8- Metastasis

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

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

• 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

• Localized invasion > loss of basement membrane/ stroma invasion • Intravasation > getting into vessel • Transport through circulation • Extravasation > leaving vessel • Seeding > formation of a micrometastasis/ minimal residual disease • Colonization > formation of a macrometastasis

• 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

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

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

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

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)

• 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

• 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)

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

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

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

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

• progression of micrometastases > large/ clinically detectable macrometastases

Colonization (last/ most complex step)

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

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

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

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

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

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

• loss of microvilli • basal membrane disassembly