Tamoxifen inhibition of prolactin action in the mouse mammary gland.

Binding of lactogenic hormones to particulate and solubilized microsomal membranes isolated from mammary glands of lactating mice is inhibited by direct addition of 10(-10) M or greater concentrations of triphenylethylene antiestrogens [i.e. tamoxifen (TAM), 4-hydroxy-tamoxifen, and Nafoxidine] to the binding assays. Estradiol and other antiestrogens such as BPEA (-2-(4-tert-butyl-phenoxy) ethyl diethylamine hydrochloride, LY117018, and LY 156758 do not have this effect. The triphenylethylene antiestrogens bind to the membrane-associated antiestrogen binding sites (AEBS). Effectiveness of binding to the AEBS parallels the effectiveness of inhibition of the lactogen binding. The effect is selective in that binding of epidermal growth factor and insulin to these same membranes is unaffected by the antiestrogens. Binding of PRL to membranes prepared from the livers of the lactating mice is also unaffected. Both the PRL receptor and AEBS are primarily localized to the microsomal membrane fraction of cells. Maximal inhibition of PRL binding by TAM is observed in the light microsomes that contain plasma membranes. In addition to inhibition of PRL binding, TAM also prevents the PRL-induced accumulation of caseins by cultured mouse mammary explants. Thus it appears that the triphenylethylene antiestrogens, acting through the AEBS, act as antilactogens in the normal mammary gland.