Stern Kathryn A, Place Trenton L, Lill Nancy L
Department of Pharmacology and the Holden Comprehensive Cancer Center, The Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, IA 52242, USA.
Biochem J. 2008 Mar 15;410(3):585-94. doi: 10.1042/BJ20071505.
EGF-R [EGF (epidermal growth factor) receptor] ligands can promote or inhibit cell growth. The biological outcome of receptor activation is dictated, at least in part, by ligand-specified patterns of endocytic trafficking. EGF-R trafficking downstream of the ligands EGF and TGF-alpha (transforming growth factor-alpha) has been investigated extensively. However, less is known about EGF-R fates induced by the ligands BTC (betacellulin) and AR (amphiregulin). We undertook comparative analyses to identify ligand-specific molecular events that regulate EGF-R trafficking and degradation. EGF (17 nM) and BTC (8.5 nM) induced significant EGF-R degradation, with or without ectopic expression of the ubiquitin ligase Cbl. Human recombinant AR (17 nM) failed to affect receptor degradation in either case. Notably, levels of ligand-induced EGF-R ubiquitination did not correlate strictly with receptor degradation. Dose-response experiments revealed that AR at a saturating concentration was a partial agonist at the EGF-R, with approx. 40% efficacy (relative to EGF) at inducing receptor tyrosine phosphorylation, ubiquitination and association with Cbl. EGF-R down-regulation and degradation also were compromised upon cell stimulation with AR (136 nM). These outcomes correlated with decreased degradation of the Cbl substrate and internalization inhibitor hSprouty2. Downstream of the hSprouty2 checkpoint in AR-stimulated cells, Cbl-free EGF-R was incorporated into endosomes from which Cbl-EGF-R complexes were excluded. Our results suggest that the AR-specific EGF-R fate results from decreased hSprouty2 degradation and reduced Cbl recruitment to underphosphorylated EGF-R, two effects that impair EGF-R trafficking to lysosomes.
表皮生长因子受体（EGF-R）[表皮生长因子（EGF）受体]配体可促进或抑制细胞生长。受体激活的生物学结果至少部分取决于配体指定的内吞运输模式。EGF和转化生长因子α（TGF-α）这两种配体下游的EGF-R运输已得到广泛研究。然而，关于β细胞素（BTC）和双调蛋白（AR）这两种配体诱导的EGF-R命运却知之甚少。我们进行了比较分析，以确定调节EGF-R运输和降解的配体特异性分子事件。无论有无泛素连接酶Cbl的异位表达，EGF（17 nM）和BTC（8.5 nM）均诱导显著的EGF-R降解。在这两种情况下，人重组AR（17 nM）均未能影响受体降解。值得注意的是，配体诱导的EGF-R泛素化水平与受体降解并不严格相关。剂量反应实验表明，饱和浓度的AR是EGF-R的部分激动剂，在诱导受体酪氨酸磷酸化、泛素化以及与Cbl结合方面，其效力约为EGF的40%。用AR（136 nM）刺激细胞后，EGF-R的下调和降解也受到损害。这些结果与Cbl底物和内化抑制剂hSprouty2降解减少相关。在AR刺激的细胞中，hSprouty2检查点下游，不含Cbl的EGF-R被纳入内体，而Cbl-EGF-R复合物被排除在外。我们的结果表明，AR特异性的EGF-R命运是由于hSprouty2降解减少以及Cbl募集到磷酸化不足的EGF-R上减少所致，这两种效应损害了EGF-R向溶酶体的运输。
