La résistance aux inhibiteurs de BRAF

La voie de signalisation MAPK/ERK est une voie centrale de la signalisation intracellulaire. Sa dérégulation participe à la transformation et la progression tumorales. Dans plusieurs cancers, la découverte de mutations activatrices de BRAF, à l’origine de l’activation de cette voie, a ouvert de nouvelles perspectives thérapeutiques avec le développement d’inhibiteurs spécifiques de la protéine. Selon les cancers, ces inhibiteurs ont cependant montré soit une efficacité insuffisante, due à la résistance primaire des cellules tumorales, soit une efficacité transitoire, due à l’apparition d’une résistance acquise. Dans cette revue, nous revenons sur les découvertes qui ont conduit au développement de ces inhibiteurs de BRAF. Nous détaillons également les mécanismes moléculaires et cellulaires de la résistance à ces inhibiteurs observée dans différents types de cancers. Comprendre ces mécanismes est en effet primordial pour développer des stratégies thérapeutiques qui soient plus efficaces.

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.

PTPN13 induces cell junction stabilization and inhibits mammary tumor invasiveness

Clinical data suggest that the protein tyrosine phosphatase PTPN13 exerts an anti-oncogenic effect. Its exact role in tumorigenesis remains, however, unclear due to its negative impact on FAS receptor-induced apoptosis.

Methods: We crossed transgenic mice deleted for PTPN13 phosphatase activity with mice that overexpress human HER2 to assess the exact role of PTPN13 in tumor development and aggressiveness. To determine the molecular mechanism underlying the PTPN13 tumor suppressor activity we developed isogenic clones of the aggressive human breast cancer cell line MDA-MB-231 overexpressing either wild type or a catalytically-inactive mutant PTPN13 and subjected these to phosphoproteomic and gene ontology analyses.

We investigated the PTPN13 consequences on cell aggressiveness using wound healing and Boyden chamber assays, on intercellular adhesion using videomicroscopy, cell aggregation assay and immunofluorescence.

Results: The development, growth and invasiveness of breast tumors were strongly increased by deletion of the PTPN13 phosphatase activity in transgenic mice. We observed that PTPN13 phosphatase activity is required to inhibit cell motility and invasion in the MDA-MB-231 cell line overexpressing PTPN13. In vivo, the negative PTPN13 effect on tumor invasiveness was associated with a mesenchymal-to-epithelial transition phenotype in athymic mice xenografted with PTPN13-overexpressing MDA-MB-231 cells, as well as in HER2-overexpressing mice with wild type PTPN13, compared to HER2-overexpressing mice that lack PTPN13 phosphatase activity. Phosphoproteomic and gene ontology analyses indicated a role of PTPN13 in the regulation of intercellular junction-related proteins. Finally, protein localization studies in MDA-MB-231 cells and HER2-overexpressing mice tumors confirmed that PTPN13 stabilizes intercellular adhesion and promotes desmosome formation.

Conclusions: These data provide the first evidence for the negative role of PTPN13 in breast tumor invasiveness and highlight its involvement in cell junction stabilization.

Network Reconstruction and Significant Pathway Extraction Using Phosphoproteomic Data from Cancer Cells

Protein phosphorylation acts as an efficient switch controlling deregulated key signaling pathway in cancer. Computational biology aims to address the complexity of reconstructed networks but overrepresents well-known proteins and lacks information on less-studied proteins. A bioinformatic tool to reconstruct and select relatively small networks that connect signaling proteins to their targets in specific contexts is developed. It enables to propose and validate new signaling axes of the Syk kinase. To validate the potency of the tool, it is applied to two phosphoproteomic studies on oncogenic mutants of the well-known phosphatidyl-inositol 3-kinase (PIK3CA) and the unfamiliar Src-related tyrosine kinase lacking C-terminal regulatory tyrosine and N-terminal myristoylation sites (SRMS) kinase. By combining network reconstruction and signal propagation, comprehensive signaling networks from large-scale experimental data are built and multiple molecular paths from these kinases to their targets are extracted. Specific paths from two distinct PIK3CA mutants are retrieved, and their differential impact on the HER3 receptor kinase is explained. In addition, to address the missing connectivities of the SRMS kinase to its targets in interaction pathway databases, phospho-tyrosine and phospho-serine/threonine proteomic data are integrated. The resulting SRMS-signaling network comprises casein kinase 2, thereby validating its currently suggested role downstream of SRMS. The computational pipeline is publicly available, and contains a user-friendly graphical interface (http://doi.org/10.5281/zenodo.3333687).

Abstract 1978: The tyrosine kinase Syk plays a role in the maintenance of intercellular adhesive junctions in human breast cancer cells

The non-receptor spleen tyrosine kinase Syk has mainly been studied in hematopoietic cells in which it plays a key role in the immune-response signalling. We previously demonstrated that Syk is also present in mammary epithelial cells and that its expression is lost during malignant progression of breast cancer cell lines. Clinical studies confirmed loss of Syk expression, not only in breast tumors but also in other carcinomas and melanoma. Xenotransplantation animal experiments evidenced that Syk acts as a tumor and metastasis suppressor in breast cancer and melanoma. The mechanisms by which Syk exerts its anti-oncogenic activity remain, however, unrevealed. Using a quantitative phospho-proteomic SILAC approach in breast cancer cells, we identified new potential Syk substrates involved in intercellular adhesion and epithelial cell polarity, both characters of cell differentiation that are lost during tumor progression.

Using in vitro kinase assays, we demonstrated that E-cadherin and various catenins are direct Syk substrates. The tyrosine residues phosphorylated by Syk were identified by mass spectrometry and phospho-peptide-specific antibodies were generated. By immunofluorescence, we observed that endogenous Syk and E-cadherin colocalize at adherens junctions and that transient DsRed-Syk transfection induces phospho-E-cadherin- and phospho-catenin epitopes at cell-cell contacts. Immunoprecipitation experiments indicate that phosphorylated E-cadherin and catenins are associated in a complex. Syk-mediated phosphorylation of E-cadherin seems to be important for the proper localization of p120-catenin at adherens junctions. Syk knockdown in breast cancer cells partially inhibits the re-aggregation of cells and increases invasion in Matrigel. Using Syk knockdown and overexpression we explored its potential role in the maintenance of an epithelial phenotype, preventing EMT.

In conclusion, Syk might play a role in the establishment and maintenance of intercellular junctions via the phosphorylation of the E-cadherin/catenin complex. Loss of Syk expression or function might lead to the destabilization of these complexes and promote invasion (supported by Fondation ARC SL220110603480)

Citation Format: Toufic Kassouf, Philippe Montcourrier, Romain Larive, Nadir Bettache, Anne Morel, Fabrice Merezegue, Serge Urbach, Peter J. Coopman. The tyrosine kinase Syk plays a role in the maintenance of intercellular adhesive junctions in human breast cancer cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1978. doi:10.1158/1538-7445.AM2014-1978

Abstract 2959: The Syk tyrosine kinase negatively affects cell cycle progression through phosphorylation of the Cdk1 kinase in response to DNA damage

The non-receptor spleen tyrosine kinase Syk has mainly been studied in hematopoietic cells in which it plays a key role in the immune-response signalling. We previously demonstrated that Syk is also present in mammary epithelial cells and that its expression is lost during malignant progression of breast cancer cell lines. Clinical studies confirmed loss of Syk expression, not only in breast tumors but also in other carcinomas and melanoma. Xenotransplantation animal experiments evidenced that Syk acts as a tumor and metastasis suppressor in breast cancer and melanoma. The mechanisms by which Syk exerts its anti-oncogenic activity remain, however, unrevealed. We previously demonstrated that Syk localizes to the centrosomes and negatively affects cell division. The centrosomal Syk substrates and possible effectors of its anti-oncogenic activity remain, nevertheless, unknown. In this study, we demonstrate that Syk may negatively affect cell cycle progression through the phosphorylation of the Cdk1 kinase, a key protein involved in the control of mitosis.

Using immunofluorescence, we observed that endogenous Syk and Cdk1 colocalize at the centrosomes and that transient DsRed-Syk transfection induces pY15-Cdk1-positive epitopes at the centrosomes. In vitro kinase assays demonstrate that Syk can phosphorylate Cdk1 in the presence of different cyclins. Mass spectrometry allowed identification of four different Cdk1 tyrosine residues phosphorylated by Syk, amongst which the Y15 is known to negatively affect mitotic entry when phosphorylated by the Wee1 kinase. Transient overexpression of DsRed-Syk or GFP-Cdk1 carrying phospho-mimicking mutations resulted in a cell cycle block as observed by FACS analysis and led to a senescence-like growth arrest. Finally, we demonstrate that DNA damage-inducing agents induce Syk activation and Cdk1 tyrosine phosphorylation in a Syk-dependent manner, as evidenced by the use of a Syk-targeting shRNA.

In conclusion, our results reveal that Syk overexpression or Syk activation by genotoxic agents induce a cell cycle arrest through the phosphorylation of Cdk1. These novel aspects of Syk function will contribute to a better understanding of its onco-suppressive activity and, more generally, of the DNA damage-induced signalling.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2959. doi:10.1158/1538-7445.AM2011-2959

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.

In vitro invasion assays: phagocytosis of the extracellular matrix

Degradation of the extracellular matrix is an essential component of phagocytosis by tumor cells and can be correlated with their invasive capacity. This unit presents flow cytometry based assays for rapid quantitative assessment of both proteolysis and internalization or internalization alone. These assays provide high-throughput, low-cost methods to compare cell lines and test the efficacy of treatments designed to stimulate or inhibit invasion.

The Syk tyrosine kinase: a new negative regulator in tumor growth and progression

The spleen tyrosine kinase Syk was long thought to be a hematopoietic cell-specific signaling molecule. Recent evidence demonstrated that it is also expressed by many non-hematopoietic cell types and that it plays a negative role in cancer. A significant drop in its expression was first observed during breast cancer progression, but an anomalous Syk expression has now also been evidenced in many other tumor types. Mechanistic studies using Syk re-expression demonstrated its suppressive function in tumorigenesis and metastasis formation, which is surprising for a tyrosine kinase. Loss of Syk expression is regulated, albeit not exclusively, by its promoter hypermethylation. The molecular mechanism of its tumor-suppressive function remains largely unknown; the identification of its activators and effectors in non-hematopoietic cells will be a challenge for the years to come. An increasing number of clinical studies reveal a correlation between reduced Syk expression and an increased risk for metastasis formation, and assign Syk as a potential new prognostic marker in different tumor types.

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

We showed previously that the spleen tyrosine kinase Syk is expressed by mammary epithelial cells and that it suppresses malignant growth of breast cancer cells. The exact molecular mechanism of its tumor-suppressive activity remains, however, to be identified. Here, we show that Syk colocalizes and copurifies with the centrosomal component gamma-tubulin and exhibits a catalytic activity within the centrosomes. Moreover, its centrosomal localization depends on its intact kinase activity. Centrosomal Syk expression is persistent in interphase but promptly drops during mitosis, obviously resulting from its ubiquitinylation and proteasomal degradation. Conversely, unrestrained exogenous expression of a fluorescently tagged Discosoma sp. red fluorescent protein (DsRed)-Syk chimera engenders abnormal cell division and cell death. Transient DsRed-Syk overexpression triggers an abrupt cell death lacking hallmarks of classic apoptosis but reminiscent of mitotic catastrophe. Surviving stable DsRed-Syk-transfected cells exhibit multipolar mitotic spindles and contain multiple abnormally sized nuclei and supernumerary centrosomes, revealing anomalous cell division. Taken together, these results show that Syk is a novel centrosomal kinase that negatively affects cell division. Its expression is strictly controlled in a spatiotemporal manner, and centrosomal Syk levels need to decline to allow customary progression of mitosis.

The Syk tyrosine kinase suppresses malignant growth of human breast cancer cells

Syk is a protein tyrosine kinase that is widely expressed in haematopoietic cells. It is involved in coupling activated immunoreceptors to downstream signalling events that mediate diverse cellular responses including proliferation, differentiation and phagocytosis. Syk expression has been reported in cell lines of epithelial origin, but its function in these cells remains unknown. Here we show that Syk is commonly expressed in normal human breast tissue, benign breast lesions and low-tumorigenic breast cancer cell lines. Syk messenger RNA and protein, however, are low or undetectable in invasive breast carcinoma tissue and cell lines. Transfection of wild-type Syk into a Syk-negative breast cancer cell line markedly inhibited its tumour growth and metastasis formation in athymic mice. Conversely, overexpression of a kinase-deficient Syk in a Syk-positive breast cancer cell line significantly increased its tumour incidence and growth. Suppression of tumour growth by the reintroduction of Syk appeared to be the result of aberrant mitosis and cytokinesis. We propose that Syk is a potent modulator of epithelial cell growth and a potential tumour suppressor in human breast carcinomas.

Dual actions of sphingosine-1-phosphate: extracellular through the Gi-coupled receptor Edg-1 and intracellular to regulate proliferation and survival

Sphingosine-1-phosphate (SPP), a bioactive lipid, acts both intracellularly and extracellularly to cause pleiotropic biological responses. Recently, we identified SPP as a ligand for the G protein-coupled receptor Edg-1 (Lee, M.-J., J.R. Van Brocklyn, S. Thangada, C.H. Liu, A.R. Hand, R. Menzeleev, S. Spiegel, and T. Hla. 1998. Science. 279:1552-1555). Edg-1 binds SPP with remarkable specificity as only sphinganine-1-phosphate displaced radiolabeled SPP, while other sphingolipids did not. Binding of SPP to Edg-1 resulted in inhibition of forskolin-stimulated cAMP accumulation, in a pertussis toxin-sensitive manner. In contrast, two well-characterized biological responses of SPP, mitogenesis and prevention of apoptosis, were clearly unrelated to binding to Edg-1 and correlated with intracellular uptake. SPP also stimulated signal transduction pathways, including calcium mobilization, activation of phospholipase D, and tyrosine phosphorylation of p125(FAK), independently of edg-1 expression. Moreover, DNA synthesis in Swiss 3T3 fibroblasts was significantly and specifically increased by microinjection of SPP. Finally, SPP suppresses apoptosis of HL-60 and pheochromocytoma PC12 cells, which do not have specific SPP binding or expression of Edg-1 mRNA. Conversely, sphinganine-1-phosphate, which binds to and signals via Edg-1, does not have any significant cytoprotective effect. Thus, SPP is a prototype for a novel class of lipid mediators that act both extracellularly as ligands for cell surface receptors and intracellularly as second messengers.

Phagocytosis of cross-linked gelatin matrix by human breast carcinoma cells correlates with their invasive capacity

During invasion and metastasis, cancer cells interact closely with the extracellular matrix molecules by attachment, degradation, and migration. We demonstrated previously the local degradation of fluorescently labeled gelatin matrix by cancer cells at invasive membrane protrusions, called invadopodia. Using the newly developed quantitative fluorescence-activated cell sorting-phagocytosis assay and image analysis of localized degradation of fluorescently labeled matrix, we document here that degradation and site-specific removal of cross-linked gelatin matrix is correlated with the extent of phagocytosis in human breast cancer cells. A higher phagocytic capacity is generally associated with increasing invasiveness, documented in other invasion and motility assays as well. Gelatin phagocytosis is time and cell density dependent, and it is mediated by the actin cytoskeleton. Most of the intracellular gelatin is routed to actively acidified vesicles, as demonstrated by the fluorescent colocalization of gelatin with acidic vesicles, indicating the intracellular degradation of the phagocytosed matrix in lysosomes. We show here that normal intracellular routing is blocked after treatment with acidification inhibitors. In addition, the need for partial proteolytic degradation of the matrix prior to phagocytosis is demonstrated by the inhibition of gelatin phagocytosis with different serine and metalloproteinase inhibitors and its stimulation by conditioned medium containing the matrix metalloproteinases MMP-2 and MMP-9. Our results demonstrate that phagocytosis of extracellular matrix is an inherent feature of breast tumor cells that correlates with and may even directly contribute to their invasive capacity. This assay is useful for screening and evaluating potential anti-invasive agents because it is fast, reproducible, and versatile.

Integrin alpha 3 beta 1 participates in the phagocytosis of extracellular matrix molecules by human breast cancer cells

The mechanisms and receptors involved in phagocytosis by nonhematopoietic cells are not well understood. The involvement of the alpha 3 beta 1 integrin in phagocytosis of the extracellular matrix by human breast cancer cells was studied. The possible role of this integrin was suggested since alpha 3 and beta 1 but not alpha 2 subunits are concentrated at membrane sites where local degradation of fluorescently labeled gelatin occurs. Strikingly, anti-alpha 3 integrin monoclonal antibodies (mAbs) stimulate the phagocytosis of fluorescently labeled gelatin films, gelatin beads, and Matrigel films in a quantitative phagocytosis assay. Stimulation of the gelatin uptake by the anti-alpha 3 mAb is dose responsive, saturable, and time dependent. Antibodies against other integrin subunits have a lower stimulatory effect (anti-beta 1) or no significant effect (anti-alpha 2, -alpha 5, -alpha 6, and -alpha v) on gelatin phagocytosis. The synthetic HGD-6 human laminin peptide that binds specifically the alpha 3 beta 1 integrin, but not the scrambled HSGD-6 control peptide, also markedly stimulates gelatin uptake in a dose-responsive way. Furthermore, the stimulatory effects of the HGD-6 peptide and the anti-alpha 3 mAb are additive, suggesting that they might promote phagocytosis in different ways. Other laminin (YIGSR, IKVAV) and fibronectin (GRGDS) peptides have no effect on gelatin phagocytosis. Immunofluorescence shows that the alpha 3 and the beta 1, but not the alpha 2 integrin subunit, concentrate into patches on the cell surface after treatment with their respective mAbs. And, both gelatin and the alpha 3 beta 1 but not the alpha 2 beta 1 integrin are cointernalized and routed to acidic vesicles such as lysosomes. In conclusion, we demonstrate that human breast cancer cells locally degrade and phagocytose the extracellular matrix and show for the first time that the alpha 3 beta 1 integrin participates in this phagocytosis. We hypothesize that the anti-alpha 3 antibodies and the laminin peptide HGD-6 activate the alpha 3 beta 1 integrin, which results in a downstream signaling cascade stimulating phagocytosis.

Influence of basement membrane molecules on directional migration of human breast cell lines in vitro

Spheroidal cell aggregates were prepared from four tumorigenic human breast cell lines (HBL-100 and three MCF-7 variants). Cells from these aggregates were allowed to migrate towards lanes of basement membrane components coated on a glass substratum. Matrigel (reconstituted basement membrane) lanes permanently arrested the migration of one MCF-7 cell line, while migration of the others was permitted. Amongst several purified basement membrane constituents only laminin, not collagen type IV or fibronectin, was found to cause the same arrest of migration. Within the laminin molecule only the pepsin P1, not the elastase E8 fragment, efficiently arrested migration of that cell line. Although migration was inhibited by these components, time-lapse video recordings revealed that arrested cells still proliferated and actively ruffled on top of the coatings. These data suggest that, amongst several basement membrane components, laminin can function as a stop signal for cell migration. Within laminin, this activity seems to be mainly associated with the P1 fragment. We conclude that laminin is the major determinant of the barrier-function of the basement membrane, to which some cell types have become insensitive.