The proto-oncogene product p120(cbl) links c-Src and phosphatidylinositol 3'-kinase to the integrin signaling pathway

Common genetic denominators for Ca++-based skeleton in Metazoa: role of osteoclast-stimulating factor and of carbonic anhydrase in a calcareous sponge

Calcium-based matrices serve predominantly as inorganic, hard skeletal systems in Metazoa from calcareous sponges [phylum Porifera; class Calcarea] to proto- and deuterostomian multicellular animals. The calcareous sponges form their skeletal elements, the spicules, from amorphous calcium carbonate (ACC). Treatment of spicules from Sycon raphanus with sodium hypochlorite (NaOCl) results in the disintegration of the ACC in those skeletal elements. Until now a distinct protein/enzyme involved in ACC metabolism could not been identified in those animals. We applied the technique of phage display combinatorial libraries to identify oligopeptides that bind to NaOCl-treated spicules: those oligopeptides allowed us to detect proteins that bind to those spicules. Two molecules have been identified, the (putative) enzyme carbonic anhydrase and the (putative) osteoclast-stimulating factor (OSTF), that are involved in the catabolism of ACC. The complete cDNAs were isolated and the recombinant proteins were prepared to raise antibodies. In turn, immunofluorescence staining of tissue slices and qPCR analyses have been performed. The data show that sponges, cultivated under standard condition (10 mM CaCl₂) show low levels of transcripts/proteins for carbonic anhydrase or OSTF, compared to those animals that had been cultivated under Ca²⁺-depletion condition (1 mM CaCl₂). Our data identify with the carbonic anhydrase and the OSTF the first two molecules which remain conserved in cells, potentially involved in Ca-based skeletal dissolution, from sponges (sclerocytes) to human (osteoclast).

Interaction of c-Cbl with myosin IIA regulates Bleb associated macropinocytosis of Kaposi's sarcoma-associated herpesvirus

KSHV is etiologically associated with Kaposi's sarcoma (KS), an angioproliferative endothelial cell malignancy. Macropinocytosis is the predominant mode of in vitro entry of KSHV into its natural target cells, human dermal microvascular endothelial (HMVEC-d) cells. Although macropinocytosis is known to be a major route of entry for many viruses, the molecule(s) involved in the recruitment and integration of signaling early during macropinosome formation is less well studied. Here we demonstrate that tyrosine phosphorylation of the adaptor protein c-Cbl is required for KSHV induced membrane blebbing and macropinocytosis. KSHV induced the tyrosine phosphorylation of c-Cbl as early as 1 min post-infection and was recruited to the sites of bleb formation. Infection also led to an increase in the interaction of c-Cbl with PI3-K p85 in a time dependent manner. c-Cbl shRNA decreased the formation of KSHV induced membrane blebs and macropinocytosis as well as virus entry. Immunoprecipitation of c-Cbl followed by mass spectrometry identified the interaction of c-Cbl with a novel molecular partner, non-muscle myosin heavy chain IIA (myosin IIA), in bleb associated macropinocytosis. Phosphorylated c-Cbl colocalized with phospho-myosin light chain II in the interior of blebs of infected cells and this interaction was abolished by c-Cbl shRNA. Studies with the myosin II inhibitor blebbistatin demonstrated that myosin IIA is a biologically significant component of the c-Cbl signaling pathway and c-Cbl plays a new role in the recruitment of myosin IIA to the blebs during KSHV infection. Myosin II associates with actin in KSHV induced blebs and the absence of actin and myosin ubiquitination in c-Cbl ShRNA cells suggested that c-Cbl is also responsible for the ubiquitination of these proteins in the infected cells. This is the first study demonstrating the role of c-Cbl in viral entry as well as macropinocytosis, and provides the evidence that a signaling complex containing c-Cbl and myosin IIA plays a crucial role in blebbing and macropinocytosis during viral infection and suggests that targeting c-Cbl could lead to a block in KSHV infection.

KSHV is etiologically associated with Kaposi's sarcoma (KS), the most common AIDS related neoplasm. The first key step in KSHV infection is its initial contact with target cells and entry. While it is known that KSHV uses macropinocytosis for its infectious entry into its natural target cells, HMVEC-d cells, we know little about the molecule(s) involved in this event. Here, we show that the adaptor protein c-Cbl plays a major role in regulating bleb associated macropinocytosis of KSHV. The results demonstrate that c-Cbl protein functions as an adaptor for the myosin II hexameric complex in macropinocytic events. Knocking down c-Cbl by shRNA induces defects in myosin II dependent blebbing and KSHV entry, indicating that c-Cbl uses myosin II to coordinate signaling pathways, resulting in bleb formation and bleb retraction. This work provides a clear understanding of the role of c-Cbl in the recruitment and integration of signaling molecules around the macropinosome during virus infection, and identifies potential targets to intervene in KSHV infection.

The diverse functions of Src family kinases in macrophages

Macrophages are key components of the innate immune response. These cells possess a diverse repertoire of receptors that allow them to respond to a host of external stimuli including cytokines, chemokines, and pathogen-associated molecules. Signals resulting from these stimuli activate a number of macrophage functional responses such as adhesion, migration, phagocytosis, proliferation, survival, cytokine release and production of reactive oxygen and nitrogen species. The cytoplasmic tyrosine kinase Src and its family members (SFKs) have been implicated in many intracellular signaling pathways in macrophages, initiated by a diverse set of receptors ranging from integrins to Toll-like receptors. However, it has been difficult to implicate any given member of the family in any specific pathway. SFKs appear to have overlapping and complementary functions in many pathways. Perhaps the function of these enzymes is to modulate the overall intracellular signaling network in macrophages, rather than operating as exclusive signaling switches for defined pathways. In general, SFKs may function more like rheostats, influencing the amplitude of many pathways.

Dual role of Cbl links critical events in BCR endocytosis

Receptor endocytosis down-regulates ligand-induced signaling in a timely manner and depends on cytoskeletal remodeling. In B lymphocytes, internalization of B cell receptors (BCRs) is also critical to antigen presentation. However, the mechanisms underlying BCR endocytosis are not fully understood. Similarly, the molecular mechanisms linking endocytosis to cytoskeletal remodeling remain poorly defined. We used flow cytometry, pull-down assays, immunochemistry and fluorescence microscopy to investigate BCR internalization in the DT40 B cell line. We demonstrate that ablation of Cbl impacts BCR endocytosis and the underlying cytoskeletal dynamics. Specifically, we demonstrate that ligand-induced endocytosis is impaired in Cbl-/- cells and that the ubiquitin ligase activity is required for Cbl to promote endocytosis. We also show that phosphorylation of CrkII, activation of Rac downstream of CrkII and BCR capping require Cbl. Furthermore, we show that the association of Cbl and CrkII requires phosphorylation of Cbl, but not its ubiquitin ligase activity. Our data indicate that Cbl promotes BCR endocytosis and attenuates ligand-induced signaling by virtue of its ability to orchestrate receptor ubiquitylation and cytoskeletal dynamics.

Ligand binding induces Cbl-dependent EphB1 receptor degradation through the lysosomal pathway

Eph receptor tyrosine kinases play a critical role in embryonic patterning and angiogenesis. In the adult, they are involved in carcinogenesis and pathological neovascularization. However, the mechanisms underlying their role in tumor formation and metastasis remain to be defined. Here, we demonstrated that stimulation of EphB1 with ephrinB1/Fc led to a marked downregulation of EphB1 protein, a process blocked by the lysosomal inhibitor bafilomycin. Following ephrinB1 stimulation, the ubiquitin ligase Cbl was recruited by EphB1 and then phosphorylated. Both Cbl phosphorylation and EphB1 ubiquitination were blocked by the Src inhibitor PP2. Overexpression of wild-type Cbl, but not of 70Z mutant lacking ligase activity, enhanced EphB1 ubiquitination and degradation. This negative regulation required the tyrosine kinase activity of EphB1 as kinase-dead EphB1-K652R was resistant to Cbl. Glutathione S-transferase binding experiments showed that Cbl bound to EphB1 through its tyrosine kinase-binding domain. In aggregate, we demonstrated that Cbl induces the ubiquitination and lysosomal degradation of activated EphB1, a process requiring EphB1 and Src kinase activity. To our knowledge, this is the first study dissecting the molecular mechanisms leading to EphB1 downregulation, thus paving the way to new means of modulating their angiogenic and tumorigenic properties.

The Cbl interactome and its functions

Cbl proteins are ubiquitin ligases and multifunctional adaptor proteins that are implicated in the regulation of signal transduction in various cell types and in response to different stimuli. Cbl-associated proteins can assemble together at a given time or space inside the cell, and such an interactome can form signal competent networks that control many physiological processes. Dysregulation of spatial or temporal constraints in the Cbl interactome results in the development of human pathologies such as immune diseases, diabetes and cancer.

Epigallocatechin Gallate, a Green Tea Polyphenol, Mediates NO-dependent Vasodilation Using Signaling Pathways in Vascular Endothelium Requiring Reactive Oxygen Species and Fyn

Green tea consumption is associated with reduced cardiovascular mortality in some epidemiological studies. Epigallocatechin gallate (EGCG), a bioactive polyphenol in green tea, mimics metabolic actions of insulin to inhibit gluconeogenesis in hepatocytes. Because signaling pathways regulating metabolic and vasodilator actions of insulin are shared in common, we hypothesized that EGCG may also have vasodilator actions to stimulate production of nitric oxide (NO) from endothelial cells. Acute intra-arterial administration of EGCG to mesenteric vascular beds isolated ex vivo from WKY rats caused dose-dependent vasorelaxation. This was inhibitable by L-NAME (NO synthase inhibitor), wortmannin (phosphatidylinositol 3-kinase inhibitor), or PP2 (Src family kinase inhibitor). Treatment of bovine aortic endothelial cells (BAEC) with EGCG (50 microm) acutely stimulated production of NO (assessed with NO-specific fluorescent dye DAF-2) that was inhibitable by l-NAME, wortmannin, or PP2. Stimulation of BAEC with EGCG also resulted in dose- and time-dependent phosphorylation of eNOS that was inhibitable by wortmannin or PP2 (but not by MEK inhibitor PD98059). Specific knockdown of Fyn (but not Src) with small interfering RNA inhibited both EGCG-stimulated phosphorylation of Akt and eNOS as well as production of NO in BAEC. Treatment of BAEC with EGCG generated intracellular H(2)O(2) (assessed with H(2)O(2)-specific fluorescent dye CM-H(2)DCF-DA), whereas treatment with N-acetylcysteine inhibited EGCG-stimulated phosphorylation of Fyn, Akt, and eNOS. We conclude that EGCG has endothelial-dependent vasodilator actions mediated by intracellular signaling pathways requiring reactive oxygen species and Fyn that lead to activation of phosphatidylinositol 3-kinase, Akt, and eNOS. This mechanism may explain, in part, beneficial vascular and metabolic health effects of green tea consumption.

RAFTK/Pyk2 regulates EGF-induced PC12 cell spreading and movement

The protein tyrosine kinase RAFTK, also termed Pyk2, is a member of the focal adhesion kinase (FAK) subfamily. In this report, we show the role of RAFTK in neuroendocrine PC12 cells upon epidermal growth factor (EGF) stimulation. Following EGF treatment, we observed that RAFTK was tyrosine-phosphorylated in a time- and dose-dependent manner, while FAK was constitutively phosphorylated and primarily regulated by cell adhesion. Moreover, we found that RAFTK associated with the phosphorylated EGF receptor (EGFR) upon EGF stimulation. RAFTK phosphorylation was mediated primarily through PLCgamma-IP3-Ca(2+) signaling and partially through PI3-Kinase. Furthermore, overexpression of PRNK, a specific dominant-negative construct of RAFTK, was sufficient to block EGF-induced cell spreading and movement. Paxillin, a key modulator of the actin cytoskeleton and an RAFTK substrate, was also phosphorylated following EGF treatment. EGF induced a dynamic reorganization of RAFTK and paxillin at neuronal adhesion sites, with the specific localization of paxillin at the inner juxtaposition of RAFTK. Additionally, we observed that RAFTK associated with the scaffold protein c-Cbl and mediated its phosphorylation. Our data demonstrate that while FAK mediated cell adhesion, RAFTK was localized at the cytoplasm where it mediated inside-out signaling through intracellular Ca(2+), thus leading to cell spreading and movement upon EGF stimulation.

c-Cbl-facilitated cytoskeletal effects in v-Abl-transformed fibroblasts are regulated by membrane association of c-Cbl

The multi-functional protein c-Cbl is an important modulator of actin cytoskeletal dynamics in diverse biological systems. We had previously reported that c-Cbl facilitates cell spreading and adhesion and suppresses anchorage-independent growth of v-Abl-transformed fibroblasts. To assess the importance of membrane localization of c-Cbl for the observed effects of c-Cbl in v-Abl-3T3 cells, we first mapped the membrane interactive domain(s) of c-Cbl. Our studies indicate that localization of c-Cbl to the membrane is likely to be mediated by the tyrosine kinase binding (TKB) domain and the proline-rich region of c-Cbl, whereas C-terminal tyrosine phosphorylation does not play a role. The association of v-Cbl, which encompasses the TKB domain, with the membrane was unusual as it was not entirely dependent on SH2-phosphotyrosine interactions. Our studies further demonstrate that Src-like adaptor protein (SLAP), which binds to v-Cbl in a tyrosine phosphorylation-independent manner, facilitates membrane association of Cbl. The interaction between c-Cbl and SLAP in v-Abl-3T3 cells positively influenced c-Cbl-mediated spreading and adhesion of these cells. SLAP appears to exert its effects not simply by increasing the amount of c-Cbl in the membrane but by facilitating binding of p85-phosphatidylinositol-3-kinase (PI3K) with membrane-associated c-Cbl.

The Cbl family proteins: ring leaders in regulation of cell signaling

The proto-oncogenic protein c-Cbl was discovered as the cellular form of v-Cbl, a retroviral transforming protein. This was followed over the years by important discoveries, which identified c-Cbl and other Cbl-family proteins as key players in several signaling pathways. c-Cbl has donned the role of a multivalent adaptor protein, capable of interacting with a plethora of proteins, and has been shown to positively influence certain biological processes. The identity of c-Cbl as an E3 ubiquitin ligase unveiled the existence of an important negative regulatory pathway involved in maintaining homeostasis in protein tyrosine kinase (PTK) signaling. Recent years have also seen the emergence of novel regulators of Cbl, which have provided further insights into the complexity of Cbl-influenced pathways. This review will endeavor to provide a summary of current studies focused on the effects of Cbl proteins on various biological processes and the mechanism of these effects. The major sections of the review are as follows: Structure and genomic organization of Cbl proteins; Phosphorylation of Cbl; Interactions of Cbl; Localization of Cbl; Mechanism of effects of Cbl: (a) Ubiquitylation-dependent events: This section elucidates the mechanism of Cbl-mediated downregulation of EGFR and details the PTK and non-PTKs targeted by Cbl. In addition, it addresses the functional requirements for E3 Ubiquitin ligase activity of Cbl and negative regulation of Cbl-mediated downregulation of PTKs, (b) Adaptor functions: This section discusses the mechanisms of adaptor functions of Cbl in mitogen-activated protein kinase (MAPK) activation, insulin signaling, regulation of Ras-related protein 1 (Rap1), PI-3' kinase signaling, and regulation of Rho-family GTPases and cytoskeleton; Biological functions: This section gives an account of the diverse biological functions of Cbl and includes the role of Cbl in transformation, T-cell signaling and thymus development, B-cell signaling, mast-cell degranulation, macrophage functions, bone development, neurite growth, platelet activation, muscle degeneration, and bacterial invasion; Conclusions and perspectives.

A role of the protein Cbl in FGF-2-induced angiogenesis in murine brain endothelial cells

The Cbl protein functions both as a multivalent adaptor and a negative regulator of receptor tyrosine kinases (RTKs), the latter by directing polyubiquitination of RTKs. To study the function of Cbl in endothelial cell signalling and angiogenesis, wild-type Cbl and tyrosine kinase binding (TKB) domain mutated Cbl (G306E) were overexpressed in murine immortalised brain endothelial (IBE) cells. Wild-type Cbl cells exhibited enhanced proliferation in low serum compared with the control and G306E Cbl cells. Furthermore, up-regulated phosphorylation of fibroblast growth factor receptor 1 (FGFR-1) and Akt were observed in wild-type Cbl cells upon FGF-2 stimulation. A Cbl TKB domain mutant, G306E, disrupted the phosphorylation of the FGFR-1 but not that of FRS2. In the tubular morphogenesis assay, cells expressing wild-type Cbl initially formed tubular structures. These showed decreased stability and converted into cell aggregates, possibly due to a failure to cease proliferating. Our data support the idea that the wild-type Cbl cells exhibit enhanced proliferation, and thus lose their ability to differentiate appropriately. The present study reveals a role of the Cbl protein in FGF-2 dependent signalling in endothelial cells by its destabilisation of tubular structures.

Nef enhances c-Cbl phosphorylation in HIV-infected CD4+ T lymphocytes

The multifunctional HIV-1 protein Nef possesses several motifs that interact with signaling molecules in infected T cells. In order to determine whether Nef influences T cell activation, cells were infected with Nef-positive and Nef-negative clones of HIV. CD28 expression and changes in tyrosine phosphorylation were monitored. We observed no Nef-dependent changes in CD28 expression or function. However, infection with Nef-positive virus led to changes in tyrosine phosphorylation. This Nef-induced phosphorylation was observed in unstimulated cells, and c-Cbl was identified as one of the proteins whose phosphorylation was upregulated by Nef. Furthermore, Lck is required for Nef-mediated c-Cbl tyrosine phosphorylation. These results suggest that Nef modifies T cell signaling in the absence of T cell receptor engagement and co-stimulation.

Cbl-mediated ubiquitination of alpha5 integrin subunit mediates fibronectin-dependent osteoblast detachment and apoptosis induced by FGFR2 activation

Fibroblast growth factor receptor signaling is an important mechanism regulating osteoblast function. To gain an insight into the regulatory role of FGF receptor-2 (FGFR2) signaling in osteoblasts, we investigated integrin-mediated attachment and cell survival in human calvarial osteoblasts expressing activated FGFR2. FGFR2 activation reduced osteoblast attachment on fibronectin. This was associated with reduced expression of the alpha5 integrin subunit normally expressed in human calvarial osteoblasts in vivo. Treatment with lactacystin, a potent inhibitor of proteasome, restored alpha5 integrin levels in FGFR2 mutant osteoblasts. Immunoprecipitation analysis showed that alpha5 integrin interacts with both the E3 ubiquitin ligase Cbl and ubiquitin. Immunocytochemistry revealed that alpha5 integrin colocalizes with FGFR2 and Cbl at the leading edge in membrane ruffle regions. Transfection with the 70Z-Cbl mutant lacking the RING domain required for Cbl-ubiquitin interaction, or with the G306E Cbl mutant that abolishes the binding ability of Cbl phosphotyrosine-binding domain restored alpha5 integrin levels. This suggests that Cbl-mediated ubiquitination plays an essential role in alpha5 integrin proteasome degradation induced by FGFR2 activation. Reduced alpha5 integrin expression was associated with an increased Bax/Bcl-2 ratio and increased caspase-9 and -3 activities in FGFR2 mutant osteoblasts. Forced expression of alpha5 integrin rescued cell attachment and corrected both the Bax/Bcl-2 ratio and caspase-3 and caspase-9 activities in FGFR2 mutant osteoblasts. We show that Cbl recruitment induced by FGFR2 activation triggers alpha5 integrin degradation by the proteasome, which results in reduced osteoblast attachment on fibronectin and caspase-dependent apoptosis. This identifies a functional role of the alpha5 integrin subunit in the induction of apoptosis triggered by FGFR2 activation in osteoblasts, and reveals that a Cbl-dependent mechanism is involved in the coordinated regulation of cell apoptosis induced by alpha5 integrin degradation.

The inhibitory receptor PIR-B negatively regulates neutrophil and macrophage integrin signaling

The Ig-like receptor family member, PIR-B, has been shown to play an inhibitory role in receptor signaling within B cells, mast cells, and dendritic cells. As it has been implicated in integrin-mediated responses, we investigated the effect of loss of the PIR-B protein on integrin-mediated signaling in primary murine myeloid cells. The pir-b-/- neutrophils displayed enhanced respiratory burst, secondary granule release, and a hyperadhesive phenotype when plated on surfaces coated with either extracellular matrix proteins or cellular adhesion molecules in the presence or absence of the soluble inflammatory agonist TNF-alpha. The pir-b-/- and wild-type cells responded equivalently when stimulated with TNF-alpha in suspension, indicating that the hyperresponsive phenotype of the pir-b-/- cells during adhesion was due to enhanced integrin signaling. Both wild-type and pir-b-/- neutrophils expressed similar levels of integrin subunits. Primary bone marrow-derived macrophages from pir-b-/- mice were also hyperadhesive and spread more rapidly than wild-type cells following plating on surfaces that cross-linked cellular beta2 integrins. Biochemical analysis of macrophages from pir-b-/- mice revealed enhanced phosphorylation and activation of proteins involved in integrin signaling. These observations point to a nonredundant role for PIR-B in the regulation of leukocyte integrin signaling.

Signaling and Transcriptional Changes Critical for Transformation of Human Cells by Simian Virus 40 Small Tumor Antigen or Protein Phosphatase 2A B56γ Knockdown

One set of genes sufficient for transformation of primary human cells uses the combination of Ha-Ras-V12, the telomerase catalytic subunit hTERT, SV40 large tumor antigen (LT), and SV40 small tumor antigen (ST). Whereas SV40 LT inactivates the retinoblastoma protein and p53, the contribution of ST is poorly understood. The essential helper function of ST requires a functional interaction with protein phosphatase 2A (PP2A). Here we have identified changes in gene expression induced by ST and show that ST mediates these changes through both PP2A-dependent and PP2A-independent mechanisms. Knockdown of PP2A B56gamma subunit can substitute for ST expression to fully transform cells expressing LT, hTERT, and Ras-V12. We also identify those genes affected similarly in two cell lines that have been fully transformed from a common parental line by two alternative mechanisms, namely ST expression or PP2A B56gamma subunit knockdown. ST altered expression of genes involved in proliferation, apoptosis, integrin signaling, development, immune responses, and transcriptional regulation. ST reduced surface expression of MHC class I molecules, consistent with a need for SV40 to evade immune detection. ST expression enabled cell cycle progression in reduced serum and src phosphorylation in anchorage-independent media, whereas B56gamma knockdown required normal serum levels for these phenotypes. Inhibitors of integrin and src signaling prevented anchorage-independent growth of transformed cells, suggesting that integrin and src activation are key ST-mediated events in transformation. Our data support a model in which ST promotes survival through constitutive integrin signaling, src phosphorylation, and nuclear factor kappaB activation, while inhibiting cell-cell adhesion pathways.

Recruitment of Pyk2 and Cbl to lipid rafts mediates signals important for actin reorganization in growing neurites

Protein tyrosine kinase Pyk2 and multifunctional adaptor protein Cbl are implicated in the regulation of the cytoskeleton in several cell types. We report that Pyk2 and Cbl form a signaling complex that is translocated to lipid rafts and is enriched in growth cones of differentiating PC12 cells following growth factor stimulation. We found that Pyk2 and Cbl interacted with the adaptor protein ArgBP2, which also bound to flotillin-1, a component of lipid raft microdomains. These interactions contributed to recruitment of the Pyk2/Cbl complex to lipid raft compartments. In addition, Pyk2, Cbl and ArgBP2 were found co-localized with actin in axons and growth cones of differentiated PC12 cells. Moreover, co-expression of Pyk2, ArgBP2 and Cbl facilitated growth factor-induced formation of lamellipodia at the tip of neurites. Formation of these growth cone lamellipodia was dependent on intact lipid rafts and the Cbl-associated effectors Crk and phosphatidylinositol 3 (PI 3)-kinase. Our results indicate that recruitment of Pyk2/Cbl complexes to lipid rafts participates in growth factor-induced regulation of the actin cytoskeleton in growing neurites.

The integrin beta1 subunit transmembrane domain regulates phosphatidylinositol 3-kinase-dependent tyrosine phosphorylation of Crk-associated substrate

Our previous studies on the transmembrane domain of human integrin subunits have shown that a conserved basic amino acid in both subunits of integrin heterodimers is positioned in the plasma membrane in the absence of interacting proteins. To investigate the possible functional role of the lipid-embedded lysine in the mouse integrin beta1 subunit, this amino acid was replaced with leucine, and the mutated beta1 subunit (beta1A(K756L)) was stably expressed in beta1-deficient GD25 cells. The extracellular domain of beta1A(K756L) integrins possesses a competent conformation for ligand binding as determined by the ability to mediate cell adhesion, and by the presence of the monoclonal antibody 9EG7 epitope. However, the spreading of GD25-beta1A(K756L) cells on fibronectin and laminin-1 was impaired, and the rate of migration of GD25-beta1A(K756L) cells on fibronectin was reduced compared with GD25-beta1A cells. Phosphorylation of tyrosines in focal adhesion kinase (FAK) and the Y416 in c-Src in response to beta1A(K756L)-mediated adhesion was similar to that induced by wild-type beta1. The tyrosine phosphorylation level of paxillin, a downstream target of FAK/Src, was unaffected by the beta1 mutation, whereas tyrosine phosphorylation of CAS was strongly reduced. The results demonstrate that CAS is a target for phosphorylation both by FAK-dependent and -independent pathways after integrin ligation. The latter pathway was inhibited by wortmannin and LY294002, implicating that it required an active phosphatidylinositol 3-kinase. Furthermore, the K756L mutation in the beta1 subunit was found to interfere with beta1-induced activation of Akt. The results from this study identify phosphatidylinositol 3-kinase as an early component of a FAK-independent integrin signaling pathway triggered by the membrane proximal part of the beta1 subunit.

Selectins and anti-CD15 (Lewis x/a) antibodies transmit activation signals in Hodgkin's lymphoma-derived cell lines

OBJECTIVE

The CD15 (Lewis x) cell surface oligosaccharide moiety is expressed in a variety of normal and tumor cells and recognized by selectins. The detection of CD15 on malignant Hodgkin-Reed-Sternberg (HRS) cells serves as a diagnostic marker of Hodgkin's lymphoma (HL). Retrospective studies suggest that the expression of nonsialylated CD15 molecules on HRS cells has a positive prognostic value while presence of sialylated CD15 may correlate with a poor outcome. However, the relevance of the CD15 antigen expression to the pathobiology of the disease is not clear. In this work, we studied the contribution of CD15 to cell adhesion and the activation of signaling cascades in two HL-derived cell lines, KMH-2 and L428.

METHODS

Immobilized anti-CD15 monoclonal antibodies and recombinant E- and P-selectins were used to activate KMH-2 and L428 cells. Immunoblotting, immunoprecipitation, and the electrophoretic mobility shift assay were performed to detect tyrosine phosphorylation of c-Cbl, c-Jun nuclear translocation, and AP-1 DNA binding.

RESULTS

Treatment of cells with antibodies against the sialylated and nonsialylated forms of CD15, or with immobilized selectins, induced changes in cell morphology. Tyrosine phosphorylation of c-Cbl, together with tyrosine phosphorylation of multiple protein substrates, was also induced. In addition, binding of the CD15 molecules induced nuclear translocation of c-Jun and an increase in AP-1 DNA binding activity.

CONCLUSIONS

We suggest that CD15 has a dual physiological role, both as an adhesion molecule recognized by selectins and as a regulatory molecule upstream to specific intracellular signaling cascades with implications to the pathogenesis of HL.

Aggregation of beta2 integrins activates human neutrophils through the IkappaB/NF-kappaB pathway

Neutrophils are now considered central to the pathogenesis of most forms of acute lung injury. Neutrophils do not cause damage while suspended in the bloodstream; however, a release of cytotoxic agents occurs when neutrophils are adherent to endothelium, epithelium, or extracellular matrix proteins in the interstitium. Such neutrophil adherence is mediated predominantly through beta(2) integrins (CD11/CD18) on its surface. This study was undertaken to investigate whether the IkappaB/nuclear factor (NF)-kappaB cascade is involved in this beta(2) integrin-mediated activation of human neutrophils. beta(2) Integrin Mac-1 (CD11b/CD18) aggregation was induced by antibody cross-linking of the integrins on the cell surface. beta(2) Integrin aggregation induced interleukin-1beta and tumor necrosis factor-alpha production, which suggests the activation of neutrophils by beta(2) integrin. IkappaBalpha was markedly degraded at 1 h, and NF-kappaB-DNA-binding activity markedly increased 2 h after beta(2) integrin aggregation, which activated IkappaB kinase activity at 1 h. beta(2) Integrin-induced cytokine production was suppressed by MG132 or SN50 pretreatment, which blocked the activation of NF-kappaB. These findings suggest that the activation of human neutrophils by beta(2) integrin aggregation is mediated through the activation of the IkappaB/NF-kappaB pathway.

Deletion of the gene encoding c-Cbl alters the ability of osteoclasts to migrate, delaying resorption and ossification of cartilage during the development of long bones

During development of the skeleton, osteoclast (OC) recruitment and migration are required for the vascular invasion of the cartilaginous anlage and the ossification of long bones. c-Cbl lies downstream of the vitronectin receptor and forms a complex with c-Src and Pyk2 in a signaling pathway that is required for normal osteoclast motility. To determine whether the decreased motility we observed in vitro in c-Cbl(-/-) OCs translated into decreased cell migration in vivo, we analyzed the long bones of c-Cbl(-/-) mice during development. Initiation of vascularization and replacement of cartilage by bone were delayed in c-Cbl(-/-) mice, due to decreased osteoclast invasion of the hypertrophic cartilage through the bone collar. Furthermore, c-Cbl(-/-) mice show a delay in the formation of secondary centers of ossification, a thicker hypertrophic zone of the growth plate, and a prolonged presence of cartilaginous remnants in the spongiosa, confirming a decrease in resorption of the calcified cartilage. Thus, the decrease in motility of c-Cbl(-/-) osteoclasts observed in vitro results in a decreased ability of osteoclasts to invade and resorb bone and mineralized cartilage in vivo. These results confirm that c-Cbl plays an important role in osteoclast motility and resorbing activity.

A role for Syk-kinase in the control of the binding cycle of the beta2 integrins (CD11/CD18) in human polymorphonuclear neutrophils

A fine control of beta(2) integrin (CD11/CD18)-mediated firm adhesion of human neutrophils to the endothelial cell monolayer is required to allow ordered emigration. To elucidate the molecular mechanisms that control this process, intracellular protein tyrosine signaling subsequent to beta(2) integrin-mediated ligand binding was studied by immunoprecipitation and Western blotting techniques. The 72-kDa Syk-kinase, which was tyrosine-phosphorylated upon adhesion, was found to coprecipitate with CD18, the beta-subunit of the beta(2) integrins. Moreover, inhibition of Syk-kinase by piceatannol enhanced adhesion and spreading but diminished N-formyl-Met-Leu-Phe-induced chemotactic migration. The enhancement of adhesiveness was associated with integrin clustering, which results in increased integrin avidity. In contrast, piceatannol had no effect on the surface expression or on the affinity of beta(2) integrins. Altogether, this suggests that Syk-kinase controls alternation of beta(2) integrin-mediated ligand binding with integrin detachment.

Expression and tyrosine phosphorylation of Cbl regulates macrophage chemokinetic and chemotactic movement

Primary macrophages isolated from hck(-/-)fgr(-/-) mice display altered morphology and F-actin cytoskeletal structures and reduced migration. The ability of phorbol myristyl acetate (PMA), a protein kinase C activator that has been reported to increase macrophage spreading and carcinoma cell motility, to rescue these hck(-/-)fgr(-/-) defects was tested. Although PMA-treated wild-type and hck(-/-)fgr(-/-) macrophages exhibited a similar flattened, spread phenotype, PMA did not rescue the hck(-/-)fgr(-/-) macrophage migration defect. Instead, both PMA-treated wild type and hck(-/-)fgr(-/-) macrophages were defective in spontaneous and chemotactic migration and tyrosine phosphorylation of the Cbl protooncoprotein was decreased in both. Moreover, c-cbl(-/-) macrophages displayed the same impairment of motility as hck(-/-)fgr(-/-) macrophages and a similar morphology with less polarization and more dorsal ruffling than wild-type macrophages. As Hck and Fgr expression and activity were not decreased in c-cbl(-/-) macrophages, these results suggest that Cbl is likely to be an important downstream mediator of the Src family kinase-regulated macrophage motility pathway.

Fgr but not Syk tyrosine kinase is a target for beta 2 integrin-induced c-Cbl-mediated ubiquitination in adherent human neutrophils

An early and critical event in beta(2) integrin signalling during neutrophil adhesion is activation of Src tyrosine kinases and Syk. In the present study, we report Src kinase-dependent beta(2) integrin-induced tyrosine phosphorylation of Cbl occurring in parallel with increased Cbl-associated tyrosine kinase activity. These events concurred with activation of Fgr and, surprisingly, also with dissociation of this Src tyrosine kinase from Cbl. Moreover, the presence of the Src kinase inhibitor PP1 in an in vitro assay had only a limited effect on the Cbl-associated kinase activity. These results suggest that an additional active Src-dependent tyrosine kinase associates with Cbl. The following observations imply that Syk is such a kinase: (i) beta(2) integrins activated Syk in a Src-dependent manner, (ii) Syk was associated with Cbl much longer than Fgr was, and (iii) the Syk inhibitor piceatannol (3,4,3',5'-tetrahydroxy- trans -stilbene) abolished the Cbl-associated kinase activity in an in vitro assay. Effects of the mentioned interactions between these two kinases and Cbl may be related to the finding that Cbl is a ubiquitin E3 ligase. Indeed, we detected beta(2) integrin-induced ubiquitination of Fgr that, similar to the phosphorylation of Cbl, was abolished in cells pretreated with PP1. However, the ubiquitination of Fgr did not cause any apparent degradation of the protein. In contrast with Fgr, Syk was not modified by the E3 ligase. Thus Cbl appears to be essential in beta(2) integrin signalling, first by serving as a matrix for a subsequent agonist-induced signalling interaction between Fgr and Syk, and then by mediating ubiquitination of Fgr which possibly affects its interaction with Cbl.

Activation of Rac and tyrosine phosphorylation of cytokine receptors induced by cross-linking of integrin alpha4beta1 and cell adhesion in hematopoietic cells

Adhesion of hematopoietic cells, mainly through alpha4beta1 and alpha5beta1 integrins, to the bone marrow microenvironment may play important roles in regulation of hematopoiesis. However, the mechanisms for signaling, outside-in signaling, have largely remained to be established. We demonstrate here that cross-linking of alpha4beta1 by anti-alpha4 antibody induces tyrosine phosphorylation of Pyk2, Shc, and Cbl as well as binding of the adaptor protein CrkL with Cbl in a murine hematopoietic cell line, 32D/EpoR-Wt. Furthermore, cross-linking of alpha4beta1 induced activation of the Rho family small GTPase Rac, which was enhanced by induced overexpression of CrkL and was inhibited by the phosphatidylinositol 3(')-kinase (PI3K) inhibitor LY294002. In addition, adhesion of 32D/EpoR-Wt cells to immobilized H-296, a recombinant fibronectin peptide specific for alpha4beta1, induced tyrosine phosphorylation of Jak2, the erythropoietin receptor (EpoR), and the IL-3 receptor beta subunit as well as Pyk2, Shc, and Cbl. Tyrosine phosphorylation of Jak2 and EpoR was also induced in a human leukemic cell line, UT-7, by adhesion to immobilized H-296. However, adhesion of 32D/EpoR-PM4 cells, expressing the W282R mutant EpoR defective in coupling with Jak2, to immobilized H-296 failed to induce tyrosine phosphorylation of the mutant EpoR. These results implicate CrkL in PI3K-dependent activation of Rac by outside-in signaling from alpha4beta1 and suggest that adhesion through alpha4beta1 further activates cytokine receptor-associated Jak2 to induce phosphorylation of these receptors.

Alpha6beta1 integrin induces proteasome-mediated cleavage of erbB2 in breast cancer cells

ErbB2 and alpha6 integrin have been implicated in malignancy of breast cancer cells. Here we have determined the influence of alpha6beta1 integrin on erbB2 signaling in anchorage-independent growth, using MDA-MB435 breast cancer cells. Firstly, we transfected the cells with erbB2 cDNA, and isolated cells with high or low levels of alpha6beta1 integrin by cell sorting (alpha6H-ErbB and alpha6L-ErbB). We found that an erbB ligand, heregulin beta1, enhanced growth activity of alpha6L-ErbB cells, but not alpha6H-ErbB cells. Secondly, we established cells expressing a beta4 integrin deletion mutant (beta4-deltacyt), which selectively inhibited alpha6beta1 integrin expression and adhesion to laminin-1. Again, heregulin beta1 enhanced the growth of erbB2 cDNA-transfected beta4-deltacyt cells, but not mock cells. Western blot analysis revealed that heregulin beta1 stimulated phosphorylation of Akt and its downstream molecules, GSK3beta and p70S6kinase, and that the extent of phosphorylation was greater in ErbB2/beta4-deltacyt cells than ErbB2/mock cells. Furthermore, we found that the erbB2 cytoplasmic domain was truncated in ErbB2/mock cells, which was independent of ligand stimulation and adhesion, and was suppressed by proteasome inhibitors. These results suggest that alpha6beta1 integrin inhibits erbB2 signals by inducing proteasome-dependent proteolytic cleavage of the erbB2 cytoplasmic domain, and may thereby contribute to the regulation of tumor growth.

Interplay between integrins and FLK-1 in shear stress-induced signaling

Blood flow can modulate vascular cell functions. We studied interactions between integrins and Flk-1 in transducing the mechanical shear stress due to flow. This application of a step shear stress caused Flk-1. Casitas B-lineage lymphoma (Cbl) activation (Flk-1. Cbl association, tyrosine phosphorylation of the Cbl-bound Flk-1, and tyrosine phosphorylation of Cbl) in bovine aortic endothelial cells (BAECs). The activation of integrins by plating BAECs on vitronectin or fibronectin also induced this Flk-1. Cbl activation. The shear-induced Flk-1. Cbl activation was blocked by inhibitory antibodies for alphavbeta3- or beta1-integrin, suggesting that it is mediated by integrins. Inhibition of Flk-1 by SU1498 also abolished this shear-induced Flk-1. Cbl activation. In contrast to the requirement of integrins for Flk-1. Cbl activation, the Flk-1 blocker SU1498 had no detectable effect on the shear-induced integrin activation, suggesting that integrins and Flk-1 play sequential roles in the signal transduction hierarchy induced by shear stress. Integrins are essential for the mechanical activation of Flk-1 by shear stress but not for the chemical activation of Flk-1 by VEGF.

Expression of the palmitoylation-deficient CD151 weakens the association of alpha 3 beta 1 integrin with the tetraspanin-enriched microdomains and affects integrin-dependent signaling

Transmembrane proteins of the tetraspanin superfamily are assembled in multimeric complexes on the cell surface. Spatial orientation of tetraspanins within these complexes may affect signaling functions of the associated transmembrane receptors (e.g. integrins, receptor-type tyrosine kinases). The structural determinants that control assembly of the tetraspanin complexes are unknown. We have found that various tetraspanins and the alpha(3) integrin subunit are palmitoylated. The stability and molecular composition of the palmitoylated alpha(3)beta(1)-tetraspanin complexes are not affected by adhesion. To assess the significance of palmitoylation in the function of the alpha(3)beta(1)-tetraspanin complexes we mapped the sites of palmitoylation for CD151. Mutation of six cysteines, Cys(11), Cys(15), Cys(79), Cys(80), Cys(242), and Cys(243) was necessary to completely abolish palmitoylation of CD151. The association of the palmitoylation-deficient mutant of CD151 (CD151Cys8) with CD81 and CD63 was markedly decreased, but the interaction of the alpha(3)beta(1)-CD151Cys8 complex with phosphatidylinositol 4-kinase was not affected. Ectopic expression of CD151Cys8 in Rat-1 cells impaired the interactions of the endogenous CD63 and CD81 with the alpha(3)beta(1) integrin. Although the expression of the palmitoylation-deficient CD151 does not change cell spreading on the extracellular matrix, the number of focal adhesions increased. Adhesion-induced phosphorylation of PKB/c-Akt is markedly increased in cells expressing a palmitoylation-deficient mutant, thereby providing direct evidence for the role of the tetraspanin microdomains in regulation of the integrin-dependent phosphatidylinositol 3-kinase signaling pathway. In contrast, activation of FAK and ERK1/2 were not affected by the expression of CD151Cys8. Our results demonstrate that palmitoylation of tetraspanins is critical not only for the organization of the integrin-tetraspanin microdomains but also has a specific role in modulation of adhesion-dependent signaling.

c-Cbl is a negative regulator of GH-stimulated STAT5-mediated transcription

We have previously demonstrated that cellular stimulation with GH results in the formation of a multiprotein signaling complex. One component of this multiprotein signaling complex is the adapter molecule c-Cbl. Here we have examined the role of c-Cbl in the mechanism of GH signal transduction. Forced expression of c-Cbl in NIH3T3 cells did not alter GH-stimulated Janus kinase 2 tyrosine phosphorylation nor GH-stimulated p44/42 MAPK activation and consequent Elk-1- mediated transcription. c-Cbl overexpression did, however, result in enhanced and prolonged GH-stimulated activation of phosphatidylinositol 3-kinase. Forced expression of c-Cbl did not affect GH-stimulated STAT5 tyrosine phosphorylation, nuclear translocation, nor binding to DNA but markedly abrogated GH-stimulated STAT5-mediated transactivation. c-Cbl overexpression resulted in increased ubiquitination and proteosomal degradation of STAT5 and increased degradation of GH-stimulated tyrosine phosphorylated STAT5. Cellular pretreatment with the proteosomal inhibitor MG132 reversed the effect of c-Cbl overexpression with prolonged duration of GH-stimulated STAT5 tyrosine phosphorylation and restoration of STAT5-mediated transcription. Thus, c-Cbl is a negative regulator of GH-stimulated STAT5-mediated transcription by direction of STAT5 for proteosomal degradation.

Inhibition of Src family kinases blocks epidermal growth factor (EGF)-induced activation of Akt, phosphorylation of c-Cbl, and ubiquitination of the EGF receptor

Stimulation of T47D cells with epidermal growth factor (EGF) results in the activation of the intrinsic tyrosine kinases of the receptor and the phosphorylation of multiple cellular proteins including the receptor, scaffold molecules such as c-Cbl, adapter molecules such as Shc, and the serine/threonine protein kinase Akt. We demonstrate that EGF stimulation of T47D cells results in the activation of the Src protein-tyrosine kinase and that the Src kinase inhibitor PP1 blocks the EGF-induced phosphorylation of c-Cbl but not the activation/phosphorylation of the EGF receptor itself. PP1 also blocks EGF-induced ubiquitination of the EGF receptor, which is presumably mediated by phosphorylated c-Cbl. Src is associated with c-Cbl, and we have previously demonstrated that the Src-like kinase Fyn can phosphorylate c-Cbl at a preferred binding site for the p85 subunit of phosphatidylinositol 3'-kinase. PP1 treatment blocks EGF-induced activation of the anti-apoptotic protein kinase Akt suggesting that Src may regulate activation of Akt, perhaps by a Src --> c-Cbl --> phosphatidylinositol 3'-kinase --> Akt pathway.

APS facilitates c-Cbl tyrosine phosphorylation and GLUT4 translocation in response to insulin in 3T3-L1 adipocytes

APS is a Cbl-binding protein that is tyrosine phosphorylated by the insulin receptor kinase. Insulin-stimulated phosphorylation of tyrosine 618 in APS is necessary for its association with c-Cbl and the subsequent tyrosine phosphorylation of Cbl by the insulin receptor in both 3T3-L1 adipocytes and CHO-IR cells. When overexpressed in these cells, wild-type APS but not an APS/Y(618)F mutant facilitated the tyrosine phosphorylation of coexpressed Cbl and its association with Crk upon insulin stimulation. APS-facilitated phosphorylation occurred on tyrosines 371, 700, and 774 in the Cbl protein. APS also interacted directly with the c-Cbl-associated protein (CAP) and colocalized with the protein in cells. The association was dependent on the SH3 domains of CAP and was independent of insulin treatment. Overexpression of the APS/Y(618)F mutant in 3T3-L1 adipocytes blocked the insulin-stimulated tyrosine phosphorylation of endogenous Cbl and binding to Crk. Moreover, the translocation of GLUT4 from intracellular vesicles to the plasma membrane was also inhibited by overexpression of the APS/Y(618)F mutant. These data suggest that APS serves as an adapter protein linking the CAP/Cbl pathway to the insulin receptor and, further, that APS-facilitated Cbl tyrosine phosphorylation catalyzed by the insulin receptor is a crucial event in the stimulation of glucose transport by insulin.

Role of Cbl in shear-activation of PI 3-kinase and JNK in endothelial cells

Fluid shear stress can activate PI-3 kinase and JNK in vascular endothelial cells. This study was designed to establish the role of Cbl as an upstream molecule in the shear stress activation of PI-3 kinase and JNK. Confluent monolayers of bovine aortic endothelial cells (BAECs) were subjected to a shear stress of 12 dyn/cm(2) over intervals ranging from 0.5 to 30 min. Shear stress increased Cbl phosphorylation to 2.9-fold of control and Cbl association with the regulatory PI-3 kinase subunit p85 to 5.4-fold. The PI-3 kinase activity measured in Cbl-immunoprecipitated complexes increased to 11.7-fold in response to shear, suggesting that the shear stress activation of PI-3 kinase involves its association with Cbl. Furthermore, the shear stress induction of JNK was attenuated by a negative mutant of Cbl. Finally, shear stress caused an activation of PI 3-kinase only in BAECs seeded onto fibronectin, vitronectin, or laminin, but not poly-l-lysine. Our results suggest that Cbl plays a critical role in the shear stress induction of PI 3-kinase and JNK activities, and that this shear-induced activation requires the interaction of endothelial integrins with extracellular matrix proteins.

Activated Fes protein tyrosine kinase induces terminal macrophage differentiation of myeloid progenitors (U937 cells) and activation of the transcription factor PU.1

The c-fps/fes proto-oncogene encodes a 92-kDa protein tyrosine kinase that is preferentially expressed in myeloid and endothelial cells. Fes is believed to play a role in vascular development and myelopoiesis and in the inflammatory responses of granulocytes and macrophages. To help define the biological role of this kinase and identify its downstream targets, we have developed a gain-of-function allele of Fes that has potent biological activity in myeloid cell progenitors. Introduction of constitutively active Fes into bipotential U937 cells induced the appearance of fully differentiated macrophages within 6 to 12 days. The Fes-expressing differentiated cells became adherent, had distinctive macrophage morphology, and exhibited increased expression of myelomonocytic differentiation markers, including CD11b, CD11c, CD18, CD14, and the macrophage colony-stimulating factor receptor. These cells acquired phagocytic properties and exhibited NADPH oxidase and nonspecific esterase activities, confirming that they were functionally active macrophages. Concomitantly, there was downregulation of the granulocytic marker granulocyte colony-stimulating factor receptor, indicating that the biological activity of Fes was coordinated in a lineage-specific manner. A constitutively active Src did not induce macrophage morphology or upregulation of myelomonocytic markers in U937 cells, suggesting that the biological activity we observed was not a general consequence of expression of an activated nonreceptor tyrosine kinase. Analysis of possible downstream targets of Fes revealed that this kinase activated the ets family transcription factor PU.1, which is essential for macrophage development. Our results strongly implicate Fes as a key regulator of terminal macrophage differentiation and identify PU.1 as a transcription factor that may mediate some of its biological activities in myeloid cells.

Lipid rafts and insulin signaling

Lipid rafts are domains within the plasma membrane that are enriched in cholesterol and lipids with saturated acyl chains. Specific proteins, including many signaling proteins, segregate into lipid rafts, and this process is important for certain signal transduction events in a variety of cell types. Within the past decade, data have emerged from many laboratories that implicate lipid rafts as critical for proper compartmentalization of insulin signaling in adipocytes. A subset of lipid rafts, caveolae, are coated with membrane proteins of the caveolin family. Direct interactions between resident raft proteins (caveolins and flotillin-1) and insulin-signaling molecules may organize these molecules in space and time to ensure faithful transduction of the insulin signal, at least with respect to the glucose-dependent actions of insulin in adipocytes. The in vivo relevance of this model remains to be determined.

Beyond the RING: CBL proteins as multivalent adapters

Following discovery of c-Cbl, a cellular form of the transforming retroviral protein v-Cbl, multiple Cbl-related proteins have been identified in vertebrate and invertebrate organisms. c-Cbl and its homologues are capable of interacting with numerous proteins involved in cell signaling, including various molecular adapters and protein tyrosine kinases. It appears that Cbl proteins play several functional roles, acting both as multivalent adapters and inhibitors of various protein tyrosine kinases. The latter function is linked, to a substantial extent, to the E3 ubiquitin-ligase activity of Cbl proteins. Experimental evidence for these functions, interrelations between them, and their biological significance are addressed in this review, with the main accent placed on the adapter functions of Cbl proteins.

Soluble E-selectin induces monocyte chemotaxis through Src family tyrosine kinases

Cellular adhesion molecules such as E-selectin function to recruit leukocytes into the inflammatory lesions of diseases such as rheumatoid arthritis (RA) and atherosclerosis. Monocytes are the key components of the cellular infiltrates present in these disorders. We hypothesized that soluble E-selectin (sE-selectin) might mediate the chemotaxis of monocytes. In this report, we show that sE-selectin induced normal human peripheral blood monocyte migration in the nanomolar range in a concentration-dependent manner. Neutralization studies using RA human joint synovial fluids and anti-E-selectin antibody showed a mean 31% reduction in RA synovial fluid-mediated monocyte chemotaxis (p < 0.05), indicating that sE-selectin is a major monocyte recruiter in RA. Next, we investigated the role of tyrosine phosphorylation pathways in sE-selectin-induced monocyte chemotaxis. Human peripheral blood monocytes stimulated with sE-selectin showed a time-dependent increase in the tyrosine phosphorylation of a broad range of cellular proteins, predominantly in the molecular size range of Src family kinases (50-60 kDa) and mitogen-activated protein kinases (MAPKs). Western blot analysis of Src family kinases showed a time-dependent increase in Src, Hck, and Lyn phosphorylation. The pretreatment of monocytes with the Src inhibitor AG1879: 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolol[3,4-d]pyrimidine (PP2) prior to stimulation with sE-selectin markedly inhibited Hck and Lyn phosphorylation, whereas the phosphorylation of Src was partially inhibited. In addition, the sE-selectin stimulation of monocytes resulted in the increased phosphorylation of extracellular signal-related kinase (ERK1/2) and p38 MAPK. The pretreatment of monocytes with PP2 showed 89 and 83% inhibition of ERK1/2 and p38 MAPK phosphorylation, respectively. sE-selectin also showed a time-dependent activation of Ras kinase. Furthermore, the pretreatment of monocytes with PP2 completely inhibited sE-selectin-mediated monocyte chemotaxis. Taken together, our data demonstrate a novel function for sE-selectin as a monocyte chemotactic agent and suggest that sE-selectin might be mediating its biological functions through the Src-MAPK pathway.

Cbl: many adaptations to regulate protein tyrosine kinases

Responses to extracellular stimuli are often transduced from cell-surface receptors to protein tyrosine kinases which, when activated, initiate the formation of protein complexes that transmit signals throughout the cell. A prominent component of these complexes is the product of the proto-oncogene c-Cbl, which specifically targets activated protein tyrosine kinases and regulates their signalling. How, then, does this multidomain protein shape the responses generated by these signalling complexes?

c-Cbl facilitates fibronectin matrix production by v-Abl-transformed NIH3T3 cells via activation of small GTPases

The protooncogenic protein c-Cbl has been shown to act as a multivalent adaptor and a negative regulator of protein tyrosine kinase-mediated signaling. Recent studies have implicated it in the regulation of cell adhesion-related events. We have previously shown that c-Cbl facilitates adhesion and spreading of v-Abl-transformed fibroblasts, and that these effects are dependent on its tyrosine phosphorylation. However, the mechanisms mediating effects of c-Cbl on fibroblast adhesion remain poorly understood. In this study we demonstrate that the tyrosine phosphorylation-dependent effect of c-Cbl on adhesion of v-Abl-transformed fibroblasts is primarily mediated by an increase in fibronectin matrix deposition by these cells. This increase in fibronectin matrix deposition and, hence, in cell adhesion is dependent on cytoskeletal rearrangements induced by RhoA, Rac1 and, possibly, Rap1 activation caused by c-Cbl. The observed activation of these GTPases is mediated by the recruitment of phosphatidylinositol-3' kinase, CrkL and Vav2 to the C-terminal tyrosine residues of c-Cbl. The results of this study also demonstrate that ubiquitination is essential for the observed effects of c-Cbl on fibronectin matrix production and cell adhesion.

Suppression of apoptosis induced by growth factor withdrawal by an oncogenic form of c-Cbl

The v-Cbl oncogene induces myeloid and B-cell leukemia; however, the mechanism by which transformation occurs is not understood. An oncogenic form of c-Cbl (Cbl-DeltaY371) was expressed in the interleukin-3 (IL-3)-dependent cell line 32Dcl3 to determine whether it was able to induce growth factor-independent proliferation. We were unable to isolate clones of transfected 32Dcl3 cells expressing Cbl-DeltaY371 that proliferated in the absence of IL-3. In contrast, 32Dcl3/Cbl-DeltaY371 cells did not undergo apoptosis like parental 32Dcl3 cells when cultured in the absence of IL-3. Both 32Dcl3 and 32D/CblDeltaY371 cells arrested in G(1) when cultured in the absence of IL-3. Approximately 18% of the 32Dcl3 cells cultured in the absence of IL-3 for 24 h were present in a sub-G(1) fraction, while only 4% of the 32D/Cbl-DeltaY371 and 2% of the 32D/Bcl-2 cells were found in a sub-G(1) fraction. There was no difference in the pattern of tyrosine-phosphorylated proteins observed following stimulation of either cell type with IL-3. The phosphorylation of JAK2, STAT5, and endogenous c-Cbl was identical in both cell types. No differences were detected in the activation of Akt, ERK1, or ERK2 in unstimulated or IL-3-stimulated 32D/Cbl-DeltaY371 cells compared with parental 32Dcl3 cells. Likewise, there was no difference in the pattern of phosphorylation of JAK2, STAT5, ERK1, ERK2, or Akt when 32Dcl3 and 32D/CblDY371 cells were withdrawn from medium containing IL-3. The protein levels of various Bcl-2 family members were examined in cells grown in the absence or presence of IL-3. We observed a consistent increased amount of Bcl-2 protein in five different clones of 32D/Cbl-DeltaY317 cells. These data suggest that the Cbl-DeltaY371 mutant may suppress apoptosis by a mechanism that involves the overexpression of Bcl-2. Consistent with this result, activation of caspase-3 was suppressed in 32D/Cbl-DeltaY371 cells cultured in the absence of IL-3 compared with 32Dcl3 cells cultured under the same conditions.

The adapter type protein CMS/CD2AP binds to the proto-oncogenic protein c-Cbl through a tyrosine phosphorylation-regulated Src homology 3 domain interaction

CMS/CD2AP is a cytoplasmic protein critical for the integrity of the kidney glomerular filtration and the T cell function. CMS contains domains and motifs characteristic for protein-protein interactions, and it is involved in the regulation of the actin cytoskeleton. We report here that the individual SH3 domains of CMS bind to phosphotyrosine proteins of approximately 80, 90, and 180 kDa in cell lysates stimulated with epidermal growth factor. The second SH3 domain of CMS bound specifically to a tyrosine-phosphorylated protein of 120 kDa, which we identified as the proto-oncoprotein c-Cbl. The c-Cbl-binding site for CMS mapped to the carboxyl terminus of c-Cbl and is different from the proline-rich region known to bind SH3-containing proteins. CMS binding to c-Cbl was markedly attenuated in a tyrosine phosphorylation-defective c-Cbl mutant indicating that this interaction is dependent on the tyrosine phosphorylation of CMS. It also implies that CMS interacts with c-Cbl in an inducible fashion upon stimulation of a variety of cell-surface receptors. Immunofluorescence analysis revealed that both proteins colocalize at lamellipodia and leading edges of cells, and we propose that the interaction of CMS with c-Cbl offers a mechanism by which c-Cbl associates and regulates the actin cytoskeleton.

Cbl associates with Pyk2 and Src to regulate Src kinase activity, alpha(v)beta(3) integrin-mediated signaling, cell adhesion, and osteoclast motility

The signaling events downstream of integrins that regulate cell attachment and motility are only partially understood. Using osteoclasts and transfected 293 cells, we find that a molecular complex comprising Src, Pyk2, and Cbl functions to regulate cell adhesion and motility. The activation of integrin alpha(v)beta(3) induces the [Ca(2+)](i)-dependent phosphorylation of Pyk2 Y402, its association with Src SH2, Src activation, and the Src SH3-dependent recruitment and phosphorylation of c-Cbl. Furthermore, the PTB domain of Cbl is shown to bind to phosphorylated Tyr-416 in the activation loop of Src, the autophosphorylation site of Src, inhibiting Src kinase activity and integrin-mediated adhesion. Finally, we show that deletion of c Src or c-Cbl leads to a decrease in osteoclast migration. Thus, binding of alpha(v)beta(3) integrin induces the formation of a Pyk2/Src/Cbl complex in which Cbl is a key regulator of Src kinase activity and of cell adhesion and migration. These findings may explain the osteopetrotic phenotype in the Src(-/-) mice.

VCAM-1 activates phosphatidylinositol 3-kinase and induces p120Cbl phosphorylation in human airway smooth muscle cells

VCAM-1 is a member of the Ig superfamily of receptors the expression of which is up-regulated on human airway smooth muscle (ASM) cells following stimulation with inflammatory mediators. The function of these receptors in adhesion is well known, but there is growing recognition that they also possess "outside-in" signaling functions, such as cytoskeletal reorganization, calcium mobilization, and cytokine release. The present study examined the activation of extracellular signal-regulated kinase and phosphatidylinositol 3-kinase (PI3K) in ASM cells following VCAM-1 engagement. VCAM-1 ligation activated extracellular signal-regulated kinase 2 and resulted in increased expression of cyclin D1, yet there was neither p27(kip1) degradation nor an increase in smooth muscle cell DNA synthesis. VCAM-1 ligation, however, augmented the proliferative response to submitogenic concentrations of epidermal growth factor. VCAM-1 engagement also stimulated a rapid increase in PI3K activity. This was associated with phosphorylation of the adapter protein p120(Cbl) and an increase in Cbl-associated PI3K activity. These studies suggest that VCAM-1 is linked to multiple signaling pathways in human ASM cells and may function to augment growth factor-induced responses.

Cbl-transforming variants trigger a cascade of molecular alterations that lead to epithelial mesenchymal conversion

Dispersal of epithelial cells is an important aspect of tumorigenesis, and invasion. Factors such as hepatocyte growth factor induce the breakdown of cell junctions and promote cell spreading and the dispersal of colonies of epithelial cells, providing a model system to investigate the biochemical signals that regulate these events. Multiple signaling proteins are phosphorylated in epithelial cells during hepatocyte growth factor-induced cell dispersal, including c-Cbl, a protooncogene docking protein with ubiquitin ligase activity. We have examined the role of c-Cbl and a transforming variant (70z-Cbl) in epithelial cell dispersal. We show that the expression of 70z-Cbl in Madin-Darby canine kidney epithelial cells resulted in the breakdown of cell-cell contacts and alterations in cell morphology characteristic of epithelial-mesenchymal transition. Structure-function studies revealed that the amino-terminal portion of c-Cbl, which corresponds to the Cbl phosphotyrosine-binding/Src homology domain 2, is sufficient to promote the morphological changes in cell shape. Moreover, a point mutation at Gly-306 abrogates the ability of the Cbl Src homology domain 2 to induce these morphological changes. Our results identify a role for Cbl in the regulation of epithelial-mesenchymal transition, including loss of adherens junctions, cell spreading, and the initiation of cell dispersal.

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.

A role for phosphoinositides in tyrosine phosphorylation of p125 focal adhesion kinase in rat pancreatic acini

Previous studies have shown that different agonists increase tyrosine phosphorylation of the focal adhesion related proteins p125(FAK), p130(Cas), and paxillin in different cell types and that tyrosine phosphorylation depends on the integrity of the actin cytoskeleton. Because phosphoinositides are important for the maintenance of the cytoskeleton, the role of phosphoinositides in the tyrosine phosphorylation of these proteins in response to occupancy of m3 muscarinic and CCK(A) receptors has been investigated in pancreatic acini. Addition of carbachol or CCK-8 to pancreatic acini resulted in rapid increases in the tyrosine phosphorylation of p125(FAK), p130(Cas), and paxillin. Pretreatment of pancreatic acini with LY294002 or wortmannin resulted in a concentration-dependent inhibition of tyrosine phosphorylation of p125(FAK), p130(Cas), and paxillin stimulated by carbachol or CCK-8. Carbachol- or CCK-8-stimulated tyrosine phosphorylation of these proteins was not inhibited by rapamycin, PD 98059 or SB 203580, and thus it was dissociated from the activation of p70 S6 or MAP kinases. These results indicate that m3 muscarinic and CCK(A) receptor-mediated increase in p125(FAK), p130(Cas), and paxillin tyrosine phosphorylation in pancreatic acini depends on the ability of these cells to synthesise phosphoinositides.

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.

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.

The adapter protein Crkl links Cbl to C3G after integrin ligation and enhances cell migration

Crkl, an SH2-SH3-SH3 adapter protein, is one of the major tyrosine phosphoproteins detected in cells from patients with chronic myelogenous leukemia. Crkl binds to BCR/ABL through its N-terminal SH3 domain and is known to interact with several signaling proteins that have been implicated in integrin signaling, including Cbl, Cas, Hef-1, and paxillin. We have previously shown that overexpression of Crkl enhances adhesion to extracellular matrix proteins through beta(1) integrins. In this study, the effects of Crkl on spontaneous and chemokine-directed migration of the hematopoietic cell line Ba/F3 were examined. Full-length, SH2-, and SH3(N)-domain deletion mutants of Crkl were expressed transiently as fusion proteins with green fluorescent protein. Successfully transfected cells were isolated by fluorescence-activated cell sorting. The ability of these cells to migrate across a fibronectin-coated membrane, either spontaneously or in response to the chemokine stromal-derived factor-1alpha, was determined. Cells expressing green fluorescent protein alone were not distinguishable from untransfected or mock transfected Ba/F3 cells. However, Ba/F3 cells overexpressing full-length Crkl were found to have an increase in spontaneous migration of 2.8 +/- 0.6-fold in seven independent assays. The enhancement of migration required both the SH2 domain and the N-terminal SH3 domain. Migration in response to stromal-derived factor-1alpha was not significantly enhanced by overexpression of Crkl. Overexpression of Crkii also augmented spontaneous migration but to a lesser degree than did Crkl. Because the SH2 domain was required for enhanced migration, we looked for changes in phosphotyrosine containing proteins coprecipitating with Crkl, but not Crkl DeltaSH2, after integrin cross-linking. Full-length Crkl, but not CrklDeltaSH2, coprecipitated with a single major tyrosine phosphoprotein with an M(r) of approximately 120 kDa, identified as Cbl. The major Crkl SH3-binding protein in these cells was found to be the guanine nucleotide exchange factor, C3G. Interestingly, overexpression of C3G also enhanced migration, suggesting that a Cbl-Crkl-C3G complex may be involved in migration signaling in Ba/F3 cells. These data suggest that Crkl is involved in signaling pathways that regulate migration, possibly through a complex with Cbl and C3G.

Met-induced JNK activation is mediated by the adapter protein Crk and correlates with the Gab1 - Crk signaling complex formation

Constitutive activation of the Met tyrosine kinase results in transformation of cells of diverse origin. Recent studies have demonstrated a role for the c-Jun N-terminal kinase (JNK) in Met-induced transformation, but little is known about the molecular mechanisms that connect Met to JNK activation. Our studies show that activated Met associates with, and phosphorylates, the docking protein Gab1, which in turn binds to the src homology 2 (SH2)-domain of the adapter protein Crk and recruits Crk to the Met signaling complex. Formation of the Gab1 - Crk complex correlates with Met-induced JNK activation, and mutant forms of Met that fail to induce the complex formation also fail to activate JNK. Importantly, expression of a loss-of-function mutant of Crk severely impairs activation of the JNK pathway by Met. We also show here that Met controls the transcription of the matrix metalloproteinase-1 (MMP-1) gene in carcinoma cells and that this transcriptional regulation occurs in a Crk - JNK-dependent manner through an AP-1 element in the MMP-1 promoter. Taken together, our data implicate the Gab1 - Crk signaling complex in Met-induced JNK activation and suggest that the Gab1 - Crk complex formation may be an important event in regulating the tumorigenic phenotype of Met-transformed cells.