Cell Signalling and Cancer

Question 1

How many times do tyrosine kinase receptors span the membrane?

Answer 1

Once.

Question 2

What is the function of the tyrosine-kinase (intracellular) domain of a tyrosine kinase receptor?

Answer 2

To phosphorylate target proteins in the cell.

Question 3

List 9 tyrosine kinase receptors.

Answer 3

1 - Epidermal growth factor receptor (EGF).

2 - Insulin receptor.

3 - Insulin-like growth factor receptor (IGF).

4 - Tropomyosin receptor kinase A (Trk A).

5 - Platelet-derived growth factor receptors (PDGF).

6 - Macrophage colony-stimulating factor receptors (MCSF).

7 - Fibroblast growth factor receptors (FGF).

8 - Vascular endothelial growth factor receptors (VEGF).

9 - Ephrin receptors.

Question 4

Which class of receptors trigger the mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3 kinase (PI3K) pathways?

Answer 4

Tyrosine kinase receptors.

Question 5

Which class of signalling molecules trigger the MAP pathway?

Answer 5

Growth factors.

Question 6

What is the outcome of the MAPK pathway?

Answer 6

Cell proliferation.

Question 7

Describe the action of kinases.

Answer 7

Kinases catalyse the transfer of the terminal phosphate group of ATP to specific serine, threonine or tyrosine residues on target molecules.

Question 8

What is the effect of phosphorylation on target molecules?

Answer 8

Usually activation, but sometimes inhibition.

Question 9

Describe the action of phosphatases.

Answer 9

Phosphatases cleave a phosphate group from the serine, threonine or tyrosine residues on target molecules.

Question 10

Give an overview of the MAPK pathway.

Answer 10

1 - Signalling molecule.

2 - Tyrosine kinase receptor.

3 - GRB2.

4 - Sos.

5 - Ras.

6 - Raf.

7 - Mek.

8 - Erk.

9 - Cell growth.

Question 11

Upon binding to a ligand, what conformational change occurs in a tyrosine kinase receptor?

Why is this important?

Answer 11

• Dimerisation of the two receptor molecules, known as homodimerisation.
• This enables kinase domains of the neighbouring receptor molecules to cross-phosphorylate each other on tyrosine molecules.
• This creates high affinity binding sites for many proteins.

Question 12

In the MAPK pathway, what is the function of GRB2?

Which domains are important for its function?

Answer 12

• It acts as an adaptor protein using the SH2 and SH3 domains:
• SH2 recognises specific phosphorylated tyrosine residues and binds to the high affinity binding site of a dimerised tyrosine kinase receptor.
• SH3 binds to a motif in sos proteins. Binding to SH3 enables sos to recruit and activate ras protein.

Question 13

What is ras protein?

How does sos activate ras?

Answer 13

• A small G-protein with GTPase activity bound to the plasma membrane by a covalently attached lipid group.
• Sos exchanges GDP for GTP in ras to activate ras.

Question 14

To which class of proteins does sos belong?

Answer 14

Guanine nucleotide exchange factors (GEFs).

Question 15

What is the effect of GTPase-activating proteins (GAPs) on activated ras?

Answer 15

• They inactivate ras by stimulating its GTPase activity, which cleaves a phosphate group from GTP to form GDP.
• This terminates the signalling event.

Question 16

What is the role of ras in the MAPK pathway?

Answer 16

It activates raf by inducing a conformational change.

Question 17

What type of molecule is raf?

What is its target?

Answer 17

• A kinase.

- It phosphorylates mek.

Question 18

What type of molecule is mek?

What is its target?

Answer 18

• A kinase.

- It phosphorylates erk.

Question 19

What type of molecule is erk?

What is its target?

Answer 19

• A kinase.
• It phosphorylates transcription factors in the nucleus such as:

1 - C-jun.

2 - C-fos.

3 - C-mys.

4 - C-myb.

5 - Cyclin D.

Question 20

What is the function of erk?

Answer 20

To induce changes in gene expression that increase cell proliferation.

Question 21

List 4 ways by which the MAPK pathway can be turned off.

Answer 21

1 - Removal of the extracellular signal.

2 - Switching off activated tyrosine kinase receptors by way of protein tyrosine phosphatases such as SHP-1 and SHP-2.

3 - Ras GAPs such as p120 and neurofibromin.

4 - Dephosphorylation of target proteins by phosphatases.

Question 22

List the outcomes of the PI3K pathway.

Answer 22

1 - Cell survival.

2 - Cell growth.

Question 23

Give an overview of the PI3K pathway.

Answer 23

1 - Signalling molecule.

2 - Tyrosine kinase receptor.

3 - PI3 kinase.

4 - PIP3.

5 - PDK1.

6 - Akt (PKB).

7 - Bad.

8 - mTOR.

Question 24

What is PI3 kinase?

What is its function?

Answer 24

• A lipid kinase in the PI3K pathway that mirrors the function of GRB2:
• It becomes active upon binding to high affinity binding sites of tyrosine kinase receptors.
• Activation of PI3 kinase enables PI3 kinase to phosphorylate PIP2 into PIP3.

Question 25

What is the source of PIP2?

Answer 25

It is derived from phosphatidylinositol (PI), which is a membrane lipid.

Question 26

Describe the structure of phosphatidylinositol.

Answer 26

In order: 1 - Fatty acid chain. 2 - Glycerol. 3 - Phosphate. 4 - Inositol (a sugar).

Question 27

How is phosphatidylinositol used to produce PIP2?

Answer 27

- It is phosphorylated at the 4 position on inositol to produce PIP. - PIP is phosphorylated at the 5 position on inositol to form PIP2.

Question 28

What is the function of PIP3?

Answer 28

It acts as a docking site for PDK1 and Akt.

Question 29

Describe the interaction between PDK1 and akt. Which other molecule is involved in this interaction?

Answer 29

- Upon binding to PIP3, PDK1 phosphorylates, and therefore activates, akt. - Activated Akt moves away from the plasma membrane. - mTOR assists in the activation of Akt.

Question 30

What is bad protein? How is it activated? What is the outcome of bad activation?

Answer 30

- A protein that binds with apoptosis inhibitory proteins, rendering them inactive. - It is activated by Akt. - Bad activation promotes cell survival.

Question 31

What is the function of 14-3-3 protein?

Answer 31

It binds, and therefore sequesters, inactive bad protein in the cytoplasm.

Question 32

How is mTOR activated?

Answer 32

mTOR is activated by Akt. *NB mTOR also assists in the activation of Akt as previously mentioned.

Question 33

List the two functionally distinct protein complexes of mTOR.

Answer 33

1 - mTOR complex 1. 2 - mTOR complex 2.

Question 34

What is the function of mTOR complex 1? How does it do this?

Answer 34

- To stimulate cell growth. 1 - By promoting ribosome production, and therefore protein synthesis. 2 - By inhibiting protein degranulation.

Question 35

What is the function of mTOR complex 2?

Answer 35

- To stimulate cell survival and cell growth. - It does this by activating Akt, which results in more active mTOR complex 1 (increases cell growth) and bad protein (increases cell survival).

Question 36

List 4 ways by which the PI3K pathway can be turned off.

Answer 36

1 - Removal of the extracellular signal. 2 - Switching off activated tyrosine kinase receptors by way of protein tyrosine phosphatases such as SHP-1 and SHP-2. 3 - PTEN, an inositol lipid phosphatase which removes phosphate from PIP3 so that it no longer acts as a docking site for PDK1 and Akt. 4 - Dephosphorylation of target proteins by phosphatases.

Question 37

Define stratified medicine.

Answer 37

A more effective way of treating cancer by grouping patients according to the specific genetic lesion.

Question 38

What is HER?

Answer 38

- A gene that encodes EGF receptors: | - HER1 encodes EGFR1 and HER2 encodes EGFR2.

Question 39

How does the function of EGFR1 differ from that of EGFR2?

Answer 39

- EGFR1 works like a normal tyrosine kinase receptor. - However there is no known ligand for EGFR2. - EGFR2 heterodimerises with EGFR1. - It can then respond to the EGFR1 ligand, EGF.

Question 40

In what proportion of breast cancers is HER2 overexpressed?

Answer 40

HER2 is overexpressed in 30% of breast cancers.

Question 41

How is EGFR2 alone able to produce cancer?

Answer 41

- EGFR2 is able to homodimerise when overexpressed. | - It is able to do this despite the lack of a ligand for EGFR2.

Question 42

What is the treatment for HER2 overexpression?

Answer 42

- Trastuzumab (herceptin) is an anti-HER2 antibody. - It causes internalisation and degradation of EGFR2. - It also causes an antibody-dependent cellular cytotoxicity (ADCC) response, where the antibody triggers an immune response to destroy the cancer cell.

Question 43

What causes chronic myelogenous leukaemia?

Answer 43

- Presence of the philadelphia chromosome, which forms as a result of a translocation between chromosomes 22 and 9. - The breakage and rejoining occurs at the sites of the BCR and ABL genes creating a hybrid BCR-ABL fusion gene. * BCR isnt B cell receptor. Both BCR and ABL encode kinase proteins.

Question 44

Where is ABL kinase located?

Answer 44

In the cytoplasm (compared to the tyrosine kinase receptors which are membrane-bound).

Question 45

How does the BCR-ABL fusion gene cause chronic myelogenous leukaemia?

Answer 45

- The BCR gene translocates with the regulatory sequences of ABL. - The kinase encoded by BCR-ABL is therefore constitutively active. - This stimulates inappropriate proliferation of haematopoietic precursor cells, preventing them from dying by apoptosis.

Question 46

What is the treatment for ABL-BCR fusion gene expression?

Answer 46

- Imatinib (gleevec) is a synthetic ABL inhibitor that blocks the nucleotide binding pocket (where GTP binds) of the tyrosine kinase domain of the BCR-ABL protein. - This inhibits phosphorylating capability of the BCR-ABL protein.

Question 47

Why do patients show relapse to imatinib (gleevec)?

Answer 47

Because they eventually develop resistance due to secondary mutations to the tyrosine kinase domain.