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(Circulation. 2003;108:661.)
© 2003 American Heart Association, Inc.
Brief Rapid Communications |
From the Departments of Medicine (Y.Z., M.R., D.J.R.) and Cell and Developmental Biology (J.M.G.) and the Center for Experimental Therapeutics (Y.Z., M.R., D.J.R.), University of Pennsylvania School of Medicine, Philadelphia, Pa; and the Childrens Hospital of Philadelphia (I.Z., G.H.R.), Philadelphia, Pa.
Correspondence to Daniel J. Rader, MD, University of Pennsylvania Medical Center, 654 Biomedical Research Building II/III, 421 Curie Blvd, Philadelphia, PA 19104. E-mail rader{at}mail.med.upenn.edu
Received March 11, 2003; revision received June 18, 2003; accepted June 19, 2003.
| Abstract |
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Method and Results J774 macrophages were loaded with cholesterol by incubation with acetylated LDL, labeled with 3H-cholesterol, and then injected intraperitoneally into mice. Plasma and feces were collected at 24 hours and 48 hours, when mice were exsanguinated, tissues were harvested, and all were analyzed for tracer counts. 3H-cholesterol was found in the plasma, liver, and feces. For apoA-I overexpression, mice were injected intravenously with apoA-I adenovirus (1011 particles per animal) 3 days before labeled macrophages were injected. ApoA-I overexpression led to significantly higher 3H-cholesterol in plasma, liver, and feces. The amount of 3H-tracer in the liver was 35% higher (P<0.05) and the 3H-tracer excreted into feces over 48 hours was 63% higher (P<0.05) in apoA-Iexpressing mice than in control mice.
Conclusion Injection of 3H-cholesterollabeled macrophage foam cells is a method of measuring reverse cholesterol transport specifically from macrophages to feces in vivo, and apoA-I overexpression promotes macrophage-specific reverse cholesterol transport.
Key Words: cholesterol cholesterol, reverse transport apolipoproteins macrophages atherosclerosis
| Introduction |
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Therefore, despite continued enthusiasm for the concept of reverse cholesterol transport as a major mechanism by which apoA-I overexpression protects against atherosclerosis, no direct proof of this concept yet exists, which has created substantial doubt as to whether HDL and apoA-I actually promote the rate of reverse cholesterol transport.10 However, the methods used previously have estimated rates of reverse cholesterol transport from entire peripheral tissue and not specifically from macrophages, the most important cholesterol-accumulating cells in atherosclerosis. Macrophages are particularly reliant on the ABCA1 pathway, which is promoted by apoA-I, to rid themselves of excess cholesterol. We therefore developed a novel approach to measure reverse transport of labeled cholesterol specifically from macrophages to the liver and feces in vivo and have applied this approach to investigate the ability of apoA-I overexpression to promote macrophage-specific reverse cholesterol transport.
| Methods |
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In Vivo Studies
Twenty male C57BL/6 mice (Jackson Laboratories, Bar Harbor, Me) were injected intravenously with recombinant adenoviral vector encoding human apoA-I (AdapoA-I, n=10)3 or a control adenovirus containing no transgene (Adnull, n=10) (1011 particles per animal). Three days after vector injection, 3H-cholesterollabeled J774 foam cells (7.8x106 cells containing 3.8x106 counts per minute [cpm] in 0.5 mL minimum essential medium) were injected intraperitoneally. Blood was collected at 24 hours (retro-orbital plexus) and 48 hours (vena cava), and plasma was used for liquid scintillation counting and lipoprotein analysis. Feces were collected continuously from 0 to 24 hours and 24 hours to 48 hours and were stored at -20°C until extraction of cholesterol and bile acid. At 48 hours, mice were anesthetized, and the vasculature was perfused with cold phosphate-buffered saline. Liver, spleen, and lungs were removed and stored at -20°C until lipid extraction. An additional apoA-I overexpression experiment was performed in C57BL/6 female mice (n=8; Jackson Laboratories).
Fecal Cholesterol and Bile Acid Extraction
Fecal cholesterol and bile acid were extracted as described previously.13 The levels are expressed as counts per minute in total feces by wet weight.
Tissue Lipid Extraction
Tissue lipids were extracted using the method of Bligh and Dyer14 and expressed as counts per minute in total organ. The distribution of cholesterol between free and esterified forms in liver, spleen, and lung were measured by both TLC11 and GLC.12
Plasma Lipid Analysis
Plasma total cholesterol, HDL cholesterol, phospholipid, and human apoA-I levels were measured on a Cobas Fara (Roche Diagnostics Systems, Inc) using Sigma Diagnostics reagents as described previously,15 and the levels were expressed as milligrams per liter. Serum lipoproteins were isolated by ultracentrifugation16 and analyzed for the distribution of 3H-cholesterol among lipoprotein fractions.
Statistical Analysis
Values are presented as mean±SEM. Results were analyzed by ANOVA and Students t test with the use of GraphPad Prism Software.
| Results |
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| Discussion |
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Although we cannot fully rule out the migration of intact macrophages from the peritoneum to the liver, an examination of the distribution of 3H-cholesterol between free and esterified pools speaks against this. In our pilot experiment, we measured the distribution of free and esterified cholesterol in 3H-cholesterollabeled J774 foam cells and in mouse liver, spleen, and lung by TLC and also determined their masses of free and esterified cholesterol. The majority of cellular cholesterol of labeled J774 foam cells was esterified (
70%), whereas
15%, 13%, and 11% of the 3H-cholesterol in the liver, spleen, and lung was esterified. This suggests that the 3H-sterol in tissues was not transported there by macrophages. The abundant 3H-cholesterol detected in plasma and the fact that 75% of the tracer in plasma was in the HDL fraction suggests a model in which 3H-cholesterol was available for efflux from macrophages in situ and was transported through the plasma compartment to the liver, mainly on HDL particles.
It has been widely assumed that apoA-I overexpression inhibits atherosclerosis at least in part by reverse cholesterol transport. However, data proving this have been lacking, raising the question as to whether apoA-I overexpression actually promotes the rate of reverse cholesterol transport. The methods used previously have estimated rates of reverse cholesterol transport from entire peripheral tissue and not specifically from macrophages. Our studies demonstrate for the first time that apoA-I overexpression promotes macrophage-specific reverse cholesterol transport in vivo. It is possible that macrophages are more responsive to increases in plasma concentrations of apoA-I than are other cell types, and this tracer approach may allow more effective measurement of reverse cholesterol transport than mass-based approaches.
In summary, we report the development of a novel approach for tracing reverse transport of labeled cholesterol specifically from macrophages to the liver and feces in vivo. Using this approach, we demonstrate for the first time that apoA-I overexpression promotes macrophage-specific reverse cholesterol transport in vivo. This method may be applied to other questions about the roles of specific genes in regulating the rate of reverse cholesterol transport.
| Acknowledgments |
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| Footnotes |
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| References |
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