L04 - Cell Signalling & Cancer Flashcards
How many times do receptor tyrosine kinases span the membrane?
Once
What is the function of the tyrosine-kinase (intracellular) domain of a tyrosine kinase receptor?
To phosphorylate tyrosine residues on target proteins
Which class of receptors trigger the mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3 kinase (PI3K) pathways?
Tyrosine kinase receptors
Describe the action of kinases.
Kinases catalyse the transfer of the terminal phosphate group of ATP to specific serine (Ser), threonine (Thr) or tyrosine (Tyr) residues on target proteins
What is the effect of phosphorylation on target molecules?
Usually activation, but sometimes inhibition
Describe the action of phosphatases.
Phosphatases cleave a phosphate group from the serine, threonine or tyrosine residues on target molecules
Upon binding to a ligand, what conformational change occurs in a tyrosine kinase receptor?
Why is this important?
- Dimerisation of two receptor molecules, known as homodimerisation
- This enables kinase domains of the neighbouring receptor molecules to cross-phosphorylate each other on multiple tyrosine residues (autophosphorylation)
- This creates high affinity binding sites for many proteins e.g. Grb-2 & PI 3-kinase
What is the outcome of the MAPK pathway?
Cell proliferation
Give a brief overview of the MAPK pathway.
1 - Signalling molecule
2 - Tyrosine kinase receptor
3 - Grb-2
4 - Sos
5 - Ras
6 - Raf
7 - Mek
8 - Erk
9 - Cell growth/proliferation
In the MAPK pathway, what is the function of Grb-2?
Which domains are important for its function?
- It acts as an adaptor protein using its SH2 and SH3 domains:
- SH2 recognises specific phosphorylated tyrosine residues on RTKs 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
What is Ras protein and how is it activated?
- A small G-protein with GTPase activity bound to the plasma membrane by a covalently attached lipid group
- Sos stimulates the exchange of GDP for GTP in Ras to activate Ras
To which class of proteins does Sos belong?
Guanine nucleotide exchange factors (GEFs)
What is the effect of GTPase-activating proteins (GAPs) on activated Ras?
- They inactivate ras by stimulating its GTPase activity, which cleaves a phosphate group from GTP to form GDP
- This terminates the signalling event
What is the role of Ras in the MAPK pathway?
It activates Raf by inducing a conformational change, localised at the cell membrane
What is Raf and what is its function?
- A serine threonine kinase
- It phosphorylates Mek
What is Mek and what is its function?
- A serine threonine kinase
- It phosphorylates Erk
What is Erk and what is its function?
- A serine threonine kinase
- It phosphorylates transcription factors in the nucleus and regulates gene expression such as:
1 - C-jun
2 - C-fos
3 - C-myc
4 - C-myb
5 - Cyclin D
=> cell growth/proliferation
In what 4 ways can the MAPK pathway can be turned off?
1 - Removal of the extracellular signal
2 - Switching off activated RTKs by protein tyrosine phosphatases (PTPs) e.g. SHP-1 & -2
3 - Ras GAPs e.g. p120GAP, neurofibromin
4 - Dephosphorylation of target proteins by serine/threonine phosphatases
What are the outcomes of the PI3K pathway?
1 - Cell survival
2 - Cell growth/proliferation
Give a brief overview of the PI3K pathway.
1 - Signalling molecule
2 - Tyrosine kinase receptor
3 - PI 3-kinase enzyme activation
4 - PI(4,5)P2 to PI(3,4,5)P3
5 - PDK1 & Akt
6a - Protein BAD
6b - Kinase mTOR
7a - Prevention of apoptosis
7b - ↑ ribosome production & protein synthesis, ↓ protein degradation, Akt activation
What is PI3 kinase and what is its function?
- A lipid kinase
- It is activated by binding to phosphorylated Tyr residues on RTKs and catalyses the phosphorylation of PI(4,5)P2 to PI(3,4,5)P3
What is the source of PIP2?
- PI is a phospholipid in cell membranes
- It is phosphorylated to PI(4)P
- This is phosphorylated to PI(4,5)P2
Describe the structure of phosphatidylinositol.
Phosphatidylinositol is classified as a glycerophospholipid that contains a glycerol backbone, two non-polar fatty acid tails, a phosphate group substituted with an inositol polar head group
What is the function of PIP3?
- PIP3 acts as a docking site for PDK1 & Akt
- PDK1 = phosphatidylinositol dependent kinase & Akt = protein kinase B
Describe the interaction between PDK1 and Akt.
Upon binding to PIP3, PDK1 phosphorylates & activates Akt
What is Bad protein and what are its functions?
- Bad is a cytoplasmic protein that complexes with apoptosis inhibitory proteins, inactivating them
- Akt phosphorylates BAD, which causes the release of death inhibitory proteins & the prevention of apoptosis
How is mTOR activated and what are its functions?
- Akt phosphorylates kinase mTOR
- mTOR complex 1 stimulates cell growth by:
1 - Promoting ribosome production & protein synthesis
2 - Inhibiting protein degradation
- mTOR complex 2 stimulates cell survival by helping activate Akt
In what 4 ways can the PI3K pathway can be turned off?
1 - Removal of the extracellular signal
2 - Switching off activated RTKs by protein tyrosine phosphatases (PTPs)
3 - Dephosphorylation of target proteins by serine/threonine phosphatases
4 - PTEN = inositol lipid phosphatase which removes phosphate from PIP3 (forming PIP2), so it can no longer act as docking site for PDK1 & Akt
What is HER2?
- An oncoprotein with an orphan receptor (no known ligand)
- Activated by heterodimerisation with other EGFR family members
What is the treatment for HER2 overexpression?
Trastuzumab (Herceptin) – humanised anti-HER2 antibody, which causes HER2 internalisation/ degradation & ADCC
What causes chronic myelogenous leukaemia?
- Chronic myelogenous leukaemia (CML) is usually associated with the Philadelphia chromosome, which forms from translocation between chromosomes 22 & 9
- Breakage & re-joining occurs at the sites of BCR & ABL genes, creating a hybrid gene encoding hybrid BCR-ABL fusion protein
- BCR-ABL stimulates inappropriate proliferation of haematopoietic precursors & prevents apoptosis, leading to an accumulation of excessive WBCs in the blood
What is the treatment for ABL-BCR fusion gene expression?
Imatinib (Gleevec) – a synthetic ABL kinase inhibitor that blocks the ATP binding site of Tyr kinase domain of BCR-ABL
Why do patients show response then relapse with imatinib (Gleevec)?
Patients show response then relapse due to resistance via secondary mutations in Tyr kinase domain (preventing the drug binding)
