CXCL12 Induces Connective Tissue Growth Factor Expression in Human Lung Fibroblasts through the Rac1/ERK,JNK, and AP-1 Pathways

CXCL12 (stromal cell-derived factor-1, SDF-1) is a potent chemokine for homing of CXCR4⁺ fibrocytes to injury sites of lung tissue, which contributes to pulmonary fibrosis. Overexpression of connective tissue growth factor (CTGF) plays a critical role in pulmonary fibrosis. In this study, we investigated the roles of Rac1, extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and activator protein-1 (AP-1) in CXCL12-induced CTGF expression in human lung fibroblasts. CXCL12 caused concentration- and time-dependent increases in CTGF expression and CTGF-luciferase activity. CXCL12-induced CTGF expression was inhibited by a CXCR4 antagonist (AMD3100), small interfering RNA of CXCR4 (CXCR4 siRNA), a dominant negative mutant of Rac1 (RacN17), a mitogen-activated protein kinase (MAPK) kinase (MEK) inhibitor (PD98059), a JNK inhibitor (SP600125), a p21-activated kinase inhibitor (PAK18), c-Jun siRNA, and an AP-1 inhibitor (curcumin). Treatment of cells with CXCL12 caused activations of Rac1, Rho, ERK, and c-Jun. The CXCL12-induced increase in ERK phosphorylation was inhibited by RacN17. Treatment of cells with PD98059 and SP600125 both inhibited CXCL12-induced c-Jun phosphorylation. CXCL12 caused the recruitment of c-Jun and c-Fos binding to the CTGF promoter. Furthermore, CXCL12 induced an increase in α-smooth muscle actin (α-SMA) expression, a myofibroblastic phenotype, and actin stress fiber formation. CXCL12-induced actin stress fiber formation and α-SMA expression were respectively inhibited by AMD3100 and CTGF siRNA. Taken together, our results suggest that CXCL12, acting through CXCR4, activates the Rac/ERK and JNK signaling pathways, which in turn initiates c-Jun phosphorylation, and recruits c-Jun and c-Fos to the CTGF promoter and ultimately induces CTGF expression in human lung fibroblasts. Moreover, overexpression of CTGF mediates CXCL12-induced α-SMA expression.     

Apoptosis signal-regulating kinase 1 mediates denbinobin-induced apoptosis in human lung adenocarcinoma cells

In the present study, we explore the role of apoptosis signal-regulating kinase 1 (ASK1) in denbinobin-induced apoptosis in human lung adenocarcinoma (A549) cells. Denbinobin-induced cell apoptosis was attenuated by an ASK1 dominant-negative mutant (ASK1DN), two antioxidants (N-acetyl-L-cysteine (NAC) and glutathione (GSH)), a c-Jun N-terminal kinase (JNK) inhibitor (SP600125), and an activator protein-1 (AP-1) inhibitor (curcumin). Treatment of A549 cells with denbinobin caused increases in ASK1 activity and reactive oxygen species (ROS) production, and these effects were inhibited by NAC and GSH. Stimulation of A549 cells with denbinobin caused JNK activation; this effect was markedly inhibited by NAC, GSH, and ASK1DN. Denbinobin induced c-Jun phosphorylation, the formation of an AP-1-specific DNA-protein complex, and Bim expression. Bim knockdown using a bim short interfering RNA strategy also reduced denbinobin-induced A549 cell apoptosis. The denbinobin-mediated increases in c-Jun phosphorylation and Bim expression were inhibited by NAC, GSH, SP600125, ASK1DN, JNK1DN, and JNK2DN. These results suggest that denbinobin might activate ASK1 through ROS production to cause JNK/AP-1 activation, which in turn induces Bim expression, and ultimately results in A549 cell apoptosis.     

Proteomic analysis of CTGF-activated lung fibroblasts: identification of IQGAP1 as a key player in lung fibroblast migration

Connective tissue growth factor (CTGF, CCN2) is overexpressed in lung fibroblasts isolated from patients with interstitial lung disease (ILD) and systemic sclerosis (SSc, scleroderma) and is considered to be a molecular marker of fibrosis. To understand the significance of elevated CTGF, we investigated the changes in lung fibroblast proteome in response to CTGF overexpression. Using 2-dimensional gel electrophoresis followed by in-gel proteolytic digestion and mass spectrometric analysis, we identified 13 proteins affected by CTGF. Several of the CTGF-induced proteins, such as pro-alpha (I) collagen and cytoskeletal proteins vinculin, moesin, and ezrin, are known to be elevated in pulmonary fibrosis, whereas 9 of 13 proteins have not been studied in pulmonary fibrosis and are, therefore, novel CTGF-responsive molecules that may have important roles in ILD. Our study demonstrates that 1 of the novel CTGF-induced proteins, IQ motif containing GTPase activating protein (IQGAP) 1, is elevated in lung fibroblasts isolated from scleroderma patients with ILD. IQGAP1 is a scaffold protein that plays a pivotal role in regulating migration of endothelial and epithelial cells. Scleroderma lung fibroblasts and normal lung fibroblasts treated with CTGF demonstrated increased rate of migration in a wound healing assay. Depletion of IQGAP1 expression by small interfering RNA inhibited CTGF-induced migration and MAPK ERK1/2 phosphorylation in lung fibroblasts. MAPK inhibitor U0126 decreased CTGF-induced cell migration and did not interfere with CTGF-induced IQGAP1 expression, suggesting that MAPK pathway is downstream of IQGAP1. These findings further implicate the importance of CTGF in lung tissue repair and fibrosis and propose that CTGF-induced migration of lung fibroblasts to the damaged tissue is mediated via IQGAP1 and MAPK signaling pathways.     

Thrombin induces NF-kappaB activation and IL-8/CXCL8 expression in lung epithelial cells by a Rac1-dependent PI3K/Akt pathway

We previously showed that thrombin induces interleukin (IL)-8/CXCL8 expression via the protein kinase C (PKC)α/c-Src-dependent IκB kinase α/β (IKKα/β)/NF-κB signaling pathway in human lung epithelial cells. In this study, we further investigated the roles of Rac1, phosphoinositide 3-kinase (PI3K), and Akt in thrombin-induced NF-κB activation and IL-8/CXCL8 expression. Thrombin-induced IL-8/CXCL8 release and IL-8/CXCL8-luciferase activity were attenuated by a PI3K inhibitor (LY294002), an Akt inhibitor (1-L-6-hydroxymethyl-chiro-inositol-2-((R)-2-O-methyl-3-O-octadecylcarbonate)), and the dominant negative mutants of Rac1 (RacN17) and Akt (AktDN). Treatment of cells with thrombin caused activation of Rac and Akt. The thrombin-induced increase in Akt activation was inhibited by RacN17 and LY294002. Stimulation of cells with thrombin resulted in increases in IKKα/β activation and κB-luciferase activity; these effects were inhibited by RacN17, LY294002, an Akt inhibitor, and AktDN. Treatment of cells with thrombin induced Gβγ, p85α, and Rac1 complex formation in a time-dependent manner. These results imply that thrombin activates the Rac1/PI3K/Akt pathway through formation of the Gβγ, Rac1, and p85α complex to induce IKKα/β activation, NF-κB transactivation, and IL-8/CXCL8 expression in human lung epithelial cells.     

Cigarette smoke extract regulates cytosolic phospholipase A2 expression via NADPH oxidase/MAPKs/AP-1 and p300 in human tracheal smooth muscle cells

Up-regulation of cytosolic phospholipase A(2) (cPLA(2)) by cigarette smoke extract (CSE) may play a critical role in airway inflammatory diseases. However, the mechanisms underlying CSE-induced cPLA(2) expression in human tracheal smooth muscle cells (HTSMCs) were not completely understood. Here, we demonstrated that CSE-induced cPLA(2) protein and mRNA expression was inhibited by pretreatment with the inhibitors of AP-1 (tanshinone IIA) and p300 (garcinol) or transfection with siRNAs of c-Jun, c-Fos, and p300. Moreover, CSE also induced c-Jun and c-Fos expression, which were inhibited by pretreatment with the inhibitors of NADPH oxidase (diphenyleneiodonium chloride and apocynin) and the ROS scavenger (N-acetyl-L-cysteine) or transfection with siRNAs of p47(phox) and NADPH oxidase (NOX)2. CSE-induced c-Fos expression was inhibited by pretreatment with the inhibitors of MEK1 (U0126) and p38 MAPK (SB202190) or transfection with siRNAs of p42 and p38. CSE-induced c-Jun expression and phosphorylation were inhibited by pretreatment with the inhibitor of JNK1/2 (SP600125) or transfection with JNK2 siRNA. CSE-stimulated p300 phosphorylation was inhibited by pretreatment with the inhibitors of NADPH oxidase and JNK1/2. Furthermore, CSE-induced p300 and c-Jun complex formation was inhibited by pretreatment with diphenyleneiodonium chloride, apocynin, N-acetyl-L-cysteine or SP600125. These results demonstrated that CSE-induced cPLA(2) expression was mediated through NOX2-dependent p42/p44 MAPK and p38 MAPK/c-Fos and JNK1/2/c-Jun/p300 pathways in HTSMCs.     

Phosphoinositide-3 kinase-Rac1-c-Jun NH2-terminal kinase signaling mediates collagen I-induced cell scattering and up-regulation of N-cadherin expression in mouse mammary epithelial cells

During epithelial-to-mesenchymal transitions (EMTs), cells must change their interactions with one another and with their extracellular matrix in a synchronized manner. To characterize signaling pathways cells use to coordinate these changes, we used NMuMG mammary epithelial cells. We showed that these cells become fibroblastic and scattered, with increased N-cadherin expression when cultured on collagen I. Rac1 and c-Jun NH2-terminal kinase (JNK) were activated when cells were plated on collagen I, and dominant inhibitory Rac1 (RacN17) or inhibition of JNK signaling prevented collagen I-induced morphological changes and N-cadherin up-regulation. Furthermore, inhibiting phosphoinositide-3 kinase (PI3K) activity prevented Rac1 and JNK activation as well as collagen I-induced N-cadherin up-regulation. These data implicate PI3K-Rac1-JNK signaling in collagen I-induced changes in NMuMG cells. To establish a role for N-cadherin in collagen I-induced cell scattering, we generated N-cadherin overexpressing and knockdown NMuMG cells and showed that knocking down N-cadherin expression prevented collagen I-induced morphological changes. Motility assays showed that cells overexpressing N-cadherin were significantly more motile than mock-transfected cells and that N-cadherin-mediated motility was collagen I dependent. In addition, we showed that cord formation and branching in three-dimensional culture (EMT-dependent events) required N-cadherin expression and PI3K-Rac1-JNK signaling.     

JunB forms the majority of the AP-1 complex and is a target for redox regulation by receptor tyrosine kinase and G protein-coupled receptor agonists in smooth muscle cells

To understand the role of redox-sensitive mechanisms in vascular smooth muscle cell (VSMC) growth, we have studied the effect of N-acetylcysteine (NAC), a thiol antioxidant, and diphenyleneiodonium (DPI), a potent NADH/NADPH oxidase inhibitor, on serum-, platelet-derived growth factor BB-, and thrombin-induced ERK2, JNK1, and p38 mitogen-activated protein (MAP) kinase activation; c-Fos, c-Jun, and JunB expression; and DNA synthesis. Both NAC and DPI completely inhibited agonist-induced AP-1 activity and DNA synthesis in VSMC. On the contrary, these compounds had differential effects on agonist-induced ERK2, JNK1, and p38 MAP kinase activation and c-Fos, c-Jun, and JunB expression. NAC inhibited agonist-induced ERK2, JNK1, and p38 MAP kinase activation and c-Fos, c-Jun, and JunB expression except for platelet-derived growth factor BB-induced ERK2 activation. In contrast, DPI only inhibited agonist-induced p38 MAP kinase activation and c-Fos and JunB expression. Antibody supershift assays indicated the presence of c-Fos and JunB in the AP-1 complex formed in response to all three agonists. In addition, cotransfection of VSMC with expression plasmids for c-Fos and members of the Jun family along with the AP-1-dependent reporter gene revealed that AP-1 with c-Fos and JunB composition exhibited a higher transactivating activity than AP-1 with other compositions tested. All three agonists significantly stimulated reactive oxygen species production, and this effect was inhibited by both NAC and DPI. Together, these results strongly suggest a role for redox-sensitive mechanisms in agonist-induced ERK2, JNK1, and p38 MAP kinase activation; c-Fos, c-Jun, and JunB expression; AP-1 activity; and DNA synthesis in VSMC. These results also suggest a role for NADH/NADPH oxidase activity in some subset of early signaling events such as p38 MAP kinase activation and c-Fos and JunB induction, which appear to be important in agonist-induced AP-1 activity and DNA synthesis in VSMC.     

Akt inhibits MLK3/JNK3 signaling by inactivating Rac1: a protective mechanism against ischemic brain injury

The overall goal of this study was to determine the molecular basis by which mixed-lineage kinase 3 (MLK3) kinase and its signaling pathways are negatively regulated by the pro-survival Akt pathway in cerebral ischemia. We demonstrated that tyrosine phosphorylation of the phosphatase and tensin homolog deleted on chromosome 10 (PTEN) underlies the increased Akt-Ser473 phosphorylation by orthovanadate. Co-immunoprecipitation analysis revealed that endogenous Akt physically interacts with Rac1 in the hippocampal CA1 region, and this interaction is promoted on tyrosine phosphatase inhibition. The elevated Akt activation can deactivate MLK3 by phosphorylation at the Ser71 residue of Rac1, a small Rho family of guanidine triphosphatases required for MLK3 autophosphorylation. Subsequently, inhibition of c-Jun N-terminal kinase 3 (JNK3) results in decreased serine phosphorylation of 14-3-3, a cytoplasmic anchor of Bax, and prevents ischemia-induced mitochondrial translocation of Bax, release of cytochrome c and activation of caspase 3. At the same time, the expression of Fas-ligand decreases in the CA1 region after inhibition of c-Jun activation. The neuroprotective effect of Akt activation is significant in the CA1 region after global cerebral ischemia. Our results suggest that the activation of the pro-apoptotic MLK3/JNK3 cascade induced by ischemic stress can be suppressed through activation of the anti-apoptotic phosphatidylinositol 3-kinase/Akt pathway, which provides a direct link between Akt and the family of stress-activated kinases.     

Oxidized LDL induces transcription factor activator protein-1 in rat mesangial cells

It has been shown that oxidized low-density lipoprotein (ox-LDL), through the activation of glomerular cells, stimulates pathobiological processes involved in monocyte infiltration into the mesangium. The underlying molecular mechanisms are not fully understood. The present study showed that ox-LDL strongly induced AP-1 binding activity in rat mesangial cells (RMCs) in a dose- and time-dependent manner, reaching the maximal activation at 250 microg ml(-1) within 24 h. The results from mobility shift assays and Western blotting analysis revealed that this AP-1 binding increase involved c-Jun, but not c-Fos. Moreover, this ox-LDL-increased AP-1 binding was inhibited by several protein kinase (PK) inhibitors: the protein kinase C (PKC) inhibitor Bisindolylmaleimide I, the cAMP-dependent PK (PKA) inhibitor H89, and the tyrosine PK (PTK) inhibitor genistein. Protein phosphorylation represents mitogen-activated protein kinase (MAPK) activity. Therefore, we examined the role of ox-LDL on the activation of mesangial cell JNK/SAPK, the only recognized protein kinase that catalyses phosphorylation of c-Jun. The incubation of mesangial cells with ox-LDL induced phosphorylation of JNK1/SAPK dose dependently, with the maximal response at 150 microg ml(-1). This study demonstrates that multiple kinase activities are involved in the mechanism of ox-LDL-induced AP-1 activation in mesangial cells, and ox-LDL stimulates AP-1 through JNK-c-Jun other than MEK-c-Fos signalling pathway.     

Activation of c-Jun N-terminal kinase 1 by UV irradiation is inhibited by wortmannin without affecting c-iun expression

Activation of c-Jun N-terminal kinases (JNKs)/stress-activated protein kinases is an early response of cells upon exposure to DNA-damaging agents. JNK-mediated phosphorylation of c-Jun is currently understood to stimulate the transactivating potency of AP-1 (e.g., c-Jun/c-Fos; c-Jun/ATF-2), thereby increasing the expression of AP-1 target genes. Here we show that stimulation of JNK1 activity is not a general early response of cells exposed to genotoxic agents. Treatment of NIH 3T3 cells with UV light (UV-C) as well as with methyl methanesulfonate (MMS) caused activation of JNK1 and an increase in c-Jun protein and AP-1 binding activity, whereas antineoplastic drugs such as mafosfamide, mitomycin C, N-hydroxyethyl-N-chloroethylnitrosourea, and treosulfan did not elicit this response. The phosphatidylinositol 3-kinase inhibitor wortmannin specifically blocked the UV-stimulated activation of JNK1 but did not affect UV-driven activation of extracellular regulated kinase 2 (ERK2). To investigate the significance of JNK1 for transactivation of c-jun, we analyzed the effect of UV irradiation on c-jun expression under conditions of wortmannin-mediated inhibition of UV-induced stimulation of JNK1. Neither the UV-induced increase in c-jun mRNA, c-Jun protein, and AP-1 binding nor the activation of the collagenase and c-jun promoters was affected by wortmannin. In contrast, the mitogen-activated protein kinase/ERK kinase inhibitor PD98056, which blocked ERK2 but not JNK1 activation by UV irradiation, impaired UV-driven c-Jun protein induction and AP-1 binding. Based on the data, we suggest that JNK1 stimulation is not essential for transactivation of c-jun after UV exposure, whereas activation of ERK2 is required for UV-induced signaling leading to elevated c-jun expression.     

Sphingosine-1-Phosphate Mediates ICAM-1-Dependent Monocyte Adhesion through p38 MAPK and p42/p44 MAPK-Dependent Akt Activation

Up-regulation of intercellular adhesion molecule-1 (ICAM-1) is frequently implicated in lung inflammation. Sphingosine-1-phosphate (S1P) has been shown to play a key role in inflammation via adhesion molecules induction, and then causes lung injury. However, the mechanisms underlying S1P-induced ICAM-1 expression in human pulmonary alveolar epithelial cells (HPAEpiCs) remain unclear. The effect of S1P on ICAM-1 expression was determined by Western blot and real-time PCR. The involvement of signaling pathways in these responses was investigated by using the selective pharmacological inhibitors and transfection with siRNAs. S1P markedly induced ICAM-1 expression and monocyte adhesion which were attenuated by pretreatment with the inhibitor of S1PR1 (W123), S1PR3 (CAY10444), c-Src (PP1), EGFR (AG1478), PDGFR (AG1296), MEK1/2 (U0126), p38 MAPK (SB202190), JNK1/2 (SP600125), PI3K (LY294002), or AP-1 (Tanshinone IIA) and transfection with siRNA of S1PR1, S1PR3, c-Src, EGFR, PDGFR, p38, p42, JNK1, c-Jun, or c-Fos. We observed that S1P-stimulated p42/p44 MAPK and p38 MAPK activation was mediated via a c-Src/EGFR and PDGFR-dependent pathway. S1P caused the c-Src/EGFR/PDGFR complex formation. On the other hand, we demonstrated that S1P induced p42/p44 MAPK and p38 MAPK-dependent Akt activation. In addition, S1P-stimulated JNK1/2 phosphorylation was attenuated by SP600125 or PP1. Finally, S1P enhanced c-Fos mRNA levels and c-Jun phosphorylation. S1P-induced c-Jun activation was reduced by PP1, AG1478, AG1296, U0126, SP600125, SB202190, or LY294002. These results demonstrated that S1P-induced ICAM-1 expression and monocyte adhesion were mediated through S1PR1/3/c-Src/EGFR, PDGFR/p38 MAPK, p42/p44 MAPK/Akt-dependent AP-1 activation.     

AP-1 activation through endogenous H(2)O(2) generation by alveolar macrophages

Reactive oxygen species released during the respiratory burst are known to participate in cell signaling. Here we demonstrate that hydrogen peroxide produced by the respiratory burst activates AP-1 binding. Stimulation of the macrophage cell line NR8383 with respiratory burst agonists ADP and C5a increased AP-1 binding activity. Importantly, this increase in binding was blocked by catalase, confirming mediation by endogenous H₂O₂. Moreover, exogenously added H₂O₂ mimicked the agonists, and also activated AP-1. Antibodies revealed that the activated AP-1 complex is composed predominantly of c-Fos/c-Jun heterodimers. Treatment of the cells with ADP, C5a and H₂O₂ (100 μM) all increased the phosphorylation of c-Jun. c-Fos protein was increased in cells treated with C5a or high dose (200 μM) H₂O₂, but not in cells treated with ADP. The MEK inhibitor, PD98059, partially blocked the C5a-mediated increase in AP-1 binding. A novel membrane-permeable peptide inhibitor of JNK, JNKi, also inhibited AP-1 activation. Together these data suggest that C5a-mediated AP-1 activation requires both the activation of the ERK and JNK pathways, whereas activation of the JNK pathway is sufficient to increase AP-1 binding with ADP. Thus, AP-1 activation joins the list of pathways for which the respiratory burst signals downstream events in the macrophage.