Hypoxia induces netrin-1 and Unc5b in atherosclerotic plaques: mechanism for macrophage retention and survival.

Objective-Hypoxia is intimately linked to atherosclerosis and has become recognized as a primary impetus of inflammation. We recently demonstrated that the neuroimmune guidance cue netrin-1 (Ntn1) inhibits macrophage emigration from atherosclerotic plaques, thereby fostering chronic inflammation. However, the mechanisms governing netrin-1 expression in atherosclerosis are not well understood. In this study, we investigate the role of hypoxia in regulating expression of netrin-1 and its receptor uncoordinated-5-B receptor (Unc5b) in plaque macrophages and its functional consequences on these immune cells. Approach and Results-We show by immunostaining that netrin-1 and Unc5b are expressed in macrophages in hypoxia-rich regions of human and mouse plaques. In vitro, Ntn1 and Unc5b mRNA are upregulated in macrophages treated with oxidized low-density lipoprotein or inducers of oxidative stress (CoCl2, dimethyloxalylglycine, 1% O-2). These responses are abrogated by inhibiting hypoxia-inducible transcription factor (HIF)-1 alpha, indicating a causal role for this transcription factor in regulating Ntn1 and Unc5b expression in macrophages. Indeed, using promoter-luciferase reporter genes, we show that Ntn1- and Unc5b-promoter activities are induced by oxidized low-density lipoprotein and require HIF-1 alpha. Correspondingly, J774 macrophages overexpressing active HIF-1 alpha show increased netrin-1 and Unc5b expression and reduced migratory capacity compared with control cells, which was restored by blocking the effects of netrin-1. Finally, we show that netrin-1 protects macrophages from apoptosis under hypoxic conditions in a HIF-1 alpha-dependent manner. Conclusions-These findings provide a molecular mechanism by which netrin-1 and its receptor Unc5b are expressed in atherosclerotic plaques and implicate hypoxia and HIF-1 alpha-induced netrin-1/Unc5b in sustaining inflammation by inhibiting the emigration and promoting the survival of lesional macrophages.