Down-regulation of tristetraprolin expression results in enhanced IL-12 and MIP-2 production and reduced MIP-3alpha synthesis in activated macrophages

Role of Tristetraprolin in the Resolution of Inflammation

Simple Summary

Chronic inflammatory diseases account for up to 60% of deaths worldwide and, thus, are considered a great threat for human health by the World Health Organization. Nevertheless, acute inflammatory reactions are an integral part of the host defense against invading pathogens or injuries. To avoid excessive damage due to the persistence of a highly reactive environment, inflammations need to resolve in a coordinate and timely manner, ensuring for the immunological normalization of the affected tissues. Since post-transcriptional regulatory mechanisms are essential for effective resolution, the present review discusses the key role of the RNA-binding and post-transcriptional regulatory protein tristetraprolin in establishing resolution of inflammation.

Abstract

Inflammation is a crucial part of immune responses towards invading pathogens or tissue damage. While inflammatory reactions are aimed at removing the triggering stimulus, it is important that these processes are terminated in a coordinate manner to prevent excessive tissue damage due to the highly reactive inflammatory environment. Initiation of inflammatory responses was proposed to be regulated predominantly at a transcriptional level, whereas post-transcriptional modes of regulation appear to be crucial for resolution of inflammation. The RNA-binding protein tristetraprolin (TTP) interacts with AU-rich elements in the 3′ untranslated region of mRNAs, recruits deadenylase complexes and thereby facilitates degradation of its targets. As TTP regulates the mRNA stability of numerous inflammatory mediators, it was put forward as a crucial post-transcriptional regulator of inflammation. Here, we summarize the current understanding of the function of TTP with a specific focus on its role in adding to resolution of inflammation.

Tristetraprolin Regulates TH17 Cell Function and Ameliorates DSS-Induced Colitis in Mice

T_H17 cells have been extensively investigated in inflammation, autoimmune diseases, and cancer. The precise molecular mechanisms for T_H17 cell regulation, however, remain elusive, especially regulation at the post-transcriptional level. Tristetraprolin (TTP) is an RNA-binding protein important for degradation of the mRNAs encoding several proinflammatory cytokines. With newly generated T cell-specific TTP conditional knockout mice (CD4^CreTTP^f/f), we found that aging CD4^CreTTP^f/f mice displayed an increase of IL-17A in serum and spontaneously developed chronic skin inflammation along with increased effector T_H17 cells in the affected skin. TTP inhibited T_H17 cell development and function by promoting IL-17A mRNA degradation. In a DSS-induced colitis model, CD4^CreTTP^f/f mice displayed severe colitis and had more T_H17 cells and serum IL-17A compared with wild-type mice. Furthermore, neutralization of IL-17A reduced the severity of colitis. Our results reveal a new mechanism for regulating T_H17 function and T_H17-mediated inflammation post-transcriptionally by TTP, suggests that TTP might be a novel therapeutic target for the treatment of T_H17-mediated diseases.

Enhancing tristetraprolin activity reduces the severity of cigarette smoke-induced experimental chronic obstructive pulmonary disease

Objective

Chronic obstructive pulmonary disease (COPD) is a progressive disease that causes significant mortality and morbidity worldwide and is primarily caused by the inhalation of cigarette smoke (CS). Lack of effective treatments for COPD means there is an urgent need to identify new therapeutic strategies for the underlying mechanisms of pathogenesis. Tristetraprolin (TTP) encoded by the Zfp36 gene is an anti‐inflammatory protein that induces mRNA decay, especially of transcripts encoding inflammatory cytokines, including those implicated in COPD.

Methods

Here, we identify a novel protective role for TTP in CS‐induced experimental COPD using Zfp36^aa/aa mice, a genetically modified mouse strain in which endogenous TTP cannot be phosphorylated, rendering it constitutively active as an mRNA‐destabilising factor. TTP wild‐type (Zfp36^+/+) and Zfp36^aa/aa active C57BL/6J mice were exposed to CS for four days or eight weeks, and the impact on acute inflammatory responses or chronic features of COPD, respectively, was assessed.

Results

After four days of CS exposure, Zfp36^aa/aa mice had reduced numbers of airway neutrophils and lymphocytes and mRNA expression levels of cytokines compared to wild‐type controls. After eight weeks, Zfp36^aa/aa mice had reduced pulmonary inflammation, airway remodelling and emphysema‐like alveolar enlargement, and lung function was improved. We then used pharmacological treatments in vivo (protein phosphatase 2A activator, AAL_(S), and the proteasome inhibitor, bortezomib) to promote the activation and stabilisation of TTP and show that hallmark features of CS‐induced experimental COPD were ameliorated.

Conclusion

Collectively, our study provides the first evidence for the therapeutic potential of inducing TTP as a treatment for COPD.

Roles of Tristetraprolin in Tumorigenesis

Genetic loss or mutations in tumor suppressor genes promote tumorigenesis. The prospective tumor suppressor tristetraprolin (TTP) has been shown to negatively regulate tumorigenesis through destabilizing the messenger RNAs of critical genes implicated in both tumor onset and tumor progression. Regulation of TTP has therefore emerged as an important issue in tumorigenesis. Similar to other tumor suppressors, TTP expression is frequently downregualted in various human cancers, and its low expression is correlated with poor prognosis. Additionally, disruption in the regulation of TTP by various mechanisms results in the inactivation of TTP protein or altered TTP expression. A recent study showing alleviation of Myc-driven lymphomagenesis by the forced expression of TTP has shed light on new therapeutic avenues for cancer prevention and treatment through the restoration of TTP expression. In this review, we summarize key oncogenes subjected to the TTP-mediated mRNA degradation, and discuss how dysregulation of TTP can contribute to tumorigenesis. In addition, the control mechanism underlying TTP expression at the posttranscriptional and posttranslational levels will be discussed.

A balancing act: RNA binding protein HuR/TTP axis in endometriosis patients

Endometriosis, a major reproductive pathology affecting 8-10% of women is characterized by chronic inflammation and immune dysfunction. Human antigen R (HuR) and Tristetraprolin (TTP) are RNA binding proteins that competitively bind to cytokines involved in inflammation including: tumor necrosis factor alpha (TNF-α), granulocyte macrophage colony stimulating factor (GM-CSF), interleukin 6 (IL-6) among others, and stabilize and destabilize them, respectively. The aim of this study was to examine RNA binding protein (RNABP) HuR/TTP axis in endometriosis patients compared to menstrual stage matched healthy fertile controls in hopes of better understanding their contribution to the pathogenesis of endometriosis. Additionally, using a targeted in vitro siRNA approach, we examined whether knock-down of TTP can play a functional role on other RNABPs that competitively bind to inflammatory targets of TTP in both endometriotic and endometrial epithelial cell lines. Our results suggest that RNABPs TTP and HuR are dysregulated in endometriotic lesions compared to matched eutopic patient samples as well endometrium from healthy controls. Silencing of TTP in endometriotic and endometrial epithelial cells revealed differential response to inflammatory cytokines and other RNABPs. Our results suggest potential involvement of HuR/TTP RNA binding protein axis in regulation of inflammation in endometriosis.

Effect of the p53-tristetraprolin-stathmin-1 pathway on trophoblasts at maternal-fetal interface

Problem

To reveal the effect of p53–tristetraprolin–stathmin-1 signaling on trophoblasts and recurrent spontaneous abortion (RSA).

Method of study

Stathmin-1 (STMN1), p53, and tristetraprolin (TTP) expression in paraffin-embedded villus tissue was determined using immunohistochemistry. HTR-8/SVneo cells were treated with doxorubicin to activate p53; STMN1 and TTP levels were detected by quantitative reverse transcription–PCR and western blotting. Western blotting and immunofluorescence were used to investigate STMN1 expression after TTP overexpression or knockdown in HTR-8 cells.

Results

STMN1 was downregulated and p53 was upregulated in the villus tissue from patients with RSA. Doxorubicin decreased STMN1 expression but enhanced TTP expression in HTR-8 cells. In vitro, TTP overexpression inhibited STMN1 production; TTP knockdown promoted it. TTP downregulated STMN1 expression in trophoblasts by directly binding its 3ʹ untranslated region.

Conclusions

TTP modulates trophoblast function and interacts with STMN1 and p53, and is related to pregnancy outcomes.

Gain-of-Function Mutation of Tristetraprolin Impairs Negative Feedback Control of Macrophages In Vitro yet Has Overwhelmingly Anti-Inflammatory Consequences In Vivo

The mRNA-destabilizing factor tristetraprolin (TTP) binds in a sequence-specific manner to the 3' untranslated regions of many proinflammatory mRNAs and recruits complexes of nucleases to promote rapid mRNA turnover. Mice lacking TTP develop a severe, spontaneous inflammatory syndrome characterized by the overexpression of tumor necrosis factor and other inflammatory mediators. However, TTP also employs the same mechanism to inhibit the expression of the potent anti-inflammatory cytokine interleukin 10 (IL-10). Perturbation of TTP function may therefore have mixed effects on inflammatory responses, either increasing or decreasing the expression of proinflammatory factors via direct or indirect mechanisms. We recently described a knock-in mouse strain in which the substitution of 2 amino acids of the endogenous TTP protein renders it constitutively active as an mRNA-destabilizing factor. Here we investigate the impact on the IL-10-mediated anti-inflammatory response. It is shown that the gain-of-function mutation of TTP impairs IL-10-mediated negative feedback control of macrophage function in vitro However, the in vivo effects of TTP mutation are uniformly anti-inflammatory despite the decreased expression of IL-10.

Hallmarks of cancer and AU-rich elements

Post‐transcriptional control of gene expression is aberrant in cancer cells. Sustained stabilization and enhanced translation of specific mRNAs are features of tumor cells. AU‐rich elements (AREs), cis‐acting mRNA decay determinants, play a major role in the posttranscriptional regulation of many genes involved in cancer processes. This review discusses the role of aberrant ARE‐mediated posttranscriptional processes in each of the hallmarks of cancer, including sustained cellular growth, resistance to apoptosis, angiogenesis, invasion, and metastasis. WIREs RNA 2017, 8:e1368. doi: 10.1002/wrna.1368

For further resources related to this article, please visit the WIREs website.

Tristetraprolin and its role in regulation of airway inflammation

Chronic inflammatory diseases, such as asthma and chronic obstructive pulmonary disease (COPD), are clinically and socioeconomically important diseases globally. Currently the mainstay of anti-inflammatory therapy in respiratory diseases is corticosteroids. Although corticosteroids have proven clinical efficacy in asthma, many asthmatic inflammatory conditions (e.g., infection, exacerbation, and severe asthma) are not responsive to corticosteroids. Moreover, despite an understanding that COPD progression is driven by inflammation, we currently do not have effective anti-inflammatory strategies to combat this disease. Hence, alternative anti-inflammatory strategies are required. p38 mitogen-activated protein kinase (MAPK) has emerged as an important signaling molecule driving airway inflammation, and pharmacological inhibitors against p38 MAPK may provide potential therapies for chronic respiratory disease. In this review, we discuss some of the recent in vitro and in vivo studies targeting p38 MAPK, but suggest that p38 MAPK inhibitors may prove less effective than originally considered because they may block anti-inflammatory molecules along with proinflammatory responses. We propose that an alternative strategy may be to target an anti-inflammatory molecule farther downstream of p38 MAPK, i.e., tristetraprolin (TTP). TTP is an mRNA-destabilizing, RNA-binding protein that enhances the decay of mRNAs, including those encoding proteins implicated in chronic respiratory diseases. We suggest that understanding the molecular mechanism of TTP expression and its temporal regulation will guide future development of novel anti-inflammatory pharmacotherapeutic approaches to combat respiratory disease.

Suppression of IL-12 production by tristetraprolin through blocking NF-kcyB nuclear translocation

Tristetraprolin (TTP), an mRNA-binding protein, plays a significant role in regulating the expression of adenylate-uridylate-rich elements containing mRNAs. Mice deficient of TTP (TTP(-/-)) develop a systemic autoimmune inflammatory syndrome characterized by cachexia, conjunctivitis, and dermatitis. IL-12 plays a crucial role in immune defense against infectious and malignant diseases. In this study, we found increased production of IL-12 during endotoxic shock and enhanced Th1 cells in TTP knockout mice. The levels of IL-12 p70 and p40 protein as well as p40 and p35 mRNA were also increased in activated macrophages deficient of TTP. In line with these findings, overexpression of TTP suppressed IL-12 p35 and p40 expression at the mRNA and promoter level, whereas it surprisingly had little effects on their mRNA stability. Our data showed that the inhibitory effects of TTP on p35 gene transcription were completely rescued by overexpression of NF-кB p65 and c-Rel but not by the p50 in activated macrophages. Our data further indicated that TTP acquired its inhibition on IL-12 expression through blocking nuclear translocation of NF-кB p65 and c-Rel while enhancing p50 upon stimulation. In summary, our study reveals a novel pathway through which TTP suppresses IL-12 production in macrophages, resulting in suppression of Th1 cell differentiation. This study may provide us with therapeutic targets for treatment of inflammatory and autoimmune disorders.

Tristetraprolin (TTP): interactions with mRNA and proteins, and current thoughts on mechanisms of action

Changes in mRNA stability and translation are critical control points in the regulation of gene expression, particularly genes encoding growth factors, inflammatory mediators, and proto-oncogenes. Adenosine and uridine (AU)-rich elements (ARE), often located in the 3' untranslated regions (3'UTR) of mRNAs, are known to target transcripts for rapid decay. They are also involved in the regulation of mRNA stability and translation in response to extracellular cues. This review focuses on one of the best characterized ARE binding proteins, tristetraprolin (TTP), the founding member of a small family of CCCH tandem zinc finger proteins. In this survey, we have reviewed the current status of TTP interactions with mRNA and proteins, and discussed current thinking about TTP's mechanism of action to promote mRNA decay. We also review the proposed regulation of TTP's functions by phosphorylation. Finally, we have discussed emerging evidence for TTP operating as a translational regulator. This article is part of a Special Issue entitled: RNA Decay mechanisms.

Genetic polymorphisms in RNA binding proteins contribute to breast cancer survival

The RNA-binding proteins TTP and HuR control expression of numerous genes associated with breast cancer pathogenesis by regulating mRNA stability. However, the role of genetic variation in TTP (ZFP36) and HuR (ELAVL1) genes is unknown in breast cancer prognosis. A total of 251 breast cancer patients (170 Caucasians and 81 African-Americans) were enrolled and followed up from 2001 to 2011 (or until death). Genotyping was performed for 10 SNPs in ZFP36 and 7 in ELAVL1 genes. On comparing both races with one another, significant differences were found for clinical and genetic variables. The influence of genetic polymorphisms on survival was analyzed by using Cox-regression, Kaplan-Meier analysis and the log-rank test. Univariate (Kaplan-Meier/Cox-regression) and multivariate (Cox-regression) analysis showed that the TTP gene polymorphism ZFP36*2 A > G was significantly associated with poor prognosis of Caucasian patients (HR = 2.03; 95% CI = 1.09-3.76; p = 0.025; log-rank p = 0.022). None of the haplotypes, but presence of more than six risk genotypes in Caucasian patients, was significantly associated with poor prognosis (HR=2.42; 95% CI = 1.17-4.99; p = 0.017; log-rank p = 0.007). The effect of ZFP36*2 A > G on gene expression was evaluated from patients' tissue samples. Both TTP mRNA and protein expression was significantly decreased in ZFP36*2 G allele carriers compared to A allele homozygotes. Conversely, upregulation of the TTP-target gene COX-2 was observed ZFP36*2 G allele carriers. Through its ability to attenuate TTP gene expression, the ZFP36*2 A > G gene polymorphism has appeared as a novel prognostic breast cancer marker in Caucasian patients.

Posttranscriptional Gene Regulation: Novel Pathways for Glucocorticoids' Anti-inflammatory Action

Posttranscriptional gene regulation (PTR) is a fundamental biological process that integrates with the master transcriptional control of gene expression, in ways that only in the last decade have been increasingly understood [1, 2]. While epigenetic and transcriptional events shape cell response qualitatively, deciding the pattern of gene expression to 'switch on or off' in response to endogenous or environmental triggers, the key task of PTR is to act as a 'rheostat' and rapidly adapt the cellular response by providing the appropriate amplitude and timing to the protein expression patterns [3, 4]. The pivotal role of this mechanism comes to the forefront in inflammatory and immune response, where the changes in amplitude and duration in the expression of dangerous and protective genes are in delicate balance, and are critical in determining either the successful resolution of the immune response or its chronic overexpression [5]. This brief review introduces members of the main classes of molecules mediating the cytoplasmic arm of gene regulation, namely RNA-binding proteins and micro-RNA (miRNA), and summarizes experimental data that underscore the role of these molecules in the pathophysiology of chronic inflammation, as well as their promising value as mechanisms conveying the anti-inflammatory effect of synthetic glucocorticoids.

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.

IL-35 is a novel responsive anti-inflammatory cytokine--a new system of categorizing anti-inflammatory cytokines

It remains unknown whether newly identified anti-inflammatory/immunosuppressive cytokine interleukin-35 (IL-35) is different from other anti-inflammatory cytokines such as IL-10 and transforming growth factor (TGF)-β in terms of inhibition of inflammation initiation and suppression of full-blown inflammation. Using experimental database mining and statistical analysis methods we developed, we examined the tissue expression profiles and regulatory mechanisms of IL-35 in comparison to other anti-inflammatory cytokines. Our results suggest that in contrast to TGF-β, IL-35 is not constitutively expressed in human tissues but it is inducible in response to inflammatory stimuli. We also provide structural evidence that AU-rich element (ARE) binding proteins and microRNAs target IL-35 subunit transcripts, by which IL-35 may achieve non-constitutive expression status. Furthermore, we propose a new system to categorize anti-inflammatory cytokines into two groups: (1) the house-keeping cytokines, such as TGF-β, inhibit the initiation of inflammation whereas (2) the responsive cytokines including IL-35 suppress inflammation in full-blown stage. Our in-depth analyses of molecular events that regulate the production of IL-35 as well as the new categorization system of anti-inflammatory cytokines are important for the design of new strategies of immune therapies.

Downregulation of the AU-rich RNA-binding protein ZFP36 in chronic HBV patients: implications for anti-inflammatory therapy

Inflammation caused by chronic hepatitis B virus (HBV) infection is associated with the development of cirrhosis and hepatocellular carcinoma; however, the mechanisms by which HBV infection induces inflammation and inflammatory cytokine production remain largely unknown. We analyzed the gene expression patterns of lymphocytes from chronic HBV-infected patients and found that the expression of ZFP36, an AU-rich element (ARE)-binding protein, was dramatically reduced in CD4(+) and CD8(+) T lymphocytes from chronic HBV patients. ZFP36 expression was also reduced in CD14(+) monocytes and in total PBMCs from chronic HBV patients. To investigate the functional consequences of reduced ZFP36 expression, we knocked down ZFP36 in PBMCs from healthy donors using siRNA. siRNA-mediated silencing of ZFP36 resulted in dramatically increased expression of multiple inflammatory cytokines, most of which were also increased in the plasma of chronic HBV patients. Furthermore, we found that IL-8 and RANTES induced ZFP36 downregulation, and this effect was mediated through protein kinase C. Importantly, we found that HBsAg stimulated PBMCs to express IL-8 and RANTES, resulting in decreased ZFP36 expression. Our results suggest that an inflammatory feedback loop involving HBsAg, ZFP36, and inflammatory cytokines may play a critical role in the pathogenesis of chronic HBV and further indicate that ZFP36 may be an important target for anti-inflammatory therapy during chronic HBV infection.

The role of tristetraprolin in cancer and inflammation

Messenger RNA decay is a critical mechanism to control the expression of many inflammation- and cancer-associated genes. These transcripts are targeted for rapid degradation through AU-rich element (ARE) motifs present in the mRNA 3' untranslated region (3'UTR). Tristetraprolin (TTP) is an RNA-binding protein that plays a significant role in regulating the expression of ARE-containing mRNAs. Through its ability to bind AREs and target the bound mRNA for rapid degradation, TTP can limit the expression of a number of critical genes frequently overexpressed in inflammation and cancer. Regulation of TTP occurs on multiple levels through cellular signaling events to control transcription, mRNA turnover, phosphorylation status, cellular localization, association with other proteins, and proteosomal degradation, all of which impact TTP's ability to promote ARE-mediated mRNA decay along with decay-independent functions of TTP. This review summarizes the current understanding of post-transcriptional regulation of ARE-containing gene expression by TTP and discusses its role in maintaining homeostasis and the pathological consequences of losing TTP expression.

Tristetraprolin-driven regulatory circuit controls quality and timing of mRNA decay in inflammation

Inflammatory gene activation must be rigorously controlled to ensure a rapid, but transient, response. In this work, a regulatory circuit is revealed that governs the destabilization of inflammatory mRNAs and plays an essential role in re-establishing immune homeostasis after inflammatory stimulus.

We describe a regulatory circuit that governs the sequential destabilization of inflammatory mRNAs. This circuit limits potentially deleterious inflammatory mRNA accumulation, yet it prevents premature removal of those mRNAs that are still needed.

We show that the sequential destabilization of inflammatory mRNAs is driven by the continuous inverse coupling of p38 MAPK activity profile with the mRNA-destabilizing function of tristetraprolin (TTP) during the entire inflammatory response. This control mechanism ensures that with time, the TTP-dependent mRNA decay gradually spreads resulting in cumulative elimination of 30% of inflammation-induced unstable mRNAs in macrophages.

We generated mice with myeloid cell-specific TTP deletion to provide evidence for the function of this regulatory circuit in vivo. These animals are hypersensitive to LPS and display a dysbalanced cytokine production whose pattern is agreement with our model of sequential destabilization the individual mRNAs by TTP.

We propose that myeloid TTP is critically involved in the re-installment of immune homeostasis after inflammatory stimulus rather than in the maintenance of steady-state immune homeostasis.

For a successful yet controlled immune response, cells need to specifically destabilize inflammatory mRNAs but prevent premature removal of those still used. The regulatory circuits controlling quality and timing in the global inflammatory mRNA decay are not understood. Here, we show that the mRNA-destabilizing function of the AU-rich element-binding protein tristetraprolin (TTP) is inversely regulated by the p38 MAPK activity profile such that after inflammatory stimulus the TTP-dependent decay is initially limited to few mRNAs. With time, the TTP-dependent decay gradually spreads resulting in cumulative elimination of one third of inflammation-induced unstable mRNAs in macrophages in vitro. We confirmed this sequential decay model in vivo since LPS-treated mice with myeloid TTP ablation exhibited similar cytokine dysregulation profile as macrophages. The mice were hypersensitive to LPS but otherwise healthy with no signs of hyperinflammation seen in conventional TTP knockout mice demonstrating the requirement for myeloid TTP in re-installment but not maintenance of immune homeostasis. These findings reveal a TTP- and p38 MAPK-dominated regulatory mechanism that is vital for balancing acute inflammation by a temporally and qualitatively controlled mRNA decay.

A novel concept in antiangiogenic and antitumoral therapy: multitarget destabilization of short-lived mRNAs by the zinc finger protein ZFP36L1

Angiogenesis inhibitors have shown clinical benefits in patients with advanced cancer, but further therapeutic improvement is needed. We have previously shown that the zinc finger protein 36, C3H type-like 1 (ZFP36L1) enhances vascular endothelial growth factor (VEGF) mRNA decay through its interaction with AU-rich elements within VEGF 3'-untranslated region. In this study, we evaluated the possibility to develop an antiangiogenic and antitumoral strategy using the mRNA-destabilizing activity of ZFP36L1. We engineered a cell-penetrating ZFP36L1, by fusing it to the protein transduction domains (PTDs) TAT derived from HIV, or the polyarginine peptides R7 or R9. PTD-ZFP36L1 fusion proteins were expressed in bacterial cells and affinity-purified to homogeneity. TAT-, R7- and R9-ZFP36L1 were efficiently internalized into living cells and decreased both endogenous VEGF mRNA half-life and VEGF protein levels in vitro. Importantly, a single injection of R9-TIS11b fusion protein into a high-VEGF expressing tissue in vivo (in this study, the mouse adrenal gland) markedly decreased VEGF expression. We further evaluated the effect of R9-ZFP36L1 on tumor growth using Lewis Lung Carcinoma (LL/2) cells implanted subcutaneously into nude mice. Intratumoral injection of R9-ZFP36L1 significantly reduced tumor growth and markedly decreased the expression of multiple angiogenic and inflammatory cytokines, including VEGF, acidic fibroblast growth factor, tumor necrosis factor α, interleukin (IL)-1α and IL-6, with a concomitant obliteration of tumor vascularization. These findings indicate that R9-ZFP36L1 fusion protein may represent a novel antiangiogenic and antitumoral agent, and supports the emerging idea that modulation of mRNA stability represents a promising therapeutic approach to treat cancer.

A combination of hypoxia and lipopolysaccharide activates tristetraprolin to destabilize proinflammatory mRNAs such as tumor necrosis factor-alpha

Inflammation is often accompanied by hypoxia because of the high oxygen consumption of invading bacteria and immune cells. During resolution of inflammation, the formation of inflammatory mediators such as tumor necrosis factor-alpha (TNF-alpha), which is produced by macrophages, needs to be terminated. We show in RAW264.7 macrophages that TNF-alpha mRNA as well as intracellular and secreted TNF-alpha protein levels are reduced after prolonged incubations with lipopolysaccharide (LPS) under hypoxic conditions. The decrease in TNF-alpha was mediated by destabilization of TNF-alpha mRNA via a 3'-untranslated region-dependent mechanism. Specifically, the RNA-binding protein tristetraprolin (TTP) increased at mRNA and protein levels after 16-hour incubations with LPS under hypoxia. Interestingly, TTP accumulated in a dephosphorylated and active form, and this accumulation was attributable to reduced p38 mitogen-activated protein kinase activity under these conditions. Knockdown of TTP by small interfering RNA abolished destabilization of TNF-alpha mRNA. Prolonged incubations with LPS under hypoxia also reduced mRNA amounts and stability of other proinflammatory mediators such as macrophage inflammatory protein-2, interleukin-6, and granulocyte macrophage colony-stimulating factor. Therefore, we propose that hypoxia plays a key role during resolution of inflammation by activating posttranscriptional, TTP-dependent regulatory mechanisms.

Inflammation: cytokines and RNA-based regulation

The outcome of an inflammatory response depends upon the coordinated regulation of a variety of both pro-inflammatory and anti-inflammatory cytokines and other proteins. Regulation of these inflammation mediators can occur at multiple levels, including transcription, mRNA translation, post-translational modifications, and mRNA degradation. Post-transcriptional regulation has been shown to play an important role in controlling the expression of these mediators, allowing for normal initiation and resolution of the inflammatory response. Many inflammatory mediators have unstable mRNAs due, in part, to the presence of AU-rich elements in their 3'-untranslated regions. Increasing numbers of RNA-binding proteins have been identified that can bind to these AU-rich elements and then regulate the stability and/or translation of the mRNA. This review summarizes current knowledge about the role of several RNA-binding proteins that act through AU-rich elements to post-transcriptionally regulate the biosynthesis of proteins involved in inflammation.

Inhibition of protein kinase Cdelta reduces tristetraprolin expression by destabilizing its mRNA in activated macrophages

Tristetraprolin (TTP) binds to AU-rich elements within the mRNAs of several inflammatory genes and causes destabilization of the target mRNAs. The protein kinase C (PKC) pathway represents a major signalling system in inflammation and PKCdelta is one of the key isoenzymes in the regulation of inflammatory processes. In the present study, we investigated the role of PKCdelta in the regulation of the expression of tristetraprolin in activated macrophages by using the PKCdelta inhibitor, rottlerin, and by downregulating PKCdelta expression by using PKCdelta siRNA. TTP protein and mRNA expression were measured by Western blotting and quantitative RT-PCR, respectively. TTP and TNFalpha mRNA decays were studied by the actinomycin D assay. In addition, we measured nuclear translocation of transcription factors believed to be important for TTP transcription, i.e. NF-kappaB, AP-2, SP1 and EGR1. Downregulation of PKCdelta by siRNA decreased significantly TTP expression in activated macrophages. Rottlerin also decreased TTP expression in wild type cells but not in cells in which PKCdelta had been downregulated by siRNA. Rottlerin decreased TTP mRNA half-life as measured by actinomycin D assay but it did not affect the nuclear translocation of transcription factors NF-kappaB, Sp1, AP-2 or EGR1 (which have been shown to be involved in TTP transcription). In addition, rottlerin reduced the decay of TNFalpha mRNA, which is an important target of TTP. The results suggest that PKCdelta up-regulates the expression of TTP by stabilizing its mRNA which may serve as a feed-back loop to regulate the inflammatory response.

TIS11 family proteins and their roles in posttranscriptional gene regulation

Posttranscriptional regulation of gene expression of mRNAs containing adenine-uridine rich elements (AREs) in their 3′ untranslated regions is mediated by a number of different proteins that interact with these elements to either stabilise or destabilise them. The present review concerns the TPA-inducible sequence 11 (TIS11) protein family, a small family of proteins, that appears to interact with ARE-containing mRNAs and promote their degradation. This family of proteins has been extensively studied in the past decade. Studies have focussed on determining their biochemical functions, identifying their target mRNAs, and determining their roles in cell functions and diseases.

CD36 regulates oxidative stress and inflammation in hypercholesterolemic CKD

Scavenger receptors play a central role in atherosclerosis by processing oxidized lipoproteins and mediating their cellular effects. Recent studies suggested that the atherogenic state correlates with progression of chronic kidney disease (CKD); therefore, scavenger receptors are candidate mediators of renal fibrogenesis. Here, we investigated the role of CD36, a class B scavenger receptor, in a hypercholesterolemic model of CKD. We placed CD36-deficient mice and wild-type male mice on a high-fat Western diet for 7 to 8 wk and then performed either sham or unilateral ureteral obstruction surgery. CD36-deficient mice developed significantly less fibrosis compared with wild-type mice at days 3, 7, and 14 after obstruction. Compared with wild-type mice, CD36-deficient mice had significantly more interstitial macrophages at 7 d but not at 14 d. CD36-deficient mice exhibited reduced levels of activated NF-kappaB and oxidative stress (assessed by measuring fatty acid-derived hydroxyoctadecadienoic acid and protein carbonyl content) and decreased accumulation of interstitial myofibroblasts compared with wild-type mice. These data suggest that CD36 is a key modulator of proinflammatory and oxidative pathways that promote fibrogenesis in CKD.

Targeting mRNA stability arrests inflammatory bone loss

Many proinflammatory cytokines contain adenylate-uridylate-rich elements (AREs) within the 3'-untranslated region (UTR) that confer rapid mRNA destabilization. During the inflammatory response, cytokine mRNA are stabilized via complex interactions with RNA-binding proteins controlled by phosphorylation via multiple signaling pathways including the mitogen-activated protein kinases (MAPKs). In the absence of inflammation, a key cytokine-regulating RNA-binding protein, tristetraprolin (TTP), shuttles mRNA transcripts to degradation machinery in order to maintain low levels of inflammatory cytokines. Using this general model of mRNA decay, over expression of TTP was evaluated in an experimental model of inflammatory bone loss to determine whether altering cytokine mRNA stability has an impact in pathological bone resorption. Using adenoviral-delivered TTP, significant reductions of interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-alpha), and prostaglandin (PG)E(2) were observed in vitro through a mechanism consistent with targeting mRNA stability. In vivo analysis indicates a significant protective effect from inflammation-induced bone loss and inflammatory infiltrate in animals overexpressing TTP compared with reporter controls. These findings provide experimental evidence that mRNA stability is a valid therapeutic target in inflammatory bone loss.

Role of the RNA-binding protein tristetraprolin in glucocorticoid-mediated gene regulation

Glucocorticoids (GCs) are the mainstay of anti-inflammatory therapy. Modulation of posttranscriptional regulation (PTR) of gene expression by GCs is a relevant yet poorly characterized mechanism of their action. The RNA-binding protein tristetraprolin (TTP) plays a central role in PTR by binding to AU-rich elements in the 3'-untranslated region of proinflammatory transcripts and accelerating their decay. We found that GCs induce TTP expression in primary and immortalized human bronchial epithelial cells. To investigate the importance of PTR and the role of TTP in GC function, we compared the effect of GC treatment on genome-wide gene expression using mouse embryonic fibroblasts (MEFs) obtained from wild-type and TTP(-/-) mice. We confirmed that GCs induce TTP in MEFs and observed in TTP(-/-) MEFs a striking loss of up to 85% of GC-mediated gene expression. Gene regulation by TNF-alpha was similarly affected, as was the antagonistic effect of GC on TNF-alpha-induced response. Inflammatory genes, including cytokines and chemokines, were among the genes whose sensitivity to GCs was affected by lack of TTP. Silencing of TTP in WT MEFs by small interfering RNA confirmed loss of GC response in selected targets. Immunoprecipitation of ribonucleoprotein complexes revealed binding of TTP to several validated transcripts. Changes in the rate of transcript degradation studied by actinomycin D were documented for only a subset of transcripts bound to TTP. These results reveal a strong and previously unrecognized contribution of PTR to the anti-inflammatory action of GCs and point at TTP as a key factor mediating this process through a complex mechanism of action.

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.

Salbutamol increases tristetraprolin expression in macrophages

Tristetraprolin (TTP) is a tandem zinc finger protein that can bind to AU-rich elements (AREs) in the 3'-untranslated regions (3'-UTR) in mRNAs of transiently expressed genes, e.g. tumor necrosis factor-alpha (TNF-alpha) and granulocyte macrophage colony-stimulating factor (GM-CSF). TTP increases the turnover rate of the target mRNAs, thereby reducing, for example, the expression of TNF-alpha and GM-CSF. We examined the role of beta(2)-agonists, cAMP analogs, and forskolin (an activator of adenylate cyclase) on TTP mRNA and protein expression by quantitative real-time RT-PCR and Western blotting in J774 murine macrophages and THP-1 human macrophages. All of these agents increased TTP expression. A nonspecific inhibitor of phosphodiesterases (PDEs) 3-isobutyl-1-methylxanthine (IBMX) and type IV PDE-inhibitor rolipram further enhanced the increase in TTP expression levels, suggesting a cAMP-mediated effect. A possible mediator of these effects is transcription factor activator protein 2 (AP-2), whereas nuclear factor kappaB (NF-kappaB) seemed not to play any role. This mechanism may, at least in part, explain the anti-inflammatory effects which beta(2)-agonists have been reported to have in macrophages.