A beta 1 integrin signaling pathway involving Src-family kinases, Cbl and PI-3 kinase is required for macrophage spreading and migration

Adhesion to fibronectin via beta1 integrins regulates p27kip1 levels and contributes to cell adhesion mediated drug resistance (CAM-DR)

The tumor cell environment may influence drug response through interactions with the extracellular matrix (ECM). We recently reported that adhesion of myeloma cells to fibronectin (FN) via beta1 integrins is associated with a cell adhesion mediated drug resistance (CAM-DR). Activation of beta1 integrins is known to influence both apoptosis and cell growth. We hypothesized that the FN mediated cytoprotection may be in part due to perturbations in cell cycle progression. In this report we demonstrate that adhesion of myeloma cells to FN results in a G1 arrest associated with increased p27kip1 protein levels and inhibition of cyclin A and E associated kinase activity. Disruption of cells from FN adhesion resulted in a rapid recruitment of cells into S phase, a decrease in p27kip1 levels, and reversion to a drug sensitive phenotype. Treatment of cells with p27Kip1 antisense oligonucleotides did not affect FN adhesion; however, p27Kip1 protein levels were reduced and cells became sensitive to cytotoxic drugs. These studies demonstrate that beta1 mediated adhesion of myeloma cells to FN regulates p27kip1 levels and that p27kip1 levels are causally related to CAM-DR. Disruption of beta1 integrin mediated FN adhesion may represent a potential target for the potentiation of drug induced apoptosis.

p50(Cdc37) can buffer the temperature-sensitive properties of a mutant of Hck

Genetic studies have previously revealed that Cdc37p is required for the catalytic competence of v-Src in yeast. We have reasoned that temperature-sensitive mutants of Src family kinases might be more sensitive to the cellular level of p50(Cdc37), the mammalian homolog of Cdc37p, than their wild-type counterpart, thus potentially providing a unique opportunity to elucidate the involvement of p50(Cdc37) in the folding and stabilization of Src family kinases. A temperature-sensitive mutant of a constitutively active form of Hck (i.e., tsHck499F) was created by mutating two amino acids within the kinase domain of Hck499F. Significantly, overexpression of p50(Cdc37) rescues the catalytic activity of tsHck499F at 33 degrees C, while partially buffering it against inactivation at higher temperatures (e.g., 37 and 39 degrees C). Hsp90 function is required for tsHck499F activity and its stabilization by p50(Cdc37), but overexpression of Hsp90 is not sufficient to stabilize tsHck499F. Overexpression of p50(Cdc37) promotes the association of tsHck499F with Hsp90, suggesting that the cellular level of p50(Cdc37) might be the rate-limiting step in the association of tsHck499F with Hsp90. A truncation mutant of p50(Cdc37) that cannot bind Hsp90 still has a limited capacity to rescue the catalytic activity of tsHck499F and promote its association with Hsp90. This is a particularly important observation, since it argues that rather than solely acting as a passive adapter protein to tether tsHck499F to Hsp90, p50(Cdc37) may also act allosterically to enhance the association of tsHck499F with Hsp90. The findings presented here might also have implications for our understanding of the evolution of protein kinases and tumor development.

The proto-oncogene c-Cbl is a positive regulator of Met-induced MAP kinase activation: a role for the adaptor protein Crk

Hepatocyte growth factor triggers a complex biological program leading to invasive cell growth by activating the c-Met receptor tyrosine kinase. Following activation, Met signaling is elicited via its interactions with SH2-containing proteins, or via the phosphorylation of the docking protein Gab1, and the subsequent interaction of Gab1 with additional SH2-containing effector molecules. We have previously shown that the interaction between phosphorylated Gab1 and the adaptor protein Crk mediates activation of the JNK pathway downstream of Met. We report here that c-Cbl, which is a Gab1-like docking protein, also becomes tyrosine-phosphorylated in response to Met activation and serves as a docking molecule for various SH2-containing molecules, including Crk. We further show that Cbl is similarly capable of enhancing Met-induced JNK activation, and several lines of experimentation suggests that it does so by interacting with Crk. We also show that both Cbl and Gab1 enhance Met-induced activation of another MAP kinase cascade, the ERK pathway, in a Crk-independent manner. Taken together, our studies demonstrate a previously unidentified functional role for Cbl in Met signaling and suggest that Met utilizes at least two docking proteins, Gab1 and Cbl, to activate downstream signaling pathways. Oncogene (2000) 19, 4058 - 4065.

Generation and characterization of monoclonal antibodies to the Src-family kinase Hck

Hck, a member of the Src-family of protein tyrosine kinases, is expressed primarily in hematopoietic cells of the myeloid and B-lymphocyte lineages. Hybridoma cell lines were established that secrete monoclonal antibodies (MAbs) to Hck. Three of the MAbs were extensively characterized and designated H7, H34, and H42. The MAbs H7 and H34 recognized an epitope within the SH3 domain of Hck, while the epitope recognized by the H42 MAb resides within the Unique domain. All three MAbs specifically recognized the p59 and p56 isoforms of Hck in transiently transfected 293T cells and in a murine macrophage cell line. Notably, the antibodies did not cross-react with other Src-family kinases tested. Under native conditions, the MAbs H34 and H42 efficiently immunoprecipitated Hck from transfected cells. Both MAbs were also successfully used for the immunofluorescent staining of Hck in intact cells.Thus, the MAbs described herein should be useful in studies of Hck function and expression.

Activation of mitogen-activated protein kinases is required for alpha1-adrenergic agonist-induced cell scattering in transfected HepG2 cells

Activation of alpha1B-adrenergic receptors ((alpha1B)AR) by phenylephrine (PE) induces scattering of HepG2 cells stably transfected with the (alpha1B)AR (TFG2 cells). Scattering was also observed after stimulation of TFG2 cells with phorbol myristate acetate (PMA) but not with hepatocyte growth factor/scatter factor, epidermal growth factor, or insulin. PMA but not phenylephrine rapidly activated PKCalpha in TFG2 cells, and the highly selective PKC inhibitor bisindolylmaleimide (GFX) completely abolished PMA-induced but not PE-induced scattering. PE rapidly activated p44/42 mitogen-activated protein kinase (MAPK), p38 MAPK, c-Jun N-terminal kinase (JNK), and AP1 (c-fos/c-jun). Selective blockade of p42/44 MAPK activity by PD98059 or by transfection of a MEK1 dominant negative adenovirus significantly inhibited the PE-induced scattering of TFG2 cells. Selective inhibition of p38 MAPK by SB203850 or SB202190 also blocked PE-induced scattering, whereas treatment of TFG2 cells with the PI3 kinase inhibitors LY294002 or wortmannin did not inhibit PE-induced scattering. Blocking JNK activation with a dominant negative mutant of JNK or blocking AP1 activation with a dominant negative mutant of c-jun (TAM67) significantly inhibited PE-induced cell scattering. These data indicate that PE-induced scattering of TFG2 cells is mediated by complex mechanisms, including activation of p42/44 MAPK, p38 MAPK, and JNK. Cell spreading has been reported to play important roles in wound repair, tumor invasion, and metastasis. Therefore, catecholamines acting via the (alpha1)AR may modulate these physiological and pathological processes.

The mechanism of mononuclear cell infiltration in oral lichen planus: the role of cytokines released from keratinocytes

To clarify the pathogenesis of oral lichen planus (OLP), we investigated the roles of keratinocytes (KC) in mononuclear cell infiltration. When peripheral blood mononuclear cells (PBMC) obtained from healthy donors were cultured in the presence of culture supernatants of KC separated from the noninflamed gingivae (Nor-KC) and cheek mucosae of patients with OLP (OLP-KC), the number of migrated PBMC across monolayered human umbilical vein endothelial cells (HUVEC) were increased to about 1.3-fold and 1.5-fold of the control level, respectively, with increases of the expression of CD11a, CD11b, CD18, and CD49d on PBMC and intracellular adhesion molecule-1, vascular cell adhesion molecule-1, and endothelial-leukocyte adhesion molecule-1 on HUVEC. The number of migrated PBMC was reduced to about 60% of the control level by pretreatment of PBMC with anti-CD11a or anti-CD18 MAb and reduced to about 70% by pretreatment of HUVEC with anti-CD54 MAb. The pretreatment of PBMC with genistein, H-7, wortmannin, or exoenzyme C3 decreased the migrated PBMC by about 70 to 90%. In agreement with these results, the culture supernatants of OLP-KC up-regulated tyrosine phosphorylation of 62-kDa, 70-kDa, and 102-kDa proteins, phosphatidylinositol-3 kinase, and protein kinase C activities and activated Rho protein level more so than did those of Nor-KC. Additionally, actin reorganization with the formation of membrane ruffles and lamellipodia was distinctly induced by the culture supernatants of OLP-KC. These results indicate that cytokines generated by KC transduce their signals in PBMC, up-regulating the expression of cell surface adhesion molecules and migration activity with reorganization of actin filaments.

Differential involvement of Src family kinases in Fc gamma receptor-mediated phagocytosis

The tyrosine phosphorylation cascade originated from Fc gamma receptors (Fc gamma Rs) is essential for macrophage functions including phagocytosis. Although the initial step is ascribed to Src family tyrosine kinases, the role of individual kinases in phagocytosis signaling is still to be determined. In reconstitution experiments, we first showed that expression in the RAW 264.7 cell line of C-terminal Src kinase (Csk) inhibited and that of a membrane-anchored, gain-of-function Csk abolished the Fc gamma R-mediated signaling that leads to phagocytosis in a kinase-dependent manner. We next tested reconstruction of the signaling in the membrane-anchored, gain-of-function Csk-expressing cells by introducing Src family kinases the C-terminal negative regulatory sequence of which was replaced with a c-myc epitope. Those constructs derived from Lyn and Hck (a-Lyn and a-Hck) that associated with detergent-resistant membranes successfully reconstructed Fc gamma R-mediated Syk activation, filamentous actin rearrangement, and phagocytosis. In contrast, c-Src-derived construct (a-Src), that was excluded from detergent-resistant membranes, could not restore the series of phagocytosis signaling. Tyrosine phosphorylation of Vav and c-Cbl was restored in common by a-Lyn, a-Hck, and a-Src, but Fc gamma RIIB tyrosine phosphorylation, which is implicated in negative signaling, was reconstituted solely by a-Lyn and a-Hck. These findings suggest that Src family kinases are differentially involved in Fc gamma R-signaling and that selective kinases including Lyn and Hck are able to fully transduce phagocytotic signaling.

Tyrosine phosphorylation/dephosphorylation controls capping of Fcgamma receptor II in U937 cells

In the capping of cell-surface receptors two stages can be distinguished: 1) clustering of the receptors (patching) induced by cross-linking with specific antibodies and 2) subsequent assembly of patches into a cap which is driven by the actin-based cytoskeleton. We found that patching of Fcgamma receptor II in U937 cells was correlated with tyrosine phosphorylation of certain proteins, most prominently those of 130, 110, 75 and 28 kDa. The phosphotyrosine-bearing proteins were accumulated at the receptor patches. Formation of the receptor caps was coincident with dephosphorylation of these proteins. Inhibition of protein tyrosine kinases with herbimycin A and genistein attenuated the protein tyrosine hyperphosphorylation and blocked capping in a dose-dependent manner. Phenylarsine oxide and pervanadate, inhibitors of protein tyrosine phosphatases, also suppressed capping of Fcgamma receptor II in a concentration-dependent fashion. Simultaneously, tyrosine hyperphosphorylation of proteins occurred. In the presence of the tyrosine kinase and phosphatase inhibitors the receptors were arrested at the patching stage. In contrast, okadaic acid, a serine/threonine phosphatase blocker, did not affect assembly of the receptor caps. The inhibitory effect of phenylarsine oxide was rapidly reversed by dithiols, 2,3-dimercapto-1-propanoldithiol and dithiotreitol, and was coincident with dephosphorylation of protein tyrosine residues. Extensive washing of pervanadate-exposed cells also resulted in progressive restoration of the cap assembly. Using streptolysin O-permeabilized cells we confirmed regulatory function played by dephosphorylation of tyrosine residues in capping of Fcgamma receptor II. Exogenous phosphatases, applied to permeabilized cells in which activity of endogenous tyrosine phosphatases was blocked, evoked dephosphorylation of protein tyrosine residues that was accompanied by recovery of capping ability in the cells.

In vitro and in vivo macrophage function can occur independently of SLP-76

Expression of SH2 domain-containing leukocyte-specific phosphoprotein of 76 kDa (SLP-76), a hematopoietic cell-specific adapter protein, is required to couple Syk family tyrosine kinase activation to downstream mediators such as phospholipase C (PLC)-gamma following TCR, platelet collagen receptor and mast cell Fc epsilon R stimulation. In addition to T cells, mast cells and platelets, SLP-76 is expressed in monocytes and macrophages. To determine the role of SLP-76 in Fc gamma R-stimulated signaling pathways in macrophages, we examined cultured bone marrow-derived macrophages (BMM) from SLP-76(-/-) and wild-type mice. In this study, we show that Fc gamma R cross-linking rapidly induces tyrosine phosphorylation of SLP-76 in wild-type BMM. Surprisingly, however, BMM from SLP-76(-/-) mice activate ERK2 and phosphorylate PLC-gamma 2 following Fc gamma R ligation. Furthermore, SLP-76(-/-) BMM display normal Fc gamma R-dependent phagocytic function and reactive oxygen intermediate production. SLP-76(-/-) and SLP-76(+/+) BMM secrete comparable levels of IL-12 in response to lipopolysaccharide and IFN-gamma. To examine macrophage function in vivo, SLP-76(-/-) mice were challenged i.v. with Listeria monocytogenes. SLP-76(-/-) mice survive and efficiently contain the acute phase of infection similar to wild-type mice but exhibit a stable chronic infection attributed to the lack of mature T cells. These data show that, although SLP-76 is required to couple Syk family PTK activity to downstream mediators and effector functions in Fc gamma R-induced pathways in some cell types, activation of Fc gamma R-dependent pathways occurs independently of SLP-76 in BM

Hck enhances the adherence of lipopolysaccharide-stimulated macrophages via Cbl and phosphatidylinositol 3-kinase

Src family tyrosine kinases have previously been proposed to mediate some of the biological effects of lipopolysaccharide on macrophages. Accordingly, we have sought to identify substrates of Src family kinases in lipopolysaccharide-stimulated macrophages. Stimulation of Bac1.2F5 macrophage cells with lipopolysaccharide was found to induce gradual and persistent tyrosine phosphorylation of Cbl in an Src family kinase-dependent manner. Immunoprecipitation experiments revealed that Cbl associates with Hck in Bac1.2F5 cells, while expression of an activated form of Hck in Bac1.2F5 cells induces tyrosine phosphorylation of Cbl in the absence of lipopolysaccharide stimulation. The Src homology 3 domain of Hck can directly bind Cbl, and this interaction is important for phosphorylation of Cbl. Association of the p85 subunit of phosphatidylinositol (PI) 3-kinase with Cbl is enhanced following lipopolysaccharide stimulation of Bac1.2F5 cells, and transient expression experiments indicate that phosphorylation of Cbl by Hck can facilitate the association of p85 with Cbl. Lipopolysaccharide treatment also stimulates the partial translocation of Hck to the cytoskeleton of Bac1.2F5 cells. Notably, lipopolysaccharide enhances the adherence of Bac1.2F5 cells, an effect that is dependent on the activity of Src family kinases and PI 3-kinase. Thus, we postulate that Hck enhances the adherence of lipopolysaccharide-stimulated macrophages, at least in part, via Cbl and PI 3-kinase.

The Arf GTPase-activating protein ASAP1 regulates the actin cytoskeleton

Arf family GTP-binding proteins are best characterized as regulators of membrane traffic, but recent studies indicate an additional role in cytoskeletal organization. An Arf GTPase-activating protein of the centaurin beta family, ASAP1 (also known as centaurin beta4), binds Arf and two other known regulators of the actin cytoskeleton, the tyrosine kinase Src and phosphatidylinositol 4,5-bisphosphate. In this paper, we show that ASAP1 localizes to focal adhesions and cycles with focal adhesion proteins when cells are stimulated to move. Overexpression of ASAP1 altered the morphology of focal adhesions and blocked both cell spreading and formation of dorsal ruffles induced by platelet-derived growth factor (PDGF). On the other hand, ASAP1, with a mutation that disrupted GTPase-activating protein activity, had a reduced effect on cell spreading and increased the number of cells forming dorsal ruffles in response to PDGF. These data support a role for an Arf GTPase-activating protein, ASAP1, as a regulator of cytoskeletal remodeling and raise the possibility that the Arf pathway is a target for PDGF signaling.

Autocrine human growth hormone enhancement of human mammary carcinoma cell spreading is Jak2 dependent

We investigated the role of autocrine production of human (h) GH in the attachment and spreading of mammary carcinoma cells in vitro. We used a previously described model system for the study of the autocrine/paracrine role of GH in which the hGH gene (MCF-hGH) or a translation-deficient hGH gene (MCF-MUT) was stably transfected into MCF-7 cells. No differences in attachment to a collagen matrix between MCF-hGH and MCF-MUT cells were observed in either serum-free medium (SFM) or medium containing exogenous hGH, 5% serum, or 10% serum. In contrast, MCF-hGH cells spread more rapidly on a collagen matrix than did MCF-MUT cells. Exogenous hGH and 10% serum interacted with autocrine production of hGH in an additive manner to increase cell spreading. MCF-hGH cells formed filipodia and stress fibers earlier than MCF-MUT cells during the process of cell spreading and possessed marked differences in morphology after spreading. MCF-MUT cells displayed uniform and symmetrical formation of stress fibers, whereas MCF-hGH cells displayed irregular and elongated stress fiber formation. The level of cytoplasmic phosphotyrosine was increased in MCF-hGH compared with MCF-MUT cells during spreading and displayed colocalization with Janus kinase 2 (JAK2). Basal JAK2 tyrosine phosphorylation was increased, and it increased further on spreading in MCF-hGH cells compared with MCF-MUT cells. Transient transfection of JAK2 complementary DNA resulted in interaction with autocrine hGH to increase the rate of cell spreading in MCF-hGH cells compared with MCF-MUT cells. Treatment with a selective JAK2 tyrosine kinase inhibitor (AG 490) reduced the rate of MCF-hGH cell spreading to the rate of MCF-MUT cell spreading. Thus, we conclude that autocrine production of hGH enhances the rate of mammary carcinoma cell spreading in a JAK2-dependent manner.

Src family kinases negatively regulate platelet-derived growth factor alpha receptor-dependent signaling and disease progression

We tested the hypothesis that Src family kinases (SFK) contribute to c-Cbl-mediated degradation of the platelet-derived growth factor (PDGF) alpha receptor (alphaPDGFR). Using either a receptor mutant that does not engage SFKs (F72/74), or cells that that lack SFKs, we found that SFKs contributed to degradation of the alphaPDGFR. Overexpression of c-Cbl also reduced the receptor half-life, but only if the receptor was able to engage SFKs. In cultured cells, prolonging the half-life of the receptor correlated with enhanced signaling and more efficient S phase entry, whereas accelerating receptor degradation had the opposite effect. Consistent with these tissue culture findings, there was a statistically significant increase in the onset of a proliferative retinal disease when animals were injected with cells expressing the F72/74 receptor, as compared with cells expressing the WT receptor. Our findings suggest that SFKs cooperate with c-Cbl to negatively regulate the alphaPDGFR, and that the SFK/c-Cbl suppression of alphaPDGFR output is relevant to the onset and progression of a proliferative disease.

Activation of Akt kinase by granulocyte colony-stimulating factor (G-CSF): evidence for the role of a tyrosine kinase activity distinct from the janus kinases

Activation of the serine/threonine kinase Akt has been shown to be a critical component for growth factor and cytokine stimulation of cell survival. Although some of the immediate upstream activators of Akt have been defined, the roles of tyrosine kinases in the activation of Akt are not well delineated. Granulocyte colony-stimulating factor (G-CSF) regulates the proliferation, differentiation, and survival of neutrophilic granulocytes. G-CSF exerts its actions by stimulating several signaling cascades after binding its cell surface receptor. Both Jak (Janus) and Src families of tyrosine kinases are stimulated by incubation of cells with G-CSF. In this report, we show that G-CSF stimulation of cells leads to activation of Akt. The membrane-proximal 55 amino acids of the G-CSF receptor cytoplasmic domain are sufficient for mediating Akt activation. However, activation of Akt appears to be downregulated by the receptor's carboxy-terminal region of 98 amino acids, a region that has been shown to be truncated in some patients with acute myeloid leukemia associated with severe congenital neutropenia. Furthermore, we demonstrate that G-CSF–induced activation of Akt requires the activities of Src family kinases but can be clearly dissociated from G-CSF–stimulated activation of Stats (signal transducers and activators of transcripton) by the Jak kinases. Thus, cytokine activation of the Jak/Stat and other signaling cascades can be functionally separated.

Negative regulation of phagocytosis in murine macrophages by the Src kinase family member, Fgr

Ingestion of opsonized pathogens by professional phagocytes results in the generation and release of microbicidal products that are essential for normal host defense. Because these products can result in significant tissue injury, phagocytosis must be regulated to limit damage to the host while allowing for optimal clearance and destruction of opsonized pathogens. To pursue negative regulation of phagocytosis, we assessed the effect of the Src kinase family member, Fgr, on opsonin-dependent phagocytosis by mouse macrophages. We chose Fgr because it is present in high concentrations in circulating phagocytes but is not essential for Fcgamma receptor-mediated ingestion by mouse macrophages. Although expression of Fgr both in a macrophage cell line and in primary macrophages significantly attenuates ingestion mediated by Fcgamma receptors and CR3, it does not affect macropinocytosis or receptor-mediated endocytosis. This selective effect of Fgr is independent of its tyrosine kinase function. After Fcgamma receptor cross-linking, Fgr becomes associated with the immunoreceptor tyrosine-based inhibition motif (ITIM)-containing receptor, SIRPalpha (a member of the signal-regulatory protein family, also known as Src homology 2 domain-containing protein tyrosine phosphatase [SHP] substrate 1 [SHPS-1], brain immunoglobulin-like molecule with tyrosine-based activation motifs [BIT], and P84) and potentiates the association of the phosphatase SHP-1 with SIRPalpha. This association is responsible, at least in part, for decreasing positive signaling essential for optimal phagocytosis. These data demonstrate an important negative regulatory role for this Src kinase family member and suggest that this homeostatic function must be overcome for optimal uptake and clearance of opsonized pathogens.

The Cbl proto-oncogene product negatively regulates the Src-family tyrosine kinase Fyn by enhancing its degradation

Fyn is a prototype Src-family tyrosine kinase that plays specific roles in neural development, keratinocyte differentiation, and lymphocyte activation, as well as roles redundant with other Src-family kinases. Similar to other Src-family kinases, efficient regulation of Fyn is achieved through intramolecular binding of its SH3 and SH2 domains to conserved regulatory regions. We have investigated the possibility that the tyrosine kinase regulatory protein Cbl provides a complementary mechanism of Fyn regulation. We show that Cbl overexpression in 293T embryonic kidney and Jurkat T-lymphocyte cells led to a dramatic reduction in the active pool of Fyn; this was seen as a reduction in Fyn autophosphorylation, reduced phosphorylation of in vivo substrates, and inhibition of transcription from a Src-family kinase response element linked to a luciferase reporter. Importantly, a Fyn mutant (FynY528F) relieved of intramolecular repression was still negatively regulated by Cbl. The Cbl-dependent negative regulation of Fyn did not appear to be mediated by inhibition of Fyn kinase activity but was correlated with enhanced protein turnover. Consistent with such a mechanism, elevated levels of Fyn protein were observed in cell lines derived from Cbl(-/-) mice compared to those in wild-type controls. The effects of Cbl on Fyn were not observed when the 70ZCbl mutant protein was analyzed. Taken together, these observations implicate Cbl as a component in the negative regulation of Fyn and potentially other Src-family kinases, especially following kinase activation. These results also suggest that protein degradation may be a general mechanism for Cbl-mediated negative regulation of activated tyrosine kinases.

Association of Cbl with Fms and p85 in response to macrophage colony-stimulating factor

Tyrosine phosphorylation of Cbl and its association with signal-transducing molecules in response to macrophage colony-stimulating factor (M-CSF) were analyzed by using cell lines which express the wild-type and a mutant M-CSF receptor, Fms. We found that in a clone, F723 TF-1 cells expressing mutant Fms in which tyrosine 723 had been substituted with phenylalanine, the M-CSF stimulation-dependent association between Cbl and Fms was markedly impaired. However, phosphorylation of Cbl and its association with the p85 subunit of phosphatidylinositol 3-kinase were induced in these mutant cells as seen in the wild-type fms transfectant. These results suggest that phosphorylation of tyrosine 723 is particularly important for the recruitment of Cbl to the M-CSF receptor, but is not required for the phosphorylation and binding of Cbl to signal-transducing molecules such as p85.

c-Cbl localizes to actin lamellae and regulates lamellipodia formation and cell morphology

Adhesive and locomotive properties of cells have key roles in normal physiology and disease. Cell motility and adhesion require the assembly and organization of actin microfilaments into stress fibers, lamellipodia and filopodia, and the formation of these structures is mediated by signalling through Rho; GTPases. Here we identify c-Cbl (a multi-adaptor proto-oncogene product involved in protein tyrosine kinase signalling) as an important regulator of the actin cytoskeleton. By immunofluorescence microscopy we have determined that c-Cbl co-localizes with the adaptor protein Crk to submembranous actin lamellae in NIH 3T3 fibroblasts and that c-Cbl's actin localization requires specific SH3-binding sequences. Further, we have found that truncation of this SH3-binding domain in c-Cbl profoundly alters the morphology of NIH 3T3 fibroblasts by inhibiting the formation of actin lamellae, lamellipodia and membrane ruffles. The induction of lamellipodia and membrane ruffles are also inhibited during cell spreading and migration, conditions when these structures are normally most prominent. The inhibitory effect of truncated c-Cbl expression on lamellipodia formation can be reversed by mutational inactivation of its divergent SH2 domain, by the co-expression of constitutively active Rac or by the overexpression of c-Cbl. This study therefore identifies a cytoskeletal role for c-Cbl which may involve the regulation of Crk and Rac, and which is dependent on targeting of c-Cbl to actin lamellae and the ability to recruit signalling protein(s) associated with its divergent SH2 domain.

Role of phosphoinositide 3-kinase and the Cbl adaptor protein in coupling the alpha4beta1 integrin to mitogen-activated protein kinase signalling

Cell adhesion mediated by beta1 integrin receptors leads to the initiation of intracellular signals that affect cell differentiation and survival. Here we have analysed the mechanism by which the alpha4beta1 integrin activates the mitogen-activated protein kinase pathway in HL60 cells, a myelomonocytic cell line that lacks the expression of focal adhesion kinase. A role for phosphoinositide 3-kinase (PI-3K) in alpha4 integrin-mediated activation of extracellular signal-regulated protein kinase 2 (ERK2) is suggested by the ability of PI-3K inhibitors and a dominant-negative form of the p85 subunit of PI-3K to block the activation of ERK2 by integrin. Stimulation of alpha4beta1 integrins on HL60 cells also leads to increased tyrosine phosphorylation of the 120 kDa adaptor protein Cbl. PI-3K activity associated with Cbl also increases on the stimulation of alpha4beta1 integrins, although immunodepletion experiments suggest that Cbl-associated PI-3K does not account for all of the PI-3K activity induced on the stimulation of integrins in these cells. The expression of wild-type Cbl or the 70Z/3 Cbl mutant enhances basal ERK2 activity in transfectants with a minimal effect on alpha4 integrin-mediated ERK2 activity. In contrast, overexpression of the Hut Cbl truncation mutant, which does not associate with p85, has no effect on the ERK2 pathway. These results suggest that PI-3K has a major role in coupling alpha4beta1 integrins to ERK2 activation in myeloid cells and that the Cbl adaptor protein has a role in basal, but not alpha4beta1 integrin-mediated, activation of ERK2.

Compartmentalized signaling by GPI-anchored ephrin-A5 requires the Fyn tyrosine kinase to regulate cellular adhesion

Eph receptor tyrosine kinases and their corresponding surface-bound ligands, the ephrins, provide cues to the migration of cells and growth cones during embryonic development. Here we show that ephrin-A5, which is attached to the outer leaflet of the plasma membrane by a glycosyl-phosphatidylinositol-anchor, induces compartmentalized signaling within a caveolae-like membrane microdomain when bound to the extracellular domain of its cognate Eph receptor. The physiological response induced by this signaling event is concomitant with a change in the cellular architecture and adhesion of the ephrin-A5-expressing cells and requires the activity of the Fyn protein tyrosine kinase. This study stresses the relevance of bidirectional signaling involving the ephrins and Eph receptors during brain development.

Multiple roles for Src in a PDGF-stimulated cell

The observation that platelet-derived growth factor (PDGF) increases the catalytic activity of Src family members (Src) suggests that they contribute to PDGF-dependent responses. The role of Src in PDGF-dependent cell cycle progression, phosphorylation of proteins, and chemotaxis has been tested by investigators using a variety of cell types and approaches, and it appears that the contribution of Src is highly variable. This idea is perhaps best illustrated by the finding that Src plays radically different roles downstream of the PDGF alpha- and beta-receptor subunits. Hence, Src is a versatile signal relay enzyme, whose contribution to a signaling cascade depends on variables such as the nature of the receptor via which the cell is activated, as well as the cell type itself.

Molecular cloning and characterization of a novel cbl-family gene, cbl-c

We have cloned a novel gene, cbl-c, of mammalian cbl-family. The cbl-c gene is predicted to encode a protein of 52 kDa that has a phosphotyrosine-binding domain, a RING finger and a proline-rich region. Cbl-c shows 50% homology to the amino-terminal sequences of Cbl and Cbl-b, but a sequence corresponding to the carboxy-terminal half of Cbl and Cbl-b is largely missing in Cbl-c. The expression of cbl-c mRNA is distinct from that of cbl and cbl-b mRNAs, being high in the colon and small intestine, but undetectable in brain and lymphoid tissues. The cbl-c gene is mapped in 19q13.2-13.3. Finally, the 52 kDa Cbl-c protein binds to the EGF receptor and Fyn tyrosine kinase. We conclude that Cbl-c is a novel Cbl-family adaptor protein that would regulate intracellular signaling mediated by various tyrosine kinases.

Integrin signalling in neutrophils and macrophages

Integrins have been characterized extensively as adhesion receptors capable of transducing signals inside the cell. In myelomonocytic cells, integrin-mediated adhesive interactions regulate different selective cell responses, such as transmigration into the inflammatory site, cytokine secretion, production or reactive oxygen intermediates, degranulation and phagocytosis. In the last few years, great progress has been made in elucidating mechanisms of signal transduction by integrins in neutrophils and macrophages. This review summarises the current information on the role of integrins in regulating myelomonocytic cell functions and highlights the signalling pathways activated by integrin engagement in these cells. Also, exploiting the current knowledge of mechanisms of integrin signal transduction in other cell types, we propose a model to explain how integrins transduce signals inside neutrophils and macrophages, and how signaling pathways leading to regulation of selective cell functions may be coordinated.

Independent SH2-binding sites mediate interaction of Dok-related protein with RasGTPase-activating protein and Nck

A murine embryonic cDNA library was screened for potential substrates of the Src family kinase, Lyn, using a phosphorylation-screening strategy. One cDNA that we identified encodes Dok-related protein (DokR), a protein with homology to p62(dok) (Dok), and members of the insulin receptor substrate-1 family of proteins. Analysis of murine tissue extracts with DokR-specific antisera revealed that DokR protein is expressed at highest levels in lymphoid tissues. Co-expression of a FLAG epitope-tagged form of DokR (FLAG-DokR) with Lyn in embryonic kidney 293T cells resulted in constitutive phosphorylation of FLAG-DokR on tyrosine residues and consequential physical association with RasGTPase-activating protein (GAP) and the Nck adaptor protein. Stimulation of BaF/3 hematopoietic cells co-expressing the epidermal growth factor (EGF) receptor tyrosine kinase and FLAG-DokR with EGF also induced phosphorylation of FLAG-DokR and promoted its association with GAP. Immunoprecipitation experiments using DokR-specific antibodies revealed an interaction between endogenous DokR and a 150-kDa protein that is tyrosine-phosphorylated in EGF-stimulated BaF/3 cells. The molecular basis of the interactions involving DokR with GAP and Nck was investigated using a novel glutathione S-transferase fusion protein binding assay and/or site-directed mutagenesis. Tandem SH2-binding sites containing Tyr-276 and Tyr-304 were shown to mediate binding of DokR to GAP, whereas Tyr-351 mediated the binding of DokR to Nck. These results suggest that DokR participates in numerous signaling pathways.

Tyrosine phosphorylation of C-Cbl facilitates adhesion and spreading while suppressing anchorage-independent growth of V-Abl-transformed NIH3T3 fibroblasts

The protooncogenic protein c-Cbl becomes tyrosine phosphorylated in normal cells in response to a variety of external stimuli, as well as in cells transformed by oncogenic protein tyrosine kinases. Tyrosine phosphorylation of c-Cbl upregulates its binding to multiple crucial signaling molecules. However, the biological consequences of c-Cbl-mediated signaling are insufficiently understood. To analyse the biological functions of c-Cbl, we overexpressed wild-type c-Cbl and its tyrosine phosphorylation-defective mutant form in v-Abl-transformed NIH3T3 fibroblasts. In this system, wild-type c-Cbl facilitated adhesion and spreading of v-Abl-transformed fibroblasts on the extracellular matrix, while reducing anchorage independence of these cells, as measured by their colony-forming efficiency in soft agar. Therefore, overexpression of wild-type c-Cbl exhibits an overall transformation-suppressing effect. By contrast, overexpression of a tyrosine phosphorylation-defective form of c-Cbl increases neither adhesion nor anchorage dependence of v-Abl-transformed fibroblasts. Analysis of the role of individual tyrosine phosphorylation sites of c-Cbl in these phenomena indicates that both phosphatidylinositol-3' kinase and the CrkL adaptor protein may be involved in the observed effects of c-Cbl. To summarize, the results presented in this report indicate that c-Cbl is involved in regulation of cell adhesion and cytoskeletal rearrangements, and that these effects of c-Cbl are dependent on its tyrosine phosphorylation.

Perturbed Regulation of ZAP-70 and Sustained Tyrosine Phosphorylation of LAT and SLP-76 in c-Cbl-Deficient Thymocytes

Recent studies indicate that c-Cbl and its oncogenic variants can modulate the activity of protein tyrosine kinases. This finding is supported by studies showing that c-Cbl interacts directly with a negative regulatory tyrosine in ZAP-70, and that the levels of tyrosine-phosphorylated ZAP-70 and numerous other proteins are increased in TCR-stimulated thymocytes from c-Cbl-deficient mice. Here, we demonstrate that this enhanced phosphorylation of ZAP-70 and that of two substrates, LAT and SLP-76, is not due to altered protein levels but is the consequence of two separate events. First, we find increased expression of tyrosine-phosphorylated TCRζ chain in c-Cbl-deficient thymocytes, which results in a higher level of ζ-chain-associated ZAP-70 that is initially accessible for activation. Thus, more ZAP-70 is activated and more of its substrates (LAT and SLP-76) become tyrosine-phosphorylated after TCR stimulation. However, an additional mechanism of ZAP-70 regulation is evident at a later time poststimulation. At this time, ZAP-70 from both normal and c-Cbl−/− thymocytes becomes hyperphosphorylated; however, only in normal thymocytes does this correlate with ZAP-70 down-regulation and a diminished ability to phosphorylate LAT and SLP-76. In contrast, c-Cbl-deficient thymocytes display altered phosphorylation kinetics, for which LAT phosphorylation is increased and SLP-76 phosphorylation is sustained. Thus, the ability to down-regulate the phosphorylation of two ZAP-70 substrates is impaired in c-Cbl−/− thymocytes. These findings provide evidence that c-Cbl is involved in the negative regulation of the phosphorylation of LAT and SLP-76 by ZAP-70.

Signaling through focal adhesion kinase

Integrin receptor binding to extracellular matrix proteins generates intracellular signals via enhanced tyrosine phosphorylation events that are important for cell growth, survival, and migration. This review will focus on the functions of the focal adhesion kinase (FAK) protein-tyrosine kinase (PTK) and its role in linking integrin receptors to intracellular signaling pathways. FAK associates with several different signaling proteins such as Src-family PTKs, p130Cas, Shc, Grb2, PI 3-kinase, and paxillin. This enables FAK to function within a network of integrin-stimulated signaling pathways leading to the activation of targets such as the ERK and JNK/mitogen-activated protein kinase pathways. Focus will be placed on the structural domains and sites of FAK tyrosine phosphorylation important for FAK-mediated signaling events and how these sites are conserved in the FAK-related PTK, Pyk2. We will review what is known about FAK activation by integrin receptor-mediated events and also non-integrin stimuli. In addition, we discuss the emergence of a consensus FAK substrate phosphorylation sequence. Emphasis will also be placed on the role of FAK in generating cell survival signals and the cleavage of FAK during caspase-mediated apoptosis. An in-depth discussion will be presented of integrin-stimulated signaling events occurring in the FAK knockout fibroblasts (FAK-) and how these cells exhibit deficits in cell migration. FAK re-expression in the FAK- cells confirms the role of this PTK in the regulation of cell morphology and in promoting cell migration events. In addition, these results reinforce the potential role for FAK in promoting an invasive phenotype in human tumors.

Homing and mobilization in the stem cell niche

All mature blood cells are derived from the haemopoietic stem cell (HSC). In common with all other haemopoietic cells, stem cells are mobile, and it is this property of mobility that has allowed bone marrow transplantation to become a routine clinical option. Successful transplantation requires haemopoietic stem cells to home to the bone marrow, leave the peripheral circulation and become stabilized in regulatory niches in the extravascular space of the bone marrow cavity. This homing and tethering process is reversible - haemopoietic stem cells can be released from their bone marrow tethering through changes in molecular interactions, which are also important in homing following transplantation. The molecular mechanisms regulating this two-way flow of stem cells are beginning to be elucidated, and much recent data has emerged that sheds light on the processes and molecules involved in these complex physiological events. This article reviews current knowledge of the adhesive, homing and proliferative influences acting on HSCs and progenitor cells.

Urokinase receptor-dependent and -independent p56/59(hck) activation state is a molecular switch between myelomonocytic cell motility and adherence

Anchorage-independent myelomonocytic cells acquire adherence within minutes of differentiation stimuli, such as the proteolytically inactive N-terminal fragment of urokinase binding to its cognate glycosylphosphatidylinositol (GPI)-anchored receptor. Here, we report that urokinase-treated differentiating U937 monocyte-like cells exhibit a rapid and transient inhibition of p56/59(hck) and p55(fgr) whereas no changes in the activity of other Src family kinases, such as p53/56(lyn) and p59(fyn) were observed. U937 transfectants expressing a kinase-defective (Lys267 to Met) p56/59(hck) variant exhibit enhanced adhesiveness and a marked F-actin redistribution in thin protruding structures. Conversely, urokinase as well as expression of wild-type or constitutively active (Tyr499 to Phe) p56/59(hck) stimulates the directional migration of uninduced U937 cells. Accordingly, expression of constitutively active or kinase inactive p56/59(hck) selectively prevents urokinase receptor-dependent induction of either adhesion or motility, indicating that a specific activation state of p56/59(hck) is required for each cell response. In conclusion, modulation of the intracellular p56/59(hck) tyrosine kinase activity switches cell motility towards adherence, providing a mutually exclusive mechanism to regulate these properties during monocyte/macrophage differentiation in vivo.

The proto-oncogene p120(Cbl) is a downstream substrate of the Hck protein-tyrosine kinase

Hematopoietic cell kinase (Hck) is a member of the Src-family of protein tyrosine kinases. We have found that upon enzymatic activation of Hck by the heavy metal mercuric chloride, there was a rapid increase in the levels of tyrosine phosphorylation of several proteins including the proto-oncogene p120(Cbl). Fibroblasts that are transformed with an activated allele of Hck exhibit constitutive Cbl phosphorylation. Upon Fcgamma receptor activation, a more physiologically relevant extracellular signal, Cbl is tyrosine phosphorylated and the Src-family selective inhibitor, PP1, can prevent this phosphorylation on Cbl. Hck phosphorylates Cbl in vitro and the interaction between Cbl and Hck is direct, requiring Hck's unique, SH3 and SH2 domains for optimal binding. Using a novel estrogen-regulated chimera of Hck we have shown a hormone-dependent association between Hck and Cbl in murine fibroblasts. This work suggests that Cbl serves as a key mediator of Hck induced signalling in hematopoietic cells.

A role for caveolin and the urokinase receptor in integrin-mediated adhesion and signaling

The assembly of signaling molecules surrounding the integrin family of adhesion receptors remains poorly understood. Recently, the membrane protein caveolin was found in complexes with beta1 integrins. Caveolin binds cholesterol and several signaling molecules potentially linked to integrin function, e.g., Src family kinases, although caveolin has not been directly implicated in integrin-dependent adhesion. Here we report that depletion of caveolin by antisense methodology in kidney 293 cells disrupts the association of Src kinases with beta1 integrins resulting in loss of focal adhesion sites, ligand-induced focal adhesion kinase (FAK) phosphorylation, and adhesion. The nonintegrin urokinase receptor (uPAR) associates with and stabilizes beta1 integrin/caveolin complexes. Depletion of caveolin in uPAR-expressing 293 cells also disrupts uPAR/integrin complexes and uPAR-dependent adhesion. Further, beta1 integrin/caveolin complexes could be disassociated by uPAR-binding peptides in both uPAR-transfected 293 cells and human vascular smooth muscle cells. Disruption of complexes by peptides in intact smooth muscle cells blocks the association of Src family kinases with beta1 integrins and markedly impairs their migration on fibronectin. We conclude that ligand-induced signaling necessary for normal beta1 integrin function requires caveolin and is regulated by uPAR. Caveolin and uPAR may operate within adhesion sites to organize kinase-rich lipid domains in proximity to integrins, promoting efficient signal transduction.

Heparan sulfate-modified CD44 promotes hepatocyte growth factor/scatter factor-induced signal transduction through the receptor tyrosine kinase c-Met

CD44 has been implicated in tumor progression and metastasis, but the mechanism(s) involved is as yet poorly understood. Recent studies have shown that CD44 isoforms containing the alternatively spliced exon v3 carry heparan sulfate side chains and are able to bind heparin-binding growth factors. In the present study, we have explored the possibility of a physical and functional interaction between CD44 and hepatocyte growth factor/scatter factor (HGF/SF), the ligand of the receptor tyrosine kinase c-Met. The HGF/SF-c-Met pathway mediates cell growth and motility and has been implicated in tumor invasion and metastasis. We demonstrate that a CD44v3 splice variant efficiently binds HGF/SF via its heparan sulfate side chain. To address the functional relevance of this interaction, Namalwa Burkitt's lymphoma cells were stably co-transfected with c-Met and either CD44v3 or the isoform CD44s, which lacks heparan sulfate. We show that, as compared with CD44s, CD44v3 promotes: (i) HGF/SF-induced phosphorylation of c-Met, (ii) phosphorylation of several downstream proteins, and (iii) activation of the MAP kinases ERK1 and -2. By heparitinase treatment and the use of a mutant HGF/SF with greatly decreased affinity for heparan sulfate, we show that the enhancement of c-Met signal transduction induced by CD44v3 was critically dependent on heparan sulfate moieties. Our results identify heparan sulfate-modified CD44 (CD44-HS) as a functional co-receptor for HGF/SF which promotes signaling through the receptor tyrosine kinase c-Met, presumably by concentrating and presenting HGF/SF. As both CD44-HS and c-Met are overexpressed on several types of tumors, we propose that the observed functional collaboration might be instrumental in promoting tumor growth and metastasis.

Tyrosine kinases in neutrophils

Nonreceptor tyrosine kinases have been shown to represent essential components of several signal transduction pathways implicated in neutrophil activation. Engagement of phagocytic, cytokine, chemoattractant, and adhesion receptors transduces intracellular signals via distinct tyrosine kinases. This article discusses the role of distinct tyrosine kinases in mediating neutrophil responses.

Identification of a novel integrin signaling pathway involving the kinase Syk and the guanine nucleotide exchange factor Vav1

BACKGROUND

. Integrins induce the formation of large complexes of cytoskeletal and signaling proteins, which regulate many intracellular processes. The activation and assembly of signaling complexes involving focal adhesion kinase (FAK) occurs late in integrin signaling, downstream from actin polymerization. Our previous studies indicated that integrin-mediated activation of the non-receptor tyrosine kinase Syk in hematopoietic cells is independent of FAK and actin polymerization, and suggested the existence of a distinct signaling pathway regulated by Syk.

RESULTS

. Multiple proteins were found to be activated by Syk, downstream of engagement of the platelet/megakaryocyte-specific integrin alphaIIbbeta3. The guanine nucleotide exchange factor Vav1 was inducibly phosphorylated in a Syk-dependent manner in cells following their attachment to fibrinogen. Together, Syk and Vav1 triggered lamellipodia formation in fibrinogen-adherent cells and both Syk and Vav1 colocalized with alphaIIbbeta3 in lamellipodia but not in focal adhesions. Additionally, Syk and Vav1 cooperatively induced activation of Jun N-terminal kinase (JNK), extracellular-signal-regulated kinase 2 (ERK2) and the kinase Akt, and phosphorylation of the oncoprotein Cbl in fibrinogen-adherent cells. Activation of all of these proteins by Syk and Vav1 was not dependent on actin polymerization.

CONCLUSIONS

. Syk and Vav1 regulate a unique integrin signaling pathway that differs from the FAK pathway in its proximity to the integrin itself, its localization to lamellipodia, and its activation, which is independent of actin polymerization. This pathway may regulate multiple downstream events in hematopoietic cells, including Rac-induced lamellipodia formation, tyrosine phosphorylation of Cbl, and activation of JNK, ERK2 and the phosphatidylinositol 3'-kinase-regulated kinase Akt.