MicroRNAs (miRNAs) are endogenous ∼23 nt RNAs that play important gene-regulatory roles in animals and plants by pairing to the mRNAs of protein-coding genes to direct their posttranscriptional repression. This review outlines the current understanding of miRNA target recognition in animals and discusses the widespread impact of miRNAs on both the expression and evolution of protein-coding genes.

MicroRNAs: Target Recognition and Regulatory Functions

Many RNA species have been identified as important players in the development of chronic diseases, including cancer. Over the past decade, numerous studies have highlighted how regulatory RNAs such as microRNAs (miRNAs) and long noncoding RNAs (lncRNAs) play crucial roles in the development of a disease state. It is clear that the aberrant expression of miRNAs promotes tumor initiation and progression, is linked with cardiac dysfunction, allows for the improper physiological response in maintaining glucose and insulin levels, and can prevent the appropriate integration of neuronal networks, resulting in neurodegenerative disorders. Because of this, there has been a major effort to therapeutically target these noncoding RNAs. In just the past 5 years, over 100 antisense oligonucleotide-based therapies have been tested in phase I clinical trials, a quarter of which have reached phase II/III. Most notable are fomivirsen and mipomersen, which have received FDA approval to treat cytomegalovirus retinitis and high blood cholesterol, respectively. The continued improvement of innovative RNA modifications and delivery entities, such as nanoparticles, will aid in the development of future RNA-based therapeutics for a broader range of chronic diseases. Here we summarize the latest promises and challenges of targeting noncoding RNAs in disease.

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Targeting noncoding RNAs in disease

Long noncoding RNAs (lncRNAs) are non-protein coding transcripts longer than 200 nucleotides. Aberrant expression of lncRNAs has been found associated with gastric cancer, one of the most malignant tumors. By complementary base pairing with mRNAs or forming complexes with RNA binding proteins (RBPs), some lncRNAs including GHET1, MALAT1, and TINCR may mediate mRNA stability and splicing. Other lncRNAs, such as BC032469, GAPLINC, and HOTAIR, participate in the competing endogenous RNA (ceRNA) network. Under certain circumstances, ANRIL, GACAT3, H19, MEG3, and TUSC7 exhibit their biological roles by associating with microRNAs (miRNAs). By recruiting histone-modifying complexes, ANRIL, FENDRR, H19, HOTAIR, MALAT1, and PVT1 may inhibit the transcription of target genes in cis or trans. Through these mechanisms, lncRNAs form RNA-dsDNA triplex. CCAT1, GAPLINC, GAS5, H19, MEG3, and TUSC7 play oncogenic or tumor suppressor roles by correlated with tumor suppressor P53 or onco-protein c-Myc, respectively. In conclusion, interaction with DNA, RNA and proteins is involved in lncRNAs' participation in gastric tumorigenesis and development.

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Molecular mechanisms of long noncoding RNAs on gastric cancer

Non-coding RNA (ncRNA) has been shown to regulate diverse cellular processes and functions through controlling gene expression. Long non-coding RNAs (lncRNAs) act as a competing endogenous RNAs (ceRNAs) where microRNAs (miRNAs) and lncRNAs regulate each other through their biding sites. Interactions of miRNAs and lncRNAs have been reported to trigger decay of the targeted lncRNAs and have important roles in target gene regulation. These interactions form complicated and intertwined networks. Certain lncRNAs encode miRNAs and small nucleolar RNAs (snoRNAs), and may regulate expression of these small RNAs as precursors. SnoRNAs have also been reported to be precursors for PIWI-interacting RNAs (piRNAs) and thus may regulate the piRNAs as a precursor. These miRNAs and piRNAs target messenger RNAs (mRNAs) and regulate gene expression. In this review, we will present and discuss these interactions, cross-talk, and co-regulation of ncRNAs and gene regulation due to these interactions.

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Interaction and cross-talk between non-coding RNAs

Extracellular communication within the tumor microenvironment plays a critical role in tumor progression. Although exosomes can package into long non-coding RNAs (lncRNAs) to mediate extracellular communication, the role of exosomal lncRNA PTENP1 in bladder cancer (BC) remains unclear. We detected PTENP1 expression between patients with BC and healthy controls; the expression occurred in tissues and exosomes from plasma. We assessed the diagnostic accuracy by the receiver operating characteristic curve (ROC) and the area under curve (AUC). Cell phenotypes and animal experiments were performed to determine the effect of exosomal PTENP1. PTENP1 was significantly reduced in BC tissues and in exosomes from plasma of patients with BC (P < 0.05). We found that PTENP1 was mainly wrapped by exosomes. Exosomal PTENP1 could distinguish patients with BC from healthy controls (AUC = 0.743; 95% confidence interval (CI) = 0.645–0.840). Normal cells secreted exosomal PTENP1 and transmitted it to BC cells, thus inhibiting the biological malignant behavior of BC cells by increasing cell apoptosis and reducing the ability to invade and migrate (P < 0.05). Exosomal PTENP1 could suppress tumor growth in vivo. Furthermore, exosomal PTENP1 mediated the expression of PTEN by competitively binding to microRNA-17. Exosomal PTENP1 is a promising novel biomarker that can be used for the clinical detection of BC. Exosomes derived from normal cells transfer PTENP1 to BC cells, which reduce the progression of BC both in vitro and in vivo and suggest that exosomal PTENP1 participates in normal-cell-to-bladder-cell communication during the carcinogenesis of BC.

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Exosome–transmitted long non-coding RNA PTENP1 suppresses bladder cancer progression

The long non-coding RNA MEG3 has been reported to be a tumor suppressor in a number of malignant tumors including gastric cancer. Several studies have shown that the regulation of MEG3 may attribute to the promoter hypermethylation. However, the mechanism of MEG3 regulation in gastric cancer is still not well understood. MiR-148a can suppress gastric tumorigenesis through regulating the expression of target genes such as DNA methyltransferase 1(DNMT-1). We examined the expression of MEG3 in 52 gastric cancer samples using quantitative real-time PCR and found the down-regulation of MEG3 in both gastric cancer tissues and cell lines. The positive correlation of MEG3 and miR-148a was further confirmed in SGC-7901 and BGC-823 gastric cancer cell lines. Hypermethylation of MEG3 differentially methylated regions was identified by methylation-specific PCR, and MEG3 expression was increased with the inhibition of methylation with siRNA to DNMT-1 in gastric cancer cells. In addition, transfection of MEG3 siRNA into gastric cancer cells diminished the suppression of proliferation induced by overexpression of miR-148a. Our results suggest that the suppression of miR-148a may contribute to the down-regulation of MEG3 in gastric cancer by modulation of DNMT-1.

MiR-148a regulates MEG3 in gastric cancer by targeting DNA methyltransferase 1.

Introduction: As it is not clear whether growth arrest-specific 5 (GAS5) inhibits gastric cancer (GC) cell proliferation by regulating cell cycle, we analyzed the

GAS5 Inhibits Gastric Cancer Cell Proliferation Partly by Modulating CDK6.

PTENP1 has been demonstrated to function as a tumor suppressor in several cancercells. However, its expression and biological roles in gastric cancer (GC) have not yet beeninvestigated. In this study, we demonstrated that PTENP1 was frequently decreased in GCtissues and cell lines, which might be partly associated with DNA hypermethylation, and lowerPTENP1 expression was associated with larger tumor size, more advanced stage, deeperinvasion depth and lymphatic metastasis. In addition, our data suggested that PTENP1 couldregulate GC cell proliferation, apoptosis, migration and invasion in vitro. Furthermore, we demonstrated that PTENP1 could modulatethe PTEN protein expression. Taken together, these results suggest that PTENP1 functions as a novel tumor suppressor in GC and itssuppressive ability may be involved in the modulation of PTEN.

Zheng Liang

Papers

MiR-148a regulates MEG3 in gastric cancer by targeting DNA methyltransferase 1.

GAS5 Inhibits Gastric Cancer Cell Proliferation Partly by Modulating CDK6.

Pseudogene PTENP1 Suppresses Gastric Cancer Progression by Modulating PTEN.