Selective modulation of the endocytic uptake of ricin and fluid phase markers without alteration in transferrin endocytosis.

Cytochalasin D was found to reduce the endocytosis of ricin and the fluid phase markers [14C]sucrose and Lucifer Yellow in Vero cells without reducing the uptake of transferrin. The number of coated pits at the plasma membrane was not affected by the treatment. Cytochalasin D also reduced the endocytosis of ricin in cells where uptake of transferrin from coated pits was blocked by low cytosolic pH. Colchicine had a similar effect as cytochalasin D. Both drugs inhibited the exocytosis of ricin from the cells, and they reduced the rate by which ricin intoxicated the cells. Cytochalasin D had essentially no effect on the ability of the cells to bind transferrin, whereas colchicine reduced the binding to some extent. Epidermal growth factor (EGF) and 12-O-tetradecanoylphorbol-13-acetate (TPA) increased the endocytic uptake of ricin in A431 cells both under normal culture conditions and when the coated pit/coated vesicle pathway was blocked by acidification of the cytosol. In contrast, EGF and TPA had no stimulatory effect on the uptake of transferrin at normal cytoplasmic pH, and they did not abolish the ability of low cytoplasmic pH to inhibit endocytic uptake of transferrin. The results indicate that cytochalasin D and colchicine selectively inhibit endocytic uptake from non-clathrin-coated areas of the cell membrane whereas EGF and TPA stimulate it. The data support the view that there are different endocytic mechanisms, and they indicate that at least in some cell types the non-clathrin-coated endocytosis can be modulated.