The transmembrane adaptor Cbp/PAG1 controls the malignant potential of human non-small cell lung cancers that have c-src upregulation

The role of phosphoprotein associated with glycosphingolipid-enriched microdomains 1 (PAG1) in regulating the progression of oral squamous cell carcinoma

OBJECTIVE

The aim of this study was to explore the role of the tumor suppressor phosphoprotein associated with glycosphingolipid-enriched microdomains 1 (PAG1) on oral squamous cell carcinoma (OSCC) and its molecular mechanism.

DESIGN

Immunohistochemistry detected the expression of PAG1 in normal and tumor tissues. The PAG1 overexpressed OSCC cell lines were constructed by lentivirus transfection. Cell Counting Kit-8 assay (CCK-8), clone formation and flow cytometry evaluated the impact of PAG1 on the proliferation and apoptosis of OSCC cells. RNA sequencing (RNA-seq) detected the changes in intracellular genes, and transmission electron microscope (TEM) was used to compare the number of autophagosomes in OSCC cells between Negative and PAG1 group. Quantitative reverse transcription-polymerase chain reaction (RT-qPCR) and Western blot were used to determine the expression of signaling pathway-related mRNA and proteins respectively.

RESULTS

In contrast to the normal tissues, PAG1 expression was significantly downregulated in tumor tissues. Treatment with lentivirus transfection, the expression of PAG1 in the OSCC cell lines was increase. Notably, transfected with PAG1-overexpressing lentivirus cells inhibited the proliferation of OSCC cells and promoted OSCC cells apoptosis. RNA-seq revealed that PAG1 mainly modulated the mitophagy and autophagy pathway, and many autophagosomes were observed in the PAG1 group using TEM. Mechanistically, we found that PAG1 upregulated the expression of autophagy related factors through inhibiting PI3K/Akt/mTOR signal pathway activation.

CONCLUSION

Overexpression of PAG1 inhibited OSCC progression by activating autophagy, its mechanism might be related to inhibition of PI3K/Akt/mTOR signal pathway phosphorylation.

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.

Cell Entry of Avian Reovirus Modulated by Cell-Surface Annexin A2 and Adhesion G Protein-Coupled Receptor Latrophilin-2 Triggers Src and p38 MAPK Signaling Enhancing Caveolin-1- and Dynamin 2-Dependent Endocytosis

The specifics of cell receptor-modulated avian reovirus (ARV) entry remain unknown. By using a viral overlay protein-binding assay (VOPBA) and an in-gel digestion coupled with liquid chromatography-tandem mass spectrometry (LC-MS/MS), we determined that cell-surface annexin A2 (AnxA2) and adhesion G protein-coupled receptor Latrophilin-2 (ADGRL2) modulate ARV entry. Direct interaction between the ARV σC protein and AnxA2 and ADGRL2 in Vero and DF-1 cells was demonstrated in situ by proximity ligation assays. By using short hairpin RNAs (shRNAs) to silence the endogenous AnxA2 and ADGRL2 genes, ARV entry could be efficiently blocked. A significant decrease in virus yields and the intracellular specific signal for σC protein was observed in Vero cells preincubated with the specific AnxA2 and ADGRL2 monoclonal antibodies, indicating that AnxA2 and ADGRL2 are involved in modulating ARV entry. Furthermore, we found that cells pretreated with the AnxA2/S100A10 heterotetramer (A2t) inhibitor A2ti-1 suppressed ARV-mediated activation of Src and p38 mitogen-activated protein kinase (MAPK), demonstrating that Src and p38 MAPK serve as downstream molecules of cell-surface AnxA2 signaling. Our results reveal that suppression of cell-surface AnxA2 with the A2ti-1 inhibitor increased Csk-Cbp interaction, suggesting that ARV entry suppresses Cbp-mediated relocation of Csk to the membrane, thereby activating Src. Furthermore, reciprocal coimmunoprecipitation assays revealed that σC can interact with signaling molecules, lipid raft, and vimentin. The current study provides novel insights into cell-surface AnxA2- and ADGRL2-modulated cell entry of ARV which triggers Src and p38 MAPK signaling to enhance caveolin-1-, dynamin 2-, and lipid raft-dependent endocytosis. IMPORTANCE By analyzing results from VOPBA and LC-MS/MS, we have determined that cell-surface AnxA2 and ADGRL2 modulate ARV entry. After ARV binding to receptors, Src and p38 MAPK signaling were triggered and, in turn, increased the phosphorylation of caveolin-1 (Tyr14) and upregulated dynamin 2 expression to facilitate caveolin-1-mediated and dynamin 2-dependent endocytosis. In this work, we demonstrated that ARV triggers Src activation by impeding Cbp-mediated relocation of Csk to the membrane in the early stages of the life cycle. This work provides better insight into cell-surface AnxA2 and ADGRL2, which upregulate Src and p38MAPK signaling pathways to enhance ARV entry and productive infection.

Dissection of the catalytic and regulatory structure-function relationships of Csk protein tyrosine kinase

Protein tyrosine kinases (PTKs) are a large enzyme family that regulates many cellular processes. The key to their broad role in signaling is their tunable substrate specificity and regulatory mechanisms that allow each to respond to appropriate regulatory signals and phosphorylate the correct physiological protein substrates. Thus, in addition to the general PTK catalytic platform, each PTK acquires unique structural motifs that confer a unique combination of catalytic and regulatory properties. Understanding the structural basis for these properties is essential for understanding and manipulating the PTK-based signaling networks in normal and cancer cells. C-terminal Src kinase (Csk) and its homolog, Csk-homologous kinase (Chk), phosphorylate Src family kinases on a C-terminal Tyr residue and negatively regulate their kinase activity. While this regulatory function is biologically essential, Csk and Chk have also been excellent model PTKs for dissecting the structural basis of PTK catalysis and regulation. In this article, we review the structure-function studies of Csk and Chk that shed light on the regulatory and catalytic mechanisms of protein tyrosine kinases in general.

SH3-domain mutations selectively disrupt Csk homodimerization or PTPN22 binding

The kinase Csk is the primary negative regulator of the Src-family kinases (SFKs, e.g., Lck, Fyn, Lyn, Hck, Fgr, Blk, Yes), phosphorylating a tyrosine on the SFK C-terminal tail that mediates autoinhibition. Csk also binds phosphatases, including PTPN12 (PTP-PEST) and immune-cell PTPN22 (LYP/Pep), which dephosphorylate the SFK activation loop to promote autoinhibition. Csk-binding proteins (e.g., CBP/PAG1) oligomerize within membrane microdomains, and high local concentration promotes Csk function. Purified Csk homodimerizes in solution through an interface that overlaps the phosphatase binding footprint. Here we demonstrate that Csk can homodimerize in Jurkat T cells, in competition with PTPN22 binding. We designed SH3-domain mutations in Csk that selectively impair homodimerization (H21I) or PTPN22 binding (K43D) and verified their kinase activity in solution. Disruption of either interaction in cells, however, decreased the negative-regulatory function of Csk. Csk W47A, a substitution previously reported to block PTPN22 binding, had a secondary effect of impairing homodimerization. Csk H21I and K43D will be useful tools for dissecting the protein-specific drivers of autoimmunity mediated by the human polymorphism PTPN22 R620W, which impairs interaction with Csk and with the E3 ubiquitin ligase TRAF3. Future investigations of Csk homodimer activity and phosphatase interactions may reveal new facets of SFK regulation in hematopoietic and non-hematopoietic cells.

CADM1 suppresses c-Src activation by binding with Cbp on membrane lipid rafts and intervenes colon carcinogenesis

Cell adhesion molecules act as tumor suppressors primarily by cell attachment activity, but additional mechanisms modifying signal transduction are suggested in some cases. Cell adhesion molecule 1 (CADM1), a membrane-spanning immunoglobulin superfamily, mediates intercellular adhesion by trans-homophilic interaction and acts as a tumor suppressor. Here, we investigated CADM1-associated proteins comprehensively using proteomic analysis of immune-precipitates of CADM1 by mass spectrometry and identified a transmembrane adaptor protein, Csk-binding protein (Cbp), known to suppress Src-mediated transformation, as a binding partner of CADM1. CADM1 localizes to detergent-resistant membrane fractions and co-immunoprecipitated with Cbp and c-Src. Suppression of CADM1 expression using siRNA reduces the amount of co-immunoprecipitated c-Src with Cbp and activates c-Src in colon cancer cells expressing both CADM1 and Cbp. On the other hand, co-replacement of CADM1 and Cbp in colon cancer cells lacking CADM1 and Cbp expression suppresses c-Src activation, wound healing and tumorigenicity in nude mice. Furthermore, expression of Cbp and CADM1 was lost in 55% and 83% of human colon cancer, respectively, preferentially in tumors with larger size and/or lymph node metastasis. CADM1 would act as a colon tumor suppressor by intervening oncogenic c-Src signaling through binding with Cbp besides its authentic cell adhesion activity.

SUMOylation of Csk Negatively Modulates its Tumor Suppressor Function

Csk, a non-receptor tyrosine kinase, serves as an indispensable negative regulator of the Src family kinases (SFKs). However, little is known about regulation of Csk expression so far. SUMOylation, a reversible post-translational modification, has been shown to regulate many biological processes especially in tumor progression. Here we report that Csk is covalently modified by SUMO1 at lysine 53 (K53) both in vitro and in vivo. Treatment with hydrogen peroxide inhibited this modification to a certain extent, but PIAS3, identified as the main specific SUMO E3 ligase for Csk, could significantly enhance SUMO1-Csk level. In addition, phosphorylation at Ser364, the active site in Csk, had no effect on this modification. Ectopic expression of SUMO-defective mutant, Csk ^K53R, inhibited tumor cell growth more potentially than Csk wild-type. Consistent with the biological phenotype, the SUMO modification of Csk impaired its activity to interact with Cbp (Csk binding protein) leading to decreased c-Src phosphorylation at Y527. Our results suggest that SUMOylation of Csk mainly at lysine 53 negatively modulates its tumor suppressor function by reducing its binding with Cbp and consequently, inducing c-Src activation.

Identification of Integrin β1 as a Novel PAG1-Interacting Protein Involved in the Inherent Radioresistance of Human Laryngeal Carcinoma

Inherent radioresistance plays a crucial role in the failure of radiotherapy. Using the inherent radioresistant (Hep-2max) and radiosensitive (Hep-2min) cell lines established from the parental cell line Hep-2, we previously reported that phosphoprotein associated with glycosphingolipid-enriched microdomains 1(PAG1) overexpression in laryngeal carcinoma cells was correlated with inherent radioresistant phenotypes. However, the underlying mechanisms of this effect remain unknown. In the present study, we performed a proteomic screen to investigate the interactome of PAG1 in Hep-2max cells resulting in the identification of several interaction partners. Bioinformatic analysis and immunofluorescence experiments indicated the integrin β1 to be a crucial interaction partner of PAG1. PAG1 was also highly expressed in laryngeal carcinoma radioresistant tissues and showed co-localization with integrin β1. In addition, we demonstrated that integrin β1's binding to PAG1 could be interrupted by MβCD, an inhibitor of lipid rafts formation. Moreover, knockdown of integrin β1 by RNA interference sensitized radioresistant cells to irradiation. Importantly, we identified 2 potential interaction sites (Pro²¹⁶-Arg²³² and Asn³⁵⁶-Gly³⁷⁷) in the cytoplasmic domain of PAG1 using high throughput peptide arrays. Taken together, these results suggest that the binding of PAG1 to integrin β1 in lipid rafts is essential for inherent radioresistance of human laryngeal carcinoma.

Overexpression of Csk-binding protein/phosphoprotein associated with glycosphingolipid-enriched microdomains induces cluster of differentiation 59-mediated apoptosis in Jurkat cells

Csk-binding protein/phosphoprotein associated with glycosphingolipid-enriched microdomains (CBP/PAG) is a membrane-bound adaptor protein that downregulates the activation of Src family kinases present in lipid rafts. To elucidate the role of CBP/PAG in human T cell activation, a cell line overexpressing CBP/PAG was constructed and the function of CBP/PAG in Jurkat cells was examined. The present study revealed that increased CBP/PAG expression in T cells significantly enhanced their apoptosis and reduced cellular activation and proliferation. Overexpression of CBP/PAG suppressed the growth of Jurkat cells by recruiting c-Src and its negative regulator, C-terminal Src kinase (CSK), to lipid rafts. The negative regulation of CBP/PAG was enhanced in the presence of anti-cluster of differentiation (CD)59 monoclonal antibodies. In addition, a significant association was revealed between the location of CBP/PAG and CD59, which were co-expressed in the same region of the cell membrane, implicating a potential overlap of the elicited signaling pathways. These results indicate that CBP/PAG functions as a negative regulator of cell signal transduction and suggest that CD59 may strengthen the role of negative feedback regulation.

Transmembrane adaptor protein PAG1 is a novel tumor suppressor in neuroblastoma

(NB) is the most common extracranial pediatric solid tumor with high mortality rates. The tyrosine kinase c-Src has been known to play an important role in differentiation of NB cells, but the mechanism of c-Src regulation has not been defined. Here, we characterize PAG1 (Cbp, Csk binding protein), a central inhibitor of c-Src and other Src family kinases, as a novel tumor suppressor in NB. Clinical cohort analysis demonstrate that low expression of PAG1 is a significant prognostic factor for high stage disease, increased relapse, and worse overall survival for children with NB. PAG1 knockdown in NB cells promotes proliferation and anchorage-independent colony formation with increased activation of AKT and ERK downstream of c-Src, while PAG1 overexpression significantly rescues these effects. In vivo, PAG1 overexpression significantly inhibits NB tumorigenicity in an orthotopic xenograft model. Our results establish PAG1 as a potent tumor suppressor in NB by inhibiting c-Src and downstream effector pathways. Thus, reactivation of PAG1 and inhibition of c-Src kinase activity represents an important novel therapeutic approach for high-risk NB.

Upregulation of PAG1/Cbp contributes to adipose-derived mesenchymal stem cells promoted tumor progression and chemoresistance in breast cancer

C-terminal Src kinase (Csk)-binding protein (Cbp) is a ubiquitously expressed transmembrane adaptor protein which regulating Src family kinase (SFK) activities. Although SFKs are well known for their involvement in breast cancer, the function of Cbp in breast carcinogenesis upon the adipose-tumor microenvironment has not been investigated. Here, we reported that adipose-derived mesenchymal stem cells (ASCs) induced increased expression of Cbp accompanied by enhanced cell proliferation and chemotherapy resistance in breast cancer cell MCF-7/ADR. Depletion of Cbp in breast cancer cell by RNA interference led to remarkable inhibition of cell proliferation, invasion as well as synergy with adriamycin hydrochloride to suppress the tumor growth. Furthermore, silencing of Cbp concomitantly inhibited the expression of phosphoryl of Src, AKT and mTOR signals. Our study highlights the underlying mechanism of cross interaction between ASCs and breast cancer cells, and indicates that PAG1/Cbp in breast cancer cell may modulate tumor progression and acquired chemoresistance in the ASCs-associated breast cancer microenvironment through Src and AKT/mTOR pathways.

BMS-754807 is cytotoxic to non-small cell lung cancer cells and enhances the effects of platinum chemotherapeutics in the human lung cancer cell line A549

Background

Despite advances in targeted therapy for lung cancer, survival for patients remains poor and lung cancer remains the leading cause of cancer-related deaths worldwide. The type I insulin-like growth factor receptor (IGF-IR) has emerged as a potential target for lung cancer treatment, however, clinical trials to date have provided disappointing results. Further research is needed to identify if certain patients would benefit from IGF-IR targeted therapies and the ideal approach to incorporate IGF-IR targeted agents with current therapies.

Methods

The dual IGF-IR/insulin receptor inhibitor, BMS-754807, was evaluated alone and in combination with platinum-based chemotherapeutics in two human non-small cell lung cancer (NSCLC) cell lines. Cell survival was determined using WST-1 assays and drug interaction was evaluated using Calcusyn software. Proliferation and apoptosis were determined using immunofluorescence for phospho-histone H3 and cleaved caspase 3, respectively.

Results

Treatment with BMS-754807 alone reduced cell survival and wound closure while enhancing apoptosis in both human lung cancer cell lines. These effects appear to be mediated through IGF-IR/IR signaling and, at least in part, through the PI3K/AKT pathway as administration of BMS-754807 to A549 or NCI-H358 cells significantly suppressed IGF-IR/IR and AKT phosphorylation. In addition of BMS-754807 enhanced the cytotoxic effects of carboplatin or cisplatin in a synergistic manner when given simultaneously to A549 cells.

Conclusions

BMS-754807 may be an effective therapeutic agent for the treatment of NSCLC, particularly in lung cancer cells expressing high levels of IGF-IR.

The role of proteases in regulating Eph/ephrin signaling

Proteases regulate a myriad of cell functions, both in normal and disease states. In addition to protein turnover, they regulate a range of signaling processes, including those mediated by Eph receptors and their ephrin ligands. A variety of proteases is reported to directly cleave Ephs and/or ephrins under different conditions, to promote receptor and/or ligand shedding, and regulate receptor/ligand internalisation and signaling. They also cleave other adhesion proteins in response to Eph-ephrin interactions, to indirectly facilitate Eph-mediated functions. Proteases thus contribute to Eph/ephrin mediated changes in cell-cell and cell-matrix interactions, in cell morphology and in cell migration and invasion, in a manner which appears to be tightly regulated by, and co-ordinated with, Eph signaling. This review summarizes the current literature describing the function and regulation of protease activities during Eph/ephrin-mediated cell signaling.

Overexpression of Csk-binding protein decreases growth, invasion, and migration of esophageal carcinoma cells by controlling Src activation

AIM: To investigate the mechanisms by which Csk-binding protein (CBP) inhibits tumor progression in esophageal carcinoma.

METHODS: A CBP overexpressing esophageal carcinoma cell line (TE-1) was established. The growth, invasion, and migration of CBP-TE-1 cells, as well as the expression of Src were then determined and compared with those in normal TE-1 cells.

RESULTS: The expression of Src was decreased by the overexpression of CBP in TE-1 cells. The growth, invasion, and migration of TE-1 cells were decreased by the overexpression of CBP.

CONCLUSION: This study indicates that CBP may decrease the metastasis of esophageal carcinoma by inhibiting the activation of Src. CBP may be a potential tumor suppressor and targeting the CBP gene may be an alternative strategy for the development of therapies for esophageal carcinoma.

Roles of raft-anchored adaptor Cbp/PAG1 in spatial regulation of c-Src kinase

The tyrosine kinase c-Src is upregulated in numerous human cancers, implying a role for c-Src in cancer progression. Previously, we have shown that sequestration of activated c-Src into lipid rafts via a transmembrane adaptor, Cbp/PAG1, efficiently suppresses c-Src-induced cell transformation in Csk-deficient cells, suggesting that the transforming activity of c-Src is spatially regulated via Cbp in lipid rafts. To dissect the molecular mechanisms of the Cbp-mediated regulation of c-Src, a combined analysis was performed that included mathematical modeling and in vitro experiments in a c-Src- or Cbp-inducible system. c-Src activity was first determined as a function of c-Src or Cbp levels, using focal adhesion kinase (FAK) as a crucial c-Src substrate. Based on these experimental data, two mathematical models were constructed, the sequestration model and the ternary model. The computational analysis showed that both models supported our proposal that raft localization of Cbp is crucial for the suppression of c-Src function, but the ternary model, which includes a ternary complex consisting of Cbp, c-Src, and FAK, also predicted that c-Src function is dependent on the lipid-raft volume. Experimental analysis revealed that c-Src activity is elevated when lipid rafts are disrupted and the ternary complex forms in non-raft membranes, indicating that the ternary model accurately represents the system. Moreover, the ternary model predicted that, if Cbp enhances the interaction between c-Src and FAK, Cbp could promote c-Src function when lipid rafts are disrupted. These findings underscore the crucial role of lipid rafts in the Cbp-mediated negative regulation of c-Src-transforming activity, and explain the positive role of Cbp in c-Src regulation under particular conditions where lipid rafts are perturbed.

Global phosphoproteomic profiling reveals distinct signatures in B-cell non-Hodgkin lymphomas

Deregulation of signaling pathways controlled by protein phosphorylation underlies the pathogenesis of hematological malignancies; however, the extent to which deregulated phosphorylation may be involved in B-cell non-Hodgkin lymphoma (B-NHL) pathogenesis is largely unknown. To identify phosphorylation events important in B-NHLs, we performed mass spectrometry-based, label-free, semiquantitative phosphoproteomic profiling of 11 cell lines derived from three B-NHL categories: Burkitt lymphoma, follicular lymphoma, and mantle-cell lymphoma. In all, 6579 unique phosphopeptides, corresponding to 1701 unique phosphorylated proteins, were identified and quantified. The data are available via ProteomeXchange with identifier PXD000658. Hierarchical clustering highlighted distinct phosphoproteomic signatures associated with each lymphoma subtype. Interestingly, germinal center-derived B-NHL cell lines were characterized by phosphorylation of proteins involved in the B-cell receptor signaling. Of these proteins, phosphoprotein associated with glycosphingolipid-enriched microdomains 1 (PAG1) was identified with the most phosphorylated tyrosine peptides in Burkitt lymphoma and follicular lymphoma. PAG1 knockdown resulted in perturbation of the tyrosine phosphosignature of B-cell receptor signaling components. Significantly, PAG1 knockdown increased cell proliferation and response to antigen stimulation of these germinal center-derived B-NHLs. These data provide a detailed annotation of phosphorylated proteins in human lymphoid cancer. Overall, our study revealed the utility of unbiased phosphoproteome interrogation in characterizing signaling networks that may provide insights into pathogenesis mechanisms in B-cell lymphomas.

Non-receptor protein tyrosine kinases signaling pathways in normal and cancer cells

Protein tyrosine kinases (PTKs) are enzymes that transfer phosphate groups to tyrosine residues on protein substrates. Phosphorylation of proteins causes changes in their function and/or enzymatic activity resulting in specific biological responses. There are two classes of PTKs: the transmembrane receptor PTKs and the cytoplasmic non-receptor PTKs (NRTKs). NRTKs are involved in transduction of signals originating from extracellular clues, which often interact with transmembrane receptors. Thus, they are important components of signaling pathways which regulate fundamental cellular functions such as cell differentiation, apoptosis, survival, and proliferation. The activity of NRTKs is tightly regulated, and de-regulation and/or overexpression of NRTKs has been implicated in malignant transformation and carcinogenesis. Research on NRTKs has shed light on the mechanisms of a number of cellular processes including those involved in carcinogenesis. Not surprisingly, several tyrosine kinase inhibitors are in use as treatment for a number of malignancies, and more are under investigation. This review deals with the structure, function, and signaling pathways of nine main families of NRTKs in normal and cancer cells.

c-Src-induced activation of ceramide metabolism impairs membrane microdomains and promotes malignant progression by facilitating the translocation of c-Src to focal adhesions

We found that c-Src up-regulation perturbs sphingolipid/cholesterol-enriched membrane microdomains by activating ceramide synthesis, which contributes to malignant progression by promoting the translocation of c-Src from microdomains to focal adhesions/podosomes.

Identification of a new interaction mode between the Src homology 2 domain of C-terminal Src kinase (Csk) and Csk-binding protein/phosphoprotein associated with glycosphingolipid microdomains

Proteins with Src homology 2 (SH2) domains play major roles in tyrosine kinase signaling. Structures of many SH2 domains have been studied, and the regions involved in their interactions with ligands have been elucidated. However, these analyses have been performed using short peptides consisting of phosphotyrosine followed by a few amino acids, which are described as the canonical recognition sites. Here, we report the solution structure of the SH2 domain of C-terminal Src kinase (Csk) in complex with a longer phosphopeptide from the Csk-binding protein (Cbp). This structure, together with biochemical experiments, revealed the existence of a novel binding region in addition to the canonical phosphotyrosine 314-binding site of Cbp. Mutational analysis of this second region in cells showed that both canonical and novel binding sites are required for tumor suppression through the Cbp-Csk interaction. Furthermore, the data indicate an allosteric connection between Cbp binding and Csk activation that arises from residues in the βB/βC loop of the SH2 domain.

A broad view of scaffolding suggests that scaffolding proteins can actively control regulation and signaling of multienzyme complexes through allostery

Enzymes often work sequentially in pathways; and consecutive reaction steps are typically carried out by molecules associated in the same multienzyme complex. Localization confines the enzymes; anchors them; increases the effective concentration of substrates and products; and shortens pathway timescales; however, it does not explain enzyme coordination or pathway branching. Here, we distinguish between metabolic and signaling multienzyme complexes. We argue for a central role of scaffolding proteins in regulating multienzyme complexes signaling and suggest that metabolic multienzyme complexes are less dependent on scaffolding because they undergo conformational control through direct subunit-subunit contacts. In particular, we propose that scaffolding proteins have an essential function in controlling branching in signaling pathways. This new broadened definition of scaffolding proteins goes beyond cases such as the classic yeast mitogen-activated protein kinase Ste5 and encompasses proteins such as E3 ligases which lack active sites and work via allostery. With this definition, we classify the mechanisms of multienzyme complexes based on whether the substrates are transferred through the involvement of scaffolding proteins, and outline the functional merits to metabolic or signaling pathways. Overall, while co-localization topography helps multistep pathways non-specifically, allosteric regulation requires precise multienzyme organization and interactions and works via population shift, either through direct enzyme subunit-subunit interactions or through active involvement of scaffolding proteins. This article is part of a Special Issue entitled: The emerging dynamic view of proteins: Protein plasticity in allostery, evolution and self-assembly.

Regulation of the SRC family kinases by Csk

The non-receptor tyrosine kinase Csk serves as an indispensable negative regulator of the Src family tyrosine kinases (SFKs) by specifically phosphorylating the negative regulatory site of SFKs, thereby suppressing their oncogenic potential. Csk is primarily regulated through its SH2 domain, which is required for membrane translocation of Csk via binding to scaffold proteins such as Cbp/PAG1. The binding of scaffolds to the SH2 domain can also upregulate Csk kinase activity. These regulatory features have been elucidated by analyses of Csk structure at the atomic levels. Although Csk itself may not be mutated in human cancers, perturbation of the regulatory system consisting of Csk, Cbp/PAG1, or other scaffolds, and certain tyrosine phosphatases may explain the upregulation of SFKs frequently observed in human cancers. This review focuses on the molecular bases for the function, structure, and regulation of Csk as a unique regulatory tyrosine kinase for SFKs.

Identification of the PAG1 gene as a novel target of inherent radioresistance in human laryngeal carcinoma cells

Laryngeal carcinoma, as a malignant tumor that occurs in the head and neck region, is widely treated by radiation, but in some cases, the cancer is radioresistant to the radiotherapy. The reason for the radioresistant response needs to be clinically understood. We designed our present study to identify the molecules that may be involved in this radioresistant response. In this study, we initially established the inherent radioresistant (Hep-2max) and radiosensitive (Hep-2min) cell lines from the parental laryngeal cancer cell line Hep-2. Furthermore, using microarray analysis, we identified a novel inherent radioresistance-related gene, phosphoprotein associated with glycosphingolipid-enriched microdomains1 (PAG1). We showed that siRNA directed against PAG1 in a radioresistant (Hep-2max) cell line dramatically enhanced the radiosensitivity and IR-induced cell death. On the contrary, ectopic expression of PAG1 in radiosensitive (Hep-2min) cell lines led to radioresistance and suppressed the IR-induced cell death. Taken together, our results indicate that the PAG1 gene may be a novel, promising radiosensitization target for laryngeal carcinoma.

Antitumor activity of Src inhibitor saracatinib (AZD-0530) in preclinical models of biliary tract carcinomas

Biliary tract carcinoma (BTC) has a poor prognosis due to limited treatment options. There is, therefore, an urgent need to identify new targets and to design innovative therapeutic approaches. Among potential candidate molecules, we evaluated the nonreceptor tyrosine kinase Src, observing promising antitumor effects of its small-molecule inhibitor saracatinib in BTC preclinical models. The presence of an active Src protein was investigated by immunohistochemistry in 19 surgical samples from patients with BTC. Upon saracatinib treatment, the phosphorylation of Src and of its downstream transducers was evaluated in the BTC cell lines TFK-1, EGI-1, HuH28, and TGBC1-TKB. The effect of saracatinib on proliferation and migration was analyzed in these same cell lines, and its antitumor activity was essayed in EGI-1 mouse xenografts. Saracatinib-modulated transcriptome was profiled in EGI-1 cells and in tumor samples of the xenograft model. Src was activated in about 80% of the human BTC samples. In cultured BTC cell lines, low-dose saracatinib counteracted the activation of Src and of its downstream effectors, increased the fraction of cells in G(0)-G(1) phase, and inhibited cell migration. At high concentrations (median dose from 2.26-6.99 μmol/L), saracatinib was also capable of inhibiting BTC cell proliferation. In vivo, saracatinib treatment resulted in delayed tumor growth, associated with an impaired vascular network. Here, we provide a demonstration that the targeted inhibition of Src kinase by saracatinib is of therapeutic benefit in preclinical models of BTC. We propose our results as a basis for the design of saracatinib-based clinical applications.

Targeting protein lipidation in disease

Fatty acids and/or isoprenoids are covalently attached to a variety of disease-related proteins. The distinct chemical properties of each of these hydrophobic moieties allow lipid modification to serve as a mechanism to regulate protein structure, localization and function. This review highlights recent progress in identifying inhibitors of protein lipidation and their effects on human disease. Myristoylation inhibitors have shown promise in blocking the action of human pathogens. Although inhibitors that block prenylation of Ras proteins have not yet been successful for cancer treatment, they may be efficacious in the rare premature aging syndrome progeria. Agents that alter the palmitoylation status of Ras, Wnt and Hh proteins have recently been discovered, and represent the next generation of potential chemotherapeutics.