Smooth Muscle Cells Contribute the Majority of Foam Cells in ApoE (Apolipoprotein E)-Deficient Mouse Atherosclerosis

Objective- Smooth muscle cells (SMCs) are the most abundant cells in human atherosclerotic lesions and are suggested to contribute at least 50% of atheroma foam cells. In mice, SMCs contribute fewer total lesional cells. The purpose of this study was to determine the contribution of SMCs to total foam cells in apolipoprotein E-deficient (ApoE^-/-) mice, and the utility of these mice to model human SMC foam cell biology and interventions. Approach and Results- Using flow cytometry, foam cells in the aortic arch of ApoE^-/- mice were characterized based on the expression of leukocyte-specific markers. Nonleukocyte foam cells increased from 37% of total foam cells in 27-week-old to 75% in 57-week-old male ApoE^-/- mice fed a chow diet and were ≈70% in male and female ApoE^-/- mice following 6 weeks of Western diet feeding. A similar contribution to total foam cells by SMCs was found using SMC-lineage tracing ApoE^-/- mice fed the Western diet for 6 or 12 weeks. Nonleukocyte foam cells contributed a similar percentage of total atheroma cholesterol and exhibited lower expression of the cholesterol exporter ABCA1 (ATP-binding cassette transporter A1) when compared with leukocyte-derived foam cells. Conclusions- Consistent with previous studies of human atheromas, we present evidence that SMCs contribute the majority of atheroma foam cells in ApoE^-/- mice fed a Western diet and a chow diet for longer periods. Reduced expression of ABCA1, also seen in human intimal SMCs, suggests a common mechanism for formation of SMC foam cells across species, and represents a novel target to enhance atherosclerosis regression.