Background
Angelica gigas Nakai (i.e., Korean angelica or Dang Gui) is used as a traditional medicinal herb in East Asian countries. Decursin and decursinol angelate are major coumarinic components of the
A. gigas root, which has anti-cancer [
1‐
3], neuroprotective [
4], anti-platelet [
5], prevention of obesity [
6] and bone-loss [
7], and anti-inflammatory [
8,
9] properties. Angelan (
A. gigas peptic polysaccharide) is obtained from water-soluble fraction of
A. gigas extracts [
10]. It has immuno-stimulatory effects through the activation of the innate and adaptive immune systems [
11,
12].
Angelan induces splenic lymphocyte proliferation and increases interferon (IFN)-γ production and the immuno-stimulatory cytokine interleukin (IL)-6 during the early stages of treatment [
12]. Therefore, macrophages and natural killer (NK) cells in splenocytes might be the main cellular targets directly affected by angelan. Angelan also activates dendritic cell (DC) maturation via the toll-like receptor 4 (TLR4) signaling pathways [
11]. Its mechanism of action in lipopolysaccharide (LPS)-induced macrophage activation through the mitogen-activated protein kinase (MAPK) and NF-κB/Rel is well-understood [
13]. Angelan also prevents tumor growth and metastasis [
14], but the mechanisms via which cells are directly involved in anti-cancer activity are poorly understood. Angelan increases the migration of DCs to lymph nodes; these DCs enhance the anti-tumor activity of the lymphocytes [
15]. Release of IL-12 cytokine is one of the effector cell functions of active DCs and macrophages. IL-12 is required for the activation of NK and natural killer T (NKT) cells [
16,
17]. NK and NKT cells have major roles in the anti-cancer activity of innate immunity. Infiltration of NK and NKT cells into tumors is closely associated with augmented cytotoxicity against tumor cells, and a much higher survival rate in mice [
18,
19].
During the development of natural ingredients for functional food, we separated the water-soluble polysaccharide fraction of
A. gigas that has immuno-stimulating effects (immuno-stimulatory fraction of
A. gigas; ISAg). The polysaccharide composition of ISAg is similar to that of angelan [
10]. However, ISAg contains a higher fraction of glucose (44.7% of total polysaccharides), which is involved in the TLR4 signaling pathways of macrophages. The objective of this study was to investigate the possible roles of ISAg in induction of the innate immune response and stimulation of the anti-cancer activity of NK and NKT cells.
Methods
Angelica gigas Nakai root was obtained from Gangwon province, Korea. The voucher specimen (
Y.D. Kim et al.
TG-20090258) was deposited at the Herbarium of Hallym University (Chuncheon, Korea). The ISAg was prepared by adding five times greater
v/v % water to
A. gigas root and extracting twice at 80 °C for 6 h, and then filtered (pore size, 0.45 μm). The resulting extract was concentrated in vacuo and dissolved in 5 to 8 times 70% ethanol at 55 °C for 2 h with stirring. The ethanol-insoluble precipitates were obtained after centrifugation. The phenol-sulfuric acid method was used to measure the total carbohydrate content of the ISAg [
20]. Briefly, 200 μl ISAg was mixed with 1 ml 5% phenol; 5 ml H
2SO
4 was then added and mixed well on a vortex mixer. After a 20-min incubation, the color intensity was measured at 490 nm using a Microplate reader (Thermo Fisher Scientific, Waltham, MA, USA). To investigate the constituent sugars, the ISAg was hydrolyzed with H
2SO
4 and subjected to anion-exchange high performance liquid chromatography (ICS-5000, Dionex Co., USA) for quantitative analysis.
Mice and chemical reagents
Wild-type (WT) C57BL/6 (B6), C3H/HeN (TLR4-WT), and C3H/HeJ (TLR4-mutant) mice were obtained from Jung Ang Lab Animal Inc. (Seoul, Korea). IL-12p40 reporter (Yet40) and IL-12p35 knockout (KO) B6 were provided by Dr. R. Locksley (University of California at San Francisco, CA, USA). All mice used in this study were maintained at Hallym University or Sejong University. The animal experiments were approved by the Institutional Animal Care and Use Committee (IACUC) at Hallym University (Hallym 2016–34) and Sejong University (SJ-20160705). All experiments were performed blindly and randomly using age- and sex-matched mice. For sacrifice, mice were euthanized by CO2 asphyxiation. The CpG oligodeoxynucleotides (CpG ODN type B 1826) were manufactured by Bioneer (Daejeon, Korea). LPS was obtained from Sigma-Aldrich (St. Louis, MO, USA). Alpha-galactosylceramide (α-GalCer) was obtained from Enzo Life Sciences (Farmingdale, NY, USA).
Cell culture and cell viability determination
Murine macrophage, RAW264.7 cells were grown in Dulbecco’s modified Eagle’s medium (DMEM; Gibco, Carlsbad, CA, USA) containing 10% fetal bovine serum (FBS, Gibco) supplemented with 2 mM glutamine and 100 units/mL penicillin-streptomycin. Cell viability was measured by using CellTiter 96® AQueous assay kit (Promega, Fitchburg, WI, USA). The cultured cells (5 × 104 cells/well) on 96-well plates were treated with serial dilutions of ISAg for 24 h. MTS tetrazolium was added to the plates and incubated at 37 °C for 1 h. Absorbance was measured at 490 nm using a microplate reader.
Nitrite assay and enzyme-linked immunosorbent assay (ELISA)
RAW264.7 cells were incubated with LPS (1 μg/mL) or various amounts of ISAg (0.125–2 μg/mL) at 37 °C for 24 h. The amount of nitrite (NO2−) in the culture supernatant was measured by Griess Reagent System (Promega). The amounts of IL-6, tumor necrosis factor (TNF)-α, and IL-1β secreted to the culture medium were quantified using an ELISA kits (KOMA Biotech, Seoul, Korea).
Quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR)
Total RNA from RAW264.7 cells was isolated with TRIzol reagent (Invitrogen, Waltham, MA, USA). The relative amount of specific mRNA was assessed by RT-PCR using amfiRivert cDNA Synthesis Platinum Master Mix (GenDEPOT, Baker, TX, USA). Quantification of mRNA was performed using qRT-PCR. The AccuPower® 2X GreenStar™ qPCR Master Mix (Bioneer) and the Exicycler™ 96 PCR system (Bioneer) were used according to the manufacturer’s instructions. The sequences of the sense- and antisense-strand primers used for PCR amplification were inducible nitric oxide synthase (iNOS), 5’-GCTACCACATTGAAGAAGCTGGTG-3′, 5’-CCATAGGAAAAGACTGCACCGAAG-3′; cyclooxygenase-2 (COX-2), 5’-GTCTCTCAATGAGTACCGCAAACG-3′, 5’-CTACCATGGTCTCCCCAAAGATAG-3′; IL-6, 5’-GCCAGAGTCCTTCAGAGAGATACA-3′, 5’-ATTGGATGGTCTTGGTCCTTAGCC-3′; IL-1β, 5’-CCTGTGTAATGAAAGACGGCACAC-3′, 5’-CTTGTGAGGTGCTG ATGTACCAGT-3′; TNF-α, 5’-TCTCATCAGTTCTATGGCCCAGAC-3′, 5’-GGCACCA CTAGTTGGTTGTCTTTG-3′. As a control, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene was also amplified using 5’-GACATCAAGAAGGTGGTGAAGCAG-3′, 5’-CCCTGTTGCTGTAGCCGTATTCAT-3′.
Immunoblot analysis of MAPK
Total cell lysates were extracted using CytoBuster™ Protein Extraction Reagent (Novagen). Equal amounts of protein were separated on 10 to 15% sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) gels at 300 mA for 20 min. They were then transferred to Immobilon-P polyvinylidene difluoride (PVDF) membranes (Millipore Sigma, Billerica, MA, USA) using a trans-blot SD Semi-Dry Transfer Cell (Bio-Rad, Hercules, CA, USA). After transfer, the membranes were incubated in Tris buffered saline (TBS), 5% dry milk, and 0.2% Tween 20 for 1 h. They were then further incubated in TBS and 0.2% Tween 20 with specific antibodies at 4 °C overnight. After washing, a horseradish peroxidase (HRP)-labeled secondary antibody was applied and the membranes were incubated for 2 h. Immunoblot detection was performed using Immobilon Western HRP Substrate (Millipore Sigma). The following antibodies from Cell Signaling Technology were used: anti-protein kinase B (PBK/Akt), anti-phospho-Akt (Ser473), anti-c-jun N-terminal kinase (JNK), anti-phospho-JNK, anti-p38, anti-phospho-p38, anti-p44/42 extracellular signal-related kinase (ERK), anti-phospho-p44/42 ERK, and anti-GAPDH.
Generation of bone marrow-derived DCs (BMDCs)
BMDCs were generated from the bone marrow (BM) cells of Yet40 B6 mice, as previously described [
21]. Briefly, BM cells were harvested from femurs and tibiae of mice by flushing with Roswell Park Memorial Institute (RPMI) 1640 medium (Gibco). Red blood cells were removed by adding ACK lysis buffer (0.15 M NH
4Cl, 10 mM KHCO
3, and 2 mM EDTA in distilled water), and remaining cells were washed with phosphate buffered saline (PBS) and cultured at a concentration of 1 × 10
6 cells/well in complete RPMI 1640 medium containing recombinant mouse fms-related tyrosine kinase 3 ligand (Flt3L) (100 ng/ml; R&D Systems, Minneapolis, MN, USA). Fresh cytokine-included culture medium was added on day 5 to generate BMDCs. Five days later, the BMDCs were harvested and stimulated for 16 h with vehicle or ISAg (125 to 2000 ng/ml). The purity of cluster of differentiation (CD)11c
+ cells of these cultures was > 92%.
Flow cytometry and intracellular cytokine staining
The following monoclonal antibodies (mAbs) were obtained from BD Biosciences (San Jose, CA, USA): PE-Cy7-, or allophycocyanin (APC)-conjugated anti-CD11c (clone HL3); phycoerythrin (PE)-, or APC-conjugated anti-NK1.1 (clone PK-136); biotin-conjugated anti-TRAIL; PE-Cy7- or APC-conjugated anti-CD3ε (clone 145-2C11); PE-Cy7-conjugated anti-CD11b (clone M1/70); biotin-conjugated anti-CD45 (clone PC61); PE-conjugated anti-IL-12p40 (clone C15.6); biotin-conjugated CD86 (clone GL1); PE-conjugated anti-TNF-α (clone XP6-XT22); PE-conjugated anti-IgG1 (κ isotype control). The following mAbs from Thermo Fisher Scientific were used: PE-conjugated anti-FasL (clone MFL3); APC-conjugated anti-F4/80 (clone BM8); PE-conjugated anti-perforin (clone eBioOMAK-D); PE-conjugated anti-IFN-γ (clone XMG1.2). Flow cytometric data were acquired using a FACSCalibur (Becton Dickinson Inc., San Jose, CA, USA) and were analyzed with FlowJo analysis tool (Tree Star Inc., Ashland, OR, USA). For surface antibody staining, the cells were collected and washed twice with fluorescence-activated cell sorting (FACS) buffer (PBS containing 0.5% bovine serum albumin). The cells were pre-incubated with purified anti-CD16/CD32 mAbs (BD Bioscience, Bedford, MA, USA) on ice for 10 min for blocking non-specific binding to Fc receptors, and were then stained with fluorescence-labeled mAbs. For intracellular staining, splenocytes were incubated with brefeldin A in RPMI 1640 medium (10 μg/ml) at 37 °C for 2 h. Cells were stained for specific surface markers and fixed with 1% paraformaldehyde. After permeabilization with 0.5% saponin (Sigma-Aldrich), cells were stained with the indicated mAbs (PE-conjugated anti-IL-12p40, anti-TNF-α, anti-IFN-γ, and ant-Perforin; PE-conjugated isotype control rat IgG mAbs) for 30 min. More than 5000 events per sample were acquired using the FACSCalibur.
Cell enrichment using magnetic activated cell sorting (MACS) and cell culture
NK1.1+ cells were enriched from total splenocytes isolated from B6 mice using positive selection with anti-APC MACS (Miltenyi Biotec, Bergisch Gladbach, Germany), after staining with APC-conjugated anti-NK1.1 mAb. The cell populations included > 92% NK1.1+ cells among all the MACS-purified populations. Splenocytes were prepared as single cell suspensions and cultured in complete RPMI 1640 medium containing 10% FBS supplemented with 5 mM 2-mercaptoethanol, 2 mM L-glutamine, 10 mM HEPES, and 100 units/mL penicillin-streptomycin.
Cytotoxicity assay
The flow cytometric 7-amino actinomycin D (7-AAD)/carboxyfluorescein succinimidyl ester (CFSE) cytotoxicity assay was used as previously described [
22]. NK1.1
+ cells were isolated as described above and were suspended in complete RPMI 1640 medium. B16 melanoma cells (3 × 10
6) were stained with CFSE (50 μM) in a 2 ml Hanks’ Balanced Salt Solution at 37 °C for 10 min. NK1.1
+ cells were incubated with the CFSE-labeled target cells (20,000 cells) at different effector/target (E:T) ratios (27:1, 9:1, 3:1, and 1:1). After a 10-h incubation, the cells were stained with 0.25 μg/ml 7-AAD and were incubated at 37 °C for 10 min. After two washes with PBS containing 1% FBS, cells were resuspended in FACS buffer and their cytotoxicity was evaluated using flow cytometry.
In vivo ISAg injection procedure
To evaluate the dose-dependent effects of ISAg on innate immune responses, Yet40 B6 mice were given ISAg via oral gavage at doses of 0.5, 1, 2, or 4 mg/mouse, three times per week for 4 weeks. In addition, to measure the time-dependent effects of ISAg on innate immune responses, Yet40 B6 mice were received oral ISAg (4 mg/injection) three times per week for the indicated times (1 to 4 weeks). For experiments to test the involvement of TLR4 signaling in ISAg-mediated innate immune responses, C3H/HeN and C3H/HeJ mice were orally administered ISAg (4 mg/injection) three times per week for 4 weeks. Besides, to investigate whether ISAg-mediated innate immune responses are IL-12 dependent, WT or IL-12p35 KO mice were orally administered ISAg (4 mg/injection) three times per week for 4 weeks. In all the aforementioned experiments, LPS (2 μg/mouse) was used as a positive control to inject mice intraperitoneally (i.p.) once a week for a total of 4 weeks.
Tumor injection and isolation of tumor-infiltrating leukocytes
To examine in vivo anti-tumor effects of ISAg, Yet40 mice (n = 5/group) received subcutaneous (s.c.) injections of 5 × 105 B16 melanoma cells. One week later, these mice were treated orally using oral gavage with either ISAg (4 mg/injection) or PBS three times per week for the following 2 weeks. As a positive control for anti-tumor immune responses, α-GalCer (2 μg) was injected via the intraperitoneal route two times per week starting 7 days after tumor injection, for a total of 2 weeks. On day 21 after tumor injection, groups of mice were euthanized and tumor tissues were excised for immunological analysis. Tumor-infiltrating leukocytes were isolated using the following procedure: small pieces tumor tissues were digested (15 min, 37 °C) using 2.5 mg/ml collagenase type IV (Sigma-Aldrich) and 1 mg/ml DNase I (Promega). After incubation, the digested tissues were dissociated into single-cell suspensions using a gentleMACS Dissociator and C Tubes (Miltenyi). The cell-containing suspension was passed through a 70-μm pore nylon cell strainer (BD Bioscience, Bedford, MA, USA), and put on ice. The tumor-infiltrating leukocytes were harvested from the interface of a 40/70% Percoll (GE Healthcare, Little Chalfont, UK) gradient after centrifugation at 1000 g for 20 min.
Statistical analysis
Statistical significance was analyzed using the Excel statistical analysis tool (Microsoft, Redmond, WA, USA). The comparison of two groups was performed by the Student’s t-test. Values of *
P < 0.05 and **
P < 0.01 were considered to indicate a statistically significant result in the Student’s t-test. VassarStats statistical software (
http://vassarstats.net/anova2u.html) was used for two-way analysis of variance (ANOVA). Values of
#P < 0.05 and
##P < 0.01 indicates a statistically significant result in the two-way ANOVA.
Discussion
Natural compounds obtained from the same plant using different kinds of solvents (usually water-soluble versus water-insoluble) often have opposite effects on human and experimental animal physiology [
8,
11]. However, contradictory (immuno-stimulatory or inhibitory) regulation of immune responses induced by these extracts can achieve the same goal of anti-cancer activity. Decursin induces direct cytotoxic effects on tumor cells through cell cycle arrest [
23] or activation of the protein kinase C and reactive oxygen species signaling pathways [
24]. Our study found that the polysaccharide component of
A. gigas induced the death of target tumor cells through activation of the immune system.
Depending on the microenvironment, macrophages can be polarized into either an M1 phenotype with anti-tumor properties or an alternative M2 phenotype with pro-tumor properties [
25]. Components of
A. gigas extract can induce distinct immune responses that are pro-inflammatory or anti-inflammatory [
11,
26]; our study revealed that ISAg treatment preferentially differentiated macrophages toward the M1 phenotype (producer of TNF-α and IL-12), but not the M2 phenotype (producer of IL-10). These results indicated that the M1 polarization of macrophages by ISAg treatment is a mechanism that elicits an optimal anti-tumor response. Angelan increases the maturation of DCs via TLR4 [
11], but the signaling pathway of ISAg is unclear because the major components of ISAg polysaccharide are quite different. In conclusion, we found that macrophages and DCs were activated by ISAg through TLR4 signaling pathway and subsequently secreted IL-12 cytokine.
NK and NKT cells are activated via ISAg-induced IL-12, which results in increased levels of cytolytic molecules including perforin, TRAIL, and FasL. Thus, TLR4- and IL-12-dependent NK and NKT cell activation by ISAg ultimately results in enhanced cytotoxicity against tumor cells. Previous studies found that angelan induces proliferation of splenic B lymphocytes [
12], but T-cells can be indirectly activated [
15]. Regulatory T (Treg) cells are required for suppression of the innate anti-tumor immunity of NK and NKT cells [
27,
28]. Inversely, activated NK and NKT cells can inhibit the development of Treg cells via IFN-γ production [
29,
30]. Consistent with the results of previous studies, our results indicated that ISAg increased IFN-γ production by NK and NKT cells, resulting in a significant decrease in the Treg population within the tumor tissue (data not shown). This result suggested that enhanced anti-tumor function by ISAg treatment might be related to the suppression of Treg cells resulting from activation of NK and NKT cells. Moreover, our previous results indicated that anti-tumor effects of human T helper type 1 (Th1)-type cytokine (IL-32γ) are attributed to induction of DC maturation followed by consequent enhancement of NK and NKT cell cytotoxicity [
31] via DC-derived IL-12 manner [
16]. Therefore, it will be of interest to apply ISAg as an immune adjuvant or functional food ingredient to boost anti-cancer immune responses.