Abstract
Melanoma is the most lethal form of skin cancer, with New Zealand having one of the highest rates of incidence and mortality worldwide. Despite the emergence of targeted melanoma therapies, the development of drug resistance remains a major issue. A better understanding of the mechanisms that underly the acquisition of treatment resistance will help identify alternative therapies that overcome or prevent resistance.
Increased oxidative stress has been linked to the development and progression of melanoma, and more recently it has been implied in targeted therapy resistance. Vemurafenib is a specific inhibitor of the BRAF V600E mutant protein that is used for late-stage melanoma treatment. Melanoma cells resistant to vemurafenib are reported to have undergone metabolic reprogramming, as demonstrated by upregulated oxidative phosphorylation that is associated with increased oxidant production. An increased reliance on antioxidant defences is suggested as a mechanism for the development of vemurafenib resistance, therefore targeting antioxidant systems may sensitise resistant melanoma cells to cell death.
This study has quantified the redox status of cellular peroxiredoxins in a panel of metastatic melanoma cell lines. Peroxiredoxins are a family of peroxidase enzymes that are highly reactive with hydrogen peroxide and are sensitive indicators of cytosolic and mitochondrial oxidative stress. Assessment of peroxiredoxin oxidation revealed that metastatic melanoma cells are more highly oxidized, potentially indicative of the cells being under increased oxidative stress. Thioredoxin reductase plays an important role in the reduction of oxidized thiols, including peroxiredoxins. Thioredoxin reductase activity was assessed in the same panel of metastatic melanoma cell lines, along with the response to the thioredoxin reductase inhibitors, auranofin and TRi-1. Metastatic melanoma cells were highly sensitive to inhibition of proliferation by these compounds.
Changes in redox status of vemurafenib-resistant metastatic melanoma cells were also assessed. Thioredoxin reductase activity was increased two-fold in vemurafenib-resistant metastatic melanoma cells, however there was no consistent association with peroxiredoxin redox status. Vemurafenib-resistant metastatic melanoma cells were significantly more sensitive to inhibition of cell growth when treated with a combination of thioredoxin reductase inhibitors and vemurafenib. This was maintained even when vemurafenib had been removed from the resistant cells.
These results suggest that vemurafenib-resistant metastatic melanoma cells may be reliant on thioredoxin reductase to acquire resistance. Thioredoxin reductase inhibitors may be useful in combination with vemurafenib to prevent or treat vemurafenib-resistant metastatic melanoma cells.