Proteinases are isoform-specific regulators of the binding of transforming growth factor beta to alpha 2-macroglobulin

Agonists and Antagonists of TGF-β Family Ligands

The discovery of the transforming growth factor β (TGF-β) family ligands and the realization that their bioactivities need to be tightly controlled temporally and spatially led to intensive research that has identified a multitude of extracellular modulators of TGF-β family ligands, uncovered their functions in developmental and pathophysiological processes, defined the mechanisms of their activities, and explored potential modulator-based therapeutic applications in treating human diseases. These studies revealed a diverse repertoire of extracellular and membrane-associated molecules that are capable of modulating TGF-β family signals via control of ligand availability, processing, ligand-receptor interaction, and receptor activation. These molecules include not only soluble ligand-binding proteins that were conventionally considered as agonists and antagonists of TGF-β family of growth factors, but also extracellular matrix (ECM) proteins and proteoglycans that can serve as "sink" and control storage and release of both the TGF-β family ligands and their regulators. This extensive network of soluble and ECM modulators helps to ensure dynamic and cell-specific control of TGF-β family signals. This article reviews our knowledge of extracellular modulation of TGF-β growth factors by diverse proteins and their molecular mechanisms to regulate TGF-β family signaling.

Gene regulation networks related to neural differentiation of hESC

With the unique property of self-renewal and developmental pluripotency, human embryonic stem cells (hESC) provide an opportunity to study molecular aspects of developmental biology. Understanding gene regulation of hESC pluripotency is a critical step toward directing hESC differentiation for regenerative medicine. However, currently little is known about hESC gene regulation of hESC pluripotency. Applying network analysis to microarray gene expression profiling data, we compared gene expression profiles from pluripotent hESC to hESC-derived astrocytes and identified potential gene regulation networks. These gene regulation networks suggest that hECS has stringent control of cell cycle and apoptosis. Our data reveal several potential hESC differentiation biomarkers and suggest that IGF2 and A2M could play a role in hESC pluripotency by altering the availability of cytokines at the local environment of hECS. These findings underscore the importance of network analysis among differentially expressed genes, and should facilitate future study for understanding the gene regulation of hESC pluripotency.

Modulation of extracellular matrix genes reflects the magnitude of physiological adaptation to aerobic exercise training in humans

BACKGROUND

Regular exercise reduces cardiovascular and metabolic disease partly through improved aerobic fitness. The determinants of exercise-induced gains in aerobic fitness in humans are not known. We have demonstrated that over 500 genes are activated in response to endurance-exercise training, including modulation of muscle extracellular matrix (ECM) genes. Real-time quantitative PCR, which is essential for the characterization of lower abundance genes, was used to examine 15 ECM genes potentially relevant for endurance-exercise adaptation. Twenty-four sedentary male subjects undertook six weeks of high-intensity aerobic cycle training with muscle biopsies being obtained both before and 24 h after training. Subjects were ranked based on improvement in aerobic fitness, and two cohorts were formed (n = 8 per group): the high-responder group (HRG; peak rate of oxygen consumption increased by +0.71 +/- 0.1 L min(-1); p < 0.0001) while the low-responder group (LRG; peak rate of oxygen consumption did not change, +0.17 +/- 0.1 L min(-1), ns). ECM genes profiled included the angiopoietin 1 and related genes (angiopoietin 2, tyrosine kinase with immunoglobulin-like and EGF-like domains 1 (TIE1) and 2 (TIE2), vascular endothelial growth factor (VEGF) and related receptors (VEGF receptor 1, VEGF receptor 2 and neuropilin-1), thrombospondin-4, alpha2-macroglobulin and transforming growth factor beta2.

RESULTS

neuropilin-1 (800%; p < 0.001) and VEGF receptor 2 (300%; p < 0.01) transcript abundance increased only in the HRG, whereas levels of VEGF receptor 1 mRNA actually declined in the LRG (p < 0.05). TIE1 and TIE2 mRNA levels were unaltered in the LRG, whereas transcription levels of both genes were increased by 2.5-fold in the HRG (p < 0.01). Levels of thrombospondin-4 (900%; p < 0.001) and alpha2-macroglobulin (300%, p < 0.05) mRNA increased substantially in the HRG. In contrast, the amount of transforming growth factor beta2 transcript increased only in the HRG (330%; p < 0.01), whereas it remained unchanged in the LRG (-80%).

CONCLUSION

We demonstrate for the first time that aerobic training activates angiopoietin 1 and TIE2 genes in human muscle, but only when aerobic capacity adapts to exercise-training. The fourfold-greater increase in aerobic fitness and markedly differing gene expression profile in the HRG indicates that these ECM genes may be critical for physiological adaptation to exercise in humans. In addition, we show that, without careful demonstration of physiological adaptation, conclusions derived from gene expression profiling of human skeletal muscle following exercise may be of limited value. We propose that future studies should (a) investigate the mechanisms that underlie the apparent link between physiological adaptation and gene expression and (b) use the genes profiled in this paper as candidates for population genetic studies.

THE CAPACITY OF α2-MACROGLOBULIN TO INHIBIT AN EXOGENOUS PROTEASE IS SIGNIFICANTLY INCREASED IN CRITICALLY ILL AND SEPTIC PATIENTS

The image of alpha2-macroglobulin is based on a paradigm evolved in the 1970s. During this decade alpha2-macroglobulin was shown to irreversibly entrap and thereby inhibit a maximum of two proteases. Additional binding of nonproteolytic proteins, i.e., inflammatory mediators and growth factors, is dependent on the conformational status of alpha2-macroglobulin. It was our aim to clarify whether the interaction of nonproteolytic proteins with alpha2-macroglobulin during inflammatory conditions might also modulate the capacity of alpha2-macroglobulin to inhibit proteases. To explore this possibility, bromelain, an exogenous protease, was titrated against plasma of critically ill or septic patients, whose pathophysiological conditions are defined by a massive release of inflammatory mediators. The stoichiometry of bromelain inhibition by alpha2-macroglobulin was quantified by caseolytic activity assays. The maximal alpha2-macroglobulin/bromelain inhibition ratios were significantly increased (1:6 and 1:8 in the two patient groups, P < 0.01) as compared with control groups (1:2 with purified alpha2-macroglobulin and 1:4 in healthy volunteers). The increase of alpha2-macroglobulin inhibition capacity in patients was paralleled by the appearance of a large signal on Western blots, suggesting the formation of additional complexes. Our results demonstrate alpha2-macroglobulin to have more flexibility than had been thought, and it may thereby contribute to a shift in a 30-year-old paradigm.

Functional analysis of plasma α2-macroglobulin from Alzheimer's disease patients with the A2M intronic deletion

alpha(2)-Macroglobulin (alpha(2)M) is an abundant plasma/extracellular space protein implicated in clearance of amyloid beta (Abeta), a key constituent of Alzheimer's disease (AD) plaques. alpha(2)M also regulates proteinase and growth factor activities. In recent years, there have been >30 genetic studies debating the controversial role of a five-base-pair intronic deletion in the A2M gene in late-onset AD. However, little is known about potential effects of the deletion upon alpha(2)M function. In this study, we examined the subunit and conformational structure of alpha(2)M in AD plasma samples, and its capacity to bind trypsin, transforming growth factor-beta1, and Abeta. Plasma from patients homozygous for the deletion (DD) showed normal alpha(2)M subunit size, conformation, and proteinase inhibitory activity. Interestingly, plasma alpha(2)M from two DD patients showed markedly increased TGF-beta1 binding. Moreover, methylamine-treated DD plasma samples showed modest, but significant, elevations in Abeta binding to alpha(2)M* compared with samples from patients lacking the deletion. These observations suggest a possible functional basis by which the A2M deletion may influence multifactorial AD pathogenesis.

Increased Trypanosoma cruzi invasion and heart fibrosis associated with high transforming growth factor beta levels in mice deficient in alpha(2)-macroglobulin

Trypanosoma cruzi proteinases are involved in host cell invasion in human patients and in mouse models. In mice, murine alpha(2)-macroglobulin (MAM) and murinoglobulin are circulating plasma proteinase inhibitors that also have important roles in inflammation and immune modulation. To define their role in experimental Chagas disease, we investigated the susceptibility to T. cruzi infection of mice that are deficient only in alpha2-macroglobulins (AM-KO) or in both MAM and monomeric murinoglobulin-1 (MM-KO), relative to the wild type (WT). Despite the high parasite load, parasitemia was lower in AM-KO and MM-KO mice than in WT mice. Nevertheless, we observed a significantly higher parasite load in the hearts of AM-KO and MM-KO mice, i.e., more amastigote nests and inflammatory infiltrates than in WT mice. This result demonstrates a protective role for MAM in the acute phase of murine T. cruzi infection. We further demonstrated in vitro that human alpha2-macroglobulins altered the trypomastigote morphology and motility in a dose-dependent way, and that also impaired T. cruzi invasion in cardiomyocytes. Finally, we demonstrated that the levels of transforming growth factor beta in AM-KO mice increased significantly in the third week postinfection, concomitant with high amastigote burden and important fibrosis. Combined, these in vivo and in vitro findings demonstrate that the MAM contribute to the resistance of mice to acute myocarditis induced by experimental T. cruzi infection.

Identification of the high affinity binding site in transforming growth factor-beta involved in complex formation with alpha 2-macroglobulin. Implications regarding the molecular mechanisms of complex formation between alpha 2-macroglobulin and growth factors, cytokines, and hormones

The biological activities of transforming growth factor-beta isoforms (TGF-beta(1,2)) are known to be modulated by alpha(2)-macroglobulin (alpha(2)M). alpha(2)M forms complexes with numerous growth factors, cytokines, and hormones, including TGF-beta. Identification of the binding sites in TGF-beta isoforms responsible for high affinity interaction with alpha(2)M many unravel the molecular basis of the complex formation. Here we demonstrate that among nine synthetic pentacosapeptides with overlapping amino acid sequences spanning the entire TGF-beta(1) molecule, the peptide (residues 41-65) containing Trp-52 exhibited the most potent activity in inhibiting the formation of complexes between (125)I-TGF-beta(1) and activated alpha(2)M (alpha(2)M*) as determined by nondenaturing polyacrylamide gel electrophoresis and by plasma clearance in mice. TGF-beta(2) peptide containing the homologous sequence and Trp-52 was as active as the TGF-beta(1) peptide, whereas the corresponding TGF-beta(3) peptide lacking Trp-52, was inactive. The replacement of the Trp-52 with alanine abolished the inhibitory activities of these peptides. (125)I-TGF-beta(3), which lacks Trp-52, bound to alpha(2)M* with an affinity lower than that of (125)I-TGF-beta(1). Furthermore, unlabeled TGF-beta(3) and the mutant TGF-beta(1)W52A, in which Trp-52 was replaced with alanine, were less potent than unlabeled TGF-beta(1) in blocking I(125)-TGF-beta(1) binding to alpha(2)M*. TGF-beta(1) and TGF-beta(2) peptides containing Trp-52 were also effective in inhibiting I(125)-nerve growth factor binding to alpha(2)M*. Tauhese results suggest that Trp-52 is involved in high affinity binding of TGF-beta to alpha(2)M*. They also imply that TGF-beta and other growth factors/cytokines/hormones may form complexes with alpha(2)M* via a common mechanism involving the interactions between topologically exposed Trp and/or other hydrophobic residues and a hydrophobic region in alpha(2)M*.

Trypanosoma cruzi: acute infection affects expression of alpha-2-macroglobulin and A2MR/LRP receptor differently in C3H and C57BL/6 mice

Although a complete cellular and humoral immune response is elicited in Chagas' disease, recent data suggest that other natural elements of innate immunity may also contribute to the initial host primary defense. alpha-Macroglobulins are a family of plasma proteinase inhibitors that are acute-phase reactants in Trypanosoma cruzi-infected mice and humans. Mice contain a tetrameric alpha-2-macroglobulin (MAM) and a monomeric murinoglobulin (MUG). Heterogeneity in their reactions was observed in murine T. cruzi-infected plasma A2M levels despite an overall increase. In addition, up-regulation of the A2M receptor (A2MR/LRP) was observed in peritoneal macrophages during T. cruzi infection. Here, we show that during T. cruzi infection (Y strain), the MAM and MUG hepatic mRNA levels and the corresponding plasma protein levels were up-regulated in C3H and C57BL/6 (B6) mice, but with different kinetics. On the contrary, A2MR/LRP mRNA levels increased in acutely infected C3H mice, but decreased in B6 mice, in both liver and heart. Immunocytochemistry of infected B6 heart cryosections confirmed a less intense endothelium labeling by the fluoresceinated ligand for A2MR/LRP. On the other hand, infected B6 spleen cells displayed higher F-A2M-FITC binding and MAC1 expression, confirming higher A2MR/LRP expression in macrophages. In uninfected mice, as well as after T. cruzi infection, higher A2M plasma levels were measured in C3H mice than in B6 mice. The lower tissue T. cruzi parasitism found in C3H-infected mice could reflect an inhibitory effect of A2M on parasite invasion. Our present data further contribute to clarifying aspects of the role of A2MR/LRP in a model of acute Chagas' disease in different mouse strains.

Oxidized α2-Macroglobulin (α2M) Differentially Regulates Receptor Binding by Cytokines/Growth Factors: Implications for Tissue Injury and Repair Mechanisms in Inflammation

α2M binds specifically to TNF-α, IL-1β, IL-2, IL-6, IL-8, basic fibroblast growth factor (bFGF), β-nerve growth factor (β-NGF), platelet-derived growth factor (PDGF), and TGF-β. Since many of these cytokines are released along with neutrophil-derived oxidants during acute inflammation, we hypothesize that oxidation alters the ability of α2M to bind to these cytokines, resulting in differentially regulated cytokine functions. Using hypochlorite, a neutrophil-derived oxidant, we show that oxidized α2M exhibits increased binding to TNF-α, IL-2, and IL-6 and decreased binding to β-NGF, PDGF-BB, TGF-β1, and TGF-β2. Hypochlorite oxidation of methylamine-treated α2M (α2M*), an analogue of the proteinase/α2M complex, also results in decreased binding to bFGF, β-NGF, PDGF-BB, TGF-β1, and TGF-β2. Concomitantly, we observed decreased ability to inhibit TGF-β binding and regulation of cells by oxidized α2M and α2M*. We then isolated α2M from human rheumatoid arthritis synovial fluid and showed that the protein is extensively oxidized and has significantly decreased ability to bind to TGF-β compared with α2M derived from plasma and osteoarthritis synovial fluid. We, therefore, propose that oxidation serves as a switch mechanism that down-regulates the progression of acute inflammation by sequestering TNF-α, IL-2, and IL-6, while up-regulating the development of tissue repair processes by releasing bFGF, β-NGF, PDGF, and TGF-β from binding to α2M.

Localization of the binding site for transforming growth factor-beta in human alpha2-macroglobulin to a 20-kDa peptide that also contains the bait region

alpha2-Macroglobulin (alpha2M) functions as a major carrier of transforming growth factor-beta (TGF-beta) in vivo. The goal of this investigation was to characterize the TGF-beta-binding site in alpha2M. Human alpha2M, which was reduced and denatured to generate 180-kDa subunits, bound TGF-beta1, TGF-beta2, and NGF-beta in ligand blotting experiments. Cytokine binding was not detected with bovine serum albumin that had been reduced and alkylated, and only minimal binding was detected with purified murinoglobulin. To localize the TGF-beta-binding site in alpha2M, five cDNA fragments, collectively encoding amino acids 122-1302, were expressed as glutathione S-transferase (GST) fusion proteins. In ligand blotting experiments, TGF-beta2 bound only to the fusion protein (FP3) that includes amino acids 614-797. FP3 bound 125I-TGF-beta1 and 125I-TGF-beta2 in solution, preventing the binding of these growth factors to immobilized alpha2M-methylamine (alpha2M-MA). The IC50 values were 33 +/- 5 and 26 +/- 6 nM for TGF-beta1 and TGF-beta2, respectively; these values were comparable with or lower than those determined with native alpha2M or alpha2M-MA. A GST fusion protein that includes amino acids 798-1082 of alpha2M (FP4) and purified GST did not inhibit the binding of TGF-beta to immobilized alpha2M-MA. FP3 (0.2 microM) neutralized the activity of TGF-beta1 and TGF-beta2 in fetal bovine heart endothelial (FBHE) cell proliferation assays; FP4 was inactive in this assay. FP3 also increased NO synthesis by RAW 264.7 cells, mimicking an alpha2M activity that has been attributed to the neutralization of endogenously synthesized TGF-beta. Thus, we have isolated a peptide corresponding to 13% of the alpha2M sequence that binds TGF-beta and neutralizes the activity of TGF-beta in two separate biological assays.

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.