BMP-9 promotes the expression of PD-L1 in osteosarcoma cells through FOXO1

Osteosarcoma is the most common primary malignant bone tumor, and its occurrence and development are closely related to the bone microenvironment. Therefore, it is essential to understand the main factors that promote osteosarcoma progression in the bone microenvironment. Our study focused on the effect of BMP-9 on PD-L1 expression in osteosarcoma cells. The expression of PD-L1 was detected western blot and flow cytometry. Using RNA-seq analysis, we explored the mechanism that regulated PD-L1 mRNA expression. Our findings revealed that BMP-9 promoted PD-L1 protein expression in osteosarcoma cells. Further, BMP-9 induced the transcriptional expression of PD-L1 in osteosarcoma cells. RNA sequencing and TCGA database analysis showed that FOXO1 regulates the transcriptional expression of PD-L1. FOXO1 expression was significantly increased under BMP-9 action. Importantly, BMP-9 cannot increase the transcriptional and protein expression of PD-L1 in osteosarcoma cells when FOXO1 is knocked down. Additionally, the expression of BMP-9 was positively correlated with the expression of PD-L1 in clinical samples of osteosarcoma.