p130Cas Couples the tyrosine kinase Bmx/Etk with regulation of the actin cytoskeleton and cell migration

Bmx/Etk, a member of the Tec/Btk family of nonreceptor kinases, has recently been shown to mediate cell motility in signaling pathways that become activated upon integrin-mediated cell adhesion (Chen, R., Kim, O., Li, M., Xiong, X., Guan, J. L., Kung, H. J., Chen, H., Shimizu, Y., and Qiu, Y. (2001) Nat Cell Biol. 3, 439-444). The molecular mechanisms of Bmx-induced cell motility have so far remained unknown. Previous studies by us and others have demonstrated that a complex formation between the docking protein p130Cas (Cas) and the adapter protein Crk is instrumental in connecting several stimuli to the regulation of actin cytoskeleton and cell motility. We demonstrate here that expression of Bmx leads to an interaction between Bmx and Cas at membrane ruffles, which are sites of active actin remodeling in motile cells. Expression of Bmx also enhances tyrosine phosphorylation of Cas and Cas.Crk complex formation, and coexpression of Bmx with Cas results in an enhanced membrane ruffling and haptotactic cell migration. Importantly, a mutant form of Bmx that fails to interact with Cas also fails to induce cell migration. Furthermore, expression of a dominant-negative form of Cas that is incapable of interacting with Crk inhibits Bmx-induced membrane ruffling and cell migration. These studies suggest that Bmx-Cas interaction, phosphorylation of Cas by Bmx, and subsequent Cas.Crk complex formation functionally couple Bmx to the regulation of actin cytoskeleton and cell motility.     

Regulation of cell contraction and membrane ruffling by distinct signals in migratory cells.

Cell migration and wound contraction requires assembly of actin into a functional myosin motor unit capable of generating force. However, cell migration also involves formation of actin-containing membrane ruffles. Evidence is provided that actin-myosin assembly and membrane ruffling are regulated by distinct signaling pathways in the migratory cell. Interaction of cells with extracellular matrix proteins or cytokines promote cell migration through activation of the MAP kinases ERK1 and ERK2 as well as the molecular coupling of the adaptor proteins p130CAS and c-CrkII. ERK signaling is independent of CAS/Crk coupling and regulates myosin light chain phosphorylation leading to actin-myosin assembly during cell migration and cell-mediated contraction of a collagen matrix. In contrast, membrane ruffling, but not cell contraction, requires Rac GTPase activity and the formation of a CAS/Crk complex that functions in the context of the Rac activating protein DOCK180. Thus, during cell migration ERK and CAS/Crk coupling operate as components of distinct signaling pathways that control actin assembly into myosin motors and membrane ruffles, respectively.     

CrkI and p130(Cas) complex regulates the migration and invasion of prostate cancer cells

Prostate cancer metastasis is often associated with poor prognosis. The molecular coupling of the adaptor protein Crk to the docking protein p130(Cas) serves as a switch that regulates cell migration in several invasive cancer cells and Ack appears to act upstream of CrkII to modulate the cell motility. However, the precise role of Ack, Crk and p130(Cas) complex in prostate cancer migration remains unknown. In this study we examined the expression of Crk and p130(Cas) in prostate cancer cell lines, and found that CrkI and p130(Cas) protein level was higher in highly invasive PC-3M and PC-3 cell lines than in moderately invasive DU-145 cells. Upon shRNA mediated knockdown of CrkI and p130(Cas) in PC-3M cells, cell migration and invasion were significantly inhibited as analyzed by wound healing assay and transwell invasion assay. Furthermore, co-immunoprecipitation assay showed that p130(Cas) interacted with CrkI in PC-3M cells and the stability of p130(Cas) and CrkI depended on each other. AckI interacted with both CrkI and p130(Cas) and the interaction of AckI with CrkI seemed to be independent of p130(Cas) . Taken together, our results demonstrate the high expression of CrkI and p130(Cas) in invasive prostate cancer cells and the important role of CrkI/p130(Cas) complex in the migration and invasion of prostate cancer cells. These data suggest that CrkI/p130(Cas) could be exploited as potential molecular therapeutic target for prostate cancer metastasis.     

Ack1 mediates Cdc42-dependent cell migration and signaling to p130Cas

We previously showed that activation of the small GTPase Cdc42 promotes breast cell migration on a collagen matrix. Here we further define the signaling pathways that drive this response and show that Cdc42-mediated migration relies on the adaptor molecule p130(Cas). Activated Cdc42 enhanced p130(Cas) phosphorylation and its binding to Crk. Cdc42-driven migration and p130(Cas) phosphorylation were dependent on the Cdc42 effector Ack1 (activated Cdc42-associated kinase). Ack1 formed a signaling complex that also included Cdc42, p130(Cas), and Crk, formation of which was regulated by collagen stimulation. The interaction between Ack1 and p130(Cas) occurred through their respective SH3 domains, while the substrate domain of p130(Cas) was the major site of Ack1-dependent phosphorylation. Signaling through this complex is functionally relevant, because treatment with either p130(Cas) or Ack1 siRNA blocked Cdc42-induced migration. These results suggest that Cdc42 exerts its effects on cell migration in part through its effector Ack1, which regulates p130(Cas) signaling.     

Ankyrin repeat domain 28 (ANKRD28), a novel binding partner of DOCK180, promotes cell migration by regulating focal adhesion formation

DOCK180 is a guanine exchange factor of Rac1 originally identified as a protein bound to an SH3 domain of the Crk adaptor protein. DOCK180 induces tyrosine phosphorylation of p130^Cas, and recruits the Crk-p130^Cas complex to focal adhesions. To understand the role of DOCK180 in cell adhesion and migration, we searched for DOCK180-binding proteins with a nano-LC/MS/MS system, and identified ANKRD28, a protein that contains twenty-six ankyrin domain repeats. Knockdown of ANKRD28 by RNA interference reduced the velocity of migration of HeLa cells, suggesting that this protein plays a physiologic role in the DOCK180-Rac1 signaling pathway. Furthermore, knockdown of ANKRD28 was found to alter the distribution of focal adhesion proteins such as Crk, paxillin, and p130^Cas. On the other hand, expression of ANKRD28, p130^Cas, Crk, and DOCK180 induced hyper-phosphorylation of p130^Cas, and impaired detachment of the cell membrane during migration. Consequently, cells expressing ANKRD28 exhibited multiple long cellular processes. ANKRD28 associated with DOCK180 in an SH3-dependent manner and competed with ELMO, another protein bound to the SH3 domain of DOCK180. In striking contrast to ANKRD28, overexpression of ELMO induced extensive lamellipodial protrusion around the entire circumference. These data suggest that ANKRD28 specifies the localization and the activity of the DOCK180-Rac1 pathway.     

Signaling adaptor protein v-Crk activates Rho and regulates cell motility in 3Y1 rat fibroblast cell line.

The adaptor protein Crk has been reported to associate with focal adhesions and is thought to be involved in integrin-mediated signaling pathway. However, the precise mechanism of Crk-dependent regulation of cytoskeleton still remains under investigation. In this study, we have established a v-Crk-inducible cell line in rat fibroblasts 3Y1 cells and found that v-Crk activated Rho and induced actin stress fiber formation. In addition to the induction of tyrosine-phosphorylation of p130(Cas) and paxillin, we demonstrated that v-Crk induced threonine-phosphorylated bands sized at 72/78 kDa found specifically in 3Y1 cells. Both of the inhibitors of Rho and Rho-associated kinase, C3 and Y27632, respectively, inhibited these v-Crk-induced biochemical effects. Although v-Crk-induced cells exhibited a decrease of cell motility, integrin stimulation recovered the suppression of motility. Furthermore, v-Crk enhanced motility in chemotactic assay toward fibronectin with additional activation of Rho and the increase of levels of CD44 cleavage. These results suggest that v-Crk activated Rho and induced actin stress fiber formation and CD44 cleavage leading to the regulation of cell motility.     

P130Cas Src-binding and substrate domains have distinct roles in sustaining focal adhesion disassembly and promoting cell migration

The docking protein p130Cas is a prominent Src substrate found in focal adhesions (FAs) and is implicated in regulating critical aspects of cell motility including FA disassembly and protrusion of the leading edge plasma membrane. To better understand how p130Cas acts to promote these events we examined requirements for established p130Cas signaling motifs including the SH3-binding site of the Src binding domain (SBD) and the tyrosine phosphorylation sites within the substrate domain (SD). Expression of wild type p130Cas in Cas -/- mouse embryo fibroblasts resulted in enhanced cell migration associated with increased leading-edge actin flux, increased rates of FA assembly/disassembly, and uninterrupted FA turnover. Variants lacking either the SD phosphorylation sites or the SBD SH3-binding motif were able to partially restore the migration response, while only a variant lacking both signaling functions was fully defective. Notably, the migration defects associated with p130Cas signaling-deficient variants correlated with longer FA lifetimes resulting from aborted FA disassembly attempts. However the SD mutational variant was fully defective in increasing actin assembly at the protruding leading edge and FA assembly/disassembly rates, indicating that SD phosphorylation is the sole p130Cas signaling function in regulating these processes. Our results provide the first quantitative evidence supporting roles for p130Cas SD tyrosine phosphorylation in promoting both leading edge actin flux and FA turnover during cell migration, while further revealing that the p130Cas SBD has a function in cell migration and sustained FA disassembly that is distinct from its known role of promoting SD tyrosine phosphorylation.     

Signaling mechanisms that regulate actin-based motility processes in the nervous system

Actin-based motility is critical for nervous system development. Both the migration of neurons and the extension of neurites require organized actin polymerization to push the cell membrane forward. Numerous extracellular stimulants of motility and axon guidance cues regulate actin-based motility through the rho GTPases (rho, rac, and cdc42). The rho GTPases reorganize the actin cytoskeleton, leading to stress fiber, filopodium, or lamellipodium formation. The activity of the rho GTPases is regulated by a variety of proteins that either stimulate GTP uptake (activation) or hydrolysis (inactivation). These proteins potentially link extracellular signals to the activation state of rho GTPases. Effectors downstream of the rho GTPases that directly influence actin polymerization have been identified and are involved in neurite development. The Arp2/3 complex nucleates the formation of new actin branches that extend the membrane forward. Ena/VASP proteins can cause the formation of longer actin filaments, characteristic of growth cone actin morphology, by preventing the capping of barbed ends. Actin-depolymerizing factor (ADF)/cofilin depolymerizes and severs actin branches in older parts of the actin meshwork, freeing monomers to be re-incorporated into actively growing filaments. The signaling mechanisms by which extracellular cues that guide axons to their targets lead to direct effects on actin filament dynamics are becoming better understood.     

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.     

Regulation of focal adhesion and cell migration by ANKRD28-DOCK180 interaction

DOCK180 is an atypical guanine nucleotide exchange factor of Rac1 identified originally as one of the two major proteins bound to the SH3 domain of the Crk adaptor protein. DOCK180 induces tyrosine phosphorylation of p130^Cas, and recruits the Crk-p130^Cas complex to focal adhesions. Recently, we searched for DOCK180-binding proteins with a nano-LC/MS/MS system, and found that ANKRD28, a protein with twenty-six ankyrin domain-repeats, interacts with the SH3 domain of DOCK180. Knockdown of ANKRD28 reduced the migration velocity and altered the distribution of focal adhesion proteins such as Crk, paxillin, and p130^Cas. On the other hand, the expression of ANKRD28, p130^Cas, Crk, and DOCK180 induced hyper-phosphorylation of p130^Cas, which paralleled the induction of multiple long cellular processes. Depletion of ELMO, another protein bound to the SH3 domain of DOCK180, also retarded cell migration, but its expression together with p130^Cas, Crk, and DOCK180 induced extensive lamellipodial protrusion around the entire circumference without 130^Cas hyperphosphorylation. These data suggest the dual modes of DOCK180-Rac regulation for cell migration.     

An evolutionarily conserved autoinhibitory molecular switch in ELMO proteins regulates Rac signaling

Dedicator of cytokinesis (DOCK) proteins are guanine nucleotide exchange factors (GEFs) controlling the activity of Rac1/Cdc42 during migration, phagocytosis, and myoblast fusion [1-4]. Engulfment and cell motility (ELMO) proteins bind a subset of DOCK members and are emerging as critical regulators of Rac signaling [5-10]. Although formation of a DOCK180/ELMO complex is not essential for Rac1 activation, ELMO mutants deficient in binding to DOCK180 are unable to promote cytoskeleton remodeling [11]. How ELMO regulates signaling through DOCK GEFs is poorly understood. Here, we identify an autoinhibitory switch in ELMO presenting homology to a regulatory unit described for Dia formins. One part of the switch, composed of a Ras-binding domain (RBD) and Armadillo repeats, is positioned N-terminally while the other is housed in the C?terminus. We demonstrate interaction between these fragments, suggesting autoinhibition of ELMO. Using a bioluminescence resonance energy transfer biosensor, we establish that ELMO undergoes conformational changes upon disruption of autoinhibition. We found that engagement of ELMO to RhoG, or with DOCK180, promotes the relief of autoinhibition in ELMO. Functionally, we found that ELMO mutants with impaired autoregulatory activity promote cell?elongation. These results demonstrate an unsuspected level of regulation for Rac1 signaling via autoinhibition of ELMO.     

The docking protein Cas links tyrosine phosphorylation signaling to elongation of cerebellar granule cell axons

Crk-associated substrate (Cas) is a tyrosine-phosphorylated docking protein that is indispensable for the regulation of the actin cytoskeletal organization and cell migration in fibroblasts. The function of Cas in neurons, however, is poorly understood. Here we report that Cas is dominantly enriched in the brain, especially the cerebellum, of postnatal mice. During cerebellar development, Cas is highly tyrosine phosphorylated and is concentrated in the neurites and growth cones of granule cells. Cas coimmunoprecipitates with Src family protein tyrosine kinases, Crk, and cell adhesion molecules and colocalizes with these proteins in granule cells. The axon extension of granule cells is inhibited by either RNA interference knockdown of Cas or overexpression of the Cas mutant lacking the YDxP motifs, which are tyrosine phosphorylated and thereby interact with Crk. These findings demonstrate that Cas acts as a key scaffold that links the proteins associated with tyrosine phosphorylation signaling pathways to the granule cell axon elongation.     

Functional proteomics identifies protein-tyrosine phosphatase 1B as a target of RhoA signaling

Rho GTPases are signal transduction effectors that control cell motility, cell attachment, and cell shape by the control of actin polymerization and tyrosine phosphorylation. To identify cellular targets regulated by Rho GTPases, we screened global protein responses to Rac1, Cdc42, and RhoA activation by two-dimensional gel electrophoresis and mass spectrometry. A total of 22 targets were identified of which 19 had never been previously linked to Rho GTPase pathways, providing novel insight into pathway function. One novel target of RhoA was protein-tyrosine phosphatase 1B (PTP1B), which catalyzes dephosphorylation of key signaling molecules in response to activation of diverse pathways. Subsequent analysis demonstrated that RhoA enhances post-translational modification of PTP1B, inactivates phosphotyrosine phosphatase activity, and up-regulates tyrosine phosphorylation of p130Cas, a key mediator of focal adhesion turnover and cell migration. Thus, protein profiling reveals a novel role for PTP1B as a mediator of RhoA-dependent phosphorylation of p130Cas.     

Rac mediates cytoskeletal rearrangements and increased cell motility induced by urokinase-type plasminogen activator receptor binding to vitronectin

The urokinase-type plasminogen activator receptor (uPAR) is involved in the regulation of cell motility in a variety of cell types. We show here that expression of human uPAR in growing murine fibroblasts leads to a dramatic reorganization of the actin cytoskeleton. uPAR expression induces multiple rapidly advancing protrusions that resemble the leading edge of migrating cells. The cytoskeletal changes are independent of uPA and activation of the RGD-binding activity of integrins but require uPAR binding to vitronectin (VN). The actin reorganization is blocked by coexpression of dominant negative versions of either Rac (N17Rac) or p130Cas, but not by inhibitors of Cdc42 or Rho, and is accompanied by a Rac-dependent increase in cell motility. In addition, a fourfold increase in the level of activated Rac is induced by uPAR expression. We conclude that uPAR interacts with VN both to initiate a p130Cas/Rac-dependent signaling pathway leading to actin reorganization and increased cell motility and to act as an adhesion receptor required for these responses. This mechanism may play a role in uPAR-mediated regulation of cell motility at sites where VN and uPAR are co-expressed, such as malignant tumors.     

Laminin-10/11 and fibronectin differentially regulate integrin-dependent Rho and Rac activation via p130(Cas)-CrkII-DOCK180 pathway

The alpha(5) chain-containing laminin isoforms, laminins-10 and -11 (laminin-10/11), are the major components of the basement membrane, having potent cell-adhesive activity. We examined the cell-adhesive and integrin-mediated signaling activities of laminin-10/11 in comparison to fibronectin, the best characterized extracellular adhesive ligand. We found that laminin-10/11 are more active than fibronectin in promoting cell migration and preferentially activate Rac, not Rho, via the p130(Cas)-CrkII-DOCK180 pathway. Cells adhering to fibronectin develop stress fibers and focal contacts, whereas cells adhering to laminin-10/11 do not, consistent with the high cell migration-promoting activity of laminin-10/11. Pull-down assays of GTP-loaded Rac and Rho demonstrated the preferential activation of Rac on laminin-10/11, in contrast to the activation of Rho on fibronectin. Activation of Rac by laminin-10/11 was associated with the phosphorylation of p130(Cas) and an increased formation of a p130(Cas)-CrkII-DOCK180 complex. Cell migration on laminin-10/11 was suppressed by the expression of either a dominant-negative Rac or CrkII mutants defective in p130(Cas) or DOCK180 binding. This is the first report demonstrating a distinct activation of Rho family GTPases resulting from adhesion to different extracellular ligands.     

The SH2 domain protein Shep1 regulates the in vivo signaling function of the scaffolding protein Cas

The members of the p130Cas (Cas) family are important scaffolding proteins that orchestrate cell adhesion, migration and invasiveness downstream of integrin adhesion receptors and receptor tyrosine kinases by recruiting enzymes and structural molecules. Shep1, BCAR3/AND-34 and NSP1 define a recently identified family of SH2 domain-containing proteins that constitutively bind Cas proteins through a Cdc25-type nucleotide exchange factor-like domain. To gain insight into the functional interplay between Shep1 and Cas in vivo, we have inactivated the Shep1 gene in the mouse through Cre-mediated deletion of the exon encoding the SH2 domain. Analysis of Cas tyrosine phosphorylation in the brains of newborn mice, where Shep1 is highly expressed, revealed a strong decrease in Cas substrate domain phosphorylation in knockout compared to wild-type brains. Src family kinases bind to Cas via their SH3 and SH2 domains, which contributes to their activation, and phosphorylate multiple tyrosines in the Cas substrate domain. These tyrosine-phosphorylated motifs represent docking sites for the Crk adaptor, linking Cas to the downstream Rac1 and Rap1 GTPases to regulate cell adhesion and actin cytoskeleton organization. Accordingly, we detected lower Cas–Crk association and lower phosphorylation of the Src activation loop in Shep1 knockout brains compared to wild-type. Conversely, Shep1 transfection in COS cells increases Cas tyrosine phosphorylation. The SH2 domain is likely critical for the effects of Shep1 on Cas and Src signaling because the knockout mice express Shep1 fragments that lack the amino-terminal region including the SH2 domain, presumably due to aberrant translation from internal ATG codons. These fragments retain the ability to increase Cas levels in transfected cells, similar to full-length Shep1. However, they do not affect Cas phosphorylation on their own or in the presence of co-transfected full-length Shep1. They also do not show dominant negative effects on the activity of full-length Shep1 in vivo because the heterozygous mice, which express the fragments, have a normal life span. This is in contrast to the homozygous knockout mice, most of which die soon after birth. These data demonstrate that Shep1 plays a critical role in the in vivo regulation of Src activity and Cas downstream signaling through Crk, and suggest that the SH2 domain of Shep1 is critical for these effects.     

The integrin alpha 7 cytoplasmic domain regulates cell migration, lamellipodia formation, and p130CAS/Crk coupling

The integrin alpha(7)beta(1) is the major laminin-binding integrin in skeletal, heart, and smooth muscle and is a receptor for laminin-1 and -2. It mediates myoblast migration on laminin-1 and -2 and thus might be involved in muscle development and repair. Previously we have shown that alpha(7)B as well as the alpha(7)A and -C splice variants induce cell motility on laminin when transfected into nonmotile HEK293 cells. In this study we have investigated the role of the cytoplasmic domain of alpha(7) in the laminin-induced signal transduction of alpha(7)beta(1) integrin regulating cell adhesion and migration. Deletion of the cytoplasmic domain did not affect assembly of the mutated alpha(7)Deltacyt/beta(1) heterodimer on the cell surface or adhesion of alpha(7)Deltacyt-transfected cells to laminin. The motility of these cells on the laminin-1/E8 fragment, however, was significantly reduced to the level of mock-transfected cells; lamellipodia formation and polarization of the cells were also impaired. Adhesion to the laminin-1/E8 fragment induced tyrosine phosphorylation of the focal adhesion kinase, paxillin, and p130(CAS) as well as the formation of a p130(CAS)-Crk complex in wild-type alpha(7)B-transfected cells. In alpha(7)BDeltacyt cells, however, the extent of p130(CAS) tyrosine formation was reduced and formation of the p130(CAS)-Crk complex was impaired, with unaltered levels of p130(CAS) and Crk protein levels. These findings indicate adhesion-dependent regulation of p130(CAS)/Crk complex formation by the cytoplasmic domain of alpha(7)B integrin after cell adhesion to laminin-1/E8 and imply alpha(7)B-controlled lamellipodia formation and cell migration through the p130(CAS)/Crk protein complex.     

Differential regulation of cell migration,actin stress fiber organization,and cell transformation by functional domains of Crk-associated substrate

The Crk-associated substrate (Cas) is a unique docking protein that possesses a repetitive stretch of tyrosine-containing motifs and an Src homology 3 (SH3) domain. Embryonic fibroblasts lacking Cas demonstrated resistance to Src-induced transformation along with impaired actin bundling and cell motility, indicating critical roles of Cas in actin cytoskeleton organization, cell migration, and oncogenesis. To gain further insight into roles of each domain of Cas in these processes, a compensation assay was performed by expressing a series of Cas mutants in Cas-deficient fibroblasts. The results showed that motifs containing YDxP were indispensable for actin cytoskeleton organization and cell migration, suggesting that CrkII-mediated signaling regulates these biological processes. The C-terminal Src-binding domain played essential roles in cell migration and membrane localization of Cas, although it was dispensable in the organization of actin stress fibers. Furthermore, the Src-binding domain was also a prerequisite for Src transformation possibly, because of its crucial role in the phosphorylation of Cas during transformation. Overall, differential uses of the Cas domains in individual biological processes were demonstrated.     

CrkI and CrkII function as key signaling integrators for migration and invasion of cancer cells

Crk adaptor proteins play an important role during cellular signaling by mediating the formation of protein complexes. Increased levels of Crk proteins are observed in several human cancers and overexpression of Crk in epithelial cell cultures promotes enhanced cell dispersal and invasion, implicating Crk as a regulator of invasive responses. To determine the requirement of Crk for invasive signals, we targeted the CRKI/II gene by RNA interference. Consistent knockdown of CrkI/II was observed with two small interfering RNA targeting sequences in all human cancer cell lines tested. CrkI/II knockdown resulted in a significant decrease in migration and invasion of multiple malignant breast and other human cancer cell lines (MDA-231, MDA-435s, H1299, KB, and HeLa). Moreover, CrkI/II knockdown decreased cell spreading on extracellular matrix and led to a decrease in actin stress fibers and the formation of mature focal adhesions. Using immunohistochemistry, we show elevated CrkI/II protein levels in patients with breast adenocarcinoma. Together, these studies identify Crk adaptor proteins as critical integrators of upstream signals for cell invasion and migration in human cancer cell lines and support a role for Crk in metastatic spread.     

Regulation of integrin-mediated cellular responses through assembly of a CAS/Crk scaffold

The molecular coupling of CAS and Crk in response to integrin activation is an evolutionary conserved signaling module that controls cell proliferation, survival and migration. However, when deregulated, CAS/Crk signaling also contributes to cancer progression and developmental defects in humans. Here we highlight recent advances in our understanding of how CAS/Crk complexes assemble in cells to modulate the actin cytoskeleton, and the molecular mechanisms that regulate this process. We discuss in detail the spatiotemporal dynamics of CAS/Crk assembly and how this scaffold recruits specific effector proteins that couple integrin signaling networks to the migration machinery of cells. We also highlight the importance of CAS/Crk signaling in the dual regulation of cell migration and survival mechanisms that operate in invasive cells during development and pathological conditions associated with cancer metastasis.