The Silent Catalyst: Chronic Stress-Induced Serotonin Signaling as a Novel Driver of Brain Metastasis

Background: Brain metastases represent a significant clinical challenge, particularly in triple-negative breast cancer (TNBC), a highly aggressive breast cancer subtype that lacks expression of estrogen receptors, progesterone receptors, and HER2. TNBC patients with brain metastases face severely limited therapeutic options and poor prognoses, with survival times often reduced to weeks without effective intervention. Chronic stress, prevalent in cancer patients, is increasingly acknowledged for its role in tumor progression by affecting neuroimmune interactions, neurotransmitter levels, and pathways. Among these, serotonin signaling has emerged as a critical but understudied player in the metastatic process, especially given the widespread use of selective serotonin reuptake inhibitors (SSRIs) in cancer patients to manage depression and anxiety.Objective: This study aimed to investigate the role of chronic stress-induced serotonin signaling in promoting brain metastasis formation in TNBC. We specifically sought to determine how serotonin pathway dysregulation under chronic stress conditions, amplified by SSRIs, may create a tumor-supportive brain microenvironment, impair immune surveillance, and facilitate metastatic colonization.Methods: A multi-faceted approach was utilized, combining computational, preclinical, and spatial omics methodologies. TNBC mouse models were subjected to chronic restraint stress, and brain metastasis progression was evaluated through histological analysis and behavioral assays assessing anxiety and depression-like symptoms. Bioinformatics analysis was conducted on publicly available breast cancer gene expression datasets to assess the prognostic significance of serotonin-related genes. Immune infiltration patterns were analyzed using immune cell enrichment algorithms and single-sample Gene Set Enrichment Analysis (ssGSEA). Differential gene expression profiling and Kaplan-Meier survival analyses were employed to link serotonin signaling components with brain metastasis outcomes. Additionally, spatial transcriptomics provided insights into ligand-receptor crosstalk in the brain metastatic niche under chronic stress conditions.Results: Chronic stress markedly accelerated brain metastasis formation, with 7 of 10 mice in the stress group developing brain metastases by day 15 (P = 0.022) and demonstrating significantly increased tumor growth rates (P < 0.01). Behavioral assays confirmed that stressed mice exhibited elevated anxiety and depressive-like phenotypes. Human breast cancer datasets analysis revealed that high expression levels of serotonin receptor HTR2B and serotonin transporter SLC6A4 strongly correlated with shorter brain metastasis-free survival. Immune analysis indicated that elevated serotonin signaling was inversely associated with microglial infiltration, suggesting that stress-induced serotonin dysregulation impairs the brain's natural immune defenses against metastatic colonization. Spatial transcriptomic profiling identified significant alterations in ligand-receptor interactions within the metastatic niche. This further supports the hypothesis that serotonin pathway activation under stress reprograms the brain microenvironment to favor tumor growth.Interpretation: Our findings reveal that chronic stress promotes brain metastasis and activates serotonin signaling pathways that undermine microglial-mediated immune surveillance. Moreover, the administration of SSRIs, while beneficial for mood regulation, may exacerbate metastatic progression by further elevating extracellular serotonin levels. These insights suggest that serotonin signaling acts as a 'silent catalyst' in the metastatic cascade, reshaping the tumor-immune landscape within the brain. This research has the potential to significantly impact our understanding of brain metastasis and the development of novel therapeutic strategies.Conclusion: Chronic stress and SSRI-mediated modulation of serotonin signaling significantly contribute to brain metastasis development in TNBC. Targeting serotonin pathways may represent a novel therapeutic strategy to prevent or mitigate brain metastases. Careful consideration of antidepressant selection in oncology patients may be critical to balancing mental health needs with metastatic risk.Future Directions: Future studies will test serotonin receptor antagonists and serotonin-targeted therapies in TNBC brain metastasis models. Combination therapies integrating serotonin pathway inhibitors with immunotherapies or chemotherapies will be evaluated for synergistic efficacy. Furthermore, safer antidepressant alternatives that do not enhance metastatic potential will be explored. Expanding investigations into serotonin's role across other cancers prone to brain metastasis, such as lung cancer and melanoma, may provide broader translational applications. Personalized treatment approaches targeting serotonin signaling are essential to advancing precision oncology for patients at risk of brain metastases.