MicroRNA-125b confers the resistance of breast cancer cells to paclitaxel through suppression of pro-apoptotic Bcl-2 antagonist killer 1 (Bak1) expression

MicroRNAs in cancer management and their modulation by dietary agents

MicroRNAs (miRNAs) represent a class of small (21-23 nucleotides) non-coding RNAs that emerged as key post-transcriptional gene regulators, implicated in numerous physiological and pathological processes. Currently, a main focus of miRNA research is related to the roles of miRNAs in cancer development. The biogenesis and modes of action of miRNAs have not been completely elucidated; however, miRNA-mediated translational repression is involved in the regulation of almost every cellular process. Thus, pathological alterations in miRNA expression signatures are commonly associated with disease development. This review specifically focuses on miRNAs in cancer, with an emphasis on their use as potential biomarkers for cancer diagnosis and prognosis. Then, we discuss the potential use of synthetic antisense or miRNA mimetic oligonucleotides and dietary agents to modulate miRNA expression for chemotherapy and chemoprevention of cancer, respectively.

mRNA for N-Bak, a neuron-specific BH3-only splice isoform of Bak, escapes nonsense-mediated decay and is translationally repressed in the neurons

mRNA for neuronal Bak (N-Bak), a splice variant of pro-apoptotic Bcl-2 family member Bak is expressed in the neurons. Surprisingly the endogeneous N-Bak protein cannot be demonstrated in the neurons, although the antibodies recognize N-Bak protein from in vitro translation or transiently transfected cells. As N-Bak mRNA contains premature termination codon (PTC) at 89 nucleotides upstream from the last exon–exon junction, it could be degraded by nonsense-mediated decay (NMD) during the pioneer round of translation thus explaining the absence of the protein. We show here that the endogeneous neuronal N-Bak mRNA is not the NMD substrate, as it is not accumulating by cycloheximide treatment, it has a long lifetime, and even prevention of PTC by interfering with the alternative splicing did not lead to translation of the Bak mRNA. N-Bak protein is also not revealed by proteasome inhibitors. Our data suggest strong translational arrest of N-Bak mRNA in the neurons. We show that this arrest is partially mediated by 5′-untranslated region of Bak mRNA and it is not released during mitochondrial apoptosis.

βII-Tubulin and βIII-Tubulin Mediate Sensitivity to Peloruside A and Laulimalide, but not Paclitaxel or Vinblastine, in Human Ovarian Carcinoma Cells

Increased abundance of βII- and βIII-tubulin isotypes in cancer cells confers resistance to vinca and taxoid site drugs; however, the role of these isotypes in the acquired resistance of cancer cells to non-vinca or non-taxoid site binding agents has not been described. Peloruside A (PLA) and laulimalide are the only known non-taxoid site microtubule-stabilizing agents. A human ovarian cancer cell line, 1A9-L4 (L4), previously selected in high concentrations of laulimalide, has both a single point mutation in βI-tubulin and overexpression of βII- and βIII-tubulin. The cells are highly resistant to PLA as well as laulimalide but show no cross-resistance to taxoid site drugs or drugs that bind to the vinca site on β-tubulin. To understand the functional significance of the βII- and βIII-tubulin changes in this resistant cell line, isotype-specific short interfering RNA was used to knock down the expression of the βII and βIII isotypes, and the cellular effects of PLA and laulimalide were examined before and after silencing. It was found that inhibition of βII- and βIII-tubulin partially sensitized L4 cells to PLA and laulimalide, as seen by increased potency of PLA and laulimalide for inducing growth inhibition, cellular tubulin polymerization, microtubule aberrations, and G2-M arrest in the resistant cells. The sensitivity to paclitaxel, vinblastine, ixabepilone, and cisplatin was unaffected by the inhibition of isotype expression. It was concluded that the increased βII- and βIII-tubulin contributed significantly to the resistance phenotype, along with the tubulin structural mutation, and that the altered isotype effect was binding site specific. Mol Cancer Ther; 11(2); 393–404. ©2011 AACR.

Molecular mechanisms underlying the role of microRNAs (miRNAs) in anticancer drug resistance and implications for clinical practice

Drug resistance remains a major problem in the treatment of cancer patients for both conventional chemotherapeutic and novel biological agents. Intrinsic or acquired resistance can be caused by a range of mechanisms, including increased drug elimination, decreased drug uptake, drug inactivation and alterations of drug targets. Recent data showed that other than by genetic (mutation, amplification) and epigenetic (DNA hypermethylation, histone post-translational modification) changes, drug resistance mechanisms might also be regulated by microRNAs (miRNAs). In this review we provide an overview on the role of miRNAs in anticancer drug resistance, reporting the main studies on alterations in cell survival and/or apoptosis pathways, as well as in drug targets and determinants of drug metabolism, mediated by deregulation of miRNA expression. The current status of pharmacogenetic studies on miRNA and their possible role in cancer stem cell drug resistance are also discussed. Finally, we integrated the preclinical data with clinical evidences, in lung and pancreatic cancers, showing how the study of miRNAs could help to predict resistance of individual tumours to different anticancer drugs, and guide the oncologists in the selection of rationally based tailor-made treatments.

Disruption of CTCF at the miR-125b1 locus in gynecological cancers

BACKGROUND

In cancer cells, transcriptional gene silencing has been associated with genetic and epigenetic defects. The disruption of DNA methylation patterns and covalent histone marks has been associated with cancer development. Until recently, microRNA (miRNA) gene silencing was not well understood. In particular, miR-125b1 has been suggested to be an miRNA with tumor suppressor activity, and it has been shown to be deregulated in various human cancers. In the present study, we evaluated the DNA methylation at the CpG island proximal to the transcription start site of miR-125b1 in cancer cell lines as well as in normal tissues and gynecological tumor samples. In addition, we analyzed the association of CTCF and covalent histone modifications at the miR-125b1 locus.

METHODS

To assess the DNA methylation status of the miR-125b1, genomic DNA was transformed with sodium bisulfite, and then PCR-amplified with modified primers and sequenced. The miR-125b1 gene expression was analyzed by qRT-PCR using U6 as a control for constitutive gene expression. CTCF repressive histone marks abundance was evaluated by chromatin immunoprecipitation assays.

RESULTS

The disruption of CTCF in breast cancer cells correlated with the incorporation of repressive histone marks such H3K9me3 and H3K27me3 as well as with aberrant DNA methylation patterns. To determine the effect of DNA methylation at the CpG island of miR-125b1 on the expression of this gene, we performed a qRT-PCR assay. We observed a significant reduction on the expression of miR-125b1 in cancer cells in comparison with controls, suggesting that DNA methylation at the CpG island might reduce miR-125b1 expression. These effects were observed in other gynecological cancers, including ovarian and cervical tumors.

CONCLUSIONS

A reduction of miR-125b1 expression in cancers, correlated with methylation, repressive histone marks and loss of CTCF binding at the promoter region.

The emerging important role of microRNAs in the pathogenesis, diagnosis and treatment of human cancers

MicroRNAs are small non-protein-coding RNAs which repress gene expression, through base pair matching with messenger RNA (mRNA). A single microRNA is capable of regulating hundreds of mRNA sequences. Only a small fraction of the over 1000 discovered microRNAs have currently known functions; many are crucial in the regulation of genetic signalling, including cellular processes such as cellular differentiation, growth, proliferation and death. Dysfunction in microRNA signalling is present in all cancers studied thus far, leading to overactive oncogenic and underactive tumour suppressor gene signalling. Current research is actively pursuing the potential to use microRNAs as diagnostic tools and novel therapies in a variety of diseases. This review summarises normal and abnormal maturation and function of microRNAs and their role in the pathogenesis of various human tumours and highlights how microRNAs may be used as diagnostic and treatment tools in human cancers in the future.

MicroRNAs: toward the clinic for breast cancer patients

Expression of microRNAs (miRNAs) has been found to be deregulated in all human cancers, where they may behave either as oncogenes or as tumor-suppressor genes. In the last 5 years, miRNA investigations in breast cancer represented an exciting area of discovery, which produced new knowledge on the molecular basis of this disease, tools for molecular classification, and new markers with diagnostic and prognostic relevance, as well as the discovery of novel breast cancer-predisposing genes. In this review, we describe current knowledge of the role of microRNAs in breast cancer.

Regulation of placenta growth factor by microRNA-125b in hepatocellular cancer

BACKGROUND & AIMS

microRNAs (miRNAs) are a class of small noncoding RNAs that can regulate gene expression by translation repression or mRNA degradation. Our aim was to evaluate the role of aberrantly expressed miRNAs in hepatocellular cancer (HCC).

METHODS

miRNA expression in HCC tissues and cells was evaluated by qPCR array and Taqman miRNA assay. Cell proliferation, motility, invasion, and the angiogenesis index were quantitated using commercial assays. DNA methylation status, matrix metalloproteinases (MMPs) mRNA expression was quantitated by real-time PCR analysis.

RESULTS

miRNA profiling identified a decrease in miR-125b expression in HCC tumor tissues and cell lines. The expression of miR-125b was significantly increased by the methylation inhibitor 5-aza-2'-deoxycytidine in HCC cells but not in normal controls, suggesting that the expression of miR-125b could be epigenetically modulated. Methylation-specific PCR revealed hypermethylation status of miR-125b in HCC cells compared to non-malignant controls. Cell proliferation, anchorage-independent growth, cell migration, invasion, and angiogenesis were significantly decreased by the introduction of miR-125b precursor in HCC cell lines. Placenta growth factor was identified as a target of miR-125b by bioinformatics analysis and experimentally verified using luciferase reporter constructs. Overexpression of miR-125b in HCC cells decreased PIGF expression, and altered the angiogenesis index. Furthermore, modulation of miR-125b also distorted expression of MMP-2 and -9, the mediators of enzymatic degradation of the extracellular matrix.

CONCLUSIONS

Our studies showing epigenetic silencing of miR-125b contributes to an invasive phenotype provide novel mechanistic insights and identify a potential target mechanism that could be manipulated for therapeutic benefit in HCC.

Epigenetics and genetics. MicroRNAs en route to the clinic: progress in validating and targeting microRNAs for cancer therapy

In normal cells multiple microRNAs (miRNAs) converge to maintain a proper balance of various processes, including proliferation, differentiation and cell death. miRNA dysregulation can have profound cellular consequences, especially because individual miRNAs can bind to and regulate multiple mRNAs. In cancer, the loss of tumour-suppressive miRNAs enhances the expression of target oncogenes, whereas increased expression of oncogenic miRNAs (known as oncomirs) can repress target tumour suppressor genes. This realization has resulted in a quest to understand the pathways that are regulated by these miRNAs using in vivo model systems, and to comprehend the feasibility of targeting oncogenic miRNAs and restoring tumour-suppressive miRNAs for cancer therapy. Here we discuss progress in using mouse models to understand the roles of miRNAs in cancer and the potential for manipulating miRNAs for cancer therapy as these molecules make their way towards clinical trials.

MiR-125b promotes proliferation and migration of type II endometrial carcinoma cells through targeting TP53INP1 tumor suppressor in vitro and in vivo

BACKGROUND

Our previous studies have identified that miR-125b was overexpressed in type II endometrial carcinoma (EC) cells compared with type I using microRNAs microarray. Although recent studies have shown the important role of miR-125b in several tumors and overexpression of miR-125b in advanced EC, its function in this disease has not yet been defined. In the present study, we tried to confirm the result of microRNAs microarray and further investigated the functions of miR-125b in EC, and tried to find new downstream targets of miR-125b.

METHODS

Differential expression of miR-125b was detected between type II EC cells (KLE, AN3CA) with ER negative and type I EC cells (ishikawa, RL95-2) with ER positive by qRT-PCR and northern blotting. The effects of miR-125b of on proliferation, migration, and target protein expression were evaluated by CCK8 assay, wound healing assay, transwell migration assay, western blotting, and Tumorigenicity assays in nude mice. In addition, luciferase reporter plasmid was constructed to demonstrate the direct target of miR-125b.

RESULTS

MiR-125b was overexpressed in type II EC cells compared with type I. Exogenous miR-125b expression increased proliferation and migration of ishikawa cells and abrogating expression of miR-125b suppressed proliferation, and migration of AN3CA cells in vitro. In addition, in vivo tumor formation assay confirmed that forced miR-125b expression promoted proliferation potential of ishikawa cells, and tumor suppressor gene Tumor Protein 53-Induced Nuclear Protein 1 (TP53INP1) was identified to be the direct target of miR-125b.

CONCLUSIONS

TP53INP1 was newly identified to be the direct downstream target of miR-125b. MiR-125b, which was overexpressed in type II EC cells compared with type I, contributes to malignancy of type II EC possibly through down-regulating TP53INP1.

Cholinesterase-Targeting microRNAs Identified in silico Affect Specific Biological Processes

MicroRNAs (miRs) have emerged as important gene silencers affecting many target mRNAs. Here, we report the identification of 244 miRs that target the 3′-untranslated regions of different cholinesterase transcripts: 116 for butyrylcholinesterase (BChE), 47 for the synaptic acetylcholinesterase (AChE-S) splice variant, and 81 for the normally rare splice variant AChE-R. Of these, 11 and 6 miRs target both AChE-S and AChE-R, and AChE-R and BChE transcripts, respectively. BChE and AChE-S showed no overlapping miRs, attesting to their distinct modes of miR regulation. Generally, miRs can suppress a number of targets; thereby controlling an entire battery of functions. To evaluate the importance of the cholinesterase-targeted miRs in other specific biological processes we searched for their other experimentally validated target transcripts and analyzed the gene ontology enriched biological processes these transcripts are involved in. Interestingly, a number of the resulting categories are also related to cholinesterases. They include, for BChE, response to glucocorticoid stimulus, and for AChE, response to wounding and two child terms of neuron development: regulation of axonogenesis and regulation of dendrite morphogenesis. Importantly, all of the AChE-targeting miRs found to be related to these selected processes were directed against the normally rare AChE-R splice variant, with three of them, including the neurogenesis regulator miR-132, also directed against AChE-S. Our findings point at the AChE-R splice variant as particularly susceptible to miR regulation, highlight those biological functions of cholinesterases that are likely to be subject to miR post-transcriptional control, demonstrate the selectivity of miRs in regulating specific biological processes, and open new venues for targeted interference with these specific processes.

Combined characterization of microRNA and mRNA profiles delineates early differentiation pathways of CD133+ and CD34+ hematopoietic stem and progenitor cells

MicroRNAs (miRNAs) have been shown to play an important role in hematopoiesis. To elucidate the role of miRNAs in the early steps of hematopoiesis, we directly compared donor-matched CD133⁺ cells with the more differentiated CD34⁺CD133⁻ and CD34⁻CD133⁻ cells from bone marrow on the miRNA and mRNA level. Using quantitative whole genome miRNA microarray and sequencing-based profiling, we found that between 109 (CD133⁺) and 216 (CD34⁻CD133⁻) miRNAs were expressed. Quantification revealed that the 25 highest expressed miRNAs accounted for 73% of the total miRNA pool. miR-142-3p was the highest expressed miRNA with up to 2,000 copies per cell in CD34⁺CD133⁻ cells. Eighteen miRNAs were significantly differentially expressed between CD133⁺ and CD34⁺CD133⁻ cells. We analyzed their biological role by examining the coexpression of miRNAs and its bioinformatically predicted mRNA targets and luciferase-based reporter assays. We provide the first evidence for a direct regulation of CD133 by miR-142-3p as well as tropomyosin 1 and frizzled homolog 5 by miR-29a. Overexpression of miRNAs in CD133⁺ cells demonstrated that miR-142-3p has a negative influence on the overall colony-forming ability. In conclusion, the miRNAs expressed differentially between the CD133⁺ and CD34⁺CD133⁻ cells are involved in inhibition of differentiation, prevention of apoptosis, and cytoskeletal remodeling. These results are highly relevant for stem cell-based therapies with CD133⁺ cells and delineate for the first time how the stem cell character of CD133⁺ cells is defined by the expression of specific miRNAs. Stem Cells 2011;29:847–857

Conserved regulation of p53 network dosage by microRNA-125b occurs through evolving miRNA-target gene pairs

MicroRNAs regulate networks of genes to orchestrate cellular functions. MiR-125b, the vertebrate homologue of the Caenorhabditis elegans microRNA lin-4, has been implicated in the regulation of neural and hematopoietic stem cell homeostasis, analogous to how lin-4 regulates stem cells in C. elegans. Depending on the cell context, miR-125b has been proposed to regulate both apoptosis and proliferation. Because the p53 network is a central regulator of both apoptosis and proliferation, the dual roles of miR-125b raise the question of what genes in the p53 network might be regulated by miR-125b. By using a gain- and loss-of-function screen for miR-125b targets in humans, mice, and zebrafish and by validating these targets with the luciferase assay and a novel miRNA pull-down assay, we demonstrate that miR-125b directly represses 20 novel targets in the p53 network. These targets include both apoptosis regulators like Bak1, Igfbp3, Itch, Puma, Prkra, Tp53inp1, Tp53, Zac1, and also cell-cycle regulators like cyclin C, Cdc25c, Cdkn2c, Edn1, Ppp1ca, Sel1l, in the p53 network. We found that, although each miRNA-target pair was seldom conserved, miR-125b regulation of the p53 pathway is conserved at the network level. Our results lead us to propose that miR-125b buffers and fine-tunes p53 network activity by regulating the dose of both proliferative and apoptotic regulators, with implications for tissue stem cell homeostasis and oncogenesis.

Upregulation of microRNA-125b contributes to leukemogenesis and increases drug resistance in pediatric acute promyelocytic leukemia

Background

Although current chemotherapy regimens have remarkably improved the cure rate of pediatric acute promyelocytic leukemia (APL) over the past decade, more than 20% of patients still die of the disease, and the 5-year cumulative incidence of relapse is 17%. The precise gene pathways that exert critical control over the determination of cell lineage fate during the development of pediatric APL remain unclear.

Methods

In this study, we analyzed miR-125b expression in 169 pediatric acute myelogenous leukemia (AML) samples including 76 APL samples before therapy and 38 APL samples after therapy. The effects of enforced expression of miR-125b were evaluated in leukemic cell and drug-resistant cell lines.

Results

miR-125b is highly expressed in pediatric APL compared with other subtypes of AML and is correlated with treatment response, as well as relapse of pediatric APL. Our results further demonstrated that miR-125b could promote leukemic cell proliferation and inhibit cell apoptosis by regulating the expression of tumor suppressor BCL2-antagonist/killer 1 (Bak1). Remarkably, miR-125b was also found to be up-regulated in leukemic drug-resistant cells, and transfection of a miR-125b duplex into AML cells can increase their resistance to therapeutic drugs,

Conclusions

These findings strongly indicate that miR-125b plays an important role in the development of pediatric APL at least partially mediated by repressing BAK1 protein expression and could be a potential therapeutic target for treating pediatric APL failure.

OMIT: a domain-specific knowledge base for microRNA target prediction

Identification and characterization of the important roles microRNAs (miRNAs) perform in human cancer is an increasingly active research area. Unfortunately, prediction of miRNA target genes remains a challenging task to cancer researchers. Current processes are time-consuming, error-prone, and subject to biologists' limited prior knowledge. Therefore, we propose a domain-specific knowledge base built upon Ontology for MicroRNA Targets (OMIT) to facilitate knowledge acquisition in miRNA target gene prediction. We describe the ontology design, semantic annotation and data integration, and user-friendly interface and conclude that the OMIT system can assist biologists in unraveling the important roles of miRNAs in human cancer. Thus, it will help clinicians make sound decisions when treating cancer patients.

Specific microRNAs are preferentially expressed by skin stem cells to balance self-renewal and early lineage commitment

Increasing evidence suggests that microRNAs may play important roles in regulating self-renewal and differentiation in mammalian stem cells (SCs). Here, we explore this issue in skin. We first characterize microRNA expression profiles of skin SCs versus their committed proliferative progenies and identify a microRNA subset associating with "stemness." Of these, miR-125b is dramatically downregulated in early SC progeny. We engineer an inducible mice system and show that when miR-125b is sustained in SC progenies, tissue balance is reversibly skewed toward stemness at the expense of epidermal, oil-gland, and HF differentiation. Using gain- and loss-of-function in vitro, we further implicate miR-125b as a repressor of SC differentiation. In vivo, transcripts repressed upon miR-125b induction are enriched >700% for predicted miR-125b targets normally downregulated upon SC-lineage commitment. We verify some of these miR-125b targets, and show that Blimp1 and VDR in particular can account for many tissue imbalances we see when miR-125b is deregulated.

Src family kinases and paclitaxel sensitivity

Src-family Kinases (SFKs) participate in the regulation of proliferation, differentiation, apoptosis, autophagy, adhesion, migration, invasion and angiogenesis in normal and cancer cells. Abnormal expression of SFKs has been documented in cancers that arise in breast, colon, ovary, melanocyte, gastric mucosa, head and neck, pancreas, lung, and brain. Targeting SFKs in cancer cells has been shown to be a promising therapeutic strategy in solid tumors, particularly in ovarian, colon and breast cancers. Paclitaxel is one of most widely used chemotherapeutic agents for the management of ovarian, breast, lung and head/neck cancers. As a microtubule-stabilizing agent, paclitaxel possesses both mitosis-dependent and mitosis-independent activities against cancer cells. A variety of mechanisms such as deregulation of P-glycoprotein, alteration of tubulin isotypes, alteration of microtubule-regulatory proteins, deregulation of apoptotic signaling pathways, mutation of tubulins and overexpression of copper transporters have been implicated in the development of primary or secondary resistance to paclitaxel. By affecting cancer cell survival, proliferation, autophagy, microtubule stability, motility, and/or angiogenesis, SFKs interact with mechanisms that regulate paclitaxel sensitivity. Inhibition of SFKs can potentiate the anti-tumor activity of paclitaxel by enhancing apoptosis, autophagy and microtubule stability. Based on pre-clinical observations, administration of SFK inhibitors in combination with paclitaxel could improve treatment for ovarian, breast, lung and head/neck cancers. Identification and validation of predictive biomarkers could also permit personalization of the therapy.

miR-125b confers resistance of ovarian cancer cells to cisplatin by targeting pro-apoptotic Bcl-2 antagonist killer 1

Chemotherapy is the preferred therapeutic approach for advanced ovarian cancer, but a successful long-term treatment is prevented by the development of drug resistance. Recent works have underlined the involvement of non-coding RNAs, microRNAs (miRNAs) in cancer development, with several conjectures regarding their possible involvement in the evolution of drug resistance. This study is to investigate the promoting effects and mechanism of miR-125b involved in the development of chemoresistance in ovarian cancer. The different expression of miR-125b in cisplatin-sensitive ovarian cancer cell line (OV2008) and its resistant variant (C13*) was identified by real-time PCR. An in vitro cytotoxicity assay and apoptosis assay using CCK-8 assay and flow cytometry, were carried out to detect the effect of miR-125b and Bak1 on cisplatin resistance of cells. Real-time PCR, Western blotting and luciferase reporter assay were used to detect whether Bak1 is a target of miR-125b. As compared with OV2008 cells, the expression levels of miR-125b in C13* cells were increased. It was found that the up-regulation of microRNA-125b caused a marked inhibition of cisplatin-induced cytotoxicity and apoptosis and a subsequent increase in the resistance to cisplatin in OV2008 and C13* cells. Moreover, Bak1 was a direct target of miR-125b, and down-regulation of Bak1 suppressed cisplatin-induced apoptosis and led to an increased resistance to cisplatin. Our study indicates that miR-125b has a significantly promoting effect on chemoresistance of C13* cells and up-regulation of miR-125b expression contributes to cisplatin resistance through suppression of Bak1 expression. This finding has important implications in the development of targeted therapeutics for overcoming cisplatin resistance in ovarian cancer.

The therapeutic potential of microRNAs: disease modulators and drug targets

MiRNAs are a class of small, naturally occurring RNA molecules that play critical roles in modulating numerous biological pathways by regulating gene expression. The knowledge that miRNA expression is dysregulated in many pathological disease processes, including cancer, has led to a rapidly expanding body of literature as we try to unveil their mechanism of action. Their putative role as oncogenes or tumour suppressor genes presents a wonderful opportunity to provide targeted cancer treatment strategies. Additionally, their documented function in a host of benign diseases broadens the potential market for miRNA-based therapeutics. The present review outlines the underlying rationales for considering mi(cro)RNAs as therapeutic agents or targets. We highlight the potential of manipulating miRNAs for the treatment of many common diseases, particularly cancers. Finally, we summarize the challenges that need to be overcome to fully harness the potential of miRNA-based therapies so they become the next generation of pharmaceutical products.

MicroRNA-mediated drug resistance in breast cancer

Chemoresistance is one of the major hurdles to overcome for the successful treatment of breast cancer. At present, there are several mechanisms proposed to explain drug resistance to chemotherapeutic agents, including decreased intracellular drug concentrations, mediated by drug transporters and metabolic enzymes; impaired cellular responses that affect cell cycle arrest, apoptosis, and DNA repair; the induction of signaling pathways that promote the progression of cancer cell populations; perturbations in DNA methylation and histone modifications; and alterations in the availability of drug targets. Both genetic and epigenetic theories have been put forward to explain the mechanisms of drug resistance. Recently, a small non-coding class of RNAs, known as microRNAs, has been identified as master regulators of key genes implicated in mechanisms of chemoresistance. This article reviews the role of microRNAs in regulating chemoresistance and highlights potential therapeutic targets for reversing miRNA-mediated drug resistance. In the future, microRNA-based treatments, in combination with traditional chemotherapy, may be a new strategy for the clinical management of drug-resistant breast cancers.

miR-135a contributes to paclitaxel resistance in tumor cells both in vitro and in vivo

Cancer cell resistance to paclitaxel continues to be a major clinical problem. In this study, we utilized microRNA (miRNA) arrays to screen for differentially expressed miRNAs in paclitaxel-resistant cell lines established in vitro. We observed concordant upregulation of miR-135a in paclitaxel-resistant cell lines representing three human malignancies. Subsequently, the role of miRNA-135a was evaluated in an in vivo model of paclitaxel resistance. In this model, mice were inoculated subcutaneously with a non-small cell lung carcinoma cell line and treated with paclitaxel for a prolonged period. In paclitaxel-resistant cell lines, established either in vitro or in vivo, blockage of miR-135a sensitized resistant cell lines to paclitaxel-induced cell death. We further demonstrated a correlation between paclitaxel response and miR-135a expression in paclitaxel-resistant subclones that were established in vivo. The paclitaxel-resistant phenotype of these subclones was maintained upon retransplantation in new mice, as shown by decreased tumor response upon paclitaxel treatment compared with controls. Upregulation of miR-135a was associated with reduced expression of the adenomatous polyposis coli gene (APC). APC knockdown increased paclitaxel resistance in parental cell lines. Our results indicate that paclitaxel resistance is associated with upregulation of miR-135a, both in vitro and in vivo, and is in part determined by miR-135a-mediated downregulation of APC.

MicroRNA gene dosage alterations and drug response in lung cancer

Chemotherapy resistance is a key contributor to the dismal prognoses for lung cancer patients. While the majority of studies have focused on sequence mutations and expression changes in protein-coding genes, recent reports have suggested that microRNA (miRNA) expression changes also play an influential role in chemotherapy response. However, the role of genetic alterations at miRNA loci in the context of chemotherapy response has yet to be investigated. In this study, we demonstrate the application of an integrative, multidimensional approach in order to identify miRNAs that are associated with chemotherapeutic resistance and sensitivity utilizing publicly available drug response, miRNA loci copy number, miRNA expression, and mRNA expression data from independent resources. By instigating a logical stepwise strategy, we have identified specific miRNAs that are associated with resistance to several chemotherapeutic agents and provide a proof of principle demonstration of how these various databases may be exploited to derive relevant pharmacogenomic results.

STAT3 mediates resistance to MEK inhibitor through microRNA miR-17

AZD6244 is a small molecule inhibitor of the MEK (MAP/ERK kinase) pathway currently in clinical trials. However, the mechanisms mediating intrinsic resistance to MEK inhibition are not fully characterized. To define molecular mechanisms of MEK inhibitor resistance, we analyzed responses of 38 lung cancer cell lines following AZD6244 treatment and their genome-wide gene expression profiles and identified a panel of genes correlated with sensitivity or resistance to AZD6244 treatment. In particular, ingenuity pathway analysis revealed that activation of the STAT3 pathway was associated with MEK inhibitor resistance. Inhibition of this pathway by JSI-124, a STAT3-specific small molecule inhibitor, or with STAT3-specific siRNA sensitized lung cancer cells to AZD6244 and induced apoptosis. Moreover, combining a STAT3 inhibitor with AZD6244 induced expression of BIM and PARP cleavage, whereas activation of the STAT3 pathway inhibited BIM expression and elicited resistance to MEK inhibitors. We found that the STAT3-regulated microRNA miR-17 played a critical role in MEK inhibitor resistance, such that miR-17 inhibition sensitized resistant cells to AZD6244 by inducing BIM and PARP cleavage. Together, these results indicated that STAT3-mediated overexpression of miR-17 blocked BIM expression and caused resistance to AZD6244. Our findings suggest novel approaches to overcome resistance to MEK inhibitors by combining AZD6244 with STAT3 or miR-17 inhibitors.

Enforced expression of miR-125b affects myelopoiesis by targeting multiple signaling pathways

MicroRNAs (miRNAs) are small, noncoding RNAs that regulate gene expression by sequence-specific targeting of multiple mRNAs. Although lineage-, maturation-, and disease-specific miRNA expression has been described, miRNA-dependent phenotypes and miRNA-regulated signaling in hematopoietic cells are largely unknown. Combining functional genomics, biochemical analysis, and unbiased and hypothesis-driven miRNA target prediction, we show that lentivirally over-expressed miR-125b blocks G-CSF-induced granulocytic differentiation and enables G-CSF-dependent proliferation of murine 32D cells. In primary lineage-negative cells, miR-125b over-expression enhances colony-formation in vitro and promotes myelopoiesis in mouse bone marrow chimeras. We identified Stat3 and confirmed Bak1 as miR-125b target genes with approximately 30% and 50% reduction in protein expression, respectively. However, gene-specific RNAi reveals that this reduction, alone and in combination, is not sufficient to block G-CSF-dependent differentiation. STAT3 protein expression, DNA-binding, and transcriptional activity but not induction of tyrosine-phosphorylation and nuclear translocation are reduced upon enforced miR-125b expression, indicating miR-125b-mediated reduction of one or more STAT3 cofactors. Indeed, we identified c-Jun and Jund as potential miR-125b targets and demonstrated reduced protein expression in 32D/miR-125b cells. Interestingly, gene-specific silencing of JUND but not c-JUN partially mimics the miR-125b over-expression phenotype. These data demonstrate coordinated regulation of several signaling pathways by miR-125b linked to distinct phenotypes in myeloid cells.

Cancer microRNAs: from subtype profiling to predictors of response to therapy

MicroRNAs (miRNAs) are key regulators of gene expression that regulate important oncogenes and tumor suppressors. Many miRNAs can also act as oncogenes or tumor suppressors, and thus the altered expression of miRNAs is a hallmark of many cancer types. Dysregulated miRNAs provide a potentially powerful new tool that could be used to enable the characterization of tumor environments and identify novel and important oncogenic pathways. More recently, there has been growing interest in the field of miRNAs as biomarkers of cancer risk, diagnosis and response to therapy. Understanding the associations between miRNA expression and cancer phenotypes, and the potential of miRNA profiling in clinical applications, promises to be highly rewarding in the field of cancer research.

miRNA Biomarkers in Breast Cancer Detection and Management

Breast cancer is considered as a heterogeneous disease comprising various types of neoplasms, which involves different profile changes in both mRNA and micro-RNA (miRNA) expression. Extensive studies on mRNA expression in breast tumor have yielded some very interesting findings, some of which have been validated and used in clinic. Recent miRNA research advances showed great potential for the development of novel biomarkers and therapeutic targets. miRNAs are a new class of small non-coding regulatory RNAs that are involved in regulating gene expression at the posttranscriptional level. It has been demonstrated that miRNA expression is frequently deregulated in breast cancer, which warrants further in-depth investigation to decipher their precise regulatory role in tumorigenesis. We address briefly the regulatory mechanism of miRNA, the expression of miRNAs in tumorigenesis, and their potential use as breast cancer biomarkers for early disease diagnosis and prognosis. In addition, we discuss the use of the Formalin-Fixed, Paraffin-Embedded (FFPE) tissue as an invaluable source for breast cancer biomarker discovery and validation, and the potential use of circulating miRNAs in blood for early breast cancer detection. We envision the potential use of miRNAs in breast cancer management in the near future, particularly in improving the early diagnosis, prognosis and treatment.

MicroRNA miR-125b causes leukemia

MicroRNA miR-125b has been implicated in several kinds of leukemia. The chromosomal translocation t(2;11)(p21;q23) found in patients with myelodysplasia and acute myeloid leukemia leads to an overexpression of miR-125b of up to 90-fold normal. Moreover, miR-125b is also up-regulated in patients with B-cell acute lymphoblastic leukemia carrying the t(11;14)(q24;q32) translocation. To decipher the presumed oncogenic mechanism of miR-125b, we used transplantation experiments in mice. All mice transplanted with fetal liver cells ectopically expressing miR-125b showed an increase in white blood cell count, in particular in neutrophils and monocytes, associated with a macrocytic anemia. Among these mice, half died of B-cell acute lymphoblastic leukemia, T-cell acute lymphoblastic leukemia, or a myeloproliferative neoplasm, suggesting an important role for miR-125b in early hematopoiesis. Furthermore, coexpression of miR-125b and the BCR-ABL fusion gene in transplanted cells accelerated the development of leukemia in mice, compared with control mice expressing only BCR-ABL, suggesting that miR-125b confers a proliferative advantage to the leukemic cells. Thus, we show that overexpression of miR-125b is sufficient both to shorten the latency of BCR-ABL-induced leukemia and to independently induce leukemia in a mouse model.

MicroRNA regulation of core apoptosis pathways in cancer

Recent research has demonstrated that microRNAs (miRNAs) are key regulators of many cell processes often deregulated in cancer, including apoptosis. Indeed, it is becoming clear that many miRNAs are anti-apoptotic and mediate this effect by targeting pro-apoptotic mRNAs or positive regulators of pro-apoptotic mRNAs. Conversely, many pro-apoptotic miRNAs target anti-apoptotic mRNAs or their positive regulators. We have reviewed the current knowledge in this area including evidence of miRNA involvement in cancer drug resistance.

NF-κB and cancer: a paradigm of Yin-Yang

Recent studies have clearly linked nuclear factor-kappaB (NF-κB), a transcription factor that plays a central role in regulating immune and inflammatory responses, to tumor development, progression, and metastasis as well as tumor therapy resistance. However, it still remains largely unknown on how the tightly regulated NF-κB becomes constitutively activated in tumorigenesis and how the original cancer immunosurveillance function of NF-κB is transformed to be tumorigenic. To address these important issues for cancer prevention and treatment, we discuss current understanding of the molecular mechanisms and molecules involved in the oncogenic activation of NF-κB. We also discuss current understanding of how NF-κB coordinates the inflammatory and malignant cells in tumorigenesis.

MicroRNA-125b expands hematopoietic stem cells and enriches for the lymphoid-balanced and lymphoid-biased subsets

MicroRNAs profoundly impact hematopoietic cells by regulating progenitor cell-fate decisions, as well as mature immune effector function. However to date, microRNAs that regulate hematopoietic stem cell (HSC) function have been less well characterized. Here we show that microRNA-125b (miR-125b) is highly expressed in HSCs and its expression decreases in committed progenitors. Overexpression of miR-125b in mouse HSC enhances their function, demonstrated through serial transplantation of highly purified HSC, and enriches for the previously described Slamf1(lo)CD34(-) lymphoid-balanced and the Slamf1(neg)CD34(-) lymphoid-biased cell subsets within the multipotent HSC (CD34-KLS) fraction. Mature peripheral blood cells derived from the miR-125b-overexpressing HSC are skewed toward the lymphoid lineage. Consistent with this observation, miR-125b overexpression significantly increases the number of early B-progenitor cells within the spleen and induces the expansion and enrichment of the lymphoid-balanced and lymphoid-biased HSC subset via an antiapoptotic mechanism, reducing the mRNA expression levels of two proapoptotic targets, Bmf and KLF13. The antiapoptotic effect of miR-125b is more pronounced in the lymphoid-biased HSC subset because of their intrinsic higher baseline levels of apoptosis. These effects of miR-125b are associated with the development of lymphoproliferative disease, marked by expansion of CD8(+) T lymphocytes. Taken together, these data reveal that miR-125b regulates HSC survival and can promote lymphoid-fate decisions at the level of the HSC by preferentially expanding lymphoid-balanced and lymphoid-biased HSC.

Selective impairment of CD4+CD25+Foxp3+ regulatory T cells by paclitaxel is explained by Bcl-2/Bax mediated apoptosis

Paclitaxel has become one of the most effective and widely used chemotherapeutic agents over the past decades. Although it has shown promise to selectively deplete regulatory T (Treg) cells in our previous study, the underlying molecular mechanism remains to be further elucidated. The present study focused on the effect of paclitaxel on Treg cells in 3LL Lewis tumor model and explored the possible molecular pathways involved in this process. We found that paclitaxel significantly decreased the percentage of Treg cells in CD4(+) cells and impaired their suppressive functions, but effector T (Teff) cells remained unaffected. Compared with Teff cells, Treg cells exhibited a high sensitivity to paclitaxel-mediated apoptosis in vitro. Interestingly, though paclitaxel has been characterized as a mitotic inhibitor, tubulin was not involved in the selective function of paclitaxel. Treg cells exposed to paclitaxel displayed downregulation of Bcl-2 and upregulation of Bax. Blocking the Bcl-2 pathway eliminated the difference between Treg and Teff cells responding to paclitaxel. These results suggest that Bcl-2 rather than tubulin contributes to the distinctive effect of paclitaxel on Treg cells. Therefore, we here identify a molecular pathway through which paclitaxel selectively ablates Treg cells.

Microtubule-binding agents: a dynamic field of cancer therapeutics

Microtubules are dynamic filamentous cytoskeletal proteins composed of tubulin and are an important therapeutic target in tumour cells. Agents that bind to microtubules have been part of the pharmacopoeia of anticancer therapy for decades and until the advent of targeted therapy, microtubules were the only alternative to DNA as a therapeutic target in cancer. The screening of a range of botanical species and marine organisms has yielded promising new antitubulin agents with novel properties. In the current search for novel microtubule-binding agents, enhanced tumour specificity, reduced neurotoxicity and insensitivity to chemoresistance mechanisms are the three main objectives.

Emerging biomarkers in breast cancer care

Currently, decision-making for breast cancer treatment in the clinical setting is mainly based on clinical data, histomorphological features of the tumor tissue and a few cancer biomarkers such as steroid hormone receptor status (estrogen and progesterone receptors) and oncoprotein HER2 status. Although various therapeutic options were introduced into the clinic in recent decades, with the objective of improving surgery, radiotherapy, biochemotherapy and chemotherapy, varying response of individual patients to certain types of therapy and therapy resistance is still a challenge in breast cancer care. Therefore, since breast cancer treatment should be based on individual features of the patient and her tumor, tailored therapy should be an option by integrating cancer biomarkers to define patients at risk and to reliably predict their course of the disease and/or response to cancer therapy. Recently, candidate-marker approaches and genome-wide transcriptomic and epigenetic screening of different breast cancer tissues and bodily fluids resulted in new promising biomarker panels, allowing breast cancer prognosis, prediction of therapy response and monitoring of therapy efficacy. These biomarkers are now subject of validation in prospective clinical trials.