Véronique Kruys

TIA‐1 is a translational silencer that selectively regulates the expression of TNF‐α

TIA‐1 and TIAR are related proteins that bind to an AU‐rich element (ARE) in the 3′ untranslated region of tumor necrosis factor alpha (TNF‐α) transcripts. To determine the functional significance of this interaction, we used homologous recombination to produce mutant mice lacking TIA‐1. Although lipopolysaccharide (LPS)‐stimulated macrophages derived from wild‐type and TIA‐1−/− mice express similar amounts of TNF‐α transcripts, macrophages lacking TIA‐1 produce significantly more TNF‐α protein than wild‐type controls. The half‐life of TNF‐α transcripts is similar in wild‐type and TIA‐1−/− macrophages, indicating that TIA‐1 does not regulate transcript stability. Rather, the absence of TIA‐1 significantly increases the proportion of TNF‐α transcripts that associate with polysomes, suggesting that TIA‐1 normally functions as a translational silencer. TIA‐1 does not appear to regulate the production of interleukin 1β, granulocyte–macrophage colony‐stimulating factor or interferon γ, indicating that its effects are, at least partially, transcript specific. Mice lacking TIA‐1 are hypersensitive to the toxic effects of LPS, indicating that this translational control pathway may regulate the organismal response to microbial stress.

Intracellular NAD levels regulate tumor necrosis factor protein synthesis in a sirtuin-dependent manner

Tumor necrosis factor (TNF) synthesis is known to play a major part in numerous inflammatory disorders, and multiple transcriptional and post-transcriptional regulatory mechanisms have therefore evolved to dampen the production of this key proinflammatory cytokine. The high expression of nicotinamide phosphoribosyltransferase (Nampt), an enzyme involved in the nicotinamide-dependent NAD biosynthetic pathway, in cells of the immune system has led us to examine the potential relationship between NAD metabolism and inflammation. We show here that intracellular NAD concentration promotes TNF synthesis by activated immune cells. Using a positive screen, we have identified Sirt6, a member of the sirtuin family, as the NAD-dependent enzyme able to regulate TNF production by acting at a post-transcriptional step. These studies reveal a previously undescribed relationship between metabolism and the inflammatory response and identify Sirt6 and the nicotinamide-dependent NAD biosynthetic pathway as novel candidates for immunointervention in an inflammatory setting.

B cell growth modulating and differentiating activity of recombinant human 26-kd protein (BSF-2, HuIFN-beta 2, HPGF)

The human ‘26‐kd protein’ is a secreted glycoprotein expressed, for example, in (blood) leukocytes, in epithelial cells treated with various inducers, but most strongly in interleukin‐1 (IL‐1)‐treated fibroblasts. After finding it has antiviral and 2‐5A synthetase‐inducing activity, one group of authors called this protein IFN‐beta 2. However, recently the full‐length 26‐kd cDNA sequence was shown to be identical with that of a B‐cell‐differentiating lymphokine called BSF‐2, and another report suggested that the 26‐kd protein could support the growth of some transformed murine B cell lines. To define its biological activities, we expressed the recombinant 26‐kd protein by translating in Xenopus laevis oocytes a pure, synthetic chimeric mRNA containing the 26‐kd protein coding region surrounded by Xenopus laevis beta‐globin untranslated regions. A similar construction, but containing the HuIFN‐beta cDNA coding region, was used to produce HuIFN‐beta by the same procedure. Both recombinant glycoproteins were secreted, glycosylated, and their amounts were measured by [35S]methionine incorporation by the oocyte. Here we show that the recombinant 26‐kd protein exhibits a high growth factor activity when assayed on an IL‐HP1‐dependent murine B cell hybridoma (sp. act. approximately 2 X 10(8) U/mg) as well as a potent differentiating activity on human CESS cells (sp. act. approximately 5 X 10(7) U/mg). While rHuIFN‐beta was inactive in the latter two assays, it had the expected antiviral activity of 1‐5 X 10(8) U/mg. The parallel recombinant 26‐kd protein preparations had no detectable antiviral activity (i.e. a maximal specific activity of 1‐3 X 10(2) U/mg, if any). The 26‐kd protein is thus clearly an interleukin, and considering the confusing nomenclature now in use, this factor may better be renamed ‘interleukin 6’.

Transcriptome-wide distribution and function of RNA hydroxymethylcytosine

Chemical modification of RNA for function Chemical modifications play an important role in modifying and regulating the function of DNA and RNA. Delatte et al. show that, in the fruit fly, many messenger RNAs (mRNAs) contain the modified base 5-hydroxymethylcytosine (5hmC). The chemical mark is added by the same enzyme that adds 5hmC to DNA. Because many mRNAs involved in neuronal development contain 5hmC, blocking the enzyme causes brain defects and is lethal. In vivo, RNA hydroxymethylation promotes mRNA translation. Science, this issue p. 282 Posttranscriptional modification of messenger RNAs (mRNAs) is prevalent in Drosophila and promotes mRNA translation. Hydroxymethylcytosine, well described in DNA, occurs also in RNA. Here, we show that hydroxymethylcytosine preferentially marks polyadenylated RNAs and is deposited by Tet in Drosophila. We map the transcriptome-wide hydroxymethylation landscape, revealing hydroxymethylcytosine in the transcripts of many genes, notably in coding sequences, and identify consensus sites for hydroxymethylation. We found that RNA hydroxymethylation can favor mRNA translation. Tet and hydroxymethylated RNA are found to be most abundant in the Drosophila brain, and Tet-deficient fruitflies suffer impaired brain development, accompanied by decreased RNA hydroxymethylation. This study highlights the distribution, localization, and function of cytosine hydroxymethylation and identifies central roles for this modification in Drosophila.

Interleukin-4 and -13 inhibit tumor necrosis factor-alpha mRNA translational activation in lipopolysaccharide-induced mouse macrophages.

The production of tumor necrosis factor-α (TNF-α) by lipopolysaccharide (LPS)-stimulated macrophages can be markedly inhibited by the two closely related cytokines, interleukin (IL)-4 and IL-13. To investigate the molecular mechanism of this inhibition, we analyzed the effect of the two cytokines on TNF-α production and TNF-α mRNA accumulation in the mouse macrophage cell lines RAW 264.7 and J774 stimulated by LPS. Whereas LPS-induced TNF-α production is strongly suppressed by both cytokines, TNF-α mRNA accumulation is not significantly affected, indicating that IL-4 and IL-13 induce a translational repression of TNF-α mRNA. Transfection of reporter gene constructs containing different regions of the TNF-α gene revealed that the inhibitory action of IL-4 and IL-13 is mediated by the UA-rich sequence present in the TNF-α mRNA 3′-untranslated region.

Shuttling SR proteins: more than splicing factors.

Serine–arginine (SR) proteins commonly designate a family of eukaryotic RNA binding proteins containing a protein domain composed of several repeats of the arginine–serine dipeptide, termed the arginine–serine (RS) domain. This protein family is involved in essential nuclear processes such as constitutive and alternative splicing of mRNA precursors. Besides participating in crucial activities in the nuclear compartment, several SR proteins are able to shuttle between the nucleus and the cytoplasm and to exert regulatory functions in the latter compartment. This review aims at discussing the properties of shuttling SR proteins with particular emphasis on their nucleo‐cytoplasmic traffic and their cytoplasmic functions. Indeed, recent findings have unravelled the complex regulation of SR protein nucleo‐cytoplasmic distribution and the diversity of cytoplasmic mechanisms in which these proteins are involved.

Identification of the sequence determinants mediating the nucleo-cytoplasmic shuttling of TIAR and TIA-1 RNA-binding proteins

TIAR and TIA-1 are two closely related RNA-binding proteins which possess three RNA recognition motifs (RRMs) followed by an auxiliary region. These proteins are involved in several mechanisms of RNA metabolism, including alternative hnRNA splicing and regulation of mRNA translation. Here we characterize the subcellular localization of these proteins in somatic cells. We demonstrate that TIAR and TIA-1 continuously shuttle between the cytoplasm and the nucleus and belong to the class of RNA-binding proteins whose nuclear import is transcription-dependent. We identified RRM2 and the first half of the auxiliary region as important determinants for TIAR and TIA-1 nuclear accumulation. In contrast, the nuclear export of TIAR and TIA-1 is mediated by RRM3. Both RRMs contribute to TIAR and TIA-1 nuclear accumulation or export by their RNA-binding capacity. Indeed, whereas mutations of the highly conserved RNP2 or RNP1 peptides in RRM2 redistribute TIAR to the cytoplasm, similar modifications in RRM3 abolish TIAR nuclear export. Moreover, TIAR and TIA-1 nuclear accumulation is a Ran-GTP-dependent pathway, in contrast to its nuclear export which is unaffected by Ran-GTP depletion and which is independent of the major CRM1-exporting pathway. This study demonstrates the importance of TIAR and TIA-1 RNA-binding domains for their subcellular localization and provides the first evidence for distinct functions of TIAR and TIA-1 RRMs.

Mapping of a minimal AU-rich sequence required for lipopolysaccharide-induced binding of a 55-kDa protein on tumor necrosis factor-alpha mRNA.

In monocyte/macrophage cells, the translation of tumor necrosis factor-α (TNF-α) mRNA is tightly controlled. In unstimulated cells, TNF-α mRNA is translationally repressed. However, upon stimulation of the cells with various agents (e.g. lipopolysaccharides (LPS) and viruses), this repression is overcome and translation occurs. The key element in this regulation is the AU-rich sequence present in the 3′-untranslated region of TNF-α mRNA. Several groups have described the binding of proteins on AU-rich elements (AREs). We have previously reported the binding of two cytosolic protein complexes (1 and 2) to the TNF-α mRNA ARE, one of which (complex 2) is observed only following induction of TNF-α production by LPS. In this report, we have demonstrated that complex 1 involves a long fragment of the ARE, whereas the formation of the LPS-inducible complex 2 requires a minimal sequence which corresponds to the nonanucleotide UUAUUUAUU. Furthermore, we show that the RNA-binding protein involved in complex 2 has an apparent molecular mass of 55 kDa. Finally, we tested other AREs for their ability to form complex 2. We observed that the ARE derived from granulocyte/macrophage colony-stimulating factor mRNA, which does contain the nonanucleotide, is able to sustain the LPS-induced binding of the 55-kDa protein. However, c-myc mRNA, which does not contain the nonanucleotide, is unable to promote the formation of any LPS-induced complex.

Expression of TNF-alpha by herpes simplex virus-infected macrophages is regulated by a dual mechanism: transcriptional regulation by NF-kappa B and activating transcription factor 2/Jun and translational regulation through the AU-rich region of the 3' untranslated region.

Here we have investigated the regulation of TNF-α expression in macrophages during HSV-2 infection. Despite a low basal level of TNF-α mRNA present in resting macrophages, no TNF-α protein is detectable. HSV-2 infection marginally increases the level of TNF-α mRNA and protein in resting macrophages, whereas a strong increase is observed in IFN-γ-activated cells infected with the virus. By reporter gene assay it was found that HSV infection augments TNF-α promoter activity. Moreover, treatment of the cells with actinomycin D, which totally blocked mRNA synthesis, only partially prevented accumulation of TNF-α protein, indicating that the infection lifts a block on translation of TNF-α mRNA. EMSA analysis showed that specific binding to the κB#3 site of the murine TNF-α promoter was induced within 1 h after infection and persisted beyond 5 h where TNF-α expression is down-modulated. Binding to the cAMP responsive element site was also induced but more transiently with kinetics closely following activation of the TNF-α promoter. Inhibitors against either NF-κB activation or the activating transcription factor 2 kinase p38 abrogated TNF-α expression, showing a requirement for both signals for activation of the promoter. This observation was corroborated by reporter gene assays. As to the translational regulation of TNF-α, the AU-rich sequence in the 3′ untranslated region of the mRNA was found to be responsible for this control because deletion of this region renders mRNA constitutively translationable. These results show that TNF-α production is induced by HSV-2 in macrophages through both transcriptional and translational regulation.

Alternative polyadenylation of the amyloid protein precursor mRNA regulates translation.

The sequence of several cDNAs encoding the amyloid protein precursor showed that two polyadenylation sites of the mRNA are utilized; RNA blot analysis with different riboprobes indicated that this explains the difference between the two major 3.2 and 3.4 kb mRNAs found in the human brain. These two mRNAs, which contain the whole sequence of the natural molecules, were synthesized by in vitro transcription and translated in Xenopus oocytes. The long mRNA using the second polyadenylation site produced more protein than the short mRNA. The sequence contained within the two polyadenylation sites used in the 3′ untranslated region of the amyloid protein precursor mRNA was also able to increase the production of the chicken lysozyme or the chloramphenicol acetyl transferase, as demonstrated by in vivo translation of different chimeric mRNAs obtained by in vitro transcription. This difference in protein production was also observed when chimeric cDNA constructs were transfected into Chinese hamster ovary cells. Since long mRNAs are not more stable than short mRNAs, the sequence contained within the two polyadenylation sites of the amyloid protein precursor mRNA increases the translation.