e13100 Background: Triple-negative breast cancer (TNBC) is the most aggressive and poorly prognostic breast cancer subtype, yet there are currently no biological therapies against this subtype. Our laboratory is finding the sources of novel biological targets in TNBC by studying the E2F transcription factors, which are essential for cellular proliferation and maintenance of genomic stability. While the deregulated Rb/E2F pathway signals the epithelial-to-mesenchymal transition (EMT), the underlining mechanism of how E2Fs drive EMT in TNBC remains unknown. We recently published that the E2F transcriptional activators (E2Fs) are overexpressed in the vast majority of TNBC and that their overexpression upregulates mitotic kinases such as TTK, which we have shown to induce EMT and invasion in TNBC cells. We also demonstrated that the E2Fs maintain genomic integrity in part through Shugoshin I (SGO1), which normally controls chromosome cohesion; however, the role of SGO1 in EMT in breast cancer is unknown. Our hypothesis is that E2F3 and SGO1 are highly expressed in TNBC and that their overexpression modulates EMT genes, thus promoting cell invasion. Methods: To test our hypothesis, we conducted siRNA transfection to knockdown E2F3 and SGO1 in MDA-MB-231 and Hs578t, which are TNBC cells. After 48 hours, we evaluated mRNA levels of EMT-related genes after E2F3 or SGO1 depletion using RT-PCR analysis. We also evaluated the effects of SGO1 depletion in protein localization by immunofluorescence. Furthermore, we evaluated the invasive behavior of MDA-MB-231 and Hs578t cells after SGO1 depletion using a Boyden Chamber Assay. Results: Our results demonstrate that E2F3 and SGO1 depletion decrease MMP3 mRNA levels. Moreover, E2F3 and SGO1 depletion restore E-cadherin expression and localization. Furthermore, E2F3 and SGO1 depletion significantly reduce cell invasion in MDA-MB-231 and Hs578t cells. Conclusions: Our results suggest that SGO1 is a promising drug target for breast cancer metastasis since EMT and invasion are essential early steps in breast cancer metastasis and E2F3 is presently undruggable.