CLIC-dependent internalization of caveolin-1 to lysosomal vacuoles in response to osmotic regulation

Originally thought to be a major endocytic portal, caveolae function as a membrane buffer, whereby caveolae flattening protects the plasma membrane from rupture under mechanical stress, such as hypotonic shock. However, the fate of the caveolae coat protein caveolin-1 upon caveolae flattening is not known. Here, extended hypotonic shock induces ubiquitin-independent, CLIC-dependent endocytosis of caveolin-1 to large, intracellular, CD44-positive, pH-neutral lysosomal vacuoles negative for multivesicular body markers. Caveolin-1 recycles from these vacuoles to the plasma membrane upon return to isotonic conditions. Caveolin-1 internalization occurs upon reduced cell volume due to extended hypotonic shock as well as in low-tension cells grown on reduced-stiffness hydrogels. Upon hypertonic shock, caveolin-1 internalization occurs in PC3 cells, lacking cavin-1 and caveolae, and is inhibited upon cavin-1 reintroduction. CLIC endocytosis of non-caveolar caveolin-1 to neutral pH lysosomal vacuoles identifies a non-caveolar endocytic and recycling pathway for caveolin-1 in response to reduced membrane tension.