Uptake of liposomal phosphatidylcholine by granular pneumocytes in primary culture

Reuptake of pulmonary surfactant phospholipids was investigated with rat granular pneumocytes in primary culture and L-alpha-[2-palmitoyl-9,10-3H]dipalmitoyl phosphatidylcholine:egg phosphatidylcholine:phosphatidylglycerol:cholesterol (10:5:2:3, mol/mol) liposomes. Uptake of liposomal phosphatidylcholine by granular pneumocytes increased with time and concentration of phosphatidylcholine in the medium. With 150 microM phosphatidylcholine, uptake was about 5 nmol/mg cell protein in 2 h. Phosphatidylcholine uptake was in large part due to net transfer of vesicles as indicated by uptake of [14C]sucrose encapsulated in the aqueous compartment of liposomes. Using dipalmitoyl phosphatidylcholine:cholesterol (1:1) liposomes, uptake was inhibited significantly at 26 degrees C and completely at 4 degrees C. Inhibitors and uncouplers of oxidative phosphorylation had no effect on uptake although uptake was somewhat inhibited in the presence of either 5.6 mM 2-deoxy-D-glucose or 10 mM sodium fluoride. Cell-associated lipid radioactivity decreased after treatment with 0.25% trypsin. The percent radioactivity that was trypsin releasable decreased with increasing time and phosphatidylcholine concentration. The results suggest that uptake of phospholipids by these cells is by surface binding followed by internalization. After 2 h of incubation, 65.3 +/- 3.1% of the cell-associated radioactivity was present in phosphatidylcholine, a small fraction in phosphatidylglycerol, and the remainder in lysophosphatidylcholine, free fatty acids, and other neutral lipids, suggesting metabolic degradation of internalized lipids. This process of phospholipid uptake and degradation may have a physiological role in metabolism of surfactant phospholipids in the lung.