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 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 approximately 700 kDa. Matrix-assisted laser desorption ionization-mass spectrometry and Western immunoblotting analysis identified this co-purified protein as alpha(2)-macroglobulin (alpha(2)M). Anti-alpha(2)M antibodies co-immunoprecipitated IGFBP-1 from human plasma and from (125)I-IGFBP-1.alpha(2)M complexes formed in vitro. The (125)I-IGFBP-1/alpha(2)M association could be inhibited with excess unlabeled IGFBP-1. Surface plasmon resonance analysis indicated that alpha(2)M preferentially associates with the phosphorylated isoform of IGFBP-1 and that when complexed to alpha(2)M, IGFBP-1 can still bind IGF-I. These findings have functional significance since alpha(2)M 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 alpha(2)M 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 alpha(2)M has a novel and important role in controlling the transport and biological activity of IGFs.
Westwood
M
M
Endocrine Sciences, University of Manchester, Stopford Building, Oxford Road, Manchester M13 9PT, United Kingdom. melissa.westwood@man.ac.uk
Aplin
J D
JD
Collinge
I A
IA
Gill
A
A
White
A
A
Gibson
J M
JM
D016475:3T3 Cells; D000818:Animals; D018970:Insulin-Like Growth Factor Binding Protein 1 / Q000737:chemistry / Q000378:metabolism*; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology*; D051379:Mice; D010766:Phosphorylation; D020349:Surface Plasmon Resonance; D013936:Thymidine / Q000378:metabolism; D000511:alpha-Macroglobulins / Q000737:chemistry / Q000378:metabolism*
D018970:Insulin-Like Growth Factor Binding Protein 1; D000511:alpha-Macroglobulins; D007334:Insulin-Like Growth Factor I; D013936:Thymidine