Endothelial HIF-2α as a Key Endogenous Mediator Preventing Emphysema

Rationale: Endothelial injury may provoke emphysema, but molecular pathways of disease development require further discernment. Emphysematous lungs exhibit decreased expression of HIF-2α (hypoxia-inducible factor-2α)-regulated genes, and tobacco smoke decreases pulmonary HIF-2α concentrations. These findings suggest that decreased HIF-2α expression is important in the development of emphysema.Objectives: The objective of this study was to evaluate the roles of endothelial-cell (EC) HIF-2α in the pathogenesis of emphysema in mice.Methods: Mouse lungs were examined for emphysema after either the loss or the overexpression of EC Hif-2α. In addition, SU5416, a VEGFR2 inhibitor, was used to induce emphysema. Lungs were evaluated for HGF (hepatocyte growth factor), a protein involved in alveolar development and homeostasis. Lungs from patients with emphysema were measured for endothelial HIF-2α expression.Measurements and Main Results: EC Hif-2α deletion resulted in emphysema in association with fewer ECs and pericytes. After SU5416 exposure, EC Hif-2α-knockout mice developed more severe emphysema, whereas EC Hif-2α-overexpressing mice were protected. EC Hif-2α-knockout mice demonstrated lower levels of HGF. Human emphysema lung samples exhibited reduced EC HIF-2α expression.Conclusions: Here, we demonstrate a unique protective role for pulmonary endothelial HIF-2α and how decreased expression of this endogenous factor causes emphysema; its pivotal protective function is suggested by its ability to overcome VEGF antagonism. HIF-2α may maintain alveolar architecture by promoting vascular survival and associated HGF production. In summary, HIF-2α may be a key endogenous factor that prevents the development of emphysema, and its upregulation has the potential to foster lung health in at-risk patients.

Decreased lymphatic HIF-2α accentuates lymphatic remodeling in lymphedema

Pathologic lymphatic remodeling in lymphedema evolves during periods of tissue inflammation and hypoxia through poorly defined processes. In human and mouse lymphedema, there is a significant increase of hypoxia inducible factor 1 α (HIF-1α), but a reduction of HIF-2α protein expression in lymphatic endothelial cells (LECs). We questioned whether dysregulated expression of these transcription factors contributes to disease pathogenesis and found that LEC-specific deletion of Hif2α exacerbated lymphedema pathology. Even without lymphatic vascular injury, the loss of LEC-specific Hif2α caused anatomic pathology and a functional decline in fetal and adult mice. These findings suggest that HIF-2α is an important mediator of lymphatic health. HIF-2α promoted protective phosphorylated TIE2 (p-TIE2) signaling in LECs, a process also replicated by upregulating TIE2 signaling through adenovirus-mediated angiopoietin-1 (Angpt1) gene therapy. Our study suggests that HIF-2α normally promotes healthy lymphatic homeostasis and raises the exciting possibility that restoring HIF-2α pathways in lymphedema could mitigate long-term pathology and disability.

Endothelial Hypoxia-Inducible Factor-2α Is Required for the Maintenance of Airway Microvasculature

BACKGROUND

Hypoxia-inducible factors (HIFs), especially HIF-1α and HIF-2α, are key mediators of the adaptive response to hypoxic stress and play essential roles in maintaining lung homeostasis. Human and animal genetics studies confirm that abnormal HIF correlates with pulmonary vascular pathology and chronic lung diseases, but it remains unclear whether endothelial cell HIF production is essential for microvascular health. The large airway has an ideal circulatory bed for evaluating histological changes and physiology in genetically modified rodents.

METHODS

The tracheal microvasculature of mice, with conditionally deleted or overexpressed HIF-1α or HIF-2α, was evaluated for anatomy, perfusion, and permeability. Angiogenic signaling studies assessed vascular changes attributable to dysregulated HIF expression. An orthotopic tracheal transplantation model further evaluated the contribution of individual HIF isoforms in airway endothelial cells.

RESULTS

The genetic deletion of Hif-2α but not Hif-1α caused tracheal endothelial cell apoptosis, diminished pericyte coverage, reduced vascular perfusion, defective barrier function, overlying epithelial abnormalities, and subepithelial fibrotic remodeling. HIF-2α promoted microvascular integrity in airways through endothelial angiopoietin-1/TIE2 signaling and Notch activity. In functional tracheal transplants, HIF-2α deficiency in airway donors accelerated graft microvascular loss, whereas HIF-2α or angiopoietin-1 overexpression prolonged transplant microvascular perfusion. Augmented endothelial HIF-2α in transplant donors promoted airway microvascular integrity and diminished alloimmune inflammation.

CONCLUSIONS

Our findings reveal that the constitutive expression of endothelial HIF-2α is required for airway microvascular health.

Phenotypically Silent Bone Morphogenetic Protein Receptor 2 Mutations Predispose Rats to Inflammation-Induced Pulmonary Arterial Hypertension by Enhancing the Risk for Neointimal Transformation

BACKGROUND

Bmpr2 (bone morphogenetic protein receptor 2) mutations are critical risk factors for hereditary pulmonary arterial hypertension (PAH) with approximately 20% of carriers developing disease. There is an unmet medical need to understand how environmental factors, such as inflammation, render Bmpr2 mutants susceptible to PAH. Overexpressing 5-LO (5-lipoxygenase) provokes lung inflammation and transient PAH in Bmpr2^+/- mice. Accordingly, 5-LO and its metabolite, leukotriene B₄, are candidates for the second hit. The purpose of this study was to determine how 5-LO-mediated pulmonary inflammation synergized with phenotypically silent Bmpr2 defects to elicit significant pulmonary vascular disease in rats.

METHODS

Monoallelic Bmpr2 mutant rats were generated and found phenotypically normal for up to 1 year of observation. To evaluate whether a second hit would elicit disease, animals were exposed to 5-LO-expressing adenovirus, monocrotaline, SU5416, SU5416 with chronic hypoxia, or chronic hypoxia alone. Bmpr2-mutant hereditary PAH patient samples were assessed for neointimal 5-LO expression. Pulmonary artery endothelial cells with impaired BMPR2 signaling were exposed to increased 5-LO-mediated inflammation and were assessed for phenotypic and transcriptomic changes.

RESULTS

Lung inflammation, induced by intratracheal delivery of 5-LO-expressing adenovirus, elicited severe PAH with intimal remodeling in Bmpr2^+/- rats but not in their wild-type littermates. Neointimal lesions in the diseased Bmpr2^+/- rats gained endogenous 5-LO expression associated with elevated leukotriene B₄ biosynthesis. Bmpr2-mutant hereditary PAH patients similarly expressed 5-LO in the neointimal cells. In vitro, BMPR2 deficiency, compounded by 5-LO-mediated inflammation, generated apoptosis-resistant and proliferative pulmonary artery endothelial cells with mesenchymal characteristics. These transformed cells expressed nuclear envelope-localized 5-LO consistent with induced leukotriene B₄ production, as well as a transcriptomic signature similar to clinical disease, including upregulated nuclear factor Kappa B subunit (NF-κB), interleukin-6, and transforming growth factor beta (TGF-β) signaling pathways. The reversal of PAH and vasculopathy in Bmpr2 mutants by TGF-β antagonism suggests that TGF-β is critical for neointimal transformation.

CONCLUSIONS

In a new 2-hit model of disease, lung inflammation induced severe PAH pathology in Bmpr2^+/- rats. Endothelial transformation required the activation of canonical and noncanonical TGF-β signaling pathways and was characterized by 5-LO nuclear envelope translocation with enhanced leukotriene B₄ production. This study offers an explanation of how an environmental injury unleashes the destructive potential of an otherwise silent genetic mutation.

Dominant Role for Regulatory T Cells in Protecting Females Against Pulmonary Hypertension

RATIONALE

Pulmonary arterial hypertension (PH) is a life-threatening condition associated with immune dysregulation and abnormal regulatory T cell (Treg) activity, but it is currently unknown whether and how abnormal Treg function differentially affects males and females.

OBJECTIVE

To evaluate whether and how Treg deficiency differentially affects male and female rats in experimental PH.

METHODS AND RESULTS

Male and female athymic rnu/rnu rats, lacking Tregs, were treated with the VEGFR2 (vascular endothelial growth factor receptor 2) inhibitor SU5416 or chronic hypoxia and evaluated for PH; some animals underwent Treg immune reconstitution before SU5416 administration. Plasma PGI₂ (prostacyclin) levels were measured. Lung and right ventricles were assessed for the expression of the vasoprotective proteins COX-2 (cyclooxygenase 2), PTGIS (prostacyclin synthase), PDL-1 (programmed death ligand 1), and HO-1 (heme oxygenase 1). Inhibitors of these pathways were administered to athymic rats undergoing Treg immune reconstitution. Finally, human cardiac microvascular endothelial cells cocultured with Tregs were evaluated for COX-2, PDL-1, HO-1, and ER (estrogen receptor) expression, and culture supernatants were assayed for PGI₂ and IL (interleukin)-10. SU5416-treatment and chronic hypoxia produced more severe PH in female than male athymic rats. Females were distinguished by greater pulmonary inflammation, augmented right ventricular fibrosis, lower plasma PGI₂ levels, decreased lung COX-2, PTGIS, HO-1, and PDL-1 expression and reduced right ventricular PDL-1 levels. In both sexes, Treg immune reconstitution protected against PH development and raised levels of plasma PGI₂ and cardiopulmonary COX-2, PTGIS, PDL-1, and HO-1. Inhibiting COX-2, HO-1, and PD-1 (programmed death 1)/PDL-1 pathways abrogated Treg protection. In vitro, human Tregs directly upregulated endothelial COX-2, PDL-1, HO-1, ERs and increased supernatant levels of PGI₂ and IL-10.

CONCLUSIONS

In 2 animal models of PH based on Treg deficiency, females developed more severe PH than males. The data suggest that females are especially reliant on the normal Treg function to counteract the effects of pulmonary vascular injury leading to PH.

Microhemorrhage-associated tissue iron enhances the risk for Aspergillus fumigatus invasion in a mouse model of airway transplantation

Invasive pulmonary disease due to the mold Aspergillus fumigatus can be life-threatening in lung transplant recipients, but the risk factors remain poorly understood. To study this process, we used a tracheal allograft mouse model that recapitulates large airway changes observed in patients undergoing lung transplantation. We report that microhemorrhage-related iron content may be a major determinant of A. fumigatus invasion and, consequently, its virulence. Invasive growth was increased during progressive alloimmune-mediated graft rejection associated with high concentrations of ferric iron in the graft. The role of iron in A. fumigatus invasive growth was further confirmed by showing that this invasive phenotype was increased in tracheal transplants from donor mice lacking the hemochromatosis gene (Hfe^-/- ). The invasive phenotype was also increased in mouse syngrafts treated with topical iron solution and in allograft recipients receiving deferoxamine, a chelator that increases iron bioavailability to the mold. The invasive growth of the iron-intolerant A. fumigatus double-knockout mutant (ΔsreA/ΔcccA) was lower than that of the wild-type mold. Alloimmune-mediated microvascular damage and iron overload did not appear to impair the host's immune response. In human lung transplant recipients, positive staining for iron in lung transplant tissue was more commonly seen in endobronchial biopsy sections from transplanted airways than in biopsies from the patients' own airways. Collectively, these data identify iron as a major determinant of A. fumigatus invasive growth and a potential target to treat or prevent A. fumigatus infections in lung transplant patients.

Leukotriene B4 antagonism ameliorates experimental lymphedema

Acquired lymphedema is a cancer sequela and a global health problem currently lacking pharmacologic therapy. We have previously demonstrated that ketoprofen, an anti-inflammatory agent with dual 5-lipoxygenase and cyclooxygenase inhibitory properties, effectively reverses histopathology in experimental lymphedema. We show that the therapeutic benefit of ketoprofen is specifically attributable to its inhibition of the 5-lipoxygenase metabolite leukotriene B₄ (LTB₄). LTB₄ antagonism reversed edema, improved lymphatic function, and restored lymphatic architecture in the murine tail model of lymphedema. In vitro, LTB₄ was functionally bimodal: Lower LTB₄ concentrations promoted human lymphatic endothelial cell sprouting and growth, but higher concentrations inhibited lymphangiogenesis and induced apoptosis. During lymphedema progression, lymphatic fluid LTB₄ concentrations rose from initial prolymphangiogenic concentrations into an antilymphangiogenic range. LTB₄ biosynthesis was similarly elevated in lymphedema patients. Low concentrations of LTB₄ stimulated, whereas high concentrations of LTB₄ inhibited, vascular endothelial growth factor receptor 3 and Notch pathways in cultured human lymphatic endothelial cells. Lymphatic-specific Notch1^-/- mice were refractory to the beneficial effects of LTB₄ antagonism, suggesting that LTB₄ suppression of Notch signaling is an important mechanism in disease maintenance. In summary, we found that LTB₄ was harmful to lymphatic repair at the concentrations observed in established disease. Our findings suggest that LTB₄ is a promising drug target for the treatment of acquired lymphedema.

Leukotriene B4 Activates Pulmonary Artery Adventitial Fibroblasts in Pulmonary Hypertension

A recent study demonstrated a significant role for leukotriene B4 (LTB4) causing pulmonary vascular remodeling in pulmonary arterial hypertension. LTB4 was found to directly injure luminal endothelial cells and promote growth of the smooth muscle cell layer of pulmonary arterioles. The purpose of this study was to determine the effects of LTB4 on the pulmonary adventitial layer, largely composed of fibroblasts. Here, we demonstrate that LTB4 enhanced human pulmonary artery adventitial fibroblast proliferation, migration, and differentiation in a dose-dependent manner through its cognate G-protein-coupled receptor, BLT1. LTB4 activated human pulmonary artery adventitial fibroblast by upregulating p38 mitogen-activated protein kinase as well as Nox4-signaling pathways. In an autoimmune model of pulmonary hypertension, inhibition of these pathways blocked perivascular inflammation, decreased Nox4 expression, reduced reactive oxygen species production, reversed arteriolar adventitial fibroblast activation, and attenuated pulmonary hypertension development. This study uncovers a novel mechanism by which LTB4 further promotes pulmonary arterial hypertension pathogenesis, beyond its established effects on endothelial and smooth muscle cells, by activating adventitial fibroblasts.