Reversible Inhibition by Retinoids of 3-Methylcholanthrene-induced Neoplastic Transformation in C3H/10T½ Clone 8 Cells

Abstract Several investigators have recently shown that retinoids inhibit chemically induced carcinomas in vivo and may be important in the prevention of the development of cancer. Studies were initiated using cultured C3H/10T½ clone 8 cells to study the effects of retinoids on 3-methylcholanthrene (MCA)-induced neoplastic transformation. At concentrations that did not affect cell survival (0.005 to 0.5 µg/ml), retinyl acetate was found to inhibit MCA-induced transformation. Complete inhibition of transformation was observed when MCA-treated cells were continuously treated with retinyl acetate (0.1 µg/ml) starting 7 days after MCA exposure (2.5 µg/ml; 24 hr). A brief exposure to retinyl acetate (0.5 µg/ml) on Days 7 to 14 after MCA caused a 70% inhibition in transformation, while delaying treatment with retinyl acetate up to 3 weeks after MCA still decreased transformation by 80%. Nontoxic concentrations of retinol and retinal were found to be approximately equal in potency to retinyl acetate. The inhibition of neoplastic transformation by retinoids was found to be fully reversible on maintaining initiated cultures for 3 to 5 weeks in drug-free medium. Growth of three established transformed C3H/10T½ clone 8 cell lines in the presence of normal cells was not inhibited by 1 µg/ml doses of retinyl acetate or retinol. Thus, in extension of the in vivo studies, we have shown that retinoids inhibit chemically induced neoplastic transformation in cultured mouse fibroblasts and, furthermore, that this inhibition is not related to toxicity. The results of this study indicate that retinoids are acting to delay the progression of preneoplastic cells to fully neoplastic cells.