Regulation of cell division and of tyrosinase in B16 melanoma cells by imidazole: A possible role for the concept of metabolite Gene regulation in mammalian cells

Results of hemacytometer cell counts and of tyrosinase measurements made by the Pomerantz method demonstrate that imidazole added to the medium of cultured B 16 mouse melanoma cells can stimulate tyrosinase specific activity and inhibit cell division. These effects are greater than with adenosine 3′,5′ cyclic monophosphate (cAMP) or the cAMP‐phosphodiesterase inhibitor theophylline. The effects of imidazole on cell division and tyrosinase are enhanced by theophylline and antagonized by cAMP. Cyclic AMP‐phosphodiesterase activity in cell‐free extracts can be inhibited by theophyllne and stimulated by imidazole. However, imidazole does not affect cAMP‐phosphodiesterase specific activity in vivo, nor does it affect intracellular cAMP concentrations as determined by competitive protein‐binding assays. In contrast, the specific activity of cAMP‐phosphodiesterase in vivo is stimulated by cAMP and theophylline, supporting the hypothesis that cAMP and agents which increase intracellular cAMP concentrations induce the synthesis of cAMP‐phosphodiesterase. Studies with actinomycin‐D and cycloheximide support the hypothesis that cAMP can also mediate posttranslational activation of tyrosinase. Similar experiments suggest that imidazole, or a derivative therof, can induce the synthesis of tyrosinase at the pretranslational level of control. We hypothesize that this type of regulation (pretranslational) by imidazole may define a role for the concept of “Metabolite Gene Regulation” (MGR), in mammalian cells.