Understanding the Mechanism of Insulin and Insulin-Like Growth Factor (IGF) Receptor Activation by IGF-II

BackgroundInsulin-like growth factor-II (IGF-II) promotes cell proliferation and survival and plays an important role in normal fetal development and placental function. IGF-II binds both the insulin-like growth factor receptor (IGF-1R) and insulin receptor isoform A (IR-A) with high affinity. Interestingly both IGF-II and the IR-A are often upregulated in cancer and IGF-II acts via both receptors to promote cancer proliferation. There is relatively little known about the mechanism of ligand induced activation of the insulin (IR) and IGF-1R. The recently solved IR structure reveals a folded over dimer with two potential ligand binding pockets arising from residues on each receptor half. Site-directed mutagenesis has mapped receptor residues important for ligand binding to two separate sites within the ligand binding pocket and we have recently shown that the IGFs have two separate binding surfaces which interact with the receptor sites 1 and 2. Methodology/Principal FindingsIn this study we describe a series of partial IGF-1R and IR agonists generated by mutating Glu12 of IGF-II. By comparing receptor binding affinities, abilities to induce negative cooperativity and potencies in receptor activation, we provide evidence that residue Glu12 bridges the two receptor halves leading to receptor activation. Conclusions/SignificanceThis study provides novel insight into the mechanism of receptor binding and activation by IGF-II, which may be important for the future development of inhibitors of its action for the treatment of cancer.

背景：胰岛素样生长因子-II（insulin-like growth factor-II, IGF-II）可促进细胞增殖与存活，在正常胎儿发育及胎盘功能中发挥关键作用。IGF-II可高亲和力结合胰岛素样生长因子受体-1（insulin-like growth factor receptor-1, IGF-1R）与胰岛素受体亚型A（insulin receptor isoform A, IR-A）。值得注意的是，IGF-II与IR-A在癌症组织中常出现表达上调，且IGF-II可通过这两种受体促进肿瘤细胞增殖。目前学界对配体诱导的胰岛素受体（insulin receptor, IR）与IGF-1R的激活机制仍知之甚少。近期解析的IR结构显示，其为折叠型二聚体，每个受体亚基可形成两个潜在的配体结合口袋。定点诱变研究已将配体结合口袋内两个独立位点上的受体残基确定为配体结合所必需的关键位点，而我们团队此前的研究表明，胰岛素样生长因子家族存在两个独立的结合表面，可分别与受体的1号和2号位点相互作用。

方法/主要结果：本研究通过对IGF-II的Glu12位点进行诱变改造，构建了一系列IGF-1R与IR的部分激动剂。通过比较各变体的受体结合亲和力、诱导负协同性的能力以及受体激活的效价强度，我们证实Glu12残基可桥接两个受体亚基，进而介导受体激活。

结论与意义：本研究为IGF-II介导的受体结合与激活机制提供了全新的研究视角，这一发现或可为未来开发靶向其致癌通路的抑制剂、用于癌症治疗提供重要理论依据。