This document discusses lipoprotein metabolism and transport. Key points include:
- Chylomicrons transport dietary lipids from the intestine to liver and peripheral tissues. Very low density lipoproteins (VLDL) transport lipids from liver to tissues.
- Lipoproteins contain a core of triglycerides and cholesterol esters surrounded by phospholipids and apolipoproteins that regulate metabolism.
- Chylomicron remnants are formed after lipoprotein lipase breaks down triglycerides. Remnants deliver remaining lipids to the liver via receptor-mediated endocytosis.
- High density lipoproteins (HDL) promote reverse cholesterol transport, removing cholesterol from tissues and transporting it to
2. Transport of Fat: Lipoproteins
I. Chylomicrons
II. Triglyceride storage in adipose
III. VLDL, LDL, IDL, HDL
IV. Reverse Cholesterol Transport
V. Medical implications
VI. Nutritional regulation of lipoproteins
Stipanuk 351-364
3. Overview
• Transport dietary lipids from intestine to liver
(exogenous)
• Transport lipids from liver to peripheral tissues
(endogenous)
• Lipoproteins
– Core of TG and CE
– Surface of phospholipids and some cholesterol
– Apolipoproteins (regulators of LP metabolism)
– CM, VLDL, IDL, LDL, HDL
• Clinical importance for disease
4.
5.
6. Chylomicron Assembly
-assembled in enterocyte golgi/ER
-Apolipoprotein (Apo) B organizes assembly
-B48
- Requires phospholipids
-2 forms of apo B
-B100, large- liver
-B48, smaller – intestine
- Picks up apo A,C and E in plasma
- TG composition closely resembles dietary intake
7.
8. Microsomal Transfer Protein
Lipid exchange protein
Heterodimer (55 kDa/97 kDa)
Protein disulphide isomerase
Defects in MTP
Gordon et al. Trends in Cell Biology 5:1995
13. Fat accumulation in adipose: High I/G (Fed)
Capillary endothelium
(+)
B48 insulin
CII
LPL
TG/CE
CIII
chylomicron FFA Glucose
glut4
(+) Insulin
regulated
glucose
CoA transport
G3P
Fatty acyl CoA
Triglycerides
adipose
14. Fat accumulation in adipose: Low I/G (ketogenic)
Capillary endothelium
(-)
B48 insulin
CII
LPL
TG/CE FFA-albumin (oxidation)
CIII
chylomicron FFA Glucose
(-) Insulin
glut4 regulated
glucose
CoA transport
G3P
Fatty acyl CoA
Triglycerides
adipose
15. LPL: “Metabolic Gatekeeper?”
• LPL deficiency (chylomicronaemia)
– Massive accumulation of chylomicron-TG in plasma
– Cannot clear TG normally
- Normal fat storage and body weight ???!?!?
- How?
- Knockout mice – lethal
- LPL overexpression
- Decrease plasma TG
- Increase FA uptake in skeletal muscle
- Protect against obesity when fed high-fat diet
16. Hormones and Adipose Tissue
-Adipose tissue is not just a big fat depot
-Produces a number of hormones that regulate fat storage
1. Leptin – decrease food intake/increase energy utilization
* Adequate fat store = release leptin = decrease food
intake and increase energy utilization
2. Acylating stimulating protein (ASP)
chylomicrons stimulate production of ASP
similar anabolic effects as insulin (different mechanisms)
Promote adipocyte glucose uptake and
FA reesterification
18. Regulation of Lipoprotein Lipase
Fed state - LPL synthesis and activity (adipocytes)
LPL synthesis and activity (skeletal and heart muscle)
Fasted/ - LPL synthesis and activity (adipocytes)
exercise
LPL synthesis and activity (muscle)
state
Lactating -
LPL activity
Mammary
gland
19. Plasma
Dietary Carbohydrate
LIVER
glucose pyruvate Acetyl CoA
B48 LDL receptor Acetyl CoA
E mitochondria
TG/CE
TG
CMr Cholesterol
cholesterol (endogenous)
B100
(exogenous)
CE/TG VLDL
FFA FFA TG
VLDL
20. Dietary factors affecting Chylomicron and Chylomicron remnant clearance
-elevated postprandial lipoproteins and cardiovascular disease
-Diets rich in PUFA can reduce postprandial TG response
-compared to diets rich in SFA
-Increased LPL activity = Increased TG clearance from CM
-Preferential hydrolysis of PUFA-containing CM
-Increased clearance of CMr
-Human data are less convincing than animal studies
-Omega 3 > Omega 6 > SFA
-Not much work with MUFA although may be helpful (OLIVE OIL)
23. From liver Cholesterol.
In bile LIVER
Endogenous cholesterol
B100 E
CII
CE/TG
VLDL B100 E
LDL receptor
CE/TG
IDL E
B100
LPL FA CE
FFA LDL Extrahepatic tissue
muscle adipose LDL receptor
24. Nobel Prize Alert: 1985
A Receptor-Mediated Pathway for Cholesterol Homeostasis
Michael S. Brown Joseph Goldstein
25.
26. Function of LDL receptor
• Endocytosis of LDL and other LP
• Release free cholesterol into liver
1. Incorporate into plasma membrane
2. Inhibit new LDL receptors
3. Inhibit cholesterol synthesis
4. Promote ACAT activity (FC -> CE)
• Regulated by SREBP
monitors free cholesterol
Free cholesterol = LDL receptors, chol. synthesis
ACAT
27. HDL Formation Steroidogenic
cells Cholesterol
to other
2. Cholesterol lipoproteins
for steroid
Liver synthesis 3. Cholesterol-ester
1. Cholesterol transfer protein
to liver (CETP)
A HDL
ApoA
Lecithin-cholesterol acyl
transferase (LCAT)
A A
Pre-β-HDL Cholesterol from
Liver and intestinal Pre-β-HDL
Cells via ABCA1
Discoidal/lipid poor Unesterified cholesterol-rich
28. CETP exchanges cholesterol esters in HDLs for triglycerides in B100 LPs
VLDL
CE
CETP
FFA
LPL TG
Liver IDL TG
(LDL receptor) HDL
CETP
CE
LPL
FFA TG
CETP
Liver
CE
(LDL receptor)
LDL
29. Reverse Cholesterol Transport: Indirect
Extrahepatic tissues
Liver
Cholesterol esters Cholesterol is reused
or excreted in bile
hydrolysis
Direct
Free cholesterol
ABCA1
A
LCAT CETP Cholesterol to
Pre-β-HDL A HDL
VLDL, IDL,LDL
32. Postprandial Changes in Plasma
Lipid Metabolism
Fat storage via LPL
Transfer of cholesterol from cells into plasma
reverse transport of cholesterol from peripheral tissue to liver
Exchange of cholesterol for VLDL TG in HDL (CETP)
LCAT activity = esterification of free cholesterol (HDL)
These postprandial changes are beneficial in maintaining
whole body homeostatsis of glycerides and cholesterol
33. Dietary Regulation of Lipoprotein
Synthesis
Chylomicron Synthesis VLDL Synthesis (Liver)
Chylomicron VLDL (+)
High CARB FA/TG
Insulin
(+)
Acetyl CoA
Dietary Fat
Intestinal Epithelium
(+)
Glucose
37. Hypertriglyceridemia and CHD Risk:
Associated Abnormalities
Accumulation of chylomicron remnants
Accumulation of VLDL remnants
Generation of small, dense LDL
Association with low HDL
Increased coagulability
- plasminogen activator inhibitor (PAI-1)
- factor VIIc
- Activation of prothrombin to thrombin
38. Relationship between HDL/LDL and heart disease:
One Theory
Monocyte (white blood cell)
Cholesterol to liver
LDL
vascular endothelium
(+)
differentiate Oxidized LDL
Arterial intima
Macrophage
LDL (+) (-) HDL
Foam cells (fatty streak)
39. Alcohol Increases HDL-C Level
• Alcohol increases HDL-C level in a dose-dependent
manner.
• Half bottle of wine per day (39 g alcohol) for 6 weeks
significantly increased mean HDL-C level by 7 mg/dL
in 12 healthy subjects.1
– Wine intake did not significantly affect Total-C,
Total-TG, or LDL-C.1
• One beer per day (13.5 g alcohol) for 6 weeks
significantly increased mean HDL-C level by 2 mg/dL
in 20 healthy subjects.2
– Beer intake did not significantly affect LDL-C,
VLDL-C, TG, or apolipoproteins.
1. Thornton J et al. Lancet 1983;ii:819–822
2. McConnell MV et al. Am J Cardiol 1997;80:1226–1228
40. Journal Papers and Revision
Out of 10 points
Revisions – 30 pts
Clear, concise writing
Extend discussion –
Additional references- email author w/ ?
and include in revised report
Current and future research
41. Next Week
• Feb 23 – Dr. Neile Edens – Ross Labs
• Feb 25 – Beta oxidation/Cholesterol
• Feb 27 – Exam Review/Rough Draft
revisions
Editor's Notes
Exogenous Lipid Transport Fatty acids are absorbed by the apical microvilli of muscosal cells and resterified in the enterocyte (2 monoacylglyercol pathway). Apo B48 is the structural protein of the chylomicron and contain the majority of cholesterol (as cholesteryl ester) found in chylomicrons. Reesterfied TG are added to the chylomicron percurors via the action of a TG transfer protein. Apo B48 is then added. Apo CII is also added. This apoliprotein activates LPL activity. Nascent cylomicrons are assembled in the golgi apparatus and released from the enterocyte to enter the lymphatic system. Eventually chlyomicrons enter the plasma via the left thoracic lymph duct. Triglycerides and cholesterol esters are concentrated in the core of the chylomicron.
Microsomal transfer protein helps in the assembly of protein and lipids that makeup lipoporteins. Lipoproteins are exported through a secretory pathway as water-soluble particles and circulate in blood.
Fatty acids are absorbed by the apical microvilli of muscosal cells and resterified in the enterocyte (2 monoacylglyercol pathway). Apo B48 is the structural protein of the chylomicron and contain the majority of cholesterol (as cholesteryl ester) found in chylomicrons. Reesterfied TG are added to the chylomicron percurors via the action of a TG transfer protein. Apo B48 is then added. Apo CII is also added. This apoliprotein activates LPL activity. Nascent cylomicrons are assembled in the golgi apparatus and released from the enterocyte to enter the lymphatic system. Eventually chlyomicrons enter the plasma via the left thoracic lymph duct. Triglycerides and cholesterol esters are concentrated in the core of the chylomicron.
Our chylomicron is destined for the liver. However on its journey this particle will encounter lipoprotein lipase which will hydrolyze TG present in the chylomicron. This will result in an overall reduction in the size of the chylomicron as TG is removed. CII is required for LPL activation. As the CM loses TG CII will then disassociate from the particle and LPL activity will no longer be supported. This particle is now called a chylomicron remnant and is destined for the liver. Insulin High in Fed State Fed state – Chylomicron synthesis is high. LPL activity is high. Storage of FFA as TG in adipose is high. Fasted state- Chylomicron synthesis is low. LPL activity in adipose is low while LPL activity in heart and other muscles remains steady. In addition LPL on surface of heart has a higher affinity (lower Km) for lipoprotein substrates. Therefore TG hydrolysis by the heart is determined by lipoprotein lipase levels (not the concentration of circulating lipoproteins). Heart LPL is saturated, even at low levels of circulating lipoproteins (fasted state). This ensures that the heart has preference for energy.
Fed state: High adipose LPL. Increased glucose transport into adipocyte.
During periods of low Insulin to glucagon, like a ketogenic diet (high protein. Low carbohydrate) the majority of fatty acids will be bound to albumin and utilized for B-oxidation (we will discuss b-ox during next lecture). This occurs because the reesterification of Fatty acids in the adipose is reduced due to limited G3P from glucose! Low insulin will also downregulate the glut4 receptor and lower glucose uptake into adipocyte
LPL is differentially regulated depending on the tissue. In the fed state LPL activity in adipocytes to promote storage of TG. In the fasted/exercised state, LPL activity in the muscle is activated or unchanged. The Km for muscle LPL is lower than that of LPL in adipose. Therefore substrates have a higher affinity for muscle LPL . The ensures that muscle LPL is always saturated regardless of TG levels and that cardiac tissues have first preference for circulating TGs.
Liver removes the chylomicron remnant via the remanant receptor..
The liver synthesizes VLDL primarily during the fed state. Some of the TG present in the VLDL are synthesized de novo from dietary carbohydrate. Circulating concentrations of VLDL triacylglycerol increase after a carbohydrate rich meal. Other fatty acids of VLDL originate from free fatty acids internalized from plasma by liver. The cholesterol esters present in VLDL are from de novo synthesis
VLDL from liver enters plasma. VLDL contains B100 (structural, binds to LDL receptor), apo E (binds to LDL receptor) and CII (activates lipase). LPL works on VLDL and IDL to remove FFA.
Interrelationship between lipoproteins. PLTP transfers phospholipids (lecithin) from VLDL, IDL and LDL to HDL.LCAT associated with HDL esterifies cholesterol. CETP transfers CE from HDL to VLDL, IDL and LDL
Free cholesterol and lecithin (phospholipid) are transferred from cell membranes to pre B-HDL to form discoidal HDL. Via lymph this HDL enters the plasma. Through the action of LCAT (lecithin:cholesterol acyltransferase) the discoidal HDL becomes spheroidal and cholesterol is esterified. About 1/3 of the cholesterol present in HDL is moved to B-100 containing lipoproteins (IDL/LDL/VLDL) in exchange for TG ( 2 ). Cholesterol ester transfer protein mediates this transfer. The majority of the HDL/cholesterol is internalized by liver and to a lesser extent by adrenal and gonadal cells via the SR-BI scavenger protein for use in bile acid and steroid hormone synthesis.
Dietary fat induces CM formation in intestine Increased availability of fatty acids stimulates VLDL synthesis Stimulation of fatty acid synthesis increases VLDL synthesis (carbohydrates and alcohol) Insulin release also increases VLDL production
Monocytes are mononuclear phagocytes that circulate in blood (white blood cells). Monocytes can emigrate from blood into tissues in the body and differentiate in macrophages. In response to cellular injury monocyte infiltrates arterial intima where it differentiates into a macrophage. Macrophage release cytokines and other pro-inflammatory agents. Macrophages can also accumulate lipids and form foam cells. Foam cells can release growth factors and also metalloproteinases which can lead to matrix degradation. Oxidized LDL can promote monocyte differentiation into macrophages. Oxidized LDL can also be taken up by macrophage receptors and lead to the formation of lipid rich foam cells. HDL cholesterol promotes cholesterol efflux from extrahepatic tissues and thus can reduce the transformation of macrophages into foam cells and thus reduce fatty streak (foam cell)