REGULATION OF ANNEXIN-I BY PROTEOLYSIS IN RAT ALVEOLAR EPITHELIAL TYPE-II CELLS

Limited proteolysis of annexin I occurs endogenously in intact rat lun g alveolar epithelial type II cells as revealed by immunoblot analysis . Proteolysis increases in the presence of phorbol 12-myristate 13-ace tate (PMA), terbutaline, or ATP, agents that enhance secretion of lung surfactant from type II cells. Calpain, a calcium-dependent neutral p rotease, was activated in stimulated type II cells and cleaved purifie d annexin I at the N-terminus minus 26 amino acids. Liposome aggregati on activity of truncated annexin I showed ten-fold increase in calcium sensitivity compared to the native form. The results suggest that pro teolytic modification may be a regulatory mechanism for annexin I to m ediate the secretory processes of alveolar type LI cells at physiologi cal calcium concentrations.