α2-macroglobulin: A New Component in the Insulin-like Growth Factor/Insulin-like Growth Factor Binding Protein-1 Axis

Insulin-like growth factors (IGFs) are crucial for many aspects of development, growth, and metabolism yet control of their activity by IGF-binding proteins (IGFBPs) remains controversial. The effect of IGFBP-1 depends on its phosphorylation status; phosphorylated IGFBP-1 inhibits IGF actions whereas the nonphosphorylated isoform is stimulatory. In order to understand this phenomenon, we purified phosphorylated IGFBP-1 from normal human plasma by immunoaffinity chromatography. Unexpectedly, the resulting preparation enhanced IGF-stimulated 3T3-L1 fibroblast proliferation, due to the presence of a co-purified protein of ≈700 kDa. Matrix-assisted laser desorption ionization-mass spectrometry and Western immunoblotting analysis identified this co-purified protein as α2-macroglobulin (σ2M). Anti-α2M antibodies co-immunoprecipitated IGFBP-1 from human plasma and from 125I-IGFBP-1·α2M complexes formed in vitro. The 125I-IGFBP-1/α2M association could be inhibited with excess unlabeled IGFBP-1. Surface plasmon resonance analysis indicated that α2M preferentially associates with the phosphorylated isoform of IGFBP-1 and that when complexed to α2M, IGFBP-1 can still bind IGF-I. These findings have functional significance since α 2M protects IGFBP-1 from proteolysis and abrogates the inhibitory effect of phosphorylated IGFBP-1 on IGF-I stimulated 3T3-L1 cell proliferation. We conclude that α2M is a binding protein of IGFBP-1 which modifies IGF-I/IGFBP-1 actions resulting in enhanced IGF effects. In line with its role in regulating the clearance and activity of other growth factors, we predict that α2M has a novel and important role in controlling the transport and biological activity of IGFs.