REVIEW URRENT C OPINION
Reverse cholesterol transport fluxes Marc Hellerstein a,b,c and Scott Turner a
Purpose of review Reverse cholesterol transport (RCT) is considered a significant component of the atheroprotective effects of HDL. Methods for quantifying flux through the RCT pathway have not been available until recently. There is a need to improve our understanding of HDL function, including the role of RCT in general and individual steps of RCT in particular, on atherosclerosis. This review highlights new information about cholesterol flux through the RCT pathway. Recent findings Recent clinical studies have demonstrated several important quantitative features of cholesterol fluxes in vivo, providing insight into variability and control of specific components of the RCT pathway. The findings illustrate the independent nature of individual steps in the RCT pathway and their apparently weak relationship to plasma HDL cholesterol levels. Nonclinical studies provide some mechanistic data re-enforcing the importance of apoB particles in RCT and role roles for serum albumin and erythrocytes in free cholesterol flux. These findings suggest that the HDL-centric view of RCT may need revision. Summary The constellation of known lipoproteins and other players involved in this pathway continues to increase. Further research, particularly in humans, is needed in order to understand which parts of the RCT pathway are most relevant to the pathophysiology and treatment of atherosclerosis. Keywords flux, HDL, lipoproteins, reverse cholesterol transport
INTRODUCTION Since originally proposed by Glomset [1] as the physiological function of lecithin cholesterol acyltransferase (LCAT), the concept of reverse cholesterol transport (RCT) has changed very little. The general model has been that free cholesterol moves from peripheral tissues to plasma lipoproteins, is converted to cholesteryl ester by LCAT, and then is delivered to the liver and is ultimately excreted in the feces as bile acids or neutral sterols (Fig. 1) [2,3 ,4]. Although our molecular understanding of the likely biochemical components underlying RCT has become increasingly sophisticated and complex, our understanding of which components of the transport pathway are relevant to atherosclerosis and what molecular players are necessary for each step of the pathway remains incomplete. Although it is generally agreed that removal of cholesterol from the atherosclerotic foam cell – macrophage cholesterol efflux – is both the first step and the ‘end game’ for atheroprotective RCT, there has been no consensus on how best to achieve that goal or measure it in vivo. Indeed, our understanding of HDL function and the significance of the major &
www.co-lipidology.com
steps in the RCT pathway (e.g. efflux from cells, cholesteryl ester transfer protein (CETP) or LCAT flux, fecal excretion) remains uncertain [2,5–8]. Although the term RCT is frequently applied to individual steps in the pathway ranging from cellular efflux to fecal excretion, the most common conception of RCT is as an integrated, multistep process (Fig. 1): lipid poor HDL accepts free cholesterol from tissues, including the evolving atherosclerotic plaque (efflux step), free cholesterol is esterified by LCAT, returns to the liver via the HDL particle or via CETP-mediated transfer to LDL, and is cleared from plasma by SR-B1 or the hepatic LDL receptor, respectively (esterification a KineMed, Inc., Emeryville, bDepartment of Nutritional Sciences and Toxicology, University of California, Berkeley and cDepartment of Medicine, Division of Endocrinology and Metabolism, University of California, San Francisco, California, USA
Correspondence to Marc Hellerstein MD, PhD, 5980 Horton Street Suite 470, Emeryville, CA 94608, USA. Tel: +1 510 655 6525; e-mail: [email protected] Curr Opin Lipidol 2014, 25:40–47 DOI:10.1097/MOL.0000000000000050 Volume 25 Number 1 February 2014
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Reverse cholesterol transport fluxes Hellerstein and Turner
and delivery steps), and is ultimately excreted in the feces as bile acids or cholesterol (excretion step). The state of the art of HDL biochemistry and metabolism has been extensively reviewed elsewhere [2,3 ,5,9–12]; here, we will focus on the quantitative aspects of RCT fluxes, focusing on recent advances in the understanding of steps in the pathway and their measurement.
KEY POINTS RCT is a complex process with redundant pathways. Recently, the roles of albumin, RBCs and apoB particles in RCT have been highlighted.
&
To adequately understand the importance of each pathway sophisticated translational tools are needed. New clinical tools are now available to begin testing specific pathways in human studies.
METHODS FOR ASSESSING REVERSE CHOLESTEROL TRANSPORT FLUXES IN HUMANS Ever since it was recognized that cholesterol and cholesteryl ester accumulation in the arterial wall
HDL
FC
apoA1
CE
FC
FC HDL
CE LCAT CE
B ac ile id s
TP
SR–BI
Albumin
LDL
ch Fre ol e es t ro l
LDLr
apoB–100
RBC ? ?
TICE
?
Fecal sterol excretion
FIGURE 1. Pathways of reverse cholesterol transport. The classical pathway where free cholesterol is effluxed from tissue to HDL, converted to cholesteryl ester (CE) via LCAT, and then returning to the liver vial SR-BI or the LDL receptor for excretion as fecal sterols. The novel components recently described are highlighted in grey showing albumin, red blood cells and apoB (LDL) participating in cholesterol efflux, and the TICE pathway which delivers cholesterol directly to fecal sterols bypassing the liver. 0957-9672 ß 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
www.co-lipidology.com
41
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Nutrition and metabolism
was a key component in the development of atherosclerosis, there has been interest in assessing cholesterol fluxes in vivo. Early studies by Goodman et al. [13] in humans, modeling the decay of radioactive cholesterol over several months, identified three pools of cholesterol with distinct sizes and turnover rates. Theses pools or compartments, described by the rates at which they equilibrate with plasma cholesterol, were found to comprise a rapidly equilibrating pool (plasma, red cell, liver and other visceral organs), an intermediate kinetic pool (which includes viscera and peripheral tissue cholesterol) and a slow pool (dominated by muscle, skin and adipose cholesterol). CNS cholesterol, although significant in mass is not considered in these models, as it does not exchange with other body pools of cholesterol. These elegant studies, conducted over more than a decade, quantified the half-lives of cholesterol in these three compartments. These investigators ultimately concluded that neither the size nor turnover of these pools was significantly affected by blood lipid or lipoprotein levels nor by statin treatment, and thus whole body cholesterol turnover did not correlate with the usual parameters of atherosclerotic risk. Of note, cholestyramine and niacin did increase cholesterol turnover rates [14,15]. During the same period, Schwartz et al. [16,17] developed a series of isotope models to carefully dissect the movement of free cholesterol and cholesterol ester within and through the rapidly exchanging plasma cholesterol compartment. These complex and carefully executed studies made several important observations about cholesterol fluxes and form the basis for the contemporary RCT model discussed here [18 ]. Importantly, the work of Schwartz et al. demonstrated the presence of a rapidly equilibrating pool that includes plasma lipoproteins and hepatic free cholesterol. This &
Table 1. Cholesterol pool sizes Schwartz mg/kg
g/70 kg
Liver
27.0
1.89
Blood Cells
37.0
2.59
Lipoproteins
20.3
1.42
133.7
9.36
Peripheral tissues Data from Ref. [17].
rapidly equilibrating pool could be labeled with tracers in a reasonable timeframe (e.g. hours, not weeks or months), thus allowing flux from peripheral tissues into that pool to be readily quantified in human experimental studies. The cholesterol pool sizes and flux parameters reported using these methods is summarized in Tables 1 and 2. Recently, our laboratory, in collaboration with several other groups, has published a series of articles [18 –20 ] utilizing a simplified, nonradioactive (stable) isotope-based model that builds on the work of Schwartz et al. The model attempts to quantify the three major steps in RCT separately: flux of free cholesterol out of tissues into plasma; cholesterol ester formation and clearance rates in blood; and excretion of plasma cholesterol into feces. The exchange of free cholesterol with the red blood cell (RBC) membrane cholesterol is also quantified because of its large quantitative contribution to plasma-free cholesterol fluxes. The detailed method has been published elsewhere. Briefly, the approach involves a slow, continuous intravenous infusion of free 13C-labeled cholesterol solubilized in intralipid emulsion. The rising enrichment curves of 13C-labeled cholesterol in plasma-free cholesterol, plasma cholesteryl ester and RBC-free cholesterol are modeled using SAAMII multicompartment modeling software and a &
&
Table 2. Cholesterol flux rates Schwartz et al. [17] FC between lipoproteins and liver
g/day
Net flux
16.83
28.28
CE from lipoproteins to liver
0.98
1.64
1.69
FC from peripheral tissue to lipoproteins
2.39
4.01
1.39
FC between lipoproteins and RBC
6.30
10.59
Turner et al. [18 ] &
a
mg/kg/h
mg/kg/h
0
0 g/day
Tissue cholesterol efflux
3.8
6.3
CE clearance from lipoproteins to livera
1.1
1.84
FC clearance from plasma
2.69
4.51
FC exchange between lipoproteins and RBC
3.38
5.69
CE clearance rate is determined liver is presumed to be the major tissue.
42
www.co-lipidology.com
Volume 25 Number 1 February 2014
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Reverse cholesterol transport fluxes Hellerstein and Turner
as components of the measured fluxes. Perhaps more importantly, these measurements are intended to capture the capacity and activity of the ‘RCT system’ in an individual, which may represent a marker of their HDL functional capacity (much as clotting times do not reveal how much bleeding or clot formation has actually occurred in tissues, but reveals systemic capacity for coagulation). Systemic capacity for RCT is potentially relevant as a drug target, for gene discovery and for pathophysiologic investigation. The quantitative rates revealed by this method are of interest in thinking about the control of cholesterol fluxes in the body. Tissue free cholesterol efflux was 3.8 0.9 mg/kg per hour (mean standard deviation), or approximately 6.5 g/day.
multipool model (Fig. 2). Labeled sitostanol (a nonabsorbed phytosterol) is also taken orally, as an external standard for determining total fecal sterol excretion. Fecal samples are collected for 7 days following the infusion and both the total excretion rate and the isotopic enrichments of neutral sterols and bile acids are determined. From these measurements, multiple parameters of cholesterol flux can be determined (Table 2). This model focuses on quantifying the whole body cholesterol flux into, through and out of the plasma lipoprotein compartment. Although this model does not specifically assess macrophage efflux (which represents a small percentage of whole-body cholesterol flux), nor is biliary vs. nonbiliary (transintestinal excretion) captured, these pathways do contribute quantitatively
Tissue cholesterol efflux
(a)
k(2,1) k(1,2)
V2 RBC FC
FC between lipoproteins and RBC
k(0,1)
k(0,3)
V1 Plasma lipoproteins and liver FC
k(3,1) Plasma
CE from lipoproteins to liver
V3 Plasma CE
Infusion
CE Clearance
FC Clearance
(b) s1 PlosMPE s2 RBCMPE s3 EstMPE
0.015
0.010
0.005
0.000
0
10
20
30
1 (hours)
FIGURE 2. (a) Cholesterol flux and exchange model from Turner et al. V1 is the plasma lipoprotein free cholesterol pool which also includes a significant contribution from hepatic free cholesterol which is rapidly exchanged with plasma. Tissue efflux is the sum of the FC clearance and the cholesteryl ester (CE) clearance. Infusion indicates the input of the 13C cholesterol tracer into the system. (b) 13C cholesterol enrichments and fits over time. Plasma FC, V1 (open square, green line); RBC FC V2 (small square, red line) and Plasma CE V3 (open triangle, blue line). 0957-9672 ß 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
www.co-lipidology.com
43
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Nutrition and metabolism
In comparison, the sum of dietary cholesterol absorption and endogenous synthesis rates is typically approximately 1.0–1.5 g/day. Efflux of cholesterol from tissues into the plasma compartment is therefore five times the rate of excretion of cholesterol from the whole body. RBC-derived flux into plasma-free cholesterol, representing net exchange, was 3.4 1.1 mg/kg per hour. This value is quantitatively comparable with efflux from all other tissues into plasma, consistent with a potential role of RBCs as carriers involved in cholesterol fluxes in vivo. Esterification of plasma-free cholesterol was approximately 28% of tissue-free cholesterol efflux (1.1 0.4 mg/kg per hour), representing a substantial fate of free cholesterol that is effluxed into the plasma lipoprotein compartment. Recoveries were 7 and 12% of administered [3,4-13C2]-cholesterol in fecal bile acids and neutral sterols, respectively. Another novel method for assessing RCT was recently presented at the annual Arteriosclerosis, Thrombosis, and Vascular Biology (ATVB) meeting. This method, developed by Cuchel et al. [21 ], is intended to measure macrophage-specific cholesterol efflux as well as other components of the RCT pathway. The approach was developed to deliver 3H-cholesterol/albumin complexes to hepatic Kupfer cells and other tissue-resident macrophages. Intravenous-injected 3H-cholesterol/albumin complexes are rapidly removed from plasma, presumably by macrophages of the reticuloendothelial system, and their subsequent release of labeled cholesterol onto plasma lipoproteins (HDL and nonHDL), conversion to cholesterol ester and excretion as fecal sterols over the subsequent 8 days were characterized using a multicompartment model similar to the one developed by Schwartz et al. [17]. This approach is intended to focus on macrophage cholesterol efflux and, as such, is conceptually complementary to measurements of whole body or systemic RCT capacity. In normal individuals, the particulate cholesterol was rapidly cleared from the plasma (
Reverse cholesterol transport fluxes Marc Hellerstein a,b,c and Scott Turner a
Purpose of review Reverse cholesterol transport (RCT) is considered a significant component of the atheroprotective effects of HDL. Methods for quantifying flux through the RCT pathway have not been available until recently. There is a need to improve our understanding of HDL function, including the role of RCT in general and individual steps of RCT in particular, on atherosclerosis. This review highlights new information about cholesterol flux through the RCT pathway. Recent findings Recent clinical studies have demonstrated several important quantitative features of cholesterol fluxes in vivo, providing insight into variability and control of specific components of the RCT pathway. The findings illustrate the independent nature of individual steps in the RCT pathway and their apparently weak relationship to plasma HDL cholesterol levels. Nonclinical studies provide some mechanistic data re-enforcing the importance of apoB particles in RCT and role roles for serum albumin and erythrocytes in free cholesterol flux. These findings suggest that the HDL-centric view of RCT may need revision. Summary The constellation of known lipoproteins and other players involved in this pathway continues to increase. Further research, particularly in humans, is needed in order to understand which parts of the RCT pathway are most relevant to the pathophysiology and treatment of atherosclerosis. Keywords flux, HDL, lipoproteins, reverse cholesterol transport
INTRODUCTION Since originally proposed by Glomset [1] as the physiological function of lecithin cholesterol acyltransferase (LCAT), the concept of reverse cholesterol transport (RCT) has changed very little. The general model has been that free cholesterol moves from peripheral tissues to plasma lipoproteins, is converted to cholesteryl ester by LCAT, and then is delivered to the liver and is ultimately excreted in the feces as bile acids or neutral sterols (Fig. 1) [2,3 ,4]. Although our molecular understanding of the likely biochemical components underlying RCT has become increasingly sophisticated and complex, our understanding of which components of the transport pathway are relevant to atherosclerosis and what molecular players are necessary for each step of the pathway remains incomplete. Although it is generally agreed that removal of cholesterol from the atherosclerotic foam cell – macrophage cholesterol efflux – is both the first step and the ‘end game’ for atheroprotective RCT, there has been no consensus on how best to achieve that goal or measure it in vivo. Indeed, our understanding of HDL function and the significance of the major &
www.co-lipidology.com
steps in the RCT pathway (e.g. efflux from cells, cholesteryl ester transfer protein (CETP) or LCAT flux, fecal excretion) remains uncertain [2,5–8]. Although the term RCT is frequently applied to individual steps in the pathway ranging from cellular efflux to fecal excretion, the most common conception of RCT is as an integrated, multistep process (Fig. 1): lipid poor HDL accepts free cholesterol from tissues, including the evolving atherosclerotic plaque (efflux step), free cholesterol is esterified by LCAT, returns to the liver via the HDL particle or via CETP-mediated transfer to LDL, and is cleared from plasma by SR-B1 or the hepatic LDL receptor, respectively (esterification a KineMed, Inc., Emeryville, bDepartment of Nutritional Sciences and Toxicology, University of California, Berkeley and cDepartment of Medicine, Division of Endocrinology and Metabolism, University of California, San Francisco, California, USA
Correspondence to Marc Hellerstein MD, PhD, 5980 Horton Street Suite 470, Emeryville, CA 94608, USA. Tel: +1 510 655 6525; e-mail: [email protected] Curr Opin Lipidol 2014, 25:40–47 DOI:10.1097/MOL.0000000000000050 Volume 25 Number 1 February 2014
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Reverse cholesterol transport fluxes Hellerstein and Turner
and delivery steps), and is ultimately excreted in the feces as bile acids or cholesterol (excretion step). The state of the art of HDL biochemistry and metabolism has been extensively reviewed elsewhere [2,3 ,5,9–12]; here, we will focus on the quantitative aspects of RCT fluxes, focusing on recent advances in the understanding of steps in the pathway and their measurement.
KEY POINTS RCT is a complex process with redundant pathways. Recently, the roles of albumin, RBCs and apoB particles in RCT have been highlighted.
&
To adequately understand the importance of each pathway sophisticated translational tools are needed. New clinical tools are now available to begin testing specific pathways in human studies.
METHODS FOR ASSESSING REVERSE CHOLESTEROL TRANSPORT FLUXES IN HUMANS Ever since it was recognized that cholesterol and cholesteryl ester accumulation in the arterial wall
HDL
FC
apoA1
CE
FC
FC HDL
CE LCAT CE
B ac ile id s
TP
SR–BI
Albumin
LDL
ch Fre ol e es t ro l
LDLr
apoB–100
RBC ? ?
TICE
?
Fecal sterol excretion
FIGURE 1. Pathways of reverse cholesterol transport. The classical pathway where free cholesterol is effluxed from tissue to HDL, converted to cholesteryl ester (CE) via LCAT, and then returning to the liver vial SR-BI or the LDL receptor for excretion as fecal sterols. The novel components recently described are highlighted in grey showing albumin, red blood cells and apoB (LDL) participating in cholesterol efflux, and the TICE pathway which delivers cholesterol directly to fecal sterols bypassing the liver. 0957-9672 ß 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
www.co-lipidology.com
41
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Nutrition and metabolism
was a key component in the development of atherosclerosis, there has been interest in assessing cholesterol fluxes in vivo. Early studies by Goodman et al. [13] in humans, modeling the decay of radioactive cholesterol over several months, identified three pools of cholesterol with distinct sizes and turnover rates. Theses pools or compartments, described by the rates at which they equilibrate with plasma cholesterol, were found to comprise a rapidly equilibrating pool (plasma, red cell, liver and other visceral organs), an intermediate kinetic pool (which includes viscera and peripheral tissue cholesterol) and a slow pool (dominated by muscle, skin and adipose cholesterol). CNS cholesterol, although significant in mass is not considered in these models, as it does not exchange with other body pools of cholesterol. These elegant studies, conducted over more than a decade, quantified the half-lives of cholesterol in these three compartments. These investigators ultimately concluded that neither the size nor turnover of these pools was significantly affected by blood lipid or lipoprotein levels nor by statin treatment, and thus whole body cholesterol turnover did not correlate with the usual parameters of atherosclerotic risk. Of note, cholestyramine and niacin did increase cholesterol turnover rates [14,15]. During the same period, Schwartz et al. [16,17] developed a series of isotope models to carefully dissect the movement of free cholesterol and cholesterol ester within and through the rapidly exchanging plasma cholesterol compartment. These complex and carefully executed studies made several important observations about cholesterol fluxes and form the basis for the contemporary RCT model discussed here [18 ]. Importantly, the work of Schwartz et al. demonstrated the presence of a rapidly equilibrating pool that includes plasma lipoproteins and hepatic free cholesterol. This &
Table 1. Cholesterol pool sizes Schwartz mg/kg
g/70 kg
Liver
27.0
1.89
Blood Cells
37.0
2.59
Lipoproteins
20.3
1.42
133.7
9.36
Peripheral tissues Data from Ref. [17].
rapidly equilibrating pool could be labeled with tracers in a reasonable timeframe (e.g. hours, not weeks or months), thus allowing flux from peripheral tissues into that pool to be readily quantified in human experimental studies. The cholesterol pool sizes and flux parameters reported using these methods is summarized in Tables 1 and 2. Recently, our laboratory, in collaboration with several other groups, has published a series of articles [18 –20 ] utilizing a simplified, nonradioactive (stable) isotope-based model that builds on the work of Schwartz et al. The model attempts to quantify the three major steps in RCT separately: flux of free cholesterol out of tissues into plasma; cholesterol ester formation and clearance rates in blood; and excretion of plasma cholesterol into feces. The exchange of free cholesterol with the red blood cell (RBC) membrane cholesterol is also quantified because of its large quantitative contribution to plasma-free cholesterol fluxes. The detailed method has been published elsewhere. Briefly, the approach involves a slow, continuous intravenous infusion of free 13C-labeled cholesterol solubilized in intralipid emulsion. The rising enrichment curves of 13C-labeled cholesterol in plasma-free cholesterol, plasma cholesteryl ester and RBC-free cholesterol are modeled using SAAMII multicompartment modeling software and a &
&
Table 2. Cholesterol flux rates Schwartz et al. [17] FC between lipoproteins and liver
g/day
Net flux
16.83
28.28
CE from lipoproteins to liver
0.98
1.64
1.69
FC from peripheral tissue to lipoproteins
2.39
4.01
1.39
FC between lipoproteins and RBC
6.30
10.59
Turner et al. [18 ] &
a
mg/kg/h
mg/kg/h
0
0 g/day
Tissue cholesterol efflux
3.8
6.3
CE clearance from lipoproteins to livera
1.1
1.84
FC clearance from plasma
2.69
4.51
FC exchange between lipoproteins and RBC
3.38
5.69
CE clearance rate is determined liver is presumed to be the major tissue.
42
www.co-lipidology.com
Volume 25 Number 1 February 2014
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Reverse cholesterol transport fluxes Hellerstein and Turner
as components of the measured fluxes. Perhaps more importantly, these measurements are intended to capture the capacity and activity of the ‘RCT system’ in an individual, which may represent a marker of their HDL functional capacity (much as clotting times do not reveal how much bleeding or clot formation has actually occurred in tissues, but reveals systemic capacity for coagulation). Systemic capacity for RCT is potentially relevant as a drug target, for gene discovery and for pathophysiologic investigation. The quantitative rates revealed by this method are of interest in thinking about the control of cholesterol fluxes in the body. Tissue free cholesterol efflux was 3.8 0.9 mg/kg per hour (mean standard deviation), or approximately 6.5 g/day.
multipool model (Fig. 2). Labeled sitostanol (a nonabsorbed phytosterol) is also taken orally, as an external standard for determining total fecal sterol excretion. Fecal samples are collected for 7 days following the infusion and both the total excretion rate and the isotopic enrichments of neutral sterols and bile acids are determined. From these measurements, multiple parameters of cholesterol flux can be determined (Table 2). This model focuses on quantifying the whole body cholesterol flux into, through and out of the plasma lipoprotein compartment. Although this model does not specifically assess macrophage efflux (which represents a small percentage of whole-body cholesterol flux), nor is biliary vs. nonbiliary (transintestinal excretion) captured, these pathways do contribute quantitatively
Tissue cholesterol efflux
(a)
k(2,1) k(1,2)
V2 RBC FC
FC between lipoproteins and RBC
k(0,1)
k(0,3)
V1 Plasma lipoproteins and liver FC
k(3,1) Plasma
CE from lipoproteins to liver
V3 Plasma CE
Infusion
CE Clearance
FC Clearance
(b) s1 PlosMPE s2 RBCMPE s3 EstMPE
0.015
0.010
0.005
0.000
0
10
20
30
1 (hours)
FIGURE 2. (a) Cholesterol flux and exchange model from Turner et al. V1 is the plasma lipoprotein free cholesterol pool which also includes a significant contribution from hepatic free cholesterol which is rapidly exchanged with plasma. Tissue efflux is the sum of the FC clearance and the cholesteryl ester (CE) clearance. Infusion indicates the input of the 13C cholesterol tracer into the system. (b) 13C cholesterol enrichments and fits over time. Plasma FC, V1 (open square, green line); RBC FC V2 (small square, red line) and Plasma CE V3 (open triangle, blue line). 0957-9672 ß 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
www.co-lipidology.com
43
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Nutrition and metabolism
In comparison, the sum of dietary cholesterol absorption and endogenous synthesis rates is typically approximately 1.0–1.5 g/day. Efflux of cholesterol from tissues into the plasma compartment is therefore five times the rate of excretion of cholesterol from the whole body. RBC-derived flux into plasma-free cholesterol, representing net exchange, was 3.4 1.1 mg/kg per hour. This value is quantitatively comparable with efflux from all other tissues into plasma, consistent with a potential role of RBCs as carriers involved in cholesterol fluxes in vivo. Esterification of plasma-free cholesterol was approximately 28% of tissue-free cholesterol efflux (1.1 0.4 mg/kg per hour), representing a substantial fate of free cholesterol that is effluxed into the plasma lipoprotein compartment. Recoveries were 7 and 12% of administered [3,4-13C2]-cholesterol in fecal bile acids and neutral sterols, respectively. Another novel method for assessing RCT was recently presented at the annual Arteriosclerosis, Thrombosis, and Vascular Biology (ATVB) meeting. This method, developed by Cuchel et al. [21 ], is intended to measure macrophage-specific cholesterol efflux as well as other components of the RCT pathway. The approach was developed to deliver 3H-cholesterol/albumin complexes to hepatic Kupfer cells and other tissue-resident macrophages. Intravenous-injected 3H-cholesterol/albumin complexes are rapidly removed from plasma, presumably by macrophages of the reticuloendothelial system, and their subsequent release of labeled cholesterol onto plasma lipoproteins (HDL and nonHDL), conversion to cholesterol ester and excretion as fecal sterols over the subsequent 8 days were characterized using a multicompartment model similar to the one developed by Schwartz et al. [17]. This approach is intended to focus on macrophage cholesterol efflux and, as such, is conceptually complementary to measurements of whole body or systemic RCT capacity. In normal individuals, the particulate cholesterol was rapidly cleared from the plasma (
