Polyamines modulate the subcellular localization of RNA-binding protein HuR through AMP-activated protein kinase-regulated phosphorylation and acetylation of importin alpha1

Stabilization of Cx43 mRNA via RNA-binding protein HuR regulated by polyamines enhances intestinal epithelial barrier function

Gut barrier dysfunction occurs commonly in patients with critical disorders, leading to the translocation of luminal toxic substances and bacteria to the bloodstream. Connexin 43 (Cx43) acts as a gap junction protein and is crucial for intercellular communication and the diffusion of nutrients. The levels of cellular Cx43 are tightly regulated by multiple factors, including polyamines, but the exact mechanism underlying the control of Cx43 expression remains largely unknown. The RNA-binding protein HuR regulates the stability and translation of target mRNAs and is involved in many aspects of intestinal epithelial pathobiology. Here we show that HuR directly bound to Cx43 mRNA via its 3'-untranslated region in intestinal epithelial cells (IECs) and this interaction enhanced Cx43 expression by stabilizing Cx43 mRNA. Depletion of cellular polyamines inhibited the [HuR/Cx43 mRNA] complex and decreased the level of Cx43 protein by destabilizing its mRNA, but these changes were prevented by ectopic overexpression of HuR. Polyamine depletion caused intestinal epithelial barrier dysfunction, which was reversed by ectopic Cx43 overexpression. Moreover, overexpression of checkpoint kinase 2 in polyamine-deficient cells increased the [HuR/Cx43 mRNA] complex, elevated Cx43 levels, and promoted barrier function. These findings indicate that Cx43 mRNA is a novel target of HuR in IECs and that polyamines regulate Cx43 mRNA stability via HuR, thus playing a critical role in the maintenance of intestinal epithelial barrier function.NEW & NOTEWORTHY The current study shows that polyamines stabilize the Cx43 mRNA via HuR, thus enhancing the function of the Cx43-mediated gap junction. These findings suggest that induced Cx43 by HuR plays a critical role in the process by which polyamines regulate intestinal epithelial barrier.

Acetylation regulates the nucleocytoplasmic distribution and oncogenic function of karyopherin alpha 2 in lung adenocarcinoma

Karyopherin subunit alpha 2 (KPNA2, importin α1) is a nucleoplasmic protein responsible for the nuclear import of proteins with classical nuclear localization signals. Aberrant nuclear accumulation of KPNA2 has been observed in numerous cancer tissues. AMP-activated protein kinase (AMPK) is involved in the phosphorylation and acetylation of KPNA2 in enterocytes. However, the impact of these post-translational modifications on modulating the nucleocytoplasmic distribution of KPNA2 and its oncogenic role remain unclear. Unlike nuclear accumulation of wild-type KPNA2, which promoted lung cancer cell migration, KPNA2 Lys22 acetylation-mimicking mutations (K22Q and K22Q/S105A) prevented nuclear localization of KPNA2 and reduced the cell migration ability. Cytosolic KPNA2 K22Q interacted with and restricted the nuclear entry of E2F transcription factor 1 (E2F1), an oncogenic cargo protein of KPNA2, in lung cancer cells. Intriguingly, the AMPK activator EX229 promoted the nuclear export of KPNA2 S105A. However, the CBP/p300 inhibitor CCS-1477 abolished this phenomenon, suggesting that CBP/p300-mediated acetylation of KPNA2 promoted KPNA2 nuclear export in lung cancer cells. Collectively, our findings suggest that the CBP/p300 positively regulates KPNA2 acetylation, which enhances its cytosolic localization and suppresses its oncogenic activity in lung cancer.

Polyamines in Gut Epithelial Renewal and Barrier Function

Polyamines regulate a variety of physiological functions and are involved in pathogenesis of diverse human diseases. The epithelium of the mammalian gut mucosa is a rapidly self-renewing tissue in the body, and its homeostasis is preserved through well-controlled mechanisms. Here, we highlight the roles of cellular polyamines in maintaining the integrity of the gut epithelium, focusing on the emerging evidence of polyamines in the regulation of gut epithelial renewal and barrier function. Gut mucosal growth depends on the available supply of polyamines to the dividing cells in the crypts, and polyamines are also essential for normal gut epithelial barrier function. Polyamines modulate expression of various genes encoding growth-associated proteins and intercellular junctions via distinct mechanisms involving RNA-binding proteins and noncoding RNAs. With the rapid advance of polyamine biology, polyamine metabolism and transport are promising therapeutic targets in our efforts to protect the gut epithelium and barrier function in patients with critical illnesses.

Polyamines: Functions, Metabolism, and Role in Human Disease Management

Putrescine, spermine, and spermidine are the important polyamines (PAs), found in all living organisms. PAs are formed by the decarboxylation of amino acids, and they facilitate cell growth and development via different cellular responses. PAs are the integrated part of the cellular and genetic metabolism and help in transcription, translation, signaling, and post-translational modifications. At the cellular level, PA concentration may influence the condition of various diseases in the body. For instance, a high PA level is detrimental to patients suffering from aging, cognitive impairment, and cancer. The levels of PAs decline with age in humans, which is associated with different health disorders. On the other hand, PAs reduce the risk of many cardiovascular diseases and increase longevity, when taken in an optimum quantity. Therefore, a controlled diet is an easy way to maintain the level of PAs in the body. Based on the nutritional intake of PAs, healthy cell functioning can be maintained. Moreover, several diseases can also be controlled to a higher extend via maintaining the metabolism of PAs. The present review discusses the types, important functions, and metabolism of PAs in humans. It also highlights the nutritional role of PAs in the prevention of various diseases.

Diverse Functions of Polyamines in Virus Infection

As obligate intracellular parasites, viruses rely on host cells for the building blocks of progeny viruses. Metabolites such as amino acids, nucleotides, and lipids are central to viral proteins, genomes, and envelopes, and the availability of these molecules can restrict or promote infection. Polyamines, comprised of putrescine, spermidine, and spermine in mammalian cells, are also critical for virus infection. Polyamines are small, positively charged molecules that function in transcription, translation, and cell cycling. Initial work on the function of polyamines in bacteriophage infection illuminated these molecules as critical to virus infection. In the decades since early virus-polyamine descriptions, work on diverse viruses continues to highlight a role for polyamines in viral processes, including genome packaging and viral enzymatic activity. On the host side, polyamines function in the response to virus infection. Thus, viruses and hosts compete for polyamines, which are a critical resource for both. Pharmacologically targeting polyamines, tipping the balance to favor the host and restrict virus replication, holds significant promise as a broad-spectrum antiviral strategy.

Interaction between HuR and circPABPN1 Modulates Autophagy in the Intestinal Epithelium by Altering ATG16L1 Translation

Intestinal epithelial autophagy is crucial for host defense against invasive pathogens, and defects in this process occur frequently in patients with inflammatory bowel disease (IBD) and other mucosal disorders, but the exact mechanism that activates autophagy is poorly defined. Here, we investigated the role of RNA-binding protein HuR (human antigen R) in the posttranscriptional control of autophagy-related genes (ATGs) in the intestinal epithelium. We found that targeted deletion of HuR in intestinal epithelial cells (IECs) specifically decreased the levels of ATG16L1 in the intestinal mucosa. Intestinal mucosa from patients with IBD exhibited reduced levels of both HuR and ATG16L1. HuR directly interacted with Atg16l1 mRNA via its 3' untranslated region and enhanced ATG16L1 translation, without affecting Atg16l1 mRNA stability. Circular RNA circPABPN1 blocked HuR binding to Atg16l1 mRNA and lowered ATG16L1 production. HuR silencing in cultured IECs also prevented rapamycin-induced autophagy, which was abolished by overexpressing ATG16L1. These findings indicate that HuR regulates autophagy by modulating ATG16L1 translation via interaction with circPABPN1 in the intestinal epithelium.

Inhibition of Karyopherin-α2 Augments Radiation-Induced Cell Death by Perturbing BRCA1-Mediated DNA Repair

Ionizing radiation (IR) has been widely used in the treatment of cancer. Radiation-induced DNA damage triggers the DNA damage response (DDR), which can confer radioresistance and early local recurrence by activating DNA repair pathways. Since karyopherin-α2 (KPNA2), playing an important role in nucleocytoplasmic transport, was significantly increased by IR in our previous study, we aimed to determine the function of KPNA2 with regard to DDR. Exposure to radiation upregulated KPNA2 expression in human colorectal cancer HT29 and HCT116 cells and breast carcinoma MDA-MB-231 cells together with the increased expression of DNA repair protein BRCA1. The knockdown of KPNA2 effectively increased apoptotic cell death via inhibition of BRCA1 nuclear import following IR. Therefore, we propose that KPNA2 is a potential target for overcoming radioresistance via interruption to DDR.

RNA-Binding Protein HuR Regulates Rac1 Nucleocytoplasmic Shuttling Through Nucleophosmin in the Intestinal Epithelium

BACKGROUND & AIMS

The mammalian intestinal epithelium is a rapidly self-renewing tissue in the body, and its homeostasis is tightly regulated via well-controlled mechanisms. The RNA-binding protein HuR is essential for maintaining gut epithelial integrity, and targeted deletion of HuR in intestinal epithelial cells (IECs) disrupts mucosal regeneration and delays repair after injury. Here, we defined the role of HuR in regulating subcellular distribution of small guanosine triphosphatase Rac1 and investigated the implication of nucleophosmin (NPM) as a molecular chaperone in this process.

METHODS

Studies were conducted in intestinal epithelial tissue-specific HuR knockout (IE-HuR^-/-) mice and cultured IEC-6 cells, derived from rat small intestinal crypts. Functions of HuR and NPM in vitro were investigated via their gene silencing and overexpression.

RESULTS

The abundance of cytoplasmic Rac1 in the small intestinal mucosa increased significantly in IE-HuR^-/- mice, although HuR deletion did not alter total Rac1 levels. HuR silencing in cultured IECs also increased the cytoplasmic Rac1 levels, without an effect on whole-cell Rac1 content. In addition, HuR deficiency in the intestinal epithelium decreased the levels of NPM in IE-HuR^-/- mice and cultured IECs. NPM physically interacted with Rac1 and formed the NPM/Rac1 complex. NPM silencing decreased the NPM/Rac1 association and inhibited nuclear accumulation of Rac1, along with an increase in cytoplasmic abundances of Rac1. In contrast, ectopically expressed NPM enhanced Rac1 nuclear translocation and restored Rac1 subcellular localization to near normal in HuR-deficient cells.

CONCLUSIONS

These results indicate that HuR regulates Rac1 nucleocytoplasmic shuttling in the intestinal epithelium by altering NPM expression.

Complex HuR function in pancreatic cancer cells

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

Autophagy Stimulus Promotes Early HuR Protein Activation and p62/SQSTM1 Protein Synthesis in ARPE-19 Cells by Triggering Erk1/2, p38MAPK, and JNK Kinase Pathways

RNA-binding protein dysregulation and altered expression of proteins involved in the autophagy/proteasome pathway play a role in many neurodegenerative disease onset/progression, including age-related macular degeneration (AMD). HuR/ELAVL1 is a master regulator of gene expression in human physiopathology. In ARPE-19 cells exposed to the proteasomal inhibitor MG132, HuR positively affects at posttranscriptional level p62 expression, a stress response gene involved in protein aggregate clearance with a role in AMD. Here, we studied the early effects of the proautophagy AICAR + MG132 cotreatment on the HuR-p62 pathway. We treated ARPE-19 cells with Erk1/2, AMPK, p38^MAPK, PKC, and JNK kinase inhibitors in the presence of AICAR + MG132 and evaluated HuR localization/phosphorylation and p62 expression. Two-hour AICAR + MG132 induces both HuR cytoplasmic translocation and threonine phosphorylation via the Erk1/2 pathway. In these conditions, p62 mRNA is loaded on polysomes and its translation in de novo protein is favored. Additionally, for the first time, we report that JNK can phosphorylate HuR, however, without modulating its localization. Our study supports HuR's role as an upstream regulator of p62 expression in ARPE-19 cells, helps to understand better the early events in response to a proautophagy stimulus, and suggests that modulation of the autophagy-regulating kinases as potential therapeutic targets for AMD may be relevant.

Acetylation of MKL1 by PCAF regulates pro-inflammatory transcription

Inflammation is considered a fundamental host defense mechanism and, when aberrantly activated, contributes to a host of human diseases. Previously we have reported that the transcriptional regulator megakaryocytic leukemia 1 (MKL1) plays a role programming cellular inflammatory response by modulating NF-κB activity. Here we report that MKL1 was acetylated in vivo and pro-inflammatory stimuli (TNF-α and LPS) augmented MKL1 acetylation accompanying increased MKL1 binding to NF-κB target promoters. Further analysis revealed that the lysine acetyltransferase PCAF mediated MKL1 acetylation: TNF-α and LPS promoted the interaction between MKL1 and PCAF whereas depletion of PCAF abrogated the induction of MKL1 acetylation by TNF-α and LPS. Acetylation of MKL1 was necessary for MKL1 to activate the transcription of pro-inflammatory genes because mutation of four conserved lysine residues in MKL1 attenuated its capacity as a trans-activator of NF-κB target genes. Mechanistically, MKL1 acetylation served to promote MKL1 nuclear enrichment, to enhance the MKL1-NF-κB interaction, and to stabilize the binding of MKL1 on target promoters. In conclusion, our data unveil an important pathway that contributes to the transcriptional regulation of inflammatory response.

Targeting deregulated AMPK/mTORC1 pathways improves muscle function in myotonic dystrophy type I

Myotonic dystrophy type I (DM1) is a disabling multisystemic disease that predominantly affects skeletal muscle. It is caused by expanded CTG repeats in the 3'-UTR of the dystrophia myotonica protein kinase (DMPK) gene. RNA hairpins formed by elongated DMPK transcripts sequester RNA-binding proteins, leading to mis-splicing of numerous pre-mRNAs. Here, we have investigated whether DM1-associated muscle pathology is related to deregulation of central metabolic pathways, which may identify potential therapeutic targets for the disease. In a well-characterized mouse model for DM1 (HSALR mice), activation of AMPK signaling in muscle was impaired under starved conditions, while mTORC1 signaling remained active. In parallel, autophagic flux was perturbed in HSALR muscle and in cultured human DM1 myotubes. Pharmacological approaches targeting AMPK/mTORC1 signaling greatly ameliorated muscle function in HSALR mice. AICAR, an AMPK activator, led to a strong reduction of myotonia, which was accompanied by partial correction of misregulated alternative splicing. Rapamycin, an mTORC1 inhibitor, improved muscle relaxation and increased muscle force in HSALR mice without affecting splicing. These findings highlight the involvement of AMPK/mTORC1 deregulation in DM1 muscle pathophysiology and may open potential avenues for the treatment of this disease.

Transcriptional regulation of importin-α1 by JunD modulates subcellular localization of RNA-binding protein HuR in intestinal epithelial cells

The RNA-binding protein HuR is crucial for normal intestinal mucosal regeneration by modulating the stability and translation of target mRNAs, but the exact mechanism underlying HuR trafficking between the cytoplasm and nucleus remains largely unknown. Here we report a novel function of transcription factor JunD in the regulation of HuR subcellular localization through the control of importin-α₁ expression in intestinal epithelial cells (IECs). Ectopically expressed JunD specifically inhibited importin-α₁ at the transcription level, and this repression is mediated via interaction with CREB-binding site that was located at the proximal region of importin-α₁ promoter. Reduction in the levels of importin-α₁ by JunD increased cytoplasmic levels of HuR, although it failed to alter whole cell HuR levels. Increased levels of endogenous JunD by depleting cellular polyamines also inhibited importin-α₁ expression and increased cytoplasmic HuR levels, whereas JunD silencing rescued importin-α₁ expression and enhanced HuR nuclear translocation in polyamine-deficient cells. Moreover, importin-α₁ silencing protected IECs against apoptosis, which was prevented by HuR silencing. These results indicate that JunD regulates HuR subcellular distribution by downregulating importin-α₁, thus contributing to the maintenance of gut epithelium homeostasis.

Genome-scale functional analysis of the human genes modulating p53 activity by regulating MDM2 expression in a p53-independent manner

MDM2, a critical negative regulator of p53, is often overexpressed in leukemia, but few p53 mutations are found, suggesting that p53-independent MDM2 expression occurs due to alterations in MDM2 upstream regulators. In this study, a high MDM2 transcription level was observed (41.17%) regardless of p53 expression in patient with acute myeloid leukemia (AML). Therefore, we performed genome-scale functional screening of the human genes modulating MDM2 expression in a p53-independent manner. We searched co-expression profiles of genes showing a positive or negative pattern with MDM2 expression in a DNA microarray database, selected1089 links, and composed a screening library of 368 genes. Using MDM2 P1 and P2 promoter-reporter systems, we screened clones regulating MDM2 transcriptions in a p53-independent manner by overexpression. Nine clones from the screening library showed enhanced MDM2 promoter activity and MDM2 expression in p53-deficient HCT116 cells. Among them, six clones, including NTRK2, GNA15, SFRS2, EIF5A, ELAVL1, and YWHAB mediated MAPK signaling for expressing MDM2. These results indicate that p53-independent upregulation of MDM2 by increasing selected clones may lead to oncogenesis in AML and that MDM2-modulating genes are novel potential targets for AML treatment.

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

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

Regulation of AU-Rich Element RNA Binding Proteins by Phosphorylation and the Prolyl Isomerase Pin1

The accumulation of 3' untranslated region (3'-UTR), AU-rich element (ARE) containing mRNAs, are predominantly controlled at the post-transcriptional level. Regulation appears to rely on a variable and dynamic interaction between mRNA target and ARE-specific binding proteins (AUBPs). The AUBP-ARE mRNA recognition is directed by multiple intracellular signals that are predominantly targeted at the AUBPs. These include (but are unlikely limited to) methylation, acetylation, phosphorylation, ubiquitination and isomerization. These regulatory events ultimately affect ARE mRNA location, abundance, translation and stability. In this review, we describe recent advances in our understanding of phosphorylation and its impact on conformation of the AUBPs, interaction with ARE mRNAs and highlight the role of Pin1 mediated prolyl cis-trans isomerization in these biological process.

RNA-binding proteins and microRNAs in gastrointestinal epithelial homeostasis and diseases

The epithelium of gastrointestinal (GI) mucosa is a rapidly self-renewing tissue in the body, and its homeostasis is preserved through strict regulation of cell proliferation and apoptosis. Epithelial cells originate from a small number of pluripotent stem cells, which divide to either renew themselves or become committed crypt cells. RNA-binding proteins (RBPs) and microRNAs (miRNAs) regulate gene expression at the posttranscriptional level and are recently shown to modulate GI mucosal growth and repair after injury. Here we highlight the roles of RBPs HuR, CUG-binding protein 1, AU-binding factor 1, and several GI epithelial-specific miRNAs in gut mucosal homeostasis and diseases and also further analyze the mechanisms through which RBPs and miRNAs modulate the stability and translation of target mRNAs.

Posttranscriptional Regulation of Intestinal Epithelial Tight Junction Barrier by RNA-binding Proteins and microRNAs

Intestinal epithelial tight junctions (TJs) are a specialized structure that determines the cell polarity and prevents the diffusion of toxins, allergens, and pathogens from the lumen into the tissue. TJs are highly dynamic and its constituent protein complexes undergo continuously remodeling and turnover under tight regulation by numerous extracellular and intracellular factors. RNA-binding proteins (RBPs) and microRNAs (miRNAs) regulate gene expression at the posttranscriptional level and are involved in many aspects of cellular physiology. An increasing body of evidence indicates that RBPs including HuR and CUG-binding protein 1 and miRNAs such as miR-192 modulate the stability and translation of mRNAs encoding TJ proteins and play an important role in the control of intestinal epithelial TJ barrier function. In this mini-review article, we highlight the changes in TJ expression and intestinal epithelial TJ barrier function after activation or inactivation of RBPs and miRNAs and further analyze in some detail the mechanisms through which the stability and translation of TJ mRNAs are regulated by RBPs and miRNAs.

The HILDA complex coordinates a conditional switch in the 3'-untranslated region of the VEGFA mRNA

Cell regulatory circuits integrate diverse, and sometimes conflicting, environmental cues to generate appropriate, condition-dependent responses. Here, we elucidate the components and mechanisms driving a protein-directed RNA switch in the 3'UTR of vascular endothelial growth factor (VEGF)-A. We describe a novel HILDA (hypoxia-inducible hnRNP L-DRBP76-hnRNP A2/B1) complex that coordinates a three-element RNA switch, enabling VEGFA mRNA translation during combined hypoxia and inflammation. In addition to binding the CA-rich element (CARE), heterogeneous nuclear ribonucleoprotein (hnRNP) L regulates switch assembly and function. hnRNP L undergoes two previously unrecognized, condition-dependent posttranslational modifications: IFN-γ induces prolyl hydroxylation and von Hippel-Lindau (VHL)-mediated proteasomal degradation, whereas hypoxia stimulates hnRNP L phosphorylation at Tyr(359), inducing binding to hnRNP A2/B1, which stabilizes the protein. Also, phospho-hnRNP L recruits DRBP76 (double-stranded RNA binding protein 76) to the 3'UTR, where it binds an adjacent AU-rich stem-loop (AUSL) element, "flipping" the RNA switch by disrupting the GAIT (interferon-gamma-activated inhibitor of translation) element, preventing GAIT complex binding, and driving robust VEGFA mRNA translation. The signal-dependent, HILDA complex coordinates the function of a trio of neighboring RNA elements, thereby regulating translation of VEGFA and potentially other mRNA targets. The VEGFA RNA switch might function to ensure appropriate angiogenesis and tissue oxygenation during conflicting signals from combined inflammation and hypoxia. We propose the VEGFA RNA switch as an archetype for signal-activated, protein-directed, multi-element RNA switches that regulate posttranscriptional gene expression in complex environments.

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.

Hyper conserved elements in vertebrate mRNA 3'-UTRs reveal a translational network of RNA-binding proteins controlled by HuR

Little is known regarding the post-transcriptional networks that control gene expression in eukaryotes. Additionally, we still need to understand how these networks evolve, and the relative role played in them by their sequence-dependent regulatory factors, non-coding RNAs (ncRNAs) and RNA-binding proteins (RBPs). Here, we used an approach that relied on both phylogenetic sequence sharing and conservation in the whole mapped 3'-untranslated regions (3'-UTRs) of vertebrate species to gain knowledge on core post-transcriptional networks. The identified human hyper conserved elements (HCEs) were predicted to be preferred binding sites for RBPs and not for ncRNAs, namely microRNAs and long ncRNAs. We found that the HCE map identified a well-known network that post-transcriptionally regulates histone mRNAs. We were then able to discover and experimentally confirm a translational network composed of RNA Recognition Motif (RRM)-type RBP mRNAs that are positively controlled by HuR, another RRM-type RBP. HuR shows a preference for these RBP mRNAs bound in stem-loop motifs, confirming its role as a 'regulator of regulators'. Analysis of the transcriptome-wide HCE distribution revealed a profile of prevalently small clusters separated by unconserved intercluster RNA stretches, which predicts the formation of discrete small ribonucleoprotein complexes in the 3'-UTRs.

Polyamines regulate intestinal epithelial restitution through TRPC1-mediated Ca²+ signaling by differentially modulating STIM1 and STIM2

Early epithelial restitution occurs as a consequence of intestinal epithelial cell (IEC) migration after wounding, and its defective regulation is implicated in various critical pathological conditions. Polyamines stimulate intestinal epithelial restitution, but their exact mechanism remains unclear. Canonical transient receptor potential-1 (TRPC1)-mediated Ca(2+) signaling is crucial for stimulation of IEC migration after wounding, and induced translocation of stromal interaction molecule 1 (STIM1) to the plasma membrane activates TRPC1-mediated Ca(2+) influx and thus enhanced restitution. Here, we show that polyamines regulate intestinal epithelial restitution through TRPC1-mediated Ca(2+) signaling by altering the ratio of STIM1 to STIM2. Increasing cellular polyamines by ectopic overexpression of the ornithine decarboxylase (ODC) gene stimulated STIM1 but inhibited STIM2 expression, whereas depletion of cellular polyamines by inhibiting ODC activity decreased STIM1 but increased STIM2 levels. Induced STIM1/TRPC1 association by increasing polyamines enhanced Ca(2+) influx and stimulated epithelial restitution, while decreased formation of the STIM1/TRPC1 complex by polyamine depletion decreased Ca(2+) influx and repressed cell migration. Induced STIM1/STIM2 heteromers by polyamine depletion or STIM2 overexpression suppressed STIM1 membrane translocation and inhibited Ca(2+) influx and epithelial restitution. These results indicate that polyamines differentially modulate cellular STIM1 and STIM2 levels in IECs, in turn controlling TRPC1-mediated Ca(2+) signaling and influencing cell migration after wounding.

Ornithine decarboxylase mRNA is stabilized in an mTORC1-dependent manner in Ras-transformed cells

Upon Ras activation, ODC (ornithine decarboxylase) is markedly induced, and numerous studies suggest that ODC expression is controlled by Ras effector pathways. ODC is therefore a potential target in the treatment and prevention of Ras-driven tumours. In the present study we compared ODC mRNA translation profiles and stability in normal and Ras12V-transformed RIE-1 (rat intestinal epithelial) cells. While translation initiation of ODC increased modestly in Ras12V cells, ODC mRNA was stabilized 8-fold. Treatment with the specific mTORC1 [mTOR (mammalian target of rapamycin) complex 1] inhibitor rapamycin or siRNA (small interfering RNA) knockdown of mTOR destabilized the ODC mRNA, but rapamycin had only a minor effect on ODC translation initiation. Inhibition of mTORC1 also reduced the association of the mRNA-binding protein HuR with the ODC transcript. We have shown previously that HuR binding to the ODC 3'UTR (untranslated region) results in significant stabilization of the ODC mRNA, which contains several AU-rich regions within its 3'UTR that may act as regulatory sequences. Analysis of ODC 3'UTR deletion constructs suggests that cis-acting elements between base 1969 and base 2141 of the ODC mRNA act to stabilize the ODC transcript. These experiments thus define a novel mechanism of ODC synthesis control. Regulation of ODC mRNA decay could be an important means of limiting polyamine accumulation and subsequent tumour development.

Polyamines and Gut Mucosal Homeostasis

The epithelium of gastrointestinal (GI) mucosa has the most rapid turnover rate of any tissue in the body and its integrity is preserved through the dynamic balance between cell migration, proliferation, growth arrest and apoptosis. To maintain tissue homeostasis of the GI mucosa, the rates of epithelial cell division and apoptosis must be highly regulated by various extracellular and intracellular factors including cellular polyamines. Natural polyamines spermidine, spermine and their precursor putrescine, are organic cations in eukaryotic cells and are implicated in the control of multiple signaling pathways and distinct cellular functions. Normal intestinal epithelial growth depends on the available supply of polyamines to the dividing cells in the crypts, and polyamines also regulate intestinal epithelial cell (IEC) apoptosis. Although the specific molecular processes controlled by polyamines remains to be fully defined, increasing evidence indicates that polyamines regulate intestinal epithelial integrity by modulating the expression of various growth-related genes. In this review, we will extrapolate the current state of scientific knowledge regarding the roles of polyamines in gut mucosal homeostasis and highlight progress in cellular and molecular mechanisms of polyamines and their potential clinical applications.

An emerging role of RNA-binding proteins as multifunctional regulators of lymphocyte development and function

Sequence-specific RNA-binding proteins (RBP) and the regulation of RNA decay have long been recognized as important regulators of the inflammatory response. RBP influence gene expression throughout the lifespan of the mRNA by regulating splicing, polyadenylation, cellular localization, translation, and decay. Increasing evidence now indicates that these proteins, together with the RNA decay machinery that they recruit, also regulate the development and activation of lymphocytes. The activity of RBP is regulated by the same signal transduction pathways that govern lymphocyte development and differentiation in response to antigen and cytokine receptor engagement. Roles for these proteins in regulating the diverse functions of lymphocytes are becoming increasingly apparent.

The RNA-binding protein HuR stabilizes survivin mRNA in human oesophageal epithelial cells

Overexpression of survivin, a member of the IAP (inhibitor of apoptosis) family, has been correlated with poorer outcomes in multiple malignancies, including oesophageal cancer. The regulatory mechanisms, particularly at the post-transcriptional level, involved in survivin overexpression are not well understood. Previous work from our group has shown that the RNA-binding protein HuR (Hu antigen R), which is also overexpressed in several malignancies, stabilizes the mRNA of XIAP (X-linked IAP), another IAP family member. In the present study, we demonstrate the binding of HuR to a 288 bp fragment in the 3'-UTR (untranslated region) of survivin mRNA in human oesophageal epithelial cells. Unexpectedly, overexpression of HuR led to a decrease in survivin expression. This was associated with decreased survivin mRNA and promoter activity, suggesting a decrease in transcription. Levels of p53, a negative transcriptional regulator of survivin, increased following HuR overexpression, in conjunction with enhanced p53 mRNA stability. Silencing p53 prior to HuR overexpression resulted in increased survivin protein and mRNA stability. These results demonstrate that, in the absence of p53, HuR overexpression results in increased survivin mRNA stability and protein expression. This provides an additional explanation for the increased survivin expression observed in oesophageal cancer cells that have lost p53.

Chk2-dependent HuR phosphorylation regulates occludin mRNA translation and epithelial barrier function

Occludin is a transmembrane tight junction (TJ) protein that plays an important role in TJ assembly and regulation of the epithelial barrier function, but the mechanisms underlying its post-transcriptional regulation are unknown. The RNA-binding protein HuR modulates the stability and translation of many target mRNAs. Here, we investigated the role of HuR in the regulation of occludin expression and therefore in the intestinal epithelial barrier function. HuR bound the 3′-untranslated region of the occludin mRNA and enhanced occludin translation. HuR association with the occludin mRNA depended on Chk2-dependent HuR phosphorylation. Reduced HuR phosphorylation by Chk2 silencing or by reduction of Chk2 through polyamine depletion decreased HuR-binding to the occludin mRNA and repressed occludin translation, whereas Chk2 overexpression enhanced (HuR/occludin mRNA) association and stimulated occludin expression. In mice exposed to septic stress induced by cecal ligation and puncture, Chk2 levels in the intestinal mucosa decreased, associated with an inhibition of occludin expression and gut barrier dysfunction. These results indicate that HuR regulates occludin mRNA translation through Chk2-dependent HuR phosphorylation and that this influence is crucial for maintenance of the epithelial barrier integrity in the intestinal tract.

Regulation of cyclin-dependent kinase 4 translation through CUG-binding protein 1 and microRNA-222 by polyamines

The amino acid-derived polyamines are organic cations that are essential for growth in all mammalian cells, but their exact roles at the molecular level remain largely unknown. Here we provide evidence that polyamines promote the translation of cyclin-dependent kinase 4 (CDK4) by the action of CUG-binding protein 1 (CUGBP1) and microRNA-222 (miR-222) in intestinal epithelial cells. Both CUGBP1 and miR-222 were found to bind the CDK4 mRNA coding region and 3'-untranslated region and repressed CDK4 translation synergistically. Depletion of cellular polyamines increased cytoplasmic CUGBP1 abundance and miR-222 levels, induced their associations with the CDK4 mRNA, and inhibited CDK4 translation, whereas increasing the levels of cellular polyamines decreased CDK4 mRNA interaction with CUGBP1 and miR-222, in turn inducing CDK4 expression. Polyamine-deficient cells exhibited an increased colocalization of tagged CDK4 mRNA with processing bodies; this colocalization was abolished by silencing CUGBP1 and miR-222. Together, our findings indicate that polyamine-regulated CUGBP1 and miR-222 modulate CDK4 translation at least in part by altering the recruitment of CDK4 mRNA to processing bodies.

Chemokine transcripts as targets of the RNA-binding protein HuR in human airway epithelium

HuR is a regulator of mRNA turnover or translation of inflammatory genes through binding to adenylate-uridylate-rich elements and related motifs present in the 3'untranslated region (UTR) of mRNAs. We postulate that HuR critically regulates the epithelial response by associating with multiple ARE-bearing, functionally related inflammatory transcripts. We aimed to identify HuR targets in the human airway epithelial cell line BEAS-2B challenged with TNF-α plus IFN-γ, a strong stimulus for inflammatory epithelial responses. Ribonucleoprotein complexes from resting and cytokine-treated cells were immunoprecipitated using anti-HuR and isotype-control Ab, and eluted mRNAs were reverse-transcribed and hybridized to an inflammatory-focused gene array. The chemokines CCL2, CCL8, CXCL1, and CXCL2 ranked highest among 27 signaling and inflammatory genes significantly enriched in the HuR RNP-IP from stimulated cells over the control immunoprecipitation. Among these, 20 displayed published HuR binding motifs. Association of HuR with the four endogenous chemokine mRNAs was validated by single-gene ribonucleoprotein-immunoprecipitation and shown to be 3'UTR-dependent by biotin pull-down assay. Cytokine treatment increased mRNA stability only for CCL2 and CCL8, and transient silencing and overexpression of HuR affected only CCL2 and CCL8 expression in primary and transformed epithelial cells. Cytokine-induced CCL2 mRNA was predominantly cytoplasmic. Conversely, CXCL1 mRNA remained mostly nuclear and unaffected, as CXCL2, by changes in HuR levels. Increase in cytoplasmic HuR and HuR target expression partially relied on the inhibition of AMP-dependent kinase, a negative regulator of HuR nucleocytoplasmic shuttling. HuR-mediated regulation in airway epithelium appears broader than previously appreciated, coordinating numerous inflammatory genes through multiple posttranscriptional mechanisms.

Current status of the polyamine research field

This chapter provides an overview of the polyamine field and introduces the 32 other chapters that make up this volume. These chapters provide a wide range of methods, advice, and background relevant to studies of the function of polyamines, the regulation of their content, their role in disease, and the therapeutic potential of drugs targeting polyamine content and function. The methodology provided in this new volume will enable laboratories already working in this area to expand their experimental techniques and facilitate the entry of additional workers into this rapidly expanding field.

HuR's role in gemcitabine efficacy: an exception or opportunity?

Over the next few years, research teams will focus on, and millions of dollars will be spent on, sorting through cancer genomes. Undoubtedly, 'druggable' targets and events will be discovered that will improve our understanding and, hopefully, treatment of cancer. Highlighting an alternative to this 'genome-centric' approach, this review will further explore an underappreciated mechanism of gene expression regulation, posttranscription mRNA:protein interactions. A key molecule involved in this mode of gene regulation is HuR and we have shown that HuR levels and HuR cellular distribution can predict how pancreatic cancer patients responded to the standard of care chemotherapy (i.e., gemcitabine). HuR regulates post-transcriptional processes through: (1) association with specific mRNA cargos with ARE-rich sequences and (2) stress-induced increased cytoplasmic protein levels. Over multiple laboratories and diverse tumor types, it has been shown that HuR supports tumor survival through its regulation of tumor-promoting transcripts such as VEGF. In this article, we will highlight a recent discovery that this potent post-transcriptional gene regulatory mechanism has an impact on a common chemotherapeutic, gemcitabine. Specifically, we have identified the mechanism by which HuR can regulate a key gemcitabine metabolic enzyme, deoxycytidine kinase. We will use this example to explore and hypothesize the functional roles that HuR may have on anticancer drug therapies. We will survey novel high throughput global gene expression analysis tools to discover novel HuR targets. Future multidisciplinary approaches focused on HuR biology will provide critical events and 'druggable' targets in cancer that large-scale genomic sequencing efforts will miss.

At the stem of youth and health

Cellular senescence is a specialized form of growth arrest, confined to mitotic cells, induced by various stressful stimuli and characterized by a permanent growth arrest, resistance to apoptosis, an altered pattern of gene expression and the expression of some markers that are characteristic, although not exclusive, to the senescent state. Senescent cells profoundly modify neighboring and remote cells through the production of an altered secretome, eventually leading to inflammation, fibrosis and possibly growth of neoplastic cells. Mammalian aging has been defined as a reduction in the capacity to adequately maintain tissue homeostasis or to repair tissues after injury. Tissue homeostasis and regenerative capacity are nowadays considered to be related to the stem cell pool present in every tissue. For this reason, pathological and patho-physiological conditions characterized by altered tissue homeostasis and impaired regenerative capacity can be viewed as a consequence of the reduction in stem cell number and/or function. Last, cellular senescence is a double-edged sword, since it may inhibit the growth of transformed cells, preventing the occurrence of cancer, while it may facilitate growth of preneoplastic lesions in a paracrine fashion; therefore, interventions targeting this cell response to stress may have a profound impact on many age-related pathologies, ranging from cardiovascular disease to oncology. Aim of this review is to discuss both molecular mechanisms associated with stem cell senescence and interventions that may attenuate or reverse this process.

Polyamines regulate the stability of JunD mRNA by modulating the competitive binding of its 3' untranslated region to HuR and AUF1

Polyamines critically regulate all mammalian cell growth and proliferation by mechanisms such as the repression of growth-inhibitory proteins, including JunD. Decreasing the levels of cellular polyamines stabilizes JunD mRNA without affecting its transcription, but the exact mechanism whereby polyamines regulate JunD mRNA degradation has not been elucidated. RNA-binding proteins HuR and AUF1 associate with labile mRNAs bearing AU-rich elements located in the 3' untranslated regions (3'-UTRs) and modulate their stability. Here, we show that JunD mRNA is a target of HuR and AUF1 and that polyamines modulate JunD mRNA degradation by altering the competitive binding of HuR and AUF1 to the JunD 3'-UTR. The depletion of cellular polyamines enhanced HuR binding to JunD mRNA and decreased the levels of JunD transcript associated with AUF1, thus stabilizing JunD mRNA. The silencing of HuR increased AUF1 binding to the JunD mRNA, decreased the abundance of HuR-JunD mRNA complexes, rendered the JunD mRNA unstable, and prevented increases in JunD mRNA and protein in polyamine-deficient cells. Conversely, increasing the cellular polyamines repressed JunD mRNA interaction with HuR and enhanced its association with AUF1, resulting in an inhibition of JunD expression. These results indicate that polyamines modulate the stability of JunD mRNA in intestinal epithelial cells through HuR and AUF1 and provide new insight into the molecular functions of cellular polyamines.

Cytoplasmic accumulation of the RNA-binding protein HuR stabilizes the ornithine decarboxylase transcript in a murine nonmelanoma skin cancer model

Ornithine decarboxylase (ODC) is the first and usually rate-limiting enzyme in the polyamine biosynthetic pathway. Under normal physiological conditions, polyamine content and ODC enzyme activity are highly regulated. However, the induction of ODC activity is an early step in neoplastic transformation. The studies described here use normal mouse keratinocytes (C5N cells), and spindle carcinoma cells (A5 cells) to explore the regulation of ODC in nonmelanoma skin cancer development. Previous results have shown that induction of ODC activity is both necessary and sufficient for the promotion of skin tumors. We see a marked increase in ODC enzyme activity in A5 cells compared with C5N keratinocytes, which correlates with a 4-fold stabilization of ODC mRNA. These data suggest that ODC is post-transcriptionally regulated in skin tumor development. Thus, we sought to investigate whether the ODC transcript interacts with the RNA-binding protein HuR, which is known to bind to and stabilize its target mRNAs. We show that HuR is able to bind to the ODC 3'-UTR in A5 cells but not in C5N cells. Immunofluorescence results reveal that HuR is present in both the nucleus and cytoplasm of A5 cells, whereas C5N cells exhibit strictly nuclear localization of HuR. Knockdown experiments in A5 cells showed that when HuR is depleted, ODC RNA becomes less stable, and ODC enzyme activity decreases. Together, these data support the hypothesis that HuR plays a causative role in ODC up-regulation during nonmelanoma skin cancer development by binding to and stabilizing the ODC transcript.

Post-transcriptional regulation of MEK-1 by polyamines through the RNA-binding protein HuR modulating intestinal epithelial apoptosis

MEK-1 [MAPK (mitogen-activated protein kinase) kinase-1] is an important signal transducing enzyme that is implicated in many aspects of cellular functions. In the present paper, we report that cellular polyamines regulate MEK-1 expression at the post-transcriptional level through the RNA-binding protein HuR (Hu-antigen R) in IECs (intestinal epithelial cells). Decreasing the levels of cellular polyamines by inhibiting ODC (ornithine decarboxylase) stabilized MEK-1 mRNA and promoted its translation through enhancement of the interaction between HuR and the 3'-untranslated region of MEK-1 mRNA, whereas increasing polyamine levels by ectopic ODC overexpression destabilized the MEK-1 transcript and repressed its translation by reducing the abundance of HuR-MEK-1 mRNA complex; neither intervention changed MEK-1 gene transcription via its promoter. HuR silencing rendered the MEK-1 mRNA unstable and inhibited its translation, thus preventing increases in MEK-1 mRNA and protein in polyamine-deficient cells. Conversely, HuR overexpression increased MEK-1 mRNA stability and promoted its translation. Inhibition of MEK-1 expression by MEK-1 silencing or HuR silencing prevented the increased resistance of polyamine-deficient cells to apoptosis. Moreover, HuR overexpression did not protect against apoptosis if MEK-1 expression was silenced. These results indicate that polyamines destabilize the MEK-1 mRNA and repress its translation by inhibiting the association between HuR and the MEK-1 transcript. Our findings indicate that MEK-1 is a key effector of the HuR-elicited anti-apoptotic programme in IECs.

Stabilization of XIAP mRNA through the RNA binding protein HuR regulated by cellular polyamines

The X chromosome-linked inhibitor of apoptosis protein (XIAP) is the most potent intrinsic caspase inhibitor and plays an important role in the maintenance of intestinal epithelial integrity. The RNA binding protein, HuR, regulates the stability and translation of many target transcripts. Here, we report that HuR associated with both the 3'-untranslated region and coding sequence of the mRNA encoding XIAP, stabilized the XIAP transcript and elevated its expression in intestinal epithelial cells. Ectopic HuR overexpression or elevated cytoplasmic levels of endogenous HuR by decreasing cellular polyamines increased [HuR/XIAP mRNA] complexes, in turn promoting XIAP mRNA stability and increasing XIAP protein abundance. Conversely, HuR silencing in normal and polyamine-deficient cells rendered the XIAP mRNA unstable, thus reducing the steady state levels of XIAP. Inhibition of XIAP expression by XIAP silencing or by HuR silencing reversed the resistance of polyamine-deficient cells to apoptosis. Our findings demonstrate that HuR regulates XIAP expression by stabilizing its mRNA and implicates HuR-mediated XIAP in the control of intestinal epithelial apoptosis.

Mammalian polyamine metabolism and function

Polyamines are ubiquitous small basic molecules that play multiple essential roles in mammalian physiology. Their cellular content is highly regulated and there is convincing evidence that altered metabolism is involvement in many disease states. Drugs altering polyamine levels may therefore have a variety of important targets. This review will summarize the current state of understanding of polyamine metabolism and function, the regulation of polyamine content, and heritable pathological conditions that may be derived from altered polyamine metabolism.

Augmented bladder urothelial polyamine signaling and block of BK channel in the pathophysiology of overactive bladder syndrome

Overactive bladder syndrome (OAB) is an idiopathic condition characterized by urinary urgency and urge incontinence. Detrusor overactivity has been traditionally described as the physiologic mechanism for OAB. However, the bladder urothelium (BU) may also be involved in the pathophysiology. This study measured polyamine signaling and its downstream effects on membrane conductivity in bladder urothelial cells (BUC) obtained from asymptomatic and OAB subjects. Immunohistofluorescence was used to measure ornithine decarboxylase (ODC) expression in BU. BUC, cultured from BU biopsies, were used for electrophysiologic studies. dl-alpha-Difluoromethylornithine (DFMO), spermine, or spermidine was used to modulate polyamine signaling in BUC. Results showed ODC overexpression in OAB BU. In OAB BUC, whole cell and cell-attached configuration showed significantly decreased currents. Using inside-out patches, outward currents increased significantly, suggesting a cytoplasmic source of the outward current block in OAB BUC. In control BUC, outward currents were mediated by the large-conductance calcium-activated potassium (BK) channel due to calcium dose-dependence and block by iberiotoxin. Spermidine and spermine blocked the outward current in normal BUC in dose-dependent fashion. Conversely, DFMO significantly increased (P < 0.01) outward currents in OAB BUC both in cell-attached and in whole cell configuration. The outward currents in DFMO-treated-OAB BUC could be significantly reduced (P < 0.05) by adding back spermidine and spermine. These data suggest that polyamine signaling is upregulated in OAB urothelium and OAB BUC. Furthermore, polyamines in BUC block the BK channel. Targeting of bladder urothelial polyamine signaling may represent a novel approach for OAB treatment based on pathophysiologic mechanisms.

Polyamines regulate c-Myc translation through Chk2-dependent HuR phosphorylation

All mammalian cells depend on polyamines for normal growth and proliferation, but the exact roles of polyamines at the molecular level remain largely unknown. The RNA-binding protein HuR modulates the stability and translation of many target mRNAs. Here, we show that in rat intestinal epithelial cells (IECs), polyamines enhanced HuR association with the 3'-untranslated region of the c-Myc mRNA by increasing HuR phosphorylation by Chk2, in turn promoting c-Myc translation. Depletion of cellular polyamines inhibited Chk2 and reduced the affinity of HuR for c-Myc mRNA; these effects were completely reversed by addition of the polyamine putrescine or by Chk2 overexpression. In cells with high content of cellular polyamines, HuR silencing or Chk2 silencing reduced c-Myc translation and c-Myc expression levels. Our findings demonstrate that polyamines regulate c-Myc translation in IECs through HuR phosphorylation by Chk2 and provide new insight into the molecular functions of cellular polyamines.

The role of mammalian ribonucleases (RNases) in cancer

Ribonucleases (RNases) are a group of enzymes that cleave RNAs at phosphodiester bonds resulting in remarkably diverse biological consequences. This review focuses on mammalian RNases that are capable of, or potentially capable of, cleaving messenger RNA (mRNA) as well as other RNAs in cells and play roles in the development of human cancers. The aims of this review are to provide an overview of the roles of currently known mammalian RNases, and the evidence that associate them as regulators of tumor development. The roles of these RNases as oncoproteins and/or tumor suppressors in influencing cell growth, apoptosis, angiogenesis, and other cellular hallmarks of cancer will be presented and discussed. The RNases under discussion include RNases from the conventional mRNA decay pathways, RNases that are activated under cellular stress, RNases from the miRNA pathway, and RNases with multifunctional activity.

JunD represses transcription and translation of the tight junction protein zona occludens-1 modulating intestinal epithelial barrier function

The AP-1 transcription factor JunD is highly expressed in intestinal epithelial cells, but its exact role in maintaining the integrity of intestinal epithelial barrier remains unknown. The tight junction (TJ) protein zonula occludens (ZO)-1 links the intracellular domain of TJ-transmembrane proteins occludin, claudins, and junctional adhesion molecules to many cytoplasmic proteins and the actin cytoskeleton and is crucial for assembly of the TJ complex. Here, we show that JunD negatively regulates expression of ZO-1 and is implicated in the regulation of intestinal epithelial barrier function. Increased JunD levels by ectopic overexpression of the junD gene or by depleting cellular polyamines repressed ZO-1 expression and increased epithelial paracellular permeability. JunD regulated ZO-1 expression at the levels of transcription and translation. Transcriptional repression of ZO-1 by JunD was mediated through cAMP response element-binding protein-binding site within its proximal region of the ZO-1-promoter, whereas induced JunD inhibited ZO-1 mRNA translation by enhancing the interaction of the ZO-1 3'-untranslated region with RNA-binding protein T cell-restricted intracellular antigen 1-related protein. These results indicate that JunD is a biological suppressor of ZO-1 expression in intestinal epithelial cells and plays a critical role in maintaining epithelial barrier function.

Post-transcriptional control of gene expression through subcellular relocalization of mRNA binding proteins

Eukaryotic cells have developed multiple mechanisms to respond to different physiological cues, such as cellular stress, which allow the cells to adapt themselves to their new environment. The regulation of post-transcriptional gene expression is an important component of the cellular stress response and is mediated by mRNA binding proteins (mRBPs). Recently, several studies have shown that regulated subcellular localization of mRBPs upon diverse stimuli (such as cellular stress) provides an additional level of regulation for gene expression.