Background
Cancer gene therapy approaches include the direct killing of tumor cells by injecting a therapeutic gene into the tumor cell or employing vaccine strategies to deliver an immunomodulatory gene that stimulates the immune system to recognize tumor antigens [
1].
Bifidobacteria (BF) are an important group of the human intestinal microbiota that exert a number of beneficial probiotic effects on the host, including immunomodulation [
2], antibacterial activity [
3], bacteriocin production [
4], improvement of the intestinal microbial balance [
5], and a reduction of inflammation [
6]. BF is used in the health care and food industries as a probiotic. BF can target to the hypoxic environment of solid tumors and has been considered to be an alternative strategy in tumor therapy or as a live vaccine [
7,
8].
The Herpes Simplex Virus thymidine kinase/ganciclovir (HSV-TK + GCV) system is currently one of the best-studied tumor suicide gene therapy systems [
9‐
11]. When expressed in tumors, TK converts the non-toxic precursor GCV into GCV- 3-phosphate, a toxic substance that kills tumor cells. Apoptotic signaling is initiated either through extrinsic or intrinsic stimulation, resulting in the activation of caspases [
12].
We previously found that bladder tumor growth was significantly reduced in rats treated with BI-TK + GCV after 15 days of treatment [
10]. However, the mechanism was unclear. In this research, we constructed a BF-specific plasmid pBEX as an expression vector to express TK [
8]. A colorectal cancer model was used to decipher the molecular mechanism of BF-rTK + GCV (bifidobacterial recombination thymidine kinase/ganciclovir) using a human apoptosis antibody array kit in a murine cancer model
in vivo. Another three human cancer xenograft models (gastric cancer MKN-45, liver cancer SSMC-7721 and breast cancer MDA-MB-231) were also established for survival analysis after BF or BF-rTK + GCV intratumor treatment.
Methods
Bacterial strains and growth conditions
Escherichia
coli DH5α was used as the host for molecular cloning; pBEX was constructed by MA
et al. [
8] and used as the expression vector in Bifidobacterium (BF). The
Bifidobacterium infantis strain (Collection in our laboratory) was cultured in MRS broth (Difco) containing 0.25 % (w/v) L-cysteine. HCl (pH 7.0) at 37 °C under anaerobic conditions. Ampicillin (50 mg/ml) was added to both recombinant BF and
E. coli strains when required.
Construction of BF-rTK + GCV suicide gene therapy system
HSV TK gene (accession AB032875) was PCR amplified and sub-cloned into pBEX at the BamH I and Sal I sites with an artificial signal peptide. Potential recombinants were first screened by bacterial colony PCR. The potential recombinant plasmid was transformed into competent B. infantis cells via electroporation, signatured BF-rTK were used as TK producer cells, and verified by DNA sequencing.
An intravenous (i.v.) gene therapy in nude mice indicated that 1.0 × 106 cells/ml of BFTK was the highest concentration with no adverse effects, whereas 1.0 × 104 cells/ml was the lowest effective concentration. At concentrations greater than 1.0 × 107 cells/ml, the i.v. injection resulted in venous embolisms and subsequent death. Based on these results, 2.0 × 105 cells/ml were the dosage of BF-rTK used in this study.
BF or BF-rTK (pBEX-tk) cells (0.5 ml, 2.0 × 105 cell/ml) were prepared and mixed with 1.0 ml GCV (5.0 mg/kg) respectively and PBS was added to adjust the final volume to 2.0 ml. The negative control was 1.0 ml PBS mixed with 1.0 ml GCV (5.0 mg/kg). Mixtures were incubated at 37 °C for 1.0 h and further incubated for 10 min at 95 °C to stop the reaction. To identify whether the rTK in BF-rTK cells was secreted expression, the 1.0 ml supernatant of BF-rTK culture was isolated by centrifugation for 10 min at 12,000 rpm and incubated with1.0 ml GCV (5.0 mg/kg) at 37 °C for 1.0 h and incubated for another 10 min at 95 °C. The reactants were centrifuged for 10 min at 12,000 rpm. Both supernatants were analyzed by HPLC with an octadecylsilane chemically bonded silica column. The mobile phase ratio was methanol: H2O (5:95) and the UV detection wavelength was 252 nm.
Experimental animals
Mice (Balb/c-nu) and Balb/c mice were housed at the Laboratory Animal Center of Chongqing Medical University (Chongqing, China). This study was carried out in strict accordance with the recommendations in the Guide for the Care and Use of Laboratory Animals of the National Institutes of Health. The protocol was approved by the Committee of the Ethics of Animal Experiments at the Chongqing Medical University (SYXK2012-0001). All procedures were performed under sodium pentobarbital anesthesia, and the method of euthanasia was cervical dislocation.
Cells and cell culture
Colo320 cell line was obtained from China Center for Type Culture Collection (CCTCC GDC 042), gastric cancer (MKN-45), liver cancer (SSMC-7721) and breast cancer (MDA-MB-231) were obtained from Committee of Type Culture Collection of Chinese Academy of Sciences (CTCCCAS) and maintained in complete growth medium: RPMI 1640 medium with 2 mM L-glutamine adjusted to contain 1.5 g/L sodium bicarbonate, 4.5 g/L glucose, 10 mM HEPES, and 1.0 mM sodium pyruvate, 90 %; 10 % fetal bovine serum. The cells were cultured in 100-mm culture dishes in a humidified, mixed environment of 37 °C and 5 % CO2.
Establishment of xenograft tumor models and experimental groups
Mouse model of xenograft tumor was established by injecting Colo320 cell (1.0 × 108 cells/ml) subcutaneously. Twenty-four tumor-bearing nude mice (male, 3–4 week, 20 g/mouse) were randomly divided into five groups at 7 weeks post-inoculation: the normal control PBS group (n = 3), GCV (n = 3), PBS + GCV (n = 6), BF + GCV (n = 6), and the BF-rTK + GCV group (n = 6). Each group was once off directly given PBS, GCV, PBS + GCV, BF + GCV, or BF-rTK + GCV through intratumor injections (BF or BF-rTK was 1.0 × 106 cell/tumor, GCV was 5.0 mg/kg). Three tumors were cut from sacrificed mice in each of the last three groups (PBS + GCV, BF + GCV, or BF-rTK + GCV) 48 h postinjection. From each cut out tumor, 20 % was used for immunochemistry analysis and the other 80 % of the tumors of the three mice were mixed together for protein array analysis (n =3). mRNA samples were extracted from three tumors from the last three groups for real time PCR analysis (n = 3). From the PBS and GCV groups, mRNA samples were extracted from three tumors for real time PCR analysis (n = 3).
Apoptosis array analysis
Total protein was extracted and prepared from the colo320 tumor xenograft tissues and treated with PBS + GCV, BF + GCV, and BF-rTK + GCV respectively and the proteins concentration was normalized to 10 mg/ml, following the protocol of RayBiotech human apoptosis antibody array kit (Cat# AAH-APO-1-4). The results were analyzed using the RayBiotech cytokine antibody arrays Tool and the ratio of the significant differential expression was considered to be more than 2.0 or less than 0.5.
Gene silencing and western blotting analysis
Colo320 cells were treated with commercial synthetic small interference RNA (Bim394, Bid77, Bim394+ Bid77, negative control) for 48 h respectively and then treated with or without BF-rTK + GCV for 48 h (with three replicates). Then the cells were lysed with NP40 buffer (1 % NP-40, 0.15 M NaCl, 50 mM, Tris, pH 8.0) containing protease inhibitors (Sigma). Protein quantitation was performed by BCA protein assay reagent (Pierce, USA). Equal amounts of protein from the different groups were denatured in SDS sample buffer and separated on 8–10 % polyacrylamide-SDS gel based on the protein molecular weight. Proteins were transferred to a polyvinylidene difluoride membrane. The antibodies to Bim (abcam 32158), Bid (abcam 32060), GAPDH (cell signaling technology, 14C10) were used to detect the target proteins, followed by incubation with a secondary antibody conjugated with horseradish peroxidase. The proteins of interest were detected using SuperSignal West Pico Chemiluminescent Substrate kit.
Immunohistochemistry staining
Immunohistochemistry (IHC) of XIAP (E3 ubiquitin-protein ligase XIAP), FADD (FAS-associated death domain protein), APAF-1 (apoptotic protease-activating factor 1) and cleaved Caspase-3 was conducted on five colo320 tumor xenograft tissues treated by PBS, GCV (resolved in PBS solution), BF, BF + GCV and BF-rTK + GCV, respectively (with three replicates). Retrieved tissues were fixed, decalcified in 10 % formalin and embedded in paraffin 24 h posttreatment. Serial sections of the embedded specimens were stained with hematoxylin and eosin (H & E). The fixed tissues of colo320 intestinal tumor were blocked and incubated with XIAP antibody (ab21278, abcam), FADD antibody (ab52935), APAF-1 antibody (ab32372) and cleaved Caspase-3 antibody (ab52293). After being washed, tissues were incubated with biotin-labeled secondary antibody for 30 min, followed by incubation with streptavidin-HRP conjugate for 20 min at RT. The presence of the expected protein was visualized by DAB staining and examined under a microscope. Stains with control IgG were used as negative controls.
Immunofluorescence
Immunofluorescence staining analysis of FasL (Fas ligand) expression in mouse colo320 tumor xenograft tissues was performed (with three replicates). The slides were then incubated with primary antibody diluted in PBS containing 1 % BSA for 16 h at 4 °C. The primary antibodies used were as follows: anti-FasL antibody (ab68338, 1:500). After washing three times in PBS, Alexa Fluor 55 5-conjugated anti-rabbit IgG (Invitrogen, Grand Island, NY) was added in PBS with 1 % BSA for 1 h. In the final washes, 6-diamidino-2-phenylindole (DAPI) (Sigma) was added and used as a counterstain for nuclei. Fluorescence images were acquired using a Zeiss Axioimager microscope.
RNA isolation and quantitative RT-PCR
The Caspase-3 downstream effectors (Rock-1 (Rho-associated protein kinase 1), Cad and Acinus (apoptotic chromatin condensation inducer in the nucleus)) were not contained in the apoptosis antibody array. In order to make up for the above mentioned missing in the apoptosis antibody array, total RNA was extracted from three colo320 tumor xenograft tissues from each group treated by PBS + GCV, BF + GCV and BF-rTK + GCV respectively, using TRIzol reagent (Invitrogen). The total RNA was applied to an RNase column (Qiagen, Venlo, Netherlands) for further purification and treated with DNase following the manufacturer’s protocol. cDNA was synthesized from 1 μg of total RNA using the SuperScript III reverse transcriptase kit (Invitrogen) resulting in a final volume of 20 μl. Primers were designed with the IDT SCI primer design tool (Integrated DNA Technologies, San Diego, California). Quantitative real time PCR (qRT-PCR) experiments were performed with Bio-Rad MJ MiniOption Real Time PCR System in triplicate and the data analysis was carried out by the CFX manager software version 1.5. The PCR data were normalized to GAPDH expression. The sequences of each primer pair were listed in Table
1.
Table 1
Primers and SiRNA sequences used in this study
GAPDH sense antisense Acinus sense Acinus antisense CAD sense CAD antisense ROCK-1 sense ROCK-1 antisense TK sense TK antisense
Bim394 sense
Bim394 antisence
Bid77 sense
Bid77 antisense
a
NC sense
a
NC antisence
| 5´ ACCACAGTCCATGCCATCAC 3´ 5´ TCCACCACCCTGTTGCTGTA 3´ 5´ AGGTGAGGAGAAGGAGGAAGT 3´ 5´ TCTACTGACACCTGGGGAGG 3´ 5´ CAGCCTCTATGCCAGTCTCG 3´ 5´ CTAGCTGCTCCAGGATGCTC 3´ 5´ GAATGTGACTGGTGGTCGGT 3´ 5´ CTGGTGCTACAGTGTCTCGG 3´ 5´CGCATGGATCCCATGGCTTCGTACCCCTGC 3´ 5´ ACGCGTCGACTCAGTTAGCCTCCCCCATC 3´
5´ GGUCAUUGGUGAUUAAAUATT 3´
5´ UAUUUAAUCACCAAUGACCTT 3´
5´ GGGAUGAGUGCAUCACAAATT 3´
5´ UUUGUGAUGCACUCAUCCCTT 3´
5´ UUCUCCGAACGUGUCACGUTT 3´
5´ ACGUGACACGUUCGGAGAATT 3´
|
Survival rate analysis of the other three kinds of tumor cell lines of nude mouse models in BF-rTK + GCV intratumor treatment
The other three tumor cell lines included gastric cancer (MKN-45), liver cancer (SSMC-7721) and breast cancer (MDA-MB-231). The nude mouse models of xenograft tumor (diameter ≥3.5 mm) were established by injecting the three different kinds of cancer cells (1.0 × 108 cells/ml) subcutaneously. Each positive group contained six nude mice (male, 3–4 week, 20 g/mouse) and when the xenograft tumor diameter was greater than 3.5 mm, BF-rTK (1.0 × 106 cell/mouse) was intratumorally given twice in 5 days. GCV (5.0 mg/kg, n = 6) was given via intramuscular injections every day during the five days. Each negative control group of six nude mice bearing the xenograft tumor were raised without any injections (Ctrl, n = 6). After the second BF-rTK injection (5 d), all mice were raised without any treatment. The surviving mice were counted every day. The data at the 1 d, 5 d, 17 d, 19 d, 21 d, 24 d, 27 d, 30 d, 35 d and 37 d were used to analyze survival rate. The significant difference was measured by p value.
Analysis of inflammatory marker in tumor tissue treatment by BF-rTK + GCV
IHC of TNF-α (tumor Necrosis Factor 2 A) was performed on five colo320 tumor xenograft tissues treated by PBS, GCV (resolved in PBS solution), BF, BF + GCV and BF-rTK + GCV, respectively (with three replicates). The following process was the same as the IHC assay of the apoptosis relative markers described previously. The presence of the TNF-α was visualized by DAB staining and examined under a microscope. Stains with control IgG were used as negative controls.
Effect of BF-rTK + GCV on necroptosis and autophagy protein expression
Necroptosis and autophagy relative protein markers including RIP-1 (Zinc metalloprotease Rip1), ATG5 (autophagy protein 5) and Beclin-1 were analyzed by western blot in colo320 intestinal tumor cell treated with BF + GCV or BF-rTK + GCV. The antibodies of RIP-1 (BA0346-2) and Beclin-1 (BA3123-2) were purchased from Boster (Wuhan, China) and the antibodies of ATG5 (10181-2-AP) were purchased from Proteintech (Wuhan, China).
Statistical analysis
Statistical analysis was performed using SPSS-17.0 software. Data were analyzed using one-way analysis of variance and Tukey’s HSD test was applied as a post hoc test if statistical significance was determined. Statistical significance for the two groups was assessed using Student’s t-test. The probability level at which differences were considered significant was p < 0.05.
Discussion
Conventional suicide gene therapy vectors used in cancer cases are typically based on herpes simplex virus or adenovirus [
15,
16]. There are HSV-TK + GCV-mediated gene therapy systems and adenovirus-mediated gene therapy systems and lentivirus TK + GCV gene therapy for lung cancer treatment [
9,
17‐
19]. It is clear that GCV is phosphorylated by the HSV1-TK to GCV monophosphate, and further to GCV di- and triphosphate and incorporated into proliferating tumor cell DNA, which causes DNA chain termination and induces tumor cell apoptosis [
17]. The major obstacle for wide clinical application of this approach is the insufficient amounts of the suicide gene delivered into the target tumor tissue by virus-based vectors [
17]. For example, the multiplicity of infection (MOI) of adenovirus-Rous sarcoma virus-thymidine kinase is no less than 66 in MDAH-2774 ovarian cancer cells after acyclovir treatment [
20].
Bifidobacterium (BF) is a non-pathogenic, non-toxic, and strictly anaerobic gram-positive bacterium and can target the hypoxic environment of solid tumors for its anerotaxis [
7,
10,
21]. In this research, we revealed the differences in mechanisms of BF + GCV and BF-rTK + GCV systems in inducing colo320 cell apoptosis in detail. Compared with virus-mediated vectors, the superiority of BF-rTK recombinant is that it does not invade the tumor cell and the rTK can be secreted outside of BF and thereby phosphorylates GCV. The phosphorylated GCV diffuses in the tumor tissue and functions its antitumor activity and the process remains independent of tumor cellular bio-systems. That means, BF-rTK does not target a single cancer cell, but the solid tumor as a whole. However, recombinant viruses (e.g. adenovirus-Rous sarcoma virus-thymidine kinase) have to infect and kill single cancer cells one by one. The bacteria engulfing is not necessary for BF-rTK + GCV system. The BF-rTK recombinant can be quickly reproduced outside cancer cells independently. Therefore, BF can deliver sufficient suicide genes into the target tumor tissue without MOI limitation.
Death receptors (DRs) are the members of TNF receptor superfamily including Fas/FasL, TNFRSF (DR4 (TNFRSF10A), DR5 (TNFRSF10B)) and TNF receptor (TNFR1, 2) [
22,
23]. TNF-β, lymphotoxin α, is generally described as an inflammatory and immune response factor and is signaled via TNFR1 and TNFR2. TNF-β is involved in the processes of inducing cell apoptosis when it is signaled by TNFR1. It then subjects a wide range of tumor cells to cytotoxicity [
24]. However, there are few reports about TNF-β inducing cancer cell apoptosis
via TNFR2 to date.
Compared to BF + GCV intratumor treatment, BF-rTK + GCV treatment increased four IGFBPs expression (Table
3). IGFBPs down regulate the activity of IGFs [
18], and promote apoptosis by modulating the expression of apoptosis-specific genes such as
Bcl-2 and
Bax [
24‐
26]. IGFBP-6 has a high affinity for binding IGF-2 and is able to inhibit the growth of various cancer cells and activated apoptosis pathways as an IGF-antagonist [
27‐
29]. The Bax/Bcl-2 ratio in BF-rTK + GCV was increased 1.7-fold compared to BF + GCV treatment (Tables
2 and
3), which lead to the activation of the caspase cascade. However, IGFs promote a shift in the expression of the Bcl-2 family and prevent glucose-induced Cyto C release, which is directionally blocked activation of the terminal apoptosis program and exhibits a decrease in the Bax/Bcl-2 ratio [
30]. That could explain why no detectable Cyto C was found in the BF + GCV group. Therefore, the pro-apoptosis proteins overwhelmed the anti-apoptosis proteins and the final results were tilted the balance toward apoptosis (Tables
2 and
3).
XIAP, c-IAP2, Livin and Survivin belong to the inhibitor of apoptosis family (IAP) with typical BIR (baculovirus IAP repeat) domain. IAP directly binds to Caspases as well as neutralization of Smac and further activates downstream anti-apoptotic cascades [
30]. However, the inhibitory effect of Livin on Caspase-3 and Caspase-9 is much weaker compared to that of XIAP [
31].
Bid and Bim are two important upstream target proteins up-regulated by Fas/FasL signaling and TNFR signaling in the mitochondrial control of apoptosis. Bid and/or Bim SiRNA treatment prevented colo320 intestinal tumor cells from apoptosis induced by BF-rTK + GCV in vitro as expected. The results confirmed that Fas/FasL signaling and TNFR signaling are principal pathways in BF-rTK + GCV induced colo320 intestinal tumor apoptosis in vivo. The gene silencing results suggested that these changes are causative rather than simply secondary effects of BF-rTK + GCV treatments.
Inflammation was identified as the seventh feature of cancer [
32]. Our data showed that BF-rTK + GCV system inhibited both TNF-α and its receptor, TNFR1, expression in tumor tissue, which indicated that BF-rTK + GCV inhibited inflammation induced by TNF-α/TNFR1 pathway. It was a synergistic effect of the tumor therapy. TNF-α is known to play an important role in various aspects of tumor progression. It was reported that TNF-α may promote breast cancer cell migration by inducing activation of the MAPK/ERK signaling pathway [
33]. In another study, TNF-α was found to stimulated prostate carcinogenesis in chemically induced mice by activation of the AKT/mTOR and NFkB pathway [
34]. Evidence suggested that the anti-inflammatory treatment prior to chemotherapy suppressed the acquisition of chemoresistance of breast cancer patients [
35]. Therefore, BF-rTK + GCV anti-inflammation effect was helpful for overcoming the chemoresistance of cancer.
Hopefully, the BF-rTK + GCV system might overcome drug resistance in single-target drug use in tumor therapy for its multiple targets and multiple effects. Our study highlighted the potential of BF-rTK + GCV system for solid tumor therapy.
Abbreviations
APAF-1, apoptotic peptidase activating factor 1; BF, Bifidobacterium infantis; BF-rTK + GCV, Bifidobacterium recombination thymidine kinase/ganciclovir; DAPI, 6-diamidino-2-phenylindole; FADD, Fas-associated with death domain protein; HTrA2, HtrA serine peptidase 2; IAP, inhibitor of apoptosis protein; IGF, insulin-like growth factor; IGFBP, insulin-like growth factor-binding protein; MOI, multiplicity of infection; NIK, kappaB-inducing kinase; PBS, phosphate-buffered saline; PCR, Polymerase Chain Reaction; PVDF, polyvinylidene difluoride; SDS, sodium dodecyl sulfate; siRNA, small interference-based RNAi; TNF, tumor necrosis factors; TRAF2, tumor necrosis factor receptor associated factor 2; TRAF2, tumor necrosis factor receptor associated factor-2; XIAP, X-linked inhibitor of apoptosis protein
Acknowledgements
We thank Dr. Philip Hardwidge (Kansas State University) for critical reading of the first version of manuscript. We thank Vivian Tsungai Mutsekwa (Chongqing Medical University) for proof reading of the final manuscript.