L37&38: Lipoprotein Metabolism and Dyslipidemias

Question 1

Chylomicrons (CM)

Answer 1

Transports dietary TAGs and lipid-soluble vitamins
As a structural protein: Apo B-48
As the activator of LPL enzyme: Apo C-II
As the ligand to Apo E-repecptor: Apo E

Question 2

CM Remnants

Answer 2

As a structural protein: Apo B-48
As the activator of LPL enzyme: N/A
As the ligand to Apo E-repecptor: N/A

Question 3

VLDL

Answer 3

transport of livery-synthesized TAGs
As a structural protein: Apo B-100 (dual role, also ligand for LDL receptor)
As the activator of LPL enzyme: Apo C-II
As the ligand to Apo E-repecptor: Apo E

Question 4

IDL

Answer 4

As a structural protein: Apo B-100 (dual role, also ligand for LDL receptor)
As the activator of LPL enzyme: N/A
As the ligand to Apo E-repecptor: Apo E
In the circulation: after TAG degradation by LPL, Apo C-II is returned back to HDL, and the resultant particle is called IDL (with Apo B-100 and Apo E). A major portion of IDL is endocytosed in the liver via Apo E’s binding to its hepatic Apo-E receptor.

Question 5

LDL

Answer 5

As a structural protein: Apo B-100 (dual role as ligand for LDL receptor)
As the activator of LPL enzyme: N/A
As the ligand to Apo E-repecptor: N/A
5) A small portion of IDL is further catabolized into LDL by returning its Apo E back to HDL and by further TAG hydrolysis via hepatic lipase (synthesized in the liver and exocytosed in the blood).
LDL has a long lifetime in blood (1.5-2. days compared to a few hours for other lipoproteins.
LDL is small enough to penetrate from blood vessel lumen into the intima, the subendothelial space, where it is oxidized to oxLDL.

Question 6

HDL

Answer 6

As a structural protein: Apo A-I (dual role as an activator for PCAT enzyme)
As the activator of LPL enzyme: Apo C-II
As the ligand to Apo E-repecptor: Apo E

Question 7

Lipoprotein size and density comparison

Answer 7

Size: CM>VLDL>LDL>HDL
Density: CM<HDL

Question 8

Microsomal Transfer Protein (MTP)

Answer 8

In the intestinal enterocytes: nascent chylomicron (with Apo B48) is synthesized and assembled. The dietary TAGs are transferred to the nascent CM by microsomal transfer protein (MTP). Nascent CMs carry dietary (exogenous) TAGs, C, CEs and fat-soluble vitamins and enter lymphatic circulation before entering blood circulation.
MTP also transferes Liver synthesized TAGs to the nascent VLDLs.

Question 9

Apo A-I

Answer 9

HDL structural protein and as its dual role it also functions as an activator for the PCAT enzyme

Question 10

Apo B-100

Answer 10

Structural protein for the VLDL family (VLDL, IDL and LDL. As it’s dual role if functions as a ligand for LDL receptor.

Question 11

Apo B-48

Answer 11

Structural protein for chylomicrons (CM) and CM remnants

Question 12

Apo C-II

Answer 12

As the activator of LPL enzyme. Found on CMs, VLDLs and HDLs

Question 13

Apo E

Answer 13

As the ligand to Apo E-receptor. Found on all lipoproteins except LDL.

Question 14

What happens to nascent CMs in circulation?

Answer 14

In the circulation: HDL transfers Apo C-II and Apo E to the nascent CMs, resulting in the formation of mature CMs (now with Apo B48, Apo C-II and Apo E) plus the dietary lipid components.

Question 15

Lipoprotein Lipase (LPL)

Answer 15

On the surface of endothelial lining of adipose, muscle and heart: Apo C-II (in CM) activates LPL (lipoprotein lipase); the activated LPL then degrades the TAGs which gives rise to fatty acids and glycerol. The released free fatty acids are taken up by the adjacent tissues, and will be reconstituted back to TAGs in the adipose tissue, where the glycerol returns to the liver.

Question 16

LPL Regulation

Answer 16

LPL gene expression is positively regulated by insulin; that is, LPL levels are high in individuals who are on mixed meals (~55% calories from carbohydrates). On the flip side, a person who is on a low-carbohydrate diet (a low I/G ratio) or has a poorly controlled type 1 diabetes (very low insulin levels) will have low levels of LPL.

Question 17

After TAG degradation of mature CMs by LPL on the surface of endothelial lining of adipose, muscle and heart, what is created and what does it contain? Finally, what is ultimate the fate of this molecule?

Answer 17

In the circulation: after TAG degradation by LPL, Apo C-II is returned back to HDL, and the resultant particle is called CM remnant, which is much smaller and contains the remaining lipid components such as C, CE and fat-soluble vitamins.
In the liver: CM remnants are endocytosed (taken up) by liver via Apo E’s binding to its hepatic Apo-E Receptor. The endocytosed CM remnants are degraded in the lysosomes, releasing amino acids, free cholesterol, and fatty acids into the cytosol. CM is virtually catabolized after 12 hours of fast.

Question 18

Abetalipoproteinemia (CM Retention Disease)

Answer 18

• cause: loss-of-function mutation of MTP gene. —–Affected biochemical steps: A loss of MTP means that TAGs are not transferred to the nascent CM and the nascent VLDL. As a result, nascent CM (in enterocytes) and nascent VLDL (in hepatocytes) cannot be assembled.
• Lipid profile: CM, VLDL and LDL are almost absent from plasma, resulting in hypolipidemia.
• Clinical Presentations: Dietary fats accumulated in enterocytes, failure to thrive (generalized weakness, skeletal deformations), neurological defects due to malabsorption of fat-soluble vitamins.
• Therapy: low-fat, calorie-rich diet with high dose of vitamin supplements.

Question 19

Familial Chylomicronemia (Type I Hyperlipidemia)

Answer 19

• cause: deficiency of LPL or deficiency of Apo C‐II
• Affected biochemical steps: TAG is the CM cannot be hydrolyzed & CM remains TAG-rich.
• Lipid profile: Elevated fasting CM (high TAG).The serum appears turbid and milky; after centrifugation, the creamy top layer is observed. \ Cholesterol levels are normal. Note: it is unclear why VLDL is not elevated as a result of LPL or Apo CII deficiency.
• Clinical Presentations: Eruptive xanthomata after a high fat meal. Pancreatitis, with no increase risk in cardiovascular disease.
• Therapy: the goal is the reduce CM production. So, consuming medium and short‐chain containing TAGs instead of long‐chain ones plus fat‐soluble vitamins supplementation.

Question 20

Familial Combined Hyperlipidemia Type IIb

Answer 20

(a common disorder 1/200)
-primary cause: an overproduction of Apo B‐100 (unknown etiology).
-Secondary cause: Metabolic syndrome,
obesity, insulin resistance and hypertension
-Affected biochemical steps: Excessive production of VLDLs
-Lipid profile: Elevated VLDL (high TAG) and elevated LDL (High CE). HDL is usually decreased.
-Clinical Presentations: Few clinical
manifestations. No xanthomata. There is a high risk
of premature cardiovascular disease.
-Therapy: correct the secondary causes via diet and life style changes. Combination drug therapy to reduce hypertriacylglyceridemia (niacin) and
hypercholeseremia (statins and resins).

Question 21

Familial Dysbetalipoproteinemia

Answer 21

-Primary cause: polymorphism of Apo E gene; Apo E‐2 variant binds poorly to Apo E receptor.
-Secondary Cause: High-fat diet, diabetes, obesity, hypothyroidism, estrogen deficiency & alc.
-Affected biochemical steps: In Apo E2/ApoE2 homozygous, decreased clearing of IDL and CM remnants are observed.
-Lipid profile: Elevated IDL and CM remnants (serum TAGs and cholesterol are both elevated)
clinical presentation: Tuperoeruptive and/or eruptive xanthomata. Premature coronary and peripheral vascular diseases.
-Therapy: correct the secondary causes via lifestyle and diet changes. Combo drug therapy to reduce hyperTAGemia (niacin or fibrates) and hypercholeseremia.

Question 22

Tangier Disease (alpha-lipoprotein deficiency)

Answer 22

-cause: defect in ABCA1
ABCA1 plays a key role in the reverse cholesterol transport, through which the efflux of free cholesterol from peripheral cells is transferred to the CE‐poor HDL‐3.
-affected biochemical steps: Defective ABCA1 greatly reduces cholesterol transport out of peripheral cells, which leads to an accumulation of CE in many body tissues. It also prevents the maturation of HDL (from nascent HDL to HDL-3). The nascent HDL is rapidly degraded. Apo E and Apo C-II transfer from HDL to CM and VLDL are also prevented
-lipid profile: Low HDL and LDL. Elevated Fasting CM and VLDL. (hypertriacylglycerolemia)
-clinical presentation: Extremely enlarged tonsils (diagnostic feature), Premature myocardial infarction, Clouding of the cornea (abnormal
accumulations of lipids including CE), Hepatosplenomegaly, Intermittent peripheral
neuropathy due to accumulation of CM
-therapy: Nothing specific

Question 23

Niacin (as a drug)

Answer 23

Drug used to treat hyperTAGemia and to increase HDL levels.

• lowers serum TAGs: Niacin inhibits lipolysis resulting in decreases in VLDL and LDL production.
• To increase HDL: Niacin decreases Apo-AI breakdown, extending HDL’s t1/2

Question 24

Fibrate

Answer 24

Drugs used to treat hyperTAGemia and to increase HDL levels.

• lowers serum TAGs: fibrate activates LPL, which increases VLDL clearance. It also decreases nascent VLDL secretion.
• To increase HDL: fibrate increases Apo A-I gene expression, increasing HDL production.

Question 25

Where are VLDLs produced?

Answer 25

VLDLs are produced in the liver. They are composed predominantly of TAGs (approximately
60%), and their function is to carry this lipid from the liver to the peripheral tissues.
1) In the liver: nascent VLDL (with Apo B100) is synthesized and assembled. The liver-synthesized
TAGs are transferred to the nascent VLDL by MTP and other transport proteins.
-In the fed state (a high I/G ratio), TAGs are produced via de novo lipogenesis.
-In the fasting state, TAGs are reconstituted/repackaged due to the accumulating cytosolic fatty acyl-CoAs. Keep in mind that lipolysis liberates 2X of fatty acids than the body’s
energy needs (1X).

Question 26

What happens to nascent VLDLs in circulation?

Answer 26

2) In the circulation: HDL transfers Apo C-II and Apo E to the nascent VLDLs, resulting in the
formation of mature VLDLs (now with Apo B100, Apo C-II and Apo E) plus the hepatic TAGs
and high levels of CE which are transferred from HDL.

Question 27

What happens to mature VLDLs near the surface of adipose, muscle and heart endothelial cells?

Answer 27

3) On the surface of endothelial lining of adipose, muscle and heart: Apo C-II (in VLDL) activates LPL (lipoprotein lipase); the activated LPL then degrades the TAGs which gives rise to fatty acids and glycerol. The released free fatty acids are taken up by the adjacent tissues, and will be reconstituted back to TAGs in the adipose tissue, where the glycerol returns to the liver.

Question 28

Once Apo C-II in VLDL activates LPL (lipoprotein lipase) on the surface of endothelial linings, thus degrading the TAGs on VLDL, what happens afterward in circulation?!

Answer 28

4) In the circulation: after TAG degradation by LPL, Apo C-II is returned back to HDL, and the resultant particle is called IDL (with Apo B-100 and Apo E). A major portion of IDL is endocytosed in the liver via Apo E’s binding to its hepatic Apo-E receptor.

Question 29

In the circulation after VLDL TAG degradation by LPL, and the formation of IDL, what is the fate of IDL?

Answer 29

A major portion of IDL is endocytosed in the liver via Apo E’s binding to its hepatic Apo-E receptor.
5) A small portion of IDL is further catabolized into LDL by returning its Apo E back to HDL and by further TAG hydrolysis via hepatic lipase (synthesized in the liver and exocytosed in the blood).

Question 30

How is cholesterol redistributed to peripheral tissues during metabolism of VLDL?

Answer 30

6) Peripheral tissues (30%) and liver (70%): LDL particles are endocytosed via Apo B-100 binding to the LDL Receptor (i.e. ApoB-100 receptor). This step is how cholesterol is redistributed to the peripheral tissues.

Question 31

Where is LDL oxidized and to what?

Answer 31

LDL is small enough to enter the intima of endothelial cells, where it is oxidized to oxLDL.

Question 32

What is the first main phase of atherosclerosis formation?

Answer 32

1) Chronic endothelial injury and macrophage recruitment phase: Accumulation/excess oxLDL production -> modified the vessel walls -> inflammation of endothelial cells
- Toxins, free radicals (e.g. from cigarette smoke) and glycation due to hyperglycemia promote oxidation of LDL (oxLDL). The level of oxLDL is proportional to that of LDL; i.e. oxLDL increases in the case of hypercholesterolemia. Inflammation of endothelial cells -> recruitment of blood monocytes to migrate into the intima ->maturation into macrophages. -> Overactive macrophages release ROS (reactive oxygen species), which also oxidize LDL in the intima.

Question 33

What is the second main phase of atherosclerosis?

Answer 33

2) Fatty streak formation
Macrophages phagocytosed oxLDL particles in the intima via scavenger receptors that are, in contrast to LDL receptors, not down regulated by elevated intracelluar cholesterol concentrations. Since cholesterol cannot be degraded, macrophages accumulate oxLDL-cholesterol and turn into foam
cells. Subendothelial foam cell accumulation leads to fatty streaks.

Question 34

What is the third main phase of atherosclerosis?

Answer 34

3) Plaque formation
Activated macrophages secrete pro-inflammatory cytokines, which stimulate smooth muscle cells to migrate into the subendothelial space, proliferate and synthesize fibrous material, e.g. collagen. The result is formation of a fibrous cap underneath the endothelium, narrowing the vessels.
The risk of cardiovascular diseases (CVD) increases with increased levels of Total-C (elevated LDL) in the serum.

Question 35

What molecule is a reservoir to apolipoproterins (Apo C-II and Apo E)?

Answer 35

HDL
-Apo C-II is transferred to nascent VLDL and nascent CM, and is an activator of LPL.
-Apo E is required for the hepatic Apo E receptor-mediated endocytosis of IDLs and CM
remnants.

Question 36

What is HDL’s role in cholesterol transport?

Answer 36

(1) HDL is first synthesized and assembled in the liver and in the enterocytes (containing Apo A-I, Apo C-II and Apo E).
(2) Formation of nascent HDL and reverse cholesterol transfer: HDL uptakes phospholipids (PLs) and cholesterol (C) from the peripheral tissues via ABCA1 protein, ATP binding cassette protein A1. This is called “reverse cholesterol transfer” step because the cholesterol is removed from the peripheral tissues and transported into HDL. high levels of PLs in HDL help solubilize cholesterol.
- Because HDLs can also remove cholesterol from macrophages, which slows down the development of foam cells and atherosclerosis, it is considered anti-atherogenic.
- This specific step is the reason why HDL is considered the “good cholesterol”.

Question 37

What is HDL’s role in the esterification of cholesterol?

Answer 37

(3) Esterification of cholesterol: from C to CE by PCAT enzyme
• When cholesterol is taken up by HDL, it is immediately esterified by PCAT
(phosphatidylcholine:cholesterol acyltransferase), located on the surface of HDL. PCAT is activated by Apo A-I of HDL.
• As the HDL accumulates cholesterol esters, it first becomes a relatively CE–poor HDL3
and, eventually, a CE–rich HDL2 particle.

Question 38

How is HDL involved in the transfer of CEs into VLDL by CETP and how are the subsequent HDL remnants cleared by the liver.

Answer 38

(4) Transfer of the CEs into VLDL by CETP & clearing of HDL remnant in the liver
• CETP (cholesterol ester transfer protein) moves the CEs from HDL to VLDL in exchange for TAG. At this point, VLDLs are enriched with CEs. Because VLDLs are eventually catabolized to LDL, LDL ends up with highest cholesterol ester content among the lipoproteins.
• The excess PLs are passed on to HDL3, which needs additional PLs to help expand its cell surface area.
• The HDL remnant is endocytosed by the liver for degradation in the lysosomes.

Question 39

PCAT

Answer 39

enzymatic reaction: C->CE
cellular location: on the surface of HDL
regulation: activated by Apo A-I
Physiological significance: Storing cholesterol in the inner core of HDL in the form of CE. So, the HDL can continue to reverse transfer C from the peripheral tissues to HDL.

Question 40

ACAT

Answer 40

enzymatic reaction: C->CE
cellular location: in the cytosol of most cells
regulation: positively regulated by elevated cytosolic [cholesterol]
Physiological significance: Storing excess cytosolic cholesterol in the form of CE into intracellular storage (in lipid droplets). It contributes towards the cytosolic cholesterol homeostasis.