Laursen Lisbeth S, Kjaer-Sorensen Kasper, Andersen Mikkel H, Oxvig Claus
Department of Molecular Biology, University of Aarhus, Gustav Wieds Vej 10C, DK-8000 Aarhus C, Denmark.
Mol Endocrinol. 2007 May;21(5):1246-57. doi: 10.1210/me.2006-0522. Epub 2007 Feb 20.
The biological activity of IGF-I and -II is controlled by six binding proteins (IGFBPs), preventing the IGFs from interacting with the IGF receptor. Proteolytic cleavage of IGFBPs is one mechanism by which IGF can be released to bind the receptor. The IGFBPs are usually studied individually, although the presence of more than one of the IGFBPs in most tissues suggests a cooperative function. Thus, the IGFBPs are part of regulatory networks with proteolytic enzymes in one end and the IGF receptor in the other end. We have established a model system that allows analysis of the dynamics between IGF, IGFBP-4 and -5, the IGF receptor, and the proteolytic enzyme PAPP-A, which specifically cleaves both IGFBP-4 and -5. We demonstrate different mechanisms of IGF release from IGFBP-4 and -5: cooperative binding to IGF is observed for the proteolytic fragments of IGFBP-5, but not fragments of IGFBP-4. Furthermore, we find that PAPP-A-mediated IGF-dependent cleavage of IGFBP-4 is inhibited by IGFBP-5, which sequesters IGF from IGFBP-4, and that cleavage of both IGFBP-4 and -5 is required for the release of bioactive IGF. Finally, we show that cell surface-localized proteolysis of IGFBP-4 represents the final regulatory step of efficient IGF delivery to the receptor. Our data define a regulatory system in which molar ratios between the IGFBPs and IGF and between the different IGFBPs, sequential proteolytic cleavage of the IGFBPs, and surface association of the activating proteinase are key elements in the regulation of IGF receptor stimulation.
胰岛素样生长因子-I（IGF-I）和-II（IGF-II）的生物活性受六种结合蛋白（IGFBPs）调控，这些蛋白可阻止IGFs与IGF受体相互作用。IGFBPs的蛋白水解切割是IGF得以释放并与受体结合的一种机制。通常对IGFBPs进行单独研究，尽管大多数组织中存在不止一种IGFBPs，这表明它们具有协同功能。因此，IGFBPs是调控网络的一部分，一端与蛋白水解酶相连，另一端与IGF受体相连。我们建立了一个模型系统，可用于分析IGF、IGFBP-4和-5、IGF受体以及特异性切割IGFBP-4和-5的蛋白水解酶妊娠相关血浆蛋白-A（PAPP-A）之间的动态关系。我们证明了IGF从IGFBP-4和-5释放的不同机制：观察到IGFBP-5的蛋白水解片段与IGF存在协同结合，而IGFBP-4的片段则不存在这种情况。此外，我们发现IGFBP-5可将IGF从IGFBP-4中隔离，从而抑制PAPP-A介导的IGF依赖性IGFBP-4切割，并且生物活性IGF的释放需要IGFBP-4和-5都被切割。最后，我们表明IGFBP-4在细胞表面的定位蛋白水解是IGF有效传递至受体的最终调控步骤。我们的数据定义了一个调控系统，其中IGFBPs与IGF之间以及不同IGFBPs之间的摩尔比、IGFBPs的顺序蛋白水解切割以及激活蛋白酶的表面结合是调控IGF受体刺激的关键因素。
