Nuclear Receptors Flashcards
What are examples of ligands?
- Hormones
- Growth factors
- Neurotransmitter sugars
- Lipids
- Ions
How do nuclear receptors differ from cell surface receptors?
- Cell surface; act as sensors for extracellular molecules
- Nuclear receptors; act as sensors for intracellular levels of small molecules/metabolities, potentially inducing a signal transduction cascade binding to DNA and activating gene transcription
What signals do nuclear receptors respond to?
(small) Lipid-soluble molecules (entering into the cell, into the nucleus binding in situ):
- hormones
- vitamins
- retinoids
What are the outcomes of nuclear receptor activation?
- Activating gene transcription
- Repressing gene transcription
What forms do nuclear receptors exist in?
- Homodimers; 2 molecules of the same NR protein
- Heterodimers; 2 molecules of differing nuclear receptor protein
- Monomers
What are orphan receptors?
Nuclear receptors that are thought not to have ligands (e.g. interaction w/another protein) or of which their ligands have not yet been discovered.
What does ligand binding do to a NR?
Induces a conformational change of the nuclear receptor protein e.g. in folding/shape, altering its ability to induce gene transcription switching it from an inactive form to an active form or vice versa.
What is a xenobiotic compound?
Compounds/foreign chemical substance not normally produced by the organism; e.g. PXR nuclear receptor can detect the xenobiotic and respond accordingly, drugs pollutants etc.
What do natural NR ligands all have in common?
- Lipophilic
- Lipid solubility enables them to cross cell membranes with ease by passive diffusion
What are the different classes of natural NR ligands?
- Sex steroids (cholesterol derivatives)
- Reinoids; Vitamin A
- Prostaglandins/fatty acids
- Vitamin D
- Thyroid hormone
Why do ligands have different affinities for their respective NRs?
- Steroids, vitamins etc. have high affinity for their NR as there is a lower circulating concentration
- Than metabolites etc. such as fatty acids where there is lots of availible
What effects can ligands have on their NRs?
- Agonists
- Partial agonist
- Antagonist
What processes do NRs regulate in human physiology?
- Cell growth, proliferation, apoptosis (death), homeostasis
- Tissue differentiation
- Development
- Metabolism
- Endocrine systems
- Body clock/circadian rhythms
- Reproduction
- CNS function
- Cardiovascular system
- Respiratory system
- Renal function
- GI function
- Immune system
- Stem cell renewal
When are NRs important clinical targets?
- Inflammatory disease
- Solid tumours
- Leukaemias and lymphomas
- Developmental abnormalities
- Metabolic diseases
- Growth disorders
- Diabetes
- Kidney disease
- Obesity
- Liver disease
- Endocrine disorders
- Auto-immune disease
- Retinopathies
- Thyroid disorders
- Behavioral disorders
- Alzheimer’s disease
- Parkinson’s disease
- Stroke
- CVD
- Hypogonadism
- Polycystic ovary disease
What are the top NR drug targets?
- Estrogen receptor
- Glucocorticoid receptor
- Progesterone receptor
- PPARs
What is the NR domain structure?
- 2 main functional domains
- DNA Binding Domain (DBD)
- Ligand Binding Domain (LBD)
What enables the DBD of a NR to specifically recognise sequences?
- 2 Zinc Fingers
- 2-3 alpha helices arranged to read dsDNA
- Helices interact w/bases and read sequences, allowing receptor to recognise specific sequences
What is the structure of the LBD and what does it contain?
- Makes up most of the C-terminus
- 10-12 alpha helices; NRs have conserved folding but differed enough in sequence to give specificity
- Contains activation helix
- Many contain a hydrophobic cavity known as the ligand binding pocket
- Ligand binding changes conformation of LBD
What are the functions of the DBD NR Domain?
- Binds specific DNA sequence; normally 6 base pairs long
- Normally AGGTCA
- Or steroid receptors bind AGAACA
- Dimer nature; recognise 2 copies of the sequence
- Dimerisation (on the DNA)
What are the functions of the LBD NR Domain?
- Ligand binding
- DImerisation
- Co-factor binding (needed for transcriptional activity)
- Transcriptional activation or repression
Upon ligand-binding, how does the NR become repressed/activated?
- NRs recruit respective co-factors; for an antagonist ligand/no ligand, co-repressor proteins are recruited which in turn represses gene transcription
- With an agonist ligand, co-activator proteins are recruited which allow for transcription of the desired gene (facilitating RNA polymerase etc)
How do recruited co-factors interact with NRs?
- Via conserved helical motifs, promoting NR/cofactor complexes
- Ligand binding to NR = conformational change which accommodates binding to helical motifs of cofactors
- Strong protein-protein interaction occurs
- Co-activators: LXXLL motif (3 leucines + 2 other AAs)
- Co-repressors: LXXXI/LXXXL (1 leucine 1 isoleucine + 3 AAs or 2 leucines and 3 AAs)
- They form amphipathic alpha helices with respective hydrophobic and -philic ends, leucines pointing in the same direction
How do corepressors exert their suppressive effect?
- DNA is packaged as chromatins; nucleosomes of 8 histone proteins w/200 bases of DNA wrapped around
- Corepressors contain histone deacetylases
- Enzyme removes acetyl groups from histones, which keep nucleosomes tightly packed and generates a compact chromatin structure
- This blocks the recruitment of polymerases; transcription does not occur
How do coactivators exert their agonist activity?
- DNA is packaged as chromatins; nucleosomes of 8 histone proteins w/200 bases of DNA wrapped around
- Coactivators contain histone acetylases
- These add acetyl groups to the histones; this occurs on the lysine residue which is positive - DNA is negative (normally enhancing the interaction), thus acetylating removes the positive charge relaxing interaction
- This generates an open chromatin structure, making it easier to invade thus there is recruitment of polymerases and transcription of the gene can occur
What is different about steroid receptor dimerisation at the LBD?
- Most NR LBDs function as dimers
- Steroid receptors function as homodimers, binding inverted repeats (palindromes; reading the same forwards and backwards)
How does LBD heterodimer formation contribute to greater complexity of NR function?
- Ligand-binding NRs (but not steroids) form heterodimers with RXR (retinoid X receptor; which binds 9-cis retinoic acid)
- They bind direct repeats of AGGTCA (unlike palindromic nature of steroids)
- But spacing between repeats ‘n’ differs giving rise to unique functionalities etc
