Angiogenesis in PET studies
The process of new blood vessel growth by budding and sprouting from existing vessels in microvasculature (angiogenesis) is essential for tissue growth and regeneration, and it is strictly controlled by angiogenic stimulators and inhibitors. Vascular remodelling is the process where the arrangement and structure of existing vessels is changed through cell growth, apoptosis, migration, and production or degradation of the extracellular matrix (ECM).
Endothelial cells are the main building blocks of blood vessels. Angiogenesis is often triggered by tissue hypoxia. Hypoxia-inducible factor HIFα activates expression of vascular endothelial growth factor (VEGF). Inflammation, and metabolic and mechanical stress can also induce angiogenic process. Integrins are needed in endothelial cell growth, differentiation, adhesion, and migration. Perivascular cells (smooth muscle cells and pericytes) are recruited to the developing vessel, and when they have coated the endothelial cells of the vessel they stabilize it and inhibit further proliferation and migration of the endothelial cells, for instance by releasing sphingosine-1-phosphate and angiopoietin-1. Endothelial cells can release angiopoietin-2 (natural antagonist of angiopoietin-1) to promote angiogenesis. Platelets contain and release numerous growth factors and chemokines, regulating smooth muscle and endothelial cell proliferation and migration, recruiting bone marrow derived progenitor cells to form endothelial progenitor cells, and regulating apoptosis.
Angiogenic processes can be detected with numerous PET radioligands, including labelled integrin αvβ3 antagonists, ECM matrix metalloproteinase (MPP) inhibitors, and natriuretic peptides. Radiotracers for VEGF-receptor and endoglin have also been developed (Hong et al., 2014; Hendrikx et al., 2016). For instance, PET radiopharmaceuticals targeting endoglin have been used in treatment follow-up in peripheral artery disease animal models. PSMA has protease functions, and it may play a role in forming pro-angiogenic peptides.
Arteriogenesis is not induced by hypoxia and ischemia, but it is initiated in pre-existing collateral vessels. Increased perfusion pressure and pulsatile blood shear stress induces diametrical growth via transient inflammatory response, including growth factors and cytokines such as tumour necrosis factor α.
Vasculogenesis is the de novo formation of blood vessels from progenitor cells, especially during embryogenesis. Endothelial cell precursors, angioblasts, associate to form primitive vessels; initially, only cord without the lumen is formed, followed by tubulogenesis (Xu & Cleaver, 2011).
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Updated at: 2019-02-02
Created at: 2017-09-25
Written by: Vesa Oikonen