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Department of Nutritional Sciences, University of Connecticut, Storrs, Connecticut 06269-4017
1To whom correspondence should be addressed. E-mail: maria-luz.fernandez{at}uconn.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONHepatic cholesterol metabolismLipoprotein metabolism and...Effects of dietary factors...Exercise and lipoprotein...Drug treatment studiesGender and hormonal statusDevelopmental studiesAscorbic acid and cardiovascular...AtherosclerosisConclusionsREFERENCES |
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KEY WORDS: • Guinea pigs • lipoprotein metabolism • hepatic cholesterol metabolism • soluble fiber • atherosclerosis.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONHepatic cholesterol metabolismLipoprotein metabolism and...Effects of dietary factors...Exercise and lipoprotein...Drug treatment studiesGender and hormonal statusDevelopmental studiesAscorbic acid and cardiovascular...AtherosclerosisConclusionsREFERENCES |
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There is some controversy as to whether guinea pigs (Cavia
porcellus) should be classified as rodents (Martignetti et
al. 1993
, Noguchi et al. 1994
). Although this
issue is not relevant to the present discussion, there is one aspect of
guinea pigs that makes them stand out from other rodents: the fact that
they carry the majority of their cholesterol in LDL (Fernandez and McNamara 1989
). This outstanding difference raises the
question as to whether guinea pigs have other similarities to humans in
cholesterol and lipoprotein metabolism. Researchers at our laboratory
and other investigators have found that guinea pig cholesterol
metabolism does indeed have some analogies to human cholesterol
metabolism that merit discussion.
Some of these similarities include the following:
).
).
), lecithin-cholesterol acyltransferase (LCAT)
(Douglas and Pownell 1991
) and lipoprotein lipase (LPL)
(Olivecrona and Bengsston-Olivecrona 1993
)
activities for intravascular processing of plasma lipoproteins.
) and catabolism
(Reihner et al. 1991
).
).
).
).
).
).
).
, 1992a
and 1995a
,
He and Fernandez 1998a
) and drug treatment
(Berglund et al.1989
, Hikada et al. 1992
)
by lowering plasma LDL cholesterol (LDL-C) |
Hepatic cholesterol metabolism |
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TOPABSTRACTINTRODUCTIONHepatic cholesterol metabolismLipoprotein metabolism and...Effects of dietary factors...Exercise and lipoprotein...Drug treatment studiesGender and hormonal statusDevelopmental studiesAscorbic acid and cardiovascular...AtherosclerosisConclusionsREFERENCES |
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), guinea
pigs have moderate rates of hepatic cholesterol synthesis
(Fernandez et al. 1990a
, McNamara 1984
),
esterification (Fernandez et al. 1995a
) and catabolism
(Fernandez 1995
). However, hepatic HMG-CoA
reductase, acyl coenzyme A cholesterol acyltransferase (ACAT) and
cholesterol 7
-hydroxylase (Cyp7) activities are modulated by diet,
drug treatment and gender in guinea pigs. HMG-CoA reductase (EC 1.1.1.34).
Dietary modifications that alter hepatic free cholesterol (FC) modulate
the activity of the regulatory enzyme of cholesterol synthesis,
HMG-CoA reductase. The first compensatory response to moderate
concentrations of dietary cholesterol equivalent to an absorbed amount
of half of the daily synthesis rate in guinea pigs is a significant
down-regulation of HMG-CoA reductase activity (Lin et al. 1992
). Further, HMG-CoA reductase is up-regulated
when hepatic cholesterol concentrations are reduced as a result of the
action of dietary soluble fiber in the intestinal lumen
(Fernandez et al. 1994a
). Results from these studies
suggest that there might be a threshold of hepatic cholesterol
concentrations and that when this is achieved, the immediate response
is an up-regulation of cholesterol synthesis by increases in
HMG-CoA reductase activity. In contrast, dietary fat saturation has
only moderate effects on hepatic cholesterol synthesis in guinea pigs
(Fernandez and McNamara 1994
, Ibrahim and McNamara 1988
). The effects of dietary cholesterol and dietary
pectin on HMG-CoA reductase activity in response to changes in
hepatic FC concentrations are presented in Table 1
.
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ACAT is the intracellular enzyme responsible for catalyzing the
esterification of cholesterol in various tissues, including the liver.
We have observed that dietary interventions (Fernandez et al. 1994a
) and drug treatment (Conde et al. 1996
)
modulate this enzyme activity. A very significant dose-response in
ACAT activity to dietary pectin associated with parallel decreases in
hepatic cholesterol has been demonstrated in guinea pigs fed increasing
doses of citrus pectin (Fernandez et al. 1994a
). There
was a positive correlation (r = 0.82) between hepatic
FC and ACAT activity, demonstrating that substrate availability is a
major determinant of ACAT activity. Similarly, atorvastatin treatment
reduced hepatic microsomal FC, which resulted in a parallel decrease in
ACAT activity (Conde et al. 1996
) (Fig. 1
). Other dietary treatments that lower hepatic FC in microsomes
(Vidal-Quintanar et al. 1997
) or hepatic cholesterol
(Ramjiganesh et al. 2000
) have also been correlated with
a decrease in ACAT activity.
|
).
Monounsaturated (MONO) fatty acids resulted in the highest ACAT
activity compared with saturated (SAT) or polyunsaturated (PUFA) fatty
acids. Increasing concentrations of dietary cholesterol resulted in
parallel increases in hepatic ACAT activity and microsomal FC. After
modification of microsomal lipid composition with
FC/phosphatidylcholine (PC) treatment, ACAT activities remained
significantly related to the FC-to-PC molar ratio (Sun et al. 1999
), confirming previous findings that FC or FC/PC is a major
regulator of ACAT activity (Vidal-Quintanar et al. 1997
). Cyp7 (EC 1.14.13.7).
The conversion of hepatic cholesterol to bile acids represents the
major regulatory pathway by which the body eliminates excess
cholesterol. Cholesterol hydroxylation at the 7 position is the
initial and rate-limiting step in this process. Cyp7 presents a
feedback inhibition by hepatic flux of bile acids in rats, guinea pigs
and rabbits (Nguyen et al. 1999
).
The activity of Cyp7 is up-regulated by soluble fiber in guinea
pigs (Fernandez 1995
), possibly as a compensatory
response to interruption of the enterohepatic circulation of bile
acids. In the hamster, psyllium (PSY) intake increases Cyp7 activity
and mRNA abundance (Horton et al. 1994
). In addition,
significant decreases in plasma LDL-C have been observed in both
hamsters and guinea pigs after PSY intake (Fernandez 1995
, Horton et al. 1994
). These results suggest
that soluble fiber has multiple sites of action in the liver and plasma
compartment that contribute to the lowering of plasma LDL-C. The
first modulating step in the whole process could be the
up-regulation of Cyp7. In contrast, dietary cholesterol has no
effect on hepatic Cyp7 in guinea pigs (Fernandez 1995
,
Fernandez et al. 1995a
) or hamsters (Horton et al. 1995
).
The LDL receptor.
The LDL receptor plays a central role in the metabolism of cholesterol
in humans and animals (Brown and Goldstein 1986
). The
expression of the LDL receptor gene in the liver is regulated by a
feedback mechanism that involves hepatic cholesterol. When the demand
for cholesterol increases, the liver cells express high concentrations
of LDL receptor mRNA, and when cholesterol accumulates in the cell, the
activity and expression of the LDL receptor are suppressed. In guinea
pigs, an important mechanism that regulates plasma LDL-C in
response to diet or drug treatment is the LDL or apoB/E receptor.
Intake of PUFA compared with SAT increases hepatic LDL maximal binding
(Bmax) in vitro (Fernandez et al. 1992a,
1992b
and 1993
, Fernandez and McNamara 1989
) and receptor-mediated LDL fractional catabolic rate
(FCR) in vivo (Fernandez et al. 1992a,
1992b
and 1993
),
suggesting that up-regulation of apoB/E receptors is a significant
mechanism by which PUFA decrease plasma LDL-C concentrations.
Similarly, when intake of soluble fiber [either pectin (PE), PSY or
guar gum (GG)] was compared with a control diet that contained
cellulose, a significant increase in hepatic LDL receptors was
observed, as well as increased rates of LDL turnover (Fernandez 1995
). In agreement with these observations, a significant
negative correlation was found between plasma LDL-C and hepatic LDL
Bmax in female guinea pigs fed
different sources of fiber with low and high cholesterol diets
(r = -0.70, P < 0.01;
Fernandez et al. 1995b
) (Fig. 2
).
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Lipoprotein metabolism and processing in the intravascular compartment |
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TOPABSTRACTINTRODUCTIONHepatic cholesterol metabolismLipoprotein metabolism and...Effects of dietary factors...Exercise and lipoprotein...Drug treatment studiesGender and hormonal statusDevelopmental studiesAscorbic acid and cardiovascular...AtherosclerosisConclusionsREFERENCES |
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Dietary fat saturation (Abdel-Fattah et al. 1995
),
soluble fiber (Fernandez et al. 1997a
) and simple
carbohydrate intake (Fernandez et al. 1995c
) influence
the secretion rate of VLDL and affect the composition of nascent VLDL
in guinea pigs. Soluble fiber intake results in a slower secretion of
VLDL apoB, and the nascent particles are larger. In contrast, both a
high SAT diet and a diet high in sucrose result in a faster secretion
rate of smaller VLDL particles rich in CE compared with a PUFA diet or
a diet rich in complex carbohydrates (Abdel-Fattah et al. 1995
, Fernandez et al. 1995c
).
In agreement with studies in humans (Grundy 1996
),
guinea pigs fed diets rich in complex carbohydrates have lower plasma
TAG and VLDL cholesterol concentrations than those fed diets rich in
simple carbohydrates (Fernandez et al. 1996a
). Hepatic
VLDL TAG secretion rates were not affected by carbohydrate type after
the intravenous injection of Triton WR 1339, a detergent that blocks
the action of LPL, which results in TAG accumulation in plasma.
However, the rate of apoB secretion was 1.9-fold higher in
sucrose-fed animals (P < 0.02), which explains the
higher concentrations of plasma TAG observed in guinea pigs fed simple
carbohydrates (Fernandez et al. 1995c
and 1996b
).
CETP and LCAT (2.3.1.43).
Dietary soluble fiber (Fernandez et al. 1997
), intake of
complex carbohydrates (Fernandez et al. 1995b
) and drug
treatment (Conde et al. 1996
) result in significant
alterations in the composition of plasma lipoproteins, which may be
related to altered CETP activity. Guinea pigs treated with 1, 3, 10, 20
and 40 mg atorvastatin/d had lower CETP activity than those fed the
control diet (0 mg atorvastatin) (Conde et al. 1996
).
The lower CETP activity in guinea pigs fed soluble fiber was related to
the lower concentration of CE in VLDL, which has a lower rate of
conversion into LDL and is removed faster from plasma (Fernandez et al. 1997a
).
LCAT plays a major role in the esterification process in the plasma
compartment. When guinea pigs are fed diets low in cholesterol, the
secreted nascent VLDL has a negligible amount of CE
(Abdel-Fattah et al. 1995
). In contrast, the mature VLDL
derived from guinea pigs fed low cholesterol has 
10% CE
(Fernandez et al. 1998
). In agreement with our
observations, Barter et al. (1977)
found that the
composition of guinea pig VLDL resembles that of LDL, which suggests
that, like humans, guinea pig CE in VLDL is mostly derived from LCAT
activity. Guinea pigs fed 35 or 45% total calories from fat had higher
LCAT activity than those fed 10 or 19% total calories from fat, which
correlated with the higher VLDL-C concentrations observed in the
former compared with the latter groups (Fernandez et al. 1995d
). Thus, CE in VLDL from guinea pigs may be derived from
two sources: LCAT activity and CETP-mediated CE transfer. However,
ACAT activity may also contribute to the formation of CE in VLDL
(Fernandez and McNamara 1994
), as discussed later.
LPL (3.1.1.34) and hepatic lipase (HL) (3.1.1.3).
The lipase gene family is made up of three genes that share structural
similarities and are derived from a common ancestral gene (Hide et al. 1992
). This family includes LPL, HL and the digestive
enzyme pancreatic lipase. In humans, LPL activity corresponds to two
thirds of post–heparin lipase activity whereas HL constitutes
approximately one third (Olivecrona and Bengstton-Olivecrona
1993
). Although earlier studies reported the absence of HL in
guinea pigs (Yamada et al. 1979
), Wallinder et al. (1981)
demonstrated lipase activity in guinea pig
postheparin plasma with the characteristics of HL. In later studies, HL
activity was shown to be up-regulated in guinea pigs fed high
cholesterol diets (Heller 1983
). In guinea pigs, similar
to humans, HL activity is lower than LPL activity (Yin et al.
1999
).
A major step in the metabolism of TAG-rich lipoproteins is
hydrolysis of most of their TAG by LPL. A characteristic feature of LPL
is that apoC-II enhances the activity of this enzyme
(Jackson et al. 1977
). Earlier studies postulated that
guinea pigs lacked apoC-II in plasma (Shirai et al. 1983
); however, Wallinder et al. (1979)
demonstrated the presence of an activator for LPL in guinea pig VLDL
that hydrolyzed this lipoprotein rapidly. Later, Andersson et al. (1991)
conclusively demonstrated the presence of
apoC-II in plasma and, with a human apoC-II cDNA probe, they
identified the mRNA species in guinea pig liver. Andersson et al. (1997)
also demonstrated that the low stimulation of LPL by
guinea pig plasma was due to lower absolute concentrations of
apoC-II and the presence of apoC-II containing LDL that had an
inhibitory effect on LPL activity. In contrast, apoC-III has an
important role in the inhibition of LPL. The cDNA of guinea pig
apoC-III was cloned and sequenced (Yin and Olivecrona 1999
). The two most conserved areas of guinea pig apoC-III
were found in residues 16–33 and 50–69, which have been predicted to
form amphipathic helices assumed to play important roles in the
inhibition of LPL.
LPL activity is modulated in guinea pigs by food deprivation
(Sem and Olivecrona 1986
) and by dietary fat being
higher in the dietary regimens that result in lower
concentrations of plasma LDL-C. For example, a diet rich in PUFA
resulted in higher LPL activity compared with a diet rich in SAT
(Cryer et al. 1978
). Similarly, rapeseed oil intake
resulted in lower plasma LDL-C concentrations compared with intake
of either olive oil or palm oil, and lower LDL-C also correlated
with higher LPL activity (Fernandez et al. 1996b
).
LDL.
Various animal models have been used to determine the effects of diet
on lipoprotein concentration and metabolism. Caution must be exercised
in the interpretation of some of these studies because animal species
can vary greatly in the distribution of plasma cholesterol among
lipoproteins and be totally different from humans (Nicolosi 1997
). However, dietary modifications (Kris-Etherton and Yu-Poth 1997
) and drug treatment (Nawrocki et al. 1995
) that alter LDL-C concentrations in humans result in
similar alterations in guinea pigs (Fernandez et al. 1999
). Because guinea pigs carry the majority of their
cholesterol in LDL, this is not a surprising finding. How dietary
modifications and drug treatment affect cholesterol and lipoprotein
metabolism in guinea pigs is discussed later.
Earlier studies by Chapman et al. (1972)
reported
evidence that LDL particles are formed from serum lipoproteins and are
not secreted directly into the plasma pool by the liver. Later,
Abdel-Fattah et al. (1995)
used Triton WR 1339 to
measure the effects of dietary fat saturation on VLDL secretion and
demonstrated that LDL apoB specific radioactivity was unchanged over
time. These results are consistent with the complete blockage of the
conversion of VLDL to LDL and the absence of any direct secretion of
LDL apoB in guinea pigs.
Smaller LDL subfractions have been shown to disappear from circulation
faster in guinea pigs (Fernandez 1995
, Fernandez et al. 1992a
and 1993
, Swinkels et al. 1988
).
Swinkels et al. (1988)
isolated two human LDL
subfractions with densities of 1.023–1.034 and 1.036–1.041 kg/L,
respectively. When they were injected into guinea pigs, a faster
receptor-mediated uptake was observed for the smaller fraction.
Similarly, we have reported faster clearance rates of smaller LDL
induced by PUFA (Fernandez et al. 1992a
and 1993
) or
soluble fiber intake (Fernandez 1995
). In contrast,
cholestyramine and lovastatin treatment, which results in smaller LDL
with significant compositional changes compared with LDL derived from
control guinea pigs, had a slower FCR than did control LDL
(Berglund et al. 1989
, Witztum et al. 1985
).
HDL.
HDL are present in low concentrations in guinea pig plasma
(Fernandez et al. 1994b
, Fernandez and McNamara 1991
, He and Fernandez 1998b
). ApoAI is the
major apolipoprotein in guinea pig HDL (Fernandez and McNamara 1990
, Guo et al. 1977
). Although there are some
differences in composition, guinea pig apoA-I is as potent as human
apoA-I for activating purified LCAT derived from human plasma
(Guo et al. 1977
). Few dietary and drug manipulations
have been shown to alter plasma HDL-C concentrations in guinea
pigs. Intake of 0.33% dietary cholesterol, which is equivalent to
absorbed dietary cholesterol that is 200% of the daily endogenous
cholesterol synthesis in these animals, significantly increased plasma
HDL-C (Lin et al. 1995
). Simvastatin treatment
(Matsunaga et al. 1991
), exercise (McNamara et al. 1993
) and replacement of marginal with adequate intakes of
vitamin C (Montano et al. 1998
) have also been shown to
increase plasma HDL-C concentrations in guinea pigs.
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Effects of dietary factors on cholesterol and lipoprotein metabolism |
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TOPABSTRACTINTRODUCTIONHepatic cholesterol metabolismLipoprotein metabolism and...Effects of dietary factors...Exercise and lipoprotein...Drug treatment studiesGender and hormonal statusDevelopmental studiesAscorbic acid and cardiovascular...AtherosclerosisConclusionsREFERENCES |
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Effects of fatty acids varying in chain length and degree of saturation
on cholesterol and lipoprotein metabolism have been studied in guinea
pigs. Diets high in PUFA [corn oil (CO)] lower plasma LDL-C
concentrations compared with highly SAT palm kernel oil (PK), which is
rich in short-chain fatty acids, or SAT lard, which is rich in
long-chain fatty acids (Abdel-Fattah et al. 1995
,
Fernandez et al. 1992a
and 1993
). Numerous aspects of
lipoprotein metabolism modulated by dietary fatty acids accounted for
the observed changes in plasma LDL-C. For instance, guinea pigs fed
the PK diet had the highest apoB secretion rate (Abdel-Fattah et al. 1995
). The PK group also exhibited the slowest LDL FCR in
vivo. In contrast, guinea pigs fed the CO diet had the fastest LDL FCR
and the highest number of hepatic LDL receptors (Fernandez et al. 1993
). Intake of CO also resulted in the smallest mature
VLDL particle, as measured by electron microscopy (Abdel-Fattah et al. 1998
).
In addition, significant differences in LDL composition have been
observed in guinea pigs fed PK, lard or CO diets, which may have
metabolic implications. Guinea pigs fed SAT have CE-enriched LDL,
which are associated with higher concentrations of LDL-C. In
contrast, guinea pigs fed PUFA diets have small CE-poor LDL
particles with 1.5 times faster LDL turnover in plasma than the larger
LDL from guinea pigs fed SAT fat (Fernandez et al. 1993
).
The effects on hepatic enzyme activity of fatty acids that vary in
saturation and chain length have been evaluated in guinea pigs
(Fernandez and McNamara 1994c
). Guinea pigs fed the PK
diet had the highest plasma LDL-C, followed by those fed palm oil
rich in palmitic acid. CO intake resulted in the lowest plasma
LDL-C concentrations. HMG-CoA reductase activity varied among
groups and was independent of plasma LDL-C. In contrast, ACAT
activity exhibited a positive correlation with plasma LDL-C
concentrations. Two possibilities may account for these correlations:
1) ACAT activity determines rates of incorporation of CE
into VLDL, which is subsequently converted into LDL; and 2)
ACAT activity changes with the rate of hepatic cholesterol influx. In
support of the first possibility, hypercholesterolemic diets are
related to the production of a greater number of larger LDL particles
in African green monkeys (Carr et al. 1992
) and in
guinea pigs (Fernandez et al. 1993
). The greater number
of CE molecules incorporated into newly secreted lipoproteins has been
correlated with increased ACAT and with a greater number of
atherosclerotic lesions in African green monkeys (Carr et al. 1992
). In support of the second possibility, dietary fat
saturation and chain length affect LDL absolute catabolic rate
(Fernandez et al. 1992a
and 1992b
). In a
steady-state condition, LDL flux equals total catabolic rate and,
assuming 80% LDL uptake by the liver, guinea pigs having the greatest
influx (PK-fed animals) also had the highest ACAT activity. Similar
results supporting both theories have been reported for guinea pigs fed
low (2.5%) or high (25%) fat diets (Romero and Fernandez 1996
). A comparison of different parameters of cholesterol and
lipoprotein metabolism, which account for the hypocholesterolemic
effects of PUFA versus SAT diets, is presented in Table 2
.
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), and is in agreement with proposed mechanisms reported in
studies in humans (Cortese et al. 1983
, Turner et al. 1981
). Dietary cholesterol.
Earlier reports of the deleterious effects of dietary cholesterol on
guinea pigs, including hemolytic anemia characterized by accelerated
destruction of the erythrocytes (Puppione et al. 1971
)
and death before considerable plaques developed, precluded the use of
guinea pigs as models of cholesterol and lipoprotein metabolism. In
addition, these reports focused on the appearance of abnormal
lipoproteins and changes in lipid metabolism, which were postulated to
be species specific (Guo et al. 1982
, Meng et al. 1979
, Ostwald and Shannon 1964
, Sarted et
al. 1972). These carefully controlled studies failed to stress
that the amount of cholesterol provided to guinea pigs was 1–2%,
which, based on our careful calculations, is equivalent to 7500–15,000
mg cholesterol/d in humans. Thus, remarks concerning these earlier
studies have to be taken with caution and interpretation of the results
must be made with the consideration that these experiments cannot
possibly have any clinical relevance.
The effects of dietary cholesterol on plasma lipids, lipoprotein
composition, hepatic LDL receptors and HDL metabolism have been
evaluated in guinea pigs fed 0.08, 0.17 or 0.33% dietary cholesterol,
which is equivalent to 600-2500 mg cholesterol/d in humans (Lin et al. 1992, 1994 and 1995
). These concentrations of dietary
cholesterol correspond to an amount of absorbed cholesterol equal to
half, one time and two times the endogenous cholesterol synthesis in
guinea pigs (Lin et al. 1992
). Increasing the
concentration of dietary cholesterol resulted in a dose-dependent
increase in plasma cholesterol associated with the LDL fraction, which
was independent of the dietary fat level. HMG-CoA reductase
activity was significantly down-regulated with an absorbed amount
equivalent to half the endogenous cholesterol synthesis (0.08% dietary
cholesterol) as a first compensatory mechanism. The amount of
cholesterol in the liver was also increased in a dose-dependent
manner as the concentration of dietary cholesterol increased
(Lin et al. 1992
) (Table 1)
. The hepatic LDL receptor
number was measured by incubating increasing concentrations of
125I-LDL with hepatic membranes, and the
number of receptors was decreased as the amount of dietary cholesterol
increased (Lin et al. 1994
).
In addition, guinea pigs can be hyporesponders or hyperresponders to
dietary cholesterol (Fernandez et al. 1990b
),
demonstrating that the responses to dietary cholesterol are highly
individualized, in agreement with clinical studies (McNamara et al. 1987
). Our laboratory has also demonstrated that guinea
pigs present early atherosclerotic development and fatty streak
accumulation after 12 wk when challenged with amounts of dietary
cholesterol in the range of 2000 mg/d consumption by humans (Cos et al. 2000
).
Dietary soluble fiber.
The effectiveness of several types of soluble fiber in reducing plasma
LDL-C concentrations and their specific effects on hepatic
cholesterol and lipoprotein metabolisms have been studied in guinea
pigs. Prickly pear (Opuntia sp.), a plant commonly grown in
Mexico, has been traditionally used for diabetes treatment and as a
hypocholesterolemic agent. Studies were carried out with pectin
isolated from this plant (PPP) to test its potential properties in
lowering plasma cholesterol (Fernandez et al. 1990b,
1992b
and 1994d
). Guinea pigs were fed hypercholesterolemic diets
containing 0, 1 or 2.5% PPP, and plasma lipids and some metabolic
parameters were compared. Plasma LDL-C was 33% lower in guinea
pigs fed the PPP diet compared with the control group. The hepatic
apoB/E receptor was 60% higher in guinea pigs fed the PPP diets, which
was in agreement with the twofold faster receptor-mediated LDL FCR
observed in guinea pigs fed this type of fiber. Although a 46%
reduction in hepatic free cholesterol was observed, HMG-CoA
reductase and ACAT activities were not altered by PPP (Fernandez et al. 1994d
).
In another study, guinea pigs were fed increasing concentrations of
pectin (PE) (0, 2.5, 5, 7.5, 10 and 12.5%) with low (LC, 0.04%) or
high (HC, 0.25%) concentrations of dietary cholesterol
(Fernandez et al. 1994a
). Guinea pigs fed LC had lower
concentrations of plasma LDL-C at 10 and 12.5% PE compared with
controls. In contrast, guinea pigs fed HC exhibited a PE dose–related
decrease in plasma LDL-C and a PE dose–dependent increase in
hepatic LDL receptors. HMG-CoA reductase activity was suppressed by
high dietary cholesterol and only intake of 12.5% PE reversed this
suppression. Guinea pigs fed 12.5% PE with HC diets had a complete
reversal of hyperlipidemia because plasma LDL-C and hepatic
cholesterol concentrations were similar to those of animals fed LC.
In other fiber studies, guinea pigs were fed either GG
(Fernandez et al. 1995e
) or PSY (Fernandez et al. 1995a
). Compared with the control diet, intake of GG resulted
in lower plasma LDL-C, a lower LDL cholesteryl ester-to-protein
ratio, and lower hepatic cholesterol concentrations and ACAT activity,
whereas both HMG-CoA reductase activity and hepatic LDL receptors
were up-regulated (Fernandez et al. 1995d
). Plasma
LDL-C concentrations were 30 and 54% lower and hepatic cholesterol
was reduced an average of 25%, but hepatic HMG-CoA reductase
activity was 120% higher due to PSY intake. In addition, PSY reduced
hepatic ACAT activity and up-regulated LDL receptors and Cyp7
(Fernandez 1995
). Some of the parameters contributing to
the lowering of plasma LDL-C by PSY are presented in Table 3
. Similar to reports in humans (Everson et al. 1992
), PSY
did not affect cholesterol absorption but, rather, induced fecal bile
acid secretion, as suggested by increased hepatic Cyp7 activity
(Fernandez 1995
). In addition, decreased susceptibility
of LDL to oxidation has been reported in guinea pigs fed soluble fiber
(either PE or PSY) compared with guinea pigs fed a control diet
(Vergara-Jimenez et al. 1999
), suggesting that the
smaller CE-poor LDL particles induced by fiber intake are less
susceptible to oxidation. PE and PSY in combination with high fat diets
have also been shown to reduce the apoB secretion rate and to increase
LDL turnover (Vergara-Jimenez et al. 1998
).
|
). This generates a negative balance of hepatic
cholesterol, which results in up-regulation of HMG-CoA
reductase and hepatic apoB/E receptors and down-regulation of ACAT
activity. These alterations in hepatic cholesterol homeostasis are
correlated with decreases in the number of secreted VLDL, faster VLDL
removal from the plasma compartment, decreased conversion of VLDL to
LDL and increases in LDL apoB FCR. In addition, CETP activity is
reduced (Fernandez et al. 1997a
). All of these
alterations in hepatic cholesterol metabolism result in the known
effect of soluble fiber of reducing plasma LDL-C. |
Exercise and lipoprotein metabolism |
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TOPABSTRACTINTRODUCTIONHepatic cholesterol metabolismLipoprotein metabolism and...Effects of dietary factors...Exercise and lipoprotein...Drug treatment studiesGender and hormonal statusDevelopmental studiesAscorbic acid and cardiovascular...AtherosclerosisConclusionsREFERENCES |
|---|
).
|
). Exercise training increases
LPL activity in plasma and in parenchymal cells, suggesting that
exercise has a role in increasing the capacity to clear TAG from
circulation and that LPL is involved in the restoration of muscle TAG
stores reduced by exercise (Oscai et al. 1990
). In
addition, adipose tissue has increased LPL activity after exercise
(Savard and Bouchard 1990
). In agreement with human
studies, exercised guinea pigs have been shown to have greater LPL
activity in both heart and adipose tissue (McNamara et al. 1993
) (Table 4)
.
Significant regression and less progression of coronary atherosclerosis
have been documented for patients treated with diet and exercise
compared with usual-care control patients (Schlierf et al. 1995
). These results were correlated with significant decreases
in plasma LDL-C and TAG in the treated group. We have demonstrated
that plasma lipid changes due to exercise in guinea pigs are similar to
those in humans and that these animals develop atherosclerosis
(Cos et al. 2000
). Based on these observations, the
guinea pig could be an appropriate model to further evaluate the
beneficial effects of exercise in the treatment of coronary heart
disease.
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Drug treatment studies |
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TOPABSTRACTINTRODUCTIONHepatic cholesterol metabolismLipoprotein metabolism and...Effects of dietary factors...Exercise and lipoprotein...Drug treatment studiesGender and hormonal statusDevelopmental studiesAscorbic acid and cardiovascular...AtherosclerosisConclusionsREFERENCES |
|---|
), probucol (Hikada et al. 1992
), ACAT
inhibitors (Krause et al. 1993
), HMG-CoA reductase
inhibitors such as pravastatin (Matsunaga et al. 1994
),
simvastatin (Conde et al. 1999a and 1999b
), lovastatin
(Berglund et al. 1989
, Krause et al. 1994
, Krause and Newton 1991
) and atorvastatin
(Conde et al. 1996, 1999a and 1999b).
Cholestyramine has a potent hypocholesterolemic effect in guinea pigs,
reducing plasma LDL-C concentrations by 55–75% (Fernandez et al. 2000
). Witztum et al. (1985)
measured
cholestyramine effects on LDL size and composition and how these
affected LDL FCR. Their findings suggest that LDL from
cholestyramine-treated guinea pigs, which were smaller in size, had
a slower turnover in plasma than did LDL derived from controls. The
plasma LDL-C lowering by cholestyramine was due to increases in the
LDL receptor in treated guinea pigs, suggesting that compositional
changes in LDL have profound metabolic consequences.
Guinea pigs treated with reductase inhibitors exhibited significant
decreases in plasma LDL-C concentrations (Conde et al. 1996 and 1999a
, Matsunaga et al. 1994
). Most of these
reductase inhibitors have shown that increases in the LDL receptor
represent a major mechanism for the observed hypocholesterolimia
(Berglund et al. 1989
, Conde et al. 1996 and 1999a
, Matsunaga et al. 1994
). Lovastatin
therapy has been shown to increase hepatic LDL receptor activity in
guinea pigs, and the compositional changes induced by treatment with
this reductase inhibitor had a significant effect on the removal of LDL
from plasma (Berglund et al. 1989
). LDL from
lovastatin-treated guinea pigs had a slower LDL turnover compared
with control LDL. When these studies were repeated in subjects with
hyperlipidemia, results similar to those reported for guinea pigs were
found: a decrease in particle affinity for the receptor and increases
in receptor activity (Berglund et al. 1998
). In our
studies with atorvastatin, treatment with this reductase inhibitor
decreased secretion of apoB and increased hepatic apoB/E receptors
(Conde et al. 1996 and 1999a
). Our results are in total
agreement with the reported mechanisms of LDL-C lowering in
hyperlipidemic individuals treated with lovastatin (Berglund et al. 1998
), confirming the suitability of guinea pigs for
mimicking metabolic alterations in plasma lipoproteins induced by drug
treatment.
|
Gender and hormonal status |
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). Studies have shown that
responses to dietary factors may differ between men and women
(Cobb et al. 1993
). In addition, postmenopausal women
have significantly greater risk factors for heart disease due to
increased plasma LDL-C, apoB and TAG concentrations
(Bonithon-Kopp et al. 1990
). Based on these
observations, it is important to have an animal model with
gender-associated responses to diet that are similar to humans. In
addition, this model should present similar detrimental changes in the
lipoprotein profile in the absence of estrogen, as occurs in
postmenopausal women.
Female guinea pigs are more responsive to dietary cholesterol than are
males (Fernandez et al. 1995e
), which is in agreement
with reported observations in humans (Clifton et al. 1995
). In addition, in females, fiber does not alter the
regulatory enzymes of hepatic cholesterol metabolism, as is the case in
males (Fernandez et al. 1995b
). However, a slower apoB
secretion rate and a faster LDL FCR were observed in females as
compared with males (Shen et al. 1998
), indicating
important differences in the gender response to dietary treatments. In
addition, we have observed that female guinea pigs have higher
HDL-C concentrations than do males (Roy et al. 2000
).
In a recent study using male, female and ovariectomized guinea pigs,
several important findings emerged that reinforce the suitability of
ovariectomized guinea pigs to mimic human menopause. Ovariectomized
guinea pigs had higher concentrations of plasma TAG than did either
males or females, higher concentrations of LDL-C and a more
detrimental lipoprotein profile overall (Roy et al. 2000
). In addition, ovariectomized guinea pigs exhibited a
higher susceptibility of LDL to oxidize compared with intact females.
It has been shown that estrogen exerts a protective effect against free
radical formation (Sack et al. 1994
), which might
explain the higher susceptibility of LDL to oxidize in the
ovariectomized guinea pigs. Gender and hormonal status effects on early
atherosclerosis development were also evaluated in guinea pigs. Male
guinea pigs had the greatest fatty streak accumulations in aortas,
followed by ovariectomized animals; females had the least lesion
involvement (P < 0.001) (Cos et al. 2000
).
|
Developmental studies |
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). There is a
significant accumulation of hepatic TAG in the fetus, which is
accounted for by the rate of placental fatty acid transfer
(Bohmer and Havel 1975
).
Bile acid pools are low in most mammals during birth, including guinea
pigs, and reach adult concentrations at the time of weaning (Li et al. 1979
). Feeding 0.25% cholesterol to pregnant
guinea pig dams resulted in a significant decrease in the total bile
acid pool in the neonate, a significant reduction in chenodeoxycholic
acid and an increase in the proportion of bile acids in the neonatal
liver. These are important findings relevant to earlier reports in
which feeding chenodeoxycholic acid to guinea pigs reduced the activity
of Cyp7, suggesting that in this species chenodeoxycholic acid exerts a
feedback inhibition for the synthesis of this bile acid from
cholesterol (Hassan et al. 1983
). In contrast, in rats
the activity of fetal and neonatal Cyp7 was increased after feeding of
1% cholesterol (Naseem et al. 1980
), indicating
important differences between animal models in the handling of
pharmacological doses of dietary cholesterol.
Studies have been carried out to determine prenatal interventions on
maternal and fetal cholesterol homeostasis in guinea pigs. The dams
were fed a nonpurified diet supplemented with 1.1% cholestyramine or
0.25% cholesterol. Maternal hepatic cholesterol synthesis increased
3.5-fold with cholestyramine treatment and decreased 87% with
cholesterol intake (Yount and McNamara 1991
). In
contrast, the fetus had moderate changes in cholesterol synthesis
modulated by time of gestation rather than by dietary manipulations.
These studies indicate that responses to dietary treatment do not vary
between pregnant and nonpregnant dams and that the fetus is relatively
insensitive to maternal dietary or drug treatments.
|
Ascorbic acid and cardiovascular disease |
|---|
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TOPABSTRACTINTRODUCTIONHepatic cholesterol metabolismLipoprotein metabolism and...Effects of dietary factors...Exercise and lipoprotein...Drug treatment studiesGender and hormonal statusDevelopmental studiesAscorbic acid and cardiovascular...AtherosclerosisConclusionsREFERENCES |
|---|
). Epidemiological studies have shown a negative
correlation between intake of vitamin C and plasma cholesterol
concentrations (Cerna and Ginter 1978
). In addition, the
Basel protective study documented that low plasma carotene and
ascorbate concentrations correlated with a higher risk for coronary
heart disease (Gey et al. 1993
). Studies in guinea pigs
have shown that inadequate intake of vitamin C leads to
hypercholesterolemia (Montano et al. 1998
) and
hypertriglyceridemia (Yokota et al. 1981
) and that it
may influence the pathogenesis of atherosclerosis (Ginter 1978
) and possibly increase oxidative stress (Liu and Le 1997
).
Ascorbic acid deficiency is associated with reductions in hepatic
microsomal cytochrome P450, which leads to reductions of Cyp7 activity
(Greene et al. 1985
). In agreement with earlier studies
(Holloway et al. 1981
), a significant reduction was
observed in Cyp7 activity in guinea pigs fed marginal amounts of
vitamin C (Fernandez et al. 1997b
, Holloway et al. 1981
). Reductions in the active form of HMG-CoA
reductase have also been reported (Montano et al. 1998
).
These reductions were correlated with higher concentrations of CE in
guinea pigs fed marginal amounts of vitamin C, suggesting that the
homeostatic response of the liver to maintain the free cholesterol pool
occurs by suppressing cholesterol synthesis and increasing hepatic
cholesterol stores.
A potential mechanism for the increases in plasma LDL-C observed in
cases of an inadequate supply of vitamin C could be the
down-regulation of LDL receptors. Marginal intake of vitamin C has
been related to a lower number of hepatic LDL receptors (Montano et al. 1998
) and slower LDL FCR compared with guinea pigs fed
an adequate amount of vitamin C (Fernandez et al. 1997b
,
Ginter and Jurcovicoca 1977
). Similarly, an increase in
the LDL receptor number was observed in cultured arterial smooth muscle
cells incubated with ascorbate (Auslinkas et al. 1983
).
A consistent finding associated with vitamin C deficiency is elevated
concentrations of plasma TAG. It has been postulated that a deficiency
of vitamin C reduces carnitine synthesis, thus causing more
accumulation of TAG in the liver due to a reduced transport of fatty
acids to the mitochondria for ß-oxidation (Hylse et al.
1978
). This accumulation of TAG results in an increased TAG
secretion rate. Marginal intake of vitamin C results in increased
secretion of apoB VLDL, explaining in part the hypertriglyceridemia
observed in guinea pigs fed marginal concentrations of ascorbic acid
(Montano et al. 1998
). Bobek et al.
(1980)
reported that borderline vitamin C deficiency in guinea
pigs in wk 14–16 resulted in reduced fractional plasma TAG turnover.
In addition, supplementation with ascorbic acid has been reported to
decrease endogenous oxidative damage in guinea pig liver
(Cadenas et al. 1994
), decrease the susceptibility of
LDL to oxidize in the presence of dietary PUFA (Fernandez et al. 1997b
) and reduce lipid peroxide concentrations in kidney,
heart and brain (Kaplan 1995
). These studies suggest a
possible role for this antioxidant in reducing the formation of
oxidized LDL and influencing the uptake of this lipoprotein by the
arterial wall.
|
Atherosclerosis |
|---|
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|---|
). The oxidation of LDL
probably takes place in the arterial wall, where LDL particles are
sequestered from circulating antioxidants (Witztum and Steinberg 1991
). Vazquez et al. (1998)
measured the
oxidation behavior of rat and guinea pig LDL and concluded that guinea
pig LDL composition bears a closer resemblance to that of human LDL. In
a clinical study of patients with familial hyperlipidemia, these
authors observed that human LDL oxidation parameters were more similar
to those of guinea pigs than to those of rats (Vazquez et al. 1997
).
Studies were conducted to assess whether vitamin E affects the
proteoglycan distribution and vascular permeability in the aortas of
cholesterol-fed guinea pigs. Vitamin E preserved the morphological
and functional integrity of the vascular wall and contributed to the
inhibition of atherosclerosis (Qiao et al. 1993
). This
protective effect of vitamin E against in vitro lipid peroxidation was
observed in guinea pigs even in the presence of an optimal intake of
vitamin C (Barja et al. 1996
). Similar to reports in
humans (Esterbauer et al. 1991
), the protective
concentrations of vitamin E against lipid peroxidation were found to be
higher than the concentrations needed to avoid deficiency syndromes.
Liu et al. (1997)
showed a protective effect of vitamin
C against LDL oxidation in cases of impaired vitamin E retention.
Similarly, reduced concentrations of vitamin E as a result of ethanol
intake were raised when dietary vitamin C was provided (Suresh et al. 1999
), demonstrating the protective role of ascorbic
acid in sparing vitamin E and influencing the oxidative process in
plasma.
Other studies in guinea pigs have evaluated the role of ascorbic
acid–deficient diets in atherosclerosis development. Ginter (1978)
reported edema of the vessel wall and vacuolization of
endothelial cells in guinea pigs fed a vitamin C–deficient diet.
Satinder et al. (1987)
found white patchy plaques in the
arch and proximal aorta in two thirds of guinea pigs fed a vitamin
C–deficient diet for 8 wk. Vitamin C deficiency with cholesterol
feeding has also been studied in guinea pigs (Sharma et al. 1988
). After 3 mo, there were significant increases in the
severity of aortic lesions and an additive effect on atherosclerosis
was observed with cholesterol feeding. These studies indicate that,
similar to humans, guinea pigs develop atherosclerosis due to oxidative
damages induced by ascorbic acid deficiency or by the lack of dietary
vitamin E.
The development of early atherosclerosis has been demonstrated in male,
female and ovariectomized guinea pigs fed a hypercholesterolemic diet
for 12 wk (Cos et al. 2000
). Partial substitution of
insoluble by soluble fiber resulted in less lesion development
(Table 5
). Guinea pigs fed the control diet exhibited LDL particles that were
larger and had higher concentrations of CE. A significant correlation
between LDL diameter and fatty streak area (r = 0.56,
P < 0.01) was found, suggesting that in guinea pigs,
similar to African green monkeys, the larger LDL particles induced by
hypercholesterolemic diets are correlated with lesion development
(Carr et al. 1992
, Rudel et al. 1986
). In
the study by Cos et al. (2000)
, there was a clear gender
effect because, independent of the diet, female guinea pigs exhibited
less lesion development than did male or ovariectomized animals (Table 5)
. These results are of great interest because they suggest a
protective effect of estrogen against atherosclerotic lesions.
|
|
Conclusions |
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TOPABSTRACTINTRODUCTIONHepatic cholesterol metabolismLipoprotein metabolism and...Effects of dietary factors...Exercise and lipoprotein...Drug treatment studiesGender and hormonal statusDevelopmental studiesAscorbic acid and cardiovascular...AtherosclerosisConclusionsREFERENCES |
|---|
|
FOOTNOTES |
|---|
-hydroxylase; FC, free cholesterol; FCR, fractional
catabolic rate; GG, guar gum; HC, high concentration of dietary
cholesterol; HDL-C, HDL-cholesterol; HL, hepatic lipase;
HMG-CoA, 3-hydroxy-3-methylglutaryl coenzyme A; LC, low
concentration of dietary cholesterol; LDL-C, LDL cholesterol; LCAT,
lecithin-cholesterol acyltransferase; LPL, lipoprotein lipase;
MONO, monounsaturated fatty acids; PC, phosphatidylcholine; PE, pectin;
PK, palm kernel oil; PPP, prickly pear pectin; PSY, psyllium; PUFA,
polyunsaturated fatty acids; SAT, saturated fatty acids; TAG,
triacylglycerol; VLDL-C, VLDL cholesterol. 
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONHepatic cholesterol metabolismLipoprotein metabolism and...Effects of dietary factors...Exercise and lipoprotein...Drug treatment studiesGender and hormonal statusDevelopmental studiesAscorbic acid and cardiovascular...AtherosclerosisConclusionsREFERENCES |
|---|
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