Decreased ratio of reduced/oxidized glutathione in mouse epidermal cells treated with tumor promoters.

Cellular pro-oxidant states appear to play role in the promotion phase, presumably because tumor promoter-treated cells overproduce activated forms of oxygen and/or deficient in their ability to destroy them. Since one of the earliest responses to the potent tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) may be the generation of reactive oxygen species, we have determined the effects of this compound on the natural glutathione-dependent antioxidant protective system of the epidermal cells. Here we report that eight (chemically different) tumor promoters (including the phorbol esters, hydrogen peroxide, benzoyl peroxide, anthralin and mezerein) decreases to various degrees the intracellular ratio of reduced (GSH)/oxidized (GSSG) glutathione in isolated mouse epidermal cells. TPA leads to a rapid, transient increase in GSH peroxidase activity within 20 min, concomitant with a marked decrease in the ratio of GSH/GSSG. Beyond 1 h, while the GSH/GSSG ratio remains low, the GSH peroxidase activity declines below the control level in TPA-treated epidermal cells. This sequence suggests that the GSH-dependent detoxifying system of the cell is initially turned on but then rapidly overwhelmed by the oxidative challenge linked to the tumor-promoting activity of TPA. Since free radical scavengers, GSH level-raising agents and selenium-containing compounds all inhibit the effects of TPA on both GSH metabolism and tumor promotion, it is proposed that the enhancement of the GSH-dependent antioxidant protective system of the epidermal cells during TPA treatment might inhibit skin tumor promotion.