Heterogeneity in control of mRNA stability by AU-rich elements

CXCL3: A key player in tumor microenvironment and inflammatory diseases

CXCL3 (C-X-C Motif Chemokine 3), a member of the C-X-C chemokine subfamily, operates as a potent chemoattractant for neutrophils, thereby orchestrating the recruitment and migration of leukocytes alongside eliciting an inflammatory response. Recent inquiries have shed light on the pivotal roles of CXCL3 in the context of carcinogenesis. In the tumor microenvironment, CXCL3 emanating from both tumor and stromal cells intricately modulates cellular behaviors through autocrine and paracrine actions, primarily via interaction with its receptor CXCR2. Activation of signaling cascades such as ERK/MAPK, AKT, and JAK2/STAT3 underscores CXCL3's propensity to favor tumorigenic processes. However, CXCL3 exhibits dualistic behaviors, as evidenced by its capacity to exert anti-tumor effects under specific conditions. Additionally, the involvement of CXCL3 extends to inflammatory disorders like eclampsia, obesity, and asthma. This review encapsulates the structural attributes, biological functionalities, and molecular underpinnings of CXCL3 across both tumorigenesis and inflammatory diseases.

K-Homology Type Splicing Regulatory Protein: Mechanism of Action in Cancer and Immune Disorders

K homology-type splicing regulatory protein (KSRP) is emerging as a key player in cancer biology, and immunology. As a single-strand nucleic acid binding protein it functions in both transcriptional and post-transcriptional regulation, while facilitating multiple stages of RNA metabolism to affect proliferation and control cell fate. However, it must interact with other proteins to determine the fate of its bound substrate. Here we provide an minireview of this important regulatory protein and describe its complex subcellular functions to affect RNA metabolism, stability, miRNA biogenesis and maturation, stress granule function, metastasis, and inflammatory processes.

Screening criteria of mRNA indicators for wound age estimation

Wound age estimation is a crucial and challenging problem in forensic pathology. Although mRNA is the most commonly used indicator for wound age estimation, screening criteria are lacking. In the present study, the feasibility of screening criteria using mRNA to determine injury time based on the adenylate-uridylate-rich element (ARE) structure and gene ontology (GO) categories were evaluated. A total of 78 Sprague-Dawley male rats were contused and sampled at 4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, and 48 h after inflicting injury. The candidate mRNAs were classified based on with or without ARE structure and GO category function. The mRNA expression levels were detected using qRT-PCR. In addition, the standard deviation (STD), mean deviation (MD), relative average deviation (d%), and coefficient of variation (CV) were calculated based on mRNA expression levels. The CV score (CVs) and the CV of CV (CV'CV) were calculated to measure heterogeneity. Finally, based on classic principles, the accuracy of combination of candidate mRNAs was assessed using discriminant analysis to construct a multivariate model for inferring wound age. The results of homogeneity evaluation of each group based on CVs were consistent with the MD, STD, d%, and CV results, indicating the credibility of the evaluation results based on CVs. The candidate mRNAs without ARE structure and classified as cellular component (CC) GO category (ARE-CC) had the highest CVs, showing the mRNAs with these characteristics are the most homogenous mRNAs and best suited for wound age estimation. The highest accuracy was 91.0% when the mRNAs without ARE structure were used to infer the wound age based on the discrimination model. The accuracy of mRNAs classified into CC or multiple function (MF) GO category was higher than mRNAs in the biological process (BP) category. In all subgroups, the accuracy of the composite identification model of mRNA composition without ARE structure and classified as CC was higher than other subgroups. The mRNAs without ARE structure and belonging to the CC GO category were more homogenous, showed higher accuracy for estimating wound age, and were appropriate for rat skeletal muscle wound age estimation. Supplemental data for this article is available online at https://doi.org/10.1080/20961790.2021.1986770 .

Inflammatory gene silencing in activated monocytes by a cholesterol tagged-miRNA/siRNA: a novel approach to ameliorate diabetes induced inflammation

There is a major unmet need for the development of effective therapies for diabetes induced inflammation. Increased adenosine-uridine rich elements (AREs) containing mRNAs of inflammatory molecules are reported in inflamed monocytes. Destabilizing these inflammatory mRNAs by the miR-16 could reduce inflammation. DNA microarrays and in vitro cell studies showed that exogenous miR16 and its mimic treatment, in LPS/PMA induced monocytes, significantly downregulated several ARE containing inflammatory cytokine mRNAs similar to those seen in the normal monocytes. Ingenuity pathway analyses showed exogenous miR-16 or its synthetic mimic treatment alleviates inflammatory responses. To selectively target uptake, especially to inflamed cells, one of the CD36 substrate cholesterol was tagged to miR16/siRNA. Cholesterol tagged miR-16/ARE-siRNA showed enhanced uptake in CD36 expressing inflamed cells. In LPS or PMA, treated monocytes, candidate genes expressions levels such as IL-6, IL-8, IL-12β, IP-10, and TNF-α mRNA were increased, as measured by RT-qPCR as seen in primary monocytes of diabetes patients. Exogenous miR16 or ARE-siRNA transfection reduced mRNAs of pro-inflammatory cytokines levels in monocyte, and its adhesion. Increased uptake of cholesterol tagged miR-16 through the CD36 receptor was observed. This destabilizes numerous inflammatory ARE containing mRNAs and alleviates inflammatory responses. Cholesterol-tagged miR-16 and its mimic are novel anti-inflammatory molecules that can be specifically targeted to, via through CD36 expressing, "inflamed" cells and thus serve as therapeutic candidates to alleviate inflammatory diseases.

An Evolutionarily Conserved AU-Rich Element in the 3' Untranslated Region of a Transcript Misannotated as a Long Noncoding RNA Regulates RNA Stability

One of the primary mechanisms of post-transcriptional gene regulation is the modulation of RNA stability. We recently discovered that LINC00675, a transcript annotated as a long noncoding RNA (lncRNA), is transcriptionally regulated by FOXA1 and encodes a highly conserved small protein that localizes to the endoplasmic reticulum, hence renamed as FORCP (FOXA1-regulated conserved small protein). Here, we show that the endogenous FORCP transcript is rapidly degraded and rendered unstable as a result of 3'UTR-mediated degradation. Surprisingly, although the FORCP transcript is a canonical nonsense-mediated decay (NMD) and microRNA (miRNA) target, we found that it is not degraded by NMD or miRNAs. Targeted deletion of an evolutionarily conserved region in the FORCP 3'UTR using CRISPR/Cas9 significantly increased the stability of the FORCP transcript. Interestingly, this region requires the presence of an immediate downstream 55-nt-long sequence for transcript stability regulation. Functionally, colorectal cancer cells lacking this conserved region expressed from the endogenous FORCP locus displayed decreased proliferation and clonogenicity. These data demonstrate that the FORCP transcript is destabilized via conserved elements within its 3'UTR and emphasize the need to interrogate the function of a given 3'UTR in its native context.

Malaria parasites both repress host CXCL10 and use it as a cue for growth acceleration

Pathogens are thought to use host molecular cues to control when to initiate life-cycle transitions, but these signals are mostly unknown, particularly for the parasitic disease malaria caused by Plasmodium falciparum. The chemokine CXCL10 is present at high levels in fatal cases of cerebral malaria patients, but is reduced in patients who survive and do not have complications. Here we show a Pf 'decision-sensing-system' controlled by CXCL10 concentration. High CXCL10 expression prompts P. falciparum to initiate a survival strategy via growth acceleration. Remarkably, P. falciparum inhibits CXCL10 synthesis in monocytes by disrupting the association of host ribosomes with CXCL10 transcripts. The underlying inhibition cascade involves RNA cargo delivery into monocytes that triggers RIG-I, which leads to HUR1 binding to an AU-rich domain of the CXCL10 3'UTR. These data indicate that when the parasite can no longer keep CXCL10 at low levels, it can exploit the chemokine as a cue to shift tactics and escape.

Pou1f1, the key transcription factor related to somatic growth in tilapia (Orechromis niloticus), is regulated by two independent post-transcriptional regulation mechanisms

This study aims to determine the post-transcriptional regulation mechanism of the transcription factor pou1f1 (pou class 1 homeobox 1), which is the key gene for pituitary development, somatic growth in vertebrates, and transcription of several hormone genes in teleost fish. MicroRNA miR-223-3p was identified as a bona fide target of pou1f; overexpression of miR-223-3p in primary pituitary cells led to the down-regulation of pou1f1 and downstream genes, and inhibition of miR-223-3p led to the up-regulation of pou1f1 in Nile tilapia dispersed primary pituitary cells. An adenylate-uridylate-rich element (AU-Rich element) was found in the 3'UTR of pou1f1 mRNA, and deletion of the AU-Rich element led to slower mRNA decay and therefore more protein output. A potential mutual relationship between miR-223-3p and the AU-rich element was also investigated, and the results demonstrated that with or without the AU-Rich element, miR-223-3p induced the up-regulation of a reporter system under serum starvation conditions, indicating that miR-223-3p and the AU-Rich element function independent of each other. This study is the first to investigate the post-transcriptional mechanism of pou1f1, which revealed that miR-223-3p down-regulated pou1f1 and downstream gene expressions, and the AU-Rich element led to rapid decay of pou1f1 mRNA. MicroRNA miR-223-3p and the AU-Rich element co-regulated the post-transcriptional expression of pou1f1 independently in Nile tilapia, demonstrating that pou1f1 is under the control of a dual post-transcription regulation mechanism.

Transcriptional and posttranscriptional regulation of CXCL8/IL-8 gene expression induced by connective tissue growth factor

Connective tissue growth factor (CTGF), a CCN family member, is a secreted protein regulating cellular functions, including fibrosis, apoptosis, adhesion, migration, differentiation, proliferation, angiogenesis, and chondrogenesis. CTGF increases proinflammatory factor production; however, inflammatory cytokine regulation by CTGF is poorly understood. The aim of this study was to identify novel biological functions and elucidate the functional mechanisms of CTGF. Specifically, the study focused on the ability of CTGF-primed monocytes to secrete interleukin 8 (CXCL8/IL-8) and determined the signaling pathways involved in CTGF-induced CXCL8/IL-8 gene regulation during inflammation. We transfected wild-type or mutant CXCL8/IL-8 promoter-derived luciferase reporter constructs into 293T cells to examine the effect of CTGF on the CXCL8/IL-8 promoter. The results showed that the activator protein-1 and nuclear factor κB binding sites of the CXCL8/IL-8 promoter are essential for CTGF-induced CXCL8/IL-8 transcription. Moreover, the CTGF-induced activation of p38 mitogen-activated protein kinase (MAPK), c-Jun-N-terminal kinase, and extracellular signal-regulated kinase (ERK) is involved in this process. In addition, adenosine-uridine-rich elements (AREs) of the CXCL8/IL-8 3'-untranslated region (3'-UTR) reduce CXCL8/IL-8 mRNA stability. To investigate whether CTGF regulates CXCL8/IL-8 gene expression at the posttranscriptional level, we transfected 293 cells with serial luciferase constructs containing different segments of the CXCL8/IL-8 3'-UTR and then stimulated the cells with CTGF. The results suggested that CTGF stabilized luciferase mRNA and increased luciferase activity by regulating the CXCL8/IL-8 3'-UTR. Moreover, the p38 MAPK pathway may contribute to CTGF-induced CXCL8/IL-8 mRNA stabilization.

Regulation of mRNA turnover in cystic fibrosis lung disease

Cystic fibrosis (CF) is an autosomal recessive disease due to mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, F508del-CFTR being the most frequent mutation. The CF lung is characterized by a hyperinflammatory phenotype and is regulated by multiple factors that coordinate its pathophysiology. In CF the expression of CFTR as well as proinflammatory genes are regulated at the level of messenger RNA (mRNA) stability, which subsequently affect translation. These mechanisms are mediated by inflammatory RNA-binding proteins as well as small endogenous noncoding microRNAs, in coordination with cellular signaling pathways. These regulatory factors exhibit altered expression and function in vivo in the CF lung, and play a key role in the pathophysiology of CF lung disease. In this review, we have described the role of mRNA stability and associated regulatory mechanisms in CF lung disease. WIREs RNA 2017, 8:e1408. doi: 10.1002/wrna.1408 For further resources related to this article, please visit the WIREs website.

Chemokine CXCL1 may serve as a potential molecular target for hepatocellular carcinoma

The purpose of this study was to screen for changes in chemokine and chemokine‐related genes that are expressed in hepatocellular carcinoma (HCC) as potential markers of HCC progression. Total RNA was extracted from tumor and peritumor tissues from mice with HCC and analyzed using a PCR microarray comprising 98 genes. Changes in gene expression of threefold or more were screened and subsequently confirmed by immunohistochemical analyses and western blotting. Furthermore, whether chemokine knockdown by RNA interference (RNAi) could significantly suppress tumor growth in vivo was also evaluated. Finally, total serum samples were collected from HCC patients with HBV/cirrhosis (n = 16) or liver cirrhosis (n = 16) and from healthy controls (n = 16). The serum mRNA and protein expression levels of CXCL1 in primary liver cancer patients were detected by qRT‐PCR and western blot analysis, respectively. Several genes were up‐regulated in tumor tissues during the progression period, including CXCL1, CXCL2, CXCL3, and IL‐1β, while CXCR1 expression was down‐regulated. CBRH‐7919 cells carrying CXCL1 siRNA resulted in decreased tumor growth in nude mice. The differences in serum CXCL1 mRNA and protein levels among the HCC, hepatic sclerosis (HS), and control groups were significant (P < 0.001). The mRNA and protein levels of CXCL1 in the HCC group were up‐regulated compared with the HS group or the control group (P < 0.001). Several chemokine genes were identified that might play important roles in the tumor microenvironment of HCC. These results provide new insights into human HCC and may ultimately facilitate early HCC diagnosis and lead to the discovery of innovative therapeutic approaches for HCC.

PI3-Kinase Inhibitor LY294002 Repressed the Expression of Thrombin-Activatable Fibrinolysis Inhibitor in Human Hepatoma HepG2 Cells

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a carboxypeptidase B-like proenzyme biosynthesized in the liver and released into the blood circulation. Activated TAFI (TAFIa) has been implicated as an important player in maintaining the balance between blood coagulation and fibrinolysis. In the present study, regulation of TAFI (CPB2) gene expression was investigated using cultured human hepatoma HepG2 cells. HepG2 cells were treated with the phosphoinositide 3-kinase (PI3K) inhibitor LY294002, and the levels of TAFI antigen and CPB2 mRNA were measured. HepG2 cells treated with LY29400 decreased their release of TAFI antigen into the conditioned medium (CM). In parallel, there were decreased levels of CPB2 mRNA and TAFI antigen in the cells. However, CPB2 gene promoter activity was not influenced by treatment of the cells with LY294002. The half-life of the CPB2 transcript was shortened by treatment with LY294002 compared with control. The present results suggest that the PI3K inhibitor LY294002 suppresses expression of TAFI, a prothrombotic factor, by decreasing the stability of CPB2 transcripts.

Inflammatory microenvironment and expression of chemokines in hepatocellular carcinoma

AIM: To study the inflammatory microenvironment and expression of chemokines in hepatocellular carcinoma (HCC) in nude mice.

METHODS: CBRH-7919 HCC cells were injected into the subcutaneous region of nude mice. Beginning two weeks after the challenge, tumor growth was measured every week for six weeks. The stromal microenvironment and inflammatory cell infiltration was assessed by immunohistochemistry in paired tumor and adjacent peritumoral samples, and macrophage phenotype was assessed using double-stain immunohistochemistry incorporating expression of an intracellular enzyme. A chemokine PCR array, comprised of 98 genes, was used to screen differential gene expressions, which were validated by Western blotting. Additionally, expression of identified chemokines was knocked-down by RNA interference, and the effect on tumor growth was assessed.

RESULTS: Inflammatory cell infiltrates are a key feature of adjacent peritumoral tissues with increased macrophage, neutrophil, and T cell (specifically helper and activated subsets) infiltration. Macrophages within adjacent peritumoral tissues express inducible nitric oxide synthase, suggestive of a proinflammatory phenotype. Fifty-one genes were identified in tumor tissues during the progression period, including 50 that were overexpressed (including CXCL1, CXCL2 and CXCL3) and three that were underexpressed (CXCR1, Ifg and Actb). RNA interference of CXCL1 in the CBRH-7919 cells decreased the growth of tumors in nude mice and inhibited expression of CXCL2, CXCL3 and interleukin-1β protein.

CONCLUSION: These findings suggest that CXCL1 plays a critical role in tumor growth and may serve as a potential molecular target for use in HCC therapy.

Reversal of corticosteroid insensitivity by p38 MAPK inhibition in peripheral blood mononuclear cells from COPD

BACKGROUND

Corticosteroids (CS) have limited efficacy in the treatment of chronic obstructive pulmonary disease (COPD). p38 mitogen-activated protein kinase (MAPK) activation is increased in lung macrophages of COPD. We investigated whether p38 MAPK inhibition can modulate CS insensitivity of peripheral blood mononuclear cells (PBMCs) from patients with COPD.

METHODS

PBMCs from patients with COPD (n=8) or healthy smokers (n=8) were exposed to lipopolysaccharide (LPS) with a selective p38 MAPK inhibitor (GW856553; 10(-10)-10(-6) M), with dexamethasone (10(-10)-10(-6) M), or with both. Phosphorylated glucocorticoid receptor (GR) was measured by Western blot.

RESULTS

Baseline (P<0.01) and LPS-induced (P<0.05) CXCL8 release was greater in PBMCs from COPD compared to healthy smokers. Inhibition of LPS-induced CXCL8 release by dexamethasone (10(-6) M) was reduced, and baseline and LPS-induced p38 MAPK activation increased in PBMCs of COPD. GW856553 (10(-9) and 10(-10) M) synergistically increased the inhibitory effect of dexamethasone (10(-8) and 10(-6) M) on LPS-induced CXCL8 release in COPD. Similar results were obtained for IL-6 release. GW856553 inhibited dexamethasone- and LPS-activated phosphorylation of serine 211 on GR. CS insensitivity in COPD PBMCs is reversed by inhibition of p38 MAPK activity, partly by preventing phosphorylation of GR at serine 211.

CONCLUSION

p38 MAPK inhibition may be beneficial in COPD by restoring CS sensitivity.

Uncoupling transcription from covalent histone modification

It is widely accepted that transcriptional regulation of eukaryotic genes is intimately coupled to covalent modifications of the underlying chromatin template, and in certain cases the functional consequences of these modifications have been characterized. Here we present evidence that gene activation in the silent heterochromatin of the yeast Saccharomyces cerevisiae can occur in the context of little, if any, covalent histone modification. Using a SIR-regulated heat shock-inducible transgene, hsp82-2001, and a natural drug-inducible subtelomeric gene, YFR057w, as models we demonstrate that substantial transcriptional induction (>200-fold) can occur in the context of restricted histone loss and negligible levels of H3K4 trimethylation, H3K36 trimethylation and H3K79 dimethylation, modifications commonly linked to transcription initiation and elongation. Heterochromatic gene activation can also occur with minimal H3 and H4 lysine acetylation and without replacement of H2A with the transcription-linked variant H2A.Z. Importantly, absence of histone modification does not stem from reduced transcriptional output, since hsp82-ΔTATA, a euchromatic promoter mutant lacking a TATA box and with threefold lower induced transcription than heterochromatic hsp82-2001, is strongly hyperacetylated in response to heat shock. Consistent with negligible H3K79 dimethylation, dot1Δ cells lacking H3K79 methylase activity show unimpeded occupancy of RNA polymerase II within activated heterochromatic promoter and coding regions. Our results indicate that large increases in transcription can be observed in the virtual absence of histone modifications often thought necessary for gene activation.

The proper regulation of gene expression is of fundamental importance in the maintenance of normal growth and development. Misregulation of genes can lead to such outcomes as cancer, diabetes and neurodegenerative disease. A key step in gene regulation occurs during the transcription of the chromosomal DNA into messenger RNA by the enzyme, RNA polymerase II. Histones are small, positively charged proteins that package genomic DNA into arrays of bead-like particles termed nucleosomes, the principal components of chromatin. Increasing evidence suggests that nucleosomal histones play an active role in regulating transcription, and that this is derived in part from reversible chemical (“covalent”) modifications that take place on their amino acids. These histone modifications create novel surfaces on nucleosomes that can serve as docking sites for other proteins that control a gene's expression state. In this study we present evidence that contrary to the general case, covalent modifications typically associated with transcription are minimally used by genes embedded in a specialized, condensed chromatin structure termed heterochromatin in the model organism baker's yeast. Our observations are significant, for they suggest that gene transcription can occur in a living cell in the virtual absence of covalent modification of the chromatin template.

Inflammatory responses of airway smooth muscle cells and effects of endothelin receptor antagonism

Endothelin receptor antagonists (ETRAs), authorized for pulmonary hypertension, have failed to prove their utility in chronic lung diseases with corticosteroid-resistant airway inflammation when applied at late disease stages with emphysema/fibrosis. Earlier administration might prove effective by targeting the interaction between airway inflammation and tissue remodeling. We hypothesized that human airway smooth muscle cells (HASMCs) participate in linking inflammation with remodeling and that associated genes become differentially suppressed by ambrisentan (A-receptor selective ETRA) and bosentan (nonselective/dual ETRA). Inflammatory responses of ex vivo-cultivated HASMCs to TNF-α were investigated by whole-genome microarray analyses. qRT-PCR and ELISA were used to test inflammatory and remodeling genes for sensitivity to bosentan and ambrisentan and to investigate differential sensitivities mechanistically. ETRA and corticosteroid effects were compared in HASMCs from patients with chronic obstructive pulmonary disease. TNF-α induced the expression of 18 cytokines/chemokines and five tissue remodeling genes involved in severe, corticosteroid-insensitive asthma, chronic obstructive pulmonary disease, idiopathic pulmonary fibrosis, and/or pulmonary hypertension. Thirteen cytokines/chemokines, MMP13, and WISP1 were suppressed by ETRAs. Eight genes had differential sensitivity to bosentan and ambrisentan depending on the endothelin-B receptor impact on transcriptional regulation and mRNA stabilization. Chemokine (C-C motif) ligands 2 and 5, granulocyte macrophage colony-stimulating factor, and MMP13 had increased sensitivity to bosentan or bosentan/dexamethasone combination versus dexamethasone alone. Suppression of cytokine and remodeling gene expression by ETRAs was confirmed in TNF-α-activated human bronchial epithelial cells. HASMCs and human bronchial epithelial cells participate in the interaction of inflammation and tissue remodeling. This interaction is targeted differentially by selective and nonselective ETRAs, which could be used in therapies of chronic lung diseases with corticosteroid-resistant airway inflammation at early disease stages to attenuate inflammation-induced airway remodeling.

Signaling pathways that control mRNA turnover

Cells regulate their genomes mainly at the level of transcription and at the level of mRNA decay. While regulation at the level of transcription is clearly important, the regulation of mRNA turnover by signaling networks is essential for a rapid response to external stimuli. Signaling pathways result in posttranslational modification of RNA binding proteins by phosphorylation, ubiquitination, methylation, acetylation etc. These modifications are important for rapid remodeling of dynamic ribonucleoprotein complexes and triggering mRNA decay. Understanding how these posttranslational modifications alter gene expression is therefore a fundamental question in biology. In this review we highlight recent findings on how signaling pathways and cell cycle checkpoints involving phosphorylation, ubiquitination, and arginine methylation affect mRNA turnover.

Diversity in sequence-dependent control of GRO chemokine mRNA half-life

Neutrophil trafficking to sites of injury or infection is regulated, in part, by the closely related GRO family of chemokines (CXCL1, -2, and -3). Expression of the GRO chemokine genes is known to be determined by transcriptional bursts in response to proinflammatory stimulation, but post-transcriptional mechanisms that regulate mRNA half-life are now recognized as important determinants. mRNA half-life is regulated via distinct sequence motifs and sequence-specific, RNA-binding proteins, whose function is subject to regulation by extracellular proinflammatory stimuli. Moreover, such mechanisms exhibit cell-type and stimulus dependency. We now present evidence that in nonmyeloid cells, GRO2 and GRO3 isoforms exhibit at least two patterns of mRNA instability that are distinguished by differential sensitivity to specific mRNA-destabilizing proteins and stimulus-mediated prolongation of mRNA half-life, respectively. Although the 3' UTR regions of GRO2 and GRO3 mRNAs contain multiple AREs, GRO2 has eight AUUUA pentamers, whereas GRO3 has seven. These confer quantitative differences in half-life and show sensitivity for TTP and KSRP but not SF2/ASF. Moreover, these AUUUA determinants do not confer instability that can be modulated in response to IL-1α. In contrast, IL-1α-sensitive instability for GRO2 and GRO3 is conferred by sequences located proximal to the 3' end of the 3'UTR that are independent of the AUUUA sequence motif. These regions are insensitive to TTP and KSRP but show reduced half-life mediated by SF2/ASF. These sequence-linked, post-transcriptional activities provide substantial mechanistic diversity in the control of GRO family chemokine gene expression.

Glyceraldehyde-3-phosphate dehydrogenase regulates cyclooxygenase-2 expression by targeting mRNA stability

Cyclooxygenase (COX)-2 is an inducible inflammatory protein whose expression is partially regulated at the post-transcriptional level. We investigated whether glyceraldehyde-3-phosphate dehydrogenase (GAPDH) binds to the AU-rich element (ARE) of COX-2 mRNA for its degradation. Knockdown of GAPDH in hepa1c1c7 cells significantly enhanced COX-2 expressions. Recombinant GAPDH bound to the COX-2 ARE within the first 60 nucleotides of the 3'-UTR via the NAD(+) binding domain. Interestingly, a C151S GAPDH mutant retained binding activity. Confocal microscopy observation revealed that LPS exposure reduced the localization of GAPDH in nuclei. Our results indicate that GAPDH negatively regulates COX-2 by binding to its ARE.

KSRP: a checkpoint for inflammatory cytokine production in astrocytes

Chronic inflammation in the central nervous system (CNS) is a central feature of many neurodegenerative and autoimmune diseases. As an immunologically competent cell, the astrocyte plays an important role in CNS inflammation. It is capable of expressing a number of cytokines such as tumor necrosis factor alpha (TNF-α) and interleukin-1 beta (IL-1β) that promote inflammation directly and through the recruitment of immune cells. Checkpoints are therefore in place to keep tight control over cytokine production. Adenylate/uridylate-rich elements (ARE) in the 3' untranslated region of cytokine mRNAs serve as a major checkpoint by regulating mRNA stability and translational efficiency. Here, we examined the impact of KH-type splicing regulatory protein (KSRP), an RNA binding protein which destabilizes mRNAs via the ARE, on cytokine expression and paracrine phenotypes of primary astrocytes. We identified a network of inflammatory mediators, including TNF-α and IL-1β, whose expression increased 2 to 4-fold at the RNA level in astrocytes isolated from KSRP(-/-) mice compared to littermate controls. Upon activation, KSRP(-/-) astrocytes produced TNF-α and IL-1β at levels that exceeded control cells by 15-fold or more. Conditioned media from KSRP(-/-) astrocytes induced chemotaxis and neuronal cell death in vitro. Surprisingly, we observed a prolongation of half-life in only a subset of mRNA targets and only after selective astrocyte activation. Luciferase reporter studies indicated that KSRP regulates cytokine gene expression at both transcriptional and post-transcriptional levels. Our results outline a critical role for KSRP in regulating pro-inflammatory mediators and have implications for a wide range of CNS inflammatory and autoimmune diseases.

Intron retention and 3'-UTR analysis of Arabidopsis Dicer-like 2 transcripts

Arabidopsis thaliana Dicer-like protein 2 (AtDCL2) plays an essential role in the RNA interference pathway. The function of AtDCL2 and other DCLs has been much studied but little has been done to characterize the DCLs transcripts before they are translated into proteins. Here, we investigated AtDCL2 transcripts and showed that all 21 introns of AtDCL2 except intron 9, 18, 20 and 21 could be retained although spliced sequences usually predominated. Intron 10 was more frequently retained and transient expression assays in Nicotiana benthamiana leaves showed that when AG/C at the 3' splicing site of the intron was changed to AG/G, the intron was more frequently spliced out. Conversely, a high retention of intron 18 was obtained if the AG/G at the 3' splicing site was changed to AG/C. These results suggest that the sequence at the 3' splicing site affects the efficiency of intron splicing. The 3'-UTRs of AtDCL2 had lengths between 54 and 154 nts, and the different 3'-UTRs differentially affected the transcriptional levels of fused GFP expressed transiently in N. benthamiana. Further comparisons and mutation experiments suggested that a putative SBF-1 binding site and an AU-rich element in the 3'-UTR both down-regulated expression of the upstream GFP fused to the 3'-UTR. Conversely, a second poly(A) consensus signal sequence in one 3'-UTR up-regulated gene expression. Our results provide insight into the character of AtDCL2 transcripts and demonstrate the potential complexity of factors that affect the frequency and patterns of alternative splicing.

The myxobacterial compounds spirangien a and spirangien M522 are potent inhibitors of IL-8 expression

Elevated expression of interleukin-8 (IL-8) has been implicated in inflammatory diseases, in tumor growth, and in angiogenesis. The aim of this study was to identify natural or synthetic compounds that suppress IL-8 production in response to interleukin-1 (IL-1), the natural inflammatory stimulus of the IL-8 gene. We therefore developed an IL-1-inducible cell-based screening assay by stable integration of an IL-8 reporter gene into HeLa S3 cells. The screening of heterogeneous compound libraries revealed several compounds that displayed an inhibitory effect on the reporter gene expression. Following hit validation, we focused on the most efficient compound, spirangien A, and its chemical derivate spirangien M522. Detailed analysis shows that both compounds are potent inhibitors of the endogenous IL-8 gene transcription. Furthermore, both compounds decelerate the phosphorylation and degradation of IκBα, the key regulator of the IL-1-stimulated NF-κB signaling pathway. Our study has identified the two spirangiens A and M522 as potent inhibitors of IL-1/NF-κB-mediated IL-8 gene expression.

Cell type- and stimulus-specific mechanisms for post-transcriptional control of neutrophil chemokine gene expression

mRNAs encoding inflammatory chemokines that recruit neutrophils frequently exhibit short half-lives that serve to limit their expression under inappropriate conditions but are often prolonged to ensure adequate levels during inflammatory response. Extracellular stimuli that modulate the stability of such mRNAs may be the same as the transcriptional activator, as is the case with TLR ligands, or may cooperate with independent transcriptional stimuli, as with IL-17, which extends the half-life of TNF-induced transcripts. These different stimuli engage independent signaling pathways that target different instability mechanisms distinguished by dependence on different regulatory nucleotide sequence motifs within the 3'UTRs, which involve that action of different mRNA-binding proteins. The selective use of these pathways by different stimuli and in distinct cell populations provides the potential for tailoring of chemokine expression patterns to meet specific needs in different pathophysiologic circumstances.

Toward a systematic understanding of mRNA 3' untranslated regions

Messenger RNAs (mRNAs) contain prominent untranslated regions (UTRs) that are increasingly recognized to play roles in mRNA processing, transport, stability, and translation. 3' UTRs are believed to harbor recognition sites for a diverse set of RNA-binding proteins that regulate gene expression as well as most active microRNA target sites. Although the roles of 3' UTRs in the normal and diseased lung have not yet been studied extensively, available evidence suggests important roles for 3' UTRs in lung development, inflammation, asthma, pulmonary fibrosis, and cancer. Systematic, genome-wide approaches are beginning to catalog functional elements within 3' UTRs and identify the proteins and microRNAs that interact with these elements. Application of new data sets and experimental approaches should provide powerful insights into how 3' UTR-mediated regulatory events contribute to disease and may inspire novel therapeutic approaches.

Interleukin-1 activates synthesis of interleukin-6 by interfering with a KH-type splicing regulatory protein (KSRP)-dependent translational silencing mechanism

Post-transcriptional mechanisms play an important role in the control of inflammatory gene expression. The heterogeneous nuclear ribonucleoprotein K homology (KH)-type splicing regulatory protein (KSRP) triggers rapid degradation of mRNAs for various cytokines, chemokines, and other inflammation-related proteins by interacting with AU-rich elements (AREs) in the 3'-untranslated mRNA regions. In addition to destabilizing mRNAs, AU-rich elements can restrict their translation. Evidence that KSRP also participates in translational silencing was obtained in a screen comparing the polysome profiles of cells with siRNA-mediated depletion of KSRP with that of control cells. Among the group of mRNAs showing increased polysome association upon KSRP depletion are those of interleukin (IL)-6 and IL-1α as well as other ARE-containing transcripts. Redistribution of IL-6 mRNA to polysomes was associated with increased IL-6 protein secretion by the KSRP-depleted cells. Silencing of IL-6 and IL-1α mRNAs depended on their 3'-untranslated regions. The sequence essential for translational control of IL-6 mRNA and its interaction with KSRP was located to an ARE. KSRP-dependent silencing was reversed by IL-1, a strong inducer of IL-6 mRNA and protein expression. The results identify KSRP as a protein involved in ARE-mediated translational silencing. They suggest that KSRP restricts inflammatory gene expression not only by enhancing degradation of mRNAs but also by inhibiting translation, both functions that are counteracted by the proinflammatory cytokine IL-1.

The 3'-untranslated region length and AU-rich RNA location modulate RNA-protein interaction and translational control of β2-adrenergic receptor mRNA

Posttranscriptional controls play a major role in β(2)-adrenergic receptor (β(2)-AR) expression. We recently reported that β(2)-AR mRNA translation is suppressed by elements in its 3'-untranslated region (UTR). We also identified T-cell-restricted intracellular antigen-related protein (TIAR) and HuR as prominent AU-rich (ARE) RNA-binding proteins that associate with β(2)-AR mRNA 3'-UTR. In this study, we identified a poly(U) region at the distal end of the 3'-UTR as critical for TIAR binding to β(2)-AR mRNA and for translational suppression. Here, we also report that the locations of the poly(U) and ARE sequences within the 3'-UTR are important determinants that control the translation of β(2)-AR mRNA. Consistent with this finding, a 20-nucleotide ARE RNA from the proximal 3'-UTR that did not inhibit mRNA translation in its native position was able to suppress translation when re-located to the distal 3'-UTR of the receptor mRNA. Immunoprecipitation and polysome profile analysis demonstrated the importance of 3'-UTR length and the ARE RNA location within the 3'-UTR, as key determinants of RNA/protein interactions and translational control of β(2)-AR mRNA. Further, the importance of 3'-UTR length and ARE location in TIAR and HuR association with mRNA and translational suppression was demonstrated using a chimeric luciferase reporter gene.

Contribution of transcript stability to a conserved procyanidin-induced cytokine response in γδ T cells

γδ T cells function in innate and adaptive immunity and are primed for secondary responses by procyanidin components of unripe apple peel (APP). Here we investigate the effects of APP and purified procyanidins on γ δ T cell gene expression. A microarray analysis was performed on bovine γ δ T cells treated with APP; increases in transcripts encoding GM-CSF, IL-8, and IL-17, but not markers of TCR stimulation such as IFNγ , were observed. Key responses were confirmed in human, mouse, and bovine cells by RT-PCR and/or ELISA, indicating a conserved response to procyanidins. In vivo relevance of the cytokine response was shown in mice following intraperitoneal injection of APP, which induced production of CXCL1/KC and resulted in neutrophil influx to the blood and peritoneum. In the human γ δ T cell-line, MOLT-14, GM-CSF and IL-8 transcripts were increased and stabilized in cells treated with crude APP or purified procyanidins. The ERK1/2 MAPK pathway was activated in APP-treated cells, and necessary for transcript stabilization. Our data describe a unique γ δ T cell inflammatory response during procyanidin treatment and suggest that transcript stability mechanisms could account, at least in part, for the priming phenotype.

IL-1-induced post-transcriptional mechanisms target overlapping translational silencing and destabilizing elements in IκBζ mRNA

The inflammatory cytokine IL-1 induces profound changes in gene expression. This is contributed in part by activating translation of a distinct set of mRNAs, including IκBζ, as indicated by genome-wide analysis of changes in ribosomal occupancy in IL-1α-treated HeLa cells. Polysome profiling of IκBζ mRNA and reporter mRNAs carrying its 3' UTR indicated poor translation in unstimulated cells. 3' UTR-mediated translational silencing was confirmed by suppression of luciferase activity. Translational silencing was unaffected by replacing the poly(A) tail with a histone stem-loop, but lost under conditions of cap-independent internal initiation. IL-1 treatment of the cells caused profound shifts of endogenous and reporter mRNAs to polysome fractions and relieved suppression of luciferase activity. IL-1 also inhibited rapid mRNA degradation. Both translational activation and mRNA stabilization involved IRAK1 and -2 but occurred independently of the p38 MAPK pathway, which is known to target certain other post-transcriptional mechanisms. The translational silencing RNA element contains the destabilizing element but requires additional 5' sequences and is impaired by mutations that leave destabilization unaffected. These differences in function are associated with differential changes in protein binding in vitro. Thus, rapid degradation occurs independently of the translational silencing effect. The results provide evidence for a novel mode of post-transcriptional control by IL-1, which impinges on the time course and pattern of IL-1-induced gene expression.

MAPKAP kinase 2 blocks tristetraprolin-directed mRNA decay by inhibiting CAF1 deadenylase recruitment

Tristetraprolin (TTP) directs its target AU-rich element (ARE)-containing mRNAs for degradation by promoting removal of the poly(A) tail. The p38 MAPK pathway regulates mRNA stability via the downstream kinase MAPK-activated protein kinase 2 (MAPKAP kinase 2 or MK2), which phosphorylates and prevents the mRNA-destabilizing function of TTP. We show that deadenylation of endogenous ARE-containing tumor necrosis factor mRNA is inhibited by p38 MAPK. To investigate whether phosphorylation of TTP by MK2 regulates TTP-directed deadenylation of ARE-containing mRNAs, we used a cell-free assay that reconstitutes the mechanism in vitro. We find that phosphorylation of Ser-52 and Ser-178 of TTP by MK2 results in inhibition of TTP-directed deadenylation of ARE-containing RNA. The use of 14-3-3 protein antagonists showed that regulation of TTP-directed deadenylation by MK2 is independent of 14-3-3 binding to TTP. To investigate the mechanism whereby TTP promotes deadenylation, it was necessary to identify the deadenylases involved. The carbon catabolite repressor protein (CCR)4.CCR4-associated factor (CAF)1 complex was identified as the major source of deadenylase activity in HeLa cells responsible for TTP-directed deadenylation. CAF1a and CAF1b were found to interact with TTP in an RNA-independent fashion. We find that MK2 phosphorylation reduces the ability of TTP to promote deadenylation by inhibiting the recruitment of CAF1 deadenylase in a mechanism that does not involve sequestration of TTP by 14-3-3. Cyclooxygenase-2 mRNA stability is increased in CAF1-depleted cells in which it is no longer p38 MAPK/MK2-regulated.

Differential expression of the chemokines GRO-2, GRO-3, and interleukin-8 in colon cancer and their impact on metastatic disease and survival

BACKGROUND AND AIM

Chemotactic cytokines play a role in angiogenesis and attraction of immune cells. However, their contribution to tumor formation remains incompletely understood. In a previous transcriptome study, we identified a family of structurally related chemokines of the CXC-family to be specifically up-regulated in colorectal cancer. The aim of the present study was to investigate the regulation of their expression in colon cancer cells and to test the hypothesis that altered CXC-chemokine expression is related to critical clinical parameters, such as survival or metastasis formation.

MATERIALS AND METHODS

Expression levels of interleukin-8 (CXCL-8) and growth-related oncogenes 2 and 3 (GRO-2/CXCL-2 and GRO-3/CXCL-3) were quantified using qRT-PCR in 97 patients with completely resected colon carcinoma and correlated with clinical parameters. Moreover, 16 samples of normal mucosa, nine samples of benign adenoma, and 11 samples of liver metastasis were analyzed. Next, the regulation of chemokine expression in response to various stimuli was tested in colon cancer cell lines (HT29, HCT116, CaCO2).

RESULTS

Expression of GRO-2, GRO-3, and IL-8 was significantly increased in colon cancer as compared to normal colon tissue. Expression of GRO-2 and GRO-3 was already enhanced in premalignant adenomas, and GRO-3 was significantly down-regulated in liver metastasis as compared to the primary tumor. Importantly, expression of GRO-3 was significantly higher in patients with local versus systemic disease. Moreover, IL-8 expression was significantly associated to overall post-operative survival. Finally, all chemokines were strongly induced by IL-1alpha in the colon cancer cell lines tested, indicating a potential link to inflammatory processes.

CONCLUSION

In accordance with earlier findings, we report here a significantly increased expression of GRO-2, GRO-3, and IL-8 in colon carcinoma as compared to normal tissue. Furthermore, GRO-3 was related to metastasis formation, and IL-8 was associated with survival, suggesting a potential predictive power of these markers.

Diversity in post-transcriptional control of neutrophil chemoattractant cytokine gene expression

Regulation of neutrophil chemokine gene expression represents an important feature in tissue inflammation. While chemokine gene transcription through the action of NFkappaB is recognized as an essential component of this process, it is now clear that post-transcriptional mechanisms, particularly the rates of decay of mature cytoplasmic mRNA, provides an essential component of this control. Chemokine and other cytokine mRNA half life is known to be controlled via adenine-uridine rich sequence motifs localized within 3' untranslated regions (UTRs), the most common of which contains one or more copies of the pentameric AUUUA sequence. In myeloid cells AUUUA sequences confer instability through the action of RNA binding proteins such as tristetraprolin (TTP). The resulting instability can be regulated in response to extra-cellular stimuli including Toll like receptor ligands that signal to control the function of TTP through pathways involving the activation of p38 MAP kinases. Recent findings indicate that substantial mechanistic diversity is operative in non-myeloid cells in response to alternate pro-inflammatory stimuli such as IL-17. These pathways target distinct instability sequences that do not contain the AUUUA pentamer motif, do not signal through p38 MAPK, and function independently of TTP.

IL-17 regulates CXCL1 mRNA stability via an AUUUA/tristetraprolin-independent sequence

IL-17 contributes to inflammatory response in part by promoting enhanced expression of chemokines, such as CXCL1, by prolonging the t(1/2) of this constitutively unstable mRNA. Although IL-17 is a weak stimulus for transcription of the CXCL1 gene, it strongly potentiates message accumulation via stabilization when the mRNA is transcribed in cells stimulated with TNF. In myeloid cells, LPS-induced CXCL1 mRNA stabilization is dependent on AUUUA-containing sequence motifs that are recognized by the RNA binding protein tristetraprolin (TTP). Using deletion and site-specific mutagenesis, we report that IL-17-mediated stabilization of CXCL1 mRNA in nonmyeloid cells depends on a sequence that does not contain the AUUUA motif. Furthermore, a specific two-nucleotide mutation within this region markedly abrogates sensitivity for IL-17-mediated stabilization. Consistent with this finding, the IL-17-sensitive sequence does not exhibit increased instability in the presence of TTP, and CXCL1 mRNA remains unstable and can be stabilized in response to treatment with IL-17 in embryo fibroblasts from mice in which the TTP gene has been deleted. Whereas the RNA binding protein KSRP has been shown to participate in regulating the instability of human CXCL8 mRNA, inhibitory RNA-based reduction in KSRP does not effect the instability mediated by the IL-17-sensitive sequence motif. These findings suggest that IL-17-mediated chemokine mRNA stabilization in nonmyeloid cells uses a mechanism that is distinct from that operating to control AU-rich mRNA stability in myeloid cells.

The p38 MAPK pathway inhibits tristetraprolin-directed decay of interleukin-10 and pro-inflammatory mediator mRNAs in murine macrophages

p38 mitogen-activated protein kinase (MAPK) stabilises pro-inflammatory mediator mRNAs by inhibiting AU-rich element (ARE)-mediated decay. We show that in bone-marrow derived murine macrophages tristetraprolin (TTP) is necessary for the p38 MAPK-sensitive decay of several pro-inflammatory mRNAs, including cyclooxygenase-2 and the novel targets interleukin (IL)-6, and IL-1alpha. TTP(-/-) macrophages also strongly overexpress IL-10, an anti-inflammatory cytokine that constrains the production of the IL-6 despite its disregulation at the post-transcriptional level. TTP directly controls IL-10 mRNA stability, which is increased and insensitive to inhibition of p38 MAPK in TTP(-/-) macrophages. Furthermore, TTP enhances deadenylation of an IL-10 3'-untranslated region RNA in vitro.

Tristetraprolin regulates IL-8 mRNA stability in cystic fibrosis lung epithelial cells

Cystic fibrosis (CF) is due to mutations in the CFTR gene and is characterized by hypersecretion of the proinflammatory chemokine IL-8 into the airway lumen. Consequently, this induces the highly inflammatory cellular phenotype typical of CF. Our initial studies revealed that IL-8 mRNA is relatively stable in CF cells compared with those that had been repaired with [WT]CFTR (wild-type CFTR). Relevantly, the 3'-UTR of IL-8 mRNA contains AU-rich sequences (AREs) that have been shown to mediate posttranscriptional regulation of proinflammatory genes upon binding to ARE-binding proteins including Tristetraprolin (TTP). We therefore hypothesized that very low endogenous levels of TTP in CF cells might be responsible for the relative stability of IL-8 mRNA. As predicted, increased expression of TTP in CF cells resulted in reduced stability of IL-8 mRNA. An in vitro analysis of IL-8 mRNA stability in CF cells also revealed a TTP-induced enhancement of deadenylation causing reduction of IL-8 mRNA stability. We conclude that enhanced stability of IL-8 mRNA in TTP-deficient CF lung epithelial cells serve to drive the proinflammatory cellular phenotype in the CF lung.

Database for mRNA half-life of 19 977 genes obtained by DNA microarray analysis of pluripotent and differentiating mouse embryonic stem cells

Degradation of mRNA is one of the key processes that control the steady-state level of gene expression. However, the rate of mRNA decay for the majority of genes is not known. We successfully obtained the rate of mRNA decay for 19 977 non-redundant genes by microarray analysis of RNA samples obtained from mouse embryonic stem (ES) cells. Median estimated half-life was 7.1 h and only <100 genes, including Prdm1, Myc, Gadd45 g, Foxa2, Hes5 and Trib1, showed half-life less than 1 h. In general, mRNA species with short half-life were enriched among genes with regulatory functions (transcription factors), whereas mRNA species with long half-life were enriched among genes related to metabolism and structure (extracellular matrix, cytoskeleton). The stability of mRNAs correlated more significantly with the structural features of genes than the function of genes: mRNA stability showed the most significant positive correlation with the number of exon junctions per open reading frame length, and negative correlation with the presence of PUF-binding motifs and AU-rich elements in 3′-untranslated region (UTR) and CpG di-nucleotides in the 5′-UTR. The mRNA decay rates presented in this report are the largest data set for mammals and the first for ES cells.

A single base-pair mutation in the 3'-untranslated region of HLA-G mRNA is associated with pre-eclampsia

Human leukocyte antigen-G (HLA-G) is a non-classical class I HLA molecule that is expressed by extravillous cytotrophoblast cells. This protein may play a critical role in the protection of cytotrophoblasts from maternal immune response, allowing these semi-allogeneic cells to invade the uterus unimpeded. We have demonstrated that diminished placental HLA-G expression is associated with pre-eclampsia. In order to explore fundamental mechanisms underlying this reduced HLA-G expression in pre-eclampsia, we looked for, and found by sequences analysis, a single base-pair mutation in the HLA-G gene 3'-untranslated region (3'UTR) adjacent to an AUUUA motif. This mutation is significantly associated with pre-eclampsia, the severe form being more strongly associated with homozygosity for this mutation than the mild form. Since the null allele was discovered in the HLA-G mRNA 3'UTR adjacent to an AUUUA motif, we also examined the effect of this mutation on HLA-G mRNA stability, and found that half-lives of HLA-G mRNA with the mutation were significantly shorter than without the mutation. These data provide evidence that this mutation could be one of the fundamental mechanisms for lower levels of placental HLA-G protein expression in patients with pre-eclampsia.

Distinct roles of heterogeneous nuclear ribonuclear protein K and microRNA-16 in cyclooxygenase-2 RNA stability induced by S100b, a ligand of the receptor for advanced glycation end products

Advanced glycation end products play major roles in diabetic complications. They act via their receptor RAGE to induce inflammatory genes such as cyclooxygenase-2 (COX-2). We examined the molecular mechanisms by which the RAGE ligand, S100b, induces COX-2 in monocytes. S100b significantly increased COX-2 mRNA accumulation in THP-1 monocytes at 2 h via mRNA stability. This was further confirmed by showing that S100b increased stability of luciferase-COX-2 3'-UTR mRNA. Chromatin immunoprecipitation and RNA immunoprecipitation revealed that S100b decreased occupancy of the DNA/RNA-binding protein, heterogeneous nuclear ribonuclear protein K (hnRNPK), at the COX-2 promoter but simultaneously increased its binding to the COX-2 3'-UTR. S100b treatment promoted the translocation of nuclear hnRNPK to cytoplasm, whereas a cytoplasmic translocation-deficient hnRNPK mutant inhibited S100b-induced COX-2 mRNA stability. Small interfering RNA-mediated specific knockdown of hnRNPK blocked S100b-induced COX-2 mRNA stability, whereas on the other hand, overexpression of hnRNPK increased S100b-induced COX-2 mRNA stability. S100b promoted the release of entrapped COX-2 mRNA from cytoplasmic processing bodies, sites of mRNA degradation. Furthermore, S100b significantly down-regulated the expression of a key microRNA, miR-16, which can destabilize COX-2 mRNA by binding to its 3'-UTR. MiR-16 inhibitor oligonucleotides increased, whereas, conversely, miR-16 mimic oligonucleotides decreased COX-2 mRNA stability in monocytes, further supporting the inhibitory effects of miR-16. Interestingly, hnRNPK knockdown increased miR-16 binding to COX-2 3'-UTR, indicating a cross-talk between them. These new results demonstrate that diabetic stimuli can efficiently stabilize inflammatory genes via opposing actions of key RNA-binding proteins and miRs.

Tristetraprolin regulates CXCL1 (KC) mRNA stability

mRNAs encoding proinflammatory chemokines are regulated posttranscriptionally via adenine-uridine-rich sequences (AREs) located in the 3' untranslated region of the message, which are recognized by sequence-specific RNA-binding proteins. One ARE binding protein, tristetraprolin (TTP), has been implicated in regulating the stability of several ARE-containing mRNAs, including those encoding TNF-alpha and GM-CSF. In the present report we examined the role of TTP in regulating the decay of the mouse chemokine KC (CXCL1) mRNA. Using tetR-regulated control of transcription in TTP-deficient HEK293 cells, KC mRNA half-life was markedly decreased in the presence of TTP. Deletion and site-specific mutagenesis were used to identify multiple AUUUA sequence determinants responsible for TTP sensitivity. Although a number of studies suggest that the destabilizing activity of TTP is subject to modulation in response to ligands of Toll/IL-1 family receptors, decay mediated by TTP in 293 cells was not sensitive to stimulation with IL-1alpha. Using primary macrophages from wild-type and TTP-deficient mice, KC mRNA instability was found to be highly dependent on TTP. Furthermore, LPS-mediated stabilization of KC mRNA is blocked by inhibition of the p38 MAPK in macrophages from wild-type but not TTP-deficient mice. These findings demonstrate that TTP is the predominant regulator of KC mRNA decay in mononuclear phagocytes acting via multiple 3'-untranslated region-localized AREs. Nevertheless, KC mRNA remains highly unstable in cells that do not express TTP, suggesting that additional determinants of instability and stimulus sensitivity may operate in cell populations where TTP is not expressed.

Interleukin 1alpha-induced NFkappaB activation and chemokine mRNA stabilization diverge at IRAK1

Interleukin 1alpha (IL-1alpha) is capable of driving pro-inflammatory gene expression through both the initiation of transcription and by prolonging the half-life of short-lived mRNAs. Although the signaling events linking the IL-1 receptor to the activation of NFkappaB and the initiation of transcription have been well characterized, less is known about the signaling events linking to mRNA stabilization. As a model to study the control of mRNA stability we have used the mouse chemokine KC, expression of which requires both NFkappaB-driven transcription and stabilization of the constitutively unstable mRNA. We have evaluated the role of signaling adaptors known to play a role in IL-1alpha-driven NFkappaB activation in the generation of mRNA stability. Surprisingly, although TRAF6 is essential for NFkappaB activation, it is not required for IL-1alpha-induced mRNA stabilization. IRAK1, which is recognized to function upstream of TRAF6, is required for both mRNA stabilization and activation of NFkappaB. Consistent with the previous findings, the TRAF6 interaction sites in IRAK1 are required for NFkappaB activation but do not play a role in mRNA stabilization. These findings indicate that signals from the IL-1 receptor segregate into at least two separate pathways at the level of IRAK1; one couples through TRAF6 to NFkappaB activation while a second utilizes a TRAF6-independent pathway that is responsible for mRNA stabilization.

Heat shock co-activates interleukin-8 transcription

The heat shock (HS) response is a phylogenetically ancient cellular response to stress, including heat, that shifts gene expression to a set of conserved HS protein (HSP) genes. In our earlier studies, febrile-range hyperthermia (FRH) not only activated HSP gene expression, but also increased expression of CXC chemokines in mice, leading us to hypothesize that the CXC chemokine family of genes might be HS-responsive. To address this hypothesis we analyzed the effect of HS on the expression of IL-8/CXCL-8, a member of the human CXC family of ELR(+) chemokines. HS markedly enhanced TNF-alpha-induced IL-8 secretion in human A549 respiratory epithelial-like cells and in primary human small airway epithelial cells. IL-8 mRNA was also up-regulated by HS, but the stability of IL-8 mRNA was not affected. TNF-alpha-induced reporter activity of an IL-8 promoter construct IL8(-1471/+44)-luc stably transfected in A549 cells was also enhanced by HS. Electrophoretic mobility and chromatin immunoprecipitation assays showed that the stress-activated transcription factor heat shock factor-1 (HSF-1) binds to at least two putative heat shock response elements (HSE) present in the IL-8 promoter. Deletional reporter constructs lacking either one or both of these sites showed reduced HS responsiveness. Furthermore, depletion of HSF-1 using siRNA also reduced the effects HS on TNF-alpha-induced IL-8 expression, demonstrating that HSF-1 could also act to regulate IL-8 gene transcription. We speculate that during evolution the CXC chemokine genes may have co-opted elements of the HS response to amplify their expression and enhance neutrophil delivery during febrile illnesses.

Peptidoglycan-mediated IL-8 expression in human alveolar type II epithelial cells requires lipid raft formation and MAPK activation

Staphylococcus aureus, a major sepsis-causing Gram-positive bacterium, invades pulmonary epithelial cells and causes lung diseases. In the lung, alveolar type II epithelial cells play an important role in innate immunity by secreting chemokines and antimicrobial peptides upon bacterial infection whereas type I cells mainly function in gas-exchange. In this study, we investigated the ability of S. aureus peptidoglycan (PGN) to induce expression of a chemokine, IL-8, in a human alveolar type II epithelial cell line, A549. PGN induces IL-8 mRNA and protein expression in a dose- and time-dependent manner. Supplementation of soluble CD14 further enhanced the PGN-induced IL-8 expression. Interestingly, PGN-induced IL-8 expression was inhibited by nystatin, a specific inhibitor for lipid rafts, but not by chlorpromazine, a specific inhibitor for clathrin-coated pits. Furthermore, PGN-induced IL-8 expression was attenuated by inhibitors for MAP kinases such as ERK, p38 kinase, and JNK/SAPK, whereas no inhibitory effect was observed by inhibitors for reactive oxygen species or protein kinase C. Electrophoretic mobility shift assay demonstrates that PGN increased the DNA binding of the transcription factors, AP-1 and NF-kappaB while minimally, NF-IL6, all of which are involved in the transcription of IL-8. Taken together, these results suggest that PGN induces IL-8 expression in a CD14-enhanced manner in human alveolar type II epithelial cells, through the formation of lipid rafts and the activation of MAP kinases, which ultimately leads to activation of AP-1, NF-kappaB, and NF-IL6.

Mutant Cu/Zn-superoxide dismutase associated with amyotrophic lateral sclerosis destabilizes vascular endothelial growth factor mRNA and downregulates its expression

Vascular endothelial growth factor (VEGF) plays a neuroprotective role in mice harboring mutations of copper-zinc superoxide dismutase 1 (SOD1) in familial amyotrophic lateral sclerosis (ALS). Conversely, the loss of VEGF expression through genetic depletion can give rise to a phenotype resembling ALS independent of SOD1 mutations. Here, we observe a profound downregulation of VEGF mRNA expression in spinal cords of G93A SOD1 mice that occurred early in the course of the disease. Using an in vitro culture model of glial cells expressing mutant SOD1, we demonstrate destabilization and downregulation of VEGF RNA with concomitant loss of protein expression that correlates with level of transgene expression. Using a luciferase reporter assay, we show that this molecular effect is mediated through a portion of the VEGF 3'-untranslated region (UTR) that harbors a class II adenylate/uridylate-rich element. Other mutant forms of SOD1 produced a similar negative effect on luciferase RNA and protein expression. Mobility shift assay with a VEGF 3'-UTR probe reveals an aberrantly migrating complex that contains mutant SOD1. We further show that the RNA stabilizing protein, HuR (human antigen R), is translocated from nucleus to cytoplasm in mutant SOD1 cells in vitro and mouse motor neurons in vivo. In summary, our data suggest that mutant SOD1 gains a novel function, possibly by altering the ribonucleoprotein complex with the VEGF 3'-UTR. We postulate that the resultant dysregulation of VEGF posttranscriptional processing critically reduces the level of this neuroprotective growth factor and accelerates the neurodegenerative process in ALS.

Tristetraprolin regulates TNF TNF-alpha mRNA stability via a proteasome dependent mechanism involving the combined action of the ERK and p38 pathways

Tumor necrosis factor-alpha (TNF-alpha) is a central mediator of inflammation. TNF-alpha expression is regulated by transcriptional and post-transcriptional mechanisms, including mRNA stability and translation. Post-transcriptional control operates through cis-elements in the 3' Untranslated-Region of the TNF-alpha mRNA to which trans-acting proteins bind. One of the best characterized trans-acting proteins is Tristetraprolin (TTP), which regulates TNF-alpha message stability. However, the precise mechanisms controlling TNF-alpha message stability are unclear, with data supporting a role for the proteasome, the exosome, and the RNA processing-body (P-body), as well as the involvement of the microRNAs. We examined the effect of proteasome inhibition on endogenous TNF-alpha mRNA stability, TNF-alpha 3'UTR reporter expression and TTP function in the RAW264.7 cells. These data establish that proteasome inhibition stabilized endogenous TNF-alpha mRNA, increased TTP protein levels but inhibited TTP mediated TNF-alpha mRNA decay. Importantly, proteasome inhibition stabilized the TNF-alpha message to the same degree as LPS stimulation. To further characterize the control of TTP function, we examined the combinatorial effect of p38, ERK and JNK activation on TNF-alpha post-transcriptional expression and TTP function. These data establish that TTP mediated TNF-alpha mRNA decay is inhibited by the combined activation of ERK and p38 and not by p38 activation alone. The combined activation of ERK/p38 was sufficient to stabilize endogenous TNF-alpha mRNA to the same degree as LPS stimulation. Together these data indicate that the proteasome is a critical control point for TTP mediated TNF-alpha mRNA decay and activation of both ERK and p38 is required to inhibit TTP function and stabilize TNF-alpha mRNA.

Prostaglandin E2 induces the expression of IL-1alpha in colon cancer cells

PGE(2) has been shown to exert pro-oncogenic effects in colorectal neoplasia through producing autocrine or paracrine growth factors. In the present study, we demonstrate that PGE(2) induced the expression of IL-1alpha in colon cancer cells, which plays critical roles in tumor metastasis and neoangiogenesis in a variety of cancers. PGE(2) increased the levels of both IL-1alpha mRNA and protein, suggesting a positive feedback loop between the IL-1 pathway and PGE(2) signaling. Mechanistically, PGE(2) induced the expression of IL-1alpha at both transcriptional and posttranscriptional levels. PGE(2) stimulated the transcriptional activity of the IL-1alpha promoter and significantly stabilized IL-1alpha mRNA. Moreover, we show that IL-1alpha enhanced colorectal neoplasia, stimulating cell migration and neoangiogenesis. Knockdown of the expression of IL-1alpha by small-interfering RNA resulted in a reduction of vascular endothelial growth factor secretion in colon cancer cells and an inhibition of tube formation by HUVECs. Thus, our results suggest that PGE(2) induces the expression of proinflammatory cytokine IL-1alpha, which may potentially enhance the proneoplastic actions of the cyclooxygenase-2/PGE(2) signaling pathway.