Lamellar body membrane turnover is stimulated by secretagogues

Lamellar bodies are specialized cellular organelles used for storage of sur factant by alveolar type II cells of the lung. We utilized monoclonal antib ody (MAb) 3C9, which recognizes an integral lamellar body-limiting membrane protein of 180 kDa, to follow lamellar body trafficking. I-125-labeled MAb 3C9 bound to the surface of type II cells and was internalized by the cell s in a time- and concentration-dependent manner that was inhibitable by exc ess unlabeled antibody. The internalized antibody remained undegraded over a 4-h time period. The L2, rat lung cell line that does not have lamellar b odies did not bind iodinated 3C9. Exposure of type II cells to the secretag ogues ATP, phorbol 12-myristate 13-acetate, and cAMP resulted in a 1.5- to 8-fold enhancement of binding and uptake of MAb 3C, Calphostin C inhibited phorbol 12-myristate 13-acetate-stimulated phospholipid secretion and also reduced binding and uptake of MAb 3C9 by type II cells. Treatment of type I I cells with phenylarsine oxide to obstruct clathrin-mediated endocytosis h ad no effect on the internalization of MAb 3C9 while markedly blocking the uptake of surfactant protein A and transferrin. An actin-mediated process w as:important for lamellar body membrane uptake because incubation with cyto chalasin D partially inhibited MAb 3C9 incorporation by type II cells. Thes e studies are compatible with enhanced lamellar body membrane turnover asso ciated with surfactant secretion and indicate that this process can be moni tored by the trafficking of the antigen reporter MAb 3C9.