Background
Nifuroxazide is a nitroheterocyclic drug belonging to the 5-nitrofuran’s family and used in the treatment of infectious diseases with frequent symptoms of diarrhea and colitis, caused by Salmonella spp, Escherichia coli and Klebsiella [
1]. 5-Nitrofurans although being constituents of widely used medications, may cause toxic side effects such as polyneuropathy, depression, forgetfulness, alcohol intolerance, headaches as well as gastrointestinal complications, which interfere with their use [
2]. In this context, nifuroxazide analogues were developed replacing the nitrofuran group by benzofuroxan based on the pharmacophore similarity attributed to the nitro group, and also to improve bioavailability, with higher intrinsic activity and less toxicity [
2]. Benzofuroxan as well as 5-nitrofurans are used as antibacterial and antiparasitic drugs, giving rise to free radical species [
3]. The use of antitumor agents that elevate ROS generation has been effective in many cases with low resistance to these agents [
4].
Melanoma is a deadly type of cancer that results from the malignant transformation and uncontrolled proliferation of melanocytes. It is the most invasive skin cancer with increasing incidence and poor prognosis in the metastatic stage [
5,
6]. Melanoma of the skin has an incidence of 25,2 % per 100.000 individuals with the US mortality of 3.1 % [
7,
8]. Efficient treatment requires early diagnosis, because of the poor prognosis of the disease, especially when it reaches the metastatic stage [
9]. With the recent advances in treatment, the 5-year survival rate in melanoma went up to 91 % although that of metastatic melanoma is still averaging 15–20 % of patients. Novel therapeutic approaches are still needed, especially for the metastatic form.
Cancer cells frequently exhibit changes in the redox status associated with increased basal production of ROS, and thus cannot tolerate further intracellular increase of free radicals [
4]. Drugs that interfere in redox changes are, therefore, a promising strategy to overcome cancer cell resistance to chemotherapy. Redox status changes may, indeed, exert a critical impact on necrotic/apoptotic and normal cellular processes.
In the present study, we explored the capacity of nifuroxazide analogues to induce ROS-mediated apoptosis in B16F10-Nex2 murine melanoma cells. The signaling pathway affected by the benzofuroxan compounds showed downregulation of Akt and over-expression of pro-apoptotic BIM. A therapeutic protective effect of these compounds in a melanoma syngeneic model is also reported.
Methods
Chemicals
The benzofuroxan drugs were synthetized at the Biochemical and Pharmaceutical Technology Department, University of São Paulo, as described [
10,
11]. The structure and the substituent groups are indicated below (Table
1). Stock solutions were prepared in DMSO (Sigma-Aldrich, St. Louis, USA) at 10 mM and stored in aliquots at −80 °C. For in vitro tumor cell treatment the stock solution was diluted in RPMI (Sigma-Aldrich, St. Louis, USA), and in PBS 15 % Ethanol (Merck) for in vivo experiments.
Table 1
Benzofuroxan compounds and derivatives
1
| H | H | H |
13
| N(CH3)2 | H | H |
2
| CH3 | H | H |
14
| n-C4H9 | H | H |
3
| NH2 | H | H |
15
| tert-C4H9 | H | H |
4
| OH | H | H |
16
| OC3H7 | H | H |
5
| F | H | H |
17
| Cl | H | Cl |
6
| CN | H | H |
18
| Cl | Cl | H |
7
| CH2CH3 | H | H |
19
| CF3 | H | H |
8
| OCH3 | H | H |
20
| OC4H9 | H | H |
9
| Cl | H | H |
21
| Br | H | H |
10
| COCH3 | H | H |
22
| SO2NH2 | H | H |
11
| n-C3H7 | H | H |
23
| I | H | H |
12
| Iso-C3H7 | H | H | | | | |
Cell lines
The B16F10 murine melanoma cell line is syngeneic in C57Bl/6 mice and was originally obtained from the Ludwig Institute for Cancer Research (LICR), São Paulo branch. The melanotic subline Nex2 (B16F10-Nex2) was isolated at the Experimental Oncology Unit (UNONEX) and deposited at Banco de Células do Rio de Janeiro (BCRJ) no. 0342. Human cervical carcinoma (HeLa), and human ovarian cancer (Ovcar-3) and the glioblastoma cell line (U87) were provided by Drs. Hugo P. Monteiro, UNIFESP and Osvaldo K. Okamoto, University of São Paulo, respectively. Human colon carcinoma (HCT) was from LICR and mouse colon carcinoma (CT26) was a gift from Prof. Guillermo Mazzolini, Austral University, Argentina.
Tumor cell lineages were maintained in RPMI 1640, pH 7.2, supplemented with 10 mM N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid (HEPES), 24 mM sodium bicarbonate, 10 % heat-inactivated fetal bovine serum (FBS) from Gibco (Minneapolis, MN, USA) and 40 μg/ml gentamicin sulfate (Hipolabor Farmacêutica, Sabará, MG, Brazil). Human fibroblasts (GM637 and Fibro T75) were provided by Dr. Luis F. Lima Reis of Sirio-Libanez Hospital, São Paulo, and were grown in DMEM supplemented with 10 mM HEPES, 24 mM sodium bicarbonate, 10 % FBS, and 40 μg/ml gentamicin sulfate.
Cell viability assay
Viable cells were assessed using the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) (Sigma-Aldrich, St. Louis, MO) assay. Melanoma cell line B16F10-Nex2 (1.0 × 104 cells/well) were seeded on 96-well plates in RPMI supplemented with 10 % FBS, and compounds (1.5, 3.12, 6.25, 12.5, 25.0, 50.0 or 100 μM) or fresh medium (for control cells) was added. After 16 h, MTT solution (5 mg/ml) in 1x phosphate-buffered saline (PBS) was directly added to the cells, followed by incubation for 4 h at 37 °C. Absorbance was measured at 570 nm with an automated spectrophotometric plate reader (SpectraMax-M2, Molecular Devices Software Pro 5.4, Sunnyvale, CA). The experiments were performed in triplicate. Cell viability was expressed as percent values in comparison with untreated cells.
DNA degradation
B16F10-Nex2 cells (5 × 105 cells/well) cultured on 6-well plates, were treated with benzofuroxan derivatives N-Br/N-I at IC50 and at 100 μM or left untreated at 37 °C for 24 h. Cells were lysed in TELT buffer (50 mM Tris–HCl pH 8.0, Triton X-100 0.4 %, 2.5 mM EDTA pH 9.0, and 2.5 M LiCl). The lysate was centrifuged (12,000 g) for 20 min at 4 °C. Buffer-equilibrated phenol was added (1:1, v/v), followed by centrifugation and addition of chloroform (1:1, v/v). After centrifugation (15 min; 12,000 g; 4 °C), the aqueous phase was precipitated with sodium acetate 3 M, pH 7.0 and absolute ethanol (1:0.1:2.5, v/v/v) after overnight incubation at −80 °C. Precipitated DNA was pelleted and diluted in 50 mg/ml of RNAse-A (Invitrogen, Carlsbad, CA). The extracted DNA was subjected to electrophoresis on 1 % Agarose gel with ethidium bromide (0.5 mg/ml) in TBE buffer (2 mM EDTA, 90 mM Tris–HCl, 90 mM boric acid, pH 8.0) at 100 V. One thousand base pair (1 kb) ladder molecular weight markers (Gibco, Grand Island, NY) were used. After electrophoresis, DNA was photographed by UVItec Alliance gel documentation system (UVItec, Cambridge, UK).
N-acetylcysteine (NAC) effects
B16F10-Nex2 melanoma cells (1.0 × 104/well) were seeded on 96-well plates in RPMI and 10 % FBS, and derivative compounds at 1.5, 3.12, 6.25, 12.5, 25.0, 50.0, or 100 μM were added simultaneously with NAC at 5 mM for 16 h. After incubation, MTT (5 mg/ml) in 1x phosphate-buffered saline (PBS) was directly added to the cells, followed by incubation for 4 h at 37 °C. Absorbance was measured spectrophotometrically (SpectraMax-M2, Molecular Devices Software Pro 5.4, Sunnyvale, CA) at 570 nm. Cell viability was expressed as percent values in comparison to untreated cells.
ROS determination
Superoxide anions were assessed by dihydroethidium (DHE) assay performed according to manufacturer’s instructions (Invitrogen, Carlsbad, CA). Briefly, 1.0 × 10
4cells were cultivated on Hi-Q4 dish and treated with N-Br/N-I at IC
50 for 6 h at 37 °C. Analysis by fluorescence microscopy was carried out in Nikon BioStation IM microscope. Images were processed with ImageJ (
http://rsb.info.nih.gov/ij/).
Annexin V and propidium iodide labeling
B16F10-Nex2 cells (5 × 105/well) were grown in 6-well plates and further incubated with N-Br/N-I at IC50 and at 100 μM, or with RPMI 10 % FBS (control) for 24 h at 37 °C. Treated and untreated cells were washed three times with PBS and harvested with a cell scraper. Apoptotic/necrotic cells were detected using the Annexin V-FITC Apoptosis Detection Kit (Sigma-Aldrich, St. Louis, MO). Cells were incubated with binding buffer (10 mM HEPES/NaOH, pH 7.5, 140 mM NaCl and 2.5 mM CaCl2) in presence of propidium iodide (PI) and FITC-labeled annexin V (AV) for 10 min at room temperature and analyzed by flow cytometry (BD Bioscience FACSCanto II, Franklin Lakes, NJ), using FlowJo software (TreeStar Inc., Ashland, OR).
Chromatin condensation analysis
Murine melanoma cells (1.0 × 10
4) were cultivated overnight on Hi-Q4 dish and incubated with N-Br/N-I at IC
50 for 24 h. They were then stained with 1.25 μM Hoechst 33342 (Invitrogen, Carlsbad, CA) for 10 min, washed with PBS and examined for the state of chromatin. Analysis by time-lapsed fluorescence microscopy was carried out on a Nikon BioStation IM microscope. Images were processed with ImageJ (
http://rsb.info.nih.gov/ij/).
Mitochondrial membrane potential (Δψm)
The cationic lipophilic dye tetramethylrhodamine ethyl ester (TMRE) enters the cell in the form of an ester that is subsequently hydrolyzed and converted to tetramethylrhodamine, which is reversibly accumulated in the negatively charged mitochondrial matrix depending on Δψm . B16F10-Nex2 cells (1.0 × 105) were cultured in 12-well culture plate and incubated with N-Br/N-I at IC50 for 6 h at 37 °C. Cells were washed with PBS and labeled with 20 nM TMRE (Molecular Probes, OR, USA) for 10 min at 37 °C. Cells were detached with PBS/Trypsin/EDTA and the fluorescence was measured on FACSCanto II (BD Bioscience, Franklin Lakes, NJ), using FACSDiva software (BD Bioscience, Franklin, NJ) and analyzed using FlowJo software (TreeStar Inc., Ashland, OR, USA).
Cell lysate extracts and Western blotting
B16F10-Nex2 cells (1.0 × 106) treated with N-Br/N-I at IC50 and 100 μM or with RPMI 10 % FBS (control cells) for 6 h or 16 h, were washed with PBS and lysed for protein extraction with RIPA buffer (50 mM Tris–Cl, pH 7.5, 150 mM NaCl, 1 % Nonidet P-40, 0.5 % sodium deoxycholate, and 0.1 % SDS) supplemented with protease and phosphatase inhibitors (Sigma–Aldrich, St. Louis, MO). Lysates from cells co-incubated with 5 mM NAC and exposed to derivatives at IC50 were also obtained and subjected to Western blotting as described below.
Cell lysates were transferred to microcentrifuge tubes, and kept on ice for 1 h with shaking. They were sonicated for 10 min and centrifuged at 14,000 g for 15 min. Protein concentration of lysates was determined by BSA quantification assay (Thermo Fisher Scientific, Rockford, IL). Each cell lysate (30 μg protein) was separated by SDS gel electrophoresis and transferred to nitrocellulose membrane (Millipore, Billerica, MA), blocked with TPBS (PBS, 0.05 % Tween-20) and 5 % (w/v) skim milk or BSA, washed in TPBS and incubated with primary antibody for 16 h on 4 °C. Immunoblotting was run with antibodies against Akt, phospho-Akt (thr308), BIM, caspase 9, caspase 9-cleaved, PARP, PARP-cleaved and β-actin, all purchased from Cell Signaling Technology (Beverly, MA). After washing, membranes were incubated with anti-IgG antibody conjugated with horseradish peroxidase and the immunoreactive bands were detected using Immobilon (Millipore, Billerica, MA, USA) under a chemiluminescence detection system (UVItec, Cambridge, UK). β-Actin was used as loading control.
Detection of caspase-3 activity
Active caspase 3 was determined in B16F10-Nex2 cells (2.5 × 106 cells) grown with N-Br/N-I at IC50, or RPMI 10 % FBS (control) for 8 h. After incubation, cells were tested with Caspase-3 Assay Kit, Colorimetric (Sigma-Aldrich, St. Louis, USA) according to manufacturer’s protocol. Briefly, cells were washed twice with PBS and resuspended in 25 μL of chilled Cell lysis Buffer for 20 min on ice. The lysates were centrifuged at 20,000 g for 15 min at 4 °C and 5 μL of the aqueous phase was incubated with 85 μL Assay Buffer and 10 μL caspase-3 substrate Ac-DEVD-pNA 2 mM at 37 °C for 16 h in a 96-well plate. Absorbance was read at 405 nm in a microplate reader (SpectraMax-M2, Software Pro5.4, Molecular Devices, Sunnyvale, CA).
Experimental melanoma models in vivo
C57BL/6 mice were obtained from the Center for Development of Experimental Models (CEDEME), Federal University of São Paulo (UNIFESP). Animal experiments were approved by the UNIFESP Ethics Committee for Animal Experimentation (CEUA No. 6641300114) based on international recommendations. All in vivo experiments were performed at least twice.
In the lung colonization model, male, six-to-eight week-old, C57BL/6 and NOD/SCID-IL-2R-gamma null mice were challenged endovenously with 5 × 105 syngeneic B16F10-Nex2 melanoma cells in 100 μl of PBS. Animals (n = 5 per group) were treated 1 day after tumor challenge, with seven daily intraperitoneal doses of 300 μg (12 mg/Kg) of each benzofuroxan derivative or vehicle (PBS 15 % EtOH). After 14 days, lungs were collected from animals of each group, and the melanotic lung nodules were counted.
Statistical analysis
Unless otherwise specified, two independent experiments were performed in triplicate. Experimental values are expressed as mean values ± standard deviations (S.D.). For significance analyses Student’s t-test was used, GraphPad Prism 4.0 software (La Jolla, CA). p values < 0.05 were considered significant.
Discussion
In the current study, we report on the promising antitumor effects of benxofuroxan compounds, which have been shown to induce apoptosis in B16F10-Nex2 melanoma cells. The antibacterial, antiprotozoan and anticancer effects of nifuroxazide related drugs [
1], depend on ROS generation [
21], and in the present work, we evaluated the antitumor effects of 23 compounds that have the nitrofuran system replaced by a benzofuroxan molecule.
All 23 benzofuroxan compounds exhibited significant levels of cytotoxicity against murine melanoma B16F10-Nex2 cells, and two of them, N-Br and N-I, are the most promising derivatives with antitumor activity in vitro and in vivo. N-Br and N-I exhibited potent cytotoxic activity not only in murine melanoma, but also against several other murine and human tumor cell lines, demonstrating a therapeutic potential in different types of cancers. There was little difference in the IC50 values in tumor cells and non-tumorigenic cells, including melan-a (unpublished results), but primary normal cells have not been tested. Cultured cells in the laboratory differ from cells in a living organism in many aspects. Both compounds did not exhibit toxicity in vivo at the concentrations used and yet showed definite antitumor effects in syngeneic mice.
Both N-Br and N-I compounds induced several effects on melanoma cells, related to apoptosis, such as rounding-up, reduction of cellular and nuclear volume, chromatin condensation and DNA fragmentation, and phosphatidylserine outer membrane expression [
22,
23]. The loss of ΔΨm, activation of caspase 9 and 3, followed by cleavage of PARP are characteristics of intrinsic apoptosis [
24], and were observed upon incubation of N-Br and N-I with melanoma cells.
Given the nature of the benzofuroxan compounds, they increased the levels of ROS in tumor cells [
3]. Since co-incubation with the antioxidant N-acetyl cysteine inhibited ROS generation, benzofuroxan drug cytotoxicity and the loss of ΔΨm, we conclude that production of ROS induced by both N-Br and N-I derivatives is involved in their antitumor activity in B16F10-Nex2 cells. Due to their reactive potential, ROS can oxidize DNA bases, leading to DNA strand breaks, intra-strand adducts, crosslinks and mutations [
25]. ROS can affect the integrity of proteins and/or lipids in membranes, decreasing the fluidity and increasing membrane permeability as in mitochondria [
4]. Mitochondrial dysfunction, characterized by loss of transmembrane potential and opening of mitochondrial permeability transition pores occurs via many apoptotic stimuli [
22]. The benzofuroxan derivatives caused disruption of ΔΨmin melanoma cells after 6 h incubation. Impairment of mitochondrial membrane permits the release of cytochrome c, an event known to occur after externalization of phosphatidylserine [
23], triggering the apoptotic intrinsic pathway with activation of caspase 9 and 3 [
26]. As shown, the apoptotic effects of both benzofuroxan derivatives involved caspase activation.
In addition, N-Br and N-I compounds induced Akt inactivation and expression of pro-apoptotic BH3 protein BIM. Akt is a serine/threonine protein kinase downstream target of the phosphatidylinositol 3-kinase (PI3K) signaling pathway, a central player in response to growth factors or insulin and contribute to several cellular functions including nutrient metabolism, cell growth, transcriptional regulation and cell survival [
27]. In a wide variety of cancers, Akt is frequently over activated, contributing to malignancy and related with tumor aggressiveness [
18]. Phosphorylation of Akt at Thr308 is essential for Akt catalytic activity [
28]. In melanoma, Akt promotes tumor progression, escape from apoptosis and enhanced survival [
29]. Recent works have reported that oxidative stress can modulate Akt activation [
29‐
31]. Luo et al. reported that enhanced ROS interact with AKT/FoxO3a/BIM causing inhibition of Akt and consequently nuclear accumulation of FoxO3a, thus facilitating transcription of the target gene BIM [
32].
Our results contribute to elucidate the link between Akt signaling and ROS generation. Immunoblotting analysis showed a high expression of phospho-Akt in untreated melanoma cells that was down regulated after treatment with both benzofuroxan derivatives. Co-incubation with NAC blocked these effects suggesting that ROS generation induced in melanoma cells acted as a mediator. Another intracellular signaling molecule sensitive to redox modulation was BIM, which belongs to pro-apoptotic BH3-only proteins family. Consistent with Lou et al. findings [
32], downregulation of Akt and increased expression of BIM, which binds to the Bcl-2 family members, led to the loss of ΔΨm [
24], thus triggering the intrinsic apoptosis in melanoma cells.
Competing interests
The authors declare that they have no competing interests.
Authors’contributions
FCF participated in study design, performed experiments, data analysis, and drafting of manuscript. MHM, NG and CRF performed experiments and the acquisition of data. RAA and AKF were involved in the conception, design and manuscript revisions; SDJ and LCT in the conception, design and synthesis of benzofuroxan compounds. LRT acted as general adviser, critical reviewer, and editor of the final version of the manuscript. All authors read and approved this version of the manuscript.