We have investigated the effects of glucocorticoid treatment on lipid lateral diffusion and cholesterol:phospholipid ratios in the plasma membranes of the lipopolysaccharide (LPS)-responsive murine B-cell leukemia (BCL1). Exposure to LPS for 24 h at concentrations of 50 μg/ml, 100 μg/ml, and 250 μg/ml caused a 42% reduction in the lateral diffusion of the lipid probe 3,3′-dioctadecylindocarbocyanine iodide as measured by fluorescence photobleaching recovery techniques at 37°C. In cells incubated with 50 μg/ml LPS, 3,3′-dioctadecylindocarbocyanine iodide diffusion is reduced to 2.2 × 10-9 cm2s-1 compared to freshly isolated BCL1 cells where 3,3′-dioctadecylindocarbocyanine iodide diffused at a rate of 3.8 × 10-9 cm2s-1. In BCL1 cells activated by LPS for 24 h and recultured with a pharmacological concentration of the synthetic glucocorticoid triamcinolone acetonide (TA, 10-6 m) for 6 h, lipid lateral diffusion increased to 3.5 × 10-9 cm2s-1. Concentrations of TA lower than 10-6 m had no effect on lipid lateral diffusion. Six-h treatment with 10-6 m TA had no effect on freshly isolated BCL1 cells (Time 0). However, lipid lateral diffusion increased in cells incubated 24, 48, and 72 h with LPS, and an additional 6 h with 10-5 m TA suggested that activated BCL1 cells were more glucocorticoid sensitive than cells at Time 0. Plasma membrane cholesterol and phospholipid content, analyzed at 0 and 24 h, indicated that LPS activation was associated with a 7% increase in the cholesterol:phospholipid ratio in BCL1 membranes and that glucocorticoid treatment of these LPS-activated cells for 6 h decreased the membrane cholesterol:phospholipid ratio perhaps through inhibition of de novo cholesterol synthesis.
This work was supported by NIH AI-21873 (B. G. B.), by NIH HD 23236 (D. A. R.), and by the United States Department of Agriculture Animal Health and Disease program.