Triple negative breast cancer (TNBC) is a heterogeneous group of clinically aggressive breast cancers. TNBC patients have high risk of recurrence and metastasis, and current treatment options remain limited. There is strong evidence for the involvement of Notch signaling in TNBC and in breast cancer stem-like cells (CSCs). Notch1 is highly expressed in Basal-like 1 (BL1) and especially Mesenchymal-Stem-Like (MSL) TNBCs. Expression of Notch1 and its ligand Jagged1 correlate with poor prognosis. Treatment of TNBC with dual mTORC1/2 inhibitors leads to resistance through activation of Notch1. Expression of Notch1 protein correlates with pAKT and nuclear NF-κB in TNBC. Here, we demonstrate that Notch1 promotes cell survival in MDA-MB-231 cells, representative of MSL TNBC, in part by activating NF-κB. Notch activation by Jagged1-expressing stromal cells enhances transcription of the anti-apoptotic gene cIAP-2 (BIRC3), a known NF-κB target. This event is dependent on recruitment to the cIAP-2 promoter of NF-κB subunits, IKKα and Notch1. Short term exposure of MDA-MB-231 cells (MSL, PTEN wild-type), but not MDA-MB-468 cells (BL1, PTEN-null) to recombinant Jagged1 leads to rapid AKT phosphorylation. This is suppressed by dual mTORC1/2 inhibitors, AKT inhibitors and IKKα inhibitors but not Everolimus (mTORC1-selective inhibitor). Rapid AKT phosphorylation downstream of Notch1 requires mTORC2, PI3K and IKKα, and contributes to NF-κB activation. These observations support a model where canonical and non-canonical mechanisms downstream of Notch1 trigger rapid AKT phosphorylation and NF-κB activation in PTEN wild-type TNBC cells. Both arms of this pathway require IKKα. CSCs derived from MDA-MB-231 cells have increased Notch1, pAKT and pIKKα expression. Combined pharmacological inhibition of Notch and AKT or Notch and IKKα completely blocks secondary mammosphere formation. These data and published literature suggest that: 1) IKKα connects the Notch and mTORC2/AKT pathways in some TNBC subtypes; 2) IKKα is also required for nuclear Notch1-mediated NF-kB activation and may be a critical node in the Notch signaling network; 3) A feedback mechanism may exist in some TNBC cells between mTORC2/AKT and Notch1; 4) The non-canonical Notch-IKKα-AKT pathway has a potential therapeutic role in targeting CSCs of selected TNBC subtypes.

Citation Format: Hossain F, Peng Y, Pannuti A, Backus K, Golde T, Osborne B, Miele L. A novel non-canonical Notch1-IKKα-mTORC2-AKT pathway maintains survival in triple negative breast cancer cells and cancer stem-like cells [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P5-07-06.