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UBC Theses and Dissertations

The role of lysosomal acid lipase in regulation of the ATP-binding cassette transporter A1, high density lipoprotein and reverse cholesterol transport Bowden, Kristin Louise


The key regulator of initial HDL particle formation by cells is the ATP-binding cassette transporter A1 (ABCA1). ABCA1 expression is regulated primarily by oxysterol dependent activation of the liver X receptor (LXR). We investigated the role of lysosomal cholesterol on ABCA1 regulation by studying the lysosomal disorder Cholesteryl Ester Storage Disease (CESD). CESD is caused by genetic mutations in the LIP-A gene that result in only 5% of normal activity of lysosomal acid lipase (LAL), an enzyme that hydrolyzes cholesteryl esters (CE) and triglycerides on internalized lipoproteins specifically within the lysosome. We hypothesized that the flux of unesterified cholesterol out of the lysosomes from LAL-mediated hydrolysis of LDL cholesteryl esters is a key regulator of cellular ABCA1 expression, HDL formation and reverse cholesterol transport (RCT). We found that primary skin fibroblasts derived from individuals with CESD had impaired upregulation of ABCA1 in response to LDL loading, reduced phospholipid and cholesterol efflux to apoA-I, lower production of 27-hydroxycholesterol (27-OH) production in response to LDL loading and reduced α-HDL particle formation. This defect was recapitulated in normal fibroblasts following treatment with LAL inhibitors, whereas, treatment with conditioned medium from normal fibroblasts containing secreted LAL rescued ABCA1 expression, apoA-I-mediated cholesterol efflux, HDL particle formation and production of 27-OH by CESD cells. We further investigated the role of LAL in RCT from macrophages specifically using an immortalized macrophage cell line created from LAL-deficient mouse peritoneal macrophages (LAL-/-). LAL-/- macrophages exhibited reduced basal and cholesterol-stimulated ABCA1 expression in culture, and reduced ability to support RCT in LAL-/- mice compared to wild-type (LAL⁺/⁺) macrophages injected into LAL⁺/⁺ mice. ABCA1 protein expression was reduced in LAL-/- mouse liver and mRNA expression of several LXR-dependent genes involved in reverse cholesterol transport (ABCG1, ABCG5, ABCG8, CYP7A1, SR-B1) were differentially modulated compared to LAL⁺/⁺ controls. These results indicate a critical role of LAL in promoting lysosomal flux of cholesterol for ABCA1 expression, cellular cholesterol efflux and RCT in vivo.

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