Circular RNA circ-ADD3 inhibits hepatocellular carcinoma metastasis through facilitating EZH2 degradation via CDK1-mediated ubiquitination.

Emerging evidence suggests that circular RNA (circRNA) plays a fundamental role in tumorigenesis. However, its contribution to hepatocellular carcinoma (HCC) malignancy remains largely unknown. Here, we performed circRNA microarray expression profile in four paired HCC and normal tissues, and found that circ-ADD3, a novel circRNA derived from linear ADD3 exon 4 to exon 12, was significantly downregulated in HCC, which was further validated in 112 matched HCC and paracancerous tissues. High circ-ADD3 expression was negatively correlated with vascular invasion, intrahepatic metastasis as well as distant metastasis. Moreover, it was identified as an effective biomarker for diagnosis and prognosis of HCC. Functionally, exogenous expression of circ-ADD3 dramatically weakened HCC cell invasion and metastasis both <i>in vitro</i> and <i>in vivo</i>. Mechanistically, circ-ADD3 was capable of reinforcing the interaction between CDK1 and EZH2, resulting in increased EZH2 ubiquitination and subsequent degradation via phosphorylation at Thr-345 and Thr-487 sites. The decreased EZH2 markedly increased the expression of a cohort of anti-metastatic genes, including circ-ADD3, by reducing H3K27me3 levels on their promoter regions, which formed a regulatory circuit, thereby dampening HCC metastasis. Taken together, our findings unveil the essential role of circ-ADD3 in inhibiting HCC metastasis through regulation of EZH2 stability.