Effect of low-density lipoprotein on endothelial cell membrane fluidity and mononuclear cell attachment

To determine the effects of prolonged low-density lipoprotein (LDL) exposure in vitro on cultured endothelial cell (EC) lipid dynamics and cellular function, human umbilical vein ECs were incubated in LDL concentrations [cholesterol (Chol) = 240 mg/dl] associated with the premature development of atherosclerosis. After 4 days of incubation, cells were examined for changes in cellular lipid composition and for membrane fluidity. Results indicate that LDL-EC have increased Chol content (control EC vs. LDL-EC = 22.4 ± 5.26 vs. 38.9 ± 0.24 nmol/10⁶ cells, P < 0.05) and cellular Chol-to-phospholipid ratio (0.61 ± 0.10 vs. 1.21 ± 0.10 mol/mol, P < 0.05). Augmentation of EC Chol content was accompanied by a marked decrease in EC cellular membrane fluidity as assessed by fluorescence polarization (anisotropy, r values, 0.172 ± 0.019 vs. 0.226 ± 0.014, P < 0.0001). LDL-induced changes in EC lipid dynamics were associated with enhanced EC binding of monocytes (P < 0.05) and U937 cells (P < 0.01). Both LDL-induced decreases in membrane fluidity and enhanced attachment of mononuclear cells were reversed to control levels following a 2-min incubation of LDL-EC with the membrane mobility agent, A₂C. These data therefore suggest that LDL-induced modulations in lipid dynamics play an important role in perturbation of EC function.