The S1P/S1P1 Signaling Axis Plays Regulatory Functions in the Crosstalk Between Brain-Metastasizing Melanoma Cells and Microglia.

The interaction between brain-metastasizing melanoma cells and surrounding microglia shapes the immune tumor microenvironment and influences tumor progression. Gene expression analysis revealed that sphingosine-1-phosphate receptor 1 (S1PR1), encoding the S1P1 receptor, is upregulated in microglia upon interaction with melanoma cells. Here, we investigated the functions of S1P1 in microglia and its contribution to melanoma-microglia crosstalk. We examined the effects of S1P1 inhibition on microglia and four brain-metastasizing human melanoma cell lines in monocultures and co-cultures using the selective S1P1 antagonist NIBR0213 and S1PR1 gene knockdown. We found that melanoma-secreted IL-6 upregulated S1PR1 expression in microglia. S1P1 inhibition increased expression of CD32, CD150, and CD163 in microglia; however, CD150 and CD163 upregulation was abolished in the presence of melanoma cells. S1P1 inhibition downregulated immunosuppressive and anti-inflammatory factors in microglia, including CD274, SOCS3, TGFBR1, TGFBR2, and JunB, promoting a pro-inflammatory phenotype. It also reduced viability of both melanoma and microglia cells, inducing apoptosis in melanoma-associated microglia, possibly via downregulation of CH25H, an upstream regulator of SREBPs. In co-cultures, melanoma cells were more sensitive than microglia to NIBR0213-induced growth arrest. In 3D spheroid cultures, NIBR0213 delayed melanoma-microglia aggregation. Combined treatment with the BRAF inhibitor Vemurafenib and NIBR0213 enhanced Vemurafenib efficacy in three of four melanoma lines. S1P1 contributes to the immunosuppressive phenotype of microglia. Inhibiting the S1P/S1P1 axis impairs viability and crosstalk between melanoma cells and tumor-activated microglia, offering a potential therapeutic strategy for melanoma brain metastases.