Abstract

Okadaic acid and dinophysistoxin-1 are non-12-O-tetradecanoylphorbol-13-acetate (non-TPA)-type tumor promoters, which enhance chemically induced tumorigenesis on mouse skin through a different mechanism from that of TPA. In the present study, we examined the promoting effects of these okadaic acid class tumor promoters on a two-stage transformation using BALB/3T3 cells which was designed to simulate in vivo two-stage carcinogenesis. Cells were treated first with a low dose of the initiator 3-methylcholanthrene (MCA) and then with a test chemical. Okadaic acid and dinophysistoxin-1 significantly enhanced the MCA-induced cell transformation. Okadaic acid tetramethyl ether, an inactive compound, did not affect the transformation of MCA-treated cells. The okadaic acid class of tumor promoters failed to induce transformation without pretreatment by MCA. Okadaic acid did not show initiating activity in the two-stage transformation assay in which cells were treated first with okadaic acid and then with TPA. These results indicate that this transformation assay with BALB/3T3 cells is useful to predict tumor-promoting activity of non-TPA-type as well as TPA-type tumor promoters, before long-term in vivo two-stage carcinogenesis experiments are carried out.