Application of the potent tumor-promoting agent 12-O-tetradecanoylphorbol-13-acetate (TPA) to mouse skin leads to a rapid and transient induction of epidermal ornithine decarboxylase (EC 4.1.1.17) activity. Several lines of evidence, including the finding that β-retinoic acid inhibits both phorbol ester-induced ornithine decarboxylase activity and the formation of skin papillomas, suggest that this phenotypic change is an essential component of the mechanism of skin tumor promotion. We have evaluated the capacity of 23 retinoids to inhibit phorbol esterinduced mouse epidermal ornithine decarboxylase activity. The retinoid to be tested was applied to mouse skin 1 hr prior to the topical application of 17 nmoles of TPA, and epidermal ornithine decarboxylase activity was routinely measured 4.5 hr following TPA treatment, the time point of maximum enzyme induction.

β-Retinoic acid, α-retinoic acid, 13-cis-retinoic acid, 13-cis-retinal, and 5,6-dihydroretinoic acid, as well as the dimethylmethoxyethylcyclopentenyl and dimethylacetylcyclopentenyl analogs of retinoic acid, were among the potent inhibitors; less than 1 nmole was required to give 50% inhibition. The ability of β-retinoic acid to inhibit phorbol ester-induced ornithine decarboxylase activity was decreased when its cyclohexenyl ring was replaced with a substituted phenyl ring; about 14 nmoles of the trimethylmethoxyphenyl analog of either retinoic acid or ethyl retinoate were required to achieve 50% inhibition. The trimethylhydroxyphenyl analog of ethyl retinoate, the trimethylmethoxyphenyl analog of N-ethylretinamide, and the phenyl analog of retinoic acid had minimal activity. Among the fluoro derivatives of the trimethylmethoxyphenyl analogs of retinoic acid esters tested, the 8-fluoro-trimethylmethoxyphenyl analog of methyl retinoate and the 12-fluoro-trimethylmethoxyphenyl analog of ethyl retinoate were good inhibitors; application of 0.21 and 5 nmoles, respectively, resulted in 50% inhibition of TPA-induced epidermal ornithine decarboxylase activity. In contrast, the 13-trifluoromethyl-trimethylmethoxyphenyl analog of ethyl retinoate was devoid of inhibitory potency.

A similar inhibition of enzyme activity by retinoids was observed when these were administered systemically. Furthermore, β-retinoic acid and its other active analogs dramatically inhibited phorbol ester-caused accumulation of epidermal putrescine without significantly altering the levels of spermidine, spermine, and phorbol ester-induced S-adenosyl-l-methionine decarboxylase (EC 4.1.1.50) activity.

Since phorbol ester-induced epidermal ornithine decarboxylase activity is proposed as essential for tumor promotion, inhibition of this enzyme activity by retinoids may be a simple and rapid in vivo test for assessing the potential prophylactic activity of new synthetic retinoids.

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Supported by NIH Program Project Grant CA 07175.

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