Tyrosine phosphorylation of the vav proto-oncogene product in activated B cells

Defective signalling through the T- and B-cell antigen receptors in lymphoid cells lacking the vav proto-oncogene

The product of the vav proto-oncogene, p95vav or Vav, is tyrosine phosphorylated upon stimulation of T and B cells by antigen and other receptors, and contains motifs associated with signal transduction. To determine its role in vivo, we used vav-gene-targeted embryonic stem cells and RAG-2-/- blastocyst complementation. The vav(-/-)-RAG-2-/- chimaeras displayed thymic atrophy with reduced numbers of peripheral T cells. Whereas the total number of B cells was normal, the subset of peritoneal B-1 (CD5+) cells was missing. The vav-/- T and B cells were hyporeactive when stimulated through antigen receptors, but vav-/- T cells proliferated on exposure to phorbol ester and calcium ionophore, whereas B cells responded normally to bacterial mitogen, lipopolysaccharide or the CD40 ligand. Thus, we have established here a functional role for vav in the control of T- and B-cell development and activation.

Defective T-cell receptor signalling and positive selection of Vav-deficient CD4+ CD8+ thymocytes

During lymphocyte development, cellular proliferation and positive and negative selection events ensure the production of T and B lymphocytes bearing highly diverse, but self-tolerant, repertoires of antigen receptors. These processes are initiated when engagement of growth-factor receptors, or the T and B lymphocyte antigen receptors, induces tyrosine phosphorylation of specific SH2- and SH3-domain-containing cytoplasmic proteins, including Vav. Here we show that vav-/- embryonic stem cells generate only limited numbers of immature and mature T and B lymphocytes in the RAG-2 blastocyst complementation assay. Furthermore, Vav-deficient T lymphocytes showed severely impaired antigen receptor signalling. Finally, we demonstrate that Vav-dependent signalling pathways regulate maturation, but not CD4/CD8 lineage commitment, during T-cell-receptor-mediated positive selection of immature CD4+ CD8+ precursors into mature CD4+ CD8- or CD4- CD8+ T cells.

Defective antigen receptor-mediated proliferation of B and T cells in the absence of Vav

Crosslinking of B- or T-cell antigen receptors results in the rapid tyrosine phosphorylation of a number of proteins, including Vav, a protein expressed in cells of the haematopoietic system. Vav contains an array of structural motifs that include Src-homology domains SH2/SH3 and regions of homology to the guanine-nucleotide-exchange protein Dbl, pleckstrin and protein kinase C (refs 5-9). Using the RAG-complementation approach, we have analysed in vivo differentiation and in vitro responses of B- and T-lineage cells generated by injection of embryonic stem cells homozygous for a null mutation in the vav gene into blastocysts of RAG-1- or RAG-2-deficient mice. Here we report that antigen receptor-mediated proliferative responses of B and T cells in vitro are severely reduced in the absence of Vav. We also suggest a direct link between the low proliferative response of Vav-deficient B and T cells and the reduced number of these cells in peripheral lymphoid organs of chimaeric mice.

Association of the vav proto-oncogene product with poly(rC)-specific RNA-binding proteins

We have used the yeast two-hybrid system to isolate proteins that interact with the carboxy-terminal SH3-SH2-SH3 region of Vav. One of the clones encoded heterogeneous nuclear ribonucleoprotein K (hnRNP K), a poly(rC)-specific RNA-binding protein. The interaction between Vav and hnRNP K involves the binding of the most carboxy-terminal SH3 domain of Vav to two proline-rich sequences present in the central region of hnRNP K. Overexpression of Vav in mouse fibroblasts leads to the formation of a stable complex with the endogenous hnRNP K and to the preferential redistribution of this protein to the cytoplasmic fraction. More importantly, Vav and hnRNP K proteins also interact in hematopoietic cells. In addition, Vav associates in vitro with a second 45-kDa poly(rC)-specific RNA-binding protein via its SH3-SH2-SH3 region. These results suggest that Vav plays a role in the regulation of the late steps of RNA biogenesis by modulating the function of poly(rC)-specific ribonucleoproteins.

Integral membrane protein 2 of Epstein-Barr virus regulates reactivation from latency through dominant negative effects on protein-tyrosine kinases

An Epstein-Barr virus-encoded protein, LMP2, blocks the effects of surface immunoglobulin (slg) cross-linking on calcium mobilization and on lytic reactivation of EBV in latently infected and growth-transformed primary human B lymphocytes. In wild-type EBV-transformed cells, LMP2 is constitutively tyrosine phosphorylated and is associated with Lyn and Syk protein-tyrosine kinases (PTKs). Baseline Lyn PTK activity is substantially reduced, and slg cross-linking fails to activate Lyn, Syk, Pl3-K, PLC gamma 2, Vav, Shc, and MAPK. Syk, Pl3-K, PLC gamma 2, and Vav are constitutively tyrosine phosphorylated, and their tyrosine phosphorylation does not change following slg cross-linking. In contrast, cross-linking slg on cells transformed by LMP2 null mutant EBV recombinants triggers the same protein tyrosine kinase cascade as in noninfected B lymphocytes. These data are consistent with a model in which LMP2 is a constitutive dominant negative modulator of slg receptor signaling through its effects on Lyn, Syk, or regulators of these kinases.

Structure/function relationships of Fc gamma receptors in phagocytosis

An important function of Fc gamma receptors is the ingestion or phagocytosis of IgG sensitized cells. It has been difficult to clearly define the individual function of each receptor in phagocytosis because hematopoietic cells express multiple Fc gamma receptor isoforms. To examine this issue, an in-vitro model system in COS-1 cells has been developed. When transfected with an appropriate Fc gamma receptor, COS-1 cells which lack endogeneous Fc receptors, ingest IgG-sensitized cells. Using this model, a single class of human Fc gamma receptor in the absence of other Fc receptors was observed to mediate phagocytosis. Furthermore, isoforms from each Fc gamma receptor class can mediate phagocytosis although the requirements for phagocytosis differ. Investigation of the relationship between structure and function for Fc receptor-mediated phagocytosis established the importance of the cytoplasmic tyrosines of the receptor or its associated gamma chains. For example, two cytoplasmic YXXL sequences, in a configuration similar to the conserved tyrosine containing motif found in immunoglobulin gene family receptors, are important for phagocytosis by the human Fc gamma receptor, Fc gamma RIIA. Fc gamma RI and Fc gamma RIIIA do not possess cytoplasmic tyrosines, but transmit a phagocytic signal through interaction with an associated gamma-subunit which contains two YXXL sequences in a conserved motif required for phagocytosis. The human Fc gamma RII isoforms, Fc gamma RIIB2, do not induce phagocytosis and have only a single YXXL sequence. Crosslinking of the phagocytic Fc gamma receptors induces tyrosine phosphorylation of either Fc gamma RIIA or the gamma chain and treatment with tyrosine kinase inhibitors reduces both phagocytosis and phosphorylation of the receptor tyrosine residues. The protein tyrosine kinase Syk, which is associated with the gamma chain in monocytes/macrophages, dramatically enhances phagocytosis mediated by Fc gamma RI and Fc gamma RIIIA and also induces non-phagocytic Fc gamma RI or Fc gamma RIIIA expressing cells to acquire phagocytic capability.

The vav proto-oncogene is required early in embryogenesis but not for hematopoietic development in vitro

Previous studies have suggested that the vav protooncogene plays an important role in hematopoiesis. To study this further, we have ablated the vav protooncogene by homologous recombination in embryonic stem (ES) cells. Homozygous vav (-/-) ES clones differentiate normally in culture and generate cells of erythroid, myeloid and mast cell lineages. Mice heterozygous for the targeted vav allele do not display any obvious abnormalities. However, homozygous embryos die very early during development. Crosses of vav (+/-) heterozygous mice yield apparently normal vav (-/-) E3.5 embryos but not post-implantation embryos (> or = E7.5). Furthermore, homozygous vav (-/-) blastocysts do not hatch in vitro. These results indicate that vav is essential for an early developmental step(s) that precedes the onset of hematopoiesis. Consistent with the phenotypic analysis of vav (-/-) embryos, we have identified Vav immunoreactivity in the extra-embryonic trophoblastic cell layer but not in the inner embryonic cell mass of E3.5 preimplantation embryos or in the egg cylinder of E6.5 and E7.5 post-implantation embryos. These results suggest that the vav gene is essential for normal trophoblast development and for implantation of the developing embryo.

Identification of VAV2 on 9q34 and its exclusion as the tuberous sclerosis gene TSC1

A novel widely expressed homologue of the VAV oncogene, VAV2 (53% identical residues), has been identified within the critical region for the tuberous sclerosis gene, TSC1, on human chromosome 9q34. By Southern blot analysis, analysis of allele-specific transcription, and direct sequencing of the VAV2 mRNA/cDNA from patient lymphoblastoid cell lines, we demonstrate that both alleles of this gene are expressed in TSC patients and there are no significant mutations. VAV consists of a novel array of signalling domains and is thought to play an important role in signal transduction in haematopoietic tissues where it is exclusively expressed. VAV2 is likely to serve a similar role more generally in mammalian cells, but is not the TSC1 gene.

The Janus protein tyrosine kinase family and its role in cytokine signaling

During the past 2 years, research from quite divergent areas has converged to provide the first insights into the mechanisms by which cytokines that utilize receptors of the cytokine receptor superfamily function. On the one hand, the obscure Jak family of cytoplasmic protein tyrosine kinases was independently implicated in IFN and hematopoietic growth factor signaling. Recent studies have expanded these initial observations to demonstrate that Jaks are critical to the functioning of all the receptors of the cytokine receptor superfamily. A variety of questions remain to be explored regarding the structure and function of Jaks and their interaction with receptors. It will also be important to pursue additional approaches to determine if the Jaks are necessary for various biological responses, particularly for mitogenic responses. The second major area of convergence has been the demonstration that members of the Stat family of transcription factors, initially identified in IFN-regulated gene expression, are generally involved in cytokine signaling. Clearly, a number of Stat-like activities remain to be cloned and it can be anticipated that the family contains additional members. Although a variety of genes are known to be regulated by the Stats association with IFN responses, much less is known concerning the genes regulated by the new Stats in cytokine signaling. Of particular importance is information relating to their potential contribution to mitogenic responses. From a biochemical standpoint, the Stats represent a remarkable family of proteins with regard to the ability of the modification of a single tyrosine residue to so dramatically affect cellular localization and DNA binding activity. Studies to identify the domains involved, and associated proteins that might contribute to either property, will be of considerable interest. More generally, it can hypothesized that Jaks and Stats, if important for proliferation and differentiation, may be the targets for malignant transformation. Although none of the genes map to chromosomal breakpoints that have been implicated in transformation, gain of function mutations is a likely mechanism that needs to be explored. Similarly, the Jak-Stat pathway would appear to be an excellent target for the development of drugs that affect a variety of cytokine functions.

Activation of the p21ras pathway couples antigen receptor stimulation to induction of the primary response gene egr-1 in B lymphocytes

The primary response gene egr-1 encodes a sequence-specific transcription factor whose expression is necessary for antigen receptor-stimulated activation of B lymphocytes. The molecular processes involved in linking egr-1 induction to antigen receptor signaling have not been defined. The present study demonstrates that expression of an activated form of p21ras results in egr-1 induction similar to that previously shown after antigen receptor cross-linking. In addition, both antigen receptor cross-linking and p21ras use the same element in the egr-1 promoter to exert their effects. Using dominant-negative mutants of p21ras and raf-1, we demonstrate that induction of egr-1 after antigen receptor cross-linking is mediated by activation of the p21ras/mitogen-activated protein kinase signaling pathway. While regulation of the p21ras pathway during B cell activation has been intensively studied, this report represents the first description of a biologically relevant event associated with its activation.

Signal transduction by the antigen receptors of B and T lymphocytes

B and T lymphocytes of the immune system recognize and destroy invading microorganisms but are tolerant to the cells and tissues of one's own body. The basis for this self/non-self-discrimination is the clonal nature of the B and T cell antigen receptors. Each lymphocyte has antigen receptors with a single unique antigen specificity. Multiple mechanisms ensure that self-reactive lymphocytes are eliminated or silenced whereas lymphocytes directed against foreign antigens are activated only when the appropriate antigen is present. The key element in these processes is the ability of the antigen receptors to transmit signals to the interior of the lymphocyte when they bind the antigen for which they are specific. Whether these signals lead to activation, tolerance, or cell death is dependent on the maturation state of the lymphocytes as well as on signals from other receptors. We review the role of antigen receptor signaling in the development and activation of B and T lymphocytes and also describe the biochemical signaling mechanisms employed by these receptors. In addition, we discuss how signal transduction pathways activated by the antigen receptors may alter gene expression, regulate the cell cycle, and induce or prevent programmed cell death.

Binding of Vav to Grb2 through dimerization of Src homology 3 domains

The protooncogenic protein Vav has the structure of an intracellular signal transducer. It is exclusively expressed in cells of hematopoietic lineage and plays a crucial role in hematopoietic cell differentiation. Here we report that both in cell extracts and within intact mammalian cells Vav binds to Grb2 (Sem-5/ASH/Drk), an adaptor molecule which plays a key role in Ras activation. The interaction became evident from a yeast two-hybrid screen and its specificity was demonstrated by in vitro binding assays. It is mediated by an unusual protein-protein binding reaction: dimerization of specific intact Src homology 3 domains of each of the partners. Signaling during hematopoietic lineage differentiation may therefore involve the tissue-specific signal transducer Vav linking into the ubiquitous pathway involving Grb2 and ultimately Ras.

Hematopoietic development of vav-/- mouse embryonic stem cells

The vav protooncogene product is expressed nearly exclusively in hematopoietic lineages and contains several structural motifs (SH2/SH3 domains and a dbl-oncogene homology region) typical of proteins functioning in signaling pathways. To ascertain if vav expression is required for hematopoiesis we generated vav-negative mouse embryonic stem cells by gene targeting and examined the consequences of loss of vav function on erythroid and myeloid development in vitro and in vivo. In conflict with the conclusions drawn from expression of antisense vav RNA in embryonic stem cells [Wulf, G. M., Adra, C. N. & Lim, B. (1993) EMBO J. 12, 5065-5074], we observed erythroid and myeloid development in the absence of vav. These experiments demonstrate that vav expression is not absolutely required for hematopoietic development.

The B cell antigen receptor complex: mechanisms and implications of tyrosine kinase activation

The B cell receptor is a multimeric receptor complex whose constituent chains appear to mediate distinct and possibly interrelated functions. In this review we have focused on how one chain, immunoglobulin (Ig)-alpha, may function to activate tyrosine kinases and the consequences of that activation. The cytoplasmic domain of Ig-alpha contains a consensus sequence, the antigen recognition homology 1 (ARH 1) motif, which is found in Ig-beta and other antigen recognition receptor associated chains. We argue that this conserved structure reflects an underlying conserved mechanism of secondary effector activation. Our data also indicates that the specificity of each motif (i.e., the elements which restrict secondary effector binding to particular motifs) is encoded within divergent sequences found in each ARH 1 motif. In the particular case of kinase activation by Ig-alpha, the subsequent phosphorylation of multiple tyrosines on Ig-alpha, Ig-beta, CD19, CD22 and possibly other functionally related chains form recruitment sites for a myriad of secondary signal transducers. In this model, proximal tyrosine kinases and phosphatases do not function so much to mediate the linear transfer of information as to establish and modulate an interrelated network of signal transducers capable of driving complicated cellular responses.

IL-5 receptor-mediated tyrosine phosphorylation of SH2/SH3-containing proteins and activation of Bruton's tyrosine and Janus 2 kinases

Interleukin 5 (IL-5) induces proliferation and differentiation of B cells and eosinophils by interacting with its receptor (IL-5R) which consists of two distinct polypeptide chains, alpha and beta (beta c). Although both IL-5R alpha and beta c lack a kinase catalytic domain, IL-5 is capable of inducing tyrosine phosphorylation of cellular proteins. We investigated the role of IL-5R alpha in tyrosine phosphorylation of molecules involved in IL-5 signal transduction, using an IL-5-dependent early B cell line, Y16 and transfectants expressing intact or mutant IL-5R alpha together with intact beta c. The results revealed that the transfectants expressing truncated IL-5R alpha, which entirely lacks a cytoplasmic domain, together with beta c, showed neither protein-tyrosine phosphorylation nor proliferation in response to IL-5. This confirms that IL-5R alpha plays a critical role in protein-tyrosine phosphorylation which triggers cell growth. IL-5 stimulation results in rapid tyrosine phosphorylation of beta c and proteins containing Src homology 2 (SH2) and/or SH3 domains such as phosphatidyl-inositol-3 kinase, Shc, Vav, and HS1, suggesting their involvement in IL-5-mediated signal transduction. IL-5 stimulation significantly enhanced activities of Janus 2 and B cell-specific Bruton's tyrosine kinases (JAK2 and Btk) and increased the tyrosine phosphorylation of JAK2 kinase. These results and recent data on signaling of growth factors taken together, multiple biochemical pathways driven by tyrosine kinases such as JAK2 and Btk are involved in IL-5 signal transduction.

Monocyte deactivation by interleukin 10 via inhibition of tyrosine kinase activity and the Ras signaling pathway

Activation of monocytes by bacterial lipopolysaccharides (LPSs) is a central component in the pathogenesis of septic shock syndrome. Interleukin 10 (IL-10) is a potent monocyte-deactivating factor and transcriptionally inhibits LPS-induced expression of proinflammatory mediators. The intracellular signaling pathways of LPS have been only partially characterized and mechanisms of IL-10 signaling remain unknown. We show that LPS activates the protein tyrosine kinase (PTK) p56lyn and that this is associated with tyrosine phosphorylation of the protooncogene product Vav. These events are completely blocked by the tyrosine kinase inhibitor herbimycin A. LPS also increases Ras activation in monocytes. LPS-triggered phosphorylation of mitogen-activated protein kinase is a downstream activation event that is also reduced by herbimycin A. Analysis of the IL-10 effects shows that it completely inhibits the p56lyn tyrosine kinase activation and all other subsequent events in this pathway including Ras activation. The IL-10 effects are selective since it reduced PTK-dependent cytokine mRNA expression but not the PTK independent induction of c-jun and c-fos mRNA in LPS-activated monocytes. These results identify the Ras signaling pathway as a component of intracellular signaling in LPS-stimulated monocytes and define early events in this response as targets of monocyte deactivation by IL-10.

In vitro characterization of major ligands for Src homology 2 domains derived from protein tyrosine kinases, from the adaptor protein SHC and from GTPase-activating protein in Ramos B cells

Antigen receptors of B lymphocytes transmit their activation signal to the cell interior by associating with and activation of specific non-receptor tyrosine kinases. Most of these kinases as well as other cytoplasmic effectors contain at least one Src homology 2 (SH2) domain, known to bind tyrosine-phosphorylated proteins. We examined the binding specificity of SH2 domains from different signaling molecules in B cells and found that each of the SH2 domains tested bound distinct subsets of stimulation-dependent phosphoproteins in vitro. SH2 domains from Src-like tyrosine kinases bound predominantly to the HS1 phosphoprotein. The tandem SH2 domains of the ZAP-70 tyrosine kinase bound to phosphorylated Ig-beta but only weakly to Ig-alpha. Also the SHC-derived SH2 domain formed complexes with the tyrosine-phosphorylated Ig-alpha/beta heterodimer, while the C- and N-terminal SH2 domains of GTPase-activating protein displayed completely different binding preferences. These results suggest that cytoplasmic effector molecules can be recruited to the activated B cell receptor in an SH2-phosphotyrosine-mediated manner. The data also provide a possible explanation for the notion that Ig-alpha and Ig-beta might couple to different biochemical pathways.

Direct stimulation of Vav guanine nucleotide exchange activity for Ras by phorbol esters and diglycerides

We recently identified Vav as a Ras-activating guanine nucleotide exchange factor (GEF) stimulated by a T-cell antigen receptor-coupled protein tyrosine kinase (PTK). Here, we describe a novel, protein kinase-independent alternative pathway of Vav activation. Phorbol ester, 1,2-diacylglycerol, or ceramide treatment of intact T cells, Vav immunoprecipitates, or partially purified Vav generated by in vitro translation or COS-1 cell transfection stimulated the Ras exchange activity of Vav in the absence of detectable tyrosine phosphorylation. GEF activity of gel-purified Vav was similarly stimulated by phorbol myristate acetate (PMA). Stimulation was resistant to PTK and protein kinase C inhibitors but was blocked by calphostin, a PMA and diacylglycerol antagonist. In vitro-translated Vav lacking its cysteine-rich domain, or mutated at a single cysteine residue within this domain (C528A), was not stimulated by PMA but was fully activated by p56lck. This correlated with increased binding of radiolabeled phorbol ester to COS-1 cells expressing wild-type, but not C528A-mutated, Vav. Thus, Vav itself is a PMA-binding and -activated Ras GEF. Recombinant interleukin-1 alpha stimulated Vav via this pathway, suggesting that diglyceride-mediated Vav activation may couple PTK-independent receptors which stimulate production of lipid second messengers to Ras in hematopoietic cells.

Direct stimulation of Vav guanine nucleotide exchange activity for Ras by phorbol esters and diglycerides

We recently identified Vav as a Ras-activating guanine nucleotide exchange factor (GEF) stimulated by a T-cell antigen receptor-coupled protein tyrosine kinase (PTK). Here, we describe a novel, protein kinase-independent alternative pathway of Vav activation. Phorbol ester, 1,2-diacylglycerol, or ceramide treatment of intact T cells, Vav immunoprecipitates, or partially purified Vav generated by in vitro translation or COS-1 cell transfection stimulated the Ras exchange activity of Vav in the absence of detectable tyrosine phosphorylation. GEF activity of gel-purified Vav was similarly stimulated by phorbol myristate acetate (PMA). Stimulation was resistant to PTK and protein kinase C inhibitors but was blocked by calphostin, a PMA and diacylglycerol antagonist. In vitro-translated Vav lacking its cysteine-rich domain, or mutated at a single cysteine residue within this domain (C528A), was not stimulated by PMA but was fully activated by p56lck. This correlated with increased binding of radiolabeled phorbol ester to COS-1 cells expressing wild-type, but not C528A-mutated, Vav. Thus, Vav itself is a PMA-binding and -activated Ras GEF. Recombinant interleukin-1 alpha stimulated Vav via this pathway, suggesting that diglyceride-mediated Vav activation may couple PTK-independent receptors which stimulate production of lipid second messengers to Ras in hematopoietic cells.

Signaling pathways activated by protein tyrosine phosphorylation in lymphocytes

Antigen and cytokine receptors induce rapid tyrosine phosphorylation of receptor subunits, other membrane proteins, and signaling components. Each receptor induces phosphorylation of a number of proteins. Although there is often overlap between targets of different receptors, any given receptor only induces phosphorylation of a subset of possible targets. How this choice of targets is achieved for these receptors is not yet understood. The cellular events downstream of some signaling components are beginning to come into view. Recent progress in these areas is discussed.

Syk activation by the Src-family tyrosine kinase in the B cell receptor signaling

Signaling through the B cell antigen receptor (BCR) results in rapid increases in tyrosine phosphorylation on a number of proteins. The BCR associates with two classes of tyrosine kinase: Src-family kinase (Src-protein-tyrosine kinase [PTK]; Lyn, Fyn, Blk, or Lck) and Syk kinase. We have investigated the interaction between the Src-PTK and the Syk kinase in the BCR signaling. In contrast to wild-type B cells, BCR-mediated tyrosine phosphorylation of Syk and activation of its in vitro kinase activity were profoundly reduced in lyn-negative cells. The requirement of the Src-PTK to induce tyrosine phosphorylation and activation of Syk was also demonstrated by cotransfection of syk and src-PTK cDNAs into COS cells. These results suggest that the Src-PTK associated with BCR phosphorylates the tyrosine residue(s) of Syk upon receptor stimulation, enhancing the activity of Syk.

The B cell antigen receptor in B-cell development

During B-cell development, immature and mature forms of the B cell antigen receptor complex are deployed in a regulated fashion; thus, B cell antigen receptor complexes play essential roles in the transit of cells through ontogeny. The past year has seen progress in our understanding of how antigen receptor gene assembly is controlled and in defining the requirements for antigen receptor mediated signaling at specific developmental stages. The discovery that a defective protein tyrosine kinase is responsible for X-linked agammaglobulinemia in man and X-linked immunodeficiency in the mouse is particularly interesting, as it may provide the means to link a specific intracellular signaling pathway with a particular step in B-cell development.

Oncogenes, Protein Tyrosine Kinases, and Signal Transduction

Many oncogenes encode protein tyrosine kinases (PTKs). Oncogenic mutations of these genes invariably result in constitutive activation of these PTKs. Autophosphorylation of the PTKs and tyrosine phosphorylation of their cellular substrates are essential events for transmission of the mitogenic signal into cells. The recent discovery of the characteristic amino acid sequences, of the src homology domains 2 and 3 (SH2 and SH3), and extensive studies on proteins containing the SH2 and SH3 domains have revealed that protein tyrosine-phosphorylation of PTKs provides phosphotyrosine sites for SH2 binding and allows extracellular signals to be relayed into the nucleus through a chain of protein-protein interactions mediated by the SH2 and SH3 domains. Studies on oncogenes, PTKs and SH2/SH3-containing proteins have made a tremendous contribution to our understanding of the mechanisms for the control of cell growth, oncogenesis, and signal transduction. This review is intended to provide an outline of the most recent progress in the study of signal transduction by PTKs. Copyright 1994 S. Karger AG, Basel

Activation of Ras in vitro and in intact fibroblasts by the Vav guanine nucleotide exchange protein

We recently identified Vav, the product of the vav proto-oncogene, as a guanine nucleotide exchange factor (GEF) for Ras. Vav is enzymatically activated by lymphocyte antigen receptor-coupled protein tyrosine kinases or independently by diglycerides. To further evaluate the physiological role of Vav, we assessed its GDP-GTP exchange activity against several Ras-related proteins in vitro and determined whether Vav activation in transfected NIH 3T3 fibroblasts correlates with the activity status of Ras and mitogen-activated protein (MAP) kinases. In vitro translated purified Vav activated by phorbol myristate acetate (PMA) or phosphorylation with recombinant p56lck displayed GEF activity against Ras but not against recombinant RacI, RacII, Ral, or RhoA proteins. Expression of vav or proto-vav in stably transfected NIH 3T3 cells led to a approximately 10-fold increase in basal or PMA-stimulated Ras exchange activity, respectively, in total-cell lysates and Vav immunoprecipitates. Elevated GEF activity was paralleled in each case by a significant increase in the proportion of active, GTP-bound Ras. PMA had a minimal effect on the low Ras. GTP level in untransfected control fibroblasts but increased it from 20 to 37% in proto-vav-transfected cells. vav-transfected cells displayed a constitutively elevated Ras. GTP level (35%), which was not increased further by PMA treatment. MAP kinases, known downstream intermediates in Ras-dependent signaling pathways, similarly exhibited increased basal or PMA-stimulated activity in Vav-expressing cells by comparison with normal NIH 3T3 cells. These results demonstrate a physiologic interaction between Vav and its target, Ras, leading to MAP kinase activation.

B cell antigen receptors

Antigen receptors have a dual role on the surface of B lymphocytes, namely the binding and internalization of antigens and the activation of the antigen-recognizing B cell. The structural requirements for internalization are still a matter of controversy; but as far as signalling is concerned it is becoming increasingly apparent that the antigen receptor is functionally divided into the ligand-binding immunoglobulin (Ig) molecule and the signal-transducing Ig-alpha/Ig-beta heterodimer. This knowledge has been exploited to design variant antigen receptors which combine both functions in a single molecule.

Activation of Ras in vitro and in intact fibroblasts by the Vav guanine nucleotide exchange protein

We recently identified Vav, the product of the vav proto-oncogene, as a guanine nucleotide exchange factor (GEF) for Ras. Vav is enzymatically activated by lymphocyte antigen receptor-coupled protein tyrosine kinases or independently by diglycerides. To further evaluate the physiological role of Vav, we assessed its GDP-GTP exchange activity against several Ras-related proteins in vitro and determined whether Vav activation in transfected NIH 3T3 fibroblasts correlates with the activity status of Ras and mitogen-activated protein (MAP) kinases. In vitro translated purified Vav activated by phorbol myristate acetate (PMA) or phosphorylation with recombinant p56lck displayed GEF activity against Ras but not against recombinant RacI, RacII, Ral, or RhoA proteins. Expression of vav or proto-vav in stably transfected NIH 3T3 cells led to a approximately 10-fold increase in basal or PMA-stimulated Ras exchange activity, respectively, in total-cell lysates and Vav immunoprecipitates. Elevated GEF activity was paralleled in each case by a significant increase in the proportion of active, GTP-bound Ras. PMA had a minimal effect on the low Ras. GTP level in untransfected control fibroblasts but increased it from 20 to 37% in proto-vav-transfected cells. vav-transfected cells displayed a constitutively elevated Ras. GTP level (35%), which was not increased further by PMA treatment. MAP kinases, known downstream intermediates in Ras-dependent signaling pathways, similarly exhibited increased basal or PMA-stimulated activity in Vav-expressing cells by comparison with normal NIH 3T3 cells. These results demonstrate a physiologic interaction between Vav and its target, Ras, leading to MAP kinase activation.

Signal transduction by lymphocyte antigen receptors

Despite the differences in the antigens that they recognize and in the effector functions they carry out, B and T lymphocytes utilize remarkably similar signal transduction components to initiate responses. They both use oligomeric receptors that contain distinct recognition and signal transduction subunits. Antigen receptors on both cells interact with at least two distinct families of PTKs via common sequence motifs, ARAMs, in the cytoplasmic tails of their invariant chains, which have likely evolved from a common evolutionary precursor. Coreceptors appear to serve to increase the sensitivity of both of these receptor systems through events that influence ligand binding and signal transduction. The critical role of tyrosine phosphorylation of downstream signaling components, such as phospholipase C, is the net result of changes in the balance of the action of antigen receptor-regulated PTKs and PTPases. The identification of downstream effectors, including calcineurin and Ras, that regulate cellular responses, such as lymphokine gene expression, promises the future possibility of connecting the complex pathway from the plasma membrane to the nucleus in lymphocytes. Insight gained from studies of the signaling pathways downstream of TCR and BCR stimulation is likely to contribute significantly to future understanding of mechanisms responsible for lymphocyte differentiation and for the discrimination of self from nonself in developing and mature cells.

Cytokine receptors and signal transduction

Cytokines are important regulators of hemopoiesis which exert their actions by binding to specific, high affinity, cell surface receptors. In the past several years, molecular cloning of these receptors has revealed a new superfamily referred to as the hemopoietic growth factor receptors. Members of this family are defined by a 200 amino acid conserved domain; however, it has become increasingly apparent that another characteristic of these receptors is the shared usage of a common signalling subunit among subgroups in this family. The shared signalling component explains the functional redundancy of many cytokines; however, the mechanism by which these receptors transduce a signal across the membrane is not yet clear. Studies into cytokine action have shown that many of the events that occur in response to ligand stimulation are similar to those observed for the better characterized intrinsic tyrosine kinase receptors. Thus, although the cytokine receptors do not possess intrinsic tyrosine kinase activity, these observations have led to a model of cytokine signal transduction adapted from the signalling mechanisms described for the tyrosine kinase receptors.

Membrane immunoglobulin is characterized by distinct structural subpopulations

Membrane immunoglobulins are integral proteins on B cell surfaces that bind foreign antigens and are critically involved in the regulation of the immune response. Based upon the model of serum IgG, it has been assumed that membrane immunoglobulins are essentially four chain disulfide-linked structures of the form H2L2, where H represents an immunoglobulin heavy chain, and L a light chain. We show here that membrane immunoglobulins of the mu and delta isotypes are present on spleen cell surfaces in a much more diverse group of disulfide linked structures. In some cases mIg is linked into structures as large as H5L5, while in other instances mu or delta chains appear to be linked by disulfide interactions to non-immunoglobulin molecules. These various structural complexes may represent distinct functional entities, as the association of mIg with the cytoskeleton after mIg cross-linking appears to depend upon its structural subgroup.

Human T and B lymphocytes express a structurally conserved focal adhesion kinase, pp125FAK

Clustering of beta 1-integrins on adherent cells with antibodies or ligands results in increased tyrosine phosphorylation and activation of a novel focal adhesion tyrosine kinase, pp125FAK. The genes encoding pp125FAK have been cloned previously from both chicken and mouse cDNA libraries, and the deduced amino acid sequences are nearly identical (94%). Two synthetic peptides derived from sequences at the carboxyl terminus of chicken pp125FAK were conjugated to ovalbumin to generate rabbit heteroantisera. Human pp125FAK was immunodetected in both T and B lymphocytes with these antisera. A basal state of pp125FAK tyrosine phosphorylation was observed in T and B lymphocytes, and its expression level was in general augmented among human T- and B-cell leukemia/lymphoma lines. Additionally, the full-length sequence of human T-cell pp125FAK (huT-FAK) was derived from a Jurkat T-cell cDNA library. huT-FAK is structurally identical with both mouse and chicken FAK, and shares 95% amino acid identity with chicken pp125FAK and has 97% homology with the mouse sequence. This high degree of evolutionary conservation between species suggests that pp125FAK is likely to have a crucial function in the cell. Expression of the full-length huT-FAK gene in COS cells showed an immunologically indistinct human pp125FAK protein compared with the endogenous primate pp125FAK. Taken together, the data indicate that this structurally conserved human T-cell pp125FAK likely functions in T- and B-cell lineages, and its altered expression in human lymphocyte tumor cell lines may contribute to their transformed phenotype.

Mitogenic signalling and substrate specificity of the Flk2/Flt3 receptor tyrosine kinase in fibroblasts and interleukin 3-dependent hematopoietic cells

Flk2/Flt3 is a recently identified receptor tyrosine kinase expressed in brain, placenta, testis, and primitive hematopoietic cells. The mitogenic signalling potential and biochemical properties of Flk2/Flt3 have been analyzed by using a chimeric receptor composed of the extracellular domain of the human colony-stimulating factor 1 receptor and the transmembrane and cytoplasmic domains of murine Flk2/Flt3. We demonstrate that colony-stimulating factor 1 stimulation of the Flk2/Flt3 kinase in transfected NIH 3T3 fibroblasts leads to a transformed phenotype and generates a full proliferative response in the absence of other growth factors. In transfected interleukin 3 (IL-3)-dependent Ba/F3 lymphoid cells, activation of the chimeric receptor can abrogate IL-3 requirement and sustain long-term proliferation. We show that phospholipase C-gamma 1, Ras GTPase-activating protein, the p85 subunit of phosphatidylinositol 3'-kinase, Shc, Grb2, Vav, Fyn, and Src are components of the Flk2/Flt3 signal transduction pathway. In addition, we demonstrate that phospholipase C-gamma 1, the p85 subunit of phosphatidylinositol 3'-kinase, Shc, Grb2, and Src family tyrosine kinases, but not Ras GTPase-activating protein, Vav, or Nck, physically associate with the Flk2/Flt3 cytoplasmic domain. Cell-type-specific differences in tyrosine phosphorylation of p85 and Shc are observed. A comparative analysis of the Flk2/Flt3 signal cascade with those of the endogenous platelet-derived growth factor and IL-3 receptors indicates that Flk2/Flt3 displays specific substrate preferences. Furthermore, tyrosine phosphorylation of p85 and Shc is similarly affected by totally different growth factors in the same cellular background.

Mitogenic signalling and substrate specificity of the Flk2/Flt3 receptor tyrosine kinase in fibroblasts and interleukin 3-dependent hematopoietic cells

Flk2/Flt3 is a recently identified receptor tyrosine kinase expressed in brain, placenta, testis, and primitive hematopoietic cells. The mitogenic signalling potential and biochemical properties of Flk2/Flt3 have been analyzed by using a chimeric receptor composed of the extracellular domain of the human colony-stimulating factor 1 receptor and the transmembrane and cytoplasmic domains of murine Flk2/Flt3. We demonstrate that colony-stimulating factor 1 stimulation of the Flk2/Flt3 kinase in transfected NIH 3T3 fibroblasts leads to a transformed phenotype and generates a full proliferative response in the absence of other growth factors. In transfected interleukin 3 (IL-3)-dependent Ba/F3 lymphoid cells, activation of the chimeric receptor can abrogate IL-3 requirement and sustain long-term proliferation. We show that phospholipase C-gamma 1, Ras GTPase-activating protein, the p85 subunit of phosphatidylinositol 3'-kinase, Shc, Grb2, Vav, Fyn, and Src are components of the Flk2/Flt3 signal transduction pathway. In addition, we demonstrate that phospholipase C-gamma 1, the p85 subunit of phosphatidylinositol 3'-kinase, Shc, Grb2, and Src family tyrosine kinases, but not Ras GTPase-activating protein, Vav, or Nck, physically associate with the Flk2/Flt3 cytoplasmic domain. Cell-type-specific differences in tyrosine phosphorylation of p85 and Shc are observed. A comparative analysis of the Flk2/Flt3 signal cascade with those of the endogenous platelet-derived growth factor and IL-3 receptors indicates that Flk2/Flt3 displays specific substrate preferences. Furthermore, tyrosine phosphorylation of p85 and Shc is similarly affected by totally different growth factors in the same cellular background.

B-cell antigen receptor motifs have redundant signalling capabilities and bind the tyrosine kinases PTK72, Lyn and Fyn

BACKGROUND

The 13 cell antigen receptor (BCR) is a multimeric protein complex consisting of an antigen recognition structure (membrane immunoglobulin) and two associated proteins, lg-alpha and Ig-beta It has been proposed that signalling through the BCR involves Ig-alpha and Ig-beta. Both of these proteins contain within their cytoplasmic domains an amino-acid motif that is present in a number of immune recognition receptors, including the BCR, T-cell antigen receptor and Fc receptor complexes. This motif, termed the antigen-receptor homology motif (ARH1), appears to have signal transduction ability.

RESULTS

We now show that the presence of cytoplasmic regions containing the ARM motif from either Ig-alpha or Ig-beta is sufficient to confer signalling capability on an otherwise non-functional fusion protein. Both Ig-alpha- and Ig-beta-containing chimeras induced, in an apparently redundant fashion, signalling events seen upon membrane immunoglobulin crosslinking, including tyrosine phosphorylation of particular proteins, phosphoinositicle breakdown and calcium mobilization. Furthermore, crosslinking of the chimeras resulted in tyrosine phosphorylation of the Ig-alpha and Tg-beta tails and their association with the tyrosine kinases PTK72, p53/56(lyn) and p59(fyn).

CONCLUSIONS

These observations indicate that Ig-alpha and Ig-beta are responsible for coupling membrane immunoglobulin to intracellular signalling components. Moreover, they demonstrate that a number of tyrosine kinases associate directly with the cytoplasmic domains of both Ig-alpha and Ig-beta. Stimulation of the chimeras, which results in tyrosine phosphorylation of the ig-alpha and Ig-beta tails, is a prerequisite for some of these associations. The implications of these findings for the mechanism by which the BCR initiates the signalling reactions are discussed.

Human CD38 is associated to distinct molecules which mediate transmembrane signaling in different lineages

The CD38 antigen displays restricted functional associations with surface molecules involved in immune system and complement. Capping of the CD38 molecule in normal or neoplastic T cells is followed by rapid and specific co-modulation of the CD3-T cell receptor (TcR) complex. In normal and tumor cells of B lineage, CD38 was found to be also associated with surface Ig (sIg) and with the complement receptor 2 (CR2)/CD19 complex. The CD38 molecule expressed by purified natural killer cells displayed an association with the low affinity IgG Fc receptor (Fc gamma RIII) CD16. These observations suggest that specialized areas in the plasma membrane contain co-modulating structures, including different receptors involved in the transduction of extracellular signals. We propose a model whereby TcR, CR2 and CD16 are ligand binding structures in their respective lineages, while CD38 is a molecule involved in the intracellular transduction of the signals.

Mapping of sites on the Src family protein tyrosine kinases p55blk, p59fyn, and p56lyn which interact with the effector molecules phospholipase C-gamma 2, microtubule-associated protein kinase, GTPase-activating protein, and phosphatidylinositol 3-kinase

Engagement of the B-cell antigen receptor complex induces immediate activation of receptor-associated Src family tyrosine kinases including p55blk, p59fyn, p53/56lyn, and perhaps p56lck, and this response is accompanied by tyrosine phosphorylation of distinct cellular substrates. These kinases act directly or indirectly to phosphorylate and/or activate effector proteins including p42 (microtubule-associated protein kinase) (MAPK), phospholipases C-gamma 1 (PLC gamma 1) and C-gamma 2 (PLC gamma 2), phosphatidylinositol 3-kinase (PI 3-K), and p21ras-GTPase-activating protein (GAP). Although coimmunoprecipitation results indicate that the Src family protein tyrosine kinases interact physically with some of these effector molecules, the molecular basis of this interaction has not been established. Here, we show that three distinct sites mediate the interaction of these kinases with effectors. The amino-terminal 27 residues of the unique domain of p56lyn mediate association with PLC gamma 2, MAPK, and GAP. Binding to PI 3-K is mediated through the Src homology 3 (SH3) domains of the Src family kinases. Relatively small proportions of cellular PI 3-K, PLC gamma 2, MAPK, and GAP, presumably those which are tyrosine phosphorylated, bind to the SH2 domains of these kinases. Comparative analysis of binding activities of Blk, Lyn, and Fyn shows that these kinases differ in their abilities to associate with MAPK and PI 3-K, suggesting that they may preferentially bind and subsequently phosphorylate distinct sets of downstream effector molecules in vivo. Fast protein liquid chromatography Mono Q column-fractionated MAPK maintains the ability to bind bacterially expressed Lyn, suggesting that the two kinases may interact directly.

Mapping of sites on the Src family protein tyrosine kinases p55blk, p59fyn, and p56lyn which interact with the effector molecules phospholipase C-gamma 2, microtubule-associated protein kinase, GTPase-activating protein, and phosphatidylinositol 3-kinase

Engagement of the B-cell antigen receptor complex induces immediate activation of receptor-associated Src family tyrosine kinases including p55blk, p59fyn, p53/56lyn, and perhaps p56lck, and this response is accompanied by tyrosine phosphorylation of distinct cellular substrates. These kinases act directly or indirectly to phosphorylate and/or activate effector proteins including p42 (microtubule-associated protein kinase) (MAPK), phospholipases C-gamma 1 (PLC gamma 1) and C-gamma 2 (PLC gamma 2), phosphatidylinositol 3-kinase (PI 3-K), and p21ras-GTPase-activating protein (GAP). Although coimmunoprecipitation results indicate that the Src family protein tyrosine kinases interact physically with some of these effector molecules, the molecular basis of this interaction has not been established. Here, we show that three distinct sites mediate the interaction of these kinases with effectors. The amino-terminal 27 residues of the unique domain of p56lyn mediate association with PLC gamma 2, MAPK, and GAP. Binding to PI 3-K is mediated through the Src homology 3 (SH3) domains of the Src family kinases. Relatively small proportions of cellular PI 3-K, PLC gamma 2, MAPK, and GAP, presumably those which are tyrosine phosphorylated, bind to the SH2 domains of these kinases. Comparative analysis of binding activities of Blk, Lyn, and Fyn shows that these kinases differ in their abilities to associate with MAPK and PI 3-K, suggesting that they may preferentially bind and subsequently phosphorylate distinct sets of downstream effector molecules in vivo. Fast protein liquid chromatography Mono Q column-fractionated MAPK maintains the ability to bind bacterially expressed Lyn, suggesting that the two kinases may interact directly.

Interleukin-2 induces tyrosine phosphorylation of the vav proto-oncogene product in human T cells: lack of requirement for the tyrosine kinase lck

The haematopoietic protein, p95vav, has been shown to be a tyrosine kinase substrate and to have tyrosine kinase-modulated guanine-nucleotide-releasing-factor activity. This implies a function in the control of ras or ras-like proteins. Because ras activation has been shown to be a downstream event following stimulation of the interleukin-2 (IL-2) receptor, we investigated the possibility that vav was involved in IL-2 signal transduction pathways, using human T cells as a model. We found rapid tyrosine phosphorylation of vav in response to IL-2 within 1 min, with maximum increase of phosphorylation of 5-fold occurring by 5 min after treatment in normal human T cells. IL-2 stimulation of the human T-cell line YT and a subclone of the YT cell line (YTlck-) that does not express message for the src-family kinase p56lck also results in a rapid rate of tyrosine phosphorylation of vav of more than 5-fold by 5 min. These results suggest that vav may play an important role in IL-2-stimulated signal transduction and that there is not a strict requirement for the tyrosine kinase p56lck.

Tyrosine kinase-stimulated guanine nucleotide exchange activity of Vav in T cell activation

The hematopoietically expressed product of the vav proto-oncogene, Vav, shared homology with guanine nucleotide releasing factors (GRFs) [also called guanosine diphosphate-dissociation stimulators (GDSs)] that activate Ras-related small guanosine triphosphate (GTP)-binding proteins. Human T cell lysates or Vav immunoprecipitates possessed GRF activity that increased after T cell antigen receptor (TCR)-CD3 triggering; an in vitro-translated Vav fragment that contained the putative GRF domain was also active. Vav-associated GRF stimulation after TCR-CD3 ligation paralleled its tyrosine phosphorylation; both were blocked by a protein tyrosine kinase (PTK) inhibitor. Vav also was a substrate for the p56lck PTK. Thus, Vav is a PTK-regulated GRF that may be important in TCR-CD3-initiated signal transduction through the activation of Ras.

Association of Src-family kinase Lyn with B-cell antigen receptor

Antigen is thought to cross-link membrane-bound immunoglobulins (Igs) of B cells, causing proliferation and differentiation or the inhibition of growth. Compelling evidence suggests that protein-tyrosine phosphorylation is involved in signal transduction for cell proliferation and differentiation. Indeed cross-linking of membrane-bound IgM (mIgM) induced a rapid increase in tyrosine phosphorylation of at least 10 distinct proteins in B cells. The Src-family protein tyrosine kinase Lyn (p56lyn and p53lyn) is expressed preferentially in B cells. The Lyn protein and its kinase activity could be coimmunoprecipitated with both IgM and IgD from detergent lysates. Cross-linking of membrane-bound IgM with antibody induced down-regulation of the Lyn protein. From these data we concluded that Lyn is physically associated with mIgs. Further evidence showed that cross-linking of mIgM induced rapid increase in the kinase activity of Lyn and association of Lyn with 85-kDa noncatalytic subunit of phosphatidylinositol 3-kinase. Thus, Lyn is likely to participate in B-cell antigen receptor-mediated signaling. As a novel signaling molecule downstream of Lyn, we identified src homology 3-containing, transcription factor-like molecule p75HS1.