Regulation of ornithine decarboxylase in B16 mouse melanoma cells: synergistic activation of melanogenesis by alphaMSH and ornithine decarboxylase inhibition.

Ornithine decarboxylase (ODC) is the rate-limiting enzyme in the biosynthesis of polyamines, a family of cationic compounds required for optimal cell proliferation and differentiation. Within mammalian melanocytes, the expression of genes regulating cell growth and/or differentiation can be controlled by alpha-melanocyte-stimulating hormone (alphaMSH) and other melanogenesis modulating agents. In the B16 mouse melanoma model, alphaMSH stimulates melanogenesis by upmodulation of tyrosinase (tyr) activity, whereas the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) inhibits melanin synthesis. Therefore, we analyzed the regulation of ODC by these agents, as related to changes in the melanogenic pathway. Treatment of B16 cells with TPA or alphaMSH rapidly stimulated ODC activity. The effect was stronger for TPA and appeared mainly posttranslational. Irreversible inhibition of ODC with the active site-directed inhibitor alpha-difluoromethylornithine (DFMO) did not block TPA-mediated inhibition of tyr. Conversely, prolonged treatment of B16 cells with DFMO stimulated tyr activity by a posttranslational mechanism, probably requiring polyamine depletion. Combination treatment with alphaMSH and DFMO synergistically activated tyr. Therefore, ODC induction is not involved in the melanogenic response of B16 cells to alphaMSH. Rather, increased intracellular concentrations of polyamines following ODC induction might constitute a feedback mechanism to limit melanogenesis activation by alphaMSH.

Aminoglutethimide, a steroidogenesis inhibitor, abolishes hormonal induction of ornithine decarboxylase in steroidogenic tissues: evidence for its role as cAMP-dependent protein kinase inhibitor.

Aminoglutethimide (AMG), a potent inhibitor of steroidogenesis used in the treatment of breast cancer and some adrenal pathologies, abolished the induction of ornithine decarboxylase (ODC) elicited by peptide hormones and by dibutyryl-cAMP in steroidogenic tissues. This effect seems to be related to an inhibition of cAMP-dependent protein kinase (IC50 = 287 microM) rather than blockade of the steroidogenic pathway. This inhibition may explain some of the effects observed in AMG treatment which cannot be ascribed to its direct effect on the cytochrome P450scc complex or aromatase. Taking into account that ODC, the rate-limiting enzyme in polyamine synthesis, is elevated in many types of cancer and that overexpression of this enzyme is associated with cell transformation, one may speculate that the inhibitory action of AMG on protein kinase A represents a positive colateral effect of this drug in cancer therapy.