Abstract 19780: ABCA1 Response to Atherogenic Lipid Exposure is Reduced in Cultured Human Aortic Smooth Muscle Cells Compared to Human Monocyte-Derived Macrophages
Introduction: Retention of low density (LDL) and other apolipoprotein B (apoB)-containing lipoproteins in the intima of the artery wall initiates atherosclerotic lesion formation. It has been commonly thought that the majority of the lipid overloaded “foam cells” in atherosclerosis are of leukocyte origin. Recently we reported that ≥ 50% of the foam cells in coronary artery atherosclerosis are of smooth muscle cell (SMC) origin, and that the rate-limiting cholesterol exporter, ATP-binding cassette transporter protein A1 (ABCA1), has reduced expression in intimal SMCs compared to leukocytes. Upregulation of ABCA1 expression is dependent on normal flux of lipoprotein-derived cholesterol out of lysosomes. Mechanisms by which macrophages process and store lipoprotein-derived cholesterol in lysosomal and cytosolic compartments are well described, whereas less is known about SMCs.
Hypothesis: Human aortic SMCs have reduced ABCA1 levels compared to macrophages in response to atherogenic lipoprotein exposure.
Methods: Conditions were optimized to load human aortic SMCs and macrophages with aggregated LDL for 24 hrs, followed by a 24 hr equilibration without lipids to allow lysosomal processing of lipoproteins. Cellular cholesterol and cholesteryl ester mass were measured using HPLC tandem mass spectrometry. Response levels to lipid loading were investigated by Western blot for ABCA1.
Results: Cholesteryl ester mass in macrophages increased from 1.5±1.8 to 345.4±21.6 nmol/mg protein in response to aggregated LDL, and 17.4±8.7 to 74.0±16.0 nmol/mg protein in SMCs (mean±SEM, n=9-12 replicates). In macrophages fold change in ABCA1 with exposure to aggLDL was 2.24±0.31 (mean±SEM, n=8 replicates). Preliminary data indicate no significant increase in ABCA1 in SMCs with aggLDL loading. Confocal microscopy indicates increased lipid accumulation following aggLDL loading in the lysosomes of SMCs but not macrophages.
Conclusions: These data suggest that impaired lysosomal processing of excess atherogenic lipoproteins is responsible for impaired upregulation of ABCA1 in cultured human aortic SMCs when compared to macrophages. This provides a potential mechanism for preferential formation of smooth muscle foam cells in human atherosclerosis.
Author Disclosures: J.A. Dubland: None. S. Allahverdian: None. Y. Wang: None. C.S. Pryma: None. T. Chan: None. G.A. Francis: None.
- © 2016 by American Heart Association, Inc.