Mechanism of tetrahydroxy-1,4-quinone cytotoxicity: Involvement of Ca22+ and H2O2 in the impairment of DNA replication and mitochondrial function

In this work we investigated the toxicity of a polyphenolic p-benzoquinone derivative, the tetrahydroxy-1,4-quinone (THQ) toward V79 Chinese hamster fibroblasts and analyzed the role of H202 and Ca2+ in that mechanism. The exposure of exponentially growing cultures to THQ, in the presence of 1.0 mM Ca2+, caused a dose-dependent inhibition of cell growth and DNA synthesis. Complete prevention of those effects by catalase indicated that H202-induced damages should underlie both toxic processes. Further detection of a rise in the intracellular free Ca22+ concentration ([Ca2+]i) in cells exposed to THQ plus Ca2+ together with the partial protection conferred by the intracellular Ca2+-chelator fura-2 against cell growth inhibition, indicated that a disruption of Ca2+ homeostasis is a determinant event in THQ cytotoxicity. Furthermore, the intracellular accumulation of rhodizonic acid (RDZ), the primary oxidative product of THQ, indicated that THQ, or its corresponding semiquinone form, was entering the cells and undergoing further autoxidation to RDZ. It was also evidenced that mitochondria represent an important target in the development of THQ toxicity, as shown by the disruption of the transmembrane electrical potential (Δψ) of isolated rat liver mitochondria, as well as by the Ca2+-release by mitochondria of permeabilized V79 cells. We concluded that disruption of Ca2+ homeostasis and generation of H202 are critically involved in THQ-induced impairment of DNA replication and mitochondrial functions, leading ultimately to cell growth inhibition.