Alcohol and the liver: 1994 update

GASTROENTEROLOGY 1994;106:10854lO5

Alcohol and the Liver: 1994 Update CHARLES

S. LIEBER

Section of Liver Disease and Nutrition, Alcohol Research and Treatment Center, Bronx VA Medical Center and Mount Sinai School of Medicine, New York, New York

This article reviews current concepts on the pathogenesis and treatment of alcoholic liver disease. It has been known that the hepatotoxicity of ethanol results from alcohol dehydrogenase-mediated excessive generation of hepatic nicotinamide adenine dinucleotide, reduced form, and acetaldehyde. It is now recognized that acetaldehyde is also produced by an accessory (but inducible) microsomal pathway that additionally generates oxygen radicals and activates many xenobiotics to toxic metabolites, thereby explaining the increased vulnerability of heavy drinkers to industrial solvents, anesthetics, commonly used drugs, over-thecounter medications, and carcinogens. The contribution of gastric alcohol dehydrogenase to the first-pass metabolism of ethanol and alcohol-drug interactions is discussed. Roles for hepatitis C, cytokines, sex, genetics, and age are now emerging. Alcohol also alters the degradation of key nutrients, thereby promoting deficiencies as well as toxic interactions with vitamin A and p carotene. Conversely, nutritional deficits may affect the toxicity of ethanol and acetaldehyde, as illustrated by the depletion in glutathione, ameliorated by S adenosyl-L-methionine. Other “supernutrients” include polyunsaturated lecithin, shown to correct the alcoholinduced hepatic phosphatidylcholine depletion and to prevent alcoholic cirrhosis in nonhuman primates. Thus, a better understanding of the pathology induced by ethanol is now generating improved prospects for therapy.

In

a recent prospective survey of 280 subjects with alcoholic liver injury, ’ it was found that, within 48 months of follow-up, more than half of those with cirrho-

sis, and two thirds of those with cirrhosis plus alcoholic hepatitis, had died. This dismal outcome is more severe than that of many cancers, yet it is attracting much less concern,

both

among

the public

Etiology of Liver Disease in the Alcoholic Role of Dietary Factors, Including Methionine and Phospholipids, in the Hepatotoxicity of Ethanol and Associated Membrane Abnormalities Originally

it was believed

alcoholic

was due

quently,

as reviewed

exclusively

that liver disease in the to malnutrition.

Subse-

here before’ and more recently

where,‘34 the hepatotoxicity

of ethanol

else-

has been estab-

lished by the demonstration that in the absence of dietary deficiencies, and even in the presence of protein-, vitamin-, and mineral-enriched diets, ethanol produces fatty liver with striking ultrastructural lesions both in rats and in human nonhuman

volunteers,

primates.

and fibrosis with cirrhosis

The fact remains,

however,

in

that al-

cohol is rich in energy (7.1 kcal/g) and that a large intake of alcohol can have profound effects on nutritional status. It may cause decreased food intake (and thereby primary malnutrition) by displacing other nutrients in the diet (Figure

1) not only as a result of the high energy content

of the alcoholic

beverages

but also because of associated

socioeconomic and medical disorders. Secondary malnutrition may ensue from either maldigestion or malabsorption of nutrients, caused by gastrointestinal complications associated with alcoholism involving especially the pancreas and the small intestine. As described in detail elsewhere,3 such primary

and secondary

malnutrition

can

affect virtually all nutrients. For several decades, choline deficiency has been incriminated as a primary etiologic factor in the pathogenesis of liver injury. Its role was first considered because its deficiency can produce fatty liver in growing rats. Primates, however, are far less

and the medical

profession. This may be due at least in part to the general perception that not much can be done about this major public health issue. One purpose of this review is to analyze how concepts about alcoholic liver disease have evolved since the previous review on this topic published in GASTROENTEROLOGY in 1980,* and how the present state of knowledge allows for a more optimistic outlook in terms of treatment and outcome.

Abbreviations used in this paper: ADH, alcohol dehydrogenase; GSH, glutathione (reduced); L-FABP, liver fatty acid-binding protein; L-FABPc, liver cytosolic fatty acid binding protein; MEOS, microsomal ethanol oxidizing system; NAD, nicotinamide adenine dinucleotide; PC, phosphatidylchollne; PPC, polyunsaturated phosphatldylcholine; PIIIP, procollagen Ill peptlde; TIMP, tissue inhibitor of metalloproteinase; TNF, tumor necrosis factor; SAMe, Sadenosyk-methionine; XD, xanthine dehydrogenase; X0, xanthine oxldase. 0 1994 by the American Gastroenterologlcal Association 001~5085/94/$3.00

1086

GASTROENTEROLOGY Vol. 106. No. 4

CHARLES S. LIEBER

verse effects. SAMe is the principal

methylating

various transmethylation

important

acid and protein and functions,

reactions

synthesis

including

transmission

as well as membrane the transport

of signals

only

in alcohol-induced

the methyl

reactions,

fluidity

of metabolites

across membranes.

tion of SAMe may promote mented

agent in for nucleic and

Thus,

deple-

injury

docu-

the membrane

liver damage.13 SAMe is not

donor

in almost

all transmethylation

but it also plays a key role in the synthesis

polyamines

of

and provides a source of cysteine for glutathi-

one (GSH) production,

a major natural

agent (see below). Orally administered

hepatoprotective SAMe is a precur-

sor for intracellular SAMe, both as unchanged SAMe and also by the methionine it provides. Compared with methionine, Figure 1. Interaction of direct toxicity of ethanol on liver and gut with malnutrition secondary to dietary deficiencies, maldigestion, and malabsorption, as well as impaired hepatic activation or increased degradation of nutrients.

SAMe has the advantage

deficit in SAMe synthesis above. The usefulness

of bypassing

(from methionine)

of SAMe administration

shown in the baboon”

and in various

susceptible to choline deficiency than rodents. Clinically, treatment with choline of patients with alcoholic liver

decrease in liver levels of phospholipids

injury was found to be ineffective

tation.15 The total phospholipid

in the face of continued

alcohol abuse. Furthermore, massive supplementation with choline did not prevent the fatty liver produced by alcohol in volunteer subjects.> This is not surprising because, unlike in rats, the human liver contains very little

choline

oxidase

species

differences

humans,

choline

activity,

regarding deficiency

which

may explain

the

choline

requirements.

In

(and thus a need for choline

supplementation) has been documented in only very limited circumstances of extremely restricted diets.’ Moreover, fatty liver as well as fibrosis (including cirrhosis) developed in baboons despite massive supplementation with choline,

even to the point

of toxicity.’

Methionine deficiency has also been described, and its supplementation has been considered for the treatment of alcoholic liver injury,8 but some difficulties have been encountered. Indeed, excess methionine was shown to have some adverse effects,” including a decrease in hepatic levels of adenosine triphosphate (ATP). Horowitz et al.” report that the blood clearance of methionine after an oral load of this amino acid was slowed. Because approximately half the methionine is metabolized by the liver, the above observations suggest impaired hepatic metabolism of this amino acid in patients with alcoholic liver disease. To be utilized, methionine has to be activated to S-adenosyl-L-methionine (SAMe) (Figure 2). However, Duce et al. ’* found a decrease in SAMe synthetase activity in cirrhotic livers. As a consequence, SAMe depletion ensues after long-term ethanol consumption.‘2 Potentially, such SAMe depletion may have a number of ad-

dylcholine drial

(PC); both can be corrected

membranes

is

studies,‘*

also

causes

a

and phosphatiby PC supplemen-

content

decreased,

has been

clinical

some of which are still ongoing. In primates, ethanol consumption

the

referred to

with

of the mitochona

significant

reduction in the levels of PC16 and associated striking morphologic changes.” The alterations in the phospholipid

composition

of the mitochondrial

pear to be responsible oxidase

activity

and other

duced by long-term nism

whereby

for the decrease

ethanol

long-term

biochemical consumption. ethanol

membranes

ap-

in cytochrome alterations

pro-

l6 The mecha-

consumption

alters

phospholipid levels has not been clarified but may be related to the decreased phospholipid methyltransferase activity described in cirrhotic liver,” That this is not simply

secondary

to the cirrhosis

but may in fact be a

primary defect related to alcohol is suggested by the observation that the enzyme activity is already lower

LIPID PEAOXIDATION

S-ADENOSYL-METHIONINE

-

META!oLITES

-

HOMOCYSTEINE

Figure 2. Hypothetical link between accelerated acetaldehyde production, increased free radical generation by the “induced” microsomes, enhanced lipid peroxidation, and increased a-amino-rrbutyric acid production.

ALCOHOL

April 1994

before the development

of cirrhosis.”

Another

mecha-

Spinozzi

AND

et a1.30 suggest

activation

pact

Miiller et a1.31 report no difference

on signal

transduction,

as shown

in isolated

is secondary pholipase

to the ethanol-induced

C and protein

Administration

rat

of the re-

between

this effect

liver; abnormalities

activation

of phos-

controls

in alcoholic

and patients

preparations

rich in

alcohol-induced

amounts

present

but

fibrosis and cirrhosis.

it was

found

that

in PPC has no protective

PPC

choline

in

action against

the fibrogenic

effects of ethanol

rich in linoleic

acid, but this fatty acid per se is probably

not responsible

for the protective

diet was supplemented

in the baboon.’

PPC is

effect because the basic

with linoleate

and contained

large

in lymphocyte

patients,

B or C are commonly

portal

associated

and/or

subsets fatty patients

associated,

in alcoholic

inflammation

is strongly

C virus (HCV)

long-term

of chronic alcohol

correlation

and

and the

Indeed,

antibody,

even

infection,33

and

evidence for the involvement

in the pathogenesis positive

there is no

but alcoholism

of risk factors for HCV

there is increasing

strong

role.”

lobular

with hepatitis

in the absence

After

cirrhosis,

viral hepatitis,

latter may play an important

choline,

cirrhosis),

with alcohol-induced

with alcoholic

evidence of antecedent

pure

contains

or hepatic

with cirrhosis.

viral hepatitis

fully prevent

1087

of T-lymphocyte

were found only in alcoholic

polyunsaturated

phosphatidylcholine (PPC)*’ or virtually primate, to PPC15 was found, in the nonhuman

UPDATE

liver disease (even in the

of malnutrition

In many subjects

kinase C.*O

of phospholipid

absence of evidence

1994

an alteration

nism whereby ethanol may affect phospholipid levels is via formation of phosphatidylethanol, with possible imEthanol causes desensitization hepatocytes.” ceptor-mediated phospholipase C activation;

pathways

THE LIVER:

hepatitis

of viruses

in alcoholics.

administration,

there

between

endotoxin

plasma

is a

amounts of corn oil that is rich in linoleic acid. Furthermore, this fatty acid has been incriminated as a permis-

levels and severity of liver injury.34 Whereas short-term administration of alcohol was reported to enhance endo-

sive rather

toxin hepatotoxicity when the dose of endotoxin was small, the effect of alcohol was masked when larger doses

injury.”

than as a protective

factor in alcoholic

Thus, the polyunsaturated

phospholipids

liver them-

selves appear to be responsible for protection, perhaps because of their high bioavailability and selective incor-

of endotoxin

poration into liver membranes.23 rectly affects collagen metabolism

shock and sepsis, also plays a role in alcoholic

Dietary

factors contribute

holic, but this should against the hepatotoxic

Furthermore, (see below).

to liver injury

PPC di-

tumor

Circulating

in the alco-

not be viewed as an argument nature of alcohol per se. Indeed,

were given.35

necrosis

factor

It has been

(TNF),

proposed

a mediator

that

of endotoxic

levels of TNF-cx and interleukin

hepatitis. 1 remained

elevated for up to 6 months after the diagnosis of alcoholic hepatitis, whereas interleukin-6 levels normalized in parallel

with clinical

recovery.36 Concentrations

of all

in addition to the steatosis and striking ultrastructural lesions (see above), alcohol produced even more severe lesions (including necrosis and fibrosis) when nutri-

three cytokines correlated with biochemical parameters of liver injury. Sheron et al.” also found that plasma 6 is increased in severe alcoholic hepatitis and interleukin

tionally

postulated

adequate

by continuous

liquid

infusion.24

diets were administered Furthermore,

served in 14 of a total of 67 baboons

cirrhosis

to rats was ob-

fed ethanol

with

that this may mediate

hepatic

or extrahepatic

tissue damage. On the one hand, TNF levels appear to be elevated in multiple types of experimental injury and

adequate diets usually for 5 years or more, and septal fibrosis developed in an additional 14 animals.‘5X21~25326

in alcoholic cytokines,3s

Ainley et al.*’ have challenged the capacity of ethanol to produce cirrhosis in the baboon, but it is not known

growth

liver disease, as are the levels of some other and ethanol inhibits the action of epidermal

factor in hepatocytes.*’

On the other hand,

low

were given ethanol (with the regular diet) for a period exceeding 18 months, whereas the results of other stud?I .26,28 show that a longer period of treatment is reies

physiological amounts of cytokines appear to be important for liver regeneration (and perhaps are beneficial to the organism as a whole). The task at hand is to acquire further knowledge on how cytokines and ethanol interact and to conserve the positive growth-enhancing activity of cytokines while attenuating their cytotoxic

quired

effects.

how much alcohol (or diet) was actually consumed by their animals. In addition, only two of these baboons

to consistently

produce

septal fibrosis or cirrhosis.

Role of immunological Factors, Associated Viral Infection, and Cytokines As reviewed elsewhere,*’ derangements of immune systems are present in alcoholic liver disease, but there is still some debate whether they represent a consequence or a cause of the liver injury. Whereas studies by

Effect of Sex, Age, and Genetics The average cirrhogenic dose of alcohol as well as the threshold amount is lower in women than in men: a daily alcohol intake of 40-60 g in men but only 20 g in women resulted in a statistically significant increase in the incidence of cirrhosis in a well-nourished popula-

1088

CHARLES

S. LIEBER

tion.”

There is also evidence that the progression

severe liver injury

GASTROENTEROLOGY

is accelerated

of chronic advanced

in women:

to more

the incidence

liver disease is higher among women

than among men for those with similar

histories

of alco-

hol abuse.40-4’ Sex differences

in ethanol

metabolism

ported43.44; they have been described and for the stomach

have been re-

both for the liver45

(see below). One explanation

of this

of liver function addition, vulnerability

tion

activity

than

rivatives,

both

in vivo *’ and in vitro, Of course, ADH

vitro is only one of the determinants lism in vivo, and discrepancies

at least at high

activity

measured

of ethanol

between

in

metabo-

the two are not

ways to enhance

to the hepatotoxicity

In

differences further

of ethanol.

the

For in-

in rats.59 Similarly, long-term ethanol consumpwas associated with increases in the content of a

(ADH)

concentrations4’

of women.

biochemical

toxicity

specific

and its de-

in different

number

No. 4

stance, a sex-specific cytochrome P450 has been invoked as a cause of sex- and species-related differences in drug

is the fact that the hepatic alcohol dehydrogenase by testosterone

sex-dependent

may contribute

finding

is suppressed

in a significant

some

Vol. 106.

cytochrome

in female

males

and more

so in male

the microsomal tn-hydroxylation acid was significantly greater and the rise in

of lauric

(89%)

(24%).

(P4504Al),

rats”;

was significantly

Products

higher

of o-oxidation

fatty acid-binding

protein

than

increase

(L-FABPc)

in females

liver cytosolic

content

and per-

times, reduced areas under the blood alcohol concentration time curve, and faster disappearance rates during

P-oxidation,” an alternate pathway for fatty acid disposition. Thus, the increase in a-oxidation may compensate at least in part for the deficit in fatty acid

the midluteal

oxidation

due to the ethanol-induced

chondria,4

but this “compensatory”

effect is less efficient

in women

than in men, a deficiency

compounded

uncommon4a

Some studies

menstrual

levels of progesterone,

report decreased

phase, associated

elimination

with increased

elevated progesterone/estradiol

tios, and decreased levels of follicle-stimulating

ra-

hormone

oxisomal

injury

of the mitoby the

(FSH).*” The hormonal response to ethanol in terms of prolactin and cortisol may also be sex dependent.50m52

fact that the response to ethanol of the liver fatty acidbinding protein (L-FABP) differs in men and women.

One confounding

L-FABP

in the gender

variable,

studies,

not always fully accounted

is the superimposed

altering both the response to ethanols3 metabolism. In experimental animals,

for

effect of age, and possibly its it was observed

that rates of ethanol metabolism decreased linearly with advancing age, associated with a linear decrease in hepatic ADH activity,54 whereas no such effect was found in humans. 55 However, the latter study revealed an agerelated decrease in the volume Women

of distribution

also have lower gastric

ADH

of ethanol. activity

than

is a major contributorG2

to the ethanol-induced

increase in liver cytosolic protein levels in rats.63,64 However, there is a much smaller increase of cytosolic fatty acid-binding

capacity

in female

(58%)

than

in male

(161%) rats. ” The protein responsible for fatty acid binding, the L-FABP of the cytosol (L-FABPc), also promotes esterification of the fatty acids. In keeping with the postulated increases cholesterol

role of this protein,

in hepatic

triacylglycerols,

the ethanol-induced phospholipids,

and

esters were smaller in females than in males.65

men,” at least below the age of 50 years.” As a consequence, for a given intake, their blood ethanol levels are

Thus, because of the inadequate increases in liver fatty acid-binding capacity and fatty acid esterification, cou-

higher, an increase that is compounded by differences in body composition (more fat and less water in women)

pled with a lesser increase in o-oxidation, females may have a higher risk for deleterious accumulation of fatty acids in the liver, thereby potentially contributing to

and, on the average, a lower body weight. The higher blood ethanol level, in turn, may contribute to the greater susceptibility of women to alcohol (see above). However,

their enhanced vulnerability to alcohol-induced hepatotoxicity. One can predict that the sex-related difference

older women have the same or even higher gastric ADH activities than men of similar age, because gastric ADH activity decreases with age only in men.”

in drug toxicity on the basis of difference on the expression levels of sex-specific cytochromes P450 or other proteins may also pertain to humans.

Thus, sex must be recognized as one of the factors that determines ethanol metabolism and hence severity of alcoholic liver injury, especially because male/female differences in drinking are smaller than they were a generation ago, a fact that appears to relate primarily to drinking by young women5a Another mechanism whereby the female sex potentiates alcohol-induced liver damage could relate to the hormonal status, because both endogenous and exogenous (i.e., contraceptive) female hormones have been shown to result in some impairment

alcoholism in humans has been known for decades; this factor has now been shown to play a major role in the etiology of alcoholism in women.” Individual differences in rates of ethanol metabolism also appear in part to be genetically controlled, and it is suspected that genetic factors influence the severity of alcohol-induced liver disease. Indeed, preliminary results” indicate different ADH3 allele frequencies in patients with alcohol-related end-organ damage (including cirrhosis) compared with

The possible

role of heredity

for the development

of

April 1994

ALCOHOL AND THE LIVER: 1994

UPDATE

1089

ETALDEHYDE

flgure 3. Oxidation of ethanol in the hepatocyte. Many disturbances in intermediary metabolism and toxic effects can be linked to (1) ADH-mediated generation of NADH, (2) the induction of the activity of microsomal enzymes, especially the P4502El MEOS containig (CYP2El), and (3) acetaldehyde, the product of ethanol oxidation. GSH, reduced glutathione; GSSG, oxidized glutathione. - -, pathways that are depressed by ethanol; -, stimulation or activation: -[, interference or binding.

GLUYAYAYE 4

locally matched control subjects, suggesting that genetically determined differences in alcohol metabolism may

produced substrate

in part explain

steroids, oxidation of the intermediary alcohols of the shunt pathway of mevalonate metabolism, and a-oxida-

differences

related disease (possibly

in susceptibility

through

enhanced

to alcoholgeneration

of

by fermentation in the gut.‘i ADH has a broad specificity, which includes dehydrogenation of

toxic metabolites), but this has been questioned more recently.” Similarly, a significant association of a particu-

tion of fatty acids’“; these processes may act as the “physiological” substrates for ADH.

lar restriction fragment length polymorphic (RFLP) haplotype of the COLlA2 locus and alcoholic cirrhosis has been reported by some investigators 69 but not confirmed by others.‘”

Human liver ADH is a zinc metalloenzyme with five classes of multiple molecular forms that arise from the

Pathogemesis of Alcoholic Liver Disease The hepatocyte contains three pathways for ethanol metabolism, each located in a different subcellular compartment: the ADH pathway in the cytosol (the soluble fraction of the cell), the microsomal ethanol oxidizing system (MEOS), located in the endoplasmic reticulum, and catalase, located in the peroxisomes (Figure 3). ADH and MEOS produce

specific metabolic

and toxic distur-

bances, and all three pathways result in the production of acetaldehyde, a highly toxic metabolite.

The Alcohol Dehydrogenase Pathway and Associated Disorders The major pathway for ethanol disposition involves ADH, an enzyme that catalyzes the conversion of ethanol to acetaldehyde. The raison d’&re of this enzyme might be to rid the body of the small amounts of alcohol

association

of eight

different

types of subunits,

CX, pl,

p2, p3, ~1, ‘)Q, X, and X, into active dimeric molecules. A genetic model accounts for this multiplicity as products of five gene loci, ADHl through ADHS.‘j There are three types of subunit, a, p, and ‘y, in class I. Polymorphism occurs at two loci, ADH2 and ADH3, which encode the p and y subunits. Class II isozymes migrate more anodically than class I isozymes and, unlike the latter, which generally have low Michaelis constant (KJ values for ethanol, class II (or XC)ADH has a relatively high Km (34 mmol/L) and a relative insensitivity to 4methylpyrazole inhibition. participate in the oxidation

Class III (xADH) does not of ethanol in the liver because

of its very low affinity for that substrate; it is not inhibited by 12 mmol/L 4-methylpyrazole.‘* More recently, a new isoenzyme of ADH has been purified from human stomach, so-called
1090

CHARLES

S. LIEBER

ones, these enzymes

GASTROENTEROLOGY

are mostly

inactive

at the levels of

ethanol achieved in the blood. Therefore, extrahepatic metabolism of ethanol is negligible, with the exception of the one in the stomach

to the liver have signifi-

activity.

of alcohol,“’

first pass metabolism Metabolic

investigations

shown

exist in the stomach

three

different

with either high,

ate, or low K, values for ethanol.”

forms

of

intermedi-

Because of the extraor-

No. 4

used dilute

to minimize

gastric

(see above).

effects of excessive ADH-mediated

hepatic generation of nicotinamide adenine dinuclee tide, reduced form. In ADH-mediated

Gastric ADH. At least ADH

some of the negative

concentrations

(see below) or in the kidney,

the two tissues that in addition cant low-K,,, ADH

thermore,

Vol. 106,

oxidation

of eth-

anol, acetaldehyde

is produced

and hydrogen

is trans-

ferred from ethanol

to the cofactor nicotinamide

adenine

dinucleotide

(NAD),

which

is converted

to its reduced

dinarily high gastric ethanol concentration after alcohol consumption, even the gastric ADH forms with a high

form (NADH)

K, for ethanol become active, and significant

which is released into the bloodstream. As a net result, ethanol oxidation generates an excess of reducing equiva-

gastric etha-

nol oxidation ensues,79 which has recently been confirmed in cultured gastric cells8’ Ethnic variability possibly pertains,

because

80% of Japanese

were found

to lack one

of the gastric isozymes.81 Because some isozymes require a relatively high ethanol concentration for optimal activity, the concentration amount

of alcoholic

metaboliteda2;

has only the high-K,

beverages

consequently, enzyme,

relatively

affects the

used, not much first-pass

metabolism

of reducing

equivalents

ity to maintain

as NADH.

overwhelm

redox homeostasis,

most

of

The large amounts

the hepatocyte’s and a number

abil-

of meta-

bolic disorders ensue (Figure 3),* including hyperlactacidemia, which contributes to the acidosis and also reduces the capacity

of the kidney

to excrete uric acid, leading

hyperuricemia.

Alcohol-induced

was measurable,83

low concentration

(such as

beer) undergo less gastric metabolism and result in higher alcohol blood levels than distilled beverages.*> Fasting strikingly decreases first-pass metabolism,86 most gastric

lents in the liver, primarily

to acetate,

to secondary

was observed at higher conen taken with a meal, alcowh

likely because of accelerated

and is metabolized

high concentra-

metabolism

centrations. Similarly, holic beverages of relatively

loses hydrogen

again

in the rat, which

tions are required for significant first-pass metabolism to be observed.82 Accordingly, when only 2.5% ethanol was whereas greater

(Figure 3). The formed acetaldehyde

emptying.

ketosis and

acetate-mediated enhanced ATP breakdown and purine generation”” may also promote the hyperuricemia. Hyperuricemia may be related to the common clinical observation

that excessive consumption

frequently

aggravates

of alcoholic

or precipitates

gouty

beverages

attacks.

The increased NADH/NAD ratio also increases the concentration of a-glycerophosphate, which favors hepatic triglyceride accumulation by trapping fatty acids. In addition,

excess NADH

thesis. Theoretically,

may promote

enhanced

lipogenesis

fatty acid syncan be consid-

First-pass metabolism decreases the bioavailability of ethanol and represents a “protective barrier” against systemic effects, at least when ethanol is consumed in small

ered a means of disposing of the excess hydrogen. Some hydrogen equivalents are transferred into mitochondria by various “shuttle” mechanisms. The activity of the

“social drinking” amounts. In humans, this “gastric barrier” disappears after gastrectomy” and is lost in part in the alcoholic56V*6 (Figure 4E) because of a decrease in gastric ADH activity. Similar effects may also result from gastric ADH inhibition by some commonly used drugs

citric acid cycle is depressed, partly because of a slowing of the reactions of the cycle that require NAD; the mito-

(Figure 4C). For instance, aspirins8 and some H2 blockers were found to inhibit gastric ADH activity in vitro89-9’ and to result in increased blood alcohol levels in vivo.78 Although questioned at first, such increases in blood level have now been confirmed9’X92 using a low alcohol dose of 0.15 g/kg. The Hz-blocker effect on blood alcohol levels has also been shown with higher doses of ethanol, 93-95 with an associated increase in an intoxication score,96 but these effects at higher ethanol dosage are still the subject of controversy. It must be pointed out, however, that not all subjects have an appreciable firstpass metabolism.” Published negative reports with H2 blockers do not specify whether first-pass metabolism was present to begin with in the subjects studied; fur-

chondria will use the hydrogen equivalents originating from ethanol rather than those derived from the oxidation of fatty acids that normally serve as the main energy source of the liver. Short-term

alcohol intoxication

occasionally

causes se-

vere hypoglycemia, which can result in sudden death. As reviewed elsewhere,* hypoglycemia is due in part to the block of hepatic gluconeogenesis by ethanol, again as a consequence of the increased NADH/NAD ratio in subjects whose glycogen stores are already depleted by starvation or who have pre-existing abnormalities in carbohydrate metabolism. Depending on the conditions, ethanol may accelerate rather than inhibit gluconeogenesis. Indeed, hyperglycemia may also occur in association with alcoholism. Its mechanism is still obscure, but glucose intolerance may be due at least in part to decreased peripheral glucose utilization.

April 1994

ALCOHOL AND THE LIVER: 1994

ALCOHOL

ALCOHOL DEHYDROGENASE

1

iLC&OL DEHYDROGENASE

w

+t

1 ALCOHOL

METABSbUTES

1

i

DRUGS

ACETALDEHVDE

(

1 ALCOHOL

ZEnYDROGENAS5

1 ACETALDEHYDE

ACETAliEHYOE

METABOLITES

i

ALCOHOL DEHYDROGENASE

METABOLITES

ACETALDEHYDE

1091

DRUGS

ALCOHOL DEHYDROGENASE I

, A ACETklEHYDE

UPDATE

1

1

DRUGS

1

DEHYDROGENASE

ACETALDEHYDE

METAEOLITES

Figure 4. Schematic representation of hepatic ethanol-drug interactions involving the ADH pathway and liver microsomes. (A) Hepatic metabolism of alcohol by ADH and drugs by microsomes. (9) Inhibition of hepatic microsomal drug metabolism in the presence of high concentrations of ethanol, in part through competition for a common microsomal detoxification process. (C) Inhibition of gastric ethanol metabolism by drugs. (D) Microsomal induction after chronic alcohol consumption and its contribution to accelerated hepatic metabolism of ethanol at high blood levels. (E) Decreased gastric ADH activity and gastric ethanol metabolism after chronic alcohol abuse. (F) Increased hepatic drug metabolism and xenobiotic activation because of the persisting microsomal induction after withdrawal from long-term alcohol consumption.

Hepatic steatosis and other zonal effects in the

fed alcohol over a long term consume

liver. One of the earliest pathological manifestations of alcohol abuse is the development of a fatty liver. Fatty

hanced consumption

acids of different

ent of oxygen

sources can accumulate

in the liver because of different

as triglycerides

metabolic

disturbances:

enhanced hepatic lipogenesis, decreased hepatic release of lipoproteins, increased mobilization of peripheral fat, enhanced

hepatic

important, function

uptake

decreased

of circulating

lipids,

fatty acid oxidation,

whether

of the reduced citric acid cycle activity

to the altered redox potential quence of permanent changes

and, most as a

secondary

(see above) or as a consein mitochondrial structure

and functions, ‘,* now documented by breath analysis in alcoholics.‘00 In cultured hepatocytes, the increased intracellular accumulation of triacylglycerol in the presence of ethanol

was quantitatively

fatty acid uptake, tricarboxylic

decreased

accounted

acid cycle, and decreased

lipoprotein

It was then

tension

postulated

that

the en-

of oxygen would increase the gradialong the sinusoids

to the extent

of producing anoxic injury of perivenular hepatocytes.‘02 Indeed, both in human alcoholics’o3 and in animals fed alcohol over a long term,104~‘o’ decreases in either hepatic venous oxygen saturationlo or POT”‘* and in tissue oxygen tension’05

have been found

during

the withdrawal

state. However, the changes in hepatic oxygenation found during the withdrawal state disappeared’04S1o6 or decreasedlo when alcohol was present in the blood. Shortterm administration of ethanol increased splanchnic oxygen consumption in naive baboons, but the consequences of this effect on oxygenation in the perivenular zone were

in the

offset by increased blood flow resulting in unchanged hepatic venous oxygen tension.“* In fact, ethanol induces

secre-

an increase

for by increased

fatty acid oxidation

those of controls.

more oxygen than

tion.‘O1 A characteristic feature of liver injury in the alcoholic is the predominance of steatosis and other lesions in the perivenular zone, also called centrilobular or zone 3 of the hepatic acinus. The mechanism for this zonal selectivity of the toxic effects involves several distinct and not mutually exclusive mechanisms. The hypoxia hypothesis originated from the observation that liver slices from rats

in portal

hepatic

blood

flo~.~~*~‘~~~‘~~ In

cats ‘lo and baboons”” fed alcohol over a long term, defective oxygen utilization rather than lack of blood oxygen supply characterized liver injury produced by high concentrations of ethanol. The low oxygen tension normally prevailing in perivenular zones exaggerates the redox shift produced by ethanol.“* Hypoxia, by increasing NADH, may in turn inhibit the activity of NAD+-dependent xanthine dehydrogenase (XD), thereby favoring

1092

CHARLES

S. LIEBER

GASTROENTEROLOGY

that of oxygen-dependent ure 3). Purine duction fects

of oxygen towards

Physiological

xanthine

metabolism radicals,

liver

including

for X0,

together

toxic ef-

peroxidation.

hypoxanthine,

thine, as well as AMP, significantly after ethanol,

(Fig-

may lead to the pro-

which can mediate

cells,

substrates

oxidase (X0)“’

via X0

increased

with an enhanced

isolated

from ethanol-treated

form of P450, purified

No. 4

rats. An ethanol-

from rabbit liver micro-

somes, ‘*’ catalyzed ethanol oxidation at rates much than other P450 isozymes and also had an en-

higher

and xanin the liver

urinary

fraction inducible

Vol. 106,

output

hanced

capacity

to oxidize

aniline, 126 acetaminophen,“’ N-nitrosodimethylamine.‘30 (now called CYP2El

l-butanol,

1-pentanol

and

CC14,‘*’ acetone, 1283’29and The purified human protein

or 2El) was obtained

in a catalyti-

of allantoin Allopurinol

(a final product of xanthine metabolism). pretreatment resulted in 90% inhibition of

cally active form, with a high turnover

rate for ethanol

X0

and also significantly

in-

and other

has a relatively

the

high K,, for ethanol (8- 10 mmol/L compared with 0.22 mmol/L for hepatic ADH), and thus ADH normally accounts for the bulk of ethanol oxidation at low blood

activity

decreased

duced lipid peroxidation.“’ Zonal distribution of some enzymes

ethanol

can influence

specific

selective perivenular toxicity. As discussed subsequently, proliferation of the smooth endoplasmic reticulum after

ethanol

long-term

at high ethanol

venular

ethanol

consumption

zone, with associated

lated effects. Furthermore,

tabolism

in the perivenular

increased

hepatotoxicity

(together

with

enzyme

human

shown mainly in hepatocytes venule. Thus, a presumably

viding

is maximal

in the peri-

induction

ADH

and re-

has now been

around the terminal hepatic higher level of ethanol mezone could contribute

of ethanol, the “induced”

for instance

to the by pro-

microsomal

path-

way; see below) an increased amount of the toxic metabolite acetaldehyde.‘12 However, it must also be taken into account that after long-term ethanol consumption, unlike the activity

of MEOS,

which

is induced,

that

of

ADH may not change or even decrease.‘13-116 Alcoholics may show decreased hepatic ADH activity even in the absence of liver damage.“’ The bulk of hepatic ADH

concentrations

in the hepato-

found

in biopsy

tabolism

significantly

increases prostanoid

production

in

these cells. Metabolism

of Alcohol via the MEOS

Characterization of the MEOS and its role in ethanol metabolism. Liver microsomes were found to be the site for an adaptive system of ethanol oxidation,“48’20 named the MEOS. Its distinct nature was shown by (1) isolation of a P450-containing fraction from liver microsomes that, although devoid of any ADH or catalase activity, could still oxidize ethanol as well as higher aliphatic alcohols (e.g., butanol, which is not a substrate for catalase)1219’22 and (2) reconstitution of ethanol-oxidizing activity using NADPH-cytochrome P450 reductase, phospholipid, and either partially purified or highly purified microsomal P450 from untreated’23 or phenobarbital-treated124 rats. Long-term ethanol consumption results in the induction of a unique P450, as shown by Ohnishi and Lieber123 using a liver microsomal P450

long-

40) in view of the inducibilcontrasting

with hepatic

specimens

(of subjects

who had drunk

ethanol (e.g., acetone) can also serve as 2El inducers, but their change is not essential for the ethanol effect since 2El can be induced consumption

after short-term

of ethanol,

acetonemia

or hepatic

The molecular

light

even in the absence of increased steatosis.“’

mechanism

disputed.‘34

and relatively

underlying

Investigations

2E 1 induction

using

rabbits

(and

some involving rats) appeared to have ruled out transcriptional activation of the 2El gene or stabilization of 2El messenger

me-

during

recently) using specific antibodies against this 2El and the Western blot technique.i3’ Compounds other than

cause ethanol

ethanol

4A) but not necessarily

ADH, which is not inducible in primates as well as most other animal species, a 5-lo-fold induction of 2El was

acetaldehyde

from ADH-mediated

(Figure

ity of the MEOS.“4*‘20 Indeed,

cytes, but traces are also found in lipocytes.“’ Their role was questioned until Flisiak et al.“” showed that derived

MEOS

levels (Figure 48), especially

term use of alcohol (Figure

remains is present

substrates.“’

RNA

(mRNA)

and similar

as possible agents

mechanisms

(acetone,

imidazole,

be4-

methylpyrazole, pyrazole, and pyridine) had little effect on 2El transcript content in liver.‘35-‘39 A posttranslational mechanism, namely protein stabilization, was thus proposed because (1) ethanol and imidazole were found to prevent the rapid decrease in 2El enzyme levels that occurs in rat hepatocytes upon primary culture,140 (2) acetone treatment was shown to prolong the in vivo halflife of 2El in rat liver by eliminating the fast-phase component associated with the enzyme’s normal degradation.‘41 Yet other studies support the role of enhanced de novo enzyme synthesis and/or increased mRNA levels in the 2El induction process. Kim and Novak14’ observed increased rates of [14Clleucine incorporation into 2El protein after treatment of rats with pyridine, a phenomenon attributed to the enhancement by pyridine of 2El mRNA translational efficiency.‘43 Kubota et a1.14” found that induction of 2El protein in hamsters by ethanol and pyrazole was associated with an increase in translatable 2El mRNA levels, whereas Diehl et a1.‘45 have

ALCOHOL

April 1994

described

elevated

levels of hepatic

2El mRNA

in alco-

administration

somal demethylation

that mRNA stabilization and/or patients, 146 indicating transcriptional activation are involved in ethanol-mediing both

beled 2El protein the rat, ethanol 2El synthesis

2El

of radiola-

rates also results

at induced

high steady-state

reasonable

this mRNA

levels in

rates of de novo

to assume

of de novo 2El synthesis

message

from increased

and/or

the increased

that

the

noted in ethanol-

steady-state efficiency

levels of

with which

is translated.

The conflicting

brain

of the drug.‘53 These effects may

by measur-

was found to stimulate

rats results

micro-

and enhances

Furthermore,

dation. 14’ It is therefore treated

1093

and the degradation

blood, monly

but had no effect on rates of enzyme degra-

enhancement

UPDATE

effect: it inhibits

of methadone

and liver concentrations

1994

be of clinical relevance, because approximately 50% of the patients taking methadone are alcohol abusers. The combination of ethanol with tranquilizers and barbitu-

in humans.

the synthesis

THE LIVER:

has the opposite

hol-treated rats. Enhanced levels of both hepatic 2El protein and mRNA were also found in actively drinking

ated 2El induction

AND

in increased

sometimes to dangerously high observed in successful suicides.

with regard to the 2El induc-

in the

levels,

as com-

Increased xenobiotic toxicity and carcinogenicity in alcoholics, including interactions with retinoids and carotenoids.

On occasion, the metabolites

in the microsomes compounds.

are more

toxic

Much of the medical

(and its ethanol-inducible

results

drug concentrations

oxidation

of ethanol

than

produced

the precursor

significance

of MEOS

2El) results not only from the but

also from

unusual

and

unique

or distinct

activate many xenobiotic compounds to toxic metabolites. This pertains, for instance, to carbon tetrachloride.

pounds,

mechanisms

of induction

because the commonly

by the various com-

used 2El inducing

agents

capacity of 2El to generate

the

tion process may stem from differences between species, the duration and/or manner of inducer treatment, and/

mediates

such as superoxide

It is known

to an active compound

cally most relevant

pretreatment

with alcohol

radicals (Figure

that CC14 exerts its toxicity

other than ethanol, such as acetone, pyratole, or pyridine, may differ in their mode of action. Obviously, the cliniresults are those obtained

reactive oxygen inter-

remarkably

stimulates

posttranslational mechanism at low ethanol concentrations and an additional transcriptional one at high etha-

in that

zone of the liver.‘32 A larger

organic

compounds

injurious

action

Interactions with other microsomal substrates, including drugs. The from long-term the oxidation

microsomal

alcohol consumption of ethanol

induction

resulting

not only accelerates

but also increases the metabolism

of many other microsomal thesis of triacylglycerols.‘50

substrates, including Its main interaction,

is with other drugs such as warfarin,

phenytoin,

the synhowever, tolbuta-

mide, propranolol, and rifampin.4 Ethanol administration to volunteers under metabolic ward conditions resulted in a striking meprobamate

increase in the rate of blood clearance of and pentobarbital.‘5’ The metabolic drug

tolerance persists several days to weeks after the cessation of alcohol abuse, and the duration of recovery varies with each drug.15* During that period, the dosage of these drugs has to be increased to offset the increased breakdown. Contrasting

with

the

inductive

effect of long-term

ethanol consumption, after short-term administration, inhibition of hepatic drug metabolism is found, primarily because of its direct competition for a common metabolic process involving cytochrome P4504 (Figure 4B). Methadone provides a cogent example of this dual interaction: whereas long-term ethanol consumption leads to increased hepatic microsomal metabolism of methadone and decreased levels in the brain and liver, short-term

perivenular

predominance,

by the selective

the alcoholic.

toxicity

which

and induction number

of

can be of 2El of other

were found to show such a selective

in the liver as well as other

These include

as bromobenzenei’” anesthetics

presence

and alcohol

the

cc14’55 with

nol levels.1489’49

after conversion

in the microsomes,

in humans. The dose of the inducer is also of importance, because 2El induction appears to occur via two steps, a

explained

3)154 and to

other industrial

and vinylidene

tissues

of

solvents such

chloride,“’

as well as

such as enflurane15’ and halothane.15’ Ethanol

also markedly increased K, benzene-metabolizing

the activity enzymes””

of microsomal lowand aggravated the

hemopoietic toxicity of benzene. Thus, commonly used industrial solvents and anesthetics, considered safe in normal subjects, may acquire unusual hepatoxicity in heavy drinkers. Enhanced metabolism (and toxicity) pertains also to a variety of prescribed drugs, including isoniazid and phenylbutazone.“’ The same mechanism to some over-the-counter

of hepatotoxicity medications.

also applies

Among

alcoholic

patients, hepatic injury associated with acetaminophen (paracetamol, N-acetyl-p-aminophenol) has been described following repetitive intake for headaches (including those associated with withdrawal symptoms), dental pain, or the pain of pancreatitis leading in some to high daily doses.“* There is an association between alcohol misuse and an increased incidence of upper alimentary and respiratory tract cancers.‘63 Many factors have been incriminated, one of which is the effect of ethanol on enzyme systems involved in the cytochrome P450-dependent activation

1094

CHARLES

S. LIEBER

of carcinogens.

GASTROENTEROLOGY

This effect has been shown with the use

take) has by itself no detectable

Vol. 106.

No. 4

adverse effects, but when

of microsomes derived from a variety of tissues, including the liver (the principal site of xenobiotic metabo-

combined with alcohol, it results in striking leakage of the mitochondrial enzyme glutamine dehydrogenase into

lism), ‘64*165the 1ungs,‘64*‘65 and the intestines166*‘67 (the chief portals of entry for tobacco smoke and dietary car-

the bloodstream.181 Thus, in heavy drinkers there is a narrowed therapeutic window for vitamin A, and injudi-

cinogens,

cious supplementation

respectively),

nol consumption

of cancer). Alcoholics a synergistic

are commonly

heavy smokers,

effect of alcohol consumption

on cancer development elsewhere.lG3 was found

and the esophaguslG8 (where etha-

is a major risk factor in the development

Indeed,

to enhance

has been described long-term

ethanol

the mutagenicity

the liver disease

and

for P-carotene.

and reviewed

fore there was a consensus

consumption

toxicity

of tobacco-de-

may also influence

carcinogenesis

other ways, ‘69 one of which involves consumption vitamin with

A in humans”’

diets

reflecting

vitamin

A. Ethanol

has been shown to depress hepatic containing

and in animals, large

in part accelerated

the vitamin.

amounts

levels of

that retinoic

P4502C8

of microsomal

of

retinol

that

appreciable

amounts

of

this enzyme were present in human liver microsomes. The same antibody significantly inhibited retinol metabolism in liver microsomes and in a system reconstituted with

P4502C8.

The latter

also converted

retinoic

acid

to polar metabolites.‘7s When ethanol and phenobarbital were combined, a marked potentiation of the hepatic vitamin A depletion was observed.‘76 Because alcohol abuse is often associated clinically with overuse of other drugs, and because drug use may also be associated with severe hepatic vitamin A depletion,“’ this potentiation may be meaningful in terms of vitamin A depletion in an appreciable

segment

of the population.

Both hepatic vitamin A depletion and excess cause adverse effects: depletion is associated with lysosomal lesions’78 and decreased detoxification of nitrosodimethylamine,“” whereas excess is hepatotoxic.“’ Long-term ethanol consumption enhances the latter effect, resulting in striking morphological and functional alterations of the mitochondria’8’ along with hepatic necrosis and fibrosis. “* Hypervitaminosis A itself can induce fibrosis and even cirrhosis, as reviewed elsewhere,“’ but this is an unusual occurrence necessitating very large amounts of vitamin A (in excess of 50-100 times the daily requirement) given over prolonged periods. A smaller vitamin A supplementation (e.g., five times the normal in-

that the possi-

hol, and/or other drugs is virtually uncharted at this time but cannot be excluded a priori, especially because in nonhuman

primates

the presence

enhanced

of ethanol

toxicity

and/or excretion ho1 abuse.

of p-carotene

in

has been observed.183 Thus, cau-

of vitamin

A,“]

Hereto-

p-carotene

and liver disease, alco-

with p-carotene

acid’74 and retinal”’

showed

p-carotene

existence

can serve as substrates for microsomal oxidation. Immunoblots performed with a monospecific antibody directed against human

exists. It must be noted, however, between

the

this is not the case

no obvious

in view of the possible

metabolism, inducible by either ethanol or drug administration, have been discovered.‘72”73 Furthermore, reconstituted systems with purified forms of cytochrome P450 revealed

that

tion must be exercised

degradation

to retinoids,

studies are still lacking.

even when given

microsomal

New hepatic pathways

Detailed

ble interaction

in many

hasten rather than alleviate

As opposed

of which is well established,

and smoking

rived products.“’ Alcohol

toxicity

might

process.

associated

supplementation

of a defect in utilization

with liver injury

and/or alco-

184

Interactions of ethanol with energy balance. Although ethanol is rich in energy (7.1 kcal/g), long-term consumption of substantial amounts of alcohol is not associated with the expected effect on body weight.la5 Furthermore, isocaloric substitution of carbohydrates ethanol under metabolic ward conditions resulted

by in

weight loss, and addition of ethanol to an otherwise normal diet did not produce the expected weight gain.“’ This energy deficit cannot be explained merely on the basis of maldigestion or malabsorption but has been attributed primarily to induction of the microsomal ethanol oxidizing

system (a metabolic

pathway

that oxidizes

ethanol without associated chemical energy production; Figure 3). Alternate or additional mechanisms invoked include increased sympathetic tone and associated thermogenesis, and/or enhanced ATP breakdown (with increased purine

catabolism)

secondary

to the acetate pro-

duced from ethanol (see above). Although attractive, all these hypotheses do not fully explain the lack of weight loss when alcohol is consumed with a very low-fat diet (5% of energy),133 which suggests that an alteration in the energy utilization derived from fat plays a major role in the ethanol-induced energy deficit. One possible mechanism is the uncoupling of oxidation with phosphorylation in mitochondria damaged by long-term ethanol consumption, in part because of the acetaldehyde generated, as reviewed elsewhere.18>

Role of Catalase and Nonoxidative Metabolism of Ethanol Catalase can oxidize ence of an H,02-generating

ethanol in vitro in the pressystem’“’ (Figure 3). How-

ALCOHOL

April 1994

ever, under

physiological

play no major decrease

in catalase

activity

liver injury.‘s8 It has been proposed might

be enhanced

came available

through

activity.

reduced by adding acids is inhibited

by NADH

Lange”’

enzyme

that,

out that this is

of fatty

from ethanol

me-

observed

after

compared

Laposata

with

and

controls,

LIVER:

1994

UPDATE

acetaldehyde

1095

generation

be-

see above), results in an imbal-

in

long-term

ethanol 196

mans 85 and in baboons, increase

of acetaldehyde

flecting

high tissue levels.

consumption

associated

in hepatic

in hu-

with a tremendous venous

blood,“”

re-

Promotion of lipid peroxidation through interactions with cysteine, glutathione, Lipid peroxidation holic liver injury humans.‘“’

esters in vivo, and the correpurified.‘“’

(and therefore

cause of MEOS induction;

of ADH

metabolism

and P-oxidation

produced

has been

have found

be-

of fatty acids in per-

only in the absence

nol oxidation

THE

ance between production and disposition of acetaldehyde. The latter contributes to the elevated acetaldehyde levels

of catalase of H,O,

be pointed

the rate of ethanol fatty acids,“’

a

in alcoholic

amounts

P-oxidation

tabolism via ADH.‘“’ Ethanol can form ethyl sponding

was observed

it should

to

Moreover,

that the contribution

was observed

Otherwise,

catalase appears

metabolism.

if significant

oxisomes. ‘a9 However, phenomenon

conditions,

role in ethanol

AND

radical

vitamin E, and iron.

was found to be associated both

in experimental

with alco-

animals

and in

It results not only from the increased

production

from the enhanced

by the induced generation

of acetaldehyde,

per-

of acetaldehyde fused livers. 199 In vitro, metabolism X0 or aldehyde oxidase may generate free radicals,

but

liver, heart, and adipose tissue. Because this nonoxidative

the concentration

too

ethanol

high

metabolism

occurs in humans

in the organs most

lipid peroxidation

also

shown to

in isolated

short-term-intoxicated subjects, concentrations of fatty acid ethyl esters were significantly higher in pancreas,

be capable of causing

oxygen

2E1i5*~“* but

of acetaldehyde

for this mechanism

required

via

is much

to be of significance

in vivo.

commonly injured by alcohol abuse, and because some of these organs lack oxidative ethanol metabolism, Lapo-

However,

sata and Lange’92 postulated that fatty acid ethyl esters may have a role in the production of alcohol-induced

cysteine and/or GSH liver levels of GSH.“’

injury. Further experiments are needed, however, to verify the possible role of this mechanism in the pathogene-

have

sis of liver injury.

sis and produces an increased loss from the liver.201 GSH is selectively depleted in the mitochondria”’ and may

Toxic Effects of Acetaldehyde Acetaldehyde

discussed.

with

to a decrease

in

administration

inhibits

GSH synthe-

In addition

to the

scavenging

sources of acetaldolases, pyof

commensal microorganisms to produce both ethanol and acetaldehyde from sugars.‘l Another putative source of acetaldehyde is provided by the cleavage of threonine to acetaldehyde and glycine by a threonine aldolase in the hepatic cytosol. Although this represents a minor pathway in the normal degradation of threonine,‘“’ it is conceivable that its relative role may be enhanced if liver with the major pathways,

threonine

may contribute

contribute to the striking alcohol-induced alterations of that organelle. GSH offers one of the mechanisms for the

ruvate dehydrogenase, and phosphorylphosphoethanolamine phosphorylase activities, as well as the capacity

injury were to interfere

ethanol

lipid peroxida-

of acetaldehyde

Rats fed ethanol over a long term greater rates of GSH turnover.“’

significantly

Short-term

to promote

Binding

of eth-

exogenous ethanol, there are endogenous aldehyde, such as deoxypentosephosphate

the mitochondrial

mechanism

product

is the first oxidation

anol by all three pathways

another

tion is via GSH depletion.

dehydrogenase

namely

and the cyto-

solic threonine dehydratase.‘“” The major mechanism for acetaldehyde disposition is its oxidation to acetate in hepatic mitochondria. However, long-term ethanol consumption results in a significant reduction of the capacity of rat mitochondria to The decreased capacity of mitooxidize acetaldehyde.‘“5 chondria of alcohol-fed subjects to oxidize acetaldehyde, associated with unaltered or even enhanced rates of etha-

which

of toxic free radicals,

also illustrates

as shown

how the ensuing

in Figure

enhanced

2,

GSH

utilization (and thus turnover) results in a significant increase in a-amino-n-butyric acid, shown in the blood of both humans and baboons.203 Although GSH depletion per se may not be sufficient to cause lipid peroxidation, it is generally agreed that it may favor the peroxidation produced by other factors. GSH has been shown to spare and potentiate vitamin EZO*;it is important in the protection of cells against electrophilic drug injury in general, and against reactive oxygen species in particular, especially in primates, which are more vulnerable to GSH is not only depletion than r0dents.s Lipid peroxidation a reflection

of tissue damage;

genic role, for instance tion.‘05

it may also play a patho-

by promoting

collagen

produc-

Antioxidant protective mechanisms involve both enzymatic and nonenzymatic defense systems.“’ Impairments in such defense systems have been reported in alcoholics, including alterations of ascorbic acid levels,“’ GSH (see above), selenium,208-2i0 and vitamin E.2’o-213 These changes could be a result of the direct effects of ethanol

1096

CHARLES

GASTROENTEROLOGY

S. LIEBER

or the malnutrition

associated

with

alcoholism.

Bjorneboe

patic a-tocopherol

content

ing

in rats receiving

minant term

of hepatic ethanol

level was found vitamin

and ethanol copherol

etha-

a low vitamin

content,

E diet,“’

E is an important

peroxidation

in rats receiving

feeding

E

greater after long-term

The lowest

E and ethanol;

of vitamin

lipid

vitamin

lipid

feeding.

amounts

feed-

Hepatic

in rats receiving that dietary

ethanol

of alcoholics.2’7

is significantly

nol feeding indicating

after long-term

adequate

as well as in the blood peroxidation

et al.216 report reduced he-

induced hepatic

deterby long-

a-tocopherol

a combination

of low

both low dietary vitamin

E levels

significantly

the latter

conversion

of a-tocopherol

In patients

with

cirrhosis,

reduced

hepatic

a-to-

in part because of increased to a-tocopherylquinone.2’8 diminished

hepatic

E levels have been observed.184 These deficient systems, coupled with increased acetaldehyde

vitamin defense

and oxygen

radical generation by the ethanol-induced microsomes (Figure 2), may contribute to liver damage via lipid peroxidation and also via enzyme inactivation.219 Iron overload

may play a contributory

role, because

long-term alcohol consumption results in increased iron uptake by hepatocytes220 and because ferric citrate-induced

lipid

peroxidation

is accentuated

from ethanol-fed rats. Iron overload ciency in the alcoholic in conjunction abnormalities

have been reviewed

in microsomes

as well as iron defiwith other mineral elsewhere.22’

Acetaldehyde protein adducts and effects on including repair of nucleoproenzyme activities, teins. Protein adduct formation aldehyde toxicity. Acetaldehyde

is another mode of acetbinds covalently to liver

microsomal proteins,222 including 2E 1 ,723 other macromolecules2’* such as collagen,22S~‘26 and circulating proteins: serum albumin,227 hemoglobin,228 and lipoproteins.229 It also binds to the tub&n of the microtubules. One of the key functions the intracellular transport Long-term

alcohol

feeding

of microtubules is to promote of proteins and their secretion. to rats seriously

delayed

the

secretion of proteins from the liver into the plasma and caused a corresponding hepatic retention.64 The increases in levels of lipid, protein, water, and electrolytes resulted in enlargement of the hepatocytes. Acetaldehyde adducts may also serve as neoantigens, generating an immune response in mice230 and in humans.‘3’-233 Another mode of acetaldehyde toxicity involves the interference with enzyme activities,234 possibly secondary to its binding with critical functional groups. Minute concentrations of acetaldehyde (as low as 0.05 pmol/L) were found to impair the repair of alkylated nucleoproteins.235

No. 4

Alcohol-Induced Disorders of Collagen Metabolism and Production of Cirrhosis

a-to-

copherol, the major antioxidant in the membrane, is viewed as the last line of defense against membrane lipid peroxidation.2143”s

Vol. 106,

Increased droxylase

activity

of hepatic

(a key enzyme

found in patients

peptidylproline

in collagen

with alcoholic

hy-

production)

was

cirrhosis236 and hepati-

tis237 and in all stages of alcoholic

liver disease.238 In

ethanol-fed

significant

type greater

baboons

I procollagen

who developed mRNA

(per liver RNA)

analysis.239 Whereas the relative “activated”

was significantly

as determined

by hybridization

there is still some discussion

contributions

in the production

of hepatocytes

of collagen

after long-term

fibrosis,

content

about

and lipocytes

in the liver, lipocytes

alcohol consumption

pear to play a major role.240~24’Normal

lipocytes,

isolated and cultured on plastic surfaces, undergo neous transformation into myofibroblastlike

are

and apwhen spontacells,

thereby mimicking in vitro the condition that prevails in vivo after long-term alcohol consumption.242 These cells in culture

produce

collagen.242 When

is added to these cells, they respond crease in collagen

accumulation242

acetaldehyde

with a further and increased

in-

levels

of mRNA for collagen.243 Other aldehydes (such as malonaldehyde) are produced from lipid peroxidation, and they may also stimulate collagen production. Acetaldehyde stimulates collagen synthesis in cultured myofibroblasts as we1L2** and a similar effect was observed with lactate. These cells were shown to proliferate in the perivenular zones of the liver after long-term alcohol consumption; they are similar to “activated” lipocytes, although they can be differentiated by ultrastructural and cytochemical characteristics. Collagen accumulation reflects not only enhanced synthesis but also results from an imbalance between collagen degradation and collagen production. Thus, cirrhosis might in part represent a relative failure of collagen degradation to keep pace with synthesis. Interestingly, polyunsaturated lecithin may affect this balance. Indeed, addition of polyunsaturated lecithin to transformed lipocytes was found to prevent the acetaldehyde-mediated increases in collagen accumulation, possibly by stimulation of collagenase activity.245 The active ingredient was identified as dilinoleoylphosphatidylcholine.‘S The role of collagenase was also shown indirectly in humans by the correlation of the development of alcoholic fibrosis with increased activity of the circulating tissue inhibitor of metalloproteinase (TIMP).246 Indeed, serum TIMP levels were significantly greater in patients with alcoholic cirrhosis and may not only play a role in its pathogenesis through inhibition of collagenase activity but also serve as a marker of precirrhotic and cirrhotic states, because this test was more sensitive in detecting either perivenular fibrosis or septal fibrosis and offered better discrimina-

April 1994

ALCOHOL

tion from fatty liver than serum procollagen IIP).

The

explain

stimulation

of collagenase

peptide

activity**’

at least in part why polyunsaturated

tenuates

the development

after long-term

alcohol

of fibrosis (including administration,21

firmed using more purified pointed

lecithin

lecithin

of the polyunsaturated

studies,

the control

of 12 baboons

extracts,

developed of 81% ducing

transitional

normal,

lipocytes

10

primates,

unsaturated

developed

sep-

+ 9% of their

were transformed.

Prevention and Treatment

develop

Alcoholic steatosis is completely reversible in most instances. In some extremely severe cases, alcoholic

of excessive markers

in the diet might systematically

liver,‘**

be beneficial,

in terms of medical

years, and extremely cirrhosis

but

of

the fat content this has not been

liver disease are devas-

care costs, loss of productive

poor prognosis.

and the preceding

Death secondary

major medical

to

complications

are mostly due to sequelae of scarring or fibrosis of the liver. Past treatment efforts have focused on the management of the consequences of cirrhosis such as ascites and bleeding.

The results of these studies

have improved

our

capacity to help patients cope with these major complications, but they have not decreased the prevalence of the disease, and these traditional approaches come too late for the liver to revert to normal. A better understanding of how alcohol affects the liver now enables us to contemplate more direct approaches to prevent alcohol’s Originally, treatment of liver disease complicating

effect. alco-

holism

it was

appeared

to be relatively

simple

because

attributed exclusively to associated malnutrition. Indeed, nutritional deficiencies are common in the alcoholic4 and, when present, should be corrected, as recently outlined.249 Over the last two decades, the realization of the intrinsic toxicity of ethanol shifted the emphasis of treatment from the correction of nutritional deficiencies to the control of alcohol consumption. More recently, however, the pendulum is swinging back to a more com-

of fibrosis”.“‘; users there is a

who are particularly

and its complications.

(such as cirrhosis)

alcohol

do not

there is a need for early individuals

before

their

consumption.25032s’

usefu1.2523’53 Among

the heavy drinker,

nize

in the liver,

lesions already

by biological

carbohydrate-deficient

risk, namely

which subjects

at

can recog-

as perivenular

at a very early precirrhotic to predict

trans-

individuals

the physician

such

Of the

fibrosis,

stage allows

are prone to un-

dergo rapid progression to the cirrhotic stage upon continuation of drinking.15* Perivenular fibrosis is commonly associated with perisinusoidal and pericellular fibrosis

assessed.

More severe forms of alcoholic tating

fat in the production

decreasing

of

and poly-

in humans.

is now facilitated

studied,

ferrin is particularly

the physician

fatty

alcoholism

susceptible

various

few days or weeks after cessation

of alcohol consumption.

tested

the alcohol

complications

markers

which

the alcoholic

that among

in

social or medical disintegration to prevent, rather than simply treat, their major somatic complications. Screen-

fatty liver may have a fatal outcome,247 but as a rule even those patients needing hospitalization improve within a In view of the role of dietary

for the prevention

ing for heavy drinkers

and Intervention

can be

for the treatment

of very heavy drinkers

of those

cellular

that

liver injury”

in all heavy drinkers,

detection

1097

effects are often

some agents

are now being

It is obvious

UPDATE

were found to be effective

e.g., SAME

lecithin

both compounds

1994

at a biochemical

early aspects of alcohol-induced

Because major

none of the 8 ani-

and only 48%

Early Detection

nonhuman

LIVER:

“nutritional”

Therapeutically,

at risk of developing

to collagen-pro-

and

viewed as “supernutrients”

with transformation

lipocytes

cells. By contrast,

or cirrhosis,

whereas

intertwined.

THE

Indeed,

“toxic”

subpopulation

mals fed alcohol with phosphatidylcholine tal fibrosis

In the latter

approach.

both

phosphatidylcholine

septal fibrosis or cirrhosis, + 3% of the hepatic

cirrhosis) which again

lecithin.ls

without

may at-

level,

as the active

livers remained

fed alcohol

prehensive

an effect con-

to dilinoleoylphosphatidylcholine

ingredient

(PI-

AND

and correlates

with

collagenisation

of the Disse

space,‘5s but these other changes recognize and quantify on routine

are more difficult light microscopy.

present,

to detect these precir-

rhotic

liver biopsies lesions,

second-generation

are required

to At

but hopefully improved blood tests (i.e., TIMP or PIIIP) may eventually serve

the purpose of screening for those individuals who have a greater propensity to develop alcoholic cirrhosis.

Therapy for Alcoholic Cirrhosis

Hepatitis

and

In addition to the early detection of fibrosis in vulnerable subjects and efforts at preventing its progression, various therapeutic modalities have been proposed to help the alcoholic in a more severe stage of liver disease. The role of adrenocorticosteroids in therapy for acute alcoholic hepatitis has been the subject of debate for years. Several investigators25”-‘59 have reported significant improvement in survival rates of encephalopathic patients treated with steroid, but not in those with milder illness. Some other studies, however, did not confirm these findings. More recently, in patients who had either spontaneous hepatic encephalopathy or a high hepatic discriminant function (based on elevated prothrombin time and bilirubin concentration), prednisolone (40

1098

GASTROENTEROLOGY Vol. 106. No. 4

CHARLES S. LIEBER

mg/day for 28 days) improved

Propylthiouracil

has also been suggested

ment of alcoholic

tic effect has been reported by Szilagyi

current

state of knowledge

acceptance antithyroid

for the treat-

hepatitis,262 but no beneficial

review

by another

therapeu-

group.263 In a

et a1.,264 it was concluded

that

the

does not allow unequivocal

or rejection of the role of thyroid hormone and medication in treating alcoholic hepatitis. A

more recent long-term study reports a lowered mortality rate in the treated group,265 but the beneficial effect was restricted to those in whom alcohol intake was moderate. In patients

with alcoholic

quire

parenteral

amino

acids. Such parenteral

benefit prove

alimentation,

in moderate morbidity

probably

alcoholic

(as assessed

liver function

ever, it did not improve Colchicine,

hepatitis,

anorexia

may re-

including

infusion

of

alimentation

provided

no

hepatitis,

but it did im-

by liver test results)

in severe alcoholic hepatitis;

and how-

early mortality.2””

which inhibits

collagen

synthesis

and pro-

collagen secretion in embryonic tissue,267 may provide a useful approach for the treatment of alcoholic liver injury.*“.*” H owever, these studies have raised questions regarding differences in severity between colchicinetreated and placebo-treated groups and the high dropout rate. *‘” Additional

controlled

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another study, oxandrolone therapy was associated with a beneficial effect in moderately malnourished patients.*”

trials are presently

ongo-

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increas-

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UPDATE

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Received August 20, 1993. Accepted October 25, 1993. Address requests for reprints to: Charles S. Lieber, M.D., Alcohol Research and Treatment Center, VA Medical Center, 130 West Kingsbridge Road, Bronx, New York 10468. Fax: (718) 733-6257. Original studies reviewed here were supported, in part, by U.S. Department of Health and Human Services grants AAO3508, AA09479, AA05934, AA07802, AA07275, and DK 32810, and the Department of Veterans Affairs. The author thanks J. Cohen and R. Cabell for their skillful typing of the manuscript.