(Circulation. 1999;99:105-110.)
© 1999 American Heart Association, Inc.
Basic Science Reports |
From Rhône-Poulenc RorerGencell Division, Cardiovascular Department, Centre de recherche de Vitry-Alfortville, Vitry sur Seine, and Institut Pasteur (G.C., J.C.F.), Lille, France.
Correspondence to Nicolas Duverger, Rhône-Poulenc RorerGencell Division, Cardiovascular Department, Centre de recherche de Vitry-Alfortville, 13, Quai Jules Guesde-BP 14, 94403 Vitry sur Seine Cedex, France. E-mail nicolas.duverger{at}rp-rorer.fr
| Abstract |
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Methods and ResultsThe human apoA-I gene has been transferred into different mouse models by use of a recombinant adenovirus under the control of an RSV-LTR promoter (AV RSV apoA-I). Administration of AV RSV apoA-I to C57BL/6 mice resulted in moderate expression of human apoA-I for 3 weeks, leading to a transient elevation (40% at day 11 after injection) of HDL cholesterol concentration. In contrast, administration of AV RSV apoA-I to human apoA-Itransgenic mice induced a large increase of human apoA-I and HDL cholesterol concentrations (300% and 360%, respectively, at day 14 after injection) for 10 weeks, indicating that an immune response to the transgene was one major hurdle for long-term duration of expression. Recombinant adenovirus expressing human apolipoprotein A-I (AV RSV apoA-I) was also injected into human apoA-Itransgenic/apoE-deficient mice, which are prone to develop atherosclerosis. Over a 6-week period, overexpression of human apoA-I inhibited fatty streak lesion formation by 56% in comparison with control.
ConclusionsSomatic gene transfer of human apoA-I prevents the development of atherosclerosis in the mouse model.
Key Words: genes apolipoproteins atherosclerosis
| Introduction |
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In humans, apoA-I is synthesized by both liver and intestine.4 ApoA-I represents 70% of the protein component of HDL, and its concentration is directly correlated with HDL cholesterol levels. ApoA-I plays a predominant role in the molecular architecture of HDL.5 ApoA-Icontaining lipoproteins are heterogeneous in hydrated densities and sizes. In addition to the epidemiological data, several animal studies have shown that apoA-I prevents atherosclerosis development. Infusion of apoA-Icontaining lipoproteins in rabbits inhibits lesion formation.6 7 Overexpression of human apoA-I in specific inbred strains of animals, natural cholesterolfed C57BL/6 mice8 and New Zealand White rabbits,9 or in genetically engineered strains of animals, apoE-deficient mice10 11 and human apo(a)transgenic mice,12 protects against atherosclerosis. ApoA-Icontaining lipoproteins appear to exert their antiatherogenic effect by (1) facilitating reverse cholesterol transport, during which cholesterol excess is transported away from cells of extrahepatic tissues and carried back to the liver, where it can be eliminated or reused13 14 ; (2) inhibiting lipoprotein oxidation15 16 ; and (3) directly protecting the vessel wall from damages.17 18
Recently, gene therapy approaches for atherosclerosis using adenovirus-mediated transfer of human apoA-I gene in normal mice demonstrated a transient expression of the transgene associated with elevations of HDL concentration.19 20 In these studies, human apoA-I expression was under the control of cytomegalovirus (CMV) promoter. CMV promoter has been used extensively to achieve high expression of genes. It displayed a strong transient activity when expressed in an adenoviral vector. However, its activity is greatly reduced after a short period of time; human apoA-I expression lasted only 2 weeks.19 20 Moreover, expression controlled by CMV promoter is rapidly downregulated or repressed in the liver21 and is not an attractive candidate promoter for hepatic long-term expression. Another viral promoter, the Rous sarcoma virus long-terminal repeat (RSV-LTR) promoter, which allowed high and long-duration (>1 month) expression in the liver, has also been described.22 Therefore, to increase the duration of expression and to generate conclusive data on the role of apoA-I in HDL metabolism and its effect on atherogenesis progression in animal models, we constructed an adenoviral vector encoding human apoA-I under the control of the RSV-LTR promoter. In addition, because the transgene is of human origin and thus can generate an immune response in mice,23 human apoA-Itransgenic mice were used to abolish this immune reaction and then improve the duration of the transgene expression. In this article, we demonstrate that somatic adenovirus-mediated apoA-I gene transfer inhibits the development of atherosclerosis in the mouse model.
| Methods |
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Animal Experiments
The study protocol was approved by the Animal Use Committee of
Rhône-Poulenc Rorer. C57BL/6 mice, human apoA-Itransgenic
mice,27 and apoE-deficient mice expressing human
apoA-I10 have been described previously. All mice
had a C57BL/6 background. Mice 8 to 10 weeks old were treated by
tail-vein injection of purified recombinant adenovirus stocks. Blood
was taken from the retro-orbital plexus of mice fasted for 3 hours.
Plasma was separated by centrifugation at
2800g for 20 minutes at 4°C. At the end of the experiment,
animals were killed for histological analysis.
Injections of 1x109,
3x109, and 1x1010 pfu of
adenovirus in mice resulted in expression of the transgene. Very high
inflammation in the liver and some difficulties in recovering were
observed in mice receiving the higher amount of virus, as well
as a moderate and transient
hypertriglyceridemia. This elevation of
triglyceride levels may reflect the adenovirus-induced
acute-phase response. Therefore, 3x109 pfu of
the virus was chosen as the optimal dose for mice.
Protein and Lipoprotein Analysis
Cholesterol was measured
colorimetrically with a commercially available kit
(Boehringer Mannheim). Plasma lipoprotein distribution was
assayed by analytical gel filtration chromatography,
with a Superose 6 HR 10/30 column (Pharmacia). Plasma levels of human
apoA-I were determined by rocket immunoelectrophoresis (Sebia). There
was no cross-reactivity between human and mouse apoA-I in this assay.
The size of plasma human apoA-Icontaining lipoproteins was determined
by nondenaturing gradient polyacrylamide gel
electrophoresis28 followed by a Western blot
analysis using specific antihuman apoA-I
antibodies.29 Blots were analyzed by quantitative
scanning densitometry (Hoefer GS-300).
In Vitro Cellular Cholesterol Efflux
Cellular cholesterol efflux studies were performed
as described previously,30 with the rat Fu5AH hepatoma
cells incubated with 2.5% diluted serum.
Histological Analysis
Aortic sectioning, lipid staining, and lesion scoring were
performed blindly according to the methods described
previously.31 Briefly, mouse hearts were fixed, stored in
4% formalin, and embedded in 25% gelatin. Then, 10-µm proximal
aortic sections, separated by 200 µm, were stained with oil red
O for neutral lipids. Lesion area values for each mouse were obtained
from the mean of 4 sections evaluated for their lipid-stained areas.
Livers were harvested from mice, and formalin-fixed tissues were
stained with hematoxylin and eosin.
Statistical Analysis
All data are expressed as mean±SEM. Data were evaluated by
ANOVA.
| Results |
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Adenovirus-Mediated Human ApoA-I Gene Transfer in Human
ApoA-ITransgenic Mice
To avoid the effect of the immune reaction against the human
protein, transgenic mice for human apoA-I were used. Basal levels of
human apoA-I were 143±5 mg/dL, whereas endogenous mouse
apoA-I was undetectable.32 Transgenic mice were injected
with 3x109 pfu of the AV RSV apoA-I (n=10) or
with PBS (n=5). Plasma levels of human apoA-I,
triglycerides, and HDL cholesterol were
measured on a weekly basis (Figure 2
).
Seven days after adenovirus administration, plasma levels of human
apoA-I increased from basal levels to 425±58 mg/dL (300% increase).
These high levels of the transgene were still significantly higher
(P<0.0001) in AV RSV apoA-Itreated mice than in
mock-treated mice after 6 weeks. HDL cholesterol levels
(Figure 2
) were also increased in a parallel manner and rose
from 97±8 to 346±44 mg/dL (360% increase) at 7 days. After 6 weeks,
the HDL cholesterol levels were still significantly
increased, at 289±26 mg/dL (297% of the basal level,
P<0.0001). Human apoA-I levels were still higher in AV RSV
apoA-Itreated mice than in mock-treated mice after 10 weeks (data not
shown). No variation of triglyceride concentrations (mean
levels, 138±13 mg/dL) was observed after human gene transfer for all
time points. Altogether, these data demonstrated that human apoA-I gene
delivery in mice that displayed mouse- or human-type HDL was distinct
in term of levels and duration of transgene expression.
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We investigated modifications of HDL structure after human apoA-I gene transfer. Before and after infection by the adenovirus, apoA-Icontaining lipoproteins of human apoA-Itransgenic mice had a bimodal distribution, with major peak sizes of 9.1±0.4 and 12.1±0.3 nm (Stokes diameters) corresponding to HDL3 and HDL2 fractions. These data were in agreement with those previously reported.33 These data indicated that no alteration of HDL structure occurred after human apoA-I gene delivery in human apoA-Itransgenic mice. Analysis of the distribution of human apoA-I by analytical gel filtration chromatography indicated that all human apoA-I was associated with HDL (data not shown).
Adenovirus-Mediated Human ApoA-I Gene Transfer in ApoE-Deficient
Mice Expressing Human ApoA-I
ApoE-knockout/human apoA-Itransgenic mice were created by
crossing human apoA-Itransgenic mice with apoE-deficient mice. Female
mice (9 weeks old) were infused with 3x109 pfu
of the AV RSV apoA-I (n=21), AV Empty (n=8), or PBS (n=14). Plasma
human apoA-I expression (Figure 3
) rose
at day 14, from 112±3 mg/dL (basal level) to 237±11 mg/dL, and
decreased slowly to 140±7 mg/dL at day 42. Expression of human
transgene increased HDL cholesterol level (Figure 3
)
from 60±4 to 174±10 mg/dL 14 days after injection. HDL
cholesterol concentration remained elevated in AV RSV
apoA-Iinfused mice during the experiment. HDL cholesterol
levels were correlated with human apoA-I concentrations
(P<0.005). Non-HDL cholesterol concentrations
in apoE-deficient mice expressing human apoA-I were 469±12 mg/dL
(basal level). There was no difference in non-HDL
cholesterol levels between groups of mice treated with AV
RSV apoA-I, AV Empty, or PBS during the experiment.
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Atherosclerosis progression in the proximal aorta was
determined. Analysis of fatty-streak lesions (Figure 4
) indicated that mean lesion areas were
significantly higher in the PBS (n=14) and AV Empty (n=8) groups of
mice than in the AV RSV apoA-Itreated mice (n=21) (P<0.01
and P<0.01, respectively). Mean lesion area for the PBS
group was 16 872±1897 µm2. Lesions in
mice treated with AV Empty and AV RSV apoA-I were 94.31±5.44% and
56.4±1.52% of control (PBS group), respectively. Mean lesion areas
(logarithmically calculated) were highly correlated with plasma levels
of human apoA-I and HDL cholesterol (P<0.01 for
both). Initial mean lesion areas (day 0) were 2038±403
µm2, ie, 12.08±0.64% of control (PBS group).
These data indicated that overexpression of human apoA-I retarded
atherosclerosis development in apoE-deficient mice
expressing human apoA-I.
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To gain insight into the molecular mechanism of the protective effect
mediated by human apoA-I, the promotion of cholesterol
efflux from cells was investigated. Cholesterol efflux
(Figure 5
) promoted from
cholesterol-preloaded Fu5AH cells was 1.6-fold greater
(P<0.001) with the incubation of sera (14 days after
injection) from the AV RSV apoA-Itreated mice than that from
PBS-treated mice. There was no difference between
cholesterol efflux induced by the incubation of sera from
PBS- and AV Emptytreated mice.
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| Discussion |
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Adenovirus-mediated gene delivery of human apoA-I demonstrated that it is possible to increase the levels of apoA-I and HDL cholesterol in the plasma of human apoA-Itransgenic mice by nearly 300%, whereas only moderate increases of these parameters were observed in normal mice. The dramatic difference in the level of transgene expression between transgenic and nontransgenic mice reflected the lack of a reaction against the protein of foreign origin. The levels of human apoA-I expression obtained in transgenic animals were higher than in the nontransgenic models, and the duration of expression was also increased from weeks to months. Nevertheless, an immune reaction against viral proteins may still be present with this first-generation adenoviral vector and may contribute to the extinction of transgene expression by elimination of virus-transduced cells.34 Recently, several groups have reported solutions to this problem, such as transient immunosuppression, neonatal administration of the vector, or modification of the viral vector. We showed here that immune response against the foreign protein is one major hurdle to obtaining stable expression for several months and to producing data in the study of lipoprotein metabolism and atherogenesis. In addition, the use of RSV-LTR promoter, which may appear less strong in direct transgene expression at the peak level in comparison with that of a CMV promoter,19 20 allowed a constant expression for a longer period of time without side effects such as the hypertriglyceridemia that was observed.19 20
To assess the beneficial effect of apoA-I on atherogenesis, we evaluated human apoA-I gene transfer in apoE-deficient mice expressing basal levels of human apoA-I. ApoE-knockout mice develop atherosclerotic lesions resembling those observed in humans.35 36 We demonstrated here that overexpression of human apoA-I after adenovirus-mediated transfer of the human apoA-I gene was associated with a 2-fold increase of HDL cholesterol and a 2-fold decrease in the development of atherosclerotic lesions over a 6-week period. HDL- and apoA-Icontaining lipoproteins have been proposed to protect against atherogenesis by removing excess cholesterol in peripheral cells and transporting it back to the liver.14 Although other mechanisms have been proposed for the antiatherogenic role of HDL, the present in vitro cellular cholesterol efflux study supports this hypothesis. In addition, of course, direct vascular protective effects of apoA-I may also occur.20 Gene transfer of apoE, LDL receptor, or VLDL receptor in apoE- and LDL receptordeficient animals (reviewed in Reference 3737 ) has been used successfully and resulted in a modification of plasma lipid concentrations to normal levels, ie, a correction of the lipoprotein phenotype. We showed here that overexpression of apoA-I, which did not normalize the lipoprotein profile, can reduce atherosclerotic lesion formation.
The vast majority of hypoalphalipoproteinemic patients having some residual apoA-I levels in the circulation develop premature CHD.1 Data emerging from large epidemiological surveys have suggested that each 1% increase in HDL cholesterol plasma concentration results in a 3% to 4% decrease in CHD risk.38 In addition, overproduction of human apoA-I has been found in some subjects and was associated with high HDL cholesterol levels, an absence of CHD, and longevity.39 Therefore, overexpression of apoA-I to higher-than-normal concentrations can be considered as potential therapy to increase HDL concentration and induce inhibition or regression of atherosclerotic lesions in a large population. We have recently reported another potential therapy for hypoalphalipoproteinemic patients with adenovirus-mediated transfer of the human lecithin-cholesterol acyltransferase (LCAT) gene.40 Human LCAT gene transfer in human apoA-Itransgenic mice led to an increase of HDL cholesterol and human apoA-I plasma levels, probably due to a delay in apoA-Icontaining lipoprotein catabolism. Similar constructs and animal models will be necessary to compare or combine these 2 approaches.
The present study demonstrates the potential of gene transfer strategy in the appropriate animal model to gain insight into the role of apoA-I in lipoprotein metabolism as well as in the atherosclerotic process.
| Acknowledgments |
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Received March 4, 1998; revision received August 25, 1998; accepted September 8, 1998.
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