Integrative Physiology |
From the Division of Cardiovascular Medicine (J.C.R., A.E.M., G.G.), University of California, Davis, and Department of Medicine (I.J.G.), Columbia University College of Physicians and Surgeons, New York.
Correspondence to John C. Rutledge, MD, Division of Cardiovascular Medicine, One Shields Dr, TB 172, Davis, CA 95616. E-mail jcrutledge{at}ucdavis.edu
| Abstract |
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Key Words: triglycerides atherosclerosis lipoproteins apolipoprotein B cholesterol
| Introduction |
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The mechanisms by which triglyceride-containing lipoproteins, VLDL and chylomicrons, are atherogenic are unclear. It is unlikely that these large particles directly enter the arterial wall. However, several other processes that occur during the metabolism of these lipoproteins along the arterial wall are potentially atherogenic. These include the following. (1) Unlike nascent triglyceride-rich lipoproteins, the smaller remnants may infiltrate into the vessel.4 (2) Lipolysis of triglycerides might compromise the barrier function of the endothelial layer.5 6 (3) Lipids that are dissociated from the surface of triglyceride-rich lipoproteins may accumulate within the artery.5 (4) VLDL and chylomicron remnants could be selectively retained in the artery wall.7
HDLs are inversely correlated with the development of complications of atherosclerosis, such as myocardial infarction.8 9 However, thus far, there has been no direct demonstration that HDL will remove lipids from the artery wall. Therefore, the reverse cholesterol transport pathway as a protective effect of HDL must be considered hypothetical. Additionally, HDL has a number of other protective actions in vitro: HDLs contain antioxidant enzymes such as paraoxonase,10 and HDLs may reduce LDL association with matrix proteins.11 Further, in vivo data show that HDLs are a marker for other lipoprotein abnormalities, and, in some studies, HDL is positively correlated with the removal rate of postprandial lipoproteins.12 13
Despite a large body of in vitro data on the interactions of lipoproteins with cultured cells, there are limited data directly assessing the effects of lipoproteins and lipolysis products on whole arteries. Lipoprotein lipase (LpL) is located primarily on capillary endothelial cells, and immunohistological studies also have shown this protein on macrophages and smooth muscle cells within atherosclerotic plaques.14 15 Our data show that surface lipid accumulates in the intimal layer partially in the form of small regions that have been termed "lakes." HDL abrogated lipolysis-induced changes in endothelial layer permeability and removed VLDL-derived surface lipid from the artery wall. Therefore, the first step in reverse lipid transport could be observed within an intact artery.
| Materials and Methods |
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Measurement of Macromolecule Accumulation
The perfused artery preparation and experimental
apparatus are as we have described
previously.6 17 18 Several measurements were made during
and after perfusion of each carotid artery with fluorescently
labeled molecules (Figure 1
).
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Effects of HDL on Endothelial Layer
Permeability
The actions of HDL on lipolysis-mediated increases in
endothelial layer permeability were determined by
measuring the accumulation of a water-soluble, nonlipid reference
molecule (Mr 4400 dextran, labeled with
TRITC). Initially, the rate of dextran accumulation was determined
during perfusions of dextran in 1 perfused carotid artery from a rat.
Then, VLDL was added to the perfusate and the rate of dextran
accumulation again determined. Next, LpL was added to the
perfusate and the rate of dextran accumulation again
determined. In the other artery from the same animal, a parallel
experiment was performed, but with HDL added to the VLDL
perfusate. Vessels were compared before and after LpL
administration, in the absence and presence of HDL.
An expanded Materials and Methods section is available online at http://www.circresaha.org.
| Results |
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Accumulation and Localization of VLDL Surface Label During
Lipolysis in Perfused Arteries
To examine the action of lipolysis on the VLDL surface components,
we studied DiI-VLDL perfused into individual carotid arteries from male
rats. Seven carotid arteries were taken from 4 rats (technical problems
occurred with 1 carotid artery). During 3 alternating 10-minute
perfusions of DiI-VLDL (0.056 mg/mL cholesterol) and the
nonfluorescent buffer solution, little or no accumulation of
DiI-VLDL was detected (0.15±0.014 [SEM] mV/min; 0.1 ng
cholesterol/cm2
min1). When LpL (10 µg/mL) was added to the
perfusate, the rate of DiI accumulation increased after the
10-minute perfusion with DiI-VLDL. Previous experiments have shown that
LpL in this concentration will extensively hydrolyze
VLDL.20 This effect was maintained during the 2 subsequent
10-minute perfusions of DiI-VLDL. Comparison of all control perfusions
of DiI-VLDL with all DiI-VLDL+LpL perfusions showed significantly
greater DiI accumulation after LpL was added to the perfusate
(0.33±0.048 mV/min; 2.3 times greater than control, P<0.05
by paired t test; Figure 1
). No change in lumen
diameter was observed during monitoring using our fluorescence
optical system for the duration of the experiment. Therefore, LpL
markedly increased DiI accumulation in vessels. Previous experiments
from our laboratories have documented a dose-response relationship of
LpL on LDL accumulation in the artery.6
To localize DiI-surface accumulation in the artery wall, we examined
sections of the artery at the end of the experiment. As shown in Figure 3
, DiI fluorescence accumulated
in the intima, with the endothelium and the
subendothelial layers most intensely labeled. Further,
much less intense sites of homogeneous localized
accumulations of DiI were observed below the
endothelial layer. These lakes were not seen when VLDL
protein was fluorescently labeled and treated with LpL (below).
These observations indicate that remnant particles do not contribute to
the lakes and suggest that the lakes are composed of surface lipid that
is associated with DiI. Thus, lipolysis markedly increased DiI-surface
lipid accumulation on the endothelium and in the
subendothelial space and led to collections of label
deeper in the wall.
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Accumulation and Localization of VLDL Remnants in the Artery
Wall
We next assessed whether TRITC-VLDL would accumulate in the artery
wall before and after lipolysis. Arteries perfused with TRITC-VLDL
(0.058 mg cholesterol/mL) alone had little accumulation at
control (0.197±0.046 mV/min; 1.25 ng/cm2
min1). Addition of LpL (10 µg/mL) to the
perfused artery significantly increased TRITC accumulation in the
artery wall (0.546±0.076 mV/min; 3.46 ng/cm2
hour1, a 2.7-fold increase; n=4 animals).
Localization of TRITC showed an intense accumulation in the
endothelial layer (Figure 4
). Because there was little accumulation
of TRITC-VLDL and a 2.8-fold increase in TRITC accumulation after
treatment with LpL, we assume that VLDL remnant particles account for
the large increase in TRITC accumulation. In contrast, our previously
published study showed accumulation of LDL primarily in the
subendothelial space after treatment with
LpL.6 Therefore, lipolysis increased remnant accumulation
in the artery wall in the same general area that VLDL surface
components accumulated after lipolysis. However, no TRITC accumulations
(lakes) were detected in the subendothelial space.
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Actions of HDL on Endothelial Layer
Permeability
We next examined the actions of HDL on lipolysis-mediated
increases in endothelial layer permeability by
determining the accumulation of a water-soluble, nonlipid reference
molecule (Mr 4400 dextran, labeled with
TRITC). Initially, the rate of dextran accumulation was determined
during perfusions of dextran (0.07 mg/mL) in 1 perfused carotid artery
from a rat (n=4 animals). Then, VLDL (0.1 mg
cholesterol/mL) was added to the perfusate and the
rate of dextran accumulation again determined. Next, LpL (10 µg/mL)
was added to the perfusate and the rate of dextran accumulation
again determined. In the other artery from the same animal, a parallel
experiment was performed, but with HDL (0.1 mg
cholesterol/mL) added to the VLDL perfusate. A
comparison in the rates of dextran accumulation before and after LpL
administration revealed that vessels containing HDL had reduced dextran
accumulation after LpL was added to the perfusate (Figure 5
). Comparison of the change in rate of
dextran accumulation (24% versus 3%) was significantly different
(P<0.05, n=8 arteries). Therefore, addition of HDL to the
perfusion prevented the increase in dextran accumulation seen with
lipolysis.
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Removal of Lipolysis Products From the Artery Wall by
HDL
Because our in vitro experiments showed that HDL accepted
DiI-surface lipid from VLDL after lipolysis, we reasoned that HDL would
remove DiI-labeled surface lipid from the artery wall as well. Parallel
experiments were performed in the right and left carotid arteries from
a single rat. One artery served as the control (no HDL added to
perfusate), and the other artery was perfused with HDL. First,
the artery was perfused for 10 minutes with DiI-VLDL (0.13 mg/mL
cholesterol), and the rate of DiI accumulation was
measured. Then, lipolysis was induced by adding LpL (10 µg/mL) to the
perfusate containing VLDL. After 1 hour of perfusion with the
DiI-VLDL and LpL, the DiI-containing vessel was perfused with either
buffer or buffer+HDL (0.15 mg/mL cholesterol). The decay in
fluorescence intensity was monitored during this washout period
(15 minutes). The rate of DiI efflux was determined by nonlinear
regression techniques, assuming a 2-compartment model of
fluorescent decay.6 This efflux rate constant
(d) of DiI removal from the artery wall was compared in
vessels with and without HDL. In each animal, the efflux rate of DiI
was greater with the vessel perfused with HDL (0.9±0.027 versus
0.24±0.016 hours1; P<0.05, n=8
vessels; Figure 6
). Therefore, HDL
effectively removed the DiI that accumulated in the artery.
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| Discussion |
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Our fluorescence histological studies showed that both DiI and TRITC significantly accumulated in the artery wall after treatment with LpL. We saw little or no accumulation of DiI-VLDL or TRITC-VLDL on or in the artery wall before addition of LpL. This relatively low level of accumulation before LpL is added indicates that primary VLDL-endothelial cell interactions play a relatively minor role in the large increase in DiI and TRITC accumulation. Our previous studies have shown that the great majority of VLDLs undergo lipolysis at the concentration of LpL and VLDL used in these experiments.20 Only a small percentage of the LpL binds to the cells; however, marked lipolysis occurred under these conditions. Thus, it appears that the VLDL surface component and the VLDL remnant localized to the intima.
Remnant particle localization to the intima is related, at least in part, to the large size of these particles that prevents further penetration into the artery. We expected the DiI-surface component to be either randomly distributed to the entire artery or present in a gradient with the intensity of signal correlating inversely with the distance from the lumen. Because the DiI and TRITC fluorescence were discretely localized in the intima, this suggested that the postlipolysis VLDL surface lipid and apolipoprotein B bind to specific intimal components. These high-affinity interactions may have prevented further penetration of the lipids into the artery wall. It is widely believed that lipoproteins are retained within arteries by association with negatively charged glycoconjugates, ie, vessel wall proteoglycans. If correct, then negatively charged lipids such as phospholipids, fatty acids, and lysolecithin should be repelled from these proteins. Perhaps basic regions of other matrix molecules interact with these lipids. This might prevent their further diffusion into the artery wall.
Atherosclerotic plaque contains large amounts of nonlipoprotein-associated (free) lipid3 22 ; however, the source of the free lipid is unclear. Free lipid could be derived from lipid-filled macrophages that have undergone cell death or apoptosis.23 Our study demonstrates that lipolysis products from triglyceride-rich lipoproteins can also provide free lipid that collects within the artery wall. These regions containing free lipid, we hypothesize, are the progenitors of the lipid whorls found in atherosclerotic plaques. In addition, arterial wall free lipid may initiate processes that promote atherosclerosis, such as inflammation and mitogenesis.24 For these reasons, when lipolysis occurs along the artery wall, rather than in the capillaries, it may be atherogenic.
In summary, there are a number of processes that occur during lipolysis of VLDL and chylomicrons that could be deleterious to the artery.25 Many of these effects may, in vivo, be tempered by the presence of HDL. Previous studies have shown that free fatty acids can increase transendothelial movement of albumin.26 Further, lipolysis also affects the barrier function of the endothelium,6 and our current studies demonstrate that HDL modulates endothelial permeability. This may occur because HDL scavenges lipolysis products,8 substances that may injure the endothelium and increase its permeability. The net effect may be to prevent permeability-related increases in lipoprotein transfer into the artery wall. A second major action of HDL, shown in these studies, is amelioration of the accumulation of nonlipoprotein-associated surface lipid within the artery, the so-called lakes shown in our micrographs. In our studies, HDL prevents the development of these lipid accumulations and can increase the removal rate of lipid once it is present in the vessel. This later process is a direct demonstration in an artery of "reverse lipid transport," a process previously shown to occur only in cultured cells.
| Acknowledgments |
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Received August 2, 1999; accepted February 16, 2000.
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