Sphingosine-1-Phosphate Promotes FOS Activation in Osteosarcoma Under Tumor Acidosis

The tumor microenvironment in solid tumors is characterized by extracellular acidosis, which promotes cancer aggressiveness. In osteosarcoma, the most common primary bone cancer, a highly acidic tumor microenvironment is associated with metastasis and poor prognosis, partly due to metabolic rewiring, including changes in lipid pathways such as those involving sphingosine-1-phosphate, a bioactive sphingolipid. Sphingosine-1-phosphate has been previously implicated in histone deacetylase inhibition and gene activation. Here, we investigated whether acidosis induces nuclear sphingosine-1-phosphate accumulation via sphingosine kinase 2, leading to epigenetic activation of oncogenes like FOS in osteosarcoma.

Osteosarcoma spheroids were cultured under neutral or acidic conditions. Histone H3 acetylation was assessed by capillary Western blotting. FOS expression and FOS nuclear localization were analyzed. Sphingosine-1-phosphate's role was addressed through sphingosine kinase 2 silencing and inhibition (ABC294640). Functional effects were measured using colony formation assays. Patient-derived OS tissues (n = 7) were analyzed for correlations between acidity markers (LAMP2, V-ATPase), sphingosine kinase 2, and FOS expression.

Acidosis increased both sphingosine kinase 2 mRNA expression after 24 h and histone H3 acetylation, which followed progressive FOS upregulation and nuclear FOS accumulation. Sphingosine kinase 2 inhibition or silencing reduced these effects and impaired clonogenicity. In patient tissues, sphingosine kinase 2 levels correlated with acidosis markers and FOS expression.

We identified a novel mechanism where acidosis stimulates both nuclear sphingosine kinase 2 to synthesize sphingosine-1-phosphate and histone H3 acetylation, ultimately leading to FOS transcription. Targeting this axis decreased clonogenesis, underscoring its therapeutic potential in osteosarcoma and potentially other acid-adapted cancers.