Background
Obesity has become a major global health problem [
1]. It is a major contributor to diseases such as diabetes, cardiovascular disease and hypertension [
2]. It has been established that being overweight or obese is associated with an increase in both the size and number of adipocytes along with an excessive amount of fat accumulation [
3,
4]. Although using appetite suppressant drugs is a common way to treat obesity, long-term pharmacological treatment has been reported to generate many side-effects [
5,
6]. This has led to innovative research using natural products to combat obesity in the hope of a safe and novel treatment [
7,
8].
O. indicum belongs to Bignoniaceae family, widely found in Tropical Asia including Thailand. The chemical composition of
O. indicum includes baicalein, chrysin, oroxylin A and oroxylin B [
9,
10]. Many previous studies have reported antioxidant [
11], anti-inflammatory [
12], anti-diabetic [
13] and hepatoprotective properties for
O. indicum and its isolated compounds [
14]. Animal studies suggest that
O. indicum has few toxic effects and oral doses of 250 mg/kg BW for 28 days were tolerated well in rats [
13]. It has previously been demonstrated that an extract of
O. indicum derived from the root of the plant reduced the plasma concentrations of glucose, triglycerides and total cholesterol in diabetic rats [
15]. Furthermore, stem bark extracts generated an inhibitory effect on an α-glucosidase enzyme in mature 3T3-L1 adipocytes [
13]. Whilst, Oroxylin A, an isolated compound from
O. indicum, induced both anti-adipogenesis and lipolysis in 3T3-L1 adipocytes. This was largely explained by a down-regulation of many transcriptional factors and an enhanced expression of pro-apoptotic proteins [
16].
OIE has varying pharmacological properties depending on which part of the plant it is derived. In this study, we have used an extract from the fruit pods, rather than the stem of the plant which has been previously studied by several groups. The aim of this study was to provide a simple chemical composition of the extract. Initial experiments were performed using 3T3-L1 cells to establish any potential toxic effects of OIE and its impact on lipid accumulation within adipocytes. A further aim was to employ FTIR to examine the biochemical changes observed in the differentiated cells. An in vitro study was also conducted to examine the effect of OIE on pancreatic lipase. The results indicate that OIE can inhibit the development of adipocytes. Both treated and non-treated differentiated adipocytes had distinct biochemical profiles.
Methods
Chemicals
3T3-L1 mouse embryonic fibroblasts and bovine calf serum were purchased from the American Type Culture Collection (ATCC, CL-173, USA). Insulin solution from bovine, methyl isobutyl xanthine (IBMX), lipase from porcine pancreas type 2, dimethyl sulphoxide (DMSO), 4-Nitrophenyl dodecanoate (pNP), simvastatin and orlistat were purchased from Sigma-Aldrich (St. Louis, USA). Dulbecco’s modified Eagle’s medium with high glucose (DMEM), penicillin, streptomycin, N-2-hydroxyethylpiperazine-N-2-ethane sulfonic acid (HEPES) and 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) were purchased from GIBCO Invitrogen (Grand Island, NY). Dexamethasone was acquired from G Bioscience (St. Louis, USA). Oil Red O was purchased from amresco (USA). Fetal bovine serum (FBS) was purchased from Hyclone (Logan, Utah).
Preparation of O. indicum extract
O. indicum (fruit pods) fresh samples were purchased from the local market at Wang Nam Khiao district, Nakhon Ratchasima province, Thailand. The voucher specimens (SOI0808U) were deposited at the flora of Suranaree University of Technology (SUT) Herbarium and authenticated by Dr.Santi Wattatana, a lecturer and a plant biologist at Institute of Science, SUT, Thailand. Fresh pods were washed thoroughly with tap water, cut into small pieces and then dried in the oven at 40 °C for 2 days. The dried pieces were pulverised using a mechanical grinder, and the resulting coarse powder was preserved from moisture. The O. indicum dry powder (500 g) was extracted with 95% ethanol by a soxhlation for 8 h. The extract was filtered through Whatman filter paper and concentrated using a rotary evaporator at 50 °C under vacuum to remove the ethanol. The remaining extract was stored at − 80 °C until required. Subsequently, the sample was lyophilized in a freeze dryer (LABCONCO), automatic mode, vacuum 240 × 10− 3 mBar, and collector − 55 °C. The extracted powder was stored at − 20 °C until required. The lyophilized O. indicum extract was used within 3 months of preparation. The extract was resuspended in the differentiation medium containing 0.1 v/v DMSO (vehicle) and added to the cells at concentrations ranging from 0 to 1500 μg/mL.
Phytochemical screening
The stock concentration of the lyophilized extract (10 mg/mL) was prepared and tested for the presence of bioactive phytochemical compounds, including total phenolics, flavonoids, alkaloids, steroids, glycosides, tannin, and saponins.
Test for flavonoids
Briefly, 1 mL of the OIE was mixed with 2 mL of 2%
w/
v of NaOH. A 2 mL aliquot of 10% w/v lead acetate solution was added to 1 mL of the alkaline extract. The formation of a yellow colour indicated the presence of flavonoids [
17,
18].
Test for alkaloids
The Mayer’s and Wagner’s test was performed. Briefly, 1 mL of the OIE was added to 2 mL of 1%
v/v HCl and heated. A few drops of Mayer’s and Wagner’s reagents was added to the mixture, the presence of a precipitate suggested the presence of alkaloids in the sample [
17,
18].
Test for steroids
The steroids containing in OIE were investigated. In short, 1 mL of the OIE was mixed with 2 mL each of chloroform and concentrated H
2SO
4. A red colour in the lower layer indicated the presence of steroids [
17,
18].
Test for glycoside
The detection of glycosides was performed using Salkowski’s test. In brief, 2 mg of the OIE was mixed with 2 mL of chloroform and a few drops of concentrated H
2SO
4. A reddish brown colour was associated with the presence of glycosides [
17,
18].
Test for tannin
The Gelatin test was performed to identify tannins in the extract. To summarise, 1 mL of the OIE was added to 2 mL of a 1%
w/
v gelatin solution, the formation of a white precipitate indicated the presence of tannins [
17,
18].
Test for saponin
The foam test was performed. Briefly, a 5 mL aliquot of the OIE was shaken for 5 min. If saponins were present, they formed a stable foam in the test tube [
17,
18].
Total phenolic content determination (TPC)
The total phenolic content was measured following the method outlined by Kohoude with slight modifications [
19]. In brief, 20 μL of extract (0.625 mg/mL) or a standard solution of gallic acid (0–7.5 μg/mL) were added into 100 μL of Folin-Ciocalteu reagent. The mixture was incubated at room temperature for 6 min, prior to the addition of 7.5%
w/
v of Na
2CO
3. After a 1 h incubation at room temperature, the absorbance was recorded at 760 nm against a DMSO blank. TPC of the sample was expressed as mg of gallic acid equivalents (GAE) per g of dry weight
.
Total flavonoid content determination (TFC)
The total flavonoid content was measured according to a previously published method with slight modifications [
20]. To summarise, a 25 μL of extract (3 mg/mL) or a standard solution of catechin (0–200 μg/mL) were added to 125 μL of deionised water, followed by the addition of 10 μL of 5%
w/
v NaNO
2. This mixture was incubated at room temperature, and after 6 min, 15 μL of 10% AlCl
3 was added. Upon mixing the reaction was allowed to proceed for 5 min, then 50 μL of 1 M NaOH was added. The absorbance of the mixture was determined at 595 nm versus prepared DMSO blank. Total flavonoid of the sample was expressed as mg of catechin equivalent (CE) per g of dry weight.
Cell culture and differentiation procedures
The differentiation procedures of 3T3-L1 preadipocyte were performed as following the ATCC recommended protocol. Briefly, adherent 3T3-L1 cells were cultured in DMEM containing a high glucose concentration, supplemented with 10% of bovine calf serum, 100 U/mL of penicillin and 100 μg/mL of streptomycin, until they reached 70–80% confluently. Two days after confluence (day 0), the cells were stimulated to differentiate with differentiation medium containing 10% FBS, 1.0 μM dexamethasone, 0.5 mM of IBMX, and 1.0 μg/mL of insulin in DMEM. On day 2, the differentiation medium was changed to maintain a medium consisting of 10% of FBS and 1.0 μg/mL of insulin in DMEM. The maintenance medium was replaced every 48 h for the next 8 days. On day 10, the differentiation of 3T3-L1 pre-adipocytes into adipocytes was observed. The cells were maintained in 5% CO2 incubator and at 37 °C throughout the whole process. The required doses of OIE were added to the 3T3-L1 cell culture during the differentiation (at day 0, 2, 4, 6, and day 8).
Cytotoxicity assay
The cytotoxic effects of the OIE on the proliferation of preadipocytes and adipocytes were determined by MTT assay largely following the method of Dunkhunthod et al. and Denizot [
21,
22]. In brief, the 3T3-L1 cells were seeded in 96-well plates at a density of 5 × 10
3 cells/well. Two days after reaching confluence, the dividing cells were treated with the OIE at concentrations ranging from 0 to 1500 μg/mL. Both treated and control cells were incubated for a further 48 h. At the end of the treatment period, the cell viability was assessed by using the MTT assay. The culture medium was removed, and 100 μL of MTT solution (0.5 mg/mL in phosphate buffer saline) was added, then incubated at 37 °C for 4 h. After incubation, 150 μL of DMSO was added to dissolve formazan crystal. The absorbance of the intracellular formazan is proportional to the number of viable cells present was determined at 540 nm against a blank medium (Benchmark Plus, Bio-Rad, Japan). The percentage of formazan product was calculated to determine cytotoxicity [
23]. OIE at concentrations of 0–200 μg/mL were used to assess any cytotoxic effects of mature adipocytes and Oil Red O assay for lipid accumulation. The IC
50 of the extract was also calculated from a dose-response curve using linear regression analysis.
Oil red O staining
The intracellular triglyceride content was determined using an Oil Red O staining method as previously described [
21,
24]. Briefly, on day 10, cells were washed with PBS and fixed with 1 mL/well of 10% (
v/v) formalin for 1 h at room temperature. After fixation, the cells were washed, and 500 μL of 0.5% of the Oil Red O solution was added. The cells were incubated for 30 min at room temperature. The Oil Red O solution was removed by gentle aspiration, and the cells were washed with PBS. The nucleus was then stained with 0.10% (
w/
v) haematoxylin. Fat droplets were observed under an inverted microscope at an appropriate magnification. To determine the percent of lipid accumulation, the cells were extracted with 250 μL of isopropanol and 200 μL of the eluted solution was transferred to a new 96 well plate. The absorbance was measured at 490 nm with a microplate spectrophotometer. The simvastatin at 1.67 μg/mL was used as a positive control. The 3T3-L1 cells, treated with 200 μg/mL OIE were selected for FTIR studies.
The effect of OIE on 3T3-L1 adipocyte cells using FTIR measurement was performed following the method of Dunkhunthod et al. [
21]. Briefly, on day 10, cells were collected and centrifuged at 4000×
g for 5 min, the medium was removed by gentle aspiration, and the cells were agitated and washed with 0.85%
w/
v NaCl. The cell suspensions were centrifuged at 4000×
g for 5 min. The acquired cell pellets were dropped onto a window slide (MirrIR, Kevley Technologies) and dried for 30 min in a desiccator to eliminate the excess water. The dried cells were stored in a desiccator prior to FTIR analysis.
FTIR spectra were obtained at the Synchrotron Light Research Institute (Public Organization), Thailand. FTIR spectra were acquired with a Bruker Vertex 70 spectrometer coupled with a Bruker Hyperion 2000 microscope (Bruker Optics Inc., Ettlin-Gen, Germany) equipped with nitrogen cooled MCT (HgCdTe) detector with a 36 x IR. The spectra were obtained in the reflection mode with the wavenumber range of 4000–600 cm− 1, using an aperture size of 50 μm × 50 μm, with a resolution of 6 cm− 1. Each spectrum was produced following 64 scans. OPUS 7.2 software (Bruker Optics Ltd., Ettlingen, Germany) was used to acquire FTIR spectral data and control instrument system.
The spectral ranges of biochemical interest were identified using Principal Component Analysis (PCA) as being between 3000 and 2800 cm− 1 and 1800–850 cm− 1. The preprocessing of the spectra was performed by second derivative transformations using the Savitzky-Golay algorithm (nine smoothing points) and normalised with extended multiplicative signal correction (EMSC). Score plots (3D) and loading plots were used to represent the different classes of data and relations among variables of the data set, respectively.
The FTIR spectra datasets were submitted for Unsupervised Hierarchical Cluster Analysis (UHCA), to collect similar spectra in groups or clusters, using the OPUS 7.2 software (Bruker). Cluster analysis was performed on the second derivatives, and vector normalises spectra using Ward’s algorithm.
Lipase activity
Measurement of lipase activity was performed as previously described by Guo et al. and Dunkhunthod et al. [
21,
25]. In brief, lipase of porcine pancreas type 2 was dissolved in distilled water at 5 mg/mL, the solution was centrifuged at 10,000
xg for 5 min, and the supernatant was used for the assay. A 0.1%
w/
v solution of pNP laurate was prepared in 5 mM of sodium acetate (pH 5.0) containing 1%
v/v Triton X-100. The solution was heated to 80 °C and cooled to room temperature prior to use. A 30 μL volume of the lipase was added to a 96 well plate, followed by 40 μL of reaction buffer (100 mM of Tris buffer pH 8.2). Either 20 μL of OIE or 50% v/v DMSO was added prior to the addition 30 μL of the substrate solution. The mixtures were incubated at 37 °C for 6 h and measured at 409 nm using a microplate spectrophotometer. Orlistat at 12.5 to 100 μg/mL was used as a positive control. The inhibition rate (%) was calculated using the following equation. [((OD
control – blank
control) – (OD
sample – blank
sample)) /OD
control] × 100 [
26].
Statistical analysis
All the data were expressed as a mean ± standard error of the mean (SEM). The statistical significances difference between treatment and control groups of cell viability, the amount of lipid accumulation, biomolecular changes, and lipase activity were analysed by One-way analysis of variance (ANOVA) with a Turkey’s HSD post-hoc test (SPSS v 23). Values were considered statistically significant when p < 0.05 and data were representative of at least three independent experiments (n ≥ 3). Most experiments were performed in triplicate.
Discussion
Adipocyte formation and activity appears to play a central role in the development of obesity. The generation and metabolism of adipocytes have become major targets for treating obesity [
34]. One area that has increased is the use of natural products to target obesity. Compared to the convenience and cost they are becoming more attractive propositions than synthetic drugs or surgery [
35,
36]. In this study, we investigated the effects of an OIE on the anti-adipogenic and biomolecular change in 3T3-L1 cells. Our studies indicated there was a dose-dependent effect of OIE upon the viability of preadipocyte ranging from concentrations of 250 μg/mL to 1500 μg/mL. At lower doses (0–200 μg/mL) there was no significant difference from the control (
p > 0.05). Although no major impact on the viability of the cells was observed at lower doses, there may be some toxicological changes to the cells. One further observation was that doses of OIE between 50 to 200 μg/mL indicated a dose-dependent decrease of lipid accumulation in the adipogenesis assay (Figs.
3 and
4). One explanation for the reduction of lipid could be that the chemical components of OIE may have a potent effect which inhibits the differentiation of 3T3-L1 preadipocytes. A previous study reported that the fruit of
O. indicum is rich in flavonoids such as baicalein [
37]. At a concentration of 20 μM baicalein can prevent the differentiation of preadipocyte to adipocyte during first 4 days of induction [
38]. In fact, there is evidence to support that baicalein inhibits a cell cycle regulator, promoting cell cycle arrest at the G0/G1 phase. It was also reported to suppress the m-TOR signaling pathway leading to an inhibition of adipogenic factors such as
PPARγ [
39]. OIE at 200 μg/mL lipid accumulation was not significantly different to cells treated with 1.67 μg/mL (4 μM) of simvastatin (Fig.
3,
p > 0.05). This was a very interesting observation as it confirms the observations made by Nicholson et al. They demonstrated that statins, such as pitavastatin and simvastatin at 5 μM could inhibit adipocyte differentiation by blocking
PPARγ expression and activating
pref-1 expression [
40]. These findings provide evidence that the OIE may also have the capacity to inhibit
PPARγ activity.
To further elucidate the biochemical potential of OIE, an in vitro pancreatic lipase assay was performed as part of this study. Lipase became a target for research groups attempting to prevent obesity or metabolic syndrome [
21,
25]. Pancreatic lipase is an enzyme responsible for the breakdown of triglycerides into glycerol and fatty acid in the gastrointestinal tract. When lipase activity is inhibited, triacylglycerol cannot cross the intestinal brush border membrane leading to a decrease in the uptake of lipids into the human body. Orlistat, a known lipase inhibitor is a drug for treating obesity was used in this study as a positive control. The results of this study indicated that OIE demonstrates as inhibition of pancreatic lipase between doses of 100–1250 μg/mL (IC
50 of 1062.04 ± 32.21 μg/mL), 27 times less potent than orlistat. The dose of OIE that inhibited pancreatic lipase was also a concentration which induced a decrease in viability of pre-adipocytes (IC
50 of 882.68 ± 47.99 μg/mL). However, a study by Roh and Jung indicated a number of plant extracts could inhibit pancreatic lipase but had a very little effect on the viability of 3T3-L1 cells [
41]. In the light of this publication, it would appear that OIE is not the most effective inhibitor of lipase activity. However, the potential for OIE to inhibit cell cycle progression [
39] or its effects on key adipogenic biochemical pathways [
16] still requires further investigation.
Although FTIR microspectroscopy has previously been used to characterise biochemical composition in medical research studies [
29,
42,
43]. This is the first study using FTIR to demonstrate on the biochemical profile of OIE treated adipocyte. These results indicated that the lipids, lipid esters, nucleic acids, glycogen and carbohydrates of the OIE-treated adipocytes were significantly decreased compared to the untreated adipocytes (Fig.
7a and
b). This study indicates that FTIR provided data similar to that obtained using established biochemical assays. This suggests that FTIR is a very useful technique for assessing the impact of plant extracts on established cell lines. Although FTIR supported the preliminary evidence regarding to biochemical changes during the differentiation of 3 T3-L1 cells more research is needed to clarify the mechanism of action of OIE.
Acknowledgements
The authors are grateful to the Synchrotron Light Research Institute (Public Organization), Thailand, for supporting the FTIR microspectroscopy technique.
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