The effect of hexaaza-and hexathia–macrocyclic ligands on transition metal cytotoxicity in human hepatoma-derived cultured cells

Abstract

The effect of macrocyclic ligands on cytotoxic concentrations of the transition metal ions of copper, zinc, and cadmium was investigated. For this purpose, a hexaaza-[3,6,9,17,20,23-hexaazatricyclo[23.3.1.1^11,15] triaconta–1(29),11(30),12,14,25,27–hexaene (L2)] and hexathia-chelating ligand [1,4,7,10,13,16-hexathiacyclooctadecane (L3)] were used in the human hepatoma-derived HepG2 cell line. The cytotoxicity was measured by the neutral red uptake inhibition assay. First, the NI₅₀ of the ligands, i.e., the concentration of the ligand inducing a 50% inhibition in neutral red uptake compared to control cells, was determined. In several metal/ligand combination experiments, the effects for L2 were difficult to interpret, whereas for L3 in combination with copper ions, a severe increase–and for zinc ions, a significant decrease of cell toxicity–relative to the metal control was observed. To further examine the different effects observed with L3 in combination with, respectively, Cu²⁺ and Zn²⁺, the glutathione (GSH) content was measured. The relative GSH content decreased as the concentration of L3 increased. It was proposed that the increased toxicity of the combination Cu²⁺ /L3 could be caused by the depletion of GSH and a subsequent inability to scavenge the produced reactive oxygen species (ROS). This hypothesis was supported by experiments during which vitamin E or C was added to the Cu²⁺ / L3 system.

Keywords

cadmium chelating ligands copper GSH HepG2 macrocyclic ligands metal toxicity oxidative stress ROS zinc

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