Steer C J, Weiss P, Huber B E, Wirth P J, Thorgeirsson S S, Ashwell G
Laboratory of Biochemistry and Metabolism, National Institute of Diabetes, Digestive and Kidney Diseases, Bethesda, Maryland 20892.
J Biol Chem. 1987 Dec 25;262(36):17524-9.
Ligand-induced modulation of the hepatic receptor for asialoglycoproteins has been examined in the human hepatoblastoma cell line, Hep G2. When grown to confluence, approximately 80% of the functional receptors are expressed at the cell surface. In contrast, exposure of these cells to saturating levels of galactose- or N-acetylgalactosamine-terminated ligands, for extended periods of time, resulted in a drastic reduction in the number of cell surface receptors, as determined by the binding of 125I-asialo-orosomucoid at 4 degrees C. Recovery of binding capacity was slow and incomplete despite retention of cellular viability and normal growth characteristics. Under these conditions, the decreased number of surface receptors could not be accounted for by changes in binding affinity or by internalization since the intracellular receptor number remained essentially constant. No corresponding decrease was noted in the ability of insulin or transferrin to bind to their respective cell surface receptors in the modulated cells. Similarly, Northern blot analysis revealed no changes in the steady state levels of the asialoglycoprotein receptor transcripts. However, antibody prepared against the purified human receptor bound equally well to both control and modulated cells, thereby indicating the presence of a proportionate number of inactive surface receptors on the latter cells. Two-dimensional gel electrophoresis failed to detect any abnormality in the molecular weight or isoelectric point of the modulated receptor. These findings are interpreted as indicating that the Hep G2 cells are able to regulate the functional expression of this surface receptor without altering its immunologic integrity. A defect in cell surface sialylation appears to be involved in the regulatory response.
在人肝癌细胞系Hep G2中研究了配体诱导的去唾液酸糖蛋白肝受体的调节作用。当细胞生长至汇合时，约80%的功能性受体表达于细胞表面。相反，将这些细胞长时间暴露于半乳糖或N - 乙酰半乳糖胺末端配体的饱和水平下，通过4℃时125I - 去唾液酸血清类黏蛋白的结合测定，导致细胞表面受体数量急剧减少。尽管细胞活力和正常生长特性得以保留，但结合能力的恢复缓慢且不完全。在这些条件下，表面受体数量的减少不能用结合亲和力的变化或内化来解释，因为细胞内受体数量基本保持不变。在受调节的细胞中，胰岛素或转铁蛋白与其各自细胞表面受体结合的能力未观察到相应下降。同样，Northern印迹分析显示去唾液酸糖蛋白受体转录本的稳态水平没有变化。然而，针对纯化的人受体制备的抗体与对照细胞和受调节细胞的结合效果相同，从而表明后者细胞上存在数量成比例的无活性表面受体。二维凝胶电泳未能检测到受调节受体的分子量或等电点有任何异常。这些发现被解释为表明Hep G2细胞能够调节这种表面受体的功能表达而不改变其免疫完整性。细胞表面唾液酸化缺陷似乎参与了这种调节反应。
