PRINCIPLES OF TOXICOLOGY

120 HEPATOTOXICITY: TOXIC EFFECTS ON THE LIVER
benzoquinone imine, have been identified, none as yet has been clearly shown to be instrumental in
acetaminophen-induced hepatic necrosis. Without identification of the critical target(s) for irreversible
binding for hepatotoxicants, this remains an attractive but unproven mechanism.
Loss of Calcium Homeostasis Intracellular calcium is important in regulating a variety of critical
intracellular processes, and the concentration of calcium within the cell is normally tightly regulated.
The plasma membrane actively extrudes calcium ion from the cell to maintain cytosolic concentrations
at a low level compared with the external environment (the ratio of intracellular to extracellular
concentration is about 1:10,000). Both the mitochondria and endoplasmic reticulum are capable of
sequestering and releasing calcium ion as needed to modulate calcium concentrations for normal cell
functioning. Loss of control of intracellular calcium can lead to a sustained rise in intracellular calcium
levels, which, in turn, disrupts mitochondrial metabolism and ATP synthesis, damages microfilaments
used to support cell structure, and activates degradative enzymes within the cell. These events could
easily account for cell death from hepatotoxic chemicals.
Early studies of toxic effects of chemicals on liver cells in culture suggested that an influx of calcium
from outside the cell (e.g., from plasma membrane failure) was responsible for their toxic effects. Later
experiments showed that this was probably not the case, but nonetheless supported disregulation of
intracellular calcium as a key event in toxicity. Intracellular calcium levels were observed to rise
substantially in response to a number of hepatotoxicants, apparently due to chemical effects on
mitochondria and/or the endoplasmic reticulum leading to loss of control of intracellular calcium
stores. Impaired extrusion of calcium out of the cell by the plasma membrane might also be important,
at least for some chemicals. In general, increases in intracellular calcium preceded losses of viability,
suggesting a cause–effect relationship. It is sometimes difficult, however, to discern to what extent
elevated calcium levels are the cause of, or merely the result of, cytotoxicity.
Immune Reactions This mechanism of hepatotoxicity is not common, but nonetheless important.
Characteristically, an initial exposure is required that does not produce significant hepatotoxicity—a
sensitizing event. Subsequent exposure to the drug or chemical can lead to profound liver toxicity that
may be accompanied by hepatic inflammation. Consistent with a hypersensitivity reaction, there is
little evidence of a dose–response relationship, and even small doses can trigger a reaction. This
response is usually rare and difficult to predict; hence it is often considered an idiosyncratic reaction.
Typically, this kind of hepatotoxicity for a drug or chemical is very difficult to demonstrate in laboratory
animals, and unfortunately becomes known only after widespread use or exposure in humans.
Perhaps the most familiar example of a drug or chemical producing this type of hepatoxicity is the
general anesthetic halothane. Studies suggest that halothane is metabolized to a reactive metabolite
that binds with proteins. These proteins become expressed on the cell surface where they are recognized
by the immune system as being foreign. The immune system then mounts a cell-mediated response,
resulting in destruction of the hepatocytes. This response, called halothane hepatitis, seldom occurs
(only about 1 in 10,000 anesthetic administrations in adults) but has a 50 percent mortality rate. A
similar phenomenon has been observed with other drugs, including diclofenac.
Fatty Liver
Many chemicals produce an accumulation of lipids in the liver, called fatty liver or steatosis. Examples
of chemicals that produce fatty liver are provided in Table 5.2. Just as hepatocellular necrosis
preferentially occurs in specific acinar zones in response to certain chemicals, so does fatty liver. For
example, zone 1 is the primary site of lipid accumulation from white phosphorus, while zone 3 is where
most of the lipid accumulation is observed with tetracycline and ethanol. The lipid accumulates in
vacuoles within the cytoplasm, and these vacuoles are usually present as either one large, clear vacuole
(called macrovesicular steatosis) or numerous small vacuoles (microvesicular steatosis). The type of
steatosis (macro- or microvesicular) is characteristic of specific hepatotoxicants and, in some cases,
of certain diseases or conditions. For example, microvesicular steatosis has been associated with