cell cycle regulation Flashcards

1
Q

Why is there a need for checkpoint controls?

A

To ensure the faithful replication of the cells genome during S phase, and to ensure the proper allocation of replicated DNA to daughter cells during M phase.

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2
Q

What checkpoint controls are in place in each phase of the cell cycle?

A

At the G1 to S phase transition, there is a DNA damage checkpoint. The cell will not continue into S phase until DNA is appropriately repaired. During S phase there is another DNA damage check, DNA replication is halted until damaged DNA is repaired. In G2, the transition into mitosis is blocked if DNA replication of the entire genome has not been completed. During mitosis, anaphase is blocked if chromosomes have not appropriately assembled on the mitotic spindle.

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3
Q

What happens to a cells sensitivity to mitogens and cell growth inhibitors beyond the restriction point?

A

The cell loses sensitivity. Once in S phase the cell will continue through the rest of the cell cycle, even if growth factors were removed.

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4
Q

What is the restriction point?

A

The point where the cell either commits to quiescence or to undergo DNA replication and cellular division.

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5
Q

Describe another cell cycle check at the G1 to S phase transition. How does this differ between normal and cancerous cells.

A

Following the advance through the R point, cells must attach appropriately to the ECM before entering into S phase. This is generally mediated by integrins. If this attachment does not occur then progression is halted until attachment is achieved. If this isn’t possible then the cell may undergo anoikis. Tumour cells have lost their anchorage dependence, avoiding this checkpoint altogether. This is because oncogenic ras/src are able to mislead the cell into thinking that full anchorage has been achieved.

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6
Q

What is a CDK

A

Cyclin dependent kinase

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7
Q

What is a cyclin

A

A regulatory subunit associated with CDKs. They increase the catalytic activity of CDKs (up to 400,000 fold) by inducing a conformational change in the T loop, moving it out of the substrate binding cleft). They also direct CDKs to their appropriate substrate.

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8
Q

What kind of kinase’s are CDKs

A

Serine/ threonine kinase’s

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9
Q

What ensures that the cell cycle is a unidirectional process.

A

Due to the rapid degredation of cyclins following the advance through the checkpoints that they control. This is triggered by the action of highly coordinated ubiquitin ligases. These attach polyubiquitin chains leading to cyclin degredation at the proteosome.

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10
Q

Which cyclin CDK complex’s are responsible for the G1 - S phase transition

A

CDK4/6-cyclin D, CDK2-cyclin E

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11
Q

Which cyclin CDK complex is responsible for mitosis

A

CDK1-Cyclin B

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12
Q

How does phosphorylation of cyclins increase their activity?

A

Cyclin binding alone not sufficient for complete T-loop displacement and subsequent CDK activation. Phosphorylation of a specific theonine residue in the T loop ensures complete displacement. It is mediated by CDK activating kinases.

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13
Q

What is the CDK activating kinase in mammalian cells?

A

CDK7 - Cyclin H complexes.

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14
Q

What inhibitory phosphorylation does cyclin B experience and how is this overcome.

A

During interphase when cyclin B synthesis has begun, CDK1-Cyclin B complexes are kept inactive due to phosphorylation of two adjacent residues (Thr14 and Tyr15). These are mediated by the dual specific kinases Wee1 and Myt1. At the G2 to M transition, Myt1 and Wee1 are inactivated, whilst the dual specific phosphatase CDC25 becomes active. This removes the phosphates from Thr14/Tyr15, allowing for activation of CDK1-Cyclin B complexes and subsequent entry into mitosis.

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15
Q

What is meant by a dual specificity kinase/phophatase

A

Able to phos/dephosphorylate tyrosine residues as welll as serine/threonine residues.

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16
Q

How do D cyclin expression levels differ from the other cyclins>? How does this help identify its role in the cell cycle?

A

They do not vary dramatically as the cell advances through the cell cycle. The levels of D cyclins depend on the extracellular cues the cell is receiving, specifically by mitogenic growth factors. As a result D cyclins act to convey extracellular signals to the cell cycle clock. This is because the concentration of D cyclins fluctuates alongside the levels of extracellular mitogens, so it is constantly informing the cell of the extracellular environment surrounding the cell.

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17
Q

What group of CDK inhibitors target CDK4/6 complexes and how do they inhibit them.

A

The INK4 proteins, p16, 15, 18 and 19; these bind to the catalytic subunits of the CDKs preventing their ability to bind to cyclin D.

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18
Q

Which CDK inhibitors act on the CDK cyclin complexes not found in the early stages of mitosis? How do they inhibit their action?

A

p57kip2, p27kip1, p21cip1. These bind all the CDK cyclin complexes other than CDK4/6-Cyclin D. They bind to both subunits of the complex, and are able to block their kinase activity as a result.

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19
Q

How does TGF-B effect CDK 4/6-Cyclin D complexes?

A

TGF-B induces expression of p15INK4B, this acts to inhibit the formation of cyclin D-CDK4/6 complexes, whilst also inhibiting the action of already formed complexes. As a result the cell is unable to pass through the early stages of G1 to reach the R point.

20
Q

How does p21cip1 prevent R point transition?

A

In the event of physiological stress, e.g DNA damage, p21cip1 expression is increased. This results in the inhibition of CDK2-CyclinE complexes, allowing time for the cell to repair any damaged DNA before reaching the R point.

21
Q

How do mitogens regulate p21cip1/p27kip1 expression

A

One mechanism is via the PI3K pathway. Mitogen signalling activates PI3K which in turn activates Akt/PKB. These kinases phosphorylate p21cip1 in the nucleus causing it to be exported into the cytoplasm. Similarly, p27kip1 in the cytoplasm becomes phosphorylated and is unable to be exported into the nucleus as a result. This prevents them from having any effect on the CDK-Cyclin complexes in the nucleus.

22
Q

What is the clinical consequence of high cytoplasmic concentration of p27kip1 in breast cancers?

A

In low grade mammary carcinomas, cytoplasmic p27kip1 concentration is low, meaning that it is not being phosphorylated by Akt/PKB and is able to act on CDK-Cyclin complexes in the nucleus and inhibit cell cycle progression as normal. In high grade tumours, Akt/PKB are highly active causing, phosphorylation of p27kip1 and its sequestering in the cytoplasm. This is associated, likely causally with a poorer prognosis.

23
Q

What gene family is made up of the CIP/KIP inhibitors

A

CDKN1 (p21cip1, p27kip1)

24
Q

What gene family is made up of the INK4 proteins?

A

CDKN2

25
Q

How do the CDKN2 family of proteins inhibit CDK4/6-CyclinD function?

A

Bind to CDK4/6 and prevent them from binding to their cyclin partner

26
Q

How do the CDKN1 family of proteins inhibit the action of their CDK-cyclin complex targets.

A

Bind to CDK-Cylcin complexes at separate motifs that occludes the kinase activity

27
Q

How does flow cytometry work?

A

Cells are labelled with DNA dyes to quantify the amount of DNA per cell, and get an estimate of which phase of the cell cycle the cell was in at the time of staining. G0/G1 have the lowest intensity, followed by S and then G2/M.

28
Q

How is Bivariate FACS different to univariate?

A

It adds another dimension. Cells are incubated with the hallogen BrdU which is rapidly incorporated into S phase cells. Graph therefore shows the intensity of hallogenation which will be high in S and low in G1 and G2/M phase cells.

29
Q

What is FUCCI?

A

Method to look at the cell cycle in real time. In G1 phase cells appear to be red, in S it begins to become green.

30
Q

How does pRb ensure that a cell does not go beyond the R point?

A

1) When Rb is found in its unphosphorylated and hypophosphorylated states it is associated with E2F. This complex acts as a transcriptional repressor, preventing the transcription of S phase inducing genes such as cyclin E and CDK2.
2) It also acts to stabilise p21 and p27 which bind to CDK 2/cyclin E complexes and occlude their kinase activity.

31
Q

How does a cell overcome the R point and enter the S phase?

A

When a cell receives a growth signal CDK4/6/cyclin D complexes begin to phosphorylate Rb to its hypophosphorylated state. Some E2F begins to dissociate from Rb and transcription of CDK2/cyclin E begins. These complexes phosphorylate Rb further to its hyperphosphorylated state resulting in complete dissociation of E2F. E2F induces transcription of S phase genes, more Cdk2/cyclin E complexes form and inactivate Rb forming a positive feedback loop of Rb inhibition. At this point the cell passes through the R point and commits to going through the cell cycle.

32
Q

What effect does cyclin E/Cdk2 complexes have on p27kip1?

A

The complexes phosphorylate p27 which leads to its ubiquitination and degredation. This derepresses the cdk2/cyclin E complexes that were being repressed by p27, forming a positive feedback loop.

33
Q

Which two molecules are responsible for the oscillation of p53 expression and what is the importance of this?

A

mdm2 and Wip1. Negative feedback loop forms following p53 activation that leads to p53 inactivation and expression oscillation if DNA damage persists. Continuous oscillation will induce apoptosis, whereas if the damage is overcome; p53 expression is reduced and and the cell cycle can progress.

34
Q

Which Chk is phosphorylated by ATM and as a result of what DNA damaging event. (which stage of the cell cycle does this occur in?)

A

Chk2, due to DSBs in G1.

35
Q

What is the action of ATM/Chk2

A

Both Chk2 and ATM stabilise p53 which induces p21 expression. This inhibits the expression of CDK2, preventing S phase progression. Chk2 also inhibits cdc25 which prevents dephosphorylation of CDK2/cyclin A complexes.

36
Q

What checkpoint is prevented through the action of ATR and Chk1

A

G2/M phase entry, due to SSBs.

37
Q

How do Chk1 and ATR prevent mitotic entry?

A

Chk1 activates Wee1 and also tags cdc25 for degredation. Both these inhibit the action of CDK1 (Cdc2) which is required for mitotic entry.

38
Q

Which cyclin cdk complex is required for progression from S phase to G2?

A

Cdk1/cyclin A (Cdk1 = Cdc2)

39
Q

What is the result of mutations in EGFR, Ras, Her2, Wnt, Hedgehog on the G1 cell cycle checkpoint?

A

Each leads to the upregulation of Cyclin D which consequentially induces signal independent activation of Cdk4/6

40
Q

Would a mutation in Rb in one cancer have the same phenotyopic effect as an INK4A mutation in another cancer?

A

No - Rb is negatively regulated by cyclin D/Cdk4/6 complexes which themselves are regulated by INK4 proteins as well as CDKN1 proteins. If you remove the INK4 inhibition of CDK4/6 then CDKN1 proteins can still provide some regulation and the checkpoint integrity is maintained. If Rb is lost then the entire checkpoint no longer functions - giving a far more aggressive phenotype.

41
Q

What is likely to be mutated alongside an INK4 mutation to be cancerous. Why is this the case?

A

Because the G1 checkpoint is not completely lost due to INK4 mutations; a second mutation in a separate pathway is likely to be effected too. This could be mutations upstream (growth factor signalling) or mutations in CDKN1 family members (p21, p27) or in Cyclin D.

42
Q

Which cancer type has seen the development of the first FDA approved CDK4/6 small molecule inhibitor?

A

ER positive breast cancer, used in combination with an ER inhibitor

43
Q

How do ER positive breast cancers treated with hormonal therapy and CDK4/6 inhibitors develop drug resistance resulting in their relapse

A

RB1 mutations developed in only the patients treated in combination with the CDK4/6 inhibitor compared to the ER inhibitor alone.

44
Q

Which tyrosine kinase network permits cancer cell proliferation independent of CDK4/6 signalling

A

MET and FAK:
MET (RTK) activated by stromal cues which signals via FAK to induce CIP/KIP degredation and a subsequent increase in CDK2 expression.

45
Q

What mutations are found in the S and G2/M DNA damage checkpoints.

A

ATM/ATR kinase mutations, TP53, Chk1 and Chk2

46
Q

How can the S and G2/M checkpoints be exploited to induce mitotic catastrophe. (Use mutated p53 in this example)

A

Following radio/chemotherapy, give a small molecule inhibitor of Chk1/Chk2. In normal cells p53 is still present so following DNA damage by radiotherapy p21 is upregulated and DNA repair can take place, avoiding apoptosis of normal cells.
In cancer cells with no functional p53, they are reliant on ATM/ATR pathway for DNA repair and survival; so by inhibiting this (downstream via chk1 or 2) the cell goes into mitotic catastrophe

47
Q

What is a CHK1 selective inhibitor>

A

SAR-(020106)