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Nutrient Metabolism

Trans-10, Cis-12 Conjugated Linoleic Acid Increases Fatty Acid Oxidation in 3T3-L1 Preadipocytes¹

Graduate Program in Nutrition, University of North Carolina at Greensboro, Greensboro, NC 27402 and ^* Department of Animal Science, North Carolina State University, Raleigh, NC 27695

²To whom correspondence should be addressed. E-mail: mkmcinto{at}uncg.edu.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
The purpose of this study was to examine the effect of 0–50 µmol/L trans-10, cis-12 conjugated linoleic acid (CLA) and cis-9, trans-11 CLA isomers on lipid and glucose metabolism in cultures of differentiating 3T3-L1 preadipocytes. Specifically, we investigated the effects of 6 d of CLA treatment on the following: 1) ¹⁴C-glucose and ¹⁴C-oleic acid incorporation and esterification into lipid; 2) ¹⁴C-glucose and ¹⁴C-fatty acid oxidation; and 3) basal and isoproterenol-stimulated lipolysis. Trans-10, cis-12 CLA supplementation (25 and 50 µmol/L) increased both ¹⁴C-glucose and ¹⁴C-oleic acid incorporation into the cellular lipid fraction, which was primarily triglyceride (TG), compared with bovine serum albumin (BSA) controls. Although glucose oxidation (¹⁴C-glucose to ¹⁴C-CO₂) was unaffected by CLA supplementation, oleic acid oxidation (¹⁴C-oleic acid to ¹⁴C-CO₂) was increased by 55% in the presence of 50 µmol/L trans-10, cis-12 CLA compared with BSA controls. In contrast, 50 µmol/L linoleic acid (LA) and cis-9, trans-11 CLA-treated cultures had 50% lower CO₂ production from ¹⁴C-oleic acid compared with control cultures after 6 d of fatty acid exposure. Finally, 50 µmol/L trans-10, cis-12 CLA modestly increased basal, but not isoproterenol-stimulated lipolysis compared with control cultures. Thus, the TG-lowering actions of trans-10, cis-12 CLA in cultures of 3T3-L1 preadipocytes may be via increased fatty acid oxidation, which exceeded its stimulatory effects on glucose and oleic acid incorporation into lipid.

KEY WORDS: • conjugated linoleic acid • preadipocytes • glucose and fatty acid oxidation • glucose and fatty acid incorporation • lipolysis

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Conjugated linoleic acid (CLA)³ consists of a group of positional and geometric fatty acid isomers derived from linoleic acid (LA) [18:2(n-6)]. CLA is found as a mixture of isomers (predominantly cis-9, trans-11 CLA) in pasteurized cheeses, dairy products and the meat of ruminant animals. Commercial sources of CLA used in most research studies contain predominantly cis-9, trans-11 (40%) and trans-10, cis-12 (40%) CLA. Feeding a crude mixture of CLA isomers reduces body fat (1 –14 ), while increasing lean body mass in hamsters (11 ), rodents (2 –6 ) and pigs (7 ,8 ). In humans, CLA consumption (3.4–6.8 g/d) for 3 mo reduced body fat mass of obese and overweight adult men and women (15 ). In contrast, Zambell et al. (16 ) found that CLA consumption (3 g/d mixed isomers) over 3 mo did not affect fat mass, fat-free mass, percentage of body fat or body weight in normal-weight human subjects, whereas Medina et al. (17 ) and Berven et al. (18 ) found no change in body fat or body weight when overweight or obese subjects consumed mixed CLA isomers. Thus, the effects of CLA on obesity and lean body mass in humans remain controversial.

In vitro, several studies have shown that treatment with a mixture of CLA isomers lowers the lipid content of murine (pre)adipocytes (2 ,5 ,19 ). Moreover, the trans-10, cis-12 isomer of CLA is the isomer that is believed to reduce triglyceride (TG) content (5 ,19 ,20 ). More recently, we have shown that although trans-10, cis-12 CLA lowers TG content, the isomer differentially affects peroxisome proliferator-activated receptor -2 (PPAR -2) protein expression without significantly altering the level of adipogenic fatty acids such as arachidonic acid in cultures of 3T3-L1 preadipocytes (21 ). In contrast, Choi et al. (20 ) found that trans-10, cis-12 CLA reduced stearoyl-CoA desaturase activity and mRNA levels without affecting PPAR- or adipocyte fatty acid binding protein mRNA, suggesting that CLA may be interfering with the desaturation of long-chain fatty acids and their subsequent esterification into TG.

The aforementioned studies suggest that the antiobesity actions of a crude mixture of CLA isomers may be due to the direct influence of trans-10, cis-12 CLA on lipid metabolism rather than adipocyte differentiation per se. However, the specific mechanism by which CLA reduces the TG content of (pre)adipocyte cultures remains to be determined. Several possible mechanisms include the following: 1) decreased incorporation of glucose and/or fatty acids into TG (de novo lipogenesis); 2) increased oxidation of glucose and/or fatty acids; or 3) increased lipolysis. Therefore, the purpose of this study was to examine in differentiating cultures of 3T3-L1 preadipocytes the effects of trans-10, cis-12 and cis-9, trans-11 CLA isomers on ¹⁴C glucose and ¹⁴C-oleic acid incorporation and esterification into cellular lipids (Exp. 1), ¹⁴C-glucose and ¹⁴C-oleic acid oxidation to ¹⁴C-CO₂ (Exp. 2) and basal and isoproterenol-stimulated lipolysis (Exp. 3).

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Cell model.

Postconfluent monolayers of 3T3-L1 preadipocytes were used as the cellular model for these studies. 3T3-L1 preadipocytes are a nontransformed cell line, which is a continuous substrain of Swiss albino 3T3 murine cells developed through clonal expansion (22 ). These cells can be converted from a preadipose to adipose-like phenotype when appropriately stimulated. In addition, these cells are capable of differentiating in culture in response to agents that induce adipose tissue differentiation in vivo.

Experimental designs and culture conditions.

As outlined in Figure 1 , the objective of Exp. 1 was to determine whether increasing doses of cis-9, trans-11 and trans-10, cis-12 isomers of CLA decreased the incorporation of ¹⁴C-glucose (Exp. 1a) and ¹⁴C-oleic acid (Exp. 1b) into the organic cellular fraction in cultures of 3T3-L1 preadipocytes. In Exp. 1c, the effect of CLA supplementation on ¹⁴C-glucose and ¹⁴C-oleic acid incorporation into specific neutral and polar lipid classes was examined. In Exp. 2, the ability of trans-10, cis-12 CLA to increase the oxidation of ¹⁴C-glucose and ¹⁴C-oleic acid (thereby producing increased amounts of ¹⁴C-CO₂) compared with cis-9, trans-11 CLA- or LA-treated cultures was assessed. Finally, Exp. 3 determined whether trans-10, cis-12 CLA increased basal and/or isoproterenol-stimulated lipolysis. All reagents were obtained from Sigma Chemical (St. Louis, MO) unless otherwise indicated.

View larger version (14K): [in this window] [in a new window]   Figure 1. Design of experiments. Cultures were seeded and allowed to proliferate until they reached confluency. Two days postconfluency, cultures were induced to differentiate. Fatty acids and 0.2 mmol/L -tocopherol were added fresh at each change of media. Experiments were conducted as shown above.

Cells were seeded at a density of 3.3 x 10³/cm² in 12-well plates (Falcon, Becton Dickson Labware, Franklin Lakes, NJ) and cultured in Dulbecco’s modified Eagle’s medium (DMEM), 10% bovine calf serum and antibiotics until confluent. Two days postconfluency, the cells were stimulated to differentiate with DMEM containing 10% fetal bovine serum (FBS) (charcoal stripped to remove endogenous fatty acids), 10 mg/L insulin, 0.5 mmol/L 3-isobutyl-1-methylxanthine, 0.1 µmol/L dexamethasone, 0.2 mmol/L -tocopherol and 1% antibiotics. On d 3 of differentiation, the above media was replaced with DMEM, 10% stripped FBS, 2.5 mg/L insulin, 0.2 mmol/L -tocopherol and 1% antibiotics. From d 5 onward, media containing DMEM, 10% stripped FBS, 0.2 mmol/L -tocopherol and 1% antibiotics were used. For the ¹⁴C-glucose incorporation and oxidation experiments, the media on d 5 of differentiation was replaced with low-glucose DMEM (5 mmol/L).

Cell number (Exps. 1–3).

Adherent cells were harvested in a cell counting solution (25 mmol/L glucose, 0.154 mmol/L NaCL, 0.01 mol/L NaPO₄ [monobasic], 5 mmol/L EDTA, 2% albumin, pH 7.4) and counted on a Coulter Multisizer IIE (Coulter, Miami, FL).

Glucose incorporation (Exp. 1a).

¹⁴C-glucose incorporation was determined on d 7 of differentiation after chronic treatment with 0 (BSA control), 25 or 50 µmol/L cis-9, trans-11 CLA or trans-10, cis-12 CLA. On d 7, the media were replaced with low glucose DMEM supplemented with 37 KBq [U-¹⁴C]-D-glucose/L medium [specific activity: 9.2 GBq/mmol (ICN, Costa Mesa, CA)] for 2 h at 37°C. A time-course study indicated a linear increase in radiolabeled glucose incorporation into lipid over a 4-h period (data not shown). After 2 h, media were removed, the cell monolayers were washed twice, harvested in PBS with vigorous triteration and added to glass vials containing 3.75 mL of a chloroform/methanol solution (2:1, v/v). To separate the aqueous and organic fractions, the cellular mixture was capped and mixed vigorously for 1 min. Chloroform (1.25 mL) was added and the tube mixed vigorously for 1 min. Water (1.25 mL) was added and the tube mixed vigorously for 1 min. The mixture was then centrifuged at 200 x g for 5 min and the bottom layer (organic) was transferred to a scintillation vial. The top layer trans(aqueous) was poured into another scintillation vial and 1 mL of this mixture was transferred to a scintillation vial. The organic fraction was then allowed to evaporate under N₂ gas. Scintillation fluid (5 mL; Scintisafe, Fisher Scientific, Norcross, GA) was added to the vials containing the aqueous and organic fractions and allowed to stand for 1 h. The ¹⁴C content was determined on a Beckman LS 6000 Scintillation Counter (Beckman Instruments, Palo Alto, CA). Mean glucose incorporation is expressed as nmol/(h · 10⁶ cells). To control for unincorporated residual ¹⁴C-glucose, a set of cultures was exposed to ¹⁴C-glucose and immediately washed, harvested and fractionated. The radioactivity in the lipid and aqueous fractions was subtracted from the total counts in the respective treated fractions.

Oleic acid incorporation (Exp. 1b).

¹⁴C-oleic acid incorporation was determined on d 7 of differentiation after chronic treatment of the cultures with 0, 25 or 50 µmol/L cis-9, trans-11 CLA, trans-10, cis-12 CLA or LA. On d 7, the media were supplemented with 18.5 KBq [1-¹⁴C]-oleic acid/L media [specific activity: 1.9 GBq/mmol (NEN-Perkin Elmer Life Sciences, Boston, MA)] for 2 h at 37°C. A time-course study indicated a linear increase in radiolabeled oleic acid incorporation into lipid over a 3-h period (data not shown). Unincorporated ¹⁴C-oleic acid was subtracted from each assay using the procedure described above. Mean oleic acid incorporation is expressed as nmol/(h · 10⁶ cells).

Glucose and oleic acid incorporation into lipid classes (Exp. 1c).

¹⁴C-glucose and ¹⁴C-oleic acid incorporation into polar and neutral lipids was determined on d 7 of differentiation after chronic treatment with 50 µmol/L LA, cis-9, trans-11 CLA, trans-10, cis-12 CLA or vehicle (BSA). On d 7, the media were replaced with DMEM (low-glucose DMEM for the ¹⁴C-glucose experiment) supplemented with 18.5 KBq [U-¹⁴C]-D-glucose [specific activity: 9.2 GBq/mmol (ICN)] or 18.5 KBq [1-¹⁴C]-oleic acid/L medium [specific activity: 1.9 GBq/mmol (NEN-Perkin Elmer Life Sciences)] for 2 h at 37°C. ¹⁴C incorporation into the organic fraction was then isolated as described above for ¹⁴C-glucose and ¹⁴C-oleic acid incorporation. The organic fraction was dried under N₂ gas and frozen in scintillation vials at -20°C until analysis.

Quantification of ¹⁴C-glucose and ¹⁴C-oleic acid incorporation into the neutral lipid fraction was determined using radio-HPLC. HPLC was performed using a Waters 600E multisolvent delivery system with 717 plus autosampler and 996 photodiode detector (Waters, Milford, MA) connected to an in-line ß-radiochromatography detector (Radiomatics Flo-one/Beta, Flow Scintillation Analyzer 500 TR Series, Packard Instrument, Meriden, CT). The column was a Hibar Pre-Packed Column RT 250–4 (Lichrospher silica 5 µm) purchased from Alltech Associates (Deerfield IL). Chromatographic conditions followed the procedure of Patton et al. (23 ) with slight modification. Mobile phase was composed of hexane/tetrahydro-furan/acetic acid (500:20:0.1). The flow rate was 1 mL/min for the first 5 min at which point it was increased to 1.5 mL/min. Liquid scintillation cocktail (Biosafe II, Research Products International, Mount Prospect, IL) was mixed with HPLC eluate at a ratio of 3:1 (v/v) before entering the 0.5 counting loop of the detector. TG, cholesterol ester (CE), monoglycerol and free fatty acid (FFA) peaks were identified at 205 nm based on the retention times compared with injection of each individual standard. The retention times of TG, CE and FFA were confirmed by injection of ¹⁴C-labeled triolein (2 GBq/mmol, American Radiolabeled Chemicals, St. Louis, MO). Signals from the photodiode detector were processed by computer using Millenium 2.1 HPLC software. ¹⁴C-TG content was determined by collecting the TG peak from the ß flow monitor in scintillation fluid (Biosafe II) followed by counting on a Beckman LS 6500 scintillation counter. To control for unincorporated residual ¹⁴C-glucose and ¹⁴C-oleic acid, a set of cultures was exposed to ¹⁴C-glucose or ¹⁴C-oleic acid, immediately washed, harvested and fractionated. The basal radioactivity in the lipid fraction was then subtracted from the total counts in the treated samples. Data were expressed as nmol/(h · 10⁶ cells).

¹⁴C-CO₂ production from ¹⁴C-glucose. (Exp. 2a)

¹⁴C-CO₂ production from ¹⁴C-glucose was determined on d 7 of differentiation after chronic treatment with 50 µmol/L LA, cis-9, trans-11 CLA, trans-10, cis-12 CLA, or 50 µmol/L trans-10, cis-12 CLA plus 50 µmol/L LA. Experiments were conducted in 25 cm² flasks (T25 Falcon flasks) that were supplemented with 18.5 KBq [U-¹⁴C]-D-glucose/L medium [specific activity: 9.2 GBq/mmol (ICN)] and sealed with a rubber stopper fitted with a plastic center well (Kontes, Vineland, NJ) containing filter paper. After a 90-min incubation, the filter paper was injected with 100 µL ethidium hydroxide (Sigma). After 30 min, 0.5 mL of 1 mol/L H₂SO₄^2- was injected into the monolayers to terminate the reaction and liberate the ¹⁴C-CO₂. The cultures were allowed to sit for 30 min for the ¹⁴C-CO₂ to collect on the filter paper. Wells containing the filter paper were clipped from the rubber stopper, placed in 5 mL of scintillation fluid and counted using a Beckman LS 6000 Scintillation counter. ¹⁴C-CO₂ production was calculated as dpm/(h · 10⁶ cells) and expressed as a percentage of vehicle control (BSA).

¹⁴C-CO₂ production from ¹⁴C-oleic acid. (Exp. 2b.)

¹⁴C-CO₂ production from ¹⁴C-oleic acid was determined on d 7 of differentiation after chronic treatment with 50 µmol/L LA, cis-9, trans-11 CLA, trans-10, cis-12 CLA or 50 µmol/L trans-10, cis-12 CLA plus 50 µmol/L LA. Experiments were conducted in 25 cm² flasks (T25 Falcon flasks) that were supplemented with 18.5 KBq [1-¹⁴C]-oleic acid/L medium [specific activity: 1.9 GBq/mmol (NEN-Perkin Elmer Life Sciences)] and sealed with a rubber stopper fitted with a plastic center well. ¹⁴C-CO₂ production was then determined as above, calculated as dpm/(h · 10⁶ cells), and expressed as a percentage of control (BSA).

Lipolysis (Exp. 3).

On d 5 of differentiation, the media were supplemented with 50 µmol/L LA, cis-9, trans-11 CLA, trans-10, cis-12 CLA, trans-10, cis-12 CLA plus 50 µmol/L LA or vehicle (BSA) control. After 48 h of treatment, conditioned media were removed for glycerol determination and fresh differentiation media were added containing 1 µmol/L isoproterenol to stimulate lipolysis and 10 U adenosine deaminase to prevent adenosine-induced inhibition of lipolysis at 37°C for 2 h. Conditioned media were then removed for glycerol determination. Cells were harvested using cell counting solution to determine cell number. Glycerol content was measured using a commercially available colorimetric kit (Sigma #339–10), modified for cell culture as previously described (24 ). This procedure measures glycerol by enzyme-coupled reduction of a dye that absorbs light at 520 nm and can be quantified spectrophotometrically. Free glycerol in the media was expressed as µmol/(h · 10⁶ cells).

Statistical analyses.

Data were analyzed using a commercially available software package (SuperANOVA, Abacus Concepts, Berkeley CA). For Exp. 1 a and b, a three-way least-squares ANOVA (fatty acid x dose x replicate) was conducted and the fatty acid x dose interactions were compared for significance at the P < 0.05 level. For Exps. 1c, 2 and 3, a two-way ANOVA (fatty acid x replicate) was performed, and the effect of fatty acid treatment was compared for significance at the P < 0.05 level. Each treatment combination per experiment was conducted in triplicate and repeated at least once (e.g., n = 6) unless otherwise indicated.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Exp. 1.

¹⁴C-glucose incorporation (Exp. 1a) into the lipid fraction is shown in Figure 2 . Cultures treated with 25 and 50 µmol/L trans-10, cis-12 CLA had greater glucose incorporation into the lipid fraction compared with vehicle controls. Cis-9, trans-11 CLA, on the other hand, had no effect (P > 0.05) on glucose incorporation into the lipid fraction. Approximately 40% of the initial ¹⁴C-glucose remained in the media after incubation (data not shown). There were no treatment differences (P > 0.05) in unincorporated ¹⁴C-glucose.

View larger version (13K): [in this window] [in a new window]   Figure 2. Effect of treatment with increasing doses of cis-9, trans-11 conjugated linoleic acid (CLA) (9 ,11 ), or trans-10, cis-12 CLA (10 ,12 ) for 6 d on 14C-glucose incorporation into the lipid fraction after 2 h of incubation with 37 KBq 14C-glucose. Means (±SEM; n = 6) not sharing a superscript are different, P < 0.05.

View larger version (16K): [in this window] [in a new window]   Figure 3. Effect of treatment with increasing doses of linoleic acid (LA), cis-9, trans-11 conjugated linoleic acid (CLA) (9 ,11 ) or trans-10, cis-12 (10 ,12 ) CLA for 6 d on 14C-oleic acid incorporation into the lipid fraction after 2 h of incubation with 18.5 KBq 14C-oleic acid. Means (±SEM; n = 6) not sharing a superscript are different, P < 0.05.

View larger version (29K): [in this window] [in a new window]   Figure 4. Effect of treatment with 50 µmol/L linoleic acid (LA), cis-9, trans-11 conjugated linoleic acid (CLA) (9 ,11 ), trans-10, cis-12 CLA (10 ,12 ) or bovine serum albumin (BSA; vehicle control) for 6 d on 14C-glucose incorporation into triglycerides (TG) after 2 h of incubation with 37 KBq 14C-glucose. Means (+SEM; n = 5) not sharing a superscript are different, P < 0.05.

View larger version (33K): [in this window] [in a new window]   Figure 5. Effect of treatment with 50 µmol/L linoleic acid (LA), cis-9, trans-11 conjugated linoleic acid (CLA) (9 ,11 ), trans-10, cis-12 CLA (10 ,12 ) or bovine serum albumin (BSA; vehicle control) for 6 d on 14C-oleic acid incorporation into triglycerides (TG) after 2 h of incubation with 37 KBq 14C-oleic acid. Means (+SEM; n = 5) not sharing a superscript are different, P < 0.05.

To determine the effect of trans-10, cis-12 CLA on glucose (Exp. 2a) and fatty acid (Exp. 2b) oxidation, cultures were treated with either ¹⁴C-glucose or ¹⁴C-oleic acid for 2 h and ¹⁴C-CO₂ production was measured. Cultures treated with 50 µmol/L LA produced more ¹⁴C-CO₂ after incubation with ¹⁴C-glucose compared with BSA controls (Fig. 6 ). There was no difference in ¹⁴C-CO₂ production from ¹⁴C-glucose after any of the other fatty acid treatments. In contrast, cultures treated with 50 µmol/L trans-10, cis-12 CLA and trans-10, cis-12 CLA plus 50 µmol/L LA had 55 and 85% higher rates of ¹⁴CO₂ production, respectively, after incubation with ¹⁴C-oleic acid compared with controls (Exp. 2b,Fig. 7 ). Cultures treated with LA or cis-9, trans-11 CLA, on the other hand, produced 50% less ¹⁴C-CO₂ from ¹⁴C-oleic acid compared with BSA controls.

View larger version (43K): [in this window] [in a new window]   Figure 6. Effect of treatment with 50 µmol/L linoleic acid (LA), cis-9, trans-11 conjugated linoleic acid (CLA) (9 ,11 ), trans-10, cis-12 CLA (10 ,12 ), trans-10, cis-12 CLA plus 50 µmol/L linoleic acid (10,12 + LA) or bovine serum albumin (BSA; vehicle control) for 6 d on 14C-CO2 production after incubation with 14C-glucose for 2 h. 14C-CO2 production [dpm/(h · 106 cells)] is expressed as percentage 14C-CO2 production of BSA controls. Means (+SEM; n = 6) not sharing a superscript are different, P < 0.05.

View larger version (29K): [in this window] [in a new window]   Figure 7. Effect of treatment with 50 µmol/L linoleic acid (LA), cis-9, trans-11 conjugated linoleic acid (CLA) (9 ,11 ), trans-10, cis-12 CLA (10 ,12 ), trans-10, cis-12 CLA plus 50 µmol/L linoleic acid (10,12 + LA) or bovine serum albumin (BSA; vehicle control) for 6 d on 14C-CO2 production after incubation with 14C-oleic acid for 2 h. 14C-CO2 production [dpm/(h · 106 cells)] is expressed as percentage 14C-CO2 production of BSA controls. Means (+SEM; n = 6) not sharing a superscript are different, P < 0.05.

As shown in Figure 8 , basal glycerol release was 18% higher in trans-10, cis-12 CLA and trans-10, cis-12 CLA plus LA-treated cultures compared with BSA controls. After a 2-h incubation period with 1 µmol/L isoproterenol plus 1 µmol/L adenosine deaminase, glycerol release increased by 300-fold. However, there was no difference (P > 0.05) in glycerol release among cultures treated with trans-10, cis-12 CLA, LA or trans-10, cis-12 CLA plus LA compared with controls. In contrast, cultures treated with 50 µmol/L cis-9, trans-11 CLA had 11% lower isoproterenol-stimulated release of glycerol into the medium compared with BSA controls.

View larger version (21K): [in this window] [in a new window]   Figure 8. Effect of treatment with 50 µmol/L linoleic acid (LA), cis-9, trans-11 conjugated linoleic acid (CLA) (9 ,11 ), trans-10, cis-12 CLA (10 ,12 ), trans-10, cis-12 CLA plus 50 µmol/L linoleic acid (10,12 + LA) or bovine serum albumin (BSA; vehicle control) for 48 h on glycerol release before and after 2 h of stimulation with isoproterenol in the presence of adenosine deaminase. Means (+SEM; n = 6) not sharing a superscript are different, P < 0.05.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

In another study, West et al. (13 ) found a decrease in nighttime respiratory quotient in mice consuming 1.0–1.2% (wt/wt) mixed isomers of CLA for 6 wk, indicating an increase in fat oxidization after CLA consumption. Furthermore, fatty acid analyses of various tissues have consistently found higher levels of cis-9, trans-11 CLA than trans-10, cis-12 CLA (28 –33 ), although this is not true of the serum fatty acid profile of rats fed 1% (wt/wt) mixed CLA isomers for 2 wk (34 ). However, the degree to which this stimulation in fatty acid oxidation may be responsible for the TG-lowering actions of CLA remains to be determined.

Contrary to our hypothesis, 50 µmol/L trans-10, cis-12 CLA increased ¹⁴C-glucose and ¹⁴C-oleic acid incorporation into lipid. We initially hypothesized that trans-10, cis-12 CLA would decrease glucose and/or fatty acid incorporation into lipid, thereby decreasing TG synthesis. However, Satory and Smith (35 ) also reported an increase in glucose incorporation into lipid using 3T3-L1 preadipocytes, although these researchers used a crude mixture of CLA isomers (40% cis-9, trans-11 and 40% trans-10, cis-12). Furthermore, they assessed glucose incorporation into the lipid fraction without determining incorporation into TG specifically. To our knowledge, this is the first time that the effects of individual isomers of CLA, vis-à-vis trans-10, cis-12 CLA and cis-9, trans-11 CLA, on lipogenesis in preadipocytes have been examined. In keratinocyte cultures, Jun et al. (36 ) observed an increase in ³H-acetate incorporation into cholesterol esters and ³H-glycerol into TG in cultures treated with 50–250 µmol/L mixed isomers of CLA. Thus, our observed increase in glucose and oleic acid incorporation is consistent with several CLA studies and argues against the hypothesis that CLA reduces TG in adipocytes via a reduction in lipogenesis.

However, data from radiolabeled substrate studies are subject to interpretation, given their limitations. For example, one limitation of radiolabel studies is that the unlabeled endogenous pool size is unknown; therefore it is not possible to assess the actual flux of isotopes within the cultures. Therefore, the exogenous radiolabeled oleic acid may have been a larger portion of the endogenous fatty acid pool available for esterification in the trans-10, cis-12 CLA-treated cultures (because more of the available fatty acids were either released to the media through lipolysis or oxidized by ß-oxidation). Thus, an increased proportion of ¹⁴C-oleic acid may have been incorporated into the lipid fraction, although total TG was reduced. The same may hold true for our ß-oxidation results in that ¹⁴C-oleic acid may represent a larger proportion of the fatty acids available for oxidation in the trans-10, cis-12 CLA-treated cultures. This would result in relatively more ¹⁴CO₂ produced from ¹⁴C-oleic acid, whereas cis-9, trans-11 CLA and LA-treated cultures would have a decreased amount of labeled fatty acid in proportion to the total endogenous fatty acid pool. However, the addition of LA to trans-10, cis-12 CLA-treated cultures did not reduce ¹⁴CO₂ production as expected, and the reason for this finding is unclear.

Trans-10, cis-12 CLA treatment of differentiated 3T3-L1 adipocytes also increased basal, but not isoproterenol-stimulated lipolysis. An increase in basal lipolysis was also observed in 3T3-L1 adipocytes treated with 100 µmol/L mixed CLA isomers for 48 h (2 ). However, a similar experiment with trans-10, cis-12 CLA by Park et al. (5 ) showed a 30-fold increase in basal glycerol release, whereas we observed an 18% increase in basal glycerol release. The reasons for these CLA-mediated differences in basal lipolysis are unclear.

The trans-10, cis-12 CLA-dependent increase in glucose and oleic acid incorporation, fatty acid oxidation and basal lipolysis suggests that trans-10, cis-12 CLA may increase fatty acid metabolism in adipocytes. More specifically, these results suggest that trans-10, cis-12 CLA may lower TG by increasing fatty acid turnover. In support of this concept, mice fed 1 g/kg diet mixed CLA isomers had increased adipose uncoupling protein-2 mRNA, a key regulator of the oxidative phosphorylation pathway (12 ), suggesting that CLA may increase energy expenditure by reducing the efficiency of energy metabolism. Furthermore, West et al. (13 ) found an increase in energy expenditure, but no change in de novo fatty acid synthesis, in mice consuming 1 g/kg diet mixed CLA in the presence of a high fat diet.

CLA has also been shown to increase the release of tumor necrosis factor- (TNF-) in mice fed 1% mixed CLA isomers for 8 wk (12 ). In addition to inhibiting insulin-dependent stimulation of lipogenesis, TNF- decreased the expression of acyl-CoA carboxylase, lipoprotein lipase and fatty acid synthase mRNA as well as increased delipidation of adipocytes [for review, see (37 )]. This TNF-–mediated impairment of insulin signaling, down-regulation of lipogenic enzymes and delipidation may explain the observed reduction in TG content seen in CLA-treated adipocytes [for review, see (38 )]. However, the mechanism by which CLA induces TNF- production remains to be determined.

It is also important to note that we assessed the effects of chronic CLA treatment on lipid synthesis and oxidation after 6 d of fatty acid supplementation. Thus, it is possible that the effects of acute CLA supplementation on lipid metabolism during the first few days of differentiation may differ from our chronic treatment data. Recent research suggests that adipocytes reach a limit in their ability to synthesize and store lipid [for review, see (39 )]. Thus, in vivo, once adipocytes attain a certain level of lipid storage, it is believed that lipid-synthesizing enzymes are down-regulated and lipid metabolism is shifted to the liver. Thus, it is possible that in our experiments, cultures treated with BSA alone, LA or cis-9, trans-11 CLA reached maximal lipid storing capacity and subsequently down-regulated the enzymes necessary for lipid synthesis and storage, whereas trans-10, cis-12 CLA-treated cultures were still actively synthesizing lipid at d 6 of differentiation. This would explain the increased ¹⁴C-oleic acid incorporation into lipid and TG seen with trans-10, cis-12 CLA treatment compared with LA and BSA control cultures. Therefore, future studies examining the acute effects of CLA on lipid metabolism during early differentiation are warranted.

In conclusion, we have shown that trans-10, cis-12 CLA and cis-9, trans-11 CLA have differential effects on lipid metabolism in 3T3-L1 preadipocytes. Specifically, 50 µmol/L trans-10, cis-12 CLA increased oleic acid incorporation into TG by 25% compared with controls. In addition, cultures treated with 50 µmol/L trans-10, cis-12 CLA had a 55% increase in oleic acid, but not glucose oxidation, whereas basal lipolysis was increased by 18% compared with controls. The observed mild increase in TG synthesis from ¹⁴C-oleic acid as well as the dramatic increase in fatty acid oxidation suggests that trans-10, cis-12 CLA treatment may increase metabolism and fatty acid turnover in preadipocytes. Increased fatty acid turnover may explain how trans-10, cis-12 CLA reduces TG content in 3T3-L1 preadipocytes; however, this possibility remains to be examined further.

	FOOTNOTES

 
¹ Funded by the U.S. Department of Agriculture NRICGP Award #199903513 and the North Carolina ARS Project #06520.

³ Abbreviations used: BSA, bovine serum albumin; CE, cholesterol ester, CLA, conjugated linoleic acid; CPT-1, carnitine palmitoyltransferase-1; DMEM, Dulbecco’s modified Eagle’s medium; FBS, fetal bovine serum; FFA, free fatty acid; LA, linoleic acid; PPAR -2, peroxisome proliferator activated receptor -2; TG, triglyceride; TNF-, tumor necrosis factor-.

Manuscript received 7 September 2001. Initial review completed 11 October 2001. Revision accepted 18 December 2001.

	LITERATURE CITED

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 

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