Biphasic increase in c-jun mRNA is required for induction of AP-1-mediated gene transcription: differential effects of muscarinic and thrombin receptor activation

Protease-Activated Receptors (PARs): Biology and Therapeutic Potential in Perioperative Stroke

Perioperative stroke is a devastating complication that occurs during surgery or within 30 days following the surgical procedure. Its prevalence ranges from 0.08 to 10% although it is most likely an underestimation, as sedatives and narcotics can substantially mask symptomatology and clinical presentation. Understanding the underlying pathophysiology and identifying potential therapeutic targets are of paramount importance. Protease-activated receptors (PARs), a unique family of G-protein-coupled receptors, are widely expressed throughout the human body and play essential roles in various physiological and pathological processes. This review elucidates the biology and significance of PARs, outlining their diverse functions in health and disease, and their intricate involvement in cerebrovascular (patho)physiology and neuroprotection. PARs exhibit a dual role in cerebral ischemia, which underscores their potential as therapeutic targets to mitigate the devastating effects of stroke in surgical patients.

Inhibition of miR-148a-3p resists hepatocellular carcinoma progress of hepatitis C virus infection through suppressing c-Jun and MAPK pathway

Objectives

The present study was committed to investigate the role of miR‐148a‐3p in HCC infected with hepatitis C virus (HCV) and the regulatory mechanism of miR‐148a‐3p/c‐Jun/MAPK signalling pathway.

Methods

Differential analysis and GSEA analysis were performed with R packages. QRT‐PCR and Western blot were used to detect RNA or protein level, respectively. The targeted relationship between miR‐148a‐3p and c‐Jun was predicted by TargetScan database and determined by double luciferase reporter assay. MTT assay and flow cytometry were used to evaluate cell proliferation, cell cycle and cell apoptosis, respectively.

Results

C‐Jun was up‐regulated, and MAPK signalling pathway was activated in HCV‐infected HCC cells. C‐Jun expression regulated inflammation‐related gene expression and had an influence on cell proliferation, cell cycle and cell apoptosis. MiR‐148a‐3p, down‐regulated in HCV‐infected HCC cells, could target c‐Jun mRNA to suppress c‐Jun protein expression.

Conclusions

MiR‐148a‐3p suppressed the proliferation of HCC cells infected with HCV through targeting c‐Jun mRNA.

G Protein-Coupled Receptor and RhoA-Stimulated Transcriptional Responses: Links to Inflammation, Differentiation, and Cell Proliferation

The low molecular weight G protein RhoA (rat sarcoma virus homolog family member A) serves as a node for transducing signals through G protein-coupled receptors (GPCRs). Activation of RhoA occurs through coupling of G proteins, most prominently, G12/13, to Rho guanine nucleotide exchange factors. The GPCR ligands that are most efficacious for RhoA activation include thrombin, lysophosphatidic acid, sphingosine-1-phosphate, and thromboxane A2. These ligands also stimulate proliferation, differentiation, and inflammation in a variety of cell and tissues types. The molecular events underlying these responses are the activation of transcription factors, transcriptional coactivators, and downstream gene programs. This review describes the pathways leading from GPCRs and RhoA to the regulation of activator protein-1, NFκB (nuclear factor κ-light-chain-enhancer of activated B cells), myocardin-related transcription factor A, and Yes-associated protein. We also focus on the importance of two prominent downstream transcriptional gene targets, the inflammatory mediator cyclooxygenase 2, and the matricellular protein cysteine-rich angiogenic inducer 61 (CCN1). Finally, we describe the importance of GPCR-induced activation of these pathways in the pathophysiology of cancer, fibrosis, and cardiovascular disease.

Orphan nuclear receptor Nur77 is a novel negative regulator of endothelin-1 expression in vascular endothelial cells

Endothelin-1 (ET-1) produced by vascular endothelial cells plays essential roles in the regulation of vascular tone and development of cardiovascular diseases. The objective of this study is to identify novel regulators implicated in the regulation of ET-1 expression in vascular endothelial cells (ECs). By using quantitative real-time PCR (qRT-PCR) and enzyme-linked immunosorbent assay (ELISA), we show that either ectopic expression of orphan nuclear receptor Nur77 or pharmacological activation of Nur77 by 6-mercaptopurine (6-MP) substantially inhibits ET-1 expression in human umbilical vein endothelial cells (HUVECs), under both basal and thrombin-stimulated conditions. Furthermore, thrombin-stimulated ET expression is significantly augmented in both Nur77 knockdown ECs and aort from Nur77 knockout mice, suggesting that Nur77 is a negative regulator of ET-1 expression. Inhibition of ET-1 expression by Nur77 occurs at gene transcriptional levels, since Nur77 potently inhibits ET-1 promoter activity, without affecting ET-1 mRNA stability. As shown in electrophoretic mobility shift assay (EMSA), Nur77 overexpression markedly inhibits both basal and thrombin-stimulated transcriptional activity of AP-1. Mechanistically, we demonstrate that Nur77 specially interacts with c-Jun and inhibits AP-1 dependent c-Jun promoter activity, which leads to a decreased expression of c-Jun, a critical component involved in both AP-1 transcriptional activity and ET-1 expression in ECs. These findings demonstrate that Nur77 is a novel negative regulator of ET-1 expression in vascular ECs through an inhibitory interaction with the c-Jun/AP-1 pathway. Activation of Nur77 may represent a useful therapeutic strategy for preventing certain cardiovascular diseases, such as atherosclerosis and pulmonary artery hypertension.

Thrombin induces Egr-1 expression in fibroblasts involving elevation of the intracellular Ca2+ concentration, phosphorylation of ERK and activation of ternary complex factor

Background

The serine protease thrombin catalyzes fibrin clot formation by converting fibrinogen into fibrin. Additionally, thrombin stimulation leads to an activation of stimulus-responsive transcription factors in different cell types, indicating that the gene expression pattern is changed in thrombin-stimulated cells. The objective of this study was to analyze the signaling cascade leading to the expression of the zinc finger transcription factor Egr-1 in thrombin-stimulated lung fibroblasts.

Results

Stimulation of 39M1-81 fibroblasts with thrombin induced a robust and transient biosynthesis of Egr-1. Reporter gene analysis revealed that the newly synthesized Egr-1 was biologically active. The signaling cascade connecting thrombin stimulation with Egr-1 gene expression required elevated levels of cytosolic Ca²⁺, the activation of diacylgycerol-dependent protein kinase C isoenzymes, and the activation of extracellular signal-regulated protein kinase (ERK). Stimulation of the cells with thrombin triggered the phosphorylation of the transcription factor Elk-1. Expression of a dominant-negative mutant of Elk-1 completely prevented Egr-1 expression in stimulated 39M1-81 cells, indicating that Elk-1 or related ternary complex factors connect the intracellular signaling cascade elicited by activation of protease-activated receptors with transcription of the Egr-1 gene. Lentiviral-mediated expression of MAP kinase phosphatase-1, a dual-specific phosphatase that dephosphorylates and inactivates ERK in the nucleus, prevented Elk-1 phosphorylation and Egr-1 biosynthesis in thrombin stimulated 39M1-81 cells, confirming the importance of nuclear ERK and Elk-1 for the upregulation of Egr-1 expression in thrombin-stimulated lung fibroblasts. 39M1-81 cells additionally express M₁ muscarinic acetylcholine receptors. A comparison between the signaling cascades induced by thrombin or carbachol showed no differences, except that signal transduction via M₁ muscarinic acetylcholine receptors required the transactivation of the EGF receptor, while thrombin signaling did not.

Conclusion

This study shows that stimulus-transcription coupling in thrombin-treated lung fibroblasts relies on the elevation of the intracellular Ca²⁺-concentration and the activation of PKC and ERK. In the nucleus, ternary complex factors function as key proteins linking the intracellular signaling cascade with enhanced transcription of the Egr-1 gene. This study further shows that the dominant-negative Elk-1 mutant is a valuable tool to study Elk-1-mediated gene transcription.

G protein-coupled receptors go extracellular: RhoA integrates the integrins

The identification of downstream effectors of G protein-coupled receptors (GPCRs) is critical for understanding the interactions between signaling cascades and for developing new pharmacological approaches for controlling GPCR-mediated responses. RhoA is a small G protein that serves as a proximal downstream effector of numerous GPCRs and regulates a variety of basic cell functions, including migration, survival, and proliferation. Intriguingly, GPCR ligands such as thrombin, sphingosine-1-phosphate, and lysophosphatidic acid, which signal through G(12/13) and activate RhoA, have recently been shown to induce the expression of the extracellular matrix protein Cyr61 (i.e., CCN1). Cyr61 is secreted and interacts with cell surface integrins to activate kinase and transcriptional cascades that are also known to contribute to cell migration, survival, and proliferation. The GPCR/RhoA/Cyr61/integrin pathway defines a novel convergence mechanism for integrating GPCR-and integrin-dependent signaling cascades that may contribute to sustained and pathophysiological responses to GPCR activation.

Thrombin receptor and RhoA mediate cell proliferation through integrins and cysteine-rich protein 61

A subset of G-protein coupled receptors (GPCRs), including the thrombin receptor (PAR1), elicits mitogenic responses. Thrombin also activates Ras homolog gene family member A (RhoA) and activating protein (AP-1) -mediated gene expression in 1321N1 astrocytoma cells, whereas the nonmitogenic agonist carbachol does not. Transcriptomic analysis was used to explore differential gene induction by these agonists and revealed that the matricellular protein cysteine-rich 61 (Cyr61/CCN1) is selectively induced by thrombin. The ability of GPCR agonists to induce Cyr61 parallels their ability to activate RhoA; agonist-stimulated Cyr61 expression is inhibited by C3 toxin. When Cyr61 is down-regulated using short interfering RNA (siRNA) or short-hairpin RNA (shRNA), thrombin-induced DNA synthesis is significantly attenuated. When Cyr61 expression is induced, it appears in the extracellular compartment and on the cell surface. Extracellular Cyr61 interacts with alpha(5), alpha(6), and beta(1) integrins on these cells, and monoclonal antibodies directed against alpha(5) and beta(1) integrins inhibit thrombin-induced DNA synthesis. Functional blockade of Cyr61 with soluble heparin or anti-Cyr61 antibodies also inhibits thrombin-induced DNA synthesis. Thus Cyr61 is a highly inducible, secreted extracellular factor through which GPCR and RhoA signaling pathways engage integrins that contribute to GPCR-mediated proliferation.

Signalling of the M3-muscarinic receptor to the anti-apoptotic pathway

The process of programmed cell death (or apoptosis) occurs widely in tissue maintenance and embryonic development, and is under tight regulatory control. It is now clear that one of the important regulators of apoptosis are G-protein-coupled receptors. In the present study, we investigate the regulatory mechanism employed by the Gq/11-coupled M3-muscarinic receptor in mediating an anti-apoptotic response. Using a CHO (Chinese-hamster ovary) cell model, we demonstrate that the M3-muscarinic receptor anti-apoptotic response is independent of calcium/phospholipase C signalling. This response can, however, be inhibited by the transcriptional inhibitor actinomycin D at a concentration that inhibits the rapid increase in gene transcription mediated by M3-muscarinic receptor stimulation. Furthermore, apoptosis in CHO cells induced by the DNA-damaging agent, etoposide, is associated with a fall in the levels of the anti-apoptotic Bcl-2 protein. This fall in Bcl-2 protein concentration can be attenuated by M3-muscarinic receptor stimulation. We conclude, therefore, that the M3-muscarinic receptor signals to the anti-apoptotic pathway via a mechanism that is independent of calcium/phospholipase C signalling, but in a manner that involves both gene transcription and the up-regulation of Bcl-2 protein.

Molecular modulation of expression of prion protein by heat shock

Prion diseases (also known as transmissible spongiform encephalopathies) are associated with the conversion of the normal cellular form of the prion protein (PrPC) to an abnormal scrapie-isoform (PrP(Sc). The conversion of PrP(C) to PrP(Sc) is post-translational and is owing to protein conformational change. This has led to the hypothesis that molecular chaperones may be involved in the folding of prion proteins, and hence the disease process. By treating human NT-2 cells with heat-shock stress, we found that both the mRNA levels for prion protein (PrP) and heat shock protein 70 (HSP70) increased simultaneously after heat treatment. Western-blot analysis of PrP also showed a two-fold increase in PrP protein level 3 after heat treatment. Furthermore, two heat-shock elements (HSEs) were located at the positions of -680 bp (HSE1; GGAACTATTCTTGACATTGCT), and -1653 bp (HSE2; TGAGAACTCAGGAAG) of the rat PrP (RaPrP) gene promoter. Luciferase reporter constructs of the RaPrP promoter with HSE expressed higher luciferase activity (10- to 15-fold) than those constructs without HSE. Electrophoretic gel mobility shift assay (EMSA) and super-shift assay confirmed the interaction of HSE1 and HSE2 with the heat-shock transcription factor-1 (HSTF-1). These results suggest that cellular stress up-regulates both the transcription and translation of PrP through interaction with the HSEs on the PrP gene promoter, resulting in an increase in protein synthesis.

Aggregated amyloid-beta protein induces cortical neuronal apoptosis and concomitant "apoptotic" pattern of gene induction

To gain a molecular understanding of neuronal responses to amyloid-beta peptide (Abeta), we have analyzed the effects of Abeta treatment on neuronal gene expression in vitro by quantitative reverse transcription-PCR and in situ hybridization. Treatment of cultured rat cortical neurons with Abeta1-40 results in a widespread apoptotic neuronal death. Associated with death is an induction of several members of the immediate early gene family. Specifically, we (1) report the time-dependent and robust induction of c-jun, junB, c-fos, and fosB, as well as transin, which is induced by c-Jun/c-Fos heterodimers and encodes an extracellular matrix protease; these gene inductions appear to be selective because other Jun and Fos family members, i.e., junD and fra-1, are induced only marginally; (2) show that the c-jun induction is widespread, whereas c-fos expression is restricted to a subset of neurons, typically those with condensed chromatin, which is a hallmark of apoptosis; (3) correlate gene induction and neuronal death by showing that each has a similar dose-response to Abeta; and (4) demonstrate that both cell death and immediate early gene induction are dependent on Abeta aggregation state. This overall gene expression pattern during this "physiologically inappropriate" apoptotic stimulus is markedly similar to the pattern we previously identified after a "physiologically appropriate" stimulus, i.e., the NGF deprivation-induced death of sympathetic neurons. Hence, the parallels identified here further our understanding of the genetic alterations that may lead neurons to apoptosis in response to markedly different insults.

Coupling of the thrombin receptor to G12 may account for selective effects of thrombin on gene expression and DNA synthesis in 1321N1 astrocytoma cells

In 1321N1 astrocytoma cells, thrombin, but not carbachol, induces AP-1-mediated gene expression and DNA synthesis. To understand the divergent effects of these G protein-coupled receptor agonists on cellular responses, we examined Gq-dependent signaling events induced by thrombin receptor and muscarinic acetylcholine receptor stimulation. Thrombin and carbachol induce comparable changes in phosphoinositide and phosphatidylcholine hydrolysis, mobilization of intracellular Ca2+, diglyceride generation, and redistribution of protein kinase C; thus, activation of these Gq-signaling pathways appears to be insufficient for gene expression and mitogenesis. Thrombin increases Ras and mitogen-activated protein kinase activation to a greater extent than carbachol in 1321N1 cells. The effects of thrombin are not mediated through Gi, since ribosylation of Gi/Go proteins by pertussis toxin does not prevent thrombin-induced gene expression or thrombin-stimulated DNA synthesis. We recently reported that the pertussis toxin-insensitive G12 protein is required for thrombin-induced DNA synthesis. We demonstrate here, using transfection of receptors and G proteins in COS-7 cells, that G alpha 12 selectively couples the thrombin receptor to AP-1-mediated gene expression. This does not appear to result from increased mitogen-activated protein kinase activity but may reflect activation of a tyrosine kinase pathway. We suggest that preferential coupling of the thrombin receptor to G12 accounts for the selective ability of thrombin to stimulate Ras, mitogen-activated protein kinase, gene expression, and mitogenesis in 1321N1 cells.

Hepatitis B virus HBx protein induces transcription factor AP-1 by activation of extracellular signal-regulated and c-Jun N-terminal mitogen-activated protein kinases

The HBx protein of hepatitis B virus is a dual-specificity activator of transcription, stimulating signal transduction pathways in the cytoplasm and transcription factors in the nucleus, when expressed in cell lines in culture. In the cytoplasm, HBx was shown to stimulate the Ras-Raf-mitogen-activated protein kinase (MAP kinase) cascade, which is essential for activation of transcription factor AP-1. Here we show that HBx protein stimulates two independently regulated members of the MAP kinase family when expressed transiently in cells. HBx protein stimulates the extracellular signal-regulated kinases (ERKs) and the c-Jun N-terminal kinases (JNKs). HBx activation of ERKs and JNKs leads to induction and activation of AP-1 DNA binding activity involving transient de novo synthesis of c-Fos protein and prolonged synthesis of c-Jun, mediated by N-terminal phosphorylation of c-Jun carried out by HBx-activated JNK. New c-Jun synthesis was blocked by coexpression with a dominant-negative MAP kinase kinase (MEK kinase, MEKK-1), confirming that HBx stimulates the prolonged synthesis of c-Jun by activating JNK signalling pathways. Activation of the c-fos gene was blocked by coexpression with a Raf-C4 catalytic mutant, confirming that HBx induces c-Fos by acting on Ras-Raf linked pathways. HBx activation of ERK and JNK pathways resulted in prolonged accumulation of AP-1-c-Jun dimer complexes. HBx activation of JNK and sustained activation of c-jun, should they occur in the context of hepatitis B virus infection, might play a role in viral transformation and pathogenesis.

Antitumor promotion by phenolic antioxidants: inhibition of AP-1 activity through induction of Fra expression

Induction of phase 2 detoxification enzymes by phenolic antioxidants can account for prevention of tumor initiation but cannot explain why these compounds inhibit tumor promotion. Phase 2 genes are induced through an antioxidant response element (ARE). Although the ARE resembles an AP-1 binding site, we show that the major ARE binding and activating protein is not AP-1. Interestingly, AP-1 DNA binding activity was induced by the phenolic antioxidant tert-butylhydroquinone (BHQ), but the induction of AP-1 transcriptional activity by the tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA) was inhibited by this compound. BHQ induced expression of c-jun, junB, fra-1, and fra-2, which encode AP-1 components, but was a poor inducer of c-fos and had no effect on fosB. Like c-Fos and FosB, the Fra proteins heterodimerize with Jun proteins to form stable AP-1 complexes. However, Fra-containing AP-1 complexes have low transactivation potential. Furthermore, Fra-1 repressed AP-1 activity induced by either TPA or expression of c-Jun and c-Fos. We therefore conclude that inhibitory AP-1 complexes composed of Jun-Fra heterodimers, induced by BHQ, antagonize the transcriptional effects of the tumor promoter TPA, which are mediated by Jun-Fos heterodimers. Since AP-1 is an important mediator of tumor promoter action, these findings may explain the anti-tumor-promoting activity of phenolic antioxidants.

Identification of a 68 kDa protein species as a specific DNA-binding component of the H3abp complex interacting with the histone H3.2 G1/S regulatory domain

The hamster histone H3.2 promoter contains a protein binding site (referred to as site X) required for G1/S transcriptional activation. We report here that nuclear extracts prepared from serum synchronized cells at various stages of the cell cycle show a biphasic increase in the H3.2 specific complex, H3abp, binding to site X. An increase in binding activity occurs as cells first enter the cell cycle and later at the G1/S border. The H3.2 specific binding activity is enhanced by Mg2+ and Ca2+ in vitro, but is inhibited by Zn2+. Site X resembles a Jun/AP-1 site, but previously it has been shown that the H3abp complex is immunologically distinct from the characterized AP-1 proteins. Here, we identify the size of the hamster nuclear protein(s) that bind specifically to the H3abp site by ultra-violet crosslinking and renaturation of specific protein bands following gel electrophoresis. In addition, we purify H3abp by affinity chromatography and show that the purified H3abp has a different methylation interference profile from AP-1. Our results indicate that a protein species around 68 kDa is the major DNA binding component of the H3abp complex and it binds specifically to the histone promoter site required for G1/S regulation.

Plasmin modulates the thrombin-evoked calcium response in C6 glioma cells

Extracellular proteinases may be selectively targeted to cell surfaces by specific receptors or binding sites. In previous studies, we have characterized cellular binding sites for plasminogen and plasmin on rat C6 glioma cells. In this investigation, we studied the response of C6 cells to alpha-thrombin and plasmin by measuring the rapid kinetics of free intracellular Ca2+ concentrations ([Ca2+]i). Thrombin produced a strong, concentration-dependent rise in [Ca2+]i with an onset within 3 s and peak levels achieved in less than 10 s. A similar response was also evoked by an SFLLRN-containing thrombin-agonist peptide. C6 cells did not respond to plasmin (25 nM-1.5 microM). By contrast, pretreatment of C6 cells with 100 nM plasmin significantly inhibited the [Ca2+]i response to thrombin and the thrombin-agonist peptide. The peak [Ca2+]i response to thrombin, in cells pretreated with plasmin, was reduced by approx. 50%. The effect of plasmin on the cellular response to thrombin was selective, as pretreatment of the cells with plasmin did not affect the [Ca2+]i response to platelet-activating factor. Di-isopropylphosphorylplasmin and plasminogen did not inhibit the cellular response to thrombin, indicating that plasmin activity is required and that occupancy of cellular plasmin(ogen)-binding sites alone is insufficient. These studies demonstrate that plasmin does not directly induce a response in C6 cells, but may affect cellular function by specifically modulating the thrombin response.

Persistent induction of c-fos and c-jun expression by asbestos

To investigate the mechanisms of asbestos-induced carcinogenesis, expression of c-fos and c-jun protooncogenes was examined in rat pleural mesothelial cells and hamster tracheal epithelial cells after exposure to crocidolite or chrysotile asbestos. In contrast to phorbol 12-myristate 13-acetate, which induces rapid and transient increases in c-fos and c-jun mRNA, asbestos causes 2- to 5-fold increases in c-fos and c-jun mRNA that persist for at least 24 hr in mesothelial cells. The induction of c-fos and c-jun mRNA by asbestos in mesothelial cells is dose-dependent and is most pronounced with crocidolite, the type of asbestos most pathogenic in the causation of pleural mesothelioma. Induction of c-jun gene expression by asbestos occurs in tracheal epithelial cells but is not accompanied by a corresponding induction of c-fos gene expression. In both cell types, asbestos induces increases in protein factors that bind specifically to the DNA sites that mediate gene expression by the AP-1 family of transcription factors. The persistent induction of AP-1 transcription factors by asbestos suggests a model of asbestos-induced carcinogenesis involving chronic stimulation of cell proliferation through activation of the early response gene pathway that includes c-jun and/or c-fos.