Clinical significance of HuR expression in human malignancy

Synthesis and Preliminary Evaluation of Tanshinone Mimic Conjugates for Mechanism of Action Studies

Human antigen R (HuR) is an RNA binding protein (RBP) belonging to the ELAV (Embryonic Lethal Abnormal Vision) family, which stabilizes mRNAs and regulates the expression of multiple genes. Its altered expression or localization is related to pathological features such as cancer or inflammation. Dihydrotanshinone I (DHTS I) is a naturally occurring, tetracyclic ortho-quinone inhibitor of the HuR-mRNA interaction. Our earlier efforts led to the identification of a synthetic Tanshinone Mimic (TM) 2 with improved affinity for HuR. Here we report five new TM probes 3-5 bearing a detection-promoting moiety (either photo affinity probe - PAP or biotin) as a para-substituent on the phenyl-sulphonamide for mechanism of action (MoA) studies. Biological and biochemical assays were used to characterize the novel TM conjugates 3-5. They showed similar toxic activity in HuR-expressing triple-negative breast cancer MDA-MB-231 cells, with micromolar CC50s. REMSAs revealed that photoactivatable groups (4 a and 4 b), but not biotin (5 a and 5 b), prevented conjugates' ability to disrupt rHuR-RNA complexes. Further biochemical studies confirmed that biotinylated probes, in particular 5 a, can be used to isolate rM1 M2 from solutions, taking advantage of streptavidin-coated magnetic beads, thus being the most promising HuR inhibitor to be used for further MoA studies in cell lysates.

hsa_circ_0001599 promotes odontogenic differentiation of human dental pulp stem cells by increasing ITGA2 expression and stability

Dental pulp regeneration is significantly aided by human dental pulp stem cells (hDPSCs). An increasing number of studies have demonstrated that circular RNAs (circRNAs) are crucial in the multidirectional differentiation of many mesenchymal stem cells, but their specific functions and mechanisms remain unknown. This work aimed at elucidating the molecular mechanism by which hsa_circ_0001599 works in hDPSCs during odontogenic differentiation. The expression of hsa_circ_0001599 in hDPSCs and dental pulp tissue was determined by using quantitative real-time PCR (qRT‒PCR). The role of hsa_circ_0001599 in the odontogenic differentiation of hDPSCs and its mechanism were studied using a variety of in vivo and in vitro assessments. The odontogenic differentiation of hDPSCs was facilitated by the overexpression of hsa_circ_0001599, which activated the PI3K/AKT signalling pathway in vitro. In vivo, hsa_circ_0001599 can promote the formation of new dentin-like structures. Mechanistically, hsa_circ_0001599 enhanced ITGA2 expression by sponging miR-889-3p. Furthermore, hsa_circ_0001599 interacts with the methylation reader hnRNPA2B1, promoting hnRNPA2B1 translocation from the nucleus to the cytoplasm and increasing ITGA2 mRNA stability. This research revealed the important role of hsa_circ_0001599 in odontogenic differentiation. Thus, hDPSCs engineered with hsa_circ_0001599 have the potential to be effective therapeutic targets for dental pulp repair and regeneration.

An HuR mutant, HuR-V225I, identified in adult T-cell Leukemia/Lymphoma, alters the pro-apoptotic function of HuR

The RNA-binding protein HuR regulates various cellular processes, such as proliferation, differentiation, and cell fate. Moreover, recent studies have shown that HuR modulates the expression of factors important for tumor growth and progression. Despite its prominent role in tumorigenesis, until recently, there have been no reported mutations in HuR that have been associated to cancer. Here, we show that a HuR mutation, HuR-V225I, previously identified in a patient with Adult T-cell Leukemia/Lymphoma, interferes with the pro-apoptotic function of HuR. In response to apoptosis, HuR translocates to the cytoplasm and is cleaved in a caspase-dependent manner. In cervical cancer cells, neuroblastoma cells, and T-lymphocytes, we observed a decrease in cleavage of the HuR-V225I mutant under apoptotic conditions. This effect was shown to be mediated by the nuclear retention of HuR-V225I. Finally, expression of the HuR-V225I mutant decreases the cell's response to apoptotic stimuli through the increased expression of mRNAs encoding anti-apoptotic factors, such as XIAP and BCL-2. Therefore, our data establishes that the absence of HuR cytoplasmic translocation and cleavage promotes cell viability, and that acquiring this mutation during tumorigenesis may thus reduce the efficacy of cancer therapy.

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.

ELAVL2 loss promotes aggressive mesenchymal transition in glioblastoma

Glioblastoma (GBM), the most lethal primary brain cancer, exhibits intratumoral heterogeneity and molecular plasticity, posing challenges for effective treatment. Despite this, the regulatory mechanisms underlying such plasticity, particularly mesenchymal (MES) transition, remain poorly understood. In this study, we elucidate the role of the RNA-binding protein ELAVL2 in regulating aggressive MES transformation in GBM. We found that ELAVL2 is most frequently deleted in GBM compared to other cancers and associated with distinct clinical and molecular features. Transcriptomic analysis revealed that ELAVL2-mediated alterations correspond to specific GBM subtype signatures. Notably, ELAVL2 expression negatively correlated with epithelial-to-mesenchymal transition (EMT)-related genes, and its loss promoted MES process and chemo-resistance in GBM cells, whereas ELAVL2 overexpression exerted the opposite effect. Further investigation via tissue microarray analysis demonstrated that high ELAVL2 protein expression confers a favorable survival outcome in GBM patients. Mechanistically, ELAVL2 was shown to directly bind to the transcripts of EMT-inhibitory molecules, SH3GL3 and DNM3, modulating their mRNA stability, potentially through an m6A-dependent mechanism. In summary, our findings identify ELAVL2 as a critical tumor suppressor and mRNA stabilizer that regulates MES transition in GBM, underscoring its role in transcriptomic plasticity and glioma progression.

CircNUP54 promotes hepatocellular carcinoma progression via facilitating HuR cytoplasmic export and stabilizing BIRC3 mRNA

Circular RNAs (circRNAs) have been implicated in tumorigenesis and progression of various cancers. However, the underlying mechanisms of circRNAs in hepatocellular carcinoma (HCC) have not been fully elucidated. Herein, a new oncogenic circRNA, hsa_circ_0070039 (circNUP54), was identified to be significantly upregulated in HCC through circRNA sequencing. As verified in 68 HCC samples, circNUP54 overexpression was correlated with aggressive cancerous behaviors and poor outcomes. Moreover, the function experiments showed that knockdown of circNUP54 inhibited the malignant progression of HCC in vitro and in vivo, whereas overexpression of circNUP54 had the opposite role. Mechanistic investigations carried out by RNA pull-down, RNA immunoprecipitation, and immunofluorescence revealed that circNUP54 interacted with the RNA-binding protein Hu-antigen R (HuR) and promoted its cytoplasmic export. The cytoplasmic accumulation of HuR stabilized the downstream BIRC3 mRNA through its binding to the 3' UTR region. Consequently, the encoded protein of BIRC3, cellular inhibitor of apoptosis 2 (cIAP2), proceeded to activate the NF-κB signal pathway and ultimately contributed to HCC progression. In addition, depletion of BIRC3 rescued the pro-tumorigenic effect of circNUP54 on HCC cells. Overall, this study demonstrated that circNUP54 facilitates HCC progression via regulating the HuR/BIRC3/NF-κB axis, which may serve as a promising therapeutic target for HCC treatment.

Relationship between HuR and tumor drug resistance

Human resistance protein R (HuR), also known as embryonic lethal abnormal visual-like protein (ELAVL1), is an RNA-binding protein widely expressed in vivo that affects the mRNA stability of targeted and is involved in post-transcriptional regulation. Recent studies have shown that HuR is aberrantly expressed in different human cancers and is an essential factor in poor clinical prognosis. The role of HuR in numerous tumors suggests that it could be a new target for tumor therapy and as a marker for efficacy and prognostic assessment. This review focuses on the relationship between HuR and drug resistance in different tumors and briefly describes the structure, function, and inhibitors of HuR. We summarize the mechanisms by which HuR causes tumor resistance and the molecular targets affected.

The Prognostic Role of HuR Varies Between Different Subtypes of Breast Cancer Patients: Data Mining and Retrospective Analysis

Objective

Human-antigen R (HuR) is an RNA-binding protein, which regulates the expression of several oncogenes and tumor suppressor genes through post-transcriptional mechanisms. But the role of HuR in breast cancer remains controversial. The aim of this study was to verify the association between cytoplasmic HuR level and prognosis of breast cancer patients.

Methods

Data mining from the Human Protein Atlas (HPA) and Kaplan-Meier Plotter (KMP) databases was performed. Then, 394 patients with stage I-III primary breast cancer were enrolled between January 2005 and December 2016. We investigated the association between cytoplasmic HuR level and clinicopathological characteristics or survival of these patients. Immunohistochemical analysis was performed to determine HuR expression level. SPSS 21.0 statistical software was used for analysis.

Results

In the HPA and KMP datasets, HuR protein and mRNA expression level were not significantly associated with overall survival of all breast cancer patients enrolled. Results from our 394 patients indicated that higher expression level of cytoplasmic HuR was associated with larger tumor size, lymph node positive, ER negative and triple-negative subtype. For all patients enrolled, the results indicated that compared with HuR negative patients, the DFS (disease-free survival) of HuR 1+ was longer (60.5% vs 78.8, P=0.053, HR=0.616, 95% CI: 0.378-1.005), the P value was borderline. In the triple-negative breast cancer (TNBC) subgroup, HuR positive patients had significantly longer DFS than HuR negative patients (65.5% vs 30.8%, P=0.001, HR=0.345, 95% CI: 0.180-0.658). In the HR+HER2- subgroup, HuR low (0~1+) patients had significantly longer OS than HuR high (2+~3+) patients (97.0% vs 89.5%, P=0.033, HR=2.482, 95% CI: 1.074-5.736).

Conclusion

In conclusion, our results revealed that higher expression level of HuR was related to aggressive biological characteristics which supported the findings from previous researches. In the HR+HER2- subgroup, lower HuR expression level patients had better survival time, while in the TNBC subgroup we got the opposite results. Our work indicated that HuR might play different roles in different breast cancer subtypes.

Roles of RNA-binding proteins in immune diseases and cancer

Genetic information that is transcribed from DNA to mRNA, and then translated from mRNA to protein, is regulated by complex and sophisticated post-transcriptional mechanisms. Recently, it has become clear that mRNA degradation not only acts to remove unnecessary mRNA, but is also closely associated with the regulation of translation initiation, and is essential for maintaining cellular homeostasis. Various RNA-binding proteins (RBPs) have been reported to play central roles in the mechanisms of mRNA stability and translation initiation through various signal transduction pathways, and to modulate gene expression faster than the transcription process via post-transcriptional modifications in response to intracellular and extracellular stimuli, without de novo protein synthesis. On the other hand, inflammation is necessary for the elimination of pathogens associated with infection, and is tightly controlled to avoid the overexpression of inflammatory cytokines, such as interleukin 6 (IL-6) and tumor necrosis factor (TNF). It is increasingly becoming clear that RBPs play important roles in the post-transcriptional regulation of these immune responses. Furthermore, it has been shown that the aberrant regulation of RBPs leads to chronic inflammation and autoimmune diseases. Although it has been recognized since the time of Rudolf Virchow in the 19th century that cancer-associated inflammation contributes to tumor onset and progression, involvement of the disruption of the balance between anti-tumor immunity via the immune surveillance system and pro-tumor immunity by cancer-associated inflammation in the malignant transformation of cancer remains elusive. Recently, the dysregulated expression and activation of representative RBPs involved in regulation of the production of pro-inflammatory cytokines have been shown to be involved in tumor progression. In this review, we summarize the recent progress in our understanding of the functional roles of these RBPs in several types of immune responses, and the involvement of RBP dysregulation in the pathogenesis of immune diseases and cancer, and discuss possible therapeutic strategies against cancer by targeting RBPs, coupled with immunotherapy.

Nano-based targeted drug delivery for lung cancer: therapeutic avenues and challenges

Most anticancer drugs often fail in clinical trials due to poor solubility, poor bioavailability, lack of targeted delivery and several off-target effects. Polymeric nanoparticles such as poly(lactide), poly(lactic-co-glycolic acid), ALB-loading paclitaxel (Abraxane® ABI-007), lomustine-loaded chitosan, gelatin (decorated with EGF receptor-targeted biotinylated EGF) and so on offer controlled and sustained drug-release properties, biocompatibility and promising anticancer effects. EGF, folic acid, transferrin, sigma and urokinase plasminogen activator receptors-targeting nano preparations improve bioavailability and accumulate drugs on the lung tumor cell surface. However, route of administration, size, pharmacokinetic properties, immune clearance and so on hamper nanomedicines' clinical uses. This review focuses on the benefits, avenues and challenges of nanoparticle-based drug-delivery systems for lung cancer treatment.

The RNA-Binding Protein ELAVL1 Regulates Hepatitis B Virus Replication and Growth of Hepatocellular Carcinoma Cells

Previous RNA immunoprecipitation followed by proteomic approaches successfully demonstrated that Embryonic Lethal, Abnormal Vision, Drosophila-Like 1 (ELAVL1) interacts with hepatitis B virus (HBV)-derived RNAs. Although ELAVL family proteins stabilize AU-rich element (ARE)-containing mRNAs, their role in HBV transcription remains unclear. This study conducted loss-of-function assays of ELAVL1 for inducible HBV-replicating HepAD38 cells and HBx-overexpressed HepG2 cells. In addition, clinicopathological analyses in primary hepatocellular carcinoma (HCC) surgical samples were also conducted. Lentivirus-mediated short hairpin RNA knockdown of ELAVL1 resulted in a decrease in both viral RNA transcription and production of viral proteins, including HBs and HBx, probably due to RNA stabilization by ELAVL1. Cell growth of HepAD38 cells was more significantly impaired in ELAVL1-knockdown than those in the control group, with or without HBV replication, indicating that ELAVL1 is involved in proliferation by factors other than HBV-derived RNAs. Immunohistochemical analyses of 77 paired HCC surgical specimens demonstrated that diffuse ELAVL1 expression was detected more frequently in HCC tissues (61.0%) than in non-tumor tissues (27.3%). In addition, the abundant expression of ELAVL1 tended to affect postoperative recurrence in HBV-related HCC patients. In conclusion, ELAVL1 contributes not only to HBV replication but also to HCC cell growth. It may be a potent therapeutic target for HBV-related HCC treatment.

Long noncoding RNA TNFRSF10A-AS1 promotes colorectal cancer through upregulation of HuR

BACKGROUND

Recent studies have emphasized the emerging importance of long noncoding RNAs (lncRNAs) in colorectal cancer (CRC). However, the functions and regulatory mechanisms of numerous lncRNAs in CRC have not been fully elucidated.

AIM

To explore the functional role and underlying molecular mechanisms of lncRNA TNFRSF10A-AS1 in CRC.

METHODS

TNFRSF10A-AS1 expression was measured by quantitative real-time polymerase chain reaction in CRC, and the relationship between TNFRSF10A-AS1 levels and the clinicopathological features of CRC patients was analyzed. The effect of TNFRSF10A-AS1 expression on CRC proliferation and metastasis was examined in vitro and in vivo. Mechanistically, we investigated how TNFRSF10A-AS1 is involved in CRC as a competitive endogenous RNA.

RESULTS

TNFRSF10A-AS1 was expressed at a high level in CRC and the upregulation of TNFRSF10A-AS1 was associated with advanced T grade and tumor size in CRC patients. A functional investigation revealed that TNFRSF10A-AS1 enhanced the proliferation, migration ability and invasion ability of colon cancer cells in vitro and in vivo. A mechanistic analysis demonstrated that TNFRSF10A-AS1 acted as a miR-3121-3p molecular sponge to regulate HuR expression, ultimately promoting colorectal tumorigenesis and progression.

CONCLUSION

TNFRSF10A-AS1 exerts a tumor-promoting function through the miR-3121-3p/HuR axis in CRC, indicating that it may be a novel target for CRC therapy.

lncRNA LUCAT1/ELAVL1/LIN28B/SOX2 Positive Feedback Loop Promotes Cell Stemness in Triple-Negative Breast Cancer

Background. Triple-negative breast cancer (TNBC), as a subtype of breast cancer (BC), features an aggressive nature. Long noncoding RNAs (lncRNAs) are proved to get involved in the processes of cancers. lncRNA lung cancer associated transcript 1 (LUCAT1) has been reported in multiple cancers. The role of LUCAT1 in TNBC and its latent regulatory mechanism were investigated. Methods. RT-qPCR was performed to examine LUCAT1 expression. Functional experiments were implemented to disclose the role of LUCAT1 in TNBC. The underlying regulatory mechanism of LUCAT1 in TNBC was explored by chromatin immunoprecipitation (ChIP), RNA-binding protein immunoprecipitation (RIP), luciferase reporter, and RNA pull-down assays. Results. LUCAT1 is significantly overexpressed in TNBC cells. LUCAT1 interference impedes cell stemness in TNBC cells. SRY-box transcription factor 2 (SOX2) is an active transcription factor of LUCAT1. LUCAT1 recruits ELAV-like RNA binding protein 1 (ELAVL1) protein to stabilize lin-28 homolog B (LIN28B) mRNA, thereby further modulating SOX2 expression, which forms a positive feedback loop. Conclusion. The lncRNA LUCAT1/ELAVL1/LIN28B/SOX2 positive feedback loop promotes cell stemness in TNBC. The exploration of the mechanisms underlying TNBC stemness might be beneficial to TNBC treatment.

Molecular insights into binding dynamics of tandem RNA recognition motifs (tRRMs) of human antigen R (HuR) with mRNA and the effect of point mutations in impaired HuR-mRNA recognition

Human antigen R (HuR) is a key regulatory protein with prominent roles in RNA metabolism and post-transcriptional gene regulation. Many studies have shown the involvement of HuR in plethora of human diseases, which are often manifestations of impaired HuR-RNA interactions. However, the inherent complexities of highly flexible protein-RNA interactions have limited our understanding of the structural basis of HuR-RNA recognition. In this study, we dissect the underlying molecular mechanism of interaction between N-terminal tandem RNA-recognition motifs (tRRMs) of HuR and mRNA using molecular dynamics simulation. We have also explored the effect of point mutations (T90A, R97A and R136A) of three reported critical residues in HuR-mRNA binding specificity. Our findings show that N-terminal tRRMs exhibit conformational stability upon RNA binding. We further show that R136A and R97A mutants significantly lose their binding affinity owing to the loss of critical interactions with mRNA. This may be attributed to the larger domain rearrangements in the mutant complexes, especially the β2β3 loops in both the tRRMs, leading to unfavourable conformations and loss of binding affinity. We have identified critical binding residues in tRRMs of HuR, contributing favourable binding energy in mRNA recognition. This study contributes significantly to understand the molecular mechanism of RNA recognition by tandem RRMs and provides a platform to modulate binding affinities through mutations. This may further guide in future structure-based drug-therapies targeting impaired HuR-RNA interactions.Communicated by Ramaswamy H. Sarma.

The RNA binding protein human antigen R is a gatekeeper of liver homeostasis

BACKGROUND AND AIMS

NAFLD is initiated by steatosis and can progress through fibrosis and cirrhosis to HCC. The RNA binding protein human antigen R (HuR) controls RNAs at the posttranscriptional level; hepatocyte HuR has been implicated in the regulation of diet-induced hepatic steatosis. The present study aimed to understand the role of hepatocyte HuR in NAFLD development and progression to fibrosis and HCC.

APPROACH AND RESULTS

Hepatocyte-specific, HuR-deficient mice and control HuR-sufficient mice were fed either a normal diet or an NAFLD-inducing diet. Hepatic lipid accumulation, inflammation, fibrosis, and HCC development were studied by histology, flow cytometry, quantitative PCR, and RNA sequencing. The liver lipidome was characterized by lipidomics analysis, and the HuR-RNA interactions in the liver were mapped by RNA immunoprecipitation sequencing. Hepatocyte-specific, HuR-deficient mice displayed spontaneous hepatic steatosis and fibrosis predisposition compared to control HuR-sufficient mice. On an NAFLD-inducing diet, hepatocyte-specific HuR deficiency resulted in exacerbated inflammation, fibrosis, and HCC-like tumor development. A multi-omic approach, including lipidomics, transcriptomics, and RNA immunoprecipitation sequencing revealed that HuR orchestrates a protective network of hepatic-metabolic and lipid homeostasis-maintaining pathways. Consistently, HuR-deficient livers accumulated, already at steady state, a triglyceride signature resembling that of NAFLD livers. Moreover, up-regulation of secreted phosphoprotein 1 expression mediated, at least partially, fibrosis development in hepatocyte-specific HuR deficiency on an NAFLD-inducing diet, as shown by experiments using antibody blockade of osteopontin.

CONCLUSIONS

HuR is a gatekeeper of liver homeostasis, preventing NAFLD-related fibrosis and HCC, suggesting that the HuR-dependent network could be exploited therapeutically.

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.

Research progress on RNA‑binding proteins in breast cancer (Review)

Breast cancer is the most common malignancy among women, and the abnormal regulation of gene expression serves an important role in its occurrence and development. However, the molecular mechanisms underlying gene expression are highly complex and heterogeneous, and RNA-binding proteins (RBPs) are among the key regulatory factors. RBPs bind targets in an environment-dependent or environment-independent manner to influence mRNA stability and the translation of genes involved in the formation, progression, metastasis and treatment of breast cancer. Due to the growing interest in these regulators, the present review summarizes the most influential studies concerning RBPs associated with breast cancer to elucidate the role of RBPs in breast cancer and to assess how they interact with other key pathways to provide new molecular targets for the diagnosis and treatment of breast cancer.

Divulging the Critical Role of HuR in Pancreatic Cancer as a Therapeutic Target and a Means to Overcome Chemoresistance

Simple Summary

With pancreatic cancer incidence constantly rising, constituting one of the most lethal type of cancers worldwide, the need for discovering novel therapeutic targets and approaches becomes of the utmost importance. Meanwhile, modern eating habits, hyperadiposity, mutational burden affecting core signaling pathways and the unique tumor microenvironment of pancreatic cancer tissues intermingle and form a disease that is lethal and hard to treat. The importance of HuR in pancreatic cancer has repeatedly been observed and represents a key molecule in pancreatic carcinogenesis and chemoresistance. Therefore, creating and obtaining new therapeutic skills against HuR protein could prove to be the answer to pancreatic cancer therapy.

Abstract

Pancreatic cancer is set to become the most lethal and common type of cancer worldwide. This is partly attributed to the mutational burden that affects core signaling pathways and the crosstalk of tumor cells with their surrounding microenvironment, but it is also due to modern eating habits. Hyperadiposity along with the constant rise in metabolic syndrome’s incidence contribute to a state of metaflammation that impacts immune cells and causes them to shift towards an immunosuppressive phenotype that, ultimately, allows tumor cells to evade immune control. Unfortunately, among the conventional therapeutic modalities and the novel therapeutic agents introduced, pancreatic cancer still holds one of the lowest response rates to therapy. Human antigen R (HuR), an RNA binding protein (RBP), has been repeatedly found to be implicated in pancreatic carcinogenesis and chemotherapy resistance through the posttranscriptional binding and regulation of mRNA target genes. Additionally, its overexpression has been linked to adverse clinical outcomes, in terms of tumor grade, stage, lymph node status and metastasis. These properties suggest the prospective role that HuR’s therapeutic targeting can play in facilitating pancreatic neoplasia and could provide the means to overcome chemoresistance.

Quercetin Impairs HuR-Driven Progression and Migration of Triple Negative Breast Cancer (TNBC) Cells

In the present study, we have explored the prognostic value of HuR gene as well as protein in breast cancers. Furthermore, we have also investigated the HuR therapeutic relevance in TNBCs, which is an aggressive breast cancer subtype. Using an online meta-analysis tool, we found that HuR protein overexpression positively correlates with reduced overall survival of TNBC patients (p = 0.028). Furthermore, we demonstrated that the TNBC breast cancer cell lines i.e., MDA-MB-231 and MDA-MB-468 are good model systems to study HuR protein, as they both exhibit a significant amount of cytoplasmic HuR (active form). Quercetin treatment significantly inhibited the cytoplasmic HuR in both TNBC cell lines. By using specific HuR siRNA, we established that quercetin-mediated inhibition of adhesion and migration of TNBC cells is dependent on HuR. Upon analyzing adhesion proteins i.e., β-catenin and CD44, we found that quercetin mediated effect on TNBC adhesion and migration was through the HuR-β-catenin axis and CD44, independently. Overall, the present results demonstrate that elevated HuR levels are associated with TNBC progression and relapse, and the ability of quercetin to inhibit cytoplasmic HuR protein provides a rationale for using it as an anticancer agent for the treatment of aggressive TNBCs.Supplemental data for this article is available online at at 10.1080/01635581.2021.1952628.

HuR as Therapeutic Target in Cancer: What the Future Holds

ELAV-like protein 1, or HuR (human antigen R), is an RNA-binding protein encoded by the ELAVL1 gene in humans. One of its best functions is to stabilize mRNAs in order to regulate gene expression. HuR protein overexpression has undoubtedly been linked to an increased risk of tumor growth, progression, and metastasis, rendering it a potential therapeutic target candidate in cancer. Novel agents interfering with HuR expression have been tested, both in vitro and in vivo, with promising results. The aim of this paper is to review the existing literature regarding the potential agents that could actively act on and inhibit HuR expression. HuR molecule controls the expression of various proto-oncogenes, cytokines and growth factors, representing a major player in tumor progression, invasion, and metastasis and constituting an emerging target for cancer therapy. PubMed database was thoroughly searched, and all published articles providing scientific data on molecules that can exhibit antitumorigenic effects via HuR inhibition were included. According to these data, HuR inhibition should be a promising target in cancer therapeutics.

Human antigen R promotes lung fibroblast differentiation to myofibroblasts and increases extracellular matrix production

Idiopathic pulmonary fibrosis (IPF) is a disease of progressive scarring caused by excessive extracellular matrix (ECM) deposition and activation of α-SMA-expressing myofibroblasts. Human antigen R (HuR) is an RNA binding protein that promotes protein translation. Upon translocation from the nucleus to the cytoplasm, HuR functions to stabilize messenger RNA (mRNA) to increase protein levels. However, the role of HuR in promoting ECM production, myofibroblast differentiation, and lung fibrosis is unknown. Human lung fibroblasts (HLFs) treated with transforming growth factor β1 (TGF-β1) showed a significant increase in translocation of HuR from the nucleus to the cytoplasm. TGF-β-treated HLFs that were transfected with HuR small interfering RNA had a significant reduction in α-SMA protein as well as the ECM proteins COL1A1, COL3A, and FN1. HuR was also bound to mRNA for ACTA2, COL1A1, COL3A1, and FN. HuR knockdown affected the mRNA stability of ACTA2 but not that of the ECM genes COL1A1, COL3A1, or FN. In mouse models of pulmonary fibrosis, there was higher cytoplasmic HuR in lung structural cells compared to control mice. In human IPF lungs, there was also more cytoplasmic HuR. This study is the first to show that HuR in lung fibroblasts controls their differentiation to myofibroblasts and consequent ECM production. Further research on HuR could assist in establishing the basis for the development of new target therapy for fibrotic diseases, such as IPF.

Silencing of HuR Inhibits Osteosarcoma Cell Epithelial-Mesenchymal Transition via AGO2 in Association With Long Non-Coding RNA XIST

Background

Osteosarcoma (OS) is a highly malignant and aggressive bone tumor. This study was performed to explore the mechanisms of HuR (human antigen R) in the progression of OS.

Methods

HuR expression levels in OS tissues and cells were detected by immunohistochemistry and western blotting. HuR siRNA was transfected into SJSA-1 OS cells to downregulate HuR expression, and then cell proliferation, migration, and epithelial-mesenchymal transition (EMT) were evaluated. RNA immunoprecipitation was performed to determine the association of the long non-coding RNA (lncRNA) XIST and argonaute RISC catalytic component (AGO) 2 with HuR. Fluorescence in situ hybridization analysis was performed to detect the expression of lncRNA XIST. Western blotting and immunofluorescence assays were performed to observe AGO2 expression after HuR or/and lncRNA XIST knockdown.

Results

Knockdown of HuR repressed OS cell migration and EMT. AGO2 was identified as a target of HuR and silencing of HuR decreased AGO2 expression. The lncRNA XIST was associated with HuR-mediated AGO2 suppression. Moreover, knockdown of AGO2 significantly inhibited cell proliferation, migration, and EMT in OS.

Conclusion

Our findings indicate that HuR knockdown suppresses OS cell EMT by regulating lncRNA XIST/AGO2 signaling.

HuR Protein in Hepatocellular Carcinoma: Implications in Development, Prognosis and Treatment

Hu-antigen R (HuR) is a post-transcriptional regulator that belongs to the embryonic lethal abnormal vision Drosophila-like family (ELAV). HuR regulates the stability, translation, subcellular localization, and degradation of several target mRNAs, which are implicated in carcinogenesis and could affect therapeutic options. HuR protein is consistently highly expressed in hepatocellular carcinoma (HCC) compared to the adjacent normal liver tissue and is involved in the post-transcriptional regulation of various genes implicated in liver malignant transformation. Additionally, HuR protein seems to be a putative prognosticator in HCC, predicting worse survival. This review summarizes the recent evidence regarding the role of HuR in primary liver tumors, as presented in clinical studies, in vitro experiments and in vivo animal models. In conclusion, our review supports the consistent role of HuR protein in the development, prognosis, and treatment of HCC. Additional studies are expected to expand current information and exploit its putative employment as a future candidate for more personalized treatment in these tumors.

An RNA-Binding Protein, Hu-antigen R, in Pancreatic Cancer Epithelial to Mesenchymal Transition, Metastasis, and Cancer Stem Cells

Pancreatic cancer has poor prognosis and treatment outcomes due to its highly metastatic nature and resistance to current treatments. The RNA-binding protein (RBP) Hu-antigen R (HuR) is a central player in posttranscriptional regulation of cancer-related gene expression, and contributes to tumorigenesis, tumor growth, metastasis, and drug resistance. HuR has been suggested to regulate pancreatic cancer epithelial-to-mesenchymal transition (EMT), but the mechanism was not well understood. Here, we further elucidated the role HuR plays in pancreatic cancer cell EMT, and developed a novel inhibitor specifically interrupting HuR-RNA binding. The data showed that HuR binds to the 3'-UTR of the mRNA of the transcription factor Snail, resulting in stabilization of Snail mRNA and enhanced Snail protein expression, thus promoted EMT, metastasis, and formation of stem-like cancer cells (CSC) in pancreatic cancer cells. siRNA silencing or CRISPR/Cas9 gene deletion of HuR inhibited pancreatic cancer cell EMT, migration, invasion, and inhibited CSCs. HuR knockout cells had dampened tumorigenicity in immunocompromised mice. A novel compound KH-3 interrupted HuR-RNA binding, and KH-3 inhibited pancreatic cancer cell viability, EMT, migration/invasion in vitro KH-3 showed HuR-dependent activity and inhibited HuR-positive tumor growth and metastasis in vivo.

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.

Discovery of the anti-angiogenesis effect of eltrombopag in breast cancer through targeting of HuR protein

HuR (human antigen R), an mRNA-binding protein responsible for poor prognosis in nearly all kinds of malignancies, is a potential anti-tumor target for drug development. While screening HuR inhibitors with a fluorescence polarization (FP) based high-throughput screening (HTS) system, the clinically used drug eltrombopag was identified. Activity of eltrombopag on molecular level was verified with FP, electrophoretic mobility shift assay (EMSA), simulation docking and surface plasmon resonance (SPR). Further, we showed that eltrombopag inhibited in vitro cell proliferation of multiple cancer cell lines and macrophages, and the in vivo anti-tumor activity was also demonstrated in a 4T1 tumor-bearing mouse model. The in vivo data showed that eltrombopag was efficient in reducing microvessels in tumor tissues. We then confirmed the HuR-dependent anti-angiogenesis effect of eltrombopag in 4T1 cells and RAW264.7 macrophages with qRT-PCR, HuR-overexpression and HuR-silencing assays, RNA stability assays, RNA immunoprecipitation and luciferase assays. Finally, we analyzed the in vitro anti-angiogenesis effect of eltrombopag on human umbilical vein endothelial cells (HUVECs) mediated by macrophages with cell scratch assay and in vitro Matrigel angiogenesis assay. With these data, we revealed the HuR-dependent anti-angiogenesis effect of eltrombopag in breast tumor, suggesting that the existing drug eltrombopag may be used as an anti-cancer drug.

The RNA-Binding Protein HuR in Digestive System Tumors

Human antigen R (HuR) is a member of the Hu family of RNA-binding proteins. This molecule, which was first described in tumors nearly two decades ago, has recently received much attention in tumor-related research because it regulates the expression of many tumor-associated molecules through posttranscriptional regulatory mechanisms, thereby affecting biological characteristics. It is suggested that HuR might be a novel therapeutic target and a marker for therapeutic response and prognostic assessment. Increasing evidence supports that HuR also plays critical roles in the development, therapy, and prognosis of digestive system tumors. Herein, we review the relationships between HuR and digestive system tumors, demonstrating the importance of HuR in digestive system tumor diagnosis.

Auto-regulatory feedback by RNA-binding proteins

RNA-binding proteins (RBPs) are key regulators in post-transcriptional control of gene expression. Mutations that alter their activity or abundance have been implicated in numerous diseases such as neurodegenerative disorders and various types of cancer. This highlights the importance of RBP proteostasis and the necessity to tightly control the expression levels and activities of RBPs. In many cases, RBPs engage in an auto-regulatory feedback by directly binding to and influencing the fate of their own mRNAs, exerting control over their own expression. For this feedback control, RBPs employ a variety of mechanisms operating at all levels of post-transcriptional regulation of gene expression. Here we review RBP-mediated autogenous feedback regulation that either serves to maintain protein abundance within a physiological range (by negative feedback) or generates binary, genetic on/off switches important for e.g. cell fate decisions (by positive feedback).

ECRG2, a novel transcriptional target of p53, modulates cancer cell sensitivity to DNA damage

Esophageal Cancer-Related Gene 2 (ECRG2) is a recently identified tumor suppressor, its regulation and involvement in DNA damage response are unknown. Here, we show that DNA damage-induced ECRG2 upregulation coincided with p53 activation and occurred in a p53-dependent manner. We identified two p53-binding sites within ECRG2 promoter and found the promoter activity, mRNA, and protein expression to be regulated by p53. We show that DNA damage significantly enhanced p53 binding to ECRG2 promoter at the anticipated p53-binding sites. We identified a novel natural ECRG2 promoter variant harboring a small deletion that exists in the genomes of ~38.5% of world population and showed this variant to be defective in responding to p53 and DNA-damage. ECRG2 overexpression induced cancer cell death; ECRG2 gene disruption enhanced cell survival following anticancer drug treatments even when p53 was induced. We showed that lower expression of ECRG2 in multiple human malignancies correlated with reduced disease-free survival in patients. Collectively, our novel findings indicate that ECRG2 is an important target of p53 during DNA damage-induced response and plays a critical role in influencing cancer cell sensitivity to DNA damage-inducing cancer therapeutics.

Hepatic HuR modulates lipid homeostasis in response to high-fat diet

Lipid transport and ATP synthesis are critical for the progression of non-alcoholic fatty liver disease (NAFLD), but the underlying mechanisms are largely unknown. Here, we report that the RNA-binding protein HuR (ELAVL1) forms complexes with NAFLD-relevant transcripts. It associates with intron 24 of Apob pre-mRNA, with the 3′UTR of Uqcrb, and with the 5′UTR of Ndufb6 mRNA, thereby regulating the splicing of Apob mRNA and the translation of UQCRB and NDUFB6. Hepatocyte-specific HuR knockout reduces the expression of APOB, UQCRB, and NDUFB6 in mice, reducing liver lipid transport and ATP synthesis, and aggravating high-fat diet (HFD)-induced NAFLD. Adenovirus-mediated re-expression of HuR in hepatocytes rescues the effect of HuR knockout in HFD-induced NAFLD. Our findings highlight a critical role of HuR in regulating lipid transport and ATP synthesis.

Human antigen R (HuR) is a RNA binding protein involved in the regulation of many cellular functions. Here the authors show that, hepatocyte specific deletion of HuR exacerbates high-fat diet-induced NAFLD in mice by regulating transcripts involved in lipid transport and ATP synthesis.

BOPC1 Enantiomers Preparation and HuR Interaction Study. From Molecular Modeling to a Curious DEEP-STD NMR Application

The Hu family of RNA-binding proteins plays a crucial role in post-transcriptional processes; indeed, Hu-RNA complexes are involved in various dysfunctions (i.e., inflammation, neurodegeneration, and cancer) and have been recently proposed as promising therapeutic targets. Intrigued by this concept, our research efforts aim at identifying small molecules able to modulate HuR-RNA interactions, with a focus on subtype HuR, upregulated and dysregulated in several cancers. By applying structure-based design, we had already identified racemic trans-BOPC1 as promising HuR binder. In this Letter, we accomplished the enantio-resolution, the assignment of the absolute configuration, and the recognition study with HuR of enantiomerically pure trans-BOPC1. For the first time, we apply DEEP (differential epitope mapping)-STD NMR to study the interaction of BOPC1 with HuR and compare its enantiomers, gaining information on ligand orientation and amino acids involved in the interaction, and thus increasing focus on the in silico binding site model.

The zinc finger domains in U2AF26 and U2AF35 have diverse functionalities including a role in controlling translation

Recent work has associated point mutations in both zinc fingers (ZnF) of the spliceosome component U2AF35 with malignant transformation. However, surprisingly little is known about the functionality of the U2AF35 ZnF domains in general. Here we have analysed key functionalities of the ZnF domains of mammalian U2AF35 and its paralog U2AF26. Both ZnFs are required for splicing regulation, whereas only ZnF2 controls protein stability and contributes to the interaction with U2AF65. These features are confirmed in a naturally occurring splice variant of U2AF26 lacking ZnF2, that is strongly induced upon activation of primary mouse T cells and localized in the cytoplasm. Using Ribo-Seq in a model T cell line we provide evidence for a role of U2AF26 in activating cytoplasmic steps in gene expression, notably translation. Consistently, an MS2 tethering assay shows that cytoplasmic U2AF26/35 increase translation when localized to the 5'UTR of a model mRNA. This regulation is partially dependent on ZnF1 thus providing a connection between a core splicing factor, the ZnF domains and the regulation of translation. Altogether, our work reveals unexpected functions of U2AF26/35 and their ZnF domains, thereby contributing to a better understanding of their role and regulation in mammalian cells.

Circ-HuR suppresses HuR expression and gastric cancer progression by inhibiting CNBP transactivation

BACKGROUND

Circular RNAs (circRNAs), a subclass of non-coding RNAs, play essential roles in tumorigenesis and aggressiveness. Our previous study has identified that circAGO2 drives gastric cancer progression through activating human antigen R (HuR), a protein stabilizing AU-rich element-containing mRNAs. However, the functions and underlying mechanisms of circRNAs derived from HuR in gastric cancer progression remain elusive.

METHODS

CircRNAs derived from HuR were detected by real-time quantitative RT-PCR and validated by Sanger sequencing. Biotin-labeled RNA pull-down, mass spectrometry, RNA immunoprecipitation, RNA electrophoretic mobility shift, and in vitro binding assays were applied to identify proteins interacting with circRNA. Gene expression regulation was observed by chromatin immunoprecipitation, dual-luciferase assay, real-time quantitative RT-PCR, and western blot assays. Gain- and loss-of-function studies were performed to observe the impacts of circRNA and its protein partner on the growth, invasion, and metastasis of gastric cancer cells in vitro and in vivo.

RESULTS

Circ-HuR (hsa_circ_0049027) was predominantly detected in the nucleus, and was down-regulated in gastric cancer tissues and cell lines. Ectopic expression of circ-HuR suppressed the growth, invasion, and metastasis of gastric cancer cells in vitro and in vivo. Mechanistically, circ-HuR interacted with CCHC-type zinc finger nucleic acid binding protein (CNBP), and subsequently restrained its binding to HuR promoter, resulting in down-regulation of HuR and repression of tumor progression.

CONCLUSIONS

Circ-HuR serves as a tumor suppressor to inhibit CNBP-facilitated HuR expression and gastric cancer progression, indicating a potential therapeutic target for gastric cancer.

Lessons from studying the AU-rich elements in chronic inflammation and autoimmunity

AU-rich elements (AREs) comprise one of the most widely studied families of regulatory RNA structures met in RNAs engaged in complex immunological reactions. A multitude of genetic, molecular, holistic and functional studies have been utilized for the analyses of the AREs and their interactions to proteins that bind to them. Data stemming from these studies brought forth a world of RNA-related check-points against infection, chronic inflammation, tumor associated immunity, and autoimmunity; and the interest to capitalize the interactions of AREs for clinical management and therapy. They also provided lessons on the cellular capabilities of post-transcriptional control. Originally thought as transcript-restricted regulators of turnover and translation, ARE-binding proteins do in fact harbor great versatility and interactivity across nuclear and cytoplasmic compartments; and act as functional coordinators of immune-cellular programs. Harnessing these deterministic functions requires extensive knowledge of their synergies or antagonisms at a cell-specific level; but holds great promise since it can provide the efficacy of combinatorial therapies with single agents.

The neural ELAVL protein HuB enhances endogenous proto-oncogene activation

The cytoplasmic distribution of the HuR/ELAVL1 (embryonic lethal abnormal vision 1) protein is recognized as an important prognostic factor of malignant tumors. However, the previous study suggests that exogenous over-expression of HuR is not sufficient for nuclear export. Conversely, the predominantly cytosolic distribution of neuron-specific human ELAV members, including HuB/ELAVL2, HuC/ELAVL3, and HuD/ELAVL4, has been reported. In the present study, we demonstrated the expression of HuB in several types of cancer cells, but expression of HuC and HuD was not observed. In addition, our results indicated that HuR and HuB formed a complex in the cytosolic fraction of cancer cells via the RRM3 region. Ectopic expression of HuB was capable of initiating the cytosolic translocation of HuR from the nucleus to the cytosol. Furthermore, HuB-transduced cancer cells displayed significant nuclear export of HuR, with quantitative PCR experiments revealing the simultaneous upregulation of HIF-1α, c-Fos, c-MYC, and Ets2 basal mRNA expression. Phorbol 12-myristate 13-acetate (PMA)-stimulated HuB-transduced cells demonstrated significantly enhanced activation of endogenous c-Fos and CREB dependent cascades. Finally, co-transfection of HuB with the E1 region of type 5 human adenovirus significantly enhanced E1 transformation activities but that of HuR with the E1 region did not. Collectively, our findings suggest that the neural Hu family protein HuB plays a major role in the activation of memory-related proto-oncogenes.

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.

Screening Approaches for Targeting Ribonucleoprotein Complexes: A New Dimension for Drug Discovery

RNA-binding proteins (RBPs) are pleiotropic factors that control the processing and functional compartmentalization of transcripts by binding primarily to mRNA untranslated regions (UTRs). The competitive and/or cooperative interplay between RBPs and an array of coding and noncoding RNAs (ncRNAs) determines the posttranscriptional control of gene expression, influencing protein production. Recently, a variety of well-recognized and noncanonical RBP domains have been revealed by modern system-wide analyses, underlying an evolving classification of ribonucleoproteins (RNPs) and their importance in governing physiological RNA metabolism. The possibility of targeting selected RNA-protein interactions with small molecules is now expanding the concept of protein "druggability," with new implications for medicinal chemistry and for a deeper characterization of the mechanism of action of bioactive compounds. Here, taking SF3B1, HuR, LIN28, and Musashi proteins as paradigmatic case studies, we review the strategies applied for targeting RBPs, with emphasis on the technological advancements to study protein-RNA interactions and on the requirements of appropriate validation strategies to parallel high-throughput screening (HTS) efforts.

Divergent Innate and Epithelial Functions of the RNA-Binding Protein HuR in Intestinal Inflammation

HuR is an abundant RNA-binding protein acting as a post-transcriptional regulator of many RNAs including mRNAs encoding inflammatory mediators, cytokines, death signalers and cell cycle regulators. In the context of intestinal pathologies, elevated HuR is considered to enhance the stability and the translation of pro-tumorigenic mRNAs providing the rationale for its pharmacological targeting. However, HuR also possesses specific regulatory functions for innate immunity and cytokine mRNA control which can oppose intestinal inflammation and tumor promotion. Here, we aim to identify contexts of intestinal inflammation where the innate immune and the epithelial functions of HuR converge or diverge. To address this, we use a disease-oriented phenotypic approach using mice lacking HuR either in intestinal epithelia or myeloid-derived immune compartments. These mice were compared for their responses to (a) Chemically induced Colitis; (b) Colitis- associated Cancer (CAC); (c) T-cell mediated enterotoxicity; (d) Citrobacter rodentium-induced colitis; and (e) TNF-driven inflammatory bowel disease. Convergent functions of epithelial and myeloid HuR included their requirement for suppressing inflammation in chemically induced colitis and their redundancies in chronic TNF-driven IBD and microbiota control. In the other contexts however, their functions diversified. Epithelial HuR was required to protect the epithelial barrier from acute inflammatory or infectious degeneration but also to promote tumor growth. In contrast, myeloid HuR was required to suppress the beneficial inflammation for pathogen clearance and tumor suppression. This cellular dichotomy in HuR's functions was validated further in mice engineered to express ubiquitously higher levels of HuR which displayed diminished pathologic and beneficial inflammatory responses, resistance to epithelial damage yet a heightened susceptibility to CAC. Our study demonstrates that epithelial and myeloid HuR affect different cellular dynamics in the intestine that need to be carefully considered for its pharmacological exploitation and points toward potential windows for harnessing HuR functions in intestinal inflammation.

Circular RNA circAGO2 drives cancer progression through facilitating HuR-repressed functions of AGO2-miRNA complexes

Argonaute 2 (AGO2), the core component of microRNA (miRNA)-induced silencing complex, plays a compelling role in tumorigenesis and aggressiveness. However, the mechanisms regulating the functions of AGO2 in cancer still remain elusive. Herein, we indentify one intronic circular RNA (circRNA) generated from AGO2 gene (circAGO2) as a novel regulator of AGO2-miRNA complexes and cancer progression. CircAGO2 is up-regulated in gastric cancer, colon cancer, prostate cancer, and neuroblastoma, and is associated with poor prognosis of patients. CircAGO2 promotes the growth, invasion, and metastasis of cancer cells in vitro and in vivo. Mechanistic studies reveal that circAGO2 physically interacts with human antigen R (HuR) protein to facilitate its activation and enrichment on the 3'-untranslated region of target genes, resulting in reduction of AGO2 binding and repression of AGO2/miRNA-mediated gene silencing associated with cancer progression. Pre-clinically, administration of lentivirus-mediated short hairpin RNA targeting circAGO2 inhibits the expression of downstream target genes, and suppresses the tumorigenesis and aggressiveness of xenografts in nude mice. In addition, blocking the interaction between circAGO2 and HuR by cell-penetrating inhibitory peptide represses the tumorigenesis and aggressiveness of cancer cells. Taken together, these results indicate that oncogenic circAGO2 drives cancer progression through facilitating HuR-repressed functions of AGO2-miRNA complexes.

Identification and Functional Characterization of Long Non-coding RNA MIR22HG as a Tumor Suppressor for Hepatocellular Carcinoma

Long non-coding RNAs (lncRNAs) have recently been identified as critical regulators in tumor initiation and development. However, the function of lncRNAs in human hepatocellular carcinoma (HCC) remains largely unknown. Our study was designed to explore the biological function and clinical implication of lncRNA MIR22HG in HCC. Methods: We evaluated MIR22HG expression in 52-patient, 145-patient, TCGA, and GSE14520 HCC cohorts. The effects of MIR22HG on HCC were analyzed in terms of proliferation, invasion, and metastasis, both in vitro and in vivo. The mechanism of MIR22HG action was explored through bioinformatics, luciferase reporter, and RNA immunoprecipitation analyses. Results:MIR22HG expression was significantly down-regulated in 4 independent HCC cohorts compared to that in controls. Its low expression was associated with tumor progression and poor prognosis of patients with HCC. Forced expression of MIR22HG in HCC cells significantly suppressed proliferation, invasion, and metastasis in vitro and in vivo. Mechanistically, MIR22HG derived miR-22-3p to target high mobility group box 1 (HMGB1), thereby inactivating HMGB1 downstream pathways. Additionally, MIR22HG directly interacted with HuR and regulated its subcellular localization. MIR22HG competitively bound to human antigen R (HuR), resulting in weakened expression of HuR-stabilized oncogenes, such as β-catenin. Furthermore, miR-22-3p suppression, HuR or HMGB1 overexpression rescued the inhibitory effects caused by MIR22HG overexpression. Conclusion: Our findings revealed that MIR22HG plays a key role in tumor progression by suppressing the proliferation, invasion, and metastasis of tumor cells, suggesting its potential role as a tumor suppressor and prognostic biomarker in HCC.

Studying the mechanism of PLAGL2 overexpression and its carcinogenic characteristics based on 3'-untranslated region in colorectal cancer

Pleomorphic adenoma gene like-2 (PLAGL2) is a zinc finger protein transcription factor, which is upregulated and serves an oncogenic function in multiple human malignancies, including colorectal cancer (CRC). First, reverse transcription-quantitative polymerase chain reaction (RT-qPCR) was used to detect the expression levels of PLAGL2 in CRC tissues and normal tissues. Then, bioinformatics analysis, RT-qPCR, western blotting, luciferase reporter assays and RNA-binding protein immunoprecipitation assays were performed to explore whether the underlying mechanisms, including copy number variation (CNV), microRNAs (miRNAs/miRs) and RNA-binding proteins (RBPs) led to the abnormal expression of PLAGL2. Finally, cell counting kit-8 assays, Transwell assays and xenograft models were used to detect carcinogenesis-associated characteristics based on the 3′-untranslated region (3′-UTR) of PLAGL2. In the present study, PLAGL2 was revealed to be upregulated in CRC tissues compared with normal CRC tissues. CNV was one of the causes leading to the upregulation of PLAGL2. miRNA, including downregulated miR-486-5p, and RBPs, including upregulated human antigen R (HuR), were other key underlying causes. In addition, PLAGL2 3′-UTR was revealed to promote the progression of CRC in vitro and in vivo, and to regulate the expression of C-MYC and CD44. To conclude, these results suggested that high expression of PLAGL2 in CRC was associated with CNV, miR-486-5p and HuR expression, whose 3′-UTR may promote colon carcinogenesis and serve as a novel potential biomarker for CRC therapies.

Cytoplasmic Hu-Antigen R (HuR) Expression is Associated with Poor Survival in Patients with Surgically Resected Cholangiocarcinoma Treated with Adjuvant Gemcitabine-Based Chemotherapy

BACKGROUND

Hu-antigen R (HuR) is an RNA-binding protein that regulates the stability, translation, and nucleus-to-cytoplasm translocation of messenger RNAs (mRNAs).

OBJECTIVE

The aim of this study was to investigate the prognostic significance of HuR in cholangiocarcinoma patients who received adjuvant gemcitabine-based chemotherapy (AGC) after surgical resection.

METHODS

Nuclear and cytoplasmic HuR expression was investigated immunohistochemically in 131 patients with resected cholangiocarcinoma, including 91 patients administered AGC and 40 patients who did not receive adjuvant chemotherapy. The correlation between HuR expression and survival was evaluated by statistical analysis.

RESULTS

High nuclear and cytoplasmic HuR expression was observed in 67 (51%) and 45 (34%) patients, respectively. Cytoplasmic HuR expression was significantly associated with lymph node metastasis (p < 0.01), while high cytoplasmic HuR expression was significantly associated with poor disease-free survival [DFS] (p = 0.03) and overall survival [OS] (p = 0.001) in the 91 patients who received AGC, but not in the 40 patients who did not receive AGC (DFS p = 0.17; OS p = 0.07). In the multivariate analysis of patients who received AGC, high cytoplasmic HuR expression was an independent predictor of poor DFS (hazard ratio [HR] 1.77; p = 0.04) and OS (HR 2.09; p = 0.02). Nuclear HuR expression did not affect the survival of enrolled patients.

CONCLUSIONS

High cytoplasmic HuR expression was closely associated with the efficacy of AGC in patients with cholangiocarcinoma. The current findings warrant further investigations to optimize adjuvant chemotherapy regimens for resectable cholangiocarcinoma.

Complex HuR function in pancreatic cancer cells

Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal cancers with dismal patient outcomes. The underlying core genetic drivers of disease have been identified in human tumor specimens and described in genetically engineered mouse models. These genetic drivers of PDAC include KRAS signaling, TP53 mutations, and genetic loss of the SMAD4 tumor suppressor protein. Beyond the known mutational landscape of PDAC genomes, alternative disrupted targets that extend beyond conventional genetic mutations have been elusive and understudied in the context of PDAC cell therapeutic resistance and survival. This last point is important because PDAC tumors have a unique and complex tumor microenvironment that includes hypoxic and nutrient-deprived niches that could select for cell populations that garner therapeutic resistance, explaining tumor heterogeneity in regards to response to different therapies. We and others have embarked in a line of investigation focused on the key molecular mechanism of posttranscriptional gene regulation that is altered in PDAC cells and supports this pro-survival phenotype intrinsic to PDAC cells. Specifically, the key regulator of this mechanism is a RNA-binding protein, HuR (ELAVL1), first described in cancer nearly two decades ago. Herein, we will provide a brief overview of the work demonstrating the importance of this RNA-binding protein in PDAC biology and then provide insight into ongoing work developing therapeutic strategies aimed at targeting this molecule in PDAC cells. This article is categorized under: RNA in Disease and Development > RNA in Disease.

Clinical Significance and Biological Role of HuR in Head and Neck Carcinomas

Background

Hu-antigen R (HuR) is a posttranscriptional regulator of several target mRNAs, implicated in carcinogenesis. This review aims to present the current evidence regarding the biological role and potential clinical significance of HuR in head and neck carcinomas.

Methods

The existing literature concerning HuR expression and function in head and neck carcinomas is critically presented and summarised.

Results

HuR is expressed in the majority of the examined samples, showing higher cytoplasmic levels in malignant or premalignant cases. Moreover, HuR modulates several genes implicated in biological processes important for malignant transformation, growth, and invasiveness. HuR seems to be an adverse prognosticator in patients with OSCCs, whereas a correlation with a more aggressive phenotype is reported in several types of carcinomas.

Conclusions

A consistent role of HuR in the carcinogenesis and progression of head and neck carcinomas is suggested; nevertheless, further studies are warranted to expand the present information.

The RNA-binding protein HuR inhibits expression of CCL5 and limits recruitment of macrophages into tumors

The RNA-binding protein HuR promotes tumor growth by affecting proliferation, metastasis, apoptosis, and angiogenesis. Although immune cells, especially tumor-associated macrophages, are critical components of the tumor stroma, the influence of HuR in tumors on the recruitment of immune cells remains poorly understood. In the present study, we, therefore, aimed to elucidate the impact of tumor cell HuR on the interaction between tumor cells and macrophages. To this end, we stably depleted HuR in human MCF-7 breast cancer cells. We found that HuR-deficient cells not only showed reduced proliferation, they further expressed elevated levels of the chemokine CCL5. HuR-dependent repression of CCL5 was neither caused by altered CCL5 mRNA stability, nor by changes in CCL5 translation. Instead, loss of HuR augmented transcription of CCL5, which was mediated via an interferon-stimulated response element in the CCL5 promoter. Furthermore, HuR depletion enhanced macrophage recruitment into MCF-7 tumor spheroids, an effect which was completely lost upon neutralization of CCL5. HuR expression further negatively correlated with CCL5 expression and macrophage appearance in a cohort of breast tumors. Thus, while HuR is well-characterized to support various pro-tumorigenic features in tumor cells, we provide evidence that it limits the recruitment of macrophages into tumors by repressing CCL5. As macrophage infiltration is associated with poor prognosis, our findings underline the highly cell-type and context specific role of HuR in tumorigenesis.

Epigenetic modulators of thyroid cancer

There are some well known factors involved in the etiology of thyroid cancer, including iodine deficiency, radiation exposure at early ages, or some genetic changes. However, epigenetic modulators that may contribute to development of these tumors and be helpful to for both their diagnosis and treatment have recently been discovered. The currently known changes in DNA methylation, histone modifications, and non-coding RNAs in each type of thyroid carcinoma are reviewed here.