Cytoplasmic HuR expression correlates with P-gp, HER-2 positivity, and poor outcome in breast cancer

HuR controls glutaminase RNA metabolism

Glutaminase (GLS) is directly related to cell growth and tumor progression, making it a target for cancer treatment. The RNA-binding protein HuR (encoded by the ELAVL1 gene) influences mRNA stability and alternative splicing. Overexpression of ELAVL1 is common in several cancers, including breast cancer. Here we show that HuR regulates GLS mRNA alternative splicing and isoform translation/stability in breast cancer. Elevated ELAVL1 expression correlates with high levels of the glutaminase isoforms C (GAC) and kidney-type (KGA), which are associated with poor patient prognosis. Knocking down ELAVL1 reduces KGA and increases GAC levels, enhances glutamine anaplerosis into the TCA cycle, and drives cells towards glutamine dependence. Furthermore, we show that combining chemical inhibition of GLS with ELAVL1 silencing synergistically decreases breast cancer cell growth and invasion. These findings suggest that dual inhibition of GLS and HuR offers a therapeutic strategy for breast cancer treatment.

RNA binding proteins in cancer chemotherapeutic drug resistance

Drug resistance has been a major obstacle in the quest for a cancer cure. Many chemotherapeutic treatments fail to overcome chemoresistance, resulting in tumor remission. The exact process that leads to drug resistance in many cancers has not been fully explored or understood. However, the discovery of RNA binding proteins (RBPs) has provided insight into various pathways and post-transcriptional gene modifications involved in drug tolerance. RBPs are evolutionarily conserved proteins, and their abnormal gene expression has been associated with cancer progression. Additionally, RBPs are aberrantly expressed in numerous neoplasms. RBPs have also been implicated in maintaining cancer stemness, epithelial-to-mesenchymal transition, and other processes. In this review, we aim to provide an overview of RBP-mediated mechanisms of drug resistance and their implications in cancer malignancy. We discuss in detail the role of major RBPs and their correlation with noncoding RNAs (ncRNAs) that are associated with the inhibition of chemosensitivity. Understanding and exploring the pathways of RBP-mediated chemoresistance will contribute to the development of improved cancer diagnosis and treatment strategies.

RNA-binding proteins in cancer drug discovery

RNA-binding proteins (RBPs) are crucial players in tumorigenesis and, hence, promising targets in cancer drug discovery. However, they are largely regarded as 'undruggable', because of the often noncatalytic and complex interactions between protein and RNA, which limit the discovery of specific inhibitors. Nonetheless, over the past 10 years, drug discovery efforts have uncovered RBP inhibitors with clinical relevance, highlighting the disruption of RNA-protein networks as a promising avenue for cancer therapeutics. In this review, we discuss the role of structurally distinct RBPs in cancer, and the mechanisms of RBP-directed small-molecule inhibitors (SMOIs) focusing on drug-protein interactions, binding surfaces, potency, and translational potential. Additionally, we underline the limitations of RBP-targeting drug discovery assays and comment on future trends in the field.

Immunohistochemical expression of PD-L1 and MDR1 in breast tumors: association with clinico-pathological parameters and treatment outcome

Antitumor immune evasion is a hallmark for the development and progression of cancer. Tumor cells adopt various mechanisms to escape the host immune system recognition. One such mechanism is the over expression of programmed death ligand (PD-L1), a negative T cell regulatory molecule. Because PD-L1 overexpression causes resistance to chemotherapeutic response in many cancers, herein we explored the relationship between PD-L1 and multidrug resistance protein MDR1 in breast cancer. Immunohistochemical evaluation of PD-L1 and MDR1 proteins in 194 breast cancer tissue samples were carried out. The relationship between PD-L1 and MDR1 expression on cancer cells with clinicopathological factors and prognosis was investigated. IHC showed a significant correlation between PD-L1 and MDR1 expression on tumor cells. Increased PD-L1 expression was also associated with lymph node status and tumor grade of the patient. Our results also revealed that the expression of PD-L1 and MDR1 was higher in TNBC subtype compared to other breast cancer subtypes. Therefore, a better understanding of the molecular mechanism through which PD-1/PD-L1 pathway contribute to the chemoresistance might bring forth the prognostic significance of PD-L1 and selection of patients who may benefit from immunotherapy.

ABC transporters in breast cancer: their roles in multidrug resistance and beyond

ATP-binding cassette (ABC) transporters are membrane-spanning proteins involved in cholesterol homeostasis, transport of various molecules in and out of cells and organelles, oxidative stress, immune recognition, and drug efflux. They are long implicated in the development of multidrug resistance in cancer chemotherapy. Existing clinical and molecular evidence has also linked ABC transporters with cancer pathogenesis, prognostics, and therapy. In this review, we aim to provide a comprehensive update on all ABC transporters and their roles in drug resistance in breast cancer (BC). For solid tumours such as BC, various ABC transporters are highly expressed in less differentiated subtypes and metastases. ABCA1, ABCB1 and ABCG2 are key players in BC chemoresistance. Restraining these transporters has evolved as a possible mechanism to reverse this phenomenon. Further, ABCB1 and ABCC1 are important in BC prognosis. Newer therapeutic approaches have been developed to target all these molecules to dysregulate their effect, reduce cell viability, induce apoptosis, and increase drug sensitivity. In the future, targeted therapy for specific genetic variations and upstream or downstream molecules can help improve patient prognosis.

HuR-targeted agents: An insight into medicinal chemistry, biophysical, computational studies and pharmacological effects on cancer models

The Human antigen R (HuR) protein is an RNA-binding protein, ubiquitously expressed in human tissues, that orchestrates target RNA maturation and processing both in the nucleus and in the cytoplasm. A survey of known modulators of the RNA-HuR interactions is followed by a description of its structure and molecular mechanism of action - RRM domains, interactions with RNA, dimerization, binding modes with naturally occurring and synthetic HuR inhibitors. Then, the review focuses on HuR as a validated molecular target in oncology and briefly describes its role in inflammation. Namely, we show ample evidence for the involvement of HuR in the hallmarks and enabling characteristics of cancer, reporting findings from in vitro and in vivo studies; and we provide abundant experimental proofs of a beneficial role for the inhibition of HuR-mRNA interactions through silencing (CRISPR, siRNA) or pharmacological inhibition (small molecule HuR inhibitors).

microRNA-133b Regulates Cell Proliferation and Cell Cycle Progression via Targeting HuR in Colorectal Cancer

MicroRNAs (miRNAs/miRs) have been identified to serve a key role in the development of tumors. However, the role of miR-133b in colorectal cancer (CRC) remains largely unclear. This study will investigate the role and mechanism of miR-133b in CRC. Reverse transcription-quantitative polymerase chain reaction analysis was performed to detect the level of miR-133b in CRC cell lines. Bioinformatics software TargetScan predicted the potential target genes of miR-133b, and a dual luciferase reporter assay was used to confirm this. To investigate the role of miR-133b in CRC cells, miR-133b was upregulated or downregulated in CRC cell lines (SW620 and HT-29) by transfecting with a miR-133b mimic or inhibitor, respectively. Subsequently, cell viability was analyzed using MTT assay, whereas cell apoptosis and the cell cycle distribution were analyzed by flow cytometry. In addition, the associated protein levels were detected using western blot analysis. The results demonstrated that miR-133b was significantly downregulated in CRC cell lines when compared with the normal colonic epithelial NCM-460 cell line. Human antigen R (HuR; also termed ELAVL1) was demonstrated to be a direct target of miR-133b and was negatively regulated by miR-133b. HuR was also notably upregulated in the CRC cell lines when compared with the normal control. Transfection of SW620 and HT-29 cells with the miR-133b mimic significantly inhibited cell viability, and induced cell apoptosis and G1 phase arrest, while upregulation of HuR demonstrated the opposite effects. Furthermore, the present data demonstrated that the miR-133b mimic significantly enhanced the protein levels of p21 and p27, and downregulated cyclin D1 and cyclin A levels in SW620 and HT-29 cells; the opposite effects were observed following treatment with the miR-133b inhibitor. In conclusion, the data indicate that miR-133b suppressed CRC cell growth by targeting 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.

Targeting the interaction between RNA-binding protein HuR and FOXQ1 suppresses breast cancer invasion and metastasis

Patients diagnosed with metastatic breast cancer have a dismal 5-year survival rate of only 24%. The RNA-binding protein Hu antigen R (HuR) is upregulated in breast cancer, and elevated cytoplasmic HuR correlates with high-grade tumors and poor clinical outcome of breast cancer. HuR promotes tumorigenesis by regulating numerous proto-oncogenes, growth factors, and cytokines that support major tumor hallmarks including invasion and metastasis. Here, we report a HuR inhibitor KH-3, which potently suppresses breast cancer cell growth and invasion. Furthermore, KH-3 inhibits breast cancer experimental lung metastasis, improves mouse survival, and reduces orthotopic tumor growth. Mechanistically, we identify FOXQ1 as a direct target of HuR. KH-3 disrupts HuR–FOXQ1 mRNA interaction, leading to inhibition of breast cancer invasion. Our study suggests that inhibiting HuR is a promising therapeutic strategy for lethal metastatic breast cancer.

Wu et al. identify an inhibitor to the RNA-binding protein HuR, KH-3, that disrupts the interaction between HuR and target RNAs and inhibits human cancer growth and metastasis in mouse xenograft assays. This study suggests the therapeutic potential of targeting HuR in breast cancer with HuR overexpression.

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.

Inhibition of Caspase-2 Translation by the mRNA Binding Protein HuR: A Novel Path of Therapy Resistance in Colon Carcinoma Cells?

An increased expression and cytoplasmic abundance of the ubiquitous RNA binding protein human antigen R (HuR) is critically implicated in the dysregulated control of post- transcriptional gene expression during colorectal cancer development and is frequently associated with a high grade of malignancy and therapy resistance. Regardless of the fact that HuR elicits a broad cell survival program by increasing the stability of mRNAs coding for prominent anti-apoptotic factors, recent data suggest that HuR is critically involved in the regulation of translation, particularly, in the internal ribosome entry site (IRES) controlled translation of cell death regulatory proteins. Accordingly, data from human colon carcinoma cells revealed that HuR maintains constitutively reduced protein and activity levels of caspase-2 through negative interference with IRES-mediated translation. This review covers recent advances in the understanding of mechanisms underlying HuR's modulatory activity on IRES-triggered translation. With respect to the unique regulatory features of caspase-2 and its multiple roles (e.g., in DNA-damage-induced apoptosis, cell cycle regulation and maintenance of genomic stability), the pathophysiological consequences of negative caspase-2 regulation by HuR and its impact on therapy resistance of colorectal cancers will be discussed in detail. The negative HuR-caspase-2 axis may offer a novel target for tumor sensitizing therapies.

The HuR CMLD-2 inhibitor exhibits antitumor effects via MAD2 downregulation in thyroid cancer cells

Hu antigen R (HuR) is indeed one of the most studied RNA-binding protein (RBP) since its fundamental role both in tumorigenesis and cancer progression. For this reason, downregulation in HuR protein levels or inhibition of HuR biological function are, nowadays, attractive goals in cancer research. Here, we examined the antitumor effects of CMLD-2 in four thyroid cancer cell lines (SW1736, 8505 C, BCPAP and K1). Indeed, CMLD-2 competitively binds HuR protein disrupting its interaction with RNA-targets. 35 μM CLMD-2 produced a significant downregulation in thyroid cancer cell viability, coupled to an increase in apoptosis. Moreover, CMLD-2 treatment hindered both migration and colony formation ability. MAD2 is a microtubules-associated protein known to be greatly overexpressed in cancer and correlating with tumor aggressiveness. Furthermore, MAD2 is known to be a HuR target. CMLD-2 treatment induced a strong MAD2 downregulation and rescue experiments depicted it as a key effector in HuR-mediated in cancer. Altogether, these data contributed to foster HuR inhibition as valid antineoplastic treatment in thyroid cancer, highlighting MAD2 as a novel therapeutic target.

The RNA-Binding Protein HuR Confers Oxaliplatin Resistance of Colorectal Cancer By Upregulating CDC6

Human antigen R (HuR) is an RNA-binding protein that posttranscriptionally regulates many cancer-trait genes. CDC6, a central regulator of DNA replication, is regulated by HuR. In this study, we investigated the role of HuR in colorectal cancer tumorigenesis and oxaliplatin (L-OHP) resistance, as well as the underlying mechanisms involving CDC6. We detected increased HuR and CDC6 expression, along with a positive correlation between the two in human colorectal cancer tissues. HuR overexpression increased colorectal cancer cell proliferation in vitro and xenograft tumor growth in vivo, and induced resistance to L-OHP. In contrast, HuR knockdown sensitized colorectal cancer cells to L-OHP. CDC6 overexpression increased while CDC6 knockdown decreased colorectal cancer cell malignant behaviors (growth, DNA synthesis, EMT, migration, and invasion) and L-OHP resistance in vitro Moreover, L-OHP resistance induced by HuR overexpression was reversed by CDC6 knockdown. Mechanistically, the results from our luciferase reporter and ribonucleoprotein immunoprecipitation assays indicated that HuR upregulates CDC6 by binding to CDC6 3'-UTR. Taken together, our findings identified HuR's regulation of CDC6 as an essential mechanism driving colorectal cancer tumorigenesis and L-OHP resistance, and this mechanism may represent a potential target for overcoming drug resistance in colorectal cancer.

Multiple functions of HuR in urinary tumors

PURPOSE

Human embryonic lethal abnormal visual-like protein, HuR, belongs to a member of the Hu family of RNA-binding protein and plays a critical role in urinary tumors. The purpose of this review is to summarize the current literature to demonstrate the importance of HuR in urinary tract tumors' biology and explore the potential role in therapeutic strategies aimed at targeting this molecule in cancer cells.

METHODS

The relevant literature from PubMed and Medline databases is reviewed in this article.

RESULTS

Increasing evidence supports that HuR plays a critical role in urinary tumors' biology because it regulates the expression of many urinary tumors-associated molecules through post-transcriptional regulatory mechanisms (including mRNA trafficking, mRNA decay and protein translation). Recent studies have demonstrated that HuR is associated with chemoresistance of urinary tumors, suggesting that HuR might be a novel therapeutic target and a marker for therapeutic response and prognosis assessment.

CONCLUSION

HuR is associated with various urinary tumors biological characteristics. Targeted therapy of HuR may become an attractive treatment strategy. What's more, more preclinical and clinical trials of this targeted strategy are necessary for the treatment of urinary tumors.

Oligomeric transition and dynamics of RNA binding by the HuR RRM1 domain in solution

Human antigen R (HuR) functions as a major post-transcriptional regulator of gene expression through its RNA-binding activity. HuR is composed by three RNA recognition motifs, namely RRM1, RRM2, and RRM3. The two N-terminal RRM domains are disposed in tandem and contribute mostly to HuR interaction with adenine and uracil-rich elements (ARE) in mRNA. Here, we used a combination of NMR and electrospray ionization-ion mobility spectrometry-mass spectrometry (ESI-IMS-MS) to characterize the structure, dynamics, RNA recognition, and dimerization of HuR RRM1. Our solution structure reveals a canonical RRM fold containing a 19-residue, intrinsically disordered N-terminal extension, which is not involved in RNA binding. NMR titration results confirm the primary RNA-binding site to the two central β-strands, β1 and β3, for a cyclooxygenase 2 (Cox2) ARE I-derived, 7-nucleotide RNA ligand. We show by ¹⁵N relaxation that, in addition to the N- and C-termini, the β2-β3 loop undergoes fast backbone dynamics (ps-ns) both in the free and RNA-bound state, indicating that no structural ordering happens upon RNA interaction. ESI-IMS-MS reveals that HuR RRM1 dimerizes, however dimer population represents a minority. Dimerization occurs via the α-helical surface, which is oppositely orientated to the RNA-binding β-sheet. By using a DNA analog of the Cox2 ARE I, we show that DNA binding stabilizes HuR RRM1 monomer and shifts the monomer-dimer equilibrium toward the monomeric species. Altogether, our results deepen the current understanding of the mechanism of RNA recognition employed by HuR.

BAG3 regulates stability of IL-8 mRNA via interplay between HuR and miR-4312 in PDACs

Bcl-2 associated athanogene 3 (BAG3) is highly expressed in pancreatic ductal adenocarcinoma (PDAC), and its high expression appears to be a poor prognostic factor for patients with PDAC. In this study, we show that BAG3 knockdown significantly decreases migration and invasion of PDACs via reduction of interleukine-8 (IL-8) production. BAG3 knockdown regulates IL-8 expression at the posttranscriptional levels via interplay between recruitment of RNA-binding protein HuR and miR-4312. HuR binds to the cis-elements located in the 3'-untranslational region (UTR) of the IL-8 transcript to stabilize it, whereas miR-4312-containing miRNA-induced silencing complex (miRISC) is recruited to the adjacent seed element to destabilize it. The binding of HuR prevents the recruitment of Argonaute (Ago2), overriding miR-4312-mediated translation inhibition of IL-8. BAG3 knockdown decreases cytoplasmic distribution of HuR via increasing its phosphorylation at Ser202, therefore compromising its recruitment while promoting recruitment of miR-4312 containing miRISC to IL-8 transcript. Furthermore, our data indicate that only phosphorylated Ago2 at Ser387 interacts with IL-8 transcript. BAG3 knockdown increases phosphorylation of Ago2 at Ser387, thereby further promoting loading of miR-4312 containing miRISC to IL-8 transcript. Taken together, we propose that BAG3 promotes invasion by stabilizing IL-8 transcript via HuR recruitment, and subsequently suppressing the loading of miR-4312 containing miRISC in PDACs. Our results reveal a novel pathway linking BAG3 expression to enhanced PDAC metastasis, thus making BAG3 a potential target for intervention in pancreatic cancer.

Coming of Age for Autotaxin and Lysophosphatidate Signaling: Clinical Applications for Preventing, Detecting and Targeting Tumor-Promoting Inflammation

A quarter-century after the discovery of autotaxin in cell culture, the autotaxin-lysophosphatidate (LPA)-lipid phosphate phosphatase axis is now a promising clinical target for treating chronic inflammatory conditions, mitigating fibrosis progression, and improving the efficacy of existing cancer chemotherapies and radiotherapy. Nearly half of the literature on this axis has been published during the last five years. In cancer biology, LPA signaling is increasingly being recognized as a central mediator of the progression of chronic inflammation in the establishment of a tumor microenvironment which promotes cancer growth, immune evasion, metastasis, and treatment resistance. In this review, we will summarize recent advances made in understanding LPA signaling with respect to chronic inflammation and cancer. We will also provide perspectives on the applications of inhibitors of LPA signaling in preventing cancer initiation, as adjuncts extending the efficacy of current cancer treatments by blocking inflammation caused by either the cancer or the cancer therapy itself, and by disruption of the tumor microenvironment. Overall, LPA, a simple molecule that mediates a plethora of biological effects, can be targeted at its levels of production by autotaxin, LPA receptors or through LPA degradation by lipid phosphate phosphatases. Drugs for these applications will soon be entering clinical practice.

Ancient and modern: hints of a core post-transcriptional network driving chemotherapy resistance in ovarian cancer

RNA-binding proteins (RBPs) and noncoding (nc)RNAs (such as microRNAs, long ncRNAs, and others) cooperate within a post-transcriptional network to regulate the expression of genes required for many aspects of cancer behavior including its sensitivity to chemotherapy. Here, using an RBP-centric approach, we explore the current knowledge surrounding contributers to post-transcriptional gene regulation (PTGR) in ovarian cancer and identify commonalities that hint at the existence of an evolutionarily conserved core PTGR network. This network regulates survival and chemotherapy resistance in the contemporary context of the cancer cell. There is emerging evidence that cancers become dependent on PTGR factors for their survival. Further understanding of this network may identify innovative therapeutic targets as well as yield crucial insights into the hard-wiring of many malignancies, including ovarian cancer. WIREs RNA 2018, 9:e1432. doi: 10.1002/wrna.1432 This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications Translation > Translation Mechanisms RNA in Disease and Development > RNA in Disease.

PD-1/PD-L1 interaction up-regulates MDR1/P-gp expression in breast cancer cells via PI3K/AKT and MAPK/ERK pathways

Programmed cell death ligand 1 (PD-L1) is an immunosuppressive molecule expressed on tumor cells. By interacting with programmed cell death-1 (PD-1) on T cells, it inhibits immune responses. Because PD-L1 expression on cancer cells increases their chemoresistance, we investigated the correlation between PD-L1 and multidrug resistance 1/ P-glycoprotein (MDR1/P-gp) expression in breast cancer cells. Analysis of breast cancer tissues using tissue microarrays revealed a significant correlation between PD-L1 and MDR1/P-gp protein levels. Increased expression of PD-L1 was associated with lymph node metastasis and histological tumor grade. In addition, interaction of PD-L1 with PD-1 induced phosphorylation of AKT and ERK, resulting in the activation of PI3K/AKT and MAPK/ERK pathways and increased MDR1/P-gp expression in breast cancer cells. The PD-1/PD-L1 interaction also increased survival of breast cancer cells incubated with doxorubicin. These findings suggest that the PD-1/PD-L1 inhibition may increase chemotherapy efficacy by inhibiting the MDR1/P-gp expression in breast cancer cells.

Modulation of the mRNA-binding protein HuR as a novel reversal mechanism of epirubicin-triggered multidrug resistance in colorectal cancer cells

HuR (ELAVL1), a RNA-binding protein, plays a key role in posttranscriptional regulation of multidrug resistance (MDR)-related genes. Among various HuR-regulated oncogenic transcripts, the activation of galectin-3/β-catenin survival pathway is critical to induce transcription of cyclin D1, P-glycoprotein (P-gp) and/or multidrug resistance-associated proteins (MRPs). In this study, we aim to elucidate the HuR-regulating pathways related to epirubicin-mediated resistance in human colorectal carcinoma cells. The effects and mechanisms of epirubicin treatment on the expressions of upstream survival signals (e.g., β-catenin) and downstream MDR transporters (e.g., P-gp) and anti-apoptotic pathways (e.g., Bcl-2) were assessed with or without HuR knockdown (siHuR) or overexpression (overHuR; ectopic HuR or pcDNA3/HA-HuR). Our results showed that siHuR decreased transcriptional expressions of galectin-3, β-catenin, cyclin D1, Bcl-2, P-gp, MRP1, and MRP2 in epirubicin-treated colon cancer cells. Consistently, the co-treatment of epirubicin and siHuR diminished the expressions of galectin-3, ß-catenin, c-Myc, P-gp and MRP1. HuR silencing enhanced the intracellular accumulation of epirubicin in colon cancer cells. On the other hand, overHuR abolished such effects. Furthermore, siHuR significantly intensified epirubicin-mediated apoptosis via increasing reactive oxygen species and thus promoted the cytotoxic effect of epirubicin. The combined treatments of siHuR and epirubicin significantly reduced the expression of Bcl-2, but increased the expression of Bax, as well as activity and expression levels of caspase-3 and -9. In contrast, overHuR abrogated these effects. Our findings provide insight into the mechanisms by which siHuR potentiated epirubicin-induced cytotoxicity via inhibiting galectin-3/β-catenin signaling, suppressing MDR transporters and provoking apoptosis. To our best knowledge, this is an innovative investigation linking the post-transcriptional control by HuR silencing to survival signaling repression, efflux transporter reversal and apoptosis induction. Our study thus provides a powerful regimen for circumventing MDR in colon cancer cells.

Loss of constitutive ABCB1 expression in breast cancer associated with worse prognosis

ABCB1 gene encodes an adenosine 5′-triphosphate–binding cassette transporter, which not only confers multidrug resistance phenotype in malignant cells, but is also present in several nonmalignant tissues. For the last thirty years, ABCB1 expression in breast cancer has been described by many authors, but the extent of expression differs among the studies, and there is no consensus regarding its potential role in carcinogenesis or in the tumor response to antineoplastic drugs. This study aimed to characterize the expression of ABCB1 in breast tumors as a function of genetic, clinical, and histopathological variables. The ABCB1 expression was also evaluated in nonmalignant mammary tissues adjacent to tumors and in benign lesions. The detection of ABCB1 protein was performed by immunohistochemistry in tissue specimens of excised breasts obtained from a prospective cohort of Brazilian women with breast cancer. The association of ABCB1 protein levels with ABCB1 mRNA, gene polymorphisms, and clinical and histopathological variables was also evaluated. The Kaplan–Meier curves and multivariate Cox regression analyses were conducted to identify independent predictors of disease-free survival of patients with breast cancer. ABCB1 was detected in 86.3% (656) of breast tumors, 98.8% (606) of nonmalignant mammary tissue adjacent to tumors, and 100% (28) of benign lesions. Reduced ABCB1 protein levels in breast tumors was associated with triple-negative subtype (adjusted odds ratio [OR_adj] =0.24; 95% confidence interval [CI] =0.13–0.45), lymph node status < pN2 (OR_adj =0.27; 95% CI =0.10–0.71), tumor size >2 cm (OR_adj =0.55; 95% CI =0.32–0.93), and hypertensive status (OR_adj =0.42; 95% CI =0.24–0.73), and it was significantly associated with shorter disease-free survival, either for all breast cancer patients (p log-rank =0.012; hazard ratio [HR] =3.46; 95% CI =1.21–9.91) or for those with triple-negative tumors (p log-rank =0.007; HR =11.41; 95% CI =1.29–100.67). The loss of constitutive ABCB1 expression in breast cancer, especially in triple-negative tumors, seems to indicate a subgroup of worse prognosis.

HuR promotes breast cancer cell proliferation and survival via binding to CDK3 mRNA

HuR, a ubiquitously expressed RNA-binding protein, stabilizes mRNA and regulates its translation. HuR expression was increased at all stages of breast cancer and correlated with poor clinical outcome. However, the detailed mechanisms remain unclear. Here we reported that overexpression of HuR increased CDK3 mRNA stability and thus its protein expression in MDA-MB-231 and MCF-7 cells. Mechanistically, CDK3 mRNA was identified as a target of HuR via bioinformatics and RNA binding protein immunoprecipitation (RIP) assays. Furthermore, treatment with HuR shRNA decreased CDK3 expression, inhibited cell proliferation and promoted cell apoptosis in breast cancer. More importantly, overexpression of CDK3 reversed the suppressive effects of HuR knockdown on cell growth in both MDA-MB-231 and MCF-7 cells. Finally, HuR and CDK3 expression levels were positively correlated and significantly up-regulated in breast cancer samples. And overexpression of HuR attenuated the chemotherapeutical efficiency of breast cancer. Therefore, our results indicate that ectopic expression of HuR promotes breast cancer cell proliferation and survival by directly binding to and stabilizing CDK3 mRNA.

Post-transcriptional regulation of cytokine and growth factor signaling in cancer

Cytokines and growth factors regulate cell proliferation, differentiation, migration and apoptosis, and play important roles in coordinating growth signal responses during development. The expression of cytokine genes and the signals transmitted through cytokine receptors are tightly regulated at several levels, including transcriptional and post-transcriptional levels. A majority of cytokine mRNAs, including growth factor transcripts, contain AU-rich elements (AREs) in their 3' untranslated regions that control gene expression by regulating mRNA degradation and changing translational rates. In addition, numerous proteins involved in transmitting signals downstream of cytokine receptors are regulated at the level of mRNA degradation by GU-rich elements (GREs) found in their 3' untranslated regions. Abnormal stabilization and overexpression of ARE or GRE-containing transcripts had been observed in many malignancies, which is a consequence of the malfunction of RNA-binding proteins. In this review, we briefly summarize the role of AREs and GREs in regulating mRNA turnover to coordinate cytokine and growth factor expression, and we describe how dysregulation of mRNA degradation mechanisms contributes to the development and progression of cancer.

Autotaxin Expression Is Regulated at the Post-transcriptional Level by the RNA-binding Proteins HuR and AUF1

Autotaxin (ATX) is a key enzyme that converts lysophosphatidylcholine (LPC) into lysophosphatidic acid (LPA), a lysophospholipid mediator that regulates cellular activities through its specific G protein-coupled receptors. The ATX-LPA axis plays an important role in various physiological and pathological processes, especially in inflammation and cancer development. Although the transcriptional regulation of ATX has been widely studied, the post-transcriptional regulation of ATX is largely unknown. In this study, we identified conserved adenylate-uridylate (AU)-rich elements in the ATX mRNA 3'-untranslated region (3'UTR). The RNA-binding proteins HuR and AUF1 directly bound to the ATX mRNA 3'UTR and had antagonistic functions in ATX expression. HuR enhanced ATX expression by increasing ATX mRNA stability, whereas AUF1 suppressed ATX expression by promoting ATX mRNA decay. HuR and AUF1 were involved in ATX regulation in Colo320 human colon cancer cells and the LPS-stimulated human monocytic THP-1 cells. HuR knockdown suppressed ATX expression in B16 mouse melanoma cells, leading to inhibition of cell migration. This effect was reversed by AUF1 knockdown to recover ATX expression or by the addition of LPA. These results suggest that the post-transcriptional regulation of ATX expression by HuR and AUF1 modulates cancer cell migration. In summary, we identified HuR and AUF1 as novel post-transcriptional regulators of ATX expression, thereby elucidating a novel mechanism regulating the ATX-LPA axis.

Current Evidence and Future Perspectives on HuR and Breast Cancer Development, Prognosis, and Treatment

Hu-antigen R (HuR) is an RNA-binding posttranscriptional regulator that belongs to the Hu/ELAV family. HuR expression levels are modulated by a variety of proteins, microRNAs, chemical compounds, or the microenvironment, and in turn, HuR affects mRNA stability and translation of various genes implicated in breast cancer formation, progression, metastasis, and treatment. The aim of the present review is to critically summarize the role of HuR in breast cancer development and its potential as a prognosticator and a therapeutic target. In this aspect, all the existing English literature concerning HuR expression and function in breast cancer cell lines, in vivo animal models, and clinical studies is critically presented and summarized. HuR modulates many genes implicated in biological processes crucial for breast cancer formation, growth, and metastasis, whereas the link between HuR and these processes has been demonstrated directly in vitro and in vivo. Additionally, clinical studies reveal that HuR is associated with more aggressive forms of breast cancer and is a putative prognosticator for patients' survival. All the above indicate HuR as a promising drug target for cancer therapy; nevertheless, additional studies are required to fully understand its potential and determine against which types of breast cancer and at which stage of the disease a therapeutic agent targeting HuR would be more effective.

Clinical Significance of Hu-Antigen Receptor (HuR) and Cyclooxygenase-2 (COX-2) Expression in Human Malignant and Benign Thyroid Lesions

Hu-antigen R (HuR) is considered to play a crucial role in tumor formation and growth by binding to mRNAs encoding proteins such as Cyclooxygenase-2 (COX-2) and inducing their expression via mRNA stabilization and/or altered translation. The present study aimed to evaluate the clinical significance of HuR and COX-2 proteins’ expression in human benign and malignant thyroid lesions. HuR and COX-2 proteins’ expression was assessed immunohistochemically on paraffin-embedded thyroid tissues obtained from 98 patients with benign (n = 48) and malignant (n = 50) lesions and was statistically analyzed with clinicopathological parameters, follicular cells’ proliferative capacity and recurrence risk rate. Enhanced HuR and COX-2 expression was significantly more frequently observed in malignant compared to benign thyroid lesions (p = 0.0073 and p = 0.0016, respectively), as well as in papillary carcinomas compared to hyperplastic nodules (p = 0.0039 and p = 0.0009, respectively). Positive associations of both HuR and COX-2 expression with follicular cells’ proliferation rate were also noted (p = 0.0087 and p = 0.0127, respectively). In malignant thyroid lesions, elevated COX-2 expression was significantly associated with female patients’ gender (p = 0.0381) and the presence of lymph node metastases (p = 0.0296). The present data support evidence that both HuR and COX-2 may be involved in the malignant state of thyroid neoplasia and may be utilized in the diagnosis of malignant thyroid tumors.

Oncogenic p95HER2 regulates Na+-HCO3- cotransporter NBCn1 mRNA stability in breast cancer cells via 3'UTR-dependent processes

The Na⁺-HCO₃^- cotransporter NBCn1 (SLC4A7) is up-regulated in breast cancer, important for tumor growth, and a single nucleotide polymorphism (SNP), rs4973768, in its 3' untranslated region (3'UTR) correlates with increased breast cancer risk. We previously demonstrated that NBCn1 expression and promoter activity are strongly increased in breast cancer cells expressing a constitutively active oncogenic human epidermal growth factor receptor 2 (HER2) (p95HER2). Here, we address the roles of p95HER2 in regulating NBCn1 expression via post-transcriptional mechanisms. p95HER2 expression in MCF-7 cells reduced the rate of NBCn1 mRNA degradation. The NBCn1 3'UTR down-regulated luciferase reporter expression in control cells, and this was reversed by p95HER2, suggesting that p95HER2 counteracts 3'UTR-mediated suppression of NBCn1 expression. Truncation analyses identified three NBCn1 3'UTR regions of regulatory importance. Mutation of putative miRNA-binding sites (miR-374a/b, miR-200b/c, miR-29a/b/c, miR-488) in these regions did not have significant impact on 3'UTR activity. The NBCn1 3'UTR interacted directly with the RNA-binding protein human antigen R (HuR), and HuR knockdown reduced NBCn1 expression. Conversely, ablation of a distal AU-rich element increased 3'UTR-driven reporter activity, suggesting complex regulatory roles of these sites. The cancer-associated SNP variant decreased reporter expression in T-47D breast cancer cells, yet not in MCF-7, MDA-MB-231 and SK-BR-3 cells, arguing against a general role in regulating NBCn1 expression. Finally, p95HER2 expression increased total and plasma membrane NBCn1 protein levels and decreased the rate of NBCn1 protein degradation. Collectively, this is the first work to demonstrate 3'UTR-mediated NBCn1 regulation, shows that p95HER2 regulates NBCn1 expression at multiple levels, and substantiates the central position of p95HER2-NBCn1 signaling in breast cancer.

Targeting the mRNA-binding protein HuR impairs malignant characteristics of pancreatic ductal adenocarcinoma cells

Post-transcriptional regulation is a powerful mediator of gene expression, and can rapidly alter the expression of numerous transcripts involved in tumorigenesis. We have previously shown that the mRNA-binding protein HuR (ELAVL1) is elevated in human pancreatic ductal adenocarcinoma (PDA) specimens compared to normal pancreatic tissues, and its cytoplasmic localization is associated with increased tumor stage. To gain a better insight into HuR’s role in PDA biology and to assess it as a candidate therapeutic target, we altered HuR expression in PDA cell lines and characterized the resulting phenotype in preclinical models. HuR silencing by short hairpin and small interfering RNAs significantly decreased cell proliferation and anchorage-independent growth, as well as impaired migration and invasion. In comparison, HuR overexpression increased migration and invasion, but had no significant effects on cell proliferation and anchorage-independent growth. Importantly, two distinct targeted approaches to HuR silencing showed marked impairment in tumor growth in mouse xenografts. NanoString nCounter^® analyses demonstrated that HuR regulates core biological processes, highlighting that HuR inhibition likely thwarts PDA viability through post-transcriptional regulation of diverse signaling pathways (e.g. cell cycle, apoptosis, DNA repair). Taken together, our study suggests that targeted inhibition of HuR may be a novel, promising approach to the treatment of PDA.

Improving N(6)-methyladenosine site prediction with heuristic selection of nucleotide physical-chemical properties

N(6)-methyladenosine (m(6)A) is one of the most common and abundant post-transcriptional RNA modifications found in viruses and most eukaryotes. m(6)A plays an essential role in many vital biological processes to regulate gene expression. Because of its widespread distribution across the genomes, the identification of m(6)A sites from RNA sequences is of significant importance for better understanding the regulatory mechanism of m(6)A. Although progress has been achieved in m(6)A site prediction, challenges remain. This article aims to further improve the performance of m(6)A site prediction by introducing a new heuristic nucleotide physical-chemical property selection (HPCS) algorithm. The proposed HPCS algorithm can effectively extract an optimized subset of nucleotide physical-chemical properties under the prescribed feature representation for encoding an RNA sequence into a feature vector. We demonstrate the efficacy of the proposed HPCS algorithm under different feature representations, including pseudo dinucleotide composition (PseDNC), auto-covariance (AC), and cross-covariance (CC). Based on the proposed HPCS algorithm, we implemented an m(6)A site predictor, called M6A-HPCS, which is freely available at http://csbio.njust.edu.cn/bioinf/M6A-HPCS. Experimental results over rigorous jackknife tests on benchmark datasets demonstrated that the proposed M6A-HPCS achieves higher success rates and outperforms existing state-of-the-art sequence-based m(6)A site predictors.

Influence of MDR1 methylation on the curative effect of interventional embolism chemotherapy for cervical cancer

Background

Multi-drug resistance (MDR) is the main cause of tumor failure to chemotherapy. This study aims to explore the influence of MDR1 methylation on curative effect of interventional embolism chemotherapy for cervical cancer.

Methods

Sixty-seven patients with cervical cancer receiving embolism chemotherapy were selected, and 45 normal cervical tissues were included as a control. Immunohistochemistry was used to detect the level of P-glycoprotein (P-gp) in cervical cancer, and to make an analysis compared with normal tissues. The methylation status of the MDR1 gene promoter region 16 CpG units was analyzed by using kilobase-specific cracking and matrix-assisted laser desorption ionization time of flight mass spectrometry.

Results

The results indicated that the positive expression rates of P-gp were 0% (0/45) in normal cervical tissue, and 61.19% (41/67) and 77.61% (52/67) before and after interventional embolism chemotherapy in cervical cancer tissues, respectively. There were significant differences compared with normal cervical tissues (χ²=4.2523, 0.0392). The positive expression rate of P-gp before chemotherapy was negatively correlated with efficacy of chemotherapy (r=−0.340, P=0.005). Methylation rate of 13 CpG units in normal tissues was significantly greater than cervical tissues (P<0.05). In cervical cancer tissue, methylation rate of six CpG units before interventional embolism chemotherapy was higher than after chemotherapy, but that of one CpG unit was lower than after chemotherapy (P<0.05). The methylation rate of one CpG unit with effective chemotherapy before chemotherapy was significantly higher than ineffective chemotherapy (P<0.05), and the other CpG units were similar (P>0.05).

Conclusion

P-gp expression level coded by MDR1, methylation status of partial MDR1 gene promoter regions CpG island, is closely related to the efficacy of interventional embolism chemotherapy for cervical cancer before the operation.

Can we observe changes in mRNA "state"? Overview of methods to study mRNA interactions with regulatory proteins relevant in cancer related processes

RNA binding proteins (RBP) regulate the editing, localization, stabilization, translation, and degradation of ribonucleic acids (RNA) through their interactions with specific cis-acting elements within target RNAs. Post-transcriptional regulatory mechanisms are directly involved in the control of the immune response and stress response and their alterations play a crucial role in cancer related processes. In this review, we discuss mRNAs and RNA binding proteins relevant to tumorigenesis, current methodologies for detecting RNA interactions, and last, we describe a novel method to detect such interactions, which combines peptide modified, RNA imaging probes (FMTRIPs) with proximity ligation (PLA) and rolling circle amplification (RCA). This assay detects native RNA in a sequence specific and single RNA sensitive manner, and PLA allows for the quantification and localization of protein-mRNA interactions with single-interaction sensitivity in situ.

HuR mediated post-transcriptional regulation as a new potential adjuvant therapeutic target in chemotherapy for pancreatic cancer

AIM

To investigate the expression of HuR in pancreatic ductal adenocarcinoma (PDA) and to assess the effects of HuR silencing on the expression of cyclooxygenase-2 (COX-2) and heme oxygenase-1 (HO-1) and the in vitro response to gemcitabine (GEM) treatment in pancreatic cell lines.

METHODS

We compared the expression of HuR, COX-2, and HO-1 in PDA and normal pancreatic tissue using quantitative reverse transcription polymerase chain reaction (qRT-PCR) and western blot. In addition, the HuR, COX-2 and HO-1 were analyzed in four types of cancer cell lines (MiaPaca2, Su.86.86, Capan-1, and Capan-2) with and without GEM treatment. Immunocytofluorescence analysis was used to investigate HuR localization in cells. Cell viability and response to GEM after HuR silencing were determined with the 3-(4,5-dimethylthiazol-2-Yl)-2,5-diphenyltetrazolium bromide test and the crystal violet clonogenic assay, respectively. To measure apoptosis, activation of caspases 3/7 was evaluated using immunofluorescence.

RESULTS

In PDA tissue obtained from patients not treated with GEM, HuR mRNA expression was 3.2 times lower (P < 0.05) and COX-2 and HO-1 mRNA expression was 2.3-fold and 7.2-fold higher (P < 0.05), respectively, than normal pancreatic tissue (from organ donor). qRT-PCR analysis showed that HuR, COX-2, and HO-1 mRNA were overexpressed in all cancer cell lines treated with the half maximal inhibitory concentration (IC50) dose of GEM compared with control cells (P < 0.05). Western blot analysis revealed that COX-2 and HO-1 levels were significantly decreased in cancer cells after HuR silencing. Furthermore, HuR silencing increased the response to GEM treatment and decreased cell viability by 11.6%-53.7% compared to control cell lines. Caspases 3 and 7 were activated after HuR silencing and GEM treatment in all pancreatic cancer cell lines. In comparison, treatment with GEM alone did not activate caspases 3 and 7 in the same cell lines.

CONCLUSION

HuR mediated post-transcriptional upregulation of COX-2 and HO-1 expression after GEM treatment in pancreatic cancer cells. HuR silencing significantly increased the effectiveness of GEM treatment in vitro.

Identification of Small-Molecule Inhibitors of the HuR/RNA Interaction Using a Fluorescence Polarization Screening Assay Followed by NMR Validation

The human antigen R (HuR) stabilizes many mRNAs of proto-oncogene, transcription factors, cytokines and growth factors by recognizing AU-rich elements (AREs) presented in their 3’ or 5’ untranslated region (UTR). Multiple lines of experimental evidence suggest that this process plays a key role in cancer development. Thus, destabilizing HuR/RNA interaction by small molecules presents an opportunity for cancer treatment/prevention. Here we present an integrated approach to identify inhibitors of HuR/RNA interaction using a combination of fluorescence-based and NMR-based high throughput screening (HTS). The HTS assay with fluorescence polarization readout and Z’-score of 0.8 was used to perform a screen of the NCI diversity set V library in a 384 well plate format. An NMR-based assay with saturation transfer difference (STD) detection was used for hits validation. Protein NMR spectroscopy was used to demonstrate that some hit compounds disrupt formation of HuR oligomer, whereas others block RNA binding. Thus, our integrated high throughput approach provides a new avenue for identification of small molecules targeting HuR/RNA interaction.

Identification and validation of novel small molecule disruptors of HuR-mRNA interaction

HuR, an RNA binding protein, binds to adenine- and uridine-rich elements (ARE) in the 3'-untranslated region (UTR) of target mRNAs, regulating their stability and translation. HuR is highly abundant in many types of cancer, and it promotes tumorigenesis by interacting with cancer-associated mRNAs, which encode proteins that are implicated in different tumor processes including cell proliferation, cell survival, angiogenesis, invasion, and metastasis. Drugs that disrupt the stabilizing effect of HuR upon mRNA targets could have dramatic effects on inhibiting cancer growth and persistence. In order to identify small molecules that directly disrupt the HuR-ARE interaction, we established a fluorescence polarization (FP) assay optimized for high throughput screening (HTS) using HuR protein and an ARE oligo from Musashi RNA-binding protein 1 (Msi1) mRNA, a HuR target. Following the performance of an HTS of ∼6000 compounds, we discovered a cluster of potential disruptors, which were then validated by AlphaLISA (Amplified Luminescent Proximity Homogeneous Assay), surface plasmon resonance (SPR), ribonucleoprotein immunoprecipitation (RNP IP) assay, and luciferase reporter functional studies. These compounds disrupted HuR-ARE interactions at the nanomolar level and blocked HuR function by competitive binding to HuR. These results support future studies toward chemical probes for a HuR function study and possibly a novel therapy for HuR-overexpressing cancers.

Control of pro-angiogenic cytokine mRNA half-life in cancer: the role of AU-rich elements and associated proteins

Control of mRNA half-life plays a central role in normal development and disease. Several pathological conditions, such as inflammation and cancer, tightly correlate with deregulation in mRNA stability of pro-inflammatory genes. Among these, pro-angiogenesis cytokines, which play a crucial role in the formation of new blood vessels, normally show rapid mRNA decay patterns. The mRNA half-life of these genes appears to be regulated by mRNA-binding proteins that interact with AU-rich elements (AREs) in the 3'-untranslated region of mRNAs. Some of these RNA-binding proteins, such as tristetraprolin (TTP), ARE RNA-binding protein 1, and KH-type splicing regulatory protein, normally promote mRNA degradation. Conversely, other proteins, such as embryonic lethal abnormal vision-like protein 1 (HuR) and polyadenylate-binding protein-interacting protein 2, act as antagonists, stabilizing the mRNA. The steady state levels of mRNA-binding proteins and their relative ratio is often perturbed in human cancers and associated with invasion and aggressiveness. Compelling evidence also suggests that underexpression of TTP and overexpression of HuR may be a useful prognostic and predictive marker in breast, colon, prostate, and brain cancers, indicating a potential therapeutic approach for these tumors. In this review, we summarize the main mechanisms involved in the regulation of mRNA decay of pro-angiogenesis cytokines in different cancers and discuss the interactions between the AU-rich-binding proteins and their mRNA targets.

MicroRNA-16 modulates HuR regulation of cyclin E1 in breast cancer cells

RNA binding protein (RBPs) and microRNAs (miRNAs or miRs) are post-transcriptional regulators of gene expression that are implicated in development of cancers. Although their individual roles have been studied, the crosstalk between RBPs and miRNAs is under intense investigation. Here, we show that in breast cancer cells, cyclin E1 upregulation by the RBP HuR is through specific binding to regions in the cyclin E1 mRNA 3' untranslated region (3'UTR) containing U-rich elements. Similarly, miR-16 represses cyclin E1, dependent on its cognate binding sites in the cyclin E1 3'UTR. Evidence in the literature indicates that HuR can regulate miRNA expression and recruit or dissociate RNA-induced silencing complexes (RISC). Despite this, miR-16 and HuR do not affect the other’s expression level or binding to the cyclin E1 3'UTR. While HuR overexpression partially blocks miR-16 repression of a reporter mRNA containing the cyclin E1 3'UTR, it does not block miR-16 repression of endogenous cyclin E1 mRNA. In contrast, miR-16 blocks HuR-mediated upregulation of cyclin E1. Overall our results suggest that miR-16 can override HuR upregulation of cyclin E1 without affecting HuR expression or association with the cyclin E1 mRNA.

RNA epigenetics

Mammalian messenger RNA (mRNA) and long noncoding RNA (lncRNA) contain tens of thousands of posttranscriptional chemical modifications. Among these, the N(6)-methyl-adenosine (m(6)A) modification is the most abundant and can be removed by specific mammalian enzymes. m(6)A modification is recognized by families of RNA binding proteins that affect many aspects of mRNA function. mRNA/lncRNA modification represents another layer of epigenetic regulation of gene expression, analogous to DNA methylation and histone modification.

Post-transcriptional up-regulation of PDGF-C by HuR in advanced and stressed breast cancer

Breast cancer is a heterogeneous disease characterized by multiple genetic alterations leading to the activation of growth factor signaling pathways that promote cell proliferation. Platelet-derived growth factor-C (PDGF-C) is overexpressed in various malignancies; however, the involvement of PDGF-C in breast cancers and the mechanisms underlying PDGF-C deregulation remain unclear. Here, we show that PDGF-C is overexpressed in clinical breast cancers and correlates with poor prognosis. PDGF-C up-regulation was mediated by the human embryonic lethal abnormal vision-like protein HuR, which stabilizes the PDGF-C transcript by binding to two predicted AU-rich elements (AREs) in the 3'-untranslated region (3'-UTR). HuR is up-regulated in hydrogen peroxide-treated or ultraviolet-irradiated breast cancer cells. Clinically, HuR levels are correlated with PDGF-C expression and histological grade or pathological tumor-node-metastasis (pTNM) stage. Our findings reveal a novel mechanism underlying HuR-mediated breast cancer progression, and suggest that HuR and PDGF-C are potential molecular candidates for targeted therapy of breast cancers.

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.

Regulation of the mRNA half-life in breast cancer

The control of the half-life of mRNA plays a central role in normal development and in disease progression. Several pathological conditions, such as breast cancer, correlate with deregulation of the half-life of mRNA encoding growth factors, oncogenes, cell cycle regulators and inflammatory cytokines that participate in cancer. Substantial stability means that a mRNA will be available for translation for a longer time, resulting in high levels of protein gene products, which may lead to prolonged responses that subsequently result in over-production of cellular mediators that participate in cancer. The stability of these mRNA is regulated at the 3’UTR level by different mechanisms involving mRNA binding proteins, micro-RNA, long non-coding RNA and alternative polyadenylation. All these events are tightly inter-connected to each other and lead to steady state levels of target mRNAs. Compelling evidence also suggests that both mRNA binding proteins and regulatory RNAs which participate to mRNA half-life regulation may be useful prognostic markers in breast cancers, pointing to a potential therapeutic approach to treatment of patients with these tumors. In this review, we summarize the main mechanisms involved in the regulation of mRNA decay and discuss the possibility of its implication in breast cancer aggressiveness and the efficacy of targeted therapy.

Predictive and prognostic significance of cytoplasmic expression of ELAV-like protein HuR in invasive breast cancer treated with neoadjuvant chemotherapy

Cytoplasmic HuR is associated with reduced survival in invasive breast cancer. We designed this study to determine the predictive and prognostic value of HuR expression in women with breast cancer who underwent neoadjuvant chemotherapy followed by surgical resection. We immunohistochemically analyzed cytoplasmic HuR expression in tumor biopsy cores obtained from 139 patients with invasive breast cancers who received paclitaxel and anthracycline-based neoadjuvant chemotherapy. We evaluated the relationship of HuR expression level with pathologic complete response (pCR), local recurrence-free survival (LRFS), distant recurrence-free survival (DRFS), recurrence-free survival (RFS), and overall survival (OS). Cytoplasmic HuR expression was present in 60 cases (43.2 %). The expression of cytoplasmic HuR was significantly associated with high nuclear grade (P < 0.0001) and ER (P = 0.001) and PR (P = 0.005) status. Multivariate regression analysis further revealed that high nuclear grade (P = 0.023), negative ER status (P = 0.043), and human epidermal growth factor receptor 2 (HER2) overexpression (P < 0.0001), but not cytoplasmic HuR expression, were significant independent predictors of pCR. Interestingly, multivariate Cox analysis revealed that cytoplasmic HuR expression was a strong independent predictor of reduced LRFS (P = 0.014), DRFS (P = 0.001), RFS (P < 0.0001), and OS (P = 0.019) irrespective of pCR. Furthermore, the patient group with tumors showing both expression of cytoplasmic HuR and non-pCR had a worse prognosis in LRFS (P = 0.048), DRFS (P < 0.0001), RFS (P < 0.0001), and OS (P = 0.001) than did other patient groups; patients with tumors showing negative cytoplasmic expression of HuR and pCR had the best prognosis in all RFS and OS. Cytoplasmic expression of HuR is an independent prognostic marker in breast cancer patients undergoing chemotherapy. Combination analyses of HuR expression and pCR, compared with pCR alone, can better predict clinical outcome in patients with primary breast cancer.

Cytoplasmic expression of the ELAV-like protein HuR as a potential prognostic marker in esophageal squamous cell carcinoma

Esophageal squamous cell carcinoma (ESCC) is one of the most frequent cancers and a leading cause of death from cancer in China. The human ELAV-like protein HuR has been found to contribute to cancer development and progression through stabilizing a group of cellular mRNAs of cancer-related genes. In this study, we investigated the expression of HuR in a cohort of ESCC patients using immunohistochemical staining. HuR detected in the cytoplasm of cancer cells was positive in 46.6% of 58 ESCC specimens; 75.9% of these specimens had nuclear immunoreactivity for HuR. Cytoplasmic HuR expression was higher in cancer tissues compared to 20 matched adjacent noncancerous tissues. A clinicopathological study showed that cytoplasmic HuR expression was positively associated with lymph node metastasis, depth of tumor invasion, and advanced stage, whereas nuclear HuR expression was not correlated with any clinicopathological factors. Patients positive for cytoplasmic HuR expression had a cumulative 5-year survival rate of 25.3%, whereas it was 43.8% for patients negative for cytoplasmic HuR expression. In a multivariate analysis, cytoplasmic HuR expression was an independent prognostic factor, whereas nuclear positivity for HuR was not. Our results indicate that high cytoplasmic HuR expression is associated with positive lymph node metastasis, deep tumor invasion, high stage, and poor survival in ESCC. Thus, HuR is the first mRNA stability protein whose expression is associated with poor survival in esophageal cancer.

Multiple functions of the RNA-binding protein HuR in cancer progression, treatment responses and prognosis

The human embryonic lethal abnormal vision-like protein, HuR, is a member of the Hu family of RNA-binding proteins. Over the past decade, this ubiquitously expressed protein has been extensively investigated in cancer research because it is involved in the regulation of mRNA stability and translation in many cell types. HuR activity and function is associated with its subcellular distribution, transcriptional regulation, translational and post-translational modifications. HuR regulation of target mRNAs is based on the interaction between the three specific domains of HuR protein and one or several U- or AU-rich elements (AREs) in the untranslated region of target mRNAs. A number of cancer-related transcripts containing AREs, including mRNAs for proto-oncogenes, cytokines, growth factors, and invasion factors, have been characterized as HuR targets. It has been proposed that HuR has a central tumorigenic activity by enabling multiple cancer phenotypes. In this review, we comprehensively survey the existing evidence with regard to the diverse functions of HuR in caner development and progression. The current data also suggest that HuR might be a novel and promising therapeutic target and a marker for treatment response and prognostic evaluation.