A new role for an old drug: Ambroxol triggers lysosomal exocytosis via pH-dependent Ca2+ release from acidic Ca2+ stores

Abstract

Ambroxol (Ax) is a frequently prescribed drug used to facilitate mucociliary clearance, but its mode of action is yet poorly understood. Here we show by X-ray spectroscopy that Ax accumulates in lamellar bodies (LBs), the surfactant storing, secretory lysosomes of type II pneumocytes. Using lyso- and acidotropic substances in combination with fluorescence imaging we confirm that these vesicles belong to the class of acidic Ca2+ stores. Ax lead to a significant neutralization of LB pH, followed by intracellular Ca2+ release, and to a dose-dependent surfactant exocytosis. Ax-induced Ca2+ release was significantly reduced and slowed down by pretreatment of the cells with bafilomycin A1 (Baf A1), an inhibitor of the vesicular H+ ATPase. These results could be nearly reproduced with NH3/NH4+. The findings suggest that Ax accumulates within LBs and severely affects their H+ and Ca2+ homeostasis. This is further supported by an Ax-induced change of nanostructural assembly of surfactant layers.We conclude that Ax profoundly affects LBs presumably by disordering lipid bilayers and by acting as a weak base. The pH change triggers – at least in part – Ca2+ release from stores and secretion of surfactant from type II cells. This novel mechanism of Ax as a lysosomal secretagogue may also play a role for its recently discussed use for lysosomal storage and other degenerative diseases.