Mechanism for secretagogue-induced surfactant protein A binding to lung epithelial cells

Secretagogues stimulate both secretion and reuptake of surfactant components by pulmonary type II cells as well as enhance surfactant protein A (SP-A) binding. We have evaluated the possibility that the observed increase in SP-A binding is due to the movement of SP-A receptors from an intracellular pool to the plasma membrane. We utilized an anti-idiotypic monoclonal antibody, A2R, which recognizes an SP-A binding protein on type II cell membranes. Immunocytochemistry studies showed that A2R reacted with cellular antigens on type II cell membranes and paranuclear granules. A2R inhibited cell association of 125I-SP-A to type II cells plated on Transwell membranes as well as those plated on plastic dishes and also inhibited the SP-A-stimulated incorporation of phosphatidylcholine liposomes into type II cells. On exposure to secretagogues, the binding of 125I-A2R and 125I-SP-A to type II cells increased in parallel. With permeabilized type II cells on Transwell membranes, one-sixth of the binding sites were located on the plasma membrane, with the remainder being intracellular; phorbol 12-myristate 13-acetate 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 molecular masses of approximately 32 and 60 kDa, respectively, reduced. Under nonreducing conditions, the mass of the SP-A and A2R binding protein was approximately 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.