Transcriptional control of human antigen R by bone morphogenetic protein

Lipolysis-Stimulated Lipoprotein Receptor in Proximal Tubule, BMP-SMAD Signaling, and Kidney Disease

Key Points

Background

Lipolysis-stimulated lipoprotein receptor (LSR) is a single-pass membrane protein that plays essential roles in tricellular tight junction organization in the epithelium and endothelium, but its function in kidney physiology and disease development remains unknown.

Methods

Conditional Lsr deletion mice were generated and analyzed to investigate the function of LSR in proximal tubule. Unilateral ischemia-reperfusion was used as an injury model to investigate the role of LSR in AKI and the progression to CKD. Detailed mechanistic analyses were conducted using whole-transcriptome RNA sequencing, immunofluorescence, dual-luciferase reporter gene assay, coimmunoprecipitation, RNA immunoprecipitation, and adeno-associated virus-mediated gene overexpression and knockdown.

Results

The nuclear localization of LSR was found in the kidney. Proximal tubule–specific Lsr knockout mice exhibited alleviated kidney damage and fibrosis compared with those in wild-type mice in response unilateral ischemia-reperfusion injury. Loss of LSR resulted in downregulation of Chrdl1 and activation of bone morphogenetic protein (BMP)-mothers against decapentaplegic homolog (SMAD) signaling in proximal tubules. Treatment with CHRDL1 counteracted the protective effect of LSR deletion in the unilaterally ischemic injured kidney. In addition, the systemic delivery of Chrdl1 short hairpin RNA attenuated injury-induced kidney fibrosis. LSR formed a complex with 14-3-3θ in the nucleus of proximal tubular cells, thereby reducing the interaction between human antigen R and 14-3-3θ, consequently leading to the translocation of unbound human antigen R to the cytoplasm. The absence of LSR promoted the association of 14-3-3θ with human antigen R, potentially resulting in decreased human antigen R levels in the cytoplasm. Reduced human antigen R levels impaired Chrdl1 mRNA stability, subsequently leading to the activation of BMP-SMAD signaling.

Conclusions

Deletion of LSR in proximal tubule deregulated Chrdl1 to activate BMP-SMAD signaling and ameliorated kidney disease.

Fates and functions of RNA-binding proteins under stress

Exposure to stress activates a well-orchestrated set of changes in gene expression programs that allow the cell to cope with and adapt to the stress, or undergo programmed cell death. RNA-protein interactions, mediating all aspects of post-transcriptional regulation of gene expression, play crucial roles in cellular stress responses. RNA-binding proteins (RBPs), which interact with sequence/structural elements in RNAs to control the steps of RNA metabolism, have therefore emerged as central regulators of post-transcriptional responses to stress. Following exposure to a variety of stresses, the dynamic alterations in the RNA-protein interactome enable cells to respond to intracellular or extracellular perturbations by causing changes in mRNA splicing, polyadenylation, stability, translation, and localization. As RBPs play a central role in determining the cellular proteome both qualitatively and quantitatively, it has become increasingly evident that their abundance, availability, and functions are also highly regulated in response to stress. Exposure to stress initiates a series of signaling cascades that converge on post-translational modifications (PTMs) of RBPs, resulting in changes in their subcellular localization, association with stress granules, extracellular export, proteasomal degradation, and RNA-binding activities. These alterations in the fate and function of RBPs directly impact their post-transcriptional regulatory roles in cells under stress. Adopting the ubiquitous RBP HuR as a prototype, three scenarios illustrating the changes in nuclear-cytoplasmic localization, RNA-binding activity, export and degradation of HuR in response to inflammation, genotoxic stress, and heat shock depict the complex and interlinked regulatory mechanisms that control the fate and functions of RBPs under stress. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Hu Antigen R (HuR) Protein Structure, Function and Regulation in Hepatobiliary Tumors

Simple Summary

Hepatobiliary tumors are a group of primary malignancies encompassing the liver, the intra- and extra-hepatic biliary tracts, and the gall bladder. Within the liver, hepatocellular carcinoma (HCC) is the most common type of primary cancer, which is, also, representing the third-most recurrent cause of cancer-associated death and the sixth-most prevalent type of tumor worldwide, nowadays. Although less frequent, cholangiocarcinoma (CCA) is, currently, a fatal cancer with limited therapeutic options. Here, we review the regulatory role of Hu antigen R (HuR), a ubiquitous member of the ELAV/Hu family of RNA-binding proteins (RBPs), in the pathogenesis, progression, and treatment of HCC and CCA. Overall, HuR is proposed as a valuable diagnostic and prognostic marker, as well as a therapeutic target in hepatobiliary cancers. Therefore, novel therapeutic approaches that can selectively modulate HuR function appear to be highly attractive for the clinical management of these types of tumors.

Abstract

Hu antigen R (HuR) is a 36-kDa ubiquitous member of the ELAV/Hu family of RNA-binding proteins (RBPs), which plays an important role as a post-transcriptional regulator of specific RNAs under physiological and pathological conditions, including cancer. Herein, we review HuR protein structure, function, and its regulation, as well as its implications in the pathogenesis, progression, and treatment of hepatobiliary cancers. In particular, we focus on hepatocellular carcinoma (HCC) and cholangiocarcinoma (CCA), tumors where the increased cytoplasmic localization of HuR and activity are proposed, as valuable diagnostic and prognostic markers. An overview of the main regulatory axes involving HuR, which are associated with cell proliferation, invasion, metastasis, apoptosis, and autophagy in HCC, is provided. These include the transcriptional, post-transcriptional, and post-translational modulators of HuR function, in addition to HuR target transcripts. Finally, whereas studies addressing the relevance of targeting HuR in CCA are limited, in the past few years, HuR has emerged as a potential therapeutic target in HCC. In fact, the therapeutic efficacy of some pharmacological inhibitors of HuR has been evaluated, in early experimental models of HCC. We, further, discuss the major findings and future perspectives of therapeutic approaches that specifically block HuR interactions, either with post-translational modifiers or cognate transcripts in hepatobiliary cancers.

The RNA-binding protein HuR in human cancer: A friend or foe?

The RNA-binding proteins (RBPs) are critical trans factors that associate with specific cis elements present in mRNAs whose stability and translation are subject to regulation. The RBP Hu antigen R (HuR) is overexpressed in a wide variety of human cancers and serves as a prognostic factor of poor clinical outcome. HuR promotes tumorigenesis by interacting with a subset of oncogenic mRNAs implicated in different cancer hallmarks, and resistance to therapy. Reduction of HuR levels in cancer cells leads to tumor regression in mouse xenograft models. These findings prompt a working model whereby cancer cells use HuR, a master switch of multiple oncogenic mRNAs, to drive drug resistance and promote cell survival and metastasis, thus rendering the tumor cells with high cytoplasmic HuR more progressive and resistant to therapy. This review summarizes the roles of HuR in cancer and other diseases, therapeutic potential of HuR inhibition, and the current status of drug discovery on HuR.

RNA-binding proteins and their role in kidney disease

RNA-binding proteins (RBPs) are of fundamental importance for post-transcriptional gene regulation and protein synthesis. They are required for pre-mRNA processing and for RNA transport, degradation and translation into protein, and can regulate every step in the life cycle of their RNA targets. In addition, RBP function can be modulated by RNA binding. RBPs also participate in the formation of ribonucleoprotein complexes that build up macromolecular machineries such as the ribosome and spliceosome. Although most research has focused on mRNA-binding proteins, non-coding RNAs are also regulated and sequestered by RBPs. Functional defects and changes in the expression levels of RBPs have been implicated in numerous diseases, including neurological disorders, muscular atrophy and cancers. RBPs also contribute to a wide spectrum of kidney disorders. For example, human antigen R has been reported to have a renoprotective function in acute kidney injury (AKI) but might also contribute to the development of glomerulosclerosis, tubulointerstitial fibrosis and diabetic kidney disease (DKD), loss of bicaudal C is associated with cystic kidney diseases and Y-box binding protein 1 has been implicated in the pathogenesis of AKI, DKD and glomerular disorders. Increasing data suggest that the modulation of RBPs and their interactions with RNA targets could be promising therapeutic strategies for kidney diseases.

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).

AChR β-Subunit mRNAs Are Stabilized by HuR in a Mouse Model of Congenital Myasthenic Syndrome With Acetylcholinesterase Deficiency

Collagen Q (COLQ) is a specific collagen that anchors acetylcholinesterase (AChE) in the synaptic cleft of the neuromuscular junction. So far, no mutation has been identified in the ACHE human gene but over 50 different mutations in the COLQ gene are causative for a congenital myasthenic syndrome (CMS) with AChE deficiency. Mice deficient for COLQ mimic most of the functional deficit observed in CMS patients. At the molecular level, a striking consequence of the absence of COLQ is an increase in the levels of acetylcholine receptor (AChR) mRNAs and proteins in vivo and in vitro in murine skeletal muscle cells. Here, we decipher the mechanisms that drive AChR mRNA upregulation in cultured muscle cells deficient for COLQ. We show that the levels of AChR β-subunit mRNAs are post-transcriptionally regulated by an increase in their stability. We demonstrate that this process results from an activation of p38 MAPK and the cytoplasmic translocation of the nuclear RNA-binding protein human antigen R (HuR) that interacts with the AU-rich element located within AChR β-subunit transcripts. This HuR/AChR transcript interaction induces AChR β-subunit mRNA stabilization and occurs at a specific stage of myogenic differentiation. In addition, pharmacological drugs that modulate p38 activity cause parallel modifications of HuR protein and AChR β-subunit levels. Thus, our study provides new insights into the signaling pathways that are regulated by ColQ-deficiency and highlights for the first time a role for HuR and p38 in mRNA stability in a model of congenital myasthenic syndrome.

Integrated Bioinformatics Analysis Identifies ELAVL1 and APP as Candidate Crucial Genes for Crohn's Disease

Immune imbalance and barrier destruction of intestinal mucosa are the central pathogenic factors of Crohn's disease (CD). In this study, three independent microarray studies of CD were integrated and 9912 differentially expressed genes (DEGs) were analysed by NetworkAnalyst to screen candidate crucial genes. NetworkAnalyst identified ELAV-like RNA binding protein 1 (ELAVL1) as the most crucial upregulated gene and amyloid-β precursor protein (APP) as the most crucial downregulated gene in peripheral blood of CD patients. By computing significance with hypergeometric test based on the KEGG pathway database, upregulated DEGs highlight the pathways of T cell receptor signaling and the differentiation of T helpers. Downregulated DEGs were found enriched in pathways in multiple cancers, MAPK signaling, Rap1 signaling, and PI3K-AKT signaling. Further taking all DEGs together, Gene Set Enrichment Analysis (GSEA) brought out the NOD-like receptor (NLR) signaling pathway which could be regulated by ELAVL1. xCell found decreased naïve and differentiated T cell proportions in the peripheral blood of CD patients suggesting T cell migration to the intestinal tissue and/or exhaustion. Further, ELAVL1 expression correlating with multiple T cell proportions suggests that ELAVL1 may regulate T cell activation. These findings illustrated that ELAVL1 and APP were candidate crucial genes in the peripheral blood of CD patients. ELAVL1 possibly acts as a key regulator of T cell activation via the NLR signaling pathway. APP might be a downstream effector of infliximab treatment connecting with MAPK signaling.

MicroRNA-145 suppresses gastric cancer progression by targeting Hu-antigen R

Hu-antigen R (HuR) is involved in the carcinogenesis and progression of multiple types of cancer. However, its precise role in gastric cancer (GC) and the relevant molecular mechanism remain largely unclear. In the present study, we found that HuR expression level was higher in GC tissues and cell lines than in adjacent normal tissues and normal gastric epithelial cell lines, and this elevated expression was found to have a significant association with lymph node metastasis. Moreover, silencing HuR with RNA interference inhibited cell viability and induced cell apoptosis through the apoptosis-related regulators (Bcl-2 and Bax) in GC cells. In addition, bioinformatic analysis revealed that HuR expression was inversely correlated with miR-145 expression in GC tissue samples, and HuR was identified as a direct target of miR-145 with the dual-luciferase reporter. Enforced expression of miR-145 inhibited the HuR expression at both mRNA and protein levels and induced similar biologic effects of silencing HuR in GC cells. Additionally, we also found that restoration of HuR could eliminate the effects induced by miR-145 in GC cells. Taken together, these findings demonstrate the exact role of the miR-145-HuR axis in the progression of GC and indicate a potential target for GC therapy.

The RNA-binding protein HuR contributes to neuroinflammation by promoting C-C chemokine receptor 6 (CCR6) expression on Th17 cells

In both multiple sclerosis and experimental autoimmune encephalomyelitis (EAE), the C-C chemokine receptor 6 (CCR6) is critical for pathogenic T helper 17 (Th17) cell migration to the central nervous system (CNS). Whereas many cytokines and their receptors are potently regulated via post-transcriptional mechanisms in response to various stimuli, how CCR6 expression is post-transcriptionally regulated in Th17 cells is unknown. Here, using RNA-binding protein HuR conditional knock-out (KO) and wild-type (WT) mice, we present evidence that HuR post-transcriptionally regulates CCR6 expression by binding to and stabilizing Ccr6 mRNA and by promoting CCR6 translation. We also found that HuR down-regulates several microRNA expressions, which could target the 3'-UTR of Ccr6 mRNA for decay. Accordingly, knock-out of HuR reduced CCR6 expression on Th17 cells and impaired their migration to CNS compared with the response of WT Th17 cells and thereby ameliorated EAE. Together, these findings highlight how HuR contributes to Th17 cell-mediated autoimmune neuroinflammation and support the notion that targeting HuR might be a potential therapeutic intervention for managing autoimmune disorders of the CNS.

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

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

Relationship between microRNAs and signaling pathways associated with hepatic stellate cells

Activation and transdifferentiation of hepatic stellate cells (HSCs) caused by a variety of signal transduction pathways triggered by inflammatory factors and cytokines are a key initiating event in the process of hepatic fibrosis. MicroRNAs (miRNAs) existing in a wide variety of organisms play a role by negative regulation of their target genes at the transcriptional or translational level. Research shows that several signal transduction pathways associated with HSCs can regulate miRNA transcription, processing, maturation and function. At the same time, different miRNAs also regulate HSC activation, proliferation and apoptosis-related signal transduction. This interaction can provide some ideas for the molecular target therapy of hepatic fibrosis and the exploration of its pathogenesis.

Krüppel -like factor 8 is a stress-responsive transcription factor that regulates expression of HuR

BACKGROUND/AIMS

HuR is an RNA-binding protein that regulates the post-transcriptional life of thousands of cellular mRNAs and promotes cell survival. HuR is expressed as two mRNA transcripts that are differentially regulated by cell stress. The goal of this study is to define factors that promote transcription of the longer alternate form.

METHODS

Effects of transcription factors on HuR expression were determined by inhibition or overexpression of these factors followed by competitive RT-PCR, gel mobility shift, and chromatin immunoprecipitation. Transcription factor expression patterns were identified through competitive RT-PCR and Western analysis. Stress responses were assayed in thapsigargin-treated proximal tubule cells and in ischemic rat kidney.

RESULTS

A previously described NF-κB site and a newly identified Sp/KLF factor binding site were shown to be important for transcription of the long HuR mRNA. KLF8, but not Sp1, was shown to bind this site and increase HuR mRNA levels. Cellular stress in cultured or native proximal tubule cells resulted in a rapid decrease of KLF8 levels that paralleled those of the long HuR mRNA variant.

CONCLUSIONS

These results demonstrate that KLF8 can participate in regulating expression of alternate forms of HuR mRNA along with NF-κB and other factors, depending on cellular contexts.

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

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

Adaptive and maladaptive expression of the mRNA regulatory protein HuR

The RNA-binding proteins involved in regulation of mRNA post-transcriptional processing and translation control the fates of thousands of mRNA transcripts and basic cellular processes. The best studied of these, HuR, is well characterized as a mediator of mRNA stability and translation, and more recently, as a factor in nuclear functions such as pre-mRNA splicing. Due to HuR’s role in regulating thousands of mRNA transcripts, including those for other RNA-binding proteins, HuR can act as a master regulator of cell survival and proliferation. HuR itself is subject to multiple post-translational modifications including regulation of its nucleocytoplasmic distribution. However, the mechanisms that govern HuR levels in the cell have only recently begun to be defined. These mechanisms are critical to cell health, as it has become clear in recent years that aberrant expression of HuR can lead alternately to decreased cell viability or to promotion of pathological proliferation and invasiveness. HuR is expressed as alternate mRNAs that vary in their untranslated regions, leading to differences in transcript stability and translatability. Multiple transcription factors and modulators of mRNA stability that regulate HuR mRNA expression have been identified. In addition, translation of HuR is regulated by numerous microRNAs, several of which have been demonstrated to have anti-tumor properties due to their suppression of HuR expression. This review summarizes the current state of knowledge of the factors that regulate HuR expression, along with the circumstances under which these factors contribute to cancer and inflammation.

TLR9 signaling repressed tumor suppressor miR-7 expression through up-regulation of HuR in human lung cancer cells

Background

Our recent evidence showed that Toll like receptor 9 (TLR9) signaling could enhance the growth and metastatic potential of human lung cancer cells through repressing microRNA-7 (miR-7) expression. Human antigen R (HuR) has been involved in stabilizing multiple mRNAs in cellular biology. However, whether HuR also contributed to the altered expression of miR-7 in TLR9 signaling stimulated human lung cancer cells remains to be elucidated.

Methods

The expression of HuR in human lung cancer 95D cells treated with TLR9 agonist CpG Oligonucleotides (ODNs) was detected by Real-time PCR and Western blot assay. To explore the possible role of HuR on miR-7 expression, eukaryotic expression vector encoding HuR was transiently transfected into 95D cells and then the expression of miR-7 was detected by Real-time PCR assay. Moreover, RNA interference, western blot, Real-time PCR, MTT assay, BrdU labeling, invasion assay and scratch assay were employed to examine the disrupt effect of HuR on miR-7 expression in human lung cancer cells treated with CpG ODNs. Finally, inhibitors for PI3K, Akt or Erk respectively, and western blot were performed to explore the possible signaling pathway related to HuR expression in CpG ODNs treated human lung cancer cells.

Results

Our data showed that TLR9 agonist CpG ODNs could induce the expression of HuR in human lung cancer cells. Moreover, overexpression of HuR could reduce the expression of miR-7 in lung cancer cells. Notably, down-regulation of HuR using RNA interference restored miR-7 expression in CpG ODNs treated lung cancer cells, accompanied by enhanced growth and metastatic potential. Finally, CpG ODNs could induce HuR expression through Akt pathway.

Conclusion

Our findings indicated that HuR could act as regulator in regulating TLR9 signaling associated biological effect in human lung cancer cells, which might be helpful for the understanding of the potential role of HuR in tumor biology.

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.

Functional interplay between RNA-binding protein HuR and microRNAs

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

HuR inhibits apoptosis by amplifying Akt signaling through a positive feedback loop

Human antigen R (HuR) is a post-transcriptional regulator of gene expression that plays a key role in stabilizing mRNAs during cellular stress, leading to enhanced survival. HuR expression is tightly regulated through multiple transcription and post-transcriptional controls. Although HuR is known to stabilize a subset of mRNAs involved in cell survival, its role in the survival pathway of PI3-kinase/Akt signaling is unclear. Here, we show that in renal proximal tubule cells, HuR performs a central role in cell survival by amplifying Akt signaling in a positive feedback loop. Key to this feedback loop is HuR-mediated stabilization of mRNA encoding Grb10, an adaptor protein whose expression is critical for Akt activation. Stimulation of Akt by interaction with Grb10 then activates NF-κB, which further enhances HuR mRNA and protein expression. This feedback loop is active in unstressed cells, but its effects are increased during stress. Therefore, this study demonstrates a central role for HuR in Akt signaling and reveals a mechanism by which modest changes in HuR levels below or above normal may be amplified, potentially resulting in cell death or cellular transformation.

The RNA-binding protein human antigen R controls global changes in gene expression during Schwann cell development

An important prerequisite to myelination in peripheral nerves is the establishment of one-to-one relationships between axons and Schwann cells. This patterning event depends on immature Schwann cell proliferation, apoptosis, and morphogenesis, which are governed by coordinated changes in gene expression. Here, we found that the RNA-binding protein human antigen R (HuR) was highly expressed in immature Schwann cells, where genome-wide identification of its target mRNAs in vivo in mouse sciatic nerves using ribonomics showed an enrichment of functionally related genes regulating these processes. HuR coordinately regulated expression of several genes to promote proliferation, apoptosis, and morphogenesis in rat Schwann cells, in response to NRG1, TGFβ, and laminins, three major signals implicated in this patterning event. Strikingly, HuR also binds to several mRNAs encoding myelination-related proteins but, contrary to its typical function, negatively regulated their expression, likely to prevent ectopic myelination during development. These functions of HuR correlated with its abundance and subcellular localization, which were regulated by different signals in Schwann cells.

HuR function in disease

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

The mRNA-binding protein HuR is regulated in the menstrual cycle and repressed in ectopic endometrium

Cytokines modulate turnover of the endometrium during the menstrual cycle and contribute to the pathogenesis of endometriosis. Gene expression for cytokines is often regulated by proteins that bind to adenosine- and uridine-rich elements (AREs) in their transcripts to stabilize or destabilize bound messenger RNAs (mRNAs). HuR/ELAVL1 is an RNA-binding protein that stabilizes ARE-containing mRNAs. We hypothesized that HuR might play a role in regulating cytokine expression during the menstrual cycle and in endometriosis and characterized the expression and regulation of HuR in eutopic and ectopic human endometrium. Tissue sections obtained from normal (n = 23) and ectopic (n = 16) endometrium were immunostained for HuR, and staining intensity was evaluated by HSCORE. Cultured stromal cells isolated from normal endometrium were treated with vehicle, estradiol (E2), progesterone (P), E2 + P, tumor necrosis factor-α (TNF-α), and interleukin 1β (IL-1β) for 24 hours, and HuR expression was determined by Western blot. HuR immunoreactivity was significantly lower in the early proliferative and late secretory phases (157.5 ± 11.08 and 190.0 ± 15.2, respectively), compared to the mid-late proliferative (270.0 ± 8.0) and early-mid secretory phases (256.6 ± 20.2; P < .01, analysis of variance [ANOVA]). Furthermore, HuR expression was significantly lower in ectopic endometrial cells compared to normal endometrium in mid-late proliferative and early-mid-secretory phases (P < .01). Estrogen, P, or cytokines did not alter HuR expression in cultured endometrial stromal cells. Increased HuR levels in the mid-menstrual phases are likely to contribute to reduced mid-cycle cytokine expression and enhanced cellular survival in eutopic endometrium. In ectopic endometrium, elevated cytokine levels associated with endometriosis likely reduce HuR expression.

Properties of the regulatory RNA-binding protein HuR and its role in controlling miRNA repression

Gene expression in eukaryotes is subject to extensive regulation at posttranscriptional levels. One of the most important sites of control involves mRNA 3' untranslated regions (3'UTRs), which are recognized by RNA-binding proteins (RBPs) and microRNAs (miRNAs). These factors greatly influence translational efficiency and stability of target mRNAs and often also determine their cellular localization. HuR, a ubiquitously expressed member of the ELAV family of RBPs, has been implicated in regulation of stability and translation of over one hundred mRNAs in mammalian cells. Recent data indicate that some of the effects of HuR can be explained by its interplay with miRNAs. Binding of HuR may suppress the inhibitory effect of miRNAs interacting with the 3'UTR and redirect the repressed mRNA to polysomes for active translation. However, HuR can also synergize with miRNAs. The finding that HuR is able to disengage miRNAs from the repressed mRNA, or render them inactive, provides evidence that miRNA regulation is much more dynamic then originally anticipated. In this chapter we review properties of HuR and describe examples of the cross-talk between the protein and miRNAs, with emphasis on response of the regulation to cellular stress.