Tissue oxygenation stabilizes neovessels and mitigates hemorrhages in human atherosclerosis-induced angiogenesis

Progression of atherosclerosis is associated with a maladaptive form of angiogenesis which contributes to intraplaque hemorrhage and plaque disruption. Hypoxia has been implicated in mechanisms of angiogenic neovessel fragility and atherosclerotic plaque destabilization. We used ex vivo and in vivo models to characterize the effect of oxygen (O₂) on the formation, stability and tendency to bleed of human plaque-induced neovessels. Plaque explants potently stimulated the ex vivo angiogenic response of rat aortic rings at atmospheric O₂ levels. Severe hypoxia (1% O₂) inhibited plaque-induced angiogenesis and pericyte recruitment causing neovessel breakdown, whereas increasing O₂ levels dose dependently enhanced pericyte numbers and neovessel stability. Plaque fragments implanted subcutaneously with or without aortic rings in SCID mice stimulated the host angiogenic response with plaques causing minimal or no hemorrhages and plaques co-implanted with aortic rings causing marked hemorrhages. Plaque/aortic ring-induced hemorrhages were reduced in mice exposed to moderate hyperoxia (50% O₂). Hyperoxia downregulated expression of the hypoxia-sensitive genes Ca9, Ca12 and VegfA and increased influx into implants of mesenchymal cells reactive for the pericyte marker NG2. In both ex vivo and in vivo models, O₂ promoted expression of vasostabilizing genes required for pericyte recruitment (Angpt1, Pdgfb), basement membrane assembly (Col4A1), and tight junction formation (Cldn5 and/or Ocln). Our results suggest that formation of neovessels that are stable, pericyte-coated, and resistant to bleeding requires adequate tissue oxygenation. Understanding the mechanisms by which O₂ stabilizes neovessels and mitigates neovessel bleeding may lead to new therapies for the prevention of atherosclerosis complications.