Analysis of the function, expression, and subcellular distribution of human tristetraprolin

Toll-like receptor-7 activation in CD8+ T cells modulates inflammatory mediators in patients with rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune disorder of unknown etiology with aberrant immunological responses leading to inflammation, swelling and pain of the joints. CD8⁺ T cells have been known to be one of the major immune modulators in the progression of RA and the presence of toll-like receptors (TLRs) on these cells further accentuate their role in RA. Herein, we report an increased expression of TLR7 in the endosomes of CD8⁺ T cells of RA patients correlating with disease severity. The stimulation of TLR7 with Imiquimod (IMQ) in these CD8⁺ T cells drives the signalling cascade via NFkB and pERK activation and hence an increase in the mRNA transcripts of signature cytokines and cytolytic enzymes. However, a parallel synthesis of Tristetraprolin (TTP), an mRNA destabilizing protein prevents the translation of the mRNA transcripts, leading to a rapid degeneration of the target mRNA. We thus report that a direct TLR7 ligation by its agonist increases cytokine transcript signature but not an equivalent protein surge.

Targeting Tristetraprolin Expression or Functional Activity Regulates Inflammatory Response Induced by MSU Crystals

The RNA-binding protein tristetraprolin (TTP) is an anti-inflammatory factor that prompts the mRNA decay of target mRNAs and is involved in inflammatory diseases such as rheumatoid arthritis (RA). TTP is regulated by phosphorylation, and protein phosphatase 2A (PP2A) can dephosphorylate TTP to activate its mRNA-degrading function. Some small molecules can enhance PP2A activation. Short interfering RNA (siRNA) targeting TTP expression or PP2A agonist (Arctigenin) was administered to monosodium urate (MSU) crystal-induced J774A.1 cells, and the expression of inflammatory related genes was detected by RT-PCR and Western blot assays. The effects of Arctigenin in mouse models of acute inflammation induced by MSU crystals, including peritonitis and arthritis, were evaluated. The data indicated that TTP expression levels and endogenous PP2A activity were increased in MSU-crystal treated J774A.1 cells. TTP knockdown exacerbated inflammation-related genes expression and NLRP3 inflammasome activation. However, PP2A agonist treatment (Arctigenin) suppressed MSU crystal-induced inflammation in J774A.1 cells. Arctigenin also relieved mitochondrial reactive oxygen species (mtROS) production and improved lysosomal membrane permeability in MSU crystal-treated J774A.1 cells. Moreover, TTP knockdown reversed the anti-inflammatory and antioxidant effects of Arctigenin. Oral administration of Arctigenin significantly alleviated foot pad swelling, the number of inflammatory cells in peritoneal lavage fluids and the production of IL-1β in the mouse model of inflammation induced by MSU crystals. Collectively, these data imply that targeting TTP expression or functional activity may provide a potential therapeutic strategy for inflammation caused by MSU crystals.

Tristetraprolin, Inflammation, and Metabolic Syndrome in Arab Adults: A Case Control Study

Simple Summary

Metabolic syndrome (MetS) is a common disorder characterized as a low-grade chronic inflammatory state. The association of tristetraprolin (TTP), a novel anti-inflammatory protein, and MetS remains to be explored. We evaluated circulating TTP in a group of adult males and females with and without MetS. Serum levels of TTP were higher in the MetS group than in controls. In all subjects, serum TTP was also correlated with MetS components (e.g., glucose, lipids, and obesity indices). These findings suggest that TTP may be a promising biomarker for MetS.

Abstract

Tristetraprolin (TTP) is an mRNA binding protein suggested to have a substantial role in regulating the mRNA expression of numerous inflammatory factors, but data on TTP and its association with metabolic syndrome (MetS), a chronic low-grade inflammatory disorder, are scarce. We hypothesize that TTP may modulate MetS and its components. A total of 200 Saudi adults (aged 38.6 ± 8.3 years) were included in this cross-sectional study. Anthropometrics data were collected and fasting blood glucose taken for the assessment of glycemic, lipids and inflammatory markers using commercially available assays. The National Cholesterol Education Program Adult Treatment Panel (NCEP ATP III) criteria were used to define MetS. Results showed significantly higher levels of TTP in the MetS group than in controls [288.1 pg/mL vs. 150.9 pg/mL, p < 0.001]. Circulating TTP was significantly associated with tumor necrosis factor alpha [TNF-α, R = 0.30, p < 0.05], interleukin 1β [IL-1β, R = 0.41, p < 0.01] and C-reactive protein [CRP, R = 0.36, p < 0.01], adiponectin [R = 0.36, p < 0.05], insulin [R = 0.37, p < 0.05], and insulin resistance [HOMA-IR, R = 0.40, p < 0.05]. Receiver operating characteristics (ROC) suggest a potential use of TTP as diagnostic biomarker for MetS [AUC = 0.819, p < 0.001]. The findings suggest that TTP is associated with inflammation and glycemia, which may influence MetS. TTP is a promising diagnostic biomarker for MetS which can be confirmed in larger cohorts.

Protein phosphatase 2A as a therapeutic target in inflammation and neurodegeneration

Protein phosphatase 2A (PP2A) is a highly complex heterotrimeric enzyme that catalyzes the selective removal of phosphate groups from protein serine and threonine residues. Emerging evidence suggests that it functions as a tumor suppressor by constraining phosphorylation-dependent signalling pathways that regulate cellular transformation and metastasis. Therefore, PP2A-activating drugs (PADs) are being actively sought and investigated as potential novel anti-cancer treatments. Here we explore the concept that PP2A also constrains inflammatory responses through its inhibitory effects on various signalling pathways, suggesting that PADs may be effective in the treatment of inflammation-mediated pathologies.

The RNA-Binding Protein KSRP Modulates Cytokine Expression of CD4+ T Cells

The KH-type splicing regulatory protein (KSRP) is a RNA-binding protein, which regulates the stability of many mRNAs encoding immune-relevant proteins. As KSRP regulates innate immune responses, for instance by the modulation of type I interferon mRNA stability, we were interested whether knockdown of the protein (KSRP^−/−) interferes with T cell activation and polarization. Polyclonally stimulated KSRP^−/− CD4⁺ T cells proliferated at a higher extent and higher frequency and expressed the activation marker CD25 more than wild-type T cells. In supernatants of stimulated KSRP^−/− CD4⁺ T cells, levels of IL-5, IL-9, IL-10, and IL-13 were observed to be increased compared to those of the control group. KSRP^−/− CD8⁺ T cells showed no altered proliferative capacity upon polyclonal stimulation, but supernatants contained lower levels of interferon-γ. Similar changes in the cytokine expression patterns were also detected in T cells derived from KSRP^−/− mice undergoing arthritis induction indicative of a pathophysiological role of KSRP-dependent T cell polarization. We demonstrated the direct binding of KSRP to the 3′ untranslated region of IL-13, IL-10, and IFN-γ mRNA in in vitro experiments. Moreover, since IL-4 mRNA decay was reduced in KSRP^−/− CD4⁺ T cells, we identify KSRP as a negative regulator of IL-4 expression. These data indicate that overexpression of IL-4, which constitutes the primary inducer of Th2 polarization, may cause the Th2 bias of polyclonally stimulated KSRP^−/− CD4⁺ T cells. This is the first report demonstrating that KSRP is involved in the regulation of T cell responses. We present strong evidence that T cells derived from KSRP^−/− mice favor Th2-driven immune responses.

The KH-type splicing regulatory protein (KSRP) regulates type III interferon expression post-transcriptionally

Type III interferons (IFNs) are the latest members of the IFN family. They play an important role in immune defense mechanisms, especially in antiviral responses at mucosal sites. Moreover, they control inflammatory reactions by modulating neutrophil and dendritic cell functions. Therefore, it is important to identify cellular mechanisms involved in the control of type III IFN expression. All IFN family members contain AU-rich elements (AREs) in the 3'-untranslated regions (3'-UTR) of their mRNAs that determine mRNA half-life and consequently the expressional level of these cytokines. mRNA stability is controlled by different proteins binding to these AREs leading to either stabilization or destabilization of the respective target mRNA. The KH-type splicing regulatory protein KSRP (also named KHSRP) is an important negative regulator of ARE-containing mRNAs. Here, we identify the interferon lambda 3 (IFNL3) mRNA as a new KSRP target by pull-down and immunoprecipitation experiments, as well as luciferase reporter gene assays. We characterize the KSRP-binding site in the IFNL3 3'-UTR and demonstrate that KSRP regulates the mRNA half-life of the IFNL3 transcript. In addition, we detect enhanced expression of IFNL3 mRNA in KSRP-/- mice, establishing a negative regulatory function of KSRP in type III IFN expression also in vivo Besides KSRP the RNA-binding protein AUF1 (AU-rich element RNA-binding protein 1) also seems to be involved in the regulation of type III IFN mRNA expression.

Control of cytokine mRNA degradation by the histone deacetylase inhibitor ITF2357 in rheumatoid arthritis fibroblast-like synoviocytes: beyond transcriptional regulation

BACKGROUND

Histone deacetylase inhibitors (HDACi) suppress cytokine production in immune and stromal cells of patients with rheumatoid arthritis (RA). Here, we investigated the effects of the HDACi givinostat (ITF2357) on the transcriptional and post-transcriptional regulation of inflammatory markers in RA fibroblast-like synoviocytes (FLS).

METHODS

The effects of ITF2357 on the expression and messenger RNA (mRNA) stability of IL-1β-inducible genes in FLS were analyzed using array-based qPCR and Luminex. The expression of primary and mature cytokine transcripts, the mRNA levels of tristetraprolin (TTP, or ZFP36) and other AU-rich element binding proteins (ARE-BP) and the cytokine profile of fibroblasts derived from ZFP36^+/+ and ZFP36^-/- mice was measured by qPCR. ARE-BP silencing was performed by small interfering RNA (siRNA)-mediated knockdown, and TTP post-translational modifications were analyzed by immunoblotting.

RESULTS

ITF2357 reduced the expression of 85% of the analyzed IL-1β-inducible transcripts, including cytokines (IL6, IL8), chemokines (CXCL2, CXCL5, CXCL6, CXCL10), matrix-degrading enzymes (MMP1, ADAMTS1) and other inflammatory mediators. Analyses of mRNA stability demonstrated that ITF2357 accelerates IL6, IL8, PTGS2 and CXCL2 mRNA degradation, a phenomenon associated with the enhanced transcription of TTP, but not other ARE-BP, and the altered post-translational status of TTP protein. TTP knockdown potentiated cytokine production in RA FLS and murine fibroblasts, which in the latter case was insensitive to inhibition by ITF2357 treatment.

CONCLUSIONS

Our study identifies that regulation of cytokine mRNA stability is a predominant mechanism underlying ITF2357 anti-inflammatory properties, occurring via regulation of TTP. These results highlight the therapeutic potential of ITF2357 in the treatment of RA.

The control of inflammation via the phosphorylation and dephosphorylation of tristetraprolin: a tale of two phosphatases

Twenty years ago, the first description of a tristetraprolin (TTP) knockout mouse highlighted the fundamental role of TTP in the restraint of inflammation. Since then, work from several groups has generated a detailed picture of the expression and function of TTP. It is a sequence-specific RNA-binding protein that orchestrates the deadenylation and degradation of several mRNAs encoding inflammatory mediators. It is very extensively post-translationally modified, with more than 30 phosphorylations that are supported by at least two independent lines of evidence. The phosphorylation of two particular residues, serines 52 and 178 of mouse TTP (serines 60 and 186 of the human orthologue), has profound effects on the expression, function and localisation of TTP. Here, we discuss the control of TTP biology via its phosphorylation and dephosphorylation, with a particular focus on recent advances and on questions that remain unanswered.

Tristetraprolin expression by keratinocytes controls local and systemic inflammation

Tristetraprolin (TTP, encoded by the Zfp36 gene) regulates the mRNA stability of several important cytokines. Due to the critical role of this RNA-binding protein in the control of inflammation, TTP deficiency leads to the spontaneous development of a complex inflammatory syndrome. So far, this phenotype has been largely attributed to dysregulated production of TNF and IL‑23 by myeloid cells, such as macrophages or DCs. Here, we generated mice with conditional deletion of TTP in keratinocytes (Zfp36fl/flK14-Cre mice, referred to herein as Zfp36ΔEP mice). Unlike DC-restricted (CD11c-Cre) or myeloid cell-restricted (LysM-Cre) TTP ablation, these mice developed exacerbated inflammation in the imiquimod-induced psoriasis model. Furthermore, Zfp36ΔEP mice progressively developed a spontaneous pathology with systemic inflammation, psoriatic-like skin lesions, and dactylitis. Finally, we provide evidence that keratinocyte-derived TNF production drives these different pathological features. In summary, these findings expand current views on the initiation of psoriasis and related arthritis by revealing the keratinocyte-intrinsic role of TTP.

Activation of liver X receptors promotes inflammatory cytokine mRNA degradation by upregulation of tristetraprolin

Liver X receptors (LXRs) have anti-inflammatory properties. Whether LXRs play a role in post-transcriptional control of inflammatory cytokine expression is not clear. Here, we firstly identified that the synthetic LXR agonist T0901317 promoted IL-1β, IL-6 and TNFα mRNA degradation. Moreover, T0901317 destabilized TNFα mRNA through its 3'-untranslated region. In addition, T0901317 increased the expression of tristetraprolin (TTP), while antagonizing TTP with siRNA abrogated T0901317-mediated inflammatory cytokine mRNA decay. Interestingly, T0901317 repressed LPS-induced phosphorylation of ERK1/2 and p38 mitogen-activated protein kinase (MAPK) in THP-1 macrophages. The evidence presented here confirms that LXR activation with T0901317 inhibits the phosphorylation of ERK1/2 and p38 MAPK, likely resulting in the increased expression of TTP and the decay of LPS-induce inflammatory cytokine mRNAs.

Treatment of inflammatory arthritis via targeting of tristetraprolin, a master regulator of pro-inflammatory gene expression

OBJECTIVES

Tristetraprolin (TTP), a negative regulator of many pro-inflammatory genes, is strongly expressed in rheumatoid synovial cells. The mitogen-activated protein kinase (MAPK) p38 pathway mediates the inactivation of TTP via phosphorylation of two serine residues. We wished to test the hypothesis that these phosphorylations contribute to the development of inflammatory arthritis, and that, conversely, joint inflammation may be inhibited by promoting the dephosphorylation and activation of TTP.

METHODS

The expression of TTP and its relationship with MAPK p38 activity were examined in non-inflamed and rheumatoid arthritis (RA) synovial tissue. Experimental arthritis was induced in a genetically modified mouse strain, in which endogenous TTP cannot be phosphorylated and inactivated. In vitro and in vivo experiments were performed to test anti-inflammatory effects of compounds that activate the protein phosphatase 2A (PP2A) and promote dephosphorylation of TTP.

RESULTS

TTP expression was significantly higher in RA than non-inflamed synovium, detected in macrophages, vascular endothelial cells and some fibroblasts and co-localised with MAPK p38 activation. Substitution of TTP phosphorylation sites conferred dramatic protection against inflammatory arthritis in mice. Two distinct PP2A agonists also reduced inflammation and prevented bone erosion. In vitro anti-inflammatory effects of PP2A agonism were mediated by TTP activation.

CONCLUSIONS

The phosphorylation state of TTP is a critical determinant of inflammatory responses, and a tractable target for novel anti-inflammatory treatments.

Three Residues Make an Evolutionary Switch for Folding and RNA-Destabilizing Activity in the TTP Family of Proteins

Tristetraprolin (TTP) binds to mRNA transcripts to promote their degradation. The TTP protein family in humans includes two other proteins, TIS11b and TIS11d. All three proteins contain a highly homologous RNA binding domain (RBD) that consists of two CCCH zinc fingers (ZFs). Both ZFs are folded in the absence of RNA in TIS11d and TIS11b. In TTP, however, only ZF1 adopts a stable fold. The focus of this study is to understand the origin and biological significance of the structural differences of the RBD. We identified three residues that affect the affinity for the structural Zn(2+) and determine the folding of ZF2 in the absence of RNA. We observed that the mRNA destabilizing activity of TTP was increased when the partially disordered RBD of TTP was replaced with the fully structured RBD of TIS11d, indicating that differences in the folded state of the RBD affect the activity of the proteins in the cell.

Tristetraprolin Recruits Eukaryotic Initiation Factor 4E2 To Repress Translation of AU-Rich Element-Containing mRNAs

Tristetraprolin (TTP) regulates the expression of AU-rich element-containing mRNAs through promoting the degradation and repressing the translation of target mRNA. While the mechanism for promoting target mRNA degradation has been extensively studied, the mechanism underlying translational repression is not well established. Here, we show that TTP recruits eukaryotic initiation factor 4E2 (eIF4E2) to repress target mRNA translation. TTP interacted with eIF4E2 but not with eIF4E. Overexpression of eIF4E2 enhanced TTP-mediated translational repression, and downregulation of endogenous eIF4E2 or overexpression of a truncation mutant of eIF4E2 impaired TTP-mediated translational repression. Overexpression of an eIF4E2 mutant that lost the cap-binding activity also impaired TTP's activity, suggesting that the cap-binding activity of eIF4E2 is important in TTP-mediated translational repression. We further show that TTP promoted eIF4E2 binding to target mRNA. These results imply that TTP recruits eIF4E2 to compete with eIF4E to repress the translation of target mRNA. This notion is supported by the finding that downregulation of endogenous eIF4E2 increased the production of tumor necrosis factor alpha (TNF-α) protein without affecting the mRNA levels in THP-1 cells. Collectively, these results uncover a novel mechanism by which TTP represses target mRNA translation.

Dual-Specificity Phosphatase 1 and Tristetraprolin Cooperate To Regulate Macrophage Responses to Lipopolysaccharide

Dual-specificity phosphatase (DUSP) 1 dephosphorylates and inactivates members of the MAPK superfamily, in particular, JNKs, p38α, and p38β MAPKs. It functions as an essential negative regulator of innate immune responses, hence disruption of the Dusp1 gene renders mice extremely sensitive to a wide variety of experimental inflammatory challenges. The principal mechanisms behind the overexpression of inflammatory mediators by Dusp1(-/-) cells are not known. In this study, we use a genetic approach to identify an important mechanism of action of DUSP1, involving the modulation of the activity of the mRNA-destabilizing protein tristetraprolin. This mechanism is key to the control of essential early mediators of inflammation, TNF, CXCL1, and CXCL2, as well as the anti-inflammatory cytokine IL-10. The same mechanism also contributes to the regulation of a large number of transcripts induced by treatment of macrophages with LPS. These findings demonstrate that modulation of the phosphorylation status of tristetraprolin is an important physiological mechanism by which innate immune responses can be controlled.

Tristetraprolin expression and microRNA-mediated regulation during simian immunodeficiency virus infection of the central nervous system

BACKGROUND

The RNA-binding protein tristetraprolin (TTP) participates in normal post-transcriptional control of cytokine and chemokine gene expression, dysregulation of which contributes to the HIV-associated neurocognitive disorders. Transcriptional and post-transcriptional regulation of TTP has been described, including regulation by microRNA-29a. In the simian immunodeficiency virus (SIV) model of HIV CNS disease, control of cytokine/chemokine expression coincides with the end of acute phase infection. This control is lost during progression to disease. In this study, we assessed TTP regulation and association with cytokine regulation in the brain during SIV infection.

RESULTS

Quantitation of TTP expression over the course of SIV infection revealed downregulation of TTP during acute infection, maintenance of relatively low levels during asymptomatic phase, and increased expression only during late-stage CNS disease, particularly in association with severe disease. The ability of miR-29a to regulate TTP was confirmed, and evidence for additional miRNA targeters of TTP was found. However, increased miR-29a expression in brain was not found to be significantly negatively correlated with TTP. Similarly, increased TTP during late-stage disease was not associated with lower cytokine expression.

CONCLUSIONS

TTP expression is regulated during SIV infection of the CNS. The lack of significant negative correlation of miR-29a and TTP expression levels suggests that while miR-29a may contribute to TTP regulation, additional factors are involved. Reduced TTP expression during acute infection is consistent with increased cytokine production during this phase of infection, but the increases in TTP observed during late-stage infection were insufficient to halt runaway cytokine levels. While antisense inhibitors of the post-transcriptional targeters of TTP identified here could conceivably be used further to augment TTP regulation of cytokines, it is possible that high levels of TTP are undesirable. Additional research is needed to characterize members of the miRNA/TTP/cytokine regulatory network and identify nodes that may be best targeted therapeutically to ameliorate the effects of chronic inflammation in retrovirus-associated CNS disease.

Multiple functions of tristetraprolin/TIS11 RNA-binding proteins in the regulation of mRNA biogenesis and degradation

Members of the tristetraprolin (TTP/TIS11) family are important RNA-binding proteins initially characterized as mediators of mRNA degradation. They act via their interaction with AU-rich elements present in the 3'UTR of regulated transcripts. However, it is progressively appearing that the different steps of mRNA processing and fate including transcription, splicing, polyadenylation, translation, and degradation are coordinately regulated by multifunctional integrator proteins that possess a larger panel of functions than originally anticipated. Tristetraprolin and related proteins are very good examples of such integrators. This review gathers the present knowledge on the functions of this family of RNA-binding proteins, including their role in AU-rich element-mediated mRNA decay and focuses on recent advances that support the concept of their broader involvement in distinct steps of mRNA biogenesis and degradation.

Relationship of GW/P-bodies with stress granules

Whereas P-bodies are intimately linked to the cytoplasmic RNA decay machinery, stress granules harbor stalled translation initiation complexes that accumulate upon stress-induced translation arrest. In this Chapter, we reflect on the relationship between P-bodies and stress granules. In mammalian cells, the two structures can be clearly distinguished from each other using specific protein or RNA markers, but they also share many proteins and mRNAs. While the formation of P-bodies and stress granules is coordinately triggered by stress, their assembly appears to be regulated independently by different pathways. Under certain types of stress, P-bodies frequently dock with stress granules, and overexpressing certain proteins that localize to both structures can cause P-body/stress granule fusion. Currently available data suggest that these self-assembling compartments are controlled by flux of mRNAs within the cytoplasm, and that their assembly mirrors the translation and degradation rates of their component mRNAs.

The roles of TTP and BRF proteins in regulated mRNA decay

Adenylate- and uridylate-rich element (ARE) motifs are cis-acting elements present in the 3′ untranslated region of mRNA transcripts that encode many inflammation- and cancer-associated genes. The TIS11 family of RNA-binding proteins, composed of tristetraprolin (TTP) and butyrate response factors 1 and 2 (BRF-1 and -2), plays a critical role in regulating the expression of ARE-containing mRNAs. Through their ability to bind and target ARE-containing mRNAs for rapid degradation, this class of RNA-binding proteins serves a fundamental role in limiting the expression of a number of critical genes, thereby exerting anti-inflammatory and anti-cancer effects. Regulation of TIS11 family members occurs on a number of levels through cellular signaling events to control their transcription, mRNA turnover, phosphorylation status, cellular localization, association with other proteins, and proteosomal degradation, all of which impact TIS11 members' ability to promote ARE-mediated mRNA decay along with decay-independent functions. This review summarizes our current understanding of posttranscriptional regulation of ARE-containing gene expression by TIS11 family members and discusses their role in maintaining normal physiological processes and the pathological consequences in their absence.

Cullin 4B is recruited to tristetraprolin-containing messenger ribonucleoproteins and regulates TNF-α mRNA polysome loading

TNF-α is a central mediator of inflammation and critical for host response to infection and injury. TNF-α biosynthesis is controlled by transcriptional and posttranscriptional mechanisms allowing for rapid, transient production. Tristetraprolin (TTP) is an AU-rich element binding protein that regulates the stability of the TNF-α mRNA. Using a screen to identify TTP-interacting proteins, we identified Cullin 4B (Cul4B), a scaffolding component of the Cullin ring finger ligase family of ubiquitin E3 ligases. Short hairpin RNA knockdown of Cul4B results in a significant reduction in TNF-α protein and mRNA in LPS-stimulated mouse macrophage RAW264.7 cells as well as a reduction in TTP protein. TNF-α message t(1/2) was reduced from 69 to 33 min in LPS-stimulated cells. TNF-3' untranslated region luciferase assays utilizing wild-type and mutant TTP-AA (S52A, S178A) indicate that TTP function is enhanced in Cul4B short hairpin RNA cells. Importantly, the fold induction of TNF-α mRNA polysome loading in response to LPS stimulation is reduced by Cul4B knockdown. Cul4B is present on the polysomes and colocalizes with TTP to exosomes and processing bodies, which are sites of mRNA decay. We conclude that Cul4B licenses the TTP-containing TNF-α messenger ribonucleoprotein for loading onto polysomes, and reduction of Cul4B expression shunts the messenger ribonucleoproteins into the degradative pathway.

A flexible approach to studying post-transcriptional gene regulation in stably transfected mammalian cells

The study of post-transcriptional regulation is constrained by the technical limitations associated with both transient and stable transfection of chimeric reporter plasmids examining the activity of 3'-UTR cis-acting elements. We report the adaptation of a commercially available system that enables consistent stable integration of chimeric reporter cDNA into a single genomic site in which transcription is induced by tetracycline. Using this system, we demonstrate the tight control afforded by this system and its suitability in mapping the regulatory function of defined cis-acting elements in the human TNF 3'-UTR, as well as the distinct effects of serum starvation on transiently transfected and stably integrated chimeric reporter genes.

Cytotoxicity and effects on inflammatory response of modified types of cellulose in macrophage-like THP-1 cells

The cytotoxicity and in vitro effects of six variously modified types of cellulose (OC--oxidized cellulose, NaOC--oxidized cellulose sodium salt, DAC--dialdehyde cellulose, CMC--carboxymethyl cellulose, MFC--microfibrilated cellulose, and MCC--microcrystalline cellulose) on the inflammatory response in macrophage-like THP-1 cells were examined, with special focus on their ability to influence gene expression and the production of TNF-α. The study provides evidence that DAC exerts a marked effect on the induction of TNF-α gene expression and its subsequent production in human macrophages. Thus, the use of DAC for anti-hemorrhagic or wound-healing therapy should be considered carefully with regard to its pro-inflammatory activity. On the contrary, MCC showed significant anti-inflammatory effects in the LPS-induced conditions, which might be beneficial for the treatment of non-healing chronic wounds, e.g., diabetic or venous ulcers.

Analysis of post-transcriptional regulation using the FunREG method

An increasing number of arguments, including altered microRNA expression, support the idea that post-transcriptional deregulation participates in gene disturbances found in diseased tissues. To evaluate this hypothesis, we developed a method which facilitates post-transcriptional investigations in a wide range of human cells and experimental conditions. This method, called FunREG (functional, integrated and quantitative method to measure post-transcriptional regulation), connects lentiviral transduction with a fluorescent reporter system and quantitative PCR. Using FunREG, we efficiently measured post-transcriptional regulation mediated either by selected RNA sequences or regulatory factors (microRNAs), and then evaluated the contribution of mRNA decay and translation efficiency in the observed regulation. We demonstrated the existence of gene-specific post-transcriptional deregulation in liver tumour cells, and also reported a molecular link between a transcript variant abrogating HDAC6 (histone deacetylase 6) regulation by miR-433 and a rare familial genetic disease. Because FunREG is sensitive, quantitative and easy to use, many applications can be envisioned in fundamental and pathophysiological research.

CARHSP1 is required for effective tumor necrosis factor alpha mRNA stabilization and localizes to processing bodies and exosomes

Tumor necrosis factor alpha (TNF-α) is a critical mediator of inflammation, and its production is tightly regulated, with control points operating at nearly every step of its biosynthesis. We sought to identify uncharacterized TNF-α 3' untranslated region (3'UTR)-interacting proteins utilizing a novel screen, termed the RNA capture assay. We identified CARHSP1, a cold-shock domain-containing protein. Knockdown of CARHSP1 inhibits TNF-α protein production in lipopolysaccharide (LPS)-stimulated cells and reduces the level of TNF-α mRNA in both resting and LPS-stimulated cells. mRNA stability assays demonstrate that CARHSP1 knockdown decreases TNF-α mRNA stability from a half-life (t(1/2)) of 49 min to a t(1/2) of 22 min in LPS-stimulated cells and from a t(1/2) of 29 min to a t(1/2) of 24 min in resting cells. Transfecting CARHSP1 into RAW264.7 cells results in an increase in TNF-α 3'UTR luciferase expression in resting cells and CARHSP1 knockdown LPS-stimulated cells. We examined the functional effect of inhibiting Akt, calcineurin, and protein phosphatase 2A and established that inhibition of Akt or calcineurin but not PP2A inhibits CARHSP1 function. Subcellular analysis establishes CARHSP1 as a cytoplasmic protein localizing to processing bodies and exosomes but not on translating mRNAs. We conclude CARHSP1 is a TNF-α mRNA stability enhancer required for effective TNF-α production, demonstrating the importance of both stabilization and destabilization pathways in regulating the TNF-α mRNA half-life.

Regulation of tristetraprolin expression by interleukin-1 beta and dexamethasone in human pulmonary epithelial cells: roles for nuclear factor-kappa B and p38 mitogen-activated protein kinase

The mRNA-destabilizing protein tristetraprolin (TTP) negatively regulates adenine- and uridine-rich element (ARE)-containing mRNAs. In A549 pulmonary cells, TTP mRNA and both a approximately 40- and a approximately 45-kDa phosphorylated version of TTP protein were rapidly induced in response to interleukin (IL)-1beta. Analysis with IkappaBalphaDeltaN, a dominant version of inhibitor of kappaBalpha (IkappaBalpha), as well as dominant-negative and small-molecule IkappaB kinase (IKK) inhibitors demonstrated that IL-1beta-induced TTP is nuclear factor-kappaB (NF-kappaB)-dependent. Likewise, TTP expression and formation of the approximately 45-kDa phosphorylated form of TTP are blocked by the p38 mitogen-activated protein kinase (MAPK) inhibitor 4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole (SB203580). By contrast, and despite a 3- to 4-fold induction of TTP mRNA, the anti-inflammatory glucocorticoid dexamethasone only modestly induced expression of the approximately 40-kDa form of TTP. In the context of IL-1beta, dexamethasone exerted a marginal repressive effect on TTP mRNA expression and more considerably reduced TTP protein. Given a requirement for p38 MAPK in the induction of TTP by IL-1beta, this repressive effect may be explained by repression of the p38 MAPK pathway by dexamethasone. Knockdown of TTP protein by siRNA elevated IL-1beta-induced expression of granulocyte macrophage-colony-stimulating factor (GM-CSF) and IL-8, demonstrating a role for TTP in feedback control. Likewise, knockdown of TTP increased GM-CSF expression in the presence of IL-1beta plus dexamethasone, suggesting that feedback control by TTP also occurs in the context of IL-1beta plus dexamethasone. Taken together, our data demonstrate that NF-kappaB and p38 MAPK are critical to the induction of TTP by IL-1beta and that TTP induction provides feedback control of the ARE-containing genes GM-CSF and IL-8.

Separate cis-trans pathways post-transcriptionally regulate murine CD154 (CD40 ligand) expression: a novel function for CA repeats in the 3'-untranslated region

We report a role for CA repeats in the 3'-untranslated region (3'-UTR) in regulating CD154 expression. Human CD154 is encoded by an unstable mRNA; this instability is conferred in cis by a portion of its 3'-UTR that includes a polypyrimidine-rich region and CA dinucleotide repeat. We demonstrate similar instability activity with the murine CD154 3'-UTR. This instability element mapped solely to a conserved 100-base CU-rich region alone, which we call a CU-rich response element. Surprisingly, the CA dinucleotide-rich region also regulated reporter expression but at the level of translation. This activity was associated with poly(A) tail shortening and regulated by heterogeneous nuclear ribonucleoprotein L levels. We conclude that the CD154 3'-UTR contains dual cis-acting elements, one of which defines a novel function for exonic CA dinucleotide repeats. These findings suggest a mechanism for the association of 3'-UTR CA-rich response element polymorphisms with CD154 overexpression and the subsequent risk of autoimmune disease.

Identification of TTP mRNA targets in human dendritic cells reveals TTP as a critical regulator of dendritic cell maturation

Dendritic cells provide a critical link between innate and adaptive immunity and are essential to prime a naive T-cell response. The transition from immature dendritic cells to mature dendritic cells involves numerous changes in gene expression; however, the role of post-transcriptional changes in this process has been largely ignored. Tristetraprolin is an AU-rich element mRNA-binding protein that has been shown to regulate the stability of a number of cytokines and chemokines of mRNAs. Using TTP immunoprecipitations and Affymetrix GeneChips, we identified 393 messages as putative TTP mRNA targets in human dendritic cells. Gene ontology analysis revealed that approximately 25% of the identified mRNAs are associated with protein synthesis. We also identified six MHC Class I alleles, five MHC Class II alleles, seven chemokine and chemokine receptor genes, indoleamine 2,3 dioxygenase, and CD86 as putative TTP ligands. Real-time PCR was used to validate the GeneChip data for 15 putative target genes and functional studies performed for six target genes. These data establish that TTP regulates the expression of DUSP1, IDO, SOD2, CD86, and MHC Class I-B and F via the 3'-untranslated region of each gene. A novel finding is the demonstration that TTP can interact with and regulate the expression of non-AU-rich element-containing messages. The data implicate TTP as having a broader role in regulating and limiting the immune response than previously suspected.

Genome-wide analysis identifies interleukin-10 mRNA as target of tristetraprolin

Tristetraprolin (TTP) is an RNA-binding protein required for the rapid degradation of mRNAs containing AU-rich elements. Targets regulated by TTP include the mRNAs encoding tumor necrosis factor-alpha, granulocyte-macrophage colony-stimulating factor, interleukin-2 (IL-2), and immediate early response 3. To identify novel target mRNAs of TTP in macrophages, we used a genome-wide approach that combines RNA immunoprecipitation and microarray analysis. A list was compiled of 137 mRNAs that are associated with TTP with an estimated accuracy on the order of 90%. Sequence analysis revealed a highly significant enrichment of AU-rich element motifs, with AUUUA pentamers present in 96% and UUAUUUAUU nonamers present in 44% of TTP-associated mRNAs. We further show that IL-10 is a novel target regulated by TTP. IL-10 mRNA levels were found to be elevated because of a reduced decay rate in primary macrophages from TTP(-/-) mice. Our study demonstrates the importance of experimental approaches for identifying targets of RNA-binding proteins.

Extracellular signal-regulated kinase regulation of tumor necrosis factor-alpha mRNA nucleocytoplasmic transport requires TAP-NxT1 binding and the AU-rich element

Tumor necrosis factor-alpha (TNF-alpha) production is regulated by transcriptional and posttranscriptional mechanisms. Lipopolysaccharide activates the NFkappaB pathway increasing TNF-alpha transcription. Lipopolysaccharide also activates the mitogen-activated protein kinase pathways, resulting in stabilization and enhanced translation of the TNF-alpha message. In addition, nuclear export of the TNF-alpha mRNA is a posttranscriptionally regulated process involving the Tpl2-ERK pathway and requiring the presence of the TNF-alpha AU-rich element (ARE). We demonstrate that nuclear export of the TNF-alpha message requires not only the TNF-alpha ARE but also the interaction of the proteins TAP and NxT1, both of which are involved in nucleocytoplasmic transport of mRNA. Through the use of dominant negative ERK1 and ERK2, we establish that control of TNF-alpha mRNA nuclear export operates specifically through ERK1. Finally, we examined the role of two established TNF-alpha ARE-binding proteins, HuR and tristetraprolin, that shuttle between the nucleus and cytoplasm. These data demonstrate that neither tristetraprolin nor HuR is required for TNF-alpha mRNA export. It is unclear at this time if ARE-binding protein(s) directly interact with the TAP-NxT1 complex, if each complex is independently targeted by ERK1, or if only one complex is targeted.

Tristetraprolin regulates TNF TNF-alpha mRNA stability via a proteasome dependent mechanism involving the combined action of the ERK and p38 pathways

Tumor necrosis factor-alpha (TNF-alpha) is a central mediator of inflammation. TNF-alpha expression is regulated by transcriptional and post-transcriptional mechanisms, including mRNA stability and translation. Post-transcriptional control operates through cis-elements in the 3' Untranslated-Region of the TNF-alpha mRNA to which trans-acting proteins bind. One of the best characterized trans-acting proteins is Tristetraprolin (TTP), which regulates TNF-alpha message stability. However, the precise mechanisms controlling TNF-alpha message stability are unclear, with data supporting a role for the proteasome, the exosome, and the RNA processing-body (P-body), as well as the involvement of the microRNAs. We examined the effect of proteasome inhibition on endogenous TNF-alpha mRNA stability, TNF-alpha 3'UTR reporter expression and TTP function in the RAW264.7 cells. These data establish that proteasome inhibition stabilized endogenous TNF-alpha mRNA, increased TTP protein levels but inhibited TTP mediated TNF-alpha mRNA decay. Importantly, proteasome inhibition stabilized the TNF-alpha message to the same degree as LPS stimulation. To further characterize the control of TTP function, we examined the combinatorial effect of p38, ERK and JNK activation on TNF-alpha post-transcriptional expression and TTP function. These data establish that TTP mediated TNF-alpha mRNA decay is inhibited by the combined activation of ERK and p38 and not by p38 activation alone. The combined activation of ERK/p38 was sufficient to stabilize endogenous TNF-alpha mRNA to the same degree as LPS stimulation. Together these data indicate that the proteasome is a critical control point for TTP mediated TNF-alpha mRNA decay and activation of both ERK and p38 is required to inhibit TTP function and stabilize TNF-alpha mRNA.

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.

Tristetraprolin (TTP)-14-3-3 complex formation protects TTP from dephosphorylation by protein phosphatase 2a and stabilizes tumor necrosis factor-alpha mRNA

Tumor necrosis factor (TNF)-alpha is a major cytokine produced by alveolar macrophages in response to pathogen-associated molecular patterns such as lipopolysaccharide. TNF-alpha secretion is regulated at both transcriptional and post-transcriptional levels. Post-transcriptional regulation occurs by modulation of TNF-alpha mRNA stability via the binding of tristetraprolin (TTP) to the adenosine/uridine-rich elements found in the 3'-untranslated region of the TNF-alpha transcript. Phosphorylation plays important roles in modulating mRNA stability, because activation of p38 MAPK by lipopolysaccharide stabilizes TNF-alpha mRNA. We hypothesized that the protein phosphatase 2A (PP2A) regulates this signaling pathway. Our results show that inhibition of PP2A by okadaic acid or small interference RNA significantly enhanced the stability of TNF-alpha mRNA. This result was associated with increased phosphorylation of p38 MAPK and MAPK-activated kinase 2 (MK-2). PP2A inhibition increased TTP phosphorylation and enhanced complex formation with chaperone protein 14-3-3. TTP physically interacted with PP2A in transfected mammalian cells. A functional consequence of TTP-14-3-3 complex formation appeared to be protection of TTP from dephosphorylation by inhibition of the binding of PP2A to phosphorylated TTP. Mutation of the MK-2 phosphorylation sites of TTP did not influence TNF-alpha adenosine/uridine-rich element binding and did not alter the increased TNF-alpha 3'-untranslated region-dependent luciferase activity induced by PP2A-small interference RNA silencing. Our data indicate that, although phosphorylation stabilizes TNF-alpha mRNA, PP2A regulates the mRNA stability by modulating the phosphorylation state of members of the p38/MK-2/TTP pathway.

Glucocorticoids regulate tristetraprolin synthesis and posttranscriptionally regulate tumor necrosis factor alpha inflammatory signaling

Glucocorticoids are used to treat various inflammatory disorders, but the mechanisms underlying these actions are incompletely understood. The zinc finger protein tristetraprolin (TTP) destabilizes several proinflammatory cytokine mRNAs by binding to AU-rich elements within their 3' untranslated regions, targeting them for degradation. Here we report that glucocorticoids induce the synthesis of TTP mRNA and protein in A549 lung epithelial cells and in rat tissues. Dexamethasone treatment leads to a sustained induction of TTP mRNA expression that is abrogated by RU486. Glucocorticoid induction of TTP mRNA is also blocked by actinomycin D but not by cycloheximide, suggesting a transcriptional mechanism which has been confirmed by transcription run-on experiments. The most widely characterized TTP-regulated gene is the AU-rich tumor necrosis factor alpha (TNF-alpha) gene. Dexamethasone represses TNF-alpha mRNA in A549 cells and decreases luciferase expression of a TNF-alpha 3' untranslated region reporter plasmid in an orientation-dependent manner. Small interfering RNAs to TTP significantly prevent this effect, and a cell line stably expressing a short-hairpin RNA to TTP conclusively establishes that TTP is critical for dexamethasone inhibition of TNF-alpha mRNA expression. These studies provide the molecular evidence for glucocorticoid regulation of human TTP and reflect a novel inductive anti-inflammatory signaling pathway for glucocorticoids that acts via posttranscriptional mechanisms.

Tristetraprolin regulates Cyclin D1 and c-Myc mRNA stability in response to rapamycin in an Akt-dependent manner via p38 MAPK signaling

The differential expression of the critical cell cycle control proteins cyclin D1 and c-myc has been shown to result in Akt-dependent hypersensitivity of tumor cells to mTOR inhibitors. We have previously demonstrated that the differential utilization of internal ribosome entry sites within the mRNAs of these transcripts allows maintenance of protein synthesis in the face of rapamycin (rapa) exposure in an Akt-dependent manner. Here, we demonstrate that in addition to this mechanism, cyclin D1 and c-myc mRNA stability is also coordinately regulated following rapa treatment depending on Akt activity status. We identify A/U-rich response elements within the 3' untranslated regions (UTRs) of these transcripts, which confer the observed differential stabilities and show that the RNA-binding protein, tristetraprolin (TTP), interacts with these elements. We also present evidence that TTP accumulates in response to rapa exposure, binds to the cis-acting elements within the cyclin D1 and c-myc 3' UTRs and is differentially serine phosphorylated in an Akt-dependent manner. Furthermore, the differential phosphorylation status of TTP results in its sequestration by 14-3-3 proteins in quiescent Akt-containing cells. Finally, siRNA-mediated knockdown of TTP expression or inhibiting a known regulator of TTP phosphorylation, p38 MAP kinase, abolishes the effects on cyclin D1 and c-myc mRNA stability. We assume that the differential control of cyclin D1 and c-myc mRNA stability and translational efficiency constitutes a coordinate response to rapa contributing to the maintenance of expression of these determinants in rapa-resistant quiescent Akt-containing cells following exposure.

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.

Structure/function analysis of tristetraprolin (TTP): p38 stress-activated protein kinase and lipopolysaccharide stimulation do not alter TTP function

Tristetraprolin (TTP) is the only trans-acting factor shown to be capable of regulating AU-rich element-dependent mRNA turnover at the level of the intact animal; however, the mechanism by which TTP mediated RNA instability is unknown. Using an established model system, we performed structure/function analysis with TTP as well as examined the current hypothesis that TTP function is regulated by p38-MAPKAP kinase 2 (MK2) activation. Deletion of either the N- or C-terminal domains inhibited TTP function. Extensive mutagenesis, up to 16%, of serines and threonines, some of which were predicted to mediate proteasomal targeting, did not alter human TTP function. Mutation of the conserved MK2 phosphorylation sites enhanced human TTP function in both resting and p38-stress-activated protein kinase-MK2-activated cells. However, p38-stress-activated protein kinase-MK2 activation did not alter the activity of either wild-type or mutant TTP. TTP localized to the stress granules, with arsenite treatment reducing this localization. In contrast, arsenite treatment enhanced stress granule localization of the MK2 mutant, consistent with the involvement of additional pathways regulating this event. Finally, we determined that, in response to LPS stimulation, human TTP moves onto the polysomes, and this movement occurs in the absence of 14-3-3. Taken together, these data indicate that, although p38 activation alters TTP entry into the stress granule, it does not alter TTP function. Moreover, the interaction of TTP with 14-3-3, which may limit entry into the stress granule, is not involved in the downstream message stabilization events.

Involvement of KSRP in the post-transcriptional regulation of human iNOS expression-complex interplay of KSRP with TTP and HuR

We purified the KH-type splicing regulatory protein (KSRP) as a protein interacting with the 3′-untranslated region (3′-UTR) of the human inducible nitric oxide (iNOS) mRNA. Immunodepletion of KSRP enhanced iNOS 3′-UTR RNA stability in in vitro-degradation assays. In DLD-1 cells overexpressing KSRP cytokine-induced iNOS expression was markedly reduced. In accordance, downregulation of KSRP expression increases iNOS expression by stabilizing iNOS mRNA. Co-immunoprecipitations showed interaction of KSRP with the exosome and tristetraprolin (TTP). To analyze the role of KSRP binding to the 3′-UTR we studied iNOS expression in DLD-1 cells overexpressing a non-binding mutant of KSRP. In these cells, iNOS expression was increased. Mapping of the binding site revealed KSRP interacting with the most 3′-located AU-rich element (ARE) of the human iNOS mRNA. This sequence is also the target for HuR, an iNOS mRNA stabilizing protein. We were able to demonstrate that KSRP and HuR compete for this binding site, and that intracellular binding to the iNOS mRNA was reduced for KSRP and enhanced for HuR after cytokine treatment. Finally, a complex interplay of KSRP with TTP and HuR seems to be essential for iNOS mRNA stabilization after cytokine stimulation.

Tristetraprolin regulates the expression of the human inducible nitric-oxide synthase gene

The expression of human inducible NO synthase (iNOS) is regulated both by transcriptional and post-transcriptional mechanisms. Stabilization of mRNAs often depends on activation of p38 mitogen-activated protein kinase (p38 MAPK). In human DLD-1 cells, inhibition of p38 MAPK by the compound 4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole (SB203580) or by overexpression of a dominant-negative p38 MAPKalpha protein resulted in a reduction of human iNOS mRNA and protein expression, whereas human iNOS promoter activity was not affected. An important RNA binding protein regulated by the p38 MAPK pathway and involved in the regulation of the stability of several mRNAs is tristetraprolin. RNase protection, quantitative real-time polymerase chain reaction, and Western blot experiments showed that cytokines used to induce iNOS expression in DLD-1 cells also enhanced tristetraprolin expression. SB203580 incubation reduced cytokine-mediated enhancement of tristetraprolin expression. Overexpression or down-regulation of tristetraprolin in stably transfected DLD-1- or A549/8 cells consistently resulted in enhanced or reduced iNOS expression by modulating iNOS-mRNA stability. In UV cross-linking experiments, recombinant tristetraprolin did not interact with the human iNOS mRNA. However, coimmunoprecipitation experiments showed interaction of tristetraprolin with the KH-type splicing regulatory protein (KSRP), which is known to recruit mRNAs containing AU-rich elements to the exosome for degradation. This tristetraprolin-KSRP interaction was enhanced by cytokines and reduced by SB203580 treatment. We conclude that tristetraprolin positively regulates human iNOS expression by enhancing the stability of human iNOS mRNA. Because tristetraprolin does not directly bind to the human iNOS mRNA but interacts with KSRP, tristetraprolin is likely to stabilize iNOS mRNA by capturing the KSRP-exosome complex.

Recruitment and activation of mRNA decay enzymes by two ARE-mediated decay activation domains in the proteins TTP and BRF-1

In human cells, a critical pathway in gene regulation subjects mRNAs with AU-rich elements (AREs) to rapid decay by a poorly understood process. AREs have been shown to directly activate deadenylation, decapping, or 3'-to-5' exonucleolytic decay. We demonstrate that enzymes involved in all three of these mRNA decay processes, as well as 5'-to-3' exonucleolytic decay, associate with the protein tristetraprolin (TTP) and its homolog BRF-1, which bind AREs and activate mRNA decay. TTP and BRF-1 each contain two activation domains that can activate mRNA decay after fusion to a heterologous RNA-binding protein, and inhibit ARE-mediated mRNA decay when overexpressed. Both activation domains employ trans-acting factors to trigger mRNA decay, and the N-terminal activation domain functions as a binding platform for mRNA decay enzymes. Our data suggest that the TTP protein family functions as a molecular link between ARE-containing mRNAs and the mRNA decay machinery by recruitment of mRNA decay enzymes, and help explain how deadenylation, decapping, and exonucleolytic decay can all be independently activated on ARE-containing mRNAs. This describes a potentially regulated step in activation of mRNA decay.

Expression, purification, and biochemical characterization of the antiinflammatory tristetraprolin: a zinc-dependent mRNA binding protein affected by posttranslational modifications

Tristetraprolin (TTP) is a hyperphosphorylated protein that destabilizes mRNA by binding to an AU-rich element (ARE). Mice deficient in TTP develop a severe inflammatory syndrome. The biochemical properties of TTP have not been adequately characterized, due to the difficulties in protein purification and lack of a high-titer antiserum. Full-length human TTP was expressed in human HEK293 cells and purified to at least 70% homogeneity. The purified protein was free of endogenous ARE binding activity, and was used for investigating its size, zinc dependency, and binding kinetics for tumor necrosis factor alpha mRNA ARE. A high-titer rabbit antiserum was raised against the MBP-hTTP fusion protein expressed in Escherichia coli. Cellular localization studies of the transfected cells indicated that approximately 80% of the expressed TTP was in the cytosol, with 20% in the nuclei. TTP from both locations bound to the ARE and formed similar complexes. The purified TTP was shown to be intact by N-terminal His-tag purification, C-terminal peptide sequencing, and mass spectrometry analysis. Results from size exclusion chromatography are consistent with the predominant form of active TTP being a tetramer. TTP's ARE binding activity was increased by 10 microM Zn(2+). The half-maximal binding of TTP from HEK293 cells was approximately 30 nM in assays containing 10 nM ARE. This value was about twice that of TTP from E. coli. TTP from HEK293 cells was highly phosphorylated, and its electrophoretic mobility was increased by alkaline phosphatase treatment and somewhat by T271A mutation, but not by PNGase F or S186A mutation. The gel mobility of TTP from E. coli was decreased by in vitro phosphorylation with p42/ERK2 and p38 mitogen-activated protein kinases. These results suggest that TTP's zinc-dependent ARE binding affinity is reduced by half by posttranslational modifications, mainly by phosphorylation but not by glycosylation, in mammalian cells. The results support a model in which each subunit of the TTP tetramer binds to one of the five overlapping UUAUUUAUU sequences of the ARE, resulting in a stable TTP-ARE complex.

Differential expression of proteins in rat plasma exposed to benzene

Benzene, a ubiquitous environmental contaminant, is an important solvent in the chemical industry and is also known as a constituent of petroleum. It has been reported that benzene is associated with hematotoxicity including leukemia in humans and cancer in laboratory animals. To study protein expression alterations in rat plasma exposed to benzene, rats were exposed to levels of 1, 10, 100 ppm benzine for 6 h/day and 5 d/week for 2 or 6 weeks. Two-dimensional gel electrophoresis of rat plasma was carried out, and approximately 1000 protein spots were detected on the gels. The 11 spots which showed significantly different expression were selected and identified with matrix-assisted laser desorption/ionization-time of flight-mass spectrometry. Analyzing the targeted 11 spots, there was no correlation between the 2 and 6 weeks benzene-inhaled groups on up-regulated proteins (zinc finger protein, and tristetraprolin) and on down-regulated proteins (cAMP-regulated guanine nucleotide exchange factor II, protein kinase and unknown protein). The overexpressed proteins (inhibitor of kappaB-like protein, GTP-binding protein rab14, T-cell receptor alpha chain, and somatostatin transactivating factor-1) were detected only in groups inhaling benzene for 6 weeks. Among them the expression level of T-cell receptor alpha chain was confirmed by Western blot.

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.

The Role of mRNA Turnover in the Regulation of Tristetraprolin Expression: Evidence for an Extracellular Signal-Regulated Kinase-Specific, AU-Rich Element-Dependent, Autoregulatory Pathway

Tristetraprolin (TTP) is a regulator of TNF-alpha mRNA stability and is the only trans-acting factor shown to be capable of regulating AU-rich element-dependent mRNA turnover at the level of the intact animal. Using the THP-1 myelomonocytic cell line, we demonstrated for the first time that TTP is encoded by an mRNA with a short half-life under resting conditions. Using pharmacologic inhibitors of the mitogen-activated protein kinase pathways, we show that the induction of TTP by LPS activation is mediated through changes in transcription, mRNA stability, and translation. A coordinate increase in both TTP and TNF-alpha mRNA stability occurs within 15 min of LPS treatment, but is transduced through different mitogen-activated protein kinase pathways. This regulation of TTP and TNF-alpha mRNA stability is associated with the finding that TTP binds these mRNA under both resting and LPS-activated conditions in vivo. Finally, we demonstrate that TTP can regulate reporter gene expression in a TTP 3' untranslated region-dependent manner and identify three distinct AU-rich elements necessary to mediate this effect. Thus, TTP regulates its own expression in a manner identical to that seen with the TNF-alpha 3' untranslated region, indicating that this autoregulation is mediated at the level of mRNA stability. In this manner, TTP is able to limit the production of its own proteins as well as that of TNF-alpha and thus limit the response of the cell to LPS.

Gene expression arrays reveal a rapid return to normal homeostasis in immunologically-challenged trophoblast-like JAR cells

The immunologic adaptations of pregnancy have come under increasing scrutiny in the past 15 years. Existing experimental evidence clearly demonstrates that placental trophoblasts play an important role in regulating immunologic/inflammatory responses at the maternal-fetal interface. We used a well-developed gene expression array to examine in greater detail the physiologic response of trophoblast-like choriocarcinoma cells to a model immunologic 'challenge.' We co-cultured PHA-activated or resting peripheral blood mononuclear cells (PBMC) with the human choriocarcinoma cell line JAR for time periods ranging from 2 to 18 h. Messenger RNA expression in the JAR cells was then assessed using a 21,329-gene microarray and novel biostatistical analyses that we have previously published. Patterns of differential gene expression were assessed using a commercial pathway analysis software program. Differences in gene expression between JAR cells cultured with activated PBMC (experimental samples) and JAR cells cultured with resting PBMC (control samples) were seen only at the 2h time point. That is, multiple genes were transcribed in JAR cells in response to activated PBMC, but expression levels of the genes had all returned to baseline by 6h. Molecular modeling demonstrated that the differentially expressed genes were largely associated with cell growth and differentiation. This model was confirmed by noting a two-fold increase in CD10/neutral endopeptidase expression (a marker for cell differentiation) in JAR cells incubated with media from activated PBMC compared with JAR cells incubated with resting PBMC. These findings support the hypothesis that there is a delicate immunologic milieu at the maternal-fetal interface that must be maintained. Immunologic/inflammatory challenge at the maternal-fetal interface is compensated by cellular mechanisms that work to reduce inflammation and rapidly restore immunologic balance.

MK2-induced tristetraprolin:14-3-3 complexes prevent stress granule association and ARE-mRNA decay

Stress granules (SGs) are dynamic cytoplasmic foci at which stalled translation initiation complexes accumulate in cells subjected to environmental stress. SG-associated proteins such as TIA-1, TIAR and HuR bind to AU-rich element (ARE)-containing mRNAs and control their translation and stability. Here we show that tristetraprolin (TTP), an ARE-binding protein that destabilizes ARE-mRNAs, is recruited to SGs that are assembled in response to FCCP-induced energy deprivation, but not arsenite-induced oxidative stress. Exclusion of TTP from arsenite-induced SGs is a consequence of MAPKAP kinase-2 (MK2)-induced phosphorylation at serines 52 and 178, which promotes the assembly of TTP:14-3-3 complexes. 14-3-3 binding excludes TTP from SGs and inhibits TTP-dependent degradation of ARE-containing transcripts. In activated RAW 264.7 macrophages, endogenous TTP:14-3-3 complexes bind to ARE-RNA. Our data reveal the mechanism by which the p38-MAPK/MK2 kinase cascade inhibits TTP-mediated degradation of ARE-containing transcripts and thereby contributes to lipopolysaccharide-induced TNFalpha expression.

Immunological characterization of tristetraprolin as a low abundance, inducible, stable cytosolic protein

Tristetraprolin (TTP) is a zinc finger protein that can bind to AU-rich elements within certain mRNAs, resulting in deadenylation and destabilization of those mRNAs. Its physiological targets include the mRNAs encoding the cytokines tumor necrosis factor alpha (TNF) and granulocyte-macrophage colony-stimulating factor. TTP was originally identified on the basis of its massive but transient increase in mRNA levels following mitogen stimulation of fibroblasts. It has been difficult to reconcile this transient mRNA profile with the presumed continuing "need" for TTP protein, for example, to reverse the effects of lipopolysaccharide (LPS)-stimulated TNF secretion. To investigate this and other questions concerning endogenous TTP protein in cells and tissues, we raised a high titer rabbit antiserum against full-length mouse TTP. TTP could be detected on immunoblots of mouse cytosolic tissue extracts; it was most highly expressed in spleen, but its concentration in that tissue was only about 1.5 nm. TTP could be detected readily in splenic macrophages and stromal cells from LPS-injected rats. In both LPS-treated RAW 264.7 macrophages and fetal calf serum-treated mouse embryonic fibroblasts, TTP protein was stable after induction, with minimal degradation occurring for several hours after treatment of the cells with cycloheximide. The biosynthesis of TTP was accompanied by large changes in electrophoretic mobility consistent with progressive phosphorylation. Confocal microscopy revealed that TTP accumulated in a vesicular pattern in the cytosol of the LPS-stimulated RAW 264.7 cells, and was occasionally seen in the cytosol of unstimulated dividing cells. Gel filtration of the endogenous protein suggested that its predominant structure was monomeric. TTP appears to be a low abundance, cytosolic protein in unstimulated cells and tissues, but once induced is relatively stable, in contrast to its very labile mRNA.

MAPKAP kinase 2 phosphorylates tristetraprolin on in vivo sites including Ser178, a site required for 14-3-3 binding

MAPKAP kinase 2 (MK2) is required for tumor necrosis factor synthesis. Tristetraprolin (TTP) binds to the 3'-untranslated region of tumor necrosis factor mRNA and regulates its fate. We identified in vitro and in vivo phosphorylation sites in TTP using nanoflow high pressure liquid chromatography microelectrospray ionization tandem mass spectrometry and novel methods for direct digestion of TTP bound to affinity matrices (GSH-beads or anti-Myc linked to magnetic beads). MK2Delta3B, activated in Escherichia coli by p38alpha, phosphorylates TTP in vitro at major sites Ser(52) and Ser(178) (>10-fold in abundance) as well as at several minor sites that were detected after enriching for phosphopeptides with immobilized metal affinity chromatography. MK2 phosphorylation of TTP creates a functional 14-3-3 binding site. In cells, TTP was phosphorylated at Ser(52), Ser(178), Thr(250), and Ser(316) and at SP sites in a cluster (Ser(80)/Ser(82)/Ser(85)). Anisomycin treatment of NIH 3T3 cells increased phosphorylation of Ser(52) and Ser(178). Overexpression of MK2 sufficed to increase phosphorylation of Ser(52) and Ser(178) but not Ser(80)/Ser(82)/Ser(85) or Thr(250). Thus, Ser(52) and Ser(178) are putative MK2 sites in vivo. Identified phosphosite(s) may be biologic switches controlling mRNA stability and translation.

Regulation and localization of endogenous human tristetraprolin

Tumor necrosis factor (TNF) has been implicated in the development and pathogenicity of infectious diseases and autoimmune disorders, such as septic shock and arthritis. The zinc-finger protein tristetraprolin (TTP) has been identified as a major regulator of TNF biosynthesis. To define its intracellular location and examine its regulation of TNF, a quantitive intracellular staining assay specific for TTP was developed. We establish for the first time that in peripheral blood leukocytes, expression of endogenous TTP is confined to the cytoplasm. Baseline expression of TTP was higher in monocytes than in lymphocytes or neutrophils. After in vitro incubation with lipopolysaccharide (LPS), leukocyte TTP levels increased rapidly, peaking after approximately 2 hours. Monocytes showed the greatest response to LPS stimulation and lymphocytes the least. TTP levels were also studied in leukocytes isolated from healthy volunteers infused with a bolus dose of LPS. TTP expression and initial upregulation in response to LPS infusion were consistent with the in vitro data. Neutrophil TTP levels responded first, reaching an initial peak within 1 hour, monocyte levels peaked next at 2 hours, followed by lymphocytes at 4 hours. This response paralleled plasma TNF levels, which peaked 2 hours after infusion and were no longer detectable after 12 hours. A second rise in intracellular TTP levels, which did not parallel plasma TNF levels, was observed in all leukocyte populations, starting 12 hours after infusion. These data establish the cytoplasmic location of TTP, supporting a major role for this protein in regulating TNF production, and suggest that TTP levels are not regulated solely by TNF.

Pancreatic gene expression during the initiation of acute pancreatitis: identification of EGR-1 as a key regulator

We hypothesized that genes expressed in pancreatic acinar cells during the initiation of acute pancreatitis determine the severity of the disease. Therefore, we utilized microarrays to identify those genes commonly induced in rat pancreatic acinar cells within 1-4 h in two in vivo models, caerulein and taurocholate administration. This strategy yielded 51 known genes representing a complex array of molecules, including those that are likely to either reduce or increase the severity of the disease. Novel genes identified in the current study included ATF3, BRF1, C/EBPbeta, CGRP, EGR-1, ephrinA1, villin2, ferredoxin, latexin, lipocalin, MKP-1, NGFI-B, RhoA, tissue factor (TF), and syndecan. To validate these microarray results, the role of EGR-1 was further investigated using quantitative RT-PCR, Western blotting, and immunocytochemistry. EGR-1 expression occurred within acinar cells and correlated with the development of caerulein-induced acute pancreatitis in rats. Furthermore, the levels of the inflammation-related genes MCP-1, PAI, TF, IL-6, and ICAM-1 and the extent of lung inflammation were reduced during the initiation of caerulein-induced acute pancreatitis in EGR-1-deficient mice. Thus this study identified EGR-1 and several other novel genes likely to be important in the development and severity of acute pancreatitis.

Tristetraprolin binds to the 3'-untranslated region of cyclooxygenase-2 mRNA. A polyadenylation variant in a cancer cell line lacks the binding site

In human colorectal adenocarcinoma cell lines, we found two major transcripts of cyclooxygenase-2, the full-length mRNA and a short polyadenylation variant (2577 kb) lacking the distal segment of the 3'-untranslated region. Tristetraprolin, an mRNA-binding protein that promotes message instability, was shown to bind the cyclooxygenase-2 mRNA in the region of the 3'-untranslated region between nucleotides 3125 and 3432 and to reduce levels of the full-length mRNA. During cell growth and confluence, the expression of tristetraprolin mRNA was inversely correlated with that of the full-length cyclooxygenase-2 transcript, and transfection of tristetraprolin into HCA-7 cells reduced the level of full-length cyclooxygenase-2 mRNA. However, the truncated transcript escaped tristetraprolin binding and downregulation.