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We here demonstrate that mastoparan, fluoride, forskolin, cholera toxin, and 8-bromoadenosine-3'-5'-cyclic monophosphate all selectively stimulate apical endocytosis of the protein toxin ricin without increasing the uptake at the basolateral side of polarized Madin-Darby canine kidney cells. Activation of adenylyl cyclase and an increased level of cAMP seem to increase ricin endocytosis by a clathrin-independent mechanism since stimulation of endocytosis by cholera toxin and 8-bromoadenosine-3'-5'-cyclic monophosphate occurred even when the clathrin-dependent pathway was blocked by low cytosolic pH. The data suggest that mastoparan stimulates apical endocytosis by interacting with heterotrimeric G proteins, and also this stimulation of endocytosis appears to be clathrin independent since the uptake of transferrin at the apical side was strongly inhibited by mastoparan after brefeldin A-induced missorting of the transferrin receptor to this pole of the cell. In addition, mastoparan stimulated apical endocytosis when clathrin-mediated endocytosis was blocked by acidification of the cytosol. Furthermore, we provide evidence for the existence of clathrin-independent endocytosis on both the apical and the basolateral surface of control Madin-Darby canine kidney cells. Our results suggest that endocytosis at the apical pole of epithelial cells can be regulated selectively by a physiological signal.
Eker et al. (1994) studied this question.