Epithelial and mesenchymal cancer cells have to invade:
- Basement membrane
- Capsule (if present)
- Penetrate surrounding stroma resulting in local invasion
After penetration, metastasis and distant spread takes place.
The tumor cells have to break through the primary mass, enter blood vessels or lymphatics and produce secondary growth at distant site.
The metastasis cascade is divided into:
- Invasion of extracellular matrix
- Vascular dissemination and homing of tumor cells
i. Invasion of Extracellular Matrix
Extracellular matrix consists of:
- Basement membrane
- Interstitial connective tissue
- Detachment of tumor cells from each other (problem in adhesion molecules)
- Degradation of extracellular matrix/basement membrane
- Attachment to matrix components
Migration of tumor cells
1. Detachment of tumor cells from each other
a. Down regulation of E. cadherin expression
Normal cells are attached to each other with adhesion molecules:
- Cadherin family
Normal functioning of adhesion molecules is required for survival of epithelial cells, if there is some problem, they detach from each other.
So tumor cells are detached from each other when adhesion molecules are not expressed properly.
b. Mutations in genes for catenin
Catenin is a protein that lies under cell membrane.
It is linked with E cadherin, which then functions properly.
Mutations in gene for catenin also reduce the function of E cadherin.
2. Degradation of ECM/basement membrane
a. Release of proteolytic enzymes by tumor cells
- Cathepsin D
- Plasminogen activator
- Matrix metalloproteinases (type IV collagenases)
Normally inactivated by anti-proteases. Balance between proteases and anti-proteases is disturbed.
b. Role of collagenases (MMPs)
Several invasive carcinomas, sarcomas produce high levels of these enzymes.
In situ carcinoma and adenoma produce less collagenases.
Inhibition of collagenases activity reduces metastasis in experimental animals.
c. Cells involved in secretion of proteolytic enzymes
By tumor cells themselves.
Tumor cells induce other cells to secrete these proteolytic enzymes.
These are stromal fibroblasts and macrophages.
d. Migration by amoeboid movement –An alternative mechanism to pass through ECM
No need for proteolysis.
Squeeze through gaps.
3. Changes in attachment to matrix components
a. Normal epithelial cell receptors
Normal epithelial cells express integrin receptors for basement membrane laminin and collagen, (integrin laminin bind each other)
Loss of adhesion will lead to apoptosis of cells.
b. Modification of receptors for survival of tumor cells
Loss of adhesion molecules but no apoptosis. Matrix metalloproteinases generate more sites over collagen and laminin which stimulate tumor cells propagation and migration through matrix.
4. Migration of tumor cells
a. Movement of tumor cells through matrix
Cells detach from matrix, contract the actin cytoskeleton and re-attach at leading edge.
b. Mediators involved in migration
- Tumor cells derived cytokines -autocrine motility factor.
- Break down products from matrix destruction, chemotactic factor for tumor cells
- Growth factors released from stromal cells (fibroblasts) –hepatocyte growth factor –scatter factor.
Under their action, tumor cells make a path between loose ECM and migrate.
ii. Vascular Dissemination and Homing of Tumor Cells
Intravasation of tumor cells
Intravasation of tumor cells involves:
- Adhesion molecules
- Proteolysis (of basement membrane of blood vessels)
Tumor cells in circulation are
- Vulnerable to destruction and apoptosis due to loss of adhesion (anoikis)
Anoikis is the phenomenon in which tumor cells can undergo death when come into circulation.
Formation of tumor emboli within vessels
Tumor cells tend to aggregate with each other by:
- Homotypic adhesion among tumor cells
- Heterotypic adhesion between tumor cells and blood cells, particularly platelets.
Extravasation of tumor emboli involves:
- Adhesion to vascular endothelium
- Adhesion molecules i.e.
- CD44 receptors
Migration through basement membrane is by proteolytic enzymes.
Reason for organ selectivity
1. Mechanistic theory
Determined by pattern of blood flow
2. “Seed and soil” theory
The provision of a fertile environment in which compatible tumor cells could grow.
Formation of secondary deposits
1. Along the natural pathway
First capillary bed available
2. By organ tropism
- Affinity of an organ for neoplastic cells
- Endothelial cells of vascular beds of certain organs may express ligands for tumor cells receptors
- Some target organs may liberate chemoattractants that invite tumor cells at that site.
- Some target tissues may be unpermissive for tumor cells to grow there e.g. skeletal muscles.
Due to release of mediators by skeletal muscles, secondary deposits are not usually seen there.
Pathways of Spread
1. Direct spread to body cavities
Body cavities include the pleural, peritoneal, pericardial cavities and the joint spaces.
Common examples are appendix, ovaries cancers.
Adenocarcinoma of ovary spreads to liver, lungs etc.
2. Lymphatic vessels
Breast carcinoma spreads via axillary, internal mammary nodes.
Sentinel lymph nodes
1st lymph node in drainage. Dye is injected into breast, lymph node taking dye first is the sentinel lymph node. When a lymph node is involved, the whole group is actually involved, even if not enlarged (micro-deposits may be present).
3. Hematogenous spread
Typical of sarcomas but carcinomas also show vascular invasion.
- Due to thicker walls, some cancer tumor cells follow venous drainage and deposit in 1st capillary bed they encounter.
- Lungs and liver are the most common sites
- Less commonly involved due to thicker walls.