Evaluation of the role of xCT, the specific subunit of the cystine/glutamate antiporter system xc-, in the immune system and in breast cancer progression

Breast cancer remains one of the leading causes of cancer-related mortality worldwide, largely due to metastatic progression and therapy resistance. Tumor cells adapt to hostile conditions through several mechanisms, including the maintenance of redox homeostasis. The cystine/glutamate antiporter xCT, a key regulator of intracellular glutathione synthesis, is frequently upregulated in breast cancer and associated with tumor aggressiveness, metastatic dissemination, and therapy resistance. However, its precise role in metastasis, metabolic adaptation, and tumor–host interactions remains incompletely defined. The first part of this thesis examines the role of xCT in tumor development and immune competence. Using xCT-null mice and mammary cancer–prone BALB-neuT models, xCT is shown to be dispensable for immune system development, effective immune responses, and primary tumor initiation. In contrast, tumor cell–specific xCT depletion significantly reduces migratory capacity in vitro and lung metastatic dissemination in vivo, without affecting primary tumor growth. This effect is associated with altered immune cell recruitment in pre-metastatic and metastatic niches, indicating a specific role for xCT in metastatic seeding rather than tumor initiation. The second part focuses on the mechanisms underlying the reduced metastatic potential of xCT-deficient cells. In murine and human triple-negative breast cancer models, xCT regulates metastasis through both cell-autonomous and non–cell-autonomous mechanisms. xCT deficiency induces redox imbalance, oxidative stress, and metabolic reprogramming, ultimately impairing invasive behavior. These findings are supported by a human organ-on-a-chip platform that recapitulates key steps of the metastatic cascade, where xCT downregulation markedly limits metastatic dissemination. In parallel, tumor-derived extracellular vesicles mediate xCT-dependent non–cell-autonomous effects, as vesicles from xCT-expressing cells promote invasion, lung colonization, and immune cell recruitment, whereas those from xCT-deficient cells do not. The final part addresses the role of xCT in resistance to doxorubicin. While xCT inhibition alone does not increase chemotherapy-induced cytotoxicity, combined targeting of xCT and the immune signaling receptor TLR2 synergizes with doxorubicin, reducing cancer stem cell viability and restoring apoptotic responses. In vivo, xCT-targeted therapeutic vaccination enhances chemotherapy efficacy and anti-tumor immune responses. Overall, this thesis identifies xCT as a central regulator of metastatic progression, metabolic adaptation, extracellular communication, and therapy resistance in breast cancer, supporting its potential as a safe and promising therapeutic target and highlighting the value of combinatorial strategies integrating metabolic targeting, immunotherapy, and chemotherapy