Abstract
BACKGROUND: Nucleus accumbens-1 (NAC-1) is highly expressed in a variety of tumors, including colon cancer, and is closely associated with tumor recurrence, metastasis, and invasion. AIM: To determine whether and how NAC-1 affects antitumor immunity in colon cancer. METHODS: NAC-1-siRNA was transfected into RKO colon cancer cells to knock down NAC expression; tumor cells with or without knockdown of NAC-1 were treated with CD8(+) T cells to test their cytocidal effect. The level of the immune checkpoint programmed death receptor-1 ligand (PD-L1) in colon cancer cells with or without knockdown of NAC-1 was analyzed using Quantitative real-time polymerase chain reaction and Western blotting. A double luciferase reporter assay was used to examine the effects of NAC-1 on the transcription of PD-L1. Mice bearing MC-38-OVA colon cancer cells expressing NAC-shRNA or control-shRNA were treated with OT-I mouse CD8(+) T cells to determine the tumor response to immunotherapy. Immune cells in the tumor tissues were analyzed using flow cytometry. NAC-1, PD-L1 and CD8(+) T cells in colon cancer specimens from patients were examined using immunohistochemistry staining. RESULTS: Knockdown of NAC-1 expression in colon cancer cells significantly enhanced the cytocidal effect of CD8(+) T cells in cell culture experiments. The sensitizing effect of NAC-1 knockdown on the antitumor action of cytotoxic CD8(+) T cells was recapitulated in a colon cancer xenograft animal model. Furthermore, knockdown of NAC-1 in colon cancer cells decreased the expression of PD-L1 at both the mRNA and protein levels, and this effect could be rescued by transfection of an RNAi-resistant NAC-1 expression plasmid. In a reporter gene assay, transient expression of NAC-1 in colon cancer cells increased the promoter activity of PD-L1, indicating that NAC-1 regulates PD-L1 expression at the transcriptional level. In addition, depletion of tumoral NAC-1 increased the number of CD8(+) T cells but decreased the number of suppressive myeloid-derived suppressor cells and regulatory T cells. CONCLUSION: Tumor expression of NAC-1 is a negative determinant of immunotherapy.