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The Journal of Nutrition Vol. 128 No. 3 March 1998,
pp. 512-519
,, 3, and
* Institute of Experimental Endocrinology, Slovak Academy of Sciences, 83306 Bratislava, Slovakia; INSERM U. 338, F-67084 Strasbourg, France; and ** INSERM U. 311, F-67065 Strasbourg, France
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ABSTRACT |
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Abstract
Introduction
Methods
Results
Discussion
References
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The influence of dietary (n-3) compared with (n-6) polyunsatured fatty acids (PUFA) on the lipid composition and metabolism of adipocytes was evaluated in rats over a period of 1 week. Isocaloric diets comprised 16.3 g/100 g protein, 53.8 g/100 g carbohydrate and 21.4 g/100 g lipids, the latter containing either (n-3) PUFA (32.4 mol/100 mol) or (n-6) PUFA (37.8 mol/100 mol) but having identical contents of saturated, monounsaturated and total unsaturated fatty acids and identical polyunsaturated to saturated fatty acid ratios and double bond indexes. Despite comparable food intake, significantly smaller body weight increments and adipocyte size were observed in rats of the (n-3) diet group after feeding for 1 wk. Rats fed the (n-3) diet also had significantly lower concentrations of serum triglycerides, cholesterol and insulin compared with those fed the (n-6) diet, although levels of serum glucose and free fatty acids did not differ in the two dietary groups. In the (n-6) diet group, the (n-6) and (n-3) PUFA contents of plasma triglycerides, free fatty acids and phospholipids were 30-60% higher and 60-80% lower, respectively, than in the (n-3) diet group, whereas adipocyte plasma membrane phospholipids showed a significantly higher unsaturated to saturated fatty acid ratio and greater fluidity. Glycerol release in response to noradrenaline was significantly higher in the adipocytes of rats fed the (n-3) diet, whereas the antilipolytic effect of insulin generally did not differ in the two groups. Finally, insulin stimulated the transport of glucose and its incorporation into fatty acids to a lesser extent in adipocytes of (n-3) diet fed rats compared with (n-6) diet fed rats. This reduction in the metabolic effects of insulin in rats fed a (n-3) diet for 1 wk could be related to smaller numbers and a lower binding capacity of the insulin receptors on adipocytes and/or to a lesser degree of phosphorylation of the 95 kDa beta subunit of the receptor. In conclusion, dietary intake for 1 wk of (n-3) rather than (n-6) PUFA is sufficient to induce significant differences in the lipid composition and metabolic responses to insulin of rat adipocytes.
KEY WORDS: rats · adipocytes · polyunsaturated fatty acids · insulin · insulin receptors
Adipose tissue serves as the main fuel and energy supply for the whole body, and its metabolic activity is the main contributor to the development of obesity, followed by or concomitant with insulin resistance and cardiovascular disease. The fatty acid profile of adipocytes is determined by the composition of the dietary lipids (Field and Clandinin 1984 Animals and experimental diets.
Male Wistar rats (Centre de Neurochimie du CNRS, Strasbourg, France) (180-230 g initial body mass) had free access to the experimental diets for 1 wk. Diets were prepared by adding either sunflower oil [(n-6) rich diet] or MaxEpa oil (R. P. Scherer, Benheim, France) [(n-3) rich diet] to the standard powdered diet A 04 (UAR, Villemoisson, France). The final basic composition was (g/100 g): 16.3 protein (vegetable and fish origin), 21.4 lipids, 53.8 carbohydrate, 4.8 cellulose and minerals (mix UAR 205)5 and 3.7 vitamins (mix UAR 200)5. Average concentrations of other nutrients were calculated from the manufacturer's data as follows: DL-methionine 300 mg/100 g, choline 500 mg/100 g,
Isolation of adipocytes and plasma membranes.
Adipocytes were isolated from epididymal fat tissue by collagenase digestion (collagenase Type II, 2 mg/g of adipose tissue, Sigma, St. Louis, MO) according to Rodbell (1964) Extraction and analysis of lipids.
Total lipid extracts (from diets, serum and adipocytes) were obtained by the method of Folch et al. (1957) Adipocyte membrane fluidity.
Adipocyte membrane fluidity was estimated by steady-state fluorescence polarization with the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH) and an Aminco SPF 500 spectrofluorimeter. Results are expressed as fluorescence anisotropy (Lentz 1989 Lipolytic studies.
Adipocytes were incubated for 1 h at 37°C in Krebs-Ringer bicarbonate buffer pH 7.4 containing 40 g/L bovine serum albumin (BSA; A 6003, essentially fatty acid-free, Sigma) and 5 mmol/L glucose. Lipolytic activity was determined by glycerol release (enzyme assay kit, Boehringer, Mannheim, Germany), under basal conditions and in the presence of 1 × 10
Glucose transport.
Glucose transport assays were performed according to Cherqui et al. (1989) Lipogenic studies.
Adipocytes (1 ml, 5% lipocrit) were incubated for 1 h in Krebs-Ringer buffer pH 7.4 supplemented with 20 g/L BSA and 1 mmol/L glucose. Incorporation of D-[14C(U)] glucose (Du Pont NEN, Bad Homburg, Germany) into fatty acids under basal conditions and in the presence of insulin (1 µmol/L) was determined after lipid extraction and saponification. Previous studies have shown that under these conditions the incorporation of radioactive glucose into lipids is linear with time for Insulin binding.
The binding of mono-(Tyr-A-14) 125I insulin (Zorad et al. 1985 Tyrosine phosphorylation.
Tyrosine phosphorylation of the insulin receptor in adipocytes was examined by means of an immunoblot technique with antiphosphotyrosine antibodies as described by Mooney et al. (1989) Other methods.
All determinations of serum variables were performed on samples from overnight unfed rats. Glucose concentrations were measured by the glucose oxidase method (kit 510-A, Sigma), serum insulin levels by radioimmunoassay (Insulin kit, Sorin Biomedica, Saluggia, Italy), serum triglycerides by a technique using lipoprotein lipase (kit 334-A, Sigma) and total serum cholesterol (esterified and free forms) by an enzymatic assay (kit 352-20, Sigma). Adipocyte size was determined in an aliquot of a cell suspension isolated from the pooled tissue of two to four rats by measuring the diameter of 100 cells under a light microscope equipped with an ocular micrometer after staining of the cells with crystal violet (Sigma) (Belzung et al. 1993
Food intake, body mass increment, adipose tissue mass and adipocyte size.
Whereas rats of the two dietary groups consumed comparable amounts of food and hence had similar energy and fat intakes, the mean body mass increment was significantly lower in the (n-3) diet group (Table 2). These rats also had less epidydimal adipose tissue and 9% smaller adipocytes than the (n-6) diet group (P < 0.05).
Serum metabolites.
Significantly lower serum concentrations of triglycerides and free or esterified cholesterol were observed in the (n-3) diet group compared with the (n-6) diet group, although levels of NEFA did not differ between the two groups. The PUFA composition of the diet did not influence glycemia, but insulinemia was significantly lower in rats of the (n-3) diet group (Table 3). After 1 wk of treatment, the level of saturated, monounsaturated and unsaturated FA in serum triglycerides, NEFA and phospholipids did not differ in the two dietary groups (Table 4), and no differences were observed in unsaturated to saturated FA ratios. A significant effect of dietary lipids was apparent only in the different contents of (n-6) and (n-3) PUFA in all serum lipid fractions, consistent with the compositions of the diets.
Composition and properties of adipocyte plasma membranes.
Because 1 wk of intake of the PUFA modified diets affected the composition of serum FA, we examined the effects of these diets on adipocyte lipids. The FA composition of adipocyte triglycerides (data not shown) was similar to that in serum. The only significant differences induced by the dietary treatment were the higher contents of (n-3) PUFA in the phospholipid fatty acids of the (n-3) diet group (P < 0.001) and of (n-6) PUFA in the (n-6) diet group (P < 0.01; Table 5). Consequently, in phospholipids of the (n-3) diet group, the ratio of (n-3) to (n-6) FA was 2.4-fold higher and the unsaturated to saturated FA ratio lower than in those of the (n-6) diet group. The degree of FA unsaturation expressed as the double bond index did not differ between groups, while the cholesterol level (total and free forms) and the cholesterol-to-phospholipid ratio in plasma membranes were likewise not significantly different. Fluorescence polarization studies with the DPH probe revealed that its anisotropy was significantly greater in the plasma membranes of adipocytes from the (n-3) diet group compared with the (n-6) diet group (P < 0.05), indicating a lower lipid fluidity in (n-3)-rich membranes.
Biological effects of insulin on adipocytes.
The next step was to examine the effects of the diets on the major effects of insulin in adipocytes: inhibition of noradrenaline-induced lipolysis and stimulation of glucose transport and lipogenesis. In the absence of noradrenaline, there was no difference between the two groups in the lipolytic activity of adipocytes [(n-6) diet group 36.9 ± 7.8 vs. (n-3) diet group 40.0 ± 7.7 nmol glycerol released·h
Insulin binding and tyrosine phosphorylation.
Scatchard analyzis demonstrated significant differences only in the total number of receptors (0.19 ± 0.03 vs. 0.09 ± 0.02 pmol/106 adipocytes for the (n-6) and (n-3) diet groups, respectively, P < 0.05; Fig. 3). Immunoblots of cell extracts incubated with antiphosphotyrosine antibodies revealed the presence of different phosphorylated proteins in the 30- to 40-kDa and 50- to 70-kDa regions of the gel, together with 95-, 120- and ~180-kDa bands (Fig. 4). These results are comparable with those reported by Mooney et al. (1989)
As frequently reported in the literature (Harris 1996
INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References
, Gavino and Gavino 1991). Although dietary recommendations for the prevention of obese diseases remain controversial, the replacement of saturated fatty acids by polyunsaturated fatty acids (PUFA)4 of plant origin [(n-6) series] is already a well documented and widely accepted strategy. More recently, the effects of (n-3) PUFA (mainly from salt-water fish) have been studied in view of the potency of these compounds in reducing plasma triglycerides. The majority of the dietary studies, in both humans and animals, investigated the effects of modified diets consumed over long time periods (weeks, months). These long-term dietary manipulations were based on the assumption that the fatty acid composition of white adipose tissue adapts only slowly to that of the diet (Brockerhoff et al. 1967, Christakis et al. 1965, Field et al. 1989, Luo et al. 1996). However, a recent study of rats fed a fish-oil-enriched diet for 1 wk revealed major changes in the fatty acid composition of white adipocyte phospholipids (Leray et al. 1995). In the literature, the numerous investigations to date generally have focused on the relative proportions of saturated, monounsaturated and (n-6) and (n-3) polyunsaturated fatty acids (FA) in the diet. A wide range of lipid concentrations (20-60%), together with variable ratios of unsaturated to saturated FA and values of the double bond index (DBI), have been employed in experimental diets. Therefore, we compared two isocaloric diets identical in their basic FA composition (saturated and monounsaturated FA, DBI and unsaturated to saturated FA ratio). The only difference was in their content of (n-6) or (n-3) PUFA, thus allowing us to compare the effects of these FA on cellular composition and function.
MATERIALS AND METHODS
Abstract
Introduction
Methods
Results
Discussion
References
-tocopherol to mg/100 g and selenium 15 µg/100 g. The energy content of the two diets was 19.5 kJ/g distributed as (% energy) 13.8 protein, 40.7 lipids and 45.5 carbohydrate, whereas the lipid composition (Table 1) was designed to maintain constant proportions of saturated and monounsaturated fatty acids and to differ only in the ratio of (n-3) to (n-6) fatty acids. Diets were prepared weekly and kept at 
20 °C. Rats were caged individually, the amount of food consumed was recorded daily and food from the previous day was replaced with a fresh supply. The animals were killed by decapitation between 09:00 and 09:30 after one night of food deprivation, serum was prepared by centrifugation of the blood collected and samples were stored at -70°C until analysis. Ethical approval of the protocol was obtained in conformity with regulations for the care and use of laboratory animals.
View this table:
Table 1.
Fatty acid composition of the two experimental diets
. Plasma membranes were prepared from adipocytes by separation through a Percoll gradient (Belsham et al. 1980) and used for measurements of cholesterol and phospholipid content and diphenylhexatriene fluorescence anisotropy.
. Triglycerides, free and esterified cholesterol, free fatty acids and phospholipids were separated by TLC on silica gel G (Merck, Darmstadt, Germany) with hexane/diethyl ether/acetic acid (70:30:1) as the developing solvent. Fatty acids were determined by direct methylation with BF3/methanol of spots scraped from TLC plates, relative to heptadecanoate run as an internal standard. Fatty acid methyl esters were analyzed by gas-liquid chromatography (Leray et al. 1993), whereas cholesterol was determined by reverse-phase high-pressure liquid chromatography (HPLC) with light scattering detection (DDL 21, Eurosep, France) (Leray et al. 1997).
).
5 mol/L noradrenaline (Sigma). The antilipolytic effect of insulin (pork monocomponent, Novo, Bagsvaerd, Denmark) was assayed at various concentrations in noradrenaline activated adipocytes as described in the legend to Figure 1.
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Fig 1.
Dose-response curves for the antilipolytic effect of insulin in adipocytes from rats fed diets enriched in (n-3) or (n-6) fatty acids for 1 wk. Adipocytes were incubated with 1 × 10 5 mol/L noradrenaline, alone or in the presence of varying concentrations of insulin. Data represent the amount of glycerol released into the incubation medium minus basal values released in the absence of noradrenaline, and points are mean values (± SEM) from 5 independent assays. Rats were treated during the same week and samples were processed simultaneously. Means for insulin concentrations >1010 mol/L differed significantly.
. Adipocytes (20% lipocrit) were incubated for 5 min with 2-deoxy-D-3H glucose (Amersham, Little Chalfont, UK), in Krebs-Ringer buffer supplemented with 12.5 mmol/L N-2-hydroxyethylpiperazine-N
-2-ethanesulfonic acid and 20 g/L BSA. The stimulatory effect of insulin was assessed by preincubation of adipocytes with various concentrations of hormone for 30 min at 37°C, the reaction was stopped by rapid addition of ice-cold phloretin (0.4 mmol/L, Sigma) and the cells were separated by centrifugation through silicone oil (SC 200, 350 cst, Serva). All data were corrected for extracellular trapping and passive diffusion by measuring glucose transport in the presence of 25 µmol/L cytochalasin B (Sigma).
1.5 h.
) to adipocytes was estimated by the method of Kasuga et al. (1977). Competition binding curves were analyzed according to a two-site model using the Ligand program (Munson and Rodbard 1980).
. Briefly, adipocytes were incubated for 2 min in the absence (basal values) or presence of 0.1 µmol/L insulin and the solubilized cell extracts (~25 µg protein/sample) were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and electrophoretic transfer to nitrocellulose sheets. Phosphorylated tyrosine bands were labeled using monoclonal antiphosphotyrosine antibodies (clone 4G10, UBI, Lake Placid, NY) followed by peroxidase-coupled anti-mouse antibodies (Amersham, UK), labeled bands were revealed by the ECL procedure (Amersham) and the images were scanned and analyzed on a computer (Scan Analysis, Biosoft, Cambridge, UK).
).
View this table:
Table 4.
Fatty acid composition of serum triglycerides, non-esterified fatty acids and phospholipids in rats fed diets enriched
in (n-3) or (n-6) fatty acids for 1 wk1
RESULTS
Abstract
Introduction
Methods
Results
Discussion
References
View this table:
Table 2.
Food intake, weight gain and adipocyte size in rats fed diets enriched in (n-3) or (n-6) fatty acids for 1 wk1
View this table:
Table 3.
Serum concentrations of lipids, glucose and insulin in rats fed diets enriched in (n-3) or (n-6) fatty acids for 1 wk1
View this table:
Table 5.
Phospholipid fatty acids, cholesterol level, cholesterol-to-phospholipid ratio and fluorescence anisotropy (DPH) of the plasma membranes of adipocytes from rats fed diets enriched in (n-3) or (n-6) fatty acids for 1 wk1
1·106 cells1]. However, addition of 10 µmol/L noradrenaline induced a significantly higher lipolytic response in adipocytes from (n-3) fed rats [(n-6) diet group 111.6 ± 5.6 vs. (n-3) diet group 141.9 ± 6.9 nmol·h1·106 cells1, P < 0.005]. The insulin inhibition of glycerol release from noradrenaline activated cells followed comparable dose-response curves in the two dietary groups (Fig. 1), the response to insulin being significantly lower in adipocytes of the (n-3) diet group only for insulin concentrations exceeding 1010 mol/L.
1 × 109 mol/L. This was not due to the smaller size of adipocytes from the (n-3) diet group because a significant difference also was observed when the glucose transport data were expressed per unit cell surface area (not shown). Otherwise, the sensitivity to insulin did not differ between the two dietary groups [EC50: 3.49 ± 1.07 × 1010 mol/L (n-3) diet group and 3.32 ± 0.94 × 1010 mol/L (n-6) diet group].
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Fig 2.
Stimulatory effect of insulin on glucose transport in adipocytes from rats fed diets enriched in (n-3) or (n-6) fatty acids for 1 wk. Dose-response curves display the net effect of insulin above basal glucose transport activity. Points are mean values (± SEM) from 5 independent assays in which the two dietary groups were tested simultaneously. *Significant difference (P < 0.05) as calculated by one-way analysis of variance followed by a least-squares difference test.
View this table:
Table 6.
Incorporation of 14C-glucose into the fatty acids of adipocytes isolated from rats fed diets enriched
in (n-3) or (n-6) fatty acids for 1 wk1
. The principal effect of insulin (1 × 107 mol/L) was an increase in labeling of the ~60-, 95- and ~180-kDa bands. Densitometric scanning of the 95-kDa band, known to correspond to the beta-subunit of the insulin receptor, showed that insulin stimulated its labeling by factors of 3.7 (n = 6) in the (n-6) diet group and 2.0 (n = 6) in the (n-3) diet group. This difference is roughly the same as the observed difference in insulin receptor content.
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Fig 3.
Specific 125I-insulin binding to adipocytes from rats fed diets enriched in (n-3) or (n-6) fatty acids for 1 wk. Competition binding experiments were performed on adipocytes isolated from rats fed diets rich in (n-3) or (n-6) fatty acids. Points are mean values (± SEM) from 8 independent assays for each dietary group.
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Fig 4.
Western blot analysis of tyrosine-phosphorylated proteins in adipocytes from rats fed diets enriched in (n-3) or (n-6) fatty acids for 1 wk. Adipocyte suspensions were exposed or not to insulin for 2 min (see Materials and Methods). The cells then were harvested rapidly and solubilized and identical amounts of protein were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred to nitrocellulose. Phosphotyrosine containing proteins were labeled with a monoclonal antiphosphotyrosine antibody followed by a peroxidase-coupled anti-mouse antibody and visualized by chemiluminescence. One blot typical of six similar experiments is shown. IRS 1, insulin receptor substrate 1.
DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References
), a significant reduction in serum triglycerides was obtained by ingestion of (n-3) PUFA rather than (n-6) fatty acids. In this study, in order to lower serum cholesterol, we observed a high potency of dietary (n-3) PUFA in both total and esterified forms, compared with dietary (n-6) PUFA. These changes in plasma lipid composition occurred within a surprisingly short time. There exist previous reports of the capacity of (n-3) PUFA to lower free fatty acid levels in serum. Otto et al. (1992) clearly demonstrated that this effect could be observed only when rats were given free access to food and that food deprivation masked the effects of dietary fish oil. Therefore, we analyzed the serum of rats deprived of food overnight; this explains the comparable free FA levels in our two dietary groups. The presence of a lower insulinemia in rats fed the (n-3) PUFA diet also supports previous findings in humans and animals (Klimes et al. 1993). Although the precise mechanism is not known, the specific effect of (n-3) PUFA is likely at the level of insulin release from the islets or through higher receptor mediated hormone clearance. Despite the difference in insulinemia, both values still lie within the physiological range and rats of the two dietary groups had similar glycemia. This would indicate a compensatory or independent increase in peripheral insulin sensitivity in (n-3) PUFA fed rats. Because the metabolic effects of insulin and numbers of insulin receptors were reduced in adipocytes of the (n-3) diet fed group, the supposed sensitivity increase would appear to occur in the muscles and/or liver, as previously reported by Storlien et al. (1987).
) and normal Wistar rats (Pan and Storlien 1993). This ability of dietary (n-3) PUFA to reduce body mass gain and adipose tissue mass could be related to the preferential oxidation of these long-chain unsaturated FA (Jandacek et al. 1991, Jones and Schoeller 1988). Alternatively, it could also result from a "leaky membrane effect" and increased energy expenditure, as proposed by Else and Hulbert (1987) with concomitant alterations in metabolic rate (Pan and Storlien 1993). The capacity of fish oil to limit adipocyte size and the hypertrophy of abdominal adipose tissue is now well documented (Belzung et al. 1993, Hill et al. 1993, Parrish et al. 1991). Although a discussion of the mechanism of this effect is beyond the scope of the present study, we nevertheless give here the first demonstration of a short-term influence of dietary (n-3) PUFA on adipocyte size.
, who observed a stronger lipolytic response in the adipocytes of rats fed a fish oil diet for 3 wk compared with rats fed a lard diet. These changes in lipolytic response also took place within a relatively short time. Current results thus support the hypothesis of a general effect of (n-3) PUFA in modulating the lipolytic pathway. The specific loci affected could include any step of the lipolytic cascade as, for instance, adrenergic receptors, adenylate cyclase, phosphodiesterase activity or hormone-sensitive lipase, whereas, as will be discussed later, alterations of membrane properties could participate in the modulation of membrane proteins and membrane bound processes.
). Whereas under basal conditions there were no differences in the transport of glucose or its incorporation into fatty acids, the stimulatory effect of insulin on glucose transport was lower in adipocytes of the (n-3) diet group. The plasma insulin concentration is also an important regulator of the transport and intracellular metabolism of glucose (Kobayashi and Olefsky 1979, Olefsky 1978). Recently, the regulation of GLUT 4 protein by insulinemia has been proposed (Pedersen et al. 1991, Sebokova et al. 1993). Thus a lower insulinemia in rats fed the (n-3) PUFA diet could contribute to a slower rate of insulin stimulated glucose transport due to a reduction in levels of the glucose transporter protein. Translocation of GLUT 4 is influenced by the membrane lipid domain, with unsaturated phospholipids stimulating and saturated phospholipids inhibiting glucose transport (Sandra et al. 1984). Higher rates of transport are linked to increased membrane fluidity, which facilitates the penetration of GLUT4 into the membrane (Spector and Yorek 1985) and/or induces conformational changes in the quaternary structure of the transporter protein. This is compatible with our data because the lower insulin-stimulated glucose transport in adipocytes of the (n-3) PUFA group was accompanied by a reduction in plasma membrane fluidity. A slower rate of insulin-stimulated glucose transport could be responsible further for the lesser effect of insulin on glucose incorporation into FA in adipocytes of the (n-3) PUFA group. Modifications at the enzyme level, however, cannot be excluded as certain dietary PUFA have been reported to inhibit the expression of lipogenic enzyme genes (Clarke et al. 1977).
, Fickova et al. 1992, Hubert et al. 1991). As the decline in phosphorylation of the 95-kDa beta subunit of the insulin receptor in the (n-3) diet group compared with the (n-6) diet group (~50%) was similar to the fall in total number of receptors, it is unlikely that the nutritional modification of the FA composition of plasma membrane phospholipids had a specific effect on the coupling of insulin binding to the kinase activity of its receptor.
). This relationship may be expressed simply by the generalization that higher membrane fluidity is associated with higher numbers of insulin receptors and/or greater sensitivity to insulin. However, the correlation between cell membrane fluidity such as that induced by dietary PUFA and insulin receptor binding is not universal (Gould and Ginsberg 1985). Our finding of higher numbers of insulin receptors and greater membrane fluidity in adipocytes of the (n-6) PUFA group is in accordance with the usual relationship, and the same is true for the positive correlation (r = 0.556, n = 14, P < 0.05) between insulin receptors and the unsaturated-to-saturated FA ratio.
0.37, n = 14, P = 0.80). Insulin receptors in adipocytes were not correlated with the DBI of phospholipids. Because the DBI was similar in both dietary groups, membrane fluidity may not depend on the total number of double bonds as formerly proposed by Neuringer and Connor (1989) but most probably on the total content of unsaturated FA. Our results indicate that a high content of (n-3) PUFA does not necessarily result in greater fluidity of the lipid component of plasma membranes. In this study, the positive linear correlation between insulin receptors and the unsaturated-to-saturated FA ratio supports a relation between insulin binding in adipocytes and the ratio of polyunsaturated to saturated FA in the diet, in agreement with the previous findings of Clandinin et al. (1993). The higher unsaturated to saturated FA ratio in phospholipids of the (n-6) diet group could imply that dietary (n-6) PUFA favor the preferential incorporation of unsaturated FA, including monounsaturated FA, into membrane phospholipids. Such alterations could contribute substantially to increase both fluidity and numbers of insulin receptors as compared to the (n-3) PUFA group. Field et al. (1989) observed that a high ratio of PUFA to saturated FA in phospholipids induced by dietary manipulation increased not only insulin binding but also coupling between insulin receptors and glucose transport, glucose oxidation and lipid synthesis in adipocytes. Because the main PUFA in the diets of these investigators were those of the (n-6) series, our findings of higher insulin binding and responsiveness in adipocytes of the (n-6) PUFA group is in accordance with the general effects of dietary (n-6) PUFA on the metabolic activity of insulin. In conclusion, the different series of dietary PUFA differently influence the membrane properties of adipocytes and induce concomitant changes in transmembrane-located insulin receptors. The effects of the various FA and in particular of (n-3) and (n-6) PUFA on lipid metabolism have been assessed in only a few studies. It commonly is accepted that dietary (n-3) PUFA are effective in improving the insulin resistance induced by high fat diets (Storlien et al. 1987). However, this effect has been described only in skeletal muscles and liver, while in the same experiment, a reduction in the insulin stimulated glucose metabolism of white adipose tissue was observed.
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FOOTNOTES |
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Manuscript received 12 November 1996. Initial reviews completed 8 January 1997. Revision accepted 23 November 1997.
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LITERATURE CITED |
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Abstract
Introduction
Methods
Results
Discussion
References
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