Delineation of mRNA export pathways by the use of cell-permeable peptides

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

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

hnRNP A1: the Swiss army knife of gene expression

Eukaryotic cells express a large variety of RNA binding proteins (RBPs), with diverse affinities and specificities towards target RNAs. These proteins play a crucial role in almost every aspect of RNA biogenesis, expression and function. The heterogeneous nuclear ribonucleoproteins (hnRNPs) are a complex and diverse family of RNA binding proteins. hnRNPs display multiple functions in the processing of heterogeneous nuclear RNAs into mature messenger RNAs. hnRNP A1 is one of the most abundant and ubiquitously expressed members of this protein family. hnRNP A1 plays multiple roles in gene expression by regulating major steps in the processing of nascent RNA transcripts. The transcription, splicing, stability, export through nuclear pores and translation of cellular and viral transcripts are all mechanisms modulated by this protein. The diverse functions played by hnRNP A1 are not limited to mRNA biogenesis, but extend to the processing of microRNAs, telomere maintenance and the regulation of transcription factor activity. Genomic approaches have recently uncovered the extent of hnRNP A1 roles in the development and differentiation of living organisms. The aim of this review is to highlight recent developments in the study of this protein and to describe its functions in cellular and viral gene expression and its role in human pathologies.

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.

Potential effects of CRM1 inhibition in mantle cell lymphoma

Mantle cell lymphoma (MCL) is an aggressive histotype of B-cell non-Hodgkin lymphoma. The disease has no known cure, which prompts the urgent need for novel therapeutic agents. Chromosomal region maintenance 1 (CRM1) may play a role in human neoplasia and serve as a novel target of cancer treatment. This study summarizes MCL pathogenesis and determines the involvement of CRM1 in the regulation of several vital signaling pathways contributing to MCL pathogenesis, including the pathways of cell cycle progression, DNA damage response, phosphoinositide kinase-3, nuclear factor-κB activation, and chromosomal stability. A preclinical study is also presented to compare the CRM1 status in MCL cell lines and primary MCL cells with normal B cells, as well as the therapeutic efficiency of CRM1 inhibition in MCL in vitro and in vivo, which make these agents potential targets of novel MCL treatments.

Novel selective inhibitors of nuclear export CRM1 antagonists for therapy in mantle cell lymphoma

Overexpression of the cellular nuclear exportin 1, more commonly called chromosomal region maintenance 1 (CRM1), has been associated with malignant progression and mortality. Therefore, activation of nuclear export can play a significant etiologic role in some forms of human neoplasia and serve as a novel target for the treatment of these cancers. Mantle cell lymphoma (MCL) is an aggressive histotype of B-cell non-Hodgkin lymphoma that remains incurable. The objective of this study was to investigate the functional significance of CRM1 in MCL by evaluating the therapeutic efficacy of CRM1 inhibition in MCL in vitro and in vivo. Our results showed that CRM1 is highly expressed in MCL cells and is involved in regulating growth and survival mechanisms through the critical nuclear factor-κB survival pathway, which is independent of p53 status. Inhibition of CRM1 by two novel selective inhibitors of nuclear export (SINE), KPT-185 and KPT-276, in MCL cells resulted in significant growth inhibition and apoptosis induction. KPT-185 also induced CRM1 accumulation in the nucleus, resulting in CRM1 degradation by the proteasome. Oral administration of KPT-276 significantly suppressed tumor growth in an MCL-bearing severe combined immunodeficient mouse model, without severe toxicity. Our data suggest that SINE CRM1 antagonists are a potential novel therapy for patients with MCL, particular in relapsed/refractory disease.

Regulation of ARE-mRNA Stability by Cellular Signaling: Implications for Human Cancer

During recent years, it has become clear that regulation of mRNA stability is an important event in the control of gene expression. The stability of a large class of mammalian mRNAs is regulated by AU-rich elements (AREs) located in the mRNA 3' UTRs. mRNAs with AREs are inherently labile but as a response to different cellular cues they can become either stabilized, allowing expression of a given gene, or further destabilized to silence their expression. These tightly regulated mRNAs include many that encode growth factors, proto-oncogenes, cytokines, and cell cycle regulators. Failure to properly regulate their stability can therefore lead to uncontrolled expression of factors associated with cell proliferation and has been implicated in several human cancers. A number of transfactors that recognize AREs and regulate the translation and degradation of ARE-mRNAs have been identified. These transfactors are regulated by signal transduction pathways, which are often misregulated in cancers. This chapter focuses on the function of ARE-binding proteins with an emphasis on their regulation by signaling pathways and the implications for human cancer.

Apoptotic-induced cleavage shifts HuR from being a promoter of survival to an activator of caspase-mediated apoptosis

Little is known about the cellular mechanisms modulating the shift in balance from a state of survival to cell death by caspase-mediated apoptosis in response to a lethal stress. Here we show that the RNA-binding protein HuR has an important function in mediating this switch. During caspase-mediated apoptosis, HuR is cleaved to generate two cleavage products (CPs). Our data demonstrate that the cleavage of HuR switches its function from being a prosurvival factor under normal conditions to becoming a promoter of apoptosis in response to a lethal stress. In the absence of an apoptotic stimuli, HuR associates with and promotes the expression of caspase-9 and prothymosin α (ProT) mRNAs, and pro- and antiapoptotic factors, respectively, both of which have been characterized as important players in determining cell fate. During the early steps of caspase-mediated apoptosis, however, the level of caspase-9 protein increases, while ProT remains unchanged. Under these conditions, the two HuR-CPs selectively bind to and stabilize caspase-9 mRNA, but do not bind to ProT. Hence, taken together, our data show that by maintaining a threshold of expression of proapoptotic factors such as caspase-9 in response to a lethal stress, the HuR-CPs help a cell to switch from resisting death to undergoing apoptosis.

Bile salts increase epithelial cell proliferation through HuR-induced c-Myc expression

BACKGROUND

Bile salts increase intestinal mucosal proliferation through an increase in c-Myc, a transcription factor that controls the expression of numerous translation regulatory proteins. HuR is an RNA-binding protein that regulates translation of target mRNAs. RNA-binding proteins can control mRNA stability by binding to AU- and U-rich elements located in the 3'-untranslated regions (3'-UTRs) of target mRNAs.

AIM

To determine how bile salt-induced c-Myc stimulates enterocyte proliferation.

METHODS

Enterocyte proliferation was measured both in vivo using C57Bl6 mice and in vitro using IEC-6 cells after taurodeoxycholate (TDCA) supplementation. HuR and c-Myc protein expression was determined by immunoblot. c-Myc mRNA expression was determined by PCR. HuR expression was inhibited using specific small interfering RNA. HuR binding to c-Myc mRNA was determined by immunoprecipitation.

RESULTS

TDCA increased enterocyte proliferation in vivo and in vitro. TDCA stimulates translocation of HuR from the nucleus to the cytoplasm. Cytoplasmic HuR regulates c-Myc translation by HuR binding to the 3'-UTR of c-Myc mRNA. Increased TDCA-induced c-Myc increases enterocyte proliferation.

CONCLUSIONS

Bile salts have beneficial effects on the intestinal epithelial mucosa, which are important in maintaining intestinal mucosal integrity and function. These data further support an important beneficial role of bile salts in regulation of mucosal growth and repair. Decreased enterocyte exposure to luminal bile salts, as occurs during critical illness, liver failure, starvation, and intestinal injury, may have a detrimental effect on mucosal integrity.

Two in-and-out modulation strategies for endoplasmic reticulum stress-linked gene expression of pro-apoptotic macrophage-inhibitory cytokine 1

Excessive and persistent insults during endoplasmic reticulum (ER) stress lead to apoptotic cell death that is implicated in a range of chronic inflammatory diseases and cancers. Macrophage inhibitory cytokine 1 (MIC-1), a member of the transforming growth factor-β superfamily, is diversely linked to the pathogenesis of cancer. To investigate the precise molecular mechanisms of MIC-1 gene regulation, ER stress and its related signals were studied in human colon cancer cells. Functionally, MIC-1 played pivotal roles in ER stress-linked apoptotic death, which was also influenced by C/EBP homologous protein, a well known apoptotic mediator of ER stress. ER stress enhanced MIC-1 mRNA stability instead of transcriptional activation, and there were two mechanistic translocations critical for mRNA stabilization. First, C/EBP homologous protein triggered protein kinase C-linked cytosolic translocation of the HuR/ELAVL1 (Elav-like RNA-binding protein 1) RNA-binding protein, which bound to and stabilized MIC-1 transcript. As the second critical in-and-out regulation, ER stress-activated ERK1/2 signals contributed to enhanced stabilization of MIC-1 transcript by controlling the extended holding of the nucleated mRNA in the stress granules fusing with the mRNA-decaying processing body. We propose that these two sequential in-and-out modulations can account for stabilized transcription and subsequent translation of pro-apoptotic MIC-1 gene in human cancer cells under ER stress.

Inducible nitric oxide synthase (iNOS) in muscle wasting syndrome, sarcopenia, and cachexia

Muscle atrophy—also known as muscle wasting—is a debilitating syndrome that slowly develops with age (sarcopenia) or rapidly appears at the late stages of deadly diseases such as cancer, AIDS, and sepsis (cachexia). Despite the prevalence and the drastic detrimental effects of these two syndromes, there are currently no widely used, effective treatment options for those suffering from muscle wasting. In an attempt to identify potential therapeutic targets, the molecular mechanisms of sarcopenia and cachexia have begun to be elucidated. Growing evidence suggests that inflammatory cytokines may play an important role in the pathology of both syndromes. As one of the key cytokines involved in both sarcopenic and cachectic muscle wasting, tumor necrosis factor α (TNFα) and its downstream effectors provide an enticing target for pharmacological intervention. However, to date, no drugs targeting the TNFα signaling pathway have been successful as a remedial option for the treatment of muscle wasting. Thus, there is a need to identify new effectors in this important pathway that might prove to be more efficacious targets. Inducible nitric oxide synthase (iNOS) has recently been shown to be an important mediator of TNFα-induced cachectic muscle loss, and studies suggest that it may also play a role in sarcopenia. In addition, investigations into the mechanism of iNOS-mediated muscle loss have begun to reveal potential therapeutic strategies. In this review, we will highlight the potential for targeting the iNOS/NO pathway in the treatment of muscle loss and discuss its functional relevance in sarcopenia and cachexia.

The complex world of post-transcriptional mechanisms: is their deregulation a common link for diseases? Focus on ELAV-like RNA-binding proteins

Post-transcriptional mechanisms are key determinants in the modulation of the expression of final gene products. Within this context, fundamental players are RNA-binding proteins (RBPs), and among them ELAV-like proteins. RBPs are able to affect every aspect in the processing of transcripts, from alternative splicing, polyadenylation, and nuclear export to cytoplasmic localization, stability, and translation. Of interest, more than one RBP can bind simultaneously the same mRNA; therefore, since each RBP is endowed with different properties, the balance of these interactions dictates the ultimate fate of the transcript, especially in terms of both stability and rate of translation. Besides RBPs, microRNAs are also important contributors to the post-transcriptional control of gene expression. Within this general context, the present review focuses on ELAV-like proteins describing their roles in the nucleus and in the cytoplasm, also highlighting some examples of interactions with other RBPs and with microRNAs. We also examine the putative role and the observed changes of ELAV-like proteins and of their interactions with other regulatory elements in Alzheimer's disease, cancer, and inflammation. The changes in the expression of proteins involved in these diseases are examples of how a derangement in the mRNA stabilization process may be associated with disease development and contribute to pathology. Overall, we hope that the topics handled in the present manuscript provide a hint to look at ELAV-like-mediated mRNA stabilization as a mechanism relevant to disease as well as a novel putative drug target.

CRM1-dependent transport supports cytoplasmic accumulation of adenoviral early transcripts

The life cycle of adenoviruses is divided by convention into early and late phases, separated by the onset of viral genome replication. Early events include virus adsorption, transport of the genome into the nucleus, and the expression of early genes. After the onset of viral DNA replication, transcription of the major late transcription unit (MLTU) and thereby synthesis of late proteins is induced. These steps are controlled by an orchestra of regulatory processes and require import of the genome and numerous viral proteins into the nucleus, as well as active transport of viral transcripts and proteins from the nucleus to the cytoplasm. The latter is achieved by exploiting the shuttling functions of cellular transport receptors, which normally stimulate the nuclear export of cellular mRNA and protein cargos. A set of adenoviral early and late proteins contains a leucine-rich nuclear export signal of the HIV-1 Rev type, known to be recognized by the cellular export receptor CRM1. However, a role for CRM1-dependent export in supporting adenoviral replication has not been established. To address this issue in detail, we investigated the impact of two different CRM1 inhibitors on several steps of the adenoviral life cycle. Inhibition of CRM1 led to a reduction in viral early and late gene expression, viral genome replication, and progeny virus production. For the first time, our findings indicate that CRM1-dependent shuttling is required for the efficient export of adenoviral early mRNA.

G-substrate: the cerebellum and beyond

The discovery of nitric oxide (NO) as an activator of soluble guanylate cyclase (sGC) has stimulated extensive research on the NO-sGC-3':5'-cyclic guanosine monophosphate (cGMP)-cGMP-dependent protein kinase (PKG) pathway. However, the restricted localization of pathway components and the lack of information on PKG substrates have hindered research seeking to examine the physiological roles of the NO-sGC-cGMP-PKG pathway. An excellent substrate for PKG is the G-substrate, which was originally discovered in the cerebellum. The role of G-substrate in the cerebellum and other brain structures has been revealed in recent years. This review discusses the relationship between the G-substrate and other components of the NO-sGC-cGMP-PKG pathway and describes the characteristics of the G-substrate gene and protein related to diseases. Finally, we discuss the physiological role of G-substrate in the cerebellum, where it regulates cerebellum-dependent long-term memory, and its role in the ventral tegmental area and retina, where it acts as an effective neuroprotectant.

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.

Expression of HuR, COX-2, and survivin in lung cancers; cytoplasmic HuR stabilizes cyclooxygenase-2 in squamous cell carcinomas

Hu antigen R (HuR) is a member of the human family of embryonic-lethal, abnormal vision-like proteins, which serves as an mRNA-binding protein. In the cytoplasm, HuR can stabilize the mRNA of cyclooxygenase-2 (COX-2), an enzyme that catalyses the synthesis of prostaglandins and is associated with promotion of carcinogenesis and tumor cell resistance to apoptosis. Intracellular (cytoplasmic and nuclear) localization of survivin has a prognostic significance as an apoptosis inhibitor and a regulator of cell division in tumors. Patients with 151 squamous cell carcinomas and 93 adenocarcinomas underwent lobectomy or pneumonectomy with hilar and mediastinal lymph node sampling. Paraffin-embedded tumor sections were retrieved for evaluation of nuclear and cytoplasmic staining of survivin and HuR, and cytoplasmic staining of COX-2. In squamous cell carcinomas, COX-2 expression was correlated with a difference of survivin (cytoplasmic-nuclear; P=0.004), cytoplasmic HuR (P=0.018), total HuR (cytoplasmic+nuclear; P=0.009), and difference of HuR (P=0.020). COX-2 was inversely correlated with nuclear survivin (P=0.006). In a univariate analysis by log-rank test, survival was associated with cytoplasmic survivin (adenocarcinoma, P<0.001; squamous cell carcinoma, P=0.005), difference of survivin (adenocarcinoma, P<0.001; squamous cell carcinoma, P=0.014), and COX-2 (squamous cell carcinoma, P=0.001). Survival was inversely associated with nuclear survivin (adenocarcinoma, P=0.006, squamous cell carcinoma, P=0.014). In a multivariate survival analysis, cytoplasmic survivin (adenocarcinoma, P=0.002; squamous cell carcinoma, P=0.015) and COX-2 (squamous cell carcinoma, P=0.020) were determined as independent prognostic factors. Cytoplasmic HuR expression is associated with COX-2 expression in squamous cell carcinomas. The expression of COX-2 in squamous cell carcinomas, and cytoplasmic survivin in adenocarcinomas and squamous cell carcinomas could be useful independent prognostic markers.

Transportin 2 regulates apoptosis through the RNA-binding protein HuR

In response to severe stress, apoptotic cell death is engaged. Apoptosis is a well orchestrated process that involves the activation and implication of many factors. In this study, we identified a role for the nuclear trafficking factor TRN2 (transportin 2) in cell death. TRN2 is normally responsible for the nuclear import of the RNA-binding protein HuR. During apoptosis, however, HuR accumulates in the cytoplasm. This is due to the caspase-mediated cleavage of the cytoplasmic fraction of HuR. One of the cleavage fragments generated by this processing of HuR interacts with TRN2 and thus blocks the re-import of HuR into the nucleus. This concentrates HuR in the cytoplasm, advancing apoptosis. Therefore, increasing or decreasing the levels of TRN2 has an inverse consequential effect on cell death, demonstrating for the first time the role of a nucleocytoplasmic transport factor in apoptosis.

HuR controls lung branching morphogenesis and mesenchymal FGF networks

Lung development is controlled by regulatory networks governing mesenchymal-epithelial interactions. Transcription factors and signaling molecules are known to participate in this process, yet little is known about the post-transcriptional regulation of these networks. Here we demonstrate that the RNA-binding protein (RBP) HuR is an essential regulator of mesenchymal responses during lung branching. Its epiblast-induced deletion blocked the morphogenesis of distal bronchial branches at the initiation of the pseudoglandular stage. The phenotype originated from defective mesenchymal responses since the conditional restriction of HuR deletion in epithelial progenitors did not affect distal branching or the completion of lung maturation. The loss of HuR resulted in the reduction of the key inducer of bud outgrowth and endodermal branching, FGF10 and one of its putative transcriptional regulators, Tbx4. Furthermore, exogenous FGF10 could rescue the branching defect of affected lung buds. HuR was found to bind and control the Fgf10 and Tbx4 mRNAs; as a result its deletion abolished their inducible post-transcriptional regulation by the mesenchymal regulator FGF9. Our data reveals HuR as the first RBP identified to play a dominant role in lung development and as a key post-transcriptional regulator of networks guiding tissue remodeling during branching morphogenesis.

The multifaceted poliovirus 2A protease: regulation of gene expression by picornavirus proteases

After entry into animal cells, most viruses hijack essential components involved in gene expression. This is the case of poliovirus, which abrogates cellular translation soon after virus internalization. Abrogation is achieved by cleavage of both eIF4GI and eIF4GII by the viral protease 2A. Apart from the interference of poliovirus with cellular protein synthesis, other gene expression steps such as RNA and protein trafficking between nucleus and cytoplasm are also altered. Poliovirus 2A^pro is capable of hydrolyzing components of the nuclear pore, thus preventing an efficient antiviral response by the host cell. Here, we compare in detail poliovirus 2A^pro with other viral proteins (from picornaviruses and unrelated families) as regard to their activity on key host factors that control gene expression. It is possible that future analyses to determine the cellular proteins targeted by 2A^pro will uncover other cellular functions ablated by poliovirus infection. Further understanding of the cellular proteins hydrolyzed by 2A^pro will add further insight into the molecular mechanism by which poliovirus and other viruses interact with the host cell.

Role of RNA binding protein HuR in ductal carcinoma in situ of the breast

HuR is a ubiquitously expressed RNA-binding protein that modulates gene expression at the post-transcriptional level. It is predominantly nuclear, but can shuttle between the nucleus and the cytoplasm. While in the cytoplasm HuR can stabilize its target transcripts, many of which encode proteins involved in carcinogenesis. While cytoplasmic HuR expression is a marker of reduced survival in breast cancer, its role in precursor lesions of malignant diseases is unclear. To address this we explored HuR expression in atypical ductal hyperplasia (ADH) and in ductal in situ carcinomas (DCIS). We show that cytoplasmic HuR expression is elevated in both ADH and DCIS when compared to normal controls, and that this expression associated with high grade, progesterone receptor negativity and microinvasion and/or tumour-positive sentinel nodes of the DCIS. To study the mechanisms of HuR in breast carcinogenesis, HuR expression was silenced in an immortalized breast epithelial cell line (184B5Me), which led to reduction in anchorage-independent growth, increased programmed cell death and inhibition of invasion. In addition, we identified two novel target transcripts (CTGF and RAB31) that are regulated by HuR and that bind HuR protein in this cell line. Our results show that HuR is aberrantly expressed at early stages of breast carcinogenesis and that its inhibition can lead to suppression of this process. ArrayExpress Accession No. E-MEXP-3035.

The p53 target Wig-1: a regulator of mRNA stability and stem cell fate?

Wig-1 is a transcriptional target of the tumor suppressor p53 and encodes an unusual zinc-finger protein involved in post-transcriptional gene regulation. Wig-1 is expressed in all cell types investigated so far, with the highest levels in the brain, and is enriched in stem cells as compared with more differentiated cells of the same lineage. Wig-1 binds to both long double-stranded (ds) RNA and short microRNA-like dsRNA. We have shown that Wig-1 acts in a positive feedback loop that stabilizes p53 mRNA through an AU-rich element (ARE) in the p53 3'untranslated region. Our preliminary data indicate a more general effect of Wig-1 on ARE-containing mRNA. Here we shall summarize current knowledge about Wig-1 and discuss possible implications on p53 function and other cellular processes.

HuR and myogenesis: being in the right place at the right time

The process of muscle cell differentiation into myotubes, termed myogenesis, depends on a complex coordination of myogenic factors, many of which are regulated post-transcriptionally. HuR, an mRNA-binding protein, is responsible for regulating the expression of several such myogenic factors by stabilizing their mRNAs. The critical role for HuR in myogenesis also involves the nucleocytoplasmic shuttling ability of this protein. Indeed, in order to perform its stabilizing functions, HuR must accumulate in the cytoplasm. This requires its dissociation from the import factor Transportin 2 (TRN2) which is actually caused by the cleavage of a portion of cytoplasmic HuR. In this review, we describe the roles of HuR during myogenesis, and the mechanisms regulating its cytoplasmic accumulation. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Viral-mediated stabilization of AU-rich element containing mRNA contributes to cell transformation

E4orf6 is one of the oncogene products of adenovirus, and it also has an important role for transportation of cellular and viral messenger RNA (mRNA) during the late phase of virus infection. We previously revealed that E4orf6 controls the fate of AU-rich element (ARE) containing mRNA by perturbing the chromosome maintenance region 1-dependent export mechanism. Here, we show that E4orf6 stabilizes ARE-mRNA through the region required for its oncogenic activity and ubiquitin E3 ligase assembly. Cells that failed to stabilize ARE-mRNA after HuR knockdown were unable to produce colonies in soft agar, even when E4orf6 was expressed. Furthermore, the stabilized ARE-mRNA induced the transformation of rodent immortalized cells. These findings indicate that stabilized ARE-mRNA is necessary, if not all, for the oncogenic activity of E4orf6 and has the potential to transform cells, at least under a certain condition.

Global dissociation of HuR-mRNA complexes promotes cell survival after ionizing radiation

Ionizing radiation (IR) triggers adaptive changes in gene expression. Here, we show that survival after IR strongly depends on the checkpoint kinase Chk2 acting upon its substrate HuR, an RNA-binding protein that stabilizes and/or modulates the translation of target mRNAs. Microarray analysis showed that in human HCT116 colorectal carcinoma cells (WT), IR-activated Chk2 triggered the dissociation of virtually all of HuR-bound mRNAs, since IR did not dissociate HuR target mRNAs in Chk2-null (CHK2-/-) HCT116 cells. Accordingly, several HuR-interacting mRNAs encoding apoptosis- and proliferation-related proteins (TJP1, Mdm2, TP53BP2, Bax, K-Ras) dissociated from HuR in WT cells, but remained bound and showed altered post-transcriptional regulation in CHK2-/- cells. Use of HuR mutants that were not phosphorylatable by Chk2 (HuR(3A)) and HuR mutants mimicking constitutive phosphorylation by Chk2 (HuR(3D)) revealed that dissociation of HuR target transcripts enhanced cell survival. We propose that the release of HuR-bound mRNAs via an IR-Chk2-HuR regulatory axis improves cell outcome following IR.

Export of adenoviral late mRNA from the nucleus requires the Nxf1/Tap export receptor

One important function of the human adenovirus E1B 55-kDa protein is induction of selective nuclear export of viral late mRNAs. This protein interacts with the viral E4 Orf6 and four cellular proteins to form an infected-cell-specific E3 ubiquitin ligase. The assembly of this enzyme is required for efficient viral late mRNA export, but neither the relevant substrates nor the cellular pathway that exports viral late mRNAs has been identified. We therefore examined the effects on viral late gene expression of inhibition of the synthesis or activity of the mRNA export receptor Nxf1, which was observed to colocalize with the E1B 55-kDa protein in infected cells. When production of Nxf1 was impaired by using RNA interference, the efficiency of viral late mRNA export was reduced to a corresponding degree. Furthermore, synthesis of a dominant-negative derivative of Nxf1 during the late phase of infection interfered with production of a late structural protein. These observations indicate that the Nxf1 pathway is responsible for export of viral late mRNAs. As the infected-cell-specific E3 ubiquitin ligase targets its known substrates for proteasomal degradation, we compared the concentrations of several components of this pathway (Nxf1, Thox1, and Thoc4) in infected cells that did or did not contain this enzyme. Although the concentration of a well-established substrate, Mre11, decreased significantly in cells infected by adenovirus type 5 (Ad5), but not in those infected by the E1B 55-kDa protein-null mutant Hr6, no E1B 55-kDa protein-dependent degradation of the Nxf1 pathway proteins was observed.

RNA-binding motif protein 24 regulates myogenin expression and promotes myogenic differentiation

The formation of muscle fibers involves sequential expression of many proteins that regulate key steps during myoblast-to-myotube transition. Myogenin is a major player in the initiation and maintenance of myogenic differentiation in a mouse myoblast cell line, C2C12. RNA-binding proteins bind to specific target RNA sequences and regulate gene expression in a post-transcriptional manner. This study demonstrates that RNA-binding motif protein 24 (Rbm24) interacts with the 3'-untranslated region of myogenin mRNA and affects its half-life in C2C12 myogenesis. Knockdown of Rbm24 expression by RNA interference significantly decreased myogenin expression associated with the inhibition of myogenesis. In contrast, the overexpression of Rbm24 by stable transfection of a plasmid increased myogenin expression and had a positive effect on myogenic differentiation. Ectopic expression of myogenin was also able to restore myogenic differentiation in Rbm24-knockdown cells. Together, our results suggest that Rbm24 binds to myogenin mRNA and regulates its stability in C2C12 cells. Rbm24 plays a crucial role in myogenic differentiation at least in part through a myogenin-dependent post-transcriptional regulatory pathway.

Nucleocytoplasmic mRNP export is an integral part of mRNP biogenesis

Nucleocytoplasmic export and biogenesis of mRNPs are closely coupled. At the gene, concomitant with synthesis of the pre-mRNA, the transcription machinery, hnRNP proteins, processing, quality control and export machineries cooperate to release processed and export competent mRNPs. After diffusion through the interchromatin space, the mRNPs are translocated through the nuclear pore complex and released into the cytoplasm. At the nuclear pore complex, defined compositional and conformational changes are triggered, but specific cotranscriptionally added components are retained in the mRNP and subsequently influence the cytoplasmic fate of the mRNP. Processes taking place at the gene locus and at the nuclear pore complex are crucial for integrating export as an essential part of gene expression. Spatial, temporal and structural aspects of these events have been highlighted in analyses of the Balbiani ring genes.

Cap and cap-binding proteins in the control of gene expression

The 5' mRNA cap structure is essential for efficient gene expression from yeast to human. It plays a critical role in all aspects of the life cycle of an mRNA molecule. Capping occurs co-transcriptionally on the nascent pre-mRNA as it emerges from the RNA exit channel of RNA polymerase II. The cap structure protects mRNAs from degradation by exonucleases and promotes transcription, polyadenylation, splicing, and nuclear export of mRNA and U-rich, capped snRNAs. In addition, the cap structure is required for the optimal translation of the vast majority of cellular mRNAs, and it also plays a prominent role in the expression of eukaryotic, viral, and parasite mRNAs. Cap-binding proteins specifically bind to the cap structure and mediate its functions in the cell. Two major cellular cap-binding proteins have been described to date: eukaryotic translation initiation factor 4E (eIF4E) in the cytoplasm and nuclear cap binding complex (nCBC), a nuclear complex consisting of a cap-binding subunit cap-binding protein 20 (CBP 20) and an auxiliary protein cap-binding protein 80 (CBP 80). nCBC plays an important role in various aspects of nuclear mRNA metabolism such as pre-mRNA splicing and nuclear export, whereas eIF4E acts primarily as a facilitator of mRNA translation. In this review, we highlight recent findings on the role of the cap structure and cap-binding proteins in the regulation of gene expression. We also describe emerging regulatory pathways that control mRNA capping and cap-binding proteins in the cell.

Turnover of AU-rich-containing mRNAs during stress: a matter of survival

Cells undergo various adaptive measures in response to stress. Among these are specific changes in the posttranscriptional regulation of various genes. In particular, the turnover of mRNA is modified to either increase or decrease the abundance of certain target messages. Some of the best-studied mRNAs that are affected by stress are those that contain adenine/uridine-rich elements (AREs) in their 3'-untranslated regions. ARE-containing mRNAs are involved in many important cellular processes and are normally labile, but in response to stress they are differentially regulated through the concerted efforts of ARE-binding proteins (AUBPs) such as HuR, AUF1, tristetraprolin, BRF1, and KSRP, along with microRNA-mediated effects. Additionally, the fate of ARE-containing mRNAs is modified by inducing their localization to stress granules or mRNA processing bodies. Coordination of these various mechanisms controls the turnover of ARE-containing mRNAs, and thereby enables proper responses to cellular stress. In this review, we discuss how AUBPs regulate their target mRNAs in response to stress, along with the involvement of cytoplasmic granules in this process.

ICAM-1 cytoplasmic tail regulates endothelial glutathione synthesis through a NOX4/PI3-kinase-dependent pathway

We previously reported that ICAM-1 expression modulates endothelial intracellular glutathione (GSH) metabolism through unknown mechanisms. Here we report that the cytoplasmic tail of ICAM-1 is critically involved in governing intracellular GSH production. Peptides containing the antennapedia cell-permeative sequence (AP) or an AP peptide linked to the transmembrane and cytosolic tail of ICAM-1 (AP-ICAM) were synthesized and used to measure alterations in redox status in cultured endothelial cells and determine their biological effect. Treatment with AP-ICAM significantly increased GSH concentrations and glutamate-cysteine ligase (GCL) activity over time. Measuring reactive oxygen species (ROS) production with DCF revealed a rapid increase in ROS generation after AP-ICAM treatment. Measurement of superoxide production with hydroethidium revealed biphasic production at 30 min and 6h after treatment with AP-ICAM. Apocynin, DPI, catalase, or SOD attenuated AP-ICAM-dependent ROS production, GCL activity, and GSH production, implicating superoxide production and dismutation to peroxide. Consistent with these findings, NOX4 siRNA knockdown blocked AP-ICAM peptide increases in GSH or GCL activity, demonstrating the importance of NADPH oxidase. Last, inhibition of PI3-kinase activity with LY 294002 or wortmannin blocked AP-ICAM GSH induction and ROS production. These data reveal that the ICAM-1 cytoplasmic tail regulates production of endothelial GSH through a NOX4/PI3-kinase-dependent redox-sensitive pathway.

Posttranscriptional regulation of cancer traits by HuR

Cancer-related gene expression programs are strongly influenced by posttranscriptional mechanisms. The RNA-binding protein HuR is highly abundant in many cancers. Numerous HuR-regulated mRNAs encode proteins implicated in carcinogenesis. Here, we review the collections of HuR target mRNAs that encode proteins responsible for implementing five major cancer traits. By interacting with specific mRNA subsets, HuR enhances the levels of proteins that (1) promote cell proliferation, (2) increase cell survival, (3) elevate local angiogenesis, (4) help the cancer cell evade immune recognition, and (5) facilitate cancer cell invasion and metastasis. We propose that HuR exerts a tumorigenic function by enabling these cancer phenotypes. We discuss evidence that links HuR to several specific cancers and suggests its potential usefulness in cancer diagnosis, prognosis, and therapy.

Heterogeneous nuclear ribonucleoproteins (hnRNPs) in cellular processes: Focus on hnRNP E1's multifunctional regulatory roles

Heterogeneous nuclear ribonucleoproteins (hnRNPs) comprise a family of RNA-binding proteins. The complexity and diversity associated with the hnRNPs render them multifunctional, involved not only in processing heterogeneous nuclear RNAs (hnRNAs) into mature mRNAs, but also acting as trans-factors in regulating gene expression. Heterogeneous nuclear ribonucleoprotein E1 (hnRNP E1), a subgroup of hnRNPs, is a KH-triple repeat containing RNA-binding protein. It is encoded by an intronless gene arising from hnRNP E2 through a retrotransposition event. hnRNP E1 is ubiquitously expressed and functions in regulating major steps of gene expression, including pre-mRNA processing, mRNA stability, and translation. Given its wide-ranging functions in the nucleus and cytoplasm and interaction with multiple proteins, we propose a post-transcriptional regulon model that explains hnRNP E1's widespread functional diversity.

Regulation of HuR by DNA Damage Response Kinases

As many DNA-damaging conditions repress transcription, posttranscriptional processes critically influence gene expression during the genotoxic stress response. The RNA-binding protein HuR robustly influences gene expression following DNA damage. HuR function is controlled in two principal ways: (1) by mobilizing HuR from the nucleus to the cytoplasm, where it modulates the stability and translation of target mRNAs and (2) by altering its association with target mRNAs. Here, we review evidence that two main effectors of ataxia-telangiectasia-mutated/ATM- and Rad3-related (ATM/ATR), the checkpoint kinases Chk1 and Chk2, jointly influence HuR function. Chk1 affects HuR localization by phosphorylating (hence inactivating) Cdk1, a kinase that phosphorylates HuR and thereby blocks HuR's cytoplasmic export. Chk2 modulates HuR binding to target mRNAs by phosphorylating HuR's RNA-recognition motifs (RRM1 and RRM2). We discuss how HuR phosphorylation by kinases including Chk1/Cdk1 and Chk2 impacts upon gene expression patterns, cell proliferation, and survival following genotoxic injury.

Protein kinase RNA/FADD/caspase-8 pathway mediates the proapoptotic activity of the RNA-binding protein human antigen R (HuR)

The RNA-binding protein human antigen R (HuR) has been implicated in apoptosis in multiple ways. Several studies have shown that in response to a variety of stresses HuR promotes the expression of proapoptotic mRNAs, whereas others reported its regulatory effect on antiapoptotic messages. We recently showed that in response to severe stress, HuR is cleaved to generate two cleavage products (CPs), HuR-CP1 (24 kDa) and HuR-CP2 (8 kDa), by which it promotes apoptotic cell death. Here, we show that this cleavage event is dependent on protein kinase RNA (PKR). Surprisingly, although in response to the apoptotic inducer staurosporine PKR itself is not phosphorylated, PKR triggers the cleavage of HuR via its downstream effector FADD that in turn activates the caspase-8/caspase-3 pathway. This effect, however, does not require the phosphorylation of the eukaryotic translation initiation factor 2alpha. Additionally, we observed that these HuR-CPs are sufficient to trigger cell death in the absence of activation of the PKR pathway. Therefore, our results support a model whereby in response to lethal stress, PKR, without being phosphorylated, activates the FADD/caspase-8/caspase-3 pathway to trigger HuR cleavage, and the HuR-CPs are then capable of promoting apoptosis.

Dynamic nucleocytoplasmic shuttling of an Arabidopsis SR splicing factor: role of the RNA-binding domains

Serine/arginine-rich (SR) proteins are essential nuclear-localized splicing factors. We have investigated the dynamic subcellular distribution of the Arabidopsis (Arabidopsis thaliana) RSZp22 protein, a homolog of the human 9G8 SR factor. Little is known about the determinants underlying the control of plant SR protein dynamics, and so far most studies relied on ectopic transient overexpression. Here, we provide a detailed analysis of the RSZp22 expression profile and describe its nucleocytoplasmic shuttling properties in specific cell types. Comparison of transient ectopic- and stable tissue-specific expression highlights the advantages of both approaches for nuclear protein dynamic studies. By site-directed mutagenesis of RSZp22 RNA-binding sequences, we show that functional RNA recognition motif RNP1 and zinc-knuckle are dispensable for the exclusive protein nuclear localization and speckle-like distribution. Fluorescence resonance energy transfer imaging also revealed that these motifs are implicated in RSZp22 molecular interactions. Furthermore, the RNA-binding motif mutants are defective for their export through the CRM1/XPO1/Exportin-1 receptor pathway but retain nucleocytoplasmic mobility. Moreover, our data suggest that CRM1 is a putative export receptor for mRNPs in plants.

HuR knockdown changes the oncogenic potential of oral cancer cells

HuR binds to AU-rich element-containing mRNA to protect them from rapid degradation. Here, we show that knockdown of HuR changes the oncogenic properties of oral cancer cells. Oral squamous cell carcinoma cell lines, HSC-3 and Ca9.22, which express HuR protein and cytoplasmic AU-rich element mRNA more abundantly than normal cells, were subjected to HuR knockdown. In the HuR-knockdown cancer cells, the cytoplasmic expression of c-fos, c-myc, and COX-2 mRNAs was inhibited compared with those in cells that had been transfected with a control small interfering RNA, and the half-lives of these mRNAs were shorter than those of their counterparts in the control cells. HuR-knockdown cells failed to make colonies in soft agar, suggesting that the cells had lost their ability for anchorage-independent cell growth. Additionally, the motile and invasive activities of the cells decreased remarkably by HuR knockdown. Furthermore, the expression of cell cycle-related proteins, such as cyclin A, cyclin B1, cyclin D1, and cyclin-dependent kinase 1, was reduced in HuR-knockdown cancer cells, and HuR bound to cdk1 mRNA to stabilize it. These findings suggest that HuR knockdown changes the features of oral cancer cells, at least in part, by affecting their cell cycle and shows potential as an effective therapeutic approach.

RNA surveillance: molecular approaches in transcript quality control and their implications in clinical diseases

Production of mature mRNAs that encode functional proteins involves highly complex pathways of synthesis, processing and surveillance. At numerous steps during the maturation process, the mRNA transcript undergoes scrutiny by cellular quality control machinery. This extensive RNA surveillance ensures that only correctly processed mature mRNAs are translated and precludes production of aberrant transcripts that could encode mutant or possibly deleterious proteins. Recent advances in elucidating the molecular mechanisms of mRNA processing have demonstrated the existence of an integrated network of events, and have revealed that a variety of human diseases are caused by disturbances in the well-coordinated molecular equilibrium of these events. From a medical perspective, both loss and gain of function are relevant, and a considerable number of different diseases exemplify the importance of the mechanistic function of RNA surveillance in a cell. Here, mechanistic hallmarks of mRNA processing steps are reviewed, highlighting the medical relevance of their deregulation and how the understanding of such mechanisms can contribute to the development of therapeutic strategies.

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.

Nucleocytoplasmic shuttling of p62/SQSTM1 and its role in recruitment of nuclear polyubiquitinated proteins to promyelocytic leukemia bodies

p62, also known as sequestosome1 (SQSTM1), A170, or ZIP, is a multifunctional protein implicated in several signal transduction pathways. p62 is induced by various forms of cellular stress, is degraded by autophagy, and acts as a cargo receptor for autophagic degradation of ubiquitinated targets. It is also suggested to shuttle ubiquitinated proteins for proteasomal degradation. p62 is commonly found in cytosolic protein inclusions in patients with protein aggregopathies, it is up-regulated in several forms of human tumors, and mutations in the gene are linked to classical adult onset Paget disease of the bone. To this end, p62 has generally been considered to be a cytosolic protein, and little attention has been paid to possible nuclear roles of this protein. Here, we present evidence that p62 shuttles continuously between nuclear and cytosolic compartments at a high rate. The protein is also found in nuclear promyelocytic leukemia bodies. We show that p62 contains two nuclear localization signals and a nuclear export signal. Our data suggest that the nucleocytoplasmic shuttling of p62 is modulated by phosphorylations at or near the most important nuclear localization signal, NLS2. The aggregation of p62 in cytosolic bodies also regulates the transport of p62 between the compartments. We found p62 to be essential for accumulation of polyubiquitinated proteins in promyelocytic leukemia bodies upon inhibition of nuclear protein export. Furthermore, p62 contributed to the assembly of proteasome-containing degradative compartments in the vicinity of nuclear aggregates containing polyglutamine-expanded Ataxin1Q84 and to the degradation of Ataxin1Q84.

Cytoplasmic expression of HuR is related to cyclooxygenase-2 expression in colon cancer

PURPOSE

HuR, human family embryonic-lethal abnormal vision-like protein, can bind to mRNA and stabilizes the nucleic acid in the cytoplasm, resulting in more efficient translation. HuR is predominantly present in the nucleus and shuttles between the nucleus and cytoplasm. HuR stabilizes cyclooxygenase-2 (Cox-2) mRNA in several cancers, including breast, stomach, lung and brain cancer.

MATERIALS AND METHODS

We investigated the expression and cellular location of HuR, as well as evaluated Cox-2 expression in 79 colorectal cancer patients with the use of immunohistochemical methods. The biological implications of HuR localization and Cox-2 expression in colorectal carcinoma were evaluated.

RESULTS

Nuclear HuR expression was observed in 59 (74.7%) tumors and cytoplasmic HuR expression was seen in 25 (31.6%) tumors. Cox-2 immunoreactivity was noted in 42 (53%) tumors. The expression of cytoplasmic HuR was significantly associated with Cox-2 expression (p=0.004). Cytoplasmic expression of HuR showed a correlation with lymphatic invasion (p=0.025) and the presence of a lymph node metastasis (p=0.027). The presence of nuclear HuR showed no correlation with Cox-2 expression or any other of the clinicopathological parameters that were examined.

CONCLUSION

These results suggest that cytoplasmic translocation of HuR is associated with Cox-2 expression for some colorectal carcinomas.

Control of thymic T cell maturation, deletion and egress by the RNA-binding protein HuR

HuR emerged as a posttranscriptional regulator of mRNAs involved in cellular control, stress, and immunity but its role in governing such responses remains elusive. In this study, we assessed HuR's role in the staged progression of thymic T cell differentiation by means of its genetic ablation. Mice with an early deletion of HuR in thymocytes possess enlarged thymi but display a substantial loss of peripheral T cells. We show that this discordant phenotype related to specific defects in thymic cellular processes, which demonstrated HuR's involvement in: 1) intrinsic checkpoint signals suppressing the cell cycle of immature thymocyte progenitors, 2) TCR and antigenic signals promoting the activation and positive selection of mature thymocytes, 3) antigenic and death-receptor signals promoting thymocyte deletion, and 4) chemokine signals driving the egress of postselection thymocytes to the periphery. The cellular consequences of HuR's dysfunction were underlined by the aberrant expression of selective cell cycle regulators, TCR, and death-receptor signaling components. Our studies reveal the signal-dependent context of HuR's cellular activities in thymocytes and its importance in the generation of a physiological T cell pool.

HuR is exported to the cytoplasm in oral cancer cells in a different manner from that of normal cells

HuR, a ubiquitously expressed member of the Hu protein family that binds and stabilizes an AU-rich element (ARE)-containing mRNAs, is known to shuttle between the nucleus and the cytoplasm via several export pathways. When normal cells were treated with heat shock, HuR was exported to the cytoplasm in a chromosome maintenance region 1 (CRM1)-dependent manner. However, in this study, we demonstrate that HuR is exported to the cytoplasm in oral cancer cells even if the cells were treated with the inhibitor of the CRM1-independent export pathway. Immunohistochemical and biochemical analyses showed that HuR existed in both the cytoplasm and the nucleus in oral cancer cells, such as HSC-3 and Ca9.22, but existed entirely inside the nucleus in normal cells. AU-rich element-mRNAs were also exported to the cytoplasm and stabilised in the oral cancer cells, which were inhibited by HuR knockdown. This export of HuR was not affected by at least 7 h of treatment of leptomycin B (LMB), which is an inhibitor of the CRM1-dependent export pathway. These findings suggest that HuR is exported to the cytoplasm in oral carcinoma cells in a different manner from that of normal cells, and is likely to occur through the perturbation of a normal export pathway.

The RNA-binding protein HuR regulates DNA methylation through stabilization of DNMT3b mRNA

The molecular basis underlying the aberrant DNA-methylation patterns in human cancer is largely unknown. Altered DNA methyltransferase (DNMT) activity is believed to contribute, as DNMT expression levels increase during tumorigenesis. Here, we present evidence that the expression of DNMT3b is post-transcriptionally regulated by HuR, an RNA-binding protein that stabilizes and/or modulates the translation of target mRNAs. The presence of a putative HuR-recognition motif in the DNMT3b 3'UTR prompted studies to investigate if this transcript associated with HuR. The interaction between HuR and DNMT3b mRNA was studied by immunoprecipitation of endogenous HuR ribonucleoprotein complexes followed by RT-qPCR detection of DNMT3b mRNA, and by in vitro pulldown of biotinylated DNMT3b RNAs followed by western blotting detection of HuR. These studies revealed that binding of HuR stabilized the DNMT3b mRNA and increased DNMT3b expression. Unexpectedly, cisplatin treatment triggered the dissociation of the [HuR-DNMT3b mRNA] complex, in turn promoting DNMT3b mRNA decay, decreasing DNMT3b abundance, and lowering the methylation of repeated sequences and global DNA methylation. In summary, our data identify DNMT3b mRNA as a novel HuR target, present evidence that HuR affects DNMT3b expression levels post-transcriptionally, and reveal the functional consequences of the HuR-regulated DNMT3b upon DNA methylation patterns.

Temporally regulated traffic of HuR and its associated ARE-containing mRNAs from the chromatoid body to polysomes during mouse spermatogenesis

Background

In mammals, a temporal disconnection between mRNA transcription and protein synthesis occurs during late steps of germ cell differentiation, in contrast to most somatic tissues where transcription and translation are closely linked. Indeed, during late stages of spermatogenesis, protein synthesis relies on the appropriate storage of translationally inactive mRNAs in transcriptionally silent spermatids. The factors and cellular compartments regulating mRNA storage and the timing of their translation are still poorly understood. The chromatoid body (CB), that shares components with the P. bodies found in somatic cells, has recently been proposed to be a site of mRNA processing. Here, we describe a new component of the CB, the RNA binding protein HuR, known in somatic cells to control the stability/translation of AU-rich containing mRNAs (ARE-mRNAs).

Methodology/Principal Findings

Using a combination of cell imagery and sucrose gradient fractionation, we show that HuR localization is highly dynamic during spermatid differentiation. First, in early round spermatids, HuR colocalizes with the Mouse Vasa Homolog, MVH, a marker of the CB. As spermatids differentiate, HuR exits the CB and concomitantly associates with polysomes. Using computational analyses, we identified two testis ARE-containing mRNAs, Brd2 and GCNF that are bound by HuR and MVH. We show that these target ARE-mRNAs follow HuR trafficking, accumulating successively in the CB, where they are translationally silent, and in polysomes during spermatid differentiation.

Conclusions/Significance

Our results reveal a temporal regulation of HuR trafficking together with its target mRNAs from the CB to polysomes as spermatids differentiate. They strongly suggest that through the transport of ARE-mRNAs from the CB to polysomes, HuR controls the appropriate timing of ARE-mRNA translation. HuR might represent a major post-transcriptional regulator, by promoting mRNA storage and then translation, during male germ cell differentiation.

Ubiquitin-mediated proteolysis of HuR by heat shock

The RNA-binding protein HuR regulates the stability and translation of numerous mRNAs encoding stress-response and proliferative proteins. Although its post-transcriptional influence has been linked primarily to its cytoplasmic translocation, here we report that moderate heat shock (HS) potently reduces HuR levels, thereby altering the expression of HuR target mRNAs. HS did not change HuR mRNA levels or de novo translation, but instead reduced HuR protein stability. Supporting the involvement of the ubiquitin-proteasome system in this process were results showing that (1) HuR was ubiquitinated in vitro and in intact cells, (2) proteasome inhibition increased HuR abundance after HS, and (3) the HuR kinase checkpoint kinase 2 protected against the loss of HuR by HS. Within a central, HS-labile approximately 110-amino-acid region, K182 was found to be essential for HuR ubiquitination and proteolysis as mutant HuR(K182R) was left virtually unubiquitinated and was refractory to HS-triggered degradation. Our findings reveal that HS transiently lowers HuR by proteolysis linked to K182 ubiquitination and that HuR reduction enhances cell survival following HS.

The RNA-binding protein Elavl1/HuR is essential for placental branching morphogenesis and embryonic development

HuR is an RNA-binding protein implicated in a diverse array of pathophysiological processes due to its effects on the posttranscriptional regulation of AU- and U-rich mRNAs. Here we reveal HuR's requirement in embryonic development through its genetic ablation. Obligatory HuR-null embryos exhibited a stage retardation phenotype and failed to survive beyond midgestation. By means of conditional transgenesis, we restricted HuR's mutation in either embryonic or endothelial compartments to demonstrate that embryonic lethality is consequent to defects in extraembryonic placenta. HuR's absence impaired the invagination of allantoic capillaries into the chorionic trophoblast layer and the differentiation of syncytiotrophoblast cells that control the morphogenesis and vascularization of the placental labyrinth and fetal support. HuR-null embryos rescued from these placental defects proceeded to subsequent developmental stages but displayed defects in skeletal ossification, fusions in limb elements, and asplenia. By coupling gene expression measurements, data meta-analysis, and HuR-RNA association assays, we identified transcription and growth factor mRNAs controlled by HuR, primarily at the posttranscriptional level, to guide morphogenesis, specification, and patterning. Collectively, our data demonstrate the dominant role of HuR in organizing gene expression programs guiding placental labyrinth morphogenesis, skeletal specification patterns, and splenic ontogeny.

Enhancement of p53 expression in keratinocytes by the bioflavonoid apigenin is associated with RNA-binding protein HuR

We have reported previously that apigenin, a naturally occurring nonmutagenic flavonoid, increased wild-type p53 protein expression in the mouse keratinocyte 308 cell line by a mechanism involving p53 protein stabilization. Here we further demonstrated that the increase in p53 protein level induced by apigenin treatment of 308 keratinoyctes was not the result of enhanced transcription, mRNA stabilization or cytoplasmic export of p53 mRNA. Instead, biosynthetic labeling showed that apigenin increased nascent p53 protein synthesis by enhancing p53 translation. The AU-rich element (ARE) within the 3'-untranslated region (UTR) of p53 mRNA was found to be responsible for apigenin's ability to increase p53 translation, as demonstrated in studies wherein the 3'-UTR of p53 mRNA containing the ARE was fused downstream of a luciferase reporter gene. Furthermore, apigenin treatment increased the level of association of the RNA binding protein HuR with endogenous p53 mRNA. Apigenin treatment also augmented HuR translocation into the cytoplasm. Overexpression of HuR enhanced apigenin-induced p53 protein expression in 308 keratinocytes, whereas siRNA-mediated HuR reduction suppressed apigenin-induced p53 protein expression and de novo translation of p53. Moreover, apigenin treatment of cells induced p16 protein expression, which in turn was correlated with cytoplasmic localization of HuR induced by apigenin. Overall, these findings indicate that, in addition to modulating p53 protein stability, one of the mechanisms by which apigenin induces p53 protein expression is enhancement of translation through the RNA binding protein HuR.

miRNA regulation of cytokine genes

In this review we discuss specific examples of regulation of cytokine genes and focus on a new mechanism involving post-transcriptional regulation via miRNAs. The post-transcriptional regulation of cytokine genes via the destabilizing activity of AU-rich elements [AREs] and miRNAs is a pre-requisite for regulating the half-life of many cytokines and achieving the temporal and spatial distributions required for regulation of these genes.