Functional specificity of cytoplasmic and transmembrane tyrosine kinases: identification of 130- and 75-kilodalton substrates of c-fps/fes tyrosine kinase in macrophages

LMW-PTP associates and dephosphorylates STAT5 interacting with its C-terminal domain

Hematopoietic cells, particularly megakaryoblastic ones, display a high level of low M(r) phosphotyrosine protein phosphatase (LMW-PTP) expression; nevertheless, the role of this PTP in such cellular lineages has been scarcely investigated. Here, we demonstrate that LMW-PTP is able to associate and dephosphorylate signal transducer and activator of transcription-5 (STAT5) in DAMI megakaryocytic cells. Numerous researchers repeatedly hypothesized the association of a regulatory phosphotyrosine protein phosphatase with STAT5 C-terminus, but such phosphotyrosine protein phosphatase remained unknown. We show evidence indicating that the association of STAT5 and LMW-PTP does not exclusively involve the phosphatase active site and phosphotyrosine residue of STAT5, and we individuate an essential region of interaction at STAT5 C-terminus, coinciding with the previously hypothesized PTP-associating domain.

Fes tyrosine kinase promotes survival and terminal granulocyte differentiation of factor-dependent myeloid progenitors (32D) and activates lineage-specific transcription factors

The c-fps/fes proto-oncogene encodes a 92-kDa protein-tyrosine kinase that is involved in myeloid cell development and function. We have recently shown that expression of an activated allele of Fes (Fes(act)) in monocyte precursors resulted in their differentiation into functional macrophages through the activation of lineage-specific transcription factors. We now report that this kinase also plays a role in the survival and terminal differentiation of granulocyte progenitors. The expression of Fes(act) in factor-dependent 32D cells prevented their apoptotic death after interleukin-3 removal, but Fes(act)-expressing cells remained factor-dependent for proliferation. Removal of interleukin-3 from the Fes(act)-expressing cells was followed by granulocytic differentiation in the absence of granulocyte colony-stimulating factor within 4-8 days. The differentiated cells had distinctive granulocyte morphology and there was up-regulation of CD11b, Gr-1, and late differentiation markers such as lactoferrin, suggesting that this kinase induced terminal granulocytic differentiation. Concomitantly, Fes(act) down-regulated the macrophage marker F4/80, suggesting that the biological activity of Fes was coordinated in a lineage-specific manner. Further analysis showed that Fes(act) caused activation of CCAAT/enhancer-binding protein-alpha and STAT3, two transcription factors that are involved in granulocyte differentiation. Our results provide evidence that Fes may be a key component of the granulocyte differentiation machinery, and suggest a potential mechanism by which this kinase may regulate granulocyte-specific gene expression.

Low Mr phosphotyrosine protein phosphatase associates and dephosphorylates p125 focal adhesion kinase, interfering with cell motility and spreading

Low M(r) phosphotyrosine protein phosphatase interferes in vivo with the activation of several growth factor receptors and is transiently redistributed, following cell stimulation with platelet-derived growth factor, from the cytosol to the cytoskeleton. We demonstrate here that this phosphatase also participates in the regulation of cell spreading and migration, pointing to its involvement in cytoskeleton organization. Low M(r) phosphotyrosine protein phosphatase-overexpressing fibroblasts are, indeed, less spread than controls and display a significantly decreased number of focal adhesions and increased cell motility. Furthermore, p125 focal adhesion kinase is associated to, and dephosphorylated by, low M(r) phosphotyrosine protein phosphatase both in vitro and in vivo. This event is consistent with an altered association of pp60(src) with focal adhesion kinase. The activation of extracellular signal-regulated kinase, another well known event downstream of the focal adhesion kinase, is also affected. On the other hand, cells overexpressing the dominant-negative form of low M(r) phosphotyrosine protein phosphatase exhibit hyperphosphorylated focal adhesion kinase, reduced motility, and an increased number of focal adhesions, which are distributed all over the ventral cell surface. Taken together, the results reported here are in keeping with low M(r) phosphotyrosine protein phosphatase participation in FAK-mediated focal adhesion remodeling.

Opposite effects of different doses of MCSF on ERK phosphorylation and cell proliferation in macrophages

We had previously shown that murine macrophages expressing v-Fes, the oncogenically activated counterpart of the c-Fes cytoplasmic tyrosine kinase, proliferate independently of Macrophage Colony-Stimulating Factor (MCSF) and that the Extracellular signal-Regulated Kinase (ERK) pathway mediates the mitogenic effect of v-Fes. In this study, the response of c-fes- and v-fes-overexpressing cells to MCSF was investigated. A critical modulation of the activation of Mitogen-activated ERK Kinase (MEK) and ERK based on the MCSF dose was characterized. ERK activation was increased by MCSF doses capable to elicit a mitogenic response (2-5 U/ml). On the contrary, MCSF doses as low as 0.05 U/ml markedly reduced ERK phosphorylation and nuclear content and moderately but significantly reduced cell proliferation. The reduction of MEK and ERK phosphorylation was very rapid, suggesting the involvement of cytosolic phosphatases. Among these, phospho-tyrosine protein phosphatases and phosphoserine/threonine protein phosphatase-2A were found involved. These findings represent the first observation that different doses of the same growth factor, MCSF in particular, can exert opposite effects on cell proliferation by switching on or off ERK signaling.

The inositol 5-phosphatase SHIP is expressed as 145 and 135 kDa proteins in blood and bone marrow cells in vivo, whereas carboxyl-truncated forms of SHIP are generated by proteolytic cleavage in vitro

The inositol polyphosphate 5-phosphatase SHIP plays an important role in negative signalling in B cells and mast cells and in the down-regulation of cytokine receptor-mediated signals in myeloid cells. SHIP is expressed as a 145 kDa full-length protein and an isoform of 135 kDa due to alternative splicing. Additional smaller forms of SHIP which are truncated at the carboxy terminus have been described in bone marrow and peripheral blood mononuclear cells (PBMC). Our data demonstrate that human bone marrow cells and PBMC from healthy donors and patients with acute myeloid leukemia express the 145 kDa form of SHIP and low amounts of a 135 kDa form of SHIP in vivo whereas C-terminal-truncated SHIP proteins are generated by a PMSF-sensitive protease during the preparation of cell lysates in vitro. We have further characterized this protease and identified a proteolytic cleavage site in the human SHIP protein C-terminal to tryptophan residue 941. These data support a physiological role for the 145 and 135 kDa forms of SHIP in bone marrow and peripheral blood cells from normal donors and patients with acute myeloid leukemia.

Constitutive activation of the MAPK pathway mediates v-fes–induced mitogenesis in murine macrophages

Fes is a nonreceptor tyrosine kinase expressed at the highest level in macrophages. We previously showed that the overexpression of c-fes in murine macrophages of the BAC-1.2F5 cell line renders these cells independent of macrophage colony-stimulating factor (MCSF) for survival and proliferation, although no direct relationship could be established between tyrosine-phosphorylated substrates of Fes- and MCSF receptor–dependent signaling and mitogenesis. In this study, we investigated whether the mitogen-activated protein kinase (MAPK) pathway is involved in the growth factor–independent growth of v-fes–overexpressing macrophages. We found a constitutively increased phosphorylation of extracellularly regulated kinase (ERK) in v-fes–overexpressing macrophages as compared with mock-infected cells. This finding was associated with activation of mitogen/extracellular signal–regulated kinase (MEK) and with constitutive localization of ERK in the nucleus. Treatment of v-fes–overexpressing cells with the MEK-specific inhibitor PD98059 markedly reduced cell growth, hyperphosphorylation, and nuclear localization of ERK, indicating that the MAPK pathway mediates the mitogenic effect of v-fes.

Constitutive activation of the MAPK pathway mediates v-fes–induced mitogenesis in murine macrophages

Fes is a nonreceptor tyrosine kinase expressed at the highest level in macrophages. We previously showed that the overexpression of c-fes in murine macrophages of the BAC-1.2F5 cell line renders these cells independent of macrophage colony-stimulating factor (MCSF) for survival and proliferation, although no direct relationship could be established between tyrosine-phosphorylated substrates of Fes- and MCSF receptor–dependent signaling and mitogenesis. In this study, we investigated whether the mitogen-activated protein kinase (MAPK) pathway is involved in the growth factor–independent growth of v-fes–overexpressing macrophages. We found a constitutively increased phosphorylation of extracellularly regulated kinase (ERK) in v-fes–overexpressing macrophages as compared with mock-infected cells. This finding was associated with activation of mitogen/extracellular signal–regulated kinase (MEK) and with constitutive localization of ERK in the nucleus. Treatment of v-fes–overexpressing cells with the MEK-specific inhibitor PD98059 markedly reduced cell growth, hyperphosphorylation, and nuclear localization of ERK, indicating that the MAPK pathway mediates the mitogenic effect of v-fes.

The nonreceptor protein-tyrosine kinase c-Fes is involved in fibroblast growth factor-2-induced chemotaxis of murine brain capillary endothelial cells

Fibroblast growth factor-2 (FGF-2)-induced migration of endothelial cells is involved in angiogenesis in vivo. However, signal transduction pathways leading to FGF-2-induced chemotaxis of endothelial cells are largely unknown. Previous studies have shown that the cytoplasmic protein-tyrosine kinase c-Fes is expressed in vascular endothelial cells and may influence angiogenesis in vivo. To investigate the contribution of c-Fes to FGF-2 signaling, we expressed wild-type or kinase-inactive human c-Fes in the murine brain capillary endothelial cell line, IBE (Immortomouse brain endothelial cells). Wild-type c-Fes was tyrosine-phosphorylated upon FGF-2-stimulation in transfected cells, whereas kinase-inactive c-Fes was not. Overexpression of wild-type c-Fes promoted FGF-2-independent tube formation of IBE cells. Tube formation was not observed with endothelial cells expressing kinase-inactive c-Fes, indicating a requirement for c-Fes kinase activity in this biological response. Expression of kinase-defective c-Fes suppressed endothelial cell migration following FGF-2 treatment, suggesting that activation of endogenous c-Fes may be required for the chemotactic response. Expression of either wild-type c-Fes or the kinase-inactive mutant did not affect the tyrosine phosphorylation FRS2, Shc, or phospholipase C-gamma, nor did it influence the kinetics of mitogen-activated protein kinase activation. These results implicate c-Fes in FGF-2-induced chemotaxis of endothelial cells through signaling pathways not linked to mitogenesis.

YopH dephosphorylates Cas and Fyn-binding protein in macrophages

The tyrosine phosphatase YopH is an essential virulence effector of pathogenic Yersinia spp. YopH, which is translocated from extracellularly located bacteria into interacting target cells, blocks phagocytosis by professional phagocytes. We show here that immunoprecipitation of YopH from lysates of J774 cells infected with Y. pseudotuberculosis expressing an inactive form of YopH resulted in co-precipitation of certain phosphotyrosine proteins. The association between the inactive YopH and phosphotyrosine proteins in the 120 kDa range was rapid and could be detected after 2 min of infection. The proteins were identified as the docking proteins Cas and Fyn-binding protein (FYB). Upon infection of J774 cells with Y. pseudotuberculosis lacking YopH expression both of these proteins became tyrosine phosphorylated. Moreover, this infection caused recruitment of Cas to peripheral focal complexes, and FYB was relocalized to areas surrounding these structures. Both Cas and FYB became dephosphorylated upon infection with Y. pseudotuberculosis expressing active YopH, and this was associated with disruption of focal complexes. With regard to the previous identification of Cas and focal complexes as targets of YopH in HeLa cells, the present study supports an important role for these targets in a general mechanism of bacterial uptake.

Growth factor-dependent phosphorylation of the actin-binding protein cortactin is mediated by the cytoplasmic tyrosine kinase FER

Previous characterization of the nonreceptor tyrosine kinase FER identified a tight physical association with the catenin pp120 and led to the suggestion that FER may be involved in cell-cell signaling. To further understand the function of FER, we have continued our analyses of the interaction of FER with pp120 and other proteins. The majority of FER is localized to the cytoplasmic fraction where it forms a complex with the actin-binding protein cortactin. The Src homology 2 sequence of FER is required for directly binding cortactin, and phosphorylation of the FER-cortactin complex is up-regulated in cells treated with peptide growth factors. Using a dominant-negative mutant of FER, we provided evidence that FER kinase activity is required for the growth factor-dependent phosphorylation of cortactin. These data suggest that cortactin is likely to be a direct substrate of FER. Our observations provide additional support for a role of FER in mediating signaling from the cell surface, via growth factor receptors, to the cytoskeleton. The nature of the FER-cortactin interaction, and their putative enzyme-substrate relationship, support the previous proposal that one of the functions of the Src homology 2 sequences of nonreceptor tyrosine kinases is to provide a binding site for their preferred substrates.

A Tyrosine-Phosphorylated Protein of 140 kD Is Constitutively Associated With the Phosphotyrosine Binding Domain of Shc and the SH3 Domains of Grb2 in Acute Myeloid Leukemia Cells

The Shc gene encodes three proteins that have been implicated as mediators of signal transduction from growth factor receptors and nonreceptor tyrosine kinases to Ras. Overexpression of Shc in established murine fibroblasts results in oncogenic transformation, indicating that Shc has oncogenic potential. Shc proteins contain a carboxy terminal SH2 domain and a novel non-SH2 phosphotyrosine-binding (PTB) domain that specifically recognizes a phosphorylated NPXpY motif in target proteins such as the epidermal growth factor receptor. We show here that Shc is constitutively tyrosine-phosphorylated in all primary acute myeloid leukemias analyzed and that, in some of these leukemias, Shc is associated through its PTB domain with a tyrosinephosphorylated protein of 140 kD (p140) in vivo. In factor-dependent cells, this 140-kD protein can be tyrosine-phosphorylated in vitro in response to cytokines involved in myeloid proliferation and differentiation, ie, granulocyte-macrophage colony-stimulating factor and colony-stimulating factor-1. A similar or identical protein of 140 kD is constitutively bound to the C-terminal SH3 domain of Grb2 in the same acute myeloid leukemias. In addition to p140, other tyrosine-phosphorylated proteins of 61 and 200 kD are constitutively associated with Shc in some of the leukemias analyzed. Our results implicate Shc, Grb2, p140, and additional tyrosine-phosphorylated proteins of 61 and 200 kD in signalling of acute myeloid leukemia cells.

The Fes protein-tyrosine kinase phosphorylates a subset of macrophage proteins that are involved in cell adhesion and cell-cell signaling

The c-fps/fes proto-oncogene encodes a 92-kDa protein-tyrosine kinase that is expressed at high levels in macrophages. We have previously shown that overexpression of c-fps/fes in a CSF-1-dependent macrophage cell line (BAC1.2F5) partially released these cells from their factor dependence and that this correlated with the tyrosine phosphorylation of a subset of proteins in a tissue-specific manner. We have now identified one of the macrophage substrates of Fes as the crk-associated substrate (Cas) and a second substrate as a 130-kDa protein that has been previously described as a T cell activation-dependent substrate and is unrelated to Cas. Both of these proteins, which have optimal consensus sequences for phosphorylation by Fes, were tightly associated with this kinase through its SH2 domain, suggesting that they were direct substrates of Fes. Remarkably, when the Fes SH2 domain was used as an affinity reagent to identify potential substrates of endogenous Fes in control BAC1.2F5 cells, the phosphotyrosyl proteins that were recognized were the same as those that were specifically phosphorylated when Fes was overexpressed in the same cells. We conclude that the substrates we identified may be structurally related or identical to the physiological targets of this kinase in macrophages. The known functions of Cas and p130 suggest that Fes kinase may play a role in signaling triggered by cell adhesion and cell-cell interactions during immune responses of macrophages.

Antisense Inhibition of c-fes Proto-oncogene Blocks PMA-Induced Macrophage Differentiation in HL60 and in FDC-P1/MAC-11 Cells

To gain some insight into the role of c-fes in macrophage differentiation, we have analyzed the ability of HL60 leukemic promyelocytic cells and FDC-P1/MAC-11 murine myeloid precursor cells to differentiate in response to phorbol esters after inhibition of c-fes function. Fes inactivation has been obtained by using oligodeoxynucleotides (ODN) complementary to the 5′ region of c-fes mRNA and to 5′ splice junctions of c-fes primary transcript. After 5 days (d) in culture, in several separate experiments performed with different ODN preparations, a complete inhibition of c-fes expression was observed in HL60 and in FDC-P1/MAC-11 cells. No perturbation of cell growth was evident in our experimental conditions in both cell lines after c-fes inhibition. Furthermore, in HL60 cells lacking c-fes product, an almost complete downregulation of the α4β1 fibronectin receptor occurred. However, in both cell lines, the induction of macrophage differentiation by phorbol esters resulted in an almost complete maturation arrest as evaluated by morphological, cytochemical, immunological criteria, and by the cytofluorimetric cell cycle analysis. A loss of the adhesion capacity of both myeloid cell lines, when compared to terminally differentated macrophages, was also observed. These results suggest that HL60 and FDC-P1/MAC-11 cells, when treated with phorbol 12-myristate 13-acetate, require c-fes protein expression to activate the genetic program underlying macrophage differentiation.

Antisense Inhibition of c-fes Proto-oncogene Blocks PMA-Induced Macrophage Differentiation in HL60 and in FDC-P1/MAC-11 Cells

To gain some insight into the role of c-fes in macrophage differentiation, we have analyzed the ability of HL60 leukemic promyelocytic cells and FDC-P1/MAC-11 murine myeloid precursor cells to differentiate in response to phorbol esters after inhibition of c-fes function. Fes inactivation has been obtained by using oligodeoxynucleotides (ODN) complementary to the 5′ region of c-fes mRNA and to 5′ splice junctions of c-fes primary transcript. After 5 days (d) in culture, in several separate experiments performed with different ODN preparations, a complete inhibition of c-fes expression was observed in HL60 and in FDC-P1/MAC-11 cells. No perturbation of cell growth was evident in our experimental conditions in both cell lines after c-fes inhibition. Furthermore, in HL60 cells lacking c-fes product, an almost complete downregulation of the α4β1 fibronectin receptor occurred. However, in both cell lines, the induction of macrophage differentiation by phorbol esters resulted in an almost complete maturation arrest as evaluated by morphological, cytochemical, immunological criteria, and by the cytofluorimetric cell cycle analysis. A loss of the adhesion capacity of both myeloid cell lines, when compared to terminally differentated macrophages, was also observed. These results suggest that HL60 and FDC-P1/MAC-11 cells, when treated with phorbol 12-myristate 13-acetate, require c-fes protein expression to activate the genetic program underlying macrophage differentiation.

Identification of a new adapter protein that may link the common beta subunit of the receptor for granulocyte/macrophage colony-stimulating factor, interleukin (IL)-3, and IL-5 to phosphatidylinositol 3-kinase

Binding of human granulocyte/macrophage colony-stimulating factor (hGM-CSF) to its receptor induces the rapid activation of phosphatidylinositol-3 kinase (PI 3-kinase). As hGM-CSF receptor (hGMR) does not contain a consensus sequence for binding of PI 3-kinase, hGMR must use a distinct mechanism for its association with and activation of PI 3-kinase. Here, we describe the identification of a tyrosine-phosphorylated protein of 76-85 kDa (p80) that associates with the common beta subunit of hGMR and with the SH2 domains of the p85 subunit of PI 3-kinase in hGM-CSF-stimulated cells. Src/Yes and Lyn were tightly associated with the p80.PI 3-kinase complex, suggesting that p80 and other phosphotyrosyl proteins present in the complex were phosphorylated by Src family kinases. Tyrosine phosphorylation of p80 was only detected in hGM-CSF or human interleukin-3-stimulated cells, suggesting that activation of p80 might be specific for signaling via the common beta subunit. We postulate that p80 functions as an adapter protein that may participate in linking the hGM-CSF receptor to the PI 3-kinase signaling pathway.

Recognition and specificity in protein tyrosine kinase-mediated signalling

There are several factors that contribute to the specificities of protein tyrosine kinases (PTKs) in signal transduction pathways. While protein-protein interaction domains, such as the Src homology (SH2 and SH3) domains, regulate the cellular localization of PTKs and their substrates, the specificities of PTKs are ultimately determined by their catalytic domains. The use of peptide libraries has revealed the substrate specificities of SH2 domains and PTK catalytic domains, and has suggested cross-talk between these domains.

Phosphorylation of a Fes-related protein in response to granulocyte-macrophage colony stimulating factor

Previous work has suggested that a 97-kDa protein (p97) is involved in the signal transduction pathway of granulocyte-macrophage colony stimulating factor (GM-CSF) as well as interleukin 3, erythropoietin, and interleukin 2. We have examined the relationship of p97 to the protein tyrosine kinase Fes in the GM-CSF signal transduction pathway in erythroid and myeloid cell lines. GM-CSF stimulation of three different cell lines induced tyrosine phosphorylation of p97 as well as a number of other phosphotyrosylproteins. Although each cell line expressed the proto-oncogene product Fes, antisera specific for Fes did not recognize p97 in immunoblotting experiments. Furthermore, immunodepletion of Fes did not reduce the amount of p97 in GM-CSF-treated cells. Two-dimensional gel electrophoresis demonstrated that p97 and Fes have similar charge to mass ratios, and limited proteolytic mapping of p97 and Fes suggested that these proteins may be related but are not identical. Our studies demonstrate that p97 is not Fes but is probably a Fes-related protein.

Catalytic specificity of protein-tyrosine kinases is critical for selective signalling

How do distinct protein-tyrosine kinases activate specific down-stream events? Src-homology-2 (SH2) domains on tyrosine kinases or targets of tyrosine kinases recognize phosphotyrosine in a specific sequence context and thereby provide some specificity. The role of the catalytic site of tyrosine kinases in determining target specificity has not been fully investigated. Here we use a degenerate peptide library to show that each of nine tyrosine kinases investigated has a unique optimal peptide substrate. We find that the cytosolic tyrosine kinases preferentially phosphorylate peptides recognized by their own SH2 domains or closely related SH2 domains (group I; ref. 3), whereas receptor tyrosine kinases preferentially phosphorylate peptides recognized by subsets of group III SH2 domains. The importance of these findings for human disease is underscored by our observation that a point mutation in the RET receptor-type tyrosine kinase, which causes multiple endocrine neoplasia type 2B, results in a shift in peptide substrate specificity.