5: Gene Regulation By ARs And ERs Flashcards
What distance do signals act for the below systems:
- autocrine
- paracrine
- endocrine
- autocrine: affects cell producing them (eg GFs)
- paracrine: diffuse short distance to affect nearby cells (eg neurotransmitters)
- endocrine: acts on target sites far distance from site of synthesis (eg hormones)
What are the 4 characteristics of a hormone?
- They are biochemical agents
- Synthesized and secreted by glands
- Circulate in the blood to other parts of the body
- Stimulate specific (target) tissues
What are the 2types of hormones?
Lipid soluble and water soluble
Compare water soluble to lipid soluble hormones
- lipid soluble diffuse through plasma membrane, water soluble bind to receptors at surface of cell (cannot diffuse)
- water soluble travel easily in blood, lipid soluble are transported by carrier proteins
- lipid soluble alters expression of genes at nuclear level, water soluble results in series of intracellular events
Where are the 9 areas that produce hormones in the body?
- pineal
- hypothalamus
- pituitary
- thyroid
- parathyroid
- adrenals
- pancreas
- testis
- ovary
Describe the difference in molecular structure between testosterone and estrogen
They differ only by a H3C group and double O bond in testosterone, single OH in estrogen
Describe the general molecular structure of a nuclear receptor
AF-1 (activating function 1) domain, located at N-terminus
DBD (DNA binding domain) = “zinc fingers”
LBD (ligand binding domain)
AF-2 domain, located at C-terminal
AFs help switch on transcription, and are important for coactivator interaction
Hinge region: provides flexibility between the DBD and LBD
Fill in the blanks:
NRs activate transcription by preferentially binding to _____ _____ _____ of target genes.
Response elements act as _____ of receptor function.
Ligand binding induces _____ recruitment and ______ ______.
- Regulatory response elements
- Modulators
- Coactivator
- Transcriptional activation
What are the 3 types of nuclear receptors?
Type I: ‘steroid’ receptors, generally homodimeric (AR, ERa, ERb, PR, GR)
Type II: repressions in absense of agonist, heterodimeric with RXR (retinoid X receptor) (TR, RAR, PPARy, RXR)
Type III: orphan nuclear receptors, heterodimeric with RXR (unknown ligands)
Describe the action of steroid (Type 1) nuclear receptors using androgen receptor as an example
- AR associated with HSP complexes in the cytoplasm to maintain inactive state
- Binding of testosterone or dihydrotestosterone (DHT) causes dissociation of HSPs, activating AR
- Homodimerization of activated ARs
- Translocation of homodimerised ARs to nucleus and bind to androgen response elements in DNA
- Recruitment of coactivators (eg p160, HMTs, p300/CBP, etc) to remodel chromatin via LxxLL motifs
- AR/CoAct complex recruits mediator complex, interacting with RNA polII ad TAFs
- Assembly of transcriptional machinery at promoter region initiates genes transcription
What is the role of p160 steroid receptor coactivators (SRC-1/2/3)
- posses histone acetyltransferase (HAT) activity
- acetylation of histones to relax chromatin structure, facilitating transcription
What are some examples of histone methyl transferases and what do they do?
Eg: CARM1, PRMT
Methylae histones, contributing to chromatin remodeling
What are p300 & CBP
Lysine acetyltransferases
Acetylate histone and non-histone proteins, enhancing transcriptional activity
What are some examples of histone demethylases and what do they do
LSD1, JMJD2A
Remove methyl groups from histones, reversing repressive methyl marks and promoting gene activation
What is the role of the LxxLL motif?
Contains leucine (L) residues separated by 2 unspecified amino acids (X).
Interacts with AF2 domain of nuclear receptors
Binds coactivators such as SRC-1/2/3, p300/CB etc
Coactivators possess HAT activity, which remodel chromatin, allowing transcription of DNA
Fill in the blanks:
Type II nuclear receptors, such as ____ ____ ____ (____s) form heterodimers with ____ ____ ____ (____s). They are located in the ____ even in the absence of their ligand.
In the absence of a ligand (such as ____-____ ____, ____) these receptors _____ transcription.
- Retinoic acid receptors (RARs)
- Retinoic X receptors (RXRs)
- Nucleus
- All-trans Retinoic acid (ATRA)
- Repress
What do these stand for:
- HDACs
- KMTs
- KDMs
HDACs: histone deacetylases
KMTs: lysine methyltransferases
KDMs: lysine demethylases
Describe the repressive action of unliganded type II nuclear receptors
- RARs heterodimerise with RXRs in the absence of ligand (eg ATRA) in the nucleus
- CoRepressors (eg SMRT/NCoR) remove acetyl groups from histones, condensing the chromatin
- Repressive methyltransferase complexes add repressive epigenetic methyl marks to histones
- ATP-dependent chromatin remodeling complexes remodel chromatin to a repressed state
- all actions together result in transcriptional repression at Retinoic acid response elements (RAREs)
What are the 2 types of ligands for type II nuclear receptors?
- ATRA: all-trans-retinoic acid, RARa/b/y
- 9-cis-RA, RXRa/b/y
Describe the action of type II nuclear receptors
- ligand binding (eg ATRA/9-cis-RA) relives repression of receptor
- CoRepressors are displaced, and coactivators (eg SRC1/2/3, HMTs, HDMs, KATs) are recruited
- coactivator complex recruits mediator complex, interacts with RNA polII and TATA-binding protein associate factors (TAFs)
- initiates transcription at RAREs, resulting in gene activation
T or F
- Helix 12 is re-orientated upon binding (eg of DHT/T/THG)
- H12 interacts with LxxLL motif of coactivators
- LBD facilitates DNA dimerisation
- T
- T
- F, LBD facilitates RECEPTOR dimerisation
Describe the function of the LBD in coregulator interactions
- LBD involves 12a helix arrangement
- facilitates receptor dimerisation
- in a sense of agonist/ presence of antagonist, H12 interacts with CoRepressors complexes
- agonist binding induces formation of H11, and realignment of H12, allows accessible surface to bind to coactivators
(The side which is accessible changes which complexes interact: CoRepressors/coactivators) - H12 binds to LxxLL motif of coactivators
What are DR3 and DR5 and how do they contribute to nuclear receptor specificity?
Direct repeat 3/5 base pair spacing.
There are 2 similar/indentical sequences (half-sites) separated by 3 or 5 base pars.
Nuclear receptors recognise and bind to these repeats
Eg for DR3: ERE, ARE, and GRE
For DR5: RARE
What are the 2 roles of pioneer factors?
- Active role: actively ope chromatin to make it accessible for TFs and CoActs, enabling transcription
- Passive role: partially bound to inactive enhancers, maintaining a stat of readiness for rapid induction upon signal
