Heterodimerization between RXR and orphan receptors

Heterodimerization between RXR and orphan
receptors
A large number of orphan receptors dimerize with
RXR and allow ligand binding to RXR, thereby switching
RXR from a silent to an active heterodimerization
partner. The ability of orphan receptors to allow RXR
to bind and be activated by ligand (9-cis retinoic acid)
increases the diversity of 9-cis retinoic acid signalling
through RXR. The many different orphan receptors
confer DNA-specifi c binding of the heterodimer to
a variety of response elements, including DR1, DR4
and DR5 types.
COUP-TF–RXR
The COUP-TFs inhibit retinoid hormone-mediated
transcription on several response elements that are
activated by RAR–RXR heterodimers or RXR homodimers
(Tran et al., 1992). Possible mechanisms
of this repressive effect are discussed in Section 6.8.7.
DR1 response elements have been the most extensively
studied. COUP-TFs are expressed only in selected
tissues or cell types, such as liver and enterocytes.
In the developing embryo, they are expressed in a
spatially and temporally regulated manner. Furthermore,
COUP-TFs antagonize only certain subsets of
retinoid receptors; for instance, they do not repress
RARβ2-mediated transcription (Tran et al., 1992).
Overall, the ability of COUP-TFs to block transcription
from certain RAREs and RXREs provides an important
mechanism of restricting the broad spectrum
of responses to retinoid hormones.
A member of the apolipoprotein (apo) gene family,
apoAI, is an example of a gene containing a DR1
RXRE. The encoded protein, apoAI, is the major
protein constituent of plasma HDL (high-density
lipoprotein), which is involved in the transport of
cholesterol and other lipids in the plasma. Widom et
al. (1992) studied the effects of ARP-1 (apoAI regulatory
protein, a homologue of COUP-TFII) on the
responsiveness of the apoAI gene in human liver cells
to RXRα and its ligand 9-cis retinoic acid. ARP-1 and
RXRα were found to bind individually to the RXRE
with similar affi nity. These receptors also bound to
the same site as heterodimers with an affi nity approximately
10 times greater than that of either ARP-
1 or RXRα alone. Repression of gene transcription by
ARP-1 was overcome by over-expression of RXRα in
the presence of 9-cis retinoic acid. These observations
indicate that regulation of apoAI gene expression is
controlled by the balance of the intracellular levels of
ARP-1 and liganded RXRα. High plasma HDL concentrations
are believed to protect against premature
atherosclerosis. This demonstration that retinoic
acid transactivates apoAI gene expression raises the
possibility that vitamin A plays an important role in
atherosclerosis prevention.
Retinoic acid (either all-trans or 9-cis) induces
high levels of inhibitory COUP-TFI and II during
differentiation of P19 EC cells (Jonk et al., 1994), thus
creating a negative feedback system involved in the
down-regulation of retinoic acid-activated genes.
LXR–RXR
The orphan receptor LXR in heterodimeric complex
with RXR binds to a specifi c DR-4-type response element,
the LXRE. The complex is activated by the RXR
ligand 9-cis retinoic acid, there being no observable
repression in the absence of ligand. Ligand binding
induces a conformational change in the entire
heterodimer complex, exposing the AF-2 of LXR,
and allowing transcription through interaction with
coactivator proteins (Willy & Mangelsdorf, 1997).
OR1–RXR
The orphan receptor OR1 is also known as ubiquitous
receptor (UR), RIP15 and NER. Heterodimeric inter-
Vitamin A: retinoids and carotenoids 169
action between OR1 and RXRα on a DR4 response
element activates transcription in both the absence
and presence of 9-cis retinoic acid. The activation
potential is mainly dependent on the AF-2 of OR1,
which is unmasked by a dimerization-induced conformational
change (Wiebel & Gustafsson, 1997). The
DR4-type response element is shared by the TR–RXR
heterodimer and therefore OR1 has the potential to
modulate the thyroid hormone signal pathway.
RLD-1–RXR
The particular response element recognized by the
RLD-1–RXR heterodimer is a unique DR4, which
is also capable of binding RAR–RXR and TR–RXR
heterodimers. Retinoids and thyroid hormone can
induce transcription through the RLD-1 response
element in the presence of RAR–RXR or TR–RXR
heterodimers, but only in the absence of TRs or RARs
can the RLD-1–RXR heterodimer constitutively mediate
transcription. In contrast to OR1, the presence
of 9-cis retinoic acid inhibits the activity of RLD-1–
RXR (Apfel et al., 1994). These observations suggest a
highly specifi c role for RLD-1 within the network of
gene regulation by the TR/RAR subfamily.
NGFI-B–RXR
The orphan receptors NGFI-B (also called nur77) and
the closely related nurr-1 can initiate transcription
on binding both as monomers and heterodimers to
DNA. NGFI-B is found in a variety of tissues, including
the muscle, brain, heart, ovary and testis, while
nurr-1 expression is restricted to the brain. NGFI-B
binds as a monomer to a specifi c response element
that contains a single extended half-site, 5´-AAAGGTCA-
3´. The receptor’s zinc fi ngers interact with
the consensus half-site (AGGTCA), while an unusual
amino acid motif (the A box) interacts with the two A/
T base pairs immediately 5´ of the consensus half-site
(Wilson et al., 1993). NGFI-B and nurr-1, as monomers,
are constitutively active, but in heterodimeric
complex with RXR they require 9-cis retinoic acid in
order to activate transcription (Perlmann & Jansson,
1995). The heterodimers bind selectively to DR-5 response
elements, with RXR occupying the 5´ half-site.
NGFI-B and nurr-1 are encoded by primary response
genes, which are ultimately induced by various growth
factors. From a physiological point of view, the heterodimeric
interaction of these receptors with RXR
indicates a novel mechanism for convergence between
retinoid and growth factor signalling pathways.
MB67–RXR
Another orphan receptor, MB67, which occurs predominantly
in liver, can heterodimerize with RXR and
activate transcription from DR-5 response elements
in the absence of a ligand. Baes et al. (1994) demonstrated
this constitutive activity in RAREs that control
expression of the RARβ2 and alcohol dehydrogenase
3 genes. Activation of the βRARE by the MB67–RXR
heterodimer is weaker than that conferred by RAR–
RXR heterodimers, possibly because MB67 essentially
lacks the A/B domain, which contains amino acid
sequences necessary for full transcriptional activation.
The lack of dependence upon a retinoid ligand
suggests that MB67 acts to stimulate expression of
retinoid-responsive genes (i.e. those containing a
DR-5 element) when retinoic acid levels are low. In
this respect, MB67 activation of the βRARE could
contribute to the basal level of expression of RARβ2
mRNA observed in the livers of retinol-defi cient rats
(Haq et al., 1991).
FXR–RXR
Some nuclear receptors, in heterodimeric association
with RXR, are activated by non-hormonal intracellular
metabolites, thereby transducing metabolic
cues into genomic responses. Although referred to as
orphan receptors, the metabolites may possibly act as
their ligands. One such receptor, FXR, is activated by
farnesol and related metabolites, the transcriptional
response being determined by the FXR component
of the FXR–RXR complex (Forman et al., 1995).
Farnesyl pyrophosphate is a key intermediate in the
mevalonate pathway leading to the biosynthesis of
cholesterol.
Transcriptional synergy between RXR and PPAR
The peroxisome proliferator-activated receptor
(PPAR), of which there are three known subtypes (α, β
and γ), regulates expression of genes involved in modulation
of lipid homeostasis, including the metabolism
of long-chain fatty acids and conversion of cholesterol
to bile salts. PPAR functions as a heterodimer with
RXR on peroxisome proliferator response elements
(PPREs), which are exclusively of the DR1 type. RXR
occupies the 3´ half-site of the DR1 element, which is
170 Vitamins: their role in the human body
consistent with the idea that replacement of RAR by
PPAR over the 5´ half-site switches RXR from a silent
heterodimeric partner to a ligand-dependent activator.
The PPAR–RXR heterodimer permits dual activation
by the RXR ligand (9-cis retinoic acid) and PPARspecifi
c ligands (peroxisome proliferators) in a unique
synergistic fashion (Schulman et al., 1998). Peroxisome
proliferators increase both the size and number
of peroxisomes. The enzyme acyl coenzyme A oxidase
is widely used as a marker of peroxisome proliferator
action and is the rate-limiting step in peroxisomal
fatty acid β-oxidation. Certain polyunsaturated fatty
acids are potent activators of PPAR and thereby exert
positive feedback regulation of fatty acid β-oxidation.
Thus, the PPAR–RXR heterodimer represents a single
bifunctional transcription factor that integrates peroxisome
proliferator and retinoid signalling pathways.
A physiological role for 9-cis retinoic acid as a hypolipidaemic
agent is therefore evident.