Insulin-Like Growth Factor Binding Protein 1 Stimulates Cell Migration and Binds to the α_5β_1 Integrin by Means of its Arg-Gly-Asp Sequence

Insulin-like growth factor (IGF)-binding protein 1 (IGFBP-1) contains an Arg-Gly-Asp (RGD) integrin recognition sequence. In vitro mutagenesis was used to alter this RGD sequence to Trp-Gly-Asp (WGD). Migration of Chinese hamster ovary (CHO) cells expressing the wild-type protein was more than 3-fold greater in 48 hr compared with cells expressing the WGD mutant form of IGFBP-1. Similarly, wild-type IGFBP-1 added to the media of control CHO cells stimulated migration 2-fold compared with the WGD protein. A synthetic RGD-containing peptide, when added to the medium with wild-type IGFBP-1, blocked the effect of IGFBP-1 on cell migration. The addition of IGF-I to the culture medium had no effect on the migration of cells expressing IGFBP-1 or vector alone. Affinity chromatography of 125I-labeled CHO cell membrane proteins, using IGFBP-1 coupled to agarose, identified the alpha 5 beta 1 integrin (fibronectin receptor) as the only cell surface molecule capable of binding IGFBP-1 in an RGD-dependent manner. Furthermore, wild-type IGFBP-1, but not the WGD mutant form, could be coprecipitated from CHO cells with an antibody directed against the alpha 5 integrin subunit. These studies demonstrate that IGFBP-1 stimulates CHO cell migration and binds to the alpha 5 beta 1 integrin receptor, both by an RGD-dependent mechanism. The effect of IGFBP-1 on migration is independent of IGF-I and is probably mediated through the alpha 5 beta 1 integrin.