Cytoplasmic signalling and cancer Lecture 6 Flashcards
Describe the types of receptors
G protein coupled recptors or Receptor tyrosine kinases
Pathways actiated by G protein coupled receptors:
MAP kinase pathways, Cyclin AMP/PKA pathway, PI3 kinase pathway.
Phosphorylation and dephosphorlyation modifiy the activity of many target proteins - Protien kinases add phosphate onto specific Ser,Thr,Tyr residues on target proteins. Phosphorlyation can either activate or inactivate proteins.
Reeptor tyrosine kinases (enzyme coupled receptors)
Single pass membrane spanning proteins, lots of extracellular and intracellular domains which allow interaction with different protteins, all have cytoplasmic tyrosine kinase domains.
Describe the PI3-kinase pathway
- Causes cell survival and cell growth through PKB pathway
- Activation is by binding of phosphorylated Tyr residues on RTKs
- PI 3-kinase catalyses the phosphorylation of PIP2 -> PIP3
Receptor tyrosine kinase➡ PI3-kinase➡ PI(3,4,5)P3➡ PDK1➡ Akt➡ mTOR Cell survival and proliferation
How do receptor tyrosine kinases (RTKs) produce a docking site for other proteins?
- Binding of ligand induces receptor dimerisation with another bound RTK (receptor tyrosine kinases)
- They cross-phosphorylate each other on multiple Tyr residues
- This creates high-affinity docking sites for a variety of proteins:
PI 3-kinase
Grb-2 (MAP kinase pathway)
What are phosphatidylinositol (PI) and How is PIP3 formed and what does it do?
- PI is phospholipid found in eukaryotic cell membranes
- It is phosphorylated into PIP, then again to PIP2
- PI 3-Kinase turns PIP2 -> PIP3
- PIP3 acts a docking site for PDK1 and Akt
- Upon binding, PDK1 phosphorylates and activates Akt
What is the action of Akt?
Survival
-Phosphorylates BAD which releases death inhibitory proteins and prevents apoptosis
Cell Growth
-Targets mTOR, which exists in two protein complexes
mTOR complex 1 - sensitive to rapamycin and stimulates cell growth by promoting ribosome production and inhibiting protein degradation
mTOR complex 2 - insensitive to rapamycin and helps Akt which stimulates cell survival
How is mTOR complex 1 activated?
- Activate GTP-bound Rheb is required to activate mTOR
- In absence of active Akt, Rheb is inactivated by Tsc2 so mTOR is not activated
- In presence of Akt, Tsc2 is inactivated so Rheb is active and is able to activate mTOR complex 1 which then promotes cell growth
How do you turn the growth and survival signal off?
Removal of receptor ligand
Switching the activated RTK off using protein tyrosine phosphatases (PTPs- SHP1, SHP2)
Dephosphorylation of target proteins by serine/threonine phosphatatases
What is PTEN
- Removes phosphate from PIP3 so it can no longer act as a docking site for PDK1 and Akt
- Tumour suppressor gene which is mutated in 50% of all cancers
Cancer can be treated more effectively by:
- By stratifying patients according to their genetic mutation
- Some drugs will be ineffective against certain mutations
Describe the importance of the EGF receptor family in cancer
- EGFR1 is activated by EGF and TGFalpha
- EGFR2 (HER2) is an orphan receptor (no ligand) and is activated by heterodimerisation with other family EGFRs
They can Trigger PI 3-kinase and MAP kinase pathways by:
- Grb2 binds to phosphotyrosine residues on RTKs
- Sos (bound to Grb2) activates Ras which activates Raf etc. -> cell growth
How can activation of EGFRs occur
EGFR1 homodimerises and cross-phosphorylates upon EGF binding, triggering PI3 kinase pathway
EGFR2 is an orphan receptor so heterodimerises with another EGFR member
- Both over amplified in cancer
- They both begin to homodimerise in absence of ligand, triggering cell growth and survival
Describe the treatment of cancer with monoclonal antibodies
Cetuximab- humanised anti-EGFR1 antibody. Blocks ligand binding or inhibits recptor dimerisation
Only approved in patients with wildtype Ras
Used to treat colon, lung, head, neck and pancreas cancer
Trastuzumab (herceptin)- humanised anti-HER2 antibody
Used in patients over expressing HET2
Used in breast cancer
How does the Philadelphia chromosome cause cancer and how can it be treated
Forms as a result of translocation between C22 and 9
- Joins BCR and ABL genes, creating a fusion protein
- ABL is a cytoplasmic Tyr Kinase, meaning BCR is constitutively activated
- BCR-ABL stimulates inappropriate proliferation of haematopoietic precursor cells, preventing them from dying -> WBCs accumulate in blood stream
Treatment
Imatinib
- Synthetic ABL kinase inhibitor that blocks the ATP binding pocket of the Tyr kinase domain of BCR-ABL
- Results are less promising in patients who have progressed to acute phase CML (blast crisis) as secondary mutations cause resistance.
