Signalling Mechanisms in Growth and Division Flashcards
What transcription factor is stimulated by growth factor signalling and is vital to starting the cell cycle?
c-Myc
Give an example of an anti-cancer drug that targets tyrosine kinase receptors.
Herceptin – inhibits the Her2 tyrosine kinase receptor (important in many tumours e.g. breast)
Broadly speaking, how might Ras signalling be different in cancer?
Ras could be permanently switched on (in the GTP bound form), thus it constantly signals cell division
What is the family that the ERK cascade belongs to called?
MAPK cascade (Mitogen-activated protein kinase cascade)
What are the three kinases involved in the ERK cascade?
Raf
MEK
ERK
What important gene is turned on by the kinase cascade?
c-Myc
What type of kinase are cyclin-dependent kinases (Cdks)?
Serine-threonine kinases
What conditions do Cdks require to become activated?
Binding to cyclin
Phosphorylation (activating phosphorylation and removal of inhibitory phosphorylation)
What does the mitosis-promoting factor (MPF) consist of?
Cdk1 + cyclin B
What are the requirements, in terms of phosphorylation, for MPF to become active?
Cdk activating kinase (CAK) adds an activating phosphate and Wee1 adds an inhibitory phosphate at the same time
Cdc25 then removes the inhibitory phosphate
What activates MPF at the end of interphase?
Removal of the inhibitory phosphorylation by Cdc25
Describe the positive feedback loop that is formed by MPF activation.
Removal of the inhibitory phosphorylation by Cdc25 produces active MPF, which then phosphorylates Cdc25 and increases its activity meaning that more MPF can be activated
Which Cdk/cyclin is required for G1/S phase?
Cdk2-cyclin E
Which Cdk/cyclin is required for S phase?
Cdk2-cyclin A
How can the same Cdk be used for two different stages?
Cyclin binding alters the substrate specificity of Cdk
Also, different substrates are available at different stages of the cell cycle
What is one of the most important transcription targets of c-Myc?
Cyclin D
What is the first Cdk/cyclin complex that is formed when a cell goes from G0 to G1?
Cdk4/6-cyclin D
This Cdk/cyclin complex then stimulates the expression of the next cyclin in the cell cycle. What properties does this system give to the cell cycle?
This gives the cell cycle direction and timing (because the Cdk-cyclin complexes must reach a certain concentration before they can trigger the next stage of the cycle)
Give an example of a phosphorylation target of MPF that allows the cell cycle to progress.
Phosphorylation of nuclear lamins allows breakdown of the nuclear envelope
What is start kinase and what is one of its most important targets?
Start kinase = Cdk2-cyclin E
Retinoblastoma
Describe the role of retinoblastoma in the quiescent G0 state.
Retinoblastoma is unphosphorylated and binds to and sequesters a group of transcription factors called E2F
Role of Cdk4/6-D
It multiply phosphorylates retinoblastoma – as it becomes phosphorylated it loses its affinity for E2F and releases E2F
This means that the E2F transcription factors can regulate gene expression and promote progression of the cell cycle
What is one of the main targets of E2F?
Cyclin E
What type of gene is retinoblastoma?
Tumour suppressor gene (it acts a break on the cell cycle)
The initial release of E2F allows transcription of cyclin E leading to the formation of Cdk2-cyclin E. What effect does this complex have on retinoblastoma?
Cdk2-cyclin E further phosphorylates retinoblastoma so more E2F is released and the concentration of E2F increases
What is the significance of the increasing concentration of E2F?
This means that E2F can now bind to targets with a lower affinity (e.g. cyclin A gene promoter isn’t activated until the E2F concentration is high enough)
What are the two families of Cdk inhibitors?
NK4
CIP/KIP
During which phase do each of the NK4 and CIP/KIP act and how do they inhibit Cdk?
INK4 – G1 phase – it displaces cyclin D from the Cdk4/6-cyclin D complex
CIP/KIP – S phase – it binds to the Cdk/cyclin complexes and inhibits them
NOTE: these inhibitors need to be degraded at various stages for the cell cycle to progress
State some common and important oncogenes.
EGFR/HER2 Ras Cyclin D1 B-Raf c-Myc
State some important tumour suppressor genes.
Rb – inactivated in many cancers
p27KIP1–under-expression correlates with poor prognosis in many malignancies
When are Cdks present
Present throughout cell cycle in proliferating cells to control the cycle
Life of cyclins
Transiently expressed
Levels regulated by levels of expression
Are quickly synthesised then degraded
What happens when growth factor binds to GFR and where does ligand normally come from
Dimerisation with another ligand bound receptor- this allows trans-autophosphorylation of the others tyrosine residue
What type of receptor is GFR
Tyrosine kinsae
What does phosphorylation of tyrosine residue of GFR allow
Docking of Grb2 protein via its SH2 region
Describe structure of Grb2 protein
Two SH3 regions surrounding central SH2
What does SH3 region of Grb2 bind to
Proline rich region of SOS
What does SOS do
It is an exchange factor- exchanges GDP on RAS for GTP thus activating it
RAS is deactivated by what
GTPase activating protein
Role of Ras
To phosphorylate Raf
Where are Ras and Raf found
They are membrane bound
Phopsphorylating chain
Ras
Raf
MEK
ERK
What do Raf and Mek use to phosphorylate their next protein
ATP
What happens to ERK after it is phosphorylated
Goes to nucleus and phosphorylates C-Myc protein
