Chorioallantoic fusion defects and embryonic lethality resulting from disruption of Zfp36L1, a gene encoding a CCCH tandem zinc finger protein of the Tristetraprolin family

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.

Von Hippel-Lindau gene product modulates TIS11B expression in renal cell carcinoma: impact on vascular endothelial growth factor expression in hypoxia

TIS11B belongs to a group of RNA-binding proteins (including TIS11/tristetraprolin and TIS11D) that share characteristic tandem CCCH-type zinc-finger domains and can be rapidly induced by multiple stimuli. TIS11B has been shown to regulate vascular endothelial growth factor (VEGF) mRNA stability in adrenocorticotropic hormone-stimulated primary adrenocortical cells. TIS11B has also been documented as a negative regulator of VEGF during development, but nothing has yet been reported in the context of human cancers. The Von Hippel-Lindau (VHL) tumor suppressor protein regulates VEGF gene expression at both the transcriptional and post-transcriptional levels in normoxia. However, whether it can do so in hypoxia is still unclear. Here, we report a unique regulatory function of VHL in VEGF expression in hypoxia that is mediated through modulation of TIS11B protein levels in renal cancer cells. In normoxia, we detected increased expression of the microRNA hsa-miR-29b in the VHL-overexpressing renal cancer cell line 786-O. We also show that this increased expression of hsa-miR-29b decreased TIS11B protein expression by post-transcriptional regulation in normoxia. In contrast, in hypoxia, increased TIS11B expression paralleled an increased TIS11B mRNA stability in VHL-overexpressing 786-O cells. This VHL-mediated TIS11B up-regulation in hypoxia may be important for TIS11B-regulated gene expression: we observed a down-regulation of VEGF mRNA in hypoxia in VHL-overexpressing cells compared with parental 786-O cells, and this effect was reversible by silencing TIS11B expression.

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.

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

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

Stimulation of polo-like kinase 3 mRNA decay by tristetraprolin

Polo-like protein kinase 3 (Plk3) has been proposed to regulate entry into S phase and promote apoptosis in response to oxidative stress. Its mRNA contains three AU-rich elements (AREs) in its 3' untranslated region (3'-UTR) that can contribute to the rapid degradation of labile transcripts. We investigated the possibility that tristetraprolin (TTP), a tandem CCCH zinc finger protein, could promote the decay of Plk3 transcripts. TTP is known to stimulate the deadenylation and decay of mRNAs possessing one or more copies of the consensus nonamer motif UUAUUUAUU. In stable mouse fibroblast cell lines derived from wild-type and TTP knockout littermates, the decay of Plk3 transcripts after serum stimulation was slowed in the absence of TTP. The specificity of TTP for promoting the degradation of Plk3 was demonstrated by the unaltered decay of Plk3 mRNA in cell lines deficient in the TTP family members ZFP36L1 and ZFP36L2. We also found that the AREs present in the Plk3 transcript were essential for both the binding of TTP to the 3'-UTR and promoting the destruction of target transcripts in cotransfection experiments. The regulation of Plk3 mRNA stability by TTP may influence the control of the cell cycle by this protein kinase.

cAMP-dependent posttranscriptional regulation of steroidogenic acute regulatory (STAR) protein by the zinc finger protein ZFP36L1/TIS11b

Star is expressed in steroidogenic cells as 3.5- and 1.6-kb transcripts that differ only in their 3'-untranslated regions (3'-UTR). In mouse MA10 testis and Y-1 adrenal lines, Br-cAMP preferentially stimulates 3.5-kb mRNA. ACTH is similarly selective in primary bovine adrenocortical cells. The 3.5-kb form harbors AU-rich elements (AURE) in the extended 3'-UTR, which enhance turnover. After peak stimulation of 3.5-kb mRNA, degradation is seen. Star mRNA turnover is enhanced by the zinc finger protein ZFP36L1/TIS11b, which binds to UAUUUAUU repeats in the extended 3'-UTR. TIS11b is rapidly stimulated in each cell type in parallel with Star mRNA. Cotransfection of TIS11b selectively decreases cytomegalovirus-promoted Star mRNA and luciferase-Star 3'-UTR reporters harboring the extended 3'-UTR. Direct complex formation was demonstrated between TIS11b and the extended 3'-UTR of the 3.5-kb Star. AURE mutations revealed that TIS11b-mediated destabilization required the first two UAUUUAUU motifs. HuR, which also binds AURE, did not affect Star expression. Targeted small interfering RNA knockdown of TIS11b specifically enhanced stimulation of 3.5-kb Star mRNA in bovine adrenocortical cells, MA-10, and Y-1 cells but did not affect the reversals seen after peak stimulation. Direct transfection of Star mRNA demonstrated that Br-cAMP stimulated a selective turnover of 3.5-kb mRNA independent of AURE, which may correspond to these reversal processes. Steroidogenic acute regulatory (STAR) protein induction was halved by TIS11b knockdown, concomitant with decreased cholesterol metabolism. TIS11b suppression of 3.5-kb mRNA is therefore surprisingly coupled to enhanced Star translation leading to increased cholesterol metabolism.

Regulation of B-cell differentiation by microRNAs and RNA-binding proteins

Post-transcriptional control of gene expression is an important mechanism for maintaining cellular homoeostasis and regulating the immune response to infection. It allows control of mRNA abundance, translation and localization. Mechanisms for post-transcriptional control involve RNA-binding proteins and miRNAs (microRNAs). The TTP(tristetraprolin) family of proteins recognize and bind AU-rich elements. Deletion of TTP led to a systemic autoimmune syndrome with excess circulating TNFα (tumour necrosis factor α) and GM-CSF (granulocyte/macrophage colony-stimulating factor) due to aberrantly stabilized mRNA. The family may also have a role in control of lymphocyte development and function. miRNAs regulate gene expression by promoting decay or inhibiting translation of transcripts with base pair complementarity. The importance of miRNAs in lymphocytes is highlighted by the T-cell-specific deletion of Dicer, an enzyme required for miRNA-mediated processing and from the phenotype of bic (B-cell integration cluster)/miR-155 (miRNA 155)-deficient mice.

Insulin increases tristetraprolin and decreases VEGF gene expression in mouse 3T3-L1 adipocytes

OBJECTIVES

Tristetraprolin (TTP) family proteins (TTP/ZFP36; ZFP36L1, ZFP36L2, ZFP36L3) destabilize adenylate uridylate-rich element-containing mRNAs encoding cytokines, such as tumor necrosis factor (TNF) and vascular endothelial growth factor (VEGF). Little is known about the expression and insulin regulation of TTP and related genes in adipocytes. We analyzed the relative abundance of TTP family mRNAs in 3T3-L1 adipocytes compared to RAW264.7 macrophages and investigated insulin effects on the expression of 43 genes in 3T3-L1 adipocytes.

METHODS AND PROCEDURES

Insulin was added to mouse 3T3-L1 adipocytes. Relative abundance of mRNA levels was determined by quantitative real-time PCR. TTP and ZFP36L1 proteins were detected by immunoblotting.

RESULTS

Zfp36l1 and Zfp36l2 genes were expressed at eight- to tenfold higher than Ttp in adipocytes. Zfp36l3 mRNA was detected at approximately 1% of Ttp mRNA levels in adipocytes and its low level expression was confirmed in RAW cells. Insulin at 10 and 100 nmol/l increased Ttp mRNA levels by five- to sevenfold, but decreased those of Zfp36l3 by 40% in adipocytes after a 30-min treatment. Immunoblotting showed that insulin induced TTP but did not affect ZFP36L1 protein levels in adipocytes. Insulin decreased mRNA levels of Vegf and a number of other genes in adipocytes.

DISCUSSION

Insulin induced Ttp mRNA and protein expression and decreased Vegf mRNA levels in adipocytes. Zfp36l3 mRNA was detected, for the first time, in cells other than mouse placenta and extraembryonic tissues. This study established a basis for the investigation of TTP and VEGF genes in the regulation of obesity and suggested that Vegf mRNA may be a target of TTP in fat cells.

Transcription factor HNF1beta and novel partners affect nephrogenesis

Heterozygous mutations of the tissue-specific transcription factor hepatocyte nuclear factor (HNF)1beta, cause maturity onset diabetes of the young (MODY5) and kidney anomalies including agenesis, hypoplasia, dysplasia and cysts. Because of these renal anomalies, HNF1beta is classified as a CAKUT (congenital anomalies of the kidney and urinary tract) gene. We searched for human fetal kidney proteins interacting with the N-terminal region of HNF1beta using a bacterial two-hybrid system and identified five novel proteins along with the known partner DCoH. The interactions were confirmed for four of these proteins by GST pull-down assays. Overexpression of two proteins, E4F1 and ZFP36L1, in Xenopus embryos interfered with pronephros formation. Further, in situ hybridization showed overlapping expression of HNF1beta, E4F1 and ZFP36L1 in the developing pronephros. HNF1beta is present largely in the nucleus where it colocalized with E4F1. However, ZFP36L1 was located predominantly in the cytoplasm. A nuclear function for ZFP36L1 was shown as it was able to reduce HNF1beta transactivation in a luciferase reporter system. Our studies show novel proteins may cooperate with HNF1beta in human metanephric development and propose that E4F1 and ZFP36L1 are CAKUT genes. We searched for mutations in the open reading frame of the ZFP36L1 gene in 58 patients with renal anomalies but found none.

A unique C-terminal repeat domain maintains the cytosolic localization of the placenta-specific tristetraprolin family member ZFP36L3

Members of the tristetraprolin family of CCCH tandem zinc finger proteins bind to AU-rich elements in certain cellular mRNAs, leading to their deadenylation and destabilization. Studies in knock-out mice demonstrated roles for three of the family members, tristetraprolin, ZFP36L1, and ZFP36L2, in inflammation, chorioallantoic fusion, and early embryonic development, respectively. However, little is known about a recently discovered placenta-specific tristetraprolin family member, ZFP36L3. Tristetraprolin, ZFP36L1, and ZFP36L2 have been shown to shuttle between the nucleus and cytoplasm, using typical hydrophobic amino acid-rich nuclear export sequences, and nuclear localization sequences located within the tandem zinc finger domain. In contrast, we previously showed that green fluorescent protein-labeled ZFP36L3, expressed in HEK 293 cells, remained cytosolic, even in the presence of the nuclear export blocker leptomycin B. We show here that the conserved tandem zinc finger domain contains an active nuclear localization signal. However, the sequence corresponding to the nuclear export signal in the other family members was nonfunctional, and thus did not contribute to the cytosolic localization. The unique C-terminal repeat domain could override the activity of the nuclear localization sequence, preventing the import of ZFP36L3 into the nucleus. Immunostaining of mouse placenta demonstrated that ZFP36L3 was located only in the cytoplasm of trophoblast cells. Thus, in contrast to the other mammalian members of this protein family, ZFP36L3 is a "full-time" cytosolic protein, rather than a nucleocytoplasmic shuttling protein. The significance of this difference in subcellular localization to the physiology of placental trophoblast cells, where ZFP36L3 is selectively expressed, remains to be determined.

Production and characterization of ZFP36L1 antiserum against recombinant protein from Escherichia coli

Tristetraprolin/zinc finger protein 36 (TTP/ZFP36) family proteins are anti-inflammatory. They bind and destabilize some AU-rich element-containing mRNAs such as tumor necrosis factor mRNA. In this study, recombinant ZFP36L1/TIS11B (a TTP homologue) was overexpressed in E. coli, purified, and used for polyclonal antibody production in rabbits. The antiserum recognized nanograms of the antigen on immunoblots. This antiserum and another antiserum developed against recombinant mouse TTP were used to detect ZFP36L1 and TTP in mouse 3T3-L1 adipocytes and RAW264.7 macrophages. Immunoblotting showed that ZFP36L1 was stably expressed with a size corresponding to the lower mass size of ZFP36L1 expressed in transfected human embryonic kidney 293 cells, but TTP was induced by cinnamon extract and not by lipopolysaccharide (LPS) in adipocytes. In contrast, ZFP36L1 was undetectable, but TTP was strongly induced in LPS-stimulated RAW cells. Quantitative real-time polymerase chain reaction confirmed the higher levels of ZFP36L1 mRNA in adipocytes and TTP mRNA in RAW cells. Low levels of ZFP36L1 expression were also confirmed by Northern blotting in mouse embryonic fibroblasts. These results demonstrate that ZFP36L1 antiserum is useful in the detection of this protein and that TTP and ZFP36L1 are differentially expressed and regulated at the mRNA and protein levels in mouse adipocytes and macrophages.

Phosphorylation site analysis of the anti-inflammatory and mRNA-destabilizing protein tristetraprolin

Tristetraprolin (TTP) is a member of the CCCH zinc finger proteins and is an anti-inflammatory protein. Mice deficient in TTP develop a profound inflammatory syndrome with erosive arthritis, autoimmunity and myeloid hyperplasia. TTP binds to mRNA AU-rich elements with high affinity for UUAUUUAUU nucleotides and causes destabilization of those mRNA molecules. TTP is phosphorylated extensively in vivo and is a substrate for multiple protein kinases in vitro. A number of approaches have been used to identify its phosphorylation sites. This article highlights the recent progress and different approaches utilized for the identification of phosphorylation sites in mammalian TTP. Important but limited results are obtained using traditional methods, including in vivo labeling, site-directed mutagenesis, phosphopeptide mapping and protein sequencing. Mass spectrometry (MS), including MALDI/MS, MALDI/MS/MS, liquid chromatography/MS/MS, immobilized metal ion affinity chromatography (IMAC)/MALDI/MS/MS and multidimensional protein identification technology has led the way in identifying TTP phosphorylation sites. The combination of these approaches has identified multiple phosphorylation sites in mammalian TTP, some of which are predicted by motif scanning to be phosphorylated by several protein kinases. This information should provide the molecular basis for future investigation of TTP's regulatory functions in controlling proinflammatory cytokines.

Strong induction of the Tis11B gene in myogenic differentiation

TIS11B is a zinc-finger protein of the tristetraprolin (TTP) family. Using cDNA microarray analysis, we could identify the Tis11B gene based on its differential expression in myogenesis. Here, we demonstrate that expression of the Tis11B gene is strongly induced during differentiation of the murine myoblast cell line C2C12. By contrast, expression of Ttp itself was not induced in myogenesis. Pretreatment of the cells with the translation inhibitor cycloheximide demonstrated that Tis11B was a primary response gene in this process. In addition, pretreatment with the transcription inhibitor actinomycin D demonstrated that gene expression was regulated at the transcriptional level. Since specific inhibitors of p38 MAP kinase completely blocked Tis11B induction, we conclude that expression of the Tis11B gene is regulated at least in part by this signaling pathway which plays a central role in myogenesis. Induction of Tis11B expression was also observed in primary myoblasts isolated from two different mouse strains, indicating physiological relevance of our results. In addition, TIS11B might also be an important player during myogenic differentiation and regeneration in vivo, as we detected a marked decrease in expression in several muscle tissues of the dystrophic mdx mouse, a model for continuous muscle degeneration and regeneration. These data suggest that TIS11B is an important regulator of myogenesis.

The murine allantois: emerging paradigms in development of the mammalian umbilical cord and its relation to the fetus

The fertilized egg of the mammal gives rise to the embryo and its extraembryonic structures, all of which develop in intimate relation with each other. Yet, whilst the past several decades have witnessed a vast number of studies on the embryonic component of the conceptus, study of the extraembryonic tissues and their relation to the fetus have been largely ignored. The allantois, precursor tissue of the mature umbilical cord, is a universal feature of all placental mammals that establishes the vital vascular bridge between the fetus and its mother. The allantois differentiates into the umbilical blood vessels, which become secured onto the chorionic component of the placenta at one end and onto the fetus at the other. In this way, fetal blood is channeled through the umbilical cord for exchange with the mother. Despite the importance of this vascular bridge, little is known about how it is made. The aim of this review is to address current understanding of the biology of the allantois in the mouse and genetic control of its features and functions, and to highlight new paradigms concerning the developmental relationship between the fetus and its umbilical cord.

Mouse mutants with neural tube closure defects and their role in understanding human neural tube defects

BACKGROUND

The number of mouse mutants and strains with neural tube closure defects (NTDs) now exceeds 190, including 155 involving known genes, 33 with unidentified genes, and eight "multifactorial" strains.

METHODS

The emerging patterns of mouse NTDs are considered in relation to the unknown genetics of the common human NTDs, anencephaly, and spina bifida aperta.

RESULTS

Of the 150 mouse mutants that survive past midgestation, 20% have risk of either exencephaly and spina bifida aperta or both, parallel to the majority of human NTDs, whereas 70% have only exencephaly, 5% have only spina bifida, and 5% have craniorachischisis. The primary defect in most mouse NTDs is failure of neural fold elevation. Most null mutations (>90%) produce syndromes of multiple affected structures with high penetrance in homozygotes, whereas the "multifactorial" strains and several null-mutant heterozygotes and mutants with partial gene function (hypomorphs) have low-penetrance nonsyndromic NTDs, like the majority of human NTDs. The normal functions of the mutated genes are diverse, with clusters in pathways of actin function, apoptosis, and chromatin methylation and structure. The female excess observed in human anencephaly is found in all mouse exencephaly mutants for which gender has been studied. Maternal agents, including folate, methionine, inositol, or alternative commercial diets, have specific preventative effects in eight mutants and strains.

CONCLUSIONS

If the human homologs of the mouse NTD mutants contribute to risk of common human NTDs, it seems likely to be in multifactorial combinations of hypomorphs and low-penetrance heterozygotes, as exemplified by mouse digenic mutants and the oligogenic SELH/Bc strain.

Comparative expression of tristetraprolin (TTP) family member transcripts in normal human tissues and cancer cell lines

The tristetraprolin (TTP) family of tandem zinc finger proteins comprises three members in man and most other mammals, with a fourth expressed in rodents. In mice, gene disruption of TTP itself leads to a systemic inflammatory syndrome that is mediated in large part by over-expression of tumor necrosis factor alpha (TNF). This increased expression is secondary to stabilization of the TNF mRNA in the TTP KO mice, a finding that led to the characterization of TTP as an mRNA binding protein that can promote the removal of the poly(A) tail from selected mRNAs and facilitate their nucleolytic destruction. The other human family members behave similarly to TTP in over-expression studies of transfected cells, but gene disruption experiments have implicated them in different physiological processes. In the present study, we developed a real-time PCR assay for all three human family members that allowed for comparative measurements of all three family members in the same tissues and cells. We used this assay to quantitate expression levels of all three transcripts in a variety of normal human tissues, as well as in the ;;NCI 60", a well characterized panel of human tumor cell lines. Although studies in fibroblasts and macrophages derived from knockout mice have failed to demonstrate compensatory expression of the family members in terms of transcript levels, it remains possible that the different family members can function as ;;TTP equivalents" in certain physiological or pathological circumstances.

TTP and BRF proteins nucleate processing body formation to silence mRNAs with AU-rich elements

In mammalian cells, mRNAs with AU-rich elements (AREs) are targeted for translational silencing and rapid degradation. Here we present evidence that in human cells the proteins Tristetraprolin (TTP) and BRF-1 deliver ARE-mRNAs to processing bodies (PBs), cytoplasmic assemblies of mRNAs, and associated factors that promote translational silencing and mRNA decay. First, depletion of endogenous TTP and BRF proteins, or overexpression of dominant-negative mutant TTP proteins, impairs the localization of reporter ARE-mRNAs in PBs. Second, TTP and BRF-1 localize tethered mRNAs to PBs. Third, TTP can nucleate PB formation on untranslated mRNAs even when other mRNAs are trapped in polysomes by cycloheximide treatment. ARE-mRNA localization in PBs is mediated by the TTP N- and C-terminal domains and occurs downstream from mRNA polysome release, which in itself is not sufficient for mRNA PB localization. The accumulation of ARE-mRNAs in PBs is strongly enhanced when the mRNA decay machinery is rendered limiting by mRNA decay enzyme depletion or TTP/BRF-1 overexpression. Based on these observations, we propose that the PB functions as a reservoir that sequesters ARE-mRNAs from polysomes, thereby silencing ARE-mRNA function even when mRNA decay is delayed. This function of the PB can likely be extended to other mRNA silencing pathways, such as those mediated by microRNAs, premature termination codons, and mRNA deadenylation.

A cassette system to study embryonic stem cell differentiation by inducible RNA interference

Although differentiation of pluripotent embryonic stem cells is restricted by a hierarchy of transcription factors, little is known about whether post-transcriptional mechanisms similarly regulate early embryoid differentiation. We developed a system where small hairpin (sh)RNAs can be induced in embryonic stem (ES) cells from a defined locus following integration by Flp recombinase-mediated DNA recombination. To verify the system, the key transcription factor Stat3, which maintains pluripotency, was downregulated by shRNA, and the expected morphological and biochemical markers of differentiation were observed. Induction of shRNA specific for the post-transcriptional regulator Brf1 (Zfp36L1) amplified the cardiac markers with strong stimulation of cardiomyocyte formation within embryoid bodies. These findings identify Brf1 as a novel potential regulator of cardiomyocyte formation and suggest that post-transcriptional mechanisms are of importance to early development and, possibly, to regenerative medicine. The inducible RNA interference system presented here should also allow assignment of function for candidate genes with suspected roles in ES cell development. Disclosure of potential conflicts of interest is found at the end of this article.

Green tea increases anti-inflammatory tristetraprolin and decreases pro-inflammatory tumor necrosis factor mRNA levels in rats

Background

Tristetraprolin (TTP/ZFP36) family proteins have anti-inflammatory activity by binding to and destabilizing pro-inflammatory mRNAs such as Tnf mRNA, and represent a potential therapeutic target for inflammation-related diseases. Tea has anti-inflammatory properties but the molecular mechanisms have not been completely elucidated. We hypothesized that TTP and/or its homologues might contribute to the beneficial effects of tea as an anti-inflammatory product.

Methods

Quantitative real-time PCR was used to investigate the effects of green tea (0, 1, and 2 g solid extract/kg diet) on the expression of Ttp family genes (Ttp/Tis11/Zfp36, Zfp36l1/Tis11b, Zfp36l2/Tis11d, Zfp36l3), pro-inflammatory genes (Tnf, Csf2/Gm-csf, Ptgs2/Cox2), and Elavl1/Hua/Hur and Vegf genes in liver and muscle of rats fed a high-fructose diet known to induce insulin resistance, oxidative stress, inflammation, and TNF-alpha levels.

Results

Ttp and Zfp36l1 mRNAs were the major forms in both liver and skeletal muscle. Ttp, Zfp36l1, and Zfp36l2 mRNA levels were more abundant in the liver than those in the muscle. Csf2/Gm-csf and Zfp36l3 mRNAs were undetectable in both tissues. Tea (1 g solid extract/kg diet) increased Ttp mRNA levels by 50–140% but Tnf mRNA levels decreased by 30% in both tissues, and Ptgs2/Cox2 mRNA levels decreased by 40% in the muscle. Tea (2 g solid extract/kg diet) increased Elavl1/Hua/Hur mRNA levels by 40% in the liver but did not affect any of the other mRNA levels in liver or muscle.

Conclusion

These results show that tea can modulate Ttp mRNA levels in animals and suggest that a post-transcriptional mechanism through TTP could partially account for tea's anti-inflammatory properties. The results also suggest that drinking adequate amounts of green tea may play a role in the prevention of inflammation-related diseases.

Post-transcriptional Regulators in Inflammation: Exploring New Avenues in Biological Therapeutics

The biosynthesis of inflammatory mediators relies on controlling the biogenesis and utilization of their corresponding messenger RNAs (mRNAs). These latter "utilization steps" encompass post-transcriptional mechanisms that gradually and variably impose a series of flexible-rate limiting controls to modify the abundance of an mRNA and the rate of its translation to protein in response to environmental signals. Mechanistically, post-transcriptional machines comprise networks of RNA binding proteins (RBPs), which recognize, passively or inducibly, secondary or tertiary ribonucleotide structures located on their target RNAs. The outcome of these interactions is the stringent control of mRNA maturation, localization, turnover and translation. It is conceivable that if these post-transcriptional interactions fail, they may perturb cellular re-sponses to provide the impetus for chronic disease. Such is the case of the signal-responsive mechanisms affecting inflammatory mRNAs containing the AU-rich family of elements (AREs), which are recognized by a specific subset of RBPs. Intense research in this area has yielded important insight on the specific signals and mechanisms affecting the utilization of ARE-containing mRNAs. Here, we indicate briefly the inflammatory relevance of ARE-related mechanisms to highlight their importance in pathophysiology and their potential in the development of future biological therapies.

Upregulation of the tumor suppressor gene menin in hepatocellular carcinomas and its significance in fibrogenesis

The molecular mechanisms underlying the progression of cirrhosis toward hepatocellular carcinoma were investigated by a combination of DNA microarray analysis and literature data mining. By using a microarray screening of suppression subtractive hybridization cDNA libraries, we first analyzed genes differentially expressed in tumor and nontumor livers with cirrhosis from 15 patients with hepatocellular carcinomas. Seventy-four genes were similarly recovered in tumor (57.8% of differentially expressed genes) and adjacent nontumor tissues (64% of differentially expressed genes) compared with histologically normal livers. Gene ontology analyses revealed that downregulated genes (n = 35) were mostly associated with hepatic functions. Upregulated genes (n = 39) included both known genes associated with extracellular matrix remodeling, cell communication, metabolism, and post-transcriptional regulation gene (e.g., ZFP36L1), as well as the tumor suppressor gene menin (multiple endocrine neoplasia type 1; MEN1). MEN1 was further identified as an important node of a regulatory network graph that integrated array data with array-independent literature mining. Upregulation of MEN1 in tumor was confirmed in an independent set of samples and associated with tumor size (P = .016). In the underlying liver with cirrhosis, increased steady-state MEN1 mRNA levels were correlated with those of collagen alpha2(I) mRNA (P < .01). In addition, MEN1 expression was associated with hepatic stellate cell activation during fibrogenesis and involved in transforming growth factor beta (TGF-beta)-dependent collagen alpha2(I) regulation. In conclusion, menin is a key regulator of gene networks that are activated in fibrogenesis associated with hepatocellular carcinoma through the modulation of TGF-beta response.

The RNA binding proteinZfp36l1is required for normal vascularisation and post-transcriptionally regulates VEGF expression

The Zfp36l1 gene encodes a zinc finger-containing mRNA binding protein implicated in the posttranscriptional control of gene expression. Mouse embryos homozygous for a targeted mutation in the Zfp36l1 locus died mid-gestation and exhibited extraembryonic and intraembryonic vascular abnormalities and heart defects. In the developing placenta, there was a failure of the extraembryonic mesoderm to invaginate the trophoblast layer. The phenotype was associated with an elevated expression of vascular endothelial growth factor (VEGF)-A in the embryos and in embryonic fibroblasts cultured under conditions of both normoxia and hypoxia. VEGF-A overproduction by embryonic fibroblasts was not a consequence of changes in Vegf-a mRNA stability; instead, we observed enhanced association with polyribosomes, suggesting Zfp36l1 influences translational regulation. These data implicate Zfp36l1as a negative regulator of Vegf-a gene activity during development.

Novel mRNA targets for tristetraprolin (TTP) identified by global analysis of stabilized transcripts in TTP-deficient fibroblasts

Tristetraprolin (TTP) is a tandem CCCH zinc finger protein that was identified through its rapid induction by mitogens in fibroblasts. Studies of TTP-deficient mice and cells derived from them showed that TTP could bind to certain AU-rich elements in mRNAs, leading to increases in the rates of mRNA deadenylation and destruction. Known physiological target mRNAs for TTP include tumor necrosis factor alpha, granulocyte-macrophage colony-stimulating factor, and interleukin-2beta. Here we used microarray analysis of RNA from wild-type and TTP-deficient fibroblast cell lines to identify transcripts with different decay rates, after serum stimulation and actinomycin D treatment. Of 250 mRNAs apparently stabilized in the absence of TTP, 23 contained two or more conserved TTP binding sites; nine of these appeared to be stabilized on Northern blots. The most dramatically affected transcript encoded the protein Ier3, recently implicated in the physiological control of blood pressure. The Ier3 transcript contained several conserved TTP binding sites that could bind TTP directly and conferred TTP sensitivity to the mRNA in cell transfection studies. These studies have identified several new, physiologically relevant TTP target transcripts in fibroblasts; these target mRNAs encode proteins from a variety of functional classes.

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

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

Messenger RNA turnover and its regulation in herpesviral infection

The ability to regulate cellular gene expression is a key aspect of the lifecycles of a diverse array of viruses. In fact, viral infection often results in a global shutoff of host cellular gene expression; such inhibition serves not only to ensure maximal viral gene expression without competition from the host for essential machinery and substrates but also aids in evasion of immune responses detrimental to successful viral replication and dissemination. Within the herpesvirus family, host shutoff is a prominent feature of both the alpha- and gamma-herpesviruses. Intriguingly, while both classes of herpesviruses block cellular gene expression by inducing decay of messenger RNAs, the viral factors responsible for this phenotype as well as the mechanisms by which it is achieved are quite distinct. However, data suggest that the host shutoff functions of alpha- and gamma-herpesviruses are likely achieved both through the activity of virally encoded nucleases as well as via modulation of cellular RNA degradation pathways. This review highlights the processes governing normal cellular messenger RNA decay and then details the mechanisms by which herpesviruses promote accelerated RNA turnover. Parallels between the viral and cellular degradation systems as well as the known interactions between viral host shutoff factors and the cellular RNA turnover machinery are highlighted.

Defects in yolk sac vasculogenesis, chorioallantoic fusion, and embryonic axis elongation in mice with targeted disruption of Yap65

YAP is a multifunctional adapter protein and transcriptional coactivator with several binding partners well described in vitro and in cell culture. To explore in vivo requirements for YAP, we generated mice carrying a targeted disruption of the Yap gene. Homozygosity for the Yap(tm1Smil) allele (Yap-/-) caused developmental arrest around E8.5. Phenotypic characterization revealed a requirement for YAP in yolk sac vasculogenesis. Yolk sac endothelial and erythrocyte precursors were specified as shown by histology, PECAM1 immunostaining, and alpha globin expression. Nonetheless, development of an organized yolk sac vascular plexus failed in Yap-/- embryos. In striking contrast, vasculogenesis proceeded in both the allantois and the embryo proper. Mutant embryos showed patterned gene expression domains along the anteroposterior neuraxis, midline, and streak/tailbud. Despite this evidence of proper patterning and tissue specification, Yap-/- embryos showed developmental perturbations that included a notably shortened body axis, convoluted anterior neuroepithelium, caudal dysgenesis, and failure of chorioallantoic fusion. These results reveal a vital requirement for YAP in the developmental processes of yolk sac vasculogenesis, chorioallantoic attachment, and embryonic axis elongation.

Identification of the anti-inflammatory protein tristetraprolin as a hyperphosphorylated protein by mass spectrometry and site-directed mutagenesis

Tristetraprolin (TTP) is a zinc-finger protein that binds to AREs (AU-rich elements) within certain mRNAs and causes destabilization of those mRNAs. Mice deficient in TTP develop a profound inflammatory syndrome with erosive arthritis, autoimmunity and myeloid hyperplasia. Previous studies showed that TTP is phosphorylated extensively in intact cells. However, limited information is available about the identities of these phosphorylation sites. We investigated the phosphorylation sites in human TTP from transfected HEK-293 cells by MS and site-directed mutagenesis. A number of phosphorylation sites including Ser66, Ser88, Thr92, Ser169, Ser186, Ser197, Ser218, Ser228, Ser276 and Ser296 were identified by MS analyses using MALDI (matrix-assisted laser-desorption-ionization)-MS, MALDI-tandem MS, LC (liquid chromatography)-tandem MS and multidimensional protein identification technology. Mutations of Ser197, Ser218 and Ser228 to alanine in the human protein significantly increased TTP's gel mobility (likely to be stoichiometric), whereas mutations at the other sites had little effect on its gel mobility. Dephosphorylation and in vivo labelling studies showed that mutant proteins containing multiple mutations were still phosphorylated, and all were able to bind to RNA probes containing AREs. Confocal microscopy showed a similar cytosolic localization of TTP among the various proteins. Ser197, Ser218 and Ser228 are predicted by motif scanning to be potential sites for protein kinase A, glycogen synthase kinase-3 and extracellular-signal-regulated kinase 1 (both Ser218 and Ser228) respectively. The present study has identified multiple phosphorylation sites in the anti-inflammatory protein TTP in mammalian cells and should provide the molecular basis for further studies on the function and regulation of TTP in controlling pro-inflammatory cytokines.

Posttranscriptional mechanisms regulating the inflammatory response

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

Zfp36l3, a Rodent X Chromosome Gene Encoding a Placenta-Specific Member of the Tristetraprolin Family of CCCH Tandem Zinc Finger Proteins

Members of the tristetraprolin (TTP) family of CCCH tandem zinc finger (TZF) proteins can bind directly to AU-rich elements (ARE) in mRNA, causing deadenylation and destabilization of the transcripts to which they bind. We describe here a novel fourth mammalian member of the TTP protein family, designated ZFP36L3, which could also bind directly to ARE-containing RNAs and could promote the deadenylation and degradation of ARE-containing target RNAs. Zfp36l3 transcript expression was detected only in placenta and extraembryonic tissues in the mouse. It was expressed throughout development in the placenta and was particularly highly expressed in the cells of the labyrinthine layer of the trophoblastic placenta. Unlike the other family members, the expression of a ZFP36L3-green fluorescent protein fusion protein was entirely cytoplasmic when expressed in 293 cells, even in the presence of the CRM1-dependent nuclear export inhibitor leptomycin B. Zfp36l3 was located on the mouse X chromosome; a similar predicted gene was present on the rat X chromosome, but there was no evidence for a similar gene in humans. ZFP36L3 may thus be a rodent-specific or even murine-specific member of the TTP protein family. Its presumed role in placental physiology may be unique to rodents or murine rodents, but this role may be subsumed by other family members in nonrodents.

Development of structures and transport functions in the mouse placenta

The placenta is essential for sustaining the growth of the fetus during gestation, and defects in its function result in fetal growth restriction or, if more severe, fetal death. Several molecular pathways have been identified that are essential for development of the placenta, and mouse mutants offer new insights into the cell biology of placental development and physiology of nutrient transport.

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.

Differential host cell gene expression regulated by the porcine reproductive and respiratory syndrome virus GP4 and GP5 glycoproteins

The porcine reproductive and respiratory syndrome virus (PRRSV) GP4 and GP5 proteins are two membrane-associated viral glycoproteins that have been shown to induce neutralizing antibodies. In the present study, the host cell gene expression profiles altered by the GP4 and GP5 proteins were investigated by the use of DNA microarrays. Sublines of Marc-145 and HeLa cells were established by stable transfection with open reading frame (ORF)4 and ORF5 of PRRSV, respectively, and differential gene expressions were studied using microarray chips embedded with 1718 human-expressed sequence tags. The genes for protein degradation, protein synthesis and transport, and various other biochemical pathways were identified. No genes involved in the apoptosis pathway appeared to be regulated in GP5-expressing cells. The microarray data may provide insights into the specific cellular responses to the GP4 and GP5 proteins during PRRSV infection.

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

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

The tandem CCCH zinc finger protein tristetraprolin and its relevance to cytokine mRNA turnover and arthritis

Tristetraprolin (TTP) is the best-studied member of a small family of three proteins in humans that is characterized by a tandem CCCH zinc finger (TZF) domain with highly conserved sequences and spacing. Although initially discovered as a gene that could be induced rapidly and transiently by the stimulation of fibroblasts with growth factors and mitogens, it is now known that TTP can bind to AU-rich elements in mRNA, leading to the removal of the poly(A) tail from that mRNA and increased rates of mRNA turnover. This activity was discovered after TTP-deficient mice were created and found to have a systemic inflammatory syndrome with severe polyarticular arthritis and autoimmunity, as well as medullary and extramedullary myeloid hyperplasia. The syndrome seemed to be due predominantly to excess circulating tumor necrosis factor-α (TNF-α), resulting from the increased stability of the TNF-α mRNA and subsequent higher rates of secretion of the cytokine. The myeloid hyperplasia might be due in part to increased stability of granulocyte–macrophage colony-stimulating factor (GM-CSF). This review highlights briefly the characteristics of the TTP-deficiency syndrome in mice and its possible genetic modifiers, as well as recent data on the characteristics of the TTP-binding site in the TNF-α and GM-CSF mRNAs. Recent structural data on the characteristics of the complex between RNA and one of the TTP-related proteins are reviewed, and used to model the TTP-RNA binding complex. We review the current knowledge of TTP sequence variants in humans and discuss the possible contributions of the TTP-related proteins in mouse physiology and in human monocytes. The TTP pathway of TNF-α and GM-CSF mRNA degradation is a possible novel target for anti-TNF-α therapies for rheumatoid arthritis, and also for other conditions proven to respond to anti-TNF-α therapy.

The CCCH tandem zinc-finger protein Zfp36l2 is crucial for female fertility and early embryonic development

The CCCH tandem zinc finger protein, Zfp36l2, like its better-known relative tristetraprolin (TTP), can decrease the stability of AU-rich element-containing transcripts in cell transfection studies; however, its physiological importance is unknown. We disrupted Zfp36l2 in mice,resulting in decreased expression of a truncated protein in which the N-terminal 29 amino acids had been deleted (ΔN-Zfp36l2). Mice derived from different clones of ES cells exhibited complete female infertility,despite evidence from embryo and ovary transplantation experiments that they could gestate and rear wild-type young. ΔN-Zfp36l2 females apparently cycled and ovulated normally, and their ova could be fertilized; however, the embryos did not progress beyond the two-cell stage of development. These mice represent a specific model of disruption of the earliest stages of embryogenesis, implicating Zfp36l2, a probable mRNA-binding and destabilizing protein, in the physiological control of female fertility at the level of early embryonic development. This newly identified biological role for Zfp36l2 may have implications for maternal mRNA turnover in normal embryogenesis, and conceivably could be involved in some cases of unexplained human female infertility.