Hepatoxicity

Hepatotoxin - of plant, animal or natural origin

Hepatotoxicant - of human origin

Categories of Hepatotoxicants

Type I (intrinsic)

Type II (idiosyncratic)

Characteristics of Type I Hepatotoxins/Hepatotoxicants

• High incidence in the exposed population

• Good correspondence between human and experimental animal lesions - lesions typical of human injury are produced in experimental animals

• Predictable dose-response relationship

•Incidence increases with dose

• Severity increases with dose

• Time-response relationship is predictable

• Predictable lesions

• Lesion type is predictable

• Lesion location is predictable

Characteristics of Type II Hepatotoxins/Hepatotoxicants

• Low incidence in the exposed population

• Poor correspondence between human and experimental animal lesions - lesions typical of human injury are not reliably produced in experimental animals

• Unpredictable dose-response relationship

•Incidence may not increase with dose

• Severity may not increase with dose

• Time-response relationship may not be predictable

• Unpredictable lesions

• Lesion type may not be predictable

• Lesion location may not be predictable

Types of Lesions Caused by Type I toxins/toxicants

NecrosisSteatosisCholestatisSinusoidal damage

Types of Lesions Caused by Type II toxins/toxicants

NecrosisSteatosisCholestatisSinusoidal damage

Categories of Lesions

1. Steatosis

2. Necrosis

3. Cholestasis

4. Cirrhosis

5. Sinusoidal damage

6. Tumors

1. Steatosis (aka ‘fatty liver’)

• Characterized by accumulation of lipids in the liver, more specifically, in the hepatocyte

• Accumulation of triglyceride results in the formation of fat droplets in the liver

• Liver weight increases with steatosis – increase is indication of severity – in humans with severe steatosis, liver weight can increase by 30 – 50 % and in very severe cases, liver weight can double.

1. Steatosis (aka ‘fatty liver’)

Two principal types

a. microvesicular- hepatocytes filled with numerous tiny vesicles that contain fat- the fat vesicles can be so small they are overlooked under light microscopy and require special fat stains to be observed- less common than macrovesicular (?)

b. macrovesicular- hepatocytes contain several large droplets of fat

that can become so large the nucleus is displaced

- observed on hemoxylin and eosin stained sections as vacuoles – the fixing/staining process removes fat, leaving the empty vesicle – hence the term vacuolar degeneration

1. Steatosis (aka ‘fatty liver’)

Possible mechanisms (chemical dependent)• Imbalance between triglyceride rate of synthesis

and rate of release• Decreased protein synthesis• Decreased phospholipid synthesis• Nonassociation of triglyceride and protein

• Oversupply of free fatty acids to liver

Common response to many hepatotoxic agents.

Usually reversible (does not cause hepatocyte death)exceptions to this rule

2. Necrosis

Necrosis is cell death. Typically, considering hepatocyte death. Necrosis is the sum of morphological changes that occur with cell death inside a living tissue.

Necrosis Apoptosis

cell enlargement (swelling) cell shrinkage

non-rapid removal rapid removal

influx of inflammatory cells lack of inflammatory response

nuclear disintegration nuclear fragmentation

plasma membrane leakage ‘lack’ of membrane leakage

Plasma membrane leakage results in elevated serum levels of AST (aspartate aminotransferase), ALT (alanine aminotransferase) and SDH (sorbitol dehydrogenase)

2. Necrosis - Characterization of Necrosis by Location

a. Zonal Necrosis – distribution characterized by where in the lobule or acinus damage is observed

Centrilobular (zone 3) – most frequently observed type of necrosis

Midzonal (zone 2) – the ill-defined zone between the centrilobular and the periportal regions. Rare in humans

Periportal (zone 1) – less common than centrilobular but more common than midzonal

Generally, but not always, Type I hepatoxicants produce zonal lesions

2. Necrosis -Characterization of Necrosis by Location

b. Focal (diffuse) Necrosis – characterized by single cell necrosis or small clusters of dead cells that occur randomly throughout the lobule w/o a distinct zonal relationship

Generally, but not always, Type II hepatoxicants produce focal lesions

c. Massive (panacinar, panlobular) Necrosis – necrosis that encompasses entire lobules (acini) and even lobes.

Massive, diffuse and zonal necrosis appear to have different pathogenesis.

3. Cholestasis

Cholestasis is stoppage or slowing of the flow of bile.

This results in an increase in the liver and blood concentration of bilirubin – hyperbilirubinemia

Serum bilirubin:

normal: 0.01 – 1.0 mg/dl serum

jaundice: 2 – 4 mg/dl serum

Jaundice may have many different underlying mechanisms but the common element is increased serum bilirubin.

3. Cholestasis

Two major types of cholestasisintrahepatic - stoppage or slowing of bile

flow within the liverextrahepatic – stoppage or slowing of bile

flow external to the liver

3. Cholestasis

Intrahepatic cholestasis

Mechanismsdecreased cellular ATPdamage to the canalicular membranedamage to the intrahepatic bile ducts

(cholangiodestructive cholestasis)conformational changes to carrier proteinsbinding of chemicals to carrier proteins

Canalicular cholestasis – a form of intrahepatic cholestasis. It is characterized by a decrease in the volume of bile formed or impaired secretion of compounds into bile.

3. Cholestasis

Extrahepatic cholestasis mechanisms

most commonly due to mechanical blockage of bile flow

gallstones

tumors of the extrahepatic biliary tree

fibrosis of the Sphincter of Oddi

4. Cirrhosis

Cirrhosis is scarring of the liverNo universally accepted definition of cirrhosis

Cirrhosis: fibrosis and the associated conversion of normal liver architecture into structurally abnormal nodules

If liver injury not too severe, the liver recovers.Mechanisms of recovery are hypertrophy and hyperplasia

When injury becomes too severe or chronic, fibrosis occurs.

4. Cirrhosis

Fibrosis is replacement of hepatocytes with collagen. Collagen is a dense tough protein that supports and holds tissue together. Secreted by fibroblasts and Ito cells.

Fibrous tissue lacks the functions of hepatocytes.

Fibrous tissue lacks the flexibility and elastic properties of normal tissue

The liver has a large reserve capacity, so at first fibrosis results in a decrease in the functional reserve capacity.

Eventually, sufficient fibrosis occurs for overt cirrhosis to be recognized.

4. Cirrhosis

Consequences of cirrhosis

•Hepatic failure – complete or almost complete shutdown of the functions of the liver

• Portal hypertension – increased resistance to venous blood flow from the GI tract through the liver.

Typically P(portal vein) <<< P(coronary vein)

In portal hypertension

P(portal vein) > P(coronary vein)

With the pressure gradient reversed, blood can’t flow from the coronary vein to the portal vein

4. Cirrhosis

Under portal hypertension, collateral circulation develops. Blood that normally flows through the liver is shunted into small blood vessels (anastomoses) that connect the arterial and the venous blood systems.

Development of Esophageal Varices

Normal Blood Flow

Coronary vein > portal vein > liver > inferior vena cava > heart

Collateral Circulation

Coronary vein > esophageal varices > azygous vein > superior vena cava > heart

Correlation of Serum Enzymes with Liver Damage

Factors That Influence Hepatotoxicity

• Experimental Animal (F-344, SD, Long-Evans rats, strain/stock of mouse)

• Gavage Volume

• Gavage Vehicle (aqueous vs corn oil)

• Other chemicals – Sequential Exposure– Concurrent Exposure

Factors that Influence the Magnitude of Nonadditive Response

• Sequence of Exposure

Ethanol before either CCl4 or CHCl3 results in an ethanol-mediated increase in both the liver and kidney toxicity that result from CCl4 and CHCl3 exposure

Ethanol after either CCl4 or CHCl3 has no detectable effect on CCl4- and CHCl3-mediated liver and kidney toxicity

(Klaassen and Plaa, 1966)

Factors that Influence the Magnitude of Nonadditive Response

• Sequence of Exposure

Ethanol before either CCl4 or CHCl3 results in an ethanol-mediated increase in both the liver and kidney toxicity that result from CCl4 and CHCl3 exposure

Ethanol after either CCl4 or CHCl3 has no detectable effect on CCl4- and CHCl3-mediated liver and kidney toxicity

(Klaassen and Plaa, 1966)

Factors that Influence the Magnitude of Nonadditive Response

Time Between Temporally Separated Exposures

Factors that Influence the Magnitude of Nonadditive Response

Dose Level: Dose-Dependent Interactions