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ANIMAL PRODUCTS |
Division of Nutritional Biochemistry, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire, LE12 5RD, United Kingdom
Abstract
Sheep adipose tissue explants were maintained in culture for 24 h in the presence of insulin, dexamethasone, or insulin and dexamethasone, and stearoyl-CoA desaturase (SCD) messenger RNA (mRNA) levels and fatty acid synthesis were measured. Insulin increased SCD mRNA levels (P = 0.008) and synthesis of both saturated (P = 0.07) and unsaturated (P < 0.001) fatty acids but had the greatest effect on unsaturated fatty acid synthesis, resulting in the overall production of a greater (P < 0.001) proportion of monounsaturated fat. Dexamethasone, alone, had the opposite effect but actually potentiated the effect of insulin in stimulating SCD expression and both saturated and monounsaturated fatty acid synthesis, without affecting the relative proportions of each. Across adipose tissue depots, the effect of hormones was similar, although the increase in SCD mRNA levels (P = 0.008) and monounsaturated fatty acid synthesis (P < 0.001) was greater in subcutaneous adipose tissue than in the internal (omental and perirenal) depots. These data clearly show that, in ovine adipose tissue, changes in SCD gene expression in response to insulin and dexamethasone are associated with changes in monounsaturated fatty acid synthesis and suggest that it may be possible to develop strategies to manipulate sheep tissues to produce a less-saturated fatty acid profile.
Key Words: Hormones • Monounsaturated Fatty Acids • Saturated Fatty Acids • Sheep • Stearoyl-CoA Desaturase
Introduction
Ruminants in general, and sheep in particular, have high ratios of saturated:monounsaturated fatty acids in their lipids (Christie, 1981
); lamb meat contains approximately 22% palmitic acid, 18% stearic acid, and 32% oleic acid (Rhee, 1992; Enser et al., 1996). In the nonlactating ruminant, adipose tissue is the principle site for de novo fatty acid synthesis, and a major product is palmitic acid, which can further be elongated to stearic acid (Vernon, 1980). Desaturation of stearic acid to oleic acid is catalyzed by stearoyl-CoA desaturase (SCD; Enoch et al., 1976), and, because the rate of elongation is greater than the rate of desaturation (Chang et al., 1992), it is believed that the step catalyzed by SCD is rate-limiting in the production of oleic acid.
In sheep, SCD is produced from a single gene, whose expression is increased when adipose tissue explants from lactating sheep are treated with insulin plus dexamethasone (Ward et al., 1998). Oleic acid content of adipose tissue correlates well with depot-specific expression of ovine SCD (Barber et al., 2000). Thus, increasing the activity of SCD in ovine adipose tissue may significantly improve the nutritional quality of lamb by decreasing the saturated fatty acid content and increasing the oleic acid content. Insulin increases lipogenesis and, although inhibiting lipogensis in the absense of insulin, dexamethasone is required to sustain the effects of insulin for periods in excess of 24 h in cultured sheep adipose tissue (Vernon and Finley, 1988; Vernon et al., 1991; de la Hoz and Vernon, 1993). It is, therefore, of interest to determine whether any changes in oleic acid content occur independently of total fatty acid synthesis. Therefore, the aim of this study was to investigate the hormonal control of SCD gene expression and oleic acid synthesis to determine whether there is any potential for the manipulation of the fatty acid composition of sheep adipose tissue.
Materials and Methods
Culture of Adipose Tissue Explants
All animal studies were conducted under the requirements of the U.K. Animals (Scientific Procedures) Act 1986. Five Mule x Charolais ewes (8 to 10 mo) were used, except for the determination of messenger RNA (mRNA) levels in tissue treated with insulin and dexamethasone, which used two wethers (10 mo). All animals were housed in individual pens in a metabolism unit, and fed a diet containing (as-fed basis) 450 g/kg oats, 225 g/kg barley, 225 g/kg grass meal, and 100 g/kg Nutramol 30 (Rumenco, Burton-on-Trent, U.K.) for at least 1 mo before sampling. Sheep were stunned and exsanguinated, and, using a sterile scalpel, samples of adipose tissue were immediately removed from the subcutaneous (above the base of the tail), perirenal (around the kidneys), and omental (around the rumen) depots and maintained at 40°C in supplemented medium, M199 (Sigma, St. Louis, MO). Tissue was rapidly transferred to a sterile lamina flow cabinet.
Adipose tissue explants were maintained in culture media M199 supplemented with sodium hydrogen carbonate (2.2 g/L), 2 mM sodium acetate, BSA (1 g/L), and the antibiotics penicillin G (20 mg/L), streptomycin (20 mg/L), and gentamycin (50 mg/L) (adapted from Robertson et al., 1982). Tissue was transferred to a sterile petri dish containing supplemented M199, connective tissue was carefully removed, and the remaining tissue chopped into approximately 5-mg pieces. To each treatment dish, containing 25 mL of supplemented M199, an appropriate amount of tissue (a total of 40 mg for measurement of fatty acid synthesis and a total of 1 g for determination of mRNA levels) was transferred, and the plate was preincubated at 39°C for 24 h. After this period, the media were refreshed with supplemented M199 with, or without, hormones and incubated for a further 24 h. As indicated, insulin (bovine) was added at 20 nM and dexamethasone at 10 nM, and all treatments were performed in duplicate.
Determination of Stearoyl-CoA Desaturase (SCD) and Acetyl-CoA Carboxylase (ACC) mRNA Levels
After culture, tissue was washed with prewarmed PBS and total RNA was isolated using the acidified phenol-chloroform-guanidine thiocyanate method (Chomczynski and Sacchi, 1987), and RNA concentrations were determined by measuring absorbance at 260 nm using the LKB Ultrospec (Amersham Biosciences, Piscataway, NJ). A 392-nucleotide NcoI-EcoRV fragment corresponding to nucleotides 622 to 1,014 of the ovine SCD cDNA was subcloned into pGEM-7zf+ (Promega, Madison, WI) and used to generate an antisense transcript. Similarly, a 438-nucleotide EcoR1-BamH1 fragment of ovine acetyl coenzyme A carboxylase (ACC) gene (nucleotides 3,854 to 4,292) was used to produce an RNA probe for ACC. Using the Riboprobe In Vitro Transcription System (Promega), with SP6 RNA polymerase and 
-[32P] UTP for both, and purification through a Sephadex G50 column, ovine SCD and ACC riboprobes were generated. Levels of SCD and ACC mRNA were quantified in 10 µg of total RNA using the Ambion RPAII kit (Ambion Inc., Austin, TX) according to manufacturer’s instructions and protected fragments resolved on a 6% polyacrylamide 7 M urea sequencing gel with Tris/borate/EDTA buffer. All the samples from each animal were run on the same gel and a standard sample was run in triplicate on each gel to which all samples were normalized, allowing comparison between samples run on different gels. Gels were exposed to a phosphor screen (Kodak, Rochester, NY) for 2 h, and the resulting images were scanned using the Molecular Imager FX Pro Plus MultiImager System (Biorad, Hercules, CA) and the intensity of individual bands was obtained using Quantity One image analysis software (Biorad, Hercules, CA).
Measurement of Fatty Acid Synthesis
After incubation, adipose tissue pieces were transferred to an Erlenmeyer flask containing 3 mL of supplemented M199 containing appropriate hormone additions, sodium acetate to give a concentration of 5 mM, and 0.02 MBq/mL 14C-labeled acetate. Flasks were sealed and placed in a shaking water bath at 39°C for 2 h, after which the flasks were removed and placed on ice. All tissue pieces were removed from the flask, blotted dry, weighed and frozen in a solvent-resistant tube at -20°C until analysis. Preliminary studies (data not shown) showed that 14C-labeled acetate incorporation had reached a plateau well before an acetate concentration of 5 mM.
The lipid was extracted by the method of Folch et al. (1957), by which samples were saponified and transmethylated in one step by heating at 90°C for 2.5 h in 5 mL/100 mL sulfuric acid in methanol. Petroleum ether (boiling point range 40°C to 60°C) was used to extract the fatty acid methyl esters (FAME), which was then evaporated to dryness. The rest of the sample was rinsed with petroleum ether, dried, and resuspended in 0.5 mL of petroleum ether. Saturated and monounsaturated fatty acids were separated on silver-impregnated Florisil columns, which were prepared by a method adapted from Wilner (1965) and Anderson and Hollenback (1965). A 50 g/100 mL solution of silver nitrate in distilled water was prepared, 50 mL of the silver nitrate solution was added to 25 g of acid-washed Florisil, and the solution stirred and dried at 95°C. The silver-impregnated Florosil (6 g) was placed into columns and washed with 10 mL of petroleum ether prior to the 0.5-mL sample being loaded. Saturated fatty acids were eluted with 32 mL of 3 mL/100 mL diethyl ether in petroleum ether. Thirty-two millimeters of a diethyl ether:petroleum ether (1:1) solution was then used to elute the monounsaturated fatty acids from the column. Column flow rate was maintained at approximately 3 mL/min. Saturated and monounsaturated fractions were evaporated to dryness, and 8 mL of Insta Fluor scintillant (Perkin Elmer, Boston, MA) was added to each sample before counting for 5 min using a Packard Tri-Carb 19000 CA liquid scintillation analyzer (Packard Instrument Co., Meriden, CT). Results are expressed as nanomoles of acetate incorporated into each fraction per 106 adipocytes per 2 h. The percentage of monounsaturated fatty acid (MUFA) synthesized was the ratio of MUFA to total fatty acid synthesis (saturated fatty acids + MUFA), expressed as a percentage.
Determination of Adipocyte Number
The mass of lipid per gram of adipose tissue was measured following chloroform/methanol extraction (Folch et al., 1957) from a known mass of tissue (approximately 200 mg). Adipocyte volume was determined using the method of Pond et al. (1984), in which five thin slices (<0.25 g) of adipose tissue were placed in a drop of PBS on a microscope slide and the cover slip pressed down lightly to squash the tissue. The tissue was then examined under a microscope (Nikon, Melville, NY/Nikon U.K., Ltd., Kingston upon Thames, Surrey, U.K.) and camera (JVC, Yokohama, Japan), which was linked to a computer with an image capture and measurement software package (Optimas-6, Bothell, WA). The software was used to measure the diameters of 50 individual intact adipocytes, and, as adipocytes were assumed to be spherical, the diameter was used to determine the volume of the cells. Number of adipocytes per gram of adipose tissue was calculated from the mean adipocyte volume, total lipid per gram, and an assumed lipid density of 0.915 g/mL (Taylor et al., 1973; Dawson et al., 1993).
Statistical Analysis
Measurements were made on individual sheep, and data for two or three sheep were combined for analysis using a statistical computer package, Genstat release 6.1. for Windows (Lawes Agricultural Trust, Hertfordshire, U.K.). To determine the effect of treatment on mRNA levels, one-way ANOVA was used, blocked for animal. For comparison of treatments in different adipose tissue depots, two-way ANOVA with error of variation split into between depot and between plates from the same depot was used to compare the effects of treatment between plates, and the differences between depots within sheep. To compare effect of individual treatments, Dunnett’s test (Dunnett, 1955) was used to compare each treatment mean with the control mean within a sheep or depot, or if there was no interaction to compare the treatment mean with the control mean overall.
Results
Hormonal Control of mRNA Levels in Cultured Ovine Adipose Tissue
Treatment with insulin (20 nM) increased the amount of SCD mRNA in all three adipose tissue depots studied, and, although the omental and perirenal adipose tissue depots behaved similarly, the increase in SCD mRNA levels in the subcutaneous depot was greater (P = 0.008; Figure 1). Adipose tissue explants were also treated with insulin, dexamethasone, and the two hormones in combination, and there was an effect of treatment on SCD (P < 0.001) and ACC mRNA levels (P = 0.036), as shown in Figure 2. Again, insulin increased (P < 0.01) the levels of SCD mRNA, whereas dexamethasone, alone, caused a decrease (P < 0.05), compared to control. However, when the hormones were added together, the effects of insulin predominated (P < 0.01). Although there was no effect (P > 0.05) of insulin or dexamethasone, when the hormones were added together the concentration of ACC mRNA was higher (P < 0.05) than the control.
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Saturated and monounsaturated fatty acid synthesis from 14C-labeled acetate in cultured adipose tissue is shown in Table 1. The directions of hormonal effects on saturated and monounsaturated fatty acid synthesis differed between depot, resulting in treatment x fat depot interactions (P = 0.07 and P < 0.001, respectively). Under each condition, the subcutaneous depot continued to synthesize relatively more monounsaturated fatty acids and, therefore, a greater proportion of monounsaturated fat than the two internal depots, which seem to behave in a similar way. Insulin increased (P < 0.01 for subcutaneous and omental adipose tissue depots) saturated and monounsaturated fatty acid synthesis but had a greater effect on the latter, resulting in an increase (P < 0.01 in all depots) in the proportion of monounsaturated fat. Dexamethasone, alone, decreased the synthesis of both types of fatty acid, although not significant at P = 0.05. However, the decrease in unsaturation was numerically greater than the decrease in saturated fatty acid synthesis resulting in a decrease (P < 0.01) in the proportion of monounsaturated fat in the subcutaneous adipose tissue. Dexamethasone was added in combination with insulin and the syntheses of both saturated and monounsaturated fatty acids were potentiated, resulting in an increase (P < 0.01 in all depots) in the proportion of monounsaturated fat synthesized, although this was not numerically different from treatment with insulin alone.
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A strong, positive relationship exists between plasma total cholesterol and the risk of cardiovascular disease (Department of Health, 1994), and a major risk factor for increased plasma cholesterol is excessive levels of fat consumption, particularly saturated fatty acids (Salter and White, 1996). Palmitic acid, a major product of fatty acid synthesis, can be further elongated to stearic acid and then desaturated to oleic acid by the rate-limiting enzyme SCD (Enoch et al, 1976). Increasing the activity of SCD may significantly improve the nutritional quality of lamb by altering the saturated:unsaturated fatty acid content of the tissue.
Effects of hormones on SCD were determined, and it was found that adipose tissue explants SCD mRNA levels were decreased when incubated with dexamethasone but, in both male and female sheep, SCD mRNA levels were increased in the presence of insulin. However, when adipose tissue explants were incubated with insulin in the presence of dexamethasone, there was also an increase in SCD mRNA levels, indicating that the positive insulin effect is greater than the inhibiting effect of dexamethasone. This observation is supported by work done in rodents (Ntambi, 1992; Waters and Ntambi, 1994; Ntambi et al., 1996), which showed a positive effect of insulin on SCD mRNA expression. Previous work in our laboratory also demonstrated that, in the presence of insulin plus dexamethasone, SCD mRNA abundance is increased in adipose tissue explants from lactating sheep (Ward et al., 1998). Treatment with insulin increased and treatment with dexamethasone decreased ACC mRNA concentrations in ovine adipose tissue, although both failed to reach statistical significance (P = 0.05). However, when the hormones were added together, the concentration of ACC mRNA was higher than the control, a similar effect seen with SCD. This agrees with data presented by Travers et al. (1997), in which the culture of explants from the subcutaneous depot of lactating sheep with insulin plus dexamethasone resulted in a sevenfold increase in ACC mRNA. These results imply that, in nonlactating sheep, the effects of insulin on ACC mRNA concentrations are not as great as the effects on SCD mRNA.
Previous work in our laboratory has shown a strong, positive correlation between SCD mRNA and oleic acid content of sheep tissues (unpublished data), and so it was of interest to determine whether the effects of insulin on SCD expression were associated with changes in fatty acid composition. A major aim of the present study was to investigate whether the synthesis of monounsaturated fatty acids could be altered independently of the total synthesis of fatty acids using cultured adipose tissue explants in the presence, or absence, of hormones. This method has been used extensively in both nonruminant and ruminant species to investigate the effects of hormones on total lipogenic capacity of the tissue. However, to our knowledge, it has not been used previously to determine saturated and monounsaturated fatty acid synthesis separately. Methods used separated fatty acids on the basis of the number of double bonds present. Thus, all saturated fatty acids (predominantly palmitic and stearic acids) were isolated together, as were all the monounsaturated fats. By far the most predominant monounsaturate is oleic acid, with much smaller quantities of palmitoleic acid. This is because the preferred substrate of SCD is stearic rather than palmitic acid (Enoch et al., 1976). This is evident from our previous findings, which showed that less than 2% of the total fatty acids found in the adipose tissue depots studied was palmitoleic (Barber et al., 2000). Thus, the results presented can be assumed to approximate the total amount of saturated (palmitic and stearic) and monounsaturated (oleic acid) fat formed.
Studies from both lactating and nonlactating sheep have shown that insulin induces an increase in the rate of lipogenesis. However, although inhibiting lipogenesis in the absence of insulin, dexamethasone is actually required to sustain the effects of insulin for periods in excess of 24 h (Vernon and Finley, 1988; Vernon et al., 1991; de la Hoz and Vernon, 1993). Here, the effect of insulin on lipogenesis was demonstrated by the increase in the synthesis of both monounsaturated and saturated fatty acid. Conversely, dexamethasone, a synthetic glucocorticoid, had the opposite effect and inhibited total fatty acid synthesis. Insulin and dexamethasone in combination potentiated the insulin effects observed, and, although this is in agreement with previously mentioned studies, the mechanism of potentiation is unknown. The magnitude of the effects on the synthesis of monounsaturated and saturated fatty acids were not, however, equal. Insulin stimulated monounsaturated fatty acid synthesis to a greater extent than that of saturated fat, resulting in an increase in the proportion of monounsaturate produced. On the other hand, dexamethasone tended to decrease monounsaturated fatty acid synthesis to a greater extent than saturated fatty acids, thereby decreasing the proportion of monounsaturated fat synthesized. The potentiation of the insulin effect by dexamethasone, however, appeared to effect the production of the two types of fatty acid equally, such that there was no overall change in the proportion of monounsaturated fatty acids produced when compared to insulin alone.
We have previously shown that subcutaneous adipose tissue contains less stearic acid and more oleic acid than the internal adipose tissue depots (Barber et al, 2000). Comparing the controls (no hormone additions), there were no differences in the levels of SCD mRNA between depots; however, as with the fatty acid synthesis data, the results do show that different depots respond differently to insulin treatment. The increase in SCD mRNA concentrations of the omental and perirenal are similar, whereas that in the subcutaneous depot was greater. Such differences in depot expression are supported by work in both sheep (Barber et al., 2000) and cattle (St. John et al., 1991; Chang et al., 1992; Cameron et al., 1994), where it has been shown that SCD mRNA concentrations were highest in subcutaneous adipose tissue. This would suggest that SCD activity is lower in the internal depots, which is supported by the lower proportion of monounsaturated fat synthesized by explants cultured from these depots compared to those from the subcutaneous depot shown here. Although each depot responded to hormones in a similar manner, the proportion of monounsaturated fatty acids was always greater in the subcutaneous depot, suggesting that in addition to hormones, depot factors may also play a role in regulating its synthesis.
Implications
Expression of the ovine stearoyl-CoA desaturase gene is under hormonal control, and is regulated in a depot-specific manner in adipose tissue. In sheep, stearoyl-CoA desaturase influences the desaturation of fatty acids independently of total fat synthesis, and this strongly suggests that there may be potential for specifically increasing the levels of oleic acid in ruminant tissues. This has to be balanced against the recent suggestion that increasing stearoyl-CoA desaturase 1 (SCD1) gene expression may, in obese mice, be causative in increasing adiposity.
Footnotes
1 This work was funded by a Biotechnology and Biological Sciences Research Council studentship. 
2 The authors acknowledge M. C. Barber, M. T. Travers, and R. G. Vernon (Hannah Research Institute, Ayr, KA6 5HL, U.K.) for kindly donating the SCD expression vector, and J. Craigon (University of Nottingham) for statistical advice.
3 Correspondence—phone: 44-115-9516121; fax: 44-115-9516122; e-mail: peter.buttery{at}nottingham.ac.uk.
Received for publication May 8, 2003. Accepted for publication September 5, 2003.
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