Phosphatidylcholine-specific phospholipase C and D in stimulation of RAW264.7 mouse macrophage-like cells by lipopolysaccharide

Diacylglycerol kinase is a keystone regulator of signaling relevant to the pathophysiology of asthma

Signal transduction by G protein-coupled receptors (GPCRs), receptor tyrosine kinases (RTKs) and immunoreceptors converge at the activation of phospholipase C (PLC) for the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). This is a point for second-messenger bifurcation where DAG via protein kinase C (PKC) and IP3 via calcium activate distinct protein targets and regulate cellular functions. IP3 signaling is regulated by multiple calcium influx and efflux proteins involved in calcium homeostasis. A family of lipid kinases belonging to DAG kinases (DGKs) converts DAG to phosphatidic acid (PA), negatively regulating DAG signaling and pathophysiological functions. PA, through a series of biochemical reactions, is recycled to produce new molecules of PIP2. Therefore, DGKs act as a central switch in terminating DAG signaling and resynthesis of membrane phospholipids precursor. Interestingly, calcium and PKC regulate the activation of α and ζ isoforms of DGK that are predominantly expressed in airway and immune cells. Thus, DGK forms a feedback and feedforward control point and plays a crucial role in fine-tuning phospholipid stoichiometry, signaling, and functions. In this review, we discuss the previously underappreciated complex and intriguing DAG/DGK-driven mechanisms in regulating cellular functions associated with asthma, such as contraction and proliferation of airway smooth muscle (ASM) cells and inflammatory activation of immune cells. We highlight the benefits of manipulating DGK activity in mitigating salient features of asthma pathophysiology and shed light on DGK as a molecule of interest for heterogeneous diseases such as asthma.

Tricyclodecan-9-yl-Xanthogenate (D609): Mechanism of Action and Pharmacological Applications

Tricyclodecan-9-yl xanthogenate (D609) is a synthetic tricyclic compound possessing a xanthate group. This xanthogenate compound is known for its diverse pharmacological properties. Over the last three decades, many studies have reported the biological activities of D609, including antioxidant, antiapoptotic, anticholinergic, anti-tumor, anti-inflammatory, anti-viral, anti-proliferative, and neuroprotective activities. Its mechanism of action is extensively attributed to its ability to cause the competitive inhibition of phosphatidylcholine (PC)-specific phospholipase C (PC-PLC) and sphingomyelin synthase (SMS). The inhibition of PCPLC or SMS affects secondary messengers with a lipidic nature, i.e., 1,2-diacylglycerol (DAG) and ceramide. Various in vitro/in vivo studies suggest that PCPLC and SMS inhibition regulate the cell cycle, block cellular proliferation, and induce differentiation. D609 acts as a pro-inflammatory cytokine antagonist and diminishes Aβ-stimulated toxicity. PCPLC enzymatic activity essentially requires Zn²⁺, and D609 might act as a potential chelator of Zn²⁺, thereby blocking PCPLC enzymatic activity. D609 also demonstrates promising results in reducing atherosclerotic plaque formation, post-stroke cerebral infarction, and cancer progression. The present compilation provides a comprehensive mechanistic insight into D609, including its chemistry, mechanism of action, and regulation of various pharmacological activities.

Potential role of diacylglycerol kinases in immune-mediated diseases

The mechanism promoting exacerbated immune responses in allergy and autoimmunity as well as those blunting the immune control of cancer cells are of primary interest in medicine. Diacylglycerol kinases (DGKs) are key modulators of signal transduction, which blunt diacylglycerol (DAG) signals and produce phosphatidic acid (PA). By modulating lipid second messengers, DGK modulate the activity of downstream signaling proteins, vesicle trafficking and membrane shape. The biological role of the DGK α and ζ isoforms in immune cells differentiation and effector function was subjected to in deep investigations. DGK α and ζ resulted in negatively regulating synergistic way basal and receptor induced DAG signals in T cells as well as leukocytes. In this way, they contributed to keep under control the immune response but also downmodulate immune response against tumors. Alteration in DGKα activity is also implicated in the pathogenesis of genetic perturbations of the immune function such as the X-linked lymphoproliferative disease 1 and localized juvenile periodontitis. These findings suggested a participation of DGK to the pathogenetic mechanisms underlying several immune-mediated diseases and prompted several researches aiming to target DGK with pharmacologic and molecular strategies. Those findings are discussed inhere together with experimental applications in tumors as well as in other immune-mediated diseases such as asthma.

Diacylglycerol Kinase ζ Regulates Macrophage Responses in Juvenile Arthritis and Cytokine Storm Syndrome Mouse Models

Dysregulation of monocyte and macrophage responses are often observed in children with systemic juvenile idiopathic arthritis (sJIA) and cytokine storm syndrome (CSS), a potentially fatal complication of chronic rheumatic diseases. Both conditions are associated with activation of TLR signaling in monocyte and macrophage lineage cells, leading to overwhelming inflammatory responses. Despite the importance of TLR engagement in activating proinflammatory macrophages, relatively little is known about activation of intrinsic negative regulatory pathways to attenuate excessive inflammatory responses. In this study, we demonstrate that loss of diacylglycerol (DAG) kinase (Dgk) ζ, an enzyme which converts DAG into phosphatidic acid, limits inflammatory cytokine production in an arthritic mouse model dependent on TLR2 signaling and in a CSS mouse model dependent on TLR9 signaling. In vitro, Dgkζ deficiency results in reduced production of TNF-α, IL-6, and IL-1β and in limited M1 macrophage polarization. Mechanistically, Dgkζ deficiency decreases STAT1 and STAT3 phosphorylation. Moreover, Dgkζ levels are increased in macrophages derived from mice with CSS or exposed to plasma from sJIA patients with active disease. Our data suggest that Dgkζ induction in arthritic conditions perpetuates systemic inflammatory responses mediated by macrophages and highlight a potential role of Dgkζ-DAG/phosphatidic acid axis as a modulator of inflammatory cytokine production in sJIA and CSS.

The triangle of death of neurons: Oxidative damage, mitochondrial dysfunction, and loss of choline-containing biomolecules in brains of mice treated with doxorubicin. Advanced insights into mechanisms of chemotherapy induced cognitive impairment ("chemobrain") involving TNF-α

Cancer treatments are developing fast and the number of cancer survivors could arise to 20 million in United State by 2025. However, a large fraction of cancer survivors demonstrate cognitive dysfunction and associated decreased quality of life both shortly, and often long-term, after chemotherapy treatment. The etiologies of chemotherapy induced cognitive impairment (CICI) are complicated, made more so by the fact that many anti-cancer drugs cannot cross the blood-brain barrier (BBB). Multiple related factors and confounders lead to difficulties in determining the underlying mechanisms. Chemotherapy induced, oxidative stress-mediated tumor necrosis factor-alpha (TNF-α) elevation was considered as one of the main candidate mechanisms underlying CICI. Doxorubicin (Dox) is a prototypical reactive oxygen species (ROS)-generating chemotherapeutic agent used to treat solid tumors and lymphomas as part of multi-drug chemotherapeutic regimens. We previously reported that peripheral Dox-administration leads to plasma protein damage and elevation of TNF-α in plasma and brain of mice. In the present study, we used TNF-α null (TNFKO) mice to investigate the role of TNF-α in Dox-induced, oxidative stress-mediated alterations in brain. We report that Dox-induced oxidative stress in brain is ameliorated and brain mitochondrial function assessed by the Seahorse-determined oxygen consumption rate (OCR) is preserved in brains of TNFKO mice. Further, we show that Dox-decreased the level of hippocampal choline-containing compounds and brain phospholipases activity are partially protected in TNFKO group in MRS study. Our results provide strong evidence that Dox-targeted mitochondrial damage and levels of brain choline-containing metabolites, as well as phospholipases changes decreased in the CNS are associated with oxidative stress mediated by TNF-α. These results are consistent with the notion that oxidative stress and elevated TNF-α in brain underlie the damage to mitochondria and other pathological changes that lead to CICI. The results are discussed with reference to our identifying a potential therapeutic target to protect against cognitive problems after chemotherapy.

The Immunomodulatory Functions of Diacylglycerol Kinase ζ

The generation of diacylglycerol (DAG) is critical for promoting immune cell activation, regulation, and function. Diacylglycerol kinase ζ (DGKζ) serves as an important negative regulator of DAG by enzymatically converting DAG into phosphatidic acid (PA) to shut down DAG-mediated signaling. Consequently, the loss of DGKζ increases DAG levels and the duration of DAG-mediated signaling. However, while the enhancement of DAG signaling is thought to augment immune cell function, the loss of DGKζ can result in both immunoactivation and immunomodulation depending on the cell type and function. In this review, we discuss how different immune cell functions can be selectively modulated by DGKζ. Furthermore, we consider how targeting DGKζ can be potentially beneficial for the resolution of human diseases by either promoting immune responses important for protection against infection or cancer or dampening immune responses in immunopathologic conditions such as allergy and septic shock.

Lipopolysaccharide binds to and activates A1 adenosine receptors on human pulmonary artery endothelial cells

Previously, it was reported that A1 adenosine receptor antagonists prevent endotoxin-inducedacute lung injury and pulmonary arterial endothelial cell damage. In competition radioligand binding experiments in membranes prepared from human pulmonary artery endothelial cells (PAECs), lipopolysaccharides (LPSs) of Escherichia coli, Salmonella typhimurium, Klebsiella pneumoniae, and Pseudomonas aeruginosa displaced the binding of a selective A adenosine receptor antagonist [125I]-BWA844U (IC 50 values: 195 ng/ml, 290 ng/ml, 602 ng/ml, and 6931 ng/ml, respectively)in a dose-dependent, competitive manner. There was no displacement of this radioligand by enterotoxin (≤ 10 μg/ml), diphosphoryl lipid A (≤ 10μg/ml), and glycolipids, monosialoganglioside(≤ 1μg/ml), lactocerebroside (≤ 100μg/ml), or NBD galactocerebroside (≤ 100 μg/ml). Based on calculated IC values, LPS ( E. coli, IC50 111 ng/ml) 50 6 displaced the selective A1 adenosine receptor agonist, [3H]-2-chloro, N -cyclopentyladenosine (CCPA) in human PAECs with a potency profile, CCPA > LPS > 2-phenylaminoadenosine (CV 1808), a selective A2 adenosine receptor agonist. The potency profile for displacement of the selective A μ 2a adenosine receptor agonist [ 3H]-CGS 21680 was CV 1808 > CCPA. LPS ( E. coli 0.1 pg/ml—10 g/ml) did not displace [3H]-CGS 21680 binding. In human PAECs, IL-6 and TXA2 release induced by LPS (0—1 μg/ml) or CCPA (0—1 μM) at high doses was significantly reduced by the selective A1 adenosine receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine(DPCPX; 1 μM). These data suggest that LPS binds to and activates A1 adenosine receptors on human PAECs to induce the release of IL-6 and TXA 2. Activation of A1 adenosine receptors on human PAECs by LPS, may contribute to the pathophysiology of acute lung injury associated with Gram-negative septicemia and endotoxemia.

PLD1 activation mediates Amb a 1-induced Th2-associated cytokine expression via the JNK/ATF-2 pathway in BEAS-2B cells

The purpose of this study was to identify the role of phospholipase D1 (PLD1) in Amb a 1-induced IL-5 and IL-13 expression. When BEAS-2B cells were stimulated with Amb a 1, PLD activity increased, and knockdown of PLD1 decreased Amb a 1-induced IL-5 and IL-13 expression. Amb a 1 also activated the PLCγ/p70S6K/JNK pathway. Furthermore, Amb a 1-induced PLD activation was also attenuated by PLCγ inhibition, and knockdown of PLD1 decreased Amb a 1-induced activation of P70S6K and JNK. When ATF-2 activity was blocked with ATF-2 siRNA, Amb a 1-induced IL-5 and IL-13 expression was completely abolished, indicating that ATF-2 is a transcriptional factor required for the expression of IL-5 and IL-13 in response to Amb a 1. Taken together, we suggest that PLD1 acts as an important regulator in Amb a 1-induced expression of IL-5 and IL-13 via a PLCγ/p70S6K/JNK/ATF-2 pathway in BEAS-2B cells.

Anti-proliferative effects of tricyclodecan-9-yl-xanthogenate (D609) involve ceramide and cell cycle inhibition

Tricyclodecan-9-yl-xanthogenate (D609) inhibits phosphatidylcholine (PC)-phospholipase C (PLC) and/or sphingomyelin (SM) synthase (SMS). Inhibiting SMS can increase ceramide levels, which can inhibit cell proliferation. Here, we examined how individual inflammatory and glia cell proliferation is altered by D609. Treatment with 100-μM D609 significantly attenuated the proliferation of RAW 264.7 macrophages, N9 and BV-2 microglia, and DITNC(1) astrocytes, without affecting cell viability. D609 significantly inhibited BrdU incorporation in BV-2 microglia and caused accumulation of cells in G(1) phase with decreased number of cells in the S phase. D609 treatment for 2 h significantly increased ceramide levels in BV-2 microglia, which, following a media change, returned to control levels 22 h later. This suggests that the effect of D609 may be mediated, at least in part, through ceramide increase via SMS inhibition. Western blots demonstrated that 2-h treatment of BV-2 microglia with D609 increased expression of the cyclin-dependent kinase (Cdk) inhibitor p21 and down-regulated phospho-retinoblastoma (Rb), both of which returned to basal levels 22 h after removal of D609. Exogenous C8-ceramide also inhibited BV-2 microglia proliferation without loss of viability and decreased BrdU incorporation, supporting the involvement of ceramide in D609-mediated cell cycle arrest. Our current data suggest that D609 may offer benefit after stroke (Adibhatla and Hatcher, Mol Neurobiol 41:206-217, 2010) through ceramide-mediated cell cycle arrest, thus restricting glial cell proliferation.

Role of sphingomyelin synthesis in pulmonary endothelial cell cytoskeletal activation and endotoxin-induced lung injury

Sphingomyelin (SM), a major sphingolipid in the lipid raft microdomains of the cell membrane, is synthesized by plasma membrane-bound sphingomyelin synthase 2 (SMS2). SMS2 is required for the maintenance of plasma membrane microdomain fluidity and receptor-mediated responses to inflammation in macrophages. However, the exact mechanism of SMS2 activation in endothelial barrier disruption and lung injury is not fully understood. To define the role of SMS activation in lung injury, we hypothesized that the inhibition of SM synthesis may provide protection against acute lung injury (ALI) by preserving endothelial barrier function. Using SMS2-silencing RNA (siRNA) treatment in human pulmonary endothelial cells (HPAECs) and tricyclodecan-9-yl-xanthogenate (D609), a competitive inhibitor of SMS, and phosphatidylcholine-specific phospholipase C in a murine model of bacterial LPS injury, we studied the role of sphingomyelin synthesis in ALI. Results show that pretreating mice with D609 significantly attenuated LPS-induced lung injury, as measured by a significant decrease in wet to dry ratio, bronchoalveolar lavage fluid cell and protein counts, and myeloperoxidase activity in lung tissue. Similarly, LPS-induced endothelial barrier disruption was significantly reduced in HPAECs pretreated with D609 or SMS2 siRNA, as demonstrated by an increase in paracellular integrity on an FITC-dextran assay, by the inhibition of LPS-induced stress fibers, and by the formation of cortical actin rings and lamellipodia at the periphery. These results indicate that D609 attenuates LPS-mediated endothelial barrier dysfunction and lung injury in mice through inhibition of SMS, suggesting a novel and essential role of SMS inhibition in modulating endothelial barrier integrity via actin cytoskeletal activation, with a potential therapeutic role in ALI.

Tricyclodecan-9-yl-xanthogenate (D609) mechanism of actions: a mini-review of literature

Tricyclodecan-9-yl-xanthogenate (D609) is known for its antiviral and antitumor properties. D609 actions are widely attributed to inhibiting phosphatidylcholine (PC)-specific phospholipase C (PC-PLC). D609 also inhibits sphingomyelin synthase (SMS). PC-PLC and/or SMS inhibition will affect lipid second messengers 1,2-diacylglycerol (DAG) and/or ceramide. Evidence indicates either PC-PLC and/or SMS inhibition affected the cell cycle and arrested proliferation, and stimulated differentiation in various in vitro and in vivo studies. Xanthogenate compounds are also potent antioxidants and D609 reduced Aß-induced toxicity, attributed to its antioxidant properties. Zn²⁺ is necessary for PC-PLC enzymatic activity; inhibition by D609 might be attributed to its Zn²⁺ chelation. D609 has also been proposed to inhibit acidic sphingomyelinase or down-regulate hypoxia inducible factor-1α; however these are down-stream events related to PC-PLC inhibition. Characterization of the mammalian PC-PLC is limited to inhibition of enzymatic activity (frequently measured using Amplex red assay with bacterial PC-PLC as a standard). The mammalian PC-PLC has not been cloned; sequenced and structural information is unavailable. D609 showed promise in cancer studies, reduced atherosclerotic plaques (inhibition of PC-PLC) and cerebral infarction after stroke (PC-PLC or SMS). D609 actions as an antagonist to pro-inflammatory cytokines have been attributed to PC-PLC. The purpose of this review is to comprehensively evaluate the literature and summarize the findings and relevance to cell cycle and CNS pathologies.

Lack of alpha-synuclein modulates microglial phenotype in vitro

Alpha (α)-synuclein neuronal effects are continually being defined although its role in regulating glial phenotypes remains unclear. An ability to regulate microglial activation was investigated using primary cultures from wild type and α-synuclein deficient mice (Snca-/-). Snca-/- microglia demonstrated increased secretion of the cytokine tumor necrosis factor-alpha (TNF-α), impaired phagocytic ability, elevated prostaglandin levels, and increased protein levels of key enzymes in lipid-mediated signaling events, cytosolic phospholipase (cPLA(2)), cyclooxygenase-2 (Cox-2) and phospholipase D2 (PLD2) when compared to wild type cells. Increased cytokine secretion and cPLA(2) and Cox-2 levels in Snca-/- microglia were partially attenuated by inhibiting PLD-dependent signaling with n-butanol treatment.

Nitric oxide and calcium signaling regulate myocardial tumor necrosis factor-α expression and cardiac function in sepsis

Myocardial tumor necrosis factor-alpha (TNF-alpha), a proinflammatory cytokine, is a critical inducer of myocardial dysfunction in sepsis. The purpose of this review is to summarize the mechanisms through which TNF-alpha production is regulated in cardiomyocytes in response to lipopolysaccharide (LPS), a key pathogen-associated molecular pattern (PAMP) in sepsis. These mechanisms include Nox2-containing NAD(P)H oxidase, phospholipase C (PLC)gamma1, and Ca2+ signaling pathways. Activation of these pathways increases TNF-alpha expression via activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) and p38 mitogen-activated protein kinase (MAPK). Conversely, activation of c-Jun NH2-terminal kinase 1 (JNK1) negatively regulates TNF-alpha production through inhibition of ERK1/2 and p38 MAPK activity. Interestingly, endothelial nitric oxide synthase (eNOS) promotes TNF-alpha expression by enhancing p38 MAPK activation, whereas neuronal NOS (nNOS) inhibits TNF-alpha production by reducing Ca2+-dependent ERK1/2 activity. Therefore, the JNK1 and nNOS inhibitory pathways represent a "brake" that limits myocardial TNF-alpha expression in sepsis. Further understanding of these signal transduction mechanisms may lead to novel pharmacological therapies in sepsis.

Protection by D609 through cell-cycle regulation after stroke

Expressions of cell-cycle regulating proteins are altered after stroke. Cell-cycle inhibition has shown dramatic reduction in infarction after stroke. Ceramide can induce cell-cycle arrest by up-regulation of cyclin-dependent kinase (Cdk) inhibitors p21 and p27 through activation of protein phosphatase 2A (PP2A). Tricyclodecan-9-yl-xanthogenate (D609)-increased ceramide levels after transient middle cerebral artery occlusion (tMCAO) in spontaneously hypertensive rat (SHR) probably by inhibiting sphingomyelin synthase (SMS). D609 significantly reduced cerebral infarction and up-regulated Cdk inhibitor p21 and down-regulated phospho-retinoblastoma (pRb) expression after tMCAO in rat. Others have suggested bFGF-induced astrocyte proliferation is attenuated by D609 due to an increase in ceramide by SMS inhibition. D609 also reduced the formation of oxidized phosphatidylcholine (OxPC) protein adducts. D609 may attenuate generation of reactive oxygen species and formation of OxPC by inhibiting microglia/macrophage proliferation after tMCAO (please also see note added in proof: D609 may prevent mature neurons from entering the cell cycle at the early reperfusion, however may not interfere with later proliferation of microglia/ macrophages that are the source of brain derived neurotrophic factor (BDNF) and insulin-like growth factor (IGF-1) in offering protection). It has been proposed that D609 provides benefit after tMCAO by attenuating hypoxia-inducible factor-1alpha and Bcl2/adenovirus E1B 19 kDa interacting protein 3 expressions. Our data suggest that D609 provides benefit after stoke through inhibition of SMS, increased ceramide levels, and induction of cell-cycle arrest by up-regulating p21 and causing hypophosphorylation of Rb (through increased protein phosphatase activity and/or Cdk inhibition).

Phospholipase D2 acts as an important regulator in LPS-induced nitric oxide synthesis in Raw 264.7 cells

The purpose of this study was to identify the role of phospholipase D2 (PLD2) in lipopolysaccharide (LPS)-induced nitric oxide (NO) synthesis. LPS enhanced NO synthesis and inducible nitric oxide synthase (iNOS) expression in macrophage cell line, Raw 264.7 cells. When Raw 264.7 cells were stimulated with LPS, the expressions of PLDs were increased. Thus, to investigate the role of PLD in NO synthesis, we transfected PLD1, PLD2, and their dominant negative forms to Raw 264.7 cells, respectively. Interestingly, only PLD2 overexpression, but not that of PLD1, increased NO synthesis and iNOS expression. Moreover, LPS-induced NO synthesis and iNOS expression were blocked by PLD2 siRNA, suggesting that LPS upregulates NO synthesis through PLD2. Next, we investigated the S6K1-p42/44 MAPK-STAT3 signaling pathway in LPS-induced NO synthesis mechanism. Knockdown of PLD2 with siRNA also decreased phosphorylation of S6K1, p42/44 MAPK and STAT3 induced by LPS. Furthermore, we found that STAT3 bound with the iNOS promoter, and their binding was mediated by PLD2. Taken together, our results demonstrate the importance of PLD2 for LPS-induced NO synthesis in Raw 264.7 cells with involvement of the S6K1-p42/44 MAPK-STAT3 pathway.

Phospholipase C, phosphatidylinositol 3-kinase, and intracellular [Ca(2+)] mediate the activation of chicken HD11 macrophage cells by CpG oligodeoxynucleotide

The activation of phospholipases is one of the earliest key events in receptor-mediated cellular responses to a number of extracellular signaling molecules. Oligodeoxynucleotides containing CpG motifs (CpG ODN) mimic microbial DNA and are immunostimulatory to most vertebrate species. In the present study, we used the production of nitric oxide (NO) as an indicator to evaluate the involvement of the signaling cascades of phospholipases and phosphatidylinositol 3-kinase (PI3K) in the activation of chicken HD11 macrophage cells by CpG ODN. Using selective inhibitors, we have identified the involvement of phosphatidylinositol (PI)-phospholipase C (PI-PLC), but not phosphatidylcholine (PC)-phospholipase C (PC-PLC) and PC-phospholipase D (PC-PLD), in CpG ODN-induced NO production in HD11 cells. Preincubation with PI-PLC selective inhibitors (U-73122) completely abrogated CpG ODN-induced NO production in HD11 cells, whereas PC-PLC inhibitor (D609) and PC-PLD inhibitor (n-butanol) had no inhibitory effects. Additionally, inhibition of PI3K and protein kinase C (PKC) with selective inhibitors and chelation of intracellular [Ca(2+)] also significantly attenuated NO production in CpG ODN-activated HD11 cells. Our results demonstrate that PI-PLC, PI3 K, PKC, and intracellular [Ca(2+)] are important components of the CpG ODN-induced signaling pathway that leads to the production of NO in avian macrophage cells.

Oxidative stress, lipid rafts, and macrophage reprogramming

Oxidant stress, induced under a variety of conditions, is known to lead to the molecular reprogramming of the tissue-fixed macrophage. This reprogramming is associated with an altered response to subsequent inflammatory stimuli, such as lipopolysaccharide (LPS), leading to enhanced liberation of proinflammatory chemokines and cytokines. Due to this altered response, dysregulated immunity ensues, leading to the development of clinical syndromes such as multiple organ dysfunction syndrome (MODS). Although the mechanisms responsible for this altered macrophage activity by oxidant stress remains complex and poorly elucidated, it appears, based on recent research, that early and direct alterations within lipid rafts are responsible. This early and direct interaction with lipid rafts by oxidants leads to the mobilization of annexin VI from lipid raft constructs, leading to the release of calcium. This increased cytosolic concentration of this secondary messenger, in turn, results in the activation of calcium-dependent kinases, leading to further alterations in lipid raft lipids and eventually lipid raft proteins. Due to these lipid raft compositional changes, preassembly of receptor complexes occur, leading to enhanced proinflammatory activation. Within this review, the complexity of oxidant-induced reprogramming within the tissue fixed macrophage as currently understood is explained.

Diacylglycerol kinase zeta regulates microbial recognition and host resistance to Toxoplasma gondii

Mammalian Toll-like receptors (TLRs) recognize microbial pathogen-associated molecular patterns and are critical for innate immunity against microbial infection. Diacylglycerol (DAG) kinases (DGKs) regulate the intracellular levels of two important second messengers involved in signaling from many surface receptors by converting DAG to phosphatidic acid (PA). We demonstrate that the ζ isoform of the DGK family (DGKζ) is expressed in macrophages (Mφ) and dendritic cells. DGKζ deficiency results in impaired interleukin (IL) 12 and tumor necrosis factor α production following TLR stimulation in vitro and in vivo, increased resistance to endotoxin shock, and enhanced susceptibility to Toxoplasma gondii infection. We further show that DGKζ negatively controls the phosphatidylinositol 3–kinase (PI3K)–Akt pathway and that inhibition of PI3K activity or treatment with PA can restore lipopolysaccharide-induced IL-12 production by DGKζ-deficient Mφ. Collectively, our data provide the first genetic evidence that an enzyme involved in DAG/PA metabolism plays an important role in innate immunity and indicate that DGKζ promotes TLR responses via a pathway involving inhibition of PI3K.

Silica induces macrophage cytokines through phosphatidylcholine-specific phospholipase C with hydrogen peroxide

Silica particle-associated inflammation is implicated in the genesis of several pulmonary diseases, including silicosis and lung cancer. In this study we investigated the role of phosphatidylcholine-specific phospholipase C (PC-PLC) in silica-stimulated induction of TNF-alpha and IL-1beta and how PC-PLC activity is regulated by silica in a rat alveolar macrophage model. We demonstrated that inhibition of PC-PLC, which was achieved with tricychodecan-9-yl-xanthate (D609), blocked the silica-stimulated induction of TNF-alpha and IL-1beta in alveolar macrophage, suggesting that PC-PLC is involved in the silica-associated inflammatory response. PC-PLC activity was increased significantly by silica exposure, and this could be inhibited by MnTBAP, which catalyzes both the dismutation of O2.- to O2 and H2O2 and the dismutation of H2O2 to O2 and H2O, revealing that PC-PLC activity is regulated in a redox-dependent manner. This is further confirmed by the finding that PC-PLC activity was increased by exogenous H2O2. The intracellular calcium chelator BAPTA blocked the H2O2-increased PC-PLC activity, while the calcium ionophore, A23187, enhanced PC-PLC activity. The data indicate that PC-PLC plays critical roles in the silica-associated inflammatory response and that PC-PLC is regulated through redox- and calcium-dependent manners in alveolar macrophages.

Involvement of phosphatidylinositol-phospholipase C in immune response to Salmonella lipopolysacharide in chicken macrophage cells (HD11)

The activation of phospholipases is one of the earliest key events in receptor-mediated cellular responses to a number of extracellular signaling molecules. Lipopolysaccharide (LPS) is a principle component of the outer membrane of Gram-negative bacteria and a prime target for recognition by the innate immune system. In the present study, we evaluated the role of specific phospholipase in the activation of a chicken macrophage cell line HD11 by LPS. Activation of HD11 cells by LPS results in induction of nitric oxide (NO). Using selective inhibitors, we have identified that phosphatidylinositol (PI)-phospholipase C (PI-PLC), but not phosphatidylcholine (PC)-phospholipase C (PC-PLC) nor PC-phospholipase D (PC-PLD), was required for LPS-induced NO production. Preincubation with PI-PLC selective inhibitors (U-73122 and ET-18-OCH3) abrogated LPS-induced NO production in HD11 cells, whereas PC-PLC inhibitor (D609), phosphatide phosphohydrolase inhibitor (propranolol), and PC-PLD inhibitor (n-butanol) had no inhibitory effects. We also showed that inhibition of protein kinase C (PKC) by selective inhibitors Ro 31-8220 and calphostin C and chelating intracellular Ca2+ by BAPTA-AM significantly reduced NO production in LPS-stimulated HD11 cells. Our results demonstrate that PI-PLC plays a critical role, most likely through activation of PKC pathway, in TLR4 mediated immune responses of avian macrophage cells to LPS.

Lipopolysaccharide-induced cyclooxygenase-2 expression in human U937 macrophages is phosphatidic acid phosphohydrolase-1-dependent

Cyclooxygenase (COX) has two isoforms, COX-1 and -2, which catalyze the key step in the conversion of cellular arachidonic acid into prostaglandins. In recent years, interest in COX-2 has significantly increased since it has been a target for the development of specific non-steroidal anti-inflammatory drugs. We report that COX-2 expression is up-regulated in phorbol ester (phorbol myristate acetate, PMA)-differentiated human U937 macrophage-like cells stimulated with lipopolysaccharide (LPS), whereas COX-1 is not up-regulated. We show that the LPS-induced up-regulation of COX-2 depends on the activity of the Mg(+2)-dependent phosphatidic acid phosphohydrolase 1 (PAP-1). Inhibition of PAP-1 by bromoenol lactone, propranolol, or ethanol resulted in a decrease in LPS-induced COX-2 mRNA transcript production, COX-2 protein expression, and prostaglandin E(2) release from U937 macrophages. To ensure that these results did not arise because of PMA treatment of the U937 cells, similar experiments were conducted with the P388D(1) cell line, which does not require PMA differentiation. LPS increased the levels of endogenous cellular diacylglycerol (DAG) within 2 min of stimulation. This increase was observed to be sensitive to the PAP-1 inhibitors. Furthermore, phosphatidic acid phosphohydrolase activity assays showed that the bromoenol lactone-sensitive PAP-1 activity was translocated from the cytosolic fraction to the membrane fraction within 2 min of LPS exposure. Finally, DAG add-back experiments demonstrate that LPS-induced COX-2 expression is enhanced by the addition of exogenous DAG.

Unheated Cannabis sativa extracts and its major compound THC-acid have potential immuno-modulating properties not mediated by CB1 and CB2 receptor coupled pathways

There is a great interest in the pharmacological properties of cannabinoid like compounds that are not linked to the adverse effects of Delta(9)-tetrahydrocannabinol (THC), e.g. psychoactive properties. The present paper describes the potential immuno-modulating activity of unheated Cannabis sativa extracts and its main non-psychoactive constituent Delta(9)-tetrahydrocanabinoid acid (THCa). By heating Cannabis extracts, THCa was shown to be converted into THC. Unheated Cannabis extract and THCa were able to inhibit the tumor necrosis factor alpha (TNF-alpha) levels in culture supernatants from U937 macrophages and peripheral blood macrophages after stimulation with LPS in a dose-dependent manner. This inhibition persisted over a longer period of time, whereas after prolonged exposure time THC and heated Cannabis extract tend to induce the TNF-alpha level. Furthermore we demonstrated that THCa and THC show distinct effects on phosphatidylcholine specific phospholipase C (PC-PLC) activity. Unheated Cannabis extract and THCa inhibit the PC-PLC activity in a dose-dependent manner, while THC induced PC-PLC activity at high concentrations. These results suggest that THCa and THC exert their immuno-modulating effects via different metabolic pathways.

PKC alpha depletion in RAW264.7 macrophages following microbial/IFNgamma stimulation is PC-PLC-mediated

Under chronic inflammatory conditions, monocytes/macrophages often exhibit a desensitized phenotype, which is characterized by attenuated reactive oxygen species (ROS) production in close association with depletion of protein kinase C alpha (PKC alpha). This behavior has been observed in monocytes derived from septic blood although the stimulus responsible for initiating these alterations remained obscure. Using RAW264.7 macrophages, we provide evidence that components of neither gram-negative nor gram-positive bacteria deplete PKC alpha, whereas the T(H)1 cytokine interferon-gamma (IFNgamma) does. As shown by western blot analysis, lipopolysaccharide, as well as lipoteichoic acid, did not alter PKC alpha expression, but IFNgamma dose-dependently decreased PKC alpha protein level. Taking into consideration that diacylglycerol and Ca2+ as established PKC alpha activators are released in response to phospholipase C activation, we pretreated cells with the phosphatidylcholine-specific phospholipase C (PC-PLC) inhibitor tricyclodecan-9-yl potassium xanthate (D609) and the phosphatidylinositol-specific phospholipase C inhibitor 1-(6-(17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione (U73122). In cells preincubated with D609, IFNgamma-mediated PKC alpha depletion was attenuated, whereas U73122 did not impair this process. Moreover, phorbol 12-myristate 13-acetate-initiated ROS formation, which was attenuated in macrophages pretreated with IFNgamma, was restored in the presence of the PC-PLC inhibitor. These results suggest that IFNgamma causes PC-PLC stimulation, diacylglycerol release, Ca2+ influx, and concomitant PKC alpha activation, which subsequently depletes PKC alpha. Strategies to antagonize IFNgamma might be helpful to prevent monocyte/macrophage desensitization.

Analysis of the role of the PAC1 receptor in neutrophil recruitment, acute-phase response, and nitric oxide production in septic shock

Infections caused by Gram-negative bacteria constitute one of the major causes of septic shock, which results from the inability of the immune system to limit bacterial spread during the ongoing infection. In the last decade, it has been demonstrated that vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are two endogenous immunopeptides, which together with three G protein-coupled receptors (VPAC1, VPAC2, and PAC1) exert a significant, therapeutic effect attenuating the deleterious consequences of septic shock by balancing pro- and anti-inflammatory factors. We have recently shown PAC1 receptor involvement in vivo as an anti-inflammatory receptor, at least in part, by attenuating lipopolysaccharide-induced production of proinflammatory interleukin-6. The present study deepens in the protective role of PAC1 receptor in septic shock, elucidating its involvement in the modulation of neutrophil recruitment and in the expression of different molecular sensors such as intercellular adhesion molecule-1, vascular cell adhesion molecule-1, fibrinogen, serum amyloid A, and nitric oxide as important, systemic players of the development of septic shock. Our results, using a mice deficient in PAC1 and a PAC1 antagonist, show that VIP and PACAP as well as the PAC1 receptor are involved in neutrophil recruitment in different target organs, in adhesion molecules expression, and in coagulation-related molecule fibrinogen synthesis. Thus, this study provides some important insights with respect to the involvement of PAC1 into the complexities of sepsis and represents an advantage for the design of more specific drugs complementing standard intensive care therapy in severe sepsis, confirming VIP and PACAP as candidates for multitarget therapy of septic shock.

Phosphatidic acid regulates systemic inflammatory responses by modulating the Akt-mammalian target of rapamycin-p70 S6 kinase 1 pathway

Macrophages are pivotal effector cells in the innate immune system. When microbial products bind to pathogen recognition receptors, macrophages are activated and release a broad array of mediators, such as cytokines, that orchestrate the inflammatory responses of the host. Phosphatidic acid (PA) has been implicated as an important metabolite of phospholipid biosynthesis and in membrane remodeling and has been further suggested to be a crucial second messenger in various cellular signaling events. Here we show that PA is an essential regulator of inflammatory response. Deleterious effects of PA are associated with the secretion of proinflammatory cytokines, such as tumor necrosis factor-alpha, interleukin-1beta, interleukin-6, and the production of nitric oxide, prostaglandin E2, which are predominantly released by macrophage Raw264.7 cells. Furthermore, the administration of PA to mice increased the serum cytokine level. Moreover, direct or lipopolysaccharide-induced PA accumulation by macrophages led to the Akt-dependent activation of the mammalian target of rapamycin-p70 S6 kinase 1, a process required for the induction of inflammatory mediators. These findings demonstrate the importance of the role of PA in systemic inflammatory responses, and provide a potential usefulness as specific targets for the development of therapies.

Fcgamma-receptors induce Mac-1 (CD11b/CD18) mobilization and accumulation in the phagocytic cup for optimal phagocytosis

Functional interactions between Fcgamma-receptors (FcgammaR) and the beta2 integrin Mac-1 (CD11b/CD18) have been described, but the molecular basis of this relationship remains unclear. Although the glycosylphosphatidylinositol-linked receptor FcgammaRIIIB of human neutrophils is constitutively associated with Mac-1, we found no evidence for direct physical association between Mac-1 and the FcgammaR of mouse macrophages, which are transmembrane proteins. Nevertheless, Mac-1 accumulated in the phagocytic cup following engagement of FcgammaR by IgG-opsonized particles. Blocking the CD18 chains of beta2 integrins by using specific antibodies reduced Mac-1 accumulation in the cup. These antibodies or the addition of the recombinant CD11b I-domain inhibited the ingestion of IgG-opsonized particles. FcgammaR cross-linking stimulated cell adhesion to surfaces coated with Mac-1 ligands and in addition enabled macrophages to bind C3bi-opsonized particles, indicating that FcgammaR-derived signals induce activation of Mac-1. Measurements of fluorescence recovery after photobleaching revealed that whereas most (>80%) of Mac-1 is immobile in resting cells, stimulation of FcgammaR markedly increases the mobile fraction of the integrin. Activation of Mac-1 by FcgammaR required the activity of Src family tyrosine kinases, phosphatidylinositol 3-kinase and phospholipase C, with the release of diacylglycerol and stimulation of protein kinase C. Because elevated cytosolic Ca2+ was not required, we suggest that novel protein kinase C isoforms are involved in Mac-1 activation. These results suggest that FcgammaR stimulation promotes Mac-1 clustering into high avidity complexes in phagocytic cups by releasing the integrin from cytoskeletal constraints and enhancing its lateral diffusion. FcgammaR can enhance host defense by activating Mac-1 (and possibly other integrins), having a synergistic effect on pathogen engulfment and promoting the adherence of phagocytes at sites of infection.

Phospholipase C-dependent hydrolysis of phosphatidylcholine hydroperoxides to diacylglycerol hydroperoxides and its reduction by phospholipid hydroperoxide glutathione peroxidase

We have shown that 1,2-diacylglycerol hydroperoxides activate protein kinase C (PKC) as efficiently as does phorbol ester [Takekoshi S, Kambayashi Y, Nagata H, Takagi T, Yamamoto Y, Watanabe K. Activation of protein kinase C by oxidized diacylglycerol. Biochem Biophys Res Commun 1995; 217: 654-660]. 1,2-Diacylglycerol hydroperoxides also stimulate human neutrophils to release superoxide whereas their hydroxides do not [Yamamoto Y, Kambayashi Y, Ito T, Watanabe K, Nakano M. 1,2-Diacylglycerol hydroperoxides induce the generation and release of superoxide anion from human polymorphonuclear leukocytes. FEBS Lett 1997; 412: 461-464]. One of the proposed mechanisms for the formation of 1,2-diacylglycerol hydroperoxides is the hydrolysis of phosphatidylcholine hydroperoxides by phospholipase C (PLC). To confirm this hypothesis, we incubated 1-palmitoyl-2-linoleoyl-phosphatidylcholine (PLPC) liposomes containing PLPC hydroperoxides (PLPC-OOH) with Bacillus cereus PLC and found 1-palmitoyl-2-linoleoylglycerol (PLG) and its hydroperoxide (PLG-OOH) were produced. PLC hydrolyzed the two substrates without preference, as the yields of PLG and PLG-OOH were the same even though cholesterol was incorporated into liposomes to increase bilayer integrity. Phospholipid hydroperoxide glutathione peroxidase (PHGPX) reduced PLG-OOH to its hydroxide in the presence of glutathione while the conventional cytosolic glutathione peroxidase did not. These data suggest that PLC hydrolyzes oxidized biomembranes to give 1,2-diacylglycerol hydroperoxides for PKC stimulation but PHGPX may prevent neutrophil stimulation by reducing 1,2-diacylglycerol hydroperoxides to their hydroxides.