Allopurinol inhibits CXC chemokine expression and leukocyte adhesion in endotoxemic liver injury

Macrophage Xanthine Oxidoreductase Links LPS Induced Lung Inflammatory Injury to NLRP3 Inflammasome Expression and Mitochondrial Respiration

Acute lung injury (ALI) and the acute respiratory distress syndrome (ARDS) remain poorly treated inflammatory lung disorders. Both reactive oxygen species (ROS) and macrophages are involved in the pathogenesis of ALI/ARDS. Xanthine oxidoreductase (XOR) is an ROS generator that plays a central role in the inflammation that contributes to ALI. To elucidate the role of macrophage-specific XOR in endotoxin induced ALI, we developed a conditional myeloid specific XOR knockout in mice. Myeloid specific ablation of XOR in LPS insufflated mice markedly attenuated lung injury demonstrating the essential role of XOR in this response. Macrophages from myeloid specific XOR knockout exhibited loss of inflammatory activation and increased expression of anti-inflammatory genes/proteins. Transcriptional profiling of whole lung tissue of LPS insufflated XOR fl/fl//LysM-Cre mice demonstrated an important role for XOR in expression and activation of the NLRP3 inflammasome and acquisition of a glycolytic phenotype by inflammatory macrophages. These results identify XOR as an unexpected link between macrophage redox status, mitochondrial respiration and inflammatory activation.

Xanthine oxidase inhibitors and sepsis

Xanthine oxidase activation occurs in sepsis and results in the generation of uric acid (UrAc) and reactive oxygen species (ROS). We aimed to evaluate the effect of xanthine oxidase inhibitors (XOis) in rats stimulated with lipopolysaccharide (LPS). LPS (10 mg/kg) was administered intraperitoneally (i.p.) immediately after allopurinol (Alo, 2 mg/kg) or febuxostat (Feb, 1 mg/kg) every 24 h for 3 days. To increase UrAc levels, oxonic acid (Oxo) was administered by gavage (750 mg/kg per day) for 5 days. Animals were divided into the following 10 groups (n = 6 each): (1) Control, (2) Alo, (3) Feb, (4) LPS, (5) LPSAlo, (6) LPSFeb, (7) Oxo, (8) OxoLPS, (9) OxoLPSAlo, and (10) OxoLPSFeb. Feb with or without Oxo did not aggravate sepsis. LPS administration (with or without Oxo) significantly decreased the creatinine clearance (ClCr) in LPSAlo (60%, P < 0.01) versus LPS (44%, P < 0.05) and LPSFeb (35%, P < 0.05). Furthermore, a significant increase in mortality was observed with LPSAlo (28/34, 82%) compared to LPS treatment alone (10/16, 63%) and LPSFeb (11/17, 65%, P < 0.05). In addition, increased levels of thiobarbituric acid reactive substances (TBARS), tumor necrosis factor (TNF)-α, interleukin (IL)-6, and IL-10 were observed at 72 h compared to the groups that received LPS and LPSFeb with or without Oxo. In this study, coadministration of Alo in LPS-induced experimental sepsis aggravated septic shock, leading to mortality, renal function impairment, and high ROS and proinflammatory IL levels. In contrast, administration of Feb did not potentiate sepsis, probably because it did not interfere with other metabolic events.

XOR inhibition with febuxostat accelerates pulmonary endothelial barrier recovery and improves survival in lipopolysaccharide-induced murine sepsis

Sepsis is a leading cause of death among patients in the intensive care unit, resulting from multi-organ failure. Activity of xanthine oxidoreductase (XOR), a reactive oxygen species (ROS) producing enzyme, is known to be elevated in nonsurvivors of sepsis compared to survivors. We have previously demonstrated that XOR is critical for ventilator-induced lung injury. Using febuxostat, a novel nonpurine inhibitor of XOR, we sought to determine the role of XOR inhibition in a murine model of sepsis-induced lung injury and mortality. C57BL/6J mice were subjected to intravenous (IV) lipopolysaccharide (LPS) for various time points, and lungs were harvested for analyses. Subsets of mice were treated with febuxostat, pre or post LPS exposure, or vehicle. Separate groups of mice were followed up for mortality after LPS exposure. After 24 hr of IV LPS, mice exhibited an increase in XOR activity in lung tissue and a significant increase in pulmonary endothelial barrier disruption. Pretreatment of animals with febuxostat before exposure to LPS, or treatment 4 h after LPS, resulted in complete abrogation of XOR activity. Inhibition of XOR with febuxostat did not prevent LPS-induced pulmonary vascular permeability at 24 h, however, it accelerated recovery of the pulmonary endothelial barrier integrity in response to LPS exposure. Furthermore, treatment with febuxostat resulted in significant reduction in mortality. Inhibition of XOR with febuxostat accelerates recovery of the pulmonary endothelial barrier and prevents LPS-induced mortality, whether given before or after exposure to LPS.

The role of inflammation in hypoxic pulmonary hypertension: from cellular mechanisms to clinical phenotypes

Hypoxic pulmonary hypertension (PH) comprises a heterogeneous group of diseases sharing the common feature of chronic hypoxia-induced pulmonary vascular remodeling. The disease is usually characterized by mild to moderate pulmonary vascular remodeling that is largely thought to be reversible compared with the progressive irreversible disease seen in World Health Organization (WHO) group I disease. However, in these patients, the presence of PH significantly worsens morbidity and mortality. In addition, a small subset of patients with hypoxic PH develop "out-of-proportion" severe pulmonary hypertension characterized by pulmonary vascular remodeling that is irreversible and similar to that in WHO group I disease. In all cases of hypoxia-related vascular remodeling and PH, inflammation, particularly persistent inflammation, is thought to play a role. This review focuses on the effects of hypoxia on pulmonary vascular cells and the signaling pathways involved in the initiation and perpetuation of vascular inflammation, especially as they relate to vascular remodeling and transition to chronic irreversible PH. We hypothesize that the combination of hypoxia and local tissue factors/cytokines ("second hit") antagonizes tissue homeostatic cellular interactions between mesenchymal cells (fibroblasts and/or smooth muscle cells) and macrophages and arrests these cells in an epigenetically locked and permanently activated proremodeling and proinflammatory phenotype. This aberrant cellular cross-talk between mesenchymal cells and macrophages promotes transition to chronic nonresolving inflammation and vascular remodeling, perpetuating PH. A better understanding of these signaling pathways may lead to the development of specific therapeutic targets, as none are currently available for WHO group III disease.

Allopurinol ameliorates thioacetamide-induced acute liver failure by regulating cellular redox-sensitive transcription factors in rats

Oxidative stress plays important role in the development of acute liver failure. In this study, we investigated effects of allopurinol (AP) upon thioacetamide (TAA)-induced liver injury and the potential mechanisms leading to amelioration in inflammation with AP treatment. Acute liver failure was induced by intraperitoneal administration of TAA (300 mg/kg/day for 2 days). Thirty-five rats were divided into five groups as control (group 1), TAA (group 2), TAA + 25AP (group 3), TAA + 50 AP (group 4), and TAA + 100AP (group 5). The number of animals in each group was seven. At the end of the study, histopathological, biochemical, and western blot analysis were done. TAA treatment significantly increased serum levels of aminotransferases, liver malondialdehyde (MDA), nuclear factor-kappa B (NF-қB ), activator protein-1 (AP-1), tumor necrosis factor-alpha (TNF-α), cyclooxygenase-2 (COX-2) and interleukin-6 (IL-6) levels, and the necro-inflammation scores. Nevertheless, nuclear factor E2-related factor-2 and heme oxygenase-1 (HO-1) expressions in the liver were decreased by TAA. AP treatment significantly lowered the serum levels of aminotransferases (P < 0.01) and liver MDA, NF-κB, AP-1, TNF-α, COX-2, and IL-6 expressions (P < 0.05). Moreover, AP restored the liver Nrf2 and HO-1 expressions and improved the necro-inflammation scores significantly. AP improves oxidative stress-induced liver damage by regulating cellular redox-sensitive transcriptor factors and expression of pro-inflammatory and antioxidant defense mechanisms. AP probably exerts these beneficiary features by its free radical scavenging ability in a dose-dependent manner.

The G-protein-coupled bile acid receptor, Gpbar1 (TGR5), negatively regulates hepatic inflammatory response through antagonizing nuclear factor κ light-chain enhancer of activated B cells (NF-κB) in mice

Gpbar1 (TGR5), a membrane-bound bile acid receptor, is well known for its roles in regulation of energy homeostasis and glucose metabolism. TGR5 also displays strong attenuation of macrophage reactivity in vitro, but the physiological roles of TGR5 in inflammatory response, and its mechanism, is unknown. Here, we demonstrate that TGR5 is a negative modulator of nuclear factor kappa light-chain enhancer of activated B cells (NF-κB)-mediated inflammation. TGR5 activation suppresses the phosphorylation of nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor, alpha (IκBα), the translocation of p65, NF-κB DNA-binding activity, and its transcription activity. Furthermore, TGR5 activation enhances the interaction of IκBα and β-arrestin2. Suppression of NF-κB transcription activity and its target gene expression by TGR5 agonist are specifically abolished by the expression of anti-β-arrestin2 small interfering RNA. These results show that TGR5 suppresses the NF-κB pathway by mediation of the interaction between IκBα and β-arrestin2. In a lipopolysaccharide (LPS)-induced inflammation model, TGR5(-/-) mice show more severe liver necroses and inflammation, compared with wild-type (WT) mice. Activation of TGR5 by its agonist ligand inhibits the expression of inflammatory mediators in response to NF-κB activation induced by LPS in WT, but not TGR5(-/-), mouse liver.

CONCLUSION

These findings identify TGR5 as a negative mediator of inflammation that may serve as an attractive therapeutic tool for immune and inflammatory liver diseases.

Protective effect of Gö6976, a PKD inhibitor, on LPS/D: -GalN-induced acute liver injury in mice

OBJECTIVE

Protein kinase D (PKD) is a newly described serine/threonine protein kinase that plays a pivotal role in inflammatory response. In the present study, we examined the protective effect of Gö6976, a PKD inhibitor, on lipopolysaccharide (LPS) and D: -galactosamine (D: -GalN)-induced acute liver injury in mice.

MATERIALS AND METHODS

Mice were pretreated intraperitoneally with Gö6976 30 min before LPS/D: -GalN administration . The mortality and degree of hepatic injury was subsequently assessed.

RESULTS

The results indicated that LPS/D: -GalN administration markedly induced hepatic PKD activation, lethality and liver injury, while pretreatment of the PKD inhibitor Gö6976 significantly inhibited LPS-induced PKD activation, improved the survival of LPS/D: -GalN-administered mice and attenuated LPS/D: -GalN-induced liver injury, as evidenced by reduced levels of serum aminotransferases as well as reduced histopathological changes. In addition, the protective effects of Gö6976 were paralleled by suppressed activation of mitogen-activated protein kinases (MAPKs), decreased expression of tumor necrosis factor-α (TNF-α) and adhesion molecules, and reduced apoptosis and myeloperoxidase (MPO) activity in liver.

CONCLUSIONS

Our experimental data indicated that Gö6976, a PKD inhibitor, could effectively prevent LPS/D: -GalN-induced acute liver injury by inhibition of MAPKs activation to reduce TNF-α production. This suggests the potential pharmacological value of PKD inhibitors in the intervention of inflammation-based liver diseases.

Xanthine oxidoreductase promotes the inflammatory state of mononuclear phagocytes through effects on chemokine expression, peroxisome proliferator-activated receptor-{gamma} sumoylation, and HIF-1{alpha}

The protective effects of pharmacological inhibitors of xanthine oxidoreductase (XOR) have implicated XOR in many inflammatory diseases. Nonetheless, the role played by XOR during inflammation is poorly understood. We previously observed that inhibition of XOR within the inflammatory mononuclear phagocytes (MNP) prevented neutrophil recruitment during adoptive transfer demonstrating the role of XOR in MNP-mediated neutrophil recruitment. To further explore the role of XOR in the inflammatory state of MNP, we studied MNP isolated from inflammatory lungs combined with analyses of MNP cell lines. We demonstrated that XOR activity was increased in inflammatory MNP following insufflation of Th-1 cytokines in vivo and that activity was specifically increased by MNP differentiation. Inhibition of XOR reduced levels of CINC-1 secreted by MNP. Expression of peroxisome proliferator-activated receptor γ (PPARγ) in purified rat lung MNP and MNP cell lines reflected both the presence of PPARγ isoforms and PPARγ SUMOylation, and XOR inhibitors increased levels of SUMO-PPARγ in MNP cell lines. Both ectopic overexpression of XOR cDNA and uric acid supplementation reduced SUMO-PPARγ in MNP cells. Levels of the M2 markers CD36, CD206, and arginase-1 were modulated by uric acid and oxonic acid, whereas siRNA to SUMO-1 or PIAS-1 also reduced arginase-1 in RAW264.7 cells. We also observed that HIF-1α was increased by XOR inhibitors in inflammatory MNP and in MNP cell lines. These data demonstrate that XOR promotes the inflammatory state of MNP through effects on chemokine expression, PPARγ SUMOylation, and HIF-1α and suggest that strategies for inhibiting XOR may be valuable in modulating lung inflammatory disorders.

The hepatic microcirculation: mechanistic contributions and therapeutic targets in liver injury and repair

The complex functions of the liver in biosynthesis, metabolism, clearance, and host defense are tightly dependent on an adequate microcirculation. To guarantee hepatic homeostasis, this requires not only a sufficient nutritive perfusion and oxygen supply, but also a balanced vasomotor control and an appropriate cell-cell communication. Deteriorations of the hepatic homeostasis, as observed in ischemia/reperfusion, cold preservation and transplantation, septic organ failure, and hepatic resection-induced hyperperfusion, are associated with a high morbidity and mortality. During the last two decades, experimental studies have demonstrated that microcirculatory disorders are determinants for organ failure in these disease states. Disorders include 1) a dysregulation of the vasomotor control with a deterioration of the endothelin-nitric oxide balance, an arterial and sinusoidal constriction, and a shutdown of the microcirculation as well as 2) an overwhelming inflammatory response with microvascular leukocyte accumulation, platelet adherence, and Kupffer cell activation. Within the sequelae of events, proinflammatory mediators, such as reactive oxygen species and tumor necrosis factor-alpha, are the key players, causing the microvascular dysfunction and perfusion failure. This review covers the morphological and functional characterization of the hepatic microcirculation, the mechanistic contributions in surgical disease states, and the therapeutic targets to attenuate tissue injury and organ dysfunction. It also indicates future directions to translate the knowledge achieved from experimental studies into clinical practice. By this, the use of the recently introduced techniques to monitor the hepatic microcirculation in humans, such as near-infrared spectroscopy or orthogonal polarized spectral imaging, may allow an early initiation of treatment, which should benefit the final outcome of these critically ill patients.

Inhalation of carbon monoxide prevents liver injury and inflammation following hind limb ischemia/reperfusion

The induction of heme oxygenase (HO), the rate limiting enzyme in the conversion of heme into carbon monoxide (CO) and biliverdin, limits liver injury following remote trauma such as hind limb ischemia/reperfusion (I/R). Using intravital video microscopy, we tested the hypothesis that inhaled CO (250 ppm) would mimic HO-derived liver protection. Hind limb I/R significantly decreased sinusoidal diameter and volumetric flow, increased leukocyte accumulation within sinusoids, increased leukocyte rolling and adhesion within postsinusoidal venules, and significantly increased hepatocyte injury compared with naive animals. Inhalation of CO alone did not alter any microcirculatory or inflammatory parameters. Inhalation of CO following I/R restored volumetric flow, decreased stationary leukocytes within sinusoids, decreased leukocyte rolling and adhesion within postsinusoidal venules, and significantly reduced hepatocellular injury following hind limb I/R. HO inhibition did not alter microcirculatory parameters in naive mice, but did increase inflammation, as well as increase hepatocyte injury following hind limb I/R. Inhalation of CO during HO inhibition significantly reduced such microcirculatory deficits, hepatic inflammation, and injury in response to hind limb I/R. In conclusion, these results suggest that HO-derived hepatic protection is mediated by CO, and inhalation of low concentrations of CO may represent a novel therapeutic approach to prevent remote organ injury during systemic inflammatory response syndrome, or SIRS.

Critical role of CXC chemokines in endotoxemic liver injury in mice

Tissue accumulation of leukocytes constitutes a rate-limiting step in endotoxin-induced tissue injury. Chemokines have the capacity to regulate leukocyte trafficking. However, the role of CXC chemokines, i.e., macrophage inflammatory protein-2 (MIP-2) and cytokine-induced neutrophil chemoattractant (KC), in leukocyte recruitment, microvascular perfusion failure, cellular injury, and apoptosis in the liver remains elusive. Herein, mice were challenged with lipopolysaccharide (LPS) in combination with D-galactosamine, and intravital microscopy of the liver microcirculation was conducted 6 h later. It was found that immunoneutralization of MIP-2 and KC did not reduce LPS-induced leukocyte rolling and adhesion in postsinusoidal venules. In contrast, pretreatment with monoclonal antibodies against MIP-2 and KC abolished (83% reduction) extravascular recruitment of leukocytes in the livers of endotoxemic mice. Notably, endotoxin challenge increased the expression of CXC chemokines, which was mainly confined to hepatocytes. Moreover, endotoxin-induced increases of liver enzymes and hepatocellular apoptosis were decreased by more than 82% and 68%, respectively, and sinusoidal perfusion was restored in mice passively immunized against MIP-2 and KC. In conclusion, this study indicates that intravascular accumulation of leukocytes in the liver is independent of CXC chemokines in endotoxemic mice. Instead, our novel data suggest that CXC chemokines are instrumental in regulating endotoxin-induced transmigration and extravascular tissue accumulation of leukocytes. Indeed, these findings demonstrate that interference with MIP-2 and KC functions protects against septic liver damage and may constitute a potential therapeutic strategy to control pathological inflammation in endotoxemia.