Metabolism and Protective Properties of HDL
February 23, 2016
Khalid Al-Rasadi, BSc, MD, FRCPC
Head of Biochemistry Department, SQU
Head of Lipid and LDL-Apheresis Unit, SQUH
President of Oman society of Lipid & Atherosclerosis (OSLA)
DISCLOSURE
AstraZeneca
Pfizer
MSD
Abbott
Aegerion
Sanofi
Presentation Outlines
• HDL biogenesis and metabolism
• HDL function and cardioprotective properties
• HDL dysfunction
• Functional and compositional assessment of HDL
HDL biogenesis and metabolism
Structure of HDL
Surface monolayer ofphospholipidsand free cholesterol
Hydrophobic core of triglycerideand cholesteryl esters
apoA-I
apoA-II
Note: LFA1 denotes Lipid Free Apo Lipoprotein A1r HDL denotes Reconstituted HDL
HDL Consists of Heterogeneous
Particles, but Their Clinical Relevance
Remains to Be Established
HDL are Heterogenes (Contents)
PhospholipidsTriglycerids
Esterified cholesterol
Apo A1Free cholesterol
> 1000 different lipids(Phospholipid species, Cholesterol Ester, Trigylcerides, …)
70 different proteins(ApoA1, PON-1, ApoA2, ApoCIII,
ApoE, ApoH, ….)
CholesterolEster
PON-1
0
10
20
30
sn-2 Fatty Acid
sn-1 Fatty Acid
% O
ccu
rren
ce
16:0
PL
PO
PA
16:0
18:1
18:2
20:3
20:4
20:5
22:6
18:0
18:1
18:2
20:3
20:4
20:5
SL
18:2
18:1
18:2
20:4
20:5
18:1
18:2
Phospholipid Composition
PD
Discoidal
Spherical
Globular
Adapted from Barter PJ. Atheroscler Suppl. 2002;3:39-47.
HDL are Heterogeneous(Particle Shape)
HDL are Heterogeneous
(Surface Charge and Mobility)
Origin -Migration-Migration Pre--Migration
(E)HDL
Lipid-poor/Lipid-free apoA-I
Discoidal HDL
(A-I)HDL
(A-I/A-II)HDL
+Anode
-Cathode
HDL2aHDL2b HDL3cHDL3bHDL3a
HDL Particle Size/Electrophoretic Mobility
Apolipoprotein ContentParticle Shape
Discoidal
Spherical
A-I HDL A-I/A-II HDL
Lipid-poor apoA-I
HDL Subpopulations
+other apos: A-IV, C, D, E, etc.
+non-apo proteins: inflam, thromb, etc.
--------------------------Alpha-migrating---------------------------Pre-beta-migrating--
Globular Discoidal
Globular
Adapted from Barter PJ. Atheroscler Suppl. 2002;3:39-47.
The HDL heterogeneity is the result of activity of several factors that
assemble and remodel HDL in plasma
apoA-I
(lipid-free)
Origin of ApoA-1
Chylomicrons
Lipolysis
Factors that assemble and remodel HDL in plasma
• ATP-binding cassette transporter (ABCA1)
• Lecithin—cholesterol acyltransferase (LCAT)
• Cholesteryl Ester Transfer Protein (CETP)
• Hepatic Lipase (HL)
• Phospholipid Transfer Protein (PLTP)
• Scavenger receptor class B type I (SR-B1)
Generates discoidal HDL by effluxing
phospholipids and cholesterol from cell
membranes to lipid-free/lipid-poor apoA-I
ABCA1
NC
Lipid-free/lipid-poor apoA-I
Discoidal HDL
• Generates most of the cholesteryl esters in plasma
•Activated by apoA-I and, to a lesser extent,
apoE and apoA-IV
•ApoA-II does not activate LCAT
• Remodels discoidal HDL into spherical HDL
LCAT
Spherical (A-I)HDL
UC
LCAT
Lyso-PCPC
LCAT Converts Discoidal HDL into Spherical HDL
Discoidal HDL
Spherical (A-I)HDL
UC
LCAT
Lyso-PCPC
LCAT Converts Discoidal HDL into Spherical HDL
Discoidal HDL
UC(concentration
gradient)
Peripheral cells
• Generates most of the cholesteryl esters in plasma
•Activated by apoA-I and, to a lesser extent,
apoE and apoA-IV
•ApoA-II does not activate LCAT
• Remodels discoidal HDL into spherical HDL
LCAT
Discoidal (A-I)HDL Discoidal (A-II)HDL
LCAT/LDL
Spherical (A-I/A-II)HDLClay et al. J. Biol Chem. 2000 275:9019-9025
Formation of (A-I/A-II)HDL by LCAT
• Transfers core lipids between HDL, VLDL and LDL
• Remodels HDL into large and small particles
• Mediates the dissociation of apoA-I from HDL
CETP
Barter et al; Biochem J. 1982; 2081:1, Swenson et al. J. Biol. Chem. 1988;263:5150, Tall. J. Lipid res. 1993; 34:1255
Role of CETP in plasma cholesterol transport
• Hydrolyses HDL phospholipids and triglycerides
• Remodelling is enhanced by CETP
• Mediates the dissociation of apoA-I from HDL
• Remodels HDL into small particles
Hepatic Lipase
TGCE
VLDL
Large Spherical HDLCE
TG
CETP
Lipid-free
apoA-I
HL
Small Spherical HDL
TGCE
CETG
•Transfers phospholipids between HDL and VLDL
•Remodels HDL into large and small particles
•Mediates the dissociation of apoA-I from HDL
PLTP
Scavenger receptor type B1 ( SRB1)
• SRB1 promotes the selective hepatic uptake of HDL cholesteryl esters
• SRB1 thus also remodels HDL
HDL function and cardioprotective properties
39
39
Mechanisms of Cellular Cholesterol Effluxto HDL particles
Extracellular space Cell membrane
FC
FC
FC
FC
ABCA1
Diffusion
SR-B1
Diffusion
SR-B1ABCG1
Diffusion
SR-B1ABCG1
Lipid-poor ApoA-I
Discoidal HDL
Small spherical HDL
Larger spherical HDL
LCAT
LCAT
40
Current Model of HDL-C Metabolism: Reverse Cholesterol Transport
BCE
Bile
SR-BILDLReceptor
Liver
FCCE LCAT
A-1
A-1
CEHL, EL
Macrophage
Mature HDL-C
Cuchel M et al. ATVB. 2003;23:1710–1712; Assmann G et al. Circulation. 2004;109(23 suppl 1):III-8–III-14.
FC
Nascent HDL
CE
TP
VLDL/LDL-C
Net transfer of cholesterol
FC
SR-BI
ABCG1
CE
Potential Antiatherogenic Actions of HDL
MCP-1 = monocyte chemoattractant protein-1
Adapted from Barter PJ et al. Circ Res. 2004;95:764-772.
Monocyte
Macrophage
Foam
cell
Vessel Lumen
Endothelium
IntimaCytokines
Adhesion
molecule
Oxidized LDL
LDL
LDL
HDL inhibits expression of endothelial cell adhesion
molecules and MCP-1
MCP-1
HDL inhibits
oxidation of LDL-C
HDL promotes efflux of
cholesterol from foam cells
Drew et al. Circulation 2009;119:2103-2111
ABCA1 RPKA
RPKA
PKA
PKA
R
R cAMP
cAMP
Lipid-free apoA-I
Ca2+
Ca2+Ca2+
Ca2+
InsulinAAAAA
AC AC
Adenylyl cyclase
cAMP
cAMP
cAMP
cAMP
cAMP
Insulin release
PKA
FOXO1
T24 S256
S319
HDL dysfunction
Functional and compositional
assessment of HDL
48
HDL: New Dimensions
QUANTITYHDL-C / Apo AI
QUALITYParticle structure
Lipidome, Proteome
Functionality
Kontush A, Chapman MJ. Pharmacol Rev. 2006.
Functional and Compositional Assessment of HDL
• Cholesterol efflux
• Antioxidant activity
• Anti-inflammatory activity
• Proteomics/lipidomics
Note: these are research tools w/o known clinical
relevance of application
Rosenson RS, Brewer HB Jr, Chapman MJ, Fazio S, Hussain MM, Kontush A, Krauss RM, Otvos JD, Remaley AT, Schaefer EJ.
Clin Chem. 2011;57(3):392-410.
Beyond HDL-C: Laboratory assessment of reverse cholesterol efflux (by plasma)
Study design: Endothelial effects of HDL -endothelial bioassays
Isolation of HDL2/3(by sequential ultracentrifugation)
Endothelial Function
(Endothelial cell NO production and vasoreactivity)
ESR spectroscopyOrgan chamber
ESR spectroscopy
Patients with acute coronary syndrome (n=25)Patients with stable coronary disease (n=25)
Healthy control subjects (n=25)
Vascular effects
Anti-oxidanteffects
(Endothelial cell superoxide production)
Anti-inflammatoryeffects
(Endothelial cell inflammatory activation)
Monocyte adhesion VCAM-1 expression
Effects on Re-Endothelialization
Carotid artery injury model in nude mice
Anti-thromboticeffects
Tissue factor Arterial thrombosis
HDL Proteome
Proposed Term Very Large HDL (HDL-VL)
Large HDL-V (HDL-L)
Medium HDL (HDL-M)
Small HDL (HDL-S)
Very Small HDL (VS-HDL)
Density range, g/mL 1.063-1.087 1.088-1.110 1.110-1.129 1.129-1.154 1.154-1.21
Size range, nm 12.9-9.7 9.7-8.8 8.8-8.2 8.2-7.8 7.8-7.2
Density gradient
ultracentrifugation HDL2b HDL2a HDL3a HDL3b HDL3c
Density range, g/mL 1.063-1.087 1.088-1.110 1.110-1.129 1.129-1.154 1.154-1.170
Gradient gel
electrophoresis HDL2b HDL2a HDL3a HDL3b HDL3c
Size range, nm 12.9-9.7 9.7-8.8 8.8-8.2 8.2-7.8 7.8-7.2
2D gel electrophoresis Alpha-1 Alpha-2 Alpha-3 Alpha-4 Preβ-1 HDL
Size range, nm 11.2-10.8 9.4-9.0 8.5-7.5 7.5-7.0 6.0-5.0
NMR Large HDL-P Medium HDL-P Small HDL-P
Size range, nm 12.9-9.7 9.7-8.8 8.8-8.2 8.2-7.8 7.8-7.2
Ion mobility HDL2b HDL2a + 3
Size range, nm 14.5-10.5 10.5-7.65
Separation of HDL by Physical Properties
Rosenson RS, Brewer HB Jr, Chapman MJ, Fazio S, Hussain MM, Kontush A, Krauss RM, Otvos JD, Remaley AT, Schaefer EJ.
Clin Chem. 2011;57(3):392-410.
Burning questions to resolve the puzzles with HDL?
• Can we relate HDL associated function to unique HDL subpopulations?
• Can distinct function of HDL subpopulations be accounted by specific protein components?
• Which of the known HDL functions protect against CVD?
Answers to these questions require much more research