A role for Shc, Grb2, and Raf-1 in FcgammaRI signal relay

Elucidating the interplay between IgG-Fc valency and FcγR activation for the design of immune complex inhibitors

Autoantibody immune complex (IC) activation of Fcγ receptors (FcγRs) is a common pathogenic hallmark of multiple autoimmune diseases. Given that the IC structural features that elicit FcγR activation are poorly understood and the FcγR system is highly complex, few therapeutics can directly block these processes without inadvertently activating the FcγR system. To address these issues, the structure activity relationships of an engineered panel of multivalent Fc constructs were evaluated using sensitive FcγR binding and signaling cellular assays. These studies identified an Fc valency with avid binding to FcγRs but without activation of immune cell effector functions. These observations directed the design of a potent trivalent immunoglobulin G-Fc molecule that broadly inhibited IC-driven processes in a variety of immune cells expressing FcγRs. The Fc trimer, Fc3Y, was highly efficacious in three different animal models of autoimmune diseases. This recombinant molecule may represent an effective therapeutic candidate for FcγR-mediated autoimmune diseases.

The proapoptotic and antimitogenic protein p66SHC acts as a negative regulator of lymphocyte activation and autoimmunity

The ShcA locus encodes 3 protein isoforms that differ in tissue specificity, subcellular localization, and function. Among these, p66Shc inhibits TCR coupling to the Ras/MAPK pathway and primes T cells to undergo apoptotic death. We have investigated the outcome of p66Shc deficiency on lymphocyte development and homeostasis. We show that p66Shc−/− mice develop an age-related lupus-like autoimmune disease characterized by spontaneous peripheral T- and B-cell activation and proliferation, autoantibody production, and immune complex deposition in kidney and skin, resulting in autoimmune glomerulonephritis and alopecia. p66Shc−/− lymphocytes display enhanced proliferation in response to antigen receptor engagement in vitro and more robust immune responses both to vaccination and to allergen sensitization in vivo. The data identify p66Shc as a negative regulator of lymphocyte activation and show that loss of this protein results in breaking of immunologic tolerance and development of systemic autoimmunity.

Possible role of factor XIII subunit A in Fcγ and complement receptor-mediated phagocytosis

Besides its traditional role in hemostasis, factor XIII subunit A (FXIII-A) is supposed to function as a cellular transglutaminase and to be involved in certain intracellular processes, including cytoskeletal remodeling. To investigate its intracellular role, the aim of the present study was to follow changes in FXIII-A production in combination with the receptor-mediated phagocytic activities of monocytes/macrophages and to examine the phagocytic functions of monocytes in patients with FXIII-A deficiency. Human blood monocytes were isolated from the buffy coats of healthy volunteers and cultured for 4 days. The FcgammaR-mediated phagocytosis of sensitized erythrocytes (EA) and the complement receptor (CR)-mediated phagocytosis of complement-coated yeast particles were studied during monocyte/macrophage differentiation. Changes in the gene expression of FXIII-A were detected by real-time quantitative RT-PCR. FXIII-A protein production was investigated with fluorescent image analysis at single cell level and Western immunoblot analysis. Both the FcgammaR and CR-mediated phagocytosis increased during culturing, which peaked on day 3. The phagocytic activity of the cells could be markedly inhibited with monodansylcadaverine, an inhibitor of the transglutaminase-induced crosslinking of proteins. The phagocytosis of EA, complement-coated and uncoated yeast particles was found to be strongly diminished in monocytes of FXIII-A deficient patients. The phagocytic functions of cultured cells showed a change in parallel with the alterations in FXIII-A mRNA expression, as well as with that in FXIII-A in protein synthesis detected by image and Western immunoblot analyses in concert. Our results suggest that FXIII-A plays a role in the Fcgamma and complement receptor-mediated phagocytic activities of monocytes/macrophages.

SHP-1 regulates Fcγ receptor–mediated phagocytosis and the activation of RAC

Fcγ receptor–mediated phagocytosis is a complex process involving the activation of protein tyrosine kinases, events that are potentially down-regulated by protein tyrosine phosphatases. We used the J774A.1 macrophage cell line to examine the roles played by the protein tyrosine phosphatase SHP-1 in the negative regulation of Fcγ receptor–mediated phagocytosis. Stimulation with sensitized sheep red blood cells (sRBCs) induced tyrosine phosphorylation of CBL and association of CBL with CRKL. These events were completely or partially abrogated by PP1 or the heterologous expression of dominant-negative SYK, respectively. Heterologous expression of wild-type but not catalytically inactive SHP-1 also completely abrogated the phagocytosis of IgG-sensitized sRBCs. Most notably, overexpressed SHP-1 associates with CBL and this association led to CBL dephosphorylation, loss of the CBL-CRKL interaction, and the suppression of Rac activation. These data represent the first direct evidence that SHP-1 is involved in the regulation of Fcγ receptor–mediated phagocytosis and suggest that activating signals mediated by SRC family kinases SYK, CBL, phosphatidyl inositol-3 (PI-3) kinase, and Rac are directly opposed by inhibitory signals through SHP-1.

IFN-gamma pretreatment augments immune complex-induced matrix metalloproteinase-1 expression in U937 histiocytes

We reported recently that immune complexes (ICs) induced matrix metalloproteinase-1 (MMP-1) expression in U937 histiocytes. The present study was undertaken to determine the effect of pretreatment of U937 cells with interferon-gamma (IFN-gamma) on IC-induced MMP-1 expression. Our flow cytometry studies showed that IFN-gamma upregulated the surface expression of FcgammaRI, but not FcgammaRII. Results also showed that pretreatment of the cells with IFN-gamma augmented LDL-containing IC (LDL-IC)-induced MMP-1 secretion in a dose- and time-dependent manner. Furthermore, Northern blot analysis revealed that IFN-gamma pretreatment led to a marked increase in MMP-1 mRNA. Finally, we demonstrated that PD98059 was able to block LDL-IC-induced MMP-1 secretion, regardless of whether the cells were pretreated with IFN-gamma or not, suggesting that IFN-gamma pretreatment did not alter the essential role of the ERK signaling pathway in LDL-IC-induced MMP-1 expression. In conclusion, the present study has demonstrated that IFN-gamma pretreatment augments LDL-IC-induced MMP-1 expression in U937 cells, thus elucidating an immune mechanism potentially involved in plaque destabilization.

PTEN controls immunoreceptor (immunoreceptor tyrosine-based activation motif) signaling and the activation of Rac

Fcgamma receptor-mediated phagocytosis is a model for the study of immunoreceptor (immunoreceptor tyrosine-based activation motif [ITAM]) signaling and involves the activation of protein tyrosine kinases, protein tyrosine phosphatases, and downstream effectors including phosphatidylinositol-3 (PI-3) kinase. Relatively little is known of the role of lipid phosphatases in the control of ITAM signaling and inflammation. A heterologous COS7 cell system was used to examine the roles played by PI-3 kinase and the dual-specificity phosphatase, phosphatase and tensin homolog deleted on chromosome 10 (PTEN), in the signal transduction pathway leading to Fcgamma receptor IIA-mediated phagocytosis and the activation of Rac. The expression of wildtype PTEN completely abrogated the phagocytosis of immunoglobulin-G-sensitized sheep red blood cells, as compared with the catalytically inactive mutant of PTEN, which had no effect. This is the first direct evidence that PTEN, an inositol 3' phosphatase, regulates Fcgamma receptor-mediated phagocytosis, an ITAM-based signaling event. The data suggest that PTEN exerts control over phagocytosis potentially by controlling the downstream conversion of guanosine diphosphate-Rac to guanosine triphosphate-Rac following ITAM stimulation.

Caspase-3 apoptotic signaling following injury to the central nervous system

Apoptotic cell death is a fundamental and highly regulated biological process in which a cell is instructed to participate actively in its own demise. This process of cellular suicide is activated by developmental and environmental cues and normally plays an essential role in eliminating superfluous, damaged, and senescent cells of many tissue types. In recent years, a number of experimental studies have provided evidence of widespread neuronal and glial apoptosis following injury to the central nervous system (CNS). These studies indicate that injury-induced apoptosis can be detected from hours to days following injury and may contribute to neurological dysfunction. Given these findings, understanding the biochemical signaling events controlling apoptosis is a first step towards developing therapeutic agents which would target this cell death process. This review will focus on the molecular cell death pathways responsible for generating the apoptotic phenotype, summarize what is currently known about apoptotic signals activated in the injured CNS, and what potential strategies might be pursued to reduce this cell death process as a means to promote functional recovery.

Small GTP-binding proteins

Small GTP-binding proteins (G proteins) exist in eukaryotes from yeast to human and constitute a superfamily consisting of more than 100 members. This superfamily is structurally classified into at least five families: the Ras, Rho, Rab, Sar1/Arf, and Ran families. They regulate a wide variety of cell functions as biological timers (biotimers) that initiate and terminate specific cell functions and determine the periods of time for the continuation of the specific cell functions. They furthermore play key roles in not only temporal but also spatial determination of specific cell functions. The Ras family regulates gene expression, the Rho family regulates cytoskeletal reorganization and gene expression, the Rab and Sar1/Arf families regulate vesicle trafficking, and the Ran family regulates nucleocytoplasmic transport and microtubule organization. Many upstream regulators and downstream effectors of small G proteins have been isolated, and their modes of activation and action have gradually been elucidated. Cascades and cross-talks of small G proteins have also been clarified. In this review, functions of small G proteins and their modes of activation and action are described.

The requirement of both extracellular regulated kinase and p38 mitogen-activated protein kinase for stimulation of cytosolic phospholipase A(2) activity by either FcgammaRIIA or FcgammaRIIIB in human neutrophils. A possible role for Pyk2 but not for the Grb2-Sos-Shc complex

The signal transduction pathways initiated by opsonized zymosan (OZ) leading to activation of cytosolic phospholipase A(2) (cPLA(2)) in human neutrophils remain obscure. In a previous study, we showed that the activation of cPLA(2) by OZ is tyrosine kinase-dependent. The present study demonstrates that the signals initiated by OZ involve activation of tyrosine kinase Pyk2 but not the formation of the adhesion protein complex, Shc-Grb2-Sos. Stimulation of cPLA(2) activity by OZ is mediated by Fc gamma receptors (FcgammaRs) and not by complement receptors for the C3b protein. Cross-linking of FcgammaRIIA or FcgammaRIIIB induces p38 mitogen-activated protein (MAP) kinase and extracellular regulated kinase (ERK) phosphorylation. The kinetics of cPLA(2) activity stimulated by either of the FcgammaRs or by both is similar to that of p38 MAP kinase and was detected as early as 15 s after stimulation, maintained a plateau for 10 min, and decreased thereafter. ERK activation was detected also within 15 s but decreased significantly 5 min after stimulation. The MEK inhibitor, PD-098059, or the p38 MAP kinase inhibitor, SB-203580, caused a partial inhibition during the time course of cPLA(2) activity, whereas their combination caused a total inhibition. Thus, although ERK activation is significantly shorter than that of p38 MAP kinase, it is equally required for activation and maintenance of cPLA(2) activity by occupancy of a single receptor, FcgammaRIIA or FcgammaRIIIB.

High Affinity IgG Receptor Activation of Src Family Kinases Is Required for Modulation of the Shc-Grb2-Sos Complex and the Downstream Activation of the Nicotinamide Adenine Dinucleotide Phosphate (Reduced) Oxidase

We used the U937 cell line to examine the modulation of adaptor protein interactions (Shc, Grb2, and Cbl) after high affinity IgG receptor (FcγRI) cross-linking, leading to the formation of the Grb2-Sos complex, the activation of Ras, and the regulation of the respiratory burst. Cross-linking of FcγRI induced the conversion of GDP-Ras to GTP-Ras reaching a maximum 5 min after stimulation. Concomitant with Ras activation, Sos underwent an electrophoretic mobility shift and the Sos-Grb2 association was increased (6-fold). The Grb2-Sos complex was present only in the membrane fraction and was augmented after FcγRI stimulation. Tyrosine-phosphorylated Shc, mainly the p52 isoform, was observed to transiently onload to the membrane Grb2-Sos complex on FcγRI stimulation. Cross-linking of FcγRI induces the tyrosine phosphorylation of Cbl, which forms a complex with Grb2 and Shc via the Cbl C terminus. Kinetic experiments confirm that Cbl-Grb2 is relatively stable, whereas Grb2-Sos, Grb2-Shc, and Cbl-Shc interactions are highly inducible. The Src family tyrosine kinase inhibitor, PP1, was shown to completely inhibit Shc tyrosine phosphorylation, the Shc-Grb2 interaction, and the FcγR-induced respiratory burst. Our results provide the first evidence that the upstream activation of Src kinases is required for the modulation of the Shc-Grb2 interaction and the myeloid NADPH oxidase response.

Role of Src in the Modulation of Multiple Adaptor Proteins in FcRI Oxidant Signaling

Cross-linking of Fc receptors for IgA, FcR (CD89), on monocytes/macrophages is known to enhance phagocytic activity and generation of oxygen free radicals. We provide evidence here that the FcR signals through the γ subunit of FcɛRI in U937 cells differentiated with interferon γ (IFNγ). Our results provide the first evidence that FcR-mediated signals modulate a multimolecular adaptor protein complex containing Grb2, Shc, SHIP, CrkL, Cbl, and SLP-76. Cross-linking of FcRI using anti-FcRI induces the phosphorylation of the γ subunit as detected by mobility retardation on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Stimulation of FcRI induced the tyrosine phosphorylation of Shc and increased the association of Grb2 with Shc and CrkL. Grb2 associates constitutively with Sos, and the latter undergoes mobility shift upon FcRI stimulation. The complex adapter proteins, Cbl and SLP-76, are physically associated in myeloid cells and both proteins undergo tyrosine phosphorylation upon FcR stimulation. These data indicate that the stimulation of FcR results in the modulation of adaptor complexes containing tyrosine-phosphorylated Cbl, Shc, SHIP, Grb2, and Crkl. Experiments performed with the Src kinase inhibitor, PP1, provide the first evidence that Src kinase activation is required for FcRI-induced production of superoxide anions and provide insight into the mechanism for FcR-mediated activation of downstream oxidant signaling in myeloid cells.

Role of Src in the Modulation of Multiple Adaptor Proteins in FcRI Oxidant Signaling

Cross-linking of Fc receptors for IgA, FcR (CD89), on monocytes/macrophages is known to enhance phagocytic activity and generation of oxygen free radicals. We provide evidence here that the FcR signals through the γ subunit of FcɛRI in U937 cells differentiated with interferon γ (IFNγ). Our results provide the first evidence that FcR-mediated signals modulate a multimolecular adaptor protein complex containing Grb2, Shc, SHIP, CrkL, Cbl, and SLP-76. Cross-linking of FcRI using anti-FcRI induces the phosphorylation of the γ subunit as detected by mobility retardation on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Stimulation of FcRI induced the tyrosine phosphorylation of Shc and increased the association of Grb2 with Shc and CrkL. Grb2 associates constitutively with Sos, and the latter undergoes mobility shift upon FcRI stimulation. The complex adapter proteins, Cbl and SLP-76, are physically associated in myeloid cells and both proteins undergo tyrosine phosphorylation upon FcR stimulation. These data indicate that the stimulation of FcR results in the modulation of adaptor complexes containing tyrosine-phosphorylated Cbl, Shc, SHIP, Grb2, and Crkl. Experiments performed with the Src kinase inhibitor, PP1, provide the first evidence that Src kinase activation is required for FcRI-induced production of superoxide anions and provide insight into the mechanism for FcR-mediated activation of downstream oxidant signaling in myeloid cells.

Membrane-targeting of signalling molecules by SH2/SH3 domain-containing adaptor proteins

SH2/SH3 domain-containing adaptor proteins play a critical role in regulating tyrosine kinase signalling pathways. The major function of these adaptors, such as Grb2, Nck, and Crk, is to recruit proline-rich effector molecules to tyrosine-phosphorylated kinases or their substrates. In recent years dozens of novel proteins have emerged that are capable of associating with the SH2 and the SH3 domains of adaptors. In this review, the author attempts to summarise these novel binding partners of Grb2, Nck, and Crk, and to discuss current controversies regarding function and regulation of protein multicomplexes held together by SH2/SH3 adaptor molecules at the plasma membrane.

Tyrosine phosphorylation of the proto-oncoprotein Raf-1 is regulated by Raf-1 itself and the phosphatase Cdc25A

There is a growing body of evidence demonstrating that Raf-1 is phosphorylated on tyrosines upon stimulation of a variety of receptors. Although detection of Raf-1 tyrosine phosphorylation has remained elusive, genetic analyses have demonstrated it to be important for Raf-1 activation. Here we report new findings which indicate that Raf-1 tyrosine phosphorylation is regulated in vivo. In both a mammalian and baculovirus expression system, a kinase-inactive allele of Raf-1 was found to be tyrosine phosphorylated at levels much greater than that of wild-type Raf-1. The level of tyrosine phosphate on Raf-1 was markedly increased upon treatment with phosphatase inhibitors either before or after cell lysis. Cdc25A was found to dephosphorylate Raf-1 on tyrosines that resulted in a significant decrease in Raf-1 kinase activity. In NIH 3T3 cells, coexpression of wild-type Raf-1 and phosphatase-inactive Cdc25A led to a marked increase in Raf-1 tyrosine phosphorylation in response to platelet-derived growth factor. These data suggest that the tyrosine phosphorylation of Raf-1 is regulated not only by itself but also by Cdc25A.

Oxidized LDL-containing immune complexes induce Fc gamma receptor I-mediated mitogen-activated protein kinase activation in THP-1 macrophages

Our previous studies have shown that Fc gamma receptor (FcgammaR)-mediated uptake of LDL-containing immune complexes (oxLDL-ICs) by human monocyte-derived macrophages leads to not only transformation of macrophages into foam cells but also macrophage activation and release of cytokines. It has been shown that cross-linking of FcgammaR triggers activation of signal transduction pathways that alter gene expression in macrophages. In this study, we determined whether engagement of FcgammaR by oxLDL-ICs leads to activation of mitogen-activated protein (MAP) kinase pathway, a signaling cascade serving many important functions, including the regulation of gene expression, in THP-1 macrophage-like cells. Our results from immunoblotting, using specific anti-phosphorylated MAP kinase antibodies, showed that oxLDL-ICs induced extracellular signal regulated kinase 2 (ERK2) MAP kinase phosphorylation in THP-1 macrophage-like cells in time- and dose-dependent manners. Cholesterol loading before stimulation led to a longer phosphorylation of ERK2. Nuclear translocation of phosphorylated ERK was markedly increased after the stimulation. Moreover, our data showed that oxLDL-IC induction of MAP kinase was prevented by human monomeric IgG1, suggesting that the specific engagement of type I FcgammaR by oxLDL-IC is responsible for the MAP kinase activation. Finally, we showed that human anti-oxLDL autoantibody-containing immune complexes immobilized on type I collagen induced MAP kinase activation in THP-1 cells. These results strongly suggest that oxLDL-IC, which has been detected in atherosclerotic plaques, may play an important role in macrophage activation and atherogenesis.

Vav and SLP-76 recruitment by cross-linking of FcgammaRIIa1 in promyelocytic HL-60 cells

The signaling events induced upon cross-linking of the human FcgammaRIIa1 (CD32) which contains an immune receptor tyrosine-based activation motif (ITAM) in its intracellular region, were investigated in the promyelocytic HL-60 cells. It is shown here that the FcgammaRIIa1 engagement recruits the Ras pathway in these cells, as evidenced by the tyrosine-phosphorylation of the Shc adaptator protein and of the extracellular signal-regulated kinase 2 (ERK2). However, p95vav, a molecule able to interact with Rac-1 and to regulate cytoskeletal reorganization, was also found to be phosphorylated. In addition, co-immunoprecipitation experiments demonstrated that Vav is associated with SLP-76 upon FcgammaRIIa1 activation. A strong phosphorylation of p120cbl was also observed. The phosphorylation of molecules such as p95vav, SLP-76 and p120cbl suggests that FcgammaRIIa1 triggering also activates signaling pathways other than the Ras pathway.

The ankyrin repeat-containing adaptor protein Tvl-1 is a novel substrate and regulator of Raf-1

Tvl-1 is a 269-amino acid ankyrin repeat protein expressed primarily in thymus, lung, and testes that was identified by screening a murine T-cell two-hybrid cDNA library for proteins that associate with the serine-threonine kinase Raf-1. The interaction of Tvl-1 with Raf-1 was confirmed by co-immunoprecipitation of the two proteins from COS-1 cells transiently transfected with Tvl-1 and Raf-1 expression constructs as well as by co-immunoprecipitation of the endogenous proteins from CV-1 and NB2 cells. Tvl-1 interacts with Raf-1 via its carboxyl-terminal ankyrin repeat domain. The same domain also mediates Tvl-1 homodimerization. Tvl-1 was detected by immunofluorescence in both the cytoplasm and the nucleus suggesting that in addition to Raf-1 it may also interact with nuclear proteins. Activated Raf-1 phosphorylates Tvl-1 both in vitro and in vivo. In baculovirus-infected Sf9 insect cells, Tvl-1 potentiates the activation of Raf-1 by Src and Ras while in COS-1 cells it potentiates the activation of Raf-1 by EGF. These data suggest that Tvl-1 is both a target as well as a regulator of Raf-1. The human homologue of Tvl-1 maps to chromosome 19p12, upstream of MEF2B with the two genes in a head to head arrangement.

Immunoreceptor tyrosine-based inhibition motif-bearing receptors regulate the immunoreceptor tyrosine-based activation motif-induced activation of immune competent cells

ITIM-bearing receptors, a family which only recently has been recognized, play a key role in the regulation of the ITAM-induced activation of immune competent cells. The mechanism of ITM-mediated regulation in various cells was recently clarified. The present review focuses on ITIM bearing membrane proteins that negatively regulate the activation of cells when co-crosslinked with ITAM containing receptors, illustrates the inhibitory processes by the negative regulation of B-, NK-, T-cells and mast cells and summarizes current views on the mechanism of ITIM-mediated inhibition.

Fcγ Receptor-Mediated Activation of Phospholipase D Regulates Macrophage Phagocytosis of IgG-Opsonized Particles

Receptors for the Fc portion of IgG (FcγRs) integrate the innate and acquired components of immunity by coupling the specific recognition of IgG Abs to the activation of phagocytic leukocytes. Knowledge of the molecular mechanisms that regulate phagocyte stimulation by FcγRs may permit therapeutic modulation to augment immunoprotective aspects and minimize damage to host tissues in diverse inflammatory diseases. Since phospholipase D (PLD) has been linked to the stimulation of cytotoxic leukocyte responses, we characterized FcγR-dependent activation of PLD in human macrophages. IgG-coated SRBCs (EIgG) stimulated a 9.4-fold increase in PLD activity compared with SRBCs treated with control Ab (p < 0.001), determined by formation of the PLD-specific product phosphatidylethanol in the presence of 0.5% ethanol. Levels of phosphatidic acid, the physiologic product of PLD-mediated catalyzis, were significantly increased in the absence of ethanol (6.4-fold, p < 0.001). PLD activity was also stimulated by immune complex-coated latex beads or cross-linking of Abs specific for FcγRI, FcγRII, or FcγRIII. Phagocytosis of EIgG was reduced by two inhibitors of PLD-mediated signaling, 2,3-diphosphoglycerate or 1-butanol. Addition of purified PLD restored control levels of phagocytosis in cells in which endogenous PLD was inhibited. The tyrosine kinase inhibitors genistein and herbimycin A caused concordant reductions in FcγR-stimulated PLD activity and phagocytosis. These studies demonstrate that FcγR-mediated phagocytosis is accompanied by tyrosine kinase-dependent activation of PLD and support the hypothesis that stimulation of PLD functions to regulate the ingestion of IgG-opsonized particles.

Association of Shc, Cbl, Grb2, and Sos following treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin in primary rat hepatocytes

c-Src kinases and p21 Ras are known to be implicated in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated signal transduction. However, the effects of TCDD on the molecular interaction of adaptor complex in the protein tyrosine kinase signaling cascade have not been reported. The present study is designed to clarify whether TCDD modulates the molecular interactions of Shc, Cbl, Grb2, and Sos in primary rat hepatocytes. TCDD causes an electrophoretic mobility shift of Sos and increases Sos/Grb2 association. Tyrosine phosphorylated Shc, mainly p52, unloads to the Grb2/Sos complex upon TCDD stimulation. Interestingly, TCDD decreases the tyrosine phosphorylation level of Cbl, although Cbl recruits more Grb2 and Shc proteins by TCDD. These results indicate that TCDD modulates the molecular interaction of adaptor complex proteins including Shc, Grb2, Sos, and Cbl. Furthermore, tyrosine phosphorylation of Cbl may not be critical for interaction of the protein with Grb2 and Shc in the TCDD signaling pathway in primary rat hepatocytes.

Characterization of Cbl-Nck and Nck-Pak1 interactions in myeloid FcgammaRII signaling

Fc receptors modulate inflammatory processes, including phagocytosis, serotonin and histamine release, superoxide production, and secretion of cytokines. Aggregation of FcgammaRIIa, the low-affinity receptor for monomeric IgG, activates nonreceptor protein tyrosine kinases such as Lyn, Hck, and Syk, potentially driving the phosphorylation of the downstream adaptor proteins, including Cbl and/or Nck. Previous work from our laboratory using interferon-gamma-differentiated U937 (U937IF) myeloid cells investigated mechanisms which regulate Fcgamma receptor-induced assembly of adaptor complexes. Herein we report that FcgammaRII receptor signaling in U937IF and HEL cells involves Cbl and Nck, suggesting that Cbl-Nck interactions may link FcgammaRII to downstream activation of Pak kinase. FcgammaRII crosslinking induced the phosphorylation of Cbl and Nck on tyrosine. The alphaCbl immunoprecipitations revealed constitutive binding of Nck and Grb2 to Cbl and FcgammaRII-inducible binding of CrkL to Cbl. The interactions of Cbl with Nck and CrkL were phosphorylation dependent since dephosphorylation of cellular proteins with potato acid phosphatase abrogated binding. GST-Nck fusion protein pulldown experiments show that Cbl and Pak1 bind to the second SH3 domain of Nck. A specific Src inhibitor, PP1, was shown to completely abrogate the FcgammaR-induced superoxide response, correlating with a decrease in Cbl and Nck tyrosine phosphorylation. Our results provide the first evidence that Src is required for FcgammaR activation of the respiratory burst in myeloid cells and suggest that Cbl-Nck, Cbl-Pak1, and Nck-Pak1 interactions may regulate this response.

Differential Interaction of Crkl with Cbl or C3G, Hef-1, and γ Subunit Immunoreceptor Tyrosine-Based Activation Motif in Signaling of Myeloid High Affinity Fc Receptor for IgG (FcγRI)

Cbl-Crkl and Crkl-C3G interactions have been implicated in T cell and B cell receptor signaling and in the regulation of the small GTPase, Rap1. Recent evidence suggests that Rap1 plays a prominent role in the regulation of immunoreceptor tyrosine-based activation motif (ITAM) signaling. To gain insight into the role of Crkl in myeloid ITAM signaling, we investigated Cbl-Crkl and Crkl-C3G interactions following FcγRI aggregation in U937IF cells. FcγRI cross-linking of U937IF cells results in the tyrosine phosphorylation of Cbl, Crkl, and Hef-1, an increase in the association of Crkl with Cbl via direct SH2 domain interaction and increased Crkl-Hef-1 binding. Crkl constitutively binds to the guanine nucleotide-releasing protein, C3G, via direct SH3 domain binding. Our data show that distinct Cbl-Crkl and Crkl-C3G complexes exist in myeloid cells, suggesting that these complexes may modulate distinct signaling events. Anti-Crkl immunoprecipitations demonstrate that the ITAM-containing γ subunit of FcγRI is induced to form a complex with the Crkl protein, and Crkl binds to the cytoskeletal protein, Hef-1. The induced association of Crkl with Cbl, Hef-1, and FcγRIγ after FcγRI activation and the constitutive association between C3G and Crkl provide the first evidence that a FcγRIγ-Crkl-C3G complex may link ITAM receptors to the activation of Rap1 in myeloid cells.

SLP-76 Expression Is Restricted to Hemopoietic Cells of Monocyte, Granulocyte, and T Lymphocyte Lineage and Is Regulated During T Cell Maturation and Activation

The leukocyte-specific adapter protein SLP-76 is known to augment the transcriptional activity of nuclear factor of activated T cells and AP-1 following TCR ligation. A role for SLP-76 in additional receptor-mediated signaling events is less clear. To define the pattern of SLP-76 expression during murine hemopoiesis, we stained cells isolated from various tissues with a combination of surface markers followed by intracellular staining with a fluorochrome-labeled SLP-76-specific Ab. In the bone marrow, SLP-76 expression is largely restricted to cells of granulocyte and monocyte lineage. Heterogeneous SLP-76 expression is first detected in the CD44+CD25− subset within the CD3−CD4−CD8− thymocyte population. Interestingly, SLP-76 expression increases as thymocyte maturation progresses within the CD4−CD8− compartment but decreases as cells mature to a CD4+CD8+ phenotype. SLP-76 expression is then up-regulated following selection and concomitant with maturation to a CD4+ or CD8+ phenotype. In the periphery, SLP-76 is expressed in T lymphocytes with no detectable expression in the B cell compartment. Exposure to the superantigen staphylococcal enterotoxin B augments SLP-76 expression in the reactive T cell subset. Furthermore, in vitro stimulation with TCR-specific Abs augments the existing levels of SLP-76. These data reveal that SLP-76 expression is coordinately regulated with surface expression of a pre-TCR or mature TCR complex during thymocyte development and that TCR ligation elicits signals that result in increased expression of SLP-76.

SLP-76-Cbl-Grb2-Shc Interactions in FcγRI Signaling

SLP-76 and Cbl are complex adapter proteins that have the capacity to bind to smaller adapter proteins, such as Grb2, which subsequently binds the nucleotide exchange protein Sos in the transmission of intracellular signals. SLP-76, Cbl, Shc, and Grb2 have been implicated in immunoreceptor tyrosine-based activation motif (ITAM) signaling, leading to activation of Ras. However, their mechanism of action has not been determined. To date, there have been no reports of SLP-76 involvement in FcγRI-receptor signaling and no data exist for an interaction between Cbl, Shc, and SLP-76 in vivo. We provide evidence that SLP-76, Cbl, and Shc are tyrosine phosphorylated on FcγRI-receptor stimulation and are associated with the adapter protein Grb2 in γ-interferon–differentiated U937 cells (U937IF). The interactions between SLP-76 and Cbl and SLP-76 and Grb2 are present in resting U937IF cells. However, the interaction between SLP-76 and Grb2 becomes augmented twofold on FcγRI-receptor aggregation. Our results provide the first evidence for a phosphorylation-dependent interaction between SLP-76 and Shc, induced at least 10-fold on FcγRI receptor stimulation. Our data indicate that a significant portion of a multimolecular complex containing Cbl, SLP-76, Shc, and Grb2 is distinct from a trimolecular complex containing the Ras guanine nucleotide exchanger Sos, Shc, and Grb2. FcγRI-induced tyrosine phosphorylation of SLP-76, Cbl, Shc, and the highly induced SLP-76-Shc interaction provide the first evidence that SLP-76 and Cbl are involved in FcγRI signaling and suggest a functional significance for these interactions in FcγRI signal relay in the control of Ras in myeloid cells.

© 1998 by The American Society of Hematology.

FcgammaRIa-gamma-chain complexes trigger antibody-dependent cell-mediated cytotoxicity (ADCC) in CD5+ B cell/macrophage IIA1.6 cells

Most receptors for immunoglobulins exist as multi-subunit complexes, with unique ligand binding alpha-chains, combined with accessory signalling (gamma-, beta-, or zeta-) chains. The myeloid class I receptor for IgG (FcgammaRIa) has been shown to be dependent on the FcR gamma-chain for surface expression in vivo. In this study we assess the capacity of FcgammaRIa-gamma-chain complexes expressed in IIA1.6 cells to trigger phagocytosis and ADCC. An intact immunoreceptor tyrosine-based activation motif (ITAM) signalling motif proved essential for triggering of biological function via the FcgammaRIa receptor complex. Both the FcR gamma-chain and the FcgammaRIIa-ITAM proved active in directing phagocytosis of Staphylococcus aureus and ADCC of erythrocytes, triggered by the FcgammaRIa complex. The capacity of FcgammaRIa to trigger phagocytic and cytolytic activity by IIA1.6 cells, both considered 'professional phagocyte' functions, motivated us to re-evaluate the cell lineage and developmental stage of IIA1.6 cells. Although originally described as mouse B lymphocytes, the IIA1.6 cells proved positive for non-specific esterase activity and expressed the CD5 antigen. These combined characteristics place the IIA1.6 cells within a unique CD5+ B cell/macrophage lineage, optimally suited for cell biological analyses of phagocyte receptors.

SLP-76-Cbl-Grb2-Shc Interactions in FcγRI Signaling

SLP-76 and Cbl are complex adapter proteins that have the capacity to bind to smaller adapter proteins, such as Grb2, which subsequently binds the nucleotide exchange protein Sos in the transmission of intracellular signals. SLP-76, Cbl, Shc, and Grb2 have been implicated in immunoreceptor tyrosine-based activation motif (ITAM) signaling, leading to activation of Ras. However, their mechanism of action has not been determined. To date, there have been no reports of SLP-76 involvement in FcγRI-receptor signaling and no data exist for an interaction between Cbl, Shc, and SLP-76 in vivo. We provide evidence that SLP-76, Cbl, and Shc are tyrosine phosphorylated on FcγRI-receptor stimulation and are associated with the adapter protein Grb2 in γ-interferon–differentiated U937 cells (U937IF). The interactions between SLP-76 and Cbl and SLP-76 and Grb2 are present in resting U937IF cells. However, the interaction between SLP-76 and Grb2 becomes augmented twofold on FcγRI-receptor aggregation. Our results provide the first evidence for a phosphorylation-dependent interaction between SLP-76 and Shc, induced at least 10-fold on FcγRI receptor stimulation. Our data indicate that a significant portion of a multimolecular complex containing Cbl, SLP-76, Shc, and Grb2 is distinct from a trimolecular complex containing the Ras guanine nucleotide exchanger Sos, Shc, and Grb2. FcγRI-induced tyrosine phosphorylation of SLP-76, Cbl, Shc, and the highly induced SLP-76-Shc interaction provide the first evidence that SLP-76 and Cbl are involved in FcγRI signaling and suggest a functional significance for these interactions in FcγRI signal relay in the control of Ras in myeloid cells.

© 1998 by The American Society of Hematology.

CBL-GRB2 Interaction in Myeloid Immunoreceptor Tyrosine Activation Motif Signaling

In this study, we provide the first evidence for role of the CBL adapter protein interaction in FcγRI receptor signal transduction. We study the FcγRI receptor, an immunoreceptor tyrosine activation motif (ITAM)-linked signaling pathway, using IFN-γ-differentiated U937 myeloid cells, termed U937IF cells. CBL is constitutively associated with both GRB2 and the ITAM-containing receptor subunit, FcγRIγ of FcγRI, providing direct evidence that CBL functions in myeloid ITAM signaling. FcγRI cross-linking of U937IF cells induces the tyrosine phosphorylation of CBL that is associated with an altered CBL-GRB2 interaction. Both GRB2-SH3 and SH2 domains bind CBL in resting cell lysates; upon FcγRI stimulation, phosphorylated CBL binds exclusively to the GRB2-SH2 domain. Glutathione-S-transferase fusion protein data demonstrate that the constitutive interaction of CBL with GRB2 and CRKL is mediated via two discrete regions of the CBL C terminus. The proximal C terminus (residues 461–670) binds to GRB2 constitutively, and under conditions of receptor activation binds to the tyrosine-phosphorylated SHC adapter molecule. The distal C terminus of CBL (residues 671–906) binds the CRKL adapter protein. The data demonstrate that the CBL-GRB2 and GRB2-SOS protein complexes are distinct and mutually exclusive in U937IF cells, supporting a model by which the CBL-GRB2 and GRB2-SOS complexes function in separate pathways for myeloid FcγRI signaling.

Microtubule integrity regulates src-like and extracellular signal-regulated kinase activities in human pro-monocytic cells. Importance for interleukin-1 production

We have demonstrated previously that microtubule depolymerization by colchicine in human monocytes induces selective production of interleukin-1 (IL-1) (Manié, S., Schmid-Alliana, A., Kubar, J., Ferrua, B., and Rossi, B. (1993) J. Biol. Chem. 268, 13675-13681). Here, we provide evidence that disruption of the microtubule structure rapidly triggers extracellular signal-regulated kinase (ERK) activation, whereas it was without effect on SAPK2 activity, which is commonly acknowledged to control pro-inflammatory cytokine production. This process involves the activation of the entire cascade including Ras, Raf-1, MEK1/2, ERK1, and ERK2. Activation of ERKs is followed by their nuclear translocation. Although other SAPK congeners might be activated upon microtubule depolymerization, the activation of ERK1 and ERK2 is mandatory for IL-1 production as shown by the blocking effect of PD 98059, a specific MEK1/2 inhibitor. Additionally, we provide evidence that microtubule disruption also induces the activation of c-Src and Hck activities. The importance of Src kinases in the mediation of the colchicine effect is underscored by the fact that CP 118556, a specific inhibitor of Src-like kinase, abrogates both the colchicine-induced ERK activation and IL-1 production. This is the first evidence that ERK activation is an absolute prerequisite for induction of this cytokine. Altogether, our data lend support to a model where the status of microtubule integrity controls the level of Src activities that subsequently activate the ERK kinase cascade, thus leading to IL-1 production.

Protein tyrosine phosphatase inhibitors in Fc gamma RI-induced myeloid oxidant signaling

Fc-receptor stimulation in myeloid cells results in increased oxygen consumption, termed the respiratory burst, which is coupled to a rapid and transient increase in tyrosine phosphorylation of cellular proteins. In a previous paper in this journal we showed that the protein tyrosine phosphatase (PTPase) inhibitors sodium orthovanadate and phenylarsine oxide (PAO) block the Fc gamma RI-induced respiratory burst in interferon-gamma-differentiated U937 cells (U937IF) while augmenting the Fc gamma RI-induced tyrosine phosphorylation of cellular proteins. Herein we examine the effects of PTPase inhibitors on specific molecules involved in Fc gamma RI signaling. We show that orthovanadate and PAO augmented the Fc gamma RI-induced tyrosine phosphorylation of the adaptor protein CBL. CBL interactions with other phosphoproteins, among them SHC and CRKL, were also augmented in response to pretreatment with the PTPase inhibitors. SHC was tyrosine phosphorylated in response to Fc gamma RI stimulation of U937IF cells and bound to the SH2 domain of GRB2 in a stimulation-dependent manner. In fusion protein pull-down experiments the interaction of SHC with the SH2 domain of GRB2 was increased in PTPase inhibitor pretreated U937IF cells in response to Fc gamma RI stimulation. Our data support the hypothesis that a tyrosine dephosphorylation event is required for effective transmission of the Fc gamma RI signal to result in activation of the myeloid respiratory burst response.

The uptake of LDL-IC by human macrophages: predominant involvement of the Fc gamma RI receptor

The incubation of human macrophages with antigen antibody complexes prepared with rabbit anti-LDL and human LDL (LDL-IC) is followed by ingestion of those immune complexes (IC), massive cholesterol ester accumulation, cytokine release and overexpression of the LDL receptor. The massive accumulation of cholesterol esters and overexpression of the native LDL receptor are specifically induced by immune complexes containing native or modified LDL, but not by any other type of IC. We report the results of a series of experiments aimed at defining the receptor preferentially involved in LDL-IC uptake. Flow cytometry studies using CD16, CD32 and CD64 monoclonal antibodies showed a sharp reduction on the expression of CD64 (Fc gamma RI) both by human monocyte-derived macrophages and THP-1 cells after incubation with LDL-IC, suggesting preferential engagement of this type of Fc receptor. Blocking experiments with aggregate-free IgG1 and CD32 monoclonal antibody confirmed that blocking Fc gamma RI prevented both LDL-IC uptake and the upregulation of LDL receptors on THP-1 cells. In contrast, blocking Fc gamma RII did not affect either the uptake of LDL-IC or the expression of LDL receptors on the same cells. The preferential engagement of Fc gamma R-I by LDL-IC suggests a biological difference of LDL-IC relative to other types of IC and opsonized particles. The precise molecular mechanism(s) responsible for the paradoxical upregulation of LDL receptor after the uptake of LDL-IC remain to be elucidated.

The complexity of Raf-1 regulation

The activation of the serine/threonine kinase Raf-1 is proving to be an intricate multistep process. Recent advances in elucidating how Raf-1 becomes activated in response to signaling events have emphasized the role of phosphorylation and protein interactions in Raf-1 regulation. The picture clearly emerging is that Raf-1 activity can be regulated by multiple mechanisms.