Anoctamin-1/TMEM16A is the major apical iodide channel of the thyrocyte.

Iodide is captured by thyrocytes through the Na(+)/I(-) symporter (NIS) before being released into the follicular lumen, where it is oxidized and incorporated into thyroglobulin for the production of thyroid hormones. Several reports point to pendrin as a candidate protein for iodide export from thyroid cells into the follicular lumen. Here, we show that a recently discovered Ca(2+)-activated anion channel, TMEM16A or anoctamin-1 (ANO1), also exports iodide from rat thyroid cell lines and from HEK 293T cells expressing human NIS and ANO1. The Ano1 mRNA is expressed in PCCl3 and FRTL-5 rat thyroid cell lines, and this expression is stimulated by thyrotropin (TSH) in rat in vivo, leading to the accumulation of the ANO1 protein at the apical membrane of thyroid follicles. Moreover, ANO1 properties, i.e., activation by intracellular calcium (i.e., by ionomycin or by ATP), low but positive affinity for pertechnetate, and nonrequirement for chloride, better fit with the iodide release characteristics of PCCl3 and FRTL-5 rat thyroid cell lines than the dissimilar properties of pendrin. Most importantly, iodide release by PCCl3 and FRTL-5 cells is efficiently blocked by T16Ainh-A01, an ANO1-specific inhibitor, and upon ANO1 knockdown by RNA interference. Finally, we show that the T16Ainh-A01 inhibitor efficiently blocks ATP-induced iodide efflux from in vitro-cultured human thyrocytes. In conclusion, our data strongly suggest that ANO1 is responsible for most of the iodide efflux across the apical membrane of thyroid cells.

Extinction of the tumor necrosis factor locus, and of genes encoding the lipopolysaccharide signaling pathway.

The tumor necrosis factor (TNF-alpha or TNF) gene is activated by both lipopolysaccharide (LPS) and cycloheximide in RAW 264.7 macrophages, whereas neither stimulus activates the gene in 3T3 fibroblasts. Moreover, the pattern of CG methylation within the TNF gene is readily distinguishable in DNA derived from cells of these two types. These findings would suggest that the TNF gene has been rendered inaccessible to transcription in the 3T3 cell environment. When RAW 264.7 cells are fused with 3T3 cells, an immortal pentaploid hybrid results. In the hybrid cell, all three TNF genes contributed by the RAW 264.7 cell parent become highly methylated according to the pattern observed in the 3T3 cell parent. Permanently transfected chloramphenicol acetyl transferase (CAT) reporter constructs, bearing 2.2 kb of upstream sequence (including the entire TNF promoter and 5'-untranslated region [UTR]) as well as 1.0 kb of downstream sequence (including the entire TNF 3'-UTR and termination sequence), are accessible in both RAW 264.7 cells and 3T3 cells, but are silenced in transition from the RAW 264.7 cell to the hybrid cell environment. Moreover, the endotoxin signaling pathway is abrogated, as assessed by transient transfection of hybrid cells with LPS-responsive CAT reporter constructs. It would therefore appear that the fusion of 3T3 cells and RAW 264.7 cells activates a system that silences the TNF gene, as well as the LPS signaling pathway. This system may operate to determine TNF gene accessibility and LPS responsiveness in the course of cell differentiation. The DNA sequences targeted within the TNF gene are included in the CAT reporter construct; therefore, the silencing element has been circumscribed to a region of DNA 3.2 kb in length.

Translational blockade imposed by cytokine-derived UA-rich sequences.

The messenger RNAs specifying certain proteins involved in the inflammatory response and certain oncoproteins contain a conserved UA-rich sequence in the 3' untranslated region. This sequence, which is composed of several interspersed repeats of the octanucleotide UUAUUUAU, has been shown to destabilize mRNA in some eukaryotes. However, this effect is not seen when mRNAs are transferred to Xenopus oocytes, which made it possible to separate stability from translational regulation. For interferon, granulocyte-macrophage colony-stimulating factor, and c-fos RNAs, the UA-rich sequence was observed to preclude mRNA translation.

Analysis of tumor necrosis factor promoter responses to ultraviolet light.

Ultraviolet (UV) light induces the biosynthesis of chloramphenicol acetyltransferase (CAT) in the skin of mice bearing the CATTNF reporter transgene. Moreover, nuclear run-on assays indicate that UV light induces transcription of the TNF gene in RAW 264.7 macrophages. These observations suggest that the TNF gene (and/or its mRNA product) responds to signals elicited by UV light. To identify transcriptional UV response elements within the TNF promoter, and to determine whether a posttranscriptional response might also exist, a series of reporter constructs using a CAT coding sequence attached to various portions of the TNF promoter and 3' untranslated region were devised and transfected into several cultured cell lines. All cells tested were found to be UV responsive, and in NIH 3T3 cells, induction was found to depend upon two general regions of the promoter. The more distal region encompassed nucleotides (nt) -1059 through -451 with respect to the cap site, while the more proximal region spanned nt -403 through -261. A negative element, blocking the UV response, was interposed (nt -451 through -403). As with the response to LPS, the response to UV irradiation appears to involve translational activation in macrophages. However, the UV and LPS signaling pathways have little in common with one another, as indicated by three observations. First, no difference in responsiveness was observed on comparison of TNF gene induction in macrophages derived from C3H/HeN as opposed to C3H/HeJ mice. Second, cell fusion studies showed that while the LPS signaling pathway is extinguished by fusion of RAW 264.7 cells with NIH 3T3 cells, the UV signaling pathway remained intact. Finally, induction did not depend upon the NF-kappa B binding sites that are known to be required for LPS response in macrophages, since mutation of these sites did not impair the UV response.

Translation of the human c-myc P0 tricistronic mRNA involves two independent internal ribosome entry sites.

The human c-myc proto-oncogene is transcribed from four alternative promoters (P0, P1, P2, and P3) giving rise to mRNAs having 5' leader sequences of various length. The c-myc P0 mRNA contains three open reading frames (ORFs), the last one encoding c-Myc1 and c-Myc2 proteins generated by alternative translation initiated at CUG and AUG codons. The middle ORF (MYCHEX1) and the 5' ORF (ORF1) code for proteins 188 and 114 amino acids in length, respectively. We and others previously identified an internal ribosome entry site (IRES) in P0 and P2 c-myc mRNAs, promoting the cap-independent translation of c-Myc1 and c-Myc2. Here, we report the presence of a second IRES (named IRES1) promoting the cap-independent translation of MYCHEX1 in c-myc P0 mRNA. Using deletion analysis, we mapped an 80-nt region essential for IRES1 activity. c-myc P0 mRNA is thus the first eukaryotic polycistronic mRNA described for which translation initiation of two different open reading frames (MYCHEX1 and c-Myc1/c-Myc2) involves internal ribosome entry.

Identification of a translation inhibitory element (TIE) in the 3' untranslated region of the human interferon-beta mRNA.

We have previously reported that the 3' untranslated region (UTR) of the human interferon-beta mRNA has an inhibitory effect on the mRNA translation both in vitro, in a rabbit reticulocyte lysate, and in vivo, in the Xenopus oocyte. In the present study, we identify the sequence in the 3' UTR which is responsible for this translation inhibition. We show that this sequence is located between the 100th and 161st nucleotides downstream from the translation stop codon. It contains several repeats of the A + U-rich consensus octanucleotide UUAUUUAU, which is also present in the 3' UTR of several mRNAs involved in the inflammatory response. We also demonstrate here that the inhibitory effect of the sequence on the mRNA translation does not depend on its position in relation to the termination codon. However, no inhibition of translation is observed when this sequence is inserted in the 5' UTR of the mRNA. The removal of the translation inhibitory sequence not only improves the mRNA translation in Xenopus oocytes but it also strongly decreases the IFN-beta mRNA stability in those cells. This suggests that, in this system at least, the mRNA degradation is linked to its translational efficiency.

Fertilization of Xenopus eggs imposes a complete translational arrest of mRNAs containing 3'UUAUUUAU elements.

The early embryonic development of Xenopus is mainly governed by post-transcriptional regulations until the mid-blastula transition. In this report, we present evidence demonstrating that fertilization of Xenopus eggs triggers a complete translational arrest of mRNAs containing UA-rich elements in their 3'-untranslated region. This control is maintained at least until the mid-blastula transition. Neither maturation nor pseudo-fertilization of the egg is sufficient for triggering this control, suggesting that components originating from the male gamete are involved in the mechanism. Moreover, this control is exerted whether the mRNA is polyadenylated or not.

Translational control of cytokine expression by 3' UA-rich sequences.

Several messenger RNAs which are transiently expressed contain a conserved uridine-adenosine-rich sequence in their 3' untranslated region. Many of these mRNas encode cytokines, growth factors or oncoproteins. This UA-rich sequence is composed of several interpsersed repeats of the octanucleotide UUAUUUAU and plays a key role in the post-transcriptional regulation of these mRNAs. Known as instability determinants, these UA-rich elements can also strongly affect mRNA translational efficiency. In this report, we review the data which illustrate this translational regulation and give insight the underlying mechanism.

Tumor necrosis factor-alpha production induced by viruses and by lipopolysaccharides in macrophages: similarities and differences.

Tumor necrosis factor (TNF)-a gene expression can be induced primarily in cells of the monocyte-macrophage lineage by a variety of inducers, including lipopolysaccharides (LPS), phorbol esters, ultraviolet (UV) light, and viruses. In this paper, we analyzed the regulatory mechanisms of TNF-alpha production induced by infection with the Sendai" virus in RAW 264.7 macrophages. We show that in these cells TNF-a synthesis results mainly from TNF-alpha mRNA translational activation. Using CAT reporter genes, we identified the UA- rich (UAR) sequences localized in the TNF-alpha mRNA 3' untranslated region (UTR) as the main sequence involved in this regulation. This sequence has been previously shown to be the essential regulatory element involved in LPS- induced translational activation of TNF mRNA. Activation of TNF gene expression by viral infection presents other similarities with those induced by LPS. First, TNF production in response to viral infection is inhibited by the protein-tyrosine kinase inhibitor herbimycin A as it is in response to LPS. More specifically, we show here that TNF mRNA translational activation induced by viral infection or by LPS is inhibited by pretreating the cells with herbimycin A. Second, TNF production in response to viruses is tissue-specific and is abrogated in RAW 264.7x NIH3T3 hybrid cells, which lack the ability to produce TNF in response to LPS, as a consequence of a defect in the LPS signaling pathway. However, viral infection induces TNF production in LPS- unresponsive C3H/HeJ mouse-derived peritoneal macro phages indicating that viruses and LPS signaling pathways differ for at least one intermediate which is the product of the Lps gene. Finally, we show that this regulatory mechanism can be triggered by different classes of viruses.

Defective translation of tumor necrosis factor mRNA in lipopolysaccharide-tolerant macrophages.

Macrophage activation by lipopolysaccharide (LPS) results in the translational activation of tumor necrosis factor (TNF) mRNA. The initial phase of macrophage activation is followed by a refractory state called LPS tolerance characterized by an impaired TNF production in response to a secondary LPS challenge. LPS-tolerant macrophages contain high amounts of TNF mRNA, suggesting a translational regulation of TNF biosynthesis. The induction of LPS tolerance was studied in RAW 264.7 macrophages stably transfected with a chloramphenicol acetyl-transferase (CAT) reporter gene construct driven by a constitutive cytomegalovirus promoter and containing the 3' untranslated region of the murine TNF gene. We found that primary stimulation of transfected cells by LPS (1 ng/ml, 12 hr) resulted in a marked suppression (80%) of CAT accumulation in response to a secondary LPS challenge (1 microgram/ml, 6 hr). In contrast, the accumulation of CAT mRNA was not influenced by LPS tolerance. Using the same CAT reporter, we observed that the serine/threonine phosphatases 1 and 2A inhibitor okadaic acid induced TNF mRNA translation and that this activation was not inhibited by LPS-tolerance. In conclusion, these data indicate that deficient production of TNF in LPS-tolerant macrophages in response to a second LPS challenge is characterized by a defective translation of TNF mRNA. However, this hyporesponsiveness to LPS is specific, since translation of TNF mRNA induced by okadaic acid is not inhibited in LPS-tolerant macrophages.

The HTLV-1 Tax protein inhibits formation of stress granules by interacting with histone deacetylase 6.

Human T cell leukemia virus type-1 (HTLV-1) is the causative agent of a fatal adult T-cell leukemia. Through deregulation of multiple cellular signaling pathways the viral Tax protein has a pivotal role in T-cell transformation. In response to stressful stimuli, cells mount a cellular stress response to limit the damage that environmental forces inflict on DNA or proteins. During stress response, cells postpone the translation of most cellular mRNAs, which are gathered into cytoplasmic mRNA-silencing foci called stress granules (SGs) and allocate their available resources towards the production of dedicated stress-management proteins. Here we demonstrate that Tax controls the formation of SGs and interferes with the cellular stress response pathway. In agreement with previous reports, we observed that Tax relocates from the nucleus to the cytoplasm in response to environmental stress. We found that the presence of Tax in the cytoplasm of stressed cells prevents the formation of SGs and counteracts the shutoff of specific host proteins. Unexpectedly, nuclear localization of Tax promotes spontaneous aggregation of SGs, even in the absence of stress. Mutant analysis revealed that the SG inhibitory capacity of Tax is independent of its transcriptional abilities but relies on its interaction with histone deacetylase 6, a critical component of SGs. Importantly, the stress-protective effect of Tax was also observed in the context of HTLV-1 infected cells, which were shown to be less prone to form SGs and undergo apoptosis under arsenite exposure. These observations identify Tax as the first virally encoded inhibitory component of SGs and unravel a new strategy developed by HTLV-1 to deregulate normal cell processes. We postulate that inhibition of the stress response pathway by Tax would favor cell survival under stressful conditions and may have an important role in HTLV-1-induced cellular transformation.

Lipopolysaccharide signal transduction, regulation of tumor necrosis factor biosynthesis, and signaling by tumor necrosis factor itself.

In recent years, the chain of events that connects introduction of bacterial endotoxin (lipopolysaccharide; LPS) into a mammalian host, and the syndrome of organ damage and vascular collapse that ensues, have come into sharper focus. Several of the molecules that engage LPS, and a rough outline of the signaling cascade that leads to cytokine release from mononuclear cells, have been elucidated. The principal cytokines that mediate the untoward effects of LPS have also been identified. The most important of these is tumor necrosis factor (TNF), which elicits biologic responses from virtually every type of cell to which it binds. Two distinct receptors transduce the TNF signal. Mechanisms of TNF receptor action are becoming increasing clear, and there is reason to hope that, through intervention at many distinct levels, the devastating effects of LPS might be attenuated or averted.

Translational control mediated by UA-rich sequences.

Several messenger RNAs coding for cytokines, growth factors or oncoproteins contain a conserved UA-rich sequence in their 3' untranslated regions. This sequence, which is composed of several interspersed repeats of the octanucleotide UUAUUUAU, is a key element in the posttranscriptional regulation of these mRNAs. Previously shown to be involved in mRNA destabilization, these UA-rich elements can also strongly influence mRNA translation. A translation inhibition mediated by these sequences has indeed been observed in vitro as well as in frog oocyte or in somatic cells.

AU-rich element-mediated translational control: complexity and multiple activities of trans-activating factors.

Tumour necrosis factor (TNF)-alpha mRNA contains an AU-rich element (ARE) in its 3' untranslated region (3'UTR), which determines its half-life and translational efficiency. In unstimulated macrophages, TNF-alpha mRNA is repressed translationally, and becomes efficiently translated upon cell activation. Gel retardation experiments and screening of a macrophage cDNA expression library with the TNF-alpha ARE allowed the identification of TIA-1-related protein (TIAR), T-cell intracellular antigen-1 (TIA-1) and tristetraprolin (TTP) as TNF-alpha ARE-binding proteins. Whereas TIAR and TIA-1 bind the TNF-alpha ARE independently of the activation state of macrophages, the TTP-ARE complex is detectable upon stimulation with lipopolysaccharide (LPS). Moreover, treatment of LPS-induced macrophage extracts with phosphatase significantly abrogates TTP binding to the TNF-alpha ARE, indicating that TTP phosphorylation is required for ARE binding. Carballo, Lai and Blackshear [(1998) Science 281, 1001-1005] showed that TTP was a TNF-alpha mRNA destabilizer. In contrast, TIA-1, and most probably TIAR, acts as a TNF-alpha mRNA translational silencer. A two-hybrid screening with TIAR and TIA-1 revealed the capacity of these proteins to interact with other RNA-binding proteins. Interestingly, TIAR and TIA-1 are not engaged in the same interaction, indicating for the first time that TIAR and TIA-1 can be functionally distinct. These findings also suggest that ARE-binding proteins interact with RNA as multimeric complexes, which might define their function and their sequence specificity.

The 3' untranslated region of the human interferon-beta mRNA has an inhibitory effect on translation.

In vitro-transcribed human interferon-beta (IFN-beta) mRNA, which contains all the sequence of the natural molecule, is poorly translated in a reticulocyte lysate or when injected in Xenopus oocytes. This low level of translation is due to an inhibition by the 5' and 3' untranslated regions (UTRs). Indeed, the replacement of these sequences by those of Xenopus beta-globin mRNA dramatically increases the translational efficiency of the mRNA, especially in oocytes. This phenomenon is not due to a difference in mRNA stability since both native and chimeric mRNAs remain undegraded, at least during the translation period considered. Construction of different chimeric molecules having various combinations of 5' and 3' UTRs from IFN-beta or Xenopus beta-globin mRNA or a small sequence of SP6 polylinker as 5' UTR has revealed that the 3' UTR of IFN-beta in itself has a pronounced inhibitory effect on translation in the two translation systems from animal cells. Indeed, the addition of this 3' UTR at the 3' end of the coding region of a chicken lysozyme mRNA also causes a large decrease of its translational capacity in both systems. However, the nature of the 5' noncoding sequence influences the degree of translation inhibition exerted by the 3' UTR. Remarkably, we observed no difference in translation level when the different mRNAs were tested in a wheat germ extract.

Constitutive activity of the tumor necrosis factor promoter is canceled by the 3' untranslated region in nonmacrophage cell lines; a trans-dominant factor overcomes this suppressive effect.

The role of the mouse tumor necrosis factor (TNF) promoter, 5' untranslated region (UTR), and 3' UTR in TNF gene expression has been examined in three nonmacrophage cell lines (HeLa, NIH 3T3, and L-929). The TNF promoter is not macrophage-specific. On the contrary, it constitutively drives reporter gene expression in all three cell lines. Not only the full-length promoter but also truncated versions of the promoter, lacking NF-kappa B binding motifs, are active in each type of cell. The TNF 3' UTR effectively cancels reporter gene expression in HeLa cells and in NIH 3T3 cells but fails to block expression in L-929 cells. L-929 cells contain a factor that overcomes the inhibitory influence of the TNF 3' UTR. Its action depends upon the presence of sequences found in the TNF 5' UTR. Cell-fusion experiments reveal that this activator is trans-dominant. These studies highlight the essential role played by the TNF 3' UTR, which silences the TNF gene in cells that might otherwise express TNF. They also reveal the existence of an escape mechanism whereby inappropriate synthesis of TNF might occur.

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-alpha expression in macrophages during HSV-2 infection. Despite a low basal level of TNF-alpha mRNA present in resting macrophages, no TNF-alpha protein is detectable. HSV-2 infection marginally increases the level of TNF-alpha mRNA and protein in resting macrophages, whereas a strong increase is observed in IFN-gamma-activated cells infected with the virus. By reporter gene assay it was found that HSV infection augments TNF-alpha promoter activity. Moreover, treatment of the cells with actinomycin D, which totally blocked mRNA synthesis, only partially prevented accumulation of TNF-alpha protein, indicating that the infection lifts a block on translation of TNF-alpha mRNA. EMSA analysis showed that specific binding to the kappaB#3 site of the murine TNF-alpha promoter was induced within 1 h after infection and persisted beyond 5 h where TNF-alpha 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-alpha promoter. Inhibitors against either NF-kappaB activation or the activating transcription factor 2 kinase p38 abrogated TNF-alpha 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-alpha, 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-alpha production is induced by HSV-2 in macrophages through both transcriptional and translational regulation.

Regulated control by granulocyte-macrophage colony-stimulating factor AU-rich element during mouse embryogenesis.

In vitro studies have indicated that the granulocyte-macrophage colony-stimulating factor (GM-CSF) gene expression is regulated at the posttranscriptional level by the AU-rich element (ARE) sequence present in its 3' untranslated region (UTR). This study investigated the importance of the ARE in the control of GM-CSF gene expression in vivo. For this purpose, transgenic mice bearing GM-CSF gene constructs containing or lacking the ARE (GM-CSF AU(+) or GM-CSF AU(-), respectively) were generated. Both transgenes were under the transcriptional control of the immediate early promoter of the cytomegalovirus (CMV) to ensure their early, widespread, and constitutive expression. The regulation imposed by the ARE was revealed by comparing transgene expression at day 14 of embryonic development (E14); only the ARE-deleted but not the ARE-containing construct was expressed. Although GM-CSF AU(+) embryos were phenotypically normal, overexpression of GM-CSF in E14 GM-CSF AU(-) embryos led to severe hematopoietic alterations such as abnormal proliferation of granulocytes and macrophages accompanied by an increased number of peroxidase-expressing cells, their putative progenitor cells. These abnormalities compromise development because no viable GM-CSF AU(-) transgenic pups could be obtained. Surprisingly, by E18, significant accumulation of transgene messenger RNA was also observed in GM-CSF AU(+) embryos leading to similar phenotypic abnormalities. Altogether, these observations reveal that GM-CSF ARE is a developmentally controlled regulatory element and highlight the consequences of GM-CSF overexpression on myeloid cell proliferation and differentiation.

Tumor necrosis factor-alpha mRNA remains unstable and hypoadenylated upon stimulation of macrophages by lipopolysaccharides.

TNF-alpha gene expression is regulated at transcriptional and post-transcriptional levels in mouse macrophages. The post-transcriptional regulation is mediated by the AU-rich element (ARE) located in the TNF-alpha mRNA 3' untranslated region (UTR), which controls its translation and stability. In resting macrophages, the ARE represses TNF-alpha mRNA translation. Activation of macrophages with various agents [for example lipopolysaccharide (LPS), viruses] results in translational derepression, leading to the production of high levels of TNF-alpha. TNF-alpha ARE has also been shown to confer mRNA instability as its deletion from the mouse genome leads to an increase in the TNF-alpha mRNA half-life [Kontoyiannis, D., Pasparakis, M., Pizzaro, T., Cominelli, F. & Kollias, G. (1999) Immunity 10, 387-398]. In this study, we measured the half-life as well as the poly(A) tail length of TNF-alpha mRNA in the course of macrophage activation by LPS. We report that TNF-alpha mRNA is short lived even in conditions of maximal TNF-alpha synthesis. Moreover, TNF-alpha mRNA is hypoadenylated in a constitutive manner. These results reveal that TNF-alpha mRNA rapid turnover does not constitute a regulatory step of TNF-alpha biosynthesis in macrophages and that TNF-alpha mRNA translational activation upon LPS stimulation is not accompanied by a change of poly(A) tail length.