Lecture 35 - Lipid Transport

Question 1

Lipids are transported from .. (3)

Answer 1

Gut to Liver
Liver to non-hepatic tissues (inc adipocytes)
Back to liver

Question 2

Fat/Lipids in circulation (6)

Answer 2

Free fatty acids - 5%
Cholesterol esters - 15%
Phopholipids - 35%
Triacylglycerols - 45%
% in human plasma varies with nutritional state.
Insoluble in water.
Polyunstaurated FA.

Question 3

Polyunsaturated FA (5)

Answer 3

Are ligands for TF involved in energy metabolism.
Role in regulation of insulin metabolism.
Upregulate = Lipid oxidation in liver.
Downregulate = Genes involved in lipogenesis in the liver/adipose tissue.
Increase expression of UCP-2 and 3 in mitochondria = increasing thermogenesis.

Question 4

Free FA (5)

Answer 4

1) Formed from triacylglycerides stored in adipose tissue.
2) Circulates bound to protein (e.g. albumin) as Na+ salt. As unbound FA would act as a detergent.
3) Saturation occurs at about 2mM FA molecules.
4) FA enter cells by simple diffusion.
5) Intracellular concentration FFA kept low.

Question 5

Lipoproteins (6)

Answer 5

Carried in blood plasma.
Density is dependent on their composition.
Consists of a large protein with phospholipids, single layer.
In the hydrophobic core is hydrophobic compounds - cholesterol esters and triacylglycerols.
Amounts in single particle vary depending on where in the circulation the lipoprotein is.
Apoproteins/TG helps to distinguish lipoproteins.

Question 6

Five types of lipoprotein (5)

Answer 6

Chylomicrons - Low density, high TG (90-95%), low protein (1-2%).
Very Low Density Lipoproteins (VLDL)
High Density Lipoproteins (HDL) - High density, low TG (2%), high protein (49%).
Low Density Lipoproteins (LDL) - High cholesterol (50%), low TG (4%).
Intermediate Density Lipoproteins (IDL)

Question 7

Apolipoproteins/Apoproteins - Functions (5,4)

Answer 7

o Act as an agonist (a substance which initiates a physiological response when combined with a receptor).
o Structural
o To solubilise lipids
o Act as enzymes or enzyme cofactors
 Apo C2 for lipoprotein lipase
 Apo A1 for lecithin: cholesterol acyltransferase (transferring lipids between particles).
o Tissue targeting
 Apo B100 and Apo E bind to the LDL receptor
Apo E binds to the HDL receptor

Question 8

Apoproteins in Lipoproteins (6)

Answer 8

Chylomicrons - B48, Apo C2/C3/E
Very Low Density Lipoproteins (VLDL) - B100, Apo C1/2/3/E
High Density Lipoproteins (HDL) -Apo A1/A2/C1/C3/D/E
Low Density Lipoproteins (LDL) - B100
Intermediate Density Lipoproteins (IDL) - B100, Apo E

VLDL made in the liver is released with B100, as it cirxulates it picks up Apo C/E from HDL.

Question 9

Dietary lipids (8)

Answer 9

• Low density due to high TG.
• Apo C2, C3, E and B48 added in the SER.
• Secreted by reverse pinocytosis in to the lymphatics.

Triacylglycerols broken down (by lipases in gut) to FA + Monoacylglycerols. Move from gut lumen into gut epithelial cells converted back into Triacylglycerols.
TG then assemble into chylomicron with other lipids (phospholipid + cholesterol) and proteins.
Chylomicron released into lymphatics carrying them via the thoracic duct to SVC.
In this pathway dietary fats avoid direct delivery to liver and instead made available to extra hepatic tissue (have first call). DIFFERENT to digested proteins/carbs which are released into the portal vein and delivered directly to liver.

Question 10

Chlyomicron (9)

Answer 10

Content reflects meal composition.
Low density due to TG.
Contain fat soluble vitamins A/E (antioxidant preventing oxidation of lipids which are associated with heart disease).
Circulate for 1 hour (TG half life is 5 min). Rapidly modified by lipoprotein lipases (LPL) in the circulation which breaks down TG causing uptake of FA by tissue.
As they circulate the density of the particles increases (TG removed), these are called chylomicron reminants and are removed by the liver they do this by interaction of Apo E with receptors on hepatocytes.
Km of LPL isoform in adipocytes > muscle.
LPL on adipocytes is stimulated by insulin.
LPL found ontop of endothelilal cells, activated by apoprotein, Apo C2 (found on lipoprotein).
When LPL is activated the TG is broken down and FFA diffuse across membrane.

Question 11

Chylomicron - Disorders of lipid transport (4)

Answer 11

Hyperlipidaemia
Type 1 = Deficiency in LPL/Apo C2, high plasma TG.
Type 2 = High HDL, caused by genetic defect in synthesis processing or functioning of LDL receptor.
Type 4 = Most common type, raised VLDL concentrations due to obesity/alcohol abuse.

Question 12

VLDL (11)

Answer 12

Transports lipids derived from liver around the body.
Made in liver/ER/golgi body.
Released with Apo B100 (as a nascent particle) and they acquire Apo E/C2 from HDLs.
Lipids associated with VLDLs are on a greater transport system.
Interact with endothelial layer and are metabolised by LPL, the TG have a longer half-life (15-60mins).
Interaction leads to VLDLs getting smaller and smaller. Resulting in two pathways:
1) Go back to liver as VLDL remnants (50% do this) - removed by Apo E.
2) Converted into IDL through removal of more TGs.
IDLs can be removed by the liver OR converted into LDLs (reaction that occurs in liver sinusoids).

LDL (rich in cholesterol) can be..
removed by the liver (60%)
removed by non-hepatic tissue for steroid biosynthesis (40%)

Question 13

LDL (7)

Answer 13

Major carrier of cholesterol.
Long half-life.
Metabolised slowly (remain in circulation for 3 days).
Carry cholesterol to periphery and regulate denovo synthesis of cholesterol, through Apo B100 (only 1 per LDL) acting on specific receptors on hepatocytes.
60% of VLDL remnants are removed by the liver.
40% removed by adrenal/gonadal tissue and the cholesterol is used for hormone production. Both use Apo B100 to bind to LDL receptor.
Too much LDL, the receptors are saturated the excess can be removed via a low affinity scavenger receptor.

Question 14

LDL - Loss of LDL receptor function (12)

Answer 14

Familial Hypercholesterolemia (FH)
Homozygous individuals:
o High serum cholesterol (800mg/ml, normal is 200mg/ml)
o Develop blocked arteries (atherosclerosis)
o Die young from heart attacks
o De novo synthesis is not regulated by LDL (as cell thinks it has less cholesterol)
Single amino acid substitution that prevents localisation of the LDL receptor to the coated pits.
Present on
o Endothelial cells
o Macrophages
o VSMC
Low affinity active when plasma LDL high or oxidised.
Not regulated by cholesterol.

Question 15

De novo synthesis

Answer 15

Refers to the synthesis of complex molecules from simple molecules such as sugars or amino acids, as opposed to recycling after partial degradation.

Question 16

HDL (5)

Answer 16

Apoproteins are obtained from VLDLs and chylomicrons.
Reverse cholesterol transport (take up cholesterol from circulation (from VLDLs) or dead and dying cells (mopping up) and take cholesterol back to the liver and steroid producing cells. Transfer cholesterol to VLDLs/LDLs - cholesterol ester transfer.

HDLs made in the liver/intestine, they are circulating reservoir of apoporteins. Made by budding from VLDL/chylomicrons. From free Apo A1.
Contain the enzyme lecithin cholesterol acetyltransferase (LCAT) which esterifies cholesterol.
HDL binds to lipoproteins and cells via Apo E important in cholesterol transfer.

Question 17

LDL vs HDL (5)

Answer 17

Low Density Lipoproteins (LDL) - High cholesterol (50%), low TG (4%).
“Bad” cholesterol.

High Density Lipoproteins (HDL) - High density, low TG (2%), high protein (49%).
“Good” cholesterol.

Correct balance of both. Ratio is used to assess susceptibility to heart disease.
Cholesterol ratio is calculated by dividing your total cholesterol by your HDL number. Ratio should be 3.5.

Question 18

Lipoprotein table

Answer 18

Flashcard

Question 19

Receptor mediated endocytosis

Answer 19

• Mechanism of selective uptake of material by animal cells in which a macromolecule binds to a receptor in the plasma membrane and enters the cell in a clarthin-coated vesicle.

Question 20

What regulates cholesterol uptake and synthesis? (3)

Answer 20

• Cholesterol regulates its own uptake and synthesis.
• ↑ cholesterol inhibits HMG-CoA reductase activity (rate-limiting step in cholesterol synthesis).
• ↓ cholesterol ↑ LDL receptor expression.
• HMG-CoA is a target for therapy.

Question 21

What effect does an increase in HMG CoA reductase have on the level of expression of LDL receptors?

Answer 21

The synthesis of the LDL receptor and its expression at the cell surface is negatively regulated by the intracellular concentration of cholesterol. When the intracellular concentration of cholesterol decreases the synthesis of cholesterol from acetyl-CoA and the LDL receptor increase. HMGCoA reductase is the rate limiting step in the synthesis of cholesterol.

Question 22

What effect do statins have on this relationship?

Answer 22

Statins inhibit HMGCoA reductase and therefore decrease cholesterol synthesis within cells this will result in an increase in the synthesis and expression of the LDL receptor at the cell surface and so increase LDL-cholesterol uptake and therefore lowing circulating cholesterol.

Question 23

How do statins work? (3)

Answer 23

• Stains inhibit HMGCoA reductase/ competitive inhibitor. (Vmax same, Km increased).
• Low intracellular Cholesterol increases LDL receptor at the cell surface. Leading to increase uptake of LDL.
• Lower circulating LDL.

Question 24

LDL - receptor mediated endocytosis (5)

Answer 24

1) LDLs in the circulation can bind to specific receptors expressed on endothelial cells.
2) The receptor and LDL are the endocytosed.
3) Vesicle fuses with endosomes, the endosomes contain enzymes responsible for breakdown of protein and metabolism of lipids – receptor goes off and is re-inserted into membrane.
4) There will be a further fusion with a lysosome to provide more enzymes, hydrolytic ones.
5) The receptor and apolipoprotein may be broken down to give amino acids, the phospholipids, FFA and triglycerides to give lipids to be used for various things and cholesterol esters can be stored and utilised.

Question 25

Scavenger receptor ()

Answer 25

• Present on
o Endothelial cells
o Macrophages
o VSMCs
• Low affinity active when plasma LDL high or oxidised.
• Not regulated by cholesterol.
• Function to take up LDL from plasma when it is elevated (so have a normal function).
Only active when circulating levels of plasma LDL are very high. They are completely unregulated however and will continue to take up LDL independent of cellular need. They also have a role in pathology due to the following reason.
• The intracellular concentration of cholesterol is ultimately detrimental to the cell, short term low levels can be dealt with, but high levels cannot be dealt with and cause problems. Which is why cells associated with scavenger receptors are associated with problems.
• Therapy for treating high cholesterol is combined, inhibiting de novo synthesis of cholesterol and inhibit absorption of cholesterol from gut.

Question 26

Regulation of lipoproteins (9)

Answer 26

•	Hormonal regulation 
o	Insulin 
o	Cortisol 
o	Thyroid hormones
•	Nutritional status
o	Decreased synthesis during fasting
o	Increased by dietary fats – unsaturated fats best
•	LDL expression
o	Oestrogen

Question 27

Abnormalities of lipid transport (8)

Answer 27

• Diabetes Mellitus
o Increased FFA mobilisation
o Decreased Chylomicron and VLDL utilisation
• Gene defects
o Apolipoproteins, enzymes or receptors.
o Leading to: Hypercholesterolaemia, atherosclerosis
• Obesity
o Hypertension, NIDDM, hyperlipidaemia and hyperglycaemia