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

Abstract

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 IGFstimulated 3T3-L1 fibroblast proliferation, due to the presence of a co-purified protein of Ϸ700 kDa. Matrixassisted laser desorption ionization-mass spectrometry and Western immunoblotting analysis identified this copurified protein as The insulin-like growth factors (IGFs) 1 have a key role in the metabolism, development, growth, and maintenance of many tissues and organs (1). IGF bioavailability is controlled by a number of binding proteins (IGFBPs) and one of these, IG-FBP-1, is capable of either inhibiting (2-4) or potentiating (5-9) IGF activity at the cellular level.IGFBP-1 inhibits IGF actions by competing with the type 1 IGF receptor for IGF binding, however, the mechanism by which IGFBP-1 enhances IGF activity is less certain. Early work to address this phenomenon resulted in the isolation of two IGFBP-1 isoforms from amniotic fluid, which had similar physicochemical properties but markedly different effects on IGF activity (5). IGFBP-1 association with the cell surface was suggested as an explanation of these findings since only the stimulatory isoform was found to bind to cell membranes. It is now known that IGFBP-1 binds to ␣ 5 ␤ 1 integrin via its RGD site and disruption of this interaction leads to inhibition of the subsequent cellular response (6). Polymerization of IGFBP-1 was also postulated as a mechanism for enhancing IGF action (7) and this has been confirmed recently (8).Many studies have also focused on the influence of phosphorylation in relation to IGFBP-1 effects on IGF activity. The inhibitory isoform purified by Busby et al. (5) was subsequently shown to be phosphorylated (9) whereas the stimulatory preparation contained nonphosphorylated IGFBP-1. Highly phosphorylated IGFBP-1, which is the only form found in plasma (10) has a high affinity for IGF-I (9, 11) and can therefore inhibit IGF-I actions by sequestering it from cell surface receptors. Nonphosphorylated IGFBP-1, which has a relatively low affinity for IGF (9, 11) is thought to allow more IGF/IGF receptor interactions and this hypothesis has been supported by numerous in vitro studies (9,(12)(13)(14). However, it is unclear whether in vivo alteration of IGFBP-1 phosphorylation status represents an important mechanism for regulating IGF bioavailability in the non-pregnant adult, since non-and lesser phosphorylated isoforms of IGFBP-1 are only present at high concentrations during pregnancy (10,15).In the light of the above findings, we were surprised to observe that phosphorylated IGFBP-1 purified from ...

Hepcidin Bound to α2-Macroglobulin Reduces Ferroportin-1 Expression and Enhances Its Activity at Reducing Serum Iron Levels

“…9). Similar functions have been indicated for other cytokines/hormones that bind to ␣ 2 M/ ␣ 2 M-MA (8,10,11,24,62).…”

“…The consequence of these interactions with ␣ 2 M/␣ 2 M-MA can be passive (24) or stimulatory (10). However, the effects of binding to ␣ 2 M/␣ 2 M-MA can also be inhibitory (9,(11)(12)(13) via the prevention of hormone-receptor binding (9,13,25). This effect can also differ between ␣ 2 M and ␣ 2 M-MA (10).…”

“…defensin (HNP1) (17), transforming growth factor-␤1/2, etc. (7)(8)(9)(10)(11)(12)(13)24). The consequence of these interactions with ␣ 2 M/␣ 2 M-MA can be passive (24) or stimulatory (10).…”

“…The transport of hepcidin within the blood is of interest as: 1) many peptide hormones are bound by carriers that influence their function and distribution (7)(8)(9)(10)(11)(12)(13); 2) a greater understanding of the mechanism of hepcidin delivery to the cell could provide further information on its regulatory role; and 3) understanding the binding of hepcidin to a carrier protein would facilitate development of quantitative hepcidin assays.…”

iTRAQ analysis of Singapore grouper iridovirus infection in a grouper embryonic cell line

“…4). Furthermore, alpha-2-macroglobulin isoform 1, which participates in protease inhibition (Westwood et al, 2001), was upregulated in SGIV-infected GEC. It is likely that the virus can protect its proteins by inhibiting the host proteases and ubiquitin expression.…”