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Importance of different pathways of cellular cholesterol efflux
Patricia G Yancey1, Anna E Bortnick, Ginny Kellner-Weibel
1Division of Gastroenterology and Nutrition, Department of Pediatrics, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, PA 19104-4318, USA.
Insights
High-density lipoprotein (HDL) removes excess cholesterol from cells, a process crucial for preventing atherosclerosis. Scavenger receptor class-B type I (SR-BI) and ATP-binding cassette transporter A1 (ABCA1) mediate distinct cholesterol efflux pathways.
Area of Science:
- Biochemistry
- Cell Biology
- Cardiovascular Research
Background:
- Cellular cholesterol homeostasis is vital, with high-density lipoprotein (HDL) playing a key role in removing excess free cholesterol.
- Atherosclerosis development is linked to impaired cholesterol removal, often associated with low HDL cholesterol levels.
- Understanding cholesterol efflux mechanisms is critical for addressing cardiovascular disease.
Purpose of the Study:
- To elucidate the mechanisms of cellular cholesterol removal, focusing on the roles of scavenger receptor class-B type I (SR-BI) and ATP-binding cassette transporter A1 (ABCA1).
- To compare the characteristics of SR-BI-mediated and ABCA1-mediated cholesterol efflux.
- To assess the potential contribution of these pathways to preventing atherosclerotic plaque development.
Main Methods:
- Investigated cholesterol efflux via aqueous diffusion, SR-BI-mediated transport, and ABCA1-mediated transport.
- Characterized the directionality and acceptors for each efflux pathway.
- Considered the influence of protein expression levels and cholesterol acceptor availability.
Main Results:
- Aqueous diffusion mediates inefficient, bidirectional cholesterol efflux.
- SR-BI facilitates bidirectional cholesterol efflux to phospholipid-containing acceptors like HDL.
- ABCA1 mediates unidirectional cholesterol and phospholipid efflux to lipid-poor apolipoproteins.
Conclusions:
- SR-BI and ABCA1 represent distinct pathways for cellular cholesterol removal.
- The relative importance of SR-BI and ABCA1 in preventing atherosclerosis is currently unknown.
- Future research should focus on the interplay between these efflux pathways, protein expression, and available cholesterol acceptors.
Abstract:
The removal of excess free cholesterol from cells by HDL or its apolipoproteins is important for maintaining cellular cholesterol homeostasis. This process is most likely compromised in the atherosclerotic lesion because the development of atherosclerosis is associated with low HDL cholesterol. Multiple mechanisms for efflux of cell cholesterol exist. Efflux of free cholesterol via aqueous diffusion occurs with all cell types but is inefficient. Efflux of cholesterol is accelerated when scavenger receptor class-B type I (SR-BI) is present in the cell plasma membrane. Both diffusion-mediated and SR-BI-mediated efflux occur to phospholipid-containing acceptors (ie, HDL and lipidated apolipoproteins); in both cases, the flux of cholesterol is bidirectional, with the direction of net flux depending on the cholesterol gradient. The ATP-binding cassette transporter AI (ABCA1) mediates efflux of both cellular cholesterol and phospholipid. In contrast to SR-BI-mediated flux, efflux via ABCA1 is unidirectional, occurring to lipid-poor apolipoproteins. The relative importance of the SR-BI and ABCA1 efflux pathways in preventing the development of atherosclerotic plaque is not known but will depend on the expression levels of the two proteins and on the type of cholesterol acceptors available.