Potent suppressive effect of a Japanese edible seaweed, Enteromorpha prolifera (Sujiao-nori) on initiation and promotion phases of chemically induced mouse skin tumorigenesis

Potent antigenotoxic and anti-tumor promoting activities of a Japanese edible seaweed, Enteromorpha prolifera (Sujiao-nori in Japanese) were previously identified using an in vitro cell culture experiment (Y. Okai, K. Higashi-Okai, S. Nakamura, Y. Yano, S. Otani, Cancer Lett. 87 (1994) 25–32). However, in vivo anti-carcinogenic activity of this seaweed has not been elucidated until now. In the present study, the anticarcinogenic activity of E. prolifera was analyzed using an initiation and promotion model experiment of mouse skin tumorigenesis caused by 7,12-dimethylbenz[a]anthracene (initiator) and 12-O-tetradecanoylphorbol-13-acetate (promoter). (1) Application of the extract of E. prolifera prior to the treatment with a tumor initiator or promoter caused a significant suppression against skin tumorigenesis, and the combined application of the extract prior to both treatments with initiator and promoter exhibited much stronger suppression against the same skin tumorigenesis. (2) As a possible active principle for the anticarcinogenic activity of the extract, we propose a chlorophyll-related compound, pheophytin-a, which has been recently identified in the extract of this alga as an antigenotoxic substance (Y. Okai, K. Higashi-Okai, J. Sci. Food Agric. 74 (1997) 531–535), and showed significant suppressive effects in the same tumorigenesis experiment. These results suggest that E. prolifera has a potent suppressive activity against chemically induced mouse skin tumorigenesis through the suppression at the initiation and promotion phases, and that pheophytin-a might be partially associated with the in vivo anticarcinogenic activity.