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Protein phosphatase 2A regulates estrogen receptor alpha (ER) expression through modulation of ER mRNA stability.

Protein phosphatase 2A (PP2A) is a ubiquitously expressed member of the serine-threonine phosphatase family that is involved in regulation of many cellular processes including transcription, translation, cellular metabolism, and apoptosis. Because of a correlation between PP2A and estrogen receptor alpha (ER) expression in several human breast cancer cell lines, the effect of PP2A on regulation of ER expression in the human breast cancer cell line MCF-7 was studied. Inhibition of PP2A using the pharmacologic inhibitor okadaic acid at 250 nm for 16 h resulted in a 60% reduction in PP2A activity in MCF-7 cells concurrent with a 75% reduction in ER mRNA and protein expression. Similar results were obtained with a small interfering RNA probe that specifically inhibited PP2A expression. ER promoter studies showed that regulation of ER through the PP2A pathway did not occur through transcriptional activation. Rather, PP2A mediated ER expression through modulation of ER mRNA stability through degradation of ER mRNA, reversible with concomitant treatment with the proteasomal inhibitor MG 132. These data suggest a novel pathway controlling ER expression resulting from the activation of PP2A, potentially providing a novel therapeutic target.

3' Untranslated Regions↗

Regulation of CD154 (CD40 ligand) mRNA stability during T cell activation.

The CD154 protein (CD40 ligand), which is critical to the regulation of both humoral and cellular immune responses, is expressed transiently on the surface of activated CD4+ T cells. To determine whether control of mRNA stability contributes to the highly regulated expression of CD154 during T cell activation, CD4+ T cells were isolated from human peripheral blood and stimulated for various lengths of time with plate-bound anti-CD3 mAb. At early times after anti-CD3 activation, the CD154 message was found to be very unstable, however, the stability measurably increased after 24-48 h of activation. Similar analyses of TNF-alpha and c-myc mRNA decay throughout a time course of T cell activation revealed patterns of regulation that were distinct from CD154. Similar to the effect on TNF-alpha mRNA, stimulation of T cells with PMA + ionomycin greatly increased the stability of CD154 message. However, CD154 message stability was only modestly increased in T cells coactivated with anti-CD3 and anti-CD28 at 5 h and not increased by costimulation at 24 h. Finally, an analysis of both mRNA and surface protein expression over a time course of T cell activation with anti-CD3 revealed a rapid induction of expression early after activation. This induction was followed by a more gradual decrease in expression over the next 48 h. Together, these data support a role for posttranscriptional regulation in the control and overall expression of CD154 in activated T cells.

Antibodies, Monoclonal↗

mRNA stability and localisation of the low-temperature-responsive barley gene family blt14.

Transcription and translation inhibitors have been used to investigate the role of mRNA stability in the low-temperature-regulated expression of the post-transcriptionally controlled low temperature responsive barley gene family, blt14. Genomic clones (blt14.1, blt14.2) representing additional members of the blt14 gene family have been isolated and sequenced. Gene specific probes have been used to analyse the spatial expression of each individual member of the blt14 gene family. Findings indicate that all of the genes are responsive to low temperature, but the organ distribution is different for each gene. The results indicate that blt14.0 mRNA is stabilised by a low-temperature-dependent protein factor. Taken together, the results suggest that organ-specific post-transcriptional mechanisms are important in the low-temperature regulation of blt14 gene expression.

Amino Acid Sequence↗

Interleukin-5 mRNA stability in human T cells is regulated differently than interleukin-2, interleukin-3, interleukin-4, granulocyte/macrophage colony-stimulating factor, and interferon-gamma.

Interleukin-5 (IL-5) transcriptional activation and mRNA stability were investigated in a human TH0 T-cell clone (SP-B21) and in nonclonal CD4 TH2 cells, differentiated in vitro from peripheral blood T cells. Cells were stimulated with alpha-CD3 monoclonal antibody (mAb) with and without alpha-CD28 mAb. Comparison to other cytokine genes revealed aspects of mRNA regulation unique to IL-5. The half-life (t1/2) of IL-5 mRNA, determined by addition of actinomycin D (ActinoD) or cyclosporin A (CSA) was longer (by >= 2 h) than that of IL-2, IL-3, IL-4, interferon-gamma, or granulocyte/macrophage colony-stimulating factor. With the exception of IL-5, t1/2 values were significantly shorter with CSA as the transcriptional inhibitor than with ActinoD. The t1/2 value of IL-5 mRNA, but not the other cytokine transcripts, determined with either ActinoD or CSA, was longer than predicted from the kinetics of steady-state mRNA decline. Co-stimulation of both cell types with alpha-CD28 mAb increased the stability of cytokine transcripts weakly, and IL-5 remained the most stable transcript. Thus, the degradation pathway that targets IL-5 is distinct from the other cytokine transcripts measured and involves proteins whose transcription is blocked by ActinoD and CSA. From examination of the levels of transcription initiation (nuclear run-on assay) and steady-state mRNA attained in cultures stimulated in the presence of the protein synthesis inhibitor, cycloheximide, only IL-5 transcription initiation had an absolute dependency on new protein synthesis.

Antibodies, Monoclonal↗

Selective modulation of IFN-gamma mRNA stability by IL-12/NKSF.

We investigated the role of IL-12 in regulating IL-2 and IFN-gamma production in primary culture of human T cells. Addition of neutralizing antiserum against the 40-kDa subunit of IL-12 to PHA-stimulated PBMC markedly reduced both IFN-gamma protein production and mRNA accumulation and stability. Moreover, concurrent treatment of partially purified T cells (> 90% CD3+) with PHA and rIL-12 selectively enhanced IFN-gamma mRNA stability and protein production, while IL-2 protein and mRNA levels were unaffected. These studies also show that IFN-gamma and IL-2 mRNA stability are temporally dissociated during the course of T cell activation, and we propose that this dissociation may be mediated through the production of IL-12. The effect of IL-12 on modulation of IFN-gamma mRNA turnover is not associated with detectable changes in either the levels or affinity of cytoplasmic RNA-binding proteins capable of recognizing AU-rich sequences in the 3'UTR of IFN-gamma mRNA.

Base Sequence↗

Effects of starvation for exogenous carbon on functional mRNA stability and rate of peptide chain elongation in Escherichia coli.

The decay rate of the potential to synthesize proteins after complete inhibition of transcription by rifampicin was analyzed to determine the functional mRNA stability of exponentially growing and glucose-starved Escherichia coli B and K12 cells. We found the following: (i) The half-life of the mRNA pool increased 2.2-fold during a period of 2 h of starvation (from 1.8 min in exponentially growing cells to 4.0 min for cells starved for 2 h); (ii) the effect on transcript stability appeared to be global since transcripts of genes that were induced, repressed or unaltered in their expression during starvation exhibited more or less the same increased stability; (iii) the rate of peptide chain elongation, as measured by the synthesis time for beta-galactosidase, decreased 1.9-fold during the starvation period studied and may, at least in part, account for the global stabilization of transcripts in starved cells.

Carbon↗

Global mRNA stabilization preferentially linked to translational repression during the endoplasmic reticulum stress response.

The stability of mRNAs undergoing translation has long been a controversial question. Here, we systematically investigate links between mRNA turnover and translation during the endoplasmic reticulum (ER) stress response, a process during which protein synthesis is potently regulated. cDNA array-based approaches to assess the stability and translational status of each mRNA were devised. First, ER stress-triggered changes in mRNA stability were studied by comparing differences in steady-state mRNA levels with differences in gene transcription. Second, changes in translational status were monitored by studying ER stress-induced shifts in the relative distribution of each mRNA along sucrose gradients. Together, the array-derived data reveal complex links between mRNA stability and translation, with all regulatory groups represented: both stabilized and destabilized mRNAs were found among translationally induced as well as translationally suppressed mRNA collections. Remarkably, however, the subset of stabilized mRNAs was prominently enriched in translationally suppressed transcripts, suggesting that ER stress was capable of causing the stabilization of mRNAs associated with a global reduction in protein synthesis. The cDNA array-based approach described here can be applied to global analyses of mRNA turnover and translation and can serve to investigate subsets of mRNAs subject to joint posttranscriptional control.

Blotting, Northern↗

p38 mitogen-activated protein kinase (p38MAPK) upregulates catalase levels in response to low dose H2O2 treatment through enhancement of mRNA stability.

V79 fibroblasts were repetitively stressed through multiple exposures to a low dose (30 microM) H2O2 in culture for 4 weeks. Catalase activity, protein levels and mRNA levels increased markedly (5-6-fold) during this time and these augmentations were inhibited by the simultaneous presence of SB203580, an inhibitor of p38 mitogen-activated protein kinase (p38MAPK). p38MAPK became dually phosphorylated and ATF-2, a p38MAPK substrate also became increasingly phosphorylated over the repetitive stress period. Short interfering RNA that induced effective silencing of p38MAPK, was used to silence p38MAPK in V79 fibroblasts. Silencing of p38MAPK drastically hindered the elevation in catalase (protein and mRNA) levels observed after a single low dose (50 microM) of H2O. The rise in catalase mRNA levels induced by low concentration (single and multiple dose) H2O2 treatment was established to be unconnected with transcriptional upregulation but was brought forth primarily by an enhancement in catalase mRNA stability through the action of p38MAPK. Therefore, our data strongly indicate that activation of p38MAPK is a key controlling step in the upregulation of catalase levels by low dose H2O2 treatment.

Activating Transcription Factor 2↗

IgG-coated erythrocytes augment LPS-stimulated TNF-alpha secretion, TNF-alpha mRNA levels, and TNF-alpha mRNA stability in macrophages.

Previous studies have shown that IgG-coated erythrocytes (EIgG) augment the LPS-stimulated increase in serum TNF-alpha levels in animals and the LPS-stimulated secretion of TNF-alpha by isolated macrophages. The present study evaluated the mechanism for the effect of EIgG on LPS-stimulated TNF-alpha secretion in the murine macrophage cell line, RAW 264.7. Incubation of the macrophages with EIgG or IgG-coated glass beads caused a dose-dependent augmentation of LPS-stimulated TNF-alpha secretion. The addition of EIgG increased the rate of LPS-stimulated TNF-alpha protein secretion between 2 and 4 hr after LPS. Accordingly, EIgG increased the levels of TNF-alpha mRNA at 2 and 3 hr after LPS. The increase in the LPS-stimulated TNF-alpha mRNA levels caused by EIgG was associated with an increase in TNF-alpha mRNA stability. Thus, the augmentation of LPS-stimulated TNF-alpha secretion by EIgG was associated with an increase in TNF-alpha mRNA levels which at least partly resulted from an increase in the stability of TNF-alpha mRNA.

Animals↗

Agonist regulation of human beta2-adrenergic receptor mRNA stability occurs via a specific AU-rich element.

Prolonged agonist stimulation of beta2-adrenergic receptors results in receptor down-regulation, which is closely associated with a reduction of the corresponding mRNA, an effect mediated in part by changes in mRNA stability. Transfection experiments with human beta2-adrenergic receptor cDNAs bearing or lacking the untranslated regions suggested that the essential agonist sensitivity of the mRNA resides within the 3'-untranslated region. The importance of this region was further confirmed in gel shift experiments; cytosolic preparations from agonist-stimulated DDT1-MF2 smooth muscle cells caused a shift of beta2-adrenergic receptor mRNAs containing the 3'-untranslated region. Progressive 3'-terminal truncations of the receptor cDNA led to the identification of an AU-rich element at positions 329-337 of the 3'-untranslated region as the responsible cis-acting element. Substitution of this motif by cytosine residues almost completely abolished mRNA down-regulation and inhibited the formation of the RNA-protein complex. Even though the beta2-adrenergic receptor AU-rich element showed two U --> A transitions compared with the recently proposed AU-rich element consensus sequence, it revealed an almost identical destabilizing potency. Fusion of the beta2-adrenergic receptor 3'-untranslated region to the beta-globin coding sequence dramatically reduced the half-life of the chimeric transcript in an agonist- and cAMP-dependent manner. This suggests that the agonist-induced beta2-adrenergic receptor mRNA destabilization is regulated by cAMP-dependent RNA-binding protein(s) via a specific AU-rich element.

Adrenergic beta-2 Receptor Agonists↗

[Changes in the mechanisms controlling mRNA stability during the process of liver regeneration in rats].

If the protein biosynthesis was inhibited by cycloheximide, an elevated degradation of total mRNA, including mRNA of membrane-bound and free polyribosomes, occurred in the liver cells at early steps of regeneration, within 48 hrs after partial hepatectomy. Simultaneously, distinct increase in activity of alkaline RNAases was observed in cytoplasm as well as of endo-RNAase--in the membrane-bound and free polyribosomes. As shown in experiments with-p-chloromercuribenzoate, the increase in activity of alkaline RNAases in treatment with cycloheximide was due to a decrease in content of short-living proteins, which are inhibitors of RNAases, in cytoplasm of 48 hr regenerating liver cells. At the later period of liver tissue regeneration within 5 days after partial hepatectomy), under conditions of inhibition of protein biosynthesis using cycloheximide, stabilization of mRNA from membrane-bound polyribosomes was observed while the mRNA from free polyribosomes was in a state of degradation. Within this period the endo-RNAase activity was markedly decreased in membrane-bound polyribosomes but the enzymatic activity was increased in free polyribosomes. The mechanisms, controlling the mRNA stability, were apparently changed during the regeneration of rat liver tissue.

Animals↗

Agonist-mediated destabilization of m1 muscarinic acetylcholine receptor mRNA. Elements involved in mRNA stability are localized in the 3'-untranslated region.

The effects of chronic agonist exposure on receptor number (down-regulation) have been shown, in part, to be due to effects on mRNA levels. Agonist-mediated effects on muscarinic acetylcholine receptor (mAChR) mRNA were investigated in Chinese hamster ovary (CHO) cells stably transfected with m1 mAChR gene constructs containing the open reading frame and a series of deletions of the flanking 3'-untranslated region (3'-UTR). Carbachol (CBC) down-regulated m1 mAChRs encoded by the construct m1C1, an m1 mAChR transcript containing the entire flanking 3'UTR (nucleotides 1526-2622), in a time-dependent fashion with maximal decreases occurring by 12 h. Steady-state levels of m1C1 mRNA declined in a parallel fashion beginning 6 h after CBC pretreatment. Similar findings were obtained with m1C2, a construct which is missing all but 261 bases of flanking 3'-UTR (nucleotides (nt) 1526-1786). Since the rate of mRNA degradation represents an important potential regulatory mechanism to control the level of gene expression, we investigated the effects of CBC treatment on m1C1 and m1C2 mRNA stability. The half-life of either transcript in untreated cells was approximately 14 h, whereas m1C1 and m1C2 transcript half-lives decreased to approximately 3 h in cells treated with CBC. Agonist-induced destabilization of m1C2 mRNA could be mimicked by phorbol esters in a concentration-dependent manner and blocked by the protein kinase inhibitor, H-7. In contrast, m1 mAChR mRNA constructs missing nt 1526-1786 of the 3'-UTR (m1C3 and m1C4) did not undergo agonist- or phorbol ester-induced destabilization. In the neuroblastoma cell line IMR-32, endogenous m1 mAChR mRNA was down-regulated and destabilized following CBC treatment. These results demonstrate that agonist-induced mRNA destabilization is a potential mechanism for regulating m1 mAChR levels. Furthermore, deletion studies identify a 261 base region of the 3'-UTR having the potential to form stable stem-loop structures which likely harbors element(s) responsible for message destabilization.

Animals↗

Metallothionein mRNA stability in chicken and mouse cells.

Northern blot analysis revealed that metallothionein (MT) mRNAs accumulate after inhibition of protein synthesis with cycloheximide (CHX) in primary cultures of chick embryo hepatocytes and fibroblasts, as well as in an established mouse hepatoma cell line. Inhibition of RNA synthesis with actinomycin D (AMD) led to rapid loss of MT mRNAs in these cells, whereas CHX dramatically retarded the rate of MT mRNA decay (t1/2 greater than 24 h). These results suggest that CHX causes MT mRNA accumulation primarily by increasing stability of MT mRNA. Thus, changes in MT mRNA turn-over rates may play an important role in regulating the accumulation of MT mRNA. The half-lives of MT mRNAs in chicken and mouse cells were determined by oligodeoxyribonucleotide excess solution hybridization with RNA samples extracted after different periods of exposure to AMD. The half-life of chicken MT (cMT) mRNA in uninduced chicken embryo hepatocytes was 3.6 h. Induction of cMT mRNA by pretreatment of these cells with zinc (Zn) prior to exposure to AMD, did not alter the half-life of cMT mRNA significantly. In contrast, cadmium (Cd) induction led to a 2.5-fold increase in the stability of this mRNA. In uninduced chicken embryo fibroblasts, cMT mRNA levels were too low to allow accurate determination of half-life using the methods employed here. However, the half-life of this mRNA in Zn-induced chicken embryo fibroblasts was 6.2 h, whereas it was 9.3 h in Cd-induced cells. Thus, the turn-over rate of cMT mRNA after Cd-induction is very similar in chick embryo fibroblasts and hepatocytes. These data suggest that the accumulation of MT mRNA in chicken cells may reflect, in part, metal-specific effects on MT mRNA stability. The half-lives of mouse MT-I and MT-II (mMT-I and mMT-II) mRNAs in uninduced BNL hepatoma cells were identical (9.2 h), and were not effectively altered after induction by metals (Zn, Cd) or interleukin-1 beta (IL-1 beta). However, mMT mRNAs in pachytene spermatocytes and round spermatids, freshly isolated from the adult testes, were 2.2- to 4.5-fold more stable than in hepatoma cells. These results suggest that cell-type specific accumulation of mMT mRNAs may be regulated, in part, by mRNA stability.

Animals↗

Nitric oxide increases IL-8 gene transcription and mRNA stability to enhance IL-8 gene expression in lung epithelial cells.

Interleukin (IL)-8, a C-X-C chemokine, is a potent chemoattractant and an activator for neutrophils, T cells, and other immune cells. The airway and respiratory epithelia play important roles in the initiation and modulation of inflammatory responses via production of cytokines and surfactant. The association between elevated levels of nitric oxide (NO) and IL-8 in acute lung injury associated with sepsis, acute respiratory distress syndrome, respiratory syncytial virus infection in infants, and other inflammatory diseases suggested that NO may play important roles in the control of IL-8 gene expression in the lung. We investigated the role of NO in the control of IL-8 gene expression in H441 lung epithelial cells. We found that a variety of NO donors significantly induced IL-8 mRNA levels, and the increase in IL-8 mRNA was associated with an increase in IL-8 protein. NO induction of IL-8 mRNA was due to increases in IL-8 gene transcription and mRNA stability. NO induction of IL-8 mRNA levels was not inhibited by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one and KT-5823, inhibitors of soluble guanylate cyclase and protein kinase G, respectively, and 8-bromo-cGMP did not increase IL-8 mRNA levels. This indicated that NO induces IL-8 mRNA levels independently of changes in the intracellular cGMP levels. NO induction of IL-8 mRNA was significantly reduced by inhibitors of extracellular regulated kinase and protein kinase C. IL-8 induction by NO was also reduced by hydroxyl radical scavengers such as dimethyl sulfoxide and dimethylthiourea, indicating the involvement of hydroxyl radicals in the induction process. NO induction of IL-8 gene expression could be a significant contributing factor in the initiation and induction of inflammatory response in the respiratory epithelium.

Adenocarcinoma↗

The 5' coding sequence of IL-2 receptor alpha chain mRNA mediates mRNA stabilization by HTLV-1 Rex.

The interleukin-2 receptor alpha chain (IL-2R alpha) is a T-cell growth factor receptor and is known to be induced in helper T cells by infection with human T-cell leukaemia virus type-1 (HTLV-1). The Rex protein of HTLV-1 has been shown to stabilize IL-2R alpha mRNA. Although the 3' untranslated region of many RNA has been regarded as a key element for stabilization, we found that the first 300 bases of the IL-2R alpha protein coding sequence were necessary for stabilization of the mRNA. As the first 201 bases were not sufficient for this effect, we conclude that the bases at position 201-300 downstream of the AUG start are important for stabilization.

Cell Culture Techniques↗

The control of mRNA stability in Escherichia coli: manipulation of the degradation pathway of the polycistronic atp mRNA.

The physical and functional stabilities of genes in the atp operon fall into two classes. The first two genes, atpI and atpB, are rapidly inactivated and degraded at the mRNA level. The remaining seven genes are more stable. In order to investigate how these stabilities are determined, DNA sequences encoding mRNA structures that influence degradative events in other systems, including RNAse III sites and REP sequences, were subcloned or synthesized and inserted into non-coding regions of the operon. The effects of insertion of an RNAse III site depended on whether cleavage left an unstable 3' end or a stabilizing stem-loop upstream of the cutting point. Generation of an unstable 3' end destabilized the neighbouring upstream atp gene, thus modifying the course and rate control of degradation. Removal of the atp transcriptional terminator attenuated expression of the last gene of the operon, atpC. This effect was reversed by substitution of an alternative stem-loop for the terminator. REP sequences inserted into intercistronic regions apparently could not influence rate-controlling steps. The reported data shed light on the factors controlling the inactivation and degradation of genes in the polycistronic atp mRNA, and are discussed in relation to the general role of degradation processes in the control of gene expression.

Base Composition↗

Mechanisms of IFN-gamma induction by natural killer cell stimulatory factor (NKSF/IL-12). Role of transcription and mRNA stability in the synergistic interaction between NKSF and IL-2.

We have investigated the molecular mechanisms regulating IFN-gamma production in human T lymphocytes stimulated by NK cell stimulatory factor (NKSF/IL-12). We show that NKSF synergizes with IL-2 and phorbol diesters inducing the accumulation of IFN-gamma mRNA in PHA-activated T cell blasts. NKSF regulates IFN-gamma mRNA expression in PHA blasts and the T leukemia cell line, TALL-103/2, at both the transcriptional and posttranscriptional levels. NKSF increases the transcriptional rate for IFN-gamma in both these cell types, as determined by nuclear run-on analysis. However, synergy between NKSF and IL-2 can be demonstrated only at the level of mRNA stability, and both cytokines are required to increase IFN-gamma mRNA half-life.

Blotting, Northern↗

The Ams (altered mRNA stability) protein and ribonuclease E are encoded by the same structural gene of Escherichia coli.

The in vitro and in vivo analysis of the ribonuclease E-deficient (rne-) and the altered mRNA stability protein-deficient (ams-) strains of Escherichia coli has demonstrated that they carry mutations in the same structural gene. Strains encoding either thermolabile RNase E (rne-3071) or Ams protein (ams-1) are defective in both rRNA processing and mRNA turnover. Immediately after a shift to the nonpermissive temperature, the chemical decay rate of bulk mRNA is slowed 2- to 3-fold, and within 70 min, precursors to 5S rRNA begin to accumulate. In addition, all of the phenotypes associated with either the rne-3071 or the ams-1 alleles were complemented by a recombinant plasmid carrying ams+. When taken together with previous genetic studies, these results suggest that the role of ribonuclease E in mRNA turnover involves endonucleolytic cleavages at the proposed ACAG(A/U)AUUUG consensus sequence.

Bacterial Proteins↗