Csk defines the ability of integrin-mediated cell adhesion and migration in human colon cancer cells: implication for a potential role in cancer metastasis

CSK-mediated signalling by integrins in cancer

Cancer progression and metastasis are processes heavily controlled by the integrin receptor family. Integrins are cell adhesion molecules that constitute the central components of mechanosensing complexes called focal adhesions, which connect the extracellular environment with the cell interior. Focal adhesions act as key players in cancer progression by regulating biological processes, such as cell migration, invasion, proliferation, and survival. Src family kinases (SFKs) can interplay with integrins and their downstream effectors. SFKs also integrate extracellular cues sensed by integrins and growth factor receptors (GFR), transducing them to coordinate metastasis and cell survival in cancer. The non-receptor tyrosine kinase CSK is a well-known SFK member that suppresses SFK activity by phosphorylating its specific negative regulatory loop (C-terminal Y⁵²⁷ residue). Consequently, CSK may play a pivotal role in tumour progression and suppression by inhibiting SFK oncogenic effects in several cancer types. Remarkably, CSK can localise near focal adhesions when SFKs are activated and even interact with focal adhesion components, such as phosphorylated FAK and Paxillin, among others, suggesting that CSK may regulate focal adhesion dynamics and structure. Even though SFK oncogenic signalling has been extensively described before, the specific role of CSK and its crosstalk with integrins in cancer progression, for example, in mechanosensing, remain veiled. Here, we review how CSK, by regulating SFKs, can regulate integrin signalling, and focus on recent discoveries of mechanotransduction. We additionally examine the cross talk of integrins and GFR as well as the membrane availability of these receptors in cancer. We also explore new pharmaceutical approaches to these signalling pathways and analyse them as future therapeutic targets.

Regulation, targets and functions of CSK

The Src family kinases (SFK) plays an important role in multiple signal transduction pathways. Aberrant activation of SFKs leads to diseases such as cancer, blood disorders, and bone pathologies. By phosphorylating and inactivating SFKs, the C-terminal Src kinase (CSK) serves as the key negative regulator of SFKs. Similar to Src, CSK is composed of SH3, SH2, and a catalytic kinase domain. However, while the Src kinase domain is intrinsically active, the CSK kinase domain is intrinsically inactive. Multiple lines of evidence indicate that CSK is involved in various physiological processes including DNA repair, permeability of intestinal epithelial cells (IECs), synaptic activity, astrocyte-to-neuron communication, erythropoiesis, platelet homeostasis, mast cell activation, immune and inflammation responses. As a result, dysregulation of CSK may lead to many diseases with different underlying molecular mechanisms. Furthermore, recent findings suggest that in addition to the well-established CSK-SFK axis, novel CSK-related targets and modes of CSK regulation also exist. This review focuses on the recent progress in this field for an up-to-date understanding of CSK.

ELTD1 Promotes Gastric Cancer Cell Proliferation, Invasion and Epithelial-Mesenchymal Transition Through MAPK/ERK Signaling by Regulating CSK

Purpose

Patients with gastric cancer (GC) often die from metastasis. However, the exact molecular mechanism underlying GC metastasis is complicated and still remains elusive. Epidermal growth factor, latrophilin and seven-transmembrane domain-containing 1 (ELTD1), has been reported to be involved in cancer metastasis, but its role in GC is still missing.

Patients and Methods

We first analyzed the expression of ELTD1 in GC using public databases (TCGA, Oncomine, and GEO) and our clinical samples. The functions of ELTD1 in GC proliferation, invasion and metastasis were determined by in vitro and in vivo experiments. The functional mechanism of ETLD1 in GC was also investigated. Finally, the association between ELTD1 expression and the overall survival of GC patients was analyzed using public databases.

Results

ELTD1 is significantly upregulated in GC tissues. Knockdown of ELTD1 inhibits GC cell proliferation, migration and invasion in vitro as well as tumor growth and metastasis in vivo, while ELTD1 overexpression obtains opposite results. Moreover, ELTD1 could promote epithelial to mesenchymal transition (EMT) in GC. Mechanistically, ELTD1 exerts its tumor-promoting effect by activating MAPK/ERK signaling. Subsequent studies demonstrated that ELTD1 could interact with C-terminal Src kinase (CSK) and inhibit its expression, which finally lead to MAPK/ERK activation. Data from TGCA and GEO both revealed that GC patients with high ELTD1 expression had poorer prognosis and the combination of ELTD1 with CSK showed better predictive performance.

Conclusion

ELTD1 plays an oncogene role in GC through MAPK/ERK signaling via inhibiting CSK, which may be a useful prognostic predictor and potential therapeutic target for GC.

Regulation of Src Family Kinases during Colorectal Cancer Development and Its Clinical Implications

Src family kinases (SFKs) are non-receptor kinases that play a critical role in the pathogenesis of colorectal cancer (CRC). The expression and activity of SFKs are upregulated in patients with CRC. Activation of SFKs promotes CRC cell proliferation, metastases to other organs and chemoresistance, as well as the formation of cancer stem cells (CSCs). The enhanced expression level of Src is associated with decreased survival in patients with CRC. Src-mediated regulation of CRC progression involves various membrane receptors, modulators, and suppressors, which regulate Src activation and its downstream targets through various mechanisms. This review provides an overview of the current understanding of the correlations between Src and CRC progression, with a special focus on cancer cell proliferation, invasion, metastasis and chemoresistance, and formation of CSCs. Additionally, this review discusses preclinical and clinical strategies to improve the therapeutic efficacy of drugs targeting Src for treating patients with CRC.

Drosophila Jak/STAT Signaling: Regulation and Relevance in Human Cancer and Metastasis

Over the past three-decades, Janus kinase (Jak) and signal transducer and activator of transcription (STAT) signaling has emerged as a paradigm to understand the involvement of signal transduction in development and disease pathology. At the molecular level, cytokines and interleukins steer Jak/STAT signaling to transcriptional regulation of target genes, which are involved in cell differentiation, migration, and proliferation. Jak/STAT signaling is involved in various types of blood cell disorders and cancers in humans, and its activation is associated with carcinomas that are more invasive or likely to become metastatic. Despite immense information regarding Jak/STAT regulation, the signaling network has numerous missing links, which is slowing the progress towards developing drug therapies. In mammals, many components act in this cascade, with substantial cross-talk with other signaling pathways. In Drosophila, there are fewer pathway components, which has enabled significant discoveries regarding well-conserved regulatory mechanisms. Work across species illustrates the relevance of these regulators in humans. In this review, we showcase fundamental Jak/STAT regulation mechanisms in blood cells, stem cells, and cell motility. We examine the functional relevance of key conserved regulators from Drosophila to human cancer stem cells and metastasis. Finally, we spotlight less characterized regulators of Drosophila Jak/STAT signaling, which stand as promising candidates to be investigated in cancer biology. These comparisons illustrate the value of using Drosophila as a model for uncovering the roles of Jak/STAT signaling and the molecular means by which the pathway is controlled.

Protein Tyrosine Signaling and its Potential Therapeutic Implications in Carcinogenesis

Protein tyrosine phosphorylation is a crucial signaling mechanism that plays a role in epithelial carcinogenesis. Protein tyrosine kinases (PTKs) control various cellular processes including growth, differentiation, metabolism, and motility by activating major signaling pathways including STAT3, AKT, and MAPK. Genetic mutation of PTKs and/or prolonged activation of PTKs and their downstream pathways can lead to the development of epithelial cancer. Therefore, PTKs became an attractive target for cancer prevention. PTK inhibitors are continuously being developed, and they are currently used for the treatment of cancers that show a high expression of PTKs. Protein tyrosine phosphatases (PTPs), the homeostatic counterpart of PTKs, negatively regulate the rate and duration of phosphotyrosine signaling. PTPs initially were considered to be only housekeeping enzymes with low specificity. However, recent studies have demonstrated that PTPs can function as either tumor suppressors or tumor promoters, depending on their target substrates. Together, both PTK and PTP signal transduction pathways are potential therapeutic targets for cancer prevention and treatment.

Inhibitory Effects of PEI-RGD/125I-(αv) ASODN on Growth and Invasion of HepG2 Cells

Background

To investigate the in vitro inhibitory effects of PEI-RGD/¹²⁵I-(α_V)ASODN (PEI, polyethylenimine; RGD, Arg-Gly-Asp; ASODN, antisense oligodeoxynucleotide) on the growth and invasion of HepG2 cells.

Material/Methods

ASODN of the integrin α_V-subunit was marked with ¹²⁵I and underwent complexation with PEI-RGD, a PEI derivative. Next, PEI-RGD/¹²⁵I-(α_V) ASODN was introduced into HepG2 cells via receptor-mediated transfection, and its inhibition rate on HepG2 cell growth was tested using the methyl thiazolyl tetrazolium (MTT) method. The effects of PEI-RGD/¹²⁵I-(α_V) ASODN on HepG2 cell invasion ability were evaluated using the Boyden chamber assay.

Results

1) The ¹²⁵I marking rate of (α_V) ASODN was 73.78±4.09%, and the radiochemical purity was 96.68±1.38% (greater than 90% even after a 48-h incubation period at 37°C), indicating high stability. 2) The cytotoxicity assays showed that the cell inhibition rates did not differ significantly between the PEI-RGD/¹²⁵I-(α_V)ASODN group and the PEI-RGD/(α_V) ASODN group, but they were both significantly higher than in the other groups and were positively correlated (r=0.879) with the dosage within a certain range. 3) The invasion assays showed that the inhibition rate was significantly greater in the PEI-RGD/¹²⁵I-(α_V) ASODN group compared to the other groups.

Conclusions

PEI-RGD/¹²⁵I-(α_V) ASODN can efficiently inhibit the growth and proliferation of HepG2 cells and can also weaken their invasive ability.

FYNagling divergent adhesive functions for Fyn in keratinocytes

Fyn, a member of the Src family kinases (SFKs), has been shown to play important yet contradictory roles in keratinocyte (KC) adhesion. During KC differentiation, physiological activation of Fyn results in the formation of adherens junctions, recruiting junctional components and inducing signaling pathways that control the differentiation program. However, in KC transformation and oncogenesis, increased Fyn activity has been implicated in the dissolution of adhesion structures and an increased migratory phenotype. Fyn activity is also associated with both the formation and dissolution of focal adhesions, and to a lesser extent hemidesmosomes and desmosomes. This viewpoint article aims to reconcile these disparate bodies of literature regarding Fyn's role in cell-cell and cell-matrix adhesion by proposing several alternative, testable hypotheses that unify Fyn's fractured functions.

Targeting Fyn in Ras-transformed cells induces F-actin to promote adherens junction-mediated cell-cell adhesion

Fyn, a member of the Src family kinases (SFK), is an oncogene in murine epidermis and is associated with cell-cell adhesion turnover and induction of cell migration. Additionally, Fyn upregulation has been reported in multiple tumor types, including cutaneous squamous cell carcinoma (cSCC). Introduction of active H-Ras(G12V) into the HaCaT human keratinocyte cell line resulted in upregulation of Fyn mRNA (200-fold) and protein, while expression of other SFKs remained unaltered. Transduction of active Ras or Fyn was sufficient to induce an epithelial-to-mesenchymal transition in HaCaT cells. Inhibition of Fyn activity, using siRNA or the clinical SFK inhibitor Dasatinib, increased cell-cell adhesion and rapidly (5-60 min) increased levels of cortical F-actin. Fyn inhibition with siRNA or Dasatinib also induced F-actin in MDA-MB-231 breast cancer cells, which have elevated Fyn. F-actin co-localized with adherens junction proteins, and Dasatinib-induced cell-cell adhesion could be blocked by Cytochalasin D, indicating that F-actin polymerization was a key initiator of cell-cell adhesion through the adherens junction. Conversely, inhibiting cell-cell adhesion with low Ca(2+) media did not block Dasatinib-induced F-actin polymerization. Inhibition of the Rho effector kinase ROCK blocked Dasatinib-induced F-actin and cell-cell adhesion, implicating relief of Rho GTPase inhibition as a mechanism of Dasatinib-induced cell-cell adhesion. Finally, topical Dasatinib treatment significantly reduced total tumor burden in the SKH1 mouse model of UV-induced skin carcinogenesis. Together these results identify the promotion of actin-based cell-cell adhesion as a newly described mechanism of action for Dasatinib and suggest that Fyn inhibition may be an effective therapeutic approach in treating cSCC.

Chemopreventive activity of plant flavonoid isorhamnetin in colorectal cancer is mediated by oncogenic Src and β-catenin

Analysis of the Polyp Prevention Trial showed an association between an isorhamnetin-rich diet and a reduced risk of advanced adenoma recurrence; however, the mechanism behind the chemoprotective effects of isorhamnetin remains unclear. Here, we show that isorhamnetin prevents colorectal tumorigenesis of FVB/N mice treated with the chemical carcinogen azoxymethane and subsequently exposed to colonic irritant dextran sodium sulfate (DSS). Dietary isorhamnetin decreased mortality, tumor number, and tumor burden by 62%, 35%, and 59%, respectively. MRI, histopathology, and immunohistochemical analysis revealed that dietary isorhamnetin resolved the DSS-induced inflammatory response faster than the control diet. Isorhamnetin inhibited AOM/DSS-induced oncogenic c-Src activation and β-catenin nuclear translocation, while promoting the expression of C-terminal Src kinase (CSK), a negative regulator of Src family of tyrosine kinases. Similarly, in HT-29 colon cancer cells, isorhamnetin inhibited oncogenic Src activity and β-catenin nuclear translocation by inducing expression of csk, as verified by RNA interference knockdown of csk. Our observations suggest the chemoprotective effects of isorhamnetin in colon cancer are linked to its anti-inflammatory activities and its inhibition of oncogenic Src activity and consequential loss of nuclear β-catenin, activities that are dependent on CSK expression.

Fulvestrant-induced cell death and proteasomal degradation of estrogen receptor α protein in MCF-7 cells require the CSK c-Src tyrosine kinase

Fulvestrant is a representative pure antiestrogen and a Selective Estrogen Receptor Down-regulator (SERD). In contrast to the Selective Estrogen Receptor Modulators (SERMs) such as 4-hydroxytamoxifen that bind to estrogen receptor α (ERα) as antagonists or partial agonists, fulvestrant causes proteasomal degradation of ERα protein, shutting down the estrogen signaling to induce proliferation arrest and apoptosis of estrogen-dependent breast cancer cells. We performed genome-wide RNAi knockdown screenings for protein kinases required for fulvestrant-induced apoptosis of the MCF-7 estrogen-dependent human breast caner cells and identified the c-Src tyrosine kinase (CSK), a negative regulator of the oncoprotein c-Src and related protein tyrosine kinases, as one of the necessary molecules. Whereas RNAi knockdown of CSK in MCF-7 cells by shRNA-expressing lentiviruses strongly suppressed fulvestrant-induced cell death, CSK knockdown did not affect cytocidal actions of 4-hydroxytamoxifen or paclitaxel, a chemotherapeutic agent. In the absence of CSK, fulvestrant-induced proteasomal degradation of ERα protein was suppressed in both MCF-7 and T47D estrogen-dependent breast cancer cells whereas the TP53-mutated T47D cells were resistant to the cytocidal action of fulvestrant in the presence or absence of CSK. MCF-7 cell sensitivities to fulvestrant-induced cell death or ERα protein degradation was not affected by small-molecular-weight inhibitors of the tyrosine kinase activity of c-Src, suggesting possible involvement of other signaling molecules in CSK-dependent MCF-7 cell death induced by fulvestrant. Our observations suggest the importance of CSK in the determination of cellular sensitivity to the cytocidal action of fulvestrant.

Cell-surface GRP78 facilitates colorectal cancer cell migration and invasion

Glucose regulated protein 78 (GRP78) is predominantly located in the endoplasmic reticulum as a molecular chaperone. It has also been found on the membranes of some cancer cells, acting as a receptor for a wide variety of ligands. However, its presence on colorectal cancer (CRC) cell surface and its role in CRC metastatic progression remain elusive. Here we reported that GRP78 was predominantly present in the form of clustering aggregates on CRC cell surfaces, and its surface abundance was strongly correlated with CRC differentiation stage. Interestingly, we observed that cell-surface GRP78 had an interaction with the ECM adhesion molecule β1-integrin and was involved in cell-matrix adhesion through regulation of focal adhesion kinase (FAK). Moreover, the present data also implicated that surface GRP78 promoted the cell invasion process, and this effect was at least partly mediated through its association with uPA-uPAR protease system. Together, our data suggests that surface GRP78 promotes CRC cell migration and invasion by regulating cell-matrix adhesion and ECM degradation, which is independent of its signaling receptor function.

Regulation of adherens junction dynamics by phosphorylation switches

Adherens junctions connect the actin cytoskeleton of neighboring cells through transmembrane cadherin receptors and a network of adaptor proteins. The interactions between these adaptors and cadherin as well as the activity of actin regulators localized to adherens junctions are tightly controlled to facilitate cell junction assembly or disassembly in response to changes in external or internal forces and/or signaling. Phosphorylation of tyrosine, serine, or threonine residues acts as a switch on the majority of adherens junction proteins, turning "on" or "off" their interactions with other proteins and/or their enzymatic activity. Here, we provide an overview of the kinases and phosphatases regulating phosphorylation of adherens junction proteins and bring examples of phosphorylation events leading to the assembly or disassembly of adherens junctions, highlighting the important role of phosphorylation switches in regulating their dynamics.

Lyn kinase mediates cell motility and tumor growth in EGFRvIII-expressing head and neck cancer

PURPOSE

EGF receptor variant III (EGFRvIII) has been detected in several cancers in which tumors expressing this truncated growth factor receptor show more aggressive behavior. The molecular mechanisms that contribute to EGFRvIII-mediated tumor progression that are amenable to targeted therapy are incompletely understood. The present study aimed to better define the role of Src family kinases (SFKs) in EGFRvIII-mediated cell motility and tumor growth of head and neck squamous cell carcinomas (HNSCC).

EXPERIMENTAL DESIGN

HNSCC models expressing EGFRvIII were treated with dasatinib, a pharmacologic inhibitor of SFKs.

RESULTS

SFK inhibition significantly decreased cell proliferation, migration, and invasion of EGFRvIII-expressing HNSCC cells. Administration of dasatinib to mice bearing EGFRvIII-expressing HNSCC xenografts resulted in a significant reduction of tumor volume compared with controls. Immunoprecipitation with anti-c-Src, Lyn, Fyn, and Yes antibodies followed by immunoblotting for phosphorylation of the SFK activation site (Y416) showed specific activation of Lyn kinase in EGFRvIII-expressing HNSCC cell lines and human HNSCC tumor specimens. Selective inhibition of Lyn using siRNA decreased cell migration and invasion of EGFRvIII-expressing HNSCCs compared with vector control cells.

CONCLUSIONS

These findings show that Lyn mediates tumor progression of EGFRvIII-expressing HNSCCs in which strategies to inhibit SFK may represent an effective therapeutic strategy.

Cooperative role of caveolin-1 and C-terminal Src kinase binding protein in C-terminal Src kinase-mediated negative regulation of c-Src

In the present study, we assessed the cooperative roles of C-terminal Src kinase (Csk) binding protein (Cbp) and Caveolin-1 (Cav-1) in the mechanism of Src family tyrosine kinase (SFK) inhibition by Csk. SFKs are inactivated by phosphorylation of their C-terminal tyrosine by Csk. Whereas SFKs are membrane-associated, Csk is a cytoplasmic protein and therefore requires membrane adaptors such as Cbp or Cav-1 for recruitment to the plasma membrane to mediate SFK inhibition. To determine the specific role of Cav-1 and Cbp in SFK inhibition, we measured c-Src activity in the absence of each membrane adaptor. It is noteworthy that in lungs and fibroblasts from Cav-1(-/-) mice, we observed increased expression of Cbp compared with wild-type (WT) controls. However, both c-Src activity and Csk localization at the membrane were similar between Cav-1(-/-) fibroblasts and WT cells. Likewise, Cbp depletion by small interfering RNA (siRNA) treatment of WT cells had no effect on basal c-Src activity, but it increased the phosphorylation state of Cav-1. Immunoprecipitation then confirmed increased association of Csk with phosphomimicking Cav-1. Knockdown of Cbp by siRNA in Cav-1(-/-) cells revealed increased basal c-Src activity, and re-expression of WT Cav-1 in the same cells reduced basal c-Src activity. Taken together, these results indicate that Cav-1 and Cbp cooperatively regulate c-Src activity by recruiting Csk to the membrane where it phosphorylates c-Src inhibitory tyrosine 529. Furthermore, when either Cav-1 or Cbp expression is reduced or absent, there is a compensatory increase in the phosphorylation state or expression level of the other membrane-associated Csk adaptor to maintain SFK inhibition.

Rack1 promotes epithelial cell-cell adhesion by regulating E-cadherin endocytosis

E-cadherin and its cytoplasmic partners, catenins, mediate epithelial cell-cell adhesion. Disruption of this adhesion allows cancer cells to invade and metastasize. Aberrant activation of the Src tyrosine kinase disrupts cell-cell contacts through an E-cadherin/catenin-dependent mechanism. Previously we showed that Rack1 regulates the growth of colon cells by suppressing Src activity at G(1) and mitotic checkpoints, and in the intrinsic apoptotic and Akt cell survival pathways. Here we show that Rack1, partly by inhibiting Src, promotes cell-cell adhesion and reduces the invasive potential of colon cancer cells. Rack1 stabilizes E-cadherin and catenins at cell-cell contacts by inhibiting the Src phosphorylation of E-cadherin, the ubiquitination of E-cadherin by the E3 ligase Hakai and the endocytosis of E-cadherin. Upon depletion and restoration of extracellular calcium, Rack1 facilitates the re-assembly of E-cadherin-containing cell-cell contacts. Rack1 also blocks HGF-induced endocytosis of E-cadherin, disruption of cell-cell contacts and cell scatter. Our results uncover a novel function of Rack1 in maintaining the junctional homeostasis of intestinal epithelial cells by regulation of the Src- and growth factor-induced endocytosis of E-cadherin.

The NMDA receptor NR1 subunit is critically involved in the regulation of NMDA receptor activity by C-terminal Src kinase (Csk)

Previous studies have shown that Csk plays critical roles in the regulation of neural development, differentiation and glutamate-mediated synaptic plasticity. It has been found that Csk associates with the NR2A and 2B subunits of N-methyl-D-aspartate receptors (NMDARs) in a Src activity-dependent manner and serves as an intrinsic mechanism to provide a "brake" on the induction of long-term synaptic potentiation (LTP) mediated by NMDARs. In contrast to the NR2A and 2B subunits, no apparent tyrosine phosphorylation is found in the NR1 subunit of NMDARs. Here, we report that Csk can also associate with the NR1 subunit in a Src activity-dependent manner. The truncation of the NR1 subunit C-tail which contains only one tyrosine (Y837) significantly reduced the Csk association with the NR1-1a/NR2A receptor complex. Furthermore, we found that either the truncation of NR2A C-tail at aa 857 or the mutation of Y837 in the NR1-1a subunit to phenylalanine blocked the inhibition of NR1-1a/NR2A receptors induced by intracellular application of Csk. Thus, both the NR1 and NR2 subunits are required for the regulation of NMDAR activity by Csk.

SRC: a century of science brought to the clinic

The SRC family kinases are the largest family of nonreceptor tyrosine kinases and one of the best-studied targets for cancer therapy. SRC, arguably the oldest oncogene, has been implicated in pathways regulating proliferation, angiogenesis, invasion and metastasis, and bone metabolism. More recently, researchers have proposed that the transforming ability of SRC is linked to its ability to activate key signaling molecules in these pathways, rather than through direct activity. It has been hypothesized that blocking SRC activation may inhibit these pathways, resulting in antitumor activity. However, successfully targeting SRC in a clinical setting remains a challenge, and SRC inhibitors have only recently begun to move through clinical development. Preclinical studies have identified specific molecular "subgroups" and histologies that may be more sensitive to SRC inhibition. In addition, other studies have demonstrated synergistic interactions between SRC inhibitors and other targeted therapies and cytotoxics. In this review, we summarize SRC biology and how it has been applied to the clinical development of SRC inhibitors. The status of SRC inhibitors, including dasatinib, saracatinib, and bosutinib, which are in phase 1, 2, and 3 trials, is highlighted.

PTP-PEST controls motility, adherens junction assembly, and Rho GTPase activity in colon cancer cells

An important step in carcinoma progression is loss of cell-cell adhesion leading to increased invasion and metastasis. We show here that the protein tyrosine phosphatase, PTP-PEST, is a critical regulator of cell-cell junction integrity and epithelial cell motility. Using colon carcinoma cells, we show that the expression level of PTP-PEST regulates cell motility. Either transient small interfering RNA or stable short hairpin RNA knockdown of PTP-PEST enhances haptotactic and chemotactic migration of KM12C colon carcinoma cells. Furthermore, KM12C cells with stably knocked down PTP-PEST exhibit a mesenchymal-like phenotype with prominent membrane ruffles and lamellae. In contrast, ectopic expression of PTP-PEST in KM20 or DLD-1 cells, which lack detectable endogenous PTP-PEST expression, suppresses haptotactic migration. Importantly, we find that PTP-PEST localizes in adherens junctions. Concomitant with enhanced motility, stable knockdown of PTP-PEST causes a disruption of cell-cell junctions. These effects are due to a defect in junctional assembly and not to a loss of E-cadherin expression. Adherens junction assembly is impaired following calcium switch in KM12C cells with stably knocked down PTP-PEST and is accompanied by an increase in the activity of Rac1 and a suppression of RhoA activity in response to cadherin engagement. Taken together, these results suggest that PTP-PEST functions as a suppressor of epithelial cell motility by controlling Rho GTPase activity and the assembly of adherens junctions.

Downregulation of Rap1GAP in human tumor cells alters cell/matrix and cell/cell adhesion

Rap1GAP expression is decreased in human tumors. The significance of its downregulation is unknown. We show that Rap1GAP expression is decreased in primary colorectal carcinomas. To elucidate the advantages conferred on tumor cells by loss of Rap1GAP, Rap1GAP expression was silenced in human colon carcinoma cells. Suppressing Rap1GAP induced profound alterations in cell adhesion. Rap1GAP-depleted cells exhibited defects in cell/cell adhesion that included an aberrant distribution of adherens junction proteins. Depletion of Rap1GAP enhanced adhesion and spreading on collagen. Silencing of Rap expression normalized spreading and restored E-cadherin, beta-catenin, and p120-catenin to cell/cell contacts, indicating that unrestrained Rap activity underlies the alterations in cell adhesion. The defects in adherens junction protein distribution required integrin signaling as E-cadherin and p120-catenin were restored at cell/cell contacts when cells were plated on poly-l-lysine. Unexpectedly, Src activity was increased in Rap1GAP-depleted cells. Inhibition of Src impaired spreading and restored E-cadherin at cell/cell contacts. These findings provide the first evidence that Rap1GAP contributes to cell/cell adhesion and highlight a role for Rap1GAP in regulating cell/matrix and cell/cell adhesion. The frequent downregulation of Rap1GAP in epithelial tumors where alterations in cell/cell and cell/matrix adhesion are early steps in tumor dissemination supports a role for Rap1GAP depletion in tumor progression.

Caenorhabditis elegans fibroblast growth factor receptor signaling can occur independently of the multi-substrate adaptor FRS2

The components of receptor tyrosine kinase signaling complexes help to define the specificity of the effects of their activation. The Caenorhabditis elegans fibroblast growth factor receptor (FGFR), EGL-15, regulates a number of processes, including sex myoblast (SM) migration guidance and fluid homeostasis, both of which require a Grb2/Sos/Ras cassette of signaling components. Here we show that SEM-5/Grb2 can bind directly to EGL-15 to mediate SM chemoattraction. A yeast two-hybrid screen identified SEM-5 as able to interact with the carboxy-terminal domain (CTD) of EGL-15, a domain that is specifically required for SM chemoattraction. This interaction requires the SEM-5 SH2-binding motifs present in the CTD (Y(1009) and Y(1087)), and these sites are required for the CTD role of EGL-15 in SM chemoattraction. SEM-5, but not the SEM-5 binding sites located in the CTD, is required for the fluid homeostasis function of EGL-15, indicating that SEM-5 can link to EGL-15 through an alternative mechanism. The multi-substrate adaptor protein FRS2 serves to link vertebrate FGFRs to Grb2. In C. elegans, an FRS2-like gene, rog-1, functions upstream of a Ras/MAPK pathway for oocyte maturation but is not required for EGL-15 function. Thus, unlike the vertebrate FGFRs, which require the multi-substrate adaptor FRS2 to recruit Grb2, EGL-15 can recruit SEM-5/Grb2 directly.

Ca2+ signaling in airway epithelial cells facilitates leukocyte recruitment and transepithelial migration

In airway cells, TLR2 stimulation by bacterial products activates Ca2+ fluxes that signal leukocyte recruitment to the lung and facilitates transepithelial migration into the airway lumen. TLR2 is apically displayed on airway cells, where it senses bacterial stimuli. Biochemical and genetic approaches demonstrate that TLR2 ligands stimulate release of Ca2+ from intracellular stores by activating TLR2 phosphorylation by c-Src and recruiting PI3K and PLCgamma to affect Ca2+ release through IP3Rs. This Ca2+ release plays a pivotal role in signaling TLR2-dependent NF-kappaB activation and chemokine expression to recruit PMNs to the lung. In addition, TLR2-initiated Ca2+ release activates Ca2+-dependent proteases, calpains, which cleave the transmembrane proteins occludin and E-cadherin to promote PMN transmigration. This review highlights recent findings that demonstrate a central role for Ca2+ signaling in airway epithelial cells to induce proinflammatory gene transcription and to initiate junctional changes that accommodate transmigration of recruited PMNs.

Src activity alters alpha3 integrin expression in colon tumor cells

Src kinase has been linked to increased motility in the progression and metastasis of human colon cancer, although the mechanisms are not fully understood. Integrins are involved in metastasis by mediating attachment and migration of cells, as well as through transducing signals. This study examines the link between Src and integrin activity in the metastatic process in colon cancer cells. To determine Src involvement in integrin expression, the human colon cancer cell line, HCT116, was transfected with an activated Src construct and assayed for its ability to attach to and migrate across collagen and laminin. These cells attached more readily and migrated less rapidly on the extracellular matrix (ECM) than did cells transfected with empty vector. Examination of integrin levels showed a decrease in the alpha3 subunit in Src transfected cells as well as decreased cell surface localization of alpha3 integrin. The downregulation of alpha3 integrin was reversed by inhibition of Src and by inhibition of MAP kinase. Inhibition of alpha3 integrin using shRNA resulted in decreased MMP7 secretion, a possible cause of decreased invasion with low alpha3 integrin expression. This study shows that Src overexpression downregulates alpha3 integrin total protein expression and localization to the cell surface of HCT116 colon cancer cells. This indicates that Src activity may enhance metastasis by altering alpha3 integrin expression.

Antiproliferative effect of sulindac in colonic neoplasia prevention: role of COOH-terminal Src kinase

Although the nonsteroidal anti-inflammatory drugs (NSAID) protection against colorectal cancer is well established, the molecular mechanisms remain unclear. We show herein that induction of the tumor suppressor gene COOH-terminal Src kinase (Csk) by NSAID is important for their antiproliferative and hence chemopreventive effects. In the azoxymethane-treated rat model of experimental colon carcinogenesis, sulindac treatment markedly induced Csk with a corresponding increase in inhibitory phosphorylation of Src (Tyr(527)). Sulindac-mediated Csk induction was replicated in the human colorectal cancer cell line HT-29, with a corresponding suppression of both Src kinase activity (63% of vehicle; P < 0.05) and E-cadherin tyrosine phosphorylation (an in vivo Src target). To determine the importance of Csk in NSAID antiproliferative activity, we stably transfected a Csk-specific short hairpin RNA (shRNA) vector into HT-29 cells, thereby blunting the sulindac-mediated Csk induction. These transfectants were significantly less responsive to the antiproliferative effect of sulindac sulfide (suppression of proliferating cell nuclear antigen was 21 +/- 2.3% in transfectants versus 45 +/- 4.23% in wild-type cells), with a corresponding mitigation of the sulindac-mediated G(1)-S-phase arrest (S-phase cells 48 +/- 3.6% versus 14 +/- 2.8% of vehicle respectively). Importantly, the Csk shRNA cells had a marked decrease in the cyclin-dependent kinase inhibitor p21(cip/waf1), a critical regulator of G(1)-S-phase progression (49% of wild-type cells). Moreover, although sulindac-mediated induction of p21(cip/waf1) was 113% in wild-type HT-29, this induction was alleviated in the Csk shRNA transfectants (65% induction; P < 0.01). Thus, this is the first demonstration that the antiproliferative activity of NSAID is modulated, at least partly, through the Csk/Src axis.

FAK/src-family dependent activation of the Ste20-like kinase SLK is required for microtubule-dependent focal adhesion turnover and cell migration

Cell migration involves a multitude of signals that converge on cytoskeletal reorganization, essential for development, immune responses and tissue repair. Using knockdown and dominant negative approaches, we show that the microtubule-associated Ste20-like kinase SLK is required for focal adhesion turnover and cell migration downstream of the FAK/c-src complex. Our results show that SLK co-localizes with paxillin, Rac1 and the microtubules at the leading edge of migrating cells and is activated by scratch wounding. SLK activation is dependent on FAK/c-src/MAPK signaling, whereas SLK recruitment to the leading edge is src-dependent but FAK independent. Our results show that SLK represents a novel focal adhesion disassembly signal.

Translational control of C-terminal Src kinase (Csk) expression by PRL3 phosphatase

Phosphatase of regenerating liver 3 (PRL3) is up-regulated in cancer metastases. However, little is known of PRL3-mediated cellular signaling pathways. We previously reported that elevated PRL3 expression increases Src kinase activity, which likely contributes to the increased tumorigenesis and metastasis potential of PRL3. PRL3-induced Src activation is proposed to be indirect through down-regulation of Csk, a negative regulator of Src. Given the importance of PRL3 in tumor metastasis and the role of Csk in controlling Src activity, we addressed the mechanism by which PRL3 mediates Csk down-regulation. PRL3 is shown to exert a negative effect on Csk protein synthesis, rather than regulation of Csk mRNA levels or protein turnover. Interestingly, the preferential decrease in Csk protein synthesis is a consequence of increased eIF2 phosphorylation resulting from PRL3 expression. Reduced Csk synthesis also occurs in response to cellular stress that induces eIF2 phosphorylation, indicating that this regulatory mechanism may occur in response to a wider spectrum of cellular conditions known to direct translational control. Thus, we have uncovered a previously uncharacterized role for PRL3 in the gene-specific translational control of Csk expression.

Proteomic identification of ZO-1/2 as a novel scaffold for Src/Csk regulatory circuit

To elucidate the regulatory mechanism of cell transformation induced by c-Src tyrosine kinase, we performed a proteomic analysis of tyrosine phosphorylated proteins that interact with c-Src and/or its negative regulator Csk. The c-Src interacting proteins were affinity-purified from Src transformed cells using the Src SH2 domain as a ligand. LC-MS/MS analysis of the purified proteins identified general Src substrates, such as focal adhesion kinase and paxillin, and ZO-1/2 as a transformation-dependent Src target. The Csk binding proteins were analyzed by a tandem affinity purification method. In addition to the previously identified Csk binding proteins, including Cbp/PAG, paxillin, and caveolin-1, we found that ZO-1/2 could also serve as a major Csk binding protein. ZO-2 was phosphorylated concurrently with Src transformation and specifically bound to Csk in a Csk SH2 dependent manner. These results suggest novel roles for ZO proteins as Src/Csk scaffolds potentially involved in the regulation of Src transformation.

Focal adhesion kinase mediates defects in the force-dependent reinforcement of initial integrin-cytoskeleton linkages in metastatic colon cancer cell lines

Micro-environmental clues, including the biophysical interpretation of the extracellular matrix, are critical to proliferation, apoptosis and migration. Here, we show that metastatic human colon cancer cell lines display altered matrix interaction. Interaction of colon cancer cells with collagen I depends on integrins (mainly alpha(1)/beta(1)) but metastatic cells display delayed spreading and reduced extension of lamellipodia. In addition, cells show defective strengthening of integrin-cytoskeleton linkages upon mechanical stimulation, as determined by laser trapping experiments and binding of large beads to the cell surface. However, adhesion to pliable surfaces is ameliorated in metastatic variants. These changes are caused by constitutive activation of focal adhesion kinase (FAK) and can be modulated by changing expression and/or activity of FAK via RNA-interference or expression of inhibitory constructs, respectively. In addition, consistent with defective strengthening of integrin-cytoskeleton linkages, metastatic cell lines show reduced random motility. Taken together these data suggest that constitutive activation of FAK causes defects in spreading, reinforcement of integrin-cytoskeleton linkages and migration and at the same time could ameliorate the adhesion of metastatic cells to suboptimal surfaces.

C-terminal Src kinase controls development and maintenance of mouse squamous epithelia

Carboxy-terminal Src kinase (Csk) is a negative regulator of Src family kinases, which play pivotal roles in controlling cell adhesion, migration, and cancer progression. To elucidate the in vivo role of Csk in epithelial tissues, we conditionally inactivated Csk in squamous epithelia using the keratin-5 promoter/Cre-loxP system in mice. The mutant mice developed apparent defects in the skin, esophagus, and forestomach, with concomitant hyperplasia and chronic inflammation. Histology of the mutant epidermis revealed impaired cell-cell adhesion in basal cell layers. Analysis of primary keratinocytes showed that the defective cell-cell adhesion was caused by cytoskeletal remodeling via activation of the Rac1 pathway. Mutant keratinocytes also showed elevated expression of mesenchymal proteins, matrix metalloproteinases (MMPs), and the proinflammatory cytokine TNF-alpha. Inhibition of the expression of TNF-alpha and MMP9 by the anti-inflammatory reagent FK506 could cure the epidermal hyperplasia, suggesting a causal link between inflammation and epidermal hyperplasia. These observations demonstrate that the Src/Csk circuit plays crucial roles in development and maintenance of epithelia by controlling cytoskeletal organization as well as phenotypic conversion linked to inflammatory events.

PRL3 promotes cell invasion and proliferation by down-regulation of Csk leading to Src activation

Phosphatase of regenerating liver 3 (PRL3) is overexpressed in a variety of tumors, and high levels of PRL3 expression are associated with tumorigenesis and metastasis. Consistent with an oncogenic role for PRL3, we show that ectopic PRL3 expression promotes cell proliferation and invasion. However, little is known about the molecular basis for PRL3 function. Obtaining this knowledge is vital for understanding PRL3-mediated disease processes and for the development of novel anticancer therapies targeted to PRL3. Here we report that up-regulation of PRL3 activates the Src kinase, which initiates a number of signal pathways culminating in the phosphorylation of ERK1/2, STAT3, and p130(Cas). The activation of these pathways likely contributes to the increased cell growth and motility of PRL3 cells. We provide evidence that PRL3 induces Src activation through down-regulation of Csk, a negative regulator of Src. Importantly, Src activation and Csk down-regulation are also observed in colon cancer cells expressing a higher level of PRL3. Thus, we have revealed a biochemical mechanism for the PRL3-mediated cell invasion and proliferation in which elevated PRL3 expression causes a reduction in Csk level, leading to Src activation.

COOH-terminal Src kinase-mediated c-Jun phosphorylation promotes c-Jun degradation and inhibits cell transformation

The oncoprotein c-Jun is a component of the activator protein-1 transcription factor complex, which is involved in cellular proliferation, transformation, and death. The stabilization of c-Jun is critically important for its function. The phosphorylation of c-Jun by c-Jun NH(2)-terminal kinase 1 and extracellular signal-regulated protein kinases reduces c-Jun ubiquitination resulting in increased stabilization of c-Jun. In this report, we showed that COOH-terminal Src kinase (CSK) binds with and phosphorylates c-Jun at Y26 and Y170. Phosphorylation of c-Jun by CSK, in opposition to c-Jun NH(2)-terminal kinase 1 and extracellular signal-regulated protein kinases, promoted c-Jun degradation and reduced stability. By promoting c-Jun degradation, CSK helps to maintain a low steady-state level of c-Jun, thereby inhibiting activator protein-1 activity and cell transformation caused by c-Jun. These results indicated that this function of CSK controls cell proliferation under normal growth conditions and may have implications for CSK loss of function in carcinogenesis.

Functional development of Src tyrosine kinases during evolution from a unicellular ancestor to multicellular animals

The Src family of tyrosine kinases play pivotal roles in regulating cellular functions characteristic of multicellular animals, including cell-cell interactions, cell-substrate adhesion, and cell migration. To investigate the functional alteration of Src kinases during evolution from a unicellular ancestor to multicellular animals, we characterized Src orthologs from the unicellular choanoflagellate Monosiga ovata and the primitive multicellular sponge Ephydatia fluviatilis. Here, we show that the src gene family and its C-terminal Src kinase (Csk)-mediated regulatory system already were established in the unicellular M. ovata and that unicellular Src has unique features relative to multicellular Src: It can be phosphorylated by Csk at the negative regulatory site but still exhibits substantial activity even in the phosphorylated form. Analyses of chimera molecules between M. ovata and E. fluviatilis Src orthologs reveal that structural alterations in the kinase domain are responsible for the unstable negative regulation of M. ovata Src. When expressed in vertebrate fibroblasts, M. ovata Src can induce cell transformation irrespective of the presence of Csk. These findings suggest that a structure of Src required for the stable Csk-mediated negative regulation still is immature in the unicellular M. ovata and that the development of stable negative regulation of Src may correlate with the evolution of multicellularity in animals.

Dasatinib (BMS-354825) tyrosine kinase inhibitor suppresses invasion and induces cell cycle arrest and apoptosis of head and neck squamous cell carcinoma and non-small cell lung cancer cells

PURPOSE

Epithelial tumors, including non-small cell lung cancer (NSCLC) and head and neck squamous cell carcinoma (HNSCC), present clinical challenges. One potential target for systemic therapy is Src family nonreceptor tyrosine kinases, which are overexpressed in these tumors and induce pleiotropic effects, including increased proliferation, enhanced survival, stimulation of angiogenesis, and changes in motility. Dasatinib (BMS-354825), an ATP-competitive, small molecule tyrosine kinase inhibitor, suppresses the activity of these kinases at subnanomolar concentrations. Therefore, we tested the antitumor effects of this inhibitor in vitro to determine whether in vivo analyses were warranted.

EXPERIMENTAL DESIGN

The antitumor effects of dasatinib on HNSCC and NSCLC cells were evaluated using assays to measure cell cycle progression, apoptosis, migration, and invasion. Western blotting was used to monitor its effects on cell signaling.

RESULTS

Dasatinib inhibited migration and invasion in all cell lines and induced cell cycle arrest (blocking the G1-S transition) and apoptosis in some lines. The effects on migration and invasion correlated with the inhibition of Src and downstream mediators of adhesion [e.g., focal adhesion kinase (FAK), p130, and paxillin], and the cell cycle effects and apoptosis correlated with the induction of p27 and the dephosphorylation of Rb. Dasatinib also induced morphologic changes that were consistent with an upstream role for Src in regulating focal adhesion complexes.

CONCLUSIONS

This study showed that Src inhibition in HNSCC and NSCLC has antitumor effects in vitro. This suggests that dasatinib would have therapeutic activity against these tumors. Clinical studies in these tumor types are warranted.

Src and FAK signalling controls adhesion fate and the epithelial-to-mesenchymal transition

Src kinase controls cellular adhesions, including cadherin-based intercellular adhesions and integrin-mediated cell-matrix adhesions. In epithelial cells, Src activation, or increased signalling from migratory growth factor receptors via Src, induces an adhesion switch that enhances dynamic cell-matrix adhesions and migratory capacity while suppressing intercellular contact. Moreover, Src and the associated tyrosine kinase FAK are at the heart of the recently identified crosstalk between integrin- and cadherin-mediated adhesions of epithelial cells, particularly during the epithelial-to-mesenchymal transition.

c-src activating mutation analysis in Chinese patients with colorectal cancer

AIM: To investigate the occurrence of cellular src (c-src) activating mutation at codon 531 in colorectal cancer patients from Chinese mainland.

METHODS: Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assay followed by sequencing and single-strand conformation polymor-phism analysis were carried out to screen 110 samples of primary colorectal cancer and 20 colorectal liver metastases.

RESULTS: Only one sample showed PCR-RFLP-positive results and carried somatic codon 531 mutations. No additional mutation of c-src exon 12 was found.

CONCLUSION: c-src codon 531 mutation in colorectal cancer is not the cause of c-src activation.