Restoration of miR-101 suppresses lung tumorigenesis through inhibition of DNMT3a-dependent DNA methylation

The deregulation of miR-101 and DNMT3a has been implicated in the pathogenesis of multiple tumor types, but whether and how miR-101 silencing and DNMT3a overexpression contribute to lung tumorigenesis remain elusive. Here we show that miR-101 downregulation associates with DNMT3a overexpression in lung cancer cell lines and patient tissues. Ectopic miR-101 expression remarkably abrogated the DNMT3a 3′-UTR luciferase activity corresponding to the miR-101 binding site and caused an attenuated expression of endogenous DNMT3a, which led to a reduction of global DNA methylation and the re-expression of tumor suppressor CDH1 via its promoter DNA hypomethylation. Functionally, restoration of miR-101 expression suppressed lung cancer cell clonability and migration, which recapitulated the DNMT3a knockdown effects. Interestingly, miR-101 synergized with decitabine to downregulate DNMT3a and to reduce DNA methylation. Importantly, ectopic miR-101 expression was sufficient to trigger in vivo lung tumor regression and the blockage of metastasis. Consistent with these phenotypes, examination of xenograft tumors disclosed an increase of miR-101, a decrease of DNMT3a and the subsequent DNA demethylation. These findings support that the loss or suppression of miR-101 function accelerates lung tumorigenesis through DNMT3a-dependent DNA methylation, and suggest that miR-101-DNMT3a axis may have therapeutic value in treating refractory lung cancer.Owing to a high propensity for recurrence and a high rate of metastasis at the advanced stages,^{1, 2, 3} lung cancer remains the leading cause of cancer-related mortality. DNA methylation is a major epigenetic rule controlling chromosomal stability and gene expression.^{4, 5} It is under control of DNA methyltransferases (DNMTs), whose overexpression in lung cancer cells predicts worse outcomes.^{6, 7} It is postulated that DNMT overexpression induces DNA hypermethylation and silencing of tumor suppressor genes (TSGs), leading to an aggressive lung cancer. Indeed, enforced expression of DNMT1 or DNMT3a increases DNA methylation, while the abolition of DNMT expression by genetic depletion, microRNAs (miRs) or small molecules reduces genome-wide and gene-specific DNA methylation and restores TSG expression.^{8, 9, 10, 11, 12, 13} As TSGs are the master controllers for cell multiplicity and their silencing predicts poor prognosis,^{14, 15} TSG re-expression via promoter DNA hypomethylation inhibits cell proliferation and induces cell differentiation.^{13, 16} Thus, DNMT gene abundance could serve as a target for anticancer therapy, but how DNMT upregulation occurs in lung cancer is incompletely understood.MiRs are small non-coding RNAs that crucially regulate target gene expression. Up to 30% of all protein-coding genes are predicted to be targeted by miRs,^{17, 18} supporting the key roles of miRs in controlling cell fate.^{19, 20, 21, 22} Research is showing that certain miRs are frequently dysregulated in cancers, including lung cancer.^{7, 23, 24} As miR targets can promote or inhibit cancer cell expansion, miRs have huge potential for acting as bona fide oncogenes (i.e., miR-21) or TSGs (i.e., miR-29b).^{7, 25} We and others demonstrated that the levels of DNMT1 or DNMT3a or DNMT3b are regulated by miR-29b, miR-148, miR-152 or miR-30c,^{7, 13, 26, 27} and overexpression of these miRs results in DNA hypomethylation and TSG reactivation with the concurrent blockage of cancer cell proliferation.^{7, 13} These findings underscore the importance of miRs as epigenetic modulators and highlight their therapeutic applications.MiR-101 is frequently silenced in human cancers^{28, 29, 30, 31} and, importantly, exhibits antitumorigenic properties when overexpressed. Mechanistically, miR-101 inactivation by genomic loss causes the overexpression of EZH2, a histone methyltransferase, via 3′-UTR targeting, which is followed by histone hypermethylation and aggressive tumorigenesis.^{29, 30, 32} However, whether and how miR-101 silencing contributes to DNA hypermethylation patterning in lung cancer is unclear. In this study, we explore the role of miR-101 in regulating DNMT3a expression and the impacts of miR-101-DNMT3a nexus on lung cancer pathogenesis. We showed that the expression of miR-101 and DNMT3a was negatively correlated in lung cancer. We presented evidence that ectopic miR-101 expression decreased DNMT3a levels, reduced global DNA methylation and upregulated CDH1 via its promoter DNA demethylation. The biological significance of miR-101-mediated DNA hypomethylation and CDH1 re-expression was evident by its inhibition of lung tumor cell growth in vitro and in vivo. Thus, our findings mechanistically and functionally link miR-101 silencing to DNA hypermethylation in lung cancer cells.     

The novel miR-9500 regulates the proliferation and migration of human lung cancer cells by targeting Akt1

MicroRNAs have crucial roles in lung cancer cell development. They regulate cell growth, proliferation and migration by mediating the expression of tumor suppressor genes and oncogenes. We identified and characterized the novel miR-9500 in human lung cancer cells. The miR-9500 forms a stem-loop structure and is conserved in other mammals. The expression levels of miR-9500 were reduced in lung cancer cells and lung cancer tissues compared with normal tissues, as verified by TaqMan miRNA assays. It was confirmed that the putative target gene, Akt1, was directly suppressed by miR-9500, as demonstrated by a luciferase reporter assay. The miR-9500 significantly repressed the protein expression levels of Akt1, as demonstrated via western blot, but did not affect the corresponding mRNA levels. Akt1 has an important role in lung carcinogenesis, and depletion of Akt1 has been shown to have antiproliferative and anti-migratory effects in previous studies. In the current study, the overexpression of miR-9500 inhibited cell proliferation and the expression of cell cycle-related proteins. Likewise, the overexpression of miR-9500 impeded cell migration in human lung cancer cells. In an in vivo assay, miR-9500 significantly suppressed Fluc expression compared with NC and ASO-miR-9500, suggesting that cell proliferation was inhibited in nude mice. Likewise, miR-9500 repressed tumorigenesis and metastasis by targeting Akt1. These data indicate that miR-9500 might be applicable for lung cancer therapy.MicroRNAs (miRNAs) are small, non-coding RNAs, 18–25 nucleotides (nt) in length that regulate gene expression by binding to the 3′-untranslated region (UTR) of their target genes,^{1, 2} and these RNAs are processed from introns, exons or intergenic regions.³ First, miRNAs are transcribed by RNA polymerase II into primary miRNA (pri-miRNA) molecules that contain several thousand nucleotides. The pri-miRNAs are then sequentially processed by a microprocessor, such as Drosha RNase III endonuclease and DiGeorge syndrome region gene 8 protein (DGCR8), to form ∼70 nt-stem-loop intermediates known as miRNA precursors (pre-miRNAs).^{4, 5} The pre-miRNAs are then exported from the nucleus into the cytoplasm via Exportin-5 (EXP5), with its cofactor Ran-GTP; in the cytoplasm, these pre-miRNAs are processed into 18–25 nt mature miRNA duplexes by the RNase III endonuclease Dicer.^{6, 7} The mature miRNA duplexes, along with the Argonaute proteins, are integrated as single-stranded RNAs into an RNA-induced silencing complex, which induces either the cleavage or the translational inhibition of the targeted mRNAs.^{8, 9, 10} miRNAs have been implicated in a variety of biological processes associated with cancer development, including cell proliferation and invasion,¹¹ and miRNA expression is deregulated in many forms of cancer.¹²Cancer is a major public health problem worldwide. Lung cancer represents one of the most predominant types of cancer, with high mortality rates in both men and women. Epithelial lung cancer can be categorized into one of two types: small cell lung cancer (SCLC) or non-small cell lung cancer (NSCLC). NSCLC accounts for ∼80% of lung cancer cases, and these cases can be further categorized as adenocarcinoma (40%), squamous cell carcinoma (30–35%), and large cell carcinoma (5–15%). NSCLC has a 5-year survival rate of only 16%.^{13, 14, 15} Current studies have shown that miRNAs are deregulated in various cancers, including NSCLC, and may act as oncogenes or tumor suppressor genes.¹⁶ For example, the Let-7 family,¹⁷ miR-15a/16,¹⁸ miR-17-92,¹⁹ miR-107 and miR-185,²⁰ are deregulated in lung cancer.Some studies have reported that phosphatidylinositol 3-kinase (PI3K) signaling is activated in human cancers^{21, 22} and has an important role in the progression of NSCLC. The PI3K pathway modulates several cellular mechanisms, such as cell survival, proliferation, migration and motility, and thereby significantly affects the growth of tumors.^{23, 24} The primary regulator of the PI3K pathway is Akt, a protein kinase B that mediates cell survival, cell death,²⁵ cell growth, cell migration and angiogenesis.^{26, 27, 28} The silencing of the Akt1 gene has been shown to inhibit the proliferation of gastric cancer cells both in vitro and in vivo.²⁹ Other studies have shown that aberrant AKT activation has a critical role in tumorigenesis.³⁰In this study, we identified small RNAs in lung cancer cells. To analyze a novel miRNA signature, we examined the structure and sequence of the small RNAs, analyzed the expression patterns of the novel miRNAs in lung cancer tissues and assessed the miRNA target genes. Our data revealed that miR-9500 regulates certain human lung cancer cell functions, including cell growth, proliferation, and migration.