Involvement of protein kinase A in the induction of arginase in murine bone marrow-derived macrophages

cGAS knockdown promotes microglial M2 polarization to alleviate neuroinflammation by inhibiting cGAS-STING signaling pathway in cerebral ischemic stroke

Inflammation plays a pivotal role in promoting the pathophysiology of ischemic stroke (IS). Microglia is the major immunocompetent cells involved in different neuropathologies. The activation of cyclic GMP-AMP synthase (cGAS) and its downstream signaling protein-stimulator of interferon genes (STING) is increasingly recognized as a crucial determinant of neuropathophysiology. However, the mechanisms underlying cGAS-STING signaling regulating inflammatory response during IS remains to be elucidated. In this study, HT22 cells was used to establish an oxygen-glucose deprivation (OGD) cell model in vitro, and then this cell culture supernatant containing OGD-induced DAMPs (OIDs) was employed to stimulate BV2 microglia. Furthermore, a middle cerebral artery occlusion (MCAO) mouse model was established. Cells and MCAO mice were treated with si-cGAS or si-NC lentivirus. The expression levels of STING, cGAS and p-IRF3 in BV2 cells or MCAO mouse brain; the microglial M1/M2 polarization of BV2 microglia or isolated microglial cells from MCAO mouse brain; the contents of iNOS, TNF-α, TGF-β and IL-10 in the culture medium of BV2 cells or in murine brain homogenates, were all detected. In addition, the severity of cerebral infarction with or without the knockdown of cGAS in a MCAO mouse model was also determined by TTC staining. Results showed that OGD-induced DAMPs strongly activated cGAS-STING pathway and triggered microglia polarization in BV2 cells, reflecting as the accumulation of a plethora of pro-inflammatory factors in activated microglia. However, these effects could be inhibited by cGAS knockdown. In the MCAO mouse model, the inhibition of cGAS-STING pathway resulted from cGAS knockdown could effectively diminish cell apoptosis in mouse brain stimulated by MIDs (MCAO-induced DAMPs), reduced the area ratio of cerebral infarction and ultimately improved the injured nerve function during IS. Taken together, our elucidation of underlying mechanisms involved in the microglial inflammatory response, triggered by cGAS-STING signaling, highlights this pathway as a potential therapeutic target in IS.

Myeloid Cell-Derived Arginase in Cancer Immune Response

Amino acid metabolism is a critical regulator of the immune response, and its modulating becomes a promising approach in various forms of immunotherapy. Insufficient concentrations of essential amino acids restrict T-cells activation and proliferation. However, only arginases, that degrade L-arginine, as well as enzymes that hydrolyze L-tryptophan are substantially increased in cancer. Two arginase isoforms, ARG1 and ARG2, have been found to be present in tumors and their increased activity usually correlates with more advanced disease and worse clinical prognosis. Nearly all types of myeloid cells were reported to produce arginases and the increased numbers of various populations of myeloid-derived suppressor cells and macrophages correlate with inferior clinical outcomes of cancer patients. Here, we describe the role of arginases produced by myeloid cells in regulating various populations of immune cells, discuss molecular mechanisms of immunoregulatory processes involving L-arginine metabolism and outline therapeutic approaches to mitigate the negative effects of arginases on antitumor immune response. Development of potent arginase inhibitors, with improved pharmacokinetic properties, may lead to the elaboration of novel therapeutic strategies based on targeting immunoregulatory pathways controlled by L-arginine degradation.

Interleukin-10 contributes to PGE2 signalling through upregulation of EP4 via SHIP1 and STAT3

Macrophage cells form part of our first line defense against pathogens. Macrophages become activated by microbial products such as lipopolysaccharide (LPS) to produce inflammatory mediators, such as TNFα and other cytokines, which orchestrate the host defense against the pathogen. Once the pathogen has been eradicated, the activated macrophage must be appropriately deactivated or inflammatory diseases result. Interleukin-10 (IL10) is a key anti-inflammatory cytokine which deactivates the activated macrophage. The IL10 receptor (IL10R) signals through the Jak1/Tyk2 tyrosine kinases, STAT3 transcription factor and the SHIP1 inositol phosphatase. However, IL10 has also been described to induce the activation of the cyclic adenosine monophosphate (cAMP) regulated protein kinase A (PKA). We now report that IL10R signalling leads to STAT3/SHIP1 dependent expression of the EP4 receptor for prostaglandin E₂ (PGE₂). In macrophages, EP4 is a G_αs-protein coupled receptor that stimulates adenylate cyclase (AC) production of cAMP, leading to downstream activation of protein kinase A (PKA) and phosphorylation of the CREB transcription factor. IL10 induction of phospho-CREB and inhibition of LPS-induced phosphorylation of p85 PI3K and p70 S6 kinase required the presence of EP4. These data suggest that IL10R activation of STAT3/SHIP1 enhances EP4 expression, and that it is EP4 which activates cAMP-dependent signalling. The coordination between IL10R and EP4 signalling also provides an explanation for why cAMP elevating agents synergize with IL10 to elicit anti-inflammatory responses.

Arginine Metabolism in Myeloid Cells Shapes Innate and Adaptive Immunity

Arginine metabolism has been a key catabolic and anabolic process throughout the evolution of the immune response. Accruing evidence indicates that arginine-catabolizing enzymes, mainly nitric oxide synthases and arginases, are closely integrated with the control of immune response under physiological and pathological conditions. Myeloid cells are major players that exploit the regulators of arginine metabolism to mediate diverse, although often opposing, immunological and functional consequences. In this article, we focus on the importance of arginine catabolism by myeloid cells in regulating innate and adaptive immunity. Revisiting this matter could result in novel therapeutic approaches by which the immunoregulatory nodes instructed by arginine metabolism can be targeted.

Pharmacologic agents elevating cAMP prevent arginase II expression and proliferation of pulmonary artery smooth muscle cells

Arginase II has been shown to be involved in the hypoxia-induced proliferation of human pulmonary artery smooth muscle cells (hPASMCs). The signal transduction pathways responsible for the induction of arginase II are poorly understood. Cyclic AMP is involved in many intracellular processes, and cAMP levels are regulated by a balance between production via adenylate cyclases and degradation via phosphodiesterases. The purpose of this study was to determine the effects of cAMP on hypoxia-induced arginase expression, activity, and proliferation in hPASMCs. We found that the cAMP analog 8-Bromo-cAMP (8-Br-cAMP), the adenylate cyclase activator forskolin, and the phosphodiesterase 3 inhibitor cilostamide prevented the hypoxic induction of arginase II mRNA and protein expression in hPASMCs. The inhibition of arginase II protein was found to be mediated by exchange protein directly activated by cAMP. Arginase activity was decreased by 8-Br-cAMP, as evidenced by significantly lower V(max) for arginase in normoxia and hypoxia. The hypoxia-induced hPASMC proliferation was completely prevented by the addition of 8-Br-cAMP, forskolin, or cilostamide. These data are the first to describe the inhibitory effect of cAMP on arginase activity, expression, and resultant proliferation of hypoxic hPASMCs.

In vivo treatment with progestogens causes immunosuppression of carp Cyprinus carpio leucocytes by affecting nitric oxide production and arginase activity

In this study, carp Cyprinus carpio were injected with various steroid compounds, including synthetic and natural progestogens and the glucocorticoid cortisol, to investigate effects on leucocytes isolated from their kidneys. Injection of cortisol led to an increased spleeno-somatic index (I(S)) on day 21 post-injection (pi) and immunosuppressive effects measured as decreased nitric oxide (NO) production and increased arginase activity in isolated leucocytes on days 14 and 21 pi, respectively. Moreover, reduced NO production was also observed after injection of the synthetic progestogens, levonorgestrel (LEV) and medroxyprogesterone acetate. In addition, LEV influenced arginase activity in head kidney cells on day 14 and day 21 pi. This study is the first demonstration in fishes that the application of these steroid compounds in vivo affects NO production and arginase activity of isolated leucocytes.

Involvement of adenosine A2A receptors in engulfment-dependent apoptotic cell suppression of inflammation

Efficient execution of apoptotic cell death followed by efficient clearance mediated by professional macrophages is a key mechanism in maintaining tissue homeostasis. Removal of apoptotic cells usually involves three central elements: 1) attraction of phagocytes via soluble "find me" signals, 2) recognition and phagocytosis via cell surface-presenting "eat me" signals, and 3) suppression or initiation of inflammatory responses depending on additional innate immune stimuli. Suppression of inflammation involves both direct inhibition of proinflammatory cytokine production and release of anti-inflammatory factors, which all contribute to the resolution of inflammation. In the current study, using wild-type and adenosine A(2A) receptor (A2AR) null mice, we investigated whether A2ARs, known to mediate anti-inflammatory signals in macrophages, participate in the apoptotic cell-mediated immunosuppression. We found that macrophages engulfing apoptotic cells release adenosine in sufficient amount to trigger A2ARs, and simultaneously increase the expression of A2ARs, as a result of possible activation of liver X receptor and peroxisome proliferators activated receptor δ. In macrophages engulfing apoptotic cells, stimulation of A2ARs suppresses the NO-dependent formation of neutrophil migration factors, such as macrophage inflammatory protein-2, using the adenylate cyclase/protein kinase A pathway. As a result, loss of A2ARs results in elevated chemoattractant secretion. This was evident as pronounced neutrophil migration upon exposure of macrophages to apoptotic cells in an in vivo peritonitis model. Altogether, our data indicate that adenosine is one of the soluble mediators released by macrophages that mediate engulfment-dependent apoptotic cell suppression of inflammation.

Arginase in parasitic infections: macrophage activation, immunosuppression, and intracellular signals

A type 1 cytokine-dependent proinflammatory response inducing classically activated macrophages (CaMϕs) is crucial for parasite control during protozoan infections but can also contribute to the development of immunopathological disease symptoms. Type 2 cytokines such as IL-4 and IL-13 antagonize CaMϕs inducing alternatively activated macrophages (AaMϕs) that upregulate arginase-1 expression. During several infections, induction of arginase-1-macrophages was showed to have a detrimental role by limiting CaMϕ-dependent parasite clearance and promoting parasite proliferation. Additionally, the role of arginase-1 in T cell suppression has been explored recently. Arginase-1 can also be induced by IL-10 and transforming growth factor-β (TGF-β) or even directly by parasites or parasite components. Therefore, generation of alternative activation states of macrophages could limit collateral tissue damage because of excessive type 1 inflammation. However, they affect disease outcome by promoting parasite survival and proliferation. Thus, modulation of macrophage activation may be instrumental in allowing parasite persistence and long-term host survival.

Arginase: an emerging key player in the mammalian immune system

The enzyme arginase metabolizes L-arginine to L-ornithine and urea. Besides its fundamental role in the hepatic urea cycle, arginase is also expressed the immune system of mice and man. While significant interspecies differences exist regarding expression, subcellular localization and regulation of immune cell arginase, associated pathways of immunopathology are comparable between species. Arginase is induced in murine myeloid cells mainly by Th2 cytokines and inflammatory agents and participates in a variety of inflammatory diseases by down-regulation of nitric oxide synthesis, induction of fibrosis and tissue regeneration. In humans, arginase I is constitutively expressed in polymorphonuclear neutrophils and is liberated during inflammation. Myeloid cell arginase-mediated L-arginine depletion profoundly suppresses T cell immune responses and this has emerged as a fundamental mechanism of inflammation-associated immunosuppression. Pharmacological interference with L-arginine metabolism is a novel promising strategy in the treatment of cancer, autoimmunity or unwanted immune deviation.

Echinacea increases arginase activity and has anti-inflammatory properties in RAW 264.7 macrophage cells, indicative of alternative macrophage activation

ETHNOPHARMACOLOGICAL RELEVANCE

The genus Echinacea is a popular herbal immunomodulator. Recent reports indicate that Echinacea products inhibit nitric oxide (NO) production in activated macrophages.

AIM OF THE STUDY

In the present study we determined the inhibitory effects of alcohol extracts and individual fractions of alcohol extracts of Echinacea on NO production, and explored the mechanism underlying the pharmacological anti-inflammatory activity.

MATERIALS AND METHODS

Alcohol extracts of three medicinal Echinacea species, Echinacea angustifolia, Echinacea pallida and Echinacea purpurea, were prepared using Soxhlet apparatus and fractionated using HPLC. NO production by LPS activated RAW 264.7 macrophage cells was measured using a Griess reagent and iNOS detected using immunoblotting. In addition, effects on arginase activity were measured in RAW 264.7 cells stimulated with 8-bromo-cAMP +/- LPS.

RESULTS

Alcohol extracts of all three Echinacea species significantly inhibited NO production by lipopolysaccharide (LPS)-activated the RAW 264.7 macrophage cell line; among them Echinacea pallida was the most active. The Echinacea-mediated decrease in NO production was unlikely due to a direct scavenging of NO because the extracts did not directly inhibit NO released from an NO donor, sodium nitroprusside. An immunoblotting assay demonstrated that the extract of Echinacea pallida inhibited inducible nitric oxide synthase (iNOS) protein expression in LPS-treated macrophages. The enzymes iNOS and arginase metabolize a common substrate, l-arginine, but produce distinct biological effects. While iNOS is involved in inflammatory response and host defense, arginase participates actively in anti-inflammatory activation. Arginase activity of RAW 264.7 cells stimulated with 8-bromo-cAMP was significantly increased by alcohol extracts of all three Echinacea species. The polar fraction containing caffeic acid derivatives enhanced arginase activity, while the lipophilic fraction containing alkamides exhibited a potential of inhibiting NO production and iNOS expression.

CONCLUSIONS

These results suggest that the anti-inflammatory activity of Echinacea might be due to multiple active metabolites, which work together to switch macrophage activation from classical activation towards alternative activation.

Inhibition of NADPH oxidase by apocynin inhibits lipopolysaccharide (LPS) induced up-regulation of arginase in rat alveolar macrophages

Reactive oxygen species participate in the pathogenesis of inflammatory airway diseases, in which increased arginase may play a role by interfering with nitric oxide (NO) synthesis and providing substrate for collagen synthesis. Therefore a modulatory role of reactive oxygen species for arginase was explored in alveolar macrophages using the NADPH oxidase inhibitor apocynin. The effects of lipopolysacharides (LPS) and apocynin on nitrite accumulation, arginase activity and mRNA for inducible NO synthase (iNOS), arginase I and II were determined. Superoxide anion (O(2)(-)) release was analysed by the iodonitrotetrazolium (INT) formazan assay. LPS (1 microg/ml) caused a 55%, transient increase in INT formation, i.e. O(2)(-) release which was inhibited by apocynin (500 microM). LPS caused a 2 fold increase in arginase activity and a marked increase in mRNA encoding arginase I, the predominant isoenzyme. Both effects were largely attenuated by apocynin. Apocynin did not affect the stability of arginase I mRNA, but accelerated the decline of arginase activity when protein synthesis was inhibited by cycloheximide. Apocynin also reduced LPS-induced nitrite accumulation (by 30%) and iNOS mRNA expression, but the magnitude of these effects was smaller than that on arginase I. Arginase I mRNA was also increased following exposure to hydrogen peroxide (H(2)O(2), 200 muM). In conclusion, inhibition of NADPH oxidase in alveolar macrophages causes down-regulation of arginase, indicating that reactive oxygen species exert stimulatory effects on arginase. Enhanced transcription of arginase mRNA and prolongation of the life time of the active enzyme appear to contribute to the enhanced arginase activity.

Transcriptional responses of murine macrophages to the adenylate cyclase toxin of Bordetella pertussis

Three different recombinant forms of CyaA were used to investigate transcriptional responses of murine bone marrow-derived macrophages (BMMs) using Affymetrix Mouse Genome GeneChips. These forms were enzymically active, invasive CyaA, non-enzymically active, invasive CyaA (CyaA*) and non-enzymically active, non-invasive CyaA (proCyaA*). BMMs, treated with 20 ng/ml of CyaA for 24h, showed over 1000 significant changes in gene transcription compared with control cells. CyaA caused an increase in transcription of many inflammatory genes and genes associated with various signalling cascades such as those involved in cyclic AMP-dependent protein kinase A signalling. Most strikingly, CyaA caused down-regulation of numerous genes involved in cell proliferation. CyaA* at 20 ng/ml significantly up-regulated the transcription of only twelve genes after 24h whereas proCyaA* at this concentration significantly increased the transcription of only two genes.

The role of cyclic-AMP on arginase activity by a murine macrophage cell line (RAW264.7) stimulated with lipopolysaccharide from Actinobacillus actinomycetemcomitans

AIMS

The aim of the present study was to determine the role of cyclic adenosine monophosphate (cAMP) on arginase activity in a murine macrophage cell line (RAW264.7 cells) stimulated with lipopolysaccharide (LPS) from Actinobacillus actinomycetemcomitans.

MATERIALS AND METHODS

The cells were treated with A. actinomycetemcomitans LPS for 24 h. The effects of SQ22536 (an adenylyl cyclase inhibitor), ODQ (a guanylyl cyclase inhibitor), dibutyryl cAMP (a cAMP analog), 8-bromo cyclic guanosine monophosphate (a cGMP analog), forskolin (an adenylyl cylase activator), and cycloheximide (a protein synthesis inhibitor) on arginase activity in A. actinomycetemcomitans LPS-stimulated RAW264.7 cells were also determined. Arginase activity was assessed in LPS-stimulated cells in the presence of 3-isobutyl-1-methylxanthine (IBMX), siguazodan and rolipram [phosphodiesterase (PDE) inhibitors] as well as KT5720 [a protein kinase A (PKA) inhibitor].

RESULTS

Arginase activity in A. actinomycetemcomitans LPS-stimulated RAW264.7 cells was suppressed by SQ22536 but not ODQ. Enhancement of arginase activity was observed in the presence of cAMP analog or forskolin but not cGMP analog. Cycloheximide blocked arginase activity in the cells in the presence of cAMP analog or forskolin with or without A. actinomycetemcomitans LPS. IBMX augmented arginase activity in A. actinomycetemcomitans LPS-stimulated cells. Rolipram (a PDE4 inhibitor) increased the levels of arginase activity higher than siguazodan (a PDE3 inhibitor) in the antigen-stimulated cells. The effect of cAMP analog or forskolin on arginase activity in the presence or absence of A. actinomycetemcomitans LPS was blocked by the PKA inhibitor (KT5720).

CONCLUSION

The results of the present study suggest that A. actinomycetemcomitans LPS may stimulate arginase activity in murine macrophages (RAW264.7 cells) in a cAMP-PKA-dependent pathway.

The metabolic coupling of arginine metabolism to nitric oxide generation by astrocytes

Arginine, the only known precursor of nitric oxide, enters the brain parenchyma from the blood through the endothelial cells or from the cerebral spinal fluid through the ependymal cells. Astrocytes, whose processes abut the endothelium and ependymum, take up arginine through cationic amino acid transporters and release arginine through this transport system to the synapses that astrocytes shield. Some of these synapses are excitatory, and liberate glutamate into the synaptic cleft. Glutamate induces arginine release from astrocytes, making it available to the neuron. Neurons can take up arginine to be used in nitric oxide-mediated processes, such as neurotransmission. Thus, neural and nonneural cells act in concert to affect neuron physiology in an elegantly integrated system. This review focuses on the components of the interaction between astrocytes and neurons in nitric oxide biology.

Arginine metabolism during macrophage autocrine activation and infection with mouse hepatitis virus 3

In contrast to BALB/c mouse macrophages (Mphi), Mphi from the A/J mouse strain, upon activation by exogenous interferon gamma (IFNgamma), develop an anti-mouse hepatitis virus 3 (MHV3) state which correlates with resistance to virus infection. To investigate the autocrine activation of BALB/c and A/J Mphi, we activated them with interleukin-12 (IL-12) and/or IL-18, and quantified IFNgamma production, the anti-MHV3 state and arginine metabolism. Synergistic activation by IL-12/IL-18 induced the expression of the IFNgamma gene in Mphi from both mouse strains. In bone marrow (BM) or peritoneal (P) Mphi of specific pathogen-free (spf) mice of both strains, IFNgamma synthesis occurred only with a synergistic IL-12/IL-18 activation and showed increasing levels from 24 to 72 h of activation. In contrast, when non-spf mice were used in the assay, their PMphi synthesized higher IFNgamma levels upon activation with only IL-12 or only IL-18 or both. The BALB/c Mphi were always capable of synthesizing higher amounts of IFNgamma than the A/J Mphi. An anti-MHV3 state was observed only in A/J Mphi upon activation with IL-12/IL-18 or IFNgamma regardless of their origin from the peritoneum or bone marrow. Arginine metabolism in activated and/or virus infected BMMphi was investigated through nitric oxide (NO) and arginase induction as well as the consumption of arginine and synthesis of citrulline, ornithine and spermine. The results showed that both BALB/c and A/J BMMphi populations released NO only after activation with IL-12/IL-18 or IFNgamma. Arginase was not induced in BMMphi from both strains by IL-12/IL-18 or IFNgamma but only by IL-4/IL-10. Higher arginine consumption was observed in BMMphi from both strains upon activation with IL-4 or IFNgamma which further increased, in this case, when the cells were infected with MHV3. As a consequence of nitric oxide synthase synthesis and arginine consumption in IFNgamma activated BMMphi, we observed a higher synthesis of citrulline. High levels of ornithine were induced only upon IL-4 activation. Polyamine synthesis was higher in A/J BMMphi than in BALB/c ones, which correlated with the slightly lower levels of ornithine observed. Upon infection with MHV3, we observed a higher synthesis of spermine. IL-12/IL-18 or IFNgamma activation, mainly in MHV3 infected cells, led to a decreased synthesis of polyamines, notably spermine, only in A/J BMMphi. Difluoromethylornithine treatment, which leads to inhibition of polyamine synthesis, induced a decreased MHV3 multiplication in both BALB/c and A/J BMMphi. Altogether these data show the relevance of IFNgamma, from the autocrine or paracrine pathway, and arginine metabolism for the control of MHV3 replication in Mphi of a resistant mouse strain.

Effects of phosphodiesterase inhibitors on L-arginine pathways in rat alveolar macrophages

Effects of phosphodiesterase inhibitors on L-arginine-dependent pathways in rat alveolar macrophages, inducible nitric oxide (NO) synthase (iNOS) and arginase, were studied. Culture of rat alveolar macrophages in the presence of lipopolysaccharides (20 h) caused an increase of arginase activity (by 135%) and nitrite concentration (fourfold). The nonselective phosphodiesterase inhibitor IBMX (2-isobutyl-1-methylxanthine) enhanced arginase activity by 35% and nitrite accumulation by 130%. IBMX caused a clear increase in iNOS protein levels and a relatively smaller increase in iNOS mRNA. The effect of IBMX on nitrite accumulation was largely attenuated by the protein kinase A inhibitor K 5720. The phosphodiesterase 4 inhibitor rolipram enhanced nitrite accumulation more effectively than the phosphodiesterase 3 inhibitor siguadozan (about 50% and 20% of IBMX effect, respectively), whereas the phosphodiesterase 3/4 inhibitor benzafendrine was as effective as IBMX. In conclusion, in rat alveolar macrophages, phosphodiesterase 4 and, to a smaller extent, phosphodiesterase 3 play a role in the control of iNOS-mediated NO synthesis.

Regulation of enzymes of the urea cycle and arginine metabolism

The urea cycle is comprised of five enzymes but also requires other enzymes and mitochondrial amino acid transporters to function fully. The complete urea cycle is expressed in liver and to a small degree also in enterocytes. However, highly regulated expression of several enzymes present in the urea cycle occurs also in many other tissues, where these enzymes are involved in synthesis of nitric oxide, polyamines, proline and glutamate. Glucagon, insulin, and glucocorticoids are major regulators of the expression of urea cycle enzymes in liver. In contrast, the "urea cycle" enzymes in nonhepatic cells are regulated by a wide range of pro- and antiinflammatory cytokines and other agents. Regulation of these enzymes is largely transcriptional in virtually all cell types. This review emphasizes recent information regarding roles and regulation of urea cycle and arginine metabolic enzymes in liver and other cell types.

Arginase I induction in macrophages, triggered by Th2-type cytokines, supports the growth of intracellular Leishmania parasites

Leishmania spp. are intracellular protozoan parasites that invade and replicate within macrophages. In a previous report, we have demonstrated that the growth of intracellular amastigotes could be controlled by inhibition of arginase. This enzyme, induced in host cells by Th2 cytokines, synthesizes L-ornithine which can be used by parasites to generate polyamines and proliferate. In this study, we have designed experiments to better analyse the dependence of parasite proliferation on arginase induction in infected macrophages. Treatment of Leishmania major-infected BALB/c macrophages with interleukin (IL)-4, IL-10 or transforming growth factor-beta, which are all inducers of arginase I in murine macrophages, led to a proportional increase in the number of intracellular amastigotes. Moreover, parasite proliferation and arginase activity levels in macrophages from the susceptible BALB/c mice were significantly higher than those from infected C57BL/6 cells when treated with identical doses of these cytokines, indicating that a strong correlation exist between the permissibility of host cells to L. major infection and the induction of arginase I in macrophages. Specific inhibition of arginase by N(omega)-hydroxy-nor-L-arginine (nor-LOHA) reverted growth, while L-ornithine and putrescine promoted parasite proliferation, indicating that the parasite cell division depends critically on the level of L-ornithine available in the host. Therefore, arginase induction in the context of a Th2 predominant response might be a contributor to susceptibility in leishmaniasis.

Il-10 up-regulates macrophage expression of the S100 protein S100A8

The murine calcium binding protein S100A8 (A8) is a leukocyte chemoattractant, but high levels may be protective and scavenge hypochlorite. A8 is induced by LPS, IFN-gamma, and TNF in elicited macrophages. Th2 cytokines generally suppress proinflammatory gene expression, and IL-4 and IL-13 partially decreased A8 induction in macrophages and endothelial cells stimulated by LPS or IFN. In contrast, IL-10 synergized with LPS and IFN to increase mRNA levels > or =9-fold and secreted A8 levels approximately 4-fold. IL-10 decreased the optimal time of mRNA expression induced by LPS from 24 to 8 h. Blocking experiments indicated that endogenous IL-10 contributes to gene induction by LPS. Cooperation between IL-10 and LPS was not due to altered mRNA stability but was dependent on de novo protein synthesis. Transfection analysis with A8 luciferase constructs confirmed that synergy was due to increased transcription. The region of the promoter involved was localized to a 178-bp fragment flanking the transcription start site of the gene. This region was also responsible for the suppressive effects of IL-4 and IL-13. Forskolin, CTP-cAMP, and PGE(2) also enhanced LPS- and IFN-induced A8 mRNA, whereas indomethacin significantly reduced synergy between IL-10 and LPS. Mitogen-activated protein kinase/cyclooxygenase 2/cAMP pathways involving CCAAT-enhancing binding protein, located within the active promoter, may mediate A8 gene up-regulation in a manner mechanistically distinct to genes regulated by IL-10 via the STAT pathway. A8 exhibits pleiotropic effects, and the high levels secreted as a result of IL-10 synergy may regulate untoward inflammatory damage by virtue of its an antioxidant capacity.

Glucocorticoids inhibit lipopolysaccharide-induced up-regulation of arginase in rat alveolar macrophages

1. As arginase by limiting nitric oxide (NO) synthesis may play a role in airway hyperresponsiveness and glucocorticoids are known to induce the expression of arginase I in hepatic cells, glucocorticoid effects on arginase in alveolar macrophages (AM Phi) were studied. 2. Rat AM Phi were cultured in absence or presence of test substances. Thereafter, nitrite accumulation, arginase activity, and the expression pattern of inducible NO synthase, arginase I and II mRNA (RT - PCR) and proteins (immunoblotting) were determined. 3. Lipopolyssacharides (LPS, 20 h) caused an about 2 fold increase in arginase activity, whereas interferon-gamma (IFN-gamma), like LPS a strong inducer of NO synthesis, had no effect. 4. Dexamethasone decreased arginase activity by about 25% and prevented the LPS-induced increase. Mifepristone (RU-486) as partial glucocorticoid receptor agonist inhibited LPS-induced increase by 45% and antagonized the inhibitory effect of dexamethasone. 5. Two different inhibitors of the NF-kappa B-pathway also prevented LPS-induced increase in arginase activity. 6. Rat AM Phi expressed mRNA and protein of arginase I and II, but arginase I expression was stronger. Arginase I mRNA and protein was not affected by IFN-gamma, but increased by LPS and this effect was prevented by dexamethasone. Both, LPS and IFN-gamma enhanced the levels of arginase II mRNA and protein, effects also inhibited by dexamethasone. As IFN-gamma did not affect total arginase activity, arginase II may represent only a minor fraction of total arginase activity. 7. In rat AM Phi glucocorticoids inhibit LPS-induced up-regulation of arginase activity, an effect which may contribute to the beneficial effects of glucocorticoids in the treatment of inflammatory airway diseases.

Phosphodiesterase inhibitors and forskolin Up-regulate arginase activity in rabbit alveolar macrophages

Alveolar macrophages (AMsmall ef, Cyrillic) express considerable arginase activity which can be modulated by various mediators. As inhibitors of phosphodiesterase (PDE) play an increasing role in the treatment of chronic inflammatory and obstructive airway disease, we tested whether PDE inhibitors affect arginase activity in AMsmall ef, Cyrillic. Isolated rabbit AMsmall ef, Cyrillic were cultured for 20 h in the absence or presence of bacterial lipopolysaccharides (LPS) and/or different test substances. Thereafter arginase activity was determined by measuring the formation of [(3)H]-L-ornithine during 1 h incubation with [(3)H]-L-arginine. Lipopolysaccharide-enhanced (0. 01-5 microg/ml) maximal arginase activity by about 2.5-fold. The non-selective PDE inhibitor IBMX and the PDE4 selective inhibitor rolipram (each up to 30 microM) caused a 2.4-fold increase in arginase activity, and these effects were additive to those of LPS. The PDE3-selective inhibitor siguazodan had only marginal effects. Forskolin (10 microM) also enhanced arginase activity in the absence and presence of LPS. The effect of forskolin was almost prevented by cycloheximide (30 microM) and largely attenuated by the protein kinase A inhibitor KT 5720 (300 nM). In conclusion, inhibition of the cAMP-specific PDE4, like direct activation of adenylyl cyclase, causes an up-regulation of arginase activity in rabbit AMsmall ef, Cyrillic.

Uptake of apoptotic cells drives the growth of a pathogenic trypanosome in macrophages

After apoptosis, phagocytes prevent inflammation and tissue damage by the uptake and removal of dead cells. In addition, apoptotic cells evoke an anti-inflammatory response through macrophages. We have previously shown that there is intense lymphocyte apoptosis in an experimental model of Chagas' disease, a debilitating cardiac illness caused by the protozoan Trypanosoma cruzi. Here we show that the interaction of apoptotic, but not necrotic T lymphocytes with macrophages infected with T. cruzi fuels parasite growth in a manner dependent on prostaglandins, transforming growth factor-beta (TGF-beta) and polyamine biosynthesis. We show that the vitronectin receptor is critical, in both apoptotic-cell cytoadherence and the induction of prostaglandin E2/TGF-beta release and ornithine decarboxylase activity in macrophages. A single injection of apoptotic cells in infected mice increases parasitaemia, whereas treatment with cyclooxygenase inhibitors almost completely ablates it in vivo. These results suggest that continual lymphocyte apoptosis and phagocytosis of apoptotic cells by macrophages have a role in parasite persistence in the host, and that cyclooxygenase inhibitors have potential therapeutic application in the control of parasite replication and spread in Chagas' disease.

Differential regulation of arginases and inducible nitric oxide synthase in murine macrophage cells

Activated macrophages avidly consume arginine via the action of inducible nitric oxide synthase (iNOS) and/or arginase. In contrast to our knowledge regarding macrophage iNOS expression, the stimuli and mechanisms that regulate expression of the cytosolic type I (arginase I) or mitochondrial type II (arginase II) isoforms of arginase in macrophages are poorly defined. We show that one or both arginase isoforms may be induced in the RAW 264.7 murine macrophage cell line and that arginase expression is regulated independently of iNOS expression. For example, 8-bromo-cAMP strongly induced both arginase I and II mRNAs but not iNOS. Whereas interferon-gamma induced iNOS but not arginase, 8-bromo-cAMP and interferon-gamma mutually antagonized induction of iNOS and arginase I mRNAs. Dexamethasone, which did not induce either arginase or iNOS, almost completely abolished induction of arginase I mRNA by 8-bromo-cAMP but enhanced induction of arginase II mRNA. Lipopolysaccharide (LPS) induced arginase II mRNA, but 8-bromo-cAMP plus LPS resulted in synergistic induction of both arginase I and II mRNAs. In all cases, increases in arginase mRNAs were sufficient to account for the increases in arginase activity. These complex patterns of expression suggest that the arginase isoforms may play distinct, although partially overlapping, functional roles in macrophage arginine metabolism.

Alternative Metabolic States in Murine Macrophages Reflected by the Nitric Oxide Synthase/Arginase Balance: Competitive Regulation by CD4+ T Cells Correlates with Th1/Th2 Phenotype

Activated murine macrophages metabolize l-arginine via two main pathways that are catalyzed by the inducible enzymes nitric oxide synthase (iNOS) and arginase. We have previously shown that CD4+ T cell-derived cytokines regulate a competitive balance in the expression of both enzymes in macrophages; Th1-type cytokines induce iNOS while they inhibit arginase, whereas the reverse is the case for Th2-type cytokines. Here we addressed the regulation of both metabolic pathways by CD4+ T cells directly. Macrophages were used as APCs for established Th1 and Th2 T cell clones as well as for in vitro polarized Th1 or Th2 T cells of transgenic mice bearing an MHC class II-restricted TCR. Both systems revealed a similar dichotomy in the macrophages; Th1 T cells led to an exclusive induction of iNOS, whereas Th2 T cells up-regulated arginase without inducing iNOS. Arginase levels induced by Th2 T cells far exceeded those inducible by individual Th2 cytokines. Similarly, high arginase levels could be induced by supernatants of Th2 cells stimulated in various ways. Ab blocking experiments revealed the critical importance of IL-4 and IL-10 for arginase up-regulation. Finally, strong synergistic effects between IL-4/IL-13 and IL-10 were observed, sufficient to account for the extraordinarily high arginase activity induced by Th2 cells. Our results suggest that the iNOS/arginase balance in macrophages is competitively regulated in the context of Th1- vs Th2-driven immune reactions, most likely by cytokines without the requirement for direct cell interaction.