INFLUENCE OF ESTROGEN STRUCTURE ON NUCLEAR-BINDING AND PROGESTERONE-RECEPTOR INDUCTION BY THE RECEPTOR COMPLEX

The relationship between steroid structure, estrogen receptor (ER) bin ding affinity, nuclear binding of the ER complex, and induction of pro gesterone receptor (PgR) have been examined. The level of ER in membra ne-free homogenates of MCF-7 cells was found to be 10.0 +/- 0.5 fmol/m ug of DNA by utilizing an enzyme immunoassay (EIA). However, only 2.5 +/- 0.2 fmol of ER complex/mug of DNA was bound by nuclei during maxim al stimulation of PgR synthesis (2.9 +/- 0.2 fmol of PgR/mug of DNA; m easured by EIA) following a pulse with 10(-10) M E2. Except at micromo lar concentrations, estratriene was an ineffective estrogen. The addit ion of a hydroxyl group to either position 3 or position 17beta of est ratriene yielded ligands which were capable of causing nuclear binding and processing of ER as well as PgR induction. D-ring regioisomers of estradiol (E2) had lower affinity for receptor than E2. However, rece ptor complexed with these estrogens was fully capable of binding to nu clear material, undergoing processing, and inducing PgR. On the other hand, A-ring regioisomers of E2 displayed significant differences in t heir ability to mediate nuclear binding of receptor complex and induct ion of PgR. Although 1-hydroxyestratrien-17beta-ol was weakly bound by ER, this dihydroxyestrogen was capable of bringing about nuclear bind ing and processing of ER and the stimulation of PgR synthesis. In cont rast, 2- and 4-hydroxyestratrien-17beta-ol, which caused extensive nuc lear binding of ER (5-7 fmol/mug of DNA), were incapable of significan t PgR induction. Provided that the A-ring hydroxyl group was positione d correctly (3beta) on 5alpha-androstanediols or 5-androstenediol, an aromatic ring was not required for nuclear binding of the ER complex a nd stimulation of PgR synthesis. With the exception of 2- and 4-hydrox yestratrien-17beta-ol, induction of PgR by structurally altered estrog ens correlated with the affinity of ligand for ER. Electrostatic model s generated from this data were found to be useful in the characteriza tion of electronegative isopotential regions of the estrogen (or andro stanediol) molecules which were important in modulating the gene regul atory properties of ER.