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.

New member of the trefoil factor family of proteins is an alpha-macroglobulin protease inhibitor

The amino acid sequence of the monomeric alpha-macroglobulin (alphaM) from the American bullfrog, Rana catesbiana, was determined. The mature protein consisted of 1469 amino acid residues and shared sequence identity with other members of the alphaM family of protein. The central portion of the frog monomeric alphaM contained Cys residues positioned analogously to the Cys residues in human alpha(2)-macroglobulin (alpha(2)M), known to be involved in disulfide bridges. Additionally, the frog monomeric alphaM contained six Cys residues in a approximately 60 residue COOH-terminal extension not present in previously characterized alphaMs. The spacing of the Cys residues and the overall sequence identity of this COOH-terminal extension were consistent with a trefoil motif. This is the first time a member of the trefoil factor family has been identified in the circulatory system. The "bait region" was located between Arg(675)-Lys(685) and contained mainly basic amino acid residues. The COOH-terminal receptor-binding domain was not exposed prior to proteolysis of this highly susceptible region. The proximity of the receptor-binding and trefoil domains implied that the trefoil domain is similarly concealed before bait region cleavage.

Tissue factor is the receptor for plasminogen type 1 on 1-LN human prostate cancer cells

Tissue factor (TF), the initiator of the extrinsic pathway of coagulation, binds plasminogen (Pg) with high affinity through an interaction between kringles 1-3 of Pg and the extracellular domain of TF. We investigated the binding of Pg type 1 (Pg 1) and Pg type 2 (Pg 2) to highly invasive, TF-expressing, 1-LN human prostate tumor cells and to TF isolated from 1-LN cell membranes. Pg 1, containing both N-linked and O-linked oligosaccharide chains, bound to isolated TF with high affinity, whereas Pg 2, containing only one O-linked oligosaccharide chain, did not bind to TF. Although Pg 1 and Pg 2 bind to 1-LN cells, only anti-TF antibodies inhibited the binding of Pg 1, suggesting that TF functions as the receptor for Pg 1 on 1-LN cells. Binding of Pg 1 to isolated TF was inhibited by 6-aminohexanoic acid and alpha-methylmannoside, suggesting that Pg 1 L-lysine binding sites and the biantennary, mannose-containing N-linked oligosaccharide chain are involved in this interaction. Binding of Pg 1 to 1-LN cells promoted activation by receptor-bound urinary-type Pg activator (u-PA) and initiated a Ca(++) signaling cascade. In previous studies we demonstrated that the Pg 2 O-linked carbohydrate chain is essential for its binding to CD26 on 1-LN cells. The current studies suggest that Pg oligosaccharide chains regulate the binding of Pg 1 and Pg 2 to separate receptors on the cell surface.

Increased immunogenicity of HIV envelope subunit complexed with alpha2-macroglobulin when combined with monophosphoryl lipid A and GM-CSF

Critical to the success of HIV-1 subunit vaccines is the development of strategies to augment vaccine immunogenicity. Successful adjuvants must not only improve immunogenicity above current adjuvant levels, but must also decrease the dose of immunogen required for optimal immunogenicity. We have evaluated activated alpha2-macroglobulin (alpha2M*) and a squalene-based stable emulsion containing monophosphoryl lipid A (MPL-SE) with granulocyte-macrophage colony stimulating factor (GM-CSF) as adjuvants to enhance the immunogencity of candidate HIV immunogens. Balb/c mice were subcutaneously immunized on days 0, 14 and 28 with 100-0.1 microg of HIV-1 envelope gp120 C4-V3 immunogens from either HIV IIIB (C4-V3(IIIB)) or SHIV 89.6P (C4-V3(89.6P)). Immunogens were tested covalently coupled to alpha2M*, formulated with MPL-SE/GM-CSF, or as a combination of both. Using CFA/IFA, only 50 and 100 microg, but not lower doses of C4-V3(IIIB) peptides, induced antibody responses. In contrast, peak antibody responses were detected in mice immunized with 10 microg of C4-V3 peptide coupled to alpha2M* (alpha2M*-peptide). Similar to CFA/IFA, MPL-SE/GM-CSF induced optimal antibody responses at 50 and 100 microg of C4-V3 immunogen. However, the combination of MPL-SE/GM-CSF with alpha2M*-C4-V3 peptide decreased the dose of C4-V3 required for optimal response to 5 microg for C4-V3(IIIB), and to 0.1 microg for C4-V3(89.6P). Taken together, HIV envelope gp120 C4-V3 peptides covalently complexed with alpha2M* and formulated with MPL-SE/GM-CSF resulted in a subunit HIV immunogen capable of inducing anti-HIV envelope antibody responses at doses up to 100-fold less than those needed with CFA/IFA or MPL-SE/GM-CSF alone.

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.

The mechanism of action of angiostatin: can you teach an old dog new tricks?

What is angiostatin? In 1994, Folkman and colleagues published a landmark paper describing anti-tumor effects in mice with a purified fragment of plasminogen they named angiostatin (1). Although many papers have been published describing activities of cryptic polypeptides derived from plasminogen fragments, this was the first report which associated plasminogen kringles 1-4 as a suppressor of metastasis development. This review will describe what is known about the mechanism of action of angiostatin from the current literature.

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.

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.

Covalent complexes of antigen and alpha(2)-macroglobulin: evidence for dramatically-increased immunogenicity

A safe, effective, more potent adjuvant than currently available would be beneficial in developing new therapeutics and diagnostic reagents. We report here a technique for the rapid, efficient incorporation of non-proteolytic antigens into alpha(2)-macroglobulin (alpha(2)M; tradename, SynerVax), allowing us to covalently couple much larger subunit antigens to alpha(2)M than previously possible. Our goal was to determine if incorporation of HB, the monomeric form of Hepatitis B virus (HBV) surface antigen (HBsAg), into alpha(2)M would result in increased immune reactivity. Earlier attempts to immunize animals using HB did not generate significant levels of antibodies. Using HB complexes prepared with alpha(2)M we now report dramatically-increased immunogenicity of HB in BALB/c mice. Combining these soluble complexes with a depot-generating agent (alum), titers>1:1,000,000 are obtained with a single injection. This novel adjuvant technology should provide a valuable tool for the development of either prophylactic and therapeutic vaccines, or monoclonal antibodies against hitherto poorly-immunogenic subunit antigens.

Kinetics of nonproteolytic incorporation of a protein ligand into thermally activated alpha 2-macroglobulin: evidence for a novel nascent state

We have previously shown that antigens complexed to the receptor-recognized form of alpha(2)-macroglobulin (alpha(2)M*) demonstrate enhanced immune responsiveness mediated by the low density lipoprotein receptor-related protein LRP/CD91. Recently, we developed a proteinase-independent method to covalently bind antigens to alpha(2)M*. Given the potential applications of this chemistry, we analyzed the kinetics, thermodynamics, and pH dependence of this reaction. The incorporation of lysozyme into alpha(2)M* was a mixed bimolecular second-order reaction with a specific rate constant of 91.0 +/- 6.9 m(-1) s(-1), 50.0 degrees C, pH 7.4. The activation energy, activation entropy, and Gibbs' free energy at 50.0 degrees C were 156 kJ mol(-1), 266 J mol(-1) K(-1), and 70 kJ mol(-1), respectively. The rate of incorporation increased as a function of pH from pH 5.0 to 7.0 and was unchanged thereafter. Furthermore, the reaction between alpha(2)M* and lysozyme was irreversible. The data are consistent with a two-step mechanism. In the first step, alpha(2)M* reforms its thiol ester bond, entering a reactive state that mimics the proteolytically induced "nascent state." In the rate-limiting second step, the reformed bond quickly undergoes nucleophilic attack by lysozyme. The kinetic equations derived in this study are the basis for optimizing the formation of stable alpha(2)M*.antigen complexes.

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.

Characterization of human serum dipeptidyl peptidase IV (CD26) and analysis of its autoantibodies in patients with rheumatoid arthritis and other autoimmune diseases

OBJECTIVES

To assess the serum levels, specific activity and other characteristics of dipeptidylpeptidase IV (DPP IV/CD26), an ectoenzyme that plays a critical role in the modulation and expression of autoimmune and inflammatory diseases, from patients with rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), primary Sjögren syndrome (SS) and normal controls. To study the possible underlying molecular basis if significant differences were found.

METHODS

Serum DPP IV was purified by ion-exchange and affinity chromatography techniques and its specific activity and sera levels were determined by an enzyme-linked assay (ELISA). The enzyme was further analyzed for its sialic acid content, its adenosine deaminase binding capacity and its electrophoretic mobility. The levels of circulating IgA, IgG, and IgM anti-DPP IV autoantibodies were determined by an ELISA technique.

RESULTS

The median serum levels of DPP IV in RA patients was similar to controls (0.85 microg/ml versus 1.03 microg/ml, p = n.s.); in SLE and SS patients the enzyme serum levels were reduced to nearly one half of controls (p < 0.001). DPP IV specific activity was significantly reduced in serafrom RA patients when compared with those of SLE, SS and normal sera (12.24 versus 16.5, 19.69 and 16.34 mol pNA x 10(-4)/min/mol respectively, p < 0.005). Both RA and SLE enzymes were hypersialylated, but only RA DPP IV augmented its specific activity to close to control values after desialylation with V. cholerae neuraminidase. Sera from all patient groups contained anti-DPP IV autoantibodies, but only those of the IgA isotype were significantly higher than those found in normal subjects.

CONCLUSION

The specific activity of serum DPP IV was decreased only in RA patients, although its levels were similar to normal controls. While both RA and SLE DPP IV were hypersialylated, desialylation restored the specific activity only of RA DPP IV. This finding suggests that different specific glycosylation sites in the enzyme might be involved as the underlying mechanism of the decreased enzyme specific activity of RA patients. The differences in DPP IV levels observed between RA and SLE patients seem to reflect a different status of T cell activation in both diseases.

Apolipoprotein E and mimetic peptide initiate a calcium-dependent signaling response in macrophages

Apolipoprotein E (ApoE) is a 34-kDa cholesterol transport protein that also possesses immunomodulatory properties. In this study, we demonstrate that ApoE initiates a signaling cascade in murine peritoneal macrophages that leads to increased production of inositol triphosphate with mobilization of intracellular Ca(2+) stores. This cascade is inhibited by pretreatment with receptor-associated protein and Ni(2+), and it is mediated by a pertussis toxin-sensitive G protein. These properties are characteristic of signal transduction induced via ligand binding to the cellular receptor, lipoprotein receptor-related protein. A peptide derived from the receptor-binding region of ApoE also initiates signal transduction in a manner similar to that of the intact protein, suggesting that this isolated region is sufficient for signal transduction. The ApoE-mimetic peptide competed for binding with the intact protein, confirming that they both interact with the same site. ApoE-dependent signal transduction might play a role in mediating the functional properties of this lipoprotein.

alpha(2)-Macroglobulin from rheumatoid arthritis synovial fluid: functional analysis defines a role for oxidation in inflammation

A hallmark of inflammation is the release of oxidants, proteinases, and cytokines, all important mediators of the inflammatory cascade. alpha(2)-Macroglobulin (alpha(2)M) is a high-affinity, broad-specificity proteinase inhibitor that also binds and regulates the biological activities of a number of cytokines. We demonstrated recently that hypochlorite-oxidized alpha(2)M has decreased ability to inhibit proteinases and regulate cytokines in vitro. The role of oxidation in regulating alpha(2)M functions in vivo is largely unknown. To determine the extent and biological consequence of in vivo alpha(2)M oxidation, we measured the degree of oxidative alpha(2)M modification from rheumatoid arthritis (RA) synovial fluid and compared this with osteoarthritis (OA) as noninflammatory controls. We found that RA synovial fluid alpha(2)M is significantly more oxidized than that from OA. RA synovial fluid also contains a twofold higher median alpha(2)M level than OA, while having only half the alpha(2)M-proteinase inhibitory activity. Detailed biochemical analysis demonstrates proteolytically degraded alpha(2)M in RA greater than in OA synovial fluid. Additionally, the hypochlorite-mediated oxidation product, chlorotyrosine, is present in RA more than in OA or plasma alpha(2)M samples. Taken together, these findings confirm a role for oxidative regulation of inflammation by altering the functions of extracellular mediators such as alpha(2)M.

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)

Endothelial cell surface F1-F0 ATP synthase is active in ATP synthesis and is inhibited by angiostatin

Angiostatin blocks tumor angiogenesis in vivo, almost certainly through its demonstrated ability to block endothelial cell migration and proliferation. Although the mechanism of angiostatin action remains unknown, identification of F(1)-F(O) ATP synthase as the major angiostatin-binding site on the endothelial cell surface suggests that ATP metabolism may play a role in the angiostatin response. Previous studies noting the presence of F(1) ATP synthase subunits on endothelial cells and certain cancer cells did not determine whether this enzyme was functional in ATP synthesis. We now demonstrate that all components of the F(1) ATP synthase catalytic core are present on the endothelial cell surface, where they colocalize into discrete punctate structures. The surface-associated enzyme is active in ATP synthesis as shown by dual-label TLC and bioluminescence assays. Both ATP synthase and ATPase activities of the enzyme are inhibited by angiostatin as well as by antibodies directed against the alpha- and beta-subunits of ATP synthase in cell-based and biochemical assays. Our data suggest that angiostatin inhibits vascularization by suppression of endothelial-surface ATP metabolism, which, in turn, may regulate vascular physiology by established mechanisms. We now have shown that antibodies directed against subunits of ATP synthase exhibit endothelial cell-inhibitory activities comparable to that of angiostatin, indicating that these antibodies function as angiostatin mimetics.

Interaction of plasminogen with dipeptidyl peptidase IV initiates a signal transduction mechanism which regulates expression of matrix metalloproteinase-9 by prostate cancer cells

Both plasminogen (Pg) activation and matrix metalloproteinases (MMPs) are involved in the proteolytic degradation of extracellular matrix components, a requisite event for malignant cell metastasis. The highly invasive 1-LN human prostate tumour cell line synthesizes and secretes large amounts of Pg activators and MMPs. We demonstrate here that the Pg type 2 (Pg 2) receptor in these cells is composed primarily of the membrane glycoprotein dipeptidyl peptidase IV (DPP IV). Pg 2 has six glycoforms that differ in their sialic acid content. Only the highly sialylated Pg 2gamma, Pg 2delta and Pg 2epsilon glycoforms bind to DPP IV via their carbohydrate chains and induce a Ca(2+) signalling cascade; however, Pg 2epsilon alone is also able to significantly stimulate expression of MMP-9. We further demonstrate that the Pg-mediated invasive activity of 1-LN cells is dependent on the availability of Pg 2epsilon. This is the first demonstration of a direct association between the expression of MMP-9 and the Pg activation system.

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.

Downregulation of microglial activation by apolipoprotein E and apoE-mimetic peptides

Apolipoprotein E plays an important role in recovery from acute brain injury and risk of developing Alzheimer's disease. We demonstrate that biologically relevant concentrations of apoE suppress microglial activation and release of TNFalpha and NO in a dose-dependent fashion. Peptides derived from the apoE receptor-binding region mimic the effects of the intact protein, whereas deletion of apoE residues 146-149 abolishes peptide bioactivity. These results are consistent with the hypothesis that apoE modulates microglial function by binding specific cell surface receptors and that the immunomodulatory effects of apoE in the central nervous system may account for its role in acute and chronic neurological disease.

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.

The conformation-dependent interaction of alpha 2-macroglobulin with vascular endothelial growth factor. A novel mechanism of alpha 2-macroglobulin/growth factor binding

alpha(2)-Macroglobulin (alpha(2)M) is a highly conserved proteinase inhibitor present in human plasma at high concentration (2-4 mg/ml). alpha(2)M exists in two conformations, a native form and an activated, receptor-recognized form. While alpha(2)M binds to numerous cytokines and growth factors, in most cases, the nature of the alpha(2)M interaction with these factors is poorly understood. We examined in detail the interaction between alpha(2)M and vascular endothelial growth factor (VEGF) and found a novel and unexpected mechanism of interaction as demonstrated by the following observations: 1) the binding of VEGF to alpha(2)M occurs at a site distinct from the recently characterized growth factor binding site; 2) VEGF binds different forms of alpha(2)M with distinct spatial arrangement, namely to the interior of methylamine or ammonia-treated alpha(2)M and to the exterior of native and proteinase-converted alpha(2)M; and 3) VEGF (molecular mass approximately 40 kDa) can access the interior of receptor-recognized alpha(2)M in the absence of a proteinase trapped within the molecule. VEGF bound to receptor-recognized forms of alpha(2)M is internalized and degraded by macrophages via the alpha(2)M receptor, the low density lipoprotein receptor-related protein. Oxidation of both native and receptor-recognized alpha(2)M results in significant inhibition of VEGF binding. We also examined the biological significance of this interaction by studying the effect of alpha(2)M on VEGF-induced cell proliferation and VEGF-induced up-regulation of intracellular Ca(2+) levels. We demonstrate that under physiological conditions, alpha(2)M does not impact the ability of VEGF to induce cell proliferation or up-regulate Ca(2+).

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.

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.