点击显示 收起
1 From the Department of Nutrition and Food Science, the University of País Vasco, Vitoria, Spain (VMR, MPP, and MTM), and the Department of Biologia Fonamental i Ciènces de la Salut, Universitat de les Illes Balears, Palma de Mallorca, Spain (CP and AP).
2 Supported by the Government of the País Vasco (grant PI 96/22 to MPP), DGICYT, Ministerio de Educación y Ciencia, Spain (grant PM97-0094 to AP) and by the European Commission DGXII (COST 918 to AP). VMR is a recipient of a fellowship from the Spanish Government. 3 Address reprint requests to MP Portillo, Nutrición y Bromatología, Facultad de Farmacia, Paseo de la Universidad, 7.01006 Vitoria, Spain. E-mail: knppobam{at}vc.ehu.es.
ABSTRACT
Background: Some nutrients, such as carotenoids, retinoic acid, and certain types of fatty acids, increase thermogenic capacity.
Objective: The influence of 4 dietary lipid sources (olive oil, sunflower oil, palm oil, and beef tallow) on the content of uncoupling proteins 1, 2, and 3 (UCP1, UCP2, and UCP3) and their messenger RNA (mRNA) expression in several tissues of rats was compared.
Design: Wistar rats were randomly divided into 4 groups and fed ad libitum diets containing 40% of energy as fat. UCP1, UCP2, and UCP3 mRNA and protein were assessed by Northern blot and Western blot, respectively. Oxygen consumption in tissues was measured by polarography. Total-body oxygen consumption was assessed in an open-circuit chamber system. Circulating fuels (fatty acids and glucose) and hormones (triiodothyronine, thyroxine, corticosterone, and insulin) were measured.
Results: Olive oil feeding induced the highest UCP1, UCP2, and UCP3 mRNA expression in interscapular brown adipose tissue. An analogous effect was observed in gastrocnemius muscle UCP3 mRNA. No significant differences were observed in perirenal white adipose tissue UCP2 mRNA. Changes in mRNAs were not accompanied by close changes in the protein content of UCPs and were not associated with changes in adipose tissue oxygen consumption. Nevertheless, total-body oxygen consumption was higher in rats fed olive oil than in those fed the other 3 diets. No significant differences were found in body and tissue weights or in serum indexes.
Conclusion: Olive oil induced an up-regulating effect on UCP mRNA that was probably not mediated by systemic metabolic changes, but rather related to a local effect on interscapular brown adipose tissue and skeletal muscle.
Key Words: Uncoupling proteins • olive oil • messenger RNA • brown adipose tissue • skeletal muscle • total-body oxygen consumption • rats
INTRODUCTION
The regulation of body fat in mammals is a complex process involving the control of both energy intake and energy expenditure (1–3). An important component of energy expenditure is nonshivering thermogenesis, which in rodents takes place mostly in brown adipose tissue (BAT) and is based on the functionality of uncoupling protein 1 (UCP1), a member of the inner mitochondrial membrane transporters (4–6). This protein allows heat production by uncoupling respiration from ATP synthesis.
In addition to UCP1, new uncoupling proteins showing a high degree of sequence similarity with UCP1 (55–60%) have been more recently identified in both rodents and humans: UCP2, which is expressed widely among tissues (7, 8), and UCP3, which is expressed mainly in human skeletal muscle and BAT (9, 10). UCP2 and UCP3 have been shown to lower mitochondrial membrane potential when ectopically expressed in yeast (11, 12) and mammal cells, thus supporting the hypothesis that these UCP1 homologues can also uncouple oxidative phosphorylation and contribute together with UCP1 to thermogenesis in vivo.
On the other hand, the respiratory chain in mitochondria from skeletal muscle lacking UCP3 (from UCP3 knockout mice) is more coupled (thus, proton leak is low and more ATP is synthesized), indicating that UCP3 has uncoupling activity (13, 14). Recently, direct evidence for the thermogenic role of skeletal muscle UCP3 was provided by showing that transgenic mice overexpressing UCP3 in skeletal muscle are hyperphagic but weigh less than do their wild-type littermates (15). In contrast, other observations such as the increased expression of UCP2 and UCP3 messenger RNA (mRNA) in skeletal muscle in response to food deprivation goes against a putative thermogenic function of these UCP1 homologues and is more consistent with a role in the regulation of lipids as a fuel substrate (16, 17).
Some nutrients, such as naturally occurring carotenoids (18) and retinoic acid, the active form of vitamin A (19, 20), have been shown to increase in vivo and in vitro thermogenic capacity. In addition, it has been reported that capacity can be affected by dietary fatty acid composition. Thus, certain types of high-fat diets rich in polyunsaturated fatty acids (21–23) or medium-chain fatty acids (24) could enhance diet-induced nonshivering thermogenesis.
Thus, the hypothesis arose that some diets may be particularly suitable for enhancing thermogenesis. The aim of the present work was to compare the influence of an animal fat (beef tallow, rich in saturated fatty acids), a vegetable fat (palm oil, rich in saturated fatty acids), and 2 vegetable oils (olive oil, rich in monounsaturated fatty acids, and sunflower oil, rich in polyunsaturated fatty acids) on oxygen consumption and on the content of UCP1, UCP2, and UCP3 and their mRNA expression in several tissues of rats.
MATERIALS AND METHODS
Animals, diets, and experimental design
The experiment was conducted with male Wistar rats purchased from IFFA-Credo (Barcelona, Spain) and was in accordance with the institution's guide for the care and use of laboratory animals. The rats were housed individually in polycarbonate metabolic cages (Tecniplast, Gazzada, Italy) in a temperature- (22 ± 2°C) and humidity- (50%) controlled room with 12-h light and dark cycles, with the lights on at 0800.
When the animals (n = 40) had reached a body weight of 240 ± 2 g, they were randomly divided into 4 groups (n = 10/group). Rats had free access to water and diets that provided olive oil, sunflower oil, palm oil, or beef tallow as the lipid source for 4 wk. Food intake was measured daily.
The experimental diets were freshly prepared once a week, gassed with nitrogen, and stored at 0–4°C to avoid rancidity. The composition of all diets is described in Table 1. All diets contained 40% of total energy as fat. The dietary supply of vitamins, minerals, and protein was in accordance with the recommended dietary allowances for rats from the American Institute of Nutrition (AIN-93; 25). The fatty acid composition of the diets was determined by gas chromatography and is shown in Table 2. The beef tallow diet was supplemented with 1% sunflower oil to maintain an adequate intake of linoleic acid and to avoid growth alterations and changes in basal metabolism associated with linoleic acid deficiency (26). Olive oil and sunflower oil were obtained from Koipe SA (Jaén, Spain), palm oil and beef tallow were acquired locally, vitamins and minerals were purchased from Dyets Inc (Bethlehem, PA), and casein was purchased from Sigma (St Louis).
View this table:
TABLE 1 . Composition of the experimental diets
View this table:
TABLE 2 . Fatty acid composition of the dietary fats1
At the end of the feeding period and after the animals had been deprived of food overnight, the rats were weighed and decapitated. Blood was collected and centrifuged (3000 x g, 4°C, 10 min), and serum was frozen and stored (-80°C) until analyzed. Perirenal white adipose tissue (WAT), interscapular BAT (IBAT), and both gastrocnemius muscles were removed and weighed. Tissue fragments were used to analyze oxygen consumption. The rest was immediately frozen in liquid nitrogen and stored at -80°C for subsequent analyses.
Fatty acid composition of dietary lipid sources
Dietary fats were transmethylated with methanol in sulfuric acid. Fatty acid composition was analyzed with use of an HP 6890 series II gas chromatograph (Hewlett-Packard, Palo Alto, CA) equipped with a flame ionization detector and a 30 m x 320 µm 19091N-213 polyethylene glycol capillary column (HP Innowax; Hewlett-Packard). Nitrogen was used as the carrier gas at a flow rate of 1 mL/min. The temperature of the oven and the injection port were maintained at 170°C and 225°C, respectively. Peaks were identified by using fatty acid methyl ester standards obtained from Sigma. All samples were analyzed in quadruplicate. The replicate error (CV) was 5% of the mean for all fatty acids.
Northern blot analysis
Tissue RNA was extracted with Tripure Isolation Reagent (Roche Diagnostics, Mannheim, Germany). Twenty micrograms total RNA was size-fractionated on a denaturing gel containing formaldehyde (18%) and agarose (1.2%). The RNA was transferred to nylon membranes (N+; Roche Diagnostics) with 20X saline sodium citrate (SSC; 3 mmol NaCl/L, 0.3 mol sodium citrate/L) and cross-linked by exposure to ultraviolet light.
Membranes were prehybridized at 42°C with DYG Easy Hyb solution (Roche Diagnostics) and hybridized overnight at 42°C in the same solution containing specific UCP1, UCP2, UCP3, or 18S probes [previously described (27)] labeled with digoxigenin. The membranes were then washed twice with 2X SSC containing 0.1% sodium dodecyl sulfate at room temperature and then twice with 0.1X SSC containing 0.1% sodium dodecyl sulfate at 48°C. Membranes were washed with blocking reagent solution (1% in maleic acid buffer; Roche Diagnostics) and then with blocking reagent solution containing phosphatase-alkaline-labeled anti-digoxigenin (Roche Diagnostics).
Finally, CDP-Star substrate (Roche Diagnostics) was used to produce a luminescent signal, and membranes were exposed to high-performance chemiluminescence film (Amersham Pharmacia Biotech, Buckinghamshire, United Kingdom) and developed with Kodak GBX developer and fixer solutions purchased from Sigma. Signals obtained were quantified by using BIO IMAGE software (Millipore, Bedford, MA).
Western blot analysis
Samples were homogenized with cellular phosphate-buffered saline (PBS) and their protein concentrations measured. Protein (30 µg for IBAT UCP1 and 50 µg for IBAT UCP2 and UCP3 and gastrocnemius UCP3) was separated on 10%-polyacrylamide reducing gels at 70 V overnight and then electroblotted to nitrocellulose membranes (Amersham Pharmacia Biotech). Blots were blocked with 5% powdered milk in PBS containing 0.1% Tween 20 (PBS-Tween; Sigma) for 1 h and incubated with specific rabbit antibodies for UCP1 (1:1000), UCP2 (1:1000), or UCP3 (1:2000). Antibodies for UCP1 and UCP2 were purchased from Alpha Diagnostics (San Antonio, TX). Antibodies for UCP3 were obtained from Alpha Diagnostics and RDI (Flanders, NJ); similar results were obtained with both antibodies. Only the data obtained with the antibodies from RDI are presented. The specificity of primary antibodies purchased from Alpha Diagnostics was ascertained and confirmed by Sivitz et al (28).
Blots were washed in PBS-Tween and incubated with antirabbit peroxidase-conjugated secondary antibody (1:5000) for 1 h. Next, blots were washed in PBS-Tween and developed by chemiluminescent detection with use of a standard kit (Amersham Pharmacia Biotech). UCP subtype expression was quantified by densitometry with a scanner equipped with a transiluminator and BIO IMAGE analysis software (Millipore).
Measurement of total-body oxygen consumption
Resting oxygen consumption (VdotO2) was measured in each animal over 2 h in an open-circuit chamber system (Oxymax Monitoring System; Columbus Instruments, Columbus, OH) the day before the rats were killed (4 rats/d, one rat per group). To avoid the influence of an exploratory-type behavior, rats were transferred from their habitual cage to the calorimeter chamber 1 h before measurements were started. VdotO2 was calculated as the product of the air flow rate (1 L/min) through the system multiplied by the difference in oxygen concentration between the chamber postexchange and room air, corrected for changes in humidity (50%) and temperature (22 ± 2°C). Total-body oxygen consumption is expressed as mmol O2·kg-1·h-1.
In vitro tissue respiration rates
The respiratory rates of tissue fragments were measured as described by Barde et al (29). Fragments of IBAT and perirenal WAT were placed in a thick-walled, transparent thermoplastic chamber containing bubble-free Krebs-Ringer bicarbonate buffer (pH 7.4; 6 mmol glucose/L). The oxygen partial pressure of the buffer was measured by a Clark O2 electrode connected to a polarographic circuit, whose output voltage is directly proportional to oxygen partial pressure (YSI 5300 Biological Oxygen Monitor and YSI 5331 Oxygen probe; YSI Incorporated, Dayton, OH). The rate of tissue consumption was determined at 37°C and is expressed as nmol O2·g-1·min-1.
Serum analyses
Fatty acid and glucose concentrations were measured spectrophotometrically and colorimetrically by using commercial kits (Free Fatty Acids Half Micro Test and TC D-Glucose; Roche Diagnostics). Thyroxine, triiodothyronine, and corticosterone were assessed by radioimmunoassay with commercial kits (T4 and T3 Radioimmunoassay Kits; Farmos Diagnostica, Espoo, Finland, and Rat Corticosterone RIA; DRG Instruments GmbH, Marburg, Germany). Insulin was measured by enzyme-linked immunosorbent assay with a commercial kit (Rat Insulin ELISA; DRG Instruments GmbH).
Statistical analysis
Values are presented as means ± SEMs. Analyses of variance (ANOVA) was used to test the significance of differences (P < 0.05) among groups with subsequent Tukey's test for mean comparisons. Analyses were performed with SPSS software (version 8.0; SPSS Inc, Chicago).
RESULTS
Rats fed the different lipid sources gained comparable amounts of weight during the experimental period. No significant differences were found among groups in IBAT, perirenal WAT, or gastrocnemius muscle weights (Table 3). There was a main effect of the type of fat on food intake, with rats eating saturated fats having higher food intakes (22.7 ± 0.3 g/d in the palm oil group and 23.3 ± 0.5 g/d in the beef tallow group) than did rats eating unsaturated fats (21.1 ± 0.5 g/d in the olive oil group and 20.7 ± 0.4 g/d in the sunflower oil group) (P < 0.05).
View this table:
TABLE 3 . Body weight and tissue weights in rats fed high-fat diets with different fatty acid compositions for 4 wk ad libitum1
Olive oil feeding induced the highest UCP1, UCP2, and UCP3 mRNA expression in IBAT (Figures 1–4). An analogous effect was observed on UCP3 mRNA expression in gastrocnemius muscle (Figure 3). In contrast, no significant differences in the expression of UCP2 mRNA in perirenal WAT were observed (Figure 2). The changes in mRNAs in IBAT were not accompanied by close changes in UCP1, UCP2, and UCP3 protein expression. Thus, no significant differences were observed in IBAT UCP1 and UCP3. With regard to UCP2, protein expression was significantly higher in tissues from rats fed olive oil and sunflower oil than in tissues from rats fed beef tallow. Rats fed olive oil had significantly higher contents of UCP3 in gastrocnemius muscle than did rats fed the other 3 diets (Table 4 and Figure 5).
FIGURE 1. . Mean (±SEM) uncoupling protein 1 (UCP1) messenger RNA (mRNA) expression in interscapular brown adipose tissue (IBAT) (n = 5). Results are expressed as arbitrary units, with the level in rats fed olive oil set to 100 and the SEM adjusted proportionally. Bars with different superscript letters are significantly different, P < 0.05 (ANOVA).
FIGURE 2. . Mean (±SEM) uncoupling protein 2 (UCP2) messenger RNA (mRNA) expression in interscapular brown adipose tissue (IBAT) and perirenal white adipose tissue (WAT) (n = 5). Results are expressed as arbitrary units, with the level in rats fed olive oil set to 100 and the SEM adjusted proportionally. Bars with different superscript letters are significantly different, P < 0.05 (ANOVA).
FIGURE 3. . Mean (±SEM) uncoupling protein 3 (UCP3) messenger RNA (mRNA) expression in interscapular brown adipose tissue (IBAT) and gastrocnemius muscle (n = 5). Results are expressed as arbitrary units, with the level in rats fed olive oil set to 100 and the SEM adjusted proportionally. Bars with different superscript letters are significantly different, P < 0.05 (ANOVA).
FIGURE 4. . Representative Northern blots for messenger RNAs of uncoupling proteins (UCPs) in interscapular brown adipose tissue (IBAT), gastrocnemius muscle, and perirenal white adipose tissue (WAT). Total RNA (20 µg per lane) was analyzed. The corresponding blot for the 18S ribosomal RNA is also shown.
View this table:
TABLE 4 . Uncoupling protein (UCP) content in interscapular brown adipose tissue (IBAT) and gastrocnemius muscle in rats fed high-fat diets with different fatty acid compositions for 4 wk ad libitum1
FIGURE 5. . Representative Western blots for uncoupling proteins (UCPs) in interscapular brown adipose tissue (IBAT) and gastrocnemius muscle. Extracts of 30 µg (IBAT UCP1) and 50 µg (IBAT UCP2 and UCP3 and gastrocnemius UCP3) total protein were analyzed per line. M, molecular mass.
Total-body oxygen consumption, an indicator of resting metabolic rate, was significantly higher in rats fed olive oil than in rats fed the other 3 high-fat diets (Table 5). No significant differences among groups were found in oxygen consumption in isolated IBAT or perirenal WAT.
View this table:
TABLE 5 . Total-body and tissue oxygen consumption in rats fed high-fat diets with different fatty acid compositions for 4 wk ad libitum1
Fasting serum fatty acids, glucose, insulin, corticosterone, and triiodothyronine concentrations were not significantly different among groups. The concentration of thyroxine was lower in rats fed beef tallow than in those fed sunflower oil or palm oil (Table 6).
View this table:
TABLE 6 . Serum variables in rats fed high-fat diets with different fatty acid compositions for 4 wk ad libitum1
DISCUSSION
The main finding of the present study was an up-regulating effect of olive oil on UCP mRNA expression. The expression of the mRNAs of the 3 UCPs in IBAT and of UCP3 mRNA in gastrocnemius muscle was significantly higher after olive oil feeding.
The influence of monounsaturated fatty acids on UCP expression has been studied little. Samec et al (30) compared olive oil with other dietary fats rich in saturated and polyunsaturated fatty acids (lard, coconut oil, safflower oil, and fish oil). Their results differed from ours, but so did the experimental design used in their study. They examined post-starvation gene transcription of muscle UCP2 and UCP3 and energy expenditure during refeeding with high-fat diets varying in fat type. Under these experimental conditions, they observed that muscle UCP2 and UCP3 mRNA expression in rats fed olive oil was not significantly different from the expression observed in rats fed the other fat types. In contrast, olive oil refeeding resulted in lower energy expenditure than did refeeding with coconut oil or safflower oil.
More information is available on the effects of polyunsaturated fatty acids on UCPs and thermogenesis (22, 31, 32). Our results agree well with the studies of Kawada et al (31) and Takahashi and Ide (32), who found no significant differences between rats fed sunflower oil rich in n-6 polyunsaturated fatty acids and rats fed beef tallow or palm oil rich in saturated fatty acids. The discrepancies among different studies may be due to differences in experimental conditions related to the age of the animals, the overall amount of fat in the diet, the duration of the feeding period, and energy intake. Thus, significant effects of a surplus of polyunsaturated fatty acids may be observed under certain experimental conditions but not others.
In our work, the changes in mRNA expression were not accompanied by similar changes in UCP protein content in IBAT. This finding may be related to the lack of effect of fatty acid type on IBAT oxygen consumption. Discrepancies between UCP mRNA expression and protein expression were also observed by others (28). Rats fed olive oil had significantly higher expression of both UCP3 mRNA and UCP3 protein in gastrocnemius muscle than did rats fed the other 3 diets. Oxygen consumption in gastrocnemius muscle could not be measured accurately because muscle fragmentation induces lesions (release of electrolytes, fiber despolarization, etc) that can considerably modify the tissue respiration rate.
Total-body oxygen consumption, an index of resting metabolic rate, was significantly higher in rats fed olive oil than in the other 3 groups. This difference was not explained by changes in IBAT and perirenal WAT oxygen consumption. This apparent discrepancy may be due, at least in part, to the experimental conditions used for these determinations. Total-body oxygen consumption was assessed in vivo, ie, taking into account neuroendocrine influences; in contrast, tissue oxygen consumption was measured in vitro. On the other hand, possible differences in the respiration rate of some components of fat-free mass should not be overlooked.
It could be argued that the higher total-body oxygen consumption observed in the group fed olive oil was the result of higher energy intake in this group than in the other groups. In our study, however, this was not the case. Nevertheless, data concerning energy intake can be different from data on energy absorption because saturated fats are less well absorbed than are unsaturated fats (33). The lack of difference in weight gain and perirenal WAT weight with olive oil feeding despite the higher total-body oxygen consumption may be explained by the short time period of study: 4 wk may not be long enough to observe the changes in weight induced by a higher resting metabolic rate.
To determine whether the effects of olive oil on UCP mRNA were associated with metabolic changes induced by dietary fat, serum concentrations of some hormones and metabolites were measured. Thyroid hormones are deeply implicated in the regulation of thermogenesis and basal metabolic rate (34, 35). Some studies showed that triiodothyronine increases the expression of UCP1, UCP2, and UCP3 (11, 36, 37). The involvement of triiodothyronine in the up-regulating effect of olive oil described in the present work is not evident because no significant differences in serum concentrations were found among the 4 dietary groups. Some authors reported changes in UCP1 expression induced by a local increase in BAT triiodothyronine concentrations, without significant modifications in plasma triiodothyronine concentrations. These changes were due to an increased conversion of thyroxine to its more active metabolite triiodothyronine by type II thyroxine deiodinase (38), which is activated by the sympathetic nervous system (39). Taking these findings into account, the possibility that a similar situation may have taken place under our experimental conditions, and then caused the up-regulating effect on UCPs in the group fed olive oil, cannot be ruled out. In rodents, thyroxine deiodinase is expressed in BAT but not in skeletal muscle (40); thus, the differential regulation of UCP3 by cold in BAT and skeletal muscle could be attributed to this difference. In our study, UCP3 mRNA was regulated in a similar way by olive oil in both IBAT and gastrocnemius skeletal muscle. The assumption that this up-regulating effect may be caused by a local increase in triiodothyronine concentration seems unlikely.
Circulating glucocorticoid and insulin concentrations are known to be potent regulators of fuel metabolism and gene transcription (11). In our study, insulin and corticosterone, the major glucocorticoid found in the plasma of rats, were not affected by dietary fats, thus giving no indication of any involvement of this hormone in the up-regulating effect of olive oil on UCPs.
Several authors have proposed that fatty acids represent an important signal for the regulation of UCP activity (41). Weigle et al (42) showed that the elevation of circulating fatty acids in rats caused by the infusion of Intralipid (Pharmacia, Clayton, NC) plus heparin strongly stimulates UCP3 expression in skeletal muscle. Nevertheless, there are important discrepancies concerning the notion of a cause-effect relation between elevated fatty acids during starvation and up-regulation of muscle UCPs. In this context, Samec et al (43) observed that administration of the antilipolytic agent nicotinic acid to prevent the surge in circulating fatty acids during fasting prevented an increase in UCP2 and UCP3 mRNA expression in the soleus muscle, but had little or no effect on the elevated UCP2 and UCP3 mRNA expression in the gastrocnemius and tibiales anterior muscles. These authors also observed that the restoration of serum fatty acids to similar concentrations as in controls during the course of weight recovery was not accompanied by a restoration of mRNAs of UCP homologues in the gastrocnemius muscle (16). Finally, it was shown that differences in plasma fatty acid concentrations induced by refeeding with diets high in various type of fat did not correlate with muscle UCPs (30, 44).
In our study, the effects of dietary fat on UCP3 gene expression in gastrocnemius muscle (predominantly fast-glycolytic fibers) were not related to modifications in circulating fatty acids because no significant differences in fatty acids were found among our experimental groups. These results agree well with those obtained in the previously mentioned studies.
The explanation of the effects of olive oil is not clear. It seems that these effects are not mediated by systemic metabolic changes, but rather may be related to a local effect produced by oleic acid on IBAT and gastrocnemius muscle. In our laboratory, we observed that, after a 4-wk olive oil feeding period, oleic acid concentrations were increased in the stored triacylglycerols (45) and also incorporated into the plasma membrane phospholipids (46) in both perirenal and subcutaneous WAT. Although not measured, it can be expected that the same would be true for the triacylglycerols stored in IBAT, skeletal muscle, and mitochondrial membranes. This potential increase in oleic acid might modify the response of IBAT and gastrocnemius muscle to adrenaline. Furthermore, the modifications in membrane phospholipids could lead to modifications of the membrane-receptor interactions of the transduction of the hormonal signal (23, 47–49).
Fatty acids have been reported to act as transcriptional regulators of the expression of lipid-related genes in rodents (23, 50). This effect is likely mediated by proliferator-activated receptors (PPARs). In addition, expression and activation of PPAR was proposed as a mechanism for induction of BAT differentiation and UCP1, UCP2, and UCP3 gene expression without affecting UCP2 in WAT (51, 52). These findings together suggest that an activation of PPAR by oleic acid could be responsible for the up-regulating effect of olive oil on UCP in BAT. This hypothesis is supported by the study published by Hwang and Lane (53), who observed that oleic acid activated the expression of UCP3 mRNA in differentiated C2C12 muscle myotubes in a time- and concentration-dependent manner. However, even if this hypothesis is accepted, the effect of olive oil on UCP3 expression in gastrocnemius muscles would remain unclear because it was suggested that the activation of PPAR leads to a down-regulation of UCP3 mRNA in skeletal muscles (54) or causes no effect (52, 55).
In summary, a 4-wk period of olive oil feeding resulted in an up-regulation of UCP3 mRNA expression in gastrocnemius muscles and of UCP1, UCP2, and UCP3 mRNA expression in IBAT. In contrast, UCP2 mRNA expression in perirenal WAT was not modified, indicating different in vivo gene regulation. Further investigation is required to elucidate of the exact mechanism responsible for the effects of olive oil. The possibility that the effects of olive oil feeding can also be elicited in human tissues, particularly in skeletal muscles, gives new stimuli for research in this field to find new strategies for obesity treatment.
REFERENCES