Point mutation in FGF receptor eliminates phosphatidylinositol hydrolysis without affecting mitogenesis

Late signals are required for the stimulation of DNA synthesis in rat mammary fibroblasts by growth factors

Maximal stimulation of DNA synthesis in quiescent rat mammary (Rama) 27 fibroblasts is elicited by epidermal growth factor (EGF) or basic fibroblast growth factor (bFGF) 18 h after the initial addition of the growth factors-the 'lag' period. At maximally-stimulating concentrations, EGF and bFGF are interchangeable 9 h after their initial addition. When the initial concentration of growth factor is below that required to elicit a maximal response, it is possible to increase the level of DNA synthesis by increasing the concentration of growth factor 9 h after its initial addition. When the initial concentration of growth factor is high, substitution by a lower concentration of growth factor after 9 h allows a greater proportion of cells to synthesize DNA than would be expected from a continuous low dose of growth factor. Similar results are obtained when both the growth factor and its concentration are changed 9 h after the initial addition of growth factor. However, when EGF at a low concentration is substituted for a high concentration of EGF or bFGF the resulting increase in the levels of DNA synthesis is greater when EGF rather than bFGF is added for a second time. The half-life of the growth-stimulatory signals delivered by EGF and by bFGF 9 h after their initial addition is 1-2 h. These results suggest that to stimulate DNA synthesis: (i) EGF or bFGF must deliver a signal(s) continuously; (ii) the initial signals produced by EGF and bFGF are equivalent; (iii) the signals produced between 9-18 h by EGF may be different to those produced by bFGF.

Inhibition of FGF-stimulated phosphatidylinositol hydrolysis and neurite outgrowth by a cell-membrane permeable phosphopeptide

BACKGROUND

Activated receptor tyrosine kinases bind downstream effector molecules with high affinity. Provided that they can be introduced into cells, peptides corresponding to these high-affinity sites should be able to compete for the interaction and thereby inhibit specific signal transduction cascades. The high-affinity binding site for phospholipase C gamma (PLCgamma) on the activated fibroblast growth factor receptor (FGFR) is centred around the tyrosine at position 766 (766Tyr), and peptides corresponding to this site inhibit PLCgamma binding to the receptor in vitro. A 16 amino-acid peptide from the third helix of the Antennapedia homeodomain protein has recently been shown to be able to act as an internalization vector that can deliver other peptides into cells. Here, we have designed a peptide that contains both the internalization sequence and the FGFR high-affinity binding site for PLCgamma, and tested it in cultures of cerebellar neurons for its ability to inhibit the activation of PLCgamma by basic FGF.

RESULTS

The peptide containing the FGFR high-affinity binding site for PLCgamma inhibited phospholipid hydrolysis stimulated by basic FGF with a maximal effect at 1 microg ml-1. Phosphorylation of 766Tyr was required for this effect. The phosphorylated peptide had no effect on phospholipid hydrolysis stimulated by platelet-derived growth factor, neurotrophin-3 and bradykinin. The phosphorylated peptide also inhibited neurite outgrowth stimulated by FGF, but had no effect on neurite outgrowth stimulated by agents that activate the FGFR signal transduction cascade downstream from the activation of PLCgamma.

CONCLUSIONS

Cell-permeable peptides can be designed that inhibit the function of receptor tyrosine kinases. In this context we have developed a peptide that prevents the FGFR from activating PLCgamma, and have used this peptide to obtain the first direct evidence that activation of PLCgamma is required for the neurite outgrowth response stimulated by basic FGF.

Phospholipase C-gamma1 interacts with conserved phosphotyrosyl residues in the linker region of Syk and is a substrate for Syk

Antigen receptor ligation on lymphocytes activates protein tyrosine kinases and phospholipase C-gamma (PLC-gamma) isoforms. Glutathione S-transferase fusion proteins containing the C-terminal Src-homology 2 [SH2(C)] domain of PLC-gamma1 bound to tyrosyl phosphorylated Syk. Syk isolated from antigen receptor-activated B cells phosphorylated PLC-gamma1 on Tyr-771 and the key regulatory residue Tyr-783 in vitro, whereas Lyn from the same B cells phosphorylated PLC-gamma1 only on Tyr-771. The ability of Syk to phosphorylate PLC-gamma1 required antigen receptor ligation, while Lyn was constitutively active. An mCD8-Syk cDNA construct could be expressed as a tyrosyl-phosphorylated chimeric protein tyrosine kinase in COS cells, was recognized by PLC-gamma1 SH2(C) in vitro, and induced tyrosyl phosphorylation of endogenous PLC-gamma1 in vivo. Substitution of Tyr-525 and Tyr-526 at the autophosphorylation site of Syk in mCD8-Syk substantially reduced the kinase activity and the binding of this variant chimera to PLC-gamma1 SH2(C) in vitro; it also failed to induce tyrosyl phosphorylation of PLC-gamma1 in vivo. In contrast, substitution of Tyr-348 and Tyr-352 in the linker region of Syk in mCD8-Syk did not affect the kinase activity of this variant chimera but almost completely eliminated its binding to PLC-gamma1 SH(C) and completely eliminated its ability to induce tyrosyl phosphorylation of PLC-gamma1 in vivo. Thus, an optimal kinase activity of Syk and an interaction between the linker region of Syk with PLC-gamma1 are required for the tyrosyl phosphorylation of PLC-gamma1.

Nerve growth factor stimulates the tyrosine phosphorylation of endogenous Crk-II and augments its association with p130Cas in PC-12 cells

The cellular homologs of the v-Crk oncogene product consist primarily of Src homology region 2 (SH2) and 3 (SH3) domains. v-Crk overexpression causes cell transformation and elevation of tyrosine phosphorylation in fibroblasts and accelerates differentiation of PC-12 cells in response to nerve growth factor (NGF). To further explore the role of Crk in NGF-induced PC-12 cell differentiation, we found that both NGF and epidermal growth factor stimulate the tyrosine phosphorylation of endogenous Crk II. Moreover, hormone stimulation enhanced the specific association of Crk proteins with the tyrosine-phosphorylated p130Cas, the major phosphotyrosine-containing protein in cells transformed with v-Crk. This interaction is mediated by the SH2 domain of Crk and can be inhibited with a phosphopeptide containing the Crk-SH2 binding motif. Furthermore, the Crk-SH2 domain binds tyrosine-phosphorylated paxillin, a cytoskeletal protein, following treatment of PC-12 cells with NGF or epidermal growth factor. These data suggest that Crk functions in a number of signaling processes in PC-12 cells.

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

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

Role of the nuclear localization sequence in fibroblast growth factor-1-stimulated mitogenic pathways

Fibroblast growth factor-1 (FGF-1) is a potent mitogen for mesoderm- and neuroectoderm-derived cell types in vitro. However, a mutant FGF-1 with deletion in its nuclear localization sequence (NLS, residues 21-27) is not mitogenic in vitro. We demonstrated that synthetic peptides containing this NLS were able to stimulate DNA synthesis in a FGF receptor-independent manner after they were delivered into living NIH 3T3 cells by a cell-permeable peptide import technique. The stimulation of maximal DNA synthesis by these peptides required the presence of peptides during the entire G1 phase of the cell cycle. The mitogenic effect was specific for the NLS of FGF-1 because a peptide with double point mutations at lysine residues was inactive in stimulating DNA synthesis. Our results suggest that the NLS plays an important role in the mitogenic pathway initiated by exogenous FGF-1 by its direct involvement in the nuclear transport and signaling of internalized FGF-1.

Identification of p90, a prominent tyrosine-phosphorylated protein in fibroblast growth factor-stimulated cells, as 80K-H

Tyrosine phosphorylation of cellular proteins occurs rapidly upon treatment of fibroblasts with acidic or basic fibroblast growth factors (aFGF, bFGF), suggesting a role for protein phosphorylation in the FGF signaling pathway. Stimulation of Swiss 3T3 cells and MRC-5 fibroblasts with bFGF results in the tyrosine phosphorylation of several proteins, of which the most prominent has been designated as p90. The phosphorylation of p90 is observed within 30 s of treating the cells with FGF but not with other growth factors. Microsequencing of p90 resolved on two-dimensional polyacrylamide gel electrophoresis indicated an N-terminal amino acid sequence which corresponded to a protein previously named as 80K-H. Polyclonal antibodies raised against the predicted C terminus of 80K-H recognized p90 on all Western blots. p90 was found to bind specifically to GRB-2-glutathione S-transferase fusion protein and to be immunoreactive with 80K-H antibody. In addition, anti-phosphotyrosine antibodies immunoprecipitated 80K-H from cell lysates of FGF-stimulated but not from control fibroblasts. The biological function of 80K-H is yet unknown. However, from this study and a previous observation of the obligatory dependence of p90 phosphorylation on FGF receptor occupation, it appears that 80K-H is involved in FGF signaling.

Identification of six novel autophosphorylation sites on fibroblast growth factor receptor 1 and elucidation of their importance in receptor activation and signal transduction

Fibroblast growth factor receptor (FGFR) activation leads to receptor autophosphorylation and increased tyrosine phosphorylation of several intra cellular proteins. We have previously shown that autophosphorylated tyrosine 766 in FGFR1 serves as a binding site for one of the SH2 domains of phospholipase Cy and couples FGFR1 to phosphatidylinositol hydrolysis in several cell types. In this report, we describe the identification of six additional autophosphorylation sites (Y-463, Y-583, Y-585, Y-653, Y-654 and Y-730) on FGFR1. We demonstrate that autophosphorylation on tyrosines 653 and 654 is important for activation of tyrosine kinase activity of FGFR1 and is therefore essential for FGFR1-mediated biological responses. In contrast, autophosphorylation of the remaining four tyrosines is dispensable for FGFR1-mediated mitogen-activated protein kinase activation and mitogenic signaling in L-6 cells as well as neuronal differentiation of PC12 cells. Interestingly, both the wild-type and a mutant FGFR1 (FGFR1-4F) are able to phosphorylate Shc and an unidentified Grb2-associated phosphoprotein of 90 kDa (pp90). Binding of the Grb2/Sos complex to phosphorylated Shc and pp90 may therefore be the key link between FGFR1 and the Ras signaling pathway, mito-genesis, and neuronal differentiation.

Correlation between the effects of bombesin antagonists on cell proliferation and intracellular calcium concentration in Swiss 3T3 and HT-29 cell lines

Bombesin (BN) acts as an autocrine mitogen in various human cancers. Several pseudononapeptide BN-(6-14) analogs with a reduced peptide bond between positions 13 and 14 have been shown to suppress the mitogenic activity of BN or gastrin-releasing peptide (GRP) when assessed by radioreceptor or proliferation assays and may have significant clinical applications. The search for potent and safe BN antagonists requires the evaluation of a large series of analogs in radioreceptor and proliferation assays. In this paper, we report that the ability of BN analogs to inhibit BN-induced calcium transients in Swiss 3T3 cells shows a high correlation with their inhibitory potency as evaluated by classical proliferation tests. The assay of calcium transients allows a rapid characterization of new BN analogs (in terms of minutes rather than days) and can be adapted as a labor and cost-effective screening step in the selection of potentially relevant BN antagonists for further characterization in cell proliferation systems. We also observed that results from the assay of calcium transients in Swiss 3T3 cells can be correlated with the results of the proliferative response in HT-29 cells, a cell line that does not seem to use the same early transmembrane ionic signal system. This result suggests that the calcium pathway is not mandatory for triggering cell division by the BN receptor.

Mitogenic inhibition by phorbol esters is associated with decreased phosphatidylinositol-3 kinase activation

In contrast to their role as potent tumor promoters, phorbol esters can cause inhibition of cell growth. Because the effect of phorbol esters occurs through activation of protein kinase C (PKC) and because activated PKC is translocated to the membrane placing it in a position to act on the intracellular portion of the growth factor receptor, we asked whether this inhibitory effect is mediated through the action of phorbol 12-myristate 13-acetate (PMA) on receptor association with the signal transfer proteins. When added to rat vascular smooth muscle (VSM) cells concurrently with basic fibroblast growth factor (bFGF), PMA at 100 ng/ml completely inhibits bFGF-stimulated DNA synthesis. Under the same growth-inhibitory conditions of PMA addition, aggregation of phosphatidylinositol 3-kinase (PI3K) to the fibroblast growth factor receptor and tyrosine phosphorylation of the 85-kDa regulatory component of the signal transfer protein PI3K are reduced by 94 and 79%, respectively. PI3K catalytic activity, as measured by conversion of phosphatidylinositol to phosphatidylinositol 3-phosphate, is decreased 88% by PMA addition. This effect is not specific to PI3K, since aggregation of phospholipase C-gamma 1 to the activated bFGF receptor is also decreased by PMA treatment. In addition, the PI3K inhibitor wortmannin markedly attenuates bFGF-stimulated VSM cell growth in a dose-dependent manner. These data suggest that the site of growth inhibition by PMA in VSM cells lies upstream of signal transfer particle aggregation and that such growth arrest may be mediated through inhibition of activation of PI3K.

Phosphoinositide-specific phospholipase C and mitogenic signaling

The importance of PLC activation in cell proliferation is evident from the fact that the hydrolysis of PtdIns(4,5)P2 is one of the early events that follow the interaction of many growth factors and mitogens with their respective receptors. However, the importance of PLC activation is not restricted to proliferation; it is one of the most common transmembrane signaling events elicited by receptors that regulate many other cellular processes, including differentiation, metabolism, secretion, contraction, and sensory perception. It is also clear that cell proliferation signaling does not always require PLC, as indicated by the fact that growth factors such as insulin and CSF-1 do not appear to elicit the hydrolysis of PtdIns(4,5)P2, even though the intracellular domains of their receptors carry a PTK domain and the receptors show topologies very similar to those of the PLC-activating growth factors PDGF, EGF, and FGF. The growth factor-dependent activation of PLC is initiated by the formation of a complex between the receptor PTK and PLC-gamma; the formation of this complex is mediated by a specific interaction between a tyrosine phosphate residue on the intracellular domain of PTK and the SH2 domain of PLC-gamma. The receptor PTK subsequently phosphorylates PLC-gamma, of which two distinct isozymes, PLC-gamma 1 and PLC-gamma 2, have been identified. Proliferation of T cells and B cells in response to the aggregation of their respective cell surface receptors is also accompanied by the activation of PLC-gamma isozymes at an early stage. Unlike growth factor receptors, the T cell and B cell receptors lack intrinsic PTK activity but associate with several non-receptor PTKs of the Src and Syk families. Although the specific kinases are not known, one or more of these enzymes phosphorylate and activate PLC-gamma 1 and PLC-gamma 2. Transduction of growth signals by G protein-coupled receptors such as those for thrombin or bombesin also requires PtdIns(4,5)P2 hydrolysis, which, in this instance, is mediated by PLC-beta isozymes. The PLC-beta subfamily consists of four distinct members: PLC-beta 1, PLC-beta 2, PLC-beta 3, and PLC-beta 4. Agonist interaction with specific G protein-coupled receptors causes the dissociation of Gq proteins into G alpha and G beta gamma subunits and the exchange of GDP bound to G alpha for GTP. The resulting GTP-bound G alpha subunit then activates PLC-beta isoforms by binding to the carboxyl-terminal region of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

The GC factor regulates the expression of the insulin-like growth factor-I receptor

We have transfected a plasmid expressing the transcriptional regulator GC Factor (GCF) into cell lines and have found that the GCF: 1 causes a decrease in the levels of insulin-like growth factor I receptor (IGF-IR) mRNA; 2 causes a decrease in the number of IGF-IRs; and 3 represses the activity of the IGF-IR promoter. In addition, we show that the regulation of IGF-IR expression by GCF plays a physiological role in the control of cellular proliferation in vitro.

An FGF receptor signaling pathway is required for the normal cell migrations of the sex myoblasts in C. elegans hermaphrodites

The sex myoblasts (SMs) in C. elegans hermaphrodites undergo anteriorly directed cell migrations that allow for the proper localization of the egg-laying muscles. These migrations are controlled in part by a signal emanating from gonadal cells that allows the SMs to be attracted to their precise final positions flanking the center of the gonad. Mutations in egl-15 alter the nature of the interaction between the gonad and the SMs, resulting in the posterior displacement of the SMs. Here we show that egl-15 encodes a receptor tyrosine kinase of the fibroblast growth factor receptor (FGFR) subfamily with multiple roles in development. Three genes were identified that behave genetically as activators or mediators of egl-15 activity. One of these genes, sem-5, encodes an adaptor molecule that transduces signals from a variety of receptor tyrosine kinases. Like egl-15 and sem-5, the other two genes may similarly act in FGFR signaling pathways in C. elegans.

Angiotensin II stimulates calcium-dependent activation of c-Jun N-terminal kinase

In GN4 rat liver epithelial cells, angiotensin II (Ang II) and other agonists which activate phospholipase C stimulate tyrosine kinase activity in a calcium-dependent, protein kinase C (PKC)-independent manner. Since Ang II also produces a proliferative response in these cells, we investigated downstream signaling elements traditionally linked to growth control by tyrosine kinases. First, Ang II, like epidermal growth factor (EGF), stimulated AP-1 binding activity in a PKC-independent manner. Because increases in AP-1 can reflect induction of c-Jun and c-Fos, we examined the activity of the mitogen-activated protein (MAP) kinase family members Erk-1 and -2 and the c-Jun N-terminal kinase (JNK), which are known to influence c-Jun and c-Fos transcription. Ang II stimulated MAP kinase (MAPK) activity but only approximately 50% as effectively as EGF; again, these effects were independent of PKC. Ang II also produced a 50- to 200-fold activation of JNK in a PKC-independent manner. Unlike its smaller effect on MAPK, Ang II was approximately four- to sixfold more potent in activating JNK than EGF was. Although others had reported a lack of calcium ionophore-stimulated JNK activity in lymphocytes and several other cell lines, we examined the role of calcium in GN4 cells. The following results suggest that JNK activation in rat liver epithelial cells is at least partially Ca(2+) dependent: (i) norepinephrine and vasopressin hormones that increase inositol 1,4,5-triphosphate stimulated JNK; (ii) both thapsigargin, a compound that produces an intracellular Ca(2+) signal, and Ca(2+) ionophores stimulated a dramatic increase in JNK activity (up to 200-fold); (iii) extracellular Ca(2+) chelation with ethylene glycol tetraacetic acid (EGTA) inhibited JNK activation by ionophore and intracellular chelation with 1,2-bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl-ester (BAPTA-AM) partially inhibited JNK activation by Ang II or thapsigargin; and (iv) JNK activation by Ang II was inhibited by pretreatment of cells with thapsigargin and EGTA, a procedure which depletes intracellular Ca(2+) stores. JNK activation following Ang II stimulation did not involve calmodulin; either W-7 nor calmidizolium, in concentrations sufficient to inhibit Ca(2+)/calmodulin-dependent kinase II, blocked JNK activation by Ang II. In contrast, genistein, in concentrations sufficient to inhibit Ca(2+)-dependent tyrosine phosphorylation, prevented Ang II and thapsigargin-induced JNK activation. In summary, in GN4 rat liver epithelial cells, Ang II stimulates JNK via a novel Ca(2+)-dependent pathway. The inhibition by genistein suggest that Ca(2+)-dependent tyrosine phosphorylation may modulate the JNK pathway in a cell type-specific manner, particularly in cells with a readily detectable Ca(2+)-regulated tyrosine kinase.

pp60c-src is a positive regulator of growth factor-induced cell scattering in a rat bladder carcinoma cell line

The NBT-II rat carcinoma cell line exhibits two mutually exclusive responses to FGF-1 and EGF, entering mitosis at cell confluency while undergoing an epithelium-to-mesenchyme transition (EMT) when cultured at subconfluency. EMT is characterized by acquisition of cell motility, modifications of cell morphology, and cell dissociation correlating with the loss of desmosomes from cellular cortex. The pleiotropic effects of EGF and FGF-1 on NBT-II cells suggest that multiple signaling pathways may be activated. We demonstrate here that growth factor activation is linked to at least two intracellular signaling pathways. One pathway leading to EMT involves an early and sustained stimulation of pp60c-src kinase activity, which is not observed during the growth factor-induced entry into the cell cycle. Overexpression of normal c-src causes a subpopulation of cells to undergo spontaneous EMT and sensitizes the rest of the population to the scattering activity of EGF and FGF-1 without affecting their mitogenic responsiveness. Addition of cholera toxin, a cAMP-elevating agent, severely perturbs growth factor induction of EMT without altering pp60c-src activation, therefore demonstrating that cAMP blockade takes place downstream or independently of pp60c-src. On the other hand, overexpression of a mutated, constitutively activated form of pp60c-src does not block cell dispersion while strongly inhibiting growth factor-induced entry into cell division. Moreover, stable transfection of a dominant negative mutant of c-src inhibits the scattering response without affecting mitogenesis induced by the growth factors. Altogether, these results suggest a role for pp60c-src in epithelial cell scattering and indicate that pp60c-src might contribute unequally to the two separate biological activities engendered by a single signal.

FGF inhibits neurite outgrowth over monolayers of astrocytes and fibroblasts expressing transfected cell adhesion molecules

We have cultured cerebellar neurons on monolayers of cortical astrocytes in control medium or medium containing recombinant basic fibroblast growth factor (FGF). FGF was found to inhibit neurite outgrowth, with a significant effect seen at 0.5 ng/ml and a maximal effect at 10 ng/ml. FGF increased the production of arachidonic acid (AA) in cerebellar neurons, and when added directly to cultures or generated endogenously via activation of phospholipase A2 using melittin, this second messenger could mimic the inhibitory effect of FGF. FGF and AA could also specifically inhibit neurite outgrowth stimulated by three cell adhesion molecules (NCAM, N-cadherin and L1) expressed in transfected fibroblasts, or in the case of L1 bound to a tissue culture substratum. These data demonstrate that, in certain cellular contexts, FGF can act as an inhibitory cue for axonal growth and that arachidonic acid is the second messenger responsible for this activity. We discuss the possibility that arachidonic acid inhibits neurite outgrowth by desensitising the second messenger pathway underlying neuronal responsiveness to cell adhesion molecules.

Shc and a novel 89-kDa component couple to the Grb2-Sos complex in fibroblast growth factor-2-stimulated cells

A major pathway for mitogenicity is gated via the small GTP-binding protein Ras. Receptor tyrosine kinases couple to Ras through the Src homology 2 (SH2) domain protein Grb2. The activated fibroblast growth factor receptor-1 (FGFR-1) expressed in L6 myoblasts did not bind Grb2 directly, but indirectly, through the small adaptor protein Shc, which was tyrosine-phosphorylated in response to fibroblast growth factor-2 (FGF-2) stimulation. A FGFR-1 mutant in which Tyr766, a known autophosphorylation site, was changed to Phe, mediated less efficient tyrosine phosphorylation of Shc. FGF-2 stimulation of mutant FGFR-1-expressing cells still allowed formation of complexes containing Shc, Grb2, and the nucleotide exchange factor Sos and mediation of a mitogenic signal. Another pool of Grb2 was found in complex with a tyrosine-phosphorylated 89-kDa component after FGF-2 stimulation. Stimulation with other growth factors did not lead to tyrosine phosphorylation of p89. As shown by "far-Western" analysis, p89 bound directly to the Grb2 SH2 domain, and this interaction was inhibited by a peptide containing the Y(P)-X-N motif. Tyrosine-phosphorylated p89 was found exclusively in the membrane fraction, indicating its role in bringing Grb2, as well as Sos, to the plasma membrane. These data support the concept of growth factor-specific coupling of Grb2 to the Ras pathway.

Requirement of tyrosine residues 333 and 338 of the growth hormone (GH) receptor for selected GH-stimulated function

We have examined the involvement of tyrosine residues 333 and 338 of the growth hormone (GH) receptor in the cellular response to GH. Stable Chinese hamster ovary (CHO) cell clones expressing a receptor with tyrosine residues at position 333 and 338 of the receptor substituted for phenylalanine (CHO-GHR1-638 Y333F, Y338F) were generated by cDNA transfection. Compared with the wild type receptor the Y333F,Y338F mutant possessed normal high affinity ligand binding, hormone internalization, and ligand-induced receptor down-regulation. GH activation of mitogen-associated protein kinase was also similar in CHO clones expressing similar wild type and Y333F,Y338F receptor number. However, two GH-regulated cellular events (lipogenesis, and protein synthesis) were deficient in the tyrosine substituted receptor. In contrast, transcriptional regulation by GH (as evidenced by chloramphenicol acetyltransferase cDNA expression driven by the GH-responsive region of the SPI 2.1 gene) was not affected by Y333F,Y338F substitution. Thus we provide the first experimental evidence that specific tyrosine residues of the GH receptor are required for selected cellular responses to GH.

Interaction of the Flt-1 tyrosine kinase receptor with the p85 subunit of phosphatidylinositol 3-kinase. Mapping of a novel site involved in binding

We have examined the interactions of the p85 regulatory subunit of phosphatidylinositol 3-kinase with the endothelium-specific Flt-1 receptor tyrosine kinase using the yeast two-hybrid system. We find that both the amino- and carboxyl-terminal SH2 domains of p85 bind to Flt-1. We have performed site-directed mutagenesis on the carboxyl-terminal tail of the Flt-1 receptor in order to identify the site(s) that is responsible for the p85 interactions. A single tyrosine to phenylalanine change at position 1213 inhibits the binding of both p85 SH2 domains. Phosphopeptide mapping of the wild type and mutant protein expressed in insect cells verifies that this amino acid is a target for autophosphorylation. The amino acids following this tyrosine are VNA and thus define a novel binding site for p85.

Structural requirements of the epidermal growth factor receptor for tyrosine phosphorylation of eps8 and eps15, substrates lacking Src SH2 homology domains

Phosphorylation of two newly identified epidermal growth factor (EGF) receptor substrates, eps8 and eps15, which do not possess Src homology (SH2) domains, was investigated using EGF receptor mutants of the autophosphorylation sites and deletion mutants of the carboxyl-terminal region. Two mutants, F5, in which all five tyrosine autophosphorylation sites substituted by phenylalanine, and Dc 123F, in which four tyrosines were removed by deletion and the fifth (Tyr-992) was mutated into phenylalanine, phosphorylated eps8 and eps15 as efficiently as the wild-type receptor. In contrast, SH2-containing substrates, phospholipase C gamma, the GTPase-activating protein of Ras, the p85 subunit of phosphatidylinositol 3 kinase, and the Src and collagen homology protein, are not phosphorylated by the F5 and Dc 123F mutants. A longer EGF receptor deletion mutant, Dc 214, lacking all five autophosphorylation sites, was unable to phosphorylate eps15 but phosphorylated eps8 13-fold more than the wild-type receptor. To determine the EGF receptor region important for phosphorylation of eps8 and eps15, progressive deletion mutants lacking the final 123, 165, 196, and 214 COOH-terminal residues were used. eps8 phosphorylation was progressively increased in Dc 165, Dc 196, and Dc 214 EGF receptor mutants, indicating that removal of the final 214 COOH-terminal residues increases the phosphorylation of this substrate by the EGF receptor. In contrast, eps15 was phosphorylated by Dc 123 and Dc 165 EGF receptor mutants but not by Dc 196 and Dc 214 mutants. This indicates that a region of 30 residues located between Dc 165 and Dc 196 is essential for eps15 phosphorylation. This is the first demonstration of structural requirements in the EGF receptor COOH terminus for efficient phosphorylation of non-SH2-containing substrates. In addition, enhanced eps8 phosphorylation correlates well with the increased transforming potential of EGF receptor deletion mutants Dc 196 and Dc 214, suggesting that this substrate may be involved in mitogenic signaling.

Activation of mitogen-activated protein kinases by vascular endothelial growth factor and basic fibroblast growth factor in capillary endothelial cells is inhibited by the antiangiogenic factor 16-kDa N-terminal fragment of prolactin

A number of factors both stimulating and inhibiting angiogenesis have been described. In the current work, we demonstrate that the angiogenic factor vascular endothelial growth factor (VEGF) activates mitogen-activated protein kinase (MAPK) as has been previously shown for basic fibroblast growth factor. The antiagiogenic factor 16-kDa N-terminal fragment of human prolactin inhibits activation of MAPK distal to autophosphorylation of the putative VEGF receptor, Flk-1, and phospholipase C-gamma. These data show that activation and inhibition of MAPK may play a central role in the control of angiogenesis.

A truncated K-sam product lacking the distal carboxyl-terminal portion provides a reduced level of autophosphorylation and greater resistance against induction of differentiation

The K-sam gene was originally cloned from KATO-III human gastric cancer cells and is identical to the bek or keratinocyte growth factor (KGF) receptor (KGFR) or fibroblast growth factor receptor 2 gene. K-sam generates several variant transcripts by alternative splicing, and the most abundant K-sam transcript in KATO-III cells was cloned as the K-sam-IIC3 cDNA, which has the KGF-binding motif and a short carboxyl terminus lacking a putative phospholipase C-gamma 1 association site, Tyr-769. The K-sam-IIC3 cDNA was distinct from the K-sam-IIC1 cDNA, which was the same as the previously reported KGFR cDNA. The K-sam-IIC1 product contains a long carboxyl terminus with Tyr-769. K-sam-IIC3 showed greater transforming activity in NIH 3T3 cells than did K-sam-IIC1, and in gastric cancer cell lines in general, the level of K-sam-IIC3 mRNA was greater than that of K-sam-IIC1 mRNA. Here we report that the K-sam-IIC3 product was less autophosphorylated than the K-sam-IIC1 product in NIH 3T3 transfectants. K-sam-IIC3-transfected keratinocytes showed a stronger mitogenic response to KGF than did K-sam-IIC1 transfectants. Moreover, K-sam-IIC3-transfected L6 myoblast cells hardly differentiated when cultured in differentiation-inducing medium and growth was not significantly affected, while K-sam-IIC1 transfectants showed a differentiated phenotype with a reduced growth rate. These data indicate the difference in the signal transduction mediated by two KGFR-type K-sam variants generated by alternative splicing which might be involved in certain differentiation and carcinogenesis scenarios.

Ligation of the alpha 2-macroglobulin signalling receptor on macrophages induces protein phosphorylation and an increase in cytosolic pH

We have recently described an alpha 2-macroglobulin (alpha 2M) signalling receptor which is distinct from the low-density lipoprotein-related protein/alpha 2M receptor (LRP/alpha 2MR). Ligation of the macrophage signalling receptor by alpha 2M-methylamine stimulates production of several second messengers and involves a pertussis toxin-insensitive G-protein. We now report that binding of alpha 2M-methylamine, or the cloned M(r) = 20,000 receptor-binding fragment from rat alpha 1M, to macrophage alpha 2M signalling receptors induces protein phosphorylation. By use of a monoclonal antibody to phospholipase C gamma l (PLC gamma l) we were able to identify it as one target for protein phosphorylation. Phosphorylation was time and concentration dependent, being optimal at about 60 s of incubation and a 100-200 nM ligand concentration. By use of a second monoclonal antibody directed against phosphotyrosine, we were able to demonstrate that at least a portion of the label was incorporated into one or more tyrosine residues. PLC gamma l phosphorylation was then studied in membrane preparations at 4 degrees C in order to minimize serine or threonine modification. Preincubation of macrophage membranes with genistein, a tyrosine kinase inhibitor, drastically reduced phosphorylation of PLC gamma l. Receptor-associated protein, which blocks alpha 2M binding to LRP/alpha 2MR but not to the alpha 2M signalling receptor, had no effect on alpha 2M-methylamine-induced tyrosine phosphorylation of PLC gamma l. Binding of lactoferrin to LRP/alpha 2MR failed to induce phosphorylation of PLC gamma l, further supporting the hypothesis that the alpha 2M signalling receptor and LRP/alpha 2MR are distinct entities. Growth factors which induce tyrosine phosphorylation typically cause a rise in cytosolic pH. Binding of a2M-methylamine to macrophages also gradually increased the intracellular pH in a concentration-dependent manner, being optimal at a 200 nM ligand concentration. The increase in pH was amiloride sensitive. We propose that receptor-recognized forms of a2M may function like growth factors with regard to macrophage regulation.

Insulin-like growth factor I receptor is required for the mitogenic and transforming activities of the platelet-derived growth factor receptor

R- cells are 3T3-like cells derived from mouse embryos in which the insulin-like growth factor I (IGF-I) receptor (IGF-IR) genes have been disrupted by targeted homologous recombination. These cells cannot grow in serum-free medium supplemented by the growth factors that sustain the growth of other 3T3 cell lines, and cannot be transformed by oncogenes that easily transform wild type mouse embryo cells. We have used these cells to study the role of the IGF-IR in the growth and transformation of cells overexpressing the platelet-derived growth factor (PDGF)-beta beta receptor. We report that an overexpressed PDGF-beta beta receptor fails to induce mitogenesis or transformation in cells lacking the IGF-IR, while capable of doing so in cells expressing the IGF-IR. We conclude that the ability of the activated PDGF-beta beta receptor to stimulate cell proliferation and transformation requires a functional IGF-IR.

Functions of fibroblast growth factors and their receptors

Fibroblast growth factors were first characterized twenty years ago as mitogens of cultured fibroblasts. Despite a wealth of data from experiments in vitro, insights have begun to emerge only recently on the normal function of these growth factors in mice and humans, as a result of studies of natural and experimental mutations in the factors and their receptors.

Antagonistic effects of different members of the fibroblast and platelet-derived growth factor families on adipose conversion and NADPH-dependent H2O2 generation in 3T3 L1-cells

3T3 L1-cells, which undergo adipose conversion in vitro, possess a stimulus-sensitive H2O2-generating system in their plasma membrane, and its properties are virtually identical with those of the insulin-sensitive human fat-cell oxidase [Krieger-Brauer and Kather (1992) J. Clin. Invest. 89, 1006-1013]. Insulin and insulin-like growth factor I were found to be active stimulators of NADPH-dependent H2O2 generation. Surprisingly, the acidic (a) and basic (b) isoforms of fibroblast growth factor (FGF) as well as the AA and BB homodimers of platelet-derived growth factor (PDGF) had antagonistic effects on NADPH-dependent H2O2 generation in plasma membranes which were parallelled by corresponding changes in H2O2 accumulation in intact cells. bFGF and PDGF BB (which inhibit NADPH-dependent H2O2 generation) prevented the adipose conversion of 3T3 L1-preadipocytes, and this effect could be reversed by exogenously supplied H2O2. Conversely, aFGF and PDGF AA, which stimulated H2O2 generation, accelerated adipocyte conversion in the presence of insulin and were adipogenic in themselves. Consistently, expression of the adipocyte phenotype induced by insulin, dexamethasone and isobutylmethylxanthine was enhanced in the presence of exogenous hypoxanthine/xanthine oxidase, whereas antioxidants, such as N-acetylcysteine or ascorbate, suppressed the process of differentiation. It is concluded that the H2O2 produced in response to hormones and cytokines may contribute to the development and maintenance of the differentiated state.

Measurement of the binding of tyrosyl phosphopeptides to SH2 domains: a reappraisal

Src homology 2 (SH2) domain-mediated interactions with phosphotyrosine residues are critical in many intracellular signal transduction pathways. Attempts to understand the determinants of specificity and selectivity of these interactions have prompted many binding studies that have used several techniques. Some discrepancies, in both the absolute and relative values of the dissociation constants for particular interactions, are apparent. To establish the correct dissociation constants and to understand the origin of these differences, we have analyzed three previously determined interactions using the techniques of surface plasmon resonance and isothermal titration calorimetry. We find that the binding of SH2 domains to phosphopeptides is weaker than generally presumed. A phosphopeptide based on the hamster polyoma middle tumor antigen interacts with the SH2 domain from Src with an equilibrium dissociation constant (Kd) of 600 nM; a phosphopeptide based on one binding site from the platelet-derived growth factor receptor binds to the N-terminal SH2 domain of the 1-phosphatidylinositol 3-kinase p85 subunit with a Kd of 300 nM; and a phosphopeptide based on the C terminus of Lck binds to the SH2 domain of Lck with a Kd of 4 microM. In addition, we demonstrate that avidity effects that result from the dimerization of glutathione S-transferase fusion proteins with SH2 domains could be responsible for overestimates of affinities for these interactions previously studied by surface plasmon resonance.

Differential regulation of primary response gene expression in skeletal muscle cells through multiple signal transduction pathways

One of the earliest cellular responses to growth factors is the rapid induction of primary response genes. One group of such genes was originally isolated as tetradecanoyl phorbol acetate (TPA) inducible sequences (TIS genes) from mouse 3T3 cells. Proteins encoded by the TIS genes include two transcription factors: TIS8 (also known as egr1/NGFIA/zif268) and TIS1 (also known as NGFIB/nur77/N10). We have examined the inducibility of these two genes in a skeletal muscle cell line in response to agents that have been reported to block muscle differentiation. We report here that basic fibroblast growth factor (bFGF) induced the expression of both TIS1 and TIS8 in mouse C2C12cells. Both genes were also inducible by TPA while forskolin which activates the cAMP-dependent pathway induced TIS1 but not TIS8. Down-regulation of protein kinase C (PKC) activity by TPA pretreatment repressed the bFGF induction of TIS1 but had little effect on the bFGF-stimulated expression of TIS8. Moreover, while both TPA and bFGF stimulated the hyperphosphorylation of c-RAF and the activity of MAP kinase, TPA pretreatment failed to block RAF phosphorylation or the stimulation of MAP kinase activity by bFGF. Induction of the two TIS genes in skeletal myoblasts therefore appeared to be dependent to different extents on the activation of protein kinase A (PKA), PKC and MAP kinase.

Significance of PIP2 hydrolysis and regulation of phospholipase C isozymes

Phosphatidylinositol 4,5-bisphosphate (PIP2) is an important component of several intracellular signaling pathways. It serves as a substrate for phospholipase C, which produces the second messengers inositol 1,4,5-trisphosphate and diacylglycerol. It is also a substrate for a phosphatidylinositol 3-kinase, and regulates the function of a number of actin-binding proteins. PIP2 has been shown recently to serve as a cofactor for a phosphatidylcholine-specific phospholipase D and as a membrane-attachment site for many signaling proteins containing pleckstrin homology domains. The need to stringently regulate the cellular concentration of PIP2 is reflected in part by the fact that there are at least ten distinct mammalian phospholipase C isozymes and multiple mechanisms linking these isozymes to various receptors.

Demonstration of functionally different interactions between phospholipase C-gamma and the two types of platelet-derived growth factor receptors

Phosphorylated tyrosine residues in receptor tyrosine kinases serve as binding sites for signal transduction molecules. We have identified two autophosphorylation sites, Tyr-988 and Tyr-1018, in the platelet-derived growth factor (PDGF) alpha-receptor carboxyl-terminal tail, which are involved in binding of phospholipase C-gamma (PLC-gamma). The capacities of the Y988F and Y1018F mutant PDGF alpha-receptors, expressed in porcine aortic endothelial cells, to bind PLC-gamma are 60 and 5% of that of the wild-type receptor, respectively. Phosphorylated but not unphosphorylated peptides containing Tyr-1018 are able to compete with the intact receptor for binding to immobilized PLC-gamma SH2 domains; a phosphorylated Tyr-988 peptide competes 10 times less efficiently. The complex between PLC-gamma and the PDGF alpha-receptor is more stable than that of PLC-gamma and the PDGF beta-receptor. However, PDGF stimulation results in a smaller fraction of tyrosine-phosphorylated PLC-gamma and a smaller accumulation of inositol trisphosphate in cells expressing the alpha-receptor as compared with cells expressing the beta-receptor. We conclude that phosphorylated Tyr-988 and Tyr-1018 in the PDGF alpha-receptor carboxyl-terminal tail bind PLC-gamma, but this association leads to only a relatively low level of tyrosine phosphorylation and activation of PLC-gamma.

Characterization of fibroblast growth factor receptor 1 RNA expression in the embryonic mouse heart

We used reverse transcriptase-polymerase chain reaction (RT-PCR) to clone fibroblast growth factor receptor (FGFR) 1 isoforms from embryonic mouse heart and as a more sensitive method to characterize FGFR1 RNA expression in embryonic and adult mouse hearts. We describe the cloning of both full-length short (2259 base pairs) and long (2526 base pairs) FGFR1 isoform cDNAs which generated 86 and 102 kilodalton proteins, respectively, following in vitro translation. An assessment of FGFR1 RNA indicates that FGFR1-IIIc is the major form in both the embryonic and adult heart but there is an approximately 8.5-fold decrease in RNA levels in the adult. Differential RNA blotting as well as RT-PCR analyses are consistent with a switch in the relative expression of the short versus long FGFR1 isoforms during heart development. The long isoforms are more abundant in the embryo and the short isoforms predominate in the adult. This may be important in the regulation of growth and development of the heart.

Reduced activation of RAF-1 and MAP kinase by a fibroblast growth factor receptor mutant deficient in stimulation of phosphatidylinositol hydrolysis

Signaling via the fibroblast growth factor receptor 1 (FGFR1, flg) was analyzed in Ba/F3 hematopoietic cells expressing either wild-type or a mutant FGF receptor (Y766F) unable to activate phospholipase C-gamma (PLC-gamma) and stimulate phosphatidylinositol (PI) hydrolysis. Stimulation of cells expressing wild-type or mutant FGFR with acidic FGF (aFGF) caused similar activation of Ras. However, an approximately 3-fold reduced activation of Raf-1 and MAP kinase was observed in aFGF-stimulated cells expressing mutant receptors as compared to cells expressing wild-type FGF receptors. Comparison of phosphopeptide maps of Raf-1 immunoprecipitated from the two cell types activated by either aFGF or the phorbol ester (12-O-tetradecanoylphorbol-13-acetate) suggests that Raf-1 is phosphorylated by both Ras-dependent and PLC-gamma-dependent mechanisms. In spite of the differential effect on Raf-1 and MAP kinase activation, aFGF stimulated similar proliferation of cells expressing wild-type or mutant receptors indicating that Ras-dependent activation of Raf-1 and MAP kinase is sufficient for transduction of FGFR mitogenic signals. Ras may also activate signal transduction pathways that are complementary or parallel to the MAP kinase pathway to stimulate cell proliferation.

Activation of cytosolic phospholipase A2 by basic fibroblast growth factor via a p42 mitogen-activated protein kinase-dependent phosphorylation pathway in endothelial cells

Basic fibroblast growth factor (FGF) stimulates the proliferation, differentiation, and motility of multiple cell types. Signal transduction by FGF is mediated by high affinity FGF receptors that have autophosphorylating tyrosine kinase activity and also elicit the release of low molecular weight signaling molecules, including inositol 1,4,5-trisphosphate, diacylglycerol, and arachidonate. We have shown previously that basic FGF-stimulated, phospholipase A2 (PLA2)-mediated arachidonate release regulates endothelial cell (EC) motility (Sa, G., and Fox, P.L. (1994) J. Biol. Chem. 269, 3219-3225). Here we identify the phospholipase responsible for basic FGF-mediated arachidonate release as cytosolic PLA2 (cPLA2) by demonstrating in EC lysates a requirement for micromolar Ca2+, dithiothreitol insensitivity, and inactivation by anti-cPLA2 antiserum. The role of cPLA2 is also indicated by the observed mechanisms of activation which show a requirement for p42 mitogen-activated protein kinase activity, cPLA2 phosphorylation, and cPLA2 translocation from cytosol to membranes. Phosphorylation of cPLA2, arachidonate release from prelabeled intact cells, and cell motility all have similar concentration dependencies on basic FGF. Since arachidonate release is required for basic FGF-stimulated motility of EC, our results show that p42 mitogen-activated protein kinase activation of cPLA2 may be a regulatory event in stimulation of cellular release of this important eicosanoid precursor during cellular responses to basic FGF.

Expression of basic fibroblast growth factor and its receptors in human fetal microglia cells

The presence of basic fibroblast growth factor (bFGF) and FGF receptors was investigated in microglia cells derived from human fetal brain long-term cultures. Production of bFGF was suggested through the capability of microglial extracts to stimulate plasminogen activator (PA) synthesis in endothelial cells. The identity of PA-stimulating activity with bFGF was confirmed by its high affinity for heparin and its cross-reactivity with polyclonal antibodies to human recombinant bFGF. These antibodies recognized a cell-associated M(r) 18,000 protein as well as trace amounts of the M(r) 24,000 bFGF isoform in Western blot. All microglial cells showed bFGF immunoreactivity in the cytoplasm and, sometimes, in the nucleus. Scatchard plot analysis of 125I-bFGF binding data revealed the presence of low affinity heparansulphate proteoglycans (380,000 +/- 60,000 sites/cell; Kd = 730 +/- 200 nM) and of high affinity tyrosine-kinase receptors (10,300 + 2500 sites/cell; Kd = 30 +/- 9 pM). Immunocytochemistry confirmed the presence of FGF receptor (1/flg) on the cell surface of some, but not all microglial cells, with prevalent association to ameboid microglia. Transcripts for FGF receptors 1, 2, 3 and 4 were found in microglia by Northern blot analysis. Co-expression of bFGF and its receptors in human fetal microglia suggests an autocrine role of bFGF in these cells.

EGF receptor-mediated signals are differentially modulated by concanavalin A

NIH 3T3 cells expressing high levels of the human epidermal growth factor (EGF) receptor were used to examine the effects of the lectin concanavalin A (Con A) on EGF-mediated signaling events. Proliferation of NIH 3T3 cells expressing high levels of the human EGF receptor was inhibited in a dose-dependent manner by Con A. At the same time, Con A also inhibited both dimerization and tyrosine phosphorylation of the EGF receptor. Tyrosine phosphorylation of the enzyme phospholipase C-gamma, a substrate of the phosphorylated EGF receptor kinase, was also inhibited. In contrast, EGF-stimulated changes in pH, calcium, and levels of inositol phosphates were unaffected by the presence of Con A. These results indicate that certain signals (changes in the levels of intracellular calcium, pH, and inositol phosphates) mediated by EGF binding to its receptor still occur when receptor dimerization and phosphorylation are dramatically decreased, suggesting that multiple independent signals are transmitted by the binding of EGF to its receptor.

The role of fibroblast growth factor-2 (FGF-2) in hematopoiesis

Basic fibroblast growth factor (bFGF or FGF-2) is an angiogenic and pleiotropic growth factor involved in the proliferation and differentiation of numerous cell types. It is expressed mostly in tissues of mesoderm and neuroectoderm origin, and is thought to play an important role in the mesoderm induction. Although hematopoietic cells derive from the mesoderm, relatively few studies have, until recently, addressed the role of FGF-2 in hematopoiesis. FGF-2 is expressed in cells of the bone marrow including stromal cells, and possibly cells from several hematopoietic cell lineages. It is stored in the bone marrow extra-cellular matrix and released by enzymes such as heparanase, plasmin, or phospholipase C and D. FGF-receptors (FGF-Rs) are expressed in leukemic cell lines and in hematopoietic cells. FGF-2 positively regulates hematopoiesis, by acting on stromal cells, on early and committed hematopoietic progenitors, and possibly on some mature blood cells. The action of FGF-2 is most likely indirect since its action, on megakaryocytopoiesis for example, is abrogated by anti-IL6 antibodies. It synergizes with hematopoietic cytokines, or antagonizes the negative regulatory effects of TGF-beta. Taken together, these results demonstrate that FGF-2 is a potent hematopoietic growth factor that is likely to play an important role in physiological and pathological hematopoiesis.

Murine FGFR-1 is required for early postimplantation growth and axial organization

We have explored the role of fibroblast growth factor receptor 1 (FGFR-1) in early embryonic development using three experimental systems: genetically deficient mice, in vitro blastocyst culture, and FGFR-1-deficient embryonic stem cells. Using these systems, we demonstrate that FGFR-1 is required for proper embryonic cell proliferation and for the correct axial organization of early postimplantation embryos but not for mesoderm formation. FGFR-1-deficient embryos display severe growth retardation both in vitro and in vivo and die prior to or during gastrulation. Although these mutants can form nonaxial tissues, such as the allantois, amnion, and yolk sac mesoderm, they display defective patterning of the primitive streak and other axial structures, and frequently exhibit truncations or disorganization of posterior embryonic regions. Such abnormalities are unlikely to be caused by intrinsic blocks in mesodermal differentiation, as FGFR-1-deficient ES cell lines form teratomas consisting of many mesodermal cell types.

Bombesin stimulates the in vitro growth of a human gastric cancer cell line

Bombesin (BBS) and its mammalian equivalent, gastrin-releasing peptide (GRP), exhibit diverse biological functions, including that of a neurotransmitter, a regulator of gastrointestinal hormone release, and a trophic factor for various normal and neoplastic tissues. Bombesin stimulates the growth of normal cells of the stomach, pancreas, and bronchial epithelium as well as cells in breast cancer, gastrinoma, and small cell lung cancer. The purpose of this study was to determine whether BBS regulates the growth of a human gastric cancer cell line (SIIA) in vitro, and if so, to examine the mechanisms of signal-transduction that are involved. We found that BBS stimulated the growth of SIIA cells in vitro. The GRP receptor antagonists, BIM 26189 and BIM 26226, had no effect on growth of SIIA cells. Although these antagonists blocked the BBS-induced increase of [Ca2+]i, they failed to block the growth-stimulatory effect of BBS. BBS stimulated intracellular tyrosine phosphorylation of multiple proteins, with a predominant protein of apparent molecular weight of 125 kDa. Inhibition of intracellular tyrosine kinases by tyrphostin blocked the growth-stimulatory effect of BBS on SIIA cells. These results indicate that BBS exerts its trophic effect on SIIA cells through a receptor(s) linked to tyrosine kinase pathway, but not to the phospholipase C (PLC) pathway.

Cell transformation by fibroblast growth factors can be suppressed by truncated fibroblast growth factor receptors

Ligand-induced dimerization and transphosphorylation are thought to be important events by which receptor tyrosine kinases generate cellular signals. We have investigated the ability of signalling-defective, truncated fibroblast growth factor (FGF) receptors (FGFR-1 and FGFR-2) to block the FGF response in cells that express both types of endogenous FGF receptors. When these dominant negative receptors are expressed in NIH 3T3 cells transformed by the secreted FGF-4, the transformed properties of the cells can be reverted to various degrees, with better reversion phenotype correlating with higher levels of truncated receptor expression. Furthermore, truncated FGFR-2 is significantly more efficient at producing reversion than FGFR-1, indicating that FGF-4 preferentially utilizes the FGFR-2 signalling pathway. NIH 3T3 clones expressing these truncated receptors are more resistant to FGF-induced mitogenesis and also exhibit reduced tyrosine phosphorylation upon treatment with FGF. The block in FGF-signalling, however, can be overcome by the addition of excess growth factor. The truncated receptors have binding affinities that are four- to eightfold lower than those of wild-type receptors, as measured by Scatchard analysis. We also observed a partial specificity in the responses of truncated-receptor-expressing clones to FGF-2 or FGF-4. Our results suggest that the block to signal transduction produced by kinase-negative FGF receptors is achieved through a combination of dominant negative effects and competition for growth factor binding with functional receptors.

Cell transformation by fibroblast growth factors can be suppressed by truncated fibroblast growth factor receptors

Ligand-induced dimerization and transphosphorylation are thought to be important events by which receptor tyrosine kinases generate cellular signals. We have investigated the ability of signalling-defective, truncated fibroblast growth factor (FGF) receptors (FGFR-1 and FGFR-2) to block the FGF response in cells that express both types of endogenous FGF receptors. When these dominant negative receptors are expressed in NIH 3T3 cells transformed by the secreted FGF-4, the transformed properties of the cells can be reverted to various degrees, with better reversion phenotype correlating with higher levels of truncated receptor expression. Furthermore, truncated FGFR-2 is significantly more efficient at producing reversion than FGFR-1, indicating that FGF-4 preferentially utilizes the FGFR-2 signalling pathway. NIH 3T3 clones expressing these truncated receptors are more resistant to FGF-induced mitogenesis and also exhibit reduced tyrosine phosphorylation upon treatment with FGF. The block in FGF-signalling, however, can be overcome by the addition of excess growth factor. The truncated receptors have binding affinities that are four- to eightfold lower than those of wild-type receptors, as measured by Scatchard analysis. We also observed a partial specificity in the responses of truncated-receptor-expressing clones to FGF-2 or FGF-4. Our results suggest that the block to signal transduction produced by kinase-negative FGF receptors is achieved through a combination of dominant negative effects and competition for growth factor binding with functional receptors.

Differential activation by platelet-derived growth factor-BB of mitogen activated protein kinases in starved or nonstarved AKR-2B fibroblasts

More than 90% of serum-deprived (starved) AKR-2B mouse fibroblasts are stimulated to divided by the addition of platelet-derived growth factor (PDGF)-BB. In density-arrested (nonstarved) cells, PDGF-BB affords protection from cell death without stimulation of cell division. In both cultivation conditions the cells express similar amounts of PDGF beta-receptors and the receptor kinase activity was identical as judged by its autophosphorylation capacity. Three signaling pathways were studied in detail: 1) Phospholipase C-gamma (PLC-gamma) and [Ca2+]i increase, 2) activation of the phosphatidylinositol-3 kinase (PI-3 kinase), and 3) activation of mitogen activated kinases I and II (MAP kinases I and II). There was no difference in starved or nonstarved cells regarding PLC-gamma activation, increase of [Ca2+]i, and stimulation of PI-3 kinase activity. But most remarkably the activation of MAP-I was largely suppressed in nonstarved cells. The implications of these signaling pathways in cell protection or cell division are discussed.

Heparan sulfate proteoglycans as transducers of FGF-2 signalling

The fibroblast growth factor-2 (FGF-2) low-affinity binding sites, heparan sulfate proteoglycans (HSPGs), function as modulators of FGF-2 activity. It is noteworthy that HSPG binding protects FGF-2 from denaturation and proteolytic degradation, provides a matrix-bound or cell-surface reservoir of this factor for the cells and is required for the activation of FGF high-affinity receptors. In our study we investigated the biological meaning of FGF-2 internalization mediated through its low-affinity binding sites, HSPGs. Using as model system L6 myoblasts lacking endogenous FGF receptors (FGFRs), we demonstrated that these cells internalize FGF-2 efficiently through an HSPG-mediated pathway. FGF-2 internalization occurring through HSPGs was paralleled by an increase in the activity of urokinase plasminogen activator (u-PA). The u-PA-inducing activity of FGF-2 was strictly correlated to its internalization, as chlorate treatment, which causes a strong inhibition of FGF-2 internalization, abrogated the u-PA-inducing activity of FGF-2. In addition, expression of functional FGF high-affinity receptors (FGFR-1) did not enhance u-PA in L6 myoblasts upon FGF-2 stimulation. According to our results we propose that FGF-2 internalization mediated through HSPGs may transduce FGF-2 signalling such as u-PA-activity stimulation. Thus, HSPGs may act as direct transducers of FGF signalling and indeed, different FGF-signalling pathways must exist.

Limited proteolysis of phospholipase C-gamma 1 indicates stable association of X and Y domains with enhanced catalytic activity

Phospholipase C-gamma 1 (PLC-gamma 1) was treated with Staphylococcus aureus V8 protease (V8) and the digestion products were analysed with site-specific antibodies. V8 treatment generated three immunodetectable PLC-gamma 1 fragments of 120, 97, and 39 kDa. The 39 kDa fragment is derived from the C-terminus of PLC-gamma 1 and includes the conserved Y domain present in all PLC isoenzymes. The 120 and 97 kDa fragments are derived from the N-terminus of PLC-gamma 1, possess the conserved X domain common to all PLC isoenzymes, and the src-homology domains unique to PLC-gamma 1 and -gamma 2. It is likely that the 97 kDa fragment is a V8 product of the 120 kDa fragment. As the C-terminal 39 kDa fragment, and either of the N-terminal 120 or 97 kDa fragments, were precipitable with antibody specific to a sequence present in only the 39 kDa fragment, the data indicate co-precipitation of separate polypeptide chains that remain associated after V8 proteolysis. Importantly, V8 treatment increased the activity of PLC-gamma 1 and did not alter the calcium requirement. The influence of other modulators of PLC-gamma 1 activity, however, was lost following V8 treatment. These results suggest the stable association of the X and Y domains within PLC-gamma 1, and demonstrate that proteolysis in the region of PLC-gamma 1 that is subject to tyrosine phosphorylation can enhance catalytic activity.

Hierarchy of binding sites for Grb2 and Shc on the epidermal growth factor receptor

We analyzed the binding site(s) for Grb2 on the epidermal growth factor (EGF) receptor (EGFR), using cell lines overexpressing EGFRs containing various point and deletion mutations in the carboxy-terminal tail. Results of co-immunoprecipitation experiments suggest that phosphotyrosines Y-1068 and Y-1173 mediate the binding of Grb2 to the EGFR. Competition experiments with synthetic phosphopeptides corresponding to known autophosphorylation sites on the EGFR demonstrated that phosphopeptides containing Y-1068, and to a lesser extent Y-1086, were able to inhibit the binding of Grb2 to the EGFR, while a Y-1173 peptide did not. These findings were confirmed by using a dephosphorylation protection assay and by measuring the dissociation constants of Grb2's SH2 domain to tyrosine-phosphorylated peptides, using real-time biospecific interaction analysis (BIAcore). From these studies, we concluded that Grb2 binds directly to the EGFR at Y-1068, to a lesser extent at Y-1086, and indirectly at Y-1173. Since Grb2 also binds Shc after EGF stimulation, we investigated whether Y-1173 is a binding site for the SH2 domain of Shc on the EGFR. Both competition experiments with synthetic phosphopeptides and dephosphorylation protection analysis demonstrated that Y-1173 and Y-992 are major and minor binding sites, respectively, for Shc on the EGFR. However, other phosphorylation sites in the carboxy-terminal tail of the EGFR are able to compensate for the loss of the main binding sites for Shc. These analyses reveal a hierarchy of interactions between Grb2 and Shc with the EGFR and indicate that Grb2 can bind the tyrosine-phosphorylated EGFR directly, as well as indirectly via Shc.

Isoforms of the avian TrkC receptor: a novel kinase insertion dissociates transformation and process outgrowth from survival

TrkC receptor isoforms have been identified by cDNA cloning and RT-PCR analysis of embryonic chick brain RNA. An N-terminal truncation motif is missing from the signal sequence and first cysteine cluster of the extracellular domain. Within the cytoplasmic dimain, a kinase truncation motif retains part of the kinase domain, but appeared to lack activity. Finally, a kinase insert (KI) motif introduces a 25 amino acid sequence distinct from the known mammalian inserts. KI receptors, like full-length receptors, were tyrosine phosphorylated in response to NT-3 and mediated the transformation of chick embryo fibroblasts and process outgrowth from rat PC12 cells. However, KI receptors supported little, if any, survival of serum-deprived PC12 cells. These results indicate that alternative splicing of trkC transcripts is an important mechanism for regulating cellular responses to NT-3.

Hierarchy of binding sites for Grb2 and Shc on the epidermal growth factor receptor

We analyzed the binding site(s) for Grb2 on the epidermal growth factor (EGF) receptor (EGFR), using cell lines overexpressing EGFRs containing various point and deletion mutations in the carboxy-terminal tail. Results of co-immunoprecipitation experiments suggest that phosphotyrosines Y-1068 and Y-1173 mediate the binding of Grb2 to the EGFR. Competition experiments with synthetic phosphopeptides corresponding to known autophosphorylation sites on the EGFR demonstrated that phosphopeptides containing Y-1068, and to a lesser extent Y-1086, were able to inhibit the binding of Grb2 to the EGFR, while a Y-1173 peptide did not. These findings were confirmed by using a dephosphorylation protection assay and by measuring the dissociation constants of Grb2's SH2 domain to tyrosine-phosphorylated peptides, using real-time biospecific interaction analysis (BIAcore). From these studies, we concluded that Grb2 binds directly to the EGFR at Y-1068, to a lesser extent at Y-1086, and indirectly at Y-1173. Since Grb2 also binds Shc after EGF stimulation, we investigated whether Y-1173 is a binding site for the SH2 domain of Shc on the EGFR. Both competition experiments with synthetic phosphopeptides and dephosphorylation protection analysis demonstrated that Y-1173 and Y-992 are major and minor binding sites, respectively, for Shc on the EGFR. However, other phosphorylation sites in the carboxy-terminal tail of the EGFR are able to compensate for the loss of the main binding sites for Shc. These analyses reveal a hierarchy of interactions between Grb2 and Shc with the EGFR and indicate that Grb2 can bind the tyrosine-phosphorylated EGFR directly, as well as indirectly via Shc.

Alternative splicing in fibroblast growth factor receptor 2 is associated with induced epithelial-mesenchymal transition in rat bladder carcinoma cells

We described previously that acidic fibroblast growth factor (aFGF), but not basic fibroblast growth factor (bFGF), can induce the rat carcinoma cell line NBT-II to undergo a rapid and reversible transition from epithelial to mesenchymal phenotype (EMT). We now find that NBT-II EMT is stimulated by keratinocyte growth factor (KGF) in cells grown at low density. Accordingly, a high-affinity receptor showing 98% homology to mouse FGF receptor 2b/KGF receptor was cloned and sequenced from NBT-II cells. Northern analysis indicated that mRNA for FGF receptor 2b/KGF receptor was drastically down-regulated within 1 wk in aFGF-induced mesenchymal NBT-II cells. This decrease coincided with an up-regulation of FGF receptor 2c/Bek, a KGF-insensitive, alternatively spliced form of FGF receptor 2b/KGF receptor. Functional studies confirmed that KGF could not maintain EMT induction on mesenchymal NBT-II cells. FGF receptor 1 and FGF receptor 2c/Bek could also support EMT induction when transfected into NBT-II cells in response to aFGF or bFGF. Such transfected cells could bind bFGF as well as aFGF. Therefore, EMT can be induced through different FGF receptors, but EMT may also regulate FGF receptor expression itself.