Abstract 4382: N-acetylcysteine interacts with Cu++ to induce cancer cell death through the generation of hydrogen peroxide

Abstract

Metal ionophores that transport copper or zinc into cells are toxic to cancer cells. Loading of metals into cells results in loss of protective intracellular thiols, which can contribute to cell death. We postulated that the metal-thiol interaction might also lead to the generation of cytotoxic compounds. We tested this hypothesis by adding N-acetylcysteine (NAC) to cancer cell lines in the presence of copper or zinc. NAC itself, at concentrations up to 10 mM, was not toxic to a variety of human cancer cell lines, including A2780 (ovarian), MCF-7 (breast), Panc-1 (pancreas), and A375 (melanoma). When added to cells in the presence of Cu++ (3-30 µM), NAC significantly enhanced the cytotoxicity induced by CuCl2 alone. The cytotoxicity of the NAC/copper combination was blocked by the copper-binding compounds tetrathiomolybdate and bathocuproinedisulfonic acid. NAC was not cytotoxic in the presence of zinc. This finding suggested that the redox activity of copper was an important determinant of cytotoxicity and raised the possibility that H2O2 was being generated. This was confirmed in vitro using a colorimetric assay (Biovision). The ability of other thiols to promote H2O2 generation when exposed to Cu++ was then examined. Reduced lipoic acid and L-cysteine generated higher level of H2O2 than NAC did; reduced glutathione was able to generate comparable levels of H2O2 as NAC, while oxidized glutathione generated significantly lower levels of H2O2; oxidized cysteine did not generate H2O2. To determine if H2O2 played a role in the observed cytotoxicity, catalase was added to cells treated with NAC/copper. Its addition completely blocked cell killing, indicating that significant amounts of extracellular H2O2 were being generated. These findings indicate that the interaction of copper with thiols leads to cytotoxic concentrations of H2O2. Thiols may differ in their reactivity with redox active compounds such as copper. Since the copper concentration of plasma is greater than that of tissue culture media, NAC's effects in vivo may differ from those seen in vitro. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4382.