Surfactant protein A regulates uptake of pulmonary surfactant by lung type II cells on microporous membranes.

We have investigated the internalization of surfactant by type II cells in primary culture. Previously, we demonstrated that type II cells cultured on microporous membranes [Transwell membranes (TM)] maintained the morphological characteristics of lung pneumocytes and took up surfactant protein and phospholipid in a fashion similar to that described for the uptake of surfactant by whole lung. In the present study, cells cultured on TM and exposed to equivalent amounts of phospholipid (PL) as either natural surfactant or liposomes incorporated fivefold greater amounts of natural surfactant PL. The evidence supported an important role for surfactant protein A (SP-A), as the incorporation of surfactant into type II cells on TM was reduced by anti-SP-A antisera and by pretreatment of the surfactant with beta-mercaptoethanol. In addition, the uptake of liposomes into type II cells on TM was augmented by SP-A. With the use of iodinated bovine SP-A reconstituted in rat surfactant, cells cultured on TM showed a 2.6-fold increase in binding and a 3.2-fold stimulation in uptake of SP-A over that seen with cells cultured on plastic. The change in the slope of the total binding curve of 125I-labeled bovine SP-A reconstituted with bovine surfactant to type II cells on the two substrates occurred at the same concentration of SP-A (17 micrograms/mg), but maximal binding was approximately eight times higher for cells on TM than for cells on plastic dishes. Thus, in a more physiological environment, i.e., SP-A in surfactant and cells on microporous membranes, SP-A plays an important role in the uptake of phospholipids by alveolar pneumocytes.