Collagen breakdown products and lung collagen metabolism: an in vitro study on fibroblast cultures

Four-day pulse of sodium cromoglycate modulates pulmonary vessel wall remodeling during 21-day hypoxia in rats

AIM OF THE STUDY

Remodeling of pulmonary resistance arteries in rats due to 4-day hypoxia could be successfully suppressed by sodium cromoglycate. In this study, we tested the difference in the suppression between two distinct time patterns of cromoglycate administration during 21-day hypoxia. In the experiment, we focused on some details in both smooth muscle cells and extracellular matrix of pulmonary arterial walls.

METHODS

During 21-day hypoxia, rats were treated with sodium cromoglycate either in the first four days or in the last four days. The first four days were chosen to test efficiency of an initial pulse of cromoglycate to suppress pulmonary vascular remodeling. The last four-day administration tested possibility to block remodeling post hoc.

RESULTS

Initial pulse reduced and modified remodeling in all levels of pulmonary arteries, which comprises neomuscularization of prealveolar arteries, asymmetrical hypertrophy of tunica media in muscular pulmonary arteries and hypertrophy of tunica media and tunica adventitia in large conduit arteries. Terminal pulse had only negligible effect.

CONCLUSIONS

Only the initial cromoglycate therapy led to significant morphological suppression of remodeling. We therefore assume important role of initial remodeling influencing during long time hypoxia experiment.

May the fibrosis be with you: Is discoidin domain receptor 2 the receptor we have been looking for?

In a recent issue of Journal of Molecular and Cellular Cardiology, George et al. [1] identified discoidin domain receptor 2 (DDR2) as a positive modulator of collagen production in cardiac fibroblasts stimulated with angiotensin II (Ang II). DDR2 is a tyrosine kinase collagen receptor and is associated with pathological scarring of multiple organs; nevertheless, the functional role of DDR2 in the myocardium remains unclear. George et al. present evidence for the first time that Ang II induces cardiac fibrosis by enhancing DDR2 expression in cardiac fibroblasts via p38 mitogen activated protein kinase (p38 MAPK)-mediated activation of nuclear factor-κB (NF-κB).

Building a better infarct: Modulation of collagen cross-linking to increase infarct stiffness and reduce left ventricular dilation post-myocardial infarction

Matrix metalloproteinase-9 (MMP-9) deletion attenuates collagen accumulation and dilation of the left ventricle (LV) post-myocardial infarction (MI); however the biomechanical mechanisms underlying the improved outcome are poorly understood. The aim of this study was to determine the mechanisms whereby MMP-9 deletion alters collagen network composition and assembly in the LV post-MI to modulate the mechanical properties of myocardial scar tissue. Adult C57BL/6J wild-type (WT; n=88) and MMP-9 null (MMP-9(-/-); n=92) mice of both sexes underwent permanent coronary artery ligation and were compared to day 0 controls (n=42). At day 7 post-MI, WT LVs displayed a 3-fold increase in end-diastolic volume, while MMP-9(-/-) showed only a 2-fold increase (p<0.05). Biaxial mechanical testing revealed that MMP-9(-/-) infarcts were stiffer than WT infarcts, as indicated by a 1.3-fold reduction in predicted in vivo circumferential stretch (p<0.05). Paradoxically, MMP-9(-/-) infarcts had a 1.8-fold reduction in collagen deposition (p<0.05). This apparent contradiction was explained by a 3.1-fold increase in lysyl oxidase (p<0.05) in MMP-9(-/-) infarcts, indicating that MMP-9 deletion increased collagen cross-linking activity. Furthermore, MMP-9 deletion led to a 3.0-fold increase in bone morphogenetic protein-1, the metalloproteinase that cleaves pro-collagen and pro-lysyl oxidase (p<0.05) and reduced fibronectin fragmentation by 49% (p<0.05) to enhance lysyl oxidase activity. We conclude that MMP-9 deletion increases infarct stiffness and prevents LV dilation by reducing collagen degradation and facilitating collagen assembly and cross-linking through preservation of the fibronectin network and activation of lysyl oxidase.

Extracellular matrix turnover and signaling during cardiac remodeling following MI: causes and consequences

The concept that extracellular matrix (ECM) turnover occurs during cardiac remodeling is a well-accepted paradigm. To date, a multitude of studies document that remodeling is accompanied by increases in the synthesis and deposition of ECM components as well as increases in extracellular proteases, especially matrix metalloproteinases (MMPs), which break down ECM components. Further, soluble ECM fragments generated from enzymatic action serve to stimulate cell behavior and have been proposed as candidate plasma biomarkers of cardiac remodeling. This review briefly summarizes our current knowledge base on cardiac ECM turnover following myocardial infarction (MI), but more importantly extends discussion by defining avenues that remain to be explored to drive the ECM remodeling field forward. Specifically, this review will discuss cause and effect roles for the ECM changes observed following MI and the potential role of the ECM changes that may serve as trigger points to regulate remodeling. While the pattern of remodeling following MI is qualitatively similar but quantitatively different from various types of injury, the basic theme in remodeling is repeated. Therefore, while we use the MI model as the prototype injury model, the themes discussed here are also relevant to cardiac remodeling due to other types of injury.

Acute and chronic hypoxia as well as 7-day recovery from chronic hypoxia affects the distribution of pulmonary mast cells and their MMP-13 expression in rats

Chronic hypoxia results in pulmonary hypertension due to vasoconstriction and structural remodelling of peripheral lung blood vessels. We hypothesize that vascular remodelling is initiated in the walls of prealveolar pulmonary arteries by collagenolytic metalloproteinases (MMP) released from activated mast cells. Distribution of mast cells and their expression of interstitial collagenase, MMP-13, in lung conduit, small muscular, and prealveolar arteries was determined quantitatively in rats exposed for 4 and 20 days to hypoxia as well as after 7-day recovery from 20-day hypoxia (10% O2). Mast cells were identified using Toluidine Blue staining, and MMP-13 expression was detected using monoclonal antibody. After 4, but not after 20 days of hypoxia, a significant increase in the number of mast cells and their MMP-13 expression was found within walls of prealveolar arteries. In rats exposed for 20 days, MMP-13 positive mast cells accumulated within the walls of conduit arteries and subpleurally. In recovered rats, MMP-13 positive mast cells gathered at the prealveolar arterial level as well as in the walls of small muscular arteries; these mast cells stayed also in the conduit part of the pulmonary vasculature. These data support the hypothesis that perivascular pulmonary mast cells contribute to the vascular remodelling in hypoxic pulmonary hypertension in rats by releasing interstitial collagenase.

N-acetylcysteine inhibits hypoxic pulmonary hypertension most effectively in the initial phase of chronic hypoxia

Exposure to chronic hypoxia results in hypoxic pulmonary hypertension (HPH). In rats HPH develops during the first two weeks of exposure to hypoxia, then it stabilizes and does not increase in severity. We hypothesize that free radical injury to pulmonary vascular wall is an important mechanism in the early days of the hypoxic exposure. Thus antioxidant treatment just before and at the beginning of hypoxia should be more effective in reducing HPH than antioxidant therapy of developed pulmonary hypertension. We studied adult male rats exposed for 4 weeks to isobaric hypoxia (F(iO2) = 0.1) and treated with the antioxidant, N-acetylcysteine (NAC, 20 g/l in drinking water). NAC was given "early" (7 days before and the first 7 days of hypoxia) or "late" (last two weeks of hypoxic exposure). These experimental groups were compared with normoxic controls and untreated hypoxic rats (3-4 weeks hypoxia). All animals kept in hypoxia had significantly higher mean pulmonary arterial blood pressure (PAP) than normoxic animals. PAP was significantly lower in hypoxic animals with early (27.1 +/- 0.9 mmHg) than late NAC treatment (30.5 +/- 1.0 mmHg, P < 0.05; hypoxic without NAC 32.6 +/- 1.2 mmHg, normoxic controls 14.9 +/- 0.7 mmHg). Early but not late NAC treatment inhibited hypoxia-induced increase in right ventricle weight and muscularization of distal pulmonary arteries assessed by quantitative histology. We conclude that release of free oxygen radicals in early phases of exposure to hypoxia induces injury to pulmonary vessels that contributes to their structural remodeling and development of HPH.

Mechanics, nonlinearity, and failure strength of lung tissue in a mouse model of emphysema: possible role of collagen remodeling

Enlargement of the respiratory air spaces is associated with the breakdown and reorganization of the connective tissue fiber network during the development of pulmonary emphysema. In this study, a mouse (C57BL/6) model of emphysema was developed by direct instillation of 1.2 IU of porcine pancreatic elastase (PPE) and compared with control mice treated with saline. The PPE treatment caused 95% alveolar enlargement ( P = 0.001) associated with a 29% lower elastance along the quasi-static pressure-volume curves ( P < 0.001). Respiratory mechanics were measured at several positive end-expiratory pressures in the closed-chest condition. The dynamic tissue elastance was 19% lower ( P < 0.001), hysteresivity was 9% higher ( P < 0.05), and harmonic distortion, a measure of collagen-related dynamic nonlinearity, was 33% higher in the PPE-treated group ( P < 0.001). Whole lung hydroxyproline content, which represents the total collagen content, was 48% higher ( P < 0.01), and α-elastin content was 13% lower ( P = 0.16) in the PPE-treated group. There was no significant difference in airway resistance ( P = 0.7). The failure stress at which isolated parenchymal tissues break during stretching was 40% lower in the PPE-treated mice ( P = 0.002). These findings suggest that, after elastolytic injury, abnormal collagen remodeling may play a significant role in all aspects of lung functional changes and mechanical forces, leading to progressive emphysema.

Overexpression of Urokinase by Macrophages or Deficiency of Plasminogen Activator Inhibitor Type 1 Causes Cardiac Fibrosis in Mice

Several studies implicate elevated matrix metalloproteinase activity as a cause of cardiac fibrosis. However, it is unknown whether other proteases can also initiate cardiac fibrosis. Because absence of urokinase plasminogen activator (uPA) prevents development of cardiac fibrosis after experimental myocardial infarction in mice, we hypothesized that elevated activity of uPA or deficiency of the uPA inhibitor plasminogen activator inhibitor-1 (PAI-1) might cause cardiac fibrosis. We used mice with scavenger-receptor (SR)-directed, macrophage-targeted uPA overexpression (SR-uPA+/0 mice) and PAI-1 null mice to test these hypotheses. Our studies revealed that SR-uPA+/0 mice developed cardiac fibrosis beginning between 5 and 10 weeks of age. Fibrosis was preceded by cardiac macrophage accumulation, implicating uPA-secreting macrophages as important contributors to development of fibrosis. A key role for uPA-secreting macrophages in development of cardiac fibrosis was supported by experiments in which recipients of bone marrow transplants from SR-uPA+/0 donors but not nontransgenic donors developed cardiac macrophage accumulation and fibrosis. SR-uPA+/0 mice and recipients of SR-uPA+/0 bone marrow had neither macrophage accumulation nor fibrosis in other major organs despite the presence of higher levels of uPA in these organs than in hearts. PAI-1 null mice but not congenic, age-matched controls also developed macrophage accumulation and fibrosis in hearts but not in other organs. We conclude: (1) either elevated macrophage uPA expression or PAI-1 deficiency is sufficient to cause cardiac macrophage accumulation and fibrosis; (2) macrophages are important contributors to the development of cardiac fibrosis; and (3) the heart is particularly sensitive to the effects of excess uPA activity.

Metalloproteinase inhibition by Batimastat attenuates pulmonary hypertension in chronically hypoxic rats

Chronic hypoxia induces lung vascular remodeling, which results in pulmonary hypertension. We hypothesized that a previously found increase in collagenolytic activity of matrix metalloproteinases during hypoxia promotes pulmonary vascular remodeling and hypertension. To test this hypothesis, we exposed rats to hypoxia (fraction of inspired oxygen = 0.1, 3 wk) and treated them with a metalloproteinase inhibitor, Batimastat (30 mg/kg body wt, daily ip injection). Hypoxia-induced increases in concentration of collagen breakdown products and in collagenolytic activity in pulmonary vessels were inhibited by Batimastat, attesting to the effectiveness of Batimastat administration. Batimastat markedly reduced hypoxic pulmonary hypertension: pulmonary arterial blood pressure was 32 +/- 3 mmHg in hypoxic controls, 24 +/- 1 mmHg in Batimastat-treated hypoxic rats, and 16 +/- 1 mmHg in normoxic controls. Right ventricular hypertrophy and muscularization of peripheral lung vessels were also diminished. Batimastat had no influence on systemic arterial pressure or cardiac output and was without any effect in rats kept in normoxia. We conclude that stimulation of collagenolytic activity in chronic hypoxia is a substantial causative factor in the pathogenesis of pulmonary vascular remodeling and hypertension.

Matrix metalloproteinases in the progression of heart failure: potential therapeutic implications

Matrix metalloproteinases (MMPs) are a family of functionally related zinc-containing enzymes that denature and degrade fibrillar collagens and other components of the extracellular matrix. Myocardial extracellular matrix remodelling and fibrosis regulated by MMPs are believed to be important contributors to the progression of heart failure. The role of MMPs in cardiac fibrosis and the progression of heart failure, along with the possibility of halting the progression of heart failure by modulating extracellular matrix remodelling are important issues under intense study. MMPs are increased in the failing hearts of both animal models and patients with heart failure. MMP inhibition may therefore modulate extracellular matrix remodelling and the progression of heart failure. It is a great advantage that various MMP inhibitors have been developed initially for the treatment of cancer, arthritis and other diseases believed to be associated with increased MMP activity. Several preclinical studies have shown that treatment of heart failure in animal models with MMP inhibitors results in less collagen matrix damage, favourable extracellular matrix remodelling, and improved cardiac structure and function. The results suggest that modulation of MMP activity can prevent myocardial dysfunction and the progression of heart failure through alterations in the remodelling process of extracellular matrix and the left ventricle. Although these promising results suggest potential benefits of MMP inhibition for human heart failure, no clinical data evaluating MMP inhibitors in heart failure have been reported. As the preclinical evidence continues to grow and the potential of MMP inhibition for the treatment of heart failure continues to unfold, MMP inhibition may prove to be an effective treatment for heart failure.

UVA light stimulates the production of cathepsin G and elastase-like enzymes by dermal fibroblasts: a possible contribution to the remodeling of elastotic areas in sun-damaged skin

Solar elastosis is characterized by accumulation of large amounts of material staining similarly to elastin in the dermis. The nature of this material and the process responsible for its accumulation are still unknown. Elastolytic proteases have important functions in the catabolism of the interstitial matrix and can also generate, by the digestion of the interstitial proteins, soluble peptides which can induce collagen and elastin synthesis and deposition. We investigated whether (i) elastolytic enzymes can be detected in samples from sun-exposed and non-exposed skin, and (ii) ultraviolet (UV) rays influence the production of elastolytic activities in cultured dermal fibroblasts. Immunoelectron microscopy showed a positive reaction for neutrophil elastase and cathepsin G in fibroblast-like cells from specimens of sun-exposed areas. Little or no reaction was found in biopsies of sun-protected skin. Fibroblast cultures from sun-exposed skin expressed higher levels of hydrolytic activity against synthetic substrates of elastases and cathepsin G than those obtained from sun-protected areas. Irradiation with UVA strongly stimulated the production of these activities in fibroblasts from sun-protected sites. No significant change was detected in parallel sets of cultures after UVB irradiation. Inhibition experiments indicated that the elastase-like activity expressed by fibroblasts can be attributed to at least two enzymes.

Disruption of the myocardial extracellular matrix leads to cardiac dysfunction

MMP activity with disruption of structural collagen has been implicated in the pathophysiology of dilated cardiomyopathy. To examine the role of this enzyme in cardiac function, a transgenic mouse was created that constitutively expressed human collagenase (MMP-1) in the heart. At 6 months of age, these animals demonstrated compensatory myocyte hypertrophy with an increase in the cardiac collagen concentration due to elevated transcription of type III collagen. Chronic myocardial expression of MMP-1 produced loss of cardiac interstitial collagen coincident with a marked deterioration of systolic and diastolic function at 12 months of age. This is the first animal model demonstrating that direct disruption of the extracellular matrix in the heart reproduces the changes observed in the progression of human heart failure.

Role of nitric oxide in the pathogenesis of chronic pulmonary hypertension

Chronic pulmonary hypertension is a serious complication of a number of chronic lung and heart diseases. In addition to vasoconstriction, its pathogenesis includes injury to the peripheral pulmonary arteries leading to their structural remodeling. Increased pulmonary vascular synthesis of an endogenous vasodilator, nitric oxide (NO), opposes excessive increases of intravascular pressure during acute pulmonary vasoconstriction and chronic pulmonary hypertension, although evidence for reduced NO activity in pulmonary hypertension has also been presented. NO can modulate the degree of vascular injury and subsequent fibroproduction, which both underlie the development of chronic pulmonary hypertension. On one hand, NO can interrupt vascular wall injury by oxygen radicals produced in increased amounts in pulmonary hypertension. NO can also inhibit pulmonary vascular smooth muscle and fibroblast proliferative response to the injury. On the other hand, NO may combine with oxygen radicals to yield peroxynitrite and other related, highly reactive compounds. The oxidants formed in this manner may exert cytotoxic and collagenolytic effects and, therefore, promote the process of reparative vascular remodeling. The balance between the protective and adverse effects of NO is determined by the relative amounts of NO and reactive oxygen species. We speculate that this balance may be shifted toward more severe injury especially during exacerbations of chronic diseases associated with pulmonary hypertension. Targeting these adverse effects of NO-derived radicals on vascular structure represents a potential novel therapeutic approach to pulmonary hypertension in chronic lung diseases.

17beta-oestradiol enhances release of matrix metalloproteinase-2 from human vascular smooth muscle cells

Vascular remodelling occurs during all stages of atherosclerotic progression. Anti-atherosclerotic drugs may function by restoring regulation of the processes involved in remodelling of the extracellular matrix. A key group of enzymes involved in these processes are the matrix metalloproteinases (MMPs). Oestrogens have been demonstrated to possess anti-atherosclerotic properties at low concentrations while being associated with lesion formation at high concentrations. We examined the effect of 17beta-oestradiol on MMP-2 expression in human coronary artery (CAVSMC) and umbilical artery vascular smooth muscle cells (UAVSMC). MMP-2 expression was measured by chemiluminescent immunoblotting and quantified by laser densitometry. pro-MMP-2 was secreted by VSMCs and increasing levels of 17beta-oestradiol, from physiological through supraphysiological, were associated with significant dose-dependent increases in MMP-2 levels in culture media. This effect was dependent on de novo protein synthesis and could be antagonised by the oestrogen receptor antagonist, tamoxifen, and the specific receptor antagonist ICI 182, 780. 17beta-Oestradiol appears to be a specific stimulator of MMP-2 release from human vascular cells. The concentration dependence of this effect suggests a basis for the differential effects of low and high oestrogen levels on vascular integrity.

Exposure to chronic hypoxia induces qualitative changes of collagen in the walls of peripheral pulmonary arteries

Qualitative changes of vascular wall matrix collagens in chronic hypoxic pulmonary hypertension were studied by gel electrophoresis. Male adult rats (n = 12) were exposed to hypoxia (FiO2 = 0.1, 3 wks). Control rats (n = 13) were kept in air. Samples of peripheral pulmonary arteries (PPA, diam. 100-400 microm), main branches of pulmonary artery, and aorta were dissected. Arterial samples were treated with 4M guanidine-HCl to remove noncollagenous moieties and the collagenous stroma was dissolved by limited pepsin digestion at low pH. Low molecular mass peptides (M. W. approx. 76 and 66 kD) were detected in the gel electrophoretic profile of collagen peptides of PPA of the chronically hypoxic animals and in aorta of both hypoxic and normoxic groups. These peptides were absent in the PPA of normoxic rats. Since the 76 kD peptide bound anticollagen type I antibodies, it appears to be of collagenous nature and it may be the result of collagenolytic activity in PPA isolated from hypoxic lungs. This was confirmed by zymography. We conclude that exposure of rats to chronic hypoxia results in the presence of low molecular mass peptides in the wall matrix of PPA which resemble those found in aorta of normoxic animals. Collagenolytic activity in the walls of peripheral pulmonary arteries may participate in the mechanism of lung vascular remodelling in chronic hypoxia.

Elastin and collagen remodeling in emphysema. A scanning electron microscopy study

The relationship between elastin degradation and emphysema is well known. Recent evidence suggests that a complex process of pulmonary remodeling occurs within the emphysematous lung. The aim of this study was to assess the extent of extracellular matrix remodeling in emphysema by ultrastructural examination of elastin and collagen templates in an animal model of emphysema and in human emphysematous lungs. Emphysema was induced in rats by the intratracheal administration of porcine pancreatic elastase. Human lung samples were obtained at surgical resection for lung carcinoma. Emphysema was confirmed morphometrically and quantitated using the mean linear intercept. Matching sections were treated with sodium hydroxide and formic acid to expose collagen and elastin templates, respectively. Scanning electron microscopy with stereo-pair imaging allowed three-dimensional visualization of the exposed templates. In emphysematous lungs from both sources, sheets of elastin were disrupted and perforated with multiple fenestrations. In elastase-induced emphysema, this disintegration was accompanied by a marked increase in thickness of collagen fibrils, which contrasted with the fine fibrillar network of control lungs. Similarly, a pattern of thickened fibrils and disorganized deposition of collagen was observed in human lungs. In conclusion, these findings support the novel concept of increased collagen deposition and aberrant collagen remodeling in the pathogenesis of emphysema.

Ca2+ channel blockers modulate metabolism of collagens within the extracellular matrix

The extracellular matrix (ECM) is an intricate network composed of an array of macromolecules capable of regulating the functional responsiveness of cells. Its composition greatly varies among different types of tissue, and dysregulation of its metabolism may contribute to vascular remodeling during the pathogenesis of various diseases, including atherosclerosis. In view of their antiatherosclerotic effects, the role of Ca2+ channel blockers in the metabolism of ECM was examined. Nanomolar concentrations of the five Ca2+ channel blockers amlodipine, felodipine, manidipine, verapamil, or diltiazem significantly decreased both the constitutive and platelet-derived growth factor BB-dependent collagen deposition in the ECM formed by human vascular smooth muscle cells and fibroblasts. The drugs inhibited the expression of fibrillar collagens type I and III and of basement membrane type IV collagen. Furthermore, Ca2+ channel blockers specifically increased the proteolytic activity of the 72-kDa type IV collagenase as shown by gelatin zymography and inhibited the transcription of tissue inhibitor of metalloproteinases-2.