MiR-23b targets cyclin G1 and suppresses ovarian cancer tumorigenesis and progression

MiRNAs regulate their target genes by affecting base pairing to their 3? UTRs, resulting in mRNA degradation or inhibition of translation [11]. An increasing number of studies have revealed that miRNAs are promising diagnostic and prognostic molecular biomarkers as well as therapeutic targets in cancer [17, 18]. Series of studies has reported that miR-23b acts as a tumor suppressor in different cancers [19]. Majid et al. showed that miR-23b has diagnostic/prognostic significance and directly targets the oncogenic ZEB1 in bladder cancer [14]. He et al. reported that miR-23b expression levels in prostate carcinoma (PCa) tissues was significantly correlated with that of peroxiredoxin 3 (PRDX3) and that miR-23b may be involved in the response of PCa cells to hypoxic stress, therefore gene therapy using miRNA mimics may be useful as PCa therapy [20]. Our results show that MIR23B mRNA expression was significantly lower in ovarian carcinomas and borderline tumors than in normal ovarian tissues and benign tumors, and the expression among age and pathological subtypes (mucinous vs. other types) was significantly different. These findings indicate that miR-23b might affect ovarian epithelial carcinogenesis and the subsequent progression. Therefore, we explored the function and molecular mechanism of miR-23b in ovarian cancer cell lines.

Ovarian cancer cells transfected with miR-23b had significantly slower growth than the negative control– and mock-transfected cells, and there was significantly induced G₁ arrest and apoptosis and reduced cell invasion and migration, suggesting miR-23b may inhibit ovarian carcinoma tumorigenesis and progression. Moreover, the predicted seed region showed that miR-23b targets CCNG1 3? UTR, which was convinced by the dual-luciferase reporter assay. We also found that miR-23b transfection decreased CCNG1 mRNA and protein expression. CCNG1 was first identified as a p53-regulated transcript induced by DNA damage. It has been proposed that these events underpin CCNG1 participation in the enforcement of the p53-dependent G₁–S and G₂ checkpoints responsive to DNA damage [21]. Some have suggested that CCNG1 might function as an oncogenic protein [22, 23] and play a pivotal role in the initiation and metastasis of hepatocellular carcinoma [24]. Russell et al. reported that CCNG1 amplification is associated with significantly shorter postsurgical survival in patients with ovarian cancer who have received adjuvant chemotherapy with taxanes and platinum compounds [21]. These results suggest that miR-23b may inhibit ovarian cancer tumorigenesis and progression by targeting CCNG1.

In this study, we also found that miR-23b overexpression downregulated uPA expression, which is in line with the findings of Salvi et al. [15], who reported that miR-23b overexpression leads to uPA downregulation and decreased migration and proliferation ability in hepatocellular carcinoma cells. Furthermore, miR-23b overexpression also downregulated the expression of P70S6K, survivin, Bcl-xL, and MMP9 mRNA and protein. It has been established that uPA is integral to cell differentiation, migration, tissue remodeling under physiological and pathological conditions, and may be a potential diagnostic biomarker and therapeutic target in cancer. Significant elevation of uPA protein levels in primary ovarian cancer tissue has been associated with poor prognosis and disease progression [25–29]. The uPA system plays an important role in many pathophysiological processes, such as cell differentiation, migration, tissue reconstruction, and matrix dissolution [30]. The aberrant expression of phosphorylated P70S6K might contribute to the pathogenesis, growth, invasion, and metastasis of cancer [31]. Wang et al. reported that uPA promoted carcinoma cell proliferation by stimulating P70S6K activation [32]. Survivin, a member of the inhibitors of apoptosis protein family, is expressed during development and in various human cancers. Lee et al. reported that downregulating survivin suppressed uPA through the transcription factor JunB [33]. Ryan et al. reported that survivin expression in breast cancer predicts clinical outcome and is associated with uPA [34]. Furthermore, Zhou et al. reported that Bcl-xL overexpression strongly enhanced uPA in pancreatic ductal adenocarcinoma cells [35]. Therefore, we suggest that miR-23b reduces cell proliferation by downregulating CCNG1 and uPA/P70S6K expression, suppresses cell migration and invasion by downregulating uPA/MMP9 expression, and induces apoptosis by downregulating survivin and Bcl-xL/uPA expression.

We found that CCNG1 mRNA expression was significantly lower in the normal ovarian tissues and benign ovarian tumors than in the borderline ovarian tumors and ovarian carcinomas. Our subsequent in vivo tumor xenograft studies showed that miR-23b inhibited tumor growth and decreased CCNG1 expression. Recent studies have reported that CCNG1 gene therapy has been developed and has undergone phase I/II clinical trials for treating colorectal and pancreatic cancer [36]. We suggest that miR-23b may also be a potential suppressor of ovarian carcinoma by targeting CCNG1.

Our findings show that miR-23b may inhibit ovarian cancer tumorigenesis and progression by downregulating CCNG1 and the expression of the relevant genes, and that miR-23b is a potentially novel application for regulating ovarian carcinoma progression.