Ganglioside GM1 levels are a determinant of the extent of caveolae/raft-dependent endocytosis of cholera toxin to the Golgi apparatus

Cholera toxin is associated with caveolae and raft domains in various cell types and previous studies have shown that cholera toxin can be internalized by caveolae/raft-dependent endocytosis as well as by other pathways. We undertook the study of cholera toxin endocytosis in CaCo-2 and HeLa cells. CaCo-2 cells do not express detectable levels of caveolin and, relative to HeLa cells, also present significantly reduced expression of ganglioside GM1, the cholera toxin receptor, that remains Triton X-100 insoluble. Amongst the HeLa cell population, caveolin expression is constant, however, GM1 expression is highly variable. Cholera toxin is internalized to the Golgi apparatus via a caveolae/raft-dependent pathway sensitive to methyl-beta-cyclodextrin and genistein in high-GM1-expressing HeLa cells but not in low-GM1 HeLa cells or in CaCo-2 cells. Limited cholera toxin endocytosis to endosomes sensitive to neither methyl-beta-cyclodextrin nor genistein is also observed in all cells and corresponds to a non-caveolae/raft endocytic pathway. Increasing cell-associated GM1 by adding GM1 to the cell media of both HeLa and CaCo-2 cells selectively enhances the methyl-beta-cyclodextrin-, genistein-sensitive delivery of cholera toxin to the Golgi apparatus but not to endosomes. GM1 expression levels are therefore a selective determinant of caveolae/raft-dependent endocytosis of cholera toxin to the Golgi apparatus and variable expression of GM1 between cells can impact on the endocytosis and choice of pathway followed by cholera toxin.