Cell signalling and cancer Flashcards
What kind of signalling molecules can bind to intracellular receptors?
Small, hydrophobic molecules that can pass through the membrane e.g. steroids / hormones
What kind of signalling molecules can bind to extracellular receptors?
Hydrophilic proteins e.g. insulin
What happens once a molecule has bound to an extracellular receptor?
It induces an intracellular signal transduction to alter target proteins for metabolism, gene expression and structure.
What is a receptor Tyrosine Kinase?
Enzyme-coupled receptor on the cell surface, that spans the membrane once.
The extracellular domain may vary
The intracellular domain is a cytoplasmic TK.
What does TK do?
TK phosphorylates tyrosine (Tyr) residues by catalysing the transfer of a terminal phosphate group from ATP onto self ser, Thr or Tyr.
Give 3 examples of signalling proteins that will activate RTK?
EGF, Insulin, PDGF
What pathway is used by RTK?
MAP kinases pathway (mitogen activated)
PI3-kinase pathway (Phosphatidylinositol)
What does TK activity result in?
Either activates or inhibits the target protein.
Is phosphate addition reversible?
Yes with phosphatases
What happens when a signalling molecule binds to RTK?
Induces receptor DIMERISATION. The TK domains then cross-phosphorylate each other at multiple points for AUTOPHOSPHORYLATION.
What does the phosphorylation of Tyr residues allow?
Creates a high affinity binding site for cytoplasmic proteins such as:
Grb-2 for MAP kinase pathway
PI3-kinase
What induces the MAP kinase pathway? What does the pathway result in?
GF
GROWTH and PROLIFERATION
Give the overall order of the proteins involved in MAP kinase pathway.
Signalling molecule —> RTK —> Grb-2 —> Ras —> Raf —> Mek —> Erk = Cell GROWTH and PROLIFERATION
What is Grb-2?
A GF receptor bound protein. It has a SH2 domain that can recognise phosphorylated Tyr residues on RTK. The SH3 domain binds to Sos through its proline rich motif.
What results from the formation of receptor - Grb2 - Sos complex?
Recruits and activates Ras.
What is Ras?
A G protein with intrinsic GTPase activity. It is an oncogene attached to the plasma membrane.
What is Sos?
GEF - guanine nucleotide exchange factor that activates Ras by stimulating it to exchange GDP for GTP.
What inactivates Ras?
GAPs - GTPase activating proteins stimulate Ras’ intrinsic activity to hydrolyse GTP to GDP.
What happens after Ras has been activated?
It activates Raf by changing its formation. Raf is a kinase that goes on to activate Mek and Erk.
What are Raf, Mek and Erk?
They are serine 3 kinases also referred to as MAP kinases.
What is the function of Erk?
Erk phosphorylates target proteins such as kinases, gene regulatory proteins and TF in the nucleus to induce changes in cell behaviour.
Which gene regulatory proteins are activated by Erk?
c-Myc, cyclin D, c-Myb, c-fos, c-jun
How is MAP kinase pathway switched off and why is it important to do so?
Removal of extracellular signal.
Protein tyrosine PHOSPHATASES switch off RTK.
GAPs inactivate Ras.
Dephosphorylation of target proteins by phosphatases.
Allows tight control to prevent constitutive activation.
Which genes involved in this pathway are oncogenes that predispose to cancer?
RTKs, Ras, jun, fos, Myc
What is the result in a cell from the activation of PI3 kinase pathway?
Cell SURVIVAL, GROWTH and PROLIFERATION.
Give an overview in order of the proteins involved in the PI3 kinase pathway.
Signalling molecule —> RTK —> PI3 kinase —> PIP3 —> PDK1 —> Akt —> Survival or mTor —> Growth and Proliferation
How is PI3 kinase activated and what is its function?
By binding to phosphorylated Tyr residues on RTKs. It catalyses the phosphorylation of PIP2 into PIP3.
What is PIP3?
PI(3,4,5)P3 is a phospholipid in cell membranes. It produces a high affinity docking site for PDK1 and Akt.
Why must both PDK1 and Akt bind to PIP3?
PDK1 phosphorylates Akt.
What is the function of Akt when active?
It dissociates from the membrane to phosphorylate BAD = release of anti-apoptotic molecules for survival.
Targets mTOR.
What are the two types of mTOR and their functions?
Complex 1 = stimulates growth by promoting ribosome production and protein synthesis. Inhibits protein degradation.
Complex 2 = Stimulates cell survival and growth by helping PDK1 and activate Akt.
How is PI3 kinase pathway regulated?
Tyrosine PHOSPHATASES will switch off RTK and dephosphorylate target proteins.
pTEN is a tumour suppresspor inositol phosphatase that removes phosphate from PIP3 so it cannot act as a docking site.
What is HER2?
An oncogenic protein of the EGF family. It is an orphan receptor with no known ligand and so responds to ligand of EGFR.
What happens to EGFR1 when it recieves a GF signal?
Homodimerises with another EGFR1.
What happens to HER2 when it recieves a GF signal?
It heterodimerises with EGFR1 for growth and survival through PI3 kinase and MAP kinase pathway.
What happens if HER2 is overexpressed?
Overexpression due to gene amplification or mutation, allows HER2 to HOMOdimerise when in excess and signal in the ABSENCE of a GF. This causes constitutive signalling and abnormal growth.
In what cancer is HER2 overexpressed?
Breast Cancer
What therapy is used to target HER2 constitutive signalling in cancer?
Anti-HER2 antibody e.g. Transtuzumab, Herceptin.
Causes HER2 internalisation and degradation and ADCC (antibody dependent cellular cytotoxicity).
In what kind of cancers are anti-HER2 antibodies effective?
In cancers that are HER2 +ve.
How is the HER2 antibody produced to minimise immune response on administration?
Monoclonal Ig produced in mice but humanised by switching the sequences.
How does the Philadelphia chromosome form?
Through translocation of chromosome 22 and 9. It rejoins at the sites of BCR on 22 and Abl on 9.
What does the Philadelphia chromosome result in?
Creates a hybrid gene that encodes for a BCR-ABL fusion/chimeric protein.
What is the normal function of Abl gene?
Cytoplasmic TK for survival and growth
What happens to Abl when in the chimeric protein?
The N-terminus of Abl is substituted for BCR sequences to constitutively activate Abl TK activity. This stimulates inappropriate proliferation haematopoietic precursor cells and prevents apoptosis.
What type of cancer is the BCR-Abl mutation associated with?
Chronic Myeloid Leukaemia - Uncontrolled proliferation of haematopoietic stem cells leading to excessive WBCs accumulating in the blood stream.
What treatment is used in CML?
Abl Kinase inhibitor e.g. Imatinib. Blocks ATP binding pocket on the TK domain of BCR-Abl to prevent phoshorylation of proteins.
Why do only 80% of patients respond to Imatinib?
Pts in acute phase CML with show an initial response but the relapse as a result of resistance and second mutations in the TK domain to prevent the drug from binding in the pocket.
