We found that two markers of differentiation, tyrosinase (monophenol, dihydroxyphenylalanine:oxygen oxidoreductase, EC 126.96.36.199) activity and melanin synthesis, are induced by diazepam in B16/C3 mouse melanoma cells. We also demonstrated high-affinity binding sites for [3H]diazepam in these cells by radioreceptor assay, and we visualized binding to the cell surface by fluorescence microscopy with a benzodiazepine analog conjugated to a fluorescein-labeled protein. Our studies also showed that there are differences between the binding characteristics in intact cells and in membrane fractions prepared from these cells. Scatchard analysis of the binding data from membrane fractions gave a linear plot (Kd = 9.1 X 10(-8) M). With intact cells, a curvilinear Scatchard plot was obtained. This was resolved into two components defining binding sites with affinity constants of 1.7 X 10(-9) M and 4.6 X 10(-7) M. Thus, it appears that [3H]diazepam binding in intact cells is more complex than in isolated membranes. Several related benzodiazepines, including flunitrazepam, Ro-5-4864, nitrazepam, oxazepam, lorazepam, Ro-5-3072, chlordiazepoxide, and clonazepam also induced melanogenesis. When these compounds were tested for their ability to inhibit [3H]diazepam binding, flunitrazepam, diazepam, and Ro-5-4864 were found to be the most effective inhibitors. These three compounds were also the most potent in inducing melanogenesis. Our results suggest that the benzodiazepines modulate cell differentiation. The presence of high-affinity binding sites in this homogeneous, easily grown cell line may provide a useful model for studies on the mechanism of action of these compounds.
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