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

Thrombopoietin and Thrombin Induce Tyrosine Phosphorylation of Vav in Human Blood Platelets

Thrombopoietin has an essential role in megakaryopoiesis and thrombopoiesis. To investigate the signaling processes induced by thrombopoietin, we have employed human platelets and recently demonstrated that thrombopoietin induces rapid tyrosine phosphorylation of Jak-2, Tyk2, Shc, Stat3, Stat5, p120c-cbl and other proteins in human platelets. Because the apparent molecular weight of a major tyrosine phosphorylated protein in platelets stimulated by thrombopoietin is approximately 85 to 95 kD, we examined the possibility that this could be Vav, a 95-kD proto-oncogene product. Specific antisera against Vav recognized the same 95 kD protein in lysates of Jurkat cells, which are known to express Vav, and platelets, indicating that platelets have Vav. Thrombopoietin induced rapid tyrosine phosphorylation of Vav in platelets without an elevation in cytosolic free calcium concentration or activation of protein kinase C. Vav was also tyrosine phosphorylated upon treatment of platelets with thrombin, collagen, or U46619, which activate phospholipase C, leading to an increased ionized calcium concentration and activation of protein kinase C. Ionomycin or phorbol 12-myristate 13-acetate (PMA) also induces tyrosine phosphorylation of Vav, suggesting that an increase in ionized calcium concentration or activation of protein kinase C may lead to phosphorylation of Vav. Thrombopoietin also induced tyrosine phosphorylation of Vav in FDCP-2 cells, genetically engineered to express human c-Mpl (FDCP-hMpl5). However, neither ionomycin nor PMA induced an increase in tyrosine phosphorylation of Vav in FDCP-hMpl5 cells, suggesting that the calcium and protein kinase C pathways of Vav phosphorylation may be unique to platelets. Further, Vav became incorporated into the Triton X-100 insoluble 10,000g sedimentable residue in an aggregation-dependent manner, suggesting that it may have a regulatory role in platelet cytoskeletal processes. Vav was constitutively associated with a 28-kD adapter protein, Grb2, which is also incorporated into the cytoskeleton in an aggregation-dependent fashion. Lastly, we found that Vav is cleaved when there is activation of calpain, a protease that may have a role in postaggregation signaling processes. Our data suggest that thrombopoietin and other agonists may induce tyrosine phosphorylation of Vav by different mechanisms and Vav may also be involved in signaling during platelet aggregation by its redistribution to the cytoskeleton.

Coexpression of flt-3 ligand/flt-3 and SCF/c-kit signal transduction system in bile-duct-ligated SI and W mice

Stem cell factor (SCF) and its receptor c-kit constitute an important signal transduction system regulating cell growth and differentiation in hematopoiesis, gametogenesis, and melanogenesis. Recently, we have demonstrated that both SCF and c-kit are expressed in the bile duct epithelial cells of the rat liver and are highly up-regulated during activation of the normally dormant hepatic stem cell compartment. In the present study, we used sl/sld and w/wv mice, which have mutation of either SCF or c-kit, to study the possible involvement of the SCF/c-kit system in the bile duct proliferation. Bile duct ligation was performed to induce the proliferation of bile duct epithelial cells. The transcripts for both SCF and c-kit were clearly increased after bile duct ligation in both control and mutant mice. Moreover, both Sl and W mice responded to the bile duct ligation, similar to the control mice, by developing new bile ducts. Recently, a novel tyrosine kinase receptor, flt-3 receptor, has been identified in the fetal liver. It has been reported that the flt-3 ligand (FL)/flt-3 system can synergize with the SCF/c-kit system and stimulate the proliferation of hematopoietic cells. Therefore, we hypothesized that the FL/flt-3 system might compensate for the compromised SCF/c-kit system in the liver of Sl and W mice. The expression of both FL and flt-3 were significantly increased in bile duct-ligated liver from both normal and mutant mice, and the transcripts for the flt-3 receptor were selectively located on bile duct epithelial cells. Based on these results, we postulate the existence of a compensatory/additive function between the FL/flt-3 and the SCF/c-kit signal transduction systems in hepatic cell biology.

Structure and function of vav

The proto-oncogene vav is expressed solely in cells of hematopoietic origin regardless of their differentiation lineage. However, recently an homologue of vav, which is widely expressed (vav2) has been identified. Vav is a complicated and interesting molecule that contains a number of structural features found in proteins involved in cell signaling. Vav has a leucine-rich region, a leucine-zipper, a calponin homology domain, an acidic domain, a Dbl-homology domain, a pleckstrin homology domain, a cysteine-rich domain, two Src homology 3 domains, with a proline-rich region in the amino-SH3 domain, and finally an Src homology 2 domain. These domains have been implicated in protein protein interactions and strongly suggest that vav is involved in signaling events. vav is also rapidly and transiently tyrosine phosphorylated through the activation of multiple receptors on hematopoietic cells. Furthermore, vav is tyrosine phosphorylated upon the activation of several cytokines and growths factors. Recently, the generation of nice vav-/- showed that vav has an essential role in proliferation/activation of T and B cells. The purpose of this review is to summarize the current knowledge on vav and to evaluate the roles of vav in cellular functions.

Phosphatidylinositol-3' kinase is not required for mitogenesis or internalization of the Flt3/Flk2 receptor tyrosine kinase

Flt3/Flk2 is a receptor tyrosine kinase that is expressed on early hematopoietic progenitor cells. Flt3/Flk2 belongs to a family of receptors, including Kit and colony-stimulating factor-1R, which support growth and differentiation within the hematopoietic system. The Flt3/Flk2 ligand, in combination with other growth factors, stimulates the proliferation of hematopoietic progenitors of both lymphoid and myeloid lineages in vitro. We report that phosphatidylinositol 3'-kinase (PI3K) binds to a unique site in the carboxy tail of murine Flt3/Flk2. In distinction to Kit and colony-stimulating factor-1R, mutant receptors unable to couple to PI3K and expressed in rodent fibroblasts or in the interleukin 3-dependent cell line Ba/F3 provide a mitogenic signal comparable to wild-type receptors. Flt3/Flk2 receptors that do not bind to PI3K also normally down-regulate, a function ascribed to PI3K in the context of other receptor systems. These data point to the existence of other unidentified pathways that, alone or in combination with PI3K, transduce these cellular responses following the activation of Flt3/Flk2.

Vav: function and regulation in hematopoietic cell signaling

Vav, a 95 kDa proto-oncogene product expressed specifically in hematopoietic cells, was originally isolated as a transforming human oncogene. Vav contains an array of functional domains that are involved in interactions with other proteins and, possibly, with lipids. These include, among others, a putative guanine nucleotide exchange domain, a cysteine-rich region similar to the phorbol ester/diacylglycerol-binding domain of protein kinase C, a pleckstrin-homology domain, and Src-homology 2 and 3 (SH2 and SH3, respectively) domains. The presence of these domains, the transforming activity of the vav oncogene, and the rapid increase in tyrosine phosphorylation of Vav induced by triggering of diverse receptors indicate that it plays an important role in hematopoietic cell signaling pathways. Such a role is supported by recent studies using "knockout" mice and transiently transfected T cells, in which Vav deletion or overexpression, respectively, had marked effects on lymphocyte development or activation. The presence of a putative guanine nucleotide exchange domain, the prototype of which is found in the dbl oncogene product, implies that Vav functions as a guanine nucleotide exchange factor (GEF) for one (or more) members of the Ras-like family of small GTP-binding proteins. In support of such a role, Vav preparations were found in some (but not other) studies to mediate in vitro-specific GEF activity for Ras. Additional studies are required to identify the physiological regulators and targets of Vav, and its exact role in hematopoietic cell development and signaling.

Thrombin receptor activation and integrin engagement stimulate tyrosine phosphorylation of the proto-oncogene product, p95vav, in platelets

The vav proto-oncogene product, p95vav or Vav, is primarily expressed in hematopoietic cells and has been shown to be a substrate for tyrosine kinases. Although its function is unknown, Vav shares a region of homology with DBL, an exchange factor for the Rho family of GTP-binding proteins. The presence of this domain and the observation that cells transformed with Vav display prominent stress fibers and focal adhesions similar to those that are observed in RhoA transformed cells suggests that Vav may play a role in regulating the actin cytoskeleton. We have, therefore, examined Vav phosphorylation in platelets, which undergo dramatic cytoskeletal reorganization in response to agonists. Two potent platelet agonists, thrombin (via its G protein-coupled receptor) and collagen (via its interaction with the alpha2beta1 integrin), caused Vav to become phosphorylated on tyrosine. Weaker platelet agonists, including ADP, epinephrine and the thromboxane A2 analog, U46619, did not. The phosphorylation of Vav in response to thrombin was maximal within 15 s and was unaffected by aspirin, inhibitors of aggregation, or the presence of the ADP scavenger, apyrase. Vav phosphorylation was also observed when platelets became adherent to immobilized collagen (via integrin alpha2beta1), fibronectin (via integrin alpha5beta1), and fibrinogen (via integrin alphaIIbbeta3). These results show that Vav phosphorylation by tyrosine kinases 1) occurs during platelet activation by potent agonists, 2) also occurs when platelets adhere to biologically relevant matrix proteins, 3) requires neither platelet aggregation nor the release of secondary agonists such as ADP and TxA2, and 4) can be initiated by at least some members of two additional classes of receptors, G protein-coupled receptors and integrins, providing further evidence that both of these can couple to tyrosine kinases.

Targeted disruption of the flk2/flt3 gene leads to deficiencies in primitive hematopoietic progenitors

The flk2 receptor tyrosine kinase has been implicated in hematopoietic development. Mice deficient in flk2 were generated. Mutants developed into healthy adults with normal mature hematopoietic populations. However, they possessed specific deficiencies in primitive B lymphoid progenitors. Bone marrow transplantation experiments revealed a further deficiency in T cell and myeloid reconstitution by mutant stem cells. Mice deficient for both c-kit and flk2 exhibited a more severe phenotype characterized by large overall decreases in hematopoietic cell numbers, further reductions in the relative frequencies of lymphoid progenitors, and a postnatal lethality. Taken together, the data suggest that flk2 plays a role both in multipotent stem cells and in lymphoid differentiation.

Tissue- and transformation-specific phosphotyrosyl proteins in v-erbB-transformed cells

To understand the mechanism of tissue-specific and transformation-specific signaling by the v-ErbB oncoprotein, we have investigated signaling pathways downstream of this transmembrane tyrosine kinase. In this report, we describe tissue-specific patterns of phosphotyrosyl proteins in three distinct cell types transformed by the v-erbB oncogene: fibroblasts, erythroblasts, and endothelial cells. In addition, we describe transformation-specific tyrosine phosphorylation events and signal complex formation in v-erbB-transformed fibroblasts. Two patterns of phosphotyrosyl proteins have been detected in v-erbB-transformed cells. The first is a fibroblast-specific pattern which includes unique phosphotyrosyl proteins of 170 kDa (c-ErbB1), 158 kDa, and 120 kDa (the catenin-like protein p120cas). The second is an erythroblast/endothelial cell-specific pattern which includes a prominent unidentified phosphotyrosyl protein of 120 kDa. Evaluation of the phosphotyrosyl proteins p120cas and SHC in chicken embryo fibroblasts infected with transforming and nontransforming v-erbB mutants reveals transformation-specific patterns of tyrosine phosphorylation. One corollary of these phosphorylation events in v-erbB-transformed fibroblasts is the formation of a complex involving SHC, growth factor receptor-bound protein 2, and a novel 75-kDa phosphotyrosyl protein. The results of these studies suggest that the v-ErbB oncoprotein can couple to multiple signal transduction pathways, that these pathways are tissue specific, and that v-erbB-mediated transformation involves specific tyrosine phosphorylation events.

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.

Identification and characterization of a functional murine FLT3 isoform produced by exon skipping

The FLT3 gene encodes an hematopoietic receptor related to the receptors for colony-stimulating factor 1, FMS, and for Steel factor, KIT. The extracellular part of these molecules is exclusively composed of five immunoglobulin (Ig)-like domains, designated 1 to 5, from the amino terminus to the carboxyl terminus of the extracellular region. We have isolated a unique murine FLT3 cDNA that codes for a variant isoform of FLT3, devoid of the fifth Ig-like domain, by comparison with the prototypic form. The corresponding mRNA is the result of a splicing event that leads to the elimination of two coding exons. mRNA coding for this variant was detected in almost all the tissues expressing the mRNA coding for the prototypic molecule, although at a lower level. Ligand-induced tyrosine phosphorylation of the two isoforms was equivalent in COS-1 transfected cells, indicating that the fifth Ig-like domain is not strictly necessary for either ligand-binding or kinase activation.