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Conjugated linoleic acid (CLA), a family of fatty acids found in beef, dairy foods, and dietary supplements, reduces adiposity in several animal models of obesity and in some humans. However, the isomer-specific antiobesity mechanisms of action of CLA are unclear, and its use in humans is controversial. This review will summarize in vivo and in vitro findings from the literature regarding potential mechanisms by which CLA reduces adiposity including its impact on 1) energy metabolism, 2) adipogenesis, 3) inflammation, 4) lipid metabolism, and 5) apoptosis.
Conjugated linoleic acid (CLA) refers to a group of conjugated octadecadienoic acid isomers derived from linoleic acid, a fatty acid that contains 18 carbons and two double bonds in the cis configuration at the 9th and 12th carbons (i.e., cis-9, cis-12 octadecadienoic acid). Microbes in the gastrointestinal tract of ruminant animals convert linoleic acid into different isoforms of CLA through biohydrogenation. This process changes the position and configuration of the double bonds, resulting in a single bond between one or both of the two double bonds (i.e., cis-9, trans-11 (9,11) or trans-10, cis-12 (10,12) octadecadienoic acid).
Commercial preparations of CLA are made from the linoleic acid of safflower or sunflower oils under alkaline conditions. This type of processing yields a CLA mixture containing approximately 40% of the 9,11 isomer and 44% of the 10,12 isomer [reviewed in 1]. Commercial preparations also contain approximately 4% to 10% trans-9, trans-11 CLA and trans-10, trans-12 CLA, as well as trace amounts of other isomers.
The 9,11 isomer, also known as rumenic acid, is the predominant form of CLA found in naturally occurring foods. 9,11 CLA comprises approximately 90% of CLA found in ruminant meats and dairy products and the 10,12 isomer comprises the remaining 10%. Although several other isoforms of CLA have been identified (i.e., trans-9, trans-11; cis-9, cis-11; trans-10, trans-11; and cis-10, cis-12), the 9,11 and 10,12 isomers appear to be the most biologically active .
The proportion of CLA ranges from 0.34% to 1.07% of the total fat in dairy products, and from 0.12% to 0.68% of the total fat in raw or processed beef products [reviewed in 3 and 4, 5]. However, the CLA content of food is dependent on several factors including the season and the animal’s breed, nutritional status, and age [reviewed in 3]. The average daily intake of CLA is approximately 152 mg to 212 mg for non-vegetarian women and men, respectively , and human serum levels range from 10 µmol/L to 70 µmol/L [7,8].
CLA was initially discovered in 1987 by Pariza and colleagues, and it was first identified as an anti-carcinogen . Subsequently, CLA was shown to exhibit anti-atherosclerotic [reviewed in 10] and antiobesity properties [reviewed in 11]. Due to the substantial rise in obesity prevalence over the past 30 years , interest in CLA as a weight loss treatment has been increasing. Supplementation with a CLA mixture (i.e., equal concentrations of the 10,12 and 9,11 isomers) or the 10,12 isomer alone decreases body fat mass (BFM) in many animal and some human studies [reviewed in 11 and 13]. Of the two major isomers of CLA, the 10,12 isomer specifically is responsible for the antiobesity effects [14–18].
Park et al.  were the first to demonstrate that CLA modulated body composition. In this study, male and female mice given a 0.5% (w/w) CLA mixture had 57% and 60% lower BFM, respectively, compared to controls. Other researchers have subsequently demonstrated that CLA supplementation consistently reduces BFM in mice, rats, and pigs [20–24]. For example, dietary supplementation with 1% (wt/wt) CLA mixture for 28 days decreased body weight and periuteral white adipose tissue (WAT) mass in C57BL/6J mice . Similarly, a 1.0% to 1.5% (wt/wt) mixture of CLA for 3 to 4 weeks decreased body weight and WAT mass in male ob/ob  and ICR  obese mice.
Studies investigating CLA’s effects on BFM reduction in humans have produced less consistent results. Whereas some studies show that CLA decreases BFM and increases lean body mass (LBM) [reviewed in 13 and 16], others have shown no effect of CLA supplementation on body composition in humans [reviewed in 13]. For example, supplementation of a CLA mixture in overweight and obese people (3–4 g/day for 24 weeks) decreased BFM and increased LBM . On the other hand, supplementation of CLA mixture in yogurt in healthy adults (3.76 g/day for 14 weeks) had no effect on body composition . In addition, Larsen et al.  investigated the potential role of CLA for preventing body weight regain in moderately obese subjects who had lost approximately 10 kg after an 8-week dietary intervention on a low calorie diet. Supplementation for 1 year with a CLA mixture did not prevent body weight regain compared to controls in that study. Finally, supplementation with 3.2 g/day of a CLA mixture decreased total BFM and trunk fat compared to placebo in overweight subjects, but not obese subjects . These contradictory findings among human studies may be due to the following differences in experimental design 1) CLA isomer combination versus individual isomers, 2) CLA dose and duration of treatment, and 3) gender, weight, age and metabolic status of the subjects.
The primary discrepancy between animal and human studies appears to be the dose of CLA administered. For example, moderately overweight humans with an average weight of 72.5 kg supplemented with a CLA mixture (3.76 g/day for 14 weeks) experienced no decrease in body weight, BMI, or BFM . In contrast, C57BL/6 mice supplemented with 1.5% (w/w) CLA mixture for 4 weeks weighed significantly less and had reduced adiposity compared to controls . However, while subjects in the human study received approximately 0.05 g CLA/kg body weight, the mice received 1.07 g CLA/kg body weight, which was 20 times the human dose based on body weight. Supplementing humans with higher doses of CLA would address this dosing issue.
Because CLA has the potential to reduce BFM when given at high enough doses and is being taken as a supplement for that purpose, it is important to understand its mechanism of action. Therefore, this review will examine potential mechanisms by which the CLA mixture or 10,12 CLA alone reduces adiposity, with particular emphasis on its effects on WAT. Potential mechanisms to be discussed include regulation of 1) energy metabolism, 2) adipogenesis, 3) inflammation, 4) lipid metabolism, and 5) apoptosis.
Energy balance is a function of energy intake relative to energy expenditure. When energy intake exceeds energy expenditure, body weight and BFM increase, and vice-versa. Accordingly, potential mechanisms by which CLA reduces BFM include decreasing energy intake or increasing energy expenditure. Park et al.  demonstrated that mice supplemented with a CLA mixture or enriched 10,12 CLA for 4 weeks reduced their food intake. A number of subsequent studies in rodents produced similar results [16, 33–36]. No studies to date have demonstrated that CLA decreases food intake in humans [37–40].
A mechanism for this reduced food intake was suggested by So et al. , who reported that food intake was reduced by 23.6% in mice fed a low-fat diet supplemented with 10,12 CLA. In this study, mice receiving 10,12 CLA had a decreased gene expression ratio of proopiomelanocortin to neuropeptide Y (NPY) in the hypothalamus. These results suggest that CLA exerts an effect on hypothalamic appetite-regulating genes. In support of this hypothesis, injection of mixed isomers of CLA to the rat hypothalamus reduced the expression of NPY and agouti-related protein, neuropeptides that robustly increase food intake . Alternatively, CLA supplementation may reduce food intake by affecting the palatability of the diet, but so far there are no reports to support this hypothesis.
A number of studies have reported reduced adiposity without changes in energy intake following administration of a CLA mixture in mice [42–45]. For example, supplementation of mice with a CLA mixture for 42 days decreased total body weight without reducing food intake . These data indicate that CLA effects on body fat are not solely dependent on reductions of food or energy intake. Thus, although several studies show that CLA decreases energy intake, others show no effect, suggesting that CLA can decrease body fat independent of reducing energy intake.
Energy expenditure is a function of basal metabolic rate (BMR), adaptive thermogenesis, and physical activity. CLA has been proposed to reduce adiposity by elevating energy expenditure via increased BMR, thermogenesis, or lipid oxidation in animals [33, 43–47]. For example, in BALB/c male mice fed mixed isomers of CLA for 6 weeks, body fat was decreased by 50% compared to controls and was accompanied by increased BMR . Enhanced thermogenesis may be associated with an upregulation of uncoupling proteins (UCPs), which facilitate proton transport over the inner mitochondrial membrane, thereby diverting energy from ATP synthesis to heat production. UCP1 was the first member of the family to be isolated and is exclusively expressed in brown adipose tissue; UCP3 is expressed in muscle and a number of other tissues; and UCP2 is expressed in a variety of tissues, including WAT, and is the most highly-expressed UCP. Supplementation with a CLA mixture or 10,12 CLA in rodents has been shown to induce UCP2 transcription in WAT [27, 35, 48–50], but whether this plays a role in energy dissipation is unclear. CLA also increased the expression of another mitochondrial protein, carnitine palmitoyltransferase 1 (CPT1) in WAT of 10,12 CLA-treated mice [50, 51]. CPT1 is involved in mitochondrial FA uptake and catalyzes the rate-limiting step of FA oxidation. Consistent with these findings, 10,12 CLA increased beta-oxidation in differentiating 3T3-L1 mouse preadipocytes . Similarly, CLA supplementation has been shown to increase UCP expression and beta oxidation in rodent muscle and liver [35, 53–57].
On the other hand, results from human studies concerning CLA regulation of energy expenditure are mixed. For example, a recent investigation of human supplementation with a CLA mixture (3.9 g/day for 12 weeks) revealed no change in BMR or BFM . Similar results have been reported in other studies of humans supplemented with a CLA mixture [37, 58]. In contrast, healthy, moderately-overweight humans consuming a CLA mixture in yogurt (3.76 g/day for 14 weeks) exhibited higher BMR, although body weight was not affected . Similarly, supplementation with a CLA mixture for 13 weeks increased the resting metabolic rate and fat-free mass in human subjects without a corresponding effect on BFM . Thus far, only one human study has demonstrated both increased energy expenditure and decreased body weight in humans. In this study by Close et al., subjects supplemented with a CLA mixture (4 g/day for 6 months) had decreased body weight, and exhibited increased fat oxidation and energy expenditure while sleeping .
Other studies have demonstrated that CLA supplementation increases LBM, which is associated with higher levels of energy expenditure. For example, mixed CLA isomers (6.4 g/day for 12 weeks) increased LBM by 0.64 kg in healthy obese humans compared to controls . Similarly, mice fed a 0.4% (w/w) CLA mixture exhibited increased LBM compared to controls . Proposed mechanisms by which CLA increases LBM are via increased bone or muscle mass, which is supported by evidence from rodent studies. A 10-week 10,12 CLA supplementation (0.5% (wt/wt) mixed isomers) increased bone mineral density and muscle mass in C57BL/6 female mice . CLA supplementation is thought to increase bone mineral density by upregulating osteogenic gene expression and downregulating osteoclast bone resorbing activity [62, 63]. Similarly, CLA supplementation alone or with exercise increased bone mineral density in middle-aged female mice compared to controls .
Alternatively, CLA may suppress the adipogenesis of pluripotent mesenchymal stem cells (MSCs) in bone marrow, and instead enhance their commitment to become osteoblasts (bone-forming cells). Indeed, 10,12 CLA has been shown to preferentially promote the differentiation of human MSCs into osteoblasts in culture . In contrast, 9,11 CLA increased adipocyte differentiation and decreased osteoblast differentiation. Consistent with these in vitro data, CLA mixture supplementation of rats treated with corticosteroids, which decrease muscle and bone mass, prevented reductions in LBM, bone mineral density, and bone mineral content . Collectively, these findings suggest CLA may reduce adiposity through increased energy expenditure via increased mitochondrial uncoupling and fatty acid oxidation in WAT, or via increased muscle or bone mass. However, the extent to which CLA regulates BMR or LBM, and how this contributes to the reduction in body weight or fat in humans, remains to be determined.
The conversion of preadipocytes to adipocytes involves the activation of key transcription factors such as peroxisome proliferator-activated receptor (PPAR)γ and CAAT/ enhancer binding proteins (C/EBPs). During the differentiation process, increased C/EBPβ and C/EBPδ activity induces the transcription of C/EBPα and PPARγ , the master regulators of adipocyte differentiation (Fig. 1). There is much evidence showing that CLA suppresses preadipocyte differentiation in animal [18, 52, 67–71] and human [72, 73] preadipocytes. 10,12 CLA treatment has been reported to reduce adipogenesis and lipogenesis specifically by attentuating PPARγ, C/EBPα, sterol regulatory element binding protein 1c (SREBP-1c), liver X receptor (LXRα), and adipocyte fatty acid binding protein (aP2) expression [27, 71–75].
In rodents, 10,12 CLA supplementation decreased the expression of PPARγ and its target genes [24, 50, 71, 76]. In mature, in vitro-differentiated primary human adipocytes or in mature 3T3-L1 adipocytes, 10,12 CLA treatment leads to a substantial decrease in the expression and activity of PPARγ [18, 77], and a decrease in PPARγ target genes and lipid content . These and numerous other studies show that 10,12 CLA specifically is not only able to inhibit, but can reverse the adipogenic process and that this may, in part, be mediated by suppression of PPARγ activity (Fig. 2). The decrease in PPARγ target gene expression may be due to reduced PPARγ expression or to posttranslational inhibition of PPARγ activity per se. Because PPARγ directly or indirectly induces its own expression, decreased PPARγ activity would be expected to suppress PPARγ expression, which makes it difficult to determine the level at which the inhibition occurs. Time course experiments conducted in our laboratories, however, indicate that inhibition of PPARγ activity occurs prior to a decrease in PPARγ expression , suggesting that inhibition of PPARγ activity may be a primary event.
Many mechanisms exist by which CLA may posttranslationally regulate PPARγ. Transient transfection assays with ectopically-expressed PPARγ have been employed to assess CLA isomers as potential ligands for PPARγ. In such assays, both CLA isomers have been shown to only modestly activate PPARγ, even at high concentrations. However, they are able to effectively inhibit the action of full agonists such as rosiglitazone and darglitazone [18, 70, 77, 78], indicating that CLA may act as a low affinity partial agonist. Nonetheless, this mechanism cannot fully account for the 10,12 CLA-specific repression of PPARγ activity.
PPARγ activity may also be regulated by phosphorylation (Fig. 2), which can be mediated by the mitogen activated protein kinase (MAPK) pathway [79–81]. Ser-112 phosphorylation of PPARγ2, the form of PPARγ required for adipocyte differentiation, may decrease its activity via ubiquination and proteasome degradation , and by reducing both its ligand-dependent and ligand-independent transactivating functions [80, 83–85]. We have demonstrated that 24-hour treatment of 10,12 CLA increases PPARγ phosphorylation  without significantly decreasing its protein levels, suggesting that the subsequent downregulation of PPARγ target genes is due to decreased transactivating function. Intriguingly, robust extracellular signal-regulated kinase (ERK) phosphorylation is also observed 24 hours after CLA stimulation, suggesting a role for ERK in PPARγ phosphorylation and inactivation. Consistent with these data, we demonstrated that ERK activation is a key player in CLA’s suppression of adipogenic gene expression and insulin-stimulated glucose uptake . Therefore, it is tempting to speculate that CLA antagonizes PPARγ activity via activation of MAPKs like ERK, thereby leading to inhibition of PPARγ target genes (Fig. 2).
Finally, CLA may interfere with PPARγ activity by virtue of its proinflammatory effects in adipocytes (Fig. 3). We have shown that 10,12 CLA induces NFκB activation in adipocytes, and that this induction leads to increased expression of proinflammatory cytokines [86, 87]. In addition, NFκB or other proinflammatory transcription factors may interfere directly with PPARγ activation of target genes (Fig. 1–Fig. 3). This will be discussed in more detail below.
Although the primary function of WAT is energy storage, it also has the ability to produce a number of pro-inflammatory cytokines. These adipokines (i.e., cytokines produced by adipose tissue) can cause insulin resistance (Fig. 4), thereby suppressing lipid synthesis and increasing lipolysis in adipocytes (Fig. 5). Induction of these inflammatory genes is dependent on various cellular kinases including MAPK, and is driven by transcription factors like NFκB, which have been reported to directly antagonize PPARγ (Fig. 1–Fig. 3). Tumor necrosis factor (TNF)α in particular exerts potent antiadipogenic effects [88, 89], and interleukin (IL)-1β and interferon (IFN)γ have been observed to induce delipidation of human adipocytes . Treatment with 10,12 CLA has also been shown to increase the expression or secretion of IL-6 and IL-8 from murine [24, 50] and human [73, 77, 86] adipocyte cultures, as well as TNFα and IL-1β suppressing PPARγ activity and insulin sensitivity [32, 76, 77, 91].
In human subjects, 10,12 CLA supplementation also increases the levels of inflammatory prostaglandins (PG)s [58, 92]. For example, women supplemented with mixed CLA isomers (5.5 g/day for 16 weeks) exhibited higher levels of C-reactive protein in serum and the prostaglandin 8-iso-PGF2α in urine . Accordingly, the expression of cyclooxygenase 2 (COX-2), an enzyme involved in the synthesis of PGs, was elevated in cultures of newly differentiated human adipocytes treated with 10,12 CLA . Furthermore, 10,12 CLA increased PGF2α secretion from human adipocytes .
Inflammatory PGs like PGF2α have been reported to inhibit adipogenesis via phosphorylation of PPARγ by MAPKs , and via induction of the normoxic activation of the hypoxia inducible factor-1 (HIF-1). HIF-1 decreases PPARγ and C/EBPα expression by upregulating transcriptional repressor DEC1 [95, 96]. In addition, PGF2α may inhibit adipogenesis by inducing pro-inflammatory transcription factors that antagonize PPARγ.
Notably, data from our laboratory show that ERK and NFκB activation play a critical role in 10,12 CLA’s suppression of adipogenic genes and insulin-stimulated glucose uptake [73, 86]. The molecular mechanisms by which NFκB and other inflammatory transcription factors inhibit PPARγ activity are not completely understood, but results from a study in the bone marrow stromal cell line ST2 suggest that NFκB interacts directly with PPARγ, preventing it from binding DNA [97, 98]. In a different study using chromatin immunoprecipitation, the DNA binding activity of PPARγ did not appear to be affected by TNFα stimulation in 3T3-L1 adipocytes or human embryonic kidney 293 cells. Instead suppression of PPARγ activity involved IKK activation, leading to IκBα degradation and nuclear localization of histone deacetylase (HDAC)3, a component of the PPARγ corepressor complex [99, 100]. NFκB may also repress PPARγ activity via interaction with the DNA-bound retinoid X receptor (RXR)-PPARγ heterodimer, thereby interfering with coactivator recruitment.
Taken together, these data suggest that 10,12 CLA antagonizes PPARγ activity via inflammatory mediators such as MAPKs and NFκB, and/or via induction of the inflammatory PG and adipocytokine production that in turn antagonizes PPARγ activity.
Storage of FA as TG is a major function of adipocytes. Numerous proteins involved in lipogenesis, such as lipoprotein lipase (LPL), acetyl-CoA carboxylase (ACC), fatty acid synthase (FAS), and stearoyl-CoA desaturase (SCD), have been found to be decreased by supplementation with mixed isomers of CLA or 10,12 CLA alone [50, 72, 73, 101]. PPARγ is a major activator of many lipogenic genes including glycerol-3-phosphate dehydrogenase (GPDH), LPL, and lipin, as well as many genes encoding lipid droplet-associated proteins such as perilipin, adipocyte differentiation-related protein (ADRP), cell death-inducing DFFA-like effector c (CIDEC), and S3-12 . Thus, 10, 12 CLA may exert its anti-lipogenic effects, in part, through its ability to inhibit PPARγ activity. CLA repression of the lipogenic transcription factor SREBP-1 and its target genes may also play an important role. Finally, CLA suppression of insulin signaling may also affect the activation or abundance of a number of lipogenic proteins including LPL, ACC, FAS, SCD-1, and the insulin-dependent glucose transporter 4 (GLUT4).
Interestingly, 10,12 CLA or a CLA mixture have also been reported to reduce levels of monounsaturated FAs in rodents [27, 51] and in primary human adipocyte cultures . This may be due to the ability of CLA to repress SCD1 expression  and function required for monosaturated FA synthesis [103, 104]. However, 10,12 CLA supplementation was able to reduce body weight even in SCD-1 knockout mice, simultaneously increasing the ratio of 16:0/16:1 fatty acids and decreasing the ratio of 18:0/18:1 fatty acids . These results suggest that the antiobesity properties of CLA may also rely on other desaturases, which may include the isoenzyme SCD-2. For instance, body fat loss in mice fed a CLA mixture has been shown to require Δ6-desaturase .
Insulin-stimulated glucose uptake in WAT is mediated via GLUT4 (Fig. 4). Defects in insulin signaling or suppression of GLUT4 translocation to the plasma membrane are primary causes of insulin resistance in adipocytes (Fig. 4, ,5).5). Insulin resistance has been reported in overweight or obese mice  or humans [92, 106–108] and in cultures of 3T3-L1  or human adipocytes [72, 86, 87] following supplementation with a CLA mixture or 10,12 CLA alone. Moreover, supplementation with a CLA mixture or 10,12 CLA has been shown to induce hyperinsulinemia, associated with insulin resistance in animals and humans [reviewed in 13]. CLA may inhibit insulin signaling by 1) activating inflammatory pathways and stress kinases, and 2) downregulating expression of genes involved in the insulin signaling and glucose uptake pathways.
In addition, some studies in 3T3-L1 cells  and cultures of newly differentiated human adipocytes  have suggested that CLA inhibits insulin signaling via increased expression of suppressor of cytokine signaling (SOCS)-3. SOCS-3 impairs insulin signaling and glucose uptake by promoting the phosphorylation of the inhibitory serine 307 on insulin receptor substrate (IRS-1), leading to its ubiquination and proteasome degradation . CLA appears to induce SOCS-3 indirectly via inflammatory cytokines such as TNFα and IL-6 [73, 110]. 10,12 CLA treatment has also been demonstrated to decrease the protein levels of insulin receptor (IR)β  and IRS-1 [24, 86], signaling proteins critical for insulin sensitivity. In addition, 10,12 CLA treatment reduced tyrosine phosphorylation (i.e., activation) of insulin receptor (IR)β and IRS-1 in 3T3-L1 adipocytes .
10,12 CLA may also directly impair the uptake of glucose and fructose by suppressing the expression of their transporters. 10,12 CLA decreased GLUT4 gene and protein expression [71, 73, 86] in cultures of newly differentiated human adipocytes. In addition, CLA reduced the gene expression of GLUT4 and the glucose/fructose transporter SLC2A5 in WAT and 3T3-L1 adipocytes supplemented with 10,12 CLA .
CLA may also cause insulin resistance via its effects on the insulin-sensitizing hormone adiponectin. Adiponectin mRNA levels were decreased following supplementation with 10,12 CLA in mice  and in cultures of human adipocytes . Consistent with these data, 10,12 CLA or a CLA mixture decreased adiponectin assembly or secretion in cultures of murine adipocytes, respectively [18, 112]. Because adiponectin is a target gene of PPARγ , its suppression may be due, in part, to 10,12 CLA antagonizing PPARγ activity. Accordingly, the PPARγ agonist rosiglitazone was able to prevent CLA-induced suppression of adiponectin serum levels and insulin resistance in mice . However, another PPARγ agonist, troglitazone, did not prevent the 10,12 CLA suppression of adiponectin expression, although it prevented 10,12 CLA suppression of TG levels and adiponectin oligomer assembly in 3T3-L1 adipocytes . These results indicate that CLA may also suppress adiponectin expression by a PPARγ-independent mechanism.
Lipolysis is the process by which stored TG is mobilized, releasing free fatty acids (FFA) and glycerol through the action of hormone-sensitive lipase (HSL). Typically, when energy demand is increased, lipolysis is upregulated via cAMP-mediated signaling. CLA may induce lipolysis in WAT through its activation of proinflammatory pathways, thereby liberating FFA for uptake in metabolically-active tissues (i.e., liver and muscle) (Fig. 5). Acute treatment with mixed CLA isomers or 10,12 CLA alone increased lipolysis in 3T3-L1 [19, 101, 114] and newly differentiated human adipocytes . Furthermore, LaRosa et al.  observed increased mRNA levels of HSL in C57BL/6J mice fed 10,12 CLA for 3 days; however, HSL levels subsequently decreased following chronic (17 day) treatment.
Numerous studies in other species have investigated the effect of long-term CLA supplementation on lipolysis. Studies with mice or hamsters have demonstrated that chronic supplementation with a mixture of CLA has no effect on lipolysis [116, 117, 118]. In contrast chronic treatment with CLA (1–200 µmol/L mixed isomers) reduced glycerol release from isolated rat adipocytes (112). Consistent with these data, FFA levels have been reported to be lower in the serum of OLETF rats supplemented with a CLA mixture (1.0% (w/w) for 4 weeks) compared to controls . The lack of a chronic lipolysis effect may be due to depleted TG stores in WAT, which can lead to ectopic lipid accumulation seen in lipodystrophy syndromes. For example, supplementation with a CLA mixture or 10,12 CLA alone was shown to increase lipid accumulation in the liver of mice [120, 121] and hamsters [122, 123]. In summary, CLA induces inflammatory adipokines that likely impair insulin signaling, thereby decreasing TG synthesis and increasing lipolysis, leading to decreased WAT mass (Fig. 5).
Apoptosis is another mechanism by which CLA may be able to reduce BFM. Studies using mice [33, 34, 50] or 3T3-L1 murine adipocytes [52, 114] supplemented with 10,12 CLA or a CLA mixture have reported apoptosis in adipocytes. For example, mice fed a high-fat diet containing a 1.5% (w/w) CLA mixture had an increased ratio of BAX relative to Bcl2, an inducer and suppressor of apoptosis in the mitochondrial apoptotic pathway, respectively . Furthermore, supplementation of C57BL/6J mice with a 1% (w/w) CLA mixture reduced BFM and increased apoptosis and TNFα gene expression in WAT . TNFα gene expression and secretion have also been reported to be induced in mice by 10,12 CLA alone [16, 24]. TNFα is a potent inducer of apoptosis  and plays a critical role in adipocyte function [126)] TNFα gene expression was likewise induced by 10,12 CLA in cultures of newly differentiated human adipocytes, although its secretion was not detected [73, 91].
Besides the TNFα/death receptor and mitochondrial pathways, apoptosis can occur via activation of the integrated stress response (ISR) (Fig. 6). Microarray analysis revealed that 10,12 CLA treatment of mice [1% (w/w)] and 3T3-L1 adipocytes (100 µmol/L) increased the mRNA levels of genes involved in the ISR, such as activating transcription factor 3 (ATF3), C/EBP homologous protein (CHOP), pseudokinase Tribbles 3/SKIP 3 (TRIB3), X-box binding protein (XBP-1), and growth arrest and DNA damage inducible protein (GADD34) . CHOP is known to possess apoptotic characteristics and activation of this protein can lead to induction of GADD34 and TRIB3 [127, 128]. Notably, CLA-induced ISR activation in adipocytes was preceded by the induction of inflammatory genes such as IL-6 and IL-8 . In mouse mammary tumor cells, 10,12 CLA treatment (20–40 µmol/L) increased CHOP expression and ER stress leading to apoptosis [129, 130]. Collectively, these data suggest that CLA may induce adipocyte apoptosis via ER stress and the ISR, depending on the dose and isomer used. In vivo studies are needed to investigate whether the apoptotic effects of CLA in humans are specific to WAT.
Supplementation with a mixture of CLA isomers or 10,12 CLA alone reduces adiposity consistently in animal models, especially in rodents, but has been shown to reduce adiposity in only some human studies. Potential reasons for these species differences include 1) the CLA isomers used, 2) the dosage administered, and 3) age, body weight, body fat, or metabolic status of the animals or subjects. Of the major isomers, only 10,12 CLA reduces adiposity or TG content of WAT. Dosage differences among species can be considerable; rodent studies generally use ~20 times more CLA/kg body weight compared to human studies.
Potential mechanisms responsible for these antiobesity properties of 10,12 CLA include 1) decreasing energy intake by suppressing appetite 2) increasing energy expenditure in WAT, muscle, and liver tissue, or LBM, 3) decreasing lipogenesis or adipogenesis, 4) increasing lipolysis or delipidation, and 5) apoptosis via adipocyte stress, inflammation, and/or insulin resistance.
Based on these data, we propose the following working model (Fig. 7) depicting the mechanisms by which 10,12 CLA decreases WAT mass. We speculate that 10,12 CLA binds to a cell surface FA receptor, or diffuses or flip-flops into adipocytes, thereby activating upstream signals. These upstream signals induce an ISR, FFA release, and activation of NFκB and MAPKs that may directly antagonize PPARγ activity. Increased release of PGs and cytokines may further antagonize PPARγ activity, leading to insulin resistance and delipidation. The resulting FFA accumulation in blood, liver, and muscle increases FFA oxidation and FFA-induced insulin resistance in these tissues. If energy expenditure is not sufficient to completely oxidize these elevated levels of FFAs, hyperlipidemia, hyperglycemia, and lipodystrophy can result.
Future studies are needed to identify potential upstream mediators of this proposed stress cascade in adipocytes. Elucidating these mechanisms will provide valuable information on the efficacy, specificity, and potential side effects of CLA isomers as dietary strategies for weight loss or maintenance. Such knowledge is essential for the effective and safe use of CLA supplements to control obesity.
Support provided by NIH R01 DK063070-07 to MKM and SM, and from the NIH F31DK076208 and the United Negro College Fund-Merck predoctoral fellowships to AK.
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