Hormonal regulation of gene expression Flashcards
Hydrophilic vs hydrophobic hormones
Hydrophilic Hormones: interact at the plasma membrane,
and have their effect on gene expression through second
messengers
* Hydrophobic Hormones: diffuse through the membrane,
bind to their receptor and interact with their HRE on the
gene of interest
Describe classic steroid signalling
In the classic model of steroid receptor signalling, steroids
enter cells through the plasma membrane
- Bind to their receptors localized in the cytoplasm
(androgen receptor (AR) and glucocorticoid receptor (GR))
or in the nucleus (oestrogen receptor-α (ER)
HSP- heat shock protein AP-1 Activator protein 1
SRE- steroid response element SP1- Steroid protein 1 - Steroid-bound receptors
translocate to the nucleus
(cytoplasm receptors) and
bind DNA to regulate gene
transcription - This drives changes in
mRNA expression, protein
expression and cell biology
What is nuclear receptor isolation
The isolation of the first complete steroid receptor cDNAs,
the glucocorticoid and oestrogen receptors, was
transformative (Hollenberg et al., 1985)
* Conserved evolutionary structure, allowed structural and
functional features of the nuclear receptor superfamily
* Transcriptional regulation by hormone-receptor complexes
was shown to be a fundamental process
* Cloning of the first steroid receptors revealed that dozens
of other evolutionarily related proteins exist
* ‘‘orphan’’ receptors were shown to be conserved
throughout metazoan evolution, although it should be
noted that nuclear receptors are absent in protozoans,
fungi, and plants.
What are orphan receptors
That there were orphan receptors immediately suggested
the existence of a host of previously unknown signalling
pathways regulated by many undiscovered ligands
- Co-transfection assays provided a quantitative and highly
efficient tool for screening, and as it was extremely
sensitive to small-molecule ligands, it became the
mainstay of ‘‘deorphaning’’ efforts - Early results showed that the DNA- and ligand-binding
domains of the receptors function autonomously - While, isolation of thyroid hormone in 1914 required 3.5
tons of bovine thyroid gland, molecular approaches have
led to a massive increase in new signalling systems.
What is the retinoid receptor
Applying the reverse endocrinology approach to the
retinoid X receptor (RXR) led to the identification of the
first endogenous ligand for an orphan nuclear receptor (9-
cis retinoic acid, a metabolite of vitamin A), establishing
RXR as the founding member of the orphan class
(Mangelsdorf et al., 1990)
* The discovery of RXR and its ligand resulted in the
genesis of two key concepts in the nuclear receptor field:
1. Novel signalling pathway, proof of concept that could
link other orphan receptors to specific ligands
2. The discovery of RXR heterodimerization defining a
novel feature of multiple interacting signalling
pathways.
What are nuclear receptors
Approximately 50 human nuclear receptors
* Similar protein structure, especially within their DNAbinding domains (DBDs) and the ligand-binding
domains (LBDs)
* Essential functions as highly conserved
* NRs function, in part, through ligand binding and
subsequent activation of their transcriptional activity
(i.e. their ability to regulate target gene expression).
What is nuclear localisation
The NRs, like all cellular proteins, are synthesized on
ribosomes that reside outside the nucleus
* Import of the NRs into the nucleus requires the nuclear
localization signal (NLS), which is located near the border
of the C and D domains
* As a result of their NLSs, most of the NRs reside in the
nucleus, with or without their ligands
* Hormone binding to the GR induces a conformational
change that results in dissociation of the chaperone
complex, allowing the hormone-activated GR to
translocate to the nucleus
by means of its NLS.
What are hormone response elements
HRE DNA sequence to bind to a specific hormone
receptor complex and regulate transcription
* Commonly a pair of inverted repeats separated by three
nucleotides (as receptors binds as a dimers)
* The activated steroid receptor is the transcription factor
binding the HRE
* A gene may have many different response elements,
allowing complex control to be exerted over the level and
rate of transcription
Describe the NR family
Family III NRs are the classic steroid hormone receptors:
glucocorticoid receptor (GR), mineralocorticoid receptor
(MR), progesterone receptor (PR), androgen receptor
(AR), and oestrogen receptor (ER)
* In the absence of ligand, these NRs are in non-functional
complexes with heat-shock proteins thus transcriptionally
inactive
* Ligand activation, the nuclear receptors bind DNA as
homodimers to response elements configured as
palindromes composed of two nucleotide sequences
separated by 3bp’s.
- Family II receptors comprise ‘orphan nuclear receptors’
(oNRs) because they lack known physiological ligands,
except RXR.
Family I, non-steroid receptors (i.e., RAR, VDR, and TR),
bind preferentially to response elements composed of two
half-sites arranged as tandem repeats (Koenig et al.,
1987)
* These receptors form heterodimers with the retinoid X
receptor (RXR), even in the absence of ligand, and exert a
repressive silencing effect on basal promoter activity that
is reversed upon ligand binding
* Specificity of receptor/DNA interactions is encoded by the
spacing between the two half-sites repeats
What are RXR heterodimers
Receptor heterodimer can be activated by either the RXR
ligand or the partner receptor ligand
* Permissive heterodimers are those that can be
activated by ligands of either RXR or its partner
* Non-permissive heterodimers are those that can only
be activated by the partner’s ligand while RXR is silent
* Simultaneous presence of both RXR and partner receptor
ligands results in a larger response compared to binding of
only a single receptor ligand
- An important regulatory feature of
permissive receptor partners are
Peroxisome Proliferator-Activated
Receptors (PPARs).
What is permissive/non-permissive
From a pharmacologic perspective, a number of potent
synthetic RXR ligands (called ‘‘rexinoids’’) have also been
described, because of their ability to simultaneously
activate several heterodimers, such panagonists may have
utility against multiple therapeutic targets
* Small changes in the ligand partner can be amplified via
RXR heterodimerization
* By silencing RXR activity and responding only to their own
ligands, non-permissive receptors permit transcriptional
regulation that is directly proportional to the level of the
hormone, thereby meeting the requirements of endocrine
physiology.
What are cytoplasmic nuclear receptors
Since the 1950’s steroid hormones responses that are too
rapid (measured in seconds rather than minutes) to be
mediated through transcriptional regulation have been
described in the scientific literature
* Steroid receptors can also be found at the plasma
membrane
* Although most steroid receptors are located in the
nucleus, ~ 5% of steroid receptors localizes to the plasma
membrane, including classic steroid receptors (ERs, PR,
and AR).
What are selective oestrogen receptor modulators
SERMs block the effects of oestrogen in the breast tissue
* SERM/oestrogen receptor binds leaving no room for
oestrogen
* No oestrogen binding – no signal to grow and multiply
* Cells in other tissues i.e. bones, uterus, also have
oestrogen receptors
* But each oestrogen receptor has a slightly different
structure, depending on the cell expressing it
* So breast cell oestrogen receptors are different from bone
cell oestrogen receptors and both of those oestrogen
receptors are different from uterine oestrogen receptors
SERMs are “selective” – this means that a SERM that
blocks oestrogen’s action in breast cells can activate
oestrogen’s action in other cells, such as bone, liver, and
uterine cells
Three key SERMs:
* tamoxifen
* raloxifene
* toremifene
Tamoxifen is the oldest, most well-known, and mostprescribed SERM.
