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

Tissue-dependent regulation of protein tyrosine kinase activity during embryonic development

Protein tyrosine kinase activity was assayed in a variety of chicken tissues during embryonic development and in the adult. In some tissues protein tyrosine kinase activity decreased during embryonic development; however, in other tissues it remained high throughout development, it contrast to the level of protein tyrosine phosphorylation, which decreased during development. The highest levels of tyrosine kinase activity were detected in 17-d embryonic brain although only low levels of protein tyrosine phosphorylation were observed in this tissue. Several alternatives were examined in an effort to determine the mechanism responsible for the low levels of tyrosine phosphorylated proteins in most older embryonic and adult chicken tissues despite the presence of highly active tyrosine kinases. The results show that the regulation of protein tyrosine phosphorylation during embryonic development is complex and varies from tissue to tissue. Furthermore, the results suggest that protein tyrosine phosphatases play an important role in regulating the level of phosphotyrosine in proteins of many older embryonic and adult tissues.

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.

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.

Expression cDNA cloning of the KGF receptor by creation of a transforming autocrine loop

An expression cloning strategy was devised to isolate the keratinocyte growth factor (KGF) receptor complementary DNA. NIH/3T3 fibroblasts, which secrete this epithelial cell-specific mitogen, were transfected with a keratinocyte expression complementary DNA library. Among several transformed foci identified, one demonstrated the acquisition of specific high-affinity KGF binding sites. The pattern of binding competition by related fibroblast growth factors (FGFs) indicated that this receptor had high affinity for acidic FGF as well as KGF. The rescued 4.2-kilobase complementary DNA was shown to encode a predicted membrane-spanning tyrosine kinase related to but distinct from the basic FGF receptor. This expression cloning approach may be generally applicable to the isolation of genes that constitute limiting steps in mitogenic signaling pathways.

Molecular cloning of a human basic fibroblast growth factor receptor cDNA and expression of a biologically active extracellular domain in a baculovirus system

A cDNA clone encoding a human fibroblast growth factor (FGF) receptor was isolated from a hepatoma cell line cDNA library. The cDNA encodes a three immunoglobulinlike-domain FGF receptor that is similar to a human placental FGF receptor cDNA but lacks two amino acids. The variation observed at these two amino acids, also seen in the two immunoglobulinlike-domain FGF-receptors, can be explained by an alternate splicing mechanism. We have used a baculovirus expression system to produce high levels of a soluble, extracellular domain form of the FGF receptor (EC-FGF receptor). Spodoptera frugiperda (Sf9) insect cells infected with recombinant EC-FGF receptor viruses synthesized and secreted an EC-FGF receptor of apparent Mr = 58,000. The EC-FGF receptor purified from conditioned media of infected Sf9 cells by lentil lectin affinity chromatography was shown to bind basic FGF with high affinity (Kd = 1-5 nM), to inhibit the binding of radioiodinated basic FGF to its high affinity receptor and to inhibit endothelial cell proliferation. Furthermore, binding of basic FGF to the EC-FGF receptor was shown to be significantly enhanced by heparin. The availability of biologically active FGF receptors will allow an analysis of their interaction with members of the FGF family of proteins and viruses of the herpes family that have been shown to use the FGF receptor system for cell entry.

cDNA cloning and expression of a human FGF receptor which binds acidic and basic FGF

We have isolated and characterized a cDNA clone, phFGFR, encoding a human fibroblast growth factor (FGF) receptor. phFGFR contains an open reading frame which encodes an 820 amino acid polypeptide with three immunoglobulin-like domains in the extracellular part and an intracellular split tyrosine kinase domain. Transient expression in COS-1 cells and immunoprecipitation using an antiserum raised against a C-terminal peptide, gave rise to two components, representing mature (130 kDa) and precursor (115 kDa) forms of the phFGFR encoded polypeptide, which was denoted hFGFR-1. Crosslinking of iodinated acidic FGF (aFGF) and basic FGF (bFGF) to transiently expressing COS-1 cells revealed a major band of 95 kDa, which was competed for by both aFGF and bFGF. From Scatchard analyses, the Kd:s for binding of aFGF and bFGF to hFGFR-1 were estimated to 25 pM and 41 pM, respectively. Thus, phFGFR encodes a human FGF receptor with high affinity for both aFGF and bFGF.

Proteoheparan sulfates contribute to the binding of basic FGF to its high affinity receptors on bovine adrenocortical cells

Bovine adrenocortical cells in primary culture express the basic fibroblast growth factor (bFGF) gene and their proliferation is stimulated by this growth factor. We report here the characterization of bFGF receptors on these cells. Binding studies revealed the presence of two bFGF receptor types: a limited number (4,300 sites/cell) of high affinity sites (Kd congruent to 2 pM) and a larger number (230,000 sites/cell) of lower affinity sites (Kd congruent to 400 pM). Cross-linking of 125I-bFGF to adrenocortical cells revealed two bands at 145 kDa and 125 kDa which are attributed to molecular complexes between the high affinity receptors and their ligand. These high affinity receptors possess N-linked carbohydrate chains that are important for proper cell surface expression but are devoid of glycosaminoglycan chains. The low-affinity (2 M NaCl-sensitive) binding sites are totally degraded by heparitinase treatment of adrenocortical cells indicating that low-affinity sites are borne by heparan sulfate proteoglycans. However, heparitinase treatment also reduced partially the binding of bFGF to high-affinity (2 M NaCl-resistant) sites. This argues for a contribution of heparan sulfate proteoglycans to the binding of bFGF to high-affinity receptors. Exogenous soluble heparin or heparan sulfate did not restore normal high affinity bFGF binding onto heparitinase-treated cells, suggesting that heparan sulfate proteoglycans either must be membrane-anchored or must contain specific structural features to enhance FGF binding to high-affinity receptors. Taken together with previous reports, this work supports the hypothesis that bFGF may act as an autocrine growth factor in the adrenal cortex.

High affinity receptors to acidic and basic fibroblast growth factor (FGF) are detected mainly in adult brain membrane preparations but not in liver, kidney, intestine, lung or stomach

We have previously shown that only adult brain contained a detectable amount of high affinity receptors for basic Fibroblast growth factor (bFGF) whereas adult liver, kidney, lung, intestine or stomach showed only low affinity binding sites. We now have studied and compared the distribution of the receptors for acidic Fibroblast growth factor (aFGF) with that of bFGF receptors in the same tissues. Membrane binding of 125I-aFGF was time dependent, reversible and displaced by an excess of unlabeled aFGF. Scatchard analyses of binding data obtained with all tissue membrane preparations revealed the presence of at least one class of low affinity/high capacity interaction sites characterized by apparent Kd values ranging from 3.9 to 6.9 x 10(-8) M. Interestingly and as for bFGF, high affinity receptors for aFGF could be detected only in adult brain membranes. Cross-linking and Scatchard analyses indicate that this family of interaction was characterized by four molecular species of 175, 125, 95 and 70 kDa and by an apparent Kd value of 1.8 x 10(-10) M. Moreover, cross-competition binding assay revealed that these brain high affinity receptors were common for both acidic and basic FGF. These results suggest that these growth factors may share identical functions mediated by the same receptors highly expressed in the brain. Using a cDNA probe for the Bek form of FGF receptors, we were able to show that all the tissues studied expressed this mRNA (4.5 kb transcript) but probably not in sufficient amounts to account for the number of high affinity receptors that we detected only in the brain.

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.

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.

Ligand-affinity cloning and structure of a cell surface heparan sulfate proteoglycan that binds basic fibroblast growth factor

Expression cloning of cDNAs encoding a basic fibroblast growth factor (FGF) binding protein confirms previous hypotheses that this molecule is a cell-surface heparan sulfate proteoglycan. A cDNA library constructed from a hamster kidney cell line rich in FGF receptor activity was transfected into a human lymphoblastoid cell line. Clones expressing functional basic FGF binding proteins at their surfaces were enriched by panning on plastic dishes coated with human basic FGF. The amino acid sequence deduced from the isolated cDNAs revealed several interesting features, including hydrophobic signal and transmembrane domains that flank an extracellular region containing six potential attachment sites for glycosaminoglycan side chains. The structure also contains a short hydrophilic cytoplasmic tail sequence homologous to previously reported actin binding domains. Binding of basic FGF to cells expressing the binding protein could be inhibited by heparin and heparan sulfate but not by chondroitin sulfate, dermatan sulfate, or keratan sulfate. In addition to binding basic FGF, this protein or related surface proteins may function as an initial cellular attachment site for other growth factors and for viruses, such as herpes simplex virus.

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.

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.

Turnover of functional basic fibroblast growth factor receptors on the surface of BHK and NIH 3T3 cells

The recovery of functional cell-surface bFGF receptors after trypsin treatment was studied in BHK cells and NIH 3T3 cells. Restoration of functional bFGF receptors occurred at an approximately linear rate with 50% of the high-affinity binding capacity restored after 4 hr. Restoration of functional receptors required protein synthesis but not RNA synthesis. Upon exposure of BHK cells to bFGF, cell-surface receptors were rapidly lost, with only 25% remaining after 1 hr. When the bFGF was removed, down-regulated BHK and NIH 3T3 cells recovered cell-surface receptors at about the same rate observed in trypsin-treated cells. The recovery of receptors after down-regulation was inhibited by protein synthesis inhibitors. Addition of the protein synthesis inhibitor cycloheximide to unperturbed cultures of BHK or NIH 3T3 cells resulted in a time-dependent loss of cell-surface bFGF receptors, demonstrating that the receptors turn over constantly in the absence of ligand. These results suggest that bFGF receptors do not recycle and must be continuously synthesized.