Syndecan-1 modulates β-integrin-dependent and interleukin-6-dependent functions in breast cancer cell adhesion, migration, and resistance to irradiation.

Syndecan-1 is a cell surface heparan sulfate proteoglycan with various biological functions relevant to tumor progression and inflammation, including cell-cell adhesion, cell-matrix interaction, and cytokine signaling driving cell proliferation and motility. Syndecan-1 is a prognostic factor in breast cancer, and has a predicitive value for neodadjuvant chemotherapy. It is still poorly understood how syndecan-1 integrates matrix-dependent and cytokine-dependent signaling processes in the tumor microenvironment. Here, we evaluated the potential role of syndecan-1 in modulating matrix-dependent breast cancer cell migration in the presence of interleukin-6, and its potential involvement in resistance to irradiation in vitro. MDA-MB-231 breast cancer cells were transiently transfected with syndecan-1 small interfering RNA or control reagents, and this was followed by stimulation with interleukin-6 or irradiation. Cellular responses were monitored by adhesion, migration and colony formation assays, as well as analysis of cell signaling. Syndecan-1 depletion increased cell adhesion to fibronectin. Increased migration on fibronectin was significantly suppressed by interleukin-6, and GRGDSP peptides inhibited both adhesion and migration. Interleukin-6-induced activation of focal adhesion kinase and reduction of constitutive nuclear factor kappaB signaling were decreased in syndecan-1-deficient cells. Focal adhesion kinase hyperactivation in syndecan-1-depleted cells was associated with dramatically reduced radiation sensitivity. We conclude that loss of syndecan-1 leads to enhanced activation of β1 -integrins and focal adhesion kinase, thus increasing breast cancer cell adhesion, migration, and resistance to irradiation. Syndecan-1 deficiency also attenuates the modulatory effect of the inflammatory microenvironment constituent interleukin-6 on cancer cell migration.