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

MicroRNA and Cancer: Tiny Molecules with Major Implications

Cancer is currently a major public health problem and, as such, emerging research is making significant progress in identifying major players in its biology. One recent topic of interest involves microRNAs (miRNAs) which are small, non-coding RNA molecules that inhibit gene expression post-transcriptionally. They accomplish this by binding to the 3’ untranslated region (3’UTR) of target messengerRNA (mRNA), resulting in either their degradation or inhibition of translation, depending on the degree of complementary base pairing. They are transcribed by RNA polymerase II and are formed into mature miRNAs via two steps, each catalyzed by a different ribonuclease III (RNaseIII). Cross-species comparisons demonstrate that miRNAs are evolutionarily conserved and play important roles in a wide array of normal biological processes. Importantly, aberrant miRNA expression is correlated with human disease, especially in the development of cancer. Recent research has identified targets and functions of miRNAs, illustrating that some are oncogenic in nature while others show tumor suppressor activity. The miRNAs have also been characterized as having high potential in the clinical arena and, as such, have been a target for exploitation toward cancer therapy. Not only has it been shown that miRNA expression profiles may prove useful as diagnostic and prognostic markers in cancer, various miRNA-based therapies show promise as well. It is anticipated that further research will elucidate the benefits of using miRNAs as clinical agents in the battle against cancer and other chronic diseases.

Prognostic significance of miR-215 in colon cancer

BACKGROUND

We have previously shown that miR-215 suppressed the expression of key targets such as thymidylate synthase (TS), dihydrofolate reductase, and denticleless protein homolog (DTL) in colon cancer. miR-215 is a tumor suppressor candidate due to the upregulation of p53 and p21 by targeting DTL. However, high levels of miR-215 conferred chemoresistance due to cell cycle arrest and reduced cell proliferation by suppressing DTL. In this study, the clinical significance of miR-215 was further investigated as a potential prognostic biomarker in colon cancer patients.

METHODS

Total RNAs were extracted from 34 paired normal and colon (stage II and III) tumor specimens using the Trizol-based approach. The levels of miR-215 and a closely related miR-192 were quantified using quantitative real-time polymerase chain reaction (qRT-PCR) expression analysis. The expression of DTL mRNA and protein were quantified by real time qRT-PCR and immunohistochemistry.

RESULTS

The expression levels of miR-192 (P = .0008) and miR-215 (P < .0001) were significantly decreased in colon tumors compared with normal tissues. DTL was significantly over-expressed and was inversely correlated with miR-215, further suggesting an in vivo physiologic relevance of miR-215 mediated DTL suppression. Kaplan-Meier survival analysis by Cox regression revealed that high levels of miR-215 expression (hazard ratio, 3.516; 95% confidence interval, 1.007-12.28, P = .025) are closely associated with poor patient's overall survival. Furthermore, an elevated expression of a miR-215 target protein DTL was detected in colon cancer tissues whereas no expression was present in normal tissues.

CONCLUSION

miR-215 has a unique potential as a prognostic biomarker in stage II and III colon cancer.

MicroRNAs in Development and Disease

MicroRNAs (miRNAs) are a class of posttranscriptional regulators that have recently introduced an additional level of intricacy to our understanding of gene regulation. There are currently over 10,000 miRNAs that have been identified in a range of species including metazoa, mycetozoa, viridiplantae, and viruses, of which 940, to date, are found in humans. It is estimated that more than 60% of human protein-coding genes harbor miRNA target sites in their 3′ untranslated region and, thus, are potentially regulated by these molecules in health and disease. This review will first briefly describe the discovery, structure, and mode of function of miRNAs in mammalian cells, before elaborating on their roles and significance during development and pathogenesis in the various mammalian organs, while attempting to reconcile their functions with our existing knowledge of their targets. Finally, we will summarize some of the advances made in utilizing miRNAs in therapeutics.

MicroRNA delivery by cationic lipoplexes for lung cancer therapy

Lung cancer is the leading cause of cancer deaths in western countries and carries a poor overall five year survival rate. Several studies demonstrate that microRNAs (miRNAs or miRs) are actively involved in tumor development by serving as tumor suppressors, oncogenes or both. In lung cancer, miRNAs may serve as both diagnostic and prognostic biomarkers as well as regulate in vitro and in vivo tumor progression. However, miRNA-based therapy is faced with several challenges including lack of tissue specificity, lack of optimal delivery systems, poor cellular uptake and risk of systemic toxicity. Here, we report a cationic lipid based miRNA delivery system to address some of these challenges. Among many lung cancer related miRNAs, miR-133b, a tumor suppressor, was selected as a therapeutic target because it directly targets the prosurvival gene MCL-1 thus regulating cell survival and sensitivity of lung cancer cells to chemotherapeutic agents. The efficacy of pre-miR-133b containing lipoplexes was evaluated in A549 non-small cell lung cancer (NSCLC) cells. Compared with siPORT NeoFX transfection agent, lipoplexes delivered pre-miR-133b in a more efficient manner with ~2.3-fold increase in mature miR-133b expression and ~1.8-fold difference in MCL-1 protein downregulation in vitro. In the in vivo biodistribution study, lipoplexes achieved ~30% accumulation in lung tissue, which was ~50-fold higher than siPORT NeoFX transfection agent. Mice treated with pre-miR-133b containing lipoplexes had mature miR-133b expression in lung ~52-fold higher than untreated mice. Our results demonstrated that cationic lipoplexes are a promising carrier system for the development of miRNA-based therapeutics in lung cancer treatment.

Epigenetic markers for chemosensitivity and chemoresistance in pancreatic cancer--a review

Adjuvant first-line gemcitabine monochemotherapy presents a standard treatment for patients with advanced pancreatic adenocarcinoma and improves overall survival in chemosensitive patients. Nonetheless, 6-month progression-free survival remains below 15%, despite interdisciplinary approaches. The success of gemcitabine treatment is disappointing and-in the absence of reliable tumor markers--challenging to quantify. Epigenetic alterations have been recently identified to take on important roles in cancer development and possibly cancer treatment. In this context, microRNAs are becoming increasingly acknowledged as useful biomarkers for classifying cancers and providing information on their chemo- and radiosensitivity. This review illustrates the potential of genetic and epigenetic markers in the prediction of chemosensitivity in pancreatic cancer patients and in the monitoring of their response rates to adjuvant therapy.

Effects of KRAS mutation and polymorphism on the risk and prognosis of oral squamous cell carcinoma

BACKGROUND

Mutations or single nucleotide polymorphism (SNP) of relevant genes may affect the risk and prognosis of malignancies. The purpose of this study was to analyze whether the KRAS polymorphisms and mutations can be useful prognostic or risk markers in oral squamous cell carcinoma (OSCC).

METHODS

DNA was extracted from tumor tissues of 47 patients with OSCC and blood cells of 84 normal controls and subjected to sequencing for the KRAS.

RESULTS

No mutation in the KRAS was found in 47 OSCC samples. However, 2 polymorphisms (rs1137282 and rs712) were detected. Individuals with KRAS SNP rs712 genotypes of G/T or T/T have a reduced risk for OSCC than those with genotype G/G (hazard ratio [HR], 0.26; 95% confidence interval [CI], 0.10-0.60; p = .004). The overall survival between different SNPs were not statistically significant (p = .147 for rs1137282 and p = .202 for rs712).

CONCLUSION

These data demonstrate a role for rs712 polymorphism of the KRAS in susceptibility of OSCC.

Tumour-suppressor microRNAs let-7 and mir-101 target the proto-oncogene MYCN and inhibit cell proliferation in MYCN-amplified neuroblastoma

Background:

MicroRNAs (miRNAs) regulate expression of many cancer-related genes through posttranscriptional repression of their mRNAs. In this study we investigate the proto-oncogene MYCN as a target for miRNA regulation.

Methods:

A luciferase reporter assay was used to investigate software-predicted miRNA target sites in the 3′-untranslated region (3′UTR) of MYCN. The miRNAs were overexpressed in cell lines by transfection of miRNA mimics or miRNA-expressing plasmids. Mutation of the target sites was used to validate MYCN 3′UTR as a direct target of several miRNAs. To measure miRNA-mediated suppression of endogenous N-myc protein, inhibition of proliferation and inhibition of clonogenic growth, miRNAs were overexpressed in a MYCN-amplified neuroblastoma cell line.

Results:

The results from this study show that MYCN is targeted by several miRNAs. In addition to the previously shown mir-34a/c, we experimentally validate mir-449, mir-19a/b, mir-29a/b/c, mir-101 and let-7e/mir-202 as direct MYCN-targeting miRNAs. These miRNAs were able to suppress endogenous N-myc protein in a MYCN-amplified neuroblastoma cell line. The let-7e and mir-202 were strong negative regulators of MYCN expression. The mir-101 and the let-7 family miRNAs let-7e and mir-202 inhibited proliferation and clonogenic growth when overexpressed in Kelly cells.

Conclusion:

The tumour-suppressor miRNAs let-7 and mir-101 target MYCN and inhibit proliferation and clonogenic growth of MYCN-amplified neuroblastoma cells.

MicroRNA-372 is down-regulated and targets cyclin-dependent kinase 2 (CDK2) and cyclin A1 in human cervical cancer, which may contribute to tumorigenesis

MicroRNAs are a class of noncoding RNAs that are ~22 nucleotides in length. MicroRNAs have been shown to play important roles in cell differentiation and in cancer. Recently, studies have shown that miR-372 is tumorigenic in human reproductive system cancers. However, we provide evidence that miR-372 acts as a tumor suppressor gene in cervical carcinoma. miR-372 was found down-regulated in cervical carcinoma tissues as compared with adjacent normal cervical tissues. Growth curve and FACS assays indicated that ectopic expression of miR-372 suppressed cell growth and induced arrest in the S/G₂ phases of cell cycle in HeLa cells. We used bioinformatic predictions to determine that CDK2 and cyclin A1 were possible targets of miR-372 and confirmed this prediction using a fluorescent reporter assay. Taken together, these findings indicate that an anti-oncogenic role of miR-372 may be through control of cell growth and cell cycle progression by down-regulating the cell cycle genes CDK2 and cyclin A1.

Regeneration of Hair Cells: Making Sense of All the Noise

Hearing loss affects hundreds of millions of people worldwide by dampening or cutting off their auditory connection to the world. Current treatments for sensorineural hearing loss (SNHL) with cochlear implants are not perfect, leaving regenerative medicine as the logical avenue to a perfect cure. Multiple routes to regeneration of damaged hair cells have been proposed and are actively pursued. Each route not only requires a keen understanding of the molecular basis of ear development but also faces the practical limitations of stem cell regulation in the delicate inner ear where topology of cell distribution is essential. Improvements in our molecular understanding of the minimal essential genes necessary for hair cell formation and recent advances in stem cell manipulation, such as seen with inducible pluripotent stem cells (iPSCs) and epidermal neural crest stem cells (EPI-NCSCs), have opened new possibilities to advance research in translational stem cell therapies for individuals with hearing loss. Despite this, more detailed network maps of gene expression are needed, including an appreciation for the roles of microRNAs (miRs), key regulators of transcriptional gene networks. To harness the true potential of stem cells for hair cell regeneration, basic science and clinical medicine must work together to expedite the transition from bench to bedside by elucidating the full mechanisms of inner ear hair cell development, including a focus on the role of miRs, and adapting this knowledge safely and efficiently to stem cell technologies.

LIN28B fosters colon cancer migration, invasion and transformation through let-7-dependent and -independent mechanisms

Lin28b is an RNA-binding protein that inhibits biogenesis of let-7 microRNAs. LIN28B is overexpressed in diverse cancers, yet a specific role in the molecular pathogenesis of colon cancer has yet to be elucidated. We have determined that human colon tumors exhibit decreased levels of mature let-7 isoforms and increased expression of LIN28B. In order to determine LIN28B's mechanistic role in colon cancer, we expressed LIN28B in immortalized colonic epithelial cells and human colon cancer cell lines. We found that LIN28B promotes cell migration, invasion, and transforms immortalized colonic epithelial cells. In addition, constitutive LIN28B expression increases expression of intestinal stem cell markers LGR5 and PROM1 in the presence of let-7 restoration. This may occur as a result of Lin28b protein binding LGR5 and PROM1 mRNA, suggesting that a subset of LIN28B functions are independent of its ability to repress let-7. Our findings establish a new role for LIN28B in human colon cancer pathogenesis, and suggest LIN28B post-transcriptionally regulates LGR5 and PROM1 through a let-7 independent mechanism.

Fifth Educational Symposium of the Spanish Lung Cancer Group: report on the Molecular Biology Workshop

The majority of non-small-cell lung cancer (NSCLC) patients present with locally advanced (35%) or metastatic disease (40%); in this setting, it is of the utmost importance to balance efficacy with toxicity. However, with platinum combinations, survival has reached a "plateau", with median overall survival times of a mere 10-12 months, making it mandatory to search for new strategies and to identify more effective treatment. Molecular characteristics can be more informative than clinical features in predicting clinical benefit, and the identification of molecular markers can help define subgroups of patients who are likely to respond to different treatments, thus avoiding unnecessary toxicities and costs and providing the maximum benefit to each patient. Here we review research on biomarker assessment that was presented during the Molecular Biology Workshop held in Palma de Mallorca on 25 November 2010, during the Fifth Educational Symposium of the Spanish Lung Cancer Group.

MicroRNA miR-335 is crucial for the BRCA1 regulatory cascade in breast cancer development

The expression of microRNAs is altered in various cancer types, leading to their definition as onco- and tumor-suppressor microRNAs. In our study, we investigated the role of miR-335 in the formation of sporadic human breast cancer and its involvement in the regulatory network of the breast cancer susceptibility gene BRCA1. To validate single components of the BRCA1 cascade, microRNA overexpression was performed in a cell culture model with subsequent protein analysis and luciferase reporter assays. Here, we were able to identify miR-335 as simultaneously regulating the known BRCA1 activators ERα, IGF1R, SP1 and the repressor ID4, including a feedback regulation of miR-335 expression by estrogens. Overexpression of miR-335 resulted in an upregulation of BRCA1 mRNA expression, suggesting a functional dominance of ID4 signaling. The relevance of the miR-335 regulation for human breast cancer was confirmed in primary sporadic breast cancer specimens with significantly decreased miR-335 levels (p < 0.05) in comparison to normal controls. Interestingly, the microRNA expression level correlated positively to the BRCA1 transcript level, supporting the hypothesis of a miR-335-mediated regulation of the tumor suppressor gene. Functionally, overexpression of miR-335 led to decreased cell viability and an increase in apoptosis, supporting its tumor-suppressive function. In summary, our data indicate that miR-335 affects different targets in the upstream BRCA1-regulatory cascade with impact on key cellular functions such as proliferation and apoptosis. Deregulation of the microRNA during breast cancer development and progression may thereby lead to an increased tumorigenic potential by inactivating crucial tumor-suppressive signals.

MicroRNA let-7a: a potential marker for selection of paclitaxel in ovarian cancer management

OBJECTIVES

Let-7 is a family of small non-coding RNAs regulating the expression of many genes that control important cellular activities. Let-7 is shown in vitro to sensitize cancer cells to platinum, but induce ovarian cancer resistance to paclitaxel. This study aims to investigate the effect of let-7a expression on survival outcomes of epithelial ovarian cancer (EOC) patients treated with different chemotherapy.

METHODS

Let-7a expression was measured with qRT-PCR in ovarian tumors of 178 EOC patients who received platinum-based chemotherapy with and without paclitaxel after surgery. Survival analysis was performed to assess the effects of let-7a and chemotherapy on disease outcomes.

RESULTS

Let-7a expression was detectable in the EOC samples, but the expression was not associated with disease stage, tumor grade, histology and debulking results. Patients who responded to platinum with paclitaxel had significantly lower let-7a than those who did not. Survival analyses showed that patients with high let-7a had better survival compared to those with low let-7a when they were treated with platinum without paclitaxel. The hazards ratios (HRs) for death and disease progression were 0.52 (95% CI: 0.29-0.96) and 0.48 (0.26-0.89) for high let-7a when compared to low let-7a, respectively. However, when patients were treated with platinum and paclitaxel, high let-7a was associated with worse progression-free and overall survival. The HRs for death and disease progression were 3.87 (95% CI: 1.28-11.66) and 3.48 (95% CI: 1.25-9.67) for high let-7a when compared to low let-7a, respectively. Further studies showed that among patients with low let-7a, those treated with paclitaxel in addition to platinum survived better than those treated without paclitaxel [adjusted-HRs were 0.31 (95% CI: 0.15-0.66) for death and 0.40 (95% CI: 0.22-0.75) for disease], while among those with high let-7a, the two types of treatment made no difference in patient survival.

CONCLUSIONS

The study suggests that the beneficial impact of the addition of paclitaxel on EOC survival was significantly linked to let-7a levels, and that miRNAs such as let-7a may be a useful marker for selection of chemotherapeutic agents in EOC management.

MicroRNA-126 inhibits ischemia-induced retinal neovascularization via regulating angiogenic growth factors

To investigate the potential transcriptional regulation and signal pathway of a single microRNA in ischemia-induced retinal neovascularization (NV), we used oxygen-induced retinopathy (OIR) in establishing retinal NV model, and quantitative real-time reverse transcriptase PCR analyzing a microRNA (miR-126) alteration. The mice were treated with plasmid pCMV-MIR-126/liposome mixture intravitreal injection, using pCMV-MIR/liposome mixture as control. The expression levels of VEGF, IGF-2 and HIF-1α, and the level changes of total and phosphorylated p38, ERK in retina from OIR mice were determined by western blot analysis. The effects of miR-126 on retinal NV in OIR mice were identified with fluoresecin angiography and H & E staining. No effect of miR-126 intravitreal injection on retinal vessels was performed with CD31 stained retinal sections. Our results showed that miR-126 was significantly decreased in retina from OIR mice. We confirmed that restoration of miR-126 in retina overcame the high levels of VEGF, IGF-2 and HIF-1α through downregulating p38 and ERK signaling molecules in OIR model, and that miR-126 intravitreal injection reduced retinal NV in OIR model. These results suggest that miR-126 might play a potential transcriptional role in the pathogenesis in diabetic retinopathy.

Differential miRNA expression in omental adipose tissue and in the circulation of obese patients identifies novel metabolic biomarkers

BACKGROUND

Omental fat accumulation is associated with development of the metabolic syndrome, although its molecular characteristics are poorly understood. Mi(cro)RNAs (miRNAs), a class of small noncoding RNAs, are known to regulate various metabolic processes, although their role in obesity and the metabolic syndrome is not clearly defined. This study sought to characterize the miRNA expression in omentum, sc fat and in the circulation of obese and nonobese individuals. Their potential as noninvasive metabolic biomarkers was also explored.

METHODS

miRNA was extracted from paired omentum and sc fat tissues, and blood samples, from a total of 50 obese and nonobese patients. A miRNA microarray was performed and a panel of differentially expressed miRNAs validated using RQ-PCR.

RESULTS

The miRNA expression profiles were unique for omentum and paired sc fat; no correlation in miRNA expression was observed between these two fat depots. Expression of two miRNAs (miR-17-5p and miR-132) differed significantly between obese and nonobese omental fat (P = 0.048 and P = 0.016). This expression pattern was reflected in the circulation in which these same two miRNAs were also significantly dysregulated in blood from obese subjects. The miRNA expression in omental fat and blood from obese patients correlated significantly with body mass index, fasting blood glucose, and glycosylated hemoglobin.

CONCLUSION

This study demonstrates that candidate metabolic miRNAs are altered in adipose tissue and circulation of the obese. Omental fat tissue and systemic miRNA levels reflect components of the metabolic syndrome, highlighting their potential as novel biomarkers for this complex syndrome.

MicroRNA therapeutics

MicroRNAs (miRNAs) provide new therapeutic targets for many diseases, while their myriad roles in development and cellular processes make them fascinating to study. We still do not fully understand the molecular mechanisms by which miRNAs regulate gene expression nor do we know the complete repertoire of mRNAs each miRNA regulates. However, recent progress in the development of effective strategies to block miRNAs suggests that anti-miRNA drugs may soon be used in the clinic.

Predicting response to chemotherapy with early-stage lung cancer

A recent meta-analysis of 11,107 patients with non-small cell lung cancer who had undergone surgical resection showed that the 5-year survival benefit of adjuvant chemotherapy was 4%, and that of adjuvant chemoradiotherapy was 5%. Two trials have shown a trend toward improved survival with adjuvant paclitaxel plus carboplatin. However, the benefit of adjuvant treatment remains suboptimal. We must distinguish between patients who will not relapse-and who can thus be spared adjuvant treatment-and those who will-for whom adjuvant treatment must be personalized. Several gene expression signatures, generally containing nonoverlapping genes, provide similar predictive information on clinical outcome, and a model combining several signatures did not perform better than did each of the signatures separately. The invasiveness gene signature, containing 186 genes, includes genes involved in the nuclear factor κB pathway, the RAS-mitogen-activated protein kinase pathway, and epigenetic control of gene expression. A 15-gene signature has identified JBR.10 patients who are more sensitive to adjuvant chemotherapy.

MicroRNA in lung cancer diagnostics and treatment

MicroRNAs (miRs) are short RNA chains that regulate gene expression by inhibition of mRNA translation. Their expression is often deranged in cancer. Increasing evidence indicates that these molecules play an important role in oncogenesis and cancer progression. This review focuses on the clinical application of miRs in lung cancer, with the emphasis on detection of early lung cancer, prognostication and chemotherapy sensitivity prediction. Methodological aspects of studies on prognostic markers in early NSCLC are outlined in detail. Finally, modulation of miR expression in lung cancer as a therapeutic possibility is discussed.

Shielding the messenger (RNA): microRNA-based anticancer therapies

It has been a decade since scientists realized that microRNAs (miRNAs) are not an oddity invented by worms to regulate gene expression at post-transcriptional levels. Rather, many of these 21-22-nucleotide-short RNAs exist in invertebrates and vertebrates alike and some of them are in fact highly conserved. miRNAs are now recognized as an important class of non-coding small RNAs that inhibit gene expression by targeting mRNA stability and translation. In the last ten years, our knowledge of the miRNAs world was expanding at vertiginous speed, propelled by the development of computational engines for miRNA identification and target prediction, biochemical tools and techniques to modulate miRNA activity, and last but not least, the emergence of miRNA-centric animal models. One important conclusion that has emerged from this effort is that many microRNAs and their cognate targets are strongly implicated in cancer, either as oncogenes or tumor and metastasis suppressors. In this review we will discuss the diverse role that miRNAs play in cancer initiation and progression and also the tools with which miRNA expression could be corrected in vivo. While the idea of targeting microRNAs towards therapeutic ends is getting considerable traction, basic, translational, and clinical research done in the next few years will tell whether this promise is well-founded.

MicroRNAs, the DNA damage response and cancer

Many carcinogenic agents such as ultra-violet light from the sun and various natural and man-made chemicals act by damaging the DNA. To deal with these potentially detrimental effects of DNA damage, cells induce a complex DNA damage response (DDR) that includes DNA repair, cell cycle checkpoints, damage tolerance systems and apoptosis. This DDR is a potent barrier against carcinogenesis and defects within this response are observed in many, if not all, human tumors. DDR defects fuel the evolution of precancerous cells to malignant tumors, but can also induce sensitivity to DNA damaging agents in cancer cells, which can be therapeutically exploited by the use of DNA damaging treatment modalities. Regulation of and coordination between sub-pathways within the DDR is important for maintaining genome stability. Although regulation of the DDR has been extensively studied at the transcriptional and post-translational level, less is known about post-transcriptional gene regulation by microRNAs, the topic of this review. More specifically, we highlight current knowledge about DNA damage responsive microRNAs and microRNAs that regulate DNA damage response genes. We end by discussing the role of DNA damage response microRNAs in cancer etiology and sensitivity to ionizing radiation and other DNA damaging therapeutic agents.

Development of smart nanoparticle-aptamer sensing technology

Quantum dots (QDs) are excellent donors in Förster resonance energy transfer (FRET) based sensors because of their broad absorption and narrow symmetric emission. However, the strict requirement of a short donor-acceptor distance to achieve high FRET (hence sensitivity) has posed a significant challenge for QD-FRET-based sensors due to challenges associated with the preparation of QD conjugates that are both compact and highly stable. Consequently, most robust QD-FRET sensors are often too bulky to produce FRET efficiently, especially at low target-to-QD copy numbers. They have largely relied on increasing the target:QD ratio to achieve high FRET, making them undesirable and inefficient in situations of low target:QD copy numbers. Herein we report our work on the preparation of stable, compact and water-soluble QDs and their subsequent use in making compact, functional QD-DNA-based smart nanoparticle sensors for labelled and label-free DNA and protein detection. We have developed two strategies to prepare QD-DNA sensors: 1) via QD-thiolated DNA self-assembly, and 2) via covalent coupling between DNA and a QD surface ligand functional group. We found that thiolated DNA (fluorophore labelled) can self-assemble onto a 3-mercaptopropionic acid-capped QDs to produce highly efficient FRET (~80%) at a DNA:QD ratio of 1 : 1. However, this system suffers from strong non-specific adsorption and the self-assembled single-stranded (ss) DNA target is unable to hybridise to its complementary probe. More recently, we found that a dihydrolipoic acid-capped QD-ssDNA self-assembled system can hybridise to a labelled complementary probe to produce efficient FRET that can be exploited for labelled DNA probe quantification. Further, incorporating an anti-thrombin DNA aptamer to this system leads to a QD-DNA aptamer sensor that can specifically detect a 10 nM unlabelled protein probe (thrombin). The non-specific adsorption problem can be eliminated by introducing a poly(ethylene glycol) (PEG) linker to the QD capping ligands or by capping the QD with a chelating dendritic ligand. The resulting QD-DNA sensors can specifically detect 1 nM unlabelled or 35 pM labelled DNA probes using QD-sensitised dye FRET signals on a conventional fluorimeter. Extension of the DNA target to other functional DNAs or DNA/RNA aptamers should allow the development of a multi-functional QD-DNA platform suitable for biosensing, disease diagnosis and therapeutic applications.

Molecular marks for epigenetic identification of developmental and cancer stem cells

Epigenetic regulations of genes by reversible methylation of DNA (at the carbon-5 of cytosine) and numerous reversible modifications of histones play important roles in normal physiology and development, and epigenetic deregulations are associated with developmental disorders and various disease states, including cancer. Stem cells have the capacity to self-renew indefinitely. Similar to stem cells, some malignant cells have the capacity to divide indefinitely and are referred to as cancer stem cells. In recent times, direct correlation between epigenetic modifications and reprogramming of stem cell and cancer stem cell is emerging. Major discoveries were made with investigations on reprogramming gene products, also known as master regulators of totipotency and inducer of pluoripotency, namely, OCT4, NANOG, cMYC, SOX2, Klf4, and LIN28. The challenge to induce pluripotency is the insertion of four reprogramming genes (Oct4, Sox2, Klf4, and c-Myc) into the genome. There are always risks of silencing of these genes by epigenetic modifications in the host cells, particularly, when introduced through retroviral techniques. In this contribution, we will discuss some of the major discoveries on epigenetic modifications within the chromatin of various genes associated with cancer progression and cancer stem cells in comparison to normal development of stem cell. These modifications may be considered as molecular signatures for predicting disorders of development and for identifying disease states.

ELECTRONIC SUPPLEMENTARY MATERIAL

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MicroRNAs as therapeutic targets in cancer

Cancer remains a worldwide epidemic. An improved understanding of the underlying molecular mechanisms and development of effective targeted therapies are still required for many deadly cancers. The discovery of microRNAs (miRNAs or miRs) nearly 20 years ago introduced a new layer of complexity to gene regulation, but it also afforded us the opportunity to further our understanding of the molecular pathogenesis of cancers. Dysregulation of miRNAs is fundamental to the pathogenesis of many cancers based on their involvement in basic cellular functions. In addition, these previously underrecognized, noncoding RNAs have the capacity to target tens to hundreds of genes simultaneously. Thus, they are attractive candidates as prognostic biomarkers and therapeutic targets in cancer. However, several challenges remain in translating our current understanding of miRNAs to clinical therapies. Herein, we provide a review of the current knowledge of miRNAs in both solid and hematological malignancies with a focus on their potential application as therapeutic targets in cancer.

Involvement of microRNAs in lung cancer biology and therapy

MicroRNAs (miRNAs) are a class of small RNAs that regulate gene expression. Expression profiles of specific miRNAs have improved cancer diagnosis and classification as well as provided prognostic information in many human cancers, including lung cancer. Tumor-suppressive and oncogenic miRNAs were uncovered in lung carcinogenesis. The biological functions of these miRNAs in lung cancer were validated recently in well-characterized cellular, murine transgenic as well as transplantable lung cancer models, and in human paired normal-malignant lung tissue banks and tissue arrays. Tumor-suppressive and oncogenic miRNAs that were identified in lung cancer will be reviewed here. Emphasis is placed on highlighting those functionally validated miRNAs that are not only biomarkers of lung carcinogenesis but also candidate pharmacologic targets. How these miRNA findings advance an understanding of lung cancer biology and how they could improve lung cancer therapy are discussed in this article.

Systemic delivery of tumor suppressor microRNA mimics using a neutral lipid emulsion inhibits lung tumors in mice

MicroRNAs (miRNAs) are emerging as potential cancer therapeutics, but effective delivery mechanisms to tumor sites are a roadblock to utility. Here we show that systemically delivered, synthetic miRNA mimics in complex with a novel neutral lipid emulsion are preferentially targeted to lung tumors and show therapeutic benefit in mouse models of lung cancer. Therapeutic delivery was demonstrated using mimics of the tumor suppressors, microRNA-34a (miR-34a) and let-7, both of which are often down regulated or lost in lung cancer. Systemic treatment of a Kras-activated autochthonous mouse model of non-small cell lung cancer (NSCLC) led to a significant decrease in tumor burden. Specifically, mice treated with miR-34a displayed a 60% reduction in tumor area compared to mice treated with a miRNA control. Similar results were obtained with the let-7 mimic. These findings provide direct evidence that synthetic miRNA mimics can be systemically delivered to the mammalian lung and support the promise of miRNAs as a future targeted therapy for lung cancer.

Silencing of microRNA families by seed-targeting tiny LNAs

The challenge of understanding the widespread biological roles of animal microRNAs (miRNAs) has prompted the development of genetic and functional genomics technologies for miRNA loss-of-function studies. However, tools for exploring the functions of entire miRNA families are still limited. We developed a method that enables antagonism of miRNA function using seed-targeting 8-mer locked nucleic acid (LNA) oligonucleotides, termed tiny LNAs. Transfection of tiny LNAs into cells resulted in simultaneous inhibition of miRNAs within families sharing the same seed with concomitant upregulation of direct targets. In addition, systemically delivered, unconjugated tiny LNAs showed uptake in many normal tissues and in breast tumors in mice, coinciding with long-term miRNA silencing. Transcriptional and proteomic profiling suggested that tiny LNAs have negligible off-target effects, not significantly altering the output from mRNAs with perfect tiny LNA complementary sites. Considered together, these data support the utility of tiny LNAs in elucidating the functions of miRNA families in vivo.

The oncogenic and tumour suppressive roles of microRNAs in cancer and apoptosis

MicroRNAs (miRNAs) are small, non-coding, endogenous RNAs that regulate gene expression at the post-transcriptional level. MiRNAs play important roles in regulating a variety of biological process such as proliferation, differentiation and apoptosis. It has been demonstrated that miRNAs have a crucial function in oncogenesis by regulating cell proliferation and apoptosis as oncogenes or tumour suppressors. As several reports have underlined the possible contribution of miRNAs to promote or evade apoptosis, it seems that the dysregulation of miRNAs involved in apoptosis may provide a mechanism for cancer development. Given emerging evidence that points to oncogenic and tumour suppressive roles of miRNAs in cancer and apoptosis, it is thought that manipulating miRNA expression level may be a potential therapeutic strategy for curing cancer.

Characterization and functional analysis of the human microRNA let-7a2 promoter in lung cancer A549 cell lines

Recent studies have revealed that microRNAs have a strong association with cancer in humans. The miRNA let-7 is highly expressed in normal lung tissue, but frequently expressed at reduced levels in lung cancers. Let-7a2 is a member of the let-7 family. So far, little is known about the transcriptional regulation of let-7a2. Our study is focused on the characterization and functional analysis of the promoter of the human miRNA let-7a2 in A549 cell lines. Firstly, 5' rapid amplification of cDNA ends (5' RACE) was carried out and a 2.8 kb fragment in the upstream of let-7a2 gene was then cloned into pGL3-basic vector. Sequence analysis with the MatInspector database revealed that there were putative binding sites for some important transcriptional factors in the promoter region of let-7a2, such as p53, c-Myc, Ras, CEBPα, RORA, RXR, TCF, and GR. Additionally, a series of transfection and luciferase reporter assays were carried out to test let-7a2 promoter activity. RT-PCR and transfection of let-7a target sequence-reporter plasmid were performed to detect transcription levels of the let-7a2 gene in A549 cells treated with 9-cis-RA, all-trans-RA, lithium chloride or dexamethasone. Our results showed that the recombinant pGL3-p7a2 could acts as a promoter. The promoter activity of the 2.8 kb fragment could be downregulated by transfection with CEBPα or treatment with lithium chloride and enhanced by 9-cis-RA or all-trans-RA treatment. Furthermore, the results of RT-PCR analysis and transfection of let-7a target sequence-reporter plasmid showed that 9-cis-RA and all-trans-RA both upregulated let-7a2 expression, while lithium chloride downregulated its expression. Our results suggest that 9-cis-RA, all-trans-RA,lithium chloride and CEBPα might play important regulatory roles in let-7a2 gene expression in A549 cells.

Low-level expression of let-7a in gastric cancer and its involvement in tumorigenesis by targeting RAB40C

Gastric cancer is the fourth most common cancer and the second leading cause of cancer mortality worldwide but the underlying molecular mechanism is not entirely clear. The objective of this study was to explore the role of let-7a microRNA (miRNA) in gastric tumorigenesis and the possible correlation between RAB40C and let-7a miRNA in gastric cancer. We found that expression of let-7a is reduced in human gastric cancer tissues and cell lines and there was a significant correlation between the level of let-7a expression and the stage of differentiation. Overexpression of let-7a resulted in a decrease in cell proliferation and G(1) arrest, significantly suppressed anchorage-dependent growth in vitro and the tumorigenicity of gastric cancer cells in a nude mouse xenograft model. Furthermore, we demonstrated that RAB40C is regulated directly by let-7a and plays an essential role as a mediator of the biological effects of let-7a in gastric tumorigenesis. This study revealed that let-7a is significant in suppressing gastric cancer growth in vivo and in vitro and provided the first evidence that RAB40C is negatively regulated by let-7a at the posttranscriptional level via binding to the 3'-untranslated region of RAB40C messenger RNA in gastric cancer. The results of this study suggest that let-7a and RAB40C are potentially useful targets for gastric cancer diagnosis and therapy.

MicroRNAs and the cell cycle

The control of cell proliferation by microRNAs (miRNAs) is well established and the alteration of these small, non-coding RNAs may contribute to tumor development by perturbing critical cell cycle regulators. Oncogenic miRNAs may facilitate cell cycle entry and progression by targeting CDK inhibitors or transcriptional repressors of the retinoblastoma family. On the other hand, tumor suppressor miRNAs induce cell cycle arrest by downregulating multiple components of the cell cycle machinery. Recent data also suggest that miRNAs act co-ordinately with transcriptional factors involved in cell cycle regulation such as c-MYC, E2F or p53. These miRNAs not only can potentiate the function of these factors but they may also limit the excessive translation of cell cycle proteins upon mitogenic or oncogenic stimuli to protect cells from replicative stress. The implications of these regulatory networks in cell proliferation and human disease are discussed.

Non-coding RNAs as regulators of embryogenesis

Non-coding RNAs (ncRNAs) are emerging as key regulators of embryogenesis. They control embryonic gene expression by several means, ranging from microRNA-induced degradation of mRNAs to long ncRNA-mediated modification of chromatin. Many aspects of embryogenesis seem to be controlled by ncRNAs, including the maternal-zygotic transition, the maintenance of pluripotency, the patterning of the body axes, the specification and differentiation of cell types and the morphogenesis of organs. Drawing from several animal model systems, we describe two emerging themes for ncRNA function: promoting developmental transitions and maintaining developmental states. These examples also highlight the roles of ncRNAs in ensuring a robust commitment to one of two possible cell fates.

The guardians of the genome (p53, TA-p73, and TA-p63) are regulators of tumor suppressor miRNAs network

The tumor suppressor p53 homologues, TA-p73, and p63 have been shown to function as tumor suppressors. However, how they function as tumor suppressors remains elusive. Here, I propose a number of tumor suppressor pathways that illustrate how the TA-p73 and p63 could function as negative regulators of invasion, metastasis, and cancer stem cells (CSCs) proliferation. Furthermore, I provide molecular insights into how TA-p73 and p63 could function as tumor suppressors. Remarkably, the guardians--p53, p73, and p63--of the genome are in control of most of the known tumor suppressor miRNAs, tumor suppressor genes, and metastasis suppressors by suppressing c-myc through miR-145/let-7/miR-34/TRIM32/PTEN/FBXW7. In particular, p53 and TA-p73/p63 appear to upregulate the expression of (1) tumor suppressor miRNAs, such as let-7, miR-34, miR-15/16a, miR-145, miR-29, miR-26, miR-30, and miR-146a; (2) tumor suppressor genes, such as PTEN, RBs, CDKN1a/b/c, and CDKN2a/b/c/d; (3) metastasis suppressors, such as Raf kinase inhibitory protein, CycG2, and DEC2, and thereby they enlarge their tumor suppressor network to inhibit tumorigenesis, invasion, angiogenesis, migration, metastasis, and CSCs proliferation.

PDGF induced microRNA alterations in cancer cells

Platelet derived growth factor (PDGF) regulates gene transcription by binding to specific receptors. PDGF plays a critical role in oncogenesis in brain and other tumors, regulates angiogenesis, and remodels the stroma in physiologic conditions. Here, we show by using microRNA (miR) arrays that PDGFs regulate the expression and function of miRs in glioblastoma and ovarian cancer cells. The two PDGF ligands AA and BB affect expression of several miRs in ligand-specific manner; the most robust changes consisting of let-7d repression by PDGF-AA and miR-146b induction by PDGF-BB. Induction of miR-146b by PDGF-BB is modulated via MAPK-dependent induction of c-fos. We demonstrate that PDGF regulates expression of some of its known targets (e.g. cyclin D1) through miR alterations and identify the epidermal growth factor receptor (EGFR) as a new PDGF-BB target. We show that its expression and function are repressed by PDGF-induced miR-146b and that mir-146b and EGFR correlate inversely in human glioblastomas. We propose that PDGF-regulated gene transcription involves alterations in non-coding RNAs and provide evidence for a miR-dependent feedback mechanism balancing growth factor receptor signaling in cancer cells.

Vascular smooth muscle cell proliferation is influenced by let-7d microRNA and its interaction with KRAS

BACKGROUND

Several microRNAs (miRNAs) have been reported to regulate cardiovascular biological and pathological processes through inhibiting the translation of certain RNA transcripts. However, little is known about the association between miRNAs and vascular smooth muscle cell (VSMC) proliferation. The aim was to investigate the role of miRNAs in VSMC growth and the potential mechanism.

METHODS AND RESULTS

Primary VSMCs were isolated from the medial layer of the thoracic aorta obtained from spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY). miRNA microarrays were used to analyze the difference in miRNA expression between VSMCs of SHR and WKY rats and were validated using TaqMan real-time PCR. Of the potentially related genes under the influence of let-7d identified through literature search, KRAS was verified by western blot and functionally analyzed using miRNA mimics transfection and analysis of transfectants by cell enumeration was made using CCK-8 and flow cytometric analysis of cell cycle progression. let-7d-transfected VSMCs from SHR, WKY and human coronary arteries expressed significantly lower amounts of KRAS protein, displayed reduced cell growth and led to a greater number of cells in the G1 phase than the G2/M phases of the cell cycle.

CONCLUSIONS

let-7d was significantly downregulated in VSMCs as an important regulator of cell proliferation. RAS might be involved in the proliferation regulation by let-7d.

IMP-1 displays cross-talk with K-Ras and modulates colon cancer cell survival through the novel proapoptotic protein CYFIP2

Insulin-like growth factor 2 mRNA-binding protein-1 (IMP-1) is an oncofetal protein that binds directly to and stabilizes oncogenic c-Myc and regulates, in turn, its posttranscriptional expression and translation. In contrast to normal adult tissue, IMP-1 is reexpressed and/or overexpressed in human cancers. We show that knockdown of c-Myc in human colon cancer cell lines increases the expression of mature let-7 miRNA family members and downregulates several of its mRNA targets: IMP-1, Cdc34, and K-Ras. We further show that loss of IMP-1 inhibits Cdc34, Lin-28B, and K-Ras, suppresses SW-480 cell proliferation and anchorage-independent growth, and promotes caspase- and lamin-mediated cell death. We also found that IMP-1 binds to the coding region and 3'UTR of K-Ras mRNA. RNA microarray profiling and validation by reverse transcription PCR reveals that the p53-inducible proapoptotic protein CYFIP2 is upregulated in IMP-1 knockdown SW480 cells, a novel finding. We also show that overexpression of IMP-1 increases c-Myc and K-Ras expression and LIM2405 cell proliferation. Furthermore, we show that loss of IMP-1 induces Caspase-3- and PARP-mediated apoptosis, and inhibits K-Ras expression in SW480 cells, which is rescued by CYFIP2 knockdown. Importantly, analysis of 228 patients with colon cancers reveals that IMP-1 is significantly upregulated in differentiated colon tumors (P ≤ 0.0001) and correlates with K-Ras expression (r = 0.35, P ≤ 0.0001) relative to adjacent normal mucosa. These findings indicate that IMP-1, interrelated with c-Myc, acts upstream of K-Ras to promote survival through a novel mechanism that may be important in colon cancer pathogenesis.

let-7 and miR-17-92: small-sized major players in lung cancer development

MicroRNA (miRNA)-encoding small non-coding RNA have been recognized as important regulators of a number of biological processes that inhibit the expression of hundreds of genes. Accumulating evidence also indicates the involvement of miRNA alterations in various types of human cancer, including lung cancer, which has long been the leading cause of cancer death in economically well-developed countries, including Japan. We previously found that downregulation of members of the tumor-suppressive let-7 miRNA family and overexpression of the oncogenic miR-17-92 miRNA cluster frequently occur in lung cancers, and molecular insight into how these miRNA alterations may contribute to tumor development has been rapidly accumulating. The present review summarizes recent advances in elucidation of the molecular functions of these miRNA in relation to their roles in the pathogenesis of lung cancer. Given the crucial roles of miRNA alterations, additional studies are expected to provide a better understanding of the underlying molecular mechanisms of disease development, as well as a foundation for novel strategies for cancer diagnosis and treatment of this devastating disease.

The microRNA miR-34a inhibits prostate cancer stem cells and metastasis by directly repressing CD44

Cancer stem cells (CSCs) or tumor progenitor cells are involved in tumor progression and metastasis1. MicroRNAs (miRNAs) regulate both normal stem cells and CSCs2–5 and miRNA dysregulation has been implicated in tumorigenesis6. CSCs in many tumors, including cancers of the breast7, pancreas8, head and neck9, colon10,11, small intestine12, liver13, stomach14, bladder15, and ovary16 have been identified using adhesion molecule CD44, either individually or in combination with other marker(s). Prostate cancer (PCa) stem/progenitor cells with enhanced clonogenic17 and tumor-initiating and metastatic18,19 capacities are also enriched in the CD44⁺ cell population, but whether miRNAs regulate the CD44⁺ PCa cells and PCa metastasis remains unclear. Here we show, through expression analysis, that miR-34a, a p53 target20–24, was under-expressed in CD44⁺ PCa cells purified from xenograft and primary tumors. Enforced expression of miR-34a in bulk PCa cells inhibited clonogenic expansion and tumor development. miR-34a re-expression in CD44⁺ PCa cells blocked whereas miR-34a antagomirs in CD44⁻ PCa cells promoted tumor regeneration and metastasis. Systemically delivered miR-34a inhibited PCa metastasis and extended animal survival. Of significance, CD44 was identified and validated as a direct and functional target of miR-34a and CD44 knockdown phenocopied miR-34a over-expression in inhibiting PCa regeneration and metastasis. Our study reveals miR-34a as a critical negative regulator of CD44⁺ PCa cells and establishes a strong rationale for developing miR-34a as a novel therapeutic against prostate CSCs.

MicroRNAs (miRNAs) in cancer invasion and metastasis: therapeutic approaches based on metastasis-related miRNAs

The management of tumor cell invasion and metastasis is instrumental in cancer therapy, since metastases are the prime reason for cancer patient mortality. Various cellular mechanisms and underlying molecular pathways relevant for metastasis have been identified so far, providing a basis for antimetastatic drugs. MicroRNAs (miRNAs) are highly conserved, small noncoding RNA molecules that have been shown to regulate various cellular processes by interfering with protein expression through posttranscriptional repression or mRNA degradation. More importantly, beyond their roles in physiological processes, many miRNAs are aberrantly expressed in various pathologies including cancer and regulate tumor- and metastasis-associated genes. Their pivotal role in metastasis has emerged only recently. After an introduction into the mechanisms of miRNA action, this review article describes the roles of miRNAs in cancer invasion and metastasis. Various miRNAs are discussed with regard to their upstream regulators, downstream target genes, and pro-/antimetastatic effects. A table provides a comprehensive overview of miRNAs that are misregulated/relevant in metastasis and the current knowledge regarding their underlying molecular effects. Furthermore, therapeutic approaches based on miRNAs, either as drugs or as therapeutic targets, are described prior to the discussion of the delivery of miRNA-based therapeutics as novel strategy in antimetastatic treatment.

Let-7/miR-98 regulate Fas and Fas-mediated apoptosis

Fas is ubiquitously expressed on a variety of cells and triggers apoptosis, which have critical roles in the immune system. MicroRNAs (miRNAs) have been recently identified as regulators that modulate target gene expression and are involved in diverse biological processes, such as cell proliferation and apoptosis. This study was undertaken to investigate the contribution of miRNA in the regulation of Fas expression and Fas-mediated apoptosis. Bioinformatics analysis indicated that Fas was a potential target of let-7/miR-98 family. Indeed ectopic expression of let-7/miR-98 reduced, whereas knockdown of endogenous let-7/miR-98 increased the expression of Fas at both mRNA and protein levels. Let-7/miR-98 was verified to target Fas 3' untranslated region directly by site-directed gene mutagenesis and reporter gene assay. More importantly, introduction of let-7/miR-98 could decrease the sensitivity to Fas-induced apoptosis. Furthermore, let-7/miR-98 expression was reduced in activation-induced cell death process, accompanied by increased expression of Fas. In conclusion, our study first demonstrated that let-7/miR-98 regulated Fas expression and the sensitivity of Fas-mediated apoptosis.

Small players with big roles: microRNAs as targets to inhibit breast cancer progression

As modulators of gene expression, microRNAs (miRNAs) are essential for normal development. Not surprisingly, aberrant expression of miRNAs is associated with many diseases, including cancer. Studies of various breast cancer subtypes have demonstrated that, like gene expression profiles and pathological differences, miRNA profiles can distinguish various tumor subtypes. Over the last few years, roles for miRNAs during many stages of breast cancer progression have been established. This includes potential breast cancer associated polymorphisms in miRNA target sites or miRNAs themselves, miRNAs that can act as tumor suppressors or oncogenes, and miRNAs that can modulate metastatic spread. Recent studies have also suggested key roles for miRNAs in regulating cancer stem cells. Thus, miRNAs have now become important therapeutic targets. This can be achieved by replacing miRNA expression where it has been lost or decreased, or conversely by inhibiting miRNA expression where it has been amplified or overexpressed in cancers. Ultimately, miRNAs should provide both important prognostic biomarkers as well as new targetable molecules for the treatment of breast cancer.

IGF-II is regulated by microRNA-125b in skeletal myogenesis

miR-125b is identified as a myogenic miRNA that regulates skeletal muscle differentiation by targeting IGF-II under the control of mTOR signaling.

MicroRNAs (miRNAs) have emerged as key regulators of skeletal myogenesis, but our knowledge of the identity of the myogenic miRNAs and their targets remains limited. In this study, we report the identification and characterization of a novel myogenic miRNA, miR-125b. We find that the levels of miR-125b decline during myogenesis and that miR-125b negatively modulates myoblast differentiation in culture and muscle regeneration in mice. Our results identify IGF-II (insulin-like growth factor 2), a critical regulator of skeletal myogenesis, as a direct and major target of miR-125b in both myocytes and regenerating muscles, revealing for the first time an miRNA mechanism controlling IGF-II expression. In addition, we provide evidence suggesting that miR-125b biogenesis is negatively controlled by kinase-independent mammalian target of rapamycin (mTOR) signaling both in vitro and in vivo as a part of a dual mechanism by which mTOR regulates the production of IGF-II, a master switch governing the initiation of skeletal myogenesis.

miRNAs in human cancer

Mature microRNAs (miRNAs) are single-stranded RNA molecules of 20-23 nucleotide (nt) length that control gene expression in many cellular processes. These molecules typically reduce the stability of mRNAs, including those of genes that mediate processes in tumorigenesis, such as inflammation, cell cycle regulation, stress response, differentiation, apoptosis and invasion. miRNA targeting is mostly achieved through specific base-pairing interactions between the 5' end ('seed' region) of the miRNA and sites within coding and untranslated regions (UTRs) of mRNAs; target sites in the 3' UTR lead to more effective mRNA destabilization. Since miRNAs frequently target hundreds of mRNAs, miRNA regulatory pathways are complex. To provide a critical overview of miRNA dysregulation in cancer, we first discuss the methods currently available for studying the role of miRNAs in cancer and then review miRNA genomic organization, biogenesis and mechanism of target recognition, examining how these processes are altered in tumorigenesis. Given the critical role miRNAs play in tumorigenesis processes and their disease-specific expression, they hold potential as therapeutic targets and novel biomarkers.

Implications of microRNAs in colorectal cancer development, diagnosis, prognosis, and therapeutics

MicroRNAs (miRNAs) are a class of non-coding small RNAs with critical regulatory functions as post-transcriptional regulators. Due to the fundamental importance and broad impact of miRNAs on multiple genes and pathways, dysregulated miRNAs have been associated with human diseases, including cancer. Colorectal cancer (CRC) is among the most deadly diseases, and miRNAs offer a new frontier for target discovery and novel biomarkers for both diagnosis and prognosis. In this review, we summarize the recent advancement of miRNA research in CRC, in particular, the roles of miRNAs in CRC stem cells, epithelial-to-mesenchymal transition, chemoresistance, therapeutics, diagnosis, and prognosis.

Modulation of microRNAs for potential cancer therapeutics

MicroRNA (miRNA) is a nonprotein coding small RNA molecule that negatively regulates gene expression by degradation of mRNA or suppression of mRNA translation. MiRNA plays important roles in physiological processes such as cellular development, differentiation, proliferation, apoptosis, and stem cell self-renewal. Studies show that the deregulation of miRNA expression is closely associated with tumorigenicity, invasion, and metastasis. The functionality of miRNAs may act as oncogenes or tumor suppressors during tumor initiation and progression. miRNomes in almost all types of cancers started to develop the regulatory network of miRNA::mRNA interaction in the view of systems biology. Experimental evidence demonstrates that the modulation of specific miRNA alterations in cancer cells using miRNA replacement or anti-miRNA technologies can restore miRNA activities and repair the gene regulatory network and signaling pathways, in turn, reverse the phenotype of cancerous cells. Numerous animal studies for miRNA-based therapy offer the hope of targeting miRNAs as alternative cancer treatment. Developing the small molecules to interfere with miRNAs could be of great pharmaceutical interest in the future. Interestingly, specific miRNA is capable of reprogramming the cancer cells into a pluriopotent embryonic stem cell-like state (mirPS), which could be induced into tissue-specific mature cell types. This chapter will present the various strategies of modulation of miRNAs in vitro and in vivo.

Tissular and soluble miRNAs for diagnostic and therapy improvement in digestive tract cancers

Digestive cancers (e.g., gastric, colorectal, pancreatic or hepatocarcinoma) are among the most frequently reported cancers in the world, and are characterized by invasivity, metastatic potential and poor outcomes. This group includes some of the most critical cancers (among them, are those ranked second to forth in cancer-related mortality) and, despite all sustained efforts, they maintain a profile of low survival rates and lack successful therapies. Discovery of biomarkers that improve disease characterization may make optimized or personalized therapy possible. Novel biomarkers are expected to provide, hopefully, less-invasive or noninvasive diagnostic tools that make possible earlier detection of disease. Also, they may provide a more reliable selection instrument in the drug discovery process. miRNAs, short noncoding RNAs, have emerged in the last few years as significant regulators of cellular activities, controlling protein expression at the post-transcriptional level, with a significant implication in pathology in general and, of most relevance, in cancers. Deregulation of miRNA expression levels and some genetic alterations were demonstrated in various cancers, including digestive cancers. Investigations in tissue samples have provided a considerable amount of knowledge, identifying altered expressions of miRNAs associated with tumorigenesis and tumor progression. Overexpression of some tumor-inducing or tumor-promoting miRNAs was demonstrated, as well as the downregulation of tumor-suppressor miRNAs. Both individual miRNAs, as well as sets of multiple miRNAs, were set up as candidate biomarkers for diagnostics or monitoring, offering relevant insights into tumorigenic mechanisms. Circulating miRNAs were demonstrated as valuable instruments in tumor diagnosis and the prognosis of digestive cancers (affecting the esophagus, stomach, intestine, colorectum, liver and pancreas), and are being investigated thoroughly in order to generate and validate less-invasive diagnostic tools with enhanced sensitivity.

Role of ribosomal protein RPS2 in controlling let-7a expression in human prostate cancer

We discovered that an inverse relationship exists in the expression of ras/c-myc and ribosomal protein RPS2 with pre-let-7a-1/let-7a/let-7f miRNA and prostate tumor cell malignancy. Nonmalignant IBC-10a cells expressed low levels of ras/RPS2 and elevated pre-let-7a-1/let-7a/let-7f miRNA, whereas the reverse occurred in malignant PCa-20a and PC-3ML cells. Stable transfection of IBC-10a cells with pBABE.ras and pBABE.RPS2 induced ras, c-myc, and RPS2 expression, whereas the levels of let-7a/let-7f miRNA dropped to near zero. Conversely, in pBABE.pre-let-7a-1 transfected PCa-20a and PC-3ML clones, let-7a/let-7f increased whereas ras, RPS2, and c-myc dropped greater than 5-fold. Electrophoretic mobility shift assays, antibody "supershift" assays and immunoprecipitation assays revealed that RPS2 specifically binds pre-let-7a-1 to block RNA processing. Immunoflourescent studies and Northern blots confirmed that RPS2 complexes with pre-let-7a-1 (i.e., in episomal structures) to block processing to let-7a/let-7f, indicating RPS2 may prevent let-7a miRNA expression to indirectly promote oncogene expression. Functional studies further showed that the colony-forming ability (CFA) and invasive activities of IBC-10a cells were significantly enhanced in pBABE-ras.IBC-10a and pBABE-RPS2-IBC-10a clones. Conversely, with the "knockdown" of ras and RPS2 in malignant PC-3ML cells (i.e., in pLKO.TRC.shRNA.ras.PC3-ML, pLKO.TRC.shRNA.RPS2.PC-3ML transfected cells), there was both a loss of these functions and a loss of tumorigenesis in SCID mice. Likewise, with the overexpression of let-7a/let-7f in pBABE.pre-let-7a-1.PC-3ML clones (and PCa-20a clones), CFAs, invasive activities in vitro, and tumorigenesis in vivo were significantly reduced. These results show for the first time that RPS2 blocks pre-let-7a-1 processing to enable ras and c-myc expression and the transformation of primary tumor cells.