Hypoxic activation of adventitial fibroblasts: role in vascular remodeling

Hypoxia, leukocytes, and the pulmonary circulation

Data are rapidly accumulating in support of the idea that circulating monocytes and/or mononuclear fibrocytes are recruited to the pulmonary circulation of chronically hypoxic animals and that these cells play an important role in the pulmonary hypertensive process. Hypoxic induction of monocyte chemoattractant protein-1, stromal cell-derived factor-1, vascular endothelial growth factor-A, endothelin-1, and tumor growth factor-beta(1) in pulmonary vessel wall cells, either directly or indirectly via signals from hypoxic lung epithelial cells, may be a critical first step in the recruitment of circulating leukocytes to the pulmonary circulation. In addition, hypoxic stress appears to induce release of increased numbers of monocytic progenitor cells from the bone marrow, and these cells may have upregulated expression of receptors for the chemokines produced by the lung circulation, which thus facilitates their specific recruitment to the pulmonary site. Once present, macrophages/fibrocytes may exert paracrine effects on resident pulmonary vessel wall cells stimulating proliferation, phenotypic modulation, and migration of resident fibroblasts and smooth muscle cells. They may also contribute directly to the remodeling process through increased production of collagen and/or differentiation into myofibroblasts. In addition, they could play a critical role in initiating and/or supporting neovascularization of the pulmonary artery vasa vasorum. The expanded vasa network may then act as a conduit for further delivery of circulating mononuclear cells to the pulmonary arterial wall, creating a feedforward loop of pathological remodeling. Future studies will need to determine the mechanisms that selectively induce leukocyte/fibrocyte recruitment to the lung circulation under hypoxic conditions, their direct role in the remodeling process via production of extracellular matrix and/or differentiation into myofibroblasts, their impact on the phenotype of resident smooth muscle cells and adventitial fibroblasts, and their role in the neovascularization observed in hypoxic pulmonary hypertension.

[Physiopathology of pulmonary arterial hypertension. Cellular and molecular aspects]

The combined effects of vasoconstriction, remodelling of the pulmonary vessel walls and in situ thrombosis contribute to the increase in pulmonary vascular resistance during pulmonary arterial hypertension. Vascular remodelling involves all the sheaths of the vessel wall and all the cell types of which it is composed (endothelial cells, smooth muscle cells, fibroblasts, inflammatory cells and platelets). Excessive vasoconstriction has been related to a defect in the function of expression of the potassium channels and endothelial dysfunction. This leads to chronic insufficiency in the production of vasodilators, notably nitrogen monoxide and prostacyclin and the excessive production of vasoconstrictors such as endotheline-1. These defects contribute to the increase in vascular tonus and pulmonary vascular remodelling and represent pertinent pharmacological targets. Certain growth factors, including those of the super-family of transforming growth factor beta, angiopoietine-1 and serotonin, may play a part in the pathogenesis of pulmonary arterial hypertension.

Hypoxia-driven proliferation of human pulmonary artery fibroblasts: cross-talk between HIF-1alpha and an autocrine angiotensin system

Pulmonary artery adventitial fibroblasts (FBPA) may play a central role in lung vascular remodeling under conditions of hypoxia and inflammation, the result being pulmonary hypertension and cor pulmonale. In cultured human FBPA, both angiotensin II (Ang II) and hypoxia promoted cell cycle progression and cell proliferation and suppressed apoptosis. These effects were further enhanced when both stimuli were applied simultaneously. Hypoxia elevated the expression of hypoxia-inducible factor 1alpha (HIF-1alpha) and increased the expression of genes regulated by the hypoxia-responsive element (HRE). Up-regulation of both angiotensin-converting enzyme (ACE) and Ang II receptor type 1 (AT1) was also observed. Exogenous Ang II further increased HIF/HRE-dependent signaling in FBPA, whereas suppression of the autocrine ACE-Ang II-AT1 loop with inhibitors of ACE, AT1, and phosphatidylinositol 3-kinase (PI3K) reduced the proliferative response to both hypoxia and exogenous Ang II. Overexpression of HIF-1alpha by transient transfection caused the same proliferative effect and up-regulation of AT1 expression that were observed under hypoxic conditions. In contrast, small interfering RNA targeting HIF-1alpha inhibited hypoxia-induced ACE and AT1 expression. Our studies indicate that the ACE-Ang II-AT1 system serves as a positive feedback loop and fosters FBPA proliferation under hypoxic conditions, with the PI3K-HIF-HRE axis as the central effector pathway. This pathway may thus facilitate vascular remodeling under hypoxic conditions.

Vascular changes after intra-amniotic endotoxin in preterm lamb lungs

Chorioamnionitis is associated with preterm delivery and bronchopulmonary dysplasia (BPD), characterized by impaired alveolar and pulmonary vascular development and vascular dysfunction. To study the vascular effects in a model of chorioamnionitis, preterm lambs were exposed to 20 mg of intra-amniotic endotoxin or saline for 1, 2, 4, or 7 days and delivered at 122 days gestational age (term = 150 days). This intra-amniotic endotoxin dose was previously shown to induce lung maturation. The effect of intra-amniotic endotoxin on expression of endothelial proteins was evaluated. Muscularization of the media and collagen deposition in adventitia of small pulmonary arteries was used to assess vascular remodeling. Compared with controls, bronchoalveolar lavage fluid protein content was increased 2 days after intra-amniotic endotoxin exposure. Vascular endothelial growth factor (VEGF) 165 isoform mRNA decreased 2–4 days after intra-amniotic endotoxin. VEGF, VEGF receptor-2, endothelial nitric oxide synthase (eNOS), platelet endothelial cell adhesion molecule-1, and Tie-2 protein expression in the lung coordinately decreased 1–7 days after intra-amniotic endotoxin. Intra-amniotic endotoxin appeared to selectively decrease eNOS expression in small pulmonary vessels compared with large vessels. Medial smooth muscle hypertrophy and increased adventitial fibrosis were observed 4 and 7 days after intra-amniotic endotoxin. These results demonstrate that, in the preterm lamb lung, antenatal inflammation inhibits endothelial cell protein expression followed by vascular remodeling changes in small pulmonary arteries. Exposure to antenatal inflammation may cause vascular remodeling and contribute to the development of BPD.

Linked mechanical and biological aspects of remodeling in mouse pulmonary arteries with hypoxia-induced hypertension

Right heart failure due to pulmonary hypertension causes significant morbidity and mortality. To study the linked vascular mechanical and biological changes that are induced by pulmonary hypertension, we mechanically tested isolated left main pulmonary arteries from mice exposed to chronic hypobaric hypoxia and performed histological assays on contralateral vessels. In isolated vessel tests, hypoxic vessels stretched less in response to pressure than controls at all pressure levels. Given the short length and large diameter of the pulmonary artery, the tangent Young's modulus could not be measured; instead, an effective elastic modulus was calculated that increased significantly with hypoxia [(280 kPa (SD 53) and 296 kPa (SD 50) for 10 and 15 days, respectively, vs. 222 kPa (SD 35) for control; P < 0.02)]. Hypoxic vessels also had higher damping coefficients [(0.063 (SD 0.017) and 0.054 (SD 0.014) for 10 and 15 days, respectively, vs. 0.033 (SD 0.016) for control; P < 0.002)], indicating increased energy dissipation. The increased stiffness with hypoxia correlated with an increase in collagen thickness (percent collagen multiplied by wall thickness) as well as the sum of elastin and collagen thicknesses measured histologically in the artery wall. These results highlight the mechanobiological changes in the pulmonary vasculature that occur in response to hypoxia-induced pulmonary hypertension. Furthermore, they demonstrate significant vascular mechanical and biological changes that would increase pulmonary vascular impedance, leading to right heart failure.

Extracellular ATP-induced proliferation of adventitial fibroblasts requires phosphoinositide 3-kinase, Akt, mammalian target of rapamycin, and p70 S6 kinase signaling pathways

Extracellular nucleotides are increasingly recognized as important regulators of growth in a variety of cell types. Recent studies have demonstrated that extracellular ATP is a potent inducer of fibroblast growth acting, at least in part, through an ERK1/2-dependent signaling pathway. However, the contributions of additional signaling pathways to extracellular ATP-mediated cell proliferation have not been defined. By using both pharmacologic and genetic approaches, we found that in addition to ERK1/2, phosphatidylinositol 3-kinase (PI3K), Akt, mammalian target of rapamycin (mTOR), and p70 S6K-dependent signaling pathways are required for ATP-induced proliferation of adventitial fibroblasts. We found that extracellular ATP acting in part through G(i) proteins increased PI3K activity in a time-dependent manner and transient phosphorylation of Akt. This PI3K pathway is not involved in ATP-induced activation of ERK1/2, implying activation of independent parallel signaling pathways by ATP. Extracellular ATP induced dramatic increases in mTOR and p70 S6K phosphorylation. This activation of the mTOR/p70 S6 kinase (p70 S6K) pathway in response to ATP is because of independent contributions of PI3K/Akt and ERK1/2 pathways, which converge on the level of p70 S6K. ATP-dependent activation of mTOR and p70 S6K also requires additional signaling inputs perhaps from pathways operating through Galpha or Gbetagamma subunits. Collectively, our data demonstrate that ATP-induced adventitial fibroblast proliferation requires activation and interaction of multiple signaling pathways such as PI3K, Akt, mTOR, p70 S6K, and ERK1/2 and provide evidence for purinergic regulation of the protein translational pathways related to cell proliferation.

Activation of phosphatidylinositol 3-kinase, Akt, and mammalian target of rapamycin is necessary for hypoxia-induced pulmonary artery adventitial fibroblast proliferation

In contrast to cell types in which exposure to hypoxia causes a general reduction of metabolic activity, a remarkable feature of pulmonary artery adventitial fibroblasts is their ability to proliferate in response to hypoxia. Previous studies have suggested that ERK1/2, phosphatidylinositol 3-kinase (PI3K), Akt, and mammalian target of rapamycin (mTOR) are activated by hypoxia and play a role in a variety of cell responses. However, the pathways involved in mediating hypoxia-induced proliferation are largely unknown. Using pharmacological inhibitors, we established that PI3K-Akt, mTOR-p70 ribosomal protein S6 kinase (p70S6K), and EKR1/2 signaling pathways play a critical role in hypoxia-induced adventitial fibroblast proliferation. We found that exposure of serum-starved fibroblasts to 3% O2 resulted in a time-dependent activation of PI3K and transient phosphorylation of Akt. However, activation of PI3K was not required for activation of ERK1/2, implying a parallel involvement of these pathways in the proliferative response of fibroblasts to hypoxia. We found that hypoxia induced significant increases in mTOR, p70S6K, 4E-BP1, and S6 ribosomal protein phosphorylation, as well as dramatic increases in p70S6K activity. The activation of p70S6K/S6 pathway was sensitive to inhibition by rapamycin and LY294002, indicating that mTOR and PI3K/Akt are upstream signaling regulators. However, the magnitude of hypoxia-induced p70S6K activity and phosphorylation suggests involvement of additional signaling pathways. Thus our data demonstrate that hypoxia-induced adventitial fibroblast proliferation requires activation and interaction of PI3K, Akt, mTOR, p70S6K, and ERK1/2 and provide evidence for hypoxic regulation of protein translational pathways in cells exhibiting the capability to proliferate under hypoxic conditions.

Egr-1 antisense oligonucleotides inhibit hypoxia-induced proliferation of pulmonary artery adventitial fibroblasts

In most mammalian species, chronic exposure to hypoxia leads to pulmonary hypertension and vascular remodeling. The adventitial fibroblast, because of its ability to proliferate in response to hypoxia, is thought to be a critical cell in the remodeling process. However, the transcription factors driving hypoxia-induced fibroblast proliferation have yet to be elucidated. The early growth response-1 (Egr-1) transcription factor has been shown to be upregulated by hypoxia in pulmonary artery adventitial fibroblasts. We therefore hypothesized that Egr-1 is directly involved in hypoxia-induced adventitial fibroblast proliferation. Immunohistochemical analysis of in vivo lung tissue from animals exposed to chronic hypoxia revealed increased expression of Egr-1 in the pulmonary artery fibroblasts vs. expression shown in normoxic controls. In fibroblasts cultured from chronically hypoxic animals, exposure to 1% oxygen upregulated Egr-1 protein and cell proliferation. To evaluate the role of Egr-1 in hypoxia-induced proliferation, we employed an Egr-1 antisense strategy. Addition of antisense Egr-1 oligonucleotides, but not sense oligonucleotides, attenuated the hypoxia-induced upregulation of Egr-1 protein and reduced hypoxia-induced DNA synthesis by 50%. Cell proliferation was also significantly inhibited by the addition of antisense Egr-1 oligonucleotides but not the sense oligonucleotides. In addition, hypoxia-induced upregulations of cyclin D and epidermal growth factor receptor were attenuated by Egr-1 antisense oligonucleotides. We conclude that Egr-1 protein expression is very sensitive to upregulation by hypoxia in pulmonary artery adventitial fibroblasts and that it plays an important role in the autonomous growth phenotype induced by hypoxia in these cells.

Synergy between A2B adenosine receptors and hypoxia in activating human lung fibroblasts

Chronic inflammatory airway diseases, such as asthma, chronic obstructive pulmonary disease and pulmonary fibrosis, are associated with subepithelial fibroblast activation, myofibroblast hyperplasia, hypoxia, and increase in interstitial adenosine concentrations. The goal of this study was to determine the effect of adenosine and its receptors on activation of human lung fibroblasts under normoxia (21% O2) and hypoxia (5% O2). Under the normoxic condition, adenosine and its stable analog, 5'-(N-ethylcarboxamido)-adenosine, via activation of A2B adenosine receptors, increased the release of interleukin (IL)-6 by 14-fold and induced the differentiation of human lung fibroblasts to myofibroblasts. This latter effect of 5'-(N-ethylcarboxamido)-adenosine was abolished by an IL-6-neutralizing antibody. Hypoxia increased the release of IL-6 by 2.8-fold, and there was a synergy between hypoxia and activation of A2B adenosine receptors to increase the release of IL-6 and to induce differentiation of fibroblasts into myofibroblasts. Hypoxia increased the expression of A2B adenosine receptors by 3.4-fold. Altogether, these data suggest that hypoxia amplifies the effect of adenosine on the release of IL-6 and cell differentiation by upregulating the expression of A2B adenosine receptors. Our findings provide a novel mechanism whereby adenosine participates in the remodeling process of inflammatory lung diseases.

Cellular and molecular mechanisms of pulmonary vascular remodeling: role in the development of pulmonary hypertension

Pulmonary artery vasoconstriction and vascular remodeling greatly contribute to a sustained elevation of pulmonary vascular resistance (PVR) and pulmonary arterial pressure (PAP) in patients with pulmonary arterial hypertension (PAH). The development of PAH involves a complex and heterogeneous constellation of multiple genetic, molecular, and humoral abnormalities, which interact in a complicated manner, presenting a final manifestation of vascular remodeling in which fibroblasts, smooth muscle and endothelial cells, and platelets all play a role. Vascular remodeling is characterized largely by medial hypertrophy due to enhanced vascular smooth muscle cell proliferation or attenuated apoptosis and to endothelial cell over-proliferation, which can result in lumen obliteration. In addition to other factors, cytoplasmic Ca2+ in particular seems to play a central role as it is involved in both the generation of force through its effects on the contractile machinery, and the initiation and propagation of cell proliferation via its effects on transcription factors, mitogens, and cell cycle components. This review focuses on the role played by cellular factors, circulating factors, and genetic molecular signaling factors that promote a proliferative, antiapoptotic, and vasoconstrictive physiological milieu leading to vascular remodeling.

Adipose tissue angiogenesis

A review of adipose tissue angiogenesis includes the morphological and cytochemical development of adipose tissue vasculature and the concept of primitive fat organs. Spatial and temporal relationships between fetal vascular and fat cell development are discussed, including depot- and genetic-dependent arteriolar differentiation. The relationship between connective tissue deposition and elaboration of adipose tissue vasculature is discussed with respect to regulating adipocyte development in a depot-dependent manner. In vitro studies indicated that depot-dependent vascular traits may be attributable to intrinsic growth characteristics of adipose tissue endothelial cells. These studies indicate that adipogenesis may be regulated by factors that drive angiogenesis. Fundamental aspects of angiogenesis, including basement membrane breakdown, vasculogenesis, angiogenic remodeling, vessel stabilization, and vascular permeability were reviewed. Critical angiogenic factors include vascular endothelial growth factor (VEGF), VEGF receptors, angiopoietins (Ang), ephrins, matrix metalloproteinases, and the plasminogen enzymatic system. Vascular endothelial growth factor is the most critical factor because it initiates the formation of immature vessels and disruption of a single VEGF allele leads to embryonic lethality in mice. Expression of VEGF is influenced by hypoxia, insulin, growth factors, and several cytokines. Angiogenic factors secreted and/or produced by adipocytes or preadipocytes are discussed. Vascular endothelial growth factor expression and secretion by adipocytes is regulated by insulin and hypoxia, and is associated with adipose tissue accretion. Vascular endothelial growth factor accounts for most of the angiogenic activity of adipose tissue. The proposed role of leptin as an adipogenic factor is reviewed with respect to efficacy on various aspects of angiogenesis relative to other angiogenic factors. The VEGF and leptin genes are both hypoxia inducible, but potential links between VEGF and leptin gene expression have not been examined. Finally, several studies including a study of mice treated with antiangiogenic factors indicate that adipose tissue accretion can be controlled through the vasculature per se.

Cellular and molecular pathobiology of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) has a multifactorial pathobiology. Vasoconstriction, remodeling of the pulmonary vessel wall, and thrombosis contribute to increased pulmonary vascular resistance in PAH. The process of pulmonary vascular remodeling involves all layers of the vessel wall and is complicated by cellular heterogeneity within each compartment of the pulmonary arterial wall. Indeed, each cell type (endothelial, smooth muscle, and fibroblast), as well as inflammatory cells and platelets, may play a significant role in PAH. Pulmonary vasoconstriction is believed to be an early component of the pulmonary hypertensive process. Excessive vasoconstriction has been related to abnormal function or expression of potassium channels and to endothelial dysfunction. Endothelial dysfunction leads to chronically impaired production of vasodilators such as nitric oxide and prostacyclin along with overexpression of vasoconstrictors such as endothelin (ET)-1. Many of these abnormalities not only elevate vascular tone and promote vascular remodeling but also represent logical pharmacological targets. Recent genetic and pathophysiologic studies have emphasized the relevance of several mediators in this condition, including prostacyclin, nitric oxide, ET-1, angiopoietin-1, serotonin, cytokines, chemokines, and members of the transforming-growth-factor-beta superfamily. Disordered proteolysis of the extracellular matrix is also evident in PAH. Future studies are required to find which if any of these abnormalities initiates PAH and which ones are best targeted to cure the disease.

ATP-induced mitogenesis is mediated by cyclic AMP response element-binding protein-enhanced TRPC4 expression and activity in human pulmonary artery smooth muscle cells

Extracellular ATP and intracellular cyclic AMP response element-binding protein (CREB, a transcription factor) promote cell proliferation in many cell types. The canonical transient receptor potential (TRPC) channels, which putatively participate in forming store- and receptor-operated Ca2+ channels, have been implicated in the pulmonary vascular remodeling processes. A link between extracellular ATP, CREB activation, and TRPC4 channel expression and activity has not been shown in human pulmonary artery smooth muscle cells (PASMC). Long-term (24-48 h) treatment of human PASMC with a low dose (100 microM) of ATP, which did not trigger a transient rise in free cytosolic Ca2+ concentration ([Ca2+]i) when applied acutely to the cells, caused marked increases in CREB phosphorylation and TRPC4 protein expression. The time course indicated that the ATP-mediated CREB phosphorylation preceded TRPC4 upregulation, whereas transfection of a nonphosphorylatable CREB mutant abolished ATP-mediated TRPC4 expression. Furthermore, treatment of human PASMC with ATP also enhanced the amplitude of capacitative Ca2+ entry (CCE) induced by passive store depletion, whereas the small interfering RNA specifically targeting TRPC4 attenuated ATP-mediated increases in TRPC4 expression and CCE amplitude and inhibited ATP-induced PASMC proliferation. These data suggest that low-dose ATP exerts part of its mitogenic effect in human PASMC via CREB-mediated upregulation of TRPC4 channel expression and activity and the subsequent increase in CCE and [Ca2+]i.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Fatal pulmonary hypertension associated with pertussis in infants: does extracorporeal membrane oxygenation have a role?

OBJECTIVE

The deaths of 4 infants who had confirmed pertussis infection at our hospital during the past year motivated us to review their cases as well as the use of extracorporeal membrane oxygenation (ECMO) in respiratory failure caused by pertussis.

METHODS

Retrospective chart reviews of the 4 infants who had pertussis and died at Vanderbilt University Medical Center (VUMC) from May 2001 to May 2002 were conducted. The Extracorporeal Life Support Organization (ELSO) database is an international voluntary registry established in 1986 to compile passive reports of adult and pediatric patients placed on ECMO. We searched this database for pertussis cases and analyzed the clinical outcomes by age, ventilatory management, and measurements of cardiopulmonary status.

RESULTS

All 4 infants who died from pertussis infection at VUMC were younger than 3 months and had severe pulmonary hypertension, and ECMO therapy was considered for respiratory failure. Review of the international ELSO database, focusing only on children from 1986 to July 2002, revealed a total of 23,970 patients placed on ECMO. Since the first pertussis case treated with ECMO in 1990, a total of 61 children with pertussis have been treated with ECMO, representing an increase from 0.09% to 1% of the total cases. Mean age of the pertussis patients placed on ECMO was 88 days (1 day-2.7 years). The overall mortality was 70.5% (43 of 61) but was significantly higher for infants who were younger than 6 weeks (84%) compared with infants who were older than 6 weeks (61%). When evaluating pre-ECMO management, survivors had received significantly higher mean positive end expiratory pressures than nonsurvivors (11.1 +/- 4.5 vs 7.3 +/- 3.1 cmH(2)O) and had significantly higher serum pH than nonsurvivors (7.31 +/- 0.14 vs 7.14 +/- 0.19). There was no effect of duration of ECMO, positive inspiratory pressure, mean arterial pressure, ECMO mode (venoarterial vs venovenous mode), or sex on mortality.

CONCLUSIONS

Although the ECMO registry is not an active surveillance system, it suggests that the numbers of children who require ECMO for pertussis have significantly increased. The high fatality rates of pertussis patients who have placed on ECMO are alarming and should stimulate reevaluation of whether this high-risk intervention is beneficial to patients with pertussis. This review, coupled with the 4 infants who died of pertussis at VUMC, suggests that improved measures are needed to prevent pertussis in infants.

Hypoxia induces differentiation of pulmonary artery adventitial fibroblasts into myofibroblasts

Activation of the alpha-smooth muscle actin (alpha-SMA) gene during the conversion of fibroblasts into myofibroblasts is an essential feature of various fibrotic conditions. Microvascular compromise and thus local environmental hypoxia are important components of the fibrotic response. The present study was thus undertaken to test the hypothesis that hypoxia can induce transdifferentiation of vascular fibroblasts into myofibroblasts and also to evaluate potential signaling mechanisms governing this process. We found that hypoxia significantly upregulates alpha-SMA protein levels in bovine pulmonary artery adventitial fibroblasts. Increased alpha-SMA expression is controlled at the transcriptional level because the alpha-SMA gene promoter activity, assayed via a luciferase reporter, was markedly increased in transfected fibroblasts exposed to hypoxia. Hypoxic induction of the alpha-SMA gene was mimicked by overexpression of constitutively active Galphai2 (alphai2Q205L) but not Galpha16 (alpha-16Q212L). Blockade of hypoxia-induced alpha-SMA expression with pertussis toxin, a Galphai antagonist, confirmed a role for Galphai in the hypoxia-induced transdifferentiation process. c-Jun NH2-terminal kinase (JNK) inhibitor II and SB202190, but not U0126, also attenuated alpha-SMA expression in hypoxic fibroblasts, suggesting the importance of JNK in the differentiation process. Hypoxia-induced increase in bromodeoxyuridine incorporation, which occurred concomitantly with hypoxia-induced differentiation, was blocked by U0126, suggesting that DNA synthesis and alpha-SMA expression take place through simultaneously activated parallel signaling pathways. Neutralizing antibody against transforming growth factor-beta1 blocked only 30% of the hypoxia-induced alpha-SMA promoter activity. Taken together, our results suggest that hypoxia induces differentiation of vascular fibroblasts into myofibroblasts by upregulating the expression of alpha-SMA, and this increase in alpha-SMA level occurs through Galphai- and JNK-dependent signaling pathways.