Cell Signalling And Cancer Flashcards
Which class of receptors is important in for cancer cells?
Receptor tyrosine kinase
What do these receptors do? Why are they important to cancer cells
These receptors trigger mitogen activated protein (MAP) kinase and phosphatidylinositol (PI)3 kinase pathways are triggered by signal molecules after interaction with the receptors
MAP KINASE PATHWAY
Check poster for more detail
Signal molecule -> RTK-> Grb-2 -> Ras-Gef -> Ras -> raf -> Mek -> Erk -> cell growth
Cell growth is the main output but also cell differentiation
Modification of target proteins is by
The phosphorylation and dephosphorylation
Phosphorylation can either activate or inhibit target proteins
What does protein kinase do?
It catalyses the transfer of the terminal phosphate group of ATP to specific Ser, Thr or Tyr residues on target proteins
Step 1 of activation of RTK
Binding of the signalling molecule induces receptor dimerisation.
This enables domains of neighbouring receptors to cross phosphorylate each other on multiple Tyr residues (auto phosphorylation)
Step 2 what does this activation of RTK create
After the dimerisation and auto phosphorylation of the Tyr
The phosphorylation creates a docking site for a variety of proteins
- Grb-2 ( MAP kinase pathway)
- PI 3-kinase (PI 3-kinase pathway)
What does Grb-2 stand for?
Growth factor receptor-bound protein 2
What domains does Grb-2 have?
SH2 domain (Src homology 2) which docks to the phosphorylation Tyr residues on the RTK SH3 domain (Src homology 3) which is binds to protein called Sos Sos and Grb-2 complex together
What does the Sos and Grb-2 complex do
The complex enables Sos to recruit and activate Ras.
What is Ras?
Ras is a small G-protein with GTPase activity
Contains a covalently attached lipid group that attaches it to the plasma membrane.
It is an oncogene and is found mutated in 30% of cancers
What is Sos?
Sos stands for son of seven less
It is a guanine nucleotide exchange factor (GEF)
It acts on Ras exchange GDP for GTP activating it
Ras can then be inactivated by a GTPase activating protein by stimulating its GTPase activity which then removes a Pi to make a GDP which is inactivated Ras
Ras activation cascade of proteins
Activate raf -> Mek -> Erk
What happens at the end of the cascade
The phosphorykated erk - map kinase can then activate transcription factors,
Activate other kinases or affect gene regulation
Resulting changes result in growth and proliferation
Genes activated can be c-fos c-Jun c-myc
How to turn off the signal to RTK
Remove the extracellular signal
Switching off activated receptor tyrosine kinases by protein tyrosine phosphatatses
Ras GAPs to turn back to Ras-GDP
Ddephosphorylate target proteins by serine/threonine phosphates
Why is this tight regulatory control essential?
Turning the protein on all the time like Ras when RTK is on all the time can result in gene mutations which predispose to cancer
So c-myc, c-jun, c-fos, being constantly switched on can increase risk of mutated proteins
What is the PI 3-kinase pathway
Signal molecule -> RTK -> PI 3-kinase -> PIP3 -> PDK1 -> Akt (PKB) -> cell survival OR -> mTOR -> cell growth
What happens once the RTK is dimerised and Tyr residues are autophosphorylated
The PI 3-kinase binds to the phsphorylated Tyr residues on the RTKs
PI 3-kinase catalysts the phosphorylation of PIP2 to PIP3
What is PI - phosphatidylinositol
It is a phospholipid found in eukaryotic cell membranes
It is phosphorylated to form PIP1 -> PIP2 –> PIP3
What happens after PIP 3 formation
PIP3 acts as a docking station for 2 proteins, PDK1, and Akt
Upon binding PKD1 phosphorylates and activates Akt.
What does active Akt do?
It phosphorylates target proteins such as the protein BAD
BAD is phosphorylated and then releases death inhibitory proteins and prevents apoptosis
Leading to cell survival
What is the other target of Akt and then what happens when that is activated.
Other target is mTOR - mammalian target of rapamycin
It exists in two functionally distinct complexes :
mTOR complex 1 stimulates cell growth by-
Promoting ribosome production and protein synthesis and inhibiting protein degradation
mTOR complex 2 stimulate cell survival and cell growth by:
Helping to activate aKt
How to turn the PI 3kinase signal off
Remove the extracellular signal
Switch off the activated RTK by protein tyrosine phosphatases
Dephosphorylate target proteins by serine/ threonine phosphatases
PTEN which is a tumour suppressor is an inositol lipid phosphatase which removes phosphate from PIP3 to make a PIP2 which means PKD1 and Akt can no longer use it as docking site.
What is stratified/ personalised medicine ?
More effective way of treating cancer by grouping patients according to the genetic lesion or signalling pathway to which it contributes
Example HER2 - what kind of receptor is it?
Member of the EGFR family
It is an orphan receptor- no known ligand
What is HER2 activated by?
Heterodimerisation with other EGFR family members
Overexpression - percentage and what happens when it’s over expressed?
30% of breast cancers
Over expression/amplification/ mutation leads to constitutive signalling
What does HER2 positive status mean for the patient?
Shortens survival
Positive: 3 years
Neg: 6-7 years
Intervention called and what it does?
Herceptin - trastuzumab monoclonal Ab
Targets the HER2 causing internalisation and degradation of the receptor
Antibody dependent cellular cytotoxicity (ADCC)
Effective treatment for HER pos patients not for HER beg
Chronic myeloid leukaemia is associated with?
Philadelphia hormone - translocation between chromosome 22 and 9 - abnormally fused together so the BCR and ABL fused.
Hybrid gene codes for a hybrid protein
What is ABL?
It is a cytoplasmic Tyr kinase that promotes cell survival and cell growth.
How is ABL different?
The normal N terminus of ABL is substituted with BCR constitutively activating ABL Tyr kinase activity.
What does BRC-ABL cause ?
It stimulates inappropriate proliferation of haematopoietic precursor cells, and prevents then from dying from apoptosis. As a result excessive numbers accumulate in the, blood stream, producing CML.
Intervention for BRC-ABL
Imatinib Gleevec- a synthetic ABL kinase inhibitor that blocks the ATP binding pocket of the Tyr kinase domain of BRC-ABL
Effects in 80% of CML patients
Results less good in patients that have progressed to the active phase they showed response and relapse as a result of resistance due to secondary mutations in the Tyr kinase binding domain that prevent the drug from binding.
Features of intracellular signalling pathways
STAR
S - specificity provided by molecular interactions - proteins recognise specific ligands
T - transduce signals across the plasma membrane and into the cell - changes in 3 D structure of the receptor inside and outside the cell
A - amplify the external signal within the cell - done via cascades
R - regulate cellular function - this is done via feedback cycles
5 layers of signalling pathways
Signal Receptor Signal transduction Intracellular targets Cellular response
What affect can signalling have
Which pathways are important in cancer
Proliferation - mitogenesis
Uncontrolled continuous cell division
Leads to increased cell number
- causes formation of a tumour mass
Survival - anti apoptosis
Loss of a cells programmed cell death ability after a given number of divisions or after losing contact with its substrate
Motility - invasion and metastasis
The ability of a cell to move from the primary tumour mass
Into the blood stream or lymph system
And back out again at a secondary site
Types of mitogenic agonists
Polypeptide GF - EDGF, PDGF, VEGF
Their receptors have intrinsic tyrosine kinase activity
Transduction of signals occurs via phosphorylation of effectors
Peptide lipids
LPA bomesin
GPCR activity
Cytokines
TNF alpha, SCF, GMCSF
Bind to specific transducers
4 signalling pathways which are important in cancer
MAIM
proliferation - mitogenesis
Survival - anti-apoptosis
Motility - invasion and metastasis
Define apoptosis
Programmed cells death when stimulated by the appropriate trigger, may result when the cell is no longer needed or or becomes a threat to the organisms health
Define anoikis
A form of programmed cell death which is induced by anchorage - dependent cells detaching from the surrounding ECM
Define necrosis
A form of cell death that results from injury, disease, or other pathological state.
What goes wrong to initiate cancer development
Protein mutation - activating
A mutation resulting in a protein in the pathway generating an output signal without. Ring told to do so by an input signal
Protein mutation - inactivating/deletion
Removing a signal that is usually present to block or slow down a pathway
Protein addition
Adding an input signal to a pathway that is complimentary but not usually present
Protein amplification
Generating more output signals from low or normal input signal levels
What are second messenger and what do they do
Amplify signalling pathway as one upstream signal results in the generation release of many downstream second messenger signals
They are rapidly generated/ released in high numbers
Rapidly diffusible
Rapidly removed
What is Sos
GEF
Guanine nucleotide exchanged factor
