Induction of cell shape changes through activation of the interleukin-3 common beta chain receptor by the RON receptor-type tyrosine kinase

[Novel Tyrosine Phosphorylation Signals in the Nucleus and on Mitotic Spindle Fibers and Lysosomes Revealed by Strong Inhibition of Tyrosine Dephosphorylation]

Protein-tyrosine phosphorylation is one of the posttranslational modifications and plays critical roles in regulating a wide variety of cellular processes, such as cell proliferation, differentiation, adhesion, migration, survival, and apoptosis. Protein-tyrosine phosphorylation is reversibly regulated by protein-tyrosine kinases and protein-tyrosine phosphatases. Strong inhibition of protein-tyrosine phosphatase activities is required to undoubtedly detect tyrosine phosphorylation. Our extremely careful usage of Na₃VO₄, a potent protein-tyrosine phosphatase inhibitor, has revealed not only the different intracellular trafficking pathways of Src-family tyrosine kinase members but also novel tyrosine phosphorylation signals in the nucleus and on mitotic spindle fibers and lysosomes. Furthermore, despite that the first identified oncogene product v-Src is generally believed to induce transformation through continuous stimulation of proliferation signaling by its strong tyrosine kinase activity, v-Src-driven transformation was found to be caused not by continuous proliferation signaling but by v-Src tyrosine kinase activity-dependent stochastic genome alterations. Here, I summarize our findings regarding novel tyrosine phosphorylation signaling in a spatiotemporal sense and highlight the significance of the roles of tyrosine phosphorylation in transcriptional regulation inside the nucleus and chromosome dynamics.

Src Acts as an Effector for Ku70-dependent Suppression of Apoptosis through Phosphorylation of Ku70 at Tyr-530

Src-family tyrosine kinases are widely expressed in many cell types and participate in a variety of signal transduction pathways. Despite the significance of Src in suppression of apoptosis, its mechanism remains poorly understood. Here we show that Src acts as an effector for Ku70-dependent suppression of apoptosis. Inhibition of endogenous Src activity promotes UV-induced apoptosis, which is impaired by Ku70 knockdown. Src phosphorylates Ku70 at Tyr-530, being close to the possible acetylation sites involved in promotion of apoptosis. Src-mediated phosphorylation of Ku70 at Tyr-530 decreases acetylation of Ku70, whereas Src inhibition augments acetylation of Ku70. Importantly, knockdown-rescue experiments with stable Ku70 knockdown cells show that the nonphosphorylatable Y530F mutant of Ku70 reduces the ability of Ku70 to suppress apoptosis accompanied by augmentation of Ku70 acetylation. Our results reveal that Src plays a protective role against hyperactive apoptotic cell death by reducing apoptotic susceptibility through phosphorylation of Ku70 at Tyr-530.

Phosphorylation of KRAB-associated protein 1 (KAP1) at Tyr-449, Tyr-458, and Tyr-517 by nuclear tyrosine kinases inhibits the association of KAP1 and heterochromatin protein 1α (HP1α) with heterochromatin

Protein tyrosine phosphorylation regulates a wide range of cellular processes at the plasma membrane. Recently, we showed that nuclear tyrosine phosphorylation by Src family kinases (SFKs) induces chromatin structural changes. In this study, we identify KRAB-associated protein 1 (KAP1/TIF1β/TRIM28), a component of heterochromatin, as a nuclear tyrosine-phosphorylated protein. Tyrosine phosphorylation of KAP1 is induced by several tyrosine kinases, such as Src, Lyn, Abl, and Brk. Among SFKs, Src strongly induces tyrosine phosphorylation of KAP1. Nucleus-targeted Lyn potentiates tyrosine phosphorylation of KAP1 compared with intact Lyn, but neither intact Fyn nor nucleus-targeted Fyn phosphorylates KAP1. Substitution of the three tyrosine residues Tyr-449/Tyr-458/Tyr-517, located close to the HP1 binding-motif, into phenylalanine ablates tyrosine phosphorylation of KAP1. Immunostaining and chromatin fractionation show that Src and Lyn decrease the association of KAP1 with heterochromatin in a kinase activity-dependent manner. KAP1 knockdown impairs the association of HP1α with heterochromatin, because HP1α associates with KAP1 in heterochromatin. Intriguingly, tyrosine phosphorylation of KAP1 decreases the association of HP1α with heterochromatin, which is inhibited by replacement of endogenous KAP1 with its phenylalanine mutant (KAP1-Y449F/Y458F/Y517F, KAP1-3YF). In DNA damage, KAP1-3YF repressed transcription of p21. These results suggest that nucleus-localized tyrosine kinases, including SFKs, phosphorylate KAP1 at Tyr-449/Tyr-458/Tyr-517 and inhibit the association of KAP1 and HP1α with heterochromatin.

Nuclear ErbB4 signaling through H3K9me3 that is antagonized by EGFR-activated c-Src

The ErbB family of receptor tyrosine kinases comprises four members: EGFR (epidermal growth factor receptor)/ErbB1, HER2/ErbB2, ErbB3 and ErbB4, and plays roles in signal transduction at the plasma membrane upon ligand stimulation. Stimulation with neuregulin-1 (NRG-1) cleaves ErbB4 and releases the ErbB4 intracellular domain (4ICD) that translocates into the nucleus to control gene expression. However, little is known about the regulation of 4ICD nuclear signaling through tyrosine phosphorylation. We show here that 4ICD nuclear signaling is antagonized by EGF-induced c-Src activation via EGFR. Generation of 4ICD by NRG-1 leads to increased levels of trimethylated histone H3 on lysine 9 (H3K9me3) in a manner dependent on 4ICD's nuclear accumulation and its tyrosine kinase activity. Once EGF activates c-Src downstream of EGFR concomitantly with NRG-1-induced ErbB4 activation, c-Src associates with phospho-Tyr950 and phospho-Tyr1056 on 4ICD, thereby decreasing nuclear accumulation of 4ICD and inhibiting an increase of H3K9me3 levels. Moreover, 4ICD-induced transcriptional repression of the human telomerase reverse transcriptase (hTERT) is inhibited by EGF-EGFR-Src signaling. Thus, our findings reveal c-Src-mediated inhibitory regulation of ErbB4 nuclear signaling upon EGFR activation.

The Lyn kinase C-lobe mediates Golgi export of Lyn through conformation-dependent ACSL3 association

The Src-family tyrosine kinase Lyn has a role in signal transduction at the cytoplasmic face of the plasma membrane upon extracellular ligand stimulation. After synthesis in the cytoplasm, Lyn accumulates on the Golgi and is subsequently transported to the plasma membrane. However, the mechanism of Lyn trafficking remains elusive. We show here that the C-lobe of the Lyn kinase domain is associated with long-chain acyl-CoA synthetase 3 (ACSL3) on the Golgi in a manner that is dependent on Lyn conformation but is independent of its kinase activity. Formation of a closed conformation by CSK prevents Lyn from associating with ACSL3, resulting in blockade of Lyn export from the Golgi. Overexpression and knockdown of ACSL3 accelerates and blocks Golgi export of Lyn, respectively. The post-Golgi route of Lyn, triggered by ACSL3, is distinct from that of vesicular stomatitis virus glycoprotein (VSV-G) and of caveolin. Moreover, an ACSL3 mutant lacking the LR2 domain, which is required for the catalytic activity, retains the ability to associate with Lyn and accelerate Golgi export of Lyn. These results suggest that initiation of Golgi export of Lyn involves association of ACSL3 with the Lyn C-lobe, which is exposed to the molecular surface in an open conformation.

Differential trafficking of Src, Lyn, Yes and Fyn is specified by the state of palmitoylation in the SH4 domain

Src-family tyrosine kinases (SFKs), which participate in a variety of signal transduction events, are known to localize to the cytoplasmic face of the plasma membrane through lipid modification. Recently, we showed that Lyn, an SFK member, is exocytosed to the plasma membrane via the Golgi region along the secretory pathway. We show here that SFK trafficking is specified by the palmitoylation state. Yes is also a monopalmitoylated SFK and is biosynthetically transported from the Golgi pool of caveolin to the plasma membrane. This pathway can be inhibited in the trans-Golgi network (TGN)-to-cell surface delivery by temperature block at 19 degrees C or dominant-negative Rab11 GTPase. A large fraction of Fyn, a dually palmitoylated SFK, is directly targeted to the plasma membrane irrespective of temperature block of TGN exit. Fyn(C6S), which lacks the second palmitoylation site, is able to traffic in the same way as Lyn and Yes. Moreover, construction of Yes(S6C) and chimeric Lyn or Yes with the Fyn N-terminus further substantiates the importance of the dual palmitoylation site for plasma membrane targeting. Taken together with our recent finding that Src, a nonpalmitoylated SFK, is rapidly exchanged between the plasma membrane and late endosomes/lysosomes, these results suggest that SFK trafficking is specified by the palmitoylation state in the SH4 domain.

Transactivation of RON receptor tyrosine kinase by interaction with PDGF receptor beta during steady-state growth of human mesangial cells

Although it is well known that platelet-derived growth factor (PDGF) causes mesangial cell proliferation (presumably contributing to progression of glomerular disease), targeted inhibition of the PDGF receptor system has shown only limited efficacy against glomerular diseases. To examine whether this discrepancy is due to the involvement of other pathways, we used phosphorylated receptor tyrosine kinase arrays and found that RON (recepteur d'origine nantais) was phosphorylated while the PDGF receptor was dephosphorylated (thus inactive) in human mesangial cells (HMCs) at the time of cell cycle entry. Further, RON remained active during steady-state growth. Activation of RON was independent of its canonical ligand, macrophage-stimulating protein, but was mediated by transactivation from the PDGF-engaged PDGF receptor. Following stimulation with PDGF we found that the two receptors physically interacted. Knockdown of RON by siRNA increased the number of apoptotic cells without affecting the rate of DNA synthesis, suggesting that RON has anti-apoptotic functions. Immunohistochemical analysis found phosphorylated RON in glomerular lesions of patients with IgA nephropathy but not those with minimal change nephrotic syndrome, a disease not associated with mesangial proliferation. These results suggest that RON is involved in mesangial cell proliferation under both physiological and pathological conditions, and may be a relevant target for therapeutic intervention.

Nuclear localization of Lyn tyrosine kinase mediated by inhibition of its kinase activity

Src-family kinases, cytoplasmic enzymes that participate in various signaling events, are found at not only the plasma membrane but also subcellular compartments, such as the nucleus, the Golgi apparatus and late endosomes/lysosomes. Lyn, a member of the Src-family kinases, is known to play a role in DNA damage response and cell cycle control in the nucleus. However, it is still unclear how the localization of Lyn to the nucleus is regulated. Here, we investigated the mechanism of the distribution of Lyn between the cytoplasm and the nucleus in epitheloid HeLa cells and hematopoietic THP-1 cells. Lyn was definitely detected in purified nuclei by immunofluorescence and immunoblotting analyses. Nuclear accumulation of Lyn was enhanced upon treatment of cells with leptomycin B (LMB), an inhibitor of Crm1-mediated nuclear export. Moreover, Lyn mutants lacking the sites for lipid modification were highly accumulated in the nucleus upon LMB treatment. Intriguingly, inhibition of the kinase activity of Lyn by SU6656, Csk overexpression, or point mutation in the ATP-binding site induced an increase in nuclear Lyn levels. These results suggest that Lyn being imported into and rapidly exported from the nucleus preferentially accumulates in the nucleus by inhibition of the kinase activity and lipid modification.

Met-related receptor tyrosine kinase Ron in tumor growth and metastasis

The Ron receptor is a member of the Met family of cell surface receptor tyrosine kinases and is primarily expressed on epithelial cells and macrophages. The biological response of Ron is mediated by binding of its ligand, hepatocyte growth factor-like protein/macrophage stimulating-protein (HGFL). HGFL is primarily synthesized and secreted from hepatocytes as an inactive precursor and is activated at the cell surface. Binding of HGFL to Ron activates Ron and leads to the induction of a variety of intracellular signaling cascades that leads to cellular growth, motility and invasion. Recent studies have documented Ron overexpression in a variety of human cancers including breast, colon, liver, pancreas, and bladder. Moreover, clinical studies have also shown that Ron overexpression is associated with both worse patient outcomes as well as metastasis. Forced overexpression of Ron in transgenic mice leads to tumorigenesis in both the lung and the mammary gland and is associated with metastatic dissemination. While Ron overexpression appears to be a hallmark of many human cancers, the mechanisms by which Ron induces tumorigenesis and metastasis are still unclear. Several strategies are currently being undertaken to inhibit Ron as a potential therapeutic target; current strategies include the use of Ron blocking proteins, small interfering RNA (siRNA), monoclonal antibodies, and small molecule inhibitors. In total, these data suggest that Ron is a critical factor in tumorigenesis and that inhibition of this protein, alone or in combination with current therapies, may prove beneficial in the treatment of cancer patients.

Nuclear localization of magphinins, alternative splicing products of the human trophinin gene

Human magphinin proteins are translation products of differentially spliced transcripts from the 5' region of the human trophinin gene (TRO), whose 3' region encodes trophinin, a unique cell adhesion molecule involved in human embryo implantation. Magphinins belong to the MAGE (melanoma-associated antigen) family, and a previous study of mouse magphinins showed their expression in male and female germ cells, suggesting a role in germ cell development. Here, we characterized the structure and subcellular localization of human magphinins. Confocal microscopy analysis of ectopically expressed magphinins revealed that magphinin-alpha and -beta localize in the cytoplasm, whereas magphinin-gamma lacking the peptide encoded by exon-3 is nuclear. Following Triton X-100 extraction, DNA digestion, and high salt extraction magphinin-gamma remained nuclear, suggesting strong association with the nuclear matrix. A series of magphinin-gamma deletion mutants were generated and assayed for localization, which showed that the N-terminal region of the MAGE homology domain is necessary for nuclear localization. When magphinin-gamma was expressed in NIH3T3 cells, cells underwent G1 arrest. These results suggest that human magphinin-gamma inhibits cell cycle progression through nuclear activity.

Role of Src-family kinases in formation of the cortical actin cap at the dorsal cell surface

Protein-tyrosine phosphorylation is regulated by protein-tyrosine kinases and protein-tyrosine phosphatases (PTPs). Src-family tyrosine kinases (SFKs) participate in the regulation of the actin cytoskeleton. Actin filaments can be accumulated in a cap at the dorsal cell surface, which is called the cortical actin cap. Here, we show that SFKs play an important role in formation of the cortical actin cap. HeLa cells normally exhibit the cortical actin cap, one of the major sites of tyrosine phosphorylation. The cortical actin cap is disrupted by SFK inhibitors or overexpression of the Lyn SH3 domain. Csk-knockout cells form the cortical actin cap when the level of tyrosine phosphorylation is increased by Na(3)VO(4), a PTP inhibitor, and the formation of the cortical actin cap is inhibited by SFK inactivation with re-introduction of Csk. SYF cells lacking SFKs minimally exhibit the cortical actin cap even in the presence of Na(3)VO(4), and transfection with Lyn restores the cortical actin cap in the presence of Na(3)VO(4). Disruption of the cortical actin cap by dominant-negative Cdc42 causes loss of tyrosine phosphorylation at the cell top. These results suggest that SFK(s) is involved in formation of the cortical actin cap, which may serve as a platform of tyrosine phosphorylation signaling.

Differential mitotic activation of endogenous c-Src, c-Yes, and Lyn in HeLa cells

Src-family tyrosine kinases (SFKs) play an important role in mitosis. Despite overlapping expression of multiple SFK members, little is known about how individual SFK members are activated in M phase. Here, we examined mitotic activation of endogenous c-Src, c-Yes, and Lyn, which are co-expressed in HeLa cells. c-Src, c-Yes, and Lyn were activated at different levels in M phase, and the activation was inhibited by Cdc2 inactivation. Mitotic c-Src and c-Yes exhibited normal- and retarded-electrophoretic-mobility forms on SDS-polyacrylamide gels, whereas Lyn did not show mobility retardation. Like c-Src, the retardation of electrophoretic mobility of c-Yes was caused by Cdc2-mediated phosphorylation. The normal- and retarded-mobility forms of c-Src were comparably activated, but activation of the retarded-mobility form of c-Yes was higher than that of the normal-mobility form of c-Yes. Thus, these results suggest that endogenous c-Src, c-Yes, and Lyn are differentially activated through Cdc2 activation during M phase.

The Ron receptor tyrosine kinase regulates acute lung injury and suppresses nuclear factor kappaB activation

Emerging information implies that the Ron receptor tyrosine kinase may play a role in the inflammatory response. However, the manner in which this receptor contributes to the response is not well understood. In the present studies, we investigated the role of the Ron receptor in the acute lung inflammatory response. Wild-type and mutant mice lacking the tyrosine kinase domain of Ron (Ron TK-/-) were subjected to acute lung injury induced by intranasal administration of bacterial lipopolysaccharide (LPS). Wild-type mice showed increased lung injury after LPS administration, as determined by the leakage of albumin into the lung and by histopathological changes. Ron TK-/- mice had more than twice the amount of albumin leak and much greater thickening of the alveolar septae. Lipopolysaccharide administration caused neutrophil recruitment into the lungs, as measured by myeloperoxidase. However, Ron TK-/- mice had much higher baseline levels of myeloperoxidase, which did not increase further after LPS. Lung injury in wild-type mice occurred with activation of the transcription factor, nuclear factor kappaB (NF-kappaB), and subsequent increases in intrapulmonary generation of tumor necrosis factor alpha. In TK-/- mice, there was far less IkappaB-alpha and IkappaB-beta protein and greater activation of NF-kappaB. This was associated with substantially increased production of tumor necrosis factor alpha and the nitric oxide (NO) by-product, nitrite. The data suggest that the Ron receptor tyrosine kinase plays an important regulatory role in acute inflammatory lung injury by suppressing signals leading to activation of NF-kappaB.

Rapid trafficking of c-Src, a non-palmitoylated Src-family kinase, between the plasma membrane and late endosomes/lysosomes

Src-family kinases (SFKs) are co-expressed with multiple combinations of each member in a single cell and involved in various signalings. Recently, we showed by sucrose-density gradient fractionation that the subcellular distribution of c-Src is distinct from that of Lyn. However, little is known about the trafficking of c-Src in living cells. Here, we show by time-lapse monitoring combined with photobleaching techniques that c-Src, a non-palmitoylated SFK, is rapidly exchanged between the plasma membrane and intracellular organelles representing late endosomes/lysosomes possibly through its cytosolic release. Although Lyn, a palmitoylated SFK, is exocytosed to the plasma membrane via the Golgi apparatus along the secretory pathway, lack of palmitoylation directs Lyn away from the exocytotic transport to the c-Src-type trafficking between the plasma membrane and late endosomes/lysosomes. Intriguingly, c-Src and a non-palmitoylated Lyn mutant are efficiently delivered and immobilized to focal adhesions when their SH2 domains are able to mediate protein-protein interactions in place of intramolecular bindings. However, palmitoylation of Lyn inhibits its recruitment to focal adhesions. These results suggest that palmitoylation of SFKs is critical for SFK localization and trafficking and implicate that two distinct trafficking pathways for SFKs may be involved in SFKs' specific functions.

Role of Src-family kinases in formation and trafficking of macropinosomes

Src-family kinases that localize to the cytoplasmic side of cellular membranes through lipid modification play a role in signaling events including membrane trafficking. Macropinocytosis is an endocytic process for solute uptake by large vesicles called macropinosomes. Although macropinosomes can be visualized following uptake of fluorescent macromolecules, little is known about the dynamics of macropinosomes in living cells. Here, we show that constitutive c-Src expression generates macropinosomes in a kinase-dependent manner. Live-cell imaging of GFP-tagged c-Src (Src-GFP) reveals that c-Src associates with macropinosomes via its N-terminus continuously from their generation at membrane ruffles, through their centripetal trafficking, to fusion with late endosomes and lysosomes. Fluorescence recovery after photobleaching (FRAP) of Src-GFP shows that Src-GFP is rapidly recruited to macropinosomal membranes from the plasma membrane and intracellular organelles through vesicle transport even in the presence of a protein synthesis inhibitor. Furthermore, using a HeLa cell line overexpressing inducible c-Src, we show that following stimulation with epidermal growth factor (EGF), high levels of c-Src kinase activity promote formation of macropinosomes associated with the lysosomal compartment. Unlike c-Src, Lyn and Fyn, which are palmitoylated Src kinases, only minimally induce macropinosomes, although a Lyn mutant in which the palmitoylation site is mutated efficiently induces macropinocytosis. We conclude that kinase activity of nonpalmitoylated Src kinases including c-Src may play an important role in the biogenesis and trafficking of macropinosomes.

Src Signaling Regulates Completion of Abscission in Cytokinesis through ERK/MAPK Activation at the Midbody

Src family non-receptor-type tyrosine kinases regulate a wide variety of cellular events including cell cycle progression in G(2)/M phase. Here, we show that Src signaling regulates the terminal step in cytokinesis called abscission in HeLa cells. Abscission failure with an unusually elongated intercellular bridge containing the midbody is induced by treatment with the chemical Src inhibitors PP2 and SU6656 or expression of membrane-anchored Csk chimeras. By anti-phosphotyrosine immunofluorescence and live cell imaging, completion of abscission requires Src-mediated tyrosine phosphorylation during early stages of mitosis (before cleavage furrow formation), which is subsequently delivered to the midbody through Rab11-driven vesicle transport. Treatment with U0126, a MEK inhibitor, decreases tyrosine phosphorylation levels at the midbody, leading to abscission failure. Activated ERK by MEK-catalyzed dual phosphorylation on threonine and tyrosine residues in the TEY sequence, which is strongly detected by anti-phosphotyrosine antibody, is transported to the midbody in a Rab11-dependent manner. Src kinase activity during the early mitosis mediates ERK activation in late cytokinesis, indicating that Src-mediated signaling for abscission is spatially and temporally transmitted. Thus, these results suggest that recruitment of activated ERK, which is phosphorylated by MEK downstream of Src kinases, to the midbody plays an important role in completion of abscission.

Mammary-specific Ron receptor overexpression induces highly metastatic mammary tumors associated with beta-catenin activation

Activated growth factor receptor tyrosine kinases (RTK) play pivotal roles in a variety of human cancers, including breast cancer. Ron, a member of the Met RTK proto-oncogene family, is overexpressed or constitutively active in 50% of human breast cancers. To define the significance of Ron overexpression and activation in vivo, we generated transgenic mice that overexpress a wild-type or constitutively active Ron receptor in the mammary epithelium. In these animals, Ron expression is significantly elevated in mammary glands and leads to a hyperplastic phenotype by 12 weeks of age. Ron overexpression is sufficient to induce mammary transformation in all transgenic animals and is associated with a high degree of metastasis, with metastatic foci detected in liver and lungs of >86% of all transgenic animals. Furthermore, we show that Ron overexpression leads to receptor phosphorylation and is associated with elevated levels of tyrosine phosphorylated beta-catenin and the up-regulation of genes, including cyclin D1 and c-myc, which are associated with poor prognosis in patients with human breast cancers. These studies suggest that Ron overexpression may be a causative factor in breast tumorigenesis and provides a model to dissect the mechanism by which the Ron induces transformation and metastasis.

Involvement of the N-terminal unique domain of Chk tyrosine kinase in Chk-induced tyrosine phosphorylation in the nucleus

Chk tyrosine kinase phosphorylates Src-family kinases and suppresses their kinase activity. We recently showed that Chk localizes to the nucleus as well as the cytoplasm and inhibits cell proliferation. In this study, we explored the role of the N-terminal unique domain of Chk in nuclear localization and Chk-induced tyrosine phosphorylation in the nucleus. In situ binding experiments showed that the N-terminal domain of Chk was associated with the nucleus and the nuclear matrix. The presence of the N-terminal domain of Chk led to a fourfold increase in cell population exhibiting Chk-induced tyrosine phosphorylation in the nucleus. Expression of Chk but not kinase-deficient Chk induced tyrosine phosphorylation of a variety of proteins ranging from 23 kDa to approximately 200 kDa, especially in Triton X-100-insoluble fraction that included chromatin and the nuclear matrix. Intriguingly, in situ subnuclear fractionations revealed that Chk induced tyrosine phosphorylation of proteins that were associated with the nuclear matrix. These results suggest that various unidentified substrates of Chk, besides Src-family kinases, may be present in the nucleus. Thus, our findings indicate that the importance of the N-terminal domain to Chk-induced tyrosine phosphorylation in the nucleus, implicating that these nuclear tyrosine-phosphorylated proteins may contribute to inhibition of cell proliferation.

Involvement of Golgi-associated Lyn tyrosine kinase in the translocation of annexin II to the endoplasmic reticulum under oxidative stress

Src-family tyrosine kinases, known to participate in signaling pathways of a variety of receptors at the plasma membrane, are found in cellular endomembranes such as the Golgi apparatus and endosomes. Recently, we showed that Lyn, a member of the Src kinases, accumulates on the Golgi apparatus and then traffics to the plasma membrane. We show here that a majority of endogenous Lyn but not c-Src is accumulated in Golgi-enriched heavy-membrane fractions on a sucrose-density gradient, whereas a small amount of endogenous Lyn is present in light-membrane fractions containing the plasma membrane. Inducible expression of kinase-active Lyn, which biosynthetically reaches the Golgi apparatus, triggers tyrosine phosphorylation of proteins including annexin II. Coimmunoprecipitation analyses reveal that Lyn physically associates with annexin II, and an in vitro kinase assay shows that Lyn phosphorylates annexin II directly. Furthermore, stimulation of cells with H2O2 induces tyrosine phosphorylation of annexin II on the Golgi apparatus in a manner that is dependent on the kinase activity of Src kinases, leading to the translocation of annexin II from the Golgi apparatus to the endoplasmic reticulum. Thus, these results suggest that endomembranes containing the Golgi apparatus where Lyn is anchored can serve as a signaling platform under oxidative stress.

A promiscuous liaison between IL-15 receptor and Axl receptor tyrosine kinase in cell death control

Discrimination between cytokine receptor and receptor tyrosine kinase (RTK) signaling pathways is a central paradigm in signal transduction research. Here, we report a 'promiscuous liaison' between both receptors that enables interleukin (IL)-15 to transactivate the signaling pathway of a tyrosine kinase. IL-15 protects murine L929 fibroblasts from tumor necrosis factor alpha (TNFalpha)-induced cell death, but fails to rescue them upon targeted depletion of the RTK, Axl; however, Axl-overexpressing fibroblasts are TNFalpha-resistant. IL-15Ralpha and Axl colocalize on the cell membrane and co-immunoprecipitate even in the absence of IL-15, whereby the extracellular part of Axl proved to be essential for Axl/IL-15Ralpha interaction. Most strikingly, IL-15 treatment mimics stimulation by the Axl ligand, Gas6, resulting in a rapid tyrosine phosphorylation of both Axl and IL-15Ralpha, and activation of the phosphatidylinositol 3-kinase/Akt pathway. This is also seen in mouse embryonic fibroblasts from wild-type but not Axl-/- or IL-15Ralpha-/- mice. Thus, IL-15-induced protection from TNFalpha-mediated cell death involves a hitherto unknown IL-15 receptor complex, consisting of IL-15Ralpha and Axl RTK, and requires their reciprocal activation initiated by ligand-induced IL-15Ralpha.

Ron receptor signaling augments mammary tumor formation and metastasis in a murine model of breast cancer

The tyrosine kinase receptor Ron has been implicated in several types of cancer, including overexpression in human breast cancer. This is the first report describing the effect of Ron signaling on tumorigenesis and metastasis in a mouse model of breast cancer. Mice with a targeted deletion of the Ron tyrosine kinase signaling domain (TK-/-) were crossed to mice expressing the polyoma virus middle T antigen (pMT) under the control of the mouse mammary tumor virus promoter. Both pMT-expressing wild-type control (pMT+/- TK+/+) and pMT+/- TK-/- mice developed mammary tumors and lung metastases. However, a significant decrease in mammary tumor initiation and growth was found in the pMT+/- TK-/- mice compared with controls. An examination of mammary tumors showed that there was a significant decrease in microvessel density, significantly decreased cellular proliferation, and a significant increase in terminal deoxynucleotidyl transferase-mediated nick end labeling-positive staining in mammary tumor cells from the pMT+/- TK-/- mice compared with the pMT+/- TK+/+ mice. Biochemical analyses on mammary tumor lysates showed that whereas both the pMT-expressing TK+/+ and TK-/- tumors have increased Ron expression compared with normal mammary glands, the pMT-expressing TK-/- tumors have deficits in mitogen-activated protein kinase and AKT activation. These results indicate that Ron signaling synergizes with pMT signaling to induce mammary tumor formation, growth, and metastasis. This effect may be mediated in part through the regulation of angiogenesis and through proliferative and cell survival pathways regulated by mitogen-activated protein kinase and AKT.

Multi-lobulation of the nucleus in prolonged S phase by nuclear expression of Chk tyrosine kinase

Chk tyrosine kinase phosphorylates Src-family tyrosine kinases and suppresses their kinase activity. We recently showed that Chk localizes to the nucleus as well as the cytoplasm and inhibits cell proliferation. To investigate the role of nuclear Chk in proliferation, various Chk mutants were constructed and expressed. Nuclear localization of Chk-induced dynamic multi-lobulation of the nucleus and prolonged S phase of the cell cycle. The N-terminal domain of Chk and a portion of its kinase domain but not the kinase activity were responsible for induction of the multi-lobulation. Cell sorting analysis revealed that nuclear multi-lobulated cells were enriched in late S phase. Multi-lobulated nuclei were surrounded with lamin B1 that was particularly concentrated in concave regions of the nuclei. Furthermore, treatment with nocodazole or taxol disrupted multi-lobulation of the nucleus. These results suggest that nuclear multi-lobulation in late S phase, which is dependent on polymerization and depolymerization of microtubules, may be involved in nuclear Chk-induced inhibition of proliferation.

Trafficking of Lyn through the Golgi caveolin involves the charged residues on alphaE and alphaI helices in the kinase domain

Src-family kinases, known to participate in signaling pathways of a variety of surface receptors, are localized to the cytoplasmic side of the plasma membrane through lipid modification. We show here that Lyn, a member of the Src-family kinases, is biosynthetically transported to the plasma membrane via the Golgi pool of caveolin along the secretory pathway. The trafficking of Lyn from the Golgi apparatus to the plasma membrane is inhibited by deletion of the kinase domain or Csk-induced “closed conformation” but not by kinase inactivation. Four residues (Asp346 and Glu353 on αE helix, and Asp498 and Asp499 on αI helix) present in the C-lobe of the kinase domain, which can be exposed to the molecular surface through an “open conformation,” are identified as being involved in export of Lyn from the Golgi apparatus toward the plasma membrane but not targeting to the Golgi apparatus. Thus, the kinase domain of Lyn plays a role in Lyn trafficking besides catalysis of substrate phosphorylation.

Tyrosine kinase receptor RON functions downstream of the erythropoietin receptor to induce expansion of erythroid progenitors

Erythropoietin (EPO) is required for cell survival during differentiation and for progenitor expansion during stress erythropoiesis. Although signaling pathways may couple directly to docking sites on the EPO receptor (EpoR), additional docking molecules expand the signaling platform of the receptor. We studied the roles of the docking molecules Grb2-associated binder-1 (Gab1) and Gab2 in EPO-induced signal transduction and erythropoiesis. Inhibitors of phosphatidylinositide 3-kinase and Src kinases suppressed EPO-dependent phosphorylation of Gab2. In contrast, Gab1 activation depends on recruitment and phosphorylation by the tyrosine kinase receptor RON, with which it is constitutively associated. RON activation induces the phosphorylation of Gab1, mitogen-activated protein kinase (MAPK), and protein kinase B (PKB) but not of signal transducer and activator of transcription 5 (Stat5). RON activation was sufficient to replace EPO in progenitor expansion but not in differentiation. In conclusion, we elucidated a novel mechanism specifically involved in the expansion of erythroblasts involving RON as a downstream target of the EpoR.

Cross-talk between the receptor tyrosine kinases Ron and epidermal growth factor receptor

Heterogeneous receptor-receptor interactions may play a role in intracellular signaling. Accordingly, the interaction of two dissimilar tyrosine kinase receptors, Ron and epidermal growth factor receptor (EGFR) was investigated. The functional interaction of Ron and EGFR in cell scatter and oncogenic transformation was investigated in vivo. Transfection of a dominant negative form of EGFR into human embryonic kidney cells stably expressing Ron (293-Ron) dramatically reduced the scatter response induced by the Ron ligand hepatocyte growth factor-like protein/macrophage stimulating protein (HGFL). The scatter response of the 293-Ron cells was also attenuated by treatment of the cells with the specific EGFR inhibitor AG 1478. Co-transfection of Ron and dominant-negative EGFR, or co-transfection of EGFR and a dominant-negative form of Ron reduced focus formation in NIH/3T3 cells. Western analysis of NIH/3T3 cells overexpressing murine Ron and expressing endogenous levels of EGFR was used to demonstrate that Ron and EGFR co-immunoprecipitate. Stimulation of the cells in vitro with the Ron ligand HGFL or with the EGFR ligand epidermal growth factor (EGF) appeared to induce phosphorylation of both receptors. Co-immunoprecipitation and phosphorylation of phosphatidyl inositol 3-kinase (PI3-K) was also observed. This novel finding of a functional and biochemical interaction between Ron and EGFR suggests that heterologous tyrosine kinase receptor interactions may play a role in cellular processes such as scatter and transformation.

Oncogenic and invasive potentials of human macrophage-stimulating protein receptor, the RON receptor tyrosine kinase

The product of the RON (recepteur d'origine nantais) gene belongs to the MET proto-oncogene family, a distinct subfamily of receptor tyrosine kinases. The ligand of RON was identified as macrophage-stimulating protein (MSP), a member of the plasminogen-related growth factor family. RON is mainly expressed in cells of epithelial origin and is required for embryonic development. In vitro RON activation results in epithelial cell dissociation, migration and matrix invasion, suggesting that RON might be involved in the pathogenesis of certain epithelial cancers in vivo. Indeed, recent studies have shown that RON expression is significantly altered in several primary human cancers, including those of the breast and colon. Truncation of the RON protein has also been found in primary tumors from the gastrointestinal tract. These alterations lead to constitutive activation of RON that causes cell transformation in vitro, induces neoplasm formation in athymic nude mice, and promotes tumor metastasis into the lung. Studies employing transgenic models further demonstrated that over-expression of RON in lung epithelial cells results in multiple tumor formation with features of large cell undifferentiated carcinoma. The oncogenic activities of RON are mediated by RON-transduced signals that promote unbalanced cell growth and transformation leading to tumor development. Thus, abnormal accumulation and activation of RON could play a critical role in vivo in the progression of certain malignant human epithelial cancers.

The Ron proto-oncogene product is a phenotypic marker of renal oncocytoma

The proto-oncogene product Ron is the receptor for macrophage stimulating protein, a scatter factor that stimulates cell proliferation, prevents apoptosis, and induces an invasive cell phenotype. We investigated the expression of Ron, Ki-67 (proliferation index), p53, and bcl-2 (proapoptotic and antiapoptotic proteins, respectively) in 50 renal tumors (19 clear cell carcinomas, 18 oncocytomas, 7 papillary cell carcinomas, 5 chromophobe cell carcinomas, and 1 carcinoma with sarcomatoid areas). In addition, we studied Ron in normal kidney and in the renal carcinoma cell line Caki-1. Immunostaining and Western blot showed Ron in normal kidney and in all oncocytomas but never in renal cell carcinomas or in Caki-1. In addition, Western blot showed that Ron was expressed in phosphorylated, i.e., active, form. Bcl-2 was strongly expressed in oncocytomas, whereas Ki-67 and p53 were much less expressed in oncocytomas than in carcinomas. These results indicate in Ron a marker that differentiates oncocytoma from the other renal epithelial tumors. We therefore think that Ron may prove to be a new tool for a sound and precise diagnosis of oncocytoma, a benign tumor that cannot always be distinguished from carcinomas at histologic examination. The overexpression of bcl-2, but not p53 in oncocytoma, suggests that the MSP/Ron system sustains the growth of oncocytoma by opposing apoptosis.

Activation of CR3-mediated phagocytosis by MSP requires the RON receptor, tyrosine kinase activity, phosphatidylinositol 3-kinase, and protein kinase C zeta

Macrophage-stimulating protein (MSP) promotes the phagocytosis of C3bi-coated erythrocytes by resident peritoneal macrophages, although the mechanism by which this occurs is largely unknown. We show that MSP-induced complement-mediated phagocytosis requires the RON receptor tyrosine kinase and the alphaMbeta2 integrin, as evidenced by the inability of RON-/- and alphaM-/- peritoneal macrophages to augment phagocytosis of complement-coated sheep erythrocytes in response to MSP. MSP stimulation of macrophages results in tyrosine phosphorylation and AKT activation, and inhibitor studies demonstrate a phagocytic requirement for tyrosine kinase and phosphatidylinositol 3-kinase (PI-3K) activity as well as activity of the atypical protein kinase C (PKC) isoform zeta, which localizes to MSP-induced phagosomes containing complement-coated beads. Additionally, MSP augments the ability of peritoneal macrophages to bind to intercellular adhesion molecule-1 (ICAM-1) via the alphaMbeta2 integrin. MSP-induced ICAM-1 adhesion is also dependent on tyrosine kinase activity, PI-3K, and PKC zeta, indicating that these signaling requirements are upstream of complement receptor 3 activation.

Receptor tyrosine kinase Ron is expressed in mouse reproductive tissues during embryo implantation and is important in trophoblast cell function

Ron is a receptor tyrosine kinase that is activated by the binding of hepatocyte growth factor-like (HGFL) protein. Mutations in the catalytic domain of this receptor result in an aggressively invasive phenotype. Conversely, deletion of the entire receptor results in an embryonic lethality by Embryonic Day 7.5. The specific cellular localization and mechanisms of action of Ron and HGFL during embryo implantation are not known. Therefore, this report characterizes the temporal and spatial distribution of this receptor during mouse embryo implantation and placentation. Reverse transcription-polymerase chain reaction analysis demonstrated the presence of Ron transcripts in the uterus, placenta, testis, and epididymis, whereas HGFL transcripts were found in the cervix, placenta, epididymis, and testis. In situ hybridization and immunohistochemical analyses demonstrated that Ron was present in the cells of the ectoplacental cone and trophoblast giant cell regions surrounding the implanting embryo. Ron expression was also observed in SM9-1, SM9-2, and SM-10 murine trophoblast cell lines. To determine the effects of Ron activation on trophoblast function, Matrigel invasion and cell survival assays were performed using the SM9-1 and SM-10 trophoblast cell lines. The HGFL stimulation of these cells increased invasion and enhanced cell survival. These observations suggest that activation of the Ron receptor by HGFL binding may aid in implantation by way of trophoblast function and viability.

Redistribution of ERK/MAP kinase to uropod-like structures in interleukin-3-induced cell shape changes

Interleukin-3 (IL-3) is one of the cytokines of significance for the regulation of hematopoiesis and inflammation. Recently, we established IL-3-dependent Ba/F3 pro-B cells ectopically expressing RON tyrosine kinase, a receptor for macrophage-stimulating protein (MSP), and showed that MSP stimulation specifically promoted cell morphological changes through tyrosine phosphorylation of the IL-3 common beta-chain receptor subunit (betac) by activated RON kinase without activation of JAK2 tyrosine kinase. Here we investigate the IL-3 signaling pathway leading to morphological changes through tyrosine phosphorylation of betac. Treatment of RON-expressing cells with PD98059 or U0126, inhibitors of mitogen-activated protein kinase kinase activity, blocked both IL-3- and MSP-induced morphological changes. Upon stimulation with IL-3 or MSP, extracellular-regulated kinase (ERK) and F-actin were redistributed in uropod-like structures. ERK and F-actin were colocalized within uropod-like structures, and a majority of F-actin were localized around the peripheries of accumulated ERK. Tyrosine phosphorylation of ERK was detected after stimulation with IL-3 or MSP, whereas treatment with U0126 specifically inhibited IL-3- or MSP-induced ERK phosphorylation but not tyrosine phosphorylation of betac. These results suggest that the activation and localization of ERK to uropod-like structures play a role in IL-3-induced morphological changes.

Macrophage stimulating protein (MSP) evokes superoxide anion production by human macrophages of different origin

1. Macrophage Stimulating Protein (MSP), a serum factor related to Hepatocyte Growth Factor, was originally discovered to stimulate chemotaxis of murine resident peritoneal macrophages. MSP is the ligand for Ron, a member of the Met subfamily of tyrosine kinase receptors. The effects of MSP on human macrophages and the role played in human pathophysiology have long been elusive. 2. We show here that human recombinant MSP (hrMSP) evokes a dose-dependent superoxide anion production in human alveolar and peritoneal macrophages as well as in monocyte-derived macrophages, but not in circulating human monocytes. Consistently, the mature Ron protein is expressed by the MSP responsive cells but not by the unresponsive monocytes. The respiratory burst evoked by hrMSP is quantitatively higher than the one induced by N-formylmethionyl-leucyl-phenylalanine and similar to phorbol myristate acetate-evoked one. 3. To investigate the mechanisms involved in NADPH oxidase activation, leading to superoxide anion production, different signal transduction inhibitors were used. By using the non selective tyrosine kinase inhibitor genistein, the selective c-Src inhibitor PP1, the tyrosine phosphatase inhibitor sodium orthovanadate, the phosphatidylinositol 3-kinase inhibitor wortmannin, the p38 inhibitor SB203580, the MEK inhibitor PD098059, we demonstrate that hrMSP-evoked superoxide production is mediated by tyrosine kinase activity, requires the activation of Src but not of PI 3-kinase. We also show that MAP kinase and p38 signalling pathways are involved. 4. These results clearly indicate that hrMSP induces the respiratory burst in human macrophages but not in monocytes, suggesting for the MSP/Ron complex a role of activator as well as of possible marker for human mature macrophages.

Point mutations and overexpression of Ron induce transformation, tumor formation, and metastasis

The receptor tyrosine kinase Ron is a member of the receptor family that includes the proto-oncogene Met and the avian oncogene Sea. The interaction of Ron with its ligand, known as hepatocyte growth factor-like protein (HGFL) or macrophage stimulating protein (MSP), induces crucial cellular responses including invasive growth, proliferation, cell scattering, and branching morphogenesis. Based on the homology and functional similarities between Met and Ron it was hypothesized that Ron may be important in tumor formation and metastasis. To test this hypothesis, wild-type mouse Ron and three mutant forms of Ron containing mutations similar to those found in the Met gene in human hereditary papillary renal carcinoma (HPRC), were expressed in NIH3T3 cells. A transformed phenotype was produced in cell lines expressing either wild-type Ron or the mutated Ron proteins. Further, these cell lines displayed oncogenic potential by exhibiting increased proliferation and constitutive phosphorylation of Ron. These cell lines were also tested for the ability to form solid tumors. Cells expressing wild-type Ron and the three proteins with single amino acid substitutions were highly tumorigenic in vivo. In a model of experimental metastasis, two of the cell lines with altered Ron protein formed highly aggressive tumors in the lungs. These results suggest that Ron may be an aggressive oncogene when either overexpressed or when activated by mutation.

Ron-mediated cytoplasmic signaling is dispensable for viability but is required to limit inflammatory responses

Ron receptor activation induces numerous cellular responses in vitro, including proliferation, dissociation, and migration. Ron is thought to be involved in blood cell development in vivo, as well as in many aspects of the immune response including macrophage activation, antigen presentation, and nitric oxide regulation. In previous studies to determine the function of Ron in vivo, mice were generated with a targeted deletion of the extracellular and transmembrane regions of this gene. Mice homologous for this deletion appear to die early during embryonic development. To ascertain the in vivo function of Ron in more detail, we have generated mice with a germline ablation of the tyrosine kinase domain. Strikingly, our studies indicate that this domain of Ron, and therefore Ron cytoplasmic signaling, is not essential for embryonic development. While mice deficient in this domain are overtly normal, mice lacking Ron signaling have an altered ability to regulate nitric oxide levels and, in addition, have enhanced tissue damage following acute and cell-mediated inflammatory responses.

Overexpression of the Csk homologous kinase (Chk tyrosine kinase) induces multinucleation: a possible role for chromosome-associated Chk in chromosome dynamics

The Csk family of non-receptor-type tyrosine kinases consists of Csk and the Csk homologous kinase Chk. Each enzyme suppresses the catalytic activity of Src family kinases by phosphorylating their C-terminal negative regulatory tyrosine residues. Ectopic and transient expression of Chk in COS-1 cells showed nuclear localization of Chk and growth inhibition. To further explore the role of Chk in cell growth, we overexpressed Chk in human immature myeloid KMT-2 cells. Chk overexpression brought about growth retardation and aberrant chromosome movement leading to multinucleation, and these events were accompanied by insufficient formation of mitotic spindles. In vitro kinase assays showed that Chk overexpression suppressed the tyrosine kinase activity of Lyn, a member of the Src family, immunoprecipitated from Triton X-100 lysates. Subcellular fractionation studies revealed that fractions of Chk and Lyn, resistant to Triton X-100 solubilization, are associated with mitotic chromosome scaffolds and spindles. Chk overexpression induced a decrease in autophosphorylation of Lyn and concomitant changes in levels of tyrosine phosphorylation of proteins associated with both fractions. These results indicate that Chk, Lyn and the tyrosine-phosphorylated proteins localize to mitotic chromosomes and spindles, suggesting that Chk-dependent tyrosine phosphorylation, presumably through Lyn, may be involved in chromosome dynamics.

A role for bovine herpesvirus 1 (BHV-1) glycoprotein E (gE) tyrosine phosphorylation in replication of BHV-1 wild-type virus but not BHV-1 gE deletion mutant virus

Bovine herpesvirus 1 (BHV-1), an alphaherpesvirus, is a major pathogen that causes respiratory and reproductive infections. We observed tyrosine phosphorylation of a 95-kDa viral protein and dephosphorylation of 55- and 103-kDa cellular proteins during the course of BHV-1 infection. We demonstrated BHV-1 glycoprotein E (gE) to be the tyrosine phosphorylated viral protein by immunoprecipitation. Inhibition of phosphorylation of BHV-1 gE by tyrosine kinase inhibitors genistein and tyrphostin AG1478 substantially lowered the viral titer in Madin-Darby bovine kidney cells. The decrease in viral titer was directly proportional to the decrease in phosphorylation of the BHV-1 gE. Interestingly, these kinase inhibitors did not inhibit the replication of the BHV-1 gE deletion mutant virion (BHV-1gEDelta3.1). Our findings suggest that the wild-type BHV-1, with a functional gE protein, uses a different pathway of signaling events than the BHV-1 gE deletion mutant in replication. Our results indicate that the tyrosine phosphorylation of the cytoplasmic tail of BHV-1 gE is an important post-translational modification of the functional protein. An application of this study may be the use of tyrosine kinase inhibitors in controlling the BHV-1 infection.

Fv2 encodes a truncated form of the Stk receptor tyrosine kinase

The Friend virus susceptibility 2 (Fv2) locus encodes a dominant host factor that confers susceptibility to Friend virus-induced erythroleukaemia in mice. We mapped Fv2 to a 1.0-Mb interval that also contained the gene (Ron) encoding the stem cell kinase receptor (Stk). A truncated form of Stk (Sf-stk), which was the most abundant form of Stk in Fv2-sensitive (Fv2ss) erythroid cells, was not expressed in Fv2 resistant (Fv2rr) cells. Enforced expression of Sf-stk conferred susceptibility to Friend disease, whereas targeted disruption of Ron caused resistance. We conclude that the Fv2 locus encodes Ron, and that a naturally expressed, truncated form of Stk confers susceptibility to Friend virus-induced erythroleukaemia.