alpha-Difluoromethylornithine, an irreversible inhibitor of ornithine decarboxylase, inhibits tumor promoter-induced polyamine accumulation and carcinogenesis in mouse skin.

Abstract

The role of ornithine decarboxylase (OrnDCase) and of the polyamines [putrescine (Put), spermidine (Spd), and spermine (Spm)] in mouse skin tumor promotion was investigated by the use of .alpha.-difluoromethylornithine (CHF2-Orn), an enzyme-activated irrversible inhibitor of OrnDCase. 12-O-Tetradecanoylphorbol 13-acetate (TPA), mezerein and ethyl phenylpropiolate (EPP) were employed as complete, stage II specific and nonpromoting agents, respectively. TPA and mezerein, but not EPP, provided for a dose-dependent increase in tissue Put accumulation. The Put level in papillomas developed by TPA (2 .mu.g) treatment was .apprxeq. 15-fold higher than that of the surrounding skin tissue; Spd accumulation was 2- to 3-fold greater in the papillomas. Put administered i.p. with TPA greatly enhanced papilloma yield. CHF2-Orn given orally or i.p. abolished the TPA-induced OrnDCase activity and Put accumulation in mouse epidermis. The reduction of polyamine accumulation by CHF2-Orn was directly proportional to reduction of tumor size. CHF2-Orn administered in a 2-stage (TPA-mezerein) promotion protocol reduced tumor size, inhibited by 65-70% the number of papillomas per mouse and decreased by 40% the percentage of mice with tumors when given with the stage II agent mezerein. CHF2-Orn provided considerably less effect on tumorigenesis when administered with the TPA portion of the protocol, and CHF2-Orn did not inhibit the induction of dark basal keratinocytes by TPA. Based on the results with CHF2-Orn, regulation of polyamine biosynthesis, particularly Put, apparently is a critical factor in stage II promotion.