The tumour microenvironment plays a crucial role in the development of breast cancer. Tenascin-C (TNC), a matricellular protein and its high molecular weight (MW) isoforms have been shown to be over-expressed in the stroma of breast cancers and are associated with poor prognosis. The aim of this study was to investigate the effects of TNC knockdown in TNC expressing invasive breast cancer cell lines on cancer cell behaviour. Small interfering RNA (siRNA) targeting different exons in TNC (24, 14

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... and 14-AD1) were designed, synthesised and transfected into the highly invasive MDA-MB-231 breast cancer cell line. The phenotypic alterations caused by TNC knockdown were analysed by Two Dimension (2D) invasion assays and proliferation assays using the mitotic marker Phispho-Histone H3 (pHH3). The siRNA targeted cells showed significant down-regulation of both total TNC (p <0.001) and high MW isoforms (p <0.001) in MDA-MB-231 cells. Moreover, knockdown of total TNC and high MW TNC isoforms significantly decreased both cell invasion (total TNC p<0.001 and TNC-14 p <0.001) and proliferation (total TNC p <0.001 and TNC-AD1 p <0.05). In conclusion, TNC knockdown significantly decreases proliferation and invasion in breast cancer cell lines, confirming its importance in breast cancer progression.