4-Methylpyrazole protects against acetaminophen hepatotoxicity in mice and in primary human hepatocytes

Liver injury due to acetaminophen (APAP) overdose is the major cause of acute liver failure in the United States. While treatment with N-acetylcysteine is the current standard of care for APAP overdose, anecdotal evidence suggests that administration of 4-methylpyrazole (4MP) may be beneficial in the clinic. The objective of the current study was to examine the protective effect of 4MP and its mechanism of action. Male C57BL/6J mice were co-treated with 300 mg/kg of APAP and 50 mg/kg of 4MP. The severe liver injury induced by APAP at 6 h as indicated by elevated plasma alanine aminotransferase activities, centrilobular necrosis, and nuclear DNA fragmentation was almost completely eliminated by 4MP. In addition, 4MP largely prevented APAP-induced activation of c-Jun N-terminal kinase (JNK), mitochondrial translocation of phospho-JNK and Bax, and the release of mitochondrial intermembrane proteins. Importantly, 4MP inhibited the generation of APAP protein adducts and formation of APAP-glutathione (GSH) conjugates and attenuated the depletion of the hepatic GSH content. These findings are relevant to humans because 4MP also prevented APAP-induced cell death in primary human hepatocytes. In conclusion, early treatment with 4MP can completely prevent liver injury after APAP overdose by inhibiting cytochrome P450 and preventing generation of the reactive metabolite.

Controversial constitutive TSHR activity: patients, physiology, and in vitro characterization

G protein-coupled receptors constitute a large family of transmembrane receptors, which activate cellular responses by signal transmission and regulation of second messenger metabolism after ligand binding. For several of these receptors it is known that they also signal ligand-independently. The G protein-coupled thyroid stimulating hormone receptor (TSHR) is characterized by a high level of constitutive activity in the wild type state. However, little is known yet concerning the physiological relevance of the constitutive wild type TSHR activity. Certainly, knowledge of the physiological relevance of constitutive wild type receptor activity is necessary to better understand thyroid physiology and it is a prerequisite for the development of better therapies for nonautoimmune hyperthyroidism and thyroid cancer. Based on a literature search regarding all published TSHR mutations, this review covers several mutations which are clearly associated with a hyperthyroidism-phenotype, but interestingly show a lack of constitutive activity determined by in vitro characterization. Possible reasons for the observed discrepancies between clinical phenotypes and in vitro characterization results for constitutive TSHR activity are reviewed. All current in vitro characterization methods for constitutive TSHR mutations are "preliminary attempts" and may well be revised by more comprehensive and even better approaches. However, a standardized approach for the determination of constitutive activity can help to identify TSHR mutations for which the investigation of additional signaling mechanisms would be most interesting to find explanations for the current clinical phenotype/in vitro discrepancies and thereby also define suitable methods to explore the physiological relevance of constitutive wild type TSHR activity.

High prevalence of TSHR/Gsα mutation-negative clonal hot thyroid nodules (HNs) in a Turkish cohort

Whereas the majority of hot thyroid nodules are caused by somatic TSH-receptor mutations, the percentage of TSH-receptor mutation negative clonal hot nodules (HN) and thus the percentage of hot nodules likely caused by other somatic mutations are still debated. This is especially the case for toxic multinodular goiter (TMNG). 35 HNs [12 solitary hot nodules (SHN), 23 TMNG] were screened for somatic TSHR mutations in the exons 9 and 10 and for Gsα mutations in the exons 7 and 8 using DGGE. Determination of X-chromosome inactivation was used for clonality analysis. Overall TSHR mutations were detected in 14 out of 35 (40%) HNs. A nonrandom X-chromosome inactivation pattern was detected in 18 out of 25 (72%) HNs suggesting a clonal origin. Of 15 TSHR or Gsα mutation negative cases 13 (86.6%) showed nonrandom X-chromosome inactivation, indicating clonal origin. The frequency of activating TSHR and/or Gsα mutations was higher in SHNs (9 of 12) than in TMNGs (6 of 23). There was no significant difference for the incidence of clonality for HNs between TMNGs or SHNs (p: 0.6396). Activating TSHR and/or Gsα mutations were more frequent in SHNs than in TMNG. However, the frequency of clonality is similar for SHN and TMNG and there is no significant difference for the presence or absence of TSHR and/or Gsα mutations of clonal or polyclonal HNs. The high percentage of clonal mutation-negative HNs in SHN and TMNG suggests alternative molecular aberrations leading to the development of TSHR mutation negative nodules.

Prolonged inappropriate TSH suppression during hypothyroidism after thyroid ablation in a patient with nonautoimmune familial hyperthyroidism

Prolonged TSH suppression was reported in a patient with nonautoimmune hyperthyroidism. These observations were made during L-thyroxine treatment and it was not possible to investigate a possible increase in serum TSH concentrations to levels observed in untreated hypothyroidism. We describe nonautoimmune familial hyperthyroidism identified in an Israeli woman, which is remarkable for the prolonged inappropriate TSH suppression after thyroid ablation. After 2 radioiodine treatments for several years, her TSH was always lower than 0.03 mU/l with 1.6 μg/kg/day (100 μg) thyroxine. 14 years after the radioiodine treatments, she discontinued thyroxine for 3.5 months and developed myxoedema with fT4 <6.0 and fT3 1.3 pmol/l and TSH of only 4.4 mU/l, which rose to only 8.6 after TRH. Genomic analysis showed a germline substitution M626I in the TSHR gene. Both exons of the thyroid-releasing hormone receptor revealed no mutations in this gene. Functional in vitro characterization of M626I showed a cell surface expression of 70% compared with the wt (100%), a significant increase of basal activity (5-fold over wt basal), which was confirmed by linear regression analysis (LRA) (slope: M626I=7, wt=1). No TRH-receptor mutation was detected. Therefore, this is the first patient with nonautoimmune hyperthyroidism with unequivocal evidence for inappropriately prolonged TSH suppression documented by a clearly insufficient TSH increase during clinical hypothyroidism. The in vitro characterization of the TSH-receptor mutation did not show any explanations for the prolonged TSH suppression. Therefore, other possible candidate genes remain to be investigated for potential explanations for this prolonged TSH suppression.

Preferences of transmembrane helices for cooperative amplification of G(alpha)s and G (alpha)q signaling of the thyrotropin receptor

The majority of constitutively activating mutations (CAMs) of the thyroid-stimulating hormone receptor display a partially activated receptor. Thus, full receptor activation requires a multiplex activation process. To define impacts of different transmembrane helices (TMHs) on cooperative signal transduction, we combined single CAMs in particular TMHs to double mutations and measured second messenger accumulation of the G(alpha)s and the G(alpha)q pathway. We observed a synergistic increase for basal activity of the G(alpha)s pathway, for all characterized double mutants except for two combinations. Each double mutation, containing CAMs in TMH2, 6 and 7 showed the highest constitutive activities, suggesting that these helices contribute most to G(alpha)s-mediated signaling. No single CAM revealed constitutive activity for the G(alpha)q pathway. The double mutations with CAMs from TMH1, 2, 3 and 6 also exhibited increase for basal G(alpha)q signaling. Our results suggest that TMH2, 6, 7 show selective preferences towards G(alpha)s signaling, and TMH1, 2, 3, 6 for G(alpha)q signaling.

An aromatic environment in the vicinity of serine 281 is a structural requirement for thyrotropin receptor function

The majority of constitutively activating human TSH receptor (hTSHR) mutations are located in the transmembrane helices as well as in the extracellular (ECLs) and intracellular loops. S(281) is one of two positions in the ectodomain in which activating hTSHR mutations have been identified in vivo (S(281)T, I, and N). To investigate the functional properties of this key residue in more detail, S(281) was replaced by each of the other 19 amino acids. Many substitutions led to constitutive receptor activation, suggesting that S(281) plays a pivotal role in maintaining the receptor in its inactive state. Strikingly, all substitutions with aromatic residues (S(281)W, F, Y, and H) show expression similar to that of wild-type hTSHR and are tolerated at this position because they maintain basal activity or express only slight constitutive activity. Three-dimensional modeling of the hTSHR suggested that S(281) and surrounding residues are in close proximity to ECL1. To investigate the possible importance of an aromatic environment between the ectodomain in the vicinity of S(281) and ECL1, aromatic residues Y(279), Y(476), H(478), Y(481), Y(482), and H(484) were replaced by alanine. Functional characterization showed impaired cell surface expression and signaling for Y(279)A and Y(481)A, in contrast to the other alanine mutants. However, substitutions of Y(279) and Y(481) with other aromatic residues exhibited surface expression and signaling comparable to wild-type hTSHR. Our results suggest that Y(279) in the extracellular domain and probably Y(481) in the ECL1 also are involved in an aromatic environment around S(281) in the hTSHR, which is important for functional receptor conformation and intramolecular receptor signaling.

Effects of CXC chemokines on neutrophil activation and sequestration in hepatic vasculature

The initiating step of neutrophil-induced cytotoxicity in the liver is the recruitment of these phagocytes into sinusoids. The aim of our study was to compare the efficacy of systemic exposure with individual inflammatory mediators on neutrophil activation and sequestration in the hepatic vasculature of C3Heb/FeJ mice as assessed by flow cytometry and histochemistry, respectively. The CXC chemokine macrophage inflammatory protein-2 (MIP-2; 20 microg/kg) induced a time-dependent upregulation of Mac-1 (318% at 4 h) and shedding of L-selectin (41% at 4 h). MIP-2 treatment caused a temporary increase of sinusoidal neutrophil accumulation at 0.5 h [97 +/- 6 polymorphonuclear leukocytes (PMN)/50 high-power fields (HPF)], which declined to baseline (8 +/- 2) at 4 h. The CXC chemokine KC was largely ineffective in activating neutrophils or recruiting them into the liver. Cytokines (tumor necrosis factor-alpha and interleukin-1alpha) and cobra venom factor substantially increased Mac-1 expression and L-selectin shedding on neutrophils and caused stable sinusoidal neutrophil accumulation (170-220 PMN/50 HPF). Only cytokines induced venular neutrophil margination. Thus CXC chemokines in circulation are less effective than cytokines or complement in activation of neutrophils and their recruitment into the hepatic vasculature in vivo.

Differential activation pattern of redox-sensitive transcription factors and stress-inducible dilator systems heme oxygenase-1 and inducible nitric oxide synthase in hemorrhagic and endotoxic shock

OBJECTIVE

To investigate the role of redox-sensitive transcription factors nuclear factor kappa-B (NF-kappaB) or activator protein-1 (AP-1) for hepatic gene expression of heme oxygenase (HO)-1 and inducible nitric oxide synthase (iNOS) in models of hemorrhagic or endotoxic shock.

DESIGN

Prospective controlled laboratory study.

SETTING

Animal research laboratory at a university hospital.

SUBJECTS

Male Sprague-Dawley rats (250-350 g).

INTERVENTIONS

After anesthesia, animals were assigned to hemorrhagic shock (mean arterial pressure 35-40 mm Hg for 60 mins), sham operation, or endotoxemia (1 mg/kg intraperitoneally). To assess the role of reactive oxygen species for activation of NF-kappaB or AP-1, animals were treated with the antioxidant trolox (6 mg/kg body weight). In additional experiments, animals were pretreated with dexamethasone (10 mg/kg body weight), an inhibitor of the transactivating function of DNA-bound AP-1 or with actinomycin-D (2 mg/kg body weight), an inhibitor of DNA-directed RNA synthesis. Activation of NF-kappaB or AP-1 was assessed by electrophoretic mobility shift assay. HO-1 and iNOS gene expression were assessed by Northern and Western blot.

MEASUREMENTS AND MAIN RESULTS

Hemorrhage and resuscitation induced hepatic HO-1 transcripts 12-fold. Induction was abolished by actinomycin-D and was attenuated by dexamethasone and the antioxidant trolox. Activation of AP-1 was observed after hemorrhagic but not after endotoxic shock. AP-1 activation was inhibitable by trolox and correlated with accumulation of HO-1 transcripts. In contrast, a weak activation of NF-kappaB was observed after hemorrhage that was not affected by trolox. A profound activation of NF-kappaB after endotoxic shock correlated with induction of iNOS but failed to induce HO-1 transcripts.

CONCLUSIONS

These data suggest that AP-1 but not NF-kappaB activation is dependent on reactive oxygen intermediates in vivo and contributes to HO-1 gene expression. Thus, AP-1-dependent HO-1 induction under oxidative stress conditions may subserve a similar function as a stress-inducible vasodilator system as does NF-kappaB-dependent iNOS expression in liver inflammation.

Differential protection with inhibitors of caspase-8 and caspase-3 in murine models of tumor necrosis factor and Fas receptor-mediated hepatocellular apoptosis

Excessive apoptosis has been implicated in a number of acute and chronic human diseases. The activation of caspases has been shown to be critical for the apoptotic process. The objective of this investigation was to evaluate the beneficial effects and mechanism of action of the caspase-8 inhibitor IETD-CHO and the caspase-3 inhibitor DEVD-CHO against tumor necrosis factor (TNF)-induced hepatocellular apoptosis in vivo and compare these results to effects of the same inhibitors against Fas-induced apoptosis. Treatment of C3Heb/FeJ mice with 700 mg/kg galactosamine/100 microg/kg endotoxin induced parenchymal apoptosis (indicated by caspase-3 activation and morphology) and severe liver injury (indicated by the increase in plasma alanine aminotransferase activities and histology) at 7 h. Treatment with IETD-CHO or DEVD-CHO (10 mg/kg at 3, 4.5, and 5.5 h) significantly attenuated caspase-3 activation and liver injury. Western analysis showed that DEVD-CHO had no effect while IETD-CHO substantially reduced procaspase-3 and procaspase-9 processing. On the other hand, caspase-3 activation and liver injury by the anti-Fas antibody Jo-2 was completely prevented by a single dose of DEVD-CHO and, as previously shown, by IETD-CHO at 90 min. Both inhibitors prevented procaspase-3 and procaspase-9 processing. Thus, there are fundamental differences in the efficacy of caspase inhibitors in these two models. We conclude that Fas may rely exclusively on caspase-8 activation and mitochondria to activate caspase-3, which can process more procaspase-8 and thus propagate the amplification of the apoptotic signal. TNF can activate a similar signaling pathway. However, alternative signaling mechanisms seem to exist, which can compensate if the main pathway is blocked.

Vascular and hepatocellular peroxynitrite formation during acetaminophen toxicity: role of mitochondrial oxidant stress

Peroxynitrite may be involved in acetaminophen-induced liver damage. However, it is unclear if peroxynitrite is generated in hepatocytes or in the vasculature. To address this question, we treated C3Heb/FeJ mice with 300 mg/kg acetaminophen and assessed nitrotyrosine protein adducts as indicator for peroxynitrite formation. Vascular nitrotyrosine staining was evident before liver injury between 0.5 and 2 h after acetaminophen treatment. However, liver injury developed parallel to hepatocellular nitrotyrosine staining between 2 and 6 h after acetaminophen. The mitochondrial content of glutathione disulfide, as indicator of reactive oxygen formation determined 6 h after acetaminophen, increased from 2.8 +/- 0.6% in controls to 23.5 +/- 5.1%. A high dose of allopurinol (100 mg/kg) strongly attenuated acetaminophen protein-adduct formation and prevented the mitochondrial oxidant stress and liver injury after acetaminophen. Lower doses of allopurinol, which are equally effective in inhibiting xanthine oxidase, were not protective and had no effect on nitrotyrosine staining and acetaminophen protein adduct formation. In vitro experiments showed that allopurinol is not a direct scavenger of peroxynitrite. We conclude that there is vascular peroxynitrite formation during the first 2 h after acetaminophen treatment. On the other hand, reactive metabolites of acetaminophen bind to intracellular proteins and cause mitochondrial dysfunction and superoxide formation. Mitochondrial superoxide reacts with nitric oxide to form peroxynitrite, which is responsible for intracellular protein nitration. The pathophysiological relevance of vascular peroxynitrite for hepatocellular peroxynitrite formation and liver injury remains to be established.

Mechanism of cell death during warm hepatic ischemia-reperfusion in rats: apoptosis or necrosis?

Reperfusion injury can cause liver dysfunction after cold storage and warm ischemia. Recently it has been suggested that more than 50% of hepatocytes and sinusoidal endothelial cells (SEC) are undergoing apoptosis during the first 24 hours of reperfusion. The aim of our study was to quantify apoptotic and necrotic hepatocytes and apoptotic SEC after 60 or 120 minutes of warm, partial no-flow ischemia and 0 to 24 hours reperfusion in male SD rats. Apoptotic cells were identified by TUNEL assay in combination with morphological criteria. After 60 minutes of ischemia and 1 hour of reperfusion there was a significant increase of apoptotic hepatocytes (0.7 +/- 0.1% vs. 0.3 +/- 0.1% in controls) and SEC (1.5 +/- 0.6% vs. 0.3 +/- 0.1% in controls). The number of apoptotic SEC and hepatocytes was not different from controls at 6 hours or 24 hours of reperfusion. In contrast, the number of necrotic hepatocytes was quantified as 12 +/- 2% at 1 hour, 34 +/- 6% at 6 hours, and 57 +/- 11% at 24 hours. These results correlated with the increase in plasma ALT levels at these time points. Longer (120 min) ischemia times did not affect the number of apoptotic cells but increased hepatocellular necrosis to 58 +/- 4% at 6 hours reperfusion. No significant increase in caspase-3 activity and processing was detectable in any of these livers. Moreover, the caspase inhibitor Z-Asp-cmk (2 mg/kg IV) had no significant effect on reperfusion injury. Our results suggest that only a small minority of SEC and hepatocytes undergo apoptosis after 60 to 120 minutes of warm ischemia followed by 0 to 24 hours of reperfusion. Oncotic necrosis appears to be the principal mechanism of cell death for both cell types.

Protection against TNF-induced liver parenchymal cell apoptosis during endotoxemia by a novel caspase inhibitor in mice

Excessive apoptotic cell death is implicated in a growing number of acute and chronic disease states. Caspases are critical for the intracellular signaling pathway leading to apoptosis. The aim of this investigation was to evaluate the efficacy and the mechanism of action of the novel caspase inhibitor CV1013 in a well-characterized model of TNF-induced apoptosis. Administration of 700 mg/kg galactosamine/100 microg/kg endotoxin (Gal/ET) induced hepatocellular apoptosis in C3Heb/FeJ mice as indicated by increased caspase-3 activity (706% above controls) and enhanced DNA fragmentation (3400% above controls) at 6 h. In addition, apoptosis was aggravated by the neutrophil-induced injury at 7 h (ALT activities: 4220 +/- 960 U/L and 48 +/- 4% necrosis). All animals died 8-12 h after Gal/ET treatment from shock and liver failure. A dose of 10 or 1 mg/kg of CV1013 administered three times (3, 4.5, and 5.5 h after Gal/ET) effectively prevented caspase-3 activation and parenchymal cell apoptosis at 6 h as well as the subsequent neutrophil-induced aggravation of the injury at 7 h after Gal/ET treatment. Animals treated with 10 mg/kg CV1013 survived for 24 h without liver injury. CV1013 reduced the processing of caspase-3 and caspase-8. This suggests that CV1013 may have inhibited the small amount of active caspase-8 generated at the receptor level. Because of the multiple amplification loops used to activate the entire caspase cascade, blocking the initial intracellular signal by CV1013 was highly effective in preventing apoptotic cell death. CV1013 has therapeutic potential for disease states with excessive apoptosis.

Pathophysiologic importance of E- and L-selectin for neutrophil-induced liver injury during endotoxemia in mice

Neutrophils can cause parenchymal cell injury in the liver during ischemia-reperfusion and endotoxemia. Neutrophils relevant for the injury accumulate in sinusoids, transmigrate, and adhere to hepatocytes. To investigate the role of E- and L-selectin in this process, C3Heb/FeJ mice were treated with 700 mg/kg galactosamine and 100 microgram/kg endotoxin (Gal/ET). Immunogold labeling verified the expression of E-selectin on sinusoidal endothelial cells 4 hours after Gal/ET injection. In addition, Gal/ET caused up-regulation of Mac-1 (CD11b/CD18) and shedding of L-selectin from circulating neutrophils. Gal/ET induced hepatic neutrophil accumulation (422 +/- 32 polymorphonuclear leukocytes [PMN]/50 high power fields [HPF]) and severe liver injury (plasma alanine transaminase [ALT] activities: 4,120 +/- 960 U/L; necrosis: 44 +/- 3%) at 7 hours. Treatment with an anti-E-selectin antibody (3 mg/kg, intravenously) at the time of Gal/ET administration did not significantly affect hepatic neutrophil accumulation and localization. However, the anti-E-selectin antibody significantly attenuated liver injury as indicated by reduced ALT levels (-84%) and 43% less necrotic hepatocytes. In contrast, animals treated with an anti-L-selectin antibody or L-selectin gene knock out mice were not protected against Gal/ET-induced liver injury. However, E-, L-, and P-selectin triple knock out mice showed significantly reduced liver injury after Gal/ET treatment as indicated by lower ALT levels (-65%) and reduced necrosis (-68%). Previous studies showed that circulating neutrophils of E-selectin-overexpressing mice are primed and activated similar to neutrophils adhering to E-selectin in vitro. Therefore, we conclude that blocking E-selectin or eliminating this gene may have protected against Gal/ET-induced liver injury in vivo by inhibiting the full activation of neutrophils during the transmigration process.

Protection against Fas receptor-mediated apoptosis in hepatocytes and nonparenchymal cells by a caspase-8 inhibitor in vivo: evidence for a postmitochondrial processing of caspase-8

Lymphocytes can kill target cells including hepatocytes during various inflammatory diseases by Fas receptor-mediated apoptosis. Caspase-8 is activated at the receptor level, thereby initiating the processing of downstream effector caspases. The aim of this study was to investigate the time course of caspase-8 activation and to evaluate the efficacy of the caspase-8 inhibitor IETD-CHO in a model of Fas-induced apoptosis in vivo. C3Heb/FeJ mice were treated with the anti-Fas antibody Jo-2 (0.6 mg/kg). Western blot analysis demonstrated increased cytochrome c in the cytosol (20 min), which was followed by the progressive activation of caspase-3, -9 (40-120 min), and caspase-8 (120 min). At 90 and 120 min, extensive hemorrhage was observed, indicating damage to sinusoidal lining cells. In addition, high plasma ALT levels (997 +/- 316 U/L) and histological evaluation indicated severe parenchymal cell injury. Parenchymal and nonparenchymal cells showed a similar increase in caspase-3 activity and DNA fragmentation. Treatment with IETD-CHO (10 mg/kg) attenuated the increase in caspase-3 activity and DNA fragmentation by 80-90% and completely prevented hemorrhage and parenchymal cell damage. IETD-CHO also prevented the early release of mitochondrial cytochrome c and the processing of caspase-3, -8, and -9. Thus, our data support the hypothesis that Fas-mediated apoptosis is dependent on caspase-8 activation in hepatocytes and nonparenchymal cells. However, the bulk of procaspase-8 is processed late, suggesting that only a small amount of procaspase-8 may actually be activated at the Fas receptor. This initial signal may be amplified by further activation of caspase-8 by effector caspases, i.e., after mitochondrial activation. Caspase-8 is a promising therapeutic target for inhibition of Fas-mediated apoptosis.

Transcriptional activation of heme oxygenase-1 and its functional significance in acetaminophen-induced hepatitis and hepatocellular injury in the rat

BACKGROUND/AIM

Glutathione depletion contributes to acetaminophen hepatotoxicity and is known to induce the oxidative stress reactant heme oxygenase-1. The metabolites of the heme oxygenase pathway, biliverdin, carbon monoxide, and iron may modulate acetaminophen toxicity. The aim of this study was to assess cell-type specific expression of heme oxygenase-1 and its impact on liver injury and microcirculatory disturbances in a model of acetaminophen-induced hepatitis.

METHODS

Gene expression of heme oxygenase-1 was studied by Northern- and Western analysis as well as immunohistochemistry. The time course of heme oxygenase-1 and -2, cytokine-induced neutrophil chemoattractant-1, and intercellular adhesion molecule-1 was studied by Northern analysis. DNA-binding activity of nuclear factor-kappaB was determined by electrophoretic mobility shift assay. Sinusoidal perfusion and leukocyte-endothelial interactions were assessed by intravital microscopy.

RESULTS

Acetaminophen caused a moderate sinusoidal perfusion failure (-15%) and infiltration of neutrophils along with activation of nuclear factor-kappaB and intercellular adhesion molecule-1 and cytokine-induced neutrophil chemoattractant-1 mRNAs. Induction of heme oxygenase-1 mRNA and protein (approximately 30-fold) in hepatocytes and non-parenchymal cells paralleled the inflammatory response. Blockade of heme oxygenase activity with tin-protoporphyrin-IX abrogated acetaminophen-induced hepatic neutrophil accumulation and nuclear factor-kappaB activation, but failed to affect sinusoidal perfusion and liver injury.

CONCLUSIONS

The inflammatory response associated with acetaminophen hepatotoxicity is modulated by the parallel induction of the heme oxygenase-1 gene. However, heme oxygenase-1 has no permissive effect on sinusoidal perfusion and does not affect liver injury in this model. These data argue against a central role of nuclear factor-kappaB activation and neutrophil infiltration as perpetuating factors of liver injury in acetaminophen toxicity.

Reactive oxygen and mechanisms of inflammatory liver injury

Reactive oxygen species (ROS) are important cytotoxic and signalling mediators in the pathophysiology of inflammatory liver diseases. They can be generated by resident and infiltrating phagocytes and/or intracellularly in every liver cell type after stimulation with cytokines. Although ROS are able to cause cell destruction by massive lipid peroxidation, in most cases, ROS are more likely to modulate signal transduction pathways by affecting redox-sensitive enzymes, organelles (e.g. mitochondria) and transcription factors. Thus, ROS can directly induce and/or regulate apoptotic and necrotic cell death. In addition, ROS can have indirect effects on the pathophysiology by supporting protease activity through inactivation of antiproteases and by modulating the formation of inflammatory mediators and adhesion molecules. Many of the effects of ROS may occur simultaneously or sequentially in the pathophysiology. Although mainly described in this review as detrimental, ROS are essential for host-defence functions of phagocytes and can modulate the formation of mediators involved in regulating sinusoidal blood flow and liver regeneration. Thus, continuous efforts are necessary to improve our understanding of the role of ROS in the pathophysiology of inflammatory liver diseases and to discover therapeutic interventions that selectively target the negative effects of reactive oxygen formation.

The hepatic inflammatory response after acetaminophen overdose: role of neutrophils

Acetaminophen overdose induces severe liver injury and hepatic failure. There is evidence that inflammatory cells may be involved in the pathophysiology. Thus, the aim of this investigation was to characterize the neutrophilic inflammatory response after treatment of C3Heb/FeJ mice with 300 mg/kg acetaminophen. A time course study showed that neutrophils accumulate in the liver parallel to or slightly after the development of liver injury. The number of neutrophils in the liver was substantial (209 +/- 64 PMN/50 high-power fields at 12 h) compared to baseline levels (7 +/- 1). Serum levels of TNF-alpha and the C-X-C chemokines KC and MIP-2 increased by 28-, 14-, and 295-fold, respectively, over levels found in controls during the injury process. In addition, mRNA expression of MIP-2 and KC were upregulated in livers of acetaminophen-treated animals as determined by ribonuclease protection assay. However, none of these mediators were generated in large enough quantities to account for neutrophil sequestration in the liver. There was no upregulation of Mac-1 (CD11b/ CD18) or shedding of L-selectin on circulating neutrophils. Moreover, an anti-CD18 antibody had no protective effect against acetaminophen overdose during the first 24 h. These results indicate that there is a local inflammatory response after acetaminophen overdose, including a substantial accumulation of neutrophils in the liver. Because of the critical importance of beta2 integrins for neutrophil cytotoxicity, these results suggest that neutrophils do not contribute to the initiation or progression of AAP-induced liver. The inflammation observed after acetaminophen overdose may be characteristic for a response sufficient to recruit neutrophils for the purpose of removing necrotic cells but is not severe enough to cause additional damage.

Role of reactive oxygen species for hepatocellular injury and heme oxygenase-1 gene expression after hemorrhage and resuscitation

Reactive oxygen species (ROS) generated during hemorrhage and subsequent resuscitation (H/R) may contribute to cellular injury but may also regulate an adaptive cellular response to stress. Heme oxygenase (HO)-1 has been recognized as an important stress-inducible gene conferring protection after H/R. The aim of this study was to determine the contribution of ROS to hepatocellular injury and to induction of HO-1 in parenchymal and nonparenchymal cells after H/R. Anesthetized Sprague-Dawley rats were subjected to reversible H/R with or without coadministration of the potent antioxidant Trolox (6 mg/kg body wt). HO-1 gene expression was determined at baseline, at the end of hemorrhagic hypotension, and after 1, 3, and 5 h of resuscitation on the messenger ribonucleic acid (mRNA) and protein level. Assessment of hepatocellular injury by alpha-glutathione-S-transferase serum levels showed a significant increase after H/R that was attenuated by Trolox (sham: 38 (26-42); H/R: 286 (150-696); Trolox: 14 (2-227) microg/L; median (25th/75th percentile) P<0.05). Injury correlated with induction of HO-1 mRNA (r2 = 0.97) on the whole organ level and with the expression pattern of HO-1-immunoreactive protein in pericentral hepatocytes after H/R. Trolox attenuated H/R-induced increase of HO-1 in hepatocytes. In contrast, nonparenchymal cells showed high constitutive levels of HO-1 mRNA and protein that were increased by sham operation and H/R to a similar extent. HO-1 steady-state transcripts in nonparenchymal cells were not modulated by Trolox. These results suggest a differential regulation of HO-1 gene expression in hepatocytes and nonparenchymal cells. ROS formation seems to contribute to early hepatocellular injury but also serves as an important trigger for HO-1 gene expression in parenchymal cells, which confers delayed protection after H/R.

Inhibition of Fas receptor (CD95)-induced hepatic caspase activation and apoptosis by acetaminophen in mice

The mechanism of liver cell injury induced by an overdose of the analgesic acetaminophen (AAP) remains controversial. Recently, it was hypothesized that a significant number of hepatocytes die by apoptosis. Since caspases have been implicated as critical signal and effector proteases in apoptosis, we investigated their potential role in the pathophysiology of AAP-induced liver injury. Male C3Heb/FeJ mice were fasted overnight and then treated with 500 mg/kg AAP. Liver injury became apparent at 4 h and was more severe at 6 h (plasma ALT activities: 4110 +/- 320 U/liter; centrilobular necrosis). DNA fragmentation increased parallel to the increase of plasma ALT values. At 6 h there was a 420% increase of DNA fragmentation and a 74-fold increase of terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL)-positive cells located predominantly around central veins. However, the activity of the proapoptotic caspase-3 was not increased at any time after AAP. In contrast, injection of the anti-Fas antibody Jo-2 (positive control) caused a 28-fold increase of caspase-3 activity and severe DNA fragmentation before significant ALT release. Treatment with the caspase inhibitor ZVAD-CHF2 had no effect on AAP toxicity but completely prevented Jo-mediated apoptosis. In contrast, Jo-induced caspase activation and apoptosis could be inhibited by AAP treatment in a time- and dose-dependent manner. We conclude that AAP-induced DNA fragmentation does not involve caspases, suggesting a direct activation of endonucleases through elevated Ca2+ levels. In addition, electrophilic metabolites of AAP may inactivate caspases or their activation pathway. This indicates that AAP metabolism has the potential to inhibit signal transduction mechanisms of receptor-mediated apoptosis.

Prevention of Kupffer cell-induced oxidant injury in rat liver by atrial natriuretic peptide

The generation of reactive oxygen species (ROS) by activated Kupffer cells contributes to liver injury following liver preservation, shock, or endotoxemia. Pharmacological interventions to protect liver cells against this inflammatory response of Kupffer cells have not yet been established. Atrial natriuretic peptide (ANP) protects the liver against ischemia-reperfusion injury, suggesting a possible modulation of Kupffer cell-mediated cytotoxicity. Therefore, we investigated the mechanism of cytoprotection by ANP during Kupffer cell activation in perfused rat livers of male Sprague-Dawley rats. Activation of Kupffer cells by zymosan (150 microgram/ml) resulted in considerable cell damage, as assessed by the sinusoidal release of lactate dehydrogenase and purine nucleoside phosphorylase. Cell damage was almost completely prevented by superoxide dismutase (50 U/ml) and catalase (150 U/ml), indicating ROS-related liver injury. ANP (200 nM) reduced Kupffer cell-induced injury via the guanylyl cyclase-coupled A receptor (GCA receptor) and cGMP: mRNA expression of the GCA receptor was found in hepatocytes, endothelial cells, and Kupffer cells, and the cGMP analog 8-bromo-cGMP (8-BrcGMP; 50 microM) was as potent as ANP in protecting from zymosan-induced cell damage. ANP and 8-BrcGMP significantly attenuated the prolonged increase of hepatic vascular resistance when Kupffer cell activation occurred. Furthermore, both compounds reduced oxidative cell damage following infusion of H2O2 (500 microM). In contrast, superoxide anion formation of isolated Kupffer cells was not affected by ANP and only moderately reduced by 8-BrcGMP. In conclusion, ANP protects the liver against Kupffer cell-related oxidant stress. This hormonal protection is mediated via the GCA receptor and cGMP, suggesting that the cGMP receptor plays a critical role in controlling oxidative cell damage. Thus ANP signaling should be considered as a new pharmacological target for protecting liver cells against the inflammatory response of activated Kupffer cells without eliminating the vital host defense function of these cells.

Mechanisms of reperfusion injury after warm ischemia of the liver

The review highlights recent advances in our understanding of basic mechanisms of reperfusion injury after warm hepatic ischemia. Kupffer cells play a central role as the initial cytotoxic cell type and as a source of many proinflammatory mediators. Subsequently, neutrophils are activated and recruited into the liver. Factors and conditions are outlined that determine whether neutrophils undergo apoptosis without causing damage or migrate out of the sinusoids and attack parenchymal cells. In addition to the inevitable inflammatory response during reperfusion, microcirculatory perfusion failure, due to an imbalance between the actions of vasodilators and vasoconstrictors, also has a serious impact on reperfusion injury. A better understanding of the basic pathophysiology will reveal potential targets for therapeutic interventions and will show us how to avoid risk factors that may aggravate reperfusion injury.

Glutathione peroxidase-deficient mice are more susceptible to neutrophil-mediated hepatic parenchymal cell injury during endotoxemia: importance of an intracellular oxidant stress

Neutrophils contribute to hepatocellular injury in a number of acute inflammatory reactions. However, the molecular mechanism of parenchymal cell injury remains controversial. To address the issue of whether or not reactive oxygen species (ROS) are important in the injury process, we used the galactosamine/endotoxin (Gal/ET) model of acute liver failure, which involves a neutrophil-mediated parenchymal cell injury. In C3Heb/FeJ mice, Gal/ET induced a significant increase of hepatic and plasma levels of glutathione disulfide (GSSG), an indicator of oxidant stress, selectively during the neutrophil-mediated injury phase. In glutathione peroxidase-deficient mice (Gpx1(-/-)), Gal/ET or Gal/tumor necrosis factor alpha (TNF-alpha) caused more severe neutrophil-mediated liver injury compared with wild-type animals. However, there was no significant difference in other critical parameters, e.g., activation of the transcription factor, nuclear factor-kappaB (NF-kappaB), and soluble intercellular adhesion molecule-1 (sICAM-1), parenchymal cell apoptosis, and neutrophil sequestration in the liver. Our results suggest that neutrophil-derived ROS are responsible for an intracellular oxidant stress in hepatocytes after Gal/ET treatment. Because of the higher susceptibility of Gpx1(-/-) mice to a neutrophil-mediated injury, we conclude that peroxides generated by neutrophils diffused into hepatocytes and contributed to parenchymal cell death in vivo. Thus, strengthening defense mechanisms against ROS in target cells can attenuate excessive inflammatory injury without affecting host defense reactions.

Parenchymal cell apoptosis as a signal for sinusoidal sequestration and transendothelial migration of neutrophils in murine models of endotoxin and Fas-antibody-induced liver injury

Endotoxin (ET) induces neutrophil sequestration in hepatic sinusoids, the activation of proinflammatory transcription factors (nuclear factor KB [NF-kappaB]) with up-regulation of adhesion molecules on sinusoidal endothelial cells and hepatocytes. However, if galactosamine (Gal) is co-administered with ET, neutrophils transmigrate and attack parenchymal cells. This suggests that a signal from parenchymal cells triggers neutrophil transmigration. In this study, we tested the hypothesis that parenchymal cell apoptosis may induce neutrophil transendothelial migration in the Gal/ET model. Treatment of C3Heb/FeJ mice with 700 mg/kg Gal and 100 microg/kg ET induced tumor necrosis factor alpha (TNF-alpha) formation (13.25 +/- 0.75 ng/mL) and hepatic NF-kappaB activation at 90 minutes; the generation of the C-X-C chemokine KC (2.86 +/- 0.30 ng/mL at 5 hours); sinusoidal neutrophil sequestration (380 +/- 21 polymorphonuclear leukocytes/50 high-power fields) and apoptosis (925% +/- 29% increase of DNA fragmentation; and a 45-fold increase of terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL)-positive cells) at 6 hours, followed by transmigration of neutrophils and development of substantial necrosis (38% +/- 3% of hepatocytes; alanine transaminase [ALT]: 1,500 +/- 300 U/L) at 7 hours. Administration of uridine (1,000 mg/kg) did not reduce plasma levels of TNF-alpha and KC, NF-kappaB activation, or polymorphonuclear leukocyte sequestration, but attenuated apoptosis by 90% to 94%. In these livers, neutrophils did not transmigrate and liver injury was prevented (necrosis: < 5%; ALT: 40 +/- 3 U/L). However, massive apoptosis and liver injury initiated by the anti-Fas antibody, Jo2, did not recruit neutrophils into the liver. We conclude that excessive parenchymal cell apoptosis represents an important signal for transmigration of primed neutrophils sequestered in sinusoids during endotoxemia in vivo. However, apoptosis per se does not cause neutrophil sequestration in the liver vasculature.

Differential effect of 2-aminoethyl-isothiourea, an inhibitor of the inducible nitric oxide synthase, on microvascular blood flow and organ injury in models of hepatic ischemia-reperfusion and endotoxemia

The vasodilator nitric oxide (NO) is involved in the regulation of systemic blood pressure and local organ blood flow. Inhibitors of the constitutively expressed nitric oxide synthase in endothelial cells (eNOS), e.g., Nomega-nitro-L-arginine methyl ester hydrochloride (L-NAME), aggravated liver injury in a variety of models. On the other hand, inhibitors of the inducible NOS (iNOS), e.g., 2-aminoethyl-isothiourea (AET), were found to be beneficial during endotoxemia. The aim of this investigation was to study the effect of AET compared with L-NAME on liver microvascular blood flow and injury in more complex models with multiple insults, i.e., ischemia (20 min)-reperfusion (8 h) in combination with .5 mg/kg endotoxin (IRE). Male Fisher rats were treated with 10 mg/kg AET or L-NAME and subjected to IRE. At 8 h, liver injury (plasma ALT: 1320+/-164 U/L) was significantly increased in AET-treated (5,018+/-1,379 U/L) and L-NAME-treated groups (2,429+/-228 U/L). Each inhibitor attenuated microvascular blood flow (assessed by laser Doppler flowmetry) to a similar degree. In striking contrast, AET completely reversed the endotoxin-induced impairment of the microvascular blood flow and significantly protected against an endotoxin-induced liver injury (plasma ALT: 3,007+/-268 U/L (ET); 460+/-39 U/L (ET+AET)). Infusion of endothelin-1 reduced microvascular blood flow by 50-60% and caused liver injury. Our data demonstrated that an inhibitor of eNOS (L-NAME) has a consistent detrimental effect on liver injury during ischemia-reperfusion and endotoxemia mainly because it can cause additional ischemia by reducing the microvascular blood flow. However, selective inhibitors of iNOS (AET) can impair hepatic blood flow and aggravate the injury or improve blood flow and attenuate organ injury depending on the experimental model. These results suggest that iNOS inhibitors may not be universally beneficial and should be tested in a variety of experimental models of sepsis/endotoxemia before used in clinical settings.

Role of PECAM-1 (CD31) in neutrophil transmigration in murine models of liver and peritoneal inflammation

Platelet endothelial cell adhesion molecule-1 (PECAM-1) is thought to be critical for transendothelial migration of leukocytes, including neutrophils. Because neutrophil-mediated liver injury during endotoxemia is dependent on transmigration, we investigated the role of PECAM-1 in the pathophysiology of endotoxin-induced liver injury. Male C3Heb/FeJ mice were treated with galactosamine (Gal) and endotoxin (ET) (700 mg/kg Gal/100 micrograms/kg ET), and liver sections were stained for PECAM-1 expression. Control livers showed the presence of PECAM-1 on endothelial cells of large vessels but not in sinusoids. Gal/ET treatment did not change the expression pattern of PECAM-1. Gal/ET-induced liver injury (area of necrosis: 38 +/- 3%) was not attenuated by treatment with 3 mg/kg of the antimurine PECAM-1 antibody 2H8. The antibody had no effect on sequestration and transmigration of neutrophils in sinusoids or the margination of neutrophils in large vessels. In contrast, 2H8 inhibited glycogen-induced neutrophil migration into the peritoneum by 74%; this effect correlated with PECAM-1 expression in the intestinal vasculature. Thus PECAM-1 is neither expressed nor inducible in hepatic sinusoids and is consequently not involved in neutrophil transmigration in the liver during endotoxemia. On the other hand, expression of PECAM-1 in mesenteric veins is critical for peritoneal neutrophil accumulation.

Activation of caspase 3 (CPP32)-like proteases is essential for TNF-alpha-induced hepatic parenchymal cell apoptosis and neutrophil-mediated necrosis in a murine endotoxin shock model

Endotoxin (ET)-induced liver failure is characterized by parenchymal cell apoptosis and inflammation leading to liver cell necrosis. Members of the caspase family have been implicated in the signal transduction pathway of apoptosis. The aim of this study was to characterize ET-induced hepatic caspase activation and apoptosis and to investigate their effect on neutrophil-mediated liver injury. Treatment of C3Heb/FeJ mice with 700 mg/kg galactosamine (Gal) and 100 microg/kg Salmonella abortus equi ET increased caspase 3-like protease activity (Asp-Val-Glu-Asp-substrate) by 1730 +/- 140% at 6 h. There was a parallel enhancement of apoptosis (assessed by DNA fragmentation ELISA and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assay). In contrast, activity of caspase 1 (IL-1beta-converting enzyme)-like proteases (Tyr-Val-Ala-Asp-substrate) did not change throughout the experiment. Caspase 3-like protease activity and apoptosis was not induced by Gal/ET in ET-resistant mice (C3H/HeJ). Furthermore, only murine TNF-alpha but not IL-1alphabeta increased caspase activity and apoptosis. Gal/ET caused neutrophil-dependent hepatocellular necrosis at 7 h (area of necrosis, 45 +/- 3%). Delayed treatment with the caspase 3-like protease inhibitor Z-Val-Ala-Asp-CH2F (Z-VAD) (10 mg/kg at 3 h) attenuated apoptosis by 81 to 88% and prevented liver cell necrosis (< or = 5%). Z-VAD had no effect on the initial inflammatory response, including the sequestration of neutrophils in sinusoids. However, Z-VAD prevented neutrophil transmigration and necrosis. Our data indicate that activation of the caspase 3 subfamily of cysteine proteases is critical for the development of parenchymal cell apoptosis. In addition, excessive hepatocellular apoptosis can be an important signal for transmigration of primed neutrophils sequestered in sinusoids.

Increased P-selectin gene expression in the liver vasculature and its role in the pathophysiology of neutrophil-induced liver injury in murine endotoxin shock

We studied the role of P-selectin, an adhesion molecule known to be important for neutrophil localization to sites of inflammation, in a model of inflammatory liver injury. Male C3Heb/FeJ (ET-sensitive) mice treated with 700 mg/kg galactosamine and 100 microg/kg Salmonella abortus equi endotoxin (Gal/ET), murine tumor necrosis factor alpha (TNF-alpha, 15 microg/kg), or interleukin-1 (IL-1, 13-23 microg/kg), showed increased P-selectin mRNA levels in the liver. In contrast, C3H/HeJ (ET-resistant) mice responded only to cytokines with P-selectin mRNA formation. Whereas no P-selectin expression was detectable in control livers, there was temporary staining of endothelium in large blood vessels but not in sinusoids between 3 and 5 h after ET, TNF-alpha, or IL-1 treatment. Severe liver injury induced by Gal/ET at 7 h was not inhibited by an anti-P-selectin antibody in C3Heb/FeJ mice or in P-selectin-deficient animals. Sequestration of neutrophils in sinusoids, i.e. those neutrophils that have been identified as critical for the injury, was not affected by the antibody or in P-selectin-deficient mice. However, the temporary margination in portal and post-sinusoidal venules was reduced by 75% in anti-P-selectin antibody-treated animals and by 51% in P-selectin-deficient mice. We conclude that hepatic P-selectin gene transcription in vivo involves cytokines. However, blocking P-selectin neither attenuated sinusoidal neutrophil sequestration nor prevented neutrophil-induced liver injury during endotoxin shock but attenuated neutrophil margination in larger vessels. Thus, our data demonstrate similarities and fundamental differences in the requirement for adhesion molecules to localize neutrophils in the liver vasculature compared to other organs during an inflammatory response.

Cell adhesion and migration. III. Leukocyte adhesion and transmigration in the liver vasculature

Leukocytes, i.e., neutrophils, monocytes, and lymphocytes, can accumulate in the hepatic vasculature and contribute to the pathophysiology of various liver diseases. Recently, significant progress has been made in the understanding of the basic mechanisms of neutrophil infiltration and cytotoxicity in the liver. However, there are a substantial number of unresolved issues. This article describes the current knowledge and the gaps in our understanding of mechanisms of neutrophil sequestration in sinusoids and venules, adhesion to endothelial cells, and transmigration and adherence to parenchymal cells. From these data, it is clear that assumptions regarding the roles of adhesion molecules in the liver may be misleading if drawn from studies of peripheral vascular beds. Greater insight into these mechanisms is critical for the development of selective therapeutic strategies that attenuate excessive inflammatory responses without compromising the vital host defense.

Cellular adhesion molecules: regulation and functional significance in the pathogenesis of liver diseases

Adhesion molecules are cell surface glycoproteins that are critical for the localization of leukocytes at sites of inflammation. This review discusses the current knowledges of adhesion molecule expression in normal liver and its upregulation on individual liver cell types during liver inflammation. Cytokines, chemokines, complement factors, and lipid-derived mediators are critical for increased gene transcription and activation of constitutively expressed adhesion molecules. The specific role of selectins, integrins, and members of the immunoglobulin gene superfamily in sinusoidal and venular leukocyte sequestration, transendothelial migration, and adherence to target cells in the liver is described. Increased understanding of these basic mechanisms of communication between resident liver cells and infiltrating leukocytes (neutrophils, lymphocytes, macrophages) not only advances our insight into the pathophysiology of hepatic inflammation but also identifies promising new targets for therapeutic interventions and expands the spectrum of diagnosis and treatment of liver diseases, including alcoholic hepatitis and cirrhosis, viral hepatitis, ischemia-reperfusion injury (transplantation, tumor resection, shock), sepsis- or endotoxin-induced liver injury, acute and chronic rejection, primary biliary cirrhosis, and primary sclerosing cholangitis.

Transcriptional activation of vascular cell adhesion molecule-1 gene in vivo and its role in the pathophysiology of neutrophil-induced liver injury in murine endotoxin shock

Polymorphonuclear leukocytes (neutrophils) can cause hepatic parenchymal cell injury during endotoxin (ET) shock. Because adhesion molecules are critical for inflammatory cell damage, the role of vascular cell adhesion molecule-1 (VCAM-1) was studied in the pathophysiology of ET shock. ET-sensitive mice (C3Heb/FeJ) were treated with 700 mg/kg galactosamine in combination with 100 microg/kg Salmonella abortus equi ET, 15 microg/kg TNF-alpha, or 13 to 23 microg/kg IL-1. VCAM-1 mRNA formation was strongly activated in animals treated with ET, TNF-alpha, or IL-1. In contrast, only TNF-alpha and IL-1, not ET, induced VCAM-1 gene transcription in livers of ET-resistant mice (C3H/HeJ). Immunohistochemistry and isolation of liver cells during endotoxemia indicated that VCAM-1 mRNA and protein were only formed in endothelial cells and Kupffer cells, not in hepatocytes. Galactosamine/ET induced neutrophil accumulation in sinusoids (515 +/- 30 neutrophils/50 high power fields) followed by transmigration at 7 h. At that time, severe liver injury was observed (necrosis, 53 +/- 5%). An anti-VCAM-1 Ab (3 mg/kg) attenuated the area of necrosis by 60%. The Ab reduced neutrophil transmigration by 84%, but had no effect on the total number of cells in the liver vasculature. Flow cytometric analysis identified the presence of very late Ag-4 on mouse peripheral neutrophils. Our data demonstrated cytokine-dependent VCAM-1 gene transcription and protein expression in the liver during endotoxemia. Neutrophils were able to use very late Ag-4/VCAM-1 interactions to transmigrate into liver parenchyma in vivo. Preventing transmigration by blocking VCAM-1 protected hepatocytes against neutrophil-induced injury.

Transcriptional activation of vascular cell adhesion molecule-1 gene in vivo and its role in the pathophysiology of neutrophil-induced liver injury in murine endotoxin shock

Polymorphonuclear leukocytes (neutrophils) can cause hepatic parenchymal cell injury during endotoxin (ET) shock. Because adhesion molecules are critical for inflammatory cell damage, the role of vascular cell adhesion molecule-1 (VCAM-1) was studied in the pathophysiology of ET shock. ET-sensitive mice (C3Heb/FeJ) were treated with 700 mg/kg galactosamine in combination with 100 microg/kg Salmonella abortus equi ET, 15 microg/kg TNF-alpha, or 13 to 23 microg/kg IL-1. VCAM-1 mRNA formation was strongly activated in animals treated with ET, TNF-alpha, or IL-1. In contrast, only TNF-alpha and IL-1, not ET, induced VCAM-1 gene transcription in livers of ET-resistant mice (C3H/HeJ). Immunohistochemistry and isolation of liver cells during endotoxemia indicated that VCAM-1 mRNA and protein were only formed in endothelial cells and Kupffer cells, not in hepatocytes. Galactosamine/ET induced neutrophil accumulation in sinusoids (515 +/- 30 neutrophils/50 high power fields) followed by transmigration at 7 h. At that time, severe liver injury was observed (necrosis, 53 +/- 5%). An anti-VCAM-1 Ab (3 mg/kg) attenuated the area of necrosis by 60%. The Ab reduced neutrophil transmigration by 84%, but had no effect on the total number of cells in the liver vasculature. Flow cytometric analysis identified the presence of very late Ag-4 on mouse peripheral neutrophils. Our data demonstrated cytokine-dependent VCAM-1 gene transcription and protein expression in the liver during endotoxemia. Neutrophils were able to use very late Ag-4/VCAM-1 interactions to transmigrate into liver parenchyma in vivo. Preventing transmigration by blocking VCAM-1 protected hepatocytes against neutrophil-induced injury.

Mechanisms of neutrophil-induced parenchymal cell injury

Neutrophils are involved in organ damage induced by an excessive acute inflammatory response after ischemia-reperfusion, trauma, and sepsis. In addition to causing vascular injury, neutrophils can transmigrate and attack parenchymal cells. This review summarizes recent advances in our understanding of neutrophil-induced parenchymal cell injury using the liver as an example. Reviewed are the mechanisms of neutrophil sequestration in the hepatic vasculature, transendothelial migration, adherence to hepatic parenchymal cells, and mechanisms of cytotoxicity. Discussed are the involvement of various adhesion molecules in these processes, the role of cytokines and chemokines in the pathophysiology, as well as the effects of proteases and reactive oxygen species released by neutrophils. The emerging understanding of the basic mechanisms of neutrophil-induced organ damage is critical for the development of therapeutic strategies to attenuate excessive acute inflammatory responses without compromising essential host defense mechanisms.

Neutrophil margination and extravasation in sinusoids and venules of liver during endotoxin-induced injury

Neutrophils contribute to liver damage during endotoxin shock. The objective of this investigation was to document where neutrophils localize in the hepatic vasculature and whether they migrate out of sinusoids or postsinusoidal venules. A well-characterized model of galactosamine and endotoxin shock and immunostaining for neutrophil-associated migration inhibition factor-related protein complex 8/14 S100 calcium-binding proteins were used. Treatment of C3Heb/FeJ mice with 100 micrograms/kg Salmonella abortus equi endotoxin alone or in combination with 700 mg/kg galactosamine induced a time-dependent increase of neutrophil margination in sinusoids and postsinusoidal venules at 4 h. The number of venular neutrophils decreased in both groups at later time points without evidence for transmigration. Extravasation of neutrophils was only observed from sinusoids in galactosamine plus endotoxin-treated animals between 4 and 7 h, which correlated with parenchymal cell injury. After endotoxin alone, large numbers of neutrophils remained sequestered in sinusoids without injury. These data suggest that neutrophils cause hepatocellular injury during endotoxemia after extravasation and are less likely to cause damage when sequestered in the vasculature. In the liver, neutrophils migrate out of sinusoids and not out of postsinusoidal venules.

Inhibition of NF-kappa B activation by dimethyl sulfoxide correlates with suppression of TNF-alpha formation, reduced ICAM-1 gene transcription, and protection against endotoxin-induced liver injury

The effect of the free radical scavenger dimethyl sulfoxide (DMSO) on activation of the nuclear transcription factor kappa B (NF-kappa B) was investigated in an experimental model of endotoxin-induced liver failure. In galactosamine-sensitized C3Heb/FeJ mice, DMSO (10 mL/kg) effectively inhibited endotoxin-induced hepatic NF-kappa B activation, suppressed TNF-alpha levels in plasma by 86%, attenuated intercellular adhesion molecule-1 (ICAM-1) mRNA formation, blocked hepatic neutrophil accumulation by 79%, and reduced liver injury by 80%. In galactosamine-sensitized mice treated with 20 micrograms/kg murine TNF-alpha, DMSO moderately reduced hepatic NF-kappa B and decreased ICAM-1 mRNA formation and liver injury by 83%, but had no significant effect on hepatic neutrophil accumulation. Thus, DMSO was able to inhibit, at least in part, two critical NF-kappa B-dependent steps in the pathophysiology, i.e., TNF-alpha formation and ICAM-1 gene transcription. Our data suggest the involvement of redox-sensitive events in the signal transduction pathway of NF-kappa B activation in the liver. Inhibition of NF-kappa B activation correlates with the reduced activation of proinflammatory genes in vivo and the subsequent attenuation of inflammatory liver injury. Thus, antioxidants that are NF-kappa B inhibitors may have therapeutic potential in endotoxin shock and sepsis.

Sequestration of neutrophils in the hepatic vasculature during endotoxemia is independent of beta 2 integrins and intercellular adhesion molecule-1

Antibodies against cellular adhesion molecules protect against neutrophil-induced hepatic injury during ischemia-reperfusion and endotoxemia. To test if beta 2 integrins on neutrophils and intercellular adhesion molecule-1 (ICAM-1) on endothelial cells are involved in neutrophil sequestration in the hepatic vasculature, neutrophil accumulation in the liver was characterized during the early phase of endotoxemia. Intravenous injection of Salmonella enteritidis endotoxin induced a dose-dependent activation of complement, tumor necrosis factor-alpha (TNF-alpha) formation, and an increase of hepatic neutrophils with maximal numbers at 5 mg/kg (90 min: 339 +/- 14 cells/50 high power fields; controls: 18 +/- 2). Administration of 15 micrograms/kg TNF-alpha and intravascular complement activation with cobra venom factor (75 micrograms/kg) had additive effects on hepatic neutrophil accumulation compared with each mediator alone. Monoclonal antibodies (2 mg/kg) to ICAM-1 and the alpha-chain (CD11a, CD11b) or the beta-chain (CD18) of beta 2 integrins had no significant effect on hepatic neutrophil count after endotoxin. In contrast, these antibodies inhibited peritoneal neutrophil infiltration in response to glycogen administration by 28% (CD11b), 60% (CD11a, ICAM-1), and 92% (CD18). Our data suggest that TNF-alpha and complement factors contribute to hepatic neutrophil sequestration during the early phase of endotoxemia. Despite the fact that these inflammatory mediators can up-regulate integrins and ICAM-1, these adhesion molecules are not necessary for neutrophil accumulation in hepatic sinusoids. The protective effect of these antibodies against neutrophil-induced liver injury appears to be due to inhibition of transendothelial migration and adherence to parenchymal cells.

Effect of endotoxin-enhanced hepatic reperfusion injury on endothelium-dependent relaxation in rat aorta

We have investigated endothelial function in a two-hit model of multiple organ failure. Male Fischer rats were subjected to 20 min of partial hepatic ischemia followed by reperfusion and administration of .5 mg/kg Salmonella enteritidis endotoxin at 30 min of reperfusion. After either 4 or 24 h of reperfusion, rings of aorta were prepared and suspended in bioassay baths, contracted with phenylephrine, and examined for endothelium-dependent relaxation in response to acetylcholine and endothelium-independent relaxation using nitroglycerin. Endothelium-dependent relaxation to acetylcholine was impaired in rings from animals exposed to endotoxin-enhanced reperfusion injury at both 4 h and 24 h. At 24 h of reperfusion the EC50 for acetylcholine relaxation was significantly increased from 45 +/- 8 nM to 258 +/- 105 nM. Endothelium-independent relaxation to nitroglycerin was not affected. The 21-aminosteroid Tirilazad mesylate (U-74006F) prevented endothelial dysfunction; at 24 h of reperfusion the EC50 for acetylcholine relaxation in U-74006F-treated animals was 55 +/- 8 nM. Thus, endothelial function is impaired in this model of multiple organ failure and this impairment is prevented by Tirilazad mesylate.

Mechanisms of inflammatory liver injury: adhesion molecules and cytotoxicity of neutrophils

During recent years, increasing experimental evidence has suggested that hepatic nonparenchymal cells, in particular Kupffer cells and neutrophils, can contribute significantly to the pathogenesis of liver injury in various chemical toxicities. Neutrophils are central to the mechanism of injury after hepatic ischemia reperfusion and endotoxemia. In this symposium summary, an overview of critical aspects of neutrophil-dependent liver injury is presented. A general introduction to the involvement of adhesion molecules in neutrophil rolling and transendothelial migration is provided. Mediators and mechanisms of neutrophil sequestration in the liver vasculature, extravasation, and adherence-dependent cytotoxicity are discussed using the examples of endotoxin-induced hepatic failure and ischemia-reperfusion injury. These processes involve a complex network of inflammatory mediators including cytokines, chemokines, and lipid-derived compounds. The role of neutrophil-derived cytotoxic mediators, e.g., reactive oxygen and proteases, in the molecular mechanisms of parenchymal cell injury is discussed. Furthermore, interactions between neutrophils and contractile, perisinusoidal, stellate cells that influence microvascular blood flow in the liver are discussed. Results of these and other investigations are leading to increased understanding of the complex interactions between neutrophils and tissues that result in injury.

The 21-aminosteroid tirilazad mesylate protects against liver injury via membrane stabilization not inhibition of lipid peroxidation

Whether tirilazad mesylate (U-74006F) protects against liver injury by inhibition of lipid peroxidation or by cell membrane stabilization was investigated. In male Fischer rats subjected to 20 min of hepatic ischemia followed by reperfusion and injection of 0.5 mg/kg Salmonella enteritidis endotoxin, developed of liver injury was accompanied by lipid peroxidation, as indicated by 81 to 184% increases in hepatic 8-, 9-, 11-, and 12-hydroxyeicosatetraenoic acid content and a 85% increase of plasma F2-isoprostane concentrations at 4 h of reperfusion. Treatment with U-74006F (two bolus doses of 3 mg/kg each; the first dose was injected i.v. 30 min before ischemia and the second dose, at the time of reflow) reduced hepatic injury by 60% but had no significant effect on either parameter of lipid peroxidation. In contrast, U-74006F treatment attenuated liver injury and lipid peroxidation at 24 h reperfusion. Pretreatment with U-74006F in vivo had no effect on lipid peroxidation and liver injury in vitro during perfusion with tert-butylhydroperoxide. However, U-74006F protected hepatocytes significantly against membrane damage induced by cell swelling due to perfusion with hypotonic medium or ischemia-reperfusion. These data support the conclusion that U-74006F enhances the resistance of liver cell membranes to injury by its membrane-stabilizing effect and not by directly scavenging free radicals in vivo. However, the cytoprotective effect of U-74006F can under certain circumstances inhibit recruitment and activation of inflammatory cells, which will then reduce the oxidant stress and lipid peroxidation in the liver.