MDA-MB-468 cells treated with docetaxel or EZH2i+AKTi II

Triple negative breast cancer (TNBC) is the most aggressive breast cancer subtype and has the highest rate of recurrence1. The predominant standard of care for advanced TNBC is systemic chemotherapy with or without immunotherapy, however responses are typically short-lived1,2. Thus, there is an urgent need to develop more effective treatments. PI3K pathway components represent plausible therapeutic targets, as approximately 70% of TNBCs have PIK3CA/AKT1/PTEN alterations3–6. However, unlike hormone receptor-positive tumors, it is still unclear if or how PI3K pathway inhibitors will be effective in triple-negative disease7. Here we identify a promising AKT inhibitor-based therapeutic combination for TNBC. Specifically, we show that AKT inhibitors potently synergize with agents that suppress the histone methyltransferase, EZH2, and promote robust tumor regression in multiple TNBC models in vivo. AKT and EZH2 inhibitors exert these effects by first cooperatively driving basal-like TNBC cells into a more differentiated, luminal-like state, which cannot be effectively induced by either agent alone. Once differentiated, these agents kill TNBCs by hijacking signals that normally drive mammary gland involution. Finally, using machine learning approach, we developed a classifier that can be used for patient selection. Together these findings identify a promising therapeutic strategy for this highly aggressive tumor type and illustrate how deregulated epigenetic enzymes can insulate tumors from oncogenic vulnerabilities. These studies also reveal how developmental tissue-specific cell death pathways may be co-opted for therapeutic benefit. Overall design: HCC38, HCC1395, and HCC1937 cells were treated EZH2i (5 uM tazemetostat), AKTi (5 uM ipatasertib) or EZH2i+AKTi. On day -5, MDA-MB-468 cells were seeded at 40% confluency and treated with 5uM tazemetostat or DMSO. On day -3, cells were passaged again at 40% confluency and maintained in DMSO or tazemetostat. On day -1, cells were seeded into 6cm plates at 250,000 cells per plate and maintained in DMSO or tazemetostat. On day 0, cells were treated in media containing 2% FBS with AKTi (ipatasertib) and/or EZH2i (tazemetostat). Cells were collected at 24 hours, cells were isolated and then prepared for ATACseq.