Plexiform lesions in pulmonary arterial hypertension composition, architecture, and microenvironment. Am J Pathol. 2011;179:167-179. [PMID: 21703400 DOI: 10.1016/j.ajpath.2011.03.040]

Pathophysiology and treatment of pulmonary hypertension in sickle cell disease

Pulmonary hypertension affects ∼10% of adult patients with sickle cell disease (SCD), particularly those with the homozygous genotype. An increase in pulmonary artery systolic pressure, estimated noninvasively by echocardiography, helps identify SCD patients at risk for pulmonary hypertension, but definitive diagnosis requires right-heart catheterization. About half of SCD-related pulmonary hypertension patients have precapillary pulmonary hypertension with potential etiologies of (1) a nitric oxide deficiency state and vasculopathy consequent to intravascular hemolysis, (2) chronic pulmonary thromboembolism, or (3) upregulated hypoxic responses secondary to anemia, low O2 saturation, and microvascular obstruction. The remainder have postcapillary pulmonary hypertension secondary to left ventricular dysfunction. Although the pulmonary artery pressure in SCD patients with pulmonary hypertension is only moderately elevated, they have a markedly higher risk of death than patients without pulmonary hypertension. Guidelines for diagnosis and management of SCD-related pulmonary hypertension were published recently by the American Thoracic Society. Management of adults with sickle-related pulmonary hypertension is based on anticoagulation for those with thromboembolism; oxygen therapy for those with low oxygen saturation; treatment of left ventricular failure in those with postcapillary pulmonary hypertension; and hydroxyurea or transfusions to raise the hemoglobin concentration, reduce hemolysis, and prevent vaso-occlusive events that cause additional increases in pulmonary pressure. Randomized trials have not identified drugs to lower pulmonary pressure in SCD patients with precapillary pulmonary hypertension. Patients with hemodynamics of pulmonary arterial hypertension should be referred to specialized centers and considered for treatments known to be effective in other forms of pulmonary arterial hypertension. There have been reports that some of these treatments improve SCD-related pulmonary hypertension.

Enhanced caveolin-1 expression in smooth muscle cells: Possible prelude to neointima formation

AIM: To study the genesis of neointima formation in pulmonary hypertension (PH), we investigated the role of caveolin-1 and related proteins.

METHODS: Male Sprague Dawley rats were given monocrotaline (M, 40 mg/kg) or subjected to hypobaric hypoxia (H) to induce PH. Another group was given M and subjected to H to accelerate the disease process (M + H). Right ventricular systolic pressure, right ventricular hypertrophy, lung histology for medial hypertrophy and the presence of neointimal lesions were examined at 2 and 4 wk. The expression of caveolin-1 and its regulatory protein peroxisome proliferator-activated receptor (PPAR) γ, caveolin-2, proliferative and anti-apoptotic factors (PY-STAT3, p-Erk, Bcl-xL), endothelial nitric oxide synthase (eNOS) and heat shock protein (HSP) 90 in the lungs were analyzed, and the results from M + H group were compared with the controls, M and H groups. Double immunofluorescence technique was used to identify the localization of caveolin-1 in pulmonary arteries in rat lungs and in human PH lung tissue.

RESULTS: In the M + H group, PH was more severe compared with M or H group. In the 4 wk M+H group, several arteries with reduced caveolin-1 expression in endothelial layer coupled with an increased expression in smooth muscle cells (SMC), exhibited neointimal lesions. Neointima was present only in the arteries exhibiting enhanced caveolin-1 expression in SMC. Lung tissue obtained from patients with PH also revealed neointimal lesions only in the arteries exhibiting endothelial caveolin-1 loss accompanied by an increased caveolin-1 expression in SMC. Reduction in eNOS and HSP90 expression was present in the M groups (2 and 4 wk), but not in the M + H groups. In both M groups and in the M + H group at 2 wk, endothelial caveolin-1 loss was accompanied by an increase in PPARγ expression. In the M + H group at 4 wk, increase in caveolin-1 expression was accompanied by a reduction in the PPARγ expression. In the H group, there was neither a loss of endothelial caveolin-1, eNOS or HSP90, nor an increase in SMC caveolin-1 expression; or any alteration in PPARγ expression. Proliferative pathways were activated in all experimental groups.

CONCLUSION: Enhanced caveolin-1 expression in SMC follows extensive endothelial caveolin-1 loss with subsequent neointima formation. Increased caveolin-1 expression in SMC, thus, may be a prelude to neointima formation.

Hyperplastic Growth of Pulmonary Artery Smooth Muscle Cells from Subjects with Pulmonary Arterial Hypertension Is Activated through JNK and p38 MAPK

Smooth muscle in the pulmonary artery of PAH subjects, both idiopathic and hereditary, is characterized by hyperplasia. Smooth muscle cells (HPASMC) isolated from subjects with or without PAH retain their in vivo phenotype as illustrated by their expression of alpha-smooth muscle actin and expression of H-caldesmon. Both non PAH and PAH HPASMC display a lengthy, approximately 94h, cell cycle. The HPASMC from both idiopathic and hereditary PAH display an abnormal proliferation characterized by continued growth under non-proliferative, non-growth stimulated conditions. This effector independent proliferation is JNK and p38 MAP kinase dependent. Blocking the activation of either abrogates the HPASMC growth. HPASMC from non PAH donors under quiescent conditions display negligible proliferation but divide upon exposure to growth factors such as PDGF-BB or FGF2 but not EGF. This growth does not involve the MAP kinases. Instead it routes via the tyrosine kinase receptor through mTOR and then 6SK. In the PAH cells PDGF-BB and FGF2 augment the dysregulated cell proliferation, also through mTOR/6SK. Additionally, blocking the activation of mTOR also modulates the MAP kinase promoted dysregulated growth. These results highlight key alterations in the growth of HPASMC from subjects with PAH which contribute to the etiology of the disease and can clearly be targeted at various regulatory points for future therapies.

Compartment-specific expression of collagens and their processing enzymes in intrapulmonary arteries of IPAH patients

Alterations in extracellular matrix (ECM) have been implicated in the pathophysiology of pulmonary hypertension. Here, we have undertaken a compartment-specific study to elucidate the expression profile of collagens and their processing enzymes in donor and idiopathic pulmonary arterial hypertension (IPAH) pulmonary arteries. Predominant intimal, but also medial and perivascular, remodeling and reduced lumen diameter were detected in IPAH pulmonary arteries. Two-photon microscopy demonstrated accumulation of collagen fibers. Quantification of collagen in pulmonary arteries revealed collagen accumulation mainly in the intima of IPAH pulmonary arteries compared with donors. Laser capture-microdissected pulmonary artery profiles (intima+media and perivascular tissue) were analyzed by real-time PCR for ECM gene expression. In the intima+media of IPAH vessels, collagens (COL4A5, COL14A1, and COL18A1), matrix metalloproteinase (MMP) 19, and a disintegrin and metalloprotease (ADAM) 33 were higher expressed, whereas MMP10, ADAM17, TIMP1, and TIMP3 were less abundant. Localization of COLXVIII, its cleavage product endostatin, and MMP10, ADAM33, and TIMP1 was confirmed in pulmonary arteries by immunohistochemistry. ELISA for collagen XVIII/endostatin demonstrated significantly elevated plasma levels in IPAH patients compared with donors, whereas circulating MMP10, ADAM33, and TIMP1 levels were similar between the two groups. Endostatin levels were correlated with pulmonary arterial wedge pressure, and established prognostic markers of IPAH, right atrial pressure, cardiac index, 6-min walking distance, NH2-terminal pro-brain natriuretic peptide, and uric acid. Expression of unstudied collagens, MMPs, ADAMs, and TIMPs were found to be significantly altered in IPAH intima+media. Elevated levels of circulating collagen XVIII/endostatin are associated with markers of a poor prognosis.

The Sugen 5416/hypoxia mouse model of pulmonary hypertension revisited: long-term follow-up

The combination of a vascular endothelial growth factor receptor antagonist, Sugen 5416 (SU5416), and chronic hypoxia is known to cause pronounced pulmonary hypertension (PH) with angioobliterative lesions in rats and leads to exaggerated PH in mice as well. We sought to determine whether weekly SU5416 injections during 3 weeks of hypoxia leads to long-term development of angioobliterative lesions and sustained or progressive PH in mice. Male C57BL/6J mice were injected with SU5416 (SuHx) or vehicle (VehHx) weekly during 3 weeks of exposure to 10% oxygen. Echocardiographic and invasive measures of hemodynamics and pulmonary vascular morphometry were performed after the 3-week hypoxic exposure and after 10 weeks of recovery in normoxia. SuHx led to higher right ventricular (RV) systolic pressure and RV hypertrophy than VehHx after 3 weeks of hypoxia. Ten weeks after hypoxic exposure, RV systolic pressure decreased but remained elevated in SuHx mice compared with VehHx or normoxic control mice, but RV hypertrophy had resolved. After 3 weeks of hypoxia and 10 weeks of follow-up in normoxia, tricuspid annular plane systolic excursion was significantly decreased, indicating decreased systolic RV function. Very few angioobliterative lesions were found at the 10-week follow-up time point in SuHx mouse lungs. In conclusion, SU5416 combined with 3 weeks of hypoxia causes a more profound PH phenotype in mice than hypoxia alone. PH persists over 10 weeks of normoxic follow-up in SuHx mice, but significant angioobliterative lesions do not occur, and neither PH nor RV dysfunction worsens. The SuHx mouse model is a useful adjunct to other PH models, but the search will continue for a mouse model that better recapitulates the human phenotype.

Modulating the dysregulated migration of pulmonary arterial hypertensive smooth muscle cells with motif mimicking cell permeable peptides

Migration of vascular smooth muscle cells is a key element in remodeling during pulmonary arterial hypertension (PAH). We are observing key alterations in the migratory characteristics of human pulmonary artery smooth muscle cells (HPASMC) isolated from transplanted lungs of subjects with PAH. Using wound migration and barrier removal assays, we demonstrate that the PAH cells migrate under quiescent growth conditions and in the absence of pro-migratory factors such as platelet derived growth factor (PDGF). Under the same conditions, in the absence of PDGF, non-PAH HPASMC show negligible migration. The dysregulated migration initiates, in part, through phosphorylation events signaled through the unstimulated PDGF receptor via focal adhesion kinase (FAK) whose total basal expression and phosphorylation at tyrosine 391 is markedly increased in the PAH cells and is inhibited by a motif mimicking cell-permeable peptide (MMCPP) targeting the Tyr751 region of the PDGF receptor and by imatinib. However, exposure of the PAH cells to PDGF further promotes migration. Inhibition of p21 activated kinases (PAK), LIM kinases (LIMK), c-Jun N-terminal kinases (JNK) and p38 mitogen-activated protein kinases (MAPK) reduces both the dysregulated and the PDGF-stimulated migration. Immunofluorescence microscopy confirms these observations showing activated JNK and p38 MAPK at the edge of the wound but not in the rest of the culture in the PAH cells. The upstream inhibitors FAK (PF-573228) and imatinib block this activation of JNK and p38 at the edge of the site of injury and correspondingly inhibit migration. MMCPP which inhibit the activation of downstream effectors of migration, cofilin and caldesmon, also limit the dysregulated migration. These results highlight key pathways which point to potential targets for future therapies of pulmonary hypertension with MMCPP.

[Aberrant reparative tissue remodeling: histopathology and molecular pathology]

Not only tumorous infiltrations can lead to destruction of parenchymal organs but also the aberrant proliferation and matrix production of mesenchymal cells and vessels during a dysregulated repair attempt. This fibrogenesis is the result of a complex pathogenesis, which can be investigated in animal models but also in situ to harvest new biomarkers. This article deals particularly with the second method and summarizes molecular pathological findings in various model diseases for aberrant reparative tissue reconstruction. These model diseases include plexiform vasculopathy in pulmonary arterial hypertension (PAH), Quilty lesions in heart transplantation, bronchiolitis obliterans (BO), inflammatory airway remodeling and Epstein-Barr virus (EBV) induced smooth muscle proliferation (posttransplantation smooth muscle tumor, PTSMT).Using in situ molecular pathology, we were able to dismiss an assumed involvement of myofibroblastic cells in vessel reconstruction of the lung in PAH. We could also for the first time perform a comprehensive molecular characterization of the vascular remodeling and prove that plexiform vasculopathy represents a complex-regulated epiphenomenon of excessive pulmonary hypertension. This method also allowed us to describe for the first time the miRNA expression in PAH in a compartment-specific manner and to draw conclusions regarding the damaged overriding regulatory mechanisms. In the same way, we were also able to describe the chimeric character of the complex neoangiogenesis in the donor organ after heart transplantation.After lung transplantation, we identified for the first time a group of tissue-based molecular markers, which can predict later occurrence of BO even in morphologically normal transbronchial biopsies. In addition, we have documented for the first time the molecular characteristics of the morphologically analogous airway reconstruction in lung-transplanted and non-transplanted patients. We could further elucidate the role of matrix metalloproteinases (MMP) and their antagonists in inflammatory airway reconstruction and deduce from this the resulting therapeutic implications. Accordingly, we were able to further clarify the origin, pathogenesis and the malignant potential of EBV-induced PTSMT and for the first time provide an evidence-based therapy recommendation and risk assessment.In summary, this article documents that in situ diagnostics can meet the requirements of the challenging parameters and issues of life sciences. It is to be expected that the technical possibilities will develop analogously to the increasing demands and the in situ method will move further into the focus of molecular pathology.

Adaptation and remodelling of the pulmonary circulation in pulmonary hypertension

Pulmonary arterial hypertension (PAH) is characterized by remodelling of pulmonary arteries caused by a proliferation/apoptosis imbalance within the vascular wall. This pathological phenotype seems to be triggered by different environmental stress and injury events such as increased inflammation, DNA damage, and epigenetic deregulation. It appears that one of the first hit to occur is endothelial cells (ECs) injury and apoptosis, which leads to paracrine signalling to other ECs, pulmonary artery smooth muscle cells (PASMCs), and fibroblasts. These signals promote a phenotypic change of surviving ECs by disturbing different signalling pathways leading to sustained vasoconstriction, proproliferative and antiapoptotic phenotype, deregulated angiogenesis, and formation of plexiform lesions. EC signalling also recruits proinflammatory cells, leading to pulmonary infiltration of lymphocytes, macrophages, and dendritic cells, sustaining the inflammatory environment and autoimmune response. Finally, EC signalling promotes proliferative and antiapoptotic PAH-PASMC phenotypes, which acquire migratory capacities, resulting in increased vascular wall thickness and muscularization of small pulmonary arterioles. Adaptation and remodelling of pulmonary circulation also involves epigenetic components, such as microRNA deregulation, DNA methylation, and histone modification. This review will focus on the different cellular and epigenetic aspects including EC stress response, molecular mechanisms contributing to PAH-PASMC and PAEC proliferation and resistance to apoptosis, as well as epigenetic control involved in adaptation and remodelling of the pulmonary circulation in PAH.

Distinct differences in gene expression patterns in pulmonary arteries of patients with chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis with pulmonary hypertension

RATIONALE

The development of pulmonary hypertension (PH) in patients with idiopathic pulmonary fibrosis (IPF) or chronic obstructive pulmonary disease (COPD) is associated with increased morbidity.

OBJECTIVES

To elucidate whether vascular remodeling in a well-characterized PH-COPD and PH-IPF patient cohort results from similar or divergent molecular changes.

METHODS

Vascular remodeling of donor, PH-COPD, and PH-IPF pulmonary arteries was assessed. Laser capture microdissected pulmonary artery profiles in combination with whole genome microarrays were performed.

MEASUREMENTS AND MAIN RESULTS

Pulmonary arteries from patients with COPD and IPF with PH exhibited remodeling of vascular layers and reduction of lumen area. Pathway analyses comparing normalized gene expression profiles obtained from patients with PH-IPF or PH-COPD revealed the retinol and extracellular matrix (ECM) receptor interaction to be the most perturbed processes. Within the ECM-receptor pathway, differential regulation of 5 out of the top 10 results (collagen, type III, α-1; tenascin C; collagen, type VI, α-3; thrombospondin 2; and von Willebrand factor) were verified by real-time polymerase chain reaction and immunohistochemical staining.

CONCLUSIONS

Despite clinical and histologic vascular remodeling in all patients with PH-COPD and PH-IPF, differential gene expression pattern was present in pulmonary artery profiles. Several genes involved in retinol metabolism and ECM receptor interaction enable discrimination of vascular remodeling in PH-IPF or PH-COPD. This suggests that pulmonary arterial remodeling in PH-COPD and PH-IPF is caused by different molecular mechanisms and may require specific therapeutic options.

Role of microRNAs in lung development and pulmonary diseases

MicroRNAs (miRNAs) are a class of small noncoding RNA which exert post-transcriptional gene regulation activity by targeting messenger RNAs. miRNAs have been found to be involved in various fundamental biological processes and deregulation of miRNAs is known to result in pathological conditions. In this review, we provide an overview of recent discoveries on the role played by this class of molecules in lung development and in pulmonary diseases, such as asthma, cystic fibrosis, chronic obstructive pulmonary disease, and pulmonary artery hypertension. Considering the relevant role of these miRNAs under physiological and pathological conditions, they represent new clinical targets as well as diagnostic and prognostic tools. Therefore, this review pays special attention to recent advances and possible future directions for the use of miRNAs for clinical applications.

Z α1-antitrypsin confers a proinflammatory phenotype that contributes to chronic obstructive pulmonary disease

RATIONALE

Severe α1-antitrypsin deficiency caused by the Z variant (Glu342Lys; ZZ-AT) is a well-known genetic cause for emphysema. Although severe lack of antiproteinase protection is the critical etiologic factor for ZZ-AT-associated chronic obstructive pulmonary disease (COPD), some reports have suggested enhanced lung inflammation as a factor in ZZ-AT homozygotes.

OBJECTIVES

To provide molecular characterization of inflammation in ZZ-AT.

METHODS

Inflammatory cell and cytokine profile (nuclear factor-κB, IL-6, tumor necrosis factor-α), intracellular polymerization of Z-AT, and endoplasmic reticulum (ER) stress markers (protein kinase RNA-like ER kinase, activator transcription factor 4) were assessed in transgenic mice and transfected cells in response to cigarette smoke, and in explanted lungs from ZZ and MM individuals with severe COPD.

MEASUREMENTS AND MAIN RESULTS

Compared with M-AT, transgenic Z-AT mice lungs exposed to cigarette smoke had higher levels of pulmonary cytokines, neutrophils, and macrophages and an exaggerated ER stress. Similarly, the ER overload response was greater in lungs from ZZ-AT homozygotes with COPD, and was particularly found in pulmonary epithelial cells. Cigarette smoke increased intracellular Z-AT polymers, ER overload response, and proinflammatory cytokine release in Z-AT-expressing pulmonary epithelial cells, which could be prevented with an inhibitor of polymerization, an antioxidant, and an inhibitor of protein kinase RNA-like ER kinase.

CONCLUSIONS

We show here that aggregation of intracellular mutant Z-AT invokes a specific deleterious cellular inflammatory phenotype in COPD. Oxidant-induced intracellular polymerization of Z-AT in epithelial cells causes ER stress, and promotes excess cytokine and cellular inflammation. This pathway is likely to contribute to the development of COPD in ZZ-AT homozygotes, and therefore merits further investigation.

Nitration of tyrosine 247 inhibits protein kinase G-1α activity by attenuating cyclic guanosine monophosphate binding

The cGMP-dependent protein kinase G-1α (PKG-1α) is a downstream mediator of nitric oxide and natriuretic peptide signaling. Alterations in this pathway play a key role in the pathogenesis and progression of vascular diseases associated with increased vascular tone and thickness, such as pulmonary hypertension. Previous studies have shown that tyrosine nitration attenuates PKG-1α activity. However, little is known about the mechanisms involved in this event. Utilizing mass spectrometry, we found that PKG-1α is susceptible to nitration at tyrosine 247 and 425. Tyrosine to phenylalanine mutants, Y247F- and Y425F-PKG-1α, were both less susceptible to nitration than WT PKG-1α, but only Y247F-PKG-1α exhibited preserved activity, suggesting that the nitration of Tyr(247) is critical in attenuating PKG-1α activity. The overexpression of WT- or Y247F-PKG-1α decreased the proliferation of pulmonary artery smooth muscle cells (SMC), increased the expression of SMC contractile markers, and decreased the expression of proliferative markers. Nitrosative stress induced a switch from a contractile to a synthetic phenotype in cells expressing WT- but not Y247F-PKG-1α. An antibody generated against 3-NT-Y247 identified increased levels of nitrated PKG-1α in humans with pulmonary hypertension. Finally, to gain a more mechanistic understanding of how nitration attenuates PKG activity, we developed a homology model of PKG-1α. This model predicted that the nitration of Tyr(247) would decrease the affinity of PKG-1α for cGMP, which we confirmed using a [(3)H]cGMP binding assay. Our study shows that the nitration of Tyr(247) and the attenuation of cGMP binding is an important mechanism regulating in PKG-1α activity and SMC proliferation/differentiation.

Endothelial injury in a transforming growth factor β-dependent mouse model of scleroderma induces pulmonary arterial hypertension

OBJECTIVE

To delineate the constitutive pulmonary vascular phenotype of the TβRIIΔk-fib mouse model of scleroderma, and to selectively induce pulmonary endothelial cell injury using vascular endothelial growth factor (VEGF) inhibition to develop a model with features characteristic of pulmonary arterial hypertension (PAH).

METHODS

The TβRIIΔk-fib mouse strain expresses a kinase-deficient transforming growth factor β (TGFβ) receptor type II driven by a fibroblast-specific promoter, leading to ligand-dependent up-regulation of TGFβ signaling, and replicates key fibrotic features of scleroderma. Structural, biochemical, and functional assessments of pulmonary vessels, including in vivo hemodynamic studies, were performed before and following VEGF inhibition, which induced pulmonary endothelial cell apoptosis. These assessments included biochemical analysis of the TGFβ and VEGF signaling axes in tissue sections and explanted smooth muscle cells.

RESULTS

In the TβRIIΔk-fib mouse strain, a constitutive pulmonary vasculopathy with medial thickening, a perivascular proliferating chronic inflammatory cell infiltrate, and mildly elevated pulmonary artery pressure resembled the well-described chronic hypoxia model of pulmonary hypertension. Following administration of SU5416, the pulmonary vascular phenotype was more florid, with pulmonary arteriolar luminal obliteration by apoptosis-resistant proliferating endothelial cells. These changes resulted in right ventricular hypertrophy, confirming hemodynamically significant PAH. Altered expression of TGFβ and VEGF ligand and receptor was consistent with a scleroderma phenotype.

CONCLUSION

In this study, we replicated key features of systemic sclerosis-related PAH in a mouse model. Our results suggest that pulmonary endothelial cell injury in a genetically susceptible mouse strain triggers this complication and support the underlying role of functional interplay between TGFβ and VEGF, which provides insight into the pathogenesis of this disease.

The interaction of endothelin-1 and TGF-β1 mediates vascular cell remodeling

BACKGROUND

Pulmonary arterial hypertension is characterized by increased thickness of pulmonary vessel walls due to both increased proliferation of pulmonary arterial smooth muscle cell (PASMC) and deposition of extracellular matrix. In patients suffering from pulmonary arterial hypertension, endothelin-1 (ET-1) synthesis is up-regulated and may increase PASMC activity and vessel wall remodeling through transforming growth factor beta-1 (TGF-β1) and connective tissue growth factor.

OBJECTIVE

To assess the signaling pathway leading to ET-1 induced proliferation and extracellular matrix deposition by human PASMC.

METHODS

PASMC were serum starved for 24 hours before stimulation with either ET-1 and/or TGF-β1. ET-1 was inhibited by Bosentan, ERK1/2 mitogen activated protein kinase (MAPK) was inhibited by U0126 and p38 MAPK was inhibited by SB203580.

RESULTS

ET-1 increased PASMC proliferation when combined with serum. This effect involved the mitogen activated protein kinases (MAPK) ERK1/2 MAPK and was abrogated by Bosentan which caused a G1- arrest through activation of p27((Kip)). Regarding the contribution of extracellular matrix deposition in vessel wall remodeling, TGF-β1 increased the deposition of collagen type-I and fibronectin, which was further increased when ET-1 was added mainly through ERK1/2 MAPK. In contrast, collagen type-IV was not affected by ET-1. Bosentan dose-dependently reduced the stimulatory effect of ET-1 on collagen type-I and fibronectin, but had no effect on TGF-β1.

CONCLUSION AND CLINICAL RELEVANCE

ET-1 alone does not induce PASMC proliferation and extracellular matrix deposition. However, ET-1 significantly up-regulates serum induced proliferation and TGF-β1 induced extracellular matrix deposition, specifically of collagen type-I and fibronectin. The synergistic effects of ET-1 on serum and TGF-β1 involve ERK1/2 MAPK and may thus present a novel mode of action in the pathogenesis of pulmonary arterial hypertension.

Signal transduction in the development of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a unique disease. Properly speaking, it is not a disease of the lung. It can be seen more as a microvascular disease occurring mainly in the lungs and affecting the heart. At the cellular level, the PAH paradigm is characterized by inflammation, vascular tone imbalance, pulmonary arterial smooth muscle cell proliferation and resistance to apoptosis and the presence of in situ thrombosis. At a clinical level, the aforementioned abnormal vascular properties alter physically the pulmonary circulation and ventilation, which greatly influence the right ventricle function as it highly correlates with disease severity. Consequently, right heart failure remains the principal cause of death within this cohort of patients. While current treatment modestly improve patients' conditions, none of them are curative and, as of today, new therapies are lacking. However, the future holds potential new therapies that might have positive influence on the quality of life of the patient. This article will first review the clinical presentation of the disease and the different molecular pathways implicated in the pathobiology of PAH. The second part will review tomorrow's future putative therapies for PAH.

Bosentan inhibits oxidative and nitrosative stress and rescues occlusive pulmonary hypertension

Pulmonary arterial hypertension (PH) is a fatal disease marked by excessive pulmonary vascular cell proliferation. Patients with idiopathic PH express endothelin-1 (ET-1) at high levels in their lungs. As the activation of both types of ET-1 receptor (ETA and ETB) leads to increased generation of superoxide and hydrogen peroxide, this may contribute to the severe oxidative stress found in PH patients. As a number of pathways may induce oxidative stress, the particular role of ET-1 remains unclear. The aim of this study was to determine whether inhibition of ET-1 signaling could reduce pulmonary oxidative stress and attenuate the progression of disease in rats with occlusive-angioproliferative PH induced by a single dose of SU5416 (200 mg/kg) and subsequent exposure to hypoxia for 21 days. Using this regimen, animals developed severe PH as evidenced by a progressive increase in right-ventricle (RV) peak systolic pressure (RVPSP), severe RV hypertrophy, and pulmonary endothelial and smooth muscle cell proliferation, resulting in plexiform vasculopathy. PH rats also had increased oxidative stress, correlating with endothelial nitric oxide synthase uncoupling and NADPH oxidase activation, leading to enhanced protein nitration and increases in markers of vascular remodeling. Treatment with the combined ET receptor antagonist bosentan (250 mg/kg/day; day 10 to 21) prevented further increase in RVPSP and RV hypertrophy, decreased ETA/ETB protein levels, reduced oxidative stress and protein nitration, and resulted in marked attenuation of pulmonary vascular cell proliferation. We conclude that inhibition of ET-1 signaling significantly attenuates the oxidative and nitrosative stress associated with PH and prevents its progression.

Pulmonary arterial lesions in explanted lungs after transplantation correlate with severity of pulmonary hypertension in chronic obstructive pulmonary disease

BACKGROUND

Pulmonary vascular findings are largely unreported in end-stage chronic obstructive pulmonary disease (COPD).

METHODS

Pulmonary vascular lesions in explanted lungs from 70 patients with COPD/emphysema or α-1-antitrypsin deficiency were analyzed retrospectively. Patients were stratified by the presence and severity of pulmonary hypertension (PH) assessed by right-heart catheterization in 3 hemodynamically distinct groups: (1) non-PH (mean pulmonary arterial pressure [mPAP]<25 mm Hg), (2) mild to moderate PH (mPAP, 25-34 mm Hg), and (3) severe PH (mPAP≥35 mm Hg). The control group comprised 18 patients with idiopathic pulmonary arterial hypertension (IPAH). Vascular lesions were graded 1-6 according to a modified Heath and Edwards (HE) scale.

RESULTS

All COPD patients were in New York Heart Association Functional Class III-IV (forced expiratory volume in 1 second, 23%±8%; total lung capacity, 128%±20% of predicted). Non-PH group: 30 patients (60% women) aged 54±6 years; mPAP, 17.2±3.8 mm Hg; median HE Grade 1 (range, 0-3), with medial hypertrophy of muscular pulmonary arteries and muscularization of pulmonary arterioles. Mild-moderate PH group: 30 patients (50% women) aged 53±6 years; mPAP, 28.6±2.8 mm Hg; median HE Grade 3 (range, 1-3), with additional cellular intimal proliferation. Severe PH group: 10 patients (50% women) aged 54±6 years; mPAP, 40.7±5.2 mm Hg; median HE Grade 3 (range, 2-5), with additional intimal fibrosis. IPAH controls: 18 patients (67% women) aged 36±4 years; mPAP,>50 mm Hg; median HE Grade 4 (range 3-6), with generalized arterial dilatation and plexiform lesions.

CONCLUSIONS

The extent of pulmonary vascular lesions in COPD correlate with the severity of PH. Morphologic lesions similar to those characteristic of IPAH can be observed as PH in COPD progresses to levels characteristic of IPAH.

From form to function: the role of Nox4 in the cardiovascular system

The NADPH oxidase (Nox) family of proteins is comprised of seven members, including Noxes1–5 and the Duoxes 1 and 2. Nox4 is readily distinguished from the other Nox isoforms by its high level of expression in cardiovascular tissues and unique enzymatic properties. Nox4 is constitutively active and the amount of reactive oxygen species (ROS) contributed by Nox4 is primarily regulated at the transcriptional level although there is recent evidence for post-translational control. Nox4 emits a different pattern of ROS and its subcellular localizations, tissue distribution and influence over signaling pathways is different from the other Nox enzymes. Previous investigations have revealed that Nox4 is involved in oxygen sensing, vasomotor control, cellular proliferation, differentiation, migration, apoptosis, senescence, fibrosis, and angiogenesis. Elevated expression of Nox4 has been reported in a number of cardiovascular diseases, including atherosclerosis, pulmonary fibrosis, and hypertension, cardiac failure and ischemic stroke. However, many important questions remain regarding the functional significance of Nox4 in health and disease, including the role of Nox4 subcellular localization and its downstream targets. The goal of this review is to summarize the recent literature on the genetic and enzymatic regulation, subcellular localization, signaling pathways, and the role of Nox4 in cardiovascular disease states.

Pulmonary artery endothelium resident endothelial colony-forming cells in pulmonary arterial hypertension

Proliferative pulmonary vascular remodeling is the pathologic hallmark of pulmonary arterial hypertension (PAH) that ultimately leads to right heart failure and death. Highly proliferative endothelial cells known as endothelial colony-forming cells (ECFC) participate in vascular homeostasis in health as well as in pathological angiogenic remodeling in disease. ECFC are distinguished by the capacity to clonally proliferate from a single cell. The presence of ECFC in the human pulmonary arteries and their role in PAH pathogenesis is largely unknown. In this study, we established a simple technique for isolating and growing ECFC from cultured pulmonary artery endothelial cells (PAEC) to test the hypothesis that ECFC reside in human pulmonary arteries and that the proliferative vasculopathy of PAH is related to greater numbers and/or more proliferative ECFC in the pulmonary vascular wall. Flow cytometric forward and side scatter properties and aggregate correction were utilized to sort unmanipulated, single PAEC to enumerate ECFC in primary PAEC cultures derived from PAH and healthy lungs. After 2 weeks, wells were assessed for ECFC formation. ECFC derived from PAH PAEC were more proliferative than control. A greater proportion of PAH ECFC formed colonies following subculturing, demonstrating the presence of more ECFC with high proliferative potential among PAH PAEC. Human androgen receptor assay showed clonality of progeny, confirming that proliferative colonies were single cell-derived. ECFC expressed CD31, von Willebrand factor, endothelial nitric oxide synthase, caveolin-1 and CD34, consistent with an endothelial cell phenotype. We established a simple flow cytometry method that allows ECFC quantification using unmanipulated cells. We conclude that ECFC reside among PAEC and that PAH PAEC contain ECFC that are more proliferative than ECFC in control cultures, which likely contributes to the proliferative angiopathic process in PAH.

Krüppel-like zinc finger proteins in end-stage COPD lungs with and without severe alpha1-antitrypsin deficiency

Background

Chronic obstructive pulmonary disease (COPD) is influenced by environmental and genetic factors. An important fraction of COPD cases harbor a major genetic determinant, inherited ZZ (Glu342Lys) α1-antitrypsin deficiency (AATD). A study was undertaken to investigate gene expression patterns in end-stage COPD lungs from patients with and without AATD.

Methods

Explanted lungs of end-stage ZZ AATD-related (treated and non-treated with AAT augmentation therapy) and “normal” MM COPD, and liver biopsies from patients suffering from liver cirrhosis with and without ZZ AATD were used for gene expression analysis by Affymetrix microarrays or RT-PCR.

Results

A total of 162 genes were found to be differentially expressed (p-value ≤ 0.05 and |FC| ≥ 2) between MM and ZZ COPD patients. Of those, 134 gene sets were up-regulated and 28 were down-regulated in ZZ relative to MM lung tissue. A subgroup of genes, zinc finger protein 165, snail homolog 1 (Drosophila) (SNAI1), and Krüppel-like transcription factors (KLFs) 4 (gut), 9 and 10, perfectly segregated ZZ and MM COPD patients. The higher expression of KLF 9 and KLF10 has been verified in the replication cohort with AATD-related end-stage lung emphysema and liver cirrhosis. Furthermore, higher expression of KLF9, SNAI1 and DEFA1 was found in ZZ COPD lungs without augmentation therapy relative to MM COPD or ZZ COPD with augmentation therapy.

Conclusions

These results reveal the involvement of transcriptional regulators of the zinc-finger family in COPD pathogenesis and provide deeper insight into the pathophysiological mechanisms of COPD with and without AATD.

Plexiform vasculopathy of severe pulmonary arterial hypertension and microRNA expression

BACKGROUND

Recent studies have revealed that microRNAs (miRNAs) play a key role in the control of angiogenesis and vascular remodeling. Specific miRNAs in plexiform vasculopathy of severe pulmonary arterial hypertension (PAH) in humans have not yet been investigated.

METHODS

We analyzed expression of miR-143/145 (vascular smooth muscle-specific), miR-126 (endothelial-specific) and related mRNAs in plexiform (PLs) and concentric lesions (CLs), which had been laser-microdissected from specimens of formalin-fixed, paraffin-embedded, explanted lungs of PAH patients (n = 12) and unaffected controls (n = 8). Samples were analyzed by real-time polymerase chain reaction, and protein expression was determined by immunohistochemistry.

RESULTS

Expression levels of miR-143/145 and its target proteins (e.g., myocardin, smooth muscle myosin heavy chain) were found to be significantly higher in CLs than in PLs, whereas miR-126 and VEGF-A were significantly up-regulated in PLs when compared with CLs, indicating a more prominent angiogenic phenotype of PL. This correlates with a down-regulation of miR-204 as well as an up-regulation of miR-21 in PLs, which in turn corresponds to enhanced cell proliferation.

CONCLUSIONS

Our findings show that morphologic changes of plexiform vasculopathy in the end-stage PAH lung are reflected by alterations at the miRNA level.

Involvement of matrix metalloproteinase-2 in medial hypertrophy of pulmonary arterioles in broiler chickens with pulmonary arterial hypertension

Medial hypertrophy of pulmonary arterioles during pulmonary arterial hypertension (PAH) in humans is associated with enhanced proliferation of smooth muscle cells (SMCs). Elevated matrix metalloproteinase (MMP)-2 has been found in pulmonary artery SMCs (PA-SMCs) in humans with idiopathic PAH, leading to the hypothesis that MMP-2 contributes to the proliferation and migration of vascular SMCs in the pathogenesis of PAH. Rapidly growing meat-type (broiler) chickens provide a model of spontaneous PAH. The present study was conducted to determine whether MMP-2 is involved in the medial hypertrophy of pulmonary arterioles in this model. Cultured PA-SMCs from normal birds were used to evaluate the effect of MMPs on cell proliferation. Gelatin zymography showed that endothelin (ET)-1-induced proliferation of PA-SMCs was concomitant with increased pro- and active MMP-2 production. Reverse transcription PCR demonstrated upregulation of MMP-2 mRNA. However, PA-SMC proliferation was inhibited by the MMP inhibitors doxycycline and cis-9-octadecenoyl-N-hydroxylamide. In vivo experiments revealed a significant increase of MMP-2 expression in hypertrophied pulmonary arterioles of PAH broiler chickens, which was positively correlated with wall thickness and medial hypertrophy. MMP-2 may contribute to medial hypertrophy in pulmonary arterioles during PAH in broiler chickens by enhancing the proliferation of vascular SMCs.

[Histopathological aspects of pulmonary hypertension]

In pulmonary hypertension there is a discrepancy between the dramatic but unspecific clinical presentation and the remodeling of mostly only limited segments of the vascular compartment of pulmonary parenchyma. Clinical diagnosis relies for the most part on invasive procedures, such as right heart catheterization. Therefore, morphology can provide a reliable etiopathogenetic classification only in close cooperation with the clinical partner disciplines involved. Moreover, the histopathological approach requires intimate knowledge of the vascular anatomy of the lungs and assessment of the parenchyma to be able to diagnose pulmonary hypertension and differentiate between the various types.