Abstract: OBJECTIVE： To explore the effect of m⁶A demethylase FTO on m⁶A methylation，proliferation，metastasis，invasion and other biological behaviors of thyroid cancer cells，and its involvement with the MAPK-JNK signaling pathway. METHODS： The FTO knockdown plasmid was transfected into human thyroid papillary carcinoma cells (TPC-1)，thyroid squamous carcinoma cells (SW-579)，and undifferentiated thyroid carcinoma cells (8505C). Expression of m⁶A in the cancer cells were measured using a m⁶A methylation quantification detection kit. Their proliferation，migration，and invasion abilities were evaluated using the colony formation，CCK-8，wound healing，and Transwell assays. Changes in expression levels of proteins in the MAPK-JNK signaling pathway were examined using Western blot analysis. RESULTS： FTO knockdown increased m⁶A expression levels in the TPC-1，SW-579，and 8505C cancer cells，and promoted their proliferation，invasion，and migration abilities (compared to cells without FTO knockdown，P<0.01). Western blot analysis revealed that FTO knockdown led to decreased expression of E-cadherin and β-catenin，increased expression of N-cadherin and vimentin，and upregulated expression of C-JUN and MLK3 in the MAPK signaling pathway (compared with cells without FTO knockdown，all P<0.05). CONCLUSION： FTO knockdown enhanced proliferation，migration，and invasion abilities of thyroid cancer cells in vitro，and activated the MAPK signaling pathway，thereby potentially influenced development of thyroid cancer. These findings provide insights into the role of FTO demethylase in m⁶A methylation and in thyroid cancer progression.

Key words: thyroid cancer, m⁶A, demethylase, FTO, MAPK signaling pathway