AbstractIn FRSK cells, a cell line derived from fetal rat epidermal cells, cyclic AMP‐elevating agents forskolin (10 μM) and cholera toxin (10 ng/ml) increased cellular cyclic AMP content and suppressed 3H thymidine incorporation. These effects of forskolin were enhanced by the phosphodiesterase inhibitor 3‐isobutyl‐***l‐methylxanthine (0.1 mM). Dibutyryl cyclic AMP (1 mM), an analog of cyclic AMP, decreased not only basal but also both tumor promoter 12‐O‐tetradecanoylphorbol‐13‐acetate***‐ and epidermal growth factor‐stimulated 3H thymidine incorporation. From these results, we suggest that cyclic AMP may be a negative regulatory factor of DNA syn
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