Hormone binding of estradiol-17β receptor: Evidence for its regulation by cytoplasmic phosphorylation and nuclear dephosphorylation. Prevention of dephosphorylation by antiestrogens

Abstract

1. Hormone binding activity of estradiol-17β receptor appears to be regulated in vitro by a phosphorylation-dephosphorylation process: receptor inactivated by a nuclear phosphatase is reactivated by cytoplasmic activity requiring ATP. 2. The enzyme activating hormone binding has been purified and characterized from calf uterus cytosol. It is absolutely dependent on ATP, it is strongly stimulated by MgCl 2 as well as by CaCl 2, it shows high affinity for inactive receptor ( K m ~ 0.3 × 10 −9 mol of estradiol-17β binding sites/I) and it sediments through sucrose density gradient at 6 S. 3. Estrogen receptor translocated into nuclei by estradiol-17β in vivo is inactivated during incubation of nuclei at 25°C whereas receptor translocated by tamoxifen and nafoxidine is stable in the same conditions. The difference in stability, previously observed in intact cells [1], is due to inability of the nuclear phosphatase to inactivate receptor complexed with antiestrogens. 4. On the basis of these and previous results [2–5] it is suggested that phosphorylation of inactive receptor in cytosol and dephosphorylation of active receptor in nuclei regulate hormone binding of receptor in vivo. Non steroidal antiestrogens seem to interfere with this regulation by preventing nuclear dephosphorylation of receptor.