Disulfiram anti-cancer efficacy without copper overload is enhanced by extracellular H(2)O(2) generation: antagonism by tetrathiomolybdate

HIGHLIGHTS: exogenous SOD increases apoptosis by sub-toxic disulfiram without copper overload. H(2)O(2) generation from glucose oxidase also potentiates disulfiram toxicity. N-acetylcysteine suppresses antitumor potentiation of DSF by H(2)O(2) generation; sub-toxic tetrathiomolybdate inhibits potentiation of DSF by SOD. BACKGROUND: Cu/Zn superoxide dismutases (SODs) like the extracellular SOD3 and cytoplasmic SOD1 regulate cell proliferation by generating hydrogen peroxide (H(2)O(2)). This pro-oxidant inactivates essential cysteine residues in protein tyrosine phosphatases (PTP) helping receptor tyrosine kinase activation by growth factor signaling, and further promoting downstream MEK/ERK linked cell proliferation. Disulfiram (DSF), currently in clinical cancer trials is activated by copper chelation, being potentially capable of diminishing the copper dependent activation of MEK1/2 and SOD1/SOD3 and promoting reactive oxygen species (ROS) toxicity. However, copper (Cu) overload may occur when co-administered with DSF, resulting in toxicity and mutagenicity against normal tissue, through generation of the hydroxyl radical (•OH) by the Fenton reaction. PURPOSE: To investigate: a) whether sub-toxic DSF efficacy can be increased without Cu overload against human melanoma cells with unequal BRAF(V600E) mutant status and Her2-overexpressing SKBR3 breast cancer cells, by increasing H(2)O(2)from exogenous SOD; b) to compare the anti-tumor efficacy of DSF with that of another clinically used copper chelator, tetrathiomolybdate (TTM) RESULTS: a) without copper supplementation, exogenous SOD potentiated sub-toxic DSF toxicity antagonized by sub-toxic TTM or by the anti-oxidant N-acetylcysteine; b) exogenous glucose oxidase, another H(2)O(2) generator resembled exogenous SOD in potentiating sub-toxic DSF. CONCLUSIONS: potentiation of sub-lethal DSF toxicity by extracellular H(2)O(2) against the human tumor cell lines investigated, only requires basal Cu and increased ROS production, being unrelated to non-specific or TTM copper chelator sequestration. SIGNIFICANCE: These findings emphasize the relevance of extracellular H(2)O(2) as a novel mechanism to improve disulfiram anticancer effects minimizing copper toxicity.