Hepatic microsomal ethanol-oxidizing system (MEOS): increased activity following propylthiouracil administration

Alcoholic Liver Disease: Alcohol Metabolism, Cascade of Molecular Mechanisms, Cellular Targets, and Clinical Aspects

Alcoholic liver disease is the result of cascade events, which clinically first lead to alcoholic fatty liver, and then mostly via alcoholic steatohepatitis or alcoholic hepatitis potentially to cirrhosis and hepatocellular carcinoma. Pathogenetic events are linked to the metabolism of ethanol and acetaldehyde as its first oxidation product generated via hepatic alcohol dehydrogenase (ADH) and the microsomal ethanol-oxidizing system (MEOS), which depends on cytochrome P450 2E1 (CYP 2E1), and is inducible by chronic alcohol use. MEOS induction accelerates the metabolism of ethanol to acetaldehyde that facilitates organ injury including the liver, and it produces via CYP 2E1 many reactive oxygen species (ROS) such as ethoxy radical, hydroxyethyl radical, acetyl radical, singlet radical, superoxide radical, hydrogen peroxide, hydroxyl radical, alkoxyl radical, and peroxyl radical. These attack hepatocytes, Kupffer cells, stellate cells, and liver sinusoidal endothelial cells, and their signaling mediators such as interleukins, interferons, and growth factors, help to initiate liver injury including fibrosis and cirrhosis in susceptible individuals with specific risk factors. Through CYP 2E1-dependent ROS, more evidence is emerging that alcohol generates lipid peroxides and modifies the intestinal microbiome, thereby stimulating actions of endotoxins produced by intestinal bacteria; lipid peroxides and endotoxins are potential causes that are involved in alcoholic liver injury. Alcohol modifies SIRT1 (Sirtuin-1; derived from Silent mating type Information Regulation) and SIRT2, and most importantly, the innate and adapted immune systems, which may explain the individual differences of injury susceptibility. Metabolic pathways are also influenced by circadian rhythms, specific conditions known from living organisms including plants. Open for discussion is a 5-hit working hypothesis, attempting to define key elements involved in injury progression. In essence, although abundant biochemical mechanisms are proposed for the initiation and perpetuation of liver injury, patients with an alcohol problem benefit from permanent alcohol abstinence alone.

Alcoholic steatohepatitis (ASH) and alcoholic hepatitis (AH): cascade of events, clinical aspects, and pharmacotherapy options

INTRODUCTION

Clinicians caring for patients with alcoholic hepatitis (AH) are often confronted with the question of the best pharmacotherapy to be used.

AREAS COVERED

This article covers metabolic aspects of alcohol as the basis of understanding pharmacotherapy and to facilitate choosing the drug therapeutic options for patients with severe AH.

EXPERT OPINION

Alcoholic steatohepatitis (ASH) and alcoholic hepatitis (AH) as terms are often used interchangeably in scientific literature but a stringent differentiation is recommended for proper clarity. As opposed to ASH, the clinical course of AH is often severe and requires an effective drug treatment strategy, in addition to absolute alcohol abstinence and nutritional support. Drug options include corticosteroids as a first choice and pentoxifylline, an inhibitor of phosphodiesterase, as a second line therapy, especially in patients with contraindications for a corticosteroid therapy such as infections or sepsis. At seven days under corticosteroids, treatment should be terminated in non-responders, and patients must then be evaluated for liver transplantation. Pentoxifylline is not effective as a rescue therapy for these patients. Other treatments such as infliximab, propylthiouracil, N-acetylcysteine, silymarin, colchicine, insulin and glucagon, oxandrolone, testosterone, and polyunsaturated lecithin are not effective in severe AH. For liver transplantation, few patients will be eligible.

Inhibitory effect of propylthiouracil on the development of metabolic tolerance to ethanol

Chronic ethanol administration (4-5 weeks) to female spontaneously hypertensive (SH) rats led to a marked increase in the rate of ethanol metabolism. This was accompanied by an increase in hepatic alcohol dehydrogenase (ADH) and by an increase in the rate of oxygen consumption in perfused livers of these animals. Treatment with the antithyroid drug 6-n-propyl-2-thiouracil (PTU) during the last 9 days (40 mg/kg/day) of the chronic administration of ethanol reduced hepatic oxygen consumption, resulting in a net diminution of the metabolic tolerance to ethanol, despite a further elevation in ADH activity. In these animals, microsomal ethanol-oxidizing system (MEOS) activity was not affected by chronic ethanol administration or by treatment with PTU. Data strongly suggest that in the female SH rat all the metabolic tolerance to ethanol proceeds via the ADH pathway, and that the increase in hepatic oxygen consumption is more important in the development of metabolic tolerance to ethanol than the increased ADH levels.

Effect of triiodothyronine on alcohol dehydrogenase and aldehyde dehydrogenase activities in rat liver. Implications for the control of ethanol metabolism

Treatment of rats with 20 micrograms of 3,3',5-triiodo-L-thyronine (T3) per 100 g body wt for a period of 6 days led to a 45% decrease in total liver alcohol dehydrogenase and a 36% decrease in total liver aldehyde dehydrogenase. Most of the latter decrease was directly attributable to a 57% fall in the level of the physiologically-important low Km mitochondrial isoenzyme. The high Km isoenzyme of the postmitochondrial and soluble fractions was much less affected by T3-treatment. T3, at concentrations up to 0.1 mM, did not inhibit the activity of aldehyde dehydrogenase in vitro. Despite these large losses of the two enzymes most intimately involved in ethanol metabolism, the rate of ethanol elimination in vivo was the same in T3-treated and control animals. Moreover, there was no difference between the two groups in the susceptibility of ethanol elimination to inhibition by 4-methylpyrazole, making it unlikely that an alternative route of ethanol metabolism had been significantly induced by treatment with T3. As it had been suggested that T3 might create a "hypermetabolic state" in which constraints normally imposed on alcohol dehydrogenase and aldehyde dehydrogenase are removed thereby compensating for any loss in total enzymic activity, 2,4-dinitrophenol (0.1 mmoles/kg body wt) was administered to rats in order to raise the general metabolic rate. However, the uncoupler proved to be lethal to T3-treated animals and did not stimulate ethanol elimination in controls. The results do not support the notion that ethanol elimination in vivo is normally governed either by the level of alcohol dehydrogenase or by that of hepatic aldehyde dehydrogenase. However, the mode of control remains unclear.

Effects of propylthiouracil on D-galactosamine hepatotoxicity in the rat. Evidence for a non-thyroidal effect

The cytoprotective effects of propylthiouracil (PTU) were studied in rats treated with the hepatotoxin D-galactosamine (D-GNH2). Five days of PTU pretreatment prior to D-GNH2 caused hypothyroidism and a significant reduction in liver injury as assessed by serum transaminase levels. When PTU was administered as a single dose with D-GNH2, significant decreases in transaminase also occurred at times when thyroid function was unchanged. Furthermore, aminopyrine oxidation showed significant impairment after D-GNH2 and was normalized by one dose of PTU. Further studies were carried out in thyroidectomized rats. PTU caused significant reductions in transaminase levels when given for 5 days pretreatment or as a single dose. Animals receiving pretreatment with PTU plus thyroxine (T4) also had significant decreases in serum transaminase. The antithyroid drug methimazole also had a hepatoprotective effect, while two other potent antithyroid compounds (2-thiouracil and 2-thiobarbituric acid) did not. These data suggest that PTU can protect against liver injury induced by D-GNH2, that the effect is independent of thyroid function, and that this effect is not common to all thiol-containing antithyroid drugs.

Effects of thyroid status on clonidine-induced hypoactivity responses in the developing rat

Clonidine-induced hypoactivity was studied in rats made neonatally hypo- and hyperthyroid (PTU or T4 administered between 1-27 days postnatally) as an indication of central presynaptic alpha 2-adrenoceptor function. Paradoxically, at 28 days postnatally both treatments caused an increase in clonidine-hypoactivity compared with untreated control animals, which was not due to sex differences amongst littermates in the various groups. It is proposed that whereas the hyperthyroid neonate data perhaps reflect an enhancement of presynaptic alpha 2 function similar to that seen in the adult rat brain, the result obtained for the hypothyroid neonates could perhaps be due to a retardation of the ontogeny of central noradrenergic neurones.

Hypermetabolic state and hypoxic liver damage

The concept of a hypermetabolic state to explain metabolic tolerance to ethanol grew from the recognition that the rate of alcohol metabolism is, in general, limited by the rate at which mitochondria can reoxidize reducing equivalents and thus by the rate at which oxygen can be consumed by the liver. This relationship appears to be most important in conditions in which the alcohol dehydrogenase (ADH)/QO2 ratio is high and is not in conflict with observations suggesting that ADH can, under certain conditions, constitute a rate-determining step for ethanol metabolism in rodents. Liver preparations from animals fed alcohol chronically, in which an increase in ethanol metabolism is shown, consume oxygen at higher rates. This effect, concerning which there is discrepancy among investigators, depends on the type of preparation. Thyroid hormones play a permissive role in the development of the hypermetabolic state, while increased circulating levels of these hormones are not required. Antithyroid drugs inhibit both metabolic tolerance in vivo and the hypermetabolic state. While the hypermetabolic state requires an increased ATP utilization in the form of an adenosine triphosphatase, or an inhibition of ATP synthesis, the different mechanisms proposed for such an effect do not quantitatively account for the increases in oxygen consumption. In humans and animals chronically exposed to ethanol, but withdrawn, oxygen tensions in blood leaving the liver are significantly reduced. In some situations, low oxygen tensions in zone 3 of the hepatic acinus can reach critical hypoxic levels and may lead to cell necrosis. Studies in which the effectiveness of propylthiouracil is tested in human alcoholic hepatitis are discussed.

Hepatic thyroid hormone levels following chronic alcohol consumption: direct experimental evidence in rats against the existence of a hyperthyroid hepatic state

To study the effect of chronic alcohol consumption on hepatic levels of thyroid hormones, female Sprague-Dawley rats (n = 24) were pair-fed nutritionally adequate liquid diets containing either ethanol (36% of total calories) or isocaloric carbohydrates for 21 days. Compared to controls, chronic alcohol consumption failed to result in a significant change of hepatic thyroid hormone levels (thyroxine: 14.7 +/- 1.81 ng per gm of liver wet weight vs. 15.0 +/- 1.59; triiodothyronine: 2.60 +/- 0.16 ng per gm of liver wet weight vs. 2.66 +/- 0.18). Similar results were obtained when the hepatic levels of thyroid hormones were expressed per total liver, per gram of liver protein or per 100 gm of body weight. Moreover, prolonged alcohol ingestion led to a significant reduction of serum total thyroxine by 31.6% (p less than 0.001), free thyroxine by 38.9% (p less than 0.02), total triiodothyronine by 40.2% (p less than 0.001) and free triiodothyronine by 56.1% (p less than 0.001) when compared to their pair-fed controls, whereas thyroid-stimulating hormone levels remained virtually unchanged. These data, therefore, clearly show that chronic alcohol consumption is incapable of creating a hyperthyroid hepatic state in rats, and limit the rationale for antithyroid treatment in patients with alcoholic liver disease.

Ethanol, thyroid hormones and acute liver injury: is there a relationship?

Alcoholic liver disease continues to be an important cause of morbidity and mortality, and the hypermetabolic hypothesis continues to be an attractive area for research. However, the current state of knowledge does not allow unequivocal acceptance or rejection of the role of thyroid hormone and antithyroid medication in alcoholic hepatitis. Clinical trials will help to establish or disprove the veracity of this hypothesis. What has been established is that chronic ethanol ingestion enhances EMR (19-22) which probably reflects the degree of hepatocellular necrosis, at least when relatively mild (25). The influence of thyroid hormone or a hyperthyroid-like state on EMR would be established if it could be shown that different antithyroid medications inhibit the enhanced EMR in chronic alcoholics. This effect has been shown in rats (125), but not in man. It is not apparent that events in the rat model can be readily applied to man. Furthermore, proof that antithyroid medications can inhibit enhanced EMR in chronic ethanol-consuming patients may allow this feature to be used to select patients who may best benefit from such treatment. A controlled randomized clinical trial using different anti-thyroid medications in alcoholic hepatitis may shed light on this important question. At the very least, demonstration of inhibition of enhanced EMR by antithyroid medications may provide the rationale for research concerning the role of thyroid hormone (or a similar hypermetabolic factor) in alcohol-mediated hepatocellular injury.

Effect of hexachlorobenzene on the activities of hepatic alcohol metabolizing enzymes

To study the effect of experimental hepatic porphyria on the activities of hepatic alcohol metabolizing enzymes, female rats received a chow diet containing 0.05% hexachlorobenzene (HCB). After long-term HCB treatment for 60 days hepatic porphyria developed as evidenced by increased hepatic delta-aminolevulinic acid synthase activity and enhanced urinary excretion of delta-aminolevulinic acid, porphobilinogen and total porphyrins. Concomitantly, the activities of the hepatic microsomal ethanol oxidizing system (MEOS) were strikingly augmented by 213% (P less than 0.05) and 177% (P less than 0.01) when expressed per g of liver wet weight or per 100 g of body weight, respectively, whereas hepatic alcohol dehydrogenase activities remained virtually unchanged. Moreover, hepatic catalase showed only a trend for a slightly lower enzymic activity under these experimental conditions. The present data therefore show that experimental hepatic porphyria is associated with alterations of hepatic MEOS activities, which in turn may be a factor for the manifestation of human hepatic porphyrias in the course of alcohol consumption.

Sex-dependency of hepatic alcohol metabolizing enzymes

In mature female rats the administration of testosterone led to a striking reduction of hepatic alcohol dehydrogenase activity, whereas the hepatic microsomal ethanol oxidizing system as well as catalase were both increased in activity under these experimental conditions. Conversely, estradiol left the activities of all hepatic alcohol metabolizing enzymes virtually unchanged. Ovariectomy also had little if any influence on the activity levels of the enzymes. There was a clear difference between the sexes in the hepatic alcohol metabolizing enzymes with higher enzymic activities of the microsomal ethanol oxidizing system and catalase in male than in female rats, whereas the opposite constellation was found for alcohol dehydrogenase activity. These data therefore indicate the sex-dependent nature of alcohol dehydrogenase, the hepatic microsomal ethanol oxidizing system and catalase activities in rat liver.