Role of actin depolymerization in the surfactant secretory response of alveolar epithelial type II cells

Alveolar epithelial type II cells (AET(2)) respond with exocytosis of surfactant containing lamellar bodies to stimulation with mechanical stretch and secretagogues, a process that is fundamental for maintaining alveolar stability and lung gas exchange. In the present study in cultured rat AET(2) we employed botulinum C-2 toxin, a binary toxin which ADP ribosylates nonmuscle C-actin, as a specific tool to probe the role of the actin microfilament system in the surfactant secretory process. Incubation of AET(2) with C-2 toxin caused a dose-dependent decay of the cellular F-actin content to a minimum of 20% of baseline, concomitant with an increase in monomeric actin. In parallel, a significant augmentation of baseline surfactant secretion up to twofold elevated levels above control was noted, as assessed by the release of prelabeled phosphatidylcholine. Pretreatment with phalloidin, which stabilized F-actin and reduced the revel of G-actin, prevented the C-2 toxin-elicited enhancement of baseline surfactant secretion. Even tow C-2 toxin concentrations, resulting in a reduction of total cellular F-actin content of approximate to 10%, sufficed to augment secretagogue (ATP) and, more impressively, mechanical stress elicited an increase in surfactant secretion; the response to the biophysical challenge more than doubled. When investigated in the absence of toxin, different secretagogues (ATP, phorbol ester, betamimetics) caused a rapid-onset, transient reduction of F-actin in the range between 15 and 25% as a consistent part of their secretory response pattern. These data suggest that the state of actin polymerization is intimately linked to the exocytosis process underlying surfactant secretion in AET(2). Microfilament system-related compartmentalization effects and/or or the impact of the state of actin assembly on signaling events may be considered as underlying events.