Steps and mechanisms of tumor angiogenesis and stromagenesisA seminar prepared by: Basant Hamdy Abu Zaid
Introduction The word “tumor” is of Latin origin and means “swelling.” W. H. Clark: a tumor (fully evolved) is a population of abnormal cells characterized by temporally unrestricted growth & the ability to grow in at least three different tissue compartments:- the original compartment;- the mesenchyme of the primary site (tumor invasion); &- a distant mesenchyme (tumor metastasis).
Introduction Although some tumors (eg, leukemias, ascites tumors) grow as cell suspensions, most tumors grow as solid masses of tissue. Solid tumors :1- parenchyma (neoplastic cells)2- stroma
Importance of tumor stroma1- Nutritional support & removal of waste products.2- To grow beyond a minimal size of 1 to 2 mm.3- Barrier that inhibits and may regulate interchange of fluids, gases, & cells. Components of tumor stroma • Cells: fibroblasts, myofibroblasts, glial, epithelial, fat, blood, vascular, & smooth muscle cells. • ECM: extracellular molecules (cytokines & growth factors), leaked plasma, plasma proteins; proteoglycans, glycosaminoglycans, collagens ..etc
Tumor stroma formation• It requires the cooperative activities of many different cytokines and inhibitors.• Among these: VPF/VEGF• Because nearly all malignant tumors express VEGF-A, there is every reason to believe that tumors use this cytokine to generate stroma.
VEGF-A and its activities• Founding member of a family of structurally related proteins (VEGF B, C, D, and E & PlGF).• Major isoforms of 189, 165, and 121 amino acids. Of these, the 164/5 isoform is generally expressed most abundantly.
VEGF-A and its activities• Vascular permeabilizing activity 50,000 times more potent than histamine.• EC mitogen (proliferation & migration).• EC survival (avert EC apoptosis & senescence).• Continuously expressed throughout entire tumor life cycle. Receptor s• Two receptor tyrosine kinases (VEGFR-1,flt-1) and VEGFR-2 (flk-1,KDR).• Non-tyrosine kinase receptor, neuropilin.
Mechanisms of tumor angiogenesis 1- Sprouting angiogenesis • secretion of angiogenic factors by tumor cells which bind to EC receptors. • activated ECs secrete proteases, heparanase, and other digestive enzymes (breakdown of vessel BM & ECM)• the junctions between ECs become altered, cell projections pass through the space created, & the newly formed sprout grows toward the source of the stimulus.• ECs invade migrate & proliferate into the tumor mass.• newly formed ECs organize into hollow tubes (canalization) and create new basement membrane for vascular stability.
Mechanisms of tumor angiogenesisIntussusception• First, the two opposing capillary walls establish a zone of contact.• Second, the EC junctions are reorganized and the vessel bilayer is perforated to allow growth factors and cells to penetrate into the lumen.• Third, a core is formed between the two new vessels at thezone of contact that is filled with pericytes and myofibroblasts.• These cells begin laying collagen fibers into the core toprovide an extra cellular matrix for growth of the vessel lumen.
Mechanisms of tumorangiogenesis2- Angioblast recruitment• During embryonic development, a common precursor of the EC lineages, the hemangioblast which gives rise to the angioblast, which is able to differentiate into ECs.• In the adult organism, angioblasts are derived from multipotent adult progenitor cells (MAPCs) in the bone marrow.• Circulating endothelial precursors (CEPs) are recruited in the circulation and the extent of their recruitment into the tumor vasculature.
Mechanisms of tumorangiogenesis 3- Cooption • Highly vascularized tissues, such as brain. • Vessels are surrounded, coopted by tumor cells and no sprouts are observed.• Coopted ECs synthesize Ang-2 and its receptor Tie-2, that results in large decrease in tumor vessel number & increase in vessel diameter.• So, the lack of vessels leads to hypoxia which upregulates VEGF expression in tumor cells.• As a consequence, strong angiogenesis develops mainly at the tumor periphery.
Mechanisms of tumorangiogenesis 4- Mosaic vessels • Tumor cells in mosaic vessels are undergoing intravasation into the lumen and stay temporarily in the capillary vessel wall.
Steps of tumorstromagenesis1- Vascular hyperpermeability and edema.2- Leakage of plasma proteins, including fibrinogen and other clotting factors.3-Vascular hyperpermeability & extravasation of plasma proteins leads to activation of the coagulation system by a tissue factor mediated mechanism.4- Clotting of fibrinogen to fibrin, and subsequent cross-linking of fibrin by activated factor XIII.5- Cross-linked fibrin behaves as a gel that causes edema by trapping extravasated plasma and provides a proangiogenic provisional stroma.6- ECs, fibroblasts, & inflammatory cells express adhesion molecules (integrins) whose interaction with fibrin allows them to move freely in tumor stroma.
Steps of tumorstromagenesis7- By 18 hours, enlarged, thin-walled, strongly VEGF receptor–positive “mother vessels” had formed within this fibrin gel:• proteolytic digestion of vascular basement membranes• pericyte detachment• spreading and thinning of endothelial cells to cover a greatly expanded surface area.• Later, they became divided into smaller channels (transluminal bridges to form smaller caliber “ daughter vessels” ).
Steps of tumorstromagenesis8- Overtime, as VEGF-A expression declined, vascular permeability returned to normally low levels and the balance shifted from fibrin accumulation to fibrinolysis.9- Eventually, fibrin was digested, small blood vessels underwent apoptosis, and fibroblasts synthesized collagen, resulting finally in dense, relatively avascular connective tissue.10- Simultaneously, other tumor cells have broken away from the original tumor site and have begun to recapitulate the same sequence of events (increased vascular permeability and new fibrin deposition).
Tumor stroma generation Vs wound healing Wound healing Tumor stroma formationPlatelets play several Many platelets functionscritical roles can be performed by tumor cells that express analogous cytokines/growth factorsVEGF & vascular VEGF & vascularpermeability are transient permeability persisttill normoxia occurs indefinitly (wounds that don’t heal)