Involvement of annexin II in exocytosis of lamellar bodies from alveolar epithelial type II cells

Abstract

Annexins are a family of Ca(2+)- and phospholipid-binding proteins that have been implicated in exocytosis. In the present study, we investigated the participation of selected annexins in exocytosis of lamellar bodies by examining their liposome aggregation property and ability to reconstitute surfactant secretion from permeabilized rat lung alveolar type II cells. Annexins I, II, III, and VI were demonstrated in type II cells by immunoblot analysis, but annexin IV and V were not found. Annexins I-IV mediated liposome aggregation in the presence of 1 mM Ca2+. However, only annexin II tetramer had aggregation activity at 10 microM Ca2+. Annexins V and VI had negligible aggregation activity at any Ca2+ concentrations (up to 1 mM Ca2+). To study reconstitution of secretion by annexins, isolated type II cells were permeabilized with 40 microM beta-escin. Under these conditions, the permeabilized cells released approximately 30-40% lactic acid dehydrogenase into the medium. An underestimated fraction of cellular annexin content was lost during permeabilization. However, lamellar bodies in the permeabilized type II cells stained appropriately with the fluorescent dyes Nile red and quinacrine, indicating that they were intact. These permeabilized cells were secretion competent, since phosphatidylcholine (PC) secretion was stimulated by 0.2-1.0 microM Ca2+. Addition of an exogenous annexin mixture enhanced PC secretion from the permeabilized type II cells with maximal stimulation at 0.5 microM Ca2+. Of six purified annexins (I-VI) tested for their ability to reconstitute secretion from permeabilized cells, only annexin II was effective. Our results suggest that annexin II is not necessary for exocytosis of lamellar bodies.