Journal of Lipid Research最新文献

The myelin is responsible for providing stability to the axons of the nerve cells, but above all to improve transmission speed of the nerve impulse in vertebrates. Over 70% of the myelin sheath is composed of lipids and the remaining portion by approximately 2,000 proteins. The myelin sheath has been constantly evolving and it is known that unusually high concentrations of GalCer and its sulfated form play a major role in the biophysical properties of the myelin. To gain insights of the evolutionary role of GalCer, we have studied two lysosomal enzymes involved in GalCer degradation, arylsulfatase A (ARSA) and galactocerebrosidase (GALC). Deficiency of ARSA or GALC causes demyelinating disorders. We conducted phylogenetic analyses of 105 ARSA and 110 GALC orthologs representing more than 600 MYA of evolution. We examined i) low values of the ratio of non-synonymous to synonymous nucleotide-substitution rates (dN/dS) indicating purifying selection, and ii) negative selection of amino acids located in the active site preventing pathogenic mutations. Gene structure analyses showed evidence of rearrangement with gain and loss of exons while there were conserved regions mainly located around the active site. We also found a limited number of sites under positive selection pressure that do not cause alterations to the overall protein structure. Our results indicate that ARSA and GALC have been highly conserved during the evolutionary process to maintain the metabolism of GalCer, which is essential for the integrity of the white matter in vertebrate species.

Lipoxygenases (ALOX) play important roles in cell differentiation and in the pathogenesis of cardio-vascular, hyperproliferative, neurodegenerative and metabolic diseases. The human genome involves six intact ALOX genes and knockout studies of the corresponding mouse orthologs indicated that the coding multiplicity of ALOX-isoforms is not an indication for functional redundancy. Despite their evolutionary relatedness human and mouse ALOX15 and ALOX15B orthologs exhibit different catalytic properties. Human ALOX15 oxygenates arachidonic acid mainly to 15-HpETE but 12-HpETE is the dominant oxygenation product of mouse Alox15. This functional difference is the results of a targeted enzyme evolution but the driving forces for this process have not been well defined. For human and mouse ALOX15B orthologs similar functional differences have been reported but for the time being it was unclear whether these differences might also be a consequence of targeted enzyme evolution. To address this question, we systematically searched the public databases for ALOX15B genes, expressed selected enzymes and characterized their functional properties. We found that functional ALOX15B genes frequently occur in Prototheria and Eutheria but orthologous genes are rare in Metatheria. The vast majority of mammalian ALOX15B orthologs constitute arachidonic acid 15-lipoxygenating enzymes and this property did not depend on the evolutionary ranking of the animals. Only several Muridae species including M. musculus, M. pahari, M. caroli, M. coucha and A. niloticus express arachidonic acid 8-lipoxygenating ALOX15B orthologs. Consequently, the difference in the reaction specificity of mouse and human ALOX15B orthologs may not be considered a functional consequence of targeted enzyme evolution.

Present study explores the role of liver lipidome in driving T2D-associated metabolic changes. Elevated liver triacylglycerols, reduced PUFAs, and 86 differentially abundant lipid species were identified in diabetes-prone mice. Of these altered lipid species 82 markedly overlap with human plasma lipids associated with T2D/CVD risk. Pathway enrichment highlighted sphingolipid metabolism, however, only five of all genes involved in the pathway were differentially expressed in the liver. Interestingly, overlap with adipose tissue transcriptome was much higher (57 genes), pointing towards an active adipose-liver interaction. Next, the integration of liver lipidome and transcriptome identified strongly correlated lipid-gene networks highlighting Cer(22:0), dCer(24:1), and TAG(58:6) playing a central role in transcriptional regulation. Putative molecular targets of Cer(22:0) were altered (Cyp3a44, Tgf-β1) in primary mouse hepatocytes treated with Cer(22:0). Early alteration of liver lipidome markedly depends on adipose tissue expression pattern and provides substantial evidence linking early liver lipidome alterations and risk of T2D.

Plasmalogens are an abundant class of glycerophospholipids with a characteristic 1-O-alk-1'-enyl double bond. While their synthesis has been extensively investigated, their degradation remains understudied. Plasmalogen deficiencies are associated with severe disorders in humans and interfering with their degradation would be a treatment option, but it remains out of reach due to limited knowledge. The plasmalogen double bond is degraded either directly by a plasmalogenase or by conversion to the 2' lyso forms by phospholipase and subsequent cleavage by lysoplasmalogenase (E.C. 3.3.2.2). Two lysoplasmalogenases are known so far, TMEM86A and TMEM86B. While TMEM86B has been expressed in bacteria, purified and shown to encode lysoplasmalogenase activity by a coupled optical assay, the closely related protein TMEM86A has not yet been purified, but its activity was shown indirectly by a lipidomics approach. Here, we present a novel assay for lysoplasmalogenase activity based on incubation with lysoplasmenylethanolamine or lysoplasmenylcholine, derivatization of the aldehyde product with dansylhydrazine and hydrazine quantification by reversed-phase HPLC with fluorescence detection. The method was sensitive enough to robustly detect lysoplasmalogenase activity in human embryonic kidney cells following transient expression of TMEM86A or TMEM86B and also suitable for the determination of lysoplasmalogenase activity in mouse tissues where highest activities were found in liver and duodenum. We introduced point mutations at positions proposed to be catalytically relevant and provided experimental evidence that all but one affected lysoplasmalogenase activity. Our novel assay allows direct and fast measurement of lysoplasmalogenase activity thereby providing a tool to advance research in the field of plasmalogen degradation.

Ferroptosis is an iron-dependent form of cell death driven by the excessive peroxidation of poly-unsaturated fatty acids (PUFAs) within membrane phospholipids (PLs). Ferroptosis is a hallmark of many diseases and preventing or inducing ferroptosis has considerable therapeutic potential. Like other forms of cell death, the pathological importance and therapeutic potential of ferroptosis is well appreciated. However, while cell death modalities such as apoptosis and necroptosis have critical physiological roles, such as in development and tissue homeostasis, whether ferroptosis has important physiological roles is largely unknown. In this regard, key questions for field are: Is ferroptosis used for physiological processes? Are certain cell-types purposely adapted to be either resistant or sensitive to ferroptosis to be able to function optimally? Do physiological perturbations such as aging and diet impact ferroptosis susceptibility? Herein, we have reviewed emerging evidence that supports the idea that being able to selectively and controllably induce or resist ferroptosis is essential for development and cell function. While several factors regulate ferroptosis, it appears that the ability of cells and tissues to control their lipid composition, specifically the abundance of PLs containing PUFAs, is crucial for cells to be able to either resist or be sensitized to ferroptosis. Finally, aging and diets enriched in specific PUFAs lead to an increase in cellular PUFA levels which may sensitize cells to ferroptosis. Such changes may impact the pathogenesis of diseases where ferroptosis is involved.

Branched-chain fatty acids (BCFAs) are predominantly saturated fatty acids with one or more methyl branches on the carbon chain, typically found in dairy products and measured in micromolar concentrations in human plasma. The biological function of BCFAs in humans remains ill-defined, but a relationship between circulating BCFAs and cardiometabolic health has been suggested. The objective of this study was to evaluate the impact of BCFAs on energy metabolism in human myotubes. The results revealed distinct effects of BCFAs. 12-Methyltetradecanoic acid (12-MTD), increased glucose uptake and glycogen synthesis, while 13-methyltetradecanoic acid (13-MTD), 14-methylhexadecanoic acid (14-MHD) and 15-methylhexadecanoic acid (15-MHD) increased oleic acid uptake and 13-MTD and 15-MHD oleic acid oxidation, indicating a more general stimulatory effect on fatty acid than glucose metabolism. Interestingly, the same BCFAs, 13-MTD, 14-MHD and 15-MHD, appeared to reduce insulin-stimulated glycogen synthesis. Insulin-stimulated phosphorylation of IRS1 was not apparent after exposure to 12-MTD, 13-MTD and 15-MHD, whereas insulin-stimulated phosphorylation of Akt was unchanged by BCFAs. Incorporation of [¹⁴C]leucine into lipids was affected, as 13-MTD increased the total lipid content, and 12-MTD altered the distribution of lipid classes. Metabolic flux analysis indicated that 14-MHD stimulated extracellular acidification. The effects of BCFAs might involve increased mRNA expression of pyruvate dehydrogenase kinase 4. In conclusion, the study demonstrates that different BCFAs have distinct effects on energy metabolism in myotubes, 12-MTD mainly affect glucose metabolism, while 13-MTD, 14-MHD and 15-MHD modulated oleic acid metabolism. These data suggest that some BCFAs might have therapeutic applications by improving energy metabolism.

Hypertriglyceridemia (HTG), particularly in combined hyperlipidemia, increases risk for atherosclerotic cardiovascular disease, but the underlying mechanisms remain incompletely understood. We sought to determine contributions of circulating monocytes to atherosclerosis associated with HTG in combined hyperlipidemia, created by transgenic expression of human apoCIII in Ldlr^-/- mice (Ldlr^-/-ApoCIIItg) fed western high-fat diet (WD). Tissue culture with THP1 and primary human monocytes was used to examine effects of triglyceride (TG)-rich lipoproteins (TGRL) on monocytes. Ldlr^-/-ApoCIIItg mice were also treated with apoCIII antisense oligonucleotide (ASO) and examined for foamy monocytes and atherosclerosis. Compared to Ldlr^-/- mice, Ldlr^-/-ApoCIIItg mice fed WD had early and persistent increases in lipid accumulation within monocytes and enhanced atherosclerosis. Ldlr^-/-ApoCIIItg mice vs Ldlr^-/- mice had higher levels of CD11c, CD36, and cytokines in foamy monocytes, with increases in foamy monocyte adhesion to VCAM-1 and oxLDL uptake. Monocytes took up TGRL in vivo and in vitro and changed phenotypes. Foamy monocytes infiltrated into atherosclerotic lesions, and specific and sustained depletion of CD11c⁺ (foamy) monocytes profoundly reduced atherosclerosis in Ldlr^-/-ApoCIIItg mice on WD. Treatment with apoCIII ASO lowered plasma TG and cholesterol levels, improved foamy monocyte phenotypes, and reduced atherosclerosis in Ldlr^-/-ApoCIIItg mice. In conclusion, HTG in combined hyperlipidemia accelerates atherosclerosis, in part, by increasing foamy monocyte formation and infiltration into atherosclerotic plaques. Treatment with apoCIII ASO is a potential new therapy for improving monocyte phenotypes and reducing atherosclerosis in combined hyperlipidemia.

Intracellular cholesterol transport is essential for maintaining cellular cholesterol homeostasis. ATP-binding cassette A1 (ABCA1) continuously moves cholesterol from the inner leaflet to the outer leaflet of the plasma membrane (PM) to maintain low inner leaflet cholesterol levels. When PM inner leaflet cholesterol levels exceed ER cholesterol levels, which are maintained at approximately 5 mol% by the complex of sterol regulatory element-binding protein (SREBP) and SREBP cleavage-activating protein (SCAP), Aster-A/GramD1a transports the excess cholesterol to the ER. Furthermore, ABCA1 removes excess PM cholesterol by promoting its efflux as nascent high-density lipoprotein (HDL) particles. Thus, cellular cholesterol homeostasis is maintained by the coordinated action of SCAP-SREBP, Aster-A/GramD1a, and ABCA1. While the regulation of SCAP-SREBP and Aster-A/GramD1a is well-understood, the mechanism governing ABCA1 activity remains less understood. In this study, we investigated the impact of PM cholesterol levels on ABCA1-mediated cholesterol and phosphatidylcholine (PC) efflux. Cells were treated with various concentrations of methyl-β-cyclodextrin (MβCD) or MβCD-cholesterol for 30 min to modulate PM cholesterol levels. We found that the initial velocities of both cholesterol and PC efflux were dependent solely on PM cholesterol levels, despite both being substrates for ABCA1. Intriguingly, when PM cholesterol levels dropped below 70% of the level observed in cells cultured in the presence of 10% FBS, both cholesterol and PC efflux ceased, even in the presence of abundant PC in the PM. Our findings suggest that ABCA1-mediated nascent HDL formation is precisely regulated to maintain optimal PM cholesterol levels.

Carotenoids, essential nutrients for eye health, are absorbed in the intestine to support vitamin A homeostasis and provide cellular protection. This process involves the lipid transporters scavenger receptor class B type 1 (SR-B1, encoded by Scarb1 gene) and Niemann-Pick C1-Like 1 (NPC1L1), which load these dietary lipids into the plasma membrane of intestinal enterocytes. However, the precise contribution of these transporters to carotenoid absorption, the putative involvement of Aster proteins in their downstream movement, and the interactions with their metabolizing enzymes, β-carotene oxygenase 1 (BCO1) and β-carotene oxygenase 2 (BCO2), remain incompletely understood. Here, we investigated carotenoid metabolism in the mouse intestine using pharmacological and genetic approaches. We observed that ezetimibe, an NPC1L1 inhibitor, reduced zeaxanthin but did not affect β-carotene absorption. Aster-C, highly expressed in enterocytes, bound zeaxanthin in biochemical assays. In mice, Aster-C deficiency led to upregulation of Gramd1b (Aster-B) expression and increased zeaxanthin bioavailability. We further showed that BCO1 directly interacted with membranes to extract β-carotene for retinoid production, indicating that vitamin A production is Aster protein-independent. This observation is consistent with the finding that the intestine-specific transcription factor ISX, the master regulator of vitamin A production, controlled Scarb1 and Bco1 expression but had no effect on Gramd1a, b, or c, encoding Aster proteins in intestinal enterocytes. Together, our study revealed distinct pathways for β-carotene and zeaxanthin absorption and metabolism, offering new insights into carotenoid bioavailability and potential strategies to optimize dietary carotenoid intake for improved eye health.

Phosphoinositides constitute a class of seven phospholipids found in cell membranes, regulating various cellular processes like trafficking and signaling. Mutations in their metabolizing enzymes are implicated in several pathologies, including X-linked myotubular myopathy, a severe myopathy caused by mutations in the MTM1 gene. MTM1 (myotubularin 1) acts as a phosphoinositide 3-phosphatase, targeting PI3P (phosphatidylinositol 3-phosphate) and phosphatidylinositol 3,5-bisphosphate, crucial for endolysosomal trafficking. Studies in X-linked myotubular myopathy animal models have demonstrated that loss of MTM1 results in PI3P accumulation in muscle. Moreover, inactivating the class II phosphoinositide 3-kinase beta rescues the pathological phenotype and decreases PI3P levels, suggesting that the normalization of PI3P levels could be responsible for that rescue mechanism. In this study, using an Mtm1-KO skeletal muscle cell line, we investigated the localization of the PI3P pool metabolized by MTM1 in endosomal compartments. Our findings reveal that MTM1 metabolizes a pool of PI3P on EEA1 (early endosome antigen 1)-positive endosomes, leading to impaired Rab4 recycling vesicle biogenesis in the absence of MTM1. Furthermore, depletion of class II phosphoinositide 3-kinase beta rescued Mtm1-KO cell phenotype, normalized PI3P level on EEA1-positive endosomes, and restored Rab4-positive vesicle biogenesis. These results indicate that MTM1 is critical for the homeostasis of endosomal trafficking, and that depletion of MTM1 potentially alters cargo recycling through Rab4-positive vesicle trafficking.