Two species of human CRK cDNA encode proteins with distinct biological activities

Crk and CrkL as Therapeutic Targets for Cancer Treatment

Crk and CrkL are cellular counterparts of the viral oncoprotein v-Crk. Crk and CrkL are overexpressed in many types of human cancer, correlating with poor prognosis. Furthermore, gene knockdown and knockout of Crk and CrkL in tumor cell lines suppress tumor cell functions, including cell proliferation, transformation, migration, invasion, epithelial-mesenchymal transition, resistance to chemotherapy drugs, and in vivo tumor growth and metastasis. Conversely, overexpression of tumor cells with Crk or CrkL enhances tumor cell functions. Therefore, Crk and CrkL have been proposed as therapeutic targets for cancer treatment. However, it is unclear whether Crk and CrkL make distinct or overlapping contributions to tumor cell functions in various cancer types because Crk or CrkL have been examined independently in most studies. Two recent studies using colorectal cancer and glioblastoma cells clearly demonstrated that Crk and CrkL need to be ablated individually and combined to understand distinct and overlapping roles of the two proteins in cancer. A comprehensive understanding of individual and overlapping roles of Crk and CrkL in tumor cell functions is necessary to develop effective therapeutic strategies. This review systematically discusses crucial functions of Crk and CrkL in tumor cell functions and provides new perspectives on targeting Crk and CrkL in cancer therapy.

Cyclophilin A Inhibitor Debio-025 Targets Crk, Reduces Metastasis, and Induces Tumor Immunogenicity in Breast Cancer

The Crk adaptor protein, a critical modifier of multiple signaling pathways, is overexpressed in many cancers where it contributes to tumor progression and metastasis. Recently, we have shown that Crk interacts with the peptidyl prolyl cis-trans isomerase, Cyclophilin A (CypA; PP1A) via a G²¹⁹P²²⁰Y²²¹ (GPY) motif in the carboxyl-terminal linker region of Crk, thereby delaying pY221 phosphorylation and preventing downregulation of Crk signaling. Here, we investigate the physiologic significance of the CypA/Crk interaction and query whether CypA inhibition affects Crk signaling in vitro and in vivo. We show that CypA, when induced under conditions of hypoxia, regulates Crk pY221 phosphorylation and signaling in cancer cell lines. Using nuclear magnetic resonance spectroscopy, we show that CypA binds to the Crk GPY motif via the catalytic PPII domain of CypA, and small-molecule nonimmunosuppressive inhibitors of CypA (Debio-025) disrupt the CypA-CrkII interaction and restores phosphorylation of Crk Y221. In cultured cell lines, Debio-025 suppresses cell migration, and when administered in vivo in an orthotopic model of triple-negative breast cancer, Debio-025 showed antitumor efficacy either alone or in combination with anti-PD-1 mAb, reducing both tumor volume and metastatic lung dispersion. Furthermore, when analyzed by NanoString immune profiling, treatment of Debio-025 with anti-PD-1 mAb increased both T-cell signaling and innate immune signaling in tumor microenvironment. IMPLICATIONS: These data suggest that pharmacologic inhibition of CypA may provide a promising and unanticipated consequence in cancer biology, in part by targeting the CypA/CrkII axis that regulates cell migration, tumor metastasis, and host antitumor immune evasion.

Adaptor protein CrkII negatively regulates osteoblast differentiation and function through JNK phosphorylation

The adaptor protein CrkII is involved in several biological activities, including mitogenesis, phagocytosis, and cytoskeleton reorganization. Previously, we demonstrated that CrkII plays an important role in osteoclast differentiation and function through Rac1 activation both in vitro and in vivo. In this study, we investigated whether CrkII also regulates the differentiation and function of another type of bone cells, osteoblasts. Overexpression of CrkII in primary osteoblasts inhibited bone morphogenetic protein (BMP) 2-induced osteoblast differentiation and function, whereas knockdown of CrkII expression exerted the opposite effect. Importantly, CrkII strongly enhanced c-Jun-N-terminal kinase (JNK) phosphorylation, and the CrkII overexpression-mediated attenuation of osteoblast differentiation and function was recovered by JNK inhibitor treatment. Furthermore, transgenic mice overexpressing CrkII under control of the alpha-1 type I collagen promoter exhibited a reduced bone mass phenotype. Together, these results indicate that CrkII negatively regulates osteoblast differentiation and function through JNK phosphorylation. Given that CrkII acts as a negative and positive regulator of osteoblast and osteoclast differentiation, respectively, the regulation of CrkII expression in bone cells may help to develop new strategies to enhance bone formation and inhibit bone resorption.

Exosomes containing ErbB2/CRK induce vascular growth in premetastatic niches and promote metastasis of bladder cancer

Locally advanced and metastatic invasive bladder cancer (BC) has a poor prognosis, and no advanced therapies beyond cisplatin‐based combination chemotherapy have been developed. Therefore, it is an urgent issue to elucidate the underlying mechanisms of tumor progression and metastasis of invasive BC for the development of new therapeutic strategies. Here, we clarified a novel role of exosomes containing ErbB2 and CRK in a formation of premetastatic niches and subsequent metastases. CRK adaptors were overexpressed in invasive UM‐UC‐3 BC cells. In an orthotopic xenograft model, metastases to lung, liver, and bone of UM‐UC‐3 cells were completely abolished by CRK elimination. Mass spectrometry analysis identified that ErbB2 was contained in UM‐UC‐3‐derived exosomes in a CRK‐dependent manner; the exosomes significantly increased proliferation and invasion properties of low‐grade 5637 BC cells and HUVECs through FAK and PI3K/AKT signaling pathways. In athymic mice educated with UM‐UC‐3‐derived exosomes, i.v. implanted UM‐UC‐3 cells were trapped with surrounding PKH67‐labeled exosomes in lung and led to development of lung metastasis with disordered vascular proliferation. In contrast, exosomes derived from CRK‐depleted BC cells failed to induce these malignant features. Taken together, we showed that CRK adaptors elevated the expression of ErbB2/3 in BC cells, and these tyrosine kinase/adaptor units were transferred from host BC cells to metastatic recipient cells by exosomes, leading to vascular leakiness and proliferation and contributing to the formation of distant metastasis. Thus, CRK intervention with ErbB2/3 blockade might be a potent therapeutic strategy for patients with ErbB2 overexpressing advanced and metastatic BC.

MicroRNAs mediated regulation of MAPK signaling pathways in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a severe problem throughout the world and requires identification of novel targets for its treatment. This multifactorial disease accounts for about 15% of the all diagnosed leukemia cases. Mitogen-activated protein kinase (MAPK) signaling pathway is crucial for the cell survival and its dysregulation is being implicated in various types of cancers. In here, we have discussed the potential role of various miRNAs that are found involved in regulating the proteins cascades of MAPK signaling pathway associated with CML. An emphasis has been paid to summarize the influence of various miRNAs in elevating or suppressing the expression level of significant proteins such as miR-203, miR-196a, miR-196b, miR-30a, miR-29b, miR-138 in BCR-ABL tyrosine kinase; miR-126, miR-221, miR-128, miR-15a, miR-188-5p, miR-17 in CRK family proteins; miR-155, miR-181a with SOS proteins; miR-155, miR-19a, with KRAS proteins; miR-19a with RAF1 protein; and miR-17, miR-19a, miR-17-92 cluster with MAPK/ERK proteins. In light of ever-increasing importance and ever-widening regulatory roles of miRNAs in cells, we have reviewed the recent progress in the field of miRNAs and have tried to suggest them as controlling targets for various protein cascades of MAPK signaling pathway. An understanding of the supervisory mechanism of MAPK by miRNAs might provide novel targets for treating CML.

Novel signaling collaboration between TGF-β and adaptor protein Crk facilitates EMT in human lung cancer

The signaling adaptor protein Crk has been shown to play an important role in various human cancers. However, its regulatory machinery is not clear. Here, we demonstrated that Crk induced EMT in A549 human lung adenocarcinoma cells through differential regulation of Rac1/Snail and RhoA/Slug, leading to decreased expression of E-cadherin and increased N-cadherin, fibronectin, and MMP2 expression. Cancer cells with mesenchymal features produced TGF-β and also increased the levels of TGF-β receptor. TGF-β increased the endogenous levels of Crk and also augmented Crk-dependent expression of Snail and Slug, and conversely TGF-β receptor inhibitor suppressed the levels of Snail and Slug. Overexpression of Crk was observed at the invasive front of human lung cancer tissues and was significantly associated with poor prognosis. Thus, TGF-β and Crk collaborate to form a positive feedback loop to facilitate EMT, which may lead to the malignancy of human cancers possibly being affected by their microenvironment.

Binding Mechanism of the N-Terminal SH3 Domain of CrkII and Proline-Rich Motifs in cAbl

The N-terminal Src homology 3 (nSH3) domain of a signaling adaptor protein, CT-10 regulator of kinase II (CrkII), recognizes proline-rich motifs (PRMs) of binding partners, such as cAbl kinase. The interaction between CrkII and cAbl kinase is involved in the regulation of cell spreading, microbial pathogenesis, and cancer metastasis. Here, we report the detailed biophysical characterizations of the interactions between the nSH3 domain of CrkII and PRMs in cAbl. We identified that the nSH3 domain of CrkII binds to three PRMs in cAbl with virtually identical affinities. Structural studies, by using x-ray crystallography and NMR spectroscopy, revealed that the binding modes of all three nSH3:PRM complexes are highly similar to each other. Van 't Hoff analysis revealed that nSH3:PRM interaction is associated with favorable enthalpy and unfavorable entropy change. The combination of experimentally determined thermodynamic parameters, structure-based calculations, and (15)N NMR relaxation analysis highlights the energetic contribution of conformational entropy change upon the complex formation, and water molecules structured in the binding interface of the nSH3:PRM complex. Understanding the molecular basis of nSH3:PRM interaction will provide, to our knowledge, new insights for the rational design of small molecules targeting the interaction between CrkII and cAbl.

Fibroblast Growth Requires CT10 Regulator of Kinase (Crk) and Crk-like (CrkL)

CT10 regulator of kinase (Crk) and Crk-like (CrkL) are the cellular counterparts of the viral oncogene v-Crk Elevated levels of Crk and CrkL have been observed in many human cancers; inhibition of Crk and CrkL expression reduced the tumor-forming potential of cancer cell lines. Despite a close relationship between the Crk family proteins and tumorigenesis, how Crk and CrkL contribute to cell growth is unclear. We ablated endogenous Crk and CrkL from cultured fibroblasts carrying floxed alleles of Crk and CrkL by transfection with synthetic Cre mRNA (synCre). Loss of Crk and CrkL induced by synCre transfection blocked cell proliferation and caused shrinkage of the cytoplasm and the nucleus, formation of adherens junctions, and reduced cell motility. Ablation of Crk or CrkL alone conferred a much more modest reduction in cell proliferation. Reintroduction of CrkI, CrkII, or CrkL individually rescued cell proliferation in the absence of the endogenous Crk and CrkL, suggesting that Crk and CrkL play overlapping functions in regulating fibroblast growth. Serum and basic FGF induced phosphorylation of Akt, MAP kinases, and S6 kinase and Fos expression in the absence of Crk and CrkL, suggesting that cells lacking Crk and CrkL are capable of initiating major signal transduction pathways in response to extracellular stimuli. Furthermore, cell cycle and cell death analyses demonstrated that fibroblasts lacking Crk and CrkL become arrested at the G₁-S transition and undergo a modest apoptosis. Taken together, our results suggest that Crk and CrkL play essential overlapping roles in fibroblast growth.

Reciprocal regulation of Abl kinase by Crk Y251 and Abi1 controls invasive phenotypes in glioblastoma

Crk is the prototypical member of a class of Src homology 2 (SH2) and Src homology 3 (SH3) domain-containing adaptor proteins that positively regulate cell motility via the activation of Rac1 and, in certain tumor types such as GBM, can promote cell invasion and metastasis by mechanisms that are not well understood. Here we demonstrate that Crk, via its phosphorylation at Tyr251, promotes invasive behavior of tumor cells, is a prominent feature in GBM, and correlating with aggressive glioma grade IV staging and overall poor survival outcomes. At the molecular level, Tyr251 phosphorylation of Crk is negatively regulated by Abi1, which competes for Crk binding to Abl and attenuates Abl transactivation. Together, these results show that Crk and Abi1 have reciprocal biological effects and act as a molecular rheostat to control Abl activation and cell invasion. Finally, these data suggest that Crk Tyr251 phosphorylation regulate invasive cell phenotypes and may serve as a biomarker for aggressive GBM.

Optimal Drift Correction for Superresolution Localization Microscopy with Bayesian Inference

Single-molecule-localization-based superresolution microscopy requires accurate sample drift correction to achieve good results. Common approaches for drift compensation include using fiducial markers and direct drift estimation by image correlation. The former increases the experimental complexity and the latter estimates drift at a reduced temporal resolution. Here, we present, to our knowledge, a new approach for drift correction based on the Bayesian statistical framework. The technique has the advantage of being able to calculate the drifts for every image frame of the data set directly from the single-molecule coordinates. We present the theoretical foundation of the algorithm and an implementation that achieves significantly higher accuracy than image-correlation-based estimations.

The adaptor protein CrkII regulates IGF-1-induced biological behaviors of pancreatic ductal adenocarcinoma

Recently, the adaptor protein CrkII has been proved to function in initiating signals for proliferation and invasion in some malignancies. However, the specific mechanisms underlying insulin-like growth factor 1 (IGF-1)-CrkII signaling-induced proliferation of pancreatic ductal adenocarcinoma (PDAC) were not unraveled. In this work, PDAC tissues and cell lines were subjected to in vitro and in vivo assays. Our findings showed that CrkII was abundantly expressed in PDAC tissues and closely correlated with tumor-node-metastasis (TNM) stage and invasion. When cells were subjected to si-CrkII, si-CrkII inhibited IGF-1-mediated PDAC cell growth. In vitro, we demonstrated the upregulation of CrkII, p-Erk1/2, and p-Akt occurring in IGF-1-treated PDAC cells. Conversely, si-CrkII affected upregulation of CrkII, p-Erk1/2, and p-Akt. In addition, cell cycle and in vivo assay identified that knockdown of CrkII inhibited the entry of G1 into S phase and the increase of PDAC tumor weight. In conclusion, CrkII mediates IGF-1 signaling and further balanced PDAC biological behaviors via Erk1/2 and Akt pathway, which indicates that CrkII gene and protein may act as an effective target for the treatment of PDAC.

Immunohistochemichal Assessment of the CrkII Proto-oncogene Expression in Common Malignant Salivary Gland Tumors and Pleomorphic Adenoma

Background and aims. Various morphologies are seen in different salivary gland tumorsor within an individual tumor, and the lesions show divers biological behaviors. Experimental results support the hypothesis that increased CrkII proto-oncogene is associated with cytokine-induced tumor initiation and progression by altering cell motility signaling pathway. The aim of this study was to assess the CrkII expression in common malignant salivary gland tumors and pleomorphic ade-noma.

Materials and methods. Immunohistochemical analysis of CrkII expression was performed on paraffin blocks of 64 car-cinomas of salivary glands, 10 pleomorphic adenomas, and 10 normal salivary glands. Biopsies were subjected to immu-nostaining with EnVision detection system using monoclonal anti-CrkII. Evaluation of immunoreactivity of CrkII was based on the immunoreaction intensity and percentage of stained tumor cells which were scored semi-quantitatively on a scale with four grades 0 to 3. Kruskal-wallis test and additional Mann-Whitney statistical test were used for analysis of CrkII expression levels.

Results. Increased expression of CrkII was seen (P=0.005) in malignant tumors including: mucoepidermoid carcinoma, adenoid cystic carcinoma, and carcinoma ex pleomorphic adenoma, but CrkII expression in acinic cell carcinoma was weak. CrkII expression in pleomorphic adenoma was weak or negative. A weak staining was sparsely seen in normal acinar serous cell.

Conclusion. Increased expression of CrkII and its higher intensity of staining in tumors with more aggressive biologic behavior in carcinomas of salivary gland is consistent with a role for this proto-oncogene in salivary gland tumorigenesis and cancer progression.

Mouse and human CRKL is dosage sensitive for cardiac outflow tract formation

The human chromosome 22q11.2 region is susceptible to rearrangements during meiosis leading to velo-cardio-facial/DiGeorge/22q11.2 deletion syndrome (22q11DS) characterized by conotruncal heart defects (CTDs) and other congenital anomalies. The majority of individuals have a 3 Mb deletion whose proximal region contains the presumed disease-associated gene TBX1 (T-box 1). Although a small subset have proximal nested deletions including TBX1, individuals with distal deletions that exclude TBX1 have also been identified. The deletions are flanked by low-copy repeats (LCR22A, B, C, D). We describe cardiac phenotypes in 25 individuals with atypical distal nested deletions within the 3 Mb region that do not include TBX1 including 20 with LCR22B to LCR22D deletions and 5 with nested LCR22C to LCR22D deletions. Together with previous reports, 12 of 37 (32%) with LCR22B-D deletions and 5 of 34 (15%) individuals with LCR22C-D deletions had CTDs including tetralogy of Fallot. In the absence of TBX1, we hypothesized that CRKL (Crk-like), mapping to the LCR22C-D region, might contribute to the cardiac phenotype in these individuals. We created an allelic series in mice of Crkl, including a hypomorphic allele, to test for gene expression effects on phenotype. We found that the spectrum of heart defects depends on Crkl expression, occurring with analogous malformations to that in human individuals, suggesting that haploinsufficiency of CRKL could be responsible for the etiology of CTDs in individuals with nested distal deletions and might act as a genetic modifier of individuals with the typical 3 Mb deletion.

CRK proteins selectively regulate T cell migration into inflamed tissues

Effector T cell migration into inflamed sites greatly exacerbates tissue destruction and disease severity in inflammatory diseases, including graft-versus-host disease (GVHD). T cell migration into such sites depends heavily on regulated adhesion and migration, but the signaling pathways that coordinate these functions downstream of chemokine receptors are largely unknown. Using conditional knockout mice, we found that T cells lacking the adaptor proteins CRK and CRK-like (CRKL) exhibit reduced integrin-dependent adhesion, chemotaxis, and diapedesis. Moreover, these two closely related proteins exhibited substantial functional redundancy, as ectopic expression of either protein rescued defects in T cells lacking both CRK and CRKL. We determined that CRK proteins coordinate with the RAP guanine nucleotide exchange factor C3G and the adhesion docking molecule CASL to activate the integrin regulatory GTPase RAP1. CRK proteins were required for effector T cell trafficking into sites of inflammation, but not for migration to lymphoid organs. In a murine bone marrow transplantation model, the differential migration of CRK/CRKL-deficient T cells resulted in efficient graft-versus-leukemia responses with minimal GVHD. Together, the results from our studies show that CRK family proteins selectively regulate T cell adhesion and migration at effector sites and suggest that these proteins have potential as therapeutic targets for preventing GVHD.

Contribution of Crk adaptor proteins to host cell and bacteria interactions

The Crk adaptor family of proteins comprises the alternatively spliced CrkI and CrkII isoforms, as well as the paralog Crk-like (CrkL) protein, which is encoded by a different gene. Initially thought to be involved in signaling during apoptosis and cell adhesion, this ubiquitously expressed family of proteins is now known to play essential roles in integrating signals from a wide range of stimuli. In this review, we describe the structure and function of the different Crk proteins. We then focus on the emerging roles of Crk adaptors during Enterobacteriaceae pathogenesis, with special emphasis on the important human pathogens Salmonella, Shigella, Yersinia, and enteropathogenic Escherichia coli. Throughout, we remark on opportunities for future research into this intriguing family of proteins.

Phosphorylation of Dok1 by Abl family kinases inhibits CrkI transforming activity

The Crk SH2/SH3 adaptor and the Abl nonreceptor tyrosine kinase were first identified as oncoproteins, and both can induce tumorigenesis when overexpressed or mutationally activated. We previously reported the surprising finding that inhibition or knockdown of Abl family kinases enhanced transformation of mouse fibroblasts by CrkI. Abl family inhibitors are currently used or are being tested for treatment of human malignancies, and our finding raised concerns that such inhibitors might actually promote the growth of tumors overexpressing CrkI. Here, we identify the Dok1 adaptor as the key effector for the enhancement of CrkI transformation by Abl inhibition. We show that phosphorylation of tyrosines 295 and 361 of Dok1 by Abl family kinases suppresses CrkI transforming activity, and that upon phosphorylation these tyrosines bind the SH2 domains of the Ras inhibitor p120 RasGAP. Knockdown of RasGAP resulted in a similar enhancement of CrkI transformation, consistent with a critical role for Ras activity. Imaging studies using a FRET sensor of Ras activation revealed alterations in the localization of activated Ras in CrkI-transformed cells. Our results support a model in which Dok1 phosphorylation normally suppresses localized Ras pathway activity in Crk-transformed cells via recruitment and/or activation of RasGAP, and that preventing this negative feedback mechanism by inhibiting Abl family kinases leads to enhanced transformation by Crk.

The adaptor protein Crk in immune response

The adaptor proteins Crk (CT10 (chicken tumor virus number 10) regulator of kinase), including CrkI, CrkII and Crk-like, are important signal molecules that regulate a variety of cellular processes. Considerable progress has been made in understanding the roles of the Crk family proteins in signal transduction, with a focus on cellular transformation and differentiation. However, since Crk was identified in 1988, very few studies have addressed how Crk regulates the immune response. Recent work demonstrates that Crk proteins function as critical signal molecules in regulating immune cell functions. Emerging data on the roles of Crk in activation and inhibitory immunoreceptor signaling suggest that Crk proteins are potential immunotherapeutic targets in cancer and infectious diseases. The aim of this review is to summarize recent key findings regarding the role of Crk in immune responses mediated by T, B and natural killer (NK) cells. In particular, the roles of Crk in NK cell functions are discussed.

PTPN4 negatively regulates CrkI in human cell lines

PTPN4 is a widely expressed non-receptor protein tyrosine phosphatase. Although its overexpression inhibits cell growth, the proteins with which it interacts to regulate cell growth are unknown. In this study, we identified CrkI as a PTPN4-interacting protein using a yeast two-hybrid, and confirmed this interaction using in vitro GST pull-down and co-immunoprecipitation and co-localization assays. We further determined the interactional regions as the SH3 domain of CrkI and the proline-rich region between amino acids 462 and 468 of PTPN4. Notably, overexpression of PTPN4 inhibits CrkI-mediated proliferation and wound healing of HEK293T cells, while knockdown of PTPN4 by siRNA in Hep3B cells enhances CrkI-mediated cell growth and motility. Moreover, our data show that ectopic expression of PTPN4 reduces the phosphorylation level of CrkI in HEK293T cells. These findings suggest that PTPN4 negatively regulates cell proliferation and motility through dephosphorylation of CrkI.

Models of crk adaptor proteins in cancer

The Crk family of adaptor proteins (CrkI, CrkII, and CrkL), originally discovered as the oncogene fusion product, v-Crk, of the CT10 chicken retrovirus, lacks catalytic activity but engages with multiple signaling pathways through their SH2 and SH3 domains. Crk proteins link upstream tyrosine kinase and integrin-dependent signals to downstream effectors, acting as adaptors in diverse signaling pathways and cellular processes. Crk proteins are now recognized to play a role in the malignancy of many human cancers, stimulating renewed interest in their mechanism of action in cancer progression. The contribution of Crk signaling to malignancy has been predominantly studied in fibroblasts and in hematopoietic models and more recently in epithelial models. A mechanistic understanding of Crk proteins in cancer progression in vivo is still poorly understood in part due to the highly pleiotropic nature of Crk signaling. Recent advances in the structural organization of Crk domains, new roles in kinase regulation, and increased knowledge of the mechanisms and frequency of Crk overexpression in human cancers have provided an incentive for further study in in vivo models. An understanding of the mechanisms through which Crk proteins act as oncogenic drivers could have important implications in therapeutic targeting.

Roles for crk in cancer metastasis and invasion

The Crk family of adaptors is implicated in regulating various biological and pathological processes such as cell proliferation, adhesion, migration, invasion, phagocytosis, and survival. A large number of studies have shown that Crk plays an important role in aggressive and malignant behaviors of human cancers. In immunohistochemical analyses and gene-expression profiles, enhanced expression of Crk has been identified in adenocarcinomas of lung, breast, and stomach and in sarcomas and glioma. Overexpression of Crk in tumor cells induces the prominent tyrosine phosphorylations of scaffolding molecules such as p130(Cas) and paxillin through Src family tyrosine kinases and stimulates the activation loop of intracellular signalling, ultimately contributing to the increased motility and aggressive potential of cancer cells. Crk proteins thus are not simply conduits for intracellular signal transduction but also can control the amplitude of signalling. This review summarizes the significance of Crk and its mediated signaling assemblies, particularly in regulating tumor metastasis and invasion, and discusses the possibilities that they are potential cancer therapeutic targets.

Functional mechanisms and roles of adaptor proteins in abl-regulated cytoskeletal actin dynamics

Abl is a nonreceptor tyrosine kinase and plays an essential role in the modeling and remodeling of F-actin by transducing extracellular signals. Abl and its paralog, Arg, are unique among the tyrosine kinase family in that they contain an unusual extended C-terminal half consisting of multiple functional domains. This structural characteristic may underlie the role of Abl as a mediator of upstream signals to downstream signaling machineries involved in actin dynamics. Indeed, a group of SH3-containing accessory proteins, or adaptor proteins, have been identified that bind to a proline-rich domain of the C-terminal portion of Abl and modulate its kinase activity, substrate recognition, and intracellular localization. Moreover, the existence of signaling cascade and biological outcomes unique to each adaptor protein has been demonstrated. In this paper, we summarize functional roles and mechanisms of adaptor proteins in Abl-regulated actin dynamics, mainly focusing on a family of adaptor proteins, Abi. The mechanism of Abl's activation and downstream signaling mediated by Abi is described in comparison with those by another adaptor protein, Crk.

Domain organization differences explain Bcr-Abl's preference for CrkL over CrkII

CrkL is a key signaling protein that mediates the leukemogenic activity of Bcr-Abl. CrkL is thought to adopt a structure that is similar to that of its CrkII homolog. The two proteins share high sequence identity and indistinguishable ligand binding preferences; yet they have distinct physiological roles. Here we show that the structures of CrkL and phosphorylated CrkL are drastically different than the corresponding structures of CrkII. As a result, the binding activities of the SH2 and SH3 domains in the two proteins are regulated in a distinct manner and to a different extent. The different structural architecture of CrkL and CrkII may account for their distinct functional roles. The data show that CrkL forms a constitutive complex with Abl thus explaining the strong preference of Bcr-Abl for CrkL. The results also highlight how the structural organization of the modular domains in adaptor proteins can control signaling outcome.

Crk adaptor proteins act as key signaling integrators for breast tumorigenesis

Introduction

CT10 regulator of kinase (Crk) adaptor proteins (CrkI, CrkII and CrkL) play a role in integrating signals for migration and invasion of highly malignant breast cancer cell lines. This has important implications, as elevated CrkI/II protein levels were observed in a small cohort of breast cancer patients, which identified a potential role for Crk proteins in breast cancer progression. Numerous in vitro studies identified a role for Crk proteins in cell motility, but little is known about how Crk proteins contribute to breast cancer progression in vivo.

Methods

The clinical significance of Crk proteins in human breast cancer was assessed by analyzing published breast cancer datasets using a gene expression signature that was generated following CrkII over-expression and by examining Crk protein expression in tissue microarrays of breast tumors (n = 254). Stable knockdown of Crk (CrkI/CrkII/CrkL) proteins was accomplished using a short hairpin RNA (shRNA)-mediated approach in two basal breast cancer cell lines, MDA-231 1833TR and SUM1315, where the former have a high affinity to form bone metastases. Both in vitro assays (cell migration, invasion, soft agar growth) and in vivo experiments (intra-cardiac, tibial and mammary fat pad injections) were performed to assess the functional significance of Crk proteins in breast cancer.

Results

A gene signature derived following CrkII over-expression correlated significantly with basal breast cancers and with high grade and poor outcome in general. Moreover, elevated Crk immunostaining on tissue microarrays revealed a significant association with highly proliferative tumors within the basal subtype. RNAi-mediated knockdown of all three Crk proteins in metastatic basal breast cancer cells established a continued requirement for Crk in cell migration and invasion in vitro and metastatic growth in vivo. Furthermore, Crk ablation suppressed anchorage independent growth and in vivo orthotopic tumor growth. This was associated with diminished cell proliferation and was rescued by expression of non-shRNA targeted CrkI/II. Perturbations in tumor progression correlated with altered integrin signaling, including decreased cell spreading, diminished p130Cas phosphorylation, and Cdc42 activation.

Conclusions

These data highlight the physiological importance of Crk proteins in regulating growth of aggressive basal breast cancer cells and identify Crk-dependent signaling networks as promising therapeutic targets.

The adaptor protein Crk controls activation and inhibition of natural killer cells

Natural killer (NK) cell inhibitory receptors recruit tyrosine phosphatases to prevent activation, induce phosphorylation and dissociation of the small adaptor Crk from cytoskeleton scaffold complexes, and maintain NK cells in a state of responsiveness to subsequent activation events. How Crk contributes to inhibition is unknown. We imaged primary NK cells over lipid bilayers carrying IgG1 Fc to stimulate CD16 and human leukocyte antigen (HLA)-E to inhibit through receptor CD94-NKG2A. HLA-E alone induced Crk phosphorylation in NKG2A(+) NK cells. At activating synapses with Fc alone, Crk was required for the movement of Fc microclusters and their ability to trigger activation signals. At inhibitory synapses, HLA-E promoted central accumulation of both Fc and phosphorylated Crk and blocked the Fc-induced buildup of F-actin. We propose a unified model for inhibitory receptor function: Crk phosphorylation prevents essential Crk-dependent activation signals and blocks F-actin network formation, thereby reducing constraints on subsequent engagement of activation receptors.

Identification of targets of c-Src tyrosine kinase by chemical complementation and phosphoproteomics

The cellular proto-oncogene c-Src is a nonreceptor tyrosine kinase involved in cell growth and cytoskeletal regulation. Despite being dysregulated in a variety of human cancers, its precise functions are not fully understood. Identification of the substrates of c-Src remains a major challenge, because there is no simple way to directly stimulate its activity. Here we combine the chemical rescue of mutant c-Src and global quantitative phosphoproteomics to obtain the first high resolution snapshot of the range of tyrosine phosphorylation events that occur in the cell immediately after specific c-Src stimulation. After enrichment by anti-phosphotyrosine antibodies, we identified 29 potential novel c-Src substrate proteins. Tyrosine phosphopeptide mapping allowed the identification of 382 nonredundant tyrosine phosphopeptides on 213 phosphoproteins. Stable isotope labeling of amino acids in cell culture-based quantitation allowed the detection of 97 nonredundant tyrosine phosphopeptides whose level of phosphorylation is increased by c-Src. A large number of previously uncharacterized c-Src putative protein targets and phosphorylation sites are presented here, a majority of which play key roles in signaling and cytoskeletal networks, particularly in cell adhesion. Integrin signaling and focal adhesion kinase signaling pathway are two of the most altered pathways upon c-Src activation through chemical rescue. In this context, our study revealed the temporal connection between c-Src activation and the GTPase Rap1, known to stimulate integrin-dependent adhesion. Chemical rescue of c-Src provided a tool to dissect the spatiotemporal mechanism of activation of the Rap1 guanine exchange factor, C3G, one of the identified potential c-Src substrates that plays a role in focal adhesion signaling. In addition to unveiling the role of c-Src in the cell and, specifically, in the Crk-C3G-Rap1 pathway, these results exemplify a strategy for obtaining a comprehensive understanding of the functions of nonreceptor tyrosine kinases with high specificity and kinetic resolution.

The adaptor protein CRK is a pro-apoptotic transducer of endoplasmic reticulum stress

Excessive demands on the protein folding capacity of the endoplasmic reticulum (ER) cause irremediable ER stress and contribute to cell loss in a number of cell degenerative diseases, including type 2 diabetes and neurodegeneration^1,2. The signals communicating catastrophic ER damage to the mitochondrial apoptotic machinery remain poorly understood^3-6. We used a biochemical approach to purify a cytosolic activity induced by ER stress that causes release of cytochrome c from isolated mitochondria. We discovered that the principal component of the purified pro-apoptotic activity is proto-oncogene CT10-regulated kinase (CRK), an adaptor protein with no known catalytic activity⁷. Crk^-/- cells are strongly resistant to ER stress-induced apoptosis. Moreover, CRK is cleaved in response to ER stress to generate an N-terminal ~14kD fragment with greatly enhanced cytotoxic potential. We identified a putative BCL2 homology-3 (BH3) domain within this N-terminal CRK fragment, which sensitizes isolated mitochondria to cytochrome c release and when mutated significantly reduces CRK's apoptotic activity in vivo. Together these results identify CRK as a pro-apoptotic protein that signals irremediable ER stress to the mitochondrial execution machinery.

Emerging roles for crk in human cancer

Adaptor proteins are named for their function in assembling complexes of cellular proteins to execute and facilitate transmission of signals. The Crk family of adaptors consists of 2 members, Crk and CrkL. Crk, which was originally isolated as an oncogene, v-Crk, that transforms CEFs, has at least 2 splice variants, CrkI and CrkII, with differing biological activities. All Crk family proteins serve to act as molecular bridges between tyrosine kinases and their substrates and also modulate the specificity and stoichiometry of signaling processes. Signaling via CrkII and CrkL can be negatively regulated via tyrosine phosphorylation-mediated autoinhibition, while such a mechanism is not known to exist for CrkI. Although v-Crk clearly functions as a bona fide oncogene, in recent years, an emerging body of evidence suggests that cellular Crk proteins are overexpressed in human tumors and the expression levels correlate with aggressive and malignant behavior of cancer cells. These properties of Crk proteins make them potential cancer prognosis markers and therapeutic targets.

Phosphorylation of Crk on tyrosine 251 in the RT loop of the SH3C domain promotes Abl kinase transactivation

Here, we report the identification and characterization of a novel tyrosine phosphorylation site in the carboxy-terminal Src Homology 3 (SH3) (SH3C) domain of the Crk adaptor protein. Y251 is located in the highly conserved RT loop structure of the SH3C, a region of Crk involved in the allosteric regulation of the Abl kinase. Exploiting kinase assays to show that Y251 is phosphorylated by Abl in vitro, we generated affinity-purified antisera against phosphorylated Y251 in Crk and showed that Abl induces phosphorylation at Y251 in vivo, and that the kinetics of phosphorylation at Y251 and the negative regulatory Y221 site in vitro are similar. Y251 on endogenous Crk was robustly phosphorylated in chronic myelogenous leukemia cell lines and in A431 and MDA-MB-468 cells stimulated with epidermal growth factor. Using streptavidin-biotin pull downs and unbiased high-throughput Src Homology 2 (SH2) profiling approaches, we found that a pY251 phosphopeptide binds specifically to a subset of SH2 domains, including Abl and Arg SH2, and that binding of pY251 to Abl SH2 induces transactivation of Abl 1b. Finally, the Y251F Crk mutant significantly abrogates Abl transactivation in vitro and in vivo. These studies point to a yet unrealized positive regulatory role resulting from tyrosine phosphorylation of Crk, and identify a novel mechanism by which an adaptor protein activates a non-receptor tyrosine kinase by SH2 domain displacement.

c-Crk proto-oncogene contributes to transcriptional repression of p120-catenin in non-small cell lung cancer cells

As a member of adherens junction, p120-catenin (p120ctn) plays a major role in cell adhesions through stabilization of E-cadherin. p120ctn is transcriptionally down-regulated in non-small cell lung cancer (NSCLC), although the molecular mechanisms underlying p120ctn repression are incompletely defined. Here we further investigated transcriptional regulation of p120ctn in NSCLC. We prepared a promoter reporter plasmid construct that contained p120ctn promoter region from position -1082 to +320 relative to transcription start site. Through serial deletion mutation analysis of the p120ctn promoter, we pinpointed cis-acting elements involved in regulation of p120ctn. We identified transcription factor SP1 as a transcriptional repressor of p120ctn that directly binds to segment (-9 to +36) of the p120ctn promoter. SP1 can receive multiple signals from several intracellular signaling pathways. Through examination of SP1 binding partners, we identified proto-oncogene c-Crk to be involved in transcriptional down-regulation of p120ctn. RNAi mediated silencing of CRK in A549, H157 and H358 cells increased p120ctn protein levels. On the other hand, over-expression of CRK-I and CRK-II in NSCLC cells down-regulated p120ctn, an effect that was abrogated by simultaneous silencing of SP1. In summary, our data provide evidence for the role of c-Crk proto-oncogene in transcriptional repression of p120ctn that further clarifies the mechanism by which this biochemical signal promotes metastasis in NSCLC.

Integrin signalling adaptors: not only figurants in the cancer story

Current evidence highlights the ability of adaptor (or scaffold) proteins to create signalling platforms that drive cellular transformation upon integrin-dependent adhesion and growth factor receptor activation. The understanding of the biological effects that are regulated by these adaptors in tumours might be crucial for the identification of new targets and the development of innovative therapeutic strategies for human cancer. In this Review we discuss the relevance of adaptor proteins in signalling that originates from integrin-mediated cell-extracellular matrix (ECM) adhesion and growth factor stimulation in the context of cell transformation and tumour progression. We specifically underline the contribution of p130 Crk-associated substrate (p130CAS; also known as BCAR1), neural precursor cell expressed, developmentally down-regulated 9 (NEDD9; also known as HEF1), CRK and the integrin-linked kinase (ILK)-pinch-parvin (IPP) complex to cancer, along with the more recently identified p140 Cas-associated protein (p140CAP; also known as SRCIN1).

Proteins that bind the Src homology 3 domain of CrkI have distinct roles in Crk transformation

The v-Crk oncogene product consists of two protein interaction modules, a Src homology 2 (SH2) domain and an SH3 domain. Overexpression of CrkI, the cellular homolog of v-Crk, transforms mouse fibroblasts, and elevated CrkI expression is observed in several human cancers. The SH2 and SH3 domains of Crk are required for transformation, but the identity of the critical cellular binding partners is not known. A number of candidate Crk SH3 binding proteins have been identified, including the nonreceptor tyrosine kinases c-Abl and Arg, and the guanine nucleotide exchange proteins C3G, SOS1 and DOCK180. The aim of this study is to determine which of these are required for transformation by CrkI. We found that shRNA-mediated knockdown of C3G or SOS1 suppressed anchorage-independent growth of NIH-3T3 cells overexpressing CrkI, while knockdown of SOS1 alone was sufficient to suppress tumor formation by these cells in nude mice. Knockdown of C3G was sufficient to revert morphological changes induced by CrkI expression. By contrast, knockdown of Abl family kinases or their inhibition with imatinib enhanced anchorage-independent growth and tumorigenesis induced by Crk. These results demonstrate that SOS1 is essential for CrkI-induced fibroblast transformation, and also reveal a surprising negative role for Abl kinases in Crk transformation.

The role of Crk/Dock180/Rac1 pathway in the malignant behavior of human ovarian cancer cell SKOV3

Small GTPases, particularly the Rho family, are key regulators of cell motility and migration. Dock180 was well known for the main target of signal adaptor protein Crk and acted as a guanine-nucleotide exchange factor for small GTPase Rac1. In the present study, Dock180 was found to combine primarily with CrkI other than CrkII, and its association with Elmo1 was also demonstrated in ovarian cancer cell SKOV3. To evaluate the role of Dock180 in human ovarian cancer cell, we performed RNAi-mediated knockdown of Dock180 in SKOV3 cells using small interfering RNA expression vector. In Dock180 knockdown cells, we found that Elmo1 expression and Rac1 activity were decreased simultaneously. By contrast, the expressions of both another Crk-combining molecule C3G and Rap1 activity were observed to increase obviously. Accordingly, all Dock180 knockdown cells present with evident change in cell morphology, reduced cell proliferation, and attenuated cell migration. Taken together, these results suggest that signal transfer of Crk/Dock180/Rac1 is implicated in actin cytoskeleton reorganization and thus in the cell proliferation, motility, invasion, and of human ovarian cancer cell line SKOV3.

CrkII transgene induces atypical mammary gland development and tumorigenesis

The v-Crk protein was originally isolated as the oncogene fusion product of the CT10 chicken retrovirus. Cellular homologues of v-Crk include Crk, which encodes two alternatively spliced proteins (CrkI and CrkII), and CrkL. Though CrkI/II proteins are elevated in several types of cancer, including breast, the question of whether these Crk adaptor proteins can promote breast cancer has not been addressed. We created a transgenic mouse model that allows the expression of CrkII through the hormonally responsive mouse mammary tumor virus promoter. During puberty, transgenic mice were found to have delayed ductal outgrowth, characterized by increased collagen surrounding the terminal end buds. In post-pubertal mice, precocious ductal branching was observed and associated with increased proliferation. Focal mammary tumors appeared in a subset of animals, with a latency of approximately 15 months. Mouse mammary tumor virus/CrkII tumors showed high levels of Crk protein as well as various cytokeratin markers characteristic of their respective tumor pathologies. This study demonstrates that the precise expression of CrkII is critical for integrating signals for ductal outgrowth and branching morphogenesis during mammary gland development. Furthermore, this study provides evidence for a potential role of CrkII in integrating signals for breast cancer progression in vivo, which has important implications for elevated CrkII observed in human cancer.

Distinct roles for Crk adaptor isoforms in actin reorganization induced by extracellular signals

Crk family adaptors, consisting of Src homology 2 (SH2) and SH3 protein-binding domains, mediate assembly of protein complexes in signaling. CrkI, an alternately spliced form of Crk, lacks the regulatory phosphorylation site and C-terminal SH3 domain present in CrkII and CrkL. We used gene silencing combined with mutational analysis to probe the role of Crk adaptors in platelet-derived growth-factor receptor beta (PDGFbetaR) signaling. We demonstrate that Crk adaptors are required for formation of focal adhesions, and for PDGF-stimulated remodeling of the actin cytoskeleton and cell migration. Crk-dependent signaling is crucial during the early stages of PDGFbetaR activation, whereas its termination by Abl family tyrosine kinases is important for turnover of focal adhesions and progression of dorsal-membrane ruffles. CrkII and CrkL preferentially activate the small GTPase Rac1, whereas variants lacking a functional C-terminal SH3 domain, including CrkI, preferentially activate Rap1. Thus, differences in the activity of Crk isoforms, including their effectors and their ability to be downregulated by phosphorylation, are important for coordinating dynamic changes in the actin cytoskeleton in response to extracellular signals.

Crk and CrkL adaptor proteins: networks for physiological and pathological signaling

The Crk adaptor proteins (Crk and CrkL) constitute an integral part of a network of essential signal transduction pathways in humans and other organisms that act as major convergence points in tyrosine kinase signaling. Crk proteins integrate signals from a wide variety of sources, including growth factors, extracellular matrix molecules, bacterial pathogens, and apoptotic cells. Mounting evidence indicates that dysregulation of Crk proteins is associated with human diseases, including cancer and susceptibility to pathogen infections. Recent structural work has identified new and unusual insights into the regulation of Crk proteins, providing a rationale for how Crk can sense diverse signals and produce a myriad of biological responses.

The rho-specific guanine nucleotide exchange factor Dbs regulates breast cancer cell migration

Dbs is a Rho-specific guanine nucleotide exchange factor (RhoGEF) that regulates neurotrophin-3-induced cell migration in Schwann cells. Here we report that Dbs regulates cell motility in tumor-derived, human breast epithelial cells through activation of Cdc42 and Rac1. Cdc42 and Rac1 are activated in T47D cells that stably express onco- or proto-Dbs, and activation is dependent upon growth of the cells on collagen I. Transient suppression of expression of Cdc42 or Rac1 by small interfering RNAs attenuates Dbs-enhanced motility. Both onco- and proto-Dbs-enhanced motility correlates with an increase in tyrosine phosphorylation of focal adhesion kinase on Tyr-397 and p130(Cas) on Tyr-410 and an increase in the abundance of the Crk.p130(Cas) complex. Suppression of expression of Cdc42 or its effector, Ack1, reduces tyrosine phosphorylation of focal adhesion kinase and p130(Cas) and disrupts the Crk.p130(Cas) complex. We further determined that suppression of expression of Cdc42, Ack1, p130(Cas), or Crk reduces Rac1 activation and cell motility in Dbs-expressing cells to a level comparable with that in vector cells. Therefore, a cascade of activation of Cdc42 and Rac1 by Dbs through the Cdc42 effector Ack1 and the Crk.p130(Cas) complex is established. Suppression of the expression of endogenous Dbs reduces cell motility in both T47D cells and MDA-MB-231 cells, which correlates with the down-regulation of Cdc42 activity. This suggests that Dbs activates Cdc42 in these two human breast cancer cell lines and that the normal function of Dbs may be required to support cell movement.

Visualization of growth signal transduction cascades in living cells with genetically encoded probes based on Förster resonance energy transfer

Fluorescence probes based on the principle of Förster resonance energy transfer (FRET) have shed new light on our understanding of signal transduction cascades. Among them, unimolecular FRET probes containing fluorescence proteins are rapidly increasing in number because these genetically encoded probes can be easily loaded into living cells and allow simple acquisition of FRET images. We have developed probes for small GTPases, tyrosine kinases, serine-threonine kinases and phosphoinositides. Images obtained with these probes have revealed that membrane protrusions such as nascent lamellipodia or neurites provide an active signalling platform in the growth factor-stimulated cells.

Synthetic ligands discovered by in vitro selection

The recognition and catalytic properties of biopolymers derive from an elegant evolutionary mechanism, whereby the genetic material encoding molecules with superior functional attributes survives a selective pressure and is propagated to subsequent generations. This process is routinely mimicked in vitro to generate nucleic-acid or peptide ligands and catalysts. Recent advances in DNA-programmed organic synthesis have raised the possibility that evolutionary strategies could also be used for small-molecule discovery, but the idea remains unproven. Here, using DNA-programmed combinatorial chemistry, a collection of 100 million distinct compounds is synthesized and subjected to selection for binding to the N-terminal SH3 domain of the proto-oncogene Crk. Over six generations, the molecular population converges to a small number of novel SH3 domain ligands. Remarkably, the hits bind with affinities similar to those of peptide SH3 ligands isolated from phage libraries of comparable complexity. The evolutionary approach has the potential to drastically simplify and accelerate small-molecule discovery.

Cardiovascular and craniofacial defects in Crk-null mice

The Crk adaptor protein, which is encoded by two splice variants termed CrkI and CrkII, contains both SH2 and SH3 domains but no catalytic region. It is thought to function in signal transduction processes involved in growth regulation, cell transformation, cell migration, and cell adhesion. Although the function of Crk has been studied in considerable detail in cell culture, its biological role in vivo is still unclear, and no Crk-knockout mouse model has been available. Therefore, we generated a complete null allele of Crk in mice by using the Cre-loxP recombination approach. The majority of Crk-null mice die at late stages of embryonic development, and the remainder succumb shortly after birth. Embryos lacking both CrkI and CrkII exhibited edema, hemorrhage, and cardiac defects. Immunohistochemical examination suggested that defects in vascular smooth muscle caused dilation and rupturing of blood vessels. Problems in nasal development and cleft palate were also observed. These data indicate that Crk is involved in cardiac and craniofacial development and that it plays an essential role in maintaining vascular integrity during embryonic development.

Interaction of CagA with Crk plays an important role in Helicobacter pylori-induced loss of gastric epithelial cell adhesion

CagA protein is a major virulence factor of Helicobacter pylori, which is delivered into gastric epithelial cells and elicits growth factor-like responses. Once within the cells, CagA is tyrosine phosphorylated by Src family kinases and targets host proteins required to induce the cell responses. We show that the phosphorylated CagA binds Crk adaptor proteins (Crk-II, Crk-I, and Crk-L) and that the interaction is important for the CagA-mediated host responses during H. pylori infection. H. pylori-induced scattering of gastric epithelial cells in culture was blocked by overexpression of dominant-negative Crk and by RNA interference-mediated knockdown of endogenous Crk. H. pylori infection of the gastric epithelium induced disruption of E-cadherin/catenin-containing adherens junctions, which was also dependent on CagA/Crk signaling. Furthermore, inhibition of the SoS1/H-Ras/Raf1, C3G/Rap1/B-Raf, or Dock180/Rac1/Wiskott-Aldrich syndrome protein family verprolin homologous protein pathway, all of which are involved downstream of Crk adaptors, greatly diminished the CagA-associated host responses. Thus, CagA targeting of Crk plays a central role in inducing the pleiotropic cell responses to H. pylori infection that cause several gastric diseases, including gastric cancer.

Cortactin and Crk cooperate to trigger actin polymerization during Shigella invasion of epithelial cells

Shigella, the causative agent of bacillary dysentery, invades epithelial cells in a process involving Src tyrosine kinase signaling. Cortactin, a ubiquitous actin-binding protein present in structures of dynamic actin assembly, is the major protein tyrosine phosphorylated during Shigella invasion. Here, we report that RNA interference silencing of cortactin expression, as does Src inhibition in cells expressing kinase-inactive Src, interferes with actin polymerization required for the formation of cellular extensions engulfing the bacteria. Shigella invasion induced the recruitment of cortactin at plasma membranes in a tyrosine phosphorylation–dependent manner. Overexpression of wild-type forms of cortactin or the adaptor protein Crk favored Shigella uptake, and Arp2/3 binding–deficient cortactin derivatives or an Src homology 2 domain Crk mutant interfered with bacterial-induced actin foci formation. Crk was shown to directly interact with tyrosine-phosphorylated cortactin and to condition cortactin-dependent actin polymerization required for Shigella uptake. These results point at a major role for a Crk–cortactin complex in actin polymerization downstream of tyrosine kinase signaling.

Involvement of crk adapter proteins in regulation of lymphoid cell functions

The Crk adapter proteins consist of Src homology 2 (SH2) SH2 and SH3 domains, which bind tyrosine-phosphorylated peptides and polyproline-rich motives, respectively. They are linked to multiple signaling pathways in different cell types, including lymphocytes, and because of their lack of catalytic activity, many studies on Crk were aimed at the identification of their binding partners and determination of the physiologic meaning of these interactions. Crk proteins were found to be involved in the early steps of lymphocyte activation through their SH2-mediated transient interaction with signal-transducing molecules, such as Cbl, ZAP-70, CasL, and STAT5. In addition, Crk proteins are constitutively associated with effector molecules that mediate cell adhesion and thereby regulate lymphocyte extravasation and recruitment to sites of inflammation. This article describes selected studies of Crk, performed predominantly in lymphocytes, and discusses their potential relevance to the role of Crk in the regulation of lymphocyte functions.

CrkII induces serum response factor activation and cellular transformation through its function in Rho activation

CrkII belongs to the adaptor protein family that plays a crucial role in signal transduction. In order to better understand the biological functions of CrkII, we focused on the regulation of gene expression by CrkII. Various transcriptional control elements were examined for their activation by CrkII-expression, and we found that CrkII selectively activates the serum response element (SRE), a transcriptional control element of immediate-early genes. This SRE activation induced by CrkII-overexpression was mediated by the serum response factor (SRF) via Rho. Indeed, we confirmed that the amount of activated Rho was increased in the CrkII-expressing cells. Moreover, we showed that when overexpressed, CrkII induces the cellular transformation of NIH 3T3 cells and that a dominant negative mutant of Rho suppresses this transformation, strongly suggesting that activation of Rho is essential for the transforming activity by CrkII. Furthermore, we also found that CrkII and Galpha12, a member of the heterotrimeric G proteins, synergistically activates Rho as well as the SRF, and that an SH3 mutant of CrkII can inhibit the Galpha12-induced activation of SRF. These results strongly suggest that CrkII is involved in the activation of Rho and SRF by Galpha12. Our study provides strong evidence that Rho activation plays a crucial role in CrkII-mediated signals to induce gene expression and cellular transformation.

CrkIII: a novel and biologically distinct member of the Crk family of adaptor proteins

As the role for adaptor proteins constantly proliferates, appreciation of their importance has never been higher. The Crk family of adaptor proteins is no exception. Currently comprising four members, v-Crk, CrkI, CrkII and Crk-like protein, we have introduced a fifth member, CrkIII. Cloned by the CORT technique, CrkIII is identical in sequence to CrkII until the second of its two SH3 domains, which is disrupted partway through and results in a nonfunctional domain and a unique C-terminal sequence. We have demonstrated the existence of native CrkIII at the message level using RT-PCR and RNAse protection assays, and at the protein level in mouse fibroblasts. We show that CrkII overexpression is capable of enhancing insulin-stimulated ERK activity, whereas CrkIII is not, thus partially characterizing a novel member of the Crk family and elucidating important effects mediated by the c-terminal SH3 domain.

Crk associates with a multimolecular Paxillin/GIT2/beta-PIX complex and promotes Rac-dependent relocalization of Paxillin to focal contacts

We have previously demonstrated that the CrkII and CrkL adapter proteins are required for the spreading of epithelial colonies and the breakdown of adherens junctions in response to hepatocyte growth factor. When overexpressed, CrkII and CrkL promote lamellipodia formation, cell spreading, and the loss of epithelial adherens junctions in the absence of hepatocyte growth factor. The exact mechanism by which Crk proteins elicit these changes is unclear. We show that the overexpression of CrkII or CrkL, but not Src homology 2 or amino-terminal Src homology 3 domain mutant Crk proteins, promotes the relocalization of Paxillin to focal contacts throughout the cell and within lamellipodia in a Rac-dependent manner. In stable cell lines overexpressing CrkII, enhanced lamellipodia formation and cell spreading correlate with an increased association of CrkII with Paxillin, GIT2 (an ARF-GAP) and beta-PIX (a Rac1 exchange factor). Mutants of Paxillin that fail to associate with Crk or GIT2, or do not target to focal adhesions inhibit Crk-dependent cell spreading and lamellipodia formation. We conclude from these studies that the association of Crk with Paxillin is important for the spreading of epithelial colonies, by influencing the recruitment of Paxillin to focal complexes and promoting the enhanced assembly of Paxillin/GIT2/beta-PIX complexes.

Translocation of CrkL to focal adhesions mediates integrin-induced migration downstream of Src family kinases

The adapter protein Crk-Like (CrkL) can associate with the Src substrate p130(Cas) (Cas). The biological role of CrkL downstream of Cas, however, has been largely obscure. Consistent with the ability of CrkL to biochemically associate with Cas, we found that Src triggers translocation of CrkL to focal adhesions (FAs) in a manner dependent on Cas. Forced localization of CRKL to FAs (FA-CRKL) by itself was sufficient to induce activation of Rac1 and Cdc42 and rescued haptotaxis defects of mouse embryonic fibroblasts (MEFs) lacking Src, Yes, and Fyn, three broadly expressed Src family members required for integrin-induced migration. Consistent with Rac1 activation, FA-CRKL induced cotranslocation of a Rac1 activator, Dock1, to focal adhesions. These results therefore indicate a role for CrkL in mediating Src signaling by activating small G proteins at focal adhesions. Furthermore, MEFs lacking CrkL show impaired integrin-induced migration despite expression of a closely related protein, Crk-II, in these cells. These results therefore provide formal evidence that CrkL plays a specific role in integrin-induced migration as a downstream mediator of Src.

Adaptor protein Crk is required for ephrin-B1-induced membrane ruffling and focal complex assembly of human aortic endothelial cells

Endothelial cell migration is an essential step in vasculogenesis and angiogenesis, in which receptor tyrosine kinases play a pivotal role. We investigated the mechanism by which ephrin-B1 promotes membrane ruffling in human aortic endothelial cells, because membrane ruffling heralds cell body migration. We especially focused on the role of Crk adaptor protein in EphB-mediated signaling. Using DsRed-tagged Crk and a fluorescent time-lapse microscope, we showed that Crk was recruited to the nascent focal complex after ephrin-B1 stimulation. Furthermore, we found that p130(Cas), but not paxillin, recruited Crk to the nascent focal complex. The necessity of Crk in ephrin-B1-induced membrane ruffling was shown both by the overexpression of dominant negative Crk mutants and by the depletion of Crk by using RNA interference. Then, we examined the role of two major downstream molecules of Crk, Rac1 and Rap1. The dominant negative mutant of Rac1 completely inhibited ephrin-B1-induced membrane ruffling and focal complex assembly. In contrast, rap1GAPII, a negative regulator of Rap1, did not inhibit ephrin-B1-induced membrane ruffling. However, in rap1GAPII-expressing cells, ephrin-B1 did not induce membrane spreading, probably due to instability of the focal complex. These results indicated that Crk plays a critical role in Rac1-induced membrane ruffling and Rap1-mediated nascent focal complex stabilization contributing to ephrin-B1-induced human aortic endothelial cells migration.

Inefficient enhancement of viral infectivity and CD4 downregulation by human immunodeficiency virus type 1 Nef from Japanese long-term nonprogressors

It has been reported that patients infected with nef-defective human immunodeficiency virus type 1 (HIV-1) do not progress to AIDS; however, mutations that abrogate Nef expression are not common in long-term nonprogressors (LTNPs). We postulated that Nef function might be impaired in LTNPs, irrespective of the presence or absence of detectable amino acid sequence anomalies. To challenge this hypothesis we compared in vitro function of nef alleles that were derived from three groups of Japanese patients: LTNPs, progressors, and asymptomatic carriers (ACs). The patient-derived nef alleles were subcloned into a nef-defective infectious HIV-1 molecular clone and an expression vector. We first examined Nef-dependent enhancement of infection in a single-round infectivity assay by the use of MAGNEF cells, in which Nef is required more strictly for the infection than in the parent MAGI cells. All nef alleles from LTNPs showed reduced enhancement in the infectivity of nef-defective HIV-1 mutants compared to the nef alleles of progressors or ACs. Second, we found that nef alleles from LTNPs were less efficient in CD4 downregulation than those of progressors or ACs. Third, all nef alleles from LTNPs, progressors, and ACs reduced the cell surface expression of major histocompatibility complex class I to a similar level. Last, there was no correlation between Hck-binding activity of Nef and clinical grouping. In conclusion, we detected inefficient enhancement of HIV-1 infectivity and CD4 downregulation by HIV-1 nef alleles of LTNPs. It awaits further study to conclude that these characteristics of nef alleles are the cause or the consequence of the long-term nonprogression after HIV-1 infection.

BCR/ABL P190 transgenic mice develop leukemia in the absence of Crkl

The Bcr/Abl fusion protein directly causes chronic myelogenous leukemia and Philadelphia-chromosome positive acute lymphoblastic leukemia. Multiple independent studies have implicated Crkl, a small adapter protein, in transduction of oncogenic signals of Bcr/Abl and Crkl tyrosine-phosphorylation is used as a diagnostic tool for Philadelphia-positive leukemia. To evaluate the contribution of Crkl to this type of leukemia, we generated mutant mice that lack Crkl expression. We found that the overall survival of P190 BCR/ABL crkl-/- mice was comparable to that of genetically matched P190 BCR/ABL crkl +/+ mice. Both genotypes developed lymphoid lineage leukemia/lymphoma. Western blot analysis of -/- and +/+ lymphomas showed that the related Crk protein was tyrosine phosphorylated and could be found complexed with Bcr-Abl P190. These data indicate that possible therapeutic approaches that target Crkl may be complicated by the presence of pathways that compensate for lack of Crkl function.