Novel tyrosine kinase identified by phosphotyrosine antibody screening of cDNA libraries

Oncogenic FGFR Fusions Produce Centrosome and Cilia Defects by Ectopic Signaling

A single primary cilium projects from most vertebrate cells to guide cell fate decisions. A growing list of signaling molecules is found to function through cilia and control ciliogenesis, including the fibroblast growth factor receptors (FGFR). Aberrant FGFR activity produces abnormal cilia with deregulated signaling, which contributes to pathogenesis of the FGFR-mediated genetic disorders. FGFR lesions are also found in cancer, raising a possibility of cilia involvement in the neoplastic transformation and tumor progression. Here, we focus on FGFR gene fusions, and discuss the possible mechanisms by which they function as oncogenic drivers. We show that a substantial portion of the FGFR fusion partners are proteins associated with the centrosome cycle, including organization of the mitotic spindle and ciliogenesis. The functions of centrosome proteins are often lost with the gene fusion, leading to haploinsufficiency that induces cilia loss and deregulated cell division. We speculate that this complements the ectopic FGFR activity and drives the FGFR fusion cancers.

Fibroblast Growth Factor Receptors (FGFRs) and Noncanonical Partners in Cancer Signaling

Increasing evidence indicates that success of targeted therapies in the treatment of cancer is context-dependent and is influenced by a complex crosstalk between signaling pathways and between cell types in the tumor. The Fibroblast Growth Factor (FGF)/FGF receptor (FGFR) signaling axis highlights the importance of such context-dependent signaling in cancer. Aberrant FGFR signaling has been characterized in almost all cancer types, most commonly non-small cell lung cancer (NSCLC), breast cancer, glioblastoma, prostate cancer and gastrointestinal cancer. This occurs primarily through amplification and over-expression of FGFR1 and FGFR2 resulting in ligand-independent activation. Mutations and translocations of FGFR1-4 are also identified in cancer. Canonical FGF-FGFR signaling is tightly regulated by ligand-receptor combinations as well as direct interactions with the FGFR coreceptors heparan sulfate proteoglycans (HSPGs) and Klotho. Noncanonical FGFR signaling partners have been implicated in differential regulation of FGFR signaling. FGFR directly interacts with cell adhesion molecules (CAMs) and extracellular matrix (ECM) proteins, contributing to invasive and migratory properties of cancer cells, whereas interactions with other receptor tyrosine kinases (RTKs) regulate angiogenic, resistance to therapy, and metastatic potential of cancer cells. The diversity in FGFR signaling partners supports a role for FGFR signaling in cancer, independent of genetic aberration.

dDYRK2: a novel dual-specificity tyrosine-phosphorylation-regulated kinase in Drosophila

Dual-specificity tyrosine-phosphorylation-regulated kinases (DYRKs) are an emerging family of protein kinases that have been identified in all eukaryotic organisms examined to date. DYRK family members are involved in regulating key developmental and cellular processes such as neurogenesis, cell proliferation, cytokinesis and cellular differentiation. Two distinct subgroups exist, nuclear and cytosolic. In Drosophila, the founding family member minibrain, whose human orthologue maps to the Down syndrome critical region, belongs to the nuclear subclass and affects post-embryonic neurogenesis. In the present paper, we report the isolation of dDYRK2, a cytosolic DYRK and the putative product of the smell-impaired smi35A gene. This is the second such kinase described in Drosophila, but the first to be characterized at the molecular and biochemical level. dDYRK2 is an 81 kDa dual-specificity kinase that autophosphorylates on tyrosine and serine/threonine residues, but appears to phosphorylate exogenous substrates only on serine/threonine residues. It contains a YXY motif in the activation loop of the kinase domain in the same location as the TXY motif in mitogen-activated protein kinases. dDYRK2 is tyrosine-phosphorylated in vivo, and mutational analysis reveals that the activation loop tyrosines are phosphorylated and are essential for kinase activity. Finally, dDYRK2 is active at all stages of fly development, with elevated levels observed during embryogenesis and pupation.

Kinase inhibitors in cancer therapy: a look ahead

The most essential kinases involved in cell membrane receptor activation, signal transduction and cell cycle control or programmed cell death and their interconnections are reviewed. In tumours, the genes of many of those kinases are mutated or amplified or the proteins are overexpressed. The use of key kinases offers the possibility to screen in vitro for synthetic small molecule kinase inhibitors. In view of the many interconnections of cellular kinases, their role in preventing or inducing programmed cell death and the possibility that a considerable number of signal transducing proteins are still unknown, cellular test systems are recommended in which the respective key kinase or one of its main partner molecules are overexpressed.

Tyrosine phosphorylation in Myxococcus xanthus, a multicellular prokaryote

Tyrosine phosphorylation is an extremely rare event in prokaryotes, occurring almost exclusively in multicellular eukaryotes. We have identified, for the first time, by the use of antiphosphotyrosine monoclonal antibody and Western blot (immunoblot) analysis, two tyrosine-phosphorylated membrane proteins in the multicellular prokaryote Myxococcus xanthus. The pattern of tyrosine phosphorylation was shown to change during development, indicating a possible role for this regulatory modification during two stages of development, i.e., aggregation and sporulation. Furthermore, the altered pattern of tyrosine phosphorylation observed in a variety of signaling mutants was shown to differ from that observed in the wild type, suggesting further the possible involvement of tyrosine phosphorylation during the development program.

Expression of fibroblast growth factor 1 is lower in breast cancer than in the normal human breast

We have measured the amount of fibroblast growth factor 1 (FGF-1) mRNA and protein in primary breast cancers and non-malignant breast tissue and have found greatly reduced levels in breast cancer compared with non-malignant tissue. A total of 116 breast cancers and 37 biopsies taken from non-malignant breast were compared for FGF-1 mRNA expression using reverse transcriptase-polymerase chain reaction (RT-PCR) and significantly lower levels were found in the cancer tissues (P < 0.001). These findings were confirmed at the protein level where four out of five breast cancers contained no detectable FGF-1 and a fifth cancer had a low level of FGF-1 compared with three samples from reduction mammoplasties. Similar results were obtained from breast cell lines in which 80% of cancer cell lines had very low levels of FGF-1, whereas all non-malignant breast cell lines contained higher levels of FGF-1. Immunohistochemical analysis indicated that FGF-1 was present in the luminal epithelial cells of the non-malignant breast but was absent from cancer cells. The decreased levels of FGF-1 in breast cancer may indicate that stimulation of cancer cells is resulting in down-regulation of FGF-1 expression or may implicate FGF-1 as a differentiation factor rather than a growth factor at its physiological concentration in the breast.

bFGF and aFGF induce membrane ruffling in breast cancer cells but not in normal breast epithelial cells: FGFR-4 involvement

Acidic and basic fibroblast growth factors (aFGF and bFGF) are growth factors which may have a physiological role in the normal breast and in breast cancer. A study of the effects of aFGF and bFGF on a variety of breast cell lines and epithelial cells purified from normal breast organoids showed that whereas normal breast cells did not exhibit membrane ruffling in response to either of these growth factors, some breast cancer cell lines did. This difference was not due to lack of receptor since all the cell lines tested were mitogenically stimulated by bFGF. Dominant negative mutations of FGF receptor 3 (FGFR-3) and the small GTP-binding protein p21rac inhibited membrane ruffling, showing that receptor dimerization and phosphorylation and p21rac activation are prerequisites for membrane ruffling in response to aFGF and bFGF. Transient transfection of individual FGFRs into cos-7 cells showed that FGFR-1, FGFR-2 and FGFR-3 could not mediate a membrane ruffling response whereas FGFR-4 could. These studies elucidate one signalling mechanism of FGF and point to differences in the response of normal and cancer breast epithelial cells which may be important in cell motility.

Prokaryotic expression cloning of a novel human tyrosine kinase

Screening of a human embryonic lung fibroblast cDNA expression library with antiphosphotyrosine antibodies led to isolation of a novel protein kinase. A clone, designated A6, contained a 3-kb cDNA insert with a predicted open reading frame of 350 amino acids. DNA sequence analysis failed to reveal any detectable similarity with previously known genes, and the predicted A6 protein lacked any of the motifs commonly conserved in the catalytic domains of protein kinases. However, the bacterially expressed beta-galactosidase-A6 fusion protein demonstrated both tyrosine and serine phosphorylation in an in vitro kinase assay and phosphorylated exogenous substrates including myelin basic protein specifically on tyrosine residues. The enzyme also displayed biochemical properties analogous to those of other protein tyrosine kinases. The A6 gene was found to be expressed widely at the transcript level in normal tissues and was evolutionarily conserved. Thus, A6 represents a novel tyrosine kinase which is highly divergent from previously described members of this important class of regulatory molecules.

Prokaryotic expression cloning of a novel human tyrosine kinase

Screening of a human embryonic lung fibroblast cDNA expression library with antiphosphotyrosine antibodies led to isolation of a novel protein kinase. A clone, designated A6, contained a 3-kb cDNA insert with a predicted open reading frame of 350 amino acids. DNA sequence analysis failed to reveal any detectable similarity with previously known genes, and the predicted A6 protein lacked any of the motifs commonly conserved in the catalytic domains of protein kinases. However, the bacterially expressed beta-galactosidase-A6 fusion protein demonstrated both tyrosine and serine phosphorylation in an in vitro kinase assay and phosphorylated exogenous substrates including myelin basic protein specifically on tyrosine residues. The enzyme also displayed biochemical properties analogous to those of other protein tyrosine kinases. The A6 gene was found to be expressed widely at the transcript level in normal tissues and was evolutionarily conserved. Thus, A6 represents a novel tyrosine kinase which is highly divergent from previously described members of this important class of regulatory molecules.

Cleavage of K-FGF produces a truncated molecule with increased biological activity and receptor binding affinity

The K-FGF/HST (FGF-4) growth factor is a member of the FGF family which is efficiently secreted and contains a single N-linked glycosylation signal. To study the role of glycosylation in the secretion of K-FGF, we mutated the human K-fgf cDNA to eliminate the glycosylation signal and the mutated cDNA was cloned into a mammalian expression vector. Studies of immunoprecipitation from the conditioned medium of cells expressing this plasmid revealed that the lack of glycosylation did not impair secretion, however the unglycosylated protein was immediately cleaved into two NH2-terminally truncated peptides of 13 and 15 kD, which appeared to be more biologically active than the wild-type protein. These two proteins also showed higher heparin binding affinity than that of wt K-FGF. We have expressed in bacteria the larger of these two proteins (K140), in which the NH2-terminal 36 amino acids present in the mature form of K-FGF have been deleted. Mitogenicity assays on several cell lines showed that purified recombinant K140 had approximately five times higher biological activity than wild-type recombinant K-FGF. Studies of receptor binding showed that K140 had higher affinity than wt K-FGF for two of the four members of FGF receptor's family, specifically for FGFR-1 (flg) and FGFR-2 (bek). K140 also had increased heparin binding ability, but this property does not appear to be responsible for the increased affinity for FGF receptors. Thus removal of the NH2-terminal 36 amino acids from the mature K-FGF produces growth factor molecules with an altered conformation, resulting in higher heparin affinity, and more efficient binding to FGF receptors. Although it is not clear whether cleavage of K-FGF to generate K140 occurs in vivo, this could represent a novel mechanism of modulation of growth factor activity.

RYK, a receptor tyrosine kinase-related molecule with unusual kinase domain motifs

By using the polymerase chain reaction with degenerate oligonucleotides based on highly conserved motifs held in common between all members of the protein tyrosine kinase (PTK) family, a PTK-related sequence was isolated from murine peritoneal macrophage cDNA. Full-length clones have been isolated that encompass the entire coding region of the mRNA, and the predicted amino acid sequence indicates that the protein encoded has the structure of a growth factor receptor PTK (RTK). We have dubbed this molecule RYK (for related to tyrosine kinase). The RYK-encoded protein bears a transmembrane domain, with a relatively small (183 amino acid) extracellular domain, containing five potential N-linked glycosylation sites. The intracellular domain of RYK is unique among the broader family of RTKs and has several unusual sequence idiosyncrasies in some of the most highly conserved elements of the PTK domain. These sequence differences call into question the potential catalytic activity of the RYK protein.

Expression cloning of a human dual-specificity phosphatase

Using an expression cloning strategy, we isolated a cDNA encoding a human protein-tyrosine-phosphatase. Bacteria expressing the kinase domain of the keratinocyte growth factor receptor (bek/fibroblast growth factor receptor 2) were infected with a fibroblast cDNA library in a phagemid prokaryotic expression vector and screened with a monoclonal anti-phosphotyrosine antibody. Among several clones showing decreased anti-phosphotyrosine recognition, one displayed phosphatase activity toward the kinase in vitro. The 4.1-kilobase cDNA encoded a deduced protein of 185 amino acids with limited sequence similarity to the vaccinia virus phosphatase VH1. The purified recombinant protein dephosphorylated several activated growth factor receptors, as well as serine-phosphorylated casein, in vitro. Both serine and tyrosine phosphatase activities were completely abolished by mutagenesis of a single cysteine residue conserved in VH1 and the VH1-related (VHR) human protein. These properties suggest that VHR is capable of regulating intracellular events mediated by both tyrosine and serine phosphorylation.

Expression of basic fibroblast growth factor, FGFR1 and FGFR2 in normal and malignant human breast, and comparison with other normal tissues

The expression of basic fibroblast growth factor (bFGF) and two of its receptors, FGFR1 and FGFR2, was detected using the polymerase chain reaction, and quantified by comparison to the relative amount of product obtained following co-amplification of the ubiquitous glyceraldehyde phosphate dehydrogenase transcript. Varying levels were found in the vast majority of both cancer and non-malignant breast biopsies as well as in samples of several other normal human tissues. Significantly less bFGF was present in cancers (P less than 0.0001). Similarly, FGFR2 product was also much less in cancer tissues (P = 0.0078), as was FGFR1 (P = 0.002). FGFR1 levels in cancers tended to be higher in those which were oestrogen receptor positive (P less than 0.06). Amplification of different coding regions showed evidence of variant forms of FGFR1 RNA. Cancers appeared to have a significantly greater proportion of PCR product corresponding to the region between the third immunoglobulin like domain and the tyrosine kinase domain (P = 0.046). Differential expression was observed in breast cell lines, with bFGF in the normal derived HBL100, HBR SV1.6.1 and 184A1 but little or none in ZR-75-1, MCF-7, T47D and MDA-MB-231. FGFR1 was present in most of these but FGFR2 was absent from T47D, MDA-MB-231 and HBL100. ZR-75-1 cells had a marked preponderance of FGFR1 variants lacking part of the coding sequence. Aberrant receptor processing may provide clues concerning the role of FGF's and their potential involvement in malignancy.

Multiple cDNAs encoding the esk kinase predict transmembrane and intracellular enzyme isoforms

A novel protein kinase, the Esk kinase, has been isolated from an embryonal carcinoma (EC) cell line by using an expression cloning strategy. Sequence analysis of two independent cDNA clones (2.97 and 2.85 kb) suggested the presence of two Esk isoforms in EC cells. The esk-1 cDNA sequence predicted an 857-amino-acid protein kinase with a putative membrane-spanning domain, while the esk-2 cDNA predicted an 831-amino-acid kinase which lacked this domain. In adult mouse cells, esk mRNA levels were highest in tissues possessing a high proliferation rate or a sizeable stem cell compartment, suggesting that the Esk kinase may play some role in the control of cell proliferation or differentiation. As anticipated from the screening procedure, bacterial expression of the Esk kinase reacted with antiphosphotyrosine antibodies on immunoblots. Furthermore, in in vitro kinase assays, the Esk kinase was shown to phosphorylate both itself and the exogenous substrate myelin basic protein on serine, threonine, and tyrosine residues, confirming that the Esk kinase is a novel member of the serine/threonine/tyrosine family of protein kinases.

Multiple cDNAs encoding the esk kinase predict transmembrane and intracellular enzyme isoforms

A novel protein kinase, the Esk kinase, has been isolated from an embryonal carcinoma (EC) cell line by using an expression cloning strategy. Sequence analysis of two independent cDNA clones (2.97 and 2.85 kb) suggested the presence of two Esk isoforms in EC cells. The esk-1 cDNA sequence predicted an 857-amino-acid protein kinase with a putative membrane-spanning domain, while the esk-2 cDNA predicted an 831-amino-acid kinase which lacked this domain. In adult mouse cells, esk mRNA levels were highest in tissues possessing a high proliferation rate or a sizeable stem cell compartment, suggesting that the Esk kinase may play some role in the control of cell proliferation or differentiation. As anticipated from the screening procedure, bacterial expression of the Esk kinase reacted with antiphosphotyrosine antibodies on immunoblots. Furthermore, in in vitro kinase assays, the Esk kinase was shown to phosphorylate both itself and the exogenous substrate myelin basic protein on serine, threonine, and tyrosine residues, confirming that the Esk kinase is a novel member of the serine/threonine/tyrosine family of protein kinases.

Characterization of the murine BEK fibroblast growth factor (FGF) receptor: activation by three members of the FGF family and requirement for heparin

The bek gene encodes a member of the high-affinity fibroblast growth factor receptor family. The BEK/FGFR-2 receptor is a membrane-spanning tyrosine kinase with the typical features of FGF receptors. We have cloned a murine bek cDNA and expressed it in receptor-negative Chinese hamster ovary cells and in 32D myeloid cells. The BEK receptor expressed in Chinese hamster ovary cells binds acidic FGF, basic FGF, and Kaposi FGF equally well but does not bind keratinocyte growth factor or FGF-5 appreciably. Upon treatment with basic FGF or Kaposi FGF, the BEK receptor is phosphorylated and a mitogenic response is achieved. Heparan sulfate proteoglycans have been shown to play an obligate role in basic FGF binding to the high-affinity FLG receptor. Unlike the BEK-expressing Chinese hamster ovary cells, 32D cells expressing the BEK receptor require the addition of exogenous heparin in order to grow in the presence of basic FGF or Kaposi FGF. We show that the addition of heparin greatly enhances the binding of radio-labeled basic FGF to the receptor. Thus the BEK receptor, like FLG, also requires an interaction with heparan sulfate proteoglycans to facilitate binding to its ligands.

K-sam gene encodes secreted as well as transmembrane receptor tyrosine kinase

K-sam was first identified as a gene amplified in the stomach cancer cell line KATO-III. The size of the major transcript of the K-sam gene was 3.5 kilobases in KATO-III cells, and we have previously shown that K-sam encodes a receptor tyrosine kinase that belongs to the heparin-binding growth factor receptor, or fibroblast growth factor receptor, gene family. The K-sam gene expresses multiple sizes of mRNAs in brain tissue, the immature teratoma cell line NCC-IT, and KATO-III. RNA blot analyses with a variety of K-sam probes indicate that there are at least four classes of K-sam mRNAs. Three types of K-sam cDNAs in addition to the previously reported type of K-sam cDNA were isolated, and their nucleotide sequences encode a full-length transmembrane receptor, a secreted receptor with a tyrosine kinase domain, and a secreted receptor without a tyrosine kinase domain.

Determination of ligand-binding specificity by alternative splicing: two distinct growth factor receptors encoded by a single gene

Expression cDNA cloning and structural analysis of the human keratinocyte growth factor receptor (KGFR) revealed identity with one of the fibroblast growth factor (FGF) receptors encoded by the bek gene (FGFR-2), except for a divergent stretch of 49 amino acids in their extracellular domains. Binding assays demonstrated that the KGFR was a high-affinity receptor for both KGF and acidic FGF, while FGFR-2 showed high affinity for basic and acidic FGF but no detectable binding by KGF. Genomic analysis of the bek gene revealed two alternative exons responsible for the region of divergence between the two receptors. The KGFR transcript was specific to epithelial cells, and it appeared to be differentially regulated with respect to the alternative FGFR-2 transcript. Thus, two growth factor receptors with different ligand-binding specificities and expression patterns are encoded by alternative transcripts of the same gene.

Aberrant elevation of tyrosine-specific phosphorylation in human gastric cancer cells

Phosphotyrosine-containing proteins in various human cancer cell lines were studied by immunoblotting with anti-phosphotyrosine antibody. Of 29 cell lines derived from oral epidermoid cancer, esophageal cancer, gastric cancer, colon cancer, pancreatic cancer, hepatocellular carcinoma and malignant melanoma, 3 of the 6 gastric cancer cells showed aberrant elevation of tyrosine-specific phosphorylation. On the other hand, both esophageal cancer cells and colon cancer cells, which were reported to have amplified epidermal growth factor receptor and activated p60v-src kinase, respectively, showed no apparent elevation of tyrosine-specific phosphorylation, and their profiles of phosphorylation were similar to that of normal human fibroblasts. Two gastric cancer cells, NUGC-4 and MKN-45, showed similar profiles of phosphorylation but their responses to growth factors differed from each other. Tyrosine phosphorylation in NUGC-4 was strongly activated by treatment with epidermal growth factor and quickly reduced by the acid treatment which is effective in removing growth factors from cellular surface receptors. On the contrary, phosphorylation in MKN-45 did not respond to either growth factor or acid treatment. These results suggest that NUGC-4 and MKN-45 have tyrosine kinases which are activated by different mechanisms but share similar substrates.

Two additional protein-tyrosine kinases expressed in human lung: fourth member of the fibroblast growth factor receptor family and an intracellular protein-tyrosine kinase

The expression of protein-tyrosine kinases (PTKs; ATP:protein-tyrosine O-phosphotransferase, EC 2.7.1.112) was studied in normal human lung and various tumors by PCR followed by molecular cloning and sequence analysis. Six known PTKs (YES, FGR, LYN, HCK, PDGFB-R, and CSF1-R), as well as two additional members of this enzyme family, were detected in lung. One of the newly discovered sequences appears to represent a group of cytosolic PTKs. The cDNA sequence of the second unknown PTK revealed that it is a fourth member of the fibroblast growth factor receptor family. It was therefore called TKF (tyrosine kinase related to fibroblast growth factor receptor). Among a wide variety of cells and tissues tested, including human lymphocytes and macrophages, TKF was only found expressed in lung. Apart from normal lung, TKF expression could be demonstrated in some tumors of lung origin, but also in malignancies not derived from lung tissues. As fibroblast growth factors are generally involved in a variety of functions such as mitogenesis, angiogenesis, and wound healing, the specific expression of a receptor-related gene in lung only may point to yet another special function of this group of proteins.

axl, a transforming gene isolated from primary human myeloid leukemia cells, encodes a novel receptor tyrosine kinase

Using a sensitive transfection-tumorigenicity assay, we have isolated a novel transforming gene from the DNA of two patients with chronic myelogenous leukemia. Sequence analysis indicates that the product of this gene, axl, is a receptor tyrosine kinase. Overexpression of axl cDNA in NIH 3T3 cells induces neoplastic transformation with the concomitant appearance of a 140-kDa axl tyrosine-phosphorylated protein. Expression of axl cDNA in the baculovirus system results in the expression of the appropriate recombinant protein that is recognized by antiphosphotyrosine antibodies, confirming that the axl protein is a tyrosine kinase. The juxtaposition of fibronectin type III and immunoglobulinlike repeats in the extracellular domain, as well as distinct amino acid sequences in the kinase domain, indicate that the axl protein represents a novel subclass of receptor tyrosine kinases.

A tyrosine-phosphorylated carboxy-terminal peptide of the fibroblast growth factor receptor (Flg) is a binding site for the SH2 domain of phospholipase C-gamma 1

Phospholipase C-gamma (PLC-gamma) is a substrate of the fibroblast growth factor receptor (FGFR; encoded by the flg gene) and other receptors with tyrosine kinase activity. It has been demonstrated that the src homology region 2 (SH2 domain) of PLC-gamma and of other signalling molecules such as GTPase-activating protein and phosphatidylinositol 3-kinase-associated p85 direct their binding toward tyrosine-autophosphorylated regions of the epidermal growth factor or platelet-derived growth factor receptor. In this report, we describe the identification of Tyr-766 as an autophosphorylation site of flg-encoded FGFR by direct sequencing of a tyrosine-phosphorylated tryptic peptide isolated from the cytoplasmic domain of FGFR expressed in Escherichia coli. The same phosphopeptide was found in wild-type FGFR phosphorylated either in vitro or in living cells. Like other growth factor receptors, tyrosine-phosphorylated wild-type FGFR or its cytoplasmic domain becomes associated with intact PLC-gamma or with a fusion protein containing the SH2 domain of PLC-gamma. To delineate the site of association, we have examined the capacity of a 28-amino-acid tryptic peptide containing phosphorylated Tyr-766 to bind to various constructs containing SH2 and other domains of PLC-gamma. It is demonstrated that the tyrosine-phosphorylated peptide binds specifically to the SH2 domain but not to the SH3 domain or other regions of PLC-gamma. Hence, Tyr-766 and its flanking sequences represent a major binding site in FGFR for PLC-gamma. Alignment of the amino acid sequences surrounding Tyr-766 with corresponding regions of other FGFRs revealed conserved tyrosine residues in all known members of the FGFR family. We propose that homologous tyrosine-phosphorylated regions in other FGFRs also function as binding sites for PLC-gamma and therefore are involved in coupling to phosphatidylinositol breakdown.

axl, a transforming gene isolated from primary human myeloid leukemia cells, encodes a novel receptor tyrosine kinase

Using a sensitive transfection-tumorigenicity assay, we have isolated a novel transforming gene from the DNA of two patients with chronic myelogenous leukemia. Sequence analysis indicates that the product of this gene, axl, is a receptor tyrosine kinase. Overexpression of axl cDNA in NIH 3T3 cells induces neoplastic transformation with the concomitant appearance of a 140-kDa axl tyrosine-phosphorylated protein. Expression of axl cDNA in the baculovirus system results in the expression of the appropriate recombinant protein that is recognized by antiphosphotyrosine antibodies, confirming that the axl protein is a tyrosine kinase. The juxtaposition of fibronectin type III and immunoglobulinlike repeats in the extracellular domain, as well as distinct amino acid sequences in the kinase domain, indicate that the axl protein represents a novel subclass of receptor tyrosine kinases.

A tyrosine-phosphorylated carboxy-terminal peptide of the fibroblast growth factor receptor (Flg) is a binding site for the SH2 domain of phospholipase C-gamma 1

Phospholipase C-gamma (PLC-gamma) is a substrate of the fibroblast growth factor receptor (FGFR; encoded by the flg gene) and other receptors with tyrosine kinase activity. It has been demonstrated that the src homology region 2 (SH2 domain) of PLC-gamma and of other signalling molecules such as GTPase-activating protein and phosphatidylinositol 3-kinase-associated p85 direct their binding toward tyrosine-autophosphorylated regions of the epidermal growth factor or platelet-derived growth factor receptor. In this report, we describe the identification of Tyr-766 as an autophosphorylation site of flg-encoded FGFR by direct sequencing of a tyrosine-phosphorylated tryptic peptide isolated from the cytoplasmic domain of FGFR expressed in Escherichia coli. The same phosphopeptide was found in wild-type FGFR phosphorylated either in vitro or in living cells. Like other growth factor receptors, tyrosine-phosphorylated wild-type FGFR or its cytoplasmic domain becomes associated with intact PLC-gamma or with a fusion protein containing the SH2 domain of PLC-gamma. To delineate the site of association, we have examined the capacity of a 28-amino-acid tryptic peptide containing phosphorylated Tyr-766 to bind to various constructs containing SH2 and other domains of PLC-gamma. It is demonstrated that the tyrosine-phosphorylated peptide binds specifically to the SH2 domain but not to the SH3 domain or other regions of PLC-gamma. Hence, Tyr-766 and its flanking sequences represent a major binding site in FGFR for PLC-gamma. Alignment of the amino acid sequences surrounding Tyr-766 with corresponding regions of other FGFRs revealed conserved tyrosine residues in all known members of the FGFR family. We propose that homologous tyrosine-phosphorylated regions in other FGFRs also function as binding sites for PLC-gamma and therefore are involved in coupling to phosphatidylinositol breakdown.

cDNA cloning and developmental expression of fibroblast growth factor receptors from Xenopus laevis

The heparin-binding growth factors constitute a family of homologous polypeptides including basic and acidic fibroblast growth factors (FGFs). These factors participate in a variety of processes, including wound healing, angiogenesis, neuronal survival, and inductive events in the early amphibian embryo. We have isolated three closely related species of cDNA clones for Xenopus FGF receptors. One of these, designated XFGFR-A1, encodes an open reading frame of 814 amino acids. A second class encodes an identical amino acid sequence with the exception of an 88-amino-acid deletion near the 5' end. This species probably arises through alternative splicing. A third class of cDNA corresponding to the shorter form of XFGFR-A1 was isolated and shown to be 95% homologous and is designated XFGFR-A2. Xenopus FGF receptors are similar to FGF receptors from other species in that they contain a transmembrane domain, a tyrosine kinase domain split by a 14-amino-acid insertion, and a unique conserved stretch of eight acidic residues in the extracellular domain. Overexpression of Xenopus FGF receptor protein by transfection of COS1 cells with the corresponding cDNA in a transient expression vector leads to the appearance of new FGF binding sites on transfected cells, consistent with these cDNAs encoding for FGF receptors. RNA gel blot analysis demonstrates that Xenopus FGF receptor mRNA is a maternal message and is expressed throughout early development. When blastula-stage ectoderm is cultured in control amphibian salt solutions, Xenopus FGF receptor mRNA declines to undetectable levels by late neurula stages. However, when cultured in the presence of FGF of XTC mesoderm-inducing factor, Xenopus FGF receptor RNA expression is maintained.

cDNA cloning and developmental expression of fibroblast growth factor receptors from Xenopus laevis

The heparin-binding growth factors constitute a family of homologous polypeptides including basic and acidic fibroblast growth factors (FGFs). These factors participate in a variety of processes, including wound healing, angiogenesis, neuronal survival, and inductive events in the early amphibian embryo. We have isolated three closely related species of cDNA clones for Xenopus FGF receptors. One of these, designated XFGFR-A1, encodes an open reading frame of 814 amino acids. A second class encodes an identical amino acid sequence with the exception of an 88-amino-acid deletion near the 5' end. This species probably arises through alternative splicing. A third class of cDNA corresponding to the shorter form of XFGFR-A1 was isolated and shown to be 95% homologous and is designated XFGFR-A2. Xenopus FGF receptors are similar to FGF receptors from other species in that they contain a transmembrane domain, a tyrosine kinase domain split by a 14-amino-acid insertion, and a unique conserved stretch of eight acidic residues in the extracellular domain. Overexpression of Xenopus FGF receptor protein by transfection of COS1 cells with the corresponding cDNA in a transient expression vector leads to the appearance of new FGF binding sites on transfected cells, consistent with these cDNAs encoding for FGF receptors. RNA gel blot analysis demonstrates that Xenopus FGF receptor mRNA is a maternal message and is expressed throughout early development. When blastula-stage ectoderm is cultured in control amphibian salt solutions, Xenopus FGF receptor mRNA declines to undetectable levels by late neurula stages. However, when cultured in the presence of FGF of XTC mesoderm-inducing factor, Xenopus FGF receptor RNA expression is maintained.

Identification of a fibroblast growth factor-binding protein in Drosophila melanogaster

As assessed by competitive binding and protein-crosslinking experiments, Drosophila melanogaster cells possess basic fibroblast growth factor (bFGF)-specific binding proteins that are similar to FGF receptors on vertebrate cells in molecular weight and binding affinity; these D. melanogaster cells, however, have no detectable binding proteins for acidic fibroblast growth factor (aFGF). Consistent with the presence of bFGF-specific binding proteins, D. melanogaster cells degrade bFGF but not aFGF. These results indicate the conservation of heparin-binding growth factors and receptors between vertebrates and D. melanogaster.

Spk1, a new kinase from Saccharomyces cerevisiae, phosphorylates proteins on serine, threonine, and tyrosine

A Saccharomyces cerevisiae lambda gt11 library was screened with antiphosphotyrosine antibodies in an attempt to identify a gene encoding a tyrosine kinase. A subclone derived from one positive phage was sequenced and found to contain an 821-amino-acid open reading frame that encodes a protein with homology to protein kinases. We tested the activity of the putative kinase by constructing a vector encoding a glutathione-S-transferase fusion protein containing most of the predicted polypeptide. The fusion protein phosphorylated endogenous substrates and enolase primarily on serine and threonine. The gene was designated SPK1 for serine-protein kinase. Expression of the Spk1 fusion protein in bacteria stimulated serine, threonine, and tyrosine phosphorylation of bacterial proteins. These results, combined with the antiphosphotyrosine immunoreactivity induced by the kinase, indicate that Spk1 is capable of phosphorylating tyrosine as well as phosphorylating serine and threonine. In in vitro assays, the fusion protein kinase phosphorylated the synthetic substrate poly(Glu/Tyr) on tyrosine, but the activity was weak compared with serine and threonine phosphorylation of other substrates. To determine if other serine/threonine kinases would phosphorylate poly(Glu/Tyr), we tested calcium/calmodulin-dependent protein kinase II and the catalytic subunit of cyclic AMP-dependent protein kinase. The two kinases had similar tyrosine-phosphorylating activities. These results establish that the functional difference between serine/threonine- and tyrosine-protein kinases is not absolute and suggest that there may be physiological circumstances in which tyrosine phosphorylation is mediated by serine/threonine kinases.

Isolation of an additional member of the fibroblast growth factor receptor family, FGFR-3

The fibroblast growth factors are a family of polypeptide growth factors involved in a variety of activities including mitogenesis, angiogenesis, and wound healing. Fibroblast growth factor receptors (FGFRs) have previously been identified in chicken, mouse, and human and have been shown to contain an extracellular domain with either two or three immunoglobulin-like domains, a transmembrane domain, and a cytoplasmic tyrosine kinase domain. We have isolated a human cDNA for another tyrosine kinase receptor that is highly homologous to the previously described FGFR. Expression of this receptor cDNA in COS cells directs the expression of a 125-kDa glycoprotein. We demonstrate that this cDNA encodes a biologically active receptor by showing that human acidic and basic fibroblast growth factors activate this receptor as measured by 45Ca2+ efflux assays. These data establish the existence of an additional member of the FGFR family that we have named FGFR-3.

STY, a tyrosine-phosphorylating enzyme with sequence homology to serine/threonine kinases

We have cloned a novel kinase (STY) from an embryonal carcinoma cell line. Sequence analysis of the STY cDNA reveals that it shares sequence homology with serine/threonine-type kinases and yet the bacterial expression product of the STY cDNA appears to have serine-, threonine-, and tyrosine-phosphorylating activities. The predicted STY protein is highly basic and contains a putative nuclear localization signal. During differentiation, two new mRNAs were detected in addition to the embryonic transcript.

Spk1, a new kinase from Saccharomyces cerevisiae, phosphorylates proteins on serine, threonine, and tyrosine

A Saccharomyces cerevisiae lambda gt11 library was screened with antiphosphotyrosine antibodies in an attempt to identify a gene encoding a tyrosine kinase. A subclone derived from one positive phage was sequenced and found to contain an 821-amino-acid open reading frame that encodes a protein with homology to protein kinases. We tested the activity of the putative kinase by constructing a vector encoding a glutathione-S-transferase fusion protein containing most of the predicted polypeptide. The fusion protein phosphorylated endogenous substrates and enolase primarily on serine and threonine. The gene was designated SPK1 for serine-protein kinase. Expression of the Spk1 fusion protein in bacteria stimulated serine, threonine, and tyrosine phosphorylation of bacterial proteins. These results, combined with the antiphosphotyrosine immunoreactivity induced by the kinase, indicate that Spk1 is capable of phosphorylating tyrosine as well as phosphorylating serine and threonine. In in vitro assays, the fusion protein kinase phosphorylated the synthetic substrate poly(Glu/Tyr) on tyrosine, but the activity was weak compared with serine and threonine phosphorylation of other substrates. To determine if other serine/threonine kinases would phosphorylate poly(Glu/Tyr), we tested calcium/calmodulin-dependent protein kinase II and the catalytic subunit of cyclic AMP-dependent protein kinase. The two kinases had similar tyrosine-phosphorylating activities. These results establish that the functional difference between serine/threonine- and tyrosine-protein kinases is not absolute and suggest that there may be physiological circumstances in which tyrosine phosphorylation is mediated by serine/threonine kinases.

cDNA cloning and characterization of eck, an epithelial cell receptor protein-tyrosine kinase in the eph/elk family of protein kinases

A human epithelial (HeLa) cDNA library was screened with degenerate oligonucleotides designed to hybridize to highly conserved regions of protein-tyrosine kinases. One cDNA from this screen was shown to contain a putative protein-tyrosine kinase catalytic domain and subsequently used to isolate another cDNA from a human keratinocyte library that encompasses the entire coding region of a 976-amino-acid polypeptide. The predicted protein has an external domain of 534 amino acids with a presumptive N-terminal signal peptide, a transmembrane domain, and a cytoplasmic domain of 418 amino acids that includes a canonical protein-tyrosine kinase catalytic domain. Molecular phylogeny indicates that this protein kinase is closely related to eph and elk and that this receptor family is more closely related to the non-receptor protein-tyrosine kinase families than to other receptor protein-tyrosine kinases. Antibodies raised against a TrpE fusion protein immunoprecipitated a 130-kDa protein that became phosphorylated on tyrosine in immune complex kinase assays, indicating that this protein is a bona fide protein-tyrosine kinase. Analysis of RNA from 13 adult rat organs showed that the eck gene is expressed most highly in tissues that contain a high proportion of epithelial cells, e.g., skin, intestine, lung, and ovary. Several cell lines of epithelial origin were found to express the eck protein kinase at the protein and RNA levels. Immunohistochemical analysis of several rat organs also showed staining in epithelial cells. These observations prompted us to name this protein kinase eck, for epithelial cell kinase.

STY, a tyrosine-phosphorylating enzyme with sequence homology to serine/threonine kinases

We have cloned a novel kinase (STY) from an embryonal carcinoma cell line. Sequence analysis of the STY cDNA reveals that it shares sequence homology with serine/threonine-type kinases and yet the bacterial expression product of the STY cDNA appears to have serine-, threonine-, and tyrosine-phosphorylating activities. The predicted STY protein is highly basic and contains a putative nuclear localization signal. During differentiation, two new mRNAs were detected in addition to the embryonic transcript.

cDNA cloning and characterization of eck, an epithelial cell receptor protein-tyrosine kinase in the eph/elk family of protein kinases

A human epithelial (HeLa) cDNA library was screened with degenerate oligonucleotides designed to hybridize to highly conserved regions of protein-tyrosine kinases. One cDNA from this screen was shown to contain a putative protein-tyrosine kinase catalytic domain and subsequently used to isolate another cDNA from a human keratinocyte library that encompasses the entire coding region of a 976-amino-acid polypeptide. The predicted protein has an external domain of 534 amino acids with a presumptive N-terminal signal peptide, a transmembrane domain, and a cytoplasmic domain of 418 amino acids that includes a canonical protein-tyrosine kinase catalytic domain. Molecular phylogeny indicates that this protein kinase is closely related to eph and elk and that this receptor family is more closely related to the non-receptor protein-tyrosine kinase families than to other receptor protein-tyrosine kinases. Antibodies raised against a TrpE fusion protein immunoprecipitated a 130-kDa protein that became phosphorylated on tyrosine in immune complex kinase assays, indicating that this protein is a bona fide protein-tyrosine kinase. Analysis of RNA from 13 adult rat organs showed that the eck gene is expressed most highly in tissues that contain a high proportion of epithelial cells, e.g., skin, intestine, lung, and ovary. Several cell lines of epithelial origin were found to express the eck protein kinase at the protein and RNA levels. Immunohistochemical analysis of several rat organs also showed staining in epithelial cells. These observations prompted us to name this protein kinase eck, for epithelial cell kinase.

Autocrine regulation of cell growth and transformation by basic fibroblast growth factor

Basic FGF (bFGF) and acidic FGF (aFGF) are multipotential factors that stimulate and support proliferation, migration and differentiation. Both bFGF and aFGF are non-secreted growth factors consistent with the lack of a signal peptide. However, bFGF and aFGF are deposited in extracellular matrix (ECM) suggesting that an alternative mechanism for FGF release exists. Four oncogenes, int-2, hst/K-fgf, FGF-5 and FGF-6 have been isolated that are highly homologous to aFGF and bFGF. Unlike bFGF and aFGF, they possess signal peptides and are secreted. These oncogenes transform cells and induce tumors, ostensibly via an autocrine mechanism. The involvement of bFGF and aFGF in autocrine transformation has been clarified by studies using FGF cDNA transfection. NIH-3T3 cells transfected with native bFGF cDNA and expressing 20 to 100 times as much bFGF as parental 3T3 cells acquire an enhanced proliferation rate and higher saturation density. NIH cells transfected with a construct in which bFGF cDNA is altered by addition of a signal peptide, undergo autocrine transformation and exhibit morphological and biochemical alterations characteristic of highly transformed cells. Injection of cells expressing native bFGF even at levels 100 times greater than parental 3T3 cells fails to induce tumors or lung metastasis in syngeneic mice. Signal peptide bFGF-transected cells on the other hand, acquire a high tumorigenic and metastatic potential with tumor incidence and numbers comparable to those induced by ras transformed cells. Acquisition of a signal peptide converts bFGF into a transforming protein analogous to FGF-related oncogenes which naturally have signal peptide sequences.

Putative tyrosine kinases expressed in K-562 human leukemia cells

Tyrosine phosphorylation is important in the transmission of growth and differentiation signals; known tyrosine kinases include several oncoproteins and growth factor receptors. Interestingly, some differentiated cell types, such as erythrocytes and platelets contain high amounts of phosphotyrosine. We analyzed tyrosine kinases expressed in the K-562 chronic myelogenous leukemia cell line, which has a bipotential erythroid and megakaryoblastoid differentiation capacity. Analysis of 359 polymerase chain reaction-amplified cDNA clones led to the identification of 14 different tyrosine kinase-related sequences (JTK1-14). Two of the clones (JTK2 and JTK4) represent unusual members of the fibroblast growth factor receptor gene family, and the clones JTK5, JTK11, and JTK14 may also belong to the family of receptor tyrosine kinases but lack a close relationship to any known tyrosine kinase. Each of these different genes has its own characteristic expression pattern in K-562 cells and several other human tumor cell lines. In addition, the JTK11 and JTK14 mRNAs are induced during the megakaryoblastoid differentiation of K-562 cells. These tyrosine kinases may have a role in the differentiation of megakaryoblasts or in the physiology of platelets.

Related fibroblast growth factor receptor genes exist in the human genome

We have isolated, from a human tumor cDNA library, a gene encoding a putative receptor-like protein-tyrosine kinase that we call TK14. The amino acid sequence of the TK14 protein is closely related to the available partial sequence of the mouse protein bek, and more distantly related to the sequences of a chicken basic fibroblast growth factor receptor (73% sequence homology) and the apparent human equivalent of this receptor, the FLG protein (encoded by the fms-like tyrosine kinase gene). Overexpression of the TK14 protein by transfection of COS-1 cells with the corresponding cDNA in a simian virus 40-based expression vector leads to the appearance of new cell-surface binding sites for both acidic and basic fibroblast growth factors. This has been demonstrated by specific binding assays and chemical cross-linking experiments using 125I-labeled growth factors. It appears, therefore, that the human genome contains at least two distinct genes, for TK14 and FLG, that code for related fibroblast growth factor receptors.

Characterization and cDNA cloning of phospholipase C-gamma, a major substrate for heparin-binding growth factor 1 (acidic fibroblast growth factor)-activated tyrosine kinase

Heparin-binding growth factors (HBGFs) bind to high-affinity cell surface receptors which possess intrinsic tyrosine kinase activity. A Mr 150,000 protein phosphorylated on tyrosine in response to class 1 HBGF (HBGF-1) was purified and partially sequenced. On the basis of this sequence, cDNA clones were isolated from a human endothelial cell library and identified as encoding phospholipase C-gamma. Phosphorylation of phospholipase C-gamma in intact cells treated with HBGF-1 was directly demonstrated by using antiphospholipase C-gamma antibodies. Thus, HBGF-1 joins epidermal growth factor and platelet-derived growth factor, whose receptor activation leads to tyrosine phosphorylation and probable activation of phospholipase C-gamma.

Cloning and expression of two distinct high-affinity receptors cross-reacting with acidic and basic fibroblast growth factors

The fibroblast growth factor (FGF) family consists of at least seven closely related polypeptide mitogens which exert their activities by binding and activation of specific cell surface receptors. Unanswered questions have been whether there are multiple FGF receptors and what factors determine binding specificity and biological response. We report the complete cDNA cloning of two human genes previously designated flg and bek. These genes encode two similar but distinct cell surface receptors comprised of an extracellular domain with three immunoglobulin-like regions, a single transmembrane domain, and a cytoplasmic portion containing a tyrosine kinase domain with a typical kinase insert. The expression of these two cDNAs in transfected NIH 3T3 cells led to the biosynthesis of proteins of 150 kd and 135 kd for flg and bek, respectively. Direct binding experiments with radiolabeled acidic FGF (aFGF) or basic FGF (bFGF), inhibition of binding with native growth factors, and Scatchard analysis of the binding data indicated that bek and flg bind either aFGF or bFGF with dissociation constants of (2-15) x 10(-11) M. The high affinity binding of two distinct growth factors to each of two different receptors represents a unique double redundancy without precedence among polypeptide growth factor-receptor interactions.

Characterization and cDNA cloning of phospholipase C-gamma, a major substrate for heparin-binding growth factor 1 (acidic fibroblast growth factor)-activated tyrosine kinase

Heparin-binding growth factors (HBGFs) bind to high-affinity cell surface receptors which possess intrinsic tyrosine kinase activity. A Mr 150,000 protein phosphorylated on tyrosine in response to class 1 HBGF (HBGF-1) was purified and partially sequenced. On the basis of this sequence, cDNA clones were isolated from a human endothelial cell library and identified as encoding phospholipase C-gamma. Phosphorylation of phospholipase C-gamma in intact cells treated with HBGF-1 was directly demonstrated by using antiphospholipase C-gamma antibodies. Thus, HBGF-1 joins epidermal growth factor and platelet-derived growth factor, whose receptor activation leads to tyrosine phosphorylation and probable activation of phospholipase C-gamma.

K-sam, an amplified gene in stomach cancer, is a member of the heparin-binding growth factor receptor genes

DNA fragments amplified in a stomach cancer-derived cell line, KATO-III, were previously identified by the in-gel DNA renaturation method, and a 0.2-kilobase-pair fragment of the amplified sequence was subsequently cloned. By genomic walking, a portion of the exon of the gene flanking this 0.2-kilobase-pair fragment was cloned, and the gene was designated as K-sam (KATO-III cell-derived stomach cancer amplified gene). The K-sam cDNAs, corresponding to the 3.5-kilobase K-sam mRNA, were cloned from the KATO-III cells. Sequence analysis revealed that this gene coded for 682 amino acid residues that satisfied the characteristics of the receptor tyrosine kinase. The K-sam gene had significant homologies with bek, FLG, and chicken basic fibroblast growth factor receptor gene. The K-sam gene was amplified in KATO-III cells with the major transcript of 3.5-kilobases in size. This gene was also expressed in some other stomach cancer cells, a small cell lung cancer, and germ cell tumors.

A distinctive family of embryonic protein-tyrosine kinase receptors

Two closely related protein-tyrosine kinases with the characteristics of growth factor receptors were identified by screening a chicken embryo cDNA expression library with anti-phosphotyrosine antibodies and were designated Cek2 and Cek3 (chicken embryo kinases 2 and 3). Cek2 and Cek3 are structurally related to Cek1, a chicken basic fibroblast growth factor receptor, and presumably represent receptors for basic fibroblast growth factor-related molecules. The identification of Cek2 and Cek3 establishes the existence of a family of protein-tyrosine kinases that includes Cek1 and that is likely to be implicated in the control of developmental processes. Among protein-tyrosine kinases, this family of receptors, which may include other as yet unknown members, is most closely related to the protooncogene product Ret and the platelet-derived growth factor receptor family.

Developmentally regulated protein-tyrosine kinase genes in Dictyostelium discoideum

Dictyostelium discoideum, an organism that undergoes development and that is amenable to biochemical and molecular genetic approaches, is an attractive model organism with which to study the role of tyrosine phosphorylation in cell-cell communication. We report the presence of protein-tyrosine kinase genes in D. discoideum. Screening of a Dictyostelium cDNA expression library with an anti-phosphotyrosine antibody identifies fusion proteins that exhibit protein-tyrosine kinase activity. Two distinct cDNAs were identified and isolated. Though highly homologous to protein kinases in general, these kinases do not exhibit many of the hallmarks of protein-tyrosine kinases of higher eucaryotes. In addition, these genes are developmentally regulated, which suggests a role for tyrosine phosphorylation in controlling Dictyostelium development.

Developmentally regulated protein-tyrosine kinase genes in Dictyostelium discoideum

Dictyostelium discoideum, an organism that undergoes development and that is amenable to biochemical and molecular genetic approaches, is an attractive model organism with which to study the role of tyrosine phosphorylation in cell-cell communication. We report the presence of protein-tyrosine kinase genes in D. discoideum. Screening of a Dictyostelium cDNA expression library with an anti-phosphotyrosine antibody identifies fusion proteins that exhibit protein-tyrosine kinase activity. Two distinct cDNAs were identified and isolated. Though highly homologous to protein kinases in general, these kinases do not exhibit many of the hallmarks of protein-tyrosine kinases of higher eucaryotes. In addition, these genes are developmentally regulated, which suggests a role for tyrosine phosphorylation in controlling Dictyostelium development.

Autocrine transformation by chimeric signal peptide-basic fibroblast growth factor: reversal by suramin

NIH 3T3 cells transfected with basic fibroblast growth factor (bFGF) fused to an immunoglobulin signal peptide sequence are transformed in vitro and tumorigenic in vivo. The transformed phenotype of chimeric signal peptide-bFGF (spbFGF) cells is characterized by an enhanced proliferation rate compared to parental NIH 3T3 cells, density- and anchorage-independent growth, a transformed morphology, and lack of cell adhesion. The rate of spbFGF cell proliferation is not diminished by anti-bFGF neutralizing antibodies. 125I-labeled bFGF receptor cross-linking and binding studies suggest that surface FGF receptors in spbFGF cells are unavailable and down-regulated. The FGF receptors are also down-regulated in K-fgf-transformed cells but not in parental 3T3, native bFGF-transfected, and ras-transformed NIH 3T3 cells. The addition of suramin to spbFGF and K-fgf cells rapidly promotes the up-regulation of FGF receptors. Suramin also induces lowering of the proliferation rate to that of parental cells, anchorage-dependent growth, assembly of cytoskeletal filaments, cellular adhesion, and spreading. These results suggest that spbFGF cells undergo autocrine transformation, possibly by an internal autocrine loop, in which there is constitutive activation of the FGF receptor. Suramin inhibits autocrine transformation, leading to a normal untransformed phenotype.

A murine fibroblast growth factor (FGF) receptor expressed in CHO cells is activated by basic FGF and Kaposi FGF

We have cloned a murine cDNA encoding a tyrosine kinase receptor with about 90% similarity to the chicken fibroblast growth factor (FGF) receptor and the human fms-like gene (FLG) tyrosine kinase. This mouse receptor lacks 88 amino acids in the extracellular portion, leaving only two immunoglobulin-like domains compared to three in the chicken FGF receptor. The cDNA was cloned into an expression vector and transfected into receptor-negative CHO cells. We show that cells expressing the receptor can bind both basic FGF and Kaposi FGF. Although the receptor binds basic FGF with a 15- to 20-fold higher affinity, Kaposi FGF is able to induce down-regulation of the receptor to the same extent as basic FGF. The receptor is phosphorylated upon stimulation with both FGFs, DNA synthesis is stimulated, and a proliferative response is produced in cells expressing the receptor, whereas cells expressing the cDNA in the antisense orientation show none of these responses to basic FGF or Kaposi FGF. Thus this receptor can functionally interact with two growth factors of the FGF family.

Two forms of the basic fibroblast growth factor receptor-like mRNA are expressed in the developing mouse brain

The embryonic neuroepithelium gives rise to the components of the central nervous system in the mature animal. To study the early development of the murine central nervous system we have sought to isolate growth factor receptors from the neuroepithelium of the neural tube of 10-day-old mouse embryos. Because many growth factor receptors are members of the protein-tyrosine kinase family, we have used the polymerase chain reaction to amplify mRNA sequences from 10-day-old mouse embryo neuroepithelium; these sequences lie between the nucleotide sequences of two highly conserved amino acid motifs from the catalytic domain of protein-tyrosine kinases. By using this technique we have isolated a clone encoding the murine basic fibroblast growth factor receptor (bFGF-R), as well as a shorter form of this mRNA. This latter cDNA comprised 75% of the bFGF-R cDNA clones isolated from the immortalized neuroepithelial cell lines. This variant mRNA, designated here as N-bFGF-R, appears to be expressed at higher levels in neuronal cells in early stages of development. The bFGF-R is a member of a multigene family, as demonstrated by Southern blot analysis and the cloning of two other members of this family.

Receptor for acidic fibroblast growth factor is related to the tyrosine kinase encoded by the fms-like gene (FLG)

We have previously isolated a human gene from an endothelial cell cDNA library encoding a putative tyrosine kinase; we have designated this gene the fms-like gene (FLG). To analyze the gene product(s) of FLG, we have generated rabbit polyclonal antibodies directed against a synthetic peptide from FLG and used it to immunoprecipitate biosynthetically labeled FLG protein from a variety of human cell lines. These antibodies specifically recognized glycoprotein(s) of 100, 120, and 135 kDa with protein cores of 90 and 110 kDa. Acidic fibroblast growth factor (aFGF) stimulated tyrosine kinase activity of FLG in vitro and in living cells, suggesting that FLG encodes the membrane receptor for aFGF. Further supporting evidence came from cross-linking experiments on intact cells with the covalent cross-linking agent disuccinimidyl suberate and 125I-labeled aFGF as a specific probe. The cross-linked 125I-labeled aFGF-aFGF receptor complex was specifically immunoprecipitated with FLG antipeptide antibodies. It appears, therefore, that the receptor(s) for aFGF is related to the FLG gene product.

Identification of a developmentally regulated protein-tyrosine kinase by using anti-phosphotyrosine antibodies to screen a cDNA expression library

To identify the protein-tyrosine kinases that are expressed during chicken embryonic development, a 10-day chicken embryo cDNA expression library was screened with anti-phosphotyrosine antibodies. Of the positive clones isolated, many encoded the same protein-tyrosine kinase, which we designate Cek1 (chicken embryo kinase 1). Its amino acid sequence suggests that the Cek1 protein is a transmembrane tyrosine kinase and presumably the receptor for an unknown ligand. Antibodies prepared to the cloned Cek1 kinase recognize, in immunoblotting experiments, two protein bands with apparent molecular weights of 100,000 and 110,000. The Cek1 protein was detected in many chicken embryonic tissues, but not in the corresponding adult tissues (with the exception of brain). The Cek1 kinase appears to be very closely related to two protein-tyrosine kinases whose partial sequences have been recently determined, human Flg and mouse Bek. Cloning using anti-phosphotyrosine antibodies has allowed us to detect, in addition to Cek1, several other protein-tyrosine kinases that are expressed during chicken embryonic development, some of which have not been previously identified.