ABSTRACT:
Haffez2017 - RAR interaction with synthetic
analogues
This model is described in the article:
The molecular basis of the
interactions between synthetic retinoic acid analogues and the
retinoic acid receptors
Hesham Haffez, David R. Chisholm,
Roy Valentine, Ehmke Pohl, Christopher Redfern and Andrew
Whiting
MedChemComm
Abstract:
All-trans-retinoic acid (ATRA) and its synthetic analogues
EC23 and EC19 direct cellular differentiation by interacting as
ligands for the retinoic acid receptor (RARα,
β and γ) family of nuclear receptor
proteins. To date, a number of crystal structures of natural
and synthetic ligands complexed to their target proteins have
been solved, providing molecular level snap-shots of ligand
binding. However, a deeper understanding of receptor and ligand
flexibility and conformational freedom is required to develop
stable and effective ATRA analogues for clinical use.
Therefore, we have used molecular modelling techniques to
define RAR interactions with ATRA and two synthetic analogues,
EC19 and EC23, and compared their predicted biochemical
activities to experimental measurements of relative ligand
affinity and recruitment of coactivator proteins. A
comprehensive molecular docking approach that explored the
conformational space of the ligands indicated that ATRA is able
to bind the three RAR proteins in a number of conformations
with one extended structure being favoured. In contrast the
biologically-distinct isomer, 9-cis-retinoic acid (9CRA),
showed significantly less conformational flexibility in the RAR
binding pockets. These findings were used to inform docking
studies of the synthetic retinoids EC23 and EC19, and their
respective methyl esters. EC23 was found to be an excellent
mimic for ATRA, and occupied similar binding modes to ATRA in
all three target RAR proteins. In comparison, EC19 exhibited an
alternative binding mode which reduces the strength of key
polar interactions in RARα/γ but is
well-suited to the larger RARβ binding pocket. In
contrast, docking of the corresponding esters revealed the loss
of key polar interactions which may explain the much reduced
biological activity. Our computational results were
complemented using an in vitro binding assay based on FRET
measurements, which showed that EC23 was a strongly binding,
pan-agonist of the RARs, while EC19 exhibited specificity for
RARβ, as predicted by the docking studies. These
findings can account for the distinct behaviour of EC23 and
EC19 in cellular differentiation assays, and additionally, the
methods described herein can be further applied to the
understanding of the molecular basis for the selectivity of
different retinoids to RARα, β and
γ.
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Hesham Haffez 
