Lecture 4 - Cell Signalling and Cancer Flashcards
What is intracellular singalling?
a set of linked biochemical events that connect a specific biological stimulus with a specific cellular response
Communication between cells control….
cell behaviour
- survival
- division
- differentiation
- apoptosis
Signal molecules bind to specific receptors.
What are the two main types of receptor?
intracellular receptors e.g. steroid receptors
cell-surface receptors e.g. for insulin
Describe briefly the intracellular signal transduction pathway
- extracellular signal molecule
- receptor protein
- intracellular signalling proteins
- target proteins e.g. metabolic enzymes, gene regulatory protein, cytoskeletal protein
- response
What are the tyrosine kinase receptors?
Describe their structure
Enzyme coupled receptors are an important class of cell surface receptor
- Single pass membrane spanning protein
- Variety of extracellular domains
- Cytoplasmic TK domain –> phosphorylates tyrosine residues on target proteins
Give examples of some molecules that act through RTKs
EGF IGF NGF MCSF FGF VEGF Eprhins
Which 2 important pathways are activated by signal molecules interacting with receptor tyrosine kinases?
MAP (mitogen activate protein) kinase pathway
PI-3-kinase pathway - phosphatidylinositol
Give the 9 simple steps of the MAP kinase pathway
signal molecule receptor tyrosine kinase Grb-2 Ras-GEF Ras MAP-kinase-kinase-kinase (Raf) MAP-kinase-kinase (Mek) MAP-kinase (Erk) Cell growth
Phosphorylation and dephosphorylation modifies the activity of many target proteins. How?
Protein kinases catalyse the transfer of the terminal phosphate group of ATP to specific Set, Thr, Tyr residues on target proteins.
Protein phosphatases dephosphyrlate proteins
Describe dimerisation and autophophorylation and their functions
- Binding of the singnalling molecule induces receptor dimerisation
- Enable kinase domains of neighbouring receptors to cross-phosphorylate each other on multiple Tyr residues (autophosphorylation)
- Phosphorylation of Tyr residues creates high-affinity docking sites for a variety of proteins: Grb-2 (MAP kinase pathway) and PI 3-kinase (PI 3-kinase pathway)
What is Grb-2?
What does it do?
- Adaptor protein with SH2 domain which recognises specific phosphorylated Tyr residues on RTK
- Grb-2 also contains SH3 domain which binds to a proline rich motif in a protein called Sos
- Assembly of the receptor-Grb2-Sos complex enables Sos to recruit and activate Ras
What is Ras?
Where is it found?
- small protein with GTPase activity
- contains a covalently attached lipid group that attaches it to the plasma membrane
What is Sos?
What does it do?
Sos is a guanine nucleotide exchange factor (GEF) which activates Ras by stimlulating it to exchange GDP for GTP
How is Ras inactivated?
GTPase activating proteins (GAP) inactivate Ras by stimulating its intrinsic GTPase activity
one word answer:
What turns Ras on?
What turns Ras off?
on = Sos off = GAP
What does Ras do?
onward transmission of signal along multiple pathways
- Ras is not a kinase but it activates Raf which is a kinase
- Raf activates Mek
- Mek activates Erk
What are Raf, Mek and Erk examples of?
serine threonine kinases
What is the role of Erk?
Where do it act?
- Erk phosphorylates a variety of target proteins including other kinases and gene regulatory proteins in the NUCLEUS.
- The resulting changes in protein activity and gene expression cause complex changes in cell behaviour including cell growth/proliferation
- Genes activated include: c-jun, c-fos, c-myc, c-myb, cyclin D = all TF’s
In what 4 ways can the signal from the MAP kinase pathway be turned off?
- removal of extracellular signal
- switching off activated receptor tyrosine kinases by protein tyrosine phosphatases e.g. SHP1 and SHP2
- Ras GAPs
- desphosphorylation of target proteins by serine/threonine phosphatases
Why is tight regulatory of the MAP kinase pathway essential?
- constitutive activation of a protein in the pathway as a result of gene mutation predisposes to cancer
- many RTKs, Ras, jun, fos, myc etc are oncogenes
Give the 7 steps of the PI-3-kinase pathway
- signal molecule
- receptor tyrosine kinase
- PI-3-kinase
- PI(3,4,5)P3
- PDK1
- Akt (PKB) –> cell survival
- Akt (PKB) –> mTOR –> cell growth
How is the PI-3-kinase enyzme activated?
What does it then do?
by binding to Try residues on RTK
- catalyses the phosphorylation of PIP2 to generate PIP3
What is phosphatidylinositol?
Where is it found?
- a phospholipid found in eukaryotic cell membranes
- it is phosphyralted to form PIP, which is phosphphorylated again to PIP2 which is activated by PI-3-kinase
What happens down stream of PIP3?
PIP3 acts as a docking site for 2 proteins, PDK1 and Akt.
Upon binding PDK1 phosphorylates and activates Akt,
How does Akt induced cell survival?
Active Akt (activated by PDK1) phosphorylates a variety of targets including the protein BAD. Active BAD results in the release of death inhibitory proteins and the prevention of apoptosis
How does Akt induced cell growth and cell survival?
via what kinase?
Akt interacts with mTOR (kinase)
mTOR complex 1: stimulates cell growth by promoting ribosome production and protein synthesis, and inhaling protein degradation
mTOR complex 2: stimulates cell survival (and cell growth) by helping to activate Akt
In what 4 ways and the PI-3-kinase pathway be switched off?
- removal of extracellular signal
- switching off activated RTKs by protein tyrosine phosphatases
- dephoshphorylation of target proteins by serine/threonine phosphatases
- PTEN, an inositol lipid phosphatase which removes phosphate from PIP3, so that it not longer acts as a docking site for PDK1 and Akt
What is stratified (personalised) medicine?
a more effective way of treating cancer by grouping people according to the genetic lesion within individuals, and then targeting that lesion or signalling pathway to which it contributes
What are the two examples of personalised medicine from this lecture?
HER2 and Herceptin
BCR-ABL and Gleevec
What family is HER2 a member of?
What is meant by it being an ‘orphan receptor’?
How is HER2 activated?
What happens if HER2 is over expressed?
- epidermal growth factor family (EGFR)
- it has no ligand
- activated by heterodimerisation with other EGFR family members
- over expression leads homodimerisation of HER2 –> constitutive signalling (in the absence of a ligand)
How does HER2 status influence breast cancer survival?
HER2 positive –> 3 years
HER2 negative –> 6-7 years
What is the medical management for HER2 positive patients?
Trastuzumab (Herceptin)
- humanised anti-HER antibody that causes HER2 internalisation/degradation or ADCC
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Explain BCR-ABL and chronic myeloid leukaemia
- CML usually associated with the Philadelphia chromosome which forms as a result of a translocation between chromosomes 22 and 9
- breakage and rejoining occurs at the sites of the BCR and ABL genes, creating a hybrid gene that encodes a hybrid BCR-ABL fusion protein
- ABL is a a cytoplasmic Tyr kinase that promotes cell survival and cell growth
- Substitution of the normal N-terminus of ABL with BCR, constituently activates ABL Tyr kinase activity
- BCR-ABL stimulates inappropriate proliferation of haemopoetic precursor cells, and prevents them from dying from apoptosis
What is the medical management for CML?
Imatinib (Gleevec)
- synthetic ABL kinase inhibitor that blocks the ATP binding pocket of the Tyrosine kinase domain of BCR-ABL (ABL can no longer phosphorylate proteins)
