A point mutation at tyrosine-809 in the human colony-stimulating factor 1 receptor impairs mitogenesis without abrogating tyrosine kinase activity, association with phosphatidylinositol 3-kinase, or induction of c-fos and junB genes

IL-12 could induce monocytic tumor cells directional differentiation

Interleukin-12 (IL-12), a member of interleukin family, plays a critical role in immune responses and anti-tumor activity. In this study, the effects of IL-12 on monocytic tumor cell lines differentiation to macrophagocyte and its likely mechanism was investigated. We examined the differentiation markers, morphological and functional changes, and possible mechanism in IL-12-treated THP-1 and U937 cells. It was found that IL-12 could up-regulated macrophage surface marker CD68 and CD11b expression in a time-dependent manner. Morphologically, after IL-12 treatment, THP-1 and U937 cells became round or irregular shape, even stretched many cell membrane protuberances; some cell nuclei became fuzzy or completely disappeared, and the chromatin appeared dense and cordlike. Furthermore, IL-12-induced monocytic tumor cell differentiation was accompanied by the growth arrest with G1-phase accumulation and S-phase reduction; apoptosis increased with anti-apoptosis protein Bcl-2 down-expression and pro-apoptosis protein Fas up-regulation, and enhanced phagocytosis function. The IL-12-induced macrophage differentiation of THP-1 and U937 cells was associated with the up-regulation of c-fms expression and the CSF-1R Tyr 809 site phosphorylation. These findings have revealed that IL-12 could induce monocytic tumor cells directional differentiation into macrophage-like cells, and its mechanism is possible connected with the up-regulation of c-fms expression and the phosphorylation of CSF-1R Tyr-809 site.

Survey of phosphorylation near drug binding sites in the Protein Data Bank (PDB) and their effects

While it is currently estimated that 40 to 50% of eukaryotic proteins are phosphorylated, little is known about the frequency and local effects of phosphorylation near pharmaceutical inhibitor binding sites. In this study, we investigated how frequently phosphorylation may affect the binding of drug inhibitors to target proteins. We examined the 453 non-redundant structures of soluble mammalian drug target proteins bound to inhibitors currently available in the Protein Data Bank (PDB). We cross-referenced these structures with phosphorylation data available from the PhosphoSitePlus database. Three hundred twenty-two of 453 (71%) of drug targets have evidence of phosphorylation that has been validated by multiple methods or labs. For 132 of 453 (29%) of those, the phosphorylation site is within 12 Å of the small molecule-binding site, where it would likely alter small molecule binding affinity. We propose a framework for distinguishing between drug-phosphorylation site interactions that are likely to alter the efficacy of drugs versus those that are not. In addition we highlight examples of well-established drug targets, such as estrogen receptor alpha, for which phosphorylation may affect drug affinity and clinical efficacy. Our data suggest that phosphorylation may affect drug binding and efficacy for a significant fraction of drug target proteins.

FMS Kinase Inhibitors: Current Status and Future Prospects

FMS, first discovered as the oncogene responsible for Feline McDonough Sarcoma, is a type III receptor tyrosine kinase that binds to the macrophage or monocyte colony-stimulating factor (M-CSF or CSF-1). Signal transduction through that binding results in survival, proliferation, and differentiation of monocyte/macrophage lineage. Overexpression of CSF-1 and/or FMS has been implicated in a number of disease states such as the growth of metastasis of certain types of cancer, in promoting osteoclast proliferation in bone osteolysis, and many inflammatory disorders. Inhibition of CSF-1 and/or FMS may help treat these pathological conditions. This article reviews FMS gene, FMS kinase, CSF-1, IL-34, and their roles in bone osteolysis, cancer biology, and inflammation. Monoclonal antibodies, FMS crystal structure, and small molecule FMS kinase inhibitors of different chemical scaffolds are also reviewed.

CSF-1 receptor structure/function in MacCsf1r-/- macrophages: regulation of proliferation, differentiation, and morphology

CSF-1 is the major regulator of tissue macrophage development and function. A GM-CSF-dependent, CSF-1 receptor (CSF-1R)-deficient F4/80(hi)Mac-1(+)Gr1(-)CD11c(+) bone marrow macrophage (BMM) line (MacCsf1r-/-) was developed to study the roles of the eight intracellular CSF-1R tyrosines phosphorylated upon receptor activation. Retroviral expression of the wild-type CSF-1R rescued the CSF-1-induced survival, proliferation, differentiation, and morphological characteristics of primary BMM. Mutation of all eight tyrosines failed to rescue, whereas the individual Y --> F mutants (544, 559, 697, 706, 721, 807, 921, 974) rescued these CSF-1-inducible phenotypes to varying degrees. The juxtamembrane domain Y559F and activation loop Y807F mutations severely compromised proliferation and differentiation, whereas Y706, Y721F, and Y974F mutations altered morphological responses, and Y706F increased differentiation. Despite their retention of significant in vitro tyrosine kinase activity, Y559F and Y807F mutants exhibited severely impaired in vivo receptor tyrosine phosphorylation, consistent with the existence of cellular mechanisms inhibiting CSF-1R tyrosine phosphorylation that are relieved by phosphorylation of these two sites. The MacCsf1r-/- macrophage line will facilitate genetic and proteomic approaches to CSF-1R structure/function studies in the major disease-related CSF-1R-expressing cell type.

M-CSF-mediated macrophage differentiation but not proliferation is correlated with increased and prolonged ERK activation

M-CSF is a cytokine essential for both the proliferation and differentiation of monocytes/macrophages. In this study, we established a new M-CSF-mediated differentiation-inducing system, and examined how the level and duration of the activation of ERK preceded M-CSF-mediated differentiation. TF-1-fms human leukemia cells rapidly proliferated in response to M-CSF. However, in the presence of a phorbol ester, TPA, TF-1-fms cells definitely switched their responsiveness to M-CSF from proliferation to differentiation, as evidenced by a more drastic morphological change and the appearance of cells with a higher level of phagocytic activity. In TF-1-fms cells expressing HIV-1 Nef protein in a conditionally active-manner, both M-CSF-mediated proliferation and M-CSF/TPA-mediated differentiation were inhibited by the activation of Nef. The Nef-active cells showed perturbed patterns of ERK activation. Under the proliferation-inducing conditions (TPA-free), parental or Nef-inactive cells showed modest ERK activation following M-CSF stimulation, whereas Nef-active cells showed an earlier and transient ERK activation, despite a decrease in their proliferation rate. Under the differentiation-inducing conditions, parental or Nef-inactive cells showed increased and prolonged ERK activation following M-CSF stimulation, whereas Nef-active cells showed transient ERK activation. These results supported the idea that the increased and prolonged ERK activation led to M-CSF-mediated macrophage differentiation but not to proliferation.

Crystal Structure of the Tyrosine Kinase Domain of Colony-stimulating Factor-1 Receptor (cFMS) in Complex with Two Inhibitors

The cFMS proto-oncogene encodes for the colony-stimulating factor-1 receptor, a receptor-tyrosine kinase responsible for the differentiation and maturation of certain macrophages. Upon binding its ligand colony-stimulating factor-1 cFMS autophosphorylates, dimerizes, and induces phosphorylation of downstream targets. We report the novel crystal structure of unphosphorylated cFMS in complex with two members of different classes of drug-like protein kinase inhibitors. cFMS exhibits a typical bi-lobal kinase fold, and its activation loop and DFG motif are found to be in the canonical inactive conformation. Both ATP competitive inhibitors are bound in the active site and demonstrate a binding mode similar to that of STI-571 bound to cABL. The DFG motif is prevented from switching into the catalytically competent conformation through interactions with the inhibitors. Activation of cFMS is also inhibited by the juxtamembrane domain, which interacts with residues of the active site and prevents formation of the activated kinase. Together the structures of cFMS provide further insight into the autoinhibition of receptor-tyrosine kinases via their respective juxtamembrane domains; additionally the binding mode of two novel classes of kinase inhibitors will guide the design of novel molecules targeting macrophage-related diseases.

Protein Engineering of the Colony-stimulating Factor-1 Receptor Kinase Domain for Structural Studies

A parallel approach to designing crystallization constructs for the c-FMS kinase domain was implemented, resulting in proteins suitable for structural studies. Sequence alignment and limited proteolysis were used to identify and eliminate unstructured and surface-exposed domains. A small library of chimeras was prepared in which the kinase insert domain of FMS was replaced with the kinase insert domain of previously crystallized receptor-tyrosine kinases. Characterization of the newly generated FMS constructs by enzymology and thermoshift assays demonstrated similar activities and compound binding to the FMS full-length cytoplasmic domain. Two chimeras were evaluated for crystallization in the presence and absence of a variety of ligands resulting in crystal structures, and leading to a successful structure-based drug design project for this important inflammation target.

SHIP1 negatively regulates proliferation of osteoclast precursors via Akt-dependent alterations in D-type cyclins and p27

Osteoclasts arise from macrophage progenitors in bone marrow (BMMs) as a consequence of signaling events elicited by M-CSF and receptor activator of NF-kappaB ligand, acting on their unique receptors, via c-Fms and receptor activator of NF-kappaB. Both receptors activate the PI3K and MAPK pathways, which promote cell proliferation and survival. SHIP1 is essential for normal bone homeostasis, as mice lacking the protein exhibit osteoporosis resulting from increased numbers of hyper-resorptive osteoclasts. In this study, we show that BMMs from SHIP1 null mice respond to M-CSF, but not receptor activator of NF-kappaB ligand, by increasing Akt activation. In consequence, there are up-regulation of D-type cyclins, down-regulation of the cyclin-dependent kinase inhibitor p27, and, therefore, increased phosphorylation of the retinoblastoma protein and cell proliferation. Surprisingly, cell survival of wild-type and knockout BMMs is unaltered. Finally, osteoclastogenesis and periarticular bone erosions are markedly increased in SHIP1(-/-) mice with inflammatory arthritis, a condition characterized by increased M-CSF expression. The SHIP1/Akt pathway therefore suppresses bone loss in pathological states associated with an excess of the cytokine.

Regulating the regulator: negative regulation of Cbl ubiquitin ligases

Cbl proteins are regulators of signal transduction through many pathways and, consequently, regulate cell function and development. They are ubiquitin ligases that ubiquitinate and target many signaling molecules for degradation. The Cbl proteins themselves are regulated by an increasingly complex network of interactions that fine-tune the effects that Cbl proteins have on signaling. The negative regulation of Cbl protein function can occur via cis-acting structural elements that prevent inappropriate ubiquitin ligase activity, degradation of the Cbl proteins, inhibition without degradation owing to interaction with other signaling proteins, deubiquitination of Cbl substrates, and regulation of assembly of the endosomal ESCRT-I complex. Defects in the regulatory mechanisms that control Cbl function are implicated in the development of immunological and malignant diseases.

Distinct role for c-kit receptor tyrosine kinase and SgIGSF adhesion molecule in attachment of mast cells to fibroblasts

Binding of stem cell factor (SCF) to c-kit receptor tyrosine kinase (KIT) transduces signals essential for mast cell development via several pathways including activation of phosphatidylinositol 3-kinase (PI3-K). When cultured mast cells (CMCs) are cocultured with fibroblasts expressing membrane-bound SCF, CMCs with normal KIT adhere to fibroblasts and proliferate, whereas CMCs lacking cell surface expression of KIT do neither. Spermatogenic immunoglobulin superfamily (SgIGSF) was identified as another molecule that participates in mast cell adhesion to fibroblasts. Since the IC-2 mast cell line expressed neither KIT nor SgIGSF, the effect of ectopic expression of KIT or SgIGSF on the adhesion of IC-2 cells was examined. Three forms of KIT with the normal ectodomain were used: wild-type (KIT-WT) and two mutant types with a phenylalanine substitution at the tyrosine residue 719 (KIT-Y719F) or 821 (KIT-Y821F). KIT-Y719F does not activate PI3-K, whereas KIT-Y821F does. Firstly, KIT or SgIGSF was expressed singly in IC-2 cells. All three forms of KIT increased the adhesion level of IC-2 cells, whereas SgIGSF did not. Secondly, SgIGSF was coexpressed with one of the three forms of KIT. Coexpression of SgIGSF with KIT-WT or KIT-Y821F increased the adhesion level more markedly than was achieved by KIT-WT or KIT-Y821F alone. The effect was abolished by an antibody that blocks SCF-KIT interaction. In contrast, coexpression of SgIGSF with KIT-Y719F did not increase the adhesion level induced by KIT-Y719F alone. In adhesion of mast cells to fibroblasts, KIT appeared to behave as an adhesion molecule and as an activator of other adhesion molecules through phosphorylating PI3-K.

Molecular cloning and expression analysis of a macrophage-colony stimulating factor receptor-like gene from rainbow trout, Oncorhynchus mykiss

The M-CSF and its receptor (M-CSFR, CSF-1R or c-fms proto-oncogene) system were initially implicated as essential in mammals for normal monocyte development as well as for pregnancy. To allow a comparison with the M-CSF and M-CSFR system of an oviparous animal, we cloned a M-CSFR-like gene from rainbow trout (Oncorhynchus mykiss). The gene was cloned from a cDNA library of head kidney. It contained an open reading frame encoding 967 amino acids with a predicted size of 109 kDa. The putative amino acid sequence of rainbow trout M-CSFR showed 54% amino acid identity to fugu (Takifugu rubripes) M-CSFR, 52% to zebrafish (Danio rerio) M-CSFR and 40% to mouse (Mus musculus) and human (Homo sapiens) M-CSFR. The M-CSFR-like gene was constitutively expressed in head kidney, kidney, intestine, spleen and blood. The gene was detected especially in the ovary of immature female rainbow trout. These results suggest that a M-CSFR-like receptor may be involved in female reproductive tracts even in an oviparous animal like fish.

Phorbol 12-myristate 13-acetate-induced release of the colony-stimulating factor 1 receptor cytoplasmic domain into the cytosol involves two separate cleavage events

The colony-stimulating factor 1 (CSF-1) receptor is a protein-tyrosine kinase that regulates cell division, differentiation, and development. In response to phorbol 12-myristate 13-acetate (PMA), the CSF-1 receptor is subject to proteolytic processing. Use of chimeric receptors indicates that the CSF-1 receptor is cleaved at least two times, once in the extracellular domain and once in the transmembrane domain. Cleavage in the extracellular domain results in ectodomain shedding while the cytoplasmic domain remains associated with the membrane. Intramembrane cleavage depends on the sequence of the transmembrane domain and results in the release of the cytoplasmic domain. This process can be blocked by gamma-secretase inhibitors. The cytoplasmic domain localizes partially to the nucleus, displays limited stability, and is degraded by the proteosome. CSF-1 receptors are continuously subject to down-modulation and regulated intramembrane proteolysis (RIP). RIP is stimulated by granulocyte-macrophage-CSF, CSF-1, interleukin-2 (IL-2), IL-4, lipopolysaccharide, and PMA and may provide the CSF-1 receptor with an additional mechanism for signal transduction.

Proteomic approaches to the analysis of early events in colony-stimulating factor-1 signal transduction

The exposure of cells to growth factors leads to the rapid tyrosine phosphorylation of proteins that play critical roles in initiating cellular responses. These proteins are associated with other nontyrosine-phosphorylated proteins. Together, they represent less than 0.02% of the total cellular protein. To study their functions in growth factor signaling it is necessary to establish their identity, post-translational modifications, and interactions. We have focused on the characterization of this group of proteins during the early response of macrophages to the macrophage growth factor, colony-stimulating factor-1 (CSF-1). We review here the development of approaches to analysis of the rapid CSF-1-induced changes in the CSF-1 receptor tyrosine kinase and phosphotyrosyl signaling complexes. Recent advances in mass spectrometry technology are greatly facilitating the characterization of such complexes. These methods strongly support and enhance genetic approaches that are being used to analyze the function of individual signaling components and pathways.

The role of the ubiquitination-proteasome pathway in breast cancer: ubiquitin mediated degradation of growth factor receptors in the pathogenesis and treatment of cancer

Aberrant activity of growth factor receptors has been implicated in the pathogenesis of a wide variety of malignancies. The negative regulation of signaling by growth factor receptors is mediated in large part by the ubiquitination, internalization, and degradation of the activated receptor. Over the past few years, considerable insight into the mechanisms that control receptor downregulation has been gained. There are also data suggesting that mutations that lead to inhibition of downregulation of growth factor receptors could play a role in the pathogenesis of cancer. Therapies directed at enhancing the degradation of growth factor receptors offer a promising approach to the treatment of malignancies.

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.

c-Cbl associates directly with the C-terminal tail of the receptor for the macrophage colony-stimulating factor, c-Fms, and down-modulates this receptor but not the viral oncogene v-Fms

The receptor for the macrophage colony-stimulating factor (CSF-1, also termed M-CSF), the tyrosine kinase c-Fms, was originally determined to be the oncogene product of the McDonough strain of feline sarcoma virus, v-Fms. The structural difference between c-Fms and v-Fms amounts to only five point mutations in the extracellular domain, two mutations in the cytoplasmic domain, and the replacement of 50 amino acids by 14 unrelated amino acids at the C-terminal tail. Here, we have identified c-Cbl as the direct binding partner for c-Fms. c-Cbl binds to phosphotyrosine residue 977 at the C-terminal end of feline c-Fms, which is absent in v-Fms. The replacement of the C-terminal end of v-Fms by the corresponding part of c-Fms (vc-Fms) restored the binding potential. As a result, vc-Fms reduced the transforming potency of v-Fms. The overexpression of Cbl did not influence the v-Fms-transformed phenotype, although c-Cbl forms a complex with v-Fms indirectly. In contrast, the expression of Cbl drastically reduced the vc-Fms-transformed phenotype and the activation of Erk and enhanced Fms ubiquitination via phosphotyrosine residue 977. Furthermore, the replacement of tyrosine 977 into phenylalanine in feline c-Fms and vc-Fms reduced the Cbl-dependent ubiquitination. These data suggest that an indirect association of c-Cbl via multimeric complex induced a different signaling pathway from the pathway induced by c-Cbl direct interaction.

C-Cbl binds the CSF-1 receptor at tyrosine 973, a novel phosphorylation site in the receptor's carboxy-terminus

The colony-stimulating factor-1 (CSF-1) receptor is a protein-tyrosine kinase that regulates the proliferation and differentiation of monocyte and macrophage precursors. Binding of CSF-1 to its receptor results in activation of the kinase domain and autophosphorylation on a number of tyrosine residues. Phosphorylated tyrosine residues function as binding sites for SH2 domain-containing signaling proteins. It is known that activated receptors are internalized and degraded, but the mechanics of this process remain largely unknown. Recently, evidence has started to emerge that the ubiquitin-protein ligase c-Cbl is involved in CSF-1 receptor degradation. In addition, there is evidence that the CSF-1 receptor carboxy-terminus is involved in down regulation of the receptor. Here we show that the c-Cbl tyrosine kinase-binding (TKB) domain binds in vitro and in vivo to the CSF-1 receptor. Binding is dependent on the receptor's protein-kinase activity. Deletion of the carboxy-terminus or mutation of Tyr 973 blocks binding. We further provide evidence that the CSF-1 receptor's carboxy-terminus is a substrate for autophosphorylation. Our observations are consistent with a model in which receptor autophosphorylation at Tyr 973 creates a binding site for c-Cbl. Association of c-Cbl with the receptor leads to ubiquitination, followed by receptor degradation.

Proteomic analysis of macrophage differentiation. p46/52(Shc) Tyrosine phosphorylation is required for CSF-1-mediated macrophage differentiation

Macrophage colony stimulating factor (M-CSF or CSF-1) acts to regulate the development and function of cells of the macrophage lineage. Murine myeloid FDC-P1 cells transfected with the CSF-1 receptor (FD/WT) adopt a macrophage-like morphology when cultured in CSF-1. This process is abrogated in FDC-P1 cells transfected with the CSF-1 receptor with a tyrosine to phenyalanine substitution at position 807 (FD/807), suggesting that a molecular interaction critical to differentiation signaling is lost (Bourette, R. P., Myles, G. M., Carlberg, K., Chen, A. R., and Rohrschneider, L. R. (1995) Cell Growth Differ. 6, 631--645). A detailed examination of lysates of CSF-1-treated FD/807 cells by two-dimensional SDS-polyacrylamide gel electrophoresis (PAGE) revealed a number of proteins whose degree of tyrosine phosphorylation was modulated by the Y807F mutation. Included in this category were three phosphorylated proteins that co-migrated with p46/52(Shc). Immunoprecipitation, Western blotting, and in vitro binding studies suggest that they are indeed p46/52(Shc). A key regulator of differentiation in a number of cell systems, ERK was observed to exhibit an activity that correlated with the relative degree of differentiation induced by CSF-1 in the two cell types. Transfection of cells with a non-tyrosine-phosphorylatable form of p46/52(Shc) prevented the normally observed CSF-1-mediated macrophage differentiation as determined by adoption of macrophage-like morphology and expression of the monocyte/macrophage lineage cell surface marker, Mac-1. These results are the first to suggest that p46/52(Shc) may play a role in CSF-1-induced macrophage differentiation. Additionally, a number of proteins were identified by two-dimensional SDS-PAGE whose degree of tyrosine phosphorylation is also modulated by the Y807F substitution. This group of molecules may contain novel signaling molecules important in macrophage differentiation.

Scaffolding protein Gab2 mediates differentiation signaling downstream of Fms receptor tyrosine kinase

Fms is the receptor for macrophage colony-stimulating factor (M-CSF) and contains intrinsic tyrosine kinase activity. Expression of exogenous Fms in a murine myeloid progenitor cell line, FDC-P1 (FD-Fms), results in M-CSF-dependent growth and macrophage differentiation. Previously, we described a 100-kDa protein that was tyrosine phosphorylated upon M-CSF stimulation of FD-Fms cells. In this report, we identify this 100-kDa protein as the recently cloned scaffolding protein Gab2, and we demonstrate that Gab2 associates with several molecules involved in M-CSF signaling, including Grb2, SHP2, the p85 subunit of phosphatidylinositol 3'-kinase, SHIP, and SHC. Tyrosine phosphorylation of Gab2 in response to M-CSF requires the kinase activity of Fms, but not that of Src. Overexpression of Gab2 in FD-Fms cells enhanced both mitogen-activated protein kinase (MAPK) activity and macrophage differentiation, but reduced proliferation, in response to M-CSF. In contrast, a mutant of Gab2 that is unable to bind SHP2 did not potentiate MAPK activity. Furthermore, overexpression of this mutant in FD-Fms cells inhibited macrophage differentiation and resulted in a concomitant increase in growth potential in response to M-CSF. These data indicate that Gab2 is involved in the activation of the MAPK pathway and that the interaction between Gab2 and SHP2 is essential for the differentiation signal triggered by M-CSF.

Colony-stimulating factor-1 receptor utilizes multiple signaling pathways to induce cyclin D2 expression

Colony-stimulating factor-1 (CSF-1) induces expression of immediate early gene, such as c-myc and c-fos and delayed early genes such as D-type cyclins (D1 and D2), whose products play essential roles in the G1 to S phase transition of the cell cycle. Little is known, however, about the cytoplasmic signal transduction pathways that connect the surface CSF-1 receptor to these genes in the nucleus. We have investigated the signaling mechanism of CSF-1-induced D2 expression. Analyses of CSF-1 receptor autophosphorylation mutants show that, although certain individual mutation has a partial inhibitory effect, only multiple combined mutations completely block induction of D2 in response to CSF-1. We report that at least three parallel pathways, the Src pathway, the MAPK/ERK kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway, and the c-myc pathway, are involved. Induction of D2 is partially inhibited in Src(-/-) bone marrow-derived macrophages and by Src inhibitor PP1 and is enhanced in v-Src-overexpressing cells. Activation of myc's transactivating activity selectively induces D2 but not D1. Blockade of c-myc expression partially blocks CSF-1-induced D2 expression. Complete inhibition of the MEK/ERK pathway causes 50% decrease of D2 expression. Finally, simultaneous inhibition of Src, MEK activation, and c-myc expression additively blocks CSF-1-induced D2 expression. This study indicates that multiple signaling pathways are involved in full induction of a single gene, and this finding may also apply broadly to other growth factor-inducible genes.

p56(dok-2) as a cytokine-inducible inhibitor of cell proliferation and signal transduction

p56(dok-2) acts as a multiple docking protein downstream of receptor or non-receptor tyrosine kinases. However, the role of p56(dok-2) in biological functions of cells is not clear. We found that transcription of the p56(dok-2) gene in macrophages was increased markedly in response to cytokines such as macrophage colony-stimulating factor (M-CSF), granulocyte/macrophage-CSF and interleukin-3 (IL-3). Forced expression of p56(dok-2) inhibited M-CSF-, granulocyte-CSF-, IL-3- and stem cell factor-induced proliferation of myeloid leukemia cells, M-NFS-60. The p56(dok-2)-overexpressing cells showed an impaired induction of c-myc but not of c-jun, junB or c-fos when stimulated with M-CSF. Consistent with these results, the peritoneal cavity of the hairless (hr/hr) strain of mutant mice, whose cells expressed less p56(dok-2) than wild-type mice, contained more macrophages than that of +/hr mice. Moreover, the inhibition of endogenous p56(dok-2) expression in macrophage-like tumor cells, J774A.1, by stable expression of antisense p56(dok-2) mRNA accelerated cell proliferation. The study identifies a novel role for p56(dok-2) as a molecule that negatively regulates signal transduction and cell proliferation mediated by cytokines in a feedback loop.

The direct association of the multiple PDZ domain containing proteins (MUPP-1) with the human c-Kit C-terminus is regulated by tyrosine kinase activity

We have identified the multiple PDZ domain containing protein (MUPP-1 or MPDZ) as a novel binding partner of the human c-Kit. c-Kit binds specifically to the 10th PDZ domain of MUPP-1 via its C-terminal sequence. Furthermore, a kinase negative-mutant receptor interacted more strongly with MUPP-1 than the wild-type c-Kit. Strikingly, a constitutively activated c-Kit (D816V-Kit) did not bind to MUPP-1, although this oncogenic form retains the PDZ binding motif 'HDDV' at the C-terminal end. Deletion of V967 of c-Kit abolished binding to MUPP-1 and drastically reduced its tyrosine kinase activity, suggesting that the structure of the C-terminal tail of c-Kit influences its enzymatic activity.

Macrophage colony-stimulating factor promotes cell survival through Akt/protein kinase B

The signaling pathways activated by the macrophage colony-stimulating factor (M-CSF) to promote survival of monocyte and macrophage lineage cells are not well established. In an effort to elucidate these pathways, we have used two cell types responsive to M-CSF: NIH 3T3 fibroblasts genetically engineered to express human M-CSF receptors (3T3-FMS cells) and human monocytes. M-CSF treatment induced M-CSF receptor tyrosine phosphorylation and recruitment of the p85 subunit of phosphatidylinositol 3-kinase (PI3K) to these receptors. These M-CSF receptor events correlated with activation of the serine/threonine kinase Akt. To clarify that PI3K products activate Akt in response to M-CSF, NIH 3T3 fibroblasts expressing mutant human M-CSF receptors (3T3-FMS(Y809F)) that fail to activate Ras in response to M-CSF also exhibit increased Akt kinase activity in response to M-CSF challenge. Furthermore, Akt appears to be the primary regulator of survival in 3T3-FMS cells, as transfection of genes encoding dominant-negative Akt isoforms into these fibroblasts blocked M-CSF-induced survival. In normal human monocytes, M-CSF increased the levels of tyrosine-phosphorylated proteins and induced Akt activation in a PI3K-dependent manner. The PI3K inhibitor LY294002 blocked M-CSF-mediated monocyte survival, an effect that was partially restored by caspase-9 inhibitors. These data suggest that M-CSF may induce cell survival through Akt-induced suppression of caspase-9 activation.

Separation and characterization of the activated pool of colony-stimulating factor 1 receptor forming distinct multimeric complexes with signalling molecules in macrophages

Colony-stimulating factor 1 (CSF-1) triggers the activation of intracellular proteins in macrophages through selective assembly of signalling complexes. The separation of multimeric complexes of the CSF-1 receptor (CSF-1R) by anion-exchange chromatography enabled the enrichment of low-stoichiometry complexes. A significant proportion of the receptor in CSF-1-stimulated cells that neither possessed detectable tyrosine kinase activity nor formed complexes was separated from the receptor pool displaying autokinase activity that formed chromatographically distinct multimeric complexes. A small pool of CSF-1R formed a multimeric complex with phosphatidylinositol-3 kinase (PI-3 kinase), SHP-1, Grb2, Shc, c-Src, Cbl, and a significant number of tyrosine-phosphorylated proteins in CSF-1-stimulated cells. The complex showed a considerable amount of CSF-1R complex-associated kinase activity. A detectable level of the complex was also present in untreated cells. PI-3 kinase in the multimeric complex displayed low lipid kinase activity despite the association with several proteins. The major pool of activated CSF-1R formed transient multimeric complexes with distinctly different tyrosine-phosphorylated proteins, which included STAT3 but also PI-3 kinase, Shc, SHP-1, and Grb2. A significant level of lipid kinase activity was detected in PI-3 kinase in the latter complexes. The different specific enzyme activities of PI-3 kinase in these complexes support the notion that the activity of PI-3 kinase is modulated by its association with CSF-1R and other associated cellular proteins. Specific structural proteins associated with the separate CSF-1R multimeric complexes upon CSF-1 stimulation and the presence of the distinct pools of the CSF-1R were dependent on the integrity of the microtubular network.

c-Src-mediated phosphorylation of the epidermal growth factor receptor on Tyr845 and Tyr1101 is associated with modulation of receptor function

Accumulating evidence indicates that interactions between the epidermal growth factor receptor (EGFR) and the nonreceptor tyrosine kinase c-Src may contribute to an aggressive phenotype in multiple human tumors. Previous work from our laboratory demonstrated that murine fibroblasts which overexpress both these tyrosine kinases display synergistic increases in DNA synthesis, soft agar growth, and tumor formation in nude mice, and increased phosphorylation of the receptor substrates Shc and phospholipase gamma as compared with single overexpressors. These parameters correlated with the ability of c-Src and EGFR to form an EGF-dependent heterocomplex in vivo. Here we provide evidence that association between c-Src and EGFR can occur directly, as shown by receptor overlay experiments, and that it results in the appearance of two novel tyrosine phosphorylations on the receptor that are seen both in vitro and in vivo following EGF stimulation. Edman degradation analyses and co-migration of synthetic peptides with EGFR-derived tryptic phosphopeptides identify these sites as Tyr845 and Tyr1101. Tyr1101 lies within the carboxyl-terminal region of the EGFR among sites of receptor autophosphorylation, while Tyr845 resides in the catalytic domain, in a position analogous to Tyr416 of c-Src. Phosphorylation of Tyr416 and homologous residues in other tyrosine kinase receptors has been shown to be required for or to increase catalytic activity, suggesting that c-Src can influence EGFR activity by mediating phosphorylation of Tyr845. Indeed, EGF-induced phosphorylation of Tyr845 was increased in MDA468 human breast cancer cells engineered to overexpress c-Src as compared with parental MDA 468 cells. Furthermore, transient expression of a Y845F variant EGFR in murine fibroblasts resulted in an ablation of EGF-induced DNA synthesis to nonstimulated levels. Together, these data support the hypothesis that c-Src-mediated phosphorylation of EGFR Tyr845 is involved in regulation of receptor function, as well as in tumor progression.

Expression of c-Myc in response to colony-stimulating factor-1 requires mitogen-activated protein kinase kinase-1

The mitogen-inducible gene c-myc is a key regulator of cell proliferation and transformation. Yet, the signaling pathway(s) that regulate its expression have remained largely unresolved. Using the mitogen-activated protein kinase kinase (MEK1/2) inhibitor PD98059 and dominant negative forms of Ras (N17) and ERK1 (K71R), we found that activation of Ras and extracellular signal-regulated kinase (ERK) is necessary for colony-stimulating factor-1 (CSF-1)-mediated c-Myc expression and DNA synthetic (S) phase entry. Quiescent NIH-3T3 cells expressing a partially defective CSF-1 receptor, CSF-1R (Y809F), exhibited impaired ERK1 activation and c-Myc expression and failed to enter the S phase of the cell division cycle in response to CSF-1 stimulation. Ectopic expression of a constitutively active form of MEK1 in cells expressing CSF-1R (Y809F) rescued c-Myc expression and S phase entry, but only in the presence of CSF-1-induced cooperating signals. Therefore, MEK1 participates in an obligate signaling pathway linking CSF-1R to c-Myc expression, but other signals from CSF-1R must cooperate with the MEK/ERK pathway to induce c-Myc expression and S phase entry in response to CSF-1 stimulation.

Complementation of defective colony-stimulating factor 1 receptor signaling and mitogenesis by Raf and v-Src

Ras-activated signal transduction pathways are implicated in the control of cell proliferation, differentiation, apoptosis, and tumorigenesis, but the molecular mechanisms mediating these diverse functions have yet to be fully elucidated. Conditionally active forms of Raf, v-Src, and MEK1 were used to identify changes in gene expression that participate in oncogenic transformation, as well as in normal growth control. Activation of Raf, v-Src, and MEK1 led to induced expression of c-Myc and cyclin D1. Induction of c-Myc mRNA by Raf was an immediate-early response, whereas the induction of cyclin D1 mRNA was delayed and inhibited by cycloheximide. Raf activation also resulted in the induction of an established c-Myc target gene, ornithine decarboxylase (ODC). ODC induction by Raf was mediated, in part, by tandem E-boxes contained in the first intron of the gene. Activation of the human colony-stimulating factor 1 (CSF-1) receptor in NIH 3T3 cells leads to activation of the mitogen-activated protein (MAP) kinase pathway and induced expression of c-Fos, c-Myc, and cyclin D1, leading to a potent mitogenic response. By contrast, a mutated form of this receptor fails to activate the MAP kinases or induce c-Myc and cyclin D1 expression and fails to elicit a mitogenic response. The biological significance of c-Myc and cyclin D1 induction by Raf and v-Src was confirmed by the demonstration that both of these protein kinases complemented the signaling and mitogenic defects of cells expressing this mutated form of the human CSF-1 receptor. Furthermore, the induction of c-Myc and cyclin D1 by oncogenes and growth factors was inhibited by PD098059, a specific MAP kinase kinase (MEK) inhibitor. These data suggest that the Raf/MEK/MAP kinase pathway plays an important role in the regulation of c-Myc and cyclin D1 expression in NIH 3T3 cells. The ability of oncogenes such as Raf and v-Src to regulate the expression of these proteins reveals new lines of communication between cytosolic signal transducers and the cell cycle machinery.

A rate limiting function of cdc25A for S phase entry inversely correlates with tyrosine dephosphorylation of Cdk2

The cdc25A phosphatase removes inhibitory phosphates from threonine-14 and tyrosine-15 of cyclin dependent kinase-2 (cdk2) in vitro, and it is therefore widely assumed that cdc25A positively regulates cyclin E- and A-associated cdk2 activity at the G1 to S phase transition of the mammalian cell division cycle. Human cdc25A was introduced into mouse NIH3T3 fibroblasts co-expressing a form of the colony-stimulating factor-1 (CSF-1) receptor that is partially defective in transducing mitogenic signals. Cdc25A enabled these cells to form colonies in semisolid medium containing serum plus human recombinant CSF-1 in a manner reminiscent of cells rescued by c-myc. However, cdc25A-rescued cells could not proliferate in chemically defined medium containing CSF-1 and continued to require c-myc function for S phase entry. When contact-inhibited cells overexpressing cdc25A were dispersed and stimulated to synchronously enter the cell division cycle, they entered S phase 2-3 h earlier than their parental untransfected counterparts. Shortening of G1 phase temporally correlated with more rapid degradation of the cdk inhibitor p27Kip1 and with premature activation of cyclin A-dependent cdk2. Paradoxically, tyrosine phosphorylation of cdk2 increased considerably as cells entered S phase, and cdc25A overexpression potentiated rather than diminished this effect. At face value, these results are inconsistent with the hypothesis that cdc25A acts directly on cdk2 to activate its S phase promoting function.

Persistent activation of mitogen-activated protein kinases p42 and p44 and ets-2 phosphorylation in response to colony-stimulating factor 1/c-fms signaling

An antibody that specifically recognized phosphothreonine 72 in ets-2 was used to determine the phosphorylation status of endogenous ets-2 in response to colony-stimulating factor 1 (CSF-1)/c-fms signaling. Phosphorylation of ets-2 was detected in primary macrophages, cells that normally express c-fms, and in fibroblasts engineered to express human c-fms. In the former cells, ets-2 was a CSF-1 immediate-early response gene, and phosphorylated ets-2 was detected after 2 to 4 h, coincident with expression of ets-2 protein. In fibroblasts, ets-2 was constitutively expressed and rapidly became phosphorylated in response to CSF-1. In both cell systems, ets-2 phosphorylation was persistent, with maximal phosphorylation detected 8 to 24 h after CSF-1 stimulation, and was correlated with activation of the CSF-1 target urokinase plasminogen activator (uPA) gene. Kinase assays that used recombinant ets-2 protein as a substrate demonstrated that mitogen-activated protein (MAP) kinases p42 and p44 were constitutively activated in both cell types in response to CSF-1. Immune depletion experiments and the use of the MAP kinase kinase inhibitor PD98059 indicate that these two MAP kinases are the major ets-2 kinases activated in response to CSF-1/c-fms signaling. In the macrophage cell line RAW264, conditional expression of raf kinase induced ets-2 expression and phosphorylation, as well as uPA mRNA expression. Transient assays mapped ets/AP-1 response elements as critical for basal and CSF-1-stimulated uPA reporter gene activity. These results indicate that persistent activation of the raf/MAP kinase pathway by CSF-1 is necessary for both ets-2 expression and posttranslational activation in macrophages.

Parathyroid hormone induces bone resorption in human peripheral blood mononuclear cells

Osteoclasts are known to derive from a macrophage colony-stimulating factor (M-CSF)-dependent precursor shared with macrophages. Cells capable of forming osteoclasts are present in peripheral blood. We characterized this population by incubating human peripheral blood mononuclear cells (PBMCs) with osteoclast-inductive UMR 106 cells, human macrophage colony stimulating factor (hM-CSF) and parathyroid hormone (PTH) or 1.25(OH)2 vitamin D3 on slices of devitalised cortical bone. We found that PBMCs were capable of substantial bone resorption, to levels comparable to those of haemopoietic tissue. Cells plated at very low densities and screened for the presence or absence of excavations revealed a linear relationship (r = 0.994) between the number of cells plated and the number of excavations formed. The limiting dilution analysis suggested that 1 in every 300-600 plated cells (0.15-0.3% of the PBMC population) had the capacity to resorb bone. The precursor was found in the rapidly adherent fraction, and typically generated very small numbers of excavations, suggesting that it was a relatively mature cell type. Co-cultures of PBMCs with UMR 106 cells would not generate osteoclasts without PTH/1.25(OH)2 vitamin D3, even with M-CSF, indicating that osteoclast-induction by stromal cells is not attributable to hormonal induction of M-CSF in UMR 106 cells, but that PTH induces some other activity, necessary for osteoclast but not macrophage formation, in UMR 106 cells. Osteoclasts did not form if PTH was omitted in the first few days of the culture period. Thus, osteoclasts appear to form not from cells committed to macrophage differentiation, but from a discrete subpopulation of relatively mature bipotential or osteoclast-committed precursors which, in the absence of an osteoclast-inductive stimulus, become irreversibly lost to the osteoclast lineage.

The Epstein-Barr virus BARF1 gene encodes a novel, soluble colony-stimulating factor-1 receptor

Epstein-Barr virus (EBV) is a ubiquitous herpesvirus associated with infectious mononucleosis and several tumors. The BARF1 gene is transcribed early after EBV infection from the BamHI A fragment of the EBV genome. Evidence shown here indicates that the BARF1 protein is secreted into the medium of transfected cells and from EBV-carrying B cells induced to allow lytic replication of the virus. Expression cloning identified colony-stimulating factor-1 (CSF-1) as a ligand for BARF1. Computer-assisted analyses indicated that subtle amino acid sequence homology exists between BARF1 and c-fins, the cellular proto-oncogene that is the receptor for CSF-1. Recombinant BARF1 protein was found to be biologically active, and it neutralized the proliferative effects of human CSF-1 in a dose-dependent fashion when assayed in vitro. Since CSF-1 is a pleiotropic cytokine best known for its differentiating effects on macrophages, these data suggest that BARF1 may function to modulate the host immune response to EBV infection.

Polyoma middle T antigen in HL-60 cells accelerates hematopoietic myeloid and monocytic cell differentiation

Expression of the polyoma virus middle T antigen in HL-60 cells accelerates their differentiation in response to both monocytic and granulocytic differentiation-inducing agents. Middle T-expressing cells treated with the granulocytic inducer retinoic acid or the monocytic inducer 1,25-dihydroxy vitamin D3 differentiated 24 h earlier than parental, mock-electroporated, or vector control cell lines. The rapid onset of differentiation correlated with an increase in the cellular level of the middle T protein as well as two known retinoic-acid-inducible markers in HL-60 cells: the paxillin and transglutaminase gene products. The accelerated functional differentiation response and expression of retinoic-acid-inducible markers indicate that middle T played a causal role in differentiation. Thus, expression of the polyoma middle T antigen in HL-60 cells enhanced a variety of molecular changes associated with cellular differentiation.

CSF-1 and interferon-gamma act synergistically to promote differentiation of FDC-P1 cells into macrophages

FDC-P1 cells expressing the wildtype CSF-1 receptor, FDwtfms, differentiate into macrophages during incubation with CSF-1. This response is amplified in the presence of interferon-gamma. Cells expressing the 807F mutant receptor, 807F cells, proliferate in response to CSF-1 and do not differentiate. However, in response to CSF-1 and interferon-gamma they differentiate as well. CSF-1 causes the activation of STAT proteins in FDwtfms cells, but not in 807F cells. Cellular differentiation correlates with a sustained activation of STAT1 and STAT3 in response to interferon-gamma over at least 40 hours. However, interferon-gamma alone did not cause differentiation of cells expressing either receptor. Other defects in response to CSF-1 of the 807F cells, such as lack of PLC gamma 2 activation, were not complemented by co-incubation of the cells with CSF-1 and interferon-gamma. It appears that a combination of signaling pathways are activated by CSF-1 and interferon-gamma which caused the shift of response from proliferation to differentiation in the 807F cells and an enhanced differentiation in the FDwtfms cells.

Complementation of growth factor receptor-dependent mitogenic signaling by a truncated type I phosphatidylinositol 4-phosphate 5-kinase

Substitution of phenylalanine for tyrosine at codon 809 (Y809F) of the human colony-stimulating factor 1 (CSF-1) receptor (CSF-1R) impairs ligand-stimulated tyrosine kinase activity, prevents induction of c-MYC and cyclin D1 genes, and blocks CSF-1-dependent progression through the G1 phase of the cell cycle. We devised an unbiased genetic screen to isolate genes that restore the ability of CSF-1 to stimulate growth in cells that express mutant CSF-1R (Y809F). This screen led us to identify a truncated form of the murine type Ibeta phosphatidylinositol 4-phosphate 5-kinase (mPIP5K-Ibeta). This truncated protein lacks residues 1 to 238 of mPIP5K-Ibeta and is catalytically inactive. When we transfected cells expressing CSF-1R (Y809F) with mPIP5K-Ibeta (delta1-238), CSF-1-dependent induction of c-MYC and cyclin D1 was restored and ligand-dependent cell proliferation was sustained. CSF-1 normally triggers the rapid disappearance of CSF-1R (Y809F) from the cell surface; however, transfection of cells with mPIP5K-Ibeta (delta1-238) stabilized CSF-1R (Y809F) expression on the cell surface, resulting in elevated levels of ligand-activated CSF-1R (Y809F). These results suggest a role for PIP5K-Ibeta in receptor endocytosis and that the truncated enzyme compensated for a mitogenically defective CSF-1R by interfering with this process.

Raf-induced proliferation or cell cycle arrest is determined by the level of Raf activity with arrest mediated by p21Cip1

The Raf family of protein kinases display differences in their abilities to promote the entry of quiescent NIH 3T3 cells into the S phase of the cell cycle. Although conditional activation of deltaA-Raf:ER promoted cell cycle progression, activation of deltaRaf-1:ER and deltaB-Raf:ER elicited a G1 arrest that was not overcome by exogenously added growth factors. Activation of all three deltaRaf:ER kinases led to elevated expression of cyclin D1 and cyclin E and reduced expression of p27Kip1. However, activation of deltaB-Raf:ER and deltaRaf-1:ER induced the expression of p21Cip1, whereas activation of deltaA-Raf:ER did not. A catalytically potentiated form of deltaA-Raf:ER, generated by point mutation, strongly induced p21Cip1 expression and elicited cell cycle arrest similarly to deltaB-Raf:ER and deltaRaf-1:ER. These data suggested that the strength and duration of signaling by Raf kinases might influence the biological outcome of activation of this pathway. By titration of deltaB-Raf:ER activity we demonstrated that low levels of Raf activity led to activation of cyclin D1-cdk4 and cyclin E-cdk2 complexes and to cell cycle progression whereas higher Raf activity elicited cell cycle arrest correlating with p21Cip1 induction and inhibition of cyclin-cdk activity. Using green fluorescent protein-tagged forms of deltaRaf-1:ER in primary mouse embryo fibroblasts (MEFs) we demonstrated that p21Cip1 was induced by Raf in a p53-independent manner, leading to cell cycle arrest. By contrast, activation of Raf in p21Cip1(-/-) MEFs led to a robust mitogenic response that was similar to that observed in response to platelet-derived growth factor. These data indicate that, depending on the level of kinase activity, Raf can elicit either cell cycle progression or cell cycle arrest in mouse fibroblasts. The ability of Raf to elicit cell cycle arrest is strongly associated with its ability to induce the expression of the cyclin-dependent kinase inhibitor p21Cip1 in a manner that bears analogy to alpha-factor arrest in Saccharomyces cerevisiae. These data are consistent with a role for Raf kinases in both proliferation and differentiation of mammalian cells.

Tyrosine phosphorylation sites at amino acids 239 and 240 of Shc are involved in epidermal growth factor-induced mitogenic signaling that is distinct from Ras/mitogen-activated protein kinase activation

Epidermal growth factor (EGF) induces tyrosine phosphorylation of the Shc adapter protein, which plays an important role in EGF-stimulated mitogenesis. Shc stimulates Ras/mitogen-activated protein kinase (MAPK) through forming a complex with Grb2 at the phosphorylated tyrosine (Y) residue 317. In this study, we identified novel phosphorylation sites of Shc, at Y239 and Y240. To define the Shc pathway further, we used NIH 3T3 cells expressing the previously characterized mutant EGF receptor (EGF-R) which lacks all known autophosphorylation sites but retains EGF-stimulated mitogenesis with selective phosphorylation of Shc. We constructed wild-type (WT) or mutant Shc cDNAs in which Y317 or/and Y239 and Y240 are replaced with phenylalanine (F) and introduced them into NIH 3T3 cells expressing WT or mutant EGF-R. In the WT EGF-R-expressing cells, the Y239/240/317F Shc, but not Y317F or Y239/240F Shc, decreased EGF-stimulated cell growth. In the mutant EGF-R-expressing cells, Y317F Shc or Y239/240F Shc decreased EGF-stimulated cell growth significantly, though Y317F was a little more potent than Y239/240F. Although cells expressing the Y317F Shc hardly activated MAPK in response to EGF, cells expressing the Y239/240F Shc fully activated MAPK. In contrast, Y239/240F Shc, but not Y317F Shc, reduced the EGF-induced c-myc message. These results suggest that Shc activates two distinct signaling pathways, Y317 to Ras/MAPK and Y239 and Y240 to another pathway including Myc, and that both are involved in EGF-induced mitogenic signaling.

Regulation of cell cycle entry and G1 progression by CSF-1

Proliferation, differentiation, and survival of monocytes, macrophages, and their immediate progenitors is regulated by the macrophage colony-stimulating factor (CSF-1). CSF-1 initiates a mitogenic response by binding to its receptor (CSF-1R), thereby activating the receptor's intrinsic tyrosine kinase activity and initiating signaling via multiple effector-mediated pathways. CSF-1 is required throughout G1 to ensure entry of bone marrow-derived macrophages into S phase, and persistent CSF-1R kinase activity is necessary to the expression of both immediate early (e.g., c-fos, c-jun, and c-myc) and delayed early (e.g., D-type cyclins) response genes. Ectopic expression of human CSF-1R in different mouse cell lines, including fibroblasts, IL-3-dependent myeloid cells, and early pre-B cells, confers CSF-1 responsiveness by replacing the cells' requirements for other mitogenic growth factors. NIH-3T3 fibroblasts engineered to express a human CSF-1 receptor point mutant (CSF-1R [Y809F]) fail to proliferate in response to CSF-1 and remain arrested in the early G1 phase of the cell cycle. Despite CSF-1-dependent transcription of fos and jun family members, c-myc, D-type, and E-type G1 cyclin mRNAs are not expressed in the latter cells in response to growth factor stimulation. However, enforced expression of c-myc or D-type cyclins, but not cyclin E, resensitizes cells bearing CSF-1R (Y809F) to the mitogenic effects of CSF-1, enabling them to proliferate continuously in liquid culture and to form colonies in agar in response to the growth factor. D-type cyclin mutants defective in binding to the retinoblastoma protein (pRB) were unable to rescue mutant receptor signaling, suggesting that the ability of D-type cyclin-dependent kinases to cancel pRB's growth-suppressive function is necessary for CSF-1-induced G1 exit. By contrast, cyclin E must function in a different pathway. Cells rescued by c-myc were prevented from entering S phase by microinjection of antibodies to cyclin D1. Conversely, cyclin D1-rescued cells were inhibited from forming CSF-1-dependent colonies in agar when challenged with either a dominant-negative c-myc mutant or mad, a transcription factor which competes with myc for max, its requisite heterodimeric partner. Thus, although the expression of c-myc and D-type cyclins is rate limiting for G1 phase progression, their functions are interdependent, with both activities being required for mitogenicity.

Association between phosphatidylinositol-3 kinase, Cbl and other tyrosine phosphorylated proteins in colony-stimulating factor-1-stimulated macrophages

Colony stimulating factor-1 (CSF-1) stimulation of the macrophage cell line BAC1.2F5 and murine bone marrow-derived macrophages resulted in tyrosine phosphorylation of phosphatidylinositol-3 kinase (PI-3 kinase) p85 alpha and its stable association with several tyrosine phosphorylated proteins, including CSF-1 receptor (p165), p120, p95 and p55-p60. p120 co-migrated with the product of the protooncogene c-cb1 in anti-p85 alpha immunoprecipitates, and associated with p85 alpha in a rapid and transient manner. Reciprocal experiments confirmed the presence of p85 alpha in anti-Cb1 immunoprecipitates on CSF-1 stimulation of macrophages. PI-3 kinase immunoprecipitates from the myeloid FDC-P1 cell line expressing mutant CSF-1 receptor (Y721F), which does not associate with PI-3 kinase, still contained Cbl. The identity of the tyrosine phosphorylated protein p95 remains unknown. The interaction between p85 alpha and the tyrosine phosphorylated proteins survived anion-exchange chromatography, suggesting perhaps the presence of a stable complex; furthermore, in CSF-1-treated BAC1.2F5 cell extracts, only one of the two pools of PI-3 kinase separated by chromatography was present in this putative complex. The association did not appear to correlate with proliferation, since a similar interaction between p85 alpha and tyrosine phosphorylated proteins was also observed in poorly proliferating resident peritoneal macrophages stimulated with CSF-1. The possible significance of these findings for CSF-1-regulated macrophage functions is discussed.

A novel pathway from phosphorylation of tyrosine residues 239/240 of Shc, contributing to suppress apoptosis by IL-3

Interleukin 3 (IL-3) not only induces DNA synthesis of haematopoietic cells but also maintains their viability by suppressing apoptosis. IL-3 stimulates tyrosine phosphorylation of the Shc adaptor protein and thereby formation of a complex of Shc with Grb2 at phosphorylated tyrosine (Y) residue 317-Shc. This pathway is implicated in Ras/mitogen-activated protein kinase (MAPK) activation towards c-fos gene expression. We examined the possible involvement of Shc in the antiapoptotic activity of IL-3. Conditional overexpression of the Shc SH2 domain, a dominant-negative mutant of Shc, was found to induce apoptosis of IL-3-dependent Ba/F3 cells along with a reduction of c-myc gene expression. Apoptosis was rescued by the exogenously introduced c-myc gene. Since we identify novel tyrosine phosphorylation sites of Shc: Y239 and Y240, their role on cell survival was tested by mutational analysis. Ba/F3 cells expressing mutant Shc Y317F, which is unable to stimulate efficiently the Ras pathway, still showed resistance to apoptosis. However, cells expressing Shc Y239/240F, which is able to stimulate the Ras pathway, were sensitive to apoptosis. In these cells, induction of the c-myc gene was reduced. These findings suggest that a new signalling pathway for cell survival is generated from Y239/240 of Shc to the nuclei involving c-myc gene expression.

Tyrosine phosphorylation of the juxtamembrane domain of the v-Fms oncogene product is required for its association with a 55-kDa protein

Tyrosine autophosphorylation of the v-Fms oncogene product results in the formation of high affinity binding sites for cellular proteins with Src homology 2 (SH2) domains that are involved in various signal cascades. Tryptic digestion of the autophosphorylated v-Fms and of its cellular counterpart, the feline c-Fms polypeptide, gave rise to at least six common major phosphopeptides, four of which have been characterized previously. Employing site-directed mutagenesis and phosphopeptide mapping of in vitro phosphorylated glutathione S-transferase v-Fms fusion proteins as well as full-length v-Fms molecules expressed in various cells, we show here that Tyr543 of the juxtamembrane domain and Tyr696 of the kinase insert domain constitute major autophosphorylation sites. Recombinant fusion proteins containing the tyrosine-phosphorylated kinase insert domain bind the growth factor receptor bound protein 2 and the p85 and p110 subunits of phosphatidylinositol 3'-kinase. In contrast, fusion proteins containing the juxtamembrane domain phosphorylated on Tyr543 fail to bind any of the known SH2 domain-containing cellular proteins but associate specifically with an as yet undefined 55-kDa cellular protein that by itself is phosphorylated on tyrosine.

Identification of the major autophosphorylation sites of Nyk/Mer, an NCAM-related receptor tyrosine kinase

Nyk/Mer receptor tyrosine kinase is a new member of the Ufo/Axl tyrosine kinase family and is characterized by its neural cell adhesion molecule-like extracellular domain. By using a vaccinia virus expression system to express a constitutively activated form of Nyk, we identified the major sites of Nyk autophosphorylation in tryptic peptide IY749SGDY753Y754R. Tyr-749, Tyr-753, and Tyr-754 in this peptide lie in the activation loop of the kinase domain. We also studied a series of Nyk mutants in which the three tyrosine residues were replaced individually, in pairs, or all together by phenylalanine. Single mutations of Tyr-749 or Tyr-753 to phenylalanine reduced Nyk kinase activity toward exogenous substrate to 39 or 10% of that of the wild type Nyk, respectively, whereas the Tyr-754 mutant is completely inactive. All of the double and triple Tyr-Phe mutants reduced Nyk kinase activity to a level below the background. Similar results were obtained when Nyk autophosphorylation levels were examined. Our studies suggest that full activity of Nyk/Mer kinase requires phosphorylation of all three tyrosine residues in the kinase domain (Tyr-749, Tyr-753, and Tyr-754) and that Nyk kinase activity is modulated by the level of autophosphorylation in the kinase domain. Given the highly conserved nature of this region among the Ufo/Axl receptor family members, the information presented in this report may provide insight to the biochemical properties of other members of this family.

A mutation at tyrosine 1062 in MEN2A-Ret and MEN2B-Ret impairs their transforming activity and association with shc adaptor proteins

Germ line mutations of the ret proto-oncogene are associated with the development of three dominantly inherited neoplastic disorders, multiple endocrine neoplasia (MEN) 2A, MEN 2B, and familial medullary thyroid carcinoma. It has been demonstrated that the mutations result in constitutive activation of the Ret protein, leading to transformation of NIH 3T3 cells. In the present study we investigated the role of tyrosine residues present in the carboxyl-terminal sequence for the transforming activity of Ret with the MEN 2A or MEN 2B mutation (MEN2A-Ret or MEN2B-Ret). Substitution of phenylalanine for tyrosine 1062 (designated Y1062F) markedly impaired the transforming activity of both MEN2A-Ret and MEN2B-Ret, whereas substitution or deletion for four other tyrosines (codons 981, 1015, 1090, and 1096) did not affect their activity. The Shc adaptor proteins bound to the MEN2A-Ret and MEN2B-Ret proteins and were phosphorylated on tyrosine in the transfectants. The binding of Shc to the Y1062F mutant proteins was reduced by approximately 80%, indicating that tyrosine 1062 is a major binding site for Shc. In addition, phosphopeptide analysis of MEN2A-Ret demonstrated that tyrosine 1062 represents an autophosphorylation site of the mutant Ret proteins.

The transactivation potential of a c-Myc N-terminal region (residues 92-143) is regulated by growth factor/Ras signaling

The colony stimulating factor-1 receptor (CSF-1R) affects mitogenic growth and gene expression in NIH 3T3 cells through signaling pathways that require the products of the c-ras and c-myc proto-oncogenes. In this work we tested the hypothesis that there is direct communication between the Ras and Myc pathways. In transient transfection assays Ras increased by 5-fold transcriptional transactivation by chimeric c-Myc-Gal4 proteins. A constitutive active form of the CSF-1R also stimulated this activity and co-expression of a dominant negative ras gene ablated receptor stimulation. Deletion analysis of the c-Myc N-terminal region demonstrated that amino acid residues between positions 92 and 143 are the targets for Ras action. Transactivation by chimeric Myc proteins that were stably expressed could be transiently enhanced by either CSF-1 or serum, with peak activity occurring 2 h after mitogen stimulation. The steady-state levels of the chimeric c-Myc transactivators were increased following stimulation with CSF-1 or serum, but this increase in steady-state protein level did not strictly correlate with the increase in transactivation activity. Thus, Ras signaling may directly affect the activity of the c-Myc N-terminal region.

Activation and association of Stat3 with Src in v-Src-transformed cell lines

STAT proteins are a group of latent cytoplasmic transcription factors which function as signal transducers and activators of transcription. Stat1 and -2 were originally identified to function in interferon signaling, and Stat1 was also found to be activated by epidermal growth factor (EGF) and other cytokines. New members of the STAT gene family are identified. Among them, Stat3 has 52.5% amino acid sequence homology with Stat1 and is activated by platelet-derived growth factor (PDGF), colony-stimulating factor 1 (CSF-1), EGF, interleukin-6, and other cytokines. Treatment of cells with EGF activates Stat1 and Stat3, which become phosphorylated on tyrosine residues to form homo - or heterodimers and translocate into the nucleus, binding to the sis-inducible element (SIE) in the c-fos promoter. Somatic cell genetic analyses demonstrated that Jaks, a family of nontransmembrane protein tyrosine kinases, are required for the activation of Stat1 and Stat2 in interferon-treated cells. However, little is known about the activation of Stat3 by growth factors. Here we report that in all v-Src-transformed cell lines examined, Stat3 is constitutively activated to bind to DNA and the phosphorylation of tyrosine on Stat3 is enhanced by the induction of v-Src expression. We also report that Src is shown to be associated with Stat3 in vivo, as well as in vitro, and phosphorylates Stat3 in vitro. Stat3 is also activated by CSF-1, possibly through CSF-1 receptor-c Src association in NIH 3T3 cells overexpressing CSF-1 receptors. Together, the data suggest that Src is involved in activation of Stat3 in growth factor signal transduction.

Systematic mapping of potential binding sites for Shc and Grb2 SH2 domains on insulin receptor substrate-1 and the receptors for insulin, epidermal growth factor, platelet-derived growth factor, and fibroblast growth factor

Multipin peptide synthesis has been employed to produce biotinylated 11-mer phosphopeptides that account for every tyrosine residue in insulin receptor substrate-1 (IRS-1) and the cytoplasmic domains of the insulin-, epidermal growth factor-, platelet-derived growth factor- and basic fibroblast growth factor receptors. These phosphopeptides have been screened for their capacity to bind to the SH2 domains of Shc and Grb in a solution phase enzyme-linked immunosorbent assay. The data revealed new potential Grb2 binding sites at Tyr-1114 (epidermal growth factor receptor (EGFR) C-tail); Tyr-743 (platelet-derived growth factor receptor (PDGFR) insert region), Tyr-1110 from the E-helix of the catalytic domain of insulin receptor (IR), and Tyr-47, Tyr-939, and Tyr-727 in IRS-1. None of the phosphopeptides from the juxtamembrane or C-tail regions of IR bound Grb2 significantly, and only one phosphopeptide from the basic fibroblast growth factor receptor (Tyr-556) bound Grb2 but with medium strength. Tyr-1068 and -1086 from the C-tail of EGFR, Tyr-684 from the kinase insert region of PDGFR, and Tyr-895 from IRS-1 were confirmed as major binding sites for the Grb2 SH2 domain. With regard to Shc binding, the data revealed new potential binding sites at Tyr-703 and Tyr-789 from the catalytic domain of EGFR and at Tyr-557 in the juxtamembrane region of PDGFR. It also identified new potential Shc binding sites at Tyr-764, in the C-tail of basic fibroblast growth factor receptor, and Tyr-960, in the juxtamembrane of IR, a residue previously known to be required for Shc phosphorylation in response to insulin. The study confirmed the previous identification of Tyr-992 and Tyr-1173 in the C-tail of EGFR and several phosphopeptides from the PDGFR as medium strength binding sites for the SH2 domain of Shc. None of the 34 phosphopeptides from IRS-1 bound Shc strongly, although Tyr-690 showed medium strength binding. The specificity characteristics of the SH2 domains of Grb2 and Shc are discussed. This systematic peptide mapping strategy provides a way of rapidly scanning candidate proteins for potential SH2 binding sites as a first step to establishing their involvement in kinase-mediated signaling pathways.

Tyrosine 807 of the v-Fms oncogene product controls cell morphology and association with p120RasGAP

Expression of the v-fms oncogene of feline sarcoma virus in fibroblasts causes surface exposure of an activated receptor tyrosine kinase, v-Fms, that is autophosphorylated at multiple sites within its cytoplasmic domain. Cellular proteins interacting with this part of v-Fms modulate the mitogenic activity and morphology of the cells. We show here that the tyrosine residue in position 807 (Y-807) of the v-Fms molecule constitutes a major autophosphorylation site. The replacement of this residue by phenylalanine (Y807F mutation) allowed us to functionally dissect v-Fms-specific mitogenic and morphogenic cascades. Cells expressing the mutant v-Fms molecule resembled wild-type (wt) v-Fms-transformed (wt-v-Fms) cells in terms of [3H]thymidine uptake rates and activation of the Ras/Raf-1 mitogenic cascade. Such cells showed, however, a flat morphology and contained intact actin cables and fibronectin network. Our studies indicate that the v-Fms molecule controls cell morphology by a cascade that involves a direct interaction with p120RasGAP and p190RhoGAP: (i) in contrast to wt v-Fms molecules, the Y807F v-Fms protein failed to associate with and phosphorylate p120RasGAP; (ii) tight complexes between p120RasGAP and p190RhoGAP as well as detectable RhoGAP activity were present exclusively in wt-v-Fms cells; and (iii) p190RhoGAP was dispersed throughout the cytoplasm of wt-v-Fms cells, whereas its distribution was restricted to perinuclear regions of cells expressing the mutant v-Fms gene.