Sphingolipids in Ventilator-Induced Lung Injury
thesisposted on 27.02.2015 by Vidyani Suryadevara
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Mechanical forces acting in various organs in the body affect the homeostasis and normal functioning. Lung is one of the most dynamic organs in the human body that is subjected to various physical and mechanical cues inferred from blood flow, breathing and surface tension; right from developmental stages and throughout life. Mechanical deformation of the lung tissue due to various forces acting on different regions of the lung, leads to over distention of the lung, resulting in a series of mechanosensing and mechanotransduction events at the cellular and organ level. This study aims to investigate the effect of mechanical stress on sphingolipid metabolism in the lungs, particularly in the alveolar epithelium, with the goal to better understand the molecular mechanisms that occur during ventilator - induced lung injury (VILI) and to identify new targets for therapy. Sphingolipids have been implicated in various lung pathologies. However, little is known about the role of sphingolipids in VILI. A rodent model of ventilator induced lung injury and in vitro model of alveolar epithelial cells subjected to pathophysiological mechanical stress were used. Mechanical ventilation at high tidal volume (30 ml/kg, 4 hrs) in mice enhanced S1P lyase (S1PL) expression, elevated ceramide levels, decreased sphingosine-1-phosphate (S1P) levels in lung tissue, thereby leading to cytoskeletal rearrangement, lung inflammation and injury and apoptosis. Accumulation of S1P in cells is a balance between its synthesis catalyzed by sphingosine kinase (SphK) 1 and 2 and catabolism mediated by S1P phosphatases and S1PL. Thus, the role of S1P Lyase and SphK1 in VILI was investigated by studies using S1PL+/- and SphK1-/- mice. Partial genetic deletion of S1P L (Sgpl1+/-) partly protected VILI in mice. On the other hand, the genetic deletion of SphK1 enhanced VILI in mice. Simulating VILI in the laboratory requires stretching of lung alveolar tissue. In vitro cyclic stretch of the cells mimics the constant collapse and reopening of the alveoli during mechanical ventilation. Mechanical cyclic stretch of human lung microvascular endothelial cells (HLMVEC) and murine alveolar type II epithelial (MLE12) cells increased S1P Lyase protein expression. Also, the pathophysiological levels of cyclic stretch i.e. 18% stretch stimulated epithelial cell apoptosis, disrupting barrier function and differential expression of various sphingoid bases, when compared to physiological relevant magnitude of stretch i.e. 5% cyclic stretch. Pre-treatment of alveolar epithelial cells with S1PL inhibitor 4-DP effectively promoted cell barrier recovery and minimized cell apoptosis caused by 18% cyclic stretch, suggesting the regulatory roles of S1P in VILI. The in vivo and in vitro results identify a novel role for intracellularly generated S1P in protection against VILI and suggest S1PL as a potential therapeutic target.