c-jun N-terminal kinase is involved in AUUUA-mediated interleukin-3 mRNA turnover in mast cells

JNK modulates RAGE/β-catenin signaling and is essential for allergic airway inflammation in asthma

As a leading cause of occupational asthma, toluene diisocyanate (TDI)-induced asthma is an inflammatory disease of the airways with one of the most significant characteristics involving inflammation, in which the receptor of advanced glycation end products (RAGE) plays an extremely important role. However, the mechanism underlying the upregulation of RAGE is still unknown. The aim of the present study was to examine whether JNK mediates β-catenin stabilization via activation of RAGE in asthma. Herein from the results by analyzing the blood from healthy donors and patients with asthma, it was found that the expression of RAGE and p-JNK is highly correlated and elevated concomitantly with the severity of bronchial asthma. Additionally, upon sensitizing and challenging the mice with TDI, we found that RAGE inhibitor (FPS-ZM1) and JNK inhibitor (SP600125) significantly reduced the TDI-induced asthma inflammation in vivo. Furthermore, SP600125 also considerably restored RAGE and p-JNK expression. Besides, the in vitro results from TDI-HSA treatment of 16HBE cells reveal that therapeutic inhibition of JNK reduced TDI driving RAGE expression and β-catenin translocation, while treatment with Anisomycin, a JNK agonist, showed the opposite effect. Moreover, genetic knockdown of RAGE does not contribute to JNK phosphorylation, indicating that JNK functions upstream of RAGE. Collectively, these findings highlight a role for JNK signaling in RAGE/β-catenin regulation and have important therapeutic implications for the treatment of TDI induced asthma.

Calcium-induced stabilization of AU-rich short-lived mRNAs is a common default response

The AU-rich element (AUUUA)n, found in the 3' noncoding region of many short-lived cytokine and proto-oncogene mRNAs, is sufficient to specifically target these mRNAs for rapid degradation in mammalian cells. The mechanism by which the AU-rich element promotes rapid mRNA decay is not known. Previous studies have shown that release of intracellular stored calcium by ionophore treatment of thymocytes and mast cells inhibits the rapid turnover of AU-rich interleukin mRNAs. Increased cytoplasmic half-life of interleukin mRNAs was linked to calcium-induced activation of the N-terminal c-Jun kinase. In this report we have characterized the calcium-induced stabilization of AU-rich mRNAs. We show that calcium induces stabilization of mRNAs with canonical AU-rich elements in all cell types tested. These results indicate that short-lived mRNA stabilization by calcium is not unique to immune cells nor interleukin mRNAs, but is a widespread default response that includes generic AU-rich mRNAs. Stabilization is shown to be rapid but transient, and to act without altering nuclear transcription or cytoplasmic translation rates. These data support the view that calcium release likely stabilizes short-lived mRNAs by altering trans-acting decay factors that promote AU-rich mRNA turnover.

5-Azacytidine engages an IRE1α-EGFR-ERK1/2 signaling pathway that stabilizes the LDL receptor mRNA

Hepatic low-density lipoprotein receptor (LDLR) is the primary conduit for the clearance of plasma LDL-cholesterol and increasing its expression represents a central goal for treating cardiovascular disease. However, LDLR mRNA is unstable and undergoes rapid turnover mainly due to the three AU-rich elements (ARE) in its proximal 3'-untranslated region (3'-UTR). Herein, our data revealed that 5-azacytidine (5-AzaC), an antimetabolite used in the treatment of myelodysplastic syndrome, stabilizes the LDLR mRNA through a previously unrecognized signaling pathway resulting in a strong increase of its protein level in human hepatocytes in culture. 5-AzaC caused a sustained activation of the inositol-requiring enzyme 1α (IRE1α) kinase domain and c-Jun N-terminal kinase (JNK) independently of endoplasmic reticulum stress. This resulted in activation of the epidermal growth factor receptor (EGFR) and extracellular signal-regulated kinase1/2 (ERK1/2) that, in turn, stabilized LDLR mRNA. Systematic mutation of the AREs (ARE1-3) in the LDLR 3'UTR and expression of each mutant coupled to a luciferase reporter in Huh7 cells demonstrated that ARE1 is required for rapid LDLR mRNA decay and 5-AzaC-induced mRNA stabilization via the IRE1α-EGFR-ERK1/2 signaling cascade. The characterization of this pathway will help to reveal potential targets to enhance plasma LDL clearance and novel cholesterol-lowering therapeutic strategies.

Transcriptional and posttranscriptional regulation of CXCL8/IL-8 gene expression induced by connective tissue growth factor

Connective tissue growth factor (CTGF), a CCN family member, is a secreted protein regulating cellular functions, including fibrosis, apoptosis, adhesion, migration, differentiation, proliferation, angiogenesis, and chondrogenesis. CTGF increases proinflammatory factor production; however, inflammatory cytokine regulation by CTGF is poorly understood. The aim of this study was to identify novel biological functions and elucidate the functional mechanisms of CTGF. Specifically, the study focused on the ability of CTGF-primed monocytes to secrete interleukin 8 (CXCL8/IL-8) and determined the signaling pathways involved in CTGF-induced CXCL8/IL-8 gene regulation during inflammation. We transfected wild-type or mutant CXCL8/IL-8 promoter-derived luciferase reporter constructs into 293T cells to examine the effect of CTGF on the CXCL8/IL-8 promoter. The results showed that the activator protein-1 and nuclear factor κB binding sites of the CXCL8/IL-8 promoter are essential for CTGF-induced CXCL8/IL-8 transcription. Moreover, the CTGF-induced activation of p38 mitogen-activated protein kinase (MAPK), c-Jun-N-terminal kinase, and extracellular signal-regulated kinase (ERK) is involved in this process. In addition, adenosine-uridine-rich elements (AREs) of the CXCL8/IL-8 3'-untranslated region (3'-UTR) reduce CXCL8/IL-8 mRNA stability. To investigate whether CTGF regulates CXCL8/IL-8 gene expression at the posttranscriptional level, we transfected 293 cells with serial luciferase constructs containing different segments of the CXCL8/IL-8 3'-UTR and then stimulated the cells with CTGF. The results suggested that CTGF stabilized luciferase mRNA and increased luciferase activity by regulating the CXCL8/IL-8 3'-UTR. Moreover, the p38 MAPK pathway may contribute to CTGF-induced CXCL8/IL-8 mRNA stabilization.

The role of mammalian MAPK signaling in regulation of cytokine mRNA stability and translation

Extracellular-regulated kinases and p38 mitogen-activated protein kinases are activated in innate (and adaptive) immunity and signal via different routes to alter the stability and translation of various cytokine mRNAs, enabling immune cells to respond promptly. This regulation involves mRNA elements, such as AU-rich motifs, and mRNA-binding proteins, such as tristetraprolin (TTP), HuR, and hnRNPK-homology (KH) type splicing regulatory protein (KSRP). Signal-dependent phosphorylation of mRNA-binding proteins often alters their subcellular localization or RNA-binding affinity. Furthermore, it could lead to an altered interaction with other mRNA-binding proteins and altered scaffolding properties for mRNA-modifying enzymes, such as deadenylases, polyadenylases, decapping enzymes, poly(A) binding proteins, exo- or endonucleases, and proteins of the exosome machinery. In many cases, this results in unstable mRNAs being stabilized, with their translational arrest being released and cytokine production being stimulated. Hence, components of these mechanisms are potential targets for the modulation of the inflammatory response.

Application of DNA microarray technology to gerontological studies

Gene expression patterns change dramatically in aging and age-related events. The DNA microarray is now recognized as a useful device in molecular biology and widely used to identify the molecular mechanisms of aging and the biological effects of drugs for therapeutic purpose in age-related diseases. Recently, numerous technological advantages have led to the evolution of DNA microarrays and microarray-based techniques, revealing the genomic modification and all transcriptional activity. Here, we show the step-by-step methods currently used in our lab to handling the oligonucleotide microarray and miRNA microarray. Moreover, we introduce the protocols of ribonucleoprotein [RNP] immunoprecipitation followed by microarray analysis (RIP-chip) which reveal the target mRNA of age-related RNA-binding proteins.

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.

Networks controlling mRNA decay in the immune system

The active control of mRNA degradation has emerged as a key regulatory mechanism required for proper gene expression in the immune system. An adenosine/uridine (AU)-rich element (ARE) is at the heart of a first regulatory system that promotes the rapid degradation of a multitude of cytokine and chemokine mRNAs. AREs serve as binding sites for a number of regulatory proteins that either destabilize or stabilize the mRNA. Several kinase pathways regulate the activity of ARE-binding proteins and thereby coordinate the expression of their target mRNAs. Small regulatory micro (mi)-RNAs represent a second system that enhances the degradation of several mRNAs encoding important components of signal transduction cascades that are activated during adaptive and innate immune responses. Specific miRNAs are important for the differentiation of T helper cells, class switch recombination in B cells, and the maturation of dendritic cells. Excitement in this area of research is fueled by the discovery of novel RNA elements and regulatory proteins that exert control over specific mRNAs, as exemplified by an endonuclease that was found to directly cleave interleukin-6 mRNA. Together, these systems make up an extensive regulatory network that controls decay rates of individual mRNAs in a precise manner and thereby orchestrates the dynamic expression of many factors essential for adaptive and innate immune responses. In this review, we provide an overview of relevant factors regulated at the level of mRNA stability, summarize RNA-binding proteins and miRNAs that control their degradation rates, and discuss signaling pathways operating within this regulatory network.

c-Jun N-terminal kinase phosphorylates DCP1a to control formation of P bodies

Cytokines and stress-inducing stimuli signal through c-Jun N-terminal kinase (JNK) using a diverse and only partially defined set of downstream effectors. In this paper, the decapping complex subunit DCP1a was identified as a novel JNK target. JNK phosphorylated DCP1a at residue S315 in vivo and in vitro and coimmunoprecipitated and colocalized with DCP1a in processing bodies (P bodies). Sustained JNK activation by several different inducers led to DCP1a dispersion from P bodies, whereas IL-1 treatment transiently increased P body number. Inhibition of TAK1-JNK signaling also affected the number and size of P bodies and the localization of DCP1a, Xrn1, and Edc4. Transcriptome analysis further identified a central role for DCP1a in IL-1-induced messenger ribonucleic acid (mRNA) expression. Phosphomimetic mutation of S315 stabilized IL-8 but not IκBα mRNA, whereas overexpressed DCP1a blocked IL-8 transcription and suppressed p65 NF-κB nuclear activity. Collectively, these data reveal DCP1a as a multifunctional regulator of mRNA expression and suggest a novel mechanism controlling the subcellular localization of DCP1a in response to stress or inflammatory stimuli.

Phosphorylation of human tristetraprolin in response to its interaction with the Cbl interacting protein CIN85

Background

Tristetraprolin (TTP) is the prototype member of a family of CCCH tandem zinc finger proteins and is considered to be an anti-inflammatory protein in mammals. TTP plays a critical role in the decay of tumor necrosis factor alpha (TNF) mRNA, among others, by binding AU-rich RNA elements in the 3′-untranslated regions of this transcript and promoting its deadenylation and degradation.

Methodology/Principal Findings

We used yeast two-hybrid analysis to identify potential protein binding partners for human TTP (hTTP). Various regions of hTTP recovered 31 proteins that fell into 12 categories based on sequence similarities. Among these, the interactions between hTTP and CIN85, cytoplasmic poly (A) binding protein (PABP), nucleolin and heat shock protein 70 were confirmed by co-immunoprecipitation experiments. CIN85 and hTTP co-localized in the cytoplasm of cells as determined by confocal microscopy. CIN85 contains three SH3 domains that specifically bind a unique proline-arginine motif (PXXXPR) found in several CIN85 effectors. We found that the SH3 domains of CIN85 bound to a PXXXPR motif located near the C-terminus of hTTP. Co-expression of CIN85 with hTTP resulted in the increased phosphorylation of hTTP at serine residues in positions 66 and 93, possibly due in part to the demonstrated association of mitogen-activated protein kinase kinase kinase 4 (MEKK4) to both proteins. The presence of CIN85 did not appear to alter hTTP's binding to RNA probes or its stimulated breakdown of TNF mRNA.

Conclusions/Significance

These studies describe interactions between hTTP and nucleolin, cytoplasmic PABP, heat shock protein 70 and CIN85; these interactions were initially discovered by two-hybrid analysis, and confirmed by co-immunoprecipitation. We found that CIN85 binding to a C-terminal motif within hTTP led to the increased phosphorylation of hTTP, possibly through enhanced association with MEKK4. The functional consequences to each of the members of this putative complex remain to be determined.

Endotoxin recognition in fish results in inflammatory cytokine secretion not gene expression

Macrophages are phagocytes that have a central role in the organization of the immune system after an infection. These cells can recognize specific molecular components of micro-organisms (pathogen-associated molecular patterns, PAMPs) via specific receptors (PRRs) and elicit specific cellular responses. In the past, the expression of immune genes in response to different PAMPs has been characterized in different fish species. However, little is known about actual cytokine release. We characterized the secretion of tumour necrosis factor (TNF)-α in primary macrophage cultures of rainbow trout (Oncorhynchus mykiss) in response to several PAMPs by Western blot and compared this to the induction of TNF-α gene expression as well as other pro-inflammatory cytokines such as interleukin (IL)-1β and IL-6 and anti-viral molecules such as INF-α and Mx protein (Mx). We show that lipopolysaccharide (LPS) and zymosan are major inducers of TNF-α secretion, which is not initially linked to the induction of TNF-α mRNA expression. The secretion of TNF-α, but intriguingly not the expression, is also stimulated by ultrapure LPS meaning that, in fish, contaminants of commercial LPS preparations are better inducers of the inflammatory response. Moreover, we have characterized the signaling pathways that are activated by different PAMPs and the link between those pathways and the final step of TNF-α secretion: TNF-α shedding by TNF-α converting enzyme (TACE/ ADAM17). For the first time, we show that, in fish macrophages, TNF-α is processed by TACE-like activity and this cleavage is dependent upon the activation of ERK, p38MAPK and JNK signaling pathways by LPS.

Crosstalk in inflammation: the interplay of glucocorticoid receptor-based mechanisms and kinases and phosphatases

Glucocorticoids (GCs) are steroidal ligands for the GC receptor (GR), which can function as a ligand-activated transcription factor. These steroidal ligands and derivatives thereof are the first line of treatment in a vast array of inflammatory diseases. However, due to the general surge of side effects associated with long-term use of GCs and the potential problem of GC resistance in some patients, the scientific world continues to search for a better understanding of the GC-mediated antiinflammatory mechanisms. The reversible phosphomodification of various mediators in the inflammatory process plays a key role in modulating and fine-tuning the sensitivity, longevity, and intensity of the inflammatory response. As such, the antiinflammatory GCs can modulate the activity and/or expression of various kinases and phosphatases, thus affecting the signaling efficacy toward the propagation of proinflammatory gene expression and proinflammatory gene mRNA stability. Conversely, phosphorylation of GR can affect GR ligand- and DNA-binding affinity, mobility, and cofactor recruitment, culminating in altered transactivation and transrepression capabilities of GR, and consequently leading to a modified antiinflammatory potential. Recently, new roles for kinases and phosphatases have been described in GR-based antiinflammatory mechanisms. Moreover, kinase inhibitors have become increasingly important as antiinflammatory tools, not only for research but also for therapeutic purposes. In light of these developments, we aim to illuminate the integrated interplay between GR signaling and its correlating kinases and phosphatases in the context of the clinically important combat of inflammation, giving attention to implications on GC-mediated side effects and therapy resistance.

Two pathways of costimulation through CD28

CD28 is recognized as the primary costimulatory molecule involved in the activation of naïve T cells. However, the biochemical signaling pathways that are activated by CD28 and how these pathways are integrated with TCR signaling are still not understood. We have recently shown that there are at least two independent activation pathways induced by CD28 costimulation. One is integrated with TCR signaling in the context of the immunological synapse and is mediated through transcriptional enhancement and the second is mediated through the induction of mRNA stability. Here, we review the immunological consequences and biochemical mechanisms associated with CD28 costimulation and discuss the major questions that need to be resolved to understand the molecular mechanisms that transduce CD28 costimulation.

Control of mRNA decay by phosphorylation of tristetraprolin

TTP (tristetraprolin) is an RNA-binding protein that suppresses inflammation by accelerating the degradation of cytokine mRNAs. TTP binds to an AU-rich element in the 3'-untranslated region of its target mRNAs. In macrophages, the induction of cytokine expression requires activation of the p38-MAPK (mitogen-activated protein kinase)-MK2 [MAPKAP (MAPK-activated protein) kinase-2] kinase cascade. MK2 directly phosphorylates TTP and thereby contributes to transient stabilization of cytokine mRNAs. In the present review, we address the target specificity of TTP, summarize TTP-interacting proteins and discuss how phosphorylation regulates the activity, localization and stability of TTP.

Egr-1 mRNA induction by medium flow involves mRNA stabilization and is enhanced by the p38 inhibitor SB203580 in osteoblast-like cells

AIM

Mechanical stimuli are important for maintaining organ structure and tissue function. To elucidate signalling pathways activated by mechanical stimuli, the contribution of mRNA stabilization to induction of egr-1 mRNA by medium flow was examined and the mechanisms responsible for stabilization were analysed. An early-response gene that encodes a transcription factor, egr-1, activates transcription of several genes in response to mechanical stimuli, and was therefore selected to resolve how early-induced signals are integrated and connected to subsequent response.

METHODS

Mouse osteoblast-like MC3T3E1 cells were stably transfected with the chloramphenicol acetyltransferase (CAT) gene linked to the egr-1 promoter, and inductions of endogenous egr-1 and transfected CAT mRNA following medium flow were compared using real-time reverse transcriptase PCR. The mechanism of induction was examined using a transcription inhibitor and mitogen-activated protein (MAP) kinase inhibitors. Activation of MAP kinases by medium flow was investigated using western blotting.

RESULTS

Induction of egr-1 mRNA by medium flow was twofold higher than CAT mRNA induction. Induction of egr-1 mRNA was also observed in cells pre-treated with transcription inhibitor. The p38 inhibitor SB203580 enhanced induction of egr-1 mRNA by medium flow. Extracellular signal regulated kinase (ERK), p38 and c-Jun N-terminal kinase (JNK) were activated by medium flow.

CONCLUSION

A considerable part of egr-1 mRNA induction by medium flow may be due to mRNA stabilization. The p38 inhibitor SB203580 enhances induction.

Protein kinase C activation stabilizes LDL receptor mRNA via the JNK pathway in HepG2 cells

LDL is the most abundant cholesterol transport vehicle in plasma and a major prognostic indicator of atherosclerosis. Hepatic LDL receptors limit circulating LDL levels, since cholesterol internalized by the liver can be excreted. As such, mechanisms regulating LDL receptor expression in liver cells are appealing targets for cholesterol-lowering therapeutic strategies. Activation of HepG2 cells with phorbol esters enhances LDL receptor mRNA levels through transcriptional and posttranscriptional mechanisms. Here, we show that 12-O-tetradecanoyl-phorbol-13-acetate (TPA)-induced stabilization of receptor mRNA requires the activity of protein kinase C and is accompanied by activation of the major mitogen activated protein kinase pathways. Inhibitor studies demonstrated that receptor mRNA stabilization is independent of the extracellular signal-regulated kinase or p38(MAPK), but requires activation of the c-Jun N-terminal kinase (JNK). An essential role for JNK in stabilizing receptor mRNA was further confirmed through small interfering RNA (siRNA) experiments and by activating JNK through two protein kinase C-independent mechanisms. Finally, prolonged JNK activation increased steady-state levels of receptor mRNA and protein, and significantly enhanced cellular LDL-binding activity. These data suggest that JNK may play an important role in posttranscriptional control of LDL receptor expression, thus constituting a novel mechanism to enhance plasma LDL clearance by liver cells.

Posttranscriptional gene regulation by RNA-binding proteins during oxidative stress: implications for cellular senescence

To respond adequately to oxidative stress, mammalian cells elicit rapid and tightly controlled changes in gene expression patterns. Besides alterations in the subsets of transcribed genes, two posttranscriptional processes prominently influence the oxidant-triggered gene expression programs: mRNA turnover and translation. Here, we review recent progress in our knowledge of the turnover and translation regulatory (TTR) mRNA-binding proteins (RBPs) that influence gene expression in response to oxidative damage. Specifically, we identify oxidant damage-regulated mRNAs that are targets of TTR-RBPs, we review the oxidant-triggered signaling pathways that govern TTR-RBP function, and we examine emerging evidence that TTR-RBP activity is altered with senescence and aging. Given the potent influence of TTR-RBPs upon oxidant-regulated gene expression profiles, we propose that the senescence-associated changes in TTR-RBPs directly contribute to the impaired responses to oxidant damage that characterize cellular senescence and advancing age.

Stabilization of IGFBP-1 mRNA by ethanol in hepatoma cells involves the JNK pathway

BACKGROUND/AIMS

Insulin-like growth factor-binding protein-1 (IGFBP-1) modulates cell growth and metabolism in a variety of physiopathological conditions. The aim of this study was to determine the molecular mechanisms involved in IGFBP-1 upregulation by ethanol.

METHODS

We studied IGFBP-1 regulation by ethanol at the protein, mRNA and gene promoter levels in the human hepatocarcinoma cell line, HepG2, which does not express significantly ethanol-metabolizing enzymes.

RESULTS

Ethanol (35-150mM) induced the IGFBP-1 mRNA and protein up to 5-fold in a dose-dependent manner. A similar effect was observed using primary cultures of human hepatocytes. Various inhibitors of ethanol metabolism and the antioxidant N-acetylcysteine did not prevent ethanol effects. While ethanol did not modify the IGFBP-1 gene promoter activity, it elicited a 2- to 3-fold increase in IGFBP-1 mRNA half-life and this stabilization required the 5' and the 3' untranslated mRNA region. Ethanol triggered a rapid activation of c-Jun N-terminal Kinase (JNK) in HepG2 cells and IGFBP-1 induction was significantly decreased by a specific inhibitor of JNK.

CONCLUSIONS

This study reveals a novel pathway of gene regulation by alcohol which involves the activation of JNK and the consequent mRNA stabilization.

mRNA stability and cancer: an emerging link?

Many oncogenes, growth factor, cytokine and cell-cycle genes are regulated post-transcriptionally. The major mechanism is by controlling the rate of mRNA turnover for transcripts bearing destabilizing cis-elements. To date, only a handful of regulatory factors have been identified that appear to control a large pool of target mRNAs, suggesting that a slight perturbation in the control mechanism may generate wide-ranging effects that could contribute to the development of a complex disorder such as cancer. In support of this view, mRNA turnover responds to signalling pathways that are often overactive in cancer, suggesting a post-transcriptional component in addition to the well-recognised transcriptional aspect of oncogenesis. Here the authors review examples of deregulated post-transcriptional control in oncogenesis, discuss post-transcriptionally regulated transcripts of oncologic significance, and consider the key role of signalling pathways in linking both processes and as an enticing therapeutic prospect.

The macrophage migration inhibitory factor-glucocorticoid dyad: regulation of inflammation and immunity

The cytokine macrophage migration inhibitory factor (MIF) occupies a unique position in physiology by its ability to directly regulate the immunosuppressive actions of glucocorticoids. We review herein the interactions between MIF and glucocorticoids within the immune system and discuss the relevance of the MIF-glucocorticoid regulatory dyad in physiology and immunopathology. Therapeutic antagonism of MIF may be an effective approach for steroid-sparing therapies in patients with refractory autoimmune or inflammatory diseases.

Post-transcriptional regulation of human inducible nitric-oxide synthase expression by the Jun N-terminal kinase

Human inducible nitric-oxide synthase (iNOS) expression is regulated both at transcriptional and post-transcriptional levels. In the present study, the effect of Jun N-terminal kinase (JNK) on human iNOS expression was investigated. In A549/8 human alveolar epithelial cells, both the inhibition of JNK by a pharmacological inhibitor anthra[1,9-cd]pyrazol-6(2H)-one1,9-pyrazoloanthrone (SP600125) and small interfering RNA (siRNA)-mediated down-regulation of JNK led to a reduction of iNOS mRNA and protein expression. iNOS promoter activity was not affected by these treatments. Hence, JNK seems to regulate iNOS expression through post-transcriptional mechanisms by stabilizing iNOS mRNA. Our laboratory has shown recently that a cytokine-induced RNA binding protein tristetraprolin (TTP) is a major positive regulator of human iNOS expression by stabilizing iNOS mRNA. Therefore, the effect of JNK inhibition by SP600125 or down-regulation by siRNA on TTP expression was investigated. Both SP600125 and siRNA targeted at JNK resulted in a reduction of TTP protein expression without affecting the amount of TTP mRNA. These data suggest a post-transcriptional control of TTP expression by JNK. Moreover, the modulation of JNK signaling by SP600125 or siRNA did not change p38 phosphorylation. In summary, the results suggest that JNK regulates human iNOS expression by stabilizing iNOS mRNA possibly by a TTP-dependent mechanism.

Modulation of mRNA stability as a novel therapeutic approach

During the last decade evidence has accumulated that modulation of mRNA stability plays a central role in cellular homeostasis, including cell differentiation, proliferation and adaptation to external stimuli. The functional relevance of posttranscriptional gene regulation is highlighted by many pathologies, wherein occurrence tightly correlates with a dysregulation in mRNA stability, including chronic inflammation, cardiovascular diseases and cancer. Most commonly, the cis-regulatory elements of mRNA decay are represented by the adenylate- and uridylate (AU)-rich elements (ARE) which are specifically bound by trans-acting RNA binding proteins, which finally determine whether mRNA decay is delayed or facilitated. Regulation of mRNA decay by RNA stabilizing and RNA destabilizing factors is furthermore controlled by different intrinsic and environmental stimuli. The modulation of mRNA binding proteins, therefore, illuminates a promising approach for the pharmacotherapy of those key pathologies mentioned above and characterized by a posttranscriptional dysregulation. Most promisingly, intracellular trafficking of many of the mRNA stability regulating factors is, in turn, regulated by some major signaling pathways, including the mitogen-activated protein kinase (MAPK) cascade, the AMP-activated kinase (AMPK) and the protein kinase (PK) C (PKC) family. In this review, we present timely examples of genes regulated by mRNA stability with a special focus on signaling pathways involved in the ARE-dependent mRNA decay. A better understanding of these processes may form the basis for the development of novel therapeutics to treat major human diseases.

BRF1 protein turnover and mRNA decay activity are regulated by protein kinase B at the same phosphorylation sites

BRF1 posttranscriptionally regulates mRNA levels by targeting ARE-bearing transcripts to the decay machinery. We previously showed that protein kinase B (PKB) phosphorylates BRF1 at Ser92, resulting in binding to 14-3-3 and impairment of mRNA decay activity. Here we identify an additional regulatory site at Ser203 that cooperates in vivo with Ser92. In vitro kinase labeling and wortmannin sensitivity indicate that Ser203 phosphorylation is also performed by PKB. Mutation of both serines to alanine uncouples BRF1 from PKB regulation, leading to constitutive mRNA decay even in the presence of stabilizing signals. BRF1 protein is labile because of proteasomal degradation (half-life, <3 h) but becomes stabilized upon phosphorylation and is less stable in PKBalpha(-/-) cells. Surprisingly, phosphorylation-dependent protein stability is also regulated by Ser92 and Ser203, with parallel phosphorylation required at these sites. Phosphorylation-dependent binding to 14-3-3 is abolished only when both sites are mutated. Cell compartment fractionation experiments support a model in which binding to 14-3-3 sequesters BRF1 through relocalization and prevents it from executing its mRNA decay activity, as well as from proteasomal degradation, thereby maintaining high BRF1 protein levels that are required to reinstate decay upon dissipation of the stabilizing signal.

Pro-inflammatory Cytokine Tumor Necrosis Factor-α Induces Bone Morphogenetic Protein-2 in Chondrocytes via mRNA Stabilization and Transcriptional Up-regulation

In articular chondrocytes, the inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) induces the expression of bone morphogenetic protein-2 (BMP-2), a growth factor known to be involved in the induction of cartilage and bone. A study was performed to clarify the mechanism(s) underlying the induction of BMP-2 in chondrogenic ATDC5 cells and primary cultured adult human articular chondrocytes. In ATDC5 cells, the endogenous BMP-2 expression was consistently low throughout the process of chondrogenic differentiation, and TNF-alpha induced BMP-2 expression only after the cells acquired the chondrogenic phenotype. The results of nuclear run-off assay and cycloheximide treatment consistently indicated that ATDC5 cells acquire the capacity to synthesize BMP-2 mRNA in the nuclei during the differentiation process. In an attempt to explain the discrepancy between the active nuclear mRNA synthesis and the observed low expression level in differentiated ATDC5 cells, the stability of BMP-2 mRNA was evaluated, and the cells were found to regulate the expression of BMP-2 at the post-transcriptional level. Human chondrocytes were confirmed to have a similar post-transcriptional regulation. The result of 3'-rapid amplification of cDNA end revealed that both human and mouse BMP-2 mRNAs contain multiple pentameric AUUUA motifs in a conserved manner in the 3'-untranslated regions, and transient transfection experiments demonstrated that TNF-alpha increases the stability of BMP-2 mRNA through the pentameric motifs. Further experiments revealed that TNF-alpha modulates mRNA stability via p38 signal transduction pathway, whereas the cytokine also augmented the expression of BMP-2 through transcriptional up-regulation via the transcriptional factor NF-kappaB.

JNK inhibitor SP600125 reduces COX-2 expression by attenuating mRNA in activated murine J774 macrophages

Inducible prostaglandin synthase (cyclooxygenase-2, COX-2) is highly expressed in inflammation. The signaling mechanisms involved in the up-regulation of COX-2 are not known in detail. In the present study we investigated the role of c-Jun NH2-terminal kinase (JNK), a member of the mitogen-activated protein kinase (MAPK) family in COX-2 expression and prostaglandin (PG) E2 production in murine J774 macrophages activated by bacterial lipopolysaccharide (LPS). LPS caused a transient activation of JNK which was followed by increased COX-2 expression. Anthra(1,9-cd)pyrazol-6(2H)-one (SP600125), an inhibitor of JNK, inhibited phosphorylation of c-Jun with an IC50 of 5-10 microM. At the same concentrations SP600125 suppressed also LPS-induced COX-2 protein levels and PGE2 production. SP600125 did not alter LPS-induced COX-2 mRNA levels when measured 3 h after addition of LPS, whereas mRNA levels were significantly reduced in SP600125-treated cells when measured 24 h after addition of LPS. LPS-induced COX-2 mRNA levels reduced faster in cells treated with SP600125 than in control cells. Cycloheximide (that is known to activate JNK) enhanced COX-2 expression and its effect was inhibited by SP600125. The present results suggest that JNK pathway is involved in the up-regulation of COX-2 expression possibly by a mechanism related to the stability of COX-2 mRNA.

Engagement of CD28 outside of the immunological synapse results in up-regulation of IL-2 mRNA stability but not IL-2 transcription

During T cell activation by APC, CD28 is colocalized with TCR in the central supramolecular activation cluster (cSMAC) region of the immunological synapse. CD28 signaling through PI3K results in the recruitment of protein kinase C (PKC)theta to the cSMAC, activation of NF-kappaB, and induction of IL-2 transcription. These results suggest that localized engagement of CD28 within the cSMAC may be required for CD28 activation and/or signal integration with TCR signals. To test this model we have examined the mechanism of CD28-mediated induction of IL-2 secretion when CD28 is engaged outside of the immunological synapse. CD4 T cells were stimulated with Ag presented by B7-negative APC and CD28 costimulation was provided in trans by anti-CD28-coated beads or by class II-negative, B7-positive cells. We show that induction of IL-2 secretion under these conditions did not require expression of PKCtheta and did not induce NF-kappaB activation or IL-2 transcription. In contrast, CD28 costimulation in trans did induce IL-2 mRNA stability, accounting for the up-regulation of IL-2 secretion. These data indicate that the ability of CD28 to up-regulate IL-2 transcription requires colocalization of TCR and CD28 at the plasma membrane, possibly within the cSMAC of the immunological synapse. In contrast, the ability of CD28 to promote IL-2 mRNA stability can be transduced from a distal site from the TCR, suggesting that signal integration occurs downstream from the plasma membrane. These data support the potential role of trans costimulation in tumor and allograft rejection, but limit the potential functional impact that trans costimulation may have on T cell activation.

JNK1 activation mediates C5b-9-induced P0 mRNA instability and P0 gene expression in Schwann cells

The protein zero (P0) glycoprotein is an important component of compact peripheral nerve myelin produced by the glial cells of the mammalian peripheral nervous system. P0 mRNA expression is reduced following exposure of Schwann cells to sublytic C5b-9, the terminal activation complex of the complement cascade. Sublytic complement treatment decreased P0 mRNA by 81% within 6 h and required C5b-9 assembly. C5b-9 induced a threefold increase in both JNK1 activity and c-jun mRNA within 20 and 30 min, respectively, compared with cells treated with either human serum depleted of complement component C7 (C7dHS) or medium alone. Sublytic C5b-9 stimulation, in the presence of the transcription inhibitor Actinomycin D, decreased P0 mRNA expression by 52%, indicating that mRNA was selectively destabilized. This effect was prevented by pretreatment with L-JNK inhibitor 1 (L-JNKI1). To study a potential inhibition of P0 gene transcription, we transfected Schwann cells with a P0 promoter-firefly luciferase construct. Sublytic C5b-9 stimulation of the transfected cells decreased luciferase activity by 82% at 6 h, and this effect was prevented by pretreatment with L-JNKI1 inhibitor. Our results indicate that the ability of C5b-9 in vitro to affect P0 gene expression is mediated via JNK1 activation that leads to enhanced mRNA decay and transcriptional repression of P0.

The role of transforming growth factor beta signaling in messenger RNA stability

Transforming growth factor beta (TGF-beta) is a biologically multipotent regulatory protein implicated in functions that include the regulation of cellular growth, differentiation, extracellular matrix formation, and wound healing. It also plays a role in the pathologies of Alzheimer's disease, cancer and autoimmune disorders. TGF-beta modulates gene expression by affecting transcriptional activation and mRNA turnover rate. Steady-state mRNA levels depend on both the transcriptional activity and mRNA half-life. The stability of mRNA can be modified by the binding of trans-acting factors to cis-elements on the message. These can protect the mRNA from cleavage by RNAses, or they may promote mRNA cleavage. Changes in mRNA stability can lead to changes in the proteome and subsequently in cellular metabolism. The SMAD family of proteins has been implicated in the transduction of the TGF-beta signal, where they regulate transcriptional activity. This review attempts to provide new insights into the role played by TGF-beta in the regulation of mRNA turnover.

p38-dependent phosphorylation of the mRNA decay-promoting factor KSRP controls the stability of select myogenic transcripts

Transcriptional and posttranscriptional processes regulate expression of genetic networks in response to environmental cues. The extracellular signal-activated p38 MAP kinase (p38) pathway plays a fundamental role in conversion of myoblasts to differentiated myocytes. p38 phosphorylates specific transcription factors and chromatin-associated proteins promoting assembly of the myogenic transcriptome. Here, we demonstrate that p38 alpha and beta isoforms also control muscle-gene expression posttranscriptionally, by stabilizing critical myogenic transcripts. KSRP, an important factor for AU-rich element (ARE)-directed mRNA decay, undergoes p38-dependent phosphorylation during muscle differentiation. KSRP phosphorylated by p38 displays compromised binding to ARE-containing transcripts and fails to promote their rapid decay, although it retains the ability to interact with the mRNA degradation machinery. Overexpression of KSRP selectively impairs induction of ARE-containing early myogenic transcripts, without affecting p38-mediated transcriptional responses. Our results uncover an unanticipated role for KSRP in establishing a biochemical link between differentiation-activated p38 signaling and turnover of myogenic mRNAs.

Posttranscriptional mechanisms regulating the inflammatory response

The inflammatory response is a complex physiologic process that requires the coordinate induction of cytokines, chemokines, angiogenic factors, effector-enzymes, and proteases. Although transcriptional activation is required to turn on the inflammatory response, recent studies have revealed that posttranscriptional mechanisms play an important role by determining the rate at which mRNAs encoding inflammatory effector proteins are translated and degraded. Most posttranscriptional control mechanisms function to dampen the expression of pro-inflammatory proteins to ensure that potentially injurious proteins are not overexpressed during an inflammatory response. Here we discuss the factors that regulate the stability and translation of mRNAs encoding pro-inflammatory proteins.

Antagonistic functions of tetradecanoyl phorbol acetate-inducible-sequence 11b and HuR in the hormonal regulation of vascular endothelial growth factor messenger ribonucleic acid stability by adrenocorticotropin

Expression of vascular endothelial growth factor (VEGF), an endothelial cell-specific mitogen and a potent angiogenic factor, is up-regulated by a variety of factors including hypoxia, growth factors, and hormones. In the adrenal cortex, regulation of VEGF expression by the pituitary hormone ACTH ensures the maintenance of the organ vasculature. We have previously shown that ACTH evokes a rapid and transient increase in VEGF mRNA levels in primary adrenocortical cells through transcription-independent mechanisms. We further demonstrated that the zinc finger RNA-binding protein Tis11b (tetradecanoyl phorbol acetate-inducible-sequence 11b) destabilizes VEGF mRNA through its 3'-untranslated region (3'-UTR) and that Tis11b is involved in the decay phase of ACTH-induced VEGF mRNA expression. In the present study, we attempted to determine the mechanisms underlying ACTH-elicited increase in VEGF mRNA levels in adrenocortical cells. We show that ACTH triggers an increase in the levels of the mRNA-stabilizing protein HuR in the cytoplasm and a concomitant decrease in the levels of HuR in the nucleus. This process is accompanied by an increased association of HuR with the nucleocytoplasmic shuttling protein pp32, indicating that ACTH induces HuR translocation from the nuclear to the cytoplasmic compartment. Leptomycin B, a specific inhibitor of CRM1-dependent nuclear export of pp32, significantly reduced ACTH-induced VEGF mRNA levels. Furthermore, RNA interference-mediated depletion of HuR in adrenocortical cells abrogated ACTH-induced VEGF mRNA expression. Finally, we show that Tis11b and HuR exert antagonistic effects on VEGF 3'-UTR in vitro. Although both proteins could bind simultaneously on VEGF 3'-UTR, Tis11b markedly decreases HuR-binding to this RNA sequence. Altogether, these results suggest that the RNA-stabilizing protein HuR is instrumental to ACTH-induced expression of VEGF mRNA and that the nuclear export of HuR is a rate-limiting step in this process. HuR appears to transiently stabilize VEGF transcripts after ACTH stimulation of adrenocortical cells, and Tis11b appears to subsequently trigger their degradation.

Limiting inflammatory responses during activation of innate immunity

The idea of the importance of mounting an inflammatory response for effective immunity is supported by a multiplicity of experimental data. It is also well understood that resolution of inflammation is essential for maintaining the balance between health and disease. When the normal regulatory mechanisms are disturbed, the potential for developing chronic inflammatory diseases is increased. Inflammation is a key element in the response of the innate immune system to a variety of challenges, including those provided by bacterial and viral infection as well as by damaged or dying host cells. Here we review elements of innate immunity that lead to inflammation and some of the host mechanisms that allow for the resolution of the inflammatory responses.

JNK and ERK MAP kinases mediate induction of IL-1beta, TNF-alpha and IL-8 following hyperosmolar stress in human limbal epithelial cells

Hyperosmolarity has been recognized to be a pro-inflammatory stress to the corneal epithelium. The cell signalling pathways linking hyperosmolar stress and inflammation have not been well elucidated. This study investigated whether exposure of human limbal epithelial cells to hyperosmotic stress activates the mitogen-activated protein kinase (MAPK) pathways and induces production of pro-inflammatory cytokines, interleukin (IL) -1beta, tumor necrosis factor (TNF) alpha, and the C-X-C chemokine IL-8. Primary human limbal epithelial cultures in normal osmolar media (312 mOsM) were exposed to media with higher osmolarity (400-500 mOsM) by adding 50-90 mM NaCl, with or without SB202190, an inhibitor of c-Jun N-terminal kinases (JNK) pathway, PD 98059, an inhibitor of extracellular-regulated kinase (ERK) pathway, dexamethasone or doxycycline for different lengths of time. The conditioned media were collected after 24 hr of treatment for ELISA. Total RNA was extracted from cultures treated for 6 hr for semi-quantitative RT-PCR. Cells treated for 15-60 min were lysed in RIPA buffer and subjected to Western blot with phospho (p)-specific antibodies against p-JNK and p-ERK. The concentrations of IL-1beta, TNF-alpha and IL-8 proteins in 24 hr conditioned media of limbal epithelial cells progressively increased as the media osmolarity increased from 312 to 500 mOsM. Active p-JNK-1/p-JNK-2 and p-ERK-1/p-ERK-2 were detected by Western blot and peaked at 60 min in cells exposed to hyperosmolar media. The levels of p-JNK-1/p-JNK-2 and p-ERK1/p-ERK2 were positively correlated with the medium osmolarity. SB202190, PD98059 and doxycycline markedly suppressed the levels of p-JNK-1/p-JNK-2 and/or p-ERK1/p-ERK2, as well as IL-1beta, TNF-alpha and IL-8 mRNAs and proteins stimulated by hyperosmolar media. These findings provide direct evidence that hyperosmolarity induces inflammation in human limbal epithelial cells by increasing expression and production of pro-inflammatory cytokines and chemokines, a process that appears to be mediated through activation of the JNK and ERK MAPK signalling pathways. The efficacy of doxycycline in treating ocular surface diseases may be due to its ability to suppress JNK and ERK signalling activation and inflammatory mediator production in the limbal epithelium.

Convergent actions of I kappa B kinase beta and protein kinase C delta modulate mRNA stability through phosphorylation of 14-3-3 beta complexed with tristetraprolin

Regulation of gene expression at the level of mRNA stability is a major topic of research; however, knowledge about the regulatory mechanisms affecting the binding and function of AU-rich element (ARE)-binding proteins (AUBPs) in response to extracellular signals is minimal. The beta1,4-galactosyltransferase 1 (beta4GalT1) gene enabled us to study the mechanisms involved in binding of tristetraprolin (TTP) as the stability of its mRNA is regulated solely through one ARE bound by TTP in resting human umbilical vein endothelial cells. Here, we provide evidence that the complex formation of TTP with 14-3-3beta is required to bind beta4GalT1 mRNA and promote its decay. Furthermore, upon tumor necrosis factor alpha stimulation, the activation of both Ikappabeta kinase and protein kinase Cdelta is involved in the phosphorylation of 14-3-3beta on two serine residues, paralleled by release of binding of TTP and 14-3-3beta from beta4GalT1 mRNA, nuclear sequestration of TTP, and beta4GalT1 mRNA stabilization. Thus, a key mechanism regulating mRNA binding and function of the destabilizing AUBP TTP involves the phosphorylation status of 14-3-3beta.

Interaction of RNA-binding proteins HuR and AUF1 with the human ATF3 mRNA 3'-untranslated region regulates its amino acid limitation-induced stabilization

ATF3 expression is induced in cells exposed to a variety of stress conditions, including nutrient limitation. Here we demonstrated that the mechanism by which the ATF3 mRNA content is increased following amino acid limitation of human HepG2 hepatoma cells is mRNA stabilization. Analysis of ATF3 mRNA turnover revealed that the half-life was increased from about 1 h in control cells to greater than 8 h in the histidine-deprived state, demonstrating mRNA stabilization in response to nutrient deprivation. Treatment of HepG2 cells with thapsigargin, which causes endoplasmic reticulum stress, also increased the half-life of ATF3 mRNA. HuR is an RNA-binding protein that regulates both the stability and cytoplasmic/nuclear localization of mRNA species containing AU-rich elements. Another RNA-binding protein, AUF1, regulates target mRNA molecules by enhancing their decay. Amino acid limitation caused a slightly elevated mRNA level for HuR and AUF1 mRNA. The nuclear HuR protein content was unchanged, and AUF1 protein increased slightly after amino acid limitation, whereas the cytoplasmic levels of both HuR and AUF1 protein increased. Immunoprecipitation of HuR-RNA complexes followed by reverse transcriptase-PCR analysis showed that HuR interacted with ATF3 mRNA in vivo and that this interaction increased following amino acid limitation. In contrast, the interaction of AUF1 with the ATF3 mRNA is decreased in histidine-deprived cells relative to control cells. Suppression of HuR expression by RNA interference partially blocked the accumulation of ATF3 mRNA following amino acid deprivation. The results demonstrated that coordinated regulation of mRNA stability by HuR and AUF1 proteins contributes to the observed increase in ATF3 expression following amino acid limitation.

Regulatory role of the 3' untranslated region (3'UTR) of rat 5' deiodinase (D1). effects on messenger RNA translation and stability

The previous findings that both a long and a short type 1 deiodinase (D1) mRNA are present in different tissues and that the D1 gene contains two potential polyA signals suggest that the two mRNAs result from differential polyA signal usage. In this study, we examined the properties of the two D1 mRNAs generated in HEK 293 cells by the alternative use of each of the poly A signals in order to ascertain the potential regulatory role of the 3'UTR of this gene. Our results showed that the long mRNA is less stable, but that it is translated more efficiently than the short mRNA. The net result of these differences is a higher D1 activity with the long message. These data suggest that the D1 3'UTR may play an important role in regulating the stability and translational efficiency of the D1 mRNA, both of which could be physiologically relevant when the demand for D1 activity is high.

Endothelial cell co-stimulation through OX40 augments and prolongs T cell cytokine synthesis by stabilization of cytokine mRNA

Human endothelial cells (ECs) constitutively express OX40L and co-stimulate memory CD4(+) T cell proliferation that is dependent upon OX40-OX40L interaction. In vivo, OX40 prolongs T cell survival; however, an unanswered question is whether it can also prolong synthesis of proliferation-sustaining cytokines such as IL-2. Here we show that EC co-stimulation results in the secretion of T cell IL-2, IL-3 and IFN-gamma and that in the absence of OX40 signals synthesis largely ceases by 12-18 h, but is prolonged up to 60 h in the presence of OX40 signaling. Blocking OX40-mediated cytokine expression at later times suppresses T cell proliferation and this can be overcome by addition of exogenous IL-2. We find that OX40 signaling has discrete effects on T cell activation as it does not affect expression of IL-10, CD25, CD69 or soluble IL-2R. Also, OX40 does not appear to alter IL-2 transcription, but rather acts to stabilize a subset of cytokine mRNAs, increasing their half-lives by 3-6-fold. We further show that OX40L induces activation of p38 mitogen-activated protein kinase (MAPK) and phosphotidyl-inositol-3-kinase (PI3K) in T cells, and using specific inhibitors, we find that increased mRNA half-life is dependent upon both these pathways but is independent of c-jun-N-terminal kinase (JNK). Thus, EC co-stimulation through OX40 leads to prolonged T cell cytokine synthesis and enhanced proliferation.

The role of mRNA stability in airway remodelling

As a consequence of long-term exposure to inflammatory mediators, the airways of asthmatics become remodelled. Airway fibrosis becomes apparent, with thickening of the lamina recticularis and increased interstitial matrix deposition being typical features of an asthmatic airway. Mucus hypersecretion occurs, airway smooth muscle mass is increased and neovascularization is evident in the subepithelial mucosa. As development of a remodelled airway is correlated with deterioration of lung function in asthmatics, there is an urgent need for therapies that reduce airway inflammation and reverse structural changes in a remodelled airway. However, in order to design efficacious anti-remodelling agents we first need a greater understanding of the molecular mechanism/s underlying the development of airway remodelling. To date, however, most studies have primarily focused on the transcriptional regulation of genes that promote airway remodelling. Post-transcriptional mechanisms, such as control of mRNA stability, remain largely unexplored. Levels of cellular mRNA transcripts are regulated by controlling the rate at which the mRNA decays, thus investigation into the mechanisms underlying mRNA stability in asthma are of critical importance. Therefore, this review will present an overview of the control of mRNA stability and examine how mRNA stability may play a role in the development of airway remodelling in asthma.

Expression of a constitutively active mutant of M-Ras in normal bone marrow is sufficient for induction of a malignant mastocytosis/mast cell leukemia, distinct from the histiocytosis/monocytic leukemia induced by expression of activated H-Ras

Expression of constitutively activated M-Ras in normal murine bone-marrow cells was sufficient to induce the factor-independent, in vitro growth and differentiation of colonies of macrophages and neutrophils, and the generation of immortal lines of factor-independent mast cells, and, upon in vivo injection of the transduced cells, a fatal mastocytosis/mast-cell leukemia. In contrast, expression of constitutively activated H-Ras in bone-marrow cells resulted in the in vitro growth, in the absence of exogenous factors, of colonies that contained only macrophages and of lines of cells resembling dendritic cells, and, upon in vivo injection of the transduced cells, a fatal histiocytosis/monocytic leukemia. Macrophages generated by bone-marrow cells expressing activated M-Ras or activated H-Ras differed morphologically, the latter appearing more activated, a difference abrogated by an inhibitor of Erk activation. Inhibition of either Erk or PI3 kinase blocked the capacity of both activated M-Ras and activated H-Ras to support proliferation and viability. However, inhibition of p38 MAPK activity suppressed proliferation of bone-marrow cells expressing activated H-Ras, but enhanced that of bone-marrow cells expressing activated M-Ras. Thus, expression of either activated M-Ras or H-Ras in normal hematopoietic cells was sufficient for transformation but each resulted in the generation of distinct lineages of cells.

Glucose deprivation increases mRNA stability of vascular endothelial growth factor through activation of AMP-activated protein kinase in DU145 prostate carcinoma

The induction of proangiogenic cytokines such as vascular endothelial growth factor (VEGF) is a critical feature of tumor angiogenesis. In the present study, we examined the mechanisms of VEGF gene expression induced by glucose deprivation in cancer cells, a role of AMP-activated protein kinase (AMPK) in the process, and the signal transduction pathway. AMPK functions as an energy sensor to provide metabolic adaptation under ATP-depleting conditions such as hypoxia and nutritional deprivation. Here, we show that glucose deprivation leads to a significant increase in the mRNA level of VEGF, GLUT1, and PFKFB3 genes in several cancer cells via a hypoxia-inducible factor-1-independent mechanism, and we demonstrate an essential role of AMPK in these gene expressions. Our data suggest that VEGF mRNA induction by glucose deprivation is due to an increase in mRNA stability, and the AMPK activity is necessary and sufficient to confer the stability to VEGF mRNA. We further show that reactive oxygen species is involved in glucose deprivation-induced AMPK activity in DU145 human prostate carcinomas, and c-Jun amino-terminal kinase acts as an upstream component in AMPK activation cascades under these conditions. LKB1, which was recently identified as a direct upstream kinase of AMPK, was not detected in DU145 cells. In conclusion, our results demonstrate a novel and major role of AMPK in the post-transcriptional regulation of VEGF, further implying its potential role in tumor angiogenesis.

The AU-Rich Transcriptome: More Than Interferons and Cytokines, and Its Role in Disease

The AU-rich elements (AREs) are among the predominant cis-acting factors that exist primarily in the 3' untranslated region (3'-UTR) of messenger RNAs (mRNAs) and regulate mRNA stability. AREs were previously believed to be restricted to relatively few mRNAs, including those of interferons (IFNs) and cytokines, growth factors, and proto-oncogenes. Our recent analysis, however, showed that ARE mRNAs represent as much as 8% of mRNAs transcribed from human genes that encode functionally diverse proteins important in many transient biologic processes. Among those processes are cell growth and differentiation, immune responses, signal transduction, transcriptional and translational control, hematopoiesis, apoptosis, nutrient transport, and metabolism. Several recent studies examined signaling pathways that regulate ARE-mediated mRNA stability, notably the p38 mitogen-activated protein kinase (MAPK) pathway. In addition, several AU-rich binding proteins that regulate the ARE mRNA pathways have been characterized. Dysregulation of regulatory signaling pathways and regulatory proteins affecting ARE mRNA stability can lead to abnormalities in many critical cellular processes and to specific disease conditions. Thus, the heterogeneity in AREs, their signaling pathways, and effector proteins contribute to the functional diversity of the ARE gene family, which encompasses more than IFNs and cytokines.

Reciprocal regulation of beta-adrenergic receptor mRNA stability by mitogen activated protein kinase activation and inhibition

Genes encoding numerous proto-oncogenes and cytokines, as well as a number of G-protein coupled receptors, are regulated post-transcriptionally at the level of mRNA stability. A common feature of all of these genes is the presence of A + U-rich elements (AREs) within their 3' untranslated regions. We, and others, have demonstrated previously that mRNAs encoding beta-adrenergic receptors (beta-ARs) are destabilized by agonist stimulation of the beta-AR/Galphas/adenylylcyclase pathway. However, in addition to PK-A, beta-ARs can also activate or inhibit mitogen activated kinase (MAPK) cascades, in a cell-type dependent basis. Recent evidence points to an important role for MAPKs in regulating the turnover of cytokine mRNAs, such as TNFalpha. We hypothesized that activation of MAPK's may also regulate beta-AR mRNA stability. The studies conducted herein demonstrate that generalized stimulation of MAPKs (JNK, p38) with anisomycin resulted in marked stabilization of beta-AR mRNA. Reciprocally, selective inhibition of JNK with SP600125 significantly decreased beta-AR mRNA half-life. Similarly, inhibition of the MEK/ERK pathway with either PD98059 or U0126 decreased beta-AR mRNA stability substantially. However, inhibition of p38 MAPK with SB203580 produced destabilization of beta-AR mRNA only at higher, non pharmacologically selective concentrations. In contrast to their effects on several other ARE containing mRNAs, inhibition of tyrosine kinases by genistein or PI3K by wortmannin, had no detectable effect on beta-AR mRNA stability. In summary, these results demonstrate for the first time that modulation of MAPK pathways can bi-directionally influence beta-AR mRNA stability.

The ARE-dependent mRNA-destabilizing activity of BRF1 is regulated by protein kinase B

Butyrate response factor (BRF1) belongs to the Tis11 family of CCCH zinc-finger proteins, which bind to mRNAs containing an AU-rich element (ARE) in their 3' untranslated region and promote their deadenylation and rapid degradation. Independent signal transduction pathways have been reported to stabilize ARE-containing transcripts by a process thought to involve phosphorylation of ARE-binding proteins. Here we report that protein kinase B (PKB/Akt) stabilizes ARE transcripts by phosphorylating BRF1 at serine 92 (S92). Recombinant BRF1 promoted in vitro decay of ARE-containing mRNA (ARE-mRNA), yet phosphorylation by PKB impaired this activity. S92 phosphorylation of BRF1 did not impair ARE binding, but induced complex formation with the scaffold protein 14-3-3. In vivo and in vitro data support a model where PKB causes ARE-mRNA stabilization by inactivating BRF1 through binding to 14-3-3.

Endonuclease-mediated mRNA decay requires tyrosine phosphorylation of polysomal ribonuclease 1 (PMR1) for the targeting and degradation of polyribosome-bound substrate mRNA

PMR1 is an endonuclease that is activated by estrogen to degrade Xenopus albumin mRNA. A previous report showed that the functional unit of endonuclease-mediated mRNA decay is a approximately 680-kDa polysome-bound complex that contains both PMR1 and substrate mRNA. PMR1 contains two domains involved in endonuclease targeting to polysomes, an N-terminal domain that lies between residues 200 and 250, and a C-terminal domain that lies within the last 100 residues. Loss of either domain inactivated PMR1 targeting to polysomes and stabilized albumin mRNA. The current study identified a phosphorylated tyrosine residue within the C-terminal polysome-targeting domain and showed that this modification is required for PMR1-mediated mRNA decay. Changing this tyrosine to phenylalanine inactivated the targeting of PMR1 to polysomes, blocked binding of PMR1 to the functional complex containing its substrate mRNA, prevented the targeting of a green fluorescent protein fusion protein to this complex, and stabilized albumin mRNA to degradation by PMR1 in vivo. A general tyrosine kinase inhibitor inhibited the phosphorylation of PMR1, which in turn inhibited PMR1-catalyzed degradation of albumin mRNA. These results indicate that one or more tyrosine kinases functions as a regulator of endonuclease-mediated mRNA decay.

A GFP-based assay for rapid screening of compounds affecting ARE-dependent mRNA turnover

A reporter transcript containing the green fluorescent protein (GFP) gene upstream of the destabilizing 3'-untranslated region (3'-UTR) of the murine IL-3 gene was inserted in mouse PB-3c-15 mast cells. The GFP-IL-3 transcript was inherently unstable due to the presence of an adenosine-uridine (AU)-rich element (ARE) in the 3'-UTR and was subject to rapid decay giving a low baseline of GFP fluorescence. Transcript stabilization with ionomycin resulted in an increase of fluorescence that is quantitated by FACS analysis of responding cells. Using this system we have identified okadaic acid as a novel stabilizing compound, and investigated the upstream signaling pathways leading to stabilization. This reporter system has the advantage of speed and simplicity over standard methods currently in use and in addition to serving as a research tool it can be easily automated to increase throughput for drug discovery.

Distinct domains of AU-rich elements exert different functions in mRNA destabilization and stabilization by p38 mitogen-activated protein kinase or HuR

AU-rich elements (AREs) control the expression of numerous genes by accelerating the decay of their mRNAs. Rapid decay and deadenylation of beta-globin mRNA containing AU-rich 3' untranslated regions of the chemoattractant cytokine interleukin-8 (IL-8) are strongly attenuated by activating the p38 mitogen-activated protein (MAP) kinase/MAP kinase-activated protein kinase 2 (MK2) pathway. Further evidence for a crucial role of the poly(A) tail is provided by the loss of destabilization and kinase-induced stabilization in ARE RNAs expressed as nonadenylated forms by introducing a histone stem-loop sequence. The minimal regulatory element in the IL-8 mRNA is located in a 60-nucleotide evolutionarily conserved sequence with a structurally and functionally bipartite character: a core domain with four AUUUA motifs and limited destabilizing function on its own and an auxiliary domain that markedly enhances destabilization exerted by the core domain and thus is essential for the rapid removal of RNA targets. A similar bipartite structure and function are observed for the granulocyte-macrophage colony-stimulating factor (GM-CSF) ARE. Stabilization in response to p38/MK2 activation is seen with the core domain alone and also after mutation of the AUUUA motifs in the complete IL-8 ARE. Stabilization by ARE binding protein HuR requires different sequence elements. Binding but no stabilization is observed with the IL-8 ARE. Responsiveness to HuR is gained by exchanging the auxiliary domain of the IL-8 ARE with that of GM-CSF or with a domain of the c-fos ARE, which results in even stronger responsiveness. These results show that distinct ARE domains differ in function with regard to destabilization, stabilization by p38/MK2 activation, and stabilization by HuR.

The Herpesvirus saimiri small nuclear RNAs recruit AU-rich element-binding proteins but do not alter host AU-rich element-containing mRNA levels in virally transformed T cells

Herpesvirus saimiri (HVS) encodes seven Sm-class small nuclear RNAs, called HSURs (for Herpesvirus saimiri U RNAs), that are abundantly expressed in HVS-transformed, latently infected marmoset T cells but are of unknown function. HSURs 1, 2, and 5 have highly conserved 5'-end sequences containing the AUUUA pentamer characteristic of AU-rich elements (AREs) that regulate the stability of many host mRNAs, including those encoding most proto-oncogenes and cytokines. To test whether the ARE-containing HSURs act to sequester host proteins that regulate the decay of these mRNAs, we demonstrate their in vivo interaction with the ARE-binding proteins hnRNP D and HuR in HVS-transformed T cells using a new cross-linking assay. Comprehensive Northern and microarray analyses revealed, however, that the levels of endogenous ARE-containing mRNAs are not altered in T cells latently infected with HVS mutants lacking HSURs 1 and 2. HSUR 1 binds the destabilizing ARE-binding protein tristetraprolin induced following activation of HVS-transformed T cells, but even in such stimulated cells, the levels of host ARE-containing mRNAs are not altered by deletion of HSURs 1 and 2. Instead, HSUR 1 itself is degraded by an ARE-dependent pathway in HVS-transformed T cells, suggesting that HVS may take advantage of the host ARE-mediated mRNA decay pathway to regulate HSUR expression. This is the first example of posttranscriptional regulation of the expression of an Sm small nuclear RNA.