Suppression of mouse skin papilloma by canthaxanthin and beta-carotene in vivo: possibility of the regression of tumorigenesis by carotenoids without conversion to retinoic acid.

Using mouse skin papilloma as a model system, we examined whether the antitumorigenic activity of carotenoids was related to their provitamin A activity. Oral administration of canthaxanthin (CX) or beta-carotene at 200 mg/kg/day for 14 days significantly reduced the cumulative size of papillomas induced on the skin by 9,10-dimethyl-1,2-benzanthracene (p < 0.05), after the accumulation of these carotenoids in the tumors. The levels of a protooncogene, c-myc, were simultaneously suppressed in papillomas in carotenoid-treated mice. Because CX cannot be converted metabolically to retinoids, these results suggested that CX directly inhibited the growth of papillomas. Neither the accumulation of retinoids nor the expression of a retinoic acid-inducible gene, retinoic acid receptor-beta, was found in papillomas of CX- and beta-carotene-treated mice, suggesting that, like CX, beta-carotene might exert the tumor-suppressing effect without being converted to retinoids. Thus a certain antitumorigenic activity of carotenoids appears not necessarily to require their provitamin A activity.