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Effects of selective estrogen receptor modulators (SERMs) on coactivator nuclear receptor (NR) box binding to
Kelli S Bramlett1, Thomas P Burris
1Gene Regulation, Bone, and Inflammation Research, Lilly Research Laboratories, Lilly Corporate Center, Indianapolis, IN 46285, USA.
Abstract:
Coactivators are required for activation of target genes by nuclear receptors. A well-studied class of coactivators, the p160 proteins, use short nuclear receptor interaction domains (NR boxes) to bind to the activated ligand-binding domain of a nuclear receptor. To investigate how selective estrogen receptor modulators (SERMs) affect NR box recruitment, we compared the recruitment of p160 NR box peptides to the estrogen receptor (ER)alpha and ER beta in the presence of 17beta-estradiol (E2), 4-OH tamoxifen (4-OH Tam), LY 117018 (a raloxifene analog), and ICI 182780 (ICI, an ER antagonist). Our coactivator interaction assay utilizes time-resolved fluorescence technology to assess the binding of the 10 NR boxes derived from the three known p160 coactivators (SRC-1, -2, -3) to the ER subtypes in the presence of each ligand. The SERMs we studied did not increase NR box binding to either ER alpha or ER beta, but instead were potent antagonists decreasing estradiol-dependent NR box binding. We also demonstrated inverse agonism for all of the SERMs tested as they dose-dependently decreased hormone-independent NR box binding to ER beta. Therefore, the SERMs studied behave as antagonists of ER alpha and ER beta NR box binding and do not increase coactivator NR box binding to either ER subtype. In addition, we examined the preference of E2-bound ER alpha and ER beta for various naturally occurring NR boxes including the 10 SRC boxes as well as the motifs from PGC-1, TRBP, TRAP220, and CBP. Interestingly, a clear preferential pattern of interaction was noted that was receptor specific.
Insights
Selective estrogen receptor modulators (SERMs) act as antagonists, decreasing coactivator binding to estrogen receptors (ERs). These compounds do not enhance, but rather inhibit, NR box recruitment to ER alpha and ER beta.
Area of Science:
- Molecular Endocrinology
- Nuclear Receptor Signaling
- Pharmacology
Background:
- Coactivators are essential for nuclear receptor-mediated gene activation.
- p160 proteins, a class of coactivators, interact with nuclear receptors via Nuclear Receptor (NR) boxes.
- Estrogen Receptors (ERs) mediate the effects of estrogen and are modulated by Selective Estrogen Receptor Modulators (SERMs).
Purpose of the Study:
- To investigate the effect of SERMs on the recruitment of p160 coactivator NR boxes to ER alpha and ER beta.
- To compare the binding affinities of various NR boxes to ER subtypes in the presence of different ligands.
Main Methods:
- Utilized a coactivator interaction assay based on time-resolved fluorescence technology.
- Assessed the binding of 10 NR boxes from SRC-1, -2, and -3 to ER alpha and ER beta.
- Tested ligands included 17beta-estradiol (E2), 4-OH tamoxifen, LY 117018, and ICI 182780.
Main Results:
- SERMs did not enhance NR box binding to ER alpha or ER beta; instead, they acted as potent antagonists, decreasing estradiol-dependent NR box binding.
- All tested SERMs exhibited inverse agonism, dose-dependently reducing hormone-independent NR box binding to ER beta.
- A clear receptor-specific preferential binding pattern was observed between E2-bound ER alpha/beta and various natural NR boxes.
Conclusions:
- The SERMs examined function as antagonists of ER alpha and ER beta NR box binding, failing to increase coactivator recruitment.
- SERMs demonstrate inverse agonism, reducing basal coactivator binding to ER beta.
- Estrogen receptor subtypes exhibit distinct preferences for interacting with different NR boxes.