The let-7 microRNA represses cell proliferation pathways in human cells

The proto-oncogene KRAS is targeted by miR-200c

The GTPase K-ras is involved in a variety of cellular processes such as differentiation, proliferation and survival. However, activating mutations, which frequently occur in many types of cancer, turn KRAS into one of the most prominent oncogenes. Likewise, miR-200c is a key player in tumorigenesis functioning as a molecular switch between an epithelial, non-migratory, chemosensitive and a mesenchymal, migratory, chemoresistant state. While it has been reported that KRAS is modulated by several tumor suppressor miRNAs, this is the first report on the regulation of KRAS by miR-200c, both playing a pivotal role in oncogenesis. We show that KRAS is a predicted target of miR-200c and that the protein expression of KRAS inversely correlates with the miR-200c expression in a panel of human breast cancer cell lines. KRAS was experimentally validated as a target of miR-200c by Western blot analyses and luciferase reporter assays. Furthermore, the inhibitory effect of miR-200c-dependent KRAS silencing on proliferation and cell cycle was demonstrated in different breast and lung cancer cell lines. Thereby, the particular role of KRAS was dissected from the role of all the other miR-200c targets by specific knockdown experiments using siRNA against KRAS. Cell lines harboring an activating KRAS mutation were similarly affected by miR-200c as well as by the siRNA against KRAS. However, in a cell line with wild-type KRAS only miR-200c was able to change proliferation and cell cycle. Our findings suggest that miR-200c is a potent inhibitor of tumor progression and therapy resistance, by regulating a multitude of oncogenic pathways including the RAS pathway. Thus, miR-200c may cause stronger anti-tumor effects than a specific siRNA against KRAS, emphasizing the potential role of miR-200c as tumor suppressive miRNA.

MicroRNAs as biomarkers of cervical cancer development: a literature review on miR-125b and miR-34a

MicroRNAs are non-coding RNAs with important functions in several biological processes, such as, regulation of cell cycle, immune response, inflammation, and apoptosis. In fact, deregulation and abnormal expression of these molecules is associated with human pathologies including cancer and several have already emerged as potential prognostic biomarkers in different neoplasias. miR-34a is directly regulated by p53 and acts as tumor suppressor while miR-125b plays a significant role in immune response and apoptosis. In cervical carcinogenesis, HPV proteins seem to interact with both miR-34a and miR-125b changing its expression and promoting persistent infection and cervical cancer development. In this review we describe the potential role of miR-125b and miR-34a in cervical carcinogenesis, including interaction with HPV and mechanism of deregulation. Additionally, their clinical applications in cervical cancer as prognostic/predictive biomarkers are also briefly discussed.

The miRNA-mediated cross-talk between transcripts provides a novel layer of posttranscriptional regulation

Endogenously expressed transcripts that are posttranscriptionally regulated by the same microRNAs (miRNAs) will, in principle, compete for the binding of their shared small noncoding RNA regulators and modulate each other's abundance. Recently, the levels of some coding as well as noncoding transcripts have indeed been found to be regulated in this way. Transcripts that engage in such regulatory interactions are referred to as competitive endogenous RNAs (ceRNAs). This novel layer of posttranscriptional regulation has been shown to contribute to diverse aspects of organismal and cellular biology, despite the number of functionally characterized ceRNAs being as yet relatively low. Importantly, increasing evidence suggests that the dysregulation of some ceRNA interactions is associated with disease etiology, most preeminently with cancer. Here we review how posttranscriptional regulation by miRNAs contributes to the cross-talk between transcripts and review examples of known ceRNAs by highlighting the features underlying their interactions and what might be their biological relevance.

Computational methods for microRNA target prediction

MicroRNAs (miRNAs) are important players in gene regulation. The final and maybe the most important step in their regulatory pathway is the targeting. Targeting is the binding of the miRNA to the mature RNA via the RNA-induced silencing complex. Expression patterns of miRNAs are highly specific in respect to external stimuli, developmental stage, or tissue. This is used to diagnose diseases such as cancer in which the expression levels of miRNAs are known to change considerably. Newly identified miRNAs are increasing in number with every new release of miRBase which is the main online database providing miRNA sequences and annotation. Many of these newly identified miRNAs do not yet have identified targets. This is especially the case in animals where the miRNA does not bind to its target as perfectly as it does in plants. Valid targets need to be identified for miRNAs in order to properly understand their role in cellular pathways. Experimental methods for target validations are difficult, expensive, and time consuming. Having considered all these facts it is of crucial importance to have accurate computational miRNA target predictions. There are many proposed methods and algorithms available for predicting targets for miRNAs, but only a few have been developed to become available as independent tools and software. There are also databases which collect and store information regarding predicted miRNA targets. Current approaches to miRNA target prediction produce a huge amount of false positive and an unknown amount of false negative results, and thus the need for better approaches is evermore evident. This chapter aims to give some detail about the current tools and approaches used for miRNA target prediction, provides some grounds for their comparison, and outlines a possible future.

MicroRNAs and the cancer phenotype: profiling, signatures and clinical implications

MicroRNAs (miRNAs) have emerged as key genetic regulators of a wide variety of biological processes, including growth, proliferation, and survival. Recent advances have led to the recognition that miRNAs can act as potent oncogenes and tumor suppressors, playing crucial roles in the initiation, maintenance, and progression of the oncogenic state in a variety of cancers. Determining how miRNA expression and function is altered in cancer is an important goal, and a necessary prerequisite to the development and adoption of miRNA-based therapeutics in the clinic. Highly promising clinical applications of miRNAs are the use of miRNA signatures as biomarkers for cancer (for example, for early detection or diagnosis), and therapeutic supplementation or inhibition of specific miRNAs to alter the cancer phenotype. In this review, we discuss the main methods used for miRNA profiling, and examine key miRNAs that are commonly altered in a variety of tumors. Current studies underscore the functional versatility and potency of miRNAs in various aspects of the cancer phenotype, pointing to their potential clinical applications. Consequently, we discuss the application of miRNAs as biomarkers, clinical agents, and therapeutic targets, highlighting both the enormous potential and major challenges in this field.

Targeted knock-in of the polymorphism rs61764370 does not affect KRAS expression but reduces let-7 levels

Understanding the role of single-nucleotide polymorphisms (SNPs) in the pathological process represents a unique experimental challenge especially when the variants occur outside of coding regions. The noncoding SNP rs61764370 located in the 3'-untranslated region of Kirsten rat sarcoma viral oncogene homolog (KRAS) has been implicated as a risk factor for the development of cancer and the response to targeted therapies. This cancer-associated variant is thought to affect the binding of the microRNA let-7, which allegedly modulates KRAS expression. Using site-specific homologous recombination, we inserted the rs61764370:T>G KRAS gene variant in the colorectal cancer cell line SW48 (SW48 +SNP) and assessed the cellular and biochemical phenotype. We observed a significant increase in cellular proliferation, as well as a reduction in the levels of the microRNA let-7a, let-7b, and let-7c. Transcriptional and biochemical analysis showed no concomitant change in the KRAS protein expression or modulation of the downstream mitogen activated kinase or PI3K/AKT signaling. These results suggest that the cancer-associated rs61764370 variant exerts a biological effect not through transcriptional modulation of KRAS but rather by tuning the expression of the microRNA let-7.

LIN28B promotes growth and tumorigenesis of the intestinal epithelium via Let-7

The RNA-binding proteins LIN28A and LIN28B have diverse functions in cellular reprogramming, growth, and oncogenesis. Madison et al. discover that intestine targeted expression of LIN28B causes intestinal hypertrophy, crypt expansion, and adenocarcinoma formation. Modulation of Let-7 levels via deletion of the mirLet7c2/mirLet7b genes recapitulated these effects, and intestine-specific Let-7 expression reversed the hypertrophy and Paneth cell depletion caused by Lin28b. These results demonstrate that Let-7 miRNAs are critical for repressing intestinal tissue growth and that LIN28B can act as an oncogene.

The RNA-binding proteins LIN28A and LIN28B have diverse functions in embryonic stem cells, cellular reprogramming, growth, and oncogenesis. Many of these effects occur via direct inhibition of Let-7 microRNAs (miRNAs), although Let-7-independent effects have been surmised. We report that intestine targeted expression of LIN28B causes intestinal hypertrophy, crypt expansion, and Paneth cell loss. Furthermore, LIN28B fosters intestinal polyp and adenocarcinoma formation. To examine potential Let-7-independent functions of LIN28B, we pursued ribonucleoprotein cross-linking, immunoprecipitation, and high-throughput sequencing (CLIP-seq) to identify direct RNA targets. This revealed that LIN28B bound a substantial number of mRNAs and modestly augmented protein levels of these target mRNAs in vivo. Conversely, Let-7 had a profound effect; modulation of Let-7 levels via deletion of the mirLet7c2/mirLet7b genes recapitulated effects of Lin28b overexpression. Furthermore, intestine-specific Let-7 expression could reverse hypertrophy and Paneth cell depletion caused by Lin28b. This was independent of effects on insulin–PI3K–mTOR signaling. Our study reveals that Let-7 miRNAs are critical for repressing intestinal tissue growth and promoting Paneth cell differentiation. Let-7-dependent effects of LIN28B may supersede Let-7-independent effects on intestinal tissue growth. In summary, LIN28B can definitively act as an oncogene in the absence of canonical genetic alterations.

MicroRNA expression profile of pulmonary artery smooth muscle cells and the effect of let-7d in chronic thromboembolic pulmonary hypertension

Abstract Chronic thromboembolic pulmonary hypertension (CTEPH) is a life-threatening condition characterized by single or recurrent pulmonary thromboemboli, which promote pulmonary vascular remodeling. MicroRNA (miRNA), is a small, noncoding RNA that is involved in multiple cell processes and functions and may participate in the pathogenesis of CTEPH. Our aims were to identify the miRNA expression signature in pulmonary artery smooth muscle cells (PASMCs) of CTEPH patients and to study the role of let-7d in CTEPH pathogenesis. The miRNA expression profile was analyzed by microarray in PASMCs of CTEPH and control patients. Differentially expressed miRNAs were selectively validated by stem-loop quantitative real-time reverse-transcription polymerase chain reaction (qRT-PCR). The role of let-7d was identified by in silico analysis, and its effect on the proliferation of PASMCs was measured by methyl thiazolyl tetrazolium (MTT). Student's unpaired t test, the Fisher exact test, and the χ(2) test were used for statistical analysis. Eighteen miRNAs were differentially expressed in PASMCs from CTEPH patients, including 12 upregulated miRNAs and 6 downregulated miRNAs; among the latter, let-7d decreased 0.58-fold in CTEPH patients, as validated by qRT-PCR. It was found that let-7d could inhibit the proliferation of PASMCs through upregulation of p21. In conclusion, PASMCs in CTEPH patients have an aberrant miRNA profile and reduced let-7d, which could promote PASMC proliferation and may be involved in the pathogenesis of CTEPH.

Long Non-Coding RNAs Embedded in the Rb and p53 Pathways

In recent years, long non-coding RNAs (lncRNAs) have gained significant attention as a novel class of gene regulators. Although a small number of lncRNAs have been shown to regulate gene expression through diverse mechanisms including transcriptional regulation, mRNA splicing and translation, the physiological function and mechanism of action of the vast majority are not known. Profiling studies in cell lines and tumor samples have suggested a potential role of lncRNAs in cancer. Indeed, distinct lncRNAs have been shown to be embedded in the p53 and Rb networks, two of the major tumor suppressor pathways that control cell cycle progression and survival. Given the fact that inactivation of Rb and p53 is a hallmark of human cancer, in this review we discuss recent evidence on the function of lncRNAs in the Rb and p53 signaling pathways.

miR-homoHSV of Singapore grouper iridovirus (SGIV) inhibits expression of the SGIV pro-apoptotic factor LITAF and attenuates cell death

Growing evidence demonstrates that various large DNA viruses could encode microRNAs (miRNAs) that regulate host and viral genes to achieve immune evasion. In this study, we report that miR-homoHSV, an miRNA encoded by Singapore grouper iridovirus (SGIV), can attenuate SGIV-induced cell death. Mechanistically, SGIV miR-homoHSV targets SGIV ORF136R, a viral gene that encodes the pro-apoptotic lipopolysaccharide-induced TNF-α (LITAF)-like factor. miR-homoHSV suppressed exogenous and endogenous SGIV LITAF expression, and thus inhibited SGIV LITAF-induced apoptosis. Meanwhile, miR-homoHSV expression was able to attenuate cell death induced by viral infection, presumably facilitating viral replication through the down-regulation of the pro-apoptotic gene SGIV LITAF. Together, our data suggest miR-homoHSV may serve as a feedback regulator of cell death during viral infection. The findings of this study provide a better understanding of SGIV replication and pathogenesis.

Rational design of microRNA-siRNA chimeras for multifunctional target suppression

MicroRNAs (miRNAs) are involved in a variety of human diseases by simultaneously suppressing many gene targets. Thus, the therapeutic value of miRNAs has been intensely studied. However, there are potential limitations with miRNA-based therapeutics such as a relatively moderate impact on gene target regulation and cellular phenotypic control. To address these issues, we proposed to design new chimeric small RNAs (aiRNAs) by incorporating sequences from both miRNAs and siRNAs. These aiRNAs not only inherited functions from natural miRNAs, but also gained new functions of gene knockdown in an siRNA-like fashion. The improved efficacy of multifunctional aiRNAs was demonstrated in our study by design and testing of an aiRNA that inherited the functions of both miR-200a and an AKT1-targeting siRNA for simultaneous suppression of cancer cell motility and proliferation. The general principles of aiRNA design were further validated by engineering new aiRNAs mimicking another miRNA, miR-9. By regulating multiple cellular functions, aiRNAs could be used as an improved tool over miRNAs to target disease-related genes, thus alleviating our dependency on a limited number of miRNAs for the development of RNAi-based therapeutics.

A p21-ZEB1 complex inhibits epithelial-mesenchymal transition through the microRNA 183-96-182 cluster

The tumor suppressor p21 acts as a cell cycle inhibitor and has also been shown to regulate gene expression by functioning as a transcription corepressor. Here, we identified p21-regulated microRNAs (miRNAs) by sequencing small RNAs from isogenic p21(+/+) and p21(-/-) cells. Three abundant miRNA clusters, miR-200b-200a-429, miR-200c-141, and miR-183-96-182, were downregulated in p21-deficient cells. Consistent with the known function of the miR-200 family and p21 in inhibition of the epithelial-mesenchymal transition (EMT), we observed EMT upon loss of p21 in multiple model systems. To explore a role of the miR-183-96-182 cluster in EMT, we identified its genome-wide targets and found that miR-183 and miR-96 repressed common targets, including SLUG, ZEB1, ITGB1, and KLF4. Reintroduction of miR-200, miR-183, or miR-96 in p21(-/-) cells inhibited EMT, cell migration, and invasion. Conversely, antagonizing miR-200 and miR-183-96-182 cluster miRNAs in p21(+/+) cells increased invasion and elevated the levels of VIM, ZEB1, and SLUG mRNAs. Furthermore, we found that p21 forms a complex with ZEB1 at the miR-183-96-182 cluster promoter to inhibit transcriptional repression of this cluster by ZEB1, suggesting a reciprocal feedback loop.

Prognostic significance of miR-194 in endometrial cancer

Endometrial cancer (EC) is the leading malignant tumor occurring in the female genital tract and some subtypes are highly invasive and metastatic. miRNAs are small non-coding RNAs that have a broad impact on cancer progression. In particular, miR-194 regulates epithelial to mesenchymal transition (EMT) by suppressing the expression of BMI-1 in EC. In this retrospective study, the clinical significance of miR-194 was investigated in archival EC specimens. We extracted total RNA from thirty-two EC samples and quantified the expression level of miR-194. We discovered that the expression level of miR-194 was significantly (P = 0.03) lower in type I EC patients with more advanced stage. In addition, patients with higher miR-194 levels have better prognosis than those with lower miR-194 levels (P = 0.0067; Cut-off value of miR-194 = 0.3). These results indicate that miR-194 has potential to serve as prognostic biomarker for EC patients.

Let-7 in cardiovascular diseases, heart development and cardiovascular differentiation from stem cells

The let-7 family is the second microRNA found in C. elegans. Recent researches have found it is highly expressed in the cardiovascular system. Studies have revealed the aberrant expression of let-7 members in cardiovascular diseases, such as heart hypertrophy, cardiac fibrosis, dilated cardiomyopathy (DCM), myocardial infarction (MI), arrhythmia, angiogenesis, atherosclerosis, and hypertension. Let-7 also participates in cardiovascular differentiation of embryonic stem cells. TLR4, LOX-1, Bcl-xl and AGO1 are by now the identified target genes of let-7. The circulating let-7b is suspected to be the biomarker of acute MI and let-7i, the biomarker of DCM. Further studies are necessary for identifying the gene targets and signaling pathways of let-7 in cardiovascular diseases. Let-7 might be a potential therapeutic target for cardiovascular diseases. This review focuses on the research progresses regarding the roles of let-7 in cardiovascular development and diseases.

The let-7/LIN-41 pathway regulates reprogramming to human induced pluripotent stem cells by controlling expression of prodifferentiation genes

Reprogramming differentiated cells into induced pluripotent stem cells (iPSCs) promotes a broad array of cellular changes. Here we show that the let-7 family of microRNAs acts as an inhibitory influence on the reprogramming process through a regulatory pathway involving prodifferentiation factors, including EGR1. Inhibiting let-7 in human cells promotes reprogramming to a comparable extent to c-MYC when combined with OCT4, SOX2, and KLF4, and persistence of let-7 inhibits reprogramming. Inhibiting let-7 during reprogramming leads to an increase in the level of the let-7 target LIN-41/TRIM71, which in turn promotes reprogramming and is important for overcoming the let-7 barrier to reprogramming. Mechanistic studies revealed that LIN-41 regulates a broad array of differentiation genes, and more specifically, inhibits translation of EGR1 through binding its cognate mRNA. Together our findings outline a let-7-based pathway that counteracts the activity of reprogramming factors through promoting the expression of prodifferentiation genes.

Functional interpretation of microRNA-mRNA association in biological systems using R

The prediction of microRNA targets is a challenging task that has given rise to several prediction algorithms. Databases of predicted targets can be used in a microRNA target enrichment analysis, enhancing our capacity to extract functional information from gene lists. However, the available tools in this field analyze gene sets one by one limiting their use in a meta-analysis. Here, we present an R system for miRNA enrichment analysis that is suitable for systems biology. These collection of R scripts and embedded data allow using predicted targets of public databases or a custom integration of them. As a proof-of-principle, we have successfully performed the challenging analysis of 2158 tumoral samples at a time. The obtained results have been summarized in a network where each cancer disease is linked to enriched miRNAs and overrepresented functions. These network connections have proven to be an invaluable resource for the study of biological and pathological causes and effects of the expression of miRNAs.

The emerging role of miR-375 in cancer

MicroRNAs (miRNAs) are evolutionarily conserved, small noncoding RNAs that are believed to play fundamental roles in various biological processes through regulation of gene expression at the level of posttranscription. MiR-375 was first identified as a pancreatic islet-specific miRNA regulating insulin secretion. However, further study revealed that miR-375 is a multifunctional miRNA participating in pancreatic islet development, glucose homeostasis, mucosal immunity, lung surfactant secretion and more importantly, tumorigenesis. Recently, miR-375 has been found significantly downregulated in multiple types of cancer, and suppresses core hallmarks of cancer by targeting several important oncogenes like AEG-1, YAP1, IGF1R and PDK1. The alteration of miR-375 in cancer is caused by a variety of mechanisms, including the dysregulation of transcription factors, aberrant promoter methylation and so on. Reduced expression of miR-375 in tissue or circulation may indicate the presence of neoplasia as well as a poor prognosis of many malignant cancers. Moreover, miR-375 stands for a promising direction for developing targeted therapies due to its capacity to inhibit tumor cell growth in vitro and in vivo. Here, we summarize the present understanding of the tumor suppressive role of miR-375 in cancer progression; the mechanisms underlying the dysregulation of miR-375; the potential use of miR-375 in prognosis and diagnosis and the therapeutic prospects of miR-375 in cancer.

ETS-1-mediated transcriptional up-regulation of CD44 is required for sphingosine-1-phosphate receptor subtype 3-stimulated chemotaxis

Sphingosine-1-phosphate (S1P)-regulated chemotaxis plays critical roles in various physiological and pathophysiological conditions. S1P-regulated chemotaxis is mediated by the S1P family of G-protein-coupled receptors. However, molecular details of the S1P-regulated chemotaxis are incompletely understood. Cultured human lung adenocarcinoma cell lines abundantly express S1P receptor subtype 3 (S1P3), thus providing a tractable in vitro system to characterize molecular mechanism(s) underlying the S1P3 receptor-regulated chemotactic response. S1P treatment enhances CD44 expression and induces membrane localization of CD44 polypeptides via the S1P3/Rho kinase (ROCK) signaling pathway. Knockdown of CD44 completely diminishes the S1P-stimulated chemotaxis. Promoter analysis suggests that the CD44 promoter contains binding sites of the ETS-1 (v-ets erythroblastosis virus E26 oncogene homolog 1) transcriptional factor. ChIP assay confirms that S1P treatment stimulates the binding of ETS-1 to the CD44 promoter region. Moreover, S1P induces the expression and nuclear translocation of ETS-1. Knockdown of S1P3 or inhibition of ROCK abrogates the S1P-induced ETS-1 expression. Furthermore, knockdown of ETS-1 inhibits the S1P-induced CD44 expression and cell migration. In addition, we showed that S1P3/ROCK signaling up-regulates ETS-1 via the activity of JNK. Collectively, we characterized a novel signaling axis, i.e., ROCK-JNK-ETS-1-CD44 pathway, which plays an essential role in the S1P3-regulated chemotactic response.

miR-888 is an expressed prostatic secretions-derived microRNA that promotes prostate cell growth and migration

MicroRNAs (MiRNAs) are a growing class of small non-coding RNAs that exhibit widespread dysregulation in prostate cancer. We profiled miRNA expression in syngeneic human prostate cancer cell lines that differed in their metastatic potential in order to determine their role in aggressive prostate cancer. miR-888 was the most differentially expressed miRNA observed in human metastatic PC3-ML cells relative to non-invasive PC3-N cells, and its levels were higher in primary prostate tumors from cancer patients, particularly those with seminal vesicle invasion. We also examined a novel miRNA-based biomarker source called expressed prostatic secretions in urine (EPS urine) for miR-888 expression and found that its levels were preferentially elevated in prostate cancer patients with high-grade disease. These expression studies indicated a correlation for miR-888 in disease progression. We next tested how miR-888 regulated cancer-related pathways in vitro using human prostate cancer cell lines. Overexpression of miR-888 increased proliferation and migration, and conversely inhibition of miR-888 activity blocked these processes. miR-888 also increased colony formation in PC3-N and LNCaP cells, supporting an oncogenic role for this miRNA in the prostate. Our data indicates that miR-888 functions to promote prostate cancer progression and can suppress protein levels of the tumor suppressor genes RBL1 and SMAD4. This miRNA holds promise as a diagnostic tool using an innovative prostatic fluid source as well as a therapeutic target for aggressive prostate cancer.

Comprehensive analysis of single nucleotide polymorphisms in human microRNAs

MicroRNAs (miRNAs) are endogenous small non-coding RNAs that repress their targets at post transcriptional level. Single Nucleotide Polymorphisms (SNPs) in miRNAs can lead to severe defects to the functions of miRNAs and might result in diseases. Although several studies have tried to identify the SNPs in human miRNA genes or only in the mature miRNAs, there are only limited endeavors to explain the distribution of SNPs in these important genes. After a genome-wide scan for SNPs in human miRNAs, we totally identified 1899 SNPs in 961 out of the 1527 reported miRNA precursors of human, which is the most complete list of SNPs in human miRNAs to date. More importantly, to explain the distributions of SNPs existed in human miRNAs, we comprehensively and systematically analyzed the identified SNPs in miRNAs from several aspects. Our results suggest that conservation, genomic context, secondary structure, and functional importance of human miRNAs affect the accumulations of SNPs in these genes. Our results also show that the number of SNPs with significantly different frequencies among various populations in the HapMap and 1000 Genome Project data are consistent with the geographical distributions of these populations. These analyses provide a better insight of SNPs in human miRNAs and the spreading of the SNPs in miRNAs in different populations.

MicroRNAs are involved in the self-renewal and differentiation of cancer stem cells

MicroRNAs (miRNAs) are small non-coding RNA molecules, whose primary function is to regulate gene expression at the post-transcriptional/translational levels. MiRNAs play crucial roles in normal biological processes and are commonly dys-regulated in human diseases. Stem cells are regarded as the "mother" cells of all types of differentiated cells that comprise tissues and organs of the body. A novel hypothesis proposes that tumors are composed of heterogeneous cells derived from cancer stem cells, which have self-renewal and differentiation capabilities similar to those of normal stem cells. Cancer stem cells have been isolated and characterized from various tumors. Given recent studies supporting the critical regulatory roles of miRNAs in the self-renewal and differentiation of cancer stem cells, better understanding the functions of miRNAs will provide invaluable insights into the prevention of tumorigenesis and tumor progression. In this review, we will summarize the research progress in the study of miRNAs involved in the self-renewal and differentiation of cancer stem cells.

Identifying cancer specific functionally relevant miRNAs from gene expression and miRNA-to-gene networks using regularized regression

Identifying microRNA signatures for the different types and subtypes of cancer can result in improved detection, characterization and understanding of cancer and move us towards more personalized treatment strategies. However, using microRNA's differential expression (tumour versus normal) to determine these signatures may lead to inaccurate predictions and low interpretability because of the noisy nature of miRNA expression data. We present a method for the selection of biologically active microRNAs using gene expression data and microRNA-to-gene interaction network. Our method is based on a linear regression with an elastic net regularization. Our simulations show that, with our method, the active miRNAs can be detected with high accuracy and our approach is robust to high levels of noise and missing information. Furthermore, our results on real datasets for glioblastoma and prostate cancer are confirmed by microRNA expression measurements. Our method leads to the selection of potentially functionally important microRNAs. The associations of some of our identified miRNAs with cancer mechanisms are already confirmed in other studies (hypoxia related hsa-mir-210 and apoptosis-related hsa-mir-296-5p). We have also identified additional miRNAs that were not previously studied in the context of cancer but are coherently predicted as active by our method and may warrant further investigation. The code is available in Matlab and R and can be downloaded on http://www.cs.toronto.edu/goldenberg/Anna_Goldenberg/Current_Research.html.

Overview of Genomic Insights into Chicken Growth Traits Based on Genome-Wide Association Study and microRNA Regulation

Over the two past decades, a significant number of studies have observed animal growth traits to examine animal genetic mechanisms due to their ease of measurement and high heritability. Chicken which has a significant impact on fundamental biology is a major source of protein worldwide, making it an ideal model for examining animal growth trait development. The genetic mechanisms of chicken growth traits have been studied using quantitative trait loci mapping through genome-scan and candidate gene approaches, genome-wide association studies (GWAS), comparative genomic strategies, microRNA (miRNA) regulation of growth development analysis, and epigenomic analysis. This review focuses on chicken GWAS and miRNA regulation of growth traits. Several recently published GWAS reports showed that most genome-wide significant single nucleotide polymorphisms are located on chromosomes 1 and 4 in chickens. Chicken growth, particularly skeletal muscle growth and development, is greatly regulated by miRNA. Using dwarf and normal chickens, let-7b was found to be involved in determining chicken dwarf phenotypes by regulating growth hormone receptor gene expression.

microRNA in the control of stem-like phenotype of cancer cells

Current therapies against metastatic tumors are still ineffective. Cancer stem cells — a small subset of cells inside the tumor that possesses a self-renewal capacity — might be responsible for the recurrence of the tumor after anti-cancer therapies. Their immortality and unique drug resistance impede their eradication during therapy. The ‘stemness’ of these cells is controlled by microRNAs. These molecules possess the ability to downregulate gene expression by binding to the target mRNA. It turns out that microRNAs control the expression of approximately 60% of the genes in human cells. MicroRNA aberrant expression can lead to cancer development and progression. Therefore, recent research has focused on unraveling the role of microRNA in maintaining a stem-like phenotype in malignant tumors and cancer stem cells. This review summarizes our current knowledge about microRNAs that control the self-renewal capacity of cancer stem cells and indicates the importance of profound research aimed at developing efficient miRNA-targeted therapies.

MicroRNAs in skeletal muscle and their regulation with exercise, ageing, and disease

Skeletal muscle makes up approximately 40% of the total body mass, providing structural support and enabling the body to maintain posture, to control motor movements and to store energy. It therefore plays a vital role in whole body metabolism. Skeletal muscle displays remarkable plasticity and is able to alter its size, structure and function in response to various stimuli; an essential quality for healthy living across the lifespan. Exercise is an important stimulator of extracellular and intracellular stress signals that promote positive adaptations in skeletal muscle. These adaptations are controlled by changes in gene transcription and protein translation, with many of these molecules identified as potential therapeutic targets to pharmacologically improve muscle quality in patient groups too ill to exercise. MicroRNAs (miRNAs) are recently identified regulators of numerous gene networks and pathways and mainly exert their effect by binding to their target messenger RNAs (mRNAs), resulting in mRNA degradation or preventing protein translation. The role of exercise as a regulatory stimulus of skeletal muscle miRNAs is now starting to be investigated. This review highlights our current understanding of the regulation of skeletal muscle miRNAs with exercise and disease as well as how they may control skeletal muscle health.

MicroRNAs as Molecular Targets for Cancer Therapy: On the Modulation of MicroRNA Expression

The discovery of small RNA molecules with the capacity to regulate messenger RNA (mRNA) stability and translation (and consequently protein synthesis) has revealed an additional level of post-transcriptional gene control. MicroRNAs (miRNAs), an evolutionarily conserved class of small noncoding RNAs that regulate gene expression post-transcriptionally by base pairing to complementary sequences in the 3' untranslated regions of target mRNAs, are part of this modulatory RNA network playing a pivotal role in cell fate. Functional studies indicate that miRNAs are involved in the regulation of almost every biological pathway, while changes in miRNA expression are associated with several human pathologies, including cancer. By targeting oncogenes and tumor suppressors, miRNAs have the ability to modulate key cellular processes that define the cell phenotype, making them highly promising therapeutic targets. Over the last few years, miRNA-based anti-cancer therapeutic approaches have been exploited, either alone or in combination with standard targeted therapies, aiming at enhancing tumor cell killing and, ideally, promoting tumor regression and disease remission. Here we provide an overview on the involvement of miRNAs in cancer pathology, emphasizing the mechanisms of miRNA regulation. Strategies for modulating miRNA expression are presented and illustrated with representative examples of their application in a therapeutic context.

Integrated genomic analysis of triple-negative breast cancers reveals novel microRNAs associated with clinical and molecular phenotypes and sheds light on the pathways they control

Background

This study focuses on the analysis of miRNAs expression data in a cohort of 181 well characterised breast cancer samples composed primarily of triple-negative (ER/PR/HER2-negative) tumours with associated genome-wide DNA and mRNA data, extensive patient follow-up and pathological information.

Results

We identified 7 miRNAs associated with prognosis in the triple-negative tumours and an additional 7 when the analysis was extended to the set of all ER-negative cases. miRNAs linked to an unfavourable prognosis were associated with a broad spectrum of motility mechanisms involved in the invasion of stromal tissues, such as cell-adhesion, growth factor-mediated signalling pathways, interaction with the extracellular matrix and cytoskeleton remodelling. When we compared different intrinsic molecular subtypes we found 46 miRNAs that were specifically expressed in one or more intrinsic subtypes. Integrated genomic analyses indicated these miRNAs to be influenced by DNA genomic aberrations and to have an overall influence on the expression levels of their predicted targets. Among others, our analyses highlighted the role of miR-17-92 and miR-106b-25, two polycistronic miRNA clusters with known oncogenic functions. We showed that their basal-like subtype specific up-regulation is influenced by increased DNA copy number and contributes to the transcriptional phenotype as well as the activation of oncogenic pathways in basal-like tumours.

Conclusions

This study analyses previously unreported miRNA, mRNA and DNA data and integrates these with pathological and clinical information, from a well-annotated cohort of breast cancers enriched for triple-negative subtypes. It provides a conceptual framework, as well as integrative methods and system-level results and contributes to elucidate the role of miRNAs as biomarkers and modulators of oncogenic processes in these types of tumours.

Lung cancer epigenetics: emerging biomarkers

Lung cancer is the leading cause of cancer-related deaths worldwide, and the 5-year survival rate is still very poor due to the scarcity of effective tools for early detection. The discovery of highly sensitive and specific biomarkers highlighting pathological changes early enough to allow clinical intervention is therefore of great importance. In the last decade, epigenetics and particularly research on DNA methylation have provided important information towards a better understanding of lung cancer pathogenesis. Novel and promising molecular biomarkers for diagnosis and prognosis of lung cancer are continuously emerging in this area, requiring further evaluation. This process includes extensive validation in prospective clinical trials before they can be routinely used in a clinical setting. This review summarizes the evidence on epigenetic biomarkers for lung cancer, focusing on DNA methylation.

TP53 and let-7a micro-RNA regulate K-Ras activity in HCT116 colorectal cancer cells

Recent reports have indicated that KRAS and TP53 mutations predict response to therapy in colorectal cancer. However, little is known about the relationship between these two common genetic alterations. Micro-RNAs (miRNAs), a class of noncoding RNA implicated in cellular processes, have been increasingly linked to KRAS and TP53. We hypothesized that lethal-7a (let-7a) miRNA regulates KRAS through TP53. To investigate the relationship between KRAS, TP53, and let-7a, we used HCT116 KRAS^mut human colorectal cancer cells with four different genotypic modifications in TP53 (TP53^−/−, TP53^+/−, TP53^mut/+, and TP53^mut/−). Using these cells we observed that K-Ras activity was higher in cells with mutant or knocked out TP53 alleles, suggesting that wild-type TP53 may suppress K-Ras activity. Let-7a was present in HCT116 KRAS^mut cells, though there was no correlation between let-7a level and TP53 genotype status. To explore how let-7a may regulate K-Ras in the different TP53 genotype cells we used let-7a inhibitor and demonstrated increased K-Ras activity across all TP53, thus corroborating prior reports that let-7a regulates K-Ras. To assess potential clinical implications of this regulatory network, we examined the influence of TP53 genotype and let-7a inhibition on colon cancer cell survival following chemoradiation therapy (CRT). We observed that cells with complete loss of wild-type TP53 alleles (^−/− or ^−/mut) were resistant to CRT following treatment with 5-fluorouracil and radiation. Further increase in K-Ras activity with let-7a inhibition did not impact survival in these cells. In contrast, cells with single or double wild-type TP53 alleles were moderately responsive to CRT and exhibited resistance when let-7a was inhibited. In summary, our results show a complex regulatory system involving TP53, KRAS, and let-7a. Our results may provide clues to understand and target these interactions in colorectal cancer.

MicroRNA-126-mediated control of cell fate in B-cell myeloid progenitors as a potential alternative to transcriptional factors

Lineage specification is thought to be largely regulated at the level of transcription, where lineage-specific transcription factors drive specific cell fates. MicroRNAs (miR), vital to many cell functions, act posttranscriptionally to decrease the expression of target mRNAs. MLL-AF4 acute lymphocytic leukemia exhibits both myeloid and B-cell surface markers, suggesting that the transformed cells are B-cell myeloid progenitor cells. Through gain- and loss-of-function experiments, we demonstrated that microRNA 126 (miR-126) drives B-cell myeloid biphenotypic leukemia differentiation toward B cells without changing expression of E2A immunoglobulin enhancer-binding factor E12/E47 (E2A), early B-cell factor 1 (EBF1), or paired box protein 5, which are critical transcription factors in B-lymphopoiesis. Similar induction of B-cell differentiation by miR-126 was observed in normal hematopoietic cells in vitro and in vivo in uncommitted murine c-Kit(+)Sca1(+)Lineage(-) cells, with insulin regulatory subunit-1 acting as a target of miR-126. Importantly, in EBF1-deficient hematopoietic progenitor cells, which fail to differentiate into B cells, miR-126 significantly up-regulated B220, and induced the expression of B-cell genes, including recombination activating genes-1/2 and CD79a/b. These data suggest that miR-126 can at least partly rescue B-cell development independently of EBF1. These experiments show that miR-126 regulates myeloid vs. B-cell fate through an alternative machinery, establishing the critical role of miRNAs in the lineage specification of multipotent mammalian cells.

Cell cycle regulation by long non-coding RNAs

The mammalian cell cycle is precisely controlled by cyclin-dependent kinases (CDKs) and related pathways such as the RB and p53 pathways. Recent research on long non-coding RNAs (lncRNAs) indicates that many lncRNAs are involved in the regulation of critical cell cycle regulators such as the cyclins, CDKs, CDK inhibitors, pRB, and p53. These lncRNAs act as epigenetic regulators, transcription factor regulators, post-transcription regulators, and protein scaffolds. These cell cycle-regulated lncRNAs mainly control cellular levels of cell cycle regulators via various mechanisms, and may provide diversity and reliability to the general cell cycle. Interestingly, several lncRNAs are induced by DNA damage and participate in cell cycle arrest or induction of apoptosis as DNA damage responses. Therefore, deregulations of these cell cycle regulatory lncRNAs may be involved in tumorigenesis, and they are novel candidate molecular targets for cancer therapy and diagnosis.

SOX2-LIN28/let-7 pathway regulates proliferation and neurogenesis in neural precursors

The transcription factor SRY (sex-determining region)-box 2 (SOX2) is an important functional marker of neural precursor cells (NPCs) and plays a critical role in self-renewal and neuronal differentiation; however, the molecular mechanisms underlying its functions are poorly understood. Using human embryonic stem cell-derived NPCs to model neurogenesis, we found that SOX2 is required to maintain optimal levels of LIN28, a well-characterized suppressor of let-7 microRNA biogenesis. Exogenous LIN28 expression rescued the NPC proliferation deficit, as well as the early but not the late stages of the neurogenic deficit associated with the loss of SOX2. We found that SOX2 binds to a proximal site in the LIN28 promoter region and regulates LIN28 promoter acetylation, likely through interactions with the histone acetyltransferase complex. Misexpression of let-7 microRNAs in NPCs reduced proliferation and inhibited neuronal differentiation, phenocopying the loss of SOX2. In particular, we identified let-7i as a novel and potent inhibitor of neuronal differentiation that targets MASH1 and NGN1, two well-characterized proneural genes. In conclusion, we discovered the SOX2-LIN28/let-7 pathway as a unique molecular mechanism governing NPC proliferation and neurogenic potential.

Maternal imprinting at the H19-Igf2 locus maintains adult haematopoietic stem cell quiescence

The epigenetic regulation of imprinted genes by monoallelic DNA methylation of either maternal or paternal alleles is critical for embryonic growth and development. Imprinted genes were recently shown to be expressed in mammalian adult stem cells to support self-renewal of neural and lung stem cells; however, a role for imprinting per se in adult stem cells remains elusive. Here we show upregulation of growth-restricting imprinted genes, including in the H19-Igf2 locus, in long-term haematopoietic stem cells and their downregulation upon haematopoietic stem cell activation and proliferation. A differentially methylated region upstream of H19 (H19-DMR), serving as the imprinting control region, determines the reciprocal expression of H19 from the maternal allele and Igf2 from the paternal allele. In addition, H19 serves as a source of miR-675, which restricts Igf1r expression. We demonstrate that conditional deletion of the maternal but not the paternal H19-DMR reduces adult haematopoietic stem cell quiescence, a state required for long-term maintenance of haematopoietic stem cells, and compromises haematopoietic stem cell function. Maternal-specific H19-DMR deletion results in activation of the Igf2-Igfr1 pathway, as shown by the translocation of phosphorylated FoxO3 (an inactive form) from nucleus to cytoplasm and the release of FoxO3-mediated cell cycle arrest, thus leading to increased activation, proliferation and eventual exhaustion of haematopoietic stem cells. Mechanistically, maternal-specific H19-DMR deletion leads to Igf2 upregulation and increased translation of Igf1r, which is normally suppressed by H19-derived miR-675. Similarly, genetic inactivation of Igf1r partly rescues the H19-DMR deletion phenotype. Our work establishes a new role for this unique form of epigenetic control at the H19-Igf2 locus in maintaining adult stem cells.

The role of microRNAs in breast cancer stem cells

The concept of the existence of a subset of cancer cells with stem cell-like properties, which are thought to play a significant role in tumor formation, metastasis, resistance to anticancer therapies and cancer recurrence, has gained tremendous attraction within the last decade. These cancer stem cells (CSCs) are relatively rare and have been described by different molecular markers and cellular features in different types of cancers. Ten years ago, a novel class of molecules, small non-protein-coding RNAs, was found to be involved in carcinogenesis. These small RNAs, which are called microRNAs (miRNAs), act as endogenous suppressors of gene expression that exert their effect by binding to the 3′-untranslated region (UTR) of large target messenger RNAs (mRNAs). MicroRNAs trigger either translational repression or mRNA cleavage of target mRNAs. Some studies have shown that putative breast cancer stem cells (BCSCs) exhibit a distinct miRNA expression profile compared to non-tumorigenic breast cancer cells. The deregulated miRNAs may contribute to carcinogenesis and self-renewal of BCSCs via several different pathways and can act either as oncomirs or as tumor suppressive miRNAs. It has also been demonstrated that certain miRNAs play an essential role in regulating the stem cell-like phenotype of BCSCs. Some miRNAs control clonal expansion or maintain the self-renewal and anti-apoptotic features of BCSCs. Others are targeting the specific mRNA of their target genes and thereby contribute to the formation and self-renewal process of BCSCs. Several miRNAs are involved in epithelial to mesenchymal transition, which is often implicated in the process of formation of CSCs. Other miRNAs were shown to be involved in the increased chemotherapeutic resistance of BCSCs. This review highlights the recent findings and crucial role of miRNAs in the maintenance, growth and behavior of BCSCs, thus indicating the potential for novel diagnostic, prognostic and therapeutic miRNA-based strategies.

Let-7c inhibits A549 cell proliferation through oncogenic TRIB2 related factors

MicroRNAs have tumor suppressive or oncogenic roles in carcinogenesis. This study aimed to investigate the mechanism of let-7c in suppressing lung cancer cell proliferation. First, let-7c was revealed to be able to inhibit lung adenocarcinoma cell proliferation significantly. TRIB2 was further demonstrated to be a novel target and negatively regulated by let-7c. As downstream signals of TRIB2, the activities of C/EBP-α and phosphorylated p38MAPK were increased obviously in let-7c-treated cells compared with controls. Our results demonstrate that, through regulating the expression of TRIB2 and its downstream factors, let-7c can effectively inhibit A549 cell proliferation in vitro and in vivo.

MicroRNA-125b inhibitor sensitizes human primary glioblastoma cells to chemotherapeutic drug temozolomide on invasion

Malignant gliomas are treated with a combination of surgery, radiation, and temozolomide (TMZ), but these therapies ultimately fail due to tumor recurrence. In this study, we aimed to identify the combined effects of miR-125b and TMZ involved in the invasive pathogenesis of glioblastoma cells. The effects of miR-125b and TMZ on cell invasion were analyzed by Transwell assays. Unexpectedly, either overexpression or downregulation of miR-125b has no function on glioblastoma cell invasion. However, knockdown of miR-125b could enhance the effects of TMZ on glioblastoma cell invasion. Conversely, overexpression of miR-125b could decrease such effects of TMZ. Further research on the mechanism demonstrated that such function of miR-125b knockdown on enhancing the effects of TMZ was involved in downregulation of Notch1. Notch1 was overexpressed in glioblastoma cells, and found by us that downregulation of Notch1 expression decreased the cell invasion of glioblastoma cells. Knockdown of miR-125b combined with TMZ enhancely downregulated Notch1 and inhibited cell invasion of malignant glioblastoma. These findings indicate that the combination of miR-125b inhibitor and TMZ treatment could effectively inhibit the glioblastoma cell invasion by inhibiting Notch1 expression.

miR-294/miR-302 promotes proliferation, suppresses G1-S restriction point, and inhibits ESC differentiation through separable mechanisms

The miR-294 and miR-302 microRNAs promote the abbreviated G1 phase of the embryonic stem cell (ESC) cell cycle and suppress differentiation induced by let-7. Here, we evaluated the role of the retinoblastoma (Rb) family proteins in these settings. Under normal growth conditions, miR-294 promoted the rapid G1-S transition independent of the Rb family. In contrast, miR-294 suppressed the further accumulation of cells in G1 in response to nutrient deprivation and cell-cell contact in an Rb-dependent fashion. We uncovered five additional miRNAs (miR-26a, miR-99b, miR-193, miR-199a-5p, and miR-218) that silenced ESC self-renewal in the absence of other miRNAs, all of which were antagonized by miR-294 and miR-302. Four of the six differentiation-inducing miRNAs induced an Rb-dependent G1 accumulation. However, all six still silenced self-renewal in the absence of the Rb proteins. These results show that the miR-294/miR-302 family acts through Rb-dependent and -independent pathways to regulate the G1 restriction point and the silencing of self-renewal, respectively.

Let-7g and miR-21 expression in non-small cell lung cancer: correlation with clinicopathological and molecular features

MicroRNAs (miRNAs) play a key role in cancer pathogenesis and are involved in several human cancers, including non-small cell lung cancer (NSCLC). This study evaluated Let-7g and miR-21 expression by quantitative real-time PCR in 80 NSCLC patients and correlated the results with their main clinicopathological and molecular features. MiR-21 expression was significantly higher in NSCLC tissues compared to non-cancer lung tissues (p<0.0001), while no significant changes in Let-7g expression were observed between the tumor and normal lung tissues. Target prediction analysis led to the identification of 26 miR-21 and 24 Let-7g putative target genes that play important roles in cancer pathogenesis and progression. No significant association was observed between the analysed miRNAs and the main clinicopathological or molecular characteristics of the NSCLC patients, although both miRNAs were downregulated in squamous cell carcinomas compared to adenocarcinomas. Noteworthy, we observed a significant association between low Let-7g expression and metastatic lymph nodes at diagnosis (p=0.046), as well as between high miR-21 expression and K-Ras mutations (p=0.0003). Survival analysis did not show any significant correlation between prognosis and the analysed miRNAs, although the patients with a high Let-7g and miR-21 expression showed a significantly lower short-term progression-free survival (p=0.01 and p=0.0003, respectively) and overall survival (p=0.023 and p=0.0045, respectively). In conclusion, we showed that Let-7g and miR-21 expression was deregulated in NSCLC and we demonstrated a strong relationship between miR-21 overexpression and K-Ras mutations. Our data indicate that Let-7g and miR-21 profiling combined with the determination of K-Ras mutational status may be considered a useful biomarker for a more effective molecular characterization and clinical management of NSCLC patients.

Let-7 represses Nr6a1 and a mid-gestation developmental program in adult fibroblasts

MicroRNAs (miRNAs) are critical to proliferation, differentiation, and development. Here, we characterize gene expression in murine Dicer-null adult mesenchymal stem cell lines, a fibroblast cell type. Loss of Dicer leads to derepression of let-7 targets at levels that exceed 10-fold to 100-fold with increases in transcription. Direct and indirect targets of this miRNA belong to a mid-gestation embryonic program that encompasses known oncofetal genes as well as oncogenes not previously associated with an embryonic state. Surprisingly, this mid-gestation program represents a distinct period that occurs between the pluripotent state of the inner cell mass at embryonic day 3.5 (E3.5) and the induction of let-7 upon differentiation at E10.5. Within this mid-gestation program, we characterize the let-7 target Nr6a1, an embryonic transcriptional repressor that regulates gene expression in adult fibroblasts following miRNA loss. In total, let-7 is required for the continual suppression of embryonic gene expression in adult cells, a mechanism that may underlie its tumor-suppressive function.

Identification of Host Kinase Genes Required for Influenza Virus Replication and the Regulatory Role of MicroRNAs

Human protein kinases (HPKs) have profound effects on cellular responses. To better understand the role of HPKs and the signaling networks that influence influenza virus replication, a small interfering RNA (siRNA) screen of 720 HPKs was performed. From the screen, 17 HPKs (NPR2, MAP3K1, DYRK3, EPHA6, TPK1, PDK2, EXOSC10, NEK8, PLK4, SGK3, NEK3, PANK4, ITPKB, CDC2L5 (CDK13), CALM2, PKN3, and HK2) were validated as essential for A/WSN/33 influenza virus replication, and 6 HPKs (CDK13, HK2, NEK8, PANK4, PLK4 and SGK3) were identified as vital for both A/WSN/33 and A/New Caledonia/20/99 influenza virus replication. These HPKs were found to affect multiple host pathways and regulated by miRNAs induced during infection. Using a panel of miRNA agonists and antagonists, miR-149* was found to regulate NEK8 expression, miR-548d-3p was found to regulate MAPK1 transcript expression, and miRs -1228 and -138 to regulate CDK13 expression. Up-regulation of miR-34c induced PLK4 transcript and protein expression and enhanced influenza virus replication, while miR-34c inhibition reduced viral replication. These findings identify HPKs important for influenza viral replication and show the miRNAs that govern their expression.

Epigenetic Therapy in Lung Cancer - Role of microRNAs

Lung cancer is the leading cause of cancer deaths worldwide. microRNAs (miRNAs) are a class of small non-coding RNA species that have been implicated in the control of many fundamental cellular and physiological processes such as cellular differentiation, proliferation, apoptosis, and stem cell maintenance. Some miRNAs have been categorized as "oncomiRs" as opposed to "tumor suppressor miRs." This review focuses on the role of miRNAs in the lung cancer carcinogenesis and their potential as diagnostic, prognostic, or predictive markers.

Possible carcinogenesis of tumor suppressor let-7

Cancer stem cells (CSCs), a group showing high capacities of sphere-forming and self renewal, are blamed for tumor initiation, recurrence and therapy resistance. Therefore, therapeutics specifically targeting and perishing CSCs may be promising. Let-7 miRNAs, one of the earliest discovered miRNAs, were considered as novel and vital agents to eliminate cancer and CSCs. However, in recent researches, many regulatory loops among let-7 and its targeted genes were noticed; the regulation of let-7 caused by its hunting mRNAs helped to form the hypotheses that hunters and preys may swap their roles when in confrontations. Besides, the evil side of let-7 was discovered occasionally, therefore, we hypothesize that dual characteristics of let-7 do exist, which will have significant impacts on anticancer research. Targeted therapies against cancer and CSCs by using let-7 or other miRNAs as weapons should be thought twice before clinical application.

LIN28 Expression in malignant germ cell tumors downregulates let-7 and increases oncogene levels

Despite their clinicopathologic heterogeneity, malignant germ cell tumors (GCT) share molecular abnormalities that are likely to be functionally important. In this study, we investigated the potential significance of downregulation of the let-7 family of tumor suppressor microRNAs in malignant GCTs. Microarray results from pediatric and adult samples (n = 45) showed that LIN28, the negative regulator of let-7 biogenesis, was abundant in malignant GCTs, regardless of patient age, tumor site, or histologic subtype. Indeed, a strong negative correlation existed between LIN28 and let-7 levels in specimens with matched datasets. Low let-7 levels were biologically significant, as the sequence complementary to the 2 to 7 nt common let-7 seed "GAGGUA" was enriched in the 3' untranslated regions of mRNAs upregulated in pediatric and adult malignant GCTs, compared with normal gonads (a mixture of germ cells and somatic cells). We identified 27 mRNA targets of let-7 that were upregulated in malignant GCT cells, confirming significant negative correlations with let-7 levels. Among 16 mRNAs examined in a largely independent set of specimens by quantitative reverse transcription PCR, we defined negative-associations with let-7e levels for six oncogenes, including MYCN, AURKB, CCNF, RRM2, MKI67, and C12orf5 (when including normal control tissues). Importantly, LIN28 depletion in malignant GCT cells restored let-7 levels and repressed all of these oncogenic let-7 mRNA targets, with LIN28 levels correlating with cell proliferation and MYCN levels. Conversely, ectopic expression of let-7e was sufficient to reduce proliferation and downregulate MYCN, AURKB, and LIN28, the latter via a double-negative feedback loop. We conclude that the LIN28/let-7 pathway has a critical pathobiologic role in malignant GCTs and therefore offers a promising target for therapeutic intervention.