Phosphorylated HuR shuttles in cycles

Phylogenomic instructed target analysis reveals ELAV complex binding to multiple optimally spaced U-rich motifs

ELAV/Hu RNA-binding proteins are gene-specific regulators of alternative pre-mRNA processing. ELAV/Hu family proteins bind to short AU-rich motifs which are abundant in pre-mRNA, making it unclear how they achieve gene specificity. ELAV/Hu proteins multimerize, but how multimerization contributes to decode degenerate sequence environments remains uncertain. Here, we show that ELAV forms a saturable complex on extended RNA. Through phylogenomic instructed target analysis we identify the core binding motif U5N2U3, which is repeated in an extended binding site. Optimally spaced short U5N2U3 binding motifs are key for high-affinity binding in this minimal binding element. Binding strength correlates with ELAV-regulated alternative poly(A) site choice, which is physiologically relevant through regulation of the major ELAV target ewg in determining synapse numbers. We further identify a stem-loop secondary structure in the ewg binding site unwound upon ELAV binding at three distal U motifs. Base-pairing of U motifs prevents ELAV binding, but N6-methyladenosine (m6A) has little effect. Further, stem-loops are enriched in ELAV-regulated poly(A) sites. Additionally, ELAV can nucleate preferentially from 3' to 5'. Hence, we identify a decisive mechanism for ELAV complex formation, addressing a fundamental gap in understanding how ELAV/Hu family proteins decode degenerate sequence spaces for gene-specific mRNA processing.

Targeting the RNA-Binding Protein HuR in Cancer

The RNA-binding protein human antigen R (HuR) is a well-established regulator of gene expression at the posttranscriptional level. Its dysregulation has been implicated in various human diseases, particularly cancer. In cancer, HuR is considered "active" when it shows increased subcellular localization in the cytoplasm, in addition to its normal nuclear localization. Cytoplasmic HuR plays a crucial role in stabilizing and enhancing the translation of prosurvival mRNAs that are involved in stress responses relevant to cancer progression, such as hypoxia, radiotherapy, and chemotherapy. In general, due to HuR's abundance and function in cancer cells compared with normal cells, it is an appealing target for oncology research. Exploiting the principles underlying HuR's role in tumorigenesis and resistance to stressors, targeting HuR has the potential for synergy with existing and novel oncologic therapies. This review aims to explore HuR's role in homeostasis and cancer pathophysiology, as well as current targeting strategies, which include silencing HuR expression, preventing its translocation and dimerization from the nucleus to the cytoplasm, and inhibiting mRNA binding. Furthermore, this review will discuss recent studies investigating the potential synergy between HuR inhibition and traditional chemotherapeutics.

The human antigen R as an actionable super-hub within the network of cancer cell persistency and plasticity

In this perspective article, a clinically inspired phenotype-driven experimental approach is put forward to address the challenge of the adaptive response of solid cancers to small-molecule targeted therapies. A list of conditions is derived, including an experimental quantitative assessment of cell plasticity and an information theory-based detection of in vivo dependencies, for the discovery of post-transcriptional druggable mechanisms capable of preventing at multiple levels the emergence of plastic dedifferentiated slow-proliferating cells. The approach is illustrated by the author's own work in the example case of the adaptive response of BRAFV600-melanoma to BRAF inhibition. A bench-to-bedside and back to bench effort leads to a therapeutic strategy in which the inhibition of the baseline activity of the interferon-γ-activated inhibitor of translation (GAIT) complex, incriminated in the expression insufficiency of the RNA-binding protein HuR in a minority of cells, results in the suppression of the plastic, intermittently slow-proliferating cells involved in the adaptive response. A similar approach is recommended for the validation of other classes of mechanisms that we seek to modulate to overcome this complex challenge of modern cancer therapy.

Role of hsa_circ_0000280 in regulating vascular smooth muscle cell function and attenuating neointimal hyperplasia via ELAVL1

The pathological proliferation of cells in vascular smooth muscle underlies neointimal hyperplasia (NIH) development during atherosclerosis. Circular RNAs (circRNAs), which represent novel functional biomarkers and RNA-binding proteins, contribute to multiple cardiovascular diseases; however, their roles in regulating the vascular smooth muscle cell cycle remain unknown. Thus, we aimed to identify the roles of circRNAs in vascular smooth muscle during coronary heart disease (CHD). Through circRNA sequencing of CHD samples and human antigen R (ELAVL1) immunoprecipitation, we identified circRNAs that are associated with CHD and interact with ELAVL1. Our results suggested that the hsa_circ_0000280 associated with CHD inhibits cell proliferation and induces ELAVL1-dependent cell cycle arrest. Gain/loss-of-function experiments and assays in vivo indicated that hsa_circ_0000280 facilitates interactions between ELAVL1 and cyclin-dependent kinase suppressor 1 (CDKN1A) mRNA and stabilization of this complex and leads to cell cycle arrest at the G1/S checkpoint, inhibiting cell proliferation of vascular smooth muscle cells in vitro and NIH in vivo. Importantly, hsa_circ_0000280 reduced neointimal thickness and smooth muscle cell proliferation in vivo. Taken together, these findings reveal a novel pathway in which hsa_circ_0000280 facilitates the regulation of ELAVL1 on CDKN1A mRNA to inhibit NIH. Therefore, measuring and modulating their expression might represent a potential diagnostic or therapeutic strategy for CHD.

CircTHBS1 drives gastric cancer progression by increasing INHBA mRNA expression and stability in a ceRNA- and RBP-dependent manner

Circular RNAs (circRNAs) play vital regulatory roles in the progression of multiple cancers. In our study, transcriptome analysis and self-organizing maps (SOM) were applied to screen backbone circRNAs in gastric cancer (GC). Upon validation of the expression patterns of screened circRNAs, gain- and loss-of-function assays were performed in vitro and in vivo. Underlying mechanisms were investigated using RNA pull-down, luciferase reporter assay and RNA immunoprecipitation. The expression of circTHBS1 was significantly increased in GC and associated with poor prognosis. CircTHBS1 facilitated the malignant behavior and epithelial-to-mesenchymal transition of GC cells. Mechanistically, circTHBS1 sponged miR-204-5p to promote the expression of Inhibin Subunit Beta A (INHBA). Moreover, circTHBS1 could enhance the HuR-mediated mRNA stability of INHBA, which subsequently activated the TGF-β pathway. Our research identified circTHBS1 as an oncogenic circRNA that enhances GC malignancy by elevating INHBA expression, providing new insight and a feasible target for the diagnosis and treatment of GC.

RNA-Binding Protein HuR Promotes Airway Inflammation in a House Dust Mite-Induced Allergic Asthma Model

Mounting evidence indicates that interleukin 17 (IL-17) is critically involved in the pathogenesis of severe asthma. We have previously reported that upon IL-17 stimulation, Act1, an IL-17-receptor-complex adaptor, directly binds to its target mRNAs and utilizes other proteins, such as HuR, to upregulate mRNA stability and translation. HuR mRNA targets include multiple asthma-related genes. In this study, we have used house dust mite (HDM), a natural allergen, to test the role of HuR in the pathogenesis of allergic asthma. We found that HuR deletion in airway epithelium diminished HDM-induced lung inflammation, including neutrophil and eosinophil infiltration. While Th2 cytokines were not altered, the production of CXCL1, CXCL5 and CCL11 chemokines was significantly diminished. Airway smooth muscle (ASM) cells contribute to the pathogenesis of allergic asthma by orchestrating inflammatory and remodeling responses. We found that IL-17 treatment of ASM cells induced translocation of HuR from nucleus to cytoplasm, where it bound directly to Cxcl1 and Ccl11 mRNA. Deletion of HuR in ASM cells decreased their proliferation as well as CXCL1 and CCL11 production in response to IL-17. Taken together, our findings demonstrate the importance of HuR-mediated regulation of gene expression to the pathogenesis of allergic asthma, in both airway epithelial and ASM cells.

AU-Rich Element RNA Binding Proteins: At the Crossroads of Post-Transcriptional Regulation and Genome Integrity

Genome integrity must be tightly preserved to ensure cellular survival and to deter the genesis of disease. Endogenous and exogenous stressors that impose threats to genomic stability through DNA damage are counteracted by a tightly regulated DNA damage response (DDR). RNA binding proteins (RBPs) are emerging as regulators and mediators of diverse biological processes. Specifically, RBPs that bind to adenine uridine (AU)-rich elements (AREs) in the 3' untranslated region (UTR) of mRNAs (AU-RBPs) have emerged as key players in regulating the DDR and preserving genome integrity. Here we review eight established AU-RBPs (AUF1, HuR, KHSRP, TIA-1, TIAR, ZFP36, ZFP36L1, ZFP36L2) and their ability to maintain genome integrity through various interactions. We have reviewed canonical roles of AU-RBPs in regulating the fate of mRNA transcripts encoding DDR genes at multiple post-transcriptional levels. We have also attempted to shed light on non-canonical roles of AU-RBPs exploring their post-translational modifications (PTMs) and sub-cellular localization in response to genotoxic stresses by various factors involved in DDR and genome maintenance. Dysfunctional AU-RBPs have been increasingly found to be associated with many human cancers. Further understanding of the roles of AU-RBP_S in maintaining genomic integrity may uncover novel therapeutic strategies for cancer.

The RNA-binding protein HuR is required for maintenance of the germinal centre response

The germinal centre (GC) is required for the generation of high affinity antibodies and immunological memory. Here we show that the RNA binding protein HuR has an essential function in GC B cells to sustain the GC response. In its absence, the GC reaction and production of high-affinity antibody is severely impaired. Mechanistically, HuR affects the transcriptome qualitatively and quantitatively. The expression and splicing patterns of hundreds of genes are altered in the absence of HuR. Among these genes, HuR is required for the expression of Myc and a Myc-dependent transcriptional program that controls GC B cell proliferation and Ig somatic hypermutation. Additionally, HuR regulates the splicing and abundance of mRNAs required for entry into and transition through the S phase of the cell cycle, and it modulates a gene signature associated with DNA deamination protecting GC B cells from DNA damage and cell death.

CCAT2 enhances autophagy-related invasion and metastasis via regulating miR-4496 and ELAVL1 in hepatocellular carcinoma

Autophagy is thought to contribute to the pathogenesis of many diseases, including cancer. Long non‐coding RNA (lncRNA) CCAT2 functions as an oncogene in a variety of tumours. However, it is still unknown whether CCAT2 is involved in autophagy and metastasis of hepatocellular carcinoma (HCC). In our study, we found that lncRNA CCAT2 expression was significantly increased in HCC tissue and was correlated with advanced stage and venous invasion. Further experiments revealed that CCAT2 induced autophagy and promoted migration and invasion in vitro and in vivo. Mechanistic investigations found that CCAT2 involved in HCC by regulating miR‐4496/Atg5 in cytoplasm. In nucleus, CCAT2 bound with ELAVL1/HuR to facilitate HCC progression. Our findings suggest that CCAT2 is an oncogenic factor in the progression of HCC with different regulatory mechanisms and may serve as a target for HCC therapy.

Human HAND1 Inhibits the Conversion of Cholesterol to Steroids in Trophoblasts

Steroidogenesis from cholesterol in placental trophoblasts is fundamentally involved in the establishment and maintenance of pregnancy. The transcription factor gene Heart And Neural crest Derivatives expressed 1 (Hand1) promotes differentiation of mouse trophoblast giant cells. However, the role of HAND1 in human trophoblasts remains unknown. Here, we report that HAND1 inhibits human trophoblastic progesterone (P4) and estradiol (E2) from cholesterol through down-regulation of the expression of steroidogenic enzymes including aromatase, P450 cholesterol side-chain cleavage enzyme (P450scc) and 3β-hydroxysteroid dehydrogenase type 1 (3β-HSD1). Mechanically, while HAND1 inhibits transcription of aromatase by directly binding to aromatase gene promoter, it restrains transcription of P450scc by up-regulation of the methylation status of P450scc gene promoter through its binding to ALKBH1, a demethylase. Unlike aromatase and P450scc, HAND1 decreases 3β-HSD1 mRNA levels by reduction of its RNA stability through binding to and subsequent destabilization of protein HuR. Finally, HAND1 suppresses circulating P4 and E2 levels derived from JEG-3 xenograft, and attenuates uterine response to P4 and E2. Thus, our results uncover a hitherto uncharacterized role of HAND1 in regulation of cholesterol metabolism in human trophoblasts, which may help pinpoint the underlying mechanisms involved in supporting the development and physiological function of the human placenta.

A mutual information-based in vivo monitoring of adaptive response to targeted therapies in melanoma

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In vivo dependencies should be quantified in adaptive resistance mechanistic studies

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Mutual information (MI) quantifies any type rather than just linear dependencies

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MI outperforms classic expression correlation coefficients

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Adaptive response to small-molecules inhibitors can be monitored in vivo using MI

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Strategies that prevent adaptive resistance can be monitored in vivo using MI

The mechanisms of adaptive resistance to genetic-based targeted therapies of solid malignancies have been the subject of intense research. These studies hold great promise for finding co-targetable hub/pathways which in turn would control the downstream non-genetic mechanisms of adaptive resistance. Many such mechanisms have been described in the paradigmatic BRAF-mutated melanoma model of adaptive response to BRAF inhibition. Currently, a major challenge for these mechanistic studies is to confirm in vivo, at the single-cell proteomic level, the existence of dependencies between the co-targeted hub/pathways and their downstream effectors. Moreover, the drug-induced in vivo modulation of these dependencies needs to be demonstrated. Here, we implement such single-cell-based in vivo expression dependency quantification using immunohistochemistry (IHC)-based analyses of sequential biopsies in two xenograft models. These mimic phase 2 and 3 trials in our own therapeutic strategy to prevent the adaptive response to BRAF inhibition. In this mechanistic model, the dependencies between the targeted Li₂CO₃-inducible hub HuR and the resistance effectors are more likely time-shifted and transient since the minority of HuR^Low cells, which act as a reservoir of adaptive plasticity, switch to a HuR^High state as they paradoxically proliferate under BRAF inhibition. Nevertheless, we show that a copula/kernel density estimator (KDE)-based quantification of mutual information (MI) efficiently captures, at the individual level, the dependencies between HuR and two relevant resistance markers pERK and EGFR, and outperforms classic expression correlation coefficients. Ultimately, the validation of MI as a predictive IHC-based metric of response to our therapeutic strategy will be carried in clinical trials.

Developmental Cytoplasmic-to-Nuclear Translocation of RNA-Binding Protein HuR Is Required for Adult Neurogenesis

Although adult neurogenesis recapitulates processes that occur during embryonic development, it exhibits distinct characteristics from the embryonic counterpart. However, the intrinsic mechanism underlying the differential regulation of neurogenesis between these two stages remains unclear. Herein, we show that the ablation of RNA-binding protein HuR in NSCs impairs adult but not embryonic neurogenesis. HuR is predominantly expressed in the cytoplasm of embryonic NSCs but translocates into the nucleus of adult NSCs. Transcriptomic analysis of HuR-deficient adult NSCs revealed that HuR primarily regulates alternative splicing of numerous premRNA transcripts, including focal adhesion kinase (FAK). HuR-deficient adult NSCs generate increased FAK mRNA isoforms with shorter 5'-UTRs, leading to enhanced FAK mRNA translation and hyperactivated FAK signaling, and inhibition of FAK ameliorates defective adult neurogenesis and impaired hippocampus-dependent learning in HuR-deficient mice. These findings provide mechanistic insights into the differential regulation of embryonic and adult neurogenesis through developmental cytoplasmic-to-nuclear translocation of HuR.

The RNA Binding Protein HuR: A Promising Drug Target for Anticancer Therapy

The stability of mRNA is one of the key factors governing the regulation of eukaryotic gene expression and function. Human antigen R (HuR) is an RNA-binding protein that regulates the stability, translation, and nucleus-to-cytoplasm shuttling of its target mRNAs. While HuR is normally localized within the nucleus, it has been shown that HuR binds mRNAs in the nucleus and then escorts the mRNAs to the cytoplasm where HuR protects them from degradation. It contains several RNA recognition motifs, which specifically bind to adenylate and uridylate-rich regions within the 3'-untranslated region of the target mRNA to mediate its effect. Many of the HuR target mRNAs encode proteins important for cell growth, tumorigenesis, angiogenesis, tumor inflammation, invasion and metastasis. HuR overexpression is known to correlate well with high-grade malignancy and poor prognosis in many tumor types. Thus, HuR has emerged as an attractive drug target for cancer therapy. Novel small molecule HuR inhibitors have been identified by high throughput screening and new formulations for targeted delivery of HuR siRNA to tumor cells have been developed with promising anticancer activity. This review summarizes the significant role of HuR in cancer development, progression, and poor treatment response. We will discuss the potential and challenges of targeting HuR therapeutically.

Connections between 3' end processing and DNA damage response: Ten years later

Ten years ago we reviewed how the cellular DNA damage response (DDR) is controlled by changes in the functional and structural properties of nuclear proteins, resulting in a timely coordinated control of gene expression that allows DNA repair. Expression of genes that play a role in DDR is regulated not only at transcriptional level during mRNA biosynthesis but also by changing steady-state levels due to turnover of the transcripts. The 3' end processing machinery, which is important in the regulation of mRNA stability, is involved in these gene-specific responses to DNA damage. Here, we review the latest mechanistic connections described between 3' end processing and DDR, with a special emphasis on alternative polyadenylation, microRNA and RNA binding proteins-mediated deadenylation, and discuss the implications of deregulation of these steps in DDR and human disease. This article is categorized under: RNA Processing > 3' End Processing RNA-Based Catalysis > Miscellaneous RNA-Catalyzed Reactions RNA in Disease and Development > RNA in Disease.

Understanding and targeting the disease-related RNA binding protein human antigen R (HuR)

Altered gene expression is a characteristic feature of many disease states such as tumorigenesis, and in most cancers, it facilitates cancer cell survival and adaptation. Alterations in global gene expression are strongly impacted by post-transcriptional gene regulation. The RNA binding protein (RBP) HuR (ELAVL1) is an established regulator of post-transcriptional gene regulation and is overexpressed in most human cancers. In many cancerous settings, HuR is not only overexpressed, but it is "overactive" as denoted by increased subcellular localization within the cytoplasm. This dysregulation of HuR expression and cytoplasmic localization allows HuR to stabilize and increase the translation of various prosurvival messenger RNA (mRNAs) involved in the pathogenesis of numerous cancers and various diseases. Based on almost 20 years of work, HuR is now recognized as a therapeutic target. Herein, we will review the role HuR plays in the pathophysiology of different diseases and ongoing therapeutic strategies to target HuR. We will focus on three ongoing-targeted strategies: (1) inhibiting HuR's translocation from the nucleus to the cytoplasm; (2) inhibiting the ability of HuR to bind target RNA; and (3) silencing HuR expression levels. In an oncologic setting, HuR has been demonstrated to be critical for a cancer cell's ability to survive a variety of cancer relevant stressors (including drugs and elements of the tumor microenvironment) and targeting this protein has been shown to sensitize cancer cells further to insult. We strongly believe that targeting HuR could be a powerful therapeutic target to treat different diseases, particularly cancer, in the near future. This article is categorized under: RNA in Disease and Development > RNA in Disease NRA Turnover and Surveillance > Regulation of RNA Stability Translation > Translation Regulation.

p16 promotes proliferation in cervical carcinoma cells through CDK6-HuR-IL1A axis

The Cyclin-Dependent Kinase Inhibitor p16 (p16) acts as a tumor suppressor in most cells, but for HPV transformed cervical cancer, in which oncoprotein E7 expressed by human papillomavirus (HPV) mediates the degradation of retinoblastoma protein (Rb), p16 exhibits oncogenic activity. Our study was conducted to study the mechanism underling p16 mediated promoting effect of cell proliferation in cervical cancer cell lines. CCK8 assay and EdU incorporation were conducted to evaluate cell proliferation. Loss-of-function assay was used to silence p16 in Ca Ski and SiHa cells. Next, western blot, qPCR, RNA silencing, luciferase activity assay, run-on assay, mRNA stability assay, RNA immunoprecipitation, co-immunoprecipitation Immunofluorescence were performed to examine the interaction between CDK6, HuR, and IL1A mRNA in p16 mediated proliferation promoting effect. Our results showed that: (1) Silencing p16 inhibited the proliferation of cervical cancer cells by decreasing the half-life of IL1A mRNA in CDK6 dependent manner; (2) The stabilization of IL1A mRNA was regulated by HuR which could be inactivated by p16/CDK6 mediated phosphorylation at Ser202; (3) IL1A mediated the oncogenic activity of p16 in cervical carcinoma cell lines. In conclusion, p16 promotes proliferation in cervical carcinoma cells through CDK6-HuR-IL1A axis.

Loss of RNA-binding protein HuR facilitates cellular senescence through posttranscriptional regulation of TIN2 mRNA

Cellular senescence can be induced by high levels of reactive oxygen species (ROS) produced by mitochondria. However, the mechanism by which elevated mitochondrial ROS levels are produced during replicative senescence is not yet fully understood. Here, we report that loss of the RNA-binding protein, human antigen R (HuR), during replicative senescence leads to an increase in ROS levels through enhanced mitochondrial localization of the telomeric protein TIN2. HuR binds to the 3′ untranslated region of TIN2 mRNA. This association decreases TIN2 protein levels by both destabilizing TIN2 mRNA and reducing its translation. Conversely, depletion of HuR levels enhances TIN2 expression, leading to increased mitochondrial targeting of TIN2. Mitochondrial localization of TIN2 increases ROS levels, which contributes to induction and maintenance of cellular senescence. Our findings provide compelling evidence for a novel role of HuR in controlling the process of cellular senescence by regulating TIN2-mediated mitochondrial ROS production, and for a useful therapeutic route for modulating intracellular ROS levels in treating both aging-related complications and cancer.

MIR100 host gene-encoded lncRNAs regulate cell cycle by modulating the interaction between HuR and its target mRNAs

Long non-coding RNAs (lncRNAs) regulate vital biological processes, including cell proliferation, differentiation and development. A subclass of lncRNAs is synthesized from microRNA (miRNA) host genes (MIRHGs) due to pre-miRNA processing, and are categorized as miRNA-host gene lncRNAs (lnc-miRHGs). Presently, the cellular function of most lnc-miRHGs is not well understood. We demonstrate a miRNA-independent role for a nuclear-enriched lnc-miRHG in cell cycle progression. MIR100HG produces spliced and stable lncRNAs that display elevated levels during the G1 phase of the cell cycle. Depletion of MIR100HG-encoded lncRNAs in human cells results in aberrant cell cycle progression without altering the levels of miRNA encoded within MIR100HG. Notably, MIR100HG interacts with HuR/ELAVL1 as well as with several HuR-target mRNAs. Further, MIR100HG-depleted cells show reduced interaction between HuR and three of its target mRNAs, indicating that MIR100HG facilitates interaction between HuR and target mRNAs. Our studies have unearthed novel roles played by a MIRHG-encoded lncRNA in regulating RNA binding protein activity, thereby underscoring the importance of determining the function of several hundreds of lnc-miRHGs that are present in human genome.

A RAS-CaMKKβ-AMPKα2 pathway promotes senescence by licensing post-translational activation of C/EBPβ through a novel 3'UTR mechanism

Oncogene-induced senescence (OIS) is an intrinsic tumor suppression mechanism that requires the p53 and RB pathways and post-translational activation of C/EBPβ through the RAS-ERK cascade. We previously reported that in transformed/proliferating cells, C/EBPβ activation is inhibited by G/U-rich elements (GREs) in its 3′UTR. This mechanism, termed “3′UTR regulation of protein activity” (UPA), maintains C/EBPβ in a low-activity state in tumor cells and thus facilitates senescence bypass. Here we show that C/EBPβ UPA is overridden by AMPK signaling. AMPK activators decrease cytoplasmic levels of the GRE binding protein HuR, which is a key UPA component. Reduced cytoplasmic HuR disrupts 3′UTR-mediated trafficking of Cebpb transcripts to the peripheral cytoplasm – a fundamental feature of UPA – thereby stimulating C/EBPβ activation and growth arrest. In primary cells, oncogenic RAS triggers a Ca⁺⁺-CaMKKβ-AMPKα2-HuR pathway, independent of AMPKα1, that is essential for C/EBPβ activation and OIS. This axis is disrupted in cancer cells through down-regulation of AMPKα2 and CaMKKβ. Thus, CaMKKβ-AMPKα2 signaling constitutes a key tumor suppressor pathway that activates a novel UPA-cancelling mechanism to unmask the cytostatic and pro-senescence functions of C/EBPβ.

Translational Dysregulation in Cancer: Molecular Insights and Potential Clinical Applications in Biomarker Development

Although transcript levels have been traditionally used as a surrogate measure of gene expression, it is increasingly recognized that the latter is extensively and dynamically modulated at the level of translation (messenger RNA to protein). Over the recent years, significant progress has been made in dissecting the complex posttranscriptional mechanisms that regulate gene expression. This advancement in knowledge came hand in hand with the progress made in the methodologies to study translation both at gene-specific as well as global genomic level. The majority of translational control is exerted at the level of initiation; nonetheless, protein synthesis can be modulated at the level of translation elongation, termination, and recycling. Sequence and structural elements and epitranscriptomic modifications of individual transcripts allow for dynamic gene-specific modulation of translation. Cancer cells usurp the regulatory mechanisms that govern translation to carry out translational programs that lead to the phenotypic hallmarks of cancer. Translation is a critical nexus in neoplastic transformation. Multiple oncogenes and signaling pathways that are activated, upregulated, or mutated in cancer converge on translation and their transformative impact "bottlenecks" at the level of translation. Moreover, this translational dysregulation allows cancer cells to adapt to a diverse array of stresses associated with a hostile microenviroment and antitumor therapies. All elements involved in the process of translation, from the transcriptional template, the components of the translational machinery, to the proteins that interact with the transcriptome, have been found to be qualitatively and/or quantitatively perturbed in cancer. This review discusses the regulatory mechanisms that govern translation in normal cells and how translation becomes dysregulated in cancer leading to the phenotypic hallmarks of malignancy. We also discuss how dysregulated mediators or components of translation can be utilized as biomarkers with potential diagnostic, prognostic, or predictive significance. Such biomarkers have the potential advantage of uniform applicability in the face of inherent tumor heterogeneity and deoxyribonucleic acid instability. As translation becomes increasingly recognized as a process gone awry in cancer and agents are developed to target it, the utility and significance of these potential biomarkers is expected to increase.

Tumor-targeted Nanoparticle Delivery of HuR siRNA Inhibits Lung Tumor Growth In Vitro and In Vivo By Disrupting the Oncogenic Activity of the RNA-binding Protein HuR

Selective downregulation of the human antigen R (HuR) protein by siRNA may provide a powerful approach for treating lung cancer. To this end, we investigated the efficacy of transferrin receptor-targeted liposomal nanoparticle-based HuR siRNA (HuR-TfNP) therapy and compared with control siRNA (C)-TfNP therapy both, in vitro and in vivo using lung cancer models. In vitro studies showed HuR-TfNP, but not C-TfNP, efficiently downregulated HuR and HuR-regulated proteins in A549, and HCC827 lung cancer cells, resulting in reduced cell viability, inhibition of cell migration and invasion, and induction of G₁ cell-cycle arrest culminating in apoptosis. However, HuR-TfNP activity in normal MRC-9 lung fibroblasts was negligible. In vivo biodistribution study demonstrated that fluorescently labeled HuR-siRNA or ICG dye-loaded TfNP localized in tumor tissues. Efficacy studies showed intratumoral or intravenous administration of HuR-TfNP significantly inhibited A549 (>55% inhibition) and HCC827 (>45% inhibition) subcutaneous tumor growth compared with C-TfNP. Furthermore, HuR-TfNP treatment reduced HuR, Ki67, and CD31 expression and increased caspase-9 and PARP cleavage and TUNEL-positive staining indicative of apoptotic cell death in tumor tissues compared with C-TfNP treatment. The antitumor activity of HuR-TfNP was also observed in an A549-luc lung metastatic model, as significantly fewer tumor nodules (9.5 ± 3.1; P < 0.001; 88% inhibition) were observed in HuR-TfNP-treated group compared with the C-TfNP-treated group (77.7 ± 20.1). Significant reduction in HuR, Ki67, and CD31 expression was also observed in the tumor tissues of HuR-TfNP-treatment compared with C-TfNP treatment. Our findings highlight HuR-TfNP as a promising nanotherapeutic system for lung cancer treatment. Mol Cancer Ther; 16(8); 1470-86. ©2017 AACR.

NOX4 Regulates CCR2 and CCL2 mRNA Stability in Alcoholic Liver Disease

Recruitment of inflammatory cells is a major feature of alcoholic liver injury however; the signals and cellular sources regulating this are not well defined. C-C chemokine receptor type 2 (CCR2) is expressed by active hepatic stellate cells (HSC) and is a key monocyte recruitment signal. Activated HSC are also important sources of hydrogen peroxide resulting from the activation of NADPH oxidase 4 (NOX4). As the role of this NOX in early alcoholic liver injury has not been addressed, we studied NOX4-mediated regulation of CCR2/CCL2 mRNA stability. NOX4 mRNA was significantly induced in patients with alcoholic liver injury, and was co-localized with αSMA-expressing activated HSC. We generated HSC-specific NOX4 KO mice and these were pair-fed on alcohol diet. Lipid peroxidation have not changed significantly however, the expression of CCR2, CCL2, Ly6C, TNFα, and IL-6 was significantly reduced in NOX4^HSCKO compared to fl/fl mice. NOX4 promoter was induced in HSC by acetaldehyde treatment, and NOX4 has significantly increased mRNA half-life of CCR2 and CCL2 in conjunction with Ser221 phosphorylation and cytoplasmic shuttling of HuR. In conclusion, NOX4 is induced in early alcoholic liver injury and regulates CCR2/CCL2 mRNA stability thereby promoting recruitment of inflammatory cells and production of proinflammatory cytokines.

RNA Binding Protein Human Antigen R Is Translocated in Astrocytes following Spinal Cord Injury and Promotes the Inflammatory Response

Inflammation plays a prominent role in the events following traumatic injury to the central nervous system (CNS). The initial inflammatory response is driven by mediators such as tumor necrosis factor α and interleukin 1β, which are produced by activated astrocytes and microglia at the site of injury. These factors are regulated post-transcriptionally by RNA binding proteins (RBP) that interact with adenylate and uridylate-rich elements (ARE) in the 3'-untranslated region of the messenger RNA (mRNA). Human antigen R (HuR) is one of these RBPs and generally functions as a positive regulator of ARE-containing mRNAs. Here, we hypothesized that HuR plays an important role in the induction of cytokine and chemokines in astrocytes following traumatic injury. Using a mouse model of spinal cord injury, we found HuR to be extensively translocated to the cytoplasm in astrocytes at the level of injury, consistent with its activation. In an in vitro stretch injury model of CNS trauma, we observed a similar cytoplasmic shift of HuR in astrocytes and an attenuation of cytokine induction with HuR knockdown. RNA kinetics and luciferase assays suggested that the effect was more related to transcription than RNA destabilization. A small molecule inhibitor of HuR suppressed cytokine induction of injured astrocytes and reduced chemoattraction for neutrophils and microglia. In summary, HuR is activated in astrocytes in the early stages of CNS trauma and positively regulates the molecular response of key inflammatory mediators in astrocytes. Our findings suggest that HuR may be a therapeutic target in acute CNS trauma for blunting secondary tissue injury triggered by the inflammatory response.

Posttranscriptional regulation of 14-3-3ζ by RNA-binding protein HuR modulating intestinal epithelial restitution after wounding

The 14‐3‐3ζ is a member of the family of 14‐3‐3 proteins and participates in many aspects of cellular processes, but its regulation and involvement in gut mucosal homeostasis remain unknown. Here, we report that 14‐3‐3ζ expression is tightly regulated at the posttranscription level by RNA‐binding protein HuR and plays an important role in early intestinal epithelial restitution after wounding. The 14‐3‐3ζ was highly expressed in the mucosa of gastrointestinal tract and in cultured intestinal epithelial cells (IECs). The 3′ untranslated region (UTR) of the 14‐3‐3ζ mRNA was bound to HuR, and this association enhanced 14‐3‐3ζ translation without effect on its mRNA content. Conditional target deletion of HuR in IECs decreased the level of 14‐3‐3ζ protein in the intestinal mucosa. Silencing 14‐3‐3ζ by transfection with specific siRNA targeting the 14‐3‐3ζ mRNA suppressed intestinal epithelial restitution as indicated by a decrease in IEC migration after wounding, whereas ectopic overexpression of the wild‐type 14‐3‐3ζ promoted cell migration. These results indicate that HuR induces 14‐3‐3ζ translation via interaction with its 3′ UTR and that 14‐3‐3ζ is necessary for stimulation of IEC migration after wounding.

Inhibiting cytoplasmic accumulation of HuR synergizes genotoxic agents in urothelial carcinoma of the bladder

HuR, an RNA-binding protein, post-transcriptionally regulates nearly 4% of encoding proteins implicated in cell survival. Here we show that HuR is required for the efficacy of chemotherapies in urothelial carcinoma of the bladder. We identify pyrvinium pamoate, an FDA-approved anthelminthic drug, as a novel HuR inhibitor that dose-dependently inhibited cytoplasmic accumulation of HuR. Combining pyrvinium pamoate with chemotherapeutic agents (e.g. cisplatin, doxorubicin, vincristine and oxaliplatin) not only led to enhanced cytotoxicity in bladder cancer cells but also synergistically suppressed the growth of patient-derived bladder tumor xenografts in mice (P < 0.001). Mechanistically, pyrvinium pamoate promoted nuclear import of HuR by activating the AMP-activated kinase/importin α1 cascade and blocked HuR nucleo-cytoplasmic translocation by inhibiting the checkpoint kinase1/cyclin-dependent kinase 1 pathway. Notably, pyrvinium pamoate-additive treatment increased DNA double-strand breaks as indicated by elevated γH2AX expression, suggesting an involvement of DNA damage response. We further found that pyrvinium pamoate dramatically downregulated several key DNA repair genes in genotoxically-stressed cells, including DNA ligase IV and BRCA2, leading to unbearable genomic instability and cell death. Collectively, our findings are the first to characterize a clinical HuR inhibitor and provide a novel therapeutically tractable strategy by targeting cytoplasmic translocation of HuR for treatment of urothelial carcinoma of the bladder.

The RNA binding protein HuR determines the differential translation of autism-associated FoxP subfamily members in the developing neocortex

Forkhead-box domain (Fox) containing family members are known to play a role in neocorticogenesis and have also been associated with disorders on the autism spectrum. Here we show that a single RNA-binding protein, Hu antigen R (HuR), dictates translation specificity of bound mRNAs and is sufficient to define distinct Foxp-characterized subpopulations of neocortical projection neurons. Furthermore, distinct phosphorylation states of HuR differentially regulate translation of Foxp mRNAs in vitro. This demonstrates the importance of RNA binding proteins within the framework of the developing brain and further confirms the role of mRNA translation in autism pathogenesis.

PKM2 uses control of HuR localization to regulate p27 and cell cycle progression in human glioblastoma cells

The M2 isoform of pyruvate kinase (PK) is upregulated in most cancers including glioblastoma. Although PKM2 has been reported to use dual kinase activities to regulate cell growth, it also interacts with phosphotyrosine (pY)-containing peptides independently of its kinase activity. The potential for PKM2 to use the binding of pY-containing proteins to control tumor growth has not been fully examined. We here describe a novel mechanism by which PKM2 interacts in the nucleus with the RNA binding protein HuR to regulate HuR sub-cellular localization, p27 levels, cell cycle progression and glioma cell growth. Suppression of PKM2 in U87, T98G and LN319 glioma cells resulted in increased p27 levels, defects in entry into mitosis, increased centrosome number, and decreased cell growth. These effects could be reversed by shRNA targeting p27. The increased levels of p27 in PKM2 knock-down cells were caused by a loss of the nuclear interaction between PKM2 and HuR, and a subsequent cytoplasmic re-distribution of HuR, which in turn led to increased cap-independent p27 mRNA translation. Consistent with these results, the alterations in p27 mRNA translation, cell cycle progression and cell growth caused by PKM2 suppression could be reversed in vitro and in vivo by suppression of HuR or p27 levels, or by introduction of forms of PKM2 that could bind pY, regardless of their kinase activity. These results define a novel mechanism by which PKM2 regulates glioma cell growth, and also define a novel set of potential therapeutic targets along the PKM2-HuR-p27 pathway.

Posttranslational control of HuR function

The RNA-binding protein HuR (human antigen R) associates with numerous transcripts, coding and noncoding, and controls their splicing, localization, stability, and translation. Through its regulation of target transcripts, HuR has been implicated in cellular events including proliferation, senescence, differentiation, apoptosis, and the stress and immune responses. In turn, HuR influences processes such as cancer and inflammation. HuR function is primarily regulated through posttranslational modifications that alter its subcellular localization and its ability to bind target RNAs; such modifications include phosphorylation, methylation, ubiquitination, NEDDylation, and proteolytic cleavage. In this review, we describe the modifications that impact upon HuR function on gene expression programs and disease states. WIREs RNA 2017, 8:e1372. doi: 10.1002/wrna.1372 For further resources related to this article, please visit the WIREs website.

Berberine Decreased Inducible Nitric Oxide Synthase mRNA Stability through Negative Regulation of Human Antigen R in Lipopolysaccharide-Induced Macrophages

Berberine, a major isoquinoline alkaloid found in medicinal herbs, has been reported to possess anti-inflammatory effects; however, the underlying mechanisms responsible for its actions are poorly understood. In the present study, we investigated the inhibitory effects of berberine and the molecular mechanisms involved in lipopolysaccharide (LPS)-treated RAW 264.7 and THP-1 macrophages and its effects in LPS-induced septic shock in mice. In both macrophage cell types, berberine inhibited the LPS-induced nitric oxide (NO) production and inducible NO synthase (iNOS) protein expression, but it had no effect on iNOS mRNA transcription. Suppression of LPS-induced iNOS protein expression by berberine occurred via a human antigen R (HuR)-mediated reduction of iNOS mRNA stability. Molecular data revealed that the suppression on the LPS-induced HuR binding to iNOS mRNA by berberine was accompanied by a reduction in nucleocytoplasmic HuR shuttling. Pretreatment with berberine reduced LPS-induced iNOS protein expression and the cytoplasmic translocation of HuR in liver tissues and increased the survival rate of mice with LPS-induced endotoxemia. These results show that the suppression of iNOS protein expression by berberine under LPS-induced inflammatory conditions is associated with a reduction in iNOS mRNA stability resulting from inhibition of the cytoplasmic translocation of HuR.

LncRNA OIP5-AS1/cyrano sponges RNA-binding protein HuR

The function of the vast majority of mammalian long noncoding (lnc) RNAs remains unknown. Here, analysis of a highly abundant mammalian lncRNA, OIP5-AS1, known as cyrano in zebrafish, revealed that OIP5-AS1 reduces cell proliferation. In human cervical carcinoma HeLa cells, the RNA-binding protein HuR, which enhances cell proliferation, associated with OIP5-AS1 and stabilized it. Tagging OIP5-AS1 with MS2 hairpins to identify associated microRNAs revealed that miR-424 interacted with OIP5-AS1 and competed with HuR for binding to OIP5-AS1. We further identified a ‘sponge’ function for OIP5-AS1, as high levels of OIP5-AS1 increased HuR-OIP5-AS1 complexes and prevented HuR interaction with target mRNAs, including those that encoded proliferative proteins, while conversely, lowering OIP5-AS1 increased the abundance of HuR complexes with target mRNAs. We propose that OIP5-AS1 serves as a sponge or a competing endogenous (ce)RNA for HuR, restricting its availability to HuR target mRNAs and thereby repressing HuR-elicited proliferative phenotypes.

AMPK/HuR-Driven IL-20 Post-Transcriptional Regulation in Psoriatic Skin

IL-20 is involved in the development of skin psoriasis. The molecular mechanisms underlying IL-20 overexpression in psoriatic epidermis remain to be elucidated. We showed that IL-20 was primarily upregulated in psoriatic skin at the post-transcriptional level. The RNA-binding protein HuR relocalized to the cytoplasm of keratinocytes (KCs) of psoriatic patients, suggesting that it stabilizes numerous transcripts, as observed in the human KC cell lines used to assess IL-20 mRNA. We characterized epidermal HuR RNA targets in psoriatic skin using ribonucleoprotein immunoprecipitation analyzed via high-throughput sequencing. Numerous transcripts that are upregulated in psoriasis were targeted by HuR, supporting the participation of HuR in pathogenic processes such as morphological changes, innate and adaptive immune responses, and metabolic inflammatory responses. Finally, we identified the metabolic sensor AMP-activated protein kinase (AMPK) as being responsible for HuR cytoplasmic relocalization because its activity was severely impaired in human psoriatic epidermis, and in vivo drug-mediated AMPK inhibition in mouse epidermis promoted HuR cytoplasmic localization, IL-20 overproduction, acanthosis, and hyperkeratosis. These results provide insights into the molecular links between metabolism and post-transcriptional networks during chronic inflammation.

RNA-stabilizing proteins as molecular targets in cardiovascular pathologies

The stability of mRNA has emerged as a key step in the regulation of eukaryotic gene expression and function. RNA stabilizing proteins (RSPs) contain several RNA recognition motifs, and selectively bind to adenylate-uridylate-rich elements in the 3' untranslated region of several mRNAs leading to altered processing, stability, and translation. These post-transcriptional gene regulations play a critical role in cellular homeostasis; therefore act as molecular switch between 'normal cell' and 'disease state.' Many mRNA binding proteins have been discovered to date, which either stabilize (HuR/HuA, HuB, HuC, HuD) or destabilize (AUF1, tristetraprolin, KSRP) the target transcripts. Although the function of RSPs has been widely studied in cancer biology, its role in cardiovascular pathologies is only beginning to evolve. The current review provides an overall understanding of the potential role of RSPs, specifically HuR-mediated mRNA stability in myocardial infarction, hypertension and hypertrophy. Also, the effect of RSPs on various cellular processes including inflammation, fibrosis, angiogenesis, cell-death, and proliferation and its relevance to cardiovascular pathophysiological processes is presented. We also discuss the potential clinical implications of RSPs as therapeutic targets in cardiovascular diseases.

Clinical significance of HuR expression in human malignancy

Hu-antigen R (HuR) is an RNA-binding protein that regulates the stability, translation, and nucleus-to-cytoplasm translocation of target mRNAs. The aim of the present review was to summarize and present the currently available information in the English literature on HuR expression in various human tumors, verifying its possible clinical significance. HuR function is directly linked to its subcellular localization. In normal cells, HuR is mostly localized in the nucleus, while in malignant cells, an increase in cytoplasmic HuR levels has been noted, in both cell lines and tissue samples. Moreover, in malignancy, elevated HuR expression levels and cytoplasmic immunohistochemical pattern have been correlated with advanced clinicopathological parameters and altered expression levels of proteins implicated in neoplasia. Additionally, elevated HuR expression levels and mainly cytoplasmic immunohistochemical pattern were correlated with decreased patients' survival rate in various human tumors. HuR is a putative drug target for cancer therapy, since it is expressed ubiquitously in malignant clinical samples and has an apparently consistent role in tumor formation and progression.

Tissue-specific promoter usage and diverse splicing variants of found in neurons, an ancestral Hu/ELAV-like RNA-binding protein gene of insects, in the direct-developing insect Gryllus bimaculatus

Hu/ELAV-like RNA-binding proteins (RBPs) are involved in the post-transcriptional regulation of RNA metabolism including splicing, transport, translational control and turnover. The Hu/ELAV-like RBP genes are predominantly expressed in neurons, and are therefore used as common neuronal markers in many animals. Although the expression patterns and functions of the Hu/ELAV-like RBP genes have been extensively studied in the model insect Drosophila melanogaster, little is known in basal direct-developing insects. In the present study, we performed an identification and expression analysis of the found in neurons (fne) gene, an ancestral insect Hu/ELAV-like RBP gene, in the cricket Gryllus bimaculatus. Contrary to expectation that the Gryllus fne transcript would be predominantly expressed in the nervous system, expression analysis revealed that the Gryllus fne gene is expressed broadly. In addition, we discovered that alternative promoter usage directs tissue-specific and embryonic stage-dependent regulation of fne expression, and that alternative splicing contributes to the generation of diverse sets of fne transcripts. Our data provide novel insights into the evolutionary diversification of the Hu/ELAV-like RBP gene family in insects.

Nox2/ROS-dependent human antigen R translocation contributes to TNF-α-induced SOCS-3 expression in human tracheal smooth muscle cells

Elevated levels of TNF-α have been detected in the airway fluids, which may induce upregulation of inflammatory proteins. Suppressors of cytokine signaling (SOCS)-3 proteins can be induced by various cytokines and negatively regulated inflammatory responses. Although TNF-α has been shown to induce SOCS-3 expression, the mechanisms underlying TNF-α-induced SOCS-3 expression in human tracheal smooth muscle cells (HTSMCs) remain unclear. Here, we showed that TNF-α induced SOCS-3 expression, which was inhibited by pretreatment with the inhibitor of transcription level (actinomycin D), translation level (cycloheximide), JNK1/2 (SP600125), MEK1/2 (U0126), NADPH oxidase (Nox; apocynin and diphenyleneiodonium chloride), or reactive oxygen species (ROS; N-acetyl-l-cysteine) and transfection with siRNA of JNK1, p47(phox), p42, Nox2, or human antigen R (HuR). In addition, TNF-α-stimulated JNK1/2 and p42/p44 MAPK phosphorylation, Nox activation, and ROS generation were inhibited by pretreatment with U0126 or SP600125 and transfection with siRNA of JNK1 or p42. We further showed that TNF-α markedly induced HuR protein expression and translocation from the nucleus to the cytosol, which could stabilize SOCS-3 mRNA. Moreover, TNF-α-enhanced HuR translocation was reduced by transfection with siRNA of p42, JNK1, or p47(phox). These results suggested that TNF-α induces SOCS-3 protein expression and mRNA stabilization via a TNFR1/JNK1/2, p42/p44 MAPK/Nox2/ROS-dependent HuR signaling in HTSMCs. Lipopolysaccharide (LPS) has been shown to play a key role in inflammation via induction of adhesion molecules and then causes airway and lung injury. Moreover, we also demonstrated that overexpression of SOCS-3 protects against LPS-induced adhesion molecules expression and airway inflammation.

The prolyl isomerase pin1 regulates mRNA levels of genes with short half-lives by targeting specific RNA binding proteins

The peptidyl-prolyl isomerase Pin1 is over-expressed in several cancer tissues is a potential prognostic marker in prostate cancer, and Pin1 ablation can suppress tumorigenesis in breast and prostate cancers. Pin1 can co-operate with activated ErbB2 or Ras to enhance tumorigenesis. It does so by regulating the activity of proteins that are essential for gene expression and cell proliferation. Several targets of Pin1 such as c-Myc, the Androgen Receptor, Estrogen Receptor-alpha, Cyclin D1, Cyclin E, p53, RAF kinase and NCOA3 are deregulated in cancer. At the posttranscriptional level, emerging evidence indicates that Pin1 also regulates mRNA decay of histone mRNAs, GM-CSF, Pth, and TGFβ mRNAs by interacting with the histone mRNA specific protein SLBP, and the ARE-binding proteins AUF1 and KSRP, respectively. To understand how Pin1 may affect mRNA abundance on a genome-wide scale in mammalian cells, we used RNAi along with DNA microarrays to identify genes whose abundance is significantly altered in response to a Pin1 knockdown. Functional scoring of differentially expressed genes showed that Pin1 gene targets control cell adhesion, leukocyte migration, the phosphatidylinositol signaling system and DNA replication. Several mRNAs whose abundance was significantly altered by Pin1 knockdown contained AU-rich element (ARE) sequences in their 3' untranslated regions. We identified HuR and AUF1 as Pin1 interacting ARE-binding proteins in vivo. Pin1 was also found to stabilize all core histone mRNAs in this study, thereby validating our results from a previously published study. Statistical analysis suggests that Pin1 may target the decay of essential mRNAs that are inherently unstable and have short to medium half-lives. Thus, this study shows that an important biological role of Pin1 is to regulate mRNA abundance and stability by interacting with specific RNA-binding proteins that may play a role in cancer progression.

Tyrosine phosphorylation of HuR by JAK3 triggers dissociation and degradation of HuR target mRNAs

In response to stress conditions, many mammalian mRNAs accumulate in stress granules (SGs) together with numerous RNA-binding proteins that control mRNA turnover and translation. However, the signaling cascades that modulate the presence of ribonucleoprotein (RNP) complexes in SGs are poorly understood. Here, we investigated the localization of human antigen R (HuR), an mRNA-stabilizing RNA-binding protein, in SGs following exposure to the stress agent arsenite. Unexpectedly, the mobilization of HuR to SGs was prevented through the activation of Janus kinase 3 (JAK3) by the vitamin K3 analog menadione. JAK3 phosphorylated HuR at tyrosine 200, in turn inhibiting HuR localization in SGs, reducing HuR interaction with targets SIRT1 and VHL mRNAs, and accelerating target mRNA decay. Our findings indicate that HuR is tyrosine-phosphorylated by JAK3, and link this modification to HuR subcytoplasmic localization and to the fate of HuR target mRNAs.

HuR cytoplasmic expression is associated with increased cyclin A expression and inferior disease-free survival in patients with gastrointestinal stromal tumours (GISTs)

AIMS

HuR is an RNA-binding protein that post-transcriptionally modulates the expression of various target genes involved in carcinogenesis, such as CCNA2, which encodes cyclin A. The aim of this study was to evaluate the significance of HuR expression and subcellular localization in a large cohort of gastrointestinal stromal tumours (GISTs).

METHODS AND RESULTS

HuR immunostaining was assessable for nuclear and cytoplasmic expression in 341 cases on tissue microarrays of primary GISTs, of which 318, 296 and 193 cases were also characterized for Ki67 labelling, cyclin A immunoexpression, and KIT and PDGFRA receptor tyrosine kinase (RTK) genotypes, respectively. The results of HuR nuclear and cytoplasmic expression were correlated with disease-free survival (DFS) and clinicopathological, immunohistochemical and RTK genotypic variables. HuR cytoplasmic expression was present in 42% of primary GISTs, and was significantly related to epithelioid histology, larger tumour size, NIH risk category, and nuclear expression of Ki67 and cyclin A. Importantly, HuR cytoplasmic expression (P < 0.001) and cyclin A overexpression (P < 0.001) were strongly associated with worse DFS. Both variables remained independently predictive of adverse outcome [P = 0.020 and risk ratio (RR) 2.605 for cytoplasmic HuR; P = 0.026 and RR 2.763 for cyclin A].

CONCLUSIONS

HuR cytoplasmic expression not only correlates with adverse prognosticators and cyclin A overexpression, but also independently predicts worse DFS, indicating a causative role in conferring tumour aggressiveness.

MPT0B098, a novel microtubule inhibitor that destabilizes the hypoxia-inducible factor-1α mRNA through decreasing nuclear-cytoplasmic translocation of RNA-binding protein HuR

Microtubule inhibitors have been shown to inhibit hypoxia-inducible factor-1α (HIF-1α) expression through inhibition translation or enhancing protein degradation. Little is known of the effect of microtubule inhibitors on the stability of HIF-1α mRNA. We recently discovered a novel indoline-sulfonamide compound, 7-aryl-indoline-1-benzene-sulfonamide (MPT0B098), as a potent microtubule inhibitor through binding to the colchicine-binding site of tubulin. MPT0B098 is active against the growth of various human cancer cells, including chemoresistant cells with IC50 values ranging from 70 to 150 nmol/L. However, normal cells, such as human umbilical vein endothelial cells (HUVEC), exhibit less susceptibility to the inhibitory effect of MPT0B098 with IC50 of 510 nmol/L. Similar to typical microtubule inhibitors, MPT0B098 arrests cells in the G2-M phase and subsequently induces cell apoptosis. In addition, MPT0B098 effectively suppresses VEGF-induced cell migration and capillary-like tube formation of HUVECs. Distinguished from other microtubule inhibitors, MPT0B098 not only inhibited the expression levels of HIF-1α protein but also destabilized HIF-1α mRNA. The mechanism of causing unstable of HIF-1α mRNA by MPT0B098 is through decreasing RNA-binding protein, HuR, translocation from the nucleus to the cytoplasm. Notably, MPT0B098 effectively suppresses tumor growth and microvessel density of tumor specimens in vivo. Taken together, our results provide a novel mechanism of inhibiting HIF-1α of a microtubule inhibitor MPT0B098. MPT0B098 is a promising anticancer drug candidate with potential for the treatment of human malignancies.

Functional interplay between RNA-binding protein HuR and microRNAs

The mammalian RNA-binding protein (RBP) HuR associates with numerous mRNAs encoding proteins with roles in cell division, cell survival, immune response, and differentiation. HuR was known to stabilize many of these mRNAs and/or modulated their translation, but the molecular processes by which HuR affected the fate of target mRNAs was largely unknown. Evidence accumulated over the past five years has revealed that the influence of HuR on many bound transcripts depends on HuR's interplay with microRNAs which associate with the same mRNAs. Here, we review the interactions of HuR and microRNAs - both competitive and cooperative - that govern expression of shared target mRNAs. Competition between HuR and microRNAs typically results in enhanced gene expression if the HuR-mRNA interaction prevails, and in repression if the microRNA remains associated. Cooperation between HuR and microRNAs leads to lower expression of the shared mRNA. We also describe the regulation of HuR levels by microRNAs as well as the regulation of microRNA levels by HuR. Finally, we discuss transcriptome-wide analyses of HuR-bound mRNAs with neighboring microRNA sites, and review the emerging mechanisms whereby microRNAs confer versatility and robustness to the post-transcriptional outcomes of HuR targets.

Insights from the HuR-interacting transcriptome: ncRNAs, ubiquitin pathways, and patterns of secondary structure dependent RNA interactions

The HuR protein regulates the expression of thousands of cellular transcripts by modulating mRNA splicing, trafficking, translation, and stability. Although it serves as a model of RNA-protein interactions, many features of HuR's interactions with RNAs remain unknown. In this report, we deployed the cryogenic RNA immunoprecipitation technique to analyze HuR-interacting RNAs with the Affymetrix all-exon microarray platform. We revealed several thousand novel HuR-interacting RNAs, including hundreds of non-coding RNAs such as natural antisense transcripts from stress responsive loci. To gain insight into the mechanisms of specificity and sensitivity of HuR's interaction with its target RNAs, we searched HuR-interacting RNAs for composite patterns of primary sequence and secondary structure. We provide evidence that secondary structures of 66-75 nucleotides enhance HuR's recognition of its specific RNA targets composed of short primary sequence patterns. We validated thousands of these RNAs by analysis of overlap with recently published findings, including HuR's interaction with RNAs in the pathways of RNA splicing and stability. Finally, we observed a striking enrichment for members of ubiquitin ligase pathways among the HuR-interacting mRNAs, suggesting a new role for HuR in the regulation of protein degradation to mirror its known function in protein translation.

Dephosphorylation of HuR protein during alphavirus infection is associated with HuR relocalization to the cytoplasm

We have demonstrated previously that the cellular HuR protein binds U-rich elements in the 3' untranslated region (UTR) of Sindbis virus RNA and relocalizes from the nucleus to the cytoplasm upon Sindbis virus infection in 293T cells. In this study, we show that two alphaviruses, Ross River virus and Chikungunya virus, lack the conserved high-affinity U-rich HuR binding element in their 3' UTRs but still maintain the ability to interact with HuR with nanomolar affinities through alternative binding elements. The relocalization of HuR protein occurs during Sindbis infection of multiple mammalian cell types as well as during infections with three other alphaviruses. Interestingly, the relocalization of HuR is not a general cellular reaction to viral infection, as HuR protein remained largely nuclear during infections with dengue and measles virus. Relocalization of HuR in a Sindbis infection required viral gene expression, was independent of the presence of a high-affinity U-rich HuR binding site in the 3' UTR of the virus, and was associated with an alteration in the phosphorylation state of HuR. Sindbis virus-induced HuR relocalization was mechanistically distinct from the movement of HuR observed during a cellular stress response, as there was no accumulation of caspase-mediated HuR cleavage products. Collectively, these data indicate that virus-induced HuR relocalization to the cytoplasm is specific to alphavirus infections and is associated with distinct posttranslational modifications of this RNA-binding protein.

Phosphoregulation of the RNA-binding protein Hu antigen R (HuR) by Cdk5 affects centrosome function

Hu antigen R (HuR) is an mRNA-binding protein belonging to the ELAV family. It is highly expressed in cancer and involved in cell survival and proliferation. The impact of post-translational regulation of HuR and resulting cellular effects are poorly understood. In the current report, we describe a direct interaction between HuR and Cdk5 in glioma. We determined that Cdk5 specifically phosphorylates HuR at the serine 202 residue in the unique hinge region. The molecular consequences of this interaction are an altered HuR ability to bind, stabilize, and promote translation of mRNAs. At the cellular level, the anomalous HuR phosphorylation at this site evokes robust defects in centrosome duplication and cohesion as well as arrest of cell cycle progression. Subcellular fractionation and immunofluorescence technique confirm a direct integration of HuR and Cdk5 with centrosomes. We propose that HuR stores mRNA in the centrosome and that HuR phosphorylation by Cdk5 controls de novo protein synthesis in near proximity to centrosomes and, thus, impacts centrosome function.

HuR thermal stability is dependent on domain binding and upon phosphorylation

Human antigen R (HuR) is a multitasking RNA binding protein involved in posttranscriptional regulation by recognizing adenine- and uracile-rich elements placed at the 3'-untranslated regions of messenger RNAs (mRNAs). The modular architecture of the protein, which consists of two N-terminal RNA recognition motifs (RRMs) in tandem spaced from a third one by a nuclear-cytoplasmic shuttling sequence, controls the stability of many mRNA targets, as well as their translation rates. A higher level of regulation comes from the fact that both localization and function of HuR are strictly regulated by phosphorylation. Here, we report how the thermal stability of RRM2 is decreased by the presence of RRM1, indicating that both domains are interacting in solution. In addition, even though no significant structural changes are observed among mutants of HuR RRM12 mimicking phosphorylated species, slight differences in stability are appreciable, which may explain the RNA binding activity of HuR.

Downregulation of HuR as a new mechanism of doxorubicin resistance in breast cancer cells

Background

HuR, an RNA binding protein involved in the post-transcriptional regulation of a wide spectrum of mRNAs, has been demonstrated to be a determinant of carcinogenesis and tumor aggressiveness in several cancer types. In this study, we investigated the role of HuR in the apoptosis and in the chemoresistance induced by the widely used anticancer drug doxorubicin in human breast cancer cells (MCF-7).

Results

We showed that HuR acts in the early phase of cell response to doxorubicin, being induced to translocate into the cytoplasm upon phosphorylation. Reducing HuR levels diminished the apoptotic response to doxorubicin. Doxorubicin-induced apoptosis was also correlated with the presence of HuR in the cytoplasm. Rottlerin, which was able to block HuR nuclear export, had correspondingly antagonistic effects with doxorubicin on cell toxicity. The proapoptotic activity of HuR was not due to cleavage to an active form, as was previously reported. In in vitro selected doxorubicin resistant MCF-7 cells (MCF-7/doxoR) overexpressing the multidrug resistance (MDR) related ABCG2 transporter, we observed a significant HuR downregulation that was paralleled by a corresponding downregulation of HuR targets and by loss of rottlerin toxicity. Restoration of HuR expression in these cells resensitized MCF-7/doxoR cells to doxorubicin, reactivating the apoptotic response.

Conclusions

The present study shows that HuR is necessary to elicit the apoptotic cell response to doxorubicin and that restoration of HuR expression in resistant cells resensitizes them to the action of this drug, thereby identifying HuR as a key protein in doxorubicin pharmacology.

HuR function in disease

The cytoplasmic events that control mammalian gene expression, primarily mRNA stability and translation, potently influence the cellular response to internal and external signals. The ubiquitous RNA-binding protein (RBP) HuR is one of the best-studied regulators of cytoplasmic mRNA fate. Through its post-transcriptional influence on specific target mRNAs, HuR can alter the cellular response to proliferative, stress, apoptotic, differentiation, senescence, inflammatory and immune stimuli. In light of its central role in important cellular functions, HuR's role in diseases in which these responses are aberrant is increasingly appreciated. Here, we review the mechanisms that control HuR function, its influence on target mRNAs, and how impairment in HuR-governed gene expression programs impact upon different disease processes. We focus on HuR's well-recognized implication in cancer and chronic inflammation, and discuss emerging studies linking HuR to cardiovascular, neurological, and muscular pathologies. We also discuss the progress, potential, and challenges of targeting HuR therapeutically.

Hu antigen R and tristetraprolin: counter-regulators of rat apical sodium-dependent bile acid transporter by way of effects on messenger RNA stability

The apical sodium-dependent bile acid transporter (ASBT, SLC10A2) mediates intestinal, renal, and cholangiocyte bile acid reclamation. Transcriptional regulation of ASBT is well described, whereas information on posttranscriptional regulation is limited. Prior studies suggested that ontogeny of ASBT is controlled in part by changes in messenger RNA (mRNA) stability. We studied the role that Hu antigen R (HuR) and tristetraprolin (TTP) play in regulating the expression of mRNA that contains the 3' untranslated region (UTR) of rat ASBT. The 3'UTR was incorporated into an SV-40 driven luciferase reporter (rASBT3-luciferase) for rapid screening of regulatory effects. Silencing HuR reduced luciferase reporter activity, whereas silencing TTP enhanced luciferase activity. Conversely, overexpression of HuR enhanced rASBT3-luciferase reporter activity. The same 3'UTR fragments of rat ASBT were incorporated into a beta-globin coding mRNA construct for analysis of mRNA stability (rASBT3-βglobin). mRNA half-life was progressively shortened by the incorporation of increasing sized fragments of the 3'UTR. Silencing HuR shortened the half-life of rASBT3-βglobin containing 0.3 kb of the rat ASBT 3'UTR. Gel shift assays revealed binding of HuR and TTP to rat ASBT 3'UTR. Endogenously expressed human ASBT mRNA half-lives and steady-state protein levels in Caco-2 cells were repressed when HuR was silenced but was enhanced when TTP was silenced. Developmental changes in HuR and TTP protein abundance correlated with previously characterized ontogenic changes in rat ileal and renal ASBT expression.

CONCLUSION

These studies not only show that ASBT expression is controlled at the level of mRNA stability by way of its 3'UTR, but also identify HuR and TTP as two key transacting factors that are involved in exerting counterregulatory effects on ASBT mRNA stability.

RNA-binding protein HuR autoregulates its expression by promoting alternative polyadenylation site usage

RNA-binding protein HuR modulates the stability and translational efficiency of messenger RNAs (mRNAs) encoding essential components of the cellular proliferation, growth and survival pathways. Consistent with these functions, HuR levels are often elevated in cancer cells and reduced in senescent and quiescent cells. However, the molecular mechanisms that control HuR expression are poorly understood. Here we show that HuR protein autoregulates its abundance through a negative feedback loop that involves interaction of the nuclear HuR protein with a GU-rich element (GRE) overlapping with the HuR major polyadenylation signal (PAS2). An increase in the cellular HuR protein levels stimulates the expression of long HuR mRNA species containing an AU-rich element (ARE) that destabilizes the mRNAs and thus reduces the protein production output. The PAS2 read-through occurs due to a reduced recruitment of the CstF-64 subunit of the pre-mRNA cleavage stimulation factor in the presence of the GRE-bound HuR. We propose that this mechanism maintains HuR homeostasis in proliferating cells. Since only the nuclear HuR is expected to contribute to the auto-regulation, our model may explain the longstanding observation that the increase in the total HuR expression in cancer cells often correlates with the accumulation of its substantial fraction in the cytoplasm.

Discovering and validating unknown phospho-sites from p38 and HuR protein kinases in vitro by Phosphoproteomic and Bioinformatic tools

Background

The mitogen activated protein kinase (MAPK) pathways are known to be deregulated in many human malignancies. Phosphopeptide identification of protein-kinases and site determination are major challenges in biomedical mass spectrometry (MS). P38 and HuR protein kinases have been reported extensively in the general principles of signalling pathways modulated by phosphorylation, mainly by molecular biology and western blotting techniques. Thus, although it has been demonstrated they are phosphorylated in different stress/stimuli conditions, the phosphopeptides and specific amino acids in which the phosphate groups are located in those protein kinases have not been shown completely.

Methods

We have combined different resins: (a) IMAC (Immobilized Metal Affinity Capture), (b) TiO₂ (Titanium dioxide) and (c) SIMAC (Sequential Elution from IMAC) to isolate phosphopeptides from p38 and HuR protein kinases in vitro.

Different phosphopeptide MS strategies were carried out by the LTQ ion Trap mass spectrometer (Thermo): (a) Multistage activation (MSA) and (b) Neutral loss MS3 (DDNLMS3).

In addition, Molecular Dynamics (MD) bioinformatic simulation has been applied in order to simulate, over a period of time, the effects of the presence of the extra phosphate group (and the associated negative charge) in the overall structure and behaviour of the protein HuR.

This study is supported by the Declaration of Helsinki and subsequent ethical guidelines.

Results

The combination of these techniques allowed for:

(1) The identification of 6 unknown phosphopeptides of these protein kinases. (2) Amino acid site assignments of the phosphate groups from each identified phosphopeptide, including manual validation by inspection of all the spectra. (3) The analyses of the phosphopeptides discovered were carried out in four triplicate experiments to avoid false positives getting high reproducibility in all the isolated phosphopeptides recovered from both protein kinases. (4) Computer simulation using MD techniques allowed us to get functional models of both structure and interactions of the previously mentioned phosphorylated kinases and the differences between their phosphorylated and un-phosphorylated forms.

Conclusion

Many research studies are necessary to unfold the whole signalling network (human proteome), which is so important to advance in clinical research, especially in the cases of malignant diseases.

HuR binding to AU-rich elements present in the 3' untranslated region of Classical swine fever virus

Background

Classical swine fever virus (CSFV) is the member of the genus Pestivirus under the family Flaviviridae. The 5' untranslated region (UTR) of CSFV contains the IRES, which is a highly structured element that recruits the translation machinery. The 3' UTR is usually the recognition site of the viral replicase to initiate minus-strand RNA synthesis. Adenosine-uridine rich elements (ARE) are instability determinants present in the 3' UTR of short-lived mRNAs. However, the presence of AREs in the 3' UTR of CSFV conserved in all known strains has never been reported. This study inspects a possible role of the ARE in the 3' UTR of CSFV.

Results

Using RNA pull-down and LC/MS/MS assays, this study identified at least 32 possible host factors derived from the cytoplasmic extracts of PK-15 cells that bind to the CSFV 3' UTR, one of which is HuR. HuR is known to bind the AREs and protect the mRNA from degradation. Using recombinant GST-HuR, this study demonstrates that HuR binds to the ARE present in the 3' UTR of CSFV in vitro and that the binding ability is conserved in strains irrespective of virulence.

Conclusions

This study identified one of the CSFV 3' UTR binding proteins HuR is specifically binding to in the ARE region.