AngiogenesisAngiogenesis

Key features of angiogenesisKey features of angiogenesis

1) Tumour growth is angiogenesis-dependent2) Microvascular endothelial cells (ECs) are genetically-stable 3) ECs release angiogenic factors that stimulate proliferation

(a)Nascent vessels consist of a tube of ECs, which mature into specialized capillaries, arteries and veins.

(b) Capillaries consist of ECs surrounded by basement membrane and a sparse layer of pericytes embedded within the EC basement membrane. Capillary endothelial layer can be continuous (muscle), fenestrated (kidney/ endocrine glands) or discontinuous (liver sinusoids). The endothelia of the blood-brain barrier or blood-retina barrier are further specialized to include tight junctions, and are thus impermeable to various molecules.

(c) Arterioles and venules have an increased coverage of mural cells compared with capillaries.

Composition of nascent and mature blood vessel wallsComposition of nascent and mature blood vessel walls

Steps in network formation and maturation during Steps in network formation and maturation during embryonic (physiological) angiogenesisembryonic (physiological) angiogenesis

Key differences in tumour vasculatureKey differences in tumour vasculature

Different flow Different flow characteristics or characteristics or blood volumeblood volume

Microvasculature Microvasculature permeabilitypermeability

Increased fractional Increased fractional volume of volume of extravascular, extravascular, extracellular spaceextracellular space

Steps in network formation and maturation during Steps in network formation and maturation during tumour angiogenesistumour angiogenesis

Cellular mechanisms of tumour angiogenesis Cellular mechanisms of tumour angiogenesis (1) host vascular network expands (1) host vascular network expands

by budding of endothelial by budding of endothelial sprouts or formation of bridges sprouts or formation of bridges (angiogenesis); (angiogenesis);

(2) tumour vessels remodel and (2) tumour vessels remodel and expand by the insertion of expand by the insertion of interstitial tissue columns into interstitial tissue columns into the lumen of pre-existing vessels the lumen of pre-existing vessels (intussusception); and (intussusception); and

(3) endothelial cell precursors (3) endothelial cell precursors (angioblasts) home from the (angioblasts) home from the bone marrow or peripheral bone marrow or peripheral blood into tumours and blood into tumours and contribute to the endothelial contribute to the endothelial lining of tumour vessels lining of tumour vessels (vasculogenesis)(vasculogenesis)

(4) Lymphatic vessels around (4) Lymphatic vessels around tumours drain the interstitial tumours drain the interstitial fluid and provide a gateway for fluid and provide a gateway for metastasizing tumour cells. metastasizing tumour cells.

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Cellular angiogenesis-overviewCellular angiogenesis-overview

Nature Reviews Drug Discovery 1, 415-426 (2002)

Balance between inhibitory factors and angiogenic factorsInhibitory – endostatin, angiostatin, thrombospondin Angiogenic - VEGF, bFGF, PDGF

Tumour cells release pro-angiogenic factors which activate receptors (VEGFR) also stimulates secretion and activation of MMPs which degrade the basement membraneThis allows activated endothelial cells (ECs) to migrate towards tumour, helped by integrinsECs deposit a new basement membrane and secrete growth factors such as platelet-derived growth factor (PDGF), which attract supporting cells to stabilize the new vessel.

VEGF – Vascular Endothelial Growth FactorbFGF - basic Fibroblast Growth FactorMMPs – Matrix MetalloProteinasesPDGF – Platelet-Derived Growth Factor

Cellular angiogenesis-overviewCellular angiogenesis-overview

MetastasisMetastasis

Invasion & MetastasisInvasion & Metastasis

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Red: E-Cadherins Green: Integrins

Integrins – the ‘velcro’ of the cellIntegrins – the ‘velcro’ of the cell The cell moves by "ruffling" it's membrane. This is done by a The cell moves by "ruffling" it's membrane. This is done by a

series of actin fibers, whose function is controlled by the series of actin fibers, whose function is controlled by the integrins. These fibers cause the cell membrane to move in integrins. These fibers cause the cell membrane to move in certain directions, and the integrins attach to the matrix as this certain directions, and the integrins attach to the matrix as this happens, pulling the cell along a micrometer at a time happens, pulling the cell along a micrometer at a time

Invasion & MetastasisInvasion & Metastasis

Red: E-Cadherins Green: Integrins

Epithelial-mesenchymal transition (EMT) Epithelial-mesenchymal transition (EMT) necessary for invasivenessnecessary for invasiveness

Then why do secondary tumours Then why do secondary tumours histopathologically resemble primary tumours?histopathologically resemble primary tumours?

EMT induced by stromal signalsEMT induced by stromal signalsEMT may be reversible depending on the stromal signals EMT may be reversible depending on the stromal signals e.g. TGF-e.g. TGF-TNF-TNF-, EGF, HGF, IGF-1., EGF, HGF, IGF-1.

Stromal signals that trigger EMTStromal signals that trigger EMT

Cell invasiveness controlled by Cell invasiveness controlled by MMatrix atrix MMetalloetallopproteinases (MMPs)roteinases (MMPs)

MMPs secreted by stromal cellsMMPs secreted by stromal cells Can be PM-bound or soluble Can be PM-bound or soluble

enzymesenzymes

MMP activation can be indirect MMP activation can be indirect E.g. via urokinase plasminogen E.g. via urokinase plasminogen activator (uPA)activator (uPA)

Cell motility regulated by RhoGTPasesCell motility regulated by RhoGTPases

Binary switches like RasBinary switches like Ras 3 sub families; Rho, Rac and cdc423 sub families; Rho, Rac and cdc42

Serum-Serum-starvedstarved

LysophosphatLysophosphatidic acididic acid

OverexpressiOverexpressing Racng Rac GEFGEF

Metastatic cells travel via lymphatic systemsMetastatic cells travel via lymphatic systems

Colonisation depends on a variety of factorsColonisation depends on a variety of factorsmetastatic tropisms (Paget’s ‘seed & soil’theory) metastatic tropisms (Paget’s ‘seed & soil’theory)

Colonisation depends on complex interactions Colonisation depends on complex interactions between metastasising cells and their between metastasising cells and their

microenvironments microenvironments E.g Osteolytic metastasis initiated by breast cancerE.g Osteolytic metastasis initiated by breast cancer

Breast cancer initiated osteolytic metastasis Breast cancer initiated osteolytic metastasis

Bone growth versus loss

ReadingReading

Chapter 13 and 14 : Biology of Cancer by R WeinbergChapter 13 and 14 : Biology of Cancer by R Weinberg

AND /ORAND /OR

Chapter 12: Cancer Biology by RJB King Chapter 12: Cancer Biology by RJB King

Angiogenesis in cancer and other diseases by Angiogenesis in cancer and other diseases by P Carmeliet P Carmeliet & RK. Jain& RK. JainNature vol 407 14 september 2000 pp 249