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Effect of nonsense mutations on PTEN mRNA stability.

Cowden disease is an autosomal dominant disorder associated with an elevated risk of breast, thyroid and skin cancers, in which germline mutations of a tumour suppressor gene (PTEN) have been identified. PTEN has a dual-specificity tyrosine phosphatase domain thought to be essential for tumour suppression. Previous genotype/phenotype correlations have identified several potential associations, for example that truncating mutations result in increased breast cancer risk. Such associations are useful in evaluating the phenotypic functions of PTEN sub-domains. However, genotype/phenotype correlations are likely to be complicated by nonsense mediated mRNA degradation. We report here that three out of four mutations do not significantly affect PTEN transcript stability. Furthermore, we show that manual sequencing methods are better than current dye-based sequencing technologies for detecting heterozygous mutations in PTEN transcripts.

DNA Mutational Analysis↗

Activity of a 40 kDa RNA-binding protein correlates with MYCN and c-fos mRNA stability in human neuroblastoma.

Subclones of neuroblastic (N) and substrate adherent (S) cells have been established from neuroblastoma tumours cultured in vitro which differ in growth characteristics and MYCN expression. N cells derived from the NBL-W cell line (W-N) express 5-fold higher levels of MYCN mRNA and 10-fold higher levels of MYCN protein than S cells (W-S), despite having the same MYCN copy number. In an effort to identify the molecular mechanisms responsible for the disparity in steady-state MYCN levels, the rate of MYCN mRNA degradation was measured in the two subclones. The half-life of MYCN mRNA in the W-N cells was approximately 45 min compared to approximately 6 min in the W-S cells. Similarly, the half-life of another labile mRNA, c-fos, differed in W-N and W-S cells (30 min versus 15 min, respectively). The turnover of labile mRNAs is thought to be mediated by the interactions of trans-acting factors with AU-rich elements within the 3' untranslated region. RNA UV cross-linking assays using W-N cell lysate demonstrated abundant quantities of a protein, 40 kDa in size (p40), that bound specifically to AU-rich elements within the MYCN and c-fos 3' untranslated region. However, p40 was barely detectable in W-S cells. Our studies suggest that p40 may play a role in determining neuroblastoma phenotype by regulating MYCN and c-fos mRNA turnover.

Binding Sites↗

Vitamin D receptor 3'-untranslated region polymorphisms: lack of effect on mRNA stability.

Allelic variation at the 3'-end of the vitamin D receptor (VDR) gene has been associated with a 3-5-fold increased risk of developing prostate cancer and with differences in bone mineralization. This genetic diversity does not alter the VDR protein structurally, but instead may be a marker(s) of other, nearby polymorphisms that influence message stability or translation. The work reported here was instigated to identify additional VDR 3'-UTR polymorphisms that may have functional significance and to then test whether these genetic variants alter message stability. Initially, four novel, frequently occurring sequence variants were identified that associated with two common haplotypes that were described previously. These common sequence variants were not found within three message-destabilizing elements that we mapped within the 3'-UTR of the vitamin D receptor mRNA. Furthermore, the two VDR 3'-UTR haplotypes conferred an identical half-life on a heterologous beta-globin reporter gene, in an in vitro assay. We therefore conclude that common polymorphisms within the VDR 3'-UTR do not influence message stability.

3' Untranslated Regions↗

Regulation of GLUT4 mRNA stability by tumor necrosis factor-alpha: alterations in both protein binding to the 3' untranslated region and initiation of translation.

Previous studies from our laboratory have demonstrated that exposure of the 3T3-L1 adipocytes to TNF resulted in a 50% decrease in the half-life of the GLUT4 mRNA from 9.5h (control) to 4.5h (treated cells). In the current study we report that coincident with the alteration of the half-life of the message, TNF treatment resulted in an increase in the binding of a protein to the proximal 200 bases of the 3'UTR. In addition the distribution of GLUT4 mRNA in polysome profiles was altered such that message accumulated as a 40S preinitiation complex, consistent with an attenuation of initiation. In summary, our data suggest that enhanced decay of the GLUT4 mRNA may occur through a mechanism involving accumulation of a 40s preinitiation complex which correlates with increased protein binding to the 3'UTR.

3T3 Cells↗

Nerve growth factor controls GAP-43 mRNA stability via the phosphoprotein ARPP-19.

The membrane phosphoprotein GAP-43 is involved in axon growth and synaptic plasticity. In PC12 pheochromocytoma cells, induction of a neuronal phenotype by nerve growth factor (NGF) is accompanied by a marked increase in GAP-43 levels. NGF regulates GAP-43 expression by altering the half-life of its mRNA. We report here that the phosphoprotein ARPP-19 mediates this regulation. In an NGF-dependent manner, ARPP-19 bound to a region in the 3' end of GAP-43 mRNA previously found to be important for regulating the half-life of the mRNA. Overexpression of wild-type ARPP-19 in PC12 cells increased the NGF-dependent expression of a reporter construct linked to the critical 3' region of GAP-43 mRNA. Mutation of serine 104, the site of phosphorylation by protein kinase A in ARPP-19, to either alanine or aspartate abolished this regulation in PC12 cells. These findings demonstrate that ARPP-19 is an important link between NGF signaling and post-transcriptional control of neuronal gene expression.

Animals↗

Induction, regulation and messenger half-life of cytochromes P450 IA1, IA2 and IIIA6 in primary cultures of rabbit hepatocytes. CYP 1A1, 1A2 and 3A6 chromosome location in the rabbit and evidence that post-transcriptional control of gene IA2 does not involve mRNA stabilization.

A study on the regulation and induction of expression of cytochromes P450-IA1, IA2 and IIIA6 genes has been undertaken using primary cultures of adult rabbit hepatocytes grown in a serum-free chemically and hormonally defined medium. In 72-h-old cultures, 50 microM beta-naphthoflavone induced both IA1 and IA2 mRNA, the maximal level being reached after 4 h and 12 h, respectively. This was shown to result from an increase in the rate of transcription of gene IA1. In contrast, gene IA2 was constitutively transcribed in untreated cells, but mRNA only accumulated in the presence of beta-naphthoflavone which, however, did not affect the rate of transcription. Actinomycin D fully blocked induction of both IA1 and IA2 mRNA in response to their inducer. In untreated cells the presence of cycloheximide allowed a 'constitutive' expression of gene IA1, while in beta-naphthoflavone-treated cells, it produced a super-induction of IA1 but no modification of IA2 gene expression. Rifampicin (50 microM) strongly increased the IA1 mRNA level and rate of transcription only in cycloheximide-treated cells. Rifampicin and dexamethasone, two prototypical inducers of P450-IIIAs, induced both large and small IIIA6 mRNAs in a time-dependent fashion, the maximum level being reached after 24 h. This was related to a large increase in the rate of transcription of the gene. Cycloheximide significantly decreased the accumulation of both IIIA6 mRNAs in response to rifampicin, while actinomycin D fully blocked induction. The half-lives of IA1, IA2 and IIIA6 mRNAs were determined by two different methods, namely actinomycin D and [3H]uridine-chase experiments. In untreated cells, the half-lives for IA1, IA2 and IIIA6 mRNAs were 14 h, 16 h and 19 h, respectively when determined by the uridine chase and 18 h, 25 h and 22 h when determined by the actinomycin-D chase. These values were not modified significantly in cells treated with beta-naphthoflavone or rifampicin, indicating that neither of these inducers affected the stability of IA1 and IA2 or IIIA6 messages, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

The role of the AUUUUA hexamer for the posttranscriptional regulation of the AT1 receptor mRNA stability.

AT1 receptor expression is mainly regulated posttranscriptionally involving modulation of RNA stability which is dependent on protein binding to the cognate sequence bases 2179-2195 within the 3' untranslated region of the AT1 receptor RNA. This region contains an AUUUUA hexamer which forms part of a stem-loop structure. To clarify the significance of the AUUUUA hexamer for AT1 receptor mRNA regulation, mutations were introduced inside, up- or downstream of it. In vitro decay assays, transfection experiments, and UV-light mRNA protein crosslink assays could demonstrate that mutations within the AUUUUA hexamer disrupted AT1 receptor mRNA degradation as well as the binding of polysomal proteins. In contrast, modification in the neighboring sequence had no effect on mRNA turnover or protein binding. Computer modelling revealed that the AUUUUA hexamer is important for the formation of a stem-loop structure which in turn is relevant for mRNA-protein interactions. These findings indicate that the AUUUUA hexamer is essential for the posttranscriptional modulation of the AT1 receptor mRNA expression.

3' Untranslated Regions↗

IS10 mRNA stability and steady state levels in Escherichia coli: indirect effects of translation and role of rne function.

Translation of the IS10 transposase gene is known to be very infrequent. We have identified mutations whose genetic properties suggest that they act directly to increase or decrease the intrinsic level of translation initiation. Also, we have analysed in detail the effects of these mutations on IS10 mRNA using one particular IS10 derivative. In this case, increases or decreases in translation are accompanied by increases or decreases in both the steady state level and the half-life of transposase mRNA; effects on steady state levels are much more dramatic than effects on message half-life. At wild-type levels of translation initiation, the rate-limiting step in physical decay of full length IS10 message for a particular IS10 derivative is shown to be rne-dependent endonucleolytic cleavage; 3' exonucleases appear to play a secondary role, degrading primary cleavage products. Analysis of interplay between translation mutations and rne function, together with the above observations, suggests that translation stabilizes messages in a general way against rne-dependent endonucleolytic cleavage, and that significant protection may be conferred by one or a few ribosomes. However, dramatic effects of translation on steady state message levels are still observed in an rne mutant and involve the 3' end of the transcript; we propose that these additional effects reflect translation-mediated stimulation of transcript release.

Bacterial Proteins↗

Codon adjustment to maximise heterologous gene expression in Streptomyces lividans can lead to decreased mRNA stability and protein yield.

The impact of the codon bias of the mouse tumour necrosis factor alpha (mTNF) gene cloned in Streptomyces lividans on the efficiency of expression and secretion was analysed. Minor codons occurring in the mTNF gene were therefore adapted to the codon bias of Streptomyces by site-directed mutagenesis. No improvement in mTNF yield could be detected. The stability of the transcript derived from the construct was shown to be more important for determining the final level of mTNF production. A strong correlation was observed between the yield of secreted biologically active mTNF and the amount of mTNF mRNA present in the cells.

Algorithms↗

Micromanagement: a role for microRNAs in mRNA stability.

Small, inhibitory RNA molecules called microRNAs cause large decreases in target protein levels through a post-transcriptional mechanism. Until recently, it was believed this mechanism operated almost exclusively at a step in translation. However, new work has revealed that microRNAs have a second, post-transcriptional mechanism that accelerates the rate of deadenylation, the initial step of mRNA decay.

Animals↗

The 3'-untranslated region of chloroplast psbA mRNA stabilizes binding of regulatory proteins to the leader of the message.

The 5'-leader and 3'-tail of chloroplast mRNAs have been suggested to play a role in posttranscriptional regulation of expression of the message. The regulation is thought to be mediated, at least in part, by regulatory proteins that are encoded by the nuclear genome and targeted to the chloroplast where they interact with chloroplast mRNAs. Previous studies identified high affinity binding of the 5'-untranslated region (UTR) of the chloroplast psbA mRNA by Chlamydomonas reinhardtii proteins. Here we tested whether the 3'-UTR of psbA mRNA alone or linked in cis with the 5'-UTR of the mRNA affects the high affinity binding of the message in vitro. We did not detect high affinity binding that is unique to the 3'-UTR. However, we show that the cis-linked 3'-UTR increases the stability of the 5'-UTR binding complex. This effect could provide a means for translational discrimination against mRNAs that are incorrectly processed.

3' Untranslated Regions↗

Post-termination ribosome interactions with the 5'UTR modulate yeast mRNA stability.

A novel form of post-transcriptional control is described. The 5' untranslated region (5'UTR) of the Saccharomyces cerevisiae gene encoding the AP1-like transcription factor Yap2 contains two upstream open reading frames (uORF1 and uORF2). The YAP2-type of uORF functions as a cis-acting element that attenuates gene expression at the level of mRNA turnover via termination-dependent decay. Release of post-termination ribosomes from the YAP2 5'UTR causes accelerated decay which is largely independent of the termination modulator gene UPF1. Both of the YAP2 uORFs contribute to the destabilization effect. A G/C-rich stop codon context, which seems to promote ribosome release, allows an uORF to act as a transferable 5'UTR-destabilizing element. Moreover, termination-dependent destabilization is potentiated by stable secondary structure 3' of the uORF stop codon. The potentiation of uORF-mediated destabilization is eliminated if the secondary structure is located further downstream of the uORF, and is also influenced by a modulatory mechanism involving eIF2. Destabilization is therefore linked to the kinetics of acquisition of reinitiation-competence by post-termination ribosomes in the 5'UTR. Our data explain the destabilizing properties of YAP2-type uORFs and also support a more general model for the mode of action of other known uORFs, such as those in the GCN4 mRNA.

5' Untranslated Regions↗

Zygotic control of maternal cyclin A1 translation and mRNA stability.

Cyclin mRNAs are unstable in the adult cell cycle yet are stable during the first 12 cell divisions in Xenopus laevis. We recently reported that cyclin A1 and B2 maternal mRNAs are deadenylated upon completion of the 12th division (Audic et al. [2001] Mol. Cell Biol. 21:1662-1671). Deadenylation is mediated by the 3' untranslated region (UTR) of the mRNA and precedes the terminal disappearance of the cyclin proteins, with both processes requiring zygotic transcription. The purpose of the current study was (1) to ask whether deadenylation leads to translational repression and/or destabilization of endogenous cyclin A1 and B2 mRNAs, and (2) to further characterize the regulatory sequences required. We show that zygote-driven deadenylation leads to translational repression and mRNA destabilization. A 99-nucleotide region of the 3'UTR of the cyclin A1 mRNA mediates both deadenylation and destabilization. Surprisingly, two AU-rich consensus elements within this region are dispensable for this activity. These results suggest that zygote-dependent deadenylation, translational repression, and mRNA destabilization by means of novel 3'UTR elements contribute to the disappearance of maternal cyclins. They also suggest that translational control of cyclins may play a role in the transition to the adult cell cycle. These data concur with previous studies in Drosophila showing that zygote-mediated degradation of maternal cdc25 mRNA may be a general mechanism whereby transition to the adult cell cycle proceeds.

3' Untranslated Regions↗

Affinity purification of ARE-binding proteins identifies polyA-binding protein 1 as a potential substrate in MK2-induced mRNA stabilization.

An important determinant for the expression level of cytokines and proto-oncogenes is the rate of degradation of their mRNAs. AU-rich sequence elements (AREs) in the 3(') untranslated regions have been found to impose rapid decay of these mRNAs. ARE-containing mRNAs can be stabilized in response to external signals which activate the p38 MAP kinase cascade including the p38 MAP kinase substrate MAPKAP kinase 2 (MK2). In an attempt to identify components downstream of MK2 in this pathway we analyzed several proteins which selectively interact with the ARE of GM-CSF mRNA. One of them, the cytoplasmic poly(A)-binding protein PABP1, co-migrated with a protein that showed prominent phosphorylation by recombinant MK2. Phosphorylation by MK2 was confirmed using PABP1 purified by affinity chromatography on poly(A) RNA. The selective interaction with an ARE-containing RNA and the phosphorylation by MK2 suggest that PABP1 plays a regulatory role in ARE-dependent mRNA decay and its modulation by the p38 MAP kinase cascade.

3' Untranslated Regions↗

Regulating expression of the gene for matrix metalloproteinase-1 (collagenase): mechanisms that control enzyme activity, transcription, and mRNA stability.

Matrix metalloproteinase-1 (MMP-1) is one of three collagenases that can degrade the interstitial collagens, types I, II, and III at neutral pH. As these collagens are the most abundant proteins in the body, collagenase plays a critical role in modeling and remodeling the extracellular matrix. Therefore, it is not surprising that MMP-1 gene expression can be regulated at multiple points. Procollagenase can be activated by mechanisms that generate an active enzyme with differing specific activities, and the active enzyme can be inhibited by complexing with either the tissue inhibitor of metalloproteinases (TIMPs) or alpha 2 macroglobulin. The activator protein-1 (AP-1) site in the collagenase promoter plays a prominent role in the transcriptional control of the collagenase gene. It is essential for basal transcription, and contributes to induction by phorbol esters, although other sites in the proximal promoter are essential. In contrast, transactivation by cytokines such as Interleukin-1 depends on sequences in more distal regions of the promoter. Posttranscriptional mechanisms also regulate gene expression, and several cytokines and growth factors increase the stability of the collagenase transcript. Finally, glucocorticoid hormones repress transcription of the collagenase gene by the interaction of glucocorticoid receptors with the AP-1 proteins, Fos and Jun. Retinoids also suppress transcription by mechanisms that involve down-regulation of fos and jun mRNA, sequestration of Fos and Jun proteins, and the formation of complexes of retinoic acid receptors (RAR/RXR heterodimers) and AP-1 proteins on the DNA. These multiple points of regulation assure precise control of collagenolytic activity in a variety of physiologic and pathologic conditions.

Amino Acid Sequence↗

The 14 bp deletion-insertion polymorphism in the 3' UT region of the HLA-G gene influences HLA-G mRNA stability.

Human leukocyte antigen (HLA)-G molecules are generated by an alternative splicing of the primary transcript of the gene and display specialized function in regulating the immune response. Although HLA-G gene polymorphism is low, it may influence levels of protein expression and, in some cases, has been associated with pregnancy diseases. The HLA-G gene exhibits 14 alleles generating six proteins with minor variations and a null allele. HLA-G allelic variants may be also characterized by a 14 bp deletion-insertion polymorphism located in the 3' UT region of the HLA-G gene. The presence of the 14 bp insertion is known to generate an additional splice whereby 92 bases are removed from the start of exon 8. To analyze the effect of this deletion on the stability of HLA-G mRNAs, we performed actinomycin D treatments on both JEG-3 choriocarcinoma cell line and M8 melanoma cell line transfected with HLA-G*010102 allele. We observed that HLA-G mRNAs having the 92-base deletion are more stable than the complete mRNA forms, suggesting that this region may be involved in the mechanisms controlling post-transcriptional regulation of HLA-G molecule associated with allelic variants.

3' Untranslated Regions↗

The yeast transcription factor genes YAP1 and YAP2 are subject to differential control at the levels of both translation and mRNA stability.

Two forms of post-transcriptional control direct differential expression of the Saccharomyces cerevisiae genes encoding the AP1-like transcription factors Yap1p and Yap2p. The mRNAs of these genes contain respectively one (YAP1 uORF) and two (YAP2 uORF1 and uORF2) upstream open reading frames. uORF-mediated modulation of post-termination events on the 5'-untranslated region (5'-UTR) directs differential control not only of translation but also of mRNA decay. Translational control is defined by two types of uORF function. The YAP1 -type uORF allows scanning 40S subunits to proceed via leaky scanning and re-initiation to the major ORF, whereas the YAP2 -type acts to block ribosomal scanning by promoting efficient termination. At the same time, the YAP2 uORFs define a new type of mRNA destabilizing element. Both post-termination ribosome scanning behaviour and mRNA decay are influenced by the coding sequence and mRNA context of the respective uORFs, including downstream elements. Our data indicate that release of post-termination ribosomes promotes largely upf -independent accelerated decay. It follows that translational termination on the 5'-UTR of a mature, non-aberrant yeast mRNA can trigger destabilization via a different pathway to that used to rid the cell of mRNAs containing premature stop codons. This route of control of non-aberrant mRNA decay influences the stress response in yeast. It is also potentially relevant to expression of the sizable number of eukaryotic mRNAs that are now recognized to contain uORFs.

Base Sequence↗

Regulation of cell surface expression of voltage-dependent Nav1.7 sodium channels: mRNA stability and posttranscriptional control in adrenal chromaffin cells.

Regulated expression of Na+ channels is indispensable to physiological events, whereas dysregulated expression of otherwise silent or even normal Na+ channel isoforms causes Na+ channelopathies; however, the regulatory mechanisms remain unknown. In quiescent cultured bovine adrenal chromaffin cells, constitutive phosphorylation/activation of extracellular signal-regulated kinase-1 (ERK1) and ERK2 destabilized Nav l.7 Na+ channel alpha-subunit mRNA and decreased its level without altering alpha-subunit gene transcription, thus negatively regulating steady-state level of Na+ channels. Activation of protein kinase C (PKC) down-regulated Na+ channels via PKC isoform-specific mechanisms; conventional PKC-alpha promoted endocytic internalization of Na+ channels, whereas novel PKC-epsilon destabilized alpha-subunit mRNA without altering its gene transcription. Long-lasting (but not short-term) increase of cytoplasmic Ca2+ down-regulated Na+ channels; a slowly-developing moderate increase of Ca2+ activated PKC-alpha and calpain, promoting internalization of Na+ channels, whereas an immediate monophasic and salient plateau increase of Ca2+ lowered alpha- and beta1-subunit mRNA levels. Calcineurin, or FK506 binding protein- and rapamycin-associated protein (FRAP), a serine/threonine protein kinase, down-regulated, whereas insulin receptor tyrosine kinase or protein kinase A (PKA) up-regulated, Na+ channels via modulating Na+ channel internalization, and/or Na+ channel externalization from the trans-Golgi network. Neuroprotective, antiepiletic, antipsychotic, and local anesthetic drugs up-regulated Na+ channels via transcriptional/translational events.

Adrenal Medulla↗