Cyclic nucleotide regulation of PAI-1 mRNA stability. Identification of cytosolic proteins that interact with an a-rich sequence

The 1H, 15N and 13C resonance assignments of the C-terminal domain of Serpine mRNA binding protein 1 (SERBP1)

SERBP1 is a multifunctional mRNA-binding protein that has been shown to play a regulatory role in a number of biological processes such as thrombosis, DNA damage repair, and the cellular response to nutrient deprivation. Additionally, SERBP1 is upregulated in glioblastoma, leukemia as well as liver, prostrate and ovarian cancers where it has been implicated in metastatic disease and poor patient outcomes. SERBP1 binds target mRNA, stabilizing and regulating the post-translational expression of the transcript. Since SERBP1 lacks canonical RNA-binding motifs such as RRM domains or zinc fingers, its target recognition and binding mechanisms are not well understood. Recent reports suggest that it is capable of recognizing both RNA sequence motifs and structured domains. Here we report the production and purification of the intrinsically disordered C-terminal domain of SERBP1, the assignment of the 1H, 13C, 15N backbone resonances of the protein by solution-state NMR, and secondary structure predictions. We show that the protein is not entirely disordered and identify an α-helix that was stable under the experimental conditions. This work is the first step toward understanding the structural basis underpinning the molecular mechanisms of SERBP1 functions, particularly interactions with mRNA targets.

The RNA-binding protein SERBP1 interacts selectively with the signaling protein RACK1

The RACK1 protein interacts with numerous proteins involved in signal transduction, the cytoskeleton, and mRNA splicing and translation. We used the 2-hybrid system to identify additional proteins interacting with RACK1 and isolated the RNA-binding protein SERBP1. SERPB1 shares amino acid sequence homology with HABP4 (also known as Ki-1/57), a component of the RNA spicing machinery that has been shown previously to interact with RACK1. Several different isoforms of SERBP1, generated by alternative mRNA splicing, interacted with RACK1 with indistinguishable interaction strength, as determined by a 2-hybrid beta-galactosidase assay. Analysis of deletion constructs of SERBP1 showed that the C-terminal third of the SERBP1 protein, which contains one of its two substrate sites for protein arginine N-methyltransferase 1 (PRMT1), is necessary and sufficient for it to interact with RACK1. Analysis of single amino acid substitutions in RACK1, identified in a reverse 2-hybrid screen, showed very substantial overlap with those implicated in the interaction of RACK1 with the cAMP-selective phosphodiesterase PDE4D5. These data are consistent with SERBP1 interacting selectively with RACK1, mediated by an extensive interaction surface on both proteins.

Posttranscriptional regulation of expression of plasminogen activator inhibitor type-1 by sphingosine 1-phosphate in HepG2 liver cells

Altered expression of plasminogen activator inhibitor type-1 (PAI-1), a major physiologic inhibitor of fibrinolysis, is implicated in the progression of atherosclerosis. Sphingosine 1-phosphate (S1P) regulates expression of diverse genes and alters expression of PAI-1 in several types of cells. However, the nature of posttranscriptional regulation of expression of PAI-1 by S1P has not yet been thoroughly elucidated. The present study was undertaken to determine whether S1P has important effects on the posttranscriptional regulation of PAI-1 expression. To evaluate this possibility, we determined promoter activity, mRNA levels, 3'-untranslated region (UTR) activity, and protein levels of PAI-1 in HepG2 cells. S1P increased PAI-1 promoter activity and the expression of PAI-1 mRNA within 4h of exposure. It decreased the expression of PAI-1 mRNA and the accumulation of PAI-1 protein into the media in 24h. Human PAI-1 mRNA exists in two subspecies (3.2 and 2.2kb). S1P decreased the baseline luciferase activity of the 1kb fragment of the 3' terminus (+2177 to 3176nt) of the 3'-UTR of the 3.2kb PAI-1 mRNA [3'-UTR (+2177-3176)]. S1P decreased expression of PAI-1 protein, presumably by regulating PAI-1 expression at the posttranscriptional level thereby affecting mRNA stability. SERPINE1 mRNA binding protein (SERBP1) and ARE3 in the 3'-UTR were involved in the posttranscriptional regulation by S1P. Our data suggest that S1P can destabilize 3.2kb PAI-1 mRNA through specific effects on the 3'-UTR. These effects appear to involve SERBP1 leading to decreased expression of PAI-1 protein.

A novel mRNA binding protein complex promotes localized plasminogen activator inhibitor-1 accumulation at the myoendothelial junction

OBJECTIVE

Plasminogen activator inhibitor-1 (PAI-1) has previously been shown to be key to the formation of myoendothelial junctions (MEJs) in normal and pathological states (eg, obesity). We therefore sought to identify the mechanism whereby PAI-1 could be selectively accumulated at the MEJ.

METHODS AND RESULTS

We identified PAI-1 protein enrichment at the MEJ in obese mice and in response to tumor necrosis factor (TNF-α) with a vascular cell coculture. However, PAI-1 mRNA was also found at the MEJ and transfection with a PAI-1-GFP with TNF-α did not demonstrate trafficking of the protein to the MEJ. We therefore hypothesized the PAI-1 mRNA was being locally translated and identified serpine binding protein-1, which stabilizes PAI-1 mRNA, as being enriched in obese mice and after treatment with TNF-α, whereas Staufen, which degrades PAI-1 mRNA, was absent in obese mice and after TNF-α application. We identified nicotinamide phosphoribosyl transferase as a serpine binding protein-1 binding partner with a functional τ-like microtubule binding domain. Application of peptides against the microtubule binding domain significantly decreased the number of MEJs and the amount of PAI-1 at the MEJ.

CONCLUSIONS

We conclude that PAI-1 can be locally translated at the MEJ as a result of a unique mRNA binding protein complex.

Involvement of miR-30c and miR-301a in immediate induction of plasminogen activator inhibitor-1 by placental growth factor in human pulmonary endothelial cells

PAI-1 (plasminogen activator inhibitor-1) is a key physiological inhibitor of fibrinolysis. Previously, we have reported PlGF (placental growth factor)-mediated transcriptional up-regulation of PAI-1 (SERPINE1) mRNA expression via activation of HIF-1α (hypoxia-inducible factor-1α) and AP-1 (activator protein-1) in HPMVECs (human pulmonary microvascular endothelial cells), which resulted in elevated PAI-1 in humans with SCA (sickle cell anaemia). In the present study, we have identified the role of post-transcriptional mechanism(s) of PlGF-mediated accumulation of PAI-1 mRNA in HPMVECs by examining the role of microRNAs (miRNAs/miRs) in PlGF-induced PAI-1 mRNA stability. Our results show reduced expression of miR-30c and miR-301a, but not of miR-99a, in response to PlGF, which have evolutionarily conserved binding sites in the 3'-UTR (3'-untranslated region) of PAI-1 mRNA. Transfection of anti-miR-30c or anti-miR-301a oligonucleotides resulted in increased PAI-1 mRNA levels, which were increased further with PlGF stimulation. Conversely, overexpression of pre-miR-30c or pre-miR-301a resulted in an attenuation of PlGF-induced PAI-1 mRNA and protein levels. Luciferase reporter assays using wild-type and mutant 3'-UTR constructs confirmed that the PAI-1 3'-UTR is indeed a direct target of miR-30c and miR-301a. Finally, plasma levels of miR-30c and miR-301a were significantly down-regulated in patients with SCA compared with normal controls. These results provide a post-transcriptional regulatory mechanism of PlGF-induced PAI-1 elevation.

Induction of tissue factor by urokinase in lung epithelial cells and in the lungs

RATIONALE

Urokinase-type plasminogen activator (uPA) regulates extracellular proteolysis in lung injury and repair. Although alveolar expression of uPA increases, procoagulant activity predominates.

OBJECTIVES

This study was designed to investigate whether uPA alters the expression of tissue factor (TF), the major initiator of the coagulation cascade, in lung epithelial cells (ECs).

METHODS

Bronchial, primary airway ECs and C57B6 wild-type, uPA-deficient (uPA(-/-)) mice were exposed to phosphate-buffered saline, uPA, or LPS. Immunohistochemistry, protein, cellular, and molecular techniques were used to assess TF expression and activity.

MEASUREMENTS AND MAIN RESULTS

uPA enhanced TF mRNA and protein expression, and TF-dependent coagulation in lung ECs. uPA-induced expression of TF involves both increased synthesis and enhanced stabilization of TF mRNA. uPA catalytic activity had little effect on induction of TF. By contrast, deletion of the uPA receptor binding growth factor domain from uPA markedly attenuated the induction of TF, suggesting that uPA receptor binding is sufficient for TF induction. Lung tissues of uPA-deficient mice expressed less TF protein and mRNA compared with wild-type mice. In addition, intratracheal instillation of mouse uPA increased TF mRNA and protein expression and accelerated coagulation in lung tissues. uPA(-/-) mice exposed to LPS failed to induce TF.

CONCLUSIONS

uPA increased TF expression and TF-dependent coagulation in the lungs of mice. We hypothesize that uPA-mediated induction of TF occurs in lung ECs to promote increased fibrin deposition in the airways during acute lung injury.

Post-transcriptional regulation of plasminogen activator inhibitor type-1 expression in human pleural mesothelial cells

The plasminogen activator inhibitor type-1 (PAI-1) effectively blocks the activities of free and receptor-bound urokinase-type plasminogen activator. Incubation of cultured human pleural mesothelial (Met5A) cells with TGF-beta increased PAI-1 protein. TGF-beta, phorbol myristate acetate, and the translation inhibitor cycloheximide induced PAI-1 mRNA and slowed its degradation, suggesting that PAI-1 mRNA could be regulated by interaction of a PAI-1 binding protein (PAI-1 mRNABp) with PAI-1 mRNA. We found that an approximately 60 kD cytoplasmic PAI-1 mRNABp is detectable in cytoplasmic extracts of MeT5A human pleural mesothelial and malignant mesothelioma cells. The PAI-1 mRNABp specifically binds to a 33-nt sequence in the 3' untranslated region of PAI-1 mRNA. Insertion of this 33-nt sequence destabilizes otherwise stable beta-globin mRNA, indicating that the binding sequence accelerates decay of endogenous PAI-1 mRNA. Competitive inhibition by overexpression of the 33-nt binding sequence in MeT5A cells reduced PAI-1 mRNA decay and increased PAI-1 protein and mRNA expression, indicating that the PAI-1 mRNABp destabilizes PAI-1 mRNA by its interaction with the endogenous 33-nt binding sequence. Incubation of Met5A cells with TGF-beta attenuated the interaction of the PAI-1 mRNABp with the 33-nt sequence. By conventional and affinity purification, we isolated the PAI-1 mRNABp and confirmed its identity as 6-phospho-d-gluconate-NADP oxidoreductase, which specifically interacts with the full-length and the 33-nt sequence of the PAI-1 mRNA 3' untranslated region. This newly recognized pathway could influence expression of PAI-1 by mesothelial or mesothelioma cells at the level of mRNA stability in the context of pleural inflammation or malignancy.

The fibrinolytic system and the regulation of lung epithelial cell proteolysis, signaling, and cellular viability

The urokinase-type plasminogen activator (uPA), its receptor (uPAR), and plasminogen activator inhibitor-1 (PAI-1) are key components of the fibrinolytic system and are expressed by lung epithelial cells. uPA, uPAR, and PAI-1 have been strongly implicated in the pathogenesis of acute lung injury (ALI) and pulmonary fibrosis. Recently, it has become clear that regulation of uPA, uPAR, and PAI-1 occurs at the posttranscriptional level of mRNA stability in lung epithelial cells. uPA further mediates its own expression in these cells as well as that of uPAR and PAI-1 through induction of changes in mRNA stability. In addition, uPA-mediated signaling controls the expression of the tumor suppressor protein p53 in lung epithelial cells at the posttranslational level. p53 has recently been shown to be a trans-acting uPA, uPAR, and PAI-1 mRNA-binding protein that regulates the stability of these mRNAs. It is now clear that signaling initiated by uPA mediates dose-dependent regulation of lung epithelial cell apoptosis and likewise involves changes in p53, uPA, uPAR, and PAI-1 expression. These findings demonstrate that the uPA-uPAR-PAI-1 system of lung epithelial cells mediates a broad repertoire of responses that encompass but extend well beyond traditional fibrinolysis, involve newly recognized interactions with p53 that influence the viability of the lung epithelium, and are thereby implicated in the pathogenesis of ALI and its repair.

Fibrinolysis, inflammation, and regulation of the plasminogen activating system

The maintenance of a given physiological process demands a coordinated and spatially regulated pattern of gene regulation. This applies to genes encoding components of enzyme cascades, including those of the plasminogen activating system. This family of proteases is vital to fibrinolysis and dysregulation of the expression pattern of one or more of these proteins in response to inflammatory events can impact on hemostasis. Gene regulation occurs on many levels, and it is apparent that the genes encoding the plasminogen activator (fibrinolytic) proteins are subject to both direct transcriptional control and significant post-transcriptional mechanisms. It is now clear that perturbation of these genes at either of these levels can dramatically alter expression levels and have a direct impact on the host's response to a variety of physiological and pharmacological challenges. Inflammatory processes are well known to impact on the fibrinolytic system and to promote thrombosis, cancer and diabetes. This review discusses how inflammatory and other signals affect the transcriptional and post-transcriptional expression patterns of this system, and how this modulates fibrinolysis in vivo.

Hepatitis C virus NS2 protein activates cellular cyclic AMP-dependent pathways

Chronic infection of the hepatitis C virus (HCV) leads to liver cirrhosis and cancer. The mechanism leading to viral persistence and hepatocellular carcinoma, however, has not been fully understood. In this study, we show that the HCV infection activates cellular cAMP-dependent pathways. Expression of a luciferase reporter gene controlled by a basic promoter with the cAMP response element (CRE) was significantly elevated in human hepatoma Huh-7 cells infected with the HCV JFH1. Analysis with viral subgenomic replicons indicated that the HCV NS2 protein is responsible for the effect. Furthermore, the level of cellular transcripts whose stability is known to be regulated by cAMP was specifically reduced in cells harboring NS2-expressing replicons. These results allude to the HCV NS2 protein having a novel function of regulating cellular gene expression and proliferation through the cAMP-dependent pathway.

Regulation of alpha-synuclein expression in alcohol-preferring and -non preferring rats

The alpha-synuclein (Snca) gene is expressed at higher levels in alcohol-naïve, inbred alcohol-preferring (iP) rats than in alcohol-non preferring (iNP) rats. Snca modulates dopamine transmission and the dopamineregic system, which play a role in mediating the rewarding properties of alcohol consumption. Thus, understanding regulation of Snca gene expression could provide insight into the relationship of Snca and alcohol consumption. To study regulation of rat Snca expression, 1,912 bp of the iP and iNP 5'-regions were cloned and sequenced. 5'-rapid amplification of cDNA ends (RACE), primer extension and RT-PCR mapped three transcription start site clusters (clusters TSS1, TSS2 and TSS3), suggesting that the Snca proximal promoter region has a complex architecture. This proximal promoter region has three TATA-less core promoters containing SP1 binding sites, initiator elements and downstream core promoter elements, which are often located in such promoters. Snca-luc constructs transiently transfected into SK-N-SH neuroblastoma cells showed that the region from - 1,912 to - 1,746 contained a strong core promoter, and that the entire approximately 2 kb region had significant promoter activity. Five polymorphisms identified between the iP and iNP in the proximal promoter region did not influence differential expression between the strains. In contrast, a single nucleotide polymorphism (SNP) at + 679 in the 3'-untranslated region (UTR) resulted in a 1.3-fold longer half-life of iP mRNA compared with iNP mRNA, which is consistent with the differential expression observed between the iP and iNP strains. These results suggest that regulation of rat Snca gene expression is complex and may contribute to alcohol preference in the iP rats.

Inhibitory effect of TNF-α on the intestinal absorption of galactose

Sepsis is a systemic response to infection in which toxins, such as bacterial lipopolysaccharide (LPS), stimulate the production of inflammatory mediators like the cytokine tumor necrosis factor alpha (TNF-alpha). Previous studies from our laboratory have revealed that LPS inhibits the intestinal absorption of L-leucine and D-fructose in rabbit when it was intravenously administered, and that TNF-alpha seems to mediate this effect on amino acid absorption. To extend this work, the present study was designed to evaluate the possible effect of TNF-alpha on D-galactose intestinal absorption, identify the intracellular mechanisms involved and establish whether this cytokine mediates possible LPS effects. Our findings indicate that TNF-alpha decreases D-galactose absorption both in rabbit intestinal tissue preparations and brush-border membrane vesicles. Western blot analysis revealed reduced amounts of the Na+/glucose cotransporter (SGLT1) protein in the plasma membrane attributable to the cytokine. On the contrary, TNF-alpha increased SGLT1 mRNA levels. Specific inhibitors of the secondary messengers PKC, PKA, the MAP kinases p38 MAP, JNK, MEK1/2 as well as the proteasome, diminished the TNF-alpha-evoked inhibitory effect. LPS inhibition of the uptake of the sugar was blocked by a TNF-alpha antagonist. In conclusion, TNF-alpha inhibits D-galactose intestinal absorption by decreasing the number of SGLT1 molecules at the enterocyte plasma membrane through a mechanism in which several protein-like kinases are involved.

Regulation of urokinase receptor mRNA stability by hnRNP C in lung epithelial cells

Increased urokinase receptor (uPAR) expression as well as stabilisation of uPAR mRNA contribute to the pathogenesis of lung inflammation and neoplasia. Post-transcriptional regulation of uPAR mRNA involves interaction of both coding and 3'-UTR sequences with regulatory uPAR mRNA binding proteins (Bps). In order to identify novel regulatory interactions, we performed gel mobility shift and UV cross-linking assays and found two distinct uPAR mRNA-protein complexes. We identified a rapidly migrating 40 kDa uPAR mRNABp that selectively bound a 110 nucleotide (nt) fragment of the uPAR mRNA 3'UTR. Chimeric beta-globin/uPAR mRNA containing the 110 nt 40 kDa protein binding fragment destabilised stable beta-globin mRNA with a rate of decay identical to that of chimeric beta-globin/uPAR containing the full uPAR 3'UTR. The 40 kDa uPAR 3'UTR Bp was purified using poly (U) sepharose and identified as heterogeneous nuclear ribonucleoprotein C (hnRNPC). Finally, we confirmed its interaction with the uPAR mRNA 3' UTR by gel mobility supershift assay using an anti-hnRNPC antibody. Direct in vivo interaction of hnRNPC with the uPAR mRNA 3'UTR was demonstrated by immunoprecipitation and combined RT PCR-Southern blotting assay. Co-transfection of hnRNPC cDNA in Beas2B cells reversed destabilisation of chimeric beta-globin/uPAR 3'UTR mRNA and its over-expression also induced uPAR protein and mRNA expression through stabilisation of uPAR mRNA. These observations indicate a novel mechanism of uPAR gene regulation in lung epithelial cells in which cis elements within a 110 nt uPAR mRNA 3'UTR sequence interact with hnRNPC to regulate uPAR mRNA stability.

Virus induction of heat shock protein 70 reflects a general response to protein accumulation in the plant cytosol

Different cytoplasmically replicating RNA viruses were shown to induce a specific subset of heat-inducible heat shock protein 70 (HSP70) genes in Arabidopsis (Arabidopsis thaliana). To identify the inducing principle, a promoterreporter system was developed for the facile analysis of differentially responding Arabidopsis HSP70 genes, by infiltration into Nicotiana benthamiana leaves. Through transient expression of individual viral cistrons or through deletion analysis of a viral replicon, we were unable to identify a unique inducer of HSP70. However, there was a positive correlation between the translatability of the test construct and the differential induction of HSP70. Since these data implied a lack of specificity in the induction process, we also expressed a random series of cytosolically targeted Arabidopsis genes and showed that these also differentially induced HSP70. Through a comparison of different promoterreporter constructs and through measurements of the steady-state levels of the individual proteins, it appeared that the HSP70 response reflected the ability of the cytosol to sense individual properties of particular proteins when expressed at high levels. This phenomenon is reminiscent of the unfolded protein response observed when the induced accumulation of proteins in the endoplasmic reticulum also induces a specific suite of chaperones.

A polymorphism in the 3' untranslated region of the gene for tumor necrosis factor receptor 2 modulates reporter gene expression

The gene encoding the human TNF alpha receptor (TNFR) 2 contains polymorphisms in the 3' untranslated region (UTR). Previous studies have shown that some variant alleles in this region are associated with obesity and insulin resistance. However, the effect of these polymorphisms on the expression of TNFR2 has not been studied to date. To examine the role played by different haplotypes in the control of TNFR2 expression (haplotypes A1-A5, referring to nucleotides 1663 G/A, 1668 T/G, and 1690 T/C), we introduced these sequences into the 3'-UTR of a heterologous reporter gene and expressed the corresponding constructs in a human T-cell line. We demonstrate that a 485-nt fragment of the TNFR2 3'-UTR that contains a U-rich region decreases reporter expression and that haplotypes A1-A4 exert a stronger effect than A5. Furthermore, time-course assays of mRNA stability using actinomycin D revealed that haplotypes A1-A4 destabilize the mRNA. The proximal TNFR2 3'-UTR, independently of haplotype differences, responded to T-cell activation by increasing mRNA decay. Electromobility shift analysis demonstrated that protein(s) found in T-cell extracts bind to the 485-nt fragment. We suggest that an increased rate of TNFR2 mRNA decay protects cells from unrestrained TNF alpha effects and that this protection is weakened in A5 subjects. These findings may explain the association of this haplotype with obesity and increased leptin levels.

Protein synthesis and urokinase mRNA metabolism

Expression of the urokinase-type plasminogen activator (uPA) is under tight regulation by hormones, cytokines and growth factors under physiological conditions. Treatment of lung epithelial (Beas2B) cells with translation inhibitors induces uPA mRNA expression, as well as early response genes. To understand the specific expression and regulation of uPA mRNA, we treated Beas2B cells with cycloheximide (CycD), anisomycin, emitine and puromycin in a time-dependent manner and measured uPA mRNA expression by Northern blotting. All these agents induced uPA mRNA by two- to seven-fold within 3 h after treatment in Beas2B cells. CycD, emitine, puromycin and anisomycin also enhanced uPA mRNA half-life by three- to five-fold in Beas2B cells treated with DRB, an inhibitor of transcription. However, run-on-transcription experiments indicated that these agents failed to induce uPA mRNA transcription indicating that they augment uPA mRNA mainly due to increased stability. Using gel mobility shift, we identified an uPA mRNA binding protein (uPA mRNABp) that selectively binds to uPA mRNA [Gyetko MR, Todd III RF, Wilkinson CC, Sitrin RG: The urokinase receptor is required for human monocyte chemotaxis in vitro. J Clin Invest 93: 1380-1387, 1994]. Binding of both cytoplasmic and nuclear uPA mRNABp to uPA mRNA was abolished after treatment with translation inhibitors, which coincides with the maximal expression of uPA mRNA. We also found a similar decline in HuR and heterogeneous nuclear ribonucleoprotein C (hnRNPC) which are known to stabilize uPA mRNA both in the nuclear and cytosolic compartments. These results strongly suggest that increased uPA mRNA stability induced by translational inhibitors involves the interaction of uPA mRNA with a degrading protein factor rather than increased interaction of proteins that are known to stabilize uPA mRNA. These data also strongly suggests that down-regulation of the uPA-uPA mRNABp interaction by translational inhibitors rather than the translocation of uPA mRNABp contributes to increased uPA mRNA stability. This pathway may regulate uPA-mediated functions of the lung epithelium in the context of inflammation or neoplasia.

Fructose modulates GLUT5 mRNA stability in differentiated Caco-2 cells: role of cAMP-signalling pathway and PABP (polyadenylated-binding protein)-interacting protein (Paip) 2

In intestinal cells, levels of the fructose transporter GLUT5 are increased by glucose and to a greater extent by fructose. We investigated the mechanism by which fructose increases GLUT5 expression. In Caco-2 cells, fructose and glucose increased activity of the -2500/+41 GLUT5 promoter to the same extent. cAMP also activated the GLUT5 promoter. However, if a protein kinase A inhibitor was used to block cAMP signalling, extensive GLUT5 mRNA degradation was observed, with no change in basal transcription levels demonstrating the involvement of cAMP in GLUT5 mRNA stability. Indeed, the half-life of GLUT5 mRNA was correlated ( R2=0.9913) with cellular cAMP levels. Fructose increased cAMP concentration more than glucose, accounting for the stronger effect of fructose when compared with that of glucose on GLUT5 production. We identified several complexes between GLUT5 3'-UTR RNA (where UTR stands for untranslated region) and cytosolic proteins that might participate in mRNA processing. Strong binding of a 140 kDa complex I was observed in sugar-deprived cells, with levels of binding lower in the presence of fructose and glucose by factors of 12 and 6 respectively. This may account for differences in the effects of fructose and glucose. In contrast, the amounts of two complexes of 96 and 48 kDa increased equally after stimulation with either glucose or fructose. Finally, PABP (polyadenylated-binding protein)-interacting protein 2, a destabilizing partner of PABP, was identified as a component of GLUT5 3'-UTR RNA-protein complexes. We conclude that the post-transcriptional regulation of GLUT5 by fructose involves increases in mRNA stability mediated by the cAMP pathway and Paip2 (PABP-interacting protein 2) binding.

Urokinase-mediated posttranscriptional regulation of urokinase-receptor expression in non small cell lung carcinoma

The urokinase-type plasminogen activator (uPA) and its cellular receptor (uPAR) are involved in the proteolytic cascade required for tumor cell dissemination and metastasis, and are highly expressed in many human tumors. We have recently reported that uPA, independently of its enzymatic activity, is able to increase the expression of its own receptor in uPAR-transfected kidney cells at a posttranscriptional level. In fact, uPA, upon binding uPAR, modulates the activity and/or the level of a mRNA-stabilizing factor that binds the coding region of uPAR-mRNA. We now investigate the relevance of uPA-mediated posttranscriptional regulation of uPAR expression in non small cell lung carcinoma (NSCLC), in which the up-regulation of uPAR expression is a prognostic marker. We show that uPA is able to increase uPAR expression, both at protein and mRNA levels, in primary cell cultures obtained from tumor and adjacent normal lung tissues of patients affected by NSCLC, thus suggesting that the enzyme can exert its effect in lung cells. We investigated the relationship among the levels of uPA, uPAR and uPAR-mRNA binding protein(s) in NSCLC. Lung tissue analysis of 35 NSCLC patients shows an increase of both uPA and uPAR in tumor tissues, as compared to adjacent normal tissues, in 27 patients (77%); 19 of these 27 patients also show a parallel increase of the level and/or binding activity of a cellular protein capable of binding the coding region of uPAR-mRNA. Therefore, in tumor tissues, a strong correlation is observed among these 3 parameters, uPA, uPAR and the level and/or the activity of a uPAR-mRNA binding protein. We then suggest that uPA regulates uPAR expression in NSCLC at a posttranscriptional level by increasing uPAR-stability through a cellular factor that binds the coding region of uPAR-mRNA.

Cell and molecular regulation of endothelin-1 production during hepatic wound healing

During hepatic wound healing, activation of key effectors of the wounding response known as stellate cells leads to a multitude of pathological processes, including increased production of endothelin-1 (ET-1). This latter process has been linked to enhanced expression of endothelin-converting enzyme-1 (ECE-1, the enzyme that converts precursor ET-1 to the mature peptide) in activated stellate cells. Herein, we demonstrate up-regulation of 56- and 62-kDa ECE-1 3'-untranslated region (UTR) mRNA binding proteins in stellate cells after liver injury and stellate cell activation. Binding of these proteins was localized to a CC-rich region in the proximal ECE-1 3' UTR base pairs (the 56-kDa protein) and to a region between 60 and 193 base pairs in the ECE-1 3' UTR mRNA (62 kDa). A functional role for the 3' UTR mRNA/protein interaction was established in a series of reporter assays. Additionally, transforming growth factor-beta1, a cytokine integral to wound healing, stimulated ET-1 production. This effect was due to ECE-1 mRNA stabilization and increased ECE-1 expression in stellate cells, which in turn was a result of de novo synthesis of the identified 56- and 62-kDa ECE-1 3' UTR mRNA binding proteins. These data indicate that liver injury and the hepatic wound healing response lead to ECE-1 mRNA stabilization in stellate cells via binding of 56- and 62-kDa proteins, which in turn are regulated by transforming growth factor-beta. The possibility that the same or similar regulatory events are present in other forms of wound healing is raised.

Induction of plasminogen activator inhibitor-1 by urokinase in lung epithelial cells

The plasminogen/plasmin system, urokinase-type plasminogen activator (uPA), its receptor (uPAR), and its inhibitor (PAI-1), influence extracellular proteolysis and cell migration in lung injury or neoplasia. In this study, we sought to determine whether tcuPA (two chain uPA) alters expression of its major inhibitor PAI-1 in lung epithelial cells. The expression of PAI-1 was evaluated at the protein and mRNA level by Western blot, immunoprecipitation, and Northern blot analyses. We found that tcuPA treatment enhanced PAI-1 protein and mRNA expression in Beas2B lung epithelial cells in a time- and concentration-dependent manner. The tcuPA-mediated induction of PAI-1 involves post-transcriptional control involving stabilization of PAI-1 mRNA. Inactivation of the catalytic activity of tcuPA had little effect on PAI-1 induction and the activity of the isolated amino-terminal fragment was comparable with full-length single- or two-chain uPA. In contrast, deletion of either the uPA receptor binding growth factor domain or kringle domain (kringle) from full-length single chain uPA markedly attenuated the induction of PAI-1. Induction of PAI-1 by exposure of lung epithelial cells to uPA is a newly recognized pathway by which PAI-1 could regulate local fibrinolysis and urokinase-dependent cellular responses in the setting of lung inflammation or neoplasia.

Coagulation, fibrinolysis, and fibrin deposition in acute lung injury

OBJECTIVES

To review: a) the role of extravascular fibrin deposition in the pathogenesis of acute lung injury; b) the abnormalities in the coagulation and fibrinolysis pathways that promote fibrin deposition in the acutely injured lung; and c) the pathways that contribute to the regulation of the fibrinolytic system via the lung epithelium, including newly recognized posttranscriptional and urokinase-dependent pathways. Another objective was to determine how novel anticoagulant or fibrinolytic strategies may be used to protect against acute inflammation or accelerated fibrosis in acute lung injury.

DATA SOURCES

Published medical literature.

DATA SUMMARY

Alveolar fibrin deposition is characteristic of diverse forms of acute lung injury. Intravascular thrombosis or disseminated intravascular coagulation can also occur in the acutely injured lung. Extravascular fibrin deposition promotes lung dysfunction and the acute inflammatory response. In addition, transitional fibrin in the alveolar compartment undergoes remodeling leading to accelerated pulmonary fibrosis similar to the events associated with wound healing, or desmoplasia associated with solid neoplasms. In acute lung injury, alveolar fibrin deposition is potentiated by consistent changes in endogenous coagulation and fibrinolytic pathways. Procoagulant activity is increased in conjunction with depression of fibrinolytic activity in the alveolar compartment. Initiation of the procoagulant response occurs as a result of local overexpression of tissue factor associated with factor VII. Depression of fibrinolytic activity occurs as a result of inhibition of urokinase plasminogen activator (uPA) by plasminogen activators, or series inhibition of plasmin by antiplasmins. Locally increased amplification of plasminogen activator inhibitor-1 (PAI-1) is largely responsible for this fibrinolytic defect. Newly described pathways by which lung epithelial cells regulate expression of uPA, its receptor uPAR, and PAI-1 at the posttranscriptional level have been identified. These pathways operate by cis-trans interactions between mRNA binding proteins; regulatory sequences within these mRNAs control their stability. The regulatory mechanisms seem to involve multiple protein-mRNA interactions, and the phosphorylation state of the proteins appears to determine whether complex formation of, or dissociation from, the regulatory sequences occurs. uPA is capable of inducing its own expression in lung epithelial cells as well as that of uPAR and PAI-1-the effects involve posttranscriptional regulatory components. These and related observations have led to the implementation of anticoagulant or fibrinolytic strategies to protect the lung against acute lung injury. The success of new fibrinolytic strategies to block pleural loculation suggests that a similar approach might be used to prevent accelerated pulmonary fibrosis, which can occur in association with many forms of acute lung injury.

CONCLUSIONS

Disordered coagulation and fibrinolysis promote extravascular fibrin deposition in acute lung injury. It is this deposition that characterizes acute lung injury and repair. Expression of uPA, uPAR, and PAI-1 by the lung epithelium, as well as the ability of uPA to induce other components of the fibrinolytic system, involves posttranscriptional regulation. These pathways may contribute to disordered fibrin turnover in the injured lung. The success of anticoagulant or fibrinolytic strategies designed to reverse the abnormalities of local fibrin turnover in acute lung injury supports the inference that abnormalities of coagulation, fibrinolysis, and fibrin deposition have a critical role in the pathogenesis of acute lung injury.

The 3' untranslated region of the new rat synaptic vesicle protein 2B mRNA transcript inhibits translational efficiency

Post-transcriptional regulatory mechanisms have been shown to play a major role in gene expression in eukaryotic cells. Sequences within the 3'-untranslated region (3'UTR) of mRNAs have been described as being important for enhancing or inhibiting message translation. Using fluorescence microscopy, Western blotting and the reverse transcription-polymerase chain reaction (RT-PCR) method, we demonstrate that the 3'UTR of the new rat transcript of synaptic vesicle protein 2B (SV2Bb) mRNA is involved in post-transcriptional regulation of SV2Bb translation. When fused to a reporter gene, this 3'UTR markedly inhibited protein synthesis in transiently transfected cells and this decreased translational efficiency did not occur through changes in mRNA stability. In conclusion, our study gives new insights into unraveling the molecular mechanisms involved in the post-transcriptional regulation of the SV2B gene.

Endothelium and disordered fibrin turnover in the injured lung: newly recognized pathways

OBJECTIVES

To review derangements of pathways of fibrin turnover that promote pathologic fibrin deposition in the acute respiratory distress syndrome and to review the contribution of the endothelium and parenchymal lung cells to the derangements. In addition, to review how these pathways can be exploited in specific clinical circumstances, including sepsis and acute lung injury. Lastly, to review newly recognized posttranscriptional and urokinase-dependent pathways by which the fibrinolytic system is regulated in the lung.

DATA SOURCES

Medical literature published in English from 1966 to present.

DATA SUMMARY

Local abnormalities of fibrin turnover in the injured lung recapitulate the systemic changes observed in sepsis. In both circumstances, the procoagulant response is increased, whereas fibrinolytic activity is concurrently depressed. The increased procoagulant activity is related to tissue factor associated with factor VII/VIIa. Fibrinolytic activity in the vasculature is mainly attributable to tissue plasminogen activator, whereas extravascular fibrinolytic activity in the lung is mainly attributable to urokinase plasminogen activator (uPA). Depressed fibrinolytic activity is in large part attributable to plasminogen activator inhibitor-1. In sepsis, activated protein C is also deficient, potentiating the inflammatory response, coagulopathy, and depressed fibrinolysis. Recombinant human activated protein C (drotrecogin alfa [activated]) was successful as an intervention for sepsis in a recent phase 3 clinical trial (PROWESS). Recently, novel posttranscriptional pathways that regulate expression of uPA, its receptor (uPAR), and plasminogen activator inhibitor-1 have been identified. The responsible mechanisms involve cis-trans interactions between newly recognized messenger RNA (mRNA) binding sequences and mRNA binding proteins. A 51 nucleotide mRNA binding sequence within the coding region of uPAR mRNA interacts with a novel 50-kDa mRNA binding protein to destabilize the message. Sequences within the 3' untranslated region of uPA or plasminogen activator inhibitor-1 mRNA interact with 30- and 60-kDa proteins, respectively, to regulate message stability. All of these pathways operate in lung epithelial cells, and endothelial cells regulate uPA expression through a similar pathway. In addition, uPA itself is capable of inducing expression of other components of the fibrinolytic system, including uPAR. This observation defines another feedback loop that could amplify local fibrinolysis and other uPA- or uPAR-mediated cellular responses, including cellular proteolysis, proliferation, and directed cellular migration.

CONCLUSIONS

Novel posttranscriptional pathways regulate expression of uPA, uPAR, and plasminogen activator inhibitor-1. uPA itself is capable of inducing other components of the fibrinolytic system. Some or all of these newly recognized pathways are operative in endothelial and parenchymal lung cells and may influence disordered fibrin turnover in the injured lung.

Post-transcriptional regulation of gene expression in the plasminogen activation system

The urokinase-mediated plasminogen activation (PA) system has been shown to play a key role in cell migration and tissue invasion by regulating both cell-associated proteolysis and cell-cell and cell-matrix interactions. The expression and activity of the components of this complex system are strictly regulated. The control of the expression occurs both at transcriptional and post-transcriptional levels. This review is focused on the post-transcriptional regulation of gene expression of all components of the PA system.

Identification of a new synaptic vesicle protein 2B mRNA transcript which is up-regulated in neurons by amyloid beta peptide fragment (1-42)

The treatment of Alzheimer's disease (AD) remains a major challenge because of the incomplete understanding of the triggering events that lead to the selective neurodegeneration characteristic of AD brains. Here we describe a new transcript of synaptic vesicle protein 2B (SV2Bb) mRNA that is up-regulated at mRNA level in neurons by amyloid beta peptide (Abeta) fragment (1-42). In comparison to SV2B this new mRNA encodes for the same protein but it has an elongated 3'-untranslated region (3'UTR) that contains several AU-rich (AUR) cis-acting elements which are probably involved in posttranscriptional regulating of SV2Bb translation. In conclusion, alteration of SV2B(b) expression appears to be involved in processes of neuronal degeneration.

Urokinase-type plasminogen activator up-regulates the expression of its cellular receptor through a post-transcriptional mechanism

We have recently reported that the urokinase-type plasminogen activator (uPA) up-regulates the cell surface expression of its own receptor (uPAR) in several cell types, independently of its enzymatic activity. uPA has no effect on kidney 293 cells which do not express uPAR and then cannot bind uPA. Kidney cells, transfected with the coding region of uPAR cDNA, express very large amounts of uPAR and respond to uPA stimulation by regulating uPAR both at mRNA and protein levels. uPA effect occurs also in the presence of the transcriptional inhibitor dichloro-ribobenzimidazole, whereas it is abolished by the protein synthesis inhibitor cycloheximide. Moreover, uPA-dependent uPAR up-regulation correlates with the increase of a complex between the coding region of uPAR mRNA and an unknown cellular factor. We then propose that uPA regulates uPAR expression at a post-transcriptional level, by promoting the binding of uPAR mRNA to a stabilizing factor.

A conserved 3'-untranslated element mediates growth regulation of DNA methyltransferase 1 and inhibits its transforming activity

Ectopic expression of DNA methyltransferase 1 (DNMT1) has been proposed to play an important role in cancer. dnmt1 mRNA is undetectable in growth-arrested cells but is induced upon entrance into the S phase of the cell cycle, and until now, the mechanisms responsible for this regulation were unknown. In this report, we demonstrate that the 3'-untranslated region (3'-UTR) of the dnmt1 mRNA can confer a growth-dependent regulation on its own message as well as a heterologous beta-globin mRNA. Our results indicate that a 54-nucleotide highly conserved element within the 3'-UTR is necessary and sufficient to mediate this regulation. Cell-free mRNA decay experiments demonstrate that this element increases mRNA turnover rates and does so to a greater extent in the presence of extracts prepared from arrested cells. A specific RNA-protein complex is formed with the 3'-UTR only in growth-arrested cells, and a UV cross-linking analysis revealed a 40-kDa protein (p40), the binding of which is dramatically increased in growth-arrested cells and is inversely correlated with dnmt1 mRNA levels as cells are induced into the cell cycle. Although ectopic expression of human DNMT1 lacking the 3'-UTR can transform NIH-3T3 cells, inclusion of the 3'-UTR prevents transformation. These results support the hypothesis that deregulated expression of DNMT1 with the cell cycle is important for cellular transformation.

A cis-dominant cyclic nucleotide-dependent regulatory domain in the 3'-untranslated region of Na(+)/glucose cotransporter (SGLT1) mRNA

A 122 nt uridine-rich sequence (URE) in the Na(+)/glucose cotransporter (SGLT1) mRNA 3'-untranslated region is critical for cAMP-dependent message stabilization. Its function was investigated in LLC-PK(1) cells stably expressing beta-globin reporter transcripts. Insertion of the SGLT1 URE downstream from an unrelated destabilizing sequence, the c-fos ARE, evoked cAMP-dependent message stabilization. Stabilization was blocked by a substitution mutation within the SLGT1 URE. These observations indicate that the SGLT1 URE is sufficient to transmit cAMP-dependent, cis-dominant mRNA stabilization in the presence of appropriate trans-acting factors and appears to function independently of the nature of the destabilizing domain.

Identification and cDNA cloning of a novel RNA-binding protein that interacts with the cyclic nucleotide-responsive sequence in the Type-1 plasminogen activator inhibitor mRNA

Incubation of HTC rat hepatoma cells with 8-bromo-cAMP results in a 3-fold increase in the rate of degradation of type-1 plasminogen activator inhibitor (PAI-1) mRNA. We have reported previously that the 3'-most 134 nt of the PAI-1 mRNA is able to confer cyclic nucleotide regulation of message stability onto a heterologous transcript. R-EMSA and UV cross-linking experiments have shown that this 134 nt cyclic nucleotide-responsive sequence (CRS) binds HTC cell cytoplasmic proteins ranging in size from 38 to 76 kDa. Mutations in the A-rich region of the CRS both eliminate cyclic nucleotide regulation of mRNA decay and abolish RN-protein complex formation, suggesting that these RNA-binding proteins may be important regulators of mRNA stability. By sequential R-EMSA and SDS-PAGE we have purified a protein from HTC cell polysomes that binds to the PAI-1 CRS. N-terminal sequence analysis and a search of protein data bases revealed identity with two human sequences of unknown function. We have expressed one of these sequences in E. coli and confirmed that the recombinant protein interacts specifically with the PAI-1 CRS. Mutation of the A-rich portion of the PAI-1 CRS reduces binding by the recombinant PAI-1 RNA-binding protein. The amino acid sequence of this protein includes an RGG box and two arginine-rich regions, but does not include other recognizable RNA binding motifs. Detailed analyses of nucleic acid and protein data bases demonstrate that blocks of this sequence are highly conserved in a number of metazoans, including Arabidopsis, Drosophila, birds, and mammals. Thus, we have described a novel RNA-binding protein that identifies a family of proteins with a previously undefined sequence motif. Our results suggest that this protein, PAI-RBP1, may play a role in regulation of mRNA stability.

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.

Reconstitution of angiotensin receptor mRNA down-regulation in vascular smooth muscle. Post-transcriptional control by protein kinase a but not mitogenic signaling directed by the 5'-untranslated region

Cell surface receptor activation generally leads to changes in mRNA abundance, which may involve regulatory targets in processes working at the post-transcriptional level. Many types of agonists down-regulate vascular smooth muscle angiotensin receptor (AT(1)-R) gene expression, but it is unclear which of these activate post-transcriptional mechanisms. To reconstitute faithfully the normal AT(1)-R mRNA regulatory environment, tetracycline-suppressible promoters drive highly accurate recombinant AT(1)-R mRNA mimics in vascular smooth muscle cells that co-express an endogenous AT(1)-R mRNA. Down-regulation of the latter occurs shortly after stimulating mitogenic receptors or by using forskolin, but only cAMP signaling reduces expression of the recombinant AT(1)-R mRNA. Transcription of the recombinant mRNA is unaffected by cAMP signaling. Deletions of the AT(1)-R mRNA 3'-untranslated region do not impair cAMP-mediated down-regulation. Both loss of function and gain of function mutants show the response is mediated by the 5'-untranslated region. These observations provide the first direct functional evidence for modulation of vascular AT(1)-R gene expression by a mechanism involving a protein kinase A-regulated post-transcriptional process.

Growth state-dependent regulation of plasminogen activator inhibitor type-1 gene expression during epithelial cell stimulation by serum and transforming growth factor-beta1

Transcription of the plasminogen activator inhibitor type-1 (PAI-1) gene appears to be growth state regulated in several cell types (e.g. , Ryan and Higgins, 1993, J Cell Physiol 155:376-384; Mu et al., 1998, J Cell Physiol 174:90-98). Transit of serum-stimulated normal rat kidney (NRK) epthelial cells through the first division cycle after release from quiescence (G(0)) provided a model system to assess the kinetics and mechanisms underlying PAI-1 expression in a growth "activated" phenotype. PAI-1 mRNA transcripts increased by more than 20-fold during the G(0)-->G(1) transition; induced expression had immediate-early response characteristics and abruptly declined prior to the onset of DNA synthesis. Transcriptional activity of the PAI-1 gene paralleled the steady-state mRNA abundance profile during this first synchronized growth cycle after release from quiescence. Although PAI-1 mRNA levels were up-regulated (approximately threefold) upon exposure to several different growth factors, neutralizing antibodies to transforming growth factor-beta1 (TGF-beta1) effectively attenuated the more than ninefold serum-associated PAI-1 inductive response by more than 70% (at both the mRNA transcript and protein levels). Similar to the metabolic requirements for serum-mediated PAI-1 transcription, PAI-1 induction upon addition of TGF-beta1 to quiescent NRK cell cultures was actinomycin D sensitive and resistant to cyclohexamide and puromycin, suggesting a primary mode of transcript control. The response to protein synthesis inhibitors, however, was complex. While cyclohexamide appeared to stabilize, or at least maintain, fetal bovine serum (FBS)- or TGF-beta1-stimulated PAI-1 mRNA levels, puromycin had no such affect. The amplitude and duration of induced PAI-1 expression were the same in either the presence or absence of puromycin. Cyclohexamide when used alone (i.e., in non-FBS- or TGF-beta1-treated cultures), moreover, effectively stimulated PAI-1 induction whereas puromycin was ineffective. Although TGF-beta1 was not a complete mitogen in the NRK cell system, incubation of quiescent renal cell cultures with TGF-beta1, prior to serum stimulation, resulted in a 10- to 12-fold increase in PAI-1 expression coincident with exit out of G(0). These data support a model in which PAI-1 gene expression is closely associated with creation of the growth-activated state and that cell cycle controls appear to be superimposed on the time course of the serum-induced expression of the PAI-1 gene.