Expression of the oncogenic transcription factor MYC is disproportionately elevated in triple-negative breast cancer (TNBC) compared to estrogen, progesterone and human epidermal growth factor 2 receptor-positive (RP) breast tumors1,2. We and others have shown that MYC alters metabolism during tumorigenesis3,4. However, the role of MYC in TNBC metabolism remains largely unexplored. We hypothesized that MYC-dependent metabolic dysregulation is essential for MYC-overexpressing (MO) TNBC and may thus identify novel therapeutic targets for this clinically challenging subset of breast cancer. Using a targeted metabolomics approach, we identified fatty acid oxidation (FAO) intermediates as being dramatically upregulated in a MYC-driven model of TNBC. A lipid metabolism gene signature was identified in patients with TNBC from The Cancer Genome Atlas (TCGA) database and multiple other clinical datasets, implicating FAO as a dysregulated pathway critical for TNBC metabolism. We find that MO-TNBC displays increased bioenergetic reliance upon fatty acid oxidation (FAO), and that pharmacologic inhibition of FAO catastrophically decreases energy metabolism of MO-TNBC, blocks growth of a MYC-driven transgenic TNBC model and that of MO-TNBC patient-derived xenografts. Our results demonstrate that inhibition of FAO is a novel therapeutic strategy against MO-TNBC.

Summary Tumor cells have an altered metabolic phenotype characterized by increased glycolysis and diminished oxidative phosphorylation. Despite the suspected importance of glycolysis in tumorigenesis, the temporal relationship between oncogene signaling, in vivo tumor formation and glycolytic pathway activity is poorly understood. Moreover, how glycolytic pathways are altered as tumors regress remains unknown. Here we use a switchable model of MYC-driven liver cancer, along with hyperpolarized 13C-pyruvate magnetic resonance spectroscopic imaging (MRSI) to visualize glycolysis in de novo tumor formation and regression. LDHA abundance and activity in tumors is tightly correlated to in vivo pyruvate conversion to lactate and is rapidly inhibited as tumors begin to regress, as are numerous glycolysis pathway genes. Conversion of pyruvate to alanine predominates in pre-cancerous tissues prior to observable morphologic or histological changes. These results demonstrate that metabolic changes precede tumor formation and regression and are directly linked to the activity of a single oncogene.

Triple-negative breast cancer (TNBC), which lacks the expression of the estrogen, progesterone, and HER2 receptors, represents the breast cancer subtype with the poorest outcome1. No targeted therapy is available against this subtype due to lack of validated molecular targets. We previously reported that MYC signaling is disproportionally elevated in triple-negative (TN) tumors compared to receptor-positive (RP) tumors2. MYC is an essential, pleiotropic transcription factor that regulates the expression of hundreds of genes3. Direct inhibition of oncogenic MYC transcriptional activity has remained challenging4,5. The present study conducted an shRNA screen against all kinases to uncover novel MYC-dependent synthetic lethal combinations, and identified PIM1, a non-essential kinase. Here we demonstrate that PIM1 expression was elevated in TN tumors and was associated with poor prognosis in patients with hormone and HER2 receptor-negative tumors. Small molecule PIM kinase inhibitors halted the growth of human TN tumors with elevated MYC expression in patient-derived tumor xenograft (PDX) and MYC-driven transgenic breast cancer models by inhibiting oncogenic transcriptional activity of MYC while simultaneously restoring the function of the endogenous cell cycle inhibitor, p27. Our findings warrant clinical evaluation of PIM kinase inhibitors in patients with TN tumors that exhibit elevated MYC expression.