Ornithine decarboxylase activity in chemically induced mouse skin papillomas

The activity levels of L-ornithine carboxy-lyase (ODC) (E.C. 4.1.1.17) and S-adenosyl-L-methionine carboxy-lyase (SAM-D) (E.C.4.1.1.50) were determined in individual papillomas induced in mouse skin by a two-stage technique, and in normal mouse epidermis. Cycloheximide treatment abolished both enzyme activities. In normal epidermis the ODC activity was barely detectable, whereas the tumors exhibited high levels of ODC. Levels of SAM-D activity above those of normal epidermis were detected in some papillomas, but in contrast to ODC the SAM-D activity levels were not consistently increased in skin tumors. By pooling a great number of papillomas, the variations in ODC and SAM-D activities between different papillomas could be minimized so that reliable measurements of the biological half-lives of ODC and SAM-D in the tumors were obtained using cycloheximide treatment. The half-life of SAM-D in squamous papillomas was 45 min, almost identical to the 41 min half-life of the 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced level of this enzyme in normal mouse epidermis. In contrast, the ODC activity of the mouse skin papillomas declined at a rate similar to that in TPA-treated epidermis for only the first 15-20 min after cycloheximide injection. Thereafter, at time points when protein synthesis was approximately 90% inhibited, the ODC activity reverted to high levels. These results show that the high level of ODC activity in squamous papillomas is stabilized. This observation is compatible with the hypothesis that the control mechanism of the ODC activity level in these tumors is severely deranged. This change in polyamine turnover pattern may be related to altered differentiation of the epidermal cells, which constitute the main bulk of cells in these tumors.