Cholesterol content but not plasma membrane fluidity influences the susceptibility of L1210 leukemia cells to merocyanine 540-sensitized irradiation.

This paper examines the relationship between lipid composition, plasma membrane fluidity, expression of dye binding sites, and susceptibility to merocyanine 540 (MC540)-sensitized irradiation in L1210 leukemia cells. Reducing the cells' cholesterol content by exchange diffusion with phosphatidylcholine liposomes or by inhibiting its biosynthesis with 25-hydroxycholesterol enhanced plasma membrane fluidity, the expression of dye binding sites, and the cells' susceptibility to MC540-sensitized irradiation. Conversely, if the cholesterol content was enhanced by exchange diffusion with cholesterol:phosphatidylcholine liposomes, the cells' susceptibility to MC540-sensitized irradiation was decreased. However, contrary to expectations, dye-binding was slightly enhanced and plasma membrane fluidity remained unchanged. Growing the cells in fatty acid-supplemented medium had profound effects on their lipid composition. Cells enriched in polyunsaturated fatty acids had more fluid plasma membranes. However, dye-binding was not significantly affected and photosensitivity was slightly reduced. These results suggest that cholesterol is one, but probably not the only, determinant of the expression of cellular dye binding sites and, consequently, the cell's susceptibility to MC540-sensitized irradiation. By contrast, plasma membrane fluidity does not appear to play a major role in the regulation of dye-binding site expression.