Identification of tyrosine 706 in the kinase insert as the major colony-stimulating factor 1 (CSF-1)-stimulated autophosphorylation site in the CSF-1 receptor in a murine macrophage cell line

The First 3D Model of the Full-Length KIT Cytoplasmic Domain Reveals a New Look for an Old Receptor

Receptor tyrosine kinases (RTKs) are key regulators of normal cellular processes and have a critical role in the development and progression of many diseases. RTK ligand-induced stimulation leads to activation of the cytoplasmic kinase domain that controls the intracellular signalling. Although the kinase domain of RTKs has been extensively studied using X-ray analysis, the kinase insert domain (KID) and the C-terminal are partially or fully missing in all reported structures. We communicate the first structural model of the full-length RTK KIT cytoplasmic domain, a crucial target for cancer therapy. This model was achieved by integration of ab initio KID and C-terminal probe models into an X-ray structure, and by their further exploration through molecular dynamics (MD) simulation. An extended (2-µs) MD simulation of the proper model provided insight into the structure and conformational dynamics of the full-length cytoplasmic domain of KIT, which can be exploited in the description of the KIT transduction processes.

Role of the colony-stimulating factor (CSF)/CSF-1 receptor axis in cancer

Cancer cells employ a variety of mechanisms to evade apoptosis and senescence. Pre-eminent among these is the aberrant co-expression of growth factors and their ligands, forming an autocrine growth loop that promotes tumour formation and progression. One growth loop whose transforming potential has been repeatedly demonstrated is the CSF-1/CSF-1R axis. Expression of CSF-1 and/or CSF-1R has been documented in a number of human malignancies, including breast, prostate and ovarian cancer and classical Hodgkin's lymphoma (cHL). This review summarizes the large body of work undertaken to study the role of this cytokine receptor system in malignant transformation. These studies have attributed a key role to the CSF-1/CSF-1R axis in supporting tumour cell survival, proliferation and enhanced motility. Moreover, increasing evidence implicates paracrine interactions between CSF-1 and its receptor in defining a tumour-permissive and immunosuppressive tumour-associated stroma. Against this background, we briefly consider the prospects for therapeutic targeting of this system in malignant disease.

Receptor tyrosine kinase alterations in AML - biology and therapy

Acute myeloid leukemia (AML) is the most common form of leukemia in adults, and despite some recent progress in understanding the biology of the disease, AML remains the leading cause of leukemia-related deaths in adults and children. AML is a complex and heterogeneous disease, often involving multiple genetic defects that promote leukemic transformation and drug resistance. The cooperativity model suggests that an initial genetic event leads to maturational arrest in a myeloid progenitor cell, and subsequent genetic events induce proliferation and block apoptosis. Together, these genetic abnormalities lead to clonal expansion and frank leukemia. The purpose of this chapter is to review the biology of receptor tyrosine kinases (RTKs) in AML, exploring how RTKs are being used as novel prognostic factors and potential therapeutic targets.

Phosphopeptide mapping and identification of phosphorylation sites

Protein phosphorylation is a common modification for many proteins in the cell. Phosphorylation can affect localization of a protein, its stability, and its ability to dimerize or form stable complexes with other molecules. To understand the underlying mechanisms behind the phosphorylation of a given protein, it is often necessary to precisely identify which amino acid residues are phosphorylated. This unit describes the technique of phosphopeptide mapping. In this procedure, a radiolabeled protein is proteolytically digested, and the resulting phosphopeptides are separated in two dimensions on a TLC plate. The phosphopeptides are also analyzed by HPLC and mass spectrometry or peptide microsequencing. Such mapping gives information about the number of phosphorylation sites present in the protein, and can also be used to find out if sites of phosphorylation on a protein change upon treatment of cells with specific agents.

Phosphopeptide mapping and identification of phosphorylation sites

This unit describes a procedure for proteolytic digestion of a (32)P-labeled protein, followed by separation of the digestion products in two dimensions on a TLC plate, which gives rise to a phosphopeptide map. Phosphopeptides present on the TLC plate are visualized by autoradiography. These maps give information about the number of phosphate-containing peptides in the digest, and this is related to the number of phosphorylation sites present in the protein. Phosphopeptide maps can also be used to find out whether the sites of phosphorylation on a protein change upon treatment of cells with certain agents. Procedures are also provided for the identification of the sites of phosphorylation from the phosphopeptide maps. A support protocol details purification of phosphopeptides by HPLC and subsequent analysis by mass spectrometry or peptide microsequencing.

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.

The inositol 5'-phosphatase SHIP-1 and the Src kinase Lyn negatively regulate macrophage colony-stimulating factor-induced Akt activity

Upon encountering macrophage colony-stimulating factor (M-CSF), human monocytes undergo a series of cellular signaling events leading to an increase in Akt activity. However, the regulation of these events is not completely understood. Because the inositol 5'-phosphatase SHIP-1 is an important regulator of intracellular levels of phosphatidylinositol 3,4,5-trisphosphate, an important second messenger necessary for Akt activation, we hypothesized that SHIP-1 was involved in the regulation of M-CSF receptor (M-CSF-R)-induced Akt activation. In the human monocytic cell line, THP-1, SHIP-1 became tyrosine-phosphorylated following M-CSF activation in a Src family kinase-dependent manner. Transfection of 3T3-Fms cells, which express the human M-CSF-R, with wild-type SHIP-1 showed that SHIP-1 was necessary for the negative regulation of M-CSF-induced Akt activation. In THP-1 cells, SHIP-1 bound Lyn, independent of the kinase activity of Lyn, following M-CSF activation. Utilizing a glutathione S-transferase fusion protein, we found that SHIP-1 bound to Lyn via the SHIP-1 Src homology 2 domain. Furthermore, transfection of THP-1 cells with a wild-type SHIP-1 construct reduced NF-kappaB-dependent transcriptional activation of a reporter gene, whereas a SHIP-1 Src homology 2 domain construct resulted in an increase in NF-kappaB activation. Additionally, in 3T3-Fms cells, Lyn enhanced the ability of SHIP-1 to regulate Akt activation by stabilizing SHIP-1 at the cellular membrane. Finally, macrophages isolated from both SHIP-1- and Lyn-deficient mice exhibited enhanced Akt phosphorylation following M-CSF stimulation. These data provide the first evidence of the involvement of both SHIP-1 and Lyn in the negative regulation of M-CSF-R-induced Akt activation.

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.

Expression of a Y559F mutant CSF-1 receptor in M1 myeloid cells: a role for Src kinases in CSF-1 receptor-mediated differentiation

We have established two M1 myeloid cell lines, M1/WT cells overexpressing the wild-type CSF-1 receptor and M1/Y559F cells expressing a specific tyrosine mutant. M1/WT cells differentiated in response to CSF-1, with a reduction in their proliferative capacity. CSF-1-mediated differentiation was partially abrogated in the M1/Y559F cells, with a less marked reduction in proliferative capacity. The Src tyrosine kinases c-Src, c-Yes, c-Fyn, and c-Hck were tyrosine phosphorylated in the M1/WT cells in response to CSF-1 and bound to the WT CSF-1R through their SH2 domains. Binding of the Src kinases to the CSF-1 receptor was greatly reduced in the M1/Y559F cells. CSF-1-mediated activation of STAT3 was also abrogated in the M1/Y559F cell line. Treatment of M1/WT cells with the Src family inhibitor PP2 resulted in an inhibition of CSF-1-mediated differentiation, equivalent to that observed in the M1/Y559F cells. These data suggest that the reduced Src binding observed in the M1/Y559F cells may contribute to their reduced ability to differentiate.

The Croonian Lecture 1997. The phosphorylation of proteins on tyrosine: its role in cell growth and disease

The reversible phosphorylation of tyrosines in proteins plays a key role in regulating many different processes in eukaryotic organisms, such as growth control, cell cycle control, differentiation cell shape and movement, gene transcription, synaptic transmission, and insulin action. Phosphorylation of proteins is brought about by enzymes called protein-tyrosine kinases that add phosphate to specific tyrosines in target proteins; phosphate is removed from phosphorylated tyrosines by enzymes called protein-tyrosine phosphatases. Phosphorylated tyrosines are recognized by specialized binding domains on other proteins, and such interactions are used to initiate intracellular signaling pathways. Currently, more than 95 protein-tyrosine kinases and more than 55 protein-tyrosine phosphatase genes are known in Homo sapiens. Aberrant tyrosine phosphorylation is a hallmark of many types of cancer and other human diseases. Drugs are being developed that antagonize the responsible protein-tyrosine kinases and phosphatases in order to combat these diseases.

cAMP enhances CSF-1-induced ERK activity and c-fos mRNA expression via a MEK-dependent and Ras-independent mechanism in macrophages

Inhibition of MAPK by elevated intracellular cAMP has often been correlated with suppression of growth factor-induced proliferation. However, in murine bone marrow-derived macrophages (BMM) we show that the cAMP analogue, 8-bromo cAMP (8BrcAMP) (1mM), despite being a dramatic G1 phase proliferation inhibitor, increased ERK activity both in the absence and presence of CSF-1; these increases were blocked by PD98059 (100 microM) suggesting MEK dependence. In contrast, CSF-1-stimulated p21Ras activity was blocked by 8BrcAMP thus correlating with the inhibition of proliferation. This is the first report to indicate that elevated intracellular cAMP can activate ERK activity while inhibiting proliferation and the data support the concept in CSF-1-treated macrophages of Ras-independent activation of ERK activity. It was also found that the acute but not the sustained elevation of c-fos mRNA expression due to 8BrcAMP was also MEK dependent indicating that there are separate pathways controlling c-fos mRNA expression in BMM.

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.

Monocyte colony-stimulating factor stimulates binding of phosphatidylinositol 3-kinase to Grb2.Sos complexes in human monocytes

Monocyte colony-stimulating factor (M-CSF) is required for the proliferation of mononuclear phagocytes. The activated M-CSF receptor associates with phosphatidylinositol 3-kinase (PI 3-kinase). In the present studies, we demonstrate that M-CSF also induces direct interaction of PI 3-kinase (p85 alpha subunit) with the SH2/SH3 adaptor protein Grb2. Tyrosine-phosphorylated PI 3-kinase interacts with the SH2 domain of Grb2. A pYRNE (pY408) site in PI 3-kinase is potentially involved in this interaction. The results also demonstrate that the PI 3-kinase.Grb2 complex associates with the guanine nucleotide exchange protein Sos. Since Sos binds to the SH3 domains of Grb2 and thereby associates with Ras at the cell membrane, formation of the PI 3-kinase.Grb2.Sos complex provides a potential mechanism for growth factor-induced interactions of PI 3-kinase and Ras.

Influence of tyrosine residues Y705 and Y807 on the transforming potency of the v-fms oncogene product of feline sarcoma virus

Cell transformation is characterized by overt changes in growth control and cell morphology. To study the role of tyrosine residues Y705 and Y807 of v-Fms of the McDonough strain of feline sarcoma virus in cell transformation we replaced them individually with phenylalanine residues. Cells expressing the mutant genes showed mitogenic properties similar to wild-type v-Fms transformed cells. However, the morphology of cells expressing the Y807F mutant remained the same as nontransformed cells. Four phosphoproteins of 190, 120, 55 and 50 kDa were detected in cells expressing the wild-type but were absent in cells expressing the mutant Y807F-v-fms gene.

Monitoring protein kinase and phosphatase reactions with matrix-assisted laser desorption/ionization mass spectrometry and capillary zone electrophoresis: comparison of the detection efficiency of peptide-phosphopeptide mixtures

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) and capillary zone electrophoresis (CZE) were evaluated for monitoring protein phosphatase and kinase reactions in vitro. Varying concentrations of peptide C (YIHLEKKYVRRDSG), peptide S (YLIEDNEYTARQGA) and kemptide (LARRSALG) mixed with their corresponding phosphorylated peptides, pC, pS and pkemptide, were analyzed. Comparison between the two techniques indicated that MALDI MS was less quantitative than CZE, showing a bias towards detection of the unphosphorylated peptide S and kemptide. In terms of sensitivity, the MALDI MS and CZE techniques are comparable. Protein kinase A phosphorylation of kemptide was monitored with both MALDI MS and CZE, whereas alkaline phosphatase dephosphorylation of pC could only be monitored with MALDI MS. The absence of inhibition with phosphatase or kinase buffers is a significant advantage of MALDI MS. In contrast to CZE, the MALDI spectra allow identification of the species analyzed by virtue of their mass. The results obtained emphasize the advantage of monitoring enzymatic reactions in buffer solutions using MALDI MS compared with CZE.

Phosphopeptide mapping and phosphoamino acid analysis by electrophoresis and chromatography on thin-layer cellulose plates

Identification of protein phosphorylation sites is essential in order to evaluate the contribution of individual sites to the regulation of a particular protein by phosphorylation. Here we review a method we have developed for the identification of phosphorylation sites based on digestion of 32P-labeled proteins with site-specific proteases and separation of the digestion products in two dimensions on thin-layer cellulose plates using electrophoresis in the first dimension followed by chromatography. This method is very sensitive, requiring only a few hundred 32P-disintegrations per minute to obtain reproducible phosphopeptide maps. We also report methods for the analysis of the phosphoamino acid content of both intact phosphoproteins and individual phosphopeptides recovered from two-dimensional separations, in which the material is subjected to partial acid hydrolysis, and the hydrolysis products are separated on thin-layer cellulose plates by electrophoresis in one or two dimensions. Finally, we describe methods for analyzing the structure of isolated phosphopeptides by secondary digestion with site-specific proteases, by manual Edman degradation, and by immunoprecipitation, and indicate how this information can be used in conjunction with the two-dimensional mobility of the peptide to deduce the identity of a phosphopeptide from the known sequence of a protein.

Cellular signalling events stimulated by myeloid haemopoietic growth factors

In haemopoietic cells, proliferation, commitment to development, lineage restriction and survival via suppression of apoptosis can all be controlled by haemopoietic growth factors. The mechanisms underlying the regulation of these events can now be studied since recombinant forms of most of these haemopoietic growth factors are now available. Recent advances in cell purification techniques and the development of multipotent cell lines (see Spangrude et al, 1988; Whetton, 1990; Heyworth et al, 1988, 1990a; Jones et al, 1990) have provided suitable cell populations on which to study the cellular signalling events associated with differentiation and lineage restriction. This process has started with the elucidation of the structure and expression of many of the myeloid growth factor receptors, which should now facilitate progress in the study of the signal transduction mechanisms these growth factors employ. Another important facet of these studies will be to determine whether a single growth factor with multiple target cell types, ranging from multipotent cells to postmitotic cells (e.g. neutrophils), employs distinct signalling mechanisms depending on the target cell in question. The cellular signalling events elicited by each of these growth factors and the ways in which they can regulate the transcriptional activation of genes associated with specific developmental events are going to be key areas of haemopoietic research in the next few years.

The kinase insert domain of colony stimulating factor-1 receptor is dispensable for CSF-1 induced phosphatidylcholine hydrolysis

Mouse NIH 3T3 fibroblasts transfected with human colony stimulating factor-1 receptor produced diacylglycerol in response to CSF1 and this correlated with elevated phosphatidylcholine hydrolyzing activity measured in an in vitro assay. Treatment of cells with the isoflavone derivative genistein attenuated PC hydrolysis in vitro suggesting a role for CSF1R tyrosine kinase activity. A CSF1R mutant lacking 67 amino acids of the kinase insert domain, which may affect the association of receptor with certain substrates, stimulated PC hydrolysis in response to CSF1. Coupling to PC hydrolysis is likely a general property of CSF1R and the kinase insert domain is dispensable for this activity.

Regulation of leukocyte migration by activation of the leukocyte adhesion molecule-1 (LAM-1) selectin

A central feature of host defence is the ability of leukocytes to enter tissues in response to immune or inflammatory stimuli. The leukocyte adhesion molecule-1 (LAM-1) regulates the migration of human leukocytes by mediating the binding both of lymphocytes to high endothelial venules of peripheral lymph nodes and of neutrophils to endothelium at inflammatory sites. As lymphocytes and neutrophils express the same LAM-1 protein, it is not clear how lineage-specific differences in leukocyte migration are controlled. We now report that the affinity of LAM-1 for a carbohydrate-based ligand, PPME, is dramatically increased following lymphocyte and neutrophil activation by lineage-specific stimuli. In addition, activation of lymphocytes by physiological stimuli enhanced LAM-1-dependent binding to high endothelial venules. Thus, transient changes in LAM-1 affinity after leukocyte stimulation probably directly influence leukocyte migration.

Structure and function of the protein tyrosine kinases

The protein tyrosine kinases (PTKs) are a large and structurally diverse family of enzymes. The conserved catalytic domain held in common by each member of this family is a self-contained 250-300 amino acid unit bearing sixteen highly conserved linear sequence elements, several of which have been shown to be important to the catalytic activity of this domain. The enzymic activity of the PTKs is clearly an evolutionarily successful theme, and at least 10 distinct morphotypes have been described. Many of these resemble cell surface receptors for growth factors, and for a small sub-set of these receptors a ligand has been discovered. The remainder are located intracellularly and presumably sense and respond to appropriate metabolic cues by exerting their physiologically powerful enzymic activity. A detailed examination of the structure/function relationships of the PTKs and their catalytic domains is particularly revealing in trying to establish the roles that these proteins play in signal transduction in eukaryotic cells.