Open Access

Eicosapentaenoic acid-enriched phospholipid ameliorates insulin resistance and lipid metabolism in diet-induced-obese mice

Contributed equally
Lipids in Health and Disease201312:109

https://doi.org/10.1186/1476-511X-12-109

Received: 8 May 2013

Accepted: 19 July 2013

Published: 23 July 2013

Abstract

Background

Over the past two decades, a striking increase in the number of people with metabolic syndrome (MS) has taken place worldwide. With the elevated risk of not only diabetes but also cardiovascular morbidity and mortality, there is urgent need for strategies to prevent this emerging global epidemic. The present study was undertaken to investigate the effects of dietary eicosapentaenoic acid-enriched phospholipid (EPA-PL) on metabolic disorders.

Methods

Male C57BL/6J mice (n = 7) were fed one of the following 4 diets for a period of 4 weeks: 1) a modified AIN-96G diet with 5% corn oil (control diet); 2) a high fat (20%, wt/wt) and high fructose (20%, wt/wt) diet (HF diet); 3) the HF diet containing 1% SOY-PL (SOY-PL diet); 4) the HF diet containing 1% EPA-PL (EPA-PL diet). The oral glucose tolerance test was performed. Plasma TG, TC, glucose, NEFA, insulin, leptin, adiponectin, TNF-α and IL-6 levels were assessed. In addition, hepatic lipid levels, lipogenic, and lipidolytic enzyme activities and gene expressions were evaluated.

Results

Both EPA-PL and SOY-PL significantly inhibited body weight gain and white adipose tissue accumulation, alleviated glucose intolerance, and lowered both serum fasting glucose and NEFA levels substantially. Only EPA-PL significantly reduced serum TNF-α and IL-6 levels, and increased serum adiponectin level. EPA-PL was more effective in reducing hepatic and serum TG and TC levels than SOY-PL. Both EPA-PL and SOY-PL reduced the activities of hepatic lipogenic enzymes, such as FAS and G6PDH, but only EPA-PL significantly increased CPT, peroxisomal β-oxidation enzymes activities and CPT-1a mRNA level. Alterations of hepatic lipogenic gene expressions, such as FAS, G6PDH, ACC, SCD-1 and SREBP-1c were consistent with changes in related enzyme activities.

Conclusions

According to our study, EPA-PL supplementation was efficacious in suppressing body fat accumulation, and alleviating insulin resistance and hepatic steatosis by modulating the secretion of adipocytokines and inflammatory cytokines, suppression of SREBP-1c mediated lipogenesis and enhancement of fatty acid β-oxidation. These results demonstrate that EPA-PL is a novel beneficial food component for the prevention and improvement of metabolic disorders.

Keywords

Eicosapentaenoic acid Phospholipid Metabolic syndrome Obesity Insulin resistance Lipid metabolism

Background

Obesity and associated diseases such as type 2 diabetes dyslipidemia, and hypertension, i.e., components of the metabolic syndrome [1], are a major public health problem. While effective pharmacological interventions for the treatment of obesity-associated diseases require the use of multiple medicines and are often associated with adverse side-effects, lifestyle modifications remain essential components of treatment strategies. In this context, dietary supplementation with long-chain n-3 polyunsaturated fatty acids (n-3 PUFAs), in particular, eicosapentaenoic acid (EPA, C20:5) and docosahexaenoic acid (DHA, C22:6), present a variety of health benefits. In addition to their ability to reduce plasma lipid levels in animals [2] and humans [3], n-3 PUFAs have been shown to prevent arrhythmias, improve hypertension, and reduce platelet aggregation [4]. Dietary supplementation with n-3 PUFAs is thus beneficial for the prevention and/or treatment of cardiovascular disease [5] and possibly other inflammatory and neurological disorders [6, 7]. Therefore, nutritional recommendations of 250 mg/day of EPA/DHA have been established in Western countries for n-3 PUFAs intake, to achieve nutrient adequacy and lower the n-6/n-3 PUFAs ratio [8].

Dietary EPA and DHA are provided mostly by fatty fish, where they are mainly present as esterified triglyceride (TG). In some other marine sources such as fish roe and krill, the major proportion of these n-3 PUFAs is present as esterified phospholipids (PLs). This may be biologically and therapeutically significant, because PL fatty acids are well absorbed by the intestine [9] and are readily incorporated into cell membranes [10]. Dietary PLs, particularly those originating from soy and safflower, have consistently been shown to reduce plasma and liver lipid levels in experimental animals [11]. Some recent studies have compared the potential effects of different n-3 PUFAs formulations on lipid metabolism. The supplementation of n-3 PUFAs in the form of PLs would exert superior biological and nutritional functions. These would include anti-inflammatory actions [12, 13] and antioxidant activity on brain lipids [14], improved memory and learning [15], reduced blood and tissue lipids [1619], increased bioavailability of EPA and DHA in plasma [20, 21], and the tendency to reduce obesity [13, 19].

The n-3 PUFA-enriched PLs used in existing studies were basically obtained from krill or fish roe, which contained 7–28% DHA and 5–12.5% EPA [1221]. A few studies have demonstrated that DHA and EPA have different effects on lipid metabolism [22, 23], vascular function [24], and inflammation [22, 25] in rodents and humans. It has also been reported that EPA ethyl ester has significant hypolipidemic effects [22], but EPA in the esterified TG form has no obvious influence on lipid metabolism [23]. Thus, the effect of EPA in the PL form on metabolic disorders deserves special attention. In our preliminary study, we found that sea cucumber, Cucumaria frondosa, was a good natural source of EPA-enriched PL (EPA-PL), containing a large amount of EPA (more than 50% of the total fatty acids) but basically no DHA. This present study was conducted to evaluate the effects of EPA-PL on insulin resistance and hepatic steatosis in high fat/high fructose diet-induced-obese C57BL/6J mice. In addition, the molecular mechanism by which EPA-PL alters the obesity-related metabolic disorders was also investigated.

Materials and methods

Preparation and characterization of EPA-PL and SOY-PL

Dietary EPA-PL was extracted from the body wall of sea cucumber, Cucumaria frondosa. Total lipids were extracted according to the modified method of Folch et al.[26] and then mixed with one-fifth volume of 0.15 M NaCl solution. Then the mixture was placed into a separatory funnel and kept for 24 h to thoroughly clear the bottom (chloroform) phase. The chloroform solution was evaporated to dryness under vacuum. Then phospholipids were separated away from neutral lipids and glycolipids by a silica-gel column chromatography using chloroform, acetone and methanol sequentially as eluents. The methanol eluent was collected and EPA-PL was obtained after removal of organic solvent under vacuum. The purity of EPA-PL was confirmed to be 93.7% according to the HPLC-ELSD analysis [27]. Dietary SOY-PL (purity > 95%) was purchased from Tianjin Bodi Chemical Holding Co., Ltd. (Tianjin, China). The fatty acid compositions of the two phospholipids were determined by the method of Lou et al.[28] and the results are given in Table 1.
Table 1

Fatty acid compositions of SOY-PL and EPA-PL

Fatty acid (%)

SOY-PL

EPA-PL

C16:0

13.1 ± 0.36

3.85 ± 0.18

C16:1

0.36 ± 0.08

3.67 ± 0.14

C18:0

2.57 ± 0.14

9.98 ± 0.34

C18:1

8.26 ± 0.31

6.86 ± 0.16

C18:2n-6

65.6 ± 1.15

0.65 ± 0.08

C18:3n-3

10.1 ± 0.23

0.98 ± 0.06

C20:0

--

3.47 ± 0.11

C20:1

--

8.65 ± 0.34

C20:4n-6

--

6.42 ± 0.27

C20:5n-3

--

50.8 ± 1.35

C22:1

--

2.77 ± 0.09

C22:6n-3

--

1.89 ± 0.11

Note: “--”, none detected.

Animals and diets

All aspects of the experiment were conducted according to guidelines provided by the ethical committee of experimental animal care at Ocean University of China (Qingdao, China). Male C57BL/6J mice aged 6 weeks were purchased from Vital River (Beijing, China). The mice were maintained in pathogen-free conditions at constant humidity of 65 ± 15% and temperature of 23 ± 2°C with a 12 h light/dark cycle. After a one-week adaptation period, the animals were divided into four groups (seven mice each): 1) CT group: a modified AIN-96G diet with 5% corn oil; 2) HF group: a high fat (20%, wt/wt) and high fructose (20%, wt/wt) diet (HF diet); 3) SOY-PL group: HF diet containing 1% SOY-PL; 4) EPA-PL group: HF diet containing 1% EPA-PL. Experimental diets were prepared according to recommendations of the American Institute of Nutrition (AIN). The ingredients and fatty acid compositions of the four experimental diets are summarized in Table 2 and Table 3. When adding SOY-PL or EPA-PL to the high fat/high fructose diet at a dose of 1%, the saturated-monounsaturated- polyunsaturated fatty acids ratios in the diets of these two phospholipid treatment groups were basically the same. After 4 weeks of feeding, mice were sacrificed after a 12 h overnight fasting. Blood was collected by orbital venipuncture and serum was separated. Liver, kidney, heart, brain and white adipose tissues were quickly excised and their weighs were obtained. All the organs and tissues were frozen in liquid nitrogen and stored at −80°C until analysis.
Table 2

Compositions of experimental diets

Ingredient (g/kg)

CT a

HF

SOY-PL

EPA-PL

Casein

245

245

245

245

Cornstarch

650

250

250

250

Fructose

--

200

200

200

Corn oil

50

50

50

50

Lard

--

200

190

190

Mineral mix

35

35

35

35

Vitamin mix

10

10

10

10

Powdered cellulose

5

5

5

5

DL-methionine

3

3

3

3

Choline bitartrate

2

2

2

2

Soybean phospholipids

--

--

10

--

EPA-enriched phospholipids

--

--

--

10

Note: “--”, none added; a modified AIN-93G.

Table 3

Fatty acid compositions of experimental diets

Fatty acid (%)

CT

HF

SOY-PL

EPA-PL

C16:0

--

26.6

25.8

25.5

C16:1

11.6

2.32

2.33

2.47

C18:0

1.31

53.6

51.1

51.4

C18:1

30.6

6.12

6.45

6.39

C18:2n-6

55.8

11.2

13.8

11.2

C18:3n-3

0.69

0.14

0.54

0.18

C20:4n-6

--

--

--

0.26

C20:5n-3

--

--

--

2.03

C22:6n-3

--

--

--

0.08

SFA

1.31

80.3

76.9

77.0

MUFA

42.2

8.44

8.78

9.32

PUFA

56.5

11.3

14.3

13.7

Note: SFA saturated fatty acid, MUFA monounsaturated fatty acid, PUFA polyunsaturated fatty acid.

Oral glucose tolerance test

At the end of 3 weeks dietary intervention, the oral glucose tolerance test (OGTT) was performed. Following a 10 h period of feed deprivation, each mouse was intragastrically perfused with a dose of 2 g/kg body weight of 0.2 g/mL D-glucose, and then 5 μL of caudal vein blood was obtained to measure the blood glucose level using the One Touch Ultra glucometer (LifeScan, USA) at 0, 30, 60, 120 min.

Analysis of serum parameters and hepatic lipids

The serum concentrations of TG, cholesterol (TC) and glucose were measured using enzymatic reagent kits (Biosino, China). The serum insulin (R&D system, USA), nonesterified fatty acid (NEFA, R&D system, USA), leptin (Boster Biological Technology., Ltd., China), adiponectin (Boster Biological Technology., Ltd., China), TNF-α (R&D system, USA) and IL-6 (R&D system, USA) levels were measured using enzyme-linked immunosorbent assay kits. Hepatic lipids were extracted with the modified method of Folch et al.[26], and the concentrations of TG and TC were analyzed with the same enzymatic kits as used in the serum analysis. Hepatic phospholipid levels were measured according to the method of Bartlett [29].

Hepatic enzyme activity assay

A piece of liver was homogenized in six volumes of a 0.25 M sucrose solution that contained 1 mM EDTA in a 10 mM Tris–HCl buffer (pH 7.4). After the nuclei fraction was precipitated, the supernatant was centrifuged at 10 000 g for 10 min at 4°C to obtain the mitochondria fraction. The resulting supernatant was recentrifuged at 125 000 g for 60 min to precipitate microsomes, and the remaining supernatant was used as the cytosol fraction. The protein concentration was determined according to the method of Lowry et al. [30]. The enzyme activities of fatty acid synthase (FAS; EC2.3.1.85) [31], glucose 6-phosphate dehydrogenase (G6PDH; EC1.1.1.49) [32] and malic enzyme (ME; EC1.1.1.40) [33] in the liver cytosol fraction, mitochondrial carnitine palmitoyl transferase (CPT; EC2.3.1.21) [34] and peroxisomal β-oxidation enzyme (EC1.3.3.6) [35] were determined as described.

Analysis of hepatic gene expression

Hepatic mRNA levels were measured by real-time polymerase chain reaction (RT-PCR). Total RNA was extracted from liver using a Trizol Reagent (Invitrogen, Japan). 1 μg total RNA were reverse transcribed into cDNA using random primer (TOYOBO, Japan). Selected genes were amplified using SYBR Green I Master Mix (Roche, Germany) in an iQ5 real-time detection system (Bio-Rad, USA) with 0.3 μM of both forward and reverse primers. PCR conditions were as follows: 1 cycle of 95°C for 10 min, 45 cycles of 95°C for 15 s, 55–60°C for 20 s and 72°C for 30 s. The purities of PCR products were assessed by melt curve analysis. Relative gene expression was quantified using the standard curve method. Results were expressed as the relative values after normalization to 18S RNA. Primer sequences were as follows: FAS (forward, 5′-TTGATGATTCAGGGAGTGG-3′; reverse, 5′-AGCAGATGAGTTGTTCTTGGAC-3′); G6PDH (forward, 5′-GTTTGGCAGCGGCAACTAA-3′; reverse, 5′-GGCATCACCCTGGTACAACTC-3′); ME (forward, 5′-TCACCTGCCCTAATGTCCCT-3′; reverse, 5′-CATGCCGTTAT CAACTTGTCC-3′); CPT-1a (forward, 5′-TCTCAGTGGGAGCGACTCT-3′; reverse, 5′-TGTGGTACACGACAATGTGCCT-3′); ACC (forward, 5′-TTGCCTATGAACTCAACAGCG-3′; reverse, 5′-AGACCATTCCGCCCATCC-3′); SCD-1 (forward, 5′-CCACTCGCCTACACCAACG-3′; reverse, 5′-GGGGTCCCTCCTCATCCT-3′); SREBP-1c (forward, 5′-AACCTCATCCGCCACCTG-3′; reverse, 5′-TGGTAGACAACAGCCGCATC-3′); PPARα (forward, 5′-TCGGAGCTGCAA GATTCAGA-3′; reverse, 5′-CAAAGCGAATTGCATTGTGTG-3′); PPARγ (forward, 5′-GTGATGGAAGACCACTCGC-3′; reverse, 5′-CCCACAGACTCGGCACTC-3′); 18S (forward, 5′-GTTGGTGGAGCGATTTGTCTG-3′; reverse, 5′-TTGCTCAATCTCGGGTGGC-3′).

Statistical analysis

All values are expressed as means ± standard errors. Comparisons were performed with an unpaired, two-tailed Student’s t test or analysis of variance (ANOVA). If the overall F was significant for the latter, comparisons between means were made with Tukey’s post hoc test. P < 0.05 was considered statistically significant.

Results

General observations

There were no differences in daily food intake between dietary groups. The body weight gain of the mice in the HF group was significantly higher than that of the control group. Compared with the HF group, the EPA-PL and SOY-PL groups displayed suppressed body weight gain by 29.7% and 26.7%, respectively, and decreased white adipose tissue weights, especially for the epididymal component (P < 0.01 and P < 0.05, respectively). The mice fed the HF diet had significantly higher liver weight than the control group (P < 0.05), which was indicative of lipid accumulation in the liver. Meanwhile, mice fed the EPA-PL and SOY-PL diets exhibited 10.7% and 11.4% decreases in liver weights, respectively. There were no significant changes in the weights of other visceral organs (Table 4).
Table 4

Growth parameters for C57BL/6J mice fed experimental diets a

 

CT

HF

SOY-PL

EPA-PL

Food intake (g/d)

2.64 ± 0.12

2.02 ± 0.12##

1.90 ± 0.09

1.91 ± 0.07

Body weight gain (g)

6.84 ± 0.80

8.90 ± 0.74##

6.52 ± 0.73**

6.26 ± 0.50**

White adipose tissue weight (g/100 g body weight)

Epididymal

1.22 ± 0.24

2.81 ± 0.17##

2.25 ± 0.23*

1.90 ± 0.15**

Perirenal

1.02 ± 0.19

1.73 ± 0.21#

1.43 ± 0.30

1.50 ± 0.23

Mesenteric

0.43 ± 0.06

0.52 ± 0.15

0.45 ± 0.08

0.42 ± 0.08

Organ weight (g/100 g body weight)

Liver

4.05 ± 0.33

4.48 ± 0.22#

3.97 ± 0.15*

4.00 ± 0.13*

Kidney

1.28 ± 0.09

1.36 ± 0.13

1.27 ± 0.09

1.28 ± 0.07

Heart

0.52 ± 0.02

0.50 ± 0.05

0.48 ± 0.03

0.47 ± 0.01

Brain

1.29 ± 0.04

1.18 ± 0.09

1.19 ± 0.06

1.24 ± 0.08

Note: a Data are presented as mean ± SEM; n = 7 mice per group. #P < 0.05, ##P < 0.01, significant difference compared to the control group determined by Student’s t test. *P < 0.05, **P < 0.01, significant difference compared to the HF group determined by ANOVA (Tukey’s test).

Oral glucose tolerance test

After 3 weeks of feeding, the mice in the HF group developed glucose intolerance as indicated by the higher area under the curve and serum glucose levels at each node of the OGTT curve (P < 0.05). The serum glucose levels for mice in the EPA-PL and SOY-PL groups were effectively brought down to normal levels at 60 min and 120 min, compared to those for the mice in the HF group. The decreased areas under the curve also demonstrated that EPA-PL and SOY-PL could reverse the glucose intolerance caused by the HF diet (Figure 1).
Figure 1

Glucose tolerance test (A) and Area under curve (B) of C57BL/6J mice fed experimental diets a . Note: a Data are presented as mean ± SEM; n = 7 mice per group. #P < 0.05, ##P < 0.05, significant difference compared to the control group determined by Student’s t test. *P < 0.05, significant difference compared to the HF group determined by ANOVA (Tukey’s test).

Serum fasting glucose, insulin, NEFA, leptin, adiponectin, TNF-α and IL-6 levels

The fasting glucose and NEFA levels of the mice in the HF group were significantly increased compared to those observed in the control group (P < 0.01, P < 0.05). In comparison to the HF diet, EPA-PL and SOY-PL supplementation significantly reduced serum fasting glucose and NEFA levels. There were no significant changes in serum insulin levels (Table 5).
Table 5

Serum parameters related to glucose metabolism and inflammation reaction in C57BL/6J mice fed experimental diets a

 

CT

HF

SOY-PL

EPA-PL

Glucose, mmol/L

5.18 ± 0.72

7.55 ± 0.78##

4.43 ± 0.34**

3.95 ± 0.29**

Insulin, ng/mL

2.24 ± 0.20

1.97 ± 0.38

1.77 ± 0.34

1.99 ± 0.11

NEFA, μmol/L

146.3 ± 9.3

173.8 ± 15.3#

113.8 ± 17.3**

131.3 ± 13.3*

Leptin, ng/mL

1.09 ± 0.24

2.56 ± 0.94#

2.37 ± 0.63

2.08 ± 0.42

Adiponectin, ng/mL

6.71 ± 0.49

8.22 ± 0.78#

8.90 ± 0.56

9.71 ± 0.66*

TNF-α, pg/mL

33.2 ± 3.7

52.9 ± 9.6##

42.1 ± 7.5

32.0 ± 6.2*

IL-6, pg/mL

20.5 ± 2.8

27.6 ± 3.1#

29.1 ± 5.2

19.0 ± 2.2*

Note: a Data are presented as mean ± SEM; n = 7 mice per group. #P < 0.05, ##P < 0.01, significant difference compared to the control group determined by Student’s t test. *P < 0.05, **P < 0.01, significant difference compared to the HF group determined by ANOVA (Tukey’s test).

Further, we evaluated the effects of EPA-PL and SOY-PL on the serum levels of adipocytokines and inflammatory cytokines. The mice fed the HF diet displayed significantly higher serum leptin level compared to the control group (P < 0.05). However, neither EPA-PL nor SOY-PL had a significant effect on serum leptin levels. In comparison to the control group, serum adiponectin, TNF-α and IL-6 levels in mice from the HF group were all significantly increased (P < 0.05, P < 0.01 and P < 0.05, respectively). Moreover, compared to the HF diet, EPA-PL supplementation could effectively increase adiponectin level (P < 0.05) and reduce TNF-α and IL-6 levels (P < 0.05 and P < 0.05, respectively), while SOY-PL supplementation did not induce these parameters.

Lipid content in serum and liver

In addition to severe insulin resistance and inflammation, HF diet-fed mice showed progressive hyperlipidemia and hepatic steatosis. Compared to the control group, the HF diet increased serum TG and TC levels by 42.7% and 36.3%, respectively. Both phospholipid treatment groups tended to have lower serum lipid levels than the HF group. In particular, EPA-PL induced significantly superior lipid lowering effects on both serum TG and TC, with a reduction of 44.1% (P < 0.01) and 10.3% (P < 0.05) compared to the HF group. In addition, EPA-PL also markedly increased HDL-c level by 23.5% (P < 0.05), while SOY-PL did not have this effect (Table 6).
Table 6

Serum and liver Lipid contents in C57BL/6J mice fed experimental diets a

 

CT

HF

SOY-PL

EPA-PL

Serum lipids

TG, mmol/L

0.89 ± 0.10

1.27 ± 0.13##

0.96 ± 0.11*

0.71 ± 0.09**

TC, mmol/L

2.34 ± 0.23

3.19 ± 0.31##

3.00 ± 0.23

2.86 ± 0.18*

HDL-c, mmol/L

1.21 ± 0.06

1.32 ± 0.11

1.35 ± 0.13

1.63 ± 0.08*

Hepatic lipids

TG, mg/g

19.0 ± 2.0

35.9 ± 1.9##

25.7 ± 2.7**

17.5 ± 2.3**

TC, mg/g

2.66 ± 0.19

2.78 ± 0.28

2.61 ± 0.10

2.16 ± 0.17**

PL, mg/g

30.3 ± 0.8

30.1 ± 1.2

31.3 ± 0.6

32.5 ± 0.9*

Note: a Data are presented as mean ± SEM; n = 7 mice per group. ##P < 0.01, significant difference compared to the control group determined by Student’s t test. *P < 0.05, **P < 0.01, significant difference compared to the HF group determined by ANOVA (Tukey’s test).

Mice fed the HF diet had higher hepatic TG (P < 0.01) and TC levels than the control group. Mice fed both the EPA-PL and SOY-PL diets displayed significantly lowered hepatic TG levels compared to those observed in the HF group (P < 0.01 and P < 0.01, respectively). Only EPA-PL effectively decreased hepatic TC level by 22.3% (P < 0.01), while SOY-PL did not induce this effect (Table 6).

Activities of hepatic enzymes related to lipid metabolism

After 4 weeks feeding of the HF diet, the enzyme activities of hepatic FAS, G6PDH and ME increased in varying degrees, while EPA-PL and SOY-PL significantly reduced hepatic FAS (32.1%, P < 0.01; 17.1%, P < 0.05, respectively)and G6PDH activities (26.3%, P < 0.01; 31.0%, P < 0.01, respectively) to normal levels (Figure 2).
Figure 2

Hepatic enzyme activities involved in fatty acid biosynthesis in C57BL/6J mice fed experimental diets a . Note: a Data are presented as mean ± SEM; n = 7 mice per group. #P < 0.05, significant difference compared to the control group determined by Student’s t test. *P < 0.05, **P < 0.01, significant difference compared to the HF group determined by ANOVA (Tukey’s test).

In contrast, mitochondrial β-oxidation, whose rate-limiting enzyme is CPT and peroxisomal β-oxidation, were significantly enhanced in the EPA-PL group, with an improvement by 29.1% (P < 0.01) and 41.0% (P < 0.01) respectively, compared to the HF group. However, these two enzyme activities in the SOY-PL group maintained the same levels as those in the HF group (Figure 3).
Figure 3

Hepatic enzyme activities involved in fatty acid oxidation in C57BL/6J mice fed experimental diets a . Note: a Data are presented as mean ± SEM; n = 7 mice per group. ##P < 0.01, significant difference compared to the control group determined by Student’s t test. **P < 0.01, significant difference compared to the HF group determined by ANOVA (Tukey’s test).

mRNA expressions of genes related to lipid metabolism in the liver

Hepatic mRNA expressions of genes that regulate lipogenesis and lipolysis were examined. The mRNA expression of ME was similar among the groups, but the expressions of other lipogenesis genes, such as FAS, G6PDH, ACC, SCD-1, and SREBP-1c were markedly decreased by these two phospholipids, especially EPA-PL (Figure 4).
Figure 4

Hepatic mRNA expressions of genes related to lipid metabolism in C57BL/6J mice fed experimental diets a . Note: a Data are presented as mean ± SEM; n = 7 mice per group. #P < 0.05, significant difference compared to the control group determined by Student’s t test. *P < 0.05, **P < 0.01, significant difference compared to the HF group determined by ANOVA (Tukey’s test).

Nonetheless, the reduction of hepatic lipids by EPA-PL could also be a result of an upregulated mRNA expression in fatty acid β-oxidation. Therefore, we investigated the expressions of PPARα and its target gene, CPT-1a, in the liver. Although mRNA levels of PPARα were not affected by either PL, the expression of CPT-1a was greatly upregulated by EPA-PL (P < 0.01), while SOY-PL did not induce this effect (Figure 4).

Discussion

The metabolic syndrome has been defined as a cluster including visceral fat obesity, impaired glucose metabolism, atherogenic dyslipidemia (high plasma triglyceride and low HDL cholesterol) and hypertension [36]. Since it raises the risk of diabetes and cardiovascular disease, there is an urgent need for strategies to prevent this emerging global epidemic. The present study showed that dietary EPA-PL supplementation effectively reduced epididymal adipose tissue weight, hyperlipidemia, and hepatic steatosis in mice fed the HF diet. In addition, EPA-PL significantly alleviated HF diet-induced glucose intolerance, increased serum adiponectin level and reduced inflammation. The biological effects of EPA-PL on lipid and glucose metabolism were superior to the common dietary SOY-PL. These data are the first to indicate that EPA-PL has beneficial effects on metabolic disorders.

Recently, it has been reported that n-3 PUFA-enriched PL extracted from marine fish tend to improve glucose intolerance in dietary obese mice [13]. In the present study, EPA-PL similarly improved HF diet-induced glucose intolerance in mice and its effect was superior to that of SOY-PL. In order to find a potential mechanism, we analyzed several serum parameters that may be related to the increase in insulin sensitivity and glucose uptake. Chronic elevation in plasma NEFA level is commonly associated with impaired insulin-mediated glucose uptake [37] and often coexists with obesity and type 2 diabetes [38]. Both EPA-PL and SOY-PL effectively inhibited the elevation of NEFA in serum, indicating that these PLs possibly ameliorate glucose metabolism in HF diet-induced-obese mice.

Adipose tissue has been recognized not only to function as a storage depot for TG but also plays an important role in regulating glucose and lipid homeostasis by secreting a variety of adipocytokines into the circulatory system. Adiponectin is the most abundant adipose-specific protein, and it plays an important role as an insulin-sensitizing adipokine whose production is decreased in obesity and in conditions associated within insulin resistance [39, 40]. Several recent reports indicate that the effects of n-3 PUFA-enriched PLs on preventing excess weight gain and development of insulin resistance associated with high-fat feeding were thought to be mediated by adiponectin [13, 19, 41]. In agreement with these previous studies, dietary EPA-PL supplementation significantly increased serum adiponectin level in mice fed a HF diet, while SOY-PL had no such effect. We suggest that increased serum adiponectin level is the main reason for EPA-PL’s superiority to SOY-PL in the reversal of HF diet-induced glucose intolerance in mice. As for other adipocytokines, it has been known that leptin regulates energy balance by suppressing appetite [42]. In the present study, neither EPA-PL nor SOY-PL had an effect on serum leptin level and this is consistent with the unchanged food intake in each group n-3 PUFAs are known to reduce metabolic inflammation in human and rodents [4345]. Our data show that the HF diet induced higher serum TNF-α and IL-6 levels compared to the control group. SOY-PL supplementation had no influence on the HF diet-induced inflammation. However, interestingly, pro-inflammatory secretion was not activated in the EPA-PL group. We suggest that EPA-PL is different from SOY-PL in its ability to inhibit low-grade inflammation in obese mice.

After 4 weeks of feeding, EPA-PL and SOY-PL significantly decreased liver weights and hepatic TG levels in the HF diet-fed mice. Consequently, obesity-induced fatty liver was alleviated by EPA-PL and SOY-PL. To further investigate the regulation of hepatic lipid metabolism, we analyzed the effect of dietary EPA-PL and SOY-PL on enzymes activities related to fatty acid synthesis and fatty acid β-oxidation. As shown in Figures 2 and 3, FAS and G6PDH activities were markedly suppressed by both PLs, whereas CPT and peroxisomal β-oxidation activities were only markedly enhanced by EPA-PL.

It is now well-recognized that PUFAs activate PPARα by binding directly to this transcription factor and thus stimulating hepatic fatty acid oxidation. At the same time, PUFAs inhibit hepatic fatty acid synthesis by suppressing SREBP-1 nuclear abundance through suppression of SREBP-1c gene transcription and enhancement of proteosomal SREBP-1 degradation and mRNA-SREBP-1c mRNA decay [46]. In the present study, both EPA-PL and SOY-PL supplementation resulted in a marked decrease in the gene expressions of SREBP-1c-regulated enzymes affecting fatty acid synthesis in the liver, namely, FAS, ACC, and SCD-1. This was associated with decreased levels of SREBP-1c mRNA in the EPA-PL group only. The mRNA level of the PPARα-regulated enzyme affecting fatty acid oxidation (CPT-1a) was significantly upregulated by EPA-PL. There were no obvious changes in the mRNA levels of PPARα between groups. Relevant data on expression of genes related to lipid metabolism in the liver was consistent with changes in related enzyme activities. Overall, we suggest that both EPA-PL and SOY-PL can suppress hepatic fatty acid synthesis, but only EPA-PL has the potential to effectively enhance hepatic fatty acid β-oxidation. This explains why EPA-PL exerts superior lipid metabolism improving effects in HF diet-induced obesity.

The present study also showed that EPA-PL increased serum HDL-c level and decreased hepatic cholesterol level. Previous studies have shown that fish oil suppresses the activity of 3-hydroxy-3-methyl-glutaryl-CoA reductase, a rate-limiting enzyme of cholesterol synthesis [47]. Additionally, it has been reported that dietary phosphatidylcholine decreases hepatic cholesterol levels through the enhancement of the secretion of bile cholesterol in hypercholesterolemic rabbits [48]. Further studies are necessary to clarify the effect of dietary EPA-PL on the synthesis and excretion of cholesterol in the liver.

Recent reports demonstrated that n-3 PUFA-enriched PLs obtained from krill [15] or fish roe [19] showed different biological effects from n-6 PUFA-enriched PLs (such as EGG-PL and SOY-PL). Of note, all of the n-3 PUFA-enriched PLs used in existing studies were a mixture of DHA-PL and EPA-PL [1221]. Numerous studies have reported that DHA and EPA showed different effects on lipid metabolism [22, 23], vascular function [24] and inflammation [22, 25] in rodents and humans. Our present research has demonstrated that EPA-PL, which contains a large amount of EPA (more than 50% of total fatty acids) but basically no DHA, was effective in ameliorating obesity-related disorders. However, the bioactivities of DHA-PL, which contains abundant DHA but basically no EPA, have not been described. Therefore, a study comparing the biological effects of DHA-PL and EPA-PL would be of great interest.

Conclusion

In conclusion, our results show that, compared with SOY-PL, EPA-PL exhibited superior effects on improving glucose and lipid metabolism. EPA-PL supplementation was efficacious in suppressing body fat accumulation, and alleviating insulin resistance and hepatic steatosis by modulating the secretion of adipocytokines and inflammatory cytokines, suppression of SREBP-1c mediated lipogenesis and enhancement of fatty acid β-oxidation. These findings suggest that EPA-PL could be used in the development of functional foods for the prevention of chronic metabolic diseases in humans.

Notes

Abbreviations

ACC: 

Acetyl-CoA carboxylase

CPT: 

Carnitine palmitoyl transferase

DHA: 

Docosahexaenoic acid (22:6n-3)

EPA: 

Eicosapentaenoic acid (20:5n-3)

EPA-PL: 

EPA-enriched phospholipid

FAS: 

Fatty acid synthase

G6PDH: 

Glucose-6-phosphate dehydrogenase

HDL-c: 

High density lipoprotein-cholesterol

ME: 

Malic enzyme

MUFA: 

Monounsaturated fatty acid

NEFA: 

Nonesterified fatty acid

OGTT: 

Oral glucose tolerance test

PL: 

Phospholipids

PPAR: 

Peroxisome proliferator-activated receptor

PUFA: 

Polyunsaturated fatty acid

RT-PCR: 

Real-time polymerase chain reaction

SCD-1: 

Stearoyl-CoA desaturase-1

SFA: 

Saturated fatty acid

SOY-PL: 

Soybean phospholipid

SREBP-1c: 

Sterol regulatory element-binding protein-1c

TC: 

Total cholesterol

TG: 

Triglyceride

Declarations

Acknowledgements

This work was supported by grants from the Public Science and Technology Research Funds Project of Ocean (No.201105029) and National Key Technology Research and Development Program of the Ministry of Science and Technology of China (No. 2012BAD33B07).

Authors’ Affiliations

(1)
College of Food Science and Engineering, Ocean University of China
(2)
Faculty of Life Science and Biotechnology, Ningbo University
(3)
Department of Health and Nutrition Science, Nishikyushu University

References

  1. Van Gaal LF, Mertens IL, De Block CE: Mechanisms linking obesity with cardiovascular disease. Nature. 2006, 444: 875-880. 10.1038/nature05487View ArticlePubMedGoogle Scholar
  2. Harris WS: n-3 fatty acids and serum lipoproteins: animal studies. Am J Clin Nutr. 1997, 65 (5): 1611S-1616S.PubMedGoogle Scholar
  3. Eslick GD, Howe PR, Smith C, Priest R, Bensoussan A: Benefits of fish oil supplementation in hyperlipidemia: a systematic review and meta-analysis. Int J Cardiol. 2009, 136 (1): 4-16. 10.1016/j.ijcard.2008.03.092View ArticlePubMedGoogle Scholar
  4. Breslow JL: n-3 fatty acids ans cardiovascular disease. Am J Clin Nutr. 2006, 83: 1477-1482.Google Scholar
  5. Harris WS, Miller M, Tighe AP, Davidson MH, Schaefer EJ: Omega-3 fatty acids and coronary heart disease risk: clinical and mechanistic perspectives. Atherosclerosis. 2008, 197 (1): 12-24. 10.1016/j.atherosclerosis.2007.11.008View ArticlePubMedGoogle Scholar
  6. Cleland LG, James MJ, Proudman SM: The role of fish oils in the treatment of rheumatoid arthritis. Drugs. 2003, 63 (9): 845-853. 10.2165/00003495-200363090-00001View ArticlePubMedGoogle Scholar
  7. Boudrault C, Bazinet RP, Ma DW: Experimental models and mechanisms underlying the protective effects of n-3 polyunsaturated fatty acids in Alzheimer’s disease. J Nutr Biochem. 2009, 20 (1): 1-10. 10.1016/j.jnutbio.2008.05.016View ArticlePubMedGoogle Scholar
  8. , : Scientific opinion on dietary reference values for fats, including saturated fatty acids, polyunsaturated fatty acids, monounsaturated fatty acids, trans fatty acids, and cholesterol. EFSA J. 2010, 8: 1461.Google Scholar
  9. Cansell M, Nacka F, Combe N: Marine lipid-based liposomes increase in vivo FA bioavailability. Lipids. 2003, 38 (5): 551-559. 10.1007/s11745-003-1341-0View ArticlePubMedGoogle Scholar
  10. Lemaitre-Delaunay D, Pachiaudi C, Laville M, Pousin J, Armstrong M, Lagarde M: Blood compartmental metabolism of docosahexaenoic acid (DHA) in humans after ingestion of a single dose of [13C]DHA in phosphatidylcholine. J Lipid Res. 1999, 40: 1867-1874.PubMedGoogle Scholar
  11. Cohn JS, Wat E, Kamili A, Tandy S: Dietary phospholipids, hepatic lipid metabolism and cardiovascular. Curr Opin Lipidol. 2008, 19 (3): 257-262. 10.1097/MOL.0b013e3282ffaf96View ArticlePubMedGoogle Scholar
  12. Morizawa KTY, Tsuchida M, Nakano Y, Hibino H, Tanaka Y: Dietary oils and phospholipids containing n-3 highly unsaturated fatty acids suppress 2, 4-dinitro-1- fluorobenzene-induced contact dermatitis in mice. J Jpn Oil Chem Soc. 2000, 49 (1): 59-65. 10.5650/jos1996.49.59.View ArticleGoogle Scholar
  13. Rossmeisl M, Jilkova ZM, Kuda O, Jelenik T, Medrikova D, Stankova B, Kristinsson B, Haraldsson GG, Svensen H, Stoknes I, Sjovall P, Magnusson Y, Balvers MG, Verhoeckx KC, Tvrzicka E, Bryhn M, Kopecky J: Metabolic effects of n-3 PUFA as phospholipids are superior to triglycerides in mice fed a high-fat diet: possible role of endocannabinoids. PLoS One. 2012, 7 (6): e38834- 10.1371/journal.pone.0038834PubMed CentralView ArticlePubMedGoogle Scholar
  14. Hiratsuka S, Ishihara K, Kitagawa T, Wada S, Yokogoshi H: Effect of dietary docosahexaenoic acid connecting phospholipids on the lipid peroxidation of the brain in mice. J Nutr Sci Vitaminol (Tokyo). 2008, 54 (6): 501-506. 10.3177/jnsv.54.501.View ArticleGoogle Scholar
  15. Hiratsuka S, Koizumi K, Ooba T, Yokogoshi H: Effects of dietary docosahexaenoic acid connecting phospholipids on the learning ability and fatty acid composition of the brain. J Nutr Sci Vitaminol (Tokyo). 2009, 55 (4): 374-380. 10.3177/jnsv.55.374.View ArticleGoogle Scholar
  16. Batetta B, Griinari M, Carta G, Murru E, Ligresti A, Cordeddu L, Giordano E, Sanna F, Bisogno T, Uda S, Collu M, Bruheim I, Di Marzo V, Banni S: Endocannabinoids may mediate the ability of (n-3) fatty acids to reduce ectopic fat and inflammatory mediators in obese Zucker rats. J Nutr. 2009, 139 (8): 1495-1501. 10.3945/jn.109.104844View ArticlePubMedGoogle Scholar
  17. Bunea R, El Farrah K, Deutsch L: Evaluation of the effects of Neptune Krill Oil on the clinical course of hyperlipidemia. Altern Med Rev. 2004, 9 (4): 420-428.PubMedGoogle Scholar
  18. Vigerust NF, Bjorndal B, Bohov P, Brattelid T, Svardal A, Berge RK: Krill oil versus fish oil in modulation of inflammation and lipid metabolism in mice transgenic for TNF-alpha. Eur J Nutr. in press.Google Scholar
  19. Shirouchi B, Nagao K, Inoue N, Ohkubo T, Hibino H, Yanagita T: Effect of dietary omega 3 phosphatidylcholine on obesity-related disorders in obese Otsuka Long-Evans Tokushima fatty rats. J Agric Food Chem. 2007, 55 (17): 7170-7176. 10.1021/jf071225xView ArticlePubMedGoogle Scholar
  20. Ulven SM, Kirkhus B, Lamglait A, Basu S, Elind E, Haider T, Berge K, Vik H, Pedersen JI: Metabolic effects of krill oil are essentially similar to those of fish oil but at lower dose of EPA and DHA, in healthy volunteers. Lipids. 2011, 46 (1): 37-46. 10.1007/s11745-010-3490-4PubMed CentralView ArticlePubMedGoogle Scholar
  21. Maki KC, Reeves MS, Farmer M, Griinari M, Berge K, Vik H, Hubacher R, Rains TM: Krill oil supplementation increases plasma concentrations of eicosapentaenoic and docosahexaenoic acids in overweight and obese men and women. Nutr Res. 2009, 29 (9): 609-615. 10.1016/j.nutres.2009.09.004View ArticlePubMedGoogle Scholar
  22. Hung P, Gu JY, Kaku S, Yunoki S, Ohkura K, Ikeda I, Tachibana H, Sugano M, Yazawa K: Dietary effects of eicosapentaenoic and docosahexaenoic acid esters on lipid metabolism and immune parameters in Sprague–Dawley rats. Biosci Biotechnol Biochem. 2000, 64 (12): 2588-2593. 10.1271/bbb.64.2588View ArticlePubMedGoogle Scholar
  23. Gotoh N, Nagao K, Onoda S, Shirouchi B, Furuya K, Nagai T, Mizobe H, Ichioka K, Watanabe H, Yanagita T, Wada S: Effects of three different highly purified n-3 series highly unsaturated fatty acids on lipid metabolism in C57BL/KsJ-db/db mice. J Agric Food Chem. 2009, 57 (22): 11047-11054. 10.1021/jf9026553View ArticlePubMedGoogle Scholar
  24. Mori TA, Watts GF, Burke V, Hilme E, Puddey IB, Beilin LJ: Differential effects of eicosapentaenoic acid and docosahexaenoic acid on vascular reactivity of the forearm microcirculation in hyperlipidemic, overweight men. Circulation. 2000, 102: 1264-1269. 10.1161/01.CIR.102.11.1264View ArticlePubMedGoogle Scholar
  25. Sierra S, Lara-Villoslada F, Comalada M, Olivares M, Xaus J: Dietary eicosapentaenoic acid and docosahexaenoic acid equally incorporate as docosahexaenoic acid but differ in inflammatory effects. Nutrition. 2008, 24 (5): 245-254.View ArticlePubMedGoogle Scholar
  26. Folch J, Lees M, Sloane Stanley GH: A simple method for the isolation and purification of total lipids from animal tissues. J Biol Chem. 1957, 226 (1): 497-509.PubMedGoogle Scholar
  27. Graeve M, Janssen D: Improved separation and quantification of neutral and polar lipid classes by HPLC-ELSD using a monolithic silica phase: application to exceptional marine lipids. J Chromatogr B. 2009, 877 (20–21): 1815-1819.View ArticleGoogle Scholar
  28. Lou QM, Wang YM, Liu XF, Xue CH: Lipid profile and fatty acid compositions in body wall of Apostichopus Japonicus (Selenka). J Food Biochem. 2012, 36 (3): 317-321. 10.1111/j.1745-4514.2011.00544.x.View ArticleGoogle Scholar
  29. Bartlett GR: Phosphorous assay in column chromatography. J Biol Chem. 1959, 234: 466-468.PubMedGoogle Scholar
  30. Lowry OH, Rosebrough NJ, Farr AL, Randall RJ: Protein measurement with the Folin phenol reagent. J Biol Chem. 1951, 193: 265-275.PubMedGoogle Scholar
  31. Kelley DS, Nelson GJ, Hunt JE: Effect of prior nutritional status on the activity of lipogenic enzymes in primary monolayer cultures of rat hepatocytes. Biochem J. 1986, 235 (1): 87-90.PubMed CentralView ArticlePubMedGoogle Scholar
  32. Kelly DS, Kletzien RF: Ethanol modulation of the hormonal and nutritional regulation of glucose 6-phosphate dehydrogenase activity in primary cultures of rat hepatocytes. Biochem J. 1984, 217 (2): 543-549.View ArticleGoogle Scholar
  33. Ochoa S: Malic enzyme. Methods in Enzymology. Edited by: Colowick SP, Kaplan NO. 1955, 1: 739-753. New York, NY: Academic Press.Google Scholar
  34. Markwell MA, McGroarty EJ, Bieber LL, Tolbert NE: The subcellular distribution of carnitine acyltransferase in mammalian liver and kidney A new peroxisomal enzyme. J Biol Chem. 1973, 248: 3426-3432.PubMedGoogle Scholar
  35. Lazarow PB: Assay of Peroxisomal β-oxidation of fatty acids. Methods in Enzymology. Edited by: Lowenstein JM. 1981, 315-319. New York, NY: Academic Press.Google Scholar
  36. Grundy SM, Cleeman JI, Daniels SR, Donato KA, Eckel RH, Franklin BA, Gordon DJ, Krauss RM, Savage PJ, Smith SC, Spertus JA, Costa F: Diagnosis and management of the metabolic syndrome: an American Heart Association/National Heart, Lung, and Blood Institute Scientific Statement. Circulation. 2005, 112: 2735-2752. 10.1161/CIRCULATIONAHA.105.169404View ArticlePubMedGoogle Scholar
  37. Frayn KN: Insulin resistance and lipid metabolism. Curr Opin Lipidol. 1993, 4 (3): 197-204. 10.1097/00041433-199306000-00004. 10.1097/00041433-199306000-00004View ArticleGoogle Scholar
  38. Reaven GM, Hollenbeck C, Jeng CY, Wu MS, Chen YD: Measurement of plasma glucose, free fatty acid, lactate, and insulin for 24 h in patients with NIDDM. Diabetes. 1988, 37 (8): 1020-1024. 10.2337/diabetes.37.8.1020View ArticlePubMedGoogle Scholar
  39. Ouchi N, Parker JL, Lugus JJ, Walsh K: Adipokines in inflammation and metabolic disease. Nat Rev Immunol. 2011, 11 (2): 85-97. 10.1038/nri2921PubMed CentralView ArticlePubMedGoogle Scholar
  40. Ohashi K, Ouchi N, Matsuzawa Y: Anti-inflammatory and anti-atherogenic properties of adiponectin. Biochimie. 2012, 94 (10): 2137-2142. 10.1016/j.biochi.2012.06.008View ArticlePubMedGoogle Scholar
  41. Tandy S, Chung RWS, Wat E, Kamili A, Berge K, Griinari M, Cohn JS: Dietary krill oil supplementation reduces hepatic steatosis, glycemia, and hypercholesterolemia in high-fat-fed mice. J Agric Food Chem. 2009, 57 (19): 9339-9345. 10.1021/jf9016042View ArticlePubMedGoogle Scholar
  42. Blundell JE, Gillett A: Control of food intake in the obese. Obes Res. 2001, 9 (S11): 263S-270S.View ArticlePubMedGoogle Scholar
  43. James MJ, Gibson RA, Cleland LG: Dietary polyunsaturated fatty acids and inflammatory mediator production. Am J Clin Nutr. 2000, 71 (1): 343S-348S.PubMedGoogle Scholar
  44. Krebs JD, Browning LM, McLean NK, Rothwell JL, Mishra GD, Moore CS, Jebb SA: Additive benefits of long chain n-3 polyunsaturated fatty acids and weight-loss in the management of cardiovascular disease risk in overweight hyperinsulinemic women. Int J Obes (Lond). 2006, 30: 1535-1544. 10.1038/sj.ijo.0803309.View ArticleGoogle Scholar
  45. Kiecolt-Glaser JK, Belury MA, Andridge R, Malarkey WB, Hwang BS, Glaser R: Omega-3 supplementation lowers inflammation in healthy middle-aged and older adults: a randomized controlled trial. Brain Behav Immun. 2012, 26 (6): 988-995. 10.1016/j.bbi.2012.05.011PubMed CentralView ArticlePubMedGoogle Scholar
  46. Jump DB, Botolin D, Wang Y, Xu J, Christian B, Demeure O: Fatty acid regulation of hepatic gene transcription. J Nutr. 2005, 135 (11): 2503-2506.PubMedGoogle Scholar
  47. Froyland L, Vaagenes H, Asiedu DK, Garras A, Lie O, Berge RK: Chronic administration of eicosapentaenoic acid and docosahexaenoic acid as ethyl ester reduced plasma cholesterol and changed the fatty acid composition in rat blood and organs. Lipids. 1996, 31 (2): 169-178. 10.1007/BF02522617View ArticlePubMedGoogle Scholar
  48. Polichetti E, Janisson A, de la Porte PL, Portugal H, Leonardi J, Luna A, La Droitte P, Chanussot F: Dietary polyenylphosphatidylcholine decreases cholesterolemia in hypercholesterolemic rabbits: role of the hepato-biliary axis. Life Sci. 2000, 67 (21): 2563-2576. 10.1016/S0024-3205(00)00840-7View ArticlePubMedGoogle Scholar

Copyright

© Liu et al.; licensee BioMed Central Ltd. 2013

This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.