Transcript and Bile Salts

Liver Transport and Metabolic
Functions II
Cindy McKinney, Ph.D.
Cell Biology and Physiology
Block 5
Gastroenterology and Endocrinology
Lecture Objectives
1.
2.
3.
4.
5.
Discuss handling of dietary lipids
Discuss cholesterol metabolism
Describe handling of cholesterol in liver
Define and describe transport mechanisms
Explain the role of the liver in exercise
Recommended Reading
• Guyton and Hall Chapter 70
“The liver as an organ”
Liver: Handling of Dietary Lipids
Handling of Dietary Lipids
EXOGENOUS PATHWAY
• Enterocytes (small intestine) process fatty acids
consumed as dietary triglycerides
Chylomicrons lympatics blood (via thoracic duct)
A chylomicron is an extremely large aggregate of
Proteolipid
• composition: 80-90% triglyceride, phospholipids,
cholesterol, several lipoproteins(E, C-II and A)
Endothelial Lipoprotein Lipase (LPL) in capillary walls
partially digests triglycerides into chylomicrons
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LPL is activated by the presence of C-II lipoproteins in chylomicrons
Triglycerides in chylomicrons are broken down in to FFA and glycerol and taken in to the cells
 Long chain FFA are swept by the circulation to the liver
Chylomicrons decrease in size and loose the Triglyceride content (↑ cholesterol content)
and they become Chylomicron remnants
Chylomicron remnants are carried by the circulation to the liver ---depleted in triglyceride and contain high
Cholesterol (High Apo E and lipoprotein A-1 content)
Handling of Dietary Lipids
EXOGENOUS PATHWAY
Chylomicron remnants arrive at liver in blood
and bind to basolateral membrane receptors
Specific LDL receptors recognize Apo E moiety
in remnants-- uptake into hepatocytes by
receptor mediated endocytosis (substrate
specific and saturable)
Receptor degraded by lysosomes
Digestion of chylomicrons yields: glycerol, FFA , smaller remnant chylomicrons
LPL generated glycerol and FFA enter adipocytes and muscle cells from circulation
Chylomicrons Summary:
• transport glycerol and FFA to peripheral tissues (muscle and adipose)
• “remnants” transport dietary triglyceride and cholesterol to hepatocytes
Hepatocytes also absorb long-chain FA (liberated by LPL) at basolateral membrane if
not captured by other tissues
Handling of Dietary Lipids
EXOGENOUS PATHWAY
After uptake of triglycerides converted to Glycerol+FFA
FFA metabolized by hepatocytes via
β –oxidation produces two acetyl-CoA

enter TCA cycle=ENERGY
Unused Acetyl-CoA condensed to
acetoacetic acid  Ketone Bodies
(Liver is the only organ to produce
acetoacetate but does not use it---used by
muscle, brain and kidney for energy
production)
FFA can be re-esterfied to glycerol=Triglycerides
1) Stored in liver
2) Exported to circulation as VLDL’s used by peripheral tissues
Very Large Density Lipoprotein (VLDL) back into plasma
Handling of Dietary
LipidsCholesterol
•
•
Primary source of cholesterol is the liver
Main cholesterol “pools” in body are:
1. cholesterol and cholesterol derivatives in bile
2. cholesterol in cell membranes
3. cholesterol carried in blood lipoproteins
4. cholesterol in cholesterol “rich” tissues
In bile and cell membranes, cholesterol is mainly free cholesterol
In plasma and some tissues, cholesterol is esterified to long chain fatty acids
Main sources of cholesterol:
1. dietary uptake---packed and secreted by enterocytes into the chylomicrons
2. de novo synthesis
3. Liver captures cholesterol from circulation by uptake of LDL + cholesterol
Cholesterol Elimination:
1. hepatic conversion and secretion as bile salts
2. excretion in feces when cells are sloughed, skin cells
3. conversion to steroid hormones
De Novo Cholesterol Synthesis
[Cholesterol]
HMG-CoA reductase activity
Cholesterol synthesis occurs in
1. liver
2. intestine
3. extrahepatic tissues
Multi-step enzymatic process
Process occurs in Smooth ER and cytosol
Feedback controls on cholesterol synthesis
1. Inhibited () by an increase in dietary cholesterol
and fasting
2. Increased ( ) by bile drainage (fistula) and by
bile duct obstruction
Liver Function: Endogenous
Loop
Exports cholesterol and other
lipids as VLDLs
Uptake in the form of LDLs
As VLDLs travel through circulation
encounter LPL in blood vessels
LPL degrades VLDL as it does chlyomicrons
VLDLs
deliver FFA and glycerol to adipose and muscle tissue
LDLs captured by liver and other tissues by receptor mediated endocytosis
LDL uptake = major pathway for cholesterol delivery to liver
Handling of Lipids by the Liver
Endogenous pathway-IDL
IDL= Intermediate Density Lipoprotein
VLDL lose triglyceride IDLs
LDL deliver cholesterol and then LDL is
moved into liver via LDL receptor
• Requires lipoprotein B-100 cofactor
Clinical Note: Familial Cholesterolemia
• Congenital absence of LDL receptors
• Increase in plasma lipid levels
Result: Atherosclerosis/MI @ age 6-11
LDLs have three potential fates:
1. Taken into liver via LDL receptor
-deliver Cholesterol processed to liver
2. LDL travel to peripheral tissues
-deliver cholesterol to peripheral tissues
-cholesterol used for membranes
3. LDL releases cholesterol
- Plaque formation in blood vessels
(VLDL gets released to cause this)
Fate 3: Plaque formation in blood vessels
 LDL is not taken into by the liver due to two causes:
1. To much LDL ---receptor saturated --can not be absorbed by the liver (travels down the circulation)
2. Presence of endothelial damage resulting in activation of inflammatory cascade
RESULT:
Oxidative stress= Oxidation of cholesterol in LDL- makes it insoluble and inert
Result: Atherosclerotic plaque = MI or Stroke
Handling of Lipids-- Endogenous pathway HDL
Important factor in cholesterol metabolism are high density lipoproteins (HDLs)
=“GOOD” Cholesterol
Two major HDL apoproteins (A-1 and A-II) made by two sources :
1. Produced by intestines (secreted as part of chylomicron)
2. Secreted by the liver
3. Carries enzyme LCAT (lecithin cholesterolacid/acyl transferase)
4. Mediates “reverse” cholesterol transport back to liver (cholesterol re-processing)
As LPL digests VLDLs on endothelial cell surface ---excess surface material (Cholesterol and
Phospholipids) from rapidly shrinking particles is transferred to HDLs
HDL scavenges cholesterol from extrahepatic tissues---extraction of unesterified cholesterol
(insoluble) from plasma membranes of peripheral cells
BAD cholesterol=oxidized cholesterol in membranes and plaques (also found in LDLs )
(LCAT) associated with HDLs removes acyl group from lecithin and esterifies to cholesterol
=Cholesterol ester (now water soluble)
HDLs transfer cholesterol esters to VLDLs, IDLs and LDL (co-factor apoprotein D or cholesterol
ester transfer protein) can be used/stored/processed in liver
Transport Mechanisms in Liver
Uptake, processing and secretion of compounds by hepatocytes
A variety of different compounds are brought to the liver by
-systemic circulation
-portal circulation
These compounds are delivered to and handled hepatocytes in 4 steps:
1) by uptake across either the basolateral or sinusoidal membrane and modified
2) intracellular and extracellular transport
3) chemical modification or degradation
4) export from hepatocyte to bile across apical or canalicular membrane
Transport external cellinternal cell occurs in several ways
1) Simple diffusion
2) Facilitated diffusion (carrier-mediated)
3) Primary active transport
4) Secondary active transport
Hepatocyte Transport: Key Cellular Reactions
Krebs cycle, FA oxidation, formation
Acetyl-CoA, part of urea cycle, part of
gluconeogenesis
Mitochondrion
Nucleus
Golgi
Degradation of complex
molecules
Synthesis and
packaging of
complex molecules:
Glycolipids,
Glycoproteins
Lipoproteins
Cytosol
Lysosomes
hepatocyte
Glycolysis, HMP shunt, Protein Synthesis
FA synthesis, part of urea cycle, part of
gluconeogenesis
Transport and Cellular Localization
Cell membrane defines some of the major differences between cell types:
-selective cell surface receptors for specialized molecules (hormones)
-controls mechanism of entry of all sorts of molecules into cells
I.
Simple Diffusion- does not require energy;
-Molecules follow their electrochemical gradient [high to low]
-Depends on permeability of membrane (diffusion will not occur if membrane is
impermeable to molecule in question
Examples:lipid soluble substances (non-polar= O2, etc) diffuse across cell plasma membranes
polar substances (Na+, K+ etc) primarily diffuse thru water filled channels located
within plasma membrane
Transport and Cellular Localization
II. Facilitated Diffusion
-Molecule moves along its electrochemical gradient but is attached to another
“carrier” molecule that “facilitates” its movement across the membrane
-No energy necessary to drive it
-Reaches a max diffusion rate that depends on [carrier]/availability
-More rapid than simple diffusion
Characteristics:
1. Sterospecificity—D-glucose moves by facilitated diffusion but the L-glucose does not
(simple diffusion would not distinguish the two isomers)
2. Saturation---transport rate increases as concentration increases until no more
carrier available for facilitation
3. Competition---structurally similar solutes compete for transport sites on carriers
galactose is a competitive inhibitor of glucose in the small intestine
Example: a polar molecule alone would not readily diffuse thru a lipid membrane
However, a polar molecule attached to a lipid soluble carrier that passes easily thru
membrane would diffuse along its gradient
Transport and Cellular Localization
III. Primary Active Transport
-Molecule moves AGAINST its electrochemical and concentration gradient
-Energy expenditure from ATP required
-Exhibits: stereospecificity, saturation and competition
Examples:
a) Na+,K+-ATPase (Na+K+ pump) Na+ intracellular extracellular AND K+ from
extracellular to intracellular therefore  intracellular [Na+] intracellular [K+]
b) H+,K+-ATP ase (proton pump) –gastric parietal cells H+ transported into lumen
of stomach against its electrochemical gradient
IV. Secondary Active Transport (symporter/antiporters)
-Movement (transport) of molecules is COUPLED one primarily moves “downhill”
Providing energy for the “uphill” movement of the other
Example: a) Na+-Glucose co-transport in intestine (Na+ moves “downhill” into cell )
higher external [Na+] to lower internal cell concentration
b) Na+-Ca+2 exchanger (Ca+2 moves low intracellular to high extracellular [])
while Na+ moves in the opposite direction
Step 1: Transport across Basolateral
Membrane
Uptake of Bile Acids (H-BA) and Bile Salts (BA-):
Portal Vein  basolateral membrane
Primary H-BA are
- Not water soluble;
- Near neutral (lipid soluble)- cross cell
membranes by diffusion
- Synthesized by hepatocytes
Some primary and secondary bile salts (H-BA-Z) (glycine or taurine) also cross the basolateral
membrane by diffusion
Uptake of Bile Salts (BA-) from portal
vein across the basolateral membrane
Bile Salts:
-water soluble (pronated acids)
-negatively charged –cannot cross membrane
by diffusion
-conjugated and unconjugated primary
/secondary (BA- and BA-Z-) transported by
NTCP
Transporter NTCP= note it is Na coupled
(Na-taurocholate transporting polypeptide)
NTCP can also transport:
• Steroids (example-progesterone)
• Cyclic oligopeptides (example verapamil)
• A variety of drugs
Uptake and secretion of organic anions
across the basolateral membrane
Organic Anion Transporter Proteins (OATP-1, OATP-2)
----act in bilirubin uptake and secretion
---- bile salts –bilirubin(Na+ independent)
---coupled with Cl- or GSH exchange
After intracellular conjugation via UGT compounds are
Secreted into bile by MRP2 (multidrug resistanceassociated protein)
Keys: basolateral processing/transport of bile
acid and salts is complex and involves
 Na+ dependent transporter NTCP
 Na+ independent transporter OATP
 Non ionic diffusion of unconjugated bile acids
Step 2: Intracellular transport in the
hepatocyte
Bile salts are transported inside the cell bound
To “binding” proteins.
Examples: Dihydrodiol dehydrogenase
Glutathione-S-transferase B
Fatty Acid Binding Protein (FABP)
Serve a cell protection role/molecular traps
Also important role in bile acid synthesis
regulation
A high bile salt load will activate the Vesicular Pathway:
bile salt specific
slower (forming and transporting) but more efficient
Step 2: Intracelluar transport in the hepatocyte
Bilirubin:
• Enters cell via basolateral side using OATP-1
• Moves to ER for conjugation
• Unconjugated bilirubin forms “blobs” ( like oil on
water interface)
• Unconjugated “blobs” floats on cytosol to ER
• Follows concentration gradient
• Conjuated bilirubin from ER is now water soluble
• Follows concentration gradient to bile canaliculi
• Secreted by MRP2 in canaliculi
Step 3: Chemical Modification
Step 4: Transport Across the Apical
Membrane (intracelluar  bile canaliculi)
Bilirubin
Conjugated bile acids/salts (modified)
Unconjugated bile acids/salts
-secreted from hepatocyte to canaliculi (apical)
-forms bile
Two apical (canalicular) transporters:
1. MRP2
2. 2. Bile salt and acid transporter (BSEP)
Role of Liver in Exercise
• Exercise requires muscular energy expenditure
• Body compensates to ensure optimal O2 and glucose
availability
• Before compensation happens
– Shortfall on O2 supply to muscles =OXYGEN DEBT
– Due to debt, initial muscle contraction is energized by
anaerobic glycolysis (G6P Pyruvate Lactate (ATP )
– Significant Lactic acid accumulation in circulation
– Lactic acid  glucose in the liver (Cori Cycle)
– Glucose is sent back to muscles
– Gradually, the O2 supply to the muscle improves---muscle switches
to aerobic oxidation of glucose (no lactic acid build-up)
Role of Liver in Exercise
Role of Liver in Exercise