Substrate fate in activated macrophages: a comparison between innate, classic, and alternative activation

Macrophages play a relevant role in innate and adaptive immunity depending on the balance of the stimuli received. From an analytical and functional point of view, macrophage stimulation can be segregated into three main modes, as follows: innate, classic, and alternative pathways. These differential activations result in the expression of specific sets of genes involved in the release of pro- or anti-inflammatory stimuli. In the present work, we have analyzed whether specific metabolic patterns depend on the signaling pathway activated. A [1,2-(13)C(2)]glucose tracer-based metabolomics approach has been used to characterize the metabolic flux distributions in macrophages stimulated through the classic, innate, and alternative pathways. Using this methodology combined with mass isotopomer distribution analysis of the new formed metabolites, the data show that activated macrophages are essentially glycolytic cells, and a clear cutoff between the classic/innate activation and the alternative pathway exists. Interestingly, macrophage activation through LPS/IFN-gamma or TLR-2, -3, -4, and -9 results in similar flux distribution patterns regardless of the pathway activated. However, stimulation through the alternative pathway has minor metabolic effects. The molecular basis of the differences between these two types of behavior involves a switch in the expression of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK2) from the liver type-PFK2 to the more active ubiquitous PFK2 isoenzyme, which responds to Hif-1alpha activation and increases fructose-2,6-bisphosphate concentration and the glycolytic flux. However, using macrophages targeted for Hif-1alpha, the switch of PFK2 isoenzymes still occurs in LPS/IFN-gamma-activated macrophages, suggesting that this pathway regulates ubiquitous PFK2 expression through Hif-1alpha-independent mechanisms.

Impaired autophagic flux is associated with increased endoplasmic reticulum stress during the development of NAFLD

The pathogenic mechanisms underlying the progression of non-alcoholic fatty liver disease (NAFLD) are not fully understood. In this study, we aimed to assess the relationship between endoplasmic reticulum (ER) stress and autophagy in human and mouse hepatocytes during NAFLD. ER stress and autophagy markers were analyzed in livers from patients with biopsy-proven non-alcoholic steatosis (NAS) or non-alcoholic steatohepatitis (NASH) compared with livers from subjects with histologically normal liver, in livers from mice fed with chow diet (CHD) compared with mice fed with high fat diet (HFD) or methionine-choline-deficient (MCD) diet and in primary and Huh7 human hepatocytes loaded with palmitic acid (PA). In NASH patients, significant increases in hepatic messenger RNA levels of markers of ER stress (activating transcription factor 4 (ATF4), glucose-regulated protein 78 (GRP78) and C/EBP homologous protein (CHOP)) and autophagy (BCN1) were found compared with NAS patients. Likewise, protein levels of GRP78, CHOP and p62/SQSTM1 (p62) autophagic substrate were significantly elevated in NASH compared with NAS patients. In livers from mice fed with HFD or MCD, ER stress-mediated signaling was parallel to the blockade of the autophagic flux assessed by increases in p62, microtubule-associated protein 2 light chain 3 (LC3-II)/LC3-I ratio and accumulation of autophagosomes compared with CHD fed mice. In Huh7 hepatic cells, treatment with PA for 8 h triggered activation of both unfolding protein response and the autophagic flux. Conversely, prolonged treatment with PA (24 h) induced ER stress and cell death together with a blockade of the autophagic flux. Under these conditions, cotreatment with rapamycin or CHOP silencing ameliorated these effects and decreased apoptosis. Our results demonstrated that the autophagic flux is impaired in the liver from both NAFLD patients and murine models of NAFLD, as well as in lipid-overloaded human hepatocytes, and it could be due to elevated ER stress leading to apoptosis. Consequently, therapies aimed to restore the autophagic flux might attenuate or prevent the progression of NAFLD.

Rojo A, Manda G, Boscá L, Cuadrado A. Inflammation in Parkinson's Disease: Mechanisms and Therapeutic Implications

Parkinson's disease (PD) is a common neurodegenerative disorder primarily characterized by the death of dopaminergic neurons that project from the substantia nigra pars compacta. Although the molecular bases for PD development are still little defined, extensive evidence from human samples and animal models support the involvement of inflammation in onset or progression. However, the exact trigger for this response remains unclear. Here, we provide a systematic review of the cellular mediators, i.e., microglia, astroglia and endothelial cells. We also discuss the genetic and transcriptional control of inflammation in PD and the immunomodulatory role of dopamine and reactive oxygen species. Finally, we summarize the preclinical and clinical approaches targeting neuroinflammation in PD.

Nitric oxide and cell viability in inflammatory cells: a role for NO in macrophage function and fate

Macrophages participate actively in the inflammatory response by releasing cytokines, chemokines and factors that recruit additional cells to sites of infection or tissue injury or alteration. In addition to this, activated macrophages rapidly activate the expression of genes responsible for the high-output synthesis of reactive oxygen and nitrogen species (NO, O2-, H2O2 and peroxynitrite, among others) and bioactive lipids derived from arachidonic acid. All of these agents contribute to the regulation of the inflammatory response. Most of these molecules, when synthesized at these high concentrations, exert pro-apoptotic effects in many cell types. Macrophages themselves are a notable and important exception, being resistant to apoptotic death upon activation. This resistance is necessary to enable these cells to perform their functional role during the early phases of an inflammatory response. However, after cumulative damage, or when the synthesis of inflammatory mediators decreases, macrophages undergo the characteristic mitochondrial-dependent cell death program, contributing in this way to the resolution of the inflammatory reaction. In the case of infectious diseases, this also helps to prevent the development of parasitic strategies by phagocytosed pathogens.

Splenic B lymphocyte programmed cell death is prevented by nitric oxide release through mechanisms involving sustained Bcl-2 levels

Incubation of ex vivo cultured mature B cells in the presence of nitric oxide or nitric oxide-donor substances delays programmed cell death as determined by the appearance of DNA laddering in agarose gel electrophoresis or by flow-cytometry analysis of DNA. Nitric oxide also rescues B cells from antigen-induced apoptosis but fails to provide a co-stimulatory signal that converts the signal elicited by the antigen into a proliferative response. The protective effects of nitric oxide against programmed cell death can be reproduced by treatment of the cells with permeant analogues of cyclic GMP. Regarding the mechanisms by which nitric oxide prevents apoptosis in B cells, we have observed that nitric oxide release prevents the drop in the expression of the protooncogene bcl-2, both at the mRNA and protein levels, suggesting the existence of an unknown pathway that links nitric oxide signaling with Bcl-2 expression.

Inhibition of IkappaB kinase and IkappaB phosphorylation by 15-deoxy-Delta(12,14)-prostaglandin J(2) in activated murine macrophages

Activation of the macrophage cell line RAW 264.7 with lipopolysaccharide (LPS) and gamma interferon (IFN-gamma) induces the expression of gene products involved in host defense, among them type 2 nitric oxide synthase. Treatment of cells with 15-deoxy-Delta(12,14)-prostaglandin J(2) (15dPGJ(2)) inhibited the LPS- and IFN-gamma-dependent synthesis of NO, a process that was not antagonized by similar concentrations of prostaglandin J(2), prostaglandin E(2), or rosiglitazone, a peroxisomal proliferator-activated receptor gamma ligand. Incubation of activated macrophages with 15dPGJ(2) inhibited the degradation of IkappaBalpha and IkappaBbeta and increased their levels in the nuclei. NF-kappaB activity, as well as the transcription of NF-kappaB-dependent genes, such as those encoding type 2 nitric oxide synthase and cyclooxygenase 2, was impaired under these conditions. Analysis of the steps leading to IkappaB phosphorylation showed an inhibition of IkappaB kinase by 15dPGJ(2) in cells treated with LPS and IFN-gamma, resulting in an impaired phosphorylation of IkappaBalpha, at least in the serine 32 residue required for targeting and degradation of this protein. Incubation of partially purified activated IkappaB kinase with 2 microM 15dPGJ(2) reduced by 83% the phosphorylation in serine 32 of IkappaBalpha, suggesting that this prostaglandin exerts direct inhibitory effects on the activity of the IkappaB kinase complex. These results show rapid actions of 15dPGJ(2), independent of peroxisomal proliferator receptor gamma activation, in macrophages challenged with low doses of LPS and IFN-gamma.

Protein kinase Cepsilon is required for macrophage activation and defense against bacterial infection

To assess directly the role of protein kinase C (PKC)epsilon in the immune system, we generated mice that carried a homozygous disruption of the PKCepsilon locus. PKCepsilon(-/-) animals appeared normal and were generally healthy, although female mice frequently developed a bacterial infection of the uterus. Macrophages from PKCepsilon(-/-) animals demonstrated a severely attenuated response to lipopolysaccharide (LPS) and interferon (IFN)gamma, characterized by a dramatic reduction in the generation of NO, tumor necrosis factor (TNF)-alpha, and interleukin (IL)-1beta. Further analysis revealed that LPS-stimulated macrophages from PKCepsilon(-/-) mice were deficient in the induction of nitric oxide synthase (NOS)-2, demonstrating a decrease in the activation of IkappaB kinase, a reduction in IkappaB degradation, and a decrease in nuclear factor (NF)kappaB nuclear translocation. After intravenous administration of Gram-negative or Gram-positive bacteria, PKCepsilon(-/-) mice demonstrated a significantly decreased period of survival. This study provides direct evidence that PKCepsilon is critically involved at an early stage of LPS-mediated signaling in activated macrophages. Furthermore, we demonstrate that in the absence of PKCepsilon, host defense against bacterial infection is severely compromised, resulting in an increased incidence of mortality.

The increase in TNF-alpha levels is implicated in NF-kappaB activation and inducible nitric oxide synthase expression in brain cortex after immobilization stress

The underlying mechanisms by which physical or psychological stress causes neurodegeneration are still unknown. We have demonstrated that the high-output and long-lasting synthesizing source of nitric oxide (NO), inducible NO synthase (iNOS), is expressed in brain cortex after three weeks of repeated stress and that its overexpression accounts for the neurodegenerative changes found in this situation. Now we have found that a short duration of stress (immobilization for 6 h) also induces the expression of iNOS in brain cortex in adult male rats. In order to elucidate the possible mechanisms involved in iNOS expression, we have studied the role of the cytokine tumor necrosis factor-alpha (TNF-alpha) released in brain during stress. We have shown that there is an increase in soluble TNF-alpha levels after 1 h of stress in cortex and that this is preceded by an increase in TNF-alpha-convertase (TACE) activity in brain cortex as soon as 30 min after immobilization. Stress-induced increase in both TACE activity and TNF-alpha levels seems to be mediated by excitatory amino acids since they can be blocked by MK-801 (dizocilpine) (0.2 mg/kg i.p.), an antagonist of the N-methyl-D-aspartate subtype of glutamate receptor. In order to study the role of TACE and TNF-alpha in iNOS induction, a group of animals were i.p. injected with the preferred TACE inhibitor BB1101 (2 and 10 mg/kg). Indeed, BB1101 inhibited iNOS expression induced by six hours of stress. In addition, we studied the role of the transcription factor nuclear factor kappaB (NF-kappaB), which is required for iNOS expression. We have found that the administration of the TACE inhibitor BB1101 inhibited the stress-stimulated translocation of NF-kappaB to the nucleus. Taken together, these findings indicate that glutamate receptor activation induces TACE up-regulation and subsequent increase in TNF-alpha levels, and this account for stress-induced iNOS expression via NF-kappaB activation, supporting a possible neuroprotective role for specific TACE inhibitors in this situation.

Chronic stress induces the expression of inducible nitric oxide synthase in rat brain cortex

Long-term exposure to stress has detrimental effects on several brain functions in many species, including humans, and leads to neurodegenerative changes. However, the underlying neural mechanisms by which stress causes neurodegeneration are still unknown. We have investigated the role of endogenously released nitric oxide (NO) in this phenomenon and the possible induction of the inducible NO synthase (iNOS) isoform. In adult male rats, stress (immobilization for 6 h during 21 days) increases the activity of a calcium-independent NO synthase and induces the expression of iNOS in cortical neurons as seen by immunohistochemical and western blot analysis. Three weeks of repeated immobilization increases immunoreactivity for nitrotyrosine, a nitration product of peroxynitrite. Repeated stress causes accumulation of the NO metabolites NO2+ NO3- (NOx-) accumulation in cortex, and these changes occur in parallel with lactate dehydrogenase (LDH) release and impairment of glutamate uptake in synaptosomes. Administration of the selective iNOS inhibitor aminoguanidine (400 mg/kg i.p. daily from days 7 to 21 of stress) prevents NOx- accumulation in cortex, LDH release, and impairment of glutamate uptake in synaptosomes. Taken together, these findings indicate that a sustained overproduction of NO via iNOS expression may be responsible, at least in part, for some of the neurodegenerative changes caused by stress and support a possible neuroprotective role for specific iNOS inhibitors in this situation.

Nitric oxide induces apoptosis via triggering mitochondrial permeability transition

Nitric oxide (NO) induces apoptosis in thymocytes, peripheral T cells, myeloid cells and neurons. Here we show that NO is highly efficient in inducing mitochondrial permeability transition, thereby causing the liberation of apoptogenic factors from mitochondria which can induce nuclear apoptosis (DNA condensation and DNA fragmentation) in isolated nuclei in vitro. In intact thymocytes, NO triggers disruption of the mitochondrial transmembrane potential, followed by hypergeneration of reactive oxygen species, exposure of phosphatidyl serine on the outer plasma membrane leaflet, and nuclear apoptosis. Inhibitors of mitochondrial permeability transition such as bongkrekic acid and a cyclophilin D-binding cyclosporin A derivative, N-methyl-Val-4-cyclosporin A, prevent the mitochondrial as well as all post-mitochondrial signs of apoptosis induced by NO including nuclear DNA fragmentation and exposure of phosphatidylserine residues on the cell surface. These findings indicate that NO can cause apoptosis via triggering of permeability transition.

HIF-1α and PFKFB3 Mediate a Tight Relationship Between Proinflammatory Activation and Anerobic Metabolism in Atherosclerotic Macrophages

OBJECTIVE

Although it is accepted that macrophage glycolysis is upregulated under hypoxic conditions, it is not known whether this is linked to a similar increase in macrophage proinflammatory activation and whether specific energy demands regulate cell viability in the atheromatous plaque.

APPROACH AND RESULTS

We studied the interplay between macrophage energy metabolism, polarization, and viability in the context of atherosclerosis. Cultured human and murine macrophages and an in vivo murine model of atherosclerosis were used to evaluate the mechanisms underlying metabolic and inflammatory activity of macrophages in the different atherosclerotic conditions analyzed. We observed that macrophage energetics and inflammatory activation are closely and linearly related, resulting in dynamic calibration of glycolysis to keep pace with inflammatory activity. In addition, we show that macrophage glycolysis and proinflammatory activation mainly depend on hypoxia-inducible factor and on its impact on glucose uptake, and on the expression of hexokinase II and ubiquitous 6-phosphofructo-2-kinase. As a consequence, hypoxia potentiates inflammation and glycolysis mainly via these pathways. Moreover, when macrophages' ability to increase glycolysis through 6-phosphofructo-2-kinase is experimentally attenuated, cell viability is reduced if subjected to proinflammatory or hypoxic conditions, but unaffected under control conditions. In addition to this, granulocyte-macrophage colony-stimulating factor enhances anerobic glycolysis while exerting a mild proinflammatory activation.

CONCLUSIONS

These findings, in human and murine cells and in an animal model, show that hypoxia potentiates macrophage glycolytic flux in concert with a proportional upregulation of proinflammatory activity, in a manner that is dependent on both hypoxia-inducible factor -1α and 6-phosphofructo-2-kinase.

Inducible nitric oxide synthase expression in brain cortex after acute restraint stress is regulated by nuclear factor kappaB-mediated mechanisms

The underlying mechanisms by which physical or psychological stress causes neurodegeneration are still unknown. We have demonstrated that the high-output and long-lasting synthesizing source of nitric oxide (NO), inducible NO synthase (iNOS), is expressed in brain cortex during stress and that its overexpression accounts for the neurodegenerative changes seen after 3 weeks of repeated stress. Now we have found that acute stress (restraint for 6 h) increases the activity of a calcium-independent NOS and induces the expression of iNOS in brain cortex in adult male rats. In order to elucidate the possible mechanisms involved in this induction, we studied the role of transcription nuclear factor kappaB (NF-kappaB), which is required for iNOS synthesis. We have observed that an acute restraint stress session stimulates the translocation of the NF-kappaB to the nucleus after 4 h and that the administration of the NF-kappaB inhibitor pyrrolidine dithiocarbamate [PDTC, 75 and 150 mg/kg intraperitoneally (i.p.)] at the onset of stress inhibits the stress-induced increase in iNOS expression. Since glutamate release and subsequent NMDA (N-methyl-D-aspartate) receptor activation has been recognized as an early change after exposure to stressful stimuli, and glutamate has been shown to induce iNOS in brain via a NF-kappaB-dependent mechanism, we studied the possible role of excitatory amino acids in the induction of iNOS in our model. Pretreatment with the NMDA receptor antagonist dizocilpine (MK-801, 0.1 and 0.3 mg/kg i.p.) inhibits the stress-induced NF-kappaB activation as well as the stress-induced increase in iNOS expression. Taken together, these findings indicate that excitatory amino acids and subsequent activation of NF-kappaB account for stress-induced iNOS expression in cerebral cortex, and support a possible neuroprotective role for specific inhibitors in this situation.

2-deoxy-2-[18F]fluoro-D-mannose positron emission tomography imaging in atherosclerosis

Progressive inflammation in atherosclerotic plaques is associated with increasing risk of plaque rupture. Molecular imaging of activated macrophages with 2-deoxy-2-[(18)F]fluoro-D-glucose ([(18)F]FDG) has been proposed for identification of patients at higher risk for acute vascular events. Because mannose is an isomer of glucose that is taken up by macrophages through glucose transporters and because mannose receptors are expressed on a subset of the macrophage population in high-risk plaques, we applied (18)F-labeled mannose (2-deoxy-2-[(18)F]fluoro-D-mannose, [(18)F]FDM) for targeting of plaque inflammation. Here, we describe comparable uptake of [(18)F]FDM and [(18)F]FDG in atherosclerotic lesions in a rabbit model; [(18)F]FDM uptake was proportional to the plaque macrophage population. Our FDM competition studies in cultured cells with 2-deoxy-2-[(14)C]carbon-D-glucose ([(14)C]2DG) support at least 35% higher [(18)F]FDM uptake by macrophages in cell experiments. We also demonstrate that FDM restricts binding of anti-mannose receptor antibody to macrophages by approximately 35% and that mannose receptor targeting may provide an additional avenue for imaging of plaque inflammation.

Increased intrahepatic cyclooxygenase 2, matrix metalloproteinase 2, and matrix metalloproteinase 9 expression is associated with progressive liver disease in chronic hepatitis C virus infection: role of viral core and NS5A proteins

BACKGROUND

Cyclooxygenase 2 (COX-2) and matrix metalloproteinases (MMPs) have been implicated in tissue injury and fibrogenesis in animal models but little is known regarding their role in hepatitis C virus (HCV) related liver disease in humans.

AIMS

To characterise the intrahepatic expression pattern of COX-2 and MMPs in chronic HCV infection and determine whether HCV core and NS5A proteins could promote their expression in cultured hepatocyte derived cell lines.

PATIENTS

Thirty two anti-HCV+ and 10 anti-HCV- patients were studied.

METHODS

Western blot, reverse transcription-polymerase chain reaction (RT-PCR), enzyme immunoassay, and immunohistochemistry were used to assess the expression pattern of COX-2 and MMPs in liver biopsy samples from all patients. COX-2 gene expression and MMP-9 protein levels were also determined by immunoblot, RT-PCR, and luciferase assays in core and NS5A transfected hepatocyte derived cells.

RESULTS

The intrahepatic expression level of COX-2, MMP-2, and MMP-9 was significantly higher in HCV+ than in HCV- patients, increasing with the fibrotic stage of liver disease. We further demonstrated that COX-2 mRNA, protein, and activity were induced in resting and activated core and NS5A transfectants. Both viral proteins induced transcriptional activity of the COX-2 gene promoter whereas core, but not NS5A, exerted an inducer effect on MMP-9 protein levels in cultured hepatocyte derived cells.

CONCLUSIONS

Intrahepatic COX-2, MMP-2, and MMP-9 overexpression is associated with progressive hepatic fibrosis in chronic HCV infection, suggesting their pathogenic role in fibrogenesis. HCV core and NS5A proteins were able to upregulate COX-2 and MMP-9 gene expression in hepatocyte derived cells, providing a potential mechanism for hepatic fibrosis during chronic HCV infection.

Induction of cyclooxygenase-2 accounts for restraint stress-induced oxidative status in rat brain

Cyclooxygenase (COX) is the rate-limiting enzyme in the metabolism of arachidonic acid into prostanoids. Although it is constitutively expressed in brain neurons, the inducible isoform (COX-2) is also upregulated in pathological conditions such as seizures, ischemia or some degenerative diseases. To assess whether COX-2 is regulated after stress, we have used adult male Wistar rats, some of which were immobilized during 6 h. An increase in PGE2 concentration occurs in brain cortex after 2-6 h of the onset of stress as well as an enhancement of COX-2 protein. Immunohistochemical studies indicate that COX-2 is expressed in the cortex and hippocampus after stress in cells with morphology of neurons. Administration of PDTC (150 mg/kg), an inhibitor of the transcription factor NF-kappaB or MK-801 (0.2 mg/kg), an N-methyl-D-aspartate receptor blocker, prevents both stress-induced increase in COX-2 activity and protein levels, suggesting an implication of these factors in the mechanism by which stress induces COX-2 in brain. To assess if COX-2 accounts for the oxidative status seen in brain after stress, a group of animals were i.p. injected with NS-398, a specific COX-2 inhibitor 1 h prior to the onset of stress. NS-398 (5 mg/kg) decreases stress-induced malondialdehyde accumulation in cortex as well as prevents the stress-induced oxidation of glutathione. Finally, NS-398 reduced Ca2+-independent inducible nitric oxide synthase (iNOS, NOS-2) activity and lowered the stress-induced accumulation of NO metabolite levels in cortex. These effects of NS-398 seem to be due to the specific inhibition of COX-2, since it has no effect on stress-induced corticosterone release, glutamate release, and NF-kappaB activation. These findings are discussed as possible damaging and/or adaptive roles for stress-induced COX-2 in the brain.

Thromboxane A2-induced inhibition of voltage-gated K+ channels and pulmonary vasoconstriction: role of protein kinase Czeta

Voltage-gated K+ channels (KV) and thromboxane A2 (TXA2) play critical roles in controlling pulmonary arterial tone under physiological and pathological conditions. We hypothesized that TXA2 might inhibit KV channels, thereby establishing a link between these two major pathogenic pathways in pulmonary hypertension. The TXA2 analogue U46619 inhibited IK(V) (Emax=56.1+/-3.9%, EC50=0.054+/-0.019 micromol/L) and depolarized pulmonary artery smooth muscle cells via activation of TP receptors. In isolated pulmonary arteries, U46619 simultaneously increased intracellular Ca2+ concentration and contractile force, and these effects were inhibited by nifedipine or KCl (60 mmol/L). U46619-induced contractions were not altered by the inhibitors of tyrosine kinase genistein or Rho kinase Y-27632 but were prevented by the nonselective protein kinase C (PKC) inhibitors staurosporine and calphostin C. Furthermore, these responses were sensitive to Gö-6983 but insensitive to bisindolylmaleimide I and Gö-6976. Based on the specificity of these drugs, we suggested a role for an atypical PKC in U46619-induced effects. Thus, treatment with a PKCzeta pseudosubstrate inhibitor markedly prevented the vasoconstriction, the inhibition of IK(V), and the depolarization induced by U46619. Western blots showed a transient translocation of PKCzeta from the cytosolic to the particulate fraction on stimulation with U46619. These results indicate that TXA2 inhibits IK(V), leading to depolarization, activation of L-type Ca2+ channels, and vasoconstriction of rat pulmonary arteries. We propose PKCzeta as a link between TP receptor activation and KV channel inhibition.

Retinoid X receptor alpha controls innate inflammatory responses through the up-regulation of chemokine expression

The retinoid X receptor alpha (RXRalpha) plays a central role in the regulation of many intracellular receptor signaling pathways and can mediate ligand-dependent transcription by forming homodimers or heterodimers with other nuclear receptors. Although several members of the nuclear hormone receptor superfamily have emerged as important regulators of macrophage gene expression, the existence in vivo of an RXR signaling pathway in macrophages has not been established. Here, we provide evidence that RXRalpha regulates the transcription of the chemokines Ccl6 and Ccl9 in macrophages independently of heterodimeric partners. Mice lacking RXRalpha in myeloid cells exhibit reduced levels of CCL6 and CCL9, impaired recruitment of leukocytes to sites of inflammation, and lower susceptibility to sepsis. These studies demonstrate that macrophage RXRalpha plays key roles in the regulation of innate immunity and represents a potential target for immunotherapy of sepsis.

Nitric oxide induces tyrosine nitration and release of cytochrome c preceding an increase of mitochondrial transmembrane potential in macrophages

Treatment of elicited peritoneal macrophages or the macrophage cell line RAW 264.7 with high concentrations of nitric oxide donors is followed by apoptotic cell death. Analysis of the changes in the mitochondrial transmembrane potential (DeltaPsi(m)) with specific fluorescent probes showed a rapid and persistent increase of DeltaPsi(m), a potential that usually decreases in cells undergoing apoptosis through mitochondrial-dependent mechanisms. Using confocal microscopy, the release of cytochrome c from the mitochondria to the cytosol was characterized as an early event preceding the rise of DeltaPsi(m). The cytochrome c from cells treated with nitric oxide donors was modified chemically, probably through the formation of nitrotyrosine residues, suggesting the synthesis of peroxynitrite in the mitochondria. These results indicate that nitric oxide-dependent apoptosis in macrophages occurs in the presence of a sustained increase of DeltaPsi(m), and that the chemical modification and release of cytochrome c from the mitochondria precede the changes of DeltaPsi(m).-Hortelano, S., Alvarez, A. M., Boscá, L. Nitric oxide induces tyrosine nitration and release of cytochrome c preceding an increase of mitochondrial transmembrane potential in macrophages.

Transition of Macrophages to Fibroblast-Like Cells in Healing Myocardial Infarction

BACKGROUND

Macrophages and fibroblasts are 2 major cell types involved in healing after myocardial infarction (MI), contributing to myocardial remodeling and fibrosis. Post-MI fibrosis progression is characterized by a decrease in cardiac macrophage content.

OBJECTIVES

This study explores the potential of macrophages to express fibroblast genes and the direct role of these cells in post-MI cardiac fibrosis.

METHODS

Prolonged in vitro culture of human macrophages was used to evaluate the capacity to express fibroblast markers. Infiltrating cardiac macrophages was tracked in vivo after experimental MI of LysM(Cre^/+);ROSA26(EYFP^/+) transgenic mice. The expression of Yellow Fluorescent Protein (YFP) in these animals is restricted to myeloid lineage allowing the identification of macrophage-derived fibroblasts. The expression in YFP-positive cells of fibroblast markers was determined in myocardial tissue sections of hearts from these mice after MI.

RESULTS

Expression of the fibroblast markers type I collagen, prolyl-4-hydroxylase, fibroblast specific protein-1, and fibroblast activation protein was evidenced in YFP-positive cells in the heart after MI. The presence of fibroblasts after MI was evaluated in the hearts of animals after depletion of macrophages with clodronate liposomes. This macrophage depletion significantly reduced the number of Mac3⁺Col1A1⁺ cells in the heart after MI.

CONCLUSIONS

The data provide both in vitro and in vivo evidence for the ability of macrophages to transition and adopt a fibroblast-like phenotype. Therapeutic manipulation of this macrophage-fibroblast transition may hold promise for favorably modulating the fibrotic response after MI and after other cardiovascular pathological processes.

Contribution of cyclopentenone prostaglandins to the resolution of inflammation through the potentiation of apoptosis in activated macrophages

Activation of the macrophage cell line RAW 264.7 with LPS and IFN-gamma induces apoptosis through the synthesis of high concentrations of NO due to the expression of NO synthase-2. In addition to NO, activated macrophages release other molecules involved in the inflammatory response, such as reactive oxygen intermediates and PGs. Treatment of macrophages with cyclopentenone PGs, which are synthesized late in the inflammatory onset, exerted a negative regulation on cell activation by impairing the expression of genes involved in host defense, among them NO synthase-2. However, despite the attenuation of NO synthesis, the percentage of apoptotic cells increased with respect to activated cells in the absence of cyclopentenone PGs. Analysis of the mechanisms by which these PGs enhanced apoptosis suggested a potentiation of superoxide anion synthesis that reacted with NO, leading to the formation of higher concentrations of peroxynitrite, a more reactive and proapoptotic molecule than the precursors. The effect of the cyclopentenone 15-deoxy-Delta(12,14)-PGJ(2) on superoxide synthesis was dependent on p38 mitogen-activated protein kinase activity, but was independent of the interaction with peroxisomal proliferator-activated receptor gamma. The potentiation of apoptosis induced by cyclopentenone PGs involved an increase in the release of cytochrome c from the mitochondria to the cytosol and in the nitration of this protein. These results suggest a role for cyclopentenone PGs in the resolution of inflammation by inducing apoptosis of activated cells.

The cyclopentenone 15-deoxy-delta 12,14-prostaglandin J2 binds to and activates H-Ras

The cyclopentenone 15-deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)) induces cell proliferation and mitogen-activated protein kinase activation. Here, we describe that these effects are mediated by 15d-PGJ(2)-elicited H-Ras activation. We demonstrate that this pathway is specific for H-Ras through the formation of a covalent adduct of 15d-PGJ(2) with Cys-184 of H-Ras, but not with N-Ras or K-Ras. Mutation of C184 inhibited H-Ras modification and activation by 15d-PGJ(2), whereas serum-elicited stimulation was not affected. These results describe a mechanism for the activation of the Ras signaling pathway, which results from the chemical modification of H-Ras by formation of a covalent adduct with cyclopentenone prostaglandins.

Lipoxin A4 impairment of apoptotic signaling in macrophages: implication of the PI3K/Akt and the ERK/Nrf-2 defense pathways

Lipoxin A(4) (LXA(4)) is an endogenous lipid mediator that requires transcellular metabolic traffic for its synthesis. The targets of LXA(4) on neutrophils are well described, contributing to attenuation of inflammation. However, effects of lipoxins on macrophage are less known, particularly the action of LXA(4) on the regulation of apoptosis of these cells. Our data show that pretreatment of human or murine macrophages with LXA(4) at the concentrations prevailing in the course of resolution of inflammation (nanomolar range) significantly inhibits the apoptosis induced by staurosporine, etoposide and S-nitrosoglutathione or by more pathophysiological stimuli, such as LPS/IFNgamma challenge. The release of mitochondrial mediators of apoptosis and the activation of caspases was abrogated in the presence of LXA(4). In addition to this, the synthesis of reactive oxygen species induced by staurosporine was attenuated and antiapoptotic proteins of the Bcl-2 family accumulated in the presence of lipoxin. Analysis of the targets of LXA(4) identified an early activation of the PI3K/Akt and ERK/Nrf-2 pathways, which was required for the observation of the antiapoptotic effects of LXA(4). These data suggest that the LXA(4), released after the recruitment of neutrophils to sites of inflammation, exerts a protective effect on macrophage viability that might contribute to a better resolution of inflammation.

Neuronal expression of inducible nitric oxide synthase after oxygen and glucose deprivation in rat forebrain slices

Nitric oxide (NO) overproduction has been postulated to contribute significantly to ischaemia-reperfusion neurotoxicity. Inducible or type II NO synthase (iNOS) synthesizes NO in large quantities for long periods of time. Therefore we investigated the expression and localization of iNOS after oxygen and glucose deprivation in rat forebrain slices. In this experimental model, calcium-independent NOS activity reached a maximum 180 min after the end of a 20 min oxygen-glucose deprivation period. During the same period of time, the calcium-independent activity was absent in control forebrain slices. To test whether this calcium-independent NOS activity was due to the expression of iNOS, the effects of the addition of dexamethasone, cycloheximide and pyrrolidine dithiocarbamate were determined. All of them inhibited the induction of the calcium-independent NOS activity measured in the rat forebrain slices after oxygen and glucose deprivation. Furthermore, oxygen and glucose deprivation caused the expression of the gene encoding iNOS in rat forebrain slices, as assessed by the detection of iNOS message and protein in these samples. A sixfold increase in the iNOS mRNA levels was observed at 180 min and the time-course of the expression of iNOS mRNA was in agreement with the temporal profile of iNOS enzymatic activity. Immunohistochemistry analysis revealed that iNOS was highly expressed in neurones, astrocytes and microglial cells. These results demonstrate for the first time that iNOS is expressed in neurones after oxygen and glucose deprivation, and that this expression occurs in short periods of time. These findings suggest that NO can play an important pathogenic role in the tissue damage that occurs after cerebral ischaemia.

Mechanisms of nitric oxide-dependent apoptosis: involvement of mitochondrial mediators

Programmed cell death occurs in several physiopathological situations in multicellular organisms and constitutes a common mechanism of cell replacement, tissue remodelling and removal of altered cells. The effectors that induce apoptosis as well as the signalling pathways involved in the process are the subjects of current work. In addition to receptor-mediated apoptosis, highly reactive molecules, such as NO, influence cell viability either by acting as a protection against apoptogenic stimuli, or by inducing apoptosis when produced at elevated concentrations. The contribution to apoptosis of mediators released by the mitochondria and involved in the activation of caspases focused attention on the functional changes caused by NO in this organelle. NO induces mitochondrial permeability transition and promotes apoptosis in cell-free systems containing mitochondria and nuclei. Moreover, NO-dependent apoptosis can be blocked in most cases through the use of permeability transition or caspase inhibitors. The intracellular pathways activated in response to NO challenge and involved in the regulation of apoptosis are analysed.

Implication of glutamate in the expression of inducible nitric oxide synthase after oxygen and glucose deprivation in rat forebrain slices

Nitric oxide synthesis by inducible nitric oxide synthase (iNOS) has been postulated to contribute to ischemia-reperfusion neurotoxicity. The expression of this enzyme has been demonstrated in cells present in the postischemic brain. The mechanisms of iNOS expression after cerebral ischemia are a subject of current research. We therefore decided to investigate whether glutamate, which is released in ischemia and is implicated in neurotoxicity, might be involved in the mechanisms by which oxygen and glucose deprivation (OGD) leads to the expression of iNOS in rat forebrain slices. In this model, we have shown previously that 20 min of OGD causes the expression of iNOS. We have now found that the NMDA receptor antagonist MK-801 blocks the expression of iNOS, suggesting that the activation of the NMDA subtype of glutamate receptor is implicated in the mechanisms that lead to the expression of this isoform. Moreover, we have found that glutamate alone could trigger the induction process, as shown by the appearance of a Ca(2+)-independent NOS activity and by the detection of iNOS mRNA and protein in slices exposed to glutamate. Glutamate-dependent iNOS expression was concentration-dependent and was blocked by EGTA and by the inhibitors of nuclear factor kappaB (NF-kappaB) activation pyrrolidine dithiocarbamate and MG132. In addition, glutamate induced NF-kappaB translocation to the nucleus, an effect that was inhibited by MG132. Taken together, our data suggest that activation of NMDA receptors by glutamate released in ischemia is involved in the expression of iNOS in rat forebrain slices via a Ca(2+)-dependent activation of the transcription factor NF-kappaB. To our knowledge, this is the first report showing an implication of excitatory amino acids in the expression of iNOS caused by ischemia.