The antioxidant ascorbic acid mobilizes nuclear copper leading to a prooxidant breakage of cellular DNA: implications for chemotherapeutic action against cancer

Abstract

Ascorbic acid is an essential micronutrient and is considered to have an antioxidant function in living systems. For the past several decades, ascorbic acid has been the subject of considerable interest as an anticancer agent. Several studies have shown that ascorbic acid is cytotoxic to a variety of cancer cells, whereas normal cells are relatively resistant to such cytotoxic action. In this study, we propose a putative molecular mechanism that accounts for the preferential cytotoxicity of ascorbic acid against cancer cells. Standard and lysed version of alkaline single-cell gel electrophoresis (Comet assay); ferrous oxidation-xylenol orange (FOX) assay. We show that ascorbic acid acts as a prooxidant and leads to oxidative DNA breakage in lymphocytes and lymphocyte nuclei. Scavengers of reactive oxygen species were able to inhibit ascorbic acid-induced DNA breakage, suggesting the involvement of reactive oxygen species in this reaction. We further show that such DNA breakage is inhibited by both iron and copper chelators in cells, whereas in nuclei, similar inhibition was achieved only by copper chelators, indicating an important role of chromatin-bound copper in the prooxidant cellular DNA breakage by ascorbic acid. We propose that the copper-dependent cellular redox status is an important element in the cytotoxic action of ascorbic acid against cancer cells. It is well established that cellular copper levels are considerably elevated in various malignancies. Therefore, cancer cells may be more subject to electron transfer between copper and ascorbate to generate reactive oxygen species. In light of these observations and those in literature, in this paper we explain that the preferential cytotoxicity of ascorbic acid against cancer cells is the result of elevated copper levels in such cells. Further, this study identifies nuclear copper as a novel molecular target for cytotoxic action of ascorbic acid, which has implications for its chemotherapeutic properties against cancer.