Dimerization of Nuclear Receptors - Methods in Cell Biology - page 21

Abstract
Multicellular organisms require specific intercellular communication to properly or-
ganize the complex body plan during embryogenesis and maintain its properties and
functions during the entire life. While growth factors, neurotransmitters, and peptide
hormones bind to membrane receptors, thereby inducing the activity of intracellular
kinase cascades or the JAK-STAT signaling pathways, other small signaling com-
pounds such as steroid hormones, certain vitamins, and metabolic intermediates en-
ter, or are generated, within the target cells and bind to members of a large family of
nuclear receptors (NRs). NRs are ligand-inducible transcription factors that control a
plethora of biological phenomena, thus orchestrating complex events like develop-
ment, organ homeostasis, immune function, and reproduction. NR-NR interactions
are of major importance in these regulatory processes, as NRs regulate their target
genes by binding to cognate DNA response elements essentially as homo- or hetero-
dimers. A number of structural and functional studies have provided significant in-
sights as to how combinatorial NRs rely on protein-protein contacts that discriminate
geometric features of their DNA response elements, thereby allowing both binding
site diversity and physiological specificity. Here, we will review our current under-
standing of NR-NR interactions and provide protocols for a number of experimental
approaches that are useful for their study.
INTRODUCTION
Nuclear receptors (NRs) are members of a large superfamily of evolutionarily related
transcription factors that regulate genetic programs involved in a broad spectrum of
physiological phenomena. The human NR family is classified by sequence into six
evolutionary groups of unequal size. The reader is referred to several databases
providing comprehensive annotation of the literature on the pharmacology of the
48 human NR gene products, together with relevant information on their structure
and function (IUPHAR-DB ( http://www.iuphar-db.org/ ) , Nuclear Receptor Signal-
ing Atlas (NURSA, http://www.nursa.org/ ), and NureXbase ( http://nurexbase.prabi.
fr ) ). Among NRs, ligands have been identified for only 24 family members. These
receptors are ligand-dependent transcriptional factors that respond directly to a large
variety of hormonal and metabolic substances that are hydrophobic, lipid-soluble,
and of small size (e.g., retinoic acid or estradiol). The other class of NRs is the group
of so-called orphan receptors, for which regulatory ligands are still unknown or may
not exist (true orphans) or for which candidates have only recently been identified
(adopted orphans). In both groups, NRs can exist as different receptor subtypes that
originate from different genes such as the retinoic acid receptors a , b , and g (RAR a
(NR1B1), RAR b (NR1B2), and RAR g (NR1B3)), which can exist themselves as sev-
eral isoforms originating from alternative splicing and differential promoter usage
(e.g., RAR b 1-RAR b 4) ( Germain, Staels, Dacquet, Spedding, & Laudet, 2006;
Gronemeyer, Gustafsson, & Laudet, 2004 ).