Increased G-CSF responsiveness of bone marrow cells from hematopoietic cell phosphatase deficient viable motheaten mice

The mouse mutation viable motheaten (me(v)) results in defects in the expression and catalytic activity of the cytoplasmic protein tyrosine phosphatase known as hematopoietic cell phosphatase (HCP). This reduction in HCP activity leads to the aberrant regulation of several myeloid and lymphoid cell lineages, including substantial increases in numbers of granulocytes. The differentiation, proliferation, and survival of cells in this lineage are normally supported by granulocyte-colony stimulating factor (G-CSF). In this study we have determined the consequences of the loss of HCP activity in me(v)/me(v) mice on the response of bone marrow cells to G-CSF. Bone marrow from these mice exhibited substantial increases in clonogenic and proliferative responses to G-CSF. These enhanced activities of G-CSF correlated with an increase in the level of immature granulocytic, G-CSF receptor positive cells in the bone marrow. These results suggested the possibility that HCP may regulate the G-CSF receptor by a direct interaction. However, under conditions where the previously described interaction between the erythropoietin receptor and HCP was readily observed, HCP did not detectably associate with the G-CSF receptor.

Multiple signaling pathways induced by granulocyte colony-stimulating factor involving activation of JAKs, STAT5, and/or STAT3 are required for regulation of three distinct classes of immediate early genes

Granulocyte colony-stimulating factor (G-CSF) is the major regulator of proliferation and differentiation of neutrophilic granulocyte precursor cells. G-CSF activates multiple signaling molecules, including the JAK1 and JAK2 kinases and the STAT transcription factors. To investigate G-CSF signaling events regulated by the JAK-STAT pathway, we have generated UT7-epo cells stably expressing either wild-type (wt) G-CSF receptor or a series of C-terminal deletion mutants. Gel mobility shift and immunoprecipitation/Western analysis showed that STAT5 is rapidly activated by G-CSF in cells expressing the wt G-CSF receptor, in addition to the previously reported STAT3 and STAT1. Mutants lacking any tyrosine residues in the cytoplasmic domain maintain their ability to activate STAT5 and STAT1 but cannot activate STAT3, implying that STAT5 and STAT1 activation does not require receptor tyrosine phosphorylation. We also observed significant changes in the ratio of STAT1:STAT3:STAT5 activated by various G-CSF receptor C-terminal deletion mutants. These mutant receptors were further used to investigate the role of JAKs and STATs in G-CSF-mediated responses in these cells. We found that JAK activation correlates with G-CSF-induced cell proliferation, whereas STAT activation is not required. We have also identified three classes of G-CSF immediate early genes, whose activation correlates with the activation of distinct JAK-STAT pathways. Our data show that, whereas c-fos is regulated through a pathway independent of STAT activation, oncostatin M, IRF-1, and egr-1 are regulated by an STAT5-dependent pathway and fibrinogen is regulated by an STAT3-dependent pathway. In conclusion, our results suggest that G-CSF regulates its complex biologic activities by selectively activating distinct early response genes through different JAK-STAT signaling molecules.

Multiple tyrosine protein kinases in rat hippocampal neurons: isolation of Ptk-3, a receptor expressed in proliferative zones of the developing brain

Tyrosine protein kinases are likely to play an important role in the maintenance and/or development of the nervous system. In this study we have used the PCR cloning technique to isolate sequences derived from tyrosine kinase genes expressed in cultured hippocampal neurons obtained from 17.5-day-old rat embryos. Nucleotide sequence analysis of 209 independent clones revealed sequences derived from 25 tyrosine kinases, of which two corresponded to previously unreported genes. One of the PCR clones, ptk-2, belongs to the Jak family of cytoplasmic tyrosine kinases. The second clone, ptk-3, was derived from a gene encoding an additional class of tyrosine kinase receptors whose extracellular domains contain regions of homology with coagulation factors V and VIII and complement component C1. Transcripts encoding the Ptk-3 receptor are present in a variety of embryonic and adult tissues with highest levels observed in brain. During development, ptk-3 transcripts are most abundant in the proliferative neuroepithelial cells of the ventricular zone, raising the possibility that this receptor may play an important role in the generation of the mammalian nervous system.

trkC encodes multiple neurotrophin-3 receptors with distinct biological properties and substrate specificities

The trkC gene product gp145trkC is a high affinity signaling receptor for neurotrophin-3 (NT-3), a member of the NGF family of neurotrophic factors. We now report that trkC encodes at least two additional tyrosine protein kinase receptors. These receptors, designated TrkC K2 and TrkC K3, have the same amino acid sequences as gp145trkC (now designated TrkC K1) except for the presence of 14 and 25 additional amino acid residues between kinase subdomains VII and VIII, just downstream from the TDYYR motif which encompasses the putative autophosphorylation site of the Trk receptor family. Upon interaction with their cognate ligand, NT-3, all three TrkC receptor isoforms become rapidly phosphorylated on tyrosine residues and induce DNA synthesis in quiescent cells. However, only TrkC K1 has mitogenic activity in NIH3T3 cells and induces neuronal differentiation of PC12 cells. The different biological properties of these TrkC receptor isoforms probably result from their engagement with different signaling pathways. Whereas TrkC K1 phosphorylates phospholipase C gamma 1 and phosphatidylinositol-3 kinase, TrkC K2 and TrkC K3 do not. TrkC K2 and transcripts encoding TrkC K3 have been identified in various structures of the adult murine brain. These observations suggest that the trophic activities of NT-3 in the mammalian nervous system might be mediated by different TrkC receptor isoforms.

Similarities and differences in the way neurotrophins interact with the Trk receptors in neuronal and nonneuronal cells

We have exploited a battery of approaches to address several controversies that have accompanied the expansion of the nerve growth factor (NGF) family of neurotrophic factors and the identification of the Trk tyrosine kinases as receptors for these factors. For example, we find that a recently cloned mammalian neurotrophin, known as either neurotrophin-4 or neurotrophin-5 and assigned widely differing receptor specificities, represents the functional counterpart of Xenopus neurotrophin-4 and is a "preferred" ligand for TrkB. However, its interactions with TrkB can be distinguished from those of brain-derived neurotrophic factor (BDNF) with TrkB. We also find that all of the Trks display similar dose responses to their "preferred" ligands in neuronal as compared with nonneuronal cells (i.e., NGF for TrkA, BDNF and NT-4/5 for TrkB, and NT-3 for TrkC), providing evidence against a role for accessory molecules expressed in neurons in generating receptors that would allow for responses to lower concentrations of the neurotrophins. However, we find that a neuronal environment does restrict the Trks in their ability to respond to their "nonpreferred" neurotrophin ligands.

Nerve growth factor mediates signal transduction through trk homodimer receptors

We have investigated the molecular nature of the high affinity nerve growth factor (NGF) receptors by using cell lines expressing gp75LNGFR and gp140trk. Our results suggest that gp75LNGFR and gp140trk interact with NGF independently and that only gp140trk mediates NGF signaling. NGF binds to gp140trk with picomolar affinity and induces its phosphorylation on tyrosine residues regardless of the presence of gp75LNGFR. NGF-gp140trk complexes display the slow dissociation rate and rapid internalization characteristics of high affinity NGF receptors. Cross-linking studies reveal the existence of gp75LNGFR and gp140trk homodimers. However, we were unable to detect gp75LNGFR-gp140trk heterodimers. Coexpression in COS cells of wild-type and kinase deficient mutants reveals that gp140trk receptors can undergo intermolecular phosphorylation, indicating the formation of functional homodimers. Moreover, these kinase deficient mutants inhibit NGF-induced signaling through wild-type gp140trk receptors. These results indicate that the functional high affinity NGF receptors consist of gp140trk homodimeric (or oligomeric) complexes.

K252a is a selective inhibitor of the tyrosine protein kinase activity of the trk family of oncogenes and neurotrophin receptors

K252a, an efficient serine/threonine protein kinase inhibitor (IC50s of 10 to 30 nM), has been shown to block the neuronal differentiation of rat pheochromocytoma PC12 cells induced by nerve growth factor (NGF). In this report, we demonstrate that K252a is a potent inhibitor (IC50 of 3 nM) of the tyrosine protein kinase activity of the NGF receptor gp140trk, the product of the trk protooncogene. K252a also inhibits the kinase activity of its transforming alleles, the trk oncogenes, and of the related neurotrophin receptors gp145trkB and gp145trkC, the products of the other known members of the trk gene family, trkB and trkC. In contrast, K252a has no effect (even at micromolar concentrations) on other tyrosine protein kinases such as the receptors for EGF and PDGF and the products of the v-src and v-fms oncogenes. In addition, K252a rapidly reverts the transformed phenotype of NIH3T3 cells transformed by either autocrine stimulation of the trk family of receptors by their cognate ligands or by expression of trk oncogenes isolated from human tumors. The selectivity of K252a for the catalytic activity of the trk family of kinases should help to establish the structural basis for the rational design of highly specific tyrosine protein kinase inhibitors.

The trkB tyrosine protein kinase is a receptor for brain-derived neurotrophic factor and neurotrophin-3

trkB is a tyrosine protein kinase gene highly related to trk, a proto-oncogene that encodes a receptor for nerve growth factor (NGF) and neurotrophin-3 (NT-3). trkB expression is confined to structures of the central and peripheral nervous systems, suggesting it also encodes a receptor for neurotrophic factors. Here we show that brain-derived neurotrophic factor (BDNF) and NT-3, but not NGF, can induce rapid phosphorylation on tyrosine of gp145trkB, one of the receptors encoded by trkB. BDNF and NT-3 can induce DNA synthesis in quiescent NIH 3T3 cells that express gp145trkB. Cotransfection of plasmids encoding gp145trkB and BDNF or NT-3 leads to transformation of recipient NIH 3T3 cells. In these assays, BDNF elicits a response at least two orders of magnitude higher than NT-3. Finally, 125I-NT-3 binds to NIH 3T3 cells expressing gp145trkB; binding can be competed by NT-3 and BDNF but not by NGF. These findings indicate that gp145trkB may function as a neurotrophic receptor for BDNF and NT-3.

The trk tyrosine protein kinase mediates the mitogenic properties of nerve growth factor and neurotrophin-3

The product of the trk proto-oncogene encodes a receptor for nerve growth factor (NGF). Here we show that NGF is a powerful mitogen that can induce resting NIH 3T3 cells to enter S phase, grow in semisolid medium, and become morphologically transformed. These mitogenic effects are absolutely dependent on expression of gp140trk receptors, but do not require the presence of the previously described low affinity NGF receptor. gp140trk also serves as a receptor for the related factor neurotrophin-3 (NT-3), but not for brain-derived neurotrophic factor. Both NGF and NT-3 induce the rapid phosphorylation of gp140trk receptors and the transient expression of c-Fos proteins. However, NT-3 appears to elicit more limited mitogenic responses than NGF. These results indicate that the product of the trk proto-oncogene is sufficient to mediate signal transduction processes induced by NGF and NT-3, at least in proliferating cells.

The role of kinase activity and the kinase insert region in ligand-induced internalization and degradation of the c-fms protein

Molecular steps in endocytosis and degradation of the c-fms protein were analyzed by following the fate of mutated c-fms molecules after M-CSF binding. A mutant c-fms protein lacking tyrosine kinase activity was rapidly internalized after M-CSF binding but not degraded. Another mutant c-fms molecule that lacked most of the kinase insert region was similarly internalized after M-CSF binding and also not degraded. This indicates that the signal for internalization is separate from that directing degradation of the receptor. It has been shown previously that a c-fms mutant in which the kinase insert domain is deleted retains tyrosine kinase activity but lacks two major sites of autophosphorylation. The degradation step therefore requires both kinase activity and the kinase insert region whereas the internalization step is independent of these factors. The major sites of tyrosine autophosphorylation within the kinase insert region were next mutated to determine whether autophosphorylation in the kinase insert region of c-fms might be the signal that triggers degradation of internalized receptors. These mutant receptors were still rapidly degraded in response to M-CSF. Therefore, ligand-induced degradation of c-fms may require tyrosine phosphorylation of a protein other than the c-fms receptor itself and the kinase insert region may be necessary for recognition of this substrate.

Macrophage colony-stimulating factor-induced tyrosine phosphorylation of c-fms proteins expressed in FDC-P1 and BALB/c 3T3 cells

The c-fms protein is a receptor for macrophage colony-stimulating factor (M-CSF) with intrinsic protein-tyrosine kinase activity. We investigated the tyrosine phosphorylation of murine c-fms proteins expressed from a retroviral vector in factor-dependent myeloid FDC-P1 cells and in BALB/c 3T3 fibroblasts transformed by the expression of the c-fms gene. FDC-P1 cells expressing c-fms were able to grow and differentiate in response to M-CSF. Their c-fms proteins were normally phosphorylated on serine and became phosphorylated on tyrosine residues contained in five tryptic peptides when the cells were exposed to M-CSF. A subset of these peptides was constitutively phosphorylated in BALB/c cells expressing c-fms, consistent with the production of M-CSF by these cells. All the peptides detected in vivo were also phosphorylated in vitro. These peptides were analyzed by susceptibility to proteases, comparison with synthetic peptides, and site-directed mutagenesis. The identities of four of the tryptic peptides were determined; they arise from three unique tyrosine phosphorylation sites. One major site of tyrosine phosphorylation at residue 697 accounted for two of the tryptic peptides. A second major site was identified at tyrosine residue 706. These two tyrosine phosphorylation sites are located within the tyrosine kinase insert region. Tyrosine 807, which has homology to the major autophosphorylation site of the p60v-src tyrosine kinase, is a minor autophosphorylation site. Possible functional roles for these phosphorylations of the c-fms protein include interactions with substrate proteins, catalytic activity, and ligand-induced degradation.

Integrins isolated from Rous sarcoma virus-transformed chicken embryo fibroblasts

Avian integrins are a complex of three integral membrane glycoproteins that are thought to have a role both in anchoring the cytoskeleton to the plasma membrane and establishing linkages to the extracellular matrix. We previously demonstrated that bands 2 and 3 of integrin are phosphorylated on both tyrosine and serine residues in chicken embryo fibroblasts (CEF) transformed with Rous Sarcoma virus (RSV) and other oncogenic retroviruses. The effects of RSV transformation on integrins from chick cells are now further characterized. The major site of tyrosine phosphorylation on band 3 in RSV transformed CEF has been identified as tyrosine 788. We also demonstrate that the product of the RSV oncogene, pp60v-src, can phosphorylate integrin in vitro, at the same residue. Tryptic peptide mapping and tunicamycin treatment indicates that a previously observed 4-8 k increase in the Mr of integrins from RSV-transformed cells can be attributed to an alteration in a post-translational modification such as glycosylation. Equilibrium gel filtration assays were used to test the ability of integrins from RSV-transformed CEF to interact with talin and fibronectin. Tyrosine phosphorylated integrins showed a decreased ability to interact with both these ligands in vitro. Conversely, integrins isolated from RSV-transformed CEF metabolically labeled in the absence of phosphatase inhibitor contained only low levels of phosphotyrosine and showed an almost normal ability to interact with ligands. Competition experiments indicated that the region of integrin containing tyrosine 788 is also important for talin binding to integrins.