HDL mediated efflux of cholesterol from
the arterial wall: Can HDL promote
removal of cholesterol from
atherosclerotic lesions?
Prof. John Chapman
INSERM, Hôpital de la Pitié
Paris, France
Current Model of HDL-C Metabolism:
Reverse Cholesterol Transport
Bile
ABCG1
Mature HDL-C
SR-BI
A-1
CE
FC
CE
LCAT
CE
SR-BI
FC
HL, EL
LDL
Receptor
CETP
A-1
Liver
FC
Nascent HDL
Macrophage
Net transfer of cholesterol
CE
B
VLDL/LDL-C
Cuchel M et al. ATVB. 2003;23:1710–1712; Assmann G et al. Circulation. 2004;109(23 suppl 1):III-8–III-14.
2
• An appropriate model of cholesterol
efflux from the atherosclerotic plaque
should reflect the complex structure
of the lesion itself.
• What is the predominant form of
cholesterol present
in the vulnerable atherosclerotic plaque ?
Lipid accounts for 40% of surface area of vulnerable plaques
Relation of Plaque Lipid Composition and Morphology to the Stability of Human Aortic Plaques.
C.V. Felton et al, ATVB 1997;17:1337-1345
In what physical forms is cholesterol
present within vulnerable plaques ?
Cholesterol is differentially distributed
between plaque components
Cholesterol is present in distinct physical forms :
- in extracellular cholesterol-rich microdomains
- in monocyte-derived macrophage foam cells (CE > FC)
- in apoptotic cellular debris, primarily within the necrotic
core (FC > CE)
FC -rich microdomains are present in regions
of human aortic tissue with lipid-containing foam cells
FOAM
CELLS
ORO
Haematoxylin
Mab
Mab58B1
58B1
+ DAPI
Autofluorescence
Control
W/O
FOAM
CELLS
Ong DS et al J Lipid Res. 2010 51:2303-13
Incubation of human monocyte-derived macrophage cultures with AcLDL
generates extracellular unesterified cholesterol-rich microdomains
1 Day
+ AcLDL
2 Days
+ AcLDL
+ Mab 58B1
Cell
nuclei:
DAPI
Control
no AcLDL
Control
Mab
Ong DS et al J Lipid Res. 2010 51:2303-13
Unesterified cholesterol-rich microdomains
can be removed by cholesterol acceptors
HDL
apoAI
Ong DS et al J Lipid Res. 2010 51:2303-13
Cholesterol is differentially distributed
between plaque components
Cholesterol is present :
- in extracellular cholesterol-rich microdomains
- in monocyte-derived macrophage foam cells (CE > FC)
- in apoptotic cellular debris, primarily within the necrotic
core (FC > CE)
a-HDL
SR-BI
Endothelium
ABCG1
1
SR-BI
a-HDL
16
LA, AA,
12SHETE
AGEs
ABCG1
a-HDL
MPO
MDA
AGEs
SR-BI
insulin
Atherosclerotic
Lesion
Golgi
ER
TLR
CE
7-KC-3-sulfate
IRF
LXR
gene
Glucose
17
PPARa
PPARg
7-KC
SR-BI gene
AGEs, Hypoxia
Oxidative stress
Zhao et al,
Curr Opin Lipidol 2010
RXR LXR
19
G
P
S
2
12
FC
FC
Endosome
/lysosme
14
CRP, Glucose
Oxidative stress
Nucleus
7-KC
4
a-HDL
SR-BI
7
ACAT
NCEH
FC
10
8
StAR
CE
Lipid
droplet
9
27-OHC
CYP27
5
a-HDL
Aqueous
diffusion
FC
FC
FC
MPO
MDA
AGEs
FC
7-KC
6
11
ABCG1
gene
ABCG1
FC
PL
FC
13
18
2
ABCG1
HMG-CoA
Reductase
a-HDL
3
Endosome
/lysosme
15
5
FC
Mitochondria
Macrophage
HDL-Mediated Cholesterol Efflux From
Cholesterol-Rich Macrophages
Two main pathways:
Rader DJ. J Clin Invest. 2006;116:3090–3100.
 Lipid-poor nascent pre-β-HDL
apo A-1
– ABCA1 transporter
 Mature HDL
apo E
– SR-BI receptor
– ABCG1 transporter
Does this pathway comprehensively account for all forms of
cholesterol which may potentially undergo efflux from the
atherosclerotic plaque ?
14
STIMULATION OF CELLULAR CHOLESTEROL EFFLUX BY LXR
AGONISTS IN HUMAN MACROPHAGES
Question:
Does the induction of cholesterol efflux to HDL by LXR agonists result from stimulation of ABCG1 expression?
Larrede S., Arterioscler Thromb Vasc Biol. 2009, 29:1930
ABCG1 IS HIGHLY EXPRESSED IN HUMAN FOAM CELLS
UPON STIMULATION BY LXR AGONISTS
Larrede S., Arterioscler Thromb Vasc Biol. 2009, 29:1930
CELLULAR CHOLESTEROL EFFLUX TO HDL IN HUMAN
MACROPHAGES IS ABCG1 INDEPENDENT
120
ABCG1 KD
Ctrl
50nM siRNA
% of ABCG1
100
80
60
40
20
0
mRNA Prot
Larrede S., Arterioscler Thromb Vasc Biol. 2009, 29:1930
STIMULATION OF CHOLESTEROL EFFLUX TO HDL BY LXR AGONISTS
REQUIRES THE ABCA1 TRANSPORTER IN HUMAN MACROPHAGES
TD: Tangier Disease Macrophage (Exon 34, 1536 Ser/Phe)
HMDM
Larrede S., Arterioscler Thromb Vasc Biol. 2009, 29:1930
CELLULAR CHOLESTEROL EFFLUX TO HDL IN HUMAN MACROPHAGES
REQUIRES THE CLA-1 RECEPTOR
NB: Non-Blocking Cla-1 antibody
B: Blocking Cla-1 antibody
Larrede S., Arterioscler Thromb Vasc Biol. 2009, 29:1930
IN HUMAN MACROPHAGES
1- ABCG1 does not promote FC efflux to HDL  mice
2- SR-BI/Cla-1 participates in free FC efflux to HDL  mice
3- Stimulation of FC efflux by LXR is an ATP-dependent transport
mediated by ABCA1 independently of ABCG1
Cholesterol efflux studies should be conducted
in human macrophages and not in mouse macrophages!!!
HDL: New Dimensions
QUALITY
QUANTITY
Particle structure
HDL-C / Apo AI
Lipidome, Proteome
Functionality
Kontush A, Chapman MJ. Pharmacol Rev. 2006.
21
22
Mechanisms of Cellular Cholesterol Efflux
to HDL particles
Extracellular space
ABCA1
Lipid-poor ApoA-I
Diffusion
Discoidal HDL
LCAT
SR-B1
Diffusion
Small spherical HDL
LCAT
Larger spherical HDL
Cell membrane
SR-B1
ABCG1
Diffusion
SR-B1
ABCG1
FC
FC
FC
FC
22
Proteolytic inactivation of pre-HDL impairs ABCA1-dependent initiation of
reverse cholesterol transport in the atherosclerotic plaque
Lee-Rueckert and Kovanen,
Curr Opin Lipidol 2011
SUMMARY
• The potential efficacy of HDL / apoAI to efflux
arterial cholesterol is dependent upon :
• the specific apoAI / HDL particle ligand(s)
• ligand quantity and quality (functionality)
• plaque structure and compartmentalisation
• plaque PG and GAG composition
• plaque protease activities
• relative expression levels of cholesterol efflux
proteins in macrophage foam cells
• What is the evidence that HDL/apoAI
mediate cholesterol efflux
from the plaque?
Long history of HDL infusion reducing atheroma
burden
• 1989: infusion of HDL
reduced rabbit athero
• Repeated in rabbits
and mice
• 1990s: transgenic
studies showed ApoAI
to be the active agent
26
• HDL Infusions :
• apoAI Milano/PL
• Reconstituted HDL (apoAI/PL/detergent?)
• Recombinant apoAI/PL (LPS?)
• …….
Impact of HDL on Plaque Evolution :
Experimental Evidence for Plaque Regression
Infusion of Recombinant Apo AI Milano/Phospholipid complexes over
5 weeks produced significant regression of coronary
atherosclerosis as estimated on the basis of atheroma volume
by IVUS in patients with acute coronary syndromes.
Nissen et al., JAMA, 2003, 290 : 2292-2300
These findings suggest that elevation of HDL/Apo AI
may enhance cholesterol efflux from plaque tissue
and may therefore be a critical component of
atherosclerotic plaque regression
Reconstituted HDL (CSL111) reduced atheroma in man
• Single infusion of CSL111
reduced plaque lipid >60%
in femoral arteries (Circ.
Res. 2008)
Reduction of plaque lipid by CSL111
80000
Oil Red O Lesion Area
(µm2)
• Four infusions of our
prototype, CSL111, reduced
the volume of coronary
atheroma in ERASE (JAMA
2007)
60000
40000
*
20000
0
*P<0.05 from placebo
29
Impact of Reconstituted HDL infusion on Peripheral Arterial disease
Recruited patients (n=10) with symptomatic PAD and lesions in SFA on
Duplex US amenable to percutaneous revascularisation. Excluded
those with Cr >0.25mmol/l or other co-morbidities such as hx of organ
transplantation, malignancy with expected survival < 12 months
Baseline Investigations :
ABI, Fasting Plasma for Inflammatory Markers, Lipid profile
Randomised to IV Placebo or IV CSL 111 (80mg/Kg) given over
4 hr infusion
5-7 days following infusion patients admitted to hospital for
revascularisation procedureInvolving atherectomy
(Foxhollow Atherectomy® ) ± Angioplasty/Stenting
Figure 3. Accumulation of lipids and macrophage size in the lesions.
Shaw J A et al. Circulation Research 2008;103:1084-1091
Copyright © American Heart Association
Impact of rHDL infusion on atherosclerotic lesions in the SFA
Lipid content
Inflammation
-45% intra-plaque VCAM-1 expression
-40% lipid content
-40% macrophage cell size
-50% circulating monocyte activation (CD11b)
+15% cellular cholesterol efflux capacity
Enhanced RCT ?
Can the cholesterol cargo in HDL
potentially contribute to plaque cholesterol
accumulation
and thence progression ?
apoE-HDL and Reverse Cholesterol Transport
Mahley et al., JCI, 2006,116:1226-29
ApoE- HDL
-is specifically bound and retained in arterial matrix
by extracellular PGs, notably biglycan
O’Brien et al, Atherosclerosis 2004: 177
O’Brien et al, Circulation 1998: 98
Ollin et al, ATVB 2001: 21
• Are these observations relevant to
emerging therapeutic approaches to raise
HDL-C and normalise its function in
dyslipidemic, high risk metabolic states ?
HDL metabolism: Impact of CETP inhibition
Macrophage
FC
apoB
ABCA1
PL
FC
VLDL
A-I
PreßHDL
apoB
CE
LCAT
HDL
LDL-R
LDL
CE
TG
Liver
CETP
Peripheral
tissues
CE,FC
FC
E CE
CE
CE
CE AI
AI
E
HDL
HDL
Chapman et al, Eur Heart J 2009
E
SR-BI
Bile acids
Cholesterol