Cell Signalling And Cancer

Question 1

Which class of receptors is important in for cancer cells?

Answer 1

Receptor tyrosine kinase

Question 2

What do these receptors do? Why are they important to cancer cells

Answer 2

These receptors trigger mitogen activated protein (MAP) kinase and phosphatidylinositol (PI)3 kinase pathways are triggered by signal molecules after interaction with the receptors

Question 3

MAP KINASE PATHWAY

Answer 3

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

Question 4

Modification of target proteins is by

Answer 4

The phosphorylation and dephosphorylation

Phosphorylation can either activate or inhibit target proteins

Question 5

What does protein kinase do?

Answer 5

It catalyses the transfer of the terminal phosphate group of ATP to specific Ser, Thr or Tyr residues on target proteins

Question 6

Step 1 of activation of RTK

Answer 6

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)

Question 7

Step 2 what does this activation of RTK create

Answer 7

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)

Question 8

What does Grb-2 stand for?

Answer 8

Growth factor receptor-bound protein 2

Question 9

What domains does Grb-2 have?

Answer 9

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

Question 10

What does the Sos and Grb-2 complex do

Answer 10

The complex enables Sos to recruit and activate Ras.

Question 11

What is Ras?

Answer 11

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

Question 12

What is Sos?

Answer 12

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

Question 13

Ras activation cascade of proteins

Answer 13

Activate raf -> Mek -> Erk

Question 14

What happens at the end of the cascade

Answer 14

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

Question 15

How to turn off the signal to RTK

Answer 15

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

Question 16

Why is this tight regulatory control essential?

Answer 16

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

Question 17

What is the PI 3-kinase pathway

Answer 17

Signal molecule -> RTK -> PI 3-kinase -> PIP3 -> PDK1 -> Akt (PKB) -> cell survival OR -> mTOR -> cell growth

Question 18

What happens once the RTK is dimerised and Tyr residues are autophosphorylated

Answer 18

The PI 3-kinase binds to the phsphorylated Tyr residues on the RTKs
PI 3-kinase catalysts the phosphorylation of PIP2 to PIP3

Question 19

What is PI - phosphatidylinositol

Answer 19

It is a phospholipid found in eukaryotic cell membranes

It is phosphorylated to form PIP1 -> PIP2 –> PIP3

Question 20

What happens after PIP 3 formation

Answer 20

PIP3 acts as a docking station for 2 proteins, PDK1, and Akt
Upon binding PKD1 phosphorylates and activates Akt.

Question 21

What does active Akt do?

Answer 21

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

Question 22

What is the other target of Akt and then what happens when that is activated.

Answer 22

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

Question 23

How to turn the PI 3kinase signal off

Answer 23

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.

Question 24

What is stratified/ personalised medicine ?

Answer 24

More effective way of treating cancer by grouping patients according to the genetic lesion or signalling pathway to which it contributes

Question 25

Example HER2 - what kind of receptor is it?

Answer 25

Member of the EGFR family | It is an orphan receptor- no known ligand

Question 26

What is HER2 activated by?

Answer 26

Heterodimerisation with other EGFR family members

Question 27

Overexpression - percentage and what happens when it's over expressed?

Answer 27

30% of breast cancers | Over expression/amplification/ mutation leads to constitutive signalling

Question 28

What does HER2 positive status mean for the patient?

Answer 28

Shortens survival Positive: 3 years Neg: 6-7 years

Question 29

Intervention called and what it does?

Answer 29

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

Question 30

Chronic myeloid leukaemia is associated with?

Answer 30

Philadelphia hormone - translocation between chromosome 22 and 9 - abnormally fused together so the BCR and ABL fused. Hybrid gene codes for a hybrid protein

Question 31

What is ABL?

Answer 31

It is a cytoplasmic Tyr kinase that promotes cell survival and cell growth.

Question 32

How is ABL different?

Answer 32

The normal N terminus of ABL is substituted with BCR constitutively activating ABL Tyr kinase activity.

Question 33

What does BRC-ABL cause ?

Answer 33

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.

Question 34

Intervention for BRC-ABL

Answer 34

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.

Question 35

Features of intracellular signalling pathways

Answer 35

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

Question 36

5 layers of signalling pathways

Answer 36

``` Signal Receptor Signal transduction Intracellular targets Cellular response ```

Question 37

What affect can signalling have | Which pathways are important in cancer

Answer 37

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

Question 38

Types of mitogenic agonists

Answer 38

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

Question 39

4 signalling pathways which are important in cancer

Answer 39

MAIM proliferation - mitogenesis Survival - anti-apoptosis Motility - invasion and metastasis

Question 40

Define apoptosis

Answer 40

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

Question 41

Define anoikis

Answer 41

A form of programmed cell death which is induced by anchorage - dependent cells detaching from the surrounding ECM

Question 42

Define necrosis

Answer 42

A form of cell death that results from injury, disease, or other pathological state.

Question 43

What goes wrong to initiate cancer development

Answer 43

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

Question 44

What are second messenger and what do they do

Answer 44

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

Question 45

What is Sos

Answer 45

GEF | Guanine nucleotide exchanged factor