Recently, a new generation of PI3K-specific inhibitors, such as GDC0941 and BKM120, are being investigated in clinical trials for treatment against tumors harboring PIK3CA mutations. Nevertheless, not all patients benefit from such treatment, suggesting that their tumors may be resistant to PI3K inhibitors. The investigation of the underlying mechanisms and efficacious personalized treatment remain a large unmet need. In this study, we revealed an IL6-STAT3 positive feedback loop that mediated the resistance to PI3K inhibitors. We found that breast cancer cells with acquired resistance to PI3K inhibitors displayed epithelial-mesenchymal transition (EMT) features and an highly enriched cancer stem cells (CSCs), secreting similar to 1000-fold more IL6 than parental cells. Further studies elucidated that activation of the IL6-STAT3 signaling effectively triggered EMT action, expanded the CSCs population, and reduced sensitivity to PI3K inhibitors. Pharmacological inhibition of STAT3 disrupted the IL6-STAT3 signaling and overcome resistance to PI3K inhibitors partially due to increased apoptosis induction. Taken together, our results demonstrated that feedback activation of the IL6-STAT3 loop lead to acquired resistance to PI3K inhibitors by promoting EMT and CSC-like features, and suggested that targeting this loop may be an efficient strategy to overcome resistance to PI3K inhibitors. (C) 2014 Elsevier Inc. All rights reserved.