Lysophosphatidylcholine Acyltransferase 3 Knockdown-mediated Liver Lysophosphatidylcholine Accumulation Promotes Very Low Density Lipoprotein Production by Enhancing Microsomal Triglyceride Transfer Protein Expression
ABSTRACT After de novo biosynthesis phospholipids undergo extensive remodeling by the Lands' cycle. Enzymes involved in phospholipid biosynthesis have been studied extensively but not those involved in reacylation of lysophosphopholipids. One key enzyme in the Lands' cycle is fatty acyl-CoA:lysophosphatidylcholine acyltransferase (LPCAT), which utilizes lysophosphatidylcholine (LysoPC) and fatty acyl-CoA to produce various phosphatidylcholine (PC) species. Four isoforms of LPCAT have been identified. In this study we found that LPCAT3 is the major hepatic isoform, and its knockdown significantly reduces hepatic LPCAT activity. Moreover, we report that hepatic LPCAT3 knockdown increases certain species of LysoPCs and decreases certain species of PC. A surprising observation was that LPCAT3 knockdown significantly reduces hepatic triglycerides. Despite this, these mice had higher plasma triglyceride and apoB levels. Lipoprotein production studies indicated that reductions in LPCAT3 enhanced assembly and secretion of triglyceride-rich apoB-containing lipoproteins. Furthermore, these mice had higher microsomal triglyceride transfer protein (MTP) mRNA and protein levels. Mechanistic studies in hepatoma cells revealed that LysoPC enhances secretion of apoB but not apoA-I in a concentration-dependent manner. Moreover, LysoPC increased MTP mRNA, protein, and activity. In short, these results indicate that hepatic LPCAT3 modulates VLDL production by regulating LysoPC levels and MTP expression.
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ABSTRACT: The fatty acyl composition of phospholipids determines the biophysical character of membranes and impacts the function of membrane proteins. Here, we define a nuclear receptor pathway for the dynamic modulation of membrane composition in response to changes in cellular lipid metabolism. Ligand activation of liver X receptors (LXRs) preferentially drives the incorporation of polyunsaturated fatty acids into phospholipids through induction of the remodeling enzyme Lpcat3. Promotion of Lpcat3 activity ameliorates endoplasmic reticulum (ER) stress induced by saturated free fatty acids in vitro or by hepatic lipid accumulation in vivo. Conversely, Lpcat3 knockdown in liver exacerbates ER stress and inflammation. Mechanistically, Lpcat3 modulates inflammation both by regulating inflammatory kinase activation through changes in membrane composition and by affecting substrate availability for inflammatory mediator production. These results outline an endogenous mechanism for the preservation of membrane homeostasis during lipid stress and identify Lpcat3 as an important mediator of LXR effects on metabolism.Cell metabolism 11/2013; 18(5):685-97. DOI:10.1016/j.cmet.2013.10.002 · 16.75 Impact Factor
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ABSTRACT: During the post-prandial phase, intestinal triglyceride-rich lipoproteins (TRL) i.e. chylomicrons are the main contributors to the serum lipid level, which is linked to coronary artery diseases. Hypertriglyceridemia can originate from decreased clearance or increased production of TRL. During lipid absorption, enterocytes produce and secrete chylomicrons and transiently store lipid droplets (LDs) in the cytosol. The dynamic fluctuation of triglycerides in cytosolic LDs suggests that they contribute to TRL production and may thus control the length and amplitude of the post-prandial hypertriglyceridemia. In this review, we will describe the recent advances in the characterization of enterocytic LDs. The role of LDs in chylomicron production and secretion as well as potential previously unsuspected functions in the metabolism of vitamins, steroids and prostaglandins and in viral infection will also be discussed.Biochimie 07/2013; 96. DOI:10.1016/j.biochi.2013.07.009 · 3.12 Impact Factor
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ABSTRACT: Pulmonary surfactant is required for lung function at birth and throughout postnatal life. Defects in the surfactant system are associated with common pulmonary disorders including neonatal respiratory distress syndrome and acute respiratory distress syndrome in children and adults. Lipogenesis is essential for the synthesis of pulmonary surfactant by type II epithelial cells lining the alveoli. This study sought to identify the role of pulmonary epithelial SREBP, a transcriptional regulator of cellular lipid homeostasis, during a critical time period of perinatal lung maturation in the mouse. Genome wide mRNA expression profiling of lung tissue from transgenic mice with epithelial-specific deletions of Scap (ScapΔ/Δ, resulting in inactivation of SREBP signaling) or Insig1 and Insig2 (Insig1/2Δ/Δ, resulting in activation of SREBP signaling) was assessed. Differentially expressed genes responding to SREBP perturbations were identified and subjected to functional enrichment analysis, pathway mapping and literature mining to predict upstream regulators and transcriptional networks regulating surfactant lipid homeostasis. Through comprehensive data analysis and integration, time dependent effects of epithelial SCAP/INSIG/SREBP deletion and defined SCAP/INSIG/SREBP-associated genes, bioprocesses and downstream pathways were identified. SREBP signaling influences epithelial development, cell death and cell proliferation at E17.5, while primarily influencing surfactant physiology, lipid/sterol synthesis, and phospholipid transport after birth. SREBP signaling integrated with the Wnt/β-catenin and glucocorticoid receptor signaling pathways during perinatal lung maturation. SREBP regulates perinatal lung lipogenesis and maturation through multiple mechanisms by interactions with distinct sets of regulatory partners.PLoS ONE 05/2014; 9(5):e91376. DOI:10.1371/journal.pone.0091376 · 3.53 Impact Factor