Nuclear Receptors, Molecular Mechanisms Of Flashcards
What kind of dimers do nuclear receptors function as?
Homo- or heterodimers (similar to ECRs)
Define: homodimer
A dimer made up of 2 identical units (macromolecules) i.e. 2 of the same receptor
Define: heterodimer
A dimer made up of 2 similar but not identical units (macromolecules) i.e. 2 different types of receptor
Define: dimer
A macromolecular complex formed by 2, usually non-covalently bound, macromolecules. = a quaternary structure of a protein = now active
How are all nuclear receptors structurally related?
They all have 3 major domains
How many receptors are there in the nuclear receptor superfamily?
Genome sequencing predicts only 48 receptors
What is an orphan receptor?
The receptor is yet to have its agonist identified
Up to half of the nuclear receptor superfamily are termed orphan receptors
What can nuclear receptors also been known as?
Ligand-activated gene regulatory proteins or transcription factors
Where are nuclear receptors found in the cell?
In the cytosol or the nucleus, not associated with the lipid membrane
How many families of nuclear receptor are there?
6
How are nuclear receptors assigned a family?
Based on sequence
Name the main 3 families of nuclear receptor
1) Thyroid Receptor-like 2) Retinoid X Receptor-like 3) Oestrogen Receptor-like
How many terminals does a nuclear receptor have?
2 - N-terminal and C-terminal
What is the function of the N-terminal/domain?
Binds the agonist/ligand
What is the function of the middle domain?
DNA binding
What does the C-terminal domain contain?
Transcription-activating domain
What does the transcription-activating domain do?
The proteins will bind other transcription factors i.e. sit on top of DNA and promote or suppress transcription of DNA
Name 4 activators of nuclear receptors.
1) Steroid hormones 2) Thyroid hormones 3) Fatty acids and prostaglandins 4) The retinoids
Name 3 examples of steroid hormones.
1) Glucocorticoids e.g. cortisol 2) The steroid sex hormones (estradiol and testosterone) 3) Vitamin D
What are steroid hormones made from?
Cholesterol - a common precursor The metabolism of cholesterol = these hormones
What are thyroid hormones made from?
Tyrosine - a common precursor (an amino acid)
Name the active forms of thyroid hormones.
Thyroxine (T4) and triiodothyronine (T3)
What are fatty acids and prostaglandins derived from?
Lipids - they are the oxidation products
Name 3 examples of fatty acids and prostaglandins.
1) Prostacyclin 2) Lysophosphatidic acid 3) Leukotriene
What are retinoids synthesised from?
Vitamin A (retinol)
Name 2 examples of retinoids.
1) Retinoic acid (vitamin A acid) 2) All trans-retinoic acid
Why are all these activators of nuclear receptors poorly soluble in water?
They are small hydrophobic molecules
Specific carrier proteins bind to the activators of NRs in the blood, why?
To increase the activators’ solubility (as they must travel in the bloodstream)
What process occurs before the activators enter the cell, to do with the carrier proteins?
Dissociation
What are the main binding proteins (for transport) for the thyroid hormones?
1) Thyroxine-binding globulin 2) Transthyretin 3) Albumin
What is the major binding protein (for transport) for sex hormones?
Sex hormone-binding globulin
Which hormones does sex hormone-binding globulin bind to?
Androgens and oestrogens
What is the major steroid-binding protein called? (for transport)
Transcortin
Which steroid hormones does transcortin bind to?
Progesterone, cortisol and other corticosteroids
Explain the stages of the binding proteins transporting the hormones/activators
1) Hormones are secreted into the bloodstream from e.g. the thyroid 2) The binding protein in the bloodstream binds to the activator and transports it to the target tissue 3) The binding protein dissociates and the activator enters the tissue
Are agonists generally hydrophobic or hydrophilic?
Hydrophobic - therefore they can freely diffuse across the plasma membrane
Where do ligands bind on the nuclear receptor?
Agonists bind to the ligand-binding domain (and act as a molecular switch)
What kind of change does the ligand-binding induce in the receptor?
A conformational change
What happens to the receptor when it undergoes a conformational change?
The receptor can dimerise - become active
Where does the nuclear receptor translocate to once dimerised?
The nucleus - where the receptor has it effects, usually through gene transcription
Explain the stages of the mechanism of action of nuclear receptors.
1) The agonist/ligand diffuses into the cell through the plasma membrane (after using the binding proteins) 2) The agonist/ligand binds to the cytosolic NR 3) The NR dimerises and translocates to the nucleus 4) The activated NR binds to the DNA in the nucleus and modifies gene expression (+ve or -ve effects), 5) This causes the secretion of molecules
What type of nuclear receptor is a target for therapy?
PPARs = peroxisome proliferator-activated receptors
How many subtypes of PPARs are there and what are they called?
3: α, β, Y All have different amino acid sequences and so therefore affect genes in different ways, all unique transcription factors
Which family of nuclear receptor do PPARs belong to?
Thyroid-like family
What is different about α and β PPARs?
α and β PPARs have distinct tissue distribution: - α = liver; regulate fatty acid oxidation - β = skeletal muscle; regulate fatty acid oxidation i.e. metabolism
Where is Y1 PPAR found?
It is almost ubiquitous (3 isoforms)
What kind of dimers are PPARs functionally active as?
Heterodimers
Which receptors do PPARs (Y1) pair up with to form heterodimers?
Retinoid X receptors and vitamin D receptors
What is the general role of PPARs (Y1) in the body?
General lipid sensors - allow body to adjust to changes in diet - change metabolism according to what is eaten
Name 4 processes that the PPARs (Y1) regulate.
1) Lipid homeostasis 2) Cellular differentiation 3) Cellular proliferation 4) The immune system
Name 1 drug therapy for PPARα
Fibrates = PPARα agonists
What are fibrates (drug therapy) used to treat?
Type II diabetes, atherosclerosis, obesity and hyperlipidaemia
How do fibrates work? (Drug therapy for PPAR alpha)
Decrease serum triglycerides, can be used with statins (type II diabetes)
Name 1 drug therapy for PPAR(Y).
Thiazolidinediones (glitazones) = PPAR(Y) agonist Pioglitazone = only drug available on the market, licensed for type II diabetes
Name another receptor (other than the PPAR) that is a target for drug therapy.
Glucocorticoid receptor
Name 2 natural glucocorticoids.
1) Cortisol 2) Corticosterone
Name 3 things that glucocorticoids are involved in
1) Inflammatory disorders of the gut 2) Rheumatoid arthritis 3) Autoimmune disorders
Why do glucocorticoid therapies have immunological effects?
Due to the upregulation of anti-inflammatory proteins e.g. lipocortins
What are lipocortins?
Anti-inflammatory proteins
What do lipocortins do?
Suppress the activity of phospholipase A2
What does phospholipase A2 do?
Phospholipase A2 contributes to inflammation by producing the key intermediate molecule, arachidonic acid
Name 1 drug aimed at the glucocorticoid receptor.
Dexamethasone - anti-inflammatory and immunosuppressant effects
What 2 genes are mammalian thyroid hormone receptors encoded by?
Alpha and beta
What are the 4 known thyroid hormone receptors called?
α1, α2, β1 and β2
What forms are thyroid hormone receptors active in?
As monomers, homodimers or as heterodimers with retinoid X receptors
What has been found to be the cause of a syndrome of thyroid hormone resistance?
Mutations in the receptor beta gene - abolishes ligand (thyroid hormone) binding to the receptor Therefore the body produces more thyroid hormone = excessive levels in the bloodstream
Mutations in the receptor β gene cause what behavioural disorder?
Attention-deficit disorder - thyroid hormones are important in brain development Mutant receptors act in a dominant negative manner = the receptor will still bind in some way and will interfere with the other normal receptors
High levels of thyroid hormone (T3 and T4) in the blood is characterised by what symptom?
A goiter = swelling in the neck resulting from the enlargement of the thyroid gland
Name a drug that acts on the thyroid receptor?
Levothyroxine = thyroid receptor agonist
What is Levothyroxine used to treat?
Hypothyroidism - makes the thyroid stop producing hormone
Define: hypothyroidism
Excessive levels of thyroid hormone in the blood due to mutant receptor beta gene which abolishes ligand binding to the thryoid receptor
How is it determined if someone has hypothyroidism?
Take a blood test, see if thyroxine levels (T3 and T4) are low
Name 3 symptoms of hypothyroidism.
1) insensitivity to cold 2) constipation 3) depression
Name 2 nuclear receptors that are targeted for cancer therapy.
1) Oestrogen receptors 2) Vitamin D receptors
What is the role of oestrogen receptors?
Oestrogen receptors are key regulators of cell growth and differentiation
How many types of oestrogen receptor are there and what are they called?
2: ERα and ERβ
What effect does oestrogen have on breast cancer?
Oestrogen promotes cancer growth (oestrogen positive cancers)
How do therapies for oestrogen aim to do?
Disrupt the oestrogen signalling pathway
Name a drug used to target oestrogen receptors.
Tamoxifen - acts as an oestrogen receptor ANTAGONIST in breast tissues Acts as an oestrogen receptor AGONIST in other tissues
Does Tamoxifen act as an oestrogen receptor agonist or antagonist?
Antagonist in breast tissue and agonist in other tissues = a selective oestrogen-receptor modulator
What determines the agonist/antagonist properties of Tamoxifen?
The ratio of co-activator:co-repressor proteins in tissues - different in breast tissue to other tissues
How does Tamoxifen have a biological effect?
Acts on the oestrogen receptors to disrupt the oestrogen signalling pathway = cancer therapy Agonist or antagonist Not an active molecule, has to be metabolised
In which tissue is the concentration of co-repressors high?
Breast tissue
What molecule has a very high concentration in the breast tissue?
Co-repressors
How does Tamoxifen act in the breast tissue and why?
Tamoxifen acts as an antagonist in the breast tissue due to the high concentration of co-repressor. As an antagonist, it shuts off gene transcription This stops the proliferation of the cancer
How does shutting off gene transcription target cancer?
It stops cell proliferation
In which tissue is the concentration of co-activators high?
All other tissues (i.e. not breast tissue)
What molecule has a very high concentration in all other tissues?
Co-activators
How does Tamoxifen act in all other tissues and why?
Tamoxifen acts as an agonist - promotes gene transcription
What is the role of vitamin D receptors?
Vitamin D receptors regulate the absorption of calcium, iron, magnesium and phosphate
Why is vitamin D used to treat Rickets?
Vitamin D is important in bone mineralisation
What bone disorder is vitamin D used to treat?
Rickets
What do some studies suggest about the link between vitamin D and cancer?
Some suggest that high intake of vitamin D may reduce the risk of colorectal cancer
Where must nuclear receptors translocate to to have an effect?
The nucleus
How long does it take to see the effects of nuclear receptors?
Days or weeks
What disease is treated by the use of therapies aimed at the PPARα?
Hyperlipidaemia
What disease is treated by the use of therapies aimed at the PPAR(Y)?
Type II diabetes
What disease is treated by the use of therapies aimed at the Glucocorticoid receptor?
Inflammatory disease
What do the therapies aimed at the oestrogen receptor aim to treat?
Breast cancer, osteoporosis, pregnancy termination
Summarise what a nuclear receptor does.
Acts as a homo or heterodimer, translocates to the nucleus, binds to the DNA and selectively regulates gene expression
