Philip M. Bauer

Nitrite Potently Inhibits Hypoxic and Inflammatory Pulmonary Arterial Hypertension and Smooth Muscle Proliferation via Xanthine Oxidoreductase–Dependent Nitric Oxide Generation

Background— Pulmonary arterial hypertension is a progressive proliferative vasculopathy of the small pulmonary arteries that is characterized by a primary failure of the endothelial nitric oxide and prostacyclin vasodilator pathways, coupled with dysregulated cellular proliferation. We have recently discovered that the endogenous anion salt nitrite is converted to nitric oxide in the setting of physiological and pathological hypoxia. Considering the fact that nitric oxide exhibits vasoprotective properties, we examined the effects of nitrite on experimental pulmonary arterial hypertension. Methods and Results— We exposed mice and rats with hypoxia or monocrotaline-induced pulmonary arterial hypertension to low doses of nebulized nitrite (1.5 mg/min) 1 or 3 times a week. This dose minimally increased plasma and lung nitrite levels yet completely prevented or reversed pulmonary arterial hypertension and pathological right ventricular hypertrophy and failure. In vitro and in vivo studies revealed that nitrite in the lung was metabolized directly to nitric oxide in a process significantly enhanced under hypoxia and found to be dependent on the enzymatic action of xanthine oxidoreductase. Additionally, physiological levels of nitrite inhibited hypoxia-induced proliferation of cultured pulmonary artery smooth muscle cells via the nitric oxide–dependent induction of the cyclin-dependent kinase inhibitor p21Waf1/Cip1. The therapeutic effect of nitrite on hypoxia-induced pulmonary hypertension was significantly reduced in the p21-knockout mouse; however, nitrite still reduced pressures and right ventricular pathological remodeling, indicating the existence of p21-independent effects as well. Conclusion— These studies reveal a potent effect of inhaled nitrite that limits pathological pulmonary arterial hypertrophy and cellular proliferation in the setting of experimental pulmonary arterial hypertension.

Thrombospondin-1 supports blood pressure by limiting eNOS activation and endothelial-dependent vasorelaxation

AIMS Thrombospondin-1 (TSP1), via its necessary receptor CD47, inhibits nitric oxide (NO)-stimulated soluble guanylate cyclase activation in vascular smooth muscle cells, and TSP1-null mice have increased shear-dependent blood flow compared with wild-type mice. Yet, the endothelial basement membrane should in theory function as a barrier to diffusion of soluble TSP1 into the arterial smooth muscle cell layer. These findings suggested that endothelial-dependent differences in blood flow in TSP1-null mice may be the result of direct modulation of endothelial NO synthase (eNOS) activation by circulating TSP1. Here we tested the hypothesis that TSP1 inhibits eNOS activation and endothelial-dependent arterial relaxation. METHODS AND RESULTS Acetylcholine (ACh)-stimulated activation of eNOS and agonist-driven calcium transients in endothelial cells were inhibited by TSP1. TSP1 also inhibited eNOS phosphorylation at serine(1177). TSP1 treatment of the endothelium of wild-type and TSP1-null but not CD47-null arteries inhibited ACh-stimulated relaxation. TSP1-null vessels demonstrated greater endothelial-dependent vasorelaxation compared with the wild type. Conversely, TSP1-null arteries demonstrated less vasoconstriction to phenylephrine compared with the wild type, which was corrected upon inhibition of eNOS. In TSP1-null mice, intravenous TSP1 blocked ACh-stimulated decreases in blood pressure, and both intravenous TSP1 and a CD47 agonist antibody acutely elevated blood pressure in mice. CONCLUSION TSP1, via CD47, inhibits eNOS activation and endothelial-dependent arterial relaxation and limits ACh-driven decreases in blood pressure. Conversely, intravenous TSP1 and a CD47 antibody increase blood pressure. These findings suggest that circulating TSP1, by limiting endogenous NO production, functions as a pressor agent supporting blood pressure.

Nitro-Fatty Acid Inhibition of Neointima Formation After Endoluminal Vessel Injury

Rationale: Fatty acid nitroalkenes are endogenously generated electrophilic byproducts of nitric oxide and nitrite-dependent oxidative inflammatory reactions. Existing evidence indicates nitroalkenes support posttranslational protein modifications and transcriptional activation that promote the resolution of inflammation. Objective: The aim of this study was to assess whether in vivo administration of a synthetic nitroalkene could elicit antiinflammatory actions in vivo using a murine model of vascular injury. Methods and Results: The in vivo administration (21 days) of nitro-oleic acid (OA-NO2) inhibited neointimal hyperplasia after wire injury of the femoral artery in a murine model (OA-NO2 treatment resulted in reduced intimal area and intima to media ratio versus vehicle- or oleic acid (OA)-treated animals, P<0.0001). Increased heme oxygenase (HO)-1 expression accounted for much of the vascular protection induced by OA-NO2 in both cultured aortic smooth muscle cells and in vivo. Inhibition of HO by Sn(IV)-protoporphyrin or HO-1 small interfering RNA reversed OA-NO2–induced inhibition of platelet-derived growth factor-stimulated rat aortic smooth muscle cell migration. The upregulation of HO-1 expression also accounted for the antistenotic actions of OA-NO2 in vivo, because inhibition of neointimal hyperplasia following femoral artery injury was abolished in HO-1−/− mice (OA-NO2–treated wild-type versus HO-1−/− mice, P=0.016). Conclusions: In summary, electrophilic nitro-fatty acids induce salutary gene expression and cell functional responses that are manifested by a clinically significant outcome, inhibition of neointimal hyperplasia induced by arterial injury.

Activated CD47 promotes pulmonary arterial hypertension through targeting caveolin-1

AIMS Pulmonary arterial hypertension (PAH) is a progressive lung disease characterized by pulmonary vasoconstriction and vascular remodelling, leading to increased pulmonary vascular resistance and right heart failure. Loss of nitric oxide (NO) signalling and increased endothelial nitric oxide synthase (eNOS)-derived oxidative stress are central to the pathogenesis of PAH, yet the mechanisms involved remain incompletely determined. In this study, we investigated the role activated CD47 plays in promoting PAH. METHODS AND RESULTS We report high-level expression of thrombospondin-1 (TSP1) and CD47 in the lungs of human subjects with PAH and increased expression of TSP1 and activated CD47 in experimental models of PAH, a finding matched in hypoxic human and murine pulmonary endothelial cells. In pulmonary endothelial cells CD47 constitutively associates with caveolin-1 (Cav-1). Conversely, in hypoxic animals and cell cultures activation of CD47 by TSP1 disrupts this constitutive interaction, promoting eNOS-dependent superoxide production, oxidative stress, and PAH. Hypoxic TSP1 null mice developed less right ventricular pressure and hypertrophy and markedly less arteriole muscularization compared with wild-type animals. Further, therapeutic blockade of CD47 activation in hypoxic pulmonary artery endothelial cells upregulated Cav-1, increased Cav-1CD47 co-association, decreased eNOS-derived superoxide, and protected animals from developing PAH. CONCLUSION Activated CD47 is upregulated in experimental and human PAH and promotes disease by limiting Cav-1 inhibition of dysregulated eNOS.

Bone Morphogenetic Protein Receptor II Is a Novel Mediator of Endothelial Nitric-oxide Synthase Activation

Activation of bone morphogenetic protein (BMP) receptor II (BMPRII) promotes pulmonary artery endothelial cell (PAEC) survival, proliferation, and migration. Mutations to BMPRII are associated with the development of pulmonary arterial hypertension (PAH). Endothelial dysfunction, including decreased endothelial nitric-oxide synthase (eNOS) activity and loss of bioactive nitric oxide (NO), plays a prominent role in the development of PAH. We hypothesized that stimulation of BMPRII promotes normal PAEC function by activating eNOS. We report that BMPRII ligands, BMP2 and BMP4, (i) stimulate eNOS phosphorylation at a critical regulatory site, (ii) increase eNOS activity, and (iii) result in canonical changes in eNOS protein-protein interactions. The stimulation of eNOS activity by BMPRII ligands was largely dependent on protein kinase A (PKA) activation, as demonstrated using the PKA inhibitors H89 and myristoylated PKI(6–22) amide. PAEC migration stimulated by BMP2 and BMP4 was inhibited by the NOS inhibitor l-nitroarginine methyl ester, providing functional evidence of eNOS activation. Furthermore, BMP2 and BMP4 failed to stimulate eNOS phosphorylation when BMPRII was knocked down by siRNA. Most important to the pathophysiology of the disease, BMP2 and BMP4 failed to stimulate eNOS phosphorylation in PAECs isolated from patients with mutations in the BMPR2 gene. These data demonstrate a new action of BMPs/BMPRII in the pulmonary endothelium and provide novel mechanistic insight into the pathogenesis of PAH.

High mobility group box 1 contributes to the pathogenesis of experimental pulmonary hypertension via activation of Toll-like receptor 4.

Survival rates for patients with pulmonary hypertension (PH) remain low, and our understanding of the mechanisms involved are incomplete. Here we show in a mouse model of chronic hypoxia (CH)-induced PH that the nuclear protein and damage-associate molecular pattern molecule (DAMP) high mobility group box 1 (HMGB1) contributes to PH via a Toll-like receptor 4 (TLR4)-dependent mechanism. We demonstrate extranuclear HMGB1 in pulmonary vascular lesions and increased serum HMGB1 in patients with idiopathic pulmonary arterial hypertension. The increase in circulating HMGB1 correlated with mean pulmonary artery pressure. In mice, we similarly detected the translocation and release of HMGB1 after exposure to CH. HMGB1-neutralizing antibody attenuated the development of CH-induced PH, as assessed by measurement of right ventricular systolic pressure, right ventricular hypertrophy, pulmonary vascular remodeling and endothelial activation and inflammation. Genetic deletion of the pattern recognition receptor TLR4, but not the receptor for advanced glycation end products, likewise attenuated CH-induced PH. Finally, daily treatment of mice with recombinant human HMGB1 exacerbated CH-induced PH in wild-type (WT) but not Tlr4−/− mice. These data demonstrate that HMGB1-mediated activation of TLR4 promotes experimental PH and identify HMGB1 and/or TLR4 as potential therapeutic targets for the treatment of PH.

Caveolin-1 regulates BMPRII localization and signaling in vascular smooth muscle cells.

Recent studies demonstrate the interaction of BMPRII and caveolin-1 in various cell types. In this study we test the hypothesis that caveolin-1 interacts with and regulates BMPRII-dependent signaling in vascular smooth muscle cells. We demonstrate that BMPRII localizes to caveolae and directly interacts with caveolin-1 in mouse aortic smooth muscle cells. We demonstrate that this interaction is mediated by the caveolin-1 scaffolding domain and is regulated by caveolin-1 phosphorylation. Downregulation of caveolin-1 via siRNA resulted in a loss of BMP-dependent SMAD phosphorylation and gene regulation. Further studies revealed that loss of caveolin-1 results in decreased BMPRII membrane localization and decreased association of BMPRII with the type I BMP receptor BMPRIa. Dominant negative caveolin-1 decreased BMPRII membrane localization suggesting a role for caveolin-1 in BMPRII trafficking. Taken together, our findings establish caveolin-1 as an important regulator of downstream signaling and membrane targeting of BMPRII in vascular smooth muscle cells.

HO-1 and CO Decrease Platelet-Derived Growth Factor-Induced Vascular Smooth Muscle Cell Migration Via Inhibition of Nox1

Objective—Heme oxygenase-1 (HO-1), via its enzymatic degradation products, exhibits cell and tissue protective effects in models of vascular injury and disease. The migration of vascular smooth muscle cells (VSMC) from the medial to the intimal layer of blood vessels plays an integral role in the development of a neointima in these models. Despite this, there are no studies addressing the effect of increased HO-1 expression on VSMC migration. Results and Methods—The effects of increased HO-1 expression, as well as biliverdin, bilirubin, and carbon monoxide (CO), were studied in in vitro models of VSMC migration. Induction of HO-1 or CO, but not biliverdin or bilirubin, inhibited VSMC migration. This effect was mediated by the inhibition of Nox1 as determined by a range of approaches, including detection of intracellular superoxide, nicotinamide adenine dinucleotide phosphate oxidase activity measurements, and siRNA experiments. Furthermore, CO decreased platelet-derived growth factor-stimulated, redox-sensitive signaling pathways. Conclusion—Herein, we demonstrate that increased HO-1 expression and CO decreases platelet-derived growth factor-stimulated VSMC migration via inhibition of Nox1 enzymatic activity. These studies reveal a novel mechanism by which HO-1 and CO may mediate their beneficial effects in arterial inflammation and injury.

High Mobility Group Box 1 Inhibits Human Pulmonary Artery Endothelial Cell Migration via a Toll-like Receptor 4- and Interferon Response Factor 3-dependent Mechanism(s)

Background: TLR4 contributes to pulmonary hypertension, a disease of endothelial dysfunction. The role of HMGB1, one endogenous ligand of TLR4, remains unexplored. Results: HMGB1 inhibited pulmonary artery endothelial cell migration, which was reversed by TLR4 or IRF3 siRNA. Conclusion: HMGB1 inhibits pulmonary artery endothelial cell migration via TLR4- and IRF3-dependent mechanism(s). Significance: HMGB1, via TLR4, inhibits a critical process for pulmonary vascular regeneration. In pulmonary hypertension the loss of precapillary arterioles results from vascular injury causing endothelial dysfunction. Endothelial cell migration and proliferation are critical for vascular regeneration. This study focused on the effect of high mobility group box 1 protein (HMGB1) on these critical processes. HMGB1 had no effect on human pulmonary artery endothelial cell (HPAEC) proliferation. In contrast, treatment of HPAECs with HMGB1 dose-dependently inhibited VEGF-stimulated HPAEC migration. The effect of HMGB1 on HPAEC migration was TLR4-dependent because it was reversed by TLR4 siRNA or TLR4-neutralizing antibody. Exposure of HPAECs to hypoxia caused translocation and release of HMGB1 and inhibition of HPAEC migration. The effect of hypoxia on HPAEC migration was mediated by HMGB1 because HMGB1-neutralizing antibody but not control IgG restored HPAEC migration. Likewise, TLR4 siRNA but not control siRNA reversed the inhibitory effect of hypoxia in HPAECs. The canonical TLR4 signaling pathway requires the adaptor protein MyD88 and leads to downstream NFκB activation. Interestingly, HMGB1 failed to stimulate NFκB translocation to the nucleus, but instead activated an alternative pathway characterized by activation of interferon response factor 3 (IRF3). This was in contrast to human umbilical vein endothelial cells in which HMGB1 stimulated nuclear translocation of NFκB but not IRF3. IRF3 siRNA, but not MyD88 siRNA, reversed the inhibitory effect of HMGB1 on HPAEC migration. These data demonstrate that HMGB1 inhibits HPAEC migration, a critical process for vascular regeneration, via TLR4- and IRF3-dependent mechanisms.

Complement C3 deficiency attenuates chronic hypoxia-induced pulmonary hypertension in mice

Background Evidence suggests a role of both innate and adaptive immunity in the development of pulmonary arterial hypertension. The complement system is a key sentry of the innate immune system and bridges innate and adaptive immunity. To date there are no studies addressing a role for the complement system in pulmonary arterial hypertension. Methodology/Principal Findings Immunofluorescent staining revealed significant C3d deposition in lung sections from IPAH patients and C57Bl6/J wild-type mice exposed to three weeks of chronic hypoxia to induce pulmonary hypertension. Right ventricular systolic pressure and right ventricular hypertrophy were increased in hypoxic vs. normoxic wild-type mice, which were attenuated in C3−/− hypoxic mice. Likewise, pulmonary vascular remodeling was attenuated in the C3−/− mice compared to wild-type mice as determined by the number of muscularized peripheral arterioles and morphometric analysis of vessel wall thickness. The loss of C3 attenuated the increase in interleukin-6 and intracellular adhesion molecule-1 expression in response to chronic hypoxia, but not endothelin-1 levels. In wild-type mice, but not C3−/− mice, chronic hypoxia led to platelet activation as assessed by bleeding time, and flow cytometry of platelets to determine cell surface P-selectin expression. In addition, tissue factor expression and fibrin deposition were increased in the lungs of WT mice in response to chronic hypoxia. These pro-thrombotic effects of hypoxia were abrogated in C3−/− mice. Conclusions Herein, we provide compelling genetic evidence that the complement system plays a pathophysiologic role in the development of PAH in mice, promoting pulmonary vascular remodeling and a pro-thrombotic phenotype. In addition we demonstrate C3d deposition in IPAH patients suggesting that complement activation plays a role in the development of PAH in humans.