Down-modulation of receptors for phorbol ester tumor promoter in primary epidermal cells

The specific [20-3H]phorbol 12,13-dibutyrate ([3H]PDBu) binding to intact epidermal cells displayed the phenomenon of down-modulation, i.e., the specific binding of [3H]PDBu to its receptors on primary epidermal cells reached a maximum within 1 h and steadily declined thereafter. The apparent down-modulation of radiolabel resulted from a partial loss in the total number of receptors; the affinity of receptors for the ligand was essentially unchanged. A number of agents such as chloroquine, methylamine, or arginine which are known to prevent clustering, down-modulation, and/or internalization of several hormone receptors did not affect the down-modulation of phorbol ester receptors. Furthermore, cycloheximide had no effect either on down-modulation or on the binding capacity of cells. The surface binding capacity of down-modulated cells following a 90-min incubation with unlabeled ligand was almost returned to normal within 1 h. The effect of the antidepressant drug chlorpromazine, which is known to interact with calmodulin, on [3H]PDBu binding was also investigated. Our data indicate that the effect of chlorpromazine on [3H]PDBu binding is probably unrelated to its calmodulin-binding activity