Cancer 4: Signalling mechanisms in growth and division

Question 1

What is Myc?

Answer 1

A transcription factor that stimulates the cell cycle genes

Question 2

What is Myc?

Answer 2

A transcription factor that controls the expression of other genes

Question 3

What happens to Myc if you trigger cell division?

Answer 3

there is a rapid and dramatic rise in levels, which then plateau to an intermediate level

Question 4

How does growth factor stimulate signalling pathways ?

Answer 4

• growth receptors bind to receptor protein tyrosine kinase
• this then acts via small GTP-binding protein (RAS) that activates a kinase cascade

this then triggers the activation of genes that are required for the progression of cells through the cell cycle

Question 5

What happens when you trigger cell division?

Answer 5

• there is a rapid and dramatic rise in Myc which then plateaus at an intermediate level
• this correlates with cells moving out of G0 and into G1

Question 6

How does growth factor stimulate mechanism growth pathways?

Answer 6

• growth factor arrives and binds to a tyrosine kinase type receptor which acts through a small GTP-binding protein (Ras)
• this triggers an enzyme cascade

Question 7

What do phosphorylated tyrosines act as?

Answer 7

docking sites for adaptor proteins

Question 8

What is the role of adaptor proteins?

Answer 8

they have no enzymatic function they bring proteins together
-they are v important in molecular recognition

Question 9

Give an example of an important adaptor protein

Answer 9

Grb2

has two domains SH2 and SH3

SH2 binds to the phosphorylated tyrosine
SH3 binds to proline rich regions of other proteins

Question 10

How is ras activated and deactivated?

Answer 10

it is a major oncogene

when bound to GTP it becomes activated but it is not in a steady state- this means it very quickly becomes deactivated as the GTP is exchanged for GDP

Question 11

Describe the process of RPTK signalling to Ras

Answer 11

• When RPTK becomes activated you get phosphorylation of the receptor
• Grbs then binds to the phosphorylated tyrosine kinase domains
• this means that SOS which is bound to the SH3 part of Grbs is close enough to Ras to activate it
• the activation of ras changes the confirmation so the signal is propagated further

Question 12

Which proteins are involved in switching off the ras protein?

Answer 12

GTPase activating proteins

responsible for hydrolysis

Question 13

WHat other way is there to switch off Ras?

Answer 13

intrinsic GTP hydrolysis capability

Question 14

What are the two forms of Ras mutations that increase the amount of active gtp-loaded ras?

Answer 14

• V12Ras prevents GAP binding

- L61Ras prevents intrinsic GTP hydrolysis

Question 15

How does the protein kinase cascade work?

Answer 15

GTP bound Ras activates binds to a kinase, then this top kinase activates the one below it and so on

Question 16

What is the specific name of the kinase cascade activated by Ras? and then the family this specific cascade is from

Answer 16

ERK
extracellular signal-regulated kinase

MAPK
mitogen-activated protein kinase cascade

Question 17

What are the three kinases specific to the ERK cascade?

Answer 17

Raf
MEK
ERK

Question 18

What occurs at the end of the kinase cascade?

Answer 18

• kinase phosphorylates a number of proteins among them are gene regulatory proteins
• one of the genes turned on by this is the c-Myc which leads to cell proliferation

Question 19

How are cyclin dependant kinases activated?

Answer 19

• binds to cyclin
• phosphorylation

they are present throughout the cell cycle

Question 20

What different cyclin-Cdk complexes are present throughout the cell cycle?

Answer 20

The M-phase promoting factor controls the progression through mitosis - the Cdk is being activated by a mitotic cyclin
Once the Cdk has fulfilled their role, the cyclin is degraded and the Cdk is turned off
At the start of DNA replication (bottom of the diagram) there is a Cdk that is turned on by binding to an S phase cyclin
Then this cyclin is degraded once the Cdk has carried out its function

Question 21

What is the regulated expression of cyclins?

Answer 21

-they are present during mitosis, they disappear during interphase and become present again for mitosis

Question 22

What do activated Cdks do?

Answer 22

they phosphorylate proteins (on serine or threonine) to drive cell cycle progression

Question 23

How are CDks regulated by phosphorylation?

Answer 23

Cdk1 binds to cyclin B but it is not yet active, it has to be phosphorylated by CAK for this to happen
An inhibitory kinase called Wee1 balances this

Cdc25 takes off the inhibitory kinase Wee1 to get an active MPF

Question 24

summarise how signalling works in the cell cycle

Answer 24

When cdk1/cycB are active mitosis is put on hold and it will not continue until metaphase has been correctly achieved
-signalling from fully attached kinetochores causes cyclin B to be degraded

This means that Cdk1 becomes inactivated, key substrates are dephosphorylated and mitosis progresses

Question 25

Which cyclins are involved with which part of the cell cycle?

Answer 25

G1 = Cdk2 and cyclin E
S= Cdk2 and cyclin A
Mitosis = M-Cdk and cyclin B

Question 26

How does growth factor stimulate G0 to G1 transition?

Answer 26

-immediate early gene transcription factors e.g. c-Myc stimulate the transcription of other genes

Question 27

What does cyclin D activate?

Answer 27

Cdk4 and Cdk6 to stimulate the synthesis of cyclin E

Question 28

What type of gene is Rb?

Answer 28

a tumour suppressor

-Cdks phosphorylate and progressively inactivate pRb

Question 29

WHat are the two types of Cdk inhibitors?

Answer 29

INK4 family (G1 phase CKIs that inhibit Cdk4/6 replacing cyclin D)
CIP family (inhibit all Cdks by binding to the Cdk complex)

CKI must be degraded to allow cell cycle progression