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The regulation of fibrosis in airway remodeling in asthma
Molecular and Cellular Endocrinology 351 (2012) 167–175
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Molecular and Cellular Endocrinology journal homepage: www.elsevier.com/locate/mce
Review
The regulation of fibrosis in airway remodeling in asthma Simon G. Royce a, Victor Cheng a, Chrishan S. Samuel b, Mimi L.K. Tang a,⇑ a b
Department of Allergy and Immunology, Murdoch Children’s Research Institute, The Royal Children’s Hospital, Melbourne 3052, Australia Department of Pharmacology, Monash University, Melbourne, Australia
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Article history: Received 19 December 2011 Accepted 4 January 2012 Available online 14 January 2012 Keywords: Asthma Fibrosis Relaxin Airway remodeling Therapy
a b s t r a c t Fibrosis is one of the key pathological features of airway remodeling in asthma. In the normal airway the amount of collagen and other extracellular matrix components is kept in equilibrium by regulation of synthesis and degradation. In asthma this homeostasis is disrupted due to genetic and environmental factors. In the airways of patients with the disease there is increased extracellular matrix deposition, particularly in the reticular basement membrane region, lamina propria and submucosa. Fibrosis is important as it can occur early in the pathogenesis of asthma, be associated with severity and resistant to therapy. In this review we will discuss current knowledge of relaxin and other key regulators of fibrosis in the airway including TGFb, Smad2/3 and matrix metalloproteinases. As fibrosis is not directly targeted or effectively treated by current asthma drugs including corticosteroids, characterization of airway fibrosis and how it is regulated will be essential for the development of novel therapies for asthma. Ó 2012 Elsevier Ireland Ltd. All rights reserved.
Contents 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
Definition of asthma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Epidemiology of asthma . . . . . . . . . . . . . . . . . . . . . . . . . . . . Etiology of asthma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Current treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Airway remodeling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reticular basement membrane fibrosis . . . . . . . . . . . . . . . . Transforming growth factor-b (TGFb1) and Smads . . . . . . . Matrix metalloproteinases (MMPs) and tissue inhibitors of Relaxin – a novel regulator of airway remodeling . . . . . . . . Receptors for relaxin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Therapeutic targeting of fibrosis in asthma . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
........................ ........................ ........................ ........................ ........................ ........................ ........................ metalloproteinases (TIMPs) . ........................ ........................ ........................ ........................ ........................
1. Definition of asthma Asthma is a chronic inflammatory disorder of the airways characterized by spontaneous, widespread, but variable bronchoAbbreviations: AAD, allergic airways disease; ASM, airway smooth muscle; ECM, extracellular matrix; EMTU, epithelial–mesenchymal trophic unit; GR, glucocorticoid receptor; MMP, matrix metalloproteinase; OVA, ovalbumin; p-Smad2, phospho-Smad2; RBM, reticular basement membrane; RXFP1, relaxin family peptide receptor-1; TGFb, transforming growth factor-b; TIMP, tissue inhibitors of metalloproteinase. ⇑ Corresponding author. Tel.: +61 3 9345 5733; fax: +61 3 9345 6348. E-mail address: [email protected] (M.L.K. Tang). 0303-7207/$ - see front matter Ó 2012 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.mce.2012.01.007
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constriction and airflow obstruction in response to a wide variety of environmental and endogenous stimuli (Holgate, 2008a,b). The clinical manifestations of asthma are episodic and can include wheezing, coughing, dyspnoea and chest tightness (AIHW Australian Centre for Asthma Monitoring, 2005). Due to the heterogeneous nature of the disorder, the severity of these manifestations can vary greatly between asthma sufferers, but in the majority of cases, asthma sufferers are asymptomatic between episodes, so asthma is widely considered to be a reversible condition, spontaneously or through treatment (Pascual and Peters, 2005). Severe asthma, on the other hand, refers to forms of therapyresistant, potentially fatal asthma. Although severe asthma is
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difficult to define, the National Institute of Health Guidelines for the Diagnosis and Management of Asthma state the following as characteristic of severe and persistent asthma: continual symptoms (especially at night) that cause limitations in physical activity; frequent exacerbations; persistent airflow obstruction with forced expiratory volume in 1 s (FEV1) and/or peak expiratory flow (PEF) of less than 60% of the predicted value; and PEF diurnal variation of greater than 30% (National Heart Lung and Blood Institute, 2007). It is also characteristically unresponsive to treatment, including systemic corticosteroids. It is estimated that severe asthma affects 5–10% of all asthma patients. 2. Epidemiology of asthma An estimated 300 million people suffer from asthma worldwide, and higher prevalence is seen in Western countries. Over the past few decades, the incidence of asthma has risen dramatically, and it is predicted that the worldwide incidence will continue to rise, with a further estimated 100 million people suffering from asthma by 2025, as populations become more Westernized and urbanized (Masoli et al., 2004). Moreover, asthma causes approximately 1 in 250 deaths per year (AIHW Australian Centre for Asthma Monitoring, 2005). However, the number of disability-adjusted life years (DALYs) lost due to asthma is approximately 15 million per year, making asthma the 25th leading cause of DALYs lost globally (similar to diabetes mellitus and liver cirrhosis) (Masoli et al., 2004).
3. Etiology of asthma An important contributing factor to the significant burden of asthma is the heterogeneous nature of its etiology (Holgate, 2008a; Borish and Culp, 2008). The wide range of environmental and endogenous stimuli that can trigger an asthma attack has been well documented. These triggers include allergic stimuli (e.g. pollen, dust mite feces, cat dander, certain drugs and occupational chemicals) (Holgate, 2008a,b; Passalacqua and Ciprandi, 2008) and non-allergic stimuli (e.g. exercise and cold) (Holgate, 2008b; McKenzie and Boulet, 2008). Despite this heterogeneity, it is widely accepted that asthma with allergic sensitisation involves a specific immune response to an environmental allergen (Passalacqua and Ciprandi, 2008), which leads to release of inflammatory mediators and subsequent inflammation (Broide, 2008). However, it is becoming increasingly clear that this is just one aspect of asthma, which is now being described as a disease that is composed of various combinations of many different underlying conditions, expressed as a common phenotype of allergic airways disease (AAD) (Holgate, 2008a,b). These underlying conditions may include disorders of epithelial integrity (permeability and repair) (Holgate, 2008a,b), IgE class switching (Passalacqua and Ciprandi, 2008), extracellular matrix deposition and degradation (Davies et al., 2003), and collagen regulation (Davies et al., 2003). The interaction between these underlying conditions and the appropriate environmental stimuli is what results in the common phenotype of airway inflammation, airway hyperresponsiveness
Fig. 1. A simplified schematic of the mechanisms behind asthma. Asthma is increasingly regarded as a disease of many disorders amalgamating in the allergic airways disease phenotype, involving a complex interplay between both genetic and environmental factors. Pre-existing genetic factors such as epithelial susceptibility, preferential IgE class switching in plasma cells and mutation of the A Disintegrin and metalloprotease-33 (ADAM33) gene predispose to epithelial damage and allergic hypersensitivity.
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(AHR) and airway remodeling seen in the airways of asthma sufferers (Fig. 1). 4. Current treatment The list of therapeutic options available to asthma sufferers today is extensive, but limited to relief and prevention of the symptoms rather than curative of the underlying pathology. Short- and long-acting b2-agonists (Donohue, 2008) and, to a lesser extent, leukotriene modifiers and theophylline, are used to relieve bronchoconstriction during an asthma attack. For sufferers of more severe asthma, inhaled glucocorticosteroids such as dexamethasone and fluticasone propionate are added as preventers of airway inflammation (Sobande and Kercsmar, 2008; Mauad et al., 2007). Omalizumab, a monoclonal antibody that binds to the IgE receptor FCeRI (preventing IgE from binding), has recently been made available and is showing therapeutic promise in the treatment of allergic asthma (Mauad et al., 2007; Prenner, 2008). There is, however, currently a lack of targeted therapies for reversing the structural changes seen in airways of asthma sufferers, collectively known as airway remodeling (Pascual and Peters, 2005). 5. Airway remodeling The term airway remodeling can refer to the normal structural changes that occur in embryonic development of the lung, which are essential for lung function (Jeffery, 2001). In asthma pathology, however, airway remodeling describes the manifestation of complex cellular and molecular interactions, which lead to abnormal transformations in the composition of the airway wall (Pei, 1996; James, 2005). The characteristic structural changes seen in the bronchial walls of asthma sufferers include epithelial denudation, reticular basement membrane (RBM) fibrosis, smooth muscle hypertrophy and hyperplasia, goblet cell metaplasia/mucous gland hypertrophy, altered composition of the deeper extracellular matrix (ECM) and angiogenesis (Mauad et al., 2007). As yet, the exact mechanism by which these events occur is unknown, although there have been suggestions of causative factors, one of which is an inflammation-driven progression to airway remodeling (Broide, 2008). The most prominent inflammatory cell types seen in asthma-affected airways are mast cells, eosinophils, TH2 cells and neutrophils, and these immune cells have been shown to have specific roles in the progression from inflammation to airway remodeling in asthma (Jeffery et al., 2000). Although TH2 cells have long been known for their role in chronic allergic inflammatory responses, it has more recently been shown that TH2 cells specifically contribute to airway remodeling by increasing smooth muscle hyperplasia and epithelial metaplasia (Broide, 2008; Camoretti-Mercado and Solway, 2005). Moreover, the eosinophilic infiltration seen in airway inflammation results in increased levels of the eosinophil-derived transforming growth factor-b (TGFb) (Broide, 2008), a potent profibrotic cytokine known to stimulate fibroblast differentiation and collagen deposition, leading to subepithelial fibrosis and subsequent stiffening of the tracheobronchial walls (Moore et al., 2008). Mast cells have also been shown to contribute to airway remodeling through the release of inflammatory mediators such as tryptase, histamine and other cytokines (Barnes, 2008). However, the primacy of an inflammation-driven progression to airway remodeling has been questioned following recent studies indicating the presence of airway remodeling changes very early in the development of asthma. Several parties have recently provided evidence of epithelial fragility and subepithelial basement membrane thickening in the airways in childhood irrespective of airway inflammation (Barbato et al., 2006; Saglani et al., 2005;
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Fedorov et al., 2005). This implies that the structural changes of airway remodeling occur at the same time as, or even before, the onset of airway inflammation. These findings have led to the suggestion that although inflammation is a central player in the pathogenesis of asthma, it does not fully account for the structural transformations seen in airway remodeling (Davies et al., 2003). Holgate has postulated that an underlying impaired epithelial repair mechanism initiates the remodeling process due to chronic injury and a failure to heal properly, and that greatly increased permeability of the epithelium facilitates access of inhaled irritants to the underlying tissue (Holgate, 2007a,b). Evidence for epithelial cell detachment from the basement membrane has been demonstrated through analysis of the sputum of asthma patients, resulting in epithelial denudation (Pascual and Peters, 2005). This increased exposure of the subepithelial tissues to potential allergens, due to pre-existing structural and functional epithelial abnormalities, would subsequently promote allergic hypersensitivity and inflammation (Holgate et al., 2004). Moreover, injury to the airway epithelium results in the reactivation of the epithelial–mesenchymal trophic unit (EMTU), a term used to describe the interaction between the epithelium and the underlying fibroblast sheath (Holgate, 2007a). This occurs through the release of fibrogenic and fibroproliferative growth factors, including fibroblast growth factor-2 (FGF-2), insulin-like growth factor-1 (IGF-1), endothelin-1, and latent and active TGFb by damaged epithelial cells (Holgate, 2008a). It has been demonstrated that the ability of the epithelium to generate these factors is increased in asthma patients compared with normal subjects (Holgate, 2000). The pro-fibrotic effects of these factors – reticular basement membrane fibrosis in particular – is likely augmented by the additional release of TGFb by inflammatory cells, especially eosinophils (Kariyawasam and Robinson, 2007), in a parallel process of allergic inflammation. The novel concept that airway inflammation is not the primary driver of asthma and airway remodeling is supported by trials showing that administration of inhaled glucocorticosteroids to children of 1–4 years of age (Bisgaard et al., 2006; Guilbert et al., 2006) had no effect on the natural history of asthma. It is further supported by the existence of corticosteroid-insensitive asthma (Szefler and Leung, 1997); the inability of anti-inflammatory agents such as corticosteroids to attenuate the progression of asthma suggests another potential causative factor. Although the natural history of airway remodeling in asthma is not fully elucidated, its characteristic features are universally recognized, and are most prominent in those patients with moderate to severe asthma. The increased amount and altered function of airway smooth muscle (ASM) is a significant contributor to the airway hyperresponsiveness seen in these patients, accounting for the increased contractility of the airway wall (Bentley and Hershenson, 2008). Morphologically, there is marked overall thickening of the airway wall, mostly due to smooth muscle hypertrophy and hyperplasia, but also fibrosis in the reticular basement membrane (RBM).
6. Reticular basement membrane fibrosis The RBM is the region situated just below the true basement membrane consisting of the lamina rara and lamina densa (Postma and Timens, 2006). The thickening of the RBM is a process that is characteristic of asthma, and is not seen in patients of chronic bronchitis or COPD (Jeffery, 2001). The RBM is a component of the airway wall that is not present in the fetus, but develops in even normal, healthy individuals. However, excessive thickening of the RBM in asthma occurs early in the asthma process (Jeffery, 2001). The extracellular matrix (ECM) components deposited in the RBM in asthma consist of collagen I, collagen III, collagen V,
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fibronectin, and tenascin (Postma and Timens, 2006). These contribute to airway wall thickening and subsequent reduced airway distensibility and increased airflow limitation. The main effectors of this excessive collagen deposition are the fibroblasts, the primary cell type in the mesenchyme of the lung (Boxall et al., 2006). Biopsy specimens from airways of asthma sufferers show significantly increased numbers of fibroblasts compared with normal subjects (Benayoun et al., 2003), and this has been correlated with increased RBM fibrosis. Under the influence of epithelial damage and cytokines such as TGFb1, fibroblasts can differentiate into the bioactive myofibroblast form, which has both secretory and contractile phenotypes and expresses a smooth muscle actin (aSMA). In addition to the secretion of collagen, myofibroblasts also produce more TGFb1 (Boxall et al., 2006), further prolonging the fibrotic process.
7. Transforming growth factor-b (TGFb1) and Smads Since the bulk of the ECM is made up of collagen subtypes (Postma and Timens, 2006), collagen homeostasis is essential to the normal composition of the ECM in the RBM. Its regulation involves an amalgam of factors each with a distinct mechanism affecting the fibrotic process. TGFb1 is a potent profibrotic cytokine, upregulating the processes involved in the deposition of collagen while downregulating its catabolism (Camoretti-Mercado and Solway, 2005). The majority of evidence supports the dogma that TGFb1 is upregulated in the airways in asthma (Redington et al., 1997; Bosse and Rola-Pleszczynski, 2007). In the airway, sources of
TGFb1 are (myo)fibroblasts, inflammatory cells and bronchial epithelial cells (Boxall et al., 2006), so it follows that epithelial damage and the subsequent inflammation would result in an increased presence of TGFb1 in the asthma-affected lung. After binding to its receptor tyrosine kinase receptors on fibroblasts, a phosphorylation cascade leads to phosphorylation of Smad2/3 (receptor Smads). These bind to Smad4 (a co-Smad) and the complex translocates to the nucleus to promote the differentiation of the fibroblast into the more bioactive myofibroblast (Boxall et al., 2006) (Fig. 2). Elevated Smad2 levels have been reported in bronchial biopsies of asthmatic subjects (Sagara et al., 2002), suggesting an increase in TGFb1 signaling in these lungs. Smad3 has been identified by a genome wide association study (GWAS) (Moffatt et al., 2010). Smad3 deficient mice have inhibition of ovalbumin-induced airway remodeling (Le et al., 2007). In addition, TGFb1 inhibits the production of matrix metalloproteinases (MMPs, matrix-degrading enzymes) by inflammatory and epithelial cells, and promotes the production of tissue inhibitors of metalloproteinases (TIMPs) by epithelial cells (Boxall et al., 2006). The resultant profibrotic environment encourages collagen deposition and thickening of the RBM in the airway walls of asthma sufferers.
8. Matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) MMPs are a large family of ECM-degrading enzymes produced by a large number of cells including inflammatory and epithelial
Fig. 2. Synthesis and action of TGFb isoforms. TGFb is synthesised particularly – but not only – by bronchial epithelial cells and myofibroblasts in a latent form. It then undergoes activation and binds to TGFb receptors (TGFb-RI, RII, RIII) and signals through the Smad signaling cascade to induce fibroblast differentiation. Although TGFb1 can directly bind to and activate TGFb-RII (bypassing TGFb-RIII), TGFb2 must initially bind TGFb-RIII to induce intracellular signaling. While Smad2, -3 and -4 are considered stimulatory Smads, Smad7 is inhibitory. TGFb Receptor Associated Peptide-1 (TRAP1) binds and inhibits TGFb-RI until phosphorylation by TGFb-RII.
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Fig. 3. The complex synthesis pathways of matrix metalloproteinases and their activation ultimately lead to the degradation of various forms of collagen. MMPs are initially synthesised as inactive zymogens, and are activated in an extracellular environment. Tissue inhibitors of metalloproteinases (TIMPs) are responsible for the inhibition of MMPs. It is often the MMP:TIMP ratio that affects collagen balance more than individual enzymes.
cells (Yao et al., 1998). As a family, they are zinc-dependent endopeptidases responsible for the proteolytic degradation of various ECM components (Lagente et al., 2005) (Fig. 3). Of particular interest are the gelatinases (MMP-2 and MMP-9), which show aberrant regulation in both human asthma (Wenzel et al., 2003) and murine experimental models of AAD (Kumagai et al., 1999). The gelatinases are capable of degrading collagens I, IV, V, VII, X, XI, XIV; gelatin; elastin; fibronectin and other ECM proteins (Lagente et al., 2005). Although both MMP-2 and MMP-9 are at increased levels in acute asthma, a difference is seen in chronic asthma. While
MMP-9 expression is still higher than baseline in chronic, severe asthma, MMP-2 expression is below baseline (Boxall et al., 2006). This implies that in chronic and severe asthma, there is a decreased capacity to degrade the ECM, which could upset collagen homeostasis and tip the balance towards a matrix-depositing phenotype. The direct inhibitors of MMPs are the profibrotic TIMPs, which are produced by a large number of cells including fibroblasts. TIMPs bind MMPs in a 1:1 stoichiometric ratio to prevent the proteolysis of their respective ECM components. TIMP-1 and TIMP-2 bind MMP-9 and MMP-2, respectively, and it has been hypothesized
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that the balance between levels of TIMPs and levels of corresponding MMPs in the lung is the key determinant of whether the ECM is being produced or degraded (Boxall et al., 2006; Wenzel et al., 2003; Kumagai et al., 1999). Indeed, it has been shown that levels of TIMP-1 are increased in bronchoalveolar lavage fluid (BALF) of asthma subjects, and also that such subjects have a lower MMP9:TIMP-1 ratio (Wenzel et al., 2003), promoting the deposition of collagen and fibrosis. 9. Relaxin – a novel regulator of airway remodeling Relaxin is a peptide hormone in the insulin superfamily, and was first described in 1926 as a reproductive hormone responsible for the softening of the pubic ligament in preparation for childbirth. The antifibrotic properties of relaxin were first considered in the 1950s and 1960s following observation of its effect on the pubic ligament (Sherwood, 2004). Interest grew in the antifibrotic properties of relaxin such that it was considered as a possible treatment for systemic sclerosis and scleroderma (Seibold et al., 2000), and clinical trials were attempted to ascertain the effectiveness of relaxin therapy, but these were ultimately unsuccessful due to lack of consistent outcomes and the phase III trials including an expanded cohort of patients, many of whom presented with end-stage disease pathogenesis which was unlikely treatable (Seibold et al., 1998). Since then, however, relaxin has been shown to have antifibrotic properties in non-reproductive tissues such as the heart (Samuel, 2005; Samuel et al., 2006; Samuel and Hewitson, 2006), liver (Williams et al., 2001), kidneys (Samuel and Hewitson, 2006) and lung (Moore et al., 2008; Mookerjee et al., 2006, 2005) in murine models of AAD. Experiments with relaxin gene knockout mice have shown that without endogenous relaxin, there is increased RBM fibrosis in response to allergen challenge, and the administration of recombinant H2 subtype relaxin leads to reversal of fibrosis in these mice (Samuel et al., 2003). Furthermore relaxin delivered systemically by mini-osmotic pump has been shown to reverse established airway remodeling changes when delivered after the ovalbumin challenge period in chronic AAD mice (Royce et al., 2009). Novel relaxin-like peptides are under development for a number of fibrotic conditions. One such peptide, CGEN25009 has been found to ameliorate peribronchial fibrosis in a mouse model of bleomycin-induced pulmonary fibrosis (Pini et al., 2010). In addition to its antifibrotic properties relaxin may have more direct impact of AHR. Relaxin was identified in the top four genes in a GWAS of bronchial hyperresponsiveness in mice (Reinhard et al., 2005; Ganguly et al., 2007). Work done in our laboratory has provided further evidence of an effect of relaxin on methacholine induced AHR in mice with AAD. Although it seems likely that ECM degradation due to relaxin may contribute to suppression of AHR (Mookerjee et al., 2006), relaxin has been shown to inhibit contractility of myofibroblasts in idiopathic pulmonary fibrosis (Huang et al., 2011). These observations establish H2 relaxin as a candidate for novel therapies targeted at reversing RBM fibrosis in human asthma. 10. Receptors for relaxin Although some of the receptor and signaling mechanisms of relaxin have been elucidated, recent research reveals that there are not only multiple receptors for relaxin, but that the downstream mechanisms for relaxin’s action are yet to be fully understood (Fig. 4). In 2002, Hsu et al. identified the receptors through which H2 relaxin signals: LGR7 and LGR8 (now RXFP1 and RXFP2, respectively) (Bathgate et al., 2006). While H2 relaxin can bind to both of these receptors, it binds to RXFP1 with highest affinity and most of its
effects result from its binding to this receptor (Hsu et al., 2002). Like relaxin, RXFP1 can be found in reproductive and nonreproductive tissues, including the lung. These G-protein coupled receptors lead to intracellular cAMP and phosphokinase A (PKA) production, and the subsequent downstream antifibrotic effects of relaxin (Sherwood, 2004). Recently, Samuel et al. demonstrated that RXFP1 gene knockout mice show a much earlier age-related progression of collagen deposition in the airway walls compared with their wild type counterparts, without allergen challenge, and that this indicates a role for RXFP1 in the homeostatic regulation of fibrosis in the airway (Samuel et al., 2009). The mechanisms by which the relaxin–RXFP1 interaction regulates the fibrotic pathway and collagen homeostasis is still unclear, but its effects in reducing fibrosis in many tissues (Samuel et al., 2007) make it a clear candidate for further research as a therapy for airway remodeling in asthma. While it is established that relaxin binds to and acts through RXFP1 and RXFP2, it is becoming clear that these are not the only receptors through which relaxin exerts is effects (Bathgate et al., 2005; Halls et al., 2007). In the same study that showed that RXFP1 has a role in homeostatic regulation of fibrosis, it was also found that RXFP1 had no effect in reducing fibrosis in the setting of AAD; RXFP1-knockout mice that underwent allergen sensitisation and challenge did not develop more fibrosis than their wild type counterparts (Samuel et al., 2009). Relaxin gene knockout mice, however, developed significantly more fibrosis than their wild type counterparts with and without undergoing the model of AAD (Samuel et al., 2003, 2005). From this study, Samuel and colleagues propose a paradigm that relaxin, as well as acting through RXFP1 and RXFP2, also acts through another receptor. In 2004, Dschietzig et al. identified relaxin as a potential glucocorticoid receptor agonist through co-immunoprecipitation experiments (Dschietzig et al., 2004). Relaxin was seen to interact with both nuclear and cytoplasmic glucocorticoid receptors independent of RXFP1 (Dschietzig et al., 2005). This finding was further supported by a recent study, which observed that RXFP1-independent relaxin administered to a THP-1 (human monocyte leukemia) cells caused a completely GR-dependent suppression of cytokine secretion (Dschietzig et al., 2009a). Furthermore, in the same year, Garibay-Tupas and colleagues demonstrated in a model system that there are glucocorticoid response elements (GRE) present on both the H1 and H2 relaxin genes, and that H1 and H2 relaxin can both be regulated by dexamethasone (a glucocorticoid) (Garibay-Tupas et al., 2004). Dschietzig and colleagues continued these investigations and found that relaxin-bound GR translocated to the nucleus and displaced known GR agonists from the relaxin gene GREs, establishing a positive autoregulatory loop (Dschietzig et al., 2009b). However, Halls showed that although relaxin indeed binds to glucocorticoid response element, this interaction takes place only in the presence of RXFP1 (Halls et al., 2007). This research deepens the current understanding of relaxin and its receptors and raises more opportunities through which we can investigate the avenues of relaxin’s action. Since glucocorticoid receptor is targeted by corticosteroids as a conventional asthma therapy (Mauad et al., 2007), it would be interesting to explore the effects of relaxin on asthma via the glucocorticoid receptor.
11. Therapeutic targeting of fibrosis in asthma To date, no therapies that specifically target fibrosis have reached the asthma clinic. However, a number of anti-cytokine monoclonal antibodies are undergoing phase I clinical trials for other fibrotic diseases and may be appropriate for asthma treatment in the future. Those most likely to be useful include
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Fig. 4. The signaling pathway of H2 relaxin. H2 relaxin signals primarily through RXFP1, with limited activity through RXFP2. The various G-protein subunits have complex interplay – with some involvement of inositol trisphosphate kinase (PI3K) and protein kinase C (PKC) – but the ultimate downstream effect is activation of adenylate cyclase and the production of cyclic adenosine monophosphate (cAMP), a marker of relaxin activity. The importance of glucocorticoid receptor (GR) is still being fully elucidated. However, the binding of GR to GR response elements (GRE) in the nucleus leads to cytokine suppression and anti-inflammatory effects.
anti-TGFb monoclonal antibody (GC-1008; Genzyme) and antiplatelet derived growth factor (PDGF; CR-002; Curagen). However it remains to be seen whether these monoclonal antibodies will be able to reverse existing fibrosis. As with Omalizumab (humanized anti-IgE monoclonal antibody), cost is also an issue limiting widespread use in chronic disease. Fibrosis and remodeling in general can now be evaluated by computed tomography scanning, potentially allowing for monitoring of disease progression and response to treatment (Aysola et al., 2008). Imaging technologies continue to improve and will greatly assist monitoring of drug response in asthma sufferers over the long term without the need for endoscope biopsy.
12. Conclusion Airway fibrosis is a key feature of asthma phenotypes, through progression of the disease. Reticular basement membrane fibrosis and subepithelial fibrosis are important pathological features of asthma. Sufferers have altered ECM composition characterized by the presence of collagen I, III, V, tenascin, fibronectin and laminin. Furthermore, key regulators of fibrosis with known roles in other
fibrotic diseases have been identified as altered in asthma patients in GWAS (e.g. Smad3 and ADAM33). It appears that the TGFb/Smad2/3 pathway plays a central role in regulating key processes such as myofibroblast differentiation and collagen synthesis. Furthermore, MMPs, TIMPs and relaxin are important regulators of fibrosis in asthma. Given that corticosteroids do not directly target fibrosis per se, there is a need to develop new therapeutics that are able to reverse and prevent fibrotic remodeling of the airways. References AIHW (Australian Centre for Asthma Monitoring), 2005. Asthma in Australia 2005, AIHW Asthma Series 2. AIHW Cat. No. ACM 6. AIHW, Canberra. Aysola, R.S., Hoffman, E.A., Gierada, D., Wenzel, S., Cook-Granroth, J., Tarsi, J., Zheng, J., Schechtman, K.B., Ramkumar, T.P., Cochran, R., Xueping, E., Christie, C., Newell, J., Fain, S., Altes, T.A., Castro, M., 2008. Airway remodeling measured by multidetector CT is increased in severe asthma and correlates with pathology. Chest 134, 1183–1191. Barbato, A., Turato, G., Baraldo, S., Bazzan, E., Calabrese, F., Panizzolo, C., Zanin, M.E., Zuin, R., Maestrelli, P., Fabbri, L.M., Saetta, M., 2006. Epithelial damage and angiogenesis in the airways of children with asthma. Am. J. Respir. Crit. Care Med. 174, 975–981. Barnes, P.J., 2008. The cytokine network in asthma and chronic obstructive pulmonary disease. J. Clin. Invest. 118, 3546–3556.
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Bathgate, R.A., Ivell, R., Sanborn, B.M., Sherwood, O.D., Summers, R.J., 2005. Receptors for relaxin family peptides. Ann. NY Acad. Sci. 1041, 61–76. Bathgate, R.A., Ivell, R., Sanborn, B.M., Sherwood, O.D., Summers, R.J., 2006. International Union of Pharmacology LVII: recommendations for the nomenclature of receptors for relaxin family peptides. Pharmacol. Rev. 58, 7– 31. Benayoun, L., Druilhe, A., Dombret, M.C., Aubier, M., Pretolani, M., 2003. Airway structural alterations selectively associated with severe asthma. Am. J. Respir. Crit. Care Med. 167, 1360–1368. Bentley, J.K., Hershenson, M.B., 2008. Airway smooth muscle growth in asthma: proliferation, hypertrophy, and migration. Proc. Am. Thorac. Soc. 5, 89–96. Bisgaard, H., Hermansen, M.N., Loland, L., Halkjaer, L.B., Buchvald, F., 2006. Intermittent inhaled corticosteroids in infants with episodic wheezing. N. Engl. J. Med. 354, 1998–2005. Borish, L., Culp, J.A., 2008. Asthma: a syndrome composed of heterogeneous diseases. Ann. Allergy Asthma Immunol. 101, 1–8 (quiz 8–11, 50). Bosse, Y., Rola-Pleszczynski, M., 2007. Controversy surrounding the increased expression of TGF beta 1 in asthma. Respir. Res. 8, 66. Boxall, C., Holgate, S.T., Davies, D.E., 2006. The contribution of transforming growth factor-beta and epidermal growth factor signalling to airway remodelling in chronic asthma. Eur. Respir. J. 27, 208–229. Broide, D.H., 2008. Immunologic and inflammatory mechanisms that drive asthma progression to remodeling. J. Allergy Clin. Immunol. 121, 560–570 (quiz 571–2). Camoretti-Mercado, B., Solway, J., 2005. Transforming growth factor-beta1 and disorders of the lung. Cell Biochem. Biophys. 43, 131–148. Davies, D.E., Wicks, J., Powell, R.M., Puddicombe, S.M., Holgate, S.T., 2003. Airway remodeling in asthma: new insights. J. Allergy Clin. Immunol. 111, 215–225 (quiz 226). Donohue, J.F., 2008. Safety and efficacy of beta agonists. Respir. Care 53, 618–622 (discussion 623–4). Dschietzig, T., Bartsch, C., Stangl, V., Baumann, G., Stangl, K., 2004. Identification of the pregnancy hormone relaxin as glucocorticoid receptor agonist. FASEB J. 18, 1536–1538. Dschietzig, T., Bartsch, C., Greinwald, M., Baumann, G., Stangl, K., 2005. The pregnancy hormone relaxin binds to and activates the human glucocorticoid receptor. Ann. NY Acad. Sci. 1041, 256–271. Dschietzig, T., Bartsch, C., Baumann, G., Stangl, K., 2009a. RXFP1-inactive relaxin activates human glucocorticoid receptor: further investigations into the relaxin–GR pathway. Regul. Pept. 154, 77–84. Dschietzig, T., Bartsch, C., Wessler, S., Baumann, G., Stangl, K., 2009b. Autoregulation of human relaxin-2 gene expression critically involves relaxin and glucocorticoid receptor binding to glucocorticoid response half-sites in the relaxin-2 promoter. Regul. Pept. 155, 163–173. Fedorov, I.A., Wilson, S.J., Davies, D.E., Holgate, S.T., 2005. Epithelial stress and structural remodelling in childhood asthma. Thorax 60, 389–394. Ganguly, K., Stoeger, T., Wesselkamper, S.C., Reinhard, C., Sartor, M.A., Medvedovic, M., Tomlinson, C.R., Bolle, I., Mason, J.M., Leikauf, G.D., Schulz, H., 2007. Candidate genes controlling pulmonary function in mice: transcript profiling and predicted protein structure. Physiol. Genomics 31, 410–421. Garibay-Tupas, J.L., Okazaki, K.J., Tashima, L.S., Yamamoto, S., Bryant-Greenwood, G.D., 2004. Regulation of the human relaxin genes H1 and H2 by steroid hormones. Mol. Cell. Endocrinol. 219, 115–125. Guilbert, T.W., Morgan, W.J., Zeiger, R.S., Mauger, D.T., Boehmer, S.J., Szefler, S.J., Bacharier, L.B., Lemanske Jr., R.F., Strunk, R.C., Allen, D.B., Bloomberg, G.R., Heldt, G., Krawiec, M., Larsen, G., Liu, A.H., Chinchilli, V.M., Sorkness, C.A., Taussig, L.M., Martinez, F.D., 2006. Long-term inhaled corticosteroids in preschool children at high risk for asthma. N. Engl. J. Med. 354, 1985–1997. Halls, M.L., Bathgate, R.A., Summers, R.J., 2007. Comparison of signaling pathways activated by the relaxin family peptide receptors, RXFP1 and RXFP2, using reporter genes. J. Pharmacol. Exp. Ther. 320, 281–290. Holgate, S.T., 2000. Epithelial damage and response. Clin. Exp. Allergy 30 (Suppl. 1), 37–41. Holgate, S.T., 2007a. Epithelium dysfunction in asthma. J. Allergy Clin. Immunol. 120, 1233–1244 (quiz 1245–6). Holgate, S.T., 2007b. The epithelium takes centre stage in asthma and atopic dermatitis. Trends Immunol. 28, 248–251. Holgate, S.T., 2008a. Pathogenesis of asthma. Clin. Exp. Allergy 38, 872–897. Holgate, S.T., 2008b. The airway epithelium is central to the pathogenesis of asthma. Allergol. Int. 57, 1–10. Holgate, S.T., Holloway, J., Wilson, S., Bucchieri, F., Puddicombe, S., Davies, D.E., 2004. Epithelial–mesenchymal communication in the pathogenesis of chronic asthma. Proc. Am. Thorac. Soc. 1, 93–98. Hsu, S.Y., Nakabayashi, K., Nishi, S., Kumagai, J., Kudo, M., Sherwood, O.D., Hsueh, A.J.W., 2002. Activation of orphan receptors by the hormone relaxin. Science 295, 671–674 [see comment]. Huang, X., Gai, Y., Yang, N., Lu, B., Samuel, C.S., Thannickal, V.J., Zhou, Y., 2011. Relaxin regulates myofibroblast contractility and protects against lung fibrosis. Am. J. Pathol. 179, 2751–2765. James, A., 2005. Airway remodeling in asthma. Curr. Opin. Pulm. Med. 11, 1–6. Jeffery, P.K., 2001. Remodeling in asthma and chronic obstructive lung disease. Am. J. Respir. Crit. Care Med. 164, S28–S38. Jeffery, P.K., Laitinen, A., Venge, P., 2000. Biopsy markers of airway inflammation and remodelling. Respir. Med. 94 (Suppl. F), S9–S15. Kariyawasam, H.H., Robinson, D.S., 2007. The role of eosinophils in airway tissue remodelling in asthma. Curr. Opin. Immunol. 19, 681–686.
Kumagai, K., Ohno, I., Okada, S., Ohkawara, Y., Suzuki, K., Shinya, T., Nagase, H., Iwata, K., Shirato, K., 1999. Inhibition of matrix metalloproteinases prevents allergen-induced airway inflammation in a murine model of asthma. J. Immunol. 162, 4212–4219. Lagente, V., Manoury, B., Nenan, S., Le Quement, C., Martin-Chouly, C., Boichot, E., 2005. Role of matrix metalloproteinases in the development of airway inflammation and remodeling. Braz. J. Med. Biol. Res. 38, 1521–1530. Le, A.V., Cho, J.Y., Miller, M., McElwain, S., Golgotiu, K., Broide, D.H., 2007. Inhibition of allergen-induced airway remodeling in Smad 3-deficient mice. J. Immunol. 178, 7310–7316. Masoli, M., Fabian, D., Holt, S., Beasley, R., 2004. The global burden of asthma: executive summary of the GINA Dissemination Committee report. Allergy 59, 469–478. Mauad, T., Bel, E.H., Sterk, P.J., 2007. Asthma therapy and airway remodeling. J. Allergy Clin. Immunol. 120, 997–1009 (quiz 1010–1). McKenzie, D.C., Boulet, L.P., 2008. Asthma, outdoor air quality and the Olympic Games. CMAJ 179, 543–548. Moffatt, M.F., Gut, I.G., Demenais, F., Strachan, D.P., Bouzigon, E., Heath, S., von Mutius, E., Farrall, M., Lathrop, M., Cookson, W.O., 2010. A large-scale, consortium-based genomewide association study of asthma. N. Engl. J. Med. 363, 1211–1221. Mookerjee, I., Solly, N.R., Royce, S.G., Tregear, G.W., Samuel, C.S., Tang, M.L., 2006. Endogenous relaxin regulates collagen deposition in an animal model of allergic airway disease. Endocrinology 147, 754–761. Mookerjee, I., Tang, M.L.K., Solly, N., Tregear, G.W., Samuel, C.S., 2005. Investigating the role of relaxin in the regulation of airway fibrosis in animal models of acute and chronic allergic airway disease. Ann. NY Acad. Sci. 1041, 194–196. Moore, B., Murphy, R.F., Agrawal, D.K., 2008. Interaction of TGF-beta with immune cells in airway disease. Curr. Mol. Med. 8, 427–436. National Heart Lung and Blood Institute, 2007. Expert Panel Report 3: Guidelines for the Diagnosis and Management of Asthma, NIH, Publication No. 07-4051. Pascual, R.M., Peters, S.P., 2005. Airway remodeling contributes to the progressive loss of lung function in asthma: an overview. J. Allergy Clin. Immunol. 116, 477– 486 (quiz 487). Passalacqua, G., Ciprandi, G., 2008. Allergy and the lung. Clin. Exp. Immunol. 153 (Suppl. 1), 12–16. Pei, L., 1996. Identification of a negative glucocorticoid response element in the rat type 1 vasoactive intestinal polypeptide receptor gene. J. Biol. Chem. 271, 20879–20884. Pini, A., Shemesh, R., Samuel, C.S., Bathgate, R.A., Zauberman, A., Hermesh, C., Wool, A., Bani, D., Rotman, G., 2010. Prevention of bleomycin-induced pulmonary fibrosis by a novel antifibrotic peptide with relaxin-like activity. J. Pharmacol. Exp. Ther. 335, 589–599. Postma, D.S., Timens, W., 2006. Remodeling in asthma and chronic obstructive pulmonary disease. Proc. Am. Thorac. Soc. 3, 434–439. Prenner, B.M., 2008. Asthma 2008: targeting immunoglobulin E to achieve disease control. J. Asthma 45, 429–436. Redington, A.E., Madden, J., Frew, A.J., Djukanovic, R., Roche, W.R., Holgate, S.T., Howarth, P.H., 1997. Transforming growth factor-beta 1 in asthma. Measurement in bronchoalveolar lavage fluid. Am. J. Respir. Crit. Care Med. 156, 642–647. Reinhard, C., Meyer, B., Fuchs, H., Stoeger, T., Eder, G., Ruschendorf, F., Heyder, J., Nurnberg, P., de Angelis, M.H., Schulz, H., 2005. Genomewide linkage analysis identifies novel genetic loci for lung function in mice. Am. J. Respir. Crit. Care Med. 171, 880–888. Royce, S.G., Miao, Y.R., Lee, M., Samuel, C.S., Tregear, G.W., Tang, M.L., 2009. Relaxin reverses airway remodeling and airway dysfunction in allergic airways disease. Endocrinology 150, 2692–2699. Sagara, H., Okada, T., Okumura, K., Ogawa, H., Ra, C., Fukuda, T., Nakao, A., 2002. Activation of TGF-beta/Smad2 signaling is associated with airway remodeling in asthma. J. Allergy Clin. Immunol. 110, 249–254. Saglani, S., Malmstrom, K., Pelkonen, A.S., Malmberg, L.P., Lindahl, H., Kajosaari, M., Turpeinen, M., Rogers, A.V., Payne, D.N., Bush, A., Haahtela, T., Makela, M.J., Jeffery, P.K., 2005. Airway remodeling and inflammation in symptomatic infants with reversible airflow obstruction. Am. J. Respir. Crit. Care Med. 171, 722–727. Samuel, C.S., 2005. Relaxin: antifibrotic properties and effects in models of disease. Clin. Med. Res. 3, 241–249. Samuel, C.S., Hewitson, T.D., 2006. Relaxin in cardiovascular and renal disease. Kidney Int. 69, 1498–1502. Samuel, C.S., Zhao, C., Bathgate, R.A.D., Bond, C.P., Burton, M.D., Parry, L.J., Summers, R.J., Tang, M.L.K., Amento, E.P., Tregear, G.W., 2003. Relaxin deficiency in mice is associated with an age-related progression of pulmonary fibrosis. FASEB J. 17, 121–123. Samuel, C.S., Zhao, C., Bathgate, R.A.D., Du, X.-J., Summers, R.J., Amento, E.P., Walker, L.L., McBurnie, M., Zhao, L., Tregear, G.W., 2005. The relaxin gene-knockout mouse: a model of progressive fibrosis. Ann. NY Acad. Sci. 1041, 173–181. Samuel, C.S., Du, X.-J., Bathgate, R.A.D., Summers, R.J., 2006. ‘Relaxin’ the stiffened heart and arteries: the therapeutic potential for relaxin in the treatment of cardiovascular disease. Pharmacol. Ther. 112, 529–552. Samuel, C.S., Hewitson, T.D., Unemori, E.N., Tang, M.L., 2007. Drugs of the future: the hormone relaxin. Cell. Mol. Life Sci. 64, 1539–1557. Samuel, C.S., Royce, S.G., Chen, B., Cao, H., Gossen, J.A., Tregear, G.W., Tang, M.L., 2009. RXFP1 protects against airway fibrosis during homeostasis, but not against fibrosis associated with chronic allergic airways disease. Endocrinology 150, 1495–1502.
S.G. Royce et al. / Molecular and Cellular Endocrinology 351 (2012) 167–175 Seibold, J.R., Clements, P.J., Furst, D.E., Mayes, M.D., McCloskey, D.A., Moreland, L.W., White, B., Wigley, F.M., Rocco, S., Erikson, M., Hannigan, J.F., Sanders, M.E., Amento, E.P., 1998. Safety and pharmacokinetics of recombinant human relaxin in systemic sclerosis. J. Rheumatol. 25, 302–307. Seibold, J.R., Korn, J.H., Simms, R., Clements, P.J., Moreland, L.W., Mayes, M.D., Furst, D.E., Rothfield, N., Steen, V., Weisman, M., Collier, D., Wigley, F.M., Merkel, P.A., Csuka, M.E., Hsu, V., Rocco, S., Erikson, M., Hannigan, J., Harkonen, W.S., Sanders, M.E., 2000. Recombinant human relaxin in the treatment of scleroderma. A randomized, double-blind, placebo-controlled trial. Ann. Intern. Med. 132, 871– 879. Sherwood, O.D., 2004. Relaxin’s physiological roles and other diverse actions. Endocr. Rev. 25, 205–234. Sobande, P.O., Kercsmar, C.M., 2008. Inhaled corticosteroids in asthma management. Respir. Care 53, 625–633 (discussion 633–4).
175
Szefler, S.J., Leung, D.Y., 1997. Glucocorticoid-resistant asthma: pathogenesis and clinical implications for management. Eur. Respir. J. 10, 1640–1647. Wenzel, S.E., Balzar, S., Cundall, M., Chu, H.W., 2003. Subepithelial basement membrane immunoreactivity for matrix metalloproteinase 9: association with asthma severity, neutrophilic inflammation, and wound repair. J. Allergy Clin. Immunol. 111, 1345–1352. Williams, E.J., Benyon, R.C., Trim, N., Hadwin, R., Grove, B.H., Arthur, M.J., Unemori, E.N., Iredale, J.P., 2001. Relaxin inhibits effective collagen deposition by cultured hepatic stellate cells and decreases rat liver fibrosis in vivo. Gut 49, 577–583. Yao, P.M., Delclaux, C., d’Ortho, M.P., Maitre, B., Harf, A., Lafuma, C., 1998. Cell– matrix interactions modulate 92-kD gelatinase expression by human bronchial epithelial cells. Am. J. Respir. Cell Mol. Biol. 18, 813–822.
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Review
The regulation of fibrosis in airway remodeling in asthma Simon G. Royce a, Victor Cheng a, Chrishan S. Samuel b, Mimi L.K. Tang a,⇑ a b
Department of Allergy and Immunology, Murdoch Children’s Research Institute, The Royal Children’s Hospital, Melbourne 3052, Australia Department of Pharmacology, Monash University, Melbourne, Australia
a r t i c l e
i n f o
Article history: Received 19 December 2011 Accepted 4 January 2012 Available online 14 January 2012 Keywords: Asthma Fibrosis Relaxin Airway remodeling Therapy
a b s t r a c t Fibrosis is one of the key pathological features of airway remodeling in asthma. In the normal airway the amount of collagen and other extracellular matrix components is kept in equilibrium by regulation of synthesis and degradation. In asthma this homeostasis is disrupted due to genetic and environmental factors. In the airways of patients with the disease there is increased extracellular matrix deposition, particularly in the reticular basement membrane region, lamina propria and submucosa. Fibrosis is important as it can occur early in the pathogenesis of asthma, be associated with severity and resistant to therapy. In this review we will discuss current knowledge of relaxin and other key regulators of fibrosis in the airway including TGFb, Smad2/3 and matrix metalloproteinases. As fibrosis is not directly targeted or effectively treated by current asthma drugs including corticosteroids, characterization of airway fibrosis and how it is regulated will be essential for the development of novel therapies for asthma. Ó 2012 Elsevier Ireland Ltd. All rights reserved.
Contents 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
Definition of asthma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Epidemiology of asthma . . . . . . . . . . . . . . . . . . . . . . . . . . . . Etiology of asthma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Current treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Airway remodeling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reticular basement membrane fibrosis . . . . . . . . . . . . . . . . Transforming growth factor-b (TGFb1) and Smads . . . . . . . Matrix metalloproteinases (MMPs) and tissue inhibitors of Relaxin – a novel regulator of airway remodeling . . . . . . . . Receptors for relaxin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Therapeutic targeting of fibrosis in asthma . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
........................ ........................ ........................ ........................ ........................ ........................ ........................ metalloproteinases (TIMPs) . ........................ ........................ ........................ ........................ ........................
1. Definition of asthma Asthma is a chronic inflammatory disorder of the airways characterized by spontaneous, widespread, but variable bronchoAbbreviations: AAD, allergic airways disease; ASM, airway smooth muscle; ECM, extracellular matrix; EMTU, epithelial–mesenchymal trophic unit; GR, glucocorticoid receptor; MMP, matrix metalloproteinase; OVA, ovalbumin; p-Smad2, phospho-Smad2; RBM, reticular basement membrane; RXFP1, relaxin family peptide receptor-1; TGFb, transforming growth factor-b; TIMP, tissue inhibitors of metalloproteinase. ⇑ Corresponding author. Tel.: +61 3 9345 5733; fax: +61 3 9345 6348. E-mail address: [email protected] (M.L.K. Tang). 0303-7207/$ - see front matter Ó 2012 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.mce.2012.01.007
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constriction and airflow obstruction in response to a wide variety of environmental and endogenous stimuli (Holgate, 2008a,b). The clinical manifestations of asthma are episodic and can include wheezing, coughing, dyspnoea and chest tightness (AIHW Australian Centre for Asthma Monitoring, 2005). Due to the heterogeneous nature of the disorder, the severity of these manifestations can vary greatly between asthma sufferers, but in the majority of cases, asthma sufferers are asymptomatic between episodes, so asthma is widely considered to be a reversible condition, spontaneously or through treatment (Pascual and Peters, 2005). Severe asthma, on the other hand, refers to forms of therapyresistant, potentially fatal asthma. Although severe asthma is
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difficult to define, the National Institute of Health Guidelines for the Diagnosis and Management of Asthma state the following as characteristic of severe and persistent asthma: continual symptoms (especially at night) that cause limitations in physical activity; frequent exacerbations; persistent airflow obstruction with forced expiratory volume in 1 s (FEV1) and/or peak expiratory flow (PEF) of less than 60% of the predicted value; and PEF diurnal variation of greater than 30% (National Heart Lung and Blood Institute, 2007). It is also characteristically unresponsive to treatment, including systemic corticosteroids. It is estimated that severe asthma affects 5–10% of all asthma patients. 2. Epidemiology of asthma An estimated 300 million people suffer from asthma worldwide, and higher prevalence is seen in Western countries. Over the past few decades, the incidence of asthma has risen dramatically, and it is predicted that the worldwide incidence will continue to rise, with a further estimated 100 million people suffering from asthma by 2025, as populations become more Westernized and urbanized (Masoli et al., 2004). Moreover, asthma causes approximately 1 in 250 deaths per year (AIHW Australian Centre for Asthma Monitoring, 2005). However, the number of disability-adjusted life years (DALYs) lost due to asthma is approximately 15 million per year, making asthma the 25th leading cause of DALYs lost globally (similar to diabetes mellitus and liver cirrhosis) (Masoli et al., 2004).
3. Etiology of asthma An important contributing factor to the significant burden of asthma is the heterogeneous nature of its etiology (Holgate, 2008a; Borish and Culp, 2008). The wide range of environmental and endogenous stimuli that can trigger an asthma attack has been well documented. These triggers include allergic stimuli (e.g. pollen, dust mite feces, cat dander, certain drugs and occupational chemicals) (Holgate, 2008a,b; Passalacqua and Ciprandi, 2008) and non-allergic stimuli (e.g. exercise and cold) (Holgate, 2008b; McKenzie and Boulet, 2008). Despite this heterogeneity, it is widely accepted that asthma with allergic sensitisation involves a specific immune response to an environmental allergen (Passalacqua and Ciprandi, 2008), which leads to release of inflammatory mediators and subsequent inflammation (Broide, 2008). However, it is becoming increasingly clear that this is just one aspect of asthma, which is now being described as a disease that is composed of various combinations of many different underlying conditions, expressed as a common phenotype of allergic airways disease (AAD) (Holgate, 2008a,b). These underlying conditions may include disorders of epithelial integrity (permeability and repair) (Holgate, 2008a,b), IgE class switching (Passalacqua and Ciprandi, 2008), extracellular matrix deposition and degradation (Davies et al., 2003), and collagen regulation (Davies et al., 2003). The interaction between these underlying conditions and the appropriate environmental stimuli is what results in the common phenotype of airway inflammation, airway hyperresponsiveness
Fig. 1. A simplified schematic of the mechanisms behind asthma. Asthma is increasingly regarded as a disease of many disorders amalgamating in the allergic airways disease phenotype, involving a complex interplay between both genetic and environmental factors. Pre-existing genetic factors such as epithelial susceptibility, preferential IgE class switching in plasma cells and mutation of the A Disintegrin and metalloprotease-33 (ADAM33) gene predispose to epithelial damage and allergic hypersensitivity.
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(AHR) and airway remodeling seen in the airways of asthma sufferers (Fig. 1). 4. Current treatment The list of therapeutic options available to asthma sufferers today is extensive, but limited to relief and prevention of the symptoms rather than curative of the underlying pathology. Short- and long-acting b2-agonists (Donohue, 2008) and, to a lesser extent, leukotriene modifiers and theophylline, are used to relieve bronchoconstriction during an asthma attack. For sufferers of more severe asthma, inhaled glucocorticosteroids such as dexamethasone and fluticasone propionate are added as preventers of airway inflammation (Sobande and Kercsmar, 2008; Mauad et al., 2007). Omalizumab, a monoclonal antibody that binds to the IgE receptor FCeRI (preventing IgE from binding), has recently been made available and is showing therapeutic promise in the treatment of allergic asthma (Mauad et al., 2007; Prenner, 2008). There is, however, currently a lack of targeted therapies for reversing the structural changes seen in airways of asthma sufferers, collectively known as airway remodeling (Pascual and Peters, 2005). 5. Airway remodeling The term airway remodeling can refer to the normal structural changes that occur in embryonic development of the lung, which are essential for lung function (Jeffery, 2001). In asthma pathology, however, airway remodeling describes the manifestation of complex cellular and molecular interactions, which lead to abnormal transformations in the composition of the airway wall (Pei, 1996; James, 2005). The characteristic structural changes seen in the bronchial walls of asthma sufferers include epithelial denudation, reticular basement membrane (RBM) fibrosis, smooth muscle hypertrophy and hyperplasia, goblet cell metaplasia/mucous gland hypertrophy, altered composition of the deeper extracellular matrix (ECM) and angiogenesis (Mauad et al., 2007). As yet, the exact mechanism by which these events occur is unknown, although there have been suggestions of causative factors, one of which is an inflammation-driven progression to airway remodeling (Broide, 2008). The most prominent inflammatory cell types seen in asthma-affected airways are mast cells, eosinophils, TH2 cells and neutrophils, and these immune cells have been shown to have specific roles in the progression from inflammation to airway remodeling in asthma (Jeffery et al., 2000). Although TH2 cells have long been known for their role in chronic allergic inflammatory responses, it has more recently been shown that TH2 cells specifically contribute to airway remodeling by increasing smooth muscle hyperplasia and epithelial metaplasia (Broide, 2008; Camoretti-Mercado and Solway, 2005). Moreover, the eosinophilic infiltration seen in airway inflammation results in increased levels of the eosinophil-derived transforming growth factor-b (TGFb) (Broide, 2008), a potent profibrotic cytokine known to stimulate fibroblast differentiation and collagen deposition, leading to subepithelial fibrosis and subsequent stiffening of the tracheobronchial walls (Moore et al., 2008). Mast cells have also been shown to contribute to airway remodeling through the release of inflammatory mediators such as tryptase, histamine and other cytokines (Barnes, 2008). However, the primacy of an inflammation-driven progression to airway remodeling has been questioned following recent studies indicating the presence of airway remodeling changes very early in the development of asthma. Several parties have recently provided evidence of epithelial fragility and subepithelial basement membrane thickening in the airways in childhood irrespective of airway inflammation (Barbato et al., 2006; Saglani et al., 2005;
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Fedorov et al., 2005). This implies that the structural changes of airway remodeling occur at the same time as, or even before, the onset of airway inflammation. These findings have led to the suggestion that although inflammation is a central player in the pathogenesis of asthma, it does not fully account for the structural transformations seen in airway remodeling (Davies et al., 2003). Holgate has postulated that an underlying impaired epithelial repair mechanism initiates the remodeling process due to chronic injury and a failure to heal properly, and that greatly increased permeability of the epithelium facilitates access of inhaled irritants to the underlying tissue (Holgate, 2007a,b). Evidence for epithelial cell detachment from the basement membrane has been demonstrated through analysis of the sputum of asthma patients, resulting in epithelial denudation (Pascual and Peters, 2005). This increased exposure of the subepithelial tissues to potential allergens, due to pre-existing structural and functional epithelial abnormalities, would subsequently promote allergic hypersensitivity and inflammation (Holgate et al., 2004). Moreover, injury to the airway epithelium results in the reactivation of the epithelial–mesenchymal trophic unit (EMTU), a term used to describe the interaction between the epithelium and the underlying fibroblast sheath (Holgate, 2007a). This occurs through the release of fibrogenic and fibroproliferative growth factors, including fibroblast growth factor-2 (FGF-2), insulin-like growth factor-1 (IGF-1), endothelin-1, and latent and active TGFb by damaged epithelial cells (Holgate, 2008a). It has been demonstrated that the ability of the epithelium to generate these factors is increased in asthma patients compared with normal subjects (Holgate, 2000). The pro-fibrotic effects of these factors – reticular basement membrane fibrosis in particular – is likely augmented by the additional release of TGFb by inflammatory cells, especially eosinophils (Kariyawasam and Robinson, 2007), in a parallel process of allergic inflammation. The novel concept that airway inflammation is not the primary driver of asthma and airway remodeling is supported by trials showing that administration of inhaled glucocorticosteroids to children of 1–4 years of age (Bisgaard et al., 2006; Guilbert et al., 2006) had no effect on the natural history of asthma. It is further supported by the existence of corticosteroid-insensitive asthma (Szefler and Leung, 1997); the inability of anti-inflammatory agents such as corticosteroids to attenuate the progression of asthma suggests another potential causative factor. Although the natural history of airway remodeling in asthma is not fully elucidated, its characteristic features are universally recognized, and are most prominent in those patients with moderate to severe asthma. The increased amount and altered function of airway smooth muscle (ASM) is a significant contributor to the airway hyperresponsiveness seen in these patients, accounting for the increased contractility of the airway wall (Bentley and Hershenson, 2008). Morphologically, there is marked overall thickening of the airway wall, mostly due to smooth muscle hypertrophy and hyperplasia, but also fibrosis in the reticular basement membrane (RBM).
6. Reticular basement membrane fibrosis The RBM is the region situated just below the true basement membrane consisting of the lamina rara and lamina densa (Postma and Timens, 2006). The thickening of the RBM is a process that is characteristic of asthma, and is not seen in patients of chronic bronchitis or COPD (Jeffery, 2001). The RBM is a component of the airway wall that is not present in the fetus, but develops in even normal, healthy individuals. However, excessive thickening of the RBM in asthma occurs early in the asthma process (Jeffery, 2001). The extracellular matrix (ECM) components deposited in the RBM in asthma consist of collagen I, collagen III, collagen V,
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fibronectin, and tenascin (Postma and Timens, 2006). These contribute to airway wall thickening and subsequent reduced airway distensibility and increased airflow limitation. The main effectors of this excessive collagen deposition are the fibroblasts, the primary cell type in the mesenchyme of the lung (Boxall et al., 2006). Biopsy specimens from airways of asthma sufferers show significantly increased numbers of fibroblasts compared with normal subjects (Benayoun et al., 2003), and this has been correlated with increased RBM fibrosis. Under the influence of epithelial damage and cytokines such as TGFb1, fibroblasts can differentiate into the bioactive myofibroblast form, which has both secretory and contractile phenotypes and expresses a smooth muscle actin (aSMA). In addition to the secretion of collagen, myofibroblasts also produce more TGFb1 (Boxall et al., 2006), further prolonging the fibrotic process.
7. Transforming growth factor-b (TGFb1) and Smads Since the bulk of the ECM is made up of collagen subtypes (Postma and Timens, 2006), collagen homeostasis is essential to the normal composition of the ECM in the RBM. Its regulation involves an amalgam of factors each with a distinct mechanism affecting the fibrotic process. TGFb1 is a potent profibrotic cytokine, upregulating the processes involved in the deposition of collagen while downregulating its catabolism (Camoretti-Mercado and Solway, 2005). The majority of evidence supports the dogma that TGFb1 is upregulated in the airways in asthma (Redington et al., 1997; Bosse and Rola-Pleszczynski, 2007). In the airway, sources of
TGFb1 are (myo)fibroblasts, inflammatory cells and bronchial epithelial cells (Boxall et al., 2006), so it follows that epithelial damage and the subsequent inflammation would result in an increased presence of TGFb1 in the asthma-affected lung. After binding to its receptor tyrosine kinase receptors on fibroblasts, a phosphorylation cascade leads to phosphorylation of Smad2/3 (receptor Smads). These bind to Smad4 (a co-Smad) and the complex translocates to the nucleus to promote the differentiation of the fibroblast into the more bioactive myofibroblast (Boxall et al., 2006) (Fig. 2). Elevated Smad2 levels have been reported in bronchial biopsies of asthmatic subjects (Sagara et al., 2002), suggesting an increase in TGFb1 signaling in these lungs. Smad3 has been identified by a genome wide association study (GWAS) (Moffatt et al., 2010). Smad3 deficient mice have inhibition of ovalbumin-induced airway remodeling (Le et al., 2007). In addition, TGFb1 inhibits the production of matrix metalloproteinases (MMPs, matrix-degrading enzymes) by inflammatory and epithelial cells, and promotes the production of tissue inhibitors of metalloproteinases (TIMPs) by epithelial cells (Boxall et al., 2006). The resultant profibrotic environment encourages collagen deposition and thickening of the RBM in the airway walls of asthma sufferers.
8. Matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) MMPs are a large family of ECM-degrading enzymes produced by a large number of cells including inflammatory and epithelial
Fig. 2. Synthesis and action of TGFb isoforms. TGFb is synthesised particularly – but not only – by bronchial epithelial cells and myofibroblasts in a latent form. It then undergoes activation and binds to TGFb receptors (TGFb-RI, RII, RIII) and signals through the Smad signaling cascade to induce fibroblast differentiation. Although TGFb1 can directly bind to and activate TGFb-RII (bypassing TGFb-RIII), TGFb2 must initially bind TGFb-RIII to induce intracellular signaling. While Smad2, -3 and -4 are considered stimulatory Smads, Smad7 is inhibitory. TGFb Receptor Associated Peptide-1 (TRAP1) binds and inhibits TGFb-RI until phosphorylation by TGFb-RII.
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Fig. 3. The complex synthesis pathways of matrix metalloproteinases and their activation ultimately lead to the degradation of various forms of collagen. MMPs are initially synthesised as inactive zymogens, and are activated in an extracellular environment. Tissue inhibitors of metalloproteinases (TIMPs) are responsible for the inhibition of MMPs. It is often the MMP:TIMP ratio that affects collagen balance more than individual enzymes.
cells (Yao et al., 1998). As a family, they are zinc-dependent endopeptidases responsible for the proteolytic degradation of various ECM components (Lagente et al., 2005) (Fig. 3). Of particular interest are the gelatinases (MMP-2 and MMP-9), which show aberrant regulation in both human asthma (Wenzel et al., 2003) and murine experimental models of AAD (Kumagai et al., 1999). The gelatinases are capable of degrading collagens I, IV, V, VII, X, XI, XIV; gelatin; elastin; fibronectin and other ECM proteins (Lagente et al., 2005). Although both MMP-2 and MMP-9 are at increased levels in acute asthma, a difference is seen in chronic asthma. While
MMP-9 expression is still higher than baseline in chronic, severe asthma, MMP-2 expression is below baseline (Boxall et al., 2006). This implies that in chronic and severe asthma, there is a decreased capacity to degrade the ECM, which could upset collagen homeostasis and tip the balance towards a matrix-depositing phenotype. The direct inhibitors of MMPs are the profibrotic TIMPs, which are produced by a large number of cells including fibroblasts. TIMPs bind MMPs in a 1:1 stoichiometric ratio to prevent the proteolysis of their respective ECM components. TIMP-1 and TIMP-2 bind MMP-9 and MMP-2, respectively, and it has been hypothesized
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that the balance between levels of TIMPs and levels of corresponding MMPs in the lung is the key determinant of whether the ECM is being produced or degraded (Boxall et al., 2006; Wenzel et al., 2003; Kumagai et al., 1999). Indeed, it has been shown that levels of TIMP-1 are increased in bronchoalveolar lavage fluid (BALF) of asthma subjects, and also that such subjects have a lower MMP9:TIMP-1 ratio (Wenzel et al., 2003), promoting the deposition of collagen and fibrosis. 9. Relaxin – a novel regulator of airway remodeling Relaxin is a peptide hormone in the insulin superfamily, and was first described in 1926 as a reproductive hormone responsible for the softening of the pubic ligament in preparation for childbirth. The antifibrotic properties of relaxin were first considered in the 1950s and 1960s following observation of its effect on the pubic ligament (Sherwood, 2004). Interest grew in the antifibrotic properties of relaxin such that it was considered as a possible treatment for systemic sclerosis and scleroderma (Seibold et al., 2000), and clinical trials were attempted to ascertain the effectiveness of relaxin therapy, but these were ultimately unsuccessful due to lack of consistent outcomes and the phase III trials including an expanded cohort of patients, many of whom presented with end-stage disease pathogenesis which was unlikely treatable (Seibold et al., 1998). Since then, however, relaxin has been shown to have antifibrotic properties in non-reproductive tissues such as the heart (Samuel, 2005; Samuel et al., 2006; Samuel and Hewitson, 2006), liver (Williams et al., 2001), kidneys (Samuel and Hewitson, 2006) and lung (Moore et al., 2008; Mookerjee et al., 2006, 2005) in murine models of AAD. Experiments with relaxin gene knockout mice have shown that without endogenous relaxin, there is increased RBM fibrosis in response to allergen challenge, and the administration of recombinant H2 subtype relaxin leads to reversal of fibrosis in these mice (Samuel et al., 2003). Furthermore relaxin delivered systemically by mini-osmotic pump has been shown to reverse established airway remodeling changes when delivered after the ovalbumin challenge period in chronic AAD mice (Royce et al., 2009). Novel relaxin-like peptides are under development for a number of fibrotic conditions. One such peptide, CGEN25009 has been found to ameliorate peribronchial fibrosis in a mouse model of bleomycin-induced pulmonary fibrosis (Pini et al., 2010). In addition to its antifibrotic properties relaxin may have more direct impact of AHR. Relaxin was identified in the top four genes in a GWAS of bronchial hyperresponsiveness in mice (Reinhard et al., 2005; Ganguly et al., 2007). Work done in our laboratory has provided further evidence of an effect of relaxin on methacholine induced AHR in mice with AAD. Although it seems likely that ECM degradation due to relaxin may contribute to suppression of AHR (Mookerjee et al., 2006), relaxin has been shown to inhibit contractility of myofibroblasts in idiopathic pulmonary fibrosis (Huang et al., 2011). These observations establish H2 relaxin as a candidate for novel therapies targeted at reversing RBM fibrosis in human asthma. 10. Receptors for relaxin Although some of the receptor and signaling mechanisms of relaxin have been elucidated, recent research reveals that there are not only multiple receptors for relaxin, but that the downstream mechanisms for relaxin’s action are yet to be fully understood (Fig. 4). In 2002, Hsu et al. identified the receptors through which H2 relaxin signals: LGR7 and LGR8 (now RXFP1 and RXFP2, respectively) (Bathgate et al., 2006). While H2 relaxin can bind to both of these receptors, it binds to RXFP1 with highest affinity and most of its
effects result from its binding to this receptor (Hsu et al., 2002). Like relaxin, RXFP1 can be found in reproductive and nonreproductive tissues, including the lung. These G-protein coupled receptors lead to intracellular cAMP and phosphokinase A (PKA) production, and the subsequent downstream antifibrotic effects of relaxin (Sherwood, 2004). Recently, Samuel et al. demonstrated that RXFP1 gene knockout mice show a much earlier age-related progression of collagen deposition in the airway walls compared with their wild type counterparts, without allergen challenge, and that this indicates a role for RXFP1 in the homeostatic regulation of fibrosis in the airway (Samuel et al., 2009). The mechanisms by which the relaxin–RXFP1 interaction regulates the fibrotic pathway and collagen homeostasis is still unclear, but its effects in reducing fibrosis in many tissues (Samuel et al., 2007) make it a clear candidate for further research as a therapy for airway remodeling in asthma. While it is established that relaxin binds to and acts through RXFP1 and RXFP2, it is becoming clear that these are not the only receptors through which relaxin exerts is effects (Bathgate et al., 2005; Halls et al., 2007). In the same study that showed that RXFP1 has a role in homeostatic regulation of fibrosis, it was also found that RXFP1 had no effect in reducing fibrosis in the setting of AAD; RXFP1-knockout mice that underwent allergen sensitisation and challenge did not develop more fibrosis than their wild type counterparts (Samuel et al., 2009). Relaxin gene knockout mice, however, developed significantly more fibrosis than their wild type counterparts with and without undergoing the model of AAD (Samuel et al., 2003, 2005). From this study, Samuel and colleagues propose a paradigm that relaxin, as well as acting through RXFP1 and RXFP2, also acts through another receptor. In 2004, Dschietzig et al. identified relaxin as a potential glucocorticoid receptor agonist through co-immunoprecipitation experiments (Dschietzig et al., 2004). Relaxin was seen to interact with both nuclear and cytoplasmic glucocorticoid receptors independent of RXFP1 (Dschietzig et al., 2005). This finding was further supported by a recent study, which observed that RXFP1-independent relaxin administered to a THP-1 (human monocyte leukemia) cells caused a completely GR-dependent suppression of cytokine secretion (Dschietzig et al., 2009a). Furthermore, in the same year, Garibay-Tupas and colleagues demonstrated in a model system that there are glucocorticoid response elements (GRE) present on both the H1 and H2 relaxin genes, and that H1 and H2 relaxin can both be regulated by dexamethasone (a glucocorticoid) (Garibay-Tupas et al., 2004). Dschietzig and colleagues continued these investigations and found that relaxin-bound GR translocated to the nucleus and displaced known GR agonists from the relaxin gene GREs, establishing a positive autoregulatory loop (Dschietzig et al., 2009b). However, Halls showed that although relaxin indeed binds to glucocorticoid response element, this interaction takes place only in the presence of RXFP1 (Halls et al., 2007). This research deepens the current understanding of relaxin and its receptors and raises more opportunities through which we can investigate the avenues of relaxin’s action. Since glucocorticoid receptor is targeted by corticosteroids as a conventional asthma therapy (Mauad et al., 2007), it would be interesting to explore the effects of relaxin on asthma via the glucocorticoid receptor.
11. Therapeutic targeting of fibrosis in asthma To date, no therapies that specifically target fibrosis have reached the asthma clinic. However, a number of anti-cytokine monoclonal antibodies are undergoing phase I clinical trials for other fibrotic diseases and may be appropriate for asthma treatment in the future. Those most likely to be useful include
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Fig. 4. The signaling pathway of H2 relaxin. H2 relaxin signals primarily through RXFP1, with limited activity through RXFP2. The various G-protein subunits have complex interplay – with some involvement of inositol trisphosphate kinase (PI3K) and protein kinase C (PKC) – but the ultimate downstream effect is activation of adenylate cyclase and the production of cyclic adenosine monophosphate (cAMP), a marker of relaxin activity. The importance of glucocorticoid receptor (GR) is still being fully elucidated. However, the binding of GR to GR response elements (GRE) in the nucleus leads to cytokine suppression and anti-inflammatory effects.
anti-TGFb monoclonal antibody (GC-1008; Genzyme) and antiplatelet derived growth factor (PDGF; CR-002; Curagen). However it remains to be seen whether these monoclonal antibodies will be able to reverse existing fibrosis. As with Omalizumab (humanized anti-IgE monoclonal antibody), cost is also an issue limiting widespread use in chronic disease. Fibrosis and remodeling in general can now be evaluated by computed tomography scanning, potentially allowing for monitoring of disease progression and response to treatment (Aysola et al., 2008). Imaging technologies continue to improve and will greatly assist monitoring of drug response in asthma sufferers over the long term without the need for endoscope biopsy.
12. Conclusion Airway fibrosis is a key feature of asthma phenotypes, through progression of the disease. Reticular basement membrane fibrosis and subepithelial fibrosis are important pathological features of asthma. Sufferers have altered ECM composition characterized by the presence of collagen I, III, V, tenascin, fibronectin and laminin. Furthermore, key regulators of fibrosis with known roles in other
fibrotic diseases have been identified as altered in asthma patients in GWAS (e.g. Smad3 and ADAM33). It appears that the TGFb/Smad2/3 pathway plays a central role in regulating key processes such as myofibroblast differentiation and collagen synthesis. Furthermore, MMPs, TIMPs and relaxin are important regulators of fibrosis in asthma. Given that corticosteroids do not directly target fibrosis per se, there is a need to develop new therapeutics that are able to reverse and prevent fibrotic remodeling of the airways. References AIHW (Australian Centre for Asthma Monitoring), 2005. Asthma in Australia 2005, AIHW Asthma Series 2. AIHW Cat. No. ACM 6. AIHW, Canberra. Aysola, R.S., Hoffman, E.A., Gierada, D., Wenzel, S., Cook-Granroth, J., Tarsi, J., Zheng, J., Schechtman, K.B., Ramkumar, T.P., Cochran, R., Xueping, E., Christie, C., Newell, J., Fain, S., Altes, T.A., Castro, M., 2008. Airway remodeling measured by multidetector CT is increased in severe asthma and correlates with pathology. Chest 134, 1183–1191. Barbato, A., Turato, G., Baraldo, S., Bazzan, E., Calabrese, F., Panizzolo, C., Zanin, M.E., Zuin, R., Maestrelli, P., Fabbri, L.M., Saetta, M., 2006. Epithelial damage and angiogenesis in the airways of children with asthma. Am. J. Respir. Crit. Care Med. 174, 975–981. Barnes, P.J., 2008. The cytokine network in asthma and chronic obstructive pulmonary disease. J. Clin. Invest. 118, 3546–3556.
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S.G. Royce et al. / Molecular and Cellular Endocrinology 351 (2012) 167–175
Bathgate, R.A., Ivell, R., Sanborn, B.M., Sherwood, O.D., Summers, R.J., 2005. Receptors for relaxin family peptides. Ann. NY Acad. Sci. 1041, 61–76. Bathgate, R.A., Ivell, R., Sanborn, B.M., Sherwood, O.D., Summers, R.J., 2006. International Union of Pharmacology LVII: recommendations for the nomenclature of receptors for relaxin family peptides. Pharmacol. Rev. 58, 7– 31. Benayoun, L., Druilhe, A., Dombret, M.C., Aubier, M., Pretolani, M., 2003. Airway structural alterations selectively associated with severe asthma. Am. J. Respir. Crit. Care Med. 167, 1360–1368. Bentley, J.K., Hershenson, M.B., 2008. Airway smooth muscle growth in asthma: proliferation, hypertrophy, and migration. Proc. Am. Thorac. Soc. 5, 89–96. Bisgaard, H., Hermansen, M.N., Loland, L., Halkjaer, L.B., Buchvald, F., 2006. Intermittent inhaled corticosteroids in infants with episodic wheezing. N. Engl. J. Med. 354, 1998–2005. Borish, L., Culp, J.A., 2008. Asthma: a syndrome composed of heterogeneous diseases. Ann. Allergy Asthma Immunol. 101, 1–8 (quiz 8–11, 50). Bosse, Y., Rola-Pleszczynski, M., 2007. Controversy surrounding the increased expression of TGF beta 1 in asthma. Respir. Res. 8, 66. Boxall, C., Holgate, S.T., Davies, D.E., 2006. The contribution of transforming growth factor-beta and epidermal growth factor signalling to airway remodelling in chronic asthma. Eur. Respir. J. 27, 208–229. Broide, D.H., 2008. Immunologic and inflammatory mechanisms that drive asthma progression to remodeling. J. Allergy Clin. Immunol. 121, 560–570 (quiz 571–2). Camoretti-Mercado, B., Solway, J., 2005. Transforming growth factor-beta1 and disorders of the lung. Cell Biochem. Biophys. 43, 131–148. Davies, D.E., Wicks, J., Powell, R.M., Puddicombe, S.M., Holgate, S.T., 2003. Airway remodeling in asthma: new insights. J. Allergy Clin. Immunol. 111, 215–225 (quiz 226). Donohue, J.F., 2008. Safety and efficacy of beta agonists. Respir. Care 53, 618–622 (discussion 623–4). Dschietzig, T., Bartsch, C., Stangl, V., Baumann, G., Stangl, K., 2004. Identification of the pregnancy hormone relaxin as glucocorticoid receptor agonist. FASEB J. 18, 1536–1538. Dschietzig, T., Bartsch, C., Greinwald, M., Baumann, G., Stangl, K., 2005. The pregnancy hormone relaxin binds to and activates the human glucocorticoid receptor. Ann. NY Acad. Sci. 1041, 256–271. Dschietzig, T., Bartsch, C., Baumann, G., Stangl, K., 2009a. RXFP1-inactive relaxin activates human glucocorticoid receptor: further investigations into the relaxin–GR pathway. Regul. Pept. 154, 77–84. Dschietzig, T., Bartsch, C., Wessler, S., Baumann, G., Stangl, K., 2009b. Autoregulation of human relaxin-2 gene expression critically involves relaxin and glucocorticoid receptor binding to glucocorticoid response half-sites in the relaxin-2 promoter. Regul. Pept. 155, 163–173. Fedorov, I.A., Wilson, S.J., Davies, D.E., Holgate, S.T., 2005. Epithelial stress and structural remodelling in childhood asthma. Thorax 60, 389–394. Ganguly, K., Stoeger, T., Wesselkamper, S.C., Reinhard, C., Sartor, M.A., Medvedovic, M., Tomlinson, C.R., Bolle, I., Mason, J.M., Leikauf, G.D., Schulz, H., 2007. Candidate genes controlling pulmonary function in mice: transcript profiling and predicted protein structure. Physiol. Genomics 31, 410–421. Garibay-Tupas, J.L., Okazaki, K.J., Tashima, L.S., Yamamoto, S., Bryant-Greenwood, G.D., 2004. Regulation of the human relaxin genes H1 and H2 by steroid hormones. Mol. Cell. Endocrinol. 219, 115–125. Guilbert, T.W., Morgan, W.J., Zeiger, R.S., Mauger, D.T., Boehmer, S.J., Szefler, S.J., Bacharier, L.B., Lemanske Jr., R.F., Strunk, R.C., Allen, D.B., Bloomberg, G.R., Heldt, G., Krawiec, M., Larsen, G., Liu, A.H., Chinchilli, V.M., Sorkness, C.A., Taussig, L.M., Martinez, F.D., 2006. Long-term inhaled corticosteroids in preschool children at high risk for asthma. N. Engl. J. Med. 354, 1985–1997. Halls, M.L., Bathgate, R.A., Summers, R.J., 2007. Comparison of signaling pathways activated by the relaxin family peptide receptors, RXFP1 and RXFP2, using reporter genes. J. Pharmacol. Exp. Ther. 320, 281–290. Holgate, S.T., 2000. Epithelial damage and response. Clin. Exp. Allergy 30 (Suppl. 1), 37–41. Holgate, S.T., 2007a. Epithelium dysfunction in asthma. J. Allergy Clin. Immunol. 120, 1233–1244 (quiz 1245–6). Holgate, S.T., 2007b. The epithelium takes centre stage in asthma and atopic dermatitis. Trends Immunol. 28, 248–251. Holgate, S.T., 2008a. Pathogenesis of asthma. Clin. Exp. Allergy 38, 872–897. Holgate, S.T., 2008b. The airway epithelium is central to the pathogenesis of asthma. Allergol. Int. 57, 1–10. Holgate, S.T., Holloway, J., Wilson, S., Bucchieri, F., Puddicombe, S., Davies, D.E., 2004. Epithelial–mesenchymal communication in the pathogenesis of chronic asthma. Proc. Am. Thorac. Soc. 1, 93–98. Hsu, S.Y., Nakabayashi, K., Nishi, S., Kumagai, J., Kudo, M., Sherwood, O.D., Hsueh, A.J.W., 2002. Activation of orphan receptors by the hormone relaxin. Science 295, 671–674 [see comment]. Huang, X., Gai, Y., Yang, N., Lu, B., Samuel, C.S., Thannickal, V.J., Zhou, Y., 2011. Relaxin regulates myofibroblast contractility and protects against lung fibrosis. Am. J. Pathol. 179, 2751–2765. James, A., 2005. Airway remodeling in asthma. Curr. Opin. Pulm. Med. 11, 1–6. Jeffery, P.K., 2001. Remodeling in asthma and chronic obstructive lung disease. Am. J. Respir. Crit. Care Med. 164, S28–S38. Jeffery, P.K., Laitinen, A., Venge, P., 2000. Biopsy markers of airway inflammation and remodelling. Respir. Med. 94 (Suppl. F), S9–S15. Kariyawasam, H.H., Robinson, D.S., 2007. The role of eosinophils in airway tissue remodelling in asthma. Curr. Opin. Immunol. 19, 681–686.
Kumagai, K., Ohno, I., Okada, S., Ohkawara, Y., Suzuki, K., Shinya, T., Nagase, H., Iwata, K., Shirato, K., 1999. Inhibition of matrix metalloproteinases prevents allergen-induced airway inflammation in a murine model of asthma. J. Immunol. 162, 4212–4219. Lagente, V., Manoury, B., Nenan, S., Le Quement, C., Martin-Chouly, C., Boichot, E., 2005. Role of matrix metalloproteinases in the development of airway inflammation and remodeling. Braz. J. Med. Biol. Res. 38, 1521–1530. Le, A.V., Cho, J.Y., Miller, M., McElwain, S., Golgotiu, K., Broide, D.H., 2007. Inhibition of allergen-induced airway remodeling in Smad 3-deficient mice. J. Immunol. 178, 7310–7316. Masoli, M., Fabian, D., Holt, S., Beasley, R., 2004. The global burden of asthma: executive summary of the GINA Dissemination Committee report. Allergy 59, 469–478. Mauad, T., Bel, E.H., Sterk, P.J., 2007. Asthma therapy and airway remodeling. J. Allergy Clin. Immunol. 120, 997–1009 (quiz 1010–1). McKenzie, D.C., Boulet, L.P., 2008. Asthma, outdoor air quality and the Olympic Games. CMAJ 179, 543–548. Moffatt, M.F., Gut, I.G., Demenais, F., Strachan, D.P., Bouzigon, E., Heath, S., von Mutius, E., Farrall, M., Lathrop, M., Cookson, W.O., 2010. A large-scale, consortium-based genomewide association study of asthma. N. Engl. J. Med. 363, 1211–1221. Mookerjee, I., Solly, N.R., Royce, S.G., Tregear, G.W., Samuel, C.S., Tang, M.L., 2006. Endogenous relaxin regulates collagen deposition in an animal model of allergic airway disease. Endocrinology 147, 754–761. Mookerjee, I., Tang, M.L.K., Solly, N., Tregear, G.W., Samuel, C.S., 2005. Investigating the role of relaxin in the regulation of airway fibrosis in animal models of acute and chronic allergic airway disease. Ann. NY Acad. Sci. 1041, 194–196. Moore, B., Murphy, R.F., Agrawal, D.K., 2008. Interaction of TGF-beta with immune cells in airway disease. Curr. Mol. Med. 8, 427–436. National Heart Lung and Blood Institute, 2007. Expert Panel Report 3: Guidelines for the Diagnosis and Management of Asthma, NIH, Publication No. 07-4051. Pascual, R.M., Peters, S.P., 2005. Airway remodeling contributes to the progressive loss of lung function in asthma: an overview. J. Allergy Clin. Immunol. 116, 477– 486 (quiz 487). Passalacqua, G., Ciprandi, G., 2008. Allergy and the lung. Clin. Exp. Immunol. 153 (Suppl. 1), 12–16. Pei, L., 1996. Identification of a negative glucocorticoid response element in the rat type 1 vasoactive intestinal polypeptide receptor gene. J. Biol. Chem. 271, 20879–20884. Pini, A., Shemesh, R., Samuel, C.S., Bathgate, R.A., Zauberman, A., Hermesh, C., Wool, A., Bani, D., Rotman, G., 2010. Prevention of bleomycin-induced pulmonary fibrosis by a novel antifibrotic peptide with relaxin-like activity. J. Pharmacol. Exp. Ther. 335, 589–599. Postma, D.S., Timens, W., 2006. Remodeling in asthma and chronic obstructive pulmonary disease. Proc. Am. Thorac. Soc. 3, 434–439. Prenner, B.M., 2008. Asthma 2008: targeting immunoglobulin E to achieve disease control. J. Asthma 45, 429–436. Redington, A.E., Madden, J., Frew, A.J., Djukanovic, R., Roche, W.R., Holgate, S.T., Howarth, P.H., 1997. Transforming growth factor-beta 1 in asthma. Measurement in bronchoalveolar lavage fluid. Am. J. Respir. Crit. Care Med. 156, 642–647. Reinhard, C., Meyer, B., Fuchs, H., Stoeger, T., Eder, G., Ruschendorf, F., Heyder, J., Nurnberg, P., de Angelis, M.H., Schulz, H., 2005. Genomewide linkage analysis identifies novel genetic loci for lung function in mice. Am. J. Respir. Crit. Care Med. 171, 880–888. Royce, S.G., Miao, Y.R., Lee, M., Samuel, C.S., Tregear, G.W., Tang, M.L., 2009. Relaxin reverses airway remodeling and airway dysfunction in allergic airways disease. Endocrinology 150, 2692–2699. Sagara, H., Okada, T., Okumura, K., Ogawa, H., Ra, C., Fukuda, T., Nakao, A., 2002. Activation of TGF-beta/Smad2 signaling is associated with airway remodeling in asthma. J. Allergy Clin. Immunol. 110, 249–254. Saglani, S., Malmstrom, K., Pelkonen, A.S., Malmberg, L.P., Lindahl, H., Kajosaari, M., Turpeinen, M., Rogers, A.V., Payne, D.N., Bush, A., Haahtela, T., Makela, M.J., Jeffery, P.K., 2005. Airway remodeling and inflammation in symptomatic infants with reversible airflow obstruction. Am. J. Respir. Crit. Care Med. 171, 722–727. Samuel, C.S., 2005. Relaxin: antifibrotic properties and effects in models of disease. Clin. Med. Res. 3, 241–249. Samuel, C.S., Hewitson, T.D., 2006. Relaxin in cardiovascular and renal disease. Kidney Int. 69, 1498–1502. Samuel, C.S., Zhao, C., Bathgate, R.A.D., Bond, C.P., Burton, M.D., Parry, L.J., Summers, R.J., Tang, M.L.K., Amento, E.P., Tregear, G.W., 2003. Relaxin deficiency in mice is associated with an age-related progression of pulmonary fibrosis. FASEB J. 17, 121–123. Samuel, C.S., Zhao, C., Bathgate, R.A.D., Du, X.-J., Summers, R.J., Amento, E.P., Walker, L.L., McBurnie, M., Zhao, L., Tregear, G.W., 2005. The relaxin gene-knockout mouse: a model of progressive fibrosis. Ann. NY Acad. Sci. 1041, 173–181. Samuel, C.S., Du, X.-J., Bathgate, R.A.D., Summers, R.J., 2006. ‘Relaxin’ the stiffened heart and arteries: the therapeutic potential for relaxin in the treatment of cardiovascular disease. Pharmacol. Ther. 112, 529–552. Samuel, C.S., Hewitson, T.D., Unemori, E.N., Tang, M.L., 2007. Drugs of the future: the hormone relaxin. Cell. Mol. Life Sci. 64, 1539–1557. Samuel, C.S., Royce, S.G., Chen, B., Cao, H., Gossen, J.A., Tregear, G.W., Tang, M.L., 2009. RXFP1 protects against airway fibrosis during homeostasis, but not against fibrosis associated with chronic allergic airways disease. Endocrinology 150, 1495–1502.
S.G. Royce et al. / Molecular and Cellular Endocrinology 351 (2012) 167–175 Seibold, J.R., Clements, P.J., Furst, D.E., Mayes, M.D., McCloskey, D.A., Moreland, L.W., White, B., Wigley, F.M., Rocco, S., Erikson, M., Hannigan, J.F., Sanders, M.E., Amento, E.P., 1998. Safety and pharmacokinetics of recombinant human relaxin in systemic sclerosis. J. Rheumatol. 25, 302–307. Seibold, J.R., Korn, J.H., Simms, R., Clements, P.J., Moreland, L.W., Mayes, M.D., Furst, D.E., Rothfield, N., Steen, V., Weisman, M., Collier, D., Wigley, F.M., Merkel, P.A., Csuka, M.E., Hsu, V., Rocco, S., Erikson, M., Hannigan, J., Harkonen, W.S., Sanders, M.E., 2000. Recombinant human relaxin in the treatment of scleroderma. A randomized, double-blind, placebo-controlled trial. Ann. Intern. Med. 132, 871– 879. Sherwood, O.D., 2004. Relaxin’s physiological roles and other diverse actions. Endocr. Rev. 25, 205–234. Sobande, P.O., Kercsmar, C.M., 2008. Inhaled corticosteroids in asthma management. Respir. Care 53, 625–633 (discussion 633–4).
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Szefler, S.J., Leung, D.Y., 1997. Glucocorticoid-resistant asthma: pathogenesis and clinical implications for management. Eur. Respir. J. 10, 1640–1647. Wenzel, S.E., Balzar, S., Cundall, M., Chu, H.W., 2003. Subepithelial basement membrane immunoreactivity for matrix metalloproteinase 9: association with asthma severity, neutrophilic inflammation, and wound repair. J. Allergy Clin. Immunol. 111, 1345–1352. Williams, E.J., Benyon, R.C., Trim, N., Hadwin, R., Grove, B.H., Arthur, M.J., Unemori, E.N., Iredale, J.P., 2001. Relaxin inhibits effective collagen deposition by cultured hepatic stellate cells and decreases rat liver fibrosis in vivo. Gut 49, 577–583. Yao, P.M., Delclaux, C., d’Ortho, M.P., Maitre, B., Harf, A., Lafuma, C., 1998. Cell– matrix interactions modulate 92-kD gelatinase expression by human bronchial epithelial cells. Am. J. Respir. Cell Mol. Biol. 18, 813–822.