Altered phospholipid composition affects endocytosis in cultured LM fibroblasts

The phospholipid polar head group composition of LM fibroblast membranes was altered by growing the cells in a chemically defined, serum-free medium containing choline, N,N'-dimethylethanolamine, N-monomethylethanolamine, or ethanolamine. The cells incorporated these bases into their membrane phospholipid such that 29-40% of the total plasma membrane phospholipids contained these polar head groups. Alteration of the phospholipid composition correlated with a depression of polystyrene bead phagocytosis by 36, 55 and 85% when the cells had been supplemented with N,N'-dimethylethanolamine, N-monoethylethanolamine, or ethanolamine, respectively. Pinocytotic uptake of horseradish peroxidase was depressed 44, 39, and 32%, respectively. The phagosomal membrane phospholipid composition qualitatively resembled that of the primary plasma membrane from which it was derived. However, enrichment of phosphatidylcholine, and other quantitative differences were noted in the phagosomal membranes as compared to the parent primary plasma membrane. Approx. 50% of the phagosomal membrane's phosphatidylethanolamine was accessible to the chemical labelling reagent trinitrobenzenesulfonate at 4 degrees C. The asymmetric distribution of phosphatidylethanolamine across the phagosomal membrane did not appear to be altered by base analogues except in the case of phagosomes from cells supplemented with ethanolamine. The data were consistent with a nonrandom site for endocytosis with regard to phospholipid composition.