The Syk tyrosine kinase localizes to the centrosomes and negatively affects mitotic progression

Mitomycin C and its analog trigger cytotoxicity in MCF-7 and K562 cancer cells through the regulation of RAS and MAPK/ERK pathways

Mitomycin C (MC) is an anti-cancer drug which functions by forming interstrand crosslinks (ICLs) between opposing DNA strands. MC analog, 10-decarbamoyl mitomycin C (DMC), unlike MC, has stronger cytotoxic effects on cancer cells with TP53 mutation. We previously demonstrated that MC/DMC could activate p21WAF1/CIP1 in MCF-7 (TP53-proficient) and K562 (TP53 deficient) cells in a TP53-independent mode. We also found that MC/DMC regulate AKT activation in a TP53-dependent manner and that AKT deactivation is not associated with the activation of p21WAF1/CIP1 in response to MC/DMC treatment. RAS proteins are known players in the upstream mediated signaling of p21WAF1/CIP1 activation that leads to control of cell proliferation and cell death. Thus, this prompted us to investigate the effect of both drugs on the expression of RAS proteins and regulation of the MAPK/ERK signaling pathways in MCF-7 and K562 cancer cells. To accomplish this goal, we performed comparative label free proteomics profiling coupled to bioinformatics/complementary phosphoprotein arrays and Western blot validations of key signaling molecules. The MAPK/ERK pathway exhibited an overall downregulation upon MC/DMC treatment in MCF-7 cells but only DMC exhibited a mild downregulation of that same pathway in TP53 mutant K562 cells. Furthermore, treatment of MCF-7 and K562 cell lines with oligonucleotides containing the interstrand crosslinks (ICLs) formed by MC or DMC shows that both ICLs had a stronger effect on the downregulation of RAS protein expression in mutant TP53 K562 cells. We discuss the implication of this regulation of the MAPK/ERK pathway in relation to cellular TP53 status.

Epigenetic upregulation of spleen tyrosine kinase in cancer cells through p53-dependent downregulation of DNA methyltransferase

Syk is a tumor suppressor gene in some solid tumors. Currently, it remains unknown how Syk gene hypermethylation is controlled by DNA methyltransferase (DNMT) and p53. In colorectal cancer HCT116 cells, we found that protein and mRNA levels of Syk were much higher in WT than in p53^-/- cells. Both p53 inhibitor PFT-α and p53 silencing can reduce the protein and mRNA expression of Syk in WT cells, while DNMT inhibitor 5-Aza-2'-dC can increase Syk expression in p53^-/- cells. Interestingly, the DNMT expression in p53^-/- HCT116 cells was higher than that in WT cells. PFT-α can not only enhance Syk gene methylation but also increase DNMT1 protein and mRNA levels in WT HCT116 cells. In metastatic lung cancer cell lines A549 and PC9, which express WT p53 and gain function of p53, respectively, PFT-α can also downregulate Syk mRNA and protein expression. However, the Syk methylation level was increased by PFT-α in A549 but not in PC9 cells. Likewise, 5-Aza-2'-dC transcriptionally increased Syk gene expression in A549 cells, but not in PC9 cells. In summary methylation of Syk promoter requires DNMT1, and p53 can upregulate Syk expression via downregulation of DNMT1 at the transcriptional level.

Targeting the splicing isoforms of spleen tyrosine kinase affects the viability of colorectal cancer cells

Spleen tyrosine kinase (Syk) expression have been both positively and negatively associated with tumorigenesis. Our goal was to evaluate the contribution of Syk and its two splice variants, full length Syk (L) and short isoform Syk (S), in the tumor biology of colorectal cancer cells (CRC). The analysis of Syk expression in primary human colorectal tumors, as well as the analysis of TCGA database, revealed a high Syk mRNA expression score in colorectal cancer tumors, suggesting a tumor promotor role of Syk in CRC. Our analysis showed that Syk (L) isoform is highly expressed in the majority of the tumor tissues and that it remains expressed in tumors in which global Syk expression is downregulated, suggesting the dependence of tumors to Syk (L) isoform. We also identified a small cluster of tumor tissues, which express a high proportion of Syk (S) isoform. This specific cluster is associated with overexpressed genes related to translation and mitochondria, and down regulated genes implicated in the progression of mitosis. For our functional studies, we used short hairpin RNA tools to target the expression of Syk in CRC cells bearing the activating K-Ras (G13D) mutation. Our results showed that while global Syk knock down increases cell proliferation and cell motility, Syk (L) expression silencing affects the viability and induces the apoptosis of the cells, confirming the dependence of cells on Syk (L) isoform for their survival. Finally, we report the promising potential of compound C-13, an original non-enzymatic inhibitor of Syk isolated in our group. In vitro studies showed that C-13 exerts cytotoxic effects on Syk-positive CRC cells by inhibiting their proliferation and their motility, and by inducing their apoptosis, while Syk-negative cell lines viability was not affected. Moreover, the oral and intraperitoneal administration of C-13 reduced the tumor growth of CRC DLD-1 cells xenografts in Nude mice in vivo.

Comparison of SYK Signaling Networks Reveals the Potential Molecular Determinants of Its Tumor-Promoting and Suppressing Functions

Spleen tyrosine kinase (SYK) can behave as an oncogene or a tumor suppressor, depending on the cell and tissue type. As pharmacological SYK inhibitors are currently evaluated in clinical trials, it is important to gain more information on the molecular mechanisms underpinning these opposite roles. To this aim, we reconstructed and compared its signaling networks using phosphoproteomic data from breast cancer and Burkitt lymphoma cell lines where SYK behaves as a tumor suppressor and promoter. Bioinformatic analyses allowed for unveiling the main differences in signaling pathways, network topology and signal propagation from SYK to its potential effectors. In breast cancer cells, the SYK target-enriched signaling pathways included intercellular adhesion and Hippo signaling components that are often linked to tumor suppression. In Burkitt lymphoma cells, the SYK target-enriched signaling pathways included molecules that could play a role in SYK pro-oncogenic function in B-cell lymphomas. Several protein interactions were profoundly rewired in the breast cancer network compared with the Burkitt lymphoma network. These data demonstrate that proteomic profiling combined with mathematical network modeling allows untangling complex pathway interplays and revealing difficult to discern interactions among the SYK pathways that positively and negatively affect tumor formation and progression.

Src-mediated tyrosine phosphorylation of PRC1 and kinastrin/SKAP on the mitotic spindle

Src-family tyrosine kinases (SFKs) play important roles in a number of signal transduction events during mitosis, such as spindle formation. A relationship has been reported between SFKs and the mitotic spindle; however, the underlying mechanisms remain unclear. We herein demonstrated that SFKs accumulated in the centrosome region at the onset of mitosis. Centrosomal Fyn increased in the G₂ phase in a microtubule polymerization-dependent manner. A mass spectrometry analysis using mitotic spindle preparations was performed to identify tyrosine-phosphorylated substrates. Protein regulator of cytokinesis 1 (PRC1) and kinastrin/small kinetochore-associated protein (kinastrin/SKAP) were identified as SFK substrates. SFKs mainly phosphorylated PRC1 at Tyr-464 and kinastrin at Tyr-87. Although wild-type PRC1 is associated with microtubules, phosphomimetic PRC1 impaired the ability to bind microtubules. Phosphomimetic kinastrin at Tyr-87 also impaired binding with microtubules. Collectively, these results suggest that tyrosine phosphorylation of PRC1 and kinastrin plays a role in their delocalization from microtubules during mitosis.

Mining Database for the Expression and Clinical Significance of NF-κB Family in Hepatocellular Carcinoma

Background

Hepatocellular carcinoma (HCC) is one of the deadliest diseases affecting humans. Its incidence has been increasing over the last decade. It is characterized by poor prognosis as well as lack of therapeutic regimens for patients in the advanced stages. It is therefore important to develop effective biomarkers for diagnosis, prognosis, and immunotherapy of HCC. Research suggests that the NF-κB family plays vital roles in immune response, inflammation, tumorigenesis, and the progress of malignancy in various cancers. However, its role in HCC remains unidentified. Methodology. The expression and clinical significance of the NF-κB family in HCC were analyzed using several bioinformatics tools including UALCAN, The Human Protein Atlas, GEPIA, GSCALite, David, GeneMANIA, and TIMER.

Results

The mRNA expression levels of RelA, RelB, NF-κB1, and NF-κB2 were significantly elevated in HCC. The mRNA levels of RelB and NF-κB2 were significantly upregulated in HCC tissues compared to normal liver tissues in subgroup analyses based on patient's race, gender, age, weight, tumor grade, cancer stage, and nodal metastasis status. Moreover, HCC patients with elevated levels of RelB and NF-κB2 had a worse overall survival and disease-free survival. Methylation downregulated the expressions of RelA, RelB, and NF-κB1 in HCC. NF-κB family was also significantly involved in various hallmark cancer-related pathways such as the apoptosis, EMT, RTK, and cell cycle pathways. Similarly, the expression of RelB and NF-κB2 was positively correlated with the abundance of immune cells and the expression of immune biomarkers. Several kinase and miRNA targets of RelB and NF-κB2 were also identified.

Conclusion

RelB and NF-κB2 are potential biomarkers for the diagnosis, prognosis, and immunotherapy of HCC.

The tubulin code and its role in controlling microtubule properties and functions

Microtubules are core components of the eukaryotic cytoskeleton with essential roles in cell division, shaping, motility and intracellular transport. Despite their functional heterogeneity, microtubules have a highly conserved structure made from almost identical molecular building blocks: the tubulin proteins. Alternative tubulin isotypes and a variety of post-translational modifications control the properties and functions of the microtubule cytoskeleton, a concept known as the 'tubulin code'. Here we review the current understanding of the molecular components of the tubulin code and how they impact microtubule properties and functions. We discuss how tubulin isotypes and post-translational modifications control microtubule behaviour at the molecular level and how this translates into physiological functions at the cellular and organism levels. We then go on to show how fine-tuning of microtubule function by some tubulin modifications can affect homeostasis and how perturbation of this fine-tuning can lead to a range of dysfunctions, many of which are linked to human disease.

Synergistic Anti-Tumour Effect of Syk Inhibitor and Olaparib in Squamous Cell Carcinoma: Roles of Syk in EGFR Signalling and PARP1 Activation

Syk is a non-receptor tyrosine kinase involved in the signalling of immunoreceptors and growth factor receptors. Previously, we reported that Syk mediates epidermal growth factor receptor (EGFR) signalling and plays a negative role in the terminal differentiation of keratinocytes. To understand whether Syk is a potential therapeutic target of cancer cells, we further elucidated the role of Syk in disease progression of squamous cell carcinoma (SCC), which is highly associated with EGFR overactivation, and determined the combined effects of Syk and PARP1 inhibitors on SCC viability. We found that pharmacological inhibition of Syk could attenuate the EGF-induced phosphorylation of EGFR, JNK, p38 MAPK, STAT1, and STAT3 in A431, CAL27 and SAS cells. In addition, EGF could induce a Syk-dependent IL-8 gene and protein expression in SCC. Confocal microscopic data demonstrated the ability of the Syk inhibitor to change the subcellular distribution patterns of EGFR after EGF treatment in A431 and SAS cells. Moreover, according to Kaplan-Meier survival curve analysis, higher Syk expression is correlated with poorer patient survival rate and prognosis. Notably, both Syk and EGFR inhibitors could induce PARP activation, and synergistic cytotoxic actions were observed in SCC cells upon the combined treatment of the PARP1 inhibitor olaparib with Syk or the EGFR inhibitor. Collectively, we reported Syk as an important signalling molecule downstream of EGFR that plays crucial roles in SCC development. Combining Syk and PARP inhibition may represent an alternative therapeutic strategy for treating SCC.

The Syk Kinase Promotes Mammary Epithelial Integrity and Inhibits Breast Cancer Invasion by Stabilizing the E-Cadherin/Catenin Complex

While first discovered in immunoreceptor signaling, the Syk protein kinase behaves as a tumor and metastasis suppressor in epithelial cells. Its reduced expression in breast and other carcinomas is correlated with decreased survival and increased metastasis risk, but its action mechanism remains largely unknown. Using phosphoproteomics we found that Syk phosphorylated E-cadherin and α-, β-, and p120-catenins on multiple tyrosine residues that concentrate at intercellular junctions. Increased Syk expression and activation enhanced E-cadherin/catenin phosphorylation, promoting their association and complex stability. In human breast cancer cells, Syk stimulated intercellular aggregation, E-cadherin recruitment and retention at adherens junctions, and promoted epithelial integrity, whereas it inhibited cell migration and invasion. Opposite effects were obtained with Syk knockdown or non-phosphorylatable mutant E-cadherin expression. Mechanistically, Syk stimulated the interaction of the E-cadherin/catenin complex with zonula occludens proteins and the actin cytoskeleton. Conditional Syk knockout in the lactating mouse mammary gland perturbed alveologenesis and disrupted E-cadherin localization at adherens junctions, corroborating the observations in cells. Hence, Syk is involved in the maintenance of the epithelial integrity of the mammary gland via the phosphorylation and stabilization of the E-cadherin/catenin adherens junction complex, thereby inhibiting cell migration and malignant tumor invasion.

Regulation of Microtubule Nucleation in Mouse Bone Marrow-Derived Mast Cells by Protein Tyrosine Phosphatase SHP-1

The antigen-mediated activation of mast cells initiates signaling events leading to their degranulation, to the release of inflammatory mediators, and to the synthesis of cytokines and chemokines. Although rapid and transient microtubule reorganization during activation has been described, the molecular mechanisms that control their rearrangement are largely unknown. Microtubule nucleation is mediated by γ-tubulin complexes. In this study, we report on the regulation of microtubule nucleation in bone marrow-derived mast cells (BMMCs) by Src homology 2 (SH2) domain-containing protein tyrosine phosphatase 1 (SHP-1; Ptpn6). Reciprocal immunoprecipitation experiments and pull-down assays revealed that SHP-1 is present in complexes containing γ-tubulin complex proteins and protein tyrosine kinase Syk. Microtubule regrowth experiments in cells with deleted SHP-1 showed a stimulation of microtubule nucleation, and phenotypic rescue experiments confirmed that SHP-1 represents a negative regulator of microtubule nucleation in BMMCs. Moreover, the inhibition of the SHP-1 activity by inhibitors TPI-1 and NSC87877 also augmented microtubule nucleation. The regulation was due to changes in γ-tubulin accumulation. Further experiments with antigen-activated cells showed that the deletion of SHP-1 stimulated the generation of microtubule protrusions, the activity of Syk kinase, and degranulation. Our data suggest a novel mechanism for the suppression of microtubule formation in the later stages of mast cell activation.

SYK Inhibition Potentiates the Effect of Chemotherapeutic Drugs on Neuroblastoma Cells in Vitro

Neuroblastoma is a malignancy arising from the developing sympathetic nervous system and the most common and deadly cancer of infancy. New therapies are needed to improve the prognosis for high-risk patients and to reduce toxicity and late effects. Spleen tyrosine kinase (SYK) has previously been identified as a promising drug target in various inflammatory diseases and cancers but has so far not been extensively studied as a potential therapeutic target in neuroblastoma. In this study, we observed elevated SYK gene expression in neuroblastoma compared to neural crest and benign neurofibroma. While SYK protein was detected in the majority of examined neuroblastoma tissues it was less frequently observed in neuroblastoma cell lines. Depletion of SYK by siRNA and the use of small molecule SYK inhibitors significantly reduced the cell viability of neuroblastoma cell lines expressing SYK protein. Moreover, SYK inhibition decreased ERK1/2 and Akt phosphorylation. The SYK inhibitor BAY 61-3606 enhanced the effect of different chemotherapeutic drugs. Transient expression of a constitutive active SYK variant increased the viability of neuroblastoma cells independent of endogenous SYK levels. Collectively, our findings suggest that targeting SYK in combination with conventional chemotherapy should be further evaluated as a treatment option in neuroblastoma.

Antimitotic activity of DY131 and the estrogen-related receptor beta 2 (ERRβ2) splice variant in breast cancer

Breast cancer remains a leading cause of cancer-related death in women, and triple negative breast cancer (TNBC) lacks clinically actionable therapeutic targets. Death in mitosis is a tumor suppressive mechanism that occurs in cancer cells experiencing a defective M phase. The orphan estrogen-related receptor beta (ERRβ) is a key reprogramming factor in murine embryonic and induced pluripotent stem cells. In primates, ERRβ is alternatively spliced to produce several receptor isoforms. In cellular models of glioblastoma, short form (ERRβsf) and beta2 (ERRβ2) splice variants differentially regulate cell cycle progression in response to the synthetic agonist DY131, with ERRβ2 driving arrest in G2/M.The goals of the present study are to determine the cellular function(s) of ligand-activated ERRβ splice variants in breast cancer and evaluate the potential of DY131 to serve as an antimitotic agent, particularly in TNBC. DY131 inhibits growth in a diverse panel of breast cancer cell lines, causing cell death that involves the p38 stress kinase pathway and a bimodal cell cycle arrest. ERRβ2 facilitates the block in G2/M, and DY131 delays progression from prophase to anaphase. Finally, ERRβ2 localizes to centrosomes and DY131 causes mitotic spindle defects. Targeting ERRβ2 may therefore be a promising therapeutic strategy in breast cancer.

Stat3 regulates centrosome clustering in cancer cells via Stathmin/PLK1

Cancer cells frequently have amplified centrosomes that must be clustered together to form a bipolar mitotic spindle, and targeting centrosome clustering is considered a promising therapeutic strategy. A high-content chemical screen for inhibitors of centrosome clustering identified Stattic, a Stat3 inhibitor. Stat3 depletion and inhibition in cancer cell lines and in tumours in vivo caused significant inhibition of centrosome clustering and viability. Here we describe a transcription-independent mechanism for Stat3-mediated centrosome clustering that involves Stathmin, a Stat3 interactor involved in microtubule depolymerization, and the mitotic kinase PLK1. Furthermore, PLK4-driven centrosome amplified breast tumour cells are highly sensitive to Stat3 inhibitors. We have identified an unexpected role of Stat3 in the regulation of centrosome clustering, and this role of Stat3 may be critical in identifying tumours that are sensitive to Stat3 inhibitors.

Reconstruction and signal propagation analysis of the Syk signaling network in breast cancer cells

The ability to build in-depth cell signaling networks from vast experimental data is a key objective of computational biology. The spleen tyrosine kinase (Syk) protein, a well-characterized key player in immune cell signaling, was surprisingly first shown by our group to exhibit an onco-suppressive function in mammary epithelial cells and corroborated by many other studies, but the molecular mechanisms of this function remain largely unsolved. Based on existing proteomic data, we report here the generation of an interaction-based network of signaling pathways controlled by Syk in breast cancer cells. Pathway enrichment of the Syk targets previously identified by quantitative phospho-proteomics indicated that Syk is engaged in cell adhesion, motility, growth and death. Using the components and interactions of these pathways, we bootstrapped the reconstruction of a comprehensive network covering Syk signaling in breast cancer cells. To generate in silico hypotheses on Syk signaling propagation, we developed a method allowing to rank paths between Syk and its targets. We first annotated the network according to experimental datasets. We then combined shortest path computation with random walk processes to estimate the importance of individual interactions and selected biologically relevant pathways in the network. Molecular and cell biology experiments allowed to distinguish candidate mechanisms that underlie the impact of Syk on the regulation of cortactin and ezrin, both involved in actin-mediated cell adhesion and motility. The Syk network was further completed with the results of our biological validation experiments. The resulting Syk signaling sub-networks can be explored via an online visualization platform.

Insights into the allosteric regulation of Syk association with receptor ITAM, a multi-state equilibrium

The phosphorylation of interdomain A (IA), a linker region between tandem SH2 domains of Syk tyrosine kinase, regulates the binding affinity for association of Syk with doubly-phosphorylated ITAM regions of the B cell receptor. The mechanism of this allosteric regulation has been suggested to be a switch from the high-affinity bifunctional binding, mediated through both SH2 domains binding two phosphotyrosine residues of ITAM, to a substantially lower-affinity binding of only one SH2 domain. IA phosphorylation triggers the switch by inducing disorder in IA and weakening the SH2-SH2 interaction. The postulated switch to a single-SH2-domain binding mode is examined using NMR to monitor site-specific binding to each SH2 domain of Syk variants engineered to have IA regions that differ in conformational flexibility. The combined analysis of titration curves and NMR line-shapes provides sufficient information to determine the energetics of inter-molecular binding at each SH2 site along with an intra-molecular binding or isomerization step. A less favorable isomerization equilibrium associated with the changes in the SH2-SH2 conformational ensemble and IA flexibility accounts for the inhibition of Syk association with membrane ITAM regions when IA is phosphorylated, and refutes the proposed switch to single-SH2-domain binding. Syk localizes in the cell through its SH2 interactions, and this basis for allosteric regulation of ITAM association proposes for the first time a phosphorylation-dependent model to regulate Syk binding to alternate receptors and other signaling proteins that differ either in the number of residues separating ITAM phosphotyrosines or by having only one phosphotyrosine, a half ITAM.

Downregulation of spleen tyrosine kinase in hepatocellular carcinoma by promoter CpG island hypermethylation and its potential role in carcinogenesis

Spleen tyrosine kinase (SYK) has predominantly been studied in hematopoietic cells, where it is involved in immunoreceptor-mediated signaling. However, SYK expression has been shown in numerous non-hematopoietic cells, and its downregulation has been shown to be involved in tumor formation and progression. SYK methylation has been demonstrated to identify a subset of hepatocellular carcinoma (HCC) cases with poor prognosis, but little is known regarding the biological role of SYK in HCC. We found that SYK methylation is a common event in HCC, and is inversely associated with its expression. We established stable HCC cell lines with inducible SYK expression vectors, and compared the differential RNA expression profiles of HCC cell lines with or without the induction of SYK. Gene ontology analysis revealed that the SYK-regulated genes were enriched for genes involved in cell adhesion. Accordingly, we found that the induction of SYK expression increased the adhesion of cells to fibronectin and decreased cell migration and invasion, and that cessation of SYK overexpression increased cell migration and invasion. Our findings suggest that SYK is involved in regulating cell to matrix adhesions, and that SYK loss affects the migration, and invasion of HCC cells.

The tubulin code: molecular components, readout mechanisms, and functions

Microtubules are cytoskeletal filaments that are dynamically assembled from α/β-tubulin heterodimers. The primary sequence and structure of the tubulin proteins and, consequently, the properties and architecture of microtubules are highly conserved in eukaryotes. Despite this conservation, tubulin is subject to heterogeneity that is generated in two ways: by the expression of different tubulin isotypes and by posttranslational modifications (PTMs). Identifying the mechanisms that generate and control tubulin heterogeneity and how this heterogeneity affects microtubule function are long-standing goals in the field. Recent work on tubulin PTMs has shed light on how these modifications could contribute to a “tubulin code” that coordinates the complex functions of microtubules in cells.

Calling in SYK: SYK's dual role as a tumor promoter and tumor suppressor in cancer

SYK (spleen tyrosine kinase) is well-characterized in the immune system as an essential enzyme required for signaling through multiple classes of immune recognition receptors. As a modulator of tumorigenesis, SYK has a bit of a schizophrenic reputation, acting in some cells as a tumor promoter and in others as a tumor suppressor. In many hematopoietic malignancies, SYK provides an important survival function and its inhibition or silencing frequently leads to apoptosis. In cancers of non-immune cells, SYK provides a pro-survival signal, but can also suppress tumorigenesis by restricting epithelial-mesenchymal transition, enhancing cell-cell interactions and inhibiting migration.

A PREVIOUSLY UNKNOWN UNIQUE CHALLENGE FOR INHIBITORS OF SYK ATP-BINDING SITE: ROLE OF SYK AS A CELL CYCLE CHECKPOINT REGULATOR

The identification of SYK as a molecular target in B-lineage leukemia/lymphoma cells prompted the development of SYK inhibitors as a new class of anti-cancer drug candidates. Here we report that induction of the SYK gene expression in human cells causes a significant down-regulation of evolutionarily conserved genes associated with mitosis and cell cycle progression providing unprecedented evidence that SYK is a master regulator of cell cycle regulatory checkpoint genes in human cells. We further show that SYK regulates the G₂ checkpoint by physically associating with and inhibiting the dual-specificity phosphatase CDC25C via phosphorylation of its S216 residue. SYK depletion by RNA interference or treatment with the chemical SYK inhibitor prevented nocodazole-treated human cell lines from activating the G₂ checkpoint via CDC25C S216-phosphorylation and resulted in polyploidy. Our study provides genetic and biochemical evidence that spleen tyrosine kinase (SYK) has a unique role in the activation of the G₂ checkpoint in both non-lymphohematopoietic and B-lineage lymphoid cells. This previously unknown role of SYK as a cell cycle checkpoint regulator represents an unforeseen and significant challenge for inhibitors of SYK ATP binding site.

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SYK is a cell cycle regulatory kinase that phosphorylates CDC25C at S216

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SYK is a master regulator of cell cycle regulatory checkpoint genes in human cells

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Inhibitors of SYK ATP binding site may increase the risk for secondary cancer

Effect of cisplatin implantation on human gastric carcinoma xenograft growth in nude mice and tumor expression of SYK and HER2

AIM: To assess the effect of cisplatin implantation on human gastric carcinoma xenograft growth and tumor expression of SYK and HER2 in nude mice, and to discuss the mechanism of action of sustained-release cisplatin.

METHODS: A xenograft model of human gastric carcinoma was established by subcutaneous injection of SGC7901 cells in Balb/c nude mice. The mice were then randomly divided into four groups to receive intravenous injection of PBS (group A), intravenous injection of cisplatin (group B), intratumor injection of cisplatin (group C), and implantation of cisplatin (group D). After drug intervention, tumor weight and tumor inhibition rate were measured. Tumor tissues were observed after HE staining. Expression of SYK/HER2 was detected by immunohistochemsitry. Besides, the cell suspensions prepared from the tumor tissues from the four groups were tested by FCM.

RESULTS: Sustained-release cisplatin could promote apoptosis and suppress tumor growth significantly, and it also could enhance the expression of SYK (group D: 73.42 ± 4.92 vs group C: 30.42 ± 3.92, P < 0.05; group B: 14.14 ± 2.84 vs group A: 5.06 ± 2.96, P < 0.05) and reduce the expression of HER2 (group D: 16.32 ± 4.82 vs group C: 34.82 ± 7.32, P < 0.05; group B: 45.8 ± 6.60 vs group A: 77.34 ± 9.04, P < 0.05).

CONCLUSION: Cisplatin implantation induces tumor cell apoptosis and exerts anticancer function possibly by enhancing the expression of SYK and reducing the expression of HER2.

Global phosphoproteomics of activated B cells using complementary metal ion functionalized soluble nanopolymers

Engagement of the B cell receptor for antigen (BCR) leads to immune responses through a cascade of intracellular signaling events. Most studies to date have focused on the BCR and protein tyrosine phosphorylation. Because spleen tyrosine kinase, Syk, is an upstream kinase in multiple BCR-regulated signaling pathways, it also affects many downstream events that are modulated through the phosphorylation of proteins on serine and threonine residues. Here, we report a novel phosphopeptide enrichment strategy and its application to a comprehensive quantitative phosphoproteomics analysis of Syk-dependent downstream signaling events in B cells, focusing on serine and threonine phosphorylation. Using a combination of the Syk inhibitor piceatannol, SILAC quantification, peptide fractionation, and complementary PolyMAC-Ti and PolyMAC-Zr enrichment techniques, we analyzed changes in BCR-stimulated protein phosphorylation that were dependent on the activity of Syk. We identified and quantified over 13,000 unique phosphopeptides, with a large percentage dependent on Syk activity in BCR-stimulated B cells. Our results not only confirmed many known functions of Syk, but more importantly, suggested many novel roles, including in the ubiquitin proteasome pathway, that warrant further exploration.

Nanomechanical property maps of breast cancer cells as determined by multiharmonic atomic force microscopy reveal Syk-dependent changes in microtubule stability mediated by MAP1B

The Syk protein-tyrosine kinase, a well-characterized modulator of immune recognition receptor signaling, also plays important, but poorly characterized, roles in tumor progression, acting as an inhibitor of cellular motility and metastasis in highly invasive cancer cells. Multiharmonic atomic force microscopy (AFM) was used to map nanomechanical properties of live MDA-MB-231 breast cancer cells either lacking or expressing Syk. The expression of Syk dramatically altered the cellular topography, reduced cell height, increased elasticity, increased viscosity, and allowed visualization of a more substantial microtubule network. The microtubules of Syk-expressing cells were more stable to nocodazole-induced depolymerization and were more highly acetylated than those of Syk-deficient cells. Silencing of MAP1B, a major substrate for Syk in MDA-MB-231 cells, attenuated Syk-dependent microtubule stability and reversed much of the effect of Syk on cellular topography, stiffness, and viscosity. This study illustrates the use of multiharmonic AFM both to quantitatively map the local nanomechanical properties of living cells and to identify the underlying mechanisms by which these properties are modulated by signal transduction machinery.

Modulation by Syk of Bcl-2, calcium and the calpain-calpastatin proteolytic system in human breast cancer cells

Syk is a 72kDa non-receptor tyrosine kinase that is best characterized in hematopoietic cells. While Syk is pro-tumorigenic in some cancer cell types, it also has been reported as a negative regulator of metastatic cell growth in others. An examination of the RelA (p65) subunit of NF-κB expressed in MCF7 breast cancer cells indicated that either treatment with pervanadate or stable expression of Syk protected RelA from calpain-mediated proteolysis. Similar results were observed with the tyrosine phosphatase, PTP1B, another sensitive calpain substrate. The activity of calpain in MCF7 cell lysates was inhibited by both treatment with hydrogen peroxide and expression of Syk, the former due to oxidative inactivation of calpain and the latter to enhanced expression of calpastatin (CAST), the endogenous calpain inhibitor. The level of CAST was elevated in the cytosolic fraction of Syk-positive breast cancer cells resulting in more CAST present in complex with calpain in cell lysates. The high levels of CAST coincided with elevated basal levels of calcium-and of intracellular calpain activity-in Syk-expressing cells resulting from decreased levels of Bcl-2, an inhibitor of IP3-receptor-mediated calcium release. The inhibition of cellular calpain stimulated the Syk-mediated enhancement of NF-κB induced by TNF-α, enhanced tyrosine phosphorylation resulting from integrin crosslinking, and increased the localization of Syk to the plasma membrane.

Serum spleen tyrosine kinase and vascular endothelial growth factor-C levels predict lymph node metastasis of oesophageal squamous cell carcinoma

OBJECTIVES

Lymph node involvement is a key feature and an independent prognostic factor of oesophageal squamous cell carcinoma. However, an accurate and robust assay to predict the lymphatic spread of oesophageal squamous cell carcinoma is unavailable. The purpose of this study was to determine whether serum vascular endothelial growth factor-C (VEGF-C) and spleen tyrosine kinase levels are potential markers of lymph node metastasis in patients with oesophageal squamous cell carcinoma.

METHODS

In the study, 108 patients with oesophageal squamous cell carcinoma and 24 healthy subjects participated. Serum spleen tyrosine kinase and VEGF-C concentrations were examined by enzyme-linked immunosorbent assays.

RESULTS

Patients with oesophageal squamous cell carcinoma had significantly elevated VEGF-C and decreased spleen tyrosine kinase in serum when compared with normal controls (P < 0.05). Pearson's correlation analysis revealed that serum spleen tyrosine kinase was negatively correlated with the serum VEGF-C level in oesophageal squamous cell carcinoma patients (r = -0.453, P = 0.000). In addition, surgical resections of the oesophageal tumour profoundly brought both serum spleen tyrosine kinase and VEGF-C closer to the normal range at 2 weeks after operation (P < 0.05). Statistical analysis showed that enhanced serum VEGF-C and reduced spleen tyrosine kinase significantly correlated with the stage of regional lymph node metastasis, tumour size and the status of primary tumour extension, but not with other clinicopathological features of the patients (P < 0.05).

CONCLUSIONS

Our data suggest that both serum spleen tyrosine kinase and VEGF-C levels are valuable in the preoperative evaluation of lymph node metastasis in patients with oesophageal squamous cell carcinoma.

Sensitive kinase assay linked with phosphoproteomics for identifying direct kinase substrates

Our understanding of the molecular control of many disease pathologies requires the identification of direct substrates targeted by specific protein kinases. Here we describe an integrated proteomic strategy, termed kinase assay linked with phosphoproteomics, which combines a sensitive kinase reaction with endogenous kinase-dependent phosphoproteomics to identify direct substrates of protein kinases. The unique in vitro kinase reaction is carried out in a highly efficient manner using a pool of peptides derived directly from cellular kinase substrates and then dephosphorylated as substrate candidates. The resulting newly phosphorylated peptides are then isolated and identified by mass spectrometry. A further comparison of these in vitro phosphorylated peptides with phosphopeptides derived from endogenous proteins isolated from cells in which the kinase is either active or inhibited reveals new candidate protein substrates. The kinase assay linked with phosphoproteomics strategy was applied to identify unique substrates of spleen tyrosine kinase (Syk), a protein-tyrosine kinase with duel properties of an oncogene and a tumor suppressor in distinctive cell types. We identified 64 and 23 direct substrates of Syk specific to B cells and breast cancer cells, respectively. Both known and unique substrates, including multiple centrosomal substrates for Syk, were identified, supporting a unique mechanism that Syk negatively affects cell division through its centrosomal kinase activity.

Migration inhibition of mammary epithelial cells by Syk is blocked in the presence of DDR1 receptors

The non-receptor tyrosine kinase Syk is a well-characterized hematopoietic signal transducer, which is also expressed in non-hematopoietic cells. In epithelial cells, the function of Syk is not wholly known. It interacts with the receptor tyrosine kinase DDR1 and is frequently lost from metastatic mammary tumors. Here, using genetic tracing, we demonstrate Syk expression in murine mammary epithelium, myoepithelium and skin epithelium, but not in intestinal or lung epithelia. Investigating possible functions of Syk, we found a substantial suppression of cell mobility that depended on Syk kinase activity in trans-well migration and wounding assays. Co-expression of DDR1 resulted in constitutive interaction and strong activation of Syk kinase. Most importantly, Syk-mediated migration inhibition was blocked in the presence of DDR1, while conversely DDR1 knockdown restored migration inhibition. Our study identifies Syk as a potent inhibitor of epithelial migration and describes a first functional consequence of the interaction with the collagen receptor DDR1.

Spleen tyrosine kinase mediates BEAS-2B cell migration and proliferation and human rhinovirus-induced expression of vascular endothelial growth factor and interleukin-8

Spleen tyrosine kinase (Syk) is an immunoregulatory tyrosine kinase that was identified originally in leukocytes. It is a key regulator of innate immunity as well as hematopoietic cell differentiation and proliferation. A role for Syk in regulating normal cellular functions in nonhematopoietic cells is increasingly recognized. We have shown previously robust Syk expression in airway epithelium, where it regulates the early inflammatory response to human rhinovirus (HRV) infections, and HRV cell entry by clathrin-mediated endocytosis. To test the hypothesis that Syk plays a role in modulating airway epithelial cell proliferation, migration, and production of vascular endothelial growth factor and interleukin-8, we studied the BEAS-2B human bronchial epithelial cell line and primary human airway epithelia from normal and asthmatic donors using Syk-specific pharmacologic inhibitors and small interfering RNA. Using an in vitro "wounding" model, we demonstrated significant impairment of "wound" closure after treatment with the Syk inhibitors N4-(2,2-dimethyl-3-oxo-4H-pyrid[1,4]oxazin-6-yl)-5-fluoro-N2-(3,4,5-trimethoxyphenyl)-2,4-pyrimidinediamine (R406) and 2-[7-(3,4-dimethoxyphenyl)-imidazo[1,2-c]pyrimidin-5-ylamino]-nicotinamide dihydrochloride (BAY61-3606), overexpression of the kinase-inactive Syk(K396R) mutant, and Syk knockdown by small interfering RNA. HRV infection also impaired wound healing, an effect that was partly Syk-dependent because wound healing was impaired further when HRV infection occurred in the presence of Syk inhibition. Further investigation of potential regulatory mechanisms revealed that inhibition of Syk suppressed HRV-induced vascular endothelial growth factor expression while promoting the activation of caspase-3, a mediator of epithelial cell apoptosis. Together, these results indicate that Syk plays a role in promoting epithelial cell proliferation and migration, while mitigating the effects of apoptosis.

Quantitative phosphoproteomics revealed interplay between Syk and Lyn in the resistance to nilotinib in chronic myeloid leukemia cells

In this study, we have addressed how Lyn kinase signaling mediates nilotinib-resistance by quantitative phospho-proteomics using Stable Isotope Labeling with Amino acid in Cell culture. We have found an increased tyrosine phosphorylation of 2 additional tyrosine kinases in nilotinib-resistant cells: the spleen tyrosine kinase Syk and the UFO family receptor tyrosine kinase Axl. This increased tyrosine phosphorylation involved an interaction of these tyrosine kinases with Lyn. Inhibition of Syk by the inhibitors R406 or BAY 61-3606 or by RNA interference restored the capacity of nilotinib to inhibit cell proliferation. Conversely, coexpression of Lyn and Syk were required to fully induce resistance to nilotinib in drug-sensitive cells. Surprisingly, the knockdown of Syk also strongly decreased tyrosine phosphorylation of Lyn and Axl, thus uncovering interplay between Syk and Lyn. We have shown the involvement of the adaptor protein CDCP-1 in resistance to nilotinib. Interestingly, the expression of Axl and CDCP1 were found increased both in a nilotinib-resistant cell line and in nilotinib-resistant CML patients. We conclude that an oncogenic signaling mediated by Lyn and Syk can bypass the need of Bcr-Abl in CML cells. Thus, targeting these kinases may be of therapeutic value to override imatinib or nilotinib resistance in CML.

Alternative splicing of SYK regulates mitosis and cell survival

Most human genes produce multiple mRNA isoforms through alternative splicing. However, the biological relevance of most splice variants remains unclear. In this study, we evaluated the functional impact of alternative splicing in cancer cells. We modulated the splicing pattern of 41 cancer-associated splicing events and scored the effects on cell growth, viability and apoptosis, identifying three isoforms essential for cell survival. Specifically, changing the splicing pattern of the spleen tyrosine kinase gene (SYK) impaired cell-cycle progression and anchorage-independent growth. Notably, exposure of cancer cells to epithelial growth factor modulated the SYK splicing pattern to promote the pro-survival isoform that is associated with cancer tissues in vivo. The data suggest that splicing of selected genes is specifically modified during tumor development to allow the expression of isoforms that promote cancer cell survival.

Advances in melanoma senescence and potential clinical application

Normal cells possess a limited proliferative life span, after which they enter a state of irreversible growth arrest, called replicative senescence, which acts as a potent barrier against transformation. Transformed cells have escaped the process of replicative senescence and theoretically can not re-enter senescence. However, recent observations showed that transformed cells, and particularly the melanoma cells, can still undergo oncogene or stress-induced senescence. This senescence state is accompanied by many of the markers associated with replicative senescence, such as flattened shape, increased acidic β-galactosidase activity, characteristic changes in gene expression and growth arrest. Interestingly, in some cancers, senescence induction following chemotherapy has been correlated with a favorable patient outcome. In this review, we gathered recent results describing senescence-like phenotype induction in melanoma cells and discuss why senescence may also be exploited as a therapeutic strategy in melanoma.

Reactivation of Syk gene by AZA suppresses metastasis but not proliferation of breast cancer cells

Spleen tyrosine kinase (Syk) is reported to be involved in the suppression of proliferation and invasion of breast cancer. Methylation-mediated Syk gene silencing is found in a subset of breast cancer. In this study, we used a DNA methyltransferase inhibitor, 5-aza-2-deoxycytidine (AZA), to restore Syk expression of breast cancer cells. Surprisingly, we found that AZA treatment could reestablish the expression of Syk, but not affect the proliferation of breast cancer cells. Moreover, tumor formation in situ by MDA-MB-435s treated with (+) or without (-) AZA in a nude mice MFP (Mammary fat pad) model did not show significant difference, too. Interestingly, pulmonary metastasis was still significantly suppressed in MDA-MB-435s(+) group (1/9 vs. 7/9). Our findings suggested Syk may be more correlated to metastasis rather than proliferation. This study implied a potential use of Syk methylation as a valuable biomarker to detect high metastatic potential cancerous lesions and the prospect of AZA to join the arsenal of drug candidates to be developed as a new reagent for management of advanced breast cancer.

Autoinhibition and adapter function of Syk

Development, survival, and activation of B lymphocytes are controlled by signals emanating from the B-cell antigen receptor (BCR). The BCR has an autonomous signaling function also known as tonic signaling that allows for long-term survival of B cells in the immune system. Upon binding of antigen to the BCR, the tonic signal is amplified and diversified, leading to alteration in gene expression and B-cell activation. The spleen tyrosine kinase (Syk) intimately cooperates with the signaling subunits of the BCR and plays a central role in the amplification and diversification of BCR signals. In this review, we discuss the molecular mechanisms by which Syk activity is inhibited and activated at the BCR. Importantly, Syk acts not only as a kinase that phosphorylates downstream substrates but also as an adapter that can bind to a diverse set of signaling proteins. Depending on its interactions and localization, Syk can signal opposing cell fate decisions such as proliferation or differentiation of B cells.

Delocalization of gamma-tubulin due to increased solubility in human breast cancer cell lines

The centrosome is the major organelle responsible for the nucleation and organization of microtubules into arrays. Recent studies demonstrate that microtubules can nucleate outside the centrosome. The molecular mechanisms controlling acentrosomal microtubule nucleation are currently poorly defined, and the function of this type of microtubule regulation in tumor cell biology is particularly unclear. Since microtubule nucleation is initiated by the gamma-tubulin protein, we examined the regulation of gamma-tubulin in a panel of human breast tumor cell lines, ranging from non-tumorigenic to highly aggressive. We have identified a more dispersive subcellular localization of gamma-tubulin in aggressive breast cancer cell lines, while gamma-tubulin localization remains largely centrosomal in non-aggressive cell lines. Delocalization of gamma-tubulin occurs independently from changes in protein expression and is therefore regulated at the post-translational level. Subcellular fractionation revealed that tumor cell lines show an aberrantly increased release of gamma-tubulin into a soluble cytoplasmic fraction, with the most dramatic changes observed in tumor cell lines of greater aggressiveness. Extraction of soluble gamma-tubulin revealed acentrosomal incorporation of gamma-tubulin in cytoplasmic microtubules and along cell junctions. Moreover, acentrosomal delocalization of gamma-tubulin yielded resistance to colchicine-mediated microtubule collapse. These findings support a model where the solubility of gamma-tubulin can be altered through post-translational modification and provides a new mechanism for microtubule dysregulation in breast cancer. Gamma-tubulin that is delocalized from the centrosome can still clearly be incorporated into filaments, and defines a novel mechanism for tumor cells to develop resistance to microtubule-targeted chemotherapies.

Calpain 2 is required for sister chromatid cohesion

Calpains form a family of Ca(2+)-dependent cysteine proteases involved in diverse cellular processes. However, the specific functions of each calpain isoform remain unknown. Recent reports have shown that calpain 2 (Capn2) is essential for cell viability. We have recently shown that Capn2 is a nuclear protease associated with chromosomes during mitosis in mammalian embryonic cells. We now report that Capn2 depletion impairs mitosis and induces apoptosis in murine cells. Low Capn2 levels induce chromosome alignment defects, the loss of histone H3 threonine 3 phosphorylation at centromeres, and premature sister chromatid separation. Thus, Capn2 may play a role in fundamental mitotic functions, such as the maintenance of sister chromatid cohesion.

Clinical significance of Syk expression in human esophageal squamous cell carcinoma

AIM: To investigate the expression of spleen tyrosine kinase (Syk) mRNA and protein in human esophageal squamous cell carcinoma (ESCC) and analyze its correlation with the malignant biological behavior of ESCC.

METHODS: The expression of Syk protein in 48 ESCC specimens and matched adjacent non-carcinoma specimens was detected by immunohistochemistry. The expression of Syk mRNA in 43 ESCC specimens and matched adjacent non-carcinoma specimens was assayed by reverse transcription-polymerase chain reaction (RT-PCR). The correlations of Syk expression with tumor size, TNM stage and lymph node metastasis were analyzed.

RESULTS: The positive rate of Syk protein expression in ESCC tissue was significantly lower than that in adjacent non-carcinoma tissue (16.67% vs 89.58%, P < 0.05). The expression of Syk protein was correlated with tumor TNM stage in ESCC (χ² = 6.713, P < 0.05). A significant difference was noted in Syk expression between ESCC with and without lymph node metastasis (3.03% vs 29.41%, P < 0.05). No significant correlation was observed between Syk expression and tumor size (χ² = 0.017, P > 0.05). RT-PCR analysis showed that the expression level of Syk mRNA was significantly lower in ESCC than in adjacent non-carcinoma tissue (t = -11.27, P < 0.05).

CONCLUSION: Downregulation of Syk gene expression may be involved in the carcinogenesis and metastasis of ESCC. Syk gene is a novel candidate tumor suppressor gene in ESCC and can be used as a molecular marker for early diagnosis and treatment of ESCC.

Syk tyrosine kinase acts as a pancreatic adenocarcinoma tumor suppressor by regulating cellular growth and invasion

We have identified the nonreceptor tyrosine kinase syk as a marker of differentiation/tumor suppressor in pancreatic ductal adenocarcinoma (PDAC). Syk expression is lost in poorly differentiated PDAC cells in vitro and in situ, and stable reexpression of syk in endogenously syk-negative Panc1 (Panc1/syk) cells retarded their growth in vitro and in vivo and reduced anchorage-independent growth in vitro. Panc1/syk cells exhibited a more differentiated morphology and down-regulated cyclin D1, akt, and CD171, which are overexpressed by Panc1 cells. Loss of PDAC syk expression in culture is due to promoter methylation, and reversal of promoter methylation caused reexpression of syk and concomitant down-regulation of CD171. Moreover, suppression of syk expression in BxPC3 cells caused de novo CD171 expression, consistent with the reciprocal expression of syk and CD171 we observe in situ. Importantly, Panc1/syk cells demonstrated dramatically reduced invasion in vitro. Affymetrix analysis identified statistically significant regulation of >2000 gene products by syk in Panc1 cells. Of these, matrix metalloproteinase-2 (MMP2) and tissue inhibitor of metalloproteinase-2 were down-regulated, suggesting that the MMP2 axis might mediate Panc1/mock invasion. Accordingly, MMP2 inhibition suppressed the in vitro invasion of Panc1/mock cells without effect on Panc1/syk cells. This study demonstrates a prominent role for syk in regulating the differentiation state and invasive phenotype of PDAC cells.

The alternative TrkAIII splice variant targets the centrosome and promotes genetic instability

The hypoxia-regulated alternative TrkAIII splice variant expressed by human neuroblastomas exhibits oncogenic potential, driven by in-frame exon 6 and 7 alternative splicing, leading to omission of the receptor extracellular immunoglobulin C(1) domain and several N-glycosylation sites. Here, we show that the TrkAIII oncogene promotes genetic instability by interacting with and exhibiting catalytic activity at the centrosome. This function depends upon intracellular TrkAIII accumulation and spontaneous interphase-restricted activation, in cytoplasmic tyrosine kinase (tk) domain orientation, predominantly within structures that closely associate with the fully assembled endoplasmic reticulum intermediate compartment and Golgi network. This facilitates TrkAIII tk-mediated binding of gamma-tubulin, which is regulated by endogenous protein tyrosine phosphatases and geldanamycin-sensitive interaction with Hsp90, paving the way for TrkAIII recruitment to the centrosome. At the centrosome, TrkAIII differentially phosphorylates several centrosome-associated components, increases centrosome interaction with polo kinase 4, and decreases centrosome interaction with separase, the net results of which are centrosome amplification and increased genetic instability. The data characterize TrkAIII as a novel internal membrane-associated centrosome kinase, unveiling an important alternative mechanism to "classical" cell surface oncogenic receptor tk signaling through which stress-regulated alternative TrkAIII splicing influences the oncogenic process.

Phosphoproteomic analysis of Syk kinase signaling in human cancer cells reveals its role in cell-cell adhesion

The spleen tyrosine kinase Syk has predominantly been studied in hematopoietic cells in which it is involved in immunoreceptor-mediated signaling. Recently, Syk expression was evidenced in numerous nonhematopoietic cells and shown to be involved in tumor formation and progression. The Syk downstream signaling effectors in nonhematopoietic cells remain, however, to be uncovered, and were investigated using MS-based quantitative phosphoproteomics. Two strategies, based on the inhibition of the Syk catalytic activity and on the loss of Syk expression were employed to identify phosphotyrosine-dependent complexes. Quantitative measurements were obtained on 350 proteins purified with phosphotyrosine affinity columns using the SILAC method. Forty-one proteins are dependent on both Syk expression and catalytic activity and were selected as signaling effectors. They are involved in a variety of biological processes such as signal transduction, cell-cell adhesion and cell polarization. We investigated the functional involvement of Syk in cell-cell adhesion and demonstrated the phosphorylation of E-cadherin and alpha-catenin. In addition, Syk is localized at cell-cell contacts, and Syk-mediated phosphorylation of E-cadherin seems to be important for the proper localization of p120-catenin at adherens junctions. Identification of the biochemical pathways regulated by Syk in human cancer cells will help to uncover its role in tumor formation and progression.

Syk and pTyr'd: Signaling through the B cell antigen receptor

The B cell receptor (BCR) transduces antigen binding into alterations in the activity of intracellular signaling pathways through its ability to recruit and activate the cytoplasmic protein-tyrosine kinase Syk. The recruitment of Syk to the receptor, its activation and its subsequent interactions with downstream effectors are all regulated by its phosphorylation on tyrosine. This review discusses our current understanding of how this phosphorylation regulates the activity of Syk and its participation in signaling through the BCR.

Alcohol consumption and decreased risk of non-Hodgkin lymphoma: role of mTOR dysfunction

Several epidemiologic studies support the emerging paradigm that current alcohol consumers have decreased risk of most types of non-Hodgkin lymphoma. The observed lower risk among people who drank alcohol does not seem to vary with beverage type. The mechanisms accounting for alcohol-induced decrease in the incidence of lymphomas remain largely unknown. We demonstrate that low-dose chronic exposure to ethanol inhibits mammalian target of rapamycin (mTOR) C1 complex formation, resulting in decreased phosphorylation events involved in mTOR pathway signaling in a lymphoid-tissue specific manner. These changes in mTOR signaling lead to a decrease in eIF4E associated with the translation initiation complex and a repression of global cap-dependent synthesis in both lymphoma cell lines and normal donor lymphocytes. We show that chronic exposure of ethanol at physiologically relevant concentrations in a xenograft model results in a striking inhibition of lymphoma growth. Our data support a paradigm in which chronic ethanol exposure inhibits mTOR signaling in lymphocytes with a significant repression of cap-dependent translation, reducing the tumorigenic capacity of non-Hodgkin lymphoma in a human xenograft model. The ethanol-mediated repression of mTOR signaling coupled with decreased in vivo lymphoma growth underscore the critical role of mTOR signaling and translation in lymphoma.

Spleen tyrosine kinase functions as a tumor suppressor in melanoma cells by inducing senescence-like growth arrest

Loss of tumor-suppressive pathways that control cellular senescence is a crucial step in malignant transformation. Spleen tyrosine kinase (Syk) is a cytoplasmic tyrosine kinase that has been recently implicated in tumor suppression of melanoma, a deadly skin cancer derived from pigment-producing melanocytes. However, the mechanism by which Syk suppresses melanoma growth remains unclear. Here, we report that reexpression of Syk in melanoma cells induces a p53-dependent expression of the cyclin-dependent kinase (cdk) inhibitor p21 and a senescence program. We first observed that Syk expression is lost in a subset of melanoma cell lines, primarily by DNA methylation-mediated gene silencing and restored after treatment with the demethylating agent 5-aza-2-deoxycytidine. We analyzed the significance of epigenetic inactivation of Syk and found that reintroduction of Syk in melanoma cells dramatically reduces clonogenic survival and three-dimensional tumor spheroid growth and invasion. Remarkably, melanoma cells reexpressing Syk display hallmarks of senescent cells, including reduction of proliferative activity and DNA synthesis, large and flattened morphology, senescence-associated beta-galactosidase activity, and heterochromatic foci. This phenotype is accompanied by hypophosphorylated retinoblastoma protein (Rb) and accumulation of p21, which depends on functional p53. Our results highlight a new role for Syk tyrosine kinase in regulating cellular senescence and identify Syk-mediated senescence as a novel tumor suppressor pathway the inactivation of which may contribute to melanoma tumorigenicity.

The protein-tyrosine kinase Syk interacts with TRAF-interacting protein TRIP in breast epithelial cells

The nonreceptor, protein-tyrosine kinase Syk is a suppressor of breast cancer progression whose expression is inversely correlated with the invasive behavior of cancer cells. In contrast, Syk has a positive function in murine mammary tumor virus-mediated tumorigenesis. A yeast two-hybrid screen using a library from human mammary gland identified tumor necrosis factor (TNF) receptor-associated factor-interacting protein (TRIP) as an Syk-binding partner. This interaction is mediated by the C-terminal region of TRIP and is enhanced by the treatment of cells with TNF and the tyrosine phosphorylation of Syk. Syk and TRIP have opposing functions in TNF-signaling pathways. Syk enhances the activation of nuclear factor-kappaB by TNF and this is antagonized by TRIP. The overexpression of TRIP sensitizes cells to TNF-induced apoptosis, an effect that can be reversed by the coexpression of Syk.

Tyr130 phosphorylation triggers Syk release from antigen receptor by long-distance conformational uncoupling

The Syk protein-tyrosine kinase plays a major role in signaling through the B cell receptor for antigen (BCR). Syk binds the receptor via its tandem pair of SH2 domains interacting with a doubly phosphorylated immunoreceptor tyrosine-based activation motif (dp-ITAM) of the BCR complex. Upon phosphorylation of Tyr-130, which lies between the two SH2 domains distant to the phosphotyrosine binding sites, Syk dissociates from the receptor. To understand the structural basis for this dissociation, we investigated the structural and dynamic characteristics of the wild type tandem SH2 region (tSH2) and a variant tandem SH2 region (tSH2(pm)) with Tyr-130 substituted by Glu to permanently introduce a negative charge at this position. NMR heteronuclear relaxation experiments, residual dipolar coupling measurements and analytical ultracentrifugation revealed substantial differences in the hydrodynamic behavior of tSH2 and tSH2(pm). Although the two SH2 domains in tSH2 are tightly associated, the two domains in tSH2(pm) are partly uncoupled and tumble in solution with a faster correlation time. In addition, the equilibrium dissociation constant for the binding of tSH2(pm) to dp-ITAM (1.8 microM) is significantly higher than that for the interaction between dp-ITAM and tSH2 but is close to that for a singly tyrosine-phosphorylated peptide binding to a single SH2 domain. Experimental data and hydrodynamic calculations both suggest a loss of domain-domain contacts and change in relative orientation upon the introduction of a negative charge on residue 130. A long-distance structural mechanism by which the phosphorylation of Y130 negatively regulates the interaction of Syk with immune receptors is proposed.

The Tom1L1-clathrin heavy chain complex regulates membrane partitioning of the tyrosine kinase Src required for mitogenic and transforming activities

Compartmentalization of Src tyrosine kinases (SFK) plays an important role in signal transduction induced by a number of extracellular stimuli. For example, Src mitogenic signaling induced by platelet-derived growth factor (PDGF) is initiated in cholesterol-enriched microdomain caveolae. How this Src subcellular localization is regulated is largely unknown. Here we show that the Tom1L1-clathrin heavy chain (CHC) complex negatively regulates the level of SFK in caveolae needed for the induction of DNA synthesis. Tom1L1 is both an interactor and a substrate of SFK. Intriguingly, it stimulates Src activity without promoting mitogenic signaling. We found that, upon association with CHC, Tom1L1 reduced the level of SFK in caveolae, thereby preventing its association with the PDGF receptor, which is required for the induction of mitogenesis. Similarly, the Tom1L1-CHC complex reduced also the level of oncogenic Src in cholesterol-enriched microdomains, thus affecting both its capacity to induce DNA synthesis and cell transformation. Conversely, Tom1L1, when not associated with CHC, accumulated in caveolae and promoted Src-driven DNA synthesis. We concluded that the Tom1L1-CHC complex defines a novel mechanism involved in negative regulation of mitogenic and transforming signals, by modulating SFK partitioning at the plasma membrane.