Abstract
Retinoids have chemopreventive activity for epithelial tumors in a variety of systems, including the two-stage tumorigenesis system of mouse skin in which only the promotion stage is inhibited. We asked whether dietary vitamin A deficiency could affect the skin tumorigenic response, prior to major changes in body weight or general health of the animals. Two regimens were tested to induce vitamin A deficiency. SENCAR mice were either (a) fed a vitamin A-deficient diet from 4 or 9 weeks of age or (b) their mothers were fed the diet from the time of birth of the experimental animals which were then weaned on the same diet. The latter regimen produced typical symptoms of vitamin A deficiency in the offspring by Weeks 12–14 and all the mice died by Week 19; the former regimen permitted sufficient accumulation of retinol and its esters to sustain life for up to 45 and 75 weeks, respectively, in the majority of mice. For our experiments, vitamin A depletion was produced by placing the mothers on the deficient diet at birth of the experimental animals. A single topical dose of 20 µg of 7,12-dimethylbenz(a)anthracene (DMBA) was used as the initiator at 3 weeks of age and 1 to 2 µg of 12-O-tetradecanoylphorbol-13-acetate (TPA) once weekly as the tumor promoter for 10 weeks (from Week 4 through 13 of the experiment). Fifty-five % of mice (n = 40) on Purina laboratory chow (mean body weight, 31.4 g) developed skin tumors (2.58 per mouse) at 12 weeks, versus 2.5% (0.05 papillomas per mouse) of mice (n = 40) kept on the purified vitamin A-deficient diet (mean body weight, 30.3 g), a 98% decrease in tumor/mouse. Retinoic acid (RA) (1–3 µg/g diet) supplementation after Week 12 caused a rapid tumorigenic response in 95% of the mice by week 22. This tumor response occurred to a reduced extent in the absence of continued TPA treatment up to Week 13. Even though tumor incidence increased within 1 week of RA and 95% of the mice showed the tumorigenic response, the number of tumors per mouse was about 50% of that observed in mice maintained on standard Purina diet. This was confirmed in an experiment in which the mice were maintained for life either on Purina or on the RA (3 µg/g) containing purified diet, the latter being the control group for the effect of vitamin A deficiency on skin tumorigenesis. Tumor incidence reached higher than 95% at about Week 20 for both groups; however maximum tumor rate (tumors/mouse) was 11.6 in the Purina group compared to 7.3 in the RA diet. In two separate experiments mice (n = 40 and 30) maintained on a vitamin A-deficient diet without RA failed to develop visible tumors and eventually lost weight and died from vitamin A deficiency. The RA-induced tumorigenic response was not observed in vitamin A-depleted RA-repleted mice treated with DMBA alone. Furthermore, RA did not function as a tumor promoter in experiments where TPA administration was limited to Weeks 4–11 or 12–13. Equimolar retinylpalmitate or β-carotene given in the purified, vitamin A-deficient diet from Week 12 were also able to elicit a tumorigenic response similar to RA. Histological examination of the tumors formed in these experiments (initiation at 3 weeks and tumor promotion between Weeks 4 through 13) revealed that the majority of the tumors were keratoacanthomas, regardless of the diet. No significant difference in epidermal thickness, number of nucleated cell layers or [3H]thymidine labeling indices were found between mice fed the RA-containing diet compared to mice fed Purina or the deficient diet. Approximately 35% of mice developed carcinomas by Week 48. We conclude that physiological amounts of retinoids or their carotenoid precursor are necessary for DMBA-initiated, TPA-promoted tumor formation in the skin of female SENCAR mice.
