Substrate phosphorylation specificity of the human c-kit receptor tyrosine kinase

Function of activation loop tyrosine phosphorylation in the mechanism of c-Kit auto-activation and its implication in sunitinib resistance

The activation of receptor tyrosine kinases (RTKs) is tightly regulated through a variety of mechanisms. Kinetic studies show that activation of c-Kit RTK occurs through an inter-molecular autophosphorylation. Phosphopeptide mapping of c-Kit reveals that 14-22 phosphates are added to each mol of wild-type (WT) c-Kit during the activation. Phosphorylation sites are found on the JM, kinase insert (KID), c-terminal domains and the activation loop (A-loop), but only the sites on the JM domain contribute to the kinase activation. The A-loop tyrosine (Y(823)) is not phosphorylated until very late in the activation (>90% completion), indicating that the A-loop phosphorylation is not required for c-Kit activation. A sunitinib-resistant mutant D816H that accelerates auto-activation by 184-fold shows no phosphorylation on the A-loop tyrosine after full activation. A loss-of-phosphorylation mutation Y823F remains fully competent in auto-activation. Similar to WT and D816H, the unactivated Y823F mutant binds sunitinib and imatinib with high affinity (K(D) = 5.9 nM). But unlike the WT and D816H where the activated enzymes lose the ability to bind the two drugs, activated Y823F binds the two inhibitors effectively. These observations suggest that the A-loop of activated Y823F remains flexible and can readily adopt unactivated conformations to accommodate DFG-out binders.

Cottontail rabbit papillomavirus E8 protein is essential for wart formation and provides new insights into viral pathogenesis

The cottontail rabbit papillomavirus (CRPV) a and b subtypes display a conserved E8 open reading frame encoding a 50-amino-acid hydrophobic protein, with structural similarities to the E5 transmembrane oncoprotein of genital human PVs (HPVs). CRPV E8 has been reported to play a role in papilloma growth but not to be essential in papilloma formation. Here we report that the knockout of E8 start codon almost prevented wart induction upon biobalistic inoculation of viral DNA onto rabbit skin. The scarce warts induced showed very slow growth, despite sustained expression of E6 and E7 oncogenes. This points to an essential role of E8 in disturbing epidermal homeostasis. Using a yeast two-hybrid screen, we found that E8 interacted with the zinc transporter ZnT1, protocadherin 1 (PCDH1), and AHNAK/desmoyokin, three proteins as yet unrelated to viral pathogenesis or cell transformation. HPV16 E5 also interacted with these proteins in two-hybrid assay. CRPV E8 mainly localized to the Golgi apparatus and the early endosomes of transfected keratinocytes and colocalized with ZnT1, PCDH1, and AHNAK. We showed that ZnT1 and PCDH1 formed a complex and that E8 disrupted this complex. CRPV E8, like HPV16 E5, increased epidermal growth factor (EGF)-dependent extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation and both the EGF-dependent and the EGF-independent activity of activating protein-1 (AP-1). Competition experiments with a nonfunctional truncated ZnT1 protein showed that E8-ZnT1 interaction was required for AP-1 activation. Our data identify CRPV E8 as a key player in papilloma induction and unravel novel cellular targets for inducing the proliferation of keratinocytes.

Stem cell factor and its receptor c-Kit as targets for inflammatory diseases

Stem cell factor (SCF), the ligand of the c-Kit receptor, is expressed by various structural and inflammatory cells in the airways. Binding of SCF to c-Kit leads to activation of multiple pathways, including phosphatidyl-inositol-3 (PI3)-kinase, phospholipase C (PLC)-gamma, Src kinase, Janus kinase (JAK)/Signal Transducers and Activators of Transcription (STAT) and mitogen activated protein (MAP) kinase pathways. SCF is an important growth factor for mast cells, promoting their generation from CD34+ progenitor cells. In vitro, SCF induces mast cells survival, adhesion to extracellular matrix and degranulation, leading to expression and release of histamine, pro-inflammatory cytokines and chemokines. SCF also induces eosinophil adhesion and activation. SCF is upregulated in inflammatory conditions both in vitro and in vivo, in human and mice. Inhibition of the SCF/c-Kit pathway leads to significant decrease of histamine levels, mast cells and eosinophil infiltration, interleukin (IL)-4 production and airway hyperresponsiveness in vivo. Taken together, these data suggest that SCF/c-Kit may be a potential therapeutic target for the control of mast cell and eosinophil number and activation in inflammatory diseases.

Ectopic expression of KitD814Yin spermatids of transgenic mice, interferes with sperm morphogenesis

Kit is a receptor tyrosine kinase that plays a fundamental role during the development of germ cells. Additionally, a truncated product, tr-kit, expressed in haploid spermatids and mature spermatozoa can induce parthenogenetic activation when microinjected into mouse eggs, through the activation of PLCgamma-1. In this work, we induced ectopic expression of a mutated Kit protein, Kit(D814Y) during germ cell development. The in vivo expression of this mutant in spermatids produced malformations in mature spermatozoa, and in the most severe cases, sterility. Ultrastructural analysis indicated that condensing spermatids in the transgenic mouse presented a mislocalization of the manchette; a structure that has a crucial role during the elongation steps of spermiogenesis. This morphogenetic phenotype was accompanied by an increased phosphorylation of PLCgamma-1 in spermatogenic cells. Interestingly, we also found that, in wild-type testis, PLCgamma-1 is specifically phosphorylated in condensing spermatids, coincident with the timing of expression of tr-kit in spermiogenesis. We propose that alterations of PLCgamma-1 activity artificially promoted by ectopic Kit(D814Y) expression are related to the abnormalities of spermiogenesis. Our observations suggest that PLCgamma-1 activity could be involved in the shaping of spermatozoa.

Signal transduction of stem cell factor in promoting early follicle development

Stem cell factor (SCF), another alternative name is kit ligand, is essential for the development of early follicles. However, the underlying molecular mechanism remains to be defined. By using cultured ovaries that are rich in primordial follicles, the action of SCF (kit ligand) on early follicular development and the activated signal transduction pathways were investigated. SCF (kit ligand) promoted early follicle development. PKC and MEK but not PKA were involved in the signal transduction of SCF (kit ligand) as indicated by results using their specific pharmacological inhibitors. SCF (kit ligand) also enhanced the phosphorylation of two MEK substrates, Erk1 and 2 (Erk1/2) in thecal-interstitial cells where PKC might play an important role indicated by results using its inhibitors. SCF (kit ligand) elevated the expression of steroidogenic factor 1 (SF-1) in thecal-interstitial cells probably through a pathway that consists of Erk1/2. These results suggest that SCF (kit ligand) promotes follicular growth by stimulating the function of thecal-interstitial cells through the Erk1/2 pathway.

Anti-apoptotic action of stem cell factor on oocytes in primordial follicles and its signal transduction

Stem cell factor (SCF) is essential for the development of primordial follicles. One of its functions is to prevent oocytes from apoptosis. However, the underlying mechanism remains largely unknown. By using cultured ovaries that are rich in primordial follicles, the anti-apoptotic action of SCF and the potential signal transduction pathways were investigated. The apoptosis was evaluated by means of in situ 3'-end labeling. The expressions of proteins were analyzed with immunohistochemistry and Western blot. The data showed that SCF significantly prevented oocytes from apoptosis in the cultured organs. Addition of a specific pharmacological inhibitor of PI3K abolished the anti-apoptotic action of SCF while that of a MEK inhibitor did not. The phosphorylation of two mitogen activated protein kinases (MAPKs) (p42 and p44) and AKT, the respective substrates of MEK and PI3K, were enhanced by SCF treatment. Not surprisingly, the MAPK activation occurred only in theca cells. The expressions of apoptosis-related gene products, the Bcl-2 family proteins, in response to SCF treatment were also investigated. While SCF up-regulated the expression of the anti-apoptotic proteins Bcl-2 and Bcl-xL, it did the opposite to the pro-apoptotic factor Bax. The PI3K inhibitor reversed the regulation of SCF on Bcl-xL and Bax but not on Bcl-2. Therefore, it seemed that SCF initiated an anti-apoptotic signal starting from its membrane receptor c-kit to Bcl-2 family members through PI3K/AKT and other signaling cascades in the oocytes of primordial follicles.

Molecular mechanisms utilized by alternative c-kit gene products in the control of spermatogonial proliferation and sperm-mediated egg activation

The c-kit proto-oncogene plays a dual role in the control of male fertility in mice through two alternative gene products: (1). c-kit [the transmembrane tyrosine kinase receptor for stem cell factor (SCF)], which is expressed and functional in differentiating spermatogonia of the postnatal testis, in which c-kit is essential for pre-meiotic proliferation; and (2). tr-kit, an intracellular protein which is specifically accumulated during spermiogenesis through the use of an alternative intronic promoter, and which is able to trigger mouse egg activation when microinjected into the cytoplasm of metaphase II arrested oocytes. Here, we summarize the most recent findings about the molecular pathways through which c-kit regulates cell cycle progression in mitotic germ cells, and those through which sperm-derived tr-kit triggers parthenogenetic completion of meiosis II and pronuclear formation in microinjected mouse eggs.

A ligand-inducible epidermal growth factor receptor/anaplastic lymphoma kinase chimera promotes mitogenesis and transforming properties in 3T3 cells

Oncogenic rearrangements of the anaplastic lymphoma kinase (ALK) gene, encoding a receptor type tyrosine kinase, are frequently associated with anaplastic large cell lymphomas. Such rearrangements juxtapose the intracellular domain of ALK to 5'-end sequences belonging to different genes and create transforming fusion proteins. To understand how the oncogenic versions of ALK contribute to lymphomagenesis, it is important to analyze the biological effects and the biochemical properties of this receptor under controlled conditions of activation. To this aim, we constructed chimeric receptor molecules in which the extracellular domain of the ALK kinase is replaced by the extracellular, ligand-binding domain of the epidermal growth factor receptor (EGFR). Upon transfection in NIH 3T3 fibroblasts, the EGFR/ALK chimera was correctly synthesized and transported to the cell surface, where it was fully functional in forming high versus low affinity EGF-binding sites and transducing an EGF-dependent signal intracellularly. Overexpression of the EGFR/ALK chimera in NIH 3T3 was sufficient to induce the malignant phenotype; the appearance of the transformed phenotype was, however, conditionally dependent on the administration of EGF. Moreover, the EGFR/ALK chimera was significantly more active in inducing transformation and DNA synthesis than the wild type EGFR when either was expressed at similar levels in NIH 3T3 cells. Comparative analysis of the biochemical pathways implicated in the transduction of mitogenic signals did not show any increased ability of the EGFR/ALK to phosphorylate PLC-gamma and MAPK compared with the EGFR. On the contrary, EGFR/ALK showed to have a consistently greater effect on phosphatidylinositol 3-kinase activity compared with the EGFR, indicating that this enzyme plays a major role in mediating the mitogenic effects of ALK in NIH 3T3 cells.

Evidence for heterotypic interaction between the receptor tyrosine kinases TIE-1 and TIE-2

The orphan receptor tyrosine kinase Tie-1 is expressed in endothelial cells and is essential for vascular development. Nothing is known about the signaling pathways utilized by this receptor. In this study we have used chimeric receptors composed of the TrkA ectodomain fused to the transmembrane and intracellular domains of Tie-1, or the related receptor Tie-2, to examine Tie-1 signaling capacity. In contrast to TrkA/Tie-2, the Tie-1 chimera was unable to phosphorylate cellular proteins or undergo autophosphorylation. Consistent with this Tie-1 exhibited negligible kinase activity. Co-immunoprecipitation analysis revealed Tie-1 was present in endothelial cells bound to Tie-2. Full-length Tie-1 and truncated receptor, formed by regulated endoproteolytic cleavage, were found to complex with Tie-2. Association was mediated by the intracellular domains of the receptors and did not require Tie-1 to be membrane-localized. Tie-1 bound to Tie-2 was not tyrosine-phosphorylated under basal conditions or following Tie-2 stimulation. This study provides the first evidence for the existence of a pre-formed complex of Tie-1 and Tie-2 in endothelial cells. The data suggest Tie-1 does not signal via ligand-induced kinase activation involving homo-oligomerization. The physical association between Tie-1 and Tie-2 is consistent with Tie-1 having a role in modulating Tie-2 signaling.

Coexpression of c-kit and stem cell factor in breast cancer results in enhanced sensitivity to members of the EGF family of growth factors

Kit, a tyrosine kinase growth factor receptor, and its ligand, stem cell factor (SCF), are commonly coexpressed in breast cancer. We have previously shown that MCF7 cells (that naturally express SCF) transfected with a c-kit expression vector exhibit enhanced growth in serum-free medium supplemented with IGF-1. Consequently, we wished to examine the interaction of Kit/SCF with additional growth factors important in the biology of breast cancer. MCF7 transfectants expressing Kit, cultured in serum-free medium supplemented with EGF, displayed more than twice the growth of controls at identical EGF concentrations. Similar responses were seen in the presence of heregulin alpha. The specificity of the Kit-mediated response was illustrated by a reduction in heregulin-stimulated growth in the presence of a monoclonal antibody directed against the Kit receptor. In addition, EGF- and heregulin-stimulated growth of the ZR75-1 cell line that naturally coexpresses Kit and SCF was also inhibited by the Kit blocking antibody. Preliminary investigations into the signal transduction pathways activated by these growth factors revealed that SCF activated both the Ras-MAP kinase and phosphatidyl-inositol-3-kinase (PI3 kinase) pathway. Both EGF and heregulin activated MAPK but to a lesser degree than SCF, and combination of SCF with these growth factors resulted in enhanced MAPK activation. Assessment of PI3K pathway activation using antiphospho-Akt antibodies revealed that EGF was a poor activator of Akt; activation of this pathway was markedly enhanced by the addition of SCF. Heregulin activated Akt and addition of SCF provided no further activation. Taken together these results suggest that coexpression of SCF and Kit may enhance responsiveness to erbB ligands by enhancing activation of the MAPK and PI3K pathways.

Phosphorylation of Shc by Src family kinases is necessary for stem cell factor receptor/c-kit mediated activation of the Ras/MAP kinase pathway and c-fos induction

In this report we show that Tyr568 and Tyr570 are phosphorylated in vivo in the Kit/stem cell factor receptor (Kit/SCFR) following ligand-stimulation. By mutation of Tyr568 and Tyr570 to phenylalanine residues and expression of the mutated receptors in porcine aortic endothelial (PAE) cells, we could demonstrate a loss of activation of members of the Src family of tyrosine kinases when Tyr568 was mutated, while mutation of Tyr570 only led to a minor decrease in activation of Src family members. Mutation of both tyrosine residues led to a complete loss of Src family kinase activation. Phosphorylation of the adapter protein Shc by growth factor receptors provides association sites for Grb2-Sos, thereby activating the Ras/MAP kinase pathway. A much lowered degree of Shc phosphorylation, Ras and Erk2 activation and c-fos induction was seen in the Y568F mutant, while in the Y570F mutant these responses were less affected. In contrast, the mitogenic response was only slightly reduced. In a mutant receptor with both Tyr568 and Tyr570 mutated to phenylalanine residues, no phosphorylation of Shc and no activation of Ras and Erk2 was seen in response to stem cell factor stimulation, very weak induction of c-fos was seen and the mitogenic response was severely depressed. These data show that Ras/MAP kinase activation and c-fos induction by Kit/SCFR are mediated by members of the Src family kinases. However, the mitogenic response is only to a minor extent dependent on Src kinase activity.

Requirement for phosphatidylinositol-3'-kinase in cytokine-mediated germ cell survival during fetal oogenesis in the mouse

Apoptosis is responsible for primordial germ cell (PGC) attrition in the developing fetal ovary. In monolayer cultures of murine PGC, stem cell factor (SCF) and leukemia inhibitory factor (LIF) independently promote survival in vitro; however, the relevance of these data to fetal ovarian oogonium and oocyte survival, as well as the intracellular events involved in transducing the antiapoptotic actions of these cytokines in germ cells, remain to be elucidated. In this report, we investigated the effects of SCF and LIF, alone and in combination, on the survival of oogonia and oocytes, and elaborated on components of the signal transduction pathway used by these molecules, after validating a method of culturing fetal mouse ovaries. We further employed this system to also test the hypothesis that insulin-like growth factor-I (IGF-I), a classic antiapoptotic molecule, and transforming growth factor-beta (TGF-beta), a classic pro-apoptotic molecule, interact with the SCF/LIF pathway and function in a reciprocal fashion to precisely regulate germ cell numbers during fetal oogenesis. Freshly isolated embryonic day 13.5 ovaries contained nonapoptotic germ cells, as determined by histologic analysis of cellular morphology and in situ 3'-end-labeling of DNA integrity. In vitro culture of fetal ovaries without tropic support for 24, 48, and 72 h resulted in a time-dependent induction of germ cell apoptosis, such that most oogonia and oocytes present after 72 h were apoptotic. Morphometric analysis of serially sectioned ovaries indicated that the numbers of nonapoptotic germ cells remaining after 24, 48, and 72 h of culture were 78%, 38%, and 10%, respectively, of the number present before culture (P < 0.05 for all time points vs. 0 h). Inclusion of SCF (100 ng/ml) together with LIF (100 ng/ml) in the culture medium significantly attenuated germ cell apoptosis, with the SCF/LIF-treated ovaries retaining 5.5-fold more oogonia and oocytes after 72 h of culture as compared with control ovaries deprived of tropic support (P < 0.05). However, SCF or LIF, when added separately, had no (SCF) or little (LIF) inhibitory effect on germ cell apoptosis. Provision of 50 ng/ml IGF-I maintained survival of approximately two-thirds of the germ cells in cultured ovaries (P < 0.05), whereas a combination of all three growth factors (SCF, LIF, IGF-I) completely preserved the fetal ovary in culture to that resembling a freshly-isolated gonad. Cotreatment with 25 ng/ml TGF-beta partially reversed the survival actions of IGF-I or SCF/LIF, such that only one-third of the starting number of oogonia/oocytes remained after 72 h of culture (P < 0.05). Lastly, the antiapoptotic effects of SCF/LIF or IGF-I were almost entirely eliminated by cotreatment of fetal ovaries with either one of two inhibitors of phosphatidylinositol-3'-kinase (PI3K), LY294002 (5 microM) or wortmannin (50 nM), whereas cotreatment with an inhibitor of p70 S6 kinase (rapamycin, 25 ng/ml) was without effect. These data indicate that the combined actions of SCF, LIF, and IGF-I are required for maximal inhibition of apoptosis in germ cells of fetal mouse ovaries, and that the PI3K signaling pathway is an essential component of cytokine-mediated female germ cell survival. Moreover, TGF-beta can partially override the antiapoptotic actions of SCF/LIF or IGF-I in oogonia and oocytes, suggesting the existence of a complex signaling network that ultimately determines fetal ovarian germ cell fate.

Involvement of phospholipase Cgamma1 in mouse egg activation induced by a truncated form of the C-kit tyrosine kinase present in spermatozoa

Microinjection of a truncated form of the c-kit tyrosine kinase present in mouse spermatozoa (tr-kit) activates mouse eggs parthenogenetically, and tr-kit- induced egg activation is inhibited by preincubation with an inhibitor of phospholipase C (PLC) (Sette, C., A. Bevilacqua, A. Bianchini, F. Mangia, R. Geremia, and P. Rossi. 1997. Development [Camb.]. 124:2267-2274). Co-injection of glutathione-S-transferase (GST) fusion proteins containing the src-homology (SH) domains of the gamma1 isoform of PLC (PLCgamma1) competitively inhibits tr-kit- induced egg activation. A GST fusion protein containing the SH3 domain of PLCgamma1 inhibits egg activation as efficiently as the whole SH region, while a GST fusion protein containing the two SH2 domains is much less effective. A GST fusion protein containing the SH3 domain of the Grb2 adaptor protein does not inhibit tr-kit-induced egg activation, showing that the effect of the SH3 domain of PLCgamma1 is specific. Tr-kit-induced egg activation is also suppressed by co-injection of antibodies raised against the PLCgamma1 SH domains, but not against the PLCgamma1 COOH-terminal region. In transfected COS cells, coexpression of PLCgamma1 and tr-kit increases diacylglycerol and inositol phosphate production, and the phosphotyrosine content of PLCgamma1 with respect to cells expressing PLCgamma1 alone. These data indicate that tr-kit activates PLCgamma1, and that the SH3 domain of PLCgamma1 is essential for tr-kit-induced egg activation.

Protection From Apoptosis by Steel Factor But Not Interleukin-3 Is Reversed Through Blockade of Calcium Influx

Steel factor (SLF), the ligand for the c-Kit receptor, protects hemopoietic progenitors and mast cells from apoptosis. We show here that protection of 32D-Kit cells or mast cells from apoptosis by SLF is abrogated through concurrent inhibition of Ca2+influx. In contrast, cell survival promoted by interleukin-3 is not affected by Ca2+ influx blockers. In the presence of blockers, increasing stimulation by SLF leads to greater levels of cell death in the population, indicating that it is the combination of activation by SLF with concurrent blockade of Ca2+ influx that results in apoptosis. The p815 mastocytoma, which expresses a mutated, constitutively active c-kit receptor, dies apoptotically in the presence of Ca2+ influx blockers alone. Ionomycin protects cells from SLF plus blocker-induced apoptosis, confirming specificity for Ca2+ ion blockade in cell death induction. Overexpression of bcl-2, which protects 32D-Kit cells from factor withdrawal, does not protect cells from apoptosis by SLF plus blocker. In contrast, caspase inhibitors YVAD-CHO, DEVD-FMK, and Boc-Asp-FMK protect cells from SLF plus blocker-induced death. These observations highlight the importance of SLF-stimulated Ca2+ influx in the protection of cells from apoptosis and demonstrate a new mechanism for inducing bcl-2 insensitive, caspase-dependent apoptosis through the combination of SLF stimulation with Ca2+ influx blockade.

Protection From Apoptosis by Steel Factor But Not Interleukin-3 Is Reversed Through Blockade of Calcium Influx

Steel factor (SLF), the ligand for the c-Kit receptor, protects hemopoietic progenitors and mast cells from apoptosis. We show here that protection of 32D-Kit cells or mast cells from apoptosis by SLF is abrogated through concurrent inhibition of Ca2+influx. In contrast, cell survival promoted by interleukin-3 is not affected by Ca2+ influx blockers. In the presence of blockers, increasing stimulation by SLF leads to greater levels of cell death in the population, indicating that it is the combination of activation by SLF with concurrent blockade of Ca2+ influx that results in apoptosis. The p815 mastocytoma, which expresses a mutated, constitutively active c-kit receptor, dies apoptotically in the presence of Ca2+ influx blockers alone. Ionomycin protects cells from SLF plus blocker-induced apoptosis, confirming specificity for Ca2+ ion blockade in cell death induction. Overexpression of bcl-2, which protects 32D-Kit cells from factor withdrawal, does not protect cells from apoptosis by SLF plus blocker. In contrast, caspase inhibitors YVAD-CHO, DEVD-FMK, and Boc-Asp-FMK protect cells from SLF plus blocker-induced death. These observations highlight the importance of SLF-stimulated Ca2+ influx in the protection of cells from apoptosis and demonstrate a new mechanism for inducing bcl-2 insensitive, caspase-dependent apoptosis through the combination of SLF stimulation with Ca2+ influx blockade.

Involvement of phosphatidylinositol 3'-kinase in stem-cell-factor-induced phospholipase D activation and arachidonic acid release

We have shown previously that the stem cell factor (SCF) receptor undergoes phosphorylation on serine residues following ligand stimulation, and that this phopshorylation is dependent mainly on the activity of protein kinase C (PKC). In the present study, we have further investigated the molecular mechanisms behind SCF-stimulated activation of PKC, and found that SCF does not activate phosphatidylinositol-specific phospholipase C. In contrast, phospholipase D (PLD) is activated in response to SCF in a dose-dependent manner. Activation of PLD was not inhibited by calphostin C, an inhibitor of PKC. On the other hand, inhibitors of phosphatidylinositol PtdIns 3'-kinase (PtdIns 3'-kinase), i.e. wortmannin and LY294002, inhibited SCF-induced PLD activation. Moreover, a mutant SCF receptor in which Tyr721, which is responsible for activation of PtdIns 3'-kinase, is mutated to a phenylalanine residue was unable to mediate activation of PLD. Thus, PtdIns 3'-kinase appears to be essential for SCF-induced PLD activation. Furthermore, we demonstrate that phosphatidic acid (PtdH), generated through the action of PLD in response to SCF, is metabolized to diacylglycerol by dephosphorylation. Diacylglycerol can then activate PKC, and, moreover, after deacylation by a diacylglycerol lipase, yield arachidonic acid, an important second messenger in cell signaling.

A simple luciferase assay for signal transduction activity detection of epidermal growth factor displayed on phage

Studies on receptor-ligand interactions are important for the design of agonists or antagonists of natural ligands. We developed a luciferase reporter assay to screen epidermal growth factor receptor (EGFR) binding molecules rapidly for their ability to stimulate or inhibit signal transduction. Human EGF displayed on fd filamentous phage presented an activity similar to soluble EGF when tested for binding to the EGFR, for induction of cell cycle progression or in the luciferase assay. Two libraries of human EGF variants displayed on phage were constructed in which the aspartic acid residue at position 46 or the arginine residue at position 41 were randomised. EGF mutants displayed on phage were screened in parallel for binding to the EGFR using an ELISA assay and for transducing activity using the luciferase assay. Regarding the 46 position, most of the mutants retained the ability to bind the EGFR and their transducing activity corresponded perfectly with their binding. For the more crucial 41 position, only the wild-type EGF was able to bind the EGFR. Our approach allowed a simple determination of crucial positions and paved the way for identification of agonists with altered transduction activity.

Mutational analysis of the nucleotide binding site of the epidermal growth factor receptor and v-Src protein-tyrosine kinases

Tyrosine kinases differ from serine/threonine kinases in sequences located at the active site where ATP and substrate bind. In the structure of cyclic AMP-dependent protein kinase, the catalytic loop contains the sequence Lys-Pro-Glu where the Lys residue contacts the gamma-phosphate of ATP and the Glu residue contacts a basic residue located in the peptide substrate. In tyrosine kinases, the analogous sequence is Ala-Ala-Arg in the receptor tyrosine kinase subfamily and Arg-Ala-Ala in the Src tyrosine kinase subfamily. To deduce the role of these residues in tyrosine kinase function, site-directed mutations were prepared in the epidermal growth factor receptor (EGFR) and in v-Src and effects on ATP binding and kinase activity were determined. Changing Arg to either Lys or Ala dramatically reduced activity of both tyrosine kinases and this correlated with loss of ATP binding. Changing the orientation of this sequence impaired activity of EGFR to a greater extent than that of v-Src but did not change substrate specificity of the two enzymes. These results support the hypothesis that Arg functions to coordinate the gamma-phosphate of ATP. Analysis of sequence inversions in the catalytic loop indicate that the active site of v-Src exhibits greater flexibility than that of EGFR.

Transphosphorylation of the neurotrophin Trk receptors

The potential for the activation of one Trk receptor by ligand binding to another Trk receptor was explored by determining if transphosphorylation on tyrosine residues can occur between receptors. For most of these experiments, functional chimeric receptors were used that contained the extracellular domain of the human type 2 tumor necrosis factor receptor and the transmembrane and cytoplasmic domains of rat TrkA, TrkB, or TrkC and that, when activated by the tumor necrosis factor, mediated the nerve growth factor-like biological activities in PC12 cells. Cotransfection experiments in COS-7 cells and fibroblasts showed that despite the presence of different extracellular regions, intermolecular transphosphorylation of homologous cytoplasmic domains occurred between TrkA or TrkB and their cognate chimeras. Heterologous transphosphorylation between TrkB and TrkC kinase domains was also observed when one partner was a chimeric receptor; however, TrkA did not transphosphorylate the TrkB or TrkC kinase domains of chimeric receptors or act as a transphosphorylation substrate for these two receptors. The failure of TrkA to take part in transphosphorylation reactions with TrkB and TrkC was confirmed using the natural receptors. Trk receptor transphosphorylation occurs in the two non-neuronal cell types, but TrkA is excluded from these reactions.

Modulation of GABAA receptors by tyrosine phosphorylation

gamma-Aminobutyric acid type-A (GABAA) receptors are the major sites of fast synaptic inhibition in the brain. They are presumed to be pentameric heteroligomers assembled from four classes of subunits with multiple members: alpha (1-6), beta (1-3), gamma (1-3) and delta (1). Here, GABAA receptors consisting of alpha 1, beta 1 and gamma 2L subunits, coexpressed in mammalian cells with the tyrosine kinase vSRC (the transforming gene product of the Rous sarcoma virus), were phosphorylated on tyrosine residues within the gamma 2L and beta 1 subunits. Tyrosine phosphorylation enhanced the whole-cell current induced by GABA. Site-specific mutagenesis of two tyrosine residues within the predicted intracellular domain of the gamma 2L subunit abolished tyrosine phosphorylation of this subunit and eliminated receptor modulation. A similar modulation of GABAA receptor function was observed in primary neuronal cultures. As GABAA receptors are critical in mediating fast synaptic inhibition, such a regulation by tyrosine kinases may therefore have profound effects on the control of neuronal excitation.

An epidermal growth factor receptor-leukocyte tyrosine kinase chimeric receptor generates ligand-dependent growth signals through the Ras signaling pathway

Leukocyte tyrosine kinase (LTK) is a receptor tyrosine kinase that belongs to the insulin receptor family. LTK is mainly expressed in pre B cells and brain. Previously we cloned the full-length cDNA of human LTK, but no ligands have so far been identified, and hence, very little is known about the physiological role of LTK. To analyze the function of the LTK kinase, we constructed chimeric receptors composed of the extracellular domain of epidermal growth factor receptor and the transmembrane and the cytoplasmic domains of LTK and established cell lines that stably express these chimeric molecules. When cultured in medium containing EGF, growth of these cell lines was stimulated, and these fusion proteins became autophosphorylated and associated with Shc in vivo in a ligand-dependent manner. By treatment with EGF, Shc was associated with the Grb2/Ash-Sos complex. Our analyses demonstrate that LTK associates with Grb2/Ash through an internal adaptor, Shc, depending on a ligand stimulation. The LTK binding site for Shc was tyrosine 862 at the carboxyl-terminal domain and to a lesser extent tyrosine 485 at the juxtamembrane domain. Both of them are located in NP/AXY motif which is consistent with binding sites for Shc. These findings demonstrate that LTK can activate the Ras pathway in a ligand-dependent manner and that at least one of the functions of this kinase is involved in the cell growth.

Identification of the major phosphorylation sites for protein kinase C in kit/stem cell factor receptor in vitro and in intact cells

The c-kit-encoded tyrosine kinase receptor for stem cell factor (Kit/SCFR) is crucial for the development of hematopoietic cells, melanoblasts, and germ cells. Ligand stimulation of Kit/SCFR leads to receptor dimerization and autophosphorylation on tyrosine residues. We recently showed, that protein kinase C (PKC) acts in an SCF-stimulated negative feedback loop, which controls Kit/SCFR tyrosine kinase activity and modulates the cellular responses to SCF (Blume-Jensen, P., Siegbahn, A., Stabel, S., Heldin, C.-H., and Rönnstrand, L. (1993) EMBO J. 12, 4199-4209). We present here the identification of the major phosphorylation sites for PKC in Kit/SCFR. Two serine residues in the kinase insert, Ser-741 and Ser-746, are PKC-dependent phosphorylation sites in vivo and account for all phosphorylation by PKC in vitro. Together they comprise more than 60% of the total SCF-stimulated receptor phosphorylation in living cells and 85-90% of its phosphorylation in resting cells. Two additional serine residues, Ser-821 close to the major tyrosine autophosphorylation site in the kinase domain and Ser-959 in the carboxyl terminus are SCF-stimulated PKC-dependent phosphorylation sites. However, they are not phosphorylated directly by PKC-alpha in vitro. Both specific receptor tyrosine autophosphorylation and specific receptor-associated phosphatidylinositide 3'-kinase activity was increased approximately 2-fold in response to SCF in PAE cells stably expressing Kit/SCFR(S741A/S746A). Furthermore, the kinase activity of Kit/SCFR(S741A/S746A) toward an exogenous substrate was increased, which was reflected as a decreased Km and an increased Vmax, in accordance with the negative regulatory role of PKC on Kit/SCFR signaling.

Inhibition of proliferation of human melanocytes by a KIT antisense oligodeoxynucleotide: implications for human piebaldism and mouse dominant white spotting (W)

KIT constitutes the cell surface transmembrane receptor protein tyrosine kinase for a growth factor variously termed steel factor (SLF), stem cell factor, mast cell growth factor, or Kit ligand. Inherited mutations of the KIT gene result in piebaldism in humans and dominant white spotting (W) in mice. Patches of hypopigmented skin and hair in these disorders represent regions lacking in melanocytes, the result of defective melanoblast differentiation, migration, proliferation, or survival during embryonic development. Here we show that incubation of normal human melanocytes with a KIT antisense oligodeoxynucleotide greatly inhibits cell proliferation in culture, whereas incubation with a KIT sense oligodeoxynucleotide has no effect. The KIT oligodeoxynucleotides also had little or no effect on cell survival.

Cell cycle-specific induction of an 89 kDa serine/threonine protein kinase activity in Trypanosoma brucei

The cell cycle compartmentalization of specific activities of the protozoan parasite Trypanosoma brucei has remained unexplored due to the lack of a cell synchronization protocol. We report here that stationary phase cells stimulated to enter the cell cycle showed significant synchrony through the first cycle. The pattern of tyrosine phosphorylated proteins, known to undergo alterations during trypanosome development, showed only moderate changes as quiescent cells entered the cycle, particularly an increase in a 77 kDa species. However, the activity of an 89 kDa protein kinase (SPK89), previously demonstrated to be restricted to the proliferative stages of the parasite's life cycle, markedly increased as the population entered S phase. Cell sorting experiments demonstrated that SPK89 activity was highest in S phase cells and moderate in G2/M cells. The entry into S phase and increased SPK89 activity did not depend on serum factors but required protein synthesis for a discrete period after stimulation. Various modulators of protein phosphorylation were tested to determine their effects on progression to S and SPK89 activity. Only staurosporine and genistein were effective. However, both of these compounds inhibited virtually all protein phosphorylation and protein synthesis in the parasites. Thus these drugs cannot be used as specific protein kinase inhibitors in trypanosomes.

Structure-function analyses of the kit receptor for the steel factor

Binding of the Steel factor (SLF) to the product of the c-kit proto-oncogene stimulates the receptor's intrinsic tyrosine kinase that phosphorylates a set of cytoplasmic signaling molecules. Germ-line mutations in the genes that encode the receptor or the ligand result in remarkably similar phenotypes that affect melanogenesis, erythropoiesis and gametogenesis in mice. We concentrated on the initial events of the signal transduction pathway that underlies these processes. The extracellular portion of Kit is comprised of five immunoglobulin-(Ig)-like domains. Ligand binding to this domain induces rapid and extensive dimerization of the receptor molecules in a mechanism that involves monovalent binding of the dimeric ligand, followed by an increase in receptors' affinity and gradual stabilization of the dimers. It thus appears that Kit has at least two functions: ligand binding and ligand-induced receptor dimerization, in addition to the kinase activity. Both functions are independent of the transmembrane and cytoplasmic domains, as a recombinant soluble ectodomain retained high affinity to SLF and ligand-dependent dimerization. In order to correlate these functions with specific structures, we employed ligand-competitive monoclonal antibodies, soluble deletion mutants of the ectodomain and chimeric human-mouse Kit proteins. These approaches indicated that the N-terminal three Ig-like domains constitute the binding site, whose core is the second domain. Further experiments suggested that a putative dimerization site is distinct from the binding cleft and may be located on the fourth Ig-like domain.

The c-kit proto-oncogene in normal and malignant human hematopoiesis

The c-kit proto-oncogene encodes a tyrosine kinase receptor (KIT) which is expressed on many types of human cells. Numerous studies attest to the importance of the c-kit receptor and its ligand, known variously as stem cell factor (SCF), mast cell growth factor (MGF), Steel factor (SF), or kit ligand (KL) (the nomenclature we prefer), in the development of human hematopoietic cells. KL, which is produced in membrane-bound and soluble forms by bone marrow stromal cells, acts on pre-colony forming units (pre-CFU) and CFU cells. In synergistic combination with other cytokines, KL enhances the growth of myeloid progenitor cells. However, using an antisense oligodeoxynucleotide strategy to disrupt c-kit function, we have demonstrated that the KL-KIT complex is of greatest importance for generation and/or proliferation of normal human erythropoietic progenitor cells. In malignant hematopoietic cells, the complex also appears to be important for growth of granulocyte/macrophage (GM) CFU as well.