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INRA, Unité de Nutrition Lipidique, BP 86510-21605 Dijon, France and * INRA, Laboratoire de Recherche sur les Arômes, 21000 Dijon, France
3To whom correspondence should be addressed .
 |
ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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-linolenic acid representing 0.2 g/100 g of the
experimental diet were incorporated into liver, platelets, aorta and
heart, at the expense of their cis homologue and of
18:2(n-6). The major isomer,
9c,12c,15t-18:3, was also
metabolized to
5c,8c,11c,14c,17t-20:5
and to an unknown compound, found in liver, platelets and aorta, which
has been identified tentatively as
7c,10c,13c,16c,19t-22:5.
The greater 20:4(n-6)/18:2(n-6) ratio in the liver, platelets and heart
of the experimental group than the control group indicated an
enhancement of desaturation activities. This induced a higher content
of long-chain (n-6) fatty acids in the experimental group. Platelet
aggregation tended to be slightly higher (P = 0.065) in the experimental group. We conclude that 0.2 g of
trans isomers of -linolenic acid per 100 g of diet
was sufficient to be incorporated and metabolized, thus altering the
fatty acid profile of rat tissues.
KEY WORDS: • dietary trans -linolenic acid • trans 22:5 • platelet aggregation • rats
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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,
Engler et al. 1991
). These data suggested a limited
acceptor pool in the 2-position of phospholipids (Brokerhoof 1967
). It was also shown that -linolenic acid inhibits the
conversion of 18:2(n-6) to 20:4(n-6) (Engler et al. 1991
) and enhances susceptibility to lipid peroxidation
(L’Abbé et al. 1991
).
In rats fed a diet enriched with 10% linseed oil heated to produce
geometrical isomers of -linolenic acid,
9c,12c,15t-18:3
(18:3
15t),4
9t,12c,15c-18:3 (18:39t)
and 9t,12c,15t-18:3
(18:39t,15t) were elongated and desaturated to
form
5c,8c,11c,14c,17t-20:5
(20:517t) (Blond et al. 1990
,
Grandgirard et al. 1989
),
5c,8c,11t,14c,17c-20:5
(20:511t) and
5c,8c,11t,14c,17t-20:5
(20:511t,17t) (Chardigny et al. 1996b
). In vitro, these isomers of eicosapentaenoic acid
inhibit platelet aggregation by decreasing the synthesis of thromboxane
A2 and hydroxyheptadecatrienoic acid and
increasing that of 12-hydroxyeicosatetraenoic acid (Loï et al. 1998a
).
In human tissues, the major geometrical isomer of 18:3(n-3), i.e.,
18:315t, was found in serum (Wolff 1995a
), platelets
(Brétillon 1998
) and plasma
(Sébédio et al. 1999
). Its metabolite,
20:517t, has been detected in platelets
(Brétillon 1998
, Chardigny et al. 1993
, Sébédio et al. 1999
); however,
it is more difficult to interpret the results of studies of the effects
of trans fatty acids on platelet aggregation in humans
compared with rats because human food also contains these geometrical
isomers. Indeed, because trans polyunsaturated fatty acids
are formed during hydrogenation or heat treatment of vegetable oils,
such as in deodorization (Ackman and Hooper 1974
) or
deep frying (Grandgirard et al. 1984
,
Sébédio et al. 1987
and 1988
), they are
subsequently present in human foods, including margarine or low energy
spreads (Wolff and Sébédio 1991
), refined
oils (Chardigny et al. 1996a
, Wolff 1992
), frying oils (Sébédio et al. 1987
) and infant formulas (Chardigny et al. 1996c
, O’Keefe et al. 1994
, Ratnayake et al. 1997
). Consumption of trans fatty acids in
France was determined to be 2.8 g/d (Wolff 1995b
) of
which 5.9% (165 mg/d) was trans-18:3(n-3).
In this study, rats were fed a more realistic diet than in previous
studies (Blond et al. 1990
, Chardigny et al. 1996b
, Croft et al. 1984
, Engler et al. 1991
, Grandgirard et al. 1998
and 1989
). The
diet contained 50 g/kg (11.6% of total energy) of different canola
oils varying in their 18:3(n-3) trans isomers. The
isomerized oil contained 4% trans 18:3(n-3), which is close
to the amount found in several human food items (Wolff 1992
). This work was performed to study the effects of oil
isomerization (trans-18:3 compared with cis-18:3)
on lipid metabolism and its physiologic consequences such as platelet
aggregation. Therefore, the diets used, which contained either normal
or isomerized canola oil, differed in both their trans
18:3(n-3) as well as in the -linolenic acid concentrations.
Incorporation and metabolism of trans isomers of (n-3) fatty
acids were evaluated in liver, the organ most involved in fatty acid
conversions, and in platelets, heart and aorta, which are organs
involved in cardiovascular diseases. Furthermore, the effects of the
diets on platelet aggregation were also examined.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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), and vitamin E was added to the HT oil to offset the loss during
heating.
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All animal procedures were conducted according to French regulations
(authorization A21200 and 3273). Two groups of weanling male Wistar
rats (n = 16/group) (Centre d’élevage
Dépré, Saint Doulchard, France) consumed freely for 8 wk
diets containing 50 g/kg (11.6% of total energy) of the LT or the HT
oil (Table 2
). The rats were housed individually in stainless steel cages in a room
with conditioned air at 21 ± 1°C (humidity 55 ± 10%,
12-h light:dark cycle).
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Rats were anesthetized under ether vapor. Venous blood was collected into tubes containing an anticoagulant (citric acid, 0.08 g/L; trisodium citrate, 0.22 g/L; D-glucose, 0.245 g/L with 1 volume:9 volumes of blood). The heart, liver and aorta were dissected, blotted on filter paper and weighed.
Platelet lipids and aggregation.
Blood platelets were isolated daily over 4 d from 4 rats from each
group according to the method of Lagarde et al. (1980)
with slight modifications (Berdeaux et al. 1996
).
Isolated platelets were resuspended in Tyrode-HEPES (pH 7.35)
buffer. Lipids from an aliquot of these platelet suspensions were
extracted according to the method of Folch et al. (1957)
.
Platelet suspensions of each group were pooled and diluted to
108 platelets/L. Platelet aggregation was studied by the
turbidimetric method of Born (1962)
. Platelet suspension
(250 µL) was placed in a glass turbidity tube and
warmed at 37°C for 1.5 min in an aggregometer (Coulter 540 VS,
Coultronics, Margency, France). A small magnetic stirring bar was
added. The 100% transmission (Tyrode-HEPES buffer) and the 0%
transmission (platelet suspension) were adjusted. After 30 s, 2
µL of a calcium chloride solution was added to obtain
a final calcium concentration of 0.5 mmol/L. After 30 s, 4
µg of collagen was added and aggregation was followed
for 4 min.
Aorta and endothelial cells.
Immediately after dissection, aortae were perfused with a saline
solution (Hank’s Balanced Salt Solution) containing heparin (25,000
IU/L, Sigma, L’Isle d’Abeau, France), antibiotics
(penicillin, streptomycin at 1%, Sigma) and antifungicidal agent (1%
Amphotericin B, Polylabo, Strasbourg, France) to remove traces of
blood. Endothelial cells were isolated according to the method of
Battle et al. (1994)
. Briefly, the adventicia was
isolated from the media and the intima. These two remaining tunicae
were then cut into rings, which were treated for 15 min at 37°C, with
gentle shaking, with a collagenase solution (0.002 g/L in PBS
containing 0.011 mmol of glucose/L). After filtration on sterile gauze,
cells were sedimented (1000 x g for 5 min) and
plated in complete Dulbecco’s modified Eagle’s medium (Battle et al. 1994
) on 25 cm2 tissue culture dishes
(Falcon, Becton Dickinson, Pont de Claix, France), at 37°C,
under an atmosphere containing 5% CO2 and 95% moisture.
After 40 min, cells were rinsed. This step allowed us to purify the
culture by 95% because muscle cells require a longer time than
endothelial cells to attach to the dish. We obtained 
3.104 cells per dish, viable at 95%.
The remaining media and adventicia were ground manually with a Potter
homogenizer. Lipids from this homogenized solution and from isolated
endothelial cells were extracted according to the method of
Folch et al. (1957)
.
Heart and liver.
The liver and heart were cut into small pieces and homogenized with an
Ultra-Turrax T25 (IKA Laboratory, Stauffen, Germany) homogenizer.
Lipids were extracted according to the method of Folch et al. (1957)
.
Fatty acid analysis.
We analyzed fatty acids in phospholipids because of the role of
phospholipids in cell structures and the effect of phospholipid fatty
acids on platelet aggregation. Phospholipids of platelets, aorta, heart
and liver were separated from neutral lipids by the method of
Juanéda and Rocquelin (1985)
before fatty acid
analysis, whereas total lipids from endothelial cells were analyzed.
These different lipid classes were converted to methyl esters
(Morrison and Smith 1964
) and analyzed by gas
chromatography (GC) (BPX70, 50 m x 0.33 mm i.d., film thickness
0.25 µm, SGE, Melbourne, Australia) using a
Hewlett-Packard (Palo Alto, CA) 5890 series II gas
chromatograph, fitted with a splitless injector and a flame
ionization detector. Both were maintained at 250°C. The oven
temperature was programmed to increase from 60 to 180°C (20°C/min)
for 40 min and from 180 to 220°C (20°C/min) and then was held at
220°C until completion of the analysis.
After GC analysis, the remaining phospholipid fatty acid methyl esters
(FAME) were separated by HPLC (column: nucleosil C18, 5
µm, 250 x 4.6, mobile phase = acetonitrile,
1 mL/min) to obtain the fraction containing the 20:5 + 22:6 FAME [=
F1, retention volume (Rv) = 6 mL] and the fraction containing the
20:4 + 22:5 FAME (= F2, Rv = 7.2 mL) from the F3 fraction (Rv
= 8.7 mL) containing the 22:4 FAME. F2 was then divided into two
samples. One was converted into dimethyloxazoline derivatives (DMOX)
(Dobson and Christie 1996
) and injected onto an HP5
wall-coated capillary column (30 m x 0.25 mm i.d., film
thickness 0.25 µm) interfaced with a MSD5970
quadrupole mass spectrometer (Hewlett- Packard). The oven temperature
was programmed to change from 50 to 240°C at a rate of 20°C/min and
held at 240°C until completion of the analysis. The second sample of
F2 was analyzed by gas chromatography (HP5890 gas chromatograph, BPX70,
30 m x 0.25 mm i.d., film thickness 0.25 µm,
SGE, Melbourne, Australia) coupled with Fourier transform infrared
spectroscopy (FTIR) (FTS 60A) and fitted with a splitless injector
maintained at 250°C. The oven temperature was programmed to increase
from 60 to 200°C (20°C/min).
Statistical analysis.
Results are expressed as means ± SD Comparisons between the low-trans and high-trans groups were made by an ANOVA (NCSS 6.0 statistical package, Kaysville, UT). Differences were considered significant at P < 0.05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The growth rate of rats in the two groups did not differ. Final body weights (LT, 383 ± 23 g; HT, 384 ± 31) and liver weights (LT, 10.9 ± 1.1 g; HT, 11.2 ± 1.0 g) also did not differ.
Incorporation and metabolism of 18:3 isomers.
The major isomers incorporated into liver, platelets, heart and aorta
were 18:315t and the 18:39t (Table 3
). Small amounts of 18:39t,15t and
18:312t also were detected in platelet phospholipids and
in heart or liver phospholipids, respectively. No
trans-18:3(n-3) was detected in endothelial cells of the HT
group (data not shown), probably because we obtained an insufficient
number of endothelial cells with which to detect
trans-18:3(n-3).
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15t, i.e.,
20:517t, was detected in liver, platelets, aorta and
heart phospholipids. Furthermore, an unidentified compound with a GC
retention time close to that of 22:5(n-3) was also detected in liver,
platelets and aorta phospholipids. This compound has been designated Y
(Grandgirard et al. 1989
and 1998
). The mass spectrum of
the DMOX derivative of this molecule showed that it is a 22:5(n-3)
(Fig. 1A
), with a characteristic fragmentation and a molecular ion of 383
m/z. But this analysis did not permit us to
verify the presence of trans double bond(s). Therefore,
compound Y was also analyzed by GC-FTIR. An absorbance at 972.7
cm-1 (for 
= CH)
in the Y spectrum revealed that it has trans ethylenic
bond(s) (Fig. 1B
).
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Greater levels of -linolenic acid and its desaturation and
elongation metabolites, mainly 20:5(n-3), were detected in rats fed the
LT diet than in those fed the HT diet (Table 3)
. In contrast, less
18:2(n-6) (significant in liver and platelets) and more 20:4(n-6)
(platelets and heart), 22:4(n-6) (liver, platelets and heart) and
22:5(n-6) (liver and heart) were observed in the HT group than in the
LT group. The (n-6) fatty acid profile in aorta was not affected,
whereas it was altered greatly in heart phospholipids by diet
treatment.
Platelet aggregation.
Table 4
presents the platelet aggregation results in response to 4
µg of collagen, and an example of platelet aggregation
curves is illustrated in Figure 2
. The platelet response tended to be higher (P = 0.065)
in the HT group than in the LT group.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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-linolenic geometrical isomers.
In contrast to previous studies in which rats were fed a diet enriched
with 10% heated linseed oil containing 28.6% of total fatty acids as
trans 18:3 (Blond et al. 1990
,
Chardigny et al. 1996b
, Grandgirard et al. 1989
and 1998
), this study was performed under more realistic
nutritional conditions in which the HT oil contained 4.1%
trans-18:3 and represented 50 g/kg dry diet. Moreover, this
work was performed to study the effects of oil isomerization, hence
those of trans-18:3 compared with cis-18:3, on
lipid metabolism and physiologic consequences such as platelet
aggregation. Therefore, the diets used, containing either normal or
isomerized canola oil, differed in both their
trans-18:3(n-3) as well as in the -linolenic acid
concentrations. This was sufficient to observe the incorporation and
metabolism of geometrical isomers of -linolenic acid. As seen in
Table 3
, 18:315t (0.10% of the diet) and
18:39t (0.08% of the diet) were the major isomers
incorporated, but only the 18:315t was metabolized into a
20:5 isomer, 20:517t. This is probably due to its
cis ethylenic bond at the 9 position, which facilitates
6 desaturation (Brenner 1971
). These results are not
consistent with a previous study (Chardigny et al. 1996b
) in which 20:511t and 20:5
11t,17t were detected in liver of rats fed
10% heated oil; however, even in that study, 20:517t was
also the major 20:5 isomer.
In a study in which 9-wk-old rats were fed 10% heated linseed
oil-enriched diet for 3 wk, Grandgirard et al. (1989
and 1998)
detected other longer 18:315t metabolites
in the liver lipids, an unknown compound designated Y, also observed in
rat platelets by Blond et al. (1990)
, and
22:619t. Under our nutritional conditions, this 22:6
isomer was not detected whereas Y was detected. Furthermore, Y has been
also detected in aorta and its proportion was greater in platelets and
aorta than in liver. It was not detected in heart phospholipids.
The mass and GC-FTIR spectra of the DMOX of compound Y indicated
that it is a 22:5(n-3) metabolite with one or several trans
double bonds. Furthermore, in previous studies (Blond et al. 1990
, Grandgirard et al. 1989
), Y was suspected
to be an intermediary fatty acid between the 20:517t and
the 22:619t, and it has been detected in bovine
aortic endothelial cells incubated for 24 h with
20:517t-enriched medium (Loï et al. 1998b
). Consequently, Y is probably
7c,10c,13c,16c,19t-22:5,
i.e., an elongation product of 20:517t. In this study,
the lack of 22:619t suggests that the conversion of
22:519t is limited by either too small an amount of
substrate or a limitation at the level of elongase because
trans-24:5 was not detected.
Influence on fatty acid composition.
Bourre et al. (1989)
reported that a dietary deficiency
in 18:3(n-3) (<0.4% of total dietary energy) decreased 22:6(n-3) and
increased 22:5(n-6) in tissues. In this study, these variations were
observed but were not due to an 18:3(n-3) deficiency because it
represented 1.29 and 0.81% of the total dietary energy in the LT and
HT groups, respectively. The lower content of 22:6(n-3), 20:5(n-3) and
22:5(n-3) found in the HT group compared with LT group may be explained
by the fact that 18:315t competes with its cis
homologue for the desaturation and elongation steps (Chardigny et al. 1997
), and in this manner is converted into
20:517t and Y.
In contrast, there was an increase in long-chain (n-6) fatty acids
in rats fed the HT diet because of a stimulated activity of
desaturation, probably induced by trans 18:3 (Blond et al. 1990
). The desaturation stimulation was indicated by an
increase in the 20:4(n-6)/18:2(n-6) ratio. Indeed, in this study, the
20:4(n-6) content was either unaffected (liver, aorta) or increased
(platelets, heart) and there was a lower amount of 18:2(n-6) in the HT
group (liver and platelets) than in the LT group. This was probably due
to incorporation of 18:315t at the expense of 18:2 as a
result of its analogous chemical structure because the trans
ethylenic bond is perceived as a single bond by the enzymatic system of
acylation (Wolff et al. 1993
). It is also of interest to
observe that the 18:3(n-3) isomer incorporation affected primarily the
heart phospholipid composition.
Platelet aggregation.
Figure 2
shows that platelets from rats fed the HT diet stimulated with
4 µg of collagen tended to aggregate more than those from
rats fed the LT diet, but no significant difference was observed. This
tendency should have been due only to the increase in 20:4(n-6) because
20:5(n-3) was replaced by 20:517t, which is as
antiaggregant as its cis homologue (Loï et al. 1998a
). Similarly, in humans that consumed a high
polyunsaturated trans fatty acid diet, platelet aggregation
did not differ from the control group (Brétillon 1998
).
In conclusion, under our experimental conditions, in which
trans-18:3 consumption more closely reflected human dietary
conditions than previous studies, geometrical isomers of -linolenic
acid representing 0.2% of the diet were incorporated into liver,
platelets, heart and aorta of rats fed for 8 wk and at the expense of
their cis homologue and of 18:2(n-6). The major isomer
incorporated into tissues was 18:315t. It was also
metabolized to 20:517t and to a compound designated Y,
tentatively identified as
7c,10c,13c,16c,19t-22:5.
This metabolite was found in liver, platelets and aorta phospholipids
but not in the heart. In contrast, 22:619t
(Grandgirard et al. 1989
and 1998
) was not detected,
suggesting a limitation in the conversion of 22:519t for
the following two reasons: 1) trans 24:5 was not
detected and 2) the occurrence of 22:619t
requires a high trans-18:3 intake, which was not the case
under these nutritional conditions. Furthermore, the HT diet increased
tissue long-chain (n-6) fatty acids, mainly in heart, and platelet
aggregation tended to be slightly higher (P = 0.065)
than in rats fed the control diet.
Further studies are required to assess the effect of trans-18:3 (n-3) metabolites on eicosanoid production in different tissues, including aorta, to estimate the consequences of such incorporation on hemostasis.
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FOOTNOTES |
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2 Funded by a fellowship from INRA and the region of Burgundy (France).
4 Abbreviations used: 18:315t: 9cis,12cis,15trans-18:3; 18:39t: 9trans,12cis,15cis-18:3; 18:312t: 9cis,12trans,15cis-18:3; 18:39t,15t: 9trans,12cis,15trans-18:3; 20:517t: 5cis,8cis,11cis,14cis,17trans,–20:5; 20:511t: 5cis,8cis,11trans,14cis,17cis,–20:5; 20:511t17t: 5cis,8cis,11trans,14cis,17trans,–20:5; 22:619t: 4cis,7cis,10cis,13cis,16cis,19trans-22:6; DMOX, dimethyloxazoline derivatives; FAME, fatty acid methyl esters; GC-FTIR, gas chromatography-Fourier transform infrared spectroscopy; HT, high trans; LT, low trans; Rv, retention volume.
Manuscript received October 19, 1999. Initial review completed January 31, 2000. Revision accepted May 31, 2000.
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