ABSTRACT: This study has identified a unique susceptibility that capitalizes on a cancer cell’s reliance on key metabolic pathways, not only to meet increased energetic demands, but also to maintain the critical balance between reactive oxygen species (ROS) and cellular antioxidant levels, necessary for cancer cell survival and growth. Specifically, we explored the use of phenformin, a metabolic drug that is being repurposed as a cancer treatment in clinical trials. We find that several breast cancer cell lines, across molecular subtypes, were sensitized to phenformin when combined with the pro-inflammatory cytokine, interferon-gamma (IFNγ), revealing a previously unidentified anti-tumor effect. Similarly, we find that an elevated inflammatory response (stimulated by PolyIC) synergizes with phenformin to attenuate tumor growth, in three different mouse models of breast cancer. Through inhibiting complex I of the electron transport chain, phenformin directly inhibits tumor growth by suppressing mitochondrial-dependent energy generation, and consequently increases levels of superoxide. Functionally, the mitochondrial ROS scavenger, MitoTempo, reverses the ability of, IFNγ and PolyIC to synergize with phenformin. Through RNA sequencing, by comparing Wild-Type and STAT1 KO cells, we identify Nqo1 as a target gene that decreases upon IFNγ treatment in a STAT1 dependent manner. Nqo1 is a 2-electron reductase that is upregulated in many tumors and has important roles in ROS scavenging and cellular stress responses. In support, targeting Nqo1 either by shRNA-mediated gene silencing or through its bioactivatable inhibitor, β-lapachone, sensitized established human and mouse cell lines, cells from breast cancer patient-derived xenografts (PDX), and in vivo xenografted tumor growth to phenformin.
