Tumour Heterogeneity

Question 1

What is the reductionistic view of cancer and how does it compare to a more realistic view?

Answer 1

The reductionist view sees cancer as a homogenous clump of cancer cells where the phenotype is a direct result of the genotype, it is driven by a cell autonomous process and all stems from the behaviour of an individual cancer cell
A more realistic view however sees cancer cells where the phenotype type is not necessarily a direct result of genotype, and something which involves both cell autonomous and non-cell autonomous (stromal) influences making cancer a disease of a tissue and not just uniform cancer cells

Question 2

What is the Caveat of selection or Darwinian selection in cancer?

Answer 2

Cancer cells have a high amount of genetic diversity both within the primary tumour and the metastasis late stage diseases
This diversification appears to outpace the process of Darwinian selection with genetically distinct subpopulations of cells appear to be generated far more rapidly than they can be eliminated

Question 3

What is the role of mutations in the natural selection of cancer?

Answer 3

Mutations can giver cancer cells new driver properties- this frequently results in resistance to drug therapies as if something like bRaf is inhibited then another clone will have a mutation allowing that pathway and inhibitor to be circumvented
Mutations can give cancer cells stem like properties allowing one clone to leave the tumour mass and then give rise to a whole new clump of tumour tissue elsewhere in the body
Mutations may give cells the ability to invade and metastasise

Question 4

What is the epithelial to mesenchymal transition?

Answer 4

This is a reactivation of the mesenchymal gene expression programme which gives cancer cells invasiveness and metastatic potential

Question 5

What are examples of tumours clearly not being simply a lump of tumour cells?

Answer 5

Hodgkin lymphoma where the cancer cells are quite rare and exist surrounded by numerous other normal cells
Adenoca in the stomach where there is not a lot of normal cells but a large amount of acellular matrix

Question 6

What are the stromal interactions seen in cancer cells?

Answer 6

Interactiosn with inflammatory cells, stromal cells such as endothelial cells, pericytes and cancer associated fibroblasts as well as interactions with the extracellular matrix

Question 7

Where are many of the stromal cells that the cancer cells will interact with derived from?

Answer 7

The blood, this includes lymohicytes which release proinflammatory cytokines and mount an adaptive immune response, monocytes which migrate to the site of injury in response to chemotatci factors, macrophages which primarily stem from ifferentiated monocytes and release proinflammatory cytokines and growthfactors with their presence profoundly affecting the surrounding microenvironment, mast cells which release histamine, cytokines and proeteases, platelets which aggregate to form a fibrin clot involved in wound healing
Neutrophils and eosinophils which are the first cells recruited to the damage tissues and invading bacteria

Question 8

What immune cells are tumour antagonizing?

Answer 8

Cytotoxic T lymphocytesand NK cells

Question 9

What immune cells are tumour promoting?

Answer 9

Macrophages
Mast cells
Neutrophils
Myeloid progenitors
Myeloid derived suppressor cells

Question 10

Why are so many cancer stromal tissues derived from the blood?

Answer 10

The defective blood vessels in cancer assists in infiltration of the tumours by inflammatory cells

Question 11

What is the role of tumour-associated macrophages in cancer?

Answer 11

These cells are attracted by cancer cells secreting colony-stimulating factor-1and are a source of proinflammatory cytokines and growth factors which support cancer cell survival and proliferation through the activation of NFkappaB
They also secrete proteases that separate tumour cells degrade matrix, release and activate growth factors
TAM can also release angiogenic factors to support the formation of new blood vessels

Question 12

How can degradation of the matrix support tumour cell growths?

Answer 12

This releases and activates growth factors which are stored by protein fibres these are typically used in the case of injury to facilitate wound healing but can be manipulated by cancer cells which cause macrophages to secrete protease

Question 13

What are the proteases secreted by tumour associated macrophages?

Answer 13

Matrix Metalloproteases 1,2,7 and 9

MMP9 is particularly active in TAMs

Question 14

What are the functions of the proteases secreted by tumour associated macrophages?

Answer 14

Digest collagen, laminin, fibronectin which form the tissue scaffold, this facilitates invasion by the cancer cell
They can separate tumour cells to assist in invasion
They can activate growth factors which assists in cell growth and proliferation

Question 15

What is the relationship between the presence of tumour associated macrophages and patient outcome?

Answer 15

If there is an increased presence of TAM then the patient is less likely to have a good prognosis

Question 16

What are carcinoma associated fibroblasts (CAFs)?

Answer 16

These are cells which are potent pro-angiogenic cells therefore they can assist in the formation of blood vessels with increased presence of these cells being associated with increased blood vessel density
they can also secrete ECM components which lead to fibrosis
They can secrete growth factors which support cancer cell survival and proliferation

Question 17

How are CAFs proangiogenic?

Answer 17

They are able to recruit endothelial cell precursors through stroma-derived factor-1 (also known as CXCL12).

Question 18

How can you tell if a fibroblast is normal or is being manipulated by the tumour?

Answer 18

Fibroblasts are cultured on a plate and then endothelial cell precursors are placed in a vessel which is in contact with the fibroblasts via a semipermeable membrane if the fibroblast is being manipulated by the tumour and has become active then it will cause the endothelial precursor cells to start to proliferate while normal fibroblasts will not

Question 19

How can the extracellular matrix interact with cancer cells?

Answer 19

This is more than just a skeleton with the molecules in this matrix having profound influence in cell behaviour and example of an interaction like this is that of integrins with integrin receptors

Question 20

What are integrins?

Answer 20

These are transmembrane proteins that attach cells to the extracellular matrix components or too other cells
However they serve not only as a glue but are also key integrators of cel function palying a critical role in the intracellular signalling cascade

Question 21

How does ECM mediated pro-survival signalling occur?

Answer 21

This occurs when integrins and their receptors meet allowing the activation of Focal Adhesion Kinase (FAK) this is an important second messenger which provides a docking site for several proteisn which affect cell survival proliferation and migration

Question 22

How does the ECM regulate cell metabolism?

Answer 22

Loss of cell attachment to the matrix causes metabolic defect where there will be a decrease in the cells ability to uptake glucose and therefore reducing its ability to produce ATP leading to cell death by homelessness or anoikis

Question 23

What is the model of cancer which catches the stromal contribution?

Answer 23

The model of cancer as wound which never heals as wounds typically have a regenerative stroma similar to that seen in the stroma of cancers making cancer like a wound where the wound healing perpetuates itself

Question 24

Why was it initially thought that targeting the stromal environment rather than cancer cells directly would be easier?

Answer 24

Because stromal cells are normal and not subject to the same genetic diversity seen in cancer cells
However unfortunately cancer cells are able to work out multiple routes to the same survival problem so most cancers have many redundancies making this therapy not really clinically effective

Question 25

What are the different therapeutic approaches which have been tried targeting the stroma?

Answer 25

Inhibitors of VEGF and FGF
Inhibitors of PDGF
Inhibitors of HGF or its c-Met receptor
Anti-tumour targeting of the immune system
Anti-lymphatic targeting
Anti-inflammatory inhibitors
Inhibitors of matrix turnover

Question 26

What may be problematic about human cancer cell lines?

Answer 26

These have been selected for their independence from stroma for their survival and proliferation
While it is useful to study the neoplastic population of cells they may not be a realistic model of cancer and they are not necessarily predictive of the behaviour of primary tumours and clinical responses to drugs