Reversal of Hypoxia in Murine Atherosclerosis Prevents Necrotic Core Expansion by Enhancing Efferocytosis
Objective—Advanced murine and human plaques are hypoxic, but it remains unclear whether plaque hypoxia is causally related to atherogenesis. Here, we test the hypothesis that reversal of hypoxia in atherosclerotic plaques by breathing hyperoxic carbogen gas will prevent atherosclerosis.
Approach and Results—Low-density lipoprotein receptor–deficient mice were fed a Western-type diet, exposed to carbogen (95% O2, 5% CO2) or air, and the effect on plaque hypoxia, size, and phenotype was studied. First, the hypoxic marker pimonidazole was detected in murine low-density lipoprotein receptor–deficient plaque macrophages from plaque initiation onward. Second, the efficacy of breathing carbogen (90 minutes, single exposure) was studied. Compared with air, carbogen increased arterial blood Po2 5-fold in low-density lipoprotein receptor–deficient mice and reduced plaque hypoxia in advanced plaques of the aortic root (−32%) and arch (−84%). Finally, the effect of repeated carbogen exposure on progression of atherosclerosis was studied in low-density lipoprotein receptor–deficient mice fed a high cholesterol diet for an initial 4 weeks, followed by 4 weeks of diet and carbogen or air (both 90 min/d). Carbogen reduced plaque hypoxia (−40%), necrotic core size (−37%), and TUNEL+ apoptotic cell content (−50%) and increased efferocytosis of apoptotic cells by MAC3+ macrophages (+36%) in advanced plaques of the aortic root. Plaque size, plasma cholesterol, hematopoiesis, and systemic inflammation were unchanged. In vitro, hypoxia hampered efferocytosis by bone marrow–derived macrophages, which was dependent on the receptor Mer tyrosine kinase.
Conclusions—Carbogen restored murine plaque oxygenation and prevented necrotic core expansion by enhancing efferocytosis, likely via Mer tyrosine kinase. Thus, plaque hypoxia is causally related to necrotic core expansion.
- Received May 20, 2014.
- Accepted September 15, 2014.
- © 2014 American Heart Association, Inc.