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Presence of Specific Binding Sites for Phorbol Ester Tumor Promoters in Human Epidermal and Dermal Cells in Culture but Lack of Down Regulation in Epidermal Cells¹

Department of Cancer Cell Research, Institute of Medical Science, University of Tokyo, Shirokanedai, Minato-ku, Tokyo 108, Japan

The presence of specific binding sites for phorbol esters was demonstrated in human epidermal and dermal cells in culture by assay of binding of [³H]phorbol-12,13-dibutyrate (PDBU) to intact cells. The specificity of the binding was shown by displacement of the binding with biologically active tumor promoters, such as 12-O-tetradecanoylphorbol-13-acetate, teleocidin B, and mezerein, but not with inactive derivatives. The equilibrium binding data were analyzed by the Scatchard method and fitted by a straight line to the model of a single class of binding sites. Human epidermal cells bound PDBU with a K_d of 28 nM at 3.7 x 10⁶ molecules per cell, while human dermal cells bound PDBU with a K_d of 27 nM at 2.1 x 10⁶ molecules per cell. These values were compared with those of epidermal and dermal cells of mice. Although mouse cells showed the same affinity as did human cells, mouse epidermal cells bound one-third as much as human epidermal cells, and mouse dermal cells bound one-fifth as much as human dermal cells. When precultured with unlabeled PDBU for 24 hr, [³H]PDBU binding decreased time dependently in all cells except human epidermal cells. Thus, the binding of phorbol esters to human epidermal cells is unique in that there are a large number of binding sites compared with mouse epidermal cells, and there is no down regulation.

¹ Supported in part by a grant for cancer research from the Ministry of Education, Science, and Culture of Japan.

² To whom requests for reprints should be addressed.

Received 1/ 7/83. Accepted 5/ 9/83.