CAF-derived BHB modulates FXR1-Kbhb and NK-cell lipid metabolism in osteosarcoma

Osteosarcoma (OS) is the most common primary malignant bone tumor in children and adolescents and is characterized by high aggressiveness and poor prognosis. Although surgery and chemotherapy have improved overall survival, outcomes for relapsed or metastatic disease remain extremely poor, thus underscoring the urgent need for novel therapeutic strategies. Natural killer (NK) cells are key cytotoxic effectors in tumor immunity, but their function is profoundly suppressed within the OS tumor microenvironment. Furthermore, the role of cancer-associated fibroblasts (CAFs) in mediating NK-cell dysfunction through metabolic regulation remains insufficiently understood. We established coculture systems of NK cells, OS cells, and CAFs and assessed NK-cell activation and cytotoxicity by using functional assays, flow cytometry, and immunofluorescence. Metabolic profiling was performed with extracellular acidification rate, oxygen consumption rate, reactive oxygen species detection, MitoTracker staining, and electron microscopy. Integrated proteomics and mechanistic studies were used to identify CAF-derived metabolites and their effects on NK-cell metabolism. CAFs markedly impaired NK-cell recognition and cytotoxicity toward OS cells, accompanied by metabolic reprogramming characterized by enhanced glycolysis, increased reactive oxygen species production, and reduced mitochondrial activity. Multiomics and functional analyses identified β-hydroxybutyrate (BHB) as a key CAF-derived metabolite that contributed to reduced NK-cell FAO and effector function. BHB treatment increased FXR1 Kbhb and reduced FAO, cytokine secretion, and tumor-cell killing, whereas FXR1 K56 mutation attenuated these functional changes. In vivo, CAF-CM treatment was accompanied by elevated circulating BHB, reduced NK-cell activation, and accelerated tumor growth. Targeting the CAF-BHB- FXR1 axis may represent a promising therapeutic strategy to restore NK cell-mediated antitumor immunity and improve treatment outcomes.