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An antiestrogen-binding protein in human tissues
Abstract:
Although nonsteroidal antiestrogens of the triphenylethylene type are generally considered to act through the estrogen receptor, some observations suggest that estrogen target tissues may also contain a binding protein specific for these compounds. The data so far reported, however, are also consistent with ligand-induced changes in conformation or in the state of aggregation of the estrogen receptor. The studies reported here demonstrate the existence of a protein in human myometrial cytosol which binds 1-[4-(2-dimethylaminoethoxy)phenyl]1,2-diphenylbut-1(Z)-ene ([3H]tamoxifen) with high affinity (Kd = 2.3 X 10(-9) M). This protein exhibits striking specificity for nonsteroidal antiestrogens. Estradiol competes weakly for bound [3H]tamoxifen, while other estrogens and nonestrogenic steroid hormones do not compete at all. Sedimentation analysis and molecular sieve chromatography indicate that the antiestrogen-binding protein is a larger species than the estrogen receptor and elutes from DEAE-Sephacel at a lower KCl concentration (0.03 M) than the estrogen receptor (0.15 M). Differential thermal stability of the estrogen receptor and the antiestrogen-binding protein was demonstrable in the absence of added ligand. The antiestrogen-binding protein was ubiquitous, being present in many tissues where estrogen receptor was undetectable. These findings support the separate existence of an antiestrogen-binding protein.
Insights
Researchers discovered a unique antiestrogen-binding protein in human tissues. This protein specifically binds nonsteroidal antiestrogens, distinct from the estrogen receptor, and is present even where estrogen receptors are undetectable.
Area of Science:
- Endocrinology
- Molecular Biology
- Pharmacology
Background:
- Nonsteroidal antiestrogens, like triphenylethylene derivatives, are typically thought to interact with the estrogen receptor.
- However, some evidence suggests the presence of a distinct binding protein for these compounds in estrogen target tissues.
- Existing data could also be explained by conformational changes or aggregation states of the estrogen receptor.
Purpose of the Study:
- To investigate the existence and characteristics of a specific binding protein for nonsteroidal antiestrogens in human myometrial cytosol.
- To differentiate this potential binding protein from the established estrogen receptor.
Main Methods:
- Binding assays using radiolabeled tamoxifen ([3H]tamoxifen) to quantify binding affinity and specificity.
- Competitive binding studies with estradiol and other steroid hormones.
- Biophysical characterization including sedimentation analysis and molecular sieve chromatography (DEAE-Sephacel).
- Differential thermal stability assays.
Main Results:
- Demonstrated a high-affinity binding protein (Kd = 2.3 X 10(-9) M) for [3H]tamoxifen in human myometrial cytosol.
- This protein showed high specificity for nonsteroidal antiestrogens, with weak competition from estradiol and no competition from other steroids.
- Sedimentation and chromatography revealed the antiestrogen-binding protein is larger and biochemically distinct from the estrogen receptor (eluting at lower KCl concentration).
- Differential thermal stability confirmed the distinct nature of the two proteins.
- The antiestrogen-binding protein was found ubiquitously, even in tissues lacking detectable estrogen receptor.
Conclusions:
- The findings provide strong evidence for the separate existence of an antiestrogen-binding protein.
- This distinct protein plays a role in the action of nonsteroidal antiestrogens.
- Its presence in various tissues, independent of estrogen receptor levels, suggests broader implications in hormonal regulation and drug action.