Signalling Mechanisms in Growth and Division Flashcards

1
Q

What transcription factor is stimulated by growth factor signalling and is vital to starting the cell cycle?

A

c-Myc

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2
Q

Describe what happens to tyrosine kinase receptors when growthfactors bind to them.

A

Tyrosine kinase receptors are usually present on membranes as inactive monomers
Most growth factors are dimers, so when they bind they bring tyrosine kinase receptors close together
This allows the tyrosine kinase receptors to cross-phosphorylate each other (using the gamma phosphate from ATP to phosphorylate tyrosine residues in proteins)
The phosphorylated domains on the tyrosine kinase receptors act as docking sites for adaptor proteins

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3
Q

Give an example of an anti-cancer drug that targets tyrosine kinase receptors.

A

Herceptin – inhibits the Her2 tyrosine kinase receptor (important in many tumours e.g. breast)

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4
Q

Name an important adaptor protein.

A

Grb2

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5
Q

Describe the structure of this protein.

A

It is modular
It has an SH2 domain, which binds to the docking sites (phosphorylated tyrosine residues on the tyrosine kinase receptors)
It has two SH3 domains, which bind to proline-rich regions of proteins

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6
Q

Describe how receptor protein tyrosine kinases can signal to Ras.

A

Grb2 is bound to an exchange factor called Sos
When the tyrosine kinase receptors become active and the dockingsites become available, Grb2 binds to the docking site and it is also attached to Sos
This brings Sos close enough to the cell membrane and Ras, to allow it to exchange the GDP on Ras for GTP
GTP bound Ras is active

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7
Q

What must the Ras protein be bound to for it to work?

A

It must be bound to the plasma membrane

NOTE: interference with the membrane binding of Ras can make a good anti-cancer drug

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8
Q

How is Ras turned off?

A

Ras has intrinsic GTP hydrolysis capability

This GTPase activity is stimulated by GTPase-activating proteins (GAPs)

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9
Q

Broadly speaking, how might Ras signalling be different in cancer?

A

Ras could be permanently switched on (in the GTP bound form), thus it constantly signals cell division

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10
Q

Describe two mutations that lead to an increase in the amount of active Ras.

A

V21Ras – glycine is replaced by valine, which means that a simple hydrogen side chain is replaced by a hydrophobic sidechain. This hydrophobic side chain doesn’t allow GAPs to bind to Ras, thus preventing inactivation of Ras.
L61Ras – glutamine is replaced by leucine (in position 61), which means that an amine side chain is replaced by a hydrophobic side chain. This inhibits the GTPase activity of Ras so Ras remains in the active, GTP bound form.

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11
Q

What cascade does Ras activate?

A

ERK cascade (Extracellular signal-regulated kinase cascade)

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12
Q

What is the family that this cascade belongs to called?

A

MAPK cascade (Mitogen-activated protein kinase cascade)

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13
Q

What are the three kinases involved in the ERK cascade?

A

Raf
MEK
ERK

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14
Q

What does the last kinase in the cascade phosphorylate?

A

It phosphorylates gene regulatory proteins (transcription factors), which go on to regulate the expression of genes involved in the cell cycle
They also phosphorylate other proteins and change their activity

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15
Q

What important gene is turned on by the kinase cascade?

A

c-Myc

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16
Q

What type of kinase are cyclin-dependent kinases (Cdks)?

A

Serine-threonine kinases

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17
Q

What conditions do Cdks require to become activated?

A

Binding to cyclin
Phosphorylation (activating phosphorylation and removal of inhibitory phosphorylation)
(+ degradation of Cdk inhibitors)

18
Q

What does the mitosis-promoting factor (MPF) consist of?

A

Cdk1 + cyclin B

19
Q

What are the requirements, in terms of phosphorylation, for MPF to become active?

A

Activating phosphorylation by CAK (Cdk activating kinase)

Removal of the inhibitory phosphorylation (that was placed by Wee1)by Cdc25

20
Q

What activates MPF at the end of interphase?

A

Removal of the inhibitory phosphorylation by Cdc25

21
Q

Describe the positive feedback loop that is formed by MPF activation.

A

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

22
Q

How does MPF put mitosis on hold before progressing to the next stage?

A

At the end of metaphase, it phosphorylates a number of key proteins and inhibits their action (thus putting mitosis on hold) until it is signalled to proceed. Then the cyclin B is degraded, Cdk is inactivated and the substrates become dephosphorylated and hence become active.

23
Q

Which Cdk/cyclin is required for G1/S phase?

A

Cdk2-cyclin E

24
Q

Which Cdk/cyclin is required for S phase?

A

Cdk2-cyclin A

25
Q

How can the same Cdk be used for two different stages?

A

Cyclin binding alters the substrate specificity of Cdk

Also, different substrates are available at different stages of the cell cycle

26
Q

What is one of the most important transcription targets of c-Myc?

A

Cyclin D

27
Q

What is the first Cdk/cyclin complex that is formed when a cell goes from G0 to G1?

A

Cdk4/6-cyclin D

28
Q

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?

A

This gives the cell cycle direction and timing (because the Cdk-cyclin complexes must reach a certain concentration before they can triggerthe next stage of the cycle)

29
Q

Give an example of a phosphorylation target of MPF that allows the cell cycle to progress.

A

Phosphorylation of nuclear lamins allows breakdown of the nuclear envelope

30
Q

What is start kinase and what is one of its most important targets?

A

Start kinase = Cdk2-cyclin E

Retinoblastoma

31
Q

Describe the role of retinoblastoma in the quiescent G0 state.

A

Retinoblastoma is unphosphorylated and binds to and sequesters a group of transcription factors called E2F

32
Q

What effect does Cdk4/6-cyclin D have on retinoblastoma?

A

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

33
Q

What is one of the main targets of E2F?

A

Cyclin E (the next cyclin in the cell cycle)

34
Q

What type of gene is retinoblastoma?

A

Tumour suppressor gene (it acts a break on the cell cycle)

35
Q

State some important genes that are regulated by E2F.

A

Proto-oncogenes – c-Myc, n-Myc
Cell cycle – E2F-1,2,3, pRb, cyclin A, cyclin E, CDK4, CDK2
DNA synthesis – thymidine kinase, thymidine synthetase, dihydrofolate reductase, DNA polymerase

36
Q

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?

A

Cdk2-cyclin E further phosphorylates retinoblastoma so more E2F is released and the concentration of E2F increases

37
Q

What is the significance of the increasing concentration of E2F?

A

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)

38
Q

What are the two families of Cdk inhibitors?

A

NK4

CIP/KIP

39
Q

During which phase do each of the families act and how do they inhibit Cdk?

A

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

40
Q

State some common and important oncogenes.

A

EGFR/HER2 – mutationally activated or over-expressed in many breast cancers
Ras – mutationally activated in many cancers
Cyclin D1 – overexpressed in 50% of breast cancers
B-Raf – mutationally activated in melanomas
c-Myc – overexpressed in many tumours

41
Q

State some important tumour suppressor genes.

A

Rb – inactivated in many cancers

p27KIP1–under-expression correlates with poor prognosis in many malignancies