Cerneus D P, Strous G J, van der Ende A
Laboratory of Cell Biology, Medical School, University of Utrecht, The Netherlands.
J Cell Biol. 1993 Sep;122(6):1223-30. doi: 10.1083/jcb.122.6.1223.
Trophoblast-like BeWo cells form well-polarized epithelial monolayers, when cultured on permeable supports. Contrary to other polarized cell systems, in which the transferrin receptor is found predominantly on the basolateral cell surface, BeWo cells express the transferrin receptor at both apical and basolateral cell surfaces (Cerneus, D.P., and A. van der Ende. 1991. J. Cell Biol. 114: 1149-1158). In the present study we have addressed the question whether BeWo cells use a different sorting mechanism to target transferrin receptors to the cell surface, by examining the biosynthetic and transcytotic pathways of the transferrin receptor in BeWo cells. Using trypsin and antibodies to detect transferrin receptors at the cell surface of filter-grown BeWo cells, we show that at least 80% of newly synthesized transferrin receptor follows a direct pathway to the basolateral surface, demonstrating that the transferrin receptor is efficiently intracellularly sorted. After surface arrival, pulse-labeled transferrin receptor equilibrates between apical and basolateral cell surfaces, due to ongoing transcytotic transport in both directions. The subsequent redistribution takes over 120 min and results in a steady state distribution with 1.5-2.0 times more transferrin receptors at the basolateral surface than at the apical surface. By monitoring the fate of surface-bound 125I-transferrin, internalized either from the apical or basolateral surface transcytosis of the transferrin receptor was studied. About 15% of 125I-transferrin is transcytosed in the basolateral to apical direction, whereas 25% is transcytosed in the opposite direction, indicated that the fraction of receptors involved in transcytosis is roughly twofold higher for the apical receptor pool, as compared to the basolateral pool. Upon internalization, both apical and basolateral receptor pools become redistributed on both surfaces, resulting in a twofold higher number of transferrin receptors at the basolateral surface. Our results indicate that in BeWo cells bidirectional transcytosis is the main factor in surface distribution of transferrin receptors on apical and basolateral surfaces, which may represent a cell type-specific, post-endocytic, sorting mechanism.
滋养层样BeWo细胞在可渗透支持物上培养时可形成极性良好的上皮单层。与其他极化细胞系统不同，在其他系统中运铁蛋白受体主要位于细胞基底外侧表面，而BeWo细胞在细胞顶端和基底外侧表面均表达运铁蛋白受体（Cerneus, D.P., and A. van der Ende. 1991. J. Cell Biol. 114: 1149 - 1158）。在本研究中，我们通过检测BeWo细胞中运铁蛋白受体的生物合成和转胞吞途径，探讨了BeWo细胞是否使用不同的分选机制将运铁蛋白受体靶向至细胞表面。使用胰蛋白酶和抗体检测滤膜生长的BeWo细胞表面的运铁蛋白受体，我们发现至少80%新合成的运铁蛋白受体直接运输至基底外侧表面，这表明运铁蛋白受体在细胞内得到了有效的分选。表面到达后，脉冲标记的运铁蛋白受体在顶端和基底外侧细胞表面之间达到平衡，这是由于双向持续的转胞吞运输所致。随后的重新分布需要超过120分钟，并导致稳态分布，基底外侧表面的运铁蛋白受体数量比顶端表面多1.5 - 2.0倍。通过监测从顶端或基底外侧表面内化的表面结合的125I - 运铁蛋白的命运，研究了运铁蛋白受体的转胞吞作用。约15%的125I - 运铁蛋白从基底外侧向顶端方向转胞吞，而25%则向相反方向转胞吞，这表明与基底外侧受体池相比，参与转胞吞作用的顶端受体池的受体比例大约高出两倍。内化后，顶端和基底外侧受体池在两个表面上重新分布，导致基底外侧表面的运铁蛋白受体数量增加两倍。我们的结果表明，在BeWo细胞中，双向转胞吞作用是运铁蛋白受体在顶端和基底外侧表面进行表面分布的主要因素，这可能代表了一种细胞类型特异性的内吞后分选机制。
