Nuclear receptors

Question 1

What are the three common lipid soluble hormones?

Answer 1

• cortisol - adrenal stress hormone
• Estrodiol - principle female sex hormone
• retinol - cell growth and differentiation and retina funciton

Question 2

What is the basic structure of a steroid?

Answer 2

4 x 17c rings. Subtle differences lie in oxidation state of side chains

Question 3

What is an orphan receptor?

Answer 3

A nuclear receptor with no identified ligand

Question 4

What is the domain organization of nuclear receptors?

Answer 4

• N terminal variable domain (activation/repression)
• DNA binding domain has 2 x C4 zinc fingers
• C terminal ligand binding domain

Question 5

How is variability of the N-terminus of nuclear receptors achieved?

Answer 5

Through alternative splicing or phosphorylation

Question 6

Discuss the interactions of nuclear receptors involving zinc fingers

Answer 6

Proximal zinc finger binds to response elements in DNA (sequence)
Distal zinc finger mediates nuclear receptor dimerisation

Question 7

What is the difference in binding between homodimeric and heterodimeric nuclear receptors?

Answer 7

Homodimeric receptors bind palindromic sequences (eg oestrogen)
Heterodimeric receptors bind inverted or direct repeat sequences. Also contain common RXR monomer

Question 8

Discuss the activation of homodimeric nuclear receptors

Answer 8

Nuclear receptors are localized in the cytoplasm in absence of ligand
Anchored in cytoplasm via heatshock proteins interacting at ligand binding site of nuclear receptor
Ligand binding releases nuclear receptor from anchoring complex
Ligand/receptor complex enters the nucleus as ligand binding domain exposes NLS (nuclear localization sequence)

Question 9

Discuss the activation of heterodimeric nuclear receptors

Answer 9

These are localized only in the nucleus
In the absence of ligand, retinoid receptor is bound to RXR repressor - repressor recruits HDACs and blocks transcription
Ligand binding displaces RXR repressor and a heterodimer is formed with the RXR receptor.
Heterodimer can recruit HAT complexes and hyperacetylates nucleosome.
Upregulates mediator complex

Question 10

What are JAKs

Answer 10

Protein kinases associated with cytokine receptors

Question 11

What is an example of a human response signal that is sent via a JAK/STAT pathway?

Answer 11

Low blood oxygen levels are sensed in the kidneys, JAK/STAT sends signal, phosphorylated STAT can bind to erythropoietin receptor which induces antiapoptotic factor BCL-XL, inducing red blood cell proliferation

Question 12

At what site are JAK2 kinases phosphorylated

Answer 12

Phosphorylated at critical tyrosine residues in the activation lip

Question 13

Discuss the signal transduction pathway after JAK kinases have been phosphorylated

Answer 13

phosphorylated JAK = lower km of substrate for ATP
Can phosphorylate several tyrosines in the receptor
Phosphorylated tyrosines interact with SH2 domain on STAT proteins (already bound at receptor)
Phosphorylated STAT can dissociate and dimerise. Enters the nucleus

Question 14

How do receptor tyrosine kinases work?

Answer 14

Have intrinsic kinase activity - ligand binding causes receptor dimerization leading to autophosphorylation

Question 15

How do the 4 receptor tyrosine kinases which respond to human epithelial growth factors bind ligands?

Answer 15

Ligand binding causes a surface loop extension which is the receptor dimerisation interface

Question 16

How does Her2 differ from Her1,3 and 4

Answer 16

Her2 does not bind a ligand, exists in preactivated state primed for activation.

Question 17

Why is Her2 of medical interest?

Answer 17

Her2 is overexpressed in roughly 25% of breast cancers. Overexpression of the receptor causes hypersensitivity and tumor cells become responsive to very low levels of EGF.

Question 18

What is Herceptin?

Answer 18

A humanized antibody which can bind to the surface loop extension of Her2, preventing dimerization and therefore acting as a therapy against tumor growth.

Question 19

How is RAS activated and deactivated?

Answer 19

When Ras binds GTP it is active - this is assisted by GEFS

when Ras binds GDP it is inactive- assisted by GAPS

Question 20

How is CREB activated?

Answer 20

Ligand binding to G-coupled receptor activates G-alpha to be released from trimeric G protein complex.
G-alpha activates membrane bound adenyl cyclase
Adenyl cyclase converts ATP to cAMP
cAMP causes release of the catalytic subunit of Protein Kinase A
This enters the nucleus and phosphorylates CREB