Hypoxia-driven angiogenesis and vascular extracellular matrix

Laurent Muller, Stéphane Germain, Catherine Monnot

Angiogenesis is a highly coordinated tissue remodeling process leading to the formation of a perfused network of blood vessels. Hypoxia triggers angiogenesis by promoting the expression of numerous genes, thereby modifying the balance between pro and anti-angiogenic factors. The upregulation of growth factors such as vascular endothelial growth factor (VEGF) leads to the angiogenic switch initiating endothelial sprouting. Activated endothelial cells acquire specialized functions: – tip cells located at the front of the growing vascular network extend numerous filopodia and lead migration toward the hypoxic microenvironment; – stalk cells proliferate and organize into luminized structures. During these cellular events, hypoxia regulates extracellular matrix composition, posttranslational modifications and assembly. The vascular extracellular microenvironnement constitutes a reservoir for growth factors and matricellular proteins and initiates biochemical and biomechanical cues essential for endothelial cell migration and capillary formation. Hypoxia-driven matrix scaffolding is thus dynamically regulated through modulation of extracellular matrix-modifying enzyme activities.

Using transcriptomic and proteomic analysis of human endothelial cells maintained in hypoxia, we have characterized the up-regulation of two extracellular matrix cross-linking enzymes, lysyl oxidase-like protein-2 (LOXL2) and transglutaminase 2 (TG2). Through different catalytic mechanisms, both enzyme activities are known to stabilize the vascular microenvironment and thus participate to blood vessel maturation. We could however show that they display opposite regulation of the angiogenic process: whereas LOXL2 stimulates capillary formation, TG2 limits endothelial sprouting. Investigating the mechanisms responsible for these effects demonstrated that LOXL2 is required for type IV collagen assembly by endothelial cells and that default of local stiffening of the microenvironment in the absence of LOXL2 could mediate capillary growth arrest. On the other hand, TG2 does not regulate extracellular matrix assembly by these cells but is involved in the modulation of VEGF signaling. These results thus highlight the fine-tuning of the angiogenic process by cross-linking enzymes. They also point out the major role of extracellular matrix scaffolding and protein interaction rather than cross-linking.

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