Abstract: Breast cancer is the most common cancer in women and the second leading cause of cancer deaths in women. Over 90% of breast cancer deaths are attributable to metastasis. An essential step in the process of metastasis is epithelial-to-mesenchymal transition (EMT), a process whereby epithelial-like cancer cells change their phenotype to have mesenchymal-like characteristics. We previously discovered a novel pathway of EMT induction in HER2-positive breast cancer whereby AKT mediates the activation of the transcription factor heat shock factor 1 (HSF1). After activation, HSF1 binds and upregulates the expression of Slug, a transcriptional repressor known to induce EMT. The discovery of this pathway added to the growing evidence for a significant role of HSF1 in tumor biology. Considering the widespread activity of the PI3K-AKT pathway in breast cancer, the activation of HSF1 in other breast cancer subtypes was investigated. AKT and HSF1 were frequently co-activated in breast cancer cell lines and patient specimens across all breast cancer subtypes. HSF1 activity was further observed to be associated with a poor metastasis-free survival in a cohort of breast cancer patients, consistent with a role in EMT. In accordance with these results, AKT and HSF1 were observed to be frequently activated in metastatic cell lines and cancer stem cells while inhibition of this pathway reduced the growth of metastatic cells and the cancer stem cell population. These results suggested that targeting this pathway may be efficacious in breast cancer. Therefore, an AKT inhibitor (MK-2206) and an HSF1 inhibitor (KRIBB11) were used in combination and found to have synergy in killing breast cancer cell lines from multiple subtypes. Tumor growth in vivo was also observed to be suppressed by combined inhibition of AKT and HSF1. These studies indicate a broad importance for AKT and HSF1 signaling in breast cancer and provides rationale for further development of this combination therapy for breast cancer patients.