title: Structural insights into selective agonist actions of tamoxifen on human estrogen receptor alpha.
abstract: Tamoxifen-an anti-estrogenic ligand in breast tissues used as a first-line treatment in estrogen receptor (ER)-positive breast cancers-is associated with the development of resistance followed by resumption of tumor growth in about 30 % of cases. Whether tamoxifen assists in proliferation in such cases or whether any ligand-independent pathway to transcription exists is not fully understood; also, no ERα mutants have been detected so far that could lead to tamoxifen resistance. Using in silico conformational analysis of the ERα ligand binding domain (LBD), in the absence and presence of selective agonist (diethylstilbestrol; DES), antagonist (Faslodex; ICI), and selective estrogen receptor modulator (SERM; 4-hydroxy tamoxifen; 4-OHT) ligands, we have elucidated ligand-responsive structural modulations of the ERα-LBD dimer in its agonist and antagonist complexes to address the issue of "tamoxifen resistance". DES and ICI were found to stabilize the dimer in their agonist and antagonist conformations, respectively. The ERα-LBD dimer without the presence of any bound ligand also led to a stable structure in agonist conformation. However, binding of 4-OHT to the antagonist structure led to a flexible conformation allowing the protein to visit conformations populated by agonists as was evident from principal component analysis and radius of gyration plots. Further, the relaxed conformations of the 4-OHT bound protein exhibited a diminished size of the co-repressor binding pocket in the LBD, thus signaling a partial blockage of the co-repressor binding motif. Thus, the ability of 4-OHT-bound ERα-LBD to assume flexible conformations visited by agonists and reduced co-repressor binding surface at the LBD provide crucial structural insights into tamoxifen-resistance that complement our existing understanding.
authors: Laboratory of Computational Biophysics & Bioengineering, Department of Physics, Tougaloo College, Tougaloo, MS 39174, USA.
authors: Sandipan
authors: S
authors: Chakraborty
authors: Pradip Kumar
authors: PK
authors: Biswas
mesh_terms: D016000:Cluster Analysis; D003198:Computer Simulation; D047628:Estrogen Receptor alpha / Q000819:agonists* / Q000037:antagonists & inhibitors / Q000737:chemistry*; D006801:Humans; D008024:Ligands; D062105:Molecular Docking Simulation; D056004:Molecular Dynamics Simulation; D025341:Principal Component Analysis; D055503:Protein Multimerization; D055550:Protein Stability; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D012097:Repressor Proteins / Q000737:chemistry; D013629:Tamoxifen / Q000737:chemistry* / Q000494:pharmacology*; D013816:Thermodynamics
chemical_list: C506487:ESR1 protein, human; D047628:Estrogen Receptor alpha; D008024:Ligands; D012097:Repressor Proteins; D013629:Tamoxifen