barcelona . spain - European Association for the Study of the Liver

BARCELONA . SPAIN
44 POSTGRADUATE COURSE SYLLABUS ALCOHOLIC LIVER DISEASE 45
APRIL 18 - 19/2012 THE INTERNATIONAL LIVER CONGRESS TM 2012
methods to diagnose this severe condition. The pattern of fibrosis is not homogeneous in all patients. It can
be pericellular, perisinusoidal, and ‘chickenwire’. Eventually, bridging fibrosis develops and the subsequent
formation of regenerative nodules defines cirrhosis.
FACTORS INFLUENCING FIBROSIS PROGRESSION IN ALD
Although a positive correlation between cumulative alcohol intake and degree of liver fibrosis has been
reported, extensive individual variability exists. The main environmental factors influencing the progression
of liver fibrosis in patients with alcohol abuse include obesity and cigarette consumption. The role of
metabolic factors such as insulin resistance is unclear, yet recent reports suggest that it may favor fibrosis
progression. Moreover, it is known that genetic factors influence the organ-specificity of alcohol-related
harmful effects. Family studies, twin concordance studies, and interethnic variations in susceptibility
suggest that genetic factors are important in determining disease risk.
Genes encoding for alcohol-metabolizing enzymes and proteins involved in liver toxicity such as
antioxidants and pro-inflammatory cytokines have been the main subject of investigations in genetic
studies. The main enzymes involved in alcohol metabolism in humans are alcohol dehydrogenase (ADH),
aldehyde dehydrogenase (ALDH), and cytochrome P450IIE1 (CYP2E1). Genetic factors influencing the
activity of these enzymes and the rate of alcohol metabolism have been extensively studied. Because of
its fibrogenic potential, variations in the generation of acetaldehyde may explain individual differences after
abusive alcohol consumption. Polymorphisms in genes encoding pro-inflammatory cytokines known to
participate in the pathogenesis of ALD have also been examined, including TNF-α, NFκB, IL-1β and IL-1
receptor antagonist, IL-2, IL-6 and IL-10. Other studies have investigated the role of genetic variations of
factors involved in LPS-induced intracellular pathways including CD14 and TLR4. Patients with genetic
variations of superoxide dismutase and gluthatione-S-transferase, powerful antioxidants, are a risk factor
for developing severe ALD. Finally, a recent study indicates that variations of the patatin-like phospholipase
domain-containing protein 3 (PNPLA3) strongly influence the development of advanced fibrosis among
Caucasians. Despite the high number of studies assessing the role of gene variations in the susceptibility
of ALD, a well-designed large study performing a genome wide association analysis is still lacking, and a
genetic test capable of identifying patients the susceptibility to develop advanced ALD is lacking.
Factors at diagnosis that predict the progression of ALD to cirrhosis are not well known. The main determinant
of disease progression is the amount of alcohol intake and sustained abstinence. Thus, recent data suggest
that higher alcohol intake is associated with a poor short-term prognosis. In addition, most observational
studies indicate that age, female gender and overweight are risk factors for cirrhosis. The existence of a
florid steatohepatitis at baseline is a major determinant of progression to cirrhosis and it also favors the
development of clinical complications.
GENERAL MOLECULAR ASPECTS OF THE FIBROGENIC PROCESS
Hepatic fibrosis is the excessive accumulation of fibrillar extracellular matrix (ECM) within the liver which
occurs in the context of chronic liver diseases and is considered the result of the persistent activation of a
‘wound healing’ response. In the setting of a chronic damage, activation of this process, evolved to protect
the tissue from damage, results in the formation of a fibrotic scar which eventually alters the cellular and
functional balance of the organ. A key role in fibrogenesis is played by myofibroblasts, which produce
extracellular matrix and in general coordinate the ‘wound healing’ response (Figure 1). Hepatic stellate cells
(HSC) are classically considered to be a major source of hepatic myofibroblasts, but other cell types inside
or outside the liver have been suggested to contribute to the expansion of the myofibroblast population
observed during injury, including portal fibroblasts and possibly cells recruited from the bone marrow. The
possibility that epithelial cells contribute to the myofibroblast pool via a process of epithelial-mesenchymal
transition has also been recently hypothesized, but its overall contribution to the fibrogenic process is
debated. It is believed that the involvement of different populations of fibrogenic cells is dependent on the
etiology of liver damage and the resulting development of distinct spatial patterns of fibrosis. In alcoholic
liver disease, fibrosis develops primarily in the pericentral areas, with a picture referred to as ‘pericellular
fibrosis’. Although chronic liver diseases share most of the profibrogenic mechanisms, the pathogenesis
of fibrosis in ALD and in other conditions this conditions may have certain specificities. For this reason,
several studies have recently focused on identify specific pathways leading to fibrosis in different etiologies
of chronic liver disease.
PATHOGENETIC MECHANISMS OF FIBROSIS SPECIFIC FOR ALD
As discussed earlier, the spectrum of ALD ranges from liver steatosis to steatohepatitis, fibrosis, cirrhosis
and hepatocellular carcinoma. The pathogenesis of steatosis includes increased fatty acid and triglyceride
synthesis and inhibited mitochondrial ß-oxidation of fatty acids and enhanced hepatic influx of free fatty
acids from adipose tissue and of chylomicrones from the intestinal mucosa. Alcoholic fatty livers can
develop inflammatory infiltrate (mainly composed by polymorphonuclear cells) and hepatocellular damage,
a prerequisite to the progression to fibrosis and cirrhosis.
Different pathogenic factors are implicating in the development of liver damage and fibrosis (Figure 2). In
this respect, ALD represents a clear example of the importance of cell-cell interaction and cross-talk within
the liver. In fact, hepatocytes, Kupffer cells, HSC, and inflammatory cells recreuited from the bloodstream
all contribute to the pathogenesis of damage and the development of fibrosis.
Products of alcohol metabolism may directly damage hepatocytes. Acetaldehyde, the major methanol
metabolic product, binds to proteins and DNA resulting in functional alterations and protein adducts, which
activate the immune system by forming autoantigens. It also causes mitochondrial damage and impairs
glutathione function, leading to oxidative stress and apoptosis. Acetaldehyde has been also shown to
interfere with TGF-beta signaling via Smad3.
In addition, reactive oxygen species (ROS) generation and the resulting lipid peroxidation play a
major role in the pathogenesis of ASH and fibrosis. Main sources of ROS include CYP2E1-dependent
MEOS, mitochondrial electron transport system of the respiratory chain, NADH-dependent cytochrome
reductase and xanthine oxidase. Moreover, chronic alcohol intake markedly up-regulates CYP2E1, which
metabolizes ethanol to acetaldehyde and parallels the generation of ROS and hydroxyl-ethyl radicals.
Cytokines and inflammation: Several studies have indicated that inflammatory pathways are activated
in the liver during ALD, including NF-κB and tumor necrosis factor-α (TNFα). Kupffer cells, activated
lymphocytes, and macrophages infiltrating the liver tissue are sources of TNFα.
More recently, a possible involvement of the chemokine system has been suggested by several experimental
data in humans and in experimental models of ALD. In alcoholic hepatitis, hepatic expression of several
chemokines has been found to be upregulated, and in particular expression of CXC components correlated
with neutrophil infiltration and prognosis. These data identify a group of novel targets for novel therapeutic
approaches to the most severe forms of alcoholic liver disease.
Endotoxin and TLRs: Alcohol abuse results in changes in colonic microbiota and increased intestinal
permeability, leading to endotoxemia. i.e. an increase in plasma lipopolysaccharide (LPS) levels, which is
released in the circulation due to an increase in intestinal permeability. A pivotal aspect of this host-bacteria
interaction is the recognition of TLR4 on Kupffer cells, that induce TNF-α production and contributes to liver
injury. Recent studies have also shown that HSC also express several members of the toll-like receptor
(TLR) subgroups, such as TLR4 and TLR9. TLR4 activation triggers multiple intracellular signaling pathways,
including NF-κB, and induces the expression of several profibrogenic cytokines. The resulting inflammatory
milieu in the alcoholic liver leads to PMN infiltration, ROS formation and hepatocellular damage, which drive
fibrogenesis. Finally, impairment in the ubiquitin-proteasome pathway leads to hepatocellular injury and
hepatic inclusions of aggregated cytokeratins (a.k.a. Mallory-Denk bodies).
Following exposure to a hepatotoxic agent such as alcohol, hepatocytes activate several signaling pathways