Visibly stressed: the role of eIF2, TIA-1, and stress granules in protein translation

HuR as a negative posttranscriptional modulator in inflammation

HuR is an RNA binding protein with an alleged role in the posttranscriptional activation of inflammatory mRNAs bearing AU-rich elements (AREs). Here, we show that the inducible increase of HuR in murine innate compartments suppresses inflammatory responses in vivo. In macrophages, HuR overexpression induced the translational silencing of specific cytokine mRNAs despite positive or nominal effects on their corresponding turnover. By using a model system of ARE dysfunction, we demonstrate that HuR does not alter the accumulation of target mRNAs in the absence of the destabilizing functions of Tristetraprolin but synergizes with the translational silencer TIA-1 to reduce the translation of cytokine mRNAs. Our data suggest that HuR acts in a pleiotropic fashion in inflammation through its functional interactions with specific mRNA subsets and negative posttranscriptional modules.

A new pharmacology--drugging stressed folding pathways

Folding in the endoplasmic reticulum (ER) must couple protein-synthesis pathways operating outside of the compartment with ER-assisted folding (ERAF) pathways in the lumen. Chaperone-mediated folding imbalances that are associated with numerous misfolding diseases, including diabetes, trigger the unfolded-protein response (UPR), using both transcriptional and translational pathways to correct the problem. Recent work suggests that small-molecule inhibitors could be useful to help rebalance protein synthesis with ERAF pathways through the ribosomal initiating factor eIF2alpha. Reprogramming stress pathways with drugs provides a potential new approach for balancing ER-protein load with cellular-folding capacity, thus correcting disease.

Novel DNA-binding properties of the RNA-binding protein TIAR

TIA-1 related protein binds avidly to uridine-rich elements in mRNA and pre-mRNAs of a wide range of genes, including interleukin (IL)-8 and vascular endothelial growth factor (VEGF). The protein has diverse regulatory roles, which in part depend on the locus of binding within the transcript, including translational control, splicing and apoptosis. Here, we observed selective and potent inhibition of TIAR–RNP complex formation with IL-8 and VEGF 3′-untranslated regions (3′-UTRs) using thymidine-rich deoxyoligonucleotide (ODN) sequences derived from the VEFG 3′-UTR. We show by ultraviolet crosslinking and electrophoretic mobility shift assays that TIAR can bind directly to single-stranded, thymidine-rich ODNs but not to double-stranded ODNs containing the same sequence. TIAR had a nearly 6-fold greater affinity for DNA than RNA (Kdapp=1.6×10−9M versus 9.4 × 10⁻⁹ M). Truncation of TIAR indicated that the high affinity DNA-binding site overlaps with the RNA-binding site involving RNA recognition motif 2 (RRM2). However, RRM1 alone could also bind to DNA. Finally, we show that TIAR can be displaced from single-stranded DNA by active transcription through the binding site. These results provide a potential mechanism by which TIAR can shuttle between RNA and DNA ligands.

IL-1beta induces stabilization of IL-8 mRNA in malignant breast cancer cells via the 3' untranslated region: Involvement of divergent RNA-binding factors HuR, KSRP and TIAR

IL-8 plays an integral role in promoting the malignant phenotype in breast cancer, and its production is directly influenced by inflammatory cytokines in the tumor microenvironment. Here, we show that activation of IL-1beta receptors on malignant HS578t and MDA-MB-231 breast cancer cells strongly induces IL-8 expression and that RNA stabilization is persistently activated at least 12-24 hr after stimulation. SB 203580 and rapamycin reversed the RNA stabilization effect of IL-1beta in a dose-dependent manner, suggesting involvement of the p38/MAP kinase and mTOR pathways. A luciferase reporter assay indicated that the stabilization effect was dependent on cis elements in the 3'-untranslated region (UTR) of the IL-8 transcript. By UV cross-linking, we identified multiple cellular factors that interact with the IL-8 3'UTR, ranging 34-76 kDa. Immunoprecipitation analysis indicated that HuR, KSRP and TIAR bound to one or more loci in the 3'UTR. While the cross-linking patterns were similar, quantitative immunoprecipitation of native IL-8 RNA from IL-1beta-stimulated cytoplasmic extract revealed a 20-fold greater association of transcript with the stabilizing factor HuR vs. the destabilizing factor KSRP. In conclusion, IL-1beta is a potent cytokine stimulus for IL-8 RNA stabilization in breast cancer cells, possibly by enhanced binding of cytoplasmic HuR to the 3'UTR.

Sequestration of TRAF2 into stress granules interrupts tumor necrosis factor signaling under stress conditions

The cellular stress response (SR) is a phylogenetically conserved protection mechanism that involves inhibition of protein synthesis through recruitment of translation factors such as eIF4G into insoluble stress granules (SGs) and blockade of proinflammatory responses by interruption of the signaling pathway from tumor necrosis factor alpha (TNF-alpha) to nuclear factor-kappaB (NF-kappaB) activation. However, the link between these two physiological phenomena has not been clearly elucidated. Here we report that eIF4GI, which is a scaffold protein interacting with many translation factors, interacts with TRAF2, a signaling molecule that plays a key role in activation of NF-kappaB through TNF-alpha. These two proteins colocalize in SGs during cellular exposure to stress conditions. Moreover, TRAF2 is absent from TNFR1 complexes under stress conditions even after TNF-alpha treatment. This suggests that stressed cells lower their biological activities by sequestration of translation factors and TRAF2 into SGs through a protein-protein interaction.

Quantifying immunohistochemical staining of phospho-eIF2alpha, heme oxygenase-2 and NADPH cytochrome P450 reductase in oligodendrocytes during experimental autoimmune encephalomyelitis

As a consequence of inflammation associated with multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis (EAE), stress responses are induced in many cells within the CNS, however, those that occur within the primary pathological target, the oligodendrocyte, are not fully established. Recently, we found that phosphorylated eukaryotic initiation factor-2alpha (eIF2alpha), an inhibitor of protein translation associated with the stress response, is expressed in a greater number of oligodendrocytes in EAE animals compared to controls. However, since numerous oligodendrocytes in control animals also expressed phospho-eIF2alpha, a method was developed to detect expression levels within oligodendrocytes that did not rely on the number of oligodendrocytes that were stained. This method utilized a high dilution of the primary antibody so that the staining density was kept below a maximum plateau which could eliminate expression differences. Furthermore, the staining density within oligodendrocytes, as determined by image analysis, was corrected by the background density or that within neurons. In either case, the density of staining was greater in oligodendrocytes from EAE animals versus controls. The expression of heme oxygenase-2 and NADPH cytochrome P450 reductase also were examined, but unlike phospho-eIF2alpha, neither was increased in oligodendrocytes from EAE animals compared to controls. In summary, a protocol involving a high dilution of primary antibody and image analysis revealed that the expression of phospho-eIF2alpha within oligodendrocytes was increased in EAE animals compared to control animals.

FAST is a survival protein that senses mitochondrial stress and modulates TIA-1-regulated changes in protein expression

The Fas-activated serine/threonine phosphoprotein (FAST) is tethered to the outer mitochondrial membrane, where it interacts with BCL-X(L) (17). Here we show that RNA interference-mediated knockdown of endogenous FAST results in apoptosis, whereas overexpressed recombinant FAST inhibits Fas- and UV-induced apoptosis, indicating that FAST is a survival protein. The antiapoptotic effects of FAST are regulated by interactions with the translational silencer TIA-1: a FAST mutant lacking its TIA-1-binding domain does not inhibit apoptosis, and overexpressed recombinant TIA-1 inhibits the antiapoptotic effects of FAST. Because the antiapoptotic effects of FAST require ongoing protein synthesis, we hypothesized that FAST might function by preventing TIA-1-mediated silencing of mRNAs encoding inhibitors of apoptosis. Consistent with this hypothesis, FAST promotes the expression of cotransfected reporter proteins, a process that requires its TIA-1-binding domain and is inhibited by overexpressed recombinant TIA-1. More compellingly, recombinant FAST increases the expression of endogenous cIAP-1 and XIAP, but not GAPDH, in transfected HeLa cells. Because FAST is released from mitochondria in cells undergoing Fas- or UV-induced apoptosis, we propose that FAST serves as a sensor of mitochondrial stress that modulates a TIA-1-regulated posttranscriptional stress response program.

Stress granule assembly is mediated by prion-like aggregation of TIA-1

TIA-1 is an RNA binding protein that promotes the assembly of stress granules (SGs), discrete cytoplasmic inclusions into which stalled translation initiation complexes are dynamically recruited in cells subjected to environmental stress. The RNA recognition motifs of TIA-1 are linked to a glutamine-rich prion-related domain (PRD). Truncation mutants lacking the PRD domain do not induce spontaneous SGs and are not recruited to arsenite-induced SGs, whereas the PRD forms aggregates that are recruited to SGs in low-level-expressing cells but prevent SG assembly in high-level-expressing cells. The PRD of TIA-1 exhibits many characteristics of prions: concentration-dependent aggregation that is inhibited by the molecular chaperone heat shock protein (HSP)70; resistance to protease digestion; sequestration of HSP27, HSP40, and HSP70; and induction of HSP70, a feedback regulator of PRD disaggregation. Substitution of the PRD with the aggregation domain of a yeast prion, SUP35-NM, reconstitutes SG assembly, confirming that a prion domain can mediate the assembly of SGs. Mouse embryomic fibroblasts (MEFs) lacking TIA-1 exhibit impaired ability to form SGs, although they exhibit normal phosphorylation of eukaryotic initiation factor (eIF)2alpha in response to arsenite. Our results reveal that prion-like aggregation of TIA-1 regulates SG formation downstream of eIF2alpha phosphorylation in response to stress.

Herpes simplex virus 1 induces cytoplasmic accumulation of TIA-1/TIAR and both synthesis and cytoplasmic accumulation of tristetraprolin, two cellular proteins that bind and destabilize AU-rich RNAs

Herpes simplex virus 1 causes a shutoff of cellular protein synthesis through the degradation of RNA that is mediated by the virion host shutoff (Vhs) protein encoded by the U(L)41 gene. We reported elsewhere that the Vhs-dependent degradation of RNA is selective, and we identified RNAs containing AU-rich elements (AREs) that were upregulated after infection but degraded by deadenylation and progressive 3'-to-5' degradation. We also identified upregulated RNAs that were not subject to Vhs-dependent degradation (A. Esclatine, B. Taddeo, L. Evans, and B. Roizman, Proc. Natl. Acad. Sci. USA 101:3603-3608, 2004). Among the latter was the RNA encoding tristetraprolin, a protein that binds AREs and is known to be associated with the degradation of RNAs containing AREs. Prompted by this observation, we examined the status of the ARE binding proteins tristetraprolin and TIA-1/TIAR in infected cells. We report that tristetraprolin was made and accumulated in the cytoplasm of wild-type virus-infected human foreskin fibroblasts as early as 2 h and in HEp-2 cells as early as 6 h after infection. The amounts of tristetraprolin that accumulated in the cytoplasm of cells infected with a mutant virus lacking U(L)41 were significantly lower than those in wild-type virus-infected cells. The localization of tristetraprolin was not modified in cells infected with a mutant lacking the gene encoding infected cell protein 4 (ICP4). TIA-1 and TIAR are two other proteins that are associated with the regulation of ARE-containing RNAs and that normally reside in nuclei. In infected cells, they started to accumulate in the cytoplasm after 6 h of infection. In cells infected with the mutant virus lacking U(L)41, TIA-1/TIAR accumulated in the cytoplasm in granular structures reminiscent of stress granules in a significant percentage of the cells. In addition, an antibody to tristetraprolin coprecipitated the Vhs protein from lysates of cells late in infection. The results indicate that the Vhs-dependent degradation of ARE-containing RNAs correlates with the transactivation, cytoplasmic accumulation, and persistence of tristetraprolin in infected cells.

Immunohistochemical localization of phosphorylated protein kinase R and phosphorylated eukaryotic initiation factor-2 alpha in the central nervous system of SJL mice with experimental allergic encephalomyelitis

Inflammatory cells enter the CNS and target myelin in multiple sclerosis (MS) and experimental allergic encephalomyelitis (EAE), a model of MS, and inflammation is thought to induce stress responses in the CNS. Protein kinase R (PKR) and eukaryotic initiation factor-2 alpha (eIF2 alpha) undergo phosphorylation in response to stress, and the phosphorylated forms of these proteins play a key role in regulating protein synthesis. The objective of this study was to investigate the expression profile of phospho-PKR and phospho-eIF2 alpha during the course of EAE in order to advance the understanding of the stress response in this disease. In control animals (no encephalitogen with no emulsion; no encephalitogen with emulsion) and in preclinical EAE animals, phospho-PKR immunoreactivity was present in oligodendrocytes and some neurons, whereas, in EAE animals with active disease there was widespread labeling of inflammatory cells, and these cells were present during the recovery period of EAE, albeit to a lesser extent. Double-labeling studies revealed that T cells and a few macrophages were phospho-PKR(+). Phospho-eIF2 alpha immunoreactivity was detected in some oligodendrocytes in hindbrain sections of control animals. In EAE animals with active disease, the number of labeled oligodendrocytes increased, and inflammatory T cells also were labeled. Insofar as phospho-PKR activates nuclear factor-kappa B, it may facilitate cytokines expression by T cells. Alternatively, phospho-PKR and phospho-eIF2 alpha may promote apoptosis as a way to regulate T-cell number in the CNS. The expression of phospho-eIF2 alpha in oligodendrocytes during EAE likely is involved with inhibition of protein translation, which is a protective mechanism used to promote cell survival in response to inflammation.

FAST is a BCL-X(L)-associated mitochondrial protein

The TIA-1-interacting protein Fas-activated serine/threonine phosphoprotein (FAST) is a component of a signaling cascade that is initiated by ligation of the Fas receptor. Immunofluorescence microscopy using affinity-purified antibodies raised against recombinant FAST reveals that the endogenous protein associates with mitochondria. Subcellular fractionation confirms that FAST is a component of mitochondria. FAST is tethered to mitochondria by a lysine/arginine-rich domain at its carboxyl terminus that is structurally similar to the mitochondrial tethering motifs of monoamine oxidase B and cytochrome b5. At the mitochondrial membrane, FAST interacts with BCL-X(L). The BCL-X(L) binding domain maps to a BCL-2-homology-3 (BH3)-related domain that is distinct from the mitochondrial-tethering domain (MTD). Although interactions between FAST and BCL-X(L) require both the BH3-related domain and the MTD, the requirement for mitochondrial tethering can be conferred by a heterologous MTD. Our results suggest that FAST-BCL-X(L) interactions are likely to regulate mitochondrial metabolism during Fas-induced apoptosis.

Radiation activates HIF-1 to regulate vascular radiosensitivity in tumors: role of reoxygenation, free radicals, and stress granules

Through a poorly understood mechanism, tumors respond to radiation by secreting cytokines capable of inhibiting apoptosis in endothelial cells, thereby diminishing treatment response by minimizing vascular damage. We reveal here that this pathway is governed by a major angiogenesis regulator, HIF-1. Following radiotherapy, tumor reoxygenation leads to: (1) nuclear accumulation of HIF-1 in response to reactive oxygen, and (2) enhanced translation of HIF-1-regulated transcripts secondary to stress granule depolymerization. The resulting increase in HIF-1-regulated cytokines enhances endothelial cell radioresistance. Inhibiting postradiation HIF-1 activation significantly increases tumor radiosensitivity as a result of enhanced vascular destruction. These data describe novel pathways contributing significantly to our understanding of HIF-1 regulation which may be major determinants of tumor radiosensitivity, potentially having high clinical relevance.

Post-transcriptional regulation of proinflammatory proteins

Post-transcriptional mechanisms play a critical role in regulating the expression of numerous proteins that promote inflammatory arthritis. The mRNAs encoding a subset of these proteins possess adenine/uridine-rich elements (AREs) in their 3'-untranslated regions that profoundly influence the rate at which mRNA is degraded and translated into protein. Tristetraprolin (TTP) and T cell intracellular antigen-1 (TIA-1) are ARE-binding proteins that dampen the expression of this class of proteins by promoting mRNA degradation and protein translation, respectively. We have discovered that TIA-1 and TTP function as arthritis-suppressor genes: TIA-1-/- mice develop mild arthritis, TTP-/- mice develop severe arthritis, and TIA-1-/-TTP-/- mice develop very severe arthritis. Paradoxically, lipopolysaccharide (LPS)-activated macrophages derived from TIA-1-/-TTP-/- macrophages produce less tumor necrosis factor alpha (TNF-alpha) than TIA-1-/- or TTP-/- macrophages. The bone marrows of these mice exhibit increased cellularity, reflecting the presence of mature neutrophils that secrete TNF-alpha in response to LPS stimulation. We hypothesize that TIA-1-/-TTP-/- neutrophils are a source of arthritigenic TNF-alpha, which promotes severe erosive arthritis in these mice.

TIA proteins are necessary but not sufficient for the tissue-specific splicing of the myosin phosphatase targeting subunit 1

We are using the tissue-specific splicing of myosin phosphatase targeting subunit (MYPT1) as a model to investigate smooth muscle phenotypic diversity. We previously identified a U-rich intronic enhancer flanking the 5' splice site (IE1), and a bipartite exonic enhancer/suppressor, that regulate splicing of the MYPT1 central alternative exon. Here we show that T-cell inhibitor of apoptosis (TIA-1) and T-cell inhibitor of apoptosis-related (TIAR) proteins bind to the IE1. Co-transfection of TIA expression vectors with a MYPT1 mini-gene construct increase splicing of the central alternative exon. TIA proteins do not enhance splicing when the palindromic exonic splicing enhancer (ESE) is mutated, indicating that TIAs are necessary but not sufficient for splicing. The ESE specifically binds SRp55 and SRp20 proteins, supporting a model in which both SR and TIA proteins binding to their cis-elements are required for the recruitment of the splicing complex to a weak 5' splice site. Inactivation of TIA proteins in the DT40 cell line (TIA-1(-/-)TIAR(+/-)) reduced the splicing of the central alternative exon of the endogenous MYPT1 as well as stably transfected MYPT1 minigene constructs. Splicing of the MYPT1 3' alternative exon and the MLC(17) alternative exon were unaffected, suggesting that TIA proteins regulate a subset of smooth muscle/nonmuscle alternative splicing reactions. Finally, reduced RNA binding and reduced expression of the TIA and SR proteins in phasic (gizzard) smooth muscle around hatching coincided with the switch from exon inclusion to exon skipping, suggesting that loss of TIA and SR enhancer activity may play a role in the developmental switch in MYPT1 splicing.

Progression of hair cell ejection and molecular markers of apoptosis in the avian cochlea following gentamicin treatment

Aminoglycoside treatment induces caspase-dependent apoptotic death in inner ear sensory hair cells. The timing of apoptotic signaling in sensory hair cells following systemic aminoglycoside treatment has not been characterized in vivo. We administered a single subcutaneous injection of the aminoglycoside gentamicin (300 mg/kg) to 12-16-day-old chicks and used immunocytochemical techniques to document the following responses in affected hair cells: T-cell restricted intracellular antigen-related protein (TIAR) translocation from the nucleus to the cytoplasm, cytochrome c release from the mitochondria, caspase-3 activation, nuclear condensation, and an orderly progression of hair cell ejection from the proximal end of the basilar papilla. Hair cells in the proximal tip exhibited TIAR translocation from the nucleus and aggregation into punctate granules in the cytoplasm 12 hours after injection and the response progressed distally. Cytochrome c release from the mitochondria into the cytoplasm and caspase-3 activation were observed in affected hair cells immediately prior to and during ejection. Hair cell ejection occurred between 30 and 54 hours after injection, beginning in the proximal tip and progressing distally. Nuclear condensation accompanied ejection while the loss of: 1) membrane integrity; 2) phalloidin labeling of F-actin; and 3) TO-PRO-1 labeling of nuclear contents occurred within 48 hours following ejection. Our results present a timeline of aminoglycoside-induced inner ear sensory hair cell apoptotic death that includes an 18-hour window between the initial apoptotic response and the later stages of programmed death signaling that accompany ejection and a gradual breakdown of hair cells following ejection.

TIA-1 or TIAR is required for DT40 cell viability

TIA-1 and TIAR are a pair of related RNA-binding proteins which have been implicated in apoptosis. We show that chicken DT40 cells with both tia-1 alleles and one tiar allele disrupted (tia-1(-/-)tiar(-/+) cells) are viable. However, their growth and survival in medium containing low serum levels is significantly reduced compared with DT40 cells. The remaining intact tiar allele in tia-1(-/-)tiar(-/+) cells can only be disrupted if TIA-1 expression is first restored to the cells by transfection of a TIA-1 expression vector. We conclude that DT40 cells require either TIA-1 or TIAR for viability. TIA-1 overexpression in tia-1(-/-)tiar(-/+) cells leads to a radical drop in TIAR levels, by inducing efficient splicing of two tiar alternative exons carrying in-frame stop codons. In wild-type DT40 cells, tiar transcripts including these exons can also be detected. These transcripts increase significantly in abundance in cycloheximide-treated cells, suggesting that splicing of the exons exposes mRNAs to nonsense-mediated mRNA decay. TIA-1 or TIAR depletion leads to a marked drop in splicing of the exons. The human tiar gene contains a corresponding pair of TIA-1-inducible alternative exons, and we show that there is very high sequence conservation between chickens and humans of the exon pair and parts of the flanking introns. The TIA-1/TIAR responsiveness of these alternative tiar exons is likely to be of physiological importance for controlling TIAR levels.