Numerous lipids are heterogeneously distributed among organelles. Most lipid trafficking between organelles is achieved by a group of lipid transfer proteins (LTPs) that carry lipids using their... Show moreNumerous lipids are heterogeneously distributed among organelles. Most lipid trafficking between organelles is achieved by a group of lipid transfer proteins (LTPs) that carry lipids using their hydrophobic cavities. The human genome encodes many intracellular LTPs responsible for lipid trafficking and the function of many LTPs in defining cellular lipid levels and distributions is unclear. Here, we created a gene knockout library targeting 90 intracellular LTPs and performed whole-cell lipidomics analysis. This analysis confirmed known lipid disturbances and identified new ones caused by the loss of LTPs. Among these, we found major sphingolipid imbalances in ORP9 and ORP11 knockout cells, two proteins of previously unknown function in sphingolipid metabolism. ORP9 and ORP11 form a heterodimer to localize at the ER-trans-Golgi membrane contact sites, where the dimer exchanges phosphatidylserine (PS) for phosphatidylinositol-4-phosphate (PI(4)P) between the two organelles. Consequently, loss of either protein causes phospholipid imbalances in the Golgi apparatus that result in lowered sphingomyelin synthesis at this organelle. Overall, our LTP knockout library toolbox identifies various proteins in control of cellular lipid levels, including the ORP9-ORP11 heterodimer, which exchanges PS and PI(4)P at the ER-Golgi membrane contact site as a critical step in sphingomyelin synthesis in the Golgi apparatus. Show less
BACKGROUND: South Asians are more prone to develop atherosclerotic cardiovascular disease (ASCVD) compared with white Caucasians, which is not fully explained by classical risk factors. We recently... Show moreBACKGROUND: South Asians are more prone to develop atherosclerotic cardiovascular disease (ASCVD) compared with white Caucasians, which is not fully explained by classical risk factors. We recently reported that the presence of aggregation-prone low-density lipoprotein (LDL) in the circulation is associated with increased ASCVD mortality.OBJECTIVE: We hypothesized that LDL of South Asians is more prone to aggregate, which may be explained by differences in their LDL lipid composition.METHODS: In this cross-sectional hypothesis-generating study, LDL was isolated from plasma of healthy South Asians (n = 12) and age- and BMI-matched white Caucasians (n = 12), and its aggregation susceptibility and lipid composition were analyzed.RESULTS: LDL from South Asians was markedly more prone to aggregate compared with white Caucasians. Among all measured lipids, sphingomyelin 24:0 and triacylglycerol 56:8 showed the highest positive correlation with LDL aggregation. In addition, LDL from South Asians was enriched in arachidonic acid containing phosphatidylcholine 38:4 and had less phosphatidylcholines and cholesteryl esters containing monounsaturated fatty acids. Interestingly, body fat percentage, which was higher in South Asians (+26%), positively correlated with LDL aggregation and highly positively correlated with triacylglycerol 56:8, sphingomyelin 24:0, and total sphingomyelin.CONCLUSIONS: LDL aggregation susceptibility is higher in healthy young South Asians compared with white Caucasians. This may be partly explained by the higher body fat percentage of South Asians, leading to sphingomyelin enrichment of LDL. We anticipate that the presence of sphingomyelin-rich, aggregation -prone LDL particles in young South Asians may increase LDL accumulation in the arterial wall and thereby contribute to their increased risk of developing ASCVD later in life. (C) 2019 National Lipid Association. Published by Elsevier Inc. Show less
