Journal of Biological Chemistry. 2001;276 (45):41668-41674.
Endocrine Sciences, University of Manchester, Stopford Building, Oxford Road, Manchester
M13 9PT, United Kingdom. firstname.lastname@example.orgInsulin-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.PMID: 11546760 [PubMed - indexed for MEDLINE]