UBC Theses and Dissertations
Low lysosomal acid lipase activity in smooth muscle cells relative to macrophages provides new insights into foam cell formation in atherosclerosis Dubland, Joshua Andrew
Smooth muscle cells (SMCs) are the predominant cell type in 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 cholesterol exporter protein ATP-binding cassette transporter A1 (ABCA1). ABCA1 expression is dependent on the flux of cholesterol out of lysosomes, generated via the hydrolysis of lipoprotein-derived cholesteryl esters (CEs) to cholesterol by lysosomal acid lipase (LAL), and subsequent generation of oxysterols such as 27-hydroxycholesterol by CYP27A1 for promotion of gene transcription via the nuclear liver X receptor (LXR). In the present studies we tested the hypothesis that SMCs have reduced lysosomal function that contributes to foam cell formation. Chapter 2 investigates and defines the role of lysosomal function in cholesterol metabolism using a mouse LAL KO peritoneal macrophage model. Chapter 3 investigates lysosomal function and cholesterol metabolism differences between human monocyte-derived macrophages (HMMs) and arterial SMCs treated with aggregated LDL (agLDL). Unlike HMMs, lipid loading of SMCs did not significantly increase 27-hydroxycholesterol or ABCA1 levels and did not decrease new cholesterol synthesis. Microscopy revealed sequestration of CEs in lysosomes of SMCs, while HMMs displayed mostly cytosolic CE accumulations. We did not find evidence of a lysosomal functional defect from lipid induced loss of acidity or loss of lysosomal proteolytic function in SMCs. Instead, LAL levels were markedly higher in macrophages compared to SMCs (LAL activity 23.4-times higher in agLDL loaded HMMs compared to SMCs, p<0.001). Treatment of SMCs with LAL containing medium decreased lysosomal lipid accumulation, decreased new cholesterol synthesis, and increased cholesterol efflux. Persistent reduction of ABCA1 response to LAL treatment in SMCs was potentially attributable to low CYP27A1 and LXR expression relative to macrophages. Overall, we find that arterial SMCs have a relative deficiency in LAL and associated defects in downstream sterol regulatory events compared to macrophages. Our results indicate low LAL activity in SMCs as a novel reason for foam cell formation and a potential target to reduce atherosclerosis development and promote regression therapeutically.
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