In absence of growth signals what do cells do?
Enter G0
- cell cycle machinery dismantled
- carry out function
Example of a gene that growth factors stimulate production of:
c-Myc
What does c-Myc do?
Transcription factor for cell cycle genes
Summarise the steps from growth factor release to cell cycle control genes being activated:
Growth factor released
Binds to receptor protein tyrosine kinases
Receptors dimerise + kinase domains phosphorylate eachother creating docking site for adapter protein (Grb2) which constitutively has Sos bound which activates Ras (GDP–>GTP)
Ras activates the protein kinase cascade (ERK cascade)
Immediate early genes are transcribed
Cell cycle control genes transcribed
What do the phosphorylation sites on the kinase domains of the receptor protein kinases form?
Docking sites for adapter proteins
Example of adapter protein:
Grb2
What is the function of adapter proteins?
brings molecules closer
How many domains does Grb2 have?
3
What is always (constitutively) bound to SH3 region of Grb2?
Sos
What is Sos?
Ras activating protein
What type of gene is Ras?
oncogene
What activates Ras?
GTP
How is Ras inactivated?
GTP–>GDP
If Ras activation needs to be faster how is this done?
GTP binding proteins
If Ras inactivation needs to be faster how is this done?
GTPase activating proteins
What does activated Ras activate?
Protein kinase cascade (ERK)
What is the protein kinase cascade also known as?
ERK cascade
2 things the protein kinase cascade causes to change and what this promotes:
cell proteins
Cell gene expression
Promotes cell cycle
ERK essentially……
phosphorylates proteins required for cell cycle
Example of genes that are phosphorylated by ERK:
immediate early genes e.g. c-Myc
How is Ras affected in cancer?
Hyperactivated
What do the V12 & L61 Ras mutations result in?
Ras can’t be inactivated
What is the key regulator of the cell cycle?
Cyclin dependent kinases (Cdks)
When are Cdks present?
In proliferating cells throughout the cell cycle
2 ways Cdk activity is regulated:
cyclins
phosphorylation
What do cyclins do?
activate Cdks
Cyclins are……
transiently expressed at specific points in cell cycle
Different cyclin-Cdk complexes trigger…….
different events in cell cycle
Examples of 2 different types of cyclins:
mitotic cyclin
S cyclin
Which cyclin-Cdk complex promotes mitosis?
Cdk 1 cyclin B
3 things that Cdk 1 requires in order to be activated:
1) cyclin must bind
2) activating phosphorylation
3) removal of inactivating phosphorylation
What signal inactivates Cdk 1 cylin B mitosis complex ?
signal from fully attached kinetochores
Example of gene that immediate early gene c-Myc cause to be transcribed?
cyclin D
What does cyclin D do?
triggers Cdk4 + 6 to be produced which stimulates cyclin E production
What are the 3 Cdk-cyclin complexes i need to know about?
Cdk 1 cyclin B = mitosis
Cdk 2 cyclin E = G1
Cdk 2cyclin A = S
What stimulates cells to go from G0 –> G1?
Growth factors–>c-Myc
What is the order of Cdk Cyclin complexes?
Cdk 4/6 cyclin D
Cdk 2 cyclin E
Cdk 2 cyclin A
Cdk 1 cyclin B
What do Cdks do?
phosphorylate proteins to drive cell cycle progression
Different cyclins have different…..
substrates
What does Cdk 1 phosphorylate?
nuclear lamins
What does Cdk 2 phosphorylate?
pRb
What does pRb act as?
Brake on cell cycle
What sort of gene is Rb?
tumour suppressor
Explain Rb’s relationship to E2F:
Active Rb binds to E2F and inactivates it
Cdk 4/6 phosphorylate Rb–>pRb to inactivate it
E2F is activated + transcribes cyclin E
What does E2F do?
transcribes cyclin E + other cell cycle genes
What are inhibitors of Cdk’s called?
Cdk inhibitors
Two Cdk inhibitor families and what they inhibit:
INK4 =inhibits G1
CIP/KIP = inhibits S phase
What commonly happens to Cdk inhibitors in cancer?
lost
What must happen to Cdk inhibitors in order for cell cycle to progress?
degraded
Other cell cycle causes of cancer:
Ras not inactivated
loss of pRB
overproduction of Cdks
