Notch-mediated processes promoting inflammation-driven mechanisms in breast cancer progression

Introduction: The triple-negative subtype of breast cancer (TNBC) accounts is a most aggressive disease subtype, emphasizing the need for improved understanding of the mechanisms leading to its progression. Stromal cells and pro-inflammatory cytokines play key roles in promoting the aggressiveness of TNBC tumors. Materials, methods and results: We took an integrative approach and determined the impact of tumor-stroma-inflammation networks on pro-metastatic phenotypes in TNBC, focusing on TNFα and IL-1β as representatives of the pro-inflammatory TME. Stimulation of TNBC:stroma co-cultures by these two cytokines has led to increased pro-metastatic activities at multiple levels, including: expression levels of the chemokines CXCL8, CCL2 and CCL5, angiogenesis, cancer cell morphology towards and EMT-phenotype, tumor cell migration and tumor cell invasion. Importantly, we found that CXCL8 was a key regulator of the pro-metastatic activities that came into play in the TNBC-stroma-inflammation networks, including angiogenesis, metastasis-related morphology, tumor cell migration and invasion of TNBC cells. Moreover, the tumor-stroma-inflammation network has led to elevated metastasis in vivo. To identify the mechanisms mediating the interactions between TNBC cells and stromal cells in the context of pro-inflammatory signals, we explored the roles of Notch receptors in regulating such cross-talks. We found that the Notch pathway was a prime regulator of tumor cell migration and invasion in the tumor-stroma-inflammation network. Also, our findings indicated that Notch1 was a key mediator of CXCL8 up-regulation, and that p65 (the NF-kB pathway) induced the expression of CXCL8 and of Notch1 activation. These findings indicate that when TNBC cells interact with stromal cells, further exposure to pro-inflammatory signals gives rise to p65 activation, leading to Notch1 activation and consequently to up-regulation of CXCL8. Then, CXCL8 promotes angiogenesis, as well as tumor cell migration and invasion. These processes eventually lead to a higher aggressiveness phenotype in TNBC in vivo. Conclusions: Our data point at complex control mechanisms that are governed by the NF-kB and Notch pathways and by CXCL8 in the setting of TNBC-stroma-inflammation triage that promotes TNBC progression. These findings propose that combined targeting of the Notch pathway and of the pro-inflammatory tumor microenvironment could be considered a relevant therapy option in TNBC.