molecular control of the cell cycle

Question 1

regulating the cell cycle: key elements

Answer 1

• The Cyclin Dependent (S/T) Kinase Family (CDKs)
(phosphorylation and proteolysis)- essentially drive the cell cycle
• Checkpoints
• Oncogenes
• Tumour suppressors

Question 2

entering G0

Answer 2

terminally differentiated (most cells in the body)- G0 permanent state

resource limitation- e.g. cells growing in culture will stop when they have used up available space (contact inhibition)

Question 3

G0: a quiescent Digression

Answer 3

1) proliferating cells e.g. stem cells, gut cells etc.
2) lack of nutrients and/or growth factors
3) terminal differentiation
4) quiescent
5) 2 main stages to resume cell cycle:
- competency
- progression/ re-entry

Unfavourable environments trigger entry to G0. Restarting cell cycle depends on positive stimulatory signals received during favourable conditions

Question 4

G0: initiating re-entry

Answer 4

Re-entry occurs in two (arguably three) phases. They are referred to competence and progression phases. During the competence phase the cell has to overcome inhibitory signals that keep it in G0, such as TGFb, transforming growth factor beta. The cell overcomes this inhibition when it receives signals that promote the expression of “immediate early factors”. These are produced in response to growth factors, including PDGF and FGF . Immediate early factors are transcription factors including c-fos, c-myc and c-jun. Their transcription is switched on by other transcription factors that relocate to the nucleus upon phosphorylation (serum response factors).

Question 5

immediate early genes

Answer 5

c-jun, c-fos and c-myc:
• Transcription factors
- regulated by phosphorylation
- activated by serum-responsive TFs and translocate to nucleus in response to growth factors (e.g. PDGF)
• Activate delayed response genes E2F (TF) and cyclin D to promote progression
• Constitutively-active mutant forms promote cancer

Question 6

G1 re-entry: growth-factors

Answer 6

example: fibroblasts.
• PDGF induces competency phase (~6h) in which mRNA & protein translation machinery is restored
• EGF permits re-entry to G1
• Progression maintained by insulin until G1 restriction point (start) reached
• At this point GFs no longer required

Question 7

cell cycle control: cyclins

Answer 7

• Cyclin D is expressed in the early stages of G1 (pre-start)

• cyclins activate their partner Cdks (cyclin-dependent kinases)
• cyclin D binds to Cdk4 & Cdk6 (separately)

Question 8

cdk/cyclin and cell cycle

Answer 8

the cell cycle is driven in one direction only, as a result of changes in cyclin levels

Question 9

cdk activation

Answer 9

• Cyclical expression of cyclins enables activation of their partner Cdks
• Activity of cyclin-Cdk complexes is further regulated (activated & inhibited) by phosphorylation

Question 10

inhibitors of Cdk Actibity (CKIs)

Answer 10

• Cdk inhibitor proteins (CKIs) bind to cyclin-Cdk complexes

* Binding changes the conformation of the active site and blocks the ATP-binding site

Question 11

the ultimate regulation: proteolysis

Answer 11

• Cyclin B (aka M-cyclin) is degraded by proteolysis at the metaphase-anaphase transition
• Addition of ubiquitin (Ub) targets the cyclin for removal

Question 12

oncogenes

Answer 12

• Oncogenes promote proliferation and therefore tumour formation
• Positive regulators of the cell cycle
• Encoded by genes required for normal cell-cycle progression – “proto-oncogenes”
• Mutations constitutively activate oncogenes so they work independently of their normal signals
• Mutations often genetically dominant

Question 13

oncogene examples

Answer 13

• Early-immediate transcription factors are proto-oncogenes, e.g.
c-fos, c-jun, c-myc
• There are viral versions of these genes that act as oncogenes (v-fos, v-jun, v-myc)
• These genes were originally identified in viruses. Viral infection can “transform” cultured cells so they can grow independently of growth factors (i.e. restriction lifted)

Question 14

oncogenes and signalling cascades

Answer 14

oncogenes can be any component of a growth signalling-cascade

4 classes:

• growth factors
• receptors
• intracellular signalling
• transcription factors

Question 15

tumour suppressors

Answer 15

• Tumour suppressors are negative regulators of the cell cycle
• Prevent cell-cycle progression in multiple ways
• Most are transcription factors or regulators of transcription factors
• pRb (or just Rb) was the first tumour suppressor identified
• Inactivated in tumours
e.g. pRb nearly all cancers
p53 >50% of all cancers

Question 16

pRb and p53 example

Answer 16

Both pRb and p53 are involved in cell-cycle arrest following DNA damage
In addition, p53 activation can lead to apoptosis