Structural insights into Resveratrol's antagonist and partial agonist actions on estrogen receptor alpha

Abstract

Background: Resveratrol, a naturally occurring stilbene, has been categorized as a phytoestrogen due to its ability to compete with natural estrogens for binding to estrogen receptor alpha (ERα) and modulate the biological responses exerted by the receptor. Biological effects of resveratrol (RES) on estrogen receptor alpha (ERα) remain highly controversial, since both estrogenic and anti-estrogenic properties were observed.

Results: Here, we provide insight into the structural basis of the agonist/antagonist effects of RES on ERα ligand binding domain (LBD). Using atomistic simulation, we found that RES bound ERα monomer in antagonist conformation, where Helix 12 moves away from the ligand pocket and orients into the co-activator binding groove of LBD, is more stable than RES bound ERα in agonist conformation, where Helix 12 lays over the ligand binding pocket. Upon dimerization, the agonistic conformation of RES-ERα dimer becomes more stable compared to the corresponding monomer but still remains less stable compared to the corresponding dimer in antagonist conformation. Interestingly, while the binding pocket and the binding contacts of RES to ERα are similar to those of pure agonist diethylstilbestrol (DES), the binding energy is much less and the hydrogen bonding contacts also differ providing clues for the partial agonistic character of RES on ERα.

Conclusions: Our Molecular Dynamics simulation of RES-ERα structures with agonist and antagonist orientations of Helix 12 suggests RES action is more similar to Selective Estrogen Receptor Modulator (SERM) opening up the importance of cellular environment and active roles of co-regulator proteins in a given system. Our study reveals that potential co-activators must compete with the Helix 12 and displace it away from the activator binding groove to enhance the agonistic activity.

Scheme 1

Chemical structures of four ERα ligands used in this study: Diethylstilbestrol, Resveratrol, 4-Hydroxytamoxifen, ICI 182,780.

Figure 1

Variation of dynamic parameters of ERα bound to different ligands obtained from MD stimulation. (A) Variations in C_α-RMSD of ERα monomer with simulation time. RES-ER_antgonist, DES-ER and ICI-ER have highly stable complexes during simulation (cyan, black and orange), RES-ER_agonist and 4OHT-ER show high fluctuations (green, red). (B) RMSF profile of RES-ERα monomer. Black and red lines represent RES bound ERα in agonist and antagonist conformation, respectively. The peak in fluctuation corresponds to Helices 8 & 9 (residues 150 to 166).

Figure 2

Secondary structure profiles with simulation time. (A) RES-ERαagonist complex; (B) RES-ERαantagonist complex. (C) Variation of structural content of ERα bound with RES during simulation. D: Variations of percentages of helical structures in RES-ERα complex with simulation time.

Figure 3

RMSD matrices of ERα bound with different ligands computed from MD trajectory. DES-ERα, RES-ERαantagonist and ICI-ERα display highly stable initial conformation of the protein. On the contrary, RES-ERαagonist complex shows similarity with 4OHT-ERα where the initial structure changes appreciably during the simulation. RMSD matrix has been computed using trajectory analysis tools available within GROMACS packages by comparing the root mean square distances of each structure in the trajectory with respect to each other structure and generate a bi-dimentional matrix.

Figure 4

Structures of RES-ERα complexes obtained from MD simulation. (A) The structures of RES-ERαagonist (green) and RES-ERαantagonist (red) are overlapped. Using VMD, the ERα is shown in New Cartoon representation, and RES is shown in VDW mode. The side chains of some of the residues whose conformations are dramatically different between the complexes are shown: Helix12 and linker region of Helices 8 and 9. (B) &(C) Details of the hydrogen bonding contacts between DES and RES with ERα agonist conformation, respectively. RES is bound within the same ERα pocket that recognizes DES and 4OHT [30,32,33]. The hydrogen-bonding interactions with the different residues are shown.

Figure 5

Variations of RMSD (A) and radius of gyration, R_g (B) of RES-ERα dimer complexes with simulation time.Black and red lines represent agonist and antagonist conformation of ERα, respectively.

Figure 6

Comparison of RES-ERα dimer characteristics. (A)Left and right figures represent RES-ERαagonist and ERαantagonist conformation of dimer, respectively. (B) Variations in Coulomb and LJ interaction energies between the two monomers of ERα bound with RES during the last 7 ns of simulation. Black and red colors represents Coulomb and LJ interaction between two monomer in agonist conformation, respectively, and gray and cyan colors represents Coulomb and LJ interaction between two monomers in antagonist conformation, respectively. (C) Variations in the number of hydrogen bonds between two monomers of ERα dimer bound with RES. Black and red line represents agonist and antagonist conformation of ERα, respectively.