Circular RNAs (circRNAs) play important roles in tumourigenesis and tumour progression. However, the expression profiles and functions of circRNAs in hepatocellular carcinoma (HCC) are largely unclear. The expression profiles of circRNAs in HCC were identified through microarray analysis and were validated through quantitative reverse transcription polymerase chain reaction (qRT-PCR). Survival curves were plotted using the Kaplan-Meier method and compared using the log-rank test. The circular structure of candidate circRNA was confirmed through Sanger sequencing, divergent primer PCR, and RNase R treatments. Proliferation of HCC cells was evaluated in vitro and in vivo. The microRNA (miRNA) sponge mechanism of circRNAs was demonstrated using dual-luciferase reporter and RNA immunoprecipitation assays. CircSETD3 (hsa_circRNA_0000567/hsa_circRNA_101436) was significantly downregulated in HCC tissues and cell lines. Low expression of circSETD3 in HCC tissues significantly predicted an unfavourable prognosis and was correlated with larger tumour size and poor differentiation of HCC in patients. In vitro experiments showed that circSETD3 inhibited the proliferation of HCC cells and induced G1/S arrest in HCC cells. In vivo studies revealed that circSETD3 was stably overexpressed in a xenograft mouse model and inhibited the growth of HCC. Furthermore, we demonstrated that circSETD3 acts as a sponge for miR-421 and verified that mitogen-activated protein kinase (MAPK)14 is a novel target of miR-421. CircSETD3 is a novel tumour suppressor of HCC and is a valuable prognostic biomarker. Moreover, circSETD3 inhibits the growth of HCC partly through the circSETD3/miR-421/MAPK14 pathway.

Previous study reported that histone methyltransferase SETD3 is upregulated in tumor tissues of hepatocellular carcinoma (HCC) and is associated with the growth of HCC. However, the clinical significance and the effect of SETD3 on HCC metastasis remain unclear. In this study, both the protein and mRNA expression levels of SETD3 were measured in a larger cohort of HCC patients. The results showed that the protein level of SETD3 in HCC tissues was significantly higher than that in non-tumorous tissues, which was inconsistent with the mRNA expression level of SETD3. The high protein level of SETD3 in HCC tissues was significantly associated with male gender, poor pathological differentiation, liver cirrhosis and unfavorable prognosis of HCC patients. Subsequently, we demonstrated that SETD3 could be regulated at post-transcriptional step by a couple of miRNAs (miR-16, miR-195 and miR-497). Additionally, in vitro and in vivo experiments revealed that SETD3 played opposing roles in proliferation and metastasis of HCC: promoting proliferation but inhibiting metastasis. Mechanistic experiments revealed that doublecortin-like kinase 1 (DCLK1) was a downstream target of SETD3. SETD3 could increase the DNA methylation level of DCLK1 promoter to inhibit the transcription of DCLK1. Further study revealed that DCLK1/PI3K/MMP-2 was an important pathway that mediated the effect of SETD3 on HCC metastasis. In conclusion, this study revealed that SETD3 is associated with tumorigenesis and is a promising biomarker for predicting the prognosis of HCC patients after surgical resection. In addition, SETD3 plays inhibitory role in HCC metastasis partly through DCLK1/PI3K/MMP-2 pathway.