An Elevated Level of Cholesterol Impairs Self-Assembly of Pulmonary Surfactant into a Functional Film

Abstract

In adult respiratory distress syndrome, the primary function of pulmonary surfactant to strongly reduce the surface tension of the air-alveolar interface is impaired, resulting in diminished lung compliance, a decreased lung volume, and severe hypoxemia. Dysfunction coincides with an increased level of cholesterol in surfactant which on its own or together with other factors causes surfactant failure. In the current study, we investigated by atomic force microscopy and Kelvin-probe force microscopy how the increased level of cholesterol disrupts the assembly of an efficient film. Functional surfactant films underwent a monolayer-bilayer conversion upon contraction and resulted in a film with lipid bilayer stacks, scattered over a lipid monolayer. Large stacks were at positive electrical potential, small stacks at negative potential with respect to the surrounding monolayer areas. Dysfunctional films formed only few stacks. The surface potential of the occasional stacks was also not different from the surrounding monolayer. Based on film topology and potential distribution, we propose a mechanism for formation of stacked bilayer patches whereby the helical surfactant-associated protein SP-C becomes inserted into the bilayers with defined polarity. We discuss the functional role of the stacks as mechanically reinforcing elements and how an elevated level of cholesterol inhibits the formation of the stacks. This offers a simple biophysical explanation for surfactant inhibition in adult respiratory distress syndrome and possible targets for treatment.

Interfacial behavior and structural properties of a clinical lung surfactant from porcine source

“…Surfacen is obtained from the organic extract of lipid/protein surfactant complexes pelleted from porcine bronchoalveolar lavage, once subjected to acetone precipitation to reduce the proportion of cholesterol to less than 20-25% of the original content of cholesterol in the source material. Considering the method of production and the lipid and protein composition, Surfacen could be considered a porcine version of BLES, a clinical surfactant obtained from bovine lung lavage, which has been extensively characterized [37][38][39][40]. Considering the starting material and the production method of Surfacen and BLES, these natural preparations should contain significant proportions of surfactant proteins SP-B and SP-C, the main protein components responsible for the formation and the dynamic properties of functional surfactant films at the interface [2,29].…”

“…It has been demonstrated that the presence of physiological proportions of cholesterol modulates the lateral segregation of ordered and disordered lipid phases in surfactant membranes [13] and films [41,42], as well as the dynamic behavior of phospholipid species in terms of their diffusion coefficient [12]. On the other hand exacerbated amounts of cholesterol have shown deleterious effects on surfactant interfacial activity [37,39], which motivated the removal of a main fraction of the cholesterol originally present in the natural materials used as a source of practically all the clinical surfactants currently in use [7]. To our knowledge, this is one of very few studies in which the structure and functional properties of a clinical surfactant has been compared in detail with those of the material from which it is derived.…”

Effect of Serum, Cholesterol and Low Density Lipoprotein on the Functionality and Structure of Lung Surfactant Films

“…These trends were consistent with a change in conformation of the lipids as a function of temperature, where an increased fluidity of the hydrocarbon chain or gauche conformation had been observed at higher temperatures. At lower temperatures [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] however, the hydrocarbon…”

“…Previous studies have shown that cholesterol is an integral component of LS, found between 7-15 in various mammalian systems 9,10,39,43,44 . However, the physiological role of cholesterol is not clear to date, since the source of the material as well as its delivery route in LS has not been firmly established.…”

Activation of Sterol-response Element-binding Proteins (SREBP) in Alveolar Type II Cells Enhances Lipogenesis Causing Pulmonary Lipotoxicity

“…Surfactant PC is relatively enriched in saturated species. Although cholesterol is present in varying quantities in mammalian surfactants, excess cholesterol impairs the surface tension reducing properties of surfactants (8). Triacylglycerols, an important storage form of lung lipids, are normally present in small amounts in alveolar tissues and are thought to serve as substrates for phospholipid synthesis by alveolar type 2 cells during development (9).…”