Jäckle S, Runquist E A, Miranda-Brady S, Havel R J
Cardiovascular Research Institute, University of California, San Francisco 94143-0130.
J Biol Chem. 1991 Jan 25;266(3):1396-402.
Hepatocytes rapidly internalize epidermal growth factor (EGF) and transferrin by receptor-mediated endocytosis. Both EGF and its receptor are thought to be targeted for destruction in lysosomes, leading to down-regulation of the receptor, whereas transferrin, after unloading iron within the cell, is thought to recycle to the cell surface bound to its receptor. Previously, we isolated three endosomal fractions from livers of estradiol-treated rats and examined their roles in cellular trafficking of low density lipoproteins (LDL) and the LDL receptor, which cycles constitutively (Belcher, J. D., Hamilton, R. L., Brady, S. E., Hornick, C. A., Jäckle, S., Schneider, W. J., and Havel, R. J., Proc. Natl. Acad. Sci. U. S. A. (1987) 84, 6785-6789). In the current study we have taken advantage of the distinct trafficking of the EGF receptor and transferrin to evaluate further the functions of these endosome fractions. Intravenous injection of a saturating amount of EGF into estradiol-treated rats induced internalization of a single population of EGF receptors, which rapidly accumulated in the endosome fraction of intermediate density ("compartment of uncoupling of receptor and ligand" (CURL)) and subsequently in the low density endosome fraction (multivesicular bodies (MVBs)). The high density endosome fraction, whose membranes contain a high concentration of recycling receptors (designated receptor-recycling compartment (RRC)), failed to accumulate EGF receptors after injection of EGF. In livers of rats not given exogenous EGF, EGF receptors were found in small but comparable concentrations in RRC, CURL, and MVB membranes, consistent with other evidence that targeting of the EGF receptor to lysosomes is mediated by ligand-induced phosphorylation. Transferrin also accumulated first in CURL and later in MVBs, but it also accumulated rapidly in the RRC fraction, consistent with the proposed function of this fraction in receptor recycling. Since transferrin is not degraded during its endocytic cycle, these observations indicate that apotransferrin and its receptor recycle from late endosomes (MVBs) located at the apical pole of hepatocytes, as well as from early endosomes near the sinusoidal pole.
肝细胞通过受体介导的内吞作用迅速内化表皮生长因子（EGF）和转铁蛋白。EGF及其受体都被认为会在溶酶体中被靶向降解，从而导致受体下调，而转铁蛋白在细胞内卸载铁后，被认为会与受体结合循环回到细胞表面。此前，我们从经雌二醇处理的大鼠肝脏中分离出了三个内体组分，并研究了它们在低密度脂蛋白（LDL）和LDL受体的细胞运输中的作用，LDL受体是持续循环的（Belcher, J. D., Hamilton, R. L., Brady, S. E., Hornick, C. A., Jäckle, S., Schneider, W. J., and Havel, R. J., Proc. Natl. Acad. Sci. U. S. A. (1987) 84, 6785 - 6789）。在当前的研究中，我们利用了EGF受体和转铁蛋白不同的运输方式，进一步评估这些内体组分的功能。向经雌二醇处理的大鼠静脉注射饱和量的EGF会诱导单一群体的EGF受体发生内化，这些受体会迅速在中等密度的内体组分（“受体与配体解偶联区室”（CURL））中积累，随后在低密度内体组分（多囊泡体（MVBs））中积累。高密度内体组分的膜含有高浓度的循环受体（称为受体循环区室（RRC）），在注射EGF后未能积累EGF受体。在未给予外源性EGF的大鼠肝脏中，在RRC、CURL和MVB膜中发现了少量但相当浓度的EGF受体，这与其他证据一致，即EGF受体靶向溶酶体是由配体诱导的磷酸化介导的。转铁蛋白也首先在CURL中积累，随后在MVBs中积累，但它也迅速在RRC组分中积累，这与该组分在受体循环中的假定功能一致。由于转铁蛋白在其胞吞循环中不会被降解，这些观察结果表明脱铁转铁蛋白及其受体从位于肝细胞顶端极的晚期内体（MVBs）以及靠近窦状极的早期内体中循环。
