Molecular mechanism of lung discases by analysis of surfactant protein A and its receptor.

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Pulmonary surfactant plays important roles in phospholipid homeostasis and in host deffence mechanism of the lung. In this research project, we focussed on surfactant ptotein A (SP-A) that belongs to collectin family which possesses the carbohydrate recognition domain, and on SP-A receptor. The purpose of this project was to investigate the molecular mechanisms of the lung discases in relation to SP-A.1. We examined the effect of SP-A on the interaction of phospholipid liposomes with plasma membrane isolated from alveolar type II cells. SP-A significantly facilitated the binding of liposomes to type II cell-derived membrane but not to liver plasma membrane. The results suggest that SP-A receptor on type II cells may be involved in facilitated effect of SP-A on liposome binding to membrane.2. Mannose-binding protein A (MBP-A) also belongs to the collectin subgroup of C-type lectins and structurally homologous to SP-A.We constructed chimeric molecules in which the SP-A region Glu^<195>-P … More he^<228> was substituted with the MBP-A region Glu^<185>-Ala^<221>. This chimera retained the ability to bind dipalmitoylphosphatidylcholine and interact with alveolar type II cells, while MBP-A isolated from rat sera failed to express these SP-A function. The results indicate that the SP-A region Glu^<195>-Phe^<228> and the MBP-A region Glu^<185>-Ala^<221> are interchangeable without loss of SP-A functions. The results from the recombinant proteins with point mutations also indicate that Arg^<199> and Lys^<201> of human SP-A and Lys^<201> and Lys^<203> of rat SP-A are not critical for the SP-A functions and that the crucial mechanism of SP-A-mediated liposome aggregation is distinct from that of SP-A-mediated lipid uptake by type II cells.3. The abnormal aggregates of SP-A oligomer derived from patients with alveolar type II cells was less effective in interaction with type II cells and exhibited abnormal phospholipid membrane organization. IgG was found to associate with the abnormal larger aggregates of SP-A oligomer but not with normal-sized SP-A.4. The analysis of SP-A gene and protein expression in cancer cells from pleural effusions of patients with lung adenocarcinomas revealed that more than three quaters of adenocarcinoma cells express SP-A,one of peripheral airway cell markers. Less