The orphan nuclear receptor NR4A1 promotes FcεRI-stimulated mast cell activation and anaphylaxis by counteracting the inhibitory LKB1/AMPK axis.

BACKGROUND: Nuclear receptor subfamily 4 group A member 1 (NR4A1), an orphan nuclear receptor, has been implicated in several biological events such as metabolism, apoptosis, and inflammation. Recent studies indicate a potential role for NR4A1 in mast cells, yet its role in allergic responses remains largely unknown. OBJECTIVES: The aim of this study was to clarify the role of NR4A1 in mast cell activation and anaphylaxis. METHODS: To evaluate the function of NR4A1 in mast cells, the impacts of siRNA knockdown, gene knockout, adenoviral overexpression, and pharmacological inhibition of NR4A1 on FcεRI signaling and effector functions in mouse bone marrow-derived mast cells (BMMCs) in vitro and on anaphylactic responses in vivo were evaluated. RESULTS: Knockdown or knockout of NR4A1 markedly suppressed degranulation and lipid mediator production by FcεRI-crosslinked BMMCs, while its overexpression augmented these responses. Treatment with a NR4A1 antagonist also blocked mast cell activation to a similar extent as NR4A1 knockdown or knockout. Moreover, mast cell-specific NR4A1-deficient mice displayed dampened anaphylactic responses in vivo. Mechanistically, NR4A1 promoted FcεRI signaling by counteracting the liver kinase B1 (LKB1)/adenosine monophosphate-activated protein kinase (AMPK) axis. Following FcεRI crosslinking, NR4A1 bound to the LKB1/AMPK complex and sequestered it in the nucleus, thereby promoting FcεRI downstream signaling pathways. Silencing or knockout of LKB1/AMPK largely abrogated the effect of NR4A1 on mast cell activation. Additionally, NR4A1 facilitated spleen tyrosine kinase activation independently of LKB1/AMPK. CONCLUSIONS: Nuclear receptor subfamily 4 group A member 1 positively regulates mast cell activation by antagonizing the LKB1-AMPK-dependent negative regulatory axis. This finding may provide a novel therapeutic strategy for the development of anti-allergic compounds.

Preischemic transfusion of old packed RBCs exacerbates early-phase warm hepatic ischemia reperfusion injury in rats.

BACKGROUND: Hepatic innate immune cells are considered to play a central role in the early phase of hepatic ischemia reperfusion (IR) injury. Transfusion of old red blood cells (RBCs) is known to prime immune cells, and transfusion before IR may exacerbate liver injury because of the expected hyperresponsiveness of immune cells. MATERIALS AND METHODS: Twenty-four Sprague-Dawley rats were divided into four groups: sham operation (Sham); hepatic IR only (IR Control); and two transfusion groups, preischemic (Pre-T) and postischemic (Post-T), in which allogeneic RBCs stored for 2 weeks were transfused before hepatic IR or after reperfusion, respectively. Partial hepatic ischemia was induced for 90 min, and reperfusion was allowed for 120 min. Serum alanine transaminase levels, area of necrosis, and apoptotic cells were then assessed. Inflammatory (tumor necrosis factor alpha, interleukin 1 beta [IL-1ß], IL-6, IL-10, and cyclooxygenase 2) and oxidative mediators (heme oxygenase 1, superoxide dismutase, and glutathione peroxidase 1) were assessed for elucidating the relevant mechanisms underlying the hepatic injury. RESULTS: Pre-T, but not Post-T, showed increased serum alanine transaminase levels than IR Control (P < 0.05). Area of necrosis was more severe in Pre-T than in IR Control or Post-T (P < 0.01), and apoptotic cells were also more abundant in Pre-T than in IR Control (P < 0.01). tumor necrosis factor alpha and IL-6 levels were higher in Pre-T than in IR Control or Post-T (P < 0.05), with no significant difference in cytoprotective protein levels. CONCLUSIONS: Preischemic transfusion of old RBCs aggravated hepatic injury. Inflammatory cytokines seemed to play a crucial role in liver injury exacerbation. Our results indicate that transfusion before hepatic ischemia may be detrimental.

Trim13 Potentiates Toll-Like Receptor 2-Mediated Nuclear Factor κB Activation via K29-Linked Polyubiquitination of Tumor Necrosis Factor Receptor-Associated Factor 6.

Ubiquitination is a versatile post-translational modification involved in nuclear factor-κB (NF-κB) activation of Toll-like receptor (TLR) signaling. Here, we demonstrated that Trim13, an E3 ubiquitin ligase, is up-regulated in macrophages upon stimulation with TLR2 ligand. Knockdown of Trim13 attenuated TLR2-mediated production of cytokines/chemokines and formation of foam cells as well as activation of NF-κB. Trim13 interacts with tumor necrosis factor receptor-associated factor 6 (TRAF6) and potentiates NF-κB activity via ubiquitination of TRAF6. Overexpression of inactive mutant (C10/13A) or really interesting new gene (RING) deletion mutant of Trim13 did not potentiate ubiquitination of TRAF6 or activation of NF-κB. These results suggest that the effects of Trim13 are dependent on its E3 ligase activity. Trim13 used K29-linked polyubiquitin chains for TRAF6 ubiquitination to promote NF-κB activity and thus potentiated activation of TLR2-mediated immune responses. Our data identify Trim13 as a positive regulator of NF-κB activation and suggest that K29-linked polyubiquitination is a specific ubiquitin-linked pattern involved in the control of TLR2 signaling.

Methionine sulfoxide reductase A protects against lipopolysaccharide-induced septic shock via negative regulation of the proinflammatory responses.

Methionine sulfoxide reductase A (MsrA) is a major antioxidant enzyme that specifically catalyzes the reduction of methionine S-sulfoxide. In this study, we used MsrA gene-knockout (MsrA-/-) mice and bone marrow-derived macrophages (BMDMs) to investigate the role of MsrA in the regulation of inflammatory responses induced by lipopolysaccharide (LPS). MsrA-/- mice were more susceptible to LPS-induced lethal shock than wild-type (MsrA+/+) mice. Serum levels of the proinflammatory cytokines IL-6 and TNF-α induced by LPS were higher in MsrA-/- than in MsrA+/+ mice. MsrA deficiency in the BMDMs also increased the LPS-induced cytotoxicity as well as TNF-α level. Basal and LPS-induced reactive oxygen species (ROS) levels were higher in MsrA-/- than in MsrA+/+ BMDMs. Phosphorylation levels of p38, JNK, and ERK were higher in MsrA-/- than in MsrA+/+ BMDMs in response to LPS, suggesting that MsrA deficiency increases MAPK activation. Furthermore, MsrA deficiency increased the expression and nuclear translocation of NF-κB and the expression of inducible nitric oxide synthase, a target gene of NF-κB, in response to LPS. Taken together, our results suggest that MsrA protects against LPS-induced septic shock, and negatively regulates proinflammatory responses via inhibition of the ROS-MAPK-NF-κB signaling pathways.

Regulation of PHLDA1 Expression by JAK2-ERK1/2-STAT3 Signaling Pathway.

Toll-like receptor 2 (TLR2)-mediated signaling cascades and gene regulation are mainly involved in diseases, such as immunity and inflammation. In this study, microarray analysis was performed using bone marrow-derived macrophages (BMDM) and Raw 264.7 cells to identify novel proteins involved in the TLR2-mediated cellular response. We found that pleckstrin homology-like domain family, member 1 (PHLDA1) is a novel gene up-regulated by TLR2 stimulation and determined the unique signaling pathway for its expression. Treatment with TLR2 agonist Pam3 CSK4 increased mRNA, protein, and fluorescence staining of PHLDA1. Induction of PHLDA1 by TLR2 stimulation disappeared from TLR2 KO mice-derived BMDM. Among janus kinase (JAK) family members, JAK2 was involved in TLR2-stimulated PHLDA1 expression. Signal transducer and activator of transcription 3 (STAT3) also participated in PHLDA1 expression downstream of the JAK2. Interestingly, ERK1/2 was an intermediate between JAK2 and STAT3. In silico analysis revealed the presence of highly conserved γ-activated sites within mouse PHLDA1 promoter and confirmed the JAK2-STAT3 pathway is important to Pam3 CSK4 -induced PHLDA1 transcription. These findings suggest that the JAK2-ERK1/2-STAT3 pathway is an important signaling pathway for PHLDA1 expression and that these proteins may play a critical role in eliciting TLR2-mediated immune and inflammatory response.

RGS2 suppresses breast cancer cell growth via a MCPIP1-dependent pathway.

Regulator of G protein signaling 2 (RGS2) is a member of a family of proteins that functions as a GTPase-activating protein (GAP) for Gα subunits. RGS2 mRNA expression is lower in breast cancerous tissues than in normal tissues. In addition, expression of RGS2 is also lower in MCF7 (cancerous breast cells) than in MCF10A (normal breast cells). Here we investigated whether RGS2 inhibits growth of breast cancer cells. RGS2 overexpression in MCF7 cells inhibited epidermal growth factor- or serum-induced proliferation. In HEK293T cells expressing RGS2, cell growth was also significantly suppressed (In addition, exogenous expression of RGS2 in HEK293T cells resulted in the significant suppression of cell growth). These results suggest that RGS2 may have a tumor suppressor function. MG-132 treatment of MCF7 cells increased endogenous or exogenous RGS2 levels, suggesting a post-transcriptional regulatory mechanism that controls RGS2 protein levels. RGS2 protein was degraded polyubiquitinated the K71 residue, but stabilized by deubiquitinase monocyte chemotactic protein-induced protein 1 (MCPIP1), and not affected by dominant negative mutant (C157A) of MCPIP1. Gene expression profiling study showed that overexpression of RGS2 decreased levels of testis specific Y encoded like protein 5 (TSPYL5), which plays a causal role in breast oncogenesis. TSPYL5 protein expression was low in MCF10A and high in MCF7 cells, showing the opposite aspect to RGS2 expression. Additionally, RGS2 or MCPIP1 overexpression in MCF7 cells decreased TSPYL5 protein level, indicating that RGS2 stabilized by MCPIP1 have diminished TSPYL5 protein levels, thereby exerting an inhibitory effect of breast cancer cell growth.

TLR2 deficiency attenuates skeletal muscle atrophy in mice.

Oxidative stress and inflammation are associated with skeletal muscle atrophy. Because the activation of toll-like receptor (TLR) 2 induces oxidative stress and inflammation, TLR2 may be directly linked to skeletal muscle atrophy. This study examined the role of TLR2 in skeletal muscle atrophy in wild-type (WT) and TLR2 knockout (KO) mice. Immobilization for 2 weeks increased the expression of cytokine genes and the levels of carbonylated proteins and nitrotyrosine in the skeletal muscle, but these increases were lower in the TLR2 KO mice. Muscle weight loss and a reduction in treadmill running times induced by immobilization were also attenuated in TLR2 KO mice. Furthermore, immobilization increased the protein levels of forkhead box O 1/3, atrogin-1 and muscle ring finger 1 in the WT mice, which was attenuated in TLR2 KO mice. In addition, immobilization-associated increases in ubiquitinated protein levels were lower in the TLR2 KO mice. Immobilization increased the phosphorylation of Akt and p70S6K similarly in WT and KO mice. Furthermore, cardiotoxin injection into the skeletal muscle increased the protein levels of atrogin-1, interleukin-6, and nitrotyrosine and increased the levels of ubiquitinated proteins, although these levels were increased to a lesser extent in TLR2 KO mice. These results suggest that TLR2 is involved in skeletal muscle atrophy, and the inhibition of TLR2 offers a potential target for preventing skeletal muscle atrophy.

Phospholipase C activator m-3M3FBS protects against morbidity and mortality associated with sepsis.

Although phospholipase C (PLC) is a crucial enzyme required for effective signal transduction and leukocyte activation, the role of PLC in polymicrobial sepsis remains unclear. In this study, we show that the direct PLC activator m-3M3FBS treatment significantly attenuates vital organ inflammation, widespread immune cell apoptosis, and mortality in a mouse sepsis model induced by lethal cecal ligation and puncture challenge. Mechanistically, m-3M3FBS-dependent protection was largely abolished by pretreatment of mice with the PLC-selective inhibitor U-73122, thus confirming PLC agonism by m-3M3FBS in vivo. PLC activation enhanced the bactericidal activity and hydrogen peroxide production of mouse neutrophils, and it also enhanced the production of IFN-γ and IL-12 while inhibiting proseptic TNF-α and IL-1ß production in cecal ligation and puncture mice. In a second model of sepsis, PLC activation also inhibited the production of TNF-α and IL-1ß following systemic LPS challenge. In conclusion, we show that agonizing the central signal transducing enzyme PLC by m-3M3FBS can reverse the progression of toxic shock by triggering multiple protective downstream signaling pathways to maintain organ function, leukocyte survival, and to enhance microbial killing.

Role of JAK2-STAT3 in TLR2-mediated tissue factor expression.

Tissue factor (TF) is a core protein with an essential function in the coagulation cascade that maintains the homeostasis of the blood vessels. TF not only participates in neointima formation, but also causes the development of atherosclerosis. This study investigated the mechanism regulating TF expression in macrophages using Pam3 CSK4 , a TLR2 ligand. Pam3 CSK4 induced TF expression in two types of macrophages (Raw264.7 and BMDM), but not in TLR2 KO mice derived BMDM. Pam3 CSK4 induced TF expression was inhibited by pretreatment with pan-JAK inhibitor or JAK2 inhibitor AG490. JAK2 knock-down by siRNA inhibited Pam3 CSK4 induced TF expression. Pam3 CSK4 stimulated STAT3 phosphorylation (S727), while STAT3 knock-down by siRNA reduced Pam3 CSK4 induced TF expression. These results suggest that Pam3 CSK4 induced TF expression is regulated by the JAK2-STAT3 signaling pathway. Pam3 CSK4 , unlike increased TF expression, significantly decreased RGS2 expression, while RGS2 overexpression decreased Pam3 CSK4 induced TF expression. Inhibition of TF by RGS2 WT did not occur in mutants with flawed RGS domains. We also investigated the correlation between RGS2 and STAT3 phosphorylation. RGS2 knock-down elevated Pam3 CSK4 induced STAT3 phosphorylation, but RGS2 overexpression had the opposite effect on STAT3 phosphorylation. These results suggest that, while Pam3 CSK4 induced TF expression is regulated by JAK2-STAT3 signaling, RGS2 is a negative regulator targeted to STAT3.

Role of formyl peptide receptor 2 on the serum amyloid A-induced macrophage foam cell formation.

Recently we demonstrated that SAA induces macrophage foam cell formation. In this study we show that SAA-induced foam cell formation is inhibited by formyl peptide receptor 2 (FPR2) antagonist WRW(4), as well as by FPR2-targeted siRNA knockdown. SAA-stimulated LOX1 expression was also mediated by FPR2. We also found that SAA-stimulated foam cell formation and LOX1 expression was pertussis toxin-insensitive. In addition, FPR2 is upregulated in peripheral blood mononuclear cells from patients with atherosclerosis. Our findings therefore suggest that SAA stimulates foam cell formation via FPR2 signaling and LOX1 induction, and thus likely contributes to atherogenesis.

Serum amyloid A stimulates macrophage foam cell formation via lectin-like oxidized low-density lipoprotein receptor 1 upregulation.

Elevated levels of serum amyloid A (SAA) is a risk factor for cardiovascular diseases, however, the role of SAA in the pathophysiology of atherosclerosis remains unclear. Here we show that SAA induced macrophage foam cell formation. SAA-stimulated foam cell formation was mediated by c-jun N-terminal kinase (JNK) signaling. Moreover, both SAA and SAA-conjugated high density lipoprotein stimulated the expression of the important scavenger receptor lectin-like oxidized low-density lipoprotein receptor 1 (LOX1) via nuclear factor-κB (NF-κB). A LOX1 antagonist carrageenan significantly blocked SAA-induced foam cell formation, indicating that SAA promotes foam cell formation via LOX1 expression. Our findings therefore suggest that SAA stimulates foam cell formation via LOX1 induction, and thus likely contributes to atherogenesis.

TLR2 stimulates ABCA1 expression via PKC-η and PLD2 pathway.

ATP-binding cassette transporter A1 (ABCA1) is a membrane-bound protein that regulates cardiovascular disease including atherosclerosis by the efflux of excess cholesterol from cells and by suppression of inflammation. Using a mouse macrophage cell line Raw264.7, we studied the importance of toll-like receptor 2 (TLR2) on ABCA1 expression and the signaling pathway responsible for TLR2-mediated ABCA1 expression. Interestingly, our data demonstrated that treatment of macrophages with TLR2 agonist Pam(3)CSK(4) significantly increased ABCA1 mRNA and protein levels. We found that ABCA1 induction is myeloid differentiation primary response gene 88 (MyD88)-dependent as well as TLR2-dependent. ABCA1 induction upon Pam(3)CSK(4) is controlled by protein kinase C-η (PKC-η) and phospholipase D2 (PLD2). Furthermore, direct treatment of dioctanoyl phosphatidic acid (diC(8)PA) into cells also induced ABCA1 mRNA and protein indicating that PLD2-mediated PA involve in the TLR2-stimulated ABCA1 expression. Cumulatively, these results demonstrate for the first time that activation of PKC-η and PLD2 signaling pathway is an important mechanism for regulation of TLR2-induced ABCA1 expression.

Sphingosylphosphorylcholine stimulates CCL2 production from human umbilical vein endothelial cells.

Sphingosylphosphorylcholine (SPC) is a component of high-density lipoprotein particles. We investigated the functional role of SPC in HUVECs. SPC stimulation induced production of the CCL2 chemokine in a PTX-sensitive G-protein-dependent manner. SPC treatment caused the activation of NF-κB and AP-1, which are essential for SPC-induced CCL2 production, and induced the activation of three MAPKs, ERK, p38 MAPK, and JNK. Inhibition of p38 MAPK or JNK by specific inhibitors caused a dramatic decrease in SPC-induced CCL2 production. The Jak/STAT3 pathway was also activated upon SPC stimulation of HUVECs. Pretreatment with a Jak inhibitor blocked not only SPC-induced p38 MAPK and JNK activation, but also NF-κB and AP-1 activation. Our results suggest that SPC stimulates HUVECs, resulting in Jak/STAT3-, NF-κB-, and AP-1-mediated CCL2 production. We also observed that SPC stimulated expression of the adhesion molecule ICAM-1 in HUVECs. Our results suggest that SPC may contribute to atherosclerosis; therefore, SPC and its unidentified target receptor offer a starting point for the development of a treatment for atherosclerosis.

miR-221/222 induce instability of p53 By downregulating deubiquitinase YOD1 in acute myeloid leukemia.

Acute myeloid leukemia (AML) is a hematological malignancy characterized by the impaired differentiation and uncontrolled proliferation of myeloid blasts. Tumor suppressor p53 is often downregulated in AML cells via ubiquitination-mediated degradation. While the role of E3 ligase MDM2 in p53 ubiquitination is well-accepted, little is known about the involvement of deubiquitinases (DUBs). Herein, we found that the expression of YOD1, among several DUBs, is substantially reduced in blood cells from AML patients. We identified that YOD1 deubiqutinated and stabilized p53 through interaction via N-terminus of p53 and OTU domain of YOD1. In addition, expression levels of YOD1 were suppressed by elevated miR-221/222 in AML cells through binding to the 3' untranslated region of YOD1, as verified by reporter gene assays. Treatment of cells with miR-221/222 mimics and inhibitors yielded the expected effects on YOD1 expressions, in agreement with the negative correlation observed between the expression levels of miR-221/222 and YOD1 in AML cells. Finally, overexpression of YOD1 stabilized p53, upregulated pro-apoptotic p53 downstream genes, and increased the sensitivity of AML cells to FLT3 inhibitors remarkably. Collectively, our study identified a pathway connecting miR-221/222, YOD1, and p53 in AML. Targeting miR-221/222 and stimulating YOD1 activity may improve the therapeutic effects of FLT3 inhibitors in patients with AML.

α-Iso-cubebene, a natural compound isolated from Schisandra chinensis fruit, has therapeutic benefit against polymicrobial sepsis.

α-Iso-cubebene, a natural compound isolated from Schisandra chinensis fruit, strongly enhanced survival rate in cecal ligation and puncture (CLP) challenge-induced sepsis. The mechanism involved the marked reduction of viable bacteria in the peritoneal fluid, by virtue of increased phagocytic activity and production of hydrogen peroxide. α-Iso-cubebene also significantly attenuated lung inflammation and widespread immune cell apoptosis in a mouse CLP sepsis model, and inhibited the production of proinflammatory cytokines including tumor necrosis factor-α, interleukin (IL)-1ß, and IL-6 in CLP mice and lipopolysaccharide-stimulated splenocytes. The results indicate that α-iso-cubebene can reverse the progression of toxic shock by triggering multiple protective downstream signaling pathways to enhance microbial killing and maintain organ function and leukocyte survival.

The agonists of formyl peptide receptors prevent development of severe sepsis after microbial infection.

Severe sepsis, a principal cause of death in intensive care units, occurs when host immune defenses fail to combat invading microbes. In this paper, we report that the administration of peptide agonists of formyl peptide receptors, including Trp-Lys-Tyr-Met-Val-D-Met (WKYMVm), protected against death by enhanced bactericidal activity and inhibition of vital organ inflammation and immune cell apoptosis in a cecal ligation and puncture (CLP) sepsis mouse model. The administration of WKYMVm also enhanced the production of type 1 (IFN-γ and IL-12) and type 17 (IL-17 and TGF-ß) cytokines in CLP mice. In contrast, the administration of WKYMVm inhibited the production of proinflammatory cytokines (TNF-α, IL-1ß, and IL-6) in the CLP mice. The therapeutic and bactericidal effects of WKYMVm were partly reversed in IFN-γ-deficient mice, whereas target organ inflammation was not. Meanwhile, the therapeutic and anti-inflammatory effects of WKYMVm were partly reversed in IL-17-deficient mice. In addition, the administration of WKYMVm also enhanced type 1 and type 17 Th cell responses in mice sensitized with LPS plus Ags. These results suggest that the agonists of formyl peptide receptors effectively prevent development of severe sepsis following microbial infection partly via augmentation of type 1 and type 17 immune responses.

Activation of CXCR2 by extracellular matrix degradation product acetylated Pro-Gly-Pro has therapeutic effects against sepsis.

RATIONALE: Acetylated Pro-Gly-Pro (Ac-PGP) is an endogenous degradation product of extracellular collagen that binds to leukocyte-expressed chemoattractant receptor CXCR2. Although certain agents that block CXCR2-mediated signaling protect against experimental sepsis, the roles of Ac-PGP and CXCR2 in sepsis are unclear. OBJECTIVES: To investigate the role of Ac-PGP and its receptor, CXCR2, in murine models of cecal ligation and puncture (CLP)-induced polymicrobial sepsis and organ injury. METHODS: The impact of in vivo Ac-PGP treatment on animal survival after induction of experimental sepsis was assessed. Vital organ inflammation and immune cell apoptosis were evaluated by histology, and the modulation of proinflammatory cytokine production and bactericidal activity by Ac-PGP in mouse and human blood leukocytes was measured. MEASUREMENTS AND MAIN RESULTS: The activation of CXCR2 by tripeptide agonist Ac-PGP dramatically improved survival in three experimental sepsis models. Ac-PGP elicited bactericidal activity via the generation of hydrogen peroxide, inhibited lung inflammation, and reduced immune cell apoptosis. Fluorescein isothiocyanate-labeled PGP bound directly to CXCR2, and the protective effect of Ac-PGP in sepsis was abolished in CXCR2-deficient mice. Ac-PGP treatment enhanced the production of type 1 cytokines (IFN-γ and IL-12) but inhibited the production of proinflammatory cytokines (tumor necrosis factor [TNF]-α, IL-1ß, and IL-6) in vivo. In vitro, Ac-PGP directly increased IFN-γ production and decreased the LPS-stimulated production of TNF-α by mouse splenocytes and human leukocytes. Furthermore, direct treatment of LPS-stimulated splenocytes with IFN-γ resulted in diminished secretion of TNF-α and IL-6. CONCLUSIONS: CXCR2 and Ac-PGP are thus novel target and starting molecules, respectively, for the development of therapeutic agents against sepsis.

Calcium-independent phospholipase A2beta-Akt signaling is involved in lipopolysaccharide-induced NADPH oxidase 1 expression and foam cell formation.

Foam cell formation is the most important process in atherosclerosis, and low density lipoprotein oxidation by reactive oxygen species (ROS) is the key step in the conversion of macrophages to foam cells. This study reveals the control mechanism of the gene for NADPH oxidase 1 (Nox1), which produces ROS in the formation of foam cells by stimulating TLR4. Treatment of macrophages by the TLR4 agonist LPS stimulated ROS production and ROS-mediated macrophage to foam cell conversion. This LPS-induced ROS production and foam cell formation could be abrogated by pretreatment of macrophages with N-acetyl cysteine or apocynin. LPS increased Nox1 promoter activity, and resultant expression of mRNA and protein. Small interfering RNA mediated inhibition of Nox1 expression decreased LPS-induced ROS production and foam cell formation. LPS-mediated Nox1 expression and the responses occurred in a calcium-independent phospholipase A(2) (iPLA(2))-dependent manner. The iPLA(2)beta-specific inhibitor S-BEL or iPLA(2)beta small interfering RNA attenuated LPS-induced Nox1 expression, ROS production, and foam cell formation. In addition, activation of iPLA(2)beta by LPS caused Akt phosphorylation and was followed by increased Nox1 expression. These results suggest that the binding of LPS and TLR4 increases Nox1 expression through the iPLA(2)beta-Akt signaling pathway, and control ROS production and foam cell formation.

Protein kinase C-eta and phospholipase D2 pathway regulates foam cell formation via regulator of G protein signaling 2.

Regulator of G protein signaling 2 (RGS2) is a GTPase-activating protein for Galpha(q), which is involved in regulating various vascular functions. To understand how RGS2 regulates foam cell formation, the present study identified signaling pathways controlled by lipopolysaccharide (LPS) and discovered new mechanisms whereby protein kinase C (PKC)-eta and phospholipase D (PLD) 2 regulate RGS2 expression. The toll-like receptor (TLR) 4 agonist LPS caused foam cell formation of Raw264.7 macrophages and dramatically decreased RGS2 mRNA expression. RGS2 down-regulation by LPS was partially recovered by TLR4 small interfering RNA (siRNA). Peritoneal macrophages were separated from wild-type and TLR4 mutant mice, and treatment with LPS showed RGS2 expression decrease in wild-type macrophages but no change in TLR4 mutant macrophages. RGS2 overexpression was suppressed, whereas RGS2 down-regulation accelerated foam cell formation by LPS. Treatment of PKC-eta pseudosubstrate weakened foam cell formation and recovered RGS2 down-regulation by LPS. In addition, LPS or phorbol 12-myristate 13-acetate stimulated PLD activity, and the pretreatment of PLD inhibitor weakened foam cell formation and recovered RGS2 down-regulation. Inhibition of PLD2 expression by siRNA also weakened foam cell formation and partially recovered LPS-mediated RGS2 down-regulation. On the other hand, PLD2 overexpression intensified RGS2 down-regulation and foam cell formation by LPS. These results suggest that LPS causes foam cell formation by increasing PKC-eta and PLD2 activity by down-regulating RGS2 expression via TLR4 dependently.

Role of NADPH oxidase-2 in lipopolysaccharide-induced matrix metalloproteinase expression and cell migration.

This study examined the hypothesis that the control of NADPH oxidase-2 (Nox2)-mediated reactive oxygen species (ROS) regulates the expression of matrix metalloproteinases (MMPs) and the migration of macrophages. Lipopolysaccharide (LPS) stimulation of Raw264.7 cells and mice peritoneal macrophages increased the expression of MMP-9, 10, 12 and 13 mRNA, and also increased Raw264.7 cell migration. Treatment with an antioxidant (N-acetyl cysteine) or Nox inhibitors strongly inhibited the expression of MMPs by LPS and inhibited cell migration. LPS caused ROS production in macrophages and increased the mRNA expression of Nox isoforms Nox1 and Nox2 by 20-fold and two-fold, respectively. While Nox1 small interfering RNA (siRNA) did not inhibit LPS-mediated expression of MMPs, Nox2 siRNA inhibited the expressions of MMP-9, 10 and 12. Neither Nox1 nor Nox2 siRNA influenced the LPS-mediated expression of MMP-13. In addition, NAC or apocynin attenuated LPS-induced ROS production and MMP-9 expression. MMP-9 expression and cell migration were controlled by ERK1/2-ROS signaling. Collectively, these results suggest that LPS stimulates ROS production via ERK and induce various types of MMPs expression and cell migration.