Single-cell transcriptomic analysis reveals fatty acid metabolic heterogeneity and identifies DKK1 as a candidate...

Osteosarcoma (OS) is an aggressive primary bone malignancy that predominantly affects children and adolescents. Although surgery and chemotherapy have improved survival, therapeutic progress has remained limited over recent decades. Aberrant fatty acid metabolism has been increasingly implicated in OS progression, yet its distribution across different cellular compartments of the tumor microenvironment remains unclear. In this study, we integrated single-cell RNA-sequencing data from 140,562 cells across 17 OS patients and characterized the metabolic heterogeneity of tumor microenvironment. Two fatty acid metabolism-associated populations were identified: IBSP+ malignant osteosarcoma cells and SPARC+ macrophages. IBSP+ malignant cells showed preferential activation of fatty acid degradation and occupied an intermediate state along the pseudo-time trajectory. Lipid uptake and cholesterol efflux programs reached their highest activity near this state, indicating substantial lipid metabolic remodeling along the inferred transition among malignant-cell states. By contrast, SPARC⁺ macrophages exhibited a lipid-adapted, immunosuppressive phenotype accompanied by elevated fatty acid degradation activity. Cell-cell communication analysis further identified IBSP⁺ malignant cells as a major source of APOE- and SPP1-related signals directed toward SPARC+ macrophages, suggesting coordinated metabolic communication between these populations. To evaluate its clinical relevance, we applied transcriptomic deconvolution to independent OS cohorts. Concurrent enrichment of IBSP+ malignant cells and SPARC+ macrophages was associated with significantly shorter overall survival. Comparison of the double-high and double-low groups subsequently identified DKK1 as a candidate effector linked to this adverse metabolic niche. Consistent with this observation, DKK1 silencing in MG-63 cells reduced proliferation, colony formation, migration, and invasion. Collectively, these findings characterize fatty acid metabolism-associated cellular heterogeneity and a clinically relevant association between IBSP⁺ malignant cells and SPARC⁺ macrophages in OS, while identifying DKK1 as a candidate prognostic and therapeutic factor associated with the double-high phenotype.