Effects of phorbol esters on multiplication and differentiation of mammary cells.

Three phorbol esters: phorbol-12,13-dibutyrate (PDB), 12-0-tetradecanoylphorbol-13-acetate (TPA) and phorbol-12,13-diacetate (PDA) were added at various concentrations (10(1)-10(4) ng/ml) to the culture medium of rabbit mammary explants. The three phorbol esters inhibited the mitogenic signals of insulin, EGF and prolactin. They also prevented the induction of casein synthesis and casein mRNA accumulation by prolactin. In all cases, the most potent inhibitor was PDB. PDB did not alter the binding of prolactin to its receptors. The three phorbol esters added to mammary cell membranes with prolactin prevented the generation of the hormonal relay which specifically stimulates casein gene transcription when added to isolated nuclei. These data suggest that phorbol esters act directly at the membrane level where they interrupt the transfer of the hormonal messages, through a mechanism which may not be related to their promoter potency.