Yong Chul Lee, Department of Internal Medicine, Chonbuk National University Medical School, San 2-20, Geumam-dong, Deokjin-gu, Jeonju 561-180, South Korea. Email: email@example.com
Asthma is a chronic inflammatory disease of airways, characterized by airway hyperresponsiveness and airflow limitation with acute bronchoconstriction, swelling of the airway wall, chronic mucus plug formation and airway wall remodelling. Functional and structural changes in the vasculature of asthmatic airways have been documented, and the signalling mechanisms are complex and have recently attracted much attention. The vascular changes may affect inflammatory cell recruitment, airway hyperresponsiveness and the regulation of airway calibre, and further, the level of disease control. Many critical factors are involved in the pathophysiological regulation of vascular changes in bronchial asthma, and the actions of these factors must be very carefully orchestrated. By better understanding the complicated actions of each factor, we may be able to advance further in asthma treatment.
Airway circulation has important roles in exchange of heat and water, clearance of inhaled materials and inflammatory mediators released from airways, conduction of inflammatory cell recruitment, airway hyperresponsiveness, and possibly the regulation of airway calibre.1–9
Asthma is a chronic inflammatory disease of the airways, characterized by recurrent episodes of airflow limitation which are usually reversible either spontaneously or with treatment, airway hyperresponsiveness, and a decline of lung functions that can lead to irreversible airflow obstruction. The pathogenesis of these alterations is based on various mechanisms, such as infiltration of inflammatory cells, release of mediators, bronchial microcirculation changes and airway remodelling.10 In fact, the functional and structural changes in asthmatic airway vasculature and the causative molecular mechanisms have attracted enormous attention of numerous investigators.
This review deals with the potential therapeutical targeting of abnormal vascularity in asthma and its molecular bases.
VASCULAR CHANGES IN PATIENTS WITH ASTHMA
Allergic inflammation exhibits several effects on blood vessels in the respiratory tract, that is functional and structural changes (vascular remodelling). Currently, several mechanisms implicated in airway inflammation are presumed to be involved in bronchial microcirculation changes in asthma. The changes are vasodilation, increased blood flow, angiogenesis, and increased vascular permeability. This structural change occurs usually in inflammation, yet little is known about the functional significance of bronchial vascular remodelling in asthma (Fig. 1).
Recent studies have demonstrated an increased airway mucosal blood flow by dilatation of resistance arteries and increased number of vessels in airways of asthma.11,12 In addition, most inflammatory mediators cause bronchial vasodilation in animal models.13–15 It is also postulated that the increase in bronchial microcirculation and airway blood flow amplifies inflammatory responses by acting as a gateway to the sub-epithelium for inflammatory cells, although some reports have demonstrated that increased airway blood flow may play a role in removing inflammatory mediators from airways.16 Moreover, the increase in the number and size of vessels can contribute to thickening of the airway wall, which in turn may lead to critical narrowing of the bronchial lumen, as bronchial smooth muscle contraction occurs.17
Microvascular leakage is an essential component of the inflammatory responses, and the majority of inflammatory mediators induce this vascular leakage in asthma.1,18–20 Inflammation of the asthmatic airway mucosa is usually accompanied by increased vascular permeability and plasma exudation. Vesicular-vascular organelles, which are present largely in venular endothelium, provide the major route of extravasation of macromolecules at sites of increased vascular permeability.21,22 In the airways, the plasma exudates may readily pass through the inflamed mucosa and reach the airway lumen through leaky epithelium. Leaked plasma proteins induce a thickened, engorged and oedematous airway wall, resulting in airway obstruction. Plasma proteins leaked also traverse the epithelium and are collected in the airway lumen, resulting in a reduction of mucus clearance and ciliary dysfunction.23
Direct visualization of airway vasculature in asthmatic patients shows a markedly increased number of vessels compared with control subjects.24,25 Angiogenesis is a complex and multifaceted process that includes endothelial cell proliferation and migration, recruitment of perivascular supporting cells (pericytes), and a maturation process for newly formed vessels. This process is tightly integrated and balanced between miscellaneous angiogenic growth factors and various anti-angiogenic factors (see Table 1).26–30 The angiogenic growth factors released are implicated in inductive hypertrophy and hyperplasia of existing blood vessels in asthma, although their respective roles have not been clearly defined.24,31 Among the angiogenic growth factors, vascular endothelial growth factor (VEGF) and angiopoietin-1 (Ang-1) have been reported to play the most relevant roles in vascular changes in the airways of asthmatic subjects.26,32,33
The time frame of vascular changes in the airways of chronic inflammatory pulmonary diseases is unclear. Studies have reported that a significant increase in the number of vessels and/or percent vascular area as well as an increased average capillary dimension can be observed in children and adults with mild to moderate asthma, suggesting that vascular changes occur at a very early stage of the disease.6,7,34–36 On the other hand, a number of studies have shown a relationship between the increased bronchial vascular remodelling and the severity of asthmatic disease.6,24,29,37,38
FACTORS THAT AFFECT AIRWAY VASCULATURE
Less well characterized are the roles of endothelial cell-derived cytokines and chemokines in allergic responses. Various cytokines generated locally from resident or inflammatory cells during an asthmatic attack act directly on the endothelium, resulting in alteration of airway vasculature. However, the molecular mechanisms of these mediators in allergic responses are not clearly understood.
Mast cell mediators
Several mast cell mediators are known to induce vascular responses pertinent to asthma. These mast cell mediators include histamine, bradykinin, lipoxygenase metabolites of arachidonic acid (prostaglandins (PG) and leucotrienes (LT) C4, D4 and E4), cyclooxygenase (COX) products (thromboxane A2), platelet activating factor and certain neurogenic peptides (neurokinin A and neurokinin B). Among these mediators, histamine seems to exert most potent effect on the airway wall, including vasodilation and an increase of vascular permeability.38 Newly generated prostanoid mediators (such as PGD2 and LTC4) are also known to cause vasodilation.38
Lipid mediators produced from the metabolism of arachidonate-containing phospholipids are PG, LT and platelet activating factor and display inflammatory properties, including angiogenesis.39,40 PGE2-mediated angiogenesis has been observed in both the normally avascular cornea and chicken chorioallantoic membranes.41–43 Moreover, PGE2 is a potent vasodilator contributing to the erythema, oedema and pain, which are symptomatic features of inflammation.44 Platelet activating factor is known to stimulate endothelial cell proliferation and affect the signalling of other pro-angiogenic factors such as VEGF, fibroblast growth factor 2 (FGF-2) and angiopoietins (Angs).39,40,45,47 Cysteinyl LT (cysLT; LTC4, LTD4 and LTE4) are involved in recruitment of inflammatory cells, airway smooth muscle contraction, airway hyperreactivity, vascular leakage, mucus hypersecretion and decrease of mucociliary clearance.48–50 Moreover, our recent work has revealed that cysLT increase vascular permeability and VEGF expression, supporting a role for these mediators in airway inflammation.50 LTC4 and LTD4 were shown to stimulate endothelial tube formation and angiogenesis.46,51
Cytokines, chemokines and cognate receptors
Other cytokines and chemokines/chemokine receptors with vascular-changing activities are summarized in Table 1. Some of these mediators are associated with airway homeostasis and/or asthma pathology.
Cell adhesion molecules
Angiogenesis involves a cascade of events that require disassembly of endothelial junctions, followed by detachment, proliferation and migration of endothelial cells, and finally the re-establishment of cell–cell and cell–matrix contacts.52,53 Endothelial junctions maintain endothelial integrity and vascular homeostasis. The endothelial junctions modulate cell trafficking into tissues, mediate cell–cell contact, and regulate endothelial survival and apoptosis. Cell adhesion is an extremely important aspect of angiogenesis, which requires spatially and temporally well-coordinated adhesive interactions among vascular endothelial cells, periendothelial mural cells, smooth muscle cells and pericytes. Various cell adhesion molecules such as integrins, members of the Ig superfamily, and selectins do not only participate in this process but also are associated with chronic lung diseases like asthma.54–56 More recently, some members of the cadherin family have also been shown to be critical to angiogenic and arteriogenic processes of expansion, maturation, branching and remodelling into a network of veins and arteries of different sizes.57 Vascular cell adhesion molecule-1 (VCAM-1), an Ig-like transmembrane glycoprotein, is an optimal target as it is virtually absent in normal human vasculature, yet readily inducible by angiogenesis.58,59 VCAM-1 is expressed on the surface of endothelial cells and promotes cell-to-cell adhesion during inflammation and cancer development.58
Intergrins are multifunctional cell adhesion molecules and composed of non-covalently associated α and β chains. Among integrins, the αvβ3 integrin is an endothelial cell receptor for extracellular matrix (ECM) proteins harbouring the arginine-glycine-aspartic acid (RGD) sequence.60 The RGD-containing ECMs include von Willenbrand factor, fibrinogen (fibrin), vitronectin, thrombospondin, osteopontin and fibronectin.60 The αvβ3 integrin is highly expressed on neovascular endothelial cells and plays a critical role in endothelial cell migration.60 Endothelial cells undergoing angiogenesis experience at least three cellular alterations which are proliferation, locomotion and endothelial cell interaction with ECM. The steps of angiogenesis are directly related to the adhesion processes of the αvβ3 integrin.61 An important characteristic of the αvβ3 integrin is that it is intrinsically associated with VEGFR-2 signalling. Upon αvβ3 integrin binding to the ECMs containing the RGD motif, there is an upregulation of VEGF signalling in cell cultures.62–65 Blocking of the αvβ3 integrin binding to its ligand attenuates VEGF signalling, proving a basis for the use of blocking agents of the αvβ3 integrin for anti-angiogenesis.66 In addition, several studies have shown that integrins play an important role in the pathogenesis of asthma, suggesting the potential for therapeutical targeting of these airway inflammatory diseases.67–70 Interestingly, expression of the αvβ3 integrin is significantly elevated in the airways of patients with chronic lung diseases including bronchial asthma, and the level is closely correlated with angiogenesis of bronchial tissues.71,72
Matrix metalloproteinases (MMP) can facilitate the angiogenic process by regulating composition of endothelial basement membranes.73,74 Moreover, MMP-2 and MMP-9 increase angiogenesis, releasing angiogenic factors such as VEGF.75 Recently, we have found that the level of MMP-9 in sputum is significantly increased in patients with stable asthma and even higher in patients with acute asthmatic subjects compared with healthy control subjects.76 Our study has also indicated that the level of MMP-9 closely correlates with level of VEGF in the sputum of asthma patients.
Vascular endothelial growth factor
Vascular endothelial growth factor plays a pivotal role in vascular remodelling and angiogenesis. An increase in VEGF levels has been observed in tissues and biological samples from individuals with asthma.26,29,77–79 Moreover, the level of VEGF in asthmatic subjects correlates closely with disease activity, and it inversely correlates with the airway calibre, that is increases bronchoconstriction. Peribronchovascular angiogenesis is believed to contribute to airway narrowing and oedema that further enhance airway obstruction in asthma. VEGF, also known as a vascular permeability factor, increases vascular permeability, so that plasma proteins can leak into the extravascular space. The plasma protein leakage induces a thickened, engorged and oedematous airway wall, resulting in the airway lumen narrowing and profound alterations in ECM.21 Furthermore, VEGF enhances microvascular permeability about 50 000 times more than histamine, and acts as a vasodilator.80 VEGF apparently increases microvascular permeability by enhancing the functional activity of vesicular-vascular organelles.21 Overexpression of VEGF in the airways of transgenic mice promotes angiogenesis but also results in allergic inflammation; enhanced allergic sensitization, subsequent upregulated T-helper-2 (Th2)-type inflammatory responses, and mucous gland hyperplasia.81,82 On the other hand, inhibition of the VEGF receptor in a murine model of asthma results in a modest decrease in airway inflammation but failed to inhibit IgE responses nor to alter the vascularity of lung parenchyma.83 These observations suggest that an increase of VEGF level may not be obligatory to the process of neo-vascularization at least during acute inflammation.
Ang-1 and Ang-2 are ligands for the endothelial receptor tyrosine kinase Tie-2 involved in angiogenesis.84 Whereas Ang-1 is widely expressed in normal adult tissues, Ang-2 is expressed mainly at sites of vascular remodelling in chronic inflammation.85 Ang-1 is known to exert a protective role in the vasculature. Transgenic mice overexpressing Ang-1 display an increased vascularization but a decreased adult vascular leakage.86 In addition, Ang-1 exhibits numerous anti-inflammatory properties including suppression of: vascular permeability, leucocyte adhesion to the endothelium, cytokine production, eosinophil chemotaxis induced by VEGF and expression of adhesion molecules in response to a variety of inflammatory mediators, and it reinforces barrier function of the endothelium.84–89 Although the involvement of Ang-1 in angiogenesis is well recognized, little information is available on its role in respiratory physiology and diseases. Ang-2 antagonizes the effects of Ang-1 on the Tie-2 receptor and acts, in some contexts, as a natural inhibitor of Ang-1.90
Studies have indicated that Ang-1 level in allergic airway inflammation is variable, whereas Ang-2 level in asthmatic patients is higher than the level in normal subjects.91–93 In asthmatics, an inverse correlation between Ang-1 level and vascular permeability index and a positive correlation between Ang-2 level and the index have been observed.92 Moreover, the investigators have suggested that the ratio of Ang-1/Ang-2 might be more critical than either of their absolute levels.92 However, the role of Angs in the control of microvascular permeability in the pathophysiology of asthma remains to be clarified.
Growth factors other than VEGF
Important cytokine growth factors associated with vascular changes include platelet-derived growth factor (PDGF), endothelial cell growth factor, epidermal growth factor (EGF), endothelin-1 (ET-1), FGF, insulin-like growth factor 1 (IGF-1), and transforming growth factors (TGF).94 The cellular source of PDGF is macrophages, and its primary targets, in addition to platelets, are smooth muscle and endothelial cells.94 Endothelial cell growth factor produced by platelets, alveolar macrophages, bronchiolar epithelial cells and fibroblasts is involved in enhancing the proliferation, growth and differentiation of endothelial cells.94
ET-1 is secreted by endothelium, smooth muscle, airway epithelium and a variety of other cells in the lung.95 ET-1 is a vasoactive factor and is also capable of inducing bronchoconstriction.96–100 The vasodilatory action of ET-1 is mediated by generation of nitric oxide via stimulation of ET receptors on the pulmonary endothelium.101,102
Fibroblast growth factor synthesized by epithelial cells, fibroblasts, alveolar macrophages, and mast cells is the prototype of a large family of growth factors involved in angiogenesis, wound healing and embryonic development, thus playing key roles in the processes of proliferation and differentiation of a wide variety of cells and tissues.103 Important functions of FGF-1 and FGF-2, which are more potent angiogenic factors than VEGF or PDGF, are the promotion of endothelial cell proliferation and the physical organization of endothelial cells into tube-like structures.104,105 Asthmatic patients show significantly higher FGF-2 immunoreactivity in the airway sub-epitheliam (lamina propria) than control subjects, and there is a positive correlation between vessel area and amounts of FGF-2.30
TGF-α is synthesized primarily in macrophages and keratinocytes. TGF-α stimulates proliferation of fibroblasts, induces epithelial development and promotes angiogenesis.
TGF-β1 is an angiogenic factor and induces production and secretion of several pro-angiogenic components such as VEGF and anti-angiogenic growth factors through the Smad 3 pathway.106–109 Therefore, TGF-β1 can also contribute to the imbalance between VEGF and Ang-1, resulting in a structural abnormality of the newly formed blood vessels in the asthmatic airways.35,110,111 TGF-β1 activity has been shown to increase vascular endothelial barrier permeability and alter vascular endothelial cell shape and cell–cell contacts (barrier integrity).112,113 TGF-β1 is also involved in the loss of pulmonary artery barrier function, which leads to vascular remodelling in pulmonary hypertension; the decreased endothelial barrier function allows cytokines and inflammatory cells to infiltrate the intimal layer of the vessels, resulting in hypertrophy and hyperplasia of smooth muscle cells.114
EGF in chronic airway diseases, is known to be involved in the vasodilatory process of vascular remodelling.115 The effects of EGF on growth, migration and tube formation of endothelial cells have been investigated using blockade of EGF receptor.116–122 These in vitro studies have revealed that human microvascular endothelial cells migrate in the presence of EGF, TGF-α and VEGF. Together with TGF-β, EGF receptor signalling appears to play a role in vessel maturation as well.52
A member of the ribonuclease superfamily, angiogenin is a normal constituent of the circulation, but under some physiological and pathological conditions its level in blood is increased.123 Lack of a fourth disulphide bond in the angiogenin molecule has a functional consequence, that is angiogenesis.124 Like VEGF and FGF, angiogenin may also have a role in asthmatic airway angiogenesis since its level is significantly increased in the airways of asthmatic patients compared with control subjects.30 The mechanisms of angiogenesis induced by angiogenin have been studied in various pathological states. Thus, angiogenin binding to endothelial cell surface receptor activates endocytosis and secondary messaging cascades, such as activation of inositol-specific phospholipase A2125 or phosphorylation of extracellular signal-related kinase 1/2 (Erk 1/2).126 At the same time, the receptor–angiogenin complex is translocated to the nucleus and accumulates in the nucleolus, which is thought to be essential for its angiogenic activity.127 Angiogenin also binds actin, forming an actin–angiogenin complex that assists the activation of proteolytic cascades.128 The complex degrades the laminin and fibronectin of the basement membranes through increasing catalytic activity of plasmin and thus prevents integration of the basement membranes into the remodeled and softened perivascular space.129–131 Overall, angiogenin contributes to both angiogenesis and inflammatory processes through stimulation of complex protease activities.
RECENT AVAILABLE PROTECTIVE DRUGS FOR THE VASCULAR CHANGES IN ASTHMA
Structural changes in the bronchial microcirculation may impact airways through recruitment of inflammatory cells into the sub-epithelium, bronchial narrowing and airway hyperreactivity.132 Hence, pharmacological control of bronchial vascular remodelling may be crucial for coherent therapeutical strategies in asthmatic patients. Nevertheless, information on asthma medication specifically relating to vascular changes is largely lacking.
Inhaled corticosteroids are the most effective medication currently available for the treatment of persistent asthma. Inhaled corticosteroids inhibit microvascular changes such as vasodilation, increased blood flow and angiogenesis in asthma.5,133,134
Corticosteroids exhibit anti-angiogenic properties through acting on mediators involved in vascular changes; treatment with glucocorticoids reduces significantly the increased levels of VEGF and Ang-1 levels in induced sputum, the expression of VEGF mRNA and protein, VEGF/Ang-2 or VEGF/endostatin ratio, and various pro-angiogenic mediators (IL-8, granulocyte-macrophage-colony stimulating factor (GM-CSF), tumour necrosis factor (TNF)-α and MMP) in vitro and in vivo as well as in asthmatic patients.77,135–140
In addition, corticosteroids may act on the vascular components of airway remodelling through its inhibitory effect on mast cells.3 Increased airway permeability in asthmatic patients is decreased after treatment with inhaled corticosteroids.141,142 The modes of action of corticosteroids on the vascular components of airway remodelling and angiogenesis are likely to be complex multistep processes yet to be fully understood.
β2-adrenergic agonists are well known as major bronchodilators. While short-acting β2-agonists act as a bronchodilator only, long-acting β2-agonists seem to have both bronchodilatory and anti-inflammatory effects.139,143,144 Treatment with salmeterol, one of the widely used long-acting β2-agonists, is associated with a decrease in IL-8 levels in BAL fluid or inhibition of GM-CSF and IL-8 release from human bronchial epithelial cells.145,146 These findings support the rationale for the combination therapy of long-acting β2-agonists and corticosteroids having a potential synergistic effect on the inhibitory responses to pro-angiogenic mediators.
Long-acting β2-agonists seem to have an effect on plasma exudation. Salmeterol has been shown to inhibit vascular permeability induced by nasal allergen challenge, and formoterol displays an inhibitory effect on histamine-induced plasma exudation in the lower airways.147–149 The mechanism of beneficial effects of the β2-agonists on allergic inflammation appears to be via inhibition of endothelial gap formation.150
Leucotriene receptor antagonists
leucotrienes, lipid mediators generated from arachidonic acid by the action of 5-lipoxygenase, play important roles in the pathogenesis of allergic airway inflammation.151 Therefore, it is not surprising that leucotriene receptor antagonists decrease vascular permeability. We have demonstrated that the administration of cysLT receptor antagonists markedly reduces plasma extravasation and VEGF levels in the lungs in allergen-induced asthma.50 These results indicate that cysLT receptor antagonists modulate vascular permeability by reducing VEGF expression and suggest that cysLT receptor antagonists can be vascular-targeting therapeutical agents in allergic airway diseases. Treatment of asthmatic patients with montelukast reduces the level of Ang-2.92 Moreover, montelukast has recently been shown to decrease airway mucosal blood flow in patients with mild intermittent asthma.49
Selective COX-2 inhibitors, prescribed for pain relief, are now being evaluated for their anti-angiogenic and anti-permeability activities. Such inhibitors include celecoxib (Celebrex; Pfizer, Inc, New York, NY, USA), rofecoxib (Vioxx; Merck & Co, Whitehouse Station, NJ, USA) and valdecoxib (Bextra; Pfizer Inc). Sawaoka and colleagues have demonstrated that COX-2 inhibition by NS-398, a selective COX-2 inhibitor, suppresses the level of VEGF protein in gastrointestinal cancer xenograft model.152 Celecoxib, a potent and selective COX-2 inhibitor, induces apoptosis by blocking Akt activation.153 Celecoxib can also inhibit angiogenesis via COX-2-independent mechanism; impaired VEGF gene expression and decreased angiogenesis result from celecoxib-induced interference with DNA binding of the Sp1 transcription factor.154 This agent also alters the balance of regulation of angiogenesis in favour of inhibition by increasing serum level of an endogenous angiogenesis inhibitor endostatin, while decreasing the release of VEGF by platelets.155 Rofecoxib inhibits neovascularization in COX-2 expressing retinal vessels in a mouse model of retinopathy.156 In asthmatic airways, COX-2 induction may promote angiogenesis through increasing VEGF production, emphasizing the potential of COX-2 inhibitors as an anti-angiogenic agent for asthmatic vascular changes.157,158
TARGETED THERAPY FOR VASCULAR CHANGES IN ASTHMA
There are numerous interacting processes involved in angiogenesis and other vascular changes, some provided targets for possible therapeutical interventions (Table 2). Several drugs have been developed to reduce tumour growth and metastasis by impairing neovascularization. Regarding VEGF and its receptors, the following inhibitors are being used in clinical trials or in preclinical investigations.
Drugs with a non-specific mechanism of action affecting angiogenesis
Drugs that block breakdown of the extracellular matrices
Synthetic matrix metalloproteinase inhibitors
Drugs acting on endothelial cells
CC1069 (thalidomide analogue)
Other specific drugs acting on vascular changes
SR2F (neutralization of TGF-β)
AZ11557272 (MMP-9/MMP-12 inhibitor)
SB239063 (inhibition of MAPK p38)
PS-1145, ML120B (inhibition of NF-κB)
SU6668 (competitive inhibition of FGFR1, Flk-1/KDR, PDGFRβ via receptor tyrosine kinase)
To date, inhibitors of the VEGF signalling pathway are the most clinically advanced among vascular targeting agents. But few data are available for the application to airway inflammatory disorders. Monoclonal antibodies targeting VEGF or the VEGF receptors (VEGFR) are available. Chimeric soluble receptors such as the ‘VEGF-Trap’ (domain 2 of VEGFR-1 and domain 3 of VEGFR-2 fused to an Fc fragment of an antibody) are also undergoing clinical trials on VEGF antagonists. Additional extracellular inhibitors are pegaptanib, aptamers that bind the heparin-binding domain of VEGF165 and ranibizumab, a humanized high-affinity anti-VEGF Fab that neutralizes all VEGF-A isoforms and proteolytic fragments.158,159 Pegaptanib has been approved by the FDA for the treatment of age-related macular degeneration.160 Various small-molecule inhibitors of VEGF receptor tyrosine kinase (RTK) have been developed. Among these inhibitors, SU11248 (Sunitinib) and Bay 43-9006 (Sorafenib) are the mostly advanced agents.161 Other VEGF RTK inhibitors are SU-5614, SU-1498 (SUGEN Inc/Hebrew University of Jerusalem), ZD-6474 (Vandetanib, AstraZeneca Plc, London, United Kingdom), PTK-787 (Vatalanib, Bayer Schering, Berlin-Wedding, Germany and Novartis, Basel, Switerland) and AG-013736 (Axitinib Pfizer, New York City, NY, USA).162–164 The VEGF RTK inhibitors, SU-5614 and SU-1498, have been investigated in toluene diisocyanate-induced and ovalbumin-induced murine models of asthma to examine the potential involvement of VEGF in asthma, and the results have revealed that all the typical pathophysiological symptoms including profound vascular exudation are reduced following administration of the inhibitors.50,165 Bevacizumab, a humanized variant of a murine anti-VEGF-A monocloncal antibody used in early proof-of-concept studies,166 is the only FDA-approved anti-angiogenic agent for cancer therapy.167,168 Moreover, with VEGF RTK inhibitors, SU-5614 and SU-1498, we have found that inhibition of VEGF receptor downregulates the expression of MMP-9.76 Additional potential strategies to inhibit VEGF signalling could include antisense and siRNA targeting the genes for VEGF-A or its receptors.
Targeting Ang-1 is a potential therapeutical approach to enhance maturation of vessels and endothelial cell survival and prevent vascular leakage. Transgenic overexpression of Ang-1 in skin results in pronounced hypervascularization.169,170 Although there are modest increases in vessel number, the most marked increase is in vessel size, implicating Ang-1 effect on circumferential growth as opposed to sproutive growth. Intravenous injection of an adenoviral vector-Ang-1 gene construct showed anti-leaking effect without enlargement of the skin microvasculature, unlike that seen in transgenic mice.86 However, production of Ang-1 is hindered by aggregation and insolubility resulting from disulfide-linked higher-order structures. Therefore, Cho and colleagues have introduced a more soluble, stable and potent Ang-1 variant, COMP-Ang-1 by replacing the N-terminal portion of Ang-1 with the short coiled-coil domain of cartilage oligomeric matrix protein (COMP).171 Intravenous administration of COMP-Ang-1 attenuated the increased plasma extravasation in a murine model of asthma.54 We have also reported that COMP-Ang-1 reduces potential angiogenic factors such as VEGF, TNF-α, IL-1, ICAM-1 and VCAM-1. These findings indicate that COMP-Ang-1 impacts airway vasculature through both direct and indirect ways. The protective effect of Ang-1 on blood vessel leakiness raises the possibility of a new strategy for reducing vascular leakage in asthma.
We have observed that administration of peroxisome proliferator activated receptor γ (PPAR-γ) agonists and an adenoviral vector-PPAR-γ gene construct to mice with airway inflammation significantly reduced the increased expressions of pro-angiogentic and pro-vascular leakage mediators, including HIF-1α, VEGF, ICAM-1 and VCAM-1, and plasma extravasation.55 These findings provide a rationale for the use of PPAR-γ agonists as vascular targeting agents to prevent and/or treat asthma.
Phosphatase and tensin homologue deleted on chromosome 10 (PTEN) can also attenuate the expression of VEGF and vascular leakage through the inhibition of HIF-1α activation mediated by PI3K/Akt pathway. Thus, examination of PTEN role in allergic airway inflammation has revealed that PTEN overexpression substantially reduced the increased VEGF level and plasma leakage in lungs of a murine model of allergen-induced airway inflammation.56
There are many critical factors involved in the physiological regulation of vascular changes in bronchial asthma, and the actions of these factors in vascular remodelling seem to be complex and finely orchestrated. Currently, it is unknown whether the vascular changes in asthma have a precise temporal sequence, although a study has revealed that airway vascular leakage is a critical pathophysiological feature of early asthma deterioration, preceding airway inflammatory cellular changes in asthma.172 Clinical relevance of remodelling and angiogenesis of bronchial vasculature remains to be fully understood. However, an increased number and/or dimension of the capillaries in the bronchus can contribute to bronchial wall thickening, oedema and exudation, and therefore promote bronchial obstruction. Thus, vascular changes in the airways are likely to impact on clinical manifestations of asthma. Overexpression of vascular growth factors has led many researchers to investigate the blockers of such signalling molecules as a therapeutical strategy in various pathological conditions. Therapy targeting VEGF or FGF has been applicable and appears well tolerated in other pathologic fields. There is no doubt that VEGF is the best-validated target for anti-angiogenic approach, based on overwhelming genetical, mechanistic and therapeutical animal data. The possibility that Ang-1 may prevent or repair damaged and leaky vessels offers therapeutical hope for an assortment of unmet clinical needs. Despite the attention devoted to a number of other putative angiogenic antagonists, most of these blockers have yet to be fully characterized, and they lack defined mechanisms of action. To take advantage of benefits of new potential drugs and to avoid adverse effects, further studies are needed to understand more fully the signalling mechanisms of vascular factors in bronchial asthma.
We thank professor Mie-Jae Im for critical reading of the manuscript. This study was supported by a grant of the Korea Healthcare technology R&D Project, Ministry for Health, Welfare and Family Affairs, Republic of Korea (A084144), by the Korea Science and Engineering Foundation (KOSEF) through the National Research Laboratory Programme funded by the Ministry of Education, Science and Technology (R0A-2005-000-10052-0(2008)), and also by Fund of Chonbuk National University Hospital Research Institute of Clinical Medicine.