Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Stem cells, Ageing and Cancer > Checkpoint control > Flashcards

Checkpoint control Flashcards

1
Q

what 2 processes does the genetic material of daughter cells depend on? what happens if these processes go wrong and how do cells avoid this?

A
  1. the faithful replication of the mother cell’s genome is S phase
  2. the proper allocation of the resulting duplicated DNA to the daughter cells during M phase

defects in either of these processes can lead to cancer, so cells must use checkpoints to ensure successful duplication

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

what are cell cycle checkpoints?

A

Surveillance mechanisms to monitor each step of the cell cycle progression
- Cells are allowed to proceed with the cell cycle only if the pre-requisite step has been completed successfully

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

what does the checkpoint trigger if the cell has not completed a phase successfully?

A

Halt to further advance until the problems have been addressed:
- If the error is fixed, the cells can progress
- If the error isn’t fixed, cells undergo apoptosis

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

what are the 5 key cell cycle checkpoints?

A
  1. R
  2. G1/S checkpoint
  3. S-phase checkpoint
  4. G2/M checkpoint
  5. spindle assembly checkpoint
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

what does the G1/S checkpoint ensure?

A

A cell will not be permitted to enter S-phase if the genome needs repair due to DNA damage
- Prevents duplication of faulty DNA

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

what does the S-phase checkpoint ensure?

A

DNA replication will be paused in response to DNA damage (this can cause the doubling of the time required to complete DNA synthesis) – extension of S phase duration

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

what does the G2/M checkpoint ensure?

A

A cell will not proceed through G2 to M until the DNA replication of S phase has been completed & entrance in M phase is blocked if the DNA is damaged
- If duplication has been delayed in S phase due to damage, cells will not progress until all DNA has been synthesised

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

what does the spindle assembly checkpoint ensure?

A

A cell is not permitted to enter anaphase until all of its chromosomes are properly assembled on the mitotic spindle during metaphase

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

how do cancer cells progress through the cell cycle, despite having faulty DNA?

A

by inactivating the checkpoints

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

how does cancer have a proliferative advantage?

A

The increased mutability of the genome provides cancer cells with proliferative advantage:
- incompatible with normal cell cycle progression
- checkpoint controls block advance through the cell cycle if DNA has been damaged
- In addition to acquiring activated oncogenes and inactivated TSGs, cancer cells have inactivated one or more checkpoint controls

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

what protein controls the R point?

A

Rb protein (pRb)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

what is pRb?

A

pRb is the molecular governor of the R point
- expressed by the Rb tumour suppressor gene
- nuclear phosphoprotein
- undergoes phosphorylation to control cell cycle progression

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

how is pRb regulated at the R point?

A

pRb is regulated by phosphorylation:
- phosphorylation of pRb is regulated as cells progress through the cycle
- if pRb is active, cells do not go through the R point
- for cells to proliferate, pRb must be inactivated, and inactivation is induced by phosphorylation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

what is the key role of pRb?

A

to prevent cells progressing through the R point
-it is a tumour suppressor

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

how is pRb inactivated? what does this enable?

A

pRb is inactivated by hyperphosphorylation:
- this means that cells can now progress through the R point and proliferate

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

what must occur to pRb for cells to progress through the R point?

A

Cells can go through the R point only if pRb becomes INACTIVATED through hyperphosphorylation: pRb = guardian of the R point gate

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

what regulates the phosphorylation of pRb?

A

the cyclin/CDK complexes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

how do cyclin/CDK complexes regulate pRb phosphorylation in G1?

A

Early and mid-G1: in presence of mitogens, cyclin D and CDK4/6 initiate pRb hypophosphorylation (low phosphorylation level):
- Not enough to fully inactivate pRb and progress through R
- Hypophosphorylation is necessary but not sufficient for pRb inactivation

Cyclin E levels increase at the R-point:
- cyclin E/CDK2 mediate pRb hyperphosphorylation
- Enough to inactivate pRb and progress through cell cycle

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What does pRb phosphorylation release to control the progression from G1 into S phase?

A

Unphosphorylated/hypophosphorylated pRb binds to E2Fs (transcription factors), while it dissociates from E2Fs when hyperphosphorylated
- Early and mid G1: hypophosphorylated pRb binds to E2Fs, preventing the transcription of E2F-dependent genes (E2F is sequestered)
- R point: pRb hyperphosphorylation means pRb cannot bind to E2F -> E2Fs are released -> transcription of genes mediating G1/S transition
- E2F active transcription is short-lived

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

why is E2F transcription at the R point short-lived?

A

they remain active for short time as they are only needed to enable cells to enter S-phase
- their activity must be inhibited soon after the cells have entered S

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

how is E2F transcription inhibited after the G1/S transition?

A

as the cells undergo G1/S transition, cyclin A/CDK2 inhibit the transcriptional activity of E2Fs, which are targeted for degradation by ubiquitination

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

how is cell cycle progression ensured to occur in one direction?

A

Via positive feedback mechanisms - irreversibility of the cell cycle ensures unidirectional advancement and rapid excecution

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

what positive feedback mechanisms ensure the unidirectional advancement of the cell cycle progression?

A
  1. when E2Fs are activated, they promote cyclin E expression:
    - increased cyclin E results in increased CDK2 activation, which leads to the further hyperphosphorylation of pRb
    - hyperphosphorylated pRb then release even more E2Fs (positive feedback loop)
  2. when cyclin E/CDK2 complex is active, they phosphorylate the CKI p27Kip1, leading to the CKIs inactivation and degradation:
    - this means more E/CDK2 is released from inhibition
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

how is pRb implicated in cancer?

A

it is a tumour suppressor gene which is inactivated in cancer
- this means that cells cannot be inhibited from progressing through the R point, as E2F is constantly active

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

what are the 3 main mechanisms of pRb inactivation in cancer?

A
  1. RB1 mutation
  2. Inactivation of pRb by deregulated hyperphosphorylation
  3. interaction with viral protein
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

how does RB1 mutation lead to pRb inactivation in cancer?

A

RB1 mutations – pRb can no longer inhibit E2F, so cell cycle progression is constantly active
- The rate of RB1 gene mutations varies significantly among different tumour types, but is highest in retinoblastoma, osteosarcoma and small cell lung cancer.
- Mutations targeting the RB1 gene directly affect pRb function by either completely abrogating its expression or by producing a non-functional protein.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

how does deregulated pRb hyperphosphorylation lead to its inactivation in cancer?

A

if pRb is constantly hyperphosphorylated, it can no longer bind and sequester E2F, meaning E2F is now constitutively active and can promote continuous cell cycle progression and proliferation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

how does interaction with viral proteins lead to pRb inactivation in cancer?

A

E7 produced by human papilloma virus displaces E2F from pRb, so E2F is no longer sequestered and can promote cell cycle progression
- leads to uncontrolled cell proliferation in cervical cancers

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

what cyclins are implicated in pRb inactivation in cancer?

A

cyclin D and cyclin E are often overexpressed in tumours, leading to pRb phosphorylation and inactivation
- this drives cell proliferation and tumour formation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

why are cyclins difficult pharmacological targets? how has this been overcome?

A

they have no kinase activity and they are located intracellularly:
- many CDK inhibitors have been developed and are now in clinical trials -these will have a knock-on effect in inhibiting cyclins
- Promising results from in vitro and mouse studies of different cancer types

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

what is Palbociclib?

A

a selective inhibitor of CDK4 and CDK6 by competing with ATP
- CDK4/6 inhibitor
- thus it inhibits cyclin D-dependent kinase activity = therapeutic value as anti-cancer treatment
- inhibits breast cancer cell growth in vitro without affecting normal cells

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
Q

why is Palbociclib (CDK4/6 inhibitor) limited?

A

CDK4/6 inhibitor resistance developed
- All patients with metastatic disease will develop resistance, via 3 main mechanisms

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
33
Q

what were the 3 main mechanisms in which metastatic cancer developed resistance against CDK4/6 inhibitor (Palbociclib)?

A
  1. activation of upstream effectors
  2. inactivating mutations in pRb and overexpression of CDK6
    - cell cycle progresses regardless of CDK4/6
  3. downstream bypass pathways
    - cyclin E overexpression causing cyclin E/CDK2 activation, which phosphorylates and inactivates pRb
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
34
Q

what genes did Palbociclib-resistant cells overexpress?

A

Overexpression of cyclin E (CCNE1) and cMyc was observed in CDK4/6 inhibitor-resistant cells:
- MYC-over-expressing cells are able to grow in the presence of CDK4/6i (Palbociclib)
- Resistant cells overexpress myc, so increase cyclin E expression
- Palbocyclib treatment -> increased Myc expression -> increased cyclin E activation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
35
Q

what treatment was developed to overcome Palbociclib-resistant cancer cells?

A

CDK2/4/6 inhibitor called PF3600:
- CDK2 inhibitor used as it will inhibit cyclin E overexpression seen in the resistant cells
- Cyclin E-overexpressing tumours are resistant to CDK4/6i (palbociclib), but sensitive to CDK2/4/6i (PF3600)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
36
Q

how was PF3600 developed? what is the limitation of this method?

A

using patient-derived xenografts (PDX):
- tumours extracted from patient and injected into mice
- tumours will form in the mice which can be treated with different drugs

limitation: Animal must be immunocompromised for injection of patient tissue into mouse

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
37
Q

what is taxol?

A

antimitotic therapy used for cancer to induce DNA damage

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
38
Q

how can CDK4/6 inhibitors be used with chemotherapy?

A

sequential treatment with CDK4/6 inhibitors (Palbociclib) following DNA-damaging chemotherapy (Taxol) enhances therapeutic benefit in pancreatic ductal adenocarcinoma (PDAC) models

39
Q

what taxol/CDK inhibitor treatment regime is effective to be used to treat PDAC?

A

Taxol -> Palbociclib regime is able to significantly reduce PDAC cell growth in vitro
- Taxol then Palbociclib causes complete inhibition of cancer cell growth
- (not P -> T or P on its own)
- this regime can completely oppose the growth of cancer

40
Q

how does taxol->palbociclab compare to other PDAC cancer treatments?

A

PDAC mouse model (driven by K-Ras and CDKn2a mutations = loss of p16INK4A):
- Animals were treated when tumours reached 10-80 mm3:
- tumours were resistant to CDK4/6i
- tumours had a modest response to paclitaxel – growth was slower but tumour still grow
- there was a significant delay in tumour growth following T -> P regime
- % of animal survival was higher with T-P treatment

sequential application of CDK4/6 inhibitors in combination with available chemotherapies as a promising therapeutic strategy in PDAC

41
Q

why is maintenance of genomic integrity crucial?

A

Maintenance of genomic integrity is crucial, as it prevents genomic instability related diseases, such as cancer
- Accumulation of DNA damage and mutations may lead to disease

42
Q

what is DDR?

A

DNA damage response
- complex network of signalling pathways which monitor DNA integrity and, in case of DNA damage, activate transient cell cycle arrest and DNA repair to ensure maintenance of genomic stability
- If repair is impossible, cells undergo apoptosis

43
Q

how is DDR implicated in cancer?

A

DDR inactivation is a hallmark of cancer: associated with transformation & contributes to carcinogenesis by increased genomic instability
- however, defects in DDR render cancer cells more dependent on the activity of the remaining intact DDR & susceptible to therapies
- Balance of working DDR and inactive DDR cause cancer survival
- Some DDR is essential for cancer to survive – this DDR can be targeted, as normal cells have multiple DDR mechanisms

44
Q

what are the 2 main signalling components involved in DDR initiation?

A
  1. ATM
  2. ATR
45
Q

what is the role of ATM in DDR initiation?

A

ATM is activated by DNA double strand breaks (DSB).
- Plays a key role in the activation of the G1/S cell cycle checkpoint, preventing cells with damaged DNA from entering S phase.

46
Q

what is the role of ATR in DDR initiation?

A

ATR is activated by DNA single strand breaks (SSB), at stalled replication forks (replicative stress).
- Plays a role in the S-phase checkpoint and is a key mediator of the G2/M checkpoint, to prevent the premature entry of cells into mitosis before DNA duplication is completed or in the presence of DNA damage

47
Q

what is replicative stress (RS)?

A

Replicative stress (RS) = stalling or slowing of replication fork progression and/or DNA synthesis during DNA replication

48
Q

how is replicative stress implicated in cancer?

A
  • RS can be caused by common events in cancer, such as overexpression or constitutive activation of oncogenes such as HRAS, MYC or cyclin E
  • Oncogenic signalling can stimulate the G1/S cell cycle transition resulting in the premature onset of S phase and insufficient levels of DNA replicating enzymes and/or nucleotides
  • Inactivation of tumour suppressor genes, such as TP53 and RB1, can also induce RS by promoting G1/S transition
49
Q

how do cancers deal with replicative stress? how can this be used therapeutically?

A

DDR response helps cancers deal with the increase in replication rate and thus RS
- DDR inhibitors have been developed to force cancer into RS

50
Q

what oncogene is overexpressed in neuroblastoma?

A

myc

51
Q

what is CHK1?

A

cell cycle checkpoint kinase 1: key regulator of DDR

52
Q

what happens when CHK1 is depleted in neuroblastoma cells compared to normal cells? what does this mean therapeutically?

A
  • when CHK1 is depleted in normal cells, there is a small amount of reduction in growth
  • when CHK1 is depleted in neuroblastoma, growth is inhibited

this means that a CHK1 inhibitor can be developed to target neuroblastoma proliferation, without having a drastic effect on normal cell function

53
Q

what does CHK1 amplification correlate with in neuroblastoma?

A

CHK1 expression correlates with Myc amplification in neuroblastoma
- MYC overexpression causes replication stress, activating ATM/ATR-CHK1-dependent DNA damage response
- CHK1 mRNA is expressed at a significantly higher level in MYCN-amplified tumours compared with tumours without MYCN amplification and in high risk tumours compared to low risk ones
- neuroblastoma survival depends on CHK1-DDR to deal with MYC overactivity

54
Q

how are CHK1 inhibitors limited?

A

after time, neuroblastoma cells become insensitive to CHK1 inhibitors:
- Dose-dependent inhibition in tumour formation when treated with CHK1 inhibitor
- Significant decrease in tumour volume over time, but then tumour does start to grow again
- CHK1 inhibitor cannot be used alone as it doesn’t completely halt tumour growth
- Clinical trials: CHK1 inhibitor has limited efficacy as monotherapy;
- combination is necessary – toxicity could be a problem

CHK1 is still a potential therapeutic target for neuroblastoma however

55
Q

what is myc?

A

myc is a TF which localises in the nucleus and promote cell proliferation and differentiation
- myc becomes uncontrolled in cancer
- more than 70% of tumours overexpress a member of the myc family (c-myc, n-myc, L-myc)

56
Q

what does myc promote in normal cells compared to cancer cells?

A

Normal: apoptosis, cell cycle progression, cell growth, metabolism, cell adhesion & migration, angiogenesis, differentiation

Deregulated: genomic instability, uncontrolled cell proliferation, escape from immune surveillance, immortalisation, independence of growth factors

57
Q

what are the 3 ways in which myc expression is increased in cancer?

A
  1. The expression is driven by its normal promoter, but the gene is amplified (elevated copy number): N-myc amplification occurs in 30% childhood neuroblastoma -> increased levels of the gene product
  2. Chromosomal translocation: c-myc under the control of a foreign transcriptional promoter
    - Burkitt lymphoma: myc under the control of the promoter of an Ig gene = very active!!!
    - Structurally normal myc protein, but abnormally high amounts due to Ig gene relentless proliferation of lymphoid cells
  3. Pro-virus integration - insertional mutagenesis:
    - ALV = avian leukosis virus -> the viral transcriptional promoter disrupted the mechanisms normally controlling the expression of myc -> increased expression, extremely high levels of myc protein

all lead to increased expression of myc protein

58
Q

what family of transcription factors does myc belong in? how do these work?

A

bHLH TFs:
- members of the bHLH family form dimers
- when in the dimer form, they associate with gene promoters to promote or inhibit transcription

59
Q

which bHLH dimers promote/inhibit proliferation/apoptosis?

A

Myc-Max complex promotes proliferation & inhibits differentiation

Mad-Max complex inhibits proliferation & promotes differentiation
a-When Mad conc is high due to differentiation, Mad displaces myc

60
Q

what are myc levels influenced by?

A

Myc levels are strongly influenced by mitogenic signals
- myc accumulates in the presence of mitogens, leading to proliferation

61
Q

how does myc promote the cell cycle?

A

Myc/Max complex regulates expression of components of the cell cycle clock:
- Myc promotes progression by activating cyclin D/CDK4 complex, leading to pRb hypophosphorylation (necessary to pass the R point)
- Myc/Max promotes E2F transcription factors, which trigger cells to enter S phase
- Myc/Miz1 represses transcription of CKIs, liberating cyclin E/CDK2 complexes from inhibition
- Myc promotes the degradation of CKI p27-Kip1

all of these enable cell progression through the R point via inactivation of pRb

62
Q

is myc necessary and sufficient to promote cell progress and growth?

A

experiment: Myc expressed as a fusion protein with the oestrogen receptor
- in the cell, receptor-myc is localised in the cytoplasm, so myc cannot promote transcription and is sequestered
- when cells are treated with oesterogen/tamoxifen and they bind to the myc-receptor, myc translocates into the nucleus and induces transcription
- Cells remain G0 in the absence of growth factors
- tamoxifen addition induces entrance in G1 and S phase

therefore myc can act alone to relieve all the constraints on proliferation
- it is necessary and sufficient alone to drive cell growth
- powerful driver of cell growth

63
Q

in what ways can myc be targeted to treat cancer?

A

MYC is an integrator of several signalling pathways and represent an attractive therapeutic target:
- Direct MYC inhibitors block the dimerization of MYC and MAX or interfere with binding of the MYC/MAX dimers with the DNA
- Indirect inhibitors interfere with MYC expression at the transcriptional level or at the level of protein turnover, or they target/prevent the transactivation function of MYC

64
Q

what is the PDAC Cre/LoxP mouse model used to study myc in tumours?

A

(PDAC) mouse model using Cre/LoxP and tetracycline (tet) responsive systems:
- Cre is a DNA recombinase that cleaves DNA at the LoxP site
- tetracycline controlled transactivator (tTa) is preceded by 2 LoxP sites
- if the Cre recombinase is not present, expression of tTa is inhibited
- Cre is inserted into the mice, under the control of the Pdx1 promoter, and this initiates the constitutive expression of tTa in pancreatic cells

65
Q

what happens when tTa is expressed in the PDAC Cre/LoxP mouse model?

A
  • The pancreas-specific expression of the tTA induces the activation of TetO-driven responder transgenes encoding c-Myc as well as the GFP and luciferase reporters.
  • In pancreas, myc is overexpressed as cre is present in pancreas
66
Q

how can the overexpression of myc be inhibited in the PDAC Cre/LoxP mouse mode?

A

The expression of the TetO responder genes can be controlled in a temporal manner through administration of doxycycline (Dox) by binding to tTa and preventing it from producing the target genes
- as a result, Myc, GFP and luciferase expression is inhibited in the presence of Dox
- Can block myc expression

67
Q

how does doxycycline treatment affect myc expression in PDAC?

A

Inhibition of myc expression by doxycycline treatment caused regression of tumours after 7 days:
- No recurrence of tumours while the animals were treated for 1 month
- Significant decline in cell proliferation and induction of autophagy
- MYC inhibitors could potentially be used for the treatment of PDAC

68
Q

what is an example of a myc pharmacological inhibiter?

A

Mycro3 = small molecule inhibitor of Myc/Max dimerisation

69
Q

how can the inhibition of myc by mycro3 inhibitor be studied?

A

MYC is overexpressed in mutant KRAS-driven PDAC in mouse models:
- Cre recombinase is under control of a pancreatic promoter, can cleave LoxP site and drive expression of KRAS-G12D
- This leads to strong upregulation of myc expression

18F-FDG = labelled version of glucose that can be imaged by PET/CT scan:
- tumours have a much higher glucose uptake compared to normal tissues, can identify tumours as bright spot in the scan
- Mouse treated with Mycro3 after tumour establishment
- mycro-treated mouse marked shrinkage of the tumours
- mycro reduced cell proliferation & increased apoptosis in the tumours

70
Q

can mycro3 reduce tumour growth in xenografts from human PDACs?

A

yes
- Orthotopic: injection of human PDAC cells into the pancreas of recipient mice
- Heterotopic: subcutaneous injection of human PDAC cells in recipient mice
- Significant reduction in tumour growth upon treatment with MYC inhibitor in both conditions

71
Q

why is heterotropic injection of the myc inhibitor more effective than the orthotopic injection?

A

Subcutaneous injection had a more significant reduction in tumour size, as pancreas tumours are very fibrotic due to ECM, so conc of inhibitor in orthotopic injection is decreased

72
Q

what is the role of TGF-beta?

A

TGFb prevents phosphorylation and inactivation of pRb, so blocks cell cycle progression
- it counteracts the activity of myc

73
Q

how does TGFb affect myc? how does this affect cell cycle progression?

A

*Normally, TGFb inhibits the expression of Myc + prevents Myc from binding/inhibiting CKI promoters:
- CKI are expressed
- CDK4/6 and CDK2 are inhibited
- pRb is not phosphorylated
- Cells don’t go through the R point

74
Q

what happens to TGFb when myc expression levels are increased in tumours?

A

Myc is no longer under the control of TGFb

75
Q

what components of the TGFb signalling pathway can be mutated in cancer?

A
  • inactivation/mutation of SMAD4
  • SMAD2 mutation
  • SMAD3mutation
  • TGFb receptor expression/activity is reduced
76
Q

what does overexpression/high activity of TGFb pathway correlate with?

A

high activity of TGFb pathway correlates with poor prognosis in a variety of cancer types

77
Q

how does TGFb activity differ in early tumourigenesis vs late tumourigenesis?

A

Tumourigenesis progression converts TGFb signalling from tumour suppressor to tumour promoter:
- In early tumorigenesis, TGFb functions as a barrier to the development of cancer hallmarks in normal cells - acts as a TSG
- Late tumorigenesis counteracts the tumour suppressive functions of TGFb
- TGFb becomes the principal player involved in regulating the acquisition of cancer hallmarks
- As cancer progresses, TGFb becomes a tumour promoter, and promotes cancer hallmarks

78
Q

how does cancer inhibit TGFb to promote cell proliferation in early tumourigenesis?

A

Cancer cells counteract TGFb-dependent inhibition of uncontrolled proliferation by:
- Dysregulated myc expression -> counteract TGFb inhibition of cell cycle progression
- Hyperactivation of PI3K/AKT pathway inactivates the cytostatic activity of TGFb
- Cancer cells acquire the ability to increase cell proliferation when stimulated with TGFb, probably through TGFb-dependent expression of cytokines and growth factors and their receptors, which in turn stimulate cell growth

79
Q

how is TGFb implicated in late tumourigenesis?

A

by promoting cancer metastasis via EMT
- Epithelial to Mesenchymal Transition (EMT) = process through which polarised epithelial cells are converted into motile fibroblast-like cells - essential for cancer cell invasion & metastasis
- TGFb is a master regulator of EMT – promotes expression of EMT-mediating genes

80
Q

why is it difficult to target TGFb as a cancer treatment?

A

in early cancers, TGFb has a tumour-suppressive function
- challenging target for therapeutic intervention
- it is essential to select patients who will benefit from TGFb inhibitors e.g. patients who are in late cancer stage where TGFb is tumour-promoting
- problem if we are inhibiting the tumour suppressive role of TGFb

81
Q

what compounds/molecules can target TGFb signalling?

A
  • Antisense oligonucleotides
  • Neutralising antibodies blocking ligand/receptor interaction
  • antibodies that sequester ligands
  • Receptor kinase inhibitors

THESE ARE NOT SELECTIVE FOR PRO-ONCOGENIC RESPONSES OF TGFb AND INHIBIT ALL TGFb – INDUCED SIGNALLING EFFECTS
- can have side effects on normal cells

82
Q

how successful are TGFb receptor inhibitors and neutralising antibodies in treating cancer?

A

TGFbR kinase inhibitors and neutralising antibodies inhibited the development of lung and bone metastasis in mouse models of breast cancer
- Clinical trials have shown both positive and negative results
- TGFb2 antisense oligonucleotide increased high grade glioma patients survival
- TGFbR inhibitors did not affect survival of glioma patients
- Further studies are needed for patient stratification & combinatorial strategies

83
Q

what is p53?

A
  • p53 is a transcription factor which promotes genes which prevent cell growth & induce apoptosis
  • The p53 tumour suppressor gene is the most frequently mutated gene in human cancers
84
Q

where do mutations frequently occur in p53?

A

Most p53 mutations are in the DNA-binding domain:
- This prevents p53 from binding from the DNA
- If it cant bind to DNA it cannot induce apoptosis, so growth is uncontrolled

85
Q

how is p53 activity induced?

A

A diverse array of sensors are responsible for monitoring the integrity of various cellular systems
- When damage is detected , there is rapid increase in p53 protein levels
- rapid increase in p53 protein levels and stability during stress:

86
Q

what are the downstream effects of p53?

A
  • Promotes cell cycle arrest
  • Promotes DNA repair
  • If damage is repaired, cells return to proliferation
  • If not, cells revert to senescence or undergo apoptosis
87
Q

how is p53 at risk of leading to cancer?

A

A variety of signals converge on a single protein – p53.
- Economic design & major disadvantage for cells
- loss of a single protein results in the inability of the cells to respond to all these different signals

88
Q

how are p53 levels controlled?

A

Control of p53 levels by Mdm2:
- No difference in mRNA levels
- Post-translational stabilisation of p53
- P53 is controlled by protein stability

89
Q

how is p53 controlled by mdm2 in normal cells?

A

absence of stress:
- p53 is ubiquitinated by Mdm2 & degraded by the proteasome in the absence of stress
- Mdm2 mediates p53 ubiquitination and degradation, p53 levels are low

stress/damage: p53 must be protected from degradation
- Phosphorylation of p53 blocks Mdm2 binding, mediated by ATM/ATR/Chk2
- ATM can phosphorylate Mdm2, leading to Mdm2 inactivation
- P53 can no longer be degraded, so rapid increase in p53 levels
- P53 mediates all 3 DNA damage checkpoints: G1/S, S phase & G2/M

90
Q

how is p53 activity altered in cancer?

A

Mitogenic and cell survival signals causes induction of Mdm2:
- Phosphorylation-induced activation of Mdm2, leading to p53 proteasomal degradation
- mitogens phosphorylate mdm2 at a different site to ATM/ATR, so in presence of growth factors, mdm2 dominates
- Suppression of p53 levels
- Growth is promoted

91
Q

how does p53 arrest the cell cycle?

A

P53 arrests the cell cycle primarily by upregulating p21Cip1, which inactivates cyclin/CDK complexes
- Regulates checkpoints as p53 enables inhibition of cyclin/CDKs

92
Q

how does p53 regulate the cell cycle in a normal cell?

A
  • P53 is stabilised in damage and binds to DNA, promotes p21, blocks cell cycle progression, promotes DNA repair
  • If damage can’t be repaired, apoptosis
93
Q

how does p53 regulate the cell cycle in a cancer cell?

A
  • P53 levels increased in damage, but p53 is mutated
  • P53 cannot bind to DNA, so faulty cell continues to progress through cell cycle, giving rise to cancers as DNA mutation/damage is passed on

Stem cells, Ageing and Cancer (8 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions