SOX9 Regulates CEACAM1 and Immune Resistance in Melanoma Cells

Malignant melanoma is a highly aggressive form of skin cancer, with few treatment options. As melanoma cells are immunogenic, they instigate an adaptive immune response and production of anti-tumor T-cells. A central factor in this interaction is CEACAM1 (carcinoembryonic antigen cell adhesion molecule 1), a transmembrane glycoprotein previously shown in our lab to protect melanoma cells from lymphocyte-mediated killing. SOX9 [SRY (sex determining region Y)-box 9] is a transcription factor with several binding sites in the CEACAM1 promoter, as shown by bioinformatics analysis. Therefore, we aim to study the role of SOX9 in regulation of CEACAM1 expression and immune resistance in melanoma cells.

SOX9 knockdown in melanoma cell lines demonstrated an up-regulation of CEACAM1 expression (protein and RNA) in comparison to control cells. Furthermore, SOX9-knockdown melanoma cells were more resistant to Tumor Infiltrating Lymphocytes (TILs) mediated killing. SOX9 overexpression showed a decrease in CEACAM1 promoter activity in luciferase reporter assays. However, deletion of putative SOX9 binding sites did not eliminate this effect, suggesting SOX9 regulates CEACAM1 indirectly. Bioinformatic analysis suggested transcription factors Sp1 and Sp3 as possible mediators of the SOX9 effect on CEACAM1. Indeed, deletion of the Sp1-Sp3 binding site in the CEACAM1 promoter partly eliminated said regulation.

We show that SOX9 regulates immune resistance in melanoma cells. We hypothesize this occurs in a CEACAM1 dependent manner. Regulation of CEACAM1 by SOX9 is at the transcriptional level, but occurs indirectly, possibly via transcription factors Sp1 and Sp3. As CEACAM1 is a pivotal protein in melanoma biology and immune crosstalk, further understanding of its regulation can provide new insights in the field and contribute to the development of novel approaches to therapy.