BOD L13 Growth regulation and signal transduction Flashcards
What are the six main hallmarks of cancer?
Sustaining proliferative signalling
Insensitivity to growth inhibitors
Enabled replicative immortality
Induced angiogenesis
Resisting apoptosis
Invasion and metastasis
What is the model for GF signalling?
EGF signalling
Family members ErbB1/2/3/4
HER1/2/3/4
How is EGF signalling subverted in cancers?
Mutation causes truncated form of protein. Means signalling is consitituitively on, even in absence of ligand.
EGF receptors are transmembrane. What set of events occurs after binding of ligand?
Binding of ligand to extracellular domain results in:
- confomrational change to receptor
- activation of cytoplasmic tail, which links to downstream pathways via adaptor proteins.
This effects cell proliferation.
Name a few signalling proteins that act via RTK’s
What are the three routes of dimerisation for TKRs?
Give three examples
Ligand induced dimerisation, receptor dimersises upon ligand binding, EGF.
Predimerisation, where receptor is dimer and ligand binding alters conformation. - IGF1
Ligand dimers - dimer ligand which causes receptor dimerisation upon binding. - PDGF
What state must RTK’s be to be active?
What event occurs to activate?
Must be dimerised to be active.
Autophosphorylation.
How is the phosphorylated tyrosine recoginsed during RTK signalling?
By proteins containing SH (Src homology) domains.
What role do SH domain containing proteins perform during RTK signalling?
Serve as adaptor proteins to couple receptors to
Ras (GTPase)
What to binding sites are seen in a SH2 domain?
Binding site for phosphotyrosine, binding site for amino acid side chain
What is the result of signalling via RTKs?
Cascade of protein serine/threonine phosphorylations.
Results in signal being transmitted from cell surface to nucelus, via Ras
How is Ras subverted in cancer?
What is it also known as?
Proto-oncogene.
Gain of function mutations to increase activity.
30% of human tumours have hyperactive Ras mutation.
Also known as p21
How is Ras activated and inactivated?
Ras is active when bound to GTP, but inactive when bound to GDP.
As a GTPase, it therefore must have continual supply of GTP to be active.
What two proteins influence activity of Ras?
Activating - GEF (guanine nucleotide exchange factors exchanges GDP for GTP.
Inactivating - GAP (GTPase-activating proteins) increases rate of GTP hydrolysis on Ras, more GDP.
Where can Ras be found within the cell?
Ras is anchored to the cytoplasmic side of the plasma membrane.
How is Ras activated by a RTK?
When RTK is activated, adaptor protein binds (with SH domain) which activates a GEF. This provides GTP for Ras activation.
In EGF signalling, what is the adaptor protein?
What is the GEF?
GRB.
SOS.
Once Ras is activated during RTK signalling, what system relays this?
MAP-kinase cascade.
MAP-K-K-K, MAP-K-K, MAP-K
What is the most common TKR up-reg in breast cancer?
HER2.
25-50 copies of the gene made, 40-100 fold increase in HER2 on cancer cells.
What are the main ligands of HER1/2/3/4
How does HER2 function?
It doesn’t have a ligand, so dimerises with other RTKs
How does HER2 impact prognosis of breast cancer?
Positive cancers are more aggressive and less sensitive to hormone therapy.
But good as can be targeted by mAb therapy (Herceptin)
How does herceptin (trastuzumab) work?
Blocks downstream signaling pathways from HER2, as well as flags cancerous cells for ADCC
What is the most characterised downstream target of Ras?
Raf kinase
What isoforms of Raf exist?
C-Raf (Raf-1)
A-Raf
B-Raf
What is Raf kinase?
Family of serine/threonine kinases.
Which isoform of Raf is implicated to be mutated in 50% of melanoma tumours?
B-RAF, V600E mutation
Oncogenic mutations of Ras mean what for its binding?
Activating mutations cause Ras to be locked in active state, with GTP bound
What are the two main effector pathways of Ras?
RAF-MEK-ERK.
PI3K.
What is the function of PI3K when activated by Ras?
Converts PIP2 into PIP3
How is PIK3C involved in cancer?
Obtains ‘gain of function’ mutation to become oncogenic and is active even in absence of RAS-GTP
How can PI3K be activated in the absence of Ras?
Stimulated by RTK directly
What is the result of PI3K action?
PDK1 activation and phos. of Atk (aka PKB)
PKB is antiapoptotic as it phosphorylates cell death protein Bad
What two members of Bcl2 family oppose one another in apoptosis?
Bad - pro-apoptosis
Bcl2 - anti-apoptosis
In proliferating cells, how is Bad inhibited?
Phosphorylated, bound to cytoplasmic component (does not play a role in inducing apoptosis)
In non-proliferating cells, how is Bad active?
Bad is dephoshphorylated and blocks action of Bcl2
How does EGF signalling prevent apoptosis?
What is the important function Atk (PKB) plays during EGF signalling?
Anti-apoptotic by phos. Bad
What is the therapeutic target against cancer involved in the PI3K signaling pathway?
mTOR - PI3K-like kinase (PIKKs)
What is a good way of targeting mTOR?
Rapamycin.
What are two alternative parters of mTOR?
Rictor and Raptor.
Rictor - Activates Akt, drives proliferation.
Raptor - Increases protein synthesis, drives cell growth
What happens to cancer cells with hyperactive PI3K when treated with rapamycin?
Cells are addicted to Atk/PKB proliferation signal.
Rapamycin blocks mTOR stimulation of Akt/PKB. The cells cannot sustain proliferation without Akt and die by apoptosis.
