Cholesterol foam cells to the intercellular acceptors mostly

Cholesterol
influx depends on the amount of modified LDL and the presence of receptors
which helps in the intake of modified LDL into macrophages. Myeloperoxidase and
lipoxygenase are the enzymes which help in the oxidation of LDL. The
functional HDL molecule which is considered to be atheroprotective has an esterase
enzyme associated with it called
paraoxonase, which plays a pivotal role in the prevention of LDL oxidation (Deakin, Bioletto et al. 2011) and act as anti-atherogenic molecule (Rye KA 2009). Cluster
of differentiation 36 (CD-36) and Scavenger receptor class A type 1 (SRA) are
the two important scavenger receptors responsible for the intake of modified
LDL by the macrophages. (16,17)

HDL
exerts its atheroprotective action by extracting the fatty materials from the
foam cells and carrying these to liver for its disposal through the transport system
termed reverse cholesterol transport.

Reverse
Cholesterol Transport proposed by Glomset in 1968 is a pathway which carries
the effluxed cholesterol from the macrophages to liver for its metabolism. The RCT pathway involves the efflux of
cholesterol and phospholipids from the foam cells to the intercellular
acceptors mostly by means of receptor/protein mediated pathway (Cucuianu, Coca
et al. 2007). The efflux of cholesterol is
mediated by the ATP binding cassette transporter A1 (ABCA1), ATP binding
cassette transporter G1 (ABCG1), Scavenger receptor B1 (SR-B1) present in
macrophages mediates cholesterol efflux . The ABCA1 transfers free cholesterol
to Apo A1 containing nascent HDL, and ABCG1 and SR-B1 transfer free cholesterol
to mature HDL. Within the mature HDL the free cholesterol transferred is
esterified by an enzyme Lecithin Cholesterol Acyl Transferase (LCAT) to form
cholesteryl ester (CE). Cholesteryl esters are transported to liver by direct
or indirect pathway. In direct pathway the HDL with CE gets attached to SR-B1
receptor. In indirect pathway the HDL transfers CE in exchange with
phospholipid to Apo-B containing lipoproteins like VLDL and LDL with subsequent
uptake in the liver via the low density lipoprotein receptor (LDLR). In the
circulation the exchange of CE and phospholipid is mediated by Cholesteryl
ester transfer protein (CETP) and Phospholipid transfer protein (PLTP). The CE
taken up by the liver will be acted upon by the enzyme hepatic lipase and the
cholesterol will be metabolized and excreted through bile or faeces
 

Novel
approaches for the treatment of atherosclerosis that are expected to reduce
cholesterol burden utilising the reverse cholesterol transport targets to act
at three levels.

1.      Decreasing
the influx and increasing the efflux of cholesterol from macrophages

2.      Increasing cholesterol transport from macrophages to
liver through plasma

3.      Increasing
cholesterol uptake by liver for metabolism and excretion

The
reduced amount of foam cell formation and increased amount of macrophage
cholesterol efflux is considered to be atheroprotective.

Studies
conducted by Dai et al(18) in 2012 showed that inhibition of SR-A in
macrophages reduced foam cell formation in apoE-/- mice. Xie etal (19)
in their studies using seven phenolic acids present in blue berries reported
that the foam cell formation was reduced by down regulation of CD36 and up
regulation of ABCA1.

Tangier’s
disease is an autosomal recessive condition which affects the lipid metabolism.
(7) Studies conducted by Bodzioch in 1999 (8) showed that Tangiers disease is
due to the mutation of the gene encoding ABCA1 transporter protein. It was also
shown that there were decreased levels of HDL an Apo A1. McNiesh et al (11) in
their studies using ABCA1 knock out mice demonstrated that absence of ABCA1
resulted in severe reduction in the HDL levels along with near absence of Apo
A1.  Studies conducted by Timmins (13)
also revealed that in ABCA1 knock out mice the HDL levels were reduced to 17%
and catabolism of ApoA1 by kidney were increased by 2 folds. Vaisman et al in
2001 (106) in their studies in transgenic mice found that overexpression of
ABCA1 increased the plasma levels of HDL and Apo A1, facilitating reverse cholesterol
transport and biliary cholesterol disposal. Rayner (101) reported the role of
miR33, an intronic micro RNA in suppressing the expression of ABCA1 and
reducing the HDL levels. They also demonstrated that treatment of mice with
anti – miR33 oligonucleotide resulted in increase of ABCA1 expression and HDL
levels thereby enhancing reverse cholesterol transport.

ABCG1
are the transporter protein responsible for the transfer of cholesterol to
mature HDL. Studies were conducted to identify the role of ABCG1 in RCT.
Kennedy et al (28) studied the critical role of ABCG1 in lipid homeostasis and demonstrated
that high fat diet  in ABCG1 knock out
mice resulted in increased accumulation of lipids in macrophages with impaired
efflux of cholesterol. They also demonstrated that over expression of human
ABCG1 resulted in the protection of tissues from lipid accumulation. The
studies also revealed the fact that cholesterol efflux to Apo A1 specifically
requires ABCA1, while that to mature HDL specifically requires ABCG1.

The
most critical step in macrophage cholesterol efflux is the conversion of free
cholesterol to cholesterol ester by the enzyme acyl coenzyme A cholesterol acyl
transferase (ACAT). Free cholesterol if present in excess is toxic to
macrophages and ACAT protects macrophages by converting free cholesterol to
cholesterol esters which helps in the formation of foam cells. ACAT inhibitors
increase the amount of free cholesterol in macrophages and if sufficient efflux
mediators are present more amount of cholesterol can be effluxed from
macrophages (21). Rodriguez studied the effect of ACAT inhibitor 58-035 on foam
cell development in monocyte derived macrophages and found that these compounds
decreased foam cell accumulation and increased free cholesterol efflux.

The
production of ABCA1 and ABCG1 are regulated by the liver X receptors LXR? and
LXR? (29). In an invitro study in macrophages by Padovani in 2010 (31), it was
shown that arsenic inhibited the transcriptional activity of LXR, the
expression of ACA1 was reduced and the ability of macrophages to efflux free
cholesterol were decreased. Studies conducted by Venkateswaran et al (107)
showed that activation of LXR by oxysterols increased the induction of ABCA1
transporter protein and macrophage cholesterol efflux. Whitney et al (108) in
their studies demonstrated that LXR agonists like GW3965 and T0901317 up regulated the
expression of LXR target genes like ABCA1 and ABCG1.

 

The
role of SR-B1 in reverse cholesterol transport was confirmed by the studies
conducted by Huszar (23) which showed that mice with decreased expression of
SR-B1 produced increased atherosclerosis, while the study by Kozarsky (24)
showed that in mice that overexpress SR-B1 there was reduced atherosclerosis.Cholesterol
influx depends on the amount of modified LDL and the presence of receptors
which helps in the intake of modified LDL into macrophages. Myeloperoxidase and
lipoxygenase are the enzymes which help in the oxidation of LDL. The
functional HDL molecule which is considered to be atheroprotective has an esterase
enzyme associated with it called
paraoxonase, which plays a pivotal role in the prevention of LDL oxidation (Deakin, Bioletto et al. 2011) and act as anti-atherogenic molecule (Rye KA 2009). Cluster
of differentiation 36 (CD-36) and Scavenger receptor class A type 1 (SRA) are
the two important scavenger receptors responsible for the intake of modified
LDL by the macrophages. (16,17)

HDL
exerts its atheroprotective action by extracting the fatty materials from the
foam cells and carrying these to liver for its disposal through the transport system
termed reverse cholesterol transport.

Reverse
Cholesterol Transport proposed by Glomset in 1968 is a pathway which carries
the effluxed cholesterol from the macrophages to liver for its metabolism. The RCT pathway involves the efflux of
cholesterol and phospholipids from the foam cells to the intercellular
acceptors mostly by means of receptor/protein mediated pathway (Cucuianu, Coca
et al. 2007). The efflux of cholesterol is
mediated by the ATP binding cassette transporter A1 (ABCA1), ATP binding
cassette transporter G1 (ABCG1), Scavenger receptor B1 (SR-B1) present in
macrophages mediates cholesterol efflux . The ABCA1 transfers free cholesterol
to Apo A1 containing nascent HDL, and ABCG1 and SR-B1 transfer free cholesterol
to mature HDL. Within the mature HDL the free cholesterol transferred is
esterified by an enzyme Lecithin Cholesterol Acyl Transferase (LCAT) to form
cholesteryl ester (CE). Cholesteryl esters are transported to liver by direct
or indirect pathway. In direct pathway the HDL with CE gets attached to SR-B1
receptor. In indirect pathway the HDL transfers CE in exchange with
phospholipid to Apo-B containing lipoproteins like VLDL and LDL with subsequent
uptake in the liver via the low density lipoprotein receptor (LDLR). In the
circulation the exchange of CE and phospholipid is mediated by Cholesteryl
ester transfer protein (CETP) and Phospholipid transfer protein (PLTP). The CE
taken up by the liver will be acted upon by the enzyme hepatic lipase and the
cholesterol will be metabolized and excreted through bile or faeces
 

Novel
approaches for the treatment of atherosclerosis that are expected to reduce
cholesterol burden utilising the reverse cholesterol transport targets to act
at three levels.

1.      Decreasing
the influx and increasing the efflux of cholesterol from macrophages

2.      Increasing cholesterol transport from macrophages to
liver through plasma

3.      Increasing
cholesterol uptake by liver for metabolism and excretion

The
reduced amount of foam cell formation and increased amount of macrophage
cholesterol efflux is considered to be atheroprotective.

Studies
conducted by Dai et al(18) in 2012 showed that inhibition of SR-A in
macrophages reduced foam cell formation in apoE-/- mice. Xie etal (19)
in their studies using seven phenolic acids present in blue berries reported
that the foam cell formation was reduced by down regulation of CD36 and up
regulation of ABCA1.

Tangier’s
disease is an autosomal recessive condition which affects the lipid metabolism.
(7) Studies conducted by Bodzioch in 1999 (8) showed that Tangiers disease is
due to the mutation of the gene encoding ABCA1 transporter protein. It was also
shown that there were decreased levels of HDL an Apo A1. McNiesh et al (11) in
their studies using ABCA1 knock out mice demonstrated that absence of ABCA1
resulted in severe reduction in the HDL levels along with near absence of Apo
A1.  Studies conducted by Timmins (13)
also revealed that in ABCA1 knock out mice the HDL levels were reduced to 17%
and catabolism of ApoA1 by kidney were increased by 2 folds. Vaisman et al in
2001 (106) in their studies in transgenic mice found that overexpression of
ABCA1 increased the plasma levels of HDL and Apo A1, facilitating reverse cholesterol
transport and biliary cholesterol disposal. Rayner (101) reported the role of
miR33, an intronic micro RNA in suppressing the expression of ABCA1 and
reducing the HDL levels. They also demonstrated that treatment of mice with
anti – miR33 oligonucleotide resulted in increase of ABCA1 expression and HDL
levels thereby enhancing reverse cholesterol transport.

ABCG1
are the transporter protein responsible for the transfer of cholesterol to
mature HDL. Studies were conducted to identify the role of ABCG1 in RCT.
Kennedy et al (28) studied the critical role of ABCG1 in lipid homeostasis and demonstrated
that high fat diet  in ABCG1 knock out
mice resulted in increased accumulation of lipids in macrophages with impaired
efflux of cholesterol. They also demonstrated that over expression of human
ABCG1 resulted in the protection of tissues from lipid accumulation. The
studies also revealed the fact that cholesterol efflux to Apo A1 specifically
requires ABCA1, while that to mature HDL specifically requires ABCG1.

The
most critical step in macrophage cholesterol efflux is the conversion of free
cholesterol to cholesterol ester by the enzyme acyl coenzyme A cholesterol acyl
transferase (ACAT). Free cholesterol if present in excess is toxic to
macrophages and ACAT protects macrophages by converting free cholesterol to
cholesterol esters which helps in the formation of foam cells. ACAT inhibitors
increase the amount of free cholesterol in macrophages and if sufficient efflux
mediators are present more amount of cholesterol can be effluxed from
macrophages (21). Rodriguez studied the effect of ACAT inhibitor 58-035 on foam
cell development in monocyte derived macrophages and found that these compounds
decreased foam cell accumulation and increased free cholesterol efflux.

The
production of ABCA1 and ABCG1 are regulated by the liver X receptors LXR? and
LXR? (29). In an invitro study in macrophages by Padovani in 2010 (31), it was
shown that arsenic inhibited the transcriptional activity of LXR, the
expression of ACA1 was reduced and the ability of macrophages to efflux free
cholesterol were decreased. Studies conducted by Venkateswaran et al (107)
showed that activation of LXR by oxysterols increased the induction of ABCA1
transporter protein and macrophage cholesterol efflux. Whitney et al (108) in
their studies demonstrated that LXR agonists like GW3965 and T0901317 up regulated the
expression of LXR target genes like ABCA1 and ABCG1.

 

The
role of SR-B1 in reverse cholesterol transport was confirmed by the studies
conducted by Huszar (23) which showed that mice with decreased expression of
SR-B1 produced increased atherosclerosis, while the study by Kozarsky (24)
showed that in mice that overexpress SR-B1 there was reduced atherosclerosis.