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To determine whether a novel marine micro-organism with anticancer properties, H31, the metabolic product of Bacillus SW31, has anti-tumor effects on head and neck cancer, and potential for apoptotic-enhancing anti-cancer treatment of affected patients.
The cell viability and apoptosis assays were performed. Changes in the signal pathway related to apoptosis were investigated. Then, the therapeutic effects of H31 were explored in mouse xenograft model and drug toxicity of H31 was examined in zebrafish model.
We identified the anticancer activity of H31, a novel metabolic product of Bacillus SW31. Bacillus SW31, a new marine micro-organism, has 70% homology with Bacillus firmus and contains potent cytotoxic bioactivity in head and neck cancer cells using MTT assay. Combined with c-JUN, p53, cytochrome C, and caspase-3, H31 induced apoptosis of KB cells, a head and neck cancer cell line. In a separate in vivo model, tumor growth in C3H/HeJ syngeneic mice was suppressed by H31. In addition, in a zebrafish model used for toxicity testing, a considerable dose of H31 did not result in embryo or neurotoxicity.
Growth inhibition and apoptosis were achieved both in vitro and in vivo in head and neck cancer cells after exposure to H31, a metabolite from the marine Bacillus species, without any significant toxicity effects even at considerable H31 dose concentrations.
Head and neck cancer is the fifth most common cancer worldwide, accounting for over 35,000 new cases of cancer each year in the United States alone (1). Despite technical advances in surgery, radiotherapy, and chemotherapy, survival rates have remained virtually unchanged over the last 50 years (2). Recently, the primary treatment modalities have increasingly shifted from surgery to chemotherapy and/or radiotherapy (3). A number of anti-tumor agents for chemotherapy have been developed and used; however, the considerable toxic effects of the current drug therapies have decreased the quality of life of the patients treated with these agents (4). The limitations of current therapy have motivated the investigation and evaluation of novel bioactive compounds, especially from natural products (5).
The ocean has always been a source for new bioactive metabolites, especially from micro-organisms (6). Over 200 novel metabolites were identified from marine micro-organisms in 2005 alone (7). Recent studies have reported that natural products from marine micro-organisms have potent cytotoxic characteristics, potentially useful for future pharmaceutical and biomedical applications (8).
One key mechanism associated with cancer cell growth is the control of apoptosis. Apoptosis is defined as programmed cell death, which occurs in response to disruption of normal homeostatic mechanisms; it is critical to the surveillance of tumor and/or malfunctioning cells (9). Induction of apoptosis among cancer cells has been considered one potential novel approach to the treatment of head and neck cancer (10).
Therefore, the goal of this study was to determine whether a novel marine micro-organism with anticancer properties, H31, the metabolic product of Bacillus SW31, has anti-tumor effects on head and neck cancer, and potential for apoptotic-enhancing anti-cancer treatment of affected patients.
Seawater samples were collected from Kanghwa Island (western coast of Korea). The seawater samples (0.1 mL) were streaked onto the surface of 1/10 marine agar (bacto tryptone 5 g, yeast extract 3 g, bacto agar 10 g, glycerol 3 mL, dissolved in 75% seawater, pH 7.5). After incubation at room temperature for 24 hours, all colonies were chosen for isolation.
Among 753 samples (from 188 strains) isolated, the individual strain that showed the strongest cytotoxic effects on head and neck cancer cells (FaDu, KB, SNU 899, SNU1066) by 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay was selected. We then performed biochemical tests to analyze the physiological and metabolic characteristics to identify the specific bacterial strain. This strain was grown at 25 under aerobic conditions for 3 days in sea water complete medium (bacto tryptone 5 g, yeast extract 3 g, glycerol 3 mL, dissolved in 75% seawater, pH 7.5). Extraction of genomic DNA and 16S rRNA gene amplification were carried out according to the method reported by Rainey et al. (11). The resulting polymerase chain reaction (PCR) product was ligated into a pGEM-T easy vector (Promega, Madison, WI, USA), sequenced using a Termination Sequencing Ready Reaction kit (Perkin Elmer, Boston, MA, USA), and analyzed using an ABI 377 genetic analyzer (Perkin Elmer). The complete 16S rDNA sequence (1,485 bp) was aligned using the CLUSTAL W software program (12). The 16S rDNA sequences used for the phylogenetic analyses were derived and compared with those of other bacterial 16S rDNA sequences available in the DDBJ/EMBL/GenBank database. Kimura's two-parameter model was used for the calculation of evolutionary distance (13). Our 16S rDNA sequencing of the strain characterized it to be most similar to Bacillus firmus (71.1% homology). We named the new marine strain Bacillus SW31 and added it to the Korean Collection of Type Cultures under accession number KCTC 11135BP. Table 1 shows the results of the biochemical tests of Bacillus SW31 using the Easy 24E Plus Kit (Komed, Seoul, Korea).
For the production of secondary metabolites, the Bacillus SW31 was cultured in Erlenmeyer flasks containing sea water complete medium. The flasks were then incubated on a shaker at 150 rev min-1 for 3 days at 25. The culture broth was centrifuged (10,000 rpm for 5 minutes at 4) to remove the cells, and then hexane 100 mL was added. The hexane was concentrated by a rotary vacuum evaporator and the new metabolic product obtained was named H31. The H31 (3 g) was further purified by reverse phase HPLC (Shimadzu, Tokyo, Japan; Cosmosil 5C18-MS column, 10×250 mm; linear gradient of MeOH in H2O containing 0.05% trifluoroacetic acid [TFA], 80-100% in 50 minutes; flow rate, 1.5 mL min-1; UV detection at 210 nm) to yield a pure cytotoxic compound (PCC).
Human FaDu and KB, SNU 899, SNU1066 (head and neck cancers), AGS (gastric cancer), HepG2 (liver cancer), and HT29 (colon cancer) were obtained from the American Type Culture Collection (Rockville, MD, USA) and Korean Cell Line Bank (Seoul, Korea). The cells were grown in Dulbecco's modified Eagle's medium (DMEM) or RPMI 1640 medium, supplemented with 10% fetal bovine serum (FBS) with penicillin-streptomycin at 37 in a humidified 5% CO2 atmosphere. SCC VII/SF cell lines were grown in RPMI 1640 medium supplemented with 10% FBS and antibiotics.
To determine cell viability, the cells were seed in 96-well plates at densities of 5×103 cells/well in 1 mL complete medium after the cells were exposed to various concentrations of H31. Then, MTT was added to 40 µL of cell suspension for 4 hours. After three washes with PBS, the insoluble formazan product was dissolved in DMSO. The optical density (OD) of each culture well was measured using a microplate reader (Bio-Tek, Winooski, VT, USA) at 540 nm.
We performed DNA fragmentation tests with the G-DEX II Genomic DNA Extraction Kit (Intron, Seoul, Korea). Briefly, the cells were plated in 6-well plates at 3×105 cell/well, incubated for 24 hours, and then treated with different H31 concentrations for 24 hours in the absence of serum. After the cells were washed twice with PBS, they were harvested then lysed in 150 µL of cell lysis buffer with 1 µL RNAase A solution for 30 minutes at 37. The cell lysates were cleared by centrifugation. After centrifugation, the supernatant was collected and treated with 150 µL of 100% isopropanol by gently mixing. Centrifuge was performed at room temperature for 5 minutes at 13,000 rpm. The supernatant was discarded and treated with 1 mL of 75% EtOH by gently mixing. After centrifugation, the supernatant was discarded and the pellet was air dried. DNA was obtained by adding precipitation solution. The DNA pellet was dissolved in 50 µL of TE buffer (100 mM Tris-Cl pH7.4 and 10 mM EDTA pH8.0) containing RNase and incubated for 1 hour at 37. The fragmented DNA was resolved on 2% agarose gels in the presence of ethidium bromide and electrophoresed for 30 minutes at 100 V and then the bands were detected by UV light.
Cells were plated at 1×106 cell/well in 6-well plates, incubated for 24 hours, and then treated with different concentrations of H31 for 24 hours. Trypsinized cells were washed with PBS and fixed in 70% ethanol. After fixation, the cells were incubated for 30 minutes with 200 mg/mL of RNase A and stained with 5 mg/mL propidium iodide (PI). The stained cells were analyzed using a flow cytometry cell sorter (Becton Dickinson, Franklin Lakes, NJ, USA). In addition, the cells were conjugated with Annexin V-FITC using a TACS Annexin V-FITC kit (Trevigen Inc., Gaithersburg, MD, USA) according to the manufacturer's protocol and analyzed by flow cytometry.
The apoptotic effects of H31 were also determined by the TUNEL method using an in situ cell detection kit (Roche Molecular Biochemicals, Mannheim, Germany). KB cells were glass cover-slipped in 24-well dishes containing growth medium. After 60-70% cell confluence was achieved, the cells were exposed to different H31 concentrations. The cells were incubated with 50 µL of TUNEL reaction mixture at 37 for 1 hour in a humid atmosphere. The cells were then stained with 5 µg/mL Hoechst 33258 for 5 minutes, and the stained cells were visualized under a fluorescence microscope.
The level of protein expression was evaluated using Western blot analysis. The KB cells were washed with PBS and isolated in RIPA (radioimmunoprecipitation) buffer. The mixture was centrifuged at 12,000 rpm for 5 minutes at 4. Then, 20 µg of total proteins per well were separated by SDS-PAGE, and were transferred to a nitrocellulose membrane (Amersham, Arlington Heights, IL, USA). The membrane was incubated with specific antibodies overnight at 4. The membrane was reacted with peroxidase-conjugated donkey anti-rabbit antibodies (Amersham, Piscataway, NJ, USA) or donkey anti-mouse antibodies (Amersham, Piscataway). The membrane was developed using the Enhanced Chemiluminescence (ECL) Detection System (Amersham, Piscataway) and X-ray film.
The animal experiments were approved by the Animal Care and Use Committee of Ajou University. For the in vivo study, we used a syngeneic mouse model (C3H/HeJ mice, SCC VII/SF cell) that is commonly used for an in vivo immunocompetent murine model of head and neck cancer (14, 15). Six-week-old female C3H/HeJ syngeneic mice (about 20 g weight) were obtained from Orient (Orient Bio, Seoul Korea) and were acclimated in pathogen-free conditions for 1 week. All mice were then injected subcutaneously with 5×105 viable SCC VII/SF cells into the right flank. Three mice were given daily intraperitoneal injections of distilled water (D.W, control group), and the other three mice received intraperitoneal injections of 0.2 mg H31 (treatment group) 5 days a week. The tumor size, in two perpendicular diameters, was measured with calipers every second day. The tumor volume was calculated by the formula V=(π/6)×(L×W2) (L: large diameter, W: smaller diameter) (16).
Growth curves were plotted using the average tumor size within each experimental group at the same points in time. The animals were euthanized and the tumors were collected and weighed at 22 days after implantation. The mice were sacrificed by cervical dislocation, and the tumors were excised and divided into two parts. For each sample, the final tumor size and weight were calculated.
Active caspase-3 IHC was performed on paraffin-embedded mouse squamous carcinoma tumor sections collected on polylysine-coated slides. After paraffin removal, using xylene, the sections were rehydrated and incubated with 0.3% hydrogen peroxide for 10 minutes to eliminate endogenous peroxidase activity. The specimens were incubated with anti-active caspase-3 mouse primary antibodies (Cell signaling Technology, Danvers, MA, USA) diluted 1:400 in blocking solution overnight at 4. The sections were incubated for 2 hours at room temperature with the streptavidin-biotin-peroxidase complex (Vectastain ABC kit, Vector Laboratories, Burlingame, CA, USA). The immunolabeling was revealed after three washes in PBS using 2,3' diaminobenzidene as substrate diluted 1:10 in buffer according to the manufacturer's instructions (Roche Molecular Biochemicals, Mannheim, Germany). Staining was performed by incubation with the 3,3-diaminobenzidine (DAB) substrate-chromogen, which results in a brown-colored precipitate at the antigen site. Measurements of active caspase-3-positive cells were performed on 10-15 images/slide identified by an independent observer blinded to the experiment and normalized to the total cell count by DAPI staining. A minimum of five sections were randomly selected for the analysis of the apoptotic index, and the number of apoptotic cells were viewed under the light microscope (Leica, Wetzlar, Germany).
Wild-type zebrafish (Danio rerio) was maintained at 28.5 on a 14 hours light/10 hours dark cycle. For the embryotoxicity evaluation, fertilized eggs were exposed to various concentrations of H31 or 100 µM cisplatin diluted with Danieu's Solution, and maintained in an incubator at 28.5 for 1 hour. After H31 treatment, evaluation for egg death, organogenesis, hatch rate, and any abnormal features was performed. For the neurotoxicity evaluation, 4 days post fertilization (dpf) larvae were placed at a density of 50 per 100 mm2 in a petri dish with embryo medium (1 mM MgSO4, 120 µM KH2PO4, 74 µM NaHPO4, 1 mM CaCl2, 500 µM KCl, 15 µM NaCl, and 500 µM NaHCO3) and placed in an incubator at 28.5. Then, 4 dpf zebrafish larvae were exposed to different concentrations of H31 diluted with Danieu's Solution, and maintained in an incubator at 28.5 for 1 hour. After the larvae were washed with 0.3×Danieu's Solution and cultured at 28.5 for 3 hours, and labeled using 2 µM YO-PRO1 (Molecular Probes, Eugene, OR, USA) for 1 hour, performed three times. The zebrafish was then anesthetized with 8 µg/mL 3-aminobenzoic acid ethyl ester methanesulfonate salt (MS222, Sigma Chemical Co., St Louis, IL, USA). After fixation, the zebrafish was mounted with methylcellulose on a slide and photographs were taken using a fluorescence microscope.
The Student t-test and one-way ANOVA were used for the statistical analyses with SPSS ver. 10.0 (SPSS Inc., Chicago, IL, USA). Statistical significance was defined at P<0.05.
Our primary aim was to investigate whether H31 had strong cytotoxic activity in four head and neck cancer cell lines. As shown in Fig. 1, the MTT assay shows that the cell viability dropped by 40% or more when the cells were treated with 10 µg/mL of H31, among all four cell lines. The antitumor activity of H31 was strongest in the FaDu cells and KB cells among the four cell lines; therefore KB cells were used for further experiments.
To investigate the mechanism of H31-induced growth inhibition of KB cells, we studied internucleosomal DNA cleavage by DNA fragmentation analysis for the detection of apoptosis. DNA fragmentation was observed at 24 hours after H31 treatment, a characteristic feature of apoptosis, which was not noted in the ethanol-treated control group (Fig. 2A). The TUNEL assay also showed that H31 increased the unclear condensation, DNA fragmentation, and perinuclear apoptotic bodies in the KB cells as detected by Hoechst 33258 staining (Fig. 2B). In addition, the amount of sub-G1 DNA was investigated, which contained less DNA than the G1 cells, to quantify the amount of apoptotic cells. Flow cytometric analysis showed that H31 treatment increased accumulation during the sub-G1 phase in a dose-dependent manner (Fig. 2C). Furthermore, annexin V analysis showed that the percentage of apoptotic cells (annexin V-positive, PI-negative, bottom right quadrants) increased in a dose-dependant manner (Fig. 2D). Therefore, H31 induced apoptosis in the KB cells
To study the mechanism of H31-induced apoptosis in the KB cells, we investigated several factors related to apoptotic pathways, including c-jun, p53, cytochrome C, and caspase-3. As shown in Fig. 3, the expression of phosphorylation of JNK and p53 was increased in a dose-dependent manner. Cytochrome C, which triggers the mitochondrial apoptotic pathway, was increasingly released from mitochondria into the cytosol by H31. In addition, the expression of the active form of caspase-3 was increased at a concentration of 100 µg/mL.
To determine whether these results could be confirmed in vivo, we used the immunocompetent syngeneic mouse model (C3H/HeJ mice, SCC VII/SF cell) and randomly divided the animals into two equal groups (control and treatment group, 0.2 mg/kg) after injection of SCC VII/SF cells. As shown in Fig. 4, the final volume (A), final weight (B), and time course volume change (C) of the SCC VII/SF tumors were significantly inhibited at 21 days after treatment with 0.2 mg H31 compared to the control group (D). For the TUNEL staining (Fig. 4E) and immunohistochemical study (Fig. 4F) of caspase-3, which was performed to observe the apoptosis induced by H31 in the tumor tissue removed from the C3H/HeJ syngeneic mice, the apoptotic index in the tumor tissue from the H31 injected mice was statistically higher than in the controls (P<0.01).
We used the zebrafish model for the toxicity assays of H31. First, we examined the embryotoxicity of H31. As shown in Fig. 5A, normal organogenesis in eggs was observed with up to 100 µM of H31, while death of all eggs was noted with 100 µM of cisplatin. Next, we investigated the neurotoxicity of H31 using the zebrafish neuromast. As shown in Fig. 5B, doses of H31 that were increased up to 100 µM did not decrease the number of hair cells of the neuromast; by contrast, there was almost total loss of the hair cells of the neuromast with 100 µM cisplatin.
We have extracted several PCCs from the crude extract of H31 using the HPLC method. Two PCCs demonstrated strong cytotoxicity among the cell lines: FaDu, KB AGC, HepG2, and HT-29 by the MTT assay (data not shown). Therefore, additional HPLC studies were carried out on one PCC among them. The MTT assay of KB cells was performed using several substances that showed possible tumor suppression. As shown in Fig. 6, the PCC that showed the highest peak in the HPLC study showed strong cytotoxicity in the KB cells at a concentration of 10 µg/mL. Therefore, this single compound appears to possess the anti-tumor capabilities of H31.
Marine micro-organisms are increasingly used as a source for anti-cancer agents (17). Cytarabine, the extract of nucleosides from the Caribbean sponge and Cryptotheca crypta, are now routinely used for the treatment of patients with leukemia and lymphoma (17). Jeong et al. (18) reported that a compound from Bacillus vallismortis showed direct cytotoxic and apoptotic effects on colon cancer cells in a dose and time dependent manner. In an attempt to identify a novel compound for cancer therapy, the results of this study show that H31, from the Bacillus SW31, had cytotoxic and apoptotic properties in head and neck cancer cells.
One therapeutic approach to the control of cancer cells involves the induction of apoptosis (19). Whether H31 could induce apoptosis in head and neck cancer cells was studied, as a potential method of growth inhibition. The results of this study showed that H31 suppressed KB cells and SCC VII/SF cell growth in vitro and in vivo, respectively, and the mechanism involved was mediated by up-regulation of p53 expression and activation of caspase-3.
Apoptosis is caused by the activation of cysteine protease, known as caspases, and the subsequent cleavage of certain cellular substrates (20-22). Of the 14 currently known human caspase family members, caspase-3 is the key factor associated with apoptosis; its active form is pivotal in the breakdown of cellular activity connected with DNA repair and regulation (23). The findings of this study showed that caspase-3 expression was up regulated when the cells were treated with H31, suggesting that the apoptotic-action of H31 was associated with caspase-3 involvement.
The tumor suppressor protein, p53, is a central controlling modulator that receives specific signals, interprets complex input, and determines regulatory output (24). The loss of p53 in HNSCC has been observed to result in a relative lack of apoptosis (25). Targeting p53 may be one approach to reduce the anti-apoptotic features associated with HNSCC. Gene therapy using adenoviral transfer of p53 is being aggressively investigated as adjuvant therapy for head neck cancer cells (26). Therefore, H31 may be an additional candidate for enhancing apoptosis of head and neck cancer cells.
Chemotherapy agents are often limited by drug toxicity (27). A new agent's toxicity profile is a very important characteristic that should be defined during the drug discovery process. Recently, the animal model using zebrafish (Danio rerio) has become a useful technique to screen for cardiotoxicity, neurotoxicity, hepatotoxicity, nephrotoxicity and genotoxicity of new candidate drugs (28-30). The results of this study showed that treatment of zebrafish with 100 µM cisplatin resulted in nearly total loss of the neuromast by immunofluorescence, while 100 µM of H31 did not decrease the number of neuromast hair cells. In addition, normal zebrafish organogenesis occurred with 100 µM H31 treatment.
In conclusion, the results of this study demonstrated that growth inhibition and apoptosis was achieved both in vitro and in vivo in head and neck cancer cells after exposure to H31, a metabolite from the marine Bacillus species. The zebrafish model did not reveal any significant toxicity effects even at considerable H31 dose concentrations. Although further characterization of the chemical structure and biological activities of HPLC purified PCC is necessary, our results suggest that H31 appears to be a safe candidate for apoptosis-inducing anti-cancer treatment of head and neck cancer.
This research was supported by CCRB through the "GRRC" Project of Gyeonggi Provincial Government, Korea (GRRCA-Ajou-2009-A03).
No potential conflict of interest relevant to this article was reported.