p130CAS forms a signaling complex with the adapter protein CRKL in hematopoietic cells transformed by the BCR/ABL oncogene

A p130Cas tyrosine phosphorylated substrate domain decoy disrupts v-crk signaling

BACKGROUND

The adaptor protein p130Cas (Cas) has been shown to be involved in different cellular processes including cell adhesion, migration and transformation. This protein has a substrate domain with up to 15 tyrosines that are potential kinase substrates, able to serve as docking sites for proteins with SH2 or PTB domains. Cas interacts with focal adhesion plaques and is phosphorylated by the tyrosine kinases FAK and Src. A number of effector molecules have been shown to interact with Cas and play a role in its function, including c-crk and v-crk, two adaptor proteins involved in intracellular signaling. Cas function is dependent on tyrosine phosphorylation of its substrate domain, suggesting that tyrosine phosphorylation of Cas in part regulates its control of adhesion and migration. To determine whether the substrate domain alone when tyrosine phosphorylated could signal, we have constructed a chimeric Cas molecule that is phosphorylated independently of upstream signals.

RESULTS

We found that a tyrosine phosphorylated Cas substrate domain acts as a dominant negative mutant by blocking Cas-mediated signaling events, including JNK activation by the oncogene v-crk in transient and stable lines and v-crk transformation. This block was the result of competition for binding partners as the chimera competed for binding to endogenous c-crk and exogenously expressed v-crk.

CONCLUSION

Our approach suggests a novel method to study adaptor proteins that require phosphorylation, and indicates that mere tyrosine phosphorylation of the substrate domain of Cas is not sufficient for its function.

Tyrosine kinase oncogenes in normal hematopoiesis and hematological disease

Tyrosine kinase oncogenes are formed as a result of mutations that induce constitutive kinase activity. Many of these tyrosine kinase oncogenes that are derived from genes, such as c-Abl, c-Fes, Flt3, c-Fms, c-Kit and PDGFRbeta, that are normally involved in the regulation of hematopoiesis or hematopoietic cell function. Despite differences in structure, normal function, and subcellular location, many of the tyrosine kinase oncogenes signal through the same pathways, and typically enhance proliferation and prolong viability. They represent excellent potential drug targets, and it is likely that additional mutations will be identified in other kinases, their immediate downstream targets, or in proteins regulating their function.

Crk family adaptors-signalling complex formation and biological roles

Crk family adaptors are widely expressed and mediate the timely formation of signal transduction protein complexes upon a variety of extracellular stimuli, including various growth and differentiation factors. Selective formation of multi-protein complexes by the Crk and Crk-like (CRKL) proteins depends on specific motifs recognized by their SH2 and SH3 domains. In the case of the first SH3 domains [SH3(1)] a P-x-x-P-x-K motif is crucial for highly selective binding, while the SH2 domains prefer motifs which conform to the consensus pY-x-x-P. Crk family proteins are involved in the relocalization and activation of several different effector proteins which include guanine nucleotide releasing proteins like C3G, protein kinases of the Abl- and GCK-families and small GTPases like Rap1 and Rac. Crk-type proteins have been found not only in vertebrates but also in flies and nematodes. Major insight into the function of Crk within organisms came from the genetic model organism C. elegans, where the Crk-homologue CED-2 regulates cell engulfment and phagocytosis. Other biological outcomes of the Crk-activated signal transduction cascades include the modulation of cell adhesion, cell migration and immune cell responses. Crk family adaptors also appear to play a role in mediating the action of human oncogenes like the leukaemia-inducing Bcr-Abl protein. This review summarizes some key findings and highlights recent insights and open questions.

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.

Functions of the adapter protein Cas: signal convergence and the determination of cellular responses

Since Cas was first identified as a highly phosphorylated 130 kilodalton protein that associated with the v-Src and v-Crk-oncoproteins, considerable effort has been made to determine its function. Its predicted role as a scaffolding molecule based on its domain structure has been largely confirmed. Through its ability to undergo rapid changes in phosphorylation, subcellular localization and association with heterologous proteins, Cas may spatially and temporally regulate the function of its binding partners. Numerous proteins have been identified that bind to Cas in vitro and/or in vivo, but in only a few cases is there an understanding of how Cas may function in these protein complexes. To date, Cas-Crk and Cas-Src complexes have been most frequently implicated in Cas function, particularly in regards to processes involving regulation of the actin cytoskeleton and proliferation. These and other Cas protein complexes contribute to the critical role of Cas in cell adhesion, migration, proliferation and survival of normal cycling cells. However, under conditions in which these processes are deregulated, Cas appears to play a role in oncogenic transformation and perhaps metastasis. Therefore, in its capacity as an adapter protein, Cas serves as a point of convergence for many distinct signaling inputs, ultimately contributing to the generation of specific cellular responses.

Cell adhesion-mediated drug resistance (CAM-DR) protects the K562 chronic myelogenous leukemia cell line from apoptosis induced by BCR/ABL inhibition, cytotoxic drugs, and gamma-irradiation

Integrin-mediated cellular adhesion to extracellular matrix (ECM) components is an important determinant of chemotherapeutic response of human myeloma cells. Here, we demonstrate that when K562 chronic myelogenous leukemia (CML) cells are adhered to fibronectin (FN), they become resistant to apoptosis induced by the BCR/ABL inhibitors AG957 and STI-571, as well as DNA damaging agents and gamma-irradiation. This phenomenon, termed cell adhesion-mediated drug resistance (CAM-DR), was induced by adhesion through the alpha5beta1 (VLA-5) integrin. Phosphotyrosine analysis demonstrates that anti-apoptotic signaling through integrins in K562 cells is independent of the tyrosine kinases activated by BCR/ABL, with the possible exception of an unknown 80 kDa protein. Cytoprotection of FN-adhered CML cells indicates that tumor-ECM interactions may be critical for the emergence of drug-resistant tumor populations and treatment failure in this disease. Antagonists of beta1 integrin-mediated adhesion or corresponding signal transduction elements may sensitize CML cells to chemotherapy and prevent resistance to the novel BCR/ABL kinase inhibitors being used for the treatment of this disease.

BCR-ABL fails to inhibit apoptosis in U937 myelomonocytic cells expressing a carboxyl-terminal truncated STAT5

Recent experimental data suggest that one of the major effects of BCR-ABL gene expression in hematopoietic cells is the inhibition of apoptosis. Although the exact mechanisms of this phenomenon are not clear, it is thought to be related to the fact that BCR-ABL induces several signalling pathways also activated by growth factors. In order to determine the anti-apoptotic role of BCR-ABL in a hematopoietic cell line and to by-pass the influence of cytokine-dependence, BCR-ABL gene was expressed in the autonomously growing myelomonocytic U937 cell line using retroviral vectors. There was no resistance to apoptosis induced by either serum deprivation or different doses of etoposide in any U937 clones expressing BCR-ABL protein. In addition to serum deprivation and etoposide, BCR-ABL-expressing clones were not protected from apoptosis induced by TNF, ceramide-C2 and FAS-cross-linking. BCL2 expression was absent in U937 cells and BAX levels were identical between Neo and BCR-ABL clones. To further investigate the mechanisms of this phenomenon, band-shift assays were performed to detect activation of STAT molecules. No constitutive activation of STATs was detected in either NeoR or BCR-ABL-U937 cells, although both IFN-gamma and GM-CSF activated STAT1 and STAT5, respectively, with similar kinetics in both NeoR and BCR-ABL-U937 cells. In addition, the GM-CSF-induced-STAT5 activation was found to be weakened in all clones expressing BCR-ABL. In both control NeoR and BCR-ABL-transfected clones, band-shift assays revealed the presence of an abnormal truncated STAT5 recognized only by an anti-N-terminal but not by an anti-C-Terminal STAT5 antibody. These findings suggest a possible link between the absence of anti-apoptotic potential of BCR-ABL and abnormalities of the STAT5 pathway, including, absence of constitutive activation of STAT5, inhibition of GM-CSF-induced STAT5 activation and expression of a carboxyl-terminal-truncated STAT5.

pp60c-src and related tyrosine kinases: a role in the assembly and reorganization of matrix adhesions

Activation of tyrosine kinases during integrin-mediated cell-matrix adhesion is involved both in the regulation of focal contact assembly and in the initiation of signaling processes at the cell-matrix adhesive interface. In order to determine the role of pp60c-src and related kinases in these processes, we have compared the dynamic reorganization of phosphotyrosine, vinculin, focal adhesion kinase and tensin in cells with altered expression of Src-family kinases. Both null cells for pp60c-src and triple knockout cells for pp60c-src, pp59fyn, and pp62c-yes exhibited decreased phosphotyrosine levels in focal contacts when compared with wild-type cells. pp60c-src-null cells also exhibited faster assembly of cell-matrix adhesions and a more exuberant recruitment of FAK to these sites. Tensin, which normally segregates into fibrillar adhesions was localized in large focal contacts in the two mutant cell lines, suggesting involvement of pp60c-src in the segregation of focal contacts and fibrillar adhesions. Moreover, treatment of wild-type cells with tyrphostin AG1007, which inhibits both pp60c-src and FAK activity, induced accumulation of tensin in peripheral focal adhesions. These findings demonstrate that Src family kinases, and pp60c-src in particular, have a central role in regulating protein dynamics at cell-matrix interfaces, both during early stages of interaction and in mature focal contacts.

Inhibition of cell growth and spreading by stomach cancer-associated protein-tyrosine phosphatase-1 (SAP-1) through dephosphorylation of p130cas

SAP-1 (stomach cancer-associated protein-tyrosine phosphatase-1) is a transmembrane-type protein-tyrosine phosphatase that is abundant in the brain and certain cancer cell lines. With the use of a "substrate-trapping" approach, p130(cas), a major focal adhesion-associated phosphotyrosyl protein, has now been identified as a likely physiological substrate of SAP-1. Expression of recombinant SAP-1 induced the dephosphorylation of p130(cas) as well as that of two other components of the integrin-signaling pathway (focal adhesion kinase and p62(dok)) in intact cells. In contrast, expression of a substrate-trapping mutant of SAP-1 induced the hyperphosphorylation of these proteins, indicating a dominant negative effect of this mutant. Overexpression of SAP-1 induced disruption of the actin-based cytoskeleton as well as inhibited various cellular responses promoted by integrin-mediated cell adhesion, including cell spreading on fibronectin, growth factor-induced activation of extracellular signal-regulated kinase 2, and colony formation. Finally, the enzymatic activity of SAP-1, measured with an immunocomplex phosphatase assay, was substantially increased by cell-cell adhesion. These results suggest that SAP-1, by mediating the dephosphorylation of focal adhesion-associated substrates, negatively regulates integrin-promoted signaling processes and, thus, may contribute to contact inhibition of cell growth and motility.

Mechanisms of transformation by the BCR/ABL oncogene

The Philadelphia chromosome generates a chimeric oncogene in which the BCR and c-ABL genes are fused. The product of this oncogene, BCR/ABL, has elevated ABL tyrosine kinase activity, relocates to the cytoskeleton, and phosphorylates multiple cellular substrates. BCR/ABL transforms hematopoietic cells and exerts a wide variety of biological effects, including reduction in growth factor dependence, enhanced viability, and altered adhesion of chronic myelocytic leukemia (CML) cells. Elevated tyrosine kinase activity of BCR/ABL is critical for activating downstream signal transduction and for all aspects of transformation. This review will describe mechanisms of transformation by the BCR/ABL oncogene and opportunities for clinical intervention with specific signal transduction inhibitors such as STI-571 in CML.

Molecular abnormalities in chronic myeloid leukemia: deregulation of cell growth and apoptosis

Chronic myeloid leukemia (CML) is a disease of the hematopoietic system, characterized by the presence of the Bcr-Abl oncoprotein. The main characteristics of this disease include adhesion independence, growth factor independence, and resistance to apoptosis. Loss or mutation of the tumor suppressor gene, p53, is one of the most frequent secondary mutations in CML blast crisis. The transition between chronic phase and blast crisis is associated with increased resistance to apoptosis correlating with poor prognosis. This review focuses on the involvement of these two oncoproteins in the development and progression of the apoptotic-resistant phenotype in CML.

Src and Cas mediate JNK activation but not ERK1/2 and p38 kinases by reactive oxygen species

c-Jun NH(2)-terminal kinase (JNK) is activated by a number of cellular stimuli such as inflammatory cytokines and environmental stresses. Reactive oxygen species also cause activation of JNK; however, the signaling cascade that leads to JNK activation remains to be elucidated. Because recent reports showed that expression of Cas, a putative Src substrate, stimulates JNK activation, we hypothesized that the Src kinase family and Cas would be involved in JNK activation by reactive oxygen species. An essential role for both Src and Cas was demonstrated. First, the specific Src family tyrosine kinase inhibitor, PP2, inhibited JNK activation by H(2)O(2) in a concentration-dependent manner but had no effect on extracellular signal-regulated kinases 1 and 2 and p38 activation. Second, JNK activation in response to H(2)O(2) was completely inhibited in cells derived from transgenic mice deficient in Src but not Fyn. Third, expression of a dominant negative mutant of Cas prevented H(2)O(2)-mediated JNK activation but had no effect on extracellular signal-regulated kinases 1 and 2 and p38 activation. Finally, the importance of Src was further supported by the inhibition of both H(2)O(2)-mediated Cas tyrosine phosphorylation and Cas.Crk complex formation in Src-/- but not Fyn-/- cells. These results demonstrate an essential role for Src and Cas in H(2)O(2)-mediated activation of JNK and suggest a new redox-sensitive pathway for JNK activation mediated by Src.

Stromal cell-derived factor-1α stimulates tyrosine phosphorylation of multiple focal adhesion proteins and induces migration of hematopoietic progenitor cells: roles of phosphoinositide-3 kinase and protein kinase C

The stromal cell-derived factor-1 (SDF-1) is an alpha chemokine that binds to the CXCR4 receptor. Knock-out studies in mice demonstrate that this ligand-receptor pair is essential in hematopoiesis. One function of SDF-1 appears to be the regulation of migration of hematopoietic progenitor cells. We previously characterized signal transduction pathways induced by SDF-1 in human hematopoietic progenitors and found tyrosine phosphorylation of focal adhesion components, including the related adhesion focal tyrosine kinase (RAFTK), the adaptor molecule p130 Cas, and the cytoskeletal protein paxillin. To better understand the functional role of signaling molecules connecting the CXCR4 receptor to the process of hematopoietic migration, we studied SDF-1–mediated pathways in a model hematopoietic progenitor cell line (CTS), as well as in primary human bone marrow CD34+cells. We observed that several other focal adhesion components, including focal adhesion kinase (FAK) and the adaptor molecules Crk and Crk-L, are phosphorylated on SDF-1 stimulation. Using a series of specific small molecule inhibitors, both protein kinase C (PKC) and phosphoinositide-3 kinase (PI-3K) appeared to be required for SDF-1–mediated phosphorylation of focal adhesion proteins and the migration of both CTS and primary marrow CD34+ cells, whereas the mitogen-activated protein kinases ERK-1 and -2 were not. These studies further delineate the molecular pathways mediating hematopoietic progenitor migration and response to an essential chemokine, SDF-1.

Stromal cell-derived factor-1α stimulates tyrosine phosphorylation of multiple focal adhesion proteins and induces migration of hematopoietic progenitor cells: roles of phosphoinositide-3 kinase and protein kinase C

The stromal cell-derived factor-1 (SDF-1) is an alpha chemokine that binds to the CXCR4 receptor. Knock-out studies in mice demonstrate that this ligand-receptor pair is essential in hematopoiesis. One function of SDF-1 appears to be the regulation of migration of hematopoietic progenitor cells. We previously characterized signal transduction pathways induced by SDF-1 in human hematopoietic progenitors and found tyrosine phosphorylation of focal adhesion components, including the related adhesion focal tyrosine kinase (RAFTK), the adaptor molecule p130 Cas, and the cytoskeletal protein paxillin. To better understand the functional role of signaling molecules connecting the CXCR4 receptor to the process of hematopoietic migration, we studied SDF-1–mediated pathways in a model hematopoietic progenitor cell line (CTS), as well as in primary human bone marrow CD34+cells. We observed that several other focal adhesion components, including focal adhesion kinase (FAK) and the adaptor molecules Crk and Crk-L, are phosphorylated on SDF-1 stimulation. Using a series of specific small molecule inhibitors, both protein kinase C (PKC) and phosphoinositide-3 kinase (PI-3K) appeared to be required for SDF-1–mediated phosphorylation of focal adhesion proteins and the migration of both CTS and primary marrow CD34+ cells, whereas the mitogen-activated protein kinases ERK-1 and -2 were not. These studies further delineate the molecular pathways mediating hematopoietic progenitor migration and response to an essential chemokine, SDF-1.

The leukaemic oncoproteins Bcr-Abl and Tel-Abl (ETV6/Abl) have altered substrate preferences and activate similar intracellular signalling pathways

Inappropriate activation of Abl family kinases plays a crucial role in different human leukaemias. In addition to the well known oncoproteins p190Bcr-Abl and p210Bcr-Abl, Tel-Abl, a novel fusion protein resulting from a different chromosomal translocation, has recently been described. In this study, the kinase specificities of the Bcr-Abl and Tel-Abl proteins were compared to the physiological Abl family kinases c-Abl and Arg (abl related gene). Using short peptides which correspond to the target epitopes in known substrate proteins of Abl family kinases, we found a higher catalytic promiscuity of Bcr-Abl and Tel-Abl. Similar to Bcr-Abl, Tel-Abl was found in complexes with the adapter protein CRKL. In addition, c-Crk II and CRKL are tyrosine phosphorylated and complexed with numerous other tyrosine phosphorylated proteins in Tel-Abl expressing Ba/F3 cells. GTPase analysis with a Ras-GTP-specific precipitation assay showed constitutive elevation of GTP-loaded Ras in cells expressing the leukaemic Abl proteins. The mitogenic MAPK/Erk kinases as well as Akt/PKB, a kinase implicated to negatively regulate apoptosis, were also constitutively activated by both Bcr-Abl and Tel-Abl. The results indicate that the leukaemic Abl-fusion proteins have catalytic specificities different from the normal kinases c-Abl and Arg and that Tel-Abl is capable to activate at least some pathways which are also upregulated by Bcr-Abl.

Caspase-mediated cleavage of p130cas in etoposide-induced apoptotic Rat-1 cells

Apoptosis causes characteristic morphological changes in cells, including membrane blebbing, cell detachment from the extracellular matrix, and loss of cell-cell contacts. We investigated the changes in focal adhesion proteins during etoposide-induced apoptosis in Rat-1 cells and found that during apoptosis, p130cas (Crk-associated substrate [Cas]) is cleaved by caspase-3. Sequence analysis showed that Cas contains 10 DXXD consensus sites preferred by caspase-3. We identified two of these sites (DVPD(416)G and DSPD(748)G) in vitro, and point mutations substituting the Asp of DVPD(416)G and DSPD(748)G with Glu blocked caspase-3-mediated cleavage. Cleavage at DVPD(416)G generated a 74-kDa fragment, which was in turn cleaved at DSPD(748)G, yielding 47- and 31-kDa fragments. Immunofluorescence microscopy revealed well-developed focal adhesion sites in control cells that dramatically declined in number in etoposide-treated cells. Cas cleavage correlated temporally with the onset of apoptosis and coincided with the loss of p125FAK (focal adhesion kinase [FAK]) from focal adhesion sites and the attenuation of Cas-paxillin interactions. Considering that Cas associates with FAK, paxillin, and other molecules involved in the integrin signaling pathway, these results suggest that caspase-mediated cleavage of Cas contributes to the disassembly of focal adhesion complexes and interrupts survival signals from the extracellular matrix.

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.

Shear stress stimulation of p130(cas) tyrosine phosphorylation requires calcium-dependent c-Src activation

Fluid shear stress (flow) modulates endothelial cell function via specific intracellular signaling events. Previously we showed that flow activated ERK1/2 in an integrin-dependent manner (Takahashi, M., and Berk, B. C. (1996) J. Clin. Invest. 98, 2623-2631). p130 Crk-associated substrate (Cas), a putative c-Src substrate, was originally identified as a highly phosphorylated protein that is localized to focal adhesions and acts as an adapter protein. Recent reports have shown that Cas is important in cardiovascular development and actin filament assembly. Flow (shear stress = 12 dynes/cm(2)) stimulated Cas tyrosine phosphorylation within 1 min in human umbilical vein endothelial cells. Phosphorylation peaked at 5 min (3.5 +/- 0.7-fold) and was sustained to 20 min. Tyrosine phosphorylation of Cas was functionally important because flow stimulated association of Cas with Crk in a time- and force-dependent manner. Flow-mediated activation of c-Src, phosphorylation of Cas, and association of Cas with Crk were all inhibited by calcium chelation and pretreatment with the Src family-specific tyrosine kinase inhibitor PP1. To determine the role of c-Src in flow-stimulated phosphorylation of Cas, we transduced cells with adenovirus encoding kinase-inactive Src. Expression of kinase-inactive Src prevented flow-induced Cas tyrosine phosphorylation but not ERK1/2 activation. Calcium-dependent activation of c-Src and tyrosine phosphorylation of Cas defines a new flow-stimulated signal pathway, different from ERK1/2 activation. This pathway may be involved in focal adhesion remodeling and actin filament assembly.

Role of abnormal integrin-cytoskeletal interactions in impaired beta1 integrin function in chronic myelogenous leukemia hematopoietic progenitors

Abnormal circulation and unregulated proliferation of chronic myelogenous leukemia (CML) progenitors is related, at least in part, to BCR/ABL induced abnormalities in beta1 integrin-mediated adhesion and signaling. The BCR/ABL oncogene has several potential interactions with cytoskeletal elements that are important for normal integrin signaling. In the present study, we evaluated whether abnormalities in beta1 integrin-cytoskeletal interactions were present in primary CML progenitors and contributed to defective integrin function. beta1 integrin-cytoskeletal interactions were studied in CML and normal CD34+ primary hematopoietic progenitors as well as BCR/ABL-transfected or mock-transfected M07e cells. In normal CD34+ progenitors, antibody-mediated cross-linking of beta1 integrins resulted in their redistribution into caps via a process requiring receptor-cytoskeletal interactions. CML CD34+ cells demonstrated significantly impaired beta1 integrin capping. This defect was related to the presence of the BCR/ABL gene, because capping also was impaired in BCR/ABL-transfected M07e cells. Defective receptor capping was not seen for non-integrin receptors. In addition, CML CD34- and M07eBCR/ABL cells also demonstrated increased actin polymerization and altered actin cytoskeletal organization. Further studies suggested that impaired beta1 integrin capping and defective integrin-mediated adhesion and proliferation inhibition in CML cells were related to abnormally enhanced integrin-cytoskeletal association and restricted receptor mobility. Finally, interferon alpha, which restores integrin-mediated adhesion and signaling in CML progenitors, also enhanced integrin capping in CD34+ cells. These studies suggest that p210BCR/ABL induces abnormal association of integrin receptors with the cytoskeleton and restricted receptor mobility and provide new insights into mechanisms underlying abnormal integrin function in CML progenitors.

Cytoskeleton-dependent tyrosine phosphorylation of the p130(Cas) family member HEF1 downstream of the G protein-coupled calcitonin receptor. Calcitonin induces the association of HEF1, paxillin, and focal adhesion kinase

HEF1 is a recently described p130(Cas)-like docking protein that contains one SH3 domain and multiple SH2 binding motifs. In B cells, HEF1 is phosphorylated by a cytoskeleton-dependent mechanism that is triggered by integrin ligation. However, the induction of HEF1 phosphorylation by G protein-coupled receptors has not been reported. We found that HEF1, but not p130(Cas), is tyrosine-phosphorylated following stimulation of the rabbit C1a calcitonin receptor stably expressed in HEK-293 cells. The calcitonin-induced tyrosine phosphorylation of HEF1 increased in a time- and dose-dependent manner. Dibutyryl cAMP and forskolin had little or no effect on HEF1 phosphorylation, and the protein kinase A inhibitor H89 failed to detectably inhibit the response to calcitonin, indicating that the G(s)/cAMP/protein kinase A pathway does not mediate the calcitonin effect. Pertussis toxin, which selectively blocks G(i/o) signaling, also had no effect. Increasing cytosolic Ca(2+) with ionomycin stimulated HEF1 phosphorylation and preventing any calcitonin-induced change in cytosolic calcium by a combination of BAPTA and extracellular EGTA completely blocked the calcitonin-induced tyrosine phosphorylation of HEF1. Phorbol 12-myristate 13-acetate also induced HEF1 tyrosine phosphorylation, and the protein kinase C inhibitor calphostin C completely inhibited both calcitonin- and phorbol 12-myristate 13-acetate-stimulated HEF1 phosphorylation. Calcitonin also induced the tyrosine phosphorylation of paxillin and focal adhesion kinase, and the association of these two proteins with HEF1. Pretreatment with cytochalasin D, which disrupts actin microfilaments, prevented the calcitonin-induced HEF1 and paxillin phosphorylation. In conclusion, the calcitonin-stimulated tyrosine phosphorylation of HEF1 is mediated by calcium- and protein kinase C-dependent mechanisms and requires the integrity of the actin cytoskeleton.

p130(Cas), an assembling molecule of actin filaments, promotes cell movement, cell migration, and cell spreading in fibroblasts

p130(Cas) (Cas) is an adaptor molecule which becomes tyrosine phosphorylated by v-Src- or v-Crk-triggered transformation and several physiological stimuli, such as cell attachment to fibronectin. We previously generated mice lacking Cas and demonstrated that Cas functions as an assembling molecule of actin filaments. To further explore Cas role in cellular function, we established Cas-deficient and Cas-re-expressing fibroblasts and compared their behaviors in response to several biological stimuli. We found that Cas-deficient fibroblasts showed significant defects in cell movement after mechanical wounding and in cell migration toward fibronectin as compared with Cas-re-expressing cells. In addition, when plated on fibronectin-coated dishes, Cas-deficient cells exhibited a significant delay in cell spreading as compared with Cas-re-expressing cells albeit that protein-tyrosine phosphorylation was similarly induced. These results demonstrated that Cas functions as a molecule promoting cell movement, cell migration, and cell spreading and suggest that Cas would be implicated in various physiological and pathological processes, such as would healing, chemotaxis, and tumor invasion.

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.

Involvement of the adapter protein CRKL in integrin-mediated adhesion

CRKL, an SH2-SH3-SH3 adapter protein, is one of the major tyrosine phosphoproteins detected in primary leukemic neutrophils from patients with CML. CRKL binds directly to BCR/ABL through its N-terminal SH3 domain, suggesting it may be involved in BCR/ABL signal transduction. However, the biological function of CRKL in either normal or leukemic cells is still largely unknown. In this study, we have examined the effects of overexpressing full length or deletion mutants of CRKL in hematopoietic cell lines. Full length, SH2- and SH3(N)-domain deletion mutants of CRKL were transfected into an interleukin-3-dependent hematopoietic cell line, Ba/F3, and 3-5 individual sublines which stably overexpressed each transgene were obtained [Ba/F-CRKL, Ba/F-CRKL deltaSH2, and Ba/F-CRKL deltaSH3(N)]. The growth properties of these transfected cells in the presence or absence of IL-3 were not different from mock transfected or untransfected Ba/F3 cells. However, Ba/F3 cells overexpressing full length CRKL, but not deletion mutants of CRKL, were found to have an increase in their ability to bind to fibronectin-coated surfaces. Further, expression of full length, but not deltaSH2- or deltaSH3-CRKL deletion mutants, was found to alter cell morphology on fibronectin-coated plates, an effect which was further enhanced by certain kinds of stress stimuli, such as ionizing radiation. Similar results were obtained when CRKL was transiently overexpressed in Ba/F3 cells, and were also obtained in a second IL-3 dependent hematopoietic cell line, 32Dcl3. Adhesion to fibronectin was blocked by anti-beta1 integrin monoclonal antibody, but overexpression of CRKL did not affect surface expression of beta1 integrins, nor did it spontaneously induce expression of the beta1 integrin 'activation' epitope recognized by the 9EG7 monoclonal antibody. These data suggest a role for CRKL in signaling pathways which regulate adhesion to fibronectin.

CrkL Activates Integrin-Mediated Hematopoietic Cell Adhesion Through the Guanine Nucleotide Exchange Factor C3G

CrkL is a member of the Crk family of adapter proteins consisting mostly of SH2 and SH3 domains. CrkL is most abundantly expressed in hematopoietic cells and has been implicated in pathogenesis of chronic myelogenous leukemia. However, its function has not been precisely defined. Here, we show that overexpression of CrkL enhances the adhesion of hematopoietic 32D cells to fibronectin. The CrkL-induced increase in cell adhesion was blocked by antibodies against VLA-4 (4β1) and VLA-5 (5β1) but was observed without changes in surface expression levels of these integrins. Studies using CrkL mutants demonstrated that the SH2 domain is partially required for enhancing cell adhesion, whereas the C-terminal SH3 domain as well as the tyrosine phosphorylation site (Y207) is dispensable. In contrast, the N-terminal SH3 domain, involved in binding C3G and other signaling molecules, was showed to play a crucial role, because a mutant defective of this domain showed an inhibitory effect on the cell adhesion to fibronectin. Furthermore, overexpression of C3G also increased the adhesion of hematopoietic cells to fibronectin, whereas a C3G mutant lacking the guanine nucleotide exchange domain abrogated the CrkL-induced increase in cell adhesion. On the other hand, a dominant negative mutant of H-Ras or that of Raf-1 enhanced the basal and CrkL-induced cell adhesion and that of R-Ras modestly decreased the adhesion. Taken together, these results indicate that the CrkL-C3G complex activates VLA-4 and VLA-5 in hematopoietic cells, possibly by activating the small GTP binding proteins, including R-Ras, through the guanine nucleotide exchange activity of C3G.

CrkL Activates Integrin-Mediated Hematopoietic Cell Adhesion Through the Guanine Nucleotide Exchange Factor C3G

CrkL is a member of the Crk family of adapter proteins consisting mostly of SH2 and SH3 domains. CrkL is most abundantly expressed in hematopoietic cells and has been implicated in pathogenesis of chronic myelogenous leukemia. However, its function has not been precisely defined. Here, we show that overexpression of CrkL enhances the adhesion of hematopoietic 32D cells to fibronectin. The CrkL-induced increase in cell adhesion was blocked by antibodies against VLA-4 (4β1) and VLA-5 (5β1) but was observed without changes in surface expression levels of these integrins. Studies using CrkL mutants demonstrated that the SH2 domain is partially required for enhancing cell adhesion, whereas the C-terminal SH3 domain as well as the tyrosine phosphorylation site (Y207) is dispensable. In contrast, the N-terminal SH3 domain, involved in binding C3G and other signaling molecules, was showed to play a crucial role, because a mutant defective of this domain showed an inhibitory effect on the cell adhesion to fibronectin. Furthermore, overexpression of C3G also increased the adhesion of hematopoietic cells to fibronectin, whereas a C3G mutant lacking the guanine nucleotide exchange domain abrogated the CrkL-induced increase in cell adhesion. On the other hand, a dominant negative mutant of H-Ras or that of Raf-1 enhanced the basal and CrkL-induced cell adhesion and that of R-Ras modestly decreased the adhesion. Taken together, these results indicate that the CrkL-C3G complex activates VLA-4 and VLA-5 in hematopoietic cells, possibly by activating the small GTP binding proteins, including R-Ras, through the guanine nucleotide exchange activity of C3G.

Identification of a Drosophila homologue to vertebrate Crk by interaction with MBC

The vertebrate adapter protein termed Crk was initially identified from the chicken CT10 retrovirus on the basis of its transforming activity (Mayer et al., 1988. Nature 332, 272-275). We have identified a Drosophila protein with homology to vertebrate Crk, termed dCRK, by interaction with the protein encoded by the Drosophila myoblast city (mbc) gene. The dCRK protein has extensive homology to the both the Crk-II form of vertebrate Crk and the Crk-related protein CRKL, and includes one SH2 domain followed by two SH3 domains. A single protein of approx. 37kDa is detected in extracts from embryos, and Northern analysis revealed a single transcript of 1.3kb. The dCrk mRNA is abundant throughout embryogenesis, declines during the larval stages, and reappears during pupation. In addition, maternally-provided transcripts have been detected. During embryogenesis, the spatial distribution of this transcript is relatively broad and appears to include all germ layers. Finally, dCrk is located on the fourth chromosome, approximately at cytological position 101F-102A.

Transmembrane tyrosine phosphatase LAR induces apoptosis by dephosphorylating and destabilizing p130Cas

BACKGROUND

LAR is a transmembrane receptor-like protein tyrosine phosphatase (PTP). Genetic studies of Drosophila LAR suggest that LAR may function to regulate cell adhesions or adhesion-mediated signal transduction. The over-expression of LAR in mammalian tissue culture cells does not affect cell adhesion but induces caspase-dependent apoptosis. This study investigates molecular mechanisms of LAR-induced apoptosis by searching for in vivo substrates of LAR which are responsible for LAR-induced apoptosis.

RESULTS

The over-expression of LAR in tissue culture cells specifically decreased the steady state protein level of p130Cas, a multifunctional signal assembly protein in signal transduction, by reducing the tyrosine phosphorylation and protein stability of p130Cas. The reduction of p130Cas protein level could be inhibited by tyrosine phosphatase inhibitors. Phosphatase domain-deleted mutant LARs had no effect on p130Cas. LAR also preferentially dephosphorylated p130Cas in vitro. Subcellularly, LAR and p130Cas were co-localized along stress fibres and at focal adhesions. LAR over-expression eliminated p130Cas from focal adhesions without affecting focal adhesion assembly. Restoring the level of p130Cas alleviated LAR-induced apoptosis.

CONCLUSIONS

p130Cas is an in vivo substrate of LAR. LAR specifically dephosphorylates and destabilizes p130Cas and may play a role in regulating cell adhesion-mediated cell survival. The function of p130Cas in focal adhesions may not be to regulate focal adhesion assembly and cell adhesion but rather to transduce the cell adhesion-generated signals which are essential for cell survival.

Expression of the focal adhesion protein paxillin in lung cancer and its relation to cell motility

Lung cancer can lead to abnormalities of the actin cytoskeleton structure which may be important in transformation. In this study, we have investigated the expression of the cytoskeletal associated protein paxillin in lung cancer. Paxillin is a 68 kDa focal adhesion protein, with four tandem LIM domains at the C-terminus, involved in growth factor receptor, integrin and oncogenic signaling such as v-src, BCR/ABL, and E6 of the papilloma virus. In non-small cell lung cancer (NSCLC) cell lines, paxillin localized to the focal adhesions. The possible role of paxillin in lung cancer cells was assessed by overexpressing green fluorescence protein (GFP)-paxillin construct in two separate NSCLC cell lines (Calu-1 and H661). Over the course of 48 h, GFP-paxillin consistently caused the cells to become round and to decrease cell motility as compared to normal controls, GFP-N-terminus paxillin, or GFP-LIM transfected cells. Because some lung cancers may be quite aggressive and metastasize quickly, which may be related to the cytoskeleton, we determined the expression of paxillin in NSCLC and small cell lung cancer (SCLC) cell lines and patient tumor tissues. Expression of paxillin in NSCLC and SCLC cell lines were determined by Northern blot and Western blot analysis. The expression of paxillin was consistently low in SCLC cell lines, whereas there was paxillin expression in NSCLC cell lines. There was a variability of expression of paxillin in NSCLC tumor tissue as compared to normal lung tissue. In contrast, by immunohistochemistry, we show that there was no detectable expression of paxillin in 5/5 SCLC patients. This data suggests that absence or low level of paxillin protein expression may cause certain lung cancers, such as SCLC, to be more motile and possibly more aggressive.

Dominant effects of the bcr-abl oncogene on Drosophila morphogenesis

We targeted expression of human/fly chimeric Bcr-Abl proteins to the developing central nervous system (CNS) and eye imaginal disc of Drosophila melanogaster. Neural expression of human/fly chimeric P210 Bcr-Abl or P185 Bcr-Abl rescued abl mutant flies from pupal lethality, indicating that P210 and P185 Bcr-Abl can substitute functionally for Drosophila Abl during axonogenesis. However, increased levels of neurally expressed P210 or P185 Bcr-Abl but not Drosophila Abl produced CNS defects and lethality. Expression of P210 or P185 in the eye imaginal disc produced a dominant rough eye phenotype that was dependent on dosage of the transgene. Drosophila Enabled, previously identified as a suppressor of the abl mutant phenotype and substrate for Drosophila Abl kinase, had markedly increased phosphotyrosine levels in Bcr-Abl expressing Drosophila, indicating that it is a substrate for Bcr-Abl as well. Drosophila, therefore, is a suitable model system to identify Bcr-Abl interactions important for signal transduction and oncogenesis.

A cdc15-like adaptor protein (CD2BP1) interacts with the CD2 cytoplasmic domain and regulates CD2-triggered adhesion

A human CD2 cytoplasmic tail-binding protein, termed CD2BP1, was identified by an interaction trap cloning method. Expression of CD2BP1 is restricted to hematopoietic tissue, being prominent in T and natural killer (NK) cells, with long (CD2BP1L) and short (CD2BP1S) variants arising by alternative RNA splicing. Both CD2BP1 molecules are homologous to Schizosaccharomyces pombe cdc15, and include a helical domain, variable length intervening PEST sequence and C-terminal SH3 domain. Although the CD2BP1 SH3 domain binds directly to the CD2 sequence, KGPPLPRPRV (amino acids 300-309), its association is augmented markedly by the CD2BP1 N-terminal segment. Upon ligand-induced clustering of surface CD2 molecules, CD2BP1 redistributes from a cytosolic to a surface membrane compartment, co-localizing with CD2. In turn, CD2-stimulated adhesion is downregulated by CD2BP1, apparently through coupling of the protein tyrosine phosphatase (PTP)-PEST to CD2. These findings offer the first molecular view into the control processes for T cell adhesion.

BCR-ABL oncoprotein is expressed by platelets from CML patients and associated with a special pattern of CrkL phosphorylation

Constitutive tyrosine phosphorylation of CrkL was recently demonstrated in platelets from chronic myelogenous leukaemia (CML) patients but BCR-ABL tyrosine kinase could not be detected in the platelet lysates. We studied platelets from 14 CML patients with different types of BCR-ABL mRNA and with maximal platelet counts ranging from 149 to 3069 x 10(9)/l. P210BCR-ABL protein was detected by Western blotting in platelet lysates of 12/13 CML patients with active disease but not in the lysate of platelets from a Ph-positive acute lymphoblastic leukaemia (ALL) patient in remission or eight BCR-ABL-negative controls including one essential thrombocythaemia (ET) patient. Immunoblotting of p210BCR-ABL-positive platelets lysates with anti-CrkL antibody revealed a CrkL triplet consisting of one unphosphorylated and two phosphorylated forms of the protein. This CrkL phosphorylation pattern was not observed in normal platelets or CML platelets treated with ABL tyrosine kinase inhibitor CGP57148B. The presence of BCR-ABL provides an explanation for the constitutive tyrosine phosphorylation of CrkL in CML platelets. As no correlation was observed between platelet counts and platelet BCR-ABL protein expression, thrombocytosis or thrombocythaemia in CML cannot be explained by constitutive BCR-ABL-mediated CrkL tyrosine phosphorylation.

Association of CrkL with STAT5 in hematopoietic cells stimulated by granulocyte-macrophage colony-stimulating factor or erythropoietin

CrkL is an adapter protein comprising Src homology (SH) 2 and SH3 domains. We investigated the molecule(s) associated with CrkL in factor-dependent cell lines. In the granulocyte-macrophage colony-stimulating factor (GM-CSF)-dependent cell lines TF-1 and UT-7, an approximately 95-kDa tyrosine-phosphorylated protein was precipitated along with CrkL after GM-CSF stimulation. The same protein was also observed when we used the erythropoietin (EPO)-dependent cell line UT-7/EPO, in an EPO stimulation-dependent manner. We identified it as STAT5 (signal transducer and activator of transcription 5, 96 kDa) by STAT5-specific antibodies. The direct binding of the SH2 domain of CrkL to STAT5 was demonstrated in far Western blotting and pull-down experiments using the glutathione S-transferase (GST) fusion construct CrkL-SH2. The addition of the oligopeptide containing phosphotyrosine 694 in STAT5A impaired the association between GST-CrkL-SH2 and STAT5. Furthermore, in a gel shift assay using prolactin-inducible element (PIE) as the probe, the DNA binding activity of STAT5 was inhibited by the interaction with GST-CrkL-SH2 in vitro. Finally, we found that STAT5 associated with CrkL did not bind to PIE sequence. These results suggest that CrkL participates in the Janus kinase (JAK)-STAT pathway by direct association with STAT5.

Uncoupling signal transducers from oncogenic MET mutants abrogates cell transformation and inhibits invasive growth

The assumption that genes encoding tyrosine kinase receptors could play a role in human cancers has been confirmed by the identification of oncogenic mutations in the kinase domain of RET and KIT. Recently, homologous residues were found mutated in MET, in papillary renal carcinomas (PRCs). The link coupling these genetic lesions to cellular transformation is still unclear. METPRC mutations result in increased kinase activity and-in some instances, i.e., M1250T substitution-in changes in substrate specificity. A direct correlation occurs between the transforming potential of METPRC mutants and their ability to constitutively associate with signal transducers through two phosphorylated tyrosines (Y1349VHVNATY1356VNV) located in the receptor tail. Substitution of these "docking tyrosines" with phenylalanines leaves unaffected the altered properties of the kinase but abrogates transformation and invasiveness in vitro. Uncoupling the receptor from signal transducers with a tyrosine-phosphorylated peptide derivative (YpVNV) inhibits invasive growth induced by METPRC mutants. These data indicate that constitutive receptor coupling to downstream signal transducers is a key mechanism in neoplastic transformation driven by mutated MET and suggest a therapeutic strategy to target neoplastic diseases associated with this oncogene.

PR-39, a syndecan-inducing antimicrobial peptide, binds and affects p130(Cas)

PR-39 is a proline-arginine-rich antimicrobial peptide and an important component of innate immunity. In addition to its antimicrobial effects, PR-39 can alter mammalian cell gene expression and behavior. To determine the mechanism through which PR-39 affects mesenchymal cells, we identify a number of binding targets for PR-39 using a biologically active fragment of PR-39 (PR-39(15)). We found that PR-39 binds NIH 3T3 in a saturable manner consistent with the existence of a binding target. Similar to full-length PR-39, PR-39(15) interacts with lipid bilayers. After interacting with the membrane, PR-39(15) rapidly enters human microvascular endothelial cells and binds a number of cytoplasmic proteins. PR-39 selectively binds recombinant SH3-containing proteins and was also found to bind a native SH3-containing protein, p130(Cas). PR-39(15) treatment of endothelial cells results in altered p130 localization. These results show that PR-39(15) binds an SH3-containing signal transduction molecule that has the potential to explain a myriad of effects PR-39 has on mammalian cell behaviors.

Structural requirements for function of the Crkl adapter protein in fibroblasts and hematopoietic cells

Crkl is an adapter protein and phosphotyrosine-containing substrate implicated in transformation by the bcr-abl oncogene and in signaling by cytokines. When phosphorylated, Crkl binds through its Src homology 2 (SH2) domain to other tyrosine phosphoproteins such as paxillin and Cbl. Overexpression of Crkl in fibroblasts induces transformation. Here we examine the role of Crkl in hematopoietic cells and find that overexpression of Crkl confers a signal leading to increased adhesion to fibronectin. In both fibroblasts and hematopoietic cells, individual mutations or deletions of each SH2 and SH3 domain abrogated transformation and adhesion, respectively, indicating that interactions with other proteins such as Cbl and paxillin (SH2 domain) and Abl, Sos, and C3G (N-terminal SH3 domain) are essential for biological activity. In vivo and in vitro tryptic phosphopeptide mapping studies show that Crkl is phosphorylated on multiple tyrosine residues when overexpressed or when activated by Bcr-Abl. Mutation at tyrosine 207, a residue conserved in c-Crk, abrogates all in vivo tyrosine phosphorylation of Crkl. Despite this loss of phosphotyrosine, mutation at this site enhanced Crkl function as measured by complex formation with SH2 binding proteins, signal transduction to Jun Kinase, and fibroblast transformation. These observations implicate Crkl in cellular adhesion and demonstrate that Y207 functions as a negative regulatory site.

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

We have used mutant macrophages which are deficient in expression of Src-family kinases to define an integrin signaling pathway that is required for macrophage adhesion and migration. Following ligation of surface integrins by fibronectin, the p120(c-cbl) (Cbl) protein rapidly becomes tyrosine phosphorylated and associated with the Src-family kinases Fgr and Lyn. In hck-/-fgr-/-lyn-/- triple mutant cells, which are defective in spreading on fibronectin-coated surfaces in vitro and show impaired migration in vivo, Cbl tyrosine phosphorylation is blocked, Cbl protein levels are low, adhesion-dependent translocation of Cbl to the membrane is impaired and Cbl-associated, membrane-localized phosphatidylinositol 3 (PI-3)-kinase activity is dramatically reduced. In contrast, adhesion-dependent activation of total cellular PI-3 kinase activity is normal in mutant cells, demonstrating that it is the membrane-associated fraction of PI-3 kinase which is most critical in regulating actin cytoskeletal rearrangements that lead to cell spreading. Treatment of wild-type cells with the Src-family-specific inhibitor PP1, Cbl antisense oligonucleotides or pharmacological inhibitors of PI-3 kinase blocks cell spreading on fibronectin surfaces. These data provide a molecular description for the role of Src-family kinases Hck, Fgr and Lyn in beta 1-integrin signal transduction in macrophages.

Phosphorylation of CrkII adaptor protein at tyrosine 221 by epidermal growth factor receptor

CrkII adaptor protein becomes tyrosine-phosphorylated upon various types of stimulation. We examined whether tyrosine 221, which has been shown to be phosphorylated by c-Abl, was phosphorylated also by other tyrosine kinases, such as epidermal growth factor (EGF) receptor. For this purpose, we developed an antibody that specifically recognizes Tyr221-phosphorylated CrkII, and we demonstrated that CrkII was phosphorylated on Tyr221 upon EGF stimulation. When NRK cells were stimulated with EGF, the tyrosine-phosphorylated CrkII was detected at the periphery of the cells, where ruffling is prominent, suggesting that signaling to CrkII may be involved in EGF-dependent cytoskeletal reorganization. The EGF-dependent phosphorylation of CrkII was also detected in a c-Abl-deficient cell line. Moreover, recombinant CrkII protein was phosphorylated in vitro by EGF receptor. These results strongly suggest that EGF receptor directly phosphorylates CrkII. Mutational analysis revealed that the src homology 2 domain was essential for the phosphorylation of CrkII by EGF receptor but not by c-Abl, arguing that these kinases phosphorylate CrkII by different phosphorylation mechanisms. Finally, we found that the CrkII protein phosphorylated upon EGF stimulation did not bind to the phosphotyrosine-containing peptide and that CrkII initiated dissociation from EGF receptor within 3 min even with the sustained tyrosine phosphorylation of EGF receptor. This result implicated phosphorylation of Tyr221 in the negative regulation of the src homology 2-mediated binding of CrkII to EGF receptor.

Direct association of protein-tyrosine phosphatase PTP-PEST with paxillin

Tyrosine phosphorylation of focal adhesion-associated proteins may be involved in the regulation of the cytoskeleton and in the control of signals for growth and survival. The focal adhesion kinase (FAK) functions in regulating tyrosine phosphorylation of several of these proteins, including paxillin, tensin, and p130(cas). Protein- tyrosine phosphatases, the counterparts of protein-tyrosine kinases, also presumably regulate phosphorylation of these proteins. We have tested the hypothesis that FAK intimately associates with a protein-tyrosine phosphatase. Protein-tyrosine phosphatase activity associated with the recombinant C-terminal domain of FAK in vitro and could be coimmunoprecipitated with both FAK and paxillin from lysates of chicken embryo cells. However, the interaction with FAK appeared to be indirect and mediated via paxillin. The protein-tyrosine phosphatase was subsequently identified as protein-tyrosine phosphatase-PEST, a nonreceptor protein-tyrosine phosphatase. The C-terminal noncatalytic domain of protein-tyrosine phosphatase-PEST directly bound to paxillin in vitro. The association of both a protein-tyrosine kinase and a protein-tyrosine phosphatase with paxillin suggests that paxillin may play a critical role in the regulation of the phosphotyrosine content of proteins in focal adhesions.

Signal Transduction by the Philadelphia Chromosome-encoded BCR/ABL Oncoproteins: Therapeutic Implications for Chronic Myeloid Leukemia and Philadelphia-positive Acute Lymphoblastic Leukemia

The Philadelphia chromosomes characteristic of chronic myeloid leukemia (CML) and Philadelphia-positive acute lymphoblastic leukemia (ALL) encode chimeric protein tyrosine kinases (PTKs) derived by fusion of the normal BCR and ABL genes. The oncogenic properties of these BCR/ABL oncoproteins are dependent on their elevated PTK activity and on their ability to interact with multiple signal transduction systems. Here we summarize some of the key pathways which are activated by normal receptors with PTK activity and which modulate cell proliferation and survival. Next, we review some of the biochemical pathways initiated by BCR/ABL oncoproteins and discuss their possible relevance to the leukemic phenotype. We finally review experimental approaches designed to suppress signalling by BCR/ABL oncoproteins and discuss their potential therapeutic applications.

BCR/ABL-induced leukemogenesis causes phosphorylation of Hef1 and its association with Crkl

BCR/ABL is considered responsible for the development of Philadelphia chromosome-positive leukemia. Experimental animal models, such as transgenic mice, have demonstrated unambiguously that Bcr/Abl is capable of inducing leukemogenesis. The adaptor molecule Crkl is a major in vivo substrate of the deregulated Bcr/Abl tyrosine kinase and functions as a molecular link with other signaling proteins. While associated in vivo with Bcr/Abl through its SH3 domain, Crkl can interact simultaneously via its SH2 domain with other tyrosine-phosphorylated proteins. Here we report the identification of prominently tyrosine-phosphorylated proteins with a molecular mass of approximately 110 kDa, which bind specifically to the Crkl SH2 domain in leukemic tissues of P190BCR/ABL transgenic mice. We demonstrate that these proteins are identical to Hef1/Cas-L, which is related to p130(Cas). The proto-oncoprotein p120(Cbl) and Hef1, but not p130(Cas), were detectably phosphorylated on tyrosine in P190Bcr/Abl-expressing leukemic cells and were found in complex with Crkl, showing the existence of protein complexes in P190Bcr/Abl leukemic cells, consisting of P190Bcr/Abl, Crkl, and Hef1 or p120(Cbl). This supports a model in which Crkl acts as mediator between Bcr/Abl and downstream effectors. Since Hef1 is involved in the beta1-integrin signaling pathway, our study demonstrates that Bcr/Abl could specifically interfere with normal beta1-integrin signaling.

Association of the Cas-like molecule HEF1 with CrkL following integrin and antigen receptor signaling in human B-cells: potential relevance to neoplastic lymphohematopoietic cells

CrkL, a cellular homologue of the v-crk oncogene, belongs to the family of adaptor proteins, containing SH2 and SH3 domains, but no catalytic domain. Stimulation of normal B-cells and B-cell lines through beta1 integrin or - cell antigen receptor (BCR) promoted the association of CrkL with a set of 105-130 kD tyrosine phosphorylated substrates. The principal substrate is a recently identified molecule known as p105HEF1 (HEF1), which is highly homologous to p130Cas (Cas), the major tyrosine-phosphorylated protein detected in fibroblasts after transformation by v-crk. Immunodepletion studies indicated that all the tyrosine phosphorylated HEF1 or Cas was complexed with CrkL. Furthermore, the guanine nucleotide exchange factor C3G, which is thought to be involved in the regulation of the ras pathway and constitutively binds to the C-terminal SH3 domain of CrkL, could be detected in HEF1 immunoprecipitates. Therefore, CrkL is involved in the formation of a HEF1-CrkL-C3G ternary complex in B-cells, suggesting that it is likely to play an important role, allowing the propagation of the stimulation initiated by both BCR and beta1 integrin ligation.

In vivo regulation of CrkII and CrkL proto-oncogenes in the uterus by insulin-like growth factor-I. Differential effects on tyrosine phosphorylation and association with paxillin

Changes in CrkII and CrkL phosphorylation are associated with insulin-like growth factor receptor activation in cultured cells. We examined whether similar changes also occur following administration of recombinant human insulin-like growth factor-I to the intact animal. In female rats starved overnight, CrkL phosphorylation was significantly increased 12 min after insulin-like growth factor-I administration. Tyrosine phosphorylation of CrkII was not detectable in either control or treated animals. Paxillin, a 65-70-kDa phosphoprotein containing high affinity binding sites common for the Src homology 2 (SH2) domains of CrkII and CrkL, was observed in both CrkII and CrkL immunoprecipitates. Insulin-like growth factor-I treatment stimulated the association of CrkII with paxillin. In contrast, the same treatment resulted in the dissociation of the CrkL-paxillin complex. Similar effects of insulin-like growth factor-I treatment on the association of CrkL with tyrosine phosphorylated paxillin were observed in fibroblasts overexpressing CrkL. This study demonstrates that the activation of the insulin-like growth factor-I receptor induces changes in the tyrosine phosphorylation and protein-protein interactions of the Crk proteins in vivo. The different responses of CrkL and CrkII to insulin-like growth factor-I receptor activation suggest distinct roles for these two adapter proteins in signal transduction.

Detection of distinct pools of the adapter protein p130CAS using a panel of monoclonal antibodies

Dynamic protein interactions are thought to play an important role in regulating a wide variety of signal transduction pathways. Adapter molecules that contribute to the assembly and disassembly of these protein complexes are likely to play a critical role in the regulation of these pathways. The function of one such adapter molecule, p130CAS (CAS), has been implicated in signaling pathways involving cell growth, adhesion, and differentiation. We report here the isolation and characterization of a panel of monoclonal antibodies that specifically recognize CAS. These antibodies are proving to be invaluable molecular reagents for defining the expression, phosphorylation, binding partners, and ultimately the function of CAS with respect to cell signaling. In addition to their utility as conventional reagents for protein isolation, a subset of these antibodies has also proven to be a sensitive tool for distinguishing between different tyrosine-phosphorylated pools of CAS in the cell. Because tyrosine phosphorylation of CAS provides a dynamic means with which to regulate protein-protein interactions, these antibodies may thus serve as molecular reagents that can discern the protein binding potential of CAS. Collectively, the antibodies described in this report provide the means with which to define specific roles for CAS in cell signaling that have been otherwise difficult to establish.

Regulation of integrin-mediated p130(Cas) tyrosine phosphorylation in human B cells. A role for p59(Fyn) and SHP2

Engagement of beta1 integrins in terminally differentiated human B cell lines, such as ARH-77, leads to prominent tyrosine phosphorylation of the p130 Crk-associated substrate (Cas). Cas regulates the assembly of several SH2 and SH3 domain-containing proteins into signaling complexes, which are potentially involved in the propagation of downstream signals. We demonstrate here that immunoprecipitated Cas from beta1 integrin-stimulated ARH-77 cells was associated with tyrosine kinase and phosphatase activities and that integrin ligation led to the recruitment of at least p59(Fyn) tyrosine kinase and SHP2 tyrosine phosphatase in Cas immune complexes. Cotransfection studies in COS-7 cells further indicated that Fyn/Cas physical interaction and Fyn-mediated Cas phosphorylation required amino acids 638-889 in the C-terminal region of Cas. This sequence contains both c-Src SH2 and SH3 domain-binding motifs. In vitro binding studies using glutathione S-transferase fusion proteins derived from the SH2 or SH3 domains of Fyn suggested that both Fyn domains can participate in Fyn/Cas interaction. These data implicate Fyn and SHP2 as potential modulators of Cas signaling complexes in B cells.

Steel factor induces tyrosine phosphorylation of CRKL and binding of CRKL to a complex containing c-kit, phosphatidylinositol 3-kinase, and p120(CBL)

Steel factor (SF) is a growth and survival factor for hematopoietic cells. The receptor for SF, c-Kit, contains intrinsic tyrosine kinase activity, and binding of SF induces rapid tyrosine phosphorylation of several cellular proteins, including c-Kit itself. Activation of c-Kit is shown here to induce tyrosine phosphorylation of CRKL, and CRKL coprecipitated with c-Kit through an interaction that required the CRKL SH3 domains and not the SH2 domain. CRKL associated with c-Kit indirectly as part of a larger complex of proteins. Two proteins in this complex were identified as the p85 regulatory subunit of phosphatidylinositol 3-kinase (p85(PI3K)) and the proto-oncoprotein p120(CBL). Because p85(PI3K) is known to bind to the activated c-Kit receptor, the possibility that CRKL interacted with c-Kit indirectly through p85(PI3K) was investigated. Far Western blotting with a CRKL-SH3 glutathione S-transferase fusion protein showed that CRKL binds directly to p85(PI3K )in vitro. However, although a small amount of CRKL was preassociated with p85(PI3K), the interaction was increased after SF stimulation, suggesting that the interactions of these three proteins are complex. We conclude that SF induces the formation of a signaling complex potentially containing CRKL and p120(CBL), both of which bind to c-Kit through p85(PI3K). These data suggest that one function of CRKL in normal cells might be to recruit signaling molecules such as CBL into a complex with PI3K. Such complexes could be important in propagating signals involving PI3K such as gene expression and adhesion.

Activation of hematopoietic growth factor signal transduction pathways by the human oncogene BCR/ABL

BCR/ABL is a human chimeric oncogene that causes chronic myelogenous leukemia (CML). The BCR/ABL oncogene is generated from the Philadelphia chromosome (Ph) translocation, t(9;22)(q34;q11), and creates a constitutively active tyrosine kinase. There is clonal expansion of hematopoietic stem cells of several different lineages in CML. CML patients in stable phase usually have high white blood counts and immature cells of granulocytic lineages. Stable phase CML evolves to a more aggressive phase typically within 3.5-5 years, where differentiation is blocked and acute leukemia ensues. The transition of CML stable phase to blast phase is reflected in the loss of growth factor requirement of CML cells and correlates with additional cytogenetic alterations. Some biological effects reported in primary CML cells include reduced apoptosis and altered adhesion to fibronectin; however, the cells are dependent on hematopoietic growth factors. On a molecular level, the BCR/ABL translocation is well characterized. However, the actual mechanism of transformation by the BCR/ABL oncogene of hematopoietic cells is largely unknown. Enhancement of the c-ABL tyrosine kinase activity in BCR/ABL appears to be crucial for transformation. This tyrosine kinase activity leads to activation of several signal transduction pathways that are also utilized by hematopoietic growth factors, including steel factor, thrombopoietin, interleukin-3, and granulocyte/macrophage-colony stimulating factor. In several model systems, BCR/ABL has overlapping biological effects with hematopoietic growth factors, and transformation of hematopoietic growth factor-dependent cell lines leads to growth factor independence. In this review, we will describe the molecular and biological abnormalities in CML and several signal transduction mechanisms utilized by BCR/ABL as compared to hematopoietic growth factors.

Involvement of p130(Cas) and p105(HEF1), a novel Cas-like docking protein, in a cytoskeleton-dependent signaling pathway initiated by ligation of integrin or antigen receptor on human B cells

The Crk-associated substrate p130(Cas) (Cas) and the recently described human enhancer of filamentation 1 (HEF1) are two proteins with similar structure (64% amino acid homology), which are thought to act as "docking" molecules in intracellular signaling cascades. Both proteins contain an N-terminal Src homology (SH), three domain and a cluster of SH2 binding motifs. Here we show that ligation of either beta1 integrin or B cell antigen receptor (BCR) on human tonsillar B cells and B cell lines promoted tyrosine phosphorylation of HEF1. In contrast, Cas tyrosine phosphorylation was observed in certain B cell lines but not in tonsillar B cells, indicating a more general role for HEF1 in B cell signaling. Interestingly, pretreatment of tonsillar B cells with cytochalasin B dramatically reduced both integrin- and BCR-induced HEF1 phosphorylation, suggesting that some component of the BCR-mediated signaling pathway is closely linked with a cytoskeletal reorganization. Both HEF1 and Cas were found to complex with the related adhesion focal tyrosine kinase (RAFTK), and when tyrosine phosphorylated, with the adapter molecule CrkL. In addition, the two molecules were detected in p53/56(Lyn) immunoprecipitates, and Lyn kinase was found to specifically bind the C-terminal proline-rich sequence of Cas in an in vitro binding assay. These associations implicate HEF1 and Cas as important components in a cytoskeleton-linked signaling pathway initiated by ligation of beta1 integrin or BCR on human B cells.