Regulation of transbilayer distribution of a fluorescent sterol in tumor cell plasma membranes

Abstract

The lipid composition and transbilayer distribution of plasma membrane isolated from primary tumor (L-929, LM, A-9 and C 3H) and nine metastatic cell lines cultured under identical conditions was examined. Cultured primary tumor and metastatic cells differed two-fold in sterol/phospholipid molar ratios. There was a direct correlation between plasma membrane anionic phospholipid (phosphatidylinositol and phosphatidylserine) content and plasma membrane sterol/phospholipid ratio. This finding may bear on the possible link between oncogenes and inositol lipids. The fluorescent sterol, dehydroergosterol, was incorporated into primary tumor and metastatic cell lines. Selective quenching of outer monolayer fluorescence by covalently linked trinitrophenyl groups demonstrated an asymmetric transbilayer distribution of sterol in the plasma membranes. The inner monolayer of the plasma membranes from both cultured primary and metastatic tumor cells was enriched in sterol as compared with the outer monolayer Consistent with this, the inner monolayer was distinctly more rigid as determined by the limiting anisotropy of 1,6-diphenyl-l,3,5-hexatriene. Dehydroergosterol fluorescence was temperature dependent and sensitive to lateral phase separations in phosphatidylcholine vesicles and in LM cell plasma membranes. Dehydroergosterol detected phase separations near 24°C in the outer monolayer and at 21°C and 37°C in the inner monolayer of LM plasma membranes. Yet, no change in transbilayer sterol distribution was detected in ascending or descending temperature scans between 4 and 45°C. Alterations in plasma membrane phospholipid polar head group composition by choline analogues ( N,N- dimethylethanolamine, N- methylethanolamine , and ethanolamine) also did not perturb transbilayer sterol asymmetry. Treatment with phenobarbital or prilocaine, drugs that selectively fluidize the outer and inner monolayer of LM plasma membranes, respectively, did not change dehydroergosterol transbilayer distribution.