III. Signal transduction and cell cycle | 37. Nuclear receptors: steroid, thyroid, retinoid and Ah receptors

Question 1

I. Nuclear receptors
1. What are the features of Nuclear receptors?

Answer 1

• Nuclear receptors are a class of proteins responsible for sensing steroid/thyroid hormones as well as other molecules.
• They have the ability to directly bind to DNA and thereby regulate the expression of adjacent genes; controlling development, homeostasis or metabolism of the organism.
• Since they can bind directly to the DNA and regulate gene expression, these nuclear receptors are classified as TFs.

Question 2

I. Nuclear receptors
2. What is the role of nuclear receptors?

Answer 2

They have the ability to directly bind to DNA and thereby regulate the expression of adjacent genes; controlling development, homeostasis or metabolism of the organism.

Question 3

I. Nuclear receptors
3. Why are nuclear receptors referred as TFs?

Answer 3

Since they can bind directly to the DNA and regulate gene expression, these nuclear receptors are classified as TFs.

Question 4

I. Nuclear receptors
4. What is the consequence of binding of a ligand?

Answer 4

Binding of a ligand usually causes a conformational change in the receptor and is therefore necessary for the activation of the receptors

Question 5

I. Nuclear receptors
5. Where are nuclear receptors found?

Answer 5

Nuclear receptors either found in the cytosol or nucleus are both referred to as nuclear receptors, since it leads to regulation of transcription in the nucleus.

Question 6

II. Ligands
1. What are the features of ligands?

Answer 6

• Since nuclear receptors are found inside the nucleus or cytoplasm, the ligands are small or hydrophobic enough to cross the plasma membrane.
• Once inside, the signal molecules bind to nuclear receptor either in the cytosol or nucleus.
• Therefore, these ligands include hydrophobic substances such as steroid hormones, thyroid hormones and so on.

Question 7

II. Ligands
2A. What is the Structure of nuclear receptors?

Answer 7

Structure of nuclear receptors (5 DOMAINS)
1. N-terminal regulatory domain
2. DNA binding domain (DBD)
3. Hinge region
4. Ligand binding domain (LBD)
5. C-terminal domain:

Question 8

II. Ligands
2B. Describe N-terminal regulatory domain in the Structure of nuclear receptors?

Answer 8

N-terminal regulatory domain contains the activation function 1 (AF-1), whose action is independent of the presence of the ligand

Question 9

II. Ligands
2C. Describe DNA binding domain (DBD) in the Structure of nuclear receptors?

Answer 9

DNA binding domain (DBD) containing 2 zinc fingers that bind to specific sequences of DNA, called HRE (hormone-response-elements)

Question 10

II. Ligands
2D. Describe Hinge region in the Structure of nuclear receptors?

Answer 10

Hinge region connects DBD + LBD, influences intracellular trafficking and subcellular distribution

Question 11

II. Ligands
2E. Describe Ligand binding domain (LBD) in the Structure of nuclear receptors?

Answer 11

1. LBD is an α-helical sandwich fold, where 3 antiparallel α-helices (the ‘’sandwich filling’’) are flanked by 2 α-helices on one side and 3 α-helices on the other side (the ‘’bread’’)
2. LBD contributes to dimerization interface of the receptor + binds co-activator and co-repressor proteins
3. Also contains the activation function 2 (AF-2), which is dependent of ligand binding

Question 12

II. Ligands
2F. Describe C-terminal domain in the Structure of nuclear receptors?

Answer 12

Highly variable in sequence between different nuclear receptors

Question 13

II. Ligands
3. How do we classify ligands?

Answer 13

Depending on where the nuclear receptors wait for the ligand, we can distinguish between two types:
- in the cytoplasm (type 1)
- in the nucleus (type 2)

Question 14

II. Ligands
4A. What are the features of ligand type 1?

Answer 14

• Anchored in the cytoplasm
• Estrogen receptor-like

Question 15

II. Ligands
4A. What are the features of ligand type 1?

Answer 15

Type 1 (Cytoplasm)
- Anchored in the cytoplasm
- Estrogen receptor-like

Question 16

II. Ligands
4B. What are the 5 examples of ligand type 1?

Answer 16

Type 1 (Cytoplasm)
1. Estrogen (ER)
2. Progesterone (PR)
3. Androgens (AR)
4. Glucorticoids
5. Mineralocorticoids (MR)

Question 17

II. Ligands
5A. What are the features of Ligand Type II?

Answer 17

Type 2 (nucleus)
- Retained in the nucleus regardless of ligand binding
- Thyroid-hormone like receptor

Question 18

II. Ligands
5B. What are the 3 examples of ligand type 2?

Answer 18

Type 2 (nucleus)
- Thyroid hormone (HR)
- All-trams retinoid acid (RAR)
- Vitamin D (VDR)

Question 19

II. Ligands
1. What are the features of Steroid receptors?

Answer 19

• Nuclear receptors that bind steroid hormones are classified as type 1 receptors
• Steroid receptors form homodimers and bind to palindromic sequences (inverted DNA repeats).
• They are intracellular receptors that are located within the cytoplasm of the target cell and move towards the cell nucleus upon activation by steroid hormone.
• Belong to the nuclear receptor family of ligand-activated TFs.

Question 20

II. Ligands
2. Where can you find steroid receptors?

Answer 20

They are intracellular receptors that are located within the cytoplasm of the target cell and move towards the cell nucleus upon activation by steroid hormone.

Question 21

II. Ligands
3. Which family of receptors do steroid receptors belong to?

Answer 21

Belong to the nuclear receptor family of ligand-activated TFs.

Question 22

II. Ligands
4. What is the mechanism of steroid receptors?

Answer 22

1) At first (in the absence of ligands) the hinge region will be covered by Hsp90 (heat shock protein 90) – a type of chaperones which bind the receptor until ligand is present
2) Ligand will then enter the cell and bind to the nuclear receptor
-> dissociation of Hsp90 + activation of receptor
3) The nuclear receptor will then undergo homodimerization (2 receptor subunits join together to form 1 functional DNA-binding unit) + translocation (active transport) from cytoplasm towards the nucleus (with the help of nuclear localization signal – NLS)
4) Activated nuclear receptor binds to HRE (Hormone response elements) of DNA
5) The complex (nuclear receptor + DNA) will recruit other proteins that transcribe DNA downstream from HRE -> mRNA -> protein, which causes a change in cell function

Question 23

II. Ligands
5. What is homodimerization?

Answer 23

2 receptor subunits join together to form 1 functional DNA-binding unit

Question 24

II. Ligands
6. What is Hormone response elements (HRE)?

Answer 24

Hormone response elements (HRE): short DNA sequence within the promoter of a gene, able to bind to specific hormone receptor complexes and thereby regulate transcription.
- Specifically responds to steroid hormones.

Question 25

II. Ligands
7. What are the features of Thyroid receptors?

Answer 25

Thyroid receptors (type 2) - NUCLEUS
- Nuclear receptors that bind thyroid hormones are classified as type 2 receptors.
- Thyroid receptors form homo-/heterodimers and bind directly to DNA repeats.
- They are located inside the nucleus and do not contain any Hsp90. They are activated by binding thyroid hormone.

Question 26

II. Ligands
8. What is the mechanism of Thyroid receptors?

Answer 26

1) At first (in the absence of ligands), the nuclear receptor is already bound to the response element of DNA, but with a co-repressor protein
2) Ligands will then pass through the cell membrane + cytoplasm and enter the nucleus, where it binds to the nuclear receptor and activates it
-> dissociation of co-repressor and recruitment of co-activator proteins
3) The DNA/thyroid receptor/co-activator complex recruits RNA polymerase which transcribes DNA into mRNA and eventually a protein, resulting in a change in cell function

Question 27

II. Ligands
9. What are the features of retinoid receptors?

Answer 27

Receptor is found in the nucleus. Basically the same functions as thyroid receptors, but the ligand is retinoid.

Question 28

II. Ligands
10. What are the features of Aryl hydrocarbon receptors?

Answer 28

• Aryl hydrocarbon receptors (type 1) - CYTOPLASM
• The aryl hydrocarbon receptor (AhR) is a ligand-activated TF involved in regulation of biological responses.
• These receptors are members of the family of basic helix-loop-helix (bHLH) TFs.
• AhR has bHLH located at the N-terminal. The basic region is involved in binding of the receptor to DNA and the HLH-region which facilitates protein-protein interactions.
• The AhR is anchored in the cytoplasm and contain Hsp90, just like type 1 nuclear receptor.

Question 29

II. Ligands
11. What do Aryl hydrocarbon receptors bind to?

Answer 29

AhR bind several exogenous ligands, such as dioxin-like compounds (carcinogenic products – smoke) and benzo[a]pyrene (halogenated aromatic hydrocarbons).

Question 30

II. Ligands
11. What do Aryl hydrocarbon receptors bind to?

Answer 30

AhR bind several exogenous ligands, such as dioxin-like compounds (carcinogenic products – smoke) and benzo[a]pyrene (halogenated aromatic hydrocarbons).

Question 31

II. Ligands
12. What is the mechanism of Aryl hydrocarbon receptors?

Answer 31

1) At first (in absence of ligands), AhR will be inactive and bound to Hsp90
2) Ligands (aromatic chemicals) enter the cell and bind to AhR -> dissociation of Hsp90 + activation of receptor
3) The AhR will then undergo translocation the nucleus + dimerization with ARNT (AhR nuclear translocator) -> changes in gene transcription
4) Binds to response element of DNA -> induces transcription (ex: of enzymes/molecules needed for proper functions of cells)

Question 32

II. Ligands
13. What is the significance of Aryl hydrocarbon receptors (type 1) - CYTOPLASM?

Answer 32

• The AhR have been shown to regulate xenobiotic-metabolizing enzymes.
• Xenobiotics are biologically active, foreign chemical substances that are not naturally produced by or normally expected to be present in the organism (medicines, poisons, pollutants).
• XRE, the response element in DNA that binds AhR, is responsible for metabolism of xenobiotics.