Abstract P2-03-04: A novel druggable target upstream of Notch: MEK5/ERK5 signaling regulates Jagged-1 and Notch1 expression in triple negative breast cancer stem cells

Abstract

Triple negative breast cancer (TNBC) is a molecularly heterogeneous, clinically aggressive disease group that is highly prevalent among African-Americans and younger patients. Standard chemo/radio therapy often produces clinical responses, but recurrence and metastasis are unfortunately common. Metastatic disease is generally incurable. Chemo/radiotherapy has been shown to induce EMT and enrich a chemo-resistant cancer stem-like cell (CSC) population in TNBC. CSCs are thought to drive disease recurrence. Notch signaling is critical for maintenance of TNBC CSC. Expression of Notch1 and its ligand Jagged1 are correlated with poor prognosis. Efforts to pharmacologically target Notch with Gamma Secretase Inhibitors (GSIs) have been impaired by the systemic toxicity of the GSIs, and by the fact that Notch1 also plays a key role in anti-tumor adaptive immunity. Therapeutic agents that indirectly and selectively target Notch signaling in breast cancer cells would be a potentially attractive strategy. However, no such agents have been identified to date. We have found that the MAPK5-ERK5 kinase pathway, which contains at least two druggable targets, functions as a master regulator of Notch signaling in TNBC cells. ERK5 knockout TNBC cells have dramatically decreased expression of Notch receptors, ligands and transcriptional targets. In vivo, these cells form barely detectable tumors that do not metastasize and express lower levels of Notch1 and its ligand Jagged1. Using in silico screening, we identified a class of compounds that selectively target MAP2K5 (MEK5) and decrease the phosphorylation of MAPK7 (ERK5). We selected compound SC-181 for further study. Consistent with ERK5KO cells, pharmacological suppression of ERK5 phosphorylation with SC-181 decreased Notch1 and Jagged1 mRNAs and proteins. SC-181 reversed EMT and reduced the CD44hi/CD24lo CSC population in TNBC cells, but had no effect on T-cell proliferation. SC-181 decreased the number and size of mammospheres in a concentration-dependent manner. Overexpression of the Notch1 intracellular domain (N1IC) in ERK5KO cells rescues their phenotype, dramatically increasing the CSC fraction and promoting EMT. Our results suggest that targeting the MEK5-ERK5 pathway is a promising new strategy to selectively modulate Notch signaling in TNBC CSC without compromising tumor immunity. Citation Format: Ucar DA, Matossian MD, Hoang-Barnes VT, Hossain FM, Gupta M, Burks HE, Wright TD, Cavanaugh J, Flaherty P, Burow ME, Miele L. A novel druggable target upstream of Notch: MEK5/ERK5 signaling regulates Jagged-1 and Notch1 expression in triple negative breast cancer stem cells [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P2-03-04.