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HDL Cholesterol No Longer Is Good Cholesterol: Emerging Genetic Theories Sunita Dodani & Janice S Dorman
University of Pittsburgh
(Study proposal) Presentation Overview Study background & significance
Basic description & function of lipids, lipoproteins and apoproteins
HDL & apoprotein A-1
New theories of LDL & HDL role in atherosclerosis
Concept of dysfunctional HDL
Hypothesized causes of dysfunctionalHDL
Study Rationale
Study objectives
Study Design & Methods
Study background & significance Fats are triacylglycerols containing saturated fatty acids - solid at room temp - usually from animal source (however, coconut & palm oil are saturated).
Oils are triacylglycerols containing mono- or polyunsaturated fatty acids - liquid at room temp
- usually from plant sources (however, fish oils are polyunsaturated).
Phospholipids are triacylglycerols that have a FA replaced with a phosphate linked FA group.
The major dietary sterol is cholesterol. Functions Of Lipids
Major components of cell membranes.
Required to solubilise fat soluble vitamins
Biosynthetic precursors (e.g. steroid hormones from cholesterol)
Protection (e.g. kidneys are shielded with fat in fed state)
Insulation Lipid transport in the circulation Lipids are insoluble in plasma. In order to be transported they are combined with specific proteins to form lipoproteins Non polar lipids in core (TAG and cholesterol esters) Proteins (apoproteins) Cholesterol
Lipoproteins Particles found in plasma that transport lipids including cholesterol
Spherical particles with a hydrophobic core (TG and esterified cholesterol)
Apolipoproteins on the surface
large: apoB (b-48 and B-100) atherogenic
smaller: apoA-I, apoC-II, apoE
Classified on the basis of density (NMR spectroscopy) and electrophoretic mobility (VLDL; LDL; IDL;HDL; Lp-a) Five classes of lipoprotein (all contain characteristic amounts TAG, cholesterol, cholesterol esters, phospholipids and Apoproteins – NMR Spectroscopy) Increasing density Composition and properties of human lipoproteins Atherogenic Particles Apolipoprotein B Non-HDL-C MEASUREMENTS: TG-rich lipoproteins VLDL VLDLR IDL LDL Small, dense LDL The Apolipoproteins
Major components of lipoproteins
Often referred to as aproteins
Classified by alphabetical designation (A thru E)
The use of roman numeral suffix describes the order in which the Apolipoproteins emerge from a chromatographic column
Responsible for recognition of particle by receptors Apoproteins of human lipoproteins A-I (28,300)- principal protein in HDL
90 –120 mg% in plasma
A-II (8,700) – occurs as dimer mainly in HDL
30 – 50 mg %; enhances hepatic lipase activity
B-48 (240,000) – found only in chylomicrons
<5 mg %; derived from apo-B-100 gene by RNA editing; lacks the LDL receptor-binding domain of apo-B-100
B-100 (500,000) – principal protein in LDL
80 –100 mg %; binds to LDL receptor
(Circulation. 2004 Jun 15;109(23 Suppl):III2-7) C-I (7,000) – found in chylomicrons, VLDL, HDL
4 – 7 mg %; may also activate LCAT
C-II (8,800) - found in chylomicrons, VLDL, HDL
3 – 8 mg %; activates lipoprotein lipase
C-III (8,800) - found in chylomicrons, VLDL, IDL, HDL
8 15 mg %; inhibits lipoprotein lipase
D (32,500) - found in HDL
8 – 10 mg %; also called cholesterol ester transfer protein (CETP)
E (34,100) - found in chylomicrons, VLDL, IDL HDL
3 – 6 mg %; binds to LDL receptor
H (50,000) – found in chylomicrons; also known as b-2-glycoprotein I (involved in TG metabolism)
Apoproteins of human lipoproteins Chylomicrons
Formed through extrusion of resynthesized triglycerides from the mucosal cells into the intestinal lacteals
Flow through the thoracic ducts into the subclavian veins
Degraded to remnants by the action of lipoprotein lipase (LpL) which is located on capillary endothelial cell surface
Remnants are taken up by liver parenchymal cells Major lipoprotein classes Major lipoprotein classes VLDL
density >1.006
diameter 30 - 80nm
endogenous triglycerides
apoB-100, apoE, apoC-II/C-III
prebeta in electrophoresis
formed in the liver as nascent VLDL (contains only triglycerides, apoE and apoB)
LDL Lipoprotein
lipase Capillary wall
(endothelial surface) Tissues This animation shows how VLDL are metabolised once they enter the circulation from the liver VLDL B100 HDL CII E CII E B100 Some LDL taken up
by liver (LDL receptors) Some LDL taken up by
other tissues (LDL receptors).
LDL delivers cholesterol and
TAG to the extra hepatic tissues. Having lost TAG to tissues LDL contains a large proportion of cholesterol/cholesterol esters LDL membrane receptor Found in clathrin coated pits (endocytosis)
After endocytosis the receptor is recycled whilst the LDL is degraded to releasing lipid cargo. Cholesterol uptake down regulates the cells own production of cholesterol and down regulates LDL receptor synthesis
Mutations in LDL receptors causes increased plasma LDL levels (i.e. increased cholesterol levels). This accelerates progress of atherosclerosis (Familial hyperlipedimias).
The cholesterol in LDL is often called “bad cholesterol”. LDL Heterogeneity: Small vs. large LDL Production of Small Dense LDL
Although size (& lipid content) changes, there is always 1 molecule apo B protein / particle Oxidation &
modification TG
CE Lipase B B (CETP)
Lipids
transferred dense large 1 2 3 VLDL VLDL
remnants IDL LDL B B Lipoprotein receptors B Tg CE B Tg CE
Relative Atherogenicity of Large and Small LDL Particles High density lipoprotiens Act as a reservoir for apoproteins which can be donated or received from other lipoproteins.
Also play a vital role in scavenging “used” cholesterol (reverse cholesterol transport): HDL Peripheral tissues HDL apoproteins “used” cholesterol
transferred to HDL and
converted to cholesterol
ester
Liver HDL receptor mediated
endocytosis by liver Cholesterol can be converted to bile salts for excretion or
repackaged in VLDL for redistribution VLDL HDL LDL LDL LDLreceptor
mediated
endocytosis some cholesterol
ester transferred to
circulating VLDL High density lipoprotiens HDL HDL carries “used” cholesterol (as CE) back to the liver. Also donate some CE to circulating VLDL for redistribution to tissues.
HDL taken up by liver and degraded. The cholesterol is excreted as bile salts or repackaged in VLDL for distribution to tissues.
Cholesterol synthesis in the liver is regulated by the cholesterol arriving through HDL (and dietary cholesterol returned by chylomicrons remnants).
Cholesterol (CE) in HDL is referred to as “good cholesterol” Helical Wheel Projection Of A Portion Of Apolipoprotein A-1 HDL functioning HDL may transfer some cholesterol esters to other lipoproteins.
Some remain associated with HDL, which may be taken up by liver & degraded.
HDL thus transports cholesterol from tissues & other lipoproteins to the liver, which can excrete excess cholesterol as bile acids.
High blood levels of HDL (the "good" cholesterol) correlate with low incidence of atherosclerosis
HDL > 40 mg/dl (NCEP ATP III)
“Independent Predictor of CAD” VLDL IDL LDL LPL LPL FFA FFA Liver
(LDL receptor) Liver
(LDL receptor) HDL CETP CE TG CETP TG CE CETP TG CE Interrelationship between lipoproteins CETP LCAT Free cholesterol hydrolysis Reverse Cholesterol Transport: Indirect
Extra hepatic tissues Cholesterol esters Pre-b-HDL A HDL A Cholesterol to
VLDL, IDL,LDL Liver Cholesterol is reused
or excreted in bile ABCA1 Direct Postprandial Changes in Plasma Lipid Metabolism Fat storage via LPL Exchange of cholesterol for VLDL TG in HDL (CETP) Transfer of cholesterol from cells into plasma
reverse transport of cholesterol from peripheral
tissue to liver LCAT activity = esterification of free cholesterol (HDL) These postprandial changes are beneficial in maintaining whole body homeostasis of glycerides and cholesterol LCAT deficiency?
CETP deficiency?
Apo AI deficiency? Dietary fat small
intestine capillaries Lipoprotein Lipase FFA Adipose, muscle chylomicrons chylomicrons
reminants VLDL Lipoprotein Lipase FFA extra hepatic
tissue IDL HDL LDL Endogenous
cholesterol Exogenous
cholesterol Bile salts Liver Progression Of Atherosclerosis Role of LDL in atherosclerosis- oxidation Oxidized LDL Concentration
Residence time in arterial wall
Opportunity to be oxidized, taken up by macrophage,
glycated and be trapped
small dense LDL is more toxic ( oxidation etc.) but has less cholesterol per particle, measuring LDL cholesterol doesn’t give
complete picture. Measuring apoB provides a better index of particle number,
and an additional discriminator.
Macrophage induced inflammation Development of Atherosclerosis Non-Specific Anti-oxidant therapy not effective Relationship between HDL/LDL and heart disease
Monocyte (white blood cell) vascular endothelium Arterial Intima Macrophage Oxidized LDL (+) LDL LDL (+) Foam cells (fatty streak) (-) HDL Cholesterol to liver differentiate Role played by Apo A1 HDL Function Removal of CE from LDL
Reverse Cholesterol transport
Apo A-1 prevent seeding of LDL
Apo A-1 prevent oxidized LDL formation HDL NO More a Good Cholesterol Recent theories
Framingham study of the incidence of coronary heart disease (CHD) & HDL:44% of the events occurred in men with HDL-cholesterol levels of 40 mg/dl and 43% of the events occurred in women with HDL-cholesterol levels of 50 mg/dl
A significant number of CHD events occur in patients with normal LDL-cholesterol levels and normal HDL-cholesterol levels.
Search for markers with better predictive value
HDL NO More Good Cholesterol Increase CHD on high HDL
Good HDL becomes bad (Navab M, 2002)
Conversion of anti-inflammatory HDL into pro-inflammatory HDL Increase risk of atherosclerosis
Dysfunctional HDL has been detected by special test of cell-free assay (Navab M, 2002)
Non-functioning Apo A1
“What makes HDL dysfunctional”?
HDL NO More Good Cholesterol Recent hypotheses
Products of an inflammatory enzyme, myeloperoxidase target main Apo A1 Converting HDL into pro-inflammatory non-functional.
Apo A1 macrophages retain increase cholesterol & cholesterol reverse transport reduced. Myeperoxidase modify tyrosine AA (Fogelman AM 2003)
Dysfunctional HDL has increases hydroperoxidase This makes it pro- inflammatory (Van Lenten et al J. Clin. Invest. 96: 2758–2767 ) HDL NO More Good Cholesterol
Role of myeloperoxidase:
Modify tyrosine AA in Apo A-1 100 X more than same AA in other protein
Study: In patients with CHD there is substantial amount of tyrosine AA in Apo A1 modified by myeloperoxidase than in controls (Zheng et al 2004)
Why this occur ???
Study Rationale Un answered Questions
Which patients are susceptible to develop dysfunctional HDL
What makes myeloperoxidase to cause change in Apo A1
What is the role of hydroperoxidase in causing dysfunctional HDL Study Rationale Un answered Questions
Which patients are susceptible to develop dysfunctional HDL
What makes myeloperoxidase to cause change in Apo A1
What is the role of hydroperoxidase in causing dysfunctional HDL Study Proposal Objectives are to:
Measure the level of functional & dysfunctional HDL in CHD cases and controls
Assess the risk factors association with dysfunctional HDL in both cases and controls
Measure the levels of myeloperoxidase, hydroperoxidase and Apo A-1 protein in both cases and controls
Study the candidate genes for myeloperoxidase, hydroperoxidase and Apo A-1in cases and controls
Study Proposal Study Design: Case-control
Study population:
South Asian Immigrant population residing in San Diego, California (total population- 1500 families)
Number of Groups
1. Hindus: from India, Nepal & SriLanka
Ethnic groups: Gujarati, Marathi, Hindi
2. Muslims: from Pakistan, India & Bangladesh
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