Role of the Protein Tyrosine Kinase Syk in Regulating Cell-Cell Adhesion and Motility in Breast Cancer Cells

Abstract

The expression of the Syk protein tyrosine kinase in breast cancer cells is inversely correlated with invasive growth and metastasis. The expression of Syk inhibits cell motility while supporting the formation of cell clusters by enhancing cell-cell contacts and promoting the redistribution of the adhesion proteins cortactin and vinculin to these contacts. Syk associates physically with cortactin and catalyzes its phosphorylation on tyrosine. The clustering of integrins leads to the phosphorylation of Syk and of numerous cellular proteins in a manner dependent on the activity of the kinase and on the presence of tyrosine 342 located in the linker B region. The ability of Syk to participate in integrin-mediated protein tyrosine phosphorylation correlates well with its ability to inhibit cell motility.

In-depth Analyses of Kinase-dependent Tyrosine Phosphoproteomes Based on Metal Ion-functionalized Soluble Nanopolymers

“…The identification of substrates important in the regulation of apoptosis also is consistent with the reported role for Syk in the enhancement of cell survival (37). In addition, 27% of the phosphotyrosine-containing proteins were annotated to the nucleus, consistent with previous studies suggesting important roles for the kinase in the nucleus (23,38,39). The most abundant category for the molecular function of substrates was protein binding (44%), which is consistent with the known role of Syk and tyrosine phosphorylation in regulating protein-protein interactions.…”

“…GO annotation of the phosphoproteome identified prominent subclasses of substrates involved in important processes that regulate cell growth, motility, and survival. The large number of tyrosine-phosphorylated cytoskeletal (17%) and plasma membrane (10%) proteins is consistent with the described roles for Syk in modulating cancer cell motility, invasion, and cytoskeletal rearrangements (23). The identification of substrates important in the regulation of apoptosis also is consistent with the reported role for Syk in the enhancement of cell survival (37).…”

“…These data indicated that although lack of identification of a phosphopeptide is not evidence for its absence it is sufficient to reflect a change of phosphorylation within a signaling pathway. For example, cortactin, a cortical actin-associated protein known to play a critical role in stabilizing the adherens junction, was recently discovered as a direct substrate of Syk in breast cancer cells, although the phosphorylation sites were not identified (23). Our analysis identified three phosphorylation sites on cortactin that were modified exclusively in the Syk-expressing MDA-MB-231 cells (supplemental Table S8).…”

“…MDA-MB-231 wild type (Syk-negative) and TRS (tetracycline (Tet) response system) Syk-inducible cell cultures were prepared, grown, and stimulated as described before (23). Briefly, both cell lines were grown to 50% confluence.…”

“…A line of highly invasive MDA-MB-231 cells, which normally lack Syk, was generated in which the expression of an enhanced green fluorescent protein (EGFP)-tagged form of the kinase could be induced by treatment with Tet (23). Because sites of tyrosine phosphorylation are much less abundant than sites of serine or threonine phosphorylation, we coupled an immunoaffinity step to the PolyMAC method to enrich for phosphotyrosine-containing peptides.…”