Balance in asthma between matrix metalloproteinases and their inhibitors

Abstract

For decades asthma has been considered a condition of reversible airflow obstruction, and in themajority of patients complete reversibility of long-standing abnormal spirometric measurementssuch as FEV1 may be observed after treatment. However, many asthmatics, both children and adults, show evidence of residual airway obstruction that may even be detected in asymptomatic patients. This clinically demonstrable irreversible component of airway obstruction was observed on pathologic findings at the turn of the century and was already proposed in the definition of asthma in 1962. However, these features were almost completely ignored for a long time and the concept of airway remodeling in asthma was only proposed in 1992. Remodeling is defined as to “model again or differently, reconstruct.” This is a critical aspect of wound repair in all organs, representing a dynamic process that associates matrix production and degradation in reaction to an inflammatory insult leading to a normal reconstruction process (model again) or a pathologic one (model differently). Connective tissue cells produce and secrete an array of macromolecules, forming a complex network filling the extracellular space of the submucosa, called the extracellular matrix (ECM). The ECM is not only a scaffolding having a mechanical role in supporting and maintaining tissue structure, but it is a complex and dynamic meshwork influencing many biologic cell functions such as development, migration, and proliferation.1Raghow R The role of extracellular matrix in postinflammatory wound healing and fibrosis.Faseb J. 1994; 8: 823-831Crossref PubMed Scopus (365) Google Scholar The ECM can be divided into 2 categories: the basement membrane and the interstitial matrix. The macromolecules that constitute the ECM are secreted locally, the composition of which depends on the cell types, their state of differentiation, and their metabolic status. Molecules comprising ECM consist of fibrous proteins (collagen, elastin) and structural or adhesive proteins (fibronectin and laminin) embedded in a hydrated polysaccharide gel containing several glycosaminoglycans, including hyaluronic acid. The ECM is a dynamic structure and an equilibrium between synthesis and degradation of ECM components is required for the maintenance of its homeostasis.2Murphy G Docherty AJ The matrix metalloproteinases and their inhibitors.Am J Respir Cell Mol Biol. 1992; 7: 120-125Crossref PubMed Scopus (536) Google Scholar Although many proteases can cleave ECM molecules, the family of Zn++ matrix metalloproteinases (MMPs) and their inhibitors are likely to be the normal physiologically relevant mediators of ECM degradation.3Shapiro SD Senior RM Matrix metalloproteinases: matrix degradation and more.Am J Respir Cell Mol Biol. 1999; 20: 1100-1102Crossref PubMed Scopus (202) Google Scholar Several subclasses of MMPs have been identified, including collagenases, gelatinases, stromelysins, and membrane-type MMPs. These can degrade many proteins including collagens, fibronectin, laminin, proteoglycans, entactins, and elastin. MMPs are secreted in a latent form, as inactive proenzymes, and are activated by the loss of the propeptide under physiologic conditions. In addition, the proteolytic activity of MMPs is precisely controlled by endogenous physiologic inhibitors, including the broad-spectrum serum inhibitor α2 -macroglobulin and a special class of tissue inhibitors of metalloproteinases (TIMPs).3Shapiro SD Senior RM Matrix metalloproteinases: matrix degradation and more.Am J Respir Cell Mol Biol. 1999; 20: 1100-1102Crossref PubMed Scopus (202) Google Scholar Four members of the TIMP family have been characterized and designated as TIMP-1, TIMP-2, TIMP-3, and TIMP-4. The major role of MMPs is basement membrane and ECM breakdown in tissue remodeling and angiogenesis. TIMP-1 and TIMP-2 are capable of inhibiting the activities of all known MMPs and as such play a key role in maintaining the balance between ECM deposition and degradation in different physiologic processes. Loss of co-ordination in the expression of proteinases and inhibitors is believed to generate tissue degradation in inflammatory diseases. The restoration of functional connective tissue is a major goal of the wound-healing process. This regenerative event requires the deposition and accumulation of collagenous and noncollagenous ECM molecules as well as the remodeling of ECM by MMPs. Excessive ECM breakdown resulting from an MMP-TIMP imbalance occurs in various pathologic processes, including inflammation, chronic degenerative diseases, and tumor invasion. Structural changes in the airway walls are essential features of asthma and include ECM remodeling, epithelial desquamation, goblet cell hyperplasia, prominent smooth muscle, vascular remodeling, collagen deposition below the basement membrane, and elastolysis. Is remodeling of the airways a proved clinical concept?4Reed CE The natural history of asthma in adults: the problem of irreversibility.J Allergy Clin Immunol. 1999; 103: 539-547Abstract Full Text Full Text PDF PubMed Scopus (131) Google Scholar It is clear that changes in the ECM and bronchial wall structure have the capacity to influence airway function in asthma. However, it is not known how each of the many changes that occur in the airway wall contribute to permanently altered airway function in some asthmatic patients. There are other clinical consequences of remodeling. The increase in smooth muscle mass can lead to a severe bronchial obstruction during an asthma attack. Mucous glands are sometimes enlarged and may induce an excessive mucus production. The continuing inflammation is linked with the persistence of exacerbations and nonspecific bronchial hyperresponsiveness. Degradation or reorganization of elastin and cartilage may result in decreased airway wall stiffness and increased airway narrowing for a given amount of force generated by the smooth muscle. In asthma, remodeling is almost always present in biopsy specimens but is not always clinically demonstrated. The basement membrane of surface epithelium is composed of several layers: the basal lamina (referred to as the “true” basement membrane) and the lamina reticularis. The thickening of the lamina reticularis is a characteristic early feature of the asthmatic bronchus. In asthma the basal lamina is of normal thickness, whereas the reticular layer is thickened associated with deposition of immunoglobulins or collagen I, III, and IV and fibronectin but not collagen types V and VII5Roche WR Beasley R Williams JH Holgate ST Subepithelial fibrosis in the bronchi of asthmatics.Lancet. 1989; 1: 520-524Abstract PubMed Scopus (961) Google Scholar nor laminin,6Altraja A Laitinen A Virtanen I et al.Expression of laminins in the airways in vahous types of asthmatic patients: a morphometric study.Am J Respir Cell Mol Biol. 1996; 15: 482-488Crossref PubMed Scopus (96) Google Scholar leading to a so-called subepithelial fibrosis of the airways. These features represent one of the most common remodeling patterns of asthma, the significance of which is, however, little understood. The thickening has not been related to the severity, duration, or etiology of asthma in many studies,7Jeffery PK Wardlaw AJ Nelson FC Collins JV Kay AB Bronchial biopsies in asthma: an ultrastructural, quantitative study and correlation with hyperreactivity.Am Rev Respir Dis. 1989; 140: 1745-1753Crossref PubMed Scopus (810) Google Scholar whereas a rather weak correlation with the severity of the disease has been observed in other studies.8Chetta A Foresi A Del-Donno M et al.Bronchial responsiveness to distilled water and methacholine and its relationship to inflammation and remodeling of the airways in asthma.Am J Respir Crit Care Med. 1996; 153: 910-917Crossref PubMed Scopus (144) Google Scholar In another article published recently, a highly significant correlation between asthma severity and the size of the basement membrane assessed by collagen III and V immunoreactivity was found. However, individual data were not presented, so it is difficult to assess the magnitude of the effect.9Hoshino M Nakamura Y Sim J Shimojo J Isogai S Bronchial subepithelial fibrosis and expression of matrix metalloproteinase-9 in asthmatic airway inflammation.J Allergy Clin Immunol. 1998; 102: 783-788Abstract Full Text Full Text PDF PubMed Scopus (264) Google Scholar The effects of anti-inflammatory drugs on the process of airway remodeling have been little studied and the study of Hoshino et al published in the August issue (Hoshino M, Takahashi M, Takai Y, Sim J. Inhaled corticosteroids decrease subepithelial collagen deposition by modulation of the balance between matrix metalloproteinase-9 and tissue inhibitor of metalloproteinase-1 expression in asthma. J Allergy Clin Immunol 1999;104:356-63) is of great interest. Inhaled steroids were not shown to reverse the thickening of this reticular layer in asthma,10Jeffery PK Godfrey RW Adelroth E Nelson F Rogers A Johansson SA Effects of treatment on airway inflammation and thickening of basement membrane reticular collagen in asthma: a quantitative light and electron microscopic study.Am Rev Respir Dis. 1992; 145: 890-899Crossref PubMed Google Scholar even after a long-term treatment course.11Lundgren R Soderberg M Horstedt P Stenling R Morphological studies of bronchial mucosal biopsies from asthmatics before and after ten years of treatment with inhaled steroids.Eur Respir J. 1988; 1: 883-889PubMed Google Scholar However, in other studies in mild asthma inhaled steroids were found to reduce this thickening.12Trigg CJ Manolitsas ND Wang J et al.Placebo-controlled immunopathologic study of four months of inhaled corticosteroids in asthma.Am J Respir Crit Care Med. 1994; 150: 17-22Crossref PubMed Scopus (277) Google Scholar, 13Olivieri D Chetta A Del-Donno M et al.Effect of short-term treatment with low-dose inhaled fluticasone propionate on airway inflammation and remodeling in mild asthma: a placebo-controlled study.Am J Respir Crit Care Med. 1997; 155: 1864-1871Crossref PubMed Scopus (293) Google Scholar, 14Hoshino M Nakamura Y Sim JJ et al.Inhaled corticosteroid reduced lamina reticularis of the basement membrane by modulation of insulin-like growth factor (IGF)-I expression in bronchial asthma.Clin Exp Allergy. 1998; 28: 568-577Crossref PubMed Scopus (124) Google Scholar In a study by Hoshino et al,14Hoshino M Nakamura Y Sim JJ et al.Inhaled corticosteroid reduced lamina reticularis of the basement membrane by modulation of insulin-like growth factor (IGF)-I expression in bronchial asthma.Clin Exp Allergy. 1998; 28: 568-577Crossref PubMed Scopus (124) Google Scholar with use of transmission electron microscopy, it was found that the size of the basement membrane was unchanged after beclomethasone dipropionate 800 μg daily for 6 months, whereas the size of the lamina reticularis was significantly decreased after this corticosteroid treatment. In the study of Hoshino et al in the August issue of the Journal, it was found that inhaled corticosteroids reduced the deposition of collagen III. Other proteins might have been studied. MMP-9 was found to be the major MMP in bronchoalveolar lavage (BAL) fluid15Mautino G Henriquet C Jaffuel D Bousquet J Capony F Tissue inhibitor of metalloproteinase-1 levels in BAL fluid from asthmatics.Am J Respir Crit Care Med. 1999; 60: 324-330Crossref Scopus (81) Google Scholar and bronchial mucosal biopsy specimens9Hoshino M Nakamura Y Sim J Shimojo J Isogai S Bronchial subepithelial fibrosis and expression of matrix metalloproteinase-9 in asthmatic airway inflammation.J Allergy Clin Immunol. 1998; 102: 783-788Abstract Full Text Full Text PDF PubMed Scopus (264) Google Scholar in asthma. In healthy subjects MMP-9 is cosecreted with TIMP-1 in 1:1 stoichiometry. In the BAL fluid of untreated stable asthmatic patients, the TIMP-1/MMP-9 molar ratio, which assesses the balance between the 2 antagonists, was more than 6 times greater than in control subjects or asthmatics receiving inhaled corticosteroids.15Mautino G Henriquet C Jaffuel D Bousquet J Capony F Tissue inhibitor of metalloproteinase-1 levels in BAL fluid from asthmatics.Am J Respir Crit Care Med. 1999; 60: 324-330Crossref Scopus (81) Google Scholar In sputum the TIMP-1/MMP-9 molar ratio was also higher in untreated stable asthmatic patients and in patients with chronic bronchitis than in control subjects.16Vignola A Riccobono L Mirabella A et al.Sputum TIMP-I to MMP-9 ratio correlates with airflow obstruction in asthma and COPD.Am J Respir Crit Care Med. 1998; 158: 1945-1950Crossref PubMed Scopus (340) Google Scholar Thus, in patients with stable asthma, there is an overproduction of TIMP-1 over MMP-9. Moreover, no activated form of MMP-9 could be detected in BAL fluids from untreated stable asthmatics, suggesting that there was enough counterinhibitor (TIMP-1) to protect the molecule against activation.15Mautino G Henriquet C Jaffuel D Bousquet J Capony F Tissue inhibitor of metalloproteinase-1 levels in BAL fluid from asthmatics.Am J Respir Crit Care Med. 1999; 60: 324-330Crossref Scopus (81) Google Scholar However, an excess of MMP-9 is found in BAL fluids from patients with uncontrolled asthma or during a severe exacerbation.17Lemjabbar H Gosset P Lamblin C et al.Contribution of 92 kDa gelatinase/type IV collagenase in bronchial inflammation during status asthmaticus.Am J Respir Crit Care Med. 1999; 159: 1298-1307Crossref PubMed Scopus (152) Google Scholar In these patients with unstable asthma, free metallogelatinolytic activity was observed in the BAL fluid. The functional consequence of the TIMP-1/MMP-9 imbalance in asthma deserves more attention. In a first study it was observed that in sputum the TIMP-1/MMP-9 molar ratio was negatively correlated with FEV1 values in untreated asthmatic patients.16Vignola A Riccobono L Mirabella A et al.Sputum TIMP-I to MMP-9 ratio correlates with airflow obstruction in asthma and COPD.Am J Respir Crit Care Med. 1998; 158: 1945-1950Crossref PubMed Scopus (340) Google Scholar In asthma MMP-9 may be secreted by macrophages,18Mautino G Henriquet C Gougat C et al.Increased expression of tissue inhibitor of metalloproteinase-1 and loss of correlation with matrix metalloproteinase-9 by macrophages in asthma.Lab Invest. 1999; 79: 39-47PubMed Google Scholar, 19Mautino G Oliver N Chanez P Bousquet J Capony F Increased release of matrix metalloproteinase-9 in bronchoalveolar lavage fluid and by alveolar macrophages of asthmatics.Am J Respir Cell Mol Biol. 1997; 17: 583-591Crossref PubMed Scopus (204) Google Scholar neutrophils,17Lemjabbar H Gosset P Lamblin C et al.Contribution of 92 kDa gelatinase/type IV collagenase in bronchial inflammation during status asthmaticus.Am J Respir Crit Care Med. 1999; 159: 1298-1307Crossref PubMed Scopus (152) Google Scholar eosinophils,20Ohno I Ohtani H Nitta Y et al.Eosinophils as a source of matrix metalloproteinase-9 in asthmatic airway inflammation.Am J Respir Cell Mol Biol. 1997; 16: 212-219Crossref PubMed Scopus (212) Google Scholar and epithelial cells9Hoshino M Nakamura Y Sim J Shimojo J Isogai S Bronchial subepithelial fibrosis and expression of matrix metalloproteinase-9 in asthmatic airway inflammation.J Allergy Clin Immunol. 1998; 102: 783-788Abstract Full Text Full Text PDF PubMed Scopus (264) Google Scholar, 17Lemjabbar H Gosset P Lamblin C et al.Contribution of 92 kDa gelatinase/type IV collagenase in bronchial inflammation during status asthmaticus.Am J Respir Crit Care Med. 1999; 159: 1298-1307Crossref PubMed Scopus (152) Google Scholar but apparently not by basophils. TIMP-1 levels in BAL fluids from untreated asthmatics were correlated with the number of alveolar macrophages, suggesting that the inhibitor may originate from these cells.18Mautino G Henriquet C Gougat C et al.Increased expression of tissue inhibitor of metalloproteinase-1 and loss of correlation with matrix metalloproteinase-9 by macrophages in asthma.Lab Invest. 1999; 79: 39-47PubMed Google Scholar In the sputum MMP-9 levels were significantly correlated with the number of macrophages and neutrophils.16Vignola A Riccobono L Mirabella A et al.Sputum TIMP-I to MMP-9 ratio correlates with airflow obstruction in asthma and COPD.Am J Respir Crit Care Med. 1998; 158: 1945-1950Crossref PubMed Scopus (340) Google Scholar The release of TIMP-1 and MMP-9 from alveolar macrophages was found to be significantly correlated in control subjects and asthmatic patients treated by inhaled corticosteroids, whereas this was not true for untreated asthmatic patients, indicating an imbalance between MMP-9 and TIMP-1 production.18Mautino G Henriquet C Gougat C et al.Increased expression of tissue inhibitor of metalloproteinase-1 and loss of correlation with matrix metalloproteinase-9 by macrophages in asthma.Lab Invest. 1999; 79: 39-47PubMed Google Scholar MMP-9 can degrade native type IV and type V collagens, denatured collagens, entactin, proteoglycans, and elastin and has been shown to play a role in the migration of monocytes-macrophages or eosinophils to inflammatory foci by increasing the migration of these cells through the basement membrane and in the disruption of the epithelial layer. Imbalances between these enzymes and inhibitors may contribute to the tissue damage and remodeling seen in inflammatory diseases. The presence of an excess of TIMP-1 over MMP-9 in stable asthma may protect the bronchi against metalloproteinase-degrading activity. This may interfere with tissue repair and also contribute to fibrosis by its inhibition of MMP-9 or other MMP in vivo. On the other hand, in patients with acute asthma free metallogelatinolytic activity can be detected in the BAL fluid,17Lemjabbar H Gosset P Lamblin C et al.Contribution of 92 kDa gelatinase/type IV collagenase in bronchial inflammation during status asthmaticus.Am J Respir Crit Care Med. 1999; 159: 1298-1307Crossref PubMed Scopus (152) Google Scholar suggesting that MMPs may be involved in different tissue degradation processes depending on the state of the disease. The in vitro effects of corticosteroids differ depending on the cell type and possibly on its activation state. Dexamethasone is a potent inhibitor of MMP-9 and TIMP-1 synthesis in alveolar macrophages of control subjects.21Shapiro SD Campbell EJ Kobayashi DK Welgus HG Dexamethasone selectively modulates basal and lipopolysaccharide-induced metalloproteinase and tissue inhibitor of metalloproteinase production by human alveolar macrophages.J Immunol. 1991; 146: 2724-2729PubMed Google Scholar However, in skin fibroblasts TIMP-1 and collagenase expression appear to be independently modulated by glucocorticoids.22Clark SD Kobayashi DK Welgus HG Regulation of the expression of tissue inhibitor of metalloproteinases and collagenase by retinoids and glucocorticoids in human fibroblasts.J Clin Invest. 1987; 80: 1280-1288Crossref PubMed Scopus (124) Google Scholar In alveolar macrophages dexamethasone decreases MMP-9 release by approximately 80% in healthy subjects and asthmatic patients, regardless of whether they were receiving inhaled corticosteroids.19Mautino G Oliver N Chanez P Bousquet J Capony F Increased release of matrix metalloproteinase-9 in bronchoalveolar lavage fluid and by alveolar macrophages of asthmatics.Am J Respir Cell Mol Biol. 1997; 17: 583-591Crossref PubMed Scopus (204) Google Scholar The serum MMP-9/TIMP-1 ratio could predict the response of oral corticosteroid therapy in asthma. The low MMP-9/TIMP-1 ratio observed in subjects with little or no FEV1 improvement with corticosteroids supports the hypothesis that, in these asthmatic subjects, bronchial fibrogenesis predominates over inflammation.23Bosse M Chakir J Rouabhia M Boulet LP Audette M Laviolette M Serum matrix metalloproteinase-9:tissue inhibitor of metalloproteinase-1 ratio correlates with steroid responsiveness in moderate to severe asthma.Am J Respir Crit Care Med. 1999; 159: 596-602Crossref PubMed Scopus (113) Google Scholar In their article of the August issue of the Journal, Hoshino et al studied asthmatic patients treated with 800 μg beclomethasone dipropionate or placebo for 6 months and are the first to demonstrate a reduction of the size of the reticular basement membrane associated with a decrease in MMP-9 immunoreactivity and an increase in TIMP-1 immunoreactivity in the bronchial biopsy specimens. The association of the reduction in the size of the reticular basement membrane, based on the measurement of collagen III deposition and the overall collagenolytic activity (MMP-9/TIMP-1 ratio) is surprising. A simplistic explanation may be proposed. Because type III collagen is a far better substrate for MMP-1 than for MMP-9, the MMP-1/TIMP-1 ratio may be better related to collagen III turnover than the MMP-9/TIMP-1 ratio. This hypothesis is supported by the association between subepithelial thickness and the proliferation of fibroblasts and a much greater production of MMP-1 than MMP-9 by these cells.3Shapiro SD Senior RM Matrix metalloproteinases: matrix degradation and more.Am J Respir Cell Mol Biol. 1999; 20: 1100-1102Crossref PubMed Scopus (202) Google Scholar However, other hypotheses can be proposed to explain the surprising results of the study of Hoshino et al. First, MMP-9 and TIMP-1 immunoreactivities do not necessarily correlate with the release of these proteins nor with the final balance, which is essential in the collagenolytic activity. Second, corticosteroids may be more effective on the production of collagen than on its degradation, and in the study of Hoshino et al in the August issue there was a significant reduction in myofibroblast numbers during corticosteroid treatment. Third, during a 6-month study there may be different phases and a single snapshot study cannot assess the chronology of events. It is clear that biopsy studies cannot be performed serially, but this serial assessment of airway inflammation may be done with sputum. Moreover, the balance between MMP-9 and TIMP-1 can be studied in sputum supernatants. Thus, although the findings of Hoshino et al are of great interest, their interpretation is still a matter of discussion and we cannot fully accept that the reduction in the reticular basement membrane size is linked with MMP-9 and TIMP-1 immunoreactivities. However, this is the first study attempting to link corticosteroid treatment with the protease-antiprotease balance and, as such, it is an important study.