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Interaction of tamoxifen with cytosolic and nuclear type II estrogen binding sites (type II EBS)
G Ferrandina1, F O Ranelletti, G Scambia
1Laboratory of Antineoplastic Pharmacology Zeneca Department of Gynecology, Catholic University L.go A. Gemelli, Rome, Italy.
Abstract:
The aim of this study was to investigate the interaction of tamoxifen (TAM) with the so-called Type II estrogen binding sites (Type II EBS) in both the cytosolic and the nuclear fraction of the ER-negative A 2780 human ovarian cancer cell line and in an ER-negative ovarian cancer tissue. Although cytosolic and nuclear Type II EBS in A 2780 cells showed substantially similar binding characteristics in terms of ligand affinity and specificity, TAM, while exhibiting the ability to displace [3H]estradiol from cytosolic Type II EBS failed to interact with nuclear Type II EBS. The ability of TAM to interact only with cytosolic Type II EBS seems also to be a characteristic of ovarian cancer tissue and to be shared by several TAM metabolites. The hypothesis that the interaction of TAM with cytosolic Type II EBS could mobilize the true endogenous ligand of Type II EBS which would become available for binding to nuclear Type II EBS was tested by incubating the nuclear fraction with the cytosolic fraction. In the presence of cytosol, TAM acquires the ability to displace the tracer from nuclear Type II EBS but when the cytosolic fraction was DCC, stripped in order to remove the endogenous ligand, the competing activity of TAM for nuclear Type II EBS was abolished. Our results suggest that TAM does not interact with nuclear Type II EBS, but can favor the nuclear binding of endogenous ligand by displacing it from cytosolic Type II EBS.
Insights
Tamoxifen (TAM) interacts with cytosolic Type II estrogen binding sites (EBS) but not nuclear ones in ovarian cancer cells. TAM indirectly promotes nuclear binding of endogenous ligands by displacing them from cytosolic Type II EBS.
Area of Science:
- Endocrinology
- Molecular Biology
- Cancer Research
Background:
- Tamoxifen (TAM) is a widely used drug in hormone-dependent cancers.
- Type II estrogen binding sites (EBS) are implicated in cellular responses to estrogens and anti-estrogens.
- The precise interaction of TAM with Type II EBS in ER-negative ovarian cancer remains unclear.
Purpose of the Study:
- To investigate the interaction of tamoxifen (TAM) with Type II estrogen binding sites (EBS) in ER-negative ovarian cancer.
- To determine if TAM directly binds to nuclear Type II EBS.
- To explore the role of cytosolic Type II EBS in TAM's interaction with nuclear Type II EBS.
Main Methods:
- Utilized ER-negative A 2780 human ovarian cancer cell line and ovarian cancer tissue.
- Performed radioligand binding assays using [3H]estradiol to assess TAM and its metabolites' interaction with cytosolic and nuclear Type II EBS.
- Investigated the effect of cytosolic fraction on TAM's interaction with nuclear Type II EBS.
Main Results:
- TAM displaced [3H]estradiol from cytosolic Type II EBS but not from nuclear Type II EBS in A 2780 cells and ovarian cancer tissue.
- TAM's interaction was specific to cytosolic Type II EBS, a characteristic shared by some TAM metabolites.
- In the presence of cytosol, TAM gained the ability to displace tracer from nuclear Type II EBS, an effect abolished upon cytosol stripping.
Conclusions:
- Tamoxifen (TAM) does not directly interact with nuclear Type II estrogen binding sites (EBS).
- TAM indirectly facilitates the nuclear binding of endogenous ligands by displacing them from cytosolic Type II EBS.
- This mechanism may contribute to TAM's therapeutic effects in ER-negative ovarian cancer.