Tumor promoter-induced ornithine decarboxylase gene expression occurs independently of AP-1 activation

Activator protein 1 (AP-1) transactivation and ornithine decarboxylase (ODC ) activity have been established as essential downstream effecters of mouse skin tumor promotion by 12-O-tetradecanoylphorbol-13-acetate (TPA), Previo us studies have shown that inhibition of either AP-1 transactivation or ODC activity suppressed tumor promoter-induced transformation. By utilizing th e JB6 mouse epidermal cell system, the present study determined whether TPA -induced ODC gene expression and activity is independent of AP-1 transactiv ation. In three independent JB6 (P+) clones, stably expressing dominant neg ative c-jun, TPA-induced ODC gene expression and activity were similar comp ared to JB6 P+ cells expressing vector-control alone, while AP-1-dependent transcription was inhibited, Transformation-insensitive JB6 (P-) cells, whi ch lack TPA-inducible c-jun expression, also exhibited similar induction of ODC activity by TPA. alpha-Difluoromethylornithine, an irreversible inhibi tor of ODC, attenuated, at an equivalent IC50, both TPA-induced ODC activit y and anchorage-independent growth of JB6 P+ cells, despite no inhibition o f AP-1 transactivation. Taken together, the results presented indicate that TPA-induced ODC gene expression and activity are independent of AP-1 trans activation. Because inhibition of either AP-1 or ODC precludes TPA-induced transformation, and because ODC is independent of AP-1, we propose that the re are at least two pathways to transformation. Each pathway is required bu t not sufficient for transformation.