BBA - Molecular and Cell Biology of Lipids (v.1781, #3)

A translational view on the biliary lipid secretory network by Giuseppe Lo Sasso; Michele Petruzzelli; Antonio Moschetta (79-96).
Bile formation springs from an extraordinary sophisticated secretory network, which combines the activity of transport proteins with the physico-chemical properties of small albeit powerful lipophilic compounds. This robust interplay is activated in response to dietary stimuli, circadian rhythms, and metabolic demands to regulate cholesterol disposal, lipid digestion and absorption in the enterohepatic system. As a result, bile flow is a complex multi-organ effort that requires an integrated flux of information between liver and intestine. A coordinate regulatory task is achieved by nuclear receptors, which are ligand activated transcription factors, responsible for the coherent activation of sets of genes involved in multiple physiological actions, including hepato-biliary homeostasis. Mastering the molecular pathways underlying functional and pharmacological modulation of bile flow has great translational value for potential future treatment of cholestasis and cholelithiasis. In this review, we focus on recent discoveries in the functional biology of bile formation with the explicit aim of underlining their putative clinical relevance.
Keywords: ABC transporters; Bile acids; Cholesterol; Nuclear receptors; Phospholipids;

Knockdown of Acyl-CoA:diacylglycerol acyltransferase 2 with antisense oligonucleotide reduces VLDL TG and ApoB secretion in mice by Ying Liu; John S. Millar; Debra A. Cromley; Mark Graham; Rosanne Crooke; Jeffrey T. Billheimer; Daniel J. Rader (97-104).
Acyl-CoA:diacylglycerol acyltransferases (DGATs) are enzymes that catalyze the formation of triglyceride (TG) from acyl-CoA and diacylglycerol. Two DGATs have been identified which belong to two distinct gene families and both are ubiquitously expressed. DGAT2 knockout mice are lipopenic and die shortly after birth. In the current study, wild type mice were treated with increasing doses (25–60 mg/kg twice weekly) of a DGAT2 gene-specific antisense oligonucleotide (ASO). Treatment resulted in a dose dependent decrease in hepatic DGAT2 gene expression (up to 80%) which was associated with a 40% decrease in hepatic DGAT2 activity and a 45% decrease in hepatic TG. Decreased levels of DGAT2 resulted in a significant dose dependent decrease in VLDL TG secretion (up to 52%) and reduced plasma TG, total cholesterol, and ApoB. Similar results were obtained when DGAT1 KO mice were treated with the DGAT2 ASO. Treatment of ob/ob mice with the DGAT2 ASO resulted in significant decreases in weight gain (10%), adipose weight (25%) and hepatic TG content (80%). Our findings indicate that the majority of TG destined for secretion by liver is synthesized by DGAT2 and suggests that DGAT2 may be a therapeutic target for treatment of hypertriglyceridemia, hepatic steatosis and obesity.
Keywords: Antisense oligonucleotide; Apolipoprotein B; Triacylglycerol synthesis; Acyl-CoA:diacylglycerol acyltransferase; VLDL secretion; Obesity;

X-linked adrenoleukodystrophy (X-ALD) is the most common peroxisomal disorder and is characterized by a striking and unpredictable variation in phenotypic expression. It ranges from a rapidly progressive and fatal cerebral demyelinating disease in childhood (CCALD), to the milder slowly progressive form in adulthood (AMN). X-ALD is caused by mutations in the ABCD1 gene that encodes a peroxisomal membrane located ABC half-transporter named ALDP. Mutations in ALDP result in reduced beta-oxidation of very long-chain fatty acids (VLCFA, > 22 carbon atoms) in peroxisomes and elevated levels of VLCFA in plasma and tissues. Previously, it has been shown that culturing skin fibroblasts from X-ALD patients in lipoprotein-deficient medium results in reduced VLCFA levels and increased expression of the functionally redundant ALD-related protein (ALDRP). The aim of this study was to further resolve the interaction between cholesterol and VLCFA metabolism in X-ALD. Our data show that the reduction in 26:0 in X-ALD fibroblasts grown in lipoprotein-deficient culture medium (free of cholesterol) is offset by a significant increase in both the level and synthesis of 26:1. We also demonstrate that cholesterol-deprivation results in increased expression of stearoyl-CoA-desaturase (SCD) and increased desaturation of 18:0 to 18:1. Finally, there was no increase in [1-14C]-26:0 beta-oxidation. Taken together, we conclude that cholesterol-deprivation reduces saturated VLCFA, but increases mono-unsaturated VLCFA. These data may have implications for treatment of X-ALD patients with lovastatin.
Keywords: X-ALD; Adrenoleukodystrophy; Cholesterol; ABCD1; Peroxisome; SCD;

Effect of rapeseed oil and dietary n-3 fatty acids on triacylglycerol synthesis and secretion in Atlantic salmon hepatocytes by M.A. Kjær; A. Vegusdal; T. Gjøen; A.C. Rustan; M. Todorčević; B. Ruyter (112-122).
Fish oil (FO) has traditionally been used as the dominating lipid component in fish feed. However, FO is a limited resource and the price varies considerably, which has led to an interest in using alternative oils, such as vegetable oils (VOs), in fish diets. It is far from clear how these VOs affect liver lipid secretion and fish health. The polyunsaturated fatty acids (PUFAs), eicosapentanoic acid (EPA) and docosahexanioc acid (DHA), reduce the secretion of lipoproteins rich in triacylglycerols (TAGs) in Atlantic salmon, as they do in humans. The mechanism by which n-3 fatty acids (FAs) in the diet reduce TAG secretion is not known. We have therefore investigated the effects of rapeseed oil (RO) and n-3 rich diets on the accumulation and secretion of 3H-glycerolipids by salmon hepatocytes. Salmon, of approximately 90 g were fed for 17 weeks on one of four diets supplemented with either 13.5% FO, RO, EPA-enriched oil or DHA-enriched oil until a final average weight of 310 g. Our results show that the dietary FA composition markedly influences the endogenous FA composition and lipid content of the hepatocytes. The intracellular lipid level in hepatocytes from fish fed RO diet and DHA diet were higher, and the expressions of the genes for microsomal transfer protein (MTP) and apolipoprotein A1 (Apo A1) were lower, than those in fish fed the two other diets. Secretion of hepatocyte glycerolipids was lower in fish fed the EPA diet and DHA diet than it was in fish fed the RO diet. Our results indicate that EPA and DHA possess different hypolipidemic properties. Both EPA and DHA inhibit TAG synthesis and secretion, but only EPA induces mitochondrial proliferation and reduce intracellular lipid. Expression of the gene for peroxisome proliferator-activated receptor alpha (PPARα) was higher in the DHA dietary group than it was in the other groups.
Keywords: Atlantic salmon; Lipid secretion; Hepatocyte; PPAR; Vegetable oil; Fish oil;

Methyl jasmonate (MeJA) and cerium (Ce4+) elicitation share common features of increasing taxol accumulation of Taxus cuspidata cells. Interestingly, Ce4+ induces programmed cell death (PCD), but this phenomenon is not observed with MeJA elicitation. Here, using a lipidomic approach to measure more than 100 membrane glycerophospholipids of T. cuspidata cells quantitatively, we discovered that lysophosphatidylcholine (LysoPC), phosphatidic acid (PA) and phosphatidylcholine were three potential lipid markers that were responsible for the differences between Ce4+-induced cells and MeJA-induced cells. Compared with MeJA elicitation, marked increase of phospholipase D (PLD) activity was observed following Ce4+ elicitation, suggesting that the PLD activation and high concentrations of PA production might mediate the PCD. Rapid increase of phospholipase A2 (PLA2) activity caused the release of fatty acids and LysoPC following Ce4+ elicitation, which enhanced endogenous jasmonic acid (JA) accumulation. In contrast, PLA2 activity was poorly induced following MeJA elicitation. PLA2 inhibitor suppressed not only JA accumulation but also taxol production, suggesting that the PLA2 activation mediated Ce4+-induced taxol production partially through a JA-dependent signaling pathway. These results demonstrate that differential alternation of glycerolphospholipids caused by phospholipases constitutes an important step in cell death response to Ce4+ and increasing taxol production.
Keywords: Lipidomics; Programmed cell death; Phospholipase A2 and D; Jasmonic acid; Elicitation; Taxus cuspidata cells;

Functional domains of the fatty acid transport proteins: Studies using protein chimeras by Concetta C. DiRusso; Dina Darwis; Thomas Obermeyer; Paul N. Black (135-143).
Fatty acid transport proteins (FATP) function in fatty acid trafficking pathways, several of which have been shown to participate in the transport of exogenous fatty acids into the cell. Members of this protein family also function as acyl CoA synthetases with specificity towards very long chain fatty acids or bile acids. These proteins have two identifying sequence motifs: The ATP/AMP motif, an approximately 100 amino acid segment required for ATP binding and common to members of the adenylate-forming super family of proteins, and the FATP/VLACS motif that consists of approximately 50 amino acid residues and is restricted to members of the FATP family. This latter motif has been implicated in fatty acid transport in the yeast FATP orthologue Fat1p. In the present studies using a yeast strain containing deletions in FAT1 (encoding Fat1p) and FAA1 (encoding the major acyl CoA synthetase (Acsl) Faa1p) as an experimental platform, the phenotypic and functional properties of specific murine FATP1–FATP4 and FATP6–FATP4 protein chimeras were evaluated in order to define elements within these proteins that further distinguish the fatty acid transport and activation functions. As expected from previous work FATP1 and FATP4 were functional in the fatty acid transport pathway, while and FATP6 was not. All three isoforms were able to activate the very long chain fatty acids arachidonate (C20:4) and lignocerate (C24:0), but with distinguishing activities between saturated and highly unsaturated ligands. A 73 amino acid segment common to FATP1 and FATP4 and between the ATP/AMP and FATP/VLACS motifs was identified by studying the chimeras, which is hypothesized to contribute to the transport function.
Keywords: FATP; Transport; Activation; Protein chimera;