Dose-time response in mouse skin tumor induction by 7, 12-dimethylbenz[a]anthracene and 12-O-tetradecanoyl-phorbol-13-acetate

The question was addressed whether the dose-response relationship derived from a carcinogenicity study can be used for mechanistic interpretation and to what extent the shape of the curve is dependent on the duration of the bioassay and the time of analysis. The mouse skin tumor model was used. It allows recording of the time of tumor appearance without interim sacrifice. Groups of 16 female NMRI mice were treated twice weekly by dermal administration with combinations of (a) 2.5 nmol 12-O-tetradecanoyl-phorbol-13-acetate (TPA) plus 0, 0.3, 1, 3, or 10 nmol 7,12-dimethylbenz[a]anthracene (DMBA) or with (b) 2.5 nmol DMBA plus 0, 0.1, 0.3, 1, 3, or 10 nmol TPA. The appearance of the first papilloma was recorded for each animal and the cumulative incidence data were analyzed in two ways: (i) With the usual dose-prevalence representation at a fixed time point, the dose response for TPA was sigmoidal, while it was linear-superlinear for DMBA. This was observed at all time points, indicating that the dose-response information may be used for a distinction between DNA-reactive and tumor-promoting mechanisms of action. (ii) When time-to-tumor and loss of tumor-free lifetime was analyzed as a function of dose, there was again a marked difference between DMBA and TPA. The tumor-free lifetime increased with each step of dose reduction but the slope was about four times larger for TPA compared with that for DMBA. Further reduction of the TPA dose could result in a situation in which the natural life span sets a limit to an observable effect. Under the conditions of this bioassay for mouse skin papilloma induction by a combination treatment with DMBA plus TPA, the findings support the idea of a no-effect low-dose threshold for the tumor-promoting agent.

DNA adducts, cell proliferation and papilloma latency time in mouse skin after repeated dermal application of DMBA and TPA

The mouse skin tumor model was used to investigate whether the level of DNA adducts and/or the rate of cell division in the epidermis are indicators of the risk of cancer formation for an individual in an outbred animal population. A high risk was considered to be reflected by a short latency period for the appearance of a papilloma. Female NMRI mice were treated twice weekly with 2.5 nmol 7,12-dimethylbenz[a]anthracene (DMBA) and 3 nmol 12-O-tetradecanoylphorbol-13-acetate (TPA) and the appearance of papillomas was registered. The first papilloma appeared after 7.5 weeks. After 17 weeks, when 12 of 14 mice had at least one papilloma, an osmotic minipump delivering 5-bromo-2'-deoxyuridine (BrdU) was implanted into each mouse for 24 h. The mice were killed after 24 h and the epidermis was analyzed for DMBA-nucleotide adducts by 32P-postlabeling, for the cell number per unit skin length, and for the labeling index for DNA synthesis. Unexpectedly, DMBA-nucleotide adduct levels were highest in those animals which showed the longest latency periods. Adduct levels were negatively correlated with the labeling index, indicating that dilution of adducts by cell division was a predominant factor in determining average adduct concentrations. Individual tumor-latency time was not correlated with either cell number or labeling index. This could be due to the fact that the measurements only provided averaged data and gave no information on the specific situation in clones of premalignant cells. Under the conditions of this assay, therefore, neither DNA adduct levels nor information on the average kinetics of cell division had a predictive value for the individual cancer risk within a group of outbred animals receiving the same treatment.