Nuclear hormone receptors as regulators of protein synthesis

6.8.1 Steroid hormones
Steroid hormones include the male sex hormones
(collectively called androgens), the female sex hormones
(oestrogens and progestins) and the hormones
secreted by the cortex of the adrenal glands (the corticosteroids).
The hormones circulate in the blood
both free and in combination with carrier proteins.
The free steroids diffuse in and out of cells but are retained
only in target cells through binding to specifi c
receptor proteins, which are constitutively present. It
was initially thought that, in the absence of hormone,
the receptors were located in the cytoplasm and, after
binding hormone, were rapidly translocated into the
nucleus. It is now known that the unoccupied receptors
are actively transported from the cytoplasm to
the nucleus, but then diffuse back into the cytoplasm.
This constant movement between nucleus and cytoplasm
is known as nucleocytoplasmic shuttling.
The binding of steroid hormones to their intracellular
receptors causes the receptors to bind as homodimers
to specifi c hormone response elements on
the DNA. These elements can be regarded as enhancers
and the receptors as ligand-inducible activator
proteins. In the absence of ligand, steroid receptors
exist in the form of large complexes with various heat
shock proteins, and it is logical to suppose that these
proteins sterically hinder DNA binding. The binding
of ligand induces a conformational change in the
steroid receptor, which allows it to dissociate from the
heat shock proteins and also to pair up with a similar
receptor to form a homodimer. Ligand activation
and dimeric DNA binding of the receptor stimulates
regulated transcription, after which the hormone
and receptor dissociate from one another and from
the DNA. The hormone is metabolized and excreted
from the cell, while the free receptor is recycled to
await binding by another hormone molecule.
The genetic control of protein synthesis and its regulation by nuclear hormone receptors 127
6.8.2 Classifi cation of nuclear hormone
receptors into types I and II
Vitamin A (as retinoic acid) and vitamin D (as 1,25-
dihydroxyvitamin D3) also induce the synthesis of
specifi c proteins through receptor-mediated regulation
of gene transcription. Receptors for vitamins A
and D share certain structural and functional properties
with steroid hormone receptors, and all may be
considered as members of a nuclear hormone receptor
superfamily. The superfamily can be conveniently
divided into two types based on functionally distinct
properties. Type I comprise receptors (R) for the
classic steroid hormones such as (o)estrogen (ER),
progesterone (PR), androgen (AR), glucocorticoid
(GR) and mineralocorticoid (MR). Type II comprise
receptors for thyroid hormone (TR), retinoic acid
(RAR and RXR), vitamin D3 (VDR) and prostanoids
(PPAR). Also included in the superfamily are orphan
receptors whose cognate ligands (if any exist) are as
yet unidentifi ed.
There are several important functional differences
between the type I and type II receptors. Type I receptors
are unable to bind DNA in the absence of ligand,
whereas type II receptors are able to do so. Type I
receptors bind to their DNA response elements as
homodimers, whereas type II receptors are able to
form stable heterodimers with the retinoid X receptor
(RXR) in vitro. It is this constitutive dimerization that
facilitates DNA binding in the absence of ligand. RAR
and TR possess a silencing function, the CoR (co-repressor)
box, which, in the absence of ligand, interacts
with a co-repressor protein. Type I receptors do not
have a CoR box but, unlike type II receptors, associate
with heat shock proteins in the absence of ligand.
6.8.3 Hormone response elements
The hormone response elements on the DNA are
binding sites for nuclear receptors and act as enhancers
to increase the transcriptional activity of an adjacent
promoter. The response elements are composed
of two half-sites or core recognition motifs, each with
six base pairs, separated by base pair spacers. The dyad
symmetry of these elements permits receptor–ligand
complexes to bind as dimers, with each monomer
recognizing one of the half-sites.
Three main features determine the specifi city of
receptor binding to hormone response elements: (1)
the precise nucleotide sequence of the half-site, (2)
the orientation of half-sites with respect to each other
and (3) the spacing between half-sites. There are only
two consensus half-site sequences. The sequence
5´-AGAACA is recognized by the GR, MR, AR and
PR; the sequence 5´-AGGTCA is recognized by the
ER, RAR, VDR and TR, as well as virtually all other
known members of the nuclear receptor superfamily.
The orientation of half-sites can be as inverted repeats,
everted repeats or direct repeats, as illustrated
in Fig. 6.16. Table 6.3 shows the features of response elements recognized by the principal nuclear receptors.
Those elements recognized by type I steroid
receptors are designated IR3 elements because the
half-sites are arranged exclusively as inverted repeats
separated by three nucleotides. Specifi city between
GR, MR and PR is determined by differences in the
individual nucleotides of the spacer.
In contrast to the strict palindromic organization
of steroid hormone response elements, the highest affi
nity binding sites for type II receptors are arranged
as direct repeats, with AGGTCA as the consensus core
recognition motif. The preferred spacing between
the direct repeats (DR) is three nucleotides (DR3)
for VDR/RXR, four nucleotides (DR4) for TR/RXR,
and fi ve nucleotides (DR5) for RAR/RXR. RAR/RXR
is also capable of binding to DR1 and DR2 elements,
but with somewhat lower affi nity than to the DR5 element.
DR1 is also a response element for an RXR homodimer
and for a PPAR/RXR heterodimer. The orphan
receptor COUP-TF in dimeric partnership with
RXR recognizes DRs with 0 to 5 spacers, although the
binding is most avid with DR1 elements.