Binding of receptor-recognized forms of alpha2-macroglobulin to the alpha2-macroglobulin signaling receptor activates phosphatidylinositol 3-kinase

Host proteins Alpha-2-Macroglobulin and LRP1 associate with Chandipura virus

Chandipura Virus is an emerging tropical pathogen with a high mortality rate among children. No mode of treatment or antivirals exists against CHPV infection, due to little information regarding its host interaction. Studying viral pathogen interaction with its host can not only provide valuable information regarding its propagation strategy, but also on which host proteins interact with the virus. Identifying these proteins and understanding their role in the infection process can provide more stable anti-viral targets. In this study, we focused on identifying host factors that interact with CHPV and may play a critical role in CHPV infection. We are the first to report the successful identification of Alpha-2-Macroglobulin (A2M), a secretory protein of the host that interacts with CHPV. We also established that LRP1 (Low-density lipoprotein receptor-related protein 1) and GRP78 (Glucose regulated protein 78), receptors of A2M, also interact with CHPV. Furthermore, we could also demonstrate that knocking out A2M has a severe effect on viral infection. We conclusively show the interaction of these host proteins with CHPV. Our findings also indicate that these host proteins could play a role in viral entry into the host cell.

Linking cell-surface GRP78 to cancer: From basic research to clinical value of GRP78 antibodies

Glucose-related protein 78 (GRP78) is a chaperone protein localized primarily in the endoplasmic reticulum (ER) lumen, where it helps in proper protein folding by targeting misfolded proteins and facilitating protein assembly. In stressed cells, GRP78 is translocated to the cell surface (csGRP78) where it binds to various ligands and triggers different intracellular pathways. Thus, csGRP78 expression is associated with cancer, involved in the maintenance and progression of the disease. Extracellular exposition of csGRP78 leads to the production of autoantibodies as observed in patients with prostate or ovarian cancer, in which the ability to target csGRP78 affects the tumor development. Present on the surface of cancer cells and not normal cells in vivo, csGRP78 represents an interesting target for therapeutic antibody strategies. Here we give an overview of the csGRP78 function in the cell and its role in oncogenesis, thereby providing insight into the clinical value of GRP78 monoclonal antibodies for cancer prognosis and treatment.

The present and future of PI3K inhibitors for cancer therapy

Phosphoinositide-3- kinase (PI3K) signaling regulates cellular proliferation, survival and metabolism, and its aberrant activation is one of the most frequent oncogenic events across human cancers. In the last few decades, research focused on the development of PI3K inhibitors, from preclinical tool compounds to the highly specific medicines approved to treat patients with cancer. Herein we discuss current paradigms for PI3K inhibitors in cancer therapy, focusing on clinical data and mechanisms of action. We also discuss current limitations in the use of PI3K inhibitors including toxicities and mechanisms of resistance, with specific emphasis on approaches aimed to improve their efficacy.

Plasminogen activator inhibitor-1 regulates the vascular expression of vitronectin

Essentials Vitronectin (VN) is produced by smooth muscle cells (SMCs) and promotes neointima formation. We studied the regulation of vascular VN expression by plasminogen activator inhibitor-1 (PAI-1). PAI-1 stimulates VN gene expression in SMCs by binding LDL receptor-related protein 1. Stimulation of VN gene expression may be a mechanism by which PAI-1 controls vascular remodeling.

SUMMARY

Background Increased expression of vitronectin (VN) by smooth muscle cells (SMCs) promotes neointima formation after vascular injury, and may contribute to chronic vascular diseases, such as atherosclerosis. However, the molecular regulation of vascular VN expression is poorly defined. Given the overlapping expression profiles and functions of VN and plasminogen activator inhibitor (PAI)-1, we hypothesized that PAI-1 regulates vascular VN expression. Objectives To determine whether PAI-1 regulates VN expression in SMCs and in vivo. Methods The effects of genetic alterations in PAI-1 expression, pharmacologic PAI-1 inhibition and recombinant PAI-1 on SMC VN expression were studied, and vascular VN expression in wild-type (WT) and PAI-1-deficient mice was assessed. Results VN expression was significantly lower in PAI-1-deficient SMCs and significantly increased in PAI-1-overexpressing SMCs. PAI-1 small interfering RNA and pharmacologic PAI-1 inhibition significantly decreased SMC VN expression. Recombinant PAI-1 stimulated VN expression by binding LDL receptor-related protein-1 (LRP1), but another LRP1 ligand, α2 -macroglobulin, did not. As compared with WT controls, carotid artery VN expression was significantly lower in PAI-1-deficient mice and significantly higher in PAI-1-transgenic mice. In a vein graft (VG) model of intimal hyperplasia, VN expression was significantly attenuated in PAI-1-deficient VGs as compared with WT controls. The plasma VN concentration was significantly decreased in PAI-1-deficient mice versus WT controls at 4 weeks, but not at 5 days or 8 weeks, after surgery. Conclusions PAI-1 stimulates SMC VN expression by binding LRP1, and controls vascular VN expression in vivo. Autocrine regulation of vascular VN expression by PAI-1 may play important roles in vascular homeostasis and pathologic vascular remodeling.

Effects of a systemic enzyme therapy in healthy active adults after exhaustive eccentric exercise: a randomised, two-stage, double-blinded, placebo-controlled trial

Background

Systemic enzyme therapy may improve symptoms of exhaustive eccentric exercise due to anti-inflammatory properties.

Methods

In a randomised, placebo-controlled, two-stage clinical trial, systemic enzyme therapy (Wobenzym) was administered for 72 hours before and 72 hours following a day on which subjects performed an exhaustive eccentric exercise (isokinetic loading of the quadriceps). Efficacy criteria (maximal strength and pain) and time points were selected to account for the multidimensional nature of exercise-induced muscle damage symptoms. Subjects were randomised in a crossover (stage I, n=28) and parallel group design (stage II, n=44).

Results

Analysis of stage I data demonstrated a significant superiority (Mann-Whitney=0.6153; p=0.0332; one sided) for systemic enzyme therapy compared with placebo. Stage II was designed as a randomised controlled parallel group comparison. Heterogeneity (I²>0.5) between stages led to separate analyses of stage I (endurance-trained subjects) and stage II (strength-trained subjects). Combined analysis resulted in no evidence for corresponding treatment effects. Analysis of pooled biomarker data, however, demonstrated significant favourable effects for systemic enzyme therapy in both stages.

Conclusion

Systemic enzyme therapy before and after exhaustive eccentric exercise resulted in higher maximal concentric strength in the less strength-trained subjects (stage I) and in significant favourable effects on biomarkers (inflammatory, metabolic and immune) in all subjects. The application of these findings needs further evaluation.

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.

Activated α2-macroglobulin binding to cell surface GRP78 induces T-loop phosphorylation of Akt1 by PDK1 in association with Raptor

PDK1 phosphorylates multiple substrates including Akt by PIP3-dependent mechanisms. In this report we provide evidence that in prostate cancer cells stimulated with activated α2-macroglobulin (α2M*) PDK1 phosphorylates Akt in the T-loop at Thr(308) by using Raptor in the mTORC1 complex as a scaffold protein. First we demonstrate that PDK1, Raptor, and mTOR co-immunoprecipitate. Silencing the expression, not only of PDK1, but also Raptor by RNAi nearly abolished Akt phosphorylation at Akt(Thr308) in Raptor-immunoprecipitates of α2M*-stimulated prostate cancer cells. Immunodepleting Raptor or PDK from cell lysates of cells treated with α2M* drastically reduced Akt phosphorylation at Thr(308), which was recovered by adding the supernatant of Raptor- or PDK1-depleted cell lysates, respectively. Studies of insulin binding to its receptor on prostate cancer cells yielded similar results. We thus demonstrate that phosphorylating the T-loop Akt residue Thr(308) by PDK1 requires Raptor of the mTORC1 complex as a platform or scaffold protein.

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.

Inhibition of NF-kappaB1 and NF-kappaB2 activation in prostate cancer cells treated with antibody against the carboxyl terminal domain of GRP78: effect of p53 upregulation

Ligation of cancer cell surface GRP78 by activated alpha2-macroglobulin (alpha2M*) triggers pro-proliferative and anti-apoptotic signaling pathways. Cancer patients who develop autoantibodies to the alpha2M* binding site in GRP78 have a poor prognosis since these antibodies are receptor agonists. The NF-kappaB family of transcription factors induces expression of genes affecting cell growth and differentiation. NF-kappaB1 plays a major regulatory role in controlling innate immunity and inflammation, whereas NF-kappaB2 plays a greater role in cancer cell proliferation. Here we report that treatment of prostate cancer cells with antibody directed against the carboxyl terminal domain of GRP78 inhibits alpha2M*-induced activation of NF-kappaB2 by approximately 50% while exerting a lesser effect of approximately 20% on NF-kappaB1 activation. Treatment of these cells nearly abolished alpha2M*-induced activation of IKKalpha involved in the activation of NF-kappaB2. This antibody also suppressed alpha2M*-induced phosphorylation of IKKalpha, IKKalpha/beta, IkappaBalpha, and IkappaBbeta as well as levels of NIK. Antibody treatment of cancer cells elevated pro-apoptotic p21WAF and p27kip while reducing cyclin D1 levels. These studies demonstrate that antibody directed against the carboxyl terminal domain of GRP78 inhibits the pro-proliferative NF-kappaB signaling cascade in cancer cells.

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.

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.

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.

Phosphatidylinositol 3-kinase regulates Ca2+ signaling in pancreatic acinar cells through inhibition of sarco(endo)plasmic reticulum Ca2+-ATPase

Calcium is a key mediator of hormone-induced enzyme secretion in pancreatic acinar cells. At the same time, abnormal Ca(2+) responses are associated with pancreatitis. We have recently shown that inhibition of phosphatidylinositol 3-kinase (PI3-kinase) by LY-294002 and wortmannin, as well as genetic deletion of PI3-kinase-gamma, regulates Ca(2+) responses and the Ca(2+)-sensitive trypsinogen activation in pancreatic acinar cells. The present study sought to determine the mechanisms of PI3-kinase involvement in Ca(2+) responses induced in these cells by CCK and carbachol. The PI3-kinase inhibitors inhibited both Ca(2+) influx and mobilization from intracellular stores induced by stimulation of acini with physiological and pathological concentrations of CCK, as well as with carbachol. PI3-kinase inhibition facilitated the decay of cytosolic free Ca(2+) concentration ([Ca(2+)](i)) oscillations observed in individual acinar cells. The PI3-kinase inhibitors decreased neither CCK-induced inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] production nor Ins(1,4,5)P(3)-induced Ca(2+) mobilization, suggesting that the effect of PI3-kinase inhibition is not through Ins(1,4,5)P(3) or Ins(1,4,5)P(3) receptors. PI3-kinase inhibition did not affect Ca(2+) mobilization induced by thapsigargin, a specific inhibitor of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA). Moreover, SERCA blockade with thapsigargin abolished the effects of pharmacological and genetic PI3-kinase inhibition on [Ca(2+)](i) signals, suggesting SERCA as a downstream target of PI3-kinase. Both pharmacological PI3-kinase inhibition and genetic deletion of PI3-kinase-gamma increased the amount of Ca(2+) in intracellular stores during CCK stimulation. Finally, addition of the PI3-kinase product phosphatidylinositol 3,4,5-trisphosphate to permeabilized acini significantly attenuated Ca(2+) reloading into the endoplasmic reticulum. The results indicate that PI3-kinase regulates Ca(2+) signaling in pancreatic acinar cells through its inhibitory effect on SERCA.

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.

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.

Low density lipoprotein receptor-related protein is a calreticulin coreceptor that signals focal adhesion disassembly

Thrombospondin (TSP) signals focal adhesion disassembly (the intermediate adhesive state) through interactions with cell surface calreticulin (CRT). TSP or a peptide (hep I) of the active site induces focal adhesion disassembly through binding to CRT, which activates phosphoinositide 3-kinase (PI3K) and extracellular signal-related kinase (ERK) through Galphai2 proteins. Because CRT is not a transmembrane protein, it is likely that CRT signals as part of a coreceptor complex. We now show that low density lipoprotein receptor-related protein (LRP) mediates focal adhesion disassembly initiated by TSP binding to CRT. LRP antagonists (antibodies, receptor-associated protein) block hep I/TSP-induced focal adhesion disassembly. LRP is necessary for TSP/hep I signaling because TSP/hep I is unable to stimulate focal adhesion disassembly or ERK or PI3K signaling in fibroblasts deficient in LRP. LRP is important in TSP-CRT signaling, as shown by the ability of hep I to stimulate association of Galphai2 with LRP. The isolated proteins LRP and CRT interact, and LRP and CRT are associated with hep I in molecular complexes extracted from cells. These data establish a mechanism of cell surface CRT signaling through its coreceptor, LRP, and suggest a novel function for LRP in regulating cell adhesion.

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.

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.

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.

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.

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.

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.

Ligation of low-density lipoprotein receptor-related protein with antibodies elevates intracellular calcium and inositol 1,4, 5-trisphosphate in macrophages

We have probed the signaling characteristics of the macrophage low-density lipoprotein receptor-related protein (LRP) with monoclonal antibody 8G1, its Fab and F(ab')(2) fragments directed against the ligand binding heavy chain, and monoclonal antibody 5A6 directed against the membrane-spanning light chain of LRP. Ligation of LRP with 8G1, its Fab and F(ab')(2) fragments, or 5A6 increased intracellular Ca(2+) levels two- to threefold. Prior ligation of LRP with 8G1 did not affect the increase in [Ca(2+)](i) observed on subsequent ligation of LRP with lactoferrin, P. exotoxin A, or lipoprotein lipase. Binding to LRP by 8G1, its Fab and F(ab')(2) fragments, or 5A6 increased inositol 1,4,5-trisphosphate (IP(3)) levels by 50 to 100%. Incubation of macrophages with guanosine 5', 3'-O(thio)-triphosphate (GTP-gamma-S) before treatment with antibody potentiated and sustained the 8G1-induced increase in IP(3) levels. Treatment of macrophages with guanyl-5'-yl thiophosphate prior to GTP-gamma-S treatment abolished the GTP-gamma-S-potentiated increase in IP(3) levels in 8G1-treated macrophages. Antibody-induced increases in IP(3) and [Ca(2+)](i) in macrophages on ligation of LRP were pertussis toxin sensitive. Binding of 8G1 or its Fab or F(ab')(2) fragments to LRP stimulated macrophage protein kinase C (PKC) activity as evaluated by histone IIIs phosphorylation by about two- to sevenfold. Staurosporin inhibited the anti-LRP antibody-induced increase in PKC activity. Ligation of LRP with 8G1 increased cellular cAMP levels about twofold. Preincubation of macrophage with the LRP-binding protein receptor-associated protein suppressed the 8G1-induced increase in cAMP levels. Thus, binding of antibodies directed against either chain of LRP triggers complex signaling cascades.

Coordinate regulation of the alpha(2)-macroglobulin signaling receptor and the low density lipoprotein receptor-related protein/alpha(2)-macroglobulin receptor by insulin

We have studied insulin-dependent regulation of macrophage alpha(2)-macroglobulin signaling receptors (alpha(2)MSR) and low density lipoprotein receptor-related protein/alpha(2)M receptors (LRP/alpha(2)MR) employing cell binding of (125)I-alpha(2)M*, inhibition of binding by receptor-associated protein (RAP) or Ni(2+), LRP/alpha(2)MR mRNA levels, and generation of second messengers. Insulin treatment increased the number of alpha(2)M* high (alpha(2)MSR) and low (LRP/alpha(2)MR) affinity binding sites from 1, 600 and 67,000 to 2,900 and 115,200 sites per cell, respectively. Neither RAP nor Ni(2+) blocked the binding of (125)I-alpha(2)M* to alpha(2)MSR on insulin- or buffer-treated cells, but they both blocked binding to LRP/alpha(2)MR. Insulin significantly increased LRP/alpha(2)MR mRNA levels in a dose- and time-dependent manner. Insulin-augmented (125)I-alpha(2)M* binding to macrophages was severely reduced by wortmannin, LY294002, PD98059, SB203580, or rapamycin. The increase in alpha(2)MSR receptor synthesis was reflected by augmented generation of IP(3) and increased [Ca(2+)](i) levels upon receptor ligation. Incubation of macrophages with wortmannin, LY294002, PD98059, SB203580, rapamycin, or antibodies against insulin receptors before insulin treatment and alpha(2)M* stimulation significantly reduced the insulin-augmented increase in IP(3) and [Ca(2+)](i) levels. Pretreatment of cells with actinomycin D or cycloheximide blocked the synthesis of new alpha(2)MSR. In conclusion, we show here that insulin coordinately regulates macrophage alpha(2)MSR and LRP/alpha(2)MR, utilizing both the PI 3-kinase and Ras signaling pathways to induce new synthesis of these receptors.

Upregulation of macrophage plasma membrane and nuclear phospholipase D activity on ligation of the alpha2-macroglobulin signaling receptor: involvement of heterotrimeric and monomeric G proteins

The effect of ligating the alpha2-macroglobulin signaling receptor (alpha2MSR) with receptor-recognized forms of alpha2M (alpha2M*) was studied with respect to phospholipase D (PLD) activity in murine macrophages, their plasma membranes, and nuclei. PLD activity in plasma membranes and nuclei increased linearly up to a ligand concentration of about 100 pM of either alpha2M* or a cloned and expressed receptor binding fragment (RBF). The RBF binding site mutant K1370A, which binds with high affinity to alpha2MSR, also increased nuclear PLD activity comparable to RBF and alpha2M*. Phorbol dibutyrate caused a two- to threefold stimulation of membrane and nuclear PLD activity, whereas PLD activity was nearly abolished by downregulation of protein kinase C; prior treatment with staurosporin, genestein, cyclosporin A, actinomycin D; or chelation of intracellular Ca2+. In permeabilized macrophages, isolated plasma membranes, and nuclei, GTP-gamma-S increased alpha2M*-stimulated PLD activity via a pertussis toxin-insensitive G protein and this effect was abolished on preincubation with GDP-beta-S. Incubation of plasma membranes with polyclonal antibody against sARFII, or the addition of cytosol which was immunoprecipitated with antibody against sARFII, greatly reduced alpha2M*-stimulated PLD activity in the presence of GTP-gamma-S. Preincubation of plasma membranes with GDP-beta-S prior to the addition of GTP-gamma-S and recombinant ARF1 significantly inhibited alpha2M*-stimulation of PLD activity. Nuclear PLD activity was maximally stimulated in the presence of both GTP-gamma-S and rARF1, whereas plasma membrane PLD activity was maximally stimulated in the presence of rARF1, GTP-gamma-S, RhoA, and ATP. In contrast, nuclear PLD activity was not affected by RhoA either alone or in combination with GTP-gamma-S or ATP.