Gilligan A, Bushmeyer S, Knowles B B
Wistar Institute of Anatomy and Biology, Philadelphia, Pennsylvania 19104.
Exp Cell Res. 1992 Jun;200(2):235-41. doi: 10.1016/0014-4827(92)90169-9.
The effect of epidermal growth factor (EGF) receptor overexpression on ligand-induced EGF receptor downregulation was examined using a hepatoma-derived cell line, PLC/PRF/5, which expresses normal amounts of the EGF receptor, and a subline, NPLC/PRF/5, which expresses 10-fold more receptors at its cell surface. PLC/PRF/5 cells efficiently downregulated surface receptor levels upon exposure to saturating and subsaturating concentrations of EGF; the rate of receptor downregulation corresponded to that of ligand-receptor internalization. Upon internalization, EGF receptors were degraded and receptor biosynthesis remained at basal levels. EGF surface receptor remained downregulated for as long as cells were exposed to EGF. By contrast, surface EGF receptor abundance in NPLC/PRF/5 cells decreased by only 5-15% after 1-4 h incubation with subsaturating doses of EGF and actually increased by 67% within 20 h. Exposure of these cells to saturating concentrations of EGF induced modest decreases in surface receptor abundance during the initial 12 h incubation, followed by a progressive decline to 30% of initial values by 24 h. Relative ligand-receptor internalization rates in NPLC/PRF/5 cells were lower than those in PLC/PRF/5, although their surface receptor population was even higher than that predicted by the decreased internalization rates. EGF receptor degradation in NPLC/PRF/5 cells was also inhibited; exposure to saturating levels of EGF for more than 16 h was necessary before significant degradation occurred. Receptor protein and mRNA biosynthesis in NPLC/PRF/5 were stimulated by 8 h exposure to EGF but when saturating concentrations of EGF were present for 16 h, receptor biosynthesis was inhibited. EGF receptor overexpression circumvents the downregulatory effect of EGF by decreasing the rate of receptor internalization, inhibiting degradation of the internalized receptor pool, and stimulating EGF receptor biosynthesis. Conversely, receptor downregulation becomes pronounced at late times when receptor degradation is high and biosynthesis is inhibited.
利用一种肝癌衍生细胞系PLC/PRF/5及其亚系NPLC/PRF/5，研究了表皮生长因子（EGF）受体过表达对配体诱导的EGF受体下调的影响。PLC/PRF/5细胞表达正常量的EGF受体，而NPLC/PRF/5亚系在其细胞表面表达的受体量是前者的10倍。PLC/PRF/5细胞在暴露于饱和及亚饱和浓度的EGF后，能有效下调表面受体水平；受体下调速率与配体-受体内化速率一致。内化后，EGF受体被降解，受体生物合成维持在基础水平。只要细胞暴露于EGF，EGF表面受体就会持续下调。相比之下，NPLC/PRF/5细胞在与亚饱和剂量的EGF孵育1 - 4小时后，表面EGF受体丰度仅降低5 - 15%，而在20小时内实际上增加了67%。将这些细胞暴露于饱和浓度的EGF后，在最初12小时的孵育期间，表面受体丰度出现适度下降，随后到24小时逐渐降至初始值的30%。NPLC/PRF/5细胞中的相对配体-受体内化速率低于PLC/PRF/5细胞，尽管其表面受体数量甚至高于内化速率降低所预测的数量。NPLC/PRF/5细胞中EGF受体的降解也受到抑制；在显著降解发生之前，需要将细胞暴露于饱和水平的EGF超过16小时。暴露于EGF 8小时可刺激NPLC/PRF/5细胞中的受体蛋白和mRNA生物合成，但当存在饱和浓度的EGF 16小时时，受体生物合成受到抑制。EGF受体过表达通过降低受体内化速率、抑制内化受体池的降解以及刺激EGF受体生物合成，规避了EGF的下调作用。相反，当受体降解率高且生物合成受到抑制时，受体下调在后期变得明显。
