Cancer Flashcards
Define metaplasia
Metaplasia is a reversible change in which one adult cell type is replaced by another cell type.
What stimulates metaplasia?
It can be due to an adaptive response to a stimulus, such as cigarette smoke, acid reflux etc. The cells are replaced by those that can withstand the adverse environment.
Metaplasia also takes place when a stem cell is reprogrammed to differentiate along a different pathway in response to signalling by cytokines, growth factors and extracellular matrix components.
What is dysplasia?
Dysplasia is an abnormal pattern of growth in which some reversible cellular and architectural features of malignancy are present. (but at a non/pre-malignant stage)
What are the cytological features of dysplasia?
- Loss in architectural orientation
- Loss in uniformity of individual cells
- Hyperchromatic, enlarged nuclei
- Mitotic figures are abundant and in places where they are not usually found.
In which tissues is dysplasia most common? And what causes them.
- Cervix – HPV Infection
- Bronchus – Smoking
- Colon – Ulcerative Colitis
- Larynx – Smoking
- Stomach – Pernicious Anaemia
- Oesophagus – Acid Reflux
What was the previous name given to dysplastic tissue?
carcinoma-in-situ
Define neoplasia
A neoplasia is an abnormal, autonomous, proliferation of cells unresponsive to normal growth control.
Define tumour
A tumour is defined as a swelling resulting from excess cell proliferation.
Define metastasis
Metastasis is a discontinuous growing colony of tumour cells, at some distance from the primary tumour.
How are tumours named?
We name tumours on their presumed site of origin and weather they are benign or malignant.
What are malignant tumours of the epithelia called?
Malignant tumours of the epithelia are carcinomas.
Give examples of benign tumours of the epithelia
Their benign counterparts end in ‘–oma’. A papilloma is a benign tumour of surface epithelia, and an adenoma is a benign glandular tumour.
What are malignant tumours of the connective called?
Malignant tumours of connective tissue are sarcomas
Give examples of malignant tumours of the connective tissue
liposarcoma (fat), osteosarcoma (bone), chondrosarcoma (cartilage) leiomyosarcoma (smooth muscle).
Differentiate between lymphoma and leukaemia
A leukaemia is a malignant tumour of a bone marrow derived cell, which circulates in blood. While a lymphoma is a malignant tumour of lymphocytes (usually) in lymph nodes.
What is a teratoma?
A teratoma is a germ cell tumour
When are teratomas malignant and when are they benign?
Gonadal teratomas in males are all malignant, while those in females tend to be benign.
What is a hamartoma?
A hamartoma is a localised overgrowth of cells and native tissue to the organ. They are benign, have a completely normal cytology (and so not a neoplasia), but architecturally abnormal (so a tumour).
How are harmartomas different from neoplasias?
Harmartomas are benign and have a completely normal cytology.
Where and when are harmartomas common?
They are common in children and should stop growing when they do. Common ones are bile duct hamartomas, haemangiomas, bronchial hamartomas, and Peutz-Jegher polyps in the gut.
List features which distinguish benign from malignant tumours
Benign:
- Does not invade nor metastasise
- Encapsulated
- Well differentiated
- Slow growth
- Normal mitosis
Malignant:
- Invades surrounding tissue
- Metastasises to distant sites
- No capsule
- Poorly differentiated
- Rapid growth
- Abnormal mitosis
When are benign tumours dangerous?
Benign tumours are not normally dangerous, except if they are in dangerous places (such as meninges) or secrete dangerous hormones (insulinomas secrete insulin which can lead to hypoglycaemia). They can also bleed, get infected, rupture, or twist/tort.
What are the five morphological features that allow you to see how well differentiated the tumour is?
- A small numbers of mitoses.
- Lack of nuclear pleomorphism
- A low nuclear-cytoplasmic ratio.
- Relatively uniform nuclei
- Close resemblance to the corresponding normal tissue
What is the grade of a tumour?
How well differentiated it is.
What is a cancer’s stage?
A cancer’s stage is how well/far it’s spread.
What is more important in predicting prognosis, stage or grade?
Stage (metastasis)
Describe the TNM staging system.
- T describes the size of the original tumour and whether it has invaded nearby tissue (TX, Tin situ, T0, T1-T4)
- N describes regional lymph nodes that are involved (Nx, N0-N3)
- M describes distant metastasis (M0, M1)
What are the reasons some cells divide at different rates?
- Maturity: embryonic cells divide at a much faster rate than adult cells
- Complexity of the system: more complex cells have a slower rate of division
- Necessity for renewal: skin and gastrointestinal epithelial cells divide every 20 hours, while hepatocytes divide every year.
- State of differentiation: some cells such as neurones don’t ever divide.
- Neoplasic cells have lost the ability to inhibit division
What does the fact that most cancer cells are aneuploidic show?
That there had been errors in cell division.
Define the cell cycle
The cell cycle is an orderly sequence of events in which a cell duplicates its contents and divides in two.
What are the two phases, and the states they are split by, of the cell cycle?
- M-phase covers mitosis and cytokinesis
- Interphase covers G0, G1, S and G2
What is the most vulnerable period in the cell cycle?
Mitosis
Give four reasons why mitosis is the most vulnerable period in the cell cycle.
- Cells are most easily killed in that state; Premature or aberrant mitosis leads to cell death.
- DNA damage acquired during this phase cannot be repaired, and is passed on to daughter cells.
- Gene transcription is silenced as chromosomes are being separated.
- Metabolism is shut down to focus on mitosis.
Describe the states interphase is split into
- G0 is when the cell cycle machinery is dismantled, and the cell is functional. Most cells are at this state.
- G1 is when the decision to divide has been made.
- S is when DNA and organelles are duplicated as well as increase in protein synthesis.
- G2 is when the cell checks everything is okay before proceeding towards mitosis.
Briefly describe the structure of a centrosome
It consists of two centrioles, a mother and daughter, which are barrels of 9 triplet microtubules.
What are the functions of the centrosome
The centrosome functions as a microtubule organising centre and mitotic spindle.
When and how is the centrosome duplicated?
The daughter centrioles separate, duplicate, and thus form two centrioles in Late G1
From where do microtubules grow and polymerise?
Microtubules grow and polymerise from nucleating sites on centrosomes.
Describe what happens in prophase
Prophase mainly involves the condensation of chromatin to form chromosomes.
In late prophase, the homologous pair of chromosomes migrate to opposite sides of the nucleus and the mitotic spindle forms outside the nucleus:
- Radial microtubules arrays (Asters) form around each centrosome.
- The radial arrays meet, determining the polar microtubules
How are chromosomes formed in prophase?
As the DNA is already duplicated, there are two sets of DNA in the nucleus. The DNA wraps around histone proteins to form chromatin, which then pack closer to form nucleosomes. These eventually coil together to form a chromatid. The sister chromatids join at the centromere to form a chromosome.
How is metaphase split?
In early prometaphase there is breakdown of the nucleus, followed by the attachment of the chromosomes to the spindle via the kinetochores on each sister chromatid.
In late prometaphase the microtubules from opposite poles are captured by sister kinetochores. Chromosomes attached to each pole congress in the middle. It is metaphase when the chromosomes are at the equator.
How is anaphase split?
Anaphase A: there is a signal to break down cohesin (the protein that holds the chromatids together), allowing the separation of the chromatids.
At anaphase B, the daughter chromosomes migrate towards the poles, while the centrosomes migrate apart.
Describe what happens in telophase
Telophase begins when the sister chromatids have reached opposite poles, arriving at the centrosome spindles. The nuclear envelope reassembles around the chromosomes at opposite ends.
A contractile filament ring assembles around the equator, forming the cleavage furrow, which will contract to split the cell in two.
Describe the process cytokinesis
Mitosis ends when new membrane is inserted between the cells, essentially separating them. The acto-myosin ring contracts to divide the cell. A midbody is left between the cells, which is composed of residual spindle fibres. After cytokinesis, interphase microtubule array reassembles, chromatin decondenses and nuclear substructures reform.
Where is the mitotic checkpoint (in terms of timing)?
There is a mitotic checkpoint between prometaphase and metaphase controlling weather a cell progresses into anaphase.
Explain the mitotic checkpoint
CENP-E sense when kinetochores are attached. BUB protein kinases then dissociate from the kinetochore when the chromosomes are properly attached to the spindle. Only when all the BUBs are dissociated, can anaphase proceed.
What word is used to describe normal attachment of chromosomes to the mitotic spindles?
Amphelic
What names are given to the types of attachment that leads to aneuploidy?
- Syntelic is when both sister chromatids are attached to the same centrosome pole.
- Merotelic is when one kinetochore is connected to two microtubules from both centrioles, resulting in a broken/lost centrosome in cell division.
- Monoletic is when only one chromatid is attached.
How can the mitotic checkpoint be manipulated to treat cancer?
- A checkpoint kinase inhibitor will allow the tumour cells to produce gross chromosome mis-segregations, which will eventually cause the cell to undergo apoptosis.
- Taxanes and Vinca Alkaloids alter the microtubule dynamics leading to unattached kinetochores. This therefore causes long-term mitotic arrest.
Which is the longest phase of the cell cycle (not G0)
G1
Where (when) are the checkpoints before the cell undergoes mitosis?
- There is a checkpoint at late G1 before entering S phase. The cell will only continue to S phase if the cell is big enough
- There is another checkpoint at G2, before the onset of mitosis. The cell can only continue if the cell size and external environment are correct, as well as accurate and complete DNA replication takes place.
What can happen if the cell doesn’t pass the checkpoint?
- Cell cycle arrest – at the checkpoints. This can be temporary, until the parameter has been fixed.
- Apoptosis – happens if DNA damage is too great, or if there are chromosomal abnormalities or toxic agents present.
What transcription factor stimulates the entry of the cell into the cell cycle?
c-Myc
What is the name given to the type of signal caused by binding of growth factors?
Mitogenic signal
How is the receptor that a growth factor binds to activated?
Growth factor peptides are dimers which bind simultaneously to two tyrosine kinase receptors, bringing them together. This allows the cytoplasmic tails of the receptor (containing the tyrosine kinases) to cross-phosphorylate each other. This creates multiple phosphorylated tyrosine residues, which act as docking sites that recruit other proteins.
How is Ras activated?
Once the tyrosine kinase receptor is activated, an adaptor protein called GRB2 hooks onto a phosphate group and recruits an exchange factor called SOS. SOS activates a membrane-bound signaling (GTP binding) protein called Ras, which is normally inactive bound to GDP. SOS catalyses the exchange of GTP from the cytoplasm to the Ras protein, changing the conformation and activating it.
How is Ras inactivated?
GTPase Activating Proteins (GAPs) turn off the Ras
Describe the nature and function of Ras
- Membrane bound
- GTP binding protein
- Normally bound to GDP and inactive
- Activated by exchange factors (SOS) which change GDP with GTP
- Activates protein kinase cascade
How is Ras often mutated in caner?
- Glycine at position 12 valine: prevents GAP binding and therefore preventing Ras inactivation
- Glutamine at position 61 leucine prevents GTP hydrolysis as glutamine is normally involved in the intrinsic GTPase activity of Ras
How does Ras lead to increased levels of c-Myc?
GTP-Ras activates a protein kinase cascade by binding to the first kinase in the ERK (Extracellular Signal-regulated Kinase) cascade. The final MAPK phosphorylates Myc
What does MAPK stand for?
Mitogen Activated Protein Kinase
Describe the MAPK cascade activated by Ras
- The first kinase (generically called MAPKKK), called Raf, phosphorylates to activate the second kinase, using ATP.
- The second kinase (generically called MAPKK), called MEK, phosphorylates to activate the second kinase, also using ATP.
- The last kinase (generally called MAPK), called ERK, phosphorylates target proteins involved in gene transcription and protein synthesis. An example of a transcription factor activated is Myc.
What proteins control progression through the cell cycle?
Cyclin-dependent kinases (Cdks) also called cyclically activated protein kinases .
What type of kinases are Cdks?
serine/threonine kinases
How are Cdks activated?
Cyclins and phosphorylation (twice, then one is removed)
When are cyclins and Cdks expressed in the cell?
Cdks are always expressed
Different cyclins are expressed transiently through the cell cycle. First cyclin D at the end of G1, then cyclin E, then cyclin A and finally cyclin B.
Describe how Cdks are activated by phosphorylation
The Cdk-cyclin complexes are further regulated by two kinases. CAK (Cdk Activating Kinase) phosphorylates one site on Cdk1 to promote Cdk1 activity – positive phosphorylation. Wee1 phosphorylates another site on to inhibit Cdk1 activity – negative phosphorylation. A Cdk with two phosphate groups is still inactive. In order for it to be catalytically active, the inhibitory phosphorylation must be removed by Cdc25, a phosphatase.
How does Cdk1-cyclinB increase due to positive feedback?
Active MPF (Cdk1-CyclinB) has a positive feedback effect on Cdc25 by stimulating phosphorylation of the inactive Cdc25 which can dephosphorylate more inactive MPF more active MPF
What is the name given to the Cdk1-cyclinB complex?
MPF (maturation promoting factor)
Describe the role of MPF (Cdk1-CyclinB)
In the early phase of mitosis, active MPF phosphorylates many substrates required for mitosis. The Cdk1-cyclinB complex holds mitosis until a signal is received that all kinetochores are attached to the spindle. When all kinetochores are attached, cyclinB degradation is triggered. This leads to the inactivation of Cdk1 and key substrates are dephosphorylated so that mitosis can proceed at the metaphase-anaphase transition.
From what points does MPF (Cdk1-CyclinB) exist in the cell?
The Cdk1-cyclinB (MPF) complex is only activated at the start of mitosis and is deactivated before anaphase
How does c-Myc kick start the cell cycle?
One of the main genes ‘activated’ by c-Myc (transcription factor stimulated by growth factor) is that which codes for cyclinD. CyclinD is able to form complexes with Cdk4 and Cdk6. (then cyclin E, then A then B)
What is the order of Cdk complexes activated in the cell?
- Cdk4/6-cyclinD from G0 to G1
- Cdk2-cyclinE from G1 to S
- Cdk2-cyclinA from S to Mitosis
- Gdk1-cyclinB from Prophase to Anaphase
How does Cdk2-cyclinE increase due to positive feedback?
CyclinE is then able to form a complex with Cdk2, which is needed to move the cell from G1 to S phase. An example of a protein phosphorylated by cyclinE is the RetinoBlastoma (RB) protein - pRB, which is inactivated by phosphorylation.
When inactivated, the conformational change allows the release of a transcription factor E2F. Among other proteins, E2F allows for the transcription of further cyclinE
Describe the nature of the RB protein
The RB protein is normally active, holding transcription factor E2F in place. When phosphorylated (twice) it is inactivated, letting lose E2F which up regulates transcription of cyclinE
How are Cdks inactivated?
- Wee1
- CKIs (Cdk Inhibitors)
What are the two families of CKIs and what Cdks do they effect?
- The INK4 family is active in G1, to inhibit Cdk4/6-cyclinD complexes by displacing cyclinD. There is a balance between
cyclin D binding to Cdk4/6 and INK4 binding to control progression through G1. - The CIP/KIP family is active in S phase to inhibit all Cdk complexes by binding to the entire complex itself
Categorise the following into TS or Oncogenes:
- EGFR/HER2
- Ras
- RB
- CyclinD
- B-raf
- CKIs
- c-Myc
The oncogenes (mutated proto-oncogenes) include:
- EGFR/HER2 (receptors) mutationally activated or overexpressed in many breast cancers
- Ras mutational activation in many cancers
- CyclinD is overexpressed in 50% of breast cancers
- B-Raf (top kinase in ERK cascade) mutationally overactivated in melanomas
- c-Myc overexpression in many tumours
Tumour suppressors include:
- RB is inactivated in many cancers
- CKI under-expression in many cancers
What are the ‘pillars of therapy’ to treat cancer?
Surgery
Chemotherapy
Radiotherapy
Immunotherapy
What are the two subtypes of chemotherapy?
- Cytotoxic chemotherapy
- Targeted therapies
List the types of cytotoxic chemotherapies
- Alkylating agents
- Anti-metabolites
- Anthracyclines
- Vinca Alkaloids and Taxanes
- Topoisomerase inhibitors
List the types of targeted chemotherapies
- Small molecule inhibitors
- Monoclonal antibodies
What is the general mechanism by which cytotoxic chemotherapies work?
Cytotoxics ‘select’ rapidly dividing cells by targeting their structures. They target DNA, apart from Vinca Alkaloids and Taxanes which target mitotic spindles. [Cytotoxic drugs are usually given intravenously, but occasionally orally.] Because of its systematic nature, they target all rapidly dividing cells.
What are the side effects of cytotoxic chemotherapies?
Side effects include: hair loss, nausea, vomiting, damaged nails, fatigue, weight loss, anorexia and immunosuppression (as bone marrow cells are also rapidly dividing, there is lowering of the neutrophil count causing neutropenia), nephrotoxicity, neurotoxicity and diarrhoea.
Define cancer treatment scenarios in terms of being neoadjuvant or adjuvant
When a treatment is given pre-operatively, it can be described as an neoadjuvant. When given post-operatively, it can be described as an adjuvant.
How do alkylating agents treat cancer?
Alkylating agents add alkyl groups (CnH2n+1) to guanine residues in DNA. This allows the formation of cross-links between the stands, entire DNA molecules and between the DNA and histones. This will ultimately lead to apoptosis via a checkpoint mechanism as the DNA cannot unwind.
What are the risks with using alkylating agents as chemotherapeutic agents?
The addition of alkyl groups to guanine residues leads t the risk of secondary malignancies as they encourage miss-pairings.
What is the difference between alkylating agents and pseudoalkylating agents?
Alkylating agents add alkyl groups to guanine residues. Pseudoalkylating agents add a platinum group to guanine residues instead.
What are the side-effects of alkylating agents?
- hair loss
- nephrotoxicity (alkylating)
- neurotoxicity
- ototoxicty (platinum)
- nausea
- vomiting
- diarrhoea
- immunosuppression
- tiredness.
How do anti-metabolites treat cancer?
Anti-metabolites masquerade as purine or pyrimidine residues. They replace bases during DNA synthesis, leading to breakages in the DNA double strand and therefore apoptosis. Also blocks transcription. They can also be folate antagonists which prevents the formation of folate acid, an important building block for many amino acids.
What are the side-effects of anti-metabolites?
alopecia, immunosuppression, nausea and vomiting, diarrhoea, palmar-plantar erythrodysesthia and fatigue
How do anthracyclins treat cancer?
Anthracyclins fit in between (intercalate) DNA strands, inhibiting transcription and replication. They also generate free radicals that can damage DNA and cell membranes. Because they block DNA repair, they are mutagenic.
What are the side effects of anthracyclins?
Side effects of antracyclins include hair loss, fatigue, vomiting and nausea, immunosuppression, skin changes, cardiac toxicity, and red urine.
How do Vinca Alkaloids and Taxanes treat cancer?
Vinca Alkaloids inhibit microtubule assembly and Taxanes inhibit microtubule disassembly.
What are the side effects of Vinca Alkaloids and Taxanes?
Side effects include: nerve damage (peripheral neuropathy, autonomic neuropathy, alopecia, nausea and vomiting, immunosuppression, arthralgia and allergy.
How do topoisomerase inhibitors treat cancer
Topoisomerases prevent DNA torsional strain during DNA replication and transcription by introducing single (topo1) or double (topo2) strand breaks in the poshphodiester backbone of DNA. Inhibiting this leads to permanent DNA breaks –> apoptosis of the cell.
What cytotoxic drugs also have anti-topoisomerase activity (besides main function)?
Anthracyclines
What are the side effects of topoisomerase inhibitors?
Side effects include alopecia, nausea and vomiting, fatigue, immunosuppression and acute cholinergic type syndrome (irinotectan) which is diarrhoea, abdominal cramps, and diaphoresis (sweating) – therefore given with atropine.
Explain why different drugs and modalities are often given in combination to prevent/overcome resistance to treatment
Cancer cells can become resistant to some of the cytotoxic drugs. They do this by acquiring mutations, which are then naturally selected for during treatment.
Therefore, drugs are often given in combination as it is unlikely the cell can squire both resistant mutations.
Give examples of ways cancers can become resistant to treatment?
- DNA repair mechanisms upregulated –> no double-stranded breaks and thus no apoptosis.
- DNA adducts are replaced by Base Extinction repair
- Drug efflux from the cell using ATP-binding cassette transporters
What are the 10 hallmarks of cancer?
- Self-sufficient: normal cells need growth signals to move into the cell cycle
- Insensitive to anti-growth signals
- Anti-apoptic
- Pro-invasive and metastatic
- Pro-angiogenic
- Non senescent
- Dysregulated metabolism
- Immune system evasion
- Unstable DNA
- Inflammation
How can monoclonal antibodies be used to treat cancer?
Monoclonal antibodies (mAbs) can bind to the growth factors to prevent them causing receptor dimerisation and thus leading to mitotic signalling pathway. mAbs can also bind to the ligand binding sites on the receptor itself, preventing ligand binding. This type of binding also causes receptor internalisation and thus down-regulating the number of expressed receptors.
Give examples of cancers that can be treated with monoclonal antibodies
Bevacizumab binds to and neutralises the VEGF receptor, improving survival in colorectal cancer. Cetuximab targets the extracellular portion of EGFR to prevent it dimerising.
How can monoclonal antibodies be administered?
Intravaenously
Outline monoclonal antibody nomenclature
- momab refers to antibodies from mice
- ximab refers to chimeric antibodies
- zumab refers to humanised antibodies
- mumab refers to fully human antibodies
How do small molecule inhibitors treat cancer?
Small molecule inhibitors bind to the intracellular portion of the tyrosine kinase receptors, preventing autophosphorylation and thus blocking downstream signalling. They can also block the receptor domain. [It can be given orally]
What is the name of the small molecule inhibitor that is very successful in treating CML?
Glivec
How is CML successfully treated with a small molecule inhibitor?
The (9,22) chromosome translocation in patients with chronic myeloid leukaemia is targeted as it was found to create a fusion protein (called Bcr-abl) containing a tyrosine kinase, which was found to cause over-production of white blood cells. Glivec is a small molecule inhibitor that only targets the Bcr-abl ATP binding region within the tyrosine kinase domain.
Why was CML treated so successfully with Glivec?
the case was an example of an ‘Oncogene addicted’ cancer. This is when a single mutation is the diving force of the tumour and can therefore be seen as it’s ‘Achilles’ Heel’
What are the advantages of using targeted therapies over systemic cytotoxic drugs?
The main advantages of using targeted therapies in the blockage of cancer hallmarks the without toxicity seen in cytotoxic drugs. Targeted therapies can also inhibit angiogenesis and anti-apoptosis.
What is the main disadvantage of using targeted therapies over systemic cytotoxic drugs?
a major disadvantage of targeted therapies is the ability for the tumour to develop resistance easily
Give examples of mutations that lead to resistance against targeted therapies
- Mutation in the ATP binding region of the Bcr-abl protein allowing only ATP to bind and not Glivec.
- Intrinsic resitance (Herceptin is only effective in 85% of HER2+ breast cancers, suggesting other driving pathways)
- Intragenic mutations
- Upregulation of downstream or parallel pathways
What are the future developments in targeted therapies?
Anti-sense oligonucleotides can hinder translation of specific mRNA. They can also recruit an enzyme to cleave the target mRNA. This is particularly useful for ‘undruggable’ targets.
RNA interference is a single stranded complementary RNA which recognises and destroys mRNA, preventing the ‘bad’ protein(s) from being synthesised. However, this technique has lagged behind anti-sense technology – especially in cancer therapy. Furthermore, all these treatments are very expensive and the NHS cannot afford them.
What types of external factors can influence the cell to divide?
Chemical – hormones, growth factors, ion concentrations, ECM, molecules on other cells, nutrients and dissolved gas concentrations
Physical – mechanical stresses, temperature and the topography of the ECM.
What are the most understood external factors that influence the cell to divide?
- Growth factors
- Cell-cell adhesion
- Cell-ECM adhesion
What is anchorage dependece?
In suspension cells do not significantly synthesise proteins or DNA; cells require ECM attachment and a certain degree of spreading (important) to begin protein synthesis and DNA replication
Why is cell spreading not a passive process?
It requires energy to modulate cell adhesion and the cytoskeleton during spreading
Why don’t cells synthesise proteins or replicate in suspension?
They require ECM attachment and cell spreading
What are the most important cell-ECM adhesion molecules?
Integrins as they have lots of functions such as a role in development, the immune system, clotting system etc.
How do integrins allow mechanical continuity between the ECM and cell interior?
Most integrins are linked to the actin cytoskeleton through actin-binding proteins. An exception to this is the α6β4 integrin which is found in epithelial hemidesmosomes (basal structures associated with epithelial cells) and is linked to the cytokeratin (Intermediary Filaments of epithelial cells) network.
Explain the structure of integrins
Integrins are heterodimeric complexes of α and β subunits that are associated by their head regions. Each of their tail regions spans the plasma membrane into the cytoplasm.
What is required for integrins to function?
The head regions must bind to divalent cations such as calcium for their function.
How many combinations of integrin subunits exist?
There are 10 α units and 8 possible β subunits to make 20 combinations of integrin molecules.
What are integrin complex clusters called?
Focal adhesions
How are integrins involved in outside-in and inside-out signalling?
Focal adhesions are a way for the cell to interpret the environment around it. Ligand binding extends the tails of the integrein complex allowing cytoplasmic signalling and actin assembly.This is called outside-in signalling and can alter the phenotype of the cell.
Inside-out signalling is when a hormone can alter the affinity of the integrin.
What term is used to describe why cells in culture stop growing after they form a confluent layer?
Density dependence (competition for external growth factors), previously thought to be related to ‘contact inhibition’
How do density dependence (GF) and anchorage dependence (ECM) signalling work synergistically to allow cell proliferation?
There is cross-talk between growth factor signalling and ECM signalling. There is a convergence in the signalling of their pathways to produce proliferation. Individually, their activation of signalling pathways are weak, but together it is strong and sustained.
What are the two types of cell-cell contact?
- Short-term contact is when the transient interactions between the cells do not form stable cell-cell junctions.
- Long-term contact is when there are stable interactions leading to the formation of cell-cell junctions.
What happens when non-epithelial cells collide?
They actually ‘repel’ one-another by paralysing motility at the site of contact. This in turn promotes the formation of a motile front at the opposite site allowing the cell to move away.
What process prevents multi-layering of cells in culture and in vivo?
contact inhibition of locomotion
What cell types are able to form long-term cell-cell contacts?
epithelial or endothelial cells
How can cell-cell junctions be arranged?
These can be arranged continuously (zonulae) or in discrete spots (maculae).
What are the consequences of long-term cell-cell contact?
Contact between endothelial cells leads to stable adhesions and mutual induction of spreading. Cell-cell contact leads to lower proliferation of the cells.
How do adherens junctions enable cell-cell contact?
Adherens junctions are one of the most important types of cell-cell junctions. They use ca2+ dependent cadherin adhesion molecules to form a complex junction. Extracellular, CADHERIN associates with with identical molecules on adjacent cells. Intracellularly, the cadherin cytoplasmic tails associate with a β-CATENIN molecule, which associates with an α-CATENIN molecule, which associates with an actin filament.
How does cell-cell contact lead to decreased cell proliferation?
In the cytoplasm, β-catenin associates with LEF-1 forming a transcription factor, leading to proliferation. When bound to cadherin, β-catenin is not available for LEF-1 association and therefore inhibits proliferation.
- also activates Rac and inhibits Rho
- some growth factor receptors are associated with cell-cell junctions
How does APC gene mutation lead to increased proliferation?
APC gene-product is a protein that normally degrades beta-catenin in the cytoplasm. Without this, more beta-catenin can bind to LEF-1 to form a transcription factor leading to proliferation.
What term is given to the limit of the number of times a cell can divide?
The Hayflick limit
What changes must an epithelial cell go through to allow invasion and then metastasis?
Metastasis of a primary carcinoma cell only works when cell-cell adhesion is down-regulated. Cells must be motile in order to spread. Degradation of the ECM must also take place, and so matrix metalloproteinase levels are increase to migrate through the basal lamina.
In what percentage of cancer is Ras mutated?
30%
What type of genes are achorage dependent or density dependent related genes?
proto-oncogenes
What are the situations in which you may see cell movement?
- organogenesis
- wound healing
- growth factors/chemoattractants
- de-differentiation (in tumours)
How is directionality of cell movement achieved?
By the polarity of the cell
What do cells ‘hold’ onto while they move?
The substratum
What are the types of motility based on directionality?
Hapoptatic motility is movement with no direction. Chemotactic motility is where the cell senses a stimuli and goes towards it.
What molecule is assembled and disassembled in movement?
Actin filaments
What are the different structures formed by actin filaments, and how are they organised?
- Filopodia are finger-like projections rich in actin filaments, used by the cell to sense the environment. The actin forms tight parallel bundles which do not contract.
- Lamellipodia are sheet like protrusions rich in actin filaments, which attach to the substratum via focal adhesions to move the cell forward. Here the actin is branched and cross-linked.
- In stress fibres, they form anti-parallel, contractile bundles (which contract to slide past each other and shorten).
- Microvilli
- Stereocilia
What are the different proteins that can regulate the shape of the actin filaments and the way they interact with themselves?
- Sequestering proteins bind to the G-actin monomers and hold them in a pore until they are required.
- Nucleating proteins are made to initiate the polymerisation of actin to generate the filament
- Motor proteins provide contraction and other proteins bind to generate a filament.
- Capping proteins are stop signals to prevent further growth
- Severing proteins are like scissors
What is actin nucleation?
Nucleation is the limited step in actin dynamics, it is the formation of trimers to initiate polymerisation of filaments.
What proteins help in the formation of actin trimers in the process of nucleation (apart from nucleating proteins)?
Arp2 and Arp3 (actin related proteins) are proteins that stabilise the point at which nucleation starts, forming an Arp23 complex which is able to bind a trimer of actin monomers.
How do Profilin and Thymosin proteins influence actin filament elongation?
Profilin exchanges ADP in G-actin to ATP which makes it easier to join the filament. Thymosin forms complexes with G-actin, preventing it from polymerising. The balance between these two proteins regulate the rate of filament growth
Give examples of proteins that cap the positive and negative ends of actin filaments
- At the positive end, these include: Cap Z, Gelosin and Fragmin/Severin.
- At the negative end, these include: Tropomodulin and Arp Complex.
