Role of the extracellular matrix in the genesis of ventilator-induced lung injury
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The extracellular matrix represents the three-dimensional scaffold of the alveolar wall, which is composed of a layer of epithelial and endothelial cells, their basal membrane, and a thin interstitial layer containing fibrous proteins, glycoproteins, glycosaminoglycans, and proteoglycans. Mechanical ventilation with low and high tidal volumes can induce proteoglycan fragmentation, which may cause activation of the inflammatory cascade, leading to the main features of ventilator-induced lung injury (VILI): alveolar edema and collagen deposition. The purpose of this article is to describe VILI pathophysiology with a special focus on the effects of mechanical ventilation on the extracellular matrix. A more complete understanding of the molecular effects induced by physical forces is required to better assess the impact of existing mechanical ventilation strategies, as well as to develop new therapeutic strategies to reduce lung damage.
KeywordsExtracellular matrix Ventilator-induced lung injury Mechanical ventilation Respiration, artificial Pulmonary alveoli
Rolle der extrazellulären Matrix in der Entstehung des beatmungsinduzierten Lungenschadens
Die extrazelluläre Matrix bildet das dreidimensionale Gerüst der Alveolenwand, die aus einer Schicht von Epithel- und Endothelzellen, ihrer Basalmembran und einem dünnen Interstitium mit Faserproteinen, Glykoproteinen, Glykosaminoglykanen und Proteoglykanen besteht. Die Beatmung mit niedrigen und hohen Atemzugvolumina kann eine Fragmentierung der Proteoglykane auslösen, die wiederum eine Aktivierung der Entzündungskaskade bewirken kann. In der Folge entwickeln sich die Hauptmerkmale des beatmungsinduzierten Lungenschadens: alveoläres Ödem und Kollagenablagerung. Im vorliegenden Beitrag soll die Pathophysiologie des beatmungsinduzierten Lungenschadens erläutert werden. Ein Schwerpunkt liegt dabei auf den Auswirkungen der Beatmung auf die extrazelluläre Matrix. Wir müssen die molekularen Effekte physikalischer Kräfte genauer verstehen, wenn wir die Auswirkungen der verfügbaren Beatmungsverfahren gründlicher bestimmen und neue Therapiestrategien zur Verringerung von Lungenschäden entwickeln wollen.
SchlüsselwörterExtrazelluläre Matrix Beatmungsinduzierter Lungenschaden Mechanische Beatmung Künstliche Beatmung Lungenalveolen
Alveolar wall integrity is crucial for the adequate maintenance of gas exchange between blood and air. It also provides a mechanical framework, protects from fluid overload, and enables interactions between different cells .
The extracellular matrix organization
The ECM during mechanical ventilation
Mechanical ventilation has become an indispensable tool in medicine, facilitating general anesthesia and supporting life in critically ill patients. Compared to spontaneous breathing, controlled mechanical ventilation presents reversed distribution of intrathoracic pressures, increased transpulmonary pressure, inhomogeneous distribution of ventilation, and reduced lymphatic drainage, which altogether can promote extracellular matrix rupture and progressive lung injury .
Some authors have shown that ventilation under physiologic tidal volumes (6–8 ml/kg) for a few hours induces proteoglycan fragmentation from the alveolar septa, which is not associated with activation of systemic or tissue MMPs, lung edema, inflammation, or collagen fiber deposition [2, 6].
The ECM and ventilator-induced lung injury
It is well known that mechanical ventilation can cause lung damage, which is known as ventilator-induced lung injury (VILI) . Several studies have shown that mechanical ventilation with low tidal volumes (VT) may cause damage to the peripheral airways and interstitium in healthy lungs [8, 9], due to abnormal stresses derived from cyclic opening and closing of peripheral airways at low end-expiratory lung volume, a phenomenon known as atelectrauma [8, 9]. On the other hand, excessive inspiratory airway pressure with increased transpulmonary pressures is associated with overdistension and mechanical stress, known as barotrauma, which can in turn cause damage to the ECM and endothelial and epithelial cells, resulting in an exacerbated inflammatory response and/or inactivation of surfactant (Fig. 1b, c, ).
Fragmented PGs modify the pulmonary inflammatory response and leukocyte migration through several mechanisms, including (1) the ability of soluble PGs or GAG fragments to activate Toll-like receptor signaling pathways and downstream secretion of pro-inflammatory mediators; (2) their highly specific binding, sequestration, and interaction with cytokines, chemokines, and growth factors; (3) the ability of PGs and hyaluronan to facilitate leukocyte adhesion and sequestration; and (4) the interactions between PGs and matrix MMPs, which alter the function of these proteases [2, 4]. Therefore, fragmentation of GAGs and breakdown of PGs may have an impact on the development of the inflammatory response seen in VILI. In this context, PG levels in blood have been analyzed in the search for a potential early marker of injury. Endocan, a DS-PG synthesized and secreted by endothelial cells, is metabolized mainly by proteases throughout the lung and by inflammatory cells. Endocan levels are increased in the serum of patients with ARDS, pneumonia, and septic shock, and any increase of this PG in serum is a surrogate marker of worsening clinical course towards organ dysfunction [17, 18]. Thus, analysis of PG fragments from the basal membrane of the alveolar epithelium and endothelium and from the interstitium might be helpful in the near future as early phase markers of lung injury in previously uninjured or injured lungs.
There is increasing evidence that cyclic mechanical stress affects the release and activation of MMPs and plays an important role in the regulation of ECM remodeling. MV with physiological VT induces increased expression and activity of MMP-2 and MMP-9 over time , while high VT induces increased expression and release of MMPs by macrophages and epithelial and endothelial cells in response to mechanical stress . Moreover, Foda et al. observed that MMPs play an important role in the development of VILI in an in vivo rat model of high-volume ventilation . In this context, some groups have been studying treatment strategies that inhibit MMP activity, with promising results .
Matrikines, peptides originating from the fragmentation of ECM proteins, should also be considered to explain part of VILI pathophysiology. [2, 20]. Elastin matrikines can lead to expression and activation of MMP-1 and MMP-3 in human fibroblasts (which, in turn, worsens lung damage) and increase MMP-2 and MMP-3 (which, in turn, degrade more elastin). Thus, breakdown of ECM proteins caused by high VT results in an exposure of matrikines, which can then act as ligands to induce subsequent biologic effects, including remodeling and inflammatory processes .
Mechanical stretch can modify the gene expression of several molecules associated with remodeling and inflammatory processes [21, 22, 23]. Ventilation with high VT and airway pressure, as well as transpleural gradients, initiates ECM remodeling . Berg et al. found that high lung inflation due to high PEEP levels increased mRNA expression of procollagen, fibronectin, and profibrogenic growth factors . Parker et al. found that ventilation with high peak airway pressures and low perfusion pressures led to greater type III procollagen mRNA expression than in unperfused lungs . Garcia et al. demonstrated that the increase in the tissue stress induced by oscillation force, but not amplitude, increases type III procollagen mRNA expression in rat lung parenchymal strips . Furthermore, Farias et al. showed that lung mechanical stretch (40 cmH2O of continuous positive airway pressure), even during a short period (40 s), results in type III procollagen mRNA expression . Along these lines, De Carvalho et al. showed that ventilator strategies associated with overdistension caused an increase in type III procollagen expression, mainly in non-dependent regions, a response that was partially attenuated by prone positioning . This suggests an effect of regional transpleural pressures that emerged due to lung heterogeneity in the context of VILI.
The primary event during protective mechanical ventilation is GAG degradation/remodeling, which activates lung injury and inflammatory response. During mechanical ventilation, reversed distribution of intrathoracic pressures, increased transpulmonary pressure, inhomogeneous distribution of ventilation, and reduced lymphatic drainage can, together, promote ECM rupture and progressive lung injury. Damage to pulmonary GAGs is worse during controlled MV as compared to spontaneous breathing, mainly in the dependent regions of the lung. PEEP is protective on pulmonary GAGs during MV, independent of tidal volume. Excessive administration of fluids can induce ECM injury. Further research is required to elucidate the mechanisms of VILI and the role of ECM in this mode of lung injury.
Publication costs were funded by the Department of Surgical Sciences and Integrated Diagnostics (DISC), University of Genoa, Genoa, Italy. This study was supported by the Carlos Chagas Filho Rio de Janeiro State Research Foundation (FAPERJ; grant number E‑26/103.118/2014), Rio de Janeiro, Brazil; and the Brazilian Council for Scientific and Technological Development (CNPq; grant number 471438/2012-0, 469716/2014-2, 465064/2014-0, and 400462/2014-1 to PRMR), Brasília, Brazil.
Conflict of interest
F. F. Cruz, P. R. M. Rocco and P. Pelosi declare that they have no competing interests.
The supplement containing this article is not sponsored by industry.
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