| Notch signaling is responsible for critical developmental events from cell-fate determination to proliferation, while the aberrant activation of Notch signaling has been associated with breast cancer malignancies. Notch activation is initiated through the binding of Notch ligands (Jagged/Serrate Family) to Notch receptors (Notch1-4), followed by successive cleavages by TACE and gamma-secretase to subsequently release the Notch intracellular domain (NICD) for nuclear transcription. Due to the crucial role of gamma-secretase in mediating the release NICD, research in gamma-secretase regulation and function in breast cancer is essential for the implementation of effective gamma-secretase inhibitors (GSIs).;Tumor hypoxia contributes to therapeutic resistance and the metastatic progression of breast cancer. It has been reported that hypoxia can promote Notch signaling through stabilizing the gamma-secretase cleaved product NICD. However, little is known if hypoxia also modulates gamma-secretase activity. In our study, we showed that gamma-secretase activity of breast cancer lines were enhanced under hypoxia. Furthermore, we demonstrated that hypoxia did not elevate the level of gamma-secretase subunits including Presenilin, Nicastrin, Aph-1 and Pen2, but rather directly increased the amount of active complex formed. We found that Hif-1alpha engaged with the active enzyme complex to regulate gamma-secretase activity. More importantly, the increased invasive potential of breast cancer cells in hypoxic setting could be diminished with gamma-secretase inhibition using a sulfonamide GSI (GSI-34), indicating that gamma-secretase plays a critical role in the migration and invasion of breast cancer. Finally, we showed that both GSI treatment and knockout of Notch signaling significantly reduced metastasis to distant lung organ in mice using 4T1 mammary fatpad metastasis model. These studies together demonstrated that the administration of GSI-34 could be used to ameliorate the progression of metastatic lesion from highly aggressive hypoxic tumor types.;In addition, breast cancer is a heterogeneous disease that can be characterized by distinct molecular signatures. To date, there are no target-based therapies to combat triple-negative Basal B breast cancer subtypes. In our study, we showed hat the highly invasive Basal B breast cancer subtypes exhibited unique properties that contribute to the aggressiveness and resistance of this cell type to therapy. First, breast cancer cells expressed Notch receptors and ligands, and most importantly, were active in Notch signaling. Secondly, we demonstrated that Basal B subtype breast cancer cells differ in gamma-secretase activity for the cleavage of Notch and APP and in gamma-secretase active site conformation. Basal B cells exhibited differential response to gamma-secretase inhibition, leading to decreased DNA synthesis and clonogenic survival.;Collectively, these above studies demonstrated that gamma-secretase is a unique target for the treatment of hypoxic and aggressive breast cancer. We also provided mechanistic evidence to support gamma-secretase mediated Notch signaling in cancer metastasis by showing the potential used of GSIs for the treatment of highly aggressive breast cancer types. These studies will ultimately provide prognostic and therapeutic value for the management of breast cancer malignancies. |
