Autophosphorylation of the Fes tyrosine kinase. Evidence for an intermolecular mechanism involving two kinase domain tyrosine residues

Non-Receptor Tyrosine Kinases: Their Structure and Mechanistic Role in Tumor Progression and Resistance

Protein tyrosine kinases (PTKs) function as key molecules in the signaling pathways in addition to their impact as a therapeutic target for the treatment of many human diseases, including cancer. PTKs are characterized by their ability to phosphorylate serine, threonine, or tyrosine residues and can thereby rapidly and reversibly alter the function of their protein substrates in the form of significant changes in protein confirmation and affinity for their interaction with protein partners to drive cellular functions under normal and pathological conditions. PTKs are classified into two groups: one of which represents tyrosine kinases, while the other one includes the members of the serine/threonine kinases. The group of tyrosine kinases is subdivided into subgroups: one of them includes the member of receptor tyrosine kinases (RTKs), while the other subgroup includes the member of non-receptor tyrosine kinases (NRTKs). Both these kinase groups function as an "on" or "off" switch in many cellular functions. NRTKs are enzymes which are overexpressed and activated in many cancer types and regulate variable cellular functions in response to extracellular signaling-dependent mechanisms. NRTK-mediated different cellular functions are regulated by kinase-dependent and kinase-independent mechanisms either in the cytoplasm or in the nucleus. Thus, targeting NRTKs is of great interest to improve the treatment strategy of different tumor types. This review deals with the structure and mechanistic role of NRTKs in tumor progression and resistance and their importance as therapeutic targets in tumor therapy.

Chemical genetics strategy to profile kinase target engagement reveals role of FES in neutrophil phagocytosis

Chemical tools to monitor drug-target engagement of endogenously expressed protein kinases are highly desirable for preclinical target validation in drug discovery. Here, we describe a chemical genetics strategy to selectively study target engagement of endogenous kinases. By substituting a serine residue into cysteine at the DFG-1 position in the ATP-binding pocket, we sensitize the non-receptor tyrosine kinase FES towards covalent labeling by a complementary fluorescent chemical probe. This mutation is introduced in the endogenous FES gene of HL-60 cells using CRISPR/Cas9 gene editing. Leveraging the temporal and acute control offered by our strategy, we show that FES activity is dispensable for differentiation of HL-60 cells towards macrophages. Instead, FES plays a key role in neutrophil phagocytosis via SYK kinase activation. This chemical genetics strategy holds promise as a target validation method for kinases.

TSLP signaling network revealed by SILAC-based phosphoproteomics

Thymic stromal lymphopoietin (TSLP) is a cytokine that plays diverse roles in the regulation of immune responses. TSLP requires a heterodimeric receptor complex consisting of IL-7 receptor α subunit and its unique TSLP receptor (gene symbol CRLF2) to transmit signals in cells. Abnormal TSLP signaling (e.g. overexpression of TSLP or its unique receptor TSLPR) contributes to the development of a number of diseases including asthma and leukemia. However, a detailed understanding of the signaling pathways activated by TSLP remains elusive. In this study, we performed a global quantitative phosphoproteomic analysis of the TSLP signaling network using stable isotope labeling by amino acids in cell culture. By employing titanium dioxide in addition to antiphosphotyrosine antibodies as enrichment methods, we identified 4164 phosphopeptides on 1670 phosphoproteins. Using stable isotope labeling by amino acids in cell culture-based quantitation, we determined that the phosphorylation status of 226 proteins was modulated by TSLP stimulation. Our analysis identified activation of several members of the Src and Tec families of kinases including Btk, Lyn, and Tec by TSLP for the first time. In addition, we report TSLP-induced phosphorylation of protein phosphatases such as Ptpn6 (SHP-1) and Ptpn11 (Shp2), which has also not been reported previously. Co-immunoprecipitation assays showed that Shp2 binds to the adapter protein Gab2 in a TSLP-dependent manner. This is the first demonstration of an inducible protein complex in TSLP signaling. A kinase inhibitor screen revealed that pharmacological inhibition of PI-3 kinase, Jak family kinases, Src family kinases or Btk suppressed TSLP-dependent cellular proliferation making them candidate therapeutic targets in diseases resulting from aberrant TSLP signaling. Our study is the first phosphoproteomic analysis of the TSLP signaling pathway that greatly expands our understanding of TSLP signaling and provides novel therapeutic targets for TSLP/TSLPR-associated diseases in humans.

Kinome analysis of receptor-induced phosphorylation in human natural killer cells

BACKGROUND

Natural killer (NK) cells contribute to the defense against infected and transformed cells through the engagement of multiple germline-encoded activation receptors. Stimulation of the Fc receptor CD16 alone is sufficient for NK cell activation, whereas other receptors, such as 2B4 (CD244) and DNAM-1 (CD226), act synergistically. After receptor engagement, protein kinases play a major role in signaling networks controlling NK cell effector functions. However, it has not been characterized systematically which of all kinases encoded by the human genome (kinome) are involved in NK cell activation.

RESULTS

A kinase-selective phosphoproteome approach enabled the determination of 188 kinases expressed in human NK cells. Crosslinking of CD16 as well as 2B4 and DNAM-1 revealed a total of 313 distinct kinase phosphorylation sites on 109 different kinases. Phosphorylation sites on 21 kinases were similarly regulated after engagement of either CD16 or co-engagement of 2B4 and DNAM-1. Among those, increased phosphorylation of FYN, KCC2G (CAMK2), FES, and AAK1, as well as the reduced phosphorylation of MARK2, were reproducibly observed both after engagement of CD16 and co-engagement of 2B4 and DNAM-1. Notably, only one phosphorylation on PAK4 was differentally regulated.

CONCLUSIONS

The present study has identified a significant portion of the NK cell kinome and defined novel phosphorylation sites in primary lymphocytes. Regulated phosphorylations observed in the early phase of NK cell activation imply these kinases are involved in NK cell signaling. Taken together, this study suggests a largely shared signaling pathway downstream of distinct activation receptors and constitutes a valuable resource for further elucidating the regulation of NK cell effector responses.

Switches, excitable responses and oscillations in the Ring1B/Bmi1 ubiquitination system

In an active, self-ubiquitinated state, the Ring1B ligase monoubiquitinates histone H2A playing a critical role in Polycomb-mediated gene silencing. Following ubiquitination by external ligases, Ring1B is targeted for proteosomal degradation. Using biochemical data and computational modeling, we show that the Ring1B ligase can exhibit abrupt switches, overshoot transitions and self-perpetuating oscillations between its distinct ubiquitination and activity states. These different Ring1B states display canonical or multiply branched, atypical polyubiquitin chains and involve association with the Polycomb-group protein Bmi1. Bistable switches and oscillations may lead to all-or-none histone H2A monoubiquitination rates and result in discrete periods of gene (in)activity. Switches, overshoots and oscillations in Ring1B catalytic activity and proteosomal degradation are controlled by the abundances of Bmi1 and Ring1B, and the activities and abundances of external ligases and deubiquitinases, such as E6-AP and USP7.

The generation of polyubiquitin chains on target proteins as a degradation signal was a landmark discovery rewarded by the 2004 Nobel Prize in Chemistry. However, emerging evidence suggests that protein ubiquitination is more versatile. Different types of ubiquitin chains serve numerous non-proteolytic functions, among them regulation of the biological activities of target proteins. Here we demonstrate a flexible role of ubiquitination in the dynamic control of Ring1B, a ubiquitin ligase that monoubiquitinates histone H2A, which in turn silences gene expression. Remarkably, Ring1B increases its own activity by self-ubiquitination. A binding partner of Ring1B, Bmi1, facilitates Ring1B self-ubiquitination and protects both proteins from rapid degradation. We use computational modeling to show that the Ring1B/Bmi1 system can act as analog-digital converter, generating abrupt switches, multistable dynamics, oscillations and excitable overshoots. For instance, an increase in Bmi1 abundance brings about an abrupt “On” switch of Ring1B monoubiquitinating activity and downregulation of H2A-controlled genes, while a decrease in Bmi1 leads to an “Off” switch. These digital responses can display hysteresis, creating the biological memory. Distinct types of Ring1B activity responses (oscillatory, bistable and excitable) facilitate signal discrimination and allow the Ring1B/Bmi1/H2A system to distinctly affect gene silencing and potentially trigger different cell fates.

FES kinase participates in KIT-ligand induced chemotaxis

FES is a cytoplasmic tyrosine kinase activated by several membrane receptors, originally identified as a viral oncogene product. We have recently identified FES as a crucial effector of oncogenic KIT mutant receptor. However, FES implication in wild-type KIT receptor function was not addressed. We report here that FES interacts with KIT and is phosphorylated following activation by its ligand SCF. Unlike in the context of oncogenic KIT mutant, FES is not involved in wild-type KIT proliferation signal, or in cell adhesion. Instead, FES is required for SCF-induced chemotaxis. In conclusion, FES kinase is a mediator of wild-type KIT signalling implicated in cell migration.

FES kinases are required for oncogenic FLT3 signaling

The closely related non-receptor tyrosine kinases FEline Sarcoma (FES) and FEs Related (FER) are activated by cell surface receptors in hematopoietic cells. Despite the early description of oncogenic viral forms of fes, v-fes, and v-fps, the implication of FES and FER in human pathology is not known. We have recently shown that FES but not FER is necessary for oncogenic KIT receptor signaling. Here, we report that both FES and FER kinases are activated in primary acute myeloid leukemia (AML) blasts and in AML cell lines. FES and FER activation is dependent on FLT3 in cell lines harboring constitutively active FLT3 mutants. Moreover, both FES and FER proteins are critical for FLT3-internal tandem duplication (ITD) signaling and for cell proliferation in relevant AML cell lines. FER is required for cell cycle transitions, whereas FES seems necessary for cell survival. We concluded that FES and FER kinases mediate essential non-redundant functions downstream of FLT3-ITD.

Chemical genetics identifies c-Src as an activator of primitive ectoderm formation in murine embryonic stem cells

Multiple Src family kinases (SFKs) are present in murine embryonic stem (mES) cells. Whereas complete inhibition of SFK activity blocks mES cell differentiation, sole inhibition of the SFK member c-Yes induces differentiation. Thus, individual SFKs may have opposing roles in the regulation of mES cell fate. To test this possibility, we generated SFK mutants with engineered resistance to a nonselective SFK inhibitor. The presence of an inhibitor-resistant c-Src mutant, but not analogous mutants of Hck, Lck, c-Yes, or Fyn, reversed the differentiation block associated with inhibitor treatment, resulting in the formation of cells with properties of primitive ectoderm. These results show that distinct SFK signaling pathways regulate mES cell fate and demonstrate that the formation of primitive ectoderm is regulated by the activity of c-Src.

Toggle switches, pulses and oscillations are intrinsic properties of the Src activation/deactivation cycle

Src-family kinases (SFKs) play a pivotal role in growth factor signaling, mitosis, cell motility and invasiveness. In their basal state, SFKs maintain a closed autoinhibited conformation, where the Src homology 2 domain interacts with an inhibitory phosphotyrosine in the C-terminus. Activation involves dephosphorylation of this inhibitory phosphotyrosine, followed by intermolecular autophosphorylation of a specific tyrosine residue in the activation loop. The spatiotemporal dynamics of SFK activation controls cell behavior, yet these dynamics remain largely uninvestigated. In the present study, we show that the basic properties of the Src activation/deactivation cycle can bring about complex signaling dynamics, including oscillations, toggle switches and excitable behavior. These intricate dynamics do not require imposed external feedback loops and occur at constant activities of Src inhibitors and activators, such as C-terminal Src kinase and receptor-type protein tyrosine phosphatases. We demonstrate that C-terminal Src kinase and receptor-type protein tyrosine phosphatase underexpression or their simultaneous overexpression can transform Src response patterns into oscillatory or bistable responses, respectively. Similarly, Src overexpression leads to dysregulation of Src activity, promoting sustained self-perpetuating oscillations. Distinct types of responses can allow SFKs to trigger different cell-fate decisions, where cellular outcomes are determined by the stimulation threshold and history. Our mathematical model helps to understand the puzzling experimental observations and suggests conditions where these different kinetic behaviors of SFKs can be tested experimentally.

Bimolecular fluorescence complementation demonstrates that the c-Fes protein-tyrosine kinase forms constitutive oligomers in living cells

The c-fes proto-oncogene encodes a unique nonreceptor protein-tyrosine kinase (c-Fes) that contributes to the differentiation of myeloid hematopoietic, vascular endothelial, and some neuronal cell types. Although originally identified as the normal cellular homologue of the oncoproteins encoded by avian and feline transforming retroviruses, c-Fes has recently been implicated as a tumor suppressor in breast and colonic epithelial cells. Structurally, c-Fes consists of a unique N-terminal region harboring an FCH domain, two coiled-coil motifs, a central SH2 domain, and a C-terminal kinase domain. In living cells, c-Fes kinase activity is tightly regulated by a mechanism that remains unclear. Previous studies have established that c-Fes forms high molecular weight oligomers in vitro, suggesting that the dual coiled-coil motifs may regulate the interconversion of inactive monomeric and active oligomeric states. Here we show for the first time that c-Fes forms oligomers in live cells independently of its activation status using a YFP bimolecular fluorescence complementation assay. We also demonstrate that both N-terminal coiled-coil regions are essential for c-Fes oligomerization in transfected COS-7 cells as well as HCT 116 colorectal cancer and K-562 myeloid leukemia cell lines. Together, these data provide the first evidence that c-Fes, unlike c-Src, c-Abl, and other nonreceptor tyrosine kinases, is constitutively oligomeric in both its repressed and active states. This finding suggests that conformational changes, rather than oligomerization, may govern its kinase activity in vivo.

Spatial recruitment and activation of the Fes kinase by ezrin promotes HGF-induced cell scattering

The remodeling of epithelial monolayers induced by hepatocyte growth factor (HGF) results in the reorganization of actin cytoskeleton and cellular junctions. We previously showed that the membrane-cytoskeleton linker ezrin plays a major role in HGF-induced morphogenic effects. Here we identified a novel partner of phosphorylated ezrin, the Fes kinase, that acts downstream of ezrin in HGF-mediated cell scattering. We found that Fes interacts directly, through its SH2 domain, with ezrin phosphorylated at tyrosine 477. We show that in epithelial cells, activated Fes localizes either to focal adhesions or cell-cell contacts depending on cell confluency. The recruitment and the activation of Fes to the cell-cell contacts in confluent cells depend on its interaction with ezrin. When this interaction is impaired, Fes remains in focal adhesions and as a consequence the cells show defective spreading and scattering in response to HGF stimulation. Altogether, these results provide a novel mechanism whereby ezrin/Fes interaction at cell-cell contacts plays an essential role in HGF-induced cell scattering and implicates Fes in the cross-talk between cell-cell and cell-matrix adhesion.

Downregulation of Fes inhibits VEGF-A-induced chemotaxis and capillary-like morphogenesis by cultured endothelial cells

The aim of this study was to determine whether the downregulation of endogenous Fes by siRNA in cultured endothelial cells affects vascular endothelial growth factor-A (VEGF-A)-induced chemotaxis and capillary-like morphogenesis, which are considered as angiogenic cellular responses in vitro. VEGF-A-treatment induced autophosphorylation of Fes in cultured endothelial cells.LY294002, a phosphoinositide 3-kinase inhibitor, significantly inhibited VEGF-A-induced chemotaxis and capillary-like morphogenesis.Downregulation of Fes attenuated these VEGF-A-induced cellular responses but LY294002 did not produce further inhibition of these responses. Downregulation of Fes neither affected VEGF-A-induced autophosphorylation of VEGF receptor 2 nor mitogen-activated protein kinase activation, but markedly decreased Akt activation.Taken together, our novel results indicate the involvement of Fes in VEGF-A-induced cellular responses by cultured endothelial cells.

The tyrosine kinase FES is an essential effector of KITD816V proliferation signal

KIT is a tyrosine kinase receptor that is aberrantly activated in several neoplasms. In human pathologies, the most frequent mutation of KIT occurs at codon 816. The resulting KIT mutant protein is activated in the absence of ligand and is resistant to the clinically available inhibitors of KIT. In this report, we provide evidence for an essential function of the cytoplasmic tyrosine kinase FES downstream of KITD816V. FES is phosphorylated on tyrosine residues in cells that carry KITD816V mutation, and this phosphorylation is KIT dependent. Reduction of FES expression using RNA interference results in decreased cell proliferation in human or murine cells harboring KITD816V or the homologous mouse mutation KITD814Y. The reduced cell growth can be rescued using another cytokine (granulocyte-macrophage colony-stimulating factor [GM-CSF]) and is not observed when the closely related fer gene is targeted. Finally, signaling downstream of KITD816V is altered in cells lacking FES expression. This study shows a major function of FES downstream of activated KIT receptor and thereby points to FES as a novel target in KIT-related pathologies.

Molecular analysis of SmFes, a tyrosine kinase of Schistosoma mansoni orthologous to the members of the Fes/Fps/Fer family

A novel protein tyrosine kinase (PTK) was identified in Schistosoma mansoni and designated SmFes. SmFes exhibits the characteristic features of Fes/Fps/Fer (fes, feline sarcoma; fps, Fujinami poultry sarcoma; fer, fes related) PTKs, containing three coiled-coil regions, an SH2 (Src-homology-2) and a TK (tyrosine kinase catalytic) domain signature. SmFes is the first gene from the Fes/Fps/Fer family identified in S. mansoni, and is a single copy gene. Phylogenetic analyses revealed that SmFes is most closely related to its invertebrate orthologues. The assembly of the SmFes cDNA and genomic sequences indicated the presence of 18 introns in SmFes. Comparison of its genomic structure with those of human Fps/Fes and Drosophila Fps indicates that intron positions are conserved within the region encoding the kinase domain. Analysis of partial cDNA clones showed the presence of a 9 bp insertion at the 3' end of exon 10, producing two different cDNA populations, pointed as an alternative splicing event. In addition, an allele of SmFes containing a 15 bp insertion was observed in the genomic sequence. Quantitative RT-PCR indicated that the overall transcription level of SmFes is rather low in all parasite developmental stages. Moreover, SmFes mRNA levels decrease progressively after cercarial transformation, consistent with a role for the corresponding protein in the early stages of infection.

The KRAB-associated co-repressor KAP-1 is a coiled-coil binding partner, substrate and activator of the c-Fes protein tyrosine kinase

The c-Fes protein tyrosine kinase is implicated in the differentiation of a number of cell types including neuronal, endothelial and myeloid cells. Structurally, Fes consists of a unique N-terminal region, followed by SH2 (Src homology domain 2) and kinase domains. Two coiled-coil (CC) domains (CC1 and CC2) located within the unique N-terminal region are critical regulators of Fes activity in vivo and may function to recruit Fes activators and/or substrates. A yeast two-hybrid screen, utilizing a K-562 cell cDNA library and the Fes CC2 domain as bait, identified an interacting clone encoding the CC domain and B-box motifs (residues 114-357) of the transcriptional co-repressor KRAB-associated protein (KAP)-1. KAP-1(114-357) interacted with full-length Fes in yeast, and the KAP-1 CC domain was sufficient to bind the Fes N-terminal region in Sf-9 cells. Co-expression of Fes with full-length KAP-1 in human 293T cells stimulated Fes autophosphorylation and led to KAP-1 tyrosine phosphorylation. Association of endogenous Fes and KAP-1 was also observed in HL-60 myeloid leukaemia cells. Together, these data identify a novel Fes-KAP-1 interaction, and suggest a dual role for KAP-1 as both a Fes activator and downstream effector.

Identification of three crucial histidine residues (His115, His132 and His297) in porcine deoxyribonuclease II

DNase II is an acid endonuclease that is involved in the degradation of exogenous DNA and is important for DNA fragmentation and degradation during cell death. In an effort to understand its catalytic mechanism, we constructed plasmids encoding nine different histidine (H)-to-leucine (L) mutants for porcine DNase II and examined the enzyme properties of the expressed mutant proteins. Of the mutants, all but H132L were secreted into the medium of expressing cells. Six of the mutated DNase II proteins (H41L, H109L, H206L, H207L, H274L and H322L) showed enzyme activity, whereas the H115L, H132L and H297L mutants exhibited very little activity. The H115L and H297L mutants were found to undergo correct protein folding, but were inactive. To further examine these mutants, we expressed H115A and H297A DNase II mutants; these mutants were inactive, but their DNase activities could be rescued with imidazole, indicating that His115 and His297 are likely to function as a general acid and a general base respectively in the catalytic centre of the enzyme. In contrast with the secreted mutants, the H132L mutant protein was found in cell lysates within 16 h after transfection. This protein was inactive, improperly folded and was drastically degraded via the proteosomal pathway after 24 h. The polypeptide of another substitution for His132 with lysine resulted in the misfolded form being retained in endoplasmic reticulum.

A growth-suppressive function for the c-fes protein-tyrosine kinase in colorectal cancer

The human c-fes locus encodes a non-receptor protein-tyrosine kinase implicated in myeloid, vascular endothelial, and neuronal cell differentiation. A recent analysis of the tyrosine kinome in colorectal cancer identified c-fes as one of only seven genes with consistent kinase domain mutations. Although four mutations were identified (M704V, R706Q, V743M, S759F), the consequences of these mutations on Fes kinase activity were not explored. To address this issue, Fes mutants with these substitutions were co-expressed with STAT3 in human 293T cells. Surprisingly, the M704V, R706Q, and V743M mutations substantially reduced Fes autophosphorylation and STAT3 Tyr-705 phosphorylation compared with wild-type Fes, whereas S759F had little effect. These mutations had a similar impact on Fes kinase activity in a yeast expression system, suggesting that they inhibit Fes by affecting kinase domain structure. We have also demonstrated for the first time that endogenous Fes is strongly expressed at the base of colonic crypts where it co-localizes with epithelial cells positive for the progenitor cell marker Musashi-1. In contrast to normal colonic epithelium, Fes expression was reduced or absent in colon tumor sections from most individuals. Fes protein levels were also low or absent in a panel of human colorectal cancer cell lines, including HT-29 and HCT 116 cells. Introduction of Fes into these lines with a recombinant retrovirus suppressed their growth in soft agar. Together, our findings strongly implicate the c-Fes protein-tyrosine kinase as a tumor suppressor rather than a dominant oncogene in colorectal cancer.

Angiopoietin 1 is mitogenic for cultured endothelial cells

The angiopoietin (Ang)/Tie2 system is implicated in blood vessel formation and maturation. However, the mitogenic effects of angiopoietins remain to be elucidated. Here, we show that Ang1 is mitogenic for cultured endothelial cells. Ang1 dose-dependently induced the proliferation and increased the labeling index of a murine brain capillary endothelial cell line, IBE cells. Ang1 also increased the labeling index of human umbilical vein endothelial cells (HUVEC). Ang1 up-regulated the expression of cyclin D1 in both of these cells. Ang1 activated mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) in IBE cells and HUVECs. Activated PI3K was associated with c-Fes protein tyrosine kinase in these cells, but not with Tie2. p70 S6 kinase (p70 S6K) was activated by Ang1-treatment, although this activation was blocked by a PI3K inhibitor, LY294002. Simultaneous treatment of cells with PD98059 (MAPK/extracellular regulated kinase kinase inhibitor) and rapamycin (mTOR inhibitor) completely blocked Ang1-induced mitogenic activity for IBE cells and HUVECs. Although Ang2 at high concentration weakly activated Tie2 and p70 S6K, it failed to activate Ras and MAPK, or to induce cell proliferation. Taken together, these findings indicate that Ang1 exerts mitogenic activity on endothelial cells, which requires activation of both MAPK and p70 S6K.

Src Family Kinase Activity Is Required for Murine Embryonic Stem Cell Growth and Differentiation

Self-renewal and differentiation of embryonic stem (ES) cells are regulated by cytokines and growth factors through tyrosine kinase-dependent signaling pathways. In murine ES cells, signals for self-renewal are generated by the leukemia inhibitory factor (LIF). LIF and other growth factors are linked to the activation of the Src family of cytoplasmic protein-tyrosine kinases (SFKs), which consists of eight members having shared structural architecture. In this article, we show that murine ES cells express seven SFKs, three of which (Hck, Src, and Fyn) exhibit constitutive activity in self-renewing ES cells. Differentiation of ES cells to embryoid bodies was associated with rapid transcriptional silencing of Hck and Lck and with the loss of the corresponding kinase proteins. The expression of other family members remained relatively constant, although some loss of Fgr and Lyn proteins was observed during differentiation. Like ES cells, embryoid bodies maintained constitutive Src and Fyn kinase activity. Partial inhibition of endogenous SFK activity with the ATP-competitive inhibitors 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine or Src kinase inhibitor-1 induced differentiation of ES cells in the presence of LIF. In contrast, suppression of all SFK activity with higher concentrations of these inhibitors, or with the more potent compound A-419259 (Bioorg Med Chem Lett 12:1683-1686, 2002) blocked differentiation in response to LIF withdrawal. It is surprising that these inhibitor-treated cells remained pluripotent despite the absence of LIF. Our results implicate individual members of the Src kinase family in distinct ES cell renewal and differentiation pathways and show that small-molecule SFK inhibitors can control ES cell fate.

Conserved residues in the HIV-1 Nef hydrophobic pocket are essential for recruitment and activation of the Hck tyrosine kinase

The Nef protein of the primate lentiviruses human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) is essential for high-titer viral replication and acquired immune deficiency syndrome (AIDS) progression. Nef binds to the macrophage-specific Src family member Hck through its SH3 domain, resulting in constitutive kinase activation capable of transforming rodent fibroblasts. Nef-Hck interaction may be essential for M-tropic HIV replication and AIDS pathogenesis, identifying this virus-host protein complex as a rational target for anti-HIV drug discovery. Here, we investigated whether interaction with Hck is a common feature of Nef alleles from different strains of HIV-1. We compared the ability of four different laboratory HIV-1 Nef alleles (SF2, LAI, ELI, and Consensus) to induce Hck activation and transformation in our Rat-2 fibroblast model. While SF2, LAI, and Consensus Nef all bound and activated Hck, ELI Nef failed to bind to the Hck SH3 domain in vitro and did not cooperate with Hck in fibroblast transformation. Molecular modeling identified three residues in the core region of SF2 Nef (Ala83, His116, and Tyr120) which are substituted in ELI with Glu, Asn, and Ile, respectively. Two of these residues (Ala83 and Tyr120) form part of the hydrophobic pocket that contacts Ile 96 in the RT loop of the Hck SH3 domain in the Nef-SH3 crystal structure. Substitution of SF2 Nef Tyr120 with Ile completely abolished Hck recruitment and activation. In a complementary experiment, substitution of ELI Ile120 with Tyr partly restored ELI Nef-induced Hck activation and transformation in Rat-2 cells. Hck activation increased further by substitution of ELI Glu83 with Ala and Asn116 with His, suggestive of a supportive role for these residues in Hck binding. This study provides the first biological evidence that the HIV-1 Nef hydrophobic pocket is critical to Hck recruitment and activation in vivo. Targeting the Nef hydrophobic pocket with a small molecule may be sufficient to disrupt Nef signaling through Hck in HIV-infected macrophages, slowing disease progression.

The human c-Fes tyrosine kinase binds tubulin and microtubules through separate domains and promotes microtubule assembly

The c-Fes protein-tyrosine kinase (Fes) has been implicated in the differentiation of vascular endothelial, myeloid hematopoietic, and neuronal cells, promoting substantial morphological changes in these cell types. The mechanism by which Fes promotes morphological aspects of cellular differentiation is unknown. Using COS-7 cells as a model system, we observed that Fes strongly colocalizes with microtubules in vivo when activated via coiled-coil mutation or by coexpression with an active Src family kinase. In contrast, wild-type Fes showed a diffuse cytoplasmic localization in this system, which correlated with undetectable kinase activity. Coimmunoprecipitation and immunofluorescence microscopy showed that the N-terminal Fes/CIP4 homology (FCH) domain is involved in Fes interaction with soluble unpolymerized tubulin. However, the FCH domain was not required for colocalization with polymerized microtubules in vivo. In contrast, a functional SH2 domain was essential for microtubule localization of Fes, consistent with the strong tyrosine phosphorylation of purified tubulin by Fes in vitro. Using a microtubule nucleation assay, we observed that purified c-Fes also catalyzed extensive tubulin polymerization in vitro. Taken together, these results identify c-Fes as a regulator of the tubulin cytoskeleton that may contribute to Fes-induced morphological changes in myeloid hematopoietic and neuronal cells.

SRC family kinases mediate epidermal growth factor receptor ligand cleavage, proliferation, and invasion of head and neck cancer cells

Head and neck squamous cell carcinomas (HNSCCs) are characterized by up-regulation of the epidermal growth factor receptor (EGFR). We previously reported that a gastrin-releasing peptide/gastrin-releasing peptide receptor (GRP/GRPR) autocrine growth pathway is activated early in HNSCC carcinogenesis. GRP can induce rapid phosphorylation of EGFR and p42/44 mitogen-activated protein kinase (MAPK) activation in part via extracellular release of transforming growth factor alpha (TGF-alpha) by matrix metalloproteinases (MMPs). It has been reported that Src family kinases are activated by G-protein-coupled receptors (GPCRs), followed by downstream EGFR and MAPK activation. To further elucidate the mechanism of activation of EGFR by GRP in HNSCC, we investigated the role of Src family kinases. Blockade of Src family kinases using an Src-specific tyrosine kinase inhibitor A-419259 decreased GRP-induced EGFR phosphorylation and MAPK activation. GRP also failed to induce MAPK activation in dominant-negative c-Src-transfected HNSCC cells. Invasion and growth assays showed that c-Src was required for GRP-induced proliferation or invasion of HNSCC cells. In addition to TGF-alpha release, GRP induced amphiregulin, but not EGF, secretion into HNSCC cell culture medium, an effect that was blocked by the MMP inhibitor marimastat. TGF-alpha and amphiregulin secretion by GRP stimulation also was inhibited by blockade of Src family kinases. These results suggest that Src family kinases contribute to GRP-mediated EGFR growth and invasion pathways by facilitating cleavage and release of TGF-alpha and amphiregulin in HNSCC.

The c-Fes tyrosine kinase cooperates with the breakpoint cluster region protein (Bcr) to induce neurite extension in a Rac- and Cdc42-dependent manner

The c-fes locus encodes a cytoplasmic protein-tyrosine kinase (Fes) previously shown to accelerate nerve growth factor (NGF)-induced neurite outgrowth in rat PC12 cells. Here, we investigated the role of the Rho family small GTPases Rac1 and Cdc42 in Fes-mediated neuritogenesis, which have been implicated in neuronal differentiation in other systems. Fes-induced acceleration of neurite outgrowth in response to NGF treatment was completely blocked by the expression of dominant-negative Rac1 or Cdc42. Expression of a kinase-active mutant of Fes induced constitutive relocalization of endogenous Rac1 to the cell periphery in the absence of NGF, and led to dramatic actin reorganization and spontaneous neurite extension. We also investigated the breakpoint cluster region protein (Bcr), which possesses the Dbl and PH domains characteristic of guanine nucleotide exchange factors for Rho family GTPases, as a possible link between Fes, Rac/Cdc42 activation, and neuritogenesis. Coexpression of a GFP-Bcr fusion protein containing the Fes binding and tyrosine phosphorylation sites (amino acids 162-413) completely suppressed neurite outgrowth triggered by Fes. Conversely, coexpression of full-length Bcr with wild-type Fes in PC12 cells induced NGF-independent neurite formation. Taken together, these data suggest that Fes and Bcr cooperate to activate Rho family GTPases as part of a novel pathway regulating neurite extension in PC12 cells, and provide more evidence for an emerging role for Fes in neuronal differentiation.

Overexpression of DRG2 suppresses the growth of Jurkat T cells but does not induce apoptosis

Developmentally regulated GTP-binding protein (DRG) is a new subfamily within the superfamily of GTP-binding proteins. Its expression is regulated during embryonic development. To investigate the effect of the expression of DRG2 on cell growth, we constructed a human Jurkat-T-cell line that overexpresses DRG2. Overexpression of DRG2 suppressed the growth and the aggregation of Jurkat cells but did not induce apoptotic cell death. We used cDNA microarray analysis to examine the global changes in gene expression induced by an overexpression of DRG2. DNA array analyses identified genes that may suppress cell growth at a number of levels in multiple signaling cascades in Jurkat cells and also several prosurvival genes that may protect cells from apoptosis.

The role of c-Fes in vascular endothelial growth factor-A-mediated signaling by endothelial cells

c-Fes plays pivotal roles in angiogenic cellular responses of endothelial cells. Here we examined the role of c-Fes in vascular endothelial growth factor-A (VEGF-A)-mediated signaling pathways in endothelial cells. We introduced either wild-type or kinase-inactive c-Fes in porcine aortic endothelial (PAE) cell lines, which endogenously express VEGF receptor (VEGFR)-1, and PAE cells ectopically expressing VEGFR-2 (denoted KDR/PAE cells) and generated stable cell lines. VEGF-A induced autophosphorylation of c-Fes only in KDR/PAE cells, suggesting that VEGFR-2 was required for its activation. Expression of kinase-inactive c-Fes failed to demonstrate dominant negative effect on VEGF-A-induced chemotaxis and capillary morphogenesis. Phosphoinositide 3-kinase (PI3-kinase) was activated in KDR/PAE cells and c-Fes contributed to this process in a kinase activity-dependent manner. However, VEGFR-2, insulin receptor substrate-1, and c-Src were also involved in VEGF-A-induced activation of PI3-kinase, resulting in the compensation in cells expressing kinase-inactive c-Fes. Interestingly, overexpression of wild-type c-Fes in PAE cells induced VEGF-A-independent capillary morphogenesis. Considered collectively, VEGF-A activated PI3-kinase partly through c-Fes and increase in c-Fes kinase activity enhanced capillary morphogenesis by yet unknown signaling pathways.

Requirement of the coiled-coil domains of p92(c-Fes) for nuclear localization in myeloid cells upon induction of differentiation

The nonreceptor tyrosine kinase Fes is implicated in myeloid cells differentiation. It has been observed that its localization can be cytoplasmic, perinuclear, or nuclear. To further characterize this point, we studied Fes subcellular localization in myeloid cell lines (HL60 and K562) and in COS1 cells. Fes was observed in both the nucleus and the cytoplasm of HL60, K562 cells overexpressing Fes and only in the cytoplasm of COS1 cells, suggesting that nuclear localization is cell context dependent. Moreover, in myeloid cells, the treatment with differentiation-inducing agents such as retinoic acid, phorbol esters and vitamin D, is followed by an increase of the oligomeric form of Fes in the nucleus. In fact, oligomerization seems to be necessary for translocation to occur, since Fes mutants missing the coiled-coil domains are not able to form oligomers and fail to localize in the nucleus. The active form of Fes is tyrosine phosphorylated; however, phosphorylation is not required for Fes to localize in the nucleus, since tyrosine kinase inhibitors do not block the translocation process.

Activation of STAT3 by the Src family kinase Hck requires a functional SH3 domain

STAT3 is a member of a family of transcription factors with Src homology 2 (SH2) domains that are activated by tyrosine phosphorylation in response to a wide variety of cytokines and growth factors. In this study, we investigated the mechanism of STAT3 activation by the Src family of nonreceptor tyrosine kinases, which have been linked to STAT activation in both normal and transformed cell types. Using Sf-9 insect cells, we demonstrate direct STAT3 tyrosine phosphorylation and stimulation of DNA binding activity by five members of the Src kinase family (Src, Hck, Lyn, Fyn, and Fgr). We also observed stable STAT3.Src family kinase complex formation in this system. Recombinant Src family kinase SH3 domains were sufficient for interaction with STAT3, suggesting a mechanistic basis for the Src kinase-STAT3 interaction. To test the contribution of Src family kinase SH3 domains to the recruitment and activation of STAT3 in vivo, we used Rat-2 fibroblasts expressing activated mutants of the myeloid Src family member Hck. Transformation of fibroblasts by an activated Hck mutant lacking the negative regulatory tail tyrosine residue (Hck-YF) induced strong DNA binding activity of endogenous STAT3. Inactivation of Hck SH3 function by Ala replacement of a conserved Trp residue (W93A mutant) completely abolished STAT3 activation by Hck-YF and reduced transforming activity by 50% without affecting Hck kinase activity. Finally, overexpression of STAT3 in Rat-2 cells transiently stimulated Hck and c-Src kinase activity in the absence of extracellular signals, an effect that was dependent upon a putative SH3 binding motif in STAT3. These results support a model in which Src family kinases recruit STAT3 through an SH3-dependent mechanism, resulting in transient kinase activation and STAT3 phosphorylation.

Polycystin-1 activation of c-Jun N-terminal kinase and AP-1 is mediated by heterotrimeric G proteins

Functional analysis of polycystin-1, the product of the gene most frequently mutated in autosomal dominant polycystic kidney disease, has revealed that this protein is involved in the regulation of diverse signaling pathways such as the activation of the transcription factor AP-1 and modulation of Wnt signaling. However, the initial steps involved in the activation of such cascades have remained unclear. We demonstrated previously that the C-terminal cytosolic tail of polycystin-1 binds and activates heterotrimeric G proteins in vitro. To test if polycystin-1 can activate cellular signaling cascades via heterotrimeric G protein subunits, polycystin-1 C-terminal tail-mediated c-Jun N-terminal kinase (JNK) and AP-1 activities were assayed in transiently transfected 293T cells in the presence of dominant-negative, G protein inhibiting constructs, and in the presence of cotransfected Galpha subunits. The results showed that polycystin-1-mediated JNK/AP-1 activation is mediated by Galpha and Gbetagamma subunits. Polycystin-1-mediated AP-1 activity could be significantly augmented by cotransfected Galpha(i), Galpha(q), and Galpha(12/13) subunits, suggesting that polycystin-1 can couple with and activate several heterotrimeric G protein families.

Tyrosine kinase oncogenes in normal hematopoiesis and hematological disease

Tyrosine kinase oncogenes are formed as a result of mutations that induce constitutive kinase activity. Many of these tyrosine kinase oncogenes that are derived from genes, such as c-Abl, c-Fes, Flt3, c-Fms, c-Kit and PDGFRbeta, that are normally involved in the regulation of hematopoiesis or hematopoietic cell function. Despite differences in structure, normal function, and subcellular location, many of the tyrosine kinase oncogenes signal through the same pathways, and typically enhance proliferation and prolong viability. They represent excellent potential drug targets, and it is likely that additional mutations will be identified in other kinases, their immediate downstream targets, or in proteins regulating their function.

Closing in on the biological functions of Fps/Fes and Fer

Fps/Fes and Fer are the only known members of a distinct subfamily of the non-receptor protein-tyrosine kinase family. Recent studies indicate that these kinases have roles in regulating cytoskeletal rearrangements and inside out signalling that accompany receptor ligand, cell matrix and cell cell interactions. Genetic analysis using transgenic mouse models also implicates these kinases in the regulation of inflammation and innate immunity.

Enhanced endotoxin sensitivity in fps/fes-null mice with minimal defects in hematopoietic homeostasis

The fps/fes proto-oncogene encodes a cytoplasmic protein tyrosine kinase implicated in growth factor and cytokine receptor signaling and thought to be essential for the survival and terminal differentiation of myeloid progenitors. Fps/Fes-null mice were healthy and fertile, displayed slightly reduced numbers of bone marrow myeloid progenitors and circulating mature myeloid cells, and were more sensitive to lipopolysaccharide (LPS). These phenotypes were rescued using a fps/fes transgene. This confirmed that Fps/Fes is involved in, but not required for, myelopoiesis and that it plays a role in regulating the innate immune response. Bone marrow-derived Fps/Fes-null macrophages showed no defects in granulocyte-macrophage colony-stimulating factor-, interleukin 6 (IL-6)-, or IL-3-induced activation of signal transducer and activator of transcription 3 (Stat3) and Stat5A or LPS-induced degradation of I kappa B or activation of p38, Jnk, Erk, or Akt.

Activated Fes protein tyrosine kinase induces terminal macrophage differentiation of myeloid progenitors (U937 cells) and activation of the transcription factor PU.1

The c-fps/fes proto-oncogene encodes a 92-kDa protein tyrosine kinase that is preferentially expressed in myeloid and endothelial cells. Fes is believed to play a role in vascular development and myelopoiesis and in the inflammatory responses of granulocytes and macrophages. To help define the biological role of this kinase and identify its downstream targets, we have developed a gain-of-function allele of Fes that has potent biological activity in myeloid cell progenitors. Introduction of constitutively active Fes into bipotential U937 cells induced the appearance of fully differentiated macrophages within 6 to 12 days. The Fes-expressing differentiated cells became adherent, had distinctive macrophage morphology, and exhibited increased expression of myelomonocytic differentiation markers, including CD11b, CD11c, CD18, CD14, and the macrophage colony-stimulating factor receptor. These cells acquired phagocytic properties and exhibited NADPH oxidase and nonspecific esterase activities, confirming that they were functionally active macrophages. Concomitantly, there was downregulation of the granulocytic marker granulocyte colony-stimulating factor receptor, indicating that the biological activity of Fes was coordinated in a lineage-specific manner. A constitutively active Src did not induce macrophage morphology or upregulation of myelomonocytic markers in U937 cells, suggesting that the biological activity we observed was not a general consequence of expression of an activated nonreceptor tyrosine kinase. Analysis of possible downstream targets of Fes revealed that this kinase activated the ets family transcription factor PU.1, which is essential for macrophage development. Our results strongly implicate Fes as a key regulator of terminal macrophage differentiation and identify PU.1 as a transcription factor that may mediate some of its biological activities in myeloid cells.

HSH2: a novel SH2 domain-containing adapter protein involved in tyrosine kinase signaling in hematopoietic cells

We isolated a cDNA clone encoding a novel Src homology (SH)2 domain-containing protein of 47 kDa from a human cDNA library. As its transcript was predominantly expressed in hematopoietic cells, this gene was termed HSH2 for hematopoietic SH2 protein. This protein contains several putative protein-binding motifs, SH3-binding proline-rich regions, and phosphotyrosine sites, but lacks enzymatic motifs. In a yeast two-hybrid screen, we identified a cytokine-regulated tyrosine kinase c-FES and an activated Cdc42-associated tyrosine kinase ACK1 as HSH2 interactors. HSH2 bound c-FES via its C-terminal region as well as its N-terminal region including the SH2 domain, whereas it bound ACK1 via its N-terminal proline-rich region. Furthermore, these two kinases bound and tyrosine-phosphorylated HSH2 in mammalian cells. Hence, we postulate that HSH2 functions as an adapter protein involved in tyrosine kinase signaling, and possibly regulates cytokine signaling and cytoskeletal reorganization, in hematopoietic cells.

A point mutation in the N-terminal coiled-coil domain releases c-Fes tyrosine kinase activity and survival signaling in myeloid leukemia cells

The c-fes locus encodes a 93-kDa non-receptor protein tyrosine kinase (Fes) that regulates the growth and differentiation of hematopoietic and vascular endothelial cells. Unique to Fes is a long N-terminal sequence with two regions of strong homology to coiled-coil oligomerization domains. We introduced leucine-to-proline substitutions into the coiled coils that were predicted to disrupt the coiled-coil structure. The resulting mutant proteins, together with wild-type Fes, were fused to green fluorescent protein and expressed in Rat-2 fibroblasts. We observed that a point mutation in the first coiled-coil domain (L145P) dramatically increased Fes tyrosine kinase and transforming activities in this cell type. In contrast, a similar point mutation in the second coiled-coil motif (L334P) was without effect. However, combining the L334P and L145P mutations reduced transforming and kinase activities by approximately 50% relative to the levels of activity produced with the L145P mutation alone. To study the effects of the coiled-coil mutations in a biologically relevant context, we expressed the mutant proteins in the granulocyte-macrophage colony-stimulating factor (GM-CSF)-dependent myeloid leukemia cell line TF-1. In this cellular context, the L145P mutation induced GM-CSF independence, cell attachment, and spreading. These effects correlated with a marked increase in L145P protein autophosphorylation relative to that of wild-type Fes. In contrast, the double coiled-coil mutant protein showed greatly reduced kinase and biological activities in TF-1 cells. These data are consistent with a role for the first coiled coil in the negative regulation of kinase activity and a requirement for the second coiled coil in either oligomerization or recruitment of signaling partners. Gel filtration experiments showed that the unique N-terminal region interconverts between monomeric and oligomeric forms. Single point mutations favored oligomerization, while the double point mutant protein eluted essentially as the monomer. These data provide new evidence for coiled-coil-mediated regulation of c-Fes tyrosine kinase activity and signaling, a mechanism unique among tyrosine kinases.

HIV-1 Nef promotes survival of myeloid cells by a Stat3-dependent pathway

Human immunodeficiency virus Nef is a small myristylated protein that plays a critical role in AIDS progression. Nef binds with high affinity to the SH3 domain of the myeloid-restricted tyrosine kinase Hck in vitro, identifying this Src-related kinase as a possible cellular target for Nef in macrophages. Here we show that Nef activates endogenous Hck in the granulocyte-macrophage colony-stimulating factor-dependent myeloid cell line, TF-1. Unexpectedly, Nef induced cytokine-independent TF-1 cell outgrowth and constitutive activation of the Stat3 transcription factor. Induction of survival required the Nef SH3 binding and membrane-targeting motifs and was blocked by dominant-negative Stat3 mutants. Nef also stimulated Stat3 activation in primary human macrophages, providing evidence for Stat3 as a Nef effector in a target cell for human immunodeficiency virus.

Transformation of myeloid leukemia cells to cytokine independence by Bcr-Abl is suppressed by kinase-defective Hck

Bcr-Abl is the constitutively active protein-tyrosine kinase expressed as a result of the Philadelphia translocation in chronic myelogenous leukemia. Bcr-Abl is coupled to many of the same signaling pathways normally regulated by hematopoietic cytokines. Recent work shows that Hck, a member of the Src tyrosine kinase family with myeloid-restricted expression, associates with and is activated by Bcr-Abl. Here we investigated the mechanism of Hck interaction with Bcr-Abl and the requirement for Hck activation in Bcr-Abl transformation signaling. Binding studies demonstrated that the Hck SH3 and SH2 domains are sufficient for interaction with Bcr-Abl in vitro. Hck binding localizes to the Abl SH2, SH3, and kinase domains as well as the distal portion of the C-terminal tail. To address the requirement for endogenous Src family kinase activation in Bcr-Abl signaling, a kinase-defective mutant of Hck was stably expressed in the cytokine-dependent myeloid leukemia cell line DAGM. Kinase-defective Hck dramatically suppressed Bcr-Abl-induced outgrowth of these cells in the absence of cytokine compared with a control cell line expressing beta-galactosidase. In contrast, kinase-defective Hck did not affect cell proliferation in response to interleukin-3, suggesting that the effect is specific for Bcr-Abl. These data show that Hck interacts with Bcr-Abl through a complex mechanism involving kinase-dependent and -independent components and that interaction with Hck or other Src family members is essential for transformation signaling by Bcr-Abl.

Interaction of the N-methyl-D-aspartic acid receptor NR2D subunit with the c-Abl tyrosine kinase

The COOH-terminal domain of the NR2D subunit of the NMDA receptor contains proline-rich regions that show striking homology to sequences known to bind to Src homology 3 (SH3) domains. To determine whether the proline-rich region of the NR2D subunit interacts with specific SH3 domains, in vitro SH3 domain binding assays were performed. A proline-rich fragment of the NR2D subunit (2D(866-1064)) bound to the Abl SH3 domain but not to the SH3 domains from Src, Fyn, Grb2, GAP, or phospholipase C-gamma (PLCgamma). Co-immunoprecipitation of NR2D with Abl suggests stable association of NR2D and Abl in transfected cells. The SH3 domain plays an important role in the negative regulation of Abl kinase activity. To determine whether the interaction of NR2D with the Abl SH3 domain alters Abl kinase activity, Abl was expressed alone or with NR2D in 293T cells. Autophosphorylation of Abl was readily observed when Abl was expressed alone. However, co-expression of Abl with 2D(866-1064) or full-length NR2D inhibited autophosphorylation. 2D(866-1064) did not inhibit DeltaSH3 Abl, indicating a requirement for the Abl SH3 domain in the inhibitory effect. Similarly, 2D(866-1064) did not inhibit the catalytic activity of Abl-PP, which contains two point mutations in the SH2-kinase linker domain that release the negative kinase regulation by the SH3 domain. In contrast, the full-length NR2D subunit partially inhibited the autokinase activity of both DeltaSH3 Abl and Abl-PP, suggesting that NR2D and Abl may interact at multiple sites. Taken together, the data in this report provide the first evidence for a novel inhibitory interaction between the NR2D subunit of the NMDA receptor and the Abl tyrosine kinase.

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.

Phosphorylation of B-Myb regulates its transactivation potential and DNA binding

The transcription factor B-Myb is a cell cycle-regulated phosphoprotein and a potent regulator of cell cycle progression. Previous studies demonstrated that B-Myb was phosphorylated at the onset of S phase, suggesting that it could be due to cyclin-dependent kinases. We identified 10 B-Myb phosphorylation sites by automated peptide radiosequencing of tryptic phosphopeptides derived from in vivo (32)P-labeled B-Myb. Each B-Myb phosphorylation site contained a phosphoserine or phosphothreonine followed by a proline, suggesting that this phosphorylation is due to a proline-directed kinase. Cyclin A-Cdk2 and cyclin E-Cdk2 complexes each phosphorylated B-Myb in a cell-free system on the same sites as in intact cells. Furthermore, the ability of B-Myb to activate a reporter plasmid was enhanced by the cotransfection of cyclin A, whereas mutagenesis of the 10 identified phosphorylation sites from B-Myb blocked the effect of cyclin A coexpression. Additional analysis revealed that the effect of phosphorylation on B-Myb transactivation potential was enhanced by phosphorylation sites in its carboxyl-terminal half. One phosphorylation site (Ser(581)) appeared to negatively regulate DNA binding, as mutation of this site enhanced the ability of B-Myb to bind a Myb-binding sequence. These data suggest that B-Myb is a target for phosphorylation by cyclin-Cdk2 and that phosphorylation of B-Myb regulates its transcriptional activity.

Regulation of c-Fes tyrosine kinase and biological activities by N-terminal coiled-coil oligomerization domains

The cytoplasmic protein-tyrosine kinase Fes has been implicated in cytokine signal transduction, hematopoiesis, and embryonic development. Previous work from our laboratory has shown that active Fes exists as a large oligomeric complex in vitro. However, when Fes is expressed in mammalian cells, its kinase activity is tightly repressed. The Fes unique N-terminal sequence has two regions with strong homology to coiled-coil-forming domains often found in oligomeric proteins. Here we show that disruption or deletion of the first coiled-coil domain upregulates Fes tyrosine kinase and transforming activities in Rat-2 fibroblasts and enhances Fes differentiation-inducing activity in myeloid leukemia cells. Conversely, expression of a Fes truncation mutant consisting only of the unique N-terminal domain interfered with Rat-2 fibroblast transformation by an activated Fes mutant, suggesting that oligomerization is essential for Fes activation in vivo. Coexpression with the Fes N-terminal region did not affect the transforming activity of v-Src in Rat-2 cells, arguing against a nonspecific suppressive effect. Taken together, these findings suggest a model in which Fes activation may involve coiled-coil-mediated interconversion of monomeric and oligomeric forms of the kinase. Mutation of the first coiled-coil domain may activate Fes by disturbing intramolecular coiled-coil interaction, allowing for oligomerization via the second coiled-coil domain. Deletion of the second coiled-coil domain blocks fibroblast transformation by an activated form of c-Fes, consistent with this model. These results provide the first evidence for regulation of a nonreceptor protein-tyrosine kinase by coiled-coil domains.

Disruption of coiled-coil domains in Fer protein-tyrosine kinase abolishes trimerization but not kinase activation

The protein-tyrosine kinase Fer and the highly homologous proto-oncoprotein Fps/Fes are implicated in signaling from a variety of growth factor and cytokine receptors. Here we examine the molecular basis of Fer kinase activation with an emphasis on the role of oligomerization. We show that Fer forms trimers in vivo and that disruption of either the first or second coiled-coil domain abolishes oligomerization, suggesting a cooperative interaction between these two domains. Although Fps/Fes also forms homotypic oligomers, probably via homologous coiled-coil domains, no heterotypic interactions were observed between Fer and Fps/Fes. Incorporation of catalytically inactive Fer peptides into the oligomeric complex caused only mild reduction of wild type Fer kinase activity, suggesting that kinase-inactive Fer would not behave as a potent dominant negative. Although oligomerization of Fer can potentiate autophosphorylation in trans at three major phosphorylation sites, these residues can likely also be phosphorylated in cis. In contrast, the testis-specific FerT isomer does not oligomerize and is able to autophosphorylate in cis at two of the same three residues autophosphorylated in Fer. These results suggest that although oligomerization potentiates autophosphorylation in trans, this is apparently not necessary for Fer activation.

Tyrosine phosphorylation enhances the SH2 domain-binding activity of Bcr and inhibits Bcr interaction with 14-3-3 proteins

The cellular Bcr protein consists of an N-terminal serine/threonine kinase domain, a central guanine nucleotide exchange factor homology region and a C-terminal GTPase-activating protein domain. Previous work in our laboratory established that Bcr is a major transformation-related substrate for the v-Fps tyrosine kinase, and tyrosine phosphorylation of Bcr induces Bcr-Grb-2/SOS association in vivo through the Src homology 2 (SH2) domain of Grb-2. In the present study, we mapped the region of Bcr tyrosine phosphorylation by c-Fes, the human homologue of v-Fps, to Bcr N-terminal amino acids 162-413 by using a baculovirus/Sf-9 cell co-expression system. Tyrosine phosphorylation of Bcr by Fes greatly enhanced the binding of Bcr to the SH2 domains of multiple signalling molecules in vitro, including Grb-2, Ras GTPase activating protein, phospholipase C-gamma, the 85,000 M(r) subunit of phosphatidylinositol 3'-kinase, and the Abl tyrosine kinase. In contrast with SH2 binding, tyrosine phosphorylation of Bcr reduced its ability to associate with the 14-3-3 protein Bap-1 (Bcr-associated protein-1), a Bcr substrate and member of a family of phosphoserine-binding adaptor proteins. These experiments provide in vitro evidence that tyrosine phosphorylation may modulate the interaction of Bcr with multiple growth-regulatory signalling pathways.

Mutation of a highly conserved aspartate residue in subdomain IX abolishes Fer protein-tyrosine kinase activity

Before the structure of cAMP-dependent protein kinase had been solved, sequence alignments had already suggested that several highly conserved peptide motifs described as kinase subdomains I through XI might play some functional role in catalysis. Crystal structures of several members of the protein kinase superfamily have suggested that the nearly invariant aspartate residue within subdomain IX contributes to the conformational stability of the catalytic loop by forming hydrogen bonds with backbone amides within subdomain VI. However, substitution of this aspartate with alanine or threonine in some protein kinases have indicated that these interactions are not essential for activity. In contrast, we show here that conversion of this aspartate to arginine abolished the catalytic activity of the Fer protein-tyrosine kinase when expressed either in mammalian cells or in bacteria. Structural modeling predicted that the catalytic loop of the FerD743R mutant was disrupted by van der Waal's repulsion between the side chains of the substituted arginine residue in subdomain IX and histidine-683 in subdomain VI. The FerD743R mutant model predicted a shift in the peptide backbone of the catalytic loop, and an outward rotation of histidine-683 and arginine-684 side chains. However, the position and orientation of the presumptive catalytic base, aspartate-685, was not substantially changed. The proposed model explains how substitutions of some, but not all residues could be tolerated at this nearly invariant aspartate in kinase subdomain IX.

Fibroblast transformation by Fps/Fes tyrosine kinases requires Ras, Rac, and Cdc42 and induces extracellular signal-regulated and c-Jun N-terminal kinase activation

The small GTP-binding proteins Ras, Rac, and Cdc42 link protein-tyrosine kinases with mitogen-activated protein kinase (MAPK) signaling cascades. Ras controls the activation of extracellular signal-regulated kinases (ERKs), while Rac and Cdc42 regulate the c-Jun N-terminal kinases (JNKs). In this study, we investigated whether small G protein/MAPK cascades contribute to signal transduction by transforming variants of c-Fes, a nonreceptor tyrosine kinase implicated in cytokine signaling and myeloid differentiation. First, we investigated the effects of dominant-negative small G proteins on Rat-2 fibroblast transformation by a retroviral homolog of c-Fes (v-Fps) and by c-Fes activated via N-terminal addition of the v-Src myristylation signal (Myr-Fes). We observed that dominant-negative Ras, Rac, and Cdc42 inhibited v-Fps- and Myr-Fes-induced growth of Rat-2 cells in soft agar, indicating that activation of these small GTP-binding proteins is required for fibroblast transformation by Fps/Fes tyrosine kinases. To determine whether MAPK pathways are activated downstream of these small G proteins, we measured ERK and JNK activity in the v-Fps- and Myr-Fes-transformed Rat-2 cells. Both ERK and JNK activities were elevated in the transformed cells, suggesting that these pathways are involved in cellular transformation. Dominant-negative mutants of Ras (but not Rac or Cdc42) specifically inhibited ERK activation by v-Fps and Myr-Fes, demonstrating that ERK activation occurs exclusively downstream of Ras. All three dominant-negative small G proteins inhibited JNK activation by v-Fps and Myr-Fes, indicating that JNK activation by these tyrosine kinases requires both Ras and Rho family GTPases. These data demonstrate that multiple small G protein/MAPK cascades are involved in downstream signal transduction by Fps/Fes tyrosine kinases.

Activation of STAT3 by the c-Fes protein-tyrosine kinase

STATs (signal transducers and activators of transcription) are transcription factors that contain SH2 domains and are activated by tyrosine phosphorylation, often in response to cytokine stimulation. Recent evidence indicates that the transforming tyrosine kinases encoded by the v-Src, v-Abl, and v-Fps oncogenes can induce STAT activation, suggesting that their normal cellular homologs may contribute to STAT activation under physiological conditions. In this report, we provide direct evidence that c-Fes, the normal human homolog of v-Fps, potently activates STAT3. Transient transfection of human 293T cells with STAT3 and Fes resulted in strong stimulation of STAT3 DNA binding activity. In contrast, only modest activation of STAT5 by Fes was observed in this system, indicative of possible selectivity. To determine whether Fes-induced STAT3 activation is dependent upon endogenous mammalian kinases, co-expression studies were also performed in Sf-9 insect cells. Fes also induced a dramatic increase in STAT3 DNA binding activity in this system, whereas no activation of STAT5 was observed. As a positive control, both STAT3 and STAT5 were shown to be activated by the Bcr-Abl tyrosine kinase in Sf-9 cells. Fes induced strong tyrosine phosphorylation of STAT3 in both expression systems, consistent with the gel-shift results. Fes and STAT3 have been independently linked to myeloid differentiation. Results presented here suggest that these proteins may cooperate to promote differentiation signaling in response to hematopoietic cytokines.

Oligomerization of the Fes tyrosine kinase. Evidence for a coiled-coil domain in the unique N-terminal region

The c-fes proto-oncogene encodes a non-receptor tyrosine kinase (Fes) that has been implicated in cytokine receptor signal transduction and myeloid differentiation. Previous work from our laboratory has shown that Fes autophosphorylates via an intermolecular mechanism more commonly associated with growth factor receptor tyrosine kinases. Analysis of the Fes amino acid sequence with the COILS algorithm indicates that the N-terminal region of the protein has a very high probability of forming coiled-coil structures often associated with oligomeric proteins. These findings suggest that oligomerization may be a prerequisite for trans-autophosphorylation and activation of Fes. To establish whether the active form of Fes is oligomeric, we performed gel-filtration experiments with recombinant Fes and found that it eluted as a single symmetrical peak of approximately 500 kDa. No evidence of the monomeric, 93-kDa form of the protein was observed. Deletion of the unique N-terminal domain (amino acids 1-450, including the coiled-coil homology region) completely abolished the formation of oligomers. Furthermore, co-precipitation assays demonstrated that an immobilized glutathione S-transferase fusion protein containing the Fes N-terminal region bound to full-length Fes but not to a mutant lacking the N-terminal region. Similarly, a recombinant Fes N-terminal domain protein was readily cross-linked in vitro, whereas the SH2 and kinase domains were refractory to cross-linking. Incubation of wild-type Fes with a kinase-inactive Fes mutant or with the isolated N-terminal region suppressed Fes autophosphorylation in vitro, suggesting that oligomerization may be essential for autophosphorylation of full-length Fes. The presence of an oligomerization function in the Fes family of tyrosine kinases suggests a novel mechanism for non-receptor protein-tyrosine kinase regulation.

Co-expression with BCR induces activation of the FES tyrosine kinase and phosphorylation of specific N-terminal BCR tyrosine residues

The human BCR gene encodes a protein with serine/threonine kinase activity and regulatory domains for the small G-proteins RAC and CDC42. Previous work in our laboratory has established that BCR is a substrate for c-FES, a non-receptor tyrosine kinase linked to myeloid growth and differentiation. Tyrosine phosphorylation led to the association of BCR with the RAS guanine nucleotide exchange complex GRB2-SOS in vivo via the GRB2 SH2 domain, linking BCR to RAS signaling (Maru, Y., Peters, K. L., Afar, D. E. H., Shibuya, M., Witte, O. N., and Smithgall, T. E. (1995) Mol. Cell. Biol. 15, 835-842). In the present study, we demonstrate that BCR Tyr-246 and at least one of the closely spaced tyrosine residues, Tyr-279, Tyr-283, and Tyr-289 (3Y cluster), are phosphorylated by FES both in vitro and in 32Pi-labeled cells. Mutagenesis of BCR Tyr-177 to Phe completely abolished FES-induced BCR binding to the GRB2 SH2 domain, identifying Tyr-177 as an additional phosphorylation site for FES. Co-expression of BCR and FES in human 293T cells stimulated the tyrosine autophosphorylation of FES. By contrast, tyrosine phosphorylation of BCR by FES suppressed BCR serine/threonine kinase activity toward the 14-3-3 protein and BCR substrate, BAP-1. These data show that tyrosine phosphorylation by FES affects the interaction of BCR with multiple signaling partners and suggest a general role for BCR in non-receptor protein-tyrosine kinase regulation and signal transduction.