INHIBITION BY 2(3)-TERT-BUTYL-4-HYDROXYANISOLE AND OTHER ANTI-OXIDANT OF EPIDERMAL ORNITHINE DECARBOXYLASE ACTIVITY INDUCED BY 12-O-TETRADECANOYLPHORBOL-13-ACETATE
The relationship between reactive oxygen and/or free radical species and tumor promotion was evaluated by investigating the inhibitory effects of 2(3)-tert-butyl-4-hydroxyanisole (BHA) and other antioxidants on the induction of ornithine decarboxylase (ODC) activity in mouse epidermis by a tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA). Mice maintained on a diet containing 0.75% BHA for 8 days showed a 50% reduction in maximal ODC induction following treatment with TPA when compared to mice fed a control diet. Topical application of BHA (55 .mu.mol) 30 min prior to TPA treatment (17 nmol) elicited an 80% inhibition of promoter-induced ODC activity, BHA was ineffective as an inhibitor when administered either 16 h before or 2 h after the promoter. The inhibition by BHA was dose dependent with a dose producing a 50% inhibition of ODC induction of 6 .mu.mol. A structure-activity study with BHA analogs (2-tert-butyl-4-hydroxyanisole, 3-tert-butyl-4-hydroxyanisole, 2-tert-butyl-1,4-dimethoxybenzene, tert-butylhydroquinone, 4-hydroxyanisole, p-hydroquinone, phenol and 2-tert-butylphenol) showed that hydroxyl and tert-butyl substituents were important determinants of inhibitory activity. A spectrum of other antioxidants were also tested. Butylated hydroxytoluene was nearly equipotent to BHA; .alpha.-tocopherol, propyl gallate and disulfiram were all less potent, and L-ascorbate was inactive. None of the antioxidants affected basal ODC activity in non-TPA-treated mice. Collectively, these results demonstrate an early and direct inhibition of TPA-induced ODC activity by lipophilic phenolic antioxidants and suggest a role for reactive oxygen and/or free radical species in tumr promotion.