Preferential induction of apoptotic cell death in melanoma cells as compared with normal keratinocytes using a non-thermal plasma torch

Selective induction of apoptosis in melanoma cells is optimal for therapeutic development. To achieve this goal, a non-thermal helium plasma torch was modified for use on cultured cells in a temperature-controlled environment. Melanoma cells were targeted with this torch (1) in parallel cultures with keratinocytes, (2) in co-culture with keratinocytes and (3) in a soft agar matrix. Melanoma cells displayed high sensitivity to reactive oxygen species generated by the torch and showed a 6-fold increase in cell death compared with keratinocytes. The extent of cell death was compared between melanoma cells and normal human keratinocytes in both short-term (5 min) co-culture experiments and longer assessments of apoptotic cell death (18-24 h). Following a 10 sec plasma exposure there was a 4.9-fold increase in the cell death of melanoma vs. keratinocytes as measured after 24 h at the target site of the plasma beam. When the treatment time was increased to 30 sec, a 98% cell death was reported for melanoma cells, which was 6-fold greater than the extent of cell death in keratinocytes. Our observations further indicate that this preferential cell death is largely due to apoptosis. In addition, we report that this non-thermal plasma torch kills melanoma cells growing in soft agar, suggesting that the plasma torch is capable of inducing melanoma cell death in 3D settings. We demonstrate that the presence of gap junctions may increase the area of cell death, likely due to the bystander effect of passing apoptotic signals between cells. Our findings provide a basis for further development of this non-invasive plasma torch as a potential treatment for melanoma.