Interaction of serum lipids with lung surfactant as models of acute respiratory distress syndrome (ARDS)

Hillier, Ashley Michelle (2012) Interaction of serum lipids with lung surfactant as models of acute respiratory distress syndrome (ARDS). Masters thesis, Memorial University of Newfoundland.

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Abstract

Lung surfactant (LS) is a lipid-protein mixture secreted by type 11 alveolar cells to the air-water interface of the alveoli. LS stabilizes the alveoli and prevents lung collapse during respiration. This occurs by surfactant forming a surface active film at the interface to lower the surface tension (γ) to very low values (~1 mN/m) at end expiration. Under certain conditions, LS can become dysfunctional. This is seen during acute respiratory distress syndrome (ARDS) and lung injury, among other lung diseases. During ARDS, serum components, possibly soluble serum proteins and lipids, or a combination of both, can leak into the alveolar space and thereby inactivate LS. Previously it was found in our laboratory that serum, as a whole entity, was two hundred times more potent an inhibitor than its individual protein components. This could suggest that there are possibly other components of serum than soluble proteins which are potent inactivators of LS. -- Interactions of a bovine lipid extract surfactant (BLES) with varying amounts of whole fetal calf serum (FCS), cholesterol, and low density lipoprotein (LDL) were studied using several biophysical techniques to examine serum lipid components and their effects on the structure and function of BLES in films and bilayers. BLES contains all lipids and hydrophobic proteins of LS except neutral lipids (like cholesterol) and the hydrophilic surfactant proteins A and D. This allowed us to study the effects of normal and excess cholesterol, lipoproteins and other serum lipidic materials on this surfactant system. Mass spectrometry and lipid Iatroscan showed LDL and serum lipid fraction to contain significant amounts of cholesterol-ester and lipids not found in BLES. From our adsorption studies both serum and low density lipoprotein (LDL) hindered the ability of BLES bilayer dispersions to rapidly form films at the air-water interface. However, low physiological amounts (5-10%) of cholesterol did not disturb the rate of adsorption of BLES. Langmuir-Blodgett films were studied using a Langmuir-Wilhelmy balance to study surface activity and structure of such adsorbed films. All serum lipid components except physiological concentrations of cholesterol (5-10% wt) significantly decreased the surface activity of the films. -- Images of these films obtained by atomic force microscopy (AFM), suggested that normal amounts of cholesterol formed gel-like condensed domains until compressed to a low γ. This domain formation was abolished at higher cholesterol concentrations. Whole serum, high cholesterol and LDL prevented BLES domain formation as well as γ reduction and in fact produced non-gel domains. Bilayer dispersions of such samples (used to form the adsorbed films), using Raman spectroscopy, suggested that serum components altered the bond vibrational modes of the fatty acyl chains (C-H, C-C) of BLES phospholipids, compared 10 serum proteins alone. The bilayer studies suggested that serum lipids components affected the fluidity and lipid packing of BLES and thereby possibly altered molecular arrangements and packing of films formed from such bilayer. The study suggests a molecular model of serum lipid component as an inactivator of LS in ARDS. -- The monolayer and bilayer studies suggested that these lipid components are far more potent inhibitors of LS than their protein counterparts and are able to disturb lipid packing and change structure and surface activity of LS.

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