Regulation of cholesterol efflux from cultured human skin fibroblasts by lipoprotein lipase and phospholipids

Removal of excess cellular cholesterol from extrahepatic tissues to the liver for degradation is essential in maintaining proper lipid balance in cells and opposing the development of atherosclerosis. The ATP-binding cassette protein Al (ABCAl), a cholesterol transporter, which unidirectionally translocates cholesterol and phospholipids from cells to cholesterol acceptors, plays a pivotal role in preventing the accumulation of cholesteryl esters in various tissues. Unlike ABCAJ, the enzyme lipoprotein lipase (LPL) was shown to be proatherogenic in atherosclerotic lesions by facilitating the formation and cellular uptake of proatherogenic lipids. The objective of the first part of our studies was to investigate the effect of LPL on ABCAl-mediated cholesterol efflux. We overexpressed LPL in human skin fibroblasts, using adenoviral delivery of the gene, and analyzed ABCAI mRNA and protein levels by reverse-transcription polymerase chain reaction (RT-PCR) and western blotting, respectively. ABCA1-mediated cholesterol efflux from the LPL-overexpressing cells was measured using radioactively labeled cholesterol. We showed that overexpression of LPL causes an increase in the levels of ABCAl mRNA and protein and augments ABCAl-mediated cholesterol efflux. Our data suggest that the enzyme may play a regulatory role m ABCAl-mediated cholesterol transport and have an antiatherogenic effect m atherosclerotic lesions. In the second part of our studies, we investigated the effect of phospholipids on ABCAl mRNA and protein levels and ABCAI-dependent cholesterol efflux. We found that treatment of cholesterol-loaded fibroblasts with phosphatidylcholine (PC) results in downregulation of ABCAl mRNA and protein levels and reduces ABCAl-mediated cholesterol efflux. Our data show that PC indirectly regulates the function of the transporter by depleting cellular cholesterol. Phospholipids are the main lipid components of high density lipoprotein (HDL), which removes cellular cholesterol from extrahepatic tissues to the liver for degradation. Our studies suggest that in vivo in addition to playing an important role in cellular cholesterol efflux phospholipids of HDL may regulate ABCA 1 on a transcriptional level.