Dietary protein enhanced mammary ornithine decarboxylase activity and tumorigenesis in rats.

Sprague-Dawley rats (F-1) were fed a normal protein (19% casein, NP) or high protein (33% casein, HP) isoenergetic diet containing 15% corn oil prior to conception. Female pups (F-2) were also fed the maternal diet after weaning. At 7 wk of age, before saline or N-nitrosomethylurea (NMU) treatment, ornithine decarboxylase (ODC, EC 4.1.1.17) activity in mammary epithelium and liver tissue was significantly higher in the HP group than in the NP group. Eight weeks after saline treatment ODC activity in mammary tissue decreased in both groups, but remained significantly higher in the HP group. NMU treatment caused a sixfold increase in ODC activity in the mammary tissue in the HP group and a significantly lower response in the NP group. Liver ODC activity had a minimal response to NMU treatment. Changing from the HP to the NP diet 4 wk after NMU treatment reduced mammary ODC induction response but not tumor burden; changing from the NP diet to HP diet produced no change in ODC activity or tumor burden. Mammary tumor burden was positively related to dietary protein and mammary epithelium ODC activity prior to and following NMU treatment.