5-Hydroxyeicosatetraenoic acid is a key intermediate of the arachidonate-dependent protective signaling in monocytes/macrophages exposed to peroxynitrite

Targeting Mammalian 5-Lipoxygenase by Dietary Phenolics as an Anti-Inflammatory Mechanism: A Systematic Review

5-Lipoxygenase (5-LOX) plays a key role in inflammation through the biosynthesis of leukotrienes and other lipid mediators. Current evidence suggests that dietary (poly)phenols exert a beneficial impact on human health through anti-inflammatory activities. Their mechanisms of action have mostly been associated with the modulation of pro-inflammatory cytokines (TNF-α, IL-1β), prostaglandins (PGE2), and the interaction with NF-κB and cyclooxygenase 2 (COX-2) pathways. Much less is known about the 5-lipoxygenase (5-LOX) pathway as a target of dietary (poly)phenols. This systematic review aimed to summarize how dietary (poly)phenols target the 5-LOX pathway in preclinical and human studies. The number of studies identified is low (5, 24, and 127 human, animal, and cellular studies, respectively) compared to the thousands of studies focusing on the COX-2 pathway. Some (poly)phenolics such as caffeic acid, hydroxytyrosol, resveratrol, curcumin, nordihydroguaiaretic acid (NDGA), and quercetin have been reported to reduce the formation of 5-LOX eicosanoids in vitro. However, the in vivo evidence is inconclusive because of the low number of studies and the difficulty of attributing effects to (poly)phenols. Therefore, increasing the number of studies targeting the 5-LOX pathway would largely expand our knowledge on the anti-inflammatory mechanisms of (poly)phenols.

Mitochondrial ascorbic acid is responsible for enhanced susceptibility of U937 cells to the toxic effects of peroxynitrite

Otherwise nontoxic levels of peroxynitrite promote toxicity in U937 cells pre-exposed to low micromolar concentrations of l-ascorbic acid (AA). This event was associated with the mitochondrial accumulation of the vitamin and with the early formation of secondary reactive oxygen species and DNA single-strand breaks. The same concentrations of peroxynitrite, however, failed to elicit detectable effects in cells pre-exposed to dehydroascorbic acid (DHA), in which mitochondrial accumulation of vitamin C did not occur despite the identical cytosolic levels. Coherently, oxidation of extracellular AA failed to affect the intracellular concentration of the vitamin, but nevertheless prevented its mitochondrial localization as well as the enhanced response to peroxynitrite. Furthermore, in cells postincubated in vitamin C-free medium, time-dependent loss of mitochondrial AA was paralleled by a progressive decline of susceptibility to peroxynitrite, under the same conditions in which cells retained about half of the initial AA. Using different experimental approaches, we finally showed that the enhancing effects of AA are mediated by events associated with peroxynitrite-dependent superoxide/H2 O2 formation in the mitochondrial respiratory chain. Collectively, these results indicate that mitochondria actively take up vitamin C as AA and respond to otherwise inactive concentrations of peroxynitrite with the mitochondrial formation of secondary species responsible for DNA damage and toxicity. DHA preloading, while leading to the accumulation of identical levels of vitamin C, fails to produce these effects because of the poor mitochondrial accumulation of the vitamin.

Clostridium difficile toxin B differentially affects GPCR-stimulated Ca2+ responses in macrophages: independent roles for Rho and PLA2

Clostridium difficile toxins cause acute colitis by disrupting the enterocyte barrier and promoting inflammation. ToxB from C. difficile inactivates Rho family GTPases and causes release of cytokines and eicosanoids by macrophages. We studied the effects of ToxB on GPCR signaling in murine RAW264.7 macrophages and found that ToxB elevated Ca(2+) responses to Galphai-linked receptors, including the C5aR, but reduced responses to Galphaq-linked receptors, including the UDP receptors. Other Rho inhibitors also reduced UDP Ca(2+) responses, but they did not affect C5a responses, suggesting that ToxB inhibited UDP responses by inhibiting Rho but enhanced C5a responses by other mechanisms. By using PLCbeta isoform-deficient BMDM, we found that ToxB inhibited Ca(2+) signaling through PLCbeta4 but enhanced signaling through PLCbeta3. Effects of ToxB on GPCR Ca(2+) responses correlated with GPCR use of PLCbeta3 versus PLCbeta4. ToxB inhibited UDP Ca(2+) signaling without reducing InsP3 production or the sensitivity of cellular Ca(2+) stores to exogenous InsP3, suggesting that ToxB impairs UDP signaling at the level of InsP3/Ca(2+)coupling. In contrast, ToxB elevated InsP3 production by C5a, and the enhancement of Ca(2+) signaling by C5a was prevented by inhibition of PLA(2) or 5-LOX but not COX, implicating LTs but not prostanoids in the mechanism. In sum, ToxB has opposing, independently regulated effects on Ca(2+) signaling by different GPCR-linked PLCbeta isoforms in macrophages.

Assessing Bad sub-cellular localization under conditions associated with prevention or promotion of mitochondrial permeability transition-dependent toxicity

Cells belonging to the monocyte/macrophage lineage are in general highly resistant to peroxynitrite, a reactive nitrogen species extensively produced by these and other cell types under inflammatory conditions. Resistance is not dependent on the scavenging of peroxynitrite but is rather associated with the prompt activation of a survival signaling in response to various molecules largely available at the inflammatory sites, as arachidonic acid and products of the 5-lipoxygenase or cyclooxygenase pathways. We detected significant levels of Bad in the mitochondria of monocytes/macrophages and found that these signaling pathways converge in Bad phosphorylation, and thus in its cytosolic accumulation. Phosphorylation inhibits binding of Bad to Bcl-2, or BclXL, and promotes its translocation to the cytosol, thereby enabling Bcl-2 and BclXL to exert effects leading to prevention of mitochondrial permeability- transition (MPT). Upstream inhibition of the survival signaling indeed promotes the mitochondrial accumulation of Bad and the rapid onset of MPT-dependent toxicity. The above results contribute to the definition of the mechanism(s) whereby monocytes/macrophages survive to peroxynitrite in inflamed tissues.

Peroxynitrite damages U937 cell DNA via the intermediate formation of mitochondrial oxidants

Eight years ago, we published in this journal the first evidence that peroxynitrite does not directly produce DNA single-strand breakage in intact U937 cells (Guidarelli et al., IUBMB Life, 50, 195-201). This event was rather attributed to the secondary reactive species produced at the mitochondrial level via a Ca2+-dependent reaction, in which ubisemiquinone serves as an electron donor. Under these conditions, electrons are directly transferred to molecular oxygen and superoxide/H2O2, and the ensuing DNA damage can therefore be produced in a time- dependent manner for at least 30 min. Formation of H2O2 and DNA single-strand breaks was therefore dependent on interference with electron transport at the complex III level as well as on mitochondrial Ca2+ accumulation. Further studies led to the demonstrations that peroxynitrite mobilizes Ca2+ from the ryanodine receptor. Finally, in U937 cells, a pro-monocytic cell line sharing with monocytes/macrophages the same signaling events to survive to peroxynitrite, mitochondrial H2O2 promotes inhibition of survival via tyrosine phosphatase activation, leading to ERK1/2 dephosphorylation and thus to upstream inhibition of the survival signaling.

Peroxynitrite-mediated lipid oxidation and nitration: mechanisms and consequences

Lipid oxidation and nitration represents a novel area of research of relevance in the understanding of inflammatory processes. Peroxynitrite, the product of the diffusion-limited reaction between nitric oxide and superoxide anion, mediates oxidative modifications in lipid systems including cell membranes and lipoproteins. In this review, we discuss the mechanisms of lipid oxidation and nitration by peroxynitrite as well as the influence of physiological molecules and cell targets to redirect peroxynitrite reactivity. We also provide evidence to support that oxidation/nitration of lipids results in the formation of novel signaling modulators of key lipid-metabolizing enzymes.

Prostaglandin E2 signals monocyte/macrophage survival to peroxynitrite via protein kinase A converging in bad phosphorylation with the protein kinase C alpha-dependent pathway driven by 5-hydroxyeicosatetraenoic acid

Monocytes/macrophages committed to death by peroxynitrite nevertheless survive with a signaling response promoting Bad phosphorylation, as well as its cytosolic localization, via upstream activation of cytosolic phospholipase A(2), 5-lipoxygenase, and protein kinase C alpha. We now report evidence for an alternative mechanism converging in Bad phosphorylation when the expression/activity of the above enzymes are suppressed. Under these conditions, also associated with peroxynitrite-dependent severe inhibition of Akt, an additional Bad kinase, Bad dephosphorylation promoted its accumulation in the mitochondria and a prompt lethal response. PGE(2) prevented toxicity via EP(2) receptor-mediated protein kinase A-dependent Bad phosphorylation. This notion was established in U937 cells by the following criteria: 1) there was a strong correlation between survival and cAMP accumulation, both in the absence and presence of phosphodiesterase inhibitors; 2) direct activation of adenylyl cyclase afforded cytoprotection; and 3) PGE(2) promoted loss of mitochondrial Bad and cytoprotection, mimicked by EP(2) receptor agonists, and prevented by EP(2) receptor antagonists or protein kinase A inhibitors. Finally, selected experiments performed in human monocytes/macrophages and in rat peritoneal macrophages indicated that the above cytoprotective pathway is a general response of cells belonging to the monocyte/macrophage lineage to both exogenous and endogenous peroxynitrite. The notion that two different pathways mediated by downstream products of arachidonic acid metabolism converge in Bad phosphorylation emphasizes the relevance of this strategy for the regulation of macrophage survival to peroxynitrite at the inflammatory sites.

Inhibition of complex III promotes loss of Ca2+ dependence for mitochondrial superoxide formation and permeability transition evoked by peroxynitrite

In intact U937 cells, peroxynitrite promotes the mitochondrial formation of superoxide via a Ca2+-dependent mechanism involving inhibition of complex III. Superoxide then readily dismutates to H2O2 causing lesions on different biomolecules, including DNA. Here we show that formation of H2O2 and DNA damage are suppressed by inhibition of complex I (by rotenone) or ubisemiquinone formation (by myxothiazol), as well as by a variety of manipulations preventing either the mobilization of Ca2+ or its mitochondrial accumulation. In addition, complex III inhibitors promoted rotenone- or myxothiazol-sensitive formation of H2O2 and DNA strand scission in cells exposed to otherwise inactive concentrations of peroxynitrite. However, under these conditions, the intra-mitochondrial concentration of Ca2+ remained unchanged and the effects of peroxynitrite therefore take place via Ca2+-independent mechanisms. H2O2 formation was paralleled by, and causally linked to, the loss of mitochondrial membrane potential associated with the mitochondrial release of cytochrome c and AIF, and with the mitochondrial accumulation of Bax. These events, although Ca2+ independent, were rapidly followed by death mediated by mitochondrial permeability transition, generally considered a typical Ca2+-dependent event. Thus, enforced inhibition of complex III promotes the loss of Ca2+ dependence of those mitochondrial mechanisms regulating superoxide formation and mitochondrial permeability transition evoked by peroxynitrite.