OSMOTIC-STRESS INDUCES BOTH SECRETION AND APOPTOSIS IN RAT ALVEOLAR TYPE-II CELLS

The aim of this study was to analyze the effects of osmotic shock and secretagogues such as ATP and 12-O-tetradecanoylphorbol 13-acetate (TP A) on various intracellular signaling pathways in primary cultures of alveolar type II cells and examine their potential role in regulating events such as secretion and apoptosis in these cells. Sorbitol-induce d osmotic stress caused the sustained release of [H-3]phosphatidylchol ine ([H-3]PC)from primary cultures of rat alveolar type II cells prela beled with [H-3]choline chloride. This release was not dependent on pr otein kinase C because downregulation of the major protein kinase C is oforms (alpha, beta(II), delta, and eta) expressed in alveolar type II cells had no effect on [H-3]PC secretion. Sorbitol, as well as the kn own secretagogues TPA and ATP, activated extracellular signal-regulate d kinase. Although an inhibitor of the extracellular signal-regulated kinase cascade, PD-98059, blocked this activation, it had no effect on the release of [H-3]PC. Sorbitol and ultraviolet C radiation, but not TPA or ATP, were also found to activate both p38 and stress-activated protein kinase/c-Jun NH2-terminal kinase. Furthermore, both sorbitol and ultraviolet C radiation induced apoptosis in alveolar type II cell s as demonstrated by Hoechst 33258 staining of the condensed nuclei, t he generation of DNA ladders, and the activation of caspases. The data indicate that multiple signaling pathways are activated by traditiona l secretagogues such as TPA and ATP and by cellular stresses such as o smotic shock and that these may be involved in regulating secretory an d apoptotic events in alveolar type II cells.