Download 6.2.3 Liver

6.2.3 Liver
As it is one of the portals of
entry to the tissues of the body,
the liver is exposed to many
potentially toxic substances
via the gastrointestinal tract
from the diet, food additives
and contaminants, and drugs
and is frequently a target in
experimental animals. In
humans, liver damage is less
common, and only around 9%
of adverse drug reactions
affect the liver. By virtue of
its
position,
structure,
function, and biochemistry,
the
liver
is
especially
vulnerable to damage from
toxic compounds. Substances
taken into the body from the
gastrointestinal
tract
are
absorbed into the hepaticportal blood system and pass
via the portal vein to the
liver.
Thus,
after
the
gastrointestinal mucosa and
blood, the liver is the next
tissue to be exposed to a
compound, and as it is prior
to dilution in the systemic
circulation, this exposure will
often
beat
a
higher
concentration than that of
other tissues.
The liver, the largest gland in
the body,
represents around 2% to 3% of
the body weight in humans and
other mammals such as the rat.
It is served by two blood
supplies, the portal vein,
which accounts for 75% of the
hepatic blood supply, and the
hepatic artery. The portal vein
drains the gastrointestinal tract,
spleen, and pancreas and
therefore supplies nutrients,
and the hepatic artery supplies
oxygenated blood (Fig. 6.2).
The liver receives around 25%
of the cardiac output, which
flows through the organ at
around 1 to 1.3 mL min_1 g_1
and drains via the hepatic vein
into the inferior vena cava. In
between the blood entering the
liver via hepatic artery and
portal vein and leaving via the
hepatic vein, the blood flows
through sinusoids (Fig. 6.3).
Sinusoids
are
specialized
capillaries with discontinuous
basement membranes, which
are lined with Kupffer cells
and endothelial cells. There
are large fenestrations in the
sinusoids, which allow large
molecules to pass-through into
the interstitial space and into
close
contact
with
the
hepatocytes (Fig. 6.4). The
liver is mainly composed of
hepatocytes arranged as
plates approximately twocells thick, each plate bounded
by a sinusoid (Fig. 6.4). The
membranes
of
adjacent
hepatocytes form the bile
canaliculi into which bile is
secreted.
The bile canaliculi form a
network, which feed into
ductules, which become bile
ducts (Fig. 6.3). The
structural and functional
unit of the liver is the
lobule, which is usually
described in terms of the
hepatic acinus (Fig. 6.5),
based
on
the
microcirculation in the
lobule. When the lobule is
considered in structural
terms, it may be described
as either a classical or a
portal
lobule
(see
“Glossary”). The acinus
comprises a unit bounded by
two portal tracts and
terminal hepatic or central
venules, where a portal tract
is composed of a portal
venule, bile ductile, and
hepatic arteriole (Fig. 6.5).
Blood flows from the portal
tract toward the central
venules, whereas bile flows
in the opposite direction.
There are three circulatory
zones in the acinus, with
zone 1 receiving blood from
the afferent venules and
arterioles first, followed by
zone 2, and finally zone 3.
Thus,
there
will
be
metabolic
differences
between the zones because
of the blood flow. Zone 1
will receive blood, which is
still rich in oxygen and
nutrients, such as fats and
other constituents. The
hepatocytes in zone 3,
however, will receive blood,
which has lost much of the
nutrients and oxygen. Zone
1 approximates to the
periportal region of the
classical lobule and zone 3
to the centrilobular region.
Zone 3, particularly where
several acini meet, is
particularly sensitive to
damage
from
toxic
compounds. The acinus is
also a secretory unit, the bile
it produces flowing into the
terminal bile ductules in the
portal tract.
The close proximity of the
blood in the sinusoids with
the
hepatocytes
allows
efficient
exchange
of
compounds,
both
endogenous and exogenous,
and consequently foreign
compounds are taken up
very
readily
into
hepatocytes. For example,
the drug propranolol is
extensively extracted in the
“first pass” through the
liver.
The liver is a target organ
for toxic substances for four
main reasons:
1. The large and diverse
metabolic capabilities of the
liver enable it to metabolize
many foreign compounds,
but as metabolism does not
always
result
in
detoxication, this may make
it a target (see sects. 7.2.1
and 7.2.4 chap. 7).
2. The liver also has an
extensive
role
in
intermediary metabolism
and
synthesis,
and
consequently, interference
with endogenous metabolic
pathways may lead to toxic
effects, as discussed in
chapter 7 (see sects. 7.8.2
and 7.8.3).
3. The secretion of bile by
the liver may also be a
factor. This may be due to
the biliary excretion of
foreign compounds leading
to high concentrations,
especially if saturated, as
occurs with the hepatotoxic
drug
furosemide.
Alternatively, enterohepatic
circulation can give rise to
prolonged
high
concentrations in the liver.
Interference with bile
production and flow as a
result of precipitation of a
compound in the canalicular
lumen or interference with
bile flow may lead to
damage to the biliary system
and
surrounding
hepatocytes.
4. The blood supply ensures
that the liver is exposed to
relatively
high
concentrations of toxic
substances absorbed from
the gastrointestinal tract.
The
hepatocytes,
or
parenchymal
cells,
represent about 80% of the
liver by volume and are the
major source of metabolic
activity. However, this
metabolic activity varies
depending on the location of
the hepatocyte. Thus, zone 1
hepatocytes
are
more
aerobic and therefore are
particularly equipped for
pathways such as the boxidation of fats, and they
also have more GSH and
GSH peroxidase. These
hepatocytes also contain
alcohol dehydrogenase and
are able to metabolize allyl
alcohol
to
the
toxic
metabolite acrolein, which
causes necrosis in zone 1.
Conversely,
zone
3
hepatocytes have a higher
level of cytochromes P-450
and NADPH cytochrome P450 reductase, and lipid
synthesis is higher in this
area. This may explain why
zone 3 is most often
damaged,
and
lipid
accumulation is a common
response
(see
“Carbon
Tetrachloride,” for instance,
chap. 7).
The Kupffer cells are
known to contain significant
peroxidase activity and also
acetyltransferase.
The
differential distribution of is
enzymes may also be a
factor in the localization of
damage.
There are various types of
toxic response, which the
liver sustains and which
reflect its structure and
function. Viewed simply,
liver injury is usually due
either to the metabolic
capabilities
of
the
hepatocyte or involves the
secretion of bile. The
various types of liver
damage, which may be
caused by toxic compounds,
are
discussed
in
the
following sections.
Fatty Liver (Steatosis)
This is the accumulation of
triglycerides in hepatocytes,
and there are a number of
mechanisms underlying this
response as is discussed below
(see the sect. “Mechanisms of
Toxicity”).
The liver has an important
role in lipid metabolism, and
triglyceride synthesis occurs
particularly in zone 3.
Consequently, fatty liver is a
common response to toxicity,
often the result of interference
with protein synthesis, and
may be the only response as
after exposure tohydrazine,
ethionine, and tetracycline, or
it may occur in combination
with necrosis as with carbon
tetrachloride. It is normally a
reversible response, which
does not usually lead to cell
death, although it can be
very serious as is the case
with
tetracycline-induced
fatty liver in humans.
Repeated
exposure
to
compounds, which cause
fatty liver, such as alcohol,
may lead to cirrhosis. The
specific
accumulation
of
phospholipids
(phospholipidosis) can occur
but it also occurs in other
organs and tissues and will be
discussed later in this chapter.
Cytotoxic Damage
Many toxic compounds
cause direct damage to the
hepatocytes, which leads to
cell death and necrosis.
This is a general toxic
response, not specific for
the liver, and there are
undoubtedly
many
mechanisms, which underlie
cytotoxicity, but most are
still poorly understood. The
mechanisms
underlying
cytotoxicity in general are
discussed below, and several
examples of hepatotoxins
are discussed in more detail
in chapter 7.
The zone of the liver
damaged may depend on the
mechanism, but may also be
the
result
of
the
microcirculation.
Damage
may be zonal, diffuse, or
massive.
For example, cocaine and
allyl alcohol cause zone 1
(periportal) necrosis. With
allyl alcohol, this is partly as a
result of the presence of
alcohol dehydrogenase and
partly because this is the first
area exposed to the compound
in the blood.
Conversely,
carbon
tetrachloride, bromobenzene,
and paracetamol cause zone 3
(centrilobular) necrosis as a
result of metabolic activation
occurring primarily in that
region (see chap. 7).
Midzonal, zone 2 necrosis is
less common, but has been
described for the natural
product
ngaione
and
beryllium.
Galactosamine
causes
diffuse
hepatic
necrosis
(seechap.
7),
presumably
because
it
interferes with a metabolic
pathway, which occurs in all
regions of the liver lobule.
The explosive trinitrotoluene
(TNT) can cause massive liver
necrosis.
Ischemia may also be a
component
of
cytotoxic
damage, and consequently
interference with liver blood
flow by toxic compounds such
as phalloidin, which causes
swelling of the endothelial
cells lining the sinusoids, may
cause or contribute toward
cytotoxicity. Other compounds
cause liver necrosis because of
biliary excretion. Thus, the
drug furosemide causes a dosedependent
centrilobular
necrosis in mice. The liver is a
target as a result of its capacity
for metabolic activation and
because furosemide is excreted
into the bile by an active
process, which is saturated
after high doses. The liver
concentration of furosemide
therefore
rises
disproportionately (chap. 3,
Fig. 34), and metabolic
activation
allows
the
production
of
a
toxic
metabolite (Fig. 6.6). The drug
proxicromil (chap. 5, Fig. 11)
caused hepatic damage in dogs
as a result of saturation of
biliary excretion and a
consequent
increase
in
hepatic exposure.
Cholestatic Damage
There are various types of
interference with the biliary
system, and this can lead to
bile stasis or damage to the
bile ducts, ductules, or
canaliculi. In some cases, such
as with chlorpromazine,
damage to the hepatocytes
may ensue. Thus, some
foreign compounds, such as
the antibiotic rifampicin,
interfere
with
bilirubin
transport and conjugation
giving
rise
to
hyperbilirubinemia.
Other
compounds, icterogenin, for
example, cause bile stasis and
bilirubin deposits in the
canaliculi. This canalicular
damage
may
also
be
accompanied by damage to
hepatocytes, such as caused
by chlorpromazine. This drug
is a surface-active agent,
which
can
reach
high
concentrations in the bile and
so directly damage the lining
cells. It can also cause
precipitation of insoluble
substances in the lumen of
the canaliculi.
Accumulation of bile and its
constituent bile salts may
indeed be the cause of damage,
and some are surface-active
agents. Consequently, if high
concentrations are reached,
the cells of the biliary system
and hepatocytes exposed can
be damaged. The secondary
bile acid lithocholate will
cause direct damage to the
canalicular membrane, for
example. Some compounds
damage the bile ducts and
ductules directly such as anaphthylisothiocyanate. The
result of the destruction of bile
duct-lining cells will be
cholestasis as debris from the
necrotic cells will block the
ductules.
Cirrhosis
This is a chronic lesion
resulting from repeated
injury
and
subsequent
repair. It may result from
either hepatocyte damage or
cholestatic
damage,
each
giving rise to a different kind
of cirrhosis. Thus, carbon
tetrachloride will cause liver
cirrhosis
after
repeated
exposure, but also compounds,
which do not cause acute
necrosis, such as ethionine and
alcohol may cause cirrhosis
after chronic exposure.
Vascular Lesions
Occasionally toxic compounds
can directly damage the
hepatic
sinusoids
and
capillaries. One such toxic
compound is monocrotaline, a
naturally
occurring
pyrrolozidine alkaloid, found
in
certain
plants
(Heliotropium, Senecio, and
Crotolaria
species).
Monocrotaline (Fig. 7.7) is
metabolized to a reactive
metabolite, which is directly
cytotoxic to the sinusoidal and
endothelial
cells,
causing
damage and occlusion of the
lumen. The blood flow in the
liver is therefore reduced and
ischemic damage to the
hepatocytes
ensues.
Centrilobular
necrosis
results, and the venous return
to the liver is blocked. Hence,
this is known as veno-
occlusive disease and results
in extensive alteration in
hepatic
vasculature
and
function.
Chronic
exposure
causes
cirrhosis.
Liver Tumors
Both benign and malignant
liver tumors may arise from
exposure to hepatotoxins and
can be derived from various
cell types. Thus, adenomas
have been associated with the
use of contraceptive steroids
and exposure to aflatoxin B1,
and dimethylnitrosamine can
produce
hepatocellular
carcinomas, whereas vinyl
chloride
causes
hemangiosarcomas derived
from the vasculature (see chap.
7).
Proliferation of Peroxisomes
A response to exposure to
certain
, which occurs
predominantly in the liver is
the
phenomenon
of
peroxisomal proliferation.
Peroxisomes (microbodies)
are organelles found in many
cell types, but especially
hepatocytes.
Repeated
exposure of rodents to certain
types of foreign compound
leads to an increase in the
number of these organelles
and an increase in the
activities of various enzymes.
As well as exposure in vivo,
exposure
of
isolated
hepatocytes in vitro will also
lead to proliferation of
peroxisomes, indicating that it
is a cellular response. The
function of the organelle is
mainly
the
oxidative
metabolism of lipids and
certain
other
oxidative
metabolic pathways. Thus, the
enzymes for the b-oxidation of
fatty acids are found in
peroxisomes. The importance
of this phenomenon in toxicity
and especially carcinogenicity
is discussed later in this
chapter.
The
types
of
compounds, which cause the
proliferation, are generally
acids or compounds, which
can be metabolized to acids.
Thus, the hypolipidemic drug
clofibrate, and a number of
similarly acting drugs, will
cause
the
proliferation.
Phthalate esters, which are
commonly
used
as
plasticizers, are another group
of compounds, which have
been shown to be active. The
results of exposure in rodents
are the following: a large
increase in the numbers of
peroxisomes (e.g., 140%); a
large increase in liver weight
(e.g., 3.9–8.5%body weight);
an increase in DNA content
(e.g., 1.5–2); increases in
RNA and protein synthesis;
large increases in the
enzymes of b-oxidation such
as palmitoyl CoA oxidase
(e.g., 6–15, butsome enzymes
maybe increased up to 150);
and
increases
in
the
cytochrome
P-450
enzymes,which
metabolize
fatty acids such as lauric acid
(CYP4A1, e.g., 5–10).
This type of response is
achieved after exposure to a
compound such as clofibrate
for 28 days.
There is clearly a modulation
of gene expression occurring
for RNA, DNA, and protein
synthesis to be increased, and
this seems to be mediated by
a
receptor
interaction.
Several possible mechanisms
have been proposed to account
for the phenomenon, and these
are not necessarily exclusive:
(i) interaction
between
peroxisome proliferators
and
a
receptor[peroxisome
proliferator–activated
receptor (PPAR)];
(ii) perturbation
by
peroxisome proliferators of
lipid metabolism, leading to
substrate overload;
(iii) action of peroxisomal
proliferators as substrates for
lipid metabolism. Thus, there
is evidence for a receptor,
which can be activated by
peroxisome proliferators in
vitro. This interaction seems
to activate genes involved
with
peroxisomal
and
microsomal
fatty
acid
oxidation.
However
a
perturbation,
such
as
inhibition,
of
lipid
metabolism may also be
involved and could lead to
increased levels of an
endogenous ligand for the
receptor. For example, fatty
acids such as oleic acid are
known to bind to and activate
the PPAR in vitro. If drugs
and other chemicals, which
are
peroxisomal
proliferators,
acted
as
substrates for enzymes, such
as those catalysing boxidation,
they
could
perturb lipid metabolism,
leading to changes in the
levels of crucial fatty acids.
Therefore,
increased
peroxisomal enzyme activity
could
be
response.
an
adaptive
The receptor protein (52
kDa) is a member of the
steroid hormone receptor
superfamily, which has a
DNA-binding as well as
ligand-binding
domain.
Another receptor, the retinoid
X receptor is also involved,
and after binding of the
peroxisome proliferator, the
two
receptors
form
a
heterodimer. This binds to a
regulatory DNA sequence
known as the peroxisome
proliferator
response
element. The end result of the
interaction
between
peroxisome proliferators and
this system is that genes are
switched on, leading to
increases
in
synthesis(induction) of both
microsomal and peroxisomal
enzymes
and
possibly
hyperplasia. Structure activity
studies in vitro have revealed
that the relative potency of
peroxisome proliferators seems
to be determined by a
combination of lipophilicity
and the calculated binding
affinity to the mouse PPARa
ligand-binding domain. The
mechanisms underlying some
of these types of injury will be
discussed in general terms
below and in chapter 7.
Detection
of
Hepatic
Damage
Simple quantitative tests can
be used such as measurement
of the liver weight/body
weight ratio. Overt damage
to the liver can be detected by
light and electron microscopy
of liver sections. However,
damage can be detected by
other noninvasive means such
as the urinaryexcretion of
conjugated bilirubin or the
amino acid taurine.Various
parameters may be measured
in plasma. Thus, determination
of
the
enzymesaspartate
transaminase
(AST)
and
alanine transaminase (ALT)
is the most common means of
detecting liver damage, the
enzymes being raised several
fold in the first 24 hours after
damage. However, there are a
number of other enzymes,
which may be used as markers.
Plasma bilirubin can also be
measured, being increased in
liver damage, and plasma
albuminis decreased by liver
damage (although also by renal
damage). Liver function may
bedetermined using the hepatic
clearance of a dye such as
sulfobromophthalein.
6.2.4 Kidney
6.2.5 Lung
6.2.6 Other Target Organs
6.3 MECHANISM AND
RESPONSE IN CELLULAR
TOXICITY
6.3.1 Primary Events
6.3.2 Secondary Events
6.3.3 Tertiary Events
6.3.4 Protective Mechanisms
6.4 PHARMACOLOGICAL,
PHYSIOLOGICAL,
AND
BIOCHEMICAL EFFECTS
6.4.1 Anoxia
6.4.2 Inhibition of Cellular
Respiration
6.4.3 Respiratory Failure
6.4.4 Disturbances of the
CNS
6.4.5 Hyper-/Hypotension
6.4.6 Hyper-/Hypoglycemia
Toxic Responses to Foreign
Compounds 235
6.4.7 Anesthesia
6.4.8 Changes in Water and
Electrolyte Balance
6.4.9 Ion Transport
6.4.10 Failure of Energy
Supply
6.4.11 Changes in Muscle
Contraction/Relaxation
6.4.12 Hypo-/Hyperthermia
6.4.13 Heightened
Sensitivity
6.5 DEVELOPMENTAL
TOXICOLOGY—
TERATOGENESIS
6.5.1 Introduction,
6.5.2 Characteristics of
Teratogenesis
6.5.3 Mechanisms of
Teratogenesis
6.5.4 Role of Metabolic
Activation
6.5.5
Transplacental
Carcinogenesis
6.6 IMMUNOTOXICITY
6.6.1 Immunosuppression
6.6.2 Immunoenhancement
6.6.3 Hypersensitivity
6.6.4
Characteristics
of
Immunological Reactions
6.7 GENETIC TOXICITY
6.7.1 Introduction
6.7.2 Mutagenesis
6.7.3 Direct Interaction with
DNA
6.7.4 Primary DNA Damage
6.7.5 Gene Mutations
6.7.6 Base Substitutions
6.7.7 Frameshift Mutations
6.7.8 Clastogenesis
6.7.9 Aneugenesis
6.7.10 DNA Repair
6.7.11
Mutagenesis
in
Mammals
6.7.12
Determination
of
Mutagenicity
and
its
Relation to Carcinogenicity
6.8
CHEMICAL
CARCINOGENESIS
6.8.1 Introduction
6.8.2
Mechanisms
Underlying Carcinogenesis
6.8.3 Initiation
6.8.4 Promotion
6.8.5 Progression
6.8.6 DNA Repair
6.8.7
Non-Genotoxic
Mechanisms
6.8.8 Cell Proliferation
6.8.9
Testing
for
Carcinogenicity and Relation
of Mutagenicity