Activating PIK3CA Mutations Induce an Epidermal Growth Factor Receptor (EGFR)/Extracellular Signal-regulated Kinase (ERK) Paracrine Signaling Axis in Basal-like Breast Cancer

PIK3CA1, the gene encoding the p110α catalytic subunit of phosphatidylinositide-3 kinase (PI3K), is one of the two most frequently mutated genes in breast cancer. Approximately 80% of these mutations occur in two hot spots in the helical domain (E545K, E542K) and in the catalytic domain (H1047R). PIK3CA activating mutations occur in ∼40% of luminal and HER2-enriched breast cancer subtypes and ∼10% of basal-like breast cancer (BLBC) (1). In this last tumor subtype, mutations in PIK3CA are the most frequent activating kinase mutation. Thus, understanding of how PIK3CA mutations operate in BLBC is important for identifying therapeutic targets in this subtype of the disease, which lacks approved targeted therapies. To elucidate mechanisms by which mutant PIK3CA transforms MECs, we used immortalized, nontumorigenic MCF10A cells, which exhibit basal-like gene expression. Although MCF10A cells require growth factors for proliferation (2), heterozygous knock-in of E545K or H1047R PIK3CA mutation allows growth factor-independent proliferation (3). These knock-in PIK3CA mutant MECs provide a robust model in which to study the impact of these mutations without the effects of random insertion and overexpression associated with ectopic gene transduction. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of these cells identified 72 proteins concordantly altered by both PIK3CA mutations. A significant fraction of these were secreted proteins, cell surface receptors or ECM interacting molecules, suggesting PIK3CA mutations induce changes involving communication with the tumor microenvironment. This analysis identified a PI3K-induced amphiregulin (AREG)-EGFR-ERK signaling pathway that was required for growth of PIK3CA-mutant cells as well as adjacent PIK3CA-WT cells. In addition, these protein changes correlated with poor clinical outcome in BLBC. EGFR antagonists inhibited growth of PIK3CA mutant BLBC tumors, suggesting a potential therapeutic strategy for patients with this molecular subtype of breast cancer.