological inhibitor BEC additionally enhanced the levels of NO. Another possible role for activated arginase is involvement in regulation of cell proliferation. However, we showed that hypoxia decreased cell proliferation and upregulated Arg II did not have an effect on cell proliferation. Since hypoxia-inducible factors (HIF) are a family of transcriptional factors activated by hypoxia, we tested the possibility of involvement of HIF-1 and HIF-2 in regulation of Arg II under hypoxia. The silencing of HIF-2 but not HIF-1 prevented the activation of Arg II by hypoxia.

Key role of the RhoA/Rho kinase system in pulmonary hypertension

Pulmonary hypertension (PH) is a general term comprising a spectrum of pulmonary hypertensive disorders which have in common an elevation of mean pulmonary arterial pressure (mPAP). The prototypical form of the disease, termed pulmonary arterial hypertension (PAH), is a rare but lethal syndrome with a complex aetiology characterised by increased pulmonary vascular resistance (PVR) and progressive elevation of mPAP; patients generally die from heart failure. Current therapies are inadequate and median survival is less than three years. PH due to chronic hypoxia (CH) is a condition separate from PAH and is strongly associated with chronic obstructive pulmonary disease (COPD). An early event in the pathogenesis of this form of PH is hypoxic pulmonary vasoconstriction (HPV), an acute homeostatic process that maintains the ventilation-perfusion ratio during alveolar hypoxia. The mechanisms underlying HPV remain controversial, but RhoA/Rho kinase (ROK)-mediated Ca²+-sensitisation is considered important. Increasing evidence also implicates RhoA/ROK in PASMC proliferation, inflammatory cell recruitment and the regulation of cell motility, all of which are involved in the pulmonary vascular remodelling occurring in all forms of PH. ROK is therefore a potential therapeutic target in treating PH of various aetiologies. Here, we examine current concepts regarding the aetiology of PAH and also PH due to CH, focusing on the contribution that RhoA/ROK-mediated processes may make to their development and on ROK inhibitors as potential therapies.

Redox regulation of responses to hypoxia and NO-cGMP signaling in pulmonary vascular pathophysiology

Pulmonary vascular responses elicited by hypoxia and NO-cGMP signaling are potentially influenced by ROS and redox mechanisms that change during the progression of disease processes. Our studies in endothelium-rubbed bovine pulmonary arteries suggest increased glucose-6-phosphate dehydrogenase levels (compared to coronary arteries) seem to maintain a tonic peroxide-mediated relaxation removed by hypoxia through NADPH fueling superoxide generation from Nox oxidase. The activities of glucose-6-phosphate dehydrogenase, oxidases (i.e., Nox4), and systems metabolizing superoxide and peroxide markedly influence hypoxic pulmonary vasoconstriction (HPV). Activation of soluble guanylate cyclase and cGMP protein kinase seems to participate in peroxide-elicited relaxation. Endogenous NO helps maintain low pulmonary arterial pressure and suppresses HPV. Multiple redox processes potentially occurring during the progression of pulmonary hypertension may also attenuate NO-mediated relaxation beyond its scavenging by superoxide, including oxidation of guanylate cyclase heme and thiols normally maintained by cytosolic NADPH redox control.

PPARgamma as a potential therapeutic target in pulmonary hypertension

Pulmonary hypertension (PH) is a progressive disorder of the pulmonary circulation associated with significant morbidity and mortality. The pathobiology of PH involves a complex series of derangements causing endothelial dysfunction, vasoconstriction and abnormal proliferation of pulmonary vascular wall cells that lead to increases in pulmonary vascular resistance and pressure. Recent evidence indicates that the ligand-activated transcription factor, peroxisome proliferator-activated receptor gamma (PPARgamma) can have a favorable impact on a variety of pathways involved in the pathogenesis of PH. This review summarizes PPARgamma biology and the emerging evidence that therapies designed to activate this receptor may provide novel approaches to the treatment of PH. Mediators of PH that are regulated by PPARgamma are reviewed to provide insights into potential mechanisms underlying therapeutic effects of PPARgamma ligands in PH.

Strategic plan for lung vascular research: An NHLBI-ORDR Workshop Report

The Division of Lung Diseases of the National Heart, Lung, and Blood Institute, with the Office of Rare Diseases Research, held a workshop to identify priority areas and strategic goals to enhance and accelerate research that will result in improved understanding of the lung vasculature, translational research needs, and ultimately the care of patients with pulmonary vascular diseases. Multidisciplinary experts with diverse experience in laboratory, translational, and clinical studies identified seven priority areas and discussed limitations in our current knowledge, technologies, and approaches. The focus for future research efforts include the following: (1) better characterizing vascular genotype-phenotype relationships and incorporating systems biology approaches when appropriate; (2) advancing our understanding of pulmonary vascular metabolic regulatory signaling in health and disease; (3) expanding our knowledge of the biologic relationships between the lung circulation and circulating elements, systemic vascular function, and right heart function and disease; (4) improving translational research for identifying disease-modifying therapies for the pulmonary hypertensive diseases; (5) establishing an appropriate and effective platform for advancing translational findings into clinical studies testing; and (6) developing the specific technologies and tools that will be enabling for these goals, such as question-guided imaging techniques and lung vascular investigator training programs. Recommendations from this workshop will be used within the Lung Vascular Biology and Disease Extramural Research Program for planning and strategic implementation purposes.

Idiopathic pulmonary arterial hypertension

Despite improved understanding of the pathobiology of pulmonary arterial hypertension (PAH), it remains a severe and progressive disease, usually culminating in right heart failure, significant morbidity and early mortality. Over the last decade, some major advances have led to substantial improvements in the management of PAH. Much of this progress was pioneered by work in animal models. Although none of the current animal models of pulmonary hypertension (PH) completely recapitulate the human disease, they do provide insight into the cellular pathways contributing to its development and progression. There is hope that future work in model organisms will help to define its underlying cause(s), identify risk factors and lead to better treatment of the currently irreversible damage that results in the lungs of afflicted patients. However, the difficulty in defining the etiology of idiopathic PAH (IPAH, previously known as primary pulmonary hypertension) makes this subset of the disease particularly difficult to model. Although there are some valuable existing models that are relevant for IPAH research, the area would value from the development of new models that more closely mimic the clinical pathophysiology of IPAH.

The estrogen paradox in pulmonary arterial hypertension

Idiopathic pulmonary arterial hypertension (PAH) is a disabling condition characterized by PA vasoconstriction and remodeling as well as in situ thrombosis and eventual right heart failure. Idiopathic PAH occurs more frequently in females than in males. The female:male ratio is 1.64 ∼ 3.88:1. Although endogenous sex hormones including estrogen have been suggested to account for the observed gender differences in PAH, a precise pathobiology for the gender differences remains uncertain. Recent studies demonstrated that estrogen exerts beneficial effects on the pulmonary vasculature. However, it seems to contradict the female predominance that is observed in idiopathic PAH. Moreover, Sweeney and Voelkel (Sweeney L and Voelkel NF. Eur J Med Res 14: 433-442, 2009) showed that early and long-term estrogen exposure might be correlated with an increased risk of the development of PAH. Here we ask the question: Is estrogen a friend or a foe? According to accumulating evidence, we postulate that the different effects of estrogens on different target cells could account for this paradox, i.e., estrogens may exert beneficial effects only on the increased muscularization of vessel walls, but not on phenotypically altered endothelial cells. The effects of estrogens on the pulmonary vasculature are potent and complex, yet not fully understood. A better mechanistic understanding may allow for future therapeutic interventions in patients with PAH.

Validation of high-resolution echocardiography and magnetic resonance imaging vs. high-fidelity catheterization in experimental pulmonary hypertension

High-frequency echocardiography and high-field-strength magnetic resonance imaging (MRI) are new noninvasive methods for quantifying pulmonary arterial hypertension (PAH) and right ventricular (RV) hypertrophy (RVH). We compared these noninvasive methods of assessing the pulmonary circulation to the gold standard, cardiac catheterization (micromanometer-tipped catheters), in rats with monocrotaline-induced PAH and normal controls. Closed-chest, Sprague-Dawley rats were anesthetized with inhaled isoflurane (25 monocrotaline, 6 age-matched controls). Noninvasive studies used 37.5-MHz ultrasound (Vevo 770; VisualSonics) or a 9.4-T MRI (Bruker BioSpin). Catheterization used a 1.4-F micromanometer-tipped Millar catheter and a thermodilution catheter to measure cardiac output (CO). We compared noninvasive measures of pulmonary artery (PA) pressure (PAP) using PA acceleration time (PAAT) and CO, using the geometric PA flow method and RV free wall (RVFW) thickness/mass with cardiac catheterization and/or autopsy. Blinded operators performed comparisons using each method within 2 days of another. In a subset of rats with monocrotaline PAH, weekly echocardiograms, catheterization, and autopsy data assessed disease progression. Heart rate was similar during all studies (>323 beats/min). PAAT shortened, and the PA flow envelope displayed systolic "notching," reflective of downstream vascular remodeling/stiffening, within 3 wk of monocrotaline. MRI and echocardiography measures of PAAT were highly correlated (r(2) = 0.87) and were inversely proportional to invasive mean PAP (r(2) = 0.72). Mean PAP by echocardiography was estimated as 58.7 - (1.21 x PAAT). Invasive and noninvasive CO measurement correlated well (r(2) >or= 0.75). Noninvasive measures of RVFW thickness/mass correlated well with postmortem measurements. We conclude that high-resolution echocardiography and MRI accurately determine CO, PAP, and RV thickness/mass, offering similar results as high-fidelity right heart catheterization and autopsy, and that PAAT accurately estimates PAP and permits serial monitoring of experimental PAH. These tools are useful for assessment of the rodent pulmonary circulation and RVH.

Formation of plexiform lesions in experimental severe pulmonary arterial hypertension

BACKGROUND

The plexiform lesion is the hallmark of severe pulmonary arterial hypertension. However, its genesis and hemodynamic effects are largely unknown because of the limited availability of lung tissue samples from patients with pulmonary arterial hypertension and the lack of appropriate animal models. This study investigated whether rats with severe progressive pulmonary hypertension developed plexiform lesions.

METHODS AND RESULTS

After a single subcutaneous injection of the vascular endothelial growth factor receptor blocker Sugen 5416, rats were exposed to hypoxia for 3 weeks. They were then returned to normoxia for an additional 10 to 11 weeks. Hemodynamic and histological examinations were performed at 13 to 14 weeks after the Sugen 5416 injection. All rats developed pulmonary hypertension (right ventricular systolic pressure approximately 100 mm Hg) and severe pulmonary arteriopathy, including concentric neointimal and complex plexiform-like lesions. There were 2 patterns of complex lesion formation: a lesion forming within the vessel lumen (stalk-like) and another that projected outside the vessel (aneurysm-like). Immunohistochemical analyses showed that these structures had cellular and molecular features closely resembling human plexiform lesions.

CONCLUSIONS

Severe, sustained pulmonary hypertension in a very late stage of the Sugen 5416/hypoxia/normoxia-exposed rat is accompanied by the formation of lesions that are indistinguishable from the pulmonary arteriopathy of human pulmonary arterial hypertension. This unique model provides a new and rigorous approach for investigating the genesis, hemodynamic effects, and reversibility of plexiform and other occlusive lesions in pulmonary arterial hypertension.

Emerging therapies for the treatment of pulmonary hypertension

Current treatment of pulmonary arterial hypertension, which includes the use of prostacyclins, endothelin receptor antagonists, and phosphodiesterase type 5 inhibitors, either alone or in combination, often leads to improvements in functional capacity and modest decreases in pulmonary artery pressure. Disappointingly, however, two recent meta-analysis reviewing the controlled trials in pulmonary arterial hypertension, using these three agents, demonstrated little or no increase in survival. Importantly, however, increasing knowledge of the cellular and molecular basis of pulmonary arterial hypertension has led to the development of new agents aimed at either reversing sustained vasoconstriction or stopping/reversing the abnormal cell and extracellular matrix accumulation that, in combination, obstruct pulmonary blood flow and ultimately cause right heart failure. Rho kinase inhibitors, vasodilator peptides (such as vasoactive intestinal peptide and adrenomedullin), and endothelial nitric oxide synthase coupling agents (cicletanine) have been shown sometimes to exert potent pulmonary vasodilatory effects in animal models and in pilot studies in humans. Tyrosine kinase inhibitors (platelet-derived growth factor and epidermal growth factor receptor inhibitors), multikinase inhibitors (tyrosine kinase and serine/threonine kinase), elastase inhibitors, metabolic modulators (e.g., dichloroacetate), survivin inhibitors, and HMG-COA reductase inhibitors have been shown to reverse pulmonary hypertension in rodent models of pulmonary hypertension through inhibition of cell proliferation and induction of apoptosis. Early success in human pulmonary arterial hypertension with tyrosine kinase inhibitors has appeared in case reports. Furthermore, anti-inflammatory/immunomodulatory agents (thiazolidinedinones, rapamycin, cyclosporine, and STAT3 inhibitors) have been demonstrated to be effective at reducing vascular remodeling in animal models. Collectively, these studies are exciting and open potential new avenues for treatment. Caution should be exercised, however, as many agents, which are successful at preventing or reversing pulmonary arterial hypertension in currently used animal models, do not result in similar long-term success in the treatment of human pulmonary arterial hypertension.