The Binding of Receptor-recognized α2-Macroglobulin to the Low Density Lipoprotein Receptor-related Protein and the α2M Signaling Receptor Is Decoupled by Oxidation

Abstract

1 is a highly conserved, homotetrameric, 720-kDa glycoprotein found in high concentration in the plasma (2-4 mg/ml). It has the unique ability to inhibit all mechanistic classes of proteinases by "entrapping" the proteinase and thereby sterically blocking the access of high molecular weight substrates (reviewed in Refs. 1 and 2). Proteinases first cleave the "bait region" of native ␣ 2 M exposing the internal ␥-glutamyl-␤-cysteinyl thioester bond. Reaction of the thioester bond with a free amino lysyl residue on the surface of the proteinase results in bond rupture and a major conformational change in native ␣ 2 M. The resulting molecule is much more compact as evidenced by faster migration on a native acrylamide gel (3), electron microscopy (4, 5), sedimentation behavior (6), and circular dichroism (7). Consequently the receptor-recognition site is exposed. Small amine nucleophiles, such as methylamine, can initiate this reaction by directly attacking the thioester bond generating the conformational change and the exposure of the receptor-recognition site without bait region cleavage. Receptor-recognized ␣ 2 M (␣ 2 M*) can rapidly eliminate the "entrapped" proteinase from the circulation by binding to a cell surface clearance receptor, the low density lipoprotein receptor-related protein (LRP) (8, 9). LRP is a multiligand receptor that binds to a wide variety of unrelated ligands (reviewed in Ref. 10). Binding of all ligands to LRP can be effectively competed by receptor-associated protein (RAP), which co-purifies with LRP. Prior investigation of ␣ 2 M* binding to LRP has shown that the binding mechanism involves a cluster of positively charged residues on ␣ 2 M* interacting with the second complement-like repeat on LRP, which contains clusters of negatively charged residues (11). Analysis of the receptor-binding site on ␣ 2 M* using monoclonal antibody (12, 13) and recombinantly expressed protein (14, 15) demonstrates that the carboxyl terminus of ␣ 2 M* is involved in receptor binding.Although LRP is the only ␣ 2 M* receptor identified to date, some important cellular regulatory functions ascribed to ␣ 2 M* suggest that an alternate receptor must exist. ␣ 2 M*, but not native ␣ 2 M, suppresses the production of superoxide anion (16), enhances the release of prostaglandin E 2 (17, 18) and platelet activating factor (19), and stimulates the proliferation of vascular smooth muscle cells (20). Moreover, our laboratory has characterized a novel signaling cascade and found that it does not appear to be LRP-mediated (21-23). Furthermore, we have identified two classes of ␣ 2 M* binding sites on peritoneal macrophages and human trabecular meshwork cells, both of which demonstrate activation of signaling cascades after exposure to ␣ 2 M* (24). The lower affinity binding site is 10 times more abundant than the high affinity binding site and has clearly been identified as LRP (25,26). The identity of the signaling receptor remains elusive; however, using site-directed mutagenesis, we have found that a lysine residue (13...

Hypochlorite-induced structural modifications enhance the chaperone activity of human α2-macroglobulin

“…Reduces Aβ Neurotoxicity. Because the hypochlorite-induced structural modifications of α 2 M can expose the LRP binding site (28) and also influence the binding of α 2 M to Aβ (Fig. 5 A and B), we examined the effect of preincubating bAβ with native or oxidized α 2 M on the subsequent binding of bAβ to RAW 264.7 macrophages.…”

“…It has been shown previously that α 2 M is highly sensitive to the effects of hypochlorite, which induces dissociation of native α 2 M tetramers into stable dimers that are no longer able to trap proteases (25)(26)(27). In contrast, exposure of α 2 M to hypochlorite enhances its binding to some cytokines and growth factors and reveals on α 2 M the cryptic binding site for the endocytic lowdensity lipoprotein receptor-related protein (LRP) (28,29). Thus, the results of previous studies suggest that modification by hypochlorite could serve as a switch to control the functions of α 2 M during innate immune system activity.…”

Overexpression of Alzheimer's Disease Amyloid-β Opposes the Age-dependent Elevations of Brain Copper and Iron

“…We hypothesize that excess binding of copper and iron to A␤ could alter the metabolism of A␤ leading to its precipitation by the constitutively high ambient zinc concentrations in the synaptic (and corticovascular) milieu. Copper and iron binding to A␤ engenders H 2 O 2 production by A␤ (25), which may inhibit LRPmediated clearance mechanisms (44), leading to A␤ accumulation. An alternative possibility is that A␤ is oxidatively modified by reaction with excess copper or iron (2) and that these modified forms of A␤ are more vulnerable to zinc-(or other metal) induced precipitation.…”

PZP and PAI-2: Structurally-diverse, functionally similar pregnancy proteins?

“…The activities of α 2 M are uniquely regulated by hypochlorite (an oxidant produced in high concentrations during inflammation), which induces the dissociation of the native α 2 M tetramer into dimers that bind to: (i) misfolded proteins (Wyatt et al, 2014); (ii) growth factors and pro-inflammatory cytokines (Wu et al, 1998), and (iii) the receptor LRP (Wu et al, 1997). It is tempting to speculate that PZP (predominately a dimer in biological fluids) will perform many of the same functions as α 2 M dimers (Fig.…”