RASAL2 activates RAC1 to promote triple-negative breast cancer progression.

Patients with triple-negative breast cancer (TNBC) have a high incidence of early relapse and metastasis; however, the molecular basis for recurrence in these individuals remains poorly understood. Here, we demonstrate that RASAL2, which encodes a RAS-GTPase-activating protein (RAS-GAP), is a functional target of anti-invasive microRNA-203 and is overexpressed in a subset of triple-negative or estrogen receptor-negative (ER-negative) breast tumors. As opposed to luminal B ER-positive breast cancers, in which RASAL2 has been shown to act as a RAS-GAP tumor suppressor, we found that RASAL2 is oncogenic in TNBC and drives mesenchymal invasion and metastasis. Moreover, high RASAL2 expression was predictive of poor disease outcomes in patients with TNBC. RASAL2 acted independently of its RAS-GAP catalytic activity in TNBC; however, RASAL2 promoted small GTPase RAC1 signaling, which promotes mesenchymal invasion, through binding and antagonizing the RAC1-GAP protein ARHGAP24. Together, these results indicate that activation of a RASAL2/ARHGAP24/RAC1 module contributes to TNBC tumorigenesis and identify a context-dependent role of RASAL2 in breast cancer.