Cell Lines: The MCF-10A and MDA-MB231 cell lines were received from American Type Culture Collection (ATCC, Manassas, VA, USA) and MCF-10A and MDA-MB231 cells were transfected with vector or cMV-NRF1-GFP construct (RG220113, OriGene Technologies). The human cerebral microvascular endothelial cell line HCMEC/D3 was received from Dr. B. Weksler, Weill Medical College of Cornell University, NY (Weksler et al., 2005); and will be referred to as wildtype endothelial cells (ECWT) to distinguish them from ESCsID3. Cells were maintained in DMEM-F12 media with B27® serum-free supplement. The astrocyte cells (AC) were obtained from ScienCell, Carlsbad, CA, USA and were maintained in DMEM-F12 with astrocyte growth supplement. Cells were cultured at 37°C in a humidified atmosphere with 5% CO2. Cell treatments were in DMEM-F12 in the absence of serum and growth factors for 3 h. Thereafter, the cells were treated with PCB153 (2,2′,4,4′,5,5′-hexachlorobiphenyl) and PCB77 (3,3′,4,4′-tetrachlorobiphenyl) for the different experimental time periods. The PCB congeners were obtained from AccuStandard (New Haven, CT) and dissolved in dimethyl sulfoxide (DMSO). Equal volumes of DMSO as in PCB treatment group were used in vehicle control with less than 0.1% percentage of DMSO in each group.
Tumorospheroid Assays: Approximately 100–150 cells per well were seeded in an ultra-low attachment 96-well plate (Corning Inc., Lowell, MA, USA) for tumorigenic spheroid formation studies. Cells were suspended in serum-free DMEM/F12 (1:1) culture medium supplemented with B27. The effect of endocrine disrupting chemicals, PCB153 (60µM) and PCB77 (60µM) were treated on the day of seeding cells. Spheroids were grown for 8 days and 16 days in liquid culture. A total of 15 spheroids with a minimum diameter of 50 mm were counted in each experimental group. Data were analyzed by ANOVA; Tukey’s HSD test was used for multiple comparisons. Cells obtained from spheroids were analyzed by immunofluorescence, FACS, or immunoblotting, as described previously (2).
Blood–brain barrier (BBB) model: We established an in vitro BBB model consisting of human ECs, breast cancer stem cells (BCSCs) and astrocyte cells (ACs). Briefly, MDA-MB231 or BCSCsNRF1 were placed on 1cm height P1000 sterile tip (cut at both ends) surrounded by coculture of AC: EC (1:1) with Matrigel (cat# 354230; BD Biosciences, Franklin Lakes, NJ, USA) diluted in DMEM-F12 with astrocyte medium. The tip was removed after 24h and the cells were grown in DMEM-F12 with astrocyte medium to study 3D invasive migration. For cell adhesion and microinvasion assay, AC were grown first in a monolayer for 24 h and the ESCsID3 were grown as layer on top of AC for 24 h; and BCSCs were cultured on top of ESCsID3 for 24h.
Cell adhesion and migration assay: For cell adhesion and tumorigenesis/spheroid formation assay, BCSCs were seeded on monolayer of ESCsID3 (24h) in serum-free media for 3days. For the transmigration assay, BCSCs were seeded on a monolayer of ESCsID3 (24h) in serum-free media in the upper chamber of a transwell insert (pore size: 8 µm; Corning, Corning, NY, USA); 10% FBS medium was added to the lower chamber as a chemoattractant. After 24h incubation, invaded cells were photomicrographed by Nikon confocal microscopy. Invaded cells were counted and scored from the lower chamber in triplicate.
Antibodies and immunoblotting: Analysis of protein expression was performed by immunoblotting as described previously (2). Western blots and were probed with the following antibodies: ID3 (Cal BioReagent) and β-actin (13E5, rabbit mAb #4970, Cell Signaling Technology, Inc.). Electrochemiluminescence (ECL) intensity of detected target proteins was imaged and quantified with a Bio-Rad Versa Doc instrument. All immunoblots were accomplished a minimum of three times for each experiment.
Exosome isolation: Total Exosome Isolation Reagent (Catalog number: 4478359, Thermofisher Scientific) were used with cell culture media to isolate and concentrate exosomal ID3 from cell culture media of ESCsID3 cells with the following protocol. Briefly, the reagent is added to the cell media sample, and the solution is incubated overnight at 2°C to 8°C. The precipitated exosomes are recovered by standard centrifugation at 10,000 x g for 60 min. The pellet is then resuspended in PBS or similar buffer, and the exosomes are ready to use in further experiments with co-culture cell media for 7 and 10 days.
Fluorescence activated cell sorting (FACS) analysis: After the staining, the cells were washed twice with stain buffer (BD Pharmingen) and analyzed using a Guava easyCyte flow cytometer (Millipore). For staining, 1 × 106 cells were pelleted and incubated for 45 min at 4°C with the following antibodies: ID3 biotin-FITC and NRF1 conjugated PE for FACS analysis and data was analyzed with the Guava easyCyte™ using the CytoSoft software program according to the manufacturer’s instructions.
Xenotransplant migration assay: Xenograft zebrafish embryo experiments were performed at Florida International University, in accordance with IACUC-approved protocols (Protocol Approval #: IACUC-19-091-CR01). WT -TU Zebrafish embryos (2-dpf) were purchased from the Zebrafish International Resource Center (ZIRC), University of Oregon facility. BCSCs and ESCs were xenografted in zebrafish using specific microinjection equipment. Experiments were conducted in triplicate, and they were incubated for an extra 72 hours at 30-32°C. Approximately, 500 cells were injected into the yolk using pulled glass needles and a micromanipulator. The spread of the GFP fluorescent dye labeled lung PCB153 treated ID3 overexpressing cells. Cell migration was captured by Nikon confocal microscopy for metastasis studies. All injected cells stained with the fluorescent dye, CellTracker™ CM-DiI (both GFP and non GFP cells) were observed in live zebrafish embryo. For BCSCs brain metastasis xenotransplantation in zebrafish embryo, BCSCs were xenografted in the zebrafish embryo (to observe metastatic spread of the GFP fluorescent dye labeled BCSCsNRF1 through blood vessels from the site of injection to the region of secondary target organs. The photomicrographs were taken by Nikon confocal microscopy. Total cells injected were stained with the fluorescent dye, CellTracker™ CM-DiI (stained both GFP and non GFP cells).
Statistical Analyses: All statistics were performed using VassarStats statistical software (Richard Lowry, Poughkeepsie, NY, USA). One-way analysis of variance (ANOVA) was achieved to detect any differences between groups. If the result of the ANOVA was significant, pair wise comparisons between the groups were made by a post-hoc test (Tukey’s HSD procedure).