Transducer of Cdc42-dependent actin assembly promotes breast cancer invasion and metastasis

Metastatic breast adenocarcinomas display activation signatures for signaling pathways that trigger cell motility and tissue invasion. Here, we report that the adaptor protein transducer of Cdc42-dependent actin assembly-1 (Toca-1) is expressed in highly invasive breast cancers and regulates their metastatic phenotypes. We show that Toca-1 localizes to the filamentous actin-rich core of invadopodial protrusions actively degrading the extracellular matrix (ECM). Toca-1 colocalizes with Cortactin, and we show that this interaction is mediated by the SH3 domain of Toca-1. Stable knockdown (KD) of Toca-1 expression in MDA-MB-231 cells led to a significant defect in epidermal growth factor (EGF)-induced cell migration and invasion. Toca-1 KD cells also showed significant defects in EGF- and Src-induced ECM digestion and formation of invadopodial membrane protrusions. To test the role of Toca-1 in metastasis, we achieved stable Toca-1 KD in both human and rat metastatic breast adenocarcinoma cell lines. Orthotopic tumor xenografting of control and Toca-1 KD cells in natural-killer /B-/T-cell-deficient mice revealed a significant defect in spontaneous lung metastases with Toca-1 silencing in vivo. In contrast, no defects in primary tumor growth or lung seeding following tail vein injection of Toca-1 KD cells was observed, suggesting that Toca-1 functions at an early step in the dissemination of metastatic breast tumor cells. Taken together, our results identify Toca-1 as a proinvasive protein in breast adenocarcinoma and a potential therapeutic target to limit tumor metastasis.