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9-Desaturase Activity Impairs Triacylglycerol Secretion in Cultured Chicken Hepatocytes1
Laboratoire de Biochimie, Ecole Nationale Supérieure Agronomique, 35000 Rennes, France
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
FOOTNOTES
LITERATURE CITED
ABSTRACT 
The relationship between endogenous oleic acid produced by hepatic 
9-desaturase and the secretion of VLDL-triglycerides was investigated in a primary culture of chicken hepatocytes. When the fatty acid compositions of the secreted and intracellular triglycerides (TG) (or triacylglycerols) were compared, an imbalance between monoenes and saturated fatty acids was observed, with the secreted TG being significantly more unsaturated than the intracellular TG. The addition of a mixture of cyclopropenic fatty acids (specific inhibitors of fatty acid desaturation) to the culture medium of cells 24 h before measurement of their 9-desaturase activity and TG secretion rate caused a significant impairment of both desaturase activity and TG secretion, without affecting total TG synthesis. However, the addition of oleic acid to the culture medium of cells treated with cyclopropenic fatty acids restored the TG secretion rate. Palmitic acid did not restore the TG secretion rate and linoleic acid partly restored the TG secretion rate. Finally, even in the presence of oleic acid in the culture medium of secreting cells, those which had been treated with cyclopropenic fatty acids had a significantly lower TG secretion rate than nontreated cells. Taken together, these results show that TG secretion is highly dependent on the 9-desaturase activity and suggest that oversecretion of VLDL-TG in chickens and subsequent fattening could originate in a high hepatic 9-desaturation of saturated fatty acids.
9-desaturase,
hepatocyte,
triacylglycerol,
sterculic acid.
INTRODUCTION
Overproduction by the liver of apolipoprotein B (apo B)3-containing lipoproteins, without a comparable increase in their catabolic rates, is responsible for a large percentage of hyperlipidemia (Kissebah et al. 1981
, Teng et al. 1986). Oleic acid increases VLDL secretion in perfused rat liver (Kohout et al. 1971, Salam et al. 1988), rat cultured hepatocytes (Davis and Boogaerts 1982, Lamb et al. 1977), HepG2 cells (Byrne et al. 1991, Dashti and Wolfbauer 1987) and in human hepatocytes (Edelstein et al. 1994). All of these reports have been obtained in vitro after massive amounts (0.5-1 mmol/L) of oleic acid (as nonesterified fatty acid-albuminic complex) had been added exogenously to perfusion or culture media. Oleic acid was the most potent of the fatty acids tested for enhancing triacylglycerol secretion (Davis and Boogaerts 1982, Kohout et al. 1971). In addition, recent studies have demonstrated an effect of oleic acid on apo B secretion at the posttranslational step by protecting newly synthesized apo B against intracellular degradation (Dixon et al. 1991). This in vitro enhancing effect of oleic acid on hepatic triacylglycerol secretion seems contradictory to the hypotriglyceridemic effect of dietary oleic acid, compared with saturated fatty acids or carbohydrate in rats (Nelson et al. 1987). This discrepancy may be explained if we consider that dietary oleic acid does not necessarily increase the availability of nonesterified oleic acid at the very site of intracellular VLDL processing. A better candidate for this role is endogenous oleic acid which arises from 9-desaturase (EC 1.14.99.5) activity in the endoplasmic reticulum of hepatocytes. In other words, hepatic 9-desaturase activity and the subsequent availability of endogenous oleic acid may have an enhancing effect on hepatic TG secretion, similar to that of extracellular preformed oleic acid used in the in vitro experiments cited above, or even a greater effect depending on the 9-desaturase activity.
A first observation was obtained in our laboratory during research carried out in two lines of fowl selected for high or low adiposity (Legrand and Hermier 1992). 9-Desaturase activity was higher in the liver of fat chickens than in lean ones, whereas no significant difference was observed for other lipogenic activities. This result was obtained in experiments using a cellular extract from either fresh liver (Legrand and Hermier 1992) or hepatocytes in primary culture (Legrand and Lemarchal 1992). Higher plasma concentrations of triacylglycerol and VLDL were also found in fat chickens compared with lean birds (Hermier et al. 1984, Legrand and Hermier 1992). Moreover, we observed a correlation between plasma VLDL-triacylglycerol (VLDL-TG) concentration and 9-desaturase activity (Legrand and Hermier 1992). Consequently, we investigated the relationship between 9-desaturase activity of the cells and TG secretion rate to determine whether this relationship was of importance in chickens in general and not only in the fat and lean lines. We first developed a hepatocyte culture system of normal commercial chickens and characterized the 9-desaturase activity (Legrand et al. 1994) and TG secretion (Legrand et al. 1996) in these cells. Then, in the present work, we supplemented the culture medium of cells from normal commercial chickens with a mixture of cyclopropenic acids, mainly sterculic acid, an inhibitor of fatty acid desaturation (Jeffcoat and Pollard 1977, Zoeller and Wood 1985), and showed that inhibiting 9-desaturation impairs TG secretion. Moreover, TG secretion can be restored by the addition of increasing amounts of oleic acid to the culture medium. The role of endogenous oleic acid (produced by the 9-desaturase) as a stimulating factor of VLDL-TG secretion is discussed.
MATERIALS AND METHODS
9-desaturase activity (panel a), oleic acid concentration (panel b) and triglyceride secretion (panel c) in cultured chicken hepatocytes. Cells were plated and cultured in medium containing 1 nmol/L insulin and 1 µmol/L dexamethasone; no fatty acid was added to the medium. For plating, 7% fetal bovine serum (FBS) was added to the medium and no further serum was used after 4 h of culture. Panel a shows 9-desaturase activity expressed as nmol/(mg protein·min); panel b gives the total oleic acid concentration in cultured cells plus culture medium expressed as µmol/mg total cellular protein; and panel c shows the rate of [3H]-glycerol incorporation into secreted triglyceride expressed as nmol/(mg protein·h). The labeled triglyceride secretion was measured at different times of culture using [3H]-glycerol incorporation during 3 h as described in Materials and Methods. In this
figure (panels a, b and c), each value is the mean of three measurements from the same cell preparation ± SEM. This figure is a representative example of four independent cell preparations.
Reagents and chemicals. Bovine serum albumin (fatty acid free) (BSA), HEPES, Williams' medium E (W 4125), insulin (bovine), boron trifluoride, collagenase and dexamethasone were purchased from Sigma (St Louis, MO). Penicillin-streptomycin antibiotic mixture and trypsin solution (2.5 g/L) were from Gibco BRL (Eragny, France). Fetal bovine serum (FBS) was obtained from J. Boy (Reims, France). [1-14C] Stearic acid and [2-3H] glycerol were purchased from Amersham-France (Les Ulis, France). Solvents and other chemicals were purchased from Prolabo (Paris, France) or Merck (Darmstadt, Germany). Falcon Primaria Petri culture dishes (60-mm diameter) were used (AES, Combourg, France). A cyclopropenic fatty acid methyl ester mixture (from Sterculia foetida seed oil) containing sterculic acid [8-(2 octyl 1-cyclopropenyl) octanoic acid] and malvalic acid [7-(2-octyl-1-cyclopropenyl) heptanoic acid] (9:1 wt/wt) was provided by the Laboratoire de Phytochimie, Faculté Saint-Jérôme (Marseille, France). To remove any polyunsaturated fatty acid, the cyclopropenic fatty acid mixture was separated on AgNO3/silica (15 g/100 g) thin layer chromatography plates, using hexane:diethylether (90:10, v/v) for development. The cyclopropenic fatty acid methyl ester mixture was then collected and saponified with 0.5 mol/L KOH in ethanol for 30 min at 70°C, dried and solubilized in BSA containing Williams' medium E for albuminic complex preparation as described below. In the text, the cyclopropenic fatty acid mixture of sterculic and malvalic acid is referred to as "sterculic acid." Preparation of fatty acid albuminic complex. Fatty acids were dissolved at pH 10 in culture medium containing 0.15 mmol/L BSA. The fatty acid:albumin molar ratio was 3.3:1. The pH was then adjusted to 7.35. The final complexed fatty acid concentration was 0.5 mmol/L unless indicated otherwise in the figure legends. Cell culture. Hepatocytes were obtained from 6-wk-old male chickens by collagenase perfusion in situ as previously described (Legrand et al. 1994
). The culture medium (Williams' E) was supplemented with 26 mmol/L NaHCO3; 12.5 mmol/L HEPES; antibiotic mixture: 50000 IU/L penicillin, 50 mg/L streptomycin; 15 µmol/L BSA; 1 µmol/L dexamethasone; and 1 nmol/L insulin. For plating only, the culture medium was supplemented with 7% fetal bovine serum (FBS). After plating, the cells were maintained in a humidified incubator at 37°C under 5% CO2 in air. After 4 h, the plating medium was changed to a serum-free culture medium. The medium was then changed every 24 h thereafter.
9-Desaturase assay.
Culture dishes were rinsed twice with phosphate-buffered saline solution (PBS: 150 mmol/L NaCl, 5 mmol/L Na phosphate, pH 7.4) and cells were harvested with a rubber policeman in PBS. Pooled cell suspensions (6 dishes) were centrifuged at 800 × g for 4 min; the supernatant was discarded and the cell pellet resuspended in 400 µL of 0.25 mol/L sucrose, 0.05 mol/L phosphate buffer solution (pH 7.4), and sonicated at 20 W for 15 s using a vibra cell VC 50 sonicator (Sonics and Materials, Danbury, CT). The sonicated solution was then centrifuged at 10000 × g for 10 min. The postmitochondrial supernatant was collected and used for 9-desaturase assay. An aliquot (80 µL) was stored at 
20°C for protein measurement. All incubations were performed in tubes placed in a shaking water bath at 37°C. Each incubation mixture (1 mL final volume) contained 200 µL of supernatant and (in µmol) phosphate buffer (pH 7.16), 102; MgCl2, 6; ATP, 7.2; coenzyme A, 0.54; NADH, 0.8. The reaction was initiated by the addition of 30 nmol [1-14C] stearic acid (440 GBq/mol) in 3 µL ethanol and stopped 15 min later by adding 1 mL of 2 mol/L KOH in ethanol. Each assay mixture was then heated for 30 min at 70°C. The fatty acids were liberated by acidification, extracted with diethylether and dried. Fatty acid methyl esters were prepared by heating (70°C) for 30 min with a methanol:H2SO4 (10:1, v/v) solution and then extracted with pentane. They were later separated by thin layer chromatography on silver nitrate impregnated silica gel H (15 g/100 g) plates using a mixture of diethylether:hexane (10:90, v/v) for development. Following their visualization by dichlorofluorescein, the spots corresponding to saturated and monoene fractions were scraped off the plates separately, eluted with diethylether and subjected to liquid scintillation counting (Packard Tri-carb 1600TR, Meriden, CT). From the amount of radioactivity found in the monoene fraction vs. the radioactivity recovered from the saturated fraction, the enzyme activity could be determined and expressed as nmol stearic acid converted to oleic acid/(min·mg protein).
: [2-3H] glycerol (specific activity 37 MBq/µmol) was diluted with nonradioactive glycerol in ethanol to a specific radioactivity of 0.37 MBq/µmol. This labeled solution was added to the culture medium (0.37 MBq/culture dish; 0.5 mmol/L total glycerol concentration in the culture medium; 2 mL culture medium per dish) and incubated for 3 h. The medium was then sucked up, centrifuged at 3000 × g for 10 min to remove any cell debris and kept for lipid extraction. The remaining cell monolayer was then incubated with the trypsin solution for 5 min at 37°C on a shaker and the cells detached. The detached cells were collected by centrifugation at 500 × g for 3 min. Lipids from the medium (secreted lipids) and those from the cells (intracellular lipids) were extracted with hexane: isopropanol (3:2, v/v). The corresponding triacylglycerols were isolated by TLC on silica gel H plates, using a mixture of hexane: diethylether: acetic acid (85:15:0.5, v/v/v) for development, collected in diethylether and counted. The amount of glycerol incorporated into cellular or secreted TG as nmol/(mg cell protein·h) was determined by dividing the radioactivity of the cellular or secreted triacylglycerols by the specific radioactivity of the glycerol in the incubation medium. The latter value represents the triacylglycerol secretion rate from the exogenous labeled precursor, and the sum of the two values (total incorporation into both cellular and secreted TG) represents the rate of triacylglycerol net synthesis in our experimental conditions.
Table 1.
Fatty acid composition of the secreted and cellular triglyceride (TG) in cultured chicken hepatocytes1,2,3
9-Desaturase inhibition procedure.
The sterculic acid complex was added to the culture medium after 24 h of culture when refeeding the cells. After 48 h of culture, when refeeding the cells, TG secretion was measured using a [3H] glycerol-containing medium for a 3-h incubation period. This new medium was sterculic acid-free. In these conditions 1) we verified that 9-desaturase was still completely inhibited at least during a 10-h period following the end of the exposure to sterculic acid; and 2) the effect of 0.5 mmol/L oleic acid or other fatty acids on TG secretion could thus be tested during this 3-h period without interfering with sterculic acid in the medium.
Triacylglycerol fatty acid analysis.
Following triglyceride isolation by TLC, fatty acids were liberated through saponification with 2 mol/L KOH and methylated with boron trifluoride. Fatty acid analysis was performed by gas chromatography of the methyl ester derivatives using a bonded silica capillary column (30 m; 0.23 mm i.d.; DB 23; J&W Scientific, Folsom, CA) under isothermal conditions (180°C), with margaric acid (C 17:0) as internal standard.
9-desaturase activity (panel a) and triglyceride (TG) secretion (panel b) in cultured chicken hepatocytes. Cells were plated and cultured in medium containing 1 nmol/L insulin and 1 µmol/L dexamethasone. No fatty acid was added to the medium. For plating, 7% fetal bovine serum (FBS) was added to the medium, and after 4 h of culture, serum was omitted. After 24 h of culture, sterculic acid was added to the culture medium in the form of an albuminic complex. 9-Desaturase activity (panel a) and TG secretion (panel b) were measured after 48 h of culture as described in Materials and Methods. Each point is the mean of three measurements from the same cell preparation ± SEM. This figure is a representative example of four independent cell preparations.
Protein measurement. Protein was determined by a modified Lowry procedure (Bensadoun and Weinstein 1976
), either in the supernatant used for the 9-desaturase assays or in the cells as total cellular protein.
Expression of results.
Values reported are means ± SEM. P values were calculated by using Student's t test for two-group comparisons and the Dunnet test for comparisons to the base-line group (Hochberg and Tamhane 1987). Differences were considered significant at P < 0.05.
RESULTS
9-desaturase activity and TG secretion.
The time course of enzyme activity is presented in Figure 1a showing a low level of activity at the beginning of culture followed by an increase. Peak activity occurred between 20 and 70 h of culture depending on the cell preparation. A decrease in activity was observed in the long term, but the cultured hepatocytes continued to exhibit 9-desaturase activity up to 8 d of culture. The increase in 9-desaturase activity during the first 2 d of culture was confirmed by direct measurement of total oleic acid content (cells plus culture medium) as a function of time (Fig. 1b). It should be kept in mind that, in these experiments, no fatty acid was present in the culture medium, and that oleic acid was thus exclusively produced by 9-desaturase de novo.
|
Table 2.
Inhibition of |
9-desaturase activity and TG secretion.
Typical results showing the influence of increasing amounts of sterculic acid added to the culture medium of a cell preparation 24 h prior to the determination of 9-desaturase activity and TG secretion are presented in Figure 3. More than 80% inhibition of 9-desaturase activity and 90% of TG secretion rate were produced when the culture medium of these cells was provided with 1 mmol/L sterculic acid. Results from four independent cell preparations that had been treated or not (control cells) with 0.5 mmol/L sterculic acid emphasized the clear inhibiting effect of sterculic acid on both 9-desaturase activity and TG secretion (Table 2).
Table 3.
Effect of oleic acid addition to the culture medium of cultured chicken hepatocytes whose
DISCUSSION 
Fig. 4.
Effect of sterculic acid on secreted triglyceride (TG), intracellular TG and net TG synthesis in cultured chicken hepatocytes; (C, control; S, sterculic acid, 0.5 mmol/L). Cells were plated and cultured in medium containing 1 nmol/L insulin and 1 µmol/L dexamethasone. No fatty acid was added to the medium. For plating, 7% fetal bovine serum (FBS) was added to the medium, and after 4 h of culture, serum was omitted. Sterculic acid was added to the culture medium after 24 h of culture in the form of an albuminic complex. After 48 h of culture, secreted TG and cellular TG were determined as described in Materials and Methods. The sum of the two values was calculated and referred to as net synthesis in our experimental conditions. Each value is the mean from seven independent cell preparations ± SEM. *Significantly different from control (P < 0.05).
[View Larger Version of this Image (10K GIF file)]
9-desaturase had been inhibited by sterculic acid.
In cultured cells in which the 9-desaturase had been inhibited by a 24-h exposure to sterculic acid, 0.5 mmol/L oleic acid was added to the culture medium during TG secretion measurement (i.e., the 3-h incubation period with labeled glycerol in a sterculic acid-free medium). Extracellular preformed oleic acid restored TG secretion at variable levels (Table 3). We then confirmed this effect with a dose-response experiment using increasing oleic acid concentrations (Fig. 5). In further experiments in the presence of 0.5 mmol/L oleic acid, we studied the comparative TG secretion rate in cells that had been treated or not with sterculic acid. Results from three independent cell preparations showed that TG secretion was also significantly lower in sterculic acid-treated cells [6.20 ± 0.95 and 3.33 ± 0.77 nmol/(mg·h) in the control and treated cells, respectively].
9-desaturase was inhibited by sterculic acid, on triglyceride (TG) secretion1
Fig. 5.
Effect of oleic acid addition to the culture medium of chicken hepatocytes in which the 9-desaturase had been inhibited by sterculic acid. Cells were plated and cultured in medium containing 1 nmol/L insulin and 1 µmol/L dexamethasone. For plating, 7% fetal bovine serum (FBS) was added to the medium, and after 4 h of culture, serum was omitted. Sterculic acid was added to the culture medium after 24 h of culture in the form of an albuminic complex. After 48 h of culture, TG secretion was measured (by [3H] glycerol incorporation during 3 h) as described in Materials and Methods. A represents the TG secretion measurement after exposure to sterculic acid. B, C and D represent the TG secretion measurement in the presence of oleic acid (B, 0.1 mmol/L; C, 0.5 mmol/L; D, 1 mmol/L), after exposure to sterculic acid. Each value is the mean of three independent cell preparations ± SEM. *Significantly different from the base-line group A (P < 0.05).
[View Larger Version of this Image (28K GIF file)]
Fig. 6.
Effect of addition of different fatty acids to the culture medium of chicken hepatocytes in which the 9-desaturase had been inhibited by sterculic acid. Cells were plated and cultured in medium containing 1 nmol/L insulin and 1 µmol/L dexamethasone. For plating, 7% fetal bovine serum (FBS) was added to the medium, and after 4 h of culture, serum was omitted. Sterculic acid was added to the culture medium after 24 h of culture in the form of an albuminic complex. After 48 h of culture, TG secretion was measured (by [3H] glycerol incorporation during 3 h) as described in Materials and Methods. A represents the TG secretion measurement after exposure to sterculic acid. B, C and D represent the TG secretion measurement in the presence of 0.5 mmol/L palmitic acid, oleic acid and linoleic acid, respectively, after exposure to sterculic acid. Each value is the mean of three independent cell preparations ± SEM. *Significantly different from the base-line group A (P < 0.05).
[View Larger Version of this Image (21K GIF file)]
9-desaturase activity and TG secretion rate. In our chicken hepatocyte culture system, 9-desaturase activity could be induced (Fig. 1a), as confirmed by the time course of oleic acid production (Fig. 1b), in the absence of any exogenous fatty acid supply. The high level of TG secretion and 9-desaturase activity observed in chicken hepatocytes in culture is consistent with the fact that the liver is the main site of de novo fatty acid synthesis in this species (Leveille et al. 1975). To obtain comparative data, we studied briefly the effect of extracellular preformed oleic acid on TG secretion (Fig. 2a) and observed in the chicken hepatocyte a moderate increase (about 30%) which was much less pronounced than the strong stimulating effect already reported in rat hepatocytes or HepG2 cells (Byrne et al. 1991, Dashti and Wolfbauer 1987, Davis and Boogaerts 1982, Edelstein et al. 1994, Kohout et al. 1971, Lamb et al. 1977, Salam et al. 1988).
9-desaturase in VLDL-TG secretion in chicken hepatocytes in primary culture.
9-desaturase activity and of oleic acid production (whole oleic acid content in cells plus medium), as seen in Figure 1. However, the most convincing evidence came from the inhibition experiments using cyclopropenic fatty acid-treated cells. In these cells, both 9-desaturase activity and TG secretion were almost totally impaired (Fig. 3, Table 2). Moreover, the 9-desaturase inhibition by sterculic acid did not affect net (i.e., total) TG synthesis, but only impaired TG secretion, as seen in Figure 4. If we consider this figure and keep in mind that the corresponding cells were cultured without any fatty acid in the medium, it is apparent that endogenous oleic acid favors the output of fatty acids which have been synthesized de novo in the cells and then incorporated into the secreted triglycerides. Finally, the causal relationship between 9-desaturase activity and TG secretion in our system was demonstrated because the addition of oleic acid to the culture medium of the sterculic acid-treated cells restored TG secretion in a dose-dependent manner (see Table 3 and Fig. 5). By contrast, palmitic acid did not restore TG secretion (Fig. 6), suggesting that the availability of unsaturated fatty acids is necessary for TG secretion by hepatocytes. Because linoleic acid tended to restore TG secretion, oleic acid may not be strictly required if "unsaturation" is provided by other long-chain unsaturated fatty acids. This is in agreement with the observation that several long-chain unsaturated fatty acids enhance hepatic TG secretion in vitro (Kohout et al. 1971). Nevertheless, because most of the polyunsaturated fatty acids are essential, or derive from essential fatty acids, and are found mainly in the phospholipids, 9-desaturation is of major importance in vivo to provide "unsaturation" for TG secretion.
9-desaturase activity by sterculic acid depresses TG secretion as well. This is in keeping with our previous reports showing that, in fat and lean lines of chickens which did not differ in food intake, fat chickens with high levels of plasma VLDL-TG exhibited a higher hepatic 9-desaturase activity than lean chickens (Legrand and Hermier 1992).
) and the Zucker rat (fa/fa) (Wang et al. 1989). In the latter model, a higher hepatic 9-desaturase activity has also been independently reported (Wahle and Radcliffe 1977).
). Second, the presence of double bonds modifies the physical state of the fatty acids (by lowering their melting point) and may thus "facilitate" their incorporation into triglycerides and subsequently into VLDL as previously suggested by Jeffcoat (1979). Our observation that secreted TG are more monounsaturated than cellular TG supports this hypothesis. More precisely, the affinity of diacylglycerol acyltransferase (DGAT) for acyl-CoA has been recently investigated (Stals et al. 1994), and the involvement of CDP choline as a key regulator of the diacylglycerol branchpoint has also been suggested (Lamb et al. 1993).
9-desaturase activity, as has already been suggested by previous research in this laboratory (Legrand and Hermier 1992). Endogenous oleic acid arising from the desaturation of saturated fatty acids particularly enhances the secretion of those TG that are synthesized de novo from carbohydrates, as was apparent in the experiments with cells cultured in the absence of extracellular preformed oleic acid in the culture medium. In the presence of extracellular preformed oleic acid, the TG secretion remained highly dependent on hepatic 9-desaturase activity as well. Thus, oversecretion of VLDL and subsequent hyperlipidemia and fattening in birds may originate mainly from high 9-desaturase activity in the liver.
FOOTNOTES
Manuscript received 8 February 1996. Initial reviews completed 8 May 1996. Revision accepted 16 October 1996.
LITERATURE CITED
9 desaturation and plasma VLDL level in genetically lean and fat chickens.
Int. J. Obes.
1992;
16:289-294
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