Lec 20- steroid receptors Flashcards
Steroid Hormones
- Lipophilic; can readily enter into the cell
- Usually transported in plasma via carrier proteins
Ligand for nuclear receptors
- Glucocorticoids, mineralocorticoids, sex hormones
- Thyroid hormones
- Retinoic acid
- Endogenous lipids (act on the PPAR’s)
- Foreign chemicals (act on PXR’s)- allow metabolism of toxic compounds the body doesn’t want
- They are nucleoreceptors ligands because they are lipophilic, and cross the cell membrane
- However they can’t move through cytosol therefore bind to nuclear receptors which transports them into the nucleus where they switch on mRNA production (DNA transcription)
Nuclear receptor structure
- Variable region (section A+B)- these give each receptor specificity
- DNA binding, zinc fingers (section C)-
- Hinge (section D)
- Ligand binding (Section E+F)
Chromosome puffing
- Blow fly has massive chromosomes and are visible outside of cell division
- When steroids are added the chromosomes expand, this shows that steroids act directly on DNA
- If we add an inhibitor of mRNA synthesis then puffing is reduced
- Puffing is therefore when the DNA unwinds to allow hormones receptors to react to allow DNA polymerase to react leading to mRNA synthesis
Receptor structure
- Mineralocorticoids e,g. aldosterone receptor
- Circle binds drug, box binds DNA (look at BB slides) (binds aldosterone)
- This produces transport proteins
- GLUCOCORTICOIDS
- Binds cortisol
- Inhibits COX-2 production
Chimeric receptor
- We can mix the 2 receptors
- e.g. a receptor that binds aldosterone but inhibits COX2 production (LOOK AT BB SLIDeS)
- These are called modular proteins (different parts of the protein function independently)
How receptors work
-Glucocorticoids travel across cell membrane
-Inside the cell they interact with the glucocorticoid receptor
-The receptor is not a single protein: SRC: HSP90 (Heat shock proteins, they bind to other proteins and stabilise there structure so stop denaturing ie by HEAT)- the HSP90 binds to receptor constantly (not just when heated) to keep it inactive and GR
-When the glucocorticoid binds to the receptor, HSP90 and SRC binds and they fall off
+SRC is a protein kinase and phosphorylate the receptor complex giving it rapid action, this way is not the normal way but I used when needed quickly
-Genomic signalling: Zinc fingers find the correct region of DNA to bind to
-Simple GRE- They normally bind as a dimer which allows activation of RNA polymerase (MAIN PATHWAY)
-nGRE- when one receptor (monomer) binds to the DNA this sends a negative signal and makes it harder for RNA polymerase
-Tethering- this is where the receptors bind to another nuclear transcription factor (NFkB), this changes its activity which reduces IL-1beta
-Composite- receptors bind to DNA but increase binding of other transcription factors
Type II nuclear receptors
- Not associated with heart shock proteins
- but apart from that its identical
ZINC fingers
- This allow the receptors to identify the correct place for binding
- Zn is at the base and there is a tip of amino acids which sticks into the DNA
- Particular Zn fingers bind various nucleotides well, therefore when put in a sequence this will allow accurate and specific binding
DNA binding sites; inverted repeats
- Steriods normally bind as a dimer, this is allowed by the response element (area on DNA)
- Some response elements lack the repeat (GATC rather than GATCCTAG)
- In this case only a single receptor binds
- Glucorticoids, this receptor then binds a protein that makes it harder for RNA polymerase to bind, causing inhibiting of RNA synthesis
transcription
- Steriod receptor binds to DNA allowing RNA synthesis to happen easier
- RNA polymerase is not very good at binding by itself and therefore needs transcription factors to bind to promotor region to help RNA polymerase the bind
- Steroid receptor dimer acts as the transcription factor
- Proximal control element- this is a site close to the coding region
- Hormone receptors bind a long way away from the promoter region
- The area of DNA between steroid receptor and RNA polymerase unwinds which forms a flexible loop, this allows the steroid receptor and RNA polymerase to become close and touch (as well as other proteins) allowing RNA polymerase or is monomer pushes RNA polymerase off
- The area of DNA that steroids bind is called hormone response element or enhancer
- We get puffing because theres lots of unwinding
Drugs acting on nuclear receptors
- Anti-inflammatory agents
- Clofibrate PPARa (Cholesterol lowing)
- Thiazolidines PPAR gamma (Type II diabetes)- the adipocytes secrete chemicals which bind to muscle cells stopping insulin working, these drugs remove the negative effects of the adipocytes
