Hypothermia attenuates oxidative/nitrosative stress, encephalopathy and brain edema in acute (ischemic) liver failure

Low-dose ketamine improves animals' locomotor activity and decreases brain oxidative stress and inflammation in ammonia-induced neurotoxicity

Ammonium ion (NH4 + ) is the major suspected molecule responsible for neurological complications of hepatic encephalopathy (HE). No specific pharmacological action for NH4 + -induced brain injury exists so far. Excitotoxicity is a well-known phenomenon in the brain of hyperammonemic cases. The hyperactivation of the N-Methyl- d-aspartate (NMDA) receptors by agents such as glutamate, an NH4 + metabolite, could cause excitotoxicity. Excitotoxicity is connected with events such as oxidative stress and neuroinflammation. Hence, utilizing NMDA receptor antagonists could prevent neurological complications of NH4 + neurotoxicity. In the current study, C57BL6/J mice received acetaminophen (APAP; 800 mg/kg, i.p) to induce HE. Hyperammonemic animals were treated with ketamine (0.25, 0.5, and 1 mg/kg, s.c) as an NMDA receptor antagonist. Animals' brain and plasma levels of NH4 + were dramatically high, and animals' locomotor activities were disturbed. Moreover, several markers of oxidative stress were significantly increased in the brain. A significant increase in brain tissue levels of TNF-α, IL-6, and IL-1β was also detected in hyperammonemic animals. It was found that ketamine significantly normalized animals' locomotor activity, improved biomarkers of oxidative stress, and decreased proinflammatory cytokines. The effects of ketamine on oxidative stress biomarkers and inflammation seem to play a key role in its neuroprotective mechanisms in the current study.

The Glymphatic System May Play a Vital Role in the Pathogenesis of Hepatic Encephalopathy: A Narrative Review

Hepatic encephalopathy (HE) is a neurological complication of liver disease resulting in cognitive, psychiatric, and motor symptoms. Although hyperammonemia is a key factor in the pathogenesis of HE, several other factors have recently been discovered. Among these, the impairment of a highly organized perivascular network known as the glymphatic pathway seems to be involved in the progression of some neurological complications due to the accumulation of misfolded proteins and waste substances in the brain interstitial fluids (ISF). The glymphatic system plays an important role in the clearance of brain metabolic derivatives and prevents aggregation of neurotoxic agents in the brain ISF. Impairment of it will result in aggravated accumulation of neurotoxic agents in the brain ISF. This could also be the case in patients with liver failure complicated by HE. Indeed, accumulation of some metabolic by-products and agents such as ammonia, glutamine, glutamate, and aromatic amino acids has been reported in the human brain ISF using microdialysis technique is attributed to worsening of HE and correlates with brain edema. Furthermore, it has been reported that the glymphatic system is impaired in the olfactory bulb, prefrontal cortex, and hippocampus in an experimental model of HE. In this review, we discuss different factors that may affect the function of the glymphatic pathways and how these changes may be involved in HE.

Hyperammonemia in Inherited Metabolic Diseases

Ammonia is a neurotoxic compound which is detoxified through liver enzymes from urea cycle. Several inherited or acquired conditions can elevate ammonia concentrations in blood, causing severe damage to the central nervous system due to the toxic effects exerted by ammonia on the astrocytes. Therefore, hyperammonemic patients present potentially life-threatening neuropsychiatric symptoms, whose severity is related with the hyperammonemia magnitude and duration, as well as the brain maturation stage. Inherited metabolic diseases caused by enzymatic defects that compromise directly or indirectly the urea cycle activity are the main cause of hyperammonemia in the neonatal period. These diseases are mainly represented by the congenital defects of urea cycle, classical organic acidurias, and the defects of mitochondrial fatty acids oxidation, with hyperammonemia being more severe and frequent in the first two groups mentioned. An effective and rapid treatment of hyperammonemia is crucial to prevent irreversible neurological damage and it depends on the understanding of the pathophysiology of the diseases, as well as of the available therapeutic approaches. In this review, the mechanisms underlying the hyperammonemia and neurological dysfunction in urea cycle disorders, organic acidurias, and fatty acids oxidation defects, as well as the therapeutic strategies for the ammonia control will be discussed.

Albumin infusion may decrease the incidence and severity of overt hepatic encephalopathy in liver cirrhosis

BACKGROUND

The role of human albumin infusion for the prevention and treatment of overt hepatic encephalopathy (HE) in liver cirrhosis remains unclear.

RESULTS

Among the 708 patients without pre-existing overt HE, albumin infusion significantly decreased the incidence of overt HE (4.20% versus 12.70%, P<0.001) and in-hospital mortality (1.70% versus 5.40%, P=0.008). Among the 182 patients with overt HE at admission or during hospitalization, albumin infusion significantly improved overt HE (84.60% versus 68.10%, P=0.009) and decreased in-hospital mortality (7.70% versus 19.80%, P=0.018). Meta-analysis of 6 studies found that albumin infusion might decrease the risk of overt HE (OR=1.63, P=0.07), but the difference was not statistically significant. Meta-analysis of 3 studies found that albumin infusion significantly improved overt HE (OR=2.40, P=0.04).

CONCLUSIONS

Based on the results of our retrospective study and meta-analysis, albumin infusion might prevent from the occurrence of overt HE and improve the severity of overt HE in cirrhosis. Our retrospective study also suggested that albumin infusion improved the outcomes of cirrhotic patients regardless of overt HE.

METHODS

Cirrhotic patients consecutively admitted between January 2010 and June 2014 were considered in a retrospective study. A 1:1 propensity score matching analysis was performed. Additionally, publications regarding albumin infusion for the management of overt HE were systematically searched. Meta-analyses were performed by random-effect model. Odds ratio (OR) was calculated.

Changes in bilirubin in infants with hypoxic-ischemic encephalopathy

Antioxidant properties of bilirubin have been reported in many studies. We hypothesized that bilirubin might be involved in neuroprotection mechanisms against oxidative stress in infants with hypoxic-ischemic encephalopathy (HIE) and that total serum bilirubin (TSB) might increase in these patients. We retrospectively studied infants with gestational age ≥ 35 weeks and birth weight ≥ 1800 g who were admitted to the neonatal intensive care unit (NICU) with a diagnosis of moderate-to-severe HIE and received or did not receive therapeutic hypothermia. We evaluated peak TSB and changes of mean TSB in these patients in comparison with a control group of infants admitted to the NICU with diagnoses other than HIE. Peak and mean TSB values were lower in the no hypothermia and hypothermia groups in comparison with the control group, while differences were not noted between infants who received hypothermia or did not. Regression analysis showed that HIE and hypothermia significantly reduced the risk of developing TSB values higher than median value (> 8.4 mg/dL) in our population.Conclusion: Peak and mean TSB values were lower in infants with moderate-to-severe HIE than in control infants. HIE and hypothermia independently decreased TSB. These results exclude a TSB increase as a neuroprotective mechanism in infants with HIE. We speculated that low TSB values in infants with HIE could be due to hypoxic repression of HO expression and represent a defensive strategy for limiting brain injuries in these patients. What is Known: • The role of oxidative stress in the pathophysiology of hypoxic-ischemic encephalopathy (HIE) has been elucidated in many studies, and other studies have demonstrated the antioxidant properties of bilirubin. • The potential neuroprotective role of bilirubin as antioxidant agent has never been evaluated in infants with HIE. What is New: • Mean total serum bilirubin (TSB) values are lower in infants with moderate-to-severe HIE than in control infants, since HIE and hypothermia independently decreased TSB. • An increase in bilirubin was not a neuroprotective mechanism in infants with HIE possibly because of hypoxic repression of HO expression as defensive strategy for limiting brain injuries.

Pathogenesis of hepatic encephalopathy in cirrhosis: the concept of synergism revisited

The concept of synergistic mechanisms as the pathophysiologic basis of hepatic encephalopathy started with the pioneering work of Les Zieve in Minneapolis some 60 years ago where synergistic actions of the liver-derived toxins ammonia, methanethiol, and octanoic acid were described. More recently, synergistic actions of ammonia and manganese, a toxic metal that is normally eliminated via the hepatobiliary route and shown to accumulate in brain in liver failure, on the glutamatergic neurotransmitter system were described. The current upsurge of interest in brain inflammation (neuroinflammation) in relation to the CNS complications of liver failure has added a third dimension to the synergy debate. The combined actions of ammonia, manganese and pro-inflammatory cytokines in brain in liver failure result in oxidative/nitrosative stress resulting from activation of glutamate (NMDA) receptors and consequent nitration of key brain proteins. One such protein, glutamine synthetase, the sole enzyme responsible for brain ammonia removal is nitrated and inactivated in brain in liver failure. Consequently, brain ammonia levels increase disproportionately resulting in alterations of brain excitability, impaired brain energy metabolism, encephalopathy and brain swelling. Experimental therapeutic approaches for which proof-of-principle has been established include the NMDA receptor antagonist memantine, N-acetyl cysteine (recently shown to have antioxidant properties at both hepatic and cerebral levels) and probiotics.

Relationship between oxidative stress and brain swelling in goldfish (Carassius auratus) exposed to high environmental ammonia

Buildups of ammonia can cause potentially fatal brain swelling in mammals, but such swelling is reversible in the anoxia- and ammonia-tolerant goldfish (Carassius auratus). We investigated brain swelling and its possible relationship to oxidative stress in the brain and liver of goldfish acutely exposed to high external ammonia (HEA; 5 mmol/l NH₄Cl) at two different acclimation temperatures (14°C, 4°C). Exposure to HEA at 14°C for 72h resulted in increased internal ammonia and glutamine concentrations in the brain, and it caused cellular oxidative damage in the brain and liver. However, oxidative damage was most pronounced in brain, in which there was a twofold increase in thiobarbituric acid-reactive substances, a threefold increase in protein carbonylation, and a 20% increase in water volume (indicative of brain swelling). Increased activities of catalase, glutathione peroxidase, and glutathione reductase in the brain suggested that goldfish upregulate their antioxidant capacity to partially offset oxidative stress during hyperammonemia at 14°C. Notably, acclimation to colder (4°C) water completely attenuated the oxidative stress response to HEA in both tissues, and there was no change in brain water volume despite similar increases in internal ammonia. We suggest that ammonia-induced oxidative stress may be responsible for the swelling of goldfish brain during HEA, but further studies are needed to establish a mechanistic link between reactive oxygen species production and brain swelling. Nevertheless, a high capacity to withstand oxidative stress in response to variations in internal ammonia likely explains why goldfish are more resilient to this stressor than most other vertebrates.

Induction of systemic oxidative stress leads to brain oedema in portacaval shunted rats

BACKGROUND & AIMS

The pathogenesis of hepatic encephalopathy (HE) is multifactorial and often associated with the development of brain oedema. In addition to ammonia playing a central role, systemic oxidative stress is believed to aggravate the neuropsychological effects of ammonia in patients with chronic liver disease (CLD). The aim of this study was to (i) induce systemic oxidative stress in hyperammonaemic portacaval anastomosed (PCA) rats by inhibiting the antioxidant glutathione using Dimethyl maleate (DEM) and (ii) investigate whether a synergistic relationship between ammonia and oxidative stress contributes to the pathogenesis of brain oedema in CLD.

METHODS

Four-week PCA and sham-operated rats received DEM (0.4-4 mg/kg/day) for the last 10 days before sacrifice when oxidative stress markers [reactive oxygen species (ROS) and malondialdehyde (MDA)] were assessed in blood and frontal cortex. Brain water content was measured using a specific gravimetric technique.

RESULTS

Dimethyl maleate induced an increase in ROS and MDA in the blood, but not in the brain, of the PCA rats, compared with non-treated PCA rats. This was accompanied with an increase in brain water content (PCA+DEM: 78.45 ± 0.13% vs. PCA: 77.38 ± 0.11%, P < 0.001). Higher doses of DEM induced systemic oxidative stress in sham-operated controls, but brain oedema did not develop.

CONCLUSIONS

Dimethyl maleate provoked systemic, not central, oxidative stress in PCA rats, resulting in the development of brain oedema. Independently, hyperammonaemia and systemic oxidative stress do not precipitate brain oedema; therefore, our findings sustain that a synergistic effect between hyperammonaemia and systemic oxidative stress is responsible for the development of brain oedema in HE.

Hypothermia activates adipose tissue to promote malignant lung cancer progression

Microenvironment has been increasingly recognized as a critical regulator of cancer progression. In this study, we identified early changes in the microenvironment that contribute to malignant progression. Exposure of human bronchial epithelial cells (BEAS-2B) to methylnitrosourea (MNU) caused a reduction in cell toxicity and an increase in clonogenic capacity when the temperature was lowered from 37°C to 28°C. Hypothermia-incubated adipocyte media promoted proliferation in A549 cells. Although a hypothermic environment could increase urethane-induced tumor counts and Lewis lung cancer (LLC) metastasis in lungs of three breeds of mice, an increase in tumor size could be discerned only in obese mice housed in hypothermia. Similarly, coinjections using differentiated adipocytes and A549 cells promoted tumor development in athymic nude mice when adipocytes were cultured at 28°C. Conversely, fat removal suppressed tumor growth in obese C57BL/6 mice inoculated with LLC cells. Further studies show hypothermia promotes a MNU-induced epithelial-mesenchymal transition (EMT) and protects the tumor cell against immune control by TGF-β1 upregulation. We also found that activated adipocytes trigger tumor cell proliferation by increasing either TNF-α or VEGF levels. These results suggest that hypothermia activates adipocytes to stimulate tumor boost and play critical determinant roles in malignant progression.

Oxidative stress: a systemic factor implicated in the pathogenesis of hepatic encephalopathy

Although ammonia is considered the main factor involved in the pathogenesis of hepatic encephalopathy (HE), it correlates well with the severity of HE in acute liver failure, but not in chronic liver disease. Oxidative stress is another factor believed to play a role in the pathogenesis of this syndrome; it represents an imbalance between the production and neutralization of reactive oxygen species, which leads to cellular dysfunction. In the setting of liver disease, oxidative stress represents a systemic phenomenon induced by several mechanisms: decreased antioxidant synthesis, increased systemic release of oxidant enzymes, generation of reactive oxygen species, and impaired neutrophil function. High ammonia concentrations induce cerebral oxidative stress, thus contributing to severe hepatic encephalopathy, as observed in acute liver failure. In chronic liver disease, significantly lower degrees of hyperammonemia (<500 μM) do not induce cerebral nor systemic oxidative stress. Data from both animal and human studies sustain that there is a synergistic effect between systemic oxidative stress, and ammonia that is implicated in the pathogenesis of hepatic encephalopathy.

Role of the heme oxygenase/carbon monoxide pathway in the pathogenesis and prevention of hepatic encephalopathy

Hepatic encephalopathy (HE) is a severe complication of liver cirrhosis and its pathogenesis has yet to be fully elucidated. Previous studies have demonstrated that heme oxygenase-1 (HO-1) is important in the induction of liver cirrhosis. The present study aimed to investigate the role of HO-1 in the pathogenesis of HE. Rats were divided into 5 treatment groups; sham, bile duct ligation (BDL), HE, zinc protoporphyrin (ZnPP) and cobalt protoporphyrin (CoPP). The levels of HO-1 were examined by western blotting and quantitative real-time PCR (qRT-PCR). Serum levels of carboxyhemoglobin (COHb), ammonia levels in the plasma and brain, brain water content and portal vein pressure (PVP) were also quantified. Aquaporin-4 expression levels were measured by immunohistochemistry and qRT-PCR. The results demonstrated that the levels of HO-1 in the brain and the serum levels of COHb were significantly increased in the HE group compared with the BDL group. Brain water content, PVP and ammonia levels in the plasma and brain were increased in the HE and CoPP groups; however, these were reduced following the treatment with ZnPP. The levels of AQP-4 expression and oxidative stress in the brain were reduced following treatment with ZnPP and increased following treatment with CoPP. In conclusion, following the inhibition of HO-1 expression, treatment with ZnPP improved HE due to reducing the expression levels of AQP-4 and oxidative stress. Therefore, ZnPP treatment may represent a novel therapeutic approach for HE.

Brain edema in acute liver failure and chronic liver disease: similarities and differences

Hepatic encephalopathy (HE) is a complex neuropsychiatric syndrome that typically develops as a result of acute liver failure or chronic liver disease. Brain edema is a common feature associated with HE. In acute liver failure, brain edema contributes to an increase in intracranial pressure, which can fatally lead to brain stem herniation. In chronic liver disease, intracranial hypertension is rarely observed, even though brain edema may be present. This discrepancy in the development of intracranial hypertension in acute liver failure versus chronic liver disease suggests that brain edema plays a different role in relation to the onset of HE. Furthermore, the pathophysiological mechanisms involved in the development of brain edema in acute liver failure and chronic liver disease are dissimilar. This review explores the types of brain edema, the cells, and pathogenic factors involved in its development, while emphasizing the differences in acute liver failure versus chronic liver disease. The implications of brain edema developing as a neuropathological consequence of HE, or as a cause of HE, are also discussed.

Combined therapy for acute hepatic failure

Acute hepatic failure (AHF) is a devastating clinical syndrome characterized by rapid impairment of liver functions and development of encephalopathy, multiple organ failure, and in most cases cerebral edema. AHF has a high mortality rate. Although advances in drug treatment, artificial liver and liver transplantation have significantly improved the prognosis of AHF, there is still a lack of effective treatment for AHF because of its complicated etiopathogenesis, rapid progression and less clinical knowledge about managing the disease. There is an urgent need to develop effective treatments for AHF. This article aims to review recent advances in the treatment of AHF.

Oxidative and nitrosative stress in ammonia neurotoxicity

Increased ammonia accumulation in the brain due to liver dysfunction is a major contributor to the pathogenesis of hepatic encephalopathy (HE). Fatal outcome of rapidly progressing (acute) HE is mainly related to cytotoxic brain edema associated with astrocytic swelling. An increase of brain ammonia in experimental animals or treatment of cultured astrocytes with ammonia generates reactive oxygen and nitrogen species in the target tissues, leading to oxidative/nitrosative stress (ONS). In cultured astrocytes, ammonia-induced ONS is invariably associated with the increase of the astrocytic cell volume. Interrelated mechanisms underlying this response include increased nitric oxide (NO) synthesis which is partly coupled to the activation of NMDA receptors and increased generation of reactive oxygen species by NADPH oxidase. ONS and astrocytic swelling are further augmented by excessive synthesis of glutamine (Gln) which impairs mitochondrial function following its accumulation in there and degradation back to ammonia ("the Trojan horse" hypothesis). Ammonia also induces ONS in other cell types of the CNS: neurons, microglia and the brain capillary endothelial cells (BCEC). ONS in microglia contributes to the central inflammatory response, while its metabolic and pathophysiological consequences in the BCEC evolve to the vasogenic brain edema associated with HE. Ammonia-induced ONS results in the oxidation of mRNA and nitration/nitrosylation of proteins which impact intracellular metabolism and potentiate the neurotoxic effects. Simultaneously, ammonia facilitates the antioxidant response of the brain, by activating astrocytic transport and export of glutathione, in this way increasing the availability of precursors of neuronal glutathione synthesis.

Nitrotyrosine and nitrate/nitrite levels in cardiac arrest survivors treated with endovascular hypothermia

The protective effect of therapeutic hypothermia in cardiac arrest survivors (CAS) has been previously well documented. Animal studies have indicated that attenuation of tissue oxidative stress (OS) may be involved in the mechanisms that lead to the beneficial effect of hypothermia. The extent of OS and nitric oxide (NO) production in adult CAS treated with endovascular hypothermia is, however, unknown. A total of 11 adult patients who experienced cardiac arrest out of hospital were included in the present study, and all were treated with mild hypothermia using the Thermogard XP (Alsius, USA) endovascular system. A target core temperature of 33 °C was maintained for 24 hours, with a subsequent rewarming rate of 0.15 °C per hour, followed by normothermia at 36.8 °C. Blood samples for the measurement of nitrotyrosine and nitrate/nitrite levels were drawn at admission and every 6 hours thereafter for two days. During the hypothermic period, the levels of nitrotyrosine and nitrates/nitrites were comparable with baseline values. During the rewarming period, serum levels of both parameters gradually increased and, during the normothermic period, the levels were significantly higher compared with hypothermic levels (nitrotyrosine, P<0.001; nitrates/nitrites, P<0.05). In our study, significantly lower levels of nitrotyrosine and nitrates/nitrites were demonstrated during hypothermia compared with levels during the normothermic period in adult CAS. These data suggest that attenuation of OS and NO production may be involved in the protective effect of hypothermia in adult CAS.

Selective brain cooling in rats ameliorates intracerebral hemorrhage and edema caused by penetrating brain injury: possible involvement of heme oxygenase-1 expression

Brain edema formation associated with trauma-induced intracerebral hemorrhage (ICH) is a clinical complication with high mortality. Studies have shown that heme oxygenase-1 (HO-1) plays an important role in ICH-induced brain edema. In order to understand the role of HO-1 in the protective effect of selective brain cooling (SBC), we investigated the time course of HO-1 changes following penetrating ballistic-like brain injury (PBBI) in rats. Samples were collected from injured and control animals at 6, 24, 48, and 72 h, and 7 days post-injury to evaluate HO-1 expression, heme concentration, brain water content, and immunohistochemistry (IHC). Following a 10% frontal PBBI, HO-1 mRNA and protein was increased at all time points studied, reaching maximum expression levels at 24-48 h post-injury. An increase in the heme concentration and the development of brain edema coincided with the upregulation of HO-1 mRNA and protein during the 7-day post-injury period. SBC significantly decreased PBBI-induced heme concentration, attenuated HO-1 upregulation, and concomitantly reduced brain water content. These results suggest that the neuroprotective effects of SBC may be partially mediated by reducing the heme accumulation, which reduced injury-mediated upregulation of HO-1, and in turn ameliorated edema formation. Collectively, these results suggest a potential value of HO-1 as a diagnostic and/or therapeutic biomarker in hemorrhagic brain injury.

Systemic oxidative stress is implicated in the pathogenesis of brain edema in rats with chronic liver failure

Chronic liver failure leads to hyperammonemia, a central component in the pathogenesis of hepatic encephalopathy (HE); however, a correlation between blood ammonia levels and HE severity remains controversial. It is believed oxidative stress plays a role in modulating the effects of hyperammonemia. This study aimed to determine the relationship between chronic hyperammonemia, oxidative stress, and brain edema (BE) in two rat models of HE: portacaval anastomosis (PCA) and bile-duct ligation (BDL). Ammonia and reactive oxygen species (ROS) levels, BE, oxidant and antioxidant enzyme activities, as well as lipid peroxidation were assessed both systemically and centrally in these two different animal models. Then, the effects of allopurinol (xanthine oxidase inhibitor, 100mg/kg for 10days) on ROS and BE and the temporal resolution of ammonia, ROS, and BE were evaluated only in BDL rats. Similar arterial and cerebrospinal fluid ammonia levels were found in PCA and BDL rats, both significantly higher compared to their respective sham-operated controls (p<0.05). BE was detected in BDL rats (p < 0.05) but not in PCA rats. Evidence of oxidative stress was found systemically but not centrally in BDL rats: increased levels of ROS, increased activity of xanthine oxidase (oxidant enzyme), enhanced oxidative modifications on lipids, as well as decreased antioxidant defense. In PCA rats, a preserved oxidant/antioxidant balance was demonstrated. Treatment with allopurinol in BDL rats attenuated both ROS and BE, suggesting systemic oxidative stress is implicated in the pathogenesis of BE. Analysis of ROS and ammonia temporal resolution in the plasma of BDL rats suggests systemic oxidative stress might be an important "first hit", which, followed by increases in ammonia, leads to BE in chronic liver failure. In conclusion, chronic hyperammonemia and oxidative stress in combination lead to the onset of BE in rats with chronic liver failure.

The Na-K-Cl cotransporter in the brain edema of acute liver failure

BACKGROUND & AIMS

Astrocyte swelling and brain edema associated with increased intracranial pressure are major complications of acute liver failure (ALF). The mechanism for such astrocyte swelling/brain edema, however, is not well understood. We recently found that ammonia, a key etiological factor in ALF, caused the activation of the Na-K-Cl cotransporter-1 (NKCC1) in cultured astrocytes, and that inhibition of such activation led to a reduction in astrocyte swelling, suggesting that NKCC1 activation may be an important factor in the mechanism of brain edema in ALF. To determine whether NKCC activation is also involved in brain edema in vivo, we examined whether NKCC activation occurs in the thioacetamide (TAA) rat model of ALF and determined whether treatment with the NKCC inhibitor bumetanide reduces the severity of brain edema in TAA-treated rats.

METHODS

Brain water content was measured using the gravimetric method. NKCC1 phosphorylation and protein expression were measured by Western blots. NKCC activity was measured in brain cortical slices.

RESULTS

NKCC activity was elevated in brain cortical slices of TAA-treated rats as compared to sham animals. Western blot analysis showed significant increases in total as well as phosphorylated (activated) NKCC1 protein expression in the cortical tissue. These findings were associated with a significant increase in brain water content which was attenuated by treatment with the NKCC inhibitor bumetanide.

CONCLUSIONS

Our studies suggest the involvement of NKCC in the development of brain edema in experimental ALF, and that targeting NKCC may represent a useful therapeutic strategy in humans with ALF.

CDK5 phosphorylates eNOS at Ser-113 and regulates NO production

Phosphorylation of endothelial nitric oxide synthase (eNOS) is key mechanism in response to various forms of cellular stimulation. Through protein nitration by peroxynitrite, eNOS is believed to be responsible for the major abnormalities in several important neurodegenerative diseases including Alzheimer's (AD) and Parkinson's diseases (PD). Recent studies provide important in vivo evidence that hyperactivation of Cdk5 by p25 plays an essential role in the cell death of neurons in experimental models of AD and PD. This study focuses on the functional regulation of eNOS by Cdk5/p35 complex in a phosphorylation dependent manner. Our results showed that Cdk5 can phosphorylate eNOS both in vitro and in vivo. In vitro kinase assay together with the bioinformatic analysis and site direct mutagenesis revealed that Ser-113 is the major phosphorylation site for Cdk5. Most interestingly, the nitrite production was significantly reduced in eNOS and Cdk5/p35 co-transfected SH-SY5Y cells when compared with co-transfection of Cdk5/p35 and S113A. Together, our data suggest that Cdk5 can phosphorylate eNOS at the Ser-113 site and down-regulate eNOS-derived NO levels.

Evidence for oxidative/nitrosative stress in the pathogenesis of hepatic encephalopathy

Hepatic encephalopathy (HE) is a serious complication of liver failure. HE manifests as a series of neuropsychiatric and neuromuscular symptoms including personality changes, sleep abnormalities, asterixis and muscle rigidity progressing through stupor to coma. The pathophysiologic basis of HE remains unclear. There is general agreement that ammonia plays a key role. In recent years, it has been suggested that oxidative/nitrosative stress constitutes part of the pathophysiologic cascade in HE. Direct evidence for oxidative/nitrosative stress in the pathogenesis of HE has been demonstrated in experimental animal models of acute or chronic liver failure. However, evidence from studies in HE patients is limited. This review summarizes this evidence for a role of oxidative/nitrosative stress in relation to ammonia toxicity and to the pathogenesis of HE.

Brain edema in acute liver failure: inhibition by L-histidine

Brain edema and the associated increase in intracranial pressure are potentially lethal complications of acute liver failure (ALF). Astrocyte swelling (cytotoxic edema) represents a significant component of the brain edema in ALF, and elevated blood and brain ammonia levels have been strongly implicated in its formation. We earlier showed in cultured astrocytes that oxidative stress (OS) and the mitochondrial permeability transition (mPT) play major roles in the mechanism of ammonia-induced astrocyte swelling. Glutamine, a byproduct of ammonia metabolism, has also been shown to induce OS, the mPT, and astrocyte swelling. Such effects of glutamine were suggested to be mediated by its hydrolysis in mitochondria, potentially yielding high levels of ammonia in this organelle and leading to OS and the mPT. L-histidine, an inhibitor of mitochondrial glutamine transport, was recently shown to mitigate OS, mPT, and cell swelling in cultured astrocytes treated with ammonia. The present study examined whether L-histidine similarly abolishes OS, the mPT, and brain edema in a rat model of ALF. Treatment of rats with thioacetamide caused a significant degree of brain edema, which was associated with induction of OS and the mPT. These changes were completely abolished by L-histidine, supporting a key role of mitochondrial glutamine transport and hydrolysis in the mechanism of the brain edema associated with ALF.

Interaction of oxidative stress, astrocyte swelling and cerebral ammonia toxicity

PURPOSE OF REVIEW

Description of the role of oxidative stress in the pathogenesis of cerebral ammonia toxicity and hepatic encephalopathy.

RECENT FINDINGS

Ammonia plays a key role in the pathogenesis of hepatic encephalopathy, which manifests as a neuropsychiatric syndrome accompanying acute and chronic liver failure. One consequence of ammonia action on the brain is astrocyte swelling, which triggers the generation of oxidative/nitrosative stress at the level of NADPH oxidase, nitric oxide synthases and the mitochondria. A self-amplifying signaling loop between oxidative stress and astrocyte swelling has been proposed. Consequences of the ammonia-induced oxidative/nitrosative stress response are protein modifications through nitration of tyrosine residues and oxidation of astrocytic and neuronal RNA. Nitrosative stress also mobilizes zinc from intracellular stores with impact on gene expression. These alterations may at least in part mediate cerebral ammonia toxicity through disturbances of intracellular and intercellular signaling and of synaptic plasticity.

SUMMARY

Oxidative/nitrosative stress and a low-grade cerebral edema as key events in the pathogenesis of ammonia toxicity and hepatic encephalopathy may offer potential new strategies for treatment. Ammonia-induced oxidation of RNA and proteins may impair postsynaptic protein synthesis, which is critically involved in learning and memory consolidation. RNA oxidation offers a novel explanation for multiple disturbances of neurotransmitter systems and gene expression and the cognitive deficits observed in hepatic encephalopathy.

Minocycline attenuates oxidative/nitrosative stress and cerebral complications of acute liver failure in rats

In the present study, the effects of minocycline on progression of encephalopathy and brain edema in rats with acute liver failure (ALF) resulting from hepatic devascularization were studied in relation to the antioxidant action of the drug. ALF rats were sacrificed at precoma and coma stages of encephalopathy along with their appropriate sham-operated controls. Minocycline-treated ALF rats were sacrificed in parallel with comatose vehicle-treated ALF controls. Microglial activation was assessed using CD11b/c (OX-42) immunohistochemistry. Nitrite/nitrate levels in plasma and brain were measured using the Griess reaction. Expression of nitric oxide synthase (NOS) isoforms and heme oxygenase-1 (HO-1) were measured using real-time quantitative PCR and Western blot analysis. Increased nitrite/nitrate levels were observed in the plasma of ALF rats at coma stage of encephalopathy compared to sham-operated controls. Increased expression of HO-1 mRNA and protein was observed in the frontal cortex of ALF rats at both precoma and coma stages of encephalopathy. Significant increases in expression of endothelial (eNOS) and inducible (iNOS) isoforms of NOS mRNA and protein occurred only at coma stages of encephalopathy accompanied by increased brain nitrite/nitrate concentrations. As expected, minocycline attenuated microglial activation as confirmed by decreased OX-42 immunoreactivity, normalized nitrite/nitrate levels in brain and significantly attenuated HO-1, eNOS and iNOS expression. These results indicate that the beneficial effect of minocycline on the neurological complications of ALF is mediated, at least in part, by reduction of oxidative/nitrosative stress.