Wortmanin-sensitive trafficking steps in the endocytic pathway in rat liver endothelial cells

Liver endothelial cells (LECs) play an important homoeostatic role by remov ing potentially harmful macromolecules from blood. The extremely efficient endocytosis in LECs makes these cells an interesting model for the study of the involvement of phosphoinositides in the different steps of the endocyt ic process. In the present investigation we have studied the effect of wort mannin, an inhibitor of phosphatidylinositol kinases, on uptake, recycling and intracellular transport of I-125-labelled ovalbumin, which is taken up in LECs via mannose-receptor-mediated endocytosis. Wortmannin was found to inhibit both uptake and degradation of ovalbumin. Further studies indicated that the reduced uptake via the mannose receptor was due both to a reducti on of the number of surface receptors and a reduction in the rate of recept or-ligand internalization. Transport of ligand from endosomes to lysosomes was prevented, leading to increased recycling of internalized ligand. Wortm annin treatment released the Rab5 effector EEA1 from the endosomes and caus ed reduced size of early endosomes.