ENVR 430 Hepatic Physiology and Toxicology Nov 17, 2006 Jane Ellen Simmons,

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 Necrosis Steatosis Cholestatis Sinusoidal damage Types of Lesions Caused by Type II toxins/toxicants Necrosis Steatosis Cholestatis Sinusoidal 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. NecrosisApoptosis cell enlargement (swelling)cell shrinkage non-rapid removalrapid removal influx of inflammatory cells lack of inflammatory response nuclear disintegrationnuclear 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 cholestasis intrahepatic - stoppage or slowing of bile flow within the liver extrahepatic – stoppage or slowing of bile flow external to the liver

3. Cholestasis Intrahepatic cholestasis Mechanisms decreased cellular ATP damage to the canalicular membrane damage to the intrahepatic bile ducts (cholangiodestructive cholestasis) conformational changes to carrier proteins binding 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 liver No 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