Treatment with IL-17 prolongs the half-life of chemokine CXCL1 mRNA via the adaptor TRAF5 and the splicing-regulatory factor SF2 (ASF).

Interleukin 17 (IL-17) promotes the expression of chemokines and cytokines via the induction of gene transcription and post-transcriptional stabilization of mRNA. We show here that IL-17 enhanced the stability of chemokine CXCL1 mRNA and other mRNAs through a pathway that involved the adaptor Act1, the adaptors TRAF2 or TRAF5 and the splicing factor SF2 (also known as alternative splicing factor (ASF)). TRAF2 and TRAF5 were necessary for IL-17 to signal the stabilization of CXCL1 mRNA. Furthermore, IL-17 promoted the formation of complexes of TRAF5-TRAF2, Act1 and SF2 (ASF). Overexpression of SF2 (ASF) shortened the half-life of CXCL1 mRNA, whereas depletion of SF2 (ASF) prolonged it. SF2 (ASF) bound chemokine mRNA in unstimulated cells, whereas the SF2 (ASF)-mRNA interaction was much lower after stimulation with IL-17. Our findings define an IL-17-induced signaling pathway that links to the stabilization of selected mRNA species through Act1, TRAF2-TRAF5 and the RNA-binding protein SF2 (ASF).

Postexposure Liponucleotide Prophylaxis and Treatment Attenuates Acute Respiratory Distress Syndrome in Influenza-infected Mice.

There is an urgent need for new drugs for patients with acute respiratory distress syndrome (ARDS), including those with coronavirus disease (COVID-19). ARDS in influenza-infected mice is associated with reduced concentrations of liponucleotides (essential precursors for de novo phospholipid synthesis) in alveolar type II (ATII) epithelial cells. Because surfactant phospholipid synthesis is a primary function of ATII cells, we hypothesized that disrupting this process could contribute significantly to the pathogenesis of influenza-induced ARDS. The goal of this study was to determine whether parenteral liponucleotide supplementation can attenuate ARDS. C57BL/6 mice inoculated intranasally with 10,000 plaque-forming units/mouse of H1N1 influenza A/WSN/33 virus were treated with CDP (cytidine 5'-diphospho)-choline (100 µg/mouse i.p.) ± CDP -diacylglycerol 16:0/16:0 (10 µg/mouse i.p.) once daily from 1 to 5 days after inoculation (to model postexposure influenza prophylaxis) or as a single dose on Day 5 (to model treatment of patients with ongoing influenza-induced ARDS). Daily postexposure prophylaxis with CDP-choline attenuated influenza-induced hypoxemia, pulmonary edema, alterations in lung mechanics, impairment of alveolar fluid clearance, and pulmonary inflammation without altering viral replication. These effects were not recapitulated by the daily administration of CTP (cytidine triphosphate) and/or choline. Daily coadministration of CDP-diacylglycerol significantly enhanced the beneficial effects of CDP-choline and also modified the ATII cell lipidome, reversing the infection-induced decrease in phosphatidylcholine and increasing concentrations of most other lipid classes in ATII cells. Single-dose treatment with both liponucleotides at 5 days after inoculation also attenuated hypoxemia, altered lung mechanics, and inflammation. Overall, our data show that liponucleotides act rapidly to reduce disease severity in mice with severe influenza-induced ARDS.

Exercise-induced alterations in phospholipid hydrolysis, airway surfactant, and eicosanoids and their role in airway hyperresponsiveness in asthma.

The mechanisms responsible for driving endogenous airway hyperresponsiveness (AHR) in the form of exercise-induced bronchoconstriction (EIB) are not fully understood. We examined alterations in airway phospholipid hydrolysis, surfactant degradation, and lipid mediator release in relation to AHR severity and changes induced by exercise challenge. Paired induced sputum (n = 18) and bronchoalveolar lavage (BAL) fluid (n = 11) were obtained before and after exercise challenge in asthmatic subjects. Samples were analyzed for phospholipid structure, surfactant function, and levels of eicosanoids and secreted phospholipase A2 group 10 (sPLA2-X). A primary epithelial cell culture model was used to model effects of osmotic stress on sPLA2-X. Exercise challenge resulted in increased surfactant degradation, phospholipase activity, and eicosanoid production in sputum samples of all patients. Subjects with EIB had higher levels of surfactant degradation and phospholipase activity in BAL fluid. Higher basal sputum levels of cysteinyl leukotrienes (CysLTs) and prostaglandin D2 (PGD2) were associated with direct AHR, and both the postexercise and absolute change in CysLTs and PGD2 levels were associated with EIB severity. Surfactant function either was abnormal at baseline or became abnormal after exercise challenge. Baseline levels of sPLA2-X in sputum and the absolute change in amount of sPLA2-X with exercise were positively correlated with EIB severity. Osmotic stress ex vivo resulted in movement of water and release of sPLA2-X to the apical surface. In summary, exercise challenge promotes changes in phospholipid structure and eicosanoid release in asthma, providing two mechanisms that promote bronchoconstriction, particularly in individuals with EIB who have higher basal levels of phospholipid turnover.

Cellular stress amplifies TLR3/4-induced CXCL1/2 gene transcription in mononuclear phagocytes via RIPK1.

The impact of environmental stressors on the magnitude of specific chemokine gene expression was examined in mouse bone marrow-derived macrophages stimulated through various TLRs. Levels of TLR-stimulated CXCL1 and CXCL2 but not CXCL10 or CCL5 mRNAs were selectively enhanced (>10-fold) in stressed macrophages. The amplification was also manifested for other proinflammatory cytokines, including TNF-α, IL-1α, and IL-6. Responses through TLR3 and TLR4 exhibited the greatest sensitivity, reflecting a requirement for Toll/IL-IR domain-containing adaptor-inducing IFN-ß (TRIF), the adaptor protein selectively associated with these TLRs. IFN regulatory factor 3, a transcription factor that is downstream of TLR4/TRIF signaling, was not required for sensitivity to stress-induced chemokine amplification. c/EBP homologous protein and X box binding protein 1 have been reported to enhance inflammatory cytokine responses but are not required for amplification of TLR3/4-induced CXCL1 expression. Rather, receptor-interacting protein kinase 1, a kinase also linked with TLR3/4/TRIF signaling, is required and involves a stress-dependent increase in its abundance and ubiquitination. Whereas NF-κB activation is necessary for TLR-induced chemokine gene transcription, this factor does not appear to be the primary mechanistic target of environmental stress. The application of stress also enhanced chemokine expression in macrophages infiltrating the peritoneal cavity but was not observed in the resident peritoneal cells or in the liver. These findings identify novel mechanisms for modulating the magnitude and duration of selective TLR-induced chemokine and cytokine gene expression and further establish the importance of cell stress pathways in coordinating the outcomes of cellular and tissue injury.

Method for depletion of mitochondria DNA in human bronchial epithelial cells.

Mitochondria are increasingly recognized to play a role in the airway inflammation of asthma. Model systems to study the role of mitochondrial gene expression in bronchial epithelium are lacking. Here, we create custom bronchial epithelial cell lines that are depleted of mitochondrial DNA. One week of ethidium bromide (EtBr) treatment led to ∼95 % reduction of mtDNA copy number (mtDNA-CN) in cells, which was further reduced by addition of 25 µM 2',3'-dideoxycytidin (ddC). Treatment for up to three weeks with EtBr and ddC led to near complete loss of mtDNA. The basal oxygen consumption rate (OCR) of mtDNA-depleted BET-1A and BEAS-2B cells dropped to near zero. Glycolysis measured by extracellular acidification rate (ECAR) increased ∼two-fold in cells when mtDNA was eliminated. BET-1A ρ0 and BEAS-2B ρ0 cells were cultured for two months, frozen and thawed, cultured for two more months, and maintained near zero mtDNA-CN. Mitochondrial DNA-depleted BET-1A ρ0 and BEAS-2B ρ0 cell lines are viable, lack the capacity for aerobic respiration, and increase glycolysis.•BET-1A and BEAS-2B cells were treated with ethidium bromide (EtBr) with or without 2',3'-dideoxycytidine (ddC) to create cells lacking mitochondrial DNA (mtDNA).•Cells' mtDNA copy number relative to nuclear DNA (nDNA) were verified by quantitative polymerase chain reaction (qPCR).•Cells were also assessed for oxidative phosphorylation by measures of oxygen consumption using the Seahorse analyzer.

Method for Depletion of Mitochondria DNA in Human Bronchial Epithelial Cells.

Introduction: Mitochondria are increasingly recognized to play a role in the airway inflammation of asthma. Model systems to study the role of mitochondrial gene expression in bronchial epithelium are lacking. Here, we create custom bronchial epithelial cell lines derived from primary airway epithelium that are depleted of mitochondrial DNA. Methods: We treated BET-1A and BEAS-2B cells with ethidium bromide (EtBr) with or without 2',3'-dideoxycytidine (ddC) to create cells lacking mitochondrial DNA (mtDNA). Cells' mtDNA copy number were verified by quantitative polymerase chain reaction (qPCR) in comparison to nuclear DNA (nDNA). Cells were also assessed for oxidative phosphorylation by measures of oxygen consumption using the Seahorse analyzer. Results: One week of EtBr treatment led to ~95% reduction of mtDNA copy number (mtDNA-CN) in cells (mtDNA-CN, mean±SE, baseline vs. treatment: BEAS-2B, 820 ± 62 vs. 56 ± 9; BET-1A, 957 ± 52 vs. 73 ± 2), which was further reduced by addition of 25 µM ddC (mtDNA-CN: BEAS-2B, 2.8; BET-1A, 47.9). Treatment for up to three weeks with EtBr and ddC led to near complete loss of mtDNA (mtDNA-CN: BEAS-2B, 0.1; BET-1A, 0.3). The basal oxygen consumption rate (OCR) of mtDNA-depleted BET-1A and BEAS-2B cells dropped to near zero. Glycolysis measured by extracellular acidification rate (ECAR) increased ~two-fold in cells when mtDNA was eliminated [ECAR (mpH/min/103 cells), baseline vs. treatment: BEAS-2B, 0.50 ± 0.03 vs. 0.94 ± 0.10 P=0.005; BET-1A, 0.80 ± 0.04 vs. 1.14 ± 0.06 P=0.001]. Conclusion: Mitochondrial DNA-depleted BET-1A ρ0 and BEAS-2B ρ0 cell lines are viable, lack the capacity for aerobic respiration, and increase glycolysis. This cell model system can be used to further test mitochondrial mechanisms of inflammation in bronchial epithelial cells.

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.

IL-17 signaling for mRNA stabilization does not require TNF receptor-associated factor 6.

IL-17 alone is a relatively weak inducer of gene expression, but cooperates with other cytokines, including TNF-alpha, to generate a strong response in part via prolongation of mRNA t(1/2). Because TNFR-associated factor 6 (TRAF6) has been reported to be essential for signaling by IL-17, we examined its involvement in IL-17-mediated mRNA stabilization. Although overexpression of TRAF6 in HeLa cells activates NF-kappaB, it does not stabilize transfected KC mRNA. Furthermore, a dominant-negative TRAF6 abrogates NF-kappaB activation, but does not block IL-17-induced chemokine mRNA stabilization. IL-17 can stabilize KC and MIP-2 mRNAs comparably in TNF-alpha-treated mouse embryo fibroblasts from TRAF6(+/+) and TRAF6(-/-) mice. TRAF6 is known to couple upstream signals with activation of p38 MAPK and mitogen activated protein kinase activated protein kinase 2, both of which have been shown to be important for Toll/IL-1R-mediated mRNA stabilization in various cell types. Inhibition of p38 MAPK, however, does not block IL-17-induced KC mRNA stabilization, and IL-17 can stabilize KC mRNA equally in mouse embryo fibroblasts from both wild-type and mitogen activated protein kinase activated protein kinase 2/3 doubly-deficient mice. Finally, IL-17 can amplify the levels of multiple TNF-alpha-stimulated mRNAs in wild-type and TRAF6-deficient cells, but not in cells from Act1(-/-) mice. Collectively, these findings demonstrate the existence of a TRAF6/p38 MAPK-independent pathway that couples the IL-17R with enhanced mRNA stability. Because the most potent effects of IL-17 on gene expression are obtained in cooperation with other cytokines such as TNF-alpha, these findings suggest that this pathway is a major contributing mechanism for response to IL-17.

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.

Three Alveolar Phenotypes Govern Lung Function in Murine Ventilator-Induced Lung Injury.

Mechanical ventilation is an essential lifesaving therapy in acute respiratory distress syndrome (ARDS) that may cause ventilator-induced lung injury (VILI) through a positive feedback between altered alveolar mechanics, edema, surfactant inactivation, and injury. Although the biophysical forces that cause VILI are well documented, a knowledge gap remains in the quantitative link between altered parenchymal structure (namely alveolar derecruitment and flooding), pulmonary function, and VILI. This information is essential to developing diagnostic criteria and ventilation strategies to reduce VILI and improve ARDS survival. To address this unmet need, we mechanically ventilated mice to cause VILI. Lung structure was measured at three air inflation pressures using design-based stereology, and the mechanical function of the pulmonary system was measured with the forced oscillation technique. Assessment of the pulmonary surfactant included total surfactant, distribution of phospholipid aggregates, and surface tension lowering activity. VILI-induced changes in the surfactant included reduced surface tension lowering activity in the typically functional fraction of large phospholipid aggregates and a significant increase in the pool of surface-inactive small phospholipid aggregates. The dominant alterations in lung structure at low airway pressures were alveolar collapse and flooding. At higher airway pressures, alveolar collapse was mitigated and the flooded alveoli remained filled with proteinaceous edema. The loss of ventilated alveoli resulted in decreased alveolar gas volume and gas-exchange surface area. These data characterize three alveolar phenotypes in murine VILI: flooded and non-recruitable alveoli, unstable alveoli that derecruit at airway pressures below 5 cmH2O, and alveoli with relatively normal structure and function. The fraction of alveoli with each phenotype is reflected in the proportional changes in pulmonary system elastance at positive end expiratory pressures of 0, 3, and 6 cmH2O.

Chemokine and chemoattractant receptor expression: post-transcriptional regulation.

The magnitude and character of the inflammatory process are determined in part via the trafficking of leukocytes into sites of injury and infection, and this process depends on proper control of the expression of genes encoding chemoattractant peptides and their receptors. Although these controls operate at multiple mechanistic levels, recent evidence indicates that post-transcriptional events governing the half-life of select mRNAs are important determinants. Adenine-uridine rich elements (AREs) located within 3' untranslated regions (UTRs) confer constitutive mRNA instability and in some cases, stabilization following stimulation by ligands of the Toll-IL-1 receptor (TIR) family. Although the importance of AREs in determining activity and mRNA half-life is well-recognized, the mechanistic scope and diversity remain poorly understood. Using the mouse KC or CXCL1 gene as a model, we have demonstrated that the abundance of mRNA and protein produced during an inflammatory response depends on multiple mechanistically distinct AREs present in the 3' UTR of the mRNA. The mRNA encoding the receptor for N-terminal formyl-methionine-containing peptides is also unstable and subject to stabilization in response to TIR ligands. These two models can, however, be readily distinguished from one another on the basis of specific stimulus sensitivity and the signaling pathways, through which such stimuli couple to the control of mRNA decay. These models demonstrate the substantial diversity operative in the post-transcriptional regulation of inflammatory gene expression.

IL-4 inhibits expression of the formyl peptide receptor gene in mouse peritoneal macrophages.

Regulation of leukocyte recruitment is an important determinant of the host response to microbial infection. Because tissue infiltration by inflammatory cells represents a potential source of unnecessary tissue damage, the process may be controlled by modulating the expression of chemoattractants and the receptors through which they promote directed leukocyte migration. In the present report, we show that expression of the receptor for chemotactic formylated peptides (FPR1)is negatively regulated in both macrophages and neutrophils by interleukin-4 (IL-4). The reduction of FPR1 mRNA occurs rapidly in response to both IL-4 and IL-13 but endures for <4 h after the removal of IL-4. As with many other responses to IL-4 and IL-13, suppression of FPR1 expression is dependent on the activation of Stat6. The inhibitory effect exhibits relative stimulus specificity in that other Stat-activating cytokines, such as interferon-gamma (IFN-gamma), IFN-beta, and IL-10, have no effect. Using nuclear run-on analysis, the rate of FPR1 gene transcription is high but is not suppressed by IL-4. Moreover, IL-4 does not appear to alter the rate of FPR mRNA decay. Nevertheless, FPR mRNA exhibits a short half-life (< or =2 h), and this appears to be a critical feature of the ability of IL-4 to reduce expression. Taken together, the data suggest that IL-4 and IL-13 suppress the expression of FPR1 mRNA via a mechanism that operates to eliminate primary transcripts prior to maturation and depends on the constitutive instability of preexisiting mRNA.

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.

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.

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.

IL-17 enhances chemokine gene expression through mRNA stabilization.

IL-17 plays an important role in host defense and autoimmunity via the induction of proinflammatory gene expression, particularly in combination with TNF-alpha. The molecular mechanisms by which IL-17 regulates such expression are not well understood. Using the mouse chemokine CXCL1 (KC) gene as a model, we have examined the effects of IL-17 alone or in combination with TNF-alpha on transcriptional and posttranscriptional events. Although treatment of mouse embryonic fibroblasts with IL-17 alone only modestly increased KC expression, the combination of IL-17 with TNF-alpha induced a synergistic response. IL-17 treatment exerted a strong posttranscriptional effect by extending the t1/2 of the highly unstable, TNF-alpha-induced KC mRNA. Using a tetracycline-regulated transgene in HeLa cells, we determined that IL-17 treatment alone promoted stabilization of KC mRNA in the absence of TNF-alpha. IL-17 treatment exerted little effect on KC transcription or NF-kappaB activation, suggesting that it primarily acts posttranscriptionally. We identified a number of other mRNAs whose t1/2 are prolonged in response to IL-17, suggesting that this is a common mechanism by which IL-17 promotes enhanced gene expression. Finally, activator of NF-kappaB1 protein (Act1), an adaptor protein recently implicated in IL-17 signaling, was necessary for IL-17-induced stabilization, and overexpression of Act1 resulted in stabilization of KC mRNA, indicating that events downstream of Act1 are sufficient to initiate this process. Thus, the synergy between TNF-alpha and IL-17 reflects their independent actions on KC gene expression; TNF-alpha serves as a stimulus to initiate transcription through activation of NF-kappaB, whereas IL-17 drives mRNA stabilization through an Act1-dependent pathway.

Lipopolysaccharide induces formyl peptide receptor 1 gene expression in macrophages and neutrophils via transcriptional and posttranscriptional mechanisms.

Bacterial infection promotes the infiltration of inflammatory leukocytes mediated in part by receptors for formyl-methionine-terminated peptides. In this study, we show that LPS can markedly enhance the expression of the formyl peptide receptor gene (FPR1) in mouse macrophages and neutrophils by enhancing transcription and by stabilization of the mRNA. In untreated cells, FPR1 mRNA exhibits a half-life of approximately 90 min and this is markedly increased (to >6 h) following stimulation with LPS. Although FPR1 mRNA levels remained elevated over baseline for >20 h after stimulation, the half-life of the message is prolonged only transiently. LPS-induced FPR1 mRNA expression is mediated in part by the intermediate production of secreted factors. First, the response to LPS is partially blocked by the translational inhibitor cycloheximide. Second, a heat-labile but polymyxin B-insensitive factor present in supernatants from LPS-treated cells stimulates enhanced expression of FPR1 mRNA and, like LPS, promotes stabilization of FPR1 mRNA. Furthermore, supernatants from LPS-treated wild-type macrophages can stimulate FPR1 mRNA expression in LPS-insensitive macrophages from TLR4-mutant mice. Elevated FPR1 mRNA expression is also induced in response to ligands for TLR2 and TLR3. TNF-alpha but not IL-1, IL-6, IFN-beta, and IFN-gamma can mimic the effects of LPS although other factors apparently also contribute. Collectively, these findings define a distinct molecular pattern of response to TLR stimulation in inflammatory phagocytes and demonstrate that regulation of FPR1 expression is achieved through both transcriptional and posttranscriptional mechanisms.

Functionally independent AU-rich sequence motifs regulate KC (CXCL1) mRNA.

Certain pro-inflammatory chemokine mRNAs containing adenine/uridine-rich sequence elements (AREs) in their 3' untranslated regions (3'-UTRs) are known to exhibit constitutive instability and sensitivity to proinflammatory stimuli resulting in the stabilization of the message. Using tetR-regulated transcription we now show that the 3'-UTR of the mouse CXCL1 (KC) mRNA contains at least two ARE motifs that are structurally and functionally distinct. A fragment of 77 nucleotides containing 4 clustered AUUUA pentamers located at the 5'-end of the KC 3'-UTR is only modestly unstable yet promotes markedly enhanced, post-transcriptional protein production in response to either interleukin-1alpha (IL-1alpha) or lipopolysaccharide (LPS), suggesting translational regulation. In contrast, a fragment containing 3 isolated AUUUA pentamers corresponding to the residual 3' 400 nucleotides of the KC 3'-UTR confers both instability and is stabilized in response to IL-1alpha. Although the clustered AUUUA pentamers in the upstream region are required for stimulus sensitivity, mutation of all three pentamers in the downstream region has little or no effect on either instability or stimulus sensitivity. The upstream region is comparably stabilized in response to either IL-1alpha or LPS, whereas the AUUUA-independent downstream determinant is differentially more sensitive to IL-1alpha. Finally, using UV-induced RNA cross-linking, these functionally independent sequences exhibit different patterns of interaction with RNA-binding proteins. Collectively, these findings document the presence of multiple independent determinants of KC mRNA function and demonstrate that these operate via distinct mechanisms.

TGFbeta inhibits LPS-induced chemokine mRNA stabilization.

The mechanisms involved in anti-inflammatory action of transforming growth factor beta (TGFbeta) have been examined by evaluating its effect on chemokine gene expression in mouse macrophages. Lipopolysaccharide (LPS)-stimulated expression of the CXC chemokines KC and MIP-2 was selectively reduced by TGFbeta in a time- and protein synthesis-dependent process. While TGFbeta had a modest effect on transcription of the KC and MIP-2 mRNAs as measured by nuclear run-on, it had no effect on LPS-stimulated luciferase expression driven by the KC promoter nor on the activation of nuclear factor kappaB (NFkappaB) DNA-binding activity and transactivation function. Interestingly, KC mRNA levels were markedly reduced by TGFbeta treatment in cells transfected with KC genomic or cDNA constructs driven from either the KC or cytomegalovirus (CMV) promoters, demonstrating the importance of sequences within the mature mRNA and suggesting that suppression may involve a posttranscriptional mechanism. In support of this possibility, LPS stimulation prolonged the half-life of KC mRNA and this stabilization response was blocked in cells treated with TGFbeta. Examination of KC mRNA expressed under control of a tetracycline-responsive promoter demonstrated that TGFbeta prevented stabilization of KC mRNA, in response to LPS but did not alter KC mRNA half-life directly. KC mRNA stabilization by LPS was dependent on activation of p38 mitogen-activated protein kinase (MAPK) activity, and TGFbeta treatment inhibited p38 MAPK activation. These findings support the hypothesis that TGFbeta-mediated suppression of chemokine gene expression involves antagonism of LPS-stimulated KC mRNA stabilization via inhibition of p38 MAPK.

Regulation of chemokine mRNA stability by lipopolysaccharide and IL-10.

IL-10 has been reported to inhibit the expression of LPS-induced proinflammatory cytokines and chemokines by altering the rate of specific mRNA decay although the molecular target(s) for its action remain unknown. In the present study, using primary peritoneal exudate macrophages and a cell culture model in which a tetracycline-responsive promoter controls transcription of CXC ligand 1 (KC) mRNA, we demonstrate that LPS promotes a time-dependent increase in KC mRNA stability. Although IL-10 had no direct effect on mRNA decay, this treatment antagonized the stabilizing action of LPS. The mechanisms involved were further explored using a cell-free mRNA degradation system. A 5'-capped, polyadenylated in vitro transcript derived from the 3'-untranslated region of KC mRNA exhibited time-dependent decay in the presence of protein extracts prepared from untreated RAW264.7 macrophages. Extracts prepared from LPS-treated RAW264.7 cells had reduced decay activity and this change was antagonized if the cells were costimulated with IL-10. A substrate in which the AU-rich element motifs were mutated exhibited minimal decay that did not vary using extracts prepared from cells treated with LPS or LPS and IL-10. A nonadenylated RNA substrate was also degraded and that activity was diminished by LPS. In concert, these findings demonstrate that KC mRNA stability is regulated by LPS-induced alterations in activities that govern both deadenylation and degradation of the mRNA body. The effects of IL-10 on KC mRNA stability reflect antagonism of the response to LPS.