Abstract
BACKGROUND: Follicular thyroid carcinoma (FTC) is the second most common cancer of the thyroid and easily develops into distant metastasis. PD-L1 is known to be associated with the carcinogenesis and progression of thyroid carcinoma. Our study aimed to investigate the biological functions of PD-L1 and to identify miRNAs that were responsible for modulating the activity of PD-L1. METHODS: A total of 72 patients with FTC at The Second Affiliated Hospital of Fujian Medical University were enrolled in this retrospective study. Immunohistochemical (IHC) assay was used to measure PD-L1 expression in FTC. The association between PD-L1 expression and clinicopathologic characteristics was evaluated. Bioinformatics analysis, RT-qPCR and western blotting were used to examine the relationships between miR-199a-5p, PD-L1 and Claudin-1. Cell proliferation, migration and invasion were evaluated by using CCK8 and Transwell migration and invasion assays. Target prediction and luciferase reporter assays were performed to verify the binding between miR-199a-5p and PD-L1. Rescue assay was performed to confirm whether PD-L1 downregulation abolished the inhibitory effect of miR-199a-5p. RESULTS: Among 72 pairs of tumor and normal specimens, the proportion of PD-L1 positive samples was higher in FTC tissues than in normal tissues. The results of ESTIMATE and CIBERSORT illustrated that there was a positive correlation between PD-L1 expression and immune infiltration, especially regulatory T cells and M1 macrophages. Prediction of immunotherapy revealed that patients with high PD-L1 expression might benefit from immune checkpoint inhibitors. Transwell migration and invasion assays showed that PD-L1 downregulation in FTC cells could significantly inhibit cell migration and invasion. The bioinformatics analysis and luciferase activity results indicated that PD-L1 was a potential target of miR-199a-5p. Knockdown of PD-L1 reversed the miR-199a-5p inhibitor mediated promotion effect. In addition, we found that PD-L1 expression was positively correlated with Claudin-1 expression and that miR-199a-5p affected the progression of FTC cells through the negative regulation of PD-L1 and Claudin-1. CONCLUSIONS: Our study revealed that PD-L1 expression was elevated in FTC and was closely associated with tumor aggressiveness and progression. MiR-199a-5p has a functional role in the progression and metastasis of FTC by regulating PD-L1 and Claudin-1 expression.