Previously, we have reported that PLK1 is overexpressed in human NSCLC tissues and the overexpression of PLK1 was correlated with poor prognosis and malignant phenotypes of NSCLC patients. However, the molecular mechanisms of PLK1 overexpression in NSCLC are still unclear. MiRNAs constitute a large family of small, approximately 21-nucleotide-long, non-coding RNAs that have emerged as key post-transcriptional regulators of gene expression, and miRNAs have been predicted to control the activity of approximately 30% of all protein-coding genes . By base pairing to mRNAs, microRNAs mediate translational repression or mRNA degradation . By performing in-silico screening using TargetScan, we found that the 3’-UTR of PLK1 gene contained binding sites for miR-100 with reasonable scores. In the present study, we showed that downregulation of miR-100 might play critical roles in the formation of malignant phenotypes by posttranscriptionally regulating PLK1 expression.
Up to date, increasing evidence shows that the dysregulation of miRNAs is correlated with tumor initiation and progression, suggesting that miRNAs may act as tumour suppressor genes or oncogenes [20, 21]. Recent studies have shown that not only can miRNAs be used to sub-classify NSCLCs but specific miRNA profiles may also predict prognosis and disease recurrence in early-stage NSCLCs [22, 23]. Since Takamizawa’ et al. firstly reported that reduced expression of the let-7 microRNAs in human lung cancers was found to be correlated with shortened postoperative survival of patients , other miRNAs were also found to be correlated with prognosis of NSCLC patients [25, 26]. In our previous study, we also found that serum miR-21 expression might be useful as a prognostic marker for NSCLC patients . With the development of research for experiment, dysregulation of miRNAs were reported to affect growth, apoptosis and chemo- or radioresistance of NSCLC cells . Xiong and his colleagues found that microRNA-7 inhibited the growth of human non-small cell lung cancer A549 cells through targeting BCL-2 . Moreover, miRNA-145 was found to inhibit non-small cell lung cancer cell proliferation by targeting c-Myc . In our previous studies, we have shown that miR-451 functions as tumor suppressor in NSCLC by targeting RAB14 gene . Meanwhile, we also found that the level of miR-451 could affect the sensitivity of NSCLC cells to cisplatin . In other researches, ectopic expression of miR-200c alters expression of EMT proteins, sensitivity to erlotinib, and migration in lung cells . The association of dysregulated miRNAs with angiogenesis and metastasis of NSCLC cells was reported. Donnem and his colleagues showed that several angiogenesis-related miRNAs (miR-21, miR-106a, miR-126, miR-155, miR-182, miR-210 and miR-424. miR-155) which were correlated significantly with fibroblast growth factor 2 (FGF2), are significantly altered in NSCLC . MicroRNA-328 along with other miRNAs was found to be associated with (non-small) cell lung cancer (NSCLC) metastasis and mediates NSCLC migration . Although miR-100 has been found to function as a tumor suppressor in nasopharyngeal cancer, epithelial ovarian cancer, bladder cancer and acute myeloid leukemia [36, 37], the expression of miR-100 and its roles in NSCLC development are unknown.
In the present study, we firstly found that miR-100 was significantly lower in NSCLC tissues than in corresponding nontumor tissues. Then, we analyzed the association of downregulated miR-100 with clinicopathologic factors of NSCLC patients. By statistical analysis, we found that miR-100 expression was significantly correlated with clinical stage, tumor classification and lymph node metastasis of NSCLC patients, suggesting that low miR-100 expression might play roles in NSLC progression. The disease-free survival showed between NSCLC patients with low miR-100 and those with high miR-100, but the overall survival of patients with high miR-100 was higher than that of patients with low miR-100. Furthermore, multivariate analysis using the Cox proportional hazard model indicated that miR-100 expression was an independent prognostic factor for NSCLC patients. Functional experiments showed that upregulation of miR-100 could inhibit growth of NSCLC cells, which might be apoptosis enhancement and cell cycle arrest in G2/M stage. Sequence analyses revealed that the 3’-UTR of PLK1 mRNA contains a putative site partially complementary to miR-100. By firefly luciferase activity assay, miR-100 could inhibit luciferase activity in the PLK1 WT but had no effect in the mutant construct. Meanwhile, miR-100 mimics or inhibitor could lead to the decreased or increased PLK1 expression in NSCLC at both transcriptional and translational levels. By functional analysis, it was shown that siRNA-mediated PLK1 downregulation could mimic the effects of miR-100 mimics on phenotypes of NSCLC cells and overexpression of PLK1 could partially reverse miR-100 mimics-induced phenotypical changes in NSCLC cells. Additionally, miR-100 expression was inversely correlated with PLK1 mRNA expression in NSCLC tissues. From these data, PLK1 is a direct and functional target gene in NSCLC. While a single miRNA can target many genes, multiple miRNAs can regulate a single gene. In acute myeloid leukemia, RBSP3 (a phosphatase-like tumor suppressor) has been validated as a bona fide target of miR-100 . Zheng and his colleagues revealed a new pathway that miR-100 regulates G1/S transition and S-phase entry and blocks the terminal differentiation by targeting RBSP3, which promoted cell proliferation. However, the functions of RBSP3 and its correlation with posttranscriptional regulation of miR-100 in NSCLC are unclear and remains to be elucidated in future research.