Analysis of thiolester bond cleavage-dependent conformational changes in binary alpha 2-macroglobulin-proteinase complexes

A derivative of the plasma protease inhibitor alpha(2)-macroglobulin regulates the response to peripheral nerve injury

Peripheral nerve injury induces endoneural inflammation, controlled by diverse cytokines and extracellular mediators. Although inflammation is coupled to axonal regeneration, fulminant inflammation may increase nerve damage and neuropathic pain. alpha(2)-Macroglobulin (alpha2M) is a plasma protease inhibitor, cytokine carrier, and ligand for cell-signaling receptors, which exists in two well-characterized conformations and in less well-characterized intermediate states. Previously, we generated an alpha2M derivative (alpha(2)-macroglobulin activated for cytokine binding; MAC) similar in structure to alpha(2)M conformational intermediates, which binds tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta), and inhibits endotoxin toxicity. In this study, we report that the continuum of cytokines that bind to MAC includes IL-6 and IL-18. MAC inhibited TNF-alpha-induced p38 mitogen-activated protein kinase activation and cell death in cultured Schwann cells. When administered by i.p. injection to mice with sciatic nerve crush injury, MAC decreased inflammation and preserved axons. Macrophage infiltration and TNF-alpha expression also are decreased. MAC inhibited TNF-alpha expression in the chronic constriction injury model of nerve injury. When MAC was prepared using a mutated recombinant alpha2M, which does not bind to the alpha2M receptor, low-density lipoprotein receptor-related protein-1, activity in the chronic constriction injury model was blocked. These studies demonstrate that an alpha2M derivative is capable of regulating the response to peripheral nerve injury by a mechanism that requires low-density lipoprotein receptor-related protein-1.

Chemical modification of alpha2-macroglobulin to generate derivatives that bind transforming growth factor-beta with increased affinity

alpha2-Macroglobulin (alpha2M) binds a number of cytokines, including transforming growth factor-beta1 (TGF-beta1) and TGF-beta2. The affinity of these interactions depends on the alpha2M conformation. In this investigation, we treated human alpha2M with cis-dichlorodiammineplatinum (II) (cis-Pt), a crosslinking reagent that partially 'locks' the alpha2M conformation, and then with methylamine to generate a preparation (alpha2M-P/M) consisting of stable alpha2M conformational intermediates. alpha2M-P/M bound TGF-beta1 and TGF-beta2 with higher affinity than any other form of alpha2M studied to date. The equilibrium dissociation constants were 14 and 2 nM for TGF-beta1 and TGF-beta2, respectively. alpha2M-P/M, at 100 nM, neutralized the activity of TGF-beta1 by about 75% in an endothelial cell proliferation assay. The equivalent concentration of native alpha2M or methylamine-modified alpha2M had no effect. These studies demonstrate that the potential of alpha2M as a cytokine carrier and neutralizer may not be fully realized in either the native or completely activated conformations.

Structural and functional analysis of the spontaneous re-formation of the thiol ester bond in human alpha 2-macroglobulin, rat alpha 1-inhibitor-3 and chemically modified derivatives

The alpha-macroglobulins are proteinase inhibitors that form part of a superfamily along with components of the complement system. Internal beta-cysteinyl-gamma-glutamyl thiol ester bonds are an important structural feature of most alpha-macroglobulins and several complement components. We have studied the reversibility of thiol ester cleavage caused by NH3 or CH3NH2 in tetrameric human alpha 2-macroglobulin (alpha 2M) and monomeric rat alpha 1-inhibitor-3 (alpha 1I3). When employing NH3 as the nucleophile, the thiol ester in alpha 1I3 re-formed spontaneously at room temperature after gel filtration to remove excess nucleophile, and an active proteinase inhibitor was regained. When CH3NH2 was employed as the nucleophile, thiol ester reversibility was more energy-demanding. With either nucleophile, alpha 2M once inactivated did not regain proteinase-inhibitory capacity at room temperature. At elevated temperatures, however, the reaction between alpha 2M and NH3 or CH3NH2 was reversible and the inhibitory capacity could be recovered. Modification of the cysteinyl groups from the thiol ester prevented its re-formation but did not prevent the heat-induced retrieval of inhibitory capacity, suggesting that conformational features rather than the thiol ester are essential for alpha 2M to function as an inhibitor. As demonstrated by non-denaturing PAGE, the conformation of native alpha 2M is restored when the proteinase-inhibitory capacity is recovered.

Binding of platelet-derived growth factor-BB and transforming growth factor-beta 1 to alpha 2-macroglobulin in vitro and in vivo: comparison of receptor-recognized and non-recognized alpha 2-macroglobulin conformations

alpha 2-Macroglobulin (alpha 2M) undergoes a major conformational change when reacting with proteinases or primary amines. This conformational change has been referred to as the 'slow' to 'fast' transformation based on the increase in alpha 2M mobility shown by non-denaturing PAGE. Previous studies demonstrated that many cytokines, including transforming growth factor beta 1 (TGF-beta 1) and interleukin-1 beta, bind preferentially or exclusively to alpha 2M which has undergone conformational change. In this study, we demonstrate that platelet-derived growth factor-BB (PDGF-BB) also binds preferentially to conformationally transformed alpha 2M (alpha 2M-methylamine, alpha 2M-trypsin) in vitro. Purified 125I-PDGF-BB-alpha 2M-methylamine complex cleared rapidly from the circulation of mice via the alpha 2M receptor/low-density-lipoprotein-receptor-related protein (alpha 2M-R/LRP). In order to determine whether PDGF-BB or TGF-beta 1 binds to native alpha 2M, we defined the native conformation by lack of interaction with alpha 2M-R/LRP instead of electrophoretic mobility. 125I-PDGF-BB was incubated with 4.3 microM native alpha 2M and 0.47 microM alpha 2M-methylamine. The 125I-PDGF-BB distributed evenly between slow-form and fast-form alpha 2M without shifting the electrophoretic mobility of either species. When the mixed preparation was injected intravenously in mice, 125I-PDGF-BB-fast-form-alpha 2M cleared rapidly and selectively from the circulation; 125I-PDGF-BB which was bound to slow-form alpha 2M was stable in the blood (apparently not recognized by alpha 2M-R/LRP). Therefore, while conformationally transformed alpha 2M binds PDGF-BB preferentially in vitro, non-alpha 2M-R/LRP-recognized alpha 2M binds PDGF-BB as well. Binding of 125I-PDGF-BB and 125I-TGF-beta 1 to alpha 2M was demonstrated in vivo by injecting the free growth factors intravenously into mice. Plasma samples which were subjected to non-denaturing PAGE and autoradiography demonstrated binding of both growth factors exclusively to the slow-form of alpha 2M. Therefore, under normal physiological conditions, native alpha 2M (non-alpha 2M-R/LRP-recognized) is the primary form of the proteinase inhibitor functioning as a carrier of PDGF-BB and TGF-beta 1 in the blood.

Independent analysis of bait region cleavage dependent and thiolester bond cleavage dependent conformational changes by cross-linking of alpha 2-macroglobulin with cis-dichlorodiammineplatinum(II) and dithiobis(succinimidyl propionate)

Treatment of the human plasma proteinase inhibitor alpha 2-macroglobulin (alpha 2M) with proteinase results in conformational changes in the inhibitor and subsequent activation and cleavage of the internal thiolester bonds of alpha 2M. Previous studies from this laboratory have shown that cross-linking the alpha 2M subunits with cis-dichlorodiammineplatinum(II) (cis-DDP) prevents the proteinase-induced conformational changes which lead to the activation and cleavage of the internal thiolester bonds of alpha 2M. In addition, cis-DDP treatment prevents the proteinase- or CH3NH2-induced conformational changes in alpha 2M which lead to a "slow" to "fast" change in nondenaturing polyacrylamide gel electrophoresis. In this paper, we demonstrate that treatment of alpha 2M with dithiobis(succinimidyl propionate) (DSP) also results in cross-linking of the subunits of alpha 2M with concomitant loss of proteinase inhibitory activity. Although proteinase is not inhibited by DSP-treated alpha 2M, bait region specific proteolysis of the alpha 2M subunits still occurs. Unlike cis-DDP-treated alpha 2M, however, incubation of DSP-treated alpha 2M with proteinase does not prevent the bait region cleavage dependent conformational changes which lead to activation and cleavage of the internal thiolester bonds in alpha 2M. On the other hand, cross-linking of alpha 2M with DSP does prevent the conformational changes which trigger receptor recognition site exposure following cleavage of the alpha 2M thiolester bonds by CH3NH2. These conformational changes, however, occur following incubation of the CH3NH2-treated protein with proteinase.(ABSTRACT TRUNCATED AT 250 WORDS)