Background
Obesity is a complex multifactorial chronic disease characterized by excess body fat and is associated with concurrent diseases that reduce life expectancy, including cardiovascular disease, stroke, hyperlipidemia, fatty liver, and diabetes [
1,
2]. Obesity caused by hypertrophy of adipose tissue as well as adipose tissue hyperplasia triggers the differentiation of preadipocytes into adipocytes [
3]. Adipocyte differentiation is regulated by crucial transcription factors such as peroxisome proliferator-activated receptor-γ (PPARγ) and CCAAT/enhancer-binding proteins α (C/EBPα). These transcription factors control the expression of many adipogenic proteins [
4‐
7]. Several studies have reported that Sterol regulatory element-binding protein 1 (SREBP-1) is a transcription factor that regulates adipogenesis, cellular cholesterol, and cholesterol synthesis proteins, such as 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR), in 3 T3-L1 adipocytes [
8‐
10]. Adenosine monophosphate-activated protein kinase (AMPK) is a key enzyme in energy metabolism and is involved in regulation of glucose levels and lipid uptake. AMPK is expressed in a number of tissues, including adipose tissue, the liver, skeletal muscle, the heart, pancreatic beta cells, and brain cells [
11,
12]. AMPK is phosphorylated and inactivates metabolic enzymes involved in fatty acid and cholesterol syntheses [
13‐
15]. AMPK also provides an upstream signal of PPARγ/CEBPα and suppresses differentiation of preadipocytes into adipocytes [
16‐
18]. Activation of AMPK decreases cellular cholesterol and fatty acids, such as SREBP-1 and HMGCR [
19,
20]. Recently, the identification of a natural compound that can exert anti-obesity effects with fewer side effects than currently available prescription medications is attracting attention [
4]. One such compound is
P. koraiensis (Korean nut pine), which is native to Korea, Japan, China, and Eastern Russia. The main chemicals in essential oil from
P. koraiensis leaves (EOPK) are camphene, D-limonene, borneol, α-pinene, 3-carene, 4-carene, β-phellandrene, and fencyl [
21]. Our previous research has shown that EOPK has anti-hyperlipidemic [
21], anti-diabetic [
22], anti-obesity [
23] and anti-cancer effects [
24]. P. koraiensis seed oil has been investigated to inhibition of lipid metabolism in rats and mice [
25,
26]. However, the biological and biochemical effects of the ethanol extract of
P. koraiensis (EPK) and its main compounds have not yet been proven. The EPK is easier to extract than EOPK. In addition, EPKs are convenient and easy to use. The purpose of this study is to investigate the anti-adipogenic effect of EPK on 3T3L-1 cells and the anti-obesity activity of EPK on high fat diet (HFD)-fed rats.
Methods
Plant materials
The leaves of
P. koraiensis were used same materials with our previous study and the details were described in our previous published study [
24].
Preparation of EPK
The EPK was prepared using the hydrodistillation method with dried and pulverized P. koraiensis leaves. In order to increase the extraction efficiency, P. koraiensis leaves and young stems under 1 cm in diameter were cut into 2–3 cm sections. P. koraiensis leaves were obtained from the agricultural corporation Beaksongyounlim (Gangwondo, Korea) and authenticated by the Department of Oriental Medicine Biotechnology at Kyung Hee University. Dried and pulverized leaves (1 kg) were immersed in 50 % ethanol (10 L) and distilled. The reflux distillation was continued for 10 h at 45 °C and was repeated twice. The EPK was prepared under reduced pressure to obtain 30 Brix.
Extraction and isolation of LA
The EPK was partitioned with EtOAc / distilled water (1:1). The water layer was suspended and partitioned with n-butanol / distilled water. Coulum chromatography of the EtOAc fraction showing anti-hyperlipidemic activity, over silica gel using an n-hexan-EtOAc-chloroform-MeOH mixture with increasing polarity, yielded 15 fractions. 15 fractions were confirmed with thin layer chromatography. Among these fractions, fr. 6 showed distinct and vivid red-purple color by TLC. Also the fr. 6 showed the most potent anti-hyperlipidemic aciticity. LA was obtained by an additional purification step from fr. 6. The structure of LA was identified by 1H-nuclear magnetic resonance (NMR) and 13C-nuclear magnetic resonance (NMR). (Additional file
1: Figure S1)
Cell culture assay
3T3L-1 preadipocytes were purchased from Korean Cell Line Bank (KCLB). The cells were cultured in Dulbecco’s Modified Eagle’s Medium (DMEM) with 4500 mg/L D-glucose, 10 % fetal bovine serum (FBS), 2 μM L-glutamine and penicillin/streptomycin (WelGene, Daegu, South Korea) in a humidified atmosphere of 5 % CO2 at 37 °C.
Cytotoxicity assay
Cytotoxicity of EPK was evaluated with the 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium bromide (MTT) (Sigma Aldrich, St Louis, MO) assay. The cells were seeded at a density of 1 × 104cells per well in a 96-well plate, cultured for 24 h, and then treated with various concentrations of EPK. After 24 h incubation, 50 μL of MTT solution (1 mg/mL) was added to each well and incubated for 2 h at 37 °C in darkness. The viable cell number was correlated with the production of formazan, which was dissolved with dimethyl sulfoxide (DMSO), and optical density (O.D.) was measured with a microplate reader (Sunrise, TECAN, Mannedorf, Switzerland) at 570 nm. Cell viability was calculated by the following equation: Cell viability (%) = [O.D.(EPK)-O.D.(blank)]/[O.D(control)-O.D.(blank)] × 100.
Differentiation induction and Oil-Red-O staining
The preadipocyte 3 T3-L1 cells were plated on 6-well plates on day 0 and incubated until confluency was achieved. For adipocyte differentiation, the confluent cells were treated with 1 μM dexamethasone, 1 μg/mL insulin, and 0.5 mM IBMX for 2 days, and the medium was replaced by fresh normal medium containing only 1 μg/mL insulin for 2 days. On day 2, the differentiated adipocyte cells were cultured in the presence or absence of EPK (25 or 50 μg/mL) for 6 days. The medium was changed every 2 days. The cells were fixed with 2 % paraformaldehyde, washed twice with PBS, and finally stained with Oil-Red-O. The cellular lipid retained Oil-Red-O in isopropanol and adipocyte expression was estimated by measuring O.D. with a microplate reader (Sunrise, TECAN, Mannedorf, Switzerland) at 510 nm.
RT-PCR analysis
The preadipocyte 3 T3-L1 cells were plated onto 6-well plates on day 0 and incubated until confluency was achieved. For adipocyte differentiation, the confluent cells were treated with 1 μM dexamethasone, 1 μg/mL insulin, and 0.5 mM IBMX for 2 days, and the medium was replaced by fresh normal medium containing only 1 μg/mL insulin for 2 days. On day 2, the differentiated adipocyte cells were cultured in the presence or absence of EPK (25 or 50 μg/mL) for 6 days. The total RNA was extracted by using TRIzol reagent (Invitrogen, Carlsbad, CA, USA) according to the manufacturer’s instructions. cDNA was synthesized from 1 μg of total RNA and subjected to PCR reaction by using a SuperScript One-Step reverse transcription-PCR (RT-PCR) kit (Invitrogen, Carlsbad, CA, USA). The PCR conditions were as follows: 30 cycles of 94 °C for 30 s, 57 °C for 30 s, and 72 °C for 30 s. The primer sequences have been provided in Table
1 (Additional file
2: Table S1). PCR products were run on 2 % agarose gels and then stained with ethidium bromide. Stained bands were visualized under UV light and photographed.
Table 1
The primer sequences
PPARγ | forward 5′-GGTGAAACTCTGGGAGATTC-3′ |
reverse 5′-CAACCATTGGGTCAGCTCTT-3′ |
C/EBPα | forward 5′-AGGTGCTGGAGTTGACCAGT-3′ |
reverse 5′-CAGCCTAGAGATCCAGCGAC-3′ |
GPDH | forward 5′-GAACTAAGGAGCAGCTCAAAGGTTC-3′ |
reverse 5′-CAGTTGACTGACTGAGCAAACATAG-3′ |
β-actin | forward 5-ACCGTGAAAAGATGACCCAG-3′ |
reverse 5′-TACGGATGACAACGTCACAC-3′ |
Western blot analysis
On day 2, the differentiated adipocyte cells were cultured in the presence or absence of EPK (25 or 50 μg/mL) or LA (200 μM) for 6 days. Cell were lysed in RIPA buffer (50 mM Tris–HCl, pH 7.4, 150 mM NaCl, 1 % NP-40, 0.25 % deoxycholic acid-Na, 1 M EDTA, 1 mM Na3VO4, 1 mM NaF, and protease-inhibitor cocktail). The proteins in the samples were quantified using Bio-Rad DC protein assay kit II (Bio-Rad, Hercules, CA), separated by electrophoresis on 8 to 10 % SDS-PAGE gels, and electrotransferred onto a Hybond ECL transfer membrane (Amersham Pharmacia, Piscataway, NJ). The membranes were blocked in 3 % nonfat skim milk and probed with primary antibodies for PPARγ (Novus, Littleton, CO, USA), C/EBPα (Cell Signaling Tech., Danvers, MA, USA), p-AMPK (Cell Signaling Tech., MA, USA), AMPK (Cell Signaling Tech., MA, USA), adiponectin (Cell Signaling Tech., MA, USA), FABP (Cell Signaling Tech., Danvers, MA, USA), HMGCR (Bioss Antibodies, MA, USA), SREBP-1 (Sigma, St. Louis, MO, USA), or β-actin (Sigma, St. Louis, MO, USA) overnight. Subsequently, they were exposed to horseradish peroxidase (HRP)-conjugated secondary anti-mouse or rabbit antibodies. Protein expression was examined by using the enhanced chemiluminescence (ECL) system (Amersham Pharmacia, Piscataway, NJ).
AMPK gene silencing
AMPK small interfering RNA (siRNA) was purchased from Cell Signaling. A control siRNA was purchased from Santa Cruz Biotechnology. To transfect the siRNA, 3 T3-L1 cells were plated at a density of 4 × 105 cells per well in a 6-well plate. The cells were transfected using 100 nM of AMPK siRNA with INTERFERin (Poly plus, France) for 48 h. After treatment, the cells were confirmed by western blot.
HPLC analysis
In order to analyze the compounds from EPK, standards for the compounds were run on HICHROM HPLC columns (5 μM, 250 × 4.6 mm, Hichrom Ltd.) using a high HPLC system (Agilent Technologies, CA). The binary mobile phase consisted of 35 % to 100 % methanol-tetrahydrofuran solvent (99.5:0.5, v/v). The solvent flow rate was 1.0 mL/min and ambient temperature was set at 30 °C. UV detection was at a wavelength of 260 nm.
Animals
Male Sprague–Dawley rats (age, 4 weeks) were purchased from Hyo-Chang Science (Daegu, Korea). The rats were maintained under specific pathogen-free conditions with a 12 h light–dark cycle, 55 % humidity, and 22 ± 2 °C. All animal procedures were approved by the institutional Animal Care and Use Committee (IACUC) of Kyungsung University (Permit Number: 2011-13A), and performed in accordance with the Policy of the Ethical Committee of Ministry of Health and Welfare, Korea.
Experimental design and EPK treatment
Forty rats were divided into four groups (10 rats per group): normal group (low fat diet), control group (HFD), and two EPK-treated groups consuming HFD. Rats were fed the low fat diet or the HFD for 6 weeks. HFD composition has been described in Table
2. For EPK treatment dissolved in 4 % tween 80/normal saline was orally administered once daily to the rats at doses of 100 and 200 mg/kg for 6 weeks from the first day of HFD-feeding, whereas PBS was orally administered to the rats in the control group.
Table 2
Composition of normal and HFD
Casein | 20.0 | 20.0 |
DL-methionine | 0.3 | 0.3 |
Corn starch | 15.0 | 15.0 |
Sucrose | 50.0 | 34.5 |
Fiber | 5.0 | 5.0 |
Corn oil | 5.0 | - |
AIN-mineral mixture | 3.5 | 3.5 |
AIN-vitamin mixture | 1.0 | 1.0 |
Choline bitartate | 0.2 | 0.2 |
Beef tallow | - | 20.5 |
Preparation of rat serum
Whole blood was collected from rats by the cardiac puncture method and serum was isolated by centrifugation at 3000 rpm for 10 min.
Measurement of serum lipids level
Total cholesterol level was measured by using a total cholesterol assay kit (AM 202-K, Asan Pharm Co., Seoul, Korea) based on Richmond’s method [
27]. Pipet 0.3 mL of the enzymatic cholesterol reagent into a test tube. Add either 20 μL of serum or glycerol standard. Mix well and incubate at 37 °C for at 30 min. Measure the absorbancesof samples and standard at 550 nm.
Triglyceride level was measured by a triglyceride assay kit (AM 157S-K, Asan Pharm Co., Seoul, Korea) based on McGowan’s method [
28]. Pipet 0.3 mL of the enzymatic triglyceride reagent into a test tube. Add either 20 μL of serum or glycerol standard. Mix well and incubate at 37 °C for at 30 min. Measure the absorbancesof samples and standard at 500 nm.
Measurement of serum HDL and LDL levels
The levels of high-density lipoprotein (HDL) and low-density lipoprotein (LDL) in serum were measured using the Roche Cobas C-111 analyzer (Roche-Diagnostics, Indianapolis, IN, USA): Atherosclerosis index (AI) was calculated by employing the following equation: AI = (total cholesterol − HDL cholesterol)/HDL cholesterol.
Measurement of body weight, retroperitoneal fat, and epididymal fat
The body weight of rats in normal (N), control (C), and EPK (100 and 200 mg/kg)-treated groups was monitored once a week for 6 weeks. The retroperitoneal and epididymal fat was also removed from EPK (100 and 200 mg/kg)-treated rats on the last day of animal study and weighed.
Immunohistochemical staining
For histopathological examination, paraffin sections (4 μM) from dissected liver and adipose tissues were stained with hematoxylin and eosin. Immunohistochemical staining of PPARγ (Novus, Littleton, CO, USA) and p-AMPK (Cell signaling Tech., MA, USA) was performed using the indirect avidin/biotin-enhanced horseradish peroxidase method. Antigen retrieval was performed after dewaxing and dehydration of the tissue sections by microwaving for 10 min in 10 mM citrate buffer. Sections were cooled to room temperature, treated with 3 % hydrogen peroxide in methanol for 10 min, and blocked with 6 % horse serum for 30 min at room temperature in a humidity chamber. The sections were then incubated with primary antibody against PPARγ (diluted 1: 200; Novus, Littleton, CO, USA) or p-AMPK (diluted 1: 150; Cell Signaling) at 4 °C overnight in a humidity chamber. The sections were washed in PBS and incubated with secondary antibody (biotinylated goat anti-rabbit antibody; diluted 1: 150; Vector Laboratories, Burlingame, CA, USA) for 30 min in the humidity chamber. After further washes, the antibodies were detected with the Vector ABC complex/horseradish peroxidase (HRP) kit (Vector Laboratories, Burlingame, CA, USA) and color developed with 3,3′-diaminobenzidine tetrahydrochloride. For semiquantitation, ten photomicrographs (200×) were taken with a CCD camera, avoiding gross necrotic areas.
Measurement of adipocyte size
Images were acquired using an Axio Imager. Adipocyte size in adipoxe tissue was analyzed using Image J (National Institutes of Health, Bethesda, MD) software.
Statistical analysis
All data are shown as mean ± SD. In vitro experiment data were analyzed by Student’s t-test. In vivo experiment data were calculated by analysis of variance (ANOVA) followed by Duncan’s multiple range test. A P value of less than 0.05 was considered statistically significant. Means in the same column with different superscript letters (a, b, c, d, e and f) are significantly different (P < 0.05) between groups.
Discussion
This study provides the first direct evidence of the anti-obesity effects of EPK and its component LA, while providing insight into the regulatory mechanisms underlying those effects in rats with HFD-induced obesity. Our findings show that EPK efficaciously inhibits adipocyte differentiation and adipogenesis in 3 T3-L1 adipocytes and in rats with HFD-induced obesity by activating AMPK. In recent years, EOPK has been reported to demonstrate anti-cancer, anti-obesity, and anti-hypolipidemic effects [
21,
23,
24]. EPK is easier to extract than EOPK. There are significant differences between EPK and EOPK with respect to their components and extraction cost of the two extracts. Moreover, EPK is convenient and easy to use.
Adipogenesis is a cellular differentiation process in which the preadipocytes are transformed into differentiated adipocytes [
31] and accumulate lipids [
33]. Therefore, controlling adipocyte differentiation is important. AMPK agonists and PPARγ antagonists appear to be involved in adipocyte differentiation and thus can be potential drugs for the treatment of obesity [
34]. EPK suppressed fat accumulation and serum triglyceride levels, decreased PPARγ, CEBPα, FABP, and GPDH expression, and increased p-AMPK expression in the differentiated 3 T3-L1 adipocytes, without any cytotoxic effect. Similarly, many natural products, including green tomato extract [
35], tiacremonone [
4,
36,
37], and ursolic acid [
38], suppress adipogenesis and improve insulin sensitivity in vitro and in vivo via AMPK and PPARγ signaling. Moreover, EPK in HFD-fed SD rats was found to reduce body weight gain without loss of appetite (Additional file
3: Figure S2). Loss of body weight is related to decrease in fat pad mass as a result of reduction of adipocyte size or triglyceride accumulation [
39]. EPK reduces the retroperitoneal and epididymal fat weight as well as serum triglyceride levels compared to those in HFD-fed rats. Our data suggest that EPK can prevent obesity via inhibition of lipid metabolism, including reduction of triglyceride levels.
Elevated cholesterol is a risk factor for obesity, stroke, and heart disease, as well as diabetes [
40]. Specifically, low levels of serum HDL cholesterol are closely related to obesity [
41]. EPK increased the level of HDL cholesterol in a dose-dependent manner compared to that in HFD-fed rats. Furthermore, EPK decreased triglyceride and cholesterol levels, suggesting that EPK regulates lipid metabolism, which was in accordance with
in vitro data. EPK inhibited SREBP-1 and HMGCR as the cholesterol related factor. Adipogenesis and cholesterol syntheses in adipocytes as well as the liver, is regulated by SREBP-1 and HMGCR [
20,
42,
43]. Several natural compounds including mevalonic acid [
42], suppressed cholesterol levels
in vitro and in vivo through AMPK, SREBP-1, and HMGCR in adipocytes, similary to our data. LA as an active compound in EPK that is responsible for anti-hyperlipidemic activity was identified by a bioactivity-guided fractionation procedure. The active fractions were characterized by reverse-phase HPLC and identified by mass spectrometry (Additional file
1: Figure S1). LA is a bioactive diterpene and is known to have anti-allergic effects [
44]. LA inhibited fat accumulation in adipocytes, induced p-AMPK expression, and inhibited PPARγ expression. Further study is required to verify the anti-obesity effects of LA in HFD-fed animal models.
Competing interests
The authors declare that there are no conflicts of interest.
Authors’ contributions
HJL conceived and designed the experiments; SMC and MSL performed the experiments; EOL analyzed the data; and MHL and SHK contributed reagents/analysis tools. All authors read and approved the final manuscript.