HDL Cholesterol No Longer Is Good Cholesterol: Emerging Genetic Theories

1
HDL Cholesterol No Longer Is Good
Cholesterol Emerging Genetic Theories

• Sunita Dodani Janice S Dorman
• University of Pittsburgh
• (Study proposal)

2
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

3
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.

4

• 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

5
Lipid transport in the circulation
Proteins (apoproteins)
Cholesterol
Non polar lipids in core (TAG and cholesterol
esters)
Lipids are insoluble in plasma. In order to be
transported they are combined with specific
proteins to form lipoproteins
 
6
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 IDLHDL Lp-a)

7
Five classes of lipoprotein(all contain
characteristic amounts TAG, cholesterol,
cholesterol esters, phospholipids and Apoproteins
NMR Spectroscopy)
Class Diameter(nm) Source and function Major apoliproteins
Chylomicrons(CM) 500 Intestine. Transport of dietary TAG A, B48, C(I,II,III) E
Very low density lipoproteins(VLDL) 43 Liver. Transport of endogenously synthesised TAG B100, C(I,II,III) , E
Low density lipoproteins (LDL) 22 Formed in circulation by partial breakdown of IDL. Delivers cholesterol to peripheral tissues B100
High density lipoproteins (HDL) 8 Liver. Removes used cholesterol from tissues and takes it to liver. Donates apolipoproteins to CM and VLDL A, C(I,II,III), D, E
Increasing density
8
Lipoprotein class Density (g/mL) Diameter (nm) Protein of dry wt Phospholipids Triacyl-glycerols of dry wt
HDL 1.063-1.21 5 15 33 29 8
LDL 1.019 1.063 18 28 25 21 4
IDL 1.006-1.019 25 - 50 18 22 31
VLDL 0.95 1.006 30 - 80 10 18 50
Chylo-microns lt 0.95 100 - 500 1 - 2 7 84
Composition and properties of human lipoproteins
9
Atherogenic Particles
MEASUREMENTS
Apolipoprotein B
Non-HDL-C
VLDL
IDL
VLDLR
Small,denseLDL
LDL
TG-rich lipoproteins

10
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

11
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
• lt5 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 15109(23 Suppl)III2-7)

12
Apoproteins of human lipoproteins

• 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)

13
Chylomicrons
Major lipoprotein classes

• 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

14
Major lipoprotein classes

• VLDL
• density gt1.006
• 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)

15
This animation shows how VLDL are metabolised
once they enter the circulation from the liver
B100
LDL
E
CII
Some LDL taken up by liver (LDL receptors)
Having lost TAG to tissues LDL contains a large
proportion of cholesterol/cholesterol esters
Some LDL taken up by other tissues (LDL
receptors). LDL delivers cholesterol and TAG to
the extra hepatic tissues.
E
CII
HDL
16
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.

17
(No Transcript)
18
(No Transcript)
19
Relative Atherogenicity of Large
and Small LDL Particles
20
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)

Liver
HDL receptor mediated endocytosis by liver
apoproteins
HDL
HDL
HDL
some cholesterol ester transferred
to circulating VLDL
used cholesterol transferred to HDL and
converted to cholesterol ester
VLDL
Cholesterol can be converted to bile salts for
excretion or repackaged in VLDL for redistribution
Peripheral tissues
LDLreceptor mediated endocytosis
LDL
LDL
21
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

22
Helical Wheel Projection Of A Portion Of
Apolipoprotein A-1
23
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 gt 40 mg/dl (NCEP ATP III)
• Independent Predictor of CAD

24
Interrelationship between lipoproteins
VLDL
CE
CETP
FFA
LPL
TG
IDL
Liver (LDL receptor)
TG
CETP
CE
HDL
LPL
TG
FFA
CETP
Liver (LDL receptor)
CE
LDL
25
Reverse Cholesterol Transport Indirect
Extra hepatic tissues
Liver
Cholesterol is reused or excreted in bile
Cholesterol esters
hydrolysis
Direct
Free cholesterol
ABCA1
LCAT
Pre-b-HDL
Cholesterol to VLDL, IDL,LDL
CETP
HDL
A
A
26
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 homeostasis of glycerides
and cholesterol
27

• LCAT deficiency?
• CETP deficiency?
• Apo AI deficiency?

28
LDL
Liver
Dietary fat
Endogenous cholesterol
Bile salts
extra hepatic tissue
Exogenous cholesterol
small intestine
chylomicrons
chylomicrons reminants
HDL
VLDL
IDL
capillaries
Lipoprotein Lipase
Lipoprotein Lipase
Adipose, muscle
FFA
FFA
29
(No Transcript)
30
Progression Of Atherosclerosis
31
Role of LDL in atherosclerosis- oxidation
32
(No Transcript)
33
Oxidized LDL
?Concentration ?Residence time in arterial
wall ?Opportunity to be oxidized, taken up by
macrophage, glycated and be trapped
34
(No Transcript)
35
small dense LDL is more toxic (? oxidation etc.)
but has less cholesterol per particle, measuring
LDL cholesterol doesnt give complete picture.
Measuring apoB provides a better index of
particle number, and an additional
discriminator.
36
Macrophage induced inflammation
37
Development of Atherosclerosis
38
Non-Specific
Anti-oxidant therapy not effective
39
Relationship between HDL/LDL and heart disease
Monocyte (white blood cell)
Cholesterol to liver
LDL
vascular endothelium
()
Oxidized LDL
differentiate
Arterial Intima
Macrophage
Role played by Apo A1
LDL ()
(-) HDL
Foam cells (fatty streak)
40
HDL Function

• Removal of CE from LDL
• Reverse Cholesterol transport
• Apo A-1 prevent seeding of LDL
• Apo A-1 prevent oxidized LDL formation

41
HDL NO More a Good Cholesterol

• Recent theories
• Framingham study of the incidence of coronary
  heart disease (CHD) HDL44 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

42
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?

43
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 27582767 )

44
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 ???

45
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

46
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

47
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

48
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

49
Study Proposal

• Cases with known CAD
• Controls without CAD
• Risk factors under study
• Traditional risk factors
• Emerging risk factors
• Myeloperoxidase levels
• Hydroperoxidase levels
• Apo A-1 levels

50
Study Proposal

• Sample size Biostatistician
• Data analysis Multiple logistic regression