Cell Proliferation I

Question 1

What do cells need to pass in order to proliferate?

Answer 1

Restriction point (R-point)

Question 2

What is the R-point?

What does it act as?

Answer 2

Restriction Point

Acts as a cellular brake, which blocks cells advancing from G1 to S-phase.

Question 3

What are proliferative signals in the form?

Answer 3

Mitogens

Question 4

What does a mitogen do?

Answer 4

Causes cell to synthesise proteins to overcome R-point

Question 5

What is MAPK?

Answer 5

Mitogen Activated Protein Kinase

It is a signalling cascade

Question 6

How do MAPK signalling cascades operate?

Answer 6

They operate downstream of many Tyrosine Kinase Receptors (RTKs) like EGFR

Question 7

ERK pathway (MAPK)

Answer 7

Raf
MEK 1/2
ERK 1/2

Question 8

p38 pathway (MAPK)

Answer 8

TAK
MKK 3/6
p38

Question 9

JNK pathway (MAPK)

Answer 9

MAP3K
MKK 4/7
JNK

Question 10

What is the ERK pathway commonly activated by? (2)

Answer 10

• mitogens
• growth factors

Question 11

What are the p38 and JNK pathways activated by? (3)

Answer 11

• cytokines
• osmotic stress
• DNA damage

Question 12

What is Ras?

Answer 12

G-protein

Question 13

What are Grb2 and Sos?

Answer 13

Guanine exchange factors (GEFs)

Question 14

Function of GEF (3)

Answer 14

• activate G-proteins
• by releasing GDP
• and adding GTP

Question 15

How does a mitogen outside the cell tell it to divide? (5)

Answer 15

1) receptor activation leads to Ras activation

2) active Ras attracts Raf to bind and become activated

3) Raf phosphorylates and activates MEK

4) MEK phosphorylates and activates ERK

5) action of ERK controls activation of protein and transcription factors

Question 16

How do cells maintain rate of cell proliferation? (2)

Answer 16

• after activation, cells need to turn off proliferative pathway
• cells need to be able to die when required

Question 17

How is mitogenic signalling turned off? (2)

Answer 17

• prevent release of ligand
• turn off receptor

Question 18

How is the release of ligand (EGF) prevented? (2)

Answer 18

• EGF is a transmembrane protein
• metalloproteinases cleave off and make EGF diffusible

Question 19

How is the receptor (EGFR) turned off? (3)

Answer 19

• cells internalise RTKs (EGFR) and send it to lysosomes to be destroyed
• EGFR is tagged with ubiquitin
• to be marked for endocytosis and degradation

Question 20

How does EGFR cause cells to proliferate? (9)

Answer 20

1) EGF binds to EGFR

2) EGFR is phosphorylated and relays signal

3) EGFR activates Grb2 and Sos

4) Sos activates Ras

5) active Ras attracts Raf to bind and become activated

6) Raf phosphorylates and activates MEK

7) MEK phosphorylates and activates ERK

8) ERK activates proteins and transcription factors

9) AP-1 transcription factors are activated to express genes needed for cell cycle progression

Question 21

Function of Cbl

Answer 21

Functions in the endocytosis of signalling recpetors

Question 22

Explain how Cbl mutation leads to cancer (4)

Answer 22

• endocytosis is required to turn off RTKs (like EGFR)
• Cbl mutation causes defect in the process of endocytosis
• this causes EGFR to remain active and continue proliferative signalling
• enabling cells to divide and proliferate uncontrollably (cancer progression)

Question 23

What is B-raf and its function? (2)

Answer 23

• subtype of Raf
• relays signal to MEK

Question 24

Explain how B-raf mutation leads to cancer (5)

Answer 24

• B-raf undergoes gain of function mutation
• this allows B-raf to be active without any mitogenic signalling
• hence continuous signalling relayed to MEK and ERK causing continuous activation of transcription factors
• causing continuous expression of proteins involved in activating the cell cycle
• hence hyper proliferation and cancer

Question 25

Apoptosis

Answer 25

programmed cell death

Question 26

What does p53 act as?

Answer 26

Transcription factor

Question 27

Function of p53 (3)

Answer 27

- induce cell cycle arrest - repair DNA damage - induce apoptosis

Question 28

How are p53 levels kept low? (3)

Answer 28

- after synthesis, p53 is bound to Mdm2 - Mdm2 serves as a marker by tagging p53 with ubiquitin - to mark it for degradation by proteasome

Question 29

What degrades RTKs?

Answer 29

Lysosomes

Question 30

What degrades p53?

Answer 30

Proteasomes

Question 31

How is the degradation of p53 prevented when needed? (3)

Answer 31

- when cells incur DNA damage, a signalling cascade is initiated which leads to the phosphorylation of p53 - prevents binding of Mdm2 - hence blocking its degradation

Question 32

Intrinsic Induction of Apoptosis (4)

Answer 32

1) Apoptotic stimulus causes activation of p53, which induces expression of p53 up-regulated modulator of apoptosis (PUMA) 2) PUMA activates Bax and Bak 3) Bax and Bak form pores in the mitochondrial membrane causing the release of cytochrome C 4) This triggers activation of capsases and apoptosis

Question 33

What blocks the action of Bax and Bak?

Answer 33

Bcl2 and BclXl

Question 34

Capsase activation (2)

Answer 34

- cleavage of actin and breakdown of the cytoskeleton - cleavage of nuclear lamina, nuclear shrinkage and DNA fragmentation

Question 35

What does p38 act as?

Answer 35

Tumour suppressor

Question 36

How is p38 be activated? (2)

Answer 36

- cellular stress - DNA damage

Question 37

What can p38 mediate?

Answer 37

Apoptosis

Question 38

How does the p38 MAPK pathway lead to apoptosis? (5)

Answer 38

1) cellular stress activates TAK 2) TAK activates MKK 3/6 3) MKK 3/6 activates p38 4) Phosphorylation of p38 activates p53 5) Phosphorylation of p53 leads to apoptosis

Question 39

Cisplastin and its function (2)

Answer 39

- chemotherapy drug - activates p38 MAPK which activates p53 for apoptosis

Question 40

How does the JNK MAPK pathway lead to immune response? (6)

Answer 40

1) Immune stimulation activates cytokines TNF-alpha 2) TNF-alpha binds to TNFR, leading to receptor activation 3) MAP3K is activated, which phosphorylates and activates MKK 4/7 4) this phosphorylates and activates JNK 5) JNK activates transcription factors 6) Immune response is induce (T-cell activation, Apoptosis, Inflammation)