n vitro comet assay with HepG2 cell line
Modified version of the alkaline protocol described by Uhl et al.13 was performed with drinking water samples. Human hepatoma cell line (HepG2 cells) was obtained from prof. dr. Knasmueller, Institute of Cancer Research of The Univeristy of Vienna. Cells were grown in multilayer culture at 37 °C in humified atmosphere of 5% CO2 in Dulbecco’s Modified Eagle’s Medium (DMEM) supplemented with 10% foetal calf serum (FCS) and antibiotic (0.1% gentamycin) in well plates for 7 days (cell density: 106–108 cells/mL). Medium was changed every 2 days. Seven days old cells were exposed to water samples, negative and positive controls (500 μM hydrogen peroxide) for 20 min and afterwards cell suspensions were prepared with 0.25% trypsin-EDTA solution, passed through injection needle for several times to achieve single cell suspension and finally resuspended in DMEM medium, supplemented with 10% FCS medium. The dye-exclusion test with Trypan blue was used to examine the viability of cells before the comet assay was performed.27
Rough microscope slides were used for the comet assay, they were first coated with up to 400 µL of 1% normal melting point agarose (NMP) the day before the test and left to air dry overnight. The supportive (second) agarose layer (0.6% NMP agarose) was solidified on ice and the collected HepG2 cells were immobilized in the third layer. Approximately 2×104 cells were mixed with 0.7% low melting point agarose (LMP) and spread over the slides as the third layer. After removing the cover glasses, the slides were covered with 500 µL of 0.5% LMP agarose (the fourth layer) to prevent nuclear DNA escaping during cell lysis and electrophoresis. One-hour incubation in alkaline lysis buffer followed. The slides were submerged in electrophoretic buffer (pH >13) to unwind the nuclear DNA for 1hr and then subjected to electrophoresis in the same buffer. The electrophoresis was carried out at 2V/cm and 300 mA; for 30 min. Following electrophoresis the gels were neutralized in 400 mM Tris-HCl pH 7.5 for 15 min. The damaged DNA traveled toward the anode during electrophoresis and formed an image of a “comet” tail. After staining the slides with ethidium bromide (20 µg/mL) the comets were detected and quantified as described below.