Alteration of translation and stability of mRNA for the poly(A)-binding protein during myogenesis

Reduced stability of mitogen-activated protein kinase kinase-2 mRNA and phosphorylation of poly(A)-binding protein (PABP) in cells overexpressing PABP

The poly(A)-binding protein (PABP) is an important regulator of mRNA translation and stability. The cellular level of PABP is controlled by regulating its mRNA translation by a feedback mechanism. The important aspect of this mechanism is that PABP binds to an adenosine-rich cis-element at the 5'-untranslated region of its own mRNA and inhibits its translation. To assess the importance of controlling the PABP level, we studied the effect of PABP overexpression on the transcription profile using the microarray technique. In PABP-overexpressing cells, 19 mRNAs showed a reduction in cellular levels due to reduced mRNA stability, and one showed an increase due to increased mRNA stability. Among these mRNAs, the MKK-2 mRNA encodes the protein kinase activator of ERK1/2 kinase, which is involved in the phosphorylation of eukaryotic initiation factor (eIF) 4E. As a result, mRNA translation may be regulated by the cellular level of MKK-2. In this study, we show that the abundance of the MKK-2 polypeptide is reduced in PABP-overexpressing cells. In these cells, the levels of phosphorylated PABP, eIF4E, and ERK2 are also reduced. Treatment of HeLa cells with the MKK-2 inhibitor U0126 reduced PABP phosphorylation, suggesting that the phosphorylation of PABP is mediated by the MKK-2/ERK signaling pathway. Thus, a novel signaling pathway involving MKK-2 and ERK1/2 may down-regulate the activity of PABP and eIF4E by controlling their phosphorylation and compensates for the effect of excess cellular PABP.

Adenosine-rich elements present in the 5'-untranslated region of PABP mRNA can selectively reduce the abundance and translation of CAT mRNAs in vivo

The poly(A)-binding protein (PABP) is a highly conserved eukaryotic protein whose synthesis is regulated at the post-transcriptional level. The binding of PABP to the poly(A)-rich element found in the 5'-untranslated region (5'UTR) of PABP mRNA specifically inhibits its own translation. In this report, we show that similar adenosine-rich elements in the 5'UTR of the chloramphenicol acetyl-transferase (CAT) gene can significantly reduce the reporter mRNA abundance and translation in human 293 cells. The reduction in mRNA level, but not CAT expression, is dependent on the size of the 5'UTR poly(A) element. Furthermore, one 5'UTR-tethered PABP molecule is enough to inhibit CAT expression without affecting its mRNA level. We propose that the control of PABP synthesis may involve mRNA decay and the repression of translation.

Feedback inhibition of poly(A)-binding protein mRNA translation. A possible mechanism of translation arrest by stalled 40 S ribosomal subunits

An adenine-rich cis element at the 5'-untranslated region (UTR) of Pabp1 mRNA is able to inhibit translation of its own mRNA. Similar inhibition of translation of a reporter beta-galactosidase mRNA is observed when the adenine-rich auto regulatory sequence (ARS) is placed within the 5'-UTR of this mRNA. For this translational control the distance of the ARS from the 5' cap is not important. However, it determines the number of 40 S ribosomal subunits bound to the translationally arrested mRNA. Inhibition of mRNA translation by this regulatory sequence occurs at the step of joining of the 60 S ribosomal subunit to the pre-initiation complex. Translational arrest of the ARS containing mRNA in a rabbit reticulocyte lysate cell-free system in the presence of exogenous Pabp1 protects the 5'-flanking region of the ARS from nuclease digestion. This protection depends on the binding of the 40 S ribosomal subunit to the mRNA. The size and the sequence of the nucleotide-protected fragment depends on the location of the ARS within the 5'-UTR. When the ARS is located at a distance of about 78 nucleotides from the 5' cap, a 40-nucleotide long region adjacent to the ARS is protected. On the other hand, when the ARS is moved further away from the 5' cap to a distance of approximately 267 nucleotides, a 100-nucleotide-long region adjacent to the ARS is protected from nuclease digestion. Nuclease protection is attributed to the presence of one or more stalled 40 S ribosomal subunits near the Pabp1-bound ARS.

Distinct regions in the 3' untranslated region are responsible for targeting and stabilizing utrophin transcripts in skeletal muscle cells

In this study, we have sought to determine whether utrophin transcripts are targeted to a distinct subcellular compartment in skeletal muscle cells, and have examined the role of the 3' untranslated region (UTR) in regulating the stability and localization of utrophin transcripts. Our results show that utrophin transcripts associate preferentially with cytoskeleton-bound polysomes via actin microfilaments. Because this association is not evident in myoblasts, our findings also indicate that the localization of utrophin transcripts with cytoskeleton-bound polysomes is under developmental influences. Transfection of LacZ reporter constructs containing the utrophin 3'UTR showed that this region is critical for targeting chimeric mRNAs to cytoskeleton-bound polysomes and controlling transcript stability. Deletion studies resulted in the identification of distinct regions within the 3'UTR responsible for targeting and stabilizing utrophin mRNAs. Together, these results illustrate the contribution of posttranscriptional events in the regulation of utrophin in skeletal muscle. Accordingly, these findings provide novel targets, in addition to transcriptional events, for which pharmacological interventions may be envisaged to ultimately increase the endogenous levels of utrophin in skeletal muscle fibers from Duchenne muscular dystrophy (DMD) patients.

Negative control of the poly(A)-binding protein mRNA translation is mediated by the adenine-rich region of its 5'-untranslated region

Translation of the mRNA for the poly(A)-binding protein (PABP) may be autoregulated by the binding of PABP to the A-rich segment of its 5'-untranslated region (UTR). To test this hypothesis, we examined the effect of different fragments of the 5'-UTR from human PABP cDNA on the translation of the beta-galactosidase (beta-Gal) reporter gene. Presence of the A-rich sequence from the 5'-UTR of PABP mRNA inhibited expression of the chimeric beta-Gal gene in transfected HeLa cells. The differences in expression of beta-Gal polypeptide was due to the translational repression of beta-Gal mRNA containing the A-rich 5'-UTR of PABP mRNA. The A-rich region of the 5'-UTR located within nucleotides 58-146 of PABP mRNA was sufficient to mediate translational control of this mRNA expression. We also examined the effect of overexpression of PABP mRNA in HeLa cells. The ectopic PABP mRNA without the A-rich 5'-UTR region was translated efficiently, whereas the translation of the endogenous PABP mRNA was substantially inhibited in the transfected cells. In contrast, the ectopic PABP mRNA containing the A-rich 5'-UTR region did not show similar effect on the translation of the endogenous PABP mRNA in these cells. These results suggest that feedback control of mRNA translation is involved in regulating PABP expression in HeLa cells.

Uncoordinated inhibition of gene expression for muscle proteins by a troponin T antisense oligodeoxynucleotide

The effect of antisense oligodeoxynucleotide to rat troponin T (TnT) mRNA on its expression in differentiated rat L6 myotubes in culture was examined. The target sequence following the initiation codon was between nucleotides 83 and 97 and is found in all mRNAs produced from the f-TNT gene. Our studies showed that chimeric oligomer with one phosphorothioate linkage at the 3'-end was considerably more resistant to nucleases than was a phosphodiester oligomer. The chimeric oligomer produced >50% inhibition of TnT polypeptide synthesis. Synthesis of myosin heavy chain (MHC), troponin I (TnI), and alpha and beta tropomyosins (Tm) was not inhibited by the anti-TnT oligomer. However, synthesis of alpha-actin and troponin C (TnC) was somewhat affected by this treatment. Furthermore, compared with the untreated control myotubes, the steady-state level of TnT mRNA was reduced by approximately 40%-50% in anti-TnT oligomer-treated myotubes. Cellular levels of three other muscle mRNAs, alpha-Tm, s-TnI, and alpha-actin were also reduced by approximately 30%-40%. In contrast, fast TnI, beta-Tm, and TnC mRNA levels were not significantly affected by this treatment. Therefore, inhibition of TnT synthesis in differentiated myotubes uncoupled the coordinated expression of muscle proteins.

Participation of the human p53 3'UTR in translational repression and activation following gamma-irradiation

p53 protein levels have been shown to increase in a number of cells after treatment with genotoxic agents through a post-transcriptional mechanism. In gamma-irradiated human cells, the accumulation of p53 protein is accompanied by an increase in the association of p53 mRNA with large polysomes without any change in the level of p53 mRNA. This redistribution of p53 mRNA on polysomes in response to irradiation is consistent with enhanced translational activity of p53 mRNA. We demonstrate that a region of the p53 3'-untranslated region (3'UTR) inhibits translation of a chimeric reporter mRNA in vivo. Induced elevation of reporter activity after gamma-irradiation was seen in cells expressing chimeric reporter-p53 3'UTR transcripts. These data taken together demonstrate translational control of p53 gene expression after gamma-irradiation and denote a previously unsuspected and novel role for the p53 3'UTR in controlling translation.

Inhibition of troponin C production without affecting other muscle protein synthesis by the antisense oligodeoxynucleotide

The effect of blocking expression of a specific gene with antisense phosphodiester oligodeoxynucleotides on the coordinate regulation of myogenesis was studied. Different regions of both fast and slow troponin C (TnC) mRNAs were targeted for binding of the antisense oligomer. The 5'-cap region of both mRNAs was found to be the most effective target for inhibiting the expression of these genes. Approximately 40%-60% inhibition of expression of a specific isoform of TnC was achieved. However, inhibition of the TnC expression did not appreciably alter the pattern of myogenesis of mouse C2C12 cells. The differentiated murine muscle cells were able to cope with this reduced level of the target gene expression by antisense phosphodiester oligomers. We have also used a phosphorothioate oligomer targeted against a common sequence within the coding region of both fast and slow TnC mRNAs. This oligomer was found to be ineffective in blocking TnC gene expression.

Translational control of poly(A)-binding protein expression

Poly(A)-binding protein (PABP) is important for translation of eukaryotic mRNA and may be involved in shortening of its poly(A) tract. In many eukaryotic cells, this mRNA is inefficiently translated. The 5' untranslated region (UTR) of PABP mRNA has several adenine-rich regions which may serve as the PABP-binding sites to control its translation by a feed-back mechanism. This postulate was tested by using in vitro transcribed PABP mRNA and a rabbit reticulocyte lysate cell-free system. Results of our studies show that removal of the putative PABP-binding sites from the 5' UTR of this mRNA enhances its translation in the rabbit reticulocyte cell-free system. Furthermore, in vitro translation of the full-length PABP mRNA was inhibited by addition of purified PABP to the cell-free system. In contrast, translation of truncated mRNA lacking the putative PABP-binding sites at the 5' UTR was not inhibited by exogenous PABP. We have also tested the ability of purified PABP to bind to the 5' UTR of PABP mRNA using ultraviolet-mediated covalent cross-linking of RNA and proteins in vitro. Our results show that exogenous PABP binds to the 5' UTR of its full-length mRNA. Furthermore, incubation of PABP mRNA in rabbit reticulocyte lysate also led to binding of the endogenous PABP within the first 223 nucleotides of the 5' UTR. The adenine-rich regions are located within this segment of PABP mRNA. Following incubation of PABP mRNA in the reticulocyte lysate cell-free system under conditions of mRNA translation, the polysomal and non-translated free mRNA fractions were separated by centrifugation. Analysis of free and polysomal mRNA-protein (mRNP) complexes following ultraviolet-induced cross-linking showed that the free mRNP population was preferentially enriched in PABP. Results of our studies, therefore, suggest that PABP mRNA translation may be repressed by a unique feed-back mechanism.

mRNA stability in mammalian cells

This review concerns how cytoplasmic mRNA half-lives are regulated and how mRNA decay rates influence gene expression. mRNA stability influences gene expression in virtually all organisms, from bacteria to mammals, and the abundance of a particular mRNA can fluctuate manyfold following a change in the mRNA half-life, without any change in transcription. The processes that regulate mRNA half-lives can, in turn, affect how cells grow, differentiate, and respond to their environment. Three major questions are addressed. Which sequences in mRNAs determine their half-lives? Which enzymes degrade mRNAs? Which (trans-acting) factors regulate mRNA stability, and how do they function? The following specific topics are discussed: techniques for measuring eukaryotic mRNA stability and for calculating decay constants, mRNA decay pathways, mRNases, proteins that bind to sequences shared among many mRNAs [like poly(A)- and AU-rich-binding proteins] and proteins that bind to specific mRNAs (like the c-myc coding-region determinant-binding protein), how environmental factors like hormones and growth factors affect mRNA stability, and how translation and mRNA stability are linked. Some perspectives and predictions for future research directions are summarized at the end.

A truncated K-sam product lacking the distal carboxyl-terminal portion provides a reduced level of autophosphorylation and greater resistance against induction of differentiation

The K-sam gene was originally cloned from KATO-III human gastric cancer cells and is identical to the bek or keratinocyte growth factor (KGF) receptor (KGFR) or fibroblast growth factor receptor 2 gene. K-sam generates several variant transcripts by alternative splicing, and the most abundant K-sam transcript in KATO-III cells was cloned as the K-sam-IIC3 cDNA, which has the KGF-binding motif and a short carboxyl terminus lacking a putative phospholipase C-gamma 1 association site, Tyr-769. The K-sam-IIC3 cDNA was distinct from the K-sam-IIC1 cDNA, which was the same as the previously reported KGFR cDNA. The K-sam-IIC1 product contains a long carboxyl terminus with Tyr-769. K-sam-IIC3 showed greater transforming activity in NIH 3T3 cells than did K-sam-IIC1, and in gastric cancer cell lines in general, the level of K-sam-IIC3 mRNA was greater than that of K-sam-IIC1 mRNA. Here we report that the K-sam-IIC3 product was less autophosphorylated than the K-sam-IIC1 product in NIH 3T3 transfectants. K-sam-IIC3-transfected keratinocytes showed a stronger mitogenic response to KGF than did K-sam-IIC1 transfectants. Moreover, K-sam-IIC3-transfected L6 myoblast cells hardly differentiated when cultured in differentiation-inducing medium and growth was not significantly affected, while K-sam-IIC1 transfectants showed a differentiated phenotype with a reduced growth rate. These data indicate the difference in the signal transduction mediated by two KGFR-type K-sam variants generated by alternative splicing which might be involved in certain differentiation and carcinogenesis scenarios.

Developmental expression of poly(A) binding protein mRNAs during spermatogenesis in the mouse

The poly(A) binding protein (PABP), a conserved protein that binds to the 3' poly(A) tail on mRNAs in eukaryotic cells, has been implicated in the regulation of mRNA stability and translation. Two PABP cDNAs with different sequences were isolated from mouse testis cDNA libraries. The predicted amino acid sequence of one, PABP1, is nearly identical (98.9%) to human liver PABP, while 80% of the amino acids of the second, PABPt, are identical to mouse and human PABPs. Northern blots reveal that there is one major PABP mRNA species in liver, muscle, kidney, and brain, two in spleen, and at least four in testis. The levels of PABP mRNA in testis are 5-10-fold higher than in these somatic tissues, but surprisingly the vast majority of all PABP mRNA size variants sediment more slowly than single ribosomes, indicating strong translational repression. Reverse transcriptase-polymerase chain reaction assays demonstrate that PABPt mRNAs are abundant only in testis. Northern blots of RNAs purified from highly enriched spermatogenic cells show that the high levels, multiple sizes of PABP mRNAs, and the PABPt mRNA are present in meiotic and early haploid spermatogenic cells, and are sharply reduced in late haploid cells. Comparison of the binding of PABP1 and PABPt to poly(A) Sepharose in vitro revealed subtle differences, even though PABPt contains substitutions for highly conserved aromatic amino acids that are thought to be necessary for binding to poly(A). The existence of two PABP isoforms in mouse spermatogenic cells could influence cytoplasmic gene expression during spermatogenesis.