Solid tumors often survive in hypoxic microenvironment, a stress condition which affects the functionality of the endoplasmic reticulum (ER), determining the "ER stress" condition. Unfolded protein response (UPR) is the adaptive response which restores protein homeostasis after ER stress, contributing to tumor survival. Recent studies highlighted the role of UPR in triple negative breast cancer (TNBC), the most aggressive breast cancer subtype, where UPR targeting, through IRE1alpha-XBP1 pathway inhibition, has been suggested as a promising therapeutic approach. XBP1 has a pivotal role in progression and invasiveness of TNBC representing a specific prognostic value for TNBC patients. For this reason, XBP1 downregulation increases the sensitivity of transformed cells to be killed by hypoxia. Aberrant Notch signaling is a common feature of pathogenesis and progression of TNBCs, leading to the employment of Notch inhibitors in TNBC therapy. Recently, in T-cell acute lymphoblastic leukemia context, we demonstrated that Notch3 sustains UPR pathway through a positive regulation of IRE1alpha-XBP1 axis. Our proposal is to demonstrate an involvement of Notch3 signaling in the regulation of the IRE1alpha-XBP1-mediated UPR in response to hypoxia in TNBCs, thus providing a rationale for using Notch3 inhibition as a therapeutic strategy to affect UPR, finally inducing TNBC cell death.
Triple-negative breast cancer (TNBC), which accounts for 15%-20% of all diagnosed BC, represents the subtype with the worst clinical outcome. The current treatments are tumor excision and conventional chemotherapy, resulting in poor prognosis. Thus, the identification of novel therapies is required. Due to their low oxygen areas (12), the intra-tumoral hypoxia is a common finding in TNBCs, representing a potent driver of tumor aggressiveness. In hypoxic microenvironment, tumor cells respond with cellular adaptations, as the activation of HIF-1alpha-dependent or independent mechanisms, aimed to invasive properties acquisition by TNBC cells. In keeping with this, hypoxia environment triggers a stress condition, which affects Endoplasmic Reticulum (ER) functionality, and a consequent adaptive pathway known as "Unfolded Protein Response" (UPR). During UPR activation the ER endoribonuclease IRE1alpha activates the X-box binding protein 1 (XBP1) (16), involved in the progression of TNBCs in hypoxic condition. For that, UPR targeting in hypoxic stress condition, through IRE1alpha-XBP1 pathway inhibition, may represent the alternative therapeutic strategy for TNBC treatment, finally improving TNBC patients outcome.
Considering that hypoxia increases Notch pathway activity in TNBC subtype, and in view of our finding supporting a Notch3 contribution in sustaining the IRE1alpha-XBP1 pathway during UPR response in T-ALLs (Giuli MV*-Giuliani E* et al, submitted to Oncogenesis, under review), our proposal aims to demonstrate, the Notch3 involvement in regulating the IRE1alpha-XBP1-mediated UPR during hypoxia in TNBC context. This could provide a rationale for Notch3 inhibition as a therapeutic strategy in TNBCs. Until now, much efforts have been focused on the use of small molecules, acting as Gamma-secretase inhibitors (GSIs), to target Notch pathway (31). Notably, GSIs act by preventing the cleavage of the active NICD of all receptors isoforms, thus failing to target an individual Notch receptor. In addition, GSI treatment results in severe side effects (32), thus making its use not feasible (33). Because of this, the Notch3 inhibition during hypoxia would represent a more specific approach for TNBC treatment.
31 Takebe N et al., 2014; 32 Strosberg, et al.,2012; 33 Ferrando, 2009.