METTL3-Induced m6A Modification Enhances Hsa_Circ_0136959 Expression to Impair the Tumor Characteristics of Papillary Thyroid Carcinoma via Accelerating Ferroptosis

The number of cases of papillary thyroid cancer (PTC) has gone up significantly in recent years, with high recurrence. Numerous reports have highlighted the participation of circular RNAs (circRNAs) in regulating the advancement of cancers, including PTC. Furthermore, recent studies suggest that N6-methyladenosine (m6A) modified circRNAs play pivotal roles in cancer progression. Hence, we studied the potential role of a novel circRNA, hsa_circ_0136959, and its regulatory mechanism on m6A modification by methyltransferase-like 3 (METTL3) in the tumor characteristics of PTC. The expressions of hsa_circ_0136959 and METTL3 were evaluated in PTC samples and cell lines via quantitative real-time polymerase chain reaction. The effect of hsa_circ_0136959 on the malignant properties of PTC was analyzed by performing Cell Counting Kit-8, colony formation, and transwell assays. In addition, its effects on the levels of markers related to ferroptosis (reactive oxygen species, Fe²⁺, and iron) in PTC cells were also assessed. Bioinformatics analysis was done to determine the hsa_circ_0136959 expression and m6A modification sites on it in PTC. The m6A level of hsa_circ_0136959 was analyzed through methylated (m6A) RNA immunoprecipitation. The hsa_circ_0136959 was observed to be downregulated in both PTC samples and cells. In vitro experiments showed that its overexpression impeded the malignant properties of PTC cells. Moreover, hsa_circ_0136959 overexpression increased the levels of ferroptosis-related markers in PTC cells. We also found that METTL3 was notably reduced in PTC samples and was positively correlated with hsa_circ_0136959. Mechanistically, METTL3 enhanced hsa_circ_0136959 expression through m6A modification. Our results demonstrate that METTL3-mediated m6A modification elevated hsa_circ_0136959 expression and subsequently restricted the tumor characteristics of PTC by accelerating ferroptosis.