Dual function lipin proteins and glycerolipid metabolism

Low doses of curcumin protect alcohol-induced liver damage by modulation of the alcohol metabolic pathway, CYP2E1 and AMPK

AIMS

This study investigated the hepatoprotective effects of low doses of curcumin against liver damage induced by chronic alcohol intake and a high-fat diet. We also examined several potential underlying mechanisms including action on alcohol metabolism, antioxidant activity, AMPK level and lipid metabolism.

MAIN METHOD

Alcohol (25% v/v, 5 g/kg body weight) was orally administered once a day for 6 weeks to mice fed a high-fat diet with or without two different doses of curcumin (0.02% and 0.05%, wt/wt).

KEY FINDINGS

Curcumin significantly decreased the plasma aspartate aminotransferase, alanine aminotransferase, lactate dehydrogenase and alkaline phosphatase activities (p<0.05) and prevented hepatic steatosis compared with the alcohol control group. Curcumin significantly reversed the alcohol-induced inhibition of the alcohol dehydrogenase, aldehyde dehydrogenase 2 and antioxidant enzyme activities as well as the activation of cytochrome P4502E1 and promotion of lipid peroxidation (p<0.05). Curcumin significantly increased the hepatic total AMPK protein level and concomitantly suppressed the fatty acid synthase and phosphatidate phosphohydrolase activities compared with the alcohol control group (p<0.05). Furthermore, curcumin significantly lowered the plasma leptin, free fatty acids and triglycerides levels and hepatic lipid levels (p<0.05).

SIGNIFICANCE

These findings indicate that low doses of curcumin may protect against liver damage caused by chronic alcohol intake and a high-fat diet partly by modulating the alcohol metabolic enzyme activity, the antioxidant activity and the lipid metabolism. Therefore, curcumin may provide a promising natural therapeutic strategy against liver disease.

Lipids of mitochondria

A unique organelle for studying membrane biochemistry is the mitochondrion whose functionality depends on a coordinated supply of proteins and lipids. Mitochondria are capable of synthesizing several lipids autonomously such as phosphatidylglycerol, cardiolipin and in part phosphatidylethanolamine, phosphatidic acid and CDP-diacylglycerol. Other mitochondrial membrane lipids such as phosphatidylcholine, phosphatidylserine, phosphatidylinositol, sterols and sphingolipids have to be imported. The mitochondrial lipid composition, the biosynthesis and the import of mitochondrial lipids as well as the regulation of these processes will be main issues of this review article. Furthermore, interactions of lipids and mitochondrial proteins which are highly important for various mitochondrial processes will be discussed. Malfunction or loss of enzymes involved in mitochondrial phospholipid biosynthesis lead to dysfunction of cell respiration, affect the assembly and stability of the mitochondrial protein import machinery and cause abnormal mitochondrial morphology or even lethality. Molecular aspects of these processes as well as diseases related to defects in the formation of mitochondrial membranes will be described.

Combination of lipid metabolism alterations and their sensitivity to inflammatory cytokines in human lipin-1-deficient myoblasts

Lipin-1 deficiency is associated with massive rhabdomyolysis episodes in humans, precipitated by febrile illnesses. Despite well-known roles of lipin-1 in lipid biosynthesis and transcriptional regulation, the pathogenic mechanisms leading to rhabdomyolysis remain unknown. Here we show that primary myoblasts from lipin-1-deficient patients exhibit a dramatic decrease in LPIN1 expression and phosphatidic acid phosphatase 1 activity, and a significant accumulation of lipid droplets (LD). The expression levels of LPIN1-target genes [peroxisome proliferator-activated receptors delta and alpha (PPARδ, PPARα), peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), acyl-coenzyme A dehydrogenase, very long (ACADVL), carnitine palmitoyltransferase IB and 2 (CPT1B and CPT2)] were not affected while lipin-2 protein level, a closely related member of the family, was increased. Microarray analysis of patients' myotubes identified 19 down-regulated and 51 up-regulated genes, indicating pleiotropic effects of lipin-1 deficiency. Special attention was paid to the up-regulated ACACB (acetyl-CoA carboxylase beta), a key enzyme in the fatty acid synthesis/oxidation balance. We demonstrated that overexpression of ACACB was associated with free fatty acid accumulation in patients' myoblasts whereas malonyl-carnitine (as a measure of malonyl-CoA) and CPT1 activity were in the normal range in basal conditions accordingly to the normal daily activity reported by the patients. Remarkably ACACB invalidation in patients' myoblasts decreased LD number and size while LPIN1 invalidation in controls induced LD accumulation. Further, pro-inflammatory treatments tumor necrosis factor alpha+Interleukin-1beta(TNF1α+IL-1ß) designed to mimic febrile illness, resulted in increased malonyl-carnitine levels, reduced CPT1 activity and enhanced LD accumulation, a phenomenon reversed by dexamethasone and TNFα or IL-1ß inhibitors. Our data suggest that the pathogenic mechanism of rhabdomyolysis in lipin-1-deficient patients combines the predisposing constitutive impairment of lipid metabolism and its exacerbation by pro-inflammatory cytokines.

Assembly of high molecular weight complexes of lipin on a supported lipid bilayer observed by atomic force microscopy

Lipins are phosphatidic acid phosphatases involved in the biosynthesis of triacylglycerols and phospholipids. They are associated with the endoplasmic reticulum but can also travel into the nucleus and alter gene expression. Previous studies indicate lipins in solution form high molecular weight complexes, possibly tetramers. This study was undertaken to determine if lipins form complexes on membranes as well. Murine lipin 1b was applied to a supported bilayer of phosphatidylcholine, phosphatidylserine, and cholesterol and examined by atomic force microscopy (AFM) over time. Lipin on bare mica appeared as a symmetric particle with a volume consistent with the size of a monomer. On the bilayer, lipin initially bound as asymmetric, curved particles that sometimes assembled into circular structures with an open center. Subsequently, lipin assemblies grew into large, symmetric particles with an average volume 12 times that of the monomer. Over time, some of the lipin assemblies were removed from the bilayer by the AFM probe leaving behind "footprints" composed of complex patterns that may reflect the substructure of the lipin assemblies. The lipin complexes appeared very flat, with a diameter 20 times their height. The footprints had a similar diameter, providing confirmation of the extensive deformation of the protein under the AFM probe. The ability of lipin to form large complexes on membranes may have significant implications for the local concentrations of the product, diacylglycerol, formed during hydrolysis of phosphatidic acid and for cooperative hormonal regulation of lipin activity through phosphorylation of one or more monomers in the complexes.

Lipid transport between the endoplasmic reticulum and mitochondria

Mitochondria are partially autonomous organelles that depend on the import of certain proteins and lipids to maintain cell survival and membrane formation. Although phosphatidylglycerol, cardiolipin, and phosphatidylethanolamine are synthesized by mitochondrial enzymes, phosphatidylcholine, phosphatidylinositol, phosphatidylserine, and sterols need to be imported from other organelles. The origin of most lipids imported into mitochondria is the endoplasmic reticulum, which requires interaction of these two subcellular compartments. Recently, protein complexes that are involved in membrane contact between endoplasmic reticulum and mitochondria were identified, but their role in lipid transport is still unclear. In the present review, we describe components involved in lipid translocation between the endoplasmic reticulum and mitochondria and discuss functional as well as regulatory aspects that are important for lipid homeostasis.

Identification of a novel lipin homologue from the parasitic protozoan Trypanosoma brucei

Background

Arginine methylation is a post-translational modification that expands the functional diversity of proteins. Kinetoplastid parasites contain a relatively large group of protein arginine methyltransferases (PRMTs) compared to other single celled eukaryotes. Several T. brucei proteins have been shown to serve as TbPRMT substrates in vitro, and a great number of proteins likely to undergo methylation are predicted by the T. brucei genome. This indicates that a large number of proteins whose functions are modulated by arginine methylation await discovery in trypanosomes. Here, we employed a yeast two-hybrid screen using as bait the major T. brucei type I PRMT, TbPRMT1, to identify potential substrates of this enzyme.

Results

We identified a protein containing N-LIP and C-LIP domains that we term TbLpn. These domains are usually present in a family of proteins known as lipins, and involved in phospholipid biosynthesis and gene regulation. Far western and co-immunoprecipitation assays confirmed the TbPRMT1-TbLpn interaction. We also demonstrated that TbLpn is localized mainly to the cytosol, and is methylated in vivo. In addition, we showed that, similar to mammalian and yeast proteins with N-LIP and C-LIP domains, recombinant TbLpn exhibits phosphatidic acid phosphatase activity, and that two conserved aspartic acid residues present in the C-LIP domain are critical for its enzymatic activity.

Conclusions

This study reports the characterization of a novel trypanosome protein and provides insight into its enzymatic activity and function in phospholipid biosynthesis. It also indicates that TbLpn functions may be modulated by arginine methylation.

Regulation of lipid droplet and membrane biogenesis by the acidic tail of the phosphatidate phosphatase Pah1p

Binding and dephosphorylation of the yeast lipin Pah1p by its phosphatase Nem1p-Spo7p is essential for its membrane targeting and is mediated by a C-terminal acidic stretch on Pah1p. This results in the recruitment of Pah1p to the vicinity of lipid droplets, where it controls triglyceride and droplet biogenesis in an acidic tail–dependent manner.

Lipins are evolutionarily conserved phosphatidate phosphatases that perform key functions in phospholipid, triglyceride, and membrane biogenesis. Translocation of lipins on membranes requires their dephosphorylation by the Nem1p-Spo7p transmembrane phosphatase complex through a poorly understood mechanism. Here we identify the carboxy-terminal acidic tail of the yeast lipin Pah1p as an important regulator of this step. Deletion or mutations of the tail disrupt binding of Pah1p to the Nem1p-Spo7p complex and Pah1p membrane translocation. Overexpression of Nem1p-Spo7p drives the recruitment of Pah1p in the vicinity of lipid droplets in an acidic tail–dependent manner and induces lipid droplet biogenesis. Genetic analysis shows that the acidic tail is essential for the Nem1p-Spo7p–dependent activation of Pah1p but not for the function of Pah1p itself once it is dephosphorylated. Loss of the tail disrupts nuclear structure, INO1 gene expression, and triglyceride synthesis. Similar acidic sequences are present in the carboxy-terminal ends of all yeast lipin orthologues. We propose that acidic tail–dependent binding and dephosphorylation of Pah1p by the Nem1p-Spo7p complex is an important determinant of its function in lipid and membrane biogenesis.

Lipins, lipinopathies, and the modulation of cellular lipid storage and signaling

Members of the lipin protein family are phosphatidate phosphatase (PAP) enzymes, which catalyze the dephosphorylation of phosphatidic acid to diacylglycerol, the penultimate step in TAG synthesis. Lipins are unique among the glycerolipid biosynthetic enzymes in that they also promote fatty acid oxidation through their activity as co-regulators of gene expression by DNA-bound transcription factors. Lipin function has been evolutionarily conserved from a single ortholog in yeast to the mammalian family of three lipin proteins-lipin-1, lipin-2, and lipin-3. In mice and humans, the levels of lipin activity are a determinant of TAG storage in diverse cell types, and humans with deficiency in lipin-1 or lipin-2 have severe metabolic diseases. Recent work has highlighted the complex physiological interactions between members of the lipin protein family, which exhibit both overlapping and unique functions in specific tissues. The analysis of "lipinopathies" in mouse models and in humans has revealed an important role for lipin activity in the regulation of lipid intermediates (phosphatidate and diacylglycerol), which influence fundamental cellular processes including adipocyte and nerve cell differentiation, adipocyte lipolysis, and hepatic insulin signaling. The elucidation of lipin molecular and physiological functions could lead to novel approaches to modulate cellular lipid storage and metabolic disease.

Differential regulation of the expressions of the PGC-1α splice variants, lipins, and PPARα in heart compared to liver

Peroxisome proliferator-activated receptor α (PPARα) and PPARγ coactivator 1α (PGC-1α) are crucial transcriptional regulators for genes involved in FA oxidation. Lipin-1 is essential for this increased capacity for β-oxidation in fasted livers, and it is also a phosphatidate phosphatase involved in triacylglycerol and phospholipid synthesis. Little is known about the regulation of these proteins in the heart during fasting, where there is increased FA esterification and oxidation. Lipin-1, lipin-2, lipin-3, carnitine palmitoyltransferase-1b (Cpt1b), and PGC-1α-b mRNA were increased by glucocorticoids and cAMP in neonatal rat cardiomyocytes. However, Cpt1b upregulation was caused by increased PPARα activation rather than expression. By contrast, the effects of PPARα in fasted livers are mediated through increased expression. During fasting, the expressions of PGC-1α-b and PGC-1α-c are increased in mouse hearts, and this is explained by increased cAMP-dependent signaling. By contrast, PGC-1α-a expression is increased in liver. Contrary to our expectations, lipin-1 expression was decreased and lipin-2 remained unchanged in hearts compared with increases in fasted livers. Our results identify novel differences in the regulation of lipins, PPARα, and PGC-1α splice variants during fasting in heart versus liver, even though the ultimate outcome in both tissues is to increase FA turnover and oxidation.

Phosphorylation of lipin 1 and charge on the phosphatidic acid head group control its phosphatidic acid phosphatase activity and membrane association

The lipin gene family encodes a class of Mg(2+)-dependent phosphatidic acid phosphatases involved in the de novo synthesis of phospholipids and triglycerides. Unlike other enzymes in the Kennedy pathway, lipins are not integral membrane proteins, and they need to translocate from the cytosol to intracellular membranes to participate in glycerolipid synthesis. The movement of lipin 1 within the cell is closely associated with its phosphorylation status. Although cellular analyses have demonstrated that highly phosphorylated lipin 1 is enriched in the cytosol and dephosphorylated lipin 1 is found on membranes, the effects of phosphorylation on lipin 1 activity and binding to membranes has not been recapitulated in vitro. Herein we describe a new biochemical assay for lipin 1 using mixtures of phosphatidic acid (PA) and phosphatidylethanolamine that reflects its physiological activity and membrane interaction. This depends on our observation that lipin 1 binding to PA in membranes is highly responsive to the electrostatic charge of PA. The studies presented here demonstrate that phosphorylation regulates the ability of the polybasic domain of lipin 1 to recognize di-anionic PA and identify mTOR as a crucial upstream signaling component regulating lipin 1 phosphorylation. These results demonstrate how phosphorylation of lipin 1 together with pH and membrane phospholipid composition play important roles in the membrane association of lipin 1 and thus the regulation of its enzymatic activity.

Sterol regulatory element binding protein-dependent regulation of lipid synthesis supports cell survival and tumor growth

Background

Regulation of lipid metabolism via activation of sterol regulatory element binding proteins (SREBPs) has emerged as an important function of the Akt/mTORC1 signaling axis. Although the contribution of dysregulated Akt/mTORC1 signaling to cancer has been investigated extensively and altered lipid metabolism is observed in many tumors, the exact role of SREBPs in the control of biosynthetic processes required for Akt-dependent cell growth and their contribution to tumorigenesis remains unclear.

Results

We first investigated the effects of loss of SREBP function in non-transformed cells. Combined ablation of SREBP1 and SREBP2 by siRNA-mediated gene silencing or chemical inhibition of SREBP activation induced endoplasmic reticulum (ER)-stress and engaged the unfolded protein response (UPR) pathway, specifically under lipoprotein-deplete conditions in human retinal pigment epithelial cells. Induction of ER-stress led to inhibition of protein synthesis through increased phosphorylation of eIF2α. This demonstrates for the first time the importance of SREBP in the coordination of lipid and protein biosynthesis, two processes that are essential for cell growth and proliferation. SREBP ablation caused major changes in lipid composition characterized by a loss of mono- and poly-unsaturated lipids and induced accumulation of reactive oxygen species (ROS) and apoptosis. Alterations in lipid composition and increased ROS levels, rather than overall changes to lipid synthesis rate, were required for ER-stress induction.

Next, we analyzed the effect of SREBP ablation in a panel of cancer cell lines. Importantly, induction of apoptosis following SREBP depletion was restricted to lipoprotein-deplete conditions. U87 glioblastoma cells were highly susceptible to silencing of either SREBP isoform, and apoptosis induced by SREBP1 depletion in these cells was rescued by antioxidants or by restoring the levels of mono-unsaturated fatty acids. Moreover, silencing of SREBP1 induced ER-stress in U87 cells in lipoprotein-deplete conditions and prevented tumor growth in a xenograft model.

Conclusions

Taken together, these results demonstrate that regulation of lipid composition by SREBP is essential to maintain the balance between protein and lipid biosynthesis downstream of Akt and to prevent resultant ER-stress and cell death. Regulation of lipid metabolism by the Akt/mTORC1 signaling axis is required for the growth and survival of cancer cells.

Gene expression phenotypes for lipid metabolism and intramuscular fat in skeletal muscle of cattle

Gene expression phenotypes were evaluated for intramuscular fat (IMF) in bovine skeletal muscle as an alternative to traditional estimates of IMF%. Gene expression data from a time course of LM development in high- and low-marbling Bos taurus cattle crosses were compared to identify genes involved in intramuscular adipocyte lipid metabolism with developmentally similar gene expression profiles. Three sets of genes were identified: triacylglyceride (TAG) synthesis and storage, fatty acid (FA) synthesis, and PPARγ-related genes. In an independent analysis in the LM of 48 Bos indicus cattle, TAG and FA gene sets were enriched in the top 100 genes of which expression was most correlated with IMF% (P = 1.2 × 10(-24) and 3.5 × 10(-9), respectively). In general, genes encoding enzymes involved in the synthesis of FA and TAG in the intramuscular adipocytes were present in the top 100 genes. In B. indicus, effects of a steroid hormone growth promotant (HGP), 2 experimental sites [New South Wales (NSW) and Western Australia (WA)], and 3 tenderness genotypes on the expression levels of genes in the TAG gene set and the correlation of gene expression with IMF% were investigated. Although correlation between expression of 12 individual TAG genes and IMF% was observed in HGP-treated animals in both experimental sites (mean r = 0.43), correlation was not observed for untreated animals at the NSW site (mean r = -0.07, P < 3 × 10(-6)). However, TAG genes showed an average 1.6-fold (P < 0.0004) reduction in expression in the LM of HGP-treated cattle relative to untreated cattle, an effect consistent across both experimental sites. Cattle possessing the favored tenderness calpain 1 and 3 and calpastatin alleles exhibited a greater (P = 0.008) reduction in expression in NSW (1.8-fold reduction, P = 0.0002) compared with WA (1.2-fold reduction, P = 0.03). Tenderness genotype had no impact (P > 0.05) on the correlation of TAG genes with IMF%. In general, the interactions among genotype, treatment and location, and TAG gene set gene expression were consistent with the interactions among the same factors and IMF% detected using 255 animals, of which the 48 in this study were a subset. Thus, the TAG gene set constitutes a gene expression phenotype able to predict effects of different genotypes and treatments on IMF% using much smaller groups than current approaches, even in animals with very low IMF%.

Ethanol administration exacerbates the abnormalities in hepatic lipid oxidation in genetically obese mice

Alcohol consumption synergistically increases the risk and severity of liver damage in obese patients. To gain insight into cellular or molecular mechanisms underlying the development of fatty liver caused by ethanol-obesity synergism, we have carried out animal experiments that examine the effects of ethanol administration in genetically obese mice. Lean wild-type (WT) and obese (ob/ob) mice were subjected to ethanol feeding for 4 wk using a modified Lieber-DeCarli diet. After ethanol feeding, the ob/ob mice displayed much more pronounced changes in terms of liver steatosis and elevated plasma levels of alanine aminotransferase and aspartate aminotransferase, indicators of liver injury, compared with control mice. Mechanistic studies showed that ethanol feeding augmented the impairment of hepatic sirtuin 1 (SIRT1)-AMP-activated kinase (AMPK) signaling in the ob/ob mice. Moreover, the impairment of SIRT1-AMPK signaling was closely associated with altered hepatic functional activity of peroxisome proliferator-activated receptor γ coactivator-α and lipin-1, two vital downstream lipid regulators, which ultimately contributed to aggravated fatty liver observed in ethanol-fed ob/ob mice. Taken together, our novel findings suggest that ethanol administration to obese mice exacerbates fatty liver via impairment of the hepatic lipid metabolism pathways mediated largely by a central signaling system, the SIRT1-AMPK axis.

Mice with an adipocyte-specific lipin 1 separation-of-function allele reveal unexpected roles for phosphatidic acid in metabolic regulation

Lipin 1 is a coregulator of DNA-bound transcription factors and a phosphatidic acid (PA) phosphatase (PAP) enzyme that catalyzes a critical step in the synthesis of glycerophospholipids. Lipin 1 is highly expressed in adipocytes, and constitutive loss of lipin 1 blocks adipocyte differentiation; however, the effects of Lpin1 deficiency in differentiated adipocytes are unknown. Here we report that adipocyte-specific Lpin1 gene recombination unexpectedly resulted in expression of a truncated lipin 1 protein lacking PAP activity but retaining transcriptional regulatory function. Loss of lipin 1-mediated PAP activity in adipocytes led to reduced glyceride synthesis and increased PA content. Characterization of the deficient mice also revealed that lipin 1 normally modulates cAMP-dependent signaling through protein kinase A to control lipolysis by metabolizing PA, which is an allosteric activator of phosphodiesterase 4 and the molecular target of rapamycin. Consistent with these findings, lipin 1 expression was significantly related to adipose tissue lipolytic rates and protein kinase A signaling in adipose tissue of obese human subjects. Taken together, our findings identify lipin 1 as a reciprocal regulator of triglyceride synthesis and hydrolysis in adipocytes, and suggest that regulation of lipolysis by lipin 1 is mediated by PA-dependent modulation of phosphodiesterase 4.

Mouse lipin-1 and lipin-2 cooperate to maintain glycerolipid homeostasis in liver and aging cerebellum

The three lipin phosphatidate phosphatase (PAP) enzymes catalyze a step in glycerolipid biosynthesis, the conversion of phosphatidate to diacylglycerol. Lipin-1 is critical for lipid synthesis and homeostasis in adipose tissue, liver, muscle, and peripheral nerves. Little is known about the physiological role of lipin-2, the predominant lipin protein present in liver and the deficient gene product in the rare disorder Majeed syndrome. By using lipin-2-deficient mice, we uncovered a functional relationship between lipin-1 and lipin-2 that operates in a tissue-specific and age-dependent manner. In liver, lipin-2 deficiency led to a compensatory increase in hepatic lipin-1 protein and elevated PAP activity, which maintained lipid homeostasis under basal conditions, but led to diet-induced hepatic triglyceride accumulation. As lipin-2-deficient mice aged, they developed ataxia and impaired balance. This was associated with the combination of lipin-2 deficiency and an age-dependent reduction in cerebellar lipin-1 levels, resulting in altered cerebellar phospholipid composition. Similar to patients with Majeed syndrome, lipin-2-deficient mice developed anemia, but did not show evidence of osteomyelitis, suggesting that additional environmental or genetic components contribute to the bone abnormalities observed in patients. Combined lipin-1 and lipin-2 deficiency caused embryonic lethality. Our results reveal functional interactions between members of the lipin family in vivo, and a unique role for lipin-2 in central nervous system biology that may be particularly important with advancing age. Additionally, as has been observed in mice and humans with lipin-1 deficiency, the pathophysiology in lipin-2 deficiency is associated with dysregulation of lipid intermediates.

Phosphatidate phosphatase, a key regulator of lipid homeostasis

Yeast Pah1p phosphatidate phosphatase (PAP) catalyzes the penultimate step in the synthesis of triacylglycerol. PAP plays a crucial role in lipid homeostasis by controlling the relative proportions of its substrate phosphatidate and its product diacylglycerol. The cellular amounts of these lipid intermediates influence the synthesis of triacylglycerol and the pathways by which membrane phospholipids are synthesized. Physiological functions affected by PAP activity include phospholipid synthesis gene expression, nuclear/endoplasmic reticulum membrane growth, lipid droplet formation, and vacuole homeostasis and fusion. Yeast lacking Pah1p PAP activity are acutely sensitive to fatty acid-induced toxicity and exhibit respiratory deficiency. PAP is distinguished in its cellular location, catalytic mechanism, and physiological functions from Dpp1p and Lpp1p lipid phosphate phosphatases that utilize a variety of substrates that include phosphatidate. Phosphorylation/dephosphorylation is a major mechanism by which Pah1p PAP activity is regulated. Pah1p is phosphorylated by cytosolic-associated Pho85p-Pho80p, Cdc28p-cyclin B, and protein kinase A and is dephosphorylated by the endoplasmic reticulum-associated Nem1p-Spo7p phosphatase. The dephosphorylation of Pah1p stimulates PAP activity and facilitates the association with the membrane/phosphatidate allowing for its reaction and triacylglycerol synthesis. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Synphilin-1 alters metabolic homeostasis in a novel Drosophila obesity model

AIMS

The pathogenesis of obesity remains incompletely understood. Drosophila have conserved neuroendocrine and digestion systems with human and become an excellent system for studying energy homeostasis. Here, we reported a novel obesity Drosophila model, in which expression of human protein, synphilin-1 (SP1), in neurons fosters positive energy balance.

SUBJECTS AND METHODS

To further understand the actions of SP1 in energy balance control, the upstream activation sequence UAS/GAL4 system was used to generate human SP1 transgenic Drosophila. We characterized a human SP1 transgenic Drosophila by assessing SP1 expression, fat lipid deposition, food intake and fly locomotor activity to determine the major behavioral changes and their consequences in the development of the obesity-like phenotype.

RESULTS

Overexpression of SP1 in neurons, but not peripheral cells, increased the body weight of flies compared with that of non-transgenic controls. SP1 increased food intake but did not affect locomotor activity. SP1 increased the levels of triacylglycerol, and the size of fat body cells and lipid droplets, indicating that SP1 increased lipid-fat disposition. Survival studies showed that SP1 transgenic flies were more resistant to food deprivation. SP1 regulated lipin gene expression that may participate in SP1-induced fat deposition and starvation resistance.

CONCLUSION

These studies demonstrate that SP1 expression affects energy homeostasis in ways that enhance positive energy balance and provide a useful obesity model for future pathogenesis and therapeutic studies.

The dynamic roles of intracellular lipid droplets: from archaea to mammals

During the past decade, there has been a paradigm shift in our understanding of the roles of intracellular lipid droplets (LDs). New genetic, biochemical and imaging technologies have underpinned these advances, which are revealing much new information about these dynamic organelles. This review takes a comparative approach by examining recent work on LDs across the whole range of biological organisms from archaea and bacteria, through yeast and Drosophila to mammals, including humans. LDs probably evolved originally in microorganisms as temporary stores of excess dietary lipid that was surplus to the immediate requirements of membrane formation/turnover. LDs then acquired roles as long-term carbon stores that enabled organisms to survive episodic lack of nutrients. In multicellular organisms, LDs went on to acquire numerous additional roles including cell- and organism-level lipid homeostasis, protein sequestration, membrane trafficking and signalling. Many pathogens of plants and animals subvert their host LD metabolism as part of their infection process. Finally, malfunctions in LDs and associated proteins are implicated in several degenerative diseases of modern humans, among the most serious of which is the increasingly prevalent constellation of pathologies, such as obesity and insulin resistance, which is associated with metabolic syndrome.

Hypoxia causes triglyceride accumulation by HIF-1-mediated stimulation of lipin 1 expression

Adaptation to hypoxia involves hypoxia-inducible transcription factors (HIFs) and requires reprogramming of cellular metabolism that is essential during both physiological and pathological processes. In contrast to the established role of HIF-1 in glucose metabolism, the involvement of HIFs and the molecular mechanisms concerning the effects of hypoxia on lipid metabolism are poorly characterized. Here, we report that exposure of human cells to hypoxia causes accumulation of triglycerides and lipid droplets. This is accompanied by induction of lipin 1, a phosphatidate phosphatase isoform that catalyzes the penultimate step in triglyceride biosynthesis, whereas lipin 2 remains unaffected. Hypoxic upregulation of lipin 1 expression involves predominantly HIF-1, which binds to a single distal hypoxia-responsive element in the lipin 1 gene promoter and causes its activation under low oxygen conditions. Accumulation of hypoxic triglycerides or lipid droplets can be blocked by siRNA-mediated silencing of lipin 1 expression or kaempferol-mediated inhibition of HIF-1. We conclude that direct control of lipin 1 transcription by HIF-1 is an important regulatory feature of lipid metabolism and its adaptation to hypoxia.

MicroRNA-217 promotes ethanol-induced fat accumulation in hepatocytes by down-regulating SIRT1

Ethanol-mediated inhibition of hepatic sirtuin 1 (SIRT1) plays a crucial role in the pathogenesis of alcoholic fatty liver disease. Here, we investigated the underlying mechanisms of this inhibition by identifying a new hepatic target of ethanol action, microRNA-217 (miR-217). The role of miR-217 in the regulation of the effects of ethanol was investigated in cultured mouse AML-12 hepatocytes and in the livers of chronically ethanol-fed mice. In AML-12 hepatocytes and in mouse livers, chronic ethanol exposure drastically and specifically induced miR-217 levels and caused excess fat accumulation. Further studies revealed that overexpression of miR-217 in AML-12 cells promoted ethanol-mediated impairments of SIRT1 and SIRT1-regulated genes encoding lipogenic or fatty acid oxidation enzymes. More importantly, miR-217 impairs functions of lipin-1, a vital lipid regulator, in hepatocytes. Taken together, our novel findings suggest that miR-217 is a specific target of ethanol action in the liver and may present as a potential therapeutic target for treating human alcoholic fatty liver disease.

Regulation of hepatic lipin-1 by ethanol: role of AMP-activated protein kinase/sterol regulatory element-binding protein 1 signaling in mice

Lipin-1 is a protein that exhibits dual functions as a phosphatidic acid phosphohydrolase enzyme in the triglyceride synthesis pathways and a transcriptional coregulator. Our previous studies have shown that ethanol causes fatty liver by activation of sterol regulatory element-binding protein 1 (SREBP-1) and inhibition of hepatic AMP-activated protein kinase (AMPK) in mice. Here, we tested the hypothesis that AMPK-SREBP-1 signaling may be involved in ethanol-mediated up-regulation of lipin-1 gene expression. The effects of ethanol on lipin-1 were investigated in cultured hepatic cells and in the livers of chronic ethanol-fed mice. Ethanol exposure robustly induced activity of a mouse lipin-1 promoter, promoted cytoplasmic localization of lipin-1, and caused excess lipid accumulation, both in cultured hepatic cells and in mouse livers. Mechanistic studies showed that ethanol-mediated induction of lipin-1 gene expression was inhibited by a known activator of AMPK or overexpression of a constitutively active form of AMPK. Importantly, overexpression of the processed nuclear form of SREBP-1c abolished the ability of 5-aminoimidazole-4-carboxamide ribonucleoside to suppress ethanol-mediated induction of lipin-1 gene-expression level. Chromatin immunoprecipitation assays further revealed that ethanol exposure significantly increased the association of acetylated histone H3 at lysine 9 with the SRE-containing region in the promoter of the lipin-1 gene.

CONCLUSION

In conclusion, ethanol-induced up-regulation of lipin-1 gene expression is mediated through inhibition of AMPK and activation of SREBP-1.

Phosphatidate phosphatase activity plays key role in protection against fatty acid-induced toxicity in yeast

The PAH1-encoded phosphatidate (PA) phosphatase in Saccharomyces cerevisiae is a pivotal enzyme that produces diacylglycerol for the synthesis of triacylglycerol (TAG) and simultaneously controls the level of PA used for phospholipid synthesis. Quantitative lipid analysis showed that the pah1Δ mutation caused a reduction in TAG mass and an elevation in the mass of phospholipids and free fatty acids, changes that were more pronounced in the stationary phase. The levels of unsaturated fatty acids in the pah1Δ mutant were unaltered, although the ratio of palmitoleic acid to oleic acid was increased with a similar change in the fatty acid composition of phospholipids. The pah1Δ mutant exhibited classic hallmarks of apoptosis in stationary phase and a marked reduction in the quantity of cytoplasmic lipid droplets. Cells lacking PA phosphatase were sensitive to exogenous fatty acids in the order of toxicity palmitoleic acid > oleic acid > palmitic acid. In contrast, the growth of wild type cells was not inhibited by fatty acid supplementation. In addition, wild type cells supplemented with palmitoleic acid exhibited an induction in PA phosphatase activity and an increase in TAG synthesis. Deletion of the DGK1-encoded diacylglycerol kinase, which counteracts PA phosphatase in controlling PA content, suppressed the defect in lipid droplet formation in the pah1Δ mutant. However, the sensitivity of the pah1Δ mutant to palmitoleic acid was not rescued by the dgk1Δ mutation. Overall, these findings indicate a key role of PA phosphatase in TAG synthesis for protection against fatty acid-induced toxicity.