MECHANISM FOR SECRETAGOGUE-INDUCED SURFACTANT PROTEIN-A BINDING TO LUNG EPITHELIAL-CELLS

Secretagogues stimulate both secretion and reuptake of surfactant comp onents by pulmonary type II cells as well as enhance surfactant protei n A (SP-A) binding. We have evaluated the possibility that the observe d increase in SP-A binding is due to the movement of SP-A receptors fr om an intracellular pool to the plasma membrane. We utilized an anti-i diotypic monoclonal antibody, A2R, which recognizes an SP-A binding pr otein on type II cell membranes. Immunocytochemistry studies showed th at A2R reacted with cellular antigens on type II cell membranes and pa ranuclear granules. A2R inhibited cell association of I-125-Sp-A to ty pe II cells plated on Transwell membranes as well as those plated on p lastic dishes and also inhibited the SP-A-stimulated incorporation of phosphatidylcholine liposomes into type II cells. On exposure to secre tagogues, the binding of I-125-A2R and I-125-Sp-A to type II cells inc reased in parallel. With permeabilized type II cells on Transwell memb ranes, one-sixth of the binding sites were located on the plasma membr ane, with the remainder being intracellular; phorbol 12-myristate 13-a cetate treatment increased the binding of A2R to the cell surface but did not affect the total binding of A2R. Ligand blots of type II cell plasma membranes showed that SP-A and A2R both bound proteins with mol ecular masses of similar to 32 and 60 kDa, respectively, reduced. Unde r nonreducing conditions, the mass of the SP-A and A2R binding protein was similar to 210 kDa, indicating that the SP-A receptor is composed of disulfide-linked subunits. The results support our hypothesis that secretagogues increase SP-A binding sites by accelerating recruitment of receptors to the cell surface.