Phospholipids co-isolated with rat surfactant protein C account for the apparent protein-enhanced uptake of liposomes into lung granular pneumocytes.

Phosphatidylethanolamine (PE) and phosphatidylglycerol (PG) were co-isolated with the low molecular weight rat surfactant-associated protein C (SP-C) of Mr approximately equal to 6,000. The contribution of these phospholipids to the incorporation of 3H-labeled phosphatidylcholine (PC) liposomes into rat alveolar type II cells stimulated by SP-C was examined. PG showed a concentration-dependent enhancement in the uptake of PC liposomes by the pneumocytes. PE alone had no effect but could inhibit the incorporation of liposomal PC stimulated by PG depending on the concentration of PG and the PG to PE ratio. SP-C augmented the cellular uptake of the PC liposomes only when the SP-C preparation had a protein to phospholipid ratio greater than 1 and a PG to PE ratio greater than 2. The results with the isolated SP-C could be reproduced using mixtures of PG and PE which reflected the phospholipid composition of the SP-C in the absence of SP-C protein. Thus, the ability of SP-C to stimulate liposomal PC uptake by rat type II cells could be accounted for by its phospholipid composition.