8- Metastasis

Question 1

What is metastasis?

Answer 1

• process of dissemination of a tumor from its site of origin to a distant organ, where it ultimately establishes one/more colonies

Question 2

What is a primary tumor?

Answer 2

• tumor that first formed/ from where tumor cells originally detached

Question 3

What is a secondary tumor?

Answer 3

• tumor mass that forms in organ different from one of origin
• can be 1 or multiple, generally multiple at later stages of disease
  = Metastastic colony/ Metastatic nodule/ Secondary nodule

Question 4

What are the steps of the invasion-metastasis cascade?

• referring to epithelial tumors (most steps same in others)

Answer 4

1. Localized invasion > loss of basement membrane/ stroma invasion
2. Intravasation > getting into vessel
3. Transport through circulation
4. Extravasation > leaving vessel
5. Seeding > formation of a micrometastasis/ minimal residual disease
6. Colonization > formation of a macrometastasis

Question 5

What happens in localized invasion?

Answer 5

• initial invasion involves degradation of basement membrane
• followed by degradation of the extracellular matrix within the stroma
  > breaks barriers/ releases growth factors
• epithelial cells can produce proteases themselves or co-opt stromal cells (macrophages/ fibroblasts) to be able to invade/ get into stoma
• epithelial localized invasion typically occurs as a group of cells
  = Collective Invasion

Question 6

What happens in intravasation?

Answer 6

• same features that allow localized invasion allows vessel invasion
  > production of proteases/ help from stromal cells (macrophages)

Question 7

What is the triad formed by cancer cells in intravasation?

Answer 7

TMEM = tumor microenvironment of metastasis
- apposed to macrophage/ endothelial cell

Question 8

What are the 4 serious challenges of transport through circulation?

Answer 8

1. Lack of substrate-dependent survival signals > Anoikis
2. Lack of growth/ survival factors initially provided by stroma
3. Hydrodynamic shear forces from blood flow (in small vessels)
4. Immune system > NK cells

Question 9

How do tumor cells adapt for transport through circulation?

Answer 9

CTCs = circulating tumor cells
- pinch off large amounts of cytoplasm > to become smaller
- avoid capillaries > travel via arterial-venous shunts
- form microthrombi > coat themselves with platelets
- EMT > acquire a plastic/ flexible cytoskeleton (like RBCs)

Question 10

Why do CTCs coat themselves with platelets?

Answer 10

• when enter vessel > shear stress promotes platelet adhesion
• platelets reduce deformation by helping distribute force on CTCs membrane more homogeneously
  > preserves the integrity of the CTC
• enhances attachment to vessel wall/ reduces rolling motion of CTCs
  > possibly favours extravasation

Question 11

What happens in extravasation?

Answer 11

• depends on complex interaction between cancer cells/ vessel walls (2 possibilities)
  1. Immediate extravasation as individual cells
  2. Platelet-assisted extravasation at later times (w/w-out proliferation)

Question 12

What is seeding?

Answer 12

• establishment of small tumor cell clumps (micrometastases) at distant sites from the primary tumor
• micrometastases referred to as “minimal residual disease” in clinic

Question 13

What can determine patient prognosis?

Answer 13

Minimal residual disease
- micrometastases are not the cause of death, but ↑ risk of developing macrometastases > ↑ risk of death

Question 14

What is metastasis dormancy?

Answer 14

• capacity of tumor cells to remain viable as single cells/ clumps for long periods of time
• dormant cells not dividing > not targeted by treatment

Question 15

What is metastasis inefficiency?

Answer 15

• failure of individual dormant cells to form micrometastases
• failure of micrometastases to form macrometastases (tumor mass)

Question 16

What is colonization?

Answer 16

• progression of micrometastases > large/ clinically detectable macrometastases

Question 17

What is the rate-limiting step of the invasion-metastasis cascade?

Answer 17

Colonization (last/ most complex step)

Question 18

Do all cells in a tumor have the same ability to metastasize?

Answer 18

no > metastasis heterogeneity
- # of metastases produced by tumor proportional to # of CTC, but most CTCs do NOT form metastases (less than 0.01%)
- not all cells in primary tumor have same ability to disseminate

Question 19

What is the most accepted model of cancer metastasis

Answer 19

Progression model
- suggests cancer is biologically heterogeneous
- metastatic capacity acquired rarely in a subpopulation of cells through genetic mutations

Question 20

Although the progression model is most accepted, what is other evidence suggesting about cancer metastasis?

Answer 20

• primary tumor gene signature predicts metastasis > Initiation model
• cell of origin dictates metastatic behaviour (environment has role)

Question 21

If there are not many mutations, how do tumor cells acquire ability to metastasize?

Answer 21

• evolution of cancer in cooperation with TME
• tumor microenvironment has a key role in metastasis cascade

Question 22

What is plasticity?

Answer 22

• tumor cells change phenotype highly influenced by microenvironment
• REactivate EMT via reciprocal tumor-microenvironment interactions

Question 23

What is EMT?

Answer 23

= Epithelial to mesenchymal transition
- process of cellular plasticity where epithelial cells lose their epithelial morphology/ acquire mesenchymal characteristics

Question 24

What are some general features of EMT?

Answer 24

• loss of microvilli
• basal membrane disassembly

Question 25

During EMT, what features are lost/ gained?

Answer 25

Epithelial Features (lost)
- tight cell-cell adhesions/ tight junctions (E-cadherin expression)
- static (non-motile) > non-invasive
- apical-basal polarity

Mesenchymal Features (gained)
- no cell to call adhesion
- motile > invasive (gain capacity of locomotion)
- upregulation of N-cadherin expression
- front-rear polarity
- fibroblast-like shape
- stem-cell characteristics (resistance to therapy/ self-renewal)

Question 26

What are 3 mechanisms of E-cadherin loss in invasive tumors? (E > N)

Answer 26

• transcriptional repression by EMT-promoting factors
  > Twist/ Snail/ Slug
• epigenetic gene silencing by gene promoter methylation
• mutations that alter the reading frame (truncated protein)

Question 27

What is a possible link between obesity/ metastasis?

Answer 27

• adipose tissue in secondary organs may promote MET

Question 28

What is required for EMT/ invasion/ metastasis?

Answer 28

• constant communication between tumor/ microenvironment

Question 29

What induces EMT?

Answer 29

• paracrine action of growth factors produced by stroma (TGF B)

Question 30

How do EMT-TFs act in a pleiotropic fashion?

Answer 30

• repress epithelial genes/ induce mesenchymal genes
• promote self-renewal/ drug resistance

Question 31

What is the take-home message for tumor metastasis?

Answer 31

• EMT TFs have pleiotropic cellular effects
  > genetic mutations are not the only driving force of metastasis

Question 32

What is the seed/ soil hypothesis?

Answer 32

• pattern of metastasis is NOT random/ can not simply be explained by mechanics of blood flow
  > preferential colonization of specific organs

Question 33

What is the take-home message on metastasis tropism?

Answer 33

• preference of specific type of tumor to colonize specific organs is driven by a combination of factors
  > pattern of blood circulation
  > presence of a favourable environment in target organ (seed/soil)
• specific gene expression patterns in tumor cells
• organ-specific growth is driven by mutual interaction between tumor cell/ organ microenvironment