Regulatory roles of pulmonary surfactant apoprotein and its receptor in metabolism of lung diseases.

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Pulmonary surfactant is a complex mixture of lipids and proteins synthesized and secreted by alveolar type II cells. Phospholipids are the major components of pulmonary surfactant. The major class of phospholipids is phosphatidylcholine, which constitutes 70-80 % of phospholipids. The hydrophilic surfactant-associated proteins, SP-A and SP-D, are believed to play important roles in phospholipid metabolism and host-defense mechanism in the lung. The purpose of the present study was to investigate the roles of SP-A and SP-D in normal and diseased lungs.1. Regulation of phospholipid metabolism by surfactant proteins.(1) Structural requirement of SP-A for its biological activity (the inhibitory effect on phospholipid secretion by type II cells via a specific receptor) was examined. The C-terminal collegenase-resistent fragment of SP-A possessed the ability to regulate phospholipid secretion and to bind a high affinity receptor. Monoclonal antibody to human SP-A that blocked the SP-A activi … More ty was found to recognize the C-terminal side from Glu^<202>, suggesting the involvement of this region with the binding to SP-A receptor.(2) Native SP-D that formed a comlex with lipid counteracted the inhibitory effect of SP-A on phospholipid secretion by alveolar type II cells.(3) The ligand binding studies using ^<125>l-labeled proteins as probes revealed that SP-A and SP-D bound to phosphatidylcholine and phosphatidylinostitol, respectively.(4) SP-A-mediated uptake of phosphatidylcholine (PC) by type II cells was investigated. When subcellular distribution of radiolabeled dipalmitoyl PC (DPPC) taken up by type II cells was analyzed, approximately 52 % of cell-associated radiolabeled DPPC was recovered in the lamellabody-rich fraction in the presence of SP-A, whereas only 19 % was found to this fraction in the absence of SP-A. The result indicates that SP-A facilitates the incorporation of DPPC into lamellar bodies.2. Binding specificities of surfactant proteins for glycolipids.The direct binding of SP-A and SP-D to various glycolipids was investigated. SP-A was found to bind to galactosylceramide and asialo GM2. SP-D bound to glucosylceramide. The binding property of surfactant proteins to glycolipids appeas important in pulmonary defense system since cell surface glycolipids serve as receptors for various microorganism.3. Appearance of SP-A in the sera from patients with idiopathic pulmonary fibrosis (IPF) and pulmonary alveolar proteinosis (PAP). Lung surfactant components are believed to be present exclusively in the alveolar spaces and not in the blood stream under normal conditions. We examined whether SP-A appears in the sera of patients with lung diseases. The serum SP-A levels in patients with IPF (205*23 ng/ml, n=32) and PAP (285*23 ng/ml, n=6) were significantly higher than those in control subjects (45*3 ng/ml, n=56) (mean*SEM, p<0.01). Less