BackgroundAnaplastic thyroid carcinoma (ATC) is considered to be one of the most aggressive cancers. Our previous study proved that highly expressed lysine acetyltransferase 5 (KAT5) in ATC is associated with a poorer prognosis. Here, this study examined the effects of a KAT5 inhibitor (NU9056) in human ATC cells.MethodsFirst, theCancerGenome Atlas (TCGA) dataset was used to detect the relationship between KAT5 expression and outcomes of thyroid carcinoma patients. Then, bothin vitroandin vivoexperiments were conducted to investigate the effects of NU9056 on normal and ATC human thyroid cells. Finally, microRNA sequencing, qPCR, and dual-luciferase reporter assay were performed to explore potential mechanisms by identifying downstream microRNA related to NU9056.ResultsKAT5 dysregulation correlated with more advanced-stage and poorer outcomes of thyroid carcinoma patients. Endogenous KAT5 protein and mRNA levels were much higher in ATC cells than in normal thyroid cells. Suppression of KAT5 by NU9056 inhibited survival, growth, migration, invasion, and tube formation, and increased radiosensitivity and chemosensitivity in ATC cells but showed no impact on normal thyroid cells. Mechanistically, microRNA-202-5p (miR-202) was identified as the most significantly decreased miRNA after NU9056 treatment. Knockdown of miR-202 suppressed ATC cell progression, while forced expression of miR-202 partially blocked the inhibitory effect of NU9056 on ATC cells. Furthermore, c-Myc was validated as the transcription factor of miR-202, and NU9056 decreased the c-Myc protein level by shortening its half-life. Finally, we proved that NU9056 inhibited ATC proliferation in vivo.ConclusionsOur results indicated that NU9056 targets KAT5, shortens c-Myc half-life, subsequently downregulates miR-202 expression, and results in the suppression of ATC cells. Overall, KAT5 could be a potential target for clinical treatment for ATC.