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The combination of chronic hypoxia and treatment of rats with the vascular endothelial growth factor (VEGF) receptor blocker, SU5416, induces pulmonary angio-obliteration, resulting in severe pulmonary arterial hypertension (PAH). Inflammation is thought to contribute to the pathology of PAH. Allergic inflammation caused by ovalbumin (OVA) immunization causes muscularization of pulmonary arteries, but not severe PAH. Whether disturbance of the immune system and allergic inflammation in the setting of lung endothelial cell apoptosis causes PAH is unknown. We investigated the effects of OVA-allergic inflammation on the development of PAH initiated by VEGF blockade-induced lung endothelial cell apoptosis. OVA-immunized rats were treated with SU5416 to induce pulmonary vascular endothelial cell apoptosis. The combination of OVA and SU5416 treatment resulted in severe angio-obilterative PAH, accompanied by increased IL-6 expression in the lungs. c-Kit(+) and Sca-1(+) cells were found in and around the lung vascular lesions. Pan-caspase inhibiton, dexamethasone treatment, and depletion of B-lymphocytes using an anti-CD20 antibody suppressed this remodeling. OVA immunization also increased lung tissue hypoxia-induced factor-1α and VEGF expression. Our results also suggest that the increased expression of hypoxia-induced factor-1α and IL-6 induced by the allergic lung inflammation may be a component of the pathogenesis of PAH.

Idiopathic pulmonary fibrosis (IPF) is a disabling disease of the lung parenchyma, characterized by progressive accumulation of scar tissue and myofibroblast activation after repetitive epithelial microinjury. The therapeutic options are limited, and patients usually die within a few years after diagnosis. Pulmonary hypertension (PH) in IPF has been increasingly recognized as a condition with relevance for the overall prognosis. Treatment trials are being designed, but to be effective, it is crucial to better understand the pathobiology of PH in IPF: the traditional concept, that hypoxic vasoconstriction and accumulation of scar tissue are mainly responsible for the development of PH in IPF, has been challenged. Recent studies, including our own in vivo research, suggest that the underlying pathobiology is much more complex, and includes a complicated interaction of epithelial cells, fibroblasts, and vascular cells. This interaction seems to be regulated by a large variety of angiogenesis promoters and inhibitors, as well as growth factors. Central components seem to be endothelial apoptosis and growth factor-induced remodeling of the pulmonary artery wall. The present review gives a conceptual overview about known and putative mechanisms that are involved in the development of PH in IPF. This report summarizes currently available therapeutic options, and also translates experimental research to discuss potential novel biomarkers and therapeutic strategies derived from new concepts in pathogenesis.

Vascular remodeling is an important pathological feature of pulmonary arterial hypertension (PAH), which leads to increased pulmonary vascular resistance, with marked proliferation of pulmonary artery smooth muscle cells (SMC) and/or endothelial cells (EC). Successful treatment of experimental PAH with a platelet-derived growth factor (PDGF) receptor tyrosine kinase inhibitor offers the perspective of "reverse remodeling" (i.e., the regression of established pulmonary vascular lesions). Here we ask the question: which forms of pulmonary vascular remodeling are reversible and can such remodeling caused by angiogenic proliferation of EC be reversed? It is important to emphasize that the report showing reduction of vascular remodeling by PDGF receptor tyrosine kinase inhibitor showed only a reduction of the pulmonary artery muscularization in chronic hypoxia and monocrotaline models, which lack the feature of clustered proliferated EC in the lumen of pulmonary arteries. The regression of vascular muscularization is an important manifestation, whereby proliferative adult SMC convert back to a nonproliferative state. In contrast, in vitro experiments assessing the contribution of EC to the development of PAH demonstrated that phenotypically altered EC generated as a consequence of a vascular endothelial growth factor receptor blockade did not reverse to normal EC. Whereas it is suggested that the proliferative state of SMC may be reversible, it remains unknown whether phenotypically altered EC can switch back to a normal monolayer-forming EC. This article reviews the pathogenetic concepts of severe PAH and explains the many forms in PAH with reversible or irreversible remodeling.

Emphysema occurs in a subgroup of patients with chronic obstructive pulmonary disease and patients with the genetic defect of alpha(1)-antitrypsin deficiency who have a smoking history of many years' duration. Emphysema is generally the result of a chronic and progressive destruction of the alveolar structures, which is believed to be driven by chronic inflammation, infections, oxidative stress, and an imbalance of protease and antiprotease activity. Here, we use microarray technology to characterize the gene expression profile of lung tissue samples obtained from patients with advanced emphysema and that obtained from healthy subjects. We hypothesized that the gene expression profile of emphysema lung tissue is distinct when compared with the expression profile of normal lungs. We report that severely emphysematous tissue is characterized by a global decrease in gene expression and by an increased abundance of transcripts encoding proteins involved in inflammation, immune responses, and proteolysis. Whereas the gene expression profile is to some degree shared between "usual" emphysema and alpha(1)-antitrypsin deficiency-related emphysema, there are statistically significant differences in the modulation of groups of genes associated with protein and energy metabolism, and immune function, which allow distinction between these two emphysema types on the lung tissue level.

We have previously demonstrated that a failure of pulmonary endothelial cell survival induced by vascular endothelial growth factor (VEGF) receptor blockade results in lung alveolar septal cell apoptosis and emphysema. Because apoptosis and oxidative stress may be pathobiologically linked, we hypothesized that oxidative stress has a central role in alveolar septal cell apoptosis and emphysema induced by VEGF receptor blockade. When compared with control animals, rats treated with the VEGF receptor blocker SU5416 showed increased alveolar enlargement, alveolar septal cell apoptosis, and expression of markers of oxidative stress, all of which were prevented by the superoxide dismutase mimetic M40419. The preservation of lung structure in SU5416+M40419-treated lungs was associated with increased septal cell proliferation, and enhanced phosphorylation of the prosurvival and antiapoptotic Akt, when compared with SU5416-treated lungs. Consistent with a positive feedback interaction between oxidative stress and apoptosis, we found that apoptosis predominated in areas of oxidative stress, and that apoptosis blockade by a broad spectrum caspase inhibitor markedly reduced the expression of markers of oxidative stress induced by SU5416 treatment. Oxidative stress and apoptosis, which cause lung cellular destruction in emphysema induced by VEGF receptor blockade, may be important mediators common to human and experimental emphysema.

Pulmonary arterial hypertension (PAH) is characterized by dysfunctional angiogenesis leading to lung vessel obliteration. PAH is widely considered a proangiogenic disease; however, the role of angiogenic factors, such as the vascular endothelial growth factor (VEGF) and its receptors, in the pathobiology of PAH remains incompletely understood. This Review attempts to untangle some of the complex multilayered actions of VEGF to provide a VEGF-centered perspective of PAH. Furthermore, we provide a cogent explanation for the paradox of VEGF receptor blockade-induced pulmonary hypertension that characterizes the SU5416-hypoxia rat model of PAH, and attempt to translate the knowledge gained from the experimental model to the human disease by postulating the potential role of endogenous (SU5416-like) VEGF inhibitors. The main objective of this Review is to promote discussion and investigation of the opposing and complementary actions of VEGF in PAH. Understanding the balance between angiogenic and antiangiogenic factors and their role in the pathogenesis of PAH will be necessary before antiangiogenic drugs can be considered for the treatment of PAH.

Prostaglandins have emerged as a therapeutic option for patients with peripheral vascular disease as well as pulmonary hypertension as a means to increase blood flow. We tested the hypothesis that prostaglandins regulate vascular endothelial growth factor (VEGF) expression in the human monocytic THP-1 cell line and in isolated perfused rat lungs. Our data show that the stable PGI2-analogue iloprost induces VEGF gene expression (predominantly VEGF121, but also VEGF165 isoforms) and VEGF protein synthesis in THP-1 cells. This effect is abolished by dexamethasone and by Rp-cAMP, a specific inhibitor of cAMP-dependent protein kinase (PKA) activation. The calcium channel blocker diltiazem has no effect on the iloprost-induced VEGF gene expression, and depletion of intracellular Ca2+ stores by long-term exposure (16 h) of THP-1 cells to thapsigargin does not inhibit iloprost-induced VEGF gene expression, suggesting that an increase in intracellular Ca2+ is not essential for VEGF gene induction by iloprost. However, an increase of intracellular Ca2+ by a short-term (2 h) exposure of THP-1 cells to thapsigargin or to the calcium-ionophore A23187 increases VEGF mRNA levels, indicating that a change in intracellular Ca2+ by itself can alter VEGF gene expression. The effects of thapsigargin or A23187 on VEGF gene expression are also mediated via cAMP-PKA since they are inhibited by Rp-cAMP. In isolated perfused rat lungs, PGI2 and PGE2 increases VEGF mRNA abundance whereas Rp-cAMP inhibits the prostaglandin-induced VEGF gene activation. Thus, our data suggest that prostaglandins stimulate VEGF gene expression in monocytic cells and in rat lungs via a cAMP-dependent mechanism.

Obliteration of the vascular lumen by endothelial cell growth is a hallmark of many forms of severe pulmonary arterial hypertension. Copper plays a significant role in the control of endothelial cell proliferation in cancer and wound-healing. We sought to determine whether angioproliferation in rats with experimental pulmonary arterial hypertension and pulmonary microvascular endothelial cell proliferation in humans depend on the proangiogenic action of copper. A copper-depleted diet prevented, and copper chelation with tetrathiomolybdate reversed, the development of severe experimental pulmonary arterial hypertension. The copper chelation–induced reopening of obliterated vessels was caused by caspase-independent apoptosis, reduced vessel wall cell proliferation, and a normalization of vessel wall structure. No evidence was found for a role of super oxide–1 inhibition or lysyl–oxidase–1 inhibition in the reversal of angioproliferation. Tetrathiomolybdate inhibited the proliferation of human pulmonary microvascular endothelial cells, isolated from explanted lungs from control subjects and patients with pulmonary arterial hypertension. These data suggest that the inhibition of endothelial cell proliferation by a copper-restricting strategy could be explored as a new therapeutic approach in pulmonary arterial hypertension. It remains to be determined, however, whether potential toxicity to the right ventricle is offset by the beneficial pulmonary vascular effects of antiangiogenic treatment in patients with pulmonary arterial hypertension.

Although alveolar type II cells in primary culture have been shown to produce eicosanoids and exposure of type II cells to silica in vitro alters eicosanoid production, the production of eicosanoids by alveolar type II cells isolated after acute lung injury in vivo has not been evaluated. Therefore, we investigated the production of arachidonic acid (AA) metabolites by alveolar type II cells isolated after silica-induced lung injury. Alveolar type II cells were isolated from rats 14 days after intratracheal silica instillation and from untreated animals. Type II cells were separated into normotrophic and hypertrophic populations by centrifugal elutriation, and secreted eicosanoids were determined under basal and stimulated conditions by enzyme immunoassay on the day of isolation and after 1 day in culture. Under basal conditions, freshly isolated type II cells from silica-treated animals produced more prostaglandin (PG) E2 than 6-keto-PGF1 alpha or thromboxane B2 (TxB2). Production of all three prostanoids increased with increasing cell size. The calcium ionophore A23187 stimulated a less than 2-fold increase in PGE2 and 6-keto-PGF1 alpha production in all groups of cells. In contrast, this calcium ionophore greatly enhanced TxB2 and leukotriene C4 (LTC4) production by normotrophic type II cells from both untreated and silica-treated animals. Incubation with exogenous AA suggested that the increased capability of the hypertrophic cells to synthesize PGE2 and TxB2 was due primarily to an increase in arachidonate availability. The hypertrophic type II cells also appear to have increased prostacyclin synthase activity. There were no differences in the catabolism of PGE2 between the normotrophic and the hypertrophic type II cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Department of Pathology, Division of Cardiopulmonary Pathology, and Division of Pulmonary and Critical Care Medicine, Johns Hopkins University School of Medicine, Baltimore, Maryland; Section of Pulmonary and Critical Care Medicine, Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut; Division of Pulmonary and Critical Care Medicine, and COPD Center, Department of Medicine, University of Colorado School of Medicine, Denver, Colorado; and Program in Cell Biology, Department of Pediatrics, National Jewish Medical and Research Center, Denver, Colorado

Chronic pulmonary hypertension is associated with significant vascular remodeling. We demonstrated recently in the monocrotaline (MCT) and chronic hypoxia rat models of pulmonary hypertension that treatment with platelet-activating factor (PAF) antagonists inhibited the development of chronic pulmonary hypertension. PAF and other lipid mediators interact with interleukin-1. We postulated that chronic treatment with a recombinant human interleukin-1 receptor antagonist (IL-1ra) would inhibit development of chronic pulmonary hypertension in animal models. Rats were either injected with (60 mg/kg) MCT or exposed to a stimulated high altitude of 16,000 feet; half of the animals were treated with twice-daily injections (2 mg/kg) of IL-1ra. At 3 wk after MCT injection or 3 wk of hypoxic exposure, pulmonary artery pressure and right heart ventricle weight/(left ventricle and septum weight), RV/(LV + S), were measured. IL-1ra treatment reduced pulmonary hypertension and right heart hypertrophy in the MCT model, but not in the chronic hypoxia model. Measurement of lung homogenate IL-1 alpha by radioimmunoassay showed elevated levels in the MCT-treated rats throughout the 3-wk observation period. IL-1ra treatment reduced the levels of IL-1 alpha in lung tissue in most of the MCT-treated rats. MCT treatment was also associated with an increase in lung mRNA for IL-1 alpha, IL-1 beta, and IL-1ra. Immunohistology, using an antibody against rat IL-1 alpha, revealed staining of alveolar structures and of vascular and bronchial smooth muscle. In situ hybridization using a human IL-1 alpha cDNA probe demonstrated increased expression of the IL-1 alpha gene in the lung cells after endotoxin or MCT treatment. Northern blot analysis demonstrated low-level expression of IL-1 alpha mRNA in extracts of normal rat lung and increased expression after endotoxin or MCT treatment. We conclude that chronic treatment with human IL-1ra inhibited the development of pulmonary hypertension in the inflammatory (MCT) model, but not in the chronically hypoxic rats. This result indicates that IL-1 participates in the pathogenesis of some forms of pulmonary hypertension.

Neonatal calves exposed to chronic hypobaric hypoxia develop severe pulmonary hypertension associated with altered vascular reactivity, cellular proliferation, and increased elastin and collagen production. We hypothesized that prostaglandin (PG) production would be decreased in the pulmonary arterial vessel wall of these calves. Further, because of the possibility that the hemodynamic stresses of hypoxic pulmonary hypertension might change along the longitudinal axis of the pulmonary circulation, we measured prostaglandin synthetic capability in tissues isolated from proximal pulmonary artery, distal pulmonary artery, and pulmonary vein. We found that PGI2 production was decreased in both proximal and distal pulmonary artery rings isolated from pulmonary hypertensive calves compared to controls. PGI2 production was greater in distal than in proximal lobar pulmonary artery. In contrast, pulmonary veins from hypertensive calves, which are protected from the hemodynamic stress of pulmonary arterial hypertension, did not demonstrate altered PGI2 production compared to controls. PGE2 production was also decreased in proximal hypertensive pulmonary arterial rings as compared to controls. To determine if this decrease in vessel wall production of prostaglandins was due to changes in cellular prostaglandin production, we studied prostaglandin production by the three major cell types comprising hypertensive and control arteries. Endothelial cells cultured from hypertensive main pulmonary artery produced less PGI2 than did those from control artery, and there appeared to be a shift from PGI2 production to PGE2 production in endothelial cells isolated from hypertensive artery. Explanted advential fibroblasts from hypertensive artery produced less PGE2 than did controls. Smooth muscle cell PGI2 production did not differ between cells isolated from hypertensive and control arteries in these brief 30-min incubations. We conclude that there is a relative deficit in PGI2 and PGE2 production in the pulmonary arteries of calves with hypoxia-induced pulmonary hypertension and speculate that this contributes to altered vascular tone and vessel remodeling.