Surface properties and behavior of lipid extracts from plasma membranes of cells cultured as monolayer and in tissue-like conditions.

The differences in the surface active properties of native lipids extracted from plasma membranes of cells cultured as a monolayer and in three-dimensional (3D) matrix were investigated. This experimental model was chosen because most of the current knowledge on cellular physiological processes is based on studies performed with conventional monolayer two-dimensional (2D) cell cultures, where cells are forced to adjust to unnaturally rigid surfaces that differ significantly from the natural matrix surrounding cells in living organisms. Differences between monolayer and 3D cells were observed in the lipid composition of plasma membranes and especially in the level of the two major microdomain-forming lipids--sphingomyelin (SM) and cholesterol, which were significantly elevated in 3D cells. The obtained results showed that culturing of cells in in vivo-like environment affected the surface active properties of plasma membrane lipids at interfaces which might influence certain membrane-associated interface processes. The detected differences in the lipid levels in 2D and 3D cell extracts affected significantly the behavior of the model lipid monolayers at the air-water interface (Langmuir monolayers) which resulted in different values of the monolayer equilibrium (gamma(eq)) and dynamic (gamma(max), gamma(min)) surface tension and surface potential. Compensation of the SM content in extracts of 2D cell cultures up to a level close to the one measured in 3D cells approximated the monolayer properties to the values observed for 3D cells. These results implied that the interactions between the cells and the surrounding medium affected the level of plasma membrane SM and other lipids, which had a strong impact on the surface properties of lipid monolayers, such as gamma(eq), gamma(max), and gamma(min), the compression/decompression curve shape, the hysteresis area during cycling of the monolayers, etc. We suggest that the elevated content of SM observed in plasma membranes of 3D fibroblasts could be responsible for an increased rigidity and possibly reduced permeability of cells cultured in 3D environment. The current results provide useful information that should be taken into account in the interpretation of the membrane physico-chemical properties of cells cultured under different conditions.