Evidence for a second alpha 2-macroglobulin receptor

Alpha-2-Macroglobulin in Inflammation, Immunity and Infections

Alpha-2-macroglobulin is an extracellular macromolecule mainly known for its role as a broad-spectrum protease inhibitor. By presenting itself as an optimal substrate for endopeptidases of all catalytic types, alpha-2-macroglobulin lures active proteases into its molecular cage and subsequently ‘flags’ their complex for elimination. In addition to its role as a regulator of extracellular proteolysis, alpha-2-macroglobulin also has other functions such as switching proteolysis towards small substrates, facilitating cell migration and the binding of cytokines, growth factors and damaged extracellular proteins. These functions appear particularly important in the context of immune-cell function. In this review manuscript, we provide an overview of all functions of alpha-2-macroglobulin and place these in the context of inflammation, immunity and infections.

Failure of in vitro-cultured schistosomes to produce eggs: how does the parasite meet its needs for host-derived cytokines such as TGF-β?

When adult schistosome worm pairs are transferred from experimental hosts to in vitro culture they cease producing viable eggs within a few days. Female worms in unisexual infections fail to mature, and when mature adult females are separated from male partners they regress sexually. Worms cultured from the larval stage are also permanently reproductively defective. The cytokine transforming growth factor beta derived from the mammalian host is considered important in stimulating schistosome female worm maturation and maintenance of fecundity. The means by which schistosomes acquire TGF-β have not been elucidated, but direct uptake in vivo seems unlikely as the concentration of free, biologically active cytokine in host blood is very low. Here we review the complexities of schistosome development and male-female interactions, and we speculate about two possibilities on how worms obtain the TGF-β they are assumed to need: (i) worms may have mechanisms to free active cytokine from the latency-inducing complex of proteins in which it is associated, and/or (ii) they may obtain the cytokine from alpha 2-macroglobulin, a blood-borne protease inhibitor to which TGF-β can bind. These ideas are experimentally testable.

Activated α2-macroglobulin binding to human prostate cancer cells triggers insulin-like responses

Ligation of cell surface GRP78 by activated α2-macroglobulin (α2M*) promotes cell proliferation and suppresses apoptosis. α2M*-treated human prostate cancer cells exhibit a 2-3-fold increase in glucose uptake and lactate secretion, an effect similar to insulin treatment. In both α2M* and insulin-treated cells, the mRNA levels of SREBP1-c, SREBP2, fatty-acid synthase, acetyl-CoA carboxylase, ATP citrate lyase, and Glut-1 were significantly increased together with their protein levels, except for SREBP2. Pretreatment of cells with α2M* antagonist antibody directed against the carboxyl-terminal domain of GRP78 blocks these α2M*-mediated effects, and silencing GRP78 expression by RNAi inhibits up-regulation of ATP citrate lyase and fatty-acid synthase. α2M* induces a 2-3-fold increase in lipogenesis as determined by 6-[(14)C]glucose or 1-[(14)C]acetate incorporation into free cholesterol, cholesterol esters, triglycerides, free fatty acids, and phosphatidylcholine, which is blocked by inhibitors of fatty-acid synthase, PI 3-kinase, mTORC, or an antibody against the carboxyl-terminal domain of GRP78. We also assessed the incorporation of [(14)CH3]choline into phosphatidylcholine and observed similar effects. Lipogenesis is significantly affected by pretreatment of prostate cancer cells with fatostatin A, which blocks sterol regulatory element-binding protein proteolytic cleavage and activation. This study demonstrates that α2M* functions as a growth factor, leading to proliferation of prostate cancer cells by promoting insulin-like responses. An antibody against the carboxyl-terminal domain of GRP78 may have important applications in prostate cancer therapy.

Citrullinated glucose-regulated protein 78 is an autoantigen in type 1 diabetes

Posttranslational modifications of self-proteins play a substantial role in the initiation or propagation of the autoimmune attack in several autoimmune diseases, but their contribution to type 1 diabetes is only recently emerging. In the current study, we demonstrate that inflammatory stress, induced by the cytokines interleukin-1β and interferon-γ, leads to citrullination of GRP78 in β-cells. This is coupled with translocation of this endoplasmic reticulum chaperone to the β-cell plasma membrane and subsequent secretion. Importantly, expression and activity of peptidylarginine deiminase 2, one of the five enzymes responsible for citrullination and a candidate gene for type 1 diabetes in mice, is increased in islets from diabetes-prone nonobese diabetic (NOD) mice. Finally, (pre)diabetic NOD mice have autoantibodies and effector T cells that react against citrullinated GRP78, indicating that inflammation-induced citrullination of GRP78 in β-cells generates a novel autoantigen in type 1 diabetes, opening new avenues for biomarker development and therapeutic intervention.

Introduction to a Special Issue of the Journal of Immunological Methods: Building global resource programs to support HIV/AIDS clinical trial studies

This Special Issue of the Journal of Immunological Methods includes 16 manuscripts describing quality assurance activities related to virologic and immunologic monitoring of six global laboratory resource programs that support international HIV/AIDS clinical trial studies: Collaboration for AIDS Vaccine Discovery (CAVD); Center for HIV/AIDS Vaccine Immunology (CHAVI); External Quality Assurance Program Oversight Laboratory (EQAPOL); HIV Vaccine Trial Network (HVTN); International AIDS Vaccine Initiative (IAVI); and Immunology Quality Assessment (IQA). The reports from these programs address the many components required to develop comprehensive quality control activities and subsequent quality assurance programs for immune monitoring in global clinical trials including: all aspects of processing, storing, and quality assessment of PBMC preparations used ubiquitously in HIV clinical trials, the development and optimization of assays for CD8 HIV responses and HIV neutralization, a comprehensive global HIV virus repository, and reports on the development and execution of novel external proficiency testing programs for immunophenotyping, intracellular cytokine staining, ELISPOT and luminex based cytokine measurements. In addition, there are articles describing the implementation of Good Clinical Laboratory Practices (GCLP) in a large quality assurance laboratory, the development of statistical methods specific for external proficiency testing assessment, a discussion on the ability to set objective thresholds for measuring rare events by flow cytometry, and finally, a manuscript which addresses a framework for the structured reporting of T cell immune function based assays. It is anticipated that this series of manuscripts covering a wide range of quality assurance activities associated with the conduct of global clinical trials will provide a resource for individuals and programs involved in improving the harmonization, standardization, accuracy, and sensitivity of virologic and immunologic testing.

Contribution of murine innate serum inhibitors toward interference within influenza virus immune assays

Please cite this paper as: Cwach et al. (2011) Contribution of murine innate serum inhibitors toward interference within influenza virus immune assays. Influenza and Other Respiratory Viruses DOI: 10.1111/j.1750‐2659.2011.00283.x.

Background  Prior to detection of an antibody response toward influenza viruses using the hemagglutination inhibition assay (HAI), sera are routinely treated to inactivate innate inhibitors using both heat inactivation (56°C) and recombinant neuraminidase [receptor‐destroying enzyme (RDE)].

Objectives  We revisited the contributions of innate serum inhibitors toward interference with influenza viruses in immune assays, using murine sera, with emphasis on the interactions with influenza A viruses of the H3N2 subtype.

Methods  We used individual serum treatments: 56°C alone, RDE alone, or RDE + 56°C, to treat sera prior to evaluation within HAI, microneutralization, and macrophage uptake assays.

Results  Our data demonstrate that inhibitors present within untreated murine sera interfere with the HAI assay in a manner that is different from that seen for the microneutralization assay. Specifically, the γ class inhibitor α₂‐Macroglobulin (A2‐M) can inhibit H3N2 viruses within the HAI assay, but not in the microneutralization assay. Based on these findings, we used a macrophage uptake assay to demonstrate that these inhibitors can increase uptake by macrophages when the influenza viruses express an HA from a 1968 H3N2 virus isolate, but not a 1997 H3N2 isolate.

Conclusions  The practice of treating sera to inactivate innate inhibitors of influenza viruses prior to evaluation within immune assays has allowed us to effectively detect influenza virus‐specific antibodies for decades. However, this practice has yielded an under‐appreciation for the contribution of innate serum inhibitors toward host immune responses against these viruses, including contributions toward neutralization and macrophage uptake.

Loss of cell surface TFII-I promotes apoptosis in prostate cancer cells stimulated with activated α₂ -macroglobulin

Receptor-recognized forms of α₂ -macroglobulin (α₂ M) bind to cell surface-associated GRP78 and initiate pro-proliferative and anti-apoptotic signaling. Ligation of GRP78 with α₂ M also upregulates TFII-I, which binds to the GRP78 promoter and enhances GRP78 synthesis. In addition to its transcriptional functions, cytosolic TFII-I regulates agonist-induced Ca(2+) entry. In this study we show that down regulation of TFII-I gene expression by RNAi profoundly impairs its cell surface expression and anti-apoptotic signaling as measured by significant reduction of GRP78, Bcl-2, and cyclin D1 in 1-Ln and DU-145 human prostate cancer cells stimulated with α₂ M. In contrast, this treatment significantly increases levels of the pro-apoptotic proteins p53, p27, Bax, and Bak and causes DNA fragmentation. Furthermore, down regulation of TFII-I expression activates agonist-induced Ca(2+) entry. In plasma membrane lysates p-PLCγ1, TRPC3, GRP78, MTJ1, and caveolin co-immunoprecipitate with TFII-I suggesting multimeric complexes of these proteins. Consistent with this hypothesis, down regulating TFII-I, MTJ1, or GRP78 expression by RNAi greatly attenuates cell surface expression of TFII-I. In conclusion, we demonstrate that not only does cell surface GRP78 regulate apoptosis, but it also regulates Ca(2+) homeostasis by controlling cell surface localization of TFII-I.

Activated α(2) macroglobulin induces matrix metalloproteinase 9 expression by low-density lipoprotein receptor-related protein 1 through MAPK-ERK1/2 and NF-κB activation in macrophage-derived cell lines

Macrophages under certain stimuli induce matrix metalloproteinase 9 (MMP-9) expression and protein secretion through the activation of MAPK-ERK and NF-κB signaling pathways. Previously, we demonstrated that activated α(2)-macroglulin (α(2)M*) through the interaction with its receptor low-density lipoprotein receptor-related protein 1 (LRP1) induces macrophage proliferation mediated by the activation of MAPK-ERK1/2. In the present work, we examined whether α(2)M*/LRP1interaction could induce the MMP-9 production in J774 and Raw264.7 macrophage-derived cell lines. It was shown that α(2)M* promoted MMP-9 expression and protein secretion by LRP1 in both macrophage-derived cell lines, which was mediated by the activation of MAPK-ERK1/2 and NF-κB. Both intracellular signaling pathways activated by α(2)M* were effectively blocked by calphostin-C, suggesting involvement of PKC. In addition, we demonstrate that α(2)M* produced extracellular calcium influx via LRP1. However, when the intracellular calcium mobilization was inhibited by BAPTA-AM, the α(2)M*-induced MAPK-ER1/2 activation was fully blocked in both macrophage cell lines. Finally, using specific pharmacological inhibitors for PKC, Mek1, and NF-κB, it was shown that the α(2)M*-induced MMP-9 protein secretion was inhibited, indicating that the MMP production promoted by the α(2)M*/LRP1 interaction required the activation of both signaling pathways. These findings may prove useful in the understanding of the macrophage LRP1 role in the vascular wall during atherogenic plaque progression.

Modulation of the unfolded protein response in prostate cancer cells by antibody-directed against the carboxyl-terminal domain of GRP78

Receptor-recognized forms of alpha(2)-macroglobulin (alpha(2)M*) bind to cancer cell surface GRP78, which functions as a signaling receptor promoting proliferation and survival. Patients with prostate, ovary, and skin cancer may develop auto-antibodies to the alpha(2)M* binding site which are receptor agonists whose presence indicates a poor prognosis. By contrast, antibodies directed against the COOH-terminal domain of GPR78 (anti-CTD antibody), are antagonists which down regulate pro-proliferative signaling and upregulate p53. Unfolded protein response (UPR) signaling plays an important role in cell survival and proliferation as well as apoptosis. We, therefore, studied the effect of anti-CTD antibody on UPR signaling in 1-LN and DU-145 prostate cancer cells. Treatment of these cells, which express GRP78 on their cell surface, with this antibody significantly downregulated IRE1-alpha, PERK, and ATF6alpha-dependent UPR signaling. By contrast, the pro-apoptotic protein GADD153 was elevated. Anti-CTD antibody treatment also elevated apoptotic components, cleaved PARP-1, and Erdj5. In general, a two to threefold effect was observed for the parameters which were studied. These studies suggest that anti-CTD antibody induces growth inhibitory and pro-apoptotic effects by modulating UPR signaling in human prostate cancer cells.

GRP78: a multifunctional receptor on the cell surface

The 78 kDa glucose-regulated protein (GRP78) is an endoplasmic reticulum chaperone, whose function is generally thought to be restricted to controlling the structural maturation of nascent glycoproteins. However, GRP78 also is expressed on the cell surface where it functions as a receptor for a wide variety of ligands, behaving as an autoantigen for several classes of autoantibodies. GRP78 is a signaling receptor for activated alpha2-macroglobulin, plasminogen kringle 5, and microplasminogen, and it plays a critical role in viral entry of coxsackie B, and dengue fever viruses. GRP78 is also implicated in the regulation of tissue factor procoagulant activity and functions as a receptor for angiogenic peptides via a mechanism independent of the VEGF receptor. Cell surface GRP78 is found associated with such diverse proteins as the voltage-dependent anion channel (VDAC), the major histocompatibility complex class I (MHC-I), the teratocarcinoma-derived growth factor I (Cripto), and the DnaJ-like protein MTJ-1. These associations suggest a unique GRP78 cell surface topography, which appears to be compartmentalized to respond differently to agonists that bind to its N- or C-terminal domains. Here, we discuss the significance of these associations, and the possible mechanisms involved in the transportation of GRP78 from the cytosol to the cell surface.

Alpha 2-macroglobulin binds CpG oligodeoxynucleotides and enhances their immunostimulatory properties by a receptor-dependent mechanism

CpG oligodeoxynucleotides (ODN) stimulate the immune system and are under evaluation as treatments and vaccine adjuvants for infectious diseases, cancer, and immune system disorders. Although they have shown promising results in numerous clinical trials, the ultimate use of CpG ODN-based therapeutics may hinge on improved pharmacokinetics and reduced systemic side-effects. CpG ODN efficacy and potency might be enhanced greatly by packaging them into particles that protect them from degradation and specifically target them for uptake by immune-competent cells. The plasma proteinase inhibitor alpha 2-macroglobulin (alpha 2M) binds numerous biologically active macromolecules, including cytokines, chemokines, and growth factors, and can modulate their activity. Molecules bound to alpha 2M are protected from interactions with neighboring macromolecules and are targeted for receptor-mediated uptake by immune-competent cells. Here, we report that activated alpha 2M (alpha 2M*) binds CpG ODN and enhances their immunostimulatory properties significantly. Murine macrophages treated with alpha 2M*-ODN complexes respond more rapidly and produce a greater cytokine response than induced by free CpG ODN. Using human PBMC, alpha 2M*-ODN complexes exhibit fourfold enhanced potency and 15-fold greater efficacy for stimulating production of inflammatory cytokines. alpha 2M* targets delivery of CpG ODN specifically to immune-competent cells, which endocytose the complexes sixfold more rapidly than free CpG ODN. CpG ODN bound to alpha 2M* are also protected from degradation by nucleases. This novel targeting technology may improve CpG ODN-based therapeutics by increasing efficacy at reduced doses, thus reducing side-effects and cost.

Prostate cancer cell proliferation in vitro is modulated by antibodies against glucose-regulated protein 78 isolated from patient serum

Circulating autoantibodies against the glucose-regulated protein of 78 kDa (GRP78) are present at high levels in prostate cancer patients and are a biomarker of aggressive tumor behavior. We purified the anti-GRP78 IgGs and examined their effect on 1-LN, PC-3, DU145, and LnCap human prostate cancer cells. We also evaluated its effects on the breast cancer MDA-MB231 and melanoma DM413 cell lines. The anti-GRP78 antibody binds only to cells expressing GRP78 on the surface, to a site also recognized by its physiologic agonist, activated alpha(2)-macroglobulin (alpha(2)M*). This antibody is completely specific for a peptide, including the primary amino acid sequence CNVKSDKSC, which contains a tertiary structural motif mimicking an epitope in GRP78. Tertiary structural analysis suggested the linear GRP78 primary amino acid sequence LIGRTWNDPSVQQDIKFL (Leu(98)-Leu(115)) as the putative binding site, containing the tertiary structual arrangement described above, which was confirmed experimentally. The anti-GRP78 antibodies from prostate cancer patients recognize almost exclusively this epitope. We produced animal antibodies against both these peptides, and they are able to mimic the effects of the human antibody. Our experiments also suggest this epitope as highly immunogenic, thereby explaining the specificity of the immune response against this epitope in GRP78, observed in humans. Using 1-LN cells as a model, we show that anti-GRP78 IgG purified from the sera of these patients mimics the proproliferative effects induced by alpha(2)M* via the common receptor, GRP78. Furthermore, increasing concentrations of human anti-GRP78 IgG show a dose-dependent protective effect on apoptosis induced by tumor necrosis factor alpha.

Activation and cross-talk between Akt, NF-kappaB, and unfolded protein response signaling in 1-LN prostate cancer cells consequent to ligation of cell surface-associated GRP78

Binding of activated forms of the proteinase inhibitor alpha2-macroglobulin (alpha2M*) to cell surface-associated GRP78 on 1-LN human prostate cancer cells causes their proliferation. We have now examined the interplay between Akt activation, regulation of apoptosis, the unfolded protein response, and activation of NF-kappaB in alpha2M*-induced proliferation of 1-LN cells. Exposure of cells to alpha2M* (50 pM) induced phosphatidylinositol 3-kinase-dependent activation of Akt by phosphorylation at Thr-308 and Ser-473 with a concomitant 60-80% increase in Akt-associated kinase activity. ERK1/2 and p38 MAPK were also activated, but there was only a marginal effect on JNK activation. Treatment of 1-LN cells with alpha2M* down-regulated apoptosis and promoted NF-kappaB activation as shown by increases of Bcl-2, p-Bad(Ser-136), p-FOXO1(Ser-253), p-GSK3beta(Ser-9), XIAP, NF-kappaB, cyclin D1, GADD45beta, p-ASK1(Ser-83), and TRAF2 in a time of incubation-dependent manner. alpha2M* treatment of 1-LN cells, however, showed no increase in the activation of caspase -3, -9, or -12. Under these conditions, we observed increased unfolded protein response signaling as evidenced by elevated levels of GRP78, IRE1alpha, XBP-1, ATF4, ATF6, p-PERK, p-eIF2alpha, and GADD34 and reduced levels of GADD153. Silencing of GRP78 gene expression by RNAi suppressed activation of Akt(Thr-308), Akt(Ser-473), and IkappaB kinase alpha kinase. The effects of alpha2M* on the NF-kappaB activation, antiapoptotic signaling, unfolded protein response signaling, and proapoptotic signaling were also reversed by this treatment. In conclusion, alpha2M* promotes cellular proliferation of 1-LN prostate cancer cells by activating MAPK and Akt-dependent signaling, down-regulating apoptotic signaling, and activating unfolded protein response signaling.

Differential CD91 dependence for calreticulin and Pseudomonas exotoxin-A endocytosis

Calreticulin (CRT) interaction with cell-surface receptors is integral to its function in escorting associated peptides into the antigen-presenting cell (APC) antigen presentation pathway. Additionally, extracellular CRT is proposed to be required for lung APC interaction with collectins. In both cases, CD91 has been proposed to act as the APC cell-surface receptor requisite for mediating these processes. However, the evidence for a CRT interaction with CD91 is indirect, predicated on partial competition of cellular binding by gp96, of which CD91 has been proposed as the unique endocytic receptor, and by the CD91 ligand alpha2-macroglobulin. Here, we directly investigate the function of CD91 in binding and trafficking CRT. We find that the ability of CRT to interact with APC does not correlate with cellular CD91 expression or function. Additionally, in the first genetic test of CD91 function regarding CRT, CD91 expression neither conferred CRT association nor did CD91-deficient (CD91-/-) and CD91-expressing cells differ in their ability to traffic CRT. Finally, cellular CRT trafficking did not parallel that of Pseudomonas exotoxin-A, an obligate CD91 ligand, by the criteria of CD91 dependence, cell-type specificity and endocytic itinerary. These data identify that CRT trafficking is not, as previously hypothesized, CD91 dependent and indicate usage of alternative cellular trafficking pathways.

Ligation of cell surface-associated glucose-regulated protein 78 by receptor-recognized forms of alpha 2-macroglobulin: activation of p21-activated protein kinase-2-dependent signaling in murine peritoneal macrophages

Previous studies of the plasma proteinase inhibitor alpha2-macroglobulin (alpha2M) demonstrated that alpha2M-proteinase complexes (alpha2M*) modulate immune responses and promotes macrophage locomotion and chemotaxis. Alpha2M* binds to cell surface-associated glucose-regulated protein 78 (GRP78), which activates downstream signaling events. The role of p21-activated protein kinase-1 and -2 (PAK-1 and -2) in promoting cellular motility is well documented. In the current study, we examined the ability of alpha2M* to activate PAK-1 and PAK-2. Upon macrophage stimulation with alpha2M*, PAK-2 is autophosphorylated, resulting in increased kinase activity; however, PAK-1 is negligibly affected. Alpha2M*-stimulated macrophages showed a marked elevation in the levels of Rac x GTP. Receptor tyrosine phosphorylation upon binding of alpha2M* to GRP78, recruits PAK-2 to the plasma membrane via the adaptor protein NCK. Consistent with this hypothesis, silencing of GRP78 gene expression greatly attenuated the levels of membrane-associated PAK-2 and NCK. PAK-2 activity was markedly decreased by inhibition of tyrosine kinases and PI3K before alpha2M* stimulation. We further demonstrate that phosphorylation of Lin-11, Isl-1, Mec-3 (LIM) kinase and cofilin is promoted by treating macrophages with alpha2M*. Thus, alpha2M* regulates activation of the PAK-2-dependent motility mechanism in these cells.

Association of plasminogen with dipeptidyl peptidase IV and Na+/H+ exchanger isoform NHE3 regulates invasion of human 1-LN prostate tumor cells

Binding of plasminogen type II (Pg 2) to dipeptidyl peptidase IV (DPP IV) on the surface of the highly invasive 1-LN human prostate tumor cell line induces an intracellular Ca2+ ([Ca2+]i) signaling cascade accompanied by a rise in intracellular pH (pHi). In endothelial cells, Pg 2 regulates intracellular pH via Na+/H+ exchange (NHE) antiporters; however, this mechanism has not been demonstrated in any other cell type including prostate cancer cells. Because the Pg 2 receptor DPP IV is associated with NHE3 in kidney cell plasma membranes, we investigated a similar association in 1-LN human prostate cancer cells and a mechanistic explanation for changes in [Ca2+]i or pHi induced by Pg 2 in these cells. Our results suggest that the signaling cascade initiated by Pg 2 and its receptor proceeds via activation of phospholipase C, which promotes formation of inositol 3,4,5-trisphosphate, an inducer of Ca2+ release from endoplasmic reticulum stores. Furthermore, our results suggest that Pg 2 may regulate pHi via an association with NHE3 linked to DPP IV in these cells. These associations suggest that Pg has the potential to simultaneously regulate calcium signaling pathways and Na+/H+ exchanges necessary for tumor cell proliferation and invasiveness.

A novel receptor function for the heat shock protein Grp78: silencing of Grp78 gene expression attenuates alpha2M*-induced signalling

The activated proteinase inhibitor alpha2-macroglobulin (alpha2M*) binds to two receptors, the low density lipoprotein receptor-related protein (LRP-1) and the alpha2M* signalling receptor (alpha2MSR). Silencing LRP-1 gene expression in macrophages by RNA interference does not block alpha2M* activation of signalling cascades. We now demonstrate that transfection of macrophages with a double-stranded RNA homologous in sequence to the Grp78 gene markedly decreased induction of inositol 1,4,5-trisphosphate (IP3) and subsequent IP3-dependent elevation of [Ca2+]i induced by alpha2M*. Concomitantly, alpha2M*-induced increase in [3H]thymidine uptake was abolished in these transfected cells. Insulin treatment significantly upregulates alpha2MSR and it also caused a marked increase in Grp78 expression which could be blocked by RNA interference. alpha2M* treatment of cells activates the Ras- and PI 3-kinase-dependent signalling pathways. Suppressing Grp78 expression leads to the loss of these activation events in transfected macrophages. We thus conclude that Grp78 is the alpha2M* signalling receptor.

Effect of alpha2-macroglobulin on retinal glial cell proliferation

BACKGROUND

Activation of the receptor for alpha2-macroglobulin (alpha2 M), the low-density lipoprotein-related protein (LRP1; CD91), has been suggested to represent a possible strategy for the inhibition of uncontrolled retinal cell proliferation via stimulation of the clearance of alpha2 M-bound growth factors and proteinases from the extracellular space. In order to prove this assumption, we investigated the effect of alpha2 M on the proliferation of Müller glial cells in vitro.

METHODS

Proliferation assays using bromodeoxyuridine were carried out on cultured Müller glial cells of the guinea pig in the absence and presence of alpha2 M.

RESULTS

Activated alpha2 M evoked a slight increase of the cell proliferation at control conditions. Addition of alpha2 M to the culture medium inhibited the proliferation evoked by agonists of G-protein-coupled receptors [adenosine 5'-triphosphate (ATP), neuropeptide Y]. However, alpha2 M did not diminish the proliferation evoked by agonists of receptor tyrosine kinases (epidermal and platelet-derived growth factors) and by serum, respectively. Inhibition of LRP1 by a neutralizing antibody did not alter the ATP-evoked proliferation while it increased the proliferation in the presence of alpha2 M.

CONCLUSION

It is concluded that alpha2 M inhibits the proliferation evoked by agonists of G-protein-coupled receptors, possibly via enhanced growth factor clearance by LRP.

The role of MTJ-1 in cell surface translocation of GRP78, a receptor for alpha 2-macroglobulin-dependent signaling

MTJ-1 associates with a glucose-regulated protein of Mr approximately 78,000(GRP78) in the endoplasmic reticulum and modulates GRP78 activity as a chaperone. GRP78 also exists on the cell surface membrane, where it is associated with a number of functions. MHC class I Ags on the cell surface are complexed to GRP78. GRP78 also serves as the receptor for alpha2-macroglobulin-dependent signaling and for uptake of certain pathogenic viruses. The means by which GRP78, lacking a transmembrane domain, can fulfill such functions is unclear. In this study we have examined the question of whether MTJ-1, a transmembrane protein, is involved in the translocation of GRP78 to the cell surface. MTJ-1 and GRP78 coimmunoprecipitated from macrophage plasma membrane lysates. Silencing of MTJ-1 gene expression greatly reduced MTJ-1 mRNA and protein levels, but also abolished cell surface localization of GRP78. Consequently, binding of the activated and receptor-recognized form of alpha2-macroglobulin to macrophages was greatly reduced, and activated and receptor-recognized form of alpha2-macroglobulin-induced calcium signaling was abolished in these cells. In conclusion, we show that in addition to assisting the chaperone GRP78 in protein quality control in the endoplasmic reticulum, MTJ-1 is essential for transport of GRP78 to the cell surface, which serves a number of functions in immune regulation and signal transduction.

Pregnancy zone protein is a carrier and modulator of placental protein-14 in T-cell growth and cytokine production

A successful pregnancy can only occur when the maternal immune system fails to attack the allogeneic fetus. Two plasma proteins with described immunoregulatory activities, pregnancy zone protein (PZP) and placental protein-14 (PP14; also known as glycodelin-A), increase dramatically during pregnancy, prompting us to examine their potential role in mediating fetal protection. First, we demonstrated that both native PZP and its receptor-recognized monoamine-activated form (MA-PZP) bound non-covalently and specifically to PP14, exhibiting K(d) values greater than 3 microM, as determined by surface plasmon resonance. Our evidence further suggests that PZP is potentially a more effective carrier of PP14 than its relative alpha2-macroglobulin. Second, we found that T-cell activation, as measured by increased proliferation and IL-2 production, was inhibited by either PZP or PP14 in a dose-dependent manner. However, when PZP and PP14 were combined, they acted synergistically to inhibit T cell proliferation and IL-2 production. Interestingly, the combination of PZP and PP14 had little effect on the production of T(H)2 cytokine, IL-4. Based upon these findings, we hypothesize that PZP and PP14 form a stable complex in the plasma of pregnant women and together act synergistically to selectively modulate T-cell activation. Mechanistically, this activity appears to be independent of the PZP receptor (CD91) or PZP's anti-proteinase activity.

Potentiation of signal transduction mitogenesis and cellular proliferation upon binding of receptor-recognized forms of alpha2-macroglobulin to 1-LN prostate cancer cells

The alpha2-macroglobulin signalling receptor is upregulated in highly metastatic 1-LN prostate cancer cells. Stimulation of 1-LN cells with activated alpha2-macroglobulin (alpha2M*) caused a two- to threefold increase in [3H]thymidine uptake and cell number. These events require the Ras-dependent MAPK and PI 3-kinase/Akt signalling cascades. Incubation of 1-LN cells with alpha2M* induced Grb2, shc, sos and Raf-1 expression, as well as phosphorylation of MEK 1/2, ERK 1/2, p38 MAPK and JNK. This treatment also increased PI 3-kinase activation, PDK1 expression, Akt phosphorylation and p70s6k phosphorylation. Levels of the early gene products c-fos protein and thymidylate synthase were comparably increased. Exposure of 1-LN cells to alpha2M* significantly raised the levels of phosphorylated CREB by about 15-20 min and phosphorylated p53 by about 60-90 min of incubation. We conclude that the growth regulatory effects of ligating the alpha2M* signalling receptor on 1-LN cells are exerted via the onset and crosstalk between the Ras-dependent MAPK and PI 3-kinase/Akt signalling cascades.

Induction of mitogenic signalling in the 1LN prostate cell line on exposure to submicromolar concentrations of cadmium+

Cadmium exposure increases the risk of prostate cancer. We now describe the effects of Cd2+ on signalling and proliferation in 1LN prostate cells. Cd2+ increased [3H]thymidine uptake and cell number twofold. Cd2+ elevated intracellular IP3, cytosolic-free Ca2+, phosphorylated MEK1/2, ERK1/2, p38 MAPK and JNK two- to threefold. Increased PDK1 and phosphorylation of the 85-kDa regulatory subunit of PI 3-kinase, Akt and p70s6k were also observed. Cd2+ treatment increased transcription factors NFkappaB and CREB, and the expression of c-fos and c-myc. Cd2+-induced increased uptake of [3H]thymidine was abolished by translational and transcriptional inhibitors, and Ca2+ channel blockers. Inhibition of phospholipase C and of Ca2+ binding to IP3 receptors inhibited Cd2+-induced DNA synthesis as did inhibition of tyrosine kinases, protein kinase C, PI 3-kinase, farnesyl transferase, MEK1/2, ERK1/2 and p38MAPK. Thus signalling events, which are triggered on exposure of 1LN cells to submicromolar concentrations of Cd2+, induce increased proliferation of these cells.

Evidence for autocrine or paracrine roles of alpha2-macroglobulin in regulation of estradiol production by granulosa cells and development of dominant follicles

alpha(2)-Macroglobulin (alpha(2)-M) inhibits proteinases and modulates the actions of growth factors and cytokines. Despite the key roles proteinases, growth factors, and cytokines have in folliculogenesis, the role of alpha(2)-M in follicular development is unknown. Our objectives were to: 1) determine whether granulosa cells produce alpha(2)-M and have alpha(2)-M receptors, 2) examine the effect of alpha(2)-M on estradiol production by granulosa cells, 3) establish whether amounts of alpha(2)-M and alpha(2)-M receptors were altered during dominant nonovulatory follicle development, and 4) examine alpha(2)-M's mechanism of action. The results demonstrated that bovine granulosa cells contain 5.2- and 15-kb mRNAs and 720- and 500-kDa proteins that correspond, respectively, to sizes of mRNAs and proteins for alpha(2)-M and the alpha(2)-M receptor. Treatment of granulosa cells with alpha(2)-M resulted in a specific dose-responsive increase in estradiol production. Cell viability, cell number, and the amount of aromatase in granulosa cells were not altered by alpha(2)-M. Treatment of granulosa cells with factors that bind alpha(2)-M or its receptor did not mimic alpha(2)-M action. Although intrafollicular amounts of alpha(2)-M remained unchanged, amounts of alpha(2)-M receptor in granulosa cells were strongly inversely associated with concentrations of estradiol in dominant and subordinate follicles. Based on these results, we concluded that alpha(2)-M may have autocrine or paracrine roles in granulosa cells potentially important for regulation of estradiol production and development of dominant follicles.

Activation of Akt/PDK signaling in macrophages upon binding of receptor-recognized forms of ?2-macroglobulin to its cellular receptor: Effect of silencing theCREB gene

Macrophage binding of receptor-recognized forms of alpha2-macrogobulin (alpha2M*) significantly increases cAMP, CREB, and activated CREB. We have now examined the participation of the PI 3-kinase/PDK/Akt/p70s6k signaling cascade in alpha2M*-induced cellular proliferation and also studied the role of CREB in these events. Exposure of cells to alpha2M* caused an approximately 2-fold increase in CREB and its phosphorylation at Ser133, phosphorylation of the regulatory subunit of PI 3-kinase, Akt phosphorylation at Ser473 or Thr308, and phosphorylated 70s6k. Silencing of the CREB gene with dsRNA homologous in sequence to the target gene, markedly reduced the levels of CREB mRNA activation of CREB, PI 3-kinase, Akt, and p70s6k in alpha2M*-stimulated macrophages. We conclude that in murine peritoneal macrophages, alpha2M*-induced increase of cAMP is involved in cellular proliferation and this process is mediated by the PI 3-kinase signaling cascade.

The role of Grp 78 in alpha 2-macroglobulin-induced signal transduction. Evidence from RNA interference that the low density lipoprotein receptor-related protein is associated with, but not necessary for, GRP 78-mediated signal transduction

The low density lipoprotein receptor-related protein (LRP) is a scavenger receptor that binds to many proteins, some of which trigger signal transduction. Receptor-recognized forms of alpha(2)-Macroglobulin (alpha(2)M*) bind to LRP, but the pattern of signal transduction differs significantly from that observed with other LRP ligands. For example, neither Ni(2+) nor the receptor-associated protein, which blocks binding of all known ligands to LRP, block alpha(2)M*-induced signal transduction. In the current study, we employed alpha(2)-macroglobulin (alpha(2)M)-agarose column chromatography to purify cell surface membrane binding proteins from 1-LN human prostate cancer cells and murine macrophages. The predominant binding protein purified from 1-LN prostate cancer cells was Grp 78 with small amounts of LRP, a fact that is consistent with our previous observations that there is little LRP present on the surface of these cells. The ratio of LRP:Grp 78 is much higher in macrophages. Flow cytometry was employed to demonstrate the presence of Grp 78 on the cell surface of 1-LN cells. Purified Grp 78 binds to alpha(2)M* with high affinity (K(d) approximately 150 pm). A monoclonal antibody directed against Grp 78 both abolished alpha(2)M*-induced signal transduction and co-precipitated LRP. Ligand blotting with alpha(2)M* showed binding to both Grp 78 and LRP heavy chains in these preparations. Use of RNA interference to silence LRP expression had no effect on alpha(2)M*-mediated signaling. We conclude that Grp 78 is essential for alpha(2)M*-induced signal transduction and that a "co-receptor" relationship exists with LRP like that seen with several other ligands and receptors such as the uPA/uPAR (urinary type plasminogen activator or urokinase/uPA receptor) system.

The role of cAMP-dependent signaling in receptor-recognized forms of alpha 2-macroglobulin-induced cellular proliferation

Ligation of alpha(2)-macroglobulin receptors by receptor-recognized forms of alpha(2)-macroglobulin (alpha(2)M*) activates various signaling cascades and promotes cell proliferation. It also elevates cAMP in murine peritoneal macrophages. We now report that a significant elevation of cAMP-response element-binding protein (CREB) occurs in alpha(2)M*-stimulated cells, and this effect is potentiated by isobutylmethylxanthine, dibutyryl-cAMP, or forskolin. An alpha(2)M* concentration-dependent rapid increase in phosphorylated CREB at Ser(133) also occurred, a necessary event in its activation. Inhibition of Ca(2+)/calmodulin kinase, protein kinases A and C, tyrosine kinases, ribosomal S6 kinase, farnesyl transferase, extracellular signal-regulated kinases 1/2, phosphatidylinositol 3-kinase, or p38 mitogen-activated protein kinase markedly reduce alpha(2)M*-induced phosphorylation of CREB, indicating a role for the p21(ras)-dependent and phosphatidylinositol 3-kinase signaling pathways in regulating CREB activation by alpha(2)M*. Finally, silencing the CREB gene by transfecting cells with a homologous gene sequence double-stranded RNA drastically reduced the expression of CREB and blocked the ability of alpha(2)M* to promote macrophage cell division. We conclude that cAMP-dependent signal transduction as well as other signaling cascades are essential for alpha(2)M*-induced cell proliferation.

Cutting edge: CD91-independent cross-presentation of GRP94(gp96)-associated peptides

GRP94(gp96) elicits CD8(+) T cell responses against its bound peptides, a process requiring access of its associated peptides into the MHC class I cross-presentation pathway of APCs. Entry into this pathway requires receptor-mediated endocytosis, and CD91 (low-density lipoprotein receptor-related protein) has been reported to be the receptor mediating GRP94 uptake into APC. However, a direct role for CD91 in chaperone-based peptide Ag re-presentation has not been demonstrated. We investigated the contribution of CD91 to GRP94 cell surface binding, internalization, and trafficking in APCs. Whereas a clear role for CD91 in alpha(2)-macroglobulin binding and uptake was readily obtained, the addition of excess CD91 ligand, activated alpha(2)-macroglobulin, or receptor-associated protein, an antagonist of all known CD91 ligands, did not affect GRP94 cell surface binding, receptor-mediated endocytosis, or peptide re-presentation. These data identify a CD91-independent, GRP94 internalization pathway that functions in peptide Ag re-presentation.

Cadmium-induced DNA synthesis and cell proliferation in macrophages: the role of intracellular calcium and signal transduction mechanisms

Cd(2+) exposure increases the risk of cancer in humans and animals. In this report, we have studied the effect of Cd(2+) on signal transduction and Ca(2+) mobilization in murine macrophages. At micromolar concentrations, Cd(2+) significantly increased cell division as judged by [3H]thymidine uptake and cell counts. Cd(2+)-treated cells continued to proliferate even after more than 4 weeks in culture. Cd(2+) (1 microM) treatment induced a 1.5- to 2-fold increase in cytosolic free Ca(2+), [Ca(2+)](i), which was transitory and/or oscillatory. The sources of this Ca(2+) included both inositol 1,4,5-trisphosphate (IP(3))-sensitive and -insensitive stores. Macrophage treatment with 1-(6-((17beta-3-methoxyestra-1,2,5(10)-triene-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione (U73122), an inhibitor of phosphatidylinositol-specific phospholipase C (PLC), decreased Cd(2+)-induced formation of IP(3) in a concentration-dependent manner (K(d) about 2 microM). This caused a concomitant, partial decrease in the effect of Cd(2+) on [Ca(2+)](i). Cd(2+) itself crosses the macrophage membrane in part via L-type Ca(2+) channels, but it also interacts with a cell surface membrane protein(s) coupled to a pertussis toxin-sensitive G protein. Use of selective inhibitors of signal transduction and the quantitation of the levels of phosphorylated MAPK/ERK-activating kinase-1 (MEK1), extracellular signal-regulated kinase-1 (ERK1), and p38 mitogen-activated protein kinase (MAPK) suggests that the effects of Cd(2+) are mediated by the p21(ras)-dependent MAPK, but not the phosphoinositide 3 (PI 3)-kinase signalling pathway. The effect of activating these pathways includes increased availability of the transcription factor NFkappaB as well as activation of the early genes c-fos and c-myc.

Cell surface antigen CD109 is a novel member of the alpha(2) macroglobulin/C3, C4, C5 family of thioester-containing proteins

Cell surface antigen CD109 is a glycosylphosphatidylinositol (GPI)-linked glycoprotein of approximately 170 kd found on a subset of hematopoietic stem and progenitor cells and on activated platelets and T cells. Although it has been suggested that T-cell CD109 may play a role in antibody-inducing T-helper function and it is known that platelet CD109 carries the Gov alloantigen system, the role of CD109 in hematopoietic cells remains largely unknown. As a first step toward elucidating the function of CD109, we have isolated and characterized a human CD109 cDNA from KG1a and endothelial cells. The isolated cDNA comprises a 4335 bp open-reading frame encoding a 1445 amino acid (aa) protein of approximately 162 kd that contains a 21 aa N-terminal leader peptide, 17 potential N-linked glycosylation sites, and a C-terminal GPI anchor cleavage-addition site. We report that CD109 is a novel member of the alpha 2 macroglobulin (alpha 2M)/C3, C4, C5 family of thioester-containing proteins, and we demonstrate that native CD109 does indeed contain an intact thioester. Analysis of the CD109 aa sequence suggests that CD109 is likely activated by proteolytic cleavage and thereby becomes capable of thioester-mediated covalent binding to adjacent molecules or cells. In addition, the predicted chemical reactivity of the activated CD109 thioester is complement-like rather than resembling that of alpha 2M proteins. Thus, not only is CD109 potentially capable of covalent binding to carbohydrate and protein targets, but the t(1/2) of its activated thioester is likely extremely short, indicating that CD109 action is highly restricted spatially to the site of its activation.

Inducible expression of the alpha2-macroglobulin signaling receptor in response to antigenic stimulation: a study of second messenger generation

Thioglycollate (TG)-elicited murine, peritoneal macrophages express two receptors for activated forms of the proteinase inhibitor alpha2-macroglobulin (alpha2M*)--namely, the low density lipoprotein receptor-related protein (LRP) and the alpha2M signaling receptor (alpha2MSR). We now report that resident peritoneal macrophages express only 400+/-50 alpha2MSR receptors/cell compared to 5000+/-500 receptor/TG-elicited macrophage. By contrast, LRP expression is only 2-2.5-fold greater on elicited cells. The low level of alpha2MSR expression by resident cells is insufficient to trigger signal transduction in contrast to TG-elicited cells which when exposed to alpha2M* demonstrate a rapid rise in inositol 1,4,5-trisphosphate and a concomitant increase in cytosolic free Ca2+. We then studied a variety of preparations injected subcutaneously for their ability to upregulate alpha2MSR. Macroaggregated bovine serum albumin (macroBSA) injection upregulated alpha2MSR and triggered signaling responses by splenic macrophages. Nonaggregated BSA injection alone or in the presence of alum, by contrast, did not alter alpha2MSR expression. Recombivax (hepatitis B antigen adsorbed to alum) injection also upregulated alpha2MSR on splenic macrophages while the alum carrier had no effect. We conclude that macrophage alpha2M* receptors are inducible and their expression may be regulated, in part, by potential antigens.

Regulation of cytosolic phospholipase A2 activity in macrophages stimulated with receptor-recognized forms of alpha 2-macroglobulin: role in mitogenesis and cell proliferation

Macrophages exposed to receptor-recognized forms of alpha(2)-macroglobulin (alpha(2)M*) demonstrate increased DNA synthesis and cell division. In the current study, we have probed the role of cytosolic phospholipase A(2) (cPLA(2)) activity in the cellular response to alpha(2)M*. Ligation of the alpha(2)M* signaling receptor by alpha(2)M*, or its receptor binding fragment, increased cPLA(2) activity 2-3-fold in a concentration and time-dependent manner. This activation required a pertussis toxin-insensitive G protein. Cellular binding of alpha(2)M* also induced transient translocation of cPLA(2) activity to nuclei and membrane fractions. Inhibition of protein kinase C activity or chelation of Ca(2+) inhibited alpha(2)M*-induced increased cPLA(2) activity. Binding of alpha(2)M* to macrophages, moreover, increased phosphorylation of MEK 1/2, ERK 1/2, p38 MAPK, and JNK. Incubation of macrophages with inhibitors of MEK 1/2 or p38 MAPK before stimulation with alpha(2)M* profoundly decreased phosphorylation of MAPKs, blocking cPLA(2) activation. alpha(2)M*-induced increase in [(3)H]thymidine uptake and cell proliferation was completely abolished if activation of cPLA(2) was prevented. The response of macrophages to alpha(2)M* requires transcription factors nuclear factor kappaB, and cAMP-responsive element-binding protein as well as expression of the proto-oncogenes c-fos and c-myc. These studies indicate that the activation of cPLA(2) plays a crucial role in alpha(2)M*-induced mitogenesis and cell proliferation.

The low density lipoprotein receptor-related protein contributes to selective uptake of high density lipoprotein cholesteryl esters by SW872 liposarcoma cells and primary human adipocytes

The concept that selective transfer of high density lipoprotein (HDL)-derived cholesteryl esters (CE) does not require lipoprotein internalization has been challenged recently by evidence that implicates HDL recycling during the selective uptake process. This has prompted us to examine the role of the low density lipoprotein receptor-related protein (LRP) in selective uptake. LRP is an endocytic receptor for lipoprotein lipase (LpL) and apolipoprotein E (apoE) ligands that are able to mediate selective uptake. We report that molecules that interfere with ligand binding to LRP, such as the receptor-associated protein (RAP), suramin, alpha(2)-macroglobulin, or lactoferrin, inhibit HDL-CE selective uptake by human primary adipocytes and SW872 liposarcoma cells by 35-50%. This partial inhibition of selective uptake from total HDL was not due to preferential inhibition of the HDL(2) or HDL(3) subfractions. Selective uptake by the scavenger receptor BI was not inhibited by RAP, excluding its involvement. Furthermore, in SW872 cells in which LRP was reduced to 14% of control levels by stable antisense expression, selective uptake was attenuated by at least 33%, confirming a role for LRP in this process. RAP, alpha(2)-macroglobulin, lactoferrin, and suramin (individually or in paired combinations) also attenuated selective uptake of HDL-CE by primary human adipocytes by about 40%. On the other hand, human skin fibroblasts express LRP abundantly but lack the capacity for selective uptake, demonstrating that other molecules are required. In SW872 cells, exogenous apoE or LpL can facilitate selective uptake but only the apoE-enhanced uptake can be inhibited by RAP, implicating apoE as a likely co-mediator. We discuss the possible mechanisms by which the endocytic receptor, LRP, can mediate selective uptake.

Receptor-associated protein binding blocks ubiquitinylation of the low density lipoprotein receptor-related protein

The low density lipoprotein receptor-related protein (LRP) consists of two subunits, M(r) approximately 515,000 and 85,000. LRP is a receptor for activated alpha2-macrogobulin (alpha2M*), Pseudomonas exotoxin A, and many other proteins. We now report that ubiquitinylation of the LRP heavy chain occurred when either Pseudomonas exotoxin A or alpha2M* bound to LRP on macrophages. Ubiquitinylation was dose-dependent and maximal about 30 min after ligation of the receptor. Addition of the proteosome inhibitor MG-132 sustained the level of ubiquitin-LRP for longer time intervals in macrophages treated with either alpha2M* or Pseudomonas exotoxin A. By contrast, when receptor associated protein (RAP) bound to LRP, ubiquitinylation did not occur. While RAP is not found in the extracellular environment it binds to LRP and is believed to function as an intracellular chaperone. The presence of RAP within the cell may, therefore, contribute to the recycling of intact LRP which has ligated and internalized its ligands.

alpha 2-Macroglobulin: a new component in the insulin-like growth factor/insulin-like growth factor binding protein-1 axis

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 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.

Induction of cyclooxygenase-2 synthesis by ligation of the macrophage alpha(2)-macroglobulin signalling receptor

We have studied the induction of cyclooxygenase-2 (COX-2) in macrophages consequent to ligating the alpha(2)-macroglobulin (alpha(2)M) signalling receptor (alpha(2)MSR) with receptor-recognized forms of alpha(2)M (alpha(2)M*). Macrophage stimulation with alpha(2)M* increased total cellular and nuclear COX-2 two- to threefold. The maximal increase in COX-2 occurred at a ligand concentration of 50-100 pM and after 2 h. Modulation of intracellular Ca(2+) levels or incubation of [35S] methionine-labelled macrophages with actinomycin D, prior to treatment with alpha(2)M*, markedly reduced the induction of total cellular and nuclear COX-2. Protein kinase C (PKC) or phospholipase A(2) (PLA(2)) inhibition in alpha(2)M*-stimulated macrophages or inhibition of the p21(ras)-dependent mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI 3-kinase) signalling pathways also significantly reduced alpha(2)M*-induced total cellular and nuclear COX-2 expression. Thus, COX-2 induction is dependent on cPLA(2) activity, Ca(2+) mobilization, and PKC activity and requires participation of both the p21(ras)-dependent MAPK and PI 3-kinase signalling pathways. COX-2 activation may mediate alpha(2)M*-induced mitogenesis, which we have previously observed in this and other cell types.

Self antigen prognostic for human immunodeficiency virus disease progression

We have recently found that an extracellular protein, alpha(1) proteinase inhibitor (alpha(1)PI; alpha(1) antitrypsin), is required for in vitro human immunodeficiency virus (HIV) infectivity outcome. We show here in a study of HIV-seropositive patients that decreased viral load is significantly correlated with decreased circulating alpha(1)PI. In the asymptomatic category of HIV disease, 100% of patients manifest deficient levels of active alpha(1)PI, a condition known to lead to degenerative lung diseases and a dramatically reduced life span. Further, HIV-associated alpha(1)PI deficiency is correlated with circulating anti-alpha(1)PI immunoglobulin G. These results suggest that preventing HIV-associated alpha(1)PI deficiency may provide a strategic target for preventing HIV-associated pathophysiology.

Conformational state and receptor recognition of the C-terminal domain of human alpha(2)-macroglobulin after dissociation into half-molecules

BACKGROUND

Dissociation of native human alpha(2)-macroglobulin (alpha(2)M) by sodium thiocyanate generates stable half-molecules with intact thiol esters. Significant conformational changes occur by the dissociation, which are similar to those occurring by transformation from native to methylamine-treated alpha(2)-macroglobulin.

METHODS

The conformational state of the receptor-binding domain of the half-molecules was investigated by receptor binding and clearance studies, and by use of a panel of 11 monoclonal antibodies (mAbs) specific for the 18-kDa C-terminal receptor-binding fragment of alpha(2)-macroglobulin.

RESULTS

The half-molecules simultaneously express epitopes specific for native, as well as epitopes specific for transformed alpha(2)-macroglobulin. While it is possible to immunochemically discriminate between the different forms of tetrameric protein, the half-molecules retain a conformational state with no observed conformational changes in the C-terminal domain following cleavage of thiol esters or bait regions. The in vivo clearance rate in mice was consequently significantly slower for the half-molecules than for the tetrameric receptor-recognized forms of alpha(2)-macroglobulin. Furthermore, half-molecules demonstrate lower affinity for binding to mouse macrophages than methylamine-treated tetrameric alpha(2)-macroglobulin in competition studies.

CONCLUSIONS

It is suggested that contact zones are functionally important for mediating conformational switches, which result in trapping and exposure of the receptor-binding sites.

Differential regulation of the fibroblast growth factor (FGF) family by alpha(2)-macroglobulin: evidence for selective modulation of FGF-2-induced angiogenesis

The fibroblast growth factor (FGF) family has an important role in processes such as angiogenesis, wound healing, and development in which precise control of proteinase activity is important. The human plasma proteinase inhibitor alpha(2)-macroglobulin (alpha(2)M) regulates cellular growth by binding and modulating the activity of many cytokines and growth factors. These studies investigate the ability of native and activated alpha(2)M (alpha(2)M*) to bind to members of the FGF family. Both alpha(2)M and alpha(2)M* bind specifically and saturably to FGF-1, -2, -4, and -6, although the binding to alpha(2)M* is of significantly higher affinity. Neither alpha(2)M nor alpha(2)M* bind to FGF-5, -7, -9, or -10. FGF-2 was chosen for more extensive study in view of its important role in angiogenesis. It was demonstrated that FGF-2 binds to the previously identified TGF-beta binding site. The alpha(2)M* inhibits FGF-2-dependent fetal bovine heart endothelial cell proliferation in a dose-dependent manner. Unexpectedly, alpha(2)M* does not affect FGF-2-induced vascular tubule formation on Matrigel basement membrane matrix or collagen gels. Further studies demonstrate that FGF-2 partitions between fluid-phase alpha(2)M* and solid-phase Matrigel or collagen. These studies suggest that the ability of alpha(2)M* to modulate the activity of FGF-2 is dependent on an interplay with extracellular matrix components. (Blood. 2001;97:3450-3457)

Ligation of the α2M* Signaling Receptor Regulates Synthesis of Cytosolic Phospholipase A2

We have studied the regulation of cytosolic phospholipase A2 (cPLA2) synthesis in macrophages stimulated with receptor-recognized forms of alpha2-macroglobulin (alpha2M*). [35S]methionine-labeled cells were stimulated with alpha2M* and [35S]cPLA2 was immunoprecipitated with a monoclonal antibody directed against cPLA2. The precipitates were electrophoresed, immunoblotted, cPLA2 detected by Enhanced Chemifluorescence, and its radioactivity determined. Stimulation of cells with alpha2M* caused a two- to threefold increase in cPLA2 synthesis compared to buffer-treated cells which was consistently maximal at 200 pM of alpha2M*. Actinomycin D or cycloheximide treatment of cells drastically reduced alpha2M*-induced cPLA2 synthesis. Likewise, inhibition of protein kinase C with chelerythrin, farnesyl transferase with manumycin A, MEK kinase with U0126, Erk1/2 kinases with PD98059, p38MAPK with SB203580, PI 3-kinase with wortmannin or LY294002, p70s6k with rapamycin, or depletion of [Ca2+]i with either BAPTA/AM or EGTA drastically reduced alpha2M* induction of cPLA2. Inhibition of NFKB activation with BAY11-7182 or PGA1 also abolished alpha2M* induction of cPLA2. We conclude that alpha2M*-induced cPLA2 synthesis is controlled by [Ca2+]i levels, tyrosine kinase activity, the p21ras-dependent MAPK and PI 3-kinase downstream signaling pathways, and regulation of NFkappaB.

Selective upregulated expression of the alpha2-macroglobulin signaling receptor in highly metastatic 1-LN prostate carcinoma cells

Cellular binding of receptor-recognized forms of alpha2-macroglobulin (alpha2M*) is mediated by the low-density lipoprotein receptor related protein (LRP) and the alpha2M signaling receptor (alpha2MSR). In nonmalignant cells, ligation of alpha2MSR promotes DNA synthesis and cellular proliferation. Here, we report that insulin treatment of highly metastatic 1-LN human prostate carcinoma selectively increases alpha2MSR expression and binding of alpha2M* to 1-LN cells. alpha2M* induces transient increases in intracellular calcium and inositol 1,4,5-trisphosphate in insulin-treated 1-LN cells, consistent with activation of alpha2MSR. Inhibition of signaling cascades activated by insulin blocks upregulation of alpha2MSR. By contrast, alpha2M* does not bind to nor induce intracellular signaling in PC-3 cells, even though 1-LN cells were subcloned from PC-3 cells. We suggest that alpha2M* behaves like a growth factor in these highly malignant cells. The 1-LN metastatic phenotype may result, in part, from aberrant expression of alpha2MSR, indicating the possible involvement of alpha2M* in tumor progression.

Involvement of alpha-2-macroglobulin receptor in clearance of interleukin 8-alpha-2-macroglobulin complexes by human alveolar macrophages

The purpose of this study was to determine if interleukin 8 (IL-8) in complex with alpha2-macroglobulin (alpha-2-M) can be taken up by human alveolar macrophages. First, we demonstrated that human alveolar macrophages have receptors for alpha-2-M but not IL-8. The binding of(125)I-labeled alpha-2-M to the cells was specific and saturable, whereas(125)I-labeled recombinant human IL-8 (rhIL-8) did not bind to macrophages. However,(125)I-rhIL-8-alpha-2-M complexes bound to macrophages, and unlabeled alpha-2-M competed for the binding. We then cultured the cells in the presence of(125)I-rhIL-8-alpha-2-M complexes,(125)I-rhIL-8 alone or buffer for 24 h. Macrophages were lysed, and the released radioactivity measured. IL-8 concentrations in supernatants and cells were also measured using an IL-8 ELISA. When the macrophages were incubated with(125)I-rhIL-8-alpha-2-M complexes there was a significant amount of IL-8 associated with the cells. However, this was not the case when the cells were incubated with(125)I- rhIL-8 alone suggesting that only these complexes were taken-up by human alveolar macrophages. Furthermore, the clearance of complexes was specifically inhibited by a monoclonal antibody against the 515-kDa subunit of the alpha-2-M receptor (alpha-2-MR) but not by an isotopic mouse IgG1. The study shows an important clearance mechanism for IL-8 in the lung.

Cytosolic phospholipase A(2) activity associated with nuclei is not inhibited by arachidonyl trifluoromethyl ketone in macrophages stimulated with receptor-recognized forms of alpha(2)-macroglobulin

We have studied the translocation of cytosolic phospholipase A(2) (cPLA(2)) to nuclei in macrophages stimulated with receptor-recognized forms of alpha(2)-macroglobulin (alpha(2)M*). Translocation of phosphorylated cPLA(2) to nuclei was determined by immunoprecipitation of cPLA(2) in (32)P(i)-labeled cells. The identity of cPLA(2) was established by comparing its mobility on gels with an authentic cPLA(2) standard. cPLA(2) activity was quantified by measuring the release of [(14)C]arachidonic acid from the substrate 1-palmitoyl-2-[1-(14)C]arachidonyl-sn-glycerophosphatidylcholine. alpha(2)M* caused a two- to threefold increase in cPLA(2) phosphorylation and its translocation to nuclei. The p38 MAPK inhibitor SB203580, PKC inhibitor chelerythrin, or depletion of intracellular Ca(2+) profoundly decreased cPLA(2) activity in nuclei isolated from agonist-stimulated cells. The requirement for Ca(2+), PKC, and p38 MAPK activation appears to be of major importance for nuclear cPLA(2) activity. In contrast to cellular cPLA(2) activity, nuclear cPLA(2) activity was not inhibited by arachidonyl trifluoromethyl ketone (AACOCF(3)) in agonist-stimulated cells. It is concluded that the association of cPLA(2) with nuclear membranes in agonist-stimulated cells modifies the activity and the sensitivity of the enzyme to inhibition by AACOCF(3) in this phospholipid environment.

Conformational variants of human alpha2-macroglobulin are reflected in a C-terminal 'switch region'

Human alpha2-macroglobulin displays extensive conformational changes when induced to transform into new quaternary structures, which are eliminated from the systemic circulation by receptor-mediated endocytosis. One major region involved in these conformational changes is located in a segment of 30 amino acids from Glu1314 to Ser1343 (-Glu-Glu-Phe-Pro-Phe-Ala-Leu-Gly-Val-Gln-Thr-Leu-Pro-Gln-Thr-Cys-Asp -Glu-Pro-Lys-Ala-His-Thr-Ser-Phe-Gln-Ile-Ser-Leu-Ser-), which we term the 'switch region' of alpha2-macroglobulin, as deduced by immunochemical techniques. Monoclonal antibodies were generated using either native, methylamine-treated or the 18-kDa C-terminal receptor-binding fragment as the immunogen. From an extensive number of obtained hybridomas, 11 mAbs were selected because of their capacity to bind to the C-terminal fragment. Irrespective of the original configuration of the antigen used for immunization, seven of the antibodies were shown to be reactive with a set of overlapping epitopes, closely positioned within the 'switch region', as confirmed by the use of synthetic peptides covering the entire C-terminal fragment. The specificities of the seven individual antibodies, as determined by ELISA and BIAcore technologies, revealed a pronounced conformational pleomorphism in the 'switch region'. The results indicate that the 'switch region' may be involved in the exposure of the receptor recognition site and can be used as an indicator region for different conformational states of alpha2-macroglobulin.

Expression of alpha(2)-macroglobulin receptor/low density lipoprotein receptor-related protein (LRP) in rat microglial cells

Low density lipoprotein receptor-related protein (LRP) participates in the uptake and degradation of several ligands implicated in neuronal pathophysiology including apolipoprotein E (apoE), activated alpha(2) -macroglobulin (alpha(2)M*) and beta-amyloid precursor protein (APP). The receptor is expressed in a variety of tissues. In the brain LRP is present in pyramidal-type neurons in cortical and hippocampal regions and in astrocytes that are activated as a result of injury or neoplasmic transformation. As LRP is expressed in the monocyte/macrophage cell system, we were interested in examining whether LRP is expressed in microglia. We isolated glial cells from the brain of neonatal rats and LRP was immunodetected both in microglial cells and in astrocytes expressing glial fibrillar acidic protein (GFAP). Microglial cells were able to bind and internalize LRP-specific ligand, alpha(2)M*. The internalization was inhibitable by RAP, with a Kd of 1.7 nM. The expression of LRP was up-regulated by dexamethasone, and down-regulated by lipopolysaccharide (LPS), gamma interferon (IFN-gamma) or a combination of both. LRP was less sensitive to dexamethasone in activated astrocytes than in microglia. We provided the first analysis of LRP expression and regulation in microglia. Our results open the possibility that microglial cells could be related to the participation of LRP and its ligands in different pathophysiological states in brain.

Potentiation of calcium levels by extracellular arachidonic acid in nuclei isolated from macrophages stimulated with receptor-recognized forms of alpha(2)-macroglobulin

Ligation of macrophage alpha(2)-macroglobulin signalling receptors (alpha(2)MSR) with activated alpha(2)-macroglobulin (alpha(2)M*) increases intracellular Ca(2+), and cytosolic phospholipase A(2) (cPLA(2)) and phospholipase D activities. In view of the relationship between cellular Ca(2+) and mitogenesis, we examined the effect of the product of cPLA(2) activity, arachidonic acid (AA), on nuclear Ca(2+) levels in macrophages stimulated with alpha(2)M*, platelet derived growth factor, and bradykinin. AA addition increased Ca(2+) levels in Fura-2/AM loaded nuclei from both buffer-treated and agonist-stimulated cells, but the increase in stimulated macrophages was 2-4-fold higher. Preincubation of Fura-2/AM loaded nuclei with EGTA or BAPTA/AM abolished AA-induced increase in nuclear Ca(2+) levels. Preincubation of nuclei with indomethacin did not affect AA-induced increase in nuclear Ca(2+) in agonist-stimulated nuclei. It is concluded that in macrophages stimulated with various agonists, AA, derived from cPLA(2)-dependent hydrolysis of phospholipids, plays a significant role in regulating nuclear Ca(2+) levels and thus nuclear functions.