Abstract 13838: Smooth Muscle Cells Are the Origin of the Majority of Foam Cells in Atherosclerotic Lesions of Apolipoprotein E-deficient Mice

Abstract

Background: Apolipoprotein E-deficient (ApoE -/- ) mice are the most frequently used model of human atherosclerosis. Foam cells in mouse lesions have been believed to be primarily macrophage-derived using traditional staining with macrophage markers. Recent studies indicated that smooth muscle cells (SMCs) comprise at least 50% of foam cells in human coronary atherosclerosis, and express reduced levels of ATP-binding cassette transporter A1 (ABCA1) when compared to intimal leucocytes. Identification of SMC-derived foam cells is challenging due to the absence of classic SMC markers and expression of macrophage markers by a high percentage of intimal SMCs in mouse and human lesions. Hypothesis: Unlike human atherosclerosis, foam cells in ApoE -/- mice are primarily macrophage- rather than SMC-derived. Methods: 8-week-old male and female ApoE -/- mice were fed a Western diet for 6 weeks to promote lesion development. Flow cytometry was used to estimate the proportion of leukocyte- and SMC-derived foam cells in cell suspensions from digested ascending aortas. Foam cells were identified by the neutral lipid dye BODIPY, and leukocyte-derived were distinguished from SMC-derived foam cells by expression of CD45 (a pan-leukocyte marker), and I-A/I-E (antigen presenting cell markers), which have not been reported to be expressed by SMCs. SMC origin was confirmed using a SMC-specific epigenetic lineage marker. ABCA1 level was also measured in the two foam cell populations. Results: 69.47±3.20 % (male) and 73.33±2.35 % (female) (mean ± SEM, n=9 each) of foam cells in ApoE -/- mouse lesions were CD45 and I-A/I-E negative, and were confirmed by SMC lineage marker to be predominantly SMC-derived. SMC-derived foam cells were thus significantly greater than leukocyte-derived foam cells in ApoE -/- mice ( P <0.01), and exhibited three times lower ABCA1 protein than leukocyte-derived foam cells in both sexes. Conclusions: Contrary to our hypothesis, SMCs contribute the majority of foam cells in ApoE -/- mouse atherosclerosis. Impaired ABCA1 expression in SMC foam cells, as seen in human intimal SMC foam cells, suggests a likely mechanism for preferential SMC foam cell development in both species, and a potential target for atherosclerosis treatment and prevention.