Point mutation in FGF receptor eliminates phosphatidylinositol hydrolysis without affecting mitogenesis

FGF signal transduction in PC12 cells: comparison of the responses induced by endogenous and chimeric receptors

Rat phaeochromocytoma (PC12) cells respond to many growth factors and produce different phenotypes, including neurite outgrowth. Receptor tyrosine kinases (RTK), which activate multiple signalling pathways in response to ligand binding, initiate many of these. One such family of receptors, the fibroblast growth factor receptor (FGFR), has four different members and expresses at least three of these in PC12 cells. A chimeric tyrosine kinase receptor, consisting of the extracellular domain of human plasma-derived growth factor receptor-beta (hPDGFR-beta) and the transmembrane and intracellular region of FGFR1 (designated PFR1), was constructed and was stably transfected into cloned PC12 cell lines. This chimera, which can be activated without stimulating endogenous RTK including other FGFR, induces neurite outgrowth in a PDGF-dependent manner. By altering the protocol for preparing the retroviral vectors, cells with a wide range of expression levels can be obtained. The amount of these chimeric receptors seems to correlate with the time and the intensity of response as observed in neurite outgrowth assays. Analysis of proteins implicated in FGFR1 signalling indicates that upon stimulation, a tyrosine phosphorylated protein designated FRS2 associates with SOS, Grb2 and the receptor. The chimeric receptor appears entirely similar to that observed for the stimulation of native PC12 cells with FGF2. These results support the view that FRS2 is the dominant FGFR1 signalling entity in PC12 cells.

Opposite phenotypes of hypomorphic and Y766 phosphorylation site mutations reveal a function for Fgfr1 in anteroposterior patterning of mouse embryos

Intercellular communication is needed for both the generation of the mesodermal germ layer and its division into distinct subpopulations. To dissect the functions of fibroblast growth factor receptor-1 (FGFR1) during mouse gastrulation as well as to gain insights into its possible roles during later embryonic development, we have introduced specific mutations into the Fgfr1 locus by gene targeting. Our results show functional dominance of one of the receptor isoforms and suggest a function for the autophosphorylation of site Y766 in the negative regulation of FGFR1 activity. Y766F and hypomorphic mutations in Fgfr1 generate opposite phenotypes in terms of homeotic vertebral transformations, suggesting a role for FGFR1 in patterning the embryonic anteriorposterior axis by way of regulation of Hox gene activity.

Novel recognition motif on fibroblast growth factor receptor mediates direct association and activation of SNT adapter proteins

Fibroblast growth factors (FGFs) stimulate tyrosine phosphorylation of a membrane-anchored adapter protein, FRS2/SNT-1, promoting its association with Shp-2 tyrosine phosphatase and upstream activators of Ras. Using the yeast two-hybrid protein-protein interaction assay, we show that FRS2/SNT-1 and a newly isolated SNT-2 protein directly bind to FGF receptor-1 (FGFR-1). A juxtamembrane segment of FGFR-1 and the phosphotyrosine-binding domain of SNTs are both necessary and sufficient for interaction in yeast and in vitro, and FGFR-mediated SNT tyrosine phosphorylation in vivo requires these segments of receptor and SNT. Our findings establish SNTs as direct protein links between FGFR-1 and multiple downstream pathways. The SNT binding motif of FGFR-1 is distinct from previously described phosphotyrosine-binding domain recognition motifs, lacking both tyrosine and asparagine residues.

Binding of Shp2 tyrosine phosphatase to FRS2 is essential for fibroblast growth factor-induced PC12 cell differentiation

FRS2 is a lipid-anchored docking protein that plays an important role in linking fibroblast growth factor (FGF) and nerve growth factor receptors with the Ras/mitogen-activated protein (MAP) kinase signaling pathway. In this report, we demonstrate that FRS2 forms a complex with the N-terminal SH2 domain of the protein tyrosine phosphatase Shp2 in response to FGF stimulation. FGF stimulation induces tyrosine phosphorylation of Shp2, leading to the formation of a complex containing Grb2 and Sos1 molecules. In addition, a mutant FRS2 deficient in both Grb2 and Shp2 binding induces a weak and transient MAP kinase response and fails to induce PC12 cell differentiation in response to FGF stimulation. Furthermore, FGF is unable to induce differentiation of PC12 cells expressing an FRS2 point mutant deficient in Shp2 binding. Finally, we demonstrate that the catalytic activity of Shp2 is essential for sustained activation of MAP kinase and for potentiation of FGF-induced PC12 cell differentiation. These experiments demonstrate that FRS2 recruits Grb2 molecules both directly and indirectly via complex formation with Shp2 and that Shp2 plays an important role in FGF-induced PC12 cell differentiation.

soc-2 encodes a leucine-rich repeat protein implicated in fibroblast growth factor receptor signaling

Activation of fibroblast growth factor (FGF) receptors elicits diverse cellular responses including growth, mitogenesis, migration, and differentiation. The intracellular signaling pathways that mediate these important processes are not well understood. In Caenorhabditis elegans, suppressors of clr-1 identify genes, termed soc genes, that potentially mediate or activate signaling through the EGL-15 FGF receptor. We demonstrate that three soc genes, soc-1, soc-2, and sem-5, suppress the activity of an activated form of the EGL-15 FGF receptor, consistent with the soc genes functioning downstream of EGL-15. We show that soc-2 encodes a protein composed almost entirely of leucine-rich repeats, a domain implicated in protein-protein interactions. We identified a putative human homolog, SHOC-2, which is 54% identical to SOC-2. We find that shoc-2 maps to 10q25, shoc-2 mRNA is expressed in all tissues assayed, and SHOC-2 protein is cytoplasmically localized. Within the leucine-rich repeats of both SOC-2 and SHOC-2 are two YXNX motifs that are potential tyrosine-phosphorylated docking sites for the SEM-5/GRB2 Src homology 2 domain. However, phosphorylation of these residues is not required for SOC-2 function in vivo, and SHOC-2 is not observed to be tyrosine phosphorylated in response to FGF stimulation. We conclude that this genetic system has allowed for the identification of a conserved gene implicated in mediating FGF receptor signaling in C. elegans.

Identification and characterization of a novel member of the fibroblast growth factor family

A new member of the fibroblast growth factor (FGF) family, FGF-13, has been molecularly cloned as a result of high throughput sequencing of a human ovarian cancer cell library. The open reading frame of the novel human gene (1419 bp) encodes for a protein of 216 a.a. with a molecular weight of 22 kDa. The FGF-13 sequence contains an amino-terminal hydrophobic region of 23 a.a. characteristic of a signal secretion sequence. FGF-13 is most homologous, 70% similarity at the amino acid level, to FGF-8. Northern hybridization analysis demonstrated prominent expression of FGF-13 in human foetal and adult brain, particularly in the cerebellum and cortex. In proliferation studies with BaF3 cells, FGF-13 preferentially activates cell clones expressing either FGF receptor variant, 3-IIIc or 4. The signal transduction pathways of FGF-13 and FGF-2 were compared in rat hippocampal astrocytes. The two FGFs induce an equivalent level of tyrosine phosphorylation of mitogen-activated protein kinase (MAPK) and c-raf activation. However, FGF-13 is more effective than FGF-2 in inducing the phosphorylation of phospholipase C-gamma (PLC-gamma). Treatment of neuronal cultures from rat embryonic cortex with FGF-13 increases the number of glutamic acid decarboxylase immunopositive neurons, the level of high-affinity gamma-aminobutyric acid (GABA) uptake, and choline acetyltransferase enzyme activity. The GABAergic neuronal response to FGF-13 treatment is rapid with a significant increase occurring within 72 h. We have identified a novel member of the FGF family that is expressed in the central nervous system (CNS) and increases the number as well as the level of phenotypic differentiation of cortical neurons in vitro.

Epidermal growth factor signaling and mitogenesis in Plcg1 null mouse embryonic fibroblasts

Gene targeting techniques and early mouse embryos have been used to produce immortalized fibroblasts genetically deficient in phospholipase C (PLC)-gamma1, a ubiquitous tyrosine kinase substrate. Plcg1(-/-) embryos die at embryonic day 9; however, cells derived from these embryos proliferate as well as cells from Plcg1(+/+) embryos. The null cells do grow to a higher saturation density in serum-containing media, as their capacity to spread out is decreased compared with that of wild-type cells. In terms of epidermal growth factor receptor activation and internalization, or growth factor induction of mitogen-activated protein kinase, c-fos, or DNA synthesis in quiescent cells, PLcg1(-/-) cells respond equivalently to PLcg1(+/+) cells. Also, null cells are able to migrate effectively in a wounded monolayer. Therefore, immortalized fibroblasts do not require PLC-gamma1 for many responses to growth factors.

Lipid metabolism in fibroblast growth factor-stimulated L6 myoblasts: a receptor mutation (Y766F) abrogates phospholipase D and diacylglycerol kinase activities

Phosphatidylcholine (PC) hydrolysis induced by basic fibroblast growth factor (bFGF) was studied in rat L6 myoblasts expressing the wild-type FGF receptor-1 (FGFR-1) or a mutant (Y766F) that is incapable of activating phospholipase C-gamma (PLCgamma). Stimulation of FGFR-1 activated phospholipase D (PLD) rapidly and transiently, but did not induce PC-specific PLC activity. Downregulation of protein kinase C blocked bFGF-induced PLD activation but not phosphatidic acid formation by diacylglycerol (DG) kinase. Only phosphoinositide (PI)-derived DG, not PC-derived DG, appeared to be a substrate for DG kinase. Stimulation of FGFR-1(Y766F) did not activate PLD or DG kinase, both of which apparently require initial PLCgamma activation. The Y766F mutation reduced mitogen-activated protein kinase activation but not cell proliferation. We conclude that both PI turnover and PC hydrolysis are dispensable for bFGF-induced mitogenesis.

Inositol trisphosphate mediates a RAS-independent response to LET-23 receptor tyrosine kinase activation in C. elegans

Activity of LET-23, the C. elegans homolog of the epidermal growth factor receptor, is required in multiple tissues. RAS activation is necessary and sufficient for certain LET-23 functions. We show that an inositol trisphosphate receptor can act as a RAS-independent, tissue-specific positive effector of LET-23. Moreover, an inositol trisphosphate kinase negatively regulates this transduction pathway. Signals transduced by LET-23 control ovulation through changes in spermathecal dilation, possibly dependent upon calcium release regulated by both IP3 and IP4. Our results demonstrate that one mechanism by which receptor tyrosine kinases can evoke tissue-specific responses is through activation of distinct signal transduction cascades in different tissues.

Novel recruitment of Shc, Grb2, and Sos by fibroblast growth factor receptor-1 in v-Src-transformed cells

In response to fibroblast growth factor (FGF), FGF receptor-1 (FGFR-1) (flg) becomes tyrosine phosphorylated and associates with phospholipase C gamma (PLC gamma) and a 90 kDa protein. We report here that in cells transformed by v-Src, FGFR-1 becomes phosphorylated on tyrosine; however, neither PLC gamma nor p90 was found to be associated with tyrosine-phosphorylated FGFR-1. Instead, there was a strong constitutive association of FGFR-1 with the adaptor proteins Shc and Grb2 and the Ras guanine nucleotide exchange factor Sos. Association with Shc and Grb2 and Sos was not observed in response to FGF. Suramin did not prevent either tyrosine phosphorylation or Shc/Grb2/Sos association, indicating a non-autocrine mechanism. Thus, in cells transformed by v-Src, tyrosine phosphorylation of FGFR-1 results not in the expected association with PLC gamma and p90, but rather in the recruitment of the Ras activating Shc/Grb2/Sos complex. These data suggest a mechanism for Ras activation by v-Src involving phosphorylation of novel tyrosine(s) on FGFR-1.

Fibroblast variants nonresponsive to fibroblast growth factor 1 are defective in its nuclear translocation

Fibroblast growth factors (FGF) elicit biological effects by binding to high affinity cell-surface receptors and activation of receptor tyrosine kinase. We previously reported that two NIH/3T3 derivatives, NR31 and NR33 (NR cells), express high levels of full-length FGF-1 and exhibit a complete spectrum of transformed phenotype. In the present study, we report that NR cells respond to the mitogenic stimulation of truncated FGF-1 but not to the full-length FGF-1. Incubation of the NR cells with either form of FGF-1 resulted in its binding to cell-surface FGF receptors, activation of mitogen-activated protein (MAP) kinase, and induction of c-fos and c-myc. These data demonstrate that the FGF receptor-mediated, MAP kinase-dependent signaling pathway is not defective in the NR cells. Our data further suggest that the activation of MAP kinase in response to full-length FGF-1 is not sufficient for mitogenesis. Subcellular distribution of exogenously added FGF-1 demonstrated that full-length FGF-1 fails to translocate to the nuclei of NR31 cells. Although the full-length FGF-1 was detected in the nuclear fractions of both NIH/3T3 and NR33 cells, its half-life is much shortened in NR33 than in NIH/3T3 cells. These observations suggest that non-responsiveness of the two NR cell lines may be due to defectiveness at different steps of nuclear translocation mechanism of FGF-1.

Wortmannin-sensitive phosphorylation, translocation, and activation of PLCgamma1, but not PLCgamma2, in antigen-stimulated RBL-2H3 mast cells

In RBL-2H3 tumor mast cells, cross-linking the high affinity IgE receptor (FcepsilonRI) with antigen activates cytosolic tyrosine kinases and stimulates Ins(1,4,5)P3 production. Using immune complex phospholipase assays, we show that FcepsilonRI cross-linking activates both PLCgamma1 and PLCgamma2. Activation is accompanied by the increased phosphorylation of both PLCgamma isoforms on serine and tyrosine in antigen-treated cells. We also show that the two PLCgamma isoforms have distinct subcellular localizations. PLCgamma1 is primarily cytosolic in resting RBL-2H3 cells, with low levels of plasma membrane association. After antigen stimulation, PLCgamma1 translocates to the plasma membrane where it associates preferentially with membrane ruffles. In contrast, PLCgamma2 is concentrated in a perinuclear region near the Golgi and adjacent to the plasma membrane in resting cells and does not redistribute appreciably after FcepsilonRI cross-linking. The activation of PLCgamma1, but not of PLCgamma2, is blocked by wortmannin, a PI 3-kinase inhibitor previously shown to block antigen-stimulated ruffling and to inhibit Ins(1,4,5)P3 synthesis. In addition, wortmannin strongly inhibits the antigen-stimulated phosphorylation of both serine and tyrosine residues on PLCgamma1 with little inhibition of PLCgamma2 phosphorylation. Wortmannin also blocks the antigen-stimulated translocation of PLCgamma1 to the plasma membrane. Our results implicate PI 3-kinase in the phosphorylation, translocation, and activation of PLCgamma1. Although less abundant than PLCgamma2, activated PLCgamma1 may be responsible for the bulk of antigen-stimulated Ins(1,4,5)P3 production in RBL-2H3 cells.

The heparan sulfate-fibroblast growth factor family: diversity of structure and function

The fibroblast growth factor (FGF) receptor complex is a ubiquitous regulator of development and adult tissue homeostasis that bridges the peri-cellular matrix and the intracellular environment. Diverse members of the FGF polypeptide family, the FGF receptor tyrosine kinase (FGFRTK) family and the FGF receptor heparan sulfate proteoglycan (FGFRHS) family combine to result in active and specific FGFR signal transduction complexes. Regulated alternate splicing and combination of variant subdomains give rise to diversity of FGFRTK monomers. Divalent cations cooperate with the FGFRHS to conformationally restrict FGFRTK trans-phosphorylation, which causes depression of kinase activity and facilitates appropriate activation of the FGFR complex by FGF. Diffusional and conformational molecular models of the oligomeric FGFR complex are presented to explain how different point mutations in the FGFRTK commonly cause craniofacial and skeletal abnormalities of graded severity by graded increases in FGF-independent activity of total FGFR complexes. The role of the FGF family in liver growth and function and in prostate tumor progression is discussed.

Requirement for phospholipase C-gamma1 enzymatic activity in growth factor-induced mitogenesis

The cytoplasmic regions of the receptors for epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) bind and activate phospholipase C-gamma1 (PLC-gamma1) and other signaling proteins in response to ligand binding outside the cell. Receptor binding by PLC-gamma1 is a function of its SH2 domains and is required for growth factor-induced cell cycle progression into the S phase. Microinjection into MDCK epithelial cells and NIH 3T3 fibroblasts of a polypeptide corresponding to the noncatalytic SH2-SH2-SH3 domains of PLC-gamma1 (PLC-gamma1 SH2-SH2-SH3) blocked growth factor-induced S-phase entry. Treatment of cells with diacylglycerol (DAG) or DAG and microinjected inositol-1,4,5-triphosphate (IP3), the products of activated PLC-gamma1, did not stimulate cellular DNA synthesis by themselves but did suppress the inhibitory effects of the PLC-gamma1 SH2-SH2-SH3 polypeptide but not the cell cycle block imposed by inhibition of the adapter protein Grb2 or p21 Ras. Two c-fos serum response element (SRE)-chloramphenicol acetyltransferase (CAT) reporter plasmids, a wild-type version, wtSRE-CAT, and a mutant, pm18, were used to investigate the function of PLC-gamma1 in EGF- and PDGF-induced mitogenesis. wtSRE-CAT responds to both protein kinase C (PKC)-dependent and -independent signals, while the mutant, pm18, responds only to PKC-independent signals. Microinjection of the dominant-negative PLC-gamma1 SH2-SH2-SH3 polypeptide greatly reduced the responses of wtSRE-CAT to EGF stimulation in MDCK cells and to PDGF stimulation in NIH 3T3 cells but had no effect on the responses of mutant pm18. These results indicate that in addition to Grb2-mediated activation of Ras, PLC-gamma1-mediated DAG production is required for EGF- and PDGF-induced S-phase entry and gene expression, possibly through activation of PKC.

Cloning and interspecies comparisons of three newt (Notophthalmus viridescens) fibroblast growth factor receptor sequences

We report the nucleotide sequences of two fibroblast growth factor receptor (FGFR) cDNAs, FGFR1 and FGFR3, from the newt species Notophthalmus viridescens. These two cDNA sequences and a previously published newt FGFR cDNA, FGFR2, were used to derive the amino acid sequences which were then compared with their homologues from other species. This comparison shows that the intracellular tyrosine kinase domain is highly conserved across the species examined with the second half of the domain slightly more conserved than the first half. The 3' portion of the carboxyl terminal tail is not very highly conserved. The comparison of the extracellular portion of FGFR2 shows a high degree of conservation among the Ig-like domains and a low degree of conservation in the region that links the third Ig-like domain with the transmembrane domain.

Phosphatidylinositol 3'-kinase-independent p70 S6 kinase activation by fibroblast growth factor receptor-1 is important for proliferation but not differentiation of endothelial cells

p70(s6k) has a role in cell cycle progression in response to specific extracellular stimuli. The signal transduction pathway leading to activation of p70(s6k) by fibroblast growth factor receptor-1 (FGFR-1) was examined in FGF-2-treated rat L6 myoblasts. p70(s6k) was activated in a biphasic and rapamycin-sensitive manner. Although phosphatidylinositol 3'-kinase was not activated in the FGF-2 treated cells, as judged from in vitro and in vivo analyses, wortmannin and LY294002 treatment inhibited p70(s6k) activation. Inhibition of protein kinase C (PKC), by bisindolylmaleimide or by chronic phorbol ester treatment of the FGFR-1 cells, suppressed but did not block p70(s6k) activation. In cells expressing a point-mutated FGFR-1, Y766F, unable to mediate PKC activation, p70(s6k) was still activated, in a bisindolylmaleimide- and phorbol ester-resistant manner. The involvement of S6 kinase in FGFR-1-dependent biological responses was examined in murine brain endothelial cells. In response to FGF-2, these cells differentiate to form tube-like structures in collagen gel cultures and proliferate when cultured on fibronectin. p70(s6k) was not activated in endothelial cells on collagen, whereas activation was observed during proliferation on fibronectin. In agreement with this finding, rapamycin inhibited the proliferative but not the differentiation response. Our results indicate that FGFR-1 mediates p70(s6k) activation by a phosphatidylinositol 3'-kinase-independent mechanism that does not require PKC activation and, furthermore, proliferation, but not differentiation of endothelial cells in response to FGF-2, is associated with p70(s6k) activation.

Interleukin-2 modulates the responsiveness to angiotensin II in cultured vascular smooth muscle cells

Preincubation with interleukin-2 (IL-2), a T cell-derived cytokine, enhanced the increase in intracellular Ca2+ ([Ca2+]i) induced by angiotensin II (AII) in vascular smooth muscle cells (VSMC). IL-2 itself did not affect the basal [Ca2+]i level or the maximal response of [Ca2+]i increase induced by AII. Furthermore, IL-2-induced enhancement was not observed in the absence of extracellular Ca2+, suggesting that IL-2 enhances Ca2+ influx induced by AII. IL-2 also enhanced the stimulation of DNA synthesis induced by AII, although IL-2 alone did not stimulate DNA synthesis. Genistein, an inhibitor of protein tyrosine kinases, significantly inhibited IL-2-induced enhancement of both Ca2+ influx and DNA synthesis induced by AII. A neutralizing antibody against heparin-binding epidermal growth factor-like growth factor (HB-EGF) partially inhibited IL-2-induced enhancement of DNA synthesis induced by AII. These findings suggest that autocrine HB-EGF is partially involved in the mechanism of IL-2-induced enhancement of DNA synthesis. On the other hand IL-2 stimulated both glycosaminoglycan (GAG) and prostacyclin syntheses and enhanced the stimulation of both GAG and prostacyclin syntheses induced by AII. Therefore, IL-2 may play important roles in the pathogenesis of atherosclerosis and vascular disease by modulating the responsiveness to AII in VSMC.

Transforming growth factor alpha treatment alters intracellular calcium levels in hair cells and protects them from ototoxic damage in vitro

To determine if transforming growth factor alpha (TGF alpha) pretreatment protects hair cells from aminoglycoside induced injury by modifying their intracellular calcium concentration, we assayed hair cell calcium levels in organ of Corti explants both before and after aminoglycoside (i.e. neomycin, 10(-3) M) exposure either with or without growth factor pretreatment. After TGF alpha (500 ng/ml) treatment, the intracellular calcium level of hair cells showed a five-fold increase as compared to the levels observed in the hair cells of control cultures. After ototoxin exposure, calcium levels in hair cells of control explants showed an increase relative to their baseline levels, while in the presence of growth factors pretreatment, hair cells showed a relative reduction in calcium levels. Pretreatment of organ of Corti explants afforded significant protection of hair cell stereocilia bundle morphology from ototoxic damage when compared to explants exposed to ototoxin alone. This study correlates a rise in hair cell calcium levels with the otoprotection of hair cells by TGF alpha in organ of Corti explants.

Regulation of proliferation and apoptosis by Ras and Rho GTPases through specific phospholipid-dependent signaling

Small GTPases are molecular switches that control signaling pathways critical for diverse cellular functions. Recent evidence indicates that multiple effector molecules can be activated by small GTPases. As a result, complex biological processes such as cell proliferation and apoptosis are turned on. Thus, rather than a single linear pathway from the membrane to the nucleus, the integration of complementary signals is required for these events to occur. In fact, the coordinated activation of small GTPases may constitute some of the critical modulators of those signals triggering either proliferation or cell death. In addition to the activation of specific kinases cascades, phospholipid-derived messengers are candidates to compose some of the most critical elements associated to regulation of signaling cascades capable of discerning among life and death. Both proliferation and apoptosis needs competence and progression signals. Phospholipase D and sphingomyelinase may be important players in this decision-maker step.

Phospholipase C-gamma1: regulation of enzyme function and role in growth factor-dependent signal transduction

Phospholipase C(gamma)1 (PLC-gamma1), a tyrosine kinase substrate, is a multi-domain molecule that modulates the intracellular levels of the second messenger molecules: Ca2+ and diacylglycerol. Although a wide variety of growth factor receptor tyrosine kinases phosphorylate and activate PLC-gamma1, the biological role and necessity of this signal transduction element in mitogenesis has remained unclear. Recent results, however, point to a more essential role than was suggested by initial studies. Also, biochemical studies have indicated a putative means for the intramolecular repression of PLC-gamma1 activity and provide a means for interpreting activation signals through a derepression mechanism.

Tumor promotion by depleting cells of protein kinase C delta

Tumor-promoting phorbol esters activate, but then deplete cells of, protein kinase C (PKC) with prolonged treatment. It is not known whether phorbol ester-induced tumor promotion is due to activation or depletion of PKC. In rat fibroblasts overexpressing the c-Src proto-oncogene, the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) induced anchorage-independent growth and other transformation-related phenotypes. The appearance of transformed phenotypes induced by TPA in these cells correlated not with activation but rather with depletion of expressed PKC isoforms. Consistent with this observation, PKC inhibitors also induced transformed phenotypes in c-Src-overexpressing cells. Bryostatin 1, which inhibited the TPA-induced down-regulation of the PKCdelta isoform specifically, blocked the tumor-promoting effects of TPA, implicating PKCdelta as the target of the tumor-promoting phorbol esters. Consistent with this hypothesis, expression of a dominant negative PKCdelta mutant in cells expressing c-Src caused transformation of these cells, and rottlerin, a protein kinase inhibitor with specificity for PKCdelta, like TPA, caused transformation of c-Src-overexpressing cells. These data suggest that the tumor-promoting effect of phorbol esters is due to depletion of PKCdelta, which has an apparent tumor suppressor function.

A lipid-anchored Grb2-binding protein that links FGF-receptor activation to the Ras/MAPK signaling pathway

Activation of the Ras/MAPK signaling cascade is essential for growth factor-induced cell proliferation and differentiation. In this report, we describe the purification, cloning, and characterization of a novel protein, designated FRS2, that is tyrosine phosphorylated and binds to Grb2/Sos in response to FGF or NGF stimulation. We find that FRS2 is myristylated and that this modification is essential for membrane localization, tyrosine phosphorylation, Grb2/Sos recruitment, and MAPK activation. FRS2 functions as a lipid-anchored docking protein that targets signaling molecules to the plasma membrane in response to FGF stimulation to link receptor activation with the MAPK and other signaling pathways essential for cell growth and differentiation. Finally, we demonstrate that FRS2 is closely related and probably indentical to SNT, the long-sought target of FGF and NGF receptors.

Positive and negative tissue-specific signaling by a nematode epidermal growth factor receptor

The major determinants of receptor tissue tyrosine kinase (RTK) signaling specificity have been proposed to be Src homology 2 (SH2) binding sites, phosphotyrosine-containing oligopeptides in the cytoplasmic domain of the receptor. The Caenorhabditis elegans epidermal growth factor receptor homologue LET-23 has multiple functions during development and has eight potential SH2-binding sites in a region carboxyl terminal to its kinase domain. By analyzing transgenic nematodes for three distinct LET-23 functions, we show that six of eight potential sites function in vivo and that they are required for most, but not all, of LET-23 activity. A single site is necessary and sufficient to promote wild-type fertility. Three other sites activate the RAS pathway and are involved only in viability and vulval differentiation. A fifth site is promiscuous and can mediate all three LET-23 functions. An additional site mediates tissue-specific negative regulation. Putative SH2 binding sites are thus key effectors of both cell-specific and negative regulation in an intact organism. We suggest two distinct mechanisms for tissue-specific RTK-mediated signaling. A positive mechanism would promote RTK function through effectors present only in certain cell types. A negative mechanism would inhibit RTK function through tissue-specific negative regulators.

Altered expression of fibroblast growth factor receptors in human pituitary adenomas

We have shown that basic fibroblast growth factor (FGF) is heterogeneously expressed by human pituitary adenomas and may be implicated as a growth stimulus for these tumors. There are four mammalian FGF receptor (FGFR) genes encoding a complex family of transmembrane tyrosine kinases. The prototypic receptor is composed of three Ig-like extracellular ligand-binding domains, a transmembrane domain, and a cytoplasmic split tyrosine kinase. Multiple forms of cell-bound or secretable isoforms of FGFR-1, -2, and -3 can be generated by cell- and tissue-specific alternative splicing, resulting in tissue-specific FGF function. Shifts in isoform expression accompany tumor progression in some systems. We examined the normal human adenohypophysis and 40 pituitary adenomas to determine the pattern of FGFR expression by reverse transcription-PCR; all tumors were characterized clinically and morphologically. Ribonucleic acid (RNA) was extracted from frozen tumor tissue and primers were used to distinguish messenger RNA of the secretable first Ig-like domain (I) and those of the transmembrane and kinase domains (K) of each FGFR subtype. The normal pituitary-expressed mRNAs for FGFR-1 I and K, FGFR-2 I and K, FGFR-3 I and K, and FGFR-4 I but not FGFR-4 K; this represents the first report of a truncated isoform of FGFR-4, indicating possible alternative polyadenylation sites in this receptor. Only 3 tumors had the same pattern of expression of the 4 FGFRs as the normal gland. Although all tumors expressed FGFR-1 I, 1 tumor did not express FGFR-1 K, suggesting the production of only a secretable form of FGFR-1 by this tumor. Four tumors were negative for FGFR-2 I and K; 6 expressed the secretable form only, and 17 expressed FGFR-2 K but not I. All tumors expressed FGFR-3 I; 14 had secretable forms only, and no tumors expressed FGFR-3 K alone. As in the normal gland, 13 tumors expressed only the secretable I form of FGFR-4. Unlike the normal pituitary, however, 22 expressed FGFR-4 I and K, indicating a possible tumor-specific transmembrane receptor. Five tumors were negative for FGFR-4 I and K. Expression of FGFR proteins was confirmed by immunohistochemical localization of the C-terminal portion of FGFR-1, -2, -3, and -4; the results correlated with the RNA data in each case. There was no correlation between tumor type, size, or aggressiveness and the expression pattern of FGFRs. Our study suggests that pituitary adenomas have altered FGFR subtype and isoform expression, which may determine their hormonal and proliferative responses to FGFs.

Essential role of the tyrosine kinase substrate phospholipase C-gamma1 in mammalian growth and development

The activation of many tyrosine kinases leads to the phosphorylation and activation of phospholipase C-gamma1 (PLC-gamma1). To examine the biological function of this protein, homologous recombination has been used to selectively disrupt the Plcg1 gene in mice. Homozygous disruption of Plcg1 results in embryonic lethality at approximately embryonic day (E) 9.0. Histological analysis indicates that Plcg1 (-/-) embryos appear normal at E 8.5 but fail to continue normal development and growth beyond E 8.5-E9.0. These results clearly demonstrate that PLC-gamma1 with, by inference, its capacity to mobilize second messenger molecules is an essential signal transducing molecule whose absence is not compensated by other signaling pathways or other genes encoding PLC isozymes.

Tyrosine phosphorylation events during coxsackievirus B3 replication

In order to study cellular and viral determinants of pathogenicity, interactions between coxsackievirus B3 (CVB3) replication and cellular protein tyrosine phosphorylation were investigated. During CVB3 infection of HeLa cells, distinct proteins become phosphorylated on tyrosine residues, as detected by the use of antiphosphotyrosine Western blotting. Two proteins of 48 and 200 kDa showed enhanced tyrosine phosphorylation 4 to 5 h postinfection (p.i.), although virus-induced inhibition of cellular protein synthesis had already occurred 3 to 4 h p.i. Subcellular fractionation experiments revealed distinct localization of tyrosine-phosphorylated proteins of 48 and 200 kDa in the cytosol and membrane fractions of infected cells, respectively. In addition, in Vero cells infected with CVB3, echovirus (EV)11, or EV12, increased tyrosine phosphorylation of a 200-kDa protein was detected 6 h p.i. Herbimycin A, a specific inhibitor of Src-like protein tyrosine kinases, was shown to inhibit virus-induced tyrosine phosphorylations and to reduce the production of progeny virions. In contrast, in cells treated with the inhibitors staurosporine and calphostin C, the synthesis of progeny virions was not affected. Immunoprecipitation experiments suggested that the tyrosine-phosphorylated 200-kDa protein in CVB3-infected cells is of cellular origin. In summary, these investigations have begun to unravel the effect of CVB3 as well as EV11 and EV12 replication on cellular tyrosine phosphorylation and support the importance of tyrosine phosphorylation events for effective virus replication. Such cellular phosphorylation events triggered in the course of enterovirus infection may enhance virus replication.

Amphibian FGF-1 is structurally and functionally similar to but antigenically distinguishable from its mammalian counterpart

Recent studies have shown that fibroblast growth factors (FGF) play an important role in the diverse cellular mechanisms involved with vertebrate development. One system which has received a great deal of attention is the developing limb in part because of the extensive epithelial-mesenchymal interactions that take place during this process. Because it closely parallels the developmental process of the limb and is a model for wound repair, the phenomenon of amphibian limb regeneration has been used to investigate the role of FGF in these processes. We have recently reported on the cloning and functional characterization of an FGF receptor (FGFR) isolated from amphibian regenerative tissue. In this report, we describe the isolation and characterization of an FGF-1 molecule from the newt, Notophthalmus viridescens. Amino acid sequence comparisons indicate that the newt FGF-1 exhibits between 79 to 83% identity with FGF-1 from mammalian and avian species. The full length cDNA of the newt FGF-1 was cloned into a prokaryotic expression vector and purified from E. coli. Although the newt FGF-1 shares a high degree of primary amino acid sequence similarity with other FGF-1 molecules, the recombinant protein was not detected in a Western blot analysis using a polyclonal antibody directed against mammalian FGF-1. Despite the antigenic divergence, the newt FGF-1 was capable of binding to NIH/3T3 and Chinese hamster ovary cells overexpressing mammalian and amphibian FGFRs with dissociation constants comparable to those reported for mammalian FGF-1. Newt FGF-1 could also be cross-linked to receptors on the surface of NIH/3T3 cells. In addition, it elicits a mitogenic response in NIH/3T3 cells indistinguishable from human recombinant FGF-1.

Down-regulation of a novel form of fibroblast growth factor receptor 1 in human breast cancer

Monoclonal antibodies against two epitopes of FGFR-1 have been used to investigate FGFR-1 expression in the normal and neoplastic human breast. Different forms are detected in the different cell types constituting the normal breast. Moreover, breast cancer cells lack one form of FGFR-1. Western blot analysis showed 115-kDa and 106-kDa forms of FGFR-1 within the human breast. The 115-kDa band corresponds to the beta form of FGFR-1, whereas the 106-kDa band is truncated at the carboxyl terminus. The 106-kDa form of FGFR-1 is the major form present in breast fibroblasts and myoepithelial cells, whereas epithelial cells contain equal amounts of the 115-kDa and 106-kDa forms. Breast cancer cells, however, appear to contain only the 115-kDa form of FGFR-1. This expression pattern is reflected in malignant and non-malignant tissue samples. Using reverse transcription polymerase chain reaction (RT-PCR) analysis, we have shown that the 106-kDa FGFR-1 isoform is not the previously described alpha 2 receptor that arises from a 25-base pair insertion in the second kinase domain. It is probable that the 106-kDa FGFR-1 has different signalling properties to the full-length receptor, having lost at least one tyrosine at amino acid 766, which is required for phospholipase C activation. This form of FGFR-1 appears to be lost in all breast cancer cells analysed and its absence may have a bearing on malignancy.

Basic fibroblast growth factor stimulates phosphatidylcholine-hydrolyzing phospholipase D in osteoblast-like cells

We examined the effect of basic fibroblast growth factor (bFGF) on the activation of phosphatidylcholine-hydrolyzing phospholipase D in osteoblast-like MC3T3-E1 cells. bFGF stimulated both the formations of choline (EC50 was 30 ng/ml) and inositol phosphates (EC50 was 10 ng/ml). Calphostin C, an inhibitor of protein kinase C (PKC), had little effect on the bFGF-induced formation of choline. bFGF stimulated the formation of choline also in PKC down regulated cells. Genistein and methyl 2,5-dihydroxycinnamate, inhibitors of protein tyrosine kinases, significantly suppressed the bFGF-induced formation of choline. Sodium orthovanadate, an inhibitor of protein tyrosine phosphatases, enhanced the bFGF-induced formation of choline. In vitro kinase assay for FGF receptors revealed that FGF receptor 1 and 2 were autophosphorylated after FGF stimulation. bFGF dose-dependently stimulated DNA synthesis of these cells. These results strongly suggest that bFGF activates phosphatidylcholine-hydrolyzing phospholipase D through the activation of tyrosine kinase, but independently of PKC activated by phosphoinositide hydrolysis in osteoblast-like cells.

Induction of urokinase-type plasminogen activator by fibroblast growth factor (FGF)-2 is dependent on expression of FGF receptors and does not require activation of phospholipase Cgamma1

The roles of heparan sulfate proteoglycans and tyrosine kinase fibroblast growth factor (FGF) receptors in mediating the induction of plasminogen activator (PA) by FGF-2 were investigated using L6 myoblast cells that normally do not express detectable FGF receptors. PA was induced by FGF-2 in a dose-dependent manner in L6 cells expressing transfected FGF receptor-1 but not in nontransfected cells or cells transfected with the vector alone. The PA produced in these cells was characterized as urokinase-type PA (uPA). Thus, expression of a tyrosine kinase FGF receptor was required for induction of uPA. Internalization of FGF through heparan sulfates does not seem to be involved in this response as soluble heparin and suramin at concentrations which inhibited FGF-2 binding to heparan sulfates but not receptors did not affect the induction of uPA by FGF-2. Mutant receptors in which the tyrosine kinase was inactivated were not able to respond to FGF-2. In contrast, mutation of the site of phospholipase Cgamma1 (PLCgamma) binding in the receptor, which causes loss of PLCgamma activation, had no effect on uPA induction by FGF-2. These results suggest that PLCgamma activation is not required for induction of uPA by FGF-2.

Phosphorylation of tyrosine 720 in the platelet-derived growth factor alpha receptor is required for binding of Grb2 and SHP-2 but not for activation of Ras or cell proliferation

Following binding of platelet-derived growth factor (PDGF), the PDGF alpha receptor (alphaPDGFR) becomes tyrosine phosphorylated and associates with a number of signal transduction molecules, including phospholipase Cgamma-1 (PLCgamma-1), phosphatidylinositol 3-kinase (PI3K), the phosphotyrosine phosphatase SHP-2, Grb2, and Src. Here, we present data identifying a novel phosphorylation site in the kinase insert domain of the alphaPDGFR at tyrosine (Y) 720. We replaced this residue with phenylalanine and expressed the mutated receptor (F720) in Patch fibroblasts that do not express the alphaPDGFR. Characterization of the F720 mutant indicated that binding of two proteins, SHP-2 and Grb2, was severely impaired, whereas PLCgamma-1 and PI3K associated to wild-type levels. In addition, mutating Y720 to phenylalanine dramatically reduced PDGF-dependent tyrosine phosphorylation of SHP-2. Since Y720 was required for recruitment of two proteins, we investigated the mechanism by which these two proteins associated with the alphaPDGFR. SHP-2 bound the alphaPDGFR directly, whereas Grb2 associated indirectly, most probably via SHP-2, as Grb2 and SHP-2 coimmunoprecipitated when SHP-2 was tyrosine phosphorylated. We also compared the ability of the wild-type and F720 alphaPDGFRs to mediate a number of downstream events. Preventing the alphaPDGFR from recruiting SHP-2 and Grb2 did not compromise PDGF-AA-induced activation of Ras, initiation of DNA synthesis, or growth of cells in soft agar. We conclude that phosphorylation of the alphaPDGFR at Y720 is required for association of SHP-2 and Grb2 and tyrosine phosphorylation of SHP-2; however, these events are not required for the alphaPDGFR to activate Ras or initiate a proliferative response. In addition, these findings reveal that while SHP-2 binds to both of the receptors, it binds in different locations: to the carboxy terminus of the betaPDGFR but to the kinase insert of the alphaPDGFR.

The relationship between human epidermal growth-like factor receptor expression and cellular transformation in NIH3T3 cells

A collection of cell lines expressing each human epidermal growth factor receptor (HER) family member alone or in all pairwise combinations in a clone of NIH3T3 cells (3T3-7d) devoid of detectable epidermal growth factor receptor family members has been generated. Transformation, as measured by growth in soft agar, occurred only in the presence of appropriate ligand and only in cells expressing two different HER family members. Transfection of oncogenic neu (Tneu), conferred ligand-independent transformation only in cells which co-expressed HER1, HER3, or HER4, but not when expressed alone or with HER2. Cell lines were also tested for their ability to form tumors in animals. None of the cell lines expressing single HER family members was able to form tumors in animals with the exception of HER1, which was weakly tumorigenic. Although unable to form tumors when expressed alone, HER2 was tumorigenic when expressed with HER1 or HER3, but not HER4. Of all complexes analyzed, cells expressing HER1 + HER2 were the most aggressive. The relationship between HER1 activation, intracellular calcium fluxes, and phospholipase Cgamma1 activation is well established. We found that activation of HER1 was required for the induction of a calcium flux and the phosphorylation of phospholipase Cgamma1. These activities were independent of, and unaffected by, the co-expression of any other family member. Further, heregulin stimulation of all cell lines including those containing HER1 did not demonstrate any effect on intracellular calcium levels or phospholipase Cgamma1 phosphorylation. This demonstrates that heregulin induced cellular transformation by activating HER3- and HER4-containing complexes does not require the activation of either phospholipase Cgamma1 or the mobilization of intracellular calcium.

Signal transduction by basic fibroblast growth factor in rat osteoblastic Py1a cells

Basic fibroblast growth factor (bFGF) is a potent mitogen for bone. In this study, we utilized the clonal rat osteoblastic cell line, Py1a, to examine signal transduction by bFGF and to determine the role of mitogen activated protein kinases (MAPK) and induction of c-fos mRNA in the mitogenic response to bFGF. Stimulation of [3H]thymidine incorporation (TDR) into DNA by bFGF was determined in the presence of phorbol myristate acetate of (PMA) to down-regulate the protein kinase C (PKC) pathway, genistein, an inhibitor of tyrosine kinase and H-7, a PKC inhibitor, bFGF 10(-8) M and PMA 10(-7) M increased TDR by 242 and 245%, respectively. Treatment with bFGF or PMA for 5 or 30 minutes increased tyrosine phosphorylation of multiple proteins, and immunoblotting with MAPK-specific antibody revealed that two of these bands were the 42 and 44 kD isoforms of MAPK. PMA and bFGF induced c-fos mRNA expression at 30 minutes. Genistein at 10 micrograms/ml blocked the mitogenic effect of bFGF and partially inhibited the mitogenic effect of PMA. Genistein at 100 micrograms/ml also blocked both bFGF- and PMA-induced increases in c-fos mRNA. A 24 h pretreatment with PMA at 10(-7) M inhibited the mitogenic response, tyrosine phosphorylation of MAPK, and induction of c-fos mRNA subsequent to the addition of PMA, but not bFGF. H-7 at 50 microM blocked bFGF-induced mitogenesis and c-fos induction, but did not inhibit bFGF-induced tyrosine phosphorylation of MAPK. In this study, we show that the signaling pathway of bFGF and PMA are similar in that they both induce tyrosine phosphorylation of MAP kinases and activate c-fos. However, the signaling pathways ultimately diverge in that once the PKC pathway is down-regulated by PMA pretreatment or blocked by the PKC inhibitor H-7, tyrosine phosphorylation of MAP kinase, c-fos induction, and the mitogenic effect of PMA is blocked. In contrast, down-regulation of the PKC pathway inhibits c-fos and the mitogenic response to bFGF, but not bFGF's effects on tyrosine phosphorylation of MAP kinase.

Structure of the FGF receptor tyrosine kinase domain reveals a novel autoinhibitory mechanism

The crystal structure of the tyrosine kinase domain of fibroblast growth factor receptor 1 (FGFR1K) has been determined in its unliganded form to 2.0 angstroms resolution and in complex with with an ATP analog to 2.3 angstrosms A resolution. Several features distinguish the structure of FGFR1K from that of the tyrosine kinase domain of the insulin receptor. Residues in the activation loop of FGFR1K appear to interfere with substrate peptide binding but not with ATP binding, revealing a second and perhaps more general autoinhibitory mechanism for receptor tyrosine kinases. In addition, a dimeric form of FGFR1K observed in the crystal structure may provide insights into the molecular mechanisms by which FGF receptors are activated. Finally, the structure provides a basis for rationalizing the effects of kinase mutations in FGF receptors that lead to developmental disorders in nematodes and humans.

Calcineurin is essential for DNA synthesis in Swiss 3T3 fibroblasts

DNA synthesis was measured 16 h after stimulation of Swiss 3T3 fibroblasts in the resting phase with various growth factors (platelet-derived growth factor, fibroblast growth factor, lysophosphatidic acid and thrombin). When extracellular Ca2+ was chelated by EGTA, or when the influx of Ca2+ from outside to inside the cell was blocked by cobalt, DNA synthesis was completely inhibited. As there was no effect whatsoever on DNA synthesis when Ca2+ was chelated, or when the influx of Ca2+ was blocked up to the first 4 h after growth stimulation, it was concluded that, at an early stage, Ca2+ influx from outside to inside the cell is not related to the transition from the G1 to the S phase. A Ca2+/calmodulin-dependent protein kinase II inhibitor (KN-62) had no effect on DNA synthesis. However, cyclosporin A and FK-506, which are inhibitors of Ca2+/calmodulin-dependent protein phosphatase 2B (calcineurin), markedly inhibited DNA synthesis stimulated by all of the growth factors. These results indicate that calcineurin plays a role, not only in activation of T-cells of the immune system in the initial phase, but also in DNA synthesis in fibroblasts. It was concluded that Ca2+ influx from outside to inside the cell during the mid-to-late G1 phase, followed by calcineurin activation, is essential as a mechanism of growth signal transduction.

Profound ligand-independent kinase activation of fibroblast growth factor receptor 3 by the activation loop mutation responsible for a lethal skeletal dysplasia, thanatophoric dysplasia type II

Thanatophoric dysplasia type II (TDII) is a neonatal lethal skeletal dysplasia caused by a recurrent Lys-650-->Glu mutation within the highly conserved activation loop of the kinase domain of fibroblast growth factor receptor 3 (FGFR3). We demonstrate here that this mutation results in profound constitutive activation of the FGFR3 tyrosine kinase, approximately 100-fold above that of wild-type FGFR3. The mechanism of FGFR3 activation in TDII was probed by constructing various point mutations in the activation loop. Substitutions at position 650 indicated that not only Glu but also Asp and, to a lesser extent, Gln and Leu result in pronounced constitutive activation of FGFR3. Additional mutagenesis within the beta10-beta11 loop region (amino acids Tyr-647 to Leu-656) demonstrated that amino acid 650 is the only residue which can activate the receptor when changed to a Glu, indicating a specificity of position as well as charge for mutations which can give rise to kinase activation. Furthermore, when predicted sites of autophosphorylation at Tyr-647 and Tyr-648 were mutated to Phe, either singly or in combination, constitutive kinase activity was still observed in response to the Lys-650-->Glu mutation, although the effect of these mutations on downstream signalling was not investigated. Our data suggest that the molecular effect of the TDII activation loop mutation is to mimic the conformational changes that activate the tyrosine kinase domain, which are normally initiated by ligand binding and autophosphorylation. These results have broad implications for understanding the molecular basis of other human developmental syndromes that involve mutations in members of the FGFR family. Moreover, these findings are relevant to the study of kinase regulation and the design of activating mutations in related tyrosine kinases.

Identification of a murine TEF-1-related gene expressed after mitogenic stimulation of quiescent fibroblasts and during myogenic differentiation

Fibroblast growth factor (FGF)-1 binding to cell surface receptors stimulates an intracellular signaling pathway that ultimately promotes the transcriptional activation of specific genes. We have used a mRNA differential display method to identify FGF-1-inducible genes in mouse NIH 3T3 fibroblasts. Here, we report that one of these genes, FGF-regulated (FR)-19, is predicted to encode a member of the transcriptional enhancer factor (TEF)-1 family of structurally related DNA-binding proteins. Specifically, the deduced FR-19 amino acid sequence has approximately89, 77, and 68% overall identity to chicken TEF-1A, mouse TEF-1, and mouse embryonic TEA domain-containing factor, respectively. Gel mobility shift experiments indicate that FR-19, like TEF-1, can bind the GT-IIC motif found in the SV40 enhancer. The FR-19 gene maps in the distal region of mouse chromosome 6, and analysis of several FR-19 cDNA clones indicates that at least two FR-19 isoforms may be expressed from this locus. FGF-1 induction of FR-19 mRNA expression in mouse fibroblasts is first detectable at 4 h after FGF-1 addition and is dependent on de novo RNA and protein synthesis. FGF-2, calf serum, platelet-derived growth factor-BB, and phorbol 12-myristate 13-acetate can also induce FR-19 mRNA levels. We have also found that FR-19 mRNA expression increases during mouse C2C12 myoblast differentiation in vitro. The FR-19 gene is expressed in vivo in a tissue-specific manner, with a relatively high level detected in lung. These results indicate that increased expression of a TEF-1-related protein may be important for both mitogen-stimulated fibroblast proliferation and skeletal muscle cell differentiation.

Basic fibroblast growth factor (FGF-2) protects rat cochlear hair cells in organotypical culture from aminoglycoside injury

Given the evidence that basic fibroblast growth factor (FGF-2) can protect neural and retinal cells from degeneration, we evaluated the potential of this growth factor to protect sensory cells in the inner ear. When sensory cells of the organ of Corti are exposed to aminoglycoside antibiotics such as neomycin either in vivo or in vitro, significant ototoxicity is observed. The in vitro cytotoxic effects of neomycin are dose and time dependent. In neonatal rat organ of Corti cultures, complete inner and outer hair cell destruction is observed at high (mM) concentrations of neomycin while inner hair cell survival and severely damaged outer hair cells are noted at moderate (microM) concentrations, with a maximal effect observed after 2 days of culture. Approximately 50% of cochlear outer hair cells are lost at a dose of 35 microM neomycin, and most surviving cells show disorganized stereocilia. Inner hair cells show primarily disorganization of their stereocilia. A significant protective effect is observed when the organ of Corti is pre-treated with FGF-2 (500 ng/ml) for 48 hours, and then FGF-2 is included with neomycin in the culture medium. A greater extent of outer hair cell survival and a significant decrease in stereociliary damage are noted with FGF-2. However, disorganization of inner hair cell stereocilia is unaffected by FGF-2. The protective effect of FGF-2 is specific, since interleukin-1B, nerve growth factor, tumor necrosis factor, and epidermal growth factor are ineffective, while retinoic acid and transforming growth factor alpha show only a moderate protective effect. These results confirm the potential of molecules like FGF-2 for preventing cell death due to a variety of causes.