INHIBITION OF APICAL BUT NOT BASOLATERAL ENDOCYTOSIS OF RICIN AND FOLATE IN CACO-2 CELLS BY CYTOCHALASIN-D

Apical and basolateral endocytic pathways in polarised Caco-2 cells we re investigated by following the uptake, recycling and transcytosis of the galactose-binding protein toxin ricin, as a membrane marker. Diff erences in the extent and kinetics of lectin uptake, recycling and tra nscytosis were observed at the apical and basolateral domains and alte red with the age of the cell monolayer. Treatment of polarised Caco-2 cells with cytochalasin D showed a domain-specific, concentration-depe ndent inhibition of apical endocytosis of ricin. Inhibition of apical endocytosis by cytochalasin D was not due to a gross change in brush b order morphology, although actin stress fibres within the cell body we re disrupted. It is not clear whether inhibition of apical endocytosis in polarized epithelial cells by cytochalasin D is caused simply by d isruption of a mechanochemical motor involving microvillar actin filam ents. The cytochalasin D effect was also observed when measuring uptak e of folate, suggesting apical domain-specific inhibition of caveolar, as well as clathrin-mediated, endocytic routes.