Résumé
The spleen tyrosine kinase Syk was mainly studied in immunoreceptor-activated signaling in hematopoietic cells. We first demonstrated that Syk is also present in mammary epithelial cells and that its expression is lost in malignant breast cancer cells. Using mouse xenograft models injected with Syk-transfected cells we and others established that Syk acts as a tumor and metastasis suppressor. Moreover, clinical studies reveal a correlation between reduced Syk expression and a decreased survival and increased metastasis risk in breast cancer and other carcinomas. The main objective of our investigations is to unravel the mechanisms of the anti-oncogenic activity of Syk. For this, identification of its substrates and signaling pathways is crucial. A quantitative phospho-proteomic approach allowed to identify novel potential Syk substrates involved in intercellular adhesion and epithelial polarity, both characteristics of cell differentiation that are lost during tumor invasion and metastasis.
Phosphorylation assays with recombinant proteins demonstrated that E-Cadherin (E-Cdh), α, β and p120-catenin (Ctn) are direct Syk substrates. Using mass spectrometry we identified the tyrosine residues phosphorylated by Syk and generated phospho-Tyr-peptide-specific (pY) antibodies. Using immuno-fluorescence Syk was shown to colocalize with E-Cdh at adherens junctions (AJ). Syk transfection increased E-Cdh, α- and β-Ctn phosphorylation at AJ in a kinase-dependent manner, whereas non-phosphorylatable mutant Syk exhibited a decreased AJ localization. This was biochemically confirmed by immunoprecipitation and Western blot experiments. Phosphorylated Ctns are still associated with E-Cdh and vice versa. In 2D culture, Syk transfection stimulated intercellular contact formation. Correspondingly, in Syk-knockdown cells the 2D and 3D cell-cell re-aggregation was decreased. Conversely, cell migration and invasion were inhibited by Syk transfection and stimulated by Syk knockdown. At the molecular level, Syk transfection increases the interaction between the E-Cdh/Ctn complex with zonula occludens proteins and the actin cytoskeleton.
In conclusion, Syk seems to play a positive role in the formation and maintenance of AJ via the phosphorylation of the E-Cdh/Ctn complex. Loss of Syk expression or function might lead to the disruption of these complexes and consequently promote invasion and metastasis. (Supported by the Fondation ARC SL220110603480 and Plan Cancer/INCa ASC14021FSA)
Citation Format: Toufic Kassouf, Philippe Montcourrier, Romain Larive, Anne Morel, Serge Urbach, Peter J. Coopman. The E-cadherin/catenin complex and the intercellular adhesion of breast cancer cells are positively regulated via phosphorylation by the Syk tyrosine kinase in breast cancer cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 917.