Increase in tryptophan oxygenase activity in alcoholic patients

Induction of the hepatic aryl hydrocarbon receptor by alcohol dysregulates autophagy and phospholipid metabolism via PPP2R2D

Disturbed lipid metabolism precedes alcoholic liver injury. Whether and how AhR alters degradation of lipids, particularly phospho-/sphingo-lipids during alcohol exposure, was not explored. Here, we show that alcohol consumption in mice results in induction and activation of aryl hydrocarbon receptor (AhR) in the liver, and changes the hepatic phospho-/sphingo-lipids content. The levels of kynurenine, an endogenous AhR ligand, are elevated with increased hepatic tryptophan metabolic enzymes in alcohol-fed mice. Either alcohol or kynurenine treatment promotes AhR activation with autophagy dysregulation via AMPK. Protein Phosphatase 2 Regulatory Subunit-Bdelta (Ppp2r2d) is identified as a transcriptional target of AhR. Consequently, PPP2R2D-dependent AMPKα dephosphorylation causes autophagy inhibition and mitochondrial dysfunction. Hepatocyte-specific AhR ablation attenuates steatosis, which is associated with recovery of phospho-/sphingo-lipids content. Changes of AhR targets are corroborated using patient specimens. Overall, AhR induction by alcohol inhibits autophagy in hepatocytes through AMPKα, which is mediated by Ppp2r2d gene transactivation, revealing an AhR-dependent metabolism of phospho-/sphingo-lipids.

Alcoholism and inflammation: neuroimmunology of behavioral and mood disorders

Alcohol abuse changes behavior and can induce major mood disorders such as depression. Recent evidence in pre-clinical rodent models and humans now supports the conclusion that the innate immune system is an important physiological link between alcoholism and major depressive disorders. Deficiency of toll-like receptor 4 (TLR4), a protein that has been known to immunologists for 50 years, not only prevents lipopolysaccharide (LPS)-induced sickness behavior but recently has been demonstrated to induce resistance to chronic alcohol ingestion. Activation of the immune system by acute administration of LPS, a TLR4 agonist, as well as chronic infection with Bacille Calmette-Guérin (BCG), causes development of depressive-like behaviors in pre-clinical rodent models. Induction of an enzyme expressed primarily in macrophages and microglia, 2,3 indoleamine dioxygenase, shunts tryptophan catabolism to form kynurenine metabolites. This enzyme is both necessary and sufficient for expression of LPS and BCG-induced depressive-like behaviors in mice. New findings have extended these concepts to humans by showing that tryptophan catabolites of 2,3 indoleamine dioxygenase are elevated in the cerebrospinal fluid of hepatitis patients treated with the recombinant cytokine interferon-α. The remarkable conservation from mice to humans of the impact of inflammation on mood emphasizes the ever-expanding role for cross-talk among diverse physiological symptoms that are likely to be involved in the pathogenesis of alcohol abuse. These findings present new and challenging opportunities for scientists who are engaged in brain, behavior and immunity research.

Tryptophan metabolism in alcoholism

Acute and chronic alcohol (ethanol) intake and subsequent withdrawal exert major effects on tryptophan (Trp) metabolism and disposition in human subjects and experimental animals. In rats, activity of the rate-limiting enzyme of Trp degradation, liver Trp pyrrolase (TP), is enhanced by acute, but inhibited after chronic, ethanol administration, then enhanced during withdrawal. These changes lead to alterations in brain serotonin synthesis and turnover mediated by corresponding changes in circulating Trp availability to the brain. A low brain-serotonin concentration characterizes the alcohol-preferring C57BL/6J mouse strain and many alcohol-preferring rat lines. In this mouse strain, liver TP enhancement causes the serotonin decrease. In man, acute ethanol intake inhibits brain serotonin synthesis by activating liver TP. This may explain alcohol-induced depression, aggression and loss of control in susceptible individuals. Chronic alcohol intake in dependent subjects may be associated with liver TP inhibition and a consequent enhancement of brain serotonin synthesis, whereas subsequent withdrawal may induce the opposite effects. The excitotoxic Trp metabolite quinolinate may play a role in the behavioural disturbances of the alcohol-withdrawal syndrome. Some abstinent alcoholics may have a central serotonin deficiency, which they correct by liver TP inhibition through drinking. Further studies of the Trp and serotonin metabolic status in long-term abstinence in general and in relation to personality characteristics, alcoholism typology and genetic factors in particular may yield important information which should facilitate the development of more effective screening, and preventative and therapeutic strategies in this area of mental health.

Impact of tryptophan metabolism on the vulnerability to alcohol-related blackouts and violent impulsive behaviours

AIMS

We examined (1) the association of SLC6A4 genotypes and alcohol dependence (AD) in a sample of alcoholics; (2) the validity of lifetime occurrence of blacked-out violent impulsive behaviour (BOVIB) during binge drinking bouts as a criterion for subtyping AD patients and (3) a mechanistic hypothesis for BOVIB involving tryptophan-2,3-dioxygenase (TDO) activity.

METHODS

Three common polymorphisms of the SLC6A4 gene (5-HTTLPR, A/G SNP of LPR region and VNTR in intron 2) were genotyped. An oral tryptophan (Trp) load (OTL) was administered to a sample of patients seeking help for AD. BOVIB history and psychological status were screened by BOVIB-Q, depression (BDI), anxiety (BAI, STAI) and personality (TCI) questionnaires. During the 7 h following Trp load, serum kynurenine (Kyn) and Trp were monitored.

RESULTS

BOVIB+ patients showed significantly higher scores on depression, anxiety and character scales but no significant association was found between SLC6A4 polymorphisms and BOVIB. Patients with a history of BOVIB (BOVIB+ subgroup) differed from those exempt from such episodes (BOVIB- subgroup) for TDO activity response to OTL assessed by the Kyn:Trp ratio (P = 0.043) and the slope of concentration increase ratio (SCIR) of serum Kyn (P = 0.043).

CONCLUSIONS

Put together, these findings support the validity of the BOVIB criterion to differentiate a sub-group of vulnerable AD subjects and suggest that OTL may help to concurrently define a specific endophenotype.

Tryptophan metabolism in alcoholism

Studies of tryptophan (Trp) metabolism in relation to the serotonin status in alcoholism are of 2 types: (1) those related to the pharmacological effects of ethanol; (2) those concerning the serotonin status in the absence of alcohol intake. In experimental animals, acute and chronic ethanol administration and subsequent withdrawal exert a variety of effects on brain serotonin synthesis and turnover mediated by corresponding changes in Trp availability to the brain secondarily mainly to modulation of liver Trp pyrrolase (TP) activity. Alcohol-preferring mice and rats exhibit a central serotonin deficiency caused by, or in some cases associated with, a higher TP activity. Liver TP also appears to be a target of ethanol in man and evidence has recently emerged that alcoholics with positive family history are serotonin-deficient because of a lower availability of circulating Trp to the brain. Acutely, ethanol depletes brain serotonin in normal subjects, which may explain alcohol-induced aggression in susceptible individuals and also the incidence of depression in alcoholism. Trp availability to the brain is increased before the appearance of the alcohol-withdrawal syndrome in man, raising the possibility that the associated behavioural disturbances may involve the excitotoxic Trp metabolite quinolinate. Further studies of the Trp and serotonin status in relation to these important clinical features of alcohol dependence and alcoholism may therefore yield fruitful results.

Pathophysiology of delirium

Hypotheses about the pathophysiology of delirium are speculative and largely based on animal research. According to the neurotransmitter hypothesis, decreased oxidative metabolism in the brain causes cerebral dysfunction due to abnormalities of various neurotransmitter systems. Reduced cholinergic function, excess release of dopamine, norepinephrine, and glutamate, and both decreased and increased serotonergic and gamma-aminobutyric acid activity may underlie the different symptoms and clinical presentations of delirium. According to the inflammatory hypothesis, increased cerebral secretion of cytokines due to a wide range of physically stressful events plays an important role in the occurrence of delirium. Since cytokines can influence the activity of various neurotransmitter systems, these mechanisms may interact. Also, more fundamental processes like intraneuronal signal transduction, second messenger systems that at the same time use neurotransmitters as first messengers and play an important role in their synthesis and release, may be disturbed. Furthermore, severe illness and physiologic stress may give rise to modification of blood-brain barrier permeability, the sick euthyroid syndrome with abnormalities of thyroid hormone concentrations, and increased activity of the hypothalamic-pituitary-adrenal axis. These circumstances possibly also contribute to changes in neurotransmitter synthesis and release of cytokines in the brain, and consequently to the occurrence of delirium. Elderly patients are more at risk for developing delirium,very likely due to age-related cerebral changes in stress-regulating neurotransmitter and intracellular signal transduction systems. This paper will expand upon these current theories and discuss their applicability to research and clinical work with elderly patients suffering from delirium.

Serotonin-immune interactions in detoxified chronic alcoholic patients without apparent liver disease: activation of the inflammatory response system and lower plasma total tryptophan

The aims of the present study were to examine (1) the inflammatory response system (IRS), through measurements of serum interleukin-6 (IL-6), soluble IL-6 receptor (sIL-6R), sgp130 (the soluble form of the IL-6 transducer signal protein), CC16 (Clara Cell protein; an endogenous anti-cytokine), IL-1R antagonist (IL-1RA), IL-8 and sCD14; and (2) the availability of plasma total tryptophan to the brain in chronic alcoholic patients without apparent liver disease (AWLD). Detoxified AWLD patients had significantly lower plasma tryptophan and serum CC16 and significantly higher serum IL-1RA and IL-8 concentrations than normal volunteers. There were significant correlations between the availability of tryptophan to the brain and serum IL-6, IL-8 and IL-1RA (all negative) and CC16 (positive). The results suggest that (1) there is, in detoxified AWLD patients, an activation of the monocytic arm of cell-mediated immunity and a lowered anti-inflammatory capacity of the serum; and that (2) lower availability of plasma tryptophan to the brain in detoxified AWLD patients is related to activation of the IRS. Lower CC16 may be one factor predisposing chronic alcoholic patients toward infectious disorders.

Contrasting effects of acute and chronic ethanol administration on rat liver tyrosine aminotransferase

We compared the effects of acute and chronic ethanol administration on the activity and synthesis of tyrosine aminotransferase (TAT) in rat liver. In acute experiments, chow-fed rats received a single dose of either ethanol (6 g/kg body wt.) or saline. In chronic studies, rats were pair-fed liquid diets containing either ethanol (36 % of calories) or isocaloric maltose-dextrin for 6-8 weeks. In rats acutely fed ethanol, the relative rate of TAT synthesis was more than twofold higher than in saline-treated controls. In rats subjected to chronic ethanol administration, both the TAT activity and synthesis rate were the same as in pair-fed controls, but both these parameters in the two groups were equal to those in animals given acute ethanol acutely. These findings indicate that whereas acute ethanol administration was associated with a stimulation of TAT synthesis, long-term ethanol administration was not. The data suggest that ethanol itself does not directly induce TAT. Rather, enzyme synthesis is regulated by one or more endogenous secondary effector(s) whose production is influenced differently by acute or chronic ethanol feeding.

Elevated tryptophan levels in post-withdrawal alcoholics

Changes in serotonin function and disturbances in tryptophan availability have been implicated in many psychiatric disorders, including alcoholism. In the present study we took serum free tryptophan samples from 31 healthy volunteer controls and from 42 DSM-III-R alcohol-dependent subjects who had abstained from alcohol for at least 2.5 weeks (range 2.5-104 weeks). We also measured the basal serum cortisol level at 09.00 hours for the same subjects and controls. There was a significant increase in the serum tryptophan level of the alcoholic subjects, by 43.7 mumol l-1 (range 29-63 mumol l-1), regardless of age of onset of alcoholism, family history of alcoholism or sociopathic traits, compared to the controls (33.0 mumol l-1, range 19-60 mumol l-1). There was also an increase in the basal serum cortisol level in the alcoholic subjects compared to the controls, but this was not related to the increase in tryptophan levels. These findings indicate a disturbance in serotonin precursor availability in post-withdrawal alcoholics, and contribute to the evidence for involvement of the serotonin system in alcoholism.

Serotonin and alcohol intake, abuse, and dependence: clinical evidence

A large body of literature has emerged concerning the role of the neurotransmitter serotonin (5-hydroxytryptamine, or 5-HT) in the regulation of alcohol intake and the development of alcoholism. Despite the wealth of information, the functional significance of this neurotransmitter remains to be fully elucidated. This paper, part one of a two-part review, summarizes the available clinical research along two lines: the effects of alcohol on serotonergic functioning and the effects of pharmacological manipulation of serotonergic functioning on alcohol intake in normal (nonalcohol dependent) and alcohol-dependent individuals. It is concluded that considerable evidence exists to support the notion that some alcoholic individuals may have lowered central serotonin neurotransmission.

Metabolic profiles of steroids in urine of alcoholics after withdrawal

The metabolic profiles of steroids in urine were analyzed in 13 male alcoholics during long-term abstinence, in most cases exceeding 3 months. The ratios of 5 beta- to 5 alpha-reduced steroid metabolites (etiocholanolone/androsterone and tetrahydrocortisol/allotetrahydrocortisol) were initially elevated but decreased slowly following withdrawal. The half-life of this normalization exceeded 3 weeks. The change was most marked in patients with signs of liver injury, and may reflect a relative decrease of the activity of hepatic 5 alpha-reductase. The ratio between cortisol metabolites carrying a 11 beta-hydroxy and an 11-oxo group was elevated in the patients and showed no tendency to normalize. This might reflect a decrease in the peripheral inactivation of cortisol.

Is quinolinic acid an endogenous excitotoxin in alcohol withdrawal?

Levels of tryptophan in brain are increased by the action of chronic ethanol, particularly in the event of compromised hepatic function. This is likely to result in elevated brain levels of the potent excitotoxin quinolinic acid (quinolinate) since levels of this tryptophan metabolite can be elevated considerably by tryptophan loading. Ethanol may also selectively increase the activity of enzymes important in the synthesis of quinolinic acid such as tryptophan oxygenase. It is concluded that ethanol may generate significant levels of the NMDA receptor agonist, quinolinic acid, possibly even toxic levels in localized brain areas, especially during ethanol withdrawal and when associated with acute or chronic hepatotoxicity.