Effect of nitrogen dioxide on surface membrane fluidity and insulin receptor binding of pulmonary endothelial cells

Nitrogen dioxide (NO2), an environmental oxidant pollutant, is known to peroxidize membrane lipids of lung cells. We evaluated the ability of NO2 to alter the surface membrane fluidity, lipid composition, and insulin receptor binding of porcine pulmonary artery endothelial cells in culture. After 3- to 24-hr exposure to 5 ppm NO2, cells were labeled with either 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH), a cationic fluorescent aromatic hydrocarbon that anchors at the lipid-water interface, or fluorescamine, a fluorescent molecular probe that covalently binds with amino groups of surface phospholipids and proteins. Membrane fluidity was measured by monitoring changes in the steady-state fluorescence anisotropies (rs) for TMA-DPH and fluorescamine. Insulin specific receptor binding was monitored by measuring time-dependent binding of 125I-insulin. Following NO2 exposure, rs values for TMA-DPH and fluorescamine were increased significantly in a time-dependent fashion, with maximum increases at 24 hr (P less than 0.001). Similar increases in rs values were observed in isolated plasma membranes as well as in lipid vesicles prepared from total lipid extracts of endothelial cells or their plasma membranes. Phosphatidylethanolamine plus phosphatidylserine content in lipid extracts from 24-hr but not 3- to 12-hr NO2-exposed cells was increased significantly (P less than 0.01) compared to control cells. Specific binding of 125I-insulin to cells exposed to NO2 for 12 and 24 hr (but not 3 and 6 hr) was reduced significantly (P less than 0.05) compared to binding in control cells. Scatchard analysis of the binding data indicated that NO2 exposure caused a 5-fold reduction in insulin receptor binding sites in endothelial cells. Recovery was achieved 24 hr after NO2 exposure with, but not without, changing culture medium. These results indicate that NO2 exposure causes reversible changes in the physical state of lipids in the superficial lipid domains of the pulmonary endothelial cell plasma membrane, and these alterations may interfere with plasma membrane-dependent functions such as receptor-ligand interaction.