Penicillin-streptomycin solution, ficoll-hypaque solution, trypsin-EDTA solution, RPMI-1640 medium, Dulbecco's modified Eagle's medium (DMEM/F-12), and 1% antibiotic-antimycotic solution were obtained from (Life Technologies GIBCO, Grand Island, NY, USA). Fetal bovine serum (FBS) was purchased from Sigma-Aldrich (St. Louis, MO).
MCF-7 human breast cancer cell line was purchased from American Type Culture Collection (ATCC, Manassas, VA, USA) and was maintained in Dulbecco's modified Eagle's medium supplemented with 10% fetal bovine serum (FBS) and 1% antibiotic-antimycotic solution. Cells were grown to confluence at 37°C, and 5% CO2 atmosphere.
Isolation of peripheral blood mononuclear cells (PBMC)
Blood from healthy human volunteers was obtained with heparinized syringes and was placed into sterile polypropylene tubes. PBMC were further isolated by hystopaque 1077 density gradient centrifugation at 400 g for 30 min at 25°C (Sigma-Aldrich, St. Louis MO, USA). PBMC were then washed twice with FBS-free medium (RPMI-1640) at 250 g for 10 min at 25°C and adjusted to 5 × 103 cells/well for analysis.
The grenetine-stabilized colloidal silver was purchased from MICRODYN (Mexico, D.F.) as a 0.35% stock solution. It was filtered and diluted to a concentration of 1.75 ng/mL with DMEM/F-12 or RPMI-1640 medium.
Cells (5 × 103 cells/well) were plated on 96 flat-bottom well plates, and incubated 24 h at 37°C in 5% CO2 atmosphere. After incubation, culture medium was removed, and colloidal silver diluted in the same medium was added at concentrations ranging from 1.75 to 17.5 ng/mL. The plates were then incubated for 5 h at 37°C, and 5% CO2 atmosphere. Thereafter, the supernatant was removed and cells were washed twice with DMEM/F-12 medium. Cell viability was determined by the trypan blue exclusion method, and cytotoxicity was expressed as the concentration of 50% (LD50) and 100% (LD100) cell growth inhibition. Results were given as the mean + SD of three independent experiments.
Mechanism of cell death analysis
Cell death type was assessed by the detection of mono-oligonucleosomes (histone-associated DNA fragments) using an ELISA kit (Cell Death Detection ELISA PLUS, Roche Applied Science, IN, USA) following the manufacturer's instructions. In brief, the cytoplasmic lysates from untreated controls and colloidal silver treated cultures were transferred to a streptavidin-coated plate supplied by the manufacturer. A mixture of anti-histone biotin and anti DNA-POD were added to cell lysates and incubated for 2 h. The complex was conjugated and then the plate was read at a wavelength of 405 nm. The increase in mono-oligonucleosomes production in cells lysates was calculated as the ratio of the absorbance of colloidal silver treated cells/absorbance of untreated control. Results were given as the mean + SD of three independent experiments.
Terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) was performed with TACS 2 TdT-DAB In Situ Apoptosis Detection kit (Trevigen, Gaithersburg, Maryland, USA), following the manufacturer's instructions. Briefly, after culture MCF-7 cells at 106 cells/well and treated with LD50 and LD100, by 5 h, the cells were digested with proteinase K at a concentration of 20 μg/mL for 15 minutes. Endogenous peroxidase activity was quenched with 2% H2O2 for 5 minutes. The cells were immersed in terminal deoxynucleotidyl transferase (TdT) buffer. TdT, 1 mM Mn2+, and biotinylated dNTP in TdT buffer were then added to cover the cells and incubated in a humid atmosphere at 37°C for 60 minutes. The cells were washed with PBS and incubated with streptavidin-horseradish peroxidase for 10 minutes. After rinsing with PBS, the cells were immersed in DAB solution. The cells were counterstained for 3 minutes with 1% methyl green. Cells containing fragmented nuclear chromatin characteristic of apoptosis will exhibit brown nuclear staining that may be very dark after labeling.
Detection of lactate dehydrogenase (LDH) activity
The conversion of lactate to pyruvate was detected using the Cytotoxicity Detection Lactate Dehydrogenase kit (Roche Applied Science, IN, USA) following the manufacturer's instructions. MCF-7 breast cancer cells and PBMC treated with colloidal silver were washed twice with ice-cold PBS, harvested by centrifugation at 250 g for 10 min at 25°C, and the supernatant was used for the activity assay according to the manufacturer's instructions. Optical densities resulting from LDH activity were measured in a microplate reader at 490 nm. Results were given as the mean + SD of three independent experiments.
Accumulation of nitrite in the supernatants of control and treated MCF-7 and PBMC cultures was used as an indicator of nitric oxide production. Cells were incubated for 5 h in DMEM/F-12 medium, in the presence or absence of colloidal silver in triplicates, in a total volume of 200 μL DMEM/F-12 medium. After incubation, supernatants were obtained and nitrite levels were determined with the Griess reagent, using NaNO2 as standard. Optical densities at 540 nm were then determined in a microplate reader (Bio-Tek Instruments, Inc.).
Determination of intracellular antioxidants
The antioxidants production was measured using the following kits: Cellular glutathione peroxidase (Gpx) assay kit (Oxford Biomedical Research, MI, USA), superoxide dismutase (SOD) assay kit (Cayman Chemical Company, MI, USA), and catalase (CAT) assay kit (Cayman Chemical Company, MI, USA) according to the manufacturer's instructions. Briefly, to determine the activity of Gpx, SOD, and CAT; MCF-7 and PBMC were incubated with LD50 (3.5 ng/mL) and LD100 (14 ng/mL) of colloidal silver for 5 h. Cells were then washed three times with PBS and sonicated on ice in a bath-type ultrasonicador (80 Watts output power) for 15-s periods for a total of 4 min; the solution was then centrifuged at 1500 g for 5 min at 4°C. The obtained supernatants were used to determine intracellular antioxidants in a microplate reader at 540 nm.
Total antioxidant (extracellular antioxidants)
The total antioxidant production was determined using the Total Antioxidant Colorimetric Assay Kit (US Biological, Massachussets, USA) following manufacturer's instructions. Briefly, MCF-7 and PBMC were treated with LD50 (3.5 ng/mL) and LD100 (14 ng/mL) of colloidal silver for 5 h. Thereafter, supernatants were used to determine antioxidants in a microplate reader at 490 nm.
Data represent the mean + SD of triplicates from three independent experiments. Statistical differences were obtained using the analysis of variance, and the Dunnett's and Turkey's tests (SPSS v. 12 program).