MACROPHAGES TRANSMIT POTENT PROANGIOGENIC EFFECTS OF oxLDL IN VITRO AND IN VIVO INVOLVING HIF-1a ACTIVATION: A NOVEL ASPECT OF ANGIOGENESIS IN ATHEROSCLEROSIS
Randolph Hutter, M.D., The Cardiovascular Institute, Mount Sinai School of Medicine, New York, NY, USA
Background: Neovascularization has been linked to the progression and vulnerability of atherosclerotic lesions. Angiogenesis is increased in lipid-rich plaque. Hypoxia-inducible factor alpha (HIF-1a) is a key transcriptional regulator responding to hypoxia and activating genes, which promote angiogenesis, among them vascular endothelial growth factor (VEGF). Oxidized low-density lipoprotein (oxLDL) is generated in lipid-rich plaque by oxidative stress. It triggers an inflammatory response and was traditionally thought to inhibit endothelial cells. New data, however, suggest that oxLDL can activate HIF-1a in monocytes in a hypoxia-independent fashion. We hypothesized that HIF-1a activation in monocyte–macrophages could transmit proangiogenic effects of oxLDL linking hyperlipidemia, inflammation, and angiogenesis in atherosclerosis.
Methods and Results: First, we examined the effect of oxLDL on HIF-1a and VEGF expression in monocyte–macrophages and on their proangiogenic effect on endothelial cells in vitro in a monocyte–macrophage/endothelial co-culture model. OxLDL strongly induced HIF-1a and VEGF in monocyte–macrophages (23±5 vs. 0.0±0 %; P<0.05) and VEGF (37±6 vs. 4±2 %; P<0.05) compared to untreated controls under normoxic conditions and significantly increased tube formation in co-cultured endothelial cells. HIF-1a inhibition with chetomin reversed this effect. Second, we demonstrated a direct proangiogenic effect of oxLDL in the in vivo matrigel plug angiogenesis assay (29±4 mm of blood vessels per high power field compared to 1±0.5 mm in growth factor-depleted matrigel alone, p<0.05). Again, HIF-1a inhibition with chetomin abrogated the proangiogenic effect of oxLDL (6±1mm vs. 29±4mm; p<0.05). Third, in a rabbit atherosclerosis model, we studied the effect of dietary lipid lowering on arterial HIF-1a and VEGF expression. The administration of low-lipid diet significantly reduced the expression of both HIF-1a and VEGF, resulting in decreased plaque neovascularization.
Conclusions: Our data point to oxLDL as a proangiogenic agent linking hyperlipidemia, inflammation, and angiogenesis in atherosclerosis. This effect is dependent on macrophages and, at least in part, on the induction of the HIF-1a pathway.