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Different nuclear binding sites for antiestrogen and estrogen receptor complexes
Abstract:
Experiments were performed to determine the in vivo effect of various estrogens and anti-estrogens on the nuclear accumulation and retention of estrogen receptors, the cytoplasmic levels of estrogen receptors, and the formation of salt-resistant and salt-extractable forms of the nuclear estrogen receptor in immature rat uteri. A 5 mug injection of estradiol-17beta (E2) or diethylstilbestrol (DES) resulted in a maximal nuclear translocation of the receptor complexes by 1 h with a subsequent rapid decrease of both the estradiol receptor complex (ERC) and diethylstilbestrol receptor complex (DRC) to levels found in uteri of saline-injected rats by 12 h. However, the antiestrogens U-11,100A, zuclomiphene and enclomphene (100 mug/injection) resulted in a slower nuclear accumulation of receptor complex which continued to increase through 24 h. The cytosol receptor levels with E2 and DES were depleted to 10--20% of control levels within 1 h, but then were replenished so that they were above control levels by 24 h. The clomiphene-type compounds also showed an initial depletion of cytosol estrogen receptor, but the antiestrogens were almost ineffective in receptor replenishment. The estrogen receptor translocated to the nuclear fraction by estrogens demonstrated both salt-extractable (0.3M KCl) and salt-resistant forms at 1--6 h, whereas the clomiphene-type compounds resulted in the formation of only a salt-extractable form of the estrogen receptor at all times. By 12--24 h after injection, the salt-resistant forms of the ERC and DRC were no longer present. The effect of varying the dosage of injected E2 (0.05 mug-5 mug) resulted in the formation of an identical amount of salt-resistant ERC at 1--2 h, whereas the total amount of nuclear ERC (salt-resistant and salt-extractable) varied with the injected dose of E2. However, at 6 h, the amount of salt resistant ERC varied with the injected dose of E2 (0.005-5 mug). These results suggest that the nuclear salt-resistant form (formed by estrogens only) of the estrogen receptor is required for true uterine growth, whereas the nuclear salt-extractable form may be only sufficient for short term estrogenic responses.
Insights
Estrogen receptor nuclear translocation differs between estrogens and anti-estrogens. Estrogens form a salt-resistant nuclear estrogen receptor (ERC) essential for uterine growth, while anti-estrogens only form a salt-extractable form.
Area of Science:
- Endocrinology
- Molecular Biology
- Reproductive Science
Background:
- Estrogen receptors (ERs) mediate the effects of estrogens.
- Understanding ER nuclear translocation and retention is key to comprehending estrogenic and anti-estrogenic actions.
- The in vivo dynamics of ERs in response to different ligands are not fully elucidated.
Purpose of the Study:
- To investigate the in vivo effects of estrogens and anti-estrogens on estrogen receptor nuclear accumulation, cytoplasmic levels, and nuclear receptor forms in immature rat uteri.
- To differentiate the roles of salt-resistant and salt-extractable nuclear estrogen receptor forms in mediating estrogenic responses.
Main Methods:
- Immature rats were injected with estradiol-17beta (E2), diethylstilbestrol (DES), or anti-estrogens (U-11,100A, zuclomiphene, enclomiphene).
- Uterine tissues were analyzed for cytoplasmic and nuclear estrogen receptor levels at various time points post-injection.
- Nuclear estrogen receptors were fractionated into salt-extractable and salt-resistant forms using 0.3M KCl.
Main Results:
- Estrogens (E2, DES) induced rapid nuclear translocation of estrogen receptor complexes (ERC, DRC) within 1 hour, followed by depletion and subsequent replenishment of cytoplasmic receptors.
- Anti-estrogens induced slower nuclear accumulation, with limited cytoplasmic receptor replenishment.
- Estrogens generated both salt-extractable and salt-resistant nuclear ERC, with the salt-resistant form correlating with uterine growth.
- Anti-estrogens primarily formed only salt-extractable nuclear receptors.
Conclusions:
- The salt-resistant nuclear estrogen receptor form, generated by estrogens, is crucial for sustained uterine growth.
- The salt-extractable nuclear estrogen receptor form may mediate short-term estrogenic effects.
- Estrogens and anti-estrogens exhibit distinct mechanisms in regulating estrogen receptor nuclear localization and function.