Transactivation of miR-34a by p53 broadly influences gene expression and promotes apoptosis

MicroRNAs in ovarian carcinomas

The molecular mechanisms involved in epithelial ovarian cancer initiation and progression are just beginning to be elucidated. In particular, it has become evident that microRNAs (miRNAs or miRs), a class of molecules that post-transcriptionally regulate gene expression, play a major role in ovarian tumorigenesis. Several microRNA profiling studies have identified changes in microRNA patterns that take place during ovarian cancer development. While most deregulated microRNAs are down-regulated in cancer, and may therefore act as tumor suppressors, others are elevated and may represent novel oncogenes in this disease. A number of microRNAs identified as aberrantly expressed in ovarian carcinoma have been shown to have important functional roles in cancer development and may therefore represent targets for therapy. In addition, some of the microRNA patterns may have prognostic significance. The identification of functional targets represents a major hurdle in our understanding of microRNA function in ovarian carcinoma, but significant progress is being made. It is hoped that a better understanding of the microRNA expression and roles in ovarian cancer may provide new avenues for the detection, diagnosis, and therapy of this deadly disease.

MicroRNA regulation of IFN-beta protein expression: rapid and sensitive modulation of the innate immune response

IFN-beta production is an inaugural event in the innate immune response to viral infections, with relatively small fold changes in IFN-beta expression resulting in the activation of important antiviral signaling cascades. In our rapid SIV/macaque model of HIV encephalitis, the virus enters the CNS within 4 d of infection, accompanied by a marked IFN-beta response that wanes as SIV replication is controlled. The centrality of IFN-beta to the innate antiviral response in the CNS combines with the potential inflammatory damage associated with long-term activation of this pathway to suggest that IFN-beta may be subject to regulatory fine-tuning in addition to well-established transcriptional and message stability mechanisms of regulation. In this paper, we present for the first time evidence that microRNAs (miRNAs), including miR-26a, -34a, -145, and let-7b, may directly regulate IFN-beta in human and macaque cells. In primary primate macrophages, the main cell type implicated in HIV and SIV infection in the CNS, specific miRNAs reduce, whereas miRNA inhibitors enhance, IFN-beta protein production. The potential biologic significance of this regulation is supported by evidence of an apparent negative feedback loop, with increased expression of three IFN-beta-regulating miRNAs by primate macrophages exposed to recombinant IFN-beta or stimulated to produce IFN-beta. Thus, miRNAs may contribute significantly to the regulation of IFN-beta in innate immune responses.

A pathway involving farnesoid X receptor and small heterodimer partner positively regulates hepatic sirtuin 1 levels via microRNA-34a inhibition

Sirtuin 1 (SIRT1) is a NAD-dependent deacetylase that is critically involved in diverse cellular processes including metabolic disease, cancer, and possibly aging. Despite extensive studies on SIRT1 function, how SIRT1 levels are regulated remains relatively unknown. Here, we report that the nuclear bile acid receptor farnesoid X receptor (FXR) inhibits microRNA-34a (miR-34a) in the liver, which results in a positive regulation of SIRT1 levels. Activation of FXR by the synthetic agonist GW4064 decreases hepatic miR-34a levels in normal mice, and consistently, hepatic miR-34a levels are elevated in FXR-null mice. FXR induces expression of small heterodimer partner (SHP), an orphan nuclear receptor and transcriptional corepressor, which in turn results in repression of p53, a key activator of the miR-34a gene, by inhibiting p53 occupancy at the promoter. MiR-34a decreased SIRT1 levels by binding to the 3'-untranslated region of SIRT1 mRNA, and adenovirus-mediated overexpression of miR-34a substantially decreased SIRT1 protein levels in mouse liver. Remarkably, miR-34a levels were elevated, and SIRT1 protein levels were reduced in diet-induced obese mice, and FXR activation in these mice reversed the miR-34a and SIRT1 levels, indicating an intriguing link among FXR activation, decreased miR-34a, and subsequently, increased SIRT1 levels. Our study demonstrates an unexpected role of the FXR/SHP pathway in controlling SIRT1 levels via miR-34a inhibition and that elevated miR-34a levels in obese mice contribute to decreased SIRT1 levels. Manipulation of this regulatory network may be useful for treating diseases of aging, such as metabolic disease and cancer.

MicroRNAs and cancer epigenetics: a macrorevolution

PURPOSE OF REVIEW

Since the first demonstration of microRNA (miRNA) roles in tumorigenesis, a multitude of studies have established a solid scaffold that supports the increased and accelerated progression in this field. The aim of this article is to comment on the most recent findings of miRNAs in cancer, particularly focusing on epigenetics and the potential clinical applications derived from comprehensive and exhaustive research carried out during the last years.

RECENT FINDINGS

A global reduction of miRNA levels is emerging as a common hallmark of cancer. Several strands of evidence have shown that one of the mechanisms responsible for this deregulation is the epigenetic silencing of miRNA genes. In turn, recent studies have revealed that some miRNAs directly repress enzymes of the epigenetic machinery, including DNA methyltransferases, histone deacetylases and histone methyltransferases. These facts broaden the promising biomedical uses of miRNAs. Apart from epigenetic mechanisms, other causes of miRNA deregulation in cancer are also discussed in this review, as well as novel clinical applications of miRNAs in cancer treatment.

SUMMARY

The ability of individual miRNAs to regulate multiple target genes, implicated in turn in several pathways, confers them an extraordinary capacity as multifunctional tools for cancer therapy. Thus, restoration of the level of a single or few pleiotropic miRNAs could eventually re-establish molecular pathways altered in cancer, providing a more effective therapeutic strategy. However, further studies will be needed to validate the preliminary successful results of miRNA-based therapy obtained in cellular and animal models. Also, it is crucial to expand our knowledge about the molecular regulation of the miRNome (global miRNA expression levels) in physiological and pathological settings.

Pediatric primary central nervous system germ cell tumors of different prognosis groups show characteristic miRNome traits and chromosome copy number variations

Background

Intracranial pediatric germ cell tumors (GCTs) are rare and heterogeneous neoplasms and vary in histological differentiation, prognosis and clinical behavior. Germinoma and mature teratoma are GCTs that have a good prognosis, while other types of GCTs, termed nongerminomatous malignant germ cell tumors (NGMGCTs), are tumors with an intermediate or poor prognosis. The second group of tumors requires more extensive drug and irradiation treatment regimens. The mechanisms underlying the differences in incidence and prognosis of the various GCT subgroups are unclear.

Results

We identified a distinct mRNA profile correlating with GCT histological differentiation and prognosis, and also present in this study the first miRNA profile of pediatric primary intracranial GCTs. Most of the differentially expressed miRNAs were downregulated in germinomas, but miR-142-5p and miR-146a were upregulated. Genes responsible for self-renewal (such as POU5F1 (OCT4), NANOG and KLF4) and the immune response were abundant in germinomas, while genes associated with neuron differentiation, Wnt/β-catenin pathway, invasiveness and epithelial-mesenchymal transition (including SNAI2 (SLUG) and TWIST2) were abundant in NGMGCTs. Clear transcriptome segregation based on patient survival was observed, with malignant NGMGCTs being closest to embryonic stem cells. Chromosome copy number variations (CNVs) at cytobands 4q13.3-4q28.3 and 9p11.2-9q13 correlated with GCT malignancy and clinical risk. Six genes (BANK1, CXCL9, CXCL11, DDIT4L, ELOVL6 and HERC5) within 4q13.3-4q28.3 were more abundant in germinomas.

Conclusions

Our results integrate molecular profiles with clinical observations and provide insights into the underlying mechanisms causing GCT malignancy. The genes, pathways and microRNAs identified have the potential to be novel therapeutic targets.

Dysregulation of protein synthesis and disease

The regulation of protein synthesis plays as important a role as transcriptional control in the control of gene expression. Once thought solely to act globally, translational control has now been shown to be able to control the expression of most genes specifically. Dysregulation of this process is associated with a range of pathological conditions, notably cancer and several neurological disorders, and can occur in many ways. These include alterations in the expression of canonical initiation factors, mutations in regulatory mRNA sequence elements in 5' and 3' untranslated regions (UTRs), such as upstream open reading frames (uORFs), internal ribosome entry segments (IRESs) and micro-RNA (miR) target sites, and the altered expression of trans-acting protein factors that bind to and regulate these elements. Translational control is increasingly open for study in both fresh and fixed tissue, and this rapidly developing field is yielding useful diagnostic and prognostic tools that will hopefully provide new targets for effective treatments.

Genome-wide transcriptome analyses reveal p53 inactivation mediated loss of miR-34a expression in malignant peripheral nerve sheath tumours

Malignant peripheral nerve sheath tumours (MPNSTs) are aggressive soft tissue tumours that occur either sporadically or in patients with neurofibromatosis type 1. The malignant transformation of the benign neurofibroma to MPNST is incompletely understood at the molecular level. We have determined the gene expression signature for benign and malignant PNSTs and found that the major trend in malignant transformation from neurofibroma to MPNST consists of the loss of expression of a large number of genes, rather than widespread increase in gene expression. Relatively few genes are expressed at higher levels in MPNSTs and these include genes involved in cell proliferation and genes implicated in tumour metastasis. In addition, a gene expression signature indicating p53 inactivation is seen in the majority of MPNSTs. Subsequent microRNA profiling of benign and malignant PNSTs indicated a relative down-regulation of miR-34a in most MPNSTs compared to neurofibromas. In vitro studies using the cell lines MPNST-14 (NF1 mutant) and MPNST-724 (from a non-NF1 individual) show that exogenous expression of p53 or miR-34a promotes apoptotic cell death. In addition, exogenous expression of p53 in MPNST cells induces miR-34a and other miRNAs. Our data show that p53 inactivation and subsequent loss of expression of miR-34a may significantly contribute to the MPNST development. Collectively, our findings suggest that deregulation of miRNAs has a potential role in the malignant transformation process in peripheral nerve sheath tumours.

miR-9 and NFATc3 regulate myocardin in cardiac hypertrophy

Myocardial hypertrophy is frequently associated with poor clinical outcomes including the development of cardiac systolic and diastolic dysfunction and ultimately heart failure. To prevent cardiac hypertrophy and heart failure, it is necessary to identify and characterize molecules that may regulate the hypertrophic program. Our present study reveals that nuclear factor of activated T cells c3 (NFATc3) and myocardin constitute a hypertrophic pathway that can be targeted by miR-9. Our results show that myocardin expression is elevated in response to hypertrophic stimulation with isoproterenol and aldosterone. In exploring the molecular mechanism by which myocardin expression is elevated, we identified that NFATc3 can bind to the promoter region of myocardin and transcriptionally activate its expression. Knockdown of myocardin can attenuate hypertrophic responses triggered by NFATc3, suggesting that myocardin can be a downstream mediator of NFATc3 in the hypertrophic cascades. MicroRNAs are a class of small noncoding RNAs that mediate post-transcriptional gene silencing. Our data reveal that miR-9 can suppress myocardin expression. However, the hypertrophic stimulation with isoproterenol and aldosterone leads to a decrease in the expression levels of miR-9. Administration of miR-9 could attenuate cardiac hypertrophy and ameliorate cardiac function. Taken together, our data demonstrate that NFATc3 can promote myocardin expression, whereas miR-9 is able to suppress myocardin expression, thereby regulating cardiac hypertrophy.

Teratogen-induced alterations in microRNA-34, microRNA-125b and microRNA-155 expression: correlation with embryonic p53 genotype and limb phenotype

Background

In a large number of studies, members of the microRNA (miRNA)-34 family such as miRNA-34a, miRNA-34b, miRNA-34c, as well as miRNA-125b and miRNA-155, have been shown to be regulators of apoptosis. The ability of these miRNAs to perform this function is mainly attributed to their ability to interact with the p53 tumor suppressor, which is a powerful regulator of the teratologic susceptibility of embryos. We chose to explore whether miRNA-34a/b/c, miRNA-125b and miRNA-155 may play a role in teratogenesis by using p53^+/- pregnant mice treated with cyclophosphamide (CP) as a model. We evaluated how CP-induced alterations in the expression of these miRNAs in the embryonic limbs correlate with embryonic p53 genotype and CP-induced limb phenotypes.

Results

The limbs of p53 positive embryos were more sensitive to CP-induced teratogenic insult than the limbs of p53 negative embryos. The hindlimbs were more severely affected than the forelimbs. Robust miRNA-34a expression was observed in the fore- and hindlimbs of p53^+/+ embryos exposed to 12.5 mg/kg CP. The dose of 20 mg/kg CP induced almost a two-fold increase in the level of miRNA-34a expression as compared to that exhibited by p53^+/+ embryos exposed to a lower dose. Increased miRNA-34b and miRNA-34c expression was also observed. Of note, this dose activated miRNA-34a and miRNA-34c in the forelimbs of p53^-/- embryos. When embryos were exposed to 40 mg/kg CP, the expression pattern of the miRNA-34a/b/c was identical to that registered in the limbs of embryos exposed to 20 mg/kg CP. However, this dose suppressed miRNA-125b and miRNA-155 expression in the fore- and hindlimbs of p53^+/+ embryos.

Conclusion

This study demonstrates that teratogen-induced limb dysmorphogenesis may be associated with alterations in miRNA-34, miRNA-125b and miRNA-155 expression. It also suggests for the first time that p53-independent mechanisms exist contributing to teratogen-induced activation of miRNA-34a and miRNA-34c. At the same time, teratogen-induced suppression of miRNA-125b and miRNA-155 expression may be p53 dependent. The analysis of correlations between the expression pattern of the tested miRNAs and CP induced limb phenotypes implies that miRNAs regulating apoptosis may differ from each other with respect to their functional role in teratogenesis: some miRNAs act to protect embryos, whereas other miRNAs boost a teratogen-induced process of maldevelopment to induce embryonic death.

Frequent downregulation of miR-34 family in human ovarian cancers

PURPOSE

The miR-34 family is directly transactivated by tumor suppressor p53, which is frequently mutated in human epithelial ovarian cancer (EOC). We hypothesized that miR-34 expression would be decreased in EOC and that reconstituted miR-34 expression might reduce cell proliferation and invasion of EOC cells.

EXPERIMENTAL DESIGNS

miR-34 expression was determined by quantitative reverse transcription-PCR and in situ hybridization in a panel of 83 human EOC samples. Functional characterization of miR-34 was accomplished by reconstitution of miR-34 expression in EOC cells with synthetic pre-miR molecules followed by determining changes in proliferation, apoptosis, and invasion.

RESULTS

miR-34a expression is decreased in 100%, and miR-34b*/c in 72%, of EOC with p53 mutation, whereas miR-34a is also downregulated in 93% of tumors with wild-type p53. Furthermore, expression of miR-34b*/c is significantly reduced in stage IV tumors compared with stage III (P = 0.0171 and P = 0.0029, respectively). Additionally, we observed promoter methylation and copy number variations at mir-34. In situ hybridization showed that miR-34a expression is inversely correlated with MET immunohistochemical staining, consistent with translational inhibition by miR-34a. Finally, miR-34 reconstitution experiments in p53 mutant EOC cells resulted in reduced proliferation, motility, and invasion, the latter of which was dependent on MET expression.

CONCLUSIONS

Our work suggests that miR-34 family plays an important role in EOC pathogenesis and reduced expression of miR-34b*/c may be particularly important for progression to the most advanced stages. Part of miR-34 effects on motility and invasion may be explained by regulation of MET, which is frequently overexpressed in EOC.

Onconase responsive genes in human mesothelioma cells: implications for an RNA damaging therapeutic agent

Background

Onconase represents a new class of RNA-damaging drugs. Mechanistically, Onconase is thought to internalize, where it degrades intracellular RNAs such as tRNA and double-stranded RNA, and thereby suppresses protein synthesis. However, there may be additional or alternative mechanism(s) of action.

Methods

In this study, microarray analysis was used to compare gene expression profiles in untreated human malignant mesothelioma (MM) cell lines and cells exposed to 5 μg/ml Onconase for 24 h. A total of 155 genes were found to be regulated by Onconase that were common to both epithelial and biphasic MM cell lines. Some of these genes are known to significantly affect apoptosis (IL-24, TNFAIP3), transcription (ATF3, DDIT3, MAFF, HDAC9, SNAPC1) or inflammation and the immune response (IL-6, COX-2). RT-PCR analysis of selected up- or down-regulated genes treated with varying doses and times of Onconase generally confirmed the expression array findings in four MM cell lines.

Results

Onconase treatment consistently resulted in up-regulation of IL-24, previously shown to have tumor suppressive activity, as well as ATF3 and IL-6. Induction of ATF3 and the pro-apoptotic factor IL-24 by Onconase was highest in the two most responsive MM cell lines, as defined by DNA fragmentation analysis. In addition to apoptosis, gene ontology analysis indicated that pathways impacted by Onconase include MAPK signaling, cytokine-cytokine-receptor interactions, and Jak-STAT signaling.

Conclusions

These results provide a broad picture of gene activity after treatment with a drug that targets small non-coding RNAs and contribute to our overall understanding of MM cell response to Onconase as a therapeutic strategy. The findings provide insights regarding mechanisms that may contribute to the efficacy of this novel drug in clinical trials of MM patients who have failed first line chemotherapy or radiation treatment.

Epigenetic inactivation of the miR-34a in hematological malignancies

miR-34a is a transcriptional target of p53 and implicated in carcinogenesis. We studied the role of miR-34a methylation in a panel of hematological malignancies including acute leukemia [acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL)], chronic leukemia [chronic lymphocytic leukemia (CLL) and chronic myeloid leukemia (CML)], multiple myeloma (MM) and non-Hodgkin's lymphoma (NHL). The methylation status of miR-34a promoter was studied in 12 cell lines and 188 diagnostic samples by methylation-specific polymerase chain reaction. miR-34a promoter was unmethylated in normal controls but methylated in 75% lymphoma and 37% myeloma cell lines. Hypomethylating treatment led to re-expression of pri-miR-34a transcript in lymphoma cells with homozygous miR-34a methylation. In primary samples at diagnosis, miR-34a methylation was detected in 4% CLL, 5.5% MM samples and 18.8% of NHL at diagnosis but none of ALL, AML and CML (P = 0.011). In MM patients with paired samples, miR-34a methylation status remained unchanged at progression. Amongst lymphoid malignancies, miR-34a was preferentially methylated in NHL (P = 0.018), in particular natural killer (NK)/T-cell lymphoma. In conclusion, amongst hematological malignancies, miR-34a methylation is preferentially hypermethylated in NHL, in particular NK/T-cell lymphoma, in a tumor-specific manner, therefore the role of miR-34a in lymphomagenesis warrants further study.

microRNA-34a expression correlates with MDM2 SNP309 polymorphism and treatment-free survival in chronic lymphocytic leukemia

In chronic lymphocytic leukemia (B-CLL), aberrations along the p53 axis lead to decreased overall survival and therapy resistance. Recent studies identified microRNA-34a (miR-34a) as a major downstream target of p53. We monitored the expression of miR-34a during disease development in a murine B-CLL model. miR-34a was up-regulated more than 20-fold during the leukemic but not during the preleukemic phase. In the human system, B-CLL cells also had 4.6-fold higher miR-34a expression compared with B cells of healthy controls. In B-CLL cells of patients with p53 aberrations, miR-34a expression was consistently low. The broad distribution of miR-34a levels in p53 wild-type patients prompted us to study the correlation between single nucleotide polymorphism 309 (SNP309) in the intronic promoter of MDM2 and miR-34a expression. B-CLL cells of patients with the SNP309 GG genotype had significantly lower miR-34a expression levels compared with patients with the TT genotype (P = .002). Low miR-34a levels were able to predict shorter time to treatment (P = .003) and were associated with an abbreviated lymphocyte doubling time. Further, overexpression of miR-34a in primary B-CLL cells induced apoptosis. These findings suggest miR-34a as a possible therapeutic avenue and a sensitive indicator of the activity of the p53 axis in B-CLL.

Novel modulators of senescence, aging, and longevity: Small non-coding RNAs enter the stage

During the last decade evidence has accumulated that the aging process is driven by limited allocation of energy to somatic maintenance resulting in accumulation of stochastic damage. This damage, affecting molecules, cells, and tissues, is counteracted by genetically programmed repair, the efficiency of which thus importantly determines the life and 'health span' of organisms. Therefore, understanding the regulation of gene expression during cellular and organismal aging as well as upon exposure to various damaging events is important to understand the biology of aging and to positively influence the health span. The recent identification of small non-coding RNAs (ncRNAs), has added an additional layer of complexity to the regulation of gene expression with the classes of endogenous small inhibitory RNAs (siRNAs), PIWI-interacting RNAs (piRNAs), QDE1-interacting RNAs (qiRNAs) and microRNAs (miRNAs). Some of these ncRNAs have not yet been identified in mammalian cells and are dependent on RNA-dependent RNA polymerases. The first mammalian enzyme with such activity has only now emerged and surprisingly consists of the catalytic subunit of telomerase (hTERT) together with RMPR, an alternative RNA component. The so far most studied small non-coding RNAs, miRNAs, however, are now increasingly found to operate in the complex network of cellular aging. Recent findings show that (i) miRNAs are regulated during cellular senescence in vitro, (ii) they contribute to tissue regeneration by regulation of stem cell function, and (iii) at least one miRNA modulates the life span of the model organism C. elegans. Additionally, (iv) they act as inhibitors of proteins mediating the insulin/IGF1 and target of rapamycin (TOR) signalling, both of which are conserved modulators of organism life span. Here we will give an overview on the current status of these topics. Since little is so far known on the functions of small ncRNAs in the context of aging and longevity, the entry of the RNA world into the field of biogerontology certainly holds additional surprises and promises. Even more so, as miRNAs are implicated in many age-associated pathologies, and as RNAi and miRNA based therapeutics are on their way to clinics.

Obesity and genetics regulate microRNAs in islets, liver, and adipose of diabetic mice

Type 2 diabetes results from severe insulin resistance coupled with a failure of b cells to compensate by secreting sufficient insulin. Multiple genetic loci are involved in the development of diabetes, although the effect of each gene on diabetes susceptibility is thought to be small. MicroRNAs (miRNAs) are noncoding 19-22-nucleotide RNA molecules that potentially regulate the expression of thousands of genes. To understand the relationship between miRNA regulation and obesity-induced diabetes, we quantitatively profiled approximately 220 miRNAs in pancreatic islets, adipose tissue, and liver from diabetes-resistant (B6) and diabetes-susceptible (BTBR) mice. More than half of the miRNAs profiled were expressed in all three tissues, with many miRNAs in each tissue showing significant changes in response to genetic obesity. Furthermore, several miRNAs in each tissue were differentially responsive to obesity in B6 versus BTBR mice, suggesting that they may be involved in the pathogenesis of diabetes. In liver there were approximately 40 miRNAs that were downregulated in response to obesity in B6 but not BTBR mice, indicating that genetic differences between the mouse strains play a critical role in miRNA regulation. In order to elucidate the genetic architecture of hepatic miRNA expression, we measured the expression of miRNAs in genetically obese F2 mice. Approximately 10% of the miRNAs measured showed significant linkage (miR-eQTLs), identifying loci that control miRNA abundance. Understanding the influence that obesity and genetics exert on the regulation of miRNA expression will reveal the role miRNAs play in the context of obesity-induced type 2 diabetes.

Obesity and genetics regulate microRNAs in islets, liver, and adipose of diabetic mice

Type 2 diabetes results from severe insulin resistance coupled with a failure of b cells to compensate by secreting sufficient insulin. Multiple genetic loci are involved in the development of diabetes, although the effect of each gene on diabetes susceptibility is thought to be small. MicroRNAs (miRNAs) are noncoding 19-22-nucleotide RNA molecules that potentially regulate the expression of thousands of genes. To understand the relationship between miRNA regulation and obesity-induced diabetes, we quantitatively profiled approximately 220 miRNAs in pancreatic islets, adipose tissue, and liver from diabetes-resistant (B6) and diabetes-susceptible (BTBR) mice. More than half of the miRNAs profiled were expressed in all three tissues, with many miRNAs in each tissue showing significant changes in response to genetic obesity. Furthermore, several miRNAs in each tissue were differentially responsive to obesity in B6 versus BTBR mice, suggesting that they may be involved in the pathogenesis of diabetes. In liver there were approximately 40 miRNAs that were downregulated in response to obesity in B6 but not BTBR mice, indicating that genetic differences between the mouse strains play a critical role in miRNA regulation. In order to elucidate the genetic architecture of hepatic miRNA expression, we measured the expression of miRNAs in genetically obese F2 mice. Approximately 10% of the miRNAs measured showed significant linkage (miR-eQTLs), identifying loci that control miRNA abundance. Understanding the influence that obesity and genetics exert on the regulation of miRNA expression will reveal the role miRNAs play in the context of obesity-induced type 2 diabetes.

Development and progression of colorectal neoplasia

A variety of genetic and molecular alterations underlie the development and progression of colorectal neoplasia (CRN). Most of these cancers arise sporadically due to multiple somatic mutations and genetic instability. Genetic instability includes chromosomal instability (CIN) and microsatellite instability (MSI), which is observed in most hereditary non-polyposis colon cancers (HNPCCs) and accounts for a small proportion of sporadic CRN. Although many biomarkers have been used in the diagnosis and prediction of the clinical outcomes of CRNs, no single marker has established value. New markers and genes associated with the development and progression of CRNs are being discovered at an accelerated rate. CRN is a heterogeneous disease, especially with respect to the anatomic location of the tumor, race/ethnicity differences, and genetic and dietary interactions that influence its development and progression and act as confounders. Hence, efforts related to biomarker discovery should focus on identification of individual differences based on tumor stage, tumor anatomic location, and race/ethnicity; on the discovery of molecules (genes, mRNA transcripts, and proteins) relevant to these differences; and on development of therapeutic approaches to target these molecules in developing personalized medicine. Such strategies have the potential of reducing the personal and socio-economic burden of CRNs. Here, we systematically review molecular and other pathologic features as they relate to the development, early detection, diagnosis, prognosis, progression, and prevention of CRNs, especially colorectal cancers (CRCs).

Cross-talk between miRNA and Notch signaling pathways in tumor development and progression

Notch signaling pathways are known to regulate many cellular processes, including cell proliferation, apoptosis, migration, invasion, and angiogenesis, and is one of the most important signaling pathway during normal development. Recently, emerging evidences suggest that microRNAs (miRNAs) can function as key regulators of various biological and pathologic processes during tumor development and progression. Notch signaling has also been reported to be regulated through cross-talk with many pathways and factors where miRNAs appears to play a major role. This article will provide a brief overview of the published evidences for the cross-talks between Notch and miRNAs. Further, we summarize how targeting miRNAs by natural agents could become a novel and safer approach for the prevention of tumor progression and treatment.

The neuronal microRNA miR-326 acts in a feedback loop with notch and has therapeutic potential against brain tumors

Little is known of microRNA interactions with cellular pathways. Few reports have associated microRNAs with the Notch pathway, which plays key roles in nervous system development and in brain tumors. We previously implicated the Notch pathway in gliomas, the most common and aggressive brain tumors. While investigating Notch mediators, we noted microRNA-326 was upregulated following Notch-1 knockdown. This neuronally expressed microRNA was not only suppressed by Notch but also inhibited Notch proteins and activity, indicating a feedback loop. MicroRNA-326 was downregulated in gliomas via decreased expression of its host gene. Transfection of microRNA-326 into both established and stem cell-like glioma lines was cytotoxic, and rescue was obtained with Notch restoration. Furthermore, miR-326 transfection reduced glioma cell tumorigenicity in vivo. Additionally, we found microRNA-326 partially mediated the toxic effects of Notch knockdown. This work demonstrates a microRNA-326/Notch axis, shedding light on the biology of Notch and suggesting microRNA-326 delivery as a therapy.

MicroRNAs regulate human hepatocyte nuclear factor 4alpha, modulating the expression of metabolic enzymes and cell cycle

Hepatocyte nuclear factor (HNF) 4alpha is a key transcription factor regulating endo/xenobiotic-metabolizing enzymes and transporters. We investigated whether microRNAs are involved in the regulation of human HNF4alpha. Potential recognition elements for miR-24 (MRE24) were identified in the coding region and the 3'-untranslated region (3'-UTR), and those for miR-34a (MRE34a) were identified in the 3'-UTR in HNF4alpha mRNA. The HNF4alpha protein level in HepG2 cells was markedly decreased by the overexpression of miR-24 and miR-34a. The HNF4alpha mRNA level was significantly decreased by the overexpression of miR-24 but not by miR-34a. In luciferase analyses in HEK293 cells, the reporter activity of plasmid containing the 3'-UTR of HNF4alpha was significantly decreased by miR-34a. The reporter activity of plasmid containing the HNF4alpha coding region downstream of the luciferase gene was significantly decreased by miR-24. These results suggest that the MRE24 in the coding region and MRE34a in the 3'-UTR are functional in the negative regulation by mRNA degradation and translational repression, respectively. The down-regulation of HNF4alpha by these microRNAs resulted in the decrease of various target genes such as cytochrome P450 7A1 and 8B1 as well as morphological changes and the decrease of the S phase population in HepG2 cells. We also clarified that the expressions of miR-24 and miR-34a were regulated by protein kinase C/mitogen-activated protein kinase and reactive oxygen species pathways, respectively. In conclusion, we found that human HNF4alpha was down-regulated by miR-24 and miR-34a, the expression of which are regulated by cellular stress, affecting the metabolism and cellular biology.

MicroRNA in pancreatic cancer: pathological, diagnostic and therapeutic implications

MicroRNAs (miRNAs) are a group of small non-coding RNA molecules of 17-25 nucleotides (nt) in length, predicted to control the activity of about 30% of all protein-coding genes in mammals. Altered expressions of miRNAs are reported in various cancers and may associate with cancer pathogenesis, apoptosis, and cell growth, thereby functioning as either tumor suppressors or oncogenes. Recent reports showed that deregulation of miRNA contribute to tumor development and progression and hence, have diagnostic and prognostic value in several human malignancies. This review discusses the current status of miRNA in pancreatic cancer development, progression, diagnosis, and therapy.

E2F1-inducible microRNA 449a/b suppresses cell proliferation and promotes apoptosis

E2F1 is a positive regulator of cell cycle progression and also a potent inducer of apoptosis, especially when activated by DNA damage. We identified E2F1-inducible microRNAs (miRNAs) by microarray hybridization and found that the levels of miRNAs 449a and 449b, as well as their host gene CDC20B, are strongly upregulated by E2F1. High miR-449 levels were found in testes, lung, and trachea, but not in testicular and other cancer cells. MiR-449a/b structurally resemble the p53-inducible miRNA 34 family. In agreement with a putative tumor-suppressive role, miR-449a as well as miR-34a reduced proliferation and strongly promoted apoptosis by at least partially p53-independent mechanisms. Both miRNAs reduced the levels of CDK6, implying miR-449 in a negative feedback mechanism for E2F1. Moreover, miR-449a and miR-34a diminished the deacetylase Sirt1 and augmented p53 acetylation. We propose that both miRNAs provide a twofold safety mechanism to avoid excessive E2F1-induced proliferation by cell cycle arrest and by apoptosis. While responding to different transactivators, miRNAs 449 and 34 each repress E2F1, but promote p53 activity, allowing efficient cross-talk between two major DNA damage-responsive gene regulators.

A genetic variant in the pre-miR-27a oncogene is associated with a reduced familial breast cancer risk

MicroRNAs (miRNAs) regulate pathways involved in cell differentiation, proliferation, development, and apoptosis by degradation of target mRNAs and/or repression of their translation. Although the single nucleotide polymorphisms (SNPs) in miRNAs target sites have been studied, the effects of SNPs in miRNAs are largely unknown. In our study, we first systematically sequenced miRNA genes reported to be involved in breast cancer to identify/verify SNPs. We analyzed four SNPs, one located in the pre-miRNA and the other three located in miRNA flanking regions, for a putative association with breast cancer risk. The SNP rs895819, located in the terminal loop of pre-miRNA-27a, showed a protective effect. In a large familial breast cancer study cohort, the rare [G] allele of rs895819 was found to be less frequent in the cases than in the controls, indicating a reduced familial breast cancer risk ([G] vs. [A]: OR = 0.88, 95% CI 0.78-0.99, P = 0.0287). Furthermore, age stratification revealed that the protective effect was mainly observed in the age group < 50 years of age ([G] vs. [A]: OR = 0.83, 95% CI 0.70-0.98, P = 0.0314), whereas no significant effect was observed in the age group >or= 50 years of age, indicating a possible hormone-related effect. It has been shown that artificial mutations in the terminal loop of miR-27a can block the maturation process of the miRNA. We hypothesize that the G-variant of rs895819 might impair the maturation of the oncogenic miR-27a and thus, is associated with familial breast cancer risk.

Analysis of microRNA expression profiles during the cell cycle in synchronized HeLa cells

Cell cycle progression is regulated by both transcriptional and post-transcriptional mechanisms. MicroRNAs (miRNAs) emerge as a new class of small non-coding RNA regulators of cell cycle as recent evidence suggests. It is hypothesized that expression of specific miRNAs oscillates orderly along with cell cycle progression. However, the oscillated expression patterns of many candidate miRNAs have yet to be determined. Here, we describe miRNA expression profiling in double-thymidine synchronized HeLa cells as cell cycle progresses. Twenty-five differentially expressed miRNAs were classified into five groups based on their cell cycle-dependent expression patterns. The cyclic expression of six miRNAs (miR-221, let-7a, miR-21, miR-34a, miR-24, miR-376b) was validated by real-time quantitative RT-PCR (qRT-PCR). These results suggest that specific miRNAs, along with other key factors are required for maintaining and regulating proper cell cycle progression. The study deepens our understanding on cell cycle regulation. [BMB reports 2009; 42(9): 593-598].

MicroRNAs in colorectal cancer: translation of molecular biology into clinical application

MicroRNAs (miRNAs) are small non-coding RNAs 18-25 nucleotides in length that downregulate gene expression during various crucial cell processes such as apoptosis, differentiation and development. Changes in the expression profiles of miRNAs have been observed in a variety of human tumors, including colorectal cancer (CRC). Functional studies indicate that miRNAs act as tumor suppressors and oncogenes. These findings significantly extend Vogelstein's model of CRC pathogenesis and have shown great potential for miRNAs as a novel class of therapeutic targets. Several investigations have also described the ability of miRNA expression profiles to predict prognosis and response to selected treatments in CRC patients, and support diagnosis of CRC among cancer of unknown primary site. miRNAs' occurrence has been repeatedly observed also in serum and plasma, and miRNAs as novel minimally invasive biomarkers have indicated reasonable sensitivity for CRC detection and compare favorably with the fecal occult blood test. In this review, we summarize the knowledge regarding miRNAs' functioning in CRC while emphasizing their significance in pathogenetic signaling pathways and their potential to serve as disease biomarkers and novel therapeutic targets.

Identification of microRNA activity by Targets' Reverse EXpression

MOTIVATION

Non-coding microRNAs (miRNAs) act as regulators of global protein output. While their major effect is on protein levels of target genes, it has been proven that they also specifically impact on the messenger RNA level of targets. Prominent interest in miRNAs strongly motivates the need for increasing the options available to detect their cellular activity.

RESULTS

We used the effect of miRNAs over their targets for the detection of miRNA activity using mRNAs expression profiles. Here we describe the method, called T-REX (from Targets' Reverse EXpression), compare it to other similar applications, show its effectiveness and apply it to build activity maps. We used six different target predictions from each of four algorithms: TargetScan, PicTar, DIANA-microT and DIANA Union. First, we proved the sensitivity and specificity of our technique in miRNA over-expression and knock-out animal models. Then, we used whole transcriptome data from acute myeloid leukemia to show that we could identify critical miRNAs in a real life, complex, clinically relevant dataset. Finally, we studied 66 different cellular conditions to confirm and extend the current knowledge on the role of miRNAs in cellular physiology and in cancer.

AVAILABILITY

Software is available at http://aqua.unife.it and is free for all users with no login requirement.

miR-449a and miR-449b are direct transcriptional targets of E2F1 and negatively regulate pRb-E2F1 activity through a feedback loop by targeting CDK6 and CDC25A

The Rb-E2F pathway drives cell cycle progression and cell proliferation, and the molecular strategies safeguarding its activity are not fully understood. Here we report that E2F1 directly transactivates miR-449a/b. miR-449a/b targets and inhibits oncogenic CDK6 and CDC25A, resulting in pRb dephosphorylation and cell cycle arrest at G1 phase, revealing a negative feedback regulation of the pRb-E2F1 pathway. Moreover, miR-449a/b expression in cancer cells is epigenetically repressed through histone H3 Lys27 trimethylation, and epigenetic drug treatment targeting histone methylation results in strong induction of miR-449a/b. Our study reveals a tumor suppressor function of miR-449a/b through regulating Rb/E2F1 activity, and suggests that escape from this regulation through an aberrant epigenetic event contributes to E2F1 deregulation and unrestricted proliferation in human cancer.

A resource for analysis of microRNA expression and function in pancreatic ductal adenocarcinoma cells

MicroRNAs (miRNAs) are 21-24 nucleotide RNA molecules that regulate the translation and stability of target messenger RNAs. Abnormal miRNA expression is a common feature of diverse cancers. Several previous studies have classified miRNA expression in pancreatic ductal adenocarcinoma (PDAC), although no uniform pattern of miRNA dysregulation has emerged. To clarify these previous findings as well as to set the stage for detailed functional analyses, we performed global miRNA expression profiling of 21 human PDAC cell lines, the most extensive panel studied to date. Overall, 39 miRNAs were found to be dysregulated and have at least two-fold or greater differential expression in PDAC cell lines compared to control nontransformed pancreatic ductal cell lines. Several of these miRNAs show comparable dysregulation in first-passage patient derived xenografts. Initial functional analyses demonstrate that enforced expression of miRNAs derived from the miR-200 family and the miR-17-92 cluster, both of which are overexpressed in PDAC cell lines, enhances proliferation. In contrast, inhibition of the miR-200 family, the miR-17-92 cluster, or miR-191 diminishes anchorage independent growth. Consistent with a known role for the miR-200 family in negatively regulating an epithelial-to-mesenchymal transition (EMT), the abundance of these miRNAs correlated positively with E-cadherin expression and negatively with the EMT-associated transcription factor and established miR-200 target ZEB1. Finally, restituted expression of miR-34a, a miRNA whose expression is frequently lost in PDAC cell lines, abrogates growth, demonstrating that the anti-proliferative activity of this miRNA is operative in PDAC. These results, and the widespread availability of PDAC cell lines wherein the aforementioned data were generated, provide a valuable resource for the pancreatic cancer research community and will greatly facilitate functional studies essential for elucidating the consequences of miRNA dysregulation in pancreatic cancer.

The microRNA signature in response to insulin reveals its implication in the transcriptional action of insulin in human skeletal muscle and the role of a sterol regulatory element-binding protein-1c/myocyte enhancer factor 2C pathway

OBJECTIVE

Factors governing microRNA expressions in response to changes of cellular environment are still largely unknown. Our aim was to determine whether insulin, the major hormone controlling whole-body energy homeostasis, is involved in the regulation of microRNA expressions in human skeletal muscle.

RESEARCH DESIGN AND METHODS

We carried out comparative microRNA (miRNA) expression profiles in human skeletal muscle biopsies before and after a 3-h euglycemic-hyperinsulinemic clamp, with TaqMan low-density arrays. Then, using DNA microarrays, we determined the response to insulin of the miRNA putative target genes in order to determine their role in the transcriptional action of insulin. We further characterized the mechanism of action of insulin on two representative miRNAs, miR-1 and miR-133a, in human muscle cells.

RESULTS

Insulin downregulated the expressions of 39 distinct miRNAs in human skeletal muscle. Their potential target mRNAs coded for proteins that were mainly involved in insulin signaling and ubiquitination-mediated proteolysis. Bioinformatic analysis suggested that combinations of different downregulated miRNAs worked in concert to regulate gene expressions in response to insulin. We further demonstrated that sterol regulatory element-binding protein (SREBP)-1c and myocyte enhancer factor 2C were involved in the effect of insulin on miR-1 and miR-133a expression. Interestingly, we found an impaired regulation of miRNAs by insulin in the skeletal muscle of type 2 diabetic patients, likely as consequences of altered SREBP-1c activation.

CONCLUSIONS

This work demonstrates a new role of insulin in the regulation of miRNAs in human skeletal muscle and suggests a possible implication of these new modulators in insulin resistance.

Sequence-non-specific effects of RNA interference triggers and microRNA regulators

RNA reagents of diverse lengths and structures, unmodified or containing various chemical modifications are powerful tools of RNA interference and microRNA technologies. These reagents which are either delivered to cells using appropriate carriers or are expressed in cells from suitable vectors often cause unintended sequence-non-specific immune responses besides triggering intended sequence-specific silencing effects. This article reviews the present state of knowledge regarding the cellular sensors of foreign RNA, the signaling pathways these sensors mobilize and shows which specific features of the RNA reagents set the responsive systems on alert. The representative examples of toxic effects caused in the investigated cell lines and tissues by the RNAs of specific types and structures are collected and may be instructive for further studies of sequence-non-specific responses to foreign RNA in human cells.

Reduced levels of miR-34a in neuroblastoma are not caused by mutations in the TP53 binding site

Neuroblastoma (NB) is the most common extracranial solid tumor in children below the age of 5 years. miR-34a, located in chromosome band 1p36, has been recently implicated as a tumor suppressor gene in NB. In addition, it has been shown that miR-34a is activated by TP53 by binding to a TP53 binding site upstream to the mature miR-34a. We studied NB tumors from 57 patients for miR-34a expression levels, 1p status, mutations in the TP53 coding region and mutations of the TP53 binding site. Reduced expression levels of miR-34a were identified in tumors harboring 1p36.3 Loss (P = 0.028). No mutations were identified in the coding region of TP53, or in the TP53 binding site. Thus, mutations in the binding site are not an additional mechanism for the inactivation of miR-34a in NB. Other regulatory mechanisms controlling miR-34a expression and its relationship to TP53 should be further explored.

The first 30 years of p53: growing ever more complex

Thirty years ago p53 was discovered as a cellular partner of simian virus 40 large T-antigen, the oncoprotein of this tumour virus. The first decade of p53 research saw the cloning of p53 DNA and the realization that p53 is not an oncogene but a tumour suppressor that is very frequently mutated in human cancer. In the second decade of research, the function of p53 was uncovered: it is a transcription factor induced by stress, which can promote cell cycle arrest, apoptosis and senescence. In the third decade after its discovery new functions of this protein were revealed, including the regulation of metabolic pathways and cytokines that are required for embryo implantation. The fourth decade of research may see new p53-based drugs to treat cancer. What is next is anybody's guess.

Causes and consequences of microRNA dysregulation in cancer

Over the past several years it has become clear that alterations in the expression of microRNA (miRNA) genes contribute to the pathogenesis of most--if not all--human malignancies. These alterations can be caused by various mechanisms, including deletions, amplifications or mutations involving miRNA loci, epigenetic silencing or the dysregulation of transcription factors that target specific miRNAs. Because malignant cells show dependence on the dysregulated expression of miRNA genes, which in turn control or are controlled by the dysregulation of multiple protein-coding oncogenes or tumour suppressor genes, these small RNAs provide important opportunities for the development of future miRNA-based therapies.

The mammalian ovary from genesis to revelation

Two major functions of the mammalian ovary are the production of germ cells (oocytes), which allow continuation of the species, and the generation of bioactive molecules, primarily steroids (mainly estrogens and progestins) and peptide growth factors, which are critical for ovarian function, regulation of the hypothalamic-pituitary-ovarian axis, and development of secondary sex characteristics. The female germline is created during embryogenesis when the precursors of primordial germ cells differentiate from somatic lineages of the embryo and take a unique route to reach the urogenital ridge. This undifferentiated gonad will differentiate along a female pathway, and the newly formed oocytes will proliferate and subsequently enter meiosis. At this point, the oocyte has two alternative fates: die, a common destiny of millions of oocytes, or be fertilized, a fate of at most approximately 100 oocytes, depending on the species. At every step from germline development and ovary formation to oogenesis and ovarian development and differentiation, there are coordinated interactions of hundreds of proteins and small RNAs. These studies have helped reproductive biologists to understand not only the normal functioning of the ovary but also the pathophysiology and genetics of diseases such as infertility and ovarian cancer. Over the last two decades, parallel progress has been made in the assisted reproductive technology clinic including better hormonal preparations, prenatal genetic testing, and optimal oocyte and embryo analysis and cryopreservation. Clearly, we have learned much about the mammalian ovary and manipulating its most important cargo, the oocyte, since the birth of Louise Brown over 30 yr ago.

PPEF/PP7 protein Ser/Thr phosphatases

PPEF/PP7 represents one of the five subfamilies of the PPP protein Ser/Thr phosphatases. Studies published in recent years point to a role of plant PP7 at a crossroad of different pathways of light and stress signalling. In animals, PPEFs are highly expressed in sensory neurons, and Drosophila PPEF phosphatase, rdgC, is essential for dephosphorylation of rhodopsin. Expression profiling suggests that mammalian PPEF may play a role in stress-protective responses, cell survival, growth, proliferation, and oncogenesis. Despite structural similarities of the catalytic domains and the fact that some of these phosphatases are involved in light perception both in animals and in plants, the plant and non-plant representatives of this group have distinct domain architecture and appear not to be orthologues.

The complexities of microRNA regulation: mirandering around the rules

MicroRNAs (miRNAs) are an important class of non-coding small RNAs that possess a large range of biological activities in a variety of organisms and are linked to human diseases such as cancer. Initially, miRNAs were thought to act solely as negative regulators of gene expression and exert their effects by binding to regions within the 3'UTR of their target protein-coding messenger RNAs (mRNAs) in a sequence dependent manner. However, recent data reveals that miRNA regulation entails a far more complex system of post-transcriptional control than initially appreciated. An evolving consensus has emerged of how miRNAs can repress as well as activate gene expression by interacting with complementary regions found in the promoter, coding region, as well as the 3'UTR of their mRNA targets. Furthermore, miRNAs are extensively regulated at the levels of miRNA promoter transcription, methylation, miRNA processing, RNA editing, and miRNA-target interactions. This review will discuss new insights into miRNA-based mechanisms and the role specific DNA- and RNA-binding factors play in fine-tuning gene expression in both negative and positive ways by directing miRNA biogenesis and activity. We will also discuss the influence that cellular context and environmental cues have on miRNA function. In the future, a clear understanding of miRNA regulation will be essential when understanding the role miRNAs play during animal development and in maintaining adult homeostasis as well as exploring the use of small RNAs for diagnostic and therapeutic purposes.

Transfection of small RNAs globally perturbs gene regulation by endogenous microRNAs

Transfection of small RNAs (such as small interfering RNAs (siRNAs) and microRNAs (miRNAs)) into cells typically lowers expression of many genes. Unexpectedly, increased expression of genes also occurs. We investigated whether this upregulation results from a saturation effect--that is, competition among the transfected small RNAs and the endogenous pool of miRNAs for the intracellular machinery that processes small RNAs. To test this hypothesis, we analyzed genome-wide transcript responses from 151 published transfection experiments in seven different human cell types. We show that targets of endogenous miRNAs are expressed at significantly higher levels after transfection, consistent with impaired effectiveness of endogenous miRNA repression. This effect exhibited concentration and temporal dependence. Notably, the profile of endogenous miRNAs can be largely inferred by correlating miRNA sites with gene expression changes after transfections. The competition and saturation effects have practical implications for miRNA target prediction, the design of siRNA and short hairpin RNA (shRNA) genomic screens and siRNA therapeutics.

Transfection of small RNAs globally perturbs gene regulation by endogenous microRNAs

Transfection of small RNAs (such as small interfering RNAs (siRNAs) and microRNAs (miRNAs)) into cells typically lowers expression of many genes. Unexpectedly, increased expression of genes also occurs. We investigated whether this upregulation results from a saturation effect--that is, competition among the transfected small RNAs and the endogenous pool of miRNAs for the intracellular machinery that processes small RNAs. To test this hypothesis, we analyzed genome-wide transcript responses from 151 published transfection experiments in seven different human cell types. We show that targets of endogenous miRNAs are expressed at significantly higher levels after transfection, consistent with impaired effectiveness of endogenous miRNA repression. This effect exhibited concentration and temporal dependence. Notably, the profile of endogenous miRNAs can be largely inferred by correlating miRNA sites with gene expression changes after transfections. The competition and saturation effects have practical implications for miRNA target prediction, the design of siRNA and short hairpin RNA (shRNA) genomic screens and siRNA therapeutics.

miR-24 Inhibits cell proliferation by targeting E2F2, MYC, and other cell-cycle genes via binding to "seedless" 3'UTR microRNA recognition elements

miR-24, upregulated during terminal differentiation of multiple lineages, inhibits cell-cycle progression. Antagonizing miR-24 restores postmitotic cell proliferation and enhances fibroblast proliferation, whereas overexpressing miR-24 increases the G1 compartment. The 248 mRNAs downregulated upon miR-24 overexpression are highly enriched for DNA repair and cell-cycle regulatory genes that form a direct interaction network with prominent nodes at genes that enhance (MYC, E2F2, CCNB1, and CDC2) or inhibit (p27Kip1 and VHL) cell-cycle progression. miR-24 directly regulates MYC and E2F2 and some genes that they transactivate. Enhanced proliferation from antagonizing miR-24 is abrogated by knocking down E2F2, but not MYC, and cell proliferation, inhibited by miR-24 overexpression, is rescued by miR-24-insensitive E2F2. Therefore, E2F2 is a critical miR-24 target. The E2F2 3'UTR lacks a predicted miR-24 recognition element. In fact, miR-24 regulates expression of E2F2, MYC, AURKB, CCNA2, CDC2, CDK4, and FEN1 by recognizing seedless but highly complementary sequences.

Reduced transcriptional activity in the p53 pathway of senescent cells revealed by the MDM2 antagonist nutlin-3

The p53 tumor suppressor plays a key role in induction and maintenance of cellular senescence but p53-regulated response to stress in senescent cells is poorly understood. Here, we use the small-molecule MDM2 antagonist, nutlin-3a, to selectively activate p53 and probe functionality of the p53 pathway in senescent human fibroblasts, WI-38. Our experiments revealed overall reduction in nutlin-induced transcriptional activity of nine p53 target genes and four p53-regulated microRNAs, indicating that not only p53 protein levels but also its ability to activate transcription are altered during senescence. Addition of nutlin restored doxorubicin-induced p53 protein and transcriptional activity in senescent cells to the levels in early passage cells but only partially restored its apoptotic activity, suggesting that changes in both upstream and downstream p53 signaling during senescence are responsible for attenuated response to genotoxic stress.

MicroRNA-34a inhibits glioblastoma growth by targeting multiple oncogenes

MicroRNA-34a (miR-34a) is a transcriptional target of p53 that is down-regulated in some cancer cell lines. We studied the expression, targets, and functional effects of miR-34a in brain tumor cells and human gliomas. Transfection of miR-34a down-regulated c-Met in human glioma and medulloblastoma cells and Notch-1, Notch-2, and CDK6 protein expressions in glioma cells. miR-34a expression inhibited c-Met reporter activities in glioma and medulloblastoma cells and Notch-1 and Notch-2 3'-untranslated region reporter activities in glioma cells and stem cells. Analysis of human specimens showed that miR-34a expression is down-regulated in glioblastoma tissues as compared with normal brain and in mutant p53 gliomas as compared with wild-type p53 gliomas. miR-34a levels in human gliomas inversely correlated to c-Met levels measured in the same tumors. Transient transfection of miR-34a into glioma and medulloblastoma cell lines strongly inhibited cell proliferation, cell cycle progression, cell survival, and cell invasion, but transfection of miR-34a into human astrocytes did not affect cell survival and cell cycle status. Forced expression of c-Met or Notch-1/Notch-2 transcripts lacking the 3'-untranslated region sequences partially reversed the effects of miR-34a on cell cycle arrest and cell death in glioma cells and stem cells, respectively. Also, transient expression of miR-34a in glioblastoma cells strongly inhibited in vivo glioma xenograft growth. Together, these findings represent the first comprehensive analysis of the role of miR-34a in gliomas. They show that miR-34a suppresses brain tumor growth by targeting c-Met and Notch. The results also suggest that miR-34a could serve as a potential therapeutic agent for brain tumors.

Coupling transcriptional and post-transcriptional miRNA regulation in the control of cell fate

miRNAs function as a critical regulatory layer in development, differentiation, and the maintenance of cell fate. Depletion of miRNAs from embryonic stem cells impairs their differentiation capacity. Total elimination of miRNAs leads to premature senescence in normal cells and tissues through activation of the DNA-damage checkpoint, whereas ablation of miRNAs in cancer cell lines results in an opposite effect, enhancing their tumorigenic potential. Here we compile evidence from the literature that point at miRNAs as key players in the maintenance of genomic integrity and proper cell fate. There is an apparent gap between our understanding of the subtle way by which miRNAs modulate protein levels, and their profound impact on cell fate. We propose that examining miRNAs in the context of the regulatory transcriptional and post-transcriptional networks they are embedded in may provide a broader view of their role in controlling cell fate.

miR-21: an androgen receptor-regulated microRNA that promotes hormone-dependent and hormone-independent prostate cancer growth

Androgen receptor (AR)-mediated oncogenic pathways have not been fully elucidated. In this study, we used high-throughput microarray analysis on two AR-positive prostate cancer (CaP) cell lines to identify 16 AR-responsive microRNAs (miRNA). We focused on miR-21 because of its previously reported oncogenic activity in other cancers. We show androgen-induced AR binding to the defined miR-21 promoter, miPPR-21, suggesting direct transcriptional regulation. Inhibition of miR-21 diminished androgen-induced CaP cell proliferation, providing new evidence that miRNAs can contribute to androgen-driven cell growth. Elevated expression of miR-21 enhanced CaP tumor growth in vivo and, surprisingly, was sufficient for androgen-dependent tumors to overcome castration-mediated growth arrest. Thus, elevated miR-21 expression alone is sufficient to impart castration resistance. Moreover, quantitative reverse transcription-PCR analysis revealed elevated miR-21 expression in CaP when compared with adjacent normal tissue. These results suggest that miR-21 may contribute to CaP pathogenesis.