CD45 regulates Src family member kinase activity associated with macrophage integrin-mediated adhesion

Structural and functional analysis of the human CD45 gene (PTPRC) upstream region: evidence for a functional promoter within the first intron of the gene

Expression of the leucocyte common antigen (CD45) in mammals is restricted to the nucleated lineages of haematopoietic cells. It appears in early progenitors in the bone marrow and is expressed at the surface of these cells throughout their differentiation. However, at least in T cells, the pattern of expression switches between different isoforms during the successive stages of differentiation in the thymus and after activation in the periphery. In order to understand the mechanisms controlling the transcription of the human CD45 gene, 2.7 kbp of the 5'-flanking region were sequenced and analysed for their ability to direct expression of a reporter gene. The only region with promoter activity was localized within the first intron of the gene. This promoter shows no tissue specificity but could be enhanced by a heterologous enhancer. Mobility shift assays showed complex but specific protein binding. The sequence in this region lacks similarity with known promoters or initiators but is highly conserved in evolution. No transcription initiation could be detected within or downstream of this region, suggesting that this might be a new type of RNA polymerase II promoter able to drive transcription from an upstream sequence. An additional exon was also found upstream of exon 1. The two exons 1 (1a and 1b) are mutually exclusive and both are spliced to exon 2. This makes the structure of the 5' region of the human CD45 gene identical to its mouse homologue.

Serum amyloid P component and C-reactive protein mediate phagocytosis through murine Fc gamma Rs

The pentraxins, serum amyloid P component (SAP) and C-reactive protein (CRP) are acute-phase serum proteins in mice and humans, respectively. Although SAP binds to DNA and chromatin and affects clearance of these autoantigens, no specific receptor for SAP has been identified. CRP is an opsonin, and we have shown that it binds to FcgammaR. Mice deficient in FcgammaR were used to assess the role of these receptors in phagocytosis by pentraxins using zymosan as a ligand. Phagocytosis of zymosan by bone marrow macrophages (BMM) was enhanced by opsonization with SAP or CRP. BMM from mice deficient in all three FcgammaR or in gamma-chain ingested unopsonized zymosan, but phagocytosis of SAP- or CRP-opsonized zymosan was not enhanced. SAP binding to BMM from gamma-chain-deficient mice was also greatly reduced, indicating little or no binding of SAP to FcgammaRII. SAP and CRP opsonized zymosan for phagocytosis by BMM from mice deficient in FcgammaRII or FcgammaRIII. SAP, but not CRP, opsonized zymosan for uptake by neutrophils that express only low levels of FcgammaRI. Together these results indicate that FcgammaRI and FcgammaRIII are receptors for SAP in the mouse. Opsonization of zymosan by CRP is mediated through FcgammaRI. Pentraxins are major proteins of the innate immune system and arose earlier in evolution than Igs. The use of FcgammaR by the pentraxins links innate and adaptive immunity and may have important consequences for processing, presentation, and clearance of the self-Ags to which these proteins bind.

Targeting signal 1 through CD45RB synergizes with CD40 ligand blockade and promotes long term engraftment and tolerance in stringent transplant models

The induction and maintenance of allograft tolerance is a daunting challenge. Although combined blockade of CD28 and CD40 ligand (CD40L)-costimulatory pathways prevents allograft rejection in some murine models, this strategy is unable to sustain engraftment in the most immunogenic allograft and strain combinations. By targeting T cell activation signals 1 and 2 with the novel combination of anti-CD45RB and anti-CD40L, we now demonstrate potent enhancement of engraftment in C57BL/6 recipients that are relatively resistant to costimulatory blockade. This combination significantly augments the induction of tolerance to islet allografts and dramatically prolongs primary skin allograft survival. Compared with either agent alone, anti-CD45RB plus anti-CD40L inhibits periislet infiltration by CD8 cells, B cells, and monocytes; inhibits Th1 cytokines; and increases Th2 cytokine expression within the graft. These data indicate that interference with activation signals one and two may provide synergy essential for prolonged engraftment in situations where costimulatory blockade is only partially effective.

Deficiency of Src homology 2-containing phosphatase 1 results in abnormalities in murine neutrophil function: studies in motheaten mice

Neutrophils, an essential component of the innate immune system, are regulated in part by signaling pathways involving protein tyrosine phosphorylation. While protein tyrosine kinase functions in regulating neutrophil behavior have been extensively investigated, little is known about the role for specific protein tyrosine phosphatases (PTP) in modulating neutrophil signaling cascades. A key role for Src homology 2 domain-containing phosphatase 1 (SHP-1), a PTP, in neutrophil physiology is, however, implied by the overexpansion and inappropriate activation of granulocyte populations in SHP-1-deficient motheaten (me/me) and motheaten viable (me(v)/me(v)) mice. To directly investigate the importance of SHP-1 to phagocytic cell function, bone marrow neutrophils were isolated from both me/me and me(v)/me(v) mice and examined with respect to their responses to various stimuli. The results of these studies revealed that both quiescent and activated neutrophils from motheaten mice manifested enhanced tyrosine phosphorylation of cellular proteins in the 60- to 80-kDa range relative to that detected in wild-type congenic control neutrophils. MOTHEATEN: neutrophils also demonstrated increased oxidant production, surface expression of CD18, and adhesion to protein-coated plastic. Chemotaxis, however, was severely diminished in the SHP-deficient neutrophils relative to control neutrophils, which was possibly attributable to a combination of defective deadhesion and altered actin assembly. Taken together, these results indicate a significant role for SHP-1 in modulating the tyrosine phosphorylation-dependent signaling pathways that regulate neutrophil microbicidal functions.

Effects of the protein tyrosine phosphatase CD45 on FcgammaRIIa signaling and neutrophil function

OBJECTIVE

Neutrophil receptors for the Fc portion of IgG (FcgammaR) trigger immune responses following cross-linking by IgG-coated foreign particles or immune complexes. Membrane-associated CD45, a protein tyrosine phosphatase termed leukocyte common antigen, has been shown to be essential for antigen receptor kinase mediated signaling in lymphocytes, and we hypothesized that CD45 may play a similar role in FcgammaR-mediated signaling and immune function in human neutrophils.

METHODS

The experimental approach was that of cell surface molecule ligation via cross-linking with specific antibodies. Antibody dependent cellular cytotoxicity (ADCC) was assessed using a single-cell plaque assay and IL-6 production measured using ELISA. Tyrosine phosphorylation levels were assessed with anti-phospho-tyrosine blots and F-actin polymerization by flow cytometry and confocal microscopy.

RESULTS

Neutrophils pretreated with anti-CD45 had a reduced ability to perform ADCC compared to untreated neutrophils. FcgammaRIIa cross-linking resulted in significantly increased concentrations of secreted IL-6 compared to untreated neutrophils, and IL-6 production was further enhanced by cocross-linking CD45 with FcgammaRIIa. Cross-linking CD45 alone also induced IL-6 production. FcgammaRIIa cross-linking resulted in increased protein tyrosine phosphorylation and F-actin polymerization in neutrophils. Cocross-linking CD45 with FcgammaRIIa resulted in abrogation of FcgammaRIIa mediated tyrosine phosphorylation and F-actin polymerization.

CONCLUSIONS

These data provide evidence that CD45 can regulate or enhance the stimulation and function of human neutrophils mediated through FcgammaR(s). In addition, CD45 ligation may play an essential role in cytokine induction pathways that lead to inflammatory reactions in vivo.

A supramolecular basis for CD45 tyrosine phosphatase regulation in sustained T cell activation

Transmembrane protein tyrosine phosphatases, such as CD45, can act as both positive and negative regulators of cellular signaling. CD45 positively modulates T cell receptor (TCR) signaling by constitutively priming p56lck through the dephosphorylation of the C-terminal negative regulatory phosphotyrosine site. However, CD45 can also exert negative effects on cellular processes, including events triggered by integrin-mediated adhesion. To better understand these opposing actions of tyrosine phosphatases, the subcellular compartmentalization of CD45 was imaged by using laser scanning confocal microscopy during functional TCR signaling of live T lymphocytes. On antigen engagement, CD45 was first excluded from the central region of the interface between the T cell and the antigen-presenting surface where CD45 would inhibit integrin activation. Subsequently, CD45 was recruited back to the center of the contact to an area adjacent to the site of sustained TCR engagement. Thus, CD45 is well positioned within a supramolecular assembly in the vicinity of the engaged TCR, where CD45 would be able to maintain src-kinase activity for the duration of TCR engagement.

The CD45 tyrosine phosphatase: a positive and negative regulator of immune cell function

The CD45 phosphotyrosine phosphatase is one of the most abundant glycoproteins expressed on immune cells. Previously, the serpentine twists and turns of the CD45 research field have tended to draw attention to CD45 either as a positive or negative regulator of immune cell function. This review draws heavily on CD45 knockout mouse data to emphasize that CD45 has both positive and negative actions in regulating receptor thresholds, and these roles vary according to cell lineage and developmental stage. Previously conflicting results are reconciled in a model suggesting how CD45 regulates the p56(lck)tyrosine kinase in T cell signalling and development.

Emerging issues in receptor protein tyrosine phosphatase function: lifting fog or simply shifting?

Transmembrane (receptor) tyrosine phosphatases are intimately involved in responses to cell-cell and cell-matrix contact. Several important issues regarding the targets and regulation of this protein family are now emerging. For example, these phosphatases exhibit complex interactions with signaling pathways involving SRC family kinases, which result from their ability to control phosphorylation of both activating and inhibitory sites in these kinases and possibly also their substrates. Similarly, integrin signaling illustrates how phosphorylation of a single protein, or the activity of a pathway, can be controlled by multiple tyrosine phosphatases, attesting to the intricate integration of these enzymes in cellular regulation. Lastly, we are starting to appreciate the roles of intracellular topology, tyrosine phosphorylation and oligomerization among the many mechanisms regulating tyrosine phosphatase activity.

Membrane cofactor protein (MCP; CD46): isoform-specific tyrosine phosphorylation

Membrane cofactor protein (MCP; CD46) is a widely expressed type 1 transmembrane glycoprotein that inhibits complement activation on host cells. It also is a receptor for several pathogens including measles virus, Streptococcus pyogenes, Neisseria gonorrhea, and Neisseria meningitidis. That MCP may have signaling capability was suggested by its microbial interactions. That is, binding of MCP on human monocytes by measles virus hemagglutinin or cross-linking by an anti-MCP Ab resulted in IL-12 down-regulation, while binding to MCP by Neisseria on epithelial cells produced a calcium flux. Through alternative splicing, MCP is expressed on most cells with two distinct cytoplasmic tails of 16 (CYT-1) or 23 (CYT-2) amino acids. These play pivotal roles in intracellular precursor processing and basolateral localization. We investigated the putative signal transduction pathway mediated by MCP and demonstrate that CYT-2, but not CYT-1, is phosphorylated on tyrosine. We examined MCP tail peptides and performed Ab cross-linking experiments on several human cell lines and MCP isoform transfectants. We found an MCP peptide of CYT-2 was phosphorylated by a src kinase system. Western blots of the cells lines demonstrated that cells bearing CYT-2 were also phosphorylated on tyrosine. Additionally, we provide genetic and biochemical evidence that the src family of kinases is responsible for the latter phosphorylation events. In particular, the src kinase, Lck, is required for phosphorylation of MCP in the Jurkat T cell line. Taken together, these studies suggest a src family-dependent pathway for signaling through MCP.

Negative regulation of phagocytosis in murine macrophages by the Src kinase family member, Fgr

Ingestion of opsonized pathogens by professional phagocytes results in the generation and release of microbicidal products that are essential for normal host defense. Because these products can result in significant tissue injury, phagocytosis must be regulated to limit damage to the host while allowing for optimal clearance and destruction of opsonized pathogens. To pursue negative regulation of phagocytosis, we assessed the effect of the Src kinase family member, Fgr, on opsonin-dependent phagocytosis by mouse macrophages. We chose Fgr because it is present in high concentrations in circulating phagocytes but is not essential for Fcgamma receptor-mediated ingestion by mouse macrophages. Although expression of Fgr both in a macrophage cell line and in primary macrophages significantly attenuates ingestion mediated by Fcgamma receptors and CR3, it does not affect macropinocytosis or receptor-mediated endocytosis. This selective effect of Fgr is independent of its tyrosine kinase function. After Fcgamma receptor cross-linking, Fgr becomes associated with the immunoreceptor tyrosine-based inhibition motif (ITIM)-containing receptor, SIRPalpha (a member of the signal-regulatory protein family, also known as Src homology 2 domain-containing protein tyrosine phosphatase [SHP] substrate 1 [SHPS-1], brain immunoglobulin-like molecule with tyrosine-based activation motifs [BIT], and P84) and potentiates the association of the phosphatase SHP-1 with SIRPalpha. This association is responsible, at least in part, for decreasing positive signaling essential for optimal phagocytosis. These data demonstrate an important negative regulatory role for this Src kinase family member and suggest that this homeostatic function must be overcome for optimal uptake and clearance of opsonized pathogens.

CD45-associated protein is not essential for the regulation of antigen receptor-mediated signal transduction

CD45 is a transmembrane protein tyrosine phosphatase required for signaling through the T-and B-cell antigen receptors. In lymphocytes, CD45 interacts with CD45-associated protein (CD45AP), a 32 000 Mr phosphoprotein, through their respective transmembrane domains. To determine whether CD45AP affects the ability of CD45 to regulate antigen receptor signaling, CD45AP-deficient mice were generated. Thymocyte development was grossly normal. Moreover, the cellularity of the thymus and spleens were normal. CD45 expression on thymocytes and splenocytes, ascertained by flow cytometry, was comparable between CD45AP-deficient mice and littermate controls. In contrast to a previous report (Matsuda et al., J. Exp. Med. 1998 187: 1863 - 1870). CD45AP-deficient and normal thymocytes and splenocytes proliferated similarly in response to various mitogens or antigen receptor cross-linking. Furthermore, thymocyte CD45-associated p56(lck) kinase activity was similar between CD45AP-deficient and normal cells. We conclude that CD45AP is not essential for the regulation of Src-family kinase activity by CD45.

Impaired integrin-mediated signal transduction, altered cytoskeletal structure and reduced motility in Hck/Fgr deficient macrophages

Integrin-mediated adhesion of monocytes and macrophages initiates a signal transduction pathway that leads to actin cytoskeletal reorganization, cell migration and immunologic activation. This signaling pathway is critically dependent on tyrosine kinases. To investigate the role of the Src-family of tyrosine kinases in integrin signal transduction, we have examined the adhesive properties of macrophages isolated from hck-/-fgr-/- double knockout mice which lack two of the three predominant Src-family kinases expressed in myeloid cells. Previous examination of polymorphonuclear leukocytes from these animals indicated that these kinases were critical in initiating the actin cytoskeletal rearrangements that lead to respiratory burst and granule secretion following integrin ligation. Double mutant peritoneal exudate macrophages demonstrated markedly reduced tyrosine phosphorylation responses compared to wild-type cells following plating on fibronectin, collagen or vitronectin-coated surfaces. Tyrosine phosphorylation of several actin-associated proteins (cortactin, paxillin, and tensin), as well as the Syk and Pyk2 tyrosine kinases, were all significantly reduced in double mutant cells. The subcellular localization of focal-adhesion associated proteins was also dramatically altered in mutant macrophages cultured on fibronectin-coated surfaces. In wild-type cells, filamentous actin, paxillin, and talin were concentrated along leading edges of the plasma membrane, suggesting that these proteins contribute to cellular polarization during migration in culture. Double mutant cells failed to show the polarized subcellular localization of these proteins. Likewise, double mutant macrophages failed to form normal filopodia under standard culture conditions. Together, these signaling and cytoskeletal defects may contribute to the reduced motility observed in in vitro assays. These data provide biochemical and morphological evidence that the Src-family kinases Hck and Fgr are required for normal integrin-mediated signal transduction in murine macrophages.

The receptor tyrosine phosphatase CRYPalpha promotes intraretinal axon growth

Retinal ganglion cell axons grow towards the optic fissure in close contact with the basal membrane, an excellent growth substratum. One of the ligands of receptor tyrosine phosphatase CRYPalpha is located on the retinal and tectal basal membranes. To analyze the role of this RPTP and its ligand in intraretinal growth and guidance of ganglion cell axons, we disrupted ligand- receptor interactions on the retinal basal membrane in culture. Antibodies against CRYPalpha strongly reduced retinal axon growth on the basal membrane, and induced a dramatic change in morphology of retinal growth cones, reducing the size of growth cone lamellipodia. A similar effect was observed by blocking the ligand with a CRYPalpha ectodomain fusion protein. These effects did not occur, or were much reduced, when axons were grown either on laminin-1, on matrigel or on basal membranes with glial endfeet removed. This indicates that a ligand for CRYPalpha is located on glial endfeet. These results show for the first time in vertebrates that the interaction of a receptor tyrosine phosphatase with its ligand is crucial not only for promotion of retinal axon growth but also for maintenance of retinal growth cone lamellipodia on basal membranes.

Integrin signalling in neutrophils and macrophages

Integrins have been characterized extensively as adhesion receptors capable of transducing signals inside the cell. In myelomonocytic cells, integrin-mediated adhesive interactions regulate different selective cell responses, such as transmigration into the inflammatory site, cytokine secretion, production or reactive oxygen intermediates, degranulation and phagocytosis. In the last few years, great progress has been made in elucidating mechanisms of signal transduction by integrins in neutrophils and macrophages. This review summarises the current information on the role of integrins in regulating myelomonocytic cell functions and highlights the signalling pathways activated by integrin engagement in these cells. Also, exploiting the current knowledge of mechanisms of integrin signal transduction in other cell types, we propose a model to explain how integrins transduce signals inside neutrophils and macrophages, and how signaling pathways leading to regulation of selective cell functions may be coordinated.

Protein tyrosine phosphatases as negative regulators of mitogenic signaling

The regulation of tyrosine phosphorylation represents a key mechanism governing cell proliferation. In fibroblasts, inputs from both growth factor and extracellular matrix receptors are required for cell division. Triggering such receptors induces a wave of tyrosine phosphorylation on key signaling molecules, culminating in the activation of cyclin-dependent kinases and cell cycle progression. In general, protein tyrosine kinases stimulate, while protein tyrosine phosphatases inhibit, such cell proliferation pathways. The role of protein tyrosine kinases in mitogenesis has been extensively studied, but the identity and targets of the protein tyrosine phosphatases that regulate cell growth are not well described. In this review, I will survey recent advances in the identification and regulation of protein tyrosine phosphatases that downregulate cell proliferation.

Regulation of Integrin-Mediated T Cell Adhesion by the Transmembrane Protein Tyrosine Phosphatase CD45

The transmembrane protein tyrosine phosphatase CD45 is required for Ag receptor signal transduction in lymphocytes. Recently, a role for CD45 in the regulation of macrophage adhesion has been demonstrated as well. To investigate further the role of CD45 in the regulation of adhesion, we examined integrin-mediated adhesion to fibronectin of two T cell lines and their CD45-deficient variants. The absence of CD45 correlated with enhanced adhesion to fibronectin via integrin α5β1 (VLA-5), but not α4β1 (VLA-4) in both cell lines. Adhesion returned to normal levels upon transfection of wild-type CD45 into the CD45-deficient lines. Transfection of chimeric or mutant molecules expressing some, but not all, CD45 domains and activities demonstrated that both the transmembrane domain and the tyrosine phosphatase activity of CD45 were required for regulation of integrin-dependent adhesion, but the highly glycosylated extracellular domain was dispensable. In contrast, only a catalytically active CD45 cytoplasmic domain was required for TCR signaling. Transfectants that restored normal levels of adhesion to fibronectin coimmunoprecipitated with the transmembrane protein known as CD45-associated protein. These studies demonstrate a novel role for CD45 in adhesion regulation and suggest a possible function for its association with CD45-associated protein.

Expression of the p56(Lck) Y505F mutation in CD45-deficient mice rescues thymocyte development

Mice deficient in the transmembrane protein tyrosine phosphatase CD45 exhibit a block in thymocyte development. To determine whether the block in thymocyte development was due to the inability to dephosphorylate the inhibitory phosphorylation site (Y505) in p56(lck) (Lck), we generated CD45-deficient mice that express transgenes for the Lck Y505F mutation and the DO11.10 T-cell antigen receptor (TCR). CD4 single-positive T cells developed and accumulated in the periphery. Treatment with antigen resulted in thymocyte apoptosis and the loss of transgenic-TCR-bearing cells. Peripheral CD45-deficient T cells from the mice expressing both transgenes responded to antigen by increasing CD69 expression, interleukin-2 production, and proliferation. These results indicate that thymocyte development requires the dephosphorylation of the inhibitory site in Lck by CD45.

Receptor protein tyrosine phosphatase alpha activates Src-family kinases and controls integrin-mediated responses in fibroblasts

BACKGROUND

Fyn and c-Src are two of the most widely expressed Src-family kinases. Both are strongly implicated in the control of cytoskeletal organization and in the generation of integrin-dependent signalling responses in fibroblasts. These proteins are representative of a large family of tyrosine kinases, the activity of which is tightly controlled by inhibitory phosphorylation of a carboxyterminal tyrosine residue (Tyr527 in chicken c-Src); this phosphorylation induces the kinases to form an inactive conformation. Whereas the identity of such inhibitory Tyr527 kinases has been well established, no corresponding phosphatases have been identified that, under physiological conditions, function as positive regulators of c-Src and Fyn in fibroblasts.

RESULTS

Receptor protein tyrosine phosphatase alpha (RPTPalpha) was inactivated by homologous recombination. Fibroblasts derived from these RPTPalpha-/- mice had impaired tyrosine kinase activity of both c-Src and Fyn, and this was accompanied by a concomitant increase in c-Src Tyr527 phosphorylation. RPTPalpha-/- fibroblasts also showed a reduction in the rate of spreading on fibronectin substrates, a trait that is a phenocopy of the effect of inactivation of the c-src gene. In response to adhesion on a fibronectin substrate, RPTPalpha-/- fibroblasts also exhibited characteristic deficiencies in integrin-mediated signalling responses, such as decreased tyrosine phosphorylation of the c-Src substrates Fak and p 130(cas), and reduced activation of extracellular signal regulated (Erk) MAP kinases.

CONCLUSIONS

These observations demonstrate that RPTPalpha functions as a physiological upstream activator of Src-family kinases in fibroblasts and establish this tyrosine phosphatase as a newly identified regulator of integrin signalling.

Effects of overexpression of PTP36, a putative protein tyrosine phosphatase, on cell adhesion, cell growth, and cytoskeletons in HeLa cells

Non-receptor-type putative protein tyrosine phosphatase-36 (PTP36), also known as PTPD2/Pez, possesses a domain homologous to the N-terminal half of band 4.1 protein. To gain insight into the biological function of PTP36, we established a HeLa cell line, HtTA/P36-9, in which the overexpression of PTP36 was inducible. PTP36 expressed in HeLa cells was enriched in the cytoskeleton near the plasma membrane. There was little endogenous PTP36 detectable in uninduced HtTA/P36-9 cells or in the parental HeLa cells. Upon induction of PTP36 overexpression, HtTA/P36-9 cells spread less well, grew more slowly, and adhered to the extracellular matrix proteins less well than uninduced cells. Moreover, decreases in the actin stress fibers and the number of focal adhesions were observed. The tyrosine phosphorylation of the focal adhesion kinase induced by lysophosphatidic acid was suppressed in the HtTA/P36-9 cells overexpressing PTP36. These results indicate that PTP36 affects cytoskeletons, cell adhesion, and cell growth, thus suggesting that PTP36 is involved in their regulatory processes.

Regulation of early events in integrin signaling by protein tyrosine phosphatase SHP-2

The nontransmembrane protein tyrosine phosphatase SHP-2 plays a critical role in growth factor and cytokine signaling pathways. Previous studies revealed that a fraction of SHP-2 moves to focal contacts upon integrin engagement and that SHP-2 binds to SHP substrate 1 (SHPS-1)/SIRP-1alpha, a transmembrane glycoprotein with adhesion molecule characteristics (Y. Fujioka et al., Mol. Cell. Biol. 16:6887-6899, 1996; M. Tsuda et al., J. Biol. Chem. 273:13223-13229). Therefore, we asked whether SHP2-SHPS-1 complexes participate in integrin signaling. SHPS-1 tyrosyl phosphorylation increased upon plating of murine fibroblasts onto specific extracellular matrices. Both in vitro and in vivo studies indicate that SHPS-1 tyrosyl phosphorylation is catalyzed by Src family protein tyrosine kinases (PTKs). Overexpression of SHPS-1 in 293 cells potentiated integrin-induced mitogen-activated protein kinase (MAPK) activation, and potentiation required functional SHP-2. To further explore the role of SHP-2 in integrin signaling, we analyzed the responses of SHP-2 exon 3(-/-) and wild-type cell lines to being plated on fibronectin. Integrin-induced activation of Src family PTKs, tyrosyl phosphorylation of several focal adhesion proteins, MAPK activation, and the ability to spread on fibronectin were defective in SHP-2 mutant fibroblasts but were restored upon SHP-2 expression. Our data suggest a positive-feedback model in which, upon integrin engagement, basal levels of c-Src activity catalyze the tyrosyl phosphorylation of SHPS-1, thereby recruiting SHP-2 to the plasma membrane, where, perhaps by further activating Src PTKs, SHP-2 transduces positive signals for downstream events such as MAPK activation and cell shape changes.

Negative Regulation of Myeloid Cell Proliferation and Function by the SH2 Domain-Containing Tyrosine Phosphatase-1

The SH2 domain containing tyrosine phosphatase SHP-1 has been implicated in the regulation of a multiplicity of signaling pathways involved in hemopoietic cell growth, differentiation, and activation. A pivotal contribution of SHP-1 in the modulation of myeloid cell signaling cascades has been revealed by the demonstration that SHP-1 gene mutation is responsible for the overexpansion and inappropriate activation of myelomonocytic populations in motheaten mice. To investigate the role of SHP-1 in regulation of myeloid leukocytes, an HA epitope-tagged dominant negative (interfering) SHP-1 (SHP-1C453S) was expressed in the myelo-monocytic cell line U937 using the pcDNA3 vector. Overexpression of this protein in SHP-1C453S transfectants was demonstrated by Western blot analysis and by detection of decreased specific activity. Growth, proliferation, and IL-3-induced proliferative responses were substantially increased in the SHP-1C453S-overexpressing cells relative to those in control cells. The results of cell cycle analysis also revealed that the proportion of cells overexpressing SHP-1C453S in S phase was greater than that of control cells. The SHP-1C453S-expressing cells also displayed diminished rates of apoptosis as detected by flow cytometric analysis of propidium iodide-stained cells and terminal deoxynucleotidyltransferase-mediated fluorescein-dUTP nick end-labeling assay. While motility and phagocytosis were not affected by SHP-1C453S overexpression, adhesion and the oxidative burst in response to PMA were enhanced in the SHP-1C453S compared with those in the vector alone transfectants. Taken together, these results suggest that SHP-1 exerts an important negative regulatory influence on cell proliferation and activation while promoting spontaneous cell death in myeloid cells.

Expression of Dominant-Negative Src-Homology Domain 2-Containing Protein Tyrosine Phosphatase-1 Results in Increased Syk Tyrosine Kinase Activity and B Cell Activation

The Src-homology domain 2 (SH2)-containing cytoplasmic tyrosine phosphatase, SHP-1 (SH2-containing protein tyrosine phosphatase-1), interacts with several B cell surface and intracellular signal transduction molecules through its SH2 domains. Mice with the motheaten and viable motheaten mutations are deficient in SHP-1 and lack most mature B cells. To define the role of SHP-1 in mature B cells, we expressed phosphatase-inactive SHP-1 (C453S) in a mature B cell lymphoma line. SHP-1 (C453S) retains the ability to bind to both substrates and appropriate tyrosine-phosphorylated proteins and therefore can compete with the endogenous wild-type enzyme. We found that B cells expressing SHP-1 (C453S) demonstrated enhanced and prolonged tyrosine phosphorylation of proteins with molecular masses of 110, 70, and 55–60 kDa after stimulation with anti-mouse IgG. The tyrosine kinase Syk was hyperphosphorylated and hyperactive in B cells expressing SHP-1 (C453S). SHP-1 and Syk were coimmunoprecipitated from wild-type K46 cells, K46 SHP-1 (C453S) cells, and splenic B cells, and SHP-1 dephosphorylated Syk. Cells expressing SHP-1 (C453S) showed increased Ca2+ mobilization, extracellular signal-regulated kinase activation, and homotypic adhesion after B cell Ag receptor engagement. Thus, SHP-1 regulates multiple early and late events in B lymphocyte activation.

Requirement of PEST domain tyrosine phosphatase PEP in B cell antigen receptor-induced growth arrest and apoptosis

Signaling events leading to B cell growth or apoptosis are beginning to be unravelled, but detailed information is still lacking. To identify signaling molecules involved in B cell antigen receptor (BCR)-initiated pathways, we used the immature B cell line, WEHI-231, to investigate protein tyrosine phosphatases (PTP) whose expression was modulated by BCR ligation. Among the PTP cloned by reverse transcription-PCR, mRNA expression of the proline-, glutamic acid-, serine- and threonine-rich (PEST) domain phosphatase (PEP) was selectively elevated 3.1-fold within 3 h after anti-IgM antibody stimulation. In contrast, expression of another PEST domain phosphatase, PTP-PEST, was unaffected. Western blot analysis revealed that 71% of PEP was located in the cytosolic fraction, while 29% was in the membrane fraction. To examine the direct contribution made by PEP to BCR-initiated signal transduction, we transfected an antisense PEP cDNA into WEHI-231 cells. Two stable clones were established in which PEP expression was reduced by 34% and 47%, respectively. Strikingly, BCR-mediated inhibition of DNA synthesis was significantly rescued in the clones, and G1 phase cell cycle arrest and apoptosis were almost completely ablated. Considered collectively, these results indicate that PEP is a positive, crucial regulator in determining B cell fate triggered by BCR engagement.

Cutting Edge: The CD45 Tyrosine Phosphatase Is an Inhibitor of Lck Activity in Thymocytes

A widely accepted model for regulation of the Lck tyrosine kinase is that it is activated by CD45-mediated dephosphorylation of its COOH-terminal negative regulatory tyrosine (Tyr505). Previous work from our laboratory, however, found that despite hyperphosphorylation of Tyr505, the activity of Lck from CD45− T cell lines was actually increased due to hyperphosphorylation of the positive regulatory tyrosine, residue 394. To avoid potential complications introduced by transformed cells, in this study we have characterized the effect of CD45 on Lck activity in normal cells. Lck in thymocytes from CD45−/− mice was hyperphosphorylated on tyrosine residues. Importantly, and in disagreement with the model that CD45 only activates Lck in vivo, the kinase activity of Lck from cells lacking CD45 was substantially increased. These results support a model in which CD45 dephosphorylates both Tyr505 and Tyr394, the net effect in normal thymocytes being a decrease in enzymatic activity.

Tyrosine kinases in neutrophils

Nonreceptor tyrosine kinases have been shown to represent essential components of several signal transduction pathways implicated in neutrophil activation. Engagement of phagocytic, cytokine, chemoattractant, and adhesion receptors transduces intracellular signals via distinct tyrosine kinases. This article discusses the role of distinct tyrosine kinases in mediating neutrophil responses.

Protein-tyrosine phosphatase alpha regulates Src family kinases and alters cell-substratum adhesion

The roles of protein-tyrosine phosphatases (PTPs) in processes such as cell growth and adhesion are poorly understood. To explore the ability of specific PTPs to regulate cell signaling pathways initiated by stimulation of growth factor receptors, we expressed the receptor-like PTP, PTPalpha, in A431 epidermoid carcinoma cells. These cells express high levels of the epidermal growth factor (EGF) receptor and proliferate in response to the autocrine production of transforming growth factor-alpha. Conversely, EGF stimulation of A431 cells in vitro leads to growth inhibition and triggers the rapid detachment of these cells from the substratum. Although PTPalpha expression did not alter the growth characteristics of either unstimulated or EGF-stimulated cells, this phosphatase was associated with increased cell-substratum adhesion. Furthermore, PTPalpha-expressing A431 cells were strikingly resistant to EGF-induced cell rounding. Overexpression of PTPalpha in A431 cells was associated with the dephosphorylation/activation of specific Src family kinases, suggesting a potential mechanism for the observed alteration in A431 cell-substratum adhesion. Src kinase activation was dependent on the D1 catalytic subunit of PTPalpha, and there was evidence of association between PTPalpha and Src kinase(s). PTPalpha expression also led to increased association of Src kinase with the integrin-associated focal adhesion kinase, pp125(FAK). In addition, paxillin, a Src and/or pp125(FAK) substrate, displayed increased levels of tyrosine phosphorylation in PTPalpha-expressing cells and was associated with elevated amounts of Csk. In view of these alterations in focal adhesion-associated molecules in PTPalpha-expressing A431 cells, as well as the changes in adhesion demonstrated by these cells, we propose that PTPalpha may have a role in regulating cell-substratum adhesion.

The Src-like tyrosine kinase Hck is activated by granulocyte colony-stimulating factor (G-CSF) and docks to the activated G-CSF receptor

Activation of the granulocyte colony-stimulating factor receptor (G-CSF-R) leads to tyrosine-phosphorylation of multiple cytoplasmic components. To date, the kinases Jak1, Jak2, Tyk2, Lyn, and Syk have been implicated in this process. However, it is unknown if other kinases might be involved in the diverse responses from the G-CSF-R, which include mitogenesis, survival, differentiation, and functional activation of responsive cells. The hematopoietic cell kinase (Hck) is a member of the Src-family of kinases known to be expressed in cells of the granulocytic lineage. It also interacts with the gp130 subunit of the LIF/IL-6 receptors, which is closely related to the G-CSF-R, and so represents a good candidate for mediating at least some of the downstream signaling from the G-CSF-R. Therefore, we investigated the activation of Hck by the G-CSF-R in intact cells as well as in vitro. These studies revealed recruitment of Hck to activated G-CSF-R, mediated by direct binding via its SH2 domain to multiple phosphotyrosines of the receptor. In addition, we show that Hck becomes activated upon G-CSF treatment and is, in turn, able to phosphorylate the G-CSF-R, indicating a clear functional and physical involvement in G-CSF signaling.

Vanadate-dependent FAK activation is accomplished by the sustained FAK Tyr-576/577 phosphorylation

Focal adhesion kinase (FAK) is a nonreceptor tyrosine kinase implicated in cell-matrix interaction and integrin signaling. It is well established that Tyr-397 is the FAK autophosphorylation site and Tyr-407, -576/577, -861, and -925 are the sites on murine FAK that are mediated by Src family kinases. To study how FAK is regulated by tyrosine phosphatase(s), cells overexpressing chicken FAK are treated with sodium vanadate. Both the phosphotyrosine content and the enzymatic activity of FAK are increased in response to vanadate. Interestingly, sustained FAK Tyr-576/577 and -863 phosphorylations are detected in vanadate-treated FAK overexpressors and are dependent on FAK autophosphorylation. Further analysis of sodium vanadate-treated FAK overexpressors reveals that the enhanced FAK kinase activity parallels its elevated Tyr-576/577 phosphorylation. Thus, we conclude that Src-mediated FAK phosphorylation is regulated by a tyrosine phosphatase(s) and may be of physioligical significance.

The protein tyrosine phosphatase SHP-1 regulates integrin-mediated adhesion of macrophages

The Src homology 2 domain phosphatase-1 (SHP-1) is a tyrosine phosphatase containing two amino-terminal SH2 domains and is expressed primarily by hematopoietic-derived cells [1]. The viable motheaten (Hcphme-v) mutant mice (mev) suffer from progressive inflammation due to a deficiency of SHP-1 enzyme activity [2,3] and die at 3-4 months of age from macrophage and neutrophil accumulation in the lung [4]. The mechanism by which SHP-1 deficiency leads to inflammation is unknown. We found that macrophages from mev mice adhered and spread to a greater extent than normal macrophages through alpha m beta 2 integrin-mediated contacts. Whereas macrophages deficient in the transmembrane tyrosine phosphatase CD45 (CD45-/-) spontaneously detached from alpha m beta 2 integrin contacts [5], cells deficient in both CD45 and SHP-1 did not. In SHP-1 deficient macrophages there was a 10-15-fold increase in D-3 phospholipid products of phosphatidylinositol (PI) 3-kinase. Concomitantly, there was a 2-5-fold increase in membrane-associated PI 3-kinase activity in mev macrophages relative to normal macrophages. Treatment of macrophages with the PI 3-kinase inhibitors wortmannin or LY294002 resulted in a dramatic detachment of cells, indicating that PI 3-kinase activity is required for adhesion. These data demonstrate that SHP-1 is necessary for detachment from alpha m beta 2 integrin-mediated contacts in primary macrophages and suggest that a defect in this pathway may contribute to inflammatory disease.

A beta 1 integrin signaling pathway involving Src-family kinases, Cbl and PI-3 kinase is required for macrophage spreading and migration

We have used mutant macrophages which are deficient in expression of Src-family kinases to define an integrin signaling pathway that is required for macrophage adhesion and migration. Following ligation of surface integrins by fibronectin, the p120(c-cbl) (Cbl) protein rapidly becomes tyrosine phosphorylated and associated with the Src-family kinases Fgr and Lyn. In hck-/-fgr-/-lyn-/- triple mutant cells, which are defective in spreading on fibronectin-coated surfaces in vitro and show impaired migration in vivo, Cbl tyrosine phosphorylation is blocked, Cbl protein levels are low, adhesion-dependent translocation of Cbl to the membrane is impaired and Cbl-associated, membrane-localized phosphatidylinositol 3 (PI-3)-kinase activity is dramatically reduced. In contrast, adhesion-dependent activation of total cellular PI-3 kinase activity is normal in mutant cells, demonstrating that it is the membrane-associated fraction of PI-3 kinase which is most critical in regulating actin cytoskeletal rearrangements that lead to cell spreading. Treatment of wild-type cells with the Src-family-specific inhibitor PP1, Cbl antisense oligonucleotides or pharmacological inhibitors of PI-3 kinase blocks cell spreading on fibronectin surfaces. These data provide a molecular description for the role of Src-family kinases Hck, Fgr and Lyn in beta 1-integrin signal transduction in macrophages.

Multiple Grb2-mediated integrin-stimulated signaling pathways to ERK2/mitogen-activated protein kinase: summation of both c-Src- and focal adhesion kinase-initiated tyrosine phosphorylation events

Fibronectin receptor integrin-mediated cell adhesion triggers intracellular signaling events such as the activation of the Ras/mitogen-activated protein (MAP) kinase cascade. In this study, we show that the nonreceptor protein-tyrosine kinases (PTKs) c-Src and focal adhesion kinase (FAK) can be independently activated after fibronectin (FN) stimulation and that their combined activity promotes signaling to extracellular signal-regulated kinase 2 (ERK2)/MAP kinase through multiple pathways upstream of Ras. FN stimulation of NIH 3T3 fibroblasts promotes c-Src and FAK association in the Triton-insoluble cell fraction, and the time course of FN-stimulated ERK2 activation paralleled that of Grb2 binding to FAK at Tyr-925 and Grb2 binding to Shc. Cytochalasin D treatment of fibroblasts inhibited FN-induced FAK in vitro kinase activity and signaling to ERK2, but it only partially inhibited c-Src activation. Treatment of fibroblasts with protein kinase C inhibitors or with the PTK inhibitor herbimycin A or PP1 resulted in reduced Src PTK activity, no Grb2 binding to FAK, and lowered levels of ERK2 activation. FN-stimulated FAK PTK activity was not significantly affected by herbimycin A treatment and, under these conditions, FAK autophosphorylation promoted Shc binding to FAK. In vitro, FAK directly phosphorylated Shc Tyr-317 to promote Grb2 binding, and in vivo Grb2 binding to Shc was observed in herbimycin A-treated fibroblasts after FN stimulation. Interestingly, c-Src in vitro phosphorylation of Shc promoted Grb2 binding to both wild-type and Phe-317 Shc. In vivo, Phe-317 Shc was tyrosine phosphorylated after FN stimulation of human 293T cells and its expression did not inhibit signaling to ERK2. Surprisingly, expression of Phe-925 FAK with Phe-317 Shc also did not block signaling to ERK2, whereas FN-stimulated signaling to ERK2 was inhibited by coexpression of an SH3 domain-inactivated mutant of Grb2. Our studies show that FN receptor integrin signaling upstream of Ras and ERK2 does not follow a linear pathway but that, instead, multiple Grb2-mediated interactions with Shc, FAK, and perhaps other yet-to-be-determined phosphorylated targets represent parallel signaling pathways that cooperate to promote maximal ERK2 activation.

Paxillin phosphorylation and association with Lck and Pyk2 in anti-CD3- or anti-CD45-stimulated T cells

Antibodies to either CD3 or CD45 have been shown to induce dramatic changes in cell morphology, increased tyrosine phosphorylation of cellular proteins, and the association of a subset of these proteins with the tyrosine kinase Lck. The current study was initiated to determine the identity of the tyrosine-phosphorylated 70-80 kDa protein that becomes Lck-associated after stimulation with anti-CD45 or anti-CD3. We demonstrate that the cytoskeletal protein paxillin becomes tyrosine-phosphorylated when cells are plated on immobilized antibodies specific for CD45 or CD3. Only tyrosine-phosphorylated paxillin is associated with Lck, suggesting that the association is through the SH2 domain of Lck. Consistent with this we demonstrate that the SH2 domain of Lck binds tyrosine-phosphorylated paxillin. In contrast, the association of paxillin with the FAK-related kinase Pyk2 was found to be constitutive and not altered by the phosphorylation of either protein. Finally, we establish that the phosphorylation of paxillin is dependent on the expression of Lck. Taken together, these results demonstrate that paxillin is physically associated with kinases from two different families in T cells and suggest that paxillin may function as an adaptor protein linking cellular signals with cytoskeletal changes during T cell activation.