Redox-active chalcogen-containing glutathione peroxidase mimetics and antioxidants inhibit tumour promoter-induced downregulation of gap junctional intercellular communication between WB-F344 liver epithelial cells.

Abstract

Evidence is mounting supporting a role for oxidative stress in the mechanism of tumour promotion in response to agents such as 12-O-tetradecanoylphorbol-13-acetate (TPA). In this paper we demonstrate that glutathione peroxidase-mimetic xenobiotics, ebselen, ebselen-glutathione, alpha-(phenylselenenyl) acetophenone and bis-(4-aminophenyl) telluride (at concentrations between 10 microM and 50 microM) all demonstrate protective effects on TPA-induced downregulation of gap-junctional intercellular communication (GJIC) between WB-F344 rat liver epithelial cells. These effects were, in each case, diminished if the cells were depleted of their intracellular glutathione, and potentiated if glutathione was supplemented into the incubations. Additionally, bis-(4-aminophenyl) selenide and several N-substituted analogues, possessing potent antioxidant activity but being devoid of GSH peroxidase-mimetic activity, demonstrated remedial activity against TPA-induced downregulation of GJIC. Structure-activity relationships between these molecules showed a strong correlation to the oxidation potential of the selenium atom in the compound as the bis-(4-nitrophenyl)- and bis-(4-cyanophenyl)- derivatives, which possess poor antioxidant capacity and a half-wave redox potential well above +1.0 V, did not affect TPA-induced effects on GJIC. Examination of the mechanism of action of these redox-active compounds demonstrated correlations between their abilities to (i) prevent TPA-induced downregulation of GJIC, (ii) abolish the accumulation of intracellular oxidants and (iii) prevent the hyper-phosphorylation and internalization of connexin 43 in the cells. The active compounds were also able to prevent the rapid, TPA-induced translocation of protein kinase C to the particulate fraction of the cells, without affecting phorbol ester binding. These data support a synergistic role for oxidants and other TPA-dependent responses within the cell in mediating the downregulation of GJIC. Such oxidative metabolism may play a role in the control of translocation of protein kinase C from the cytosol to membranes in response to TPA within these cells. Despite the nature of the in vitro test system studied, the data also clarify the molecular basis for a potential anti-tumour promotive effect of antioxidants, based on established redox chemistries of several series of structurally-related molecules.