Background
In recent years, one of the breakthroughs in the field of tumor immunology is the identification of the important role played by nonresolving inflammation (NRI) and myeloid-derived suppressor cells (MDSCs) in the tumor occurrence and development [
1,
2]. Our research group has successfully established multiple malignant transformed cell lines by transplanting glioma stem/progenitor cells to the subcutaneous tissue, cranial cavity, abdomen, liver and other parts of EGFP transgenic nude mice [
3,
4]. In the area of TME, the research on malignant transformation of non-tumor cells has formed a solid foundation. Tumor-associated dendritic cells (DCs) are an important member of MDSCs in TME. The malignant DCs used in our study is highly proliferative and prepared based on human GSC line SU3 (SU3-induced host Dendritic Cells Transformed Cell, ihDCTC). It is known that TME cells, especially fibroblasts, may facilitate tumor cell resistance to chemotherapeutic drugs in traditional tumor chemotherapy [
5]. Furthermore, the action of various inflammatory factors and chemokines in the complicated TME may also disable the immune cells to kill the tumor cells, thereby maintaining tumor angiogenesis and aggravating tumor invasion and metastasis [
6]. For example, tumor-associated macrophages (TAMs) are involved in tumor angiogenesis, matrix remodeling, invasion and metastasis, immunosuppression, and chemical resistance [
7]. However, it remains unclear about the sensitivity of transformed cells in TME,such as ihDCTC, to chemotherapeutic drugs. This paper reported that ihDCTC is not sensitive to the traditional chemotherapeutic drug cisplatin (Cis) but sensitive to resveratrol (Res) and is regulated by IL-6/p-STAT3/NF-κB.
Methods
Materials
SU3 cells and experimental animals were prepared by our research group, SU3 cells were first obtained from the department of the Second Affiliated Hospital of Soochow University. The method of obtaining the cells is previously described [
8], 4-6-week-old male and female GFP nude mice and non-fluorescent nude mice at an average weight of 22 g were provided by the Center for Experimental Animals, Soochow University (certificate No. SY X K (Su) 2007-0035) [
9]. All the animals were bred and maintained in the Specific Pathogen Free Animal Care Facility,Nasal1000 grade. The RFP lentiviral vector was purchased from Shanghai Innovation Biotechnology Co., Ltd.; hamster anti-mouse CD11c antibody from eBioscience Corporation, US; APC-labeled anti-mouse CD11c antibody and APC-labeled anti-mouse CD80 antibody from Biolegend Corporation, US; recombinant mouse granulocyte-macrophage colony-stimulating factor (rmGM-CSF) and recombinant mouse interleukin 4 (rmIL-4) from Peprotech Corporation; rabbit anti-mouse α signal protein (SIRP-α) antibody from Abcam, Inc.; CCK-8 reagent from Dojindo Chemical Technology Co., Ltd.; immunohistochemical staining and Western Blot primary antibody reagents: antibodies against IL-6, STAT3, p-STAT3, NF-κB and p-NF-κB were purchased from Abcam, Inc. DMEM medium and fetal bovine serum were purchased from Hyclone Laboratories, Inc., US; flow cytometer from Beckman Coulter, US; fluorescent inverted microscope from Olympus Corporation, Japan; microplate reader from Tecan, Switzerland. Freezing microtome was purchased from LEICA, Germany and cell incubator from SANYO, Japan. Resveratrol and cisplatin were purchased from Gibco and Qilu Pharmaceutical Co., Ltd. Respectively.
Establishment of SU3-ihDCTC cell line
Obtain immortalized DCs from co-cultured SU3-RFP and DCs
The mouse DC line was established and identified for corresponding molecular markers according to the classic Inaba method [
10]. The established glioma stem/progenitor cell line SU3 was transfected with RFP gene (SU3-RFP) according to the manufacturer’s instructions, followed by puromycin resistance screening to obtain stable cell strain SU3-RFP; the strain was then placed to DMEM/F12 stem cell culture medium that contained B27 additive, 20 ng/ml epidermal growth factor (EGF) and 20 ng/ml recombinant human basic fibroblast growth factor (bFGF) for balling, followed by digesting into single cells; the above obtained SU3-RFP and DCs were directly mixed and cultured in DMEM/high glucose medium containing 10% fetal bovine serum subject to the ratio of 1:10; the growth status of co-cultured cells was observed under the fluorescence microscope every day until EGFP + cells became clustered or colonized, followed by 0.25% trypsin digestion and single-cell suspension collection via centrifugation; in the subculture process, single cells expressing EGFP rather than RFP were screened out under the fluorescence microscope and then separately passaged in 96-well plates, where 1 strain of EGFP positive cells that could be infinitely passaged, named SU3-ihDCTC, was selected for further analysis on their cytobiological characteristics; after that, amplification and passaging were conducted.
Detection of the genetic characteristics of SU3-ihDCTC
RT-PCR and immunocytochemical staining were used to detect murine markers β-actin and EGFP in SU3-ihDCTC cells, DC molecular marker proteins D11c, CD80 [
11] and SIRP-α [
12], as well as mouse macrophage marker F4/80 [
13]. The specific steps were as follows:
RT-PCR detection. ihDCTC cells in the logarithmic growth period were selected for the test. The total RNA of the collected and treated cells were extracted using Trizol reagent, followed by reverse transcription to synthesize cDNA according to the reverse transcription kit instructions. PCR reaction was conducted under the following conditions: predegeneration at 94 °C for 5 min, 30 cycles of denaturation at 94 °C for 30s and renaturation at 72 °C for 30s, and extension at 72 °C for 7 min. β-actin was taken as the internal reference. PCR products were processed on the 1.5% agarose gel electrophoresis (AGE) at 100 V for 30 min, and the gel image processing system was used for imaging; the primers were synthesized by Suzhou Genewiz Co., Ltd. (Table
1).
Table 1
Primer name, sequence and product length
EGFP | F:GCCACAAGTTCAGCGTGTCCG R:GTTGGGGTCTTTGCTCAGGGCG | 566 bp |
RFP | F:AGGTTCTTAGCGGGTTTCTTG R:CTTCCCTGAGGGCTTCACAT | 312 bp |
CD68 | F:CTACATGGCGGTGGAATACAATG R:TAGCCTTAGAGAGAGCAGGTCAA | 175 bp |
F4/80 | F:CAGCTGTCTTAGAGGCTTCTCTT R:TGTAGCTTCCCACAGAGTTAGAG | 149 bp |
CD1a | F:GAGTTGTTTCGTCAGTTTCCATAG R:GGAGGCCCTTGGAGTTATCATT | 452 bp |
CD83 | F:CTCTACTGGGCTGTTACCTTGTT R:GAGGAGTTCACACAGAAGACCAT | 138 bp |
CD86 | F:GCCTGAGTGAGCTGGTAGTATTT R:TGTGAAGTCGTAGAGTCCAGTTG | 150 bp |
β-actin(H) | F:ACATCCGCAAAGACCTGTAC R:GCCATGCCAATCTCATCTTG | 346 bp |
β-actin(M) | F:CTTTGCAGCTCCTTCGTTG R:TGGTAACAATGCCATGTTCA | 278 bp |
Immunocytochemical staining. The cells were inoculated to the culture plate at the concentration of 2 × 104/ml for growing on the slide until the cells grew to a relatively uniform concentration; the culture medium was taken out and fixed by ice acetone for 15 min after washing by PBS, followed by 0.5% H2O2 incubation for 15 min, blocking serum incubation for 20 min, primary antibody incubation at 4 °C overnight, secondary antibody working solution incubation at 37 °C for 30 min, DAB coloring, hematoxylin counterstaining, ethanol decolorizing and mounting with gum.
SU3-ihDCTC chromosome karyotype analysis
SU3-ihDCTC cells and DCs were submitted for chromosome karyotype analysis by g-banding, followed by addition of colchicine at the final (mass) concentration of 10μg/ml and action at 37 °C for 4 h. After that, the cells were collected and processed by Giemsa staining after hypotonic treatment, fixation and slide dropping. Cell division phase was automatically searched by the platform microscope on chromosomal analyzer.
SU3-ihDCTC in vivo tumorigenic experiment
SU3-ihDCTC (1 × 106 cells/100 μl) in the logarithmic growth period was inoculated subcutaneously on the right back of 10 non-fluorescent nude mice to observe the tumor induction rate and pathological features. At 21 days after inoculation, 10 mice were anesthetized by intraperitoneal injection of 10% chloral hydrate (200 mg/kg).and were sacrificed by breaking the neck.
Detection of cell proliferation via CCK-8 assay
Both ihDCTC and SU3 cells were cultured in DMEM high glucose medium containing 10% fetal bovine serum and placed in a 5% CO2 incubator at 37 °C. The cells in the logarithmic growth period were inoculated to the 96-well plate (3000 cells/well) with 6 duplicate wells for each group; after culture in the 5% CO2 incubator at 37 °C for 24 h, media with different concentrations of Res and Cis were submitted for this procedure and cultured for 24 h, 48 h and 72 h. After that, CCK-8 reagent was added to each well in the dark, followed by culture in the incubator for 2 h and detection of OD value at 450 nm using the microplate reader. The cell survival rate and the half maximal inhibitory concentration (IC50 value) of two cell lines for two drugs were calculated. SPSS 22 software was adopted to analyze the OD value of two cell lines treated by Res and Cis, and GraphPad Prism 5 was used for plotting to demonstrate the differences between groups.
Cell cycle detection using flow cytometer
After trypsin digestion, the ihDCTC cells in the logarithmic growth period was centrifuged at 1000 rpm for 5 min; the supernatant was discarded before adding the medium; after being measured by counting chamber, the solution was diluted into single cell suspension with 106 cells and then tiled in a 6-well plate. The plate was placed in the 5% CO2 incubator at 37 °C for 24 h of adherent growth, followed by replacing by media with different concentrations of Res and Cis, where the control group was simultaneously set. After incubation for 24 h, trypsin digestion and centrifugation, the cells were washed by PBS twice before addition of 70% ice ethanol and placement at 4 °C overnight, followed by action by RNaes at 37 °C for 1 h, addition of PI and staining at 4 °C for 1 h. at last, cell cycle was detected by flow cytometer.
Apoptosis detection using flow cytometer and Hoechst 33,342 staining
Apoptosis detection using Annexin-V method
Two cell lines were treated with 10, 50 and 100 μM Res and 5, 10 and 50 μM Cis for 48 h, respectively. The cells were stained by Annexin V-FITC and PI respectively according to the manufacturer’s instructions, and flow cytometer was used to analyze the fluorescence intensity of FITC and PI, thereby quantitatively detecting the apoptosis status of different groups.
Hoechst vital cell staining
ihDCTC and SU3 were treated with 100 μM Res and 10 μM Cis respectively for 48 h; after removing the medium, Hoechst33342 was diluted by PBS subject to the ratio of 1:2 and then added to the cells for 20 min of staining. With the staining solution removed, the cells were washed by PBS twice and then photographed under the purple exciting light for counting.
Detection of protein expression via western bolt
Cells from each treatment group or tumor tissues from each group of the nude mouse subcutaneous xenograft model were collected and placed in a 1.5 mL EP tube, followed by addition of appropriate amount of lysate for thorough lysis and ultrasonic testing in the ice bath using ultrasonic processor; after centrifugation at 12000 rpm for 15 min at 4 °C, the supernatant was extracted and stored at − 20 °C for subsequent use. The concentration of proteins extracted above was quantified using BCA Protein Quantification Kit; 5× loading buffer was added to the sample, followed by boiling at 100 °C for 5 min to denature the proteins. After processing by SDS-PAGE, the proteins were transferred to the NC membrane using protein transfer device and then blocked by 5% skimmed milk powder for 1 h. After addition of IL-6, p-STAT3 and NF-κB antibodies and incubation overnight at 4 °C, the proteins were washed by TBST three times before 1 h of fluorescent secondary antibody incubation and another three times of TBST washing; Western Blot imaging software Odyssey analyzer was used to detect and produce images.
Animal experiment
ihDCTC thawed from liquid nitrogen was placed in DMEM high glucose medium containing 10% fetal bovine serum and cultured in 5% CO2 incubator to the vigorous growth period; the cells were then inoculated to 21 nude mice (age, about 6 weeks; weight, about 22 g) at the concentration of 1 × 106 cells/animal. When the tumor grew to the extent that vernier caliper could be used for measuring, the animals were divided into A) control group (n = 4); B) ihDCTC Res treatment group (n = 5); C) ihDCTC Cis treatment group (n = 4); and D) ihDCTC Res + Cis treatment group (n = 4), where the Res dose was 12.5 mg/Kg and intraperitoneally injected daily for three consecutive weeks, while Cis was injected every other day for three consecutive weeks at the dose of 2 mg/kg. The body weight and tumor size of modeled mice were measured every 3 days. Tumor volume was calculated by formula a × b2÷2, where a represented tumor long diameter, while b represented tumor short diameter, followed by plotting the proliferation curve. Blood was collected from the caudal vein weekly and submitted for blood routine. At the end of the experiment, as previously described,mice were anesthetized by intraperitoneal injection of 10% chloral hydrate (200 mg/kg).blood was sampled from the eyeball with their liver and kidney function examined,and the node mice was sacrificed.the tumor tissue was removed and weighed in the aseptic condition, and part of the tissue was cut into pieces to prepare cell suspension and perform conventional cell culture. After that, paraffin section and immunohistochemical staining were conducted to analyze the traditional pathological and molecular pathological features of xenografts.
Statistical methodology
The experiment was duplicated at least 3 times. GraphPad Prism 5 software was used for imaging analysis; one-way ANOVA and Student’s t-test were employed for data statistical processing, where the data were represented by mean ± standard deviation (x ± SD). P < 0.05 was defined as statistical significance.
Discussion
SU3-ihDCTC is a malignantly transformed DC strain derived from the bone marrow of nude mice expressing green fluorescent proteins in vitro induced by glioma stem/progenitor cell SU3-RFP, which can highly express EGFP and is featured by immortalization, heteroploid chromosomes and high tumorigenicity in nude mice. According to the mouse DC classification criteria by Stephanie et al. [
22] based on cytokine chemokine receptor (XCR1) and signal-regulated protein α (SIRP-α), SU3-ihDCTC belongs to tumor-associated SIRP+DCs and can be further used for studies focusing on nonresolving inflammation cells as a tool cell. In particular, it can be utilized in the research on new antitumor drugs as malignantly transformed cells induced in TME.
Res has been found to exist in 72 plant species distributed in 21 families and 31 genera, including Chinese herbs
Cassia tora,
Veratrum nigrum and
Polygonum cuspidatum [
23]. Interestingly, it has also been found in wines [
24], which contributes to the research enthusiasm of many scholars. Constant investigations have shown that Res can generate multiple biological effects, such as anti-oxidation, anti-inflammatory and lipid metabolism regulating, and exhibit a wide antagonism against mammalian pathogen-induced infections. Owing to the inhibitory effect on the proliferation of varying tumors at different stages like malignant glioma and melanoma, it has been used for the experimental research focusing on chemoradiotherapy and related target molecules during the past two decades [
25,
26]. Studies have suggested that Res can inhibit the growth of glioma U87 cells and promote the apoptosis [
27]; it can also permeate the blood brain barrier and be absorbed by brain tissue [
14], thereby achieving an effective plasma concentration. However, it has not been reported whether Res can inhibit the proliferation of tumor-associated cells originated from TME, especially the malignantly transformed immunotolerant inflammatory cells induced by tumors, such as ihDCTC cells. Providing that ihDCTC cells are derived from bone marrow DCs and belong to immune inflammatory cells, Res is speculated to be effective from the “anti-inflammatory” perspective, and the results of our experiment appear to be consistent with this theory.
However, the problem is that ihDCTC cells, as malignantly transformed DCs, neither have immunological function nor are immunotolerant. Despite of its nature of cancer cells, the effectiveness to them from the “anti-cancer” point of view remains to be proved compared with those malignant tumors like breast cancer, colon cancer and glioma reported in the literature [
28‐
31]. Considering the relevance research theories about MDSC and NRI in TME, the occurrence and development of almost all malignancies are related with chronic inflammation [
1,
2], where those conditions that cannot be cured either by anti-inflammatory or anti-cancer therapies are called NRI. In this regard, only drugs capable of acting against both cancer cells and NRI cells can realize the requirements for cancer treatment. Therefore, in our report, cancer cells were represented by SU3, NRI cells by ihDCTC, developed new drug by Res and traditional anticancer drug by Cis. The results of our treatment experiment indicated that 1) for Cis anticancer action, ihDCTC was more resistant than SU3, and the NRI problem remained unsolved after treatment; 2) for Res, both ihDCTC and SU3 exhibited certain sensitivity, and it may simultaneously solve the “anti-cancer” and “anti-inflammatory” tasks. In order to confirm the special double-edged features of Res for ihDCTC and SU3, its effect on the proliferation and apoptosis of ihDCTC and SU3 was investigated in this paper. The results demonstrated that Res can indeed simultaneously kill both ihDCTC and SU3 cells (Figs.
1 and
4).
Finally, it must be pointed out that the killing mechanism of Res against ihDCTC proves to be extensive, and our study mainly focuses on the process by which Res arrests the cell cycle in proliferation to the DNA synthesis period and induces massive apoptosis. This mechanism has been used in the traditional anti-cancer treatment. As mentioned above, the special point is that ihDCTC cells are related with both MDSC and NRI; the key target molecules for specific treatment should be located in their regulatory network, and their basic approach is to stimulate tumor cells to release proinflammatory factors, chemokines and other factors, which may lead to uncontrolled inflammatory cell proliferation and loss of MDSC immune function. At the same time, these two cell types can secrete cytokine IL-6 to promote tumor growth, while STAT3 and NF-κB expressed in them may be activated by IL-6 and TNF-a in TME and then released to extracellular space; the activated STAT3 and NF-κB pathways can not only promote tumor growth but also facilitate MDSC proliferation and activation. Moreover, IL-6 is bound to STAT3 and NF-κB pathways, which makes STAT3-NF-κB a key regulatory hub. Our findings suggest that after Res treatment, the reduced xenograft mass of ihDCTC-bearing mice is associated with the decrease in the protein expression of IL-6, STAT3 and NF-κB, indicating that this pathway may serve as a key target for the treatment of ihDCTC. However, more critical target molecules are required to be explored, since only the authentic and key target treatment meets the precision medical treatment mode favored by our ages. The tumor should be eventually eliminated, while our study only reported a reduction in tumor mass.
Conclusions
In conclusions,In vitro co-culture with GSC can induce the malignant transformation of bone marrow derived dendritic cells, on the one hand,which shows higher drug resistance to the traditional chemotherapeutic drug Cis than GSCs, but, on the other hand, appears to be more sensitive to Res than GSCs. In brief,ihDCTC which comes from TME was more resistant than SU3,and Res may be have capable of againsting both cancer cells and NRI cells.which can provide a broader vision not only for the further study on the correlation between TME and tumor drug resistance but also for the exploration of Res anti-cancer value.