Inactivation of oestrogen receptor in vitro by nuclear dephosphorylation

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Abstract
Nuclei of the calf uterus are endowed with an activity inactivating crude estrogen-receptor complex. This activity was partially purified. It shows a very high affinity for the estrogen-receptor complex (Km = 0.8 .times. 10-9 mol of specific [3H]estradiol-17.beta.-binding sites/l) and for the estrogen-free receptor (Km = 1.5 .times. 10-9 mol of specific [3H]estradiol-17.beta. binding sites/l). The nuclear receptor-inactivating activity is enhanced by dithiothreitol and inhibited by several phosphatase inhibitors as well as by 4-nitrophenyl phosphate, a well known phosphatase substrate. This inhibition shows that a dephosphorylation process is required for the receptor inactivation. The purified nuclear activity inactivates pure receptor, and phosphatase inhibitors prevent this inactivation. Receptor inactivation is apparently due to a nuclear phosphatase directly acting on the estrogen receptor. The nuclear localization of the receptor-inactivating activity, its high affinity for specific estrogen binding sites and its presence only in estrogen target tissues suggest that this activity is the same as that involved in the nuclear loss of the receptor observed in intact cells.