alpha(2)-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 developm ent, growth, and metabolism yet control of their activity by IGF-binding pr oteins (IGFBPs) remains controversial. The effect of IGFBP-1 depends on its phosphorylation status; phosphorylated IGFBP-1 inhibits IGF actions wherea s 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 enh anced IGF-stimulated 3T3-L1 fibroblast proliferation, due to the presence o f a co-purified protein of approximate to 700 kDa. Matrix-assisted laser de sorption ionization-mass spectrometry and Western immunoblotting analysis i dentified this copurified protein as alpha (2)-macroglobulin (alpha M-2). A nti-alpha M-2 antibodies co-immunoprecipitated IGFBP-1 from human plasma an d from I-125-IGFBP-1(.)alpha M-2 complexes formed in vitro. The I-125-IGFBP -1/alpha M-2 association could be inhibited with excess unlabeled IGFBP-1. Surface plasmon resonance analysis indicated that alpha M-2 preferentially associates with the phosphorylated isoform of IGFBP-1 and that when complex ed to alpha M-2, IGFBP-1 can still bind IGF-I. These findings have function al significance since alpha M-2 protects IGFBP-1 from proteolysis and abrog ates the inhibitory effect of phosphorylated. IGFBP-1 on IGF-I stimulated 3 T3-L1 cell proliferation. We conclude that alpha M-2 is a binding protein o f IGFBP-1 which modifies IGF-I/IG-FBP-1 actions resulting in enhanced IGF e ffects. In line with its role in regulating the clearance and activity of o ther growth factors, we predict that alpha M-2 has a novel and important ro le in controlling the transport and biological activity of IGFs.