Abstract 612: Low Expression of Lysosomal Acid Lipase in Smooth Muscle Cells Relative to Macrophages Provides New Insights into Foam Cell Lipid Accumulation

Background and Hypothesis: Smooth muscle cells (SMCs) are the predominant cell type within the intima of human atherosclerosis-prone arteries and promote initial retention of atherogenic lipoproteins in the deep intima. We previously found that ≥50% of foam cells in intermediate coronary atheromas are of SMC origin and that intimal SMCs have reduced expression of the ATP-binding cassette transporter A1 (ABCA1). Previously we also found that ABCA1 expression is acutely dependent on the flux of cholesterol out of lysosomes and subsequent generation of oxysterols for promotion of gene transcription by the nuclear liver X receptor (LXR). Hypothesis: In the present studies we tested the hypothesis that SMCs have lysosomal dysfunction that contributes to foam cell formation. Methods and Results: Human monocyte-derived macrophages (HMMs) and arterial SMCs were treated with aggregated LDL (agLDL) to increase intracellular cholesterol. Unlike HMMs, agLDL treatment failed to upregulate ABCA1 expression in SMCs and did not significantly increase 27-hydroxycholesterol levels. Also in contrast to HMMs, SMCs did not downregulate new cholesterol synthesis and displayed minimal increases in cholesteryl ester formation with agLDL loading. This data suggested retention of lipids within lysosomal compartments. Indeed, confocal microscopy revealed retention of lipids identified by the neutral lipid dye BODIPY within lysosomal compartments stained with LAMP1 in SMCs, while HMMs showed mostly cytosolic lipid droplets [Pearson correlation of colocalization of +0.3197±0.0123 in SMCs and -0.0897±0.0143 in HMMs (p<0.0001, avg±SEM, n>100 cells)]. LIPA mRNA levels and LAL protein were markedly reduced in SMCs relative to HMMs, with LAL activity being 23.4-times higher in HMMs compared to SMCs (p<0.0001, n=4). Similarly, we found reduced LAL levels in mouse SMCs relative to macrophages. Incubation of SMCs with medium containing LAL reduced lysosomal lipid accumulation, resulting in decreased new cholesterol synthesis and increased cholesterol efflux. Conclusions: We find that arterial SMCs have a relative deficiency in LAL expression resulting in defects in downstream sterol regulatory events compared to macrophages. Our results provide a novel reason for SMC foam cell formation and a potential therapeutic target.

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