PMDO - Hallmark 1+2

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G phase = grow; make more stuff e.g. proteins and lipids

S phase = copy their genetic material

M phase = divide into 2 cells

In cell cycle cells can enter G0

G0 = non-dividing cells (quiescence) - but still metabolically and functionally active

non-dividing, differentiated state

most adult cells in G0 phase

Liver cells - in G0 but can be "called back" to cell cycle by external cues

Nerve and muscle cells - highly specialised, arrested in G0 and can never divide

The mammalian cell cycle is regulated by a molecular control system:

- external signals

- internal signals - checkpoints

External signals:

*influence a cells decision to enter the active cell cycle

Mitogenic growth factors stimulate cells to grow and divide

Mitogens that stimulate growth include - platelet derived growth factor, insulin growth factor, epidermal growth factors and growth hormone/oestrogen

absence/withdrawal of mitogenic growth factors pushes cells in quiescence, G0

lack of nutrients inhibit cell division

1. growth factor

2. growth factor binds to receptor

3. receptor sets off a signal cascade to nucleus

4. target cell enters S-phase and divides

Transforming growth factor beta (TGFB)

*external anti-proliferative signal - when present cell cycle is stopped

G1 is the period in which cells are responsive to mitogenic GF and to TGF-B

R point = once cell passes R point it is committed to one round of cell cycle

Internal Regulation

* each phase of cell cycle has a 'checkpoint'

1 - G1 phase = DNA damage checkpoint, involves ATM + ATR, activates P53 (senses whether there is any damage to genome of the cell, if yes cell will repair it or if not able to repair it, it will kill the cell)

2 - S phase (DNA synthesis) = DNA synthesis is stopped by double strand breaks (looks for any damage to the double stranded brakes, will block replication if damaged)

3 - G2 phase = checks that all DNA has been replicated (if not the resulting cells will be genetically unstable) if yes it can proceed into mitosis

4 - M phase = monitors correct alignment of chromosomes and checks for spindle formation (so daughter cells have same number of chromosomes)

* R point adds another level of control to cell cycle - additional restriction point

Cell Cycle Regulation

Activators: cyclins and cyclin dependent kinases (CDK)

cyclins are a family of proteins that are synthesised and degraded in a phase specific manner

when synthesised, cyclins complex with CDKs to form an active holoenzyme *combination of an enzyme with a co-enzyme

Active CDK holoenzyme (CDK and cyclin): - active complex that can interact with target protein, resulting in phosphorylation = changing the proteins shape

- causes the phosphorylation of threonine and serine residues of target proteins

- transfer of the terminal phosphate of ATP to a hydroxyl group of specific amino acids

*rapid in way in which it turns proteins on or off

Inhibitors: Cyclin dependent kinase inhibitors (CDKI)

*put brakes on cell cycle

- when inactivated they block the binding sites = preventing kinase binding, stopping the cell cycle

pRb: function is lost in many cancers

If pRb wasn't phosphorylated cell cycle would stay in G1

*Cyclin D, CDK 4/6 = very first cyclin to be activated (un-hypo phosphorylation)

Cyclin E, CDK 2 = hypo to hyper phosphorylation

HPV 16/18 encode protein that degrades pRb in cells = dire consequences *inhibition of cell cycle function is lost

Checkpoint 1:

G1 DNA damage checkpoint = controlled by p53 tumour suppressor protein

*activates transcription = cell cycle arrest or apoptosis/cell death

in response to DNA damage ATM/ATR kinases are activated

additional kinases are activated

p53 is phosphorylated

p53 is stabilised (normally degraded rapidly)

*p53 function is lost in most cancers by mutation

G2-M transition: mitosis promoting factor

MPF causes mitosis

MPF is present in all mitotic cells

MPF appears at the G2/M interphase and then rapidly decrease

How does MPF cause mitosis?

MPF is a protein kinase

- phosphorylates histones causing chromatin condensation

- phosphorylates nuclear membrane proteins (lamins) causing membrane disruption

*MPF = cyclin B (subunit B) + protein kinase (subunit A)

Hallmark 1: self-sufficiency in growth signals

- cells require positive growth signals to progress from a quiescent to active state = growth factors

*no normal somatic adult cell can proliferate in the absence of growth signals

Cancer cells can alter the level of growth signal by synthesising their own growth signals

Cancer cells can modify growth factor receptors:

cancer cell can alter the growth factor receptor itself - synthesise more receptors or structurally alter the receptor so it is constantly "on"

upregulation of receptors - cells can respond strongly to low levels of circulating signals (ErB-2 and HER2 in mammary carcinomas)

structural alteration of receptors (truncating of the EGF receptor causes it to fire contitutively)

Cancer cells can alter downstream signalling pathway, so that the requirement for growth factor and receptor are both bypased

Ras is one of the key downstream components of the growth factor signalling

Ras is an oncogene

Mutant Ras is permanently wtiched on

Ras is mutated in 20-25% of tumours

Hallmark 2: Insensitivity to antigrowth signals

normally antigrowth signals block proliferation by forcing the cell out of proliferation cycle into quiescent state (G0)

*two main anti growth signals are p53 and pRb

Thirds anti-growth signal is TGF beta (works in opposite way of other growth factors - inhibits cell cycle = cancer cells can become insensitive)

What is the function of TGF beta?

- transforming growth factor

- prevents the phosphorylation of retinoblastoma

presence blocks the advancement of the cell cycle

*TGF beta pathway is disrupted in many cancers - cancer cells stop responding, produce mutated receptors that do not respond to the presence of TGF-beta

Summary:

•Describe the major events of the cell cycle:

–G1, G2, M, S, G0, R point, checkpoints in each phase, growth factor dependent

•Discuss the role of cyclins and cyclin dependent kinases in cell cycle progression

–cyclins synthesised in phase specific manner complex with CDKs to form active complex, complexes cause P of key proteins

–R point controlled by pRb (you should know the pathway)

–P53 controls DNA damage checkpoint via cyclins (you ahould know the pathway)

–Mitosis promoting factors control G2/M transition

•Detail how cancer cells overcome the cell cycle controls

–Hallmark 1: self sufficiency in growth signals (3 mechanisms)

–Hallmark 2: insensitivity to anti growth signals (p53, pRb and TGF beta)