Progestin-induced down regulation of nuclear estrogen receptor in uterine decidual cells: analysis of receptor synthesis and turnover by the density-shift method.

The density-shift method was used to study the effect of the synthetic progestin, R5020, (17,21-dimethyl-19-nor-4,9-pregnadiene-3,20-dione) on the turnover and synthesis of nuclear estrogen receptor in hamster decidual cells. Newly-synthesized receptor was labeled with dense [2H, 13C, 15N] amino acids and separated from pre-existing receptor by density-gradient centrifugation. Progestin increased receptor turnover within 3 h of treatment and blocked estradiol-induced receptor synthesis at 6 h and 9 h. Thus, progestin down regulates estrogen receptor by increasing receptor turnover and inhibiting estrogen-induced receptor replenishment.