Serotonin Metabolism Shapes the Tumor Immune Microenvironment and Serves as a Therapeutic Target in Lung Cancer

Introduction: Lung cancer progression involves complex interactions between metabolic pathways and the immune microenvironment. The role of serotonin, a tryptophan-derived metabolite, in immune responses to lung tumors remains unclear. Methods: An orthotopic lung cancer model was established by intravenously injecting KP (KrasG12D/p53-/-) cells into C57BL/6 mice. Metabolomic and flux analyses were conducted on tumor versus normal lung tissues. Serotonin was administered to tumor-bearing mice, followed by immunofluorescence and flow cytometry to assess immune responses. Human lung cancer datasets were analyzed to validate clinical relevance. Results: Tumor tissues exhibited a significant decrease in serotonin levels. Although tryptophan, serotonin, and kynurenine levels were decreased overall, flux analysis revealed a metabolic shift favoring kynurenine synthesis, with a ~10-fold increase in the kynurenine-to-serotonin ratio. Serotonin supplementation significantly prolonged survival and enhanced dendritic cell and CD8⁺ T cell infiltration and activation in tumors. Analysis of public datasets showed that serotonin expression positively correlated with CD8⁺ T cell activation signatures and patient prognosis. Discussion: By revealing serotonin as a potential biomarker and therapeutic target, this study paves new avenues for improving lung cancer treatment strategies through modulation of the immune microenvironment. Moreover, the precise receptor-mediated mechanisms underlying serotonin's immunomodulatory effects remain to be clarified, and translational validation in human tissues is warranted to strengthen clinical relevance. Conclusion: Serotonin deficiency in the tumor microenvironment of the lung suppresses antitumor immunity. Its restoration reverses immune dysfunction and limits tumor progression. These findings identify serotonin as a potential metabolic regulator and immunotherapeutic target in lung cancer.