β-Carotene fails to act as a tumor promoter, induces RAR expression, and prevents carcinoma formation in a two-stage model of skin carcinogenesis in male sencar mice

Clinical trials have shown a significant increase in incidence of lung cancer among smokers and asbestos workers supplemented with β-carotene, suggesting a tumor-promoting activity for this agent. We set out to test possible tumor-promoting and chemopreventive activities of dietary and topical β-carotene in the two-stage 7,12-dimethyl-benz[a]anthracene-12-O-tetradecanoylphorbol 13-acetate (TPA) model of mouse skin carcinogenesis. In the first study, the effects of three levels of dietary β-carotene (6, 60, and 600 μg/g purified diet containing no other retinoid or carotenoid) were assessed over a period of 42 weeks. Carcinoma yield was reduced by ~50% in the 600 μg/g diet group (mean 0.22 carcinomas/mouse) compared with the 6 μg/g diet group (mean 0.44 carcinomas/mouse, p = 0.003). The 60 μg/g diet group showed a pattern of inhibition similar to the 600 μg/g diet group. A protective effect (25% reduction) of β-carotene (in the 600 μg/g diet group) on papilloma formation was also found. However, the intermediate 60 μg/g diet group showed the same incidence as the low 6 μg/g diet group. This points to a lack of correlation between papilloma and carcinoma incidence, as we also found in previous work on dietary retinoids and carotenoids. The purpose of the second study was to assess whether topical β-carotene (2 μg) has tumor-promoting or chemopreventive activity in the two-stage protocol. In the absence of TPA, β-carotene had no significant tumor-promoting activity. Instead, papilloma yield induced by TPA was decreased by topical β-carotene from an average of 20 to ~10 papillomas/mouse (p = 2.5 x 10^-7). The effect of topical β-carotene persisted beyond the treatment period (Week 24) until the termination of the study at Week 42. Western blot analysis of mouse skin extracts showed that topical β-carotene upregulated retinoic acid receptor-α and -γ expression in the dorsal skin. This finding suggests that β-carotene may work as a chemopreventive agent by upregulating the expression of retinoid receptors in mouse skin. In conclusion, our data show that β-carotene prevents skin carcinoma formation, induces retinoic acid receptor expression, and fails to act as a tumor promoter in the two-stage model of skin tumorigenesis.