Redox modulation of oxidatively induced DNA damage by ascorbate enhances melanoma cancer cell DNA damage formation & cell killing

Malignant melanoma (MM) is the 5th most common cancer in the UK and the most lethal form of skin malignancies. Its incidence has more than quadrupled and mortality more than doubled over the last three decades. With a five-year survival rate for patients with advanced melanoma of ≤ 20%, there is an obvious need for a better treatment approach. Heightened genome-instability in cancer-cells suggests a model-scenario for their selective killing via the therapeutic delivery of defined levels of further genomic damage. To interrogate/exploit this model-scenario, it was proposed to investigate intracellular ascorbate's redox-modulation of oxidatively-induced DNA damage in MM cancer-cells, to selectively enhance both DNA damage and cell-killing. Alkaline comet assay (ACA) data reveals MM-cells to have higher endogenous DNA damage levels than “normal” skin cells. This in turn correlates with MM-cells having higher intracellular ROS and lower catalase activity. ACA data also shows MM-cells to be more sensitive towards the induced-damaging effects of H2O2 than “normal” skin cells, and that ascorbate further enhances this effect in MM-cells. This effect was also noted in primary melanoma cancer cells exposed to H2O2 and other oxidants (e.g. Elesclomol). A proposed model for the enhancement of H2O2-induced oxidatively-damaged DNA by ascorbate suggests that an increased local production of hydroxyl radicals (●OH) at the DNA may lead to relatively greater increases in complex lesions (i.e. DSBs) relative to single/‘isolated’ lesions (i.e. SSBs). However, correlated measures of DSBs vs. SSBs reveal only a proportional increase in DSBs relative to SSBs with ascorbate. Further data shows that ascorbate enhances oxidative-induced cell death in MM-cells and that in HaCaT cells the effect was slightly protective. Together, these results suggest that ascorbate enhances DNA damage/cell-killing through modulation of oxidative stress in MM-cells. This could increase the possibility of using ascorbate plus novel oxidant therapies to treat metastatic melanomas.