Physicochemical and genetic evidence for specific antiestrogen binding sites

Area of Science:

• Endocrinology
• Molecular Pharmacology
• Cancer Biology

Background:

• Antiestrogens like tamoxifen (Tam) and 4-hydroxytamoxifen (OH-Tam) exhibit complex biological activities.
• These compounds interact with both the estrogen receptor (ER) and other cellular components.

Purpose of the Study:

• To investigate the nature and function of antiestrogen binding sites (ABS).
• To determine the relationship between ABS and cellular responses to antiestrogens, particularly tamoxifen resistance.

Main Methods:

• Binding studies using radiolabeled antiestrogens in rat uterus and human breast cancer MCF-7 cell cytosol.
• Physical separation of ABS from ER using bioaffinity adsorbents and heparin-Sepharose chromatography.
• Sucrose gradient ultracentrifugation to analyze ABS heterogeneity.
• Comparison of ABS presence in tamoxifen-sensitive (MCF-7) and tamoxifen-resistant (RTx6) cell lines.

Main Results:

• ABS were identified and physically separated from ER, with distinct binding affinities for Tam and OH-Tam.
• ABS concentration was 10-20% of ER concentration, with a dissociation constant (Kd) of 1-2 x 10(-9) M for Tam and OH-Tam.
• ABS exhibited heterogeneous sedimentation patterns (10-40 S) and were unaffected by conditions altering ER.
• Tamoxifen-resistant RTx6 cells showed a significant lack of detectable ABS compared to sensitive MCF-7 cells.

Conclusions:

• ABS represent a distinct binding entity in cells, separate from the ER.
• The presence of ABS appears crucial for the antiestrogenic effects of tamoxifen and related compounds.
• The absence of ABS in resistant cells suggests their critical role in mediating tamoxifen sensitivity and action.