Gi Pathophysiology Lecture 2: Bile Formation, Bilirubin Metabolism, Cholestasis

Front 1

Basolateral Membrane Transporters

Back 1

NA/K ATPase

Front 2

Bile Composition

Back 2

o Isoosmotic to plasma; electrolytes in similar concentrations to blood.
o Organic anions and cations (mainly bile salts and glutathione) are highly concentrated in bile.
o Fluid content (water) formed by osmotic diffusion.

Front 3

Basolateral membrane transporters

Back 3

(1) Na/K ATPase: causes a continual extrusion of Na, creating an inwardly directed gradient for Na and an intracellular negative electrical gradient; (2) NTCP (sodium taurocholate cotransport polypeptide): selectively removes conjugated bile salts from blood in sinusoids; OATP (organic anion transporter): mediates sodium independent uptake of bile salts.

Front 4

Determinants of bile flow

Back 4

equally divided between bile salt dependent, bile salt independent, and bile duct epithelial flow with 650 to 750 cc of bile formed per day.

Front 5

Canalicular membrane transporters

Back 5

localized to apical domain; transport organic solutes against gradients; require an ATP binding cassette (ABC). ABCs include multidrug resistance proteins, cMOAT (canalicular multispecific organic anion transport), cBST (canalicular bile salt transporter), and a chloride-bicarbonate exchanger.

Front 6

Multidrug resistance proteins

Back 6

are the major class of apical membrane transporters. These P glycoproteins transport organic solutes, lipids, and toxic compounds out of hepatocytes. MDR1 acts as a hydrophobic "vacuum cleaner"; excretes anticancer drugs. MDR3 is a phosholipid transporter.

Front 7

CMOAT (MRP2)

Back 7

transports bilirubin diglucuronides GSH conjugates and heavy metals.

Front 8

cBST

Back 8

translocates bile salts and is dependent on ATP. Sister p glycoprotein (spgp) is one of these proteins.

Front 9

Chloride-bicarbonate exchanger

Back 9

prevents an increase in pH and stimulates secretion of alkaline bile.

Front 10

Intracellular Transport
3 modes of transport

Back 10

o Hydrophilic compounds often bind to cytosolic proteins, which then move by diffusion to the apical membrane.
o Other compounds may diffuse within the lipid bilayer of intracellular membranes.
o Other bulky organic solutes move within the cell by vesicular transport. These vesicles appear to move along microtubules.

Front 11

Tight Junctions.

Back 11

Barrier for paracellular exchange; establishes secretory polarity.

Front 12

Bile Duct Epithelial Cells

Back 12

o Dilute, condition, and modify bile flow.
o An acidic pH in the duodenum and to a lesser extent, fatty acids and bile salts, stimulate duodenal secretion of secretin.
o Secretin binds to G protein basolateral receptors, increasing cAMP.
o Protein kinase A phosphorylation opens CFTR (chloride channel), resulting in Cl secretion and depolarization of the bile duct cell. This activates Na/HCO3 cotransport, increasing intracellular HCO3. Apical CL/HCO3 exchange leads to net HCO3 secretion.
o CAMP also leads to the insertion of water channels (aquaporins) that increase permeability to water.

Front 13

Bilirubin Metabolism

Sources of Bilirubin.

Back 13

o 70% comes from heme moiety of red blood cells sequesterd by spleen, bone marrow, and liver.
o Liver contributes from 10-30% of bilirubin from turnover of cytochromal.
o Enzymes, catalase, and tryptophan pyrrolase.
o Remainder results from ineffective erythropoiesis.

Front 14

Formation of Bilirubin.

Back 14

o NADPH-cytochrome c reductase and heme oxygenase, localized in the endoplasmic reticulum, convert heme to biliverdin-IX-alpha.
o Biliverdin-IX-alpha is converted to unconjugated bilirubin by biliverdin reductase.
o Unconjugated bilirubin is lipid soluble and is transported in plasma bound tightly to albumin.
o Protein bound, unconjugated bilirubin is not filtered by the kidney.
o Bilirubin taken up by hepatocyte, though exact mechanism is unclear.

Front 15

Bilirubin Conjugation and Transport.

Back 15

o Bilirubin uridine-diphosphate glucuronosyltransferase (UGT) located primarily in the rough and smooth endoplasmic reticulum. Major enzyme responsible for conjugation of bilirubin to mono- and diglucuronides.
o Mechanism of transport to canalicular membrane is unclear, but appears to be delivered by cytosolic diffusion.
o Transport across canalicular membrane is an ATP dependent process involving OATP.

Front 16

Extrahepatic Processing of Bilirubin.

Back 16

o Absorption of conjugated bilirubin in intestine is negligible due to its water solubility. In the terminal ileum and colon, bacterial beta-glucuronidases hydrolyze conjugated bilirubin to unconjugated bilirubin. Unconjugated bilirubin is then further reduced by colonic bacteria to urobilinogen. Colonic absorption of urobilinogen is low; if absorbed, it is promptly reexcreted into bile. A small amount reaches the circulation and is excreted in the urine.
o Bilirubin is only excreted by the kidney with elevated levels of plasma conjugated bilirubin, since only that part that is not protein bound is available for glomerular filtration.

Front 17

Disorders of Bilirubin Metabolism.
Unconjugated hyperbilirubinemia

Back 17

Unconjugated hyperbilirubinemia can be secondary to an increased production of bilirubin. Hemolysis secondary to hereditary abnormalities of erythrocytes, Wilson’s disease, blood transfusions, liver dysfunction, and toxins can cause elevated bilirubin levels of the indirect fraction. Ineffective erythropoiesis secondary to iron deficiency anemia, pernicious anemia, or thalassemia can lead to hyperbilirubinemia due to abnormalities in heme biosynthesis.

Front 18

Disorders of Bilirubin Metabolism.
Inherited disorders of bilirubin metabolism

Back 18

Inherited disorders of bilirubin metabolism can cause a nonhemolytic unconjugated hyperbilirubinemia. Crigler-Najjar Type I Syndrome results from a severe deficiency of bilirubin-UGT activity. Severe jaundice results with short term survival unless liver transplantation is performed. Crigler-Najjar Type II Syndrome is a less severe form of Type I with mutations involving both alleles. Patients frequently survive into adulthood. Phenobarbital induces hepatic microsomal UGT and frequently leads to improvement, if not resolution of the jaundice. Gilbert’s Syndrome is a chronic benign condition characterized by intermittent hyperbilirubinemia in the absence of chronic liver disease. Bilirubin levels are usually less than 3 mg/dL, but may increase in the face of stress, fever, or fasting. Bilirubin-UGT levels are reduced.

Front 19

Disorders of Bilirubin Metabolism.
Conjugated hyperbilirubinemia

Back 19

is reflected in increased levels of direct bilirubin. Dubin-Johnson Syndrome is a benign disorder characterized by chronic or intermittent jaundice and a grossly pigmented liver. Patients are otherwise asymptomatic. Canlicular expression of MRP is lacking with another isoform of MRP localized to the lateral hepatocyte membrane. This may be responsible for the transport of glucuronide and glutathione conjugates back into the blood. Rotor’s Syndrome is a benign disorder characterized by conjugated hyperbilirubinemia and an absence of symptoms. The principal defects are in the hepatic uptake and/or storage of bilirubin and organic anions.

Front 20

Dubin-Johnson Syndrome

Back 20

a benign disorder characterized by chronic or intermittent jaundice and a grossly pigmented liver. Patients are otherwise asymptomatic. Canlicular expression of MRP is lacking with another isoform of MRP localized to the lateral hepatocyte membrane. This may be responsible for the transport of glucuronide and glutathione conjugates back into the blood.

Front 21

Rotor’s Syndrome

Back 21

a benign disorder characterized by conjugated hyperbilirubinemia and an absence of symptoms. The principal defects are in the hepatic uptake and/or storage of bilirubin and organic anions.

Front 22

Gilbert’s Syndrome

Back 22

a chronic benign condition characterized by intermittent hyperbilirubinemia in the absence of chronic liver disease. Bilirubin levels are usually less than 3 mg/dL, but may increase in the face of stress, fever, or fasting. Bilirubin-UGT levels are reduced

Front 23

Crigler-Najjar Type II Syndrome

Back 23

A less severe syndrome with mutations involving both alleles. Patients frequently survive into adulthood. Phenobarbital induces hepatic microsomal UGT and frequently leads to improvement, if not resolution of the jaundice.

Front 24

Crigler-Najjar Type I Syndrome

Back 24

syndrome that results from a severe deficiency of bilirubin-UGT activity. Severe jaundice results with short term survival unless liver transplantation is performed.

Front 25

Cholestasis

Definition and Classification.

Back 25

o Decrease or absence of bile flow.
o Extrahepatic cholestasis.
o Intrahepatic cholestasis.

Front 26

Extrahepatic Cholestasis.

Back 26

o Usually secondary to bile duct obstruction. Possible etiologies include gallstones, tumors, strictures, and extrinsic compression. Hepatocytes are profoundly modified as well. Various enzymes and carriers from the canalicular membrane are redistributed to the sinusoidal and lateral membrane. In rats, ntcp and oatp1 are reduced in bile duct obstruction, secondary to a decrease in transcription. This down regulation likely prevents the accumulation of potentially toxic bile acids.

Front 27

Intrahepatic Cholestasis.
Cystic fibrosis

Back 27

Patients have impaired function of CFTR, with a decreased chloride and bicarbonate secretion. This leads to a decrease in ductular bile flow and precipitation of mucous with obstruction of small bile ducts.

Front 28

Intrahepatic Cholestasis.
Drug-induced

Back 28

Certain drugs can cause a decrease in the activity of Na/K-ATPase. Ethinyl estradiol, chlorpromazine, and monohydroxy bile acids can decrease this activity with a decrease in membrane fluidity.

Front 29

Intrahepatic Cholestasis.
Other causes

Back 29

Interference with bile acid binding cytosolic proteins can decrease bile acid secretion. Microtubule inhibitors, such as colchicine and vinblastine, can decrease bile flow.

Front 30

Intrahepatic Cholestasis.
Other causes

Back 30

Canalicular alterations can result in cholestasis. Estrogens can decrease ATP-dependent transport of organic anions mediated by cMOAT.

Front 31

Cholestasis.
Clinical Signs and Symptoms.

Back 31

Jaundice
Pruritis
Fat soluble vitamin deficiency
Hepatic osteodystrophy

Front 32

Cholestasis
Clinical Signs and Symptoms.
* Jaundice

Back 32

is usually seen as the bilirubin approaches 3 mg/dL. When the bilirubin is conjugated, the urine is dark (conjugated bilirubin is water soluble) and the stool is light (lack of bile pigments).

Front 33

Cholestasis
Clinical Signs and Symptoms.
* Pruritis

Back 33

is a common feature of cholestasis. It may be permanent or intermittent; mild or severe to the point of making patients suicidal. The pathogenesis remains unclear. Bile acid concentrations in the skin are inconsistent and correlate poorly with symptoms. An increased concentration of central nervous system opioid receptors is possible as well.

Front 34

Cholestasis
Clinical Signs and Symptoms.
* Fat soluble vitamin deficiency

Back 34

is due to decreased intestinal bile acid concentrations and fat malabsorption.

Front 35

Cholestasis
Clinical Signs and Symptoms.
* Hepatic osteodystrophy

Back 35

an accelerated form of osteoporosis, can result in disabling vertebral body Xanthomas are lipid deposits in the skin associated with elevated cholesterol levels. Nerve compression can result if the xanthoma is placed strategically.

Front 36

Xanthomas

Back 36

are lipid deposits in the skin associated with elevated cholesterol levels

Front 37

Tight Junctions:
Occludin

Back 37

is a protein that forms an extracellular seal. Colocalize with ZO-1 and Z0-2 proteins

Front 38

Tight Junctions:
PKC, along with calcium and nitric oxide

Back 38

Increases the permeability for cations and maintains the biliary output of anions.