6. Neoplasia VI- Angiogenesis And Metastasis Flashcards
Describe the four major steps and the factors involved in the formation of new capillaries during the angiogenic process (objective)
Answer later
Describe the role of angiogenesis/neovascularization in the growth and metastasis of tumor cells (objective)
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Describe the concept of the “angiogenic switch” in a tumor and the factors that control it (objective)
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Describe the key events in the epithelial to mesenchyme transition that induce metastasis potential to a tumor (objective)
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Describe the process of metastasis and how tumors are targeted to specific organs (objective)
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Interesting Facts
1cm3 of tissue is 10^9 cells
1cm3 of tumor is 10^11
Time of detection depends on primary site
Signs/symptoms of cancer: physical pressure, organ dysfunction
Death from cancer: primary tumor (10%), metastases (90%)
Metastatic tropism (tumors originating from given organ preferentially seed in particular tissue)
Angiogenesis and Invasion Video
When cells don’t have enough oxygen, VEGF is secreted, leading to TIP cell formation (filopenol) and stalk cells
Recruit pericytes to stabilize structure
Tumor cells can have poorly assembled vessels, continues secreting VEGF and spreads
Three Types of Capillaries
Continuous
Fenestrated
Sinusoid
Continuous capillaries
Have tight occluding junctions that seal the spaces between endothelial cells. All transport must take place across the membranes.
Fenestrated capillaries
Have perforations (fenestrations) through the endothelial cells that allow exchange of small molecules with blood (ie endocrine organs, intestinal wall)
Holes in cytoplasm
Sinusoid capillaries
Have wide spaces between endothelial cells, large fenestrations, and a discontinuous basement membrane that allow for exchange of macromolecules and cells with tissues and blood (ie bone marrow, liver and spleen
Basement membrane not continuous (like tumors)
Endothelial cell, basement membrane and pericyte (schematic)
Pericyte on outside to stabilize capillary
Basement membrane on outside
Endothelial cell on inside
Tumors require access to circulation in order to grow and survive and shed waste products and carbon dioxide
Cancer cells grow preferentially around blood vessels
-tumor cells more than 0.2mm away from vessels were non-growing
-those even further were dying
0.2mm=distance oxygen can diffuse through living tissues
Hypoxia can lead to necrosis
Vessel 1- Anoxia- Vessel 2 (diagram)
PH is lowest (most acidic) in between vessels
Glucose/ATP/Oxygen is lowest in between vessels
Lactate is highest between vessels in anoxia zone
Developing vasculature through angiogenesis is essential for tissue survival and is seen in:
Embryonic development Implantation of the placenta Wound healing Many disease processes Tumorigenesis
VEGF and bFGF
Receptors display on the surfaces of endothelial cells
Stimulate endothelial cell proliferation
EGF molecules bind to tyrosine kinase which have dimerized, which then phosphorylate each other after binding
Production of VEGF is governed by availability of oxygen
HIF-1a has two paths based on oxygen
Normoxia: enzyme proline hydroxylase brings OH for degradation, recognition and binding by pVHL and other proteins, polyubiquitylation, degradation in proteasome
Hypoxia: angiogenesis- HIF-1a/b activate VEGF gene in nucleus
Endothelial cell nuclei join to form capillary lumen
Slide 16
As tumor growth continues, capillaries are increasing
- Circulating endothelial cells from bone marrow are recruited to settle in the tumor stroma and differentiate
- Capillaries are also being assembled from endothelial cells present within tumor stroma
- Other key players: TGF-beta, IL-8, angiopoietin, angiogenin 1/2.
The chaotic organization of tumor-associated vasculature
Normal tissue very organized
Tumor tissue is chaotic and more permeable (can get more nutrients and dispose of waste)
Steps in formation of new capillaries (4 steps)
- Stimulation of endothelial cells by angiogenic growth factors (VEGF)
- Degradation of the parental vessel basal lamina by activated endothelial cells to facilitate the formation of a capillary sprout
- Endothelial cell migration/proliferation
- Maturation of endothelial cells involving formation of capillary tubes, reformation of basal lamina and recruiting pericytes
Angiogenesis in Tumors
- Cells of the vasculature (endothelial cells, pericytes, and smooth muscle cells)
- Nonvascular cells (neoplastic cells, supporting cells of the stroma)
Tumors resemble wound healing sites
Slide 22
Step 1: Stimulation of Endothelial Cells by Angiogenic Growth Factors
- Basic Fibroblast Growth Factor (bFGF)
- Vascular Endothelial Growth Factor (VEGF)
Tumor expresses hypoxia, growth factors bind to endothelial
Step 2: Degradation of the Capillary Basal Lamina by Activated Endothelial Cells
Secreted Proteases destroy basement membrane:
- Matrix metalloproteinases (MMPs)
- Plasminogen Activator Urokinase (uPA)
*MMPs secreted to allow cells to migrate through the ECM
Step 3: Capillary Sprout Formation and Migration of Endothelial Cells
Regulation of Cell Migration:
- Integrins
- Extracellular Proteinases (continue degradation)
Step 4: New Vessel Maturation
- Angiopoietin 1 (Ang 1)- tell to stop growing
2. Platelet Derived Growth Factor (PDGF)- binds to pericyte precursor so that pericyte stabilizes vessel
Tripping the “angiogenic switch” is essential for tumor expansion
Many tumor cell populations initially lack the ability to attract blood vessels.
Not all tumors grow the same
Angiogenic Switch Diagram
Hyperplastic islet can recruit signals for the bone marrow to get mass cells and macrophages which secrete MMPs which activate VEGF to activate the switch.
Hyperplastic islet can undergo regular angiogenic switch to ultimately become angiogenic islet
The Angiogenic Switch
-The induction of angiogenesis is a component of the tumor phenotype that is activated during the early pre-neoplastic stages in the development of the tumor
-Most tumors arise without angiogenic activity, exist in this dormant stage (carcinoma in situ, CIS) without vascularization for long periods of time, and become vascularized when a subset of cells within the tumor switches to the angiogenic phenotype
Dormant lesion: proliferation equals apoptosis, angiogenic inducers low, inhibitors high .
Metastatic lesion: proliferation greater than apoptosis, angiogenic inducers high, inhibitors low
Angiogenic Switch: Prevascular Phase
-Angiogenic activity absent/low
-Tumors remain small with volumes in a few cubic mm (proliferation=apoptosis)
-Tumors are generally thin or flat, stable, asymptomatic and rarely metastatic
-Micrometastases may have similar pre-vascular phase
Hyperplasia remains above basement membrane
Angiogenic Switch: Vascular Phase
-Characterized by tumors which have entered phase of rapid growth, intensified invasion and increased metastatic potential
-A bi-directional paracrine relationship between the tumor cells and endothelial cells is established. Endothelial-derived growth factors stimulate the grow of tumor cells
-Hypoxic areas of tumor stimulate VEGF production and subsequent endothelial cell growth
-Associated with increased appearance of symptoms. Bleeding, local edema, inappropriate hormonal activities, hypercoagulation states and cachexias
Normal to Hyperplasia to Dysplasia (angiogenesis already started but membrane intact; try to catch this) to Carcinoma
Angiogenic Switch: Heterogeneity of Angiogenic Activity in Tumors
- Neovascularization of tumor usually originates in a subset of cells due to presence of a mixture of angiogenic and non-angiogenic cells
- Metastases derived from the angiogenic portion of tumor are already angiogenic upon arrival at the target organ and therefore have an increased chance of becoming a detectable metastasis
Anti-Angiogenic Therapy
- Specificity have less side effects
- Angiogenesis in adults limited to reproductive tissues and wound healing
- Target accessible vessel endothelial cells and not traditional tumor vasculature which is a barrier to normal drug delivery
- Endothelial cells are pretty stable with low mutation rates, so resistance is reduced
- In theory, can work on all solid tumors regardless of origin, since endothelial cells do not vary from one type of tumor to another.
Metastasis is Angiogenesis-Dependent
Slide 39
Sequence of Events in Tumor Cell Metastasis
- Initial transforming event
- Proliferation of transformed cells
- Compromised nutritional supply to the tumor mass requiring release of tumor angiogenesis factors
- Endothelial cell expansion and reorganization
- Local invasion and destruction of ECM and parenchymal cells leading to migration of tumor cells away from the primary tumor mass
- Penetration of cancers cells through the blood vessel wall (intravasation)
- Arrest of cancer cells in the lumen of small blood vessels or lymphatics
- Reverse penetration of blood vessels
- Organ colonization resulting in the formation of the secondary tumor (metastases)
Tumor Cells Can Secrete Enzymes that Degrade the Adhesion Proteins Between Cells of the Vasculature
A. Loosening of intercellular junctions
B. Degradation of ECM
C. Migration and Invasion
Epithelial-Mesenchymal Transitions in Cancer
Cytoskeleton reorganization
Membrane reorganization
Increased growth
Increased motility
Epithelial markers
E-cadherin
Mesenchymal markers
Vimentin
Fibronectin
N-cadherin
Seed and Soil Hypothesis
- Colonization of metastasized tumor cells is inefficient
- Micrometastases often seed organs, but most fail to grow into clinically observed tumors
- Organs must provide a hospitable environment/soil for the metastasized tumors to grow
Factors that Affect the Location and Efficiency of Metastasis
- Chemokines in the specific tissue that attract the tumor cells to the site
- The differentiation, developmental and growth conditions of the tissue (mitogens and growth factors)
- The nature of the trafficking circulation (blood and lymphatic) systems in the specific organ
