Apical and basolateral transferrin receptors in polarized BeWo cells recycle through separate endosomes.

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Abstract
Contrary to most other epithelia, trophoblasts in the human placenta, which form the physical barrier between the fetal and the maternal blood circulation, express high numbers of transferrin receptors on their apical cell surface. This study describes the establishment of a polarized trophoblast-like cell line BeWo, which exhibit a high expression of transferrin receptors on the apex of the cells. Cultured on permeable filter supports, BeWo cells formed a polarized monolayer with microvilli on their apical cell surface. Across the monolayer a transepithelial resistance developed of approximately 600 omega.cm2 within 4 d. Depletion of Ca2+ from the medium decreased the resistance to background levels, showing its dependence on the integrity of tight junctions. Within the same period of time the secretion of proteins became polarized. In addition, the compositions of integral membrane proteins at the apical and basolateral plasma membrane domains were distinct as determined by domain-selective iodination. Similar to placental trophoblasts, binding of 125I-labeled transferrin to BeWo monolayers revealed that the transferrin receptor was expressed at both plasma membrane domains. Apical and basolateral transferrin receptors were found in a 1:2 surface ratio and exhibited identical dissociation constants and molecular weights. After uptake, transferrin recycled predominantly to the domain of administration, indicating separate recycling pathways from the apical and basolateral domain. This was confirmed by using diaminobenzidine cytochemistry, a technique by which colocalization of endocytosed 125I-labeled and HRP-conjugated transferrin can be monitored. No mixing of the two types of ligands was observed, when both ligands were simultaneously internalized for 10 or 60 min from opposite domains, demonstrating that BeWo cells possess separate populations of apical and basolateral early endosomes. In conclusion, the trophoblast-like BeWo cell line can serve as a unique model to compare the apical and basolateral endocytic pathways of a single ligand, transferrin, in polarized epithelial cells.