Selective destabilization of short-lived mRNAs with the granulocyte-macrophage colony-stimulating factor AU-rich 3' noncoding region is mediated by a cotranslational mechanism

Calcium-induced stabilization of AU-rich short-lived mRNAs is a common default response

The AU-rich element (AUUUA)n, found in the 3' noncoding region of many short-lived cytokine and proto-oncogene mRNAs, is sufficient to specifically target these mRNAs for rapid degradation in mammalian cells. The mechanism by which the AU-rich element promotes rapid mRNA decay is not known. Previous studies have shown that release of intracellular stored calcium by ionophore treatment of thymocytes and mast cells inhibits the rapid turnover of AU-rich interleukin mRNAs. Increased cytoplasmic half-life of interleukin mRNAs was linked to calcium-induced activation of the N-terminal c-Jun kinase. In this report we have characterized the calcium-induced stabilization of AU-rich mRNAs. We show that calcium induces stabilization of mRNAs with canonical AU-rich elements in all cell types tested. These results indicate that short-lived mRNA stabilization by calcium is not unique to immune cells nor interleukin mRNAs, but is a widespread default response that includes generic AU-rich mRNAs. Stabilization is shown to be rapid but transient, and to act without altering nuclear transcription or cytoplasmic translation rates. These data support the view that calcium release likely stabilizes short-lived mRNAs by altering trans-acting decay factors that promote AU-rich mRNA turnover.

The Interplay between the RNA Decay and Translation Machinery in Eukaryotes

RNA decay plays a major role in regulating gene expression and is tightly networked with other aspects of gene expression to effectively coordinate post-transcriptional regulation. The goal of this work is to provide an overview of the major factors and pathways of general messenger RNA (mRNA) decay in eukaryotic cells, and then discuss the effective interplay of this cytoplasmic process with the protein synthesis machinery. Given the transcript-specific and fluid nature of mRNA stability in response to changing cellular conditions, understanding the fundamental networking between RNA decay and translation will provide a foundation for a complete mechanistic understanding of this important aspect of cell biology.

SIDER2 retroposon-mediated mRNA decay in Leishmania is coupled to translation

We previously reported that Short Interspersed Degenerate Retroposons of the SIDER2 subfamily predominantly located within 3' untranslated regions (UTRs) of Leishmania transcripts promote rapid turnover that is initiated by endonucleolytic cleavage. Here, we investigated whether SIDER2-mediated mRNA decay is linked to translation. We show that preventing translation initiation by inserting a hairpin structure at the 5'-end of a SIDER2-containing mRNA blocks degradation. Similarly, global inhibition of translation elongation by cycloheximide or termination by puromycin causes stabilisation of SIDER2-containing transcripts. Altogether, these findings support that the mechanism of SIDER2-mediated decay is coupled to translation, possibly through the recruitment of decay factors to elongating ribosomes.

Perspectives on the ARE as it turns 25 years old

The AU-rich element (ARE) was discovered in 1986 as a conserved mRNA sequence found in the 3' untranslated region of the TNF-α transcript and other transcripts encoding cytokines and inflammatory mediators. Shortly thereafter, the ARE was shown to function as a regulator of mRNA degradation, and AREs were later shown to regulate other posttranscriptional mechanisms such as translation and mRNA localization. AREs coordinately regulate networks of chemokine, cytokine, and growth regulatory transcripts involved in cellular activation, proliferation, and inflammation. ARE-mediated regulation is carried out by a host of ARE-binding proteins, whose activity is regulated in a cell type and activation-dependent manner. The last 25 years of ARE research has offered insight into the mechanisms and regulation of ARE-mediated mRNA decay, and has provided a road map for the discovery of additional mRNA regulatory motifs. The future of ARE research will transition from a discovery phase to a phase focused on translating basic biological findings into novel therapeutic targets. Our understanding of ARE-mediated gene regulation and posttranscriptional control has implications for many fields of study including developmental biology, neuroscience, immunobiology, and cancer biology.

hnRNP Q mediates a phase-dependent translation-coupled mRNA decay of mouse Period3

Daily mRNA oscillations of circadian clock genes largely depend on transcriptional regulation. However, several lines of evidence highlight the critical role of post-transcriptional regulation in the oscillations of circadian mRNA oscillations. Clearly, variations in the mRNA decay rate lead to changes in the cycling profiles. However, the mechanisms controlling the mRNA stability of clock genes are not fully understood. Here we demonstrate that the turnover rate of mouse Period3 (mPer3) mRNA is dramatically changed in a circadian phase-dependent manner. Furthermore, the circadian regulation of mPer3 mRNA stability requires the cooperative function of 5′- and 3′-untranslated regions (UTRs). Heterogeneous nuclear ribonucleoprotein Q (hnRNP Q) binds to both 5′- and 3′-UTR and triggers enhancement of translation and acceleration of mRNA decay. We propose the phase-dependent translation coupled mRNA decay mediated by hnRNP Q as a new regulatory mechanism of the rhythmically regulated decay of mPer3 mRNA.

The role of AUF1 in regulated mRNA decay

Messenger ribonucleic acid (mRNA) turnover is a major control point in gene expression. In mammals, many mRNAs encoding inflammatory cytokines, oncoproteins, and G-protein-coupled receptors are destabilized by the presence of AU-rich elements (AREs) in their 3'-untranslated regions. Association of ARE-binding proteins (AUBPs) with these mRNAs promotes rapid mRNA degradation. ARE/poly(U)-binding/degradation factor 1 (AUF1), one of the best-characterized AUBPs, binds to many ARE-mRNAs and assembles other factors necessary to recruit the mRNA degradation machinery. These factors include translation initiation factor eIF4G, chaperones hsp27 and hsp70, heat-shock cognate protein hsc70, lactate dehydrogenase, poly(A)-binding protein, and other unidentified proteins. Numerous signaling pathways alter the composition of this AUF1 complex of proteins to effect changes in ARE-mRNA degradation rates. This review briefly describes the roles of mRNA decay in gene expression in general and ARE-mediated decay (AMD) in particular, with a focus on AUF1 and the different modes of regulation that govern AUF1 involvement in AMD.

A conserved post-transcriptional BMP2 switch in lung cells

An ultra-conserved sequence in the bone morphogenetic protein 2 (BMP2) 3' untranslated region (UTR) markedly represses BMP2 expression in non-transformed lung cells. In contrast, the ultra-conserved sequence stimulates BMP2 expression in transformed lung cells. The ultra-conserved sequence functions as a post-transcriptional cis-regulatory switch. A common single-nucleotide polymorphism (SNP, rs15705, +A1123C), which has been shown to influence human morphology, disrupts a conserved element within the ultra-conserved sequence and altered reporter gene activity in non-transformed lung cells. This polymorphism changed the affinity of the BMP2 RNA for several proteins including nucleolin, which has an increased affinity for the C allele. Elevated BMP2 synthesis is associated with increased malignancy in mouse models of lung cancer and poor lung cancer patient prognosis. Understanding the cis- and trans-regulatory factors that control BMP2 synthesis is relevant to the initiation or progression of pathologies associated with abnormal BMP2 levels.

Homologous posttranscriptional regulation of insulin-like growth factor-I receptor level via glycogen synthase kinase-3beta and mammalian target of rapamycin in adrenal chromaffin cells: effect on tau phosphorylation

In cultured bovine adrenal chromaffin cells, approximately 24 h-treatment with insulin-like growth factor-I (IGF-I) decreased cell surface (125)I-IGF-I binding capacity and IGF-I receptor protein level by approximately 64% (EC(50) = 5.0 nM; t(1/2) = approximately 7 h). IGF-I-induced IGF-I receptor decrease was abolished by LY294002 (phosphoinositide 3-kinase inhibitor) and partially attenuated by rapamycin (an inhibitor of mammalian target of rapamycin [mTOR]). SB216763 (an inhibitor of glycogen synthase kinase-3 [GSK-3]) down-regulated IGF-I receptor, which was further decreased by IGF-I. IGF-I increased inhibitory Ser(9)-phosphorylation of GSK-3beta and stimulatory Ser(2448)-phosphorylation of mTOR. l-leucine increased phosphorylation of mTOR (but not GSK-3beta), and down-regulated IGF-I receptor, both events being abolished by rapamycin. IGF-I-induced IGF-I receptor decrease was not prevented by proteolysis inhibitors. Pulse-label with [(35)S]methionine/cysteine followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that SB216763 or L-leucine retarded synthesis of IGF-I receptor and its precursor molecule. SB216763 (but not l-leucine) destabilized IGF-I receptor mRNA and decreased its level, without changing IGF-I receptor gene transcription. In SB216763-treated cells, IGF-I-induced Tyr-autophosphorylation of IGF-I receptor was decreased by 36%, compared to nontreated cells. IGF-I attenuated constitutive Ser(396)-phosphorylation of tau by 30% in nontreated cells, but not in SB216763-treated cells. IGF-I-induced down-regulations of (125)I-IGF-I binding and IGF-I receptor, as well as IGF-I-induced phosphorylations of GSK-3beta and mTOR were restored to the control levels of nontreated cells after washout of IGF-I (10 nM for 12 h)-treated cells. Thus, IGF-I down-regulated functional IGF-I receptor via GSK-3beta inhibition and mTOR activation; constitutive activity of GSK-3beta maintained IGF-I receptor level in nonstimulated cells.

Amino acid limitation regulates the expression of genes involved in several specific biological processes through GCN2-dependent and GCN2-independent pathways

Evidence has accumulated that amino acids play an important role in controlling gene expression. Nevertheless, two components of the amino acid control of gene expression are not yet completely understood in mammals: (a) the target genes and biological processes regulated by amino acid availability, and (b) the signaling pathways that mediate the amino acid response. Using large-scale analysis of gene expression, the objective of this study was to gain a better understanding of the control of gene expression by amino acid limitation. We found that a 6 h period of leucine starvation regulated the expression of a specific set of genes: 420 genes were up-regulated by more than 1.8-fold and 311 genes were down-regulated. These genes were involved in the control of several biological processes, such as amino acid metabolism, lipid metabolism and signal regulation. Using GCN2-/- cells and rapamycin treatment, we checked for the role of mGCN2 and mTORC1 kinases in this regulation. We found that (a) the GCN2 pathway was the major, but not unique, signaling pathway involved in the up- and down-regulation of gene expression in response to amino acid starvation, and (b) that rapamycin regulates the expression of a set of genes that only partially overlaps with the set of genes regulated by leucine starvation.

Control of protein expression through mRNA stability in calcium signalling

Specific sequences (cis-acting elements) in the 3'-untranslated region (UTR) of RNA, together with stabilizing and destabilizing proteins (trans-acting factors), determine the mRNA stability, and consequently, the level of expression of several proteins. Such interactions were discovered initially for short-lived mRNAs encoding cytokines and early genes like c-jun and c-myc. However, they may also determine the fate of more stable mRNAs in a tissue and disease-dependent manner. The interactions between the cis-acting elements and the trans-acting factors may also be modulated by Ca(2+) either directly or via a control of the phosphorylation status of the trans-acting factors. We focus initially on the basic concepts in mRNA stability with the trans-acting factors AUF1 (destabilizing) and HuR (stabilizing). Sarco/endoplasmic reticulum Ca(2+) pumps, SERCA2a (cardiac and slow twitch muscles) and SERCA2b (most cells including smooth muscle cells), are pivotal in Ca(2+) mobilization during signal transduction. SERCA2a and SERCA2b proteins are encoded by relatively stable mRNAs that contain cis-acting stability determinants in their 3'-regions. We present several pathways where 3'-UTR mediated mRNA decay is key to Ca(2+) signalling: SERCA2a and beta-adrenergic receptors in heart failure, renin-angiotensin system, and parathyroid hormones. Other examples discussed include cytokines vascular endothelial growth factor, endothelin and endothelial nitric oxide synthase. Roles of Ca(2+) and Ca(2+)-binding proteins in mRNA stability are also discussed. We anticipate that these novel modes of control of protein expression will form an emerging area of research that may explore the central role of Ca(2+) in cell function during development and in disease.

Assembly of AUF1 with eIF4G-poly(A) binding protein complex suggests a translation function in AU-rich mRNA decay

An AU-rich element (ARE) located in the 3'-untranslated region of many short-lived mRNAs functions as an instability determinant for these transcripts. AUF1/hnRNP D, an ARE-binding protein family consisting of four isoforms, promotes rapid decay of ARE-mRNAs. The mechanism by which AUF1 promotes rapid decay of ARE-mRNA is unclear. AUF1 has been shown to form an RNase-resistant complex in cells with the cap-initiation complex and heat shock proteins Hsp70 and Hsc70, as well as other unidentified factors. To understand the function of the AUF1 complex, we have biochemically investigated the association of AUF1 with the components of the translation initiation complex. We used purified recombinant proteins and a synthetic ARE RNA oligonucleotide to determine the hierarchy of protein interactions in vitro and the effect of AUF1 binding to the ARE on the formation of protein complexes. We demonstrate that all four AUF1 protein isoforms bind directly and strongly to initiation factor eIF4G at a C-terminal site regardless of AUF1 interaction with the ARE. AUF1 is shown to directly interact with poly(A) binding protein (PABP), both independently of eIF4G and in a complex with eIF4G. AUF1-PABP interaction is opposed by AUF1 binding to the ARE or Hsp70 heat shock protein. In vivo, AUF1 interaction with PABP does not alter PABP stability. Based on these and other data, we propose a model for the molecular interactions of AUF1 that involves translation-dependent displacement of AUF1-PABP complexes from ARE-mRNAs with possible unmasking of the poly(A) tail.

Genomic organization, promoter activity, and expression of the human choline transporter-like protein 1

Choline transporter-like (CTL) proteins of the CTL1 family are novel transmembrane proteins implicated in choline transport for phospholipid synthesis. In this study, we characterized the 5'-flanking region of the human (h)CTL1 gene and examined some of the possible mechanisms of its regulation, including promoter activity, splicing, and expression. The transcription start site of the hCTL1 gene was mapped by 5'-rapid amplification of cDNA ends (RACE), and the presence of two splice variants, hCTL1a and hCTL1b, was investigated using isoform-specific PCR and 3'-RACE. The hCTL1 promoter region of approximately 900 bp was isolated from MCF-7 human breast cancer cells. The promoter was TATA-less and driven by a long stretch of GC-rich sequence in accordance with widespread expression of hCTL1 at both mRNA and protein levels. Deletion analyses demonstrated that a very strong promoter is contained within 500 bp of the transcription start site, and more upstream regions did not increase its activity. The core promoter that conferred the minimal transcription is within the -188/+27-bp region, and its activity varied in human breast cancer and mouse skeletal muscle cells. Multiple motifs within the promoter regulatory region bound nuclear factors from both cultured cells and normal human skeletal muscle. The motifs within the three regions [S1 (-92/-61 bp), S2 (-174/-145 bp), and S3 (-289/-260 bp)] contained overlapping binding sites for hematopoietic transcription factors and ubiquitous transcription factors, in line with the expected gene function. Genomic analyses demonstrated a high conservation of hCTL1 and mouse CTL1 proximal promoters. Accordingly, mRNA profiles demonstrated that human splice variants were expressed ubiquitously, as demonstrated for the mouse transcripts; however, they differed from the profiles of rat CTL1 transcripts, which were more restricted to neurons and intestinal tissues. The shorter hCTL1b variant contained the cytosolic COOH-terminal motif L651KKR654 for endoplasmic reticulum retrieval/retention. This retention signal was conserved in hCTL1b and rat and mouse CTL1b and is typical for transmembrane proteins of type 1 topology.

Messenger RNA turnover in eukaryotes: pathways and enzymes

The control of mRNA degradation is an important component of the regulation of gene expression since the steady-state concentration of mRNA is determined both by the rates of synthesis and of decay. Two general pathways of mRNA decay have been described in eukaryotes. Both pathways share the exonucleolytic removal of the poly(A) tail (deadenylation) as the first step. In one pathway, deadenylation is followed by the hydrolysis of the cap and processive degradation of the mRNA body by a 5' exonuclease. In the second pathway, the mRNA body is degraded by a complex of 3' exonucleases before the remaining cap structure is hydrolyzed. This review discusses the proteins involved in the catalysis and control of both decay pathways.

The AU-Rich Transcriptome: More Than Interferons and Cytokines, and Its Role in Disease

The AU-rich elements (AREs) are among the predominant cis-acting factors that exist primarily in the 3' untranslated region (3'-UTR) of messenger RNAs (mRNAs) and regulate mRNA stability. AREs were previously believed to be restricted to relatively few mRNAs, including those of interferons (IFNs) and cytokines, growth factors, and proto-oncogenes. Our recent analysis, however, showed that ARE mRNAs represent as much as 8% of mRNAs transcribed from human genes that encode functionally diverse proteins important in many transient biologic processes. Among those processes are cell growth and differentiation, immune responses, signal transduction, transcriptional and translational control, hematopoiesis, apoptosis, nutrient transport, and metabolism. Several recent studies examined signaling pathways that regulate ARE-mediated mRNA stability, notably the p38 mitogen-activated protein kinase (MAPK) pathway. In addition, several AU-rich binding proteins that regulate the ARE mRNA pathways have been characterized. Dysregulation of regulatory signaling pathways and regulatory proteins affecting ARE mRNA stability can lead to abnormalities in many critical cellular processes and to specific disease conditions. Thus, the heterogeneity in AREs, their signaling pathways, and effector proteins contribute to the functional diversity of the ARE gene family, which encompasses more than IFNs and cytokines.

Control of SERCA2a Ca2+ pump mRNA stability by nuclear proteins: role of domains in the 3′-untranslated region

Alternative splicing of the sarco/endoplasmic reticulum (SERCA2) Ca2+ pump transcript generates the two isoforms: SERCA2a in left ventricular myocytes (LVM) and SERCA2b in most tissues. Nuclear protein extracts from left ventricular myocytes can cause a decay of the 3'-region of the SERCA2a. To determine if all the domains in the 800 b SERCA2a 3'-end region (3344-4243) are equally stable, we examined in vitro decay of synthetically capped, polyadenylated overlapping RNA fragments 2A1-2A6 from the 3'-end region of SERCA2a. Whereas 2A1-2A5 RNAs were stable, the distal fragment 2A6 (4135-4243 b) decayed rapidly. Deleting the 2A6 sequence from the 800-b 3'-end region increased its stability. In mobility shift assays, 2A6 bound to protein(s) in the LVM nuclear extracts in a specific manner: unlabelled 2A6 or the 800 b 3'-region RNA competed for binding but poly A, poly U, and poly C RNA did not. Secondary structure analysis revealed three hairpin loops in 2A6. Experiments using small synthetic RNA fragments for competition with 2A6 binding to nuclear proteins were consistent with a model involving the three hairpin loops. Thus, the secondary structure of the distal domain of SERCA2a RNA may be important in regulating its stability.

Post-transcriptional regulation of gene expression by degradation of messenger RNAs

Recent evidence suggests that gene expression may be regulated, at least in part, at post-transcriptional level by factors inducing the extremely rapid degradation of messenger RNAs. These factors include reactions between adenyl-uridyl-rich elements (AREs) of the relevant mRNA and either specific proteins that bind to these elements or exosomes. This review deals with examples of the proteins (AU-rich binding proteins, AUBPs) and exosomes, which have been shown to form complexes with AREs and bring about rapid degradation of the relevant mRNA, and with certain other factors, which protect the RNA from such degradation. The biochemical and physiological factors underlying the stability of messenger RNAs carrying the ARE motifs will be reviewed in the light of their emerging significance for cell physiology, human pathology, and molecular medicine. We also consider the possible application of the results of recent insights into the mechanisms to pharmacological interventions to prevent or cure disorders, especially developmental disorders, which the suppression of gene expression may bring about. Molecular targeting of specific steps in protein degradation by synthetic compounds has already been utilized for the development of pharmacological therapies.

Analysis of the function, expression, and subcellular distribution of human tristetraprolin

OBJECTIVE

The zinc-finger protein tristetraprolin (TTP) has been demonstrated to regulate tumor necrosis factor alpha (TNFalpha) messenger RNA (mRNA) instability in murine macrophages. We sought to develop a model system to characterize the effects of human TTP (hTTP) on TNFalpha 3'-untranslated region (3'-UTR)-mediated expression. We also generated a specific polyclonal antibody against hTTP that enabled the examination of the subcellular distribution of hTTP and its RNA binding in vivo.

METHODS

Transfection of reporter gene constructs were used to functionally characterize the role of hTTP in regulating TNFalpha expression in a 3'-UTR-dependent manner. An immunoprecipitation reverse transcription-polymerase chain reaction technique, immunoblotting, immunocytochemistry, and sucrose density fractionation were used to identify and localize hTTP.

RESULTS

We found that hTTP interacted with human TNFalpha mRNA in the cytoplasm. The presence of the TNFalpha 3'-UTR was sufficient to confer binding by TTP in vivo. This interaction resulted in reduced luciferase reporter gene activity in a TNFalpha 3'-UTR adenine-uridine-rich element (ARE)-dependent manner. Immunoblotting and immunocytochemistry indicated that endogenous and transfected hTTP localized to the cytoplasm. Results of sucrose density fractionation studies were consistent with a polysomal location of hTTP. In rheumatoid synovium, hTTP expression was restricted to cells in the synovial lining layers.

CONCLUSION

Through the development of an antiserum specific for hTTP, we have been able to demonstrate that hTTP binds specifically to the TNFalpha 3'-UTR and reduces reporter gene expression in an ARE-specific manner. These studies establish that hTTP is likely to function in a similar, if not identical manner, in the posttranscriptional regulation of TNFalpha. Understanding the posttranscriptional regulation of TNFalpha biosynthesis is important for the development of novel treatment strategies in rheumatoid arthritis.

The upstream stem-loop domain of the 3' untranslated region of apolipoprotein II mRNA binds the estrogen-regulated mRNA stabilizing factor

Apolipoprotein II (apoII), a component of the very low density lipoprotein (VLDL) particle, is a yolk protein expressed in the liver in response to estrogen. Its expression is modulated by estrogen-mediated stimulation of transcription as well as stabilization of its mRNA. This stabilization is due to the estrogen-regulated mRNA stabilizing factor (E-RmRNASF) [Cell. Mol. Biol. Res. 41 (1995) 583). E-RmRNASF protects apoII mRNA from targeted endonucleolytic degradation. The expression of E-RmRNASF itself is under estrogenic control. The hepatic expression of E-RmRNASF is also modulated by certain estrogenic and anti-estrogenic non-steroidal environmental xenobiotics [Biochem. Pharmacol. 53 (1997) 1425]. Studies involving RNA affinity-based depletion of mRNA stabilization activity indicated that E-RmRNASF binds to apoII mRNA. E-RmRNASF binds apoII mRNA in a region-specific manner. The region of binding has been narrowed down to the upstream domain of stem-loop secondary structure spanning nucleotides (nt) 402-558 in the 3' untranslated region (3'UTR). A RNA affinity chromatography procedure using this portion of apoII mRNA was utilized for the purification of E-RmRNASF. A gel filtration (GF) chromatography step preceding the RNA affinity chromatography was required for additional enrichment of E-RmRNASF. A functional assay involving the in vitro stabilization of apoII mRNA from degradation was utilized to detect E-RmRNASF during chromatography. E-RmRNASF appears to be a protein of apparent molecular weight of 20-25kDa visualized by SDS polyacrylamide gel electrophoresis.

HuD, a neuronal-specific RNA-binding protein, increases the in vivo stability of MYCN RNA

MYCN amplification and consequent deregulated expression plays a crucial role in determining the clinical behavior of neuroblastoma. Enhanced expression of MYCN confers growth potential to neuroblastoma cells, and a direct link between MYCN expression and the development of neuroblastoma has been demonstrated in transgenic mice studies. Although the molecular pathways underlying the regulation of MYCN have not been fully elucidated, post-transcriptional mechanisms appear to be important. Previously, we reported that an embryonic lethal abnormal vision-like (ELAV) protein binds with high specificity to at least two AU-rich elements within the MYCN 3'-untranslated region. In this study, we characterized the ability of cis-acting elements within the MYCN 3'-untranslated region to destabilize mRNA in cells and examined the functional consequences of its interactions with the ELAV protein HuD. We show that at least 4 cis-acting elements within the MYCN 3'-untranslated region are able to signal the degradation of stable heterologous mRNA. Ectopic overexpression of HuD dramatically inhibits RNA decay mediated by the full-length MYCN 3'-untranslated region and cis-acting destabilizing elements that harbor HuD binding sites in vivo. HuD may contribute to the malignant phenotype of neuroblastoma cells by stabilizing MYCN mRNA, thereby enhancing steady-state levels of expression of this oncogene.

Characterization of CDw92 as a member of the choline transporter-like protein family regulated specifically on dendritic cells

CDw92 is a 70-kDa surface protein broadly expressed on leukocytes and endothelial cells. In this manuscript, we present the molecular cloning of the CDw92 molecule by using a highly efficient retroviral expression cloning system. Sequence analysis of the CDw92 cDNA revealed a length of 2679 bp. The 1959-bp open reading frame encodes a protein of 652 amino acids. Computational analysis of the CDw92 protein sequence indicates 10 transmembrane domains, three potential N-linked glycosylation sites, and an amino acid stretch in the C-terminal region that is related to the immunoreceptor tyrosine-based inhibitory motif. Comparison of the sequence of the CDw92 clone presented in this study with various database entries show that it is a C-terminal variant of human choline transporter-like protein 1, a member of a recently identified family of multitransmembrane surface proteins. Furthermore, we found that CDw92 is stably expressed on monocytes, PBLs, and endothelial cells, as we did not yet find modulation of expression by various stimuli on these cells. In contrast to this factor-independent expression of CDw92, we detected a specific regulation of CDw92 on monocyte-derived dendritic cells (Mo-DCs). Maturation of Mo-DCs by ionomycin or calcium ionophore resulted in down-regulation of CDw92 and incubation of these cells with IL-10 in a specific re-expression. Moreover, targeting of CDw92 on LPS-treated Mo-DCs by CDw92 mAb VIM15b augmented the LPS-induced IL-10 production 2.8-fold. Together, these data suggest a crucial role of the CDw92 protein in the biology and regulation of the function of leukocytes in particular DCs.

Suppression of the transcriptional activity and DNA binding of nuclear factor-kappa B by a paraneoplastic cerebellar degeneration-associated antigen

Paraneoplastic cerebellar degeneration (PCD) associated with gynecological malignancies is a disorder in which an autoimmune mechanism has been suggested, and both antibody- and cell-mediated immune responses exist against pcd17/cdr2, a neural protein expressed in cerebellar Purkinje neurons and brainstem neurons. In this report, we describe that pcd17 can suppress the basal or activated NF-kappaB-dependent transcriptional activity in a co-transfection study. The DNA binding of constitutive NF-kappaB complexes decreased in the nucleus of TNF-alpha-stimulated neuroblastoma cells, though pcd17 does not bind to classical NF-kappaB consensus site. These data indicate that pcd17 is a potential repressor for NF-kappaB-dependent gene transcription in neurons.

Nonsense-mediated mRNA decay in Saccharomyces cerevisiae

Cell survival depends on the precise and correct production of polypeptides. Eukaryotic cells have evolved conserved proofreading mechanisms to get rid of incomplete and potentially deleterious proteins. The nonsense-mediated mRNA decay (NMD) pathway is an example of a surveillance mechanism that monitors premature translation termination and promotes degradation of aberrant transcripts that code for nonfunctional or even harmful proteins. In this review we will describe our current knowledge of the NMD pathway, analyzing primarily the results obtained from the yeast Saccharomyces cerevisiae, but establishing functional comparisons with those obtained in higher eukaryotes. Based on these observations, we present two related working models to explain how this surveillance pathway recognizes and selectively degrades aberrant mRNAs.

Mechanisms and control of mRNA decapping in Saccharomyces cerevisiae

The process of mRNA turnover is a critical component of the regulation of gene expression. In the past few years a discrete set of pathways for the degradation of polyadenylated mRNAs in eukaryotic cells have been described. A major pathway of mRNA degradation in yeast occurs by deadenylation of the mRNA, which leads to a decapping reaction, thereby exposing the mRNA to rapid 5' to 3' exonucleolytic degradation. A critical step in this pathway is decapping, since it effectively terminates the existence of the mRNA and is the site of numerous control inputs. In this review, we discuss the properties of the decapping enzyme and how its activity is regulated to give rise to differential mRNA turnover. A key point is that decapping appears to be controlled by access of the enzyme to the cap structure in a competition with the translation initiation complex. Strikingly, several proteins required for mRNA decapping show interactions with the translation machinery and suggest possible mechanisms for the triggering of mRNA decapping.

Dietary iron induces rapid changes in rat intestinal divalent metal transporter expression

The divalent metal transporter (DMT1, also known as NRAMP2 or DCT1) is the likely target for regulation of intestinal iron absorption by iron stores. We investigated changes in intestinal DMT1 expression after a bolus of dietary iron in iron-deficient Belgrade rats homozygous for the DMT1 G185R mutation (b/b) and phenotypically normal heterozygous littermates (+/b). Immunofluorescent staining with anti-DMT1 antisera showed that DMT1 was located in the brush-border membrane. Duodenal DMT1 mRNA and protein levels were six- and twofold higher, respectively, in b/b rats than in +/b rats. At 1.5 h after dietary iron intake in +/b and b/b rats, DMT1 was internalized into cytoplasmic vesicles. At 1.5 and 3 h after iron intake in +/b and b/b rats, there was a rapid decrease of DMT1 mRNA and a transient increase of DMT1 protein. The decrease of DMT1 mRNA was specific, because ferritin mRNA was unchanged. After iron intake, an increase in ferritin protein and decrease in iron-regulatory protein binding activity occurred, reflecting elevated intracellular iron pools. Thus intestinal DMT1 rapidly responds to dietary iron in both +/b and b/b rats. The internalization of DMT1 may be an acute regulatory mechanism to limit iron uptake. In addition, the results suggest that in the Belgrade rat DMT1 with the G185R mutation is not an absolute block to iron.

Cyclic nucleotide regulation of Na+/glucose cotransporter (SGLT1) mRNA stability. Interaction of a nucleocytoplasmic protein with a regulatory domain in the 3'-untranslated region critical for stabilization

Expression of the Na(+)-coupled glucose cotransporter SGLT1 is regulated post-transcriptionally at the level of mRNA stability. We have previously demonstrated that cAMP-dependent stabilization of the SGLT1 message was correlated with the protein phosphorylation-dependent binding of cytoplasmic proteins to a uridine-rich sequence (URE) in the 3'-untranslated region (UTR). In the present study, the regulatory role of the URE was demonstrated by inserting it into the 3'-UTR of a beta-globin reporter minigene under the control of the tetracycline-regulated promoter. The resultant chimeric globin/SGLT1 mRNA expressed after transfection into LLC-PK1 cells exhibited a decreased half-life compared with the beta-globin control, indicating that the URE serves a destabilizing function. Activation of protein kinase A stabilized the chimeric message but not the beta-globin control, indicating the presence of a regulatory stabilizing sequence within the URE. A 38-kDa nucleocytoplasmic protein was identified that recognized a 12-nucleotide binding site within the URE. A mutation in this binding site that prevented protein binding assayed in vitro by UV cross-linking also prevented protein kinase A-dependent stabilization of the chimeric message assayed in vivo. These findings identify the interaction between a 38-kDa nucleocytoplasmic protein and a regulatory uridine-rich sequence in the 3'-UTR as critical for cAMP-mediated SGLT1 message stabilization.

The chicken c-Jun 5' untranslated region directs translation by internal initiation

The 5' untranslated region (UTR) of the chicken c-jun message is exceptionally GC rich and has the potential to form a complex and extremely stable secondary structure. Because stable RNA secondary structures can serve as obstacles to scanning ribosomes, their presence suggests inefficient translation or initiation through alternate mechanisms. We have examined the role of the c-jun 5' UTR with respect to its ability to influence translation both in vitro and in vivo. We find, using rabbit reticulocyte lysates, that the presence of the c-jun 5' UTR severely inhibits translation of both homologous and heterologous genes in vitro. Furthermore, translational inhibition correlates with the degree of secondary structure exhibited by the 5' UTR. Thus, in the rabbit reticulocyte lysate system, the c-jun 5' UTR likely impedes ribosome scanning resulting in inefficient translation. In contrast to our results in vitro, the c-jun 5' UTR does not inhibit translation in a variety of different cell lines suggesting that it may direct an alternate mechanism of translational initiation in vivo. To distinguish among the alternate mechanisms, we generated a series of bicistronic expression plasmids. Our results demonstrate that the downstream cistron, in the bicistronic gene, is expressed to a much higher level when directly preceded by the c-jun 5' UTR. In addition, inhibition of ribosome scanning on the bicistronic message, through insertion of a synthetic stable hairpin, inhibits translation of the first cistron but does not inhibit translation of the cistron downstream of the c-jun 5' UTR. These results are consistent with a model by which the c-jun message is translated through cap independent internal initiation. Oncogene (2000) 19, 2836 - 2845

Post-transcriptional control of cyclooxygenase-2 gene expression. The role of the 3'-untranslated region

The cyclooxygenase (COX)-2 enzyme is responsible for increased prostaglandin formation in inflammatory states and is the major target of nonsteroidal anti-inflammatory drugs. Normally COX-2 expression is tightly regulated, however, constitutive overexpression plays a key role in colon carcinogenesis. To understand the mechanisms controlling COX-2 expression, we examined the ability of the 3'-untranslated region of the COX-2 mRNA to regulate post-transcriptional events. When fused to a reporter gene, the 3'-untranslated region mediated rapid mRNA decay (t(1/2) = 30 min), which was comparable to endogenous COX-2 mRNA turnover in serum-induced fibroblasts treated with actinomycin D or dexamethasone. Deletion analysis demonstrated that a conserved 116-nucleotide AU-rich sequence element (ARE) mediated mRNA degradation. In transiently transfected cells, this region inhibited protein synthesis approximately 3-fold. However, this inhibition did not occur through changes in mRNA stability since mRNA half-life and steady-state mRNA levels were unchanged. RNA mobility shift assays demonstrated a complex of cytoplasmic proteins that bound specifically to the ARE, and UV cross-linking studies identified proteins ranging from 90 to 35 kDa. Fractionation of the cytosol showed differential association of ARE-binding proteins to polysomes and S130 fractions. We propose that these factors influence expression at a post-transcriptional step and, if dysregulated, may increase COX-2 protein as detected in colon cancer.

Regulation of mda-7 gene expression during human melanoma differentiation

Induction of irreversible growth arrest and terminal differentiation in human melanoma cells following treatment with recombinant human fibroblast interferon (IFN-beta) and mezerein (MEZ) results in elevated expression of a specific melanoma differentiation associated gene, mda-7. Experiments were conducted to define the mechanism involved in the regulation of mda-7 expression in differentiating human melanoma cells. The mda-7 gene is actively transcribed in uninduced HO-1 human melanoma cells and the rate of transcription of mda-7 is not significantly enhanced by treatment with IFN-beta, MEZ or IFN-beta+MEZ. The high basal activity of the mda-7 promoter in uninduced melanoma cells and the absence of enhancing effect upon treatment with differentiation inducers is corroborated by transfection studies using the promoter region of mda-7 linked to a luciferase reporter gene containing the SV40 polyadenylation signal sequence. RT - PCR analysis detects the presence of low levels of mda-7 transcripts in uninduced and concomitant increases in differentiation inducer treated HO-1 cells. However, steady-state mda-7 mRNA is detected only in IFN-beta+MEZ and to a lesser degree in MEZ treated cells. We show that induction of terminal differentiation of HO-1 cells with IFN-beta+MEZ dramatically increases the half-life of mda-7 mRNA while treatment with cycloheximide results in detectable mda-7 mRNA in control and inducer treated cells. These observations confirm constitutive activity of the mda-7 promoter in HO-1 cells irrespective of differentiation status suggesting posttranscriptional processes as important determinants of mda-7 expression during terminal differentiation. The 3' UTR region of mda-7 contains AU-rich elements (ARE) that contribute to rapid mda-7 mRNA turnover during proliferation and reversible differentiation, a process controlled by a labile protein factor(s). Substitution of the SV40 polyadenylation signal sequence in the luciferase reporter plasmid with the mda-7-ARE-3'-UTR renders the Luciferase message unstable when expressed in proliferating and reversibly differentiated melanoma cells. In contrast, the luciferase message is stabilized when the mda-7-ARE-3'-UTR construct is expressed in terminally differentiated HO-1 cells. These results provide compelling evidence that mda-7 expression during terminal differentiation in human melanoma cells is regulated predominantly at a posttranscriptional level.

Cortisol inhibits hepatocyte growth factor/scatter factor expression and induces c-met transcripts in osteoblasts

Hepatocyte growth factor/scatter factor (HGF/SF) is expressed by osteoblasts and has important effects on repair and bone remodeling. Because glucocorticoids regulate these two functions, we tested the effects of cortisol on the expression of HGF/SF and c-met, the protooncogene encoding the HGF/SF receptor, in cultures of osteoblast-enriched cells from 22-day fetal rat calvariae (Ob cells). Cortisol decreased HGF/SF mRNA levels and diminished the induction of HGF/SF transcripts by fibroblast growth factor-2 (FGF-2) and platelet-derived growth factor BB (PDGF BB). Cortisol also decreased FGF-2 and PDGF BB-induced HGF/SF mRNA and polypeptide levels in MC3T3 cells. In contrast, cortisol enhanced the expression of c-met transcripts in Ob cells. Cortisol did not modify the half-life of HGF/SF or of c-met mRNA in transcriptionally arrested cells, and it increased the rate of transcription of c-met. In conclusion, cortisol decreases HGF/SF transcripts in Ob cells and enhances c-met expression transcriptionally. The effects of cortisol on HGF/SF could be relevant to its inhibitory actions on bone formation and repair.

Interaction between a poly(A)-specific ribonuclease and the 5' cap influences mRNA deadenylation rates in vitro

We have used an in vitro system that reproduces in vivo aspects of mRNA turnover to elucidate mechanisms of deadenylation. DAN, the major enzyme responsible for poly(A) tail shortening in vitro, specifically interacts with the 5' cap structure of RNA substrates, and this interaction is greatly stimulated by a poly(A) tail. Several observations suggest that cap-DAN interactions are functionally important for the networking between regulated mRNA stability and translation. First, uncapped RNA substrates are inefficiently deadenylated. Second, a stem-loop structure in the 5' UTR dramatically reduces deadenylation by interfering with cap-DAN interactions. Third, the addition of cap binding protein eIF4E inhibits deadenylation in vitro. These data provide insights into the early steps of substrate recognition that target an mRNA for degradation.

Assembly of AUF1 oligomers on U-rich RNA targets by sequential dimer association

Many labile mammalian mRNAs are targeted for rapid cytoplasmic turnover by the presence of A + U-rich elements (AREs) within their 3'-untranslated regions. These elements are selectively recognized by AUF1, a component of a multisubunit complex that may participate in the initiation of mRNA decay. In this study, we have investigated the recognition of AREs by AUF1 in vitro using oligoribonucleotide substrates. Gel mobility shift assays demonstrated that U-rich RNA targets were specifically bound by AUF1, generating two distinct RNA-protein complexes in a concentration-dependent manner. Chemical cross-linking revealed the interaction of AUF1 dimers to form tetrameric structures involving protein-protein interactions in the presence of high affinity RNA targets. From these data, a model of AUF1 association with AREs involving sequential dimer binding was developed. Using fluorescent RNA substrates, binding parameters of AUF1 dimer-ARE and tetramer-ARE equilibria were evaluated in solution by fluorescence anisotropy measurements. Using two AUF1 deletion mutants, sequences C-terminal to the RNA recognition motifs are shown to contribute to the formation of the AUF1 tetramer.ARE complex but are not obligate for RNA binding activity. Kinetic studies demonstrated rapid turnover of AUF1.ARE complexes in solution, suggesting that these interactions are very dynamic in character. Taken together, these data support a model where ARE-dependent oligomerization of AUF1 may function to nucleate the formation of a trans-acting, RNA-destabilizing complex in vivo.

The Destabilization of IL-2 mRNA by a Premature Stop Codon and Its Differential Stabilization by Trans-Acting Inhibitors of Protein Synthesis Do Not Support a Role for Active Translation in mRNA Stability

To investigate the role that translation plays in the stabilization of the IL-2 mRNA, we inhibited protein synthesis in both cis and trans. To block translation in trans, we utilized the inhibitors puromycin (PUR) and cycloheximide (CHX), which differentially effect polysome structure. We found that CHX enhances the stability of IL-2 mRNA in cells stimulated with anti-TCR Ab alone, but it inhibits CD28-induced message stabilization in costimulated cells. In contrast, PUR had a minimal effect on IL-2 mRNA stability in either the presence or absence of costimulation. The differential effects of these two inhibitors suggest that: 1) CHX is unlikely to stabilize the IL-2 mRNA by inhibiting the expression of a labile RNase; 2) CD28-mediated IL-2 mRNA stabilization does not require translation; and 3) IL-2 mRNA decay is not coupled to translation. To block translation in cis, we generated sequence-tagged IL-2 genomic reporters that contain a premature termination codon (PTC). In both the presence and absence of costimulation, these PTC-containing mRNAs exhibit drastically diminished stability. Interestingly, the addition of CHX but not PUR completely restored CD28-mediated stabilization, suggesting that CHX can block the enhanced decay induced by a PTC. Finally, CHX was able to superinduce IL-2 mRNA levels in anti-TCR Ab-stimulated cells but not in CD28-costimulated cells, suggesting that CHX may also act by other mechanisms.

Post-transcriptional regulation of the DNA damage-inducible gadd45 gene in human breast carcinoma cells exposed to a novel retinoid CD437

The biologically active synthetic retinoid CD437 (6-[3-adamantyl-4-hydroxyphenyl]-2-naphthalene, AHPN) and different human breast carcinoma (HBC) cell lines were used to examine the possible mechanism(s) of gadd45 induction. Northern blot analysis of mRNA isolated from MCF-7, MDA-MB-468 and MDA-MB-231 HBC cell lines demonstrated a progressive increase in the 1.4 kb gadd45 transcript after exposure to 1 microM CD437. Western blot analysis showed increased gadd45 protein levels in MDA-MB-468 HBC cells following exposure to CD437. CD437 increased gadd45 mRNA levels by approximately 20-fold in MDA-MB-468 cells, however, the transcriptional activity was increased approximately 2-3-fold as demonstrated by the human gadd45 promoter-luciferase reporter construct and nuclear run-off assays. Sublines of MDA-MB-468 HBC cells expressing stably integrated GADD45 cDNA fragments were obtained and CD437-dependent induction of GADD45 analyzed. We report that approximately 300 nt located in the 5"-untranslated region (5"-UTR) of gadd45 mRNA are involved in the CD437-dependent 4-fold enhanced stability of gadd45 transcripts. MDA-MB-468 cells were stably transfected with either a plasmid having a CMV promoter-driven rabbit beta-globin gene or plasmids having a CMV promoter-driven chimeric gadd45 5"-UTR-rabbit beta-globin gene, where the entire gadd45 5"-UTR (from +1 to +298) or a 45 bp subfragment of the gadd45 5"-UTR (from +10 to +55) was positioned at the 5"-end of the rabbit beta-globin gene. CD437 was found to up-regulate expression of both the chimeric gadd45 -rabbit beta-globin transcripts, suggesting that cis element(s) involved in the CD437-dependent enhanced stability of gadd45 mRNA are contained in the 45 nt of the 5"-UTR of the gadd45 mRNA.

Characterization of the translation-dependent step during iron-regulated decay of transferrin receptor mRNA

Iron regulates the stability of the mRNA encoding the transferrin receptor (TfR). When iron is scarce, iron regulatory proteins (IRPs) stabilize TfR mRNA by binding to the 3'-untranslated region. High levels of iron induce degradation of TfR mRNA; the translation inhibitor cycloheximide prevents this. To distinguish between cotranslational mRNA decay and a trans effect of translation inhibitors, we designed a reporter system exploiting the properties of the selectable marker gene thymidine kinase (TK). The 3'-untranslated region of human transferrin receptor, which contains all elements necessary for iron-dependent regulation of mRNA stability, was fused to the TK cDNA. In stably transfected mouse fibroblasts, the expression of the reporter gene was perfectly regulated by iron. Introduction of stop codons in the TK coding sequence or insertion of stable stem-loop structures in the leader sequence did not affect on the iron-dependent regulation of the reporter mRNA. This implies that global translation inhibitors stabilize TfR mRNA in trans. Cycloheximide prevented the destabilization of TfR mRNA only in the presence of active IRPs. Inhibition of IRP inactivation by cycloheximide or by the specific proteasome inhibitor MG132 correlated with the stabilization of TfR mRNA. These observations suggest that inhibition of translation by cycloheximide interferes with the rate-limiting step of iron-induced TfR mRNA decay in a trans-acting mechanism by blocking IRP inactivation.

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-α 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-α 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.

The search for trans-acting factors controlling messenger RNA decay

Control of mRNA turnover is an integral component of regulated gene expression. Individual mRNAs display a wide range of stabilities, which in many cases have been linked to discrete sequence elements. The most extensively characterized determinants of rapid constitutive mRNA turnover in mammalian systems are A + U-rich elements (AREs), first identified in the 3' untranslated regions of many cytokine/lymphokine and protooncogene mRNAs. In this article, we describe recent advances in the characterization of ARE-directed mRNA turnover, including links to deadenylation kinetics and functional heterogeneity among AREs from different mRNAs. We then describe strategies employed in the search for trans-acting factors interacting with these elements. Using such techniques, an ARE-binding activity capable of accelerating c-myc mRNA turnover in vitro was identified, and named AUF1. Subsequent cloning and characterization revealed that AUF1 exists as a family of four proteins formed by alternative splicing of a common pre-mRNA and appears to function as part of a multisubunit trans-acting complex to promote ARE-directed mRNA turnover. Investigations using several systems have demonstrated that AUF1 expression and/or activity correlate with rapid decay of ARE-containing mRNAs, and that both expression and activity of AUF1 are regulated by developmental and signal transduction mechanisms.

An in vitro system using HeLa cytoplasmic extracts that reproduces regulated mRNA stability

The pathways and machinery involved in the regulated turnover of mRNAs in mammalian cells are largely unknown. We have developed an in vitro system using HeLa cytoplasmic S100 extracts and exogenous polyadenylated RNA substrates that faithfully reproduces in vivo aspects of regulated mRNA turnover. RNA substrates for use in the system that contain a poly(A) tail precisely at their 3' end can be readily prepared using a ligation-polymerase chain reaction approach. The system also uses standard cytoplasmic S100 extracts that are activated through the sequestration of poly(A)-binding proteins by the addition of cold poly(A) RNA. On incubation in the system, the poly(A) tail is removed from RNA substrates by a sequence-specific deadenylase activity and the body of the transcript is ultimately degraded in the system with no apparent intermediates by an ATP-dependent ribonulceolytic activity. AU-rich destability elements can regulate the rates of both deadenylation and degradation in the system. This in vitro system, therefore, should allow the elucidation of pathways of mRNA turnover, identification of the cellular factors involved, and insights into the mechanisms that regulate the half-life of a mRNA.

Tissue-specific and developmental regulation of transforming growth factor-beta1 expression in fetal lamb ductus arteriosus endothelial cells

We previously established that increased hyaluronan synthesis in ductus arteriosus (DA) compared with aorta (Ao) endothelial cells (EC) from early gestation fetal lambs (100-d, term = 145 d) is transforming growth factor-beta (TGF-beta)-dependent. We now address whether this is associated with tissue-specific and developmentally up-regulated expression of TGF-beta1 in the 100-d DA EC. Immunoprecipitation revealed TGF-beta synthesis doubled in DA versus Ao EC from 100-d gestation lambs (p < 0.05). In 138-d DA EC, TGF-beta protein levels were reduced (p < 0.05) and comparable to those in Ao cells. Western immunoblotting with a beta1 isoform-specific antibody confirmed these differences as being related to TGF-beta1. Northern blot analysis demonstrated that TGF-beta1 mRNA levels were slightly but not significantly increased in 100-d DA compared with Ao EC, despite its short half-life in DA (9.5 h) versus Ao EC (20 h). TGF-beta1 mRNA levels were reduced in 138-d DA and Ao EC (p < 0.05), and the mRNA half-life was comparable in DA (9 h) versus Ao (13 h). Nuclear run-on analysis confirmed increased TGF-beta1 mRNA transcription in 100-d DA versus Ao and 138-d DA EC. Thus, up-regulated TGF-beta1 expression in 100-d DA compared with Ao cells is due to increased transcription and translation of a relatively unstable mRNA, and its down-regulation in 138-d DA and Ao EC is related to reduced mRNA transcription and stability, respectively.

An interleukin-1 loop is induced in human skin fibroblasts upon stress relaxation in a three-dimensional collagen gel but is not involved in the up-regulation of matrix metalloproteinase 1

The integrin-mediated stress relaxation as it occurs in a retracting three-dimensional collagen gel (RCG) is accompanied by a large up-regulation of the interstitial collagenase, matrix metalloproteinase 1 ((MMP-1), EC 3.4.24.7), regulated notably by interleukin-1 (IL-1), phorbol esters, and cytoskeleton-disrupting drugs as cytochalasin D (CD). The repression of MMP-1 up-regulation in RCG by cycloheximide suggested the participation in the regulation process of a de novo synthesized intermediary component. We demonstrate here that culture of human skin fibroblasts in RCG or in CD- and 12-O-tetradecanoylphorbol-13-acetate (TPA)-treated monolayers resulted in the activation of an IL-1 autocrine feedback loop that was switched off by the naturally occurring IL-1 receptor antagonist (IL-1RA), a blocker of the common IL-1 receptor. The IL-1RA did not suppress the MMP-1 up-regulation induced in RCG nor in CD-treated cells, indicating that the up-regulation of MMP-1 and the IL-1 autocrine loop occurred in an independent way, while the TPA-induced MMP-1 expression was suppressed by the receptor antagonist. The RCG- as well as the TPA-, IL-1-, and CD-induced up-regulation of both MMP-1 and IL-1 was totally suppressed by protein tyrosine kinases inhibitors. In contrast bisindoylmaleimide, at a concentration (5 microM) that inhibits the TPA-induced protein kinase C activity, suppressed the CD-induced MMP-1 expression but did not or barely altered that induced in RCG or by IL-1. None of the other tested inhibitors of a variety of signaling pathways including those used by integrins was able to suppress the RCG or CD-induced MMP-1. These results point to a potent regulation of MMP-1 by mechanical stress relaxation, a process depending on de novo protein synthesis and occurring independently of the activation of an IL-1 autocrine feedback loop.

hnRNP C increases amyloid precursor protein (APP) production by stabilizing APP mRNA

We have previously shown that heterogeneous nuclear ribonucleoprotein C (hnRNP C) and nucleolin bound specifically to a 29 nt sequence in the 3'-untranslated region of amyloid precursor protein (APP) mRNA. Upon activation of peripheral blood mononuclear cells, hnRNP C and nucleolin acquired APP mRNA binding activity, concurrent with APP mRNA stabilization. These data suggested that the regulated interaction of hnRNP C and nucleolin with APP mRNA controlled its stability. Here we have directly examined the role of the cis element and trans factors in the turnover and translation of APP mRNA in vitro . In a rabbit reticulocyte lysate (RRL) translation system, a mutant APP mRNA lacking the 29 nt element was 3-4-fold more stable and synthesized 2-4-fold more APP as wild-type APP mRNA. Therefore, the 29 nt element functioned as an APP mRNA destabilizer. RNA gel mobility shift assays with the RRL suggested the presence of endogenous nucleolin, but failed to show hnRNP C binding activity. However, wild-type APP mRNA was stabilized and coded for 6-fold more APP when translated in an RRL system supplemented with exogenous active hnRNP C. Control mRNAs lacking the 29 nt element were unaffected by hnRNP C supplementation. Therefore, occupancy of the 29 nt element by hnRNP C stabilized APP mRNA and enhanced its translation.

Regulation of eukaryotic messenger RNA turnover

We have demonstrated the existence of multiple mRNA binding proteins that interact specifically with defined regions in posttranscriptionally regulated mRNAs. These domains appear to be destabilizers whose function can be attenuated by the interaction with the specific binding proteins. Thus, the ability to alter mRNA decay rates on demand, given different environmental or intracellular conditions, appears to be mediated by controlling the localization, activity, and overall function of the cognate binding protein. Based on our limited experience, we predict that most, if not all, of similarly regulated mRNAs will ultimately be found to interact with regulatory mRNA binding proteins. Under conditions whereby the mRNA binding proteins are constitutively active (e.g., tumor cell lines), abnormal mRNA decay will result, with accumulation and overtranslation. Such appears to be the case for cytokines and possibly amyloid protein precursor mRNAs in cancer and Alzheimer's disease, respectively. Conversely, mutagenesis of these critical 3' untranslated region elements will likely have comparable deleterious effects on the regulation of gene expression. To the extent that such derangements exist in human disease, attention to understanding the mechanistic detail at this level may provide insights into the development of appropriate therapeutics or treatment strategies.

The regulation mechanism of HLA class II gene expression at the level of mRNA stability

The number of major histocompatibility complex (MHC) class II antigens may be regulated at different levels. Although transcriptional regulation has been studied most intensely, evidence for control mechanisms acting on the stability of MHC class II mRNAs has been reported. We have previously shown, in fact, that the half-life of MHC class II mRNA rapidly decreases in Raji cells upon inhibition of translation by cycloheximide; further data indicated that this effect was not correlated with the inhibition of the synthesis of trans-acting protein(s) required for mRNA stability. In the present work, we developed an in vitro mRNA decay assay system to measure HLA-DRA mRNA stability and used inhibitors of protein synthesis affecting different steps of the process of translation in order to discriminate among possible mechanisms determining controlled MHC class II mRNA hydrolysis. We found that HLA-DRA mRNA associated with polysomes derived from cells treated with either puromycin (which causes dispersion of polysomes and accumulation of monosomes) or cycloheximide (which slows down translation causing ribosome stalling) is more rapidly degraded than in the absence of protein synthesis inhibitors. On the basis of our findings, we suggest that arrest of protein synthesis per se exposes the HLA-DRA mRNA molecules to degradative activities co-sedimenting with the polysomal fraction.

Posttranscriptional Regulation of the Sesbania rostrata Early Nodulin Gene SrEnod2 by Cytokinin

The mRNA from the Sesbania rostrata early nodulin gene SrEnod2 accumulates in response to cytokinin application. Nuclear run-on assays using isolated root nuclei have shown that this accumulation occurs posttranscriptionally, and northern blot analysis of nuclear and total RNA levels revealed that it occurs primarily in the cytoplasm and not in the nucleus. After cytokinin enhancement of SrEnod2 mRNA accumulation and the subsequent removal of cytokinin, the levels of SrEnod2 mRNA did not return to basal levels, but oscillated over a 36-h time course. Application of the translational inhibitor cycloheximide was found to inhibit the enhancement of SrEnod2 mRNA accumulation by cytokinin and to cause its rapid decay. Okadaic acid and staurosporine, inhibitors of protein phosphatases and kinases, respectively, also inhibited cytokinin enhancement of SrEnod2 mRNA accumulation. In addition, okadaic acid was found to cause a decrease in SrEnod2 mRNA levels. These results provide evidence for a posttranscriptional mechanism of cytokinin enhancement of SrEnod2 mRNA accumulation, which appears to require concurrent protein synthesis, to involve protein phosphatases and kinases, and to occur primarily in the cytoplasm of the plant cell.

Turnover and translation of in vitro synthesized messenger RNAs in transfected, normal cells

We have developed a novel system to examine intracellular mRNA decay pathways in the absence of transcriptional blockade. In vitro transcribed, capped, and adenylated granulocyte-macrophage colony stimulating factor (GM-CSF) or globin mRNAs were introduced by particle-mediated gene transfer into primary cultures of normal peripheral blood mononuclear cells. Transfected wild-type, human GM-CSF (hGM-AUUUA) mRNA decayed rapidly (t1/2 = 9 min), while a mutated version lacking AUUUA repeats (hGM-AUGUA) was significantly more stable (t1/2 = 30 min). A truncated GM-CSF mRNA lacking the entire 3'-UTR (hGM-Delta3'-UTR) was still more stable (t1/2 = 80 min) demonstrating the existence of non-AUUUA, 3'-UTR destabilizing domains. Transfected beta-globin mRNA was very stable, decaying with a half-life of >360 min. Transfected mRNAs were >90% polysome associated with transgenic protein detectable within 15 min of transfection. The most stable GM-CSF mRNAs were not associated with maximal GM-CSF protein production. Agents known or hypothesized to interfere with mRNA decay, including cycloheximide, phorbol ester, or actinomycin D, stabilized both hGM-AUUUA and hGM-AUGUA mRNAs. These data demonstrate the presence of 3'-UTR, destabilizing, and translational regulatory elements outside of the AUUUA repeats and unambiguously show that actinomycin D at concentrations commonly used to inhibit transcription stabilizes cytokine mRNAs.