Dual Role of Cancer Epithelial-Specific TRAF3 in Regulating Breast Cancer Cell Survival and Lymphocyte Activity.

TRAF3 (TNF Receptor Associated Factor 3) is a regulator of NF-κB signaling, acting mainly as an inhibitor of the alternative NF-κB pathway. While TRAF3 has a well-established role in immune function, mainly via B- and T-lymphocyte regulation, its roles in cancer remain unclear. Breast cancer is the most common malignancy in women and a neoplasm displaying high levels of intratumoral heterogeneity. Identifying and understanding key molecules at the interface of breast cancer cells and the immune system is crucial for advancing therapeutic strategies for breast cancer patients. Here, by employing publicly available breast cancer datasets, breast cancer cell lines stably expressing TRAF3, mass spectrometry analysis in combination with functional assays, co-culture systems, and signal pathway characterization, we sought to assess the specific role of TRAF3 in breast cancer cells and how TRAF3-expressing breast cancer cells affect their immune microenvironment. Our results indicate that TRAF3 protein overexpression inhibits colony formation through apoptosis regulation. Proteome analysis for TRAF3 interactors and over-representation analysis identified multiple protein complexes related to cell cycle, apoptosis, and immune responses. Furthermore, TRAF3-expressing breast cancer cells displayed reduced levels of PD-L1 and when co-cultured with PBMCs induced a pro-inflammatory profile with increased CD16-NK cells and higher levels of IFN-γ and TNF-α and lower IL-10 and Tregs in the culture. These findings further expand the role of TRAF3 in breast cancer, not only as a regulator of EMT and survival of cancer cells, but also as a modulator of the tumor-immune microenvironment.