MiR-126 negatively regulates PLK-4 to impact the development of hepatocellular carcinoma via ATR/CHEK1 pathway

Emerging evidence has shown that microRNA-126 (miR-126) is aberrantly downregulated and plays a vital role in carcinogenesis in various cancers, including HCC. However, the underlying biological mechanisms of miR-126 in HCC are still largely unknown. In present study, we found that miR-126 was downregulated both in HCC tissues and cell lines. Low expression level of miR-126 was associated with poor overall survival (OS), late TNM stage and the presence of recurrence. Overexpression of miR-126 significantly decreased cell proliferation, metastasis and promoted apoptosis in vitro. Additional, high miR-126 expression reduced the tumor growth in vivo. Further we discovered that PLK (polo-like kinases)-4, a critical regulator in cell cycle, was a target of miR-126. PLK-4 overexpression could rescue the inhibitory effects of miR-126 on cell proliferation and invasion. Moreover, PLK-4 mRNA and protein levels were significantly upregulated in HCC tissues and positively associated with malignancies and poor OS. Knockdown PLK-4 significantly inhibited cell proliferation, invasion and promoted cell apoptosis in vitro whereas decreased tumor growth in vivo. More importantly, bioinformatics analysis combined with validation experiments in vitro and in vivo showed that activation of the ATR/CHEK1 pathway was involved in the oncogenic functions of PLK4 in HCC. We also validated that PLK4 could directly interact with ATR through CoIP assay. Taken together, we demonstrate that miRNA-126/PLK-4 axis is critical for tumorigenesis and progression of HCC, and the newly identified PLK-4/ATR/CHEK1 pathway may be a potential therapeutic target for HCC treatment.